Scintillation noise reduction in free-space optical communications

Abstract

In some embodiments, an optical communication system may include a light source, a modulator, and an optical receiver. The light source may be configured to generate a beam including a series of optical pulses. The optical receiver may have a detection window duration of 1 nanosecond or less. When a first pulse passes through a variable refractive index medium, photons of the first pulse may be refracted and travel along different optical ray paths to reach the optical receiver according to a time distribution curve. The full width at half maximum (FWHM) value of the time distribution curve may be greater than the coherence time value of the first pulse, and the detection window of the optical receiver may be greater than the FWHM value of the time distribution curve.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This patent application is a PCT international application, which claims the benefit of U.S. Non-Provisional Ser. No. 17 / 932,364, filed September 15, 2022. The above application is incorporated herein by reference.

[0002] The subject matter described herein relates to free-space optical (FSO) wireless transmission, including optical communications, remote sensing, laser ranging, power beaming, and the like, and more particularly to optical transmission efficiency improvements that can be realized for wavelength propagation using ultrashort pulsed laser (USPL) sources for beam propagation through variable refractive index media such as the Earth's atmosphere. [Background technology]

[0003] FSO communications have the potential to significantly increase data throughput, reduce costs, and increase access to high-speed internet and other communications technologies. However, to date, FSO communications systems have had limited operational success due to atmospheric interference, which reduces the distance data can be transmitted optically and introduces bit errors. Meanwhile, alternative communications technologies, such as radio frequency and microwave communications, face significant spectrum limitations and cannot be used to provide enough data to meet demand. Currently available optical systems cannot produce data transmission results that are sufficiently accurate, reliable, and usable to reliably offload communications demand from these radio frequency and microwave systems and improve data transmission and access, nor can currently available optical systems transmit data over long distances.

[0004] Therefore, there is a demand for optical communication systems that can achieve long-distance data transmission with high reliability and high availability, as well as optical communication systems that can reliably transmit data over long distances of more than half a mile. Summary of the Invention [Problem to be solved by the invention]

[0005] The following presents a simplified summary in order to provide a basic understanding of some aspects described herein. This summary is not an extensive overview of the claimed subject matter. It is not intended to identify key or critical elements of the claimed subject matter, nor is it intended to delineate the scope thereof. [Means for solving the problem]

[0006] In some embodiments, an optical communication system for optically transmitting data through a variable-refractive-index medium may include a light source, a modulator, and a light receiver. The light source may be configured to generate a beam including a series of light pulses, each having a duration of less than 100 picoseconds. The modulator may be configured to modulate the series of light pulses in response to a data transmission signal, thereby encoding the transmitted data into the series of light pulses. The light receiver may have a detection window duration of less than 1 nanosecond and a detection threshold. The light receiver may be configured to indicate whether the light energy received during a given detection window is greater than the detection threshold. The series of light pulses may include a first light pulse having a coherence length of less than 400 microns. As the first pulse passes through the variable-refractive-index medium, photons of the first pulse may be refracted and travel along different light ray paths of different lengths to the light receiver, and the photons of the first pulse may arrive at the light receiver according to a time distribution curve that depends, at least in part, on the duration of the first pulse and the lengths of the different light ray paths taken by the photons of the first pulse to the light receiver. A full width at half maximum (FWHM) value of the time distribution curve may be at least three times as large as a coherence time value equal to the coherence length of the first pulse divided by the speed of light through the variable refractive medium, and a detection window of the receiver may be at least six times as large as the FWHM value of the time distribution curve.

[0007] In some embodiments, a laser ranging system may include a light source and a light receiver. The light source may be configured to generate a beam including a series of light pulses, each having a duration of less than 100 picoseconds. The light receiver may have a detection window duration and a detection threshold of less than 1 nanosecond. The light receiver may be configured to indicate whether the light energy received during a given detection window is greater than the detection threshold. The series of light pulses may include a first light pulse having a coherence length of less than 400 microns. As the first pulse passes through the variable refractive index medium, photons of the first pulse may refract and travel along different light ray paths of different lengths to the light receiver. The photons of the first pulse may arrive at the light receiver according to a time distribution curve that depends, at least in part, on the duration of the first pulse and the lengths of the different light ray paths followed by the photons of the first pulse to the light receiver. A full width at half maximum (FWHM) value of the time distribution curve may be at least three times as large as a coherence time value equal to a coherence length of the first pulse divided by the speed of light through the variably refractive medium, and a detection window of the receiver may be at least six times as large as the FWHM value of the time distribution curve. The laser ranging system may be configured to transmit a series of light pulses toward a surface, receive at least a portion of the series of light pulses reflected by the surface, and determine a distance of at least a portion of the surface from the laser ranging system based on the time-of-flight of the received portion of the series of light pulses.

[0008] Further variations included within the systems and methods are described below in the detailed description of the invention.

[0009] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain aspects of the subject matter disclosed herein and, together with the description, serve to explain certain principles related to the disclosed implementations. [Brief explanation of the drawings]

[0010] [Figure 1]1 illustrates an example of an optical communications platform that includes free-space coupling of a USPL source as the optical source for transmission to a remote optical receiving terminal. [Figure 2] 1 illustrates an example of an optical communications platform that includes fiber coupling of a USPL source as the light source for transmission to a remote optical receiving terminal. [Figure 3] 1 illustrates an example optical communications platform that includes fiber coupling of a USPL source to an external modulator for transmission to a remote optical receiving terminal. [Figure 4] 1 illustrates an example optical communications platform that includes fiber coupling of a USPL source to an external modulator via a fiber medium for transmission to a remote optical receiving terminal. [Figure 5] An example of a transmit and / or receive element is shown, which can be of either type, hyperbolic mirror manufacturing technology or a traditional Newtonian design. [Figure 6] 1 illustrates an example of an optical fiber amplifying element identified and used to increase optical transmit launch power for transmission to a remote optical receiving terminal. [Figure 7] 1 shows an example of a USPL laser device fiber-coupled to an external modulator for transmission in a point-to-point configuration to a remote optical receiving terminal. [Figure 8] 1 shows an example of a USPL laser device fiber-coupled to an external modulator for transport in a point-to-multipoint configuration. [Figure 9] An example of the use of a USPL source acting as a tracking and alignment (pointing) beacon source is shown. [Figure 10] An example of a USPL laser source that is polarization multiplexed onto a transmitted optical signal to provide polarization multiplexed USP-FSO (PM-USP-FSO) functionality is shown. [Figure 11A] An example of a USPL-FSO transceiver used for line-of-sight laser communications applications is shown. [Figure 11B] 1 shows an example of a USPL-FSO transceiver used for use in non-line-of-sight laser communications applications. [Figure 12] An example is shown in which light, including light from a forward propagating data signal, is backscattered by interaction with airborne particles under investigation. [Figure 13] We present examples of USPL laser light sources as optical receiving techniques for improving detection sensitivity, and USPL laser light sources and optical receiving techniques for improving detection sensitivity. [Figure 14] 1 illustrates an example of a USPL-FSO transceiver utilized and operated over the infrared wavelength range, optionally including light from a data signal as a rangefinder and spotting device for target identification purposes. [Figure 15] An example of a USPL pulse multiplier consistent with the implementation of the current subject matter is shown. [Figure 16] 1 shows another example of a device for generating a high pulse rate USPL light stream consistent with the implementation of the current subject matter. [Figure 17] 1 illustrates another example of an optical device that generates a USPL RZ data stream from conventional transport networking elements. [Figure 18] An example implementation of a USPL pulse multiplier for generating a 10x TDM type signaling system to give an output of 100 Gbps is presented. [Figure 19] We present an implementation of another type of USPL pulse multiplier to extend the pulse repetition rate for use in high-capacity networks. [Figure 20] We present an implementation of another type of USPL pulse multiplier to extend the pulse repetition rate for use in high-capacity networks. [Figure 21] An example of an active mode-locked linear fiber laser with a feedback regeneration system is presented, which consists of a fiber reflector (FR), a wavelength division multiplexer (WDM), an erbium-doped fiber (EDF), an optical coupler (OC), a photodetector (PD), a phase-locked loop (PLL), and a Mach-Zehnder modulator (MZM). [Figure 22]We present an example of a passively mode-locked linear fiber laser using a carbon nanotube saturable absorber, which is a fiber reflector (FR), a wavelength division multiplexer (WDM), an erbium-doped fiber (EDF), an optical coupler (OC), and a saturable absorber (SA). [Figure 23] We present an example of a passively mode-locked linear fiber laser using a carbon nanotube saturable absorber, which is a fiber reflector (FR), a wavelength division multiplexer (WDM), an erbium-doped fiber (EDF), an optical coupler (OC), and a saturable absorber (SA). [Figure 24] This shows an example of a time delay stabilization mechanism, which is an optical coupler (OCin, OCout), photodetector (PDin, PDout), high pass filter (HPF), low pass filter (LPF), phase locked loop (PLL), phase comparator (PC), frequency divider (1 / N), clock data recovery system (CDR), piezoelectric actuator (PZ1...PZN), and summing op amp, used to stabilize the optical pulse-to-pulse relationship generated from a USPL source. [Figure 25] 25A and 25B include schematic diagrams of an example control mechanism that utilizes an idealized PZ actuator to stabilize the output frequency of a TDM source. [Figure 26] We present an example of time-domain multiplexing (TDM) where parallel time delay channels are used to multiplex pulse trains, with the delay channels being "consistent" with each other. (The frequency of the output multiplexed pulse train is ideally as insensitive as possible to environmental changes, so that a feedback loop control system can correct the delay units for any fluctuations that compromise the stability of the output repetition rate; feedback can be provided through interconnections to a neural network.) [Figure 27] An example of the use of a fiber-based collimator in conjunction with a piezoelectric transducer to control individual MFC circuits is shown. [Figure 28] An example of the timing of TDM chips from a USPL modulated source to provide a multiplier photonic chip to Terabits per second (or higher) is shown. [Figure 29] We present an example of the timing of TDM chips from a USPL modulated source to provide a Terabit / second (or higher) multiplier photonic chip operating in a WDM configuration. [Figure 30] We present an example of the construction of a computer-aided system that can simultaneously stabilize the repetition rate of the cavity using a synchronous self-regeneration mechanism and control the pulse width of an all-fiber modelocked laser using recursive linear polarization adjustment, and also provide tunability of the repetition rate and pulse width. [Figure 31] We present an example of a modified pulse interleaving scheme with pulse multiplication techniques in which a lower repetition rate pulse train from a well-characterized, well-mode-locked laser can be coupled into an integrated optical directional coupler, where a well-determined portion of the pulses is tapped off, "recirculated" in an optical loop with an optical delay equal to the desired inter-pulse spacing in the output pulse train, and recombined into the output of the directional coupler. [Figure 32] 1 is a process flow chart illustrating features of a method consistent with implementation of the current subject matter. [Figure 33] 1 is another process flow diagram illustrating method features consistent with implementations of the current subject matter. [Figure 34] 1 is another process flow diagram illustrating method features consistent with implementations of the current subject matter. [Figure 35A] 1 illustrates exemplary nodes that can be used to transmit information. [Figure 35B] 1 illustrates exemplary nodes that may be used to receive information. [Figure 36] An exemplary configuration is shown in which data is transmitted over optical communication distance D from a first communication network 3542 to a second communication network 3544 using a transmitting node 3510 and a receiving node 3530 . [Figure 37] An exemplary beam is shown traveling through a constant refractive medium over an optical communication distance D, for example, one mile. [Figure 38] 1 provides a diagrammatic representation of photons in a beam traveling through a variably refractive medium. [Figure 39] 1 shows a diagrammatic representation of pulse broadening as the pulse travels over an optical communication distance. [Figure 40] 4 shows an exemplary time distribution curve of a short duration pulse 4010 that has traveled a significant distance through a variable refractive medium and has been spread out in time. [Figure 41] 1 shows a diagrammatic representation of a light pulse arriving at the detection window of a receiver. [Figure 42] An example of test data received over a 1-mile optical communication distance is shown below. [Figure 43] 1 illustrates an exemplary ranging node that can be used to detect objects or surfaces and determine the location of those objects relative to the node. DETAILED DESCRIPTION OF THE INVENTION

[0011] While aspects of the presently disclosed subject matter may be embodied in a variety of forms, the following description and accompanying drawings are intended to disclose merely some of these forms as illustrative examples of the subject matter, and therefore, the presently disclosed subject matter is not intended to be limited to the forms or embodiments so described and illustrated.

[0012] FIG. 1 illustrates an example of an optical communications platform 100 configured to use a USPL source as an optical source for transmission. As shown in FIG. 1, the USPL source 102 may be directly modulated by an external source element 104. Optical power from the USPL source 102 may be coupled across free space 110 to a transmission element 106, optionally via an optical telescope. The transmission element 106 may optionally include optical components formed by hyperbolic mirror fabrication techniques, traditional Newtonian designs, or the like. A reciprocating receive telescope in the receiving system may provide optical reception. Consistent with implementations of the present subject matter, each optical transmission platform may be designed to operate as a bidirectional unit. That is, the transmission element 106 of the optical communications platform 100 may also function as a receiving element. In general, unless expressly stated otherwise, a transmission element 106 as described may also function as a receiving element, and vice versa. An optical element performing both transmission and receiving functions may be referred to herein as an optical transceiver.

[0013] FIG. 2 illustrates an example optical communications system 200 including the optical communications platform 100 of FIG. 1. Also illustrated in FIG. 2 is a second, complementary receiving element 204, which may be a receiving telescope located remotely from the transmitting element 106. As noted above, both the transmitting element 106 and the receiving element 204 may be bidirectional, each functioning as both the transmitting element 106 and the receiving element 204 depending on the instantaneous direction of data transmission in the optical communications system 200. This functionality applies to all transmitting and receiving elements in this disclosure unless otherwise specified. Either or both the transmitting element 106 and the receiving element 204 may be an optical telescope or other device for transmitting and receiving optical information.

[0014] 3 illustrates an example of an optical communications platform 300 for using a USPL source 102 fiber coupled to an external modulator 302 via a fiber medium 304 and connected to a transmission element 106 via an additional transmission medium 306, which may be a fiber medium, a free space connection, etc. The USPL source 102 may be externally modulated by the external modulator 302 such that optical power from the USPL source 102 is fiber coupled to the transmission element 106 or processed via an equivalent optical telescope.

[0015] Figure 4 illustrates an example of an optical communications system 400 that includes the optical communications platform 300 of Figure 3. Also illustrated in Figure 4 is a second, complementary receive telescope 204, which may be a receive telescope located at a distance from the transmit element 106, as described above in connection with Figure 2.

[0016] Figure 5 shows an example of an optical communications architecture 500. The architecture 500 of Figure 5 may include the elements of Figure 4 and may further include a first communications network 502 connected to the first optical communications platform 300. The receiving element 204 is part of a second optical communications platform 504, which may optionally include components similar to those of the first optical communications platform 300. A second communications network 506 may be connected to the second optical communications platform 504 such that data is transmitted optically between the transmitting element 106 and the receiving element 204 or passes between the first and second communications networks 502, 506, each of which may include one or more optical and electrical networking functions.

[0017] FIG. 6 illustrates an example of an optical communications system 600. As part of an optical communications platform 602, a USPL source 102 is fiber-coupled to an external modulator 302, e.g., via an optical fiber 202 or other transmission medium. Light from the USPL source 102 propagates through a transmission element 106 in a manner similar to that described above. An optical amplifier element 604, which may optionally be an optical fiber amplifier element, may be used to increase the optical transmission launch power and may optionally be located between the external modulator 302 and the transmission element 106 and connected to one or both via an additional transmission medium 306, which may optionally be a fiber medium, a free-space connection, or the like. Also illustrated in FIG. 6 is a second, complementary receiving element 204 located a distance away from the optical communications platform 602. It will be readily understood that the second optical communications platform 504, including the receiving element 204, may also include an optical amplifier element 604. First and second communications networks 502, 506 may be connected to the two optical communications platforms 602, 504, respectively.

[0018] 7 illustrates an example of an optical communications system 700. The optical communications platform 602 illustrated in FIG. 6 can be in communication with a second optical communications platform 702, which in this implementation can include a receiving element 204 and an optical pre-amplifier 704. Other components similar to those illustrated in the optical communications platform 602 can also be included in the second optical communications platform 702, although not shown in FIG. 7. It will be understood that a bidirectional optical communications platform can include both an optical pre-amplifier 704 for amplifying a received optical signal and an optical amplification element 604 for boosting a transmitted optical signal.

[0019] Consistent with the implementation depicted in FIG. 7 and other implementations of the present subject matter, optical amplification (e.g., either or both of the optical amplification element 604 or the optical pre-amplifier 704) is included to enhance the optical budget for the data link between the transmitting element 106 and the receiving element 204 (or vice versa), for example, using one or more of an erbium-doped fiber amplifier (EDFA), a high-power erbium-ytterbium-doped fiber amplifier (Er / Yb-DFA), or equivalent, including, but not limited to, a semiconductor-optical amplifier (SOA).

[0020] FIG. 8 illustrates an example of an optical communications system 800. The optical communications platform 602 illustrated in FIG. 6 can be in communication with a second optical communications platform 802, which, in this implementation, can include a receiving element 204 and an optical pre-amplifier 704, similar to that illustrated in FIG. 7. As illustrated in FIG. 8, the second optical communications platform 802 can further include an optical receiver circuit 804 that can receive electrically recovered data received at the receiving element 204 and amplified by the optical pre-amplifier. Multiple clock sources 806 can interface to multiple remote multipoint network connections with multiple communications networks 810 as needed. Similarly, a complementary set of clock sources and multiple communications networks can operate in conjunction with the optical communications platform 602 (e.g., instead of the single communications network 502 depicted in FIG. 8).

[0021] FIG. 9 illustrates an example optical communications system 900. The optical communications platform 902, which may feature elements similar to those of the optical communications platform 602 first discussed herein with reference to FIG. 6, may also include an additional USPL source 904 that functions as a tracking and alignment (pointing) beacon source. The second optical communications platform 906 may also include an additional USPL source 910 that functions as a tracking and alignment (pointing) beacon source. The tracking and alignment (pointing) beacon sources 904, 910 may optionally originate from available communications sources used in data transfer transmissions or may be provided by separate, dedicated USPL sources. Furthermore, each USPL beacon source 904, 910 may comprise an in-band or out-of-band source, allowing for the benefit of available optical amplification sources or from dedicated optical amplification resources.

[0022] FIG. 10 illustrates an example of an FSO communications system 1000 including a dual-polarization USPL-FSO optical data link platform 1001, in which USPL sources are polarization multiplexed into transmitted optical signals, thereby providing polarization-multiplexed USP-FSO (PM-USP-FSO) functionality. Two USPL sources 102 and 1002 are fiber-coupled to either directly modulated or externally modulated modulation components 1004 and 1006, respectively. Each modulated signal is optically amplified by optical amplifier components 1010 and 1012, followed by adjustment of the optical polarization state using polarization components 1014 and 1016. The polarization state signals are fiber-coupled to a polarization-dependent multiplexer (PDM) component 1020 for interfacing with an optical launch platform component 1022, which may be similar to the transmission element 106 described above. The PDM 1020 multiplexes the light of different polarization states into a single pulse train for transmission via the optical launch platform component 1022. The USPL optical beacon 904 can be included, for example, in conjunction with or to operate in conjunction with a second USPL optical beacon 906 in a receiving platform 1024, which can include a receiving element 204 similar to that described above, to provide functionality similar to that discussed above with reference to FIG. 9. As previously mentioned, the receiving element 204, as well as other features and components of the receiving platform 1024, can generally support transmission functions such that a bidirectional link is established. The received signal recovered by the receiving element 204 can provide an optical signal interfaced to an appropriate polarization-dependent demultiplexer 1026, which can provide two signals for further optical amplification using amplification elements 1030, 1032. Each optically amplified signal provided by the amplification elements 1030, 1032 can be interfaced to an appropriate optical network 1034, 1036 for network use.

[0023] Figure 11A illustrates an example system 1100 in which a USPL-FSO transceiver can be utilized for line-of-sight optical communications (e.g., "lasercom") applications, while Figure 11B illustrates an example system 1150 in which a USPL-FSO transceiver can be utilized for non-line-of-sight lasercom applications. An advantage over some implementations of the present subject matter can be realized by scattering of the optical signal sent from the transmitting element as the transmitted light passes through the atmosphere. This scattering enables the use of non-line-of-sight communications. Furthermore, radios used in such communications systems can operate in the solar-blind portion of the UV-C band, emitting light at wavelengths between 200 and 280 nm. In this band, solar radiation is strongly attenuated by the Earth's atmosphere as it propagates through the environment. This means that the amount of background noise emitted decreases dramatically closer to the Earth, enabling low-power communications link operation. However, environmental factors such as oxygen, ozone, and water can weaken or interrupt communications broadcasts, potentially limiting their use to short-range applications.

[0024] As ultraviolet light spreads through the atmosphere, it is typically strongly scattered into various signal paths. Signal scattering is essential for UV systems operating in non-line-of-sight conditions, and communication performance can be highly dependent on the pointing of the transmit beam and the field of view of the receiver. A line-of-sight configuration 1100, such as that shown in FIG. 11A, can have a different bandwidth size than a non-line-of-sight configuration 1150, such as that shown in FIG. 11B. Ultraviolet communication can be more dependent on the transmitter beam position and the field of view of the receiver. As a result, refining the pointing vertex angle can be advantageous, for example, by experimenting with auxiliary devices to enhance the UV-C signal.

[0025] FIG. 12 illustrates an example of a remote sensing system 1200 in which a USPL source 102 is fiber-coupled to an optical emitting element 1202 capable of transmitting and receiving optical signals via an optical fiber component 202. A portion of the forward-propagating light, including light from a data signal passing through the optical emitting element 1202, is backscattered by interaction with airborne particles of interest. The optical backscattered signal is detected via the optical emitting element 1202 or a similar receiving aperture and passed for detection and spectroscopic analysis, such as through a detection circuit 1204 (see FIG. 12). The signature of particles within the atmospheric region of interest 1206 can be calibrated using predetermined spectroscopic calibration measurements based on one or more known approaches, such as ultraviolet spectroscopy, infrared spectroscopy, and Raman spectroscopy. Consistent with this implementation, the optical system can be operated as a LIDAR instrument, using a USPL laser source operating in the spectral range of interest to improve resolution and detection sensitivity. The spectral range tuning capability is useful for assessing and analyzing atmospheric chemical constituents.

[0026] Utilizing optical transmission terminals fabricated using either hyperbolic mirror fabrication techniques or traditional Newtonian designs that focus the received signal to a single ideal point, the USPL-FSO transceiver can be utilized for remote sensing and detection of airborne element signatures using ionization or non-ionization detection techniques. Specific adaptations that may be relevant to remote ionization probes include the controllable ionization that has been shown to occur at these frequencies, and the ionization process being focusable over distance to adjust the depth of atmospheric penetration, particularly with weather and clouds.

[0027] Figure 13 shows an example of the use of a USPL source to improve detection sensitivity, as well as an example of optical receiving technology. Researchers at the National Institute of Standards and Technology (NIST) have built a laser ranging system capable of pinpointing multiple objects with nanometer accuracy at distances of up to 100 km. LIDAR (light detection and ranging) systems have applications ranging from precision manufacturing on Earth to maintaining a fully formed network of satellites (Nature Photonics DOI:10.1038 / NPHOTON.2009.94). The NIST device uses two coherent broadband fiber laser frequency combs. The frequency combs output a series of stable, short pulses that also contain highly coherent carriers interspersed across the pulse train. This means that the frequency combs can be used to simultaneously perform interferometric and time-of-flight measurements, thereby enhancing analytical capabilities for application-specific situations.

[0028] In the arrangement shown in Figure 13, two phase-locked frequency combs 1301 and 1302 are used in a coherent linear optical sampling configuration, also known as multiheterodyne. One frequency comb measures both distance paths, while the other frequency comb provides the distance information encoded in the light of the first comb. Pulses from one frequency comb 1301 can be launched from a fiber and directed toward two glass plates, reference 1303 and target 1304. Plates 1303 and 1304 can reflect a specific portion of the pulse (e.g., approximately 4%) back into the fiber, effectively creating two new pulses. The time separation between the two pulses 1301 can give the distance between the movable target plate and the reference plate. The second frequency comb 1302 is tightly phase-locked with the first frequency comb but has a slightly different repetition rate. Due to the different delays between successive pulses when the light sources interfere, the second frequency comb can sample a slightly different portion of the light from the electric field of the first comb.

[0029] Using the techniques described with reference to Figure 13, two coherent broadband fiber laser sources can be replaced with two suitable USPL sources used within the outlined configuration, with each USPL source fiber coupled to a dedicated free-space optical telescope design. This can significantly improve overall efficiency, optical ranging, and accuracy.

[0030] In some embodiments, the native pulse repetition rate of a USPL laser source may be 50 MHz or less, which is undesirably low for optical data transmission, limiting the system to low data rate applications below 50 Mbps. Therefore, a system that increases the operational speed of the USPL is needed to provide a solution for data transfers above 50 Mbps.

[0031] FIG. 14 illustrates an example of a remote sensing system 1400 in which a USPL source 102 is fiber-coupled to an optical emitting element 1202 capable of transmitting and receiving optical signals via an optical fiber component 202. Light propagated forward by the optical emitting element 1202, including light from a data signal, is backscattered by interactions with known and unknown targets under investigation within an atmospheric region 1206. The optical backscattered signal, including light from the data signal, is detected via the optical emitting element 1202 or a similar receiving aperture and passed for detection analysis via the detection circuitry and spectroscopy component 1402 of FIG. 14. Signatures of particulates within the region under investigation 1206 can be calibrated, for example, where range detection analysis can be performed. A system 1400 such as that of FIG. 14 can include a USPL-FSO transceiver utilized and operated across the infrared wavelength range as a rangefinder and spotting device for target identification and interrogation applications. As used herein, the term "optical" includes at least visible, infrared, and near-infrared wavelengths.

[0032] FIG. 15 illustrates an optical pulse multiplier module 1500 capable of increasing the repetition rate of the output from a USPL source 102. An exemplary USPL may have a pulse width of 10-100 femtoseconds and a repetition rate of, for example, 50 MHz. The output from the USPL 102 can be fed as input 1502 to a USPL photonic chip pulse multiplier module 1504. In this example, the photonic chip can include a 20,000:1 splitter element 1506 that splits the input into discrete optical elements. Each optical element on the other side of the splitter element 1506 contains a 50 MHz pulse train. Each optical element then passes through a delay controller (either a fiber loop or a lens array) 1510, which delays that element's pulse train in time, for example, by a number of picoseconds. This results in successive optical elements being delayed in picoseconds. All of these pulse trains, each with its own time delay, are combined into a single pulse train in a manner similar to time-division multiplexing using a 20,000:1 optical combiner element 1512. The desired splitter and combiner ratio can be controlled to provide the optical design required for the desired application. The final output 1514 is a pulse train of 10 to 100 femtosecond pulses at a 1 THz repetition rate. This THz pulse train can be modulated by a 10 or 100 GigE signal, as shown in Figure 28, resulting in 100 femtosecond pulses per bit in a 10 GigE system and 10 femtosecond pulses per bit in a 100 GigE system. The cited applications are not limited to the specific data rates of 10 Gbps and 100 Gbps and can operate according to the requirements of the application under consideration. These figures are for illustrative purposes only. The current subject implementation can increase the USPL repetition rate to any desired repetition rate via the photonic chip pulse multiplier module 1504 using any multiplier factor. This submission provides other examples that are used to generate enhanced USPL repetition rates.

[0033] FIG. 16 illustrates a system 1600 for the generation, transmission, and reception of high pulse rate USPL optical streams. For example, an optical chip multiplexing module 1610, which may be similar to that discussed with reference to FIG. 15, may be used in this application. In this approach to achieving USPL pulse multiplication, a series of 10 GigE router connections (10 GigE is not intended to be a limiting feature) illustrated by signals 1601, 1602, 1603, and 1604 (four signals are shown in FIG. 16, but it is understood that any number is within the scope of the current subject matter) are interfaced to the optical chip multiplexing module 1610. In operation, the optical chip multiplexing module 1610 may support full duplex (Tx and Rx) for connecting to the 10 GigE routers 1601, 1602, 1603, and 1604. Optical chip multiplexing module 1610 can provide efficient modulation of incoming optical signals 1601, 1602, 1603, 1604 with USPL signal 1685 output from USPL source 1690. Optical chip multiplexing module 1610 can provide the capability to modulate and multiplex these incoming optical signals.

[0034] At a remote receiving site where a receiving device is located, all signals transmitted via transmit elements 1660 at the transmitting device can be collected using appropriate receive elements 1665. A complementary set of optical chip multiplexing modules 1675 can provide the functionality necessary to demultiplex the received data streams, as shown by the elements for distribution to a series of routers 1601', 1602', 1603', 1604' (again, the depiction of four such routers is not intended to be limiting). End-to-end network connectivity can be demonstrated through network endpoint elements.

[0035] Figure 17 shows an example of a system 1700 in which optical chips are interconnected in a wavelength division multiplexing (WDM) system. A WDM system has the advantage that each 10 GigE signal operates independently of other such signals at its own wavelength, eliminating the need for timing or synchronization with a 10 GigE (or other speed) router 1701. Timing or synchronization between the TDM optical chip and the 10 GigE router can be important with a TDM optical chip. A 10 GigE switch 1701 can provide the necessary electrical RF signal 1705 from the switch 1701 to modulate a USPL source 1702, either directly or using a USPL pulse multiplier as previously detailed in this document. A typical RZ output 1710 can be coupled to an external modulator 1720, which can be modulated using an NRZ clock source for the switch 1701, resulting in an RZ-modulated spectrum 1730. A conversion process using readily available equipment can provide the ability to introduce USPL sources and their benefits into terrestrial backhaul network spectrum.

[0036] For an optical chip system to successfully bridge between two remote 10 GigE switches, the chip may act like a simple fiber. Therefore, the timing of the TDM chip can be driven by the 10 GigE switch 1701. Both the actively mode-locked USPL (40 GHz, 1 picosecond pulse width) and the passively mode-locked USPL (50 MHz, 100 femtosecond pulse width) can be driven by an RF timing signal.

[0037] Figure 18 shows a device 1800 that can support another approach to high-pulse-repetition data-rate operation, for example, an approach for ultra-high-data-rate operation where optical chip designs can be implemented using either fiber or free-space optics. A 50 MHz USPL source 1801 can be interfaced to a series of optical delay control elements 1802, which can be designed using either a fiber loop or offset lenses, resulting in an RZ output stream of precisely 10.313 Gbps, which is the 10 GigE line rate (greater than 10 Gbps due to 64B / 66B encoding). Splitter elements 1803, along with variable optical delay lines 1804, provide the function of splitting the incoming optical signal train 1801 into (in this example) 206 paths. After sufficient delay is introduced through the design, all signals are multiplexed together through combiner elements 1805. In this way, a series of optical signals with identical or evenly spaced pulses between adjacent pulses forms a pulse continuum for modulation. Before entering the EO modulator element 1806, all optical input signals can be conditioned by pre-emphasis techniques, for example, using typical optical amplification techniques, to result in a uniform power spectrum for each output signal from the combiner element 1805. The conditioned output signals can then be coupled to the EO modulator element 1806 and modulated with an available NRZ signal from the 10 GigE signal source element 1807. The 10 GigE modulated output 1809 can be interfaced to an EDFA and then to the TX of an FSO system (or optical fiber system). The Rx side (after the detector) can be fed directly to a 10 GigE switch as modulated and amplified output 1810.

[0038] Figure 19 shows another example of a device 1900 that can be used for USPL pulse multiplication consistent with the implementation of the current subject matter. Similar to this approach, a 10x TDM system can be configured to provide a 100 Gbps output. A TDM demultiplexer chip can be placed on the receiving side of the communication link to split out the individual 10 GigE signals, including the reciprocal approach to the design shown in Figure 19.

[0039] As shown in Figure 18, a 50 MHz USPL source 1801 can be interfaced to a series of optical delay control elements 1802, which can be designed using either a fiber loop or offset lenses, resulting in a precise 10.313 Gbps RZ output stream, i.e., 10 GigE line rate (greater than 10 Gbps due to 64B / 66B encoding). Splitter element 1803, along with variable optical delay line 1804, provides the function of splitting the incoming optical signal train 1801 into 206 paths (in this example). After sufficient delay is introduced through the design, all signals are multiplexed together through combiner element 1805. However, instead of a single modulator element 1806 as shown in Figure 18, the 10.313 GHz RZ output 1901 from combiner element 1805 can be fed to a second splitter element 1910, which in this case can be a 10x splitter that splits the optical signal into 10 parallel paths. Other implementations of this design can support various splitting ratios as required by the design. The optical paths exiting the second splitter element 1910 are individually connected to designated optical delay lines 1920. Each individual delay path is connected to a dedicated optical modulator in the set of optical modulators 1930, which modulates the available NRZ signal from the 10x10 GigE signal source element 1931, resulting in a series of modulated optical signals 1935. An optical combiner, identified at 1940, provides a single optical pulse train 1950. The series of optical pulses in the single optical pulse train 1950 can be interfaced to an appropriate optical amplifier for desired optical conditioning for network use.

[0040] FIG. 20 shows another example of a device 2000 that can be used for USPL pulse multiplication consistent with implementations of the present subject matter. As shown, device 2000 can provide the ability to achieve high USPL pulse repetition data rates for network applications by modulating intra-channel pulses at low repetition rates. Direct modulation of each channel, applied to a delay controller, can beneficially create a modulation scheme that is not constrained by current speed limitations imposed by electronics technology. Implementations of the present subject matter can provide a mechanism to enhance the data transmission capacity of a system by separately modulating individual channels at current standard electronic modulation rates (at the rate of 100×10 GigE signal input 2001 in the example of FIG. 20) and time-multiplexing the channels into a single frequency, high-repetition-rate pulse stream. This approach can enhance current standards, limited by the speed of electro-optic modulators (40 Gbps), by approximately N orders of magnitude (where N is the number of channels in the time multiplexer). For example, 100-channel TDM, where each channel's amplitude is modulated at current standard data rates, can provide data rates up to 4 Tb / s. N can be limited by the width of the optical pulse itself. In the limit where information is transmitted at one bit per pulse, the time slot occupied by one bit is the width of the pulse itself (in that sense, RZ systems converge to NRZ). For example, in this scheme, a 40-fs pulse-width laser with a 40-GHz repetition rate can transmit information at rates up to 25 Tbps. This approach can be used with a modulation scheme for 40-Gbps channels (i.e., one bit every 25 ps), allowing for a capacity of N ~ 625 channels in a single transmission, which is the number of 40-fs time intervals that fit into a 25-ps time interval. A major advantage of this approach is its ability to "optically enhance" otherwise limited-data-capacity modulation schemes while interfacing with existing data-rate-limited modulators. For example, an amplitude modulator based on a Mach-Zehnder interferometer can be easily incorporated into a TDM IC package, which simply requires splitting a channel into two separate paths, adding a small phase modulator (nonlinear crystal) to one of the paths, and the ability to combine the paths for interference.

[0041] FIG. 20 includes a USPL source 2010 coupled to a multiport optical splitter element 2020. The number of optical ports identified is not limited to those described or shown herein. A series of optical delay lines 2030 provides the necessary optical delay between each parallel path from the multiport optical splitter element 2020 and can be tailored to a particular application. The optical delay paths from the optical delay lines 2030 are summed using an optical combiner element 2035. The resulting combined optical data stream emerging through element 2040 represents a multiplicative enhancement of the pulse repetition rate of the original USPL source identified by element 2010. Further enhancement of the pulse repetition rate is achieved through the use of element 2041, described by an optical splitter, in which the incoming signal 2040 is split into a series of paths not limited to those identified by element 2041. Via a second delay controller 2045, optical delays may be introduced into each path within the device, as identified by a second set of optical delay paths 2042. Each parallel path 2042 is, in turn, modulated by a modulation element 2044 with an available RF signal source element identified by signal input 2001. An optical combiner element 2050 combines all input signals into a single data stream 2060.

[0042] Optical pre-emphasis and de-emphasis techniques can be implemented within each segment of the described elements to custom tailor the optical spectrum for uniform or asymmetric optical power distribution. Pre-emphasis and de-emphasis can be achieved using commonly used optical amplifiers such as Er-doped optical amplifiers (EDFAs).

[0043] Figure 21 shows an example of a system 2100 including a mode-locked USPL source 2101, which can be used to generate the required clock and data streams as appropriate for the application. Mode-locked lasers can represent a high-performance, high-precision source option for clocks in digital communication systems. In this regard, mode-locked fiber lasers can be an attractive option because they can achieve pulse widths in the USPL source regime and repetition rates as high as GHz in either linear or ring configurations. Additionally, fiber offers compactness, low cost, low sensitivity to thermal noise, low jitter, and no issues related to diffraction or atmospheric dust contamination, to name just a few. In communication scenarios, the pulse width determines the usable bandwidth of the system, while the repetition rate limits the data rate. The pulse width can be determined by the intrinsic properties of the laser cavity, namely, the balance of the overall group velocity dispersion (GVD), the choice of saturable absorber (for passive systems), or the bandwidth of the active elements (for active mode-locked systems). The repetition rate of the pulse train is constrained by the length of the fiber. For example, in a linear laser, the fundamental mode vos of the laser can be expressed as:

number

[0044] Detailed in Figure 21 is a 980 nm pump element 2102 coupled to an optical WDM device 2105. An erbium-doped optical amplifier 2110 or equivalent can be used to create a nonlinear environment for mode-locked pulse train emission within the closed cavity established between two Faraday reflectors 2101 and 2160 at either end of the optical USPL resonator. Operation of the device can establish a self-contained train of optical pulses exceeding 100 Gbps, inherently highly synchronized at the module's output port 2170. The EDFA 2110 can be specially designed to achieve high gain in the nonlinear medium. A phase-locked loop 2130 can provide advantageous stability in operation by maintaining a synchronized clock source through modulation of signals via the self-contained high-repetition-rate pulse generator components 2120, 2130, and 2150. Achieving high repetition rates with lasers limited by their dimensions (length in the case of linear lasers, perimeter in the case of ring lasers) may require stimulating intracavity generation of multiples of the fundamental mode. In the active case, an amplitude modulator inserted into the resonator modulates the system's losses, acting as a "threshold gating" device. For this approach to be successful, the control signal to the modulator may need to be referenced to the laser's own oscillations, preventing the drive signal from "forcing" the laser to oscillate at an external frequency. This can be achieved by introducing a phase-locked loop element 2130, or a synchronous oscillator circuit that tracks and locks to the laser's repetition rate and regenerates the signal. In the case of a PLL, the RF output can be set to a multiple of the input signal (similar to the devices used in cellular telephone technology), increasing the laser's repetition rate. The signal can then be used to trigger a pulse generator or in combination with a low-pass filter. An MZ amplitude modulator 2150, located outside the laser resonator, can be used to create on-off keying (OOK) modulation in the pulse train emanating from the mode-locked laser.

[0045] FIG. 22 shows a graphical representation 2200 illustrating the effect of loss modulation introduced into an input pulse train 2201 by the presence of an amplitude modulator 2205 with a control signal NRZ signal 2210 consisting of a bit sequence as shown. The resulting signal at the output of device 2220 represents an NRZ-to-RZ converter device for use in telecommunications and scientific applications where applications can benefit from an RZ data stream. A clock signal 2201 (optical input) at a given pulse repetition rate passes through modulator 2205. Simultaneously, a control signal consisting of a sequence of 1s and 0s can be applied to the RF port of modulator element 2215. When modulator element 2215 is biased for minimum transmission, in the absence of a control signal, the loss experienced by the optical signal can be greatest. When an RF signal (1s) is present, the loss drops to a minimum (OPEN GATE), thereby functioning as an on-off keying modulation device. The pulse width of the output optical signal is typically much smaller than the time slot occupied by one bit of information (less than half a clock period in NRZ format), making the system true RZ as identified by element 2220 .

[0046] FIG. 23 illustrates an exemplary system 2300 for generating high-optical harmonic USPL pulse streams with a high pulse repetition rate using a saturable absorber (SA) device 2330. The SA device 2330 can, in some examples, include carbon nanotubes. Passively modelocked fiber lasers using carbon nanotube SAs (CNT-SAs) are another attractive option for high-repetition rate sources due to their ability to generate harmonics of the fundamental repetition rate. In the described approach, a closed, self-contained optical cavity is established, in which two Faraday reflector sources 2301 and 2350 form the optical cavity. While a high-power erbium-doped fiber amplifier (EDFA) 2310 is shown in FIG. 23, any inverted medium that creates a nonlinear optical cavity can be used. A seed laser 2315, such as a 980 nm pump laser as shown in FIG. 23, can be used to generate a high-repetition rate optical train. In particular, any suitable pump laser can be considered in terms of the required optical wavelength and pulse repetition rate. SA elements 2330 can be positioned within the resonator to establish the required optical pulse characteristics 2350 as needed through design requirements.

[0047] Figure 23 shows a schematic diagram of one example of a laser that can be used in one or more implementations of the present subject matter. Unlike the active laser shown in Figure 22, here the MZ modulator can be replaced with an SA element 2330. Techniques similar to those described herein can be utilized within fiber-based plant distribution systems or within FSO systems, either in terrestrial, submarine or airborne, space or submarine applications.

[0048] Figure 24 shows an approach to providing time-domain multiplexing (TDM) where the TDM multiplexes pulse trains using parallel time delay channels. In some cases, it can be important to operate the delay channels so that they remain "coherent" with one another. The frequency of the output multiplexed pulse trains would ideally be as insensitive as possible to environmental changes. To that end, the proposed feedback loop control system is designed to compensate the delay units for fluctuations that destabilize the output repetition rate.

[0049] Figure 24 shows a diagram of an example delay control system 2400. The control loop can be implemented in any of several ways consistent with the current subject matter. Figure 24 illustrates one possibility for illustrative purposes. An input pulse train enters the TDM and is multiplexed into N paths, each with its own delay line. If the paths are made of low-bend-loss fiber waveguides, each path can be wrapped around a cylindrical piezoelectric actuator (PZ) of radius R. The actuator typically expands radially as a result of a control voltage (Vc). This expansion, ΔR, is linearly proportional to Vc, so there is a fiber length change of ΔL = 2πNΔR, where N is the number of fiber turns around the PZ. For terahertz multiplexing, the inter-pulse (i.e., PZ1) delay needs to be 1 picosecond. This would require a length change equal to 200 microns, which corresponds to ΔR = 32.5 microns for one rotation of the PZ actuator. Most commercially available piezoelectric actuators are highly linear and operate well within this range. The control mechanism is therefore based on several PZ actuators, each with a number of turns corresponding to a multiple of the first delay, i.e., (32, 64, 96 microns, etc.), and is controlled by a single voltage Vc. The control voltage is determined by a feedback system, which compares the frequency of the output signal using a 1 / N divider with that of the input signal using a phase comparator (PC). The frequency of the "slow" input optical signal (represented by the waveform with τRT in Figure 24) is converted to an RF signal using a photodetector PDin. To reduce the effects of electronic jitter, a "differentiator" (or high-pass filter) can be applied to the RF signal to steepen the leading edge of the pulse. A phase-locked loop is used to track and lock the signal and regenerate it into a 50% duty cycle waveform. Similarly, on the output side, the optical signal is picked up by a photodetector PDout, high-pass filtered, and regenerated using the clock output port of the clock / data recovery system. The output signal, which has a frequency N times the frequency of the input signal, is sent to an N-fold frequency divider before going to the phase comparator.The phase comparator outputs a DC voltage level (similar to that used in PLL circuit architectures) representing the discrepancy between the input and output signals, which indicates the direction of the actuator correction. The low-pass filter adds a time constant to the system, making it more sensitive to spurious noise.

[0050] A CDR can be advantageously used for the output, as opposed to a PLL, where the output signal may or may not be modulated. The system can be designed to operate both unmodulated and with "in-TDM modulation" (i.e., one modulator per delay path). However, this is a completely deterministic approach to compensating for delay line length variations. Ideally, from a practical standpoint, the delay paths should all be referenced to the same "thermal level," i.e., simultaneously sensitive to the same thermal changes. If each line experiences different variations, the system cannot compensate for them in real time.

[0051] An alternative, entirely statistical approach involves summing operational amplifier circuits (S1...SN) to provide control voltages to the actuators. Using such an approach, the input voltages (V1 through VN) can be used to compensate for length mismatches between lines in a completely static sense and can otherwise be used for initial fine-tuning of the system. This approach typically requires compensating for, or at least taking into account, the bend loss requirements of the fiber used. Some new fibers just coming onto the market have critical radii of only a few millimeters.

[0052] If each path delay line senses different temperature changes or experiences uncorrelated length changes due to spurious local noise, the aforementioned approach may currently have difficulty performing real-time compensation. A more robust approach that operates in a purely statistical sense can be used, consistent with some implementations of the current subject matter. In such an approach, operational amplifier circuits (S1...SN) can be used to provide control voltages to the actuators. In this case, the input voltages (V1 through VN) can be used to compensate for length mismatches between the delay lines in a purely statistical sense; otherwise, they can only serve for initial fine-tuning (calibration) of the system.

[0053] Referring again to FIG. 24, an input USPL source, identified as element 2401, is coupled to optical coupler element 2403, such that one leg of the coupler connects to an optical photodiode selected to operate at the operating data rate of 2401. Using standard electronic filtering techniques described by elements 2404, 2405, and 2406, an electrical square wave representation of the input USPL signal is extracted and identified by element 2407. The second optical leg of coupler 2403 is interfaced to an appropriate optical splitter element, identified by 2410, which splits the input signal into 2410 into 206 parallel optical paths. Also shown are variable-rate optical delay lines established in parallel with each of the parallel branches of splitter element 2410. The parallel piezoelectric elements, identified by element 242N, are electronically controlled via a feedback circuit in the diagram. A control voltage, identified by Vc, is generated through photodiode 2485 in conjunction with electronic circuit elements 2480 and 2475. A clock data recovery (CDR) element 2475 generates the clock source used to control each PZ element. The optical paths identified as 244N are combined after appropriate delays are introduced in each leg of element 2410. This generates the pulse-multiplied USPL signal 2490.

[0054] FIG. 25A shows a schematic diagram of a fiber PZ actuator 2500, and FIG. 25B shows a graph 2590 of radius versus voltage for such an actuator. Together, these figures demonstrate the operation of a PZ actuator to increase the pulse repetition rate of an input USPL pulse train through induced optical delay. While shown for use as an element for enhancing the pulse repetition rate generation of USPL signals, the same approach can be used in other optical devices that require or benefit from optical delay. The basic structure of this device is a fiber-based PZ actuator 2501. When a voltage 2550 is applied to the electrodes 2520, a voltage-induced stress is created in the fiber, causing a time delay in the optical signal passing through the fiber. By varying the applied voltage, a performance curve of optical delay versus applied voltage is obtained, as shown in graph 2590 of FIG. 25B.

[0055] Figure 26 shows a diagram illustrating the features of an exemplary statistical corrector 2600. The coarse correction controller 2640 shown in Figure 26 corresponds to the system described in the previous section and can correct for length variations that occur simultaneously across all delay lines. As previously mentioned, these variations are expected to occur on a much slower time scale than spurious "in-delay-line" variations. This latter effect can manifest as period-to-period jitter introduced into the system. This type of jitter can be monitored using an RF spectrum analyzer (RFA), where it will show "sidelines" (or sidebands) in the system's repetition rate line, resulting from the analyzer picking up noisy frequencies resulting from uneven time intervals between successive pulses. One such pattern can be processed using an analog-to-digital converter (ADC), stored as an array of values, and fed to a neural network (NN) machine. Neural network machines are known for their remarkable adaptability, inherently learning patterns from external events by adapting to new sets of inputs and outputs. The set of inputs in this case can be generated from a series of "imperfect observations"—that is, the "noisy" output of a TDM system detected by an RFA and converted to a digital array by an ADC ({f1, f2, …, fN}, where f is the frequency component picked up by the RFA). The set of outputs can be generated from the corrections needed to remove unwanted excess frequency noise from the output frequency set due to external perturbations of the system ({V1, V2, …, VN}, where Vt is the compensation input voltage to the summing operational amplifier). With a sufficiently large number of {f, V} pairs (f, V are frequency, voltage arrays), a statistical set can be constructed to train a neural network (NN) to learn the underlying patterns associated with the presence of in-channel noise. These machines are commercially available in IC form from several manufacturers or can be implemented as software and used in conjunction with a computer feedback control mechanism. A single-layer perceptron-type neural network, or an adaptive linear neuron or later adaptive linear element (ADALINE), is sufficient to accomplish this task.

[0056] Similar to the description above in connection with Figure 24, statistical correction element 2670 may include electronic circuitry similar to, or providing similar functionality to, electrical circuit elements 2480 and 2475 and photodiode 2485 of Figure 24. In the approach illustrated in Figure 26, an RF spectrum analyzer 2695, along with neural network 2670 and coarse correction controller element 2640, is used to implement the optical delay requirements introduced in the parallel series of PZ elements 262N.

[0057] Figure 27 illustrates the concept and capabilities of an approach consistent with the current implementation, where performance, accuracy, and resolution can be improved through the replacement of piezoelectric disk (PZ) modules, identified by elements 2795 and 272N, to obtain an optical delay line using a compact microfiber-based collimator (MFC) 2795 surrounded by a ceramic disk. While demonstrating a technique for increasing the native pulse repetition rate of a USPL pulse train, the design shown is not limited to such applications and can be applied or extended to other needs within the optical sector where optical delay is required. This allows for the introduction of a more controlled amount of time delay within each MFC element of the circuit. Improvements made through the use of MFC elements can achieve improved responsiveness, improved resolution, and rapid reproduction of required voltage responses in mass-produced means. The concepts identified in Figure 27 can be incorporated into precision-manufactured elements as complementary paired units for use in reducing USPL interpulse jitter and for data encryption needs.

[0058] 27, a USPL source 2701 having a certain pulse repetition rate is split into a preselected number of optical paths 271N (which may be a number other than 206), as identified by splitter element 2705. An appropriately controlled delay 273N is introduced into each parallel leg of the split optical path 271N using elements illustrated at 2795 and 272N. The resulting delay paths 274N are summed together through optical combiner element 2760. The pulse multiplied USPL signal 2780 results.

[0059] One potential drawback of some previously available TDM designs is that the fiber is "wrapped" around the piezoelectric actuator, requiring the mechanism to comply with the bend loss requirements of the fiber being used. Some new fibers just coming onto the market have a critical radius of only a few millimeters. To correct this issue, the current subject implementation can use a micromachined air-gap U-bracket instead of the cylindrical piezoelectric elements wrapped in fiber. Figure 27 illustrates this principle. In this approach, the piezoelectric actuators (PZ1...PZN) are replaced with an air-gap U-bracket structure constructed using microfiber collimators (MFCs) and microrings made of piezoelectric material. However, in this case, the piezoelectric actuators expand longitudinally, increasing (or decreasing) the air-gap distance between the collimators depending on the control voltages (V1, V2, ..., VN). As with cylindrical piezoelectrics, a single voltage Vc can be used to drive all of the piezoelectric devices, provided that the gain of each channel (G1, G2, ..., GN) is adjusted accordingly to provide the correct expansion for each row. Ideally, excluding any bias inherent in the system (i.e., inherent differences between op amps), the gain adjustments should be G1, 2G1, 3G1, etc., to provide expansions that are multiples of τRT / N. Another way to implement such an approach is to use multiple piezoelectric rings in the channel. This way, you can have a channel with 1, 2, 3, N piezoelectric rings driven with the same voltage, all amplifiers with the same gain.

[0060] Figure 28 provides a conceptual representation of an optical chip system 2800 for successfully bridging between two remote 10 GigE switches. Ideally, such a connection could function similarly to a simple fiber. The timing of the TDM chips could be driven by the 10 GigE switches.

[0061] Referring to Figure 28, a USPL source 2805 having a predetermined native pulse repetition rate, identified by 2806, connects to an optical pulse multiplication chip 2807. Element 2807 is designed to convert the input pulse repetition rate signal 2806 to a level suitable for operation with a high-speed network Ethernet switch, as identified by 2801. Switch 2801 provides a reference signal 2802 that is used to modulate signal 2809 via a standard electro-optic modulator 2820 at the data rate of interest. The resulting RZ optical signal is generated as shown in element 2840.

[0062] Instead of running the timing from the 10GigE switch, we could use a photonic multiplier chip to configure USPL to Tb / s (or higher) and then modulate this Tb / s signal directly from the 10GigE switch. Each bit has around 100 pulses. The advantage of this approach is that we can eliminate the need to run a separate timing signal from the switch to the USPL. The USPL via the multiplier chip simply sends out Tb / s pulses. Another advantage is that the output of the multiplier chip does not need to be exactly 10.313 or 103.12 Gbps; it just needs to be around 1 Tb / s. This limitation is not an issue if each 10GigE bit has 100, 101, or 99 pulses. Another advantage is that because there are many 10 USPL per bit, the 10GigE signal benefits from atmospheric propagation (fog and scintillation). Another advantage can be realized at the receiver side. Detectors are more likely to detect bits if they have around 100 USPL pulses within that bit. This can improve receiver sensitivity and potentially improve the range of FSO systems. An added benefit is that upgrading to 100GigE is as easy as replacing a 10GigE switch with a 100GigE switch, where each bit has around 10 pulses.

[0063] From a purely signal processing perspective, this approach represents an efficient way to combine and transmit data and clocks in a single transmission stream. Similar to "sampling" bits using a stream of optical pulses, this approach has the advantage that the "size" of a bit is determined by the maximum number of pulses it transmits, thus establishing a basis for counting bits arriving at the receiver. In other words, if a bit unit has a time slot that can fit N pulses, the system's clock can be established as "one new bit of information" every 5 seconds.

[0064] Techniques similar to those described herein can be utilized within fiber-based plant distribution systems or within FSO systems for FSO systems in either terrestrial, submarine or airborne, space or submarine applications, and demonstrate for the first time how interconnection from USPL sources to optical network elements can be achieved for networking applications.

[0065] Figure 29 shows system 2900, a conceptual network extension of the design concept reflected in Figure 28. Multiple USPL sources 2901, 2902, and 2903 (while three are shown, it should be noted that any number is within the scope of the current subject matter) are configured in a WDM arrangement, each modulated via a dedicated optical switch and USPL laser multiplier chip circuit. As described with reference to Figure 28, electrical signals from each Ethernet switch can be used to modulate dedicated optical modulators 2911, 2922, and 2928 for each optical path. Optical power for each segment of the system can be provided by optical amplification elements 2931, 2932, and 2933 for amplification purposes. Each amplified USPL path can then be interfaced to an appropriate optical combiner 2940 for transport to network 2950, ​​which can be either free-space or fiber-based as needed. The output from the WDM modules can be configured to transmission element 102 for FSO transport or to fiber plant equipment.

[0066] The techniques described herein can be utilized within fiber-based plant distribution systems or within FSO systems, either for terrestrial, submarine or airborne, space or submarine applications, and show for the first time how interconnection from USPL sources to optical network elements can be achieved for networking applications.

[0067] Figure 30 shows a schematic diagram of the experimental setup for the implementation of the current subject matter, which involves building a computer-assisted system to control the pulse width of an all-fiber mode-locked laser using recursive linear polarization adjustment that simultaneously stabilizes the resonator repetition rate using a synchronous self-regeneration mechanism. This design can also provide repetition rate and pulse width adjustment capabilities.

[0068] The fiber ring laser is represented by the inner blue loop, and all intracavity fiber branches are coded in blue, while the positive high-dispersion fiber outside the loop is part of the fiber grating compressor (coded in dark brown). The outer loop represents the active system with feedback.

[0069] FIG. 30 shows a diagram of a system 3000 illustrating the features of a USPL module that provides pulse width control and pulse repetition rate control via mirrors (M1, M2), gratings (G1, G2), lengths (L1, L2), second harmonic generators (SHG), photomultiplier tubes (PMT), lock-in amplifiers (LIA), data acquisition systems (DAC), detectors (DET), clock extraction mechanisms (CLK), frequency voltage controllers (FVC), high voltage drivers (HVD), reference signals (REF), pulse generators (PGEN), amplitude modulators (AM), isolators (ISO), piezoelectric actuators (PZT), optical couplers (OC), polarizers (POL), and polarization controllers (PC), which function to provide control of pulse repetition rate and pulse width control.

[0070] A passive mode-locking mechanism can be based on nonlinear polarization rotation (NPR), which can be used in mode-locked fiber lasers. In this mechanism, a weakly birefringent single-mode fiber (SMF) can be used to generate elliptically polarized light in the propagating pulse. As the pulse travels along the fiber, nonlinear effects occur, resulting in intensity-dependent polarization rotation. By the time the pulse reaches the polarization controller (PC) 3001, the polarization state of the high-intensity portion of the pulse experiences a greater rotation than the low-intensity portion. The controller performs the function of rotating the high-intensity polarization component of the pulse, aligning it as closely as possible to the axis of the polarizer (POL). As a result, as the pulse passes through the polarizer, its low-intensity component is attenuated more than the high-intensity component. Thus, the pulse exiting the polarizer is narrowed, and the entire process acts as a fast saturable absorber (FSA). This nonlinear effect works in conjunction with the group velocity dispersion (GVD) of the loop, and after several round trips, a stable situation occurs, and passive mode-locking is achieved. The overall GVD of an optical loop can be tuned to produce a specific desired pulse width within a margin of error by using different types of fiber (single-mode, dispersion-shifted, polarization-maintaining, etc.) and summing the contributions to the average GVD of the lasers.

[0071] Active control of the linear polarization rotation from a PC can significantly improve laser performance. This can be achieved using a feedback system that tracks changes in pulse width. This system, represented by the outer loop in Figure 1, can be used to maximize compression and, consequently, the average power of the pulse. Pulses emerging from a fiber ring laser via an optical fiber converter are expected to have widths on the order of a few picoseconds. An external pulse compression scheme using a fiber grating compressor is used to narrow the pulses to the sub-100 fsec range. This technique has been widely used in many reported experiments, leading to high-energy, high-power USPL pulses. Here, the narrowed pulses are focused into a second-harmonic generator (SHG) crystal and detected using a photomultiplier tube (PMT). A lock-in amplifier (LIA) provides an output DC signal to a data acquisition card (DAC). This signal follows changes in pulse width by tracking the increase or decrease in the pulse's peak power. A similar technique has been used successfully in the past, but in those cases a spatial light modulator (SLM) was used instead. Here, a programmable servomechanism directly controls the linear polarization rotation using an actuator on a PC. Using the DC signal data provided by the DAC, decision-making software (such as, but not limited to, LABVIEW, MATLAB®, or SIMULINK®) can be developed to control the servomechanism, which adjusts the rotation angle of the input pulse relative to the polarizer axis. These adjustments, made by the actuator, are achieved using stress-induced birefringence. For example, a decrease in pulse width encourages the actuator to follow a certain direction of linear angular rotation to compensate, while an increase in pulse width acts in the opposite direction, aiming to maximize the average output power.

[0072] The round-trip time of a laser can be adjusted by a self-regenerative feedback system using a drive signal to an amplitude modulator (AM) synchronized to the repetition rate of the optical oscillation. In an active system, the AM acts as a threshold gating device by modulating the loss synchronously with the round-trip time. This technique has been successfully used to stabilize a mode-locked laser in a recent report. The signal captured from the optical coupler (OC) by a photodetector (DET) can be electronically locked and regenerated by a clock extraction mechanism (CLK), such as a phase-locked loop or a synchronous oscillator. The regenerated signal triggers a pulse generator (PGen), which is then used to drive the modulator. In a perfectly synchronized scenario, the AM "opens" at each round-trip time (TRT) as the pulse passes. As the CLK follows the TRT variations, the AM's drive signal also changes accordingly.

[0073] An external reference signal (REF) can be used to adjust the repetition rate of the cavity. This can be compared to a signal recovered from the CLK using a mixer or the output used to drive a piezoelectric (PZT) system, which can adjust the resonator length. Using a PZT system to adjust the cavity length in this way is a well-known concept, and similar designs have already been experimentally demonstrated with success. Here, a linear frequency-to-voltage converter (FVC) can be calibrated to provide the input signal to the PZT's high-voltage driver (HVD). The PZT adjusts the resonator length to match the repetition rate of the REF signal. For example, if the frequency of the REF signal increases, the output of the FVC decreases, which also reduces the HV drive level to the piezoelectric cylinder, causing it to contract and, consequently, increasing the laser's repetition rate. The opposite occurs if the repetition rate of the reference decreases.

[0074] Using a pair of negative dispersion gratings, the pulse width can be tuned to a "transform-limited" value. This chirped pulse compression technique is well established, with reports of pulse compression as narrow as 6 fs. The idea is to mount the grating pair pulse compressor on a translation stage that moves along a line that sets the separation between the gratings. As the distance changes, so does the compression ratio.

[0075] An example of a data modulation scheme consistent with the current subject matter implementation involves using a passively mode-locked laser as a source of ultrafast pulses, limiting the flexibility to change the data modulation rate. Scaling up the system's data rate requires increasing the fundamental repetition rate of the pulse source. Traditionally, the repetition rate of a passively mode-locked laser is increased by reducing the length of the laser cavity or by harmonic mode-locking the laser. Both techniques reduce the peak power of the intracavity pulses, resulting in longer pulse widths and unstable mode-locking.

[0076] One approach to solving this problem is the use of a modified pulse interleaving scheme, a technique called pulse multiplication. Figure 31 illustrates this concept. A lower-repetition-rate pulse train from a well-characterized, well-mode-locked laser 3101 is coupled to an integrated optical directional coupler 3180, where a well-determined portion of the pulses is tapped off and "recirculated" in an optical loop with an optical delay 3150 equal to the desired inter-pulse spacing of the output pulse train, before being recombined into the directional coupler's output. For example, generating a 1 GHz pulse train from a 10 MHz pulse train requires an optical delay of Ins, and the optical delay must be precisely controlled to ensure that the 100th pulse in the train coincides with the input pulse from the 10 MHz source. The optical delay loop includes optical gain 3120 to compensate for signal attenuation, dispersion compensation 3160 to restore the pulse width, and active optical delay control 3150. Once pulse multiplication occurs, the output pulse train is OOK modulated 3175 using the data stream 3182 to the generated RZ signal 3190 and amplified in an erbium-doped fiber amplifier 3185 to raise the pulse energy to the same level as the input pulse train (or to the desired output pulse energy level).

[0077] One or more of the features described herein, whether taken alone or in combination, can be included in various aspects or implementations of the present subject matter. For example, in some aspects, an optical wireless communication system can include at least one USPL laser source, which can optionally include one or more of picosecond, nanosecond, femtosecond, and attosecond laser sources. The optical wireless communication system can include a USPL source that can be fiber-coupled or free-space coupled to an optical transmission system, modulated using one or more modulation techniques for a point-to-multipoint communication system architecture, and / or utilize an optical transmission terminal or telescope manufactured through one or more of hyperbolic mirror manufacturing techniques, conventional Newtonian mirror manufacturing techniques, or other functionally equivalent or similar technologies. Aspheric optical designs can also be used to minimize, reduce, or alternatively obscure received optical signals.

[0078] A free-space optical transmission system consistent with the implementation of the present subject matter can utilize a USPL laser design that focuses the received signal to a single ideal point. In some implementations, one telescope or other optical element for focusing and delivering light can be considered the transmission element, while a second telescope or other optical element for focusing and receiving light, located remotely from the first telescope or other optical element, can function as the receiving element to create an optical data link. Either optical communications platform can optionally include components necessary to provide both transmission and reception functions and can be referred to as a USPL optical transceiver. Either or both of the telescope or other optical element for focusing and delivering light can be coupled to the transmitting USPL source via optical fiber or by free-space coupling to the transmission element. Either or both of the telescope or other optical element for focusing and receiving light can be coupled to the receiving endpoint via either optical fiber or free-space coupling to an optical receiver. A free-space optical (FSO) wireless communication system including one or more USPL sources can be used transparently within the framework of an optical communication network, in combination with an optical fiber backhaul network, and within an optical communication network (and can be modulated using on-off keying (OOK), non-return to zero (NRZ), and return to zero (RZ) in the 1550 nm optical communication band), within an optical communication network (and can be modulated using differential phase-shift keying (DPSK) modulation techniques), within an optical communication network (and can be modulated using commonly used modulation techniques for point-to-point communication system architectures using commonly used free-space optical transceiver terminals), within an optical communication network utilizing D-TEK detection techniques, within a communication network for use in combination with erbium-doped fiber amplifiers (EDFAs) and high-power erbium-ytterbium-doped fiber amplifiers (Er / Yb-DFAs), and within an optical communication network (and can be modulated using commonly used modulation techniques for point-to-multipoint communication system architectures).

[0079] In some embodiments, USPL technology can be used in auto-tracking functions and as a beacon source to provide optical tracking and beam steering to maintain terminal core alignment during operation. The recovered clock and data extracted at the receiving terminal can be used for multi-hop spans to extend network reach. Optical networks can offer similar benefits in WDM configurations, increasing the effective optical bandwidth of carrier data links. USP laser sources can also or alternatively be polarization multiplexed onto the transmitted optical signal, providing polarization multiplexed USP-FSO (PM-USP-FSO) functionality. The recovered clock and data extracted at the receiving terminal can be used for multi-hop spans to extend network reach, including a general wide bandwidth range to provide data rate-invariant operation. Optical pre-amplifiers or semiconductor optical amplifiers (SOAs) can be used before the optical receiver element, or alternatively, in combination with the recovered clock and data extracted at the receiving terminal, for multi-hop spans to extend network reach, including a general wide bandwidth range to provide data rate-invariant operation. Terminal core alignment can be maintained during operation, so significant improvements in performance and terminal core alignment can be achieved through the use of USPL technology, the use of USPL data sources, and improved methods for maintaining transceiver alignment through the use of USPL laser beacons.

[0080] USPL-FSO transceivers can be utilized in several configurations for remote sensing and detection of airborne element signatures using ionizing or non-ionizing detection techniques, utilizing optical transmission terminals fabricated using either hyperbolic mirror fabrication techniques or traditional Newtonian designs that focus the received signal to a single ideal point. USPL-FSO transceivers consistent with the current subject matter can be utilized in non-line-of-sight laser communications applications. USPL-FSO transceivers consistent with the current subject matter can also be utilized for tuning the distance at which scattering effects occur (enabling NLOS techniques), receiver techniques that improve detection sensitivity using DTech detection methods, and improved bandwidth through wideband detectors, including frequency combs. USPL-FSO transceivers consistent with the current subject matter can be utilized in conjunction with adaptive optics (AO) technology to perform input optical wavefront correction (AO-USPL-FSO). USPL-FSO transceivers consistent with the current subject matter can be utilized and operate across the entire infrared wavelength range. USPL-FSO transceivers consistent with the current subject matter can be utilized in conjunction with optical add-drop and optical multiplexing techniques in both single-mode and multimode fiber configurations. A USPL-FSO transceiver consistent with the present subject implementation can be utilized and operated across the infrared wavelength range as a rangefinder and spotting device for the purposes of target identification and interrogation applications.

[0081] In another aspect of the present subject matter, a series of switched network connections, e.g., 10 GigE, 100 GigE, etc., can be connected from one point to another via fiber or free space optics, e.g., via time division multiplexing (TDM).

[0082] A mode-locked USPL source consistent with the current subject implementation can be used to generate both clocks and data streams. Mode-locked lasers can represent a high-performance, high-precision source option for clocks in digital communication systems. In this regard, mode-locked fiber lasers can be an attractive option, as they can achieve pulse widths in the USPL source regime and repetition rates as high as GHz, in either linear or ring configurations.

[0083] High-order harmonic generation can be achieved using carbon nanotube saturable absorbers. Passively modelocked fiber lasers using carbon nanotube saturable absorbers (CNT-SA) can easily generate harmonics of the fundamental repetition rate, making them an option for high repetition rate sources.

[0084] FSO can be used in terrestrial, space and undersea applications.

[0085] Conditional path length control from the splitter to the aperture can be an important parameter. Consistent with the current implementation, TDM multiplexing can be used to control the relative time delay between paths from the aperture to the source. Each pulse train can be controlled using parallel time delay channels. This technique can be used to control conventional multiplexed FSO aperture systems employing WDM or TDM techniques. The interpulse spacing of a USPL laser can be maintained and controlled to precise time requirements in both TDM and WDM systems. The described technique can be used in both TDM and WDM fiber-based systems. The use of a TDM multiplexer as described herein can be used to implement unique encryption methods for transmitted optical signals. A complementary TDM multiplexer can be used to invert the incoming received signal and recover the unique signature of the pulse signal. The TDM multiplexer described herein can be utilized to control WDM pulse character for WDM encryption purposes. TDM multiplexers can be used in conventional FSO systems where multiple apertures connected to a common source signal can maintain a constant path length by controlling the time delay between pulses. TDM multiplexers can be used in TDM fiber-based and FSO-based systems. TDM multiplexers can be used as a technology to control the relationship of optical pulse trains from USPL sources. TDM multiplexers can be used as an optical-to-air link characterization utility through measurement of neural correction factors to obtain the same pulse relationship.

[0086] Any combination of PZ disks can be used in the transmitter, allowing for infinite encryption combinations for fiber-based and FSO-based USPL-based systems. Timing can be performed from a 10GigE switch or equivalent, and multiplier photonic chips can be used to build USPL to Terabit / s (or higher) rates, and this Terabit / s signal can be modulated directly from the 10GigE switch. While operating in a WDM configuration, interfaces to fiber-based systems or FSO network elements can be included.

[0087] The system can accept ultrafast optical pulse trains and generate optical pulse trains with the same pulse width, spectral content, and chirp characteristics as the input optical pulses, and with a pulse repetition rate that is an integer multiple of the input pulses. This is achieved by tapping a portion of the input pulse power with a 2 × 2 optical coupler with an actively controllable optical coupling coefficient, recirculating this tapped pulse through one round trip in an optical delay line with optical amplification, optical isolation, optical delay (path length) control, optical phase and amplitude modulation, and compensation for the temporal and spectral changes experienced by the optical pulses within the optical delay line, with the goal of minimizing the temporal pulse width at the device's output, and then recombining this power with the 2 × 2 optical coupler.

[0088] Passive or active optical delay control can be used, as can optical gain utilizing rare-earth doped optical fiber and / or rare-earth doped integrated optical devices and / or electrically or optically pumped semiconductor optical amplification. Dispersion compensation can be provided using fiber Bragg gratings and / or volume Bragg gratings. Wavelength division multiplexed data modulation of pulses traversing the delay line can be employed, as can pulse-coded data modulation of pulses traversing the delay line.

[0089] Tuning of conventional USPL sources by synthesis of USPL square wave pulses can be achieved using microlithographic amplitude and phase masking techniques for FSO applications. The ability to tune pulse widths using this technique, along with similar approaches to controlled and active control of pulses through this technology, can improve propagation efficiency through FSO transmission links, increasing system availability and received optical power levels.

[0090] Active programmable pulse shapers can be used to actively control the USPL pulse width, including matching atmospheric conditions in real time to maximize propagation through changing environments. In FSO applications, the optical temporal spectrum can be adapted using one or more of the following techniques: Fourier transform pulse shaping, liquid crystal modular (LCM) arrays, liquid crystal on silicon (LCOS) technology, programmable pulse shaping using acousto-optic modulators (AOMs), acousto-optic programmable dispersive filters (AOPDFs), and polarization pulse shaping.

[0091] Figure 32 shows a process flowchart 3200 illustrating method features, one or more of which may appear in an implementation of the current subject matter. At 3202, a beam of optical pulses, each having a duration of about 1 nanosecond or less, is generated. At 3204, a modulation signal is applied to the beam to generate a modulated optical signal. The modulated signal carries data for transmission to a remote receiving device. The modulated optical signal is received at an optical transceiver within an optical communications platform at 3206, and at 3210, the modulated optical signal is transmitted using the optical transceiver for reception by a second optical communications device.

[0092]

[00130] Figure 33 shows another process flowchart 3300 illustrating method features, one or more of which may appear in implementations of the current subject matter. At 3302, a beam of light pulses, each having a duration of about 1 nanosecond or less, is generated, for example, using a USPL source. The beam of light pulses is transmitted at 3304 via an optical transceiver toward a target atmospheric region. At 3306, optical information received at the optical transceiver as a result of optical backscattering of the light pulse beam from one or more objects within the target atmospheric region is analyzed.

[0093] FIG. 34 shows another process flowchart 3400 illustrating method features, one or more of which may appear in implementations of the current subject matter. In 3402, first and second beams containing optical pulses are generated, for example, by a USPL source. In 3404, a first modulation signal is applied to the first beam to generate a first modulated optical signal, and a second modulation signal is applied to the second beam to generate a second modulated optical signal. A first polarization state of the first modulated optical signal is adjusted in 3406. Optionally, a second polarization state of the second modulated optical signal may also be adjusted. In 3410, the first modulated optical signal having the adjusted first polarization state is multiplexed with a second modulated signal. In 3412, the multiplexed first modulated optical signal having the adjusted first polarization state with the second modulated signal is transmitted by an optical transceiver for reception by a second optical communication device.

[0094] 35A and 35B illustrate exemplary nodes that can be used to transmit and receive information. The transmitting node 3510 and the receiving node 3530 may be communications platforms such as those described above, including those with reference to FIGS. 1-9. Furthermore, the transmitting node 3510 is shown with components for generating and transmitting data-bearing optical signals, while the receiving node 3530 is shown with components for receiving and extracting data from the optical signals; these components may be combined into a single node configured to transmit and receive optical signals. In some embodiments, for example, the telescope 3522 may act as both an aperture for transmitting and receiving optical signals.

[0095] FIG. 35A shows an exemplary transmitting node 3510. In some embodiments, the transmitting node 3510 may include a source 3512. In some embodiments, the source 3512 may be a USPL source, a superluminescent diode, or other source. In other embodiments, the source 3512 may be a continuous wave source. Preferably, the source 3512 may be configured to generate a beam of light pulses, where each pulse has a coherence length of less than 400 microns. The coherence length of the source is determined as follows:

number

[0096] In some embodiments, the source 3512 may have a center wavelength in the infrared range. For example, the center wavelength of the source 3512 may be between 1400 nm and 1700 nm. In some embodiments, the source 3512 may be configured to output pulses at a repetition rate of at least 50 MHz, 100 MHz, 200 MHz, 500 MHz, 800 MHz, 1 GHz, 1.25 GHz, 1.5 GHz, 2 GHz, 5 GHz, or 10 GHz. The source 3512 may include a pulse multiplier (internally or externally), as generally described above, including with reference to Figures 15 and 18-20. In some embodiments, the pulse width may be less than 10 ns, less than 1 ns, less than 500 ps, ​​less than 300 ps, ​​less than 100 ps, ​​less than 50 ps, ​​less than 10 ps, ​​less than 1 ps, less than 700 fs, less than 500 fs, less than 300 fs, less than 200 fs, or less than 100 fs.

[0097] The transmitting node 3510 may optionally include a splitter 3514. The splitter 3514 may be configured to split a pulse from the source 3512 into multiple separated pulses having different wavelength bands. For example, a pulse having an original spectral width of 1500-1600 nm may be split into 25 pulses having respective spectral widths of 1500 nm-1600 nm (e.g., 1500-1504 nm, 1504-1508 nm, 1508-1512 nm, etc.). The splitter 3514 may use any known beam splitting mechanism. Each of the multiple separated pulses may have a coherence length of less than 1 mm, less than 600 microns, less than 400 microns, less than 200 microns, less than 100 microns, less than 50 microns, or less than 1 micron.

[0098] The transmitting node 3510 may include one or more modulators 3516. In some embodiments, each of the modulators 3516 may be a Mach-Zehnder modulator (MZM). The modulators 3516 may receive a data signal indicating the data to be transmitted in the optical beam and, based on the data signal, may encode the data into the pulses of the beam using on-off keying or other modulation techniques. In some embodiments, the modulator 3516 may allow pulses to pass to indicate a "1" and block or reduce the amplitude of the pulse to indicate a "0" in the bit stream. In embodiments in which the beam is split, each of the multiple separated pulses may be directed to a respective modulator 3516 of the multiple modulators. In other embodiments, each of the multiple separated pulses may be modulated by a single modulator 3516. For example, the separated pulses may be delayed or staggered relative to one another, and the modulator 3516 may encode data into each pulse at a repetition rate higher than the pulse generation repetition rate of the source. If the source 3512 generates pulses at a rate of at least 1 GHz, for example, the splitter may divide each pulse into 25 or more separate pulses, which can be modulated by one or more modulators 3516 to encode data at a rate of at least 25 Gbps. In some embodiments, the source may generate pulses at a rate of at least 1 GHz, and the splitter may divide each pulse into at least 10, at least 20, 30, at least 40, or at least 50 separate pulses to generate data rates of at least 10 Gbps, at least 20 Gbps, at least 30 Gbps, at least 40 Gbps, or at least 50 Gbps. In some embodiments, the FWHM bandwidth of the source may be at least 100 nm, at least 150 nm, or at least 200 nm, which allows the pulses to be divided into more separate pulses without reducing the coherence length of those pulses below the values ​​described below with respect to FIGS. 40 and 41.

[0099] After being modulated, the pulses (and optionally separated pulses if a splitter is used) may be passed to an optional threshold filter 3518. In some embodiments, the threshold filter may be a saturable absorber (or a different nonlinear device) that attenuates weak pulses and transmits strong pulses. The threshold filter 3518 may be configured to remove or significantly reduce pulses below a defined threshold, while allowing pulses above that threshold to pass. In some embodiments, the modulator 3516 may significantly reduce pulses intended to transmit "0"s, but it may do so imperfectly, allowing some optical energy to pass through, which, when amplified by the amplifier 3520, can create a signal strong enough to generate bit errors. The use of the threshold filter 3518 allows for more complete removal of pulses intended to be removed, thereby improving the data transmission accuracy of the system.

[0100] The modulated pulses may be passed to an amplifier 3520, which may increase the size of the pulses for transmission by a telescope 3522 (which may be, for example, an aperture and / or a lens). If a splitter is used, the separated pulses may be recombined using a recombiner (not shown) before or after passing to the amplifier 3520.

[0101] FIG. 35B illustrates an exemplary embodiment of a receiving node 3530, which may be configured to receive and extract data from the optical beam transmitted by the transmitting node 3510, for example. The receiving node 3530 may include an aperture 3532, an optional splitter 3534, and one or more optical receivers 3536, which may have specific characteristics with respect to the source, as described in more detail below. The optical receiver 3536 may include a photodiode and processing circuitry. In some embodiments, the optical receiver 3536 may be, for example, an avalanche photodiode. In some embodiments, the optical receiver's processing circuitry may determine whether the received light within a detection window exceeds a detection threshold and output bit data (e.g., a "0" or a "1") for that window based on the result of that determination. The receiving node 3530 may be an optical communications platform, as described above. In some embodiments, the components of the transmitting node 3510 and the receiving node 3530 may be included in a single transceiver node.

[0102] Aperture 3532 may be configured to receive an optical signal, such as the optical beam transmitted by transmitting node 3510, as depicted in FIG. 35A. In some embodiments, light received at aperture 3532 may pass through a filter that filters out wavelengths of light that are not near the center wavelength of the source. For example, the source in the transmitting node may have a center wavelength between 1500 nm and 1700 nm, and the filter in receiving node 3530 may block or reduce light outside the source band. For example, the filter may reduce the magnitude of the light below 1500 nm. Optionally, the filter may block light of even longer wavelengths or may set a threshold at lower wavelengths, such as 1480 nm or 1460 nm. Optionally, receiving node 3530 may include a splitter 3534, which may split pulses in the received beam into multiple separated pulses of different wavelength bands. If the pulses are split and separately modulated at transmitting node 3510, the pulses may be split into the same wavelength band by splitter 3534 in the receiving node. The pulse (or the combined pulse, if a splitter is used) may then be processed by one or more optical receivers 3536. In embodiments in which the pulse is split into multiple separated pulses, each pulse may be directed to a respective optical receiver, which may be configured to determine whether an "on" or "off" signal has been transmitted in a given detection window. In some embodiments, encoding modalities other than on-off keying, such as frequency modulation, may be used. Additional details regarding the optical receivers 3536 are provided below with respect to FIG. 41.

[0103] FIG. 36 illustrates an exemplary configuration in which data is transmitted over optical communication distance D from a first communication network 3542 to a second communication network 3544 using a transmitting node 3510 and a receiving node 3530 as described above with respect to FIGS. 35A-35B. Data may be received from the optical communication network 3542 encoded into an optical beam and transmitted across the optical communication distance D using the transmitting node 3510. The receiving node 3530 may receive the optical beam, extract the transmitted data, and pass the data to the communication network 3544. In some embodiments, data from the communication 3544 may also be transmitted from the node 3530 back to the node 3510, which may pass the data to the communication network 3542, enabling two-way communication. In some embodiments, the optical communication distance may be at least 0.5 miles, at least 1 mile, at least 2 miles, at least 3 miles, at least 5 miles, at least 7 miles, at least 10 miles, or at least 20 miles.

[0104] Figure 37 shows an exemplary beam traveling over an optical communication distance D, say one mile, through a perfectly uniform refractive index medium. Even in a perfectly constant refractive index medium, the beam will naturally spread due to diffraction, but the beam will remain the same shape and only expand by an amount proportional to the propagation distance, and there will be no beam scintillation effects in a uniform refractive index medium.

[0105] Figure 38 provides a schematic representation of photons within a beam traveling through a variable-refractive medium. The atmosphere has variations in temperature, density, pressure, humidity, aerosols, wind, convection, and other parameters that cause the atmosphere's refractive index to fluctuate. When a light beam passes through air or other variable-refractive media, such as water, photons within the beam may refract slightly differently than other photons. As shown in Figure 38, different ray paths within the beam may refract differently due to variations in the refractive index in the variable-refractive medium. As a result, in a system such as that shown in Figure 35, where a free-space optical beam is transmitted over a sufficiently large optical communication distance D and received at a receiving node, different photons within a single pulse may take paths of different lengths to reach the receiving node and arrive at different times. These path length differences and the time required for photons to travel these distances can cause coherent interference in free-space optical communication systems, degrading signal quality, if the time delay is smaller than the source's coherence length. Solutions to this problem are described herein, including those with reference to Figures 40 and 41, and those applied within systems such as those shown in Figures 35A, 35B, and 36.

[0106] In addition to path length variations, photons within a pulse may travel at different speeds due to fluctuations in atmospheric conditions such as humidity, temperature, density, etc. As different photons within a pulse travel through slightly different atmospheric conditions, the photons may travel at different speeds and arrive at different times. Furthermore, different wavelengths of light within a pulse may travel at different speeds, allowing the pulse to further broaden as it passes through a variable refractive index medium.

[0107] FIG. 39 shows a diagrammatic representation of a pulse emitted by a transmitter and received by a receiver. As shown in FIG. 39, the pulse may have a pulse width of 90 femtoseconds when transmitted by the transmitting node. The pulse then propagates along an optical transmission distance, where it may be received by a photoreceptor having a detection window 4020 of a defined duration, such as 500 picoseconds. Once the pulse is received by the receiver, its received pulse width may be broadened by passing through a variable refractive index medium, as described above with respect to FIGS. 37-38. Due to variations in the path length traveled by the beam and variations in the atmospheric conditions through which the beam passes, different photons may reach the detector at different times along a distribution curve, which may have a time duration longer than the pulse duration at emission. The degree of broadening may vary depending on the length of the optical transmission distance and atmospheric conditions such as humidity, temperature, density, and the presence or absence of aerosols such as fog. This broadening can be on the order of picoseconds or longer depending on the conditions.

[0108] The pulse can have a time distribution curve as shown. While a typical time distribution curve is shown, other pulse shapes are possible. By making the width of curve 4010 longer than the coherence length of the emitted pulse (e.g., three times longer), coherent beam interference and coherent beam scintillation can be reduced.

[0109] FIG. 40 shows an exemplary time distribution curve of a short duration (e.g., about 100 femtoseconds) pulse 4010 that travels a significant distance (e.g., 1 mile) through a variable refractive index medium and becomes temporally spread out. When the pulse reaches the receiver, it may have a FWHM duration 4030 and a coherence time 4040, which may be equal to the coherence length of the pulse divided by the speed of light through the variable refractive index medium. In some embodiments, the FWHM duration 4030 may be greater than the coherence time 4040 of the pulse. Preferably, the FWHM duration 4030 may be at least 2 times, at least 3 times, at least 4 times, at least 5 times, at least 6 times, at least 8 times, at least 10 times, or at least 12 times the coherence time 4040 of the pulse. By ensuring that the FWHM duration 4030 of the pulse received at the receiver is relatively large compared to the coherence time 4040 of the pulse 4010, interference between different ray paths of the pulse may be reduced when they arrive at the receiver at different times, reducing noise and resulting in a higher quality signal reaching the receiver.

[0110] The optical receiver may have a detection window 4020 of a specified duration. Generally, a shorter detection window results in higher data throughput. For example, in a system using on-off keying for data modulation, an optical receiver with a 1 nanosecond detection window can extract up to 1 Gbps, while an optical receiver with a 100 picosecond detection window can extract up to 10 Gbps. The optical receiver can have a repeatable detection window of less than 100 ns, less than 10 ns, less than 1 ns, less than 100 ps, ​​or less than 10 ps.

[0111] However, due to pulse length and temporal spreading, it is possible that a photon from a pulse that is intended to be received in one detection window may fall into an adjacent detection window. This phenomenon can cause a bit error if the adjacent detection window does not receive the transmitted photon (e.g., because a "0" is transmitted at that bit position). Therefore, to maximize data transmission accuracy, it is important that the FWHM duration 4030 of the pulse received at the receiver be greater than (and preferably at least three times greater than) the coherence length 4040 of the pulse; at the same time, the FWHM duration 4030 of the pulse received at the receiver must also be substantially smaller than the receiver's detection window 4020.

[0112] For example, the detection window 4020 may be at least 2 times, at least 5 times, at least 6 times, at least 7 times, at least 7 times, at least 8 times, at least 10 times, or at least 20 times larger than the FWHM duration 4030 of the pulse received at the receiver. Preferably, at least 95%, at least 99%, or at least 99.99% of the photons in the pulse that reach the receiver may arrive at respective arrival times spaced from the center 4040 of the pulse's time distribution curve by respective time differences that are less than half the receiver's detection window duration. Note that while the center 4040 of the pulse's time distribution curve is shown at the center of the detection window 4020, this is not necessarily the case, and the pulse may arrive earlier or later than the midpoint of the detection window. Preferably, the center 4040 of the time distribution curve is at or near the center of the detection window 4020, to reduce the chance of photons in the pulse spilling into adjacent detection windows. In some embodiments, the center 4040 of the time distribution curve may be less than 100 picoseconds, 50 picoseconds, 20 picoseconds, 10 picoseconds, 5 picoseconds, 1 picosecond, 800 femtoseconds, or 500 femtoseconds from the center of the detection window 4020.

[0113] By specifying the relationship between the coherence time 4040 of the pulse, the FWHM duration 4030 of the pulse arriving at the receiver, and the detection window 4020 of the receiver in the manner described herein, data transmission accuracy and effective transmission range can be significantly improved (see the discussion below regarding FIG. 42 for test results). The FWHM duration 4030 of the pulse arriving at the receiver can vary depending on the pulse length transmitted from the source, the medium through which the pulse travels (e.g., atmospheric pressure, temperature, sunlight intensity, aerosols), and the distance the pulse travels to reach the receiver. Therefore, it may be necessary to decrease the coherence time 4040 of the pulse and / or increase the detection window 4020 of one or more receivers, depending on the conditions of the optical communication system. Decreasing the coherence time 4040 and increasing the detection window 4020 may therefore improve data transmission quality, while adversely affecting data throughput. In some embodiments, the system can be configured to determine the data transmission quality of the system (e.g., measuring the bit error rate or signal values ​​above or below a detection threshold) and, depending on the determined data transmission quality, can modify either or both the coherence time 4040 of the pulse or the detection window duration 4020 of the optical receiver.

[0114] Similarly, when using a light source capable of continuously emitting light, such as a continuous-wave source or a superluminescent diode, the emitted light can be gated (or converted into pulses using data modulation or other known techniques) into pulses that occupy only a relatively small portion of the detection window duration, and those pulses can be timed to arrive at or near the center of the receiver's detection window. Gating and timing the pulses in this manner reduces the risk of photons from the "on" window (where light is intended to be transmitted) spilling into the "off" window (where light is not intended to be transmitted) and causing bit errors. Therefore, the pulse duration and position relative to the detection window described above also apply to pulses generated using a source capable of continuously emitting light. In such cases, the light source can emit continuously, but its effective power may be "off" most of the time, even during the "on" transmission window where light is intended to be transmitted, leaving enough space between the center of the pulse and the edge of the detection window to avoid spillover. For example, during the "on" bit window during which light is intended to be transmitted, the effective output from the continuous light source may be less than 50%, less than 30%, less than 20%, or less than 10% of the respective transmission bit window.

[0115] Figure 41 shows a diagrammatic representation of light pulses arriving at detection windows 4020a, 4020b, 4020c of an optical receiver. The light pulses may be of any shape and can generally be broadened to some extent by traveling the optical path through a variable refractive index medium. In the first detection window 4020a, the light pulse may arrive at or near the center of the window such that the total light received within that window exceeds the detection threshold V. th, which may be processed by the optical receiver's circuitry to indicate that a pulse was received in that window. In some embodiments, this may cause the optical receiver to output a "1" for this detection window. At the end of detection window 4020a and before detection window 4020b, the optical receiver circuitry may be reset and return to zero. In detection window 4020b, no pulse is transmitted (e.g., because a "0" was intended to be transmitted and the modulator at the transmitting node blocked the pulse), and all light received in window 4020b exceeds the detection threshold V th , which may cause the optical receiver to output a "0" for this detection window. The optical receiver circuit may again reset back to zero, and the cycle may repeat in the third window 4020c, and so on.

[0116] Detection threshold V th is high enough that the ambient light does not cause false positives, but the true pulse is detected below the detection threshold V th The detection threshold V can be configured to be low enough to ensure that the pulse is sufficiently above the noise floor so that there is sufficient signal difference between the "on" and "off" bit windows to ensure that the detection threshold V th It is important that V can be both high enough to ignore environmental noise and low enough to capture all transmitted pulses. This is particularly challenging over long distances (e.g., over 1 mile) or under less than optimal environmental conditions (e.g., partly clear skies, heavy aerosols, etc.). The relationship between pulse length, coherence time, and detection window at the receiver described herein with respect to Figures 39-41 significantly improves signal quality transmission, enabling detection thresholds V to be useful even for free-space optical systems transmitting data over optical transmission distances of 1 mile, 2 miles, 3 miles, 5 miles, or even 7 miles. th This makes it possible.

[0117] With a beam splitter and multiple receivers, each of the multiple receivers may generate a bit stream based on the separated pulses directed at that receiver, and the bit streams from each receiver may be interleaved to generate a combined bit stream having a higher data rate. The combined bit stream may be output to a communications network as described above, including with respect to FIG. 36.

[0118] FIG. 42 shows an example of test data received over a one-mile optical communication distance. The test data compares an optical signal generated using the transmitting node described above with respect to FIG. 35A with an optical signal generated using a continuous-wave source with the same average power as the USPL source. Specifically, to generate the data shown in the top row of the chart shown in FIG. 42, data was transmitted over a one-mile optical communication distance using a USPL source integrated into the transmitting node as described above with respect to FIG. 35A. The received signal was directed toward a piece of white paper, and an infrared camera was placed behind the paper to record the light that passed through the paper. To generate the data shown in the bottom row of the chart shown in FIG. 42, the same experimental setup was used, using a continuous-wave source with the same average power and optical communication distance as the USPL source. Light from both the USPL source and the CW light source was directed toward the same white paper, and two signal spots were captured in the same frame using the infrared camera. The spot size was approximately 12 inches in diameter. Background ambient light was subtracted from each pixel, and each pixel was subjected to thresholding logic such that pixels with received optical signals above the threshold were set to "white" and pixels with received optical signals below the threshold were set to "black." The four images shown for each source were taken from the same frame of the video feed, and the frames were evenly spaced at 10-second intervals. Frame A shows the received signals from the USPL and CW sources at 10 seconds, Frame B shows the received signals from the USPL and CW sources at 20 seconds, Frame C shows the received signals from the USPL and CW sources at 30 seconds, and Frame D shows the received signals from the USPL and CW sources at 40 seconds.

[0119] This data demonstrates that transmitting nodes as described herein are substantially more clustered and, within the detection field, generate ultrashort pulses that far more reliably exceed the detection threshold. When applied to communication systems employing optical receivers with the characteristics described above, including those referenced in Figures 35B-41, this significantly improves data transmission accuracy. Testing of Applicant's systems according to the present disclosure has demonstrated free-space optical communication distances of over 1 mile, 2 miles, 3 miles, 5 miles, and up to 7.4 miles with zero bit error rates measured over time intervals of at least 10 seconds, at least 30 seconds, at least 60 seconds, at least 10 minutes, at least 30 minutes, and at least 1 hour. In some embodiments, the systems described herein may transmit data over optical communication distances of at least 1 mile with measured bit error rates of less than one in a million, less than one in a billion, less than one in a trillion, or less than one in a trillion over a measurement period of at least 60 seconds. To Applicant's knowledge, no other free-space optical system has achieved optical communication distances as low as even half a mile.

[0120] Thus, the systems described herein enable significantly improved data transmission accuracy, communication link distance, and also enable the use of free-space optical communications in adverse weather conditions (e.g., rain, fog, atmospheric scintillation) that previously disabled free-space optical communications in previous systems. In some embodiments, the improved data transmission quality and range may also enable the application of free-space optical communications to systems where it was previously impossible to use effectively. For example, transmitting and / or receiving nodes according to the present disclosure may be provided on Earth-orbiting satellites to provide ground-to-space and / or space-to-ground free-space optical communications. While effective optical data transmission was not demonstrated using technology prior to the present disclosure due to the amount of atmosphere through which beams must travel between Earth ground level and space, the technology described herein can achieve effective optical communications over this distance.

[0121] FIG. 43 illustrates an exemplary ranging node 4400 that can be used to detect objects or surfaces and determine the location of those objects relative to the node. The ranging node 4400 may generally include the components of the transmitting and receiving nodes 3510, 3530 described above with respect to FIGS. 35A and 35B. For example, the ranging node 4400 may include a source 3512, a splitter, one or more modulators, amplifiers, and a telescope. These elements may be collectively configured to emit a light pulse that travels through a variably refractive medium toward the surface S. For a laser ranging node, data modulation is optional but may be included to encode information related to the pulse, the node, or other information. Photons from the light pulse can be reflected by the surface S and return to the node 4400. The total travel distance of the light pulse from transmission by the ranging node to receipt of the reflected pulse may be twice the distance from the node to the surface S. Upon returning to the node, the pulse may be received by aperture 3532, optionally split by splitter 3534, and analyzed using one or more optical receivers 3536. Each of these components may have the same characteristics and parameters as the corresponding components described above with respect to Figures 35A-41. Ranging node 4400 may further include time-of-flight (TOF) circuitry 4410, which may be configured to determine the time of flight of the pulse from reaching surface S to returning to node 4400, and thereby determine the distance of that surface S from ranging node 4400.

[0122] One or more aspects or features of the subject matter described herein can be implemented in digital electronic circuitry, integrated circuits, specially designed application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), computer hardware, firmware, software, and / or combinations thereof. These various aspects or features may include implementation in one or more computer programs executable and / or interpretable on a programmable system including at least one programmable processor, which may be special-purpose or general-purpose, coupled to receive data and instructions from, and transmit data and instructions to, a storage system, at least one input device, and at least one output device.

[0123] These computer programs, also referred to as programs, software, software applications, applications, components, or code, contain machine instructions for a programmable processor and may be implemented in high-level procedural and / or object-oriented programming languages, and / or assembly / machine languages. The term "machine-readable medium" as used herein refers to any computer program product, apparatus, and / or device used to provide machine instructions and / or data to a programmable processor, including a machine-readable medium that receives the machine instructions as a machine-readable signal, such as a magnetic disk, optical disk, memory, and programmable logic device (PLD). The term "machine-readable signal" refers to any signal used to provide machine instructions and / or data to a programmable processor. A machine-readable medium may non-transiently store such machine instructions, such as, for example, a non-transient solid-state memory, a magnetic hard drive, or any equivalent storage medium. A machine-readable medium may temporarily store such machine instructions, such as, for example, a processor cache or other random access memory associated with one or more physical processor cores.

[0124] To provide for user interaction, one or more aspects or features of the subject matter described herein can be implemented on a computer having a display device, e.g., a cathode ray tube (CRT) or liquid crystal display (LCD) or light-emitting diode (LED) monitor, for displaying information to a user, and a keyboard and pointing device, e.g., a mouse or trackball, through which a user can provide input to the computer. Other types of devices can also be used to provide for user interaction. For example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback. Input from the user can also be received in any form, including, but not limited to, acoustic input, voice input, or tactile input. Other possible input devices include, but are not limited to, touchscreens or other touch-sensitive devices, e.g., single-point or multi-point resistive or capacitive trackpads, voice recognition hardware and software, optical scanners, optical pointers, digital image capture devices and associated interpretation software, etc. The computer remote from the analytical device can be linked to the analytical device via a wired or wireless network to allow data exchange between the analytical device and the remote computer (e.g., receiving data from the analytical device at the remote computer and transmitting information such as calibration data, operating parameters, software upgrades or updates, etc.) and remote control, diagnostics, etc. of the analytical device.

[0125] While the subject matter of the present disclosure has been described and illustrated in considerable detail with reference to specific exemplary embodiments, including various combinations and subcombinations of features, those skilled in the art will readily appreciate other embodiments and variations and modifications thereof that fall within the scope of the present disclosure. Moreover, the description of such embodiments, combinations, and subcombinations is not intended to convey that the claimed subject matter requires features or combinations of features other than those expressly recited in the claims. Accordingly, the scope of the present disclosure is intended to include all modifications and variations that fall within the spirit and scope of the following appended claims.

Claims

1. 1. An optical communication system for optically transmitting data through a variable refractive index medium, comprising: a light source configured to generate a beam comprising a series of light pulses; a modulator configured to modulate the series of optical pulses in response to a data transmission signal, thereby encoding transmission data into the series of optical pulses; A receiver, a detection window duration of 1 nanosecond or less, and an optical receiver having a detection threshold configured to indicate whether the optical energy received by the optical receiver during a given detection window is greater than the detection threshold; the series of optical pulses includes a first pulse having a coherence length of less than 400 microns; As the first pulse passes through the variably refractive medium, photons of the first pulse are refracted and travel along different ray paths of different lengths to the optical receiver; the photons of the first pulse arrive at the receiver according to a time distribution curve that depends, at least in part, on a duration of the first pulse and the lengths of different ray paths taken by the photons of the first pulse to the receiver; a full width at half maximum (FWHM) value of the temporal distribution curve is greater than a coherence time value equal to the coherence length of the first pulse divided by the speed of light through the variable refractive index medium; and the detection window duration of the optical receiver is greater than the FWHM value of the time distribution curve; Optical communication system.

2. 10. The optical communication system of claim 1, wherein the light source and the optical receiver are spaced apart by a free-space optical communication distance of at least one mile, and the optical communication system has a measured bit error rate of less than one part in a billion over a measurement period of at least 60 seconds for the free-space optical communication distance of at least one mile.

3. 2. The optical communication system of claim 1, wherein the FWHM value of the time distribution curve is at least three times as large as the coherence time value, which is equal to the coherence length of the first pulse divided by the speed of light through the variable refractive index medium.

4. 4. The optical communication system of claim 3, wherein the light source is a continuous wave source, and the beam is gated into a series of light pulses each generating a respective time distribution curve traveling to the optical receiver, each time distribution curve having a full width at half maximum (FWHM) value less than 50% of the detection window duration.

5. 4. The optical communication system of claim 3, wherein the light source is a continuous wave source, and the beam is gated into a series of light pulses each generating a respective time distribution curve traveling to the optical receiver, each time distribution curve having a full width at half maximum (FWHM) value no greater than 30% of the detection window duration.

6. 2. The optical communication system of claim 1, wherein the FWHM value of the time distribution curve is at least six times as large as the coherence time value, which is equal to the coherence length of the first pulse divided by the speed of light through the variable refractive index medium.

7. 2. The optical communication system of claim 1, wherein at least 95% of the photons of the first pulse that reach the optical receiver arrive at respective arrival times that are spaced apart from the center of the time distribution curve by respective time differences that are less than half the detection window duration of the optical receiver.

8. 2. The optical communication system of claim 1, wherein the optical source is located at a ground station and the optical receiver is located at an Earth-orbiting satellite, and the optical communication system has a measured bit error rate of less than one part in a billion over a measurement period of at least 60 seconds over a free-space optical communication distance between the ground station and the Earth-orbiting satellite.

9. 10. The optical communication system of claim 1, wherein the series of optical pulses produced by the light source have a center wavelength between 1500 nm and 1700 nm, and the optical receiver is located on a detection node including a filter configured to reduce the amount of light with wavelengths less than 1500 nm reaching the optical receiver.

10. the light source is located at a transmitting node; the transmitting node includes a beam splitter configured to split a combined pulse generated by the light source into a plurality of separate pulses having different wavelength bands, the first pulse being among the plurality of separate pulses; the transmitting node is configured to separately modulate each of the plurality of separated pulses in response to the data transmission signal, thereby encoding the transmission data onto the plurality of separated pulses; each of the plurality of separated pulses having a respective coherence length of less than 400 microns; the optical receiver is located at a receiving node; the receiving node includes a beam splitter configured to direct the plurality of separated pulses to a respective optical receiver of a plurality of optical receivers, the optical receiver being among the plurality of optical receivers; each of the plurality of separated pulses includes a respective ray path that reaches the respective receiver of the plurality of receivers according to a respective time distribution curve; and each of the plurality of separated pulses has a respective FWHM value of its time distribution curve that is at least three times as large as a respective coherence time value equal to the respective coherence length of the each separated pulse divided by the speed of light through the variable refractive index medium; 2. The optical communication system according to claim 1.

11. an amplifier configured to amplify the magnitude of the series of light pulses; a threshold filter configured to receive the series of optical pulses after the transmission data is encoded by the modulator and before the series of optical pulses reaches the amplifier; the threshold filter is configured to selectively attenuate pulses having a magnitude less than a threshold of the threshold filter; 2. The optical communication system according to claim 1.

12. 1. A laser ranging system, comprising: a light source configured to generate a beam comprising a series of light pulses; A receiver, a detection window duration of 1 nanosecond or less, and a detection threshold configured to indicate whether the optical receiver receives optical energy during a given detection window greater than the detection threshold; a light receiver; the series of light pulses includes a first pulse having a coherence length of less than 400 microns; As the first pulse passes through a variable refractive index medium, photons of the first pulse are refracted and travel along different ray paths of different lengths to the receiver; the photons of the first pulse arrive at the receiver according to a time distribution curve that depends, and is at least partially dependent, on a duration of the first pulse and the lengths of different ray paths taken by the photons of the first pulse to the receiver; a full width at half maximum (FWHM) value of the temporal distribution curve is greater than a coherence time value equal to the coherence length of the first pulse divided by the speed of light through the variable refractive index medium; the detection window duration of the optical receiver is greater than the FWHM value of the time distribution curve; and the laser ranging system is configured to transmit the series of light pulses toward a surface, receive at least a portion of the series of light pulses reflected by the surface, and determine a distance of at least a portion of the surface from the laser ranging system based on a time of flight of the received portion of the series of light pulses. Laser ranging system.

13. 13. The laser ranging system of claim 12, wherein the FWHM value of the time distribution curve is at least three times as large as the coherence time value, which is equal to the coherence length of the first pulse divided by the speed of light through the variable refractive index medium.

14. 14. The laser ranging system of claim 13, wherein the light source is a continuous wave source, and the beam is gated into a series of light pulses each generating a respective time distribution curve traveling to the receiver, each time distribution curve having a full width at half maximum (FWHM) value less than or equal to 50% of the detection window duration.

15. 14. The laser ranging system of claim 13, wherein the light source is a continuous wave source, and the beam is gated into a series of light pulses each generating a respective time distribution curve traveling to the receiver, each time distribution curve having a full width at half maximum (FWHM) value no greater than 30% of the detection window duration.

16. 13. The laser ranging system of claim 12, wherein the FWHM value of the time distribution curve is at least six times as large as the coherence time value, which is equal to the coherence length of the first pulse divided by the speed of light through the variable refractive index medium.

17. 13. The laser ranging system of claim 12, wherein at least 95% of the photons of the first pulse that reach the receiver arrive at respective arrival times that are spaced apart from the center of the time distribution curve by respective time differences that are less than half of the detection window duration of the receiver.

18. 13. The laser ranging system of claim 12, wherein the light source and the receiver are located at a ground station and the surface is disposed on an Earth orbiting satellite, and the laser ranging system has a measurement bit error rate of less than one part per billion over a measurement period of at least 60 seconds at a distance of at least one mile.

19. 13. The laser ranging system of claim 12, wherein the series of light pulses produced by the light source have a center wavelength between 1500 nm and 1700 nm, and the light receiver is positioned behind a filter configured to reduce the amount of light with wavelengths below 1500 nm that reaches the light receiver.

20. the laser ranging system includes a first beam splitter configured to split a combined pulse generated by the light source into a plurality of separated pulses having different wavelength bands, the first pulse being among the plurality of separated pulses; the laser ranging system is configured to separately modulate each of the plurality of separated pulses in response to a signal, thereby encoding signal data onto the plurality of separated pulses; each of the plurality of separated pulses having a respective coherence length of less than 400 microns; the laser ranging system includes a second beam splitter configured to direct the plurality of separated pulses to a respective receiver of a plurality of receivers, the receiver being among the plurality of receivers; each of the plurality of separated pulses includes a respective ray path that reaches the respective receiver of the plurality of receivers according to a respective time distribution curve; and 13. The laser ranging system of claim 12, wherein each of the plurality of separated pulses has a respective FWHM value of its time distribution curve that is at least three times as large as a respective coherence time value equal to the respective coherence length of the each separated pulse divided by the speed of light through the variable refractive index medium.

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