Method and apparatus for ultrashort pulse laser communication through lossy media

Abstract

Free-space optical (FSO) wireless transmissions, including optical communications, remote sensing, power beaming, etc., can be enhanced by replacing conventional laser sources operating in the infrared portion of the spectrum with ultrashort pulse laser (USPL) sources with peak pulse powers of 1 kW or more and pulse lengths less than 1 picosecond. Specifically, it has been observed that under these conditions, in a lossy medium (e.g., the atmosphere), the attenuation of a USPL beam with the same average optical power as a conventional laser is significantly less than the attenuation of a conventional laser beam with lower peak pulse power and / or longer pulse width. The superior system performance when using USPL can translate into increased distances between the laser source in the transmitter and the photodetector in the receiver and / or higher reliability of system operation in adverse weather conditions.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Patent Application No. 16 / 269,106, filed on February 6, 2019, entitled “Method and Apparatus for Ultrashort Pulse Laser Communication Through Lossy Media,” the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The subject matter described herein relates to free space optics (FSO) wireless transmissions, including optical communications, remote sensing, power transmission, etc., and more particularly, to enhanced optical transmission efficiency that can be achieved for wavelength propagation using ultrashort pulse laser (USPL) sources for beam propagation through optically impaired atmospheric conditions due to various factors, which factors can include but are not limited to fog, clouds (i.e., water aerosols), atmospheric beam drift, scintillation effects, etc. Background Art

[0004] The explosive growth in demand for telecommunication services from the private as well as commercial and government sectors has placed unprecedented pressure on currently available telecommunication networks. Without alternative network transport technologies and topologies, the overall effective network speed is likely to be limited, while the occurrence of bottlenecks within the network will become increasingly frequent.

[0005] Bidirectional free space optics (FSO) communication networks can provide useful alternatives to microwave links, wire or cable system applications where feasible. Such networks can be transparent to current and future network architectures due to the sharing of a common technology platform with an optical fiber transmission system (the backbone of many modern telecommunication systems). FSO communication systems can usually share common optical fiber components, and commercial optical components can often be used for free space optics and optical fiber applications. The main difference in free space optical data links is that the medium of propagation is the atmosphere rather than optical fiber.

[0006] Utilizing current state-of-the-art fiber optic components, free space optical data links can be fully integrated into current short-haul and long-haul high-speed optical networks. Free space data links can fully reach current synchronous optical network (SONET) system architectures, such as SONET OC-48 architectures utilizing current 1550nm wavelength optical technology platforms, for example. In addition, such systems can be upgraded to higher data rates and configurations. Optical data link systems can benefit from operating in unregulated segments of the electromagnetic spectrum. Unlike microwave and RF spectrums, optical data links can typically not require special lease fees or rates to be issued. In addition, because of the operating wavelength of the system, issues related to eye safety can typically be minimized. In addition, special measures or licenses for operating free space optical data links related to ground rights of way are typically not required. The costs associated with digging and trenching for fixed wired systems can also be avoided.

[0007] Recently, FSO communication technology has taken advantage of commercial advances made within the 1550 nm optical wavelength transmission band. Erbium doped fiber amplifier (EFDA) technology has been incorporated into system design configurations to enhance the overall effective optical budget to achieve acceptable optical attenuation and thereby extend the reach of the transmission system in the atmosphere.

[0008] High power optical amplifiers are useful for terrestrial free space transmission as well as fiber optic systems. Repeater distances have been extended in terrestrial and submarine fiber optic systems and dense wavelength division multiplexing (DWDM) transmission architectures have been introduced. With the advent of high power Er / Yb optical amplifiers, similar advances as seen in fiber optic transmission have also been achieved in optical wireless and free space laser communication systems. Experimental transmission results of a single channel 1550nm free space optical data link operating at 2.5Gbps over a 2.4km transmission span have been reported, as have results of a four channel 1550nm wavelength division multiplexing (WDM) free space optical data link operating at 10Gbps over a 4.4km transmission distance.

[0009] While several commercial and military FSO communication systems have been developed and are in operation, their performance for ground applications is limited by atmosphere-related optical attenuation effects caused by adverse weather effects including fog, rain, snow, smoke, and air turbulence. These effects limit the application of FSO communications to relatively short optical transmission distances (on the order of a few hundred meters) in typical scenarios requiring high reliability and uninterrupted availability.

[0010] On August 4, 1997 a patent application assigned to Motorola Inc. (Application No. 08 / 905,760) was filed promising to mitigate the undesirable attenuation effects caused by the atmosphere, and a patent for that application was granted in 2003 (US 6,043,920). It states in column 2, lines 22 to 35: "The method and apparatus of the present invention relates to the application of these newly discovered properties of narrow pulse width signals to the field of laser communications. According to a preferred embodiment, extremely narrow width modulated laser pulses can be transmitted through lossy media without suffering significant attenuation. The method and apparatus of the present invention thus enable extremely high rate communications through media previously thought to be too lossy to facilitate high speed laser communications. For example, the method and apparatus of the present invention can be used for laser communications through air, water, water vapor, solid obstructions, suspended particles, glass fibers, and other media."

[0011] However, this assertion is supported only by the following statement (column 2, lines 16-21): "Modern laser technology has made it possible to produce narrow-width pulses, and experiments have shown that these narrow-width pulses have the property of being able to penetrate media with much less attenuation than is suffered by existing technology."

[0012] Claim 1 in the issued patent is very broad, including applicability to communications over any wireless link through any lossy medium (such as the atmosphere), using laser modulated pulses of unspecified wavelength with a pulse width of less than 200 femtoseconds.

[0013] Based on research conducted by Attochron LLC and elsewhere, it is surmised that the patent, and in particular the claim, is based on a constructive reduction to practice that has never been verified or replicated by others, even after the patent's term expired in 2017. However, there has been considerable debate over the years regarding the potential reduction of optical attenuation in various media using ultrashort optical pulses. For example, patent number US 6,583,911 B1 filed on October 6, 2000 and granted on June 24, 2003 (inventor DR Alexander,) reported extensive laboratory testing using ultrashort laser pulses with a duration of less than 100 femtoseconds and a wavelength range of 0.75 to 0.85 microns, which states: "...the data do not indicate that pulsed wave transmission is more beneficial than continuous wave." This factual data directly contradicts the assertion in the Motorola patent.

[0014] Subsequently, in 2005 and 2006, two papers were published by Ulf Osterberg et al. [UJ Gibson, U.L. Osterberg, “Optical precursors and Beer’s law violation; non-exponential propagation losses in water”, Optics Express 13 6 (2005) and A.E. Fox, U.Osterberg, “Observation of non-exponential absorption in superconducting fast pulses in water”, Optics Express 14 8 (2006)] showing that the absorption of USPL beams may not obey Beer’s law (exponential decay of beams in lossy media). The authors used these results to show that the optical absorption losses of femtosecond laser pulses are significantly lower than those of CW lasers operating at the same wavelength. Wavelengths in the range of 650 to 800 nm were reported to have been used in the experimental part of the research work. Therefore, at first, it seemed that the underlying hypothesis of the unusually low optical attenuation reported in the Motorola patent US 6,043,920 filed in 1997 may have been confirmed a posteriori by the work of Ulf Osterberg et al. Even if it is somewhat speculative and contradicts the data provided by Alexander in US 6,583,911 Bl.

[0015] However, a significant paper published in 2007 by Jane Chao Li et al. WO 2020 / 162960 PCT / US2019 / 033132 from the University of Nebraska [University of Nebraska – Lincoln; Digital Commons @ University of Nebraska – Lincoln; Electrical Engineering Department Faculty Publications, 2-2007] entitled “Propagation of Ultrashort Laser Pulses through Water” specifically refutes the above conclusions and interpretations of Osterberg et al. This paper concludes that the sub-Beers’ law (non-exponential) absorption effect observed by Osterberg et al. is simply due to the broad spectral band of USPL and has nothing to do with the shortness of the laser pulses. Li et al. point out that the same sub-Beers effect will be observed for an incoherent CW source with the same broad spectral band as USPL. Therefore, the sub-Beers’ law absorption cannot be directly attributed to the lower attenuation of femtosecond pulses in water. This again calls into question any advantage of using ultrashort pulses to enhance transmission through lossy media, as taught by the inventors at Motorola in US 6,043,920.

[0016] The most definitive answer to the question “Is there any benefit to using USPL to enhance atmospheric transmission?” seems to come from the work of Paul Corrigan et al. in a 2007 paper titled “Improving the performance of a low-power (60mW) femtosecond free-space optical communication system over conventional CW operation” [Proc. SPIE 6457, Free-space Laser Communications Technology XIX and Atmospheric Propagation of Electromagnetic Waves, 64570X (WO 2020 / 162960 PCT / US2019 / 033132 February 2007)]. The answer to the above question seems to be a marginal “yes” for light scattering and a “no, not yet” for light absorption. The authors of this paper state in their conclusion:

[0017] "Once the transmission and scattering properties of a stable substance are known (usually determined by two different measurements), the difference is the absorption. This [separating absorption from scattering effects] is particularly difficult to do in simulated fog because the simulation itself is unstable, just like real fog. However, there are no current measurements or statements indicating that this enhanced propagation is related to different physical absorption of ultrafast pulses by the atmosphere."

[0018] It should be noted here that this work by Corrigan et al. was performed at a longer wavelength (1560nm) than that used by previously reported researchers. Therefore, these results may have a different interpretation than some or all of the other results discussed above.

[0019] In the face of all the above unproven results and the subsequent contradictions and controversies, it is not surprising that the growth of FSO communications has been limited and that no commercial system based on the claims in the expired Motorola patent has been produced (U.S. 6,043,920). Clearly, if the use of ultrashort laser pulses can significantly improve terrestrial free-space optical communications under harsh conditions, it would be beneficial to establish once and for all. If the results are positive, this would represent a major breakthrough and have a significant beneficial impact on the adoption of FSO communications in the future. Summary of the invention

[0020] A turning point related to the above discussion occurred in 2013 when Isaac Kim et al. published an article in Laser Focus magazine in 2013 titled “Advances in Communications: New FSOs Provide Reliable Backhaul Connectivity of 10Gbit / sec and Beyond.” The article reported:

[0021] “Experiments conducted by Attochron, LLC at the 500m wireless test facility at the U.S. Army’s Picatinny Arsenal in Dover, New Jersey, showed that a USP laser-based FSO system achieved a 25dB improvement in received power over a conventional CW FSO system in foggy conditions.” The authors also mention that “conventional FSO systems work well in clear or hazy weather up to 1.5 km, but the presence of fog reduces the effective link distance to 200 m [II Kim, Lightwave, 26, 19-21 (2009)]”.

[0022] In this work, the USPL system and the CW laser system were operated simultaneously, side by side, with their laser beams crossing adjacent optical paths within the same test range. They go on to say:

[0023] “The output of these new USP FSO systems is generated from passively mode-locked 100fs pulses at 1550nm with an average output power of 50mW and a repetition rate of 1Gbit / s [data]. The ultrashort pulse stream is modulated externally to produce a Gigabit Ethernet signal. A 3-inch diameter telescope was used on the transmitting side and a similar 3-inch telescope on the receiving side in the Picatinny Arsenal experiment.”

[0024] The conclusion drawn by Isaac Kim et al. is: "In preliminary tests of our prototype USP laser-based FSO system, 25dB of additional margin increased the link availability of 1Gbit / s to 99.5% at [a range of] 3km".

[0025] After achieving these important results during testing of the Picatinny Arsenal in 2010 and 2011, Attochron LLC filed a patent application (Application No. 13 / 737,898) on January 16, 2013, with broad claims describing:

[0026] “An optical communication device, comprising: an ultrashort pulse laser (USPL) source, which generates a light beam including light pulses, each light pulse having a duration of about 1 nanosecond or less (claim 1), or 1 picosecond or less (claim 3), or 1 femtosecond or less (claim 4)…

[0027] However, the scope of the claims of a granted patent is substantially limited by the following clause, which is added to all claims during the prosecution of the application:

[0028] "Wherein the optical transceiver is configured to detect atmospheric elements and is capable of analyzing backscattered signals of airborne particles characteristic of the detected atmospheric elements, thereby being capable of adjusting the light beam generated by the USPL source to enhance atmospheric penetration."

[0029] In fact, the broader claims of the original application are disallowed because it is not apparent from the Attochron patent application as filed that an invention was made absent this limiting clause. Specifically, the specifications do not even mention the field test results obtained by Attochron LLC at Picatinny Arsenal. However, even if they were so introduced, the superior system performance observed and reported by Attochron LLC staff during field testing at Picatinny Arsenal was simply due to one or more well-known effects, such as (1) reduced effects of atmospheric turbulence on the USPL beam, since its coherence is known to be lower than that of the CW laser beam used as a reference during the Picatinny Arsenal testing, and / or (2) known improved signal-to-noise ratio at the time of detection of a laser beam having higher peak pulses than a CW laser beam or a laser beam having comparable energy per received bit (see L. Biovin et al., "Improvement of Receiver Sensitivity by Pulse Coding," IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 5, May 1997), or (3) other known factors.

[0030] One of the purposes of this application is to summarize all the important results observed during the testing at the Picatinny Arsenal in 2010 and 2011, as well as further results revealed in October 2017 through an extensive analysis of the resulting recorded data. These results are impressive. In foggy conditions as well as in clear warm weather conditions that lead to scintillation effects, the performance of the USPL system showed in both cases an advantage of more than 25dB in terms of received signal margin compared to a side-by-side CW laser FSO system operating in the same test range and, in fact, on exactly the same optical path as the USPL beam path. Furthermore, the observed advantage factor of at least 25dB of the USPL system over the CW system is only a lower limit and may be a larger factor that may appear when evaluating more powerful laser sources.

[0031] It should be noted that the advantage factor of at least 25 dB for the USPL system over the CW system discussed above is expressed in logarithmic terms, which is conventional for describing the performance of many types of communication systems. In linear terms, this is equivalent to a factor of 316 or more (log 316 = 10 x 2.5 = 25 dB). When expressed in this way, it is clear that this is a very large factor that is unexpected based on any reported prior art or known physics.

[0032] While there are many theories that could explain some or all of the observed results, some nonlinear optical phenomenon that is not yet understood or widely accepted could be contributing to the large observed advantage factors associated with USPL's CW laser test results. Preliminary testing by Attochron LLC also suggests this is the case. Specifically, the observed advantage factors do not appear when using ultrashort laser pulses with lower peak power levels, less than about 1 kilowatt. However, the advantage factors become evident when using ultrashort laser pulses with pulse widths less than 1 nanosecond that operate at optical peak powers of 1 kilowatt or more. These effects become apparent when peak pulse optical power levels are in the range of 5 to 10 kilowatts or more.

[0033] With this new insight into FSO communications, it becomes clear why the unproven claims in Motorola’s expired patent (6,043,920) have remained so controversial for years. The beneficial effects of using femtosecond pulses can only be realized when using a laser source with a peak optical power in the range of 1 kW or greater, preferably in the range of 10 kW or greater. Most of the claims in Motorola’s patent are limited to “applying a modulated [electrical] pulse stream to a [CW] laser [beam] to generate laser pulses, wherein each pulse of the pulse stream has a pulse width of less than 200 femtoseconds; and directing the laser light pulses through a lossy medium toward a detector.” The reality is that the only way to meet these conditions is to start with a CW laser with an average output power of at least 1 kW, which can be sculpted into pulses less than 200 femtoseconds using an external beam modulator. Such high power lasers are impractical to build and operate, and are too expensive for commercial FSO communications systems. However, the use of USPL sources with more modest average output powers but very high peak pulse powers in excess of 1 kW in FSO communications systems is the essence of the present invention. This results in an unexpected improvement in atmospheric transmission compared to the use of continuous wave (CW) lasers and all types of low-power pulsed lasers.

[0034] It should be noted that the spectral width of a pulse from a USPL can have considerable width. For example, a USPL with a central wavelength of 1560 nm may have a spectral bandwidth extending for hundreds of nanometers or more. This is due to the fact that there is a fundamental inverse relationship between the minimum spectral width Af of a pulse from any laser and its temporal pulse width At. That is, Af = 1 / At. Since the wavelength X is defined by the relationship X, = c / f, where c is the speed of light in a vacuum and f is the center frequency of the laser, differentiating this relationship yields AX, = (-c / f2)Af = -(X 2 / c)(1 / At). For example, if X,=1560nm and the pulse width, At, is 20 femtoseconds, then using this equation, the minimum spectral width, AX, will be equal to 406nm. Similarly, for a 10 femtosecond pulse width USPL, the minimum spectral width will be twice that of 812nm. For a USPL with a 100 femtosecond pulse width, the minimum spectral width is 81nm.

[0035] The following summarizes the main results observed:

[0036] 1. Unforeseen benefits of propagating USPL pulses via clear-air atmospheric effects:

[0037] The USPL link margin (relative to CW lasers) at the “intended” central wavelength (1550nm) through clear-air atmospheric effects (scintillation, beam wander, turbulence) is improved by 30dB+.

[0038] 2. Unforeseen benefits of USPL pulses propagating through water aerosols (fog, clouds, etc.):

[0039] The USPL link margin (relative to CW lasers) at the “intended” central wavelength (1550nm) through water aerosols (fog, clouds, etc.) is improved by 30dB+.

[0040] 3. Unforeseen benefits of wavelength shift to “deeper infrared” wavelengths in dense water aerosols (improved propagation):

[0041] In heavy water aerosol (sometimes “at or near” — 100 to — 150 dB attenuation per kilometer), and after the CW laser beam decayed below its detection level (i.e., below the noise floor of the optical power meter) and after the most intense portion of the USPL output spectrum (centered at 1550 nm) also decayed below its detection level, the remaining 1571 nm portion of the USPL beam (with an emission-average power 5 dB lower than the 1550 nm portion of the spectrum) was well above the detection level by 15 dB and stable (low temporal fluctuations).

[0042] 4. Unforeseen benefits of USPL pulse reception “peak-to-fade” power levels:

[0043] The received power fluctuations in the USPL beam - both during periods of lower attenuation, but most critically, during periods of increasing or decreasing attenuation due to weather effects - are detected over very short time periods (typically 1 second compared to power fluctuations in a CW laser beam), and

[0044] 5. Benefits of using true discrete return-to-zero high peak power pulses for pulse encoding:

[0045] By using ultrashort pulses as signal carriers in any type of "pulse code" modulation, the link margin of the photodetector is improved by more than 10dB.

[0046] It should also be noted that the above benefits, both individually and in combination, significantly improve the overall availability of USPL communications and sensor systems.

[0047] Depending on the desired configuration, the subject matter described herein may be embodied in systems, devices, methods and / or articles. The embodiments set forth in the foregoing description do not represent all embodiments consistent with the subject matter described herein. On the contrary, they are merely some examples consistent with aspects related to the described subject matter. Although some variants have been described in detail above, other modifications or additions are also possible. In particular, further features and / or variations may be provided in addition to those set forth herein. For example, the above-described embodiments may relate to various combinations and sub-combinations of the disclosed features and / or combinations and sub-combinations of the above-mentioned several further features. In addition, the logical flows depicted in the accompanying drawings and / or described herein do not necessarily require the specific order or order shown to achieve the desired results. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate certain aspects of the subject matter disclosed herein and, together with the description, help explain some of the principles associated with the disclosed embodiments. In all cases where USPLs are shown or discussed, it should be assumed that the peak output power of these lasers in the drawings is 1 kW or greater and their pulse width is 1 nanosecond or less. In the drawings:

[0049] Figure 1 An example of an optical communication platform including free-space coupling of a USPL source as a light source for transmission to a remote optical receiving terminal is depicted;

[0050] Figure 2 An example of an optical communication platform including a fiber-optic coupling of a USPL source as a light source for transmission to a remote optical receiving terminal is depicted;

[0051] Figure 3 An example of an optical communication platform including fiber coupling of a USPL source to an external modulator for transmission to a remote optical receiving terminal is depicted;

[0052] Figure 4 An example of an optical communication platform including fiber coupling of a USPL source to an external modulator through a fiber optic medium for transmission to a remote optical receiving terminal is depicted;

[0053] Figure 5Depicted are examples of transmitting elements and or receiving elements, which may be of a type derived from hyperbolic mirror fabrication techniques or conventional Newtonian design;

[0054] Figure 6 depicts examples of fiber amplifier elements identified and used to increase enhanced optical transmit pump power for transmission to a remote optical receive terminal;

[0055] Figure 7 depicts an example of a USPL laser device fiber-coupled to an external modulator for transmission in a point-to-point configuration for transmission to a remote optical receiving terminal;

[0056] Figure 8 depicts an example of a USPL laser device fiber-coupled to an external modulator for transmission in a point-to-multipoint configuration;

[0057] Fig. 9 Depicted are examples of the use of a USPL source as a tracking and aiming (targeting) beacon source;

[0058] Fig.10 depicts 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;

[0059] Fig.11A and Fig. 11B Depicting examples of USPL-FSO transceivers for use in line-of-sight and non-line-of-sight laser communication applications, respectively;

[0060] Fig.12 depicts an example of forward propagating light (including light from a data signal) being backscattered by interaction with airborne particles that are the subject of investigation;

[0061] Fig.13 Depicted are examples of USPL laser sources as well as optical system receiving techniques to improve detection sensitivity consistent with embodiments of the current subject matter, USPL laser sources and optical receiving techniques to improve detection sensitivity.

[0062] Fig.14 Depicted is an example of a USPL-FSO transceiver utilized and operating in the infrared wavelength range (optionally including light from data signals) as a range finder and spotting device for the purpose of target identification;

[0063] Fig.15 depicts an example of a USPL pulse multiplier apparatus consistent with embodiments of the current subject matter;

[0064] Fig.16Another example of an apparatus for generating a high pulse rate USPL optical flow consistent with embodiments of the current subject matter is depicted;

[0065] Fig.17 Another example of an optical device used to generate a USPL RZ data stream from conventional transmission networking elements is depicted;

[0066] Fig.18 Depicts an example of implementing a USPL pulse multiplier device for generating a 10 x TDM type signal system to give a 100 Gbps output;

[0067] Fig.19 depicts an example of another type of USPL pulse multiplier device for extending the pulse repetition rate for use in high capacity networks;

[0068] Fig. 20 depicts an example of another type of USPL pulse multiplier device for extending the pulse repetition rate for use in high capacity networks;

[0069] Fig.21 Depicted are examples of actively mode-locked linear fiber lasers employing feedback regeneration systems: fiber reflectors (FRs), wavelength division multiplexers (WDMs), erbium-doped fibers (EDFs), optical couplers (OCs), photodetectors (PDs), phase-locked loops (PLLs), and Mach-Zehnder modulators (MZMs);

[0070] Fig. 22 and Fig.23 Depicted are examples of passively mode-locked linear fiber lasers using carbon nanotube saturable absorbers: fiber reflectors (FR), wavelength division multiplexers (WDM), erbium-doped fibers (EDF), optical couplers (OC), and saturable absorbers (SA);

[0071] Fig.24 Depicted are examples of delay stabilization mechanisms: 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), summing operational amplifier for stabilizing the optical pulse-to-pulse relationship generated from a USPL source;

[0072] Fig.25A and Fig.25B respectively include schematic diagrams and diagrams relating to examples of control mechanisms for stabilizing the output frequency of a TDM source using an idealized PZ actuator;

[0073] Fig.26depicts an example of time domain multiplexing (TDM) where TDM multiplexes pulse trains using parallel delay channels with delay channels to be "coordinated" with respect to each other (because the frequency of the output multiplexed pulse trains is ideally as insensitive to environmental changes as possible, a feedback loop control system can correct the delay elements for any fluctuations that compromise the stability of the output repetition rate, and feedback can be provided through the interconnect to the neural network);

[0074] Fig. 27 Depicts an example of using a fiber-based collimator in conjunction with a piezoelectric transducer for controlling a separate MFC circuit;

[0075] Fig.28 Depicts an example of a TDM chip providing terabit / second (or faster) timing from a USPL modulated source using a multiplier photonic chip;

[0076] Fig.29 Depicts an example of a TDM chip providing terabit / second (or faster) timing from a USPL modulated source utilizing a multiplier photonic chip operating in a WDM configuration;

[0077] Fig.30 depicts an example of the construction of a computer-aided system capable of controlling the pulse width of a laser all-fiber mode-locked using recursive linear polarization modulation with simultaneous stabilization of the repetition rate of the cavity using a synchronized self-regeneration mechanism and also capable of providing tunability of repetition rate and pulse width;

[0078] Fig.31 An example of a modified pulse interleaving scheme is depicted by a pulse doubling technique, where a lower repetition rate pulse train of a well-characterized, well-mode-locked laser can be coupled into an integrated optical directional coupler, where a well-determined fraction of the pulses is tapped and "re-circulated" in an optical loop with an optical delay equal to the desired inter-pulse spacing in the output pulse train, and recoupled into the output of the directional coupler;

[0079] Fig.32 is a process flow diagram illustrating features of a method consistent with embodiments of the present subject matter;

[0080] Fig.33 is another process flow diagram illustrating features of a method consistent with embodiments of the present subject matter; and

[0081] Fig.34 is another process flow diagram illustrating features of a method consistent with embodiments of the present subject matter. DETAILED DESCRIPTION

[0082] Figure 1An example of an optical communication platform 100 consistent with embodiments of the current subject matter for using a USPL device free-space coupled as a transmission light source is illustrated. Figure 1 As shown, the USPL source 102 is modulated directly by an external source element 104. The optical power from the USPL source 102 can be optionally coupled to the transmitting element 106 across the free space 110 through an optical telescope. The transmitting element 106 can optionally include an optical component formed by a hyperbolic mirror manufacturing technology, a conventional Newton design, etc. An interchangeable receiving telescope at the receiver system can provide optical reception. Consistent with the embodiments of the current subject matter, each optical transmission platform can be designed as a two-way unit. In other words, the transmitting element 106 of the optical communication platform 100 can also be used as a receiving element. In general, unless explicitly stated otherwise, the transmitting element 106 as described can be used as a receiving element and vice versa. An optical element that performs both the transmitting function and the receiving function can be referred to as an optical transceiver in this article.

[0083] Figure 2 Examples include Figure 1 An example of an optical communication system 200 consistent with embodiments of the current subject matter is provided for an optical communication platform 100 of FIG. Figure 2 2 is a second complementary receiving element 204, which may be a receiving telescope located at a remote distance from the transmitting element 106. As noted above, both the transmitting element 106 and the receiving element 204 may be bidirectional and each may function as both the transmitting element 106 and the receiving element 204 depending on the instantaneous direction of data transmission in the optical communication system 200. Unless explicitly stated otherwise, this feature applies throughout this disclosure to both transmitting and receiving elements. Either or both of the transmitting element 106 and the receiving element 204 may be a telescope or other device for transmitting and receiving optical information.

[0084] Figure 3 An example of an optical communication platform 300 consistent with embodiments of the current subject matter is illustrated for using a USPL source 102 fiber-coupled to an external modulator 302 via a fiber optic medium 304 and connected to a transmit element 106 via an additional transmission medium 306, which may alternatively be a fiber optic medium, a free-space connection, etc. The USPL source 102 can be externally modulated by the external modulator 304 such that optical power from the USPL source 102 is fiber-coupled to the transmit element 106 or processed via an equivalent optical telescope.

[0085] Figure 4 Examples include Figure 3An example of an optical communication system 400 consistent with embodiments of the current subject matter is provided for an optical communication platform 300 of FIG. Figure 4 Shown in FIG. 1 is a second complementary receiving telescope 204, which is similar to the above description of Figure 2 As noted, there may be a receiving telescope located at a remote distance from the transmitting element 106 .

[0086] Figure 5 An example of an optical communication architecture 500 consistent with implementations of the current subject matter is illustrated. Figure 5 The architecture 500 includes Figure 4 , and also includes a first communication network 502 connected to the first optical communication platform 300. The receiving element 204 is part of a second optical communication platform 504, which can optionally include components similar to those of the first optical communication platform 300. A second communication network 506 can be connected to the second optical communication platform 504 so that data is optically transmitted between the transmitting element 106 and the receiving element 204, or passed between the first communication network 502 and the second communication network 506, which can each include one or more of optical and electrical networking features.

[0087] Figure 6 An example of an optical communication system 600 consistent with embodiments of the present subject matter is illustrated. As part of an optical communication platform 602, a USPL source 102 is fiber-coupled to an external modulator 302, such as via an optical fiber 202 or other transmission medium. Light from the USPL source 102 is propagated via a transmission element 106 in a similar manner as discussed above. An optical amplifier element 604 (which may optionally be a fiber amplifier element) can be used to increase the optical transmission excitation power and can optionally be disposed 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, etc. Also in FIG. Figure 6 Shown is a second complementary receiving element 206 located at a remote distance from the optical communication platform 602. It should be readily appreciated that the second optical communication platform 504 including the receiving element 204 can also include an optical amplifier element 604. The first communication network 502 and the second communication network 506 can be connected to the two optical communication platforms 602, 504, respectively.

[0088] Figure 7 An example of an optical communication system 700 consistent with implementations of the current subject matter is illustrated. Figure 6The optical communication platform 602 shown can communicate with a second optical communication platform 702, which can include the receiving element 204 and the optical preamplifier 704 in this embodiment. Other components similar to those shown in the optical communication platform 602 can also be included in the second optical communication platform 702, but they are not shown in FIG. Figure 7 It should be understood that the bidirectional optical communication platform can include both an optical preamplifier 704 for amplifying received optical signals and an optical amplifier element 604 for strengthening transmitted optical signals. Figure 7 The embodiments depicted in the drawings are consistent with other embodiments of the current subject matter, such as using an erbium doped fiber amplifier (EDFA), a high power erbium-doped ytterbium fiber amplifier (Er / Yb-DFA), or an equivalent, optical amplification (e.g., for either or both of the optical amplifier element 604 or the optical preamplifier 704) is included to improve the optical budget for the data link between the transmitting element 106 and the receiving element 204 (and vice versa), and the equivalent can include but is not limited to a semiconductor optical amplifier (SOA).

[0089] Figure 8 An example of an optical communication system 800 consistent with implementations of the current subject matter is illustrated. Figure 6 The optical communication platform 602 is shown to be capable of communicating with a second optical communication platform 802, which in this embodiment can include a receiving element 204 and a second optical communication platform 802. Figure 7 An optical preamplifier 704 similar to those shown. Figure 8 As shown, the second optical communication platform 802 can further include an optical receiver circuit 804 that can receive amplified and electrically recovered data received at the receiving element 204 and amplified by the optical preamplifier. Multiple clock sources 806 can be connected to multiple remote multi-point network connections using multiple communication networks 810 as needed. In a similar manner, it is possible to connect to the optical communication platform 602 (for example, instead of Figure 8 A single depicted communication network 502) in FIG. 1 operates a complementary set of clock sources and multiple communication networks in combination.

[0090] Fig. 9 An example of an optical communication system 900 consistent with an embodiment of the current subject matter is illustrated. An optical communication platform 902 (which can be used in the same manner as reference Figure 6The optical communication platform 602 discussed first herein (featured as similar elements to those in the optical communication platform 602) can also include an additional USPL source 904 as a tracking and alignment (aiming) beacon source. The second optical communication platform 906 can also include an additional USPL source 910 as a tracking and alignment (aiming) beacon source. The tracking and alignment (aiming) beacon sources 904, 910 can optionally be derived from an available communication source used in the data transport transmission, or can be provided by a separate dedicated USPL source. In addition, each USPL beacon source 904, 910 can include an in-band source or an out-of-band source, thereby allowing the advantages of an available optical amplification source or from a dedicated optical amplification resource.

[0091] Fig.10 An example of an FSO communication system 1000 including a dual polarization USPL-FSO optical data link platform 1001 is illustrated in which a USPL source is polarization multiplexed onto a transmit optical signal to thereby provide polarization multiplexed USP-FSO (PM-USP-FSO) functionality. Two USPL sources 102 and 1002 are fiber-coupled to directly modulated or externally modulated modulation components 1004, 1006, respectively. Each respective modulated signal is optionally amplified by an optical amplifier component 1010, 1012, followed by adjustment of the optical polarization state using polarization components 1014, 1016. The polarization state signal is fiber-coupled to a polarization-dependent multiplexer (PDM) component 1020 for docking to an optical excitation platform component 1022, which can be similar to the transmit element 106 discussed above. The PDM 1020 multiplexes light of different polarization states into a single pulse train for transmission via the optical excitation platform component 1022. USPL optical beacon 904 can be included to provide the same Fig. 9 The similar capabilities discussed above, for example, to operate along or in conjunction with the second USPL optical beacon 906 at a receiving platform 1024, the receiving platform 1024 can include a receiving element 204 similar to those described above. As previously noted, the receiving element 204 and other features and components of the receiving platform 1024 are generally capable of supporting transmission functions such that a bidirectional link is established. The received signal recovered by the receiving element 204 can provide an optical signal that is interfaced to an appropriate polarization-dependent demultiplexer 1026 that can provide two signals for further optical amplification using the amplification elements 1030, 1032. The respective optically amplified signals provided by the amplification elements 1030, 1032 can be interfaced to an appropriate optical network 1034, 1036 for network use.

[0092] Fig.11AAn example of a system 100 in which a USPL-FSO transceiver can be utilized for use in line-of-sight optical communication (eg, "laser communication") applications is shown, and Fig. 11B An example of a system 1150 in which a USPL-FSO transceiver can be utilized for use in non-line-of-sight laser communication applications is shown. The advantages of some embodiments of the current subject matter can be achieved due to the scattering of the optical signal sent from the transmitting element as the transmitted light passes through the atmosphere. This scattering can permit the use of non-line-of-sight communications. In addition, the radio equipment used in such a communication system can operate in the solar-blind portion of the UV-C band, where light is emitted at wavelengths of 200nm to 280nm. In this band, as solar radiation propagates through the environment, it is strongly attenuated by the earth's atmosphere. This means that when it is closer to the ground, the amount of background noise radiation drops significantly, and low-power communication link operation is possible. On the other hand, environmental elements such as oxygen, ozone, and water can weaken or interrupt communication broadcasts, thereby limiting the use of short-range applications.

[0093] As UV waves propagate through the atmosphere, they are typically strongly scattered into various signal paths. Signal scattering is necessary for UV systems to operate in non-line-of-sight conditions, and communication performance can be highly dependent on transmission beam aiming and the field of view of the receiver. Fig.11A The line of sight arrangement 1100 shown can be similar in bandwidth size to the Fig. 11B The non-line-of-sight arrangement 1150 shown is different. Ultraviolet communications can be more strongly dependent on the beam position of the transmitter and the field of view of the receiver. As a result, aiming at the top angle, for example, by experimenting with complementary equipment to enhance the improvement of the UV-C signal, may be advantageous.

[0094] Fig.12 An example of a remote sensing system 1200 is illustrated in which a USPL source 102 is fiber-coupled by a fiber assembly 202 to an optical excitation element 1202 capable of sending and receiving optical signals. Some of the light (including light from the data signal) propagating forward through the optical excitation element 1202 is backscattered by interacting with airborne particles that are the subject of investigation. The optical backscattered signal is detected by the optical excitation element 1202 or a similar receiving aperture and is transmitted through the optical excitation element 1202. Fig.12The detection circuit 1204 or the like in the embodiment of the present invention is transmitted to the detection circuit 1204 or the like in the embodiment of the present invention for detection and spectral analysis. The signature difference of the particle in the target atmospheric region 1206 under investigation can be calibrated by conventional methods, such as using predetermined spectral calibration measurements based on one or more of ultraviolet spectroscopy, infrared spectroscopy, Raman spectroscopy, etc. Consistent with this embodiment, the optical system can be operated as a LiDAR instrument to provide enhanced resolution and detection sensitivity performance using a USPL laser source operating over the spectral range of interest. The tunability of the spectral range can help evaluate and analyze the chemical composition of the atmosphere.

[0095] USPL-FSO transceivers can be utilized for remote sensing and detection to obtain the sign difference of the air propagation element of the optical transmission terminal manufactured by hyperbolic mirror manufacturing technology or conventional Newtonian design to focus the received signal at an ideal point using ionization or non-ionization detection techniques. And specific changes can be related to ionization detection of remote areas including controlled ionization, which has been shown to occur at these frequencies and ionization processing, which can be focused at a distance to adjust the depth of atmospheric penetration, especially in weather and clouds.

[0096] Fig.13 An example of using USPL source and optical receiving technology to improve detection sensitivity is illustrated. Researchers at the National Institute of Standards and Technology (NIST) have established a laser ranging system that can accurately find multiple objects with nanometer precision at a distance of up to 100km. LIDAR (light detection and ranging) systems can have applications from precision manufacturing in the world to maintaining a network of satellites in perfect formation (Nature Photonics DOI: 10.1038 / NPHOTON.2009.94). The NIST device uses two coherent broadband fiber-laser frequency combs. The frequency comb combs out a series of stable short pulses, which also contain highly coherent carriers that extend across the pulse train. This means that the frequency comb can be used to simultaneously perform interferometry and time-of-flight measurements, thereby enhancing analysis capabilities for special situations.

[0097] exist Fig.13In the arrangement shown, two phase-locked frequency combs 1301 and 1302 are used in a coherent linear optical sampling configuration also known as multi-heterodyne, meaning that one frequency comb measures two distance paths while the other frequency comb provides distance information encoded in view of the first comb. Pulses from one frequency comb 1301 can begin to leave the optical fiber and be directed to two glass plates, a reference 1303 and a target 1304. The plates 1303 and 1304 can reflect a certain small portion (e.g., about 4%) of the pulses returning from the optical 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 consecutive pulses when the light source interferes, the second frequency comb can sample slightly different portions of the light from the electric field of the first comb.

[0098] Use reference Fig.13 The described technique enables the replacement of two coherent broadband fiber-laser sources with two suitable USPL sources used within the scope of the configuration outlining each having a USPL source fiber coupled to a dedicated free-space optical telescope design. By doing so, the overall efficiency, optical ranging and accuracy can be substantially improved.

[0099] Currently available USPL optical pulse trains operate at the native pulse repetition rate of the USPL laser source and are typically limited to 50 MHz or less, thereby exceeding the maximum data rate for optical transmission. As a result, optical systems utilizing USPL laser sources are limited to low data rate applications of 50 MHz or less. Having a means to increase the USPL operating rate is necessary to provide a solution for data transmission beyond 50 MHz.

[0100] Fig.14 An example of a remote sensing system 1400 is illustrated in which a USPL source 102 is optically coupled by an optical fiber assembly 202 to an optical excitation element 1202 capable of sending and receiving optical signals. Light (including light from the data signal) propagating forward through the optical excitation element 1202 is backscattered by interacting with targets known and unknown to be subjects of investigation within an atmospheric region 1206. The optical backscattered signal (including light from the data signal) is detected by the optical excitation element 1202 or a similar receiving aperture and is transmitted through the optical excitation element 1202. Fig.14 The detection circuit and the spectral analysis element 1402 in the embodiment are transmitted to obtain the detection analysis. For example, in the case where the ranging analysis can be performed, the sign difference of the particles in the area 1206 under investigation can be calibrated. Fig.14The same system 1400 can include a USPL-FSO transceiver that utilizes and operates across the infrared wavelength range as a range finder and pointing device for the purpose of target identification and interrogation applications.

[0101] Fig.15 An optical pulse multiplier module 1500 capable of increasing the repetition rate of the output from the USPL source 102 is illustrated. A typical USPL with a pulse width of 10-100 femtoseconds has a repetition rate of, for example, 50MHz. The output from the USPL 102 can be fed into a USPL photonic chip pulse multiplier module 1504 as an input 1502. 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 opposite side of the splitter element 1506 contains a 50MHz pulse train. Each optical element then passes through a delay controller (fiber loop or lens array) 1510, which delays the pulse train used for the element in time by, for example, many picoseconds. Continuous optical elements are thereby delayed by incremental picoseconds. All of these pulse trains with their unique time delays 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 ratio of splitters and combiners required can be controlled to provide the necessary optical design for the desired application. The final output 1514 is a pulse train of 10-100 femtosecond pulses with a repetition rate of 1 THz. This THz pulse train can then be used by, for example, Fig.28 The 10 or 100GigE signal modulation shown is used to generate 100 femtosecond pulses per bit for the 10GigE system and 10 femtosecond pulses per bit for the 100GigE system. The referenced applications are not limited to the specific data rates of 10 and 100Gbps, but are capable of operating as required by the application under consideration. These numbers are for illustrative purposes only. Implementations of the current subject matter are capable of using any multiplier factor to increase the repetition rate of the USPL to any arbitrary repetition rate via the photonic chip multiplier module 1504. Other examples used in the generation of enhanced USPL repetition rates are illustrated within this submission.

[0102] Fig.16 A system 1600 for the generation, transmission, and reception of high pulse rate USPL optical streams is depicted. An optical chip multiplexing module 1610 can be used in the present application, which can be used, for example, with reference to Fig.15 The optical chip multiplexing module discussed is similar. In this method for realizing USPL pulse multiplication, the signals 1601, 1602, 1603, 1604 (in Fig.161604. The optical chip multiplexing module 1610 is a module that is used to connect optical signals to the optical chip multiplexing module 1610 ...

[0103] At a remote receiving site where a receiving device is located, all signals transmitted at the transmitting device via the transmitting element 1660 can be recovered using appropriate receiver elements 1665. A complementary set of optical chip multiplexing modules 1675 can provide the necessary capabilities for demultiplexing the received data streams as shown by the elements for delivery 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 the network endpoint elements.

[0104] Fig.17 An example system 1700 is depicted in which the optical chips are interconnected to a wavelength division multiplexing (WDM) system, the currently available versions of which can be very expensive. The WDM system has the advantage of not requiring timing or synchronization as required for a 10GigE (or other speed) router 1701, because each 10GigE signal runs independently of other such signals on its own wavelength. The timing or synchronization of the TDM optical chip with the 10GigE router can be important in the TDM optical chip. The GbE switch 1701 can provide the necessary electrical RF signals 1705 from the switch 1701 to modulate the USPL source 1702 either directly or by using the USPL pulse multiplier module previously described in detail in this document. The typical NRZ output 1710 can be coupled to an external modulator 1720, which can be modulated using an NRZ clock source for the switch 1701, thereby producing an RZ modulated spectrum 1730. The conversion process using readily available equipment can provide the capability for introducing USPL sources and their beneficial effects into the terrestrial backhaul network spectrum.

[0105] In order for the optical chip system to successfully bridge between two remote 10GigE switches, it must typically behave like a simple piece of optical fiber. The timing of the TDM chip can therefore be driven by the 10GigE switch 1701. Both the actively mode-locked USPL (i.e., 40 GHz, 1 picosecond pulse width) and the passively mode-locked USPL (i.e., 50 MHz, 100 femtosecond pulse width) can be driven by the RF timing signal.

[0106] Fig.18 An apparatus 1800 is illustrated that can support another approach of asymptotically high pulse repetition data rate operation (such as to obtain extremely high data rate operation that can be performed using optical fiber or free space optics to perform optical chip designs). A 50MHz USPL source 1801 is interfaced to a series of optical delay controller elements 1802, which can be designed using fiber loops or offset lenses to result in the exact generation of a 10.313Gbps RZ output stream, which is a 10GigE line rate (because 64B / 66B encoding is greater than 10Gbps). A splitter element 1803, along with a variable optical delay line 1804, provides the functionality of splitting the incoming optical signal train 1801 into (in this example) 206 paths. After sufficient delay is introduced by design, all signals are multiplexed together by a combiner element 1805. In doing so, a series of optical signals that are each identical and equally spaced between adjacent pulses form a continuous set of modulation pulses. Before entering the EO modulator element 1806, all optical incoming signals can be conditioned by pre-emphasis techniques, such as using typical optical amplification techniques, to produce a uniform power spectrum for each output signal from the combiner element 1805. The conditioned output signal is then coupled into the EO modulator element 1806 and modulated with the available NRZ signal from the 10GigE signal source element 1807. The 10GigE modulated output 1809 can be interfaced to an EDFA and then enter the TX of the FSO system (or optical fiber system). The Rx side (after the detector) can be directly fed into the 10GigE switch as a modulated and amplified output 1810.

[0107] Fig.19 Another example of an apparatus 1900 that can be used for USPL pulse multiplication consistent with embodiments of the current subject matter is illustrated. Consistent with this approach, a 10 x TDM system is configured to give a 100 Gbps output. A TDM demultiplexer chip can be on the receive side of a communication link to decompose the individual 10GigE signals, and can include, for example, Fig.19 Interchange method of the design shown.

[0108] As in Fig.18As in FIG. 1 , a 50MHz USPL source 1801 is interfaced to a series of optical delay controller elements 1802, which can be designed using fiber loops or offset lenses to result in exactly a 10.313Gbps RZ output stream, which is the 10GigE line rate (greater than 10Gbps because of 64B / 66B encoding). Splitter elements 1803 along with variable optical delay lines 1804 provide the functionality of splitting the incoming optical signal train 1801 into (in this example) 206 paths. After sufficient delay is introduced by design, all signals are multiplexed together by combiner elements 1805. However, instead of as in Fig.18 A single modulator element 1806 is shown, the 10.313 GHz RZ output 1901 from the combiner element 1805 is fed into a second splitter element 1910, which in this case can be a 10x splitter, which splits the optical signal into ten parallel paths. Other embodiments of this design can support various split ratios as required by the design, and the optical paths out of the second splitter element 1910 are individually connected to a specified optical delay line 1920. Each individual delay path is connected to a dedicated optical modulator of a set of optical modulators 1930 modulated with the available NRZ signal from the 10x10GigE signal source element 1931, producing a series of modulated optical signals 1935. The identified optical combiner 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 to obtain the desired optical conditioning for network use.

[0109] Fig. 20 Another example of an apparatus 2000 that can be used for USPL pulse multiplication consistent with embodiments of the current subject matter is illustrated. Apparatus 2000 as depicted can achieve high USPL pulse repetition data rate capabilities for network applications through modulation of pulses within low repetition rate channels. By applying direct modulation of individual channels on delay controllers, the creation of modulation schemes that are not constrained by current speed limitations from electronic technology can be advantageously achieved. Embodiments of the current subject matter can provide for the creation of high USPL pulse repetition data rate capabilities for network applications by separately modulating pulses at current standard electronic modulation speeds (at Fig. 20In the example of 100x10GigE signal input (2001 rate) the individual channels are modulated and the channels are time multiplexed into a single frequency high repetition rate pulse stream to increase the data transmission capacity of the system. In this approach, the current standard limited by the speed of the electro-optical modulator (40Gbps) can be improved by about N orders of magnitude, where N is the number of channels of the time multiplexer. For example, a 100 channel TDM in which the amplitudes of the individual channels are modulated at the current standard data rate may be able to provide data rates at speeds of up to 4Tbs. N can be limited by the width of the optical pulse itself. In the limitation that information is carried 1 bit / pulse, the time slot occupied by 1 bit is the width of the pulse itself (in that sense, RZ systems will converge to NRZ). For example, in this scheme, a 40fs pulse width laser with a 40GHz repetition rate can carry information at a maximum rate of 25Tbps. This approach can be used in 40Gbps-channel modulation schemes (i.e., 1 bit every 25ps) and can correspond to a capacity of N~625 channels in a single transmission, which can be the number of 40fs time intervals that fit into a 25ps time interval. The effective advantage of this approach is to "optically boost" an otherwise limited data capacity modulation scheme while still interfacing with existing data rate limited modulators. For example, an amplitude modulator based on a Mach-Zehnder interferometer can be easily integrated in a TDM IC package, since all that is needed is the ability to branch the channel into two separate paths, add a microphase modulator (nonlinear crystal) in one of the paths, and combine the paths to obtain interference.

[0110] Fig. 20 A USPL source 2010 is included that is coupled to a multi-port optical splitter element 2020. The number of the optical ports identified need not be limited to those described or shown herein. A series of optical delay lines 2030 provide the required optical delay from the multi-port optical splitter element 2020 between the various parallel paths, and can be customized for specific applications. The optical delay paths from the optical delay lines 2030 are summed up together using an optical combiner element 2035. The resulting combined optical data stream that appears through element 2040 represents a multiplication enhancement in the pulse repetition rate of the original USPL source identified by element 2010. Further enhancement in the pulse repetition rate is achieved by the use of the element 2041 described by the optical splitter that the incoming signal 2040 is divided into a series of paths that are not limited to those identified by element 2041. Through a second delay controller 2045, optical delays can be introduced into the various paths within the device as identified by a second group 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 output 2001. An optical combiner element 2050 integrates all incoming signals onto a single data stream 2060.

[0111] Optical pre-emphasis and de-emphasis techniques can be introduced within the various segments of the element described to tailor the optical spectrum to obtain uniform or asymmetric optical power distribution. Pre-emphasis and de-emphasis can be implemented using commonly used optical amplifiers such as Erbium Doped Optical Amplifiers (EDFA).

[0112] Fig.21 An example of a system 2100 including a mode-locked USPL source 2101 is depicted, which can be used to generate appropriately required clocks and data streams for applications. Mode-locked lasers can represent a high-performance, high-precision source of choice for clocks in digital communication systems. In this regard, mode-locked fiber lasers - in either linear or ring configurations - can make attractive selection candidates because they can achieve pulse bandwidths and repetition rates as high as GHz on the USPL source region. In addition, optical fibers offer compactness, low cost, low sensitivity to thermal noise, low jitter, and no problems associated with diffraction or airborne dust contamination, to name just a few. In communications scenarios, pulse width can determine the available bandwidth of the system, and repetition rate limits the data rate. Pulse width can be determined by the intrinsic properties of the laser cavity - namely, the balance of the overall group velocity dispersion (GVD) and the choice of saturable absorbers (in the case of passive systems) - or the bandwidth of the active element (in the case of active mode-locked systems). The repetition rate of the pulse train is constrained by the length of the optical fiber. For example, in a linear laser, the fundamental mode ν osc can be expressed as:

[0113]

[0114] where c is the speed of light in a vacuum, n g is the average group index, and L is the length of the cavity. Thus, a 10 cm long fiber laser cavity element 2110 with an average group index of 1.47 will have a repetition rate of 1 GHz. In a strictly passive system, mode locking can be achieved by using a saturable absorber. In an active laser, an amplitude modulator element 2150 can be inserted into the cavity to increase the repetition rate of the laser (harmonic mode locking). In order to achieve high repetition rates using a mode-locked USPL source, it is possible to use (i) Fig.21 One or more of an intracavity amplitude Mach-Zehnder modulator (MZM) 2150 and (ii) a low threshold saturable absorber are shown. For terrestrial, subsea or FSO systems in airborne, space or subsea applications, these techniques, known as "harmonic mode locking", can be utilized within fiber-based station distribution systems or within FSO systems.

[0115] exist Fig.21Detailed within is a 980nm 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 to obtain a mode-locked pulse train emission in a closed cavity established between two Faraday reflectors 2101 and 2160 at either end of the optical USPL cavity. Operation of the device can establish a self-contained series of optical pulses that exceed 100Gbps and are highly synchronized in nature at the output port 2170 of the module. The EDFA 2110 can be specially designed to achieve high gain nonlinear media. The phase-locked loop 2130 can provide favorable stability in operation by maintaining a synchronized clock source through modulation of the signal through the components 2120, 2130, 2150 of the self-contained high repetition rate pulse generator.

[0116] In order to achieve high repetition rates in a laser that is limited by its size (length in the case of a linear laser and circumference in the case of a ring laser), it may be necessary to stimulate the intracavity generation of multiples of the fundamental mode. In the active case, an amplitude modulator inserted into the cavity modulates the losses of the system operating as a "threshold gated" device. For this approach to succeed, it may be necessary that the control signal to the modulator references the oscillation of the laser itself to avoid driving signals of external frequencies that "force" oscillations on the laser. This can be achieved by introducing a phase-locked loop element 2130 or a synchronous oscillator circuit to track and lock to the repetition rate of the laser, and regenerate the signal. In the case of a PLL, the RF output can be set to a multiple of the input signal (much as this device is used in mobile phone technology), and the repetition rate of the laser is increased. The signal can then be used for triggering of a pulse generator, or in combination with a low-pass filter. An MZ amplitude modulator 2150 outside the laser cavity can be used to create on-off keying (OOK) modulation of the pulse train coming out of the mode-locked laser.

[0117] Fig. 22A graphical depiction 2200 illustrating the effect of loss modulation introduced into an input pulse train 2201 due to the presence of an amplitude modulator 2205 in the case where a control signal NRZ signal 2210 consists of the illustrated bit sequence is shown. The resulting signal at the output of the device 2220 represents an NRZ to RZ converter device for use in telecommunications and scientific applications, where applications can benefit from RZ data streams. The clock signal 2201 (optical input) will pass through the modulator 2205 at a given pulse repetition rate. At the same time, a control signal consisting of a sequence of 1s and 0s can be applied to the RF port of the modulator element 2215. When the modulator element 2215 is biased at minimum transmission, the loss experienced by the optical signal in the absence of the control signal can be maximized. In the presence of the RF signal (1), the loss will drop to a minimum (pass gate), thereby acting as an on-off keyed modulation device. The pulse width of the output optical signal is typically much smaller than the time slot occupied by a single bit of information (even less than half a clock cycle for an NRZ scheme), making this system purely RZ as indicated by element 2220.

[0118] Fig.23 An example system 2300 for generating high-order optical harmonic USPL pulses with high pulse repetition rate using a saturable absorber (SA) device 2330 is illustrated. The SA device 2330 can include carbon nanotubes in some examples. Passively mode-locked fiber lasers using carbon nanotube SA (CNT-SA) make attractive options for high repetition rate sources due to their ability to generate high harmonics of the fundamental repetition rate. In the described method, a self-contained optical cavity is established, with two Faraday reflectors 2301 and 2350 forming the optical cavity. Although in Fig.23 A high power erbium doped fiber amplifier (EDFA) 2310 is shown, but any anti-phase medium that produces a nonlinear optical cavity can be used. A seed laser 2315 can be used in generating a high repetition rate optical train, such as, for example, Fig.23 980 nm pump laser shown. In particular, any suitable pump laser may be considered depending on the required optical wavelength and pulse repetition rate. SA element 2330 can be placed in the cavity to establish the required optical pulse characteristics 2350 as required by design requirements.

[0119] Fig.23 A simplified diagram of an example of a laser that can be used in one or more embodiments of the current subject matter is shown. Fig. 22 The active laser shown, here the MZ modulator can be replaced with a SA element 2330. For terrestrial, subsea or FSO systems in airborne, space or subsea applications, techniques similar to those described herein can be utilized within fiber-based station distribution systems or within FSO systems.

[0120] Fig.24 A method of providing time domain multiplexing (TDM) is illustrated, wherein TDM multiplexes pulse trains using parallel delay channels. In some instances, it can become important to manipulate the delay channels so that they are "consistent" with respect to each other. The frequency of the output multiplexed pulse train can ideally be as insensitive to environmental changes as possible. Nevertheless, the proposed feedback loop control system is designed to correct the delay unit for any fluctuations that impair the stability of the output repetition rate.

[0121] Fig.24 A diagram of an example of a delay control system 2400 is shown. The control loop can be implemented in one of several ways consistent with the current subject matter. For illustration purposes Fig.24 One possibility is described. An input pulse train enters the TDM and is multiplexed into N paths, each with its own delay line. If the paths consist of low "bend loss" fiber waveguides, each path can be wound around a cylindrical piezoelectric actuator (PZ) of radius R. The actuator typically expands in the radial direction as a result of a control voltage (Vc). This expansion ΔR, which is linearly proportional to Vc, causes a change in the length of the fiber ΔL=2πNΔR, where N is the number of fiber turns around PZ. For Terahertz multiplexing, the delay between pulses (and therefore of PZ1) must be 1 picosecond. This can require a length change equal to 200 microns, which corresponds to ΔR=32.5 microns for one turn of the PZ actuator. Most commercially available piezoelectric actuators are highly linear and operate well in this range. The control mechanism can therefore be 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 controlled by a single voltage Vc. The control voltage is determined by a feedback system that compares the frequency of the output signal using the 1 / N divider with the frequency of the input signal using a phase comparator (PC). Fig.24The frequency of the optical signal (denoted by the waveform with τRT in the figure) is converted into an RF signal using a photodetector PDin. In order 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 to regenerate it into a 50% duty cycle waveform. Similarly, in the output side, the optical signal is picked up by the photodetector PDout, high-pass filtered, and regenerated using the clock output port of the clock and data recovery system. The clock of the output signal with a frequency N times that of the input signal is sent to the N-times divider before turning to the phase comparator. From the phase comparator, a DC voltage level representing the mismatch between the input signal and the output signal (almost the same as used in the architecture of the PLL circuit) indicates the direction of correction for the actuator. The low-pass filter adds a time constant to the system to enhance its insensitivity to parasitic noise.

[0122] In contrast to PLL, CDR can be advantageously used in the output so that the output signal may or may not be modulated. This system can be designed to work in both unmodulated and "modulated within TDM" (i.e. one modulator at each delay path) schemes. However, this is a completely deterministic method of compensating for variations in the length of the delay line. Ideally, and within a practical point of view, the delay paths should all be referenced to the same "thermal level", i.e. be sensitive to the same thermal changes at the same time. If the various lines sense different variations, the system will not be able to correct that in real time.

[0123] In the alternative, a fully deterministic approach can include the summation of operational amplifier circuits (S1 ... SN) to deliver the control voltage to the actuator. Using such an approach, the input voltages (V1 to VN) can be used to compensate for length deviations between the lines in a completely static sense, or they can be used for initial fine tuning of the system. The approach typically also must compensate for or at least take into account any bending loss requirements of the optical fiber used. Some new optical fibers just coming out on the market may have a critical radius of only a few millimeters.

[0124] If the individual path delay lines sense different changes in temperature and experience uncorrelated length changes due to parasitic local noise, the previously described method as is can suffer from difficulties in performing real-time corrections. A more robust method that operates in a purely statistical sense can be used consistent with some embodiments of the current subject matter. In such an approach, a summing operational amplifier circuit (S1...SN) can be used to deliver a control voltage to the actuator. In this case, the input voltages (V1 to VN) can be used to compensate for length deviations between the delay lines in a purely statistical sense, otherwise they can only be used for initial fine tuning (calibration) of the system.

[0125] Refer again Fig.24 , the incoming USPL source, identified as element 2401, is coupled to an optical coupler element 2403 such that one leg of the coupler is connected to an optical photodiode selected for operation 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 incoming USPL signal is extracted and identified by element 2407. The second optical leg of coupler 2403 is docked into an appropriate optical splitter element identified by 2410, wherein the incoming signal into 2410 is split into 206 parallel optical paths. Also illustrated is a variable rate optical delay line established in parallel for each of the parallel legs of beam splitter element 2410. Parallel piezoelectric elements are identified by element 242N and are electronically controlled by feedback circuits within the figure. The control voltage identified by Vc is generated by photodiode 2485 together with electronic circuit elements 2480 and 2475. Clock and data recovery (CDR) element 2475 generates the clock source used in controlling the various PZ elements. The optical paths designated 244N are combined after appropriate delays are introduced into the various legs of element 2410. A pulse-doubled USPL signal 2490 is thereby generated.

[0126] Fig.25A A simplified diagram of a fiber optic PZ actuator 2500 is shown, and Fig.25B A graph 2590 of radius versus voltage for such an actuator is shown. Together, these figures illustrate the operation of a PZ actuator for increasing the pulse repetition rate of an incoming USPL pulse train by induced optical delay. Although shown as an element for enhancing pulse repetition rate generation for USPL signals, the same technology can be used for other optical devices that require or benefit from optical delay. The basic structure for the device is an optical fiber based PZ actuator 2501. When a voltage 2550 is applied to the electrode 2520, voltage induced stress is generated within the optical fiber, causing a time delay in the optical signal traveling through the fiber. By varying the applied voltage, as in Fig.25B A performance curve of optical delay versus applied voltage is obtained as shown in Graph 2590 .

[0127] Fig.26 A diagram illustrating features of an example statistical corrector 2600 is shown. Fig.26The coarse correction controller 2640 shown corresponds to the system described in the previous section, which is capable of correcting length variations picked up by all delay lines simultaneously. As mentioned, these variations are expected to occur at a much slower time scale than parasitic variations "within the delay line". This late effect can manifest itself as cycle-to-cycle jitter introduced on the system. This type of jitter can be monitored using an RF spectrum analyzer (RFA), causing the repetition rate line of the system to display "edges" (or sidebands), which are the result of the analyzer beat frequency and the noise frequency generated by the non-uniform time intervals between consecutive pulses. One such pattern can be processed using an analog-to-digital converter (ADC) and saved as an array of values, which can then be fed to a neural network (NN) machine. Neural network machines are known to possess excellent adaptive properties, which allow them to learn patterns from external events, generally by adapting new sets of inputs and outputs. A set of inputs in this case can be composed of a set of "imperfect observations", i.e., as detected by the RFA and converted by the ADC into a digital array ({f 1 ,f 2 ,…,f N}, where f i The "noisy" output of the TDM system is generated by the frequency components picked up by the RFA. A set of outputs can be obtained by the correction required to get rid of the output frequency set from the undesirable excessive frequency noise ({V 1 ,V 2 ,…,V N}, where V i is the compensation input voltage of the summing operational amplifier), the excess frequency noise is due to external disturbances of the system. With a sufficiently large number of {f,V} pairs, where f, V are frequency, voltage arrays, a statistical set can be constructed to train the NN machine to learn the basic patterns associated with the presence of intra-channel noise. These machines can be found commercially in IC format from several manufacturers, or implemented as software and used in conjunction with a computer feedback control mechanism. A single layer perceptron type neural network or ADALINE (Adaptive Linear Neuron or Post-Adaptive Linear Element) should be sufficient for the task.

[0128] Regarding the above Fig.24 Similar to the description provided, the statistical corrector element 2670 can include Fig.24 The circuit elements 2480 and 2475 and the photodiode 2485 are similar to or provide Fig.24 The circuit elements 2480 and 2475 and the photodiode 2485 are similar in functionality to the electronic circuit. Fig.26In the method illustrated in FIG. 2 , an RF spectrum analyzer 2695 along with a neural network 2670 and a coarse correction controller element 2640 are used to implement the required optical delay introduced into a parallel series of PZ elements 262N.

[0129] Fig. 27 The concepts and capabilities of a method consistent with an embodiment of the present subject matter are illustrated in which performance, accuracy, and resolution can be improved by replacement of a piezoelectric disc (PZ) module identified by elements 2795 and 272N, wherein a compact micro-fiber based collimator (MFC) 2795 surrounded by a ceramic disc is used to obtain an optical delay line. Although techniques for increasing the native pulse repetition rate for USPL pulse trains are illustrated, the illustrated design is not limited to such applications, but can be applied or extended to other needs within the optical sector whenever an optical delay is required. In doing so, more controlled amounts of time delay can be introduced within the various MFC elements of the circuit. Improvements in the use of MFC elements can improve response, resolution, and implementation in mass production devices that require rapid reproduction of voltage responses. In Fig. 27 The concepts identified within can be incorporated into precision produced components that can be used as complementary paired units for use in reducing USPL pulse-to-pulse jitter and for purposes required for data encryption.

[0130] Further references Fig. 27 , a USPL source 2701 with a particular pulse repetition rate is split into a preselected number of optical paths 271N (which can be numbered differently than 206) as identified by beam splitter elements 2705. Appropriately controlled delays 273N are introduced into the various parallel branches of the split optical paths 271N using elements described by 2795 and 272N. The resulting delayed paths 274N are added together by optical combiner elements 2760. A pulse-multiplied USPL signal 2780 is obtained.

[0131] One potential drawback of some previously available TDM designs in which the optical fiber is "wrapped" around the piezoelectric actuator is that the mechanism must comply with the bending loss requirements of the optical fiber used. Some new optical fibers that are just coming out on the market have a critical radius of only a few millimeters. To correct this problem, embodiments of the current subject matter can use a micromechanical air gap U-bracket instead of a cylindrical bore around which the optical fiber is wrapped. Fig. 27exemplifies this principle. In this approach, the piezoelectric actuators (PZ1, ... PZN) can be replaced with an air gap U-shaped bracket structure constructed using a micro-fiber collimator (MFC) and a micro-ring made of piezoelectric material. However, in this case, the piezoelectric actuators expand longitudinally in response to a control voltage (V1, V2, ... VN), thereby increasing (or decreasing) the air gap distance between the collimators. As in the case of cylindrical piezoelectrics, a single voltage Vc can be used to drive all piezoelectric devices, as long as the gain of each channel (G1, G2, ... GN) is adjusted accordingly to provide the correct expansion for each line. Ideally, in addition to the inherent bias of the system (i.e., the inherent differences between operational amplifiers), the gain adjustment should be the same as G1, 2G1, 3G1, etc., in order to provide expansion as a multiple of τRT / N. Another way to implement such an approach may be to use multiple piezoelectric rings at the channel. In that way, one can have channels with 1, 2, 3, N piezoelectric rings driven by all amplifiers at the same gain with the same voltage.

[0132] Fig.28 A conceptual presentation of an optical chip system 2800 used to successfully bridge between two remote 10GigE switches is provided. Ideally, such a connection can be performed similarly to a simple piece of optical fiber. The timing of the TDM chip can be driven by the 10GigE switch.

[0133] Reference Fig.28 A USPL source 2805 having a predetermined native pulse repetition rate identified by 2806 is connected to an optical pulse multiplier chip 2807. Element 2807 is designed to convert the incoming pulse repetition rate signal 2806 to appropriate levels for operation with a high speed network Ethernet switch as identified by 2801. Switch 2801 provides a reference signal 2802 which is used to modulate a signal 2809 at the data rate of interest via a standard piezo optical modulator 2820. The resulting RZ optical signal is generated as shown at element 2840.

[0134] An alternative to having timing run from a 10GigE switch is to build a USPL to terahertz per second (or faster) with a multiplier photonic chip, and then modulate this terahertz per second signal directly from the 10GigE switch. Each bit will have 100 or so pulses. An advantage of this approach can be the elimination of the need for a separate timing signal to be run from the switch to the USPL. The USPL via the multiplier chip only has to extract the terahertz per second pulse. Another advantage is that the output of the multiplier chip does not have to be exactly 10.313 or 103.12 Gbps. It only has to be at a rate of about 1 terahertz per second. In the case where each 10GigE bit has 100 or 101 or 99 pulses, this limitation is not a problem. Another advantage is that each bit will have many 10USPLs, so the 10GigE signal will have an atmospheric propagation (fog and flicker) advantage. Another advantage can be achieved at the receiver end. It should be easier for a detector to detect a bit if it has 100 or so USPL pulses within that single bit. This can lead to improved receiver sensitivity and therefore allow improved range for FSO systems. Additional advantages can be achieved because upgrading to 100GigE may be as simple as replacing a 10GigE switch with a 100GigE switch. In this case each bit will have about 10 pulses.

[0135] From a purely signal processing point of view, this method demonstrates an efficient way to send data and clock combined in a single transmission stream. Much like the "sampling" of bits using an optical pulse stream, this method has the following advantages: the "size" of a bit is determined by the maximum number of pulses it carries, thus establishing a basis for counting bits as they arrive at the receiving end. In other words, if the bit cell has a time slot that can fit N pulses, the system's clock can be established as a "new information bit" after every 5th.

[0136] For terrestrial, subsea or FSO systems in airborne, space or subsea applications, techniques similar to those described herein can be utilized within fiber-based station distribution systems or within FSO systems, and illustrate for the first time how interconnection from USPL sources to optical network elements can be implemented for networking applications.

[0137] Fig.29 An example is shown for Fig.28 The concept of the design concept reflected in the network expansion system 2900. As multiple USPL sources 2901, 2902, 2903 (it should be noted that although three are shown, any number is within the scope of the current subject matter), each modulated by a dedicated optical switch and configured with a USPL laser multiplier chip in a WDM arrangement. Fig.28As described, electrical signals from each Ethernet switch can be used to modulate dedicated optical modulators 2911, 2922, 2928 for each optical path. The optical power for each segment of the system can be provided by optical amplifier elements 2931, 2932, 2933 for amplification purposes. Each amplified USPL path can then be docked to an appropriate optical combiner 2940 for transmission to the network 2950, ​​and can be free space or fiber-based as required. The output from the WDM module can then be configured to the transmission element 102 for FSO transmission or to the fiber station equipment.

[0138] For terrestrial, subsea or FSO systems in air, space or subsea applications, the technology described herein can be utilized within a fiber-based station distribution system or within a FSO system and illustrates for the first time how interconnection from USPL sources to optical network elements can be implemented for networking applications.

[0139] Fig.30 A simplified diagram of an experimental arrangement for an embodiment of the current subject matter to include the construction of a computer-aided system to control the pulse width of a laser all-fiber mode-locked using recursive linear polarization modulation with simultaneous stabilization of the cavity repetition rate using a synchronized self-regeneration mechanism. The design can also provide tunability of repetition rate and pulse width.

[0140] The fiber ring laser is represented by the inner blue loop with all intracavity fiber branches coded in blue except the positive high dispersion fiber outside the loop which is part of the fiber grating compressor (coded in dark brown). The outer loop represents the feedback active system.

[0141] Fig.30 A diagram of system 3000 is shown, which illustrates features of a USPL module that provides control of pulse width and pulse repetition rate control through mirrors (M1, M2), gratings (G1, G2), lengths (L1, L2), second harmonic generators (SHG), photomultiplier tubes (PMTs), lock-in amplifiers (LIA), data acquisition systems (DACs), detectors (DETs), clock extraction mechanisms (CLKs), frequency to voltage controllers (FVCs), high voltage drivers (HVDs), reference signals (REFs), pulse generators (PGENs), amplitude modulators (AMs), isolators (ISOs), piezoelectric actuators (PZTs), optical couplers (OCs), polarizers (POLs), and polarization controllers (PCs) all used to provide control of pulse repetition rate and pulse width control.

[0142] The 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 create elliptically polarized light in a propagating pulse. As the pulse travels along the fiber, it experiences a nonlinear effect in which an intensity-dependent polarization rotation occurs. By the time the pulse reaches the polarization controller (PC) 3001, the polarization state of the high-intensity portion of the pulse has undergone more rotation than the lower-intensity pulse. The controller can perform the function of rotating the high-intensity polarization component of the pulse so that its orientation is as nearly aligned as possible with the axis of the polarizer (POL). Therefore, as the pulse passes through the polarizer, its lower-intensity component experiences more attenuation than the high-intensity component. The pulse coming out of the polarizer is therefore 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 many round trips, saturation of stability occurs and passive mode-locking is achieved. The overall GVD of an optical loop can be tailored to produce a specific desired pulse width within a margin of error by using different types of fibers (such as single mode, dispersion shifted, polarization maintaining, etc.) and accumulating their contributions to the average GVD of the laser.

[0143] Active control of the linear polarization rotation based on PC can greatly improve the performance of the laser. This can be achieved using a feedback system that tracks the progression of the pulse width. Figure 1This system represented by the outer loop in can be used to maximize compression, and therefore, maximize the average power of the pulse. The pulses coming out of the fiber ring laser through the OC are expected to have a width of about a few picoseconds. An external pulse compression scheme using a fiber grating compressor is used to narrow the pulses to less than 100fsec range. This technology has been widely used in many reported experiments, resulting in high-energy, high-power USPL pulses. Here, the narrowed pulses are focused on the second harmonic generator (SHG) crystal and detected using a photomultiplier tube (PMT). The lock-in amplifier (LIA) provides the output DC signal to the data acquisition card (DAC). This signal follows the change in pulse width by tracking the increase or decrease in the peak power of the pulse. Similar techniques have been successfully used in the past, except that in that case, a spatial light modulator (SLM) was used instead. Here, a programmable servo mechanism directly uses an actuator on a PC to control the linear polarization rotation. Using the DC signal data provided by the DAC, decision software (such as but not limited to LABVIEW or MATLAB SIMULINK) can be developed to control the servo mechanism, which in turn adjusts the rotation of the input pulse relative to the angle of the axis of the polarizer. These adjustments performed by the actuator are achieved using stress-induced birefringence. For example, if the pulse width is reduced, the mechanism will cause the actuator to follow a specific direction of linear angular rotation to compensate for this, and if the pulse width is increased, it will act in the opposite direction, both aimed at maximizing the average output power.

[0144] A self-regenerative feedback system that is synchronized with the repetition rate of the optical oscillation and used as the drive signal for the amplitude modulator (AM) can regulate the round trip time of the laser. In an active system, the amplitude modulator acts as a threshold device by modulating the loss synchronously with the round trip time. This technique has been successfully able to stabilize a mode-locked laser in recent reports. The signal picked up by the photodetector (DET) from the optical coupler (OC) 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 is "turned on" whenever a pulse passes through it at each round trip time (TRT). Because the CLK follows the changes in the TRT, the drive signal for the AM will change accordingly.

[0145] An external reference signal (REF) can be used to tune the repetition rate of the cavity. It can be compared to the signal recovered from CLK using a mixer, and the output is used to drive a piezoelectric (PZT) system that can adjust the length of the cavity. Such use of a PZT system to adjust the length of the cavity is a well-known concept, and similar designs have been successfully demonstrated experimentally. A linear frequency to voltage converter (FVC) can be calibrated here to provide an input signal to the high voltage driver (HVD) of the PZT. The PZT will adjust the length of the cavity to match the repetition rate of the REF signal. If, for example, the REF signal increases its frequency, the output of the FVC will decrease, and so the HV drive level will also decrease to the piezoelectric cylinder, forcing it to contact and thus increasing the repetition rate of the laser. The opposite happens when the repetition rate of the reference decreases.

[0146] It is possible to use a pair of negative dispersion gratings to tune the width of the pulse to a "transform-limited" value. This chirped pulse compression technique is well established, and pulses as narrow as 6 fs have been reported. The idea is to mount the grating pair pulse compressor on a moving stage that translates along a line that sets the separation between the gratings. As the distance changes, the compression factor changes.

[0147] In an example of a data modulation scheme consistent with embodiments of the current subject matter, a passively mode-locked laser can be used as a source of ultrafast pulses, which limits our flexibility in changing the data modulation rate. In order to scale up the data rate of our system, we need to increase the fundamental repetition rate of our pulse source. Traditionally, the repetition rate of passively mode-locked lasers has been increased by either shortening the laser cavity length or by harmonic mode locking of the laser. Both techniques reduce the peak power of the intracavity pulses, resulting in longer pulse widths and more unstable mode locking.

[0148] One approach to solving this problem involves using a modified pulse interleaving scheme through a technique we call pulse doubling. Fig.31This concept is illustrated. A lower repetition rate pulse train of a well-characterized, well-mode-locked laser 3101 is coupled into an integrated optical directional coupler 3180, where a well-defined small portion of the pulses is tapped and "re-circulated" in an optical loop at an optical delay 3150 equal to the desired inter-pulse spacing in the output pulse train, and re-coupled to the output of the directional coupler. For example, to generate a 1 GHz pulse train from a 10 MHz pulse train, an optical delay of 1 ns is required, and in order to enable the 100th pulse in the pulse train to coincide with the input pulse from the 10 MHz source, the optical delay may have to be precisely controlled. The optical delay loop includes an optical gain 3120 to compensate for signal attenuation, a dispersion compensation 3160 to restore the pulse width and activate the optical delay control 3150. After pulse multiplication has occurred, the output pulse train is OOK modulated 3175 along with the data stream 3182 to generate an RZ signal 3190, and amplified in an erbium-doped fiber amplifier 3185 to bring pulse energy up to the same level as the output pulse train (or up to the desired output pulse energy level).

[0149] One or more of the features described herein, either alone or in combination, can be included in various aspects or embodiments of the current subject matter. For example, in some aspects, the 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, can be modulated using one or more modulation techniques for a point-to-multipoint communication system architecture, and / or can utilize an optical transmission terminal or telescope manufactured by one or more of a hyperbolic mirror manufacturing technique, a conventional Newtonian mirror manufacturing technique, or other functionally equivalent or similar techniques. Aspheric optical designs can also or alternatively be used to minimize and reduce shielding of received optical signals.

[0150] A free-space optical transmission system consistent with an embodiment of the current subject matter can utilize a USPL laser design that focuses the received signal at an ideal point. In some embodiments, a telescope or other optical element for focusing and transmitting light can be considered a transmitting element and a second telescope or other optical element for focusing and receiving light that is remotely positioned from the first telescope or other optical element can be used as a receiving element to create an optical data link. Two optical communication platforms can optionally include components necessary to provide both transmitting and receiving functions, and can be referred to as USPL optical transceivers. Any one or both of the telescopes or other optical elements for focusing and transmitting light can be coupled to a transmitting USPL source via light or by free space coupling to a transmitting element. Any one or both of the telescopes or other optical elements for focusing and receiving light can be coupled to a receiving endpoint via optical fiber or free space coupling to an optical receiver. A free space (FSO) wireless communication system including one or more USPL sources can be used for the following: within the framework of an optical communication network, in combination with a fiber optic backhaul network (and can be used transparently within an optical communication network), within an optical communication network (and can be modulated using on-off keying (OOK) non-return-to-zero (NRZ) and return-to-zero (RZ) modulation techniques within the 1550nm 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 technology, within a communication network for use in combination with erbium doped fiber amplifiers (EDFA) and high power erbium-doped-ytterbium fiber amplifiers (Er / Yb-DFA), within an optical communication network (and can be modulated using commonly used modulation techniques for point-to-multipoint communication system architectures), etc.

[0151] USPL technology can be used in some aspects as a beacon source that provides optical tracking and beam steering for use in automatic tracking capabilities and for maintaining terminal cooperative alignment during operation. The recovered clock and data extracted at the receiving terminal can be used for multi-hop spans for use when the network is accessible. Optical networks can provide similar benefits in WDM configurations, thereby increasing the size of the effective optical bandwidth of the carrier data link. The USP laser source can also or alternatively be polarization multiplexed onto the transmitted optical signal to provide polarization multiplexing USP-FSO (PM-USP-FSO) functionality. The recovered clock and data extracted at the receiving terminal can be used for multi-hop spans for use when the network is accessible, and can include a universal large bandwidth range of operation for providing data rate constant operation. An optical preamplifier or semiconductor optical amplifier (SOA) can be used before the optical receiver element and, alternatively or in combination with the recovered clock and data extracted at the receiving terminal, can be used for multi-hop spans for use in extending the reach of the network, thereby having a universal large bandwidth range of operation for providing data rate invariant operation. Terminal co-alignment can be maintained during operation, so that effective improvements in performance and terminal co-alignment can be achieved by using USPL technology, by using USPL data sources, and by providing improved methods for maintaining transceiver alignment by using USPL laser beacons.

[0152] The USPL-FSO transceiver can be utilized in some aspects for performing remote sensing and detection using ionization or non-ionization detection techniques to obtain the symbol difference of the airborne elements, using an optical transmission terminal manufactured by hyperbolic mirror manufacturing technology or conventional Newton design that focuses the received signal at an ideal point. The USPL-FSO transceiver consistent with the implementation of the current subject matter can be utilized in non-line-of-sight laser communication applications. The USPL-FSO transceiver consistent with the implementation of the current subject matter can allow the adjustment of the distance at which the scattering effect (enabling NLOS technology) occurs, allowing the receiving technology to use DTech detection schemes to improve detection sensitivity, and allowing improved bandwidth via a wideband detector including a frequency comb. The USPL-FSO transceiver consistent with the implementation of the current subject matter can be utilized in combination with adaptive optical (AO) technology for performing incoming optical wavefront correction (AO-USPL-FSO). The USPL-FSO transceiver consistent with the implementation of the current subject matter can be utilized and operate across the infrared wavelength range. USPL-FSO transceivers consistent with embodiments of the current subject matter can be utilized in conjunction with optical add / drop techniques and optical multiplexing techniques in both single mode fiber configurations as well as multimode fiber configurations. USPL-FSO transceivers consistent with embodiments of the current subject matter can be utilized and operated across the infrared wavelength range as a range finder and pointing device for the purposes of target identification and interrogation applications.

[0153] In other aspects of the current subject matter, a series of switched network connections (such as, for example, 10GigE, 100GigE, etc. connections) can be connected from one point to another through optical fiber or free space optics, for example, via time division multiplexing (TDM).

[0154] Mode-locked USPL sources consistent with embodiments of the current subject matter can be used to generate both clock and data streams. Mode-locked lasers can represent a high-performance, high-precision source of choice for clocks in digital communication systems. In this regard, mode-locked fiber lasers - in either linear or ring configurations - can make attractive candidates for selection because they can achieve pulse bandwidths in the USPL source region and repetition rates as high as GHz.

[0155] High harmonic generation can be achieved using carbon nanotube saturable absorbers. Passively mode-locked fiber lasers using carbon nanotube saturable absorbers (CNT-SA) make an option for high repetition rate sources due to their ability to easily generate high harmonics of the fundamental repetition rate.

[0156] FSO can be used in land, space and underwater applications.

[0157] Conditional path length control from beam splitter to aperture can be an important parameter. A TDM multiplexer consistent with the embodiment of the current theme can be used to control the relative time delay between the aperture and the source path. Parallel delay channels can be used to control each pulse train. This technology can be used to control conventional multi-transmission FSO aperture systems using WDM and TDM systems. USPL laser pulse to pulse interval can be maintained and controlled to the precise time requirements for both TDM and WDM systems. The described technology can be used in systems based on TDM and WDM optical fibers. The use of a TDM multiplexer as described herein can be used to implement a unique encryption device onto a transmitted optical signal. A complementary TDM multiplexer can be utilized to reverse the incoming received signal, and thereby restore the unique symbol difference of the pulse signal. The TDM multiplexer described herein can be utilized to control WDM pulse characteristics for the purpose of WDM encryption. A TDM multiplexer can be used in conventional FSO systems, wherein multiple apertures connected to a common source signal can control the time delay between pulses to maintain a constant path length. TDM multiplexers can be used in both TDM fiber based systems and FSO based systems. TDM multiplexers can be an enabling technology to control the optical pulse train relationship for USPL sources. TDM multiplexers can be used as an atmospheric link characterization tool to get the same pulse relationship across the optical link through the measurement of neural correction factors.

[0158] Any combination of PZ disks can be used in the transmitter and can have countless encryption combinations for USPL based systems or both fiber and FSO based systems. Timing can be run from a 10GigE switch or equivalent and to build up USPL to terabit / second (or faster) rates with a multiplier photonic chip and this terabit / second signal can be modulated directly from the 10GigE switch. When operating in a WDM configuration, interfaces to fiber based systems or to FSO network elements can be included.

[0159] The system is capable of accepting an ultrafast optical pulse train and is capable of generating a train of optical pulses having pulse width, spectral content, chirp characteristics identical to those of the input optical pulses and having a pulse repetition rate that is an integer multiple of the pulse repetition rate of the input pulses. This can be achieved by tapping a small portion of the input pulse power in a 2x2 optical coupler with an actively controllable optical coupling coefficient, recirculating this tapped pulse through a round trip in an optical delay line provided with optical amplification, optical isolation, optical delay (path length) control, optical phase and amplitude modulation and compression experienced by the optical pulses in the optical delay line for the purpose of minimizing the temporal pulse width at the output of the device, and recombining this power with the 2x2 optical coupler.

[0160] Passive or active optical delay control can be used as can optical gain using rare earth doped optical fiber and / or rare earth doped integrated optics and / or electrically or optically pumped semiconductor optical amplification. Dispersion compression can be provided using fiber-Bragg gratings and / or volume Bragg gratings. Wavelength division multiplexing data modulation of pulses traversing the delay line can be requested as can pulse code data modulation of pulses traversing the delay line.

[0161] For FSO applications, tailoring of conventional USPL sources through synthesis of USPL square wave pulses can be achieved using microlithographic amplitude and phase mask techniques. The ability to adjust pulse width using techniques and similar methods that employ this technique to control and actively control pulses can improve propagation efficiency through FSO transmission links, thereby improving system availability and received optical power levels.

[0162] Active programmable pulse shapers can be used to actively control USPL pulse width, which can include matching real-time atmospheric conditions to maximize propagation by changing the environment. One or more of the following techniques can be used in FSO applications to adapt the optical temporal spectrum using each technique: Fourier transform pulse shaping, liquid crystal module (LCM) arrays, liquid crystal on silicon (LCOS) technology, programmable pulse shaping using an acousto-optic modulator (AOM), acousto-optic programmable dispersion filter (AOPDF), and polarization pulse shaping.

[0163] Fig.32 A process flow diagram 3200 illustrating features of an example method, one or more of which can be present in an embodiment 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 communication platform at 3206, and the modulated optical signal is sent using the optical transceiver at 3210 for reception by a second optical communication device.

[0164] Fig.33 Another process flow diagram 3300 illustrating features of an example method, one or more of which can be present in embodiments of the current subject matter. At 3302, a beam of optical pulses, each having a duration of about 1 nanosecond or less, is generated, for example using a USPL source. The beam of optical pulses is transmitted to a target atmospheric region via an optical transceiver at 3304. At 3306, optical information received at the optical transceiver as a result of optical backscattering of the beam of optical pulses from one or more objects in the target atmospheric region is analyzed.

[0165] Fig.34Another process flow diagram 3400 illustrating features of an example method, one or more of which can be present in an embodiment of the present subject matter. At 3402, a first light beam and a second light beam comprising light pulses are generated, for example by a USPL source. At 3404, a first modulation signal is applied to the first light beam to generate a first modulated optical signal and a second modulation signal is applied to the second light beam to generate a second modulated optical signal. At 3406, a first polarization state of the first modulated optical signal is adjusted. Optionally, a second polarization state of the second modulated optical signal can also be adjusted. At 3410, the first modulated optical signal having the adjusted first polarization state is multiplexed with the second modulated signal. In 3412, the multiplexed first modulated optical signal having the adjusted first polarization state and the second modulated signal are sent by an optical transceiver for reception by a second optical communication device.

[0166] One or more aspects or features of the subject matter described herein can be implemented using digital electronic circuits, 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 can include implementation in one or more computer programs that can be executed and / or interpreted on a programmable system including at least one programmable processor, which can be special purpose or general purpose, coupled to receive data and instructions from a storage system, at least one input device, and at least one output device, and coupled to transmit data and instructions to the storage system, at least one input device, and at least one output device.

[0167] These computer programs (which can also be referred to as programs, software, software applications, applications, components or codes) include machine instructions for programmable processors and can be implemented in high-level procedural and / or object-oriented programming languages ​​and / or in assembly / machine languages. As used herein, the term "machine-readable medium" refers to any computer program product, device and / or apparatus used to provide machine instructions and / or data to a programmable processor, such as, for example, a disk, an optical disk, a memory, and a programmable logic device (PLD), including a machine-readable medium that receives machine instructions as a machine-readable signal. 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 can store such machine instructions non-temporarily, such as, for example, a non-temporary solid-state memory or a magnetic hard drive or any equivalent storage medium. A machine-readable medium can store such machine instructions alternatively or additionally in a transient manner, such as, for example, a processor cache or other random access memory associated with one or more physical processor cores.

[0168] To provide interaction with a user, one or more aspects or features of the subject matter described herein can be implemented on a computer having a display device (such as, for example, a cathode ray tube (CRT) or a liquid crystal display (LCD) or a light emitting diode (LED) monitor for displaying information to the user) and a keyboard and a pointing device (such as, for example, a mouse or trackball) that the user can use to provide input to the computer. Other types of devices can also be used to provide interaction with the user. For example, the feedback provided to the user can be any form of sensory feedback, such as, for example, visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including but not limited to sound, voice, or tactile input. Other possible input devices include, but are not limited to, touch screens or other touch-sensitive devices, such as single-point or multi-point resistive or capacitive touch pads, voice recognition hardware and software, optical scanners, optical pointers, digital image capture devices and associated interpretation software, etc. A computer remote from the analyzer can be linked to the analyzer via a wired or wireless network to enable data exchange between the analyzer and the remote computer (e.g., receiving data from the analyzer at the remote computer and transmitting information such as calibration data, operating parameters, software upgrades or updates, etc.) and remote control, diagnosis, etc. of the analyzer.

[0169] The subject matter described herein can be specifically implemented in a system, device, method and / or article mode depending on the desired configuration. The embodiments described in the above description do not represent all embodiments consistent with the subject matter described herein. Instead, they are only some examples consistent with the aspects related to the described subject matter. Although several changes have been described in detail above, other modifications and additions are possible. In particular, additional features and / or changes can be provided in addition to those described herein. For example, the embodiments described above can be directed to various combinations and sub-combinations of the disclosed features and / or combinations and sub-combinations of several additional features disclosed above. In addition, the logical flows depicted in the drawings and / or described herein do not necessarily require the specific order or sequential order shown to achieve the desired results. Other embodiments may be within the scope of the following claims.

Claims

1. An optical communication or sensor system for generating a free-space FSO beam to enhance propagation through a lossy medium, the system comprising: an ultrashort pulse laser (USPL) source and a modulator, the modulator being operably disposed inside or outside the USPL source and configured to perform data modulation to generate a data modulated optical output, the data modulated optical output comprising data modulated optical pulses, each data modulated optical pulse having a duration of 1 nanosecond or less, a peak optical pulse power of 1 kilowatt (kWatt) or greater, and a data capacity of at least 1 gigabit per second (Gbps); and a first optical transceiver configured to receive the data modulated optical output and convert it into an FSO optical beam, the FSO optical beam being transmitted through a lossy medium to an optical transmission platform located remote from the first optical transceiver, and including a second optical transceiver configured to receive the FSO optical beam and pass the received FSO optical beam to a photodetector; and wherein the lossy medium comprises at least one of water aerosol, turbulent air, and clear-air scintillation that presents optically impairing atmospheric conditions to the FSO optical beam, The data modulated light output has a pulse spectrum bandwidth of an infrared central wavelength, The lossy medium shifts the central wavelength of the FSO beam to form a second central wavelength different from the central wavelength of the USPL source output, and The second optical transceiver detects a portion of the FSO light beam at the second center wavelength using a photodetector.

2. The optical communication or sensor system according to claim 1, wherein: The data modulated optical output has a pulse spectral bandwidth at an infrared center wavelength.

3. The optical communication or sensor system according to claim 1, wherein: The pulse duration is less than one picosecond.

4. The optical communication or sensor system according to claim 1, wherein: The pulse duration is less than 100 femtoseconds.

5. The optical communication or sensor system according to claim 1, wherein: The duration is less than one femtosecond.

6. The optical communication or sensor system according to claim 1, wherein: The peak optical pulse power is equal to or greater than 5 kilowatts.

7. The optical communication or sensor system according to claim 1, wherein: The peak optical pulse power is equal to or greater than 10 kilowatts.

8. The optical communication or sensor system according to claim 7, wherein: The data modulation includes pulse code modulation.

9. The optical communication or sensor system according to claim 1, wherein: The pulse duration is equal to or less than 100 femtoseconds, and the peak optical pulse power is greater than 10 kilowatts.

10. The optical communication or sensor system of claim 1, further comprising an optical multiplexer for multiplexing more than one data channel into the FSO light beam.

11. The optical communication or sensor system of claim 1 , wherein the lossy medium comprises water aerosol, and wherein the system provides an improved signal peak-to-fall of 25 dB or more compared to a free-space beam generated using a continuous wave laser having the same average power as the ultrashort pulse laser source.

12. The optical communication or sensor system according to claim 1, wherein: The USPL source also has a repetition rate of at least 1 GHz.

13. A method of forming a free-space optical beam to enhance propagation through a lossy medium, comprising: generating, at an optical transmitter, a data modulated pulsed optical output comprising optical pulses, wherein each optical pulse has a duration of 1 nanosecond or less, a peak optical pulse power of 1 kilowatt or greater, and a data capacity of at least 1 gigabit per second (Gbps); forming a free-space light beam from the data modulated pulsed light output; transmitting a free-space optical beam through a lossy medium to an optical transmission platform having a photodetector and located at a great distance from the optical transmitter; and detecting a free-space light beam at the photodetector; wherein the lossy medium comprises at least one of water aerosol, turbulent air, and clear-air scintillation that presents optically impairing atmospheric conditions to the free-space optical beam, The data modulated pulse light output has a pulse spectrum bandwidth of an infrared central wavelength, The lossy medium shifts the central wavelength of the free-space light beam further toward the infrared to form a second central wavelength that is longer than the central wavelength output by the optical transmitter, and The method also includes detecting a portion of the free-space beam at the second center wavelength using the photodetector.

14. The method according to claim 13, wherein: The optical transmitter provides at least 10 dB improved signal peak-to-fall compared to a free-space beam generated using a continuous wave laser having the same average power as the optical transmitter.

15. The method according to claim 13, wherein: Generating the data modulated pulsed light output includes performing pulse encoding.

16. The method according to claim 13, wherein: The peak optical pulse power is: a) between 5 kW and 10 kW; or b) greater than 10 kW, with pulse duration less than 100 femtoseconds.

17. The method according to claim 13, wherein: The data modulated pulsed light output also has a repetition rate of at least 1 GHz.

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