Method and apparatus for ultra-short pulsed laser communication through lossy medium

Abstract

SOLUTION: Free-space optical (FSO) wireless transmission, including optical communications, remote-sensing, power beaming, etc., can be enhanced by replacing conventional laser sources that operate in the infrared portion of the optical spectrum with ultra-short pulsed laser (USPL) sources 102 having peak pulse powers of 1 kW or more and pulse lengths of less than 1 picosecond. Specifically, it has been observed that, under these conditions, the attenuation of an USPL beam having the same average optical power as a conventional laser in a lossy medium, such as the atmosphere, is substantially less than the attenuation of a conventional laser beam having a lower peak pulse power and / or a longer pulse width.EFFECT: The superior system performance in using a USPL can be translated into an increased distance between a laser source in a transmitter and a photodetector in a receiver and / or higher reliability of the system in inclement weather conditions.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of U.S. Patent Application No. 16 / 269,106, filed on February 6, 2019, entitled METHOD AND APPARATUS FOR ULTRA - SHORT PULSED LASER COMMUNICATION THROUGH A LOSSY MEDIUM, the content of which is incorporated herein by reference.

[0002] The subject matter described herein relates to free - space optical (FSO) wireless transmission, including optical communication, remote sensing, power beaming, etc. More particularly, it relates to enhanced optical transmission efficiency achievable for wavelength propagation using an ultrashort - pulse laser (USPL) source for beam propagation through optically impaired atmospheric conditions due to factors that may include, but are not limited to, fog, clouds (i.e., water aerosols), atmospheric beam wander, fluorescence effects, etc.

Background Art

[0003] The explosive growth in demand for telecommunications services from both the private and commercial sectors and the government sector is imposing an unprecedented burden on currently available telecommunications networks. Without alternative network delivery technologies and topologies, the overall effective network speed may be limited, while bottlenecks within the network are becoming increasingly frequent.

[0004] A bidirectional free-space optical (FSO) communication network, when feasible, can provide a useful alternative to microwave links, wires, or cable systems. Such networks can be transparent to current and future network architectures due to sharing with optical fiber transmission systems on a common technology platform and being the backbone of many modern communication systems. FSO communication systems may generally share common optical fiber components, and commercial optical components can often be utilized for both free-space optical and optical fiber applications. The main difference in free-space optical data is that the propagation medium is the atmosphere rather than an optical fiber.

[0005] By leveraging current state-of-the-art optical fiber components, free-space optical data links can be fully integrated into current short- and long-haul high-speed optical networks. Free-space data links can fully achieve a synchronous optical networking (SONET) system architecture that utilizes, for example, the current 1550 nm wavelength optical technology platform such as the SONET OC-48 architecture. Furthermore, such systems can also scale to higher data rates and configurations. Optical data link systems may benefit from operating in the unregulated portion of the electromagnetic spectrum. Unlike the microwave and RF spectrums, optical data links generally may not require the issuance of special lease fees or tariffs. Additionally, due to the operating wavelength of the system, eye safety-related issues can usually be minimized. Further, special precautions or permits are usually not required when operating free-space optical data links related to right-of-way. Also, the costs associated with plowing and trenching of fixed cable systems can be avoided.

[0006] More recently, FSO communication technology has utilized commercial progress achieved within the 1550 nm optical wavelength transmission band. Erbium fiber doped amplifier (EFDA) technology has been incorporated into system design configurations to enhance the overall effective optical budget for acceptable optical attenuation, thereby expanding the reach of transmission systems through the atmosphere.

[0007] High-power optical amplifiers are useful for terrestrial free-space transmission systems and optical fiber systems. Repeater distances have been extended in terrestrial fiber systems and undersea fiber systems, and dense wavelength division multiplexing (DWDM) transmission architectures have been introduced. With the advent of high-power Er / Yb optical amplifiers, similar advancements seen in optical fiber transmission have also been realized in optical wireless systems and free-space laser communication systems. Experimental transmission results for a single-channel 1550 nm free-space optical data link operating at 2.5 Gbps over a 2.4 km transmission span have been reported, similar to the results reported for a 4-channel 1550 nm wavelength division multiplexing (WDM) free-space optical data link operating at 10 Gbps over a 4.4 km transmission distance.

[0008] Several commercial and military FSO communication systems have been developed and are currently in operation, but their performance for terrestrial applications is limited by the optical attenuation effects associated with the atmosphere caused by adverse weather conditions including fog, rain, snow, smoke, and turbulence. These effects limit the application of FSO communication to relatively short optical transmission distances (about several hundred meters) in typical situations where high reliability and interruption-free availability are required.

[0009] On August 4, 1997, a patent application (application number 08 / 905,760) was filed and assigned to Motorola Corporation, which promised relief from the effects of undesirable attenuation caused by the atmosphere. A patent (U.S. Patent No. 6,043,920) was granted for this application in 2003. In the second paragraph, lines 22 to 35, the following is described. "The method and apparatus of the present invention apply the newly discovered characteristics of these narrow pulse width signals to the laser communication field. According to a preferred embodiment, a laser pulse of extremely narrow width and modulated can be transmitted through a lossy medium without significant attenuation. Therefore, the method and apparatus of the present invention enable extremely high-speed communication through media that were previously considered too lossy to facilitate high-speed laser communication. For example, the method and apparatus of the present invention could be used for laser communication through air, water, water vapor, solid obstacles, particle suspensions, glass fibers, and other media." However, this claim was only supported by the following description (second paragraph, lines 16 to 21). "Modern laser technology enables the generation of narrow pulses, and experiments have shown that these narrow pulses have the property of being able to penetrate a medium with substantially less attenuation than that suffered by the prior art."

[0010] Claim 1 of this issued patent is very broad and applicable to communication via any wireless link through any lossy medium (such as the atmosphere) having a modulated pulse of laser light of unspecified wavelength having a pulse width of less than 200 femtoseconds.

[0011] Based on research conducted at Attochron.LLC and elsewhere, this patent, and specifically this claim, is presumed to have been based on constructive reduction to practice that was never once verified or reproduced by others, even after the expiration of the patent term in 2017. However, over the years, there has been much discussion on the subject of the potential for reducing optical attenuation in various media using ultrashort optical pulses. For example, in U.S. Patent No. 6,583,911 B1, filed on October 6, 2000, and issued on June 24, 2003, inventor D.R. Alexander reported on extensive laboratory tests using ultrashort laser pulses having a duration of less than 100 femtoseconds and wavelengths in the range of 0.75 to 0.85 microns. “...the data did not show an indication that pulse wave transmission was more beneficial than continuous wave.” This factual data was in direct contradiction to the claims made in the Motorola patent.

[0012] Subsequently, in 2005 and 2006, Ulf Osterberg et al. published two papers [U. J. Gibson, U. L. Osterberg, “Optical Precursors and Beer’s Law Violations; Non-Exponential Propagation Losses in Water”, Optics Express 13 6 (2005), and A. E. Fox, U. Osterberg, “Observation of Non-Exponential Absorption of Ultra-fast Pules in Water”, Optics Express 14 8 (2006)] suggesting that the absorption of USPL beams may not follow Beer's law (exponential decay of an optical beam in a lossy medium). These results were used by the authors to suggest that the optical absorption losses of femtosecond laser pulses are substantially lower than those of CW lasers operating at the same wavelength. Wavelengths in the range of 650 to 800 nm were reported to be used in the experimental part of the research work. Thus, initially, it seemed that the basic presumption regarding the unusually low optical attenuation reported in the Motorola patent US 6,043,920 filed in 1997, even if it was somewhat speculative and contradicted the data presented by Alexander in US 6,583,911 B1, was perhaps retrospectively corroborated by the research of Ulf Ostenberg.

[0013] However, an important paper published in 2007 by Jian Chao Li et al. from the University of Nebraska [University of Nebraska-Lincoln; DigitalCommons@University of Nebraska-Lincoln; Faculty Publications from the Department of Electrical Engineering], entitled "Propagation of ultrashort laser pulses through water", specifically disputes the above conclusions and explanations presented by Osterberg et al. This paper concludes that the sub-Beer's law (non-exponential) observed by Osterberg et al. is merely due to the broad spectral band of USPL and is not related to the shortness of the laser pulse. Li et al. pointed out that the same sub-Beer's effect would be reported for a coherent but non-CW light source having the same broad spectral band as USPL. Therefore, the absorption of the sub-Beer's law cannot be directly attributed to the lower attenuation of femtosecond pulses in water. This also similarly casts doubt on any advantages of using ultrashort pulses for extended transmission through a lossy medium as taught by the inventors in Motorola's U.S. Patent No. 6,043,920.

[0014] The clear answer to the question "What are the advantages of using USPL for enhanced transmission through the atmosphere?" seems to be derived from the research of Paul Corrigan et al. in a 2007 paper titled "Enhanced Performance of Low-Power(60mW) Femtosecond Free Space Optical Communication Sytem Over Conventional CW Operation" [Proc. SPIE 6457, Free-Space Laser Communication Tecnologies XIX and Atmospheric Propagation of Electromagnetic Waves, 64570X (February 12, 2007)]. And the answer to the above question seems to be "yes" with a slight difference for light scattering and "no, not yet" for light absorption. The author of this paper states the following in that conclusion. "When the transmission and scattering characteristics of the stable substance are known (usually determined by two different measurements), the difference is absorption. This [separating the absorption effect from the scattering effect] is particularly difficult to do in simulated fog because the simulation itself is unstable as is the case with actual fog. However, there are no current measurements or claims indicating that this enhanced propagation is related to different physical characteristics of the absorption of ultra-fast pulses by the atmosphere. Here, it should be pointed out that this research by Corrigan et al. was carried out at a wavelength (1560 nm) longer than the wavelengths used by previously reported researchers. Therefore, there may be different explanations for these results compared to some or all of the other results mentioned above.

[0015] Facing all of the above unproven results and subsequent contradictions and debates, it is not surprising that the growth of FSO communication is limited and that no single commercial system based on the claims of the expired Motorola patent (U.S. 6,043,920) has yet been produced. Clearly, it would be beneficial to definitively prove whether using ultrashort laser pulses would substantially improve free-space optical communication in adverse conditions. And if the results are positive, this would represent a major breakthrough and would likely have a substantially favorable impact on the future adoption of FSO communication.

Summary of the Invention

Problems to be Solved by the Invention

[0016] In 2013, a turning point in this discussion occurred with the publication of an article in the 2013 issue of Laser Focus by Isaac Kim et al. titled "ADVANCES IN COMMUNICATIONS: New FSO provides reliable 10Gbps / sec and beyond backhaul connections". This article reported the following. "Experiments conducted by Attochron, LLC at the 500m wireless test facility of the U.S. Army's Picatinny Arsenal in Dover, New Jersey, demonstrated that a USP laser-based FSO system had a maximum 25dB increase in received output compared to a legacy CW FSO system in fog. " Also, these authors mention that "legacy FSO systems function well in clear or cloudy weather up to distances of up to 1.5km, but the presence of fog can reduce the effective link distance to 200m [I.I. Kim, Lightwave, 26, 19 - 21 (2009).]".

[0017] In this study, the USPL system and the CW laser system were operated side by side simultaneously, and their laser beams traveled through adjacent optical paths on the same test range. The authors further "The output of these new USP FSO systems is at 1550 nm, with an average output power of 50 mW, and is generated by passively mode-locked 100 fs pulses at a 1 Gbit / sec [data] repetition rate. The stream of ultrashort pulses is externally modulated to generate gigabit Ethernet signals. In the Picatinny Arsenal experiments, a single 3-inch diameter telescope was used on the transmit side, and a similar 3-inch telescope was also used on the receive side." continues.

[0018] The conclusions reached by Isaac Kim et al. are as follows. "In the preliminary tests of our prototype USP laser-based FSO system, an additional margin of 25 dB improves the link availability at 1 Gbit / sec to 99.5% over a range of 3 km."

[0019] Following the obtaining of these important results during the Picatinny Arsenal tests in 2010 and 2011, a patent application (Application No. 13 / 737,898) was filed on January 16, 2013, by Attochron LLC, with extensive claims that describe the following. "An optical communication device, an ultrashort pulse laser (USPL) source that generates a beam including optical pulses each 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)... and including" However, the scope of the granted patent claims was substantially limited by the following clauses added to all of the claims during the examination process of the application. "The optical transceiver is configured to detect atmospheric elements and enable analysis of the backscattered signal of the detected atmospheric element's aerosol signature to enable adjustment of the beam generated by the USPL source to enhance atmospheric penetration."

[0020] In fact, because it was not clear from the filed Attochron patent application that the invention had been made without this restrictive clause, the broader patent claims of the original application were not allowed. In particular, the field test results obtained by Attochron LLC at Picatinny Arsenal were not even mentioned in the specification. However, even if they had been so incorporated, it would not have been clear that the excellent system performance observed and reported by Attochron LLC's researchers during the field tests at Picatinny Arsenal was due to (1) a reduction in the effect of atmospheric turbulence on the USPL beam due to its lower known coherence compared to the CW laser beam used as a reference during the Picatinny Arsenal tests, and / or (2) a known improved signal-to-noise ratio when detecting a laser beam with a higher peak pulse compared to the CW laser beam or comparable energy per received bit (L. Biovin et al., "Receiver Sensitivity Improvement by Impulsive Coding", IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 5, May 1997), or (3) one or more other known factors.

[0021] One of the objectives of the present application is to summarize all of the significant results observed during the 2010 and 2011 tests at Picatinny Arsenal, and additional results revealed by an extensive analysis of the recorded data, finally completed in October 2017. These results are impressive. During both foggy conditions and sunny warm weather that causes a fluorescence effect, the performance of the USPL system showed an advantage of more than 25 dB in received signal margin over the entire same test range and, in fact, compared to a parallel CW laser FSO system operating on exactly the same optical path as the USPL beam path in both cases. Further, the observed at least 25 dB advantage factor of the USPL system over the CW system likely represents only a lower limit for a much larger factor that would seem likely when more powerful laser sources are evaluated.

[0022] Note that the at least 25 dB advantage factor of the USPL system over the CW system, as described above, is expressed in logarithmic terms, as is customary for explaining the performance of many types of communication systems. In linear terms, this is equivalent to a factor of 316 or more (log 10 316 = 10×2.5 = 25 dB). When expressed in this way, it is clear that this is a very large factor that was not predicted based on any reported prior art or known physical characteristics.

[0023] There are several theories that may explain some or all of the observed results, but some non-linear optical phenomenon that is not yet understood or widely accepted may contribute to the observed large figure of merit associated with the USPL vs CW laser test results. Preliminary tests at Attochron LLC also suggest that this may be the case. Specifically, the observed figure of merit does not appear when using ultrashort laser pulses with lower peak power levels of less than about 1 kilowatt. However, the figure of merit becomes apparent when using ultrashort laser pulses with a pulse width shorter than 1 nanosecond and operating at an optical peak power of 1 kilowatt or above. And these effects become significant when the peak pulse optical power level is in the range of 5 to 10 kilowatts or more.

[0024] Putting the advantages of this new insight into FSO communication reveals why the unproven claims of the expired Motorola patent (6,043,920) have been the subject of such debate for so many years. The beneficial effects of using femtosecond pulses can only be achieved when using a laser source having a high peak optical power in the range of 1 kilowatt or more, and preferably in the range of 10 kilowatts or more. And it should be clearly noted that all of the claims of the Motorola patent are limited to "applying the modulated [electrical] pulse stream to a [CW] laser [beam] to generate a laser pulse, wherein the pulse width of each pulse of the pulse stream is 200 femtoseconds or less, and directing the laser optical pulse through the loss medium to a detector". The reality is that the only way these conditions could be met would be for pulses less than 200 femtoseconds to originate from a CW laser having an average output power of at least 1 kilowatt that would be chopped using an external optical beam modulator. Such high-power lasers are not practical to build and operate and would be far too expensive for use in commercial FSO communication systems. However, the use of a USPL source in an FSO communication system, which has a much lower average output power but a very high peak pulse power exceeding 1 kilowatt, is the essence of the present invention. This leads to an unexpected improvement in atmospheric transmission over the use of continuous wave (CW) lasers and all types of lower-output pulsed lasers.

[0025] It should be noted that the spectral width of the USPL pulse can sometimes have a very large width. For example, a USPL with a central wavelength of 1560 nm can have a spectral bandwidth that spreads over several hundred nanometers or more. This is due to the fundamental relationship between the minimum frequency spectral width Δf of any laser pulse and its temporal pulse width Δt. That is, Δf = 1 / Δt. And since the wavelength λ is defined by this relationship, λ = c / f, where c is the speed of light in a vacuum and f is the central frequency of the laser. Differentiating this relationship gives Δλ = (-c / f2)Δf = -(λ 2 / c)(1 / Δt). For example, when λ = 1560 nm and the pulse width Δt is 20 femtoseconds, then using this equation, the minimum spectral width Δλ would be equal to 406 nm. Similarly, for a USPL with a 10 femtosecond pulse width, the minimum spectral width would be twice as large, 812 nm. And for a 100 femtosecond USPL, the pulse width would have a minimum spectral width of 81 nm.

[0026] The following summarizes the important results observed. 1. Unexpected advantages of USPL pulse propagation due to clear - sky atmospheric effects: Improvement of ~30 dB+ in USPL link margin for the 'intended' central wavelength (1550 nm) (for CW lasers) due to clear - sky atmospheric effects (scintillation, beam wander, turbulence) 2. Unexpected advantages of USPL propagation due to water aerosols (fog, clouds, etc.): Improvement of 30 dB+ in USPL link margin for the 'intended' central frequency (1550 nm) (for CW lasers) due to water aerosols (fog, clouds, etc.) 3. Unexpected advantages of wavelength shift to'more infrared' wavelengths in high - density water aerosols (improvement in propagation) Heavy water aerosol (which may attenuate to, or approach, ~100 to ~150 dB per kilometer), and after the CW laser beam is attenuated below its detection level (i.e., below the noise floor of the optical power meter), and after the most powerful part of the USPL output spectrum (centered at 1550 nm) is also attenuated below its detection level, the remaining 1571 nm part of the USPL beam, which emits an average power ~5 dB lower than the 1550 nm part of the spectrum, was 15 dB above the detection level and stable (low-level fluctuations over time). 4. Unexpected advantages at the USPL pulse received 'peak-to-fade' power level The received power fluctuations of the USPL beam - during periods of lower attenuation, but most importantly, during periods of attenuation that increase or decrease due to weather effects - were substantially less, on the order of tens of dB over a very short period (usually a period of 1 second), compared to the power fluctuations of the CW laser beam). And, 5. Advantages of impulse coding using true discrete zero-return high peak power pulses Improvements of up to 10 dB, and improvements exceeding 10 dB, in link margin at the photodetector by using ultrashort pulses as signal carriers in any kind of 'impulse coding' modulation.

[0027] Also note that the above advantages have contributed significantly to substantially enhancing the overall usability of USPL communication systems and sensor systems, either alone or in combination.

[0028] The subject matter described in this specification can be embodied in a system, apparatus, method, and / or product, depending on the desired configuration. The embodiments described in the foregoing description do not represent all embodiments consistent with the subject matter described in this specification. Instead, the embodiments are merely some examples consistent with aspects related to the subject matter described. Although some variations have been described in detail above, other modifications or additions are also conceivable. In particular, additional features and / or variations can be provided in addition to those described in this specification. For example, the embodiments described above may be directed to various combinations and sub - combinations of the disclosed features, and / or combinations and sub - combinations of some additional features disclosed above. Further, the logical flows shown in the accompanying figures and / or described in this specification do not necessarily require the particular order or sequence shown to achieve the desired result.

Brief Description of the Drawings

[0029] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate specific aspects of the subject matter disclosed in this specification and, together with the description, serve to explain some of the principles associated with the disclosed embodiments. In all cases where a USPL is illustrated or described, it should be assumed that the peak output power from these lasers is 1 kilowatt or more and that the pulse width is 1 nanosecond or less.

[0030]

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Mode for Carrying Out the Invention

[0031] FIG. 1 shows an example of an optical communication platform 100 that conforms to an embodiment of the present subject matter for using a USPL device free space coupled as a light source for transfer. As shown in FIG. 1, the USPL source 102 is directly modulated by an external source element 104. The optical power from the USPL source can optionally be coupled to a transmitting side element 106 across the free space 110 by an optical telescope. The transmitting side element 106 may optionally include optical components formed by, for example, hyperbolic mirror fabrication techniques, conventional Newtonian designs, etc. The mutual receiving telescope in the receiver system can provide optical reception. Consistent with an embodiment of the present subject matter, each optical transmission platform can be designed to operate as a bidirectional unit. In other words, the transmitting side element 106 of the optical communication platform 100 can also function as a receiving side element. Generally, unless explicitly stated otherwise, the transmitting side element 106 described can be considered to also operate as a receiving element, and vice versa. An optical element that performs both a transmitting function and a receiving function may be referred to herein as an optical transceiver.

[0032] FIG. 2 shows an example of an optical communication system 200 that conforms to an embodiment of the present subject matter, including the optical communication platform 100 of FIG. 1. Also shown in FIG. 2 is a second complementary receiving element 204, which may be a receiving side telescope located at a remote distance from the transmitting side element 106. As described above, both the transmitting side element 106 and the receiving side element 204 may be bidirectional, and each can function as both a transmitting side element 106 and a receiving side element 204 depending on the instantaneous direction of data transmission in the optical communication system 200. This feature applies throughout this disclosure to the transmitting side element and the receiving side element, unless explicitly stated otherwise. Either or both of the transmitting side element 106 and the receiving side element 204 may be an optical telescope or other device for transmitting and receiving optical information.

[0033] FIG. 3 shows an example of an optical communication platform 300 that conforms to an embodiment of the present subject matter for using a USPL source 102 fiber-coupled to an external modulator 302 through a fiber medium 304 and optionally connected to a transmitting element 106 through an additional transmission medium 306 that may be a fiber medium, a free space connection, etc. The USPL source 102 can be externally modulated by the external modulator 302 such that the optical power from the USPL source 102 is fiber-coupled to the transmitting element 106 or processed through an equivalent optical telescope.

[0034] FIG. 4 shows an example of an optical communication system 400 that conforms to an embodiment of the present subject matter and includes the optical communication platform 300 of FIG. 3. Also shown in FIG. 4 is a second complementary receiving telescope 204 that may be a receiving telescope located at a remote distance from the transmitting element 106 as described above in connection with FIG. 2.

[0035] FIG. 5 shows an example of an optical communication architecture 500 that conforms to an embodiment of the present subject matter. The architecture 500 of FIG. 5 includes the elements of FIG. 4 and further includes a first communication network 502 connected to the first optical communication platform 300. The receiving element 204 may be part of a second optical communication platform 504 that optionally includes 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 such that data is optically transmitted between the transmitting element 106 and the receiving element 204 or passed between the first communication network and the second communication networks 502, 506, each of which may include one or more of optical networking features and electrical networking features.

[0036] FIG. 6 shows an example of an optical communication system 600 that conforms to an embodiment of the present subject matter. As part of an optical communication platform 602, a USPL source 102 is fiber-coupled to an external modulator 302 through, for example, an optical fiber 202 or other transmission medium. The light from the USPL source 102 is propagated through a transmission-side element 106 as described above. An optical amplifier element 604, which may optionally be an optical fiber amplifier element, can be used to boost the optical transmission output and is optionally disposed between the external modulator 302 and the transmission-side element 106 and may be connected to one or both through an additional transmission medium 306, which may optionally be a fiber medium, a free-space connection, etc. Also shown in FIG. 6 is a second complementary receiving element 206 located at a remote distance from the optical communication platform 602. It is readily understood that a second optical communication platform 504 including a receiving-side element 204 may also include an optical amplifier element 604. First and second communication networks 502, 506 can each be connected to the two optical communication platforms 602, 504.

[0037] FIG. 7 shows an example of an optical communication system 700 consistent with an embodiment of the present subject matter. The optical communication platform 602 shown in FIG. 6 can communicate with a second optical communication platform 702 that may include a receiving element 204 and an optical preamplifier 704 in this embodiment. Also, other components similar to those shown in the optical communication platform 602 may be included in the second optical communication platform 702 although they are not shown in FIG. 7. It should also be understood that a bidirectional optical communication platform may include both an optical preamplifier 704 for amplifying received optical signals and an optical amplifier element 604 for boosting transmitted optical signals. Consistent with the embodiment shown in FIG. 7 and other embodiments of the present subject matter, one or more equivalents, such as, but not limited to, erbium-doped fiber amplifiers (EDFAs), high-power erbium-ytterbium-doped fiber amplifiers (Er / Yb-DFAs) or semiconductor optical amplifiers (SOAs), may be used to enhance the optical budget for the data link between the transmitting element 106 and the receiving element 204 (and vice versa), including optical amplification (e.g., either or both of the optical amplifier element 604 or the optical preamplifier 704).

[0038] FIG. 8 shows an example of an optical communication system 800 consistent with an embodiment of the present subject matter. The optical communication platform 602 shown in FIG. 6 may communicate with a second optical communication platform 802 that may include a receiving element 204 and an optical preamplifier 704 similar to those shown in FIG. 7 in this embodiment. As shown in FIG. 8, the second optical communication platform 802 may further include an optical receiver circuitry 804 that can receive amplified and electrically recovered data received at the receiving element 204 and amplified by the optical preamplifier. A plurality of clock sources 806 can interface, as needed, with a plurality of remote multipoint network connections to a plurality of communication networks 810. Similarly, a complementary set of clock sources and a plurality of communication networks can operate in conjunction with the optical communication platform 602 (e.g., instead of the single communication network 502 illustrated in FIG. 8).

[0039] FIG. 9 shows an example of an optical communication system 900 that is consistent with an embodiment of the present subject matter. Also, an optical communication platform 902, which may feature elements similar to those of the optical communication platform 602 first described herein with respect to FIG. 6, may include an additional USPL source 904 that serves as a tracking and alignment (pointing) beacon source. Also, a second optical communication platform 906 may include an additional USPL source 910 that serves as a tracking and alignment (pointing) beacon source. The tracking and alignment (pointing) beacon sources 904, 910 may be emitted from available communication sources optionally used for data transmission transmissions, or may be provided by separate dedicated USPL sources. Further, each USPL beacon source 904, 910 may include an in-band source or an out-of-band source, thereby enabling an advantage point from an available optical amplification source or from a dedicated optical amplification resource.

[0040] FIG. 10 shows an example of a dual-polarization USPL-FSO optical data link platform 1001 that provides a polarization multiplexed USP-FSO (PM-USP-FSO) function to the optical signals transmitted by USPL sources. Two USPL sources 102 and 1002 are fiber-coupled to modulation components 1004, 1006, either directly modulated or externally modulated, respectively. Each respective modulation signal is optically amplified by optical amplifier components 1010, 1012, and subsequently the optical polarization state is adjusted using polarization components 1014, 1016. The polarization state signals are fiber-coupled to a polarization dependent multiplexer (PDM) component 1020 to interface with an optical output platform component 1022, which may be similar to the transmission element 106 described above. The PDM 1020 multiplexes light of different polarization states into a single pulse train for transmission via the optical output platform component 1022. The USPL optical beacon 904 may be included to provide a function similar to that described above with respect to FIG. 9 to operate along or in conjunction with a second USPL optical beacon 906 at a receiving platform 1024, which may include a receiving side element 204 similar to that described above. As described above, the receiving side element 204 as well as other features and components of the receiving platform 1024 may generally be capable of supporting a transmission function such that a bi-directional link is established. The received signal recovered by the receiving side element 204 can provide an optical signal that interfaces with a suitable polarization dependent demultiplexer 1026 that can provide two elements for additional optical amplification using amplification elements 1030, 1032. Each optical amplification signal provided by the amplification elements 1030, 1032 may interface with a suitable optical network 1034, 1036 for network use.

[0041] FIG. 11A shows an example of a system 1100 that can be utilized for use of a USPL-FSO transceiver in line-of-sight optical communication (e.g., laser communication) applications, and FIG. 11B shows an example of a system 1150 that can be utilized for use of a USPL-FSO transceiver in non-line-of-sight laser communication applications. When transmitted light passes through the atmosphere, due to the scattering of the optical signal transmitted from the transmitting element, the advantages of some embodiments of the present subject matter can be realized. This scattering can enable the use of non-line-of-sight communication. Further, the wireless used in such communication systems can operate in the solar blind portion of the UV-C band where light is emitted at a wavelength of 200 to 280 nm. In this band, when solar radiation propagates through the environment, the solar radiation is strongly attenuated by the Earth's atmosphere. This means that as it approaches the ground, the amount of background noise radiation drops dramatically, enabling low-power communication link operation. On the other hand, environmental elements such as oxygen, ozone, and water can weaken or interrupt the communication broadcast and may limit the use to single-distance applications.

[0042] When UV waves diffuse through the atmosphere, the UV waves are typically strongly scattered among a variety of signal paths. Signal scattering is essential for UV systems operating in a non-line-of-sight state, and the communication performance may highly depend on the transmission beam pointing and the receiver's field of view. The line-of-sight arrangement 1100 shown in FIG. 11A may differ in bandwidth size from the non-line-of-sight arrangement 1150 shown in FIG. 11B. Ultraviolet communication may be more strongly dependent on the transmitter beam position and the receiver's field of view. As a result, it may be advantageous to refine the pointing apex angle, for example, by experimenting with supplementary equipment to enhance the UV-C signal.

[0043] FIG. 12 shows an example of a remote sensing system 1200 in which a USPL source 102 is fiber-coupled to a light emitting element 1202 that can transmit and receive optical signals via an optical fiber component 202. A portion of the forward-propagated light, including light from a data signal passing through the light emitting element 1202, is backscattered by interaction with airborne microparticles that are the subject of the investigation. The optical backscatter signal is detected through the light emitting element 1202 or a similar receiving aperture and conveyed for detection and spectroscopic analysis through a detection circuitry 1204 as shown in FIG. 12. The signature of the microparticles within the target atmosphere region 1206 where the investigation is being conducted can be calibrated by conventional techniques such as using predetermined spectroscopic calibration measurements such as ultraviolet spectroscopy, infrared spectroscopy, Raman spectroscopy, etc. Consistent with this embodiment, the optical system can operate as a LiDAR instrument that uses a USPL laser source operating in a spectral range of interest to provide enhanced performance in terms of resolution and detection sensitivity. The tunability of the spectral range may be useful for assessing and analyzing chemical components in the atmosphere.

[0044] The USPL-FSO transceiver can utilize an optical transmission terminal manufactured by either hyperbolic mirror fabrication techniques focused on the received signal at an ideal point or a conventional Newtonian design to utilize ionization detection techniques and non-ionization detection techniques for remote sensing and detection of the signature of airborne microparticles. Also, certain adaptations can be associated with ionization probing of remote regions, including controllable ionization that has been shown to occur at these frequencies and in the ionization process, which can be focused remotely, particularly to adjust the depth of atmospheric penetration through weather and clouds.

[0045] Figure 13 shows an example of the use of USPL sources and optical reception techniques to improve detection sensitivity. Researchers at the National Institute of Standards and Technology (NIST) in the United States have constructed a laser ranging system that can point to multiple objects with nanometer accuracy over distances of up to 100 km. A LIDAR (Light Detection and Ranging) system could have applications ranging from precision manufacturing on Earth to maintaining a network of satellites in a complete configuration (Nature Photonics DOI: 10.1038 / NPHOTON 2009.94). The NIST device uses two coherent broadband fiber laser frequency combs. The frequency comb outputs a stable series of short pulses that also includes a very coherent carrier extending across the pulse train. That is, the frequency comb can be used to perform both interferometric and time-of-flight measurements simultaneously, thereby enhancing the analytical capabilities for special situations in applications.

[0046] In the arrangement shown in Figure 13, two phase-locked frequency combs 1301 and 1302 are used in a coherent linear optical sampling configuration also known as a multi-heterodyne, meaning that one frequency comb measures both distance paths while the other frequency comb provides distance information encoded in view of the first comb. Pulses from one frequency comb 1301 can be emitted from within the fiber and directed towards two glass plates, namely a reference 1303 and a target 1304. Plates 1303 and 1304 can reflect a specific portion (e.g., about 4%) of the pulse back into the fiber, effectively creating two new pulses. The time separation 1301 between the two pulses can indicate the distance between the movable target plate and the reference plate. The second frequency comb 1302 is tightly phase-locked to the first frequency comb but has a slightly different repetition rate. Due to the different delays between consecutive pulses when the sources interfere, the second frequency comb can sample a slightly different portion of the light from the electric field of the first comb.

[0047] By using the technology described with reference to FIG. 13, two coherent broadband fiber laser sources can be replaced with two suitable USPL sources used within the scope of the described configuration, each USPL source fiber-coupled to a dedicated free-space optical microscope design. By doing so, the overall efficiency, optical ranging, and accuracy can be significantly improved.

[0048] The currently available USPL optical pulse trains operate at the native pulse repetition rate of the USPL laser source, typically limited to 50 MHz or less, thereby defining an upper limit on the maximum data rate of optical transmission. As a result, optical systems utilizing USPL laser sources are limited to low data rate applications below 50 Mbps. Having means to increase the operating speed of USPL is necessary to provide a solution for data transfer exceeding 50 Mbps.

[0049] FIG. 14 shows an example of a remote sensing system 1400 in which a USPL source 102 is fiber-coupled by an optical fiber component 202 to an optical emission element 1202 capable of transmitting and receiving optical signals. The light propagated forward by the optical emission element 1202, including the light from the data signal, is backscattered by interaction with known and unknown targets that are the objects of investigation within the atmosphere region 1206. The optical backscattered signal, including the light from the data signal, is detected through the optical emission element 1202 or a similar receiving aperture and conveyed for detection and analysis through the detection circuitry and spectral analysis component 1402 of FIG. 14. The signature of the particles within the region 1206 under investigation can be calibrated, for example, if range measurement analysis can be performed. The system 1400 of FIG. 14 may include a USPL-FSO transceiver utilized and operated over the infrared wavelength range as a rangefinder and spotting device for the purposes of target identification and investigation applications.

[0050] FIG. 15 shows an optical pulse multiplier module 1500 that can increase the repetition rate of the output from the USPL source 102. A typical USPL with a pulse width of 10 to 100 femtoseconds has a repetition rate of, for example, 50 MHz. The output from the USPL 102 can be fed as an input 1502 into a USPL photonic chip pulse multiplier module 1504. In this example, the photonic chip may include a 20,000:1 splitter element 1506 that divides the input into individual optical members. Each optical member on the opposite side of the splitter element 1506 contains a 50 MHz pulse train. Each optical member then passes through a delay controller (either a fiber loop or a lens array) 1510, which delays the pulse train of that member over time, for example, by several picoseconds. Successive optical members are thereby incrementally picoseconds delayed. All of these pulse trains with their respective 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 required ratios of the splitter and combiner can be controlled to provide the optical design required for the application in question. The final output 1514 is a pulse train of 10 to 100 femtosecond pulses with a repetition rate of 1 THz. This THz pulse train is then modulated by a 10 or 100 GigE signal as shown in FIG. 28, resulting in 100 femtosecond pulses per bit for a 10 GigE system and 10 femtosecond pulses per bit for a 100 GigE system. The applications cited are not limited to specific data rates of 10 and 100 Gbps, but can operate as required by the application under consideration. These numbers are for illustrative purposes only. Embodiments of the present subject matter can use any multiplier to increase the repetition rate of the USPL to any repetition rate via the photonic chip pulse multiplier module 1504. Other examples used in the generation of enhanced USPL repetition rates are shown in the present submission.

[0051] FIG. 16 shows a system 1600 for generating, transmitting, and receiving a high pulse rate USPL optical stream. For example, an optical chip multiplexing module 1610, which may be similar to that described with respect to FIG. 15, can be used in the present application. In this approach to achieve USPL pulse multiplication, a series of 10 GigE router connections (10 GigE is not intended to be a limiting feature), as explained by signals 1601, 1602, 1603, 1604 (although four signals are shown in FIG. 16, it is understood that any number is within the scope of the present subject matter), are interfaced to the optical chip multiplexing module 1610. During operation, the optical chip multiplexing module 1610 can support full duplex (Tx and Rx) for connection to the 10 GigE routers 1601, 1602, 1603, 1604. The optical chip multiplexing module 1610 can provide efficient modulation of the USPL signal 1685 output from the USPL source 1690 for the incoming optical signals 1601, 1602, 1603, 1604. The optical chip multiplexing module 1610 can provide the ability to modulate and multiplex these incoming optical signals.

[0052] At a remote receiving site where the receiving side device is located, all signals transmitted via the transmitting side element 1660 in the transmitting side device can be recovered using the appropriate receiver element 1665. A complementary set of optical chip multiplexing modules 1675 can provide the functions necessary to demultiplex the received data stream, as shown, by elements for delivery to a series of routers 1601’, 1602’, 1603’, 1604’ (again, the description of such four routers is not intended to be limiting). End-to-end network connectivity can be specified through network endpoint elements.

[0053] FIG. 17 shows a system 1700 example where an optical chip is interconnected to a wavelength division multiplexing (WDM) system where its currently available version can be very expensive. Since each 10 GigE signal operates at its own wavelength independently of other such signals, the WDM system has the advantage of not requiring timing or synchronization with a 10 GigE (or other speed) router 1701 as needed. The timing or synchronization of a TDM optical chip with a 10 GigE router can be important in a TDM optical chip. The GbE switch 1701 can provide the necessary electrical RF signal 1705 from the switch 1701 to modulate either directly or by using a pulse multiplier module detailed above within the USPL book to modulate a USPL source 1702. A typical RZ output 1710 can be coupled to an external modulator 1720, which can modulate using an NRZ clock source for the switch 1701, thereby producing an RZ modulation spectrum 1730. The conversion process using readily available equipment can provide the function for introducing the USPL source and its advantages into the terrestrial long-distance network spectrum.

[0054] For an optical chip system to successfully bridge between two remote 10 GigE switches, the optical chip system must typically operate like a simple single fiber. Thus, the timing of the TDM chip may be driven by the 10 GigE switch 1701. Both actively mode-locked USPL (i.e., 40 GHz, 1 picosecond pulse width) and passively mode-locked USPL (i.e., 50 MHz, 100 femtosecond pulse width) may be driven by an RF timing signal.

[0055] FIG. 18 shows a device 1800 that can support another approach to advancing to high pulse repetition data rate operations, such as very high data rate operations where optical chip design can be performed using either an optical fiber or free space optical components. A 50 MHz USPL source 1801 is interfaced to a series of optical delay controller elements 1802 that can be designed using either a fiber loop or an offset lens to generate an RZ output stream of exactly 10.313 Gbps, which is a 10 GigE line speed (greater than 10 Gbps for 64B / 66B encoding). A splitter element 1803, along with a variable optical delay line 1804, provides the function of splitting the incoming optical signal 1801 into (in this example) 206 paths. After sufficient delay has been introduced through the design, all the signals are multiplexed together through a combiner element 1805. In doing so, a series of optical signals, each equally spaced between identical and adjacent pulses, form a continuum of pulses for modulation. All the optical ingress signals can be conditioned by pre-emphasis techniques using, for example, typical optical amplification techniques to result in a uniform power spectrum for each egress signal from the combiner element 1805 before entering the E-O modulator element 1806. The conditioned egress signals are then coupled to the E-O modulator element 1806 and modulated with the available NRZ signal from the 10 GigE signal source element 1807. The 10 GigE modulation output 1809 can interface to an EDFA and then to the TX of an FSO system (or an optical fiber system). The Rx side (after the detector) can be fed directly into a 10 GigE switch as a modulated and amplified output 1810.

[0056] FIG. 19 shows another example of a device 1900 that can be used for USPL pulse multiplication consistent with an embodiment of the present subject matter. Consistent with this approach, a 10× TDM system is configured to provide a 100 Gbps output. The TDM de-mux chip may be on the receiving side of the communication link to split individual 10 GigE signals and may include a reciprocal approach to the design shown in FIG. 19.

[0057] As shown in FIG. 18, a 50 MHz USPL source 1801 is interfaced to a series of optical delay controller elements 1802 that can be designed using either a fiber loop or an offset lens to produce a precisely 10.313 Gbps RZ output stream at a 10 GigE line speed (greater than 10 Gbps for 64B / 66B encoding). A splitter element 1803, together with a variable optical delay line 1804, provides the function of splitting the incoming optical signal over 1801 into (in this example) 206 paths. After sufficient delay has been introduced through the design, all signals are multiplexed together through a combiner element 1805. However, instead of the single modulator element 1806 shown in FIG. 18, the 10.313 GHz RZ output 1901 from the combiner element 1805 is in this case fed into a second splitter element 1910, which may be a 10x splitter that splits the optical signal into 10 parallel paths. Other embodiments of this design can support various splitting ratios as required by the design. The optical paths emerging from 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 from a set of optical modulators 1930 modulated with the available NRZ signals from a 10×10 GigE signal source element 1931 to produce a series of modulated optical signals 1935. An identified optical combiner 1940 provides a single optical pulse train 1950. The series of optical pulses within the single optical pulse train 1950 can interface to a suitable optical amplifier for the desired optical conditioning for network use.

[0058] FIG. 20 shows another example of a device 2000 that can be used for USPL pulse multiplication consistent with an embodiment of the present subject matter. The illustrated device 2000 can provide the ability to achieve a high USPL pulse repetition data rate for network applications by modulating pulses within a low repetition rate channel. By applying direct modulation to each channel with a delay controller, it is beneficially possible to achieve the creation of a modulation scheme that is not restricted by the current speed limitations of electronics technology. Embodiments of the present subject matter can provide a mechanism for enhancing the data transmission capacity of a system by modulating individual channels separately at the current standard electronic modulation speed (in the example of FIG. 20, at the speed of 100×10 GigE signal inputs 2001) and time-division multiplexing the channels into a high repetition rate pulse stream of a single frequency. In this approach, the current standard limited by the speed of an optoelectronic modulator (40 Gbps) can be enhanced by approximately N digits, where N is the number of channels of the time-division multiplexing device. For example, 100 channel TDM with each channel amplitude modulated at the current standard data rate may be able to provide a data rate at a maximum speed of 4 Tbs. N may be limited by the width of the optical pulse itself. In the limit where information is carried at 1 bit / pulse, the time slot occupied by 1 bit is the width of the pulse itself (in that sense, an RZ system will converge to RZ). For example, in this approach, a laser with a 40 fs pulse width at a 40 GHz repetition rate can carry information at a maximum speed of 25 Tbps. This approach can be used with a 40 Gbps channel modulation scheme (i.e., 1 bit per 25 ps) and may correspond to a capacity of N - 625 channels in a single transmission, which may be the number of 40 fs time intervals that fit within a 25 ps time interval. A significant advantage of this approach is the ability to "optically enhance" a modulation scheme that would otherwise be limited in data capacity while interfacing with existing rate-limited modulators.For example, an amplitude modulator based on a Mach-Zehnder interferometer can be easily integrated into a TDM IC package, and all that is required therein is the ability to split a channel into two separate paths, add a small phase modulation period (nonlinear crystal) to one of the paths, and combine the paths for interference.

[0059] FIG. 20 includes a USPL source 2010 coupled to a multi-port optical splitter element 2020. The number of identified optical ports need not be limited to those described or illustrated herein. A series of optical delay lines 2030 provides the optical delay required between each parallel path from the multi-port optical splitter element 2030 and can be tailored to a particular application. The optical delay paths from the optical delay lines 2030 are summed together using an optical combiner element 2035. The resulting combined optical data stream, displayed through element 2040, represents an enhancement that increases multiplicatively with the pulse repetition rate of the original USPL source identified by element 2010. Further enhancement of the pulse repetition rate is achieved through the use of an element 2041, described by an optical splitter, where the incoming signal 2040 is split into a series of paths not limited to the path identified by element 2041. Through a second delay controller 2045, the optical delay can be introduced into each path within the device as identified by a second set of optical delay paths 2042. Each parallel path 2042 is similarly modulated by a modulator element 2044 with an available RF signal source element identified by a signal input 2001. An optical combiner element 2050 integrates all the incoming signals into a signal data stream 2060.

[0060] Optical pre-emphasis techniques and de-emphasis techniques can be introduced into each segment of the described elements to custom adjust the optical spectrum for a uniform or asymmetric optical power distribution. Pre-emphasis or de-emphasis can be achieved using commonly used optical amplifiers such as erbium-doped optical amplifiers (EDFAs).

[0061] FIG. 21 shows an example of a system 2100 that includes a mode-locked USPL source 2101 that can be used to generate the clock and data streams appropriately required by an application. Mode-locked lasers may represent the choice of a high-performance, high-finesse source for clocks in digital communication systems. In this regard, mode-locked fiber lasers - either in a linear configuration or a ring configuration - can be attractive candidates for selection because they can achieve pulse widths and repetition rates as high as GHz in the USPL source region. In addition, the fiber offers, to name a few, compactness, low cost, low sensitivity to thermal noise, low jitter, and no problems associated with diffraction or dust contamination in the air. In a communication scenario, the pulse width may determine the available bandwidth of the system, and the repetition rate limits the data rate. The pulse width can be determined by the inherent characteristics of the laser cavity - namely, the overall group-velocity dispersion (GVD), and (in the case of passive systems) the choice of saturable absorber - or (in the case of active mode-locked systems) the bandwidth of the active element. The repetition rate of the pulse train is constrained by the length of the fiber. For example, in the case of a linear laser, the fundamental mode V of the laser OSC can be expressed as follows. [Number]

[0062] where c is the speed of light in a vacuum, and n gis the average group refractive index, and L is the length of the cavity. Therefore, the fiber laser cavity element 2110 with a length of 10 cm and an average group refractive 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 to increase the repetition rate of the laser (harmonic mode locking). To achieve a high repetition rate clock using a mode-locked USPL source, it is possible to use (i) an intracavity amplitude Mach-Zehnder modulator (MZM) 2150 as shown in FIG. 21, and (ii) one or more low-threshold saturable absorbers. These techniques, known as "harmonic mode locking," can be utilized within a fiber-based plant dispersion system or within an FSO system for terrestrial systems, subsea systems, or FSO systems for any of airborne, space, or subsea applications.

[0063] Detailed in FIG. 21 is a 980 nm pump element 2102 coupled to an optical WDM device 2105. An erbium-doped optical amplifier 2110 or equivalent can be used to create a non-linear environment to obtain mode-locked pulse train emission within a closed cavity established between two Faraday reflectors 2101 and 2160 at either end of the optical USPL cavity. The operation of the device is capable of establishing a self-contained series of optical pulses exceeding 100 Gbps, which are essentially highly synchronized at the output port 2170 of the module. The EDFA 2110 can be specially designed to achieve a high-gain non-linear medium. A phase-locked loop 2130 can provide advantageous stability in operation by maintaining a synchronized clock source through modulation of the signals by components 2120, 2130, 2150 of a self-contained high repetition rate pulse generator.

[0064] In order to achieve a high repetition rate with a laser restricted by its dimension (length in the case of a linear laser and outer perimeter in the case of a ring laser), it may be necessary to stimulate the generation inside the cavity of multiples of the fundamental mode. In the active case, the amplitude modulator inserted into the cavity modulates the losses of a system operating as a "threshold gating" device. For this technique to succeed, it may be necessary to reference the control signal for the modulator to the oscillation of the laser itself in order to avoid the drive signal "forcing" an external frequency of oscillation on the laser. This can be achieved by the introduction of a phase-locked loop element 2130 or a synchronous oscillator circuit to track-and-lock the repetition rate of the laser and reproduce the signal. In the case of a PLL, the RF output can be set to a multiple of the input signal (just as this device is used in cellular phone technology), and the repetition rate of the laser can be increased. The signal can then be used for triggering a pulse generator or in conjunction with a low-pass filter. The MZ amplitude modulator 2150 outside the laser cavity can be used to produce on / off keying (OOK) modulation in the pulse train resulting from the mode-locked laser.

[0065] Figure 22 shows a depiction 2200 of a diagram showing the effect of loss modulation introduced into the input pulse train 2201 because the control signal NRZ signal 2210 is present in an amplitude modulator 2205 made from a bit sequence as shown. The signal resulting at the output of device 2220 represents an NRZ-to-RZ converter device for use in telecommunications and scientific applications where the application may benefit from an RZ data stream. A clock signal 2201 (optical input) at a given pulse repetition rate passes through modulator 2205. At the same time, a control signal consisting of a sequence of 1s and 0s can be applied to the RF port of modulator element 2215. When modulator element 2215 is biased at minimum transmission, the loss experienced by the optical signal, in the absence of a control signal, may be at its maximum value. When an RF signal (1) is present, the loss is reduced to a minimum (open gate) and thus functions as an on / off keying modulation device. The pulse width of the output optical signal is typically much smaller than the time slot occupied by a single N-bit of information (less than half the clock period in the NRZ format), and is made truly RZ as identified by element 2220 in this system.

[0066] FIG. 23 shows an example system 2300 for the generation of a high optical high harmonic USPL pulse stream with a high pulse repetition rate using a saturable absorber (SA) device 2330. The SA device 2330 may include carbon nanotubes in some examples. A passively mode-locked fiber laser using a carbon nanotube SA (CNT-SA) is another attractive option for a high repetition rate source due to its ability to generate harmonics at a high fundamental repetition rate. In the described approach, a closed self-contained optical cavity is established in which two Faraday reflectors 2301 and 2350 form the optical cavity. Although an output erbium-doped fiber amplifier (EDFA) 2310 is shown in FIG. 23, any gain medium that generates a nonlinear optical cavity can be used. A seed laser 2315, such as the 980 NM pump laser shown in FIG. 23, can be used in generating a high repetition rate optical train. In particular, any suitable pump laser can be considered from the perspective of the required optical wavelength and pulse repetition rate. The SA element 2330 can be placed within the cavity to establish the required optical pulse characteristics 2350, through the design requirements and as required.

[0067] FIG. 23 shows a schematic diagram of an example of a laser that can be used in one or more embodiments of the present subject matter. Different from the active laser shown in FIG. 22, here, an MZ modulator can be replaced with the SA element 2330. Techniques similar to those described herein can be utilized within a fiber-based plant dispersion system or within an FSO system for terrestrial systems, subsea systems, or FSO systems for any of airborne applications, space applications, or subsea applications.

[0068] FIG. 24 shows a technique for providing time-division multiplexing (TDM) in which TDM multiplexes a pulse train using parallel time-delay channels. In some examples, it may be important to manipulate the delay channels so that the delay channels "match" each other. The frequency of the output multiplexed pulse train may ideally be as insensitive as possible to changes in the environment. To that end, the proposed feedback loop control system is designed to correct the delay unit for any variations that would compromise the stability of the output repetition rate.

[0069] FIG. 24 shows an example diagram of a delay control system 2400. The control loop can be implemented in one of several ways consistent with the present subject matter. FIG. 24 illustrates one possibility for purposes of example. An input pulse train enters a TDM and multiplexes into N paths each having its own delay line. If the paths are made of fiber waveguides with low “bending loss”, then each path can be wound around a cylindrical piezoelectric actuator (PZ) of radius R. The actuator generally expands radially as a result of a control voltage (Vc). This expansion ΔR, which is linearly proportional to Vc, results in a change in the fiber length ΔL = 2πNΔR, where N is the number of turns of the fiber around the Pz. For terahertz multiplexing, the delay between pulses (and thus for PZ1) must be 1 picosecond. This may require a change in length equal to 200 microns, and for one turn, the PZ actuator corresponds to ΔR = 32.5 microns. Most commercially available piezoelectric actuators are very linear and operate well within this range. Thus, the control mechanism may be based on several PZ actuators each having 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 uses a 1 / N divider to divide the frequency of the output signal and a phase comparator (PC) to compare it with the frequency of the input signal. The frequency of the “slow” input optical signal (represented by the waveform having τRT in FIG. 24) is converted to an RF signal using a photodetector PDin. To reduce the effect 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 the signal into a 50% duty cycle waveform. Similarly, on the output side, the optical signal is a photodetector PD outIt is picked up by, passed through a high-pass filter, and reproduced using the clock output port of the clock and data recovery system. The clock of the output signal having a frequency N times that of the input signal is sent to a frequency divider by N before going to the phase comparator. From the phase comparator, a DC voltage level (the same as that used in the architecture of the PLL circuit) representing the mismatch between the input signal and the output signal indicates the direction of correction of the actuator. The low-pass filter adds a time constant to the system to enhance its immunity to spurious noise.

[0070] Unlike a PLL, the CDR can be advantageously used at the output such that the output signal may or may not be modulated. This system can be designed to function in both unmodulated mode and "in-TDM modulation" (i.e., one modulator per delay path) mode. However, this is a completely deterministic approach to compensating for variations in the length of the delay line. Ideally, and from a practical perspective, all delay paths should be referenced to the same "thermal level". That is, they should respond to the same thermal changes simultaneously. If each line senses different variations, this system will not be able to correct for them in real time.

[0071] As an alternative, a completely statistical approach may involve summing operational amplifier circuits (S1...SN) to deliver a control voltage to the actuator. Using such an approach, the input voltages (V1 to VN) can be used to compensate for mismatches in the length between the lines and, in a completely static sense, otherwise the input voltages can be used for an initial fine-tuning of the system. Also, the approach usually needs to compensate for or at least take into account the bending loss requirements of the fibers used. Some newly introduced fibers in the market may have a critical radius of only a few millimeters.

[0072] If each path delay line detects different fluctuations in temperature due to spurious localized noise or experiences changes in length that are not related to each other, the techniques described above may, as they are, struggle when performing real-time correction. A more robust technique that operates in a completely statistical sense can be used in accordance with some embodiments of the present subject matter. In such a technique, the 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 the length mismatch between the delay lines in a completely statistical sense; otherwise, the input voltages may only be useful for an initial fine-tuning (calibration) of the system.

[0073] Referring again to FIG. 24, the incoming USPL source identified as element 2401 is coupled to the optical coupler element 2403 such that one leg of the coupler is connected to an optical photodiode selected to operate at the operational data rate of 2401. Using the standard electronic filtering techniques described by elements 2404, 2405, and 2406, the electrical square wave representation of the incoming USPL signal is extracted and identified by element 2407. The second optical leg of the coupler 2403 interfaces with an appropriate optical splitter element identified by 2410, and the incoming signal to 2410 is split into the parallel optical paths of 206. Also shown are variable rate optical delay lines established in parallel connection to each of the parallel branches of the splitter element 2410. The parallel piezoelectric electrical elements are identified by element 242N and are electronically controlled through the feedback circuitry network in the figure. The control voltage identified by Vc is generated through the photodiode 2485 together with the electronic circuitry elements 2480 and 2475. The clock and data recovery (CDR) element 2475 generates a clock source used when controlling each of the PZ elements. The optical paths identified as 242N are combined after an appropriate delay has been introduced into each leg of the element 2410. The pulse multiplied USPL signal 2490 is thereby generated.

[0074] FIG. 25A shows a schematic view of a fiber PZ actuator 2500, and FIG. 25B shows a graph 2590 of radius versus voltage of such an actuator. Overall, these drawings show the operation of the PZ actuator to increase the pulse repetition rate of an incoming USPL pulse train through an induced optical delay. Although shown for use as an element to enhance pulse repetition rate generation for USPL signals, the same technique can be used for other optical devices that require or benefit from optical delay. The basic structure of the device is a fiber-based PZ actuator 2501. When a voltage 2550 is applied to the electrode 2520, a stress induced by the voltage occurs within the fiber, causing a time delay of the optical signal traveling through the fiber. By varying the applied voltage, a performance curve of optical delay versus applied voltage is obtained as shown in the graph 2590 of FIG. 25B.

[0075] FIG. 26 shows a diagram illustrating the features of the example statistical corrector 2600. The coarse corrector 2640 shown in FIG. 26 corresponds to the system described in the previous section that can correct for length variations picked up simultaneously by all delay lines. As described above, these variations are expected to occur on a time scale much slower than the "internal delay line" spurious variations. The latter effect may appear as jitter per period introduced into the system. This type of jitter can be monitored using a RF spectrum analyzer (RFA), and on the line of the system's repetition rate, display the "side line" (or sideband), which is the result of mixing the frequencies of the noise generated from the non-uniform time intervals between consecutive pulses detected by the analyzer. One such pattern can be processed using an analog-to-digital converter (ADC) and then saved as an array of values that can be sent to a neural network (NN) machine. The neural network machine is known to have excellent adaptive characteristics that allow the neural network machine to essentially learn patterns from external events by adapting to new sets of inputs and outputs. In this case, the set of inputs is generated from the "incomplete observations", i.e., the set of "noisy" outputs of the TDM system detected by the RFA, and can be converted into a digital array by the ADC ({f1,f2,...,f N}, where f1 is the frequency component picked up by the RFA). The set of outputs is the correction ({V1,V2,...,V N}, where V1 is the compensation input voltage for the summing operational amplifier). f, V is a frequency, voltage array, and a statistical set can be constructed to train the NN machine to learn the basic patterns associated with the presence of in-channel noise using a sufficiently large number of {f, V} pairs. These machines are commercially available in IC format from several manufacturers, or implemented as software and may be used in conjunction with a computer feedback control mechanism. A single-layer perceptron type neural network, or ADALINE (Adaptive Linear Neuron or later Adaptive Linear Element) should be sufficient to achieve the task.

[0076] Similar to the description provided above with respect to FIG. 24, the statistical corrector element 2670 may include an electrical network similar to, or providing a function similar to, the electrical network elements 2480 and 2475 and the photodiode 2485 of FIG. 24. In the case of the method shown in FIG. 26, the RF spectrum analyzer 2695 is used, together with the neural network 2670 and the coarse correction controller element 2640, to perform the optical delay requirements introduced into a series of parallel PZ elements 262N.

[0077] FIG. 27 shows the concepts and functions of a method consistent with an embodiment of the present subject matter that can improve performance, accuracy, and resolution through a piezoelectric disk (PZ) module identified by elements 2795 and 272N, where a compact microfiber-based collimator (MFC) 2795 surrounded by a ceramic disk is used to obtain an optical delay line. Although it shows a technique for increasing the original pulse repetition rate of a USPL pulse train, the design shown is not limited to such applications and can be applied or extended to other needs within an optical sector wherever optical delay is required. In doing so, a more controlled amount of time delay can be introduced into each MFC element of the circuit. The improvement by using the MFC elements utilized can improve the achievement of reproducing the required voltage response quickly in terms of response, resolution, and mass production means. The concepts identified in FIG. 27 can be incorporated into precisely manufactured elements that can function as complementary pairs for use in reducing USPL pulse-to-pulse jitter and for the needs of data encryption.

[0078] Referring further to FIG. 27, a USPL source 2701 having a specific pulse repetition rate is split into a preselected number of optical paths 271N (which may be a number other than 206) as identified by a splitter element 2705. Appropriately controlled delays 273N are introduced into each parallel leg of the split optical paths 271N using the elements described by 2795 and 272N. The resulting delayed paths 274N are added together through an optical combiner element 2760. A pulse-multiplied USPL signal 2780 is produced.

[0079] One potential disadvantage of some of the previously available TDM designs where the fiber is "wrapped around" the piezoelectric actuator is that the mechanism must conform to the bending loss requirements of the fiber being used. Some newly available fibers have a critical radius of just a few millimeters. To correct this problem, embodiments of the present subject matter can use a microfabricated air-gap U-bracket instead of a cylindrical piezoelectric element around which the fiber is wrapped. FIG. 27 illustrates this principle. In this approach, the piezoelectric actuator (PZ1,... PZ N ) can be replaced by an air-gap U-bracket structure constructed using a microfiber collimator (MFC) and a micro-ring made of piezoelectric material. However, in this case, the piezoelectric actuator expands longitudinally and, in response to the control voltages (V1, V2,..., VN), increases (or decreases) the air-gap distance between the collimators. As in the case of a cylindrical piezoelectric, a single voltage Vc can be used to drive all the piezoelectric devices if the gains (G1, G2,..., GN) of each channel are appropriately adjusted to give the correct expansion for each line. Ideally, apart from the inherent bias to the system (i.e., the essential differences between operational amplifiers), the gain adjustment should be G1, 2G1, 3G1, etc. to provide an expansion that is a multiple of τRT / N. Another way to implement such an approach may be to use multiple piezoelectric rings in the channel. In that way, 1, 2, 3, N piezoelectric rings are driven by the same voltage using all the amplifiers with the same gain.

[0080] FIG. 28 provides a conceptual rendering of an optical chip system 2800 to successfully bridge the gap between two remote 10GigE switches. Ideally, such a connection can be made to operate as simply as a single fiber. The timing of the TDM chip can be driven by the 10GigE switch.

[0081] Referring to FIG. 28, a USPL source 2805 having a predetermined original 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 a level appropriate for operation using the high-speed network Ethernet switch identified by 2801. Switch 2801 provides a reference signal 2802 that is used to modulate signal 2809 at a data rate of interest via a standard electro-optic modulator 2820. The resulting RZ optical signal is generated as shown in element 2840.

[0082] An alternative to having the timing run from a 10 GigE switch is to use a multiplier photonic chip to build the USPL at terabits / second (or faster), and then directly modulate this terabit / second signal from the 10 GigE switch. Each bit has on the order of 100 pulses. The advantage of this approach is that it may eliminate the need for a separate timing signal to be run from the switch to the USPL. The USPL via the multiplier chip simply has to send out terabit / second pulses. Another advantage is that the output of the multiplier chip does not have to be exactly 10.313 or 103.12 Gbps. The output simply has to be at a rate of about 1 terabit / second. This limitation is not a problem if each 10 GigE bit has 100 or 101 or 99 pulses. Another advantage is that since each bit has many 10 USPLs, the 10 GigE signal has an advantage for atmospheric propagation (fog and scintillation). Another advantage can be realized at the receiver end. If that bit has on the order of 100 USPL signals within that single bit, it should be easier for the detector to detect the bit. This should lead to an improvement in receiver sensitivity and thus enable an improved range for the FSO system in this way. An additional advantage can be realized in that upgrading to 100 GigE may be as simple as replacing the 10 GigE switch with a 100 GigE switch. In this case, each bit has on the order of 10 pulses.

[0083] Purely from the perspective of signal processing, this approach specifies an efficient way to transmit data and clock combined in a single transmission stream. Similar to the "sampling" of bits using an optical pulse stream, this approach has the advantage that the bit "size" is determined by the maximum number of pulses it carries, thus establishing a basis for counting the bits as they reach the receiving end. In other words, if the bit unit has a time slot that can accommodate N pulses, the system clock can be established as "one new bit of information" every fifth pulse.

[0084] Techniques similar to the techniques described herein can be utilized within a fiber-based plant distribution system or within an FSO system for terrestrial systems, subsea systems, or FSO systems for any of aerial, space, or subsea applications, showing for the first time how the interconnection from a USPL source to an optical network element can be achieved for networking applications.

[0085] Figure 29 shows a system 2900 that illustrates a conceptual network expansion for the design concepts reflected in Figure 28. The circuit is configured in a WDM arrangement, with a plurality of USPL sources 2901, 2902, 2903 (although three are shown, note that any number is within the scope of the present subject matter), each modulated through a dedicated optical switch and a USPL laser multiplier chip. As described with respect to Figure 28, the electrical signals from each Ethernet switch can be used to modulate dedicated optical modulators 2911, 2922, 2928 for each optical path. The optical power of each segment of the system can be provided by optical amplification elements 2931, 2932, 2933 for amplification purposes. Each amplified USPL path then interfaces with an appropriate optical combiner 2940 for transfer to the network 2950 and can be either free-space based or fiber-based as required. The output from the WDM module can then be configured into the transmit-side element 102 for FSO transfer or into the fiber plant facility.

[0086] The techniques described herein can be utilized within a fiber-based plant distribution system or within an FSO system for terrestrial, undersea, or FSO systems for aerial, space, or undersea applications, showing for the first time how the interconnection from a USPL source to an optical network element is achieved for networking applications.

[0087] Figure 30 shows a schematic of an experimental setup for an embodiment of the present subject matter that includes a computer-aided system configuration for controlling the pulse width of an all-fiber mode-locked laser using recursive linear polarization adjustment and simultaneously stabilizing the cavity repetition rate using a synchronous self-regeneration mechanism. The design can also provide the ability to adjust the repetition rate and the pulse width.

[0088] The fiber-ring laser is represented by the inner blue loop, and all intracavity fiber branches are coded in blue, except for the positive high-dispersion fibers outside the loop that are part of the fiber grating compressor (coded in dark brown). The outer loop represents the feedback active system.

[0089] FIG. 30 is a diagram of system 3000 showing the features of a USPL module that provides control of both the pulse repetition rate and the pulse width through a mirror (M1, M), diffraction gratings (G1, G2), lengths (L1, L2), second harmonic generator (SHG), photomultiplier tube (PMT), lock-in amplifier (LIA), data acquisition system (DAC), detector (DET), clock extraction mechanism (CLK), frequency-to-voltage controller (FVC), high-voltage driver (HVD), reference signal (REF), pulse generator (PGEN), amplitude modulator (AM), isolator (ISO), piezoelectric actuator (PZT), optical coupler (OC), polarizer (POL), and polarization controller (PC).

[0090] The passive mode-locking mechanism may be based on non-linear polarization rotation (NPR) which can be used in mode-locked fiber lasers. In this mechanism, a weakly birefringent single mode fiber (SMF) can be used to generate light that is elliptically polarized with the propagating pulse. As the pulse travels along the fiber, the pulse experiences a non-linear effect where polarization rotation dependent on intensity occurs. By the time the pulse reaches the polarization controller (PC) 3001, the polarization state of the high-intensity portion of the pulse experiences more rotation than the low-intensity portion. The controller can perform the function of rotating the high-intensity polarization component of the pulse and aligning its direction as closely as possible with the axis of the polarizer (POL). As a result, when the pulse passes through the polarizer, its lower-intensity component experiences more attenuation than the high-intensity component. Thus, the pulse emerging from the polarizer is narrowed, and the overall process functions as a fast saturable absorber (FSA). This non-linear effect functions in conjunction with the group velocity dispersion (GVD) of the loop, and after several round trips, a stable situation occurs and passive mode-locking is achieved. The overall GVD of the optical loop can be adjusted to generate a specific desired pulse width within the tolerance by using different types of fibers (single mode, dispersion shifted, polarization maintaining, etc.) and enhancing their contribution to the average GVD of the laser.

[0091] Active control of linear polarization rotation from a PC can significantly improve the performance of a laser. This can be achieved using a feedback system that finds the evolution of the pulse width. This system, represented by the outer loop in Figure 1, can be used for compression and consequently maximizing the average output of the pulse. Pulses emerging from the fiber laser through the OC are expected to have a width on the order of several picoseconds. An external pulse compression method using a fiber grating compressor is used to narrow the pulse to less than 100 fsec. This technique has been widely used in many reported experiments and leads to high-energy, high-power USPL pulses. Here, the narrowed pulse is focused on a second harmonic generation (SHG) crystal and detected using a photomultiplier tube (PMT). A lock-in amplifier (LIA) provides the output signal to a data acquisition card (DAC). This signal tracks the variation of the pulse width by following the increase or decrease in the peak power of the pulse. Similar techniques have also been successfully used in the past, except that in that case a spatial light modulator was used instead. Here, a programmable servo mechanism directly controls the linear polarization rotation using an actuator in the PC. With DC signal data provided by the DAC, decision-making software (such as, but not limited to, LABVIEW or MATLAB SIMULINK) can be developed to control a servo mechanism that adjusts the rotation angle of the input pulse with respect to the axis of the polarizer in a similar manner. These adjustments performed by the actuator are achieved using stress-induced birefringence. For example, when the pulse width decreases, the mechanism prompts the actuator to follow a linear angular rotation in a certain direction to compensate for it, and when the pulse increases, the mechanism operates in the opposite direction, both aiming to maximize the average output power.

[0092] A self-regenerative feedback system that synchronizes with the repetition rate of the optical oscillation and is used as a drive signal to an amplitude modulator (AM) can adjust the round-trip time of the laser. In an active system, the amplitude modulator acts as a threshold gate device by modulating the loss in synchronization with the round-trip time. Recent reports indicate that this technique can successfully stabilize mode-locked lasers. The signal sampled from the optical coupler (OC) by the photodetector (DET) can be electronically locked and regenerated by a clock extraction mechanism (CLK), such as a phase-locked loop or a synchronous oscillator. The regenerated signal triggers the pulse generator (PGen), which is then used to drive the modulator. In a fully synchronized situation, the AM "opens" every round-trip time (TRT) each time a pulse passes through it. Since the CLK follows the variations in the TRT, the drive signal to the AM changes accordingly.

[0093] An external reference signal (REF) can be used to adjust the cavity repetition rate. The external reference signal can be compared using a mixer to the recovered signal from the CLK and the output used to drive a piezoelectric (PZT) system that can adjust the cavity length. Such use of a PZT system to adjust the cavity length is a well-known concept, and similar designs have already been experimentally proven successful. Here, a linear frequency-voltage converter (FVC) can be calibrated to provide an input signal to the high-voltage driver (HVD) of the PZT. The PZT adjusts the cavity length to match the repetition rate of the REF signal. For example, if the REF signal increases its frequency, the output of the FVC decreases, so the HV drive level to the piezo-cylinder also decreases, forcing the piezo-cylinder to contract, resulting in an increased laser repetition rate. The exact opposite occurs when the reference repetition rate decreases.

[0094] It is possible to adjust the width of the pulse to a "transform-limited" value using a pair of negative dispersion diffraction gratings. This chirped pulse compression technique is well established, and pulses as narrow as about 6 fs have been reported. The idea is to attach the grating pair pulse compressor to a moving stage that translates along a line setting the separation between the diffraction gratings. As the distance changes, the compression factor also changes.

[0095] In an example of a data modulation scheme consistent with an embodiment of the present subject matter, a passively mode-locked laser can be used as an ultra-fast pulse source that limits the flexibility to change the data modulation rate. To scale up the data rate of the system, it is necessary to increase the fundamental repetition rate of the pulse source. Conventionally, the repetition rate of a passively mode-locked laser has been increased either by shortening the laser cavity length or by harmonic mode-locking of the laser. Both techniques reduce the pulse peak power in the cavity, leading to longer pulse widths and more unstable mode-locking.

[0096] One approach to solving this problem involves the use of a pulse interleaving scheme improved by a technique called pulse multiplication. FIG. 31 illustrates this concept. A lower repetition rate pulse train of a well-characterized and well-mode-locked laser 3101 has a sufficiently determined portion of the pulse taken out and "recirculated" within an optical loop having an optical delay 3150 equal to the desired inter-pulse spacing within the output pulse train, and is coupled to an integrated optical directional coupler 3180 where it is recombined at the output of the directional coupler. For example, to generate a 1 GHz pulse train from a 10 MHz pulse train, a 1 ns optical delay is required, and the optical delay may have to be precisely controlled in order to enable the 100th pulse in the train to coincide with the input pulse from the 10 MHz source. The optical delay loop includes an optical gain 3120 for compensating for signal attenuation, a dispersion compensation 3160 for restoring the pulse width, and an active optical delay control 3150. When pulse multiplication occurs, the output pulse train is OOK modulated 3175 with a data stream 3182 onto the generated RZ signal 3190 and amplified in an erbium-doped fiber amplifier 3185 to raise the pulse energy to the same level as (or to the desired output pulse energy level of) the input pulse train.

[0097] One or more of the features described in this specification can be included in various aspects or embodiments of the subject matter, regardless of whether they are interpreted alone or in combination. For example, in some aspects, an optical wireless communication system may optionally include one or more of picosecond-type, nanosecond-type, femtosecond-type, and attosecond-type laser sources, and may include at least one USPL laser source. The optical wireless communication system 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 may include a USPL source that can utilize an optical transmission terminal or telescope manufactured by one or more of hyperbolic mirror fabrication techniques, conventional Newton mirror fabrication techniques, or other techniques that are functionally equivalent or similar. Also or alternatively, an aspherical aspherical optical design can be used to minimize, reduce, etc., the ambiguity of the received optical signal.

[0098] A free-space optical transmission system in accordance with an embodiment of the present subject matter can utilize a USPL laser that focuses the received signal at an ideal point. In some embodiments, one of the telescopes or other optical elements for focusing and delivering light may be considered a transmitting-side element, and the other optical element, which is remotely located from the first telescope or the first telescope and other optical elements, for focusing and receiving light can function as a receiving-side element to create an optical data link. Both optical communication platforms may optionally include the components necessary to provide both transmitting and receiving functions, and may be referred to as a USPL optical transceiver. Either or both of the telescopes or other optical elements for focusing and delivering light can be coupled to the transmitting-side USPL source either through an optical fiber or through free-space coupling to the transmitting-side element. Either or both of the telescopes or other optical elements for focusing and receiving light can be coupled to the receiving endpoint either by an optical fiber or by free-space coupling to a photoreceiver. A free-space optical (FSO) wireless communication system that includes one or more USPL sources can be used within an optical communication network, in conjunction with an optical fiber long-haul network (and used transparently within the optical communication network, (using on-off keying (OOK), non-return-to-zero (NRZ), and return-to-zero (RZ) modulation techniques and can be used within the 1550 nm optical communication band), within the optical communication network (and can be modulated using differential phase shift keying (DPSK) modulation techniques), within the optical communication network (and can be modulated using modulation techniques commonly used for point-to-point communication system architectures using commonly used free-space optical transceiver terminals), within the optical communication network using D-TEK detection technology, within a communication network for use in conjunction with erbium-doped fiber amplifiers (EDFAs) and high-power erbium / ytterbium-doped fiber amplifiers (Er / Yb-DFAs), within the optical communication network (and can be modulated using modulation techniques commonly used for point-to-multipoint communication system architectures), etc.

[0099] The USPL technology can, in some aspects, be utilized as a beacon source for optical tracking and beam steering for use in an auto-tracking function and for maintaining co-alignment of the terminals during operation. The recovered clock and data extracted at the receiving terminal can be used for multi-hop spans for use in extending the network range. The optical network has similar advantages in a WDM configuration, thereby expanding the size of the effective optical bandwidth of the carrier data link. Alternatively or in addition, the USP laser source can also polarize multiplex the optical signal transmitted to provide a polarization multiplexed USP-FSO (PM-USP-FSO) function. The recovered clock and data extracted at the receiving terminal can be used for multi-hop spans for use in extending the network range and may include operations over a generally large bandwidth range to provide data rate invariant operations. An optical preamplifier or semiconductor optical amplifier (SQA) can be used in front of the optical receiver and, alternatively or in combination with the recovered clock and data extracted at the receiving terminal, can be used for multi-hop spans having operations over a generally large bandwidth version to provide data rate invariant operations for use in extending the network range. Terminal co-alignment can be maintained during operation, and as a result, the USPL technology can not only achieve significant improvements in performance and terminal co-alignment by using the USPL data source, but also provide an improved method for maintaining transceiver alignment by using the USPL laser beacon.

[0100] The USPL-FSO transceiver can be utilized to perform remote sensing and detection of signatures of airborne propagation elements that can use ionization detection technology or non-ionization detection technology, in some embodiments, using an optical transmission terminal manufactured either by hyperbolic mirror fabrication technology or a conventional Newton design that focuses the received signal at one ideal point. A USPL-FSO transceiver consistent with embodiments of the present subject matter can be utilized for line-of-sight blocked laser communication applications. A USPL-FSO transceiver consistent with embodiments of the present subject matter can enable adjustment of the distance at which a scattering effect (enabling NLOS technology) occurs, reception technology for improving detection sensitivity using a DTech detection method, and bandwidth improvement via a broadband detector including a frequency comb. A USPL-FSO transceiver consistent with embodiments of the present subject matter can be utilized in conjunction with compensation optical technology (AO) for performing incoming optical wavefront correction (AO-USPL-FSO). A USPL-FSO transceiver consistent with embodiments of the present subject matter can operate and be utilized across the entire infrared wavelength range. A USPL-FSO transceiver consistent with embodiments of the present subject matter can be utilized in conjunction with optical add-drop technology and optical multiplexing technology in both single-mode fiber configurations and multi-mode fiber configurations. A USPL-FSO transceiver consistent with embodiments of the present subject matter can operate and be utilized across the entire infrared wavelength range as a rangefinder and spotting device for hy identification and survey applications.

[0101] In other aspects of the present subject matter, a series of switched network connections, such as connections like 10GigE, 100GigE, etc., can be connected from one point to another via either an optical fiber or free space optics, for example, via time division multiplexing (TDM).

[0102] Mode-locked USPL sources that are consistent with embodiments of the present subject matter can be used to generate both a clock stream and a data stream. Mode-locked lasers may represent the choice of a high-performance, high-performance, high-finesse source for clocks in digital communication systems. In this regard, mode-locked fiber lasers - either in a linear configuration or a ring configuration - may be attractive candidates for selection since they can achieve pulse widths in the USPL source region and repetition rates as high as GHz.

[0103] Harmonic generation can be achieved using a carbon nanotube saturable absorber. Passively mode-locked fiber lasers using a carbon nanotube saturable absorber (CNT-SA) are an option for high repetition rate sources due to their ability to easily generate harmonics of the fundamental repetition rate.

[0104] FSO can be used for terrestrial, space, and undersea applications.

[0105] Conditional path length control from a splitter to an aperture may be an important parameter. A TDM multiplexer can be used in accordance with embodiments of the present subject matter to control the relative temporary time delay between paths from an aperture to a source. Each pulse train can be controlled using parallel time delay channels. This technique can be used to control conventional multiple transmission FSO aperture systems that use not only WDM systems but also TDM systems. The USPL laser pulse interval can be maintained and controlled to meet the exact time requirements of both TDM and WDM systems. The techniques described can be used in TDM fiber-based systems and WDM fiber-based systems. The use of the TDM multiplexer described herein can be used to implement unique encryption means for the transmitted optical signal. A complementary TDM multiplexer can be utilized to invert the incoming received signal and thereby recover the unique signature of the pulse signal. The TDM multiplexer described herein can be utilized to control WDM pulse characteristics for WDM encryption. The TDM multiplexer can be used in conventional FSO systems where multiple apertures connected to a common source signal can have a controlled time delay between pulses to maintain a constant path length. The TDM multiplexer can be used in TDM fiber-based systems and FSO-based systems. The TDM multiplexer may be an enabling technique for controlling the optical pulse train relationship for a USPL source. The TDM multiplexer can be used as an atmospheric link characterization utility across an optical link by measuring the neural correction rate to obtain the same pulse relationship.

[0106] Any combination of PZ disks can be used in a transmitter and can have an infinite number of encryption combinations for both fiber-based and FSO-based USPL-based systems. The timing can be performed from a 10 GigE switch or equivalent and the USPL can be built at terabit / second (or faster) speeds using a multiplier photonic chip, and this terabit / second signal can be directly modulated from a 10 GigE switch. When operating in a WDM configuration, an interface can be included either to a fiber-based system or to an FSO network element.

[0107] The system can receive an ultra-high-speed optical pulse train and generate a train of optical pulses having the same pulse width, spectral content, and chirp characteristics as 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 portion of the input pulse power in a 2×2 optical coupler having an actively controllable optical coupling coefficient, recycling this tapped pulse once through an optical delay line with compensation for the time and spectral evolution that the optical pulse experiences in the optical delay line to minimize the temporal pulse width at the output of the device, including optical amplification, optical isolation, optical delay (path length) control, modulation of the optical phase and amplitude, and recombining this power with the 2×x optical coupler.

[0108] Passive optical delay control or active optical delay control can be used, similar to how optical gain utilizing rare-earth doped optical fibers and / or rare-earth doped integrated optical elements and / or electrically or optically pumped semiconductor optical amplifiers can be used. Dispersion compensation can be provided using fiber Bragg gratings and / or volume Bragg gratings. Wavelength division multiplexed data modulation across the delay line can be invoked similar to pulse code data modulation of the pulses across the delay line.

[0109] Conventional USPL source conditioning by synthesis of USPL square wave pulses can be achieved for FSO applications by leveraging microlithography amplitude technology and phase mask technology. The ability to adjust the pulse width using techniques and similar approaches for controlling and actively controlling the pulses using this technology can improve the propagation efficiency through the FSO transmission link, thereby improving the system availability and received optical power levels.

[0110] An active programmable pulse shaper can be used to actively condition the USPL, and the pulse width may include matching the actual atmospheric conditions to maximize propagation through varying environments. In FSO applications, one or more of the following techniques can be used: 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, and the optical time spectrum is adapted using the following techniques.

[0111] FIG. 32 shows a process flow chart 3200 illustrating features of a method in which one or more may be depicted in an embodiment of the present 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 modulation signal carries data for transmission to a remote receiver. The modulated optical signal is received at an optical transceiver within an optical communication platform at 3206, and at 3210, the modulation signal is transmitted using the optical transceiver for reception by a second optical communication device.

[0112] FIG. 33 shows another process flowchart 3300 that illustrates features of a method that one or more of may appear in embodiments of the present subject matter. At 3302, a beam of optical pulses, each having a duration of about 1 nanosecond or less, is generated using, for example, a USPL source. The beam of optical pulses is transmitted at 3304 through an optical transceiver towards a target atmospheric region. 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 within the target atmospheric region is analyzed.

[0113] FIG. 34 shows another process flowchart 3400 that illustrates features of a method that one or more of may appear in embodiments of the present subject matter. At 3402, a first beam and a second beam including optical pulses are generated by, for example, a USPL source. At 3404, a first modulation signal is applied to the first beam to generate a first modulated optical signal, and a second modulation signal is applied to the second beam to generate a second modulated optical signal. The first polarization state of the first modulated optical signal is adjusted at 3406. Optionally, the 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 modulation oscillation. At 3412, the multiplexed first modulated optical signal having the adjusted first polarization state, together with the second modulation signal, is transmitted by an optical transceiver for reception by a second optical communication device.

[0114] One or more aspects or features of the subject matter described in this specification can be implemented in digital electronic circuitry, integrated circuitry, 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 be implemented in one or more computer programs that are executable and / or interpretable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device. The programmable system may be special purpose or general purpose.

[0115] These computer programs, which may also be referred to as programs, software, software applications, applications, components, or code, include machine instructions for a programmable processor and can be implemented in high-level procedural programming languages, object-oriented programming languages, and / or assembly / machine languages. As used herein, the term “machine-readable medium” refers to any computer program product, apparatus, and / or device, such as for example magnetic disks, optical disks, memory, and programmable logic devices (PLDs), used to provide machine instructions and / or data to a programmable processor that includes a machine-readable medium that receives the 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. For example, a machine-readable medium can store such machine instructions non-transitorily, as would a non-transitory solid state memory or a magnetic hard drive or any equivalent storage medium. For example, a machine-readable medium can store such machine instructions temporarily, as would a processor cache or other random access memory associated with one or more physical processor cores.

[0116] 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 a cathode ray tube (CRT), liquid crystal display (LCD), or light emitting diode (LED) monitor, for displaying information to the user, and a keyboard and a pointing device, such as a mouse or trackball, by which the user can provide input to the computer. Other types of devices can also be used for interaction with the user. For example, the feedback provided to the user can be any form of sensory feedback, such as visual feedback, auditory feedback, or tactile feedback, and the input received from the user can be in any form including, but not limited to, acoustic input, voice input, or tactile input. Other conceivable input devices include, but are not limited to, other touch sensor devices, such as touchscreens or single-point or multi-point resistive or capacitive trackpads, speech recognition hardware and software, optical scanners, light pointers, digital image capture devices, and associated translation software. A computer remote from the analyzer can be linked to the analyzer via a wired network or a 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).

[0117] The subject matter described in this specification can be embodied in a system, apparatus, method, and / or product, depending on the desired configuration. The embodiments described in the above description do not represent all embodiments that are consistent with the subject matter described in this specification. Instead, the embodiments are only some examples that are consistent with aspects related to the described subject matter. Although some variations have been described in detail above, other modifications or additions are also conceivable. In particular, in addition to what is described in this specification, additional features and / or variations can be provided. For example, the above-described embodiments may be directed to various combinations and sub-combinations of the disclosed features, and / or combinations and sub-combinations of some additional features disclosed above. Further, the logical flows shown in the accompanying figures and / or described in this specification do not necessarily require the specific order, or order, shown to achieve the desired result. Other embodiments may also be within the scope of the following claims.

Claims

1. A ranging system that uses a free-space optical beam for enhanced propagation through a lossy medium, the ranging system comprising: an ultrashort pulse laser (USPL) source that generates a beam; a modulator operably arranged either inside or outside the USPL source and configured to perform data modulation on the beam generated by the USPL source to generate a data-modulated optical output, the data-modulated optical output comprising data-modulated optical pulses each having a pulse duration of 1 nanosecond or less, a peak optical pulse power of 1 kilowatt or more, and a certain data capacity; a first optical transceiver configured to receive the data-modulated optical output and convert it into the free-space optical beam, the free-space optical beam being transmitted through the lossy medium from the first optical transceiver to a target located at a remote distance; a photodetector configured to receive the reflected portion of the free-space optical beam from the target; comprising; the ranging system determines the distance to the target based on the detected portion of the reflected free-space optical beam; the lossy medium comprises at least one of water aerosol, turbulent air flow, and clean air scintillation, which provides optically damaging atmospheric conditions to the free-space optical beam. A ranging system.

2. The ranging system according to claim 1, wherein the pulse duration is less than 1 picosecond.

3. The ranging system according to claim 1, wherein the pulse duration is less than 100 femtoseconds.

4. The ranging system according to claim 1, wherein the pulse duration is less than 1 femtosecond.

5. The ranging system according to claim 1, wherein the peak optical pulse power is 5 kilowatts or more.

6. The ranging system according to claim 1, wherein the peak optical pulse power is 10 kilowatts or more.

7. The ranging system according to claim 1, wherein the pulse duration is 100 femtoseconds or less and the peak optical pulse power is greater than 10 kilowatts.

8. The ranging system according to claim 1, further comprising an optical multiplexer that multiplexes more than one data channel onto the free-space optical beam.

9. The ranging system according to claim 6, wherein the data modulation comprises impulse coding modulation.

10. The ranging system according to claim 1, wherein the loss medium comprises a water aerosol.

11. The ranging system according to claim 1, wherein the USPL source further has a repetition rate of at least 1 GHz.

12. The ranging system according to claim 1, further comprising a polarization-dependent multiplexer component that multiplexes optical signals of different polarizations before transmitting the free-space optical beam to the second optical transceiver.

13. The ranging system according to claim 1, further comprising a polarization-dependent demultiplexer component that demultiplexes optical signals of different polarizations received as a second modulated optical signal within the second optical transceiver.

14. In the ranging system according to claim 13, the optical signals demultiplexed by the polarization-dependent demultiplexer component are each interfaced to different optical networks for network use.

15. A method comprising: generating a beam in an ultrashort pulse laser (USPL) source; performing data modulation on the beam in a modulator operably arranged either inside or outside the USPL source to generate a data-modulated optical output, wherein the data-modulated optical output comprises a data-modulated optical pulse having a pulse duration of 1 nanosecond or less, a peak optical pulse power of 1 kilowatt or more, and a certain data capacity; receiving the data-modulated optical output in a first optical transceiver and converting it into a free-space optical beam, wherein the free-space optical beam is transmitted through a loss medium from the first optical transceiver to a target located at a remote distance; receiving, in a photodetector, a reflected portion of the free-space optical beam from the target and determining the distance to the target based on the detected portion of the reflected free-space optical beam; A method comprising: The loss medium comprises at least one of a water aerosol, a turbulent air current, and a clean air scintillation that provides atmospherical conditions optically damaging to the free-space optical beam.

16. The loss medium further shifts the center wavelength of the free-space optical beam further towards the red, forming a second center wavelength that is longer than the center wavelength output by the USP source, and the method further comprises detecting, using the photodetector, a portion of the free-space optical beam at the second center wavelength. The method according to claim 15.

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