WO2011098982A1

WO2011098982A1 - Method and system for free-space optical communication with reduced scintillations

Info

Publication number: WO2011098982A1
Application number: PCT/IB2011/050593
Authority: WO
Inventors: Grigory Gelikonov
Original assignee: Grigory Gelikonov
Priority date: 2010-02-12
Filing date: 2011-02-14
Publication date: 2011-08-18

Abstract

The apparatus and the method are offered for substantial reduction in scintillations, signal fade depth, duration and probability of occurrence in a free-space optical communication system through a turbulent medium, such as atmosphere. A spatio-temporally partially coherent optical beam (STPCB) is produced by coupling a sufficiently broadband optical light into a sufficiently long multimode optical fiber or waveguide and is used for communication. The length of the fiber and the spectral width of the light are such that intermodal delay of the majority of the modes exceeds the inverse spectral width of the broadband optical source, but is smaller than the duration of the bit in the communication data stream.

Description

METHOD AND SYSTEM FOR FREE-SPACE OPTICAL COMMUNICATION WITH REDUCED SCINTILLATIONS

TECHNICAL FIELD

The present invention pertains to free-space optical communication through turbulent medium, such as atmosphere, and in particular to methods and approaches for reducing scintillations and minimizing signal fades.

BACKGROUND OF THE INVENTION

In long-range free-space optical (FSO) communication links, such as those exceeding a few kilometers and operating under turbulent conditions in the communication medium, such as atmosphere, the optical beam arrives at the receiver's input aperture highly spatially deteriorated. This is because the random fluctuations in the refractive index of air ever so slightly bend and scatter the optical beam or its portions, the effect being accumulated over the distance of propagation. Thus, the received optical beam typically becomes fragmented, consisting of a pattern of bright and dark spots, called speckles. Moreover, these spots are constantly moving and fluctuating on the time scale of the atmospheric turbulence of about one millisecond.

As a result, the signal received by the detector coupled to the optical receiver at the receiving end of the FSO communication link may strongly fluctuate in time, which decreases the performance of the link. Such fluctuations are called scintillations and are characterized by a scintillation index. Stronger fluctuations are characterized by a larger value of the scintillation index, e.g. 1 or more. Deep signal dropouts, called fades, occur more frequently and last longer when the scintillation index is large. For example these fades may occur many times per second and last for a millisecond or longer. Under such conditions a typical FSO communication link shows large error rates, low reliability or complete loss of signal. Thus, the methods and the systems designed to reduce the scintillations and minimize signal fades as much as possible are of great importance for FSO communication. Good methods and systems must also be simple, reliable and inexpensive.

Direct averaging of the scintillating signal over time cannot be used for high data rate FSO communication because of the slow rate of atmospheric fluctuations. For example, taking into account the fact that the characteristic time scale of the atmospheric turbulence is about one millisecond the effective averaging time of ten milliseconds or more is required. Ten-millisecond or longer averaging for each bit of the data stream will require the data rate of one hundred bits per second or less. Clearly, the required method and system should not only reduce scintillations, but do so in the manner applicable to the high bit-rate communication.

A number of methods to reduce scintillations in view of the above arguments were offered in the past. One of these methods is based on using adaptive optical systems. However, such systems are of limited use due to high complexity and cost, as well as the need for a feedback signal (with associated latency due to finite speed of light) to operate properly. Furthermore, such systems involve sensitive electronic components inside optical transmitter and/or receiver heads. This increases the sensitivity of the system to atmospheric electricity, such as lightning, and requires uninterrupted supply of electrical power to the optical transmitter and/or receiver head, which is often located outside the protected space, on the roof tops, towers and moving platforms. For military applications, such system would also be susceptible to electromagnetic interference (EMI) or electromagnetic pulse (EMP) attack.

Another method for scintillation reduction in FSO communication systems utilizes the MIMO (multiple-input, multiple-output) paradigm known in radiofrequency (RF) communication systems. Such approach involves the use of several independent transmitters separated in space and communicating the same data signal. Essentially the multiple transmitters used in this approach send mutually incoherent light beams through somewhat different optical paths the assumption being that the probability of a fade on all of these paths occurring simultaneously is small. This approach, however, is still rather complex, involving a large number of independent lasers and corresponding collimating optical systems, which need to be precisely aligned with each other. If the optical transmitter head is operated remotely from its electronics counterpart a large number of equal-length optical fibers connecting the transmitting lasers to their respective transmitting apertures would be required.

Still in other methods it is claimed that scintillation reduction can be achieved by using rough-surface phase masks or optical diffusers placed in the pathway of a temporally coherent light beam carrying the communication signal before it is send through atmosphere. Such phase masks or diffusers allegedly scramble the laser beam's spatial phase distribution to create a 'partially-coherent' optical beam. Though this approach indeed provides a complicated transverse phase and intensity profile of the beam that appears random, this profile remains stationary in time. In other words, the temporal coherence of the optical radiation remains unchanged after passing through the phase mask or diffuser, and is equal to the temporal coherence of the laser. The optical beam at the output of the phase mask or diffuser in this case can be considered as a set of multiple randomly distributed mutually coherent sources with stationary amplitudes and phases and can be called 'partially coherent' only to some extent.

In theoretical treatments true partial coherence is understood to imply ensemble average. This means taking multiple independent realizations of the optical beam's transverse phase and amplitude and averaging over these realizations to obtain the final result. In reality, the advantages of a true partially coherent beam can be realized only by applying the ergodicity conjecture, i.e. is the equivalence of ensemble average and the average over time. To take advantage of averaging over time what is needed then is a true partially coherent beam in both space and time with short temporal coherence. The crossection of such true partially coherent beam comprises multiple randomly distributed and mutually incoherent sources. The characteristic coherence time of such true partially coherent beam also needs to be much shorter than the duration of the averaging. For example, if it is desired to average the received signal over the duration of one bit in the communication data stream, as is done in all RF bandwidth-limited detection systems, the true partially coherent beam would need to cycle through many independent realizations of its phase and amplitude during the time of one bit. In other words, the characteristic temporal coherence time of the true partially coherent beam should be much shorter than the duration of the bit in order to take advantage of the averaging discussed above.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are provided improved method and system for free-space optical communication with reduced scintillations.

There exist at least two important conditions for high bit-rate FSO communication with reduced-scintillation between an optical transmitter and an optical receiver through turbulent medium, such as atmosphere:

1) Transmitted optical signal should reach the detector via a number of substantially independent (uncorrelated) paths through atmosphere so that the probability of fade on all of those paths is minimized, and

2) Optical signals reaching the detector via different paths should not produce detectable optical interference.

In the present invention the above conditions are satisfied by employing a spatio-temporally partially coherent optical beam (STPCB) transmitted from the optical transmitter to the optical receiver having substantially large apertures.

Prior to offering the detailed description of STPCB, its action and benefit is better illustrated by example. The exact speckle structure of the light, i.e. the position of bright and dark spots on the receiver's aperture, after propagation through turbulent atmosphere depends on both the state of the atmosphere and the spatio-temporal properties of the optical beam eradiated by the transmitter. In particular, these spatio-temporal properties include the transverse distribution of phase and amplitude of the light, as well as their variation in time. For one particular moment of time t₀ the transmitted optical beam with certain transverse phase and intensity distribution results in a certain speckle structure on the receiver aperture. For the next moment of time t₁=t₀+Delta_t, separated from the first by a delay Delta_t, another transverse phase and intensity distribution of the transmitted beam results in a substantially different speckle structure on the receiver aperture, Fig. 1A. The time delay Delta_t is chosen to be much shorter than the duration of each bit in the communication data stream. At a further delayed moment of time t₃=t₂+Delta_t the speckle structure changes yet again and so on. Because the characteristic time of speckle pattern change Delta_t is much shorter than the bit duration, the optical detector coupled to the optical receiver effectively averages multiple speckle structure realizations during each bit of the data stream, Fig. 1B. Importantly, the speckle structures discussed above are different even for 'frozen' atmosphere, which it is on the time scales of the data bit duration. The speckle structure changes with the rate of change of the transmitted beam phase and intensity distribution and not with the rate of the atmosphere. Thus, the slow signal fluctuations due to atmospheric turbulence are now replaced with fast fluctuations due to such spatio-temporally partially coherent optical beam (STPCB) action. The latter fluctuations can be averaged with a detector having finite radiofrequency (RF) bandwidth during each bit of the data stream. Provided the STPCB's rate is substantially faster than the communication data rate, or equivalently the STPCB's coherence time is substantially shorter than the duration of the bit, effective averaging is possible with the benefit of suppressing signal scintillations and deep signal fades typical for standard FSO systems.

By way of a further example, for a communication signal data rate of 1 Gbps the update rate of STPCB needs to be 10 GHz or higher to bring forth the advantages of averaging described above. The present invention teaches a very simple way of generating and using STPCBs with virtually unlimited update rates (short coherence times) suitable for high bit-rate FSO communication systems. Still, the STPCB may be modulated with communication data in a number of standard ways known in the art, e.g. with amplitude modulation.

The STPCB may be generated by coupling a broadband optical radiation into an optical system including a number of different optical paths (modes) with different delays, such as, for example, a volume diffraction element, another example being a multimode optical fiber or waveguide (MMF). There exist a certain mathematical expression relating the spectral width of the broadband optical radiation and the parameters of the MMF, most importantly its length, the fulfillment of which is required for the generation of proper STPCB, suitable for scintillation reduction in a high-rate FSO communication system. The MMF must also preferably possess an appreciable amount of intermodal dispersion, which is the difference in propagation speeds of various modes. Gradient-index MMFs typically have small amount of intermodal dispersion and thus are not preferred to be used for, although not excluded from the use in the STPCB generation. On the other hand, step-index MMFs typically have large amount of intermodal dispersion and are thus preferred for the STPCB generation. Step-index MMFs will be assumed in the following arguments.

By way of example consider the source of broadband optical radiation mentioned above to be a multi-frequency laser, such as Fabri-Perot laser, the schematic spectrum of which is shown in Fig. 2C. Such laser emits a number of distinct optical frequencies f₁, f₂,...f_n oscillating with random uncorrelated phases. When coupled into the MMF each frequency excites essentially the same set of transverse spatial modes of the MMF. These modes propagate at different velocities towards the output end of the MMF. The interference of these modes at each individual optical frequency creates a distinct transverse phase and intensity pattern at the output of the MMF. Two examples of such intensity patterns are shown in Fig. 3B and Fig. 3C corresponding to a small and a large number of modes excited in the MMF respectively.

Importantly, the optical phase accumulated by each mode at different frequencies of the laser will be different. Thus, the intermodal interference patterns produced by different frequencies of the laser after propagation through MMF will be substantially different. Each spatial pattern corresponding to each individual frequency of the laser may be considered to be an independent optical emitter with complicated transverse phase and amplitude distributions. For a large number of overlapping independent emitters operating at many different frequenciesand unrelated phases the resulting overall distribution of phase and amplitude will fluctuate rapidly and essentially randomly thus forming an STPCB. After propagation through the communication medium, such as turbulent atmosphere, the subset of optical frequencies will reach the detector. The beat note between adjacent laser frequencies will not be effectively detected by the detector due to the limited RF bandwidth of the latter. Only the data modulation signal being within the RF bandwidth of the detector will be effectively received.

A continuous-spectrum broadband optical source, such as the one shown in Fig. 2A can be viewed as a limiting case of the example considered above with the separation between neighboring frequencies reduced to zero and a corresponding increase in the number of individual frequencies - to infinity. One difference from the previous case is that the beat note between the two adjacent frequencies now falls within the RF bandwidth of the detector. However, the beat notes between multiple pairs of adjacent frequencies oscillate with random phases and effectively produce a negligible noise and continuous DC background, which is usually filtered out by the detector or otherwise do not substantially affect the ability of the detector to receive the desired communication signal.

An example of transverse spatial intensity profile of an STPCB as measured by a slow detector, for example a CCD camera, is shown in Fig. 3D. Unlike the profiles shown in Fig. 3B and Fig. 3C corresponding to a narrowband optical source or a very short length of the MMF, the STPCB intensity profile has very low contrast. This is because the slow CCD camera cannot temporally resolve all the rapid fluctuations of the light and thus produces an averaged picture. The measurement of contrast just described may, in fact, serve as an indication of the quality of the STPCB: Lower observed contrast indicates a better quality STPCB for the purpose of scintillation reduction in an FSO communication link. For completeness Fig. 3A shows the intensity profile at the output of a single-mode optical fiber. This profile is a familiar bell-shaped 2-dimentional distribution, which essentially does not depend on the spectral width of the optical source and the length of the fiber.

As was mentioned above the desired STPCB can be obtained only under certain, although easily fulfilled conditions. Specifically, detailed considerations yield the following simple relation for the case of discrete-frequency optical source:

(1)

where L is the physical length of the MMF, n is its average refractive index, NA is its numerical aperture, c is the speed of light and Delta_f is the frequency difference between the two adjacent optical frequencies of the optical source. For the case of continuous-spectrum broadband optical source an equivalent relation can be written as:

(2)

where Delta_F is the full spectral width of the optical source and M is the desired number of effective independent sources in the context of the above discussion. Larger values of M, say 100, generally results in a better-quality STPCB.

The length L of the MMF cannot be too large, however. This is because the dispersion in the MMF aside from helping to generate STPCB also temporally stretches the data bits comprising the communication signal stream. The condition expressing the requirement that this temporal stretch is small reads:

(3)

where T is the duration of the bit.

Combining the two conditions above one can write:

, for discrete-frequency optical source (4)

, for continuous-spectrum broadband optical source (5)

By way of example, for a discrete-frequency optical source with Delta_f=100GHz and a MMF with n=1.5 and NA=0.22, and for a 1Gbps data rate of the communication link from the first equation above one can obtain approximately:

which can be readily satisfied with a MMF length of about one meter.

The quality of STPCB and with it the effectiveness of temporal averaging discussed above dependson the number of paths ormodes excited in theSTPCB creating system such as MMF.It is therefore desired tohave a substantially uniform excitation of the modesof the MMF, i.e.excite as large number of modes as possible with preferably equal intensities. As known in the art, to excite a large number of modes with approximately equal intensitiesan offset splice or connection between the input fiber and the MMF may be applied.Similar result can be reached by using a mode scrambler.Further optimization of STPCB generation may be obtained by connecting several different or similarpieces of multimode fibers or waveguides. In the case of another intensity distribution is desired, such as predominantly higher number modes excitation or other distribution, an appropriate method could be applied.

It is, therefore, the objective of the present invention to teach the method and to describe the apparatus for FSO communication with reduced scintillations using simple and effective methods for generating, modulating transmitting and receiving spatio-temporally partially coherent optical beams (STPCBs). The method and the apparatus of the present invention may be applied with benefit to one-way and two-way communication for point-to-point and point-to multipoint FSO applications between stationary and moving platforms.

Other advantages of the method and the system offered herein include:

a) improved performance and reliability as compared to existing FSO communication systems;

b) increased simplicity and reduced cost of manufacturing using standard off the shelf components;

c) electric power-free operation of the optical transmitter's head;

d) insensitivity of the optical transmitter's head to natural or man-made atmospheric electricity;

e) single-fiber connectivity of the optical transmitter head

A further advantage is the ease with which the existing FSO communication systems can be upgraded to incorporate the method and the system of the present invention thus yielding immediate performance improvements described. Still further objectives and advantages will become apparent from the consideration of ensuing description and drawings.

According to one aspect of the present invention, a method for free-space optical communication with reduced scintillations is provided that includes:

(a) generating a broadband optical radiation using at least one light source;

(b) modulating said broadband optical radiation by a communication signal;

(c) generating a spatio-temporally partially coherent optical beam by propagating said broadband optical radiation through a multimode optical fiber or waveguide;

(d) collimating said spatio-temporally partially coherent optical beam to form a substantially collimated spatio-temporally partially coherent optical beam of large aperture;

(e) eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into a communication medium, such as atmosphere;

(f) receiving at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation of said substantially collimated spatio-temporally partially coherent optical beam of large aperture through said communication medium;

(g) detecting said communication signal.

According to another aspect of the present invention, an apparatus for free-space optical communication with reduced scintillations is provided that includes:

(a) at least one source of broadband optical radiation;

(b) a system for modulating said broadband optical radiation with a communication signal;

(c) a multimode optical fiber or waveguide adapted for generating a spatio-temporally partially coherent optical beam;

(d) an optical collimation system adapted for collimating said spatio-temporally partially coherent optical beam to produce substantially collimated spatio-temporally partially coherent optical beam of large aperture and adapted for eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into a communication medium, such as atmosphere;

(e) an optical receiver adapted for receiving at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation of said substantially collimated spatio-temporally partially coherent optical beam of large aperture through said communication medium;

In one embodiment, the source of broadband optical radiation is a low-coherence light source.

In another embodiment, the source of broadband optical radiation is an ultrashort-pulse laser.

In a further embodiment, the source of broadband optical radiation is a multiple-frequency optical source.

In a further embodiment, the source of broadband optical radiation is a plurality of single-frequency optical sources with distinct central optical frequencies.

In another embodiment, the system for modulating said broadband optical radiation with said communication signal is adapted for direct modulation of the output of said source of broadband optical radiation.

In still another embodiment, the system for modulating said broadband optical radiation with said communication signal comprises an external optical modulator.

In another embodiment, the apparatus further comprises an optical amplifier.

In another embodiment, the apparatus further comprises a system for a substantially uniform excitation of transverse modes of said multimode optical fiber or waveguide.

In a further embodiment, the system for achieving substantial collimation of said spatio-temporally partially coherent optical beam to obtain substantially collimated spatio-temporally partially coherent optical beam of large aperture and eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into said communication medium comprises at least one optical element with positive optical power.

In still another embodiment, the system for achieving substantial collimation of said spatio-temporally partially coherent optical beam to obtain substantially collimated spatio-temporally partially coherent optical beam of large aperture and eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into said communication medium is adapted for performing optical magnification of an optical core of said multimode optical fiber or waveguide to the transverse dimension of said large aperture.

In another embodiment, the said optical receiver further comprises a system for intercepting at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation through said communication medium and concentrating the received potion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture on an optical detector, the system for intercepting comprising at least one optical element with positive optical power.

In still another embodiment, the optical detector is a fiber-coupled optical detector.

Still other objects and aspects of the present invention will become readily apparent to those skilled in this art from the following description wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of the best modes suited for to carry out the invention. As it will be realized by those skilled in the art, the invention is capable of other different embodiments and its several details are capable of modifications in various obvious aspects all without departing from the scope of the invention. Accordingly, the drawings and description will be regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1A illustrates an example of the sequence of momentary beam intensity distributions of spatio-temporally partially coherent optical beam.

Fig. 1B illustrates an example of spatio-temporally partially coherent beam's intensity distribution averaged during a one bit-long time interval.

Fig. 2A illustrates an example of the spectrum of a continuous-spectrum broadband light source.

Fig. 2B illustrates an example of the spectrum of a source consisting of a number of narrowband spectra

Fig. 2C illustrates an example of the spectrum of a source consisting of a number of sharp lines.

Fig. 3A illustrates spatially coherent light intensity distribution at the output of a single-mode optical fiber which is essentially independent on the spectral properties of the optical source and the length of the fiber.

Fig. 3B illustrates spatially coherent multimode light intensity distribution formed by a small number of transverse modes of a multimode optical fiber excited with a narrowband optical source.

Fig. 3C illustrates spatially coherent multimode light intensity distribution formed by a large number of transverse modes of a multimode optical fiber excited with a narrowband optical source.

Fig. 3D illustrates a low-contrast intensity distribution of a spatio-temporally partially coherent optical beam formed at the output of a multimode optical fiber excited with a broadband optical source;

Fig. 4A is a schematic diagram of one embodiment of the system.

Fig. 4B is a schematic diagram of one embodiment of the system utilizing an optical amplifier.

Fig. 4C is a schematic diagram of one embodiment of the system utilizing an optical amplifier and a mode scrambler.

DETAILED DESCRIPTION OF THE INVENTION

In light of the preceding discussion the system for FSO communication with reduced scintillations according to the present invention comprises a transmitter and a receiver separated by the communication medium, such as atmosphere, preferably at the line of sight. The source of optical radiation allocated in the transmitter emits a substantially broad optical spectrum preferably in a single transverse mode. The optical spectrum of the source may be continuous, quasi-continuous or discrete within a finite bandwidth with the requirement being that the inverse width of the spectrum is substantially smaller than the duration of one bit of the digital communication signal. Specifically, but not exclusively, the broadband optical source may be a low-coherence superluminescent diode, an ultrashort-pulse laser, an optical supercontinuum source, a Fabri-Perot type multi-frequency laser, a number of single-frequency lasers with separate central frequencies, or a combination of such sources. Relatively inexpensive semiconductor optical sources of the above type are currently available and may be used in the system described herein. Several examples of optical spectra of such sources are shown in Fig. 2. In Fig. 2A is schematically shown a spectrum of a continuous-spectrum optical source with the overall spectral width Delta_F. Equation (5) above may be applied to estimate the required spectral width Delta_F for the most optimal STPCB generation. For example, for M=100 effective independent sources and MMF length not exceeding 100 cm the minimum spectral width Delta_F should be about 1 THz, which for a telecommunication-region source near 1.55 micrometer central wavelength corresponds to less than 10 nanometers on the wavelength scale. Notably, superluminescent diodes with such or larger bandwidths are inexpensively available off the shelf.

Fig. 2B shows a spectrum of quasi-continuous broadband optical source with several separated relatively broad lines. Similar analysis as outlined above can be applied to such source.

Fig. 2C schematically shows the spectrum of a multi-frequency laser, such as Fabri-Perot laser. The analysis for such laser is preferably performed using Eq. (4) above and was demonstrated above.

The communication data stream may be modulated onto the optical radiation produced by the broadband optical source discussed above by either a direct modulation of the optical source itself via its drive current modulation or by using an external optical modulator. Specifically, but not exclusively such external light modulator may be a Lithium Niobate intensity modulator, an electroabsorption modulator or a similar device that offers the modulation speeds and modulation formats desired for the FSO communication link. The current invention does not specify and is not limited to any particular modulation format. However, due to the nature of the optical beam used in the present invention direct detection formats, such as OOK, PPM will be preferred over the coherent detection formats, such as DPSK, BPSK.

Before or after modulation of the broadband optical signal with communication data the broadband optical radiation may be optionally amplified in an optical amplifier to increase the transmitted power. Specifically, but not exclusively, such amplification may be achieved using an Erbium-doped fiber amplifier (EDFA) or a semiconductor optical amplifier having sufficient optical bandwidth to not cause severe spectrum narrowing of the input broadband optical radiation.

Optically broadband radiation modulated and optionally amplified as described above is further coupled into (specifically, but not exclusively) a multimode optical fiber or waveguide (MMF) of the length L satisfying Eq. (4) or Eq. (5). An offset splice or connection is used to uniformly excite a large number of fiber modes with approximately equal intensities. Alternatively, a mode scrambler may be used for the purpose of uniform excitation of a large number of modes. At the output of the MMF the desired spatio-temporally partially coherent optical beam (STPCB) is formed. Propagation of the broadband optical radiation through a relatively short length L of MMF is usually not associated with substantial losses, making the generation of STPCB essentially penalty-free in terms of the optical power.

The STPCB produced at the output of the MMF and carrying the communication data signal is subsequently prepared for transmission through the communication medium, such as atmosphere, by enlarging its aperture to the dimension desired, for example 10 centimeters in diameter, and by properly collimating. Specifically, but not exclusively, the STPCB may be enlarged and substantially collimated using a lens, a mirror or a more complex telescope assembly known in the art. The same assembly may be used for the receiving of the optical signal propagating in the opposite direction in a full duplex FSO communication link, for which a means to separate the incoming beam from the outcoming beam, such as a beamsplitter, is used. The enlargement of the SPCB produced at the output of the MMF may also be done by imaging the optical core of the MMF with magnification to the desired dimension by using a double-lens telecentric system.

The enlarged and substantially collimated STPCB modulated with the communication data signal and optionally optically amplified as described above is further eradiated into the communication medium, such as atmosphere, towards the receiver. Optionally, techniques for pointing and tracking known in the art may be used to facilitate the directing of the transmitted STPCB.

The receiver operates in a manner typical for the art by intercepting at least a portion of the incoming optical beam. A lens or a mirror or a more complex optical telescope assembly, similar or optionally the same as the one used for transmission (in case of a full-duplex FSO communication system sharing the same optical head for transmitting and receiving optical signals), concentrates the intercepted portion of the incoming optical beam to the dimension suitable for high-speed detection. Specifically, but not exclusively, the concentrated received light is projected on the active surface of an optical detector suitable for detecting the communication data stream chosen for the FSO communication link. For example, if OOK modulation format is chosen the detector may comprise any of a number of intensity detectors known in the art, such PIN, avalanche, or photon-counting detectors. The detector may also be optical fiber-coupled.

Several embodiments of an apparatus employing the method disclosed herein are schematically shown in Fig. 4. Fig. 4A schematically illustrates a system with directly or externally modulated broadband optical source 10 coupled to MMF 11, the output of which is an STPCB, which is further substantially collimated with an optical system 12, such as a lens, spherical or parabolic mirror, an array of lenses or mirrors, a telescope, or similar system, for sending STPCB into the transmission medium 13, such as atmosphere, toward the receiving end of the communication link 14. Fig. 4B schematically illustrates a system with an optical amplifier 15 used to increase the power of the transmitted optical signal. Fig. 4C further illustrates a system in which a mode scrambler 16 or a similar device is used to enhance the excitation of a large number of modes in the MMF.

It may be reiterated again that the system offered herein possess the optional advantage of optical-only fiber-coupling the transmitter and receiver heads to the rest of the system. There is no need for any electronic components within the optical heads, which are usually located outside the protected space on the roof tops, towers and moving platforms. This feature improves the electromagnetic immunity of the overall system as previously discussed.

Furthermore, it may be observed that the simplicity of the method and the system disclosed herein allows for easy and inexpensive upgrade of existing FSO systems to include a broadband light source and a multimode optical fiber or waveguide of a certain moderate length to readily take advantage of the scintillation reduction and thus substantially improve the quality of the FSO communication link.

For the purpose of demonstrating by example a mode of practicing the invention presented herein the following steps may be taken to assemble and operate the apparatus and apply the method of the present invention:

A relatively inexpensive broadband semiconductor source of low-coherence light with central wavelength being approximately 1550 nanometers and average output power approximately 10 milliwatts, specifically a Superluminescent Diode (SLD), is chosen. Preferably the SLD incorporates an optical isolator, a polarization-maintaining single-mode fiber pigtail terminated with an FC/PC fiberoptic connector. The SLD is preferably housed in a standard 14-pin butterfly housing commonly used in the art. The SLD preferably further incorporates a thermoelectric (TE) cooler for temperature stabilization. The SLD is driven using standard techniques known in the art by supplying required electrical voltages and currents to the pins of the SLD.

For imprinting the communication data stream on the broadband light produced by the SLD a standard Lithium Niobate external optical intensity modulator is employed. The modulator has an input single-mode fiberoptic pigtail and an output single-mode fiberoptic pigtail, both terminated with FC/PC connectors. The output of the SLD is connected to the input of the modulator using a bulkhead optical FC-FC adapter. The modulator is driven by a RF communication data signal with standard means known in the art at the bit rate chosen by the user, for example 2.5 Gbps.

If larger optical power is desired a standard optical amplifier, the Erbium-doped fiber amplifier (EDFA) can be used. The maximum output power of the EDFA can be, for example, 23dBm, or 200 milliwatts. The EDFA has an input single-mode fiberoptic pigtail and an output single-mode fiberoptic pigtail, both terminated with FC/PC connectors. The output of the modulator is connected to the input of the EDFA using a bulkhead optical FC-FC adapter.

The output of the system (output fiber pigtail of the EDFA) so far described comprises a high-power broadband single spatial mode optical light intensity modulated with the communication data stream at 2.5 Gbps.

Further, for the purpose of generating STPCB a length of multimode optical fiber (MMF) is connected to the output of the EDFA. The MMF used is a standard and inexpensive step-index optical fiber with optical core diameter of 100 micrometers and a length of approximately 2 meters. The MMF can be prepared or purchased in the form of a jacketed patch cord with FC/PC connectors on both ends. The output of the EDFA is connected to the input of the MMF using a bulkhead optical FC-FC adapter. The visual image of the MMF output as obtained, for example by shining the light on the surface of an infrared-sensitive card, should be of low-contrast, as in Fig. 3D.

The output of the system (output of the MMF) so far described comprises a high-power broadband STPCB intensity modulated with the communication data stream at 2.5 Gbps.

For the purpose of transmitting the STPCB thus obtained through the communication medium, such as atmosphere, the STPCB emanating from the output of the MMF is coupled into a collimation system which also enlarges its aperture. To this end a standard double-mirror reflective telescope, for example a Newtonian telescope, can be used. The telescope is fitted with a FC/PC fiber receptacle in such a way that the output plane of the FC connector ferrule, when attached to the receptacle, approximately coincides with the focal plane of the telescope. Said receptacle can preferably have a mechanism for small adjustment of the position of the ferrule output plane with respect to the telescope's focal plane for the purpose of allowing minor tuning of the beam collimation. The output end of the MMF is connected to the telescope via said receptacle.

The output of the system (output of the telescope) so far described comprises a STPCB of large aperture, substantially collimated and intensity modulated with the communication data stream at 2.5 Gbps.

The telescope being the optical transmitter within the present discussion is further attached to a mounting system and to a steering system, which is used to steer the beam being output by the telescope in the direction of the receiver. The receiver is intended to intercept at least a portion of the STPCB eradiated by the transmitter.

The best mode receiver comprises a double-mirror Newtonian telescope very similar or identical to the one used in optical transmitter described above. Similarly, the telescope is fitted with a FC/PC fiber receptacle in such a way that the tip of the FC connector ferrule, when attached to the receptacle, approximately coincides with the focal plane of the telescope. Said receptacle can preferably have a mechanism for small adjustments of the position of the ferrule plane with respect to the telescope's focal plane for the purpose of allowing minor tuning of the coupling of the intercepted and concentrated beam into the receiving fiber. For the purpose of the best mode description the detector used to detect the incoming light carrying the communication data is chosen to have a fiber pigtail with a standard FC/PC connector at the end of the pigtail. This connector is attached to the receiving telescope's FC/PC fiber receptacle. The detector's pigtail fiber is preferably a multimode fiber with 50 micrometer diameter core or more preferably with 100 micrometer diameter core. Large-core pigtails are preferred for better coupling of the incoming light and relaxed pointing requirements of the receiving telescope. However, the length of the multimode pigtail should not be excessive as to not substantially stretch the individual data carrying bits of the communication signal due to fiber dispersion. One-meter long pigtail is preferred and is standard in the art.

The data received by the detector in the form of the electrical signal is amplified, conditioned and processed and the data signal extracted with the standard means known in the art. The decoding scheme employed at the receiving end of the FSO link should match the encoding scheme employed at the transmitting end of the FSO link, with data modulation formats and forward error correction codes, if any properly handled.

The foregoing description of the preferred embodiments of the subject application has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the subject application to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments were chosen and described to provide the best illustration of the principles of the subject application and its practical application to thereby enable one of ordinary skill in the art to use the subject application in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the subject application as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.

Claims

A method for free-space optical communication with reduced scintillations, the method comprising:

(a) generating a broadband optical radiation using at least one light source;

(b) modulating said broadband optical radiation by a communication signal;

(c) generating a spatio-temporally partially coherent optical beam by propagating said broadband optical radiation through a multimode optical fiber or waveguide;

(d) collimating said spatio-temporally partially coherent optical beam to form a substantially collimated spatio-temporally partially coherent optical beam of large aperture;

(e) eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into a communication medium, such as atmosphere;

(f) receiving at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation of said substantially collimated spatio-temporally partially coherent optical beam of large aperture through said communication medium;

(g) detecting said communication signal.
The method of claim 1 wherein said light source is a low-coherence optical source.
The method of claim 1 wherein said light source is an ultrashort-pulse laser.
The method of claim 1 wherein said light source is a multiple-frequency optical source.
The method of claim 1 wherein said light source is a plurality of single-frequency optical sources with distinct central optical frequencies.
The method of claim 1 wherein the modulation of said broadband optical radiation with said communication signal is performed by directly modulating said light source.
The method of claim 1 wherein the modulation of said broadband optical radiation with said communication signal is performed by using an external modulator.
The method of claim 1 wherein said broadband optical radiation is amplified by an optical amplifier.
The method of claim 1 wherein the generation of said spatio-temporally partially coherent optical beam in said multimode optical fiber or waveguide is performed using substantially uniform excitation of transverse modes of said multimode optical fiber or waveguide.
The method of claim 1 wherein at least one optical element with positive optical power is used for collimating said spatio-temporally partially coherent optical beam to form said substantially collimated spatio-temporally partially coherent optical beam of large aperture.
The method of claim 1 wherein collimating said spatio-temporally partially coherent optical beam to form said substantially collimated spatio-temporally partially coherent optical beam of large aperture is performed by image transfer with magnification of the optical core of said multimode optical fiber or waveguide to the dimension of said large aperture.
The method of claim 1 wherein receiving at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation of said substantially collimated spatio-temporally partially coherent optical beam of large aperture through said communication medium is performed by concentrating the received optical beam on an optical detector using an optical system comprising at least one optical element with positive optical power.
The method of claim 12 wherein said optical detector is a fiber-coupled optical detector.
An apparatus for free-space optical communication with reduced scintillations, the apparatus comprising:

(a) at least one source of broadband optical radiation;

(b) a system for modulating said broadband optical radiation with a communication signal;

(c) a multimode optical fiber or waveguide adapted for generating a spatio-temporally partially coherent optical beam;

(d) an optical collimation system adapted for collimating said spatio-temporally partially coherent optical beam to produce substantially collimated spatio-temporally partially coherent optical beam of large aperture and adapted for eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into a communication medium, such as atmosphere;

(e) an optical receiver adapted for receiving at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation of said substantially collimated spatio-temporally partially coherent optical beam of large aperture through said communication medium.
The apparatus of claim 14 wherein said source of broadband optical radiation is a low-coherence light source.
The apparatus of claim 14 wherein said source of broadband optical radiation is an ultrashort-pulse laser.
The apparatus of claim 14 wherein said source of broadband optical radiation is a multiple-frequency optical source.
The apparatus of claim 14 wherein said source of broadband optical radiation is a plurality of single-frequency optical sources with distinct central optical frequencies.
The apparatus of claim 14 wherein said system for modulating said broadband optical radiation with said communication signal is adapted for direct modulation of the output of said source of broadband optical radiation.
The apparatus of claim 14 wherein said system for modulating said broadband optical radiation with said communication signal comprises an external optical modulator.
The apparatus of claim 14 further comprising an optical amplifier.
The apparatus of claim 14 further comprising a system for a substantially uniform excitation of transverse modes of said multimode optical fiber or waveguide.
The apparatus of claim 14 wherein the system for achieving substantial collimation of said spatio-temporally partially coherent optical beam and eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into said communication medium comprises at least one optical element with positive optical power.
The apparatus of claim 14 wherein the system for achieving substantial collimation of said spatio-temporally partially coherent optical beam and eradiating said substantially collimated spatio-temporally partially coherent optical beam of large aperture into said communication medium is adapted for performing optical magnification of an optical core of said multimode optical fiber or waveguide to the transverse dimension of said substantially collimated spatio-temporally partially coherent optical beam of large aperture.
The apparatus of claim 14 wherein said optical receiver further comprises a system for intercepting at least a portion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture after propagation through said communication medium and concentrating the received potion of said substantially collimated spatio-temporally partially coherent optical beam of large aperture on an optical detector, the system for intercepting comprising at least one optical element with positive optical power.
The apparatus of claim 25 wherein said optical detector is a fiber-coupled optical detector.