Bruesselbach et al. Vol. 22, No. 2 /February 2005 /J. Opt. Soc. Am. B 347

Power-scalable phase-compensating fiber-arraytransceiver for laser communications

through the atmosphere

Hans Bruesselbach, Shuoqin Wang, Monica Minden, D. Cris Jones, and Metin Mangir

HRL Laboratories LLC, Malibu, California 90265

Received February 13, 2004; revised manuscript received June 18, 2004; accepted July 31, 2004

We report laboratory experiments demonstrating a phase-compensating 70-mm-diameter aperture transceiverthat comprises a hexagonally close-packed array of seven 23-mm-diameter fiber collimator subapertures.Other than the collimators, the transceiver uses only fiber optics, connected as a master oscillator–multipleamplifier. The master oscillator is a fiber-coupled 1.5-mm diode laser, which is split and fed to 1-W fiber am-plifiers before it exits the collimators. To obtain a phase-coherent far field we control each subaperture’sphase by adjusting the current to its amplifier’s pump diodes in a multidither arrangement, maximizing thesignal at the receiver. We achieve a diffraction-limited coherent beam combination in the far field that pro-duces 1.4 W of power in the main lobe, in agreement with theory. © 2005 Optical Society of America

OCIS codes: 010.1080, 010.1330, 060.1660, 060.2320, 060.4510.

1. INTRODUCTIONShort atmospheric links for high-data-rate laser commu-nications through the atmosphere are attractive. Futurefree-space laser communications systems will demandbandwidths approaching terahertz size. From the stand-point of a fiber optic communications network, an idealatmospheric optical link would behave just as a piece offiber and be able to transmit the full multiterahertz In-ternational Telecommunication Union grid in c and evenin l and s bands. The link that we describe, because it isall optical, can achieve such transparent performance.By ‘‘all optical’’ we mean that there is no detection or re-modulation of the 1.5-mm communications signal. Free-space links that are now available cannot be called all op-tical if they detect the incoming optical signal and thengenerate a new optical signal; the bandwidth is limited tothat of the demodulation–remodulation set. Applica-tions include urban connectivity in the absence of fiber in-frastructure in the commercial sector and numerous high-data-rate links for military applications.

Optical communication through the atmosphere is de-graded both by scattering and by refractive-index fluctua-tions in the air. Attenuation of optical signals by scatter-ing from aerosols is dependent on the local weather, andhigher transmitter power cannot overcome it when it issevere. This limit to optical link availability must beovercome by nonoptical means. Nonetheless, when me-teorological conditions are good, reasonably fade-free op-tical links can be established with properly sized simpleapertures. To improve the optical link’s performance tothe high data rates achieved by fiber optics, correction ofatmospheric aberrations, which are almost alwayspresent, needs to be undertaken. Existing systems,which demodulate at the end of the link, handle aberra-tion simply by increasing laser power, by using multipletransmitters, or most commonly by increasing the detec-tor size to be as large as the blur spot, consistent with not

0740-3224/2005/020347-07$15.00 ©

compromising bandwidth, because large-area detectorsare slower. For apertures smaller than transverse coher-ence scale size r0 , dynamic control of fine-angle tilts isalso used.

Our goal is to achieve communications bandwidths thatare comparable to fiber links by relaunching the beamthat has traversed the atmosphere back into a single-mode fiber. Such a link imposes more-stringent require-ments on tilt control and correction of aberrations thandoes a demodulation–remodulation link. Sufficient co-herence of the beam at the received end can be accom-plished only by actual correction of any tilt errors andphase aberrations introduced by the atmosphere. It canbe shown that link performance can be greatly improvedby correction of both the transmitted and the receivedbeams.

In this paper we describe an optical communicationssystem that accomplishes not only small-tilt-error correc-tion but also higher-order phase correction of the kindtypically encountered in propagation through the atmo-sphere. The system that we describe accomplishes thecorrection by dividing the transmitter apertures, the re-ceiver apertures, or both into multiple subapertures.Each subaperture consists of a fiber collimator connectedto an individual fiber amplifier. Each has its phase cor-rected by a feedback loop. Specifically, in this paper weconcentrate on describing a configuration that uses seven1-W fiber amplifiers suitable for operation with a seven-subaperture transmitter. Seven subapertures are ad-equate to demonstrate the concept and to correct atmo-spheric conditions that are representative of thosetypically found in terrestrial optical communications ap-plications. Analogous transceivers with larger numbersof subapertures are required if greater atmospheric tur-bulence must be corrected or if the system is on a movingplatform. Note that with this approach at greater than;100-km ranges the aberration-correction bandwidth of

2005 Optical Society of America

348 J. Opt. Soc. Am. B/Vol. 22, No. 2 /February 2005 Bruesselbach et al.

the link is limited by the travel time of the light betweenthe transmitter and the receiver.

In this paper we do not address amplifier development.We believe that much of the merit of our approach lies inpower scaling coming entirely from increasing the num-ber of amplifiers. Note that the resultant performanceimprovement comes not simply from increased power butmore importantly from the fact that the entire transmit-ted beam is correctly phased to put maximum coherentpower on the receiver aperture, correcting for phase er-rors introduced by the atmosphere.

Over the past decade, numerous techniques for phasingamplifiers have been discussed and some subsequentlydemonstrated.1 To our knowledge the first experimentaldemonstration of the phasing of multiple-fiber amplifierswas made in our laboratory in 2000 for a five-elementarray2 by use of a patented technique by which pump-diode current adjustments control fiber phase. We deter-mined that this effect comes mostly from the Kramers–Kronig interrelationship between gain and refractiveindex3 in the core of the fiber, and we measured that wecould obtain 6p phase shift in 0.25 ms. This speed isquite sufficient for correction of typical terrestrial atmo-spheric fluctuations. Indeed, it enables corrections to bemade even for turbulent boundary layers on moving plat-forms.

It is noteworthy that this is an all-fiber approach. Noadditional components that are not already indispensableto fiber amplifiers are required. In particular there areno independent phase modulators, which would add lossand cost. There are no optical benches. There is nosensitive-adjustment and misalignment-prone free-spaceoptical beam handling. All optical power remains insidefibers, or finally in collimators, until it exits the system.The system’s output aperture, which consists of a bundleof fiber collimators, may well weigh only a few ounces,making coarse steering mechanically simple. A gimbal,if used, requires no Coude path because the beams arerouted through flexible fibers. That is, no image-relayingoptics designed to bring the beam through the gimbalsare needed, so the gimbal is simplified. Alternatively tosteering the collimator bundle itself, a simple mirror infront of the collimators may be used for coarse steering.The mirror, if used, does not need to retain diffraction-limited flatness as it is moved, because phase errors willbe corrected. Properly packaged fibers are aircraft wor-thy, light, and rugged, and their packaging requirementsand thermal management are no more demanding thanthose for ordinary electronic components. If a few ele-ments (amplifiers or apertures) fail, the system perfor-mance degrades gracefully. Finally, and importantly, thearchitecture is scalable to higher powers by addition ofmore subapertures or higher-power individual fiber am-plifiers, or both, as they are developed.

Fiber modulation for phase locking will not interferewith the communication signal, as it is at a much lowerfrequency, ,30 kHz, and the associated amplitude modu-lation tends to zero when the loop is closed and function-ing. The fiber master oscillator–multiple amplifierphase-locking technique that we describe in this paper iscompatible with any optical communications modulationscheme. The architecture is flexible enough to permit

upgrades and injection of new communications technolo-gies within the demonstrated architecture without majormodifications.

A. Link DesignIt is informative to examine a specific design point toidentify and assess the system issues and the technologyissues and to appreciate their implications. Table 1shows point-design link margin calculations for 2.5-Gbit/sdata-rate communications over a 10-km link severely af-fected by atmospheric aberrations.

For the example in the table we assumed that 222dBm, or ;6 mW of power, would be needed at the receiverto produce a standard 1029 bit-error rate for 2.5-Gbit/sservice. We assumed 10-cm apertures and a somewhatsevere atmospheric spatial scale (r0) near the transmitterand for simplicity used only this parameter to define theatmosphere. We also simply assumed that an uncor-rected 10-cm beam diverges as though it were 1.5 cm insize (equal to r0) rather than diverging at the 10-cm ap-erture’s diffraction limit. In columns 2 and 3 of Table 1we compare two solutions to the link-loss problem: Thefirst solution entails atmospheric correction, the second,increased transmitter power. Note that even with onlyone tenth the transmitter power, the margin for the cor-rected link is 4 dB greater than the margin with a singlehigher power but an uncorrected (and same size) laser ap-erture. The ability to function with less transmittedpower is an important system improvement, particularlywhen eye safety is a concern. The example of Table 1 as-sumed that phase correction was implemented only at thetransmitter aperture, and that a relatively large detectorwas used at the receiver aperture. One could obtain fur-ther improvements and larger bandwidth by implement-ing phase correction at the receiver aperture also.

Table 1. Communications Link Power-BudgetSummary,a Showing Link Margins

Property

Is Phase Correction BeingUsed?

Yes No

Transmitter divergence 37 mrad 244 mradBeam size at range 0.4 m 2 mDiffractive beam spread loss 211 dB 228 dBAtmospheric attenuation

(20.01 dB/km)20.1 dB 20.1 dB

Window loss (85%) 20.7 dB 20.7 dBThin array sidelobe loss 25.7 dBImperfect correction (50%) 23.0 dBTotal link loss 215 dB 229 dBSubapertures 217Power per subaperture 5 mWTotal transmitter power 1 W 10 WPower collectable by receiver

aperture215 dBm 219 dbm

Power assumed needed for2.5 Gbif/ps link

222 dBm 222 dbm

Margin 7 dB 3 dB

a Wavelength, 1.5 mm; transmit aperture, 0.1 m; receive aperture, 0.1m; r0 , 15 mm; subaperture size, 6.35 mm; range, 10 km.

Bruesselbach et al. Vol. 22, No. 2 /February 2005 /J. Opt. Soc. Am. B 349

As has already been noted, a light and rugged all-fibersystem simplifies any coarse gimbal that might be used,as no Coude path is needed. Further optomechanicalsimplification comes about because the array itself doesfine tracking. The master-oscillator–multiple-amplifierfeedback loop automatically optimizes receiver signal,thereby correcting both tilt and whatever aberrations thearray can resolve. The tilt angle capability of the colli-mator array is proportional to the divergence angle of thesubapertures. For example, a 15-element-diameter ar-ray, assuming 5 elements per 2l shift, provides 3 timesdiffraction-limited, or 6100 mrad, fine-tilt correction.Experimental measurements of the array that we as-sembled, which had relatively large-diameter subaper-tures, are presented below.

B. Coherent Combining of Fiber LasersCoherent combining of fiber lasers is attractive for obtain-ing adequate laser power, but our technique of multipleapertures makes it more attractive because it reduces thepower needed for a reliable link by correcting the aberra-tions. Thus it incorporates the functionality of an adap-tive optics element as well as being a convenient way toscale power. Recent advances in the power scaling of fi-ber lasers have made these lasers much more efficientand compact than bulk crystal or glass lasers and has fa-cilitated their application to laser communications sys-tems. Individual fiber amplifier powers now easily ex-ceed 20 W (and eye-safe limits).4 Our architecture usesexisting off-the-shelf components. High-power single-fiber–single-aperture approaches in an amplified commu-nication format include the need to develop 10–100-Whigh-quality amplifier outputs (reported high-power fiberdemonstrations are mostly of oscillators, which are moreefficient); output at 1.5 rather than at 1 mm (again lessefficient, as Er-doped or Er–Yb-codoped fibers are inher-ently less efficient than Yb); and polarization-preservingoutput. These development efforts for individual high-power fibers will not be necessary if optically phased fiberamplifier arrays are used.

Phase-control feedback is a multidither system of thetype reported previously by Minden,2 chosen because itsimplementation is straightforward. Standard laboratorylock-in amplifiers,5 with 125-kHz bandwidth, are used toisolate the signals for the individual channels, which aredithered at different frequencies, all near 20 kHz. Theerror signals are sent into a modulation input on the di-ode power supplies, which also have .100-kHz band-width. The speed of the control loop is constrained by theintegration time of the lock-in amplifiers, which for theinstruments that we used could not be less than 1 ms.

The multidither approach to tagging each channel isadequate for our demonstration seven-channel system(seven modulated channels at 1 kHz each). However, fora larger number of channels, each with a larger, 10-kHzbandwidth, a form of orthogonal coding other than simplemultidither would almost certainly be preferable.

2. ATMOSPHERIC ABERRATIONSOptical propagation through the atmosphere, as dia-grammed in Fig. 1, is subject to turbulence-induced phase

distortions. These distortions may be classified by theirorigins: (1) Fast, fine-scale turbulence near the trans-mitter, i.e., the turbulent boundary layer (TBL), occurs formoving platforms. The TBL increases the transmittedbeam’s divergence, reducing the signal intensity at theother end of the link and producing much signal fluctua-tion. (2) Slower, larger-scale turbulence along the pathmanifests itself as beam wander–tilt, beam spread, andscintillation. (3) The TBL near the receiver aperturespreads and corrupts the beam profile in the plane of thedetector. Because optical distortions that are not resolv-able by the optical apertures cannot be corrected, and be-cause apertures favored for laser communications are of-ten too small to resolve distant distortions, not allaberrations can be corrected. For many situations thescale size of the turbulence is large (of the order of thesize of the apertures), and its principal consequence isfluctuating tilt error at the receiver. Indeed, it is wellknown that significant benefit is obtained even if only tiltcorrection is implemented.

The phased-array source described in this paper ad-dresses any aberrations—tilt and others—that are resolv-able at the transmitter. The TBL, which reduces thetransmitter’s effective coherent aperture size to r0 , in-creases beam divergence and can be the greatest source oflink loss for moving airborne platforms. If the phasemodulation from the TBL is large, it can also cause severemomentary fades. TBL correction can thus be of greatbenefit to optical communications links between movingplatforms.

The U.S. Air Force and others6–9 have investigated theturbulent boundary layer and the consequent degradationof resolution under a number of flight conditions and withvarious experimental methods. The degradation varieswith many parameters: the dynamic pressure (definedas the free stream density, in grams per cubic meter,times the Mach number squared), the aperture diameter,the wavelength, the distance from the front of the plane,and the presence of any fuselage features that disturb theairflow. Resolution degradations as large as 40 times thediffraction-limited performance have been measured.Strehl ratio, or peak signal relative to the diffraction-limited value, has been measured at 0.05 or less, evenwith piston and tilt errors removed. Indeed, for rear-ward facing turrets, computations10 based on aerody-namic data show that the Strehl ratio may degrade to aslow as 1024.

Two important parameters that characterize the TBLare the phase correlation length and the phase correlationtime (indicated in Fig. 1). TBL phase correlation lengthsas short as 0.5 cm have been measured at l5 632.8 nm.11 Scaling to 1.55 mm by a l6/5 rule gives 1.5cm. Output from a larger aperture will not remain co-

Fig. 1. Atmospheric turbulence effects: 1, near the transmit-ter; 2, along the path; and 3, near the receiver. Typical valuesfor the transverse coherence and the time constant are indicated.Issues are described in text.

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herent. Measured correlation times are ;100 ms,6 re-quiring an ;10-kHz adaptive optical control bandwidth.The TBL is known to degrade resolution approximatelylinearly with increasing dynamic pressure u, which isequal to the free stream density in grams per cubic metertimes the Mach number squared. In tests on board a KC-135 aircraft,14 u was varied from 0 (for ground calibra-tions) to as much as 325 for flying at an altitude of 9.1 kmat Mach 0.84 (;500 knots, or 250 m/s).

For the majority of optical communications, particu-larly on stationary platforms, however, the frequency ofthe atmospheric fluctuations is much slower. Figure 2plots recent measurements made at our range at the HRLfacility in Malibu, California. Shown is a typical Fourierspectrum of turbulence-induced received-signal intensityfluctuations under benign weather conditions, averagedover several days. Note the absence of fluctuations abovea few hundred hertz.

3. EXPERIMENTSA. Experimental SetupThe experiment reported here is diagrammed in Fig. 3.A 1.5-mm distributed-feedback laser diode is used as asource. The 10-mW signal is equally split by a 1 3 8 fi-ber splitter. One fiber is used as a signal monitor. Next,the other seven fibers are spliced into seven separate 1-Wfiber amplifiers.12 Following this, the fibers are directedthrough polarization controllers. The light in the fibersis sent to free space via seven collimators13 that useGradium13 lenses, which give Gaussian beam diameters(defined as twice the waist radius w0) DB 5 12.5 mm andhousing diameters DH 5 23 mm. The seven collimatorsare arranged in a hexagonally closely-packed array. Thishexagonal array fits within a circle that has just under a70-mm diameter; this can be considered the system’s ap-erture diameter. The subaperture diameters are 23 mm;each subaperture contains a 12.5 mm-diameter Gaussianbeam. The near-field pattern is shown as an inset inFig. 3.

Fig. 2. Measured frequency spectrum of the atmospheric-induced intensity fluctuations that occurred for a 1.5-mm signalbeam with 6.25-mm waist size (radius) transmitted over the400-m range at the HRL facility in Malibu, California. The Fou-rier transforms were calculated by an oscilloscope. At low fre-quencies the signal was sampled 50 times per second; the aver-age Fourier transform of one hundred 1000-s scans is plotted.For the higher-frequency data the signal was sampled 100,000times a second, and again one hundred Fourier transforms wereaveraged.

We obtained the far-field pattern in the laboratory byfocusing the beam with a concave mirror of 60.6-cm focallength. The focal plane was imaged onto an infraredCCD14 via a 53 microscope objective lens. The center ofthe focal plane is also relayed onto a fast photodiode (notshown in Fig. 3) for the communication link and to a slowphotodiode for the phase-control loops. Control of thephase is achieved by use of multidither feedback loops.15

The pump currents of the fiber amplifiers are ditheredwith different frequencies in the range of tens of kilo-hertz. In general, variations in the pump current lead tochanges in refractive index of doped fibers and thus tovariations of the phase of the amplified signals.16 Thephase modulation mentioned above, in turn, by coherentdetection at the slow photodiode leads to voltage modula-tions in the diode signal at that particular frequency.Lock-in amplifiers (labeled as part of the signal process-ing in Fig. 3) are used to distinguish the seven differentdither signals, and each amplifier’s output is fed back tothe appropriate current-controlled power supply. Whenthe control is turned on, the beams quickly coherentlycombine, causing the center lobe of the interference pat-tern to reach its maximum and automatically correctingthe phase errors in the fibers and along the beam paths.

B. Experimental ResultsThe far-field intensity profiles, which were frame-grabbedfrom the infrared CCD, together with line transections(lineouts) at the indicated positions, are shown in Fig. 4.As expected, when the seven beams are in phase with oneanother the far-field pattern is the two-dimensional Fou-rier transform of the near-field pattern. When the phaseis not corrected, the pattern continuously scintillates,whereas the phased-up pattern remains steady for manyhours with no significant jumps or dropouts, even when ahairdryer-style air blower is directed at the near- or far-field beam.

Samples of the temporal behavior are shown in Fig. 5.When the loop is not closed, the signal fluctuates on a

Fig. 3. Experimental layout. The inset image is of the near-field pattern of the transmitter array obtained with an infraredCCD camera. It represents the transmitted beam, which con-sists of near-Gaussian subbeams with 6.25-mm waists, and thesesubapertures have 23-mm nearest-neighbor spacing as shown.WDMs, wavelength-division modulators.

Bruesselbach et al. Vol. 22, No. 2 /February 2005 /J. Opt. Soc. Am. B 351

time scale as fast as several milliseconds. With the feed-back loop closed, the signal is five to six times larger, onaverage, and much steadier.

As another way to determine quantitatively how wellthe loop maintains phase coherence, we calculate a nu-merical integration of the intensity of the far-field patternas a function of distance from its center. We also checkthe accuracy of the numerical measurement by measuringthe power transmitted through a variable aperture in thefar field (the so-called power-in-the-bucket method). Allthese measurements are in good agreement with the re-sults of theoretical calculations by standard propagationtheory. Such calculations are straightforward for the ar-ray of Gaussian beams that we used, because Gaussianbeams maintain a Gaussian profile as they propagate.We calculated the far-field pattern by simply summingthe electric fields of an array of beams, each with its cen-ter appropriately displaced from the origin in the x –yplane, and using a standard propagation equation.17

Results based on CCD imaging, on the power-in-the-bucket method, and on theory are all drawn in Fig. 6. Ascan be seen, the central lobe includes approximately 25%of the total power, as expected based on our subaperturefill factor’s being close to 25%. Commercially available fi-ber collimators, such as those that we are now using, aredesigned to produce a minimally clipped Gaussian beam.Increasing the fill factor should enhance the energy in thecenter lobe, albeit at some compromise as a result of clip-ping. Residual discrepancies are probably due to imper-fect polarization, imperfect beam overlap in the far field,and residual phase errors.

It is important to note that in many laser communica-tions setups the receiver aperture is smaller than the

Fig. 4. Far-field intensity profiles obtained with an infraredCCD. White curves, intensity transections scanned along thedashed lines. (a) Snapshot of the intensity profile when thephase control is off; the pattern is continuously changing. (b)Phase control on; the pattern remains steady.

Fig. 5. Oscilloscope traces comparing the signal on the slow de-tector when the feedback loop was open and closed. The lower,fluctuating signal was recorded with the feedback loop discon-nected; the upper signal, with the loop closed. All seven ampli-fiers were operating.

diffraction-limited spot size that the transmitter is ca-pable of producing. Therefore it is more important thatthe peak intensity be high than that all the power be inthe central lobe. In the experiment we therefore alsomeasured the peak intensity with the feedback control onas a function of the numbers of subbeams switched on.As shown in Fig. 7, the peak intensity was enhanced ap-proximately five times when all seven beams were turnedon. One can readily calculate the maximum possible en-hancement by recognizing that it is the ratio of the coher-ent to the incoherent sum of the intensities. By ‘‘coher-ent sum’’ is meant the square of the sum of the electricfields when they are all in phase. For the experimentthis is the square of the sum of the square roots of thevoltages measured when the seven channels are turnedon one at a time. The incoherent sum is the sum of thevoltages. Theory thus indicates that the maximum en-hancement could be seven times. The discrepancy be-tween this and what was actually measured is probablydue to imperfect polarization and residual phase errors.

The experiment also demonstrates, as shown in Fig. 8,that the beams retain constructive interference and thatthe intensity peak follows movement of the detector. Theangular steering range is determined by standard Gauss-ian divergence angle 1/pw of the subapertures, where wis the waist size of the individual subbeam. Based on thespecifications of the collimator that we used, this anglewas calculated to be 675 mrad, close to our measurement.

Finally, the response to phase change of this feedbacksystem needs to be addressed. As discussed above, thephase change is due mainly to the change in refractive in-dex of the doped fibers, which occurs as we change thepump power. We have measured the phase change of abeam traversing a fiber amplifier as a function of thepump currents. The detailed results are beyond the

Fig. 6. Measurements and theory for integrated far-field inten-sity pattern as a function of distance from the pattern’s center.Filled circles, measured power transmitted through apertures ofthe indicated angular sizes (i.e., true power in the bucket); no ad-ditional power was measured for apertures even as large as 2.5cm, corresponding to ;0.5 mrad. The dashed curve was calcu-lated by numerical integration of the far-field intensity patternmeasured by the CCD camera. The solid curve was calculatedby integration of the far-field pattern of an array of Gaussians ina analytical simulation of the actual configuration. The knee atapproximately 25% power corresponds to the central lobe.

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scope of this paper; here we summarize what is relevantfor the present system. Note that we always worked inthe deep gain saturation regime (with 1-W output per fi-ber and approximately 8-W pump power for each ampli-fier), even though the phase change per current change issmaller than when the gain is not saturated. Our mea-surements indicate that thermal effects were smallenough to be ignored in Er–Yb fibers. We measured aphase change of approximately 2p/A of pump current inthe saturated regime. This is large enough for phasecontrol in our system. The output power varies by only

Fig. 7. Way in which the peak intensity, as measured by theslow photodiode, is enhanced coherently as the number ofphased-up beams is increased. The four images at the top arethe far-field patterns, with phase control on, when the indicatednumber of beams were turned on. The filled diamonds are themeasured intensity when the indicated number of fiber amplifi-ers, with phase control, were turned on. The filled circles arethe incoherent and coherent sums of the measured photodiodevoltages when the amplifiers were turned on individually. Theincoherent sum, which is a simple sum of the voltages, is fittedby a straight dashed line, and the coherent sum is fitted by a pa-rabola.

Fig. 8. Slow photodiode’s output as a function of azimuth angleof the detector. The pictures are far-field patterns grabbed bythe CCD when the slow diode was translated horizontally. Thecircles marked in the pictures indicate the locations of the detec-tor. (Note that the actual size of the detector is much smallerthan that of the optical lobes.)

;10% with a 0.5-A change in pump current. It shouldfurthermore be noted that the fact that the fiber amplifi-ers are codoped with Er31 and Yb31 has some interestingconsequences. Whereas the gain at 1.5-mm wave lengthis related to the excited-state density of Er31, the refrac-tive index depends more on the excited-state density ofthe Yb31. The saturation threshold and the responsetime of the gain versus that of the phase change are dif-ferent. The analysis of this phenomenon is complicatedbecause of the coupling between the excited states of Er31

and Yb31. It turns out that the phase response time isintrinsically determined by the fluorescence time of theexcited states of Yb31 ions in the fiber. Our previous ex-periments with these fiber amplifiers have shown that aphase change of p/2 can be obtained in 0.125 ms (4p/ms),indicating that speeds well in excess of 2 kHz are pos-sible. The electronic response of our feedback system iswhat is limiting the speed, because it has a phase re-sponse of only 0.2pr/ms, determined by the lock-in ampli-fiers now being used in the system.

4. CONCLUSIONSWe have demonstrated that a coherent fiber amplifier ar-ray can be used to scale the power of a transmitter as wellas to correct aberrations such as those produced by the at-mosphere. The experimental results that we have de-scribed in this paper are a first step toward demonstrat-ing that laser communications between terrestrial andeven moving platforms can benefit from the multiple-aperture closed-loop phase-controlled array architecturethat we have described. We showed that correcting thephases of the multiple apertures produces a near-diffraction-limited, nonfluctuating far-field optical beam.Such a beam can be efficiently relaunched into a fiber byuse of a collimator at the receiving end. This will allowthe communications signal originally in the fiber to con-tinue in the fiber without the need for demodulation andremodulation.

Receiver apertures can be similarly assembled frommultiple subapertures and can also be phase-corrected byuse of an analogous strategy. The phase control in thereceiver case would be made with the optical preamplifi-ers placed between the subapertures and the receiver andmight permit a virtually modulation-transparent link.Such a link could accommodate the optical wavelength-division multiplexing that is now routinely done in fibers.The links could have as high a bandwidth as a ground-based optical fiber link.

ACKNOWLEDGMENTSWe thank G. Duchak of the Defense Advanced ResearchProjects Agency and also E. Watson and B. Stadler of theU.S. Air Force Research Laboratory, Wright-Patterson AirForce Base, for their support. The authors thank DaveHammon for his excellent technical support.

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