1

All-fiber photonic lantern multimode optical receiverwith coherent adaptive optics beam combining

Bo Zhang, Jianfeng Sun, Chenzhe Lao, Christopher H Betters, Alexander Argyros, Yu Zhouand Sergio Leon-Saval, Member, IEEE,

Abstract—A multimode optical receiver for free space opticalcommunications (FSOC) based on a photonic lantern and adap-tive optics coherent beam combining (CBC) of the lantern’ssingle-mode outputs is proposed and demonstrated for the firsttime. The use of optical coherent combining in fiber serves toincrease the signal to noise ratio compared to similar receiversbased on electrically combined signals, and represents an all-fiberapproach to low-order adaptive optics. This optical receiver isdemonstrated using a photonic lantern with three outputs, fibrecouplers and active phase locking, and further investigated underatmospheric conditions with and without turbulence.

Index Terms—Optical receivers, communication systems, ladar.

I. INTRODUCTION

Lately, free space optical communications (FSOC) havebeen widely studied as a disruptive alternative for large ca-pacity communications in satellite and terrestrial laser links.When using multiple ground stations although, simplifiedoptical systems are desirable to allow the usage of moreflexible approaches. In terrestrial and space-to-ground lasercommunications and receivers, several methods for opticalsystem simplification are indeed possible. For instance, oneway of reducing the required power-aperture product on aspace platform is to implement effective, but costly, single-aperture ground terminals with large collection areas, e.g. largetelescope apertures. Another approach recently demonstratedfor increasing collection area and efficiency is to combinesignals from multiple apertures using digital coherent com-bination (DCC)[1]. Thus, using many small less-expensiveapertures combined to create a large effective aperture whilemaintaining good receiver sensitivity. However, the multi-aperture nature of the process imposes other constraints andrequirements on the digitization. by

Recently, a multimode single-aperture optical receiver wasproposed [2], [3] based on photonic lanterns [4], [5], [6] andthe direct coherent combination of signals in the electrical

Bo Zhang, Jianfeng Sun, Chenzhe Lao and Yu Zhou are with Key Labo-ratory of Space Laser Communication and Detection Technology, ShanghaiInstitute of Optics and Fine Mechanics, Chinese Academy of Sciences, 390Qinghe Rd., Shanghai 201800, China; and Center of Materials Science andOptoelectronics Engineering, University of Chinese Academy of Sciences,Beijing 100049, China (e-mail: zhangbo@siom.ac.cn, sunjianfengs@163.com,251728782@qq.com and sunny@mail.siom.ac.cn).

Christopher H Betters and Sergio Leon-Saval are with Sydney Astropho-tonic Instrumentation Laboratory, School of Physics, The University ofSydney, NSW 2006, Australia (e-mail: christopher.betters@sydney.edu.au,sergio.leon-saval@sydney.edu.au).

Alexander Argyros is with Matrix Education, Sydney, NSW 2000, Australia(e-mail: aarg4123@gmail.com).

Fig. 1: Structural diagram of the PL-based optical receiver: (a)using the method of digital coherent combination; (b) usingthe method of coherent beam combining.

domain. The proposed scheme is shown in Fig. 1a and involvesa photonic lantern converting a multimode signal into 𝑁

single-mode signals, which are mixed with single-mode localoscillator beams in optical hybrids. The mixed signals are thenread by 𝑁 balanced photodiodes and coherently combinedelectronically. This scheme takes advantage of the high cou-pling efficiency of a multimode signal to the multimode end ofa photonic lantern [7] and the higher tolerance in free-spacecoupling situations to tilt errors [8]. It also takes advantageof the high mixing efficiency achieved in single-mode fibre(SMF) [9], and has been used in laser detection and ranging(LIDAR) [3] and free-space optical communications [10].

However, the above method may result in lowering thesignal-to-noise ratio (SNR). This is because the original mul-timode signal is divided into 𝑁 parts, meaning the signal ateach photodiode is reduced on average by a factor of 𝑁 . Hencethe SNR at each photodiode is reduced, as is the averageover all photodiodes. Depending on the circumstances, thereduction of SNR using this DCC approach may negate theadvantages of using the photonic lantern in the first place,as compared to directly coupling to one single-mode fibreand photodiode. If the input is not diffraction limited (i.e.

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multimode, due to atmospheric turbulence), then the lanternwill be able to increase the signal through a higher couplingefficiency by capturing high order modes not guided by theSMF. However, this is only an advantage if the increase insignal compensates for the reduction in SNR. If the input isalready diffraction limited (i.e. single-mode, in the absenceof atmospheric turbulence and tilt and tip errors), the lanternprovides no advantage in terms of coupling [11] but mayreduce SNR.

Here, we propose a photonic lantern-based optical receiverthat avoids the reduction in SNR described above by usinga single photodiode and an adaptive optics in-fiber coherentbeam combining (CBC), i.e. coherently combining the signalsoptically in optical fiber couplers rather than electronically[12]. The receiver is shown in Fig. 1b, and the key differenceis that the 𝑁 single-mode outputs of the lantern are combinedinto one single-mode beam using a coherent optical combiner.The beam is then mixed with the local oscillator and detectedby a single photodiode. This retains the high coupling ef-ficiency of the lantern and high mixing efficiency and longpropagation distance bandwidth of SMFs, and eliminates thedecrease in SNR that arises from dividing the signal betweenmultiple photodiodes.

In this paper we demonstrate the operation of the CBC-based receiver described. We verify the feasibility of opti-cally combining the outputs using adaptive optics based onactive phase locking and coherent interferometry in 3-dBfiber couplers. In addition, we use a phase screen to verifythe feasibility of the optical receiver under conditions ofatmospheric turbulence.

II. COHERENT BEAM COMBINING

A. Principle of operation

Photonic lanterns with adaptive spatial mode control havebeen recently demonstrated to selectively excite only thefundamental mode in large mode area amplifiers [13], [14].To demonstrate our proposed photonic lantern-based receiverwith the coherent combination of beams in the optical domainwe applied an active phase locking architecture on the single-mode fiber pigtails combined with 3-dB fiber couplers. Thus,relatively simple feedback loops from detectors to adaptiveoptic phase elements allowed the device to adapt automati-cally to any specific input beam form [15] while coherentlycombining all single-mode output power into one single-modefiber. A lantern with three single-mode outputs was used, theschematic and microscope image are shown in Fig. 2. Thelaser signal beam passes through Lens 1 and then a phasescreen to simulate atmospheric turbulence. Lens 2 couplesthe signal into the multimode end of the photonic lanternwhich transitions adiabatically to three single-mode fibres atthe output.

The first and second output fibres are connected to a 3-dBcoupler (Coupler-1 in the Fig. 2) and the phase of the light inthe second fibre is controlled by the phase shifter PS-1, whichis controlled by the photodiode PD-1 that measures the secondoutput of the coupler. Minimising the reading on PD-1 resultsin the first and second outputs being combined coherently. This

combined output is then combined coherently with the thirdoutput from the lantern using the same method with PS-2 andPD-2.

B. Theoretical evaluation of efficiency in the system

In order to get a better understanding of the optical receiverworks in this optical coherent beam combiner architecture, itis important to evaluate the phase, optical losses and poweroutput relationships between the different components. To thispurpose we model the efficiency of the all-fiber CBC receiveras follows. The transfer matrix of the 𝑘th 3-dB fiber coupleris

𝑀𝑘 =

√︂Z𝑘

2

[1 𝑗

𝑗 1

], 𝑘 = 1, 2 (1)

where Z𝑘 is the insertion loss of the 𝑘th coupler.If we take the single-mode outputs of the 1 × 3 lantern to

be 𝑃𝑆,1, 𝑃𝑆,2 and 𝑃𝑆,3, and their phases to be \𝑆,1, \𝑆,2 and\𝑆,3, the electric field outputs (𝐸𝑂,1 and 𝐸𝑂,2) of the first 3-dBcoupler (Coupler 1) can be expressed by[

𝐸𝑂,1𝐸𝑂,2

]= 𝑀1

exp

(− 𝑗

Δ\122

)0

0 exp(𝑗Δ\12

2

)[ √︁

𝑃𝑆,1√︁b1𝑃𝑆,2

](2)

where Δ\12 = \𝑆,1 − \𝑆,2 is the phase difference between theoutput of the single-mode fibers 1 and 2, and b1 is the lossfactor of the first phase shifter (PS-1) used to control the phasedifference between fiber 1 and 2 before the first 3-dB coupler(Coupler-1). The first output of Coupler-1 and the third outputof the lantern (fibre 3) pass through the second 3-dB fibercoupler (Coupler-2). Thus, the electric field outputs of the twoarms of Coupler-2 (𝐸𝑂,3 and 𝐸𝑂,4) can be given by[

𝐸𝑂,3𝐸𝑂,4

]= 𝑀2

exp

(− 𝑗

Δ\1232

)0

0 exp(− 𝑗

Δ\1232

)[√

b2 |𝐸𝑂,1 |√︁𝑃𝑆,3

](3)

where Δ\123 = \𝑂,1−\𝑆,3, is the phase difference between thefirst output of the Coupler-1 (𝑃𝑂,1) and the fibre 3 output ofthe lantern (𝑃𝑆,3); and where \𝑂,1 is the phase value of 𝑃𝑂,1,and b2 is the loss factor of the second phase shifter (PS-2).Hence, the intensity of each output can be obtained by themagnitude of the electric field squared:

𝑃𝑂,1 =Z12

[𝑃𝑆,1 + b1𝑃𝑆,2 − 2

√︁b1𝑃𝑆,1𝑃𝑆,2 sin(Δ\12)

]𝑃𝑂,2 =

Z12

[𝑃𝑆,1 + b1𝑃𝑆,2 + 2

√︁b1𝑃𝑆,1𝑃𝑆,2 sin(Δ\12)

]𝑃𝑂,3 =

Z22

[b2𝑃𝑂,1 + 𝑃𝑆,3 − 2

√︁b2𝑃𝑂,1𝑃𝑆,3 sin(Δ\123)

]𝑃𝑂,4 =

Z22

[b2𝑃𝑂,1 + 𝑃𝑆,3 + 2

√︁b2𝑃𝑂,1𝑃𝑆,3 sin(Δ\123)

].

(4)The second outputs of the two couplers (𝑃𝑂,2 and 𝑃𝑂,4)

are directed to the photodiodes. When the phase differencesbetween the two inputs of each coupler are 3

2𝜋 + 2𝜋𝑛, for 𝑛

an integer, the first outputs of each coupler will be maximum

3

achieving coherent optical combination into one single-modefiber output. This gives

𝑃𝑂,1 =Z12

(√︁𝑃𝑆,1 +

√︁b1𝑃𝑆,2

)2

𝑃𝑂,2 =Z12

(√︁𝑃𝑆,1 −

√︁b1𝑃𝑆,2

)2

𝑃𝑂,3 =Z22

[√︂Z1b2𝑃𝑆,1

2+√︂

Z1b1b2𝑃𝑆,2

2+√︁𝑃𝑆,3

]2

𝑃𝑂,4 =Z22

[√︂Z1b2𝑃𝑆,1

2+√︂

Z1b1b2𝑃𝑆,2

2−√︁𝑃𝑆,3

]2

.

(5)

Incidentally, 𝑃𝑂,2 and 𝑃𝑂,4 are used to control the phaseof the first and second phase shifters (PS-1 and PS-2) respec-tively, and 𝑃𝑂,3 is the final coherently combined single-modesignal output. From this evaluation it can be stated that thecoherent synthesis efficiency ([) is thus

[ =𝑃𝑂,3

𝑃𝑆,1 + 𝑃𝑆,2 + 𝑃𝑆,3. (6)

III. EXPERIMENTAL WORK

The experimental assembly of the all-fiber CBC photoniclantern optical receiver is shown in Fig. 2. A 1550 nmwavelength laser was used as the source. The emitting lens(Lens 1) had a diameter of 5 cm, and the receiving lens (Lens2) of 5 mm. The spot diameter of the laser on the receivinglens was 3 cm, which is larger than the lens itself to simulatethe beam divergence that would arise in applications in whichthe signal travels over long distances like in FSOC and lasersatellite communications.

The 3×1 photonic lantern was made with Corning SMF-28fibers at the Sydney Astrophotonic Instrumentation Laboratoryat The University of Sydney. The few-mode end of thephotonic lantern (i.e. input aperture) had a 0.15 𝑁𝐴 and a corediameter of 18 `m with 0.4 dB insertion losses in the SMF tofew-mode direction. The 3-dB fiber couplers used were 2x2optical fused couplers with a loss of 0.3 dB. The phase shiftersused for phase compensation were fibre-based FPS-002 fromGeneral Photonics and had an insertion loss of 1.5 dB witha maximum phase compensation value of 65𝜋 at 1550 nm.Thorlabs APD430C photodiodes were used, and the centralprocessing unit of the digital signal processing module usedfor data acquisition and phase control was a Texas Instrumentsmodel TMS320C28346. The analog to digital converter unit ofthe digital signal processing module was an Analog Devicesmodel AD7606, and the digital to analog converter unit of thedigital signal processing module was the AD5344 model.

A. Without turbulence

The performance of the photonic lantern optical receiverwas first verified in the absence of atmospheric turbulence,and the results of the optical power outputs are shown inFigure 3. When the lantern is kept stationary and there is noatmospheric turbulence, the optical powers of the single-modeoutputs do not change [4] as expected. When the beams werecombined without phase compensation (i.e. not coherently)

the final average optical power was 𝑃𝑂,3 = 3.47 μW witha variance of 4.6853 as shown in Figure 3(d). This givesan efficiency of [ = 0.23. When the beams were coherentlycombined by using the adaptive optics phase shifter controllersin a closed loop configuration, the average power increased to𝑃𝑂,3 = 9.30 μW, increasing the efficiency to [ = 0.62, and thevariance decreased to 0.0739. This confirms the effectivenessof the all-fiber photonic lantern based optical CBC approach.

Taking the single-mode output powers of the lantern andthe loss of the various components given above into account,Eqn. (6) gives the theoretical power value of the coherentlycombined beams as 𝑃𝑂,3 = 10.17 μW. The measured outputpower is 91% of this theoretical value, we conjecture thatand the discrepancy is attributed to possible mismatches inthe polarisation of the combined beams (as the output of thelantern used SMF-28 fibre that is not polarisation maintaining).

In an ideal case where the insertion loss of all devicesare eliminated, Eqn. (6) gives the output values as 𝑃𝑂,2 =

0.03 μW, 𝑃𝑂,3 = 14.93 μW, 𝑃𝑂,4 = 0.13 μW, leading to atheoretical efficiency of [ = 0.99.

B. With turbulence

We used a rotating phase screen to simulate the atmosphericturbulence in order to verify the performance of the photoniclantern receiver. The atmospheric coherent length of the phasescreen is 𝑟𝑂 = 3 mm running at the Greenwood frequencyof the system of 𝑓𝐺 = 60 Hz with an aperture of 5 mm(Lens 2), giving us a 𝐷/𝑟𝑂 = 1.7 which corresponds to weakturbulence. The bandwidth of the receiver and the number ofmodes supported on our photonic lantern proof of conceptoptical receiver were limited, hence we worked at the weakturbulence regime to verify the feasibility of the receiver, andthe experimental results are shown in Fig. 4. Results show thatthe power of the single mode outputs of the lantern fluctuateddue to the variation of the input mode profile, caused bythe atmospheric turbulence. When the beams were combinedwithout phase compensation the final average optical powerwas 𝑃𝑂,3 = 3.58 μW, with a variance of 5.9839. This gives amean efficiency of [ = 0.27. When the beams were coherentlycombined the average power increased to 𝑃𝑂,3 = 8.58 μW,increasing the mean efficiency to [ = 0.64, and the variancedecreased to 2.1646. This confirms the effectiveness of ourproof of concept photonic lantern CBC receiver even underconditions of atmospheric turbulence.

Taking all the known losses into account, the theoreticaloutput is 𝑃𝑂,3 = 9.23 μW. The experimental value is 93% ofthis amount. The ideal case where all loss is eliminated wouldproduce 𝑃𝑂,2 = 0.06 μW, 𝑃𝑂,3 = 13.36 μW, 𝑃𝑂,4 = 0.02 μW,and an efficiency of [ = 0.99. Although the absolute valuesof these results under weak turbulence are very similar tothose without turbulence, the variance values are much highershowing a strong temporal response in overall power outputsas shown in Fig. 4.

IV. ANALYSIS OF THE SNR PERFORMANCE

An essential figure of merit to evaluate in any opticalreceiver for FSOC and laser satellite communications is the

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Fig. 2: a) Schematic diagram of the PL-based optical receiver using the method of the CBC. b) (left) Experimental configurationof the PL-based optical receiver using the method of CBC. (right) Microscope image of the few-mode end of the 3x1 PL usedin the experiment.

signal to noise ratio of the system. Here we consider theSNR of the receiver under various arrangements and a binaryphase-shift keying (BPSK) modulation, which is a two-phasemodulation scheme.

A. Optical and Digital coherent beam combining SNR deriva-tion

In the CBC method used here, the final combined single-mode signal beam with power 𝑃𝑂,3 is mixed with the single-mode local oscillator beam with power 𝑃𝐿𝑂 in the SMF mixer[9]. The beam, after optical mixing, undergoes photoelectricconversion through the balanced photodetector. For simplicityin our evaluation model we assume that the dark current of thebalanced photodetector is negligible and that 𝑃𝐿𝑂 � 𝑃𝑂,3.Under these conditions the shot noise should become thedominant source of noise, and other sources of noise can beignored. When a heterodyne detection architecture is used,the SNR of the this optical domain coherent beam combining(CBC) arrangement SNRCBC can be estimated by

SNRCBC =𝑅𝑃𝑂,3

𝑞𝐵(7)

where 𝑅 is the responsivity of the photodiode, 𝑞 is theelectronic charge and 𝐵 the noise equivalent bandwidth of thedetector.

In the DCC case of separately measuring the single-modeoutputs of the lantern and combining the signals electronically,the signal beam at the 𝑖th SMF end of the lantern has power𝑃𝑆,𝑖 and is mixed with the 𝑖th local oscillator beam in the

SMF mixer [9]. The total oscillator beam power 𝑃𝐿𝑂 isdivided equally amongst the 3 outputs of the lantern and eachbeam after optical mixing undergoes photoelectric conversionthrough balanced photodetectors. We assume, as above, thatthe shot noise is the dominant source of noise.

In this scheme, the electrical signals from all 3 photode-tectors are summed with equal weight. When heterodynedetection is used, the SNR of this electrical combining usingthe DCC method SNRDDC is given by[2]

SNRDCC =𝑅[

√︁𝑃𝑆,1 +

√︁𝑃𝑆,2 +

√︁𝑃𝑆,3]2

3𝑞𝐵. (8)

B. SNR comparison and discussion of results

Considering an ideal case of optical combining, with anefficiency of [ = 100% gives 𝑃𝑂,3 = 𝑃𝑆,1 + 𝑃𝑆,2 + 𝑃𝑆,3, and

SNRCBC =𝑅[𝑃𝑆,1 + 𝑃𝑆,2 + 𝑃𝑆,3]

𝑞𝐵(9)

in the present case of three outputs. Generalising to 𝑁 outputsgives

SNRCBC =𝑅[𝑃𝑆,1 + 𝑃𝑆,2 + · · · + 𝑃𝑆,𝑁 ]

𝑞𝐵(10)

for optical combining and

SNRDCC =𝑅[

√︁𝑃𝑆,1 +

√︁𝑃𝑆,2 + · · · +

√︁𝑃𝑆,𝑁 ]2

𝑁𝑞𝐵. (11)

for the electrical combining.

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Fig. 3: Experimental results of the optical powers in theabsence of turbulence (a) 𝑃𝑆,1; (b) 𝑃𝑆,2; (c) 𝑃𝑆,3; (d) 𝑃𝑂,3(open loop - beams combined but not coherently); (e) 𝑃𝑂,3(closed loop - beams coherently combined); (f) Means andvariances of the optical powers in the case of atmosphericturbulence and combining efficiency.

Equations (10) and (11) show that SNRCBC ≥ SNRDCC. Ifthe input is already diffraction limited (i.e. single-mode, in theabsence of atmospheric turbulence), the electrical combiningcan reduce SNR whereas the optical combining approach willnot offer a significant advantage in the case of a stable unper-turbed diffraction-limited wavefront. Furthermore, equations(10) and (11) can be applied to calculate the SNR for theexperimental results obtained with and without turbulence,shown in Figures 3 and 4. The comparison shown in Fig. 5(b)assumes an ideal case of no power loss, and shows that theoptically combined beams result in a higher SNR as expected.Figure 5(a) shows how the efficiency changes as a function ofthe relative strength of 𝑃𝑆,1, 𝑃𝑆,2, and 𝑃𝑆,3, as calculated usingEqn. (6) assuming all losses are eliminated. The idealisedexperimental results with/without turbulence are also shown.We see that when 𝑃𝑆,1/𝑃𝑆,3 < 2.5 and 𝑃𝑆,2/𝑃𝑆,3 < 2.5,as long as 𝑃𝑆,1, 𝑃𝑆,2 and 𝑃𝑆,3 are not very large or verysmall, the coherence efficiency is easily over 90%. When𝑃𝑆,1/𝑃𝑆,3 ≈ 0.5 and 𝑃𝑆,2/𝑃𝑆,3 ≈ 0.5, the coherence efficiencycan reach 99%.

In our 3 x 1 photonic lantern optical coherent receiver casestudy, the SNR improvement is moderate and the coherentefficiency greatly determined by the single-mode power dis-tributions. This can be explained by the limited number ofmodes and single-mode outputs in our proof of concept case.It is understood, that the power distribution at the single-mode

Fig. 4: Experimental results of the optical powers in the case ofatmospheric turbulence. (a) 𝑃𝑆,1; (b) 𝑃𝑆,2; (c) 𝑃𝑆,3; (d) 𝑃𝑂,3(open loop); (e) 𝑃𝑂,3 (closed loop). (f) Means and variancesof the optical powers in the case of atmospheric turbulenceand combining efficiency.

end of the lantern varies according to the input mode profile[4]. Furthermore, particularly in the case of turbulence, forthe electronic combination scheme the SNRDDC is calculatedbased on average values of the single-mode output powers.This will mask the issue of data lost due to poor SNRvariations over time. Hence, an additional consideration tothe SNR evaluation between the optical combination and theelectronic combination architectures is the effect of differentoutput power distribution amongst the single-mode end of thelantern on the overall SNR and efficiency of the coherentcombination. We see in Eqn. (10) and Eqn. (11) that the SNRfor the DCC case is always lower than for the CBC apartfrom when the power at the lantern outputs is equal. However,this is highly unlikely to occur in a real case scenario andis not observed in the present results, as seen in Fig. 5b. Toillustrate and evaluate this point, Fig. 6 shows an analysis for alantern with seven outputs (7 x 1 photonic lantern). In the casewhere the power is distributed equally amongst the outputs(corresponding to Trial 12), the DCC and CBC approachesgive the same SNR (a ratio of 𝑆𝑁𝑅𝐷𝐶𝐶/𝑆𝑁𝑅𝐶𝐵𝐶 = 1).However, if all the power is concentrated equally into twoof the seven outputs (corresponding to Trial 1), then SNR ofthe DCC case reduces to only 40% of the CBC case. Thisillustrates the superiority of the CBC approach, particularlyunder turbulent conditions when power between the lantern’soutputs is expected to fluctuate over time.

6

Fig. 5: (a) The relationship between the coherent synthesis efficiency and 𝑃𝑆,1, 𝑃𝑆,2, 𝑃𝑆,3. (b) Calculated SNR from the outputsof the receiver with and without turbulence, considered using optical and electrical combining.

123456789101112

123456789101112

1 2 3 4 5 6 7Fibre Output

0.4

0.5

0.6

0.7

0.8

0.9

SNR DC

C/S

NR

CBC

Tria

l

0 0.1 0.2 0.3 0.4 0.5Power in Fibre

Fig. 6: Illustration of difference in the SNR between DCC and CBC depending on the power distribution in the output portsof the PL. Trial 1 and 12 show two special cases where all the light is in two fibres and all the light is evenly distributedbetween the fibres respectively. In the case of trial 12, the SNR is identical for both DCC and CBC. In all other cases theSNR for the CBC case is greater than the SNR for the DCC case. The diagram is generated using Eqn. (10) and Eqn. (11).

V. CONCLUSION

We proposed a photonic lantern-based multimode opticalreceiver based on coherent beam combining in optical fibers.Compared to existing DCC technology, CBC results in ahigher SNR under realistic conditions of turbulence. Weverified the feasibility of the CBC receiver using a lanternwith only three outputs as a proof of concept, active phaselocking and 3-dB fiber couplers. The experimental resultsshowed that the coherent synthesis efficiency is 62%. Also,we used a phase screen to simulate atmospheric turbulence toverify the performance of the optical receiver and found thatthe coherent synthesis efficiency is 64% under atmospheric tur-bulence. Furthermore, the receiver performance can be furtherimproved by reducing device losses in the system and opticalcomponents. In an ideal case of negligible device losses, thecoherent synthesis efficiency could be as high as 99% in boththe presence and absence of turbulence. A theoretical analysis

and numerical modelling of the SNR in a 7 x 1 photoniclantern showed that the power distribution across the single-mode outputs of the lantern played an important role. TheSNR of the CBC approach remains constant whereas the SNRof the DCC approach decreases as the difference in poweramongst the single-mode outputs increases. This is indeed animportant factor in the presence of atmospheric turbulence andtip/tilt errors where the output of the single-mode fibers of thelantern will vary significantly with the changes in input beam.By taking this approach of active phase control in the opticalcombining system to increase SNR, this receiver representsa very versatile in-fibre approach to adaptive optics, withapplications in free space optical communications and LIDAR.

ACKNOWLEDGEMENT

Bo Zhang acknowledges the support from the UCAS JointPhD Training Program.

7

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