44 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-5, NO. 1, JANUARY 1987

Recent Advances in Coherent Optical Fiber Communication Systems

TAKANORI OKOSHI, FELLOW, IEEE

Abstruct-Research and development of coherent optical fiber com- munications have been accelerated mainly because of the possibility of receiver sensitivity improvement reaching 20 dB, and partly because of the possibility of frequency-division multiplexing (FDM) with very fine frequency separation. In this paper, recent advances in the re- search on coherent optical fiber communication systems are reviewed, with emphasis on those reported in the past two years. The bit-error rate measurements so far reported are classified and investigated in four categories: PCM-ASK, PCM-FSK, PCM-PSK, and PCM-DPSK. The states-of-the-art of polarization-state stabilization techniques is also discussed.

S I. INTRODUCTION

INCE the earliest papers on heterodyne (or homodyne) and coherent optical fiber communications appeared in

1979 in Japanese [ l ] , [2] and in 1980 in English [3]-[5] , the research in this area has expanded year by year. It is now performed in almost all of the major communications laboratories in the world. This is mainly because of the possibility of receiver-sensitivity improvement reaching 20 dB [5]-[7] and partly because of the possibility of fre- quency-division multiplexing (FDM) with very fine fre- quency separation, typically 10-100 GHz [3].

The research on the heterodyne(homodyne)/coherent optical fiber communications can be classified into three categories:

Theoretical analysis of the system including the transmitter, fiber waveguide, and receiver. The noise phenomena in the system, bit-error rate (BER) im- provement, and polarization fluctuation in the fiber lightguide have been the principal concern of the re- searchers. Research and development of special devices re- quired in the coherent systems, such as frequency- stabilized and spectrum-purified lasers, polariza- tion-maintaining fibers, polarization-state control- lers, coherent modulators, heterodyne or homodyne detectors, and demodulators. System experiments including bit-error rate (BER)

Manuscript received November 15, 1985; revised June 19, 1986. This paper describes the content of the presentation of an invited review paper read at IOOCIECOC’85, Venezia, Italy, in October 1984, by the same author and with same title. Figures, data, and references are supplemented to those in the Conference Proceedings.

The author is with the Department of Electronic Engineering, Univer- sity of Tokyo, Bunkyo-ku, Tokyo 113, Japan.

IEEE Log Number 8611060.

measurements and the development of polarization- control schemes.

In the short history of the research in this area in 1979- 1985, efforts were first paid to the theoretical system anal- ysis (item l)), and then to the improvement of the devices (item 2)). On the other hand, since late in 1981, reports began to appear on the bit-error rate (BER) measurement of heterodyne(homodyne)/coherent optical fiber commu- nication systems, taking advantage of the device technol- ogies newly developed for such systems. In the same pe- riod, various polarization-control schemes were also developed.

The purpose of this paper is to review the recent ad- vances in such systems research. The progress of the rel- evant device technologies have been reviewed in many tutorial/review papers [8]-[15], whereas the progress in the relevant BER theories and BER measurements have been summarized also in some of the previous review pa- pers [5]-[7], [15]-[17]. Therefore, as to the BER mea- surements, emphasis will be placed upon the achieve- ments in the past two years. Recent achievements in the polarization-control schemes will also be described, but rather briefly because of space limitations.

11. DEFINITION OF “COHERENT” COMMUNICATIONS

The term “coherent” in the title of this paper needs explanation. Presently the term “coherent” seems to be used in two different meanings. In the first meaning, a PCM-on-off keying (OOK) heterodyne system is classi- fied in “noncoherent heterodyne” systems, whereas a PCM-phase-shift keying (PSK) heterodyne system is clas- sified in “coherent heterodyne” systems. In the second meaning, the PCM-OOK heterodyne system is also con- sidered as “coherent” because, on the surface of the fre- quency mixing diode in this system, we take advantage of the spatial coherence of the carrier. In the title of this paper, the word “coherent” is used in the second mean- ing; i.e., we consider that all heterodyne and homodyne schemes are coherent schemes.

111. IMPROVEMENT OF EQUIVALENT RECEIVER SENSITIVITY BY COHERENT MODULATION/

DEMODULATION SCHEMES

The greatest advantage of the coherent schemes is the improvement of the equivalent receiver sensitivity, that

0733-8724/87/0100-0044$01 .OO O 1987 IEEE

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OKOSHI: ADVANCES IN COHERENT OPTICAL FIBER COMMUNICATION SYSTEMS 45

IMIDi rect Detect ion

! 10 I 2548 ilnprovement

OOK (On-Off Keying) Heterodyne

I 3dB

5dB \ 8-level FSK Het .

c PSK Heterodyne + ‘SK

3dB Homodyne

Fig. 1 . Receiver-sensitivity improvement by various heterodyne (or ho- modyne) and coherent modulationidemodulation schemes.

is, the reduction of the minimum receiving signal level for achieving a prescribed BER, for example, lo-’.

As has been described in detail in previous papers (e.g., [ 17]), this sensitivity improvement is attributed to two ef- fects. One is the improvement of the S/N at the output end of the receiver preamplifier (for a given signal level) by the use of the heterodyne or homodyne detection, as com- pared with a direct detection. The amount of this sensitiv- ity improvement is typically 10-20 dB at wavelengths of 1.3-1.6 pm [6], [17].

The other effect is the improvement of the BER (for a given S / N ) brought about by the use of a coherent mod- ulation/demodulation scheme (PSK and FSK) as com- pared with a noncoherent (OOK) scheme. The amount of this improvement reaches 3-9 dB, depending upon the type of modulation and demodulation [6], [7], [17] , as shown in Fig. 1.

IV. FREQUENCY-SELECTIVITY IMPROVEMENT BY

HETERODYNING The second advantage of a heterodyne (or homodyne)

system is that the frequency selectivity can be improved because of the good frequency selectivity of the interme- diate frequency (IF) amplifier, which is much sharper than that of an optical filter. Thus, FDM schemes with very fine carrier separation become possible [3]. This allows, for example, an efficient use of the minimum dispersion wavelength region (1.3-pm region) of silica-glass optical fibers.

However, in the present state-of-the-art, this advantage can not be emphasized for a long-haul telecommunication system, because the optical power loss in the optical mul- tiplexer at the transmitting end and demultiplexer at the receiving end will not be negligible, when the frequency separation between carriers ‘is small.

Fig. 2 illustrates the problem. Fig. 2(a) shows an ex- ample of the carrier frequency allotment in a supenvide- band FDM system discussed in one of the earliest papers [ 11. In this case ten carrier frequencies are arranged with 100 GHz separation in each carrier group (corresponding to the passband of an optical branching filter) having 2- THz bandwidth. Fig. 2(b) shows how the ten carriers are divided into ten heterodyne receivers. If the power divider

4 rlO0GHz 1

I I I

2-

Fig. 2. (a) An example of carrier frequency allotment in an FDM system using heterodyne receiving technique. (b) Power-dividing circuit in a heterodyne FDM receiver.

in Fig. 2(b) has no frequency selectivity, all the signals will undergo an appreciable branching loss, i.e., at least 10-dB loss when ten carriers are divided into ten receiv- ers.

Such a branching loss is more or less inevitable in the present state-of-the-art. In the near future, however, op- tical branching filters with 100-GHz separation (or even 10-GHz separation) might become possible by new schemes such as fiber-type double-path filters [ 181.

Some specialists assert, on the other hand, that we may take advantage of the fine frequency selectivity of the het- erodyne system in some applications even within the pre- sent state-of-the-art; these applications are supenvide band local area networks (LAN’s), CATV networks, and inte- grated-services subscriber networks to be realized in the future. Note that the multiplexing/demultiplexing loss is not very serious in such short-haul systems.

V. BIT-ERROR RATE (BER) MEASUREMENTS The first report of BER measurement appeared in 1981.

Since then more than twenty papers describing BER mea- surements have been published; Tables I-IV summarizes these reports in four categories: PCM-ASK, PCM-FSK, PCM-PSK, and PCM-DPSK, each in chronological or- ders. Table V summarizes these reports to show the gen- eral trends in the research. The investigation of the re- ports may not be thorough, particularly for 1984 and 1985. A remarkable fact, however, is that the researchers’ con- cern has been moving from ASK to more sophisticated FSK, PSK, and DPSK.

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46 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-5, NO. I , JANUARY 1987

TABLE I BIT-ERROR RATE MEASUREMENTS OF ASK HETERODYNE/HOMODYNE

SYSTEMS (1981-1985)

No. A 1 A2 A3 A4 A 5 A€

Yew

Q.E. Let t . (Cannesi t ron.Lect . 3 . L.T. t;on.Lett. o r Conf. IEEE J. Els~trm. ECOC EZOC/Elec- IEBE/OSP. OFC/?lec- Journal

Sectenbe? Ss?t. /Ocz. Decemoer Jan./F.o. December Jane Month 1981 1982 1982 1932 1983 1984

Der' Si-PO InGaAs-PIN Si-?3 InCa.As-?:N :nCaA-?IN Gr-:?D

LO Self- HeNe-GL S e l f - HeNe-Gi HeNe-GL DF3-LD (PINPET) (PINFI?)

heterodyne he te rodyre

Technlca; 1F:rst fJt F i r s t BER s i g n r f l - meas. meas. of

cance ,het.sysiem homodyne ( se l f -he t . sys t em method I

R e s u l t i d 3 d i f i - lOdB ~ n - ierence provenenr: from oyer 3.3. i t heo ry

TABLE I1 BIT-ERROR RATE MEASUREMENTS OF FSK HETERODYNE SYSTEMS (1981-

1985). A PCM-PSK-FM EXPERIMEKT IS INCLUDED. -

No. / F l E2 F3(=?5) F4 F5 F6

Y e w

L e t t . 3 . E . z c o c ' s a t r o n . L e = t . Let:. G E I V ~ ~ L C ~ ) or Canf. E lec t ron . I E E E J. O F C ' 8 4 / EC3C/Elec- E lec t ron . IOOC/ECOC J o u r n a l

193n 1984 19es May February Jan./Se?c. Sept./Nosv. Octooer October Month 1982 1933 1984

~~~~ ~~

Elze iatlOn 1:: Kimura F u ~ i t a Hodgrlnson Standley

Authors S a l t 0 S a i t o Bacnus Esura Wyart Vodhanel

Orzani- Musashlno Musasnino Heinr lch- Nippon E l . Brl t .Tele- Bell Zomir.

System FSK FSK PSd-FM 7% FSL F jR

Yamamoto Yamamota Bohnke Shlkada D.l .Sn;th Ginlnt

e t a l . P t a l . e t al. Cheung

ECL H e r t z I n s t Company com.R.L. Research

Heterodyne Heterodyne Heterodyne Heterodyne Reterodynr Hezerocyne

wavelength

B l t Rate

In t . Freq .

I F E.Y.

Distance

Devlce5 TX

325nm 8ahm 83onn

100Mb/s 100/200:4bk 7QMb/s

1,70O1&% 1,700MHz 1,500MHz

700MHz 700MHz 9OGMHr

0 0 2aom

A l G a A s - X AiGaAs-LO AXaAs-LD

Mod D i r e c t D l r e c t D l r e c t

o e t G&S-APD G~AS-APD AeD

LO AlGaPs-LD AlGaAs-LD AlGaAs-LD SI-APD

1570nm l5LOnn l5SOnm

100MWs InOMbls 560Yb/s

500wz ZlOMHz -- uaoowz --- 10Sxm 200kn ---

__

In the following, the technical significance of these pa- pers, in particular those appearing in the past 24 months, are described. Papers published before the middle of 1983 have been tabulated, and their significance discussed, in another review paper [ 171.

A. BER Measurements of ASK Heterodyne/Homodyne Systems (Table I)

Significant recent trends (after 1983) are the uses of LiNb03 waveguide-type external modulators in [A5], [A6], and the use of DFB laser diodes both as the trans- mitter and the local oscillator. These new trends have al- together brought forth high bit-rate modulation and long transmission distance. However, the number of the re- ports in this category is decreasing.

B. BER Measurement of FSK Heterodyne Systems Including a PSK-FM System (Table 11)

In this category many reports have appeared in the past two years. A PCM-PSK-FM system using a 280 MHz subcarrier [F3] seems to aim at a preliminary experiment for FDM wideband LAN's to appear in future. Direct frequency modulation of a DFB laser diode [A41 or a dou- ble-channel planar buried heterojunction (DCPBH) laser diode with an extended (10 cm long) cavity resonator [A51

are new trends aiming at the dynamically single-mode di- rectly-modulated transmitter.

The use of a new scheme called single-filter FM detec- tion [F4], the achievement of 200-km transmission dis- tance [F5], and BER measurement at a very high bit-rate (560 Mbit/s; [F6]) are the noticeable new achievements.

After Table I1 was completed, another latest report on BER measurement on an FSK system from Yokosuka ECL, NTT, drew the author's attention [F7]. It describes a BER measurement using 1.5-pm DFB laser diodes with extended cavities, 8 cm and 10 cm long for the transmitter and local oscillator, respectively.

C. BER Measurements of PSK Heterodyne/Homodyne Systems {Table 111)

Two of the recent papers [P4], [P6] dealt with PSK homodyne systems. The technical tasks are fairly differ- ent for the PSK heterodyne and PSK homodyne systems, because an optical phase-locked loop (OPLL) technique is needed in the homodyne systems, whereas the phase locking can be achieved at the intermediate-frequency stage in heterodyne systems.

The paper given in [P4] was the first to report the ho- modyne detection of a signal transmitted over a long dis- tance (30 km), but using HeNe gas lasers for both the

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OKOSHI: ADVANCES 1N COHERENT OPTICAL FIBER COMMUNICATION SYSTEMS 47

TABLE 111 BIT-ERROR RATE MEASUREMENTS OF PSK HETERODYNEIHOMODYNE

SYSTEMS (1981-1985)

Ye-

Lett. Lett. Lett. Ga?,eva ECOC'81 StUtfzBr: or Canf. Electron. Eieczron. E1ec::ar.. ECOC'83 0FC'84/ EC2C'Sn J o u r n a l

198d lDB4 Februarf May J u l y O c r m e r Jan./Sr?t. S a z c e ~ o e r Month 1283 1983 1983 18-32

Wavelength

IF B . Y .

0 40MHr 2lOMHz 2 l W H z 1.53OXHr 0 Int. Freq.

14OW;s 1Mb/s 140Mb/s ldCXb/s 70MbIs l40Xb/s Bit Race

1520nm 84Onn 1520nm 152Cnm 820nrn 10.500m

0 @ 102km 2@km 290m @ D15tanCB

-- 2MHZ _-- -- 900I.IHz ---

Devices TX HeNe-Gi AiGaAs-LD HeNe-GL HeNe-GL AiGaiis-LD C02 laser

Mod LiNb03 A.O.Mad. L1NbO3 LINb03 OlreCt A.O.MOd.

Det InGas-PIN Si-?D Incas-PIN InCaAs-PIN A?D L(gCdTe-?D

LO self- Self- I~G~AsP-LD ~eUe-2~ AiGaAs-20 CO, laser !PINfXTj !PIAFZT) (PINFZTI

lhomodyne heterodyne e Cavity

TABLE IV BIT-ERROR RATE MEASUREMENTS OF DPSK HETERODYNE SYSTEMS (1982-

1985)

~

No. I T 1 02 D3!=?31 Gd 05 36

Year Mircn October October June July June Month

lsaa 1983 1983 1983 1985

Lett. Tokyo Lett. Tokyo IECE J a p a n Venlce or Conf. Elec t ron . IOOC'82 Elec t ron . IOOC'83 Nat.Eonv. ECOC'85 Journa l

1985

Authors favre Shrkada Wyatt D.W.Smlm Enura Emura Le Guen Emura Hodgklnson Yamazakl Shlraca

Mxemura G.W.Smlth Shikada Yanazakr et a l . e: ai.

C.N.E.T. Nlppon El. Bnt.Tele- Brrt.Teie- Nippon El. Nippon El. zation cornoany c m . 3 . ~ . CW.R.L. comoany company

System DPSX DPSK PSKDPSK DPSa(ara DPSX 0PSX Hererodyne Heterocyne Heterodyne ASioher. Heceroayde Heterodyne

~~ ~~

transmitter and local oscillator. The homodyne detection of a signal transmitted over a long distance has not suc- cessfully been achieved with semiconductor lasers.

D. BER Measurement of DPSK Heterodyne Systems (Table ZV)

Improvement of spectral purity of lasers (transmitter and local oscillator) by extended-cavity (30 cm long) scheme [D5], and experiment at very high bit-rate (400 Mbit/s; [D6]), are the noticeable new achievements.

VI. SPECTRAL PURITY OF LIGHT SOURCES

As has been predicted by theory (see Fig. l) , the PSK homodyne system has exhibited the highest receiver sen- sitivity, i.e., the lowest signal level to achieve a pre- scribed BER. The greatest sensitivity improvement so far observed reaches 19 dB [Pl] with a PCM-PSK "self- homodyne" setup, i.e., using a single HeNe gas laser as both the transmitter and local oscillator.

However, it is speculated that gas lasers would not be used in practical systems because of their large size, low efficiency and short life. Semiconductor lasers will prob- ably be used despite their relatively poor spectral purity, i.e., large linewidth. This speculation has stimulated the

TABLE V GENERAL TRENDS IN BER-MEASUREMENTS REPORTED IN 1981-1985

T U . T o k y o

1 9 8 2

BTRL U . T o k y o BTRL

E C L ( M )

PSR I DPSK 1 I CNET

E C L ( M )

BTRL U . T o k y o BTRL BTRL

NEC BTRL BTRL

E C L ( Y ) BTRL

TU Wien

So far the BER theory of coherent optical communica- tions has been advancing in the following three steps: 1) BER theories assuming purely monochromatic carrier (- 1980) [5]-[7], 2) BER theories considering linewidth of carrier (and local oscillator) originating from white fre- quency-noise spectrum ( - 1983) [P2] [19], and 3) BER theories Considering also the effect of l l f frequency-noise

researchon the effect of phase noise of lasers on the BER. spectrum ( - 1985)-[20].

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48 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-5, NO. 1, JANUARY 1987

In the above second stage, the estimated requirement for the linewidth was severe; it was estimated that the linewidth must typically be less than 1 MHz for a PCM- PSK system to achieve BER = at a bit rate of 32 Mbit/s. However, it was found recently that the spectral broadening of DFB semiconductor lasers operating at high power level is principally due to the llf-type FM noise [20]. It is also predicted theoretically that when the 1/ f- type FM noise is predominant, the apparent linewidth re- quirement is much lightened (e.g., one order of magni- tude). This is because when the linewidth is mainly due to low-frequency FM noise, its effect is much less harm- ful than an equal linewidth produced by a white FM noise. Moreover, the linewidth could also be reduced by feed- back techniques when low-frequency component is pre- dominant.

VII. POLARIZATION-STATE CONTROL SCHEMES FOR HETERODYNE OR HOMODYNE OPTICAL FIBER

COMMUNICATIONS

A . Countermeasures Against Polarization-State Fluctuation

In an optical heterodyne (or homodyne) receiver, and/ or when an optical IC is used in the receiver, the fluctua- tion of the polarization state in the fiber may deteriorate the receiver sensitivity. In such cases, therefore, a polar- ization-state control scheme is indispensable [12]. The complete solution to this problem will be the use of a po- larization-maintaining fiber over the entire length of the communication channel (see [2 11, and for a more up-to- date review [22]). However, the polarization-maintaining fiber is still technologically at a prematured stage, and can not be used in practical communications at reasonable cost.

Two alternative countermeasures are: 1) the use of a polarization-state control device at re-

ceiving end, which matches the local oscillator (LO) po- larization state with that of the signal [23], and

2) the use of a polarization-diversity receiver, in which two orthogonal polarization components of the received signal are detected separately and added later (in a het- erodyne receiver, at its intermediate frequency (IF) stage) after an appropriate phase compensation [24].

In addition, various versions and combinations of these two basic schemes are being investigated.

In this paper various polarization-state control schemes proposed so far are compared, and their features are com- pared.

B. Classijication and Principles of Various Polarization-State Control Schemes

All of the polarization-state control schemes so far pro-, posed consist basically of two controlling elements, be- cause the number of freedom of a polarization state is two, i.e., the ellipticity and deflection angle. The controlling elements used in the six schemes proposed so far are, in a chronological order, electromagnetic fiber squeezers

TABLE VI

PAR (after Okoshi [ 2 3 ] ) FEATURES OF SIX POLARIZATIOK-STATE CONTROL SCHEMES PROPOSED SO

Poiarlzation-state control schemes

Insersion Endlessness Temporal MechanlcaL loss in controi ressonse fatxgue

Fiber squeezers Law No Medlurn Present (Ulrlch)

Electrooptlc crystals High (Kubota et al.)

No Fast Absent

Rotataoie flber coils Low No slow Presect (Lefevrc 1

Phase plates Meoiurn Yes slow ( h a 1 et al. 1

Small

Faraday rotators LOW No Fast Absenr; (Okoshl. Cheng. et al.)

Rotatable f;ber cranks Low Yes (Okoshi, hkaya, et al.)

-. >LOW Present

[25], electrooptic crystals [26], [27], rotatable fiber coils [28], rotatable quarter-wave and half-wave plates [29], Faraday rotators [30], and rotatable fiber cranks (RFC's) 1311. These six schemes are tabulated in Table VI.

1. Electromagnetic Fiber Squeezers: In the first pro- posal of this sort, two electromagnetic fiber squeezers were used as the polarization-state conversion elements [25]. In this scheme two electromagnets give transverse stress to the fiber in the directions of 0" and 45", so that the polarization state becomes (for example) horizontally polarized at the exit of the second squeezer. The control signals for the electromagnets are obtained from four po- larization components (Oo , 4 5 " , 90°, and 135") measured at the exit; these are used to compute the currents to be fed to the electromagnets.

2. Electrooptic Crystals: Shortly after Ulrich's pro- posal, Kubota et al. reported an electrooptic equivalence of the electromagnetic fiber squeezers. In this scheme [26], [27], the first electrooptic crystal converts, by con- trolling the voltage applied to the crystal (and hence its birefringence), the incident polarization state to an "up- right" elliptical polarization. Next, by controlling the bi- refringence of the second electrooptic crystal which is tilted by 45 " , we can convert the polarization state to a linear polarization in the horizontal or vertical direction.

3. Rotatable Fiber Coils: The device invented by Le- fevre [28], called here tentatively rotatable fiber coils, is based upon a somewhat different principle. The first coil gives, by bend-induced birefringence, a 90" phase differ- ence to two orthogonal modes, whereas the second coil gives a 180" phase difference. Any elliptical polarization can first be converted, by adjusting the tilt angle of the first coil, to a tilted linear polarization. It can then be ro- tated to another linear polarization with an arbitrary angle by controlling the tilt angle of the second coil, because a rotatable 180" phase plate functions as a simple deflec- tion-angle rotator for a linearly polarized incident light.

4. Rotatable Phase Plates: Recently Imai et al. [29] reported a new scheme in which quarter-wave and half- wave plates are used in place of the rotatable fiber coils.

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OKOSHI: ADVANCES IN COHERENT OPTICAL FIBER COMMUNICATION SYSTEMS 49

E

tb)

Fig. 3. A rotatable fiber crank (RFC) (a) Shape and dimensions,, (b) Driv- ing mechanism for “translation” movement.

The principle of its operation is entirely identical to that of the Lefevre’s device. However, the phase plates have a great advantage in that the rotation is endless, in con- trast to the limited tilt angle in the Lefevre’s device.

5 . Faraday Rotators: In the new scheme called Fara- day-rotator device [30], two Faraday rotators are con- nected in cascade, with a coil of fiber between them. When a light having an arbitrary elliptical polarization is inci- dent, we can set it upright at the exit of the first rotator by controlling the current flowing in it; Then the light passes through a fiber coil which gives a 90” phase dif- ference between the x- and y - ,polarizations to produce a linearly polarized light at its exit. Finally, this linear po- larization is rotated to horizontal (or vertical) polariza- tions at the exit of the second Faraday rotator by control- ling the current flowing in it.

6. Rotatable Fiber Cranks: In this scheme [31], the polarization-state control device consists of two fiber ele- ments, each of which is made by bending a short fiber section in a crank form, as shown in Fig. 3(a). This scheme features a negligibly small insertion loss and a capability of an endless control of polarization state like in the rotatable pnase plate.

If a short fiber is bent in a crank form (Fig. 3(a)), linear birefringence will be induced between x- and y-axes. When such a crank element is rotated around the axis A- C (see Fig. 3(b)) giving only “translation” movement to the fiber at point B , the principal axis of bending birefrin- gence also rotates without changing its magnitude. Hence, if we choose an appropriate fiber length 2E and shape so

that the bending birefringence .AB = d 2 or x, we can obtain a device equivalent to a quarter-wave plate or a half-wave plate, which have been used in the polariza- tion-state control scheme by Imai et al. [29].

C. Features of Various Polarization-State Control Schemes

It has been confirmed experimentally that all of the above schemes function satisfactorily as to the compen- sation of the polarization-state fluctuations. However, these schemes have different features as to the following technical requirements: 1) insertion loss, 2) endlessness in control, 3) temporal response, and 4) presence or ab- sence of mechanical fatigue.

1 . Insertion Loss: The requirement for low insertion loss can practically be satisfied only by all-fiber-type de- vices. This is because a polarization-state controller is most probably used in a single-mode fiber circuit. Only an all-fiber-type device can be coupled to input and output fibers with low insertion loss ( - 0.2 dB) by splicing.

2. Endlessness in Control: This is an important re- quiremefit, because “resetting” might become necessary in those attempts irl which the control range is limited. Among those devices so far reported only the phase-plate scheme and rotatable fiber cranks can satisfy this require- ment.

3. Temporal Response: All the mechanical schemes have poor temporal response as compared with all elec- tronic ones.

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50 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-5, NO. 1, JANUARY 1987

4. Presence or Absence of Mechanical Fatigue: All the mechanical schemes have more or less the possibility of mechanical fatigue.

The two schemes which have been proposed very re- cently [30], [31] feature the low insertion loss, because both of these are all-fiber devices. In addition, the Fara- day-rotator device features a relatively fast response, whereas the RFC device features the endlessness in con- trol as seen in Table VI.

VIII. CONCLUSIONS Recent advances in coherent optical fiber communica-

tion systems, in particular those in BER measurements and polarization-state stabilization techniques have been reviewed. Active research and development are still in progress; the state-of-the-art will again change drastically in coming few years. Probably the first application of the coherent optical fiber communications technology will be seen within several years, and it will become quite com- mon in the next century. Meanwhile, a number of by- products of the research toward the coherent schemes will be put into practical use in conventional optical fiber com- munications.

At IOOC-ECOC’85 at Venice, Italy, in October 1985, following the review talk upon which this paper is based, several papers were presented to report new experiments of coherent optical fiber communications [32]-[35]. Sig- nificant achievements were the transmission distance of 251 km at bit rate of 400 Mbit/s [33] and 150 km at 1 Gbit/s [34], and the achievement of bit-error rate of low9 with only 90 received photodbit [34]. However, the de- tails of these reports are omitted here and introduced else- where [36], together with discussion on technical limita- tions of the coherent schemes such as laser-noise problem.

REFERENCES [ l ] T . Okoshi, “Feasibility study of frequency-division multiplexing op-

tical fiber communication systems using optical heterodyne or ho- modyne schemes,” Pap. Tech. Group, IECE Japan, no. OQE78-139, Feb. 27, 1979 (in Japanese).

[2] Y. Yamamoto, “Study on optical digital modulation-demodulation systems,” Pap. Tech. Group, IECE Japan, no. CS79-144, Oct. 25, 1979 (in Japanese).

[3] T. Okoshi and K. Kikuchi, “Frequency stabilization of semiconduc- tor lasers for heterodyne-type optical communication schemes,” Electron Letr., vol. 16, no. 4, pp. 179-181, Feb. 28, 1980.

[4] F. Favre and D. LeGuen, “High frequency stability of laser diode for heterodyne communication systems,” Electron. Lett., vol. 16, no.

[5] Y. Yamamoto, “Receiver performance evaluation of various digital optical modulation-demodulation systems in the 0.5-IO-pm-wave- length region,” IEEE J . Quantum Electron., vol. QE-16, no. 11, pp.

[6] T. Okoshi, K. Emura, K. Kikuchi, and R. Th. Kersten, “Computa- tion of bit-error rate of various heterodyne and coherent-type optical communication schemes,” J. Opt. Commun., vol. 2, no. 3, pp. 89- 96, Sept. 1981.

[7] L. G. Kazovsky, “Optical heterodyning versus optical homodyning: A comparison,” J . Opt. Commun., vol. 6, no. 1, pp. 18-24, Mar. 1985.

[SI T. Okoshi, “Heterodyne-type optical fiber communications,” in Tech. Dig. Third IOOC Int. Con$ Integrated Opt. Optical Fiber Commun. (San Francisco, CA), Apr. 27-29, 1981, pap. no. TUB1, p. 44.

[9] T. Okoshi and K. Kikuchi, “Heterodyne-type optical fiber commu- nications,”J. Opt. Commun., vol. 2, no. 3, pp. 82-88, Sept. 1981.

18, pp. 709-710, Aug. 28, 1980.

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[IO] Y. Yamamoto and T. Kimura, “Coherent optical fiber transmission systems,” IEEE J. Quantum Electron., vol. QE-17, no. 6, pp. 919- 935, June 1981.

1111 F. Favre, L. Jeunhomme, I . Joindot, M. Monerie, and J. C. Simon, “Progress towards heterodyne-type single-mode fiber communication systems,” IEEE J . Quantum Electron., vol. QE-17, no. 6, pp. 897- 906, June 1981.

[12] T. Okoshi, “Heterodyne and coherent optical fiber communications: Recent progress,” IEEE Trans. Microwave Theory Tech., vol. MTT- 30, no. 8, pp. 1138-1149, Aug. 1982.

[13] S . Saito, T. Mukai, and T. Kimura, “Coherent optical fiber trans- mission systems,” in Tech. Dig. Fourth IOOC In?. Con$ Integrated Opt. Optical Fiber Commun. (Tokyo, Japan), June 27-30, 1983, pap. no. 3OC3-1, pp. 384-385.

[14] D. W. Smith and I. W. Stanley, “The worldwide status of coherent optical fiber transmission systems,” in Tech. Dig. ECOC’83, Ninth Europ. ConJ Opt. Commun. (Geneva, Switzerland), Oct. 23-26, 1983, pp. 263-266.

[15] R. C. Hooper, J. E. Midwinter, D. W. Smith, and I. W. Stanley, “Progress in monomode transmission techniques in the United King- dom,” J. Lightwave Technol., vol. LT-1, no. 4, pp. 596-611, Dec. 1983.

I161 T. Okoshi, “Recent progress in heterodynelcoherent optical fiber communications,” in Proc. OFC’84 Confi Opt. Fiber Commun. (New Orleans, LA), Jan. 23-25, 1984, Pap. TUK1.

[17] T. Okoshi, “Recent progress in heterodyneicoherent optical fiber communications,” J . Lightwave Technol., vol. LT-2, no. 4, pp. 341- 346, Aug. 1984.

[IS] K. Inoue, H. Toba, and K. Nosu, “Characteristics of an optical multil demultiplexer for lightwave FDM transmission,” National Conven- tion Record, IECE Japan, Pap. 2646, pp. 10-359, Mar. 1985 (in Jap- anese).

[19] K. Kikuchi, T. Okoshi, M. Nagamatsu, and N. Henmi, “Degradation of bit-error rate in coherent optical communications due to spectral spread of the transmitter and local oscillator,” J . Lightwave Technol., vol. LT-2, no. 6, pp. 1024-1032, Dec. 1984.

[20] K. Kikuchi and T. Okoshi, “Dependence of semiconductor-laser linewidth on measurement time: Evidence of predominance of llf noise,” Electron. Lett., vol. 21, no. 22, pp. 1011-1012, Oct. 24, 1985.

[21] T. Okoshi, “Review of polarization-maintaining single-mode fiber,” in Proc. IOOC’83, Fourth Int. Con$ on Integral Opt. and Optical Fiber Commun. (Tokyo, Japan), June 27-30, 1983, pap. 28A4-1.

[22] T. Okoshi, “Polarization-maintaining optical fibers,” in Opto-Elec- tronics for the Information Age, Chinlon Lin, Ed. New York: Van Nostrand Reinhold, 1987, ch. A6.

[23] T. Okoshi, “Polarization-state control schemes for heterodyne or homodyne optical fiber communications (Invited),” J . Lightwave Technol., vol. LT-3, no. 6, pp. 1232-1237, Dec. 1985.

[24] T. Okoshi, S. Ryu, and K. Kikuchi, “Polarization-diversity receiver for heterodynelcoherent optical fiber communications,” in Proc. IOOC‘83 Fourth Int. Con$ on Integrated Opt. and Optical Fiber Commun. (Tokyo, Japan), June 27-30, 1983, pap. 3OC3-2.

[25] R. Ulrich, “Polarization stabilization on single-mode fiber,” Appl. Phys. Lett., vol. 35, no. 11, pp. 840-842, Dec. 1979.

[26] M. Kubota, T. Oohara, K. Furuya, and Y. Suematsu, “Electrooptical polarisation control on single-mode optical fibres,” Electron. Lett., vol. 16, no. 15, p. 573, July 17, 1980.

[27] Y. Kidoh, Y. Suematsu, and K. Furuya, “Polarization control on outputs of single-mode optical fibers,” IEEE J. Quantum Electron., vol. QE-17, no. 6, pp. 991-994, June 1981.

[28] H. C. Lefevre, “Single-mode fiber fractional wave devices and po- larisation controllers,” Electron. Lett., vol. 16, no. 20, pp. 778-780, Sept. 25, 1980.

[29] T. Imai, K. Nosu, and H. Yamaguchi, “Optical polarisation control utilising an optical heterodyne detection scheme,” Electron. Lett., vol. 21, no. 2, pp. 52-53, Jan. 17, 1985.

1301 T. Okoshi, Y, Cheng, and K. Kikuchi, “A new polarization-control scheme for optical heterodyne receiver,” Electron. Lett., vol. 21, no.

[31] T. Okosbi, N. Fukaya, and K. Kikuchi, “ A new polarization-state control device: Rotatable fiber cranks,” Electron. Letts., vol. 21, no. 20, pp. 895-896, Sept. 26, 1985.

[32] R. S. Vodhanel, J. L. Gimlett, R. D. Standley, and N. K. Cheung, “FSK heterodyne detection experiment at 560 Mbit/s using 1550-nm DFB lasers,” in IOOC-ECOC’85 Tech. Dig. (Venice, Italy), Oct. 1- ,

18, pp. 787-788, Aug. 29, 1985.

4, 1985, pp. 339-342.

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OKOSHI: ADVANCES IN COHERENT OPTICAL FIBER COMMUNICATION SYSTEMS 51

[33] T. Matsumoto, K. Iwashita, and T. Imai, “400 Mbitis long-span op- tical FSK transmission experiment at 1.5 pm,” in IOOC-ECOC’85 Tech. Dig. (Post-Deadline Papers) (Venice, Italy), Oct. 1-4, 1985,

[34] R. A. Linke, B. L. Kasper, N. A. Olsson, R. C:Alfereness, L. L. Buhl, and A. R. McCormick, “Coherent lightwave transmission over 150 km fiber lengths at 400 Mbit/s and 1 Gbitis data rates using DPSK modulation,” in IOOC-ECOC’85 Tech. Dig.(Venice, Italy), Oct. 1-

[35] S . Yamazaki, K. Emura, M. Shikada, M. Yamaguchi, I. Mito, and K. Minemura, “140 Mbitis 243 km optical FSK heterodyne single filter detection transmission experiment using a phase tunable DFB laser diode,” in IOOC-ECOC’85 Tech. Dig. (Venice, Italy), Oct. 1-

[36] T. Okoshi, “Ultimate performance of heterodyneicoherent optical fi-

pp. 31-34.

4, 1985, pp. 35-39.

4, 1985, pp. 39-42.

ber communications,” J . Lightwave Technol., Oct. 1986.

References for Table I [PCM-ASK/BER Measurements] [All K. Kikuchi, T. Okoshi, and J. Kitano, “Measurement of bit-error

rate of heterodyne-type optical communication system-A simula- tion experiment,” IEEE J . Quantum Electron., vol. QE-17, no. 12, pp. 2266-2267, Dec. 1981.

[A21 T. G. Hodgkinson, R. Wyatt, and D. W. Smith, “Experimental as- sessment of a 140 Mbit/s coherent optical receiver at 1.52 pm,” Electron. Lett., vol. 18, no. 12, pp. 523-525, June 10, 1982.

[A31 K. Kikuchi; T. Okoshi, and K. Emura, “Achievement of nearly shot- noise-limited operation in a heterodyne-type PCM-ASK optical communication system,” in Tech. Dig. ECOC’82 Eighth Eur. Con5 Opt. Comm. (Cannes, France), Sept. 21-24, 1982, pp. 419-424.

[A41 T. G. Hodgkinson, R. Wyatt, D. J. Malyon, B. K. Nayar, R. Har- mon, and D. W. Smith, “Experimental 1.5 pm coherent optical fi- ber transmission system,” in Tech. Dig. ECOC’82 Eighth Eur. Con5 Opt. Comm. (Cannes, France), Sept. 21-24, 1982, pp. 414-418.

[A4’] T. G. Hodgkinson, D. W. Smith, and R. Wyatt, “1.5 pm optical heterodyne system operating over 30 km of monomode fiber,” Elec- tron. Lett., vol. 18, no. 21, pp. 929-930, Oct. 14, 1982.

[A51 R. C. Hooper, J. E. Midwinter, D. W. Smith, and I. W. Stanley, “Progress in monomode transmission techniques in the United Kingdom,” J. Lightwave Technol., vol. LT-1, no. 4, pp. 596-611, Dec. 1983.

[A61 M. Shikada, K. Emura, S . Fujita, M. Kitamura, M. Arai, M. Kondo, and K. Minemura, “lOO-Mbit/s ASK heterodyne detection experi- ment using 1.3-pm DFB laserdiodes,” in Tech. Dig. OFC ’84 Con5 on Opt. Fiber Commun. (New Orleans, LA), Jan. 23-25, 1984, pp.

[A6‘] M. Shikada, K. Emura, S . Fujita, M. Kitamura, M. Arai, M. Kondo, and K. Minemura, “100 Mbitis ASK heterodyne detection experi- ment using 1.3 pm DFB-laser diodes,” Electron. Lett., vol. 20, no. 4, pp. 164-165, Feb. 16, 1984.

62-64.

References for Table II [PCM-FSK BER Measurements] [Fl] S . Saito, Y. Yarnamoto, and T. Kimura, “Optical FSK signal de-

tection in a heterodyne system using semiconductor lasers,” Elec- tron. Lett., vol. 18, no, 11, pp. 470-471, May 27, 1982.

[F2] S . Saito, Y. Yamamoto, and T. Kimura, “ S I N and error-rate eval- uation for an optical FSK-heterodyne detection system using semi- conductor lasers,” IEEE J . Quantum Electron., vol. QE-19, no. 2, pp. 180-193, Feb. 1983.

[F3] E.-J. Bachus, F . Bohkke, G. Elze, W. Eutin, H. Foisel, K. Heimes, and B. Strebel, “Digital transmission of TV signals with a fiber- optic heterodyne transmission system,” in Tech. Dig. OFC’84 Con5 on Opt. Fiber Commun. (New Orleans, LA), Jan. 23-25, 1984, p. 62.

tern using a directly modulated DFB-laser diode,” Electron. Lett., vol. 20, no. 24, pp. 1022-1023, Nov. 22, 1984.

[F5] R. Wyatt, D. W. Smith, T. G. Hodgkinson, R. A. Harmon, and W. J . Devlin, “140 Mbitis optical FSK fibre heterodyne experiment at 1.54 pm,” Electron. Lett., vol. 20, no. 22, pp. 912-913, Oct. 25, 1984.

[F6] R. S . Vodhanel, J. L. Gimlett, R. D. Standley, and N. K. Cheung, “FSK heterodyne detection experiment at 560 Mbitis using 1550 nm DFB lasers,” in Proc. ECOC’85, Eleventh Eur. Con5 Opt. Com- mun. (Venezia, Italy), Oct. 1-4, 1985.

[F7] K. Iwashita, T. Imai, H. Yamaguchi, and Y. Yoshikuni, “Laser diode FSK modulation characteristics for heterodvne lightwave - transmission (in Japanese),” Nut. Conv. Rec. IECE Japan, pp. 10- 362, Mar. 1985.

References for Table 111 [PCM-PSK BER Measurements] [Pl] D. J. Malyon, T. G. Hodgkinson, D. W. Smith. R. C. Booth. and

B. D. Daymond-John, “PSK homodyne receiver sensitivity mea- surement at 1.5 pm,” Electron. Lett., vol. 19, no. 4, pp. 144-146, Feb. 17, 1983.

[P2] K. Kikuchi, T. Okoshi, M. Nagamatsu, and N. Henmi, “Bit-error rate of PSK heterodyne optical communication system and its deg- radation due to spectral spread of transmitter and local oscillator,” Electron. Lett., vol. 19, no. 11, pp. 417-418, May 26, 1983.

[P3] R. Wyatt, T. G. Hodgkinson, and D. W. Smith, “1.52 pm PSK heterodyne experiment featuring an external cavity diode laser local oscillator,” Electron. Lett., vol. 19, no. 14, pp. 550-552, July 7 , 1983.

[P4] D. J . Malyon, “Digital transmission over a 30 km monomode fibre link using optical homodyne detection,” in Proc. ECOC’83 Ninth Eur. Con5 Opt. Commun. (Geneva, Switzerland), Oct. 24-25, 1983 (post-deadline paper).

[P5] Same as [F3]. [P5’] Same as [F3’]. [P6] H. K. Philipp, A. L. Scholtz, and W. R. Leeb, “Homodyning of

subnanowatt 140 Mbitis data at X = 10.6 pm.” in Proc. ECOC’84 Tenth Eur. Con5 Opt. Commun. (Stuttgart, W . Germany), Sept. 3- 6, 1984, pp. 230-231.

References for Table IV [PCM-DPSK BER Measurements] [Dl] F. Favre and D. LeGuen, “Effect of semiconductor laser phase noise

on BER performance in an optical DPSK heterodyne-type experi- ment,” Electron. Lett., vol. 18, no. 22, pp. 964-965, Oct. 28, 1982.

[D2] M. Shikada, K. Emura, and K. Minemura, “High-sensitivity optical PSK heterodyne differential detection simulation experiment,” in Tech. Dig. IOOC’83 Fourth Int. Con5 Integrated Opt. Optical Fiber Commun. (Tokyo, Japan), June 27-30, 1983, pp. 390-391.

[D3] Same as [P3]. [D4] D. W. Smith, “Noncoherent optical systems in the 1500 nm wave-

band,” in Tech. Dig. IOOC’83 Fourth Int. Con$ Integrated Opt. Optical Fiber Commun. (Tokyo, Japan), June 27-30, 1983, pp. 394- 395.

[D5] K. Emura, S. Yamazaki, M. Shikada, I. Mito, K. Komatsu, and K. Minemura, “Performance of an optical PSK heterodyne-differential detection system using optical feedback DFB LD’s,” Nut. Conv. Rec. IECE Japan, pp. 10-361, Mar. 1985 (in Japanese).

[D6] K. Emura, M. Shikada, S. Yamazaki, K. Komatsu, I. Mito, and K. Minemura, “400 Mbit/s optical DPSK heterodyne detection experi- ments using DBR laser diodes with external optical feedback,” in ECOC’85 Eleventh Eur. Con5 Opt. Commun., (Venezia, Italy), Oct. 1-4, 1985.

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52 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. LT-5, NO. 1, JANUARY 1987

Laboratories, Inc., Murray Hill, NJ, where he was engaged in research on optical fiber communications, in particular, coherent optical fiber com- electron guns. In 1972 he joined the Technical University of Munich on a munications and photon-counting optical communications. He has written temporary basis as a Guest Professor. In January 1977 he became a Pro- twelve books including three in English entitled Three-Dimensional Im- fessor at the University of Tokyo. At present he is also a Guest Research aging Techniques (New York: Academic, 1976), Optical Fibers (Aca- Fellow of Radio Research Laboratory of Japanese Government, Commis- demic, 1982), and Planar Circuits (Heidelberg: Springer, 1984). He has sion-D Vice-chairman of URSI (International Union of Radio Science), been awarded fourteen prizes from three Japanese academic institutions. and President of the Japanese National Committee for URSI. He holds 13 Japanese Patents and 6 U.S. Patents.

The main fields of Dr. Okoshi’s present interest are optical fibers and

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