# a phase-noise-compensating transmission method for phase-modulated coherent optical systems

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A PHASE-NOISE-COMPENSATING

PHASE-MODULATED COHERENT TRANSMISSION METHOD FOR

OPTICAL SYSTEMS S. Betti, F. Curti, G. De Marchis, and E. lannone Fondazione Ugo Bordoni Viale Europa 190-001 44 Rorna Italy

KEY TERMS Phase noise, coherenr optical system, phase modularion

ABSTRAa A new method capable of overcoming the limitations in coherenf optical systems that occur when the optical source has a nonnegligible linewidth is proposed. The fransmission of a reference signal derived from a common source allows the use of high-power laser sources which are required in long-haul optical systems.

INTRODUCTION

The main problem that limits the feasibility of coherent opti- cal systems is due to the phase noise of the laser sources [l, 21. Technical solutions have been suggested in the direction of improving the source spectral characteristics. For example, a solution that has been adopted in a number of coherent systems [3, 41 is the use of an external cavity [ 5 ] for narrowing

the spectral linewidth of laser diodes. However, similar solu- tions seem to be very critical and not reliable enough for practical applications.

In this article we propose an alternative approach and demonstrate how the transmission of a reference signal de- rived from a common optical source and suitably frequency- shifted with respect to the modulated signal allows practical insensitiveness of the system to the phase noise, offering the possibility of using lasers with relatively broad linewidth. Moreover, this technique leads indirectly to an appreciable reduction of the stimulated Brillouin scattering (SBS) [6], allowing the use of high-power laser sources. The proposed system is also compatible with the use of polarization-inde- pendent detection schemes in which a neghgible power penalty has been demonstrated [7]. The system is affected by a 3-dB penalty with respect to the conventional PSK heterodyne receiver due to the power split at the transmitter, but its application to long-haul coherent optical systems, where high-power laser sources are required, seems to be very prom- ising.

SYSTEM ANALYSIS

The block diagram of the proposed system is shown in Figure 1. At the transmitter the optical carrier, generated by the laser source LS, is divided into two orthogonally polarized compo- nents by a polarization beam splitter BS: One of these is modulated by the angular modulator PM, the other is shifted

BS BS

LS

0

FIBRE

Figure 1 Block diagram of the proposed PSK transmission system

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 1 , No. 5. July 1988 161

by the frequency translator FT from the optical angular frequency w1 to w2. Before launching into the optical fiber, the two signals are recombined by means of another polarization beam splitter as this operation allows no power loss in the coupling. The received signal can therefore be written as

s ( t ) = ( x ~ , + yAl)exp[ - j ( m l t + + ( t > + 77m(t))l

+(xA, + yA1,.)exp[ - J ( W 2 f + +(I> + PI1 (1)

where x and y are the unitary vectors along the x and y axes, 2A' = 2(A: + A f ) is the received optical power, + ( r ) is the transmitter phase noise, m( r ) is the transmitted binary mes- sage, and 8 is the phase mismatch between the two trans- mitted optical signals. At the receiver the polarization beam splitter allows selection of the x and y components of the combined signal and local oscillator (LO) beams, yielding the following expressions for the signals on the electrical branches:

I , ( t > =Is,( t ) + W>I2 + f l , ( t >

( 2)

where s , , s , , I , and 1 ( = 1, ) indicate the polarization compo- nents, respectively, of the signal and the LO along the x and y axes, and n , ( t ) and n , ( t ) are the shot-noise processes on the two branches.

Neglecting the DC terms and the beat terms proportional to A t and At at angular frequency wl2 = w2 - w l , Eq (2) yields for I , ( f ) the expression:

where = wj - wl, wZ3 = w3 - w2 with w 3 , \ c ( t ) , and L, ( = L,. = L ) , respectively, the angular frequency, the phase noise, and the x-component amplitude of the LO. A similar expression can be obtained for Zr( t ) .

On each branch the two beat terms are selected by means of the ideal bandpass filters 4 and F, . The filters 4, centered at wl3 , are assumed 2 R + kB, wide while F2, centered at w23, are kB,. wide, R being the bit-rate and B , being the sum of the laser linewidths. The positive parameter k has to be chosen to transmit both the modulated and the reference signal undistorted through the filters and to limit the shot- noise. The frequency spacing between the two transmitted signals has been chosen equal to 4( R + kB,.) in order to have negligible spectra overlapping. The beat signal between the component at w23 of I , ( r , and that at of I,(,y), is ex- pressed by

where oL2 = ol3 - w23 and Nxc,.,(t) is a term due to the shot-noise, which can be considered a Gaussian process. Therefore, Eq. (4) shows that the signal is not affected by the phase noise of the optical sources so that a lot of problems of conventional PSK schemes are avoided.

As shown in Figure 1 a further mixing with a carrier at the shift frequency wl2, recovered by a PLL, provides two base- band signals. The sum of these signals gives the decision function in which the influence of the received polarization is eliminated.

10 60 110

Ps (ph lb i t )

Figure 2 Error probability P , vb. received signal power P, in photons/hit (ph/hit) for various values of the sum of R / E , ~ with the assumption L = 8

\ \

\

\ +&- +-+-

1 2 5 10 20 50 100 200 500

Figure 3 System penalt) vs R / B , ( - proposed system, --- PSK heterodyne system)

Assuming a Lorentzian laser linewidth, the signal-to-noise ratio S / N and the error probability can be calculated. The system performance in terms of the error probability P, vs. signal power Pr (in photons/bits) has been evaluated for different values of R/B,., where R is the bit rate and is shown in Figure 2, where the absence of a phase-noise-induced floor for P, can be noted. Figure 3 shows the penalty vs. R / B , with respect to the ideal PSK heterodyne system. This penalty is compared with that relative to a conventional PSK heterodyne system which is not feasible for R / B , < 100 [l], while it is confirmed that the proposed technique allows carrying out the transmission even for low values of R / B , with a penalty of a few dB's.

CONCLUSIONS

In conclusion, we propose a PSK transmission system in which a reference signal for the phase-noise compensation is transmitted. The system performances have been evaluated and have shown its feasibility even in the presence of a large laser linewidth. Moreover, the adopted technique allows a sensible reduction of stimulated Brillouin scattering [6] and is compatible with a polarization independent receiver.

162 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 1 , No. 5, July 1988

REFERENCES 1. J. Salz, Coherent Lightwave Communications, A T & T Tech.

Journul, Vol. 64, No. 10, Dec. 1985, pp. 2153-2209. 2. I. Garrett and G. Jacobsen, Theoretical Analysis of Heterodyne

Optical Receivers for Transmission Systems Using (Semiconduc- tor) Laser with Non-Negligible Linewidth, IEEE J . Lightwave Tech., LT-4, No. 3, 1986, pp. 323-334.

3. R. A. Linke, B. L. Kasper, N. A. Olson, and R. C. Alferness, Coherent Lightwave Transmission over 150 Km Fiber Lengths at 400 Mbit/s and 1 Gbit/s Data Rates Using Phase-Modulation, Electr. Lett., Vol. 22, No. 1, 1986, pp. 30-31.

4. T. Matsumoto, K. Iwashta, and T. Imai, 400 Mb/s Long-Span Optical FSK Transmission Experiment at 1.5 pm, Tech. Digest of IOOC-EOOC 85, Venice, Oct. 1-4, 1985.

5. R. Wyatt and J. Devlin, 10 kHz Linewidth 1.5 pm InGaAsP External Cavity Laser with 55 nm Tuning Range, Electron. Lett., Vol. 19, No. 3, 1983, pp. 110-112.

6. D. Cotter, Suppression of Stimulated Brillouin Scattering During Transmission of High-Power Narrowband Light in Monomode Optical Fibres, Electron. Lett., Vol. 18, No. 15, July 1982, pp.

7. B. Glance, Polarization Independent Coherent Optical Receiver, 638-640.

IEEE J . Lightwave Tech., LT-5, No. 2, 1987, pp. 214-276.

Receiued 4-18-88

Microwaoe and Optical Technolou Letters, 1/5, 161 -163 0 1988 John Wiley & Sons, Inc. CCC 0895-2477/88/$4.00

SOLUTIONS OF SOME TRANSMISSION LINES Weigan Lin Chengdu Institute of Radio Engineering Chengdu, 61 0054, Sichuan Peoples Republic of China

Wan Changhua Shanghai Research Institute of Microwave Technology Shanghai Peoples Republic of China

KEY TERMS Slab line, triangular, square, hexagonal slab line, conformal niupping

ABSTRACT Solutions of the unscreened slab line, the triungular slab line, the square slab line, and the hexagonal slab line are obtained bb, means of conformul mapping and the continuity of electric potential. Both the method and the results are new. Al l the solutions are believed to he uirtuul!y exact for all ranges of the ratio of the dimensions.

INTRODUCTION

The coaxial lines of an outer N-regular polygon concentric with an inner circle are extensively used in microwave filtering [ l , 21, microwave measurement [3], high package transmission, and impedance transformations [4, 51. How to determine their characteristic impedances is permanently the subject of great interest. Over the past years, laborous work has been done for this and remarkable achievements have been gained [6-191. The objective of this paper is to make fuller use of conformal mapping and the continuity of electric potential on the basis of ref. [17] in order to find more ac

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