Impact of local oscillator intensity noise on the performance of the optical phase-diversity fsk receiver using delay-and-multiplying discriminator

TABLE 3 Measured Efficiencies

Cascaded 1.6 14 1 26 0 88

Balanced 1 6 29 5 12 6 1 84

Novel 1 6 19 9 25 1 24

thermore, if the conventional balanced configuration is used the mismatch will be even worse than for the individual am- plifier. This is because the power reflected back to the input will be the input power amplified once and then reflected by the amplifiers to the input port. By the introduction of two

90" sections the input mismatch is reduced to the input mis- match of the individual amplifier.

MEASUREMENTS

To verify the principle the balanced, cascaded, and proposed configurations have been measured for the cases of low and high gain individual amplifiers. The same amplifier, coupler. and filter modules have been used for all configurations. Cen- ter frequency was 4.8 GHz. The loss of the filter was 0.5 dB. The results of the power measurements are shown in Fig- ures 3 and 4. The I-dB compression points are shown in Table I . The low power output of the balanced configuration is due to the I-dB gain difference between amplifiers.

The measured signal responses of the three configura- tions at center frequency are shown in Table 2, and the mea- sured efficiencies of the different configurations are shown in Table 3.

CONCLUSION

to obtain output power and efficiency comparable to that of a balanced configuration and at the same time the gain of a cascade configuration.

Received 10-22-91 Microwave and Optical Technology Letters, 514, 166-168

0 1992 John Wiley & Sons, Inc. CCC 0895-2477192/$4.00

IMPACT OF LOCAL OSCILLATOR

INTENSITY NOISE ON THE

PERFORMANCE OF THE OPTICAL

PHASE-DIVERSITY

FSK RECEIVER

USING DELAY -AN D-M U LTI P LY I N G

DISCRIMINATOR

Yang-Han Lee, Ching-Chih Kuo, and Hen-Wai 1-0 Department of Electrical Engineering

National Taiwan University Taipei, Taiwan Republic of China

KEY TERMS

Optical communication, frequency modulation, discriminator, noise ABSTRACT

The impact of local oscillator intensity noise on the performance of an optical phase-diversity FSK receiver using a delay-and-multiply-

ing discriminator is analyzed. Given the data rate, frequency devia- tion, laser linewidth, RZN noise, and thermal noise, we can obtain the minimum received signal power together with the corresponding optimal local oscillator power to achieve the required BER. Numeri- cal results with system parameters given in [ I ] show that (P&, is

-1.46 d B m for RIN = -160 d B I H z and - 6 . 4 d B m for RIN =

-150 d B I H z when the thermal noise is 9 x A'IHz.

INTRODUCTION

The local oscillator relative intensity noise (RIN), caused by random fluctuations of the optical power of both the trans- mitting and local oscillator (LO) lasers, may have a significant influence on the performance of phase-diversity receivers

The impact of local oscillator

RTN

on the performance of a phase-diversity ASK receiver has been analyzed i n [ I , 3). The phase-diversity FSK receiver [4, 51 also suffers from the LO RIN. In this letter, we use the Gaussian approximation [6) to analyze the impact of RIN on the performance of the phase-diversity FSK receiver.

For a coherent system with negligible RlN or with a bal- anced receiver structure [7], the system performance can ap- proach the shot noise limit as the LO power increases to suppress the thermal noise. However, when the RIN is not negligible, increase of LO power tends, on the one hand, to alleviate the degradation caused by the thermal noise, but, on the other hand, to increase the degradation due to the KIN. So there should exist an optimal LO power to meet the required system performance, i.e., the specified BER. RECEIVER SYSTEM MODEL

Figure 1 is the diagram of an optical phase diversity FSK receiver [4-6). The 90" optical hybrid has two inputs: One is the received optical field given by

[l, 21.

E,(t) =

a

cos(w,t

+

Q,(t)

+

Q T ( t ) ) , (1)

and the other, assumed to have the same polarization as E , ( t ) , is the LO oscillator field given by

E l ( [ ) =

a

C O S ( ( 0 , t

+

cp,(t)), (2)

where P, and P, are the received signal and LO powers, respectively. w, and oL are the frequencies of the received and LO optical carriers, respectively. @T(f) and (P,(t) are the phase noise of the transmitting and LO lasers, respectively.

a,,,([)

is the angle modulation given by

where a, is the input data stream, f d i s the frequency deviation,

p ( t ) is a pulse function satisfying the Nyquist criterion [8]:

p ( 0 ) = 1, p ( m T ) = 0 for any nonzero integer m . The raised

cosine waveform [9] is a well-known example. And T is the bit duration ( R , = 1/T: data rate).

As shown in [6], we may obtain the signal-to-noise ratio

(SNR) at the output of the LPF, as,

(4)

(fd/R*)2

SNR =

hvIaRb f Y ( ( ~ ~ I R ~ ) ~

+

AvlnR,

+

4)

+

3y2

Sample at t=mT 1 I

I

1

decision

I

.

Figure 1 The system block diagram of a phase-diversity FSK system

where y is defined as

(2qRPLIN

+

(ith)

+

(RPL)2y/N2)Rb

Y = R2P,PL

where q is the electronic charge, (i:h) represents the thermal noise spectral density of the preamplifier at the IF stage, N

is the number of receiver branches, and Av is the sum of the transmitting and LO laser linewidths. The first term in (5)

represents the shot noise. The second term represents the thermal noise. The third term represents the LO RIN where RIN = 10 log y in dB/Hz. (6) The bit error rate based on the Gaussian approximation (BER) is given as

BER = Q ( m ) , (7)

where

DERIVATION OF OPTIMUM PLo AND MINIMUM RECEIVER SENSITIVITY P.

From (4), we can express the parameter y in terms of SNR,

f d , R b , Av as

From the definition of y in ( 5 ) , we obtain the received signal sensitivity P, as

(9)

( 2 q l N

+

(i:,,)lRPL

+

R P L y l W ) R b RY

P,

=

Taking the derivative of P, in (9) with respective to P ,

and setting it to be zero, we can obtain the optimal LO

power together with the minimum receiver sensitivity, respec- tively, as

where for a specified BER (and thus SNR), y can be obtained from (8).

fd=27SMHz

109

-10 -8 -6 -4 -2 0 2 4 6 8

Lacat Oscillator Power [am]

fd=SSOMHz 1V (b) I@'

3

5

i3 lp 1IT -10 -8 -6 -4 -2 0 2 4 6 8

Local Oscillator Power [dBm]

Figure 2 Bit error rate (BER) vs the local oscillator power ( P L ) for (a) f d = 275 MHz, P, = - 57.43 dBm of RIN = - 150 dB/Hz, and

P, = -58.7 dBm of RIN = - 160 dBIHz; (b) f d = 550 MHz, P, = -57.74 dBm of RIN = -150 dB/Hz, and P, = -59.02 dBm of RIN =

-

160 dB/Hz in terms of (i5) = 9 X A21Hz and Av =

0, 5 MHz, 10 MHz, respectively

NUMERICAL EXAMPLE

Consider a typical data link: Data rate Rb = 150 Mbits/sec,

R = 0.84 A/W, At’ = 0, 5 MHz, 10 MHz, (i:,,) = 9 x lo-” A*/Hz [1], N = 2 (for a 90” optical hybrid, fd = 275 MHz

and 550 MHz. To meet BER = the SNR should be around 36 for Gaussian approximation.

The BER versus the local oscillator power ( P L ) for f d =

275 MHz, P, = - 57.43 dBm of RIN = - 150 dBIHz, and P, = - 58.7 dBm of RIN =

-

160 dB/Hz are shown in Figure 2(a); f d = 550 MHz. P, = - 57.74 dBm of RIN = - 150 dB/ Hz, and P, = - 59.02 dBm of RIN = - 160 dB/Hz are shown in Figure 2(b). In Figure 2, we use the Ps to be equal to P,,,

at BER = and A v = 0 from (11). We find there exists an optimal local power which can be found from (10).

From (8) and (11) with BER specified as lo-’, the mini- mum receiver signal power (PY),,, as a function of RIN for

f d = 275 MHz and f d = 550 MHz are shown in Figures

3(a) and 3(b), respectively. The minimum received signal power increases as the RIN and the laser phase noise increase. When f d = 275 MHz, (PI),,,,, = -58.7 dBm for Ail = 0 and RIN = - 160 dB/Hz (point A); (PJmLn = - 57.43 dBm for

AI, = 0 and RIN = - 150 dB/Hz (point B) as shown in Figure 3(a). When f d = 550 MHz, (Pf),,” = -59.02 dBm for A V =

0 and RIN = - 160 dB/Hz (point a); (PJm,, = - 57.74 dBm for Av = 0 and RIN = -150 dB/Hz (point b) as shown in Figure 3(b). The optimal LO power (P& as a function of RIN for

(iU

= 9 x A’IHz and R = 0.84 A / W are shown in Figure 4. The optimal LO power decreases as the RIN increases. It is also noted that (PL)op, is independent of

y , thus independent of f d , R,, A v , and SNR [see Eq. (7)]. We

fd=275MHz, BER=IO^-9 (SNR=36) / ’ / I ’ , p -55 , ‘

/i

I’

.I

-585 I’

.I

-585

Relative Intensity Nok (dBtHz]

f d = 5 5 O W BER=10^-9 (SNR=36)

~ -142 -140

S

L

-sg~bl

-158 -IS6 -154 -152 -150 -148 -146 -142 .I$

Rclatrvc Intensity Nok [dBfi]

Figure 3 The minimum receiver signal power Ps as a function of RIN for (a) f d = 275 MHz and (b) fd = 550 MHz in terms of

( i i ) = 9 X A’IHz and hv = 0, 5 MHz, 10 MHz, respectively

\

M

Figure 4 The optimal LO power (PL) as a function of RIN for (ia) = 9 x lo-*‘ A*IHz

can find (P&,, to be -1.46 dBm for RIN = -160 dB/Hz and - 6.46 dBm for RIN = - 150 dBIHz.

CONCLUSION

The impact of local oscillator laser intensity noise on the performance of an optical phase-diversity FSK receiver using a delay-and-multiplying discriminator is investigated with the assumption of Gaussian noise distribution. Given the fre- quency deviation, data rate, laser linewidth, the RIN, and the thermal noise, we can analytically obtain the minimum re- ceived signal power together with the corresponding optimal local oscillator power for a specified BER at

REFERENCES

1 . A . F. Elrefaie, D. A. Atlas, L. G. Kazovsky, and R . E . Wagner, “Intensity Noise in ASK Coherent Lighwave Receivers.” Elec- iron. Leti., Vol. 24, No. 3, 1988, pp. 158-159.

2. E. Patzak. and R. Langenhorst, “Sensitivity Degradation of Con- ventional and Balanced 3 x 3 Port Phase Diversity DPSK Re- ceivers due to Thermal and Local Oscillator Intensity Noise,” Electron. Lett., Vol. 25, No. 8, 1989, pp. 545-547.

3. W. H. C. Krom, “Impact of Local Oscillator Intensity Noise and the Threshold Level on the Performance of a 2 x 2 and 3 x 3 Phase-Diversity ASK Receiver,” J . Lightwave Technol., Vol 9, 4. A . W. Davis, M. J . Pettitt, J . P. King, and S. Wright, “Phase Diversity Techniques for Coherent Optical Receivers,” J. Light- wave Technol., Vol. 5 , No. 4, 1987, pp. 561-572.

5. R. Noe, W. B. Sessa, R. Welter. and L. G. Kazovsky, “New FSK Phase Diversity Receiver in a 150 MbitIs Coherent Optical Trans- mission System.” Electron. Lett., Vol. 24, No. 9, 1989, pp. 567- 568.

6. H.-W. Tsao, W. Jingshown, and Y.-H. Lee, “Performance Anal- ysis of Optical Phase Diversity FSK Receiver Using Delay-and- Multiplying Discriminators,” J Opt. Commun., Vol. 10, No. 3 , 7. L. G . Kazovsky, “Balanced Phase-Locked Loops for Optical Homodyne Receivers: Performance Analysis, Design Consider- ations and Laser Linewidth Requirements,” J. Lightwave Tech- nol., Vol. 4, No. 4, 1986, pp. 182-194.

8. R . E. Ziemer and W. H. Tranter, Principles of Communications, 2nd ed., Houghton-Mifflin, Boston, 1985, Chap. 7.

9. R. E. Ziemer and R. L. Peterson, Digital Communications and Spread Spectrum System, Macmillan, New York, 1985, Chap. 3. NO. 5, 1991, pp. 641-649.

1989, pp. 97-100.

Received 10-23-91 Microwave and Optical Technology Letters, 514, 168-170

0 1992 John Wiley & Sons, Inc. CCC 0895-2477 I92 /$4.00