194 Journal of Optical Communications 16(1995) 5 J. Opt. Commun. 16 (1995) 5, 194-196 B Journal of Optical Communications © by Fachverlag Schiele & Schön 1995 Power Penalty due to Optical Amplifier Induced Crosstalk in Noncoherent OOK Transmission System V. K. Jain, Y. N. Singh, H. M. Gupta Summary In this paper, analysis of direct detection (DD) receiver with an optical preamplifier has been carried out for OOK signalling scheme. Effect of crosstalk due to adja- cent channel has been considered along with the ampli- fier noise. It has been observed that there is a consi- derable improvement in the receiver sensitivity after the use of an optical preamplifier. Presence of crosstalk sig- nal may reduce this sensitivity gain substantially. 1 Introduction In optical communication systems, optical amplifiers can be used to amplify several channels simultaneously as long as carrier frequency of multiple channels lie with- in the amplifier bandwidth. When a semiconductor laser amplifier (SLA) is used in multichannel transmission system, several nonlinear phenomenon may induce interchannel crosstalk. Two such nonlinear phenomenon are cross-saturation and four wave mixing (FWM). In the presence of cross-saturation, gain of a particular channel is saturated not only by its own power, but also by the power of neighbouring channels. When the input light intensity is low, signal gain becomes constant and crosstalk does not occur. On the other hand, if the input intensity is high, output intensity is not proportional to input intensity i.e., amplifier is gain saturated. In this case, signal gain is dependent upon the total light inten- sity. In multichannel transmission system, total light intensity varies randomly since each channel light is independently modulated. Thus, the signal output of one channel varies according to signal gain fluctuations induced by modulation of the other channels, even when this signal input is constant. This is the crosstalk indu- ced by gain saturation in the amplifier. It is possible to avoid this crosstalk by operating SLAs in the unsatura- ted regime. It can also be avoided by using phase shift keying (PSK) or frequency shift keying (FSK) coherent systems, since for these signalling schemes power remains constant. In the on-off keyed (OOK) systems, this crosstalk will be a serious problem irrespective of whether the system is coherent or noncoherent type [1]. The SLAs in addition to crosstalk will also generate some noise of its own. When electrons and holes in SLA spon- taneously recombine, emitted light is spread over a wide wavelength (of the order of 50 nm). This light gets ampli- fied along with the main signal and generates amplified spontaneous emission (ASE). When the output from an amplifier-signal plus ASE is detected by a photodiode, different components recombine to generate what is known as beat noise. Signal-spontaneous beat noise is generated by mixing between the amplified signal and ASE components. Similarly, spontaneous-spontaneous beat noise is generated by mixing within the ASE com- ponents themselves. In addition to these two noise com- ponents, there will be shot noise components produced by the signal and the spontaneous emission. The signal- spontaneous beat noise tend to be dominant noise com- ponent at high signal input power, while the shot noise and spontaneous-spontaneous beat noise components are dominant at low input power [2]. In a practical system, amplified signal component is quite predomi- nant and therefore the main source of noise is the sig- nal-spontaneous beat noise component. When an opti- cal bandpass filter is placed between optical amplifier and photodetector, it will cut-off the ASE noise outside the passband of filter and improve the receiver sensiti- vity to some extent. In this paper, analysis of direct detection (DD) receiver with an optical preamplifier has been carried out for a two channel OOK noncoherent transmission system. In the analysis, effect of saturation induced crosstalk due to adjacent channel has been included along with the amplifier noise. Power penalty due to crosstalk has been determined. Numerical results have been presented for the practical values of the parameters and their physi- cal implications have been discussed. Address of authors: Department of Electrical Engineering Indian Institute of Technology, Delhi Hauz Khas, New Delhi-110 016 India Received 28 April 1994 Brought to you by | provisional account Unauthenticated Download Date | 6/11/15 8:08 AM