Mode Suppression Ratio of CW Laser Diode and its Influence on Optical Communication Network

The influence of mode suppression ratio on the dynamics of semiconductor laser will be investigated in this paper, especially in a high bit rate optical communication. An optical communication network of fiber length of 50 km was established, with semiconductor laser modulated at 10(Gbps). Mode suppression ratio of more than 20 dB was an optimum value for a laser diode to be considered as a single mode laser. We have taken into account in this study the following factors namely; the number of longitudinal modes, the relative intensity noise, laser mode linewidth, and chirping. The result showed that, the quality factor and bit error rates were affected by the mode suppression ratio. Relative intensity noise of (-100 dB/Hz) had an extreme drop in the Mode Suppression Ratio of CW Laser Diode and its Influence on Optical ... 87 received power and more fluctuations were found in the power output spectra. Intramodal distortion was observed in the optical layout system when chirping was considered in the laser characteristic. 1Introduction: Optical communication network usually works with laser diode as a transmitter in the wavelength 1550 nm where the fiber loss is in its minimum. Laser diode spectral linewidth will play an important role in its characteristics by bandwidth and bit rate[1]. Laser phase noise and relative intensity noise are restricting the bit rate and BER (bit error rate)[2]. Phase stability of laser diode and its influence on linewidth had been pointed out by Gliese et.al and a stable requirement for coherent optical beam characteristics of the laser diode[3]. Spectral purity of a very narrow linewidth laser transmitter allows the phase modulation of data signals had bit rate as low as a 155 Mb/s, which alleviates the need for digital signal processing in the detection circuit[4]. Laser diode injection current is another noise component and the tuning mechanism for direct modulation will convert this drive current noise as an additional frequency fluctuations “frequency jitter” components, so all noise components mentioned will contribute to the measured linewidth[5]. Mode-beating exhibits a narrow linewidth below 100 kHz, and low jitter components with high frequency correlation between these modes [6]. It has shown that amplified spontaneous emission can affect laser diode linewidth and relative intensity noise (RIN), and the enhancement linewidth factor [7]. Recently, it has been shown that for long haul optical communication system, single mode laser diode is the most cost effective transmitter due to the limited chromatic dispersion and polarization mode dispersion [8]. On the other hand, side mode rejection ratio (MSR) has influence on the coherency of laser diode [9]. 2Theory: In semiconductor lasers, the linewidth enhancement factor ( ) plays a great role in deterring its spectral linewidth. This factor can be defined as [9]: