**6. Relative intensity noise**

The relative intensity noise (RIN), intrinsic noise inherited in the device due to spontaneous emission noise, is of major importance for optical communication systems, whereas low RIN is needed to achieve high signal-to-noise ratio (SNR). It is observed that RIN shows a peak near the relaxation oscillation frequency and this would be related to the bandwidth of the laser [30]. If the injection locked slave laser could show a suppression in the spectrum of the RIN near the resonance and more damping due to the increase of injection strength, more enhancement in bandwidth will be realized. Experimentally has verified the enhancement bandwidth of sampled grating distributed Bragg reflector laser (SG-DBR), lower of intensity noise and higher resonance frequency due to strong optical injection [31]. The dependence of RIN on both bias current and detuning have been studied [32].

The fluctuations in the power spectrum can be defined as: ( ) *<sup>P</sup>* 

124 Optical Communication

modulation, since this factor plays great role in the refractive change with injected carriers.

The frequency chirping when considered in the output power of the slave laser, the detuning was assumed to be constant, but it is dependent on the optical input power, hence the CPR has to take into account this dependent. Also, the laser cavity frequency , is carrier dependent. So the effects of variations in frequency detuning, and laser cavity

( ) *<sup>o</sup> N*

Frequency chirping had been decreased with increasing injection strength and with lowered value of linewidth enhancement factor. The simulation reveals the dramatic influence of injection locking on the frequency chirping characteristics. Dynamical properties have been found, experimentally lately [28], to depend on the injection strength through the evolution of the optical and electrical spectral distribution. Substantial reduction in frequency chirping was observed in the direct modulation injection-locked laser, and this reduction was much

**Figure 8.** Frequency chirping characteristics of injection locked semiconductor showing its dependence

The relative intensity noise (RIN), intrinsic noise inherited in the device due to spontaneous emission noise, is of major importance for optical communication systems, whereas low RIN is needed to achieve high signal-to-noise ratio (SNR). It is observed that RIN shows a peak near the relaxation oscillation frequency and this would be related to the bandwidth of the laser [30]. If the injection locked slave laser could show a suppression in the spectrum of the

This influence can be realized from the relation stated in Eq. (17).

more pronounced at low modulation frequency in experiment [29].

frequency have to be included in the CPR.

on injection strength and linewidth enhancement factor.

**6. Relative intensity noise** 

$$\Theta\_p(\alpha) = < \left| \Delta \tilde{P}(\alpha) \right|^2>\tag{20}$$

This, by normalization with , would give the relative intensity noise spectrum: <sup>2</sup> ( ) *P*

$$RIN = \frac{\Theta\_p(oo)}{\left(\overline{P}\right)^2} \tag{21}$$

The relative intensity noise spectrum, RIN, for free-running laser and with strong optical injection is shown in figure 9. With injection locking, the noise peak was shifted to a higher frequency.

**Figure 9.** Relative intensity noise spectrum of free-running semiconductor laser and injection-locked for two different injection strengths.

When injection locking was increased from -18 dB to -14.4 dB, the noise peak was shifted form 10 GHz to at least 13 GHz. Hence, injection locking can enhance the bandwidth of optical communication systems. Also, a reduction 14 dB/Hz in RIN had occurred when injection strength was increased at the mentioned values above.

**Figure 10.** The influence of gain saturation coefficient on the RIN spectrum of semiconductor laser.

To illustrate how gain saturation coefficient can alter the RIN spectrum of the slave laser, figure 10 shows this effect. When the gain saturation coefficient was increased, it is noticed that the RIN spectrum at the resonance peak was suppressed and this can be interpreted as damping due to the gain saturation coefficient. We would expect that the strong injection locking phenomenon may give extra damping of RIN peak in addition to the gain saturation coefficient. This has to be verified experimentally. So, both the methods will exhibit large significant RIN suppression, and meet the demands for higher bit rates and longer optical communication network.
