A theoretical study of the dynamics of semiconductor lasers subjected to pseudorandom digital modulation at gigabit rates is presented. The eye diagram, turn-on jitter ͑TOJ͒, and power fluctuations in the modulated laser wave form are analyzed. The study is based on numerical large-signal analysis of the laser rate equations. Influences of the biasing and modulation currents on the eye diagram and TOJ are examined. The degree of eye opening is measured in terms of a Q factor of the laser signal analogous to the Q factor determining the bit-error rate in transmission systems. Influence of optimizing both the sampling and decision times on the signal Q factor is modeled. We show that the most eye opening corresponds to shortening the sampling time associated with lengthening the decision time. We also assess the relative contributions of the laser intrinsic noise and pseudorandom bit pattern to the TOJ. The results show that the bit pattern is the major contributor to the TOJ when the setting time of the relaxation oscillation is longer than the bit slot.

We introduce numerical modeling of noise performance and distortion of semiconductor lasers subject to two‐tone modulation according to the National Television Standards Committee NTSC plan for the cable television (CATV) signal transport. The waveform of the laser signal is simulated over wide ranges of the modulation index and placements of the modulation channel. The power and frequency spectra of the relative intensity noise (RIN) are characterized, and the associated second‐order harmonic distortion (2HD), and the second‐order (IMD2) and third‐order intermodulation distortions (IMD3) are calculated. The calculated data are used to predict the measures of the CATV system performance. The dynamic range of linearity of the laser is evaluated in terms of the spurious‐free dynamic range SFDR. We show that under weak modulation, the low‐frequency RIN is constant at −168 dB/Hz regardless the modulation frequency. Within this accepted noise level, IMD2 is 4 dBm higher than 2HD and 10 dBm higher than IMD3. When the modulation index exceeds 25%, the laser emits clipped signals associated with large values of signal distortions. Accepted values of the carrier to noise ratio and distortions correspond to lower modulation index and/or channel frequency. Copyright © 2015 John Wiley & Sons, Ltd.

SUMMARY A comprehensive study on semiconductor laser characteristics under gigabit‐per‐second digital modulation is presented. Comparison of the modulation characteristics under both formats of the return to zero (RZ) and non‐return to zero (NRZ) bit formats is introduced. The modulation characteristics include the eye diagram, turn‐on jitter (TOJ) and frequency chirp. The study elucidates how the laser modulation performance changes with variation of the modulation bit rate relative to the maximum bit rate and the setting bit rate of the relaxation oscillations. The relative contributions of the intrinsic noise of the laser and the pseudorandom bit‐pattern effect to the modulation characteristics are differentiated. The results showed that when the bit rate is higher than the setting bit rate, the eye diagram is partially closed and becomes completely closed when the bit rate increases beyond the maximum bit rate. When the bit rate is higher than 2.25 Gbps, the TOJ values under NRZ modulation are smaller than those under RZ modulation near the threshold level. Under both RZ and NRZ formats, chirp increases with the bit rate with the chirp under the RZ format being higher than that under the NRZ format. Copyright © 2013 John Wiley & Sons, Ltd.

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SUMMARYThis paper demonstrates numerical analysis of the dynamics and intensity noise of fiber grating semiconductor lasers (FGSLs). The induced phenomenon of strong optical feedback (OFB) is analyzed. The simulations are based on an improved time-delay rate equations model of a single-mode laser that takes into account the multiple round trips of the lasing field in the fiber cavity. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram and relative intensity noise (RIN). We explore influence of the fiber-cavity length on the dynamics and RIN. The results show that when the fiber-cavity is short, the regime of strong OFB is characterized by either continuous-wave (CW) operation or periodic pulsation. The pulsation frequency is locked at the frequency separation of either the compound-cavity modes or the external fiber-cavity modes. The corresponding RIN level is close to or higher than the level of the solitary laser depending on pulse symmetry. When the fiber cavity is long, the laser exhibits unstable dynamics over wider range of OFB. Moreover, the strong-OFB pulsation becomes beating quasi-periodic at the relaxation oscillation frequency and the fiber-cavity mode-separation frequency.