Comparative study on modulation dynamic characteristics of laser diodes using RZ and NRZ bit formats

Abstract

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.

Performance analysis of a digital fiber optic link incorporating gain‐levered laser diode transmitter

“…The result shows that the distortion increases as the number of bits transmitted per second is increased, leading to reduced eye opening. These results coincide with theory …”

“…However, the value of the Q factor can be further improved by increasing the modulation current beyond 5 I ath . But the increase in modulation current increases the intensity of bit level “1.” The difference between the photon population for bit level 0 and 1 is also high along with increased chirp in the device . Hence, the modulation current is limited to 5 I ath in our analysis.…”

“…Coding techniques generally used to represent digital data are return to zero (RZ) and non‐return to zero (NRZ) formats; however, NRZ is preferred because of its simple implementation. Further, the NRZ signal requires bandwidth less than that of RZ case . Digital modulation is realized in optical transmitters under direct and external modulation schemes.…”

“…This noise is due to photons that are generated from spontaneous emission and coupled into lasing mode. [3][4][5][6][7][8][9] Coding techniques generally used to represent digital data are return to zero (RZ) and non-return to zero (NRZ) formats; however, NRZ is preferred because of its simple implementation. Further, the NRZ signal requires bandwidth less than that of RZ case.…”

“…Further, the NRZ signal requires bandwidth less than that of RZ case. 7 Digital modulation is realized in optical transmitters under direct and external modulation schemes. However, direct modulation is preferred for data rates less than 10 Gbps because of its low cost and simplicity.…”