SOA-Booster Integrated Mach–Zehnder Modulator: Investigation of SOA Position

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Integration of a booster semiconductor optical amplifier (SOA) is an efficient way of overcoming losses in InP based Mach-Zehnder modulators. We analyze the impact of locating the SOA before and after the MZM, respectively, in terms of output power and signal integrity at 10 Gb/s, both experimentally and theoretically. For a device with 10 dB MZM loss it is found that, for a fixed power consumption, locating the SOA after the MZM provides 7-9 dB higher output power. This advantage is reduced for lower MZM losses but remains significant. With the SOA after the MZM, the gain is restricted by dynamic saturation effects (waveform distortion), which is not the case if the SOA is at the input of the MZM. The waveform distortion is accompanied by a spectral red-shift, which degrades the transmission performance. Simulations show that for MZM losses below ~4 dB, locating the SOA before the MZM can provide a higher power with no waveform distortion and negative chirp, at the cost of a higher SOA gain. For higher MZM losses it is unfeasible to locate the SOA before the MZM, due to a prohibitively large power consumption.