**3.2. Other advanced format conversions techniques**

Higher order all-optical modulation formats can be generated using the phase modulator setup of **Figures 15** and **16** as a building block. For example, all-optical OOK to quadrature phase shift keying (QPSK) converter can be constructed with nested amplitude to binary phase shift keying (BPSK) converter pairs [23]. When the overall phase difference between the two nested BPSK pairs is set to /2 rad, a QPSK signal is generated. Kang et al. [24] demonstrate the viability of this conversion scheme to generate a QPSK signal with 173 Gbps.

Furthermore, one may build a modulator for generating even higher order modulation formats, including quadrature amplitude modulation (QAM), following a similar method by which the amplitude to QPSK format converter is constructed [25]. Other methods of all-optical format conversion techniques using MZI-SOA are subject of further research in Ref. [26].
