Research & Applications Of High-Speed All-Optical Modulation-Format Conversions

All-optical signal processing is one of the promising technologies to omit optical-electric-optical converters in future all-optical networks. A lot of studies have been conducted on all-optical signal regeneration and wavelength conversion. Recently, all-optical format conversion has received much interest. It becomes a key technology to interconnect future optical networks, which could employ different modulation formats according to the network scales and applications. Much research on advanced modulation formats other than traditional non-return-to-zero (NRZ) and return-to-zero (RZ) has been done in recent years. These formats are employed in various transmission systems due to their different dispersion and nonlinearity performances.In this thesis, we first introduce the methods of generating and demodulating all kinds of optical modulation formats and analyze their transmission performances. Then, we reveal the intrinsic relations among various modulation formats. Finally, we focus on four novel schemes of format conversions as follows.1. Conversions from NRZ to (binary phase-shift-keying) PSK and quadrature phase-shift-keying (QPSK)While cost-effective NRZ is widely employed in optical metro networks, BPSK and QPSK formats may become the chosen formats in long-haul transmission systems due to their superior receiver sensitivity, high tolerance to chromatic dispersion and nonlinear impairments. Therefore, all-optical format conversion from NRZ to BPSK or QPSK would be desirable at gateway nodes between metropolitan area networks (MANs) and wide area networks. We propose and numerically verify a novel scheme of all-optical format conversion from NRZ to BPSK at 160 Gb/s using cascaded microring resonators (CMRR) on a single silicon chip. The conversion is based on large phase shift and flattened intensity-response characteristics in the CMRR. All-optical format conversion from NRZ to quadrature phase-shift-keying (QPSK) is also demonstrated based on parallel NRZ/BPSK converters in a Mach-Zehnder interferometer structure.2. Conversion from NRZ to (frequency-shift-keying) FSKAs FSK format presents constant intensity, it has received considerable attention in passive optical networks (PONs). The downstream FSK signal often serves as a carrier for upstream re-modulation at the optical network unit. Therefore, NRZ to FSK format conversion would be desirable to interconnect a MAN and a PON system. We have proposed and experimentally demonstrated an all-optical metro-access interface using NRZ-to-FSK converter based on cross-gain modulation in a semiconductor optical amplifier.3. Conversion from carrier-suppressed return-to-zero (CSRZ) to duobinaryCSRZ shows good tolerance to some linear filtering and nonlinear impairments accumulated along a long-haul transmission line. Duobinary format provides narrow spectral bandwidth and large chromatic dispersion tolerance, showing its effectiveness in dispersion uncompensated metro or local networks. We propose and experimentally demonstrate a simple format conversion scheme from CSRZ to duobinary, which was not investigated before, using a Mach-Zehnder delay interferometer, suitable for multi-channel operation. 4. Conversion from differential PSK (DPSK) to FSKFor the first time, we have proposed and simulated a scheme of DPSK-to-FSK format conversion based on two parallel Mach-Zehnder modulators, which could become a necessary technology to interface a backbone network and a PON system.