Investigation Of High-Speed All-Optical Signal Processing Based On LiNbO₃ Waveguides

All-optical signal processing is a key technology in future high-speed large-capacity all-optical communication networks. The all-optical signal processing using lithium niobate waveguides has distinct advantages of ultra-fast response and low noise, which has attracted more and more attentions in the recent years. In this dissertation, by using various second-order nonliearities and their cascading in peridocially poled lithium niobate (PPLN) waveguides, we focus on the theoretical and experimental investigations on several important all-optical signal processing applications including all-optical wavelength conversions, all-optical logic gates, all-optical format conversions, all-optical signal regeration, and all-opticl ultrawideband (UWB) signal generation. In particular, we discover a new characteristic of second-order nonlinearity and PPLN called"optical phase erasure". The detailed research contents can be found as follows.(1) The basic theories of PPLN-based all-optical signal processing are discussed. The analytical solutions to second-harmonic generation (SHG), sum-frequency generation (SFG), difference-frequency generation (DFG), cascaded second-harmonic generation and difference-frequency generation (cSHG/DFG), and cascaded sum- and difference-frequency generation (cSFG/DFG) are derived under the non-depletion approximation. The conversion efficiency and conversion bandwidth are analyzed. The conversion functions and optical phase conjugation (i.e. spectral inversion) are compared with each other.(2) We start a doubt on a traditional concept that"second-order nonlinearity and PPLN waveguide are always completely transparent". It is interesting to find that the PPLN waveguide actually also has the"non-transparent"aspect via both experimental and theoretical demonstrations. As a consequence, we discover a new characteristic of second-oder nonlinearity and PPLN waveguide capable of removing the optical phase information, which is called"optical phase erasure". With the help of such new characterictic of"optical phase erasure", we experimentally demonstrate 40 Gbit/s format conversion from carrier-suppressed return-to-zero (CSRZ) to return-to-zero (RZ), true demodulation of 40 Gbit/s differential phase-shift keying (DPSK) signals, 40 Gbit/s format conversions from optical duobinary (ODB) to non-return-to-zero (NRZ) or RZ, from alternate-mark inversion (AMI) to RZ, and the generation of single-polarity pseudo-return-to-zero (S-PRZ) signal. In addition, the"non-transparent"aspect and corresponding"optical phase erasure"characteristic of four-wave mixing (FWM) are also discovered in the experiment, based on which 40 Gbit/s all-optical format conversion from CSRZ to RZ is also experimentally demonstrated.(3) We experimentally investigate the high-speed all-optical wavelength conversions using various second-order nonlinearities and their cascading in PPLN waveguides. Based on cSFG/DFG, multi-functional all-optical wavelength conversion including fixed-in variable-out (tunable), variable-in fixed-out, and variable-in variable-out for 40 GHz picosecond pulses are carried out in the experiment. Simultaneously non-inverted and inverted wavelength conversions are observed. We design a novel PPLN-based double-ring tunble wavelength converter, with which tunable wavelength conversion for picosecond pulses is experimentally demonstrated. No external continuous-wave (CW) sources are required, which makes the configuration compact and easy to realize and effectively reduce the system complexity and cost. Based on cSHG/DFG, multicasting wavelength coversion for 40 GHz picosecond pulses, 40 Gbit/s wavelength conversions for advanced modulations formats including frequency-shift keying (FSK), ODB and AMI are successfully implemented in the experiment.(4) We theoretically and experimentally study the high-speed all-optical logic gates using various second-order nonlinearities and their cascading in PPLN waveguides. Based on SFG, 40 Gbit/s NRZ logic NOT gate is experimentally demonstrated. 40 Gbit/s dual-direction half-subtracter, logic XOR gate, logic OR gate are theoretically investigated. Basd on cSFG/DFG, single-PPLN-based 40 Gbit/s logic AND gate, logic XOR gate, logic OR gate, half-adder, and half-subtracter are theoretically studied. 20 Gbit/s and 40 Gbit/s three-input NRZ/RZ logic AND gate and three-input NRZ-DPSK/RZ-DPSK logic XOR gate are experimentally implemented. 40 Gbit/s logic AND gate for CSRZ together with CSRZ-to-RZ format conversion and 40 Gbit/s logic XOR gate for CSRZ-DPSK accompanied with CSRZ-DPSK-to-RZ-DPSK format conversion are demonstrated in the experiment. Based on cSHG/DFG, 40 Gbit/s logic AND gate for CSRZ but without format conversion is experimentally performed.(5) We theoretically and experimentally investigate the high-speed all-optical format conversions using various second-order nonlinearities and their cascading in PPLN waveguides. We propose a novel PPLN loop mirror (PPLN-LM)-based format converter. 40 Gbit/s format conversion from NRZ to RZ is theoretically studied by using SFG, cSHG/DFG, and cSFG/DFG in a PPLN-LM format converter. All-optical 10 Gbit/s and 20 Gbit/s NRZ-to-RZ format conversions are successfully demonstrated in the experiment. The synchronized pump optical clock is obtained with the help of all-optical clock recovery. We also propose and theoretically investigate all-optical 40 Gbit/s format conversions from NRZ-DPSK to RZ-DPSK, from NRZ to CSRZ, from RZ to CSRZ, from NRZ-DPSK to CSRZ-DPSK, and from RZ-DPSK to CSRZ-DPSK. In addition, experimental verifications of 40 Gbit/s NRZ-to-RZ, NRZ-to-CSRZ, and NRZ-DPSK-to-RZ-DPSK format conversions are performed in the experiment.(6) We propose and theoretically study the all-optical signal regeneration using a PPLN waveguide. It is found that the cSHG/DFG with the signal set at the SHG QPM wavelength has the ability to enhance the extinction ratio (ER), optical signal-to-noise ratio (OSNR) and Q-factor, and therefore is capable of improving the signal quality.(7) We theoretically and experimentally investigate the all-optical UWB signal generation using PPLN waveguides. A new idea is suggested to generate the arbitray-order UWB signal pulses, i.e. the UWB pulses can be regarded as the combination of"overshoots"and"undershoots". It is possible to generate"overshoots"by parametric amplification and to achieve"undershoots"by parametric attenuation. By using SFG and cSHG/DFG in cascaded PPLN waveguides, we theoreticall demonstrate the generation of UWB Monocycle, UWB Doublet, UWB Triplet, UWB Quadruplet and UWB Quintuple. Additionally, by using the parametric attenuation effect during the SFG process in single PPLN waveguide, we successfully implement the generation of UWB monocycle in the experiment.