基于铌酸锂光波导的高速全光信号处理技术研究
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摘要
全光信号处理是未来高速大容量全光通信网络的关键技术。基于铌酸锂光波导的全光信号处理具有超快、低噪声等优点,近年来受到越来越多的关注。本文围绕基于周期极化反转铌酸锂(PPLN)光波导的二阶和级联二阶非线性效应开展了全光波长转换、全光逻辑门、全光码型转换、全光信号再生和全光超宽带(UWB)信号产生等理论和实验研究,发现了二阶非线性和PPLN光波导“光相位擦除”新特性,具体内容如下:
(1)讨论了基于PPLN光波导全光信号处理的基本理论。推导了倍频(SHG)、和频(SFG)、差频(DFG)、级联倍频和差频(cSHG/DFG)、级联和频与差频(cSFG/DFG)等二阶和级联二阶非线性效应非耗尽近似条件下的解析解,分析了转换效率和转换带宽,比较了转换功能和相位共轭即光谱反转特性。
(2)对一个传统观念“二阶非线性和PPLN光波导完全透明”产生疑问并通过实验和理论论证了PPLN光波导其实也存在“不透明”的一面,并由此发现了二阶非线性和PPLN光波导“光相位擦除”的新特性。利用这一“光相位擦除”新特性实验实现了40 Gbit/s载波抑制归零码(CSRZ)到归零码(RZ)的码型转换、40 Gbit/s差分相移键控(DPSK)信号的真解调、光双二进制码(ODB)到非归零码(NRZ)和RZ的码型转换、传号交替反转码(AMI)到RZ的码型转换以及单极性伪归零码(S-PRZ)的产生。另外,实验发现了四波混频(FWM)“不透明”的一面及其“光相位擦除”新特性,同时实现了40 Gbit/s CSRZ到RZ的全光码型转换。
(3)实验研究了基于PPLN光波导二阶和级联二阶非线性效应的高速全光波长转换。基于cSFG/DFG实验实现了40 GHz皮秒脉冲固定输入-可变输出(可调谐)、可变输入-固定输出、可变输入-可变输出等多功能全光波长转换以及同时正反向波长转换。构建新颖的PPLN光波导双环腔可调谐波长转换器基于cSFG/DFG实验实现了可调谐全光波长转换,无需任何外界注入连续光源,装置结构简单易实现,有效降低了系统复杂性和成本。基于cSHG/DFG实验实现了40 GHz皮秒脉冲“广播式”波长转换和40 Gbit/s频移键控(FSK)、光双二进制(ODB)、传号交替反转(AMI)等先进高级调制格式的波长转换。
(4)理论和实验研究了基于PPLN光波导二阶和级联二阶非线性效应的高速全光逻辑门。基于SFG实验实现了40 Gbit/s NRZ非(NOT)门,理论研究了40 Gbit/s双向半减器、异或门(XOR)、或门(OR)。基于cSFG/DFG提出并理论研究了仅利用单个PPLN光波导同时实现40 Gbit/s与(AND)门、XOR门、OR门、半加器和半减器。实验实现了20 Gbit/s和40 Gbit/s三信道NRZ和RZ输入AND门以及20 Gbit/s和40 Gbit/s三信道NRZ-DPSK和RZ-DPSK输入XOR门。实验研究了40 Gbit/s CSRZ同时码型转换和AND门、CSRZ-DPSK同时码型转换和XOR门。基于cSHG/DFG实验实现了40 Gbit/s CSRZ的AND门。
(5)理论和实验研究了基于PPLN光波导二阶和级联二阶非线性效应的高速全光码型转换。提出一种新颖的PPLN光波导环形镜码型转换器,基于SFG、cSHG/DFG、cSFG/DFG理论研究了40 Gbit/s NRZ到RZ的码型转换。实验实现了基于cSHG/DFG、cSFG/DFG 10 Gbit/s和20 Gbit/s NRZ到RZ的码型转换,采用全光时钟提取的方法获得同步抽运光时钟。提出并理论研究了40 Gbit/s NRZ-DPSK到RZ-DPSK、NRZ到CSRZ、RZ到CSRZ、NRZ-DPSK到CSRZ-DPSK以及RZ-DPSK到CSRZ-DPSK的码型转换并进行了验证性实验。
(6)提出并理论研究了基于PPLN光波导的全光信号再生。基于cSHG/DFG且将信号光置于SHG过程准相位匹配(QPM)波长处,此时波长转换具有提高消光比、光信噪比和品质因子的功能,因而可以改善信号质量。
(7)理论和实验研究了基于PPLN光波导的全光UWB脉冲信号产生。提出一种新的思路和方法用于产生任意阶UWB脉冲信号,即将UWB脉冲视为“过冲”和“下冲”的合成,利用参量放大可产生“过冲”而参量衰减可产生“下冲”,基于级联PPLN光波导SFG和cSHG/DFG理论研究了UWB Monocycle、UWB Doublet、UWB Triplet、UWB Quadruplet和UWB Quintuplet的产生。基于单个PPLN光波导SFG参量衰减作用实验实现了UWB Monocycle的产生。
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.
(1)讨论了基于PPLN光波导全光信号处理的基本理论。推导了倍频(SHG)、和频(SFG)、差频(DFG)、级联倍频和差频(cSHG/DFG)、级联和频与差频(cSFG/DFG)等二阶和级联二阶非线性效应非耗尽近似条件下的解析解,分析了转换效率和转换带宽,比较了转换功能和相位共轭即光谱反转特性。
(2)对一个传统观念“二阶非线性和PPLN光波导完全透明”产生疑问并通过实验和理论论证了PPLN光波导其实也存在“不透明”的一面,并由此发现了二阶非线性和PPLN光波导“光相位擦除”的新特性。利用这一“光相位擦除”新特性实验实现了40 Gbit/s载波抑制归零码(CSRZ)到归零码(RZ)的码型转换、40 Gbit/s差分相移键控(DPSK)信号的真解调、光双二进制码(ODB)到非归零码(NRZ)和RZ的码型转换、传号交替反转码(AMI)到RZ的码型转换以及单极性伪归零码(S-PRZ)的产生。另外,实验发现了四波混频(FWM)“不透明”的一面及其“光相位擦除”新特性,同时实现了40 Gbit/s CSRZ到RZ的全光码型转换。
(3)实验研究了基于PPLN光波导二阶和级联二阶非线性效应的高速全光波长转换。基于cSFG/DFG实验实现了40 GHz皮秒脉冲固定输入-可变输出(可调谐)、可变输入-固定输出、可变输入-可变输出等多功能全光波长转换以及同时正反向波长转换。构建新颖的PPLN光波导双环腔可调谐波长转换器基于cSFG/DFG实验实现了可调谐全光波长转换,无需任何外界注入连续光源,装置结构简单易实现,有效降低了系统复杂性和成本。基于cSHG/DFG实验实现了40 GHz皮秒脉冲“广播式”波长转换和40 Gbit/s频移键控(FSK)、光双二进制(ODB)、传号交替反转(AMI)等先进高级调制格式的波长转换。
(4)理论和实验研究了基于PPLN光波导二阶和级联二阶非线性效应的高速全光逻辑门。基于SFG实验实现了40 Gbit/s NRZ非(NOT)门,理论研究了40 Gbit/s双向半减器、异或门(XOR)、或门(OR)。基于cSFG/DFG提出并理论研究了仅利用单个PPLN光波导同时实现40 Gbit/s与(AND)门、XOR门、OR门、半加器和半减器。实验实现了20 Gbit/s和40 Gbit/s三信道NRZ和RZ输入AND门以及20 Gbit/s和40 Gbit/s三信道NRZ-DPSK和RZ-DPSK输入XOR门。实验研究了40 Gbit/s CSRZ同时码型转换和AND门、CSRZ-DPSK同时码型转换和XOR门。基于cSHG/DFG实验实现了40 Gbit/s CSRZ的AND门。
(5)理论和实验研究了基于PPLN光波导二阶和级联二阶非线性效应的高速全光码型转换。提出一种新颖的PPLN光波导环形镜码型转换器,基于SFG、cSHG/DFG、cSFG/DFG理论研究了40 Gbit/s NRZ到RZ的码型转换。实验实现了基于cSHG/DFG、cSFG/DFG 10 Gbit/s和20 Gbit/s NRZ到RZ的码型转换,采用全光时钟提取的方法获得同步抽运光时钟。提出并理论研究了40 Gbit/s NRZ-DPSK到RZ-DPSK、NRZ到CSRZ、RZ到CSRZ、NRZ-DPSK到CSRZ-DPSK以及RZ-DPSK到CSRZ-DPSK的码型转换并进行了验证性实验。
(6)提出并理论研究了基于PPLN光波导的全光信号再生。基于cSHG/DFG且将信号光置于SHG过程准相位匹配(QPM)波长处,此时波长转换具有提高消光比、光信噪比和品质因子的功能,因而可以改善信号质量。
(7)理论和实验研究了基于PPLN光波导的全光UWB脉冲信号产生。提出一种新的思路和方法用于产生任意阶UWB脉冲信号,即将UWB脉冲视为“过冲”和“下冲”的合成,利用参量放大可产生“过冲”而参量衰减可产生“下冲”,基于级联PPLN光波导SFG和cSHG/DFG理论研究了UWB Monocycle、UWB Doublet、UWB Triplet、UWB Quadruplet和UWB Quintuplet的产生。基于单个PPLN光波导SFG参量衰减作用实验实现了UWB Monocycle的产生。
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.
引文
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