We investigate the properties of propagation and self-pumped phase conjugation (SPPC) of 90-fs laser pulses at 450nm in BaTiO3. Femtosecond SPPC in BaTiO3, both in air and in immersion oil, is explored; it is demonstrated that femtosecond self-pumped phase-conjugate pulses are generated from all crystallographic faces of BaTiO3 in immersion oil. We measure the duration of the pulses after propagation and SPPC by using electric field cross correlation. The duration of SPPC increases as the incident angle increases. Because of the partial compensation of the negative angular dispersion, the self-pumped phase-conjugate pulse undergoes a total dispersion smaller than that of the transmitted pulse. By managing the angular dispersion of the femtosecond phase conjugator, we can control the total dispersion and thus the duration of the self-pumped phaseconjugate pulses. The experimental results agree with the theoretical simulations.

Pulse propagation and shaping are investigated in photorefractive self-pumped phase conjugators in both transmission-and reflection-grating regimes. The dispersion properties of self-pumped phase conjugators are analyzed by taking into account both the grating dispersion and the angular dispersion. The complex transfer functions are obtained by treating the crystal as a linear dispersive medium. We show that the pulse width as a result of the self-pumped phase conjugation is much wider in the reflection regime than in the transmission regime. The experimental results are consistent with the results calculated for the transmission-grating regime, indicating that this type grating is the dominant mechanism in the case of a femtosecond self-pumped phase conjugator.

We investigate the form birefringence due to photoinduced one-and two-dimensional volume index gratings in DuPont photopolymers. The effective medium theory has been applied to the continuous volume subwavelength gratings. With the presence of a one-dimensional volume subwavelength grating, the originally isotropic photopolymer exhibits a negative form birefringence and behaves like a uniaxial dielectric. A form birefringence of 21.231023 at l5632.8nm is measured at a grating period of L5447 nm. Experimental results on the dispersion of the form birefringence versus the illumination wavelength and the grating period are presented. With the presence of a two-dimensional grating, in general, the originally isotropic photopolymer exhibits a biaxial form birefringence and behaves like a biaxial dielectric. Positive form birefringence can also be obtained by recording two volume gratings with different grating wave vectors. By recording two photoinduced volume gratings we can synthesize biaxial dielectrics with the principal axes along any desired directions. We also report the measured principal refractive indices of the medium due to the presence of two-dimensional volume gratings in DuPont photopolymer.

The effect of grating dispersion on pulse evolution in a photorefractive self-pumped phase conjugator is analyzed. The broadening factors and the time-dependent phases of self-pumped phase-conjugate pulses of unchirped and chirped Gaussian pulses are obtained. When the grating dispersion and the angular dispersion cancel the material dispersion, the self-pumped phase-conjugate pulse has a minimum width, which is determined by the first-order dispersion of the grating. The self-pumped phase conjugation of an unchirped Gaussian pulse broadens and is linearly chirped whereas the self-pumped phase conjugation of a chirped Gaussian pulse is compressed when the dispersion-induced chirp cancels the initial chirp.

We report the enhancement of the bandwidth and the response flatness of wavelength conversion through cascaded second-order nonlinear processes by sinusoidally chirped optical superlattices (SCOSs) in lithium niobate crystal. The conversion bandwidth of the SCOS is significantly larger than that of uniform and segmented gratings. The SCOS shows very flat conversion response in the 1.5-m region. The physical origin of the bandwidth enhancement is ascribed to the plentiful reciprocal wave vectors of the SCOS, which provide enough momenta to simultaneously satisfy quasi-phase matching conditions for both second-harmonic generation and difference-frequency generation.

We report difference-frequency generation (DFG)-based wavelength conversion with sinusoidally chirped optical superlattices (SCOSs) in lithium niobate waveguides. The SCOS shows much broader bandwidth, larger pump-wavelength tolerance and flatter response with some conversion efficiency penalty than the periodic grating with the same length. Especially, the conversion bandwidth of the SCOS is over the whole conventional band and long-wavelength band, which is very helpful to broadband optical communications. The main advantages of the DFG-based wavelength conversion with SCOS structures over the cascaded second-order effect-based wavelength conversion are higher conversion efficiency, broader bandwidth, better pump-wavelength tolerance and response flatness when the device is shorter. In order to ensure the conversion quality, analysis shows that the manufacture tolerance of the SCOS period should not more than 0.02lm.

We report a wavelength converter based on linearly chirped gratings in a lithium niobate crystal through cascaded second-order nonlinear processes. The in uence of the chirp coe

We propose a technique to simultaneously stabilize relative wavelengths among multiple diode lasers by using multiplexed photorefractive gratings. Each grating is designed to be Bragg matched for the counter-propagating reflection at a specific diode laser wavelength. Experimental results from stabilization of two diode laser wavelengths by using two photorefractive gratings exhibit excellent short-and long-term wavelength stability. We show that this technique can be extended to stabilize relative wavelengths of many diode lasers. The thermal stability of the photorefractive gratings and the signal-to-noise ratio (or the bit-error rate) are discussed.

Sinusoidal chirps are introduced in fiber gratings to improve their performance as dispersion compensators and multichannel filters. The sinusoidally chirped fiber gratings exhibit a flattop spectrum with steep edges and high reflectivity. The bandwidth utilization defined as the ratio of 1:30-dB bandwidth could be very high (＞0.85). This structure can be applied to the sampled fiber gratings to enhance channel uniformity. We demonstrate 264 uniform channels with a 25-GHz-spacing for high-densitywavelength multiplexing applications. Multichannel dispersion compensations with seven uniform channels of 50-GHz-spacing in short fiber gratings are also demonstrated. The impact of possible fabrication errors on the spectra of the gratings is discussed.

We investigate power-dependent phase-matched four-wave mixing FWM in wavelength division multiplexing transmission lines, in which positive and negative dispersion fibers are alternately arranged to manage the dispersion and the dispersion slope. The FWM effect shows power-independent phase matching when the channel power is low. However, it is power dependent at high power. The maximum FWM conversion efficiency is shifted away from the zero channel space in the case of powerdependent phase matching. Optimization of the dispersion system for suppression of the FWM effect is determined.