Novel design method for all-optical ultrafast gate switches using cascaded second-order nonlinear effect in quasi-phase matched LiNbO/sub 3/ devices

The authors propose a novel method of designing all-optical ultrafast gate switches that employ the cascade of the second harmonic generation and difference frequency mixing in quasi-phase matched LiNbO/sub 3/ devices. In this design method, the device length is maximized under the condition that crosstalk of the switch is maintained below a certain allowable value at a given bit rate. Following the design method, the authors find that the switching efficiency can significantly be enhanced by compensation for the walkoff between the fundamental and second harmonic pulses.     

Characteristics of all-optical ultrafast gate switches using cascade of second-harmonic generation and difference frequency mixing in quasi-phase-matched lithium niobate waveguides

In all-optical gate switches that employ the cascade of second-harmonic generation and difference frequency mixing in quasi-phase-matched (QPM) lithium niobate (LN) devices, walkoff between the fundamental and second-harmonic pulses causes crosstalk of the switch. We numerically calculate the switching performance of QPM-LN waveguide devices with consideration for the crosstalk effect, and discuss the favorable choice of the nonlinear optical tensor element for the LN crystal. In the analyses, the device length is maximized for a given bit rate under the condition that the crosstalk is maintained below a critical level that is determined by the allowable power penalty for the switched signal. It is shown that the time offset between the gate and signal pulses can compensate for the walkoff, and can significantly improve the switching efficiency. In ultrafast operation beyond 1 Tbps, the use of the nonlinear optical tensor element d/sub 31/ is found to be favorable when compared to the use of the commonly used maximum tensor element d/sub 33/.     

Optimisation of LiNbO/sub 3/ BOA waveguide switches operating at 1.3 mu m

The optimisation of X-cut LiNbO/sub 3/ BOA switches for low voltage, low insertion loss operation is described. An LiNbO/sub 3/ waveguide switch operating at 1.3 mu m with an insertion loss of 4.3 dB, switching voltage of 4.7 V and extinction ratio of 21 dB is reported.<>     

Two-section reversed /spl Delta//spl beta/ switch with uniform electrodes and domain reversal

The performance of the reversed-/spl Delta//spl beta/ LiNbO/sub 3/ optical directional coupler (DC) switch with electrodes of opposite polarity is known to be more tolerant to variations in coupling length than that of the single-section DC. However, sectioned electrodes are incompatible to a single microwave source and, therefore, preclude the use of the device in high-speed applications. In this letter, we discuss the operation and results of a modified DC that uses a domain reversal and uniform electrodes that can be readily matched to a microwave source. A crosstalk of -34 dB was measured in an experimental device, fabricated in Z-cut LiNbO/sub 3/. Factors which improve device performance are also discussed.     

New method of domain inversion in LiNbO/sub 3/ using thermal oxidation of Ti film

A new technique for fabricating domain inversion regions in -c face LiNbO/sub 3/ by thermal oxidation of Ti is reported. This technique can minimise refractive index changes in such regions, thus it is suitable for quasiphase-matching (QPM) second harmonic generation (SHG). A prototype QPM waveguide SHG device is also demonstrated.<>     

All-optical time-division demultiplexing of 160 Gbit/s signal using cascaded second-order nonlinear effect in quasi-phase matched LiNbO/sub 3/ waveguide device

Using the cascade of the second-harmonic generation and the difference frequency mixing in a 30 mm-long quasi-phase matched LiNbO/sub 3/ waveguide device, all-optical time-division demultiplexing of a 160 Gbit/s signal into 10 Gbit/s channels is demonstrated. The power penalty is as small as 2.5 dB at the bit error rate of 10/sup -9/, which is attributed to crosstalk of the switch.     

A polarization independent guided-wave LiNbO/sub 3/ electrooptic switch employing polarization diversity

A 2*2 lithium niobate optical switch is demonstrated which uses polarization diversity to obtain polarization-independent operations with good crosstalk, a simple electrical drive configuration, and low drive voltage. The switch, on Z-cut LiNbO/sub 3/, is suitable for operation at 1.3 mu m with standard single-mode fibers. Resonant metal-loaded directional coupler mode-splitters enable the processing of each polarization component to be separately optimized. Switching voltages of 7 V for transverse magnetic and 24 V for transverse electric components have been achieved using a single substrate.<>     

Optical gain in proton-exchanged Cr:LiNbO/sub 3/ waveguides

The authors report on a study of optical gain in chromium-doped LiNbO/sub 3/ waveguides formed by the proton-exchange process. Optical gain in proton-exchanged Cr:LiNbO/sub 3/ channel waveguides has been measured for the first time. These waveguides show potential for diode-laser pumping of an integrated, broadband tunable laser in the 750-1150 nm spectral range.<>     

Polarisation-switching-induced resistance change in ferroelectric Mg-doped LiNbO/sub 3/ single crystals

Reversible resistance change at room temperature by polarisation switching is reported in ferroelectric single crystals. Resistance switching up to six orders of magnitude in Mg-doped LiNbO/sub 3/ was found. The resistance change is caused by the polarisation switching and increases with Mg concentration. Based on these results, continuous switching properties and memory behaviours were demonstrated.     

Nonlinear pulse switching using cross-phase modulation and fiberBragg gratings

We propose and analyze a new all-optical nonlinear switch. The device is based on cross-phase modulation (XPM)-induced temporary frequency shift upon co-propagation of signal and control pulses with walkoff, and on the filtering effect of a fiber Bragg grating (FBG). Very high ON/OFF contrast with practically no pulse distortion can be achieved with moderate switching power. The new device possesses some important advantages over nonlinear optical-loop mirror switches     

Space and/or polarization diversity multiplexing with type II second-harmonic generation

We describe the interplay among solitary waves steering and polarization switching in a KTiOPO/sub 4/ crystal with type II second-harmonic generation in the presence of walkoff. We show that this nonlinear interaction could be used in addressing a space and/or polarization diversity device, driven by the input polarization state.     

Passive integrated optical polarisation mode-splitter in lithium niobate employing a resonant metal-loaded structure

A passive polarisation mode-splitter has been demonstrated in Z-cut Y-propagating LiNbO/sub 3/ which achieves polarisation crosstalk better than -30 dB and excess loss of less than 1 dB in both polarisation states at lambda =1.52 mu m. The technique employed simplifies the design of polarisation mode splitters for arbitrary operating wavelengths and LiNbO/sub 3/ wafer orientations.<>     

Gain-transparent SOA-switch for high-bitrate OTDM add/dropmultiplexing

We report on an all-optical interferometric optical time-division multiplexing switch that exhibits high linearity, high-switching contrast, low noise, wide bandwidth, and low crosstalk. The key element of the gain-transparent switch is a semiconductor optical amplifier (SOA), which is transparent for the data signal. However, the injection of optical control pulses in the gain wavelength region of the SOA leads to index modulations at the wavelength of the data. This variation of the refractive index can be used for interferometric switching. In the application as add/drop multiplexer, the switch has the inherent advantage of leaving the nonswitched pulses undisturbed     

Theoretical analysis on polarization deviation and switch windowoptimization in nonlinear optical loop mirror demultiplexer

This paper investigates theoretically two of the dominant issues on a nonlinear optical loop mirror (NOLM) demultiplexer: the sensibility to the polarization deviation between signal and control pulses and the optimization of the switch window width. The complete nonlinear Schrodinger equations concerning the different states of polarization between signal and control lights are firstly established to study the impact of the polarization deviation on the demultiplexed signal. Considering simultaneously the channel crosstalk and the timing jitter between signal and control pulses, the switch window width of NOLM is optimized to achieve the best demultiplexing performance. The theoretical analysis shows that the polarization deviation has to be controlled less than 20° within the bit error rate (BER) of 10^-9 s. The optimal amount of the pulse walkoff is a little less than half of the slot width of the OTDM system     

A low-voltage 8×8 Ti:LiNbO3 switch with adilated-Benes architecture

An 8×8 switch array with a dilated-Benes architecture that greatly relaxes the crosspoint extinction ratio requirements needed to achieve low overall switch array crosstalk is discussed. This, combined with the low uniform switching voltages (9.2+0.2 V) of the 48 directional coupler crosspoints, facilitates high-speed low-crosstalk operation. The crosspoints can be switched in about 1 ns. The switch array is fully packaged with permanently attached single-mode fiber pigtails. The high data transfer rate inherent in lithium niobate switches in general, combined with the low crosstalk and high switching speed of this switch array, is a good match to the requirements of time-multiplexing switching     

Performance of an 8×8 LiNbO3switch matrix as a gigahertz self-routing switching node

The performance of an LiNbO3 integrated-optic crossbar switch as a node in a gigahertz self-routing network is measured. Switch throughput supports 12.5 Gbit/s signals with a measured switching speed of 1.33 GHz. Crosstalk due to RF/ acousto-optic coupling and modulation depth are reported at 1.33 GHz. Optical self-routing of l00 Mbit/s information using the 8×8 switch is demonstrated.     

Crosstalk measurements of integrated high-speedTi:LiNbO3 Δβ-reversal switching circuits

Directional coupler crosstalk was measured at multigigahertz frequencies using an optical sampling method for improved sensitivity. At both 2- and 4-GHz switching frequency, the crosstalk in both switch states of the coupler was ⩽-19 dB for an element of a 1×4 switch array intended for time-division-multiplexing applications. The authors also report measurements of intercoupler crosstalk for this switch array     

Thermo-optic effect in LiNbO3, for light deflection and switching

Light deflection and switching in LiNbO3 via thermo-optic effect based on the temperature dependence of the refractive index are demonstrated. The switching speed of 1 ms order has been confirmed in the thermo-optically induced waveguide switch.     

1/spl times/2 and 1/spl times/4 electrooptic switches

We describe the design, fabrication, and testing of two packaged electrooptic switches built from poled LiTaO/sub 3/ crystals. The 1/spl times/2 switch requires a driving voltage of 1200 V and exhibits insertion loss of 2.4 dB and crosstalk of -39.2 dB; the 1/spl times/4 switch exhibits insertion loss and crosstalk of 2.8 dB and -40.6 dB, respectively, and operates using a 1100-V voltage source. The maximum deflection time between the channels is 86 ns.     

A novel wet-etching method using electric-field-assisted proton exchange in LiNbO/sub 3/

This paper presents a novel wet etching method for LiNbO/sub 3/ using electric-field-assisted proton exchange. By applying voltage with appropriate polarity on designed electrodes placed on both sides of substrate, the induced electric-field distribution can effectively suppress or enhance proton diffusion in the lateral and depth directions. Thus, the proton-exchanged range in LiNbO/sub 3/ can be expertly manipulated. Because the proton-exchanged region can be removed by using a mixture of HF/HNO/sub 3/ acids, the proposed wet-etching method can effectively control the shape of the etched region in the LiNbO/sub 3/ substrate. Under appropriate electrode and proton-exchange parameters, a vertical sidewall with smooth surface is successfully produced, which makes fabricating reflection mirrors and T-junctions in LiNbO/sub 3/ possible. By utilizing the proposed wet-etching method, optical integrated circuits with higher integration density can be fabricated in LiNbO/sub 3/.