Modified dilated Benes networks for photonic switching

A modified dilated Benes (1965) network composed of directional couplers is proposed. This structure is introduced to improve the signal-to-noise ratio (SNR) characteristics of dilated Benes networks. A new estimation of the SNR for dilated Benes networks is derived, and it is shown that this SNR is much worse than that previously known. The SNR for modified dilated Benes networks is estimated and compared to dilated Benes and other network architectures. Some other properties including the number of switching elements required, number of crossovers, and system attenuation are also derived and analyzed. Most of the characteristics are shown to be similar to dilated Benes networks and better than those of other well-known networks fabricated in Ti:LiNbO ₃     

Prototype 32×32 optical switch matrix

A 32×32 optical switch matrix with a banyan network architecture is demonstrated using a Ti:LiNbO₃ waveguide. The switching voltage was 24 V and extinction ratio averaged -18 dB. The prototype device showed path dependent insertion loss deviation of 10 dB. The device can be used as a building block for many functional optical switch matrices     

Optical digital cross-connect system using photonic switch matricesand optical amplifiers

This paper presents, for potential application to network failure restoration, an optical digital cross-connect system (DCS) which uses both a photonic switching network and an electric DCS. It is shown that a system constructed of LiNbO₃ 8×8 switch matrices and semiconductor traveling-wave optical amplifiers (TWA's) could be applied to metropolitan area networks. An experimental optical DCS system has been designed and fabricated which incorporates both LiNbO₃ switch matrices and TWA's, and with it, line-failure restoration, a fundamental operation of optical DCS, has been successfully demonstrated     

Optical switch matrix with simplified N×Ntree structure

Optical switch matrices with compact size and excellent crosstalk characteristics are discussed. Novel architectures for optical switch matrices are obtained by condensing the structure of N× N tree structure. 4×4 switches fabricated on LiNbO₃ substrates are discussed     

Design and realization of an integrated optic switch for crossbarswitching arrays

An electrooptic switch that is insensitive to parameter tolerances is proposed for use in crossbar switching arrays. It makes use of intrinsically mismatched and tapered directional couplers and allows the realization of switching arrays with the same switching voltage for each element. Computations are carried out to verify a realization with Ti:LiNbO₃ waveguides and to compare the latter with conventional directional couplers. Switching elements on Z-cut LiNbO₃ with a crosstalk of -27 dB for zero voltage and a switching voltage of 27.5 V have been realized for use at 1.3 μm     

Optical and physical characterization of SiO2-x-Althin-film polarizer on x-cut LiNbO3 substrate

For the purpose of a mechanical evaluation of a metal-cladding polarizer, a precise characterization of SiO_2-x-Al thin-film succession fabricated on a LiNbO₃ substrate was made as well as an experimental optimization of the SiO_2-x-Al polarizer for the Ti:LiNbO₃ waveguide. A 10-nm-thick SiO_2-x was selected as the optimized underlay of a SiO_2-x-Al polarizer for the Ti:LiNbO₃ waveguide using a wavelength of λ=1.55 μm. Results of scratch testing show that the adhesive strength of SiO_2-x-Al films was almost the same level as that of Ti-Au films on a thick SiO₂ layer, commonly used for metallic underlay of Au-plated electrodes. From observing SiO_2-x -Al film using a transmission electron microscope, it was confirmed that the 10-nm-thick SiO_2-x underlay stratified well without serious thickness fluctuation     

Analysis of end-pumped Nd:Ti:LiNbO3 microchip waveguideFabry-Perot lasers

A self-consistent analysis of end-pumped Nd:Ti:LiNbO₃ microchip waveguide lasers based on the fast Fourier transform beam propagation method (FFT-BPM) has been proposed. The algorithm of the model allows one to describe the laser gain and pump absorption in terms of the complex atomic susceptibility for the case of the Nd³⁺ ions. Considering the interference effects between the forward and backward light waves, the population inversion longitudinal and transversal spatial effects can be simulated. The laser characteristics of the Nd:Ti:LiNbO₃ waveguide laser correlate well with the experimental data and theoretical results. The design rules for the optimized microchip laser are also developed by using the proposed model     

Optical time-division multiplexing for very high bit-ratetransmission

Optical time-division multiplexing (OTDM) extends and expands the well-known techniques of electrical time-division multiplexing into the optical domain. In OTDM, optical data streams are constructed by time-multiplexing a number of lower-bit-rate optical streams. Opportunities for very high-speed transmission and switching are created by removing limitations set by the restricted bandwidth of electronics and by capitalizing on the inherent high-speed characteristics of optical devices. An overview of recent work in optical time-division multiplexing and demultiplexing is presented. Design considerations affecting system architecture are described. Emphasis on the factors that limit system performance, such as crosstalk between multiplexed channels. Examples of very high bit-rate optical time-division multiplexed system experiments using short pulses from mode-locked semiconductor lasers and high-speed Ti:LiNbO₃ waveguide switch/modulators are presented     

Wet-etched ridge waveguides in y-cut lithium niobate

By the technique of nickel indiffusion proton exchange (NIPE) and the technique of buffered proton exchange (PE) melt, wet-etched ridge waveguides in y-cut LiNbO₃ are fabricated for the first time. The fabricated ridge waveguides have smooth surfaces and are good enough for low-loss waveguides. Moreover, a ridge waveguide Mach-Zehnder modulator in y-cut LiNbO₃ is fabricated. The measured half-wave voltage is about 30% lower than that of a conventional modulator. The wet-etched ridge waveguides in y-cut and z-cut substrates are also characterized for comparison     

A high-speed 4×4 Ti:LiNbO3 integrated optic switchat 1.5 μm

The operation of a fully packaged and pigtailed polarization dependent Ti:LiNbO₃ 4×4 integrated optic switch with an operating wavelength of 1.5 μm is demonstrated. The switching matrix is fast, with a measured 3-dB small-signal bandwidth greater than 1 GHz. Unwanted cross modulation between channels due to coupling between switching elements was measured and found to be less than -20 dB of the signal strength. The switch is composed of balanced bridge switching elements having a 17-V switching voltage, an 18 dB extinction ratio, and an excess optical loss of 0.6 dB     

An improved procedure to calculate the refractive index profilefrom the measured near-field intensity

Refractive index profile of optical waveguides is reconstructed from the measured transmitted near-field intensity. A Butterworth low-pass digital filter is employed in the frequency domain to remove impulsive and high frequency fluctuations which have severe effects on the procedure to calculate the index profile from the measured power intensity. The proposed method has been applied to measure the index profile of monomode optical fiber, Ti:LiNbO₃ and buried MgTi:LiNbO₃ channel waveguides     

Ferroelectric integrated optics: recent developments

Advances in rare-earth-doped waveguide lasers in LiNbO₃, waveguided second harmonic generation through quasi-phase-matching in LiNbO₃ and LiTaO₃, and pulsed laser epitaxial growth of LiNbO₃ on Sapphire are reported. Efficient lasers operating in CW, Q-switch, and mode-lock modes were fabricated in Nd:LiNbO₃. In order to assess the potential of bulk-doped Er:LiNbO₃ samples, spectroscopic measurements have been carried out, the main limiting mechanism identified, the corresponding up-conversion coefficient evaluated, and a comprehensive gain model developed. First lasing action in this material, when pumped at 1.48 μm, is described. Quasi-phase-matched generation of blue light with conversion efficiencies up to 150%/Wcm² is also demonstrated, and finally we present the fabrication of low-loss LiNbO₃ single-crystal waveguiding layers     

Optical TDM sorting networks for high-speed switching

The general time-space-time switching problem in telecommunications requires the use of multichannel time slot interchangers. We propose two multichannel time slot sorters which sort N² time-division multiplexed (TDM) optical inputs, arranged as N frames with N time slots per frame using O(Nlog²N) optical switch elements. The TDM optical inputs are sorted in place without expanding the space-time fabric into a space-division switch. The hardware components used are 2×2 optical switches (LiNbO₃ directional couplers) and optical delay lines connected in a feedforward fashion. Two space-time variants of the spatial odd-even merge algorithm are used to design the sorters. By maintaining the number of shift-exchange operations invariant at each stage, the proposed sorters use fewer switches than previously proposed sorters using switches with feedback line delays. The use of local control at each 2×2 switch makes the proposed sorters more practical for high-speed optical inputs than Benes-based time slot permuters with global control and high latency, which affects interframe distance. Both time slot sorters support pipelining of input frames and sorted outputs are available at each time slot after an initial frame delay. The proposed sorters find practical application in the time-domain equivalents of space-division, nonblocking, self-routing packet switches using the sort-banyan architecture, such as the Starlite switch, Sunshine switch, etc     

Fluctuational crosstalk in Ti:LiNbO3 directional couplerswitches

It has been found that the fluctuations in the Ti concentration along waveguides of Ti:LiNbO₃ directional coupler switches degrade the switch extinction ratio and introduce an unwanted crosstalk between both channels. We discuss analytically the effect of effective optical index fluctuations on the transfer matrix of the optical switch. We demonstrate the sensitivity of the mismatch to the statistics of the fluctuations of the Ti concentration     

Low-drive-power asymmetric Mach-Zehnder modulator with band-limitedoperation

A novel 2×2 asymmetric Mach-Zehnder interferometric modulator is fabricated on Ti:LiNbO₃ waveguide and tested at microwave frequency. Band-limited operation was realized by using a traveling-wave periodic polarity reversed structure with the asymmetric strip line as the modulator electrode instead of the conventional coplanar waveguide. For a modulator with a three-section polarity reversed electrode, a 3-dB bandwidth of 7 GHz was obtained experimentally from 9 to 16 GHz centered at 12 GHz     

Lithium–Niobate Channel Waveguide for the Realization of Long-Period Gratings

We propose a special lithium-niobate (LiNbO₃) single-mode waveguide for the realization of long-period gratings, which consists of a channel core embedded in a thin slab cladding. We fabricated the waveguide on a z-cut LiNbO₃ substrate with a two-step proton-exchange process and demonstrated its suitability for grating application with a number of removable photoresist long-period gratings deposited on the waveguide surface. The waveguide fabrication process and the LiNbO₃ waveguide structure could be further explored for the development of electrooptic gratings for high-speed applications.     

Evaluation of absorption and emission cross sections of Er-dopedLiNbO3 for application to integrated optic amplifiers

The polarization-dependent absorption and emission spectra of the ⁴I_13/2-⁴I_15/2 transition (λ~1.5 μm) in single crystal bulk Er:LiNbO₃ have been measured. Low-temperature (10 K) measurements of the Stark split energy levels of these two manifolds indicate at least two Er³⁺ sites. McCumber theory is applied to determine the Er:LiNbO₃ absorption and emission cross sections. These values are used to calculate the gain characteristics of Er:LiNbO₃ channel waveguides. Calculations indicate that a gain of 10 dB is achievable in a waveguide of several centimeters using ~20-mW pump power     

Polarisation independent switch and polarisation splitter employingΔβ and Δκ modulation

The authors report a novel type of polarisation independent directional coupler switch on X-cut, Y-propagating Ti:LiNbO₃, with modest fabrication tolerances. The device can also be used as a polarisation splitter. The principle of operation and experimental results are given     

Metal-ferroelectric-semiconductor (MFS) FET's using LiNbO3/Si (100) structures for nonvolatile memory application

N-channel metal-ferroelectric-semiconductor field-effect-transistors (MFSFET's) by using rapid thermal annealed LiNbO₃/Si (100) structures were fabricated and demonstrated nonvolatile memory (NVM) operations of the MFSFET's. The estimated field-effect electron mobility and transconductance on a linear region of the fabricated FET were 600 cm²/V·s and 0.16 mS/mm, respectively. The I_D-V_G characteristics of MFSFET's showed a hysteresis loop due to the ferroelectric nature of the LiNbO₃ films. The drain current of the “on” state was more than four orders of magnitude larger than the “off” state current at the same “read” gate voltage of 0.5 V, which means the memory operation of the MFSFET     

Analysis of Nd:MgO:Ti:LiNbO3 waveguide lasers withnonuniform concentration distributions

We propose a theoretical analysis for the Ti-diffused Nd:MgO:LiNbO ₃ waveguide lasers with nonuniform concentration distributions by using the evolution equations of the pumping light and amplified spontaneous emission (ASE). The concept that ASE is the origin of the laser light is applied, and the evolutions of the pumping and ASE are calculated by considering the feedback from the reflectors at the waveguide ends. Furthermore, because this analysis also considers the spatial distributions of the Nd and Ti concentrations, pumping, and ASE light in the waveguide laser, it is appropriate for a case involving arbitrary Nd and Ti concentrations and waveguide profile. Also, the laser characteristics of the Ti-diffused channel waveguide structures in the Nd-diffused MgO:LiNbO₃ and Nd:MgO:LiNbO₃ crystals are analyzed and compared. Moreover, theoretical data correlate well with the experimental results reported previously