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     

Method for calculating coupling length of Ti:LiNbO3waveguide directional couplers

Reports a theory for calculating the coupling length L _c of Ti:LiNbO₃ single-mode waveguide directional couplers from process parameters and operating wavelength. Estimates are accurate to within a factor of 2 compared with published experimental results for z-cut y-propagating LiNbO₃ devices for 0.63 μm⩽λ⩽1.56 μm. Use of this formalism to assess acceptable process parameter and wavelength tolerances is demonstrated     

Buried optical waveguide polarizer by titanium indiffusion andproton-exchange in LiNbO3

Buried optical waveguide polarizers on LiNbO₃ have been realized by titanium indiffusion, followed by proton-exchange and annealing. The proton-exchange process decreases the ordinary refractive index and so modifies the index profile of the titanium indiffused waveguide. The measured intensity profile is in good agreement with calculation. An aluminum film absorbs the surface TM mode on z-cut LiNbO₃, leaving a buried nearly symmetric TE mode with lower optical loss than surface-guided TE modes. The extinction ratio obtained is estimated to be greater than 50 dB/cm at 0.633 μm     

Dependence of transient gain on gas-flow velocity and dischargecurrent in the FAF CO2 laser amplifier

Transient behaviors in which the output power amplified through the fast-axial-flow (FAF) CO₂ laser amplifier decreases temporally after initiating the discharge are discussed. The output power becomes stable within a few minutes, i.e. the transient time. These phenomena should strongly depend on input irradiance, plasma length, gas-flow velocity v, and discharge current I _dis. The small signal gain γ₀ is formulated as a function of v and I_dis and the discharge time for a partially-homogeneously-broadened, slightly saturated gain medium at individual vibrational-rotational transitions     

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     

On the number of memories that can be perfectly stored in a neuralnet with Hebb weights

Let {w_ij} be the weights of the connections of a neural network with n nodes, calculated from m data vectors v¹, ···, v^m in {1,-1}ⁿ, according to the Hebb rule. The author proves that if m is not too large relative to n and the v^k are random, then the w_ij constitute, with high probability, a perfect representation of the v^k in the sense that the v ^k are completely determined by the w_ij up to their sign. The conditions under which this is established turn out to be less restrictive than those under which it has been shown that the v^k can actually be recovered by letting the network evolve until equilibrium is attained. In the specific case where the entries of the v^k are independent and equal to 1 or -1 with probability 1/2, the condition on m is that m should not exceed n/0.7 log n     

A novel integrated acoustooptic frequency shifter

An integrated optical frequency shifter that utilizes guided-wave acoustooptic Bragg diffractions in cascade from two tilted- and counterpropagating-surface acoustic waves is reported. The doubly and frequency-shifted diffracted light propagates in a fixed direction, but spatially resolved from the incident light, irrespective of the magnitude of frequency tuning. A device fabricated in a Y-cut LiNbO₃ planar waveguide has demonstrated desirable characteristics, including single-sideband suppressed-carrier properties, a frequency shift centered at 1 GHz, a tunable bandwidth of 165 MHz, a linear dynamic range greater than 41 dB, and high efficiency at the optical wavelength of 0.63 μm. These preliminary results suggest that a compact integrated acoustooptic frequency shifter module capable of multigigahertz center frequency and gigahertz bandwidth can be realized in a common LiNBO₃ substrate 0.2 cm×1.0 cm×1.5 cm in size     

Observation of a large number of new laser lines from12CD3F gas optically pumped with a continuouslytunable high-pressure pulsed CO2 laser

An observation is reported of a large number of new laser lines from ¹²CD₃F gas optically pumped with a continuously tunable high-pressure pulsed CO₂ laser. Making use of the coincidence of the 10 μm P and R branches of CO₂ with the v₃ and v₆ vibrational-rotational absorption bands of ¹²CD₃F, 180 laser lines were found in the wavenumber range between 8 and 55 cm^-1, all of them yet unknown; these lines are studied for characteristic properties of laser action. All laser lines are assigned as pure rotational transitions in the vibrational excited or ground states     

Experimental values for the hole diffusion coefficient andcollector transit velocity in P-n-p AlGaAs/GaAs HBTs

The diffusion coefficient (D_h) and a value for the collector velocity (v_h) of holes in AlGaAs/GaAs P-n-p HBTs (heterojunction bipolar transistors) were obtained from high-frequency measurements on structures with different base and collector widths. Quantities for D_h and v _h of 5.6 cm²/s and 5.5×10⁶ cm/s, respectively, were obtained by plotting the total emitter-collector delay versus inverse emitter current and extrapolating the data to infinite emitter current to obtain the base and collector transit delays. An f_t and f_max as high as 15 and 29 GHz, respectively, were obtained for non-self-aligned (1-μm emitter mesa/base contact separation) devices with a 2.6-μm×10-μm emitter     

Integrated waveguide modulator using a LiNbO3TE&rlarr2;TM convertor for electrooptic coherence modulation of light

The authors report a wavelength independent TE-TM mode convertor used as a waveguide electrooptic modulator in a coherence modulated system. The modulation voltages thus obtained (20 V) are about 10 times lower than those previously reported in earlier coherence modulated devices. The modulator is a strip waveguide fabricated on Y-cut Z-propagation, Ti-indiffused LiNbO₃. When used in conjunction with a birefringent plate and polarizers, the modulator is shown to imprint on light a signal encoded into an optical delay greater than the source coherence length. Demonstration of the device is reported using a highly multimode laser diode at 820 nm     

Intensity noise and spontaneous emission coupling insuperluminescent light sources

A simple expression for the noise in the photocurrent generated by the detection of light from an ideal superluminescent source is derived using a quantum amplifier model. The excess noise factor X is found to be related to the photocurrent I_d, electronic charge e, spontaneous emission coupling factor K , and full width at half maximum power of the emission spectrum Δv_1/2 by the expression X=0.664 I _d/eKΔv_1/2. Implications of this result for the performance of fiber-optic gyroscopes using superluminescent-diode (SLD) light sources and for the design of low-noise SLDs are discussed     

Ocean imaging with two-antenna radars

Synthetic-aperture radar (SAR) measurements are affected both by the radar backscattering cross section (σ⁰) and by the radial velocity (V_r) of the surface. An analysis of the capabilities of two-antenna SARs using a method for forming radial-velocity images to provide a general framework for evaluating the performance of different possible measurements is presented. Several key results are derived. One can measure σ⁰ and V _r separately, given a properly designed SAR. The error in V_r depends on the thermal noise in the receivers, the spacing of the two antennas, the coherence time of the surface (τ_c) and the spatial resolution of the measurement. There is an optimal separation of the antennas proportional to vτ _c, where v is the speed of the aircraft. In cases where radial velocities are unimportant, two-antenna SARs can be used to image azimuth-traveling waves with wavenumbers larger than the usual azimuth cutoff. The authors show how phase errors affect the accuracy of the measurement of V_r. In this development, they also provide a simple explanation for velocity bunching, a subject that has caused much controversy     

Coupled-mode analysis of second harmonic generation in the form ofCerenkov radiation from a channel optical waveguide

Optical second-harmonic generation (SHG) in the form of Cerenkov radiation from a channel waveguide is analyzed by a coupled-mode theory. A simple formula is given which expresses the SHG efficiency in terms of waveguide and optical nonlinear parameters and by which one can simulate the radiation pattern of SHG. Computational examples are plotted for LiNbO₃ crystals and reasonably agree with reported results. The analysis shows that the efficiency could be increased significantly if the sign of a d-constant in a substrate were modulated with a proper period     

Microwave properties of traveling-wave electrodes in LiNbO3 electrooptic modulators

The microwave characteristics of a traveling-wave electrode in electrooptic modulators on z-cut LiNbO₃ crystals with buffer layers with various dielectric constants are calculated by a quasi-TEM analysis. The theoretical results are compared with the measured results on z-cut crystals for coplanar waveguide (CPW) electrodes. The agreement between the calculated and the measured results is good     

Tensile strain dependence of Brillouin frequency shift in silicaoptical fibers

Brillouin frequency shift in a single-mode optical fiber has been measured as a function of tensile strain. The strain coefficient of normalized Brillouin frequency shift C≡(dv_B/ dϵ)/v_B is found to be 4.4 for silica fibers. This result shows the potential of Brillouin spectroscopy to evaluate tensile strain in the fiber with the strain resolution of about 2×10^-4. The origin of the large strain coefficient is discussed     

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     

Absolute and relative nonlinear optical coefficients of KDP, KD*P,BaB2O4, LiIO3, MgO:LiNbO3,and KTP measured by phase-matched second-harmonic generation

Both absolute and relative nonlinear optical coefficients of six nonlinear materials measured by second-harmonic generation are discussed. A single-mode, injection-seeded, Q-switched Nd:YAG laser with spatially filtered output was used to generate the 1.064-μm fundamental radiation. The following results were obtained: d₃₆(KD*P)=0.38 pm/V, d₃₆(KD*P)=0.37 pm/V, |d₂₂(BaB ₂O₄)|=2.2 pm/V, d₃₁(LiIO₃ )=-4.1 pm/V, d₃₁(5%MgO:MgO LiNbO₃)=-4.7 pm/V, and d_eff(KTP)=3.2 pm/V. The accuracy of these measurements is estimated to be better than 10%     

Blue light generated by frequency doubling of laser diode light ina lithium niobate channel waveguide

Blue light was generated in a LiNbO₃ channel waveguide by frequency doubling radiation from a laser diode in a guided to guided wave interaction, utilizing first-order quasi-phase-matching in a periodically domain inverted structure. A fabrication method that does not depend on the use of titanium was used. A periodic pattern of silicon oxide on the positive c-face of LiNbO₃ was used in combination with a heat treatment to achieve a periodic outdiffusion and domain reversal in the surface layer. A channel guide was subsequently formed by proton exchange     

Adiabatic 2×2 polarization splitter on LiNbO3

The authors have demonstrated a passive 2×2 polarization splitter in x-cut LiNbO₃, using a hybrid Ti-indiffusion/proton-exchange technology. At a wavelength of 1530 nm, all paths exhibited an extinction ratio of better than 20 dB, with an average excess loss of 1 dB. This component can be integrated with optical filters, switches, and beam combiners to achieve polarization-independent operations