Active and nonlinear wave propagation devices in ultrafastelectronics and optoelectronics [and prolog]

We describe active and nonlinear wave propagation devices for generation and detection of (sub)millimeter wave and (sub)picosecond signals. Shock-wave nonlinear transmission lines (NLTL's) generate ~4-V step functions with less than 0.7-ps fall times. NLTL-gated sampling circuits for signal measurement have attained over 700-GHz bandwidth. Soliton propagation on NLTL's is used for picosecond impulse generation and broadband millimeter-wave frequency multiplication. Picosecond pulses can also be generated on traveling-wave structures loaded by resonant tunneling diodes. Applications include integration of photodetectors with sampling circuits for picosecond optical waveform measurements and instrumentation for millimeter-wave waveform and network (circuit) measurements both on-wafer and in free space. General properties of linear and nonlinear distributed devices and circuits are reviewed, including gain-bandwidth limits, dispersive and nondispersive propagation, shock-wave formation, and soliton propagation     

Highly nonlinear bismuth oxide-based glass fibres for all-opticalsignal processing

A highly nonlinear bismuth oxide (Bi₂O₃)-based glass fibre aiming at applications to all-optical signal processing is reported. This fibre features a high nonlinear coefficient γ of 64.2 W^-1 km^-1, low propagation loss of a=0.8 dB/m, low splicing loss of 0.48 dB, and high mechanical and chemical durability     

Deuterated L-arginine phosphate: a new efficient nonlinear crystal

Deuterated L-arginine phosphate (d-LAP) is a highly transparent monoclinic crystal with attractive properties for efficient frequency conversion. It is grown easily from aqueous solution, and it is phase-matchable for all nonlinear processes where KDP is phase-matchable. Over most of its usable frequency range, it is substantially more efficient than KDP. Crystals grown in large sizes (100 cm³) have a high damage threshold, excellent optical quality, are less hygroscopic than KDP, and are easily fabricated into nonlinear devices. Deuterated LAP is attractive for harmonic generation of Nd lasers and for work in the ultraviolet down to about 250 nm     

A modified beam propagation method to model second harmonicgeneration in optical fibers

A finite-difference implementation of the beam propagation method (BPM) is used to solve the paraxial, scalar wave equation with a nonlinear source term. A transparent boundary condition capable of handling asymmetric modes is incorporated in the finite-difference algorithm. This nonlinear BPM is used to model the generation and propagation of second harmonic light in an optical fiber which has been prepared for second harmonic generation (SHG) by the formation of a _χ⁽²⁾ grating. This method can be used to predict the guided mode in which the generated second harmonic light propagates based on the modes of the writing (fundamental and second harmonic) and reading (fundamental only) light. The effects of self-phase modulation (SPM) and cross-phase modulation (XPM) are included in the model     

Multiple-wavelength quasi-phase-matched nonlinear interactions

Quasi-phase-matching allows one to arbitrarily phase match a single interaction by periodic modulation of the material nonlinear coefficient. A partial extension is obtained by Fibonacci-based quasi-periodic modulation of the nonlinear coefficient. These Fibonacci-based structures allow for simultaneously phase matching two interactions, provided that their wavevector mismatch ratio obeys selection rules, which are governed by the golden ratio τ=(1+√5)/2. In this paper, we present a novel method for simultaneously phase matching any two nonlinear interactions by general quasi-periodic modulation of the nonlinear coefficient. These quasi-periodic structures, which also include the Fibonacci-based structures as a subgroup, provide greater design flexibility. Our method can be useful for various nonlinear devices, such as multiple-peak frequency doublers, frequency triplers, and frequency quadruplers. We show for two specific devices that similar efficiency, compared to a cascaded device, can be obtained. Furthermore, in contrast to some cascaded devices, these structures can be used in double-pass and standing-wave configurations, since they operate with the same efficiency in both directions of propagation     

Improved second-harmonic generation by selective Yb ion doping in anew nonlinear optical crystal YCa4O(BO3)3

We describe experiments characterizing a new nonlinear optical crystal, YCa₄O(BO₃)₃ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (~3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa₄ O(BO₃)₃, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa₄O(BO₃)₃ (RE=La³⁺, La ³⁺, Y³⁺, Sm³⁺, Gd³⁺, Er³⁺, and Nd³⁺). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb³⁺, which has larger mass, but approximately the same atomic size as Y³⁺ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa₄O(BO₃)₃ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity     

Reduction of Light Signal Pulse Distortion in Cascaded SFG–DFG Wavelength Conversion: Segmented QPM Interleaved by Short Wavelength Cut Filters

Cascaded sum-frequency-generation (SFG) and difference-frequency-generation (DFG) can implement a wavelength conversion between arbitrary combinations of input and output signal wavelengths. By using a tunable wavelength pump light, the output wavelength can be tuned to a desired wavelength. As in many wavelength conversion devices using the nonlinear optical effect, the group velocity difference between light pulses with different wavelength causes a walk-off effect deforming the output pulse shape. Thus, the device length should be kept short to avoid the walk-off effect resulting in limited conversion efficiency. In this report, we propose a method, for quasi-phase matched device, to maintain the pulse shape of the SFG light pulse along the propagation distance. The output DFG light pulse deformation is suppressed and the conversion efficiency can be increased by extending the device length.      

Performance enhancement of GaInP/GaAs heterojunction bipolarphototransistors using dc base bias

GaInP-GaAs heterojunction bipolar phototransistors grown by metal organic vapor phase epitaxy (MOVPE) and operated with frontside optical injection through the emitter are reported with high optical gain (<88) and record high frequency performance (28 GHz). Heteropassivation of the extrinsic base surface is employed using a depleted GaInP emitter layer between the nonself-aligned base contact and the emitter mesa. The phototransistor's performance is shown to improve with increasing dc base bias in agreement with predictions of a recently reported Gummel-Poon model. Experimental results are reported for devices with optical active areas of 10×10 μm², 20×20 μm², and 30×30 μm², with peak measured cutoff frequencies of 28.5, 23.1, and 18.5 GHz, respectively, obtained at collector current densities between 2×10 ³ and 6×10³ A/cm²     

TM modes guided by nonlinear dielectric slabs

Propagation of a TM₀ mode in a Kerr-type nonlinear dielectric slab is investigated. The exact analytical expression of E ²(x), E²=E_x²+E_Z ², the decoupled linear differential equation and its equivalent Volterra integral equation of E_x, and the eigenvalue equation of propagation constant β are presented. Finally, an example of approximate analytical solutions obtained by the method of successive approximation is given     

An iterative finite difference beam propagation method for modelingsecond-order nonlinear effects in optical waveguides

An iterative finite difference beam propagation method based on the Crank-Nicholson scheme is presented to simulate continuous wave (CW) second-order nonlinear effects in optical waveguides with the depletion of the pump wave taken into account. This method is an extension of the linear finite difference beam propagation method and preserves the same order of accuracy. Comparisons with the previously published explicit finite difference beam propagation method and the rectangular approximation method are presented. Quasi-phase matched difference frequency generation in AlGaAs and quasi-phase-matched second harmonic generation in LiNbO₃ are considered in the evaluation, showing that one iteration for the IFD-BPM is sufficient for the simulation with good accuracy and without increasing much computation time     

Measurement of nonlinear coefficient and phase matchingcharacteristics of AgGaS2

The nonlinear optical characteristics of AgGaS₂ were investigated by measuring visible parametric fluorescence with a pump wavelength of 600 nm. A value of d₃₆ [AgGaS₂ ]=31±5×10^-12 m/V for the nonlinear coefficient was determined. The temperature dependence of phase matching up to 100°C was studied. A significant temperature effect, although much smaller than for LiNbO₃, was found and results in a change in the infrared difference frequency generated of ~0.6 cm^-1 -°C^-1     

Differential optical switching at subnanowatt input power

An operating technique for a differential optical switching device based on a set of parallel-connected pnpn structures is discussed. The differential function at subnanowatt input powers was demonstrated with AlGaAs/GaAs pnpn devices. The highly sensitive devices are promising for applications both to optical neural networks and to optical digital computing because of the low power consumption of the light emitting devices and the large fan-in/fan-out ratio (~10⁵)     

Design of nonlinear photonic crystal fibers with ultra‐flattened zero dispersion for supercontinuum generation

The study reports on the design and performance of two air‐filled and two partial ethanol‐filled photonic crystal fiber (PCF) structures with a tetra core for supercontinuum generation. The PCFs are nonlinear with ultra‐flattened zero dispersion. Holes with smaller areas are used to create a tetra‐core PCF structure. Ethanol is filled in the holes of smaller area while the larger holes of cladding region are air‐filled. Optical properties including dispersion, effective mode area, confinement loss, normalized frequency, and nonlinear coefficient of the designed PCF structures are investigated via full vector finite difference time domain (FDTD) method. A PCF structure with lead silicate as wafer exhibits significantly better results than a PCF structure with silica as wafer. However, both structures report dispersion at a telecommunication wavelength corresponding to 1.55 μm. Furthermore, the PCF structure with lead silicate as wafer exhibits a very high nonlinear coefficient corresponding to 1375 W^?1 km^?1 at the same wavelength. This scheme can be used for optical communication systems and in optical devices by exploiting the principle of nonlinearity.     

Optical and electrooptical analysis of GaAs inverted rib phase modulators grown by vapor phase epitaxy

We present experimental and theoretical analysis of GaAs homojunction Schottky barrier phase modulators. The waveguide structures-inverted rib-were grown using the vapor-phase-epitaxy chloride process. These structures are attractive because perfectly planar devices can be realized. Besides that, single-mode operation and low losses can be achieved. The optical (propagation losses, dispersion curves), electrical (electrical field distribution, breakdown voltages) and electrooptical (modulation efficiency) parameters have been calculated using numerical two-dimensional methods. In order to optimize the modulator, various waveguide structures as well as n+ (i.e., substrate) and n- (i.e., waveguide) dopings have been considered in the modeling. Experimental results fit well with the calculated ones. A modulation efficiency of 2.3° . V^-1. mm^-1has been measured. For a completely optimized structure, an efficiency of 4° . V^-1. mm^-1is expected.     

Reliable Low-Cost Fabrication of Low-Loss $hbox{Al}_{2}hbox{O} _{3}{:}hbox{Er}^{3+}$ Waveguides With 5.4-dB Optical Gain

A reliable and reproducible deposition process for the fabrication of Al₂O₃ waveguides with losses as low as 0.1 dB/cm has been developed. The thin films are grown at ~ 5 nm/min deposition rate and exhibit excellent thickness uniformity within 1% over 50times50 mm² area and no detectable OH^- incorporation. For applications of the Al₂O₃ films in compact, integrated optical devices, a high-quality channel waveguide fabrication process is utilized. Planar and channel propagation losses as low as 0.1 and 0.2 dB/cm, respectively, are demonstrated. For the development of active integrated optical functions, the implementation of rare-earth-ion doping is investigated by cosputtering of erbium during the Al₂O₃ layer growth. Dopant levels between 0.2-5times10²⁰ cm^-3 are studied. At Er³⁺ concentrations of interest for optical amplification, a lifetime of the ⁴I_13/2 level as long as 7 ms is measured. Gain measurements over 6.4-cm propagation length in a 700-nm-thick Al₂O₃:Er³⁺ channel waveguide result in net optical gain over a 41-nm-wide wavelength range between 1526-1567 nm with a maximum of 5.4 dB at 1533 nm.     

An ARROW optical wavelength filter: design and analysis

A novel ARROW (antiresonant reflecting optical waveguide)-based optical wavelength filter is proposed. The modal characteristics of the coupled ARROW structures is analyzed rigorously by the transverse resonance method. As an alternative configuration to the conventional directional coupler filter, the proposed device features large core size compatible with fiber and nearly periodic dependence of the coupling-length on the waveguide separation, which provides more flexibility for fabrication of the device. A design procedure for this type of coupled ARROW structure used for wavelength filtering is presented. The devices designed are simulated and verified by the beam propagation method. An ARROW wavelength filter based on SiO₂/TiO₂ has a FWHM bandwidth narrower than 7 Å     

Comparison of conventional and gain-clamped semiconductor opticalamplifiers for wavelength-division-multiplexed transmission systems

We compare the output spectra and data streams of a conventional 1550-nm semiconductor optical amplifier (SOA) with its gain-clamped (GCSOA) counterpart, in order to assess the impact of gain clamping on cross-gain modulation (XGM) and difference frequency generation (DFG). Whereas the conventional SOA exhibits a large amount of crosstalk due to XGM, there is virtually no XGM present in the GCSOA. However, the XGM effect in the SOA shows evidence of diminished efficiency at moderate input levels. We observe much higher DFG levels from the GCSOA (roughly 10 dB greater than the SOA). These DFG levels are such that cascaded wavelength cross-connect devices, in-line amplifiers, and even optical gates could experience inhibited performance     

A time-domain algorithm for the analysis of second-harmonicgeneration in nonlinear optical structures

A time-domain simulator of integrated optical structures containing second-order nonlinearities is presented. The simulation algorithm is based on nonlinear wave equations representing the propagating fields and is solved using the finite-difference time-domain method. The simulation results for a continuous-wave operation are compared with beam propagation method simulations showing excellent agreement for the particular examples considered. Because the proposed algorithm does not suffer from the inaccuracies associated with the paraxial approximation, it should find application in a wide range of device structures and in the analysis of short-pulse propagation in second-order nonlinear devices     

Applications of highly nonlinear chalcogenide glass fibers inultrafast all-optical switches

Applications of chalcogenide glass fibers in ultrafast all-optical switches have been investigated. Ultrafast all-optical switching has been accomplished in an optical Kerr shutter configuration using As₂S₃-based glass fiber. The nonlinear refractive index of the As₂S₃-based glass is estimated to be n₂=4.0×10^-14 (cm²/W ), which is higher by two orders of magnitude than that of silica glass fiber. Nonlinear absorption due to two-photon absorption has been revealed to be negligible, and up to a 2π-phase shift has been obtained. Switching speed and switching power were investigated experimentally and through calculations. A switching time of 12 ps and a switching power of 5 W can be achieved using a 10-ps gate pulse and only a 1-m chalcogenide glass fiber. However, signal transformation due to cross-phase modulation and group velocity dispersion is not negligible for shorter gate pulses. Lower switching power is possible by reducing the transmission loss and the core area and by optimizing the driving conditions     

Absolute measurement of the second-harmonic generation nonlinear susceptibility of tellurium at 28.0 µm

The second-harmonic generation nonlinear optical susceptibility of trigonal tellurium has been measured at 28.0 μm by the phase-matching technique. The value obtained was|d_{11}| = (5.7 pm 1.9) cdot 10^{-10}m/V in MKS units. This value establishes the longest wavelength "standard" for the measurement of nonlinear optical susceptibilities by comparative techniques. Experimental details and data are given to justify the quoted uncertainty.