Monolithic integration of curved waveguides and channeled-substrate DH lasers by wet chemical etching

Integration of AlGaAs/GaAs curved waveguides and other two-dimensional waveguides with DH lasers and detectors is demonstrated. Devices are fabricated from LPE AlGaAs/GaAs layers by wet chemical etching processes. Differential transfer efficiencies ofeta_{t}= 5percent are routinely achieved in a structure consisting of an integrated laser, a 90° curved waveguide with 150μm radius, and a detector, for the case where one laser mirror is etched and one cleaved. This value iseta_{t}= 4percent if both mirrors are etched. A comparison of waveguide attenuation between straight and curved rib waveguides is given, along with the transfer characteristics of curved waveguides. The loss coefficient of curved rib waveguides with 150-μm radius is about two times that of a straight waveguide of the same length. The fabrication and properties of channeled-substrate crescent (CSC) lasers and detectors with transverse single-mode confinement, monolithically integrated by means of passive CSC interconnecting waveguides, is also described.     

Oscillator performance and energy extraction from a KrF laser pumped by a high-intensity relativistic electron beam

A laser cell with 21 of excitation volume was used to study the electron-beam pumped KrF laser system at excitation rates of 1.8- 7.0 MW/cm³. The system was optimized as an oscillator for various mixtures of Ar, Kr, and F2at total pressures of 1000 and 2500 torr. The resulting optimum conditions gave an intrinsic efficiency (laser energy out/electron-beam energy deposited) of 12 percent for the 1000 torr total pressure mixture with an output energy of 11 J/1. An efficiency of 10 percent with an output of 40 J/1 was obtained for the 2500 torr mixture. The system was then used as an amplifier to measure the extracted power as a function of input power for the two mixtures. The small-signal gain go, the nonsaturable absorption α, and the saturation intensity Iswere determined for the two mixtures. Analysis of the data gave g0= 16-18 percent/cm,alpha = 0.75-1.25percent/cm, and Is= 2 MW/cm²for the 1000 torr mixture and g0= 17-19 percent/cm,alpha = 1.0-1.5percent/cm, andI_{s} = 9MW/cm²for the 2500 torr mix.     

Chemically etched-mirror GaInAsP/InP lasers--Review

Detailed results on stripe GaxIn1-xAsyP1-y/InP lasers (lambda = 1.3 mum) with chemically etched-mirrors are reviewed. These devices are fabricated from GaInAsP/InP wafers grown by liquid phase epitaxy. A simple stripe laser structure with one etched mirror and one cleaved mirror is proposed. Monolithic passivation has been achieved using a Si3N4film and metal coatings on the etched facets. These processes not only increase the reflectivity of the etched mirrors, resulting in threshold currents even lower than uncoated cleaved devices, but also ease the problem of bonding of the chips on heat sinks. CW operation at room temperature has been achieved. Threshold currents of devices with 10 μm stripe electrodes were about 180-200 mA. Short cavity lasers and integrated monitoring detectors have also been demonstrated.     

Optimum low-level injection efficiency of silicon transistors with shallow arsenic emitters

The influence of As surface concentration CSEon the emitter efficiency βγand the temperature dependence of βγare reported. The theoretical model that is used to explain the variation of βγwith CSEis based upon the difference in the effective energy bandgaps in the emitter and base regionsDeltaE_{g}. Experimental measurements ofDeltaE_{g}versus CSEare presented. Measurements of βγversus CSEshow that the effective emitter doping densityQ_{E}/x_{eb}reaches a maximum value atC_{SE} cong 1.5 times 10^{20}atoms/cm³, corresponding to the threshold above whichDeltaE_{g} > 0. For the case of a constant active base doping/cm²QB, this also corresponds to an optimum in the emitter efficiency βγ. However, it is shown that in typical sequential diffusion processing of transistors, βγincreases monotonically with CSEbecauseQ_{B} = Q_{B}(C_{SE})decreases. In addition, for devices fabricated in this study,Deltabeta_{gamma}/DeltaC_{SE}atC_{SE}=2 times 10^{20}atoms/ cm³for As-diffused emitters (doped oxide) was ≈ 5 times greater than for ion-implanted-diffused As emitters, showing the superiority of implantation in controlling gain. Finally, transistors that were made withC_{SE} siml 1.4 times 10^{20}atoms/cm³(DeltaE_{g} = 0) showed βgamma(85°C)/ βγ(-15°C) ≤ 1.05.     

LaserQswitching by a PLZT shutter

The application of a polycrystalline ferroelectric ceramic Pb1-xLax(ZryTiz)1-(x/4)O3(called PLZT) electrooptic shutter is described for controlling the giant pulse emission of a ruby laser. The PLZT material hady = 65percent,z = 35percent, and the value ofxwas in the vicinity of 8 percent. Half-wave retardation switching was not possible with the PLZT shutter when 10-ns rise-time 70-ns-duration voltage pulses (fastQswitching) were applied. The increase insertion loss, due to less than half-wave retardation, resulted in laser pulsewidths that were wider and contained less energy than predicted by fastQ-switching theory. Since the PLZT material was damaged at relatively low power densities of between 5.6 to 8.5 MW/cm², respectively, its use is restricted toQ-switching small output power.     

The nitrogen ion laser pumped by charge transfer

Charge transfer from He+2to N2has been used in a variety of devices to pump significant levels of inversion of theB^{2}Sigma_{u}electronic transistion of N+2. Laser output has been achieved at three wavelengths, 391.4, 427.8, and 470.9 nm. Each has corresponded to a transition from the same upper laser level to lower levels having vibrational quantum numbers equal to 0, 1, and 2, respectively. With proper mirror sets each has been individually excited. Studies of the scaling and efficiency at which the laser output could be obtained have been made with devices pumped bye-beams, preionized avalanche discharges, and preionized resonant and traveling wave discharges. The possible efficiencies for the extraction of the energies stored in the inversions have been examined for various configurations of the coupling of the optical fields to the plasmas. Resulting devices have included self-excited oscillators, regenerative amplifiers, and master oscillator power amplifiers. The experimental results have supported a kinetic model which generally explains the unique features of the nitrogen ion laser. Of particular interest is the capacity of these systems to support the simultaneous optimization of both scale and efficiency. Withe-beam excitation optical power densities of 320 MW . l^-1have been achieved at a constant power transfer efficiency of 3 percent. With resonant discharge excitation 44 MW . l^-1have been demonstrated at power conversion efficiencies of 2 percent and overall efficiencies ranging from 0.2 to 0.4 percent. From these results the kinetic model has projected an overall efficiency of 0.9 percent to be ultimately realized in a discharge device.     

Regenerative gain in a Raman free-electron laser oscillator

A parametric study of gain in a millimeter-wave Raman free-electron laser oscillator and comparisons to linear theory are carried out. The intense (1 kA/cm²), relativistic (600-800 keV), cold [(deltagamma/gamma)_{parallel} < 1percent] electron beam employed is guided by a 9.45 kG magnetic field through a 1.45 cm period, 49.5 cm long uniform undulator. Operation at < 1 kG pump field results in a < 10 percent electron quiver velocity (upsilon_{perp}/upsilon_{parallel}) velocity. The laser power output has been mea sured at ∼ 3 MW corresponding to an efficiency of 4 percent, and tunability in the 90-170 GHz range has been achieved with a narrow linewidth (Delta lambda/lambda leq 1percent). Using a new technique, linear small-signal growth rates have been unfolded from the oscillator startup delays. Excellent agreement is found with three-dimensional small-signal calculations for both the spatial growth rate and the resonance frequency. One-dimensional theory was found to predict shorter wavelength laser output than that observed.     

High-reflectivity cavity mirrors for high-performance (GaAl) As diode lasers

Effects of the high-reflectivity mirrors on the performance of (GaAl)As diode lasers are investigated by using a laser diode model. An optimum value for the mirror reflectivity is obtained by using a figure of merit which reflects low threshold and high slope efficiency. An optimum reflectivity valueR = 0.75is obtained for a laser with a cavity lengthl= 300 mum and an internal loss coefficientgamma_{i} = 10cm^-1. A higher internal photon flux density relative to the output photon flux density is shown to occur with the high-reflectivity mirror. High-reflectivity mirrors are shown to reduce optical feedback effect from external devices and the lowered threshold value to reduce the thermal effects on the modulation characteristics and the degradation.     

Polymeric Waveguide Film With Embedded Mirror for Multilayer Optical Circuits

A polymeric waveguide film with embedded mirrors is presented for a multilayer optical circuit. For the polymeric waveguide fabrication, arrayed waveguide patterns were directly formed on a metallic master body using ultra-precision machining (UPM) method, and 45$^{circ}$ -angled mirrors as beam reflector were constructed at the master pattern ends. We have achieved excellent surface roughness of the pattern and mirror facet using UPM. The waveguide lines and 45 $^{circ}$-angled mirrors were simultaneously fabricated on an under-clad sheet using an imprint method with a mirror-integrated master. We manufactured polymer waveguide films with embedded beam reflectors and consequently measured the optical characteristics of waveguides.      

Monolithically-integrated hybrid heterostructure diode laser with dielectric-film waveguide DBR

In the monolithically-integrated hybrid (MIH) DBR diode laser, the five-layer Ga(Al)As-GaAs heterostructure waveguide of the gain region was monolithically butt-joined on a common GaAs substrate with a highly-transparent corrugated dielectric-film waveguide consisting of sputtered SiO2, Ta2O5, and evaporated (corrugated) As2S3layers. The laser operated with the first-order grating under the pulsed current pumping at 300 K. The efficient resonant mode conversion (70 percent in power) has been obtained at the interface between the heterostructure and dielectric waveguides. The fundamental transverse and single-longitudinal mode output emission was obtained up to 160 mW (Ith = 120mA) with external differential quantum efficiencyeta simeq 32percent. The advantages of a dielectric-film waveguide DBR are demonstrated. The use of such a DBR results in a high degree of sidemode suppression and stability of the spectral position of the emission line under the temperature variation, the corresponding spectral shift beinglsim 0.01Å/K.     

Continuous operation of visible-spectrum in1-xGaxP1-zAszlaser diodes (6280 Å, 77 K)

Stripe-geometry In1-xGaxP1-zAsz(x approx 0.84-0.86, z approx 0.38-0.42) double heterojunction laser diodes, grown by liquid phase epitaxy (LPE) on vapor phase epitaxy (VPE) GaAs1-yPysubstrates, are described that operate (CW, 77 K) in the visible atlambda approx 6280-6360Å with differential quantum efficiencyeta_{ext} sim 28percent and power output in the range 1-7 mW.     

Absorption in the near-ultraviolet wing of the Kr2F*(410 nm) band

An intracavity laser technique has been used to study the absorption of electron-beam pumped Ne/Kr/F2gas mixtures (196 and 300 K) in the "blue wing" of the Kr2F emission continuum. The experiments were conducted at 358 nm using theupsilon' = 0 rightarrow upsilon" = 1transition of the N2(C rightarrow B) laser. Comparing the results with the predictions of a computer model, the species primarily responsible for absorption have been identified as Ne+2, Kr+2, and Kr2F*. The photoabsorption cross sections for Ne+2and Kr2F (Kr+2F-) at 358 nm have been estimated to be8.1 cdot 10^{-19}and5.4 cdot 10^{-18}cm², respectively. The Kr2F* absorption cross section is roughly 20 percent of that reported for Kr+2at the same wavelength. The fluorescence efficiency of Kr2F* ine-beam excited 94.93 percent Ne/5 percent Kr/0.07 percent F2(P_{total} = 4000torr) gas mixtures has been found to be a factor of 2.8 higher than that of the N2(C rightarrow B) band in Ar/5 percent N2mixtures. Also, the rate constant for quenching of Kr2F* by F2was measured to be(4.1 pm 0.5) cdot 10^{-10}cm³. s^-1at 300 K and(3.0 pm 0.5) cdot 10^{-10}cm². s^-1at 196 K.     

Finite-aperture waveguide-laser resonators

A general theory of finite-aperture waveguide-laser resonators is developed which represents the external reflectors by matrices which couple linearly polarized waveguide modes having the same azimuthal symmetry. The theory allows the determination of resonator efficiency, resonator frequencies, and laser near- and far-field patterns. Computations of the coupling loss for the fundamental waveguide mode as a function of mirror curvature, separation, and aperture are in good agreement with recent infinite-aperture calculations in the limit of large apertures and indicate three low-loss configurations: 1) large radius of curvature mirrors close to the guide; 2) "large" radius of curvature mirrors centered at the guide entrance: and 3) generally smaller curvature mirrors separated by half their curvature from the guide entrance. The importance of higher order waveguide modes in determining laser output power and far-field patterns is demonstrated experimentally and compared to theoretical predictions. Design guidelines for the construction of high-efficiency CO2, CO, and He-Ne waveguide-laser resonators are summarized in tabular form.     

InGaAsP/InP double-heterostructure lasers: Simple expressions for wave confinement, beamwidth, and threshold current over wide ranges in wavelength (1.1-1.65 µm)

For InGaAsP/InP devices emitting in the wavelength range of1.1-1.65 mum, we present novel and accurate analytical approximations of the crucial parameters in laser design: the radiation confinement factorGamma; the effective refractive index Neff; and the transverse beamwidththeta_{perp}. It is found thatGammais independent of wavelength or step-index difference andtheta_{perp}becomes independent of wavelength as the active layer thicknessesddecreases below 0.15 μm. An explicit analytical expression is derived for the threshold current density Jth. The new linear gain-current relationshipg(cm^-1) = 28.5(J/d) - 50is deduced from a critical assessment of recent experimental data. A theoretical framework is provided to explain experimental observations such as: the Jthversusdcurve is independent of wavelength, the threshold has a negligible variation over the0.1-0.3 mum active-layer thickness range, and the normalized threshold current density (J_{th}/d, ford geq 0.5 mum) varies strongly (3-5 kA/cm²/μm) with cavity length.     

Second breakdown of double epitaxial transistors

This paper deals with the second breakdown of transistors with epitaxial collector, epitaxial base, and diffused emitter. Transistors were fabricated with base width WBin the range of 2 to 18 µ and resistivity in the range of 0.1 to 10 ohm . cm. The optimum values of the resistivity and the thickness of these regions were calculated by computer techniques. The devices were mounted onto a TO-63 header and the base and the emitter leads were bonded onto the device ultrasonically. The electrical characteristics, including the frequency response ftand secondary breakdownS/Bcapability, were tested. For the measurement of second breakdown current IM, forward bias condition was used. It was found that for fixed collector and emitter parameters, IMwas controlled by the product of base resistivity ρBand base width WB. The value of IMwas found to increase withrho_{B}W_{B}. However, for a specified device characteristic, an optimum value ofrho_{B}W_{B}was found to exist. For transistors withV_{CEO} =150volts,f_{t}=20mHz andh_{FE}=20, the optimum value ofrho_{B}W_{B}was found to be 6 × 10^-4ohm . cm².     

Threshold current density in solution-grown GaAs laser diodes

Threshold current density of solution-grown GaAs laser diodes with a Fabry-Perot cavity were measured at 77 and 300°K by varying the acceptor concentration in thepregion Na. Threshold current density was lower in the series of diodes with larger values of Nathan in the series of diodes with smaller values of Nafor the diode length between 0.1 and 1 mm. Through these experiments diodes with the threshold current density as low as3 times10^{2}A/cm²at 77°K and2.8 times 10^{4}A/cm²at 300°K for the diode length of 1 mm, and as low as 10³A/cm²at 77°K and4.5 times 10^{4}A/cm²at 300°K for the diode length of 0.1 mm were obtained.     

Linear and nonlinear optical properties of AgGaS2, CuGaS2, and CuInS2, and theory of the wedge technique for the measurement of nonlinear coefficients

The refractive indices of the ternaryA^{I}B^{III}C_{2}^{VI}semiconductors AgGaS2, CuGaS2, and CuInS2have been measured over the entire range of transparency of these crystals. The optical nonlinear coefficients for second-harmonic generation have also been determined. Three-frequency collinear phase matching is analyzed in detail for AgGaS2. The birefringences of CuGaS2and CuInS2are not large enough to permit three-frequency phase matching within the transparent regions. A parametric oscillator threshold calculation for a pump wavelength 0.89 μ, which is within the range of the GaAs injection laser, indicates that AgGaS2is promising for this application. The upconversion efficiency in AgGaS2for sum mixing of the CO2laser (lambda = 10.5 mu) with the xenon ion laser (lambda = 0.597 mu) is also calculated. The result indicates that, depending upon system requirements and the availability of high optical quality material, AgGaS2can be comparable to ZnGeP2for upconversion. In Appendix II, we present a theory of the wedge technique for the measurement of nonlinear coefficients. This theory takes into account losses and assumes a Gaussian beam geometry. Furthermore, a discussion of units in nonlinear optics is given.     

Gain spectra in GaInAsP/InP proton-bombarded stripe-geometry DH lasers

We have measured gain spectra for TE polarization in a GaInAsP/InP laser as a function of dc bias current below laser threshold. The measurements were made on a low-threshold device with a stripe geometry defined by proton bombardment. A number of other characteristics of this and similar devices from the same wafer are also reported. These data permit the maximum gain coefficient of the active layer gmaxto be evaluated as a function of nominal current density Jnom. We obtain the linear relationshipg_{max} = (3.1 times 10^{-2}/eta_{r})(J_{nom} - eta_{r} 5.4 times 10^{3}), where ηris the radiative quantum efficiency. Our data apply only for large gain (g gsim 150cm^-1) and large Jnombecause the active layer of the test device is thin (0.1 μm).     

Be-implanted GaInAsP/InP double heterojunction laser diodes

Beryllium ion implantation has been used to fabricate broad-area GaInAsP/InP laser diodes operating at a wavelength of 1.3 μm, which have threshold current densities comparable to those obtained using conventional Zn doping during the epitaxial growth process. Both a Be-implant schedule which results in minimal diffusion of the implanted Be and one which results in significant diffusion have been investigated. On most wafers, the average normalized threshold current density (J_{nom} = J_{th}/d) using either implant schedule has been typically 6-8 kA/cm². μm. The lowest Jnomobserved was 4.2 kA/cm². μm and was measured on a "nondiffused" implanted laser.     

Performance analysis and laser linewidth requirements for optical PSK heterodyne communications systems

An optical PSK heterodyne communications receiver is investigated. The receiver is based on the decision-directed phase-locked loop. The performance of the phase-locked loop subsystem is analyzed taking into account both shot noise and laser phase noise. It is shown that for reliable phase locking (rms phase error less than 10°), heterodyne second-order loops require at least 6771 electrons/s per volt every hertz of the laser linewidth. This number corresponds to the limit when the loop dumping factor η is infinitely large; ifeta = 0.7, then the loop needs 10 157 electrons/(s . Hz). If the detector has a unity quantum efficiency andlambda = 1.5 mum, the above quoted numberers give 0.9 pW/ kHz foreta rightarrow inftyand 1.35 pW/kHz fornu = 0.7. The loop bandwidth required is also evaluated and found to be155 Deltanu, whereDeltanuis the laser linewidth. Finally, the linewidth permitted for PSK heterodyne recievers is evaluated and found to be2.26 cdot 10^{-3} R_{b}where Rbis the system bit rate. ForR_{b}=100Mbit/s, this leads toDeltanu < 226kHz. Such and better linewidths have been demonstrated with laboratory external cavity lasers. For comparison, ASK and FSK heterodyne receivers are much more tolerant to phase noise,-they can tolerateDeltanuup to 0.09 Rb. At the same time, homodyne receivers impose much more stringent requirements on the laser linewidth (Deltanu < 3 cdot 10^{4} R_{b}).