Low-threshold distributed feedback lasers fabricated on material grown completely by LP-MOCVD

GaInAsP-InP distributed feedback (DFB) lasers emitting at 1.57 μm have been fabricated on material grown completely by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The CW threshold current of 60 mA and an output power of 6 mW per facet at room temperature have been obtained. The lasing wavelength λLunder CW operation showed a temperature coefficient (d_{lambdaL}/dT) of 0.9 Å/°C for this DFB laser over the range of10-90degC. A stable single longitudinal mode was maintained under high speed pulse modulation up to 500 ps, and sinusoidal modulation at 1 Ghz.     

Loss increase for optical fibers exposed to hydrogen atmosphere

Loss spectrum changes for optical fibers exposed to a hydrogen atmosphere in the15-200degC temperature range are measured. Loss increase due to molecular hydrogen dissolved into fibers is investigated from the loss peak at 1.24 μm, and that due to hydroxyl group formation from the loss peak at 1.41 μm. The loss increase due to molecular hydrogen is fully explained by physical solubility theory and diffusion equation. The empirical formula for time, temperature, and hydrogen-pressure dependences of the loss increase due to hydroxyl group formation is evaluated from the experimental results. The loss increase at 1.3- and 1.5-μm wavelength band at room temperature are estimated.     

Interferometric measurement of the elongation of a pulsed diode laser

An interferometric technique was developed for detecting fast displacements of small areas in beam direction by less thanlambda/8. It utilizes a modified Michelson interferometer and a He-Ne laser light source. The technique was applied to measure the temporal and spatial evolution of the thermal mirror deformation of a GaAs-Ga1-xAlxAs single heterostructure laser diode. A small spot on the facet of the laser diode formed one mirror of the interferometer. The other mirror was moved linearly with time by applying a ramp voltage to a piezoelectric transducer. The diode was pulsed during this motion. The local elongation of the laser diode was evaluated from the interferometer output signal. The measured elongation of the active region at a time 1 μs after a 150-ns pulse with current density 48 000 A/cm²was between 4 and 20 nm, indicating a temperature rise of2-11degC. The smallest elongations detected were in the range of 0.5-1 nm. The measured values are in substantial agreement with theoretical expectations from thermal model calculations.     

Externally heated copper vapor laser for parametric studies

The construction and operation of a reliable externally heated copper vapor laser for parametric studies is presented. A long-life high-temperature heating element is described. A temperature measurement method has been developed that is compatible with laser operation and has an accuracy ofpm20degC over the range0-1700degC. The laser is operated in the burst mode to eliminate effects of discharge hearing and enable measurement of the time evolution of laser behavior. An electrical circuit was developed which eliminates transient electrical behavior at the beginning of the burst. An example of parameter measurement, the dependence of output power on tube temperature under specific operating conditions, is also given.     

Simultaneous stabilization of the frequency and power of an AlGaAs semiconductor laser by use of the optogalvanic effect of krypton

The frequency of an AlGaAs semiconductor laser operated in the 0.77 μm region has been locked to the optogalvanic signal of Kr I at 7694.5401 Å. By use of a feedback loop to the injection current source, the frequency stability of5.3 times 10^{-11}has been obtained for the 1 s averaging time and of1.5 times 10^{-11}for 240 s. The output power and frequency have been simultaneously stabilized by a combined use of feedback loops to the temperature controller and the injection current source. The drift of the power level for 20 min has been less than 1 μW under the frequency-stabilized condition. The power level adjustability at the frequency-locked state has been discussed. The shift of the laser frequency caused by the power level control has been found to be within 2 MHz in the power dynamic range over 0.7-1.6 mW.     

InP-In1-xGaxAsyP1-yembedded mesa stripe lasers

A laser structure was fabricated using two-step liquid-phase epitaxy, employing the melt-back technique. The fabrication and properties of this structure are described in detail. Good linearity of the power output up to power levels of 20 mW was obtained. The threshold current density at 300 K is 9-12 kA/cm². This high value is mainly due to Zn-diffusion from the third to the buffer layer during the second growth step of the fabrication process. The external differential quantum efficiency is 30-35 percent under pulsed operation at 25°C. The pulsed threshold current has an exponential behavior with temperature whereT_{0} = 60degC. The far-field beam divergences in the directions parallel and perpendicular to the junction plane are12-15degand35-40deg, respectively. Transverse mode stabilization was improved with this laser structure.     

High-modulus low-linear-expansion-coefficient loose-jacket optical fibers

Optical fibers loosely jacketed with high-modulus low-linear-expansion-coefficient polymers have been proposed, and the mechanical, thermal, and optical properties have been investigated. The loose-jacket tube is made of highly oriented polyoxymethylene (POM), which is produced by tensile-drawing the isotropic POM tube with dielectric heating. The oriented POM exhibits Young's moduli 20-40 GPa and linear expansion coefficients of the order of10^{-5}-10^{-6}degC^-1. Owing to the low linear expansion coefficients of the material, the oriented POM loose-jacket optical fiber has no low-temperature excess losses, which is due to fiber bends caused by thermal contraction of the loose tube, in the-60-80degC temperature range.     

Design and characterization of a high temperature fiber-optic pressure transducer

We have measured the pressure response of a high temperature pressure transducer fabricated from a super alloy. This transducer contains a microbend fiber-optic sensor to measure diaphragm deflection and a reference fiber to help reduce light source fluctuations caused by perturbations in the fiber leads. The transducer was characterized at pressures up to 22.8 MPa and temperatures to 430°C. The experimental data taken at constant temperature shows a 0.58 percent (of full scale pressure) minimum deviation between the fiber-optic pressure transducer output and output of a calibrated gage pressure transducer. Over the entire temperature range of20-430degC, the fiber-optic pressure transducer output exhibits a 1.2-percent (of full scale pressure) maximum deviation from calibrated pressure.     

Temperature dependent phase-matched nonlinear optical devices using CdSe and ZnGeP2

Temperature tuning and stability of nonlinear optical devices employing CdSe and ZnGeP2are considered for the first time. From the measured refractive index data at room temperature and the corresponding appropriate temperature derivatives, Sellmeier coefficients are found for the evaluation of refractive indexes both below and above room temperature. These can be used to calculate phase-matching angles at different temperatures. We have evaluated temperature stability of phase-matching angles in tunable parametric oscillators and down-converters and also in up-converter operations which have been reported at room temperature. Temperature characteristics have been worked out for tunable generation of radiation near the 16 μm region in CdSe and the type II second-harmonic generation has been found to be possible for temperaturesgsim275degC in ZnGeP2because of its positive temperature coefficient in birefringence.     

Static characteristics of 1.5 - 1.6 µm GaInAsP/InP buried heterostructure butt-jointed built-in integrated lasers

Lasing characteristics of GaInAsP/InP buried heterostructure butt-jointed built-in distributed Bragg reflector integrated lasers (BH-BJB-DBR lasers) intended for dynamic single mode operation in the wavelength range of1.5-1.6 mum are given. In the first section, the coupling property between the active region and the butt-jointed external waveguide region is calculated to show the possibility of large fabrication tolerance. A coupling coefficient of more than 95 percent is estimated. Secondly, the GaInAsP/InP integrated lasers were fabricated and tested in the view of the static characteristics and the axial mode selection property. Lateral mode control was achieved by the use of a buried heterostructure, so that the axial and lateral modes were maintained to a fixed single mode. The lasers thus fabricated were operated in CW conditions at room temperature with a threshold current of about 100 mA. Single longitudinal mode operation was observed with a temperature dependence of about 0.13 nm/deg with the temperature range of more than50-65degC at around 0°C. Differential quantum efficiency as high as 13 percent/ facet was obtained for the laser with power output of more than 5 mW/facet. The output spectrum below threshold indicated the strong wavelength selectivity of the DBR region, and the net gain difference between the main DBR mode and the adjacent submode was measured to be about 6 cm^-1. No appreciable degradation and change of characteristics have been observed even after CW operation of more than 9770 h at 20°C.     

Microoptic grating multiplexers and optical isolators for fiber-optic communications

As new optical devices for increasing further the utility of and to expand the application of fiber-optic communications, grating multiplexers and isolators have been developed for 0.8 μm band employing microoptic approach. The development of these devices is the subject of this paper. The devices have desirable features of small size, compactness, high optical performances, and high reliability. The grating multiplexer consists of a graded-index rod, a blazed reflection grating replicated onto the graded-index rod slanting facet or a wedge facet, and an input-output fiber array. Simple calculations have been done to determine necessary element parameters for a given channel spacing. Experimental results are presented for five-channel multiplexers devised using a SELFOC^®lens. Around 3 dB insertion loss and less than -30 dB crosstalk have been obtained for about 35 nm channel spacing in overall device size of18 times 13 times 50mm. Faraday rotation optical isolators for 0.8 μm band have been miniaturized by employing an efficient paramagnetic glass Faraday rotator, a magnet with a through hole and a folded optics in the Faraday rotator. The path number in the folded optics has been optimized in terms of trading-off between the magnet size and the insertion loss. A 0.9 dB insertion loss including fiber coupling loss and 36 dB isolation have been obtained in overall device size of24 times 24.5 times 42mm. Results on the temperature and wavelength dependence of the isolation are also presented. In addition, fundamental properties of optical circulators for 0.8 μm band and optical isolators and circulators both for 1.3 μm band, developed as extended modifications of the optical isolators for 0.8 μm band, are briefly described.     

The radar cross section of ogives, double-backed cones, double-rounded cones, and cone spheres

Experimental measurements have been made of the nose-on backscattering radar cross section over a range of diameters from 0.05 to 1.0 wavelengths of a series of ogives with total included nose angles of40deg,75deg, and120deg; of cone spheres with nose angles of15deg,30deg,40deg60degand120deg; of double-rounded cones with nose angles of15deg,40deg, and75deg; and of double-backed cones with nose angles of40deg. Further, a series of ogives of approximately the same diameter and with nose angles of25deg,40deg,60deg,75deg,95deg,120deg,160degand180deghave also been measured. All the present results have been obtained using two monostatic phase-locked CW balanced-bridge model measurement radars operating at frequencies of 9 Gc and 35 Gc. For targets with maximum dimensions less than two inches, cross sections of less than10^{-7}square meters at 9Gc and10^{-6}square meters at 35 Gc can be measured with an accuracy of pm0.5 db. The experimental data is sufficiently comprehensive to provide qualitative explanations of the scattering mechanism in the Rayleigh and resonance regions, and to make rigorous tests of the various theoretical predictions. An extension of the physical optics theory of Adachi was found to predict accurately the echo area of the ogive, the double-rounded cone, and double-backed cone in the resonance region. The Rayleigh theory was found to be generally accurate for all the models considered. The impulse analysis of Kennaugh and Moffatt was found to be accurate for the cone sphere.     

Stabilization of reproducibility of frequencies of He-Ne lasers at 0.63 µ

We have investigated the frequency reproducibility of a He-Ne laser (0.63μ). We used the dependence of absorption on frequency in the strong-field standing-wave limit to provide stabilization. An absorption cell with a pure neon discharge atT = 73degC could be placed both inside and outside the resonator. In the former case, a magnetic field was used to influence the gain profile of the line in the He-Ne mixture in such a way as to increase the accuracy with which the center of the absorption line corresponded to the frequency for maximum power output of the laser oscillation. In the latter case, we used a single-mode laser to obtain saturation of absorption in the cell. The long-term reproducibility obtained wasapprox 10^{-9}. The short-term stability was much better. To obtain a very narrow dip in the center of the absorption line and consequently much higher stability (10^-13), we have investigated the vibration spectrum of absorption in the strong field of a standing wave in the CO2laser (10.6μ) and He-Ne-Ch4(3.39μ) systems.     

High-power difference-frequency generation at 5-11 µm in AgGaS2

Efficient generation of high peak power and high average power difference-frequency radiation, continuously tunable over the range of5-11 mum, has been achieved by mixing the Nd:YAG laser radiation with the output of an infrared dye laser pumped by the same Nd: YAG laser in AgGaS2. Peak pulse powers as high as 180 kW with an average power output of 14 mW were obtained around 6 μm.     

High-power difference-frequency generation at 4.4-5.7 µm in LiIO3

Efficient generation of high-power difference-frequency radiation, continuously tunable over the range of4.4-5.7 mum, has been achieved by mixing the Nd :YAG laser and Nd :YAG pumped infrared dye-laser outputs in LiIO3. Peak pulse powers as high as 550 kW with an average power output of 45 mW were obtained around 4.9μm.     

Collision effect on the saturation character of vibration-rotation transitions for 00°1-10°0 band of CO2

Experimental studies of collision effects on the saturation characteristics of vibration-rotation transitions for00deg1-10deg0 band of CO2is described. Saturation was studied in a passive absorption cell inside the laser resonator. The saturation value could be altered by varying the cell temperature and the pressure of CO2. Vibration-rotation transitions, at pressures greater than or equal to 1 mmHg, were found to be saturated homogeneously, in spite of the fact that the Lorenz width was much less than the Doppler width. This is explained by the high number of collisions during the lifetime in a vibrational state. In this case the spectrum of a single molecule corresponds to that of a Doppler profile. Cross sections for the destruction of levels of00deg1-10deg0by added gases have been obtained, which att = 800degK appeared to be σCO2-He =6 times 10^{-19};sigmaCO2-Ne =2.8 times 10^{-18}; sigmaCO2-CO2=6.6 times 10^{-18}; sigmaCO2-N2=1.2 times 10^{-17}cm²The introduction of sufficiently large absorption caused self-sustained radiation pulsation. When the field influenced the saturating system for only a short period of time, with the interaction time being commensurate with the period of time between collisions, the line was saturated nonhomogeneously. This was expressed by the fact that with the scanning of the laser frequency, a peak power output was observed, corresponding to Lamb's hole, in the center of a saturation line.     

The dielectric properties of brine in sea ice at microwave frequencies

The dc conductivity and complex dielectric constant at frequencies of 7.50, 9, 11, 30, and 40 GHz of 16 samples of sea water brine in equilibrium with sea ice with freezing temperatures ranging from -2.8degC to-25.0degC have been measured. The data is analyzed to yield parameters occurring in a Debye relaxation equation so that the dielectric constant of brine may be calculated throughout the microwave region of the electromagnetic spectrum.     

An Experimental 50 Mb/s Fiber Optic PCM Repeater

An experimental repeater for amplification and regeneration of 50 Mb/s fiber-optical pulses has been built and tested. For the receiver either Si p-i-n or avalanche photodiodes are used in conjunction with a high impedance FET input amplifier. The high voltage for the avalanche photodiode is generated internally and controlled by the received signal. This AGC circuit is capable of compensating for temperature changes of the avalanche gain over the range of-40 - +60degC. The optical transmitter consists of either a GaAs light emitting diode or a GaA1As laser diode coupled to optical fibers and directly modulated by a current driver with 30 percent electrical efficiency. For 10^-9error rate, the required average optical signal power for a pseudorandom signal is p-i-n diode: -41.5 dBm; avalanche diode: -56.6 dBm. The optical output power into a fiber with 1 percent index difference is LED: -17 dBm; GaAlAs laser: 0 dBm. The repeater power requirement is about 2 W.     

High-performance single-longitudinal-mode operation of InGaAsP/InP DFB-DC-PBH LD's

The lasing performance of InGaAsP/InP distributed feedback laser diodes with double-channel planar buried heterostructure (DFB-DC-PBH LD's) is reported for end-titled and antireflection (AR) coated configuration. High-power CW single-longitudinal-mode (SLM) operation over 55-mW light output at room temperature, high-temperature CW SLM operation over 105°C, as well as stable SLM operation under 2-Gb/s high-speed direct modulation, have been attained for 1.3-μm band DFB-DC-PBH LD's. 1.5-μm band DFB-DC-PBH LD's have also exhibited excellent DFB lasing characteristics, such as high power over 20 mW and high temperature over 75°C CW SLM operation. DFB SLM yield in the laboratory was also examined for 1.3-μm DFB-DC-PBH LD's, giving rise to a good prospect for practical use in optical-fiber communication systems.     

Low dark-current, high gain GaInAs/InP avalanche photodetectors

High performance inverted-mesa GaInAsP/InP avalanche photodiodes responding out to 1.25 μm have been fabricated. Uniform avalanche gainsM, of 700 dark-current densities of3 times 10^{-6}A/cm²atM = 10, and an excess noise factor of ∼3 atM = 10have been achieved by placing the p-n junction in the InP and using a new passivation technique. Pulse-response rise times of less than 160 ps, limited by the rise time of the mode-locked Nd:YAG laser pulse, were measured with an avalanche gain of 40.