Band-to-band Auger processes in 1.3 µm InGaAsP semiconductor lasers

Within the context of the four-band Kane model, we have analyzed the contributions of pure collision and phonon-assisted CHSH and CHCC Auger processes to the nonradiative recombination lifetime in highly excited InGaAsP. Our method incorporates refined expressions for the wavefunction overlap integrals and spectral density functions, and avoids approximations to Fermi and phase-space factors. Our results are in good agreement with recent experiments, despite a rather large uncertainty associated with the unavailability of precise values for certain band-structure parameters.     

1.5 µm range InGaAsP/InP distributed feedback lasers

Lasing characteristics of InGaAsP/InP distributed feedback (DFB) lasers in the 1.5 μm range were studied theoretically and experimentally. Wave propagation in five-layer DFB waveguides were analyzed to estimate the effect of the structural parameters on threshold conditions. A brief consideration on designing a low threshold laser and its lasing wavelength was made. DFB buried heterostructure lasers with fundamental grating emitting at 1.53 μm were prepared by liquid phase epitaxial techniques. CW operation was confirmed in the temperature rangeof -20° to 58°C, and a CW threshold current was as low as 50 mA at room temperature. A stable single longitudinal mode operation was observed both in dc condition and in modulated condition by a pseudorandom pulse current at 500 Mbits/s. No significant increase in the threshold current was observed after 1400 h continuous CW operation at 20°C.     

Frequency response of 1.3µm InGaAsP high speed semiconductor lasers

The frequency response of a group of 1.3 μm InGaAsP vapor-phase-regrown buried heterostructure lasers of various cavity lengths is analyzed by fitting the measured response curves. The dependence of resonant frequency f0and damping rateGammaon bias power is determined. The differential gain coefficient for InGaAsP is determined as3.5 times 10^{-16}cm². The damping rate is found to be proportional to the square of the resonant frequency with a proportionality factor which is independent of device geometry and facet reflectivity. The existence of such a universal relationship betweenGammaand f0and the observed magnitude of the damping rate is explained by the interband relaxation model of nonlinear gain.     

An investigation of the frequency stability and temperature characteristics of 1.5 µm coupled-cavity injection lasers

It has been demonstrated, both theoretically and experimentally, that a coupled-cavity injection laser is capable of oscillating with a single-frequency output. However, because of the nature of the constructive and destructive interference of the optical fields inside the coupled cavities, the frequency of the lasing mode is subject to change due to variations in the refractive indexes of the active media, which in turn are functions of both injected carrier density and junction temperature. In this paper, the frequency stability (mode hopping) and temperature characteristics of such lasers are computed numerically for two specific cases based on a rate equation formalism previously reported. The behavior of experimental lasers fabricated as cleaved-coupled-cavity (C³) devices showed good agreement with the computed result, except that the measured light-versus-current (L-I) curves were "kinky." Each kink was found to signify mode hopping, which is believed to be caused by incomplete electron pinning above lasing threshold. The speed of mode switching from the initial lasing mode to the adjacent lasing mode was less than 1 ns and a hysteresis was observed when both cavities were operated with large gains. Experimental results have shown that when the C³laser is operated in a steady-state single-frequency output the photon fluctuations are significantly reduced, which implies that the coupled-cavity laser can be used as a CW signal source, in conjunction with an external modulation, which is free from partition noise at gigabit per second data rates.     

Field-assisted semiconductor photoemitters for the 1—2-µm range

Photoemission data and model calculations are presented for a field-assisted semiconductor photoemitter which has achieved reflection-mode quantum efficiencies as high as 8.0 percent at 1.55 µm. The cathodes are p-p heterostructures employing lattice-matched InP-InGaAsP alloys. A thin electron semitransparent Schottky barrier provides the biasing contact for field-assisted electron emission. Parameters for optimal photoemission and sources of dark-current emission are discussed.     

Amplifying medium characteristics in optimized 190 µm/195 µm DCN waveguide lasers

Extensive study of the CW, dischaxge-pumped, DCN laser operating on the (22⁰0-09¹0) 190 and 195 μm lines has been undertaken in order to develop a new source of radiation suitable for plasma diagnostics. The optimum values of the parameters of the discharge, the unsaturated gain, and the saturation intensity are given as a function of the tube diameter. Scaling laws predicting the maximum output power as a function of the tube dimensions and of the cavity loss are established. Such lasers compare favorably with optically pumped submillimeter lasers, since 250 mW are available from a 3 m long, 5 cm diameter waveguide DCN laser.     

Spectral behavior of 1.5 µm BIG-DBR-DSM lasers

Direct modulation properties of BIG-DBR-DSM lasers were investigated by measuring both the time-averaged and the time-resolved spectra. A high side-mode-suppression ratio of more than 30 dB was obtained from time-averaged measurements with the modulation frequency in the range of 250 MHz-2 GHz. The dynamic wavelength shift at - 3 dB level obtained from both time-averaged and time-resolved measurements was less than 2 Å. Time-resolved measurements showing an oscillatory behavior of the dynamic wavelength shift with both red and blue shifts were also observed. Calculations based on a numerical solution of the rate equations, which agreed quantitatively with the measurement, indicate a line width enhancement factor estimated to lie between 3.5 and 5.5.     

Longitudinal mode behaviors of 1.5 µm range GaInAsP/InP distributed feedback lasers

Longitudinal mode behaviors of asymmetric structure distributed feedback buried heterostructure (DFB-BH) lasers are examined theoretically and experimentally. A 1.5 μm range GaInAsP/InP DFB-BH laser was fabricated by a three-step LPE growth process. We measured the stopband in the spectrum of the DFB laser. It was found that no resonance mode emission occurred in the gain spectrum and its spectrum was asymmetric with respect to the Bragg wavelength. Most of the lasing power concentrated on the DFB mode adjacent to the stop-band which was determined by the Bragg condition. The measured spectrum was explained by the calculated results of the coupled wave theory with external reflectors. The asymmetric spectrum was caused by the relative position of the cleaved facet on the corrugation grating. It was shown that the asymmetric structure DFB laser, which consisted of two end facets with different reflection coefficients, gives a stable single longitudinal mode. There was no mode jump up to 2.3 times threshold. At a modulation depth of 100 percent, the ratio of the highest nonlasing mode intensity to the lasing DFB mode was estimated to be -16.0 dB.     

1.5 µm GaInAsP traveling-wave semiconductor laser amplifier

This paper presents a theoretical and experimental study in terms of small-signal gain, signal gain saturation, and noise characteristics of a 1.5 μm GaInAsP traveling-wave amplifier (TWA), realized through the application of SiOxfilm antireflection coatings. This TWA, having a residual facet reflectivity of 0.04 percent, exhibits a wide, flat signal gain spectrum and a saturation output power of +7 dBm at a 20 dB signal gain. The TWA also has a noise figure of 5.2 dB, which is the smallest value reported for semiconductor laser amplifiers. The experimental results are confirmed to be in good agreement with the theoretical predictions based on the multimode traveling-wave rate equations in conjunction with the photon statistic master equation analysis, which takes into account the amplifier material and device structural parameters. Signal gain undulation, saturation output power, and noise figure are also theoretically evaluated as functions of the facet reflectivity. The superior performance of the TWA demonstrates that the device is favorable for use in linear optical repeaters in fiber transmission systems.     

Three- and four-layer LPE InGaAs(P) mushroom stripe lasers for λ = 1.30, 1.54, and 1.66 µm

Mushroom stripe (MS) InGaAsP/InP and InGaAs/InP lasers have been realized emitting atlambda = 1.3, 1.54,and 1.66 μm, respectively. The main advantage of these MS lasers is their technological simplicity, because only one epitaxial growth step consisting of three or four layers, respectively, is required. No contact layer and no filling layers are needed. In our phosphorus silicate glass (PSG) passivated MS lasers, current spreading is completely inhibited. The devices have very low CW threshold currents and high values of output power, external differential efficiency, and To. All these properties are equivalent to those of the much more complicated buried heterostructure lasers. CW operation in up-side-up mounted MS lasers on p-type substrates is easily achieved, because their series resistance is very low. The devices oscillate in the fundamental lateral mode for easily achievable width and thickness combinations, and tend to longitudinal monomode behavior at moderate output powers. The modulation capability is more than 1 Gbit/s RZ due to the low capacitance of the mushrooms. The commonly used antimeltback layer for lasers withlambda > 1.5 mum on     

Low-threshold InGaAsP ridge waveguide lasers at 1.3 µm

The ridge waveguide configuration is shown to provide reliable low-threshold fundamental-transverse-mode lasers that are readily fabricated. Two variants are described: in the simple ridge laser, the 1.3 μm bandgap active layer is sandwiched between InP layers and in the cladded ridge, the active layer is surrounded by 1.1 μm bandgap InGa AsP. Thresholds as low as 34 mA and efficiencies as high as 66 percent are observed. Output power is linear to more than 12 mW. Several lasers have been operated at 30°C for over 1500 h without measurable degradation. Selected lasers exhibit stabilized longitudinal mode behavior over extended temperature and current ranges. The potential manufacturability of this device is its most attractive feature.     

Distributed feedback InGaAsP/InP lasers with window region emitting at 1.5 µm range

Distributed feedback (DFB) InGaAsP/InP lasers with a window region formed at an end of the corrugated DFB region were made in order to overcome the problems inherent in the previous structures of DFB lasers with cleaved, sawed, etched, or AR-coated facets, or with an unexcited corrugation region. The window structure DFB lasers showed linear current versus output (I-L) curves, in contrast to those with a hysteresis or a kink appearing in a DFB laser with an unexcited region. Suppression of Fabry-Perot (F-P) resonances due to the two facets were sufficient enough to keep a single longitudinal mode property by DFB up to high excitation level. CW operation up to 65°C was achieved at the 1.5 μm wavelength range. Axial modes concerning the corrugated resonator were measured at about the threshold current. A stop band of a DFB laser was clearly observed with two dominant modes and much smaller submodes, which almost agreed with the axial modes predicted from a basic DFB theory.     

Gain measurements in 1.3 µm InGaAsP-InP double heterostructure lasers

The net gain per unit length (G) versus current (I) is measured at various temperatures for 1.3 μm InGaAsP-InP double heterostructure lasers.Gis found to vary linearly with the currentIat a given temperature. The gain bandwidth is found to decrease with decreasing temperature. The lasing photon energy decreases at 0.325 meV/K with increasing temperature. Also, the slopedG/dIat the lasing photon energies decreases with increasing temperature. This decrease is more rapid forT > sim210K. This faster decrease is consistent with the observed higher temperature dependence of threshold (low T0at high temperatures) of 1.3 μm InGaAsP lasers. A carrier loss mechanism, due to Auger recombination, also predicts thatdG/dIshould decrease much faster with increasing temperature at high temperatures. We also find that the slopedG/dIdecreases slowly with increasing temperature for a GaAs laser, which is consistent with the observed temperature dependence of threshold of these lasers.     

Comparison of the electrical characteristics and threshold temperature dependences of λ ~ 1.3 µm and λ ~ 1.6 µm stripe-geometry InGaAsP DH lasers

Light-current and derivative voltage-current characteristics are compared for dielectric-isolated stripe-geometry InGaAsP DH lasers operating under both pulsed and continuous excitation at wavelengths near 1.3 and 1.6 μm. At both wavelengths the lasers have comparable threshold temperature dependences, and both exhibit similar kinks and light jumps in their characteristics. The low T0values observed (sim50- 70K) cannot be attributed to drift-current losses.     

Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H2CO [5_{1,5}(upsilon = 0) rightarrow 6_{0,6}(upsilon_{5}= 1)] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H2CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H2CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was1.0times10^{-14}attau = 100s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as8.0times10^{-13} cdot tau^{-1}for 10 msleq tau leq 100s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H2CO spectrum.     

InGaAsP ridge waveguide distributed feedback lasers operating near 1.55 µm

Fabrication and performance of ridge waveguide distributed feedback lasers grown by liquid phase epitaxy for operation in the 1.50-1.58 μm spectral region have been studied. These lasers incorporate first- and second-order diffraction gratings written by the electron beam lithography and optical holography and defined by wet chemical etching and novel ion-beam-milling techniques. Ridge waveguides were formed by post-regrowth processing and heteroepitaxial ridge overgrowth. Distributed feedback ridge lasers were characterized by room temperature CW threshold currents as low as 40 mA, two facet external quantum efficiencies of up to 40 percent and stable transverse mode operation up to the output power of 10 mW. In strongly coupled devices, even with a cleaved resonator, the Bragg mode intensity exceeds that of the residual Fabry-Perot modes by a factor of 4000:1. Stable, single longitudinal mode operation was obtained under modulation rates as high as 4 GHz and error free transmission experiments over 60-km lengths of single mode fiber reproducibly performed at data rates as high as 2 Gbit/s.