Characteristics of diffused-stripe InP/InGaAsP/InP lasers emitting around 1.55 µm

Fabrication and lasing characteristics of InGaAsP/InP lasers emitting around 1.55 μm are described. Zn-diffused stripe-geometry lasers with emission wavelengths in the1.53-1.60mum range were fabricated from InP/InGaAsP/InP DH epitaxial wafers prepared by a low temperature LPE technique. The dependence of the lasing charaeteristics on the stripe width was examined. The lowest threshold current (≃160 mA under CW operation at 27°C) was obtained for a laser with a 13 μm stripe. CW operation of the laser has been achieved at a heat-sink temperature as high as 53°C. For sufficiently narrow stripe widths (simeq6 mum), fundamental-transverse mode and single-longitudinal mode operation was obtained under CW operation. Moreover, the lasers have good high-frequency performance. The lasers showed excellent dynamic properties without waveform distortion under high-frequency (800 Mbits/s) large-signal pulse modulation. The full width at half maximum of the longitudinal mode envelope was approximately 30 Å at 800 Mbits/s.     

1.064- and 1.32-µm Nd:YAG single crystal fiber lasers

We report the development of CW 1.06- and 1.32-μm fiber lasers made of short sections of Nd:YAG single crystal fibers optically end-pumped either with an Argon-ion laser (lambda = 0.5145mum) or a single laser diode (lambda = 0.817mum). High conversion efficiencies, a few milliwatt thresholds and 10-20 mW output powers are reported, as well as a good coupling efficiency to a standard single-mode fiber. Loss mechanisms and means of reduction are also investigated for a variety of fiber lasers in either guided or unguided configurations.     

New laser lines with wavelengths from λ = 61.7 µm down to λ = 27.7 µm in optically pumped CH3OH and CD3OH

25 new CW far infrared laser lines have been observed with wavelengths fromlambda = 61.7 mum down tolambda = 27.7 mum. We have significantly increased the number of known short wavelength laser lines and extended the laser line spectrum to the 30 μm region by using a BaF2outcoupling system.     

Thermal resistance and temperature distribution in double-heterostructure lasers: Calculations and experimental results

The thermal resistance and temperature distribution in double-heterostructure lasers have been calculated taking into account the characteristics of the different layers, the internal quantum efficiency, and allotment of the dissipated power, in order to optimize their structure. The influence of the different layers in the heterostructure and of the electrical contact is analyzed. Thermal resistance of CW, shallow proton-implanted lasers has been determined experimentally using the technique that relies upon a null measurement of the wavelength of a single Fabry-Perot mode. Statistical results on some hundreds of lasers with different stripe widths (6-125 mum), mounted on different heat sinks (copper, silicon, beryllium oxide) are given and compared to theoretical values. The model we propose gives good agreement with experimental results. The 6 μm width stripe laser is of special interest because this laser is transverse monomode up to an optical power of 6 mW. A value of 22° C/W has been achieved in a reproducible manner for6 times 300 mum lasers mounted on copper heat sinks. The effectiveness of the bonding technique is demonstrated. Si and BeO heat sinks are suitable for many applications because of their chemical (V grove etching in Si) and thermal properties (better linear expansion coefficient match to GaAs). We show that the increase of thermal resistance so introduced is still compatible with long CW operation.     

High power far-infrared emission from CO2laser-pumped D2S and HDS

Intense superfluorescent emission in the far infrared has been produced for the first time at wavelengths oflambda = 135.7 mum andlambda = 90.1 mum from D2S pumped with a CO2TEA laser. The pump and emission lines have been assigned to molecular transitions in the ν2vibrational mode. An additional strong line atlambda = 183.2 mum has been attributed to the HDS molecule.     

Spectral properties of strongly coupled 1.5 µm DFB laser diodes

Spectral measurements of strongly coupled DFB lasers operating at 1.5 μm are presented. The magnitude of the coupling coefficientkin these devices was determined to be 80 cm^-1for lasers withlambda = 1.12 mum cladding layers and 160 cm^-1for devices withlambda = 1.3 mum cladding layers. These values forkare believed to be the largest reported for 1.5 μm DFB lasers. CW spectral linewidths as low as 10 MHz at 15 mW output power were obtained, and the linewidth was observed to vary approximately as the inverse of the device length cubed. Spectral measurements performed under 2 Gbit/s direct modulation exhibited a side mode suppression ratio of >38 dB. The effects of transient wavelength chirping were also investigated in detail and the maximum wavelength deviation was found to be ≃1.5 Å.     

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     

CW HF/HCN and HF/DF optical resonance transfer lasers

CW laser oscillation has been observed in HCN at3.85-3.9 mum and in DF at3.8-4.0 mum. Mixtures of HF/HCN/He and HF/DF/He were irradiated by a CW multiline HF chemical laser. Vibrational excitation of HF by resonance absorption, followed by rapid V-V energy transfer to HCN or DF, produced a population inversion. The HCN gain was estimated to be between 0.08 and 0.17 percent/cm. The DF gain was greater than 0.17 percent/cm and 25 mW of power were outcoupled.     

Dispersion phenomena in hollow alumina waveguides

Significant variations in the attenuation characteristics of hollow alumina waveguides (suitable for use in the manufacture of CO2waveguide lasers) have been measured over the9-11 mum waveband. Existing waveguide transmission theory has been successfully used to explain the qualitative nature of the observed experimental results by taking into account the effects of dispersion due to active lattice absorption bands in alumina. The suprising conclusion of the work is that the major variations in attenuation are a result of a complete change in the guiding nature of the waveguide atsim 10 mum. At this point, the nature of the waveguide changes from being "leaky" at shorter wavelengths to exhibiting attenuated total internal reflection at longer wavelengths. This results in there being a distinct difference in attenuation characteristics for the two main laser emission bands centered at 9.6 and 10.6 μm, respectively.     

Effective refractive index and first-order-mode cutoff conditions in InGaAsP/InP DH laser structures (λ = 1.2-1.6 µm)

The virtual wavelength-invariance of the ratio between bandgap wavelength and the square root of the dielectric-constant step in InGaAsP/InP DH structures [i.e., (lambda_{g}/sqrt{Deltaepsilon}) = 0.95 pm 0.03for 1.2 μm< lambda_{g} < 1.6 mum] and an analytical approximation for the transverse propagation constantb, allow the derivation of an accurate, closed-form expression for the effective refractive index Neffof InGaAsP/InP planar DH lasers emitting in the1.2-1.6 mum range. Then, Neffis only a function of two readily measurable parameters: emission wavelength and active-layer thickness. Furthermore, the mode cutoff conditions for various lateral waveguides: buried-rectangular, buried-crescent, and ridge-guide, become wavelength-independent analytical expressions. First-order-mode cutoff conditions for these lateral waveguides are derived, plotted and compared to experimental data from mode-stabilized 1.3 and 1.55 μm DH lasers.     

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.     

Observations of self-focusing in stripe geometry semiconductor lasers and the development of a comprehensive model of their operation

Experimental measurements of the optical-beam parameters of conventional oxide-insulated GaAs stripe-geometry lasers as a function of stripe width have shown a marked difference in the waveguide mechanism of narrow-stripe (simeq10 mum) and wide-stripe (>20 mum) lasers. The optical wave of narrow-stripe lasers is guided by the previously reported gain-guiding mechanism. The optical wave of wide-stripe lasers is found to be guided by changes in the real part of the dielectric constant that are caused by a dip in carrier concentration along the axis of the lasing filament. This self-focused guiding has been predicted theoretically. These experimental results strongly support the hypothesis that in all cases the waveguides are formed predominantly by the naturally occurring variations in carrier concentration beneath the stripe. A new and fairly comprehensive mathematical model has been developed based on this assumption. The model predicts the carrier concentration, resultant gain, and dielectric constant profiles together with the optical-beam parameters and light/current characteristics of stripe-geometry lasers. The model is applicable over a wide range of stripe widths and device structures. The results are compared with experiment over the range of stripe widths from10-20 mum and found in reasonable agreement. The effects of narrowing the stripe width below 10 μm are calculated and found to be in qualitative agreement with recently published experimental results. In particular the light-output power at which a predicted "kink" in the light/current characteristic occurs is found to increase rapidly as the stripe width reduces.     

Laboratory-simulation experiment for optical communication through low-visibility atmosphere using a diode laser

A laboratory-simulation experiment has been performed to determine the feasibility of exploiting the scattered (in addition to the unscattered) radiation to improve optical communication through low-visibility atmosphere. A multiple field-of-views (FOV) (0.043-0.945degfull angle) optical receiver was designed which utilizes a narrow-band interference filter (Deltalambda sim 15Å) for background-light suppression. The laser transmitter was a CW GaAlAs laser diode (lambda = 0.8486 mum) capable of emitting ∼ 7 mW power output of 15° divergent beam. Both polydisperse (particle diameters,din the range ofsim0.2-5.17 mum) and monodisperse (d sim 0.2 mum andd = 3.01 mum) latex spheres in water were used for simulating haze, fog, etc. Results include signal-to-noise ratio (SNR) and scattered-to-unscattered signal ratio as a function of field-of-view of reception for various optical thickness of the medium. Also, SNR is plotted as a function of optical thickness for various FOV's. The unique feature of this simulation is that it can simulate worst case solar background where the sun enters the FOV of receiver. Finally a new technique of estimating forward-scattering efficiency and root-mean-square forward scatter angle has also been presented with examples.     

Lasing characteristics of 1.5 - 1.6 µm GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors

Lasing characteristics of1.5-1.6 mum GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors (DBR-ITG lasers) are given theoretically and experimentally. At this wavelength region the fiber loss is ultimately low, but the effect of the material dispersion is serious. It is theoretically found that single longitudinal mode operation over twice the threshold current is obtainable with high quantum efficiency by optimizing the coupling properties and configurational parameters of the DBR-ITG laser. The experimental results are also demonstrated. Single longitudinal mode operation of1.5-1.6mum DBR-ITG lasers under high-speed direct modulation was achieved with a pulsewidth of 1.5 ns.     

1.5-1.6 µm GaInAsP/InP dynamic-single-mode (DSM) lasers with distributed Bragg reflector

The theoretical bases and the experimental results of the dynamic behavior of1.5-1.6 mum GaInAsP/InP dynamic-single-mode (DSM) lasers with distributed Bragg reflector (DBR) are given. A condition for the single-mode operation of a rapidly modulated DBR laser, called a "dynamic-single-mode laser," and the dynamic spectral width were theoretically presented. Experimentally, buried-heterostructure distributed-Bragg-reflector integrated-twin-guide (BH-DBR-ITG) and buried-heterostructure butt-jointed-built-in distributed-Bragg-reflector (BH-BJB-DBR) lasers emitting at the wavelength of1.5-1.6 mum were directly modulated up to 3 GHz, and the stable single-mode operations were obtained in both types of lasers. The dynamic spectral width at the worst modulation condition was measured to be 0.27 nm, which was aboutfrac{1}{35}times smaller than that of conventional lasers.     

GaInAsP/InP DH lasers with a chemically etched facet

We describe a new configuration and novel fabrication method for GaInAsP/InP DH lasers in which one of the facet mirrors is chemically etched and the other one is formed by cleaving. The etched facet is fabricated monolithically by wet chemical etching in a solution of HCl:CH3COOH:H2O2= (1:2:1). Broad-area contact lasers of this type operating at room temperature at a wavelength ofsim 1.3 mum have been demonstrated.     

Theoretical simulation of a pulsed HF/DF optical resonance transfer laser

The theoretical performance of a pulsed HF/DF optical resonance transfer laser (ORTL) is presented. The key reaction of HF/DF is the HF-DFV-Venergy transfer reaction, but its rate constant has not been accurately determined so far. Therefore, in order to precisely predict the CW HF/DF ORTL experiment, the rate constant of the HF-DFV-Venergy transfer reaction is determined to be dependent on V². The relationship between gas concentration (HF, DF, and He) and pumping condition (pumping intensity and pulse width) is determined to maximize the DF small-signal gain. As a result, it is found that the pumping HF laser lines of2.7-3.0mum are shifted to the ORTL lines of3.9-4.3mum at a low total pulse efficiency of about 0.1 percent.     

Near-infrared In1-xGaxP1-zAszdouble-heterojunction lasers: Constant-temperature LPE growth and operation in an external-grating cavity

InP-In1-xGaxP1-zAsz-InP (x sim 0.08, z sim 0.17) double-heterojuncfion (DH) lasers emitting atlambda sim 1.0 mum (77 K) have been fabricated by constant-temperature liquid-phase epitaxy (LPE). The crystal-growth process is described and compared to previous work on visible spectrum (lambda sim 6000-Å) In1-xGaxP1-zAszdouble-heterojunction lasers. Emission spectra are presented for the near-infrared quaternary DH lasers. In particular, the observation of cavity oscillations in the low-level spontaneous-emission spectra allows the determination of the index dispersion quantity over a wide wavelength range. Finally, these In1-xGaxP1-zAszDH lasers are operated in an external-grating cavity. Such operation permits tunable, narrow-linewidth laser emission and provides information concerning radiative-recombination processes. Comparisons are made to earlier work on external-grating operation of visible-spectrum (lambda sim 6000-Å) In1-xGaxP1-zAszDH lasers.     

Long operating time CW atomic iodine probe laser at 1.315 µm

A photolytically pumped longitudinal flow CW atomic iodine laser at1.315 mum with virtually an unlimited operating time and easy operation is reported. Several weeks of continual operation is easily attainable. The prolonged lasing is achieved by employing a passive closed cycle flow system of the C3F7I laser fuel. A high pressure dc Hg arc lamp is used for the photolytic pumping. Peak CW powers of 38 mW have been obtained along with good stability (∼1 percent amplitude fluctuations) in the fundamental TEM00mode via the use of an internal lens.