Single Spatial Mode Research Articles

Index guided single stripe lasers fabricated by selective area growth in a metalorganic chemical vapor deposition system

Index guided single stripe lasers have been fabricated by using a one-step selective area growth technique in the metalorganic chemical vapor deposition system. By pyrolytically depositing SiO or SiO2 layers on GaAs substrate, and lithographically defining the stripes, single stripe heterostructure lasers can be achieved on the uncovered substrate while polycrystalline films are observed beyond the stripe regions. The device operated with a single spatial mode for up to more than three times the threshold current.

Surface-emitting distributed feedback semiconductor laser

Electrically pumped, surface-emitting distributed feedback lasers were for the first time demonstrated without any facet reflections. The devices contained second-order gratings etched into a thin p-AlGaAs cladding layer surface. A gold ohmic contact deposited directly onto the p-AlGaAs grating surface provided strong coupling to the waveguide mode. Single spatial mode devices with low divergence output beams and single frequency spectra were obtained at room temperature.

Low threshold buried heterostructure quantum well diode lasers by laser-assisted disordering

The first buried heterostructure laser diodes fabricated by laser-assisted disordering, a direct-write process, are described. In laser-assisted disordering a focused laser beam is scanned across the AlGaAs-GaAs heterostructure material to induce local melting and thereby incorporate Si from an encapsulation layer into the regrown crystal. A subsequent thermal anneal (850 °C, 0.7 h) is used to diffuse the Si deeper into the sample to enlarge the disordered region. This combination of patterned crystal regrowth and impurity-induced disordering is used to fabricate quantum well devices with lasing threshold currents as low as 6 mA (pulsed) and 8 mA (continuous). These devices have a narrow 4.5-μm-wide waveguide region and operate with a single longitudinal and spatial mode. They are the first optoelectronic devices fabricated with a direct-write laser-assisted process.

CW performance and life of 1.3μm InGaAsP/InP lasers emitting at high facet power densities

Individual stripe InGaAsP/lnP lasers emitting in a single spatial mode have been operated for times in excess of 7000 h at constant output power levels up to 40 mW (NA 0.5) with no significant increase in drive current and at 60 mW (NA 0.5) with drive current increases of typically 1.2%/1000h. Units have been operated short-term at values of output power and facet flux density of 263 mW and 20MW/cm2, respectively, and life-tested at a constant power of 90 mW (NA 0.5) for times in excess of 6300 h. These results demonstrate the resistance of this laser material to facet degradation, and show a clear advantage over GaAlAs/GaAs lasers of comparable structure which typically show catastrophic facet damage at flux densities of 2–4 MW/cm2.

A ten-element array of individually addressable channeled- substrate-planar AlGaAs diode lasers

A monolithic array of ten individually addressable channeled-substrate-planar diode lasers, each emitting 30 mW CW in a single spatial mode, has been developed with particular application to high data rate multichannel optical recording systems.

Performance of a channelled-substrate-planar high-power phase-locked array operating in the diffraction limit

We report a stable, coherent, single spatial and single longitudinal mode phase-locked 8-element CSP array operating in the diffraction limit with a phase front aberration of <λ/50. Both theoretical modelling and experimental observation indicate that the single out-of-phase coherent array mode is strongly preferred.

Transformation of statistical characteristics of picosecond laser pulses by multiple-scattering media

Higher-order (up to eighth) temporal moments to characterize non-Gaussian picosecond laser pulses were measured in a laboratory-simulation experiment for laser communication through multiple-scattering media. A technique employing deconvolution of higher-order moments (taking into account the impulse response of the detector and sampling system) is described to determine the true moments from the observed sampling scope traces of the pulse shapes. Measuring the broadening of the original pulses (~20-ps pulse width) generated with a single spatial mode GaAlAs laser diode by current modulation gain switching method, the temporal moments of order n = 2–8 were analyzed as a function of optical depth τ of the multiple-scattering medium (consisting of latex spheres suspended in water) (5.5 ≤ τ ≤ 10.31) and receiver’s field of views (FOV) ranging from 0.503 to 2.1°. Higher-order temporal moments curves are presented as a function of both optical depth and FOV showing the general trend of the moments initially to rise quickly and finally to attain saturation or limiting values. A simple empirical relationship of a polynomial function of the order of 3 is obtained to express the broadened pulse width (n = 2) in the range of optical depth and FOV considered here. The results described in this paper clearly indicate the significance of the contribution of the longer part of the non-Gaussian tails of the broadened pulses to practical design problems in all-weather atmospheric optical communications.

High peak power and gateable picosecond optical pulses from a diode array traveling-wave amplifier and a mode-locked diode laser

A traveling-wave amplifier made from an AlGaAs laser diode array has been used to selectively amplify individual pulses from a 250-MHz pulse train of a mode-locked laser diode. A coherent average output power of 35.3 mW was obtained for 14.2 mW incident on the final objective. Peak powers as high as 4.75 W have been obtained with application of both continuous and pulsed current to the amplifier. This represents a 5.7-fold enhancement of the peak power over the power of the incident signal. When the mode-locked laser source had a single spatial mode, the amplifier output was dominated by a single lobe, ∼0.82° wide.

A high-power channeled-substrate-planar AlGaAs laser

A channeled-substrate-planar AlGaAs laser has been operated to 190 mW cw, and 170 mW cw in single fundamental spatial mode. A single fundamental spectral mode is observed up to about 70 mW. Beyond 70 mW there are minor increases in spectral sideband power and the far-field pattern broadened asymmetrically with increased drive. Both these effects are attributed to spatial ‘‘hole burning.’’

A nonlinear study of Kelvin–Helmholtz stability

The Kelvin–Helmholtz stability problem is studied by employing the variational method. With the restriction to a single dominant spatial mode, a set of fully nonlinear evolution equations is derived. The limiting states of these evolution equations are discussed and their stability is analyzed. It is found that at the critical point for the linear stability problem, another stable limiting state with finite amplitude appears. A nonlinear sinusoidal wave state is also found. The nonlinear dispersion relation for this wave is derived. The wavenumber of the dominant spatial mode and its evolution with time can also be determined with the aid of the variational method.

Thermally accelerated life testing of single mode, double-heterostructure, AlGaAs laser diodes operated pulsed at 50mW peak power

Single spatial mode, double-heterostructure, channel-substrate-planar AlGaAs laser diodes have been life tested under thermally accelerated conditions to characterize the reliability of the diodes in a digital, optical communication system intended for space application. The diodes were operated pulsed under constant drive current conditions at 50 mW peak power, 25 ns pulse width, and 1 percent duty cycle in a dry, inert environment at ambient test temperatures at 40,55, and 70°C. Diode performance parameters as related to the space application, such as pulsewidth, peak power, wavelength spectrum, spatial mode, and threshold current, were periodically monitored. Tests have continued for over 14 000 h. The test results for all diodes with failure defined by power degradation alone is compared to the test results for single mode diodes with failure defined by power degradation, wavelength shift and spatial mode changes. It is found that the life test results are substantially equivalent but differ from earlier published reports for laser diodes operated CW. An activation energy of about 0.39 eV is deduced with a predicted median life of about <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5 \times 10^{4}</tex> h at 20 °C. These values are somewhat lower than those found for diodes operated CW and are attributed to the use of single mode laser diodes here. It is concluded that thermally accelerated life testing for single spatial mode laser diodes must incorporate a means to separate bulk material, current, and optical density induced degradation effects. A test scheme is proposed.

Amplification of a polarized single-photon state and determination of resulting polarization correlations

The photon statistics of a single spatial mode of the optical field (including the two polarization states) resulting from a single-photon state undergoing unsaturated amplification by a two-level medium are derived. Results are compared with those for an idealized noiseless linear amplifier and a conventional quantum-noise-limited amplifier in the limit of no gain. When the amplified field is divided for detection of different polarization states, distinctive polarization biases and correlations are evident on average when the polarization of the input state is constant. However, the results are insufficient to determine the initial polarization state based on a single measurement. If one must average over two orthogonally polarized input states (e.g., an incoherent mixture of either linearly or circularly polarized initial states) as required in the absence of hidden variable information in an Einstein–Podolsky-Rosen experiment, the distinctions are completely erased.

Single mode, high power GaAlAs/GaAs lasers

A new channeled-substrate narrow stripe laser with double current confinement and large optical cavity has been developed. This laser incorporates symmetrical waveguiding layers to form the large cavity and additional reverse biased p-n junction to confine the current. Light output up to 60 mW with stable single longitudinal and spatial modes is obtained in cw operation at room temperature. Threshold current as low as 15 mA and differential quantum efficiency as high as 70% are achieved. Furthermore, high quality beam with far-field angle of 32°×10° (full width at half-maximum) is maintained throughout the whole power range.

Optical communications research program to demonstrate 2.5 bits/detected photon

The transmission of information by optical signals over a space channel with a power efficiency of 2.5 bits/detected photon markedly increases the amount of information that can be transmitted to satellites. An account is given of the research program at the Jet Propulsion Laboratory that is attempting to demonstrate that optical signals can be used to transmit information over a space channel with this power efficiency. It is noted, however, that the ability to attain 2.5 bits/detected photon (or higher) depends heavily on the validity of the mathematical models used in the performance analysis. Therefore, verification of the channel dark current noise models is a crucial first step. Another prerequisite is a high-brightness, single-spatial mode laser emitter. It is believed that single spatial model devices with power outputs of about 1W can be achieved by coherently combining a number of GaAs lasers in what effectively amounts to a phased array.

Terraced-heterostructure large-optical-cavity AlGaAs diode laser: A new type of high-power cw single-mode device

A new terraced lateral wave confining structure is obtained by liquid phase epitaxy over channeled substrates misoriented perpendicular to the channels’ direction. Single spatial and longitudinal mode cw operation is achieved to 50 mW from one facet, in large spot sizes (2×7.5 μm, 1/e2 points in intensity) and narrow beams (6°×23°, full width half-power). At 70 °C ambient temperature cw lasing is obtained to 15 mW from one facet. Weak mode confinement in an asymmetric lateral waveguide provides discrimination against high-order mode oscillation.

High-power diode lasers for optical recording with operating lifetimes in excess of 10 000 h

Real-time lifetest data are reported which show that single spatial mode CDH-LOC diode lasers can operate stably at 25°C ambient temperature for greater than 10000 h at 40–50 mW output power and 50% duty cycle. This makes CDH-LOC lasers useful for optical disc recording, as well as for a variety of other applications that require high output power in a stable nonastigmatic farfield beam pattern.

Single-growth embedded epitaxy AlGaAs injection lasers with extremely low threshold currents

A new type of strip-geometry AlGaAs double-heterostructure laser with an embedded optical waveguide has been developed. The new structure is fabricated using a single step of epitaxial growth. Lasers with threshold currents as low as 9.5 mA (150 μm long) were obtained. These lasers exhibit operation in a single spatial and longitudinal mode, have differential quantum efficiencies exceeding 45%, and a characteristic temperature of 175° C. They emit more than 12 mW/facet of optical power without any kinks.

Stability of the output power from a cw CO laser emitting the dominant spatial mode

A comparative investigation was made of the amplitude stability of the output power from sealed CO and CO2 lasers emitting the dominant spatial mode. It is shown that by using a cw CO molecular laser (emitting simultaneously due to a large number of cascade-coupled vibrational-rotational transitions) as a coherent radiation source, it is possible to achieve a relatively high output power stability in a single spatial mode without using frequency control devices.

High-power TEM_00 tunable laser system

A 45-MW, 25-nsec duration, tunable laser operates in the near-ir and can be frequently-doubled to produce a 20-nsec duration uv output of 7.5 MW. The additional feature of a near single spatial mode makes this laser a tunable source of exceptional brightness. The beam diameter and divergence are substantially influenced by the spatial gain profile of the oscillator dye medium, by the ability to pump uniformly the dye amplifiers and, for the uv light, by the stringent angular sensitivity of the frequency-doubling crystal.

On the concept of energy-dependent buckling in multi-group diffusion theory—I

By considering the energy multi-group (or energy multi-point) neutron diffusion equation a technique is developed which allows an energy-dependent buckling to be defined. Expanding the scalar flux in terms of the eigensolutions of the Helmholtz equation produces a linear set of strongly coupled differential equations for the various spatial modes in a given energy group. Each mode has an associated buckling which is a function of the particular energy group. The approximation of representing the flux by a single spatial mode produces a simple modification to the usual diffusion equation.