Semiconductor Laser Arrays Research Articles

Frequency selectivity in laterally coupled semiconductor laser arrays

A longitudinal-mode analysis of a system of laterally coupled waveguided resonators is presented in the coupled-mode approximation. It is shown that variations in the mirror reflectivity of the individual channels result in coupling between the supermodes of the structure. This may lead to mode suppression by modulation of the threshold gain of different Fabry-Perot modes.

Highly efficient neodymium:yttrium aluminum garnet laser end pumped by a semiconductor laser array

In recent experiments, 80-mW cw power in a single mode has been achieved from a neodymium: yttrium aluminum garnet (Nd:YAG) laser with only 1 W of electrical power input to a single semiconductor laser array pump. This corresponds to an overall efficiency of 8%, the highest reported cw efficiency for a Nd:YAG laser. A tightly focused semiconductor laser end pump configuration is used to achieve high pumping intensities (on the order of 1 kW/cm2), which in turn causes the photon to photon conversion efficiency to approach the quantum efficiency (76% for Nd:YAG at 1.06 μm pumped at 0.810 μm). This is achieved despite the dual-lobed nature of the pump. Through the use of simple beam-combining schemes (e.g., polarization coupling and multireflection point pumping), output powers over 1 W and overall electrical to optical efficiencies as high as 10% are expected.

Effect of mirror imperfections on phase-locked semiconductor laser arrays

Phase-locked semiconductor laser arrays normally have far-field-radiation patterns broader than that predicted by theory. We propose a possible broadening mechanism caused by steps on the cleaved mirror facets and by misalignment of the laser guide with respect to the laser facet. In the presence of facet steps, supermodes are coupled upon reflection. To satisfy the round-trip threshold condition, a laser cavity mode has to be an admixture of supermodes. This results in a broader far-field pattern (FFP) than that of a single supermode. Asymmetry of the pattern can also be predicted by this model. For an array of 10 lasers with center-to-center spacing of 8μm, the FWHM angle in the FFP of the lowest cavity mode is broadened by 1.5 and 2.5 folds, respectively, for random steps of average heights 17 and 90 A between adjacent lasers. The broadening factor is 2.3 when a single 204 A step exists between the third and the fourth laser. If the laser cavity is not aligned perpendicular to the facet, significant change in the FFP occurs only for a civity length not much shorter than the coupling length. A general consequence of the supermode mixing is the diminution of the threshold gain difference among the cavity modes, encouraging their simultaneous excitation.

Improved high-power twin-channel laser with blocking layer

We present an improved version of the twin-channel laser as a prototype for a new concept in semiconductor laser array structures with a well controlled far-field beam pattern. The laser device constitutes the first CW semiconductor laser array structure on a p-type GaAs substrate using current-blocking layers and yields improved current utilisation and efficiency over previous structures.

High-performance index-guided phase-locked semiconductor laser arrays

Using strongly coupled index-guided ridge-waveguide structures and tailoring the gain distribution to the fundamental eigenmode intensity distribution in a five-element coupled laser array, we greatly improve the performance over previously reported results. A stable single-lobe far-field pattern has been observed up to nearly three times threshold. This result is achieved by nonuniform pumping, which tends to excite the fundamental array eigenmode only by preventing spatial hole burning, which causes the excitation of higher-order modes.

Mode stability in real index-guided semiconductor laser arrays

A model has been developed of the zero-order supermode stability with drive current for real index-guided laser arrays allowing for the effect of injected carriers on the active-layer dielectric constant. It is difficult to design simultaneously for a high output power capability and zero-order supermode stability over the full dynamic range of the array.

Gain saturation effects in supermodes of phase-locked semiconductor laser arrays

A basic modal analysis that includes gain saturation effects in phase-locked semiconductor laser arrays is presented. For a particular supermode operation, different lasers in the array emit different amounts of light, and hence their (waveguide) propagation constants are modified differently. Solving the lasers' rate equations self-consistently with the coupled-mode wave equations seems to provide an answer that is in much better agreement with experimental results than the result using only the coupled-mode analysis.

Heatsink requirements for coherent operation of high-power semiconductor laser arrays

The heatsink requirements for coherent operation of laser diode arrays are examined by thermal analysis of specific heatsinks and the phase locking of laser diodes. It is shown that mounting laser diode arrays at the edge of semi-infinite copper heatsinks results in temperature variations between individual laser diode elements that are too large to permit coherent operation except at relatively low-power levels. It is also shown that by the use of diamond and/or shaped heatsinks it is possible to operate large arrays coherently to high optical output levels.

High power coupled ridge waveguide semiconductor laser arrays

Using index-guided ridge waveguide structures, we have obtained single-lobe far-field pattern from coupled multiple-stripe semiconductor laser arrays. A stable far-field pattern has been observed up to 250-mW peak pulsed power. This desired performance of a semiconductor laser array is achieved by intentional pumping of a laser array at coupling regions, which is possible in ridge waveguide structures.

Single contact tailored gain phased array of semiconductor lasers

We demonstrate a single contact tailored gain-guided array in which the gain profile across the array is made strongly asymmetric by varying the width of the contact stripes. A proton isolated array of six (GaAl)As lasers with 5-μm separations and widths varying linearly between 3 and 8 μm had a single lobed far field 2° wide, close to the diffraction limit for a single supermode. Fabrication of this device is simple, and suited to large-scale processing techniques. We also show that in such an asymmetric gain-guided array the fundamental mode is favored over higher order modes, and that higher order modes can have single lobed far-field patterns differing only slightly from that of the fundamental.

Controlled fundamental supermode operation of phase-locked arrays of gain-guided diode lasers

Uniform semiconductor laser arrays tend to oscillate in a superposition of their supermodes, thus leading to large beam divergence and spectral spread. Discrimination among the supermodes in phase-locked arrays is discussed theoretically. It is shown that supermode discrimination in gain-guided arrays, in favor of the fundamental supermode, is made possible by the near-field interference patterns which result from the complex optical fields of the gain-guided lasers. A fundamental supermode operation is demonstrated, for the first time, in GaAlAs/GaAs gain-guided laser arrays. This is achieved by control of the current (gain) profile across the array by means of individual laser contacts.

Rate equations analysis of phase-locked semiconductor laser arrays under steady state conditions

Rate equations analysis of phase-locked semiconductor laser arrays has been carried out. It was found that for given (Laser) current densities, the photon density distribution in the array elements is that particular one which maximizes the total photon density. The results of this analysis were then combined with the waveguiding properties of the laser array waveguide, yielding a basic model of phase-locked diode laser arrays. This model explains the effects of the variation of the current combination through the array elements on its mode structure that were observed recently.

Coupling coefficient of gain-guided lasers

An analytical model is presented for the coupling coefficient for two gain-guided coupled waveguides, e.g., semiconductor laser arrays. A common parabolic gain distribution is assumed for the lasers, and the effective dielectric constant distribution is approximated in terms of the bulk refraction index, wavelength, power filling factor, and the antiguiding factor. The fundamental mode is then formulated and used in an integral for the coupling coefficient. The dependence of the coefficient of various waveguide parameters is described.

Supermode analysis of phase-locked arrays of semiconductor lasers

The optical characteristics of phase-locked semiconductor laser arrays are formulated in terms of the array supermodes, which are the eigenmodes of the composite-array waveguide, by using coupled-mode theory. These supermodes are employed to calculate the near fields, the far fields, and the difference in the longitudinal-mode oscillation wavelengths of the array. It is shown that the broadening in the far-field beam divergence, as well as the broadening of each of the longitudinal modes that were observed in phase-locked arrays, may arise from the excitation of an increasing number of supermodes at increasing pumping levels.

Coupling mechanism of gain-guided integrated semiconductor laser arrays

It is shown that a gain-guided laser array couples via propagating fields rather than the evanescent mode coupling typically responsible for directional coupling in passive (directional couplers) and active (laser array) devices. We show that these phase-locked modes exhibit an interference pattern, in the junction plane, which arises from the curvature of the phase fronts of optical fields of the interacting lasers. The experimental results are interpreted with the aid of a simple theoretical model, and the effect of the observed mode pattern on the coupling of gain-guided lasers is discussed.

Longitudinal-mode control in integrated semiconductor laser phased arrays by phase velocity matching

The spectrum of semiconductor laser arrays with separate contacts is investigated. It is demonstrated that the individual laser currents can be selected such that the array operates in a single longitudinal mode in contrast to the multimode nature of its individual constituents. Moreover, it is possible to tune the lasing frequency by varying the laser currents. Wavelength tuning range of ∼50 Å, with tuning rate of ∼5 Å/mA, is demonstrated. It is suggested that these spectral features, characteristic of lasers which are coupled in parallel, result from the strong frequency dependence of their spatial mode pattern near the phase-matching frequency of their coupled waveguides.

Phase-locked semiconductor laser array with separate contacts

A new monolithic phase-locked semiconductor laser array has been fabricated. Employing two-level metallization, each of the eight elements in the array has a separate contact, thus making it possible to compensate for device nonuniformities and control the near-field and far-field patterns. Threshold currents are approximately 60 mA for each 5-μm-wide laser in the array. Phase locking has been observed via the narrowing of the far-field pattern. Experimental results are compared to those obtained from the same arrays operated with all the lasers connected in parallel.

Semiconductor optoelectronic devices for free-space optical communications

LIGHT emitting devices based AlGaAs lasers are very useful radiation sources in free-space opticalcommunications systems. Following a brief review of the properties of individual injection lasers, more complex devices are described. These include (or are relevant to) monolithic integration configurations of the lasers with their electronic driving circuitry, power combining methods of semiconductor lasers, and electronic methods of steering the radiation patterns of semiconductor lasers and laser arrays. Fabrication of such devices is one of the major prerequisites for realizing fully the potential advantages of free-space optical communications.

Transformation of the near radiation field of semiconductor lasers by a multimode rectangular waveguide

A report is given of theoretical calculations and of an experimental demonstration of transformation of twodimensional amplitude-phase distributions of a radiation field generated by a single semiconductor injection laser or an array of such lasers. This transformation is performed by a rectangular waveguide with transverse dimensions which are large compared with the radiation wavelength. Consideration is given to ways of constructing two-dimensional light-emitting arrays of semiconductor lasers (both oscillators and amplifiers) with a coherent interaction of the fields.

Diffraction coupled phase-locked semiconductor laser array

A new monolithic, diffraction coupled phase-locked semiconductor laser array has been fabricated. Stable narrow far-field patterns (∼3°) and peak power levels of 1 W have been obtained for 100-μm-wide devices with threshold currents as low as 250 mA. Such devices may be useful in applications where high power levels and stable radiation patterns are needed.