Index-guided Semiconductor Lasers Research Articles

High brightness, index-guided parabolic bow-tie laser diodes

The category of devices of interest in the paper is that of high power semiconductor lasers that also have high brightness. However, to achieve simultaneously high output optical power and a 'good' quality (narrow single-lobed) output beam from semiconductor lasers that can be fabricated relatively simply and cheaply, it is necessary to carefully design the cavity to control the output beam characteristics. The authors present the outcome of a systematic study on compact, index-guided semiconductor lasers of different geometry to show that with suitable design of the laser cavity it is possible to achieve the desired operational characteristics. In particular, the parabolic taper geometry has been found to be well-suited to achieve high brightness. Details of the specially designed high power semiconductor material used to fabricate the devices are also presented. The main advantage of the proposed compact devices is that the narrow output beam is achieved without the use of external lenses, thereby reducing the device cost for applications involving free-space propagation. Output powers in excess of 600 mW per facet have been measured from parabolic lasers (45% wall-plug efficiency) without catastrophic optical damage, at I = 1 A = 20I/sub th/, with a full width at half maximum far field intensity profile of /spl sim/2.5/spl deg/.

Carrier dynamics in semiconductor lasers operating in the low-frequency fluctuation regime

We analyse the dynamics of a weakly index-guided semiconductor laser operating near threshold and with optical feedback, using the Lang-Kobayashi model. Lateral effects are included, taking into account lateral carrier diffusion and a lateral profile for the optical field. The laser intensity fluctuates in a sequence of picosecond pulses, and the time-averaged intensity shows sudden dropouts to a nearly zero value, followed by gradual, steplike recoveries. We find that for large carrier diffusion the averaged intensity drops less frequently to zero and recovers less noisily than for low carrier diffusion. We analyse the time evolution of the carriers, and find that at the centre of the active region it fluctuates rapidly following the fast pulsing intensity. Further from the centre of the active region, diffusion has a preponderant effect and the carriers vary more smoothly in time (following the oscillations of the time-averaged intensity). Even further from the centre of the active region, the carrier profile is not influenced by the external optical feedback. The spatial variations of the carrier distribution and of the light intensity distribution within the mode seem to anticipate the occurrence of a dropout.

Kinks induced by free-carrier absorption in weakly index guided semiconductor lasers

The time dependence of the kink power in weakly index guided high power stripe lasers in the AlGaAs system are investigated. A periodic dependence of the kink power on the length of the laser is observed in continuous and pulsed modes, but the pulsed mode kink curve is shifted with respect to the continuous one. This is observed to be proportional to 1/τ, where τ is the pulse width. The kink is due to phase locking of the modes of the waveguide resulting from a shift of the effective indexes, which is driven by self-heating due to free-carrier absorption of the laser mode.

Spatial feedback effects in narrow-stripe index-guided semiconductor lasers

We demonstrate experimentally that spatial effects of feedback are important in determining the operating characteristics of index-guided semiconductor lasers. It is in fact the inflexibility of the index-guided mode that makes the laser susceptible to small misalignments of the external cavity, which can induce power variations of nearly 40% of the solitary laser power with feedback levels of less than 2%.(-17 dB). With a simple model, we calculate the modification of the feedback wavefront as a function of the external cavity length and show that the power variations are induced through changes in the cavity loss brought on by the interference of the feedback field with the field reflected off the laser facet. By experimental comparison to a partly gain-guided laser, we conclude that these large-scale variations will be significantly reduced or absent in purely gain-guided lasers.

Kink power in weakly index guided semiconductor lasers

A periodic dependence of kink power on laser length is observed and explained. Weakly index guided high power stripe lasers in the AlGaAs, InGaAlP, and InGaAs–AlGaAs material systems are studied and oscillation periods of 100–350 μm are found. Relative kink power differences exceeding a factor of 4 are observed. Facet coatings lead to differences in the oscillation amplitude but not in the oscillation period. The observations indicate that phase-locked fundamental and first-order modes exist at certain preferred laser lengths. This general model fully explains the oscillatory behavior of the kink power and the correlated changes in lateral far field distributions at the front and rear mirrors. It is concluded that the optimum diffraction limited power output can be obtained by choosing the proper laser length.

Linear sensitivity analysis of coupled arrays of lasers under random active-region width variations

An analytical and numerical study of the sensitivity of a one-dimensional evanescently coupled array of real index-guided (no antiguiding) semiconductor lasers under random device width variation is carried out with the aim of characterizing the far-field disturbance as a function of the variance of d (device active width), the number of devices n, and the interelement coupling κ. It is shown that the sensitivity of the far-field amplitude distribution increases approximately linearly with n up to n ≃ 20 with a given κ. A simple expression is derived that relates the maximum allowable variance of device widths for a given n under the constraint of nondegeneracy of the lowest-order eigenvalues. The size of an array that may be expected to remain in the fundamental mode under given perturbations is deduced. It is predicted that if this value is exceeded the array will break up into domains of in-phase regions separated by π rad in phase, and a formula is derived that gives the size of such domains.

Nonlinear dynamics of semiconductor laser arrays: a mean field model

An analysis of the spatiotemporal dynamics of single-emitter and multiemitter index-guided semiconductor laser arrays is presented. The results are based on a propagation model that involves a pair of coupled nonlinear partial differential equations. The model describes the space-time evolution of the mutually interacting lateral field and carrier density distributions. It includes the effect of carrier diffusion and the effect of carrier-induced antiguiding. Coupling between the emitters is provided by the overlapping evanescent fields of the emitters as well as by the exchange of carriers through carrier diffusion. We show that carrier diffusion plays a significant role in the stability and the nature of bifurcations. Also we show that carrier-induced antiguiding is the principal reason of instability for singleand two-emitter arrays. We illustrate a wide variety of instabilities that demonstrate the richness of the dynamics in these devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Semiconductor laser array dynamics: numerical simulations on multistripe index-guided lasers

The space–time dynamical behavior of multistripe index-guided semiconductor laser arrays is studied by using an extension of the usual phenomenological laser model to include transverse diffraction of the counterpropagating optical fields and transverse diffusion of the excited carriers. Our results confirm that evanescently coupled multistripe lasers are a fascinating manifestation of spatiotemporal complexity in spatially extended nonlinear systems. Stabilization of the laser output can be achieved by injection locking the array with a weak external injected signal, and we show that the stability of the externally driven array depends on the transverse profile of the injected signal. A numerical algorithm is presented that takes advantage of high-performance parallel computing architectures to solve the coupled partial differential equations describing the light–matter interaction in the laser structure. Both the model and the numerical algorithm are sufficiently flexible and modular to support arbitrary laser geometries and to allow for inclusion of important many-body semiconductor effects in future studies.

Improved coupled-mode theory for the dynamics of semiconductor laser arrays

We present an improved coupled-mode model to describe the dynamics of weakly index-guided semiconductor laser arrays. The model is derived from a partial differential equation model that includes the effects of carrier diffusion and gain guiding. Results from the two models are compared, and good qualitative and quantitative agreement is obtained.

A GaInAsP/InP tapered-waveguide semiconductor laser amplifier integrated with a 1.5 mu m distributed feedback laser

A GaInAsP/InP index-guided type tapered-waveguide traveling-wave semiconductor laser amplifier (TTW-SLA) with exponential taper structure was realized for the first time and its integration with a 1.5 mu m wavelength phase adjusted DFB laser was demonstrated. A reasonable amount of the single-pass gain, which was limited by the residual reflectivity at the output end of the amplifier region, was obtained with no serious problem with the lasing spectrum. A narrow beam divergence of FWHM=5.5 degrees and stable longitudinal mode operation were obtained with the output waveguide width of 20 mu m by adopting both a window structure and 7 degrees angled facets. This value agrees well with the theoretically calculated one and proves that single transverse-mode propagation can be attained by an exponentially tapered TTW-SLA with a very wide output end. >

An examination of the lateral mode control of the laser diode

AbstractTo avoid the high light output power density which has become the problem of the high‐power laser diode, the light power is assumed to be distributed uniformly. However, the disadvantage is that the laser is apt to operate in higher‐order modes in this case. For optical communications use, the laser must operate in a single fundamental mode. This paper aims at obtaining the single fundamental lateral mode, controlling the modes of an index guided semiconductor laser diode together with the investigations of its theoretic results. The finite element method based on Galerkin's method is used for the analysis presented here. In numerical calulations, we take the broad area laser and the array laser as examples for discussions starting from the modal gain. As a result, it is shown that placing high gain in the middle part of the laser diode cross section is effective for the single fundamental lateral mode operation.

Erratum: Lasing wavelengths of index-guided AlGaInP semiconductor lasers as functions of off-angle from (100) plane of GaAs substrate

Erratum: Lasing wavelengths of index-guided AlGaInP semiconductor lasers as functions of off-angle from (100) plane of GaAs substrate

Influence of carrier nonuniformity on the phase relationship between frequency and intensity modulation in semiconductor lasers

The modulus and phase of the chirp to modulated power ratio (CPR) was measured for a 1.5- mu m buried-heterostructure laser and a 0.85- mu m channeled-substrate planar (CSP) laser. The results for the phase are inconsistent with previously published expressions including spontaneous emission and spectral hole burning. In particular, the CSP laser exhibits an abrupt phase shift in the CPR. An explanation of this behavior is presented in terms of the influence of a nonuniform carrier density on the phase-amplitude coupling, as expressed by an integral expression for the mode parameter alpha representing the linewidth enhancement factor. Using a simple model for lateral behavior which analytically incorporates diffusion and a nonuniform material alpha parameter, qualitative agreement was obtained with the CSO data. The authors demonstrate the importance of the lateral laser structure on the phase-amplitude coupling in index-guided semiconductor lasers, and the usefulness of CPR phase measurements for laser characterization.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Linewidth enhancement for DFB lasers due to longitudinal field dependence in the laser cavity

To calculate the linewidth for an index-guided semiconductor laser, one usually neglects a correction factor for the spontaneous emission rate, which is introduced by the longitudinal field distribution within the laser cavity. For FabryPerot lasers with cleaved facets the correction factor is small. However, for DFB lasers this correction factor may become quite significant, yielding a linewidth enhancement for DFB laser diodes.

Effects of spatial gain variation in an index-guided semiconductor laser array

This paper is written with two goals in mind. The first goal involves the extension of semiclassical laser theory to include the effects of gain guiding. The second goal involves the application of the more general semiclassical laser theory developed in this paper to investigate the effects of spatially varying the excitation geometry on the intracavity field of an index-guided semiconductor laser array.

Self stabilisation of the fundamental lateral mode in index-guided semiconductor lasers

Lateral mode stability in index guided semiconductor lasers is discussed. As is well known, the fundamental mode self-focuses at high power due to perturbations in the refractive index profile produced by spatial hole burning. However, the lateral waveguide is not strengthened for all modes. The first order mode, for example, is actually slightly defocused. In strong waveguides the gain profile perturbation due to hole burning is the dominant cause of the first-order mode reaching threshold. In weak wave-guides, however, the defocusing effect can compete with the gain profile perturbation effect leading to stable fundamental mode operation at high output powers.

Frequency locking in an index-guided semiconductor laser array

A theory is developed for investigating frequency locking in an index-guided semiconductor laser array. This theory, which is based on the expansion of the laser field in terms of the passive array eigenmodes, and on the use of semiclassical laser theory to treat the interaction of these modes among themselves, is valid for all values of coupling between lasers and for all array sizes. To illustrate the application of this theory, we use it to determine the lockband of a two-laser array with dissimilar waveguides.

Injection locking of an index-guided semiconductor laser array

This paper describes an analysis of the effects of an injected laser field on an index-guided semiconductor laser array. We assume an injected field that is resonant with the lowest order array mode and a free-running array that is either operating with a higher order array mode or is not frequency locked. For both cases, a sufficiently intense injected field is found to cause the array to operate predominantly in the lowest order mode. Also, according to our analysis, for certain array configurations, it is more advantageous to have the injected field present during the startup of oscillation than to introduce it after the free-running array has reached steady state. Finally, the injection-locked array is found to contain a tilt in the output beam.

Asymmetric noise and output power in semiconductor lasers with optical feedback near threshold

An asymmetry in the output power and the low-frequency noise emitted from the rear and front facets of index-guided semiconductor lasers with a large percentage of optical feedback into the front facet is reported. Transitions from coherent to incoherent feedback-induced properties by modulation of the feedback path length can be distinguished in the output power versus current (L-I) curve. A cross-over point is found only in the front facet L-I curve corresponding to a minimum in the noise. A model is presented which correctly describes the asymmetry between the L-I curve of the front and rear facets. It also predicts the observed cross-over point in the front facet L-I curve.

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.