Spatio-temporal dynamics of gain-guided semiconductor laser arrays

A continuous model based on the coupled field-matter Maxwell-Bloch equations for a two-level homogeneously broadened single mode laser is developed. The model includes a Langevin formulation to model thermal and spontaneous emission noises and accounts for carrier diffusion, optical field diffraction and current spreading. Our model is flexible enough to simulate any gain-guided longitudinally uniform laser geometry and is applied to both a single-stripe and a four-stripe gain-guided semiconductor lasers where the influence of the injection current, the interstripe distance and carrier diffusion is discussed within the context of the laser dynamics. We show that an array operating with quasi-independent stripes may be achieved at low pumps and larger interstripe distances. However, as injection current is increased or the interstripe distance is decreased, the device passes through a variety of dynamical instabilities which can be analyzed in the context of lateral cavity modes. Moreover, we also show that the array dynamics is strongly influenced by carrier diffusion which may also lead to different thresholds for each element of the array     

Nonlinear dynamics of a semiconductor laser with delayed negative optoelectronic feedback

Nonlinear dynamics of a semiconductor laser with delayed negative optoelectronic feedback are studied both numerically and experimentally. Mappings of the dynamic states and bifurcation diagrams are compared between a delayed negative optoelectronic feedback system and a delayed positive optoelectronic feedback system. Both systems follow a quasiperiodic route to chaos, where regular pulsing, quasiperiodic pulsing, and chaotic pulsing states are observed. Frequency-locked pulsing states are also found in a delayed negative optoelectronic feedback system, but not in a delayed positive optoelectronic feedback system. These frequency-locked pulsing states are experimentally observed to exhibit a harmonic frequency-locking phenomenon, where the pulsing frequency is locked to a harmonic of the delay loop frequency instead of the delay loop frequency itself. The rotation numbers of these frequency-locked pulsing states show a Devil's staircase structure.     

Nonlinear dynamics of semiconductor laser emission under variablefeedback conditions

A study was made of the behavior of external cavity semiconductor lasers with tilted external cavity mirrors providing variable feedback. The appearance of intermittent breakdowns of the light intensity induced by the tilting of the external mirror is demonstrated. The rate of the breakdowns of the light intensity is directly related to the relaxation oscillation frequency of the external cavity semiconductor laser. The authors describe the experimental observations theoretically by the coupled rate equations for the electric field density and the carrier density     

Y-Junction semiconductor laser arrays: Part II--Experiments

Laser arrays employingY-junctions to coherently couple the emitters in-phase have been fabricated by metalorganic chemical vapor deposition. The far-field patterns are stable and centered normal to the facet. They are typically multilobed in accordance with the diffraction pattern of multiple emitters. By the use of flared waveguides near the facets the near-field fill factor can be increased to greater than 80 percent. This modification concentrates the radiation in the central diffraction lobe. Power outputs of 200 mW CW have been obtained from a ten element array of 1.5μm wide emitters.     

Y-Junction semiconductor laser arrays: Part I--Theory

A general formulation for theY-junction semiconductor laser array is developed and analytic solutions are obtained for the important special case of identical waveguides. Modal thresholds, resonances, near-field, and far-field patterns are derived for the array modes. We show that the minimum threshold mode has identical modal intensities (except in the outermost waveguides) and the modal amplitudes are all in-phase. Thus the dominant mode radiates in a centered, narrow, single-lobe beam. Because the injected charges are utilized uniformly and efficiently, the mode is predicted to remain stable above threshold. All other array modes are shown to resonate at the same wavelength as the dominant mode, hut at increased thresholds.     

Low-cost packaging of semiconductor laser arrays

This article reports on a passive self-alignment technique that we developed for semiconductor waveguide array packaging. A flip-chip Si-motherboard technique is described for the packaging of 4-, 8-, and 12-channel 980 nm laser arrays with fiber ribbons. A major aspect of this development is also to show that minor modifications on an existing laser fabrication process, carried out at the IBM Zurich Research Laboratory, and a specially developed motherboard from our lab, allow a passive self-aligned packaging process     

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.     

Analysis of semiconductor laser arrays with high-intensityuniformity

It is shown that an array with a highly uniform intensity distribution can be constructed. This decreases the spatial hole burning and makes stable, high-power operation easier. An estimate of the remaining spatial hole burning is derived and it is shown how its effects can be compensated for. It is considered that laser arrays which operate in the fundamental mode even at high-power levels can be designed     

组合式二维列阵面发射半导体激光器

设计出新型二维列阵面发射激光器,获得了室温准连续大于250mW的光功率输出。     

Theory of semiconductor quantum-dot laser dynamics

A theory for describing nonequilibrium dynamics in a semiconductor quantum-dot laser is presented. This theory is applied to a microcavity laser with a gain region consisting of an inhomogeneous distribution of quantum dots, a quantum-well wetting layer, and injection pumped bulk regions. Numerical results are presented and the effects of spectral hole burning, plasma heating, and many-body effects are analyzed.     

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.     

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 Å between adjacent lasers. The broadening factor is 2.3 when a single 204 Å 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.     

Postgrowth tuning of semiconductor vertical cavities formultiple-wavelength laser arrays

Combined lateral-vertical oxidation of AlGaAs is investigated as a means of tuning the resonant wavelength of a semiconductor microcavity after the epitaxial growth. It is shown that this technique can provide arrays with a wavelength spread equal to the cavity's free spectral range with a single postgrowth processing step. Design issues for multiple-wavelength vertical-cavity laser arrays using this postgrowth tuning technique are discussed, comparing the performance of devices with all-semiconductor and partially or totally oxidized Bragg mirrors. Experimental results are presented on arrays with a 48-nm lasing span around 970 nm, using partially and totally oxidized mirrors     

Multilongitudinal-mode dynamics in a semiconductor laser subject tooptical injection

Multilongitudinal-mode dynamics in a semiconductor laser subject to optical injection are investigated both experimentally and numerically. We found that there are parameter regimes for which the slave laser hops into an adjacent longitudinal mode as we vary the detuning of the optical frequencies between the master and slave laser. A traveling wave model is used to numerically investigate the mode hopping. Good qualitative agreement is found between the numerical computations and the experimental observations     

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.     

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.     

Nonlinear space-charge domain dynamics in a semiconductor with negative differential mobility

On a semiphenomenological level, the explanation of the Gunn effect is one in terms of a time-independent, negative differential, bulk conductivity. This mechanism is based on the conduction-band structure of GaAs, which provides for two kinds of electrons, light and heavy ones. Light electrons dominate at low fields, heavy ones at high fields. Since the mobility of the heavy electrons is much lower than that of the light ones, there is a range of current decrease with increasing field, i.e., a negative conductivity. This negative conductivity leads to an electrical breakup of the crystal into alternating traveling domains of high and low fields, accompanied by alternating current. In a "mathematically perfect" crystal this instability would take the form of traveling negatively charged electron accumulation layers, separating the domains of high and low fields. In real crystals the inevitable spatial fluctuations in the impurity distribution lead to the experimentally observed dipole mode, wherein both negatively charged electron accumulation layers and positively charged electron depletion layers occur.     

Thermal characteristics of semiconductor laser phase-locked arrays with diamond film heatsink

The equation of heat conduction in the heatsink consisting of an artificial diamond film and a copper bulk is solved by the image method. The influence of the diamond thickness on the temperature rise and distribution is shown.<>     

Non-linear dynamics and chaos in semiconductor laser devices

The theoretical and experimental aspects of non-linear dynamics and chaos in semiconductor lasers are surveyed. Three novel scenaria for complex dynamical behaviour in semiconductor lasers are proposed for experimental investigation.     

Analysis of the stability of the highest-order supermode in semiconductor laser arrays

Uniform evanescently coupled laser arrays tend to oscillate in the highest-order supermode. We show by modelling that this supermode is much more stable with increasing output power than the zero-order supermode.