Emission properties of the double-proton bombarded laser--Including kinks and self-sustained oscillations

The double-proton bombarded laser is similar to the conventional proton delineated stripe laser except that it combines two proton bombardments, one shallow and the other deep, in one laser. The "strangling" effect that results from these two proton bombardments provides very good carrier confinement in the active region under the stripe. The emission properties of this laser are discussed in relation to the problems of kinks in the light-current characteristics and self-sustained oscillations in the light output of the laser. It is found that a kink always occurs, while self-sustained oscillations hardly ever occur. The good carrier confinement in the lateral plane seems to be the reason why the laser is so stable temporally. This indicates that current spreading in the lateral plane may be a major cause of self-sustained oscillations in narrow striped lasers in which there is poor carrier confinement. However, the severe kinks in the light-current characteristics which tend to move to lower levels, probably due to some annealing of the proton-bombardment near the active layer, are expected to limit the usefulness of the laser.     

A stabilized zinc diffused-proton bombarded (GaAl)As laser

A new (GaAl)As stripe laser in which a refractive index step is created by a zinc diffusion on both sides of a conventional proton stripe structure, called a diffused bombarded stripe (DBS) laser, is presented. This diffusion provides passive transverse guiding with a relatively simple technology. The index step can be adjusted by controlling the diffused zinc concentration in relation to the active region doping level. A self-alignment technique for the diffused and bombarded stripes reduces the current leakage through the diffused regions. Lasing threshold currents as low as those of conventional proton stripe lasers are obtained. A significant improvement of light-current linearity, transverse mode stability and longitudinal mode structure are observed with the DBS lasers.     

Gain and carrier lifetime measurements in AlGaAs multiquantum well lasers

We have measured the gain and the carrier lifetime at threshold in shallow proton stripe AlGaAs multiquantum well lasers with several different active layer structures. The lasers studied had active layers with two wells, four wells, six wells, and the modified multiquantum well. The net gainGis found to vary almost linearly with the injection currentIfor all the laser structures studied. The slopedG/dIis largest for the modified multiquantum well (MMQW) laser which is consistent with the observed lowest threshold current of these devices. We find that the carrier density at threshold for the MMQW laser is about a factor of 4 lower than that for a single quantum well laser. Thus, the effect of a nonradiative mechanism (e.g., Auger effect) which varies superlinearly with the injected carrier density is considerably reduced in MMQW lasers compared to that in single quantum well (SQW) lasers or the conventional double heterostructure lasers. The reduced threshold carrier density of the MMQW lasers has important implications for high temperature performance of lasers fabricated from the InGaAsP/InP material systems which are believed to have nonradiative mechanisms that vary superlinearly with carrier density, particularly for those laser structures for which the high temperature operation is not limited by leakage current.     

Self-pulsation dynamics in narrow stripe semiconductor lasers

In this paper, we address the physical origin of self-pulsation in narrow stripe edge emitting semiconductor lasers. We present both experimental time-averaged polarization-resolved near-field measurements performed with a charged-coupled device camera and picosecond time resolved near-field measurements performed with a streak camera. These results demonstrate dynamic spatial-hole burning during pulse formation and evolution. We conclude from these experimental results that the dominant process which drives the self-pulsation in this type of laser diode is carrier induced effective refractive index change induced by the spatial-hole burning.     

The effect of device geometry on lateral mode content of stripe geometry lasers

The dielectric profile of stripe geometry injection lasers is modeled with an objective of structure design requirements for fundamental lateral mode operation. Heterostructure lasers are modeled with a dielectric step profile using an effective dielectric discontinuity based on the gain/loss profile of the active layer as well as the overall geometrical structure. The analysis provides a quantitative comparison of the performance of two important double-heterostructure lasers: 1) the oxide-stripe geometry laser and 2) the channeled-substrate planar (CSP) laser. Modes of oxide-stripe lasers have lateral gain confinement, whereas, modes of CSP devices have strong lateral index confinement. To isolate the influence of geometry on the effective dielectric profile we assume that the real refractive index of the active layer is position independent. Resulting calculations show that a stripe geometry laser inherently has a depressed effective index in the active region below the metallic contact. This phenomenon alone produces index anti-guiding. In actual devices, both geometry and free carrier injection into the active region produce lateral index antiguiding. Lateral mode cut-off conditions are calculated as functions of the effective complex dielectric step and the stripe width. The results show that cutoff is related in a unique fashion to the ratio of the real and imaginary parts of the complex dielectric step; the ratio is positive for index guided modes and negative for gain guided ones.     

High-temperature operation of 650-nm wavelength AlGaInPself-pulsating laser diodes

Low-coherence self-pulsating laser diodes operating at a wavelength of 650 nm and at temperatures in excess of 70°C are required for high density optical storage systems. We report on AlGaInP lasers operating at this wavelength which exhibit stable self-pulsation up to a temperature of 100°C. The lasers are 50-μm-wide oxide-isolated stripe devices in which the saturable absorption necessary for pulsation is provided by multiple-quantum wells placed within the p-doped cladding layer. The pulsation frequency of the devices increases linearly with increasing drive current and is present up to 1.5 times, lasing threshold     

Planar stripe with waist and/or notch (SWAN) injection laser

This paper describes a novel planar stripe-geometry injection laser (referred as SWAN laser) showing a single transverse and longitudinal mode operation made by a simple fabrication method. This stripe-geometry is composed of a main stripe section with the width of around 10 μm and mode control section with the nominal width near to carrier diffusion length, less than 5 μm, a shape of which looks like waist(s) and/or notch(es) along the stripe. A "kink", which often appears in the relation between a light output and a current in conventional planar injection lasers, is not observed at the power level of more than 20 mw/ facet. The laser has advantage of controlling modulation dynamics by the shape of waist(s) and/or notch(es) along the stripe that enables control of the lateral carrier diffusion profile in the vicinity of an active layer and the amount of spontaneous emission into laser mode.     

Self-Pulsating Amplified Feedback Laser Based on a Loss-Coupled DFB Laser

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.     

Effect of longitudinal variations in current injection and carrier concentration in laser diodes

The effects of nonuniform current injection along the length of a semiconductor laser are investigated using multiple segmented stripe contact lasers. The results of the experiments are analyzed with a model for which the carrier concentration, gain, and photon density are functions of position. From the model, it was determined that nonuniform current injection caused large variations in the optical field within the laser, and the field produced a redistribution of electrical carriers in the active region. The nonlinearities in the light versus current measurements were explained by the redistribution of carriers as the optical field changed.     

(GaAl)As/GaAs质子轰击隔离条形DH激光器的退化原因

对研制的(GaAl)As/GaAs质子轰击隔离条形DH激光器的退化原因进行了实验分析。结果表明:快退化主要起因于有源区内的暗点、暗线及暗区等缺陷的增殖;腔面氧化是限制寿命在千小时的原因之一;质子轰击引入的点缺陷移入有源区是器件限制寿命在万小时的原因之一。     

Self-focusing effects in pulsating AlxGa1-xAs double-heterostructure lasers

Using proton bombarded stripe geometry lasers which emit intense optical pulses, we have measured the width of the optical beam in the plane of the junction as a function of time during the pulse. The width of the beam is qualitatively proportional to the change in the carrier density. The width increases during the quiescent period between pulses where the carrier density increases by current injection and decreases during the emission of the pulse. For one laser studied in considerable detail, the full width at half intensity decreases from 9 μm at the start of the pulse to 7.2 μm at the end of the pulse. The reduction in the width results from the self-focusing of the beam. It is due to an increase in the refractive index and the decrease in the gain distributions near the center of the stripe. The reduction in the beamwidth concentrates the mode to a region of sufficiently higher average gain to compensate for the reduction in spatial gain distribution. The self-focusing acts to reduce the damping of the relaxation oscillations, and thus enhances the effect of other nonlinearities such as saturable absorption in causing pulsations. The thermal induced refractive index distribution across the stripe is shown to play a crucial role in the gain instability caused by self-focusing.     

Thermal-gradient enhanced current spreading in d.h. lasers

The observed differences in light output against current (P/I) curves between c.w. and short-pulse operation of (GaAl)As lasers have generally been considered as arising from the increase in active-region temperature under c.w. operation. This temperature increase results in a decrease in optical output at a given current level because of the reduced carrier confinement at the heterojunctions. However, by carefully compensating the temperature rise in the active region in the c.w. mode, we have experimentally established that, for exactly the same active-region temperature and total pumping current, the laser emits significantly less light in the c.w. mode than in the pulse mode of operation. This difference becomes larger as the input power increases. The thermoelectric (Seebeck) effect can explain this observation. The temperature gradient induced by c.w. operation causes current to flow away from the stripe region, leading to excess wasted current and reduced light output.     

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.     

Dispersive self-Q-switching in self-pulsating DFB lasers

Self-pulsations reproducibly achieved in newly developed lasers with two distributed feedback sections and with an additional phase tuning section are investigated. The existence of the dispersive self-Q-switching mechanism for generating the high-frequency self-pulsations is verified experimentally for the first time. This effect is clearly distinguished from other possible self-pulsation mechanisms by detecting the single-mode type of the self-pulsation and the operation of one section near the transparency current density using it as a reflector with dispersive feedback. The operating conditions for generating this self-pulsation type are analyzed. It is revealed that the required critical detuning of the Bragg wavelengths of the two DFB sections is achieved by a combination of electronic wavelength tuning and current-induced heating. The previous reproducibility problems of self-pulsations in two-section DFB lasers operated at, in principle, suited current conditions are discussed, and the essential role of an electrical phase-control section for achieving reproducible device properties is pointed out. Furthermore, it is demonstrated that phase tuning can be used for extending the self-pulsation regime and for optimizing the frequency stability of the self-pulsation. Improved performance of the devices applied as optical clocks thus can be expected     

The effect of bonding wires on longitudinal temperature profiles oflaser diodes

The thermal effect of bonding wires in laser diodes is analyzed using the analytical temperature solution for a five-layer structure and an iteration technique. Finite element method is used to confirm the results. Due to the bonding wire, the longitudinal temperature profile of laser diodes exhibits significant reduction at the foot of the wire even with uniform longitudinal heat distribution. For lasers designed with uniform longitudinal current density, heat increases toward the laser facets because of nonradiative recombination of carriers through surface quantum states on the facets. This leads to local temperature concentration on and near the facets. The conduction of heat through the bonding wire at the top center of laser chips further enhances this temperature concentration. In use, the stripe electrode of laser diodes is at uniform voltage. Under this operation condition, the current density would increase in the higher temperature regions due to bandgap decrease, causing higher heat flux. And consequently even higher temperature. Accordingly, the location of bonding wire and the shape of stripe electrode require careful consideration in the design phase to achieve uniform longitudinal temperature profile     

Nonlinear characterization of modal gain and effective indexsaturation in channeled-substrate-planar double-heterojunction lasers

The nonlinear modal gain and effective index in AlGaAs/GaAs channel-substrate-planar double-heterojunction (CSP-DH) lasers are calculated as a function of the drive current and intracavity power. These functions are first computed using a self-consistent procedure that ties together the optical fields and the carrier distribution in the active layer. The functions are then suitably fit to a new empirical analytic expression that is useful for modeling the laser as a single device or as multiple lasers that are coherently locked. The specific analytic expressions are simple algebraic formulas that have six parameters that can be calculated using regression techniques. These parameters are computed for the CSP-DH laser as a function of the stripe contact width     

Near-field control in multistripe geometry injection lasers

Control of the intensity and position of the near field is demonstrated in an analysis of twin-stripe and triple-stripe injection lasers. Calculations of file electrooptical properties of the structures are presented, taking into account the interaction between the carrier concentration profile and the optical field. For twin-stripe devices interstripe coupling is investigated as a function of stripe separation. For stripe separation comparable with stripewidth, the properties of triple-stripe lasers become of interest. Nonlinear light-current characteristics for the twin-stripe device are obtained and lead to an interpretation of such characteristics in conventional stripe geometry lasers.     

Optical guiding properties of high-brightness parabolic bow-tie laser arrays

This paper presents the characteristics of parabolic bow-tie laser arrays (PBTLAs) which are a novel category of laser diodes specially designed to achieve high power with high brightness at 980 nm. Output powers in excess of 2.8 W/facet have been measured from five-element PBTLAs with output beam less than twice the diffraction limit, achieving high brightness of 275 MWcm/sup -2/srad/sup -1/ at 3 A (pulsed) injection current (=22 times the threshold). Changes in the achievable brightness due to changes in the optical cavity geometry and in the lateral optical guiding strength are discussed in detail, using the coupled-mode theory to interpret the experimental results. At threshold all devices operate in the highest (double lobed) array mode. At higher currents the arrays of tapered lasers change to quasi-in-phase operation when the modal gain of the fundamental array mode dominates because of the combined effect of carrier hole burning and spatial filtering from the narrow stripe central section of the device. Similar trends have been observed under continuous-wave operation. The reduction of lateral optical guiding strength is deleterious for the operational characteristics of PBTLAs and linear bow-tie arrays, and it leads to filamentation in gain-guided devices even at low currents. Theoretical results presented in this paper show that scalability is in principle possible; however, changes in the lateral gain profile due to hole-burning can significantly increase the modal gain of higher order modes and, therefore, strongly influence the optical output profile.     

The phase shift of the light output in sinusoidally modulated semiconductor lasers

The dependence of the phase shift of the light output from sinusoidally modulated semiconductor lasers was investigated as a function of the modulation current. This measurement is effective in accurately determining the short damping time constant associated with the relaxation oscillation. The frequency half width of this phase shiftDelta fwas found to be inversely proportional to the damping time constant. For narrow stripe lasers, the phase shift occurs more gradually, which corresponds to the fact that the narrow stripe lasers have shorter damping time constants. To analyze the narrow stripe effect, the recently developed time-dependent self-consistent theory was applied, considering the transverse distribution of both optical field and carrier density and including the carrier diffusion term. This theory can explain the shorter damping time constant for narrow stripe lasers compared with broader stripe lasers.     

Improved CW operation of GaAs-based QC lasers: T/sub max/= 150 K

We present a substantial improvement in the CW performance of GaAs-based quantum cascade lasers with operation up to 150 K. This has been achieved through suitable changes in device processing of a well-characterized laser. The technology optimizes the current injection in the laser by reducing the size of the active stripe whilst maintaining a strong coupling of the optical mode to preserve low current densities. The reduction of total dissipated power is critical for these lasers to operate CW. At 77 K, the maximum CW optical power is 80 mW, threshold current is 470 mA, slope efficiency is 141 mW/A, and lasing wavelength /spl lambda//spl sim/10.3 /spl mu/m.