InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm². The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8×10²⁰/cm³, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8×10^-17 cm², 9.3×10 ¹⁹ cm^-3, 5200 cm^-1, and 43 cm^-1 respectively     

Stadium and quasi-stadium laser diodes

We present theoretical and experimental studies on two-dimensional microcavity laser diodes with stadium and quasi-stadium shapes. We report a demonstration of lasing for the first time in a fully chaotic microcavity-a stadium-shaped cavity which is rigorously known to be fully chaotic. We also present systematic studies on quasi-stadium laser diodes for three types of resonator conditions: stable, marginally stable, and unstable. Morphological dependence of lasing characteristics of quasi-stadium laser diodes is elucidated. We also show examples of the application of quasi-stadium laser diodes to beam switching operation and optical signal distribution by using patterned electrodes and the phase locking phenomenon between two resonator modes.     

Dynamic characteristics of bistable laser diodes

A stability analysis of bistable laser diodes, which gives a first analytical explanation of the recently observed high-speed bistable switching and self-pulsation, is presented by introducing two key parameters. It is shown that switching and self-pulsation are associated with two time scales. Turn-on and turn-off dynamics can be characterized by two different sets of eigenvalues which are used to analytically explain the switch-off and switch-on transients observed in experiments. It is also shown that the gain function profile can significantly affect the performance of self-pulsation operation of bistable laser diodes. The analysis also proves that the existence of two critical points has an important effect on the switching time if switching occurs around one of them. The results of the present analysis agree well with those of an exact numerical simulation. In addition, the analysis can be applied to study dynamic characteristics of a bistable laser diode (BLD) with other more complicated structures such as distributed feedback (DFB) laser diodes, multiquantum-well (MQW) laser diodes, etc     

Characteristics of InGaN-AlGaN multiple-quantum-well laser diodes

We demonstrate room-temperature pulsed current-injected operation of InGaAlN heterostructure laser diodes with mirrors fabricated by chemically assisted ion beam etching. The multiple-quantum-well devices were grown by organometallic vapor phase epitaxy on c-face sapphire substrates. The emission wavelengths of the gain-guided laser diodes were in the range from 419 to 432 nm. The lowest threshold current density obtained was 20 kA/cm² with maximum output powers of 50 mW. Longitudinal Fabry-Perot modes are clearly resolved in the high-resolution optical spectrum of the lasers, with a spacing consistent with the cavity length. Cavity length studies on a set of samples indicate that the distributed losses in the structure are on the order of 30-40 cm^-1     

High-power laser diodes with dry-etched mirror facets andintegrated monitor photodiodes

High-power broad-area InGaAs-AlGaAs-GaAs single-quantum-well separate-confinement heterostructure (SQW-GRINSCH) lasers with dry-etched mirror facets and integrated monitor photodiodes have been investigated. A multilayer resist system has been employed as a mask for the chemically assisted ion-beam etching (CAIBE) process resulting in vertical and smooth laser facets. Thick electroplated gold layers on top of the ohmic contacts improve the heatsinking of the lasers leading to reasonable continuous-wave (CW) output powers even when the devices are mounted junction-side up. Monolithically integrated monitor photodiodes provide a linear response to the optical output powers of the laser diodes. The properties of broad-area lasers with dry-etched and cleaved facets are almost identical, Record values for the CW output powers of 2.59 W per uncoated facet and wall-plug efficiencies of more than 55% have been achieved with junction-side-down mounted devices     

Room-temperature lasing operation of GaInNAs-GaAssingle-quantum-well laser diodes

We have succeeded in demonstrating continuous-wave (CW) operation of GaInNAs-GaAs single-quantum-well (SQW) laser diodes at room temperature (RT). The threshold current density was about 1.4 kA/cm², and the operating wavelength was approximately 1.18 μm for a broad-stripe geometry. Evenly spaced multiple longitudinal modes were clearly observed in the lasing spectrum. The full-angle-half-power far-field beam divergence measured parallel and perpendicular to the junction plane was 4.5° and 45°, respectively. A high characteristic temperature (T₀) of 126 K under CW operation and a small wavelength shift per ambient temperature change of 0.48 nm/°C under pulsed operation were obtained. These experimental results indicate the applicability of GaInNAs to long-wavelength laser diodes with excellent high-temperature performance     

Cleaved and etched facet nitride laser diodes

Room-temperature (RT) pulsed operation of blue (420 nm) nitride-based multiquantum-well laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. Structures investigated include etched and cleaved facets as well as doped and undoped quantum wells. A combination of atmospheric and low-pressure metal organic chemical vapor deposition using a modified two-flow horizontal reactor was employed. Threshold current densities as low as 12.6 kA/cm² were observed for 10×1200 μm lasers with uncoated reactive ion etched facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Laser diodes tested under pulsed conditions operated up to 6 h at RT. Lasing was achieved up to 95°C and up to a 150-ns pulselength (RT). Threshold current increased with temperature with a characteristic temperature T₀ of 114 K     

Coupling and conversion characteristics of spot-size-converterintegrated laser diodes

The coupling and conversion characteristics of four types of spot-size converter integrated laser diodes (SS-LDs) to a single-mode fiber (SMF) in the 1.3-μm-wavelength region are numerically clarified. The SS-LDs are categorized into types with vertical tapers and types with a combination of a lateral taper and a thin-film core. The eigenmodes are obtained by the finite-element method (FEM). The semivectorial 3-D finite-difference (FD) beam propagation method (BPM) is used to calculate the propagating beams     

Optimization of 635-nm tensile strained GaInP laser diodes

The authors measure the combined affect of strain and well width on the gain and recombination mechanisms in 635-nm laser structures containing three combinations of tensile strain and well width of 0.5%/10 nm, 0.6%/12.5nm, and 0.7%/15nm using the segmented contact method. They find an improvement in the intrinsic properties with increasing strain but the dominant effect in device performance is an extrinsic effect-the overall radiative efficiency, which is found to be less than 30% for all three samples even at 200 K. The authors attribute this to nonradiative recombination within the quantum well. The intrinsic gain-spontaneous current density characteristics of all three samples exhibit similar tangential gain parameters and a decreasing transparency current density from 116 to 87 to 83 Acm/sup -2/ with increasing strain and well width. They show that the reduction occurs because of a reduction in the TE polarized spontaneous recombination due to the increased splitting of light and heavy hole subbands. The quasi-Fermi level separation required to achieve a fixed value of gain is insensitive to the particular strain/well width combination. The authors use a microscopic laser theory to model the behavior of a larger range of combinations of tensile strain and well width than were examined experimentally, having first demonstrated that the model correctly describes the experimental gain spectra of the only sample exhibiting appreciable gain in both TM and TE polarizations. The calculated data suggest that using still larger values of strain and well width produces no further benefit in performance.     

Grating overgrowth and defect structures indistributed-feedback-buried heterostructure laser diodes

Distributed-feedback (DFB)-buried heterostructure lasers incorporating a substrate grating require epitaxy of waveguide layers over the corrugated grating surfaces. Unlike epitaxy on planar (100) substrate, the corrugated substrate surface contains undesirable crystal facets which lead to an uncontrollable variation in local composition during epitaxy, and thereby results in strong localized misfit stresses. These localized misfit stresses further affect the subsequent growth of high quality layers which constitute the active device structures. To address the general issue of epitaxy on a corrugated surface, a thermodynamic analysis is performed for surface mass transport on a grating surface at normal growth temperature to show that grating wash-out is a thermodynamically favorable process. However, growth on a perfectly preserved grating is undesirable due to the composition shift on groove facets. A systematic study of substrate orientation dependent composition variation of quaternary layers indicates a composition shift generally toward In and P rich directions for orientations from (100)-(111). Conditions for a strain free waveguide layer growth are demonstrated. The commonly observed imperfections associated with a grating overgrowth are summarized, and their effects on device reliability are discussed     

High-yield external optical feedback resistant partially corrugatedwaveguide laser diodes

This paper clarifies high yield external optical feedback resistant characteristics in partially corrugated waveguide laser diodes (PC-LD's), compared to conventional distributed feedback laser diodes (DFB-LD's). Based on a novel large single dynamic analysis by combining the transient mirror loss (total net threshold gain) fluctuation with the van der Pol equations in single-mode LD's, the feedback effect of mirror loss on the external optical feedback resistance in single-mode LD's was found for the first time. Theoretical analysis predicted that in the PC-LD's, relatively stable transient mirror loss suppresses the positive feedback effect of mirror loss, as well as the optical output fluctuations under the external optical feedback. Experimental results show that the increase of the relative intensity noise in 70% PC-LD's could be suppressed to lower than -120 dB/Hz and the minimum RIN was as low as -126 dB/Hz with the external optical feedback of -20 dB     

975-nm single-mode laser source: external coherent combining of two pigtailed laser diodes

We report on the efficient coherent combining up to 85% of two single-mode fiber coupled laser diodes emitting at 975 nm. The combining is performed by an external feedback on the two laser diode cavities through an all-fiber interferometric arrangement based on standard components.     

Continuous-wave operation of InGaN multiple-quantum-well laserdiodes on copper substrates obtained by laser liftoff

The performance characteristics of continuous-wave (CW) InGaN multiple-quantum-well (MQW) laser diodes on copper substrates are reported. InGaN MQW laser diodes (LDs) grown on sapphire substrates by metal-organic chemical vapor deposition were successfully separated from the sapphire and transferred onto copper substrates by using a two-step laser liftoff (LLO) process. Continuous-wave threshold currents as low as 65 mA have been achieved with threshold voltages of 6.5 V with a backside n-contact through the Cu substrate, improved heat dissipation due to the Cu substrate allowed CW laser operation up to a heatsink temperature of 80°C. Significant improvements in light output powers were observed for devices on Cu substrates with maximum CW output power of more than 100 mW     

High-power high-efficiency 660-nm laser diodes for DVD-R/RW

A kink mechanism in 660-nm laser diodes (LDs) has been studied experimentally. The experiments revealed that the main origin of the kink is a refractive index change due to heat generation in the stripe portion, and the kink power can be increased by improving the temperature characteristics of the LD. A newly developed LD, based on this result, shows stable lateral mode operation up to 190 mW at 80/spl deg/C. This is the highest power recorded among narrow stripe LDs with a wavelength of 660 nm. This LD is suitable for the next generation of high-speed (8x-) DVD-R/RW drives necessitating 140 mW class LDs.     

Non-Markovian gain of strained-layer wurtzite GaN quantum-welllasers with many-body effects

A theoretical model for the optical gain of strained-layer wurtzite GaN quantum-well (QW) lasers is developed taking into account valence-band mixing, many-body effects and non-Markovian relaxation. The valence-band structure is calculated from a 6×6 multiband effective mass Hamiltonian for the wurtzite structure taking into account built-in strain due to lattice mismatch. The theoretical foundation for the optical processes is based on the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is predicted that the Coulomb enhancement of gain is pronounced with increasing magnitude of compressive strain in the QW     

Dislocation related issues in the degradation of GaN-based laser diodes

We investigate degraded GaN-based laser diodes (LDs) on epitaxial lateral overgrown GaN layers in terms of dislocations. Almost all of the threading dislocations that appear in the wing regions are a-type dislocations. Their origins are the lateral extension of dislocations from the seed regions that contingently bend upwards to the episurface. Comparing short-lived LDs and long-lived LDs that have almost the same power consumption, we find that the relative levels of dislocation densities in their respective active layers are different. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations are observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. The degradation rate is almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.     

Ultrabroad-band Raman amplifiers pumped and gain-equalized bywavelength-division-multiplexed high-power laser diodes

This paper reviews recent progress in broad-band Raman amplifiers for wavelength-division-multiplexed (WDM) applications. After the fundamentals of Raman amplifiers are discussed in contrast to erbium-doped fiber amplifiers, a new technique called “WDM pumping” is introduced to obtain ultrabroad and flat gain in Raman amplifiers only using WDM diode pumps. The design issues of this technique are then developed to realize outstanding performances such as 100 nm of flat gain bandwidth, 0.1 dB flatness over 80 nm, and so forth     

Integrated GaInAsP laser diodes with monitoring photodiodes throughsemiconductor/air Bragg reflector (SABAR)

A 1.3-μm GaInAsP laser diode (LD) is integrated with a monitoring photodiode (M-PD) through a semiconductor/air Bragg reflector (SABAR). Instead of conventional cleavage, the SABAR can provide not only Fabry-Perot resonance with high reflectivity, but also possibility of integration of laser with other functional devices. The design, fabrication, and some characteristics including threshold current, monitoring photocurrent, SABAR reflectivity as a function of the number of semiconductor/air pairs N are reported. The threshold current of ridge waveguide laser with SABAR (cavity length L=160 μm, ridge width W=7 μm, SABAR pairs N=3) is 20 mA. The threshold current is reduced by improving butt-coupled interface between active and passive waveguides employed in this laser and is expected 2 mA/μm. The monitoring photocurrent responds linearly with output power from the laser and 0.024 mA at laser output power of 5 mW. From the threshold characteristics, SABAR reflectivity is determined to >80%. The increase of photocurrent can be achieved by optimizing the number of SABAR pairs to N=1. We have obtained threshold current of 22 mA in the followed laser structure (L=270 μm, W=7 μm, N=1), and detector photocurrent of 1.13 mA (@5 mW). The experimental SABAR reflectivity is ~50%, which is estimated by threshold characteristics and efficiency of light output power. The laser has a mode field converter section, resulting in narrow beam divergence 11° along vertical axis. This integrated laser is very promising candidate for coming optical module in low-power consumption and low-cost access network systems     

Bistable laser diodes and their applications: state of the art

Recent progresses in research on bistable laser diodes and their applications in optical communications and photonic switching are reviewed. In addition to the conventional absorptive and dispersive bistable laser diodes, bistability in two-mode lasers via gain saturation has recently attracted attention, because of its ultra high speed. On the other hand, bistable laser diodes with saturable absorbers are mainly used in the system applications because of their stable operations at present. This paper presents the theoretical analysis of the two-mode bistable laser diodes, the stripe lasers and the vertical-cavity surface-emitting lasers (VCSELs) as the two major representatives of bistable lasers, and the profound discussion of their possible applications     

All-optical ASK/DPSK label-swapping and buffering using Fabry-Perot laser diodes

We describe an approach for optical label encoding that allows the realization of all-optical label-swapping in optical packet-switched networks. The proposed method is based on a combination of the amplitude-shift-keying (ASK) modulated payload with the differential phase-shift keying (DPSK) modulated label on the same optical carrier. We demonstrate an implementation using dual-wavelength injection locking (DWIL) of a Fabry-Perot laser diode. Bit-error-rate measurements were performed for the 10-Gb/s payload, and the 2.5-Gb/s label showed the feasibility of the proposed method. An all-optical buffer for the two-level ASK/DPSK optically labeled packets is also described. The buffer is implemented by routing the packet via a delay line if potential contention is predicted. Error-free operation was also achieved.