Uncoated Facets Research Articles

8.1 μ m-emitting InP-based quantum cascade laser grown on Si by metalorganic chemical vapor deposition

This study presents the growth and characterization of an 8.1 μm-emitting, InGaAs/AlInAs/InP-based quantum cascade laser (QCL) formed on an InP-on-Si composite template by metalorganic chemical vapor deposition (MOCVD). First, for the composite-template formation, a GaAs buffer layer was grown by solid-source molecular-beam epitaxy on a commercial (001) GaP/Si substrate, thus forming a GaAs/GaP/Si template. Next, an InP metamorphic buffer layer (MBL) structure was grown atop the GaAs/GaP/Si template by MOCVD, followed by the MOCVD growth of the full QCL structure. The top-surface morphology of the GaAs/GaP/Si template before and after the InP MBL growth was assessed via atomic force microscopy, over a 100 μm2 area, and no antiphase domains were found. The average threading dislocation density (TDD) for the GaAs/GaP/Si template was found to be ∼1 × 109 cm−2, with a slightly lower defect density of ∼7.9 × 108 cm−2 after the InP MBL growth. The lasing performance of the QCL structure grown on Si was compared to that of its counterpart grown on InP native substrate and found to be quite similar. That is, the threshold-current density of the QCL on Si, for deep-etched ridge-guide devices with uncoated facets, is somewhat lower than that for its counterpart on native InP substrate, 1.50 vs 1.92 kA/cm2, while the maximum output power per facet is 1.64 vs 1.47 W. These results further demonstrate the resilience of QCLs to relatively high residual TDD values.

1.55-μm Sidewall Grating DFB Lasers Integrated With a Waveguide Crossing for an Optical Beam Forming Network

By using asymmetric twin-waveguide technology, a 1.55-μm sidewall grating distributed feedback laser monolithically integrated with a passive waveguide crossing was for the first time demonstrated for an optical beam forming network, which needs only one metalorganic vapor-phase epitaxy step. The distributed feedback laser with uncoated facets presents a side-mode suppression ratio of >44 dB and a low 3-dB linewidth of 68 kHz. An elliptical parabolic taper waveguide crossing had a theoretical crosstalk suppression ratio of 40 dB, while the measured cross-talk suppression ratio was at least 20 dB.

Significantly improved performances of 1.3 μm InAs/GaAs QD laser by spatially separated dual-doping

We report on significantly enhanced performances of 1.3 μm InAs/GaAs quantum dot (QD) lasers by spatially separated dual-doping, including p-type modulation doping in barrier layers and n-type direct doping in QDs simultaneously. The QD lasers are a ridge waveguide of 6 × 1000 μm2 with uncoated facets, whose active region consists of eight stacked InAs QD layers. Compared with the conventional single p-type modulation doped (p-doped) QD laser, the dually doped QD laser achieves a reduced threshold current from 51.07 to 43 mA, an increased single-sided slope efficiency from 0.18 to 0.25 W/A at 25 °C, and an increased characteristic temperature T0 from 654 to 7917 K between 15 and 85 °C. Furthermore, the continuous wave output power of the dually doped QD laser exceeds 20 mW without any attenuation at 85 °C, whereas that of the p-doped one appears to be saturated at 14.08 mW. The results presented here have important implications for realizing high-performance QD lasers emitting at 1.3 μm to various applications.

Semipolar {202¯1} GaN Edge-Emitting Laser Diode on Epitaxial Lateral Overgrown Wing

Edge-emitting laser diodes (LDs) were fabricated on a reduced dislocation density epitaxial lateral overgrown (ELO) wing of a semipolar {202¯1} GaN substrate, termed an ELO wing LD. Two types of facet feasibility studies were conducted: (1) “handmade” facets, wherein lifted-off ELO wing LDs were cleaved manually, and (2) facets formed on wafers through reactive ion etching (RIE). Pulsed operation electrical and optical measurements confirmed the laser action in the RIE facet LDs with a threshold current of ~19 kAcm−2 and maximum light output power of 20 mW from a single uncoated facet. Handmade facet devices showed spontaneous, LED-like emission, confirming device layers remain intact after mechanical liftoff.

Distributed feedback lasers using surface gratings in Bragg waveguides.

This Letter presents, to the best of our knowledge, the first report of a narrow-linewidth ∼790-800nm edge-emitting semiconductor distributed feedback Bragg reflection waveguide diode laser (DFB2RL). The DFB2RLs were fabricated using a ridge waveguide structure with 5th order, surface-etched grating forming the wavelength selective element. Unbonded devices with a 500 µm cavity length exhibited continuous wave threshold currents in the region of 25 mA with an output power of 2.5 mW per (uncoated) facet at 100 mA drive current. The devices operated in a single longitudinal mode, with side-mode suppression ratio (SMSR) as high as 49 dB and linewidths as low as 207 kHz. Devices maintained single mode operation with high SMSR over a 9 nm wavelength range as the temperature was swept from 15°C to 50°C.

Optical Gain and Lasing Properties of InP/AlGaInP Quantum-Dot Laser Diode Emitting at 660 nm

We investigated the optical gain properties and lasing characteristics of a pulsed electrically pumped laser structure, which consists of a single layer of self-assembled InP quantum dots (QDs) assembled in (Al 0.10 Ga 0.90 ) 0.52 In 0.48 P barriers. The optical gain and absorption spectra were obtained by analyzing the amplified spontaneous emission as a function of the excitation length. At a current density of 1.2 kA/cm 2 , an internal optical loss value of 5 ± 2 cm -1 and a peak modal gain of 39.3 cm -1 corresponding to a material gain of approximately 2675 cm -1 per QD layer were determined at room temperature. For a 2.24-mm-long laser with uncoated facets emitting at 660 nm, a low threshold current density of 281 A/cm 2 and an external differential quantum efficiency of 34.2% were determined. The internal quantum efficiency of 66% and the transparent current density of 65.8 A/cm 2 for a single layer of QDs were also demonstrated.

High-power single-longitudinal-mode double-tapered gain-coupled distributed feedback semiconductor lasers based on periodic anodes defined by i-line lithography

In this paper, we demonstrate a regrowth-free double-tapered gain-coupled distributed feedback semiconductor laser. It is designed based on periodic surface current injection to reach a high-power and single-longitudinal mode. A continuous-wave output power of over 1.2 W/facet is achieved at 4 A. High single-longitudinal-mode output power reaches up to 0.9 W/facet at 3 A at each uncoated facet. The side mode suppression ratio is nearly 30 dB at 980 nm. The 3 dB spectral width is less than 2.7 pm. The lateral far field divergence angle is only 14.5°, the beam quality factor M2 is 1.7, achieving a near-diffraction-limit emission. Our device, produced by standard i-line lithography, enhances the output power while obtaining the pleasurable spectral and spatial properties. It has great potential in widespread commercial applications such as high efficiency pumping sources for its low-cost, easy fabrication technique and excellent performance.

Sum frequency conversion of multiline CO laser emission in BaGa2GeSe6 crystal of one cm long

Sum frequency conversion of multiline Q-switched CO laser radiation was studied in 10.4mm-long BaGa2GeSe6 crystal under noncritical spectral phase-matching conditions. Spectrum of the CO laser consisted of 51 emission lines in the wavelength range of 4.95-6.20 μm. The frequency converted spectrum contained 85 spectral lines in the wavelength range of 2.5-2.8 μm. The internal energy conversion efficiency (taking into account optical losses on uncoated facets of the crystal) was up to 1.7%.

Improved performance of InP-based 2.1 μm InGaAsSb quantum well lasers using Sb as a surfactant

We demonstrate significantly enhanced performance of 2.1 μm InGaAsSb quantum well lasers using Sb as a surfactant. The lasers are grown on an InP substrate by metal-organic vapor-phase epitaxy. Following the introduction of Sb, a double-channel waveguide laser, with uncoated facets, shows a remarkably increased continuous-wave output power of 330 mW, compared with 160 mW for a Sb-free InGaAs quantum well laser, measured at 10 °C. Moreover, the introduction of Sb improves the temperature performance of the device and doubles its wall-plug efficiency. In addition, the active region material and interface quality are investigated by transmission electron microscopy, which help to elucidate the basic physical mechanism of the Sb surfactant effect.

High order DBR GaSb based single longitude mode diode lasers at 2 μm wavelength * * Project supported by the National Basic Research Program of China (Nos. 2016YFB0402403, 2014CB643903) and the National Natural Science Foundation of China (Nos. 61790583, 61435012).

The GaSb-based distributed Bragg reflection (DBR) diode laser with 23rd-order gratings have been fabricated by conventional UV lithography and inductively coupled plasma (ICP) etching. The ICP etching conditions were optimized and the relationship among etching depth, duty ratio and side-mode suppression ratio (SMSR) was studied. The device with a ridge width of 100 μm, gratings period of 13 μm and etching depth of 1.55 μm as well as the duty ratio of 85% was fabricated, its maximum SMSR reached 22.52 dB with uncoated cavity facets under single longitudinal operation mode at room temperature.

Improved performance of 1.3-μm InAs/GaAs quantum dot lasers by direct Si doping

We demonstrate significantly enhanced performances of 1.3-μm InAs/GaAs quantum dot (QD) lasers by directly Si-doped QDs. The lasers were grown by molecular beam epitaxy. Following Si doping, the ridge waveguide laser, with uncoated facets, showed a remarkably reduced continuous-wave threshold current density of 71.6 A/cm2 (14.3 A/cm2 per QD layer), compared with 167.3 A/cm2 (33.5 A/cm2 per QD layer) for an undoped device with an identical structure, measured at 20 °C. Moreover, doping improved the single-side slope efficiency from 0.28 to 0.42 W/A. In addition, the Si-doped QD laser exhibited a higher lasing temperature of up to 140 °C compared with 120 °C for the undoped QD laser.

Sum-frequency generation of Q-switched CO laser radiation in BaGa2GeSe6 and GaSe nonlinear crystals

Sum-frequency generation of multiline Q-switched CO laser radiation is studied in the BaGa2GeSe6 (BGGSe) crystal and in the GaSe crystal. The maximal external conversion efficiency obtained in the BGGSe crystal is 0.7%, which corresponded to the internal conversion efficiency of 1% taking into account optical losses under reflection from uncoated facets of the crystal. The maximum external conversion efficiency in the GaSe crystal is 0.15% with internal efficiency 0.31%. Effective nonlinearity of BGGSe crystal is determined through the ratio between measured efficiency in BGGSe and GaSe crystals. Effective nonlinearity for sum-frequency generation of the CO laser emission in the BGGSe crystal is deff = 42 ± 14 pm/V, and nonlinear coefficient of this crystal is d11 = 49 ± 15 pm/V.

Electrically pumped continuous-wave 1.3 μm quantum-dot lasers epitaxially grown on on-axis (001) GaP/Si.

We demonstrate the first electrically pumped continuous-wave (CW) III-V semiconductor lasers epitaxially grown on on-axis (001) silicon substrates without offcut or germanium layers, using InAs/GaAs quantum dots as the active region and an intermediate GaP buffer between the silicon and device layers. Broad-area lasers with uncoated facets achieve room-temperature lasing with threshold current densities around 860 A/cm2 and 110 mW of single-facet output power for the same device. Ridge lasers designed for low threshold operations show maximum lasing temperatures up to 90°C and thresholds down to 30 mA.

CW operation of high‐power blue laser diodes with polished facets on semi‐polar GaN substrates

Continuous wave (CW) operation of high-power blue laser diodes (LDs) with polished facets on semi-polar (2021) gallium nitride (GaN) substrates is demonstrated. Ridge waveguide LDs were fabricated using indium GaN waveguiding layers and GaN cladding layers. At a lasing wavelength of 452 nm, the peak two-facet CW output power from an LD with uncoated facets was 1.71 W at a current of 3 A, corresponding to an optical power density of 32.04 MW/cm2 on each facet. The dependence of output power on current did not change with repeated CW measurements, indicating that the polished facets did not degrade under high-power CW operation. These results show that polished facets are a viable alternative to cleaved or etched facets for high-power CW semi-polar LDs.

Chemically assisted ion beam etching of laser diode facets on nonpolar and semipolar orientations of GaN

We demonstrate a vertical (<1° departure) and smooth (2.0 nm root mean square line-edge roughness (LER)) etch by chemically assisted Ar ion beam etching (CAIBE) in Cl2 chemistry that is suitable for forming laser diode (LD) facets on nonpolar and semipolar oriented III-nitride devices. The etch profiles were achieved with photoresist masks and optimized CAIBE chamber conditions including the platen tilt angle and Cl2 flow rate. Co-loaded studies showed similar etch rates of ∼60 nm min−1 for and m-plane orientations. The etched surfaces of LD facets on these orientations are chemically dissimilar (Ga-rich versus N-rich), but were visually indistinguishable, thus confirming the negligible orientation dependence of the etch. Continuous-wave blue LDs were fabricated on the semipolar plane to compare CAIBE and reactive ion etch (RIE) facet processes. The CAIBE process resulted in LDs with lower threshold current densities due to reduced parasitic mirror loss compared with the RIE process. The LER, degree of verticality, and model of the 1D vertical laser mode were used to calculate a maximum uncoated facet reflection of 17% (94% of the nominal) for the CAIBE facet. The results demonstrate the suitability of CAIBE for forming high quality facets for high performance nonpolar and semipolar III-N LDs.

InGaN laser diode with metal-free laser ridge using n+-GaN contact layers

We report on InGaN edge emitting laser diodes with a top metal electrode located beside the laser ridge. Current spreading over the ridge is achieved via a highly doped n+-type GaN layer deposited on top of the structure. The low sheet resistance of the n+-GaN layer ensures excellent lateral current spreading, while carrier injection is confined all along the ridge thanks to current tunneling at the interface between the n+-GaN top layer and the p++-GaN layer. Continuous-wave lasing at 400 nm with an output power of 100 mW is demonstrated on uncoated facet devices with a threshold current density of 2.4 kA·cm−2.

2-μm single longitudinal mode GaSb-based laterally coupled distributed feedback laser with regrowth-free shallow-etched gratings by interference lithography**Project supported by the National Key Basic Research Program of China (Grant Nos. 2014CB643903 and 2013CB932904), the National Special Funds for the Development of Major Research Equipment and Instruments, China (Grant No. 2012YQ140005), the National Natural Science Foundation of China (Grant Nos. 61435012, 61274013,

We report a type-I GaSb-based laterally coupled distributed-feedback (LC-DFB) laser with shallow-etched gratings operating a continuous wave at room temperature without re-growth process. Second-order Bragg gratings are fabricated alongside the ridge waveguide by interference lithography. Index-coupled LC-DFB laser with a cavity of 1500 μm achieves single longitudinal mode continuous-wave operation at 20 °C with side mode suppression ratio (SMSR) as high as 24 dB. The maximum single mode continuous-wave output power is about 10 mW at room temperature (uncoated facet). A low threshold current density of 230 A/cm2 is achieved with differential quantum efficiency estimated to be 93 mW/A. The laser shows a good wavelength stability against drive current and working temperature.

Reducing Thermal Carrier Spreading in InP Quantum Dot Lasers

Record low values in this material system of threshold current density, particularly at elevated temperature, are presented for InP quantum dot lasers. Lasers with Ga0.58In0.42P in the dot upper confining layer have the lowest threshold current densities, 138 A·cm−2 at 300 K, and 235 A·cm −2 at 350 K (77 °C) (2-mm lasers, uncoated facets). Gain-current density data suggests laser performance with an upper confining layer of ${\rm Ga}_{x}$ ${\rm In}_{1-x}$ P with ${\rm x} = 0.54$ , 0.56 or 0.58 would be similar if not for the very low internal optical mode loss, $\alpha _{{\rm i}}$ of samples with ${\rm x} = 0.56$ and 0.58. Gain measurements at fixed inversion level suggest that increasing x content in ${\rm Ga}_{x}$ ${\rm In}_{{1-x}}$ P increases gain at fixed inversion level but samples with ${\rm x} = 0.54$ also exhibit reduced recombination current density. The increasing recombination current density at elevated temperature due to thermal carrier spreading is significantly reduced in samples with ${\rm x} = 0.56$ and ${\rm x} = 0.58$ but measurements at common operating points attribute this largely to the reduced $\alpha _{{\rm i}}$ for these samples and given the same $\alpha _{{\rm I}}$ , samples with ${\rm x} = 0.54$ , 0.56 and 0.58 would all benefit from reduced effects due to thermal carrier spreading compared to ${\rm x} = 0.52$ .

Gain-coupled distributed feedback laser based on periodic surface anode canals

A single-longitude-mode, broad-stripe, gain-coupled, distributed-feedback laser based on periodic surface anode canals (PSACs) is demonstrated. The PSACs, produced by i-line lithography, enhance the contrast of periodic current density in the active layer without introducing effective photon coupling; calculated grating κL is only 0.026. Power of 144.6mW at 968.8nm, with spectrum linewidth less than 0.04nm on every uncoated cleavage facet, is obtained at a current of 1.2A with a side-mode suppression ratio >29 dB.

A 350-nm-band GaN/AlGaN multiple-quantum-well laser diode on bulk GaN

We have demonstrated the pulsed operation of a 350-nm-band ultraviolet laser diode with a vertical current path. The laser structure was grown on a (0001)-face bulk GaN substrate. The lasing wavelength was 356.6 nm and the peak output power reached to 10 mW from the one side of uncoated facets under pulsed current operation with a pulse duration of 10 ns and a repetition frequency of 5 kHz at room temperature. The GaN substrate is expected to provide a cleaved facet configuration leading to an excellent far-field pattern as well as an advantageous thermal management solution of the devices relative to sapphire substrates. The far-field pattern of actual device on GaN substrate has been improved dramatically compared with distorted one on that of sapphire substrates.