GaSb-based Diode Lasers Research Articles

Investigation of regime switching from mode locking to Q-switching in a 2 µm InGaSb/AlGaAsSb quantum well laser.

A two-section InGaSb/AlGaAsSb single quantum well (SQW) laser emitting at 2 μm is presented. By varying the absorber bias voltage with a fixed gain current at 130 mA, passive mode locking at ~18.40 GHz, Q-switched mode locking, and passive Q-switching are observed in this laser. In the Q-switched mode locking regimes, the Q-switched RF signal and mode locked RF signal coexist, and the Q-switched lasing and mode-locked lasing happen at different wavelengths. This is the first observation of these three pulsed working regimes in a GaSb-based diode laser. An analysis of the regime switching mechanism is given based on the interplay between the gain saturation and the saturable absorption.

Cascade Pumping of 1.9–3.3 μm Type-I Quantum Well GaSb-Based Diode Lasers

Cascade pumping of type-I quantum well gain sections was utilized to increase output power and efficiency of GaSb-based diode lasers operating in spectral region from 1.9 to 3.3 μm. Coated devices with ∼100-μm-wide aperture and 3-mm-long cavity demonstrated continuous wave (CW) output power of 1.96 W near 2 μm, 980 mW near 3 μm, 500 mW near 3.18 μm, and 360 mW near 3.25 μm at room temperature. The corresponding narrow ridge lasers with nearly diffraction limited beams operate in CW regime with tens of mW of output power up to 60 °C. Two step shallow/deep narrow/wide ridge waveguide devices showed lower threshold currents and higher slope efficiencies compared to single step narrow ridge lasers. Laterally coupled DFB lasers mounted epi-up generated above 10 mW of tunable single frequency CW power at 20 °C near 3.22 μm.

Room-temperature continuous-wave operation in the telecom wavelength range of GaSb-based lasers monolithically grown on Si

We report on electrically pumped GaSb-based laser diodes monolithically grown on Si and operating in a continuous wave (cw) in the telecom wavelength range. The laser structures were grown by molecular-beam epitaxy on 6°-off (001) substrates. The devices were processed in coplanar contact geometry. 100 μm × 1 mm laser diodes exhibited a threshold current density of 1 kA/cm−2 measured under pulsed operation at 20 °C. CW operation was achieved up to 35 °C with 10 μm × 1 mm diodes. The output power at 20 °C was around 3 mW/uncoated facet, and the cw emission wavelength 1.59 μm, in the C/L-band of telecom systems.

Low frequency noise investigation of 2–3 μm GaSb-based laser diodes

A detailed noise study of mid-infrared 2–3μm wavelenght GaSb-based type-I ridge-waveguide laser diodes (LDs) has been carried out using the low frequency noise spectroscopy. Different samples having slightly different heterostructures, differently packaged, with and without facet passivation have been investigated. The activation energies of trap levels, associated with additional generation-recombination (g–r) noise components in noise spectra at small current region before the threshold, have been determined. It was shown that at particular forward currents the mode-hopping effect dominates during LD lasing operation. It was demonstrated that small differences in LD heterostructure and waveguide layers have got the largest influence to LD noise characteristics while influence of the absence of facet passivation, linking to noise generation centers at the cleaved semiconductor interface, and package type is much weaker.

In brief

PAGE 40 A methane-hydrogen dry etching technique has been used to fabricate GaSb-based cascade diode lasers emitting near 3 µm. The process, developed by researchers in the USA, produces a diffraction-limited laser with minimal increase in threshold current density and no voltage penalty when compared with wide ridge multimode lasers. PAGE 36 A performance trade-off was found when examining different methods for inserting microstrip lines into aerospace composite structures. Research from Australia showed that embroidered microstrip lines exhibited better electromagnetic performance, whilst stencil transfer was more resistant to mechanical deformation. Cascade diode lasers fabricated using methane-hydrogen dry etching technique. PAGE 34 Researchers from the UK have designed an easy to control microstrip coupler that enhances power coupling between two microstrips. Two small slot lines are inserted in the ground plane underneath the microstrips, forcing an increased number of shared field lines between strips, thus eliminating the need to narrow the distance between microstrip lines. Microstrip lines stitched into HexPly 914 S-Glass. PAGE 7 A high-performing 3D terahertz (THz) filtering antenna was designed by researchers in China, using a low-temperature co-fired ceramic (LTCC) process. This 3D structure is more compact, has enhanced radiation performance and allows for more design flexibility compared with traditional planar designs. Slot lines underneath microstrips enhance coupling. PAGE 16 Researchers from Korea have developed a dual correlated double sampling scheme (CDS) for low-noise CMOS imagers, without the need for a power-consuming programmable gain amplifier (PGA). This scheme reduces noise by removing sampling error, and reduces power consumption by amplifying the readout channel's output. Photograph of 3D THz filtering antenna. The correlated double sampling scheme can be applied to SAR-based ADCs for CMOS imagers without the use of a PGA.

Narrow ridge GaSb‐based cascade diode lasers fabricated by methane–hydrogen reactive ion etching

GaSb-based type-I quantum wells cascade diode lasers with nearly diffraction limited output beam were fabricated using methane–hydrogen dry etching plasma process. Rapid thermal annealing was shown to be a necessary step to reverse the effect of hydrogen plasma on conductivity of AlGaAsSb cladding alloy. Annealed two-step narrow ridge lasers confined both optical field and current in lateral direction and demonstrated operating voltage, threshold current density and slope efficiency comparable to those of reference wide ridge devices.

Aluminum-based contacts for use in GaSb-based diode lasers

Aluminum-based contacts could be a good alternative to conventional gold-based contacts for a number of GaSb-based devices. In this study, the use of some Al-based contacts in GaSb-based diode lasers was investigated via the measurement of specific contact resistivity and laser output characteristics. The Al-based contacts to p-type GaSb(001) exhibited lower specific contact resistivities than the conventional Au-based contacts, whereas the opposite was the case for contacts to n-type GaSb(001). The good performance of GaSb-based laser diodes using Al-based contacts shows the applicability of this type of contact in GaSb-based devices. The contact between Al only and p-type GaSb(001), however, could suffer from a reliability problem when used in diode lasers, due to interdiffusion, in which case a diffusion barrier should be included.

Cascade Type-I Quantum Well GaSb-Based Diode Lasers

Cascade pumping of type-I quantum well gain sections was utilized to increase output power and efficiency of GaSb-based diode lasers operating in a spectral region from 1.9 to 3.3 μm. Carrier recycling between quantum well gain stages was realized using band-to-band tunneling in GaSb/AlSb/InAs heterostructure complemented with optimized electron and hole injector regions. Coated devices with an ~100-μm-wide aperture and a 3-mm-long cavity demonstrated continuous wave (CW) output power of 1.96 W near 2 μm, 980 mW near 3 μm, 500 mW near 3.18 μm, and 360 mW near 3.25 μm at 17–20 °C—a nearly or more than twofold increase compared to previous state-of-the-art diode lasers. The utilization of the different quantum wells in the cascade laser heterostructure was demonstrated to yield wide gain lasers, as often desired for tunable laser spectroscopy. Double-step etching was utilized to minimize both the internal optical loss and the lateral current spreading penalties in narrow-ridge lasers. Narrow-ridge cascade diode lasers operate in a CW regime with ~100 mW of output power near and above 3 μm and above 150 mW near 2 μm.

Plasma-assisted oxide removal from p-type GaSb for low resistivity ohmic contacts

The effect of several plasma-assisted oxide removal techniques prior to metallization of p-type GaSb was investigated. Compared to conventional chemical methods, the plasma-assisted oxide removal resulted in significant improvement of the specific contact resistivities, obtained from transfer length method measurements. Very low specific contact resistivities of less than 5 × 10−8 Ω cm2 were observed after surface pretreatment by H2/Ar sputter etching and low-ion-energy argon irradiation. By eliminating sample exposure to air, in situ Ar irradiation becomes a promising technique for high performance GaSb-based semiconductor diode lasers.

Highly Strained Mid-Infrared Type-I Diode Lasers on GaSb

We describe how growth at low temperatures can enable increased active layer strain in GaSb-based type-I quantum-well diode lasers, with emphasis on extending the emission wavelength. Critical thickness and roughening limitations typically restrict the number of quantum wells that can be grown at a given wavelength, limiting device performance through gain saturation and related parasitic processes. Using growth at a reduced substrate temperature of 350 °C, compressive strains of up to 2.8% have been incorporated into GaInAsSb quantum wells with GaSb barriers; these structures exhibited peak room-temperature photoluminescence out to 3.96 μm. Using this growth method, low-threshold ridge waveguide lasers operating at 20 °C and emitting at 3.4 μm in pulsed mode were demonstrated using 2.45% compressively strained GaInAsSb/GaSb quantum wells. These devices exhibited a characteristic temperature of threshold current of 50 K, one of the highest values reported for type-I quantum-well laser diodes operating in this wavelength range. This temperature stability is attributable to the increased valence band offset afforded by the high strain values, due to the simultaneously high quantum well indium and antimony mole fractions. Exploratory experiments using bismuth both as a surfactant during quantum well growth, as well as in dilute amounts incorporated into the crystal were also studied. Both methods appear to be promising avenues to surmount current strain-related limitations to laser performance and emission wavelength.

Passive mode locking of a GaSb-based quantum well diode laser emitting at 2.1 μm

We demonstrate passive mode locking of a GaSb-based diode laser emitting at 2.1 μm. The active region of the studied device consists in two 10-nm-thick GaInSbAs/GaAlSbAs quantum wells. Passive mode locking has been achieved in a two-section laser with one of the sections used as a saturable absorber. A microwave signal at 20.6 GHz, measured in the electrical circuit of the absorber, corresponds to the fundamental photon round-trip frequency in the laser resonator. The linewidth of this signal as low as ∼10 kHz has been observed at certain operating conditions, indicating low phase noise mode-locked operation.

Distributed feedback GaSb based laser diodes with buried grating: a new field of single-frequency sources from 2 to 3 µm for gas sensing applications.

We report on the growth, fabrication, experimental study and application in an absorption gas setup of distributed feed-back antimonide diode lasers with buried grating. First, half laser structures were grown by molecular beam epitaxy on GaSb substrates and stopped at the top of the waveguide. A second order Bragg grating was then defined by interferometric lithography on the top of the structure and dry etched by Reactive Ion Etching. The grating was, afterwards, buried thanks to an epitaxial regrowth of the top cladding layer. Finally, the wafer was processed using standard photolithography and wet etched into 10 µm-wide laser ridges. A single frequency laser emission around 2.3 µm was recorded, a maximum output power of 25 mW and a total continuous tuning range reaching 4.2 nm at fixed temperature. A device has been used to detect methane gas and shows strong potential for gas spectroscopy. This process was also replicated for a target of 3 µm laser emission. These devices showed an output power of 2.5 mW and a SMSR of at least 23 dB, with a 2.5 nm continuous tuning range at fixed temperature.

2.2–2.7 μm side wall corrugated index coupled distributed feedback GaSb based laser diodes

We report on the modeling, growth, processing, characterization and integration in a gas detection setup of side wall corrugated distributed feed-back antimonide diode lasers emitting at 2.28 and 2.67 μm. The laser structures were grown by molecular beam epitaxy on GaSb substrate. Ridge lasers were fabricated from the grown wafers according to the following process: a second order Bragg grating was defined on the sides of the ridges by interferometric lithography, optical lithography and etched in a Cl-based inductively coupled plasma reactor. The devices exhibit a power reaching 40 mW, a side mode suppression ratio better than 28 dB and a tuning range of 3 nm at room temperature. One of these devices was successfully integrated in a tunable diode laser absorption spectroscopy setup, thus demonstrating that they are suitable for gas analysis.

GaSb-based >3 µm laser diodes grown with up to 2.4% compressive strain in the quantum wells using strain compensation

We describe the growth of GaSb-based diode lasers with quinary AlInGaAsSb alloy waveguide/barriers and highly strained InGaAsSb type-I quantum wells, designed to emit at ~3.2µm at room temperature. To increase the compressive strain in the QWs above the pseudomorphic limit, we employ strain compensation in which the lower portion of the quinary alloy waveguide is slightly arsenic-rich as compared to the lattice-matched composition. Using this scheme, we fabricated a set of laser devices with incrementally increasing compressive strain in the quantum wells. With compressive strain reaching as high as ~2.4%, the device performance corroborates the notion that adding compressive strain in the quantum wells helps improve hole confinement and suppresses carrier leakage during operation near room temperature.

Distributed feedback GaSb based laser diodes with buried grating

We report on the growth, fabrication, and experimental study of distributed feed-back antimonide diode lasers with buried grating. A second order index-coupled grating was defined by interferometric lithography on the top of the laser waveguide and dry etched by reactive ion etching. The grating was then buried thanks to an overgrowth of the top cladding layer using molecular beam epitaxy. The wafer was then processed using standard photolithography and wet etching into 15 μm-wide laser ridges. Single frequency laser emission at a wavelength of 2.2 μm was measured with a side mode suppression ratio of 34 dB, a maximum output power of 30 mW, and a total continuous tuning range of 6.5 nm.

High power 2-μm room-temperature continuous-wave operation of GaSb-based strained quantum-well lasers

A high power GaSb-based laser diode with lasing wavelength at 2 μm was fabricated and optimized. With the optimized epitaxial laser structure, the internal loss and the threshold current density decreased and the internal quantum efficiency increased. For uncoated broad-area lasers, the threshold current density was as low as 144 A/cm2 (72 A/cm2 per quantum well), and the slope efficiency was 0.2 W/A. The internal loss was 11 cm−1 and the internal quantum efficiency was 27.1%. The maximum output power of 357 mW under continuous-wave operation at room temperature was achieved. The electrical and optical properties of the laser diode were improved.

Q-switched Ho:Lu_2O_3 laser at 212 μm

We report on a Q-switched Ho:Lu2O3 laser resonantly pumped by a GaSb-based laser diode stack at 1.9 μm. The maximum output energy extracted from the compact plano-plano acousto-optically Q-switched resonator was 8 mJ at a 100 Hz pulse repetition rate, while the peak power was 40 kW. The laser wavelength was 2.124 μm.

Theoretical analyses on improved beam properties of GaSb-based 2.X-μm quantum-well diode lasers with no degradation in laser parameters

An asymmetric laser heterostructure is developed to improve the beam properties of GaSb-based diode lasers with no degradation in laser parameters. Employing the semivectorial finite difference method, the dependences of beam divergence and optical confinement factor on waveguide width and refractive index step are investigated theoretically. After carefully design, a particular asymmetric laser structure is proposed. Its beam divergence in the fast axis is reduced from 61° to 34° compared with that of the broad-waveguide structure. The optical confinement factor is approximately equal to 0.0362 and comparable to that of the conventional broad-waveguide structure.

Directly diode-pumped high-energy Ho:YAG oscillator

We report on the high-energy laser operation of an Ho:YAG oscillator resonantly pumped by a GaSb-based laser diode stack at 1.9 μm. The output energy was extracted from a compact plano-concave acousto-optically Q-switched resonator optimized for low repetition rates. Operating at 100 Hz, pulse energies exceeding 30 mJ at a wavelength of 2.09 μm were obtained. The corresponding pulse duration at the highest pump power was 100 ns, leading to a maximum peak power above 300 kW. Different pulse repetition rates and output coupling transmissions of the Ho:YAG resonator were studied. In addition, intracavity laser-induced damage threshold measurements are discussed.

High-power laterally coupled distributed-feedback GaSb-based diode lasers at 2 μm wavelength

We report on GaSb-based laterally coupled distributed-feedback (DFB) diode lasers designed to operate at wavelengths near 2.05 μm. Second-order Bragg gratings were etched alongside narrow ridge waveguides to enable single-mode DFB operation in 2-mm-long laser diodes. At a heat-sink temperature of 10 °C, the lasers emit more than 40 mW continuous-wave in a single longitudinal mode, while increasing the current beyond 300 mA results in multimode operation due to spectral shifting of the laser gain with respect to the peak grating reflectivity. At −10 °C, we observe DFB operation at higher current, with single-facet emission exceeding 80 mW.