Narrow Ridge Waveguide Research Articles

High-speed GaN-based laser diode with modulation bandwidth exceeding 5 GHz for 20 Gbps visible light communication

Visible light communication (VLC) based on laser diodes demonstrates great potential for high data rate maritime, terrestrial, and aerial wireless data links. Here, we design and fabricate high-speed blue laser diodes (LDs) grown on c-plane gallium nitride (GaN) substrate. This was achieved through active region design and miniaturization toward a narrow ridge waveguide, short cavity length, and single longitudinal mode Fabry–Perot laser diode. The fabricated mini-LD has a low threshold current of 31 mA and slope efficiency of 1.02 W/A. A record modulation bandwidth of 5.9 GHz (−3 dB) was measured from the mini-LD. Using the developed mini-LD as a transmitter, the VLC link exhibits a high data transmission rate of 20.06 Gbps adopting the bit and power loading discrete multitone (DMT) modulation technique. The corresponding bit error rate is 0.003, satisfying the forward error correction standard. The demonstrated GaN-based mini-LD has significantly enhanced data transmission rates, paving the path for energy-efficient VLC systems and integrated photonics in the visible regime.

Promotion of Specific Single-Transverse-Mode Beam Characteristics for GaSb-Based Narrow Ridge Waveguide Lasers via Customized Parameter Design

GaSb-based single-transverse-mode narrow ridge waveguide (RW) lasers with high power and simultaneous good beam quality have broad application prospects in the mid-infrared wavelength region. Yet its design and formation have not been investigated systematically, while the beam characteristics that affect their suitability for specific applications remain rarely analyzed and optimized. The present work addresses these issues by theoretically establishing a waveguide parameter domain that generalizes the overall possible combinations of ridge widths and etch depths that support single-transverse-mode operation for GaSb-based RW lasers. These results are applied to develop two distinct and representative waveguide designs derived from two proposed major optimization routes of model gain expansion and index-guiding enhancement. The designs were evaluated experimentally based on prototype 1-mm cavity-length RW lasers in the 1950 nm wavelength range, which were fabricated with waveguides having perpendicular ridge and smooth side-walls realized through optimized dry etching conditions. The model gain expanded RW laser design with a relatively shallow-etched (i.e., 1.55 upmum) and wide ridge (i.e., 7 upmum) yielded the highest single-transverse-mode power to date of 258 mW with a narrow lateral divergence angle of 11.1^circ full width at half maximum at 800 mA under room-temperature continuous-wave operation, which offers promising prospects in pumping and coupling applications. Meanwhile, the index-guiding enhanced RW laser design with a relatively deeply etched (i.e., 2.05 upmum) and narrow ridge (i.e., 4 upmum) provided a highly stable and nearly astigmatism-free fundamental mode emission with an excellent beam quality of M^2 factor around 1.5 over the entire operating current range, which is preferable for seeding external cavity applications and complex optical systems.

Stable Lateral Far Field of Highly Dense Arrays of Uncoupled Narrow Stripe Ridge Waveguide 1060 nm Lasers

In order to stabilize the lateral far-field profile of a broad area laser diode at high pump currents, the design of an asymmetric InGaAs/AlGaAs/GaAs laser heterostructure was developed. Lasers with an aperture formed by ten optically uncoupled ridge waveguides (each 6.5 μm wide) were developed on its basis. To maintain the single-mode operation of each ridge waveguide without coupling between emitters, a High Dense Microstripe Array (HDMSA, the pitch is 13.5 μm) and a Low Dense Microstripe Array (LDMSA, the pitch is 40 μm) were fabricated with a coupling parameter between adjacent lateral waveguides of 2·10^−5 and 3·10^−19, respectively. The maximum optical output power was 5 W (HDMSA) in CW mode and 25 W (HDMSA) and 26 W (LDMSA) in pulsed mode (130ns/1kHz). It is shown that in both CW and pulsed modes, lasers of both designs provide a stable Gaussian-like distribution in the lateral far field (LFF) with a FWHM of 8–11 degrees for HDMSA and 6.5–10 degrees for LDMSA. It was found that the LFF broadens with a pump current in the range of low currents and stabilizes in the range of high currents for both designs. Studies of the near field in both operating modes at any pump currents showed no array modes and the uncoupling mode is maintained for ridge waveguide arrays.

High spectral purity GaSb-based blazed grating external cavity laser with tunable single-mode operation around 1940nm.

In this article, we present a tunable GaSb-based blazed grating external cavity laser (BG-ECL) with high spectral purity and high output power single-mode operation around 1940nm. The drastic increase in spectral selectivity and optical power results from the employment of a single-transverse-mode operating narrow ridge waveguide laser diode with an optimized AR coating on the front facet. The stable fundamental spatial mode output beam from the laser diode enables efficient collimation and high coupling efficiency with the blazed grating, leading to stronger wavelength-selective feedback. The AR coating with proper low reflectivity on the straight waveguide effectively suppresses the internal cavity mode lasing without causing extra optical loss. As a result, the BG-ECL device exhibits excellent comprehensive performance with a side mode suppression ratio (SMSR) over 50 dB with optical power exceeding 30 mW within a 70 nm tuning range. A maximum SMSR of 56.26 dB with 35.12 mW output power was observed in continuous-wave operation. By increasing the working temperature of the diode laser, the tuning range can be further extended to over 100 nm without noticeable degradation in spectral and output power performance.

Wavelength-Tunable, GaSb-Based, Cascaded Type-I Quantum-Well Laser Emitting Over a Range of 300 nm

We present a wavelength-tunable, external-cavity GaSb-based quantum-well laser operating near 3.2 $\mu \text{m}$ . The laser setup consists of an intra-cavity grating in Littman–Metcalf configuration and a cascade pumped GaSb-based gain chip with a narrow-ridge waveguide. The narrow-ridge waveguide has a length of 2 mm and width of 7.5 $\mu \text{m}$ . Cascade pumping is realized with three type-I quantum-wells, using one quantum-well per cascade stage. The laser provides continuous-wave output powers up to 8 mW and slope-efficiencies of 13% at room temperature. Laser operation is demonstrated over a wavelength range of more than 300 nm, using continuous-wave and pulsed operation regimes.

Single Mode 2 μm GaSb Based Laterally Coupled Distributed Feedback Quantum-Well Laser Diodes with Metal Grating**Supported by the National Basic Research Program of China under Grant No 2011CBA00608, and the Beijing Nature Science Foundation of China under Grant No 4112058.

Room-temperature operation of a GaSb based laterally coupled distributed feedback quantum-well laser diode emitting at 2 μm is demonstrated. The device exhibits single longitudinal mode characteristic as a result of the first order Cr-Bragg gratings alongside the narrow ridge waveguide. We design the laser structure to obtain a critical coupling condition corresponding to a coupling coefficient of 12 cm−1. For a 1-mm-long uncoated laser diode with a 3-μm-wide stripe, a single mode output spectrum with side mode suppression ratio as high as 28.5 dB is achieved, and the maximum single mode continuous-wave output power is about 11 mW at room temperature.

High-brightness single-mode double-tapered laser diodes with laterally coupled high-order surface grating.

Double-tapered laser diodes with 23rd order laterally coupled surface gratings are fabricated at 913nm. A narrow ridge waveguide restricts the lateral mode number, and tapered waveguides are utilized to enhance the output power. The gratings defined on the sides of ridge waveguide filter not only the longitudinal modes but also the high-order lateral modes. Experimentally, continuous wave power of over 560 mW/facet is achieved. The lateral far-field pattern exhibits a single-lobe profile and narrow divergence, and the lowest divergence angle is only twice the diffraction-limited one. The measured side-mode suppression ratio is about 27dB. This device has great potential for high-power single-mode near-diffraction-limited laser emission with easy fabrication.

Index antiguiding in narrow ridge-waveguide (In,Al)GaN-based laser diodes

The threshold current density of narrow (1.5 μm) ridge waveguide (In,Al)GaN based laser diodes is found to strongly depend on the ridge etch depth. By solving the complex-value two-dimensional waveguide equation, it is shown that, for shallow-ridge devices with a small built-in index step, the dependence of the modal gain on the material gain is strongly influenced by antiguiding effects. Taking into account the lateral extension of the material gain beyond the ridge as well as the optical mode absorption in the unpumped regions of the quantum wells, the observed differences in the threshold current density can be reproduced by simulations.

2 μm laterally coupled distributed-feedback GaSb-based metamorphic laser grown on a GaAs substrate

We report a type-I GaSb-based laterally coupled distributed-feedback (DFB) laser grown on a GaAs substrate operating continuous wave at room temperature. The laser structure was designed to operate near a wavelength of 2 μm and was grown metamorphically with solid-source molecular beam epitaxy. The device was fabricated using a 6th-order deep etch grating structure as part of the sidewalls of the narrow ridge waveguide. The DFB laser emits total output power of up to 40 mW in a single longitudinal mode operation at a heat-sink temperature of 20 °C.

InAs/GaAs submonolayer quantum-dot superluminescent diodes with active multimode interferometer configuration

With a chirped InAs/GaAs SML-QD (quantum dot) structure serving as the active region, the superluminescent diodes emitting at wavelength of around 970 nm are fabricated. By using an active multimode interferometer configuration, these devices exhibit high continue-wave output powers from the narrow ridge waveguides. At continue-wave injection current of 800 mA, an output power of 18.5 mW, and the single Gaussian-like emission spectrum centered at 972 nm with a full width at half maximum of 18 nm are obtained.

Single-mode 265 µm InGaAsSb/AlInGaAsSb laterally coupled distributed-feedback diode lasers for atmospheric gas detection

We demonstrate index-coupled distributed-feedback diode lasers at 2.65 µm that are capable of tuning across strong absorption lines of HDO and other isotopologues of H2O. The lasers employ InGaAsSb/AlInGaAsSb multi-quantum-well structures grown by molecular beam epitaxy on GaSb, and single-mode emission is generated using laterally coupled second-order Bragg gratings etched alongside narrow ridge waveguides. We verify near-critical coupling of the gratings by analyzing the modal characteristics of lasers of different length. With an emission facet anti-reflection coating, 2-mm-long lasers exhibit a typical current threshold of 150 mA at 20 °C and are capable of emitting more than 25 mW in a single longitudinal mode, which is significantly higher than the output power reported for loss-coupled distributed-feedback lasers operating at similar wavelengths.

Optical loss in bent-waveguide superluminescent diodes

The optical loss in the bent region is one of the key features for bent-waveguide superluminescent diodes which affects the device performance greatly under some conditions. In our work, this bend loss is investigated with different waveguide structural parameters such as the ridge width, etching depth and curvature radius. It is found that the bend loss is sensitive to the etching depth of the ridge. Significant loss will occur in a shallow and excessively wide or narrow ridge waveguide, and the waveguide with a small curvature radius can also enhance the loss. Effects of the loss on the device properties such as power and emission spectrum have also been investigated by using a quantum-dot device model. It is observed that as the bend loss increases, intersections will appear on the power–current curves obtained from both ends of the device, and this phenomenon is further studied with different cavity lengths and bend losses.

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.

The 3.0–3.2 µm wavelength range narrow ridge waveguide Sb-based semiconductor diode lasers operating up to 333 K

We have demonstrated Fabry–Perot single spatial mode antimonide-based type-I quantum-well ridge waveguide semiconductor diode lasers operating at 3.0–3.2 µm wavelength in continuous mode up to 333 K. Internal optical loss in narrow ridge devices was significantly reduced by using thick Si3N4 dielectric films for planarization. The fabricated lasers operate in CW mode at room temperature with output powers exceeding 5 mW and have power consumption of less than 0.2 W at the output power of 1 mW, which is the power level needed in many gas sensing applications.

Single-mode 2.4 μm InGaAsSb/AlGaAsSb distributed feedback lasers for gas sensing

Single-mode laser diodes on GaSb substrates were developed using InGaAsSb/AlGaAsSb triple quantum well active regions grown by molecular beam epitaxy. The devices were fabricated using lateral Cr gratings, with a grating pitch designed to coincide with a strong absorption feature of HF gas, deposited adjacent to a dry-etched narrow ridge waveguide. High sidemode suppression was achieved, and in 20 °C continuous-wave operation, devices with a 400 μm long cavity provided 9 mW total output power at the 2396 nm target wavelength. High-resolution direct absorption measurements of HF gas agreed with HiTran calculations, yielding an absorption linewidth of 0.030 nm.

Effects of Lateral Diffusion on the Temperature Sensitivity of the Threshold Current for 1.3-$\mu{\hbox {m}}$ Double Quantum-Well GaInNAs–GaAs Lasers

We present an experimental and theoretical investigation of the temperature dependence of the threshold current for double quantum well GaInNAs-GaAs lasers in the temperature range 10 degC-110 degC. Pulsed measurements of the threshold current have been performed on broad and narrow ridge wave guide (RWG) lasers. The narrow RWG lasers exhibit high characteristic temperatures (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) of 200 K up to a critical temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ), above which T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is reduced by approximately a factor of 2. The T0-values for broad RWG lasers are significantly lower than those for the narrow RWG lasers, with characteristic temperatures on the order of 100 (60) K below (above) T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . Numerical simulations, using a model that accounts for lateral diffusion effects, show good agreement with experimental data and reveal that a weakly temperature dependent lateral diffusion current dominates the threshold current for narrow RWG lasers.

Thermal lensing in high-power ridge-waveguide lasers

The lateral farfield characteristics of ridge-waveguide (RW) lasers emitting around 1,064 nm with an output power of 1.3 W at an injection current of 2 A were measured and modeled. Due to trenches with widths between 2.5 and 20 μm defining the narrow RW, radiation leaks out into the outer regions which suppresses higher-order modes, but which can also lead to a double-peaked lateral farfield of the fundamental mode. A simple model taking into account the thermal lensing effect explains well the lateral farfield behavior observed.

1550 nm GaInNAsSb distributed feedback laser diodes on GaAs

Singlemode laser diodes on GaAs substrates were developed using GaInNAsSb double quantum well active regions grown by molecular beam epitaxy. The distributed feedback devices were fabricated using a regrowth-free process in which lateral Cr gratings were deposited adjacent to a dry-etched narrow ridge waveguide. In continuous-wave (CW) operation the devices exhibit a lasing wavelength of 1550 nm at 10°C with a very high sidemode suppression ratio of &gt;50 dB throughout most of the operational range, and output power up to 15 mW.

Narrow ridge waveguide high power single mode 1.3-μm InAs/InGaAs ten-layer quantum dot lasers

Ten-layer InAs/In0.15Ga0.85As quantum dot (QD) laser structures have been grown using molecular beam epitaxy (MBE) on GaAs (001) substrate. Using the pulsed anodic oxidation technique, narrow (2 μm) ridge waveguide (RWG) InAs QD lasers have been fabricated. Under continuous wave operation, the InAs QD laser (2 × 2,000 μm2) delivered total output power of up to 272.6 mW at 10 °C at 1.3 μm. Under pulsed operation, where the device heating is greatly minimized, the InAs QD laser (2 × 2,000 μm2) delivered extremely high output power (both facets) of up to 1.22 W at 20 °C, at high external differential quantum efficiency of 96%. Far field pattern measurement of the 2-μm RWG InAs QD lasers showed single lateral mode operation.

Optical gain, loss, and transparency current in high performance mid-infrared interband cascade lasers

The net modal gain, optical loss, and transparency current of high-performance, narrow ridge waveguide interband cascade (IC) lasers have been measured using the Hakki–Paoli technique in the temperature range from T=78 to 270 K. In this temperature range, the optical loss of IC lasers increases from αw≈17 cm−1 at T=78 K to αw≈35 at T=270 K, the transparency current density rises from Itr=10 to 330 A∕cm2, and the differential gain decreases from gd≈2.2 cm∕A to gd≈0.06 cm∕A with a characteristic temperature of T0=130 K. The implications of these observed characteristics for IC lasers are discussed.