Ridge Waveguide Lasers Research Articles

Lateral grating DFB AlGaInN laser diodes for optical communications and atomic clocks.

AlGaInN laser diode technology is of considerable interest for telecom applications and next generation atomic optical clocks based on Sr (by using 422nm & 461nm) and Rb at 420.2nm.Very narrow linewidths (<1MHz) are required for such applications. We report lateral gratings on AlGaInN ridge waveguide laser diodes to achieve a single wavelength device with a good side mode suppression ratio (SMSR) that is suitable for atomic clock and telecom applications.

Laterally coupled distributed feedback lasers emitting at 2 μm with quantum dash active region and high-duty-cycle etched semiconductor gratings

Single-mode diode lasers on an InP(001) substrate have been developed using InAs/In0.53Ga0.47As quantum dash (Qdash) active regions and etched lateral Bragg gratings. The lasers have been designed to operate at wavelengths near 2 μm and exhibit a threshold current of 65 mA for a 600 μm long cavity, and a room temperature continuous wave output power per facet &amp;gt;5 mW. Using our novel growth approach based on the low ternary In0.53Ga0.47As barriers, we also demonstrate ridge-waveguide lasers emitting up to 2.1 μm and underline the possibilities for further pushing the emission wavelength out towards longer wavelengths with this material system. By introducing experimentally the concept of high-duty-cycle lateral Bragg gratings, a side mode suppression ratio of &amp;gt;37 dB has been achieved, owing to an appreciably increased grating coupling coefficient of κ ∼ 40 cm−1. These laterally coupled distributed feedback (LC-DFB) lasers combine the advantage of high and well-controlled coupling coefficients achieved in conventional DFB lasers, with the regrowth-free fabrication process of lateral gratings, and exhibit substantially lower optical losses compared to the conventional metal-based LC-DFB lasers.

Astigmatism-free high-brightness 1060 nm edge-emitting lasers with narrow circular beam profile.

1060 nm high-brightness vertical broad-area edge-emitting lasers providing anastigmatic high optical power into a narrow circular beam profile are demonstrated. Ridge-waveguide (RW) lasers yield record 2.2 W single-transverse mode power in the 1060-nm wavelength range under continuous-wave (cw) operation at room temperature with excellent beam quality factor M2 ≤ 2. Independent of operating current the astigmatism is only 2.5 µm. 3 mm long broad-area (BA) lasers produce a θvert as narrow as 9° full width at half maximum, which agrees well with our simulation results, being insensitive to drive current. 5 mm long BA lasers deliver highest ever reported cw 12 W multimode output power among lasers showing θvert <10° in the 1060-nm wavelength range. The emitted laser beams from both RW and BA lasers show a perfect circular shape with ≤10° divergence angle at record 2.1 W and 4.2 W cw-mode output power, respectively.

Increased diffraction efficiencies of DBR gratings in diode lasers with adiabatic ridge waveguides

The influence of the lateral layout on the diffraction efficiency of gratings in DBR lasers is presented. In this experimental study DBR ridge waveguide (RW) lasers with different ridge widths as well as straight and adiabatic waveguides are compared. The lasers are based on a vertical layer structure with an asymmetric super large optical cavity. DBR gratings of 3rd and 7th order are manufactured using electron beam lithography and dry etching. Their diffraction efficiencies are determined by measuring the optical output power emitted through the rear and front facets of unmounted devices. In comparison to a laser with a 30 μm ridge, the DBR diffraction efficiency in a laser with a 4 μm ridge is reduced by 46 percentage points. Implementing an adiabatic widening of the ridge width increases the diffraction efficiency from 35% to 72%. The latter is close to 81% achieved for the laser with 30 μm ridge. The new layout with enhanced DBR diffraction efficiency increases the optical output power of the narrow RW laser by a factor of 1.3. Similar results are obtained with 7th order gratings. All devices provide single-mode emission with spectral widths of 30 pm and side mode suppressions >60 dΒ. According to these results, implementing adiabatic waveguides is beneficial in terms of diffraction efficiency and performance of narrow RW lasers based on the applied vertical structure.

Temperature-Insensitive High-Speed Directly Modulated 1.55-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\mu \text{m}$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Quantum Dot Lasers

Modulation properties and temperature stability of short cavity ridge waveguide lasers based on high-quality InAs quantum dots exhibiting a total modal gain of ~90 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are reported. The 338-μm-long lasers show a threshold current of 20 mA at room temperature and an output powers of up to 36 mW. A maximum small signal modulation bandwidth of 15 GHz was obtained at 14 °C, which degrades to 13 GHz at 60 °C and 8 GHz at 80 °C. Digital modulation at 25 Gb/s between 15 °C and 50 °C was obtained with clear open eyes under constant drive conditions (dc and ac). The maximum data rates of 32 and 35 Gb/s were obtained for 338- and 230-μm-long lasers, respectively, at 14 °C.

Ridge waveguide laser in Nd:LiNbO3 by Zn-diffusion and femtosecond-laser structuring

Ridge waveguide lasers have been fabricated on Nd3+ doped LiNbO3 crystals. The fs-laser writing technique was used to define ridge structures on a gradient-index planar waveguide fabricated by Zn-diffusion. This planar waveguide was formed in a z-cut LiNbO3 substrate homogeneously doped with a 0.23% of Nd3+ ions. To obtain lateral light confinement, the surface was then micromachined using a multiplexed femtosecond laser writing beam, forming the ridge structures. By butting two mirrors at the channel waveguide end-facets, forming a waveguide laser cavity, TM-polarized laser action at 1085 nm was achieved by end-fire TM-pumping at 815 nm. The waveguide laser shows a threshold of 31 mW, with a 7% of slope efficiency.

Miniaturised master‐oscillator power‐amplifier emitting a single longitudinal mode with more than 300 mW at 647 nm

A miniaturised red-emitting master-oscillator power-amplifier (MOPA) was developed. The MOPA emits a single longitudinal mode with a linewidth below 10 MHz and an optical output power of more than 300 mW at a wavelength of about 647 nm. The master-oscillator is a distributed-Bragg-reflector ridge-waveguide laser which provides the single mode with an output power of about 40 mW. The light is shaped by micro-lenses and guided through a miniaturised CdMnTe-Faraday isolator as well as a truncated tapered power amplifier; it is reshaped again afterwards to produce an almost circular, nearly diffraction-limited beam. The total length of the MOPA is below 50 mm, which fitted into a sealed butterfly package with a footprint of about 20 cm2.

Generation of second harmonic light with a wavelength of 560 nm in a compact module

We demonstrate a continuous wave 133mW laser module at 560.5nm on a 50mm·10mm optical bench. The setup consists of a 1121nm distributed Bragg reflector ridge waveguide laser and a MgO:LiNbO3 quasi-phase matched ridge waveguide crystal, which are coupled by a grin lens, as well as two cylindrical lenses for beam collimation behind the crystal. A novel approach to ensure phase matching is used. The laser and the crystal are stabilized by the same heat sink and only the wavelength of the laser is tuned by heating the distributed Bragg reflector section of the laser. This reduces the influence of temperature variations on the module's performance enabling operation with output power variations <10% over a temperature range of 20K. The size and robustness against temperature variations of this setup make it an interesting candidate for future biomedical applications.

AlGaInN laser diode technology for GHz high-speed visible light communication through plastic optical fiber and water

AlGaInN ridge waveguide laser diodes are fabricated to achieve single-mode operation with optical powers up to 100 mW at ∼420 nm for visible free-space, underwater, and plastic optical fiber communication. We report high-frequency operation of AlGaInN laser diodes with data transmission up to 2.5 GHz for free-space and underwater communication and up to 1.38 GHz through 10 m of plastic optical fiber.

Watt-level red-emitting diode lasers and modules for display applications

Red-emitting lasers for display applications require high output powers and a high visibility. We demonstrate diode lasers and modules in the red spectral range based on AlGaInP with optical output powers up to 1 W and a nearly diffraction limited beam. These high-luminance light sources based on tapered lasers are well suited for laser TVs and projectors for virtual reality simulators based on the flying spot technology. Additionally, we developed diode lasers with internal distributed Bragg reflector (DBR) surface gratings. These DBR tapered lasers and master-oscillator power-amplifiers based on DBR ridge-waveguide lasers and tapered amplifiers feature high power, single mode emission with coherence lengths up to several meters, which are suitable for the next-generation 3D displays based on holography.

Low-threshold-current yellow BeZnCdSe quantum-well ridge-waveguide laser diodes under continuous-wave room-temperature operation

Low-threshold-current yellow BeZnCdSe single-quantum-well (SQW) laser diodes (LDs) have been developed by using a ridge-waveguide structure. The top p-cladding layer was etched to suppress the leakage current that flowed laterally outside of the electrode. Ridge waveguides were covered with a SiO2 layer and planarized by chemical–mechanical polishing and reactive ion etching. Room-temperature lasing under continuous-wave condition was achieved with the laser cavity formed by cleaved waveguide facets coated with high-reflectivity dielectric films. Two types of LDs with different SQW thicknesses and Cd contents were developed and compared at various waveguide widths and lengths. Yellow LDs with sub-10 mA threshold current were obtained.

Fabrication and analysis of highly-reflective metal-dielectric mirrors for high-performance semiconductor laser applications

The device performance improvement of ridge waveguide (RWG) laser diodes (LDs), operating at a wavelength (λ) of ∼960 nm, with a metal-dielectric high-reflection (HR) mirror consisting of Au/Ti–SiO2 layers on the back facet was demonstrated. To determine the optimum thickness of each layer, optical reflection calculations were performed using a rigorous coupled-wave analysis method, which leads to the resultant Au (80 nm)/Ti (5 nm)–SiO2 (164 nm) layers. The layers exhibited a broad high reflection band of >91% over a wavelength range of 920–1000 nm, indicating the reflectivity of ∼91.2% at λ ∼ 960 nm. For 2 mm-cavity RWG LDs with the Au/Ti–SiO2 HR mirror, an enhanced maximum output power (Pmax) of 499.3 mW at an injection current of 3000 mA and a decreased threshold current (Ith) of 516 mA (i.e., Pmax = 259.4 mW and Ith = 650 mA for the uncoated LDs) were obtained, showing an increased slope efficiency percentage of 82%. The external differential quantum efficiency was also increased from ∼17.1 to ∼31.1%. Also, the full widths at half maximum values of beam divergences of the device were 38° (vertical direction) and 4° (horizontal direction).

Traveling Wave Analysis for a High-Order Grating, Partially Slotted Laser

We analyze the spectral and spatial characteristics of a partially slotted, ridge-waveguide semiconductor laser utilized for single mode operation and suitable for integration with additional optical devices. In particular, we extract the photon distribution and linewidth behavior of a high-order (37th) grating via the finite-difference time-domain method and show good agreement with the experimental data. The reflection and transmission coefficients are initially calculated via the scattering matrix method and implemented within the traveling wave equations for a current-injected quantum well laser. In particular, the radiative loss of photons and coupling factor is estimated using the reflection coefficient with results demonstrating the state-of-the-art capabilities for this laser to meet industrial requirements for linewidths under 500 kHz and a side-mode suppression ratio over 45 dB.

High-Power Distributed Feedback Lasers With Surface Gratings: Theory and Experiment

Semiconductor lasers with integrated surface gratings are known to operate with narrow spectra as well as high power and efficiency. In this paper, we present a theoretical description of DFB lasers with 80th-order waveguide gratings, fabricated by etching narrow grooves into the epitaxial layer structure. The passive gratings are simulated by mode expansion and S-matrix algorithm yielding the reflection and transmission matrices in dependence on wavelength for given parameters of the grating, such as residual layer thickness and width of the grooves. The lasing condition is solved by constructing a round-trip operator and singular-value deposition for the calculation of lasing wavelength and threshold gain. A fast simulation of DFB lasers is performed within the framework of coupled-wave theory using adapted coupling coefficients. GaAs based ridge-waveguide and narrow-stripe broad-area lasers with integrated surface gratings are experimentally shown to emit output powers of 0.6 W into a single spectral line ( $\Delta \lambda _{{\rm FWHM}} pm) and $>$ 5 W into a spectrum with width $\Delta \lambda _{95\%}\approx 0.8$ nm, respectively.

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.

BeZnCdSe quantum-well ridge-waveguide laser diodes under low threshold room-temperature continuous-wave operation

Low threshold current ridge-waveguide BeZnCdSe quantum-well laser diodes (LDs) have been developed by completely etching away the top p-type BeMgZnSe/ZnSe:N short-period superlattice cladding layer, which can suppress the leakage current that flows laterally outside of the electrode. The waveguide LDs are covered with a thick SiO2 layer and planarized with chemical-mechanical polishing and a reactive ion etching process. Room-temperature lasing under continuous-wave condition is achieved with the laser cavity formed by the cleaved waveguide facets coated with high-reflectivity dielectric films. For a 4 μm-wide green LD lasing around a wavelength of 535 nm, threshold current and voltage of 7.07 mA and 7.89 V are achieved for a cavity length of 300 μm, and the internal differential quantum efficiency, internal absorption loss, gain constant, and nominal transparency current density are estimated to be 27%, 4.09 cm−1, 29.92 (cm × μm)/kA and 6.35 kA/(cm2 × μm), respectively. This compact device can realize a significantly improved performance with much lower threshold power consumption, which would benefit the potential application for ZnSe-based green LDs as light sources in full-color display and projector devices installed in consumer products such as pocket projectors.

Compact Bragg Grating Stabilized Ridge Waveguide Laser Module With a Power of 380 mW at 780 nm

A single ridge waveguide laser wavelength stabilized by a compact external Bragg grating cavity with an output power as much as 380 mW and a beam quality factor M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of 1.3 is presented. A characterization concerning tunability by current, short-term optical linewidth, and intensity fluctuations is performed. The device exhibits a wavelength tuning factor at 780 nm of 0.5 pm/mA, a short-term optical linewidth of 18 kHz, and a low-frequency relative intensity noise of -136 dB/Hz. These characteristics allow a high resolution absorption spectroscopy of the D2 line of rubidium.

Fabrication of ridge waveguide of 808 nm GaAs-based laser diodes by wet chemical etching**Project supported by the National Natural Science Foundation of China (Nos. 61474110, 61377020, 61376089, 61223005, 61176126) and the National Science Fund for Distinguished Young Scholars (No. 60925017).

The fabrication of ridge waveguide of 808 nm GaAs-based laser diodes by wet chemical etching is investigated. The etching behavior of GaAs, InGaP and AlGaInP in various solutions is evaluated. As a result, the etching solutions simultaneously corroding InGaP and AlGaInP layers are searched successfully. Effects of etching time and the concentration of mixtures on etching depth and the geometrical shape of ridge are analyzed. It is found that under proper conditions, appropriate etching depth and smooth surfaces can be obtained and the steep degree of pattern can be accepted, especially for wide ridge waveguide laser diodes.

Optimization of High Power 1.55-μm Single Lateral Mode Fabry—Perot Ridge Waveguide Lasers**Supported by the National Natural Science Foundation of China under Grant Nos 61274046 and 61474111, and the National Basic Research Program of China under Grant No 2013AA014202.

Optimization of the high power single-lateral-mode double-trench ridge waveguide semiconductor laser based on InGaAsP/InP quantum-well heterostructures with a separate confinement layer is reported. Two different waveguide structures of Fabry—Perot lasers emitting at a wavelength of 1.55 μm are fabricated. The influence of an effective lateral refractive index step on the maximum output power is investigated. A cw single mode output power of 165 mW is obtained for a 1-mm-long uncoated laser.

Comparison of two concepts for dual-wavelength DBR ridge waveguide diode lasers at 785 nm suitable for shifted excitation Raman difference spectroscopy

Two concepts of dual-wavelength 785-nm DBR ridge waveguide (RW) lasers, i.e. RW mini-arrays consisting of two DBR-RW lasers and Y-branch DBR-RW lasers, will be compared with respect to their usability as excitation light sources for shifted excitation Raman difference spectroscopy (SERDS). For both types of devices for each wavelength, output powers up to 215 mW were measured. A stable spectral distance between the laser emissions of the two resonator branches with the targeted value of 0.6 nm, i.e. 10 cm−1, is observed. In the case of the mini-array up to an output power of about 70 mW, the device shows single-mode operation. Although at higher power levels, mode hops and multi-mode operation occur, the emission width smaller than 0.15 nm still meets the requirements for Raman measurements of solids and liquids. Over the whole working range, the spectral distance between the two wavelengths is approximately constant with 0.62 nm. The near field shows two emission spots according to the dimension of the RW and their processed distance of 20 µm. The Y-branch laser shows single-mode operation up to 150 mW with a narrow spectral emission width. At higher powers also, multi-mode operation with an emission width of 0.15 nm occurs. The nearly diffraction-limited emission comes from one output aperture; the far-field emission shows a pronounced asymmetry between the two branches. Both types of devices fulfil the spectral requirements from Raman spectroscopy and SERDS up to 215 mW output power.