GaN Laser Diodes Research Articles

Output characteristics of Pr:YAlO3 and Pr:YAG lasers pumped by high-power GaN laser diodes.

We investigated whether Pr:YAlO3 and Pr:Y3Al5O12 (YAG) can work as gain media for high-power visible lasers and replace trivalent praseodymium (Pr)-doped fluoride crystals, with particular focus on thermal loading resistivity. Pr:YAlO3 exhibits a high laser gain at 747 nm, and we obtained a maximum output power of 1.2 W and a slope efficiency of 26.7% with high-power GaN laser diode pumping. Excited state absorption and large phonon energy hinder laser oscillation of Pr:YAG at room temperature. We obtained 616 nm laser oscillation of Pr:YAG at 40 K. Furthermore, we achieved a visible laser with Pr:YAG ceramics for the first time. The maximum output power is ∼30mW with a slope efficiency of ∼0.7%.

Spectral-temporal dynamics of (Al,In)GaN laser diodes.

(Al,In)GaN laser diodes have various relevant applications, especially in projection systems for virtual and augmented reality devices and in optical communication, all requiring fast modulation. This corresponds to pulses in the nanosecond to microsecond range, where a rich longitudinal mode dynamics occurs. We investigate this spectral-temporal dynamics experimentally with a streak camera system and simulate it using a longitudinal multi-mode rate equation model. We observe an interplay of effects, which have been observed selectively, such as relaxation oscillations, mode competition and inhomogeneous pumping of multiple quantum wells. A mechanism is included in the simulations to model the red-shift of the gain spectrum due to the carrier density in the quantum wells exceeding threshold density, which is amplified by inhomogeneous pumping. Mode competition leads to spectral cycles of the active mode with a noticeable jitter, which is observed in single pulse measurements in comparison to multi pulse averaged measurements where blurring occurs. Here, some statistical behavior as well as repeating patterns are investigated at the same effect. Also thermal effects as laser diode self-heating are discussed and have been measured over six orders of magnitude in time.

High-efficiency, yellow-light Dy3+-doped fiber laser with wavelength tuning from 568.7 to 581.9 nm.

We report, to the best of our knowledge, the first demonstration of a wavelength-tunable and highly efficient Dy3+-doped fiber laser operating in the yellow spectral region. A 2-m-long Dy3+:ZBLAN fiber pumped by a 447-nm GaN laser diode provides a strong down-conversion gain around 575nm. A fiber end-facet mirror and a visible reflective grating in the Littrow configuration construct the resonant cavity and introduce the wavelength tunability. A stable yellow laser with a <0.05-nm narrow linewidth is achieved and continuously tuned from 568.7nm to 581.9nm, covering more than half of the yellow spectral range. The slope efficiency is as high as 34.9%, and the maximum output power is 142mW at 576.44nm, which is 13 times higher than previously reported. It is, to the best of our knowledge, the highest power and conversion efficiency of a yellow-light Dy3+-doped fiber laser with wavelength tunability.

Performance Improvement of GaN Based Laser Diode Using Pd/Ni/Au Metallization Ohmic Contact

We report an investigation of the effects of different metal systems and surface treatment on the contact performance of GaN lasers. We found that multi-element metal alloy and surface chemical treatment are the keys to achieve good ohmic behavior contacts on GaN laser diodes. Pd/Ni/Au contact demonstrates excellent thermal stability and lowest specific contact resistivity in these metal systems. Properly adjusting the thickness of the Pd and Ni layer and pretreating with the KOH solution can further improve the ohmic contact performance. The improved ohmic behavior of the KOH solution pretreated Pd/Ni/Au contact is attributed to removing surface oxides and the reduction of the schottky barrier heights due to the metal Pd has a high work function and the interfacial reactions occurring between the Pd, Ni, Au, and GaN extends into the GaN film. As a result, a low contact resistivity of 1.66 × 10−5 Ω·cm2 can be achieved from Pd(10 nm)/Ni(10 nm)/Au(30 nm) contacts with KOH solution pretreated on top of the laser diode structure. The power of the GaN based laser diode with the Pd/Ni/Au metallization ohmic contact can be enhanced by 1.95 times and the threshold current decreased by 37% compared to that of the conventional ohmic contact Ni/Au.

Screening of quantum-confined Stark effect in nitride laser diodes and superluminescent diodes

In the present work we report on the observation of carrier-induced screening of built-in electric fields in (Al, In)GaN laser diodes and superluminescent diodes. We use the emission peak energy as a measure of the quantum-confined Stark effect and its screening by free carriers. For superluminescent diodes we observe a steady increase of screening up to the current density of 10 kA cm−2. This shows that the lasing in nitride laser diodes occurs under high electric fields, far from the flat band conditions.

(Invited) the Progress and Outlook of GaN Laser Devices

GaN laser diodes are considered the next frontier for high performance GaN optoelectronics. Currently, GaN edge-emitting laser diodes are finding specific applications in optical information storage, in large-area projection, and in automobile headlights. It is widely anticipated that further advances in GaN laser diodes could enable a much broader range of applications in lighting and display. The purpose of this presentation is to provide a high-level analysis of the state of two types of GaN laser diodes; one is the edge-emitting laser (EEL) and the other is the vertical cavity surface emitting laser (VCSEL). Commercial GaN EEL has been available for more than 15 years but the performance (wall-plug efficiency) is limited to ~35%[1]. For GaN VCSELs, in spite of various laboratory demonstrations in the past 10 years, there has not been any commercial product as a result of complexity involved in the growth and fabrication. For the discussion of GaN EELs we will analyze the factors that define and limit the contemporary GaN EELs. New approaches such as using non-polar/semipolar planes [2], quantum-dot active regions [3], higher power operation [4], and our unique approach of engineering the optical cavity confinement [5] will be discussed. Concerning GaN VCSELs, the technological bottleneck has been the formation of distributed Bragg reflectors (DBRs) for a planar microcavity. We will review the pros and cons of several approaches including epitaxial DBRs (AlGaN [6] or AlInN [7]), dielectric DBRs involving layer separation and transfer [8]–[10], and later overgrowth on dielectric DBRs [11]. Special emphasis will be given to our effort in using a nanoporous medium to form an epitaxial, conductive, and highly reflective DBRs for VCSELs [12], [13]. This work is supported the National Science Foundation (NSF) under Award ECCS-1709149 and by DARPA MTO of the United States. The facilities used were supported by the Yale SEAS cleanroom, YINQE, and NSF MRSEC DMR-1119826.

Scheimpflug Lidar for combustion diagnostics.

A portable Lidar system developed for large-scale (~1-20 m) combustion diagnostics is described and demonstrated. The system is able to perform remote backscattering measurements with range and temporal resolution. The range resolution is obtained by sharply imaging a part of the laser beam onto a CMOS-array or ICCD detector. The large focal depth required to do this is attained by placing the laser beam, the collection optics and the detector in a so-called Scheimpflug configuration. Results from simulations of the range capabilities and range resolution of the system are presented and its temporal resolution is also discussed. Various applications, important for combustion diagnostics, are also demonstrated, including Rayleigh scattering thermometry, aerosol detection and laser-induced fluorescence measurements. These measurements have been carried out using various continuous-wave GaN diode lasers, emitting in the violet-blue (405 - 450 nm) wavelength regime. It is anticipated that Scheimpflug Lidar will provide a useful and versatile diagnostic tool for combustion research, not only for fundamental studies, but in particular for applications at industrial sites.

Dynamics of a green high-power tunable external-cavity broad-area GaN diode laser

Although external-cavity feedback with a grating is widely used to achieve a tunable high-power narrow-linewidth broad-area diode laser (BAL) system, the dynamics of such a system is seldom studied. In this paper, the temporal dynamics of a tunable high-power green external-cavity diode laser system based on a GaN BAL and Littrow external-cavity is investigated experimentally. The regular pulse package oscillation (PPO) is observed just above the threshold. The oscillating period of the pulse package decreases with the increasing injected current. As the current increases further, the pulse package oscillates irregularly, and finally changes to a chaotic state. The PPO is observed, for the first time to our knowledge, in a BAL with an external-cavity grating feedback.

Compact Deep UV System at 222.5 nm Based on Frequency Doubling of GaN Laser Diode Emission

Laser light sources emitting in the deep ultraviolet wavelength range between 210 and 230 nm are of great interest for spectroscopic applications. Here, a compact DUV diode laser system emitting at a wavelength of 222.5 nm is presented. The system is based on frequency doubling of the laser radiation from a micro-integrated GaN external cavity diode laser module (μECDL) emitting at 445 nm. The μECDL has an optical pump power of 1.4 W with an emission bandwidth of 35 pm. Narrowband laser radiation in continouos wave operation with an output power of 160 μW at 222.5 nm is generated in a single-pass frequency doubling stage with a β-BaB 2 O 4 crystal. The results are suitable to address applications such as spectroscopic investigations of biological samples. The presented concept of a compact and efficient deep ultraviolet laser light source enables the realization of portable systems for which a small footprint and a low power consumption is essential.

Visible laser emission from a praseodymium-doped fluorozirconate guided-wave chip.

We report visible continuous-wave laser emission at 636 nm from a praseodymium-doped fluorozirconate glass guided-wave chip laser. This ultra-fast laser inscribed gain chip is demonstrated to be a compact and integrated laser module. The laser module, pumped by 442 nm GaN laser diodes, generates >8 mW lasing output with a beam quality of Mxy2∼1.15×1.1(±0.1). To the best of our knowledge, this is the first visible laser emission from a glass-based waveguide chip laser.

Picosecond tunable gain-switched blue pulses from GaN laser diodes with nanosecond current injections.

We investigated the gain-switching properties of GaN-based ridge-waveguide lasers on free-standing GaN substrates with low-cost nanosecond current injection. It was observed that the output pulses with intense injection consisted of an isolated short pulse with a duration of around 50 ps at the high-energy side and a long steady-state component at the lower energy side independent of the electric pulse duration. The energy separation between the short pulse and steady-state component can be over 30 meV, favoring short-pulse generation with the spectral filtering technique. The duration of the steady-state component can be tuned freely by controlling the duration and voltage of the electric pulse, which is very useful for generating pulse-width-tunable optical pulses for various applications.

High-speed maskless nanolithography with visible light based on photothermal localization

High-speed maskless nanolithography is experimentally achieved on AgInSbTe thin films. The lithography was carried out in air at room temperature, with a GaN diode laser (λ = 405 nm), and on a large sample disk of diameter 120 mm. The normal width of the written features measures 46 ± 5 nm, about 1/12 of the diffraction allowed smallest light spot, and the lithography speed reaches 6 ~ 8 m/s, tens of times faster than traditional laser writing methods. The writing resolution is instantaneously tunable by adjusting the laser power. The reason behind the significant breakthrough in terms of writing resolution and speed is found as the concentration of light induced heat. Therefore, the heat spot is far smaller than the light spot, so does the size of the written features. Such a sharp focus of heat occurs only on the selected writing material, and the phenomenon is referred as the photothermal localization response. The physics behind the effect is explained and supported with numerical simulations.

Blue Laser Diode Enables Underwater Communication at 12.4\u2009Gbps

To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (LD) directly modulated by pre-leveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of −0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m.

Performance Improvement of GaN-Based Violet Laser Diodes**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0401801, the National Natural Science Foundation of China under Grant Nos 61574135, 61574134, 61474142, 61474110, 61377020, 61376089, and 61223005, and the One Hundred Person Project of the Chinese Academy of Sciences.

The influences of InGaN/GaN multiple quantum wells (MQWs) and AlGaN electron-blocking layers (EBL) on the performance of GaN-based violet laser diodes are investigated. Compared with the InGaN/GaN MQWs grown at two different temperatures, the same-temperature growth of InGaN well and GaN barrier layers has a positive effect on the threshold current and slope efficiency of laser diodes, indicating that the quality of MQWs is improved. In addition, the performance of GaN laser diodes could be further improved by increasing Al content in the AlGaN EBL due to the fact that the electron leakage current could be reduced by properly increasing the barrier height of AlGaN EBL. The violet laser diode with a peak output power of 20 W is obtained.

Green high-power tunable external-cavity GaN diode laser at 515 nm.

A 480 mW green tunable diode laser system is demonstrated for the first time to our knowledge. The laser system is based on a GaN broad-area diode laser and Littrow external-cavity feedback. The green laser system is operated in two modes by switching the polarization direction of the laser beam incident on the grating. When the laser beam is p-polarized, an output power of 50 mW with a tunable range of 9.2 nm is achieved. When the laser beam is s-polarized, an output power of 480 mW with a tunable range of 2.1 nm is obtained. This constitutes the highest output power from a tunable green diode laser system.

Wavelength-scanning calibration of detection efficiency of single photon detectors by direct comparison with a photodiode

We present a practical calibration method of the detection efficiency (DE) of single photon detectors (SPDs) in a wide wavelength range from 480 nm to 840 nm. The setup consists of a GaN laser diode emitting a broadband luminescence, a tunable bandpass filter, a beam splitter, and a switched integrating amplifier which can measure the photocurrent down to the 100 fA level. The SPD under test with a fibre-coupled beam input is directly compared with a reference photodiode without using any calibrated attenuator. The relative standard uncertainty of the DE of the SPD is evaluated to be from 0.8% to 2.2% varying with wavelength (k = 1).

Tuning range and output power optimization of an external-cavity GaN diode laser at 455 nm.

In this paper we discuss how different feedback gratings affect the tuning range and the output power of external feedback diode laser systems. A tunable high-power narrow-spectrum external-cavity diode laser system around 455 nm is investigated. The laser system is based on a high-power GaN diode laser in a Littrow external-cavity. Both a holographic diffraction grating and a ruled diffraction grating are used as feedback elements in the external cavity. The output power, spectral bandwidth, and tunable range of the external cavity diode laser system are measured and compared with the two gratings at different injected currents. When the holographic grating is used, the laser system can be tuned over a range of 1.4 nm with an output power around 530 mW. When the ruled grating is used, the laser system can be tuned over a range of 6.0 nm with an output power around 80 mW. The results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements.

4 Gbps direct modulation of 450 nm GaN laser for high-speed visible light communication.

We demonstrate high-speed data transmission with a commercial high power GaN laser diode at 450 nm. 2.6 GHz bandwidth was achieved at an injection current of 500 mA using a high-speed visible light communication setup. Record high 4 Gbps free-space data transmission rate was achieved at room temperature.

Investigation of spectroscopic properties and energy transfer between Ce and Dy in (Lu0.2Gd0.8−x−yCexDyy)2SiO5 single crystals

In this paper we present results of spectroscopic investigations of single crystals with general formula (Lu0.2Gd0.8−x−y)2SiO5 codoped with x% of Ce3+ and y% of Dy3+ ions. Investigated materials exhibit strong optical anisotropy what can be easily observed in polarized absorption and emission spectra. Based on room temperature polarized absorption spectra calculations in framework of phenomenological Judd–Ofelt model was carried out. Intensity parameters Ωt were evaluated to be Ω2=7.08 (±0.39), Ω4=2.76 (±0.44), and Ω6=3.36 (±0.21) [10−20cm2] for sample doped with 1% of cerium and Ω2=10.72 (±0.33), Ω4=1.98 (±0.37), and Ω6=2.11 (±0.18) [10−20cm2] for sample doped with 3% of cerium. Influence of cerium admixture on Judd Ofelt intensity parameters is discussed. Value of experimental lifetime of 4F9/2 multiplet of Dy3+ ion in sample doped with 1at% Ce is 0.5ms (τrad=0.45ms), while for sample doped with 3at% of Ce, experimental lifetime is 0.45ms (τrad=0.43ms). Absorption bands located between 440 and 460nm, can be utilized for optical pumping of material by GaN laser diodes. Intense and broad emission bands at 465–495 and 560–590nm, with experimental branching ratio strongly depending on polarization, give high chance for obtaining white luminophore, due to appropiate mixing of blue and yellow luminescence. By means of a pump and probe experiment optical amplification was demonstrated in the codoped sample with 1at% of Ce and 1at% Dy at 575nm corresponding to the emission of Dy3+ with a high net gain coefficient of 34cm−1. Such high amplification was obtained under 359nm excitation (at the maximum of intense absorption band of Ce3+ ions).

Single-pass UV generation at 222.5 nm based on high-power GaN external cavity diode laser.

We demonstrate a compact system for single-pass frequency doubling of high-power GaN diode laser radiation. The deep UV laser light at 222.5 nm is generated in a β-BaB2O4 (BBO) crystal. A high-power GaN external cavity diode laser (ECDL) system in Littrow configuration with narrowband emission at 445 nm is used as pump source. At a pump power of 680 mW, a maximum UV power of 16 μW in continuous-wave operation at 222.5 nm is achieved. This concept enables a compact diode laser-based system emitting in the deep ultraviolet spectral range.