GaN based ultraviolet laser diodes

In the past few years,many groups have focused on the research and development of GaN-based ultraviolet laser diodes(UV LDs).Great progresses have been achieved even though many challenges exist.In this article,we analyze the challenges of developing GaN-based ultraviolet laser diodes,and the approaches to improve the performance of ultraviolet laser diode are reviewed.With these techniques,room temperature(RT)pulsed oscillation of AlGaN UVA(ultraviolet A)LD has been realized,with a lasing wavelength of 357.9 nm.Combining with the suppression of thermal effect,the high output power of 3.8 W UV LD with a lasing wavelength of 386.5 nm was also fabricated.

An injection-locking diode laser at 671 nm with a wide tuning range up to 6 GHz

We present a compact injection-locking diode laser module to generate 671 nm laser light with a high output power up to 150 m W.The module adopts a master-slave injection-locking scheme,and the injection-locking state is monitored using the transmission spectrum from a Fabry-Perot interferometer.Beat frequency spectrum measurement shows that the injection-locked slave laser has no other frequency components within the 150-MHz detection bandwidth.It is found that without additional electronic feedback,the slave laser can follow the master laser over a wide range of 6 GHz.All the elements of the module are commercially available,which favors fast construction of a complete 671-nm laser system for the preparation of cold^(6)Li atoms with only one research-grade diode laser as the seeding source.

Influence of laser intensity in second-harmonic detection with tunable diode laser multi-pass absorption spectroscopy

Tunable diode laser absorption spectroscopy （TDLAS） has been widely employed in atmospheric trace gases detection. The ratio of the second-harmonic signal to the intensity of laser beam incident to the multi-pass cell is proved to be proportional to the product of the path length and the gas concentration under any condition. A new calibration method based on this relation in TDLAS system for the measurement of trace gas concentration is proposed for the first time. The detection limit and the sensitivity of the system are below 110 and 31ppbv （parts-per-billion in volume）, respectively.

Room-temperature continuous-wave operation of GaN-based blue-violet laser diodes with a lifetime longer than 1000h

GaN-based continuous-wave operated blue-violet laser diodes(LDs) with long lifetime are demonstrated, which are grown on a c-plane GaN substrate by metal organic chemical vapor deposition with a 10 × 600 μm^2 ridge waveguide structure.The electrical and optical characteristics of a blue-violet LD are investigated under direct-current injection at room temperature(25 °C). The stimulated emission wavelength and peak optical power of the LD are around 413 nm and over 600 mW, respectively.In addition, the threshold current density and voltage are as small as 1.46 kA/cm^2 and 4.1 V, respectively. Moreover, the lifetime is longer than 1000 hours under room-temperature continuous-wave operation.

Effect of inhomogeneous broadening on threshold current of GaN-based green laser diodes

The inhomogeneous broadening parameter and the internal loss of green LDs are determined by experiments and theoretical fitting. It is found that the inhomogeneous broadening plays an important role on the threshold current density of green LDs. The green LD with large inhomogeneous broadening even cannot lase. Therefore, reducing inhomogeneous broadening is a key issue to improve the performance of green LDs.

A remote sensing system of vehicle emissions based on tunable diode laser technology

As being an effective real-time method of monitoring vehicle emissions on-road, a remote sensing system based on the tunable diode laser （TDL） technology was presented, and the key technologies were discussed. A field test in Guangzhou（Guangdong, China） was performed and was found that the factors, such as slope, instantaneous speed and acceleration, had significant influence on the detectable rate of the system. Based on the results, the proposal choice of testing site was presented.

Experimental Study of Nd^(3+):KGd(WO_(4))_(2) Laser Pumped by Laser Diode

This paper presents the cw output characteristics of Nd^(3+):KGd(WO_(4))_(2)(KGW)laser crystal with different NcP+concentration grown in our laboratory.The laser output at 1.067 μm of KGW crystal(3×3×6 mm)was obtained to be 68.9 mW and the slope efficiency reached 28.8%when pumped by laser diode of power 305 mW at 807 nm.

GaN-based blue laser diode with 6.0 W of output power under continuous-wave operation at room temperature

In this work,we reported the room-temperature continuous-wave operation of 6.0 W GaN-based blue laser diode(LD),and its stimulated emission wavelength is around 442 nm.The GaN-based high power blue LD is grown on a c-plane GaN substrate by metal organic chemical vapor deposition(MOCVD),and the width and length of the ridge waveguide structure are 30 and 1200μm,respectively.The threshold current is about 400 mA,and corresponding threshold current density is 1.1 kA/cm2.

Diode laser using narrow bandwidth interference filter at 852 nm and its application in Faraday anomalous dispersion optical filter

We demonstrate an 852-nm external cavity diode laser（ECDL） system whose wavelength is mainly determined by an interference filter instead of other wavelength selective elements. The Lorentzian linewidth measured by the heterodyne beating between two identical lasers is 28.3 k Hz. Moreover, we test the application of the ECDL in the Faraday atomic filter.Besides saturated absorption spectrum, the transmission spectrum of the Faraday atomic filter at 852 nm is measured by using the ECDL. This interference filter ECDL method can also be extended to other wavelengths and widen the application range of diode laser.

Arbitrary frequency stabilization of a diode laser based on visual Labview PID VI and sound card output

We report a robust method of directly stabilizing a grating feedback diode laser to an arbitrary frequency in a large range. The error signal, induced from the difference between the frequency measured by a wavelength meter and the preset target frequency, is fed back to the piezoelectric transducer module of the diode laser via a sound card in the computer. A visual Labview procedure is developed to realize a feedback system. In our experiment the frequency drift of the diode laser is reduced to 8 MHz within 25 min. The robust scheme can be adapted to realize the arbitrary frequency stabilization for many other kinds of lasers.

Thermal analysis of GaN laser diodes in a package structure

Using the finite-element method, the thermal resistances of GaN laser diode devices in a TO 56 package for both epi-up configuration and epi-down configuration are calculated. The effects of various parameters on the thermal characteristics are analysed, and the thicknesses of the AlN submount for both epi-up configuration and epi-down configuration are optimized. The obtained result provides a reference for the parameter selection of the package materials.

Reduction of Electron Leakage in a Deep Ultraviolet Nitride Laser Diode with a Double-Tapered Electron Blocking Layer

A double-tapered AlGaN electron blocking layer (EBL) is proposed to apply in a deep ultraviolet semiconductor laser diode. Compared with the inverse double-tapered EBL, the laser with the double-tapered EBL shows a higher slope efficiency, which indicates that effective enhancement in the transportation of electrons and holes is achieved. Particularly, comparisons among the double-tapered EBL, the inverse double-tapered EBL, the singletapered EBL and the inverse single-tapered EBL show that the double-tapered EBL has the best performance in terms of current leakage.

Effect of microstructure of Au80Sn20 solder on the thermal resistance TO56 packaged GaN-based laser diodes

Au80Sn20 alloy is a widely used solder for laser diode packaging.In this paper,the thermal resistance of Ga N-based blue laser diodes packaged in TO56 cans were measured by the forward voltage method.The microstructures of Au80Sn20 solder were then investigated to understand the reason for the difference in thermal resistance.It was found that the microstructure with a higher content of Au-rich phase in the center of the solder and a lower content of(Au,Ni)Sn phase at the interface of the solder/heat sink resulted in lower thermal resistance.This is attributed to the lower thermal resistance of Au-rich phase and higher thermal resistance of(Au,Ni)Sn phase.

Ⅲ-nitride based ultraviolet laser diodes

In recent years,because of their small size,high efficiency and environment-friendly advantages,Ⅲ-nitride based ultraviolet(UV)light-emitting diodes(LEDs)have been widely used in many areas to substitute for mercury lamps,such as in 3D printing,curing and sterilization.Ⅲ-nitride alloys cover the whole UV spectrum which is comprised of UV-A(320–400 nm),UV-B(280–320 nm)and UV-C(200–280 nm)by controlling Al/Ga/In content.In addition,Ⅲ-nitride based UV laser diodes(LDs)also have some potential applications in the case of high-power-density,narrow-spectrum,good-directional lighting.However,Ⅲ-nitride based UV laser diodes still have many challenges such as poor crystal quality and low hole concentration in p-type AlGaN.

Diode Laser Soldering Technology of Fine Pitch QFP Devices

The laser provides a controllable means of supplying localized energy for solder joint formation and is a valuable tool in electronics manufacture.Diode laser soldering for fine pitch QFP devices were carried out with Sn-Ag-Cu lead-free solder and Sn-Pb solder respectively,and the mechanical properties of micro-joints of the QFP devices were tested and studied by STR-1000 micro-joints tester.The results indicate that sound QFP micro-joints without bridging or solder ball are gained by means of diode laser soldering method with appropriate laser processing parameters,and the pitch of the QFP devices is as fine as to 0.4mm.Tensile strength of QFP micro-joints increases gradually with the increase of laser output power,the maximum tensile strength presents when the laser output power increase to a certain value.The results also indicate that the mechanical properties of QFP micro-joints soldered by diode laser soldering system are better than those of QFP micro-joints soldered by IR reflow soldering method.The experimental results may provide a theory guide for investigation of diode laser soldering.

Effects of diode laser photocoagulation treatment on ocular biometric parameters in premature infants with retinopathy of prematurity

AIM:To investigate the effects of diode laser treatment on ocular biometric parameters in premature infants with retinopathy of prematurity(ROP).METHODS:Premature infants who received diode laser treatment for ROP(n=68)and premature infants with spontaneous regressed ROP without treatment(n=50)were performed longitudinal ocular biometric measurements including anterior chamber depth,lens thickness and axial length as follows:1 d prior to laser treatment,and 3,6,9,and 12 mo after the laser treatment.RESULTS:The mean birth weight,gestational age and initial examination time values were 936.53±302.07 g,26.66±2.42 wk,36.26±2.73 wk in the treatment group and 959.78±260.08 g,27.28±2.10 wk,36.56±2.54 wk in the control group.There was no statistically significant difference in these demographic characteristics of the groups.Anterior chamber depth,lens thickness and axial length demonstrated statistically significant linear increases during the study period in the two groups(P<0.001 for each).There were no statistically significant differences between the two groups in terms of anterior chamber depth after laser treatment.Measurements of the lens thickness at 9 th and 12 th months(9 th month 3.70±0.22 vs 3.60±0.21 mm,P=0.017;12 th month 3.81±0.21 vs 3.69±0.22 mm,P=0.002)and the axial length at 12 th month(19.35±0.79 vs 19.13±0.54 mm,P=0.031)after laser treatment were statistically higher in the treatment group.CONCLUSION:Diode laser retinal photocoagulation treatment in premature infants seems to increase the lens thickness and axial length.

Efficient and high-power laser-diode single-end-pumped Nd：YVO4 continuous wave laser at 1342nm

A compact, efficient and high-power laser diode （LD） single-end-pumped Nd：YVO4 laser with continuous-wave emission at 1342 nm is reported. With a single crystal single-end-pumped by fibre-coupled LD array, an output power of 7.36W is obtained from the laser cavity of concave-convex shape, corresponding to an optical-to-optical efficiency of 32.8%. The laser is operated in TEM00 mode with small rms amplitude noise of 0.3%. The influences of the Nd concentration, transmissivity of the output mirror and the cavity length on the output power have been studied experimentally.

Broad bandwidth interference filter-stabilized external cavity diode laser with narrow linewidth below 100kHz

Interference filter-stabilized external cavity diode lasers （ECDLs） have properties of simple configurations, high sta- bilities, and narrow linewidths. However, the interference filter used in common ECDL designs requires an ultra-narrow bandwidth （about 0.3 nm） to achieve mode selection, that is considerably expensive and not yet available for a wide range of wavelengths. In this paper, a robust ECDL using an available broad bandwidth （about 4 nm） interference filter as the wavelength discriminator is constructed and tested. The ECDL demonstrated a narrow Lorentzian fitted linewidth of 95 kHz and a spectral purity of 2.9 MHz. The long-term frequency stability of the ECDL reaches 5.59 x 10 12.

Measurements of argon metastable density using the tunable diode laser absorption spectroscopy in Ar and Ar/O2

Densities of Ar metastable states 1s5 and 1s3 are measured by using the tunable diode laser absorption spectroscopy（TDLAS） in Ar and Ar/O2 mixture dual-frequency capacitively coupled plasma（DF-CCP）. We investigate the effects of high-frequency（HF, 60 MHz） power, low-frequency（LF, 2 MHz） power, and working pressure on the density of Ar metastable states for three different gas components（0%, 5%, and 10% oxygen mixed in argon）. The dependence of Ar metastable state density on the oxygen content is also studied at different working pressures. It is found that densities of Ar metastable states in discharges with different gas components exhibit different behaviors as HF power increases. With the increase of HF power, the metastable density increases rapidly at the initial stage, and then tends to be saturated at a higher HF power. With a small fraction（5% or 10%） of oxygen added in argon plasma, a similar change of the Ar metastable density with HF power can be observed, but the metastable density is saturated at a higher HF power than in the pure argon discharge. In the DF-CCP, the metastable density is found to be higher than in a single frequency discharge, and has weak dependence on LF power. As working pressure increases, the metastable state density first increases and then decreases,and the pressure value, at which the density maximum occurs, decreases with oxygen content increasing. Besides, adding a small fraction of oxygen into argon plasma will significantly dwindle the metastable state density as a result of quenching loss by oxygen molecules.

A 15 Gbps-NRZ, 30 Gbps-PAM4, 120 mA laser diode driver implemented in 0.15-μm GaAs E-mode pHEMT technology

This paper presents the design and testing of a 15 Gbps non-return-to-zero(NRZ),30 Gbps 4-level pulse amplitude modulation(PAM4)configurable laser diode driver(LDD)implemented in 0.15-μm GaAs E-mode pHEMT technology.The driver bandwidth is enhanced by utilizing cross-coupled neutralization capacitors across the output stage.The output transmission-line back-termination,which absorbs signal reflections from the imperfectly matched load,is performed passively with on-chip 50-Ωresistors.The proposed 30 Gbps PAM4 LDD is implemented by combining two 15 Gbps-NRZ LDDs,as the high and low amplification paths,to generate PAM4 output current signal with levels of 0,40,80,and 120 mA when driving 25-Ωlasers.The high and low amplification paths can be used separately or simultaneously as a 15 Gbps-NRZ LDD.The measurement results show clear output eye diagrams at speeds of up to 15 and 30 Gbps for the NRZ and PAM4 drivers,respectively.At a maximum output current of 120 mA,the driver consumes 1.228 W from a single supply voltage of-5.2 V.The proposed driver shows a high current driving capability with a better output power to power dissipation ratio,which makes it suitable for driving high current distributed feedback(DFB)lasers.The chip occupies a total area of 0.7×1.3 mm^(2).