Laser Diode Array Research Articles

Evaluation of subject response following treatment for pigmentation or wrinkles using a diode laser.

Pulsed diode array laser systems are utilized extensively for various aesthetic indications such as removal of unwanted hair, treatment of vascular and pigmented lesions, and wrinkle reduction. The purpose of this study was to report and assess the experience of using a diode laser system delivering pulsed infrared laser light at the near-infrared (NIR) spectrum at wavelengths of 805 and 1060nm. The study was a retrospective analysis of treatment outcomes in adult subjects treated at the clinic between January 2017 and April 2018 for wrinkles and pigmentation with a noninvasive aesthetic diode laser system. Subjects were treated at nominal wavelengths of 805nm for pigmentation and 1060nm for wrinkles reduction. Improvement in pigmentation and wrinkles, adverse events, and patient tolerability to treatment and satisfaction were evaluated. Of 44 subjects with Fitzpatrick skin types II-IV, eight were treated for pigmentation and 36 for wrinkles. For both treatments, subjects reported tolerable pain levels. All immediate responses resolved within 48hours post-treatment. Evaluation of treatment outcomes by two blinded evaluators demonstrated significant pigmentation clearance mean of 2.50±0.15, (P<.05) in subjects treated for pigmentation, as well as significant improvement mean of 0.46±0.12 (P=.005) in wrinkles in 13 subjects (41%) whose "before" and "after treatment" photographs were correctly identified by both blinded evaluators. Subjects were satisfied with the treatments. Use of the Diode laser effectively resulted in improvement in pigmentation and wrinkles, while maintaining a high safety profile with limited downtime.

Wavelength-stabilized near-field laser.

Surface-trapped electromagnetic waves can be localized at a boundary between a semiconductor distributed Bragg reflector (DBR) and a homogeneous dielectric medium or air. These waves enable a novel class of in-plane-emitting optical devices including edge-emitting lasers, disk microlasers or near-field fiber-coupled lasers. We show that the surface-trapped modes can be controlled by tuning the thickness of a single DBR layer. Diagrams in variables "wavelength - thickness of the control layer" are constructed for both TM and TE optical modes revealing the parameter domains, in which surface-trapped modes exist. The domains contain cusps, in the vicinity of which a surface-trapped optical mode is allowed only in a narrow spectral region, enabling wavelength-stabilized operation of a laser. For a structure designed for lasing at ∼1 µm, the lasing wavelength shifts upon temperature increase at a rate ∼0.08 nm/K. The fraction of the optical power of the surface-trapped mode accumulated in the homogeneous dielectric can reach ∼50%. Thus, such structure is a near-field wavelength-stabilized semiconductor laser. Further, such structure can be applied as a wavelength-stabilized semiconductor optical amplifier adjacent to a dielectric waveguide or an optical fiber, both for integrated photonics and for ultrahigh-brightness laser diodes and diode arrays and stacks.

Automated UV-Epoxy-Based Micro-Optic Assembly for Kilowatt-Class Laser-Diode Arrays and Modules

This paper outlines the numerous interdisciplinary activities required to implement two ultraviolet (UV) epoxy-based automated micro-optic assembly tools within a “smart factory” for high-power semiconductor laser manufacturing. After a brief overview of the motivation and approach, two examples of automated micro-optic assembly tools [e.g., fast-axis collimation (FAC) lens attach and mirror stripe alignment] for kilowattclass laser-diode arrays and modules are given that demonstrate the breadth of automation activities required within a modern factory. For each system, the complete assembly sequence is outlined, and the steps that presented fully automated challenges are highlighted. In contrast, to the previous published results that focused on the details of the automated micro-optic alignment, the largest challenges primarily involved automation and repeatability of the UV-epoxy dispense and attach processes for these high-power semiconductor devices. These were solved by precise and stable mechanical design, customized lighting, and machine vision solutions. Finally, the complete integration of the tools into a “smart-factory” system is outlined.

Easy method to measure the packaging-induced stress of a semiconductor laser diode by lasing wavelength shifting.

In this paper, the packaging-induced stresses are theoretically calculated by modeling multilayered structures for different packaging structures. We report a method to measure the packaging-induced stress of a laser diode array (LDA) by comparing the emission wavelength of the single emitter located in the middle of a laser bar before and after packaging. The wavelength is tested under a low duty cycle (50μs/10Hz, DC 0.05%) to eliminate the thermal effect to wavelength shifting. Experimental calculation results for the packaging-induced stress of LDAs are in good agreement with the theoretical calculations and simulation results. For a GaAs laser bar, we find the packaging stresses are compression stresses, which make the emission wavelength blue-shift in terms of 1.09×10-2 nm/MPa. We propose a mapping of packaging-induced stress distribution in laser bars on a microscopic scale by considering the emission spectra of each emitter in a laser bar. Compared to single-emitter resolved photo-current or micro-photoluminescence measurements, as proposed by other authors, we offer a much easier tool to test and map the distribution of packaging-induced stress in laser bars.

Experimental study on cooling of high-power laser diode arrays using hybrid microchannel and slot jet array heat sink

In this study, a hybrid microchannel and slot jet array heat sink is designed and fabricated to achieve a better thermal performance of the high-power laser diode arrays. A standard commercial laser bar with wavelength of 808 nm is packaged on the heat sink and experiments are performed to assess the cooling performance of the hybrid heat sink. In the experiments, the forward voltage method is used to measure the chip temperature and the structure function method is applied to obtain the thermal resistance of the heat sink. Using the deionized water as coolant, the heat sink deals with a heat flux of up to 506 W/cm2 and the thermal resistance of the heat sink is only 0.23 K/W when the flow rate is 0.8 L/min (The average jet velocity is 13.3 m/s). The optical power can be up to 135.5 W corresponding to the wall plug efficiency of 64.2%. Compared with the published solutions related to cooling similar laser chips, the thermal resistance reduces by over 15%, indicating that this hybrid heat sink is an interesting solution to improve the cooling of the high-power laser diode arrays.

Vascularized liquid metal cooling for thermal management of kW high power laser diode array

In this paper, the vascularized liquid metal cooling was introduced into the thermal management of high power laser diode array (HPLDA) to realize an excellent heat removal performance. A vascularized liquid metal heat sink, with channel number of 70 and characteristic dimension of 1.8 mm, was firstly designed. And experimental tests and numerical analysis were then conducted. The experimental results indicated that liquid metal cooling can achieve 2000 W heat dissipation of HPLDA, with maximum temperature of heat sink top surface lower than 54 °C. The numerical results showed that vascularized liquid metal cooling offers better cooling performance compared to vascularized water cooling when flow rate was bigger than 2.4 l/min. It was also found that the heat capacity resistance was a significant thermal resistance for vascularized liquid metal cooling, while it was rather small for vascularized water cooling. Finally, the optimization distribution of heat source was numerically conducted to reduce the maximum temperature and maximum temperature difference, and 40 mm * 72 mm HPLDA distribution was selected as the best choice. Overall, the advanced vascularized liquid metal convection cooling offers a novel and effective heat dissipation method for the thermal management of HPLDA.

Efficient laser based on neodymium-doped yttrium aluminum garnet with passive cooling of the active element and linear laser diode arrays

This paper considers a variant of a compact laser emitter for a rangefinder with passive (contact) cooling of the cylindrical active element and linear laser diode arrays. The density distribution of the stored energy in the cross-section of the active element was calculated when pumped from three and five sides, and the thermal fields during a 90&#x00A0;s operation of the laser emitter with a pulse repetition rate of 10&#x00A0;Hz were calculated. The results indicate that the thermal lens in the active element has less optical power than under traditional liquid flow cooling. The experimental results verifying the calculations are provided herein. The laser efficiency in the single-pulse operating regime is close to 20% at an energy pulse of 70&#x00A0;mJ.

Numerical study on cooling of high-power laser diode arrays using slot jet array impingement

Thermal design shows significant effects on energy conversion efficiency, lifespan and beam quality of high-power laser diode arrays (HPLDAs). In this study, a slot jet array heat sink (SJAHS) is designed to improve the cooling performance of HPLDAs, and numerical simulations were conducted to optimize the flow and heat transfer in the heat sink. Flow passages inside the heat sink ensured that the coolant is allocated to every jet uniformly. The jet geometry is optimized to enhance the heat transfer. When the working fluid is assumed as water at 293 K inlet temperature and at a flowrate of 800 mL/min, the heat sink is capable of removing 150 W heat over an area of 15 mm2 with a maximum temperature increase of 52.75 K from the coolant inlet to emitters. It means that the emitter-to-coolant thermal resistance is 0.35 K/W without considering the interface thermal resistance. The corresponding pressure drop is 361 kPa. Compared with the existing microchannel heat sink, the numerical results implied that this new heat sink possesses a better cooling performance.

60-W 946-nm cryogenically-cooled Nd:YAG laser

Detailed spectroscopic characterisation of the ^4I_{9/2} rightarrow ^4F_{3/2} absorption cross section of Nd:YAG at temperatures ranging from room temperature to liquid nitrogen temperature is reported. Exploiting this in-band pumping scheme, a novel volume-Bragg-grating-locked diode-laser array (DLA) operating at 869 nm was developed to end-pump a cryogenically cooled Nd:YAG crystal. As a result, the first 60-W cryogenically-cooled Nd:YAG laser operating at 946 nm is demonstrated with a slope, and optical-to-optical, efficiency of 52%.

Spectral interval compressing structure in spectral beam combination system for diode laser arrays

An efficient spectral beam combining (SBC) system for diode lasers with compressed spectral interval is proposed and demonstrated experimentally. This structure consists of a transmission grating and a reflector paralleled to the grating, which can make the beam diffracted twice by only one transmission grating. The advantage of this configuration is that it compresses the spectral interval of the output beam into half compared to the traditional single-grating setup and improves the utilization rate of the grating element compared to the previous double grating system. The experimental results demonstrate that the spectral intervals of a 3-emitter diode laser array (DLA) and a 5-emitter DLA with traditional single-grating structure are 4.36 and 4.69 nm, respectively. In the modified system, the spectral intervals of the same DLA with 3 and 5 emitters are 2.15 and 2.35 nm, respectively, which shows that this method can compress the spectrum interval effectively and will be promising for a practical SBC system.

Beam-combining scheme of high-power broad-area semiconductor lasers with Lyot-filtered reinjection: modeling, simulations, and experiments

A brightness- and power-scalable polarization beam-combining scheme for high-power, broad-area semiconductor laser diodes is investigated numerically and experimentally. To achieve the beam combining, we employ Lyot-filtered optical reinjection from an external cavity, which forces lasing of the individual diodes on interleaved frequency combs with overlapping envelopes and enables a high optical coupling efficiency. Unlike conventional spectral beam-combining schemes with diffraction gratings, the optical coupling efficiency is insensitive to thermal drifts of laser wavelengths. This scheme can be used for efficient coupling of a large number of laser diodes and paves the way toward using broad-area laser diode arrays for cost-efficient material processing, which requires high-brilliance emission and optical powers in the kW regime.

Modelling of temperature profiles and stress in CW laser diode bars

We present the results of 3D thermal regimes modelling of CW laser diode arrays (LDA) assembled on the CS-mount type heat sinks. The dependences of the LDA temperature profiles on the fill factor (FF) of the LDA aperture at different levels of thermal load in different sections on LDA, including those inaccessible for direct experimental measurement calculated and discussed.

1.2 kW quasi-steady-state diamond Raman laser pumped by an M2 = 15 beam.

An external cavity diamond Raman laser with 1.2kW output power is demonstrated for durations 7 times longer than the thermal lens time constant. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained for conversion from a 1.06μm pump to the 1.24μm first Stokes. The pump had an M2 of 15, demonstrating that efficiency is maintained at the highest levels even when using exceptionally poor quality pumps. We show that a measured decrease in the output beam quality factor from M2=2.95 to M2=1.25 as power increased is evidence for thermal lens development in the diamond. The results foreshadow development of continuous-wave kilowatt-class lasers or amplifiers based on single diamond elements and pumped efficiently by lasers having poor spatial coherence such as line-narrowed diode laser arrays.

Three-dimensional thermal model of high-power semiconductor lasers.

In this paper, 3-D, steady-state, analytical thermal models of high-power single-emitter semiconductor lasers (SEs) and laser diode arrays (LDAs) are derived, considering the heat conduction in multi-layered laser structures. Heat flow in the laser chips for an epi-down bonded SE and LDA is described using this model, and it is observed that the laser chips contribute to 8% and 6% of total heat dissipation for the SE and LDA, respectively. The submount size requirement for this model is discussed by revealing the heat flow in the submount. Through finite element simulations, the solution accuracy for the lasers with non-ideal submounts is confirmed. Finally, the performance of our proposed analytical models is verified by finite element simulations and experimental measurements based on the wavelength shift method.

Transient temperature effects on the optical power wavelength shift of a high-power laser system

The effect of junction temperature change on the wavelength shift and optical power output of high-power laser systems utilizing a fiber laser design is investigated using a mathematical model. Several laser diode array configurations, along with two different cooling fluids, are used to analyze the laser performance by numerically calculating the optical power out of the fiber laser for each case. The modeling approach presented here captures the dependence of optical power output on laser diode junction temperature by way of the optical power interactions between the laser diodes and fiber gain media components. Through the presented research, the beginning of a Multiphysics modeling approach has been created for better understanding the thermal requirements for a robust and practical high-power laser system. Future research will build off this and focus on the temperature dependent aspects of individual components (laser diodes, fiber gain media, and combining optics) in high power laser systems. In conjunction, high fidelity modeling of the listed components will provide a clear picture of the aggregate system-level thermal requirements. Gains in system-level thermal knowledge are instrumental for progress in high power laser technology and application.

Generation of high brightness diode laser by using spectral and polarization beam combination

We present a beam combination technique to improve the brightness of diode lasers based on both spectral beam combination and polarization beam combination. We use eight diode laser arrays (DLAs) and two diffraction gratings to construct two independent external cavities by using spectral beam combination, and then combine the outputs of these two cavities by using polarization beam combination. The experiment shows that the total output power can be as high as 310.2 W by using this technique with eight DLAs, which are commercial standard cm-bars, each producing ∼60 W of output power. The beam qualities of the combined beam are M2 = 13.55 and 10.27 in the slow and fast axis, respectively. The brightness of the combined output is as high as 252 MW·cm−2·sr−1.

Compact Nd:YVO4 laser 1087.5 nm pumped by an edge-emitting LD

A cheap and compact Nd:YVO4 laser at 1087.5 nm pumped by an edge-emitting laser diode (LD) array is demonstrated. By specially restraining the other lines at 1064.1, 914.0, and 1342.0 nm with a refined output mirror having low-reflectivity at 1064.1 nm (R1064.1 = 3%) and high-reflectivity (R1087.5 = 92%) at 1087.5 nm, a low-gain line at 1087.5 nm is achieved. The output power is 8.79 W at the 807.9 nm pump power of 37.43 W with an optical-optical conversion efficiency of 23.5%.

Li5 Zn8 Ga5 Ge9 O36 : Cr3+ , Ti4+ : A Long Persistent Phosphor Excited in a Wide Spectral Region from UV to Red Light for Reproducible Imaging through Biological Tissue.

Near-infrared (NIR) long-persistent phosphors (LPPs) have emerged as a potential solution for bio-imaging applications over the past few years. However, there are enormous challenges regarding their in situ application based on their dependence on short-wavelength excitation. In this paper, we report a multi-spectral excited NIR LPP, Li5 Zn8 Ga5 Ge9 O36 : 1.5 % Cr3+ , 0.5 % Ti4+ , which overcomes the limitations of functional processes in biological tissues and other complex systems. This LPP exhibits a high luminescent intensity and a long emission duration in the NIR region (700-800 nm). The applicability of this phosphor to tissue imaging is demonstrated experimentally. Its persistent luminescence (PersL) can easily penetrate approximately 2 mm of pork flesh. More importantly, this phosphor can be re-charged in situ using a red LED or laser diode array to provide renewed NIR PersL for biological tissues, which is beneficial for long-term biological tissue imaging applications with high signal-to-noise ratios. Systematic investigations of the nature of energy traps and PersL mechanisms are also reported in this paper.

Second harmonic generation of ultraviolet laser based on high power laser diode array

A 780 nm continuous-wave, which is output by extra-cavity semiconductor laser with a bulk bragg grating, is used as the fundamental input light source of the Second Harmonic Generation (SHG) system. Line width of the fundamental light is 0.13 nm. Based on the type I phase-matching and Lithium Triborate (LBO) crystal, 390 nm second harmonic is obtained. The emitted average output power is 30 W (SHG efficiency is 0.01%). We provide a new technical route for the achievement of high-power UV beams.

Non-Hermiticity and exceptional points in coherently coupled vertical cavity laser diode arrays

Coherently coupled laser arrays can be described by the temporal coupled mode theory in which the array modal behavior can be classified according to the coupling matrix, M¯¯. Accounting for a nonuniform gain/loss distribution in a laser array makes M¯¯ a non-Hermitian matrix, and experimentally we find phase-front tuning (beam steering) of the coherent supermode as a result of the non-Hermiticity. We report the experimental characterization of the supermodes in coherently coupled vertical cavity surface emitting laser diode arrays and demonstrate control of non-Hermiticity by spatially varying injection currents. Exceptional points are identified in these electrically injected microcavity diode arrays.