Tunable high-power blue external cavity semiconductor laser

The blue laser diode technology that is now emerging as the next generation high power laser source when evaluated at the system level has several distinct advantages over the infrared lasers making it an interesting candidate for high power laser development. Blue laser diodes are capable of operating at high temperatures (80C) which is a major advantage when determining the system’s size weight and power, and the shorter wavelength has a higher absorption characteristic than the infrared on all materials reducing the energy on target required to destroy the target. There are two main issues associated with using a high-power blue laser as a directed energy weapon, 1) the atmospheric transmission is challenging, especially at sea level and 2) the blue wavelength is not considered an eye safe wavelength. This paper will address the maturity of the blue laser technology, what the system operating parameters would be and the system performance under a range of atmospheric conditions. Initial studies indicate that a blue laser weapon system will indeed be very effective under a wide range of operating conditions.

A blue diode laser has a higher absorption rate than a traditional laser, while the maximum power is limited. We report the structure and laser beam profile of a 250W high-power blue laser (445nm) for material processing. The absorption rate of the blue laser system for the steel was 2.75 times that of a single-mode fiber laser system (1070nm). The characteristics of the steel after laser irradiation were determined, validating the potential of this high-power blue laser for material processing, such as heat treatment and cladding. The cost of the developed laser system was lower than that of the existing one. To the best of our knowledge, this is the first blue laser with a power as high as 250W.

We outline our project to develop a circulating polarized helium-3 system for developing of large, quasi-continuously operating neutron analyzers. The project consisted of four areas: 1) Development of robust external cavity narrowed diode laser output with spectral line width < 0.17 nm and power of 2000 W. 2) Development of large glass polarizing cells using cell surface treatments to obtain long relaxation lifetimes. 3) Refinements of the circulation system with an emphasis on gas purification and materials testing. 4) Design/fabrication of a new polarizer system. 5) Preliminary testing of the new polarizer. 1. Developed Robust High-Power Narrowed Laser The optical configuration of the laser was discussed in the proposal and will be reviewed in the body of this report. The external cavity is configured to mutually lock the wavelength of five 10-bar laser stacks. All the logistical milestones were been met and critical subsystems- laser stack manifold and power divider, external laser cavity, and output telescope- were assembled and tested at low power. Each individual bar is narrowed to ~0.05 nm; when combined the laser has a cumulative spectral width of 0.17 nm across the entire beam due to variations of the bars central wavelength by +/- 0.1 nm, which is similar to that of Volume Bragg Grating narrowed laser bars. This configuration eliminates the free-running “pedestal” that occurs in other external cavity diode lasers. The full-scale laser was completed in 2016 and was used in both the older and newer helium polarizers. This laser was operated at 75% power for periods of up to 8 hours. Once installed, the spectrum became slightly broader (~.25 nm) at full power; this is likely due to very slight misalignments that occurred during handling. 2. Developed the processes to create uniform sintered sol-gel coatings. Our work on cell development comprised: 1) Production of large GE180 cells and explore different means of cell preparation, and 2) Development of apply sol-gel coatings to the interior of both borosilicate and aluminosilicate cells. We applied six sol-gel coatings. By modifying the mixture and developing procedures to drain and dry the cell, we produced visually uniform coatings on the interior of the cells. We now have perfected that process as described below in our report. We were able to accelerate the testing of cells using an ex situ method that avoids installing each cell into a polarizer. In the project’s last year, we conducted 38 external tests of 8 different cells. We also installed two sol-gel coated cells in our polarizers. We created cell with long ex situ relaxation lifetimes, one of which exceeded 40 hours. However, when installed in the polarizer the measured lifetime is 8 hours or less. 3. Demonstrated cycling of polarized gas and ex situ cell testing We are now cycling polarized gas from the polarizer to glass vessels and back. This has allowed us, for the first time, to make ex situ T1 measurements of polarizing cells without installing them into the polarizer itself. This has greatly improved our productivity in producing cells and evaluating our cell preparation processes. We continued development of the gas handling system in parallel with fabricating new polarizer. The integrated system was tested by the end of 2016. We now regularly cycle gas into and out of the polarizer. 4. Completed new polarizer infrastructure and control systems. We completed the new polarizer infrastructure in November 2016. The polarizer subsystems are 1) the frame, 2) the oil flow system, 3) the gas handling system, 4) the pressure vessel, with embedded solenoid, 5) cell mounting hardware with heat spreaders, and 6) electrical power and instrumentation. 5. Carried out initial tests of polarizer. We completed initial testing of the polarizer in April and May of 2017. These tests were carried out for periods up to 6 hours with laser power between 750 and 1300 Watts. The laser performed well and the polarization with asymptotic to 45 percent, which was below expectations. This low value resulted from a stationary thermal inversion in the cell that caused most of the laser power to be absorbed near the laser inlet window and deprived the lower portions of the cell of pumping laser light. Possible solutions to this problem include enhanced cooling of the cell near the laser entry and slight detuning of the laser. 6. Ongoing work. Our polarizer development efforts are ongoing to pursue our interest in neutron analyzers, nuclear targets, and providing helium for medical imaging. Current tests in the pipeline include: 1. Testing cooling enhancements to improve laser penetration of spectrally narrow lasers; 2. Testing of a cell with isolation valves that minimizes diffusive contact with gas handling hardware during polarization; 3. Testing of smaller hybrid cells with reduced alkali loading; 4. Producing polarized helium-3 for MRI imaging at the University of Missouri.

Dual-module combination with littman crosstalk external cavity for narrow-linewidth blue diode laser

In this paper, a high power blue laser at 447 nm was obtained by intracavity sum-frequency-mixing of a diode-side-pumped Q-switched Nd:YAlO(3)(Nd:YAP) laser operating at 1341.4 nm. A type-I critical phase matching LiB(3)O(5) (LBO) crystal and type-II critical phase matching KTiOPO(4) (KTP) crystal were used for second harmonic generation and third harmonic generation, respectively. The phase matching condition of the KTP crystal was researched. The results show that the KTP has superiority in intracavity sum-frequency-mixing blue light generation. 4.76 W blue light output was achieved at 4.6 kHz with the pulse width of 190ns. The fluctuation of output power was better than 3% at the output power of 4.76 W during half an hour.

This work proposes a method of compressing spectral linewidth and tuning the central wavelength of multiple high-power diode laser arrays in an external cavity feedback structure based on one volume Bragg grating (VBG). Through the combination of beam collimation and spatial beam technologies, a diode laser source producing 102.1 W at an operating current of 40 A is achieved. This laser source has a central wavelength of 766 nm and a narrow spectral linewidth of 0.164 nm. Moreover, a tuning central wavelength ranging from 776–766.231 nm is realized by precisely controlling the temperature of the VBG, and the locked central wavelength as a function of temperature shifts at the rate of approximately 0.0076 nm/°C. The results further prove that the smile under 1 μm can restrain the self-excitation effect of the emitting laser, which can influence the efficiency of the potassium alkali metal vapor laser pumping.

High-power edge-emitting GaN-based blue laser have the advantages of high wall-plug efficiency, small size, thereby serving as critical optoelectronic devices for achieving high-power blue light output. However, under prolonged highcurrent operation, their reliability faces significant challenges. The methodology of this study encompassed simulating the device’s temperature and thermal stress profiles, performing electrical tests such as forward I-V characterization and lowfrequency noise measurements, and conducting microscopic failure analysis with metallographic microscope and scanning electron microscope on devices after lifetime testing. Combined simulation and experimental results reveal that the primary failure mechanisms originate from the propagation of internal defects, catastrophic optical damage (COD) at the facet, and degradation of the solder layer. This work elucidates the key mechanisms governing the reliability of high-power GaN-based blue laser diodes, thereby providing a critical foundation for performance enhancement.

Pulsed lasers with pulse durations of nanosecond to millisecond are very important tools for free-space optical communication, LADAR, laser material processing, and optical sensing. Although Q-switched solid-state lasers or gas lasers are currently the most popular light sources for these purposes, pulsed semiconductor lasers have the potential for the above applications because of their compactness, accessibility of direct modulation, and inherently large electrical to optical conversion efficiency. The drawbacks with high-power semiconductor lasers are their poor beam quality and low coherence factors. This work addresses the above issues through experimental demonstration of frequency locking, wavelength tuning, and synchronization of nanosecond pulsed broad-area semiconductor lasers. Nanosecond optical pulses with the peak power of 25 W and the repetition rates of 4 KHz to 240 KHz are generated from a broad-area laser. An external cavity with a diffractive grating is used to reduce the linewidth of the laser from over 5 nm to less than 0.1 nm. The wavelength of the pulsed laser is tunable over more than 10 nm. We have conducted injection locking of a nanosecond pulsed broad-area laser with optical injection from a frequency-locked master laser. Successful injection locking strongly support the feasibility of synchronization and beam combination of pulsed broad-area lasers.

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.

Blue nitride semiconductor laser diodes and light emitting diodes have recently been developed and are having a tremendous impact on industry as well as on fundamental research. Blue lasers with Indium gallium nitride (InGaN) multiple quantum wells as the active layer have been demonstrated under continuous-wave operation. This chapter describes blue vertical cavity surface emitting lasers (VCSELs) and blue InGaN quantum dot lasers. It explains the fabrication of these two types of blue lasers using metal organic chemical vapor deposition and the observation of lasing oscillation in these devices at room temperature. To produce practical blue VCSELs, the operation temperature must be increased from the cryogenic temperature to room temperature. A laser with quantum dots in the active layer is expected to have a lower threshold current and other superior performance characteristics when compared to quantum well lasers. The key issue for commercial blue VCSELs is still the growth of highly reflective nitride Bragg reflectors.

Highly brilliant diode lasers at 1120nm with a high optical output power, nearly diffraction limited beam and narrow spectral line width are increasingly important for non-linear frequency conversion to 560 nm. We present experimental results about edge-emitting distributed Bragg reflector (DBR) tapered diode lasers emitting at 1120 nm. The investigated lasers show an output power of up to 8W with a conversion efficiency of about 40%, and a lifetime of more than 5000 h at 5 W. The lasers also exhibit a small vertical divergence <15° full width at half maximum (FWHM), a nearly diffraction limited beam quality, and a narrow spectral line width with FWHM smaller than 10pm. These properties allow the lasers to be used for future second harmonic (SH) generation.

A stable tunable external cavity diode laser (ECDL) which uses an intracavity electro- optic deflector incorporated into a Littrow-type external cavity as a wavelength tunable element is proposed. The deflector comprises an antireflection-coated rectangle Lead Lanthanum Zirconate Titanate (PLZT) electro-optic ceramic with a pair of parallel trapezoidal electrodes on the up and down surfaces, which can simultaneously change the cavity length and the beam-grating angle to achieve mode-hop-free tuning. The theory for mode-hop-free tuning will be presented in this paper, and the model calculations predict that the laser can be tuned continuously by as much as 110GHz. This new approach is simple in structure with no mechanical moving parts and has potential configuration of compact grating-stabilized ECDL. Keywords: Laser tuning, Mode-hop-free, Electro-optic defl ectors, External cavity diode laser, Ceramics 1. INTRODUCTION Due to the merits of single-mode, narrow-linewidth, fine tuna bility and large tunable range, external cavity diode lasers (ECDLs) have found more and more applications in high resolution spectroscopy, synthetic aperture laser radar, optical communication systems, and so on [1~3]. Usually, the wavelength of ECDLs is usually tuned by rotating a diffraction grating in the Littrow configuration [4] or a mirror in the Littman–Metcalf configuration [5] via a piezo-electrical actuator. Continuous single-mode scanning without mode hopping can be achieved by synchronous tuning of external cavity modes and grating selectivity [6]. However, the modulation frequency and reliability are restricted by the movement mechanical construction of the laser system. To avoid the drawbacks referred above, using a deflector as the tuning element without moving component in the extended cavity appears to be more attractive, among which the electro-optic crystal (EOC), acousto-optic modulator (AOM), and nematic liquid-crystal (NLC) deflectors have had a large development [7~10]. However, the mode-hop-free tuning range is limited to 10 GHz because of limitation of the high driving voltage, as reported in reference [7]. Although using two AOMs or NLC can achieve wider tuning rage (including mode hops), up to 225GHz for AOMs [9] and 12nm for NLC [10], these kinds of tuning have a drawback in application for high repetition rates.

Lasers with the gain medium of gas, liquid, semiconductor, and solid could generate coherent light with rather narrow spectral linewidth, which play an important role in the fields of communication, measurement, sensing, and so on. Although free electron lasers have been realized with their unique advantages, they face challenges in narrowing the spectral linewidth, owing to electron energy fluctuation, Coulomb effect, and other mechanism. Here we demonstrate the superradiant Smith-Purcell radiation (S-SPR) in terahertz frequency band with ultra-narrow and continuously tunable linewidth in a compact device. By proposing a new effect of pump-induced stimulated S-SPR (PIS-SPR), the spectral linewidth could be reduced to 0.3 kHz @ 291.7 GHz, which is about two–six orders of magnitude narrower compared with those obtained by accelerators and other electron devices. Meanwhile, the wide range of continuously tunable spectral linewidth spanning 0.3–900 kHz is observed for the first time. This work provides a way to greatly narrow the spectral linewidth of free electron radiation and to achieve high-order harmonic of S-SPR in a compact device, and offers a platform to study the interaction between free electron bunches and different micro-&nano-structures.

Second harmonic generation (SHG) is used to get continuous wave laser with a lot of applications, it is a major way to provide pump power for generating nonclassical states, especially for squeezed states and entanglement states. High-efficiency SHG resonant on atoms lines also provides laser sources for atomic entanglement generation, light-atom interaction and high-speed quantum memory. For the frequency-doubling process at 426 nm, the major challenge of increasing the conversion efficiency is the thermal effect caused by the absorption in crystal. The degradation of mode-match efficiency induced by the severely thermal effect limits the conversion efficiency of the second harmonic generator. Furthermore, the blue light induced infrared absorption (BLIIRA) in the nonlinear crystal intensifies the thermal effect, it makes the conversion efficiency of the frequency-doubling cavity and the stability of the output blue laser worse, and it is more serious at high input power. Based on the theoretical analysis of thermal lens, we find that the thermal lens should not be placed at the center of the crystal, the location of the equivalently thermals lens has a deviation from the center of the crystal. Follow the theoretical analysis of thermal lens, we design a ring cavity with a 10 mm-long periodically poled potassium titanyle phosphate (PPKTP) crystal to reduce the thermal lens effect induced mode-mismatch. The location of nonlinear crystal is adjusted precisely to reduce the mode-mismatch caused by the thermal lens under our theoretical analysis. Finally, we realized a high conversion efficiency blue laser at 426 nm with the conversion efficiency up to 83.1% with an output power of 428 mW after the adjustment of the crystal location, corresponding to our theoretical analysis well. The measured beam quality factors (&lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; value) of the generated blue laser are &lt;inline-formula&gt;&lt;tex-math id="Z-20200217024354-1"&gt;\begin{document}$ M^2(x) = 1.05 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; and &lt;inline-formula&gt;&lt;tex-math id="Z-20200217024354-2"&gt;\begin{document}$ M^2(y) = 1.02 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="4-20191417_Z-20200217024354-2.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, respectively. The measured power stability of Generated Blue laser in 15 mins is 1.25%. The output power of the SHG is strong enough to provide pump power for the generation of the continuous variable squeezed vacuum state at 852 nm and the long-term stability of the output blue laser is also measured to be fine. To the best of our knowledge, the conversion efficiency is the highest-reported one at this wavelength. We believe that such high-performance frequency doubling system is a fundamental building block for quantum information science based non-classical states.

The lasing characteristics of a tunable-single-frequency laser operated in conjunction with a tunable external cavity are investigated. The effect of phase error in the external cavity on the lasing frequency, linewidth, and output power is calculated by deriving the characteristic equation for the FP cavity as a traveling amplifier with an effective mirror concept. The analysis suggests that the diode laser and the external cavity may be successfully tuned open-loop if the feedback strength is appropriate and the tuning nonlinearities in the active and passive cavities are not too severe. This is demonstrated by maintaining a linewidth of 4 MHz while tuning continuously over 34 GHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>