This PDF file contains the front matter associated with SPIE Proceedings Volume 7325, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

The laser performance of a resonantly pumped Er³⁺:YAG solid-state heat-capacity laser (SSHCL) as an eye-safe high-power laser was investigated experimentally and theoretically using a new analytical quasi-three-level laser model as well as a fully numerical thermo-optical model. While the analytical model is based on a new spectral theory that can predict the thermal evolution of the peak absorption and emission cross-sections for pump and laser transition in a simple mathematical form, the numerical model takes into account the full spatially resolved temperature dependence of the most important parameters of the gain medium like thermo-mechanical parameters (e.g. heat conduction and specific heat), spectroscopic and multiphonon-relaxation lifetimes of the first four manifolds and the full spectrally resolved information of emission and absorption (⁴I_15/2 ↔ ⁴I_13/2) as well as exited-state absorption and re-emission (⁴I_13/2 ↔⁴I_9/2). In the experiments up to 76 W of peak output power could be achieved with a slope efficiency of 39%. The laser could be operated for about 2 s with an integrated output energy of 55 J, currently limited by the available pump power with respect to the temperature-dependent threshold. Cavity parameters like internal losses (approximately 3.6%) and a mode-fill efficiency of up to 79% were also derived as well as the effective medium energy storage lifetime caused by fluorescence radiation trapping. The experimental data are compared with the models of the diode-pumped Er3+:YAG SSHCL with good agreement.

We have measured upconversion fluorescence resulting from the excitation of room temperature 1% and 5% Er:YAG by a 1532 nm nanosecond pump source. Measurements of the fluorescence decay from the ⁴I_13/2, ⁴I_11/2, ⁴F_9/2 and ⁴S_3/2 manifolds to the ⁴I_15/2 ground state manifold were observed over a wide range of excitation pulse fluence values. A unique set of upconversion parameters were extracted from the measurements using a modification of the traditional rate equation model. Results of calculation are compared to measurement.

Growing interest to high power lasers in the eye-safe spectral domain initiated a new wave of activity in developing solid-state lasers based on bulk Er3+-doped materials. The resonant pumping of SSL allows for shifting significant part of thermal load from gain medium itself to the pump diodes, thus greatly reducing gain medium thermal distortions deleterious to SSL power scaling with high beam quality. The two major resonant pumping bands in Er:YAG are centered around 1470 and 1532 nm. Pumping into each of these bands has its pros and contras. The best approach to resonant pumping of Er:YAG active media in terms of pump wavelength is yet to be determined. We report the investigation results of high power diode-pumped Er:YAG laser aimed at direct comparison of resonant pumping at 1470 and 1532 nm. Two sources used for pumping were: 1530-nm 10-diode bar stack (>300 W CW) and 1470-nm 10-diode bar stack (>650 W CW). Both pumps were spectrally narrowed by external volume Bragg gratings. The obtained spectral width of less than 1 nm allowed for 'in-line' pumping of Er3+ in either band. The obtained CW power of over 87 W is, to the best of our knowledge, the record high power reported for resonantly pumped Er:YAG DPSSL at room temperature.

We are investigating materials for direct blue solid-state lasers assuming UV excitation with GaN based laser diodes. Room temperature spectroscopy is reported relevant to a proposed quasi-three level laser from the ⁴F_9/2 level in trivalent dysprosium. Modeling based on these measurements suggests this is a promising new laser transition.

We report on a novel electronically controlled active heat sink for high-power laser diodes offering unparalleled capacity in high-heat flux handling and temperature control. The heat sink receives diode waste heat at high flux and transfers it at reduced flux to environment, coolant fluid, heat pipe, or structure. Thermal conductance of the heat sink is electronically adjustable, allowing for precise control of diode temperature and the diode light wavelength. When pumping solid-state or alkaline vapor lasers, diode wavelength can be precisely temperature-tuned to the gain medium absorption features. This paper presents the heat sink physics, engineering design, and performance modeling.

Even though Yb-doped fiber lasers are known to be the most powerful and most efficient among all fiber lasers, recent successes in the eye-safe ~1.5μm Yb-Er-doped fiber lasers (where Er is excited through Yb-Er energy transfer) are quite impressive. Output power of Yb-Er fiber lasers reached ~300 W level and their optical-to-optical efficiency, for somewhat lower power levels, is exceeding 40% [2]. Nevertheless, as far as real eye safety is concerned, multi-hundred Watt Yb-Er fiber lasers typically carry in their output a significant fraction of competing 1-μm Yb emission, which totally compromises an eye-safe side of the application. Ultimate efficiency and thermal management of Yb-Er approach are also suffering due to: (i), inefficiency of Yb-Er energy transfer and, (ii), gigantic ~40% quantum defect of Er-doped fiber pumped at 9XX-nm. Presented here are very recent and successful results on power scaling of resonantly pumped Yb-free Er-doped fiber lasers and amplifiers. We are reporting an Ybfree Er-doped cladding-pumped fiber power scaling to ~50 W with ~57% optical-tooptical efficiency [6] in a few first experimental steps. This is clear manifestation of scaling potential of this most efficient approach to high power eye-safe fiber laser. The only competing approach to scalable eye-safe fiber laser implements Tm³⁺-doped fibers pumped at ~790 nm while relying on well known "2-for-1" process leading to quite efficient excitation of the ~2μm Tm³⁺ laser operation [4]. This approach has operational optical-to-optical efficiency quantum limit of ~75% [4], while resonantly pumped Ybfree Er-doped fiber laser's optical-to-optical efficiency quantum limit exceeds 95% due to its low-quantum-defect (QD) pump-lase scheme. Significant scaling potential of resonantly-pumped Yb-free Er-doped fiber lasers and amplifiers sets a path to an eye-safe fiber laser concept with drastically relaxed thermal management and nearly diffraction limited beam quality at ~kW-=-class power levels as well as high electrical to optical efficiency.

We have designed and developed a grating based thulium (Tm) doped fiber laser with ~150 nm tuning range which is used as the master oscillator in a master oscillator power amplifier (MOPA) thulium fiber laser system. Due to thermal instability in the grating used for tuning, the MO could produce a power up to 4.5 W, beyond which the oscillator became unstable. Injecting the seed laser into a bidirectionally pumped large mode area (LMA) Tm fiber amplifier, a stable, tunable, narrow linewidth high beam quality amplified signal of >100 W was achieved. In the absence of stable and sufficiently high power from the seed laser, the amplifier could not be tested to its full potential. The amplifier was also, converted into an oscillator to investigate its power handling capability. An excellent beam quality and ~200 W of power were achieved by running the power amplifier as an oscillator. Operation stability of the oscillator was measured to be more than one hour with a minimum power fluctuation of 0.5%. Currently efforts are underway to increase the seed laser power to ~10 W, large enough to reduce ASE and mitigate feedback to the master oscillator to demonstrate a 200 W, tunable (150 nm) and narrow linewidth (0.15 nm) MOPA system. The MOPA system will be one of a number of new state-of-the-art high power lasers to be located at the Innovative Science & Technology Experimentation Facility, creating a unique laser range facility for next generation studies and tests across a broad range of sciences and technologies.

Progress is being made developing monolithic, all-fiber 2μm wavelength devices that operate robustly at higher power levels. This development includes the critical Tm-doped LMA fiber technology, compatible components such as pump combiners and couplers, along with the optimization of high brightness, high efficiency 790nm pump diodes. In this paper we present recent CW power scaling results and demonstrate a monolithic MOPA system operating at 400W output power with around 20% E-O efficiency.

In this paper we present design considerations, thermal and optical modeling results, and device performance for a ruggedized, compact laser transmitter that utilizes a room temperature quantum cascade (QC) laser source. The QC laser transmitter is intended for portable mid-infrared spectroscopy applications, where the 3 to 5 μm and 8 to 12 μm atmospheric transmission window is relatively free of water vapor interference and where the molecular rotational vibration absorption features can be used to detect and uniquely identify chemical compounds of interest. Initial QC laser-based sensor development efforts were constrained by the complications of cryogenic operation. However, improvements in both QC laser designs and fabrication processes have provided room-temperature devices that now enable significant miniaturization and integration potential for national security, environmental monitoring, atmospheric science, and industrial safety applications.

The mid-infrared spectral region is of significant interest due to the atmospheric absorption lines present in this region. We report the development of a compact, highly efficient, high power intra-cavity pumped all-solid-state optical parametric oscillator (OPO) producing nanosecond pulses with an output tunable from 1.5 microns to 3.4 microns with pulse energies ranging from 0.5mJ to over 4mJ over the entire range at kHz repetition rates. We have built both watercooled and air-cooled versions of the OPO for various power level requirements.

In this contribution, we demonstrate that spectral beam combining in an external cavity (EC), a technique which has been applied previously to shorter wavelength diode laser bars [1], is also applicable to mid-infrared QC lasers. Within this concept, the output of multiple emitters from a 4.6 μm emitting QC laser chip is combined in a single, collinear beam. The average power of an EC-QC laser module realized that way surpasses the output of a corresponding single emitter by more than a factor of 4. Furthermore, the EC-concept allows a certain degree of wavelength tuning during operation. The EC, consisting of a collimating lens, a grating and a partially reflecting outcoupling mirror, forces each laser to emit at a unique wavelength defined by its offset relative to the main optical axis. The EC approach further ensures the collinear directional and spatial overlap of the individual QC laser output beams forming a single combined output beam.

A simple scheme for efficient generation of mid-IR laser radiation is reported. Using a 50 W thulium fibre laser to pump a Q-switched Ho:YAG laser 27.3 W of average output power is achieved with an M² of 1.5 (optical to optical conversion efficiency 65%) at a wavelength of 2.1 μm. The holmium laser is used to pump a ZGP OPO generating 12.6 W of average output power in the 3-5 μm waveband (optical to optical conversion efficiency 52%) with a worst case M² of 2.7. These efficiencies were maintained at 25% and 50% duty cycle operation.

Commercially available quantum cascade gain media has been integrated with advanced coating and die attach technologies, mid-IR micro-optics and telecom-style assembly and packaging to yield cutting edge performance. When combined into Daylight's external-cavity quantum cascade laser (ECqcL) platform, multi-Watt output power has been obtained. Daylight will describe their most recent results obtained from this platform, including high cw power from compact hermetically sealed packages and narrow spectral linewidth devices. Fiber-coupling and direct amplitude modulation from such multi-Watt lasers will also be described. In addition, Daylight will present the most recent results from their compact, portable, battery-operated "thermal laser pointers" that are being used for illumination and aiming applications. When combined with thermal imaging technology, such devices provide significant benefits in contrast and identification.

The threat posed to aircraft by shoulder-fired missiles has increased to the point where initial testing of laser-based IRCM systems on commercial aircraft is underway. However, the laser sources used in this testing are complex and costly. Consequently, the need for a simpler, cost-effective, compact, and power-efficient laser source technology is urgent. Over the last two years, quantum cascade (QC) lasers have advanced in performance and reliability to the point where a thorough, experimentally based assessment of the applicability of these lasers as IRCM sources is called for. In this talk, I will describe Maxion's development of a wavelength-beam-combined laser source module capable of emitting, in principle, multi-Watt-levels of optical power in a single, near-diffraction-limited beam. Details of the single emitter performances in the linear QC laser array are provided as well as the power performance and optical beam characteristics of the WBC system.

Leveraging Pranalytica's fundamental research into high power and high wallplug efficiency QCL devices and high performance/reliability QCL packaging technologies, we developed several models of turn-key QCL systems for security-related applications. Our tabletop high power system produces, at room temperature, more than 2W of nominally collimated continuous wave radiation at 4.6 μm. Our flashlight-size portable illuminator at 4.6 μm produces over 100 mW average power portable illuminator at 9.6 μm produces more than 20 mW, both with runtime of ~10 hrs. These systems are opening the window of QCL acceptance into real-world security and defense applications. At chip level, we have demonstrated 3W of CW power at room temperature from a single, high reflectivity coated chip.

Frequency noise reduction of semiconductor lasers using electrical feedback from an optical frequency discriminator is an efficient and simple approach to realize narrow linewidth lasers. These lasers are of great interest for applications such as LIDAR, RF photonics and interferometric sensing. In this paper, we review the technological choices made by TeraXion for the realization of its Narrow Linewidth Laser modules. The method enables to decrease the linewidth of DFB lasers from several hundreds of kHz to a few kHz. We present the work in progress to integrate such system into a miniature package and to incorporate advanced functionalities such as multi-laser phase locking.

In this paper we present the use of high power diode arrays, spectrally stabilised using chirped Volume Bragg Gratings as a pump source for a Nd:YAG based laser. The temperature dependant performance of a series of different stabilised diodes, and the side pumped Nd:YAG slab resonator was measured over a 55°C temperature range. The best performing stabilised LDAs exhibited Q-switched output energy consistent over 80% of the temperature range, and drop off by 40% at the higher temperature extremes. Beam parameters of the laser such as divergence were found to drop in combination with input energy. Factors such as spectral drifting of the diodes are also considered and the effect on the resonator is characterised.

Compact, efficient visible lasers are important for heads up displays, pointing and illumination, undersea communications, and less than lethal threat detection. We report on high power red, green, and blue lasers with output powers above 3 watts and efficiencies greater than 20%, 15%, and 5% respectively.

We report on the progress of highly-reliable, high-efficiency 885-nm diode laser bar arrays. Conduction-cooled hardsoldered bars rated to 60W and 57% conversion efficiency demonstrate >30,000 device hours under 1-sec on, 1-sec off hard pulse conditions failure-free. Microchannel-cooled bars rated to 100W and 62% efficiency demonstrate >100,000 accelerated device hours failure-free. Integrated volume Bragg grating fast axis lenses provide wavelength stabilization at low cost. Vertically stacked arrays (seven bars each) of such configuration are demonstrated with a 0.8 nm FWHM spectral width and rated to 700W, 53% conversion efficiency.

We report on the development of ultra-high brightness laser diode modules at nLIGHT Photonics. This paper demonstrates a laser diode module capable of coupling over 100W at 976 nm into a 105 μm, 0.15 NA fiber with fiber coupling efficiency greater than 85%. The high brightness module has an optical excitation under 0.13 NA, is virtually free of cladding modes, and has been wavelength stabilized with the use of volume holographic gratings for narrow-band operation. Utilizing nLIGHT's Pearl product architecture, these modules are based on hard soldered single emitters packaged into a compact and passively-cooled package. These modules are designed to be compatible with high power 7:1 fused fiber combiners, enabling over 500W power coupled into a 220 μm, 0.22 NA fiber. These modules address the need in the market for high brightness and wavelength stabilized diode lasers for pumping fiber lasers and solid-state laser systems.

High Power Laser Diode Arrays developed and produced at SCD-SemiConductor Devices support a number of advanced defence and space programs. High efficiency and unsurpassed reliability at high operating temperatures are mandatory features for those applications. We report lifetime results of high power bar stacks, operating in QCW mode that rely on a field-proven design comprising Al-free wafer material technology and hard soldering robust packaging. A variety of packaging platforms have been implemented and tested at very harsh environmental conditions. Results include a long operational lifetime study totaling 20 billion pulses monitored in the course of several years for 808 nm QCW bar stacks.. Additionally, we report results of demanding lifetime tests for space qualification performed on these stacks at different levels of current load in a unique combination with operational temperature cycles in the range of -10 ÷60 °C. Novel solutions for highly reliable water cooled devices designed for operation in long pulses at different levels of PRF, are also discussed. The cooling efficiency of microchannel coolers is preserved while reliability is improved.

InP based diode lasers are required to realize the next generation of eyesafe applications, including direct rangefinding and HEL weapons systems. We report on the progress of high power eyesafe single spatial and longitudinal mode 1550nm MOPA devices, where we have achieved peak powers in excess of 10W with 50ns pulse widths. A conceptual model based on our recent MOPA results show the path towards scaling to high powers based on spatial beam combination with operating conditions suitable for direct rangefinding applications. We also report on the progress towards high power 14xx and 15xx nm pump lasers for eyesafe HEL systems.

Directional Infrared Countermeasures (DIRCM) is an effective technique to defeat heat-seeking missiles. The major problem of existing DIRCM is that it may work like a beacon for threats that are not susceptible to the jamming code implemented: attracting a missile instead of re-directing it away from the aircraft. Ultra-fast laser pulse technology is discussed as an alternative to a conventional laser DIRCM. An ultra-fast laser is capable of providing a different type of countermeasure which is compatible with existing laser based DIRCM pointing systems as it requires much less peak power than damage inducing systems. A foundation of ultra-fast technology is its unique ability to alter the intrinsic characteristics of the semiconductor. In this paper, we will only consider the effects of a mild lattice disturbance caused by relatively low energy ultra-fast (femto-second) and, to some extent, fast (pico-second) laser pulses.

Few cost-effective methods exist for measuring the wave front of mid- and long-wave infrared beams that span 3-5 and 8-12 μm, respectively. One obvious need within the infrared laser community is the ability to measure the degree of collimation of an infrared laser beam, e.g. that formed by a beam expanding telescope used with a CO₂ laser or a system of lenses used to collimate a Quantum Cascade Laser (QCL). An ideal approach for this type of metrology is the use of a lateral shearing interferometer (LSI). An LSI uses various methods to displace ("shear") a beam with respect to itself to create an interferogram that can subsequently be used for diagnosing the wavefront quality of the beam. Since this type of interferometer is of the common path variety, it is insensitive to vibration making it ideal for field applications, where vibration isolation may neither be possible nor practical. In this paper we present and demonstrate, through laboratory measurements and computer ray tracing simulations, a low-cost LSI using a single commercial off-the-shelf uncoated ZnSe window in conjunction with an infrared camera. This plane parallel plate LSI configuration was used to interactively collimate a LWIR beam and also provide quantitative transmitted wavefront error data using static fringe analysis software. We also present a self-contained review of the theory of lateral shearing interferometry, including the necessary design equations as applicable to this LSI configuration, to enable researchers to construct a similar beam diagnostic shearing interferometer from readily available components.

Chalcogenide fibers display a wide transmission window ranging from 2-10.6 μm, ideally suited to the development of passive and active mid-infrared (MIR) sensors. They are essential building blocks for the integration and miniaturization of laser-based MIR optical systems for terrestrial, airborne and space-based sensing platforms. Single-mode chalcogenide fibers have only recently become commercially available and therefore performance data and standard reproducible processing techniques have not been widely reported. In this paper we present a method for producing high quality cleaved facets on commercial single-mode As-Se fibers with core and cladding diameters of 28μm and 170μm respectively. The emitted beam profile from these fibers, using the 9.4μm line of a tunable CO₂ laser, showed the presence of leaky cladding modes due to waveguiding conditions created by the protective acrylate jacket. These undesirable cladding modes were easily suppressed by applying a gallium coating on the cladding near both input and output facets. We provide experimental data showing efficient mode suppression and the emission of a circular nearperfect Gaussian beam profile from the fiber. Furthermore, analyses of the beam, acquired by scanning an HgCdTe detector, yielded a 1/e² numerical aperture of 0.11 with a full width half maximum divergence of 11° for these fibers. The availability of single-mode MIR fibers, in conjunction with recent advances in room temperature quantum cascade lasers (QCL), could provide compact and light-weight transmitter solutions for several critical defense and nuclear nonproliferation needs.