This PDF file contains the front matter associated with SPIE Proceedings Volume 9466, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

We propose a vertical spiral phase corrector for ring cavity surface emitting (RCSE) quantum cascade lasers (QCLs), which will allow achievement of near-Gaussian generated beam profile. A problem with RCSE QCLs is their donutshaped intensity distribution with a node along the symmetry axis of the ring. This arises because of the π phase difference for the azimuthally polarized rays emitted from opposite elements of the ring. We theoretically demonstrate that near-Gaussian beams can be achieved with a spiral phase shifter that adds one wavelength of additional optical path in going once around the ring. Various three dimensional lithographic techniques for fabricating such a phase shifter, including a grey scale mask, electron-beam resist dose dependency, and two photon induced photopolymerization, are considered. Ring cavity QCLs with the proposed phase corrector will feature better beam quality, larger power, and better resistance to radiative damage in comparison with traditional edge-emitting QCLs.

A promising approach to stand-off detection of explosive traces is using resonance Raman spectroscopy with Deepultraviolet (DUV) light. The DUV region offers two main advantages: strong explosive signatures due to resonant and λ^{- 4} enhancement of Raman cross-section, and lack of fluorescence and solar background. For DUV Raman spectroscopy, continuous-wave (CW) or quasi-CW lasers are preferable to high peak powered pulsed lasers because Raman saturation phenomena and sample damage can be avoided. In this work we present a very compact DUV source that produces greater than 1 mw of CW optical power. The source has high optical-to-optical conversion efficiency, greater than 5 %, as it is based on second harmonic generation (SHG) of a blue/green laser source using a nonlinear crystal placed in an external resonant enhancement cavity. The laser system is extremely compact, lightweight, and can be battery powered. Using two such sources, one each at 236.5 nm and 257.5 nm, we are building a second generation explosive detection system called Dual-Excitation-Wavelength Resonance-Raman Detector (DEWRRED-II). The DEWRRED-II system also includes a compact dual-band high throughput DUV spectrometer, and a highly-sensitive detection algorithm. The DEWRRED technique exploits the DUV excitation wavelength dependence of Raman signal strength, arising from complex interplay of resonant enhancement, self-absorption and laser penetration depth. We show sensor measurements from explosives/precursor materials at different standoff distances.

The latest developments in AlGaInN laser diode technology are reviewed for defence and security applications such as underwater communications. The AlGaInN material system allows for laser diodes to be fabricated over a very wide range of wavelengths from u.v., ~380nm, to the visible ~530nm, by tuning the indium content of the laser GaInN quantum well. Thus AlGaInN laser diode technology is a key enabler for the development of new disruptive system level applications in displays, telecom, defence and other industries.

The research on high-power ultrafast thulium-doped fiber lasers at 2 μm wavelength and mid-infrared (mid-IR) supercontinuum sources at 2~5 μm wavelength is one of these hot spots because of their potential applications in eye-safe radar, material processing, and electro-optical countermeasure system, which has an irreplaceable role compared to other wavelength fiber lasers. Here, we review the recent work of our group at 2 μm high-power ultrafast thulium-doped fiber lasers and 2~5 μm high-power mid-IR supercontinuum sources. Firstly, we demonstrate a high-power picosecond-pulsed thulium-doped all-fiber master-oscillator power-amplifier (MOPA), which yielded 120 W of average output power at central wavelength of 1963 nm with pulse duration of 16 ps. And then, we demonstrate a 203 W average power all-polarization-maintaining (all-PM) thulium-doped fiber MOPA system delivering picosecond pulses without using conventional chirped pulse amplication (CPA) technique. The pulse duration of 15 ps at 611.5 MHz repetition-rate results in a peak-power of 22 kW in the final PM thulium-doped fiber power amplifier. The polarization extinction ratio (PER) at the highest average output power was measured to be >15 dB, further power scaling is limited by available pump power. Finally, we demonstrate a high-power mid-IR supercontinuum generation with up to 21.8 W average power and spectrum spanning from 1.9 to beyond 3.8 μm in a single-mode ZBLAN fiber pumped by a 2 μm amplified picosecond pulses from a single-mode thulium-doped all-fiber MOPA.

We report on a Kerr lens, self-mode-locked Er³⁺:YVO₄ laser, based on the high third order nonlinearity of the gain medium. The Er³⁺:YVO₄ is resonantly pumped by a CW Er-fiber laser into the absorption band around 1538 nm and operates at ~ 1604 nm with a 1.8 W average output power and ~ 40% slope efficiency relative to the absorbed pump. The laser yields a pulse train at a 240 MHz repetition rate. The pulsewidth is estimated to fall between 15 and 100 psec.

Power scaling using a higher order mode in a ribbon fiber has previously been proposed. However, methods of selecting the higher order mode and converting to a single lobe high brightness beam are needed. We propose using a multiplexed transmitting Bragg grating (MTBG) to convert a higher order mode into a single lobe beam. Using a ribbon fiber with core dimensions of 107.8 μm by 8.3 μm, we use the MTBG to select a higher order mode oscillating within the resonator with 51.4% efficiency, while simultaneously converting the higher order mode to a beam with diffraction limited divergence of 10.2 mrad containing 60% of the total power.

Optical phased arrays (OPAs) are being actively developed as the next generation of high power laser systems. The coherent combination of several laser beams in tiled OPA configurations, as well as their propagation through the atmosphere has been successfully demonstrated. At the same time, only limited discussions of OPA beam formation and propagation characteristics can be found in scientific literature. This paper is intended to fill the void by providing details about OPA beam formation phenomenon, as well as by presenting the key OPA beam characteristics. The influences of several OPA characteristics, including the aperture fill factor, the number of the individual beams comprising the OPA, and the shape of the individual beams on the formation and propagation of the laser radiation are presented. We also demonstrate the impact of OPA beam phase discontinuities, such as piston phase delays between the individual beams, on the resulting properties of the OPA beams.

Phosphate glasses are known to produce high gain for the Er3⁺ emission into 1540nm, especially when sensitized with Yb. Unfortunately, the phosphate glass matrix tends to be weaker than other available amorphous materials. Being that glasses are engineerable, a study was initiated in order to strengthen the glass structure of a commercially available phosphate laser glass without impacting its laser output efficiencies. Secondly, we seek to understand the impact of the various glass modifiers that drive thermal shock resistance of phosphate glasses on the Er emission manifolds. This report details a number of compositions that were designed, melted and analyzed for properties. Laser output performance results for the glasses that met the targeted parameters are presented.

We have recently demonstrated efficient high power CW laser operation in Er doped Y₂O₃ at cryogenic temperature. The selection of laser host was based on the low-phonon nature of Y₂O₃, where the ⁴I_11/2 →⁴I_13/2 transition is highly radiative. Further increases in mid-IR power scaling and efficiency require in-depth study and analysis of basic spectroscopic properties of the ⁴I_11/2 → ⁴I_13/2 laser transition, such as laser emission cross- sections, fluorescence quantum efficiency, fluorescence branching ratios and inter-ionic interactions in a wide dopant-concentration and temperature range. In this work, we report the results of experimental measurements of quantum efficiency and branching ratio of the erbium initial laser state of ⁴I_11/2 in Y₂O₃ ceramic at the temperature range of 10 - 300 K. A series of Er:Y₂O₃ samples with dopant concentration between 0.2-10 at.% were used for fluorescence and absorption measurements. The fluorescence from the energy states corresponding to the visible, IR, and mid-IR transitions were studied under shortpulsed diode laser excitation. Spectrally-narrowed fiber-coupled semiconductor laser modules emitting at ~ 808, 980, and 1530 nm with variable power density were used for fluorescence excitation of three different Er states. The energy transfer processes for both down- and up-conversion, affecting the Er:Y₂O₃ mid-IR laser operation, were analyzed. A comparison between the experimental and simulation results are presented as well.

The transient theory of stimulated Brillouin Scattering (SBS) in optical fibers is used to investigate the effects of feedback of an incident periodic phase-modulated laser field. Phase modulation with a single sine wave and a pseudo-random binary sequence (PRSB) are investigated. It is shown that a reflection as small as 0.001% (-50 dB) of a laser field from the fiber end can lead to enhancement in the reflected power and effectively reduce the SBS threshold if the laser optical spectrum overlaps with the SBS gain spectrum. These results have implications on the design of high power fiber laser systems that utilize periodic phase modulation techniques for broadening the laser field to suppress SBS.

We review our work on a femtosecond erbium-doped all-fiber laser mode-locked with graphene oxide saturable absorber, which can be conveniently obtained from natural graphite by simple oxidation and ultra-sonication process. The laser directly generated 200 fs pulses at a repetition rate of 22.9 MHz. The stable passively Q-switched operation by graphene oxide saturable absorber in the 1 μm ytterbium-, 1.5 μm erbium-, and 2 μm thulium-doped fiber lasers will be demonstrated as well. These results are comparable with those of graphene saturable absorbers and the superiority of easy fabrication and hydrophilic property of graphene oxide will facilitate its potential applications for ultrafast photonics.

We investigate the use of external phase modulation to broaden the linewidth of a laser source. We use nonlinear optimization to find phase modulations that create nearly tophat-shaped discrete spectra and thus the highest total power within a limited linewidth and a limited peak spectral power density. Such phase modulations and spectra can be realized with an arbitrary waveform generator (AWG) and are attractive for suppressing stimulated Brillouin scattering in optical fiber. Compared to alternative modulation approaches, the AWG benefits from a large number of degrees of freedom and well-controlled spectral phase in the AWG output.

A compact laser with a volume of < 8 cm³ and a weight of < 80 g finds its application in many fields from military to space based. We built a small solid-state laser that produces 1 mJ of energy per-pulse at a 1 - 20 Hz repetition rate. The laser is passively Q-switched using a Cr^4+:YAG saturable absorber to generate pulses < 10 ns. A nonlinear crystal doubles the frequency to generate light at 523 nm. The laser is side-pumped by a single bar diode laser using a unique pump cavity to homogenize the pump intensity in the laser rod. The laser components can easily be modified to change the output wavelength from UV to mid IR.

We report on the continuous wave operation of a Ho:YAP laser pumped by an all-fiber Tm-doped fiber laser, the pump laser wavelength is 1.915μm and the output laser wavelength is 2.118μm. The all fiber Tm-doped fiber laser has 70W max output power with 200W pumped power, and the output laser wavelength is 1.915μm. And this laser is used as pump laser to a Ho:YAP laser system. 20.2W CW laser power is obtained from a 0.5 at % Ho³⁺-doped YAP crystal at 2118.4nm with slope efficiency of 72%.

A Yb LMA fiber amplifier based 1030nm laser transmitter capable of operating with high average power and peak power (~500W, 9kW) is presented. The prototype, all-fiber, high TRL level laser transmitter is designed to meet all the single aperture requirements of a multi aperture deep space laser beacon system including operation with Nested pulse position modulation (PPM) format. Nested PPM format consist of an inner modulation PPM- (8,4) with 128nsec slot size and an outer modulation PPM-(2, 2) 65.5usec slot size. Here, nested PPM operation is presented for the first time. In implementing inner modulation strong pre-pulse shaping is required where PPM pattern dependent pulse energy variation (PEV) is minimized. Outer modulation is implemented by directly modulating VBG locked pump lasers for the final two gain. A sophisticated multi-stage, ultra-fast loss of signal (LOS) and backward Raman/lasing monitoring algorithm is implemented for ensuring reliable operation. Mechanical and electrical design of the delivered laser is scalable to multiple apertures.

Defense application for a chemical oxygen laser (COL) is explained. Although a COL has not yet been successful in lasing, the oscillator was estimated to produce a giant pulse with the full width at half maximum (FWHM) of ~0.05ms which makes the damage threshold for the mirrors several-order higher than that for a typical solid-state laser with a ~10ns pulse width. Therefore it has a potential to produce MJ class output considering the simple scalability of being a chemical laser. Since within 0.05ms a supersonic aircraft can move only a few centimeters which is roughly equal to the spot size of the focused beam at ~10km away using a large-diameter focusing mirror, a COL has a potential to make a damage to an enemy aircraft by a single shot without beam tracking. But since the extracted beam can propagate up to a few kilometers due to the absorption in the air, it may be suitable to use in space. While a chemical oxygen-iodine laser (COIL) can give a pulsed output with a width of ~2 ms using a high-pressure singlet oxygen generator (SOG). Therefore a pulsed COIL may also not require beam tracking if a target aircraft is approaching. Another advantage for these pulsed high-energy lasers (HELs) is that, in case of propagating in cloud or fog, much less energy is required for a laser for aerosol vaporization (LAV) than that of a LAV for a CW HEL. Considerations to use a COL as a directed energy weapon (DEW) in a point defense system are shown.

This paper summarizes the results of simulation of a module for side-pumping a Nd:YAG rod. The module consists of three laser diode arrays separated by 120° rotation angle around the laser rod, where each array contains 10 emitters producing a maximum output power of 15 W at 808 nm wavelength. This high power diode pumped solid-state (DPSS) laser system was modeled in both LASCAD and GLAD simulators. LASCAD model was used to simulate the laser output power as a function of the total input power and the output mirror reflectivity. The model predicted an output power of 140 W given 400 W total input pump power with optical efficiency of 35%, in a good agreement with the published experimental results and similar commercially available CW DPSS laser systems. LASCAD was also used to model the temperature distribution inside the rod and to examine the heat load and thermally induced mechanical stress on the rod. Simulation in GLAD enabled a detailed analysis of the beam quality, beam size, and mode stability inside the resonator. GLAD models were used to simulate the pumping light distribution in the Nd:YAG rod for a single diode element, a single diode array, and three diode arrays. The GLAD shows that a stable multi-transverse mode "top hat" beam is formed after 30 passes through the resonator of the adopted high power DPSS laser system.