This paper is a survey of recent achievements at the College of Optics and Photonics/CREOL at the University of Central Florida in the use of newly developed diffractive optical elements which are volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass. Three levels of semiconductor laser design are proposed to achieve high-power low-divergence output. The first level is the change of a mechanism of transverse mode selection from spatial selection by apertures to angular selection by PTR Bragg gratings. This approach allows increasing of aperture without increasing of length and selecting of arbitrary mode but not only a fundamental one. The second level is coherent coupling of emitters by means of PTR Bragg gratings which provide excitation of the only one common mode in a multichannel resonator. This type of phase locking automatically leads to a narrow spectral width of emission usually not exceeding a few tens of picometers. The third level is spectral beam combining by a stack of PTR Bragg gratings which re-direct radiation from several phase coupled arrays to the same direction within diffraction limited divergence. This approach allows simplifying of thermal management because the only passive device with low absorption (a PTR beam combiner) is placed in a high power laser beam.

The problem of high-brightness, narrow line semiconductor lasers sources is important for different kinds of applications. The proposed solution of the problem is the use of an external cavity with volume Bragg grating for effective angular and spectral selection. High-efficient volume Bragg gratings provide complete selection directly in space of wave vectors and serve as a diaphragm in angular space. The condition of effective selection is the provision of a substantial difference in losses for a selected mode by matching angular selectivity of a Bragg grating with divergence of the selected mode. It was proposed off-axis construction of an external cavity with a transmitting volume Bragg grating as an angular selective element and a reflecting volume Bragg grating as a spectral selective feedback. In such external cavity broad area laser diodes have shown stable near-diffraction limited generation in the wide range of pumping current. For LD with 0.5% AR-coated mirror and 150 μm stripe it was achieved 1.7 W output power with divergence of 0.62° at current exceeding six thresholds. Total LD slope efficiency in the considered external cavity is less then slope efficiency of free running diodes by 3-5% only. Spectral width of such locked LD emission was narrowed down to 250 pm in the whole range of pumping current.

Dramatic spectral narrowing of normally broad band lasers, Ti:Sapphire,Cr:LiSAF, and alexandrite was achieved by simply replacing the output mirror with a reflective, volumetric Bragg grating recorded in photo thermal refractive (PTR) glass. The output power of each laser was changed very slightly from that obtained using dielectric coated output mirrors with the same output coupling as the Bragg grating while spectral brightness increased by about three orders of magnitude.

We have successfully demonstrated white-light continuum (WLC) generation in the near-infrared (NIR) region from 1.1 μm to 2.5 μm using specially designed highly nonlinear optical fibers. A passively mode-locked diode-pumped Er:Yb:glass laser with a semiconductor saturable-absorber mirror (SESAM) successfully generates femtosecond pulses with about 90 mW average output power, which is sufficient to produce the WLC with over 40 mW power without any additional optical amplification. This WLC source is expected to be suitable for many applications, such as laser radar systems and optical gas sensing.

Recently, the advantages of Yb:YAG materials for high-power and short-pulse lasers are well recognized because of its low thermal loading and broad emission bandwidth even if it is a quasi-four-level system. A face-cooled microchip, equivalent configuration to the active mirror, can reduce the thermal problems. Additionally, it is possible to minimize the re-absorption loss in Yb:YAG due to its short active medium in compensation for the pump absorption degradation. Our approach to this problem is to employ an edge-pumping configuration. Pump light propagates from the edge of outer transparent composite ceramic YAG wave-guide to the internal single crystal Yb:YAG small core without optical loss by total internal reflection. Proper designs of core size and Yb concentration allow efficient pump absorption in the core. It should be emphasized that the absorption ratio of pump light in the core does not depend on the thickness of the microchip then thinner microchip allows higher pump power absorption intensity and higher gain in the core. The sintering method has advantages in composite structure fabrication due to its solid-solution nature. It is attractive for actual applications because of low fabrication cost by mass production and short delivery time compared with conventional diffusion bonding. In this research, we'd like to report about >300 W CW laser operation in edge-pumped 300μm-thick, single crystal Yb:YAG/ceramic YAG composite microchip. Further power scaling possibility will be discussed.

High-power laser sources are used in many applications for material processing, like annealing, welding, soldering, printing and micro-machining. Additionally, they are widely used as illumination sources for metrology - e.g. LIDAR - and vision systems based on CCD cameras. In many of these applications homogenous top-hat square or rectangular light fields as well as light lines are indispensable or add strong advantages to the application. LIMO has a unique production technology based on computer-aided design that enables the manufacture of high-precision microlens arrays with free programmable surfaces. Thus, specific beam profiles with superior uniformity and efficiency can be generated. Compact beam shaper modules with prealigned optics have been developed. These modules simply have to be placed into the collimated input beam and the required intensity profile is generated at the target without any complicated alignment. High-power diode laser systems for the NIR spectral region from 1W to several KW are available with application-specific beam shapes due to integrated micro-optics. Special attention is put on compact and solid design for rough environmental conditions.

We report on improvement from 50% to 70% power conversion efficiency on a 5-bar stack with 500 W of CW power at 25C coolant temperature resulting from a multi-pronged optimization approach. We also report on wavelength stabilization (0.07 nm/C) and emission bandwidth narrowing (0.3 nm at FWHM) of diode laser pump sources for precision pumping the upper transition levels of lasers that require narrow and stable pump sources such as Er/Yb co-doped or Yb:YAG lasers. These results have been achieved by integration of a Bragg grating inside a semiconductor laser cavity forming a low-loss, weak distributed feedback (DFB) laser, which results in record 53% wall-plug efficiency at 3 W CW operation and 25°C heatsink temperature from a 100-μm aperture diode laser and 45 W of wavelength-locked CW power from a 20% fill factor bar. This technique can be readily applied to diode laser structures for other strategic pump wavelengths.

Interest is rapidly growing in solid-state lasers emitting from 1500-nm to 2100-nm with applications in eye-safe range finding, LIDAR, infrared countermeasures, medicine, dentistry, and others. Traditionally, these solid-state lasers have been pumped by flash lamps or more recently, by semiconductor diode lasers. In the case of the latter, the diodes of choice have been those emitting below 1-μm. The sub-micron class of semiconductor diode lasers is highly mature and has enjoyed recent rapid advances in power and efficiency. Unfortunately, the quantum defect generated when converting to the desired wavelengths results in large amounts of excess heat generation leading to costly and heavy, expensive cooling systems and performance problems related to thermal lensing. System complexity adds further cost and weight when intermediaries, such as optical parametric oscillators, are required to reach the desired longer wavelengths. Recent advances in laser diodes emitting from 1400-nm to over 1900-nm now enable the near resonant pumping of such solid state media as Er:YAG, Ho:YAG and Cr:ZnSe. Record results in the peak output power and electrical-to-optical conversion efficiency of diode lasers emitting around 1470-nm, 1700-nm and 1900-nm are presented here.

We present recent advances in high power semiconductor laser bars and arrays at wavelengths from the near infrared to the eye-safe regime including increased spectral brightness with internal gratings to narrow and stabilize the spectrum, increased spatial brightness with multimode and high power single mode performance, and reduced cost architectures from high power surface emitting 2-dimensional arrays. These devices have the potential to dramatically improve diode pumped systems and enable new direct diode applications.

In many sensing systems, a highly coherent laser source is necessary to perform sensitive interferometric or coherent measurements. At TeraXion, we have built a compact laser system that provides a stable laser frequency with a very narrow linewidth using a 60 mW DFB semiconductor laser. The linewidth reduction system uses a frequency discriminator to measure the laser frequency noise and provides an electrical feedback to reduce this noise over a given bandwidth. Experimental work shows that the phase noise of the DFB semiconductor laser can be reduced by more than 4 orders of magnitude from 10 Hz to 100 kHz. We analyzed the effect of the particular frequency noise spectrum of such a laser on its degree of coherence, its linewidth and the resulting interferometric noise. The laser linewidth computed from the power spectral density of frequency noise of the laser is reduced from 570 kHz down to an equivalent of 1.8 kHz when the output signal is observed for 30 ms, and from 370 kHz to 18 Hz for 1 ms. Similarly, the coherence length is increased from 145 m up to 45 km for fringes observed over 30 ms. Each result is compared with those obtained with a fiber laser.

A study of the various channels of energy transfer from the upper lasing level of Nd:YVO₄ following direct pumping at 880nm is presented. The dependency of the heat that was generated in the laser crystals as a function of doping concentrations and different pumping and lasing conditions was measured along with the laser performance. In the absence of a laser resonator, the heat to pump power relation behaved linearly and was strongly dependent upon the Nd⁺³ doping concentration. During lasing at 1064nm, the heat to pump ratio dropped dramatically but still did increase linearly with growing concentrations. The heat to pump dependency upon different output couplers with the same laser crystal was ~linear too and increased with increasing intra-cavity power density. These experimental results suggest that different mechanisms govern the heat creation before and after laser threshold. Cross relaxation on top of the Stokes shift seems to govern heat creation before threshold, while excited state absorption and crystal impurities may be the mechanisms that govern the excess heat creation above threshold.

In 2000, Textron Systems Corporation (TSC) initiated the development of an advanced diamond cooled solid-state laser concept suitable for ultra compact medium and high-power lasers. The resulting laser configuration is applicable to laser diode pumping and a wide variety of lasing materials. In order to further improve the performance and determine the limitation of this laser concept, the detailed physical understanding of the interface between diamond and YAG disks was identified as a critical issue. Numerical analyses had been conducted for investigating the thermal-mechanical interaction in the interface between the gain medium and the diamond disks when the lasing process is in progress. Following this analyses, a computer model has been developed to simulate the phenomena of light interaction with the active medium. Subsequently, this computer model has been applied to optimize the laser design, in which the performance in terms of efficiency and compactness for a diamond-cooled laser has shown significant improvements. The understanding of the thermo-mechanical/optical issues at the interface, in general, will be beneficial to a variety of solid-state laser design activities.

Erbium doped YAG, lasing at 1645 nm from a 1532 nm pump, is an intriguing alternative to wavelength shifted 1-micron lasers for eye-safe applications. In this paper, we will report on an end-pumped Er:YAG laser that employs diamond disks for heat extraction. Using an alternating arrangement of diamond and Er:YAG thin disks, heat flows from the gain material to the diamond along the optical axis and is then radially transmitted to a circulating cooling fluid at the perimeter of the diamond disks. This architecture allows larger diameter disks or rods to be used than conventional radial cooling architectures, thus allowing for higher powers through area scaling. This architecture also provides better beam quality for a given pulse energy. We have demonstrated excellent beam quality (M²=1.3) from a 10-Watt Er:YAG laser. We will also report on results of Q-switching and oscillator-amplifier experiments in a study of the well-known up-conversion process for relatively low dopant concentrations (0.5%-1.0% a.w. of Erbium).

We have demonstrated for the first time, to the best of our knowledge, that Silicon Carbide (SiC) may be used as an efficient heat sinking material for face cooling of gain media in solid-state lasers. Comparative thermal modeling and temperature distribution measurements of diode-pumped 4 at.% Nd:YAG ceramic laser medium face cooled by undoped YAG (as a baseline), Diamond and SiC lead to the conclusion that SiC is an effective replacement for much more expensive diamond as an intracavity face cooling material. Laser performance of a SiC face-cooled 4 at.% Nd:YAG ceramic press-fit stack was demonstrated with a 24% slope efficiency with no AR coatings between the SiC and YAG.

Among the crystalline rare earth laser hosts the isotropic sesquioxides Sc₂O₃, Y₂O₃, and Lu₂O₃ (cubic bixbyite structure) are known for their superior thermo-mechanical properties. Their thermal conductivity considerably exceeds that of Y₃Al₅O₁₂ (YAG). Their low phonon energy ensures large energy storage times by minimizing non-radiative relaxation processes. Yb-doped sesquioxides exhibit somewhat broader absorption and emission bandwidths than Yb:YAG which is advantageous for uncritical diode laser pumping and short pulse generation. The splitting of the lower Yb³⁺ manifold is also larger which is important in the quasi-four-level operation scheme. Solid solutions with the isostructural Yb₂O₃ are possible but the observed strong lifetime quenching makes the sesquioxide hosts more suitable for laser geometries that profit from relatively low Yb concentrations. Lu₂O₃ is the host whose thermal conductivity is least affected by Yb-doping. The high melting point (above 2400°C) makes it difficult to grow the sesquioxides from the melt. Recently, the use of the heat-exchanger-method (HEM) allowed to considerably enhance the optical quality of the grown crystals and the available single crystal size. Here we review the properties and present laser results obtained recently with Yb-doped sesquioxide crystals in the continuous-wave (cw) and mode-locked (picosecond and femtosecond) regimes using both Ti:sapphire and diode-laser pumping. In the cw regime optical-to-optical efficiency of 62.2% and slope efficiency of 72.7% were reached with Yb:Sc₂O₃ operating at 1041.6 nm. Passive mode-locking of both Yb:Sc₂O₃ and Yb:Lu₂O₃ was achieved by semiconductor saturable absorber mirrors. Pulse durations of the order of 200 fs were obtained with intracavity dispersion compensation.

We have performed a laser evaluation of the Yb³⁺-doped Y₂O₃ ceramic laser material. The Q-CW output power in excess of 30 W was obtained with the slope efficiency of 47.3%. This output power is, to the best of our knowledge, the highest reported so far for diode-pumped Yb³⁺-doped Y₂O₃ laser.

There has been considerable interest in the trivalent rare earth-ion-doped ceramic laser materials because of its numerous advantages over melt growth methods, including faster production times, solid solution allowing the fabrication of multi-phase transition materials, highly homogeneous materials and the ability to engineer profiles and structures before sintering. Much progress has been made in improving the optical quality from ceramics, as well as exploring new materials. Successfully developed concentrated Nd:YAG ceramics was opened the way for drastic heat reduction by directly upper laser level pumping. In this present, after the spectroscopic investigation of rare-earth doped garnet materials includes ceramics, we report about the heat generation properties with the radiative quantum efficiency. Lately developed RE³⁺-ion-doped disordered laser ceramic materials, Y₃Sc_xAl_5-xO₁₂, which are a solid solution of YAG and Y₃Sc₂Al₃O₁₂ (YSAG), have been interested in because of its compositional tuning of parameter x. The disordered Y₃ScAl₄O₁₂ (YAG/YSAG) ceramics exhibit relatively low minimum pump intensity (I_min) and broad emission bandwidth. The value of I_min in the Yb:Y₃ScAl₄O₁₂ ceramics was found to be 2/3 compared with the Yb:YAG single crystal under 970nm zero-line pumping. Efficient laser oscillation of 72% slope efficiency was obtained for input power. Next, we have demonstrated passively mode-locked Yb:Y₃ScAl₄O₁₂ disordered ceramic laser by using a semiconductor saturable-absorber mirror. Pulses as short as 280 fs having an average power of 62 mW at 1035.8 nm was obtained. As a conclusion, the possibility of tailored fluorescence spectral profile in layer-by-layer type ceramic composite will be discussed.

The strongly anisotropic monoclinic double tungstates are known for their large absorption and emission cross sections and broader spectral lines of the rare earth dopants which makes them preferable for diode pumping. In the case of Tm the position of the absorption peak near 800 nm is very suitable for pumping with AlGaAs laser diodes. For the first time to our knowledge we grew Tm-doped KLu(WO₄)₂ crystals with high optical quality and obtained cw laser oscillation with a commercial 20 W diode bar. Only simple beam shaping optics was used for the 802 nm pump beam. The 2.9 mm thick, uncoated, 3 at. % Tm-doped KLu(WO₄)₂ was studied in a nearly hemispherical 50 mm long cavity with longitudinal pumping. Room temperature was maintained by water cooling the crystal. The sample was N_g-cut and the oscillating polarization was parallel to the N_m optical axis. With a 3% output coupler the polarized output at 1950 nm reached 4 W for 15 W of incident pump power. The slope efficiency with respect to the absorbed pump power amounted to 69% and the maximum optical efficiency reached 47%. It is the first time such high powers were generated with Tm-doped monoclinic double tungstates.

Optical quality polycrystalline yttrium aluminum garnet (YAG) materials suitable for laser gain application have been under development at Raytheon Advanced Materials Laboratory since late 2003. Significant progress has been achieved in the optical quality improvement, scale-up, Yb and Nd dopant incorporation, and various characterizations. This communication discusses Raytheon's ongoing developments in laser quality ceramic YAG fabrication and its characteristics in comparison to the current state of the art ceramic YAG made by Konoshima Chemical in Japan.

Middle infrared laser systems for countermeasures against heat seeking missiles are currently under development. These systems, based on optical parametric oscillators, are complex, bulky and expensive. Middle-infrared fiber lasers emitting in the 3-5μm spectral region may provide an attractive alternative to the systems under development. We have investigated luminescence of silver bromide-chloride crystals and fibers doped with rare earth ions (e.g. Pr³⁺, Tb³⁺ and Nd³⁺) in the near and middle infrared spectral ranges. The emission, excitation, and absorption spectra, as well as the kinetic parameters, were measured over a broad temperature range. The crystal doping was produced by growing from the melt. No significant differences were found between the luminescence properties in bulk crystals and in fibers. The Judd Ofelt analysis was applied to the doped crystals, and the transition rates, branching ratios, and quantum efficiencies were calculated. Good agreement was obtained between theory and experiment. The strong middle-infrared luminescence and the kinetic parameters of these crystals make them good candidates for the fabrication of fiber lasers in the 4-5.5μm spectral range.

High quality bonding of undoped yttrium aluminum garnet (YAG) and Neodymium doped YAG (Nd:YAG) single crystals is critical for thermally and mechanically robust composite high power solid state laser designs. The optical properties of composite crystals must not be significantly diminished compared to monolithic single crystals. Necessary surface preparations for optical quality bonds, and methods to measure surfaces including atomic force microscopy (AFM) were examined. The optical characteristics of composites with one or more bond interfaces in high power solid state laser (HPSSL) slab or test coupon form were analyzed. Select insertion loss, stress birefringence, transmitted wavefront distortion, and optical microscopy results will be shown. High bond mechanical strengths were verified using thermal shock and 4-point flexure tests. The flexure test results were analyzed using Weibull statistics and fractography, illustrating the medium-high to high energy failures typically observed in bonded YAG sets with characteristic strengths (≥445MPa), higher than those of a commercially available YAG bond (351 MPa) and as-grown YAG crystals (409 MPa) with no post growth anneal. Acid etching generally tightened strength distributions, but with similar decrease in measured strengths for both monolithic YAG and bonded YAG sets. Recent efforts in bonding ceramic YAG and Nd:YAG slabs are briefly summarized.

We report on adhesive-free bonded (AFB"R") CVD diamond surfaces of 2 to 3 nm rms roughness to sapphire, YAG and optically coated YAG surfaces to form robust and stress free composite components. Considering the high thermal conductivity of CVD diamond with respect to YAG or sapphire, AFB"R" diamond/sapphire and diamond /YAG composites can be ideal components for thermal management of high power solid-state laser systems of certain design configurations. We ascribe the attractive bonding forces at the interface between diamond and sapphire and other single crystals such as YAG and spinel largely to Van der Waals forces. Contrary to chemical bonds, Van der Waals forces consist of non-localized dipole-dipole interactions that manifest their long range effects by allowing relative movement between two bonded surfaces of different coefficients of thermal expansion, thus avoiding local stress during thermal cycling. CVD diamond surfaces result in a much wider compatibility range for forming stress-free or low-stress composites compared to alumina compound single crystal counterparts. Feasibility of forming AFB"R" stress- free interfaces between diamond and optically coated oxide surfaces marks an important step in widening the range of AFB"R" for solid state laser and electronic applications.

We present theoretical analysis and experimental data from a monolithic semiconductor laser and optical parametric oscillator device which generates near-infrared laser beam and converts it to a longer mid-infrared wavelength by modal phase matching. The device design exploits the strong optical nonlinearity and transparency of III-V compound semiconductors while achieving phase matching of the near-infrared pump beam to the mid-infrared product beam(s). These devices have the potential to dramatically improve the CW Mid-IR power available at room temperature from monolithic semiconductor lasers, making them ideal for a broad range of applications including infrared countermeasures, detecting chemical weapons, imaging, and fog-penetrating optical communications.

A high pulse energy widely tunable UV and IR laser is demonstrated for combustion imaging, LIDAR, etc. The laser output has superior beam quality because of rotated KTP OPO cavity, and it is highly efficient thanks to intracavity doubling and mixing.

Using CBO crystal as the final mixing nonlinear optical crystal, we generated 5mJ/pulse at 193nm. The high pulse energy at 193nm is a result of several factors. The efficiency of CBO is about three times better than that of LBO, and an order of magnitude better than BBO. Novel designs in 4^th and 5^th harmonic generators using readily available BBO crystals allow the stable, efficient and repetitive generation of high pulse energy at the 5^th harmonic of 1064nm.

Using fast switching high power diode laser driver, we are able to push the limit of accurate timing of the laser pulses from a passive Q-switched diode pumped Nd:YAG laser. Combined with prepumping technique, we could reduce the delay between pump current pulse and the laser pulse down to 2.5 micro-second, and the jitter of the delay down to 100 nanoseconds.

The efficient, eye-safe, high repetition rate, intracavity optical parametric oscillator (IOPO) inside acousto-optic Q-switched Nd:YVO₄ laser end pumped by 15-W fiber coupled diode was demonstrated. The pumping Q-switched laser gives 3-W average output power at 1064-nm wavelength and 40-kHz repetition rate. The additional separating mirror, 'x-cut' KTP crystal and the output coupler, highly reflective at 1064-nm and partially transparent at 1572-nm wavelength, form flat-flat IOPO resonator of 35-mm length. We have achieved 3-ns duration pulses for 20-mm long KTP and 4-ns duration pulses for 30-mm KTP length, respectively. Above 10-kW peak power pulses with the average power of 1.5 W at the signal wavelength for 40 kHz repetition rate were demonstrated. Due to intracavity gain guiding effect, diffraction limited signal beam was achieved. Conversion efficiency of 50% with respect to Q-switched output at 1064-nm wavelength and 11% with respect to diode pump power were achieved.

Volume Bragg gratings (VBGs) have been recognized as critical elements in various types of beam-combining applications, such as, design of super-parallel holographic optical correlators, coherent power beam-combiners and couplers, and spectral beam combiners (SBC) in which the output beams from several distinct laser sources are combined into a single-aperture beam. The obvious advantage of VBG stems from extremely narrow spectral and/or angular selectivity compared, to any other surface grating. This feature of VBG enables combining of large number of laser beams within near-diffraction-limited divergence. The VBGs recorded in a photo-thermo-refractive (PTR) glass exhibit a long-term stability of all their parameters at total CW power at a multi-kilowatt level and have shown high-efficiency combining of high-power laser beams. In order to increase the spectral capacity of such a "beam-combiner", the overall loss resulting from absorption and cross-talk between channels should be minimized. This paper considers architecture-specific SBC scheme and addresses the cross-talk minimization problem based on optimal channel positioning. A mathematical model reveals the critical parameters for high efficiency spectral beam combining.

Photo-thermo-refractive (PTR) glasses have shown high efficiency and stability for different applications in laser systems. One of the applications of diffractive elements in PTR glasses is to use them for high power laser beam control and combining which requires an increase of the size of these elements. The opportunities of recording large aperture Bragg gratings by using a translational (multi-frame exposure) technique and by using an Ar⁺ ion laser with higher power operating at 334.5 nm and 351 nm are studied. It is shown that photosensitivity of PTR glass at 334.5 nm and 351 nm is comparable to that of 325 nm. Because of higher power at 334.5 nm and 351 nm, the recording of large aperture holograms at these wavelengths is possible. Large-aperture holograms produced by multi-frame technique are demonstrated.

We report on the fabrication and spectroscopic characterization of transparent Nd³⁺:YAG ceramic, a prospective material for future laser applications.