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Highly optically transparent polycrystalline fluorapatite ceramics with hexagonal crystal structures were fabricated via a liquid-phase synthesis of fluorapatite powder, followed by spark plasma sintering (SPS). The effect of sintering temperature, as observed using a thermopile, on the optical transmittance and microstructure of the ceramics was investigated in order to determine suitable sintering conditions. As a result, high optical transmittance was obtained in the SPS temperature range of 950–1100 °C. The highest optical transmittance was obtained for the ceramic sample sintered at 1000 °C, and its average grain size was evaluated at only 134 nm. The grain size dramatically increased with temperature, and the ceramics became translucent at SPS temperatures above 1200 °C. The mechanical and thermal properties of the ceramics were measured to evaluate the thermal shock parameter, which was found to be comparable to or slightly smaller than that of single-crystal fluorapatite. This transparent polycrystalline fluorapatite ceramic material should prove useful in a wide range of applications, for example as a biomaterial or optical/laser material, in the future. Furthermore, the knowledge obtained in this study should help to promote the application of this ceramic material.

The relatively narrow choice of magneto-active materials that could be used to construct Faraday devices (such as rotators or isolators) for the mid-infrared wavelengths arguably represents a pressing issue that is currently limiting the development of the mid-infrared lasers. Furthermore, the knowledge of the magneto-optical properties of the yet-reported mid-infrared magneto-active materials is usually restricted to a single wavelength only. To address this issue, we have dedicated this work to a comprehensive investigation of the magneto-optical properties of both the emerging (Dy2O3 ceramics and CeF3 crystal) and established (Y3Fe5O12 crystal) mid-infrared magneto-active materials. A broadband radiation source was used in a combination with an advanced polarization-stepping method, enabling an in-depth analysis of the wavelength dependence of the investigated materials’ Faraday rotation. We were able to derive approximate models for the examined dependence, which, as we believe, may be conveniently used for designing the needed mid-infrared Faraday devices for lasers with the emission wavelengths in the 2-μm spectral region. In the case of Y3Fe5O12 crystal, the derived model may be used as a rough approximation of the material’s saturated Faraday rotation even beyond the 2-μm wavelengths.

Thermo-optical and magneto-optical characteristics of a uniaxial CeF3 crystal have been investigated. Its optical anisotropy parameter, magneto-optical figure of merit, and the ratio of the thermo-optical constants P and Q have been measured. The found characteristics have been compared with the corresponding values for the TGG crystal. Based on the obtained results it has been concluded that, despite its anisotropy, the CeF3 crystal is promising material for the development of Faraday isolators for high-power lasers.

Temperature and wavelength dependence of the Verdet constant of dysprosium sesquioxide (Dy2O3) transparent ceramics has been measured for the temperatures ranging from cryogenic 20K up to room temperature 297K and for the wavelength range starting from visible 0.6μm up to mid-infrared 2.3μm. Several absorption bands corresponding to the electronic transitions in Dy2O3 have been identified and removed from the data analysis due to high absorption of the matching wavelengths within these absorption bands. The Verdet constant data has been fitted by the multi-transition model function covering the whole temperature and wavelength range. Such fitted function could be used for the detailed design of magneto-optical devices as well as for the analysis of physical properties of the material itself.

We fabricated an advanced transparent Yb-doped hexagonal fluorapatite ceramic with an average grain size of only 90 nm; we also demonstrated its laser oscillation. To the best of our knowledge, this is the smallest grain size for laser ceramics. The in-line transmittance was 86.1% at 1 µm for 0.79-mm-thick ceramics (corresponding to a loss coefficient of 0.45 cm-1). Although the ceramic crystal grains were randomly oriented, scattering at the grain boundary was suppressed because the grains were considerably smaller than the wavelength. This novel ceramic possesses both a fine microstructure and densification and is expected to be the foundation of many non-cubic laser ceramics.

The relatively narrow choice of magneto-active materials that could be used to construct Faraday devices (such as rotators or isolators) for the mid-infrared wavelengths arguably represents a pressing issue that is currently limiting the development of the mid-infrared lasers. Furthermore, the knowledge of the magneto-optical properties of the yet-reported mid-infrared magneto-active materials is usually restricted to a single wavelength only. To address this issue, we have dedicated this work to a comprehensive investigation of the magneto-optical properties of both the emerging (Dy2O3 ceramics and CeF3 crystal) and established (Y3Fe5O12 crystal) mid-infrared magneto-active materials. A broadband radiation source was used in a combination with an advanced polarization-stepping method, enabling an in-depth analysis of the wavelength dependence of the investigated materials&rsquo, Faraday rotation. We were able to derive approximate models for the examined dependence, which, as we believe, may be conveniently used for designing the needed mid-infrared Faraday devices for lasers with the emission wavelengths in the 2-&mu, m spectral region. In the case of Y3Fe5O12 crystal, the derived model may be used as a rough approximation of the material&rsquo, s saturated Faraday rotation even beyond the 2-&mu, m wavelengths.

Transparent Dy:Y2O3 ceramics with various doping concentrations were fabricated by spark plasma sintering (SPS). Their optical characteristics, including emission cross-sections and nonlinear absorption properties, were evaluated. The results show that Dy:Y2O3 ceramics are promising as a new candidate for 3-μm laser gain medium and passive Q-switch material. A passive Q-switching of a 2795 nm Er:YAP laser with a Dy:Y2O3 ceramic saturable absorber was performed to demonstrate the potential of this material.

As a new bonding technique for high-power laser optics, pulsed electric current bonding (PECB) of sapphire and Nd:YAG ceramics was demonstrated. The optical properties of the composite were measured, and its microstructure at the interface and laser performance was analyzed. The optical transmittance was equal to the theoretical value, and the transmitted wavefront was λ/3 (λ=633 nm); both are appropriate values for laser applications. The microstructural analysis indicated an absence of scattering sources such as pores or non-contact points at the sapphire/Nd:YAG interface, and the distance of yttrium diffusion into the sapphire was theoretically expected to be less than 10 nm, much smaller than that of ceramic materials bonded by conventional thermal diffusion techniques. The laser performance of the composite material showed an 18% higher output power with almost the same threshold power and slope efficiency as a Nd:YAG ceramic due to the sapphire-conductive cooling effect. This new PECB technique for different transparent materials has the potential to bond large aperture optical materials over 100 mm in diameter and could be especially effective for fabricating active laser media for high-average-power lasers having both high-pulse energy and high repetition rates.

Faraday effect measurements of holmium oxide (Ho2O3) ceramics-based magneto-optical materials, highly potential material candidates for high-energy laser Faraday isolators, are presented in this paper. Temperature dependence of the Verdet constant of nondoped Ho2O3 ceramics was measured for temperatures 15–305 K at $1.064~\unicode[STIX]{x03BC}\text{m}$ wavelength. The Verdet constant dispersion for wavelengths 0.5–$1~\unicode[STIX]{x03BC}\text{m}$ and $1.064~\unicode[STIX]{x03BC}\text{m}$ was measured for both nondoped Ho2O3 ceramics and Ho2O3 ceramics doped with terbium Tb3+ (0.2 at. %) and cerium Ce3+ (0.1 at. %) ions. The results suggest that the relatively low level of doping of Ho2O3 with these ions has no significant boosting impact on the Faraday effect. Therefore, other compositions of Ho2O3 ceramics-based magneto-optical materials, as well as various doping concentrations, should be further examined.

Pulsed electric current bonding condition for Sapphire/Nd:YAG composite was studied to improve optical and laser quality. Further, bonding of a sapphire-sandwich structure was demonstrated.

The thermally induced depolarization and Verdet constant of CeFsub3/subcrystals-their most important characteristics-have been studied in the 79-293 K temperature range. It has been found that thermal effects reduce substantially upon cooling down to 79 K and the Verdet constant grows in inverse proportion to the temperature. It was shown that CeFsub3/subcrystals are not inferior to TGG as a medium for Faraday isolators, including cryogenic ones.

A monolithic composite ceramic with multi total-reflection active-mirrors has been proposed as a repeatable amplifier in diode pump for joule-class pulse energy. The new amplifier was designed under both experimental and theoretical researches with a single total-reflection active-mirror amplifier.

A total-reflection active-mirror (TRAM) amplifier is functionally designed by using a composite ceramic for high pulse energy and high average power. A chirped-pulse regenerative amplifier with a cryogenic TRAM has been successfully demonstrated as a feasibility study. A 3.5 mJ pulse energy is obtained at a repetition rate of 100 Hz. The corresponding energy fluence on the TRAM is as high as 1.5 J/cm2 for 0.46 ns short pulses. The M2-factor is below 1.1 and no significant beam distortion is observed.

We calculated intravalley and intervalley phonon scattering rates of accelerated electrons under intense midinfrared laser fields. We extended the Keldysh theory, which assumes oscillatory motion of Bloch electrons driven by intense laser fields, to phonon scattering of electrons in semiconductors. We find that phonon emission is possible even if the initial electron energy is less than the phonon energy due to multi-photon absorption processes. We also find that the scattering rates increase with electric field strength and wavelength. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have measured optical transmittance through germanium in the mid-infrared region at room temperature using the Osaka free-electron laser (FEL). In spite of the fact that germanium is transparent in the mid-infrared region, we observed strong suppression of optical transmission under high-intensity FEL excitation. We found that the observed suppression is due to optical absorption in germanium. To analyze the experimental results, we have calculated optical transmittance of germanium within Keldysh theory

The Verdet constant of a 0.8 at.% Ti-doped TAG ceramic was investigated between 5 and 296 K. A high value of the Verdet constant was obtained as 184.2 rad/Tm at 296 K for 632.8 nm.

Transparent Ho2O3 ceramics are fabricated and their magneto-optical characteristics are reported for the first time, to the best of our knowledge. The value of the Verdet constant was measured in the 560–1064 nm wavelength range, and the value at 1064 nm is 46.3 rad/Tm. This corresponds to the Verdet constant for terbium aluminum garnet (TAG), which is ~1.3 times higher than that of terbium gallium garnet (TGG). The in-line transmittance at 1 μm wavelength is only ~60%, but the optical properties can be further improved by optimizing sintering conditions. This is a new potential magneto-optic material that can be applicable for high-average-power lasers.

The amplified spontaneous emission (ASE) and parasitic lasing (PL) effects in thin disk laser with an anti-ASE cap have been investigated in detail by measuring both time-resolved radiated intensity at longer axis of elliptical pump profile (dominant ASE direction) and small signal gain (SSG) in laser amplifier. A cryogenically-cooled total-reflection active-mirror laser consisting of 9.8 at.% doped, 0.6-mm thick Yb:YAG and un-doped YAG trapezoidal ceramics cap was used as a sample. The phased transitions from spontaneous emission (SE) to ASE and from ASE to PL have been unambiguously observed. For several pump beam diameters, the ASE gain parameter g(0)l(ASE) at ASE threshold was about 3, and the SSG coefficient was down to about 65% until PL started. To the best of our knowledge, this is the first quantitative characterization of the ASE/PL effects in the thin disk laser with an anti-ASE cap.

Three key techniques have been demonstrated in small scale. Cryogenic Yb:YAG-ceramic DPSSL operated in high energy fluence at high optical efficiency for a repeatable high-pulse-energy pump source. Optical parametric chirped-pulse amplification (OPCPA) with a spatially random-phased pump beam was demonstrated assuming multi-beam pump in kilo-joule-class. Partially deuterated KDP crystals showed broad spectral gain bandwidth for few cycle pulses in non-collinear OPCPA and were successfully grown.

Liquid nitrogen (LN2) behavior on the surface of excited Yb(3+):YAG is investigated using fluorometry. From the time-resolved temperature variations and integrated fluorescence spectra intensity on this directly cooled Yb(3+):YAG surface, we observe a phase transition of LN2 from nucleate boiling to film boiling. As a result of this pool boiling, good beam quality should occur when the temperature and heat flux at an excited surface of Yb(3+):YAG are below 95 K and 15.8 W/cm2, respectively. That is, the LN2 should remain in a steady state of nucleate boiling to produce good beam quality using pool boiling.

We investigate the effect of dispersion compensation on temporal characteristics in mode-locking by nonlinear polarization rotation in an ytterbium-doped fiber (YDF) oscillator with intracavity and external grating pairs. A short fixed length YDF was spliced with a longer single-mode fiber (SMF). Using experimentally measured dispersion characteristics of the YDF, SMF and cavity optics, we control the group velocity dispersion (GVD) and spectral broadening in a cavity by changing the SMF length. As a result, the oscillator generated 29.4-fs transform-limited wing-free pulses, which are to our knowledge the shortest and cleanest pulses achieved without the use of additional optics like a prism pair for high-order dispersion compensation. The results show that a precise balance of higher order terms of the GVD and self-phase modulation is essential for shortening pulse duration.

We have studied wave-front distortion due to thermal lensing, temperature rise and birefringence of cryogenic Yb:YAG TRAM laser for >100kW ultra-high power laser systems. A novel coherent beam combining technique is also presented.

A regenerative amplifier with a chirped volume Bragg grating has been developed. A seed pulse is temporally stretched to 424ps with 1.2mJ at 100Hz. The obtained chirping ratio is as high as 2.1ns/nm.

Both high pulse energy and repeatable operation are simultaneously desired for high-power lasers in the next generation. These lasers are expected to be useful tools for various kinds of advanced fields such like inertial fusion energy, neutron sources, debris remove, PET and so on. The target of 1-kJ, 100Hz in pico seconds is set for novel laser technology developments. The most significant problem in the developments is laser materials which require high thermal strength, material size scalability and suitable emission cross section (saturation fluence). Cryogenic Yb:YAG ceramic has shown good performances via our previous basic experiments.[1,2] In addition, a total-reflection active-mirror (TRAM) amplifier has been proposed for high efficiency, compactness and high optical damage threshold. So-called active-mirror amplifiers show high efficiency due to a good spatial overlap between a pump and a laser beam. The optical damage threshold reduced due to interference on the input/output surface is, however, significant for pulse duration. The TRAM recovers it. A few temperature rise in cw operation showed considerably high thermal strength.[3] In pulse operation as following, a regenerative amplifier is developed and an additional joule-class amplifier system at 100 Hz is under construction.

The first chirped-pulse regenerative amplifier using a total-reflection active-mirror with a cryogenic Yb 3+:YAG/ YAG monolithic composite ceramic was demonstrated. 3.6 mJ of output pulse energy was obtained at 100 Hz repetition rate.

We propose a new concept for a sub-100-kW laser system based on a cryogenic Yb:YAG multiple total-reflection active-mirror design. By adjusting the thickness and doping concentrations of Yb:YAG layers, we will obtain an ideal source.

We report on a multiple total-reflection active-mirror laser with cryognic Yb:YAG ceramics using zigzag optical path. A 214W output power with 62% slope efficiency has been demonstrated.

We report on the efficient operation of directly liquid-nitrogen-cooled Yb:YAG active-mirror laser. A CW output power of 273 W with slope efficiency of 72% was demonstrated. High heat removal ability was ensured at 743 W/cm2.

We report on the efficient high-power laser operation by using a cryogenic Yb:YAG composite ceramics with a total-reflection active-mirror arrangement. A 273W CW output power was demonstrated at 65% optical efficiency and 72% slope efficiency.

We have investigated the temperature dependence of midinfrared optical absorption in germanium under intense free-electron laser fields. We observed that the absorption decreases as temperature decreases. We find that the temperature dependence of the absorption is consistent with calculations based on the Keldysh theory, in which the temperature dependence of the direct-gap energy of germanium is considered.

The temperature dependence of the Verdet constant of a 0.8 at % Ti-doped terbium aluminum garnet (TAG) ceramics was investigated using lasers with wavelengths of 632.8 and 1064 nm. A high value of the Verdet constant was obtained at 296 K – namely, 184 and 53 rad/Tm for 632.8 and 1064 nm, respectively. The Verdet constant of the Ti:TAG ceramics at 1064 nm is about 1.5 times higher than that of the terbium gallium garnet (TGG) ceramics. The transmittance of this sample was about 75% at the wavelength of 1 μm. This material represents a possible candidate for next-generation devices that utilize the magneto-optic effect.

The thermal expansion coefficient α and the thermo-optic coefficient dn/dT were measured using a He-Ne laser (λ = 632.8 nm) between 300 K and 500 K for ceramic and single-crystal terbium gallium garnet (TGG). For ceramic TGG, the experimental values of α and dn/dT at 300 K were 7.0×10−6 K−1 and 17.5×10−6 K−1, respectively. We proposed fitting relations regarding the temperature dependence of α and dn/dT. Both α and dn/dT increased with temperature, and the temperature dependencies for ceramic TGG were in agreement with those of single-crystal TGG with orientation. We then evaluated thermally induced depolarization for an average laser power of 1 kW. The depolarization was evaluated to be larger than 0.01 at room temperature. Moreover, we simulated the thermal lens focal length and found that the temperature dependence is very slight compared to its dependence on the beam radius.

We have investigated the effects of high-power laser pulses on a transparent material using a free-electron laser (FEL). We have measured optical transmittance through a germanium crystal in the midinfrared region, $\ensuremath{\lambda}=5.3--12.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ at room temperature. In spite of the fact that germanium is transparent in the midinfrared region, we have observed strong suppression of optical transmission under high-intensity FEL excitation.

We have studied the amplification characteristics of a cryogenically cooled Yb³⁺:YAG total-reflection active-mirror (TRAM) ceramic laser including wavefront distortion, birefringence loss, small signal gain (SSG), and temperature rise for developing high-performance master oscillator power amplifier (MOPA) systems. A 0.6 mm thick Yb³⁺:YAG ceramic sample was used, and maximum pump intensity ~10 kW/cm² was reached. The transmitted wavefront was measured by using a Shack-Hartmann wavefront sensor, and we evaluated the thermal lens focal length and Strehl ratio for different pump conditions. We have also observed a butterfly-like leakage profile of thermally induced birefringence loss at the maximum pump intensity. From SSG measurements, we obtained moderate laser gain of G=3 for one bounce with a near aberration-free wavefront. Gain calculations, which included also temperature dependence of the emission cross section and reabsorption of Yb³⁺:YAG, were in good agreement with the experiments. These experimental results will be useful as benchmark data for numerical simulations of temperature distribution in TRAM and for designing multikilowatt-class high-performance MOPA systems.

Spectral enhancement of a mode-locked fiber oscillator has been proposed by using gain spectral filtering technique. A wide spectral width of 83 nm at 1030 nm, with 5.2 nJ pulse energy was achieved with positive net cavity dispersion. Fiber oscillator output spectrum was analyzed numerically, showing larger gain spectral filtering effect in longer gain medium systems. We show that the gain spectral filtering plays the same role as spectral filter element inside the resonator, hence could replace it in dispersion managed lasers improving the output spectrum and simplifying fiber oscillator designs.

The temperature dependence of the thermo-optic effect in cryogenically cooled Yb:YAG ceramics was evaluated by measuring the thermo-optic coefficient (the derivative of refractive index with respect to temperature, i.e., dn/dT), thermal expansion coefficient (α), and thermal conductivity (κ) between 70 and 300 K. These parameters significantly improved at low temperature. Observed values indicated that a laser gain medium cooled to 70 K can sustain a thermal load up to 20 times higher than that at 300 K, for comparable thermo-optic effects. To our best knowledge, this is the first quantitative evaluation of the improvement in thermo-optic characteristics of cryogenically cooled Yb:YAG ceramics.

We analyzed the output power characteristics of a cryogenically cooled Yb:YAG total-reflection active-mirror (TRAM) laser oscillator including the temperature dependence of the emission cross section and the reabsorption loss of the Yb:YAG TRAM. A CW multi-transverse mode oscillation of a 9.8 at.% doped 0.6 mm thick Yb:YAG ceramic TRAM was investigated for various pump spot sizes and compared with theoretical results. The Yb:YAG temperatures were inferred from the ratio between fluorescence intensities at 1022 nm and 1027 nm which varied significantly with temperature below 200 K. Output power calculations using evaluated temperatures were in good agreement with the experimental data measured between 77 and 200 K, and the output power suppression due to the temperature rise observed above ~140 K. To the best of our knowledge, this is the first evaluation of output power for a cryogenically cooled Yb:YAG TRAM laser.

We report on a novel amplifier configuration concept for a 10 kW laser system using a zig-zag optical path based on a cryogenic Yb:YAG Total-Reflection Active-Mirror (TRAM) laser. The laser material is a compact composite ceramic, in which three Yb:YAG TRAMs are combined in series to increase the output power. Output powers of up to 214 W with a slope efficiency of 63% have been demonstrated for CW operation, even at a quite low pump intensity of less than 170 W/cm2. Further scaling could achieve output powers of more than 10 kW.

High-power solid-state mid-IR lasers are being actively developed, as they possess the key properties for a vast number of technological applications [1] (e. g. lidar, gas sensing, or pump sources optical parametric oscillators) and are in a high demand for basic research as well [2]. These potentialities have recently led to intensive investigations of new mid-IR laser gain media [3,4] and other optical elements. One of the key components under investigation is an optical isolator (OI), a magneto-optical device enabling safe operation of a laser by preventing harmful back-reflections into the laser cavity. The development of high-power OIs is particularly challenging due to the severe thermal effects which arise because of the relatively high absorption of the currently available magneto-optical elements. Therefore, a lot of scientific effort is being put into the research of novel magneto-optical materials with the desired properties: high Verdet constant, low absorption and excellent thermal properties. All of these properties enable a reduction of the heat generated in the magneto-optical element.

An efficient high-power laser operation has been demonstrated by using a cryogenic Yb:YAG composite ceramic with a total-reflection active-mirror arrangement. The composite ceramic, which had no high-reflection coating and was cooled with liquid nitrogen directly, showed four-level operation even at 67 kW/cm(3) of high pump density. A 273 W cw output power was obtained with 65% optical efficiency and 72% slope efficiency.

We report a new compact high power laser system using zig-zag optical path based on the cryogenic Yb:YAG total-reflection active-mirror laser for ten-kilowatts. The thermal analyses of the laser have also been discussed.