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We review recent progress in pulsed lasers using cryogenically-cooled Yb3+-doped gain media, with an emphasis on high average power. Recent measurements of thermo-optic properties for various host materials at both room and cryogenic temperature are presented, including thermal conductivity, coefficient of thermal expansion and refractive index. Host materials reviewed include Y2O3, Lu2O3, Sc2O3, YLF, YSO, GSAG and YVO4. We report on the performance of several cryogenic Yb lasers operating at 5-kHz pulse repetition frequency (PRF). A Q-switched Yb:YAG laser is shown to operate at 114-W average power, with 16-ns pulse duration. A chirped pulse amplifier achieves 115-W output using a Yb:YAG power amplifier. Output power of 73 W is obtained from a composite Yb:YAG/Yb:GSAG amplifier, with pulses that compress to 1.6 ps. Finally, a high-average-power femtosecond laser based on Yb:YLF is discussed, with results for a 10-W regenerative amplifier at 10-kHZ PRF.
©2011 Optical Society of America
1. Introduction
The benefits of cryogenic cooling of solid-state lasers were recognized early in their development. For example, the second solid-state laser material ever demonstrated, uranium-doped CaF2, was cooled to liquid-helium temperature in order to lower the laser threshold by reducing the thermal population in the lower laser-level [1]. The cooling of solid-state lasers to cryogenic temperatures improves both the thermo-optic and spectroscopic properties of the material, leading to direct benefits in overall laser system performance.
In this paper, we review the field of cryogenically-cooled Yb3+-doped pulsed solid-state lasers. By cryogenic temperature, we refer here to that attained by liquid nitrogen (near 77 K). We discuss the important properties associated with Yb3+-doped laser materials, as well as some recent materials measurements. In addition, we review recent and state-of-the-art work in both nanosecond-class and ultrashort pulsed lasers. For further discussion, the interested reader is referred to additional reviews of cryogenically-cooled laser technology [2,3].
A large variety of applications exist for pulsed lasers, including applications in manufacturing, defense, metrology, medicine, and spectroscopy, to name but a few. More specifically, pulsed lasers have been used for spectroscopic applications such as laser-induced breakdown spectroscopy, metrological applications such as LIDAR, material processing, and nuclear fusion. Other scientific applications include laser-driven plasma accelerators, an application that may require sub-picosecond laser systems operating with multi-Joule pulse energies and kilohertz repetition rates [4].
A major challenge in the development of high-power lasers with near-diffraction-limited beam quality centers on the removal of waste heat from the active medium. The fundamental source of waste heat is the quantum defect, the energy difference between the pump and laser wavelength which is deposited in the medium through nonradiative transitions. The quantum defect of Yb-doped materials is small (e.g., 9% for Yb:YAG, assuming a pump wavelength of 940 nm and laser wavelength of 1030 nm), meaning that the amount of generated heat is intrinsically low. For comparison, the quantum defects for Nd:YAG (assuming pump wavelength of 808 nm and laser wavelength of 1064 nm) and Ti:sapphire (assuming pump wavelength of 532 nm and laser wavelength of 800 nm) are 0.24 and 0.34, respectively. Yb-doped materials thus provide a path to efficient laser operation with potential for average power scalability, and there have been several cw demonstrations of different cryogenically-cooled materials showing the potential for average power handling, including Yb:YAG [3,5–8], Yb:YLF [9], and Yb:CaF2 [10].
There are several major reasons why cryogenic cooling has been applied to Yb-doped solid-state lasers in particular. First, at room temperature, these materials operate as quasi-three-level lasers due to a finite population in the lower laser level. At cryogenic temperature, four-level laser operation is achieved because the lower laser level becomes thermally depopulated, leading to improved efficiency and lower threshold operation [11]. Second, cryogenic cooling improves the thermo-optic properties of the gain medium (higher thermal conductivity, lower dn/dT, and lower coefficient of thermal expansion), mitigating thermo-optic effects. Finally, these materials, under cryogenic cooling, maintain sufficient absorption bandwidths suitable for direct diode pumping, meaning that the same diode arrays used to pump room-temperature crystals can also be used to pump cryogenically-cooled materials.
In our opinion, the attractive properties of cryogenic Yb lasers make them the optimum approach for simultaneous peak and average power scaling. Cryogenic Ti:sapphire systems [12,13] have also been pursued for the combination of high peak and average powers, and each approach has its advantages. Cryogenic Ti:sapphire has a larger gain bandwidth and sapphire has much higher thermal conductivity at low temperature than any of the Yb-doped host materials used to date. However, the cryogenic Yb systems have much lower thermal loading per unit output power (compensating for the lower thermal conductivity), better energy storage, much higher wallplug efficiency, and the practical availability of diodes for pumping. The high thermal loading in Ti:sapphire systems drives the requirement on the cryogenic thermal management system, and the lower efficiency of Ti:sapphire drives complexity and cost.
This paper is organized as follows. In Section 2 we report new measurements on laser materials, as part of an ongoing effort [3,14] to identify candidate materials for cryogenic cooling that would yield clear benefits for laser development. Section 3 discusses cryogenically-cooled nanosecond-class Yb-doped lasers. Typically, the radiative lifetime of the upper laser level ranges in these materials from a few hundred microseconds to a few milliseconds at cryogenic temperature, enabling a high energy storage capability for Q-switched operation. This capability, when taken together with improved thermo-optics and a large mode volume in the gain medium, can result in output pulse energies in the millijoule range. Finally, in Section 4 we review recent work in cryo-Yb ultrashort pulse lasers. We compare current performance with state-of-the-art demonstrations utilizing other active media, such as fiber, Ti:sapphire, and room-temperature Yb:YAG. We discuss how cryogenic cooling can enable a laser system to operate simultaneously in the high peak and high average power regimes.
2. Materials Properties
The key materials attributes (thermal conductivity, coefficient of thermal expansion [CTE], and the change in refractive index with temperature dn/dT) all generally scale favorably for high average power as the temperature is lowered in crystalline dielectrics, although there are only limited data below room temperature. Our previous reports [3,14] on thermo-optic properties of solid-state laser materials included YAG, Lu3Al5O12, LiYF4 (YLF), LiLuF4, YAlO3, BaY2F8, Gd3Ga5O12, Y2O3, KGd(WO4)2, KY(WO4)2, and GdVO4. The measurements have been extended to additional materials, including Y2SiO5 (YSO), Gd3Sc2Al3O12 (GSAG), Sc2O3, Yb(10%):Y2O3, Yb(10%):Lu2O3, YVO4, and highly doped (25%) Yb:YLF. The sesquioxides (Y2O3, Lu2O3, and Sc2O3) are cubic crystals and are known to have relatively good thermal conductivity. They have been made in both ceramic and single-crystal form. In this study, the Yb:Y2O3, Yb:Lu2O3, Sc2O3, and Yb:Sc2O3 are ceramic samples and thus should have somewhat lower thermal conductivity than a single-crystal sample, although essentially the same CTE and dn/dT. Ytterbium-doped YSO is of interest as a material for sub-picosecond laser operation because of its large gain bandwidth. It has monoclinic symmetry, which makes characterization of the material more complex, and consequently we report here properties only along the direction of the 2-fold symmetry axis. Also of interest is GSAG, which is a YAG isomorph and so is expected to have similar thermal expansion and dn/dT to YAG. We have recently used cryogenic Yb:GSAG in combination with Yb:YAG to provide a composite gain bandwidth that is larger than Yb:YAG alone [15].
The measurement methods are the same as described in our earlier work [3,14]. These methods have given consistent results; in particular, we have shown that the CTE and dn/dT measurements are self-consistent [3], which is not possible without good accuracy. The laser-flash method is used to measure thermal diffusivity and specific heat from which thermal conductivity can be calculated [16–18]. At cryogenic temperature, the thermal diffusivity can change rapidly with temperature in crystalline dielectrics, so to minimize errors associated with this variation, the temperature rise is kept to ~1 K or less for measurements at low temperature. Thermal expansion is measured using a double Michelson laser interferometer [19] at Precision Measurements and Instruments Corporation on ~25-mm-long samples. The change in the optical path length with temperature is also measured using interferometry, and dn/dT is computed from those two measurements.
Table 1 compiles the thermal diffusivities, and Table 2 lists the specific heat that is derived from the same measurement, along with the density of the material. Table 3 shows the thermal conductivity, as derived from the data in Tables 1 and 2. As expected, the thermal diffusivity and conductivity rise significantly and the specific heat drops at lower temperature. Also as anticipated, doping reduces the thermal conductivity, caused by the mass difference of the dopant ion.
Table 1. Thermal Diffusivity (cm2/s)
Table 2. Specific Heat (J/gK) and Density (g/cm3)
Table 3. Thermal Conductivity (W/mK)
Data to make comparisons are limited and are mostly near room temperature. Our measurements generally agree with previous reports near room temperature for the sesquioxides. Undoped Y2O3 ceramic has been reported to have thermal conductivity of 13.0 W/mK at 298 K and 52 W/mK at 92 K [3]. Single-crystal undoped Y2O3 and Yb(3%):Y2O3 were reported as 13.6 W/mK and 7.7 W/mK (30 °C) respectively [20]. The thermal conductivity of Yb(10%):Y2O3 is well below all these, as anticipated. The thermal conductivity of single-crystal undoped Lu2O3 and Yb(3%):Lu2O3 were reported as 12.5 W/mK and 11.0 W/mK (30 °C) respectively [20]. Little difference exists in the reported conductivity at room temperature, as the Yb ion’s mass is very close to Lu, so phonon scattering caused by the mass difference is mitigated. At low temperature, the highly doped Yb:Lu2O3 has significantly lower thermal conductivity than undoped Y2O3 but significantly higher than the highly doped Yb:Y2O3. It is not clear whether the lower thermal conductivity compared with undoped Y2O3 is intrinsic or is instead caused by the quality of the ceramic sample; it has been shown that the quality of ceramic materials [21] can lower the thermal conductivity compared with either high-quality ceramic or single crystals. Nevertheless, the Yb(10%):Lu2O3 measurement sets a lower bound on the thermal conductivity of the material at low temperature. The thermal conductivity of single-crystal Sc2O3 has been reported as 16.5 W/mK for undoped material and 6.6 W/mK for 3% doping at 30 °C [19], as compared with 12.4 W/mK here for the undoped ceramic sample. We attribute the only small rise in thermal conductivity at low temperature to the quality of the ceramic sample, as evidenced by the relatively large difference between the single-crystal and ceramic thermal conductivity even at room temperature. The low thermal conductivity of ceramic Yb(9%):Sc2O3 can be attributed to a combination of the large mass difference between Sc and Yb and to the quality of the ceramic material.
The thermal conductivity of YSO was reported as 4.5 W/mK [22] and 4.4 W/mK [23] at room temperature, although no orientation for the sample was given in either report, which is in excellent agreement with our value of 4.66 W/mK along the b-axis. GSAG’s thermal conductivity is significantly lower than YAG’s even though these are both Al garnets, and our measurements are consistent with earlier reports [24]. Growth of GSAG crystals is typically stoichiometric, which leads to disorder in the crystal and greater phonon scattering and lower thermal conductivity. For highly doped Yb:YLF the thermal conductivity is significantly lower than previous measurements [14] on undoped and 5% doped Yb:YLF, as expected.
The CTE measurements are fitted to a second-order polynomial of the form
and the fits are valid in the range of approximately 90–320 K. The fitted coefficients are listed in Table 4 , along with a computation from the polynomial fit at 300 K and 100 K and data from previous reports.Table 4. Coefficient of Thermal Expansion (ppm/K)
As expected, the CTE drops significantly as the temperature decreases in these materials. We are unaware of any reports of previous thermal expansion data for these materials in the range below room temperature, so the only comparisons that can be made are near room temperature. The previous measurements have been made either by dilatometry or by x-ray diffraction. Interferometry, used in our work, is generally considered to be a more accurate technique [25]. There have been numerous reports in the literature on the CTE of YVO4, but many of them have been averaged values over temperature ranges above room temperature (see [29] for a compilation). Perhaps the best comparison for the CTE of YVO4 comes from our earlier measurement for GdVO4 [3], which is expected to have similar CTE to YVO4; the measurements are in good agreement.
The dn/dT data are fit to a third-order polynomial of the same form as Eq. (1). The fitted coefficients, a computation of dn/dT at 300 K and 100 K from the fit, and previous data are shown in Table 5 . For dn/dT, we are not aware of previous measurements below room temperature, so a comparison is limited to other room-temperature data around 1-µm wavelength. For YVO4, previous measurements vary widely. Again, the best comparison is probably to our previous measurement in GdVO4 [3]; our YVO4 data are in good agreement with our GdVO4 data.
Table 5. Change in Refractive Index with Temperature dn/dT at 1.06-µm Wavelength (in ppm/K)
3. Nanosecond-Class Lasers
Solid-state lasers are very well suited to the generation of Q-switched pulses, allowing for high pulse energies in a nanosecond-class waveform. As mentioned previously, cryogenic cooling of Yb-doped gain media also offers substantial improvements to laser kinetics by increasing efficiency, increasing emission cross sections (and consequently reducing the saturation fluence or intensity), and by thermally depopulating the terminal state to achieve four-level operation.
As an example, the peak stimulated emission cross section of Yb:YAG increases by a factor of about five as temperature is reduced from room temperature to 77 K. Likewise, the product of emission cross section and upper-state lifetime increases by a similar factor [3,37]. Saturation fluence and intensity are reduced accordingly, which in turn enables efficient operation of Q-switched oscillators at lower intracavity fluence.
Reported cryo-Yb nanosecond-class lasers include acousto-optically (AO) Q-switched Yb:YAG lasers that generate 30-mJ, 32-ns pulses at 1.5-kHz pulse repetition frequency (PRF) and as much as 70-W average power at higher pulse rates (5-kHz PRF with 75-ns pulsewidth) [38]. More recently, AO Q-switching of cryo-Yb:YAG was demonstrated with 340-W output power for repetition rates in the range of 40-100 kHz [39]. At 40 kHz and 340 W, the pulse duration was 75 ± 5 ns, corresponding to a 8.5 mJ pulse energy [39]. An alternative to achieving high pulse energy in a nanosecond waveform is by using a master oscillator / power amplifier (MOPA) architecture. Kawanaka et al. demonstrated a MOPA system with a cryo-cooled ceramic Yb:YAG regenerative amplifier and a four-pass power amplifier consisting of a double-end-pumped Yb:YAG rod, also at cryogenic temperature [40]. Pulse energies exiting the regenerative amplifier were 6.5 mJ and 1.5 mJ at repetition rates of 200 Hz and 1 kHz, respectively, in a 10-ns waveform. Beam quality was near diffraction limited, with M2 < 1.1. The pulse energy at the output of the power amplifier was 140 mJ at 200-Hz PRF, with no reported beam quality.
Another way of achieving shorter duration Q-switched pulses of <20 ns full width at half-maximum (FWHM) in a high-power, high-gain cryo-Yb oscillator is by using an electro-optic Q-switch (Pockels cell) to maximize extracted pulse energy and minimize Q-switched pulse duration. Recently, we have demonstrated an electro-optically Q-switched, end-pumped, cryogenic Yb:YAG laser that simultaneously achieves 114-W average power at 5-kHz PRF, a 16-ns pulse duration, and near-diffraction-limited beam quality [41].
The laser configuration of our electro-optically Q-switched laser [41] is shown in Fig. 1 . A 1%-doped Yb:YAG crystal having an undoped endcap is single-end pumped with a fiber-coupled laser diode array at 940 nm. The crystal is 23 mm long (including a 1 mm thick endcap), with a 5 mm × 5 mm cross section. Pump light is then focused into the Yb:YAG crystal through the high-reflector (HR) mirror of the laser resonator using a 150-mm FL spherical lens. The diameter of the gain region in the Yb:YAG crystal is about 1.5 mm.
Fig. 1 Layout of electro-optic Q-switched oscillator. HR / HT: highly reflecting (1030 nm) / highly transmitting (940 nm) with 6-m radius of curvature (ROC); TFP: thin-film dielectric polarizer.
The HR mirror of the laser resonator is highly reflecting (>99.5%) at 1030 nm and highly transmitting (>98%) at the 940-nm pump wavelength. The mirror has a 6-meter convex radius of curvature. The resonator axis makes a single pass through the Yb:YAG crystal and then bounces off two thin-film dielectric polarizers. The BBO Pockels cell is a transverse-field, two-crystal device having a 5-mm clear aperture and a quarter-wave voltage of 4.4 kV at 1030 nm. The output coupler is a 10% reflecting flat mirror, and overall physical resonator length is 43 cm. Both the output coupler and resonator length were chosen to achieve a pulse duration of 16 ns at 100-W output power.
Laser performance data for the resonator, operating in both cw mode and Q-switched at 5-kHz PRF, is shown in Fig. 2 . At 244-W maximum pump power, cw power is 123 W and Q-switched average power is 114 W. Laser threshold is approximately 60 W. Average slope efficiency is 68% in cw mode and 63% in Q-switched mode. Absolute efficiency at 244-W pump power is 50% in cw mode and 47% in Q-switched mode.
Fig. 2 Continuous-wave (CW) and Q-switched (QSW) input-output data. Slope efficiencies are 68% CW and 63% QSW.
The Q-switched laser pulse at 114-W average output power is shown in Fig. 3(a) . The measured pulse width is 16 ns FWHM. The temporal profile was recorded using the full 500-MHz bandwidth of the oscilloscope and represents an average of 16 pulses.
Fig. 3 (a) Q-switched pulse shape (at 114 W, 5-kHz PRF). The FWHM of the pulse determined by a polynomial fit is 16 ns. The structure in the pulse is due to longitudinal mode beating that is only partially resolved by the oscilloscope (500-MHz bandwidth). (b) 1:1 image of the beam at the output coupler (Q-switched, 114 W). Beam diameter is 1.2 mm (1/e2). (c) Far-field beam profile at focus of 25-cm FL lens. Beam diameter (1/e2) at focus is 260 μm at 114-W output power.
Shown in Fig. 3(b) is the near-field beam profile at 114-W output power and 5-kHz PRF. The profile is a 1:1 image of the beam at the laser’s output coupler; at 114 W the beam diameter (1/e2) is 1.2 mm. The far-field beam profile at the focus of a 25-cm FL lens is shown in Fig. 3(c). Far-field beam diameter at 114 W is 260 μm. Beam quality is near diffraction limited.
4. Ultrashort-Pulse Lasers
Over the last decade, ultrashort pulse solid-state lasers with cryo-cooled Yb-doped gain media have undergone a remarkable progression. Figure 4 plots a collection of ultrashort pulse laser demonstrations as a function of peak and average power. Early cryo-Yb demonstrations were focused on operation in one of two operating regimes: high peak power or high average power [42–45]. More recent results aim to demonstrate average power scalability [46,47], as well as simultaneously scale both peak and average power [15,48–50]. Also shown, for comparison, are ultrashort pulse laser demonstrations using different gain media, including room-temperature Yb:YAG [51–54], Yb-doped fiber [55,56], and cryo-cooled Ti:sapphire [12,13].
Fig. 4 A compilation of recent cryogenically-cooled Yb-doped ultrashort pulse lasers, plotted as a function of average and peak power. Also included are representative demonstrations of different gain media, including Yb:YAG at room temperature, Yb-doped fiber, and cryogenically-cooled Ti:sapphire. For consistency, peak power is defined with respect to the FWHM pulse duration, and average power is defined prior to pulse compression.
Many ultrashort laser applications require high average and high peak power in a near-diffraction-limited beam. We define a figure of merit that is the product of the peak and average power. Loci of equal figures of merit are depicted in Fig. 4, e.g., by the line representing 1010 W2. From this standpoint, one trend emerges for the cryo-Yb demonstrations plotted in Fig. 4, the rapidity of which is noteworthy. An arrow showing the general direction towards high peak and high average power is depicted, wherein the early demonstrations with high peak power are encircled on the left of Fig. 4 and the more recent demonstrations are encircled on the right.
Ultrashort-pulse lasers must confront additional challenges to those which afflict high-average-power lasers in the CW and nanosecond-pulse regime. High peak power may result in undesired nonlinear effects as well as damage of optical components, and is usually mitigated by temporally broadening the pulses during amplification [57,58]. Even with this technique, the saturation fluence can be significantly higher than the damage threshold, limiting extraction efficiency. Cryogenic cooling typically enhances the emission cross section, to the benefit of energy extraction. However, this generally comes at the expense of a reduction in the gain bandwidth, and therefore an increase in the minimum pulse duration achievable.
For example, in Yb:YAG, the 5.3-nm FWHM bandwidth at room temperature reduces to 1.5 nm at liquid nitrogen temperature. In this Section, we present the performance of a 100-W, kHz-class cryo-Yb:YAG amplifier. We then discuss two approaches to improve upon the minimum pulse duration: (i) the use of multiple laser host materials to exploit a composite gain bandwidth (e.g., YAG and GSAG [15]); (ii) the use of laser host materials with greater bandwidth at cryogenic temperature (e.g., YLF).
4.1 A Cryo-Yb:YAG Ultrashort Laser Amplifier
We present here the results of a laser system which consists of a mode-locked Yb:KYW laser, a room-temperature Yb:YAG regenerative amplifier, and a Yb:YAG power amplifier at cryogenic temperature. The Yb:KYW laser operates with 450-mW average power at 30.5-MHz PRF with 450-fs-long output pulses. The output is fed to a grating-based stretcher and then to a regenerative amplifier, where a Pockels cell pulse picks at 5-kHz PRF. The pulse length of the stretched pulse downstream from the regenerative amplifier is about 150 ps. A schematic of the four-pass power amplifier is shown in Fig. 5 . The gain crystals are 2.5-cm-long, 1%-doped Yb:YAG (including a 3.5-mm-long, undoped Yb:YAG endcap), which are placed in series in a liquid nitrogen cryostat. Each crystal is pumped from a single end by a fiber-coupled (0.4-mm diameter, 0.22 NA) diode laser system, with the pumps imaged to 4.5-mm-diameter spots in the gain crystals. The four passes are implemented using standard polarization multiplexing. The laser beam is re-imaged from the first to the second pass (and from third to fourth).
The average output power of the stretched pulse beam as a function of incident pump power is shown in Fig. 6 , along with an inset showing the measured near-field at full power. Operation up to 115 W was achieved in a near-diffraction-limited beam.
Fig. 6 Average output power as a function of total incident pump power for the Yb:YAG power amplifier at 5-kHz PRF. The inset shows the near-field intensity profile at 110 W.
Measurements of the input and output optical spectra are shown in Fig. 7 . The spectrum from the regenerative amplifier is centered at 1029.8 nm with a bandwidth of approximately 1 nm FWHM. This center wavelength is slightly longer than the gain peak of cryo-Yb:YAG but with sufficient overlap to provide efficient extraction (cf. Fig. 9 ). After amplification, the peak has blue shifted to 1029.5 nm and narrowed, although a spectral tail to the red is still present. The recompression of the amplified pulse was not attempted, but a calculation of the transform-limited pulse duration demonstrates potential for 2-3 ps FWHM. We believe this to be the first reported laser system that simultaneously operates with an average power in excess of 100 W and a potential peak power exceeding 1 GW (see Fig. 4).
Fig. 7 Spectral performance of Yb:YAG power amplifier at full power (115 W). The black dashed curve and red solid curve show the input and output spectrum of the power amplifier, respectively.
Fig. 9 Spectral performance of Yb:YAG/Yb:GSAG power amplifier at full power (73 W). The black dotted curve shows the input spectrum to the power amplifier, and the red solid curve shows the output spectrum (at 73 W). The emission cross sections for both Yb:YAG and Yb:GSAG at 77 K are shown for reference (dash-dot blue and dash blue curves, respectively).
4.2 A Cryo-Yb:YAG/Yb:GSAG Power Amplifier
In a recent demonstration, we employed the technique of using multiple gain media to provide a larger bandwidth [15]. This approach is attractive because it directly leverages existing high-power cryogenic Yb:YAG technology. In this system, the second gain medium was Yb:GSAG, which was chosen because its properties are similar to YAG, both being oxide garnets, and its gain peak at cryogenic temperature is offset from, but overlaps with, the gain spectrum in YAG.
For this demonstration, several modifications were made to the power amplifier discussed in the previous section (see Fig. 5). One of the gain elements was replaced with a 1.5-cm-long, 2%-doped Yb:GSAG crystal. The pump beam diameter was decreased from 4.5 mm to 3 mm, and the spectrum of the regenerative amplifier was wavelength tuned to the red to better match the cryo-Yb:GSAG gain peak. Finally, the quarter-wave plate is replaced with a Faraday rotator, which provides birefringence compensation [59,60]. We estimate that uncompensated birefringence would cause a 3% depolarization loss for two passes, owing primarily to residual stress in the as-grown Yb:GSAG. Thermally induced stress is mitigated by cryogenic cooling.
The amplification results are shown in Fig. 8 (average-power performance) and Fig. 9 (spectral performance) operating at 5-kHz PRF. The power, measured at the output of the amplifier but before recompression, is shown as a function of the total pump power. The inset of Fig. 8 shows the measured near-field output. Operation up to 73-W average power was achieved.
Fig. 8 Average output power as a function of total incident pump power for the Yb:YAG/Yb:GSAG power amplifier at 5-kHz PRF. The inset shows the near-field intensity profile at 73 W.
The spectrum from the regenerative amplifier is centered at 1030.3 nm with a bandwidth of 1.1 nm FWHM, as shown in Fig. 9. This center wavelength is between the gain peaks of Yb:YAG and Yb:GSAG, closer to that of Yb:GSAG. After amplification, the peak has blue shifted to 1029.7 nm and narrowed to 0.7 nm FWHM, although there is a significant tail to the red. The compressibility of the amplified pulse using a Treacy pulse compressor consisting of multilayer dielectric gratings with a transmission of 84% was tested. The input pulse was compressed to 1.4 ps (FWHM), and the output was compressed to 1.6 ps (FWHM) at full power, yielding a compressed output power of 60 W.
4.3 A Cryo-Yb:YLF Ultrashort Regenerative Amplifier
Another material which shows promise for ultrashort high-average-power applications is Yb:YLF, which exhibits a much broader gain bandwidth than Yb:YAG or Yb:GSAG, while retaining many of the thermo-optic benefits at cryogenic temperature. In addition, YLF is naturally birefringent, avoiding the loss and beam distortion which result from thermally-induced birefringence in isotropic materials.
Pulses as short as 196 fs have been obtained from a room temperature Yb:YLF oscillator [61], while cryogenic Yb:YLF amplifiers have delivered sub-picosecond pulses at the 30-mJ level, at low average power (20-Hz PRF) [42]. Recently, pulse energies in excess of 100 mJ (at 10-Hz PRF) have been achieved [62]. High-average-power demonstrations of cryo-Yb:YLF under cw operation have also been conducted (224 W), and have resulted in excellent beam quality [9].
As part of an ongoing effort to produce high-average-power ultrashort pulses, we have recently demonstrated a regenerative amplifier that delivers 1 mJ pulses at 10 kHz. The schematic of the amplifier is shown in Fig. 10 . Seed pulses are picked from a mode-locked Ti:sapphire oscillator at 10-kHz PRF. A diffraction grating stretcher is used to stretch the pulses to 400 ps in duration. Spectral components outside of a roughly 10-nm bandwidth are lost due to the finite size of the gratings. Pulses with 1-nJ energy are injected into the amplifier cavity through a polarizer. A BBO Pockels cell is used to control the number of round trips the pulses travel before being ejected. A 1.75-mm thick crystal of 25%-doped Yb:YLF is sandwiched between two undoped YLF endcaps and mounted inside a LN2 dewar, positioned in the amplifier cavity. The crystallographic orientation results in a gain profile that is predominantly a-axis. Two 960-nm fiber-coupled laser diodes pump a 500-μm diameter region of the gain from both ends through dichroic mirrors. At 10 kHz, the pulse repetition period is shorter than the laser upper state lifetime. Pumping, therefore, is continuous rather than pulsed.
Fig. 10 Schematic layout of cryo-Yb:YLF regenerative amplifier; TFP: thin-film polarizer. FR: Faraday rotator. DM: dichroic mirror.
Performance of the regenerative amplifier is shown in Fig. 11 . For 43.5 W of pump power, the seed pulses are amplified to 1 mJ after 13 round trips, producing 10 W output power. The FWHM of the amplified spectrum is 2.22 nm and the output beam quality is nearly diffraction-limited. Further power scaling of this system is underway.
Fig. 11 (a) Optical spectrum of 1 nJ seed pulses (black-dotted line) and 1 mJ amplified pulses (red-solid line). The a-axis emission cross section of Yb:YLF (77 K) is also shown (blue-dashed line). The FWHM of the amplified spectrum is 2.22 nm. The (b) near-field and (c) far-field intensity profile of the regenerative amplifier output at 10 W operation (1 mJ).
5. Conclusion
The performance of cryogenically-cooled Yb-doped pulsed lasers has been steadily improving over the past decade. This progress is expected to continue as a number of proven techniques have yet to be applied to this class of laser.
The amplifiers highlighted here have used an end-pumped-rod gain-element geometry. While simple to fabricate, this creates a temperature distribution that results in a highly aberrated thermal lens which limits the average power. Combining cryogenic cooling with advanced geometries, such as thin disks [51,52,63], total-reflection active-mirrors [40], and grazing incidence disks [64], would likely increase the average power at which beam distortion is a limiting factor. The use of a more advanced front end for pulse generation [63] may lessen the reliance on the amplifier for gain, ameliorating the significant gain narrowing which currently limits the minimum achievable pulse duration. Similarly, spectral shaping techniques [65,66] should provide for shorter pulses, and therefore higher peak power. Kilowatt average powers and terawatt peak powers appear to be within reach in the near future.
Acknowledgments
The authors acknowledge the contributions of J. D. Hybl, J. R. Ochoa, S. E. J. Shaw, J. Daneu, P. Hassett, and P. Foti at Lincoln Laboratory. In addition, we acknowledge our collaboration with S. Hawes, H. Martin, J. Zhang, S. Sarkisyan, E. Wilson, E. Nelson-Melby, and P. Lundquist, all of Applied Energetics, Tucson, AZ. This work was sponsored by the Department of the Army and the High Energy Laser – Joint Technology Office under Air Force contract number FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors, and are not necessarily endorsed by the United States Government.
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