Nd:YVO4 high-power master oscillator power amplifier laser system for second-generation gravitational wave detectors.

Ultrastable high-power laser systems are essential components of the long baseline interferometers that detected the first gravitational waves from merging black holes and neutron stars. One way to further increase the sensitivity of current generation gravitational wave detectors (GWDs) is to increase the laser power injected into the interferometers. In this Letter, we describe and characterize a 72 W and a 114 W linearly polarized, single-frequency laser system at a wavelength of 1064 nm, each based on single-pass Nd:YVO4 power amplifiers. Both systems have low power and frequency noise and very high spatial purity with less than 10.7% and 2.9% higher order mode content, respectively. We demonstrate the simple integration of these amplifiers into the laser stabilization environment of operating GWDs and show stable low-noise operation of one of the amplifier systems in such an environment for more than 45 days.

Millijoule-level, kilohertz-rate, CPA-free linear amplifier for 2 µm ultrashort laser pulses.

The generation of millijoule-level ultrashort laser pulses at a wavelength of 2.05 µm in a compact chirped pulse amplification-free linear amplifier based on Holmium-doped YLF gain medium is presented. More than 100 MW of pulse peak power has been achieved. We show the capabilities of this laser amplifier from a 1 kHz to 100 kHz repetition rate. A detailed numerical description supports the experimental work and verifies the achieved results.

Heat generation in Nd:YAG at different doping levels.

Nd:YAG lasers with output power levels of tens of watts, a nearly diffraction-limited beam quality, and a linearly polarized continuous wave output are commonly pumped by laser diodes at a wavelength around 808 nm, where the pump light spectrum is matched well to the absorption maximum of Nd:YAG. As a consequence, low Nd(3+)-doping concentrations of the laser crystals are required in order to minimize thermally induced stress. The use of higher Neodymium concentrations requires pump wavelengths beside the 808 nm absorption maximum and will furthermore result in changed thermo-optical behavior of the material. We present simulations and experimental results on how the doping concentration of Nd(3+) influences the fraction of pump light converted into heat.

Linearly polarized single-mode Nd:YAG oscillators using [100]- and [110]-cut crystals.

The output power and efficiency of linearly polarized high power Nd:YAG lasers is limited by depolarization and bifocusing. Both effects degrade the beam quality and decrease the output power. In a single pass configuration, [100]- and [110]-cut crystals can be used to reduce the depolarization. Here, we compare [100]-, [110]- and [111]-cut crystals in an oscillator configuration. As expected it was possible to reduce the depolarization loss by using [100]-cut crystals in our configuration, while the depolarization loss was higher for [110]-cut crystals. The thermal lens establishing in these crystals is not circular, which can degrade beam quality in high power operation.

All-fiber based amplification of 40 ps pulses from a gain-switched laser diode.

Amplification of a gain-switched laser diode is demonstrated in an all-fiber based setup. The amplified spontaneous emission between two consecutive pulses was investigated quantitatively in the time domain. A maximum pulse energy of 13 µJ at a repetition rate of 1 MHz and a pulse duration of 40 ps was extracted, corresponding to a peak power of 270 KW. To the best of our knowledge, this is the highest extracted pulse energy from a laser system seeded by a gain-switched laser diode. Temporal pulse deformation due to intrapulse Raman scattering was observed in the reported system.

Intrinsic reduction of the depolarization in Nd:YAG crystals.

The output power of linearly polarized Nd:YAG lasers is typically limited by thermally induced birefringence, which causes depolarization. However, this effect can be reduced either by use of some kind of depolarization compensation or by use of crystals which are cut in [110]- and [100]-direction, instead of the common [111]-direction. Investigations of the intrinsic reduction of the depolarization by use of these crystals are presented. To our knowledge, this is the first probe beam-experiment describing a comparison between [100]-, [110]- and [111]-cut Nd:YAG crystals in a pump power regime between 100 and 200 W. It is demonstrated that the depolarization can be reduced by a factor of 6 in [100]-cut crystals. The simulations reveal that a reduction of depolarization by use of a [110]-cut crystal in comparison with a [100]-cut crystal only becomes possible at pump powers in the kW region. Analysis also shows that the bifocusing for [100]-cut is slightly smaller and more asymmetrical than for [111]-cut.

Design and comparison of composite rod crystals for power scaling of diode end-pumped Nd:YAG lasers.

A comparison of composite Nd:YAG laser rod crystals with one, two and three doped segments for high-power diode end-pumping is presented. An approach based on an expansion of the heat generation density and temperature distributions into a Fourier-Bessel basis set for solving the stationary heat conduction equation is used for choosing adequate segment lengths and dopant concentrations. A maximum laser output power of 167.5 W at an optical-to-optical efficiency of 53.6% was achieved by longitudinal pumping a crystal with three doped segments with fibre-coupled laser diodes.

End-pumped Nd:YAG laser with a longitudinal hyperbolic dopant concentration profile.

An approach for scaling end-pumped rod lasers to high output powers by employing a crystal with a continuously varying, nearly hyperbolic dopant concentration profile, resulting in a homogenization of the longitudinal temperature distribution, is presented. The crystal is characterized by determining its dopant concentration profile with an absorption measurement. The fluorescence upon spectrally narrow excitation is recorded, indicating the role of quenching of the upper laser level at high dopant concentration. The on-axis temperature distribution is calculated by employing a Fourier-Bessel approach for solving the stationary heat conduction equation, taking the temperature dependence of the heat conductivity and the dependence of the heat fraction on the dopant concentration into account. Experimentally, a maximum output power of 187 W at an optical-to-optical efficiency of 53 % has been demonstrated.

Brillouin scattering spectra in high-power single-frequency ytterbium doped fiber amplifiers.

We report on theoretical and experimental investigations on spontaneous and stimulated Brillouin scattering during operation of a high-power single-frequency polarization-maintaining ytterbium doped fiber amplifier. For different amplifier configurations with co- and counter-propagating seed and pump radiation the evolution of Brillouin scattering spectra was investigated with a heterodyne detection scheme. Spontaneous Brillouin gain spectra at low powers were additionally investigated using a pump-probe technique. The data obtained from these experiments have been compared with a theoretical model based on coupled intensity equations. A Brillouin scattering suppression has been investigated theoretically and experimentally with externally applied temperature gradients along the fiber resulting in up to 3.5 dB suppression and 115 W of amplifier output power.

Fundamental mode, single-frequency laser amplifier for gravitational wave detectors.

An amplifier design for efficient amplification of linearly polarized fundamental mode lasers is presented. The concept was verified by amplifying single-frequency input powers from 1 W to 20 W into output power ranges of 35 W up to 65 W. Beam quality measurements with a mode-analyzer cavity showed only minor beam quality degradation due to the amplification process.

250 W end-pumped Nd:YAG laser with direct pumping into the upper laser level.

A high-power longitudinally pumped Nd:YAG laser using direct pumping into the upper laser level is demonstrated. With an absorbed pump power of 438 W an output power of 250 W was realized, which results in an optical-to-optical efficiency of 57%. To the best of our knowledge, this is the first demonstration of a high-output power 885 nm pumped laser design.

Core-doped Ceramic Nd:YAG Laser.

We report on a diode end-pumped composite core-doped ceramic Nd:YAG laser. The ceramic Nd:YAG rod consists of a centrally doped region of 1.5 mm in diameter in a 3 mm rod. An output power of 144 W was achieved with an absorbed pump power of 226 W corresponding to an opt.-opt. efficiency of 64% by longitudinal pumping with fiber-coupled laser diodes.

Single-frequency master-oscillator photonic crystal fiber amplifier with 148 W output power.

We report on a high-power ytterbium doped photonic crystal fiber amplifier using a single-frequency Nd:YAG non-planar ring oscillator seed source. With a large-mode-area photonic crystal fiber, operation below the threshold of stimulated Brillouin scattering is demonstrated with up to 148 W of continuous-wave output power and a slope efficiency of 75%. At maximum output power the amplified spontaneous emission was suppressed by more than 40 dB and the polarization extinction ratio was better than 22 dB. In order to investigate the overlap of the photonic crystal fiber transverse-mode with a Gaussian fundamental mode, sensitive beam quality measurements with a Fabry-Perot ring-cavity are presented.

Comparison of crystalline and ceramic composite Nd:YAG for high power diode end-pumping.

A comparison between composite crystalline and ceramic composite Nd:YAG rods for high power diode end-pumping is presented. Laser output power characteristics as well as the thermal lensing properties of the composite laser rods were evaluated. A maximum laser output power of 121 W and an optical-to-optical efficiency of 48% were achieved by longitudinal pumping with fiber-coupled laser diodes.

Nd:YAG ring laser with 213 W linearly polarized fundamental mode output power.

We present a high power fundamental mode ring oscillator with four longitudinally diode-pumped laser rods and effective birefringence compensation as a laser source for the next generation of gravitational wave detection. With this setup a linearly polarized output power of 213 W in a nearly diffraction limited beam with an M2 value of 1.15 was demonstrated.

407 W End-pumped Multi-segmented Nd:YAG Laser.

A composite crystalline Nd:YAG rod consisting of 5 segments with different dopant concentrations for high power diode end-pumping is presented. A maximum laser output power of 407 W with an optical-to-optical efficiency of 54 % was achieved by longitudinal pumping with a high power laser diode stack.

High power fundamental mode Nd:YAG laser with efficient birefringence compensation.

Experiments on a high-power end-pumped Nd:YAG rod laser with an efficient birefringence compensation will be presented. A linearly polarized output power of 114 W with an M2-value of 1.05 was realized. Furthermore, the from our best knowledge highest injection-locked singlefrequency output power of 87 W in a nearly diffraction-limited beam was demonstrated.