Diamond Raman lasers are an emerging high-power laser technology offering kW-level beam powers with high brightness. Here we report experimental observation of a diamond thermal lens at output powers up to 1.1 kW in a quasi-steady-state regime. It is found that a thermal lens of up 16 diopters is induced, which causes the beam quality to evolve over the investigated power range. The results aid design at higher power and clarify conditions under which beam quality degradation is expected when scaling output power.
High-power single-longitudinal-mode (SLM) lasers are crucial for applications such as LIGO, sodium guide stars, isotope separation and atom cooling, but are challenging due to gain profile inhomogeneities and spatial hole burning. This work shows that a SLM 620 nm laser is firstly achieved using a standing-wave diamond Raman resonator with intracavity second harmonic generation (SHG). In addition to the benefits of the spatial hole burning-free gain medium [1], SHG provides a valuable additional mechanism for increasing gain competition and therefore enhancing SLM stability. Nascent secondary modes experience double the nonlinear loss in the SHG crystal due to the role of sum frequency generation with the primary mode [2]. A further feature of the current external standing-wave cavity design is that the cavity is non-resonant at the pump frequency, thereby allowing the laser to be pumped without mutual control of the wavelength and cavity length and by using multi-longitudinal-mode (MLM) lasers with a spacing different to the free-spectral-range of the diamond Raman laser.
A quasi-continuous-wave external cavity diamond Raman laser with 1.2 kW output power was demonstrated using gated pump pulses of 100 µs duration, which was 7 times longer than the time constant for the establishment of steady-state temperature gradients. An 83% slope efficiency and a 53% optical-to-optical efficiency were obtained in conversion from a 1.064 µm Nd:YAG pump to a 1.24 µm first Stokes. The transient Stokes behavior from the sharp turn-on was consistent with calculations for the first Stokes build-up time. A brightness enhancement factor of 56 was obtained from the M2 = 15 pump beam. An observed increase in the first Stokes beam quality from M2 = 2.95 to M2 = 1.25 with increase of the Raman laser output power indicated the presence of a steady-state thermal lens in diamond.
We demonstrate a continuous-wave diamond Brillouin laser (DBL) in a ring cavity, operating near 532 nm with a 167 GHz Stokes shift. The DBL is pumped by a narrow-line (<1 MHz), frequency-doubled ytterbium fiber laser with the intracavity power resonantly enhanced via Hänsch-Couillaud locking. The measured threshold enabled the Brillouin gain coefficient in diamond to be determined for the first time, yielding a value of 60 cm/GW for pump and Brillouin polarizations aligned parallel to the 〈111〉 crystallographic direction in diamond (determined by the cut of our diamond in this case). Analysis of diamond’s photoelastic tensor shows that for polarization aligned to 〈110〉 for maximum gain, a coefficient of 115 cm/GW can be deduced, the highest bulk Brillouin gain coefficient reported for any material. The high Brillouin gain coefficient in combination with outstanding optical and thermal properties, indicates great potential for realizing diamond lasers and stimulated Brillouin scattering-enabled devices of performance far exceeding other materials.
We report a second-Stokes diamond Raman laser in eye-safe wavelength capable of high power and large-scale-factor brightness enhancement. Using a quasi-continuous 1.06 μm pump of power 823 W (0.85% duty cycle) and M2 up to 6.4, a maximum output power of 302 W was obtained with an M2 = 1.1 providing an overall brightness enhancement factor of 6.0. The output power is the highest single-mode power reported for Er-doped and Raman fiber lasers (~300 W). The measurements are in good agreement with model calculations, which we use to optimize and predict performance over wider range of power and input beam quality. The results highlight a novel pathway to high brightness eye-safe lasers based on relatively incoherent 1.0–1.1 μm pumps. The concept may be adapted and extended to other wavelength regions by using other pumps or via higher-order (3+) cascading. For example, to generate high brightness red output near 0.62 – 0.67 m by using second harmonic pumps near 0.53 μm. A large number of wavelength options are conceivable as a result of the wide transparency of diamond (0.23–3.8 µm, and > 6 µm).
We present our recent advances on power scaling of a high-power single-pass pumped CVD-diamond Raman oscillator at 1.2 μm. The single pass scheme reduced feedback to the high gain fiber amplifier, which pumps the oscillator. The Yb-doped multi-stage fiber amplifier itself enables up to 1 kW output power at a narrow linewidth of 0.16 nm. We operate this laser in quasi-cw mode at 10% duty cycle and on-time (pulse) duration of 10 ms. With a maximum conversion efficiency of 39%, a maximum steady-state output power of 380 W and diffraction limited beam quality was achieved.
We present our recent advances on power scaling of a high-power single-pass pumped CVD-diamond Raman oscillator at 1.2 μm. The single pass scheme reduced feedback to the high gain fiber amplifier, which pumps the oscillator. The Yb-doped multi-stage fiber amplifier itself enables up to 1 kW output power at a narrow linewidth of 0.16 nm. We operate this laser in quasi-cw mode at 10% duty cycle and on-time (pulse) duration of 10 ms. With a maximum conversion efficiency of 39%, a maximum steady-state output power of 380 W and diffraction limited beam quality was achieved.
We present our recent advances in the field of Raman frequency conversion using high-optical quality CVD-diamond. Different diamond Raman lasers were developed for efficiently generating multi-Watt output at specific wavelengths from the visible to the eye-safe spectral range, while single-frequency operation was accomplished by exploiting an intrinsic mode stability mechanism.
The goal of our research is a compact Raman laser emitting short pulses with high energy and peak power in “eye-safe" region around wavelength 1.5 μm. We utilize intracavity conversion of giant pulses at wavelength 1.34 μm in a BaWO4 Raman crystal (18 mm long, AR coated). Required high energy and peak power was reached using a flash-lamp pumped Nd:YAG laser (rod 100 mm long, diameter 4 mm), Q-switched by V:YAG solid-state saturable absorber (initial transmission 37% @ 1.34 μm). The L-shaped oscillator for 1.34 μm radiation consisted of a concave mirror (r = 0.5 m, HR @ 1.3 μm, HT @ 1.06 μm), flat polarizing intracavity mirror, and output coupler (r = 1 m, HR @ 1.3 μm, R = 39 % @ 1.5 μm). The polarizing mirror ensured stable linearly polarized laser emission and prevented parasitic oscillations at 1.06 μm. The Raman laser oscillator was formed by the output coupler and another intracavity mirror (r = 0.5 m, HR @ 1.5 μm, HT @ 1.3 μm), inserted between BaWO4 and the polarizing mirror. For pumping energy of 28.2 J stable vertically polarized generation of the 1st Stokes radiation at 1528 nm was reached. In multimode operation the output energy was 20 mJ in 2.25 ns pulses. Single mode operation was possible by inserting a 1.5 mm aperture between Nd:YAG and V:YAG crystal. The output energy dropped to 9.7 mJ (even for higher pump power of 30.7 W) and output pulses were shortened to 1.87 ns.
We report external cavity Raman lasers using a 9.5-mm-long low-loss CVD diamond pumped by a 35 W q-switched
Nd:YVO4 laser with approximately 22 ns pulses at 36 kHz pulse repetition frequency. Two systems were investigated.
The first demonstrated first and second Stokes simultaneously with more than 14.5 W of combined optical power. For an
output coupler optimized for second Stokes only output (1485 nm), 11.1 W at 38% conversion efficiency was obtained.
This output power is comparable to the maximum output powers reported for competing pulsed eye-safe technologies
such as optical parametric oscillators and Er:YAG lasers.
The goal of this work was the external cavity CVD diamond Raman laser generating at wavelength of 1.63 μm which
belongs to an eye safe region. As the Raman medium a sample of CVD grown single diamond crystal was used. The
crystal was grown in the [100] direction and after cutting, the sample sides were formed by {110} planes. The diamond
crystal sample dimensions were 5 mm × 5 mm × 2 mm and it had no anti-reflection coatings. As a pumping radiation
source the compact Q-switched Nd:YAP laser operating at the wavelength 1.34 μm was used. The pulse duration was 12
ns. This pumping radiation was focused by the lens (f = 250 mm) perpendicularly on the diamond crystal face. The beam
radius inside the crystal was 180 μm. The 15 mm long Raman laser resonator was formed by a pumping mirror (HR @
1.63 μm and T = 97 % @ 1.34 μm) and an output coupler (R = 95 % @ 1.63 μm, r = 500 mm). The maximum generated
laser output energy was 18 μJ @ 1.63 μm for the pumping energy of 3.35 mJ corresponding to the conversion efficiency
of 0.5 %. The output spatial beam profile corresponds to the fundamental Gaussian beam in both axes.
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