We demonstrate the compact high-power scaling of bounce geometry lasers with a new dual-pumped folded amplifier design. A Q-switched laser oscillator built with this amplifier is shown to produce over 30 W of average power from 80 W of pump power at up to 600 kHz repetition rate. In a master-oscillator power-amplifier (MOPA) configuration using the dual-pumped amplifier, we demonstrate over 100 W of output power from 250 W of pump power. We also demonstrate very high repetition rate Q-switching (1.7 MHz) of the master oscillator.
© 2009 Optical Society of America
High-power solid-state lasers have proved to be valuable sources of intense infrared radiation for use in scientific, industrial and military applications. In recent years, diode-pumped solid-state (DPSS) lasers have begun to replace existing lamp-pumped systems, due to their superior efficiency and compactness. However, the power scaling potential of many DPSS lasers is limited by thermal issues. High-power end-pumped rods have been built with very high beam quality , but are limited in power scaling by strong thermally-induced lensing and fracture of the laser material. Diode side-pumping can overcome the thermal fracture problem by spreading the pump light over a larger area, enabling high-power scaling . Side-pumped zig-zag slab designs in particular also offer spatial averaging over the gain region to achieve high beam quality . One such design which has received particular attention recently is the grazing-incidence bounce geometry [4–12].
The bounce geometry is designed to make efficient use of gain materials which absorb strongly at the pump wavelength. In a bounce amplifier, a slab of laser material is pumped on one side by a laser diode bar, and the laser mode makes a single grazing-incidence total internal reflection (TIR) at the pump face. The grazing-incidence bounce angle provides excellent overlap with the shallow region of high inversion density near the pump face, resulting in extremely high small-signal gains (>1000). The high gain of such an amplifier enables high repetition rate Q-switching (>100 kHz) [4, 5]. Moreover, the bounce provides considerable spatial averaging of both the non-uniform gain profile and thermal gradients caused by the exponential absorption of the pump radiation. As a result, laser oscillators in this geometry can produce diffraction-limited beams at the multi-10 W level [6–10].
With the high gains achievable, the bounce geometry also lends itself well to operation as a power amplifier. Master-oscillator power-amplifier (MOPA) configurations have been the focus of several previous attempts to scale bounce geometry lasers to very high powers. High-power MOPA systems have been built using single or double bounce amplifier geometries [7–9]. Folded slab amplifiers with multiple pump faces have also been investigated as a means of achieving compact power scaling , though not in the grazing-incidence geometry.
In this paper we present a new design for a bounce geometry slab amplifier. The COmpact Face-Folded INternally (COFFIN) amplifier incorporates two pump faces into a single crystal, and is shown to allow compact power scaling to the hundred-Watt level. A Q-switched laser oscillator built with the COFFIN amplifier is shown, which produced over 40 W of average power from 80 W pump power, over a range of repetition rates of 150 - 600 kHz. Power scaling to over 100 W is demonstrated with a COFFIN power amplifier in conbination with a bounce geometry master oscillator, which was Q-switched to very high repetition rates (up to 1.7 MHz).
2. The COFFIN amplifier design
The COFFIN amplifier is designed to be a compact, simple geometry for scaling lasers to very high powers. Employing two pump faces in one amplifier allows the pump power to be spread over a larer area without the added complexity of using multiple seperate amplifier stages. As well as reducing the size of the system, this eliminates the need to align and re-shape the beam between amplifiers. The COFFIN amplifier design aims to achieve this in a small crystal without requiring a steep bounce angle at the pump faces (as in other folded slab designs [13, 14]), thus retaining the benefits of the grazing-incidence geometry.
Shown in Fig. 1, the COFFIN design consists of a five-sided slab in a folded geometry. A beam entering the input/output face undergoes four TIR bounces before emerging antiparallel to the input. The slab is pumped by diode bars on each of its two long faces, at which the laser mode makes grazing incidence bounces. The COFFIN slab used in our experiments was an a-cut Nd:YVO4 crystal with 1.1 at.% neodymium doping. The crystal had dimensions 22 × 10 × 2 mm, and was designed for an internal bounce angle of approximately 7° at the pump faces. The crystal was pumped symmetrically by two diode bars at 808nm, with the pump faces antireflection (AR) coated at this wavelength. The input/output face of the crystal was AR coated for the lasing wavelength of 1064 nm. The crystal was conduction cooled by contact with the two large 22 × 10 mm faces.
3. COFFIN oscillator
A laser oscillator was constructed with the COFFIN amplifier. The experimental set-up is shown in Fig. 2. A cavity was formed with a high reflectivity (HR at 1064 nm) planar mirror and a 30% reflectivity planar output coupler (OC). The COFFIN crystal was pumped by two 40 W diode bars fitted with fast-axis collimating lenses. These were vertically focussed by cylindrical lenses (VCLD) of 25mm focal length, creating a line focus on each pump face. To match the laser mode to this narrow pump region, a 60mm focal length cylindrical lens (VCL) was placed across the input and output beams to the crystal. An acousto-optic (AO) modulator was inserted into the output arm of the cavity to Q-switch the oscillator.
The oscillator was initially operated in continuous-wave (CW) mode in a compact configuration with arm lengths L 1 = 8 cm and L 2 = 9 cm. The output power of the oscillator as a function of total pump power is shown in Fig. 3(a). A maximum CW power of 37.7 W was produced for 80 W of pump power. The beam profile was elliptical and multimode in the horizontal axis (in the plane of Fig. 2) but low-order mode in the vertical.
To improve the beam’s horizontal spatial quality, the cavity arm lengths were modified so as to match the resonator’s fundamental mode size to the gain region. It is known that an asymmetric cavity has two zones of stability with respect to the thermal lens dioptric power . Operating in the second stability zone, the cavity was designed to optimise the fundamental mode size in the presence of a strong thermal lens, thus enabling TEM00 operation at high powers. At 80 W pump power this was achieved with arm lengths L 1 = 15 cm and L 2 = 25 cm.
The laser output power as a function of pump power in TEM00 operation is also shown in Fig. 3(a). The dip in power over approx. 50 - 75 W is due to the cavity passing through the unstable region which separates the two stability zones. At 80 W of pump power, a maximum power of 32 W was obtained in a TEM00 beam with weak horizontal wings.
The COFFIN oscillator was actively Q-switched with the AO modulator, and the pulse width and average power as a function of pulse repetition rate are shown in Fig. 3(b). Pulsed operation was achieved over a range of repetition rates of approx. 150 - 600 kHz, with corresponding pulse widths of 17 - 37 ns. An average power of up to 30.5 W was obtained from 80 W of pumping. These high repetition rates were possible because of the extremely high gain of the amplifier and the short fluorescence lifetime (90 μs) of Nd:YVO4. At repetition rates below 150 kHz the AO modulator was unable to hold off the gain, and additional lasing was seen between pulses.
4. COFFIN power amplifier
In the previous section, the COFFIN amplifier design was used as an oscillator. Here we investigate its use as a power amplifier device. A MOPA was constructed with a single-bounce oscillator and a COFFIN amplifier. The experimental set-up is shown in Fig. 4.
4.1. Master oscillator
The master oscillator was constructed with a Nd:YVO4 crystal operated in the bounce geometry, using an internal bounce angle of approximately 5°. This was pumped by a 100W diode bar, vertically focussed by a 25 mm focal length vertical cylindrical lens (VCLD) onto the pump face. The cavity was formed by a HR mirror and a 30% reflectivity output coupler (OC), with the cavity mode matched to the gain region in the vertical direction by two cylindrical lenses (VCL1) of focal length 50 mm. An AO crystal was placed in the output arm to Q-switch the cavity.
The cavity arms were optimised for TEM00 operation with asymmetric arm lengths of L 1 = 9 cm and L 2 = 30 cm. For 95 W of pump power, the oscillator produced up to 43 W of power in a TEM00 beam (M 2 < 1.5 in both directions). The pulse width and average power versus pulse repetition rate are shown in Fig. 5(a). The oscillator was pulsed over a range of 750 kHz - 1.7 MHz with corresponding pulse widths of 21 - 34 ns, and an average power of over 40 W throughout this range.
4.2. Power amplification
The output from the oscillator was then fed into the COFFIN amplifier. Horizontal and vertical cylindrical lenses (HCL and VCL2) were used to optimise the beam’s spatial overlap with the gain region to maximise extraction efficiency. An aperture was used to clip the weak horizontal wing structure which was present on the master oscillator beam. The power amplifier itself was pumped by two 100 W diode bars, focussed by 25 mm focal length vertical cylindrical lenses (VCLD) to form a line focus on each pump face.
Figure 5(b) shows the output power from the MOPA as a function of the COFFIN amplifier pump power. The power was amplified to a maximum of 104 W for a total amplifier pump power of 155 W. This represents a 45% amplifier extraction efficiency, and a 41% optical-to-optical efficiency for the MOPA as a whole.
The high beam quality of the master oscillator was maintained with little degradation up to a pump power of around 120 W. Fig. 6 shows the beam profile under amplification, with an output power of 82.5 W. This beam had M 2 < 1.8 in both directions. At higher pump powers however the beam began to degrade and additional structure was present on the beam profile.
5. Discussion and conclusions
In conclusion, we have demonstrated the compact high-power scaling of a bounce geometry laser system. A new amplifier design, the COmpact Face-Folded INternally (COFFIN) amplifier was investigated, which incorporates two pump faces into one bounce geometry amplifier. A Q-switched laser oscillator built with a COFFIN amplifier was pulsed at repetition rates from 150 to 600 kHz, with an average power of over 30 W from 80 W pump power. A master-oscillator power amplifier (MOPA) was constructed with a COFFIN power amplifier and a Q-switched bounce geometry master oscillator. The master oscillator produced up to 43 W in a near-TEM00 beam, and was Q-switched at repetition rates up to 1.7 MHz. The power was amplified to 104 W by the COFFIN amplifier, with a total amplifier pump power of 155 W.
This work shows that the bounce amplifier geometry offers high power scaling potential with good beam quality and operation to extremely high pulse rates (an order of magnitude higher than standard DPSS repetition rates). The COFFIN amplifier geometry provides a very compact and simplified architecture compared to pumping two separate bounce amplifier crystals. This increased simplicity could lead to more stable and reliable systems, although under very high thermal lensing the inability to shape the beam between pump faces is a potential limitation. In principle, grazing incidence bounce amplifier crystals could be fashioned with more than two pump faces, although manufacturing tolerances and alignment through such multiface systems would be more exacting. Alternatively, the use of multiple dual-face pumped COFFIN amplifiers could be employed for even higher power scaling.
The Authors acknowledge support from the Engineering and Physical Sciences Research Council (UK) under grant number GR/T08555/01.
References and links
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