An investigation is made into a quasi-CW (QCW) diode-pumped holographic adaptive laser utilising an ultra high gain (~104) Nd:YVO4 bounce amplifier. The laser produces pulses at 1064nm with energy ~0.6mJ, duration <3ns and peak power ~200kW, with high stability, via self-Q-switching effects due to the transient dynamics of the writing and replay of the gain hologram for each pump pulse. The system produces a near-diffraction-limited output with M2<1.3 and operates with a single longitudinal mode. In a further adaptive laser configuration, the output was amplified to obtain pulses of ~5.6mJ energy, ~7ns duration and ~1MW peak power. The output spatial quality is also M2<1.3 with SLM operation. Up to 2.9mJ pulse energy of frequency doubled green (532nm) radiation is obtained, using an LBO crystal, representing ~61% conversion efficiency. This work shows that QCW diode-pumped self-adaptive holographic lasers can provide a useful source of high peak power, short duration pulses with excellent spatial quality and narrow linewidth spectrum.
© 2007 Optical Society of America
Success in power scaling of solid-state lasers whilst maintaining good beam quality depends on the ability to find a solution to the problems associated with the heating of the laser medium caused by the pumping. Such problems include a pump dependent thermal lens, aberrations and stress-induced thermal birefringence which can degrade the spatial quality of the laser system . Investigations have shown that self-adaptive laser resonators based on formation and diffraction from optically-induced gain gratings (or holograms) offer considerable promise for high-average power scaling of solid-state lasers with maintenance of high beam quality by adaptive correction of the thermally-induced distortions [2–7].
In these systems, a gain grating is formed by spatial hole burning caused by interference of coherent beams in the laser medium and modulation of the population inversion. The adaptive resonator can be formed by gain grating formation in two ways: (a) using an input beam in a self-intersecting loop geometry or (b) as a self-starting adaptive oscillator by providing feedback from an output coupler and thus requiring no external optical input [2–4]. The gain hologram encodes the distortions and the oscillation, via diffraction from the hologram, creates a phase conjugate mode with a distortion-corrected output. Much of the initial research involved flashlamp pumping of Nd:YAG [2–4]. The advent of laser diodes allowed more efficient pumping of solid-state media as the radiation typically has a better spectral match to the absorption profile of the media. More recent investigations into gain gratings have included quasi-continuous wave (QCW) diode-pumping studies of degenerate four-wave mixing , a reciprocal dynamic holographic cavity  and efficient adaptive laser oscillators with continuous wave (CW) diode-pumping .
In this paper we present, for the first time to our knowledge, performance results of a QCW diode-pumped self-starting holographic adaptive laser with a non-reciprocal loop element and utilising an ultra-high gain Nd:YVO4 bounce amplifier. A key advantage of using pulsed diode-pumping is that the transient dynamics of gain grating formation can lead to a self-Q-switching of the adaptive laser, leading to giant-pulse formation [3,4]. We demonstrate an adaptive Nd:YVO4 laser that produces highly stable pulses of ~0.6mJ energy, <3ns duration and ~200kW peak power for each pump pulse. Pump pulse rates up to 1kHz were used. The system corrects for aberrations in the laser medium and the output mode is near-diffraction-limited with M2<1.3 and operates with a single longitudinal mode (SLM). In a slightly modified adaptive laser, the output was amplified to obtain pulses of ~5.6mJ energy, <7ns duration and ~1MW peak power, with equivalent spatial quality, SLM operation and stability. Frequency doubled green radiation was obtained with an LBO crystal producing 2.9mJ pulse energy at 532nm with ~61% conversion efficiency.
2. Concept of the self-starting adaptive laser
As stated, self-adaptive laser resonators based on formation and diffraction from optically-induced gain gratings (or holograms) offer considerable promise for high-average power scaling of solid-state lasers with maintenance of high beam quality by adaptive correction of the thermally-induced distortions. The concept of the self-starting adaptive laser with self-intersecting loop geometry is shown in Fig. 1.
The interacting fields A1-A4 are initiated by amplified spontaneous emission whose source is the high gain amplifier, G. Any initial spontaneous emission from G will induce weak gain gratings due to gain saturation in the gain medium within the loop geometry. Weak diffraction from these initial gratings will cause an enhancement of the amplified spontaneous emission, and constructive interference within the cavity will give rise to an increase in the diffraction efficiency of the gain gratings. These gratings are effectively volume gain holograms that encode the spatial distributions and wave fronts of the interacting fields and act as diffractive elements that allow build up of the intracavity fields.
In this interaction, two important gratings are the transmission grating formed by the interference term (A1A3 *+A2 *A4), shown in Fig. 1, and the reflection grating formed by the interference term (A1A4 *+A2 *A3). The spatial forms of the fields are interrelated by loop boundary conditions, feedback from the partially reflective output coupler (OC) and diffraction from the gratings. The coherent mutual growth of the fields and the gratings requires a self-consistency condition leading to mode formation. Diffraction from the transmission grating forms a ring resonator and, with sufficiently high diffraction efficiency, a threshold for laser oscillation is reached resulting in the preferential build-up of a backward mode oscillating in the anti-clockwise direction. Analysis of the self-consistency condition indicates preferential growth for phase conjugate oscillation where A2 ∝ A1 * (and A4 ∝ A3 *).
Phase conjugate (PC) oscillation means that aberrations experienced by field A1 in the gain medium and in the loop are compensated by the spatial read-out of the gain holograms. A matching of the wave front of the mode (A1 and A2 at the output coupler) to the plane output coupler leads to an output mode with a high spatial mode quality and power PPC. Optimisation of diffraction efficiency is achieved by use of the non-reciprocal transmission element (NRTE). The NRTE attenuates in the forward (clockwise) direction such that the relative strength of fields A1 and A3 may be optimised for efficient grating writing. In the QCW pumped regime, to maximise output pulse energy it is important to hold-off the formation of the gratings and hence prevent lasing until towards the end of each pump pulse. To delay this time for grating formation, the forward transmission t+ needs to be set to a low value.
The NRTE also provides a relative non-reciprocal π-phase shift between forward and backward loop directions. This phase shift compensates for the fact that the saturable transmission gain grating is in antiphase to the intensity interference pattern that forms it and hence is equivalent to the grating itself being π-phase shifted. In the backward direction the transmission t- is near unity allowing efficient build-up of the PC aberration-corrected radiation PPC. A non-phase conjugate power PNPC is emitted in the opposite output direction (see Fig. 1) with significant aberrations due to two passes through the gain medium.
3. Experimental self-starting adaptive laser system
3.1 Experimental adaptive laser arrangement
Figure 2 shows the experimental self-starting adaptive laser system. The system uses a QCW diode-pumped Nd:YVO4 crystal of 1.1 at. % neodymium doping in the form of an a-cut slab (crystal is cut perpendicular to the c-axis which is the optic axis) with dimensions 20 × 5 × 2mm. It is diode-pumped at 808nm and its main lasing transition is at 1064nm. The two 5 × 2mm end faces are anti-reflection (AR) coated for 1064nm The slab is diode-pumped on the 20 × 2mm front face, which is AR coated for 808nm, in a side-pumped configuration. The pump diode has fast axis collimation and is focused onto the front face with a f=50mm vertical cylindrical lens (VCLD) producing a line focus with dimensions ~15 × 0.5mm. The light output from the laser diode is TM polarised and is parallel to the c-axis of the Nd:YVO4 crystal, thus accessing the high absorption coefficient of ~30cm-1 for 1.1 at. % Nd:YVO4. This results in strong absorption of pump power with absorption depth ~330μm. The diode was driven in a QCW mode at repetition rates up to 1kHz (limited by diode driver).
A bounce geometry was employed in the crystal, in which laser radiation can be amplified by taking a path that experiences total internal reflection at the pump face [8–10]. The bounce angle with respect to the crystal’s pump face give considerable spatial averaging of the gain and thermal non-uniformities seen by the amplified beam in the bounce plane. However, the key significance to this geometry is its ability to produce extremely high gain (~104) . The self-adaptive laser cavity was formed by an output coupler (OC) together with a self-intersecting loop produced by high-reflectivity mirrors M1-M4. The non-reciprocal transmission element (NRTE) was formed by a Faraday rotator and half waveplate placed between a pair of polarisers. The reflectivity of the output coupler was chosen at a low value of ~0.2% and the forward transmission of the NRTE was set to a low value <1% to inhibit the onset of self-Q-switching to the end of the pump pulse.
3.2 Performance results
The system is able to operate in a pulsed output mode. For pump pulses of 80μs duration and 10.6mJ energy, output pulse energy was ~0.6mJ with duration as short as <3ns and ~200kW peak power. The pulse output was virtually independent of repetition rate up to the maximum available pump rate of 1kHz. The pulses have good stability with standard deviation, σ~1.5% in pulse energy and <0.5μs time jitter. The shortest pulses observed had a FWHM of 2.7ns, as shown in Fig. 3(a), for the pump pulse parameters stated earlier.
The low efficiency may be explained by a self-termination effect of the gain grating forcing premature pulse emission reducing extraction of the pump energy. All pulses are temporally clean and smooth without evidence of spectral modebeating. Single longitudinal mode (SLM) operation is confirmed by use of a Fabry-Perot (FP) etalon with free spectral range of 3.4GHz and finesse ~50 with single ring pattern shown in Fig. 3(b). The spatial output was characterized as shown in Fig. 4. Figure 4(a) is a spatial profile of the output and Fig. 4(b) is a graph of beam radius through focus for the horizontal and vertical components giving an M2<1.3 in both planes. The profile shows some astigmatism but no attempt was made to correct for this in the experiment. With suitable design its removal should be readily obtained, e.g. with compensation block.
4. Power scaling of an adaptive self-Q-switched laser system
There are several ways to power scale an adaptive laser system. The method chosen in this experiment was to use an adaptive laser system as a high quality source for a second high gain bounce amplifier. Figure 5 shows the experimental power scaled system.
All experimental details of the self-starting adaptive resonator (shown in a dotted-line box) are as before, apart from a small modification of intracavity elements. Two vertical cylindrical lenses (VCL1 and VCL2) in the loop of focal length f=50mm were used to match the laser mode with the gain region in the vertical. The second amplifier also uses a Nd:YVO4 crystal (1.1 at. % neodymium doping) in an a-cut slab with dimensions 25 × 5 × 2mm. The amplifier is pumped with a TM polarised diode stack which is brought to a line focus on the front face of the crystal with a f=25mm vertical cylindrical lens (VCLD2). The diodes were connected electrically in series to the same QCW driver ensuring the pump pulses to the loop and amplifier were synchronised. As the output coupler (OC) reflectivity is low (~0.2%), an isolator (ISO) was included to reduce amplified spontaneous emission (ASE) from the external amplifier adversely affecting the operation of the adaptive oscillator. Vertical cylindrical lens VCL3 (f=50mm) matches the mode from the oscillator to the gain region in the amplifier and VCL4 collimates the output to the diagnostics.
4.1 System performance
The system continues to operate in a pulsed output mode. For amplifier pump pulses of 80μs duration and 26.5mJ energy, amplified output pulse energy was ~5.6mJ with ~7ns duration and ~1MW peak power. This corresponds to an amplifier extraction efficiency of ~19%. Pulse duration is several nanoseconds longer than the pulses from the oscillator and may be caused by amplified spontaneous emission from the high-gain amplifier system parasitically affecting the loop oscillator. Pulse stability was again high with standard deviation, σ~2% in pulse energy and <0.5μs time jitter. The spectral content of the output was investigated by use of a FP etalon, showing SLM operation, in Fig. 6(a). Figure 6(b) is a graph of beam radius through focus for the horizontal and vertical components, with M2<1.3 in both planes.
4.2 Second harmonic generation with LBO crystal
A second harmonic experiment was performed to demonstrate the utility of such a high peak power pulse source with near-diffraction-limited beam quality. The amplified adaptive laser output was focused into a non-critically phase-matched LBO crystal with a f= 100mm spherical lens for second harmonic (SH) conversion into green (532nm). Figure 7 is a graph of SH output pulse energy against input pulse energy from the adaptive laser (solid squares), together with the conversion efficiency (open squares).
As shown, up to 2.9mJ of green is produced at a 1064nm input energy of 4.7mJ, corresponding to ~61% conversion efficiency.
QCW diode-pumping of a self-adaptive holographic laser with a non-reciprocal loop element is investigated for the first time. The laser produces pulses of ~0.6mJ energy, <3ns duration and ~200kW peak power via self-Q-switching effects due to the transient dynamics of writing and replay of the gain holograms for each pump pulse. The output pulses are stable with standard deviation, σ~1.5% in energy and <0.5μs time jitter. The output mode is near-diffraction-limited with M2<1.3 and operates with a SLM. The adaptive laser output was amplified to obtain pulses of ~5.6mJ energy, <7ns duration and ~1MW peak power. The amplification process maintains the output spatial quality at M2<1.3 and SLM operation. Up to 2.9mJ pulse energy of frequency doubled green (532nm) radiation was obtained, using an LBO crystal, with ~61% conversion efficiency. This shows that QCW diode-pumped adaptive holographic lasers are a useful source of high peak power, short duration pulses with high spatial quality and narrow linewidth spectrum.
The authors acknowledge support from Electromagnetic Remote Sensing Defence Technology Centre and from UK Engineering and Physical Sciences Research Council under grant number GR/T08555/01.
References and links
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