High-gain diode-pumped amplifier for generation of microjoule-level picosecond pulses

Antonio Agnesi, Luca Carrà, Federico Pirzio, Daniele Scarpa, Alessandra Tomaselli, Giancarlo Reali and Carla Vacchi

Istituto Nazionale di Fisica Nucleare and Dipartimento di Elettronica dell’Università di Pavia Via Ferrata 1 - 27100 Pavia (Italy)

antonio.agnesi@unipv.it

Caterina Braggio Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica dell’Università di Ferrara

Via del Paradiso 12 - 44100 Ferrara (Italy)

Abstract: A diode-pumped single-pass amplifier system relying on two grazing-incidence Nd:YVO4 slabs was developed to increase the energy of low-repetition-rate pulses from a decimated low-power cw mode-locked oscillator. Single-pass unsaturated gain up to 1.3×105 was achieved, and amplified pulses of 10-μJ energy and 8.0-ps duration were obtained. Efficient second harmonic generation (SHG) at 532 nm was achieved, as well as traveling-wave parametric conversion in the range 770-1020 nm (signal) and 1110-1720 nm (idler).

© 2006 Optical Society of America OCIS codes: (140.4050) Mode-locked lasers; (140.3280) Laser amplifiers; (140.3480) Lasers, diode-pumped; (190.4970) Parametric oscillators and amplifiers.

References and links

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2. M. Siebold, M. Hornung, J. Hein, G. Paunescu, R. Sauerbrey, T. Bergmann, and G. Hollemann, “A high-average power diode-pumped Nd:YVO4 regenerative laser amplifier for picosecond-pulses,” Appl. Phys. B 78, 287-290 (2004).

3. J. Kleinbauer, R. Knappe, and R. Wallenstein, “13-W picosecond Nd:GdVO4 regenerative amplifier with 200-kHz repetition rate,” Appl. Phys. B 81, 163-166 (2005).

4. A. Killi, J. Dörring, U. Morgner, M. J. Lederer, J. Frei, and D. Kopf, “High speed electro-optical cavity dumping of mode-locked laser oscillators,” Opt. Express 13, 1916-1922 (2005).

5. D. N. Papadopoulos, S. Forget, M. Delaigue, F. Druon, F. Balembois, and P. Georges, “Passively mode-locked diode-pumped Nd:YVO4 oscillator operating at an ultralow repetition rate,” Opt. Lett. 28, 1838-1840 (2003).

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7. G. Smith and M. J. Damzen, “Spatially-selective amplified spontaneous emission source derived from an ultra-high gain solid-state amplifier,” Opt. Express 14, 3318-3323 (2006).

8. Y. Ojima, K. Nawata, and T. Omatsu, “Over 10-W pico-second diffraction-limited output from a Nd:YVO4 slab amplifier with a phase conjuagate mirror,” Opt. Express 13, 8993-8998 (2005).

9. W. Koechner, Solid State Laser Engineering, (5th ed. Berlin, Germany: Springer, 1999), pp. 681-685. 10. R. L. Byer, “Optical parametric oscillators,” in Quantum Electronics: A Treatise, H. Rabin and C. L. Tang,

eds. (Academic, New York, 1975), pp. 587–702. 11. P. L. Ramazza, S. Ducci, A. Zavatta, M. Bellini, and F. T. Arecchi, “Second-harmonic generation from a

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#70549 - $15.00 USD Received 2 May 2006; revised 13 June 2006; accepted 14 June 2006

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1. Introduction

Microjoule-level picosecond pulses are interesting for a variety of applications, including micromachining and nonlinear optics. Laser systems delivering microjoule pulse energy have been reported relying either on multi-pass gain elements [1] or, more often, on regenerative amplification [2, 3]. In the usual schemes, one selects one or more picosecond pulses from a low-power cw mode-locked oscillator and uses an amplifying medium to increase the pulse energy level to the saturation energy, compatibly with optical damage constraints. The multipass technique usually requires complex setups with many passes through an amplifier with relatively modest gain per pass, and is usually chosen for the amplification of a portion of the cw pulse train, for example for subsequent synchronous pumping of optical parametric oscillators. The regenerative amplifier requires instead a relatively sophisticated electronics for fast switch-in and switch-out of a single pulse, and is the preferred choice in most applications. As a less complex alternative, sometimes cavity dumping [4], employing a high-voltage, high-speed intracavity electro-optic modulator, is also used to switch pulses with lower energy directly out of a cw mode-locked oscillator. Furthermore, extremely low-repetition-rate (≈1 MHz) cw mode-locked oscillators have been proposed for direct generation of picosecond pulses approaching 1-μJ energy [5].

This and the cavity dumping technique appear more suited to very high repetition rate sources, whereas they seem to allow relatively low pulse energy.

The development of very high gain diode-pumped grazing-incidence amplification modules offers new reliable, cheap and simple solutions also for picosecond pulse amplification. Owing to the effective gain confinement near the pump side in highly absorbing media with large stimulated emission cross section, grazing-angle side-pumped slabs offer high gain per pass as well as effective averaging of gain disomogeneity in the direction of the slow-axis of the diode array [6]. Indeed, Nd:YVO4 slab amplifiers with single-pass gain ranging from ≈103 to 105 have been reported [6, 7], that are perfectly suited to the application considered here. The first example of a cw-pumped Nd:YVO4 slab amplifier designed to extract high average power, up to 12.8 W, from a ≈300-mW picosecond oscillator was reported recently [8]. However, since the pulse period (10 ns) was much smaller than the amplifier storage time (≈100 μs), the system was clearly designed for cw amplification rather than for achieving maximum pulse peak power. Indeed, the pulse peak power amplification factor was less than 100.

Starting from a cw low-power picosecond oscillator, after pulse-picking for lower repetition rate, we have achieved instead five-order-of-magnitude amplification of energy and peak power, leading up to ≈10 μJ pulse energy, employing two side-pumped grazing-incidence Nd:YVO4 slabs. To the best of our knowledge, this represents the first application of this concept to extra-cavity quasi-cw amplification of picosecond pulses.

2. Experiments and discussion

Figure 1 shows the system layout. The low-power oscillator was based on a Nd:YVO4 2%-doped plane-Brewster crystal pumped by a 1-W cw laser diode emitting at 808 nm. Passive mode-locking was achieved with a semiconductor saturable absorber mirror with 2% loss modulation at 1064 nm. The output average power was 100 mW at a repetition frequency of 56 MHz, with 6.6-ps pulse duration. An extra-cavity acousto-optic pulse-picker AOPP (8-ns minimum risetime) was employed to select a single pulse from the train, reducing conveniently the repetition frequency. Lenses L1 and L2 served to optimize the beam size inside the AOPP in order to achieve minimum risetime, and to collimate the output beam, respectively. The zeroth-order undeflected beam was dumped out of the main laser beam pathway (BD in Fig. 1). The contrast ratio of the selected main pulse with respect to the adjacent pulses was not better than 20:1, but this was not an issue in this case since nonlinear

#70549 - $15.00 USD Received 2 May 2006; revised 13 June 2006; accepted 14 June 2006

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frequency conversion required in subsequent stages improved drastically this figure. In these conditions, the diffraction efficiency of the AOPP was estimated to be ≈30%. However, if single pulses with higher contrast were required at 1064 nm, a faster pulse picker, or two acousto-optic pulse pickers in series, or an oscillator running at a lower repetition rate might be employed. We chose to use an acousto-optic pulse picker since its driving electronics is easier to develop and customize than that for electro-optic devices, which require fast switching of high voltages and generate electromagnetic noise that is more difficult to suppress. Furthermore, acousto-optic pulse pickers are cheaper and more flexible allowing operation from single-shot to hundreds of kHz repetition rate.

We tested the laser system with pulse repetition rates up to 100 Hz, but in principle there is no limitation in frequency selection, and even user-defined sequences of pulses could be generated. The amplification stage is based on a couple of 4×2×12 mm3 Nd:YVO4 slabs, 1%-doped, 6° wedge, in a grazing-incidence configuration (internal bounce angle ≈4°). Each slab was pumped by a 150-W peak-power quasi-cw laser diode array with emitting size 10 mm × 1 μm, tuned at 808 nm and fast-axis collimated by a microlens. The input and output faces were antireflection-coated at 1064 nm, while the pump side was left uncoated. Owing to the relatively low repetition rate and the neglegible thermal effects in this case, the slabs were simply glued on their mounts and conduction-cooled. Half-wave plates were used to align the polarization of the pump diode arrays with the Nd:YVO4 c-axis (perpendicular to the plane of Fig. 1). The slabs were placed few millimeters apart for maximum compactness, furthermore the pump diodes could be connected in series with minimum parasitic inductance, and driven by a single power supply unit.

About 50% of the oscillator output power was used to provide a clock signal to a microprocessor that managed both the amplifier current pulses and the timing of the pulse-picker gate, occurring on the trailing edge of the 150-μs long squared current pulse. The gate width and the delay with respect to the clock reference were optimized for the best possible pulse contrast ratio.

Fig. 1. Layout of the diode-pumped oscillator-amplifier system. L1, L2, L3: lenses; PD: photodiode generating the 56-MHz reference clock; AOPP: acousto-optic pulse-picker; BD: beam dump; LD: quasi-cw laser diode arrays; HWP: half-wave plate; slabs: Nd:YVO4 grazing-incidence high-gain modules; LBO: SHG crystal; HS: harmonic separator; KTP: OPG crystal.

#70549 - $15.00 USD Received 2 May 2006; revised 13 June 2006; accepted 14 June 2006

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The beam diameter of the injected seed pulse was ≈1 mm (full width at 1/e2), slightly larger than the 0.8-mm thin gain sheet provided by the microlens-collimated diode arrays. The seed polarization was also parallel to the crystal c-axis, for maximum gain.

Small-signal (unsaturated) single-pass gain as high as 1.3×105 was measured at the maximum pump level with attenuated seed pulses. A maximum output energy of about 10 μJ per pulse (saturated gain 1.0×105) was obtained with the unfiltered 0.1-nJ seed pulses (measured after the AOPP). The output beam diameter was 0.9 mm. Both the pulse duration and the smooth TEM00 beam profile (M2<1.2) of the seed pulse were well preserved. Owing to the extremely high single-pass gain, the amplifier setup was prone to self-lasing with the slightest scattering along the beam path, requiring that two diaphragms had to be placed before and after the amplifier system in order to avoid such inconveniences. These are not especially dangerous as far as beam quality is concerned, but substantially subtract gain (and energy) to the amplifying process. In any case, optical isolators were not required to shield the oscillator in our case.

Figure 2 shows the non-collinear second-harmonic autocorrelation of the 8.0-ps amplified pulse and that of the 6.6-ps seed, with the correspondent spectra taken with an optical spectrum analyzer (ANDO AQ6317B). Gain narrowing during amplification slightly improved the time-bandwidth product from 0.54 to 0.47.

Fig. 2. Autocorrelation of the seed and the amplified pulse, fitted with a sech2 intensity profile (6.6 ps and 8.0 ps fwhm, respectively). The inset shows the correspondent spectra.

The high peak power of the amplified pulse (>1 MW) allows effective nonlinear

frequency conversion. For example, without any focusing, SHG with conversion efficiency as high as 55% was achieved in a 15-mm long type-I LBO crystal. The oscilloscope traces of both the amplified infrared pulse and its second harmonic are shown in Fig. 3. Sub-nanosecond InGaAs and silicon photodiodes were used for these measurements, along with a 1-GHz oscilloscope (Tektronix TDS5104B). It is worth noting that notwithstanding the weakly saturated amplification process, relatively stable pulses with amplitude fluctuations as small as 2% (one-sigma) were recorded. As anticipated, satellite pulses were virtually absent in the 532-nm pulse trace. A snapshot of the second harmonic beam, qualitatively showing the smoothness of its profile, is inset into Fig. 3.

#70549 - $15.00 USD Received 2 May 2006; revised 13 June 2006; accepted 14 June 2006

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Fig. 3. Oscilloscope traces of the undepleted fundamental and second harmonic pulses, normalized so that the ratio of the peaks corresponds to the observed conversion efficiency. Inset: beam profile of the second harmonic beam.

As the first application of this laser system will be the testing of photoconductive GaAs

switches with expected relaxation time of few tens of picoseconds and which require switching energy ≤1 nJ, a pulsewidth of few picoseconds with a wavelength near 800 nm and sufficient energy is desirable. To this end, a traveling-wave optical parametric generator (OPG) was set up using a 15-mm long KTP crystal cut for type II phase-matching in the xz-plane. Parametric superfluorescence was readily observed by pumping with 5.5-μJ energy, 532-nm pulses and using a 100-mm focal lens (L3). The spot radius was calculated to be w0 ≈ 40 μm, and the peak intensity ≈25 GW/cm2, reasonably below the threshold of surface optical damage at ≈7-ps pulsewidth (38 GW/cm2 using an inverse-square law scaling rule [9] from the specified threshold of 1 GW/cm2 at 10 ns).

Fig. 4. Spectra of the OPG pulses, obtained at several tuning angles.

#70549 - $15.00 USD Received 2 May 2006; revised 13 June 2006; accepted 14 June 2006

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With this setup the OPG was pumped at about twice its threshold level at 790 nm, and was readily tunable in the range 770 nm – 1020 nm (signal wavelength λs, see Fig. 4) and 1110 nm – 1720 nm (idler wavelength λi). The signal bandwidth was ≈1 nm, close to the resolution limit of the spectrometer used for OPG characterization (Ocean Optics USB2000), and the conversion efficiency was of few percents. The OPG tuning range was actually limited by mechanical constraints. In principles, it might be readily extended down to ≈650 nm, where the significant idler absorption starts limiting the conversion process. The pump intensity threshold condition for the OPG can be written as [10]

I2ω,th ≈ 4.6ε0 c n3 λs λ i

deff2 Leff

2 (1)

where εo is the vacuum permittivity, c is the speed of light, n≈1.8 is the average refractive index, deff≈3.2 pm/V is the nonlinear coefficient. In Eq. (1) the spatial walk-off angle ρ≈2.7° (at 790 nm) and the gaussian beam diffraction (confocal parameter b = 2π n w0

2 /λ2ω ) are included as in Ref. [11] for SHG, defining a squared effective interaction length

Leff2 = L2 b

Ltan−1 L

b

⎛ ⎝ ⎜

⎞ ⎠ ⎟ tanh

La

L

⎛ ⎝ ⎜

⎞ ⎠ ⎟ (2)

This model reasonably reproduces the experimental results if one defines a walk-off length La ≈ 2 π w0 /ρ in Eq. (2), twice the value assumed heuristically for SHG in Ref. [11]. Since the OPG operated at only twice the pump threshold value, it was not as stable as the pump pulse, but exhibited quite large (≈30%) amplitude fluctuations. This problem can be considerably alleviated by adding a second crystal for parametric amplification using the nearly undepleted pump at 532 nm. This should increase the conversion efficiency and reduce the intrinsic instability due to quantum noise seeding of the exponential amplification process in the unsaturated regime corresponding to the first pass.

In conclusion, we have demonstrated a simple, cost-effective diode-pumped amplifier design relying on extra-cavity acousto-optic pulse-picking and diode-pumped grazing-incidence Nd:YVO4 slabs, with >50 dB single-pass gain. 8.0-ps pulses of 10 μJ at 1064 nm were generated from a seed pulse energy <1 nJ, with a maximum contrast ratio of 20:1 and excellent characteristics for nonlinear frequency conversion. Significant improvements are expected by slightly more complex amplifier arrangements, such as a double-pass unsaturated pre-amplifier followed by a single-pass saturated amplifier. Preliminary numerical modeling let us anticipate pulse energy in excess of 100 μJ.

#70549 - $15.00 USD Received 2 May 2006; revised 13 June 2006; accepted 14 June 2006

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