Coherent light generators – Particular beam control device – Q-switch
Reexamination Certificate
2000-01-21
2002-04-16
Font, Frank G. (Department: 2877)
Coherent light generators
Particular beam control device
Q-switch
C372S011000, C372S068000, C372S071000, C372S098000
Reexamination Certificate
active
06373864
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to pulsed lasers, to laser amplifiers and to methods for generating and amplifying pulsed laser radiation and, more particularly, to an entirely passive laser system both for the generation and the amplification of short pulses.
BACKGROUND OF THE INVENTION
Production of short pulses with high energy per pulse is usually achieved by a combination of one oscillator and one amplifier. The oscillator is traditionally a mode-locked laser producing very short pulses of typically less than 100 ps at high frequency of typically a few tens of MHz and with a low energy per pulse of a few nJ. To increase the pulse energy to several &mgr;J, one uses an amplifier working at a lower repetition rate from a few kHz to a few hundreds of kHz, depending on the pumping configuration. These systems are complex and complicated to use because they require active modulation (acousto-optic or electro-optic modulators) with high-speed electronics for short-pulse production for the oscillator plus injection and synchronization of the pulses inside the amplifier.
Passively Q-switched lasers using Nd-doped crystals can produce high-peak-power pulses of several kW at a wavelength of 1064 nm. Depending on the experimental setup, the pulse width can vary from few tens of ns (A. Agnesi, S. Dell'Acqua, E. Piccinini, G. Reali and G. Piccinno, “Efficient wavelength conversion with high power passively Q-switched diode-pumped neodymium laser”, IEEE, J. Q. E., Vol. 34, 1480-1484, 1998) to few hundreds of ps (J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett., Vol. 19, 1427-1429, 1994). For instance pulses of 19 ns and 108 &mgr;J can be obtained at 25 kHz and 1064 nm from a diode-pumped Nd:YAG laser with a Cr
4+
:YAG saturable absorber crystal. The high peak power of these lasers allows efficient wavelength conversion into the ultra-violet (UV) range with optically nonlinear materials (A. Agnesi, S. Dell'Acqua, E. Piccinini, G. Reali and G. Piccinno, “Efficient wavelength conversion with high power passively Q-switched diode-pumped neodymium laser”, IEEE, J. Q. E., Vol. 34, 1480-1484, 1998; J. J. Zayhowski, “Diode-pumped passively Q-switched picosecond microchip lasers”, Opt. Lett., Vol. 19, 1427-1429, 1994; J. J. Zaykowski, “UV generation with passively Q-switched microchip laser”, Opt. Lett., Vol. 21, 588-590, 1996).
To reduce the pulse width with the same material combination, one must combine the active medium and the saturable absorber in a short distance to reduce the cavity length to about 1 mm typically. A microchip laser combines the two materials in a monolithic crystal (J. J. Zaykowski, “Non linear frequency conversion with passively Q-switched microchip lasers”, CLEO 96, paper CWA6, 23 6-237, 1996); the energy is then smaller, e.g., 8 &mgr;J at 1064 nm. The two materials, i.e., the laser material and the saturable absorber, can be contacted by a thermal bonding, or the saturable absorber can be grown by liquid phase epitaxy (LPE) directly on the laser material (B. Ferrand, B. Chambaz, M. Couchaud, “Liquid Phase Epitaxy: a versatile technique for the development of miniature optical components in single crystal dielectric media”, Optical Materials 11, 101, 1998). At the same time, in order to obtain sub-nanosecond pulses, the saturable absorber must be highly doped and therefore the repetition rate is lower (e.g., 6-8 kHz with Nd:YAG). The wavelengthconversion efficiency from infrared (IR) to UV is in the order of 4 %. A solution to simultaneously obtain short pulses and a high repetition rate is to combine a Nd:YVO
4
crystal, whose short fluorescence lifetime of Nd:YVO4 is well suited for a higher repetition rate, with a semiconductor-based saturable absorber in an anti-resonant Fabry-Perot structure (B. Braun, F. X. Kdarner, G. Zhang, M. Moser, U. Keller, “56 PS passively Q-switched diode-pumped microchip laser”, Opt. Lett., 22, 381-383, 1997). This structure is nevertheless complex to produce.
It is therefore difficult to simultaneously produce sub-nanosecond short pulses, at frequencies of a few tens of kHz, with several micro-Joule per pulse in a simple and compact system. The solution consists in combining a compact oscillator producing short pulses at high frequency with an amplifier to increase the pulse energy. Amplifiers have been used in the past with pulsed microlasers. After amplification, pulses with 87 nJ (small-signal gain of 3.5) at 100 kHz have been produced using a 10-W diode bar as a pump (C. Larat, M. Schwarz, J. P. Pocholle, G. Feugnet, M. Papuchon, “High repetition rate solid-state laser for space communication”, SPIE, Vol. 2381, 256-263). A small-signal gain of 16 has been obtained with an 88-pass complex structure using two 20-W diode bars as a pump (J. J. Degnan, “Optimal design of passively Q-switched microlaser transmitters for satellite laser ranging”, Tenth International Workshop on Laser Ranging Instrumentation, Shanghai, China, Nov. 11-15, 1996). In these two examples, the amplification efficiency that can be defined as the ratio between the small-signal gain and the pump power is small because the transverse pumping has a low efficiency due to the poor overlap of the gain areas with the injected beam. Furthermore, these setups use Nd:YAG crystals not suited for high-frequency pulses (the fluorescence lifetime is 230 &mgr;s).
A combination of Nd ions in two different hosts, in a oscillator-amplifier system, has been performed in the past in continuous wave (cw) (H. Plaesmann, S. A. Ré, J. J. Alonis, D. L. Vecht, W. M. Grossmann, “Multipass diode-pumped solid-state optical amplifier”, Opt. Lett., 18, 1420-1422, 1993) or pulsed mode (C. Larat, M. Schwarz, J. P. Pocholle; G. Feugnet, M. Papuchon, “High repetition rate solid-state laser for space communication”, SPIE, Vol. 2381, 256-263). In these cases, the spectral distance between the emission lines of the two different material Nd:YAG and Nd:YVO
4
limits the small-signal gain to a value tower than the obtained when only Nd:YVO
4
is used in both the oscillator and the amplifier; it lies between from 5.5 cm
−1
and 7.0 cm
−1
(J. F. Bernard, E. Mc Cullough, A. J. Alcock, “High gain, diode-pumped Nd:YVO
4
slab amplifier”, Opt. Commun., Vol. 109, 109-114, 1994).
A number of amplification schemes using Nd ions in crystals have been studied, but often end up with complex multipass setups and with low efficiency due to transverse pumping.
End-pumped single-pass or double-pass amplification schemes based on guiding structures to increase the interaction length between the pump beam and the injected beam have been studied in the past: in planar guides (D. P. Shepherd, C. T. A. Brown, T. J. Warburton, D. C. Hanna and A. C. Tropper, “A diode-pumped, high gain, planar waveguide Nd:Y
3
Al
5
O
12
amplifier”, Appl. Phys. Left., 71, 876-878, 1997) or in double-cladding fibers (E. Rockat, K. Haroud, R. Dandliker, “High power Nd-doped fiber amplifier for coherent intersatellite links”, IEEE, JQE, 35, 1419-1423, 1999; I. Zawischa, K. Plaman, C. Fallnich, H. Welling, H. Zellner, A. Tunnermann, “All solid-state neodymium band single frequency master oscillator fiber power amplifier system emitting 5.5 W of radiation at 1064 nm”, Opt. Lett., 24, p. 469-471, 1999). These schemes are, however, not suited for high-peak-power pulses because unwanted nonlinear effects, such as the Raman effect, start to appear around 1 kW of peak power.
A high small-signal gain of 240 was achieved in an end-pumped double-pass bulk Nd:YLF amplifier, but it was used with a cw laser with an expensive diode-beam shaping optical setup (G. J. Friel, W. A. Clarkson, D. C. Hanna, “High gain Nd:YLF amplifier end-pumped by a beam shaped bread-stripe diode laser”, CLEO 96, paper CTUL 28, p. 144, 1996).
SUMMARY OF THE INVENTION
The object of this invention is to provide an entirely passive laser system both for the generation and amplification of short pulses. The oscillator shall directly produce &mgr;J pulses at the required repetition rate and shall be amp
Balembois Francois
Brun Alain
Devilder Pierre Jean
Druon Frederic
Georges Patrick
Nanolase S.A.
Oppedahl & Larson LLP
Rodriguez Armando
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