Solid-state laser

Details Solid-state laser Solid-state laser

2000-10-16

2003-04-22

Leung, Quyen (Department: 2828)

Coherent light generators

Particular pumping means

Pumping with optical or radiant energy

C372S069000

Reexamination Certificate

active

06553052

ABSTRACT:

BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a solid state laser pumped by a laser diode pump.
2) Description of the Related Art
Many scientific, medical, printing, ranging and industrial applications of small lasers require the laser to be reliable and efficient with high peak to average power ratio capability, and to be able to emit near diffraction-limited short-duration pulses in a controlled manner.
In U.S. Pat. No. 4,710,940 to Sipes (1987) and in U.S. Pat. No. 4,739,507 to Byer et al (1988) miniature continuous wave (CW) Nd:YAG solid state laser oscillators end-pumped by a discrete laser diode pump are disclosed. In these miniature lasers the output of a laser diode pump, typically having a power of 0.1-1W, is imaged or focused to a size similar to that of the TEM
00
mode size of the solid state laser oscillator, that is to a focus which may be ~50-200 &mgr;m in diameter. In this way, the TEM
00
mode (which gives the highest beam quality) is preferentially excited and caused to dominate the output of the solid state laser oscillator. Sipes teaches that, by concentrating the pump beam to a power density typically in the range 1-10 kW.cm
−2
within an oscillator crystal, the laser oscillator can be very efficient. In a paper entitled, ‘Diode Laser End Pumped Neodymium Lasers: The Road to Higher Powers’, (Proc Tuneable Solid-State Laser Conf, Paper TuC6, p134-6, May 1989, publ. Optical Society of America), Fields et al report achieving a laser diode pump to solid state laser optical power efficiency of up to 61% with this type of miniature laser oscillator using Nd:YVO
4
as the oscillator material. U.S. Pat. No. 5,410,559 (1995) and U.S. Pat. 5,577,060 (1996) to Nighan et al teach that higher power performance can be achieved with larger lasers pumped at higher power where a oscillator is sufficiently long (typically 100 mm) to correspond to a large TEM
00
mode size in an oscillator crystal, and where care is taken to mitigate beam degrading effects caused by a severe thermal load in the crystal oscillator.
Pulsed output can be achieved from miniature diode pumped laser oscillators. Microsecond laser pulses, typically in the range 1-500 &mgr;s duration, may be achieved by using quasi-CW laser pump diodes ie diodes that can repetitively emit power for periods up to approximately 500 &mgr;s. Nanosecond duration laser pulses can be achieved by adding a controlled optical Q-switch to a laser oscillator. Pulses shorter than 1 ns may be achieved by adding instead a passive Q-switch. In U.S. Pat. No. 4,761,786 to Baer (1988) the use of a miniature acousto-optic modulator as the Q-switch in a CW pumped laser is taught to allow use of a short solid state laser oscillator and to provide fast optical pulse dynamics. Baer teaches the production of pulses in the range 10-50 ns duration and of 10-20 &mgr;J energy from miniature oscillators using Nd:YAG and Nd:YLF as the laser crystal oscillators. In a paper entitled, Q-switching of a diode-pumped Nd:YVO
4
Laser Using a Quadrupole E-O Deflector (Appl Phys B, Vol 67, p267-70, 1998), Friel et al report operating a short laser oscillator (around 15 mm long) and the production of 10-20 &mgr;J pulses of 1-2 ns duration and of the order of 10 kW peak power. If short pulses are required, and synchronisation is not important, a simple fast passive Q-switch (which can be very small) can be used and the oscillator made even shorter. This typically results in the generation of sub-nanosecond microjoule pulses at kHz repetition rate. In a review article entitled, “Q-Switched Microchip Lasers Find Real-World Application”. (Laser Focus World, August 1999, P129-36, PennWell Pub, USA), Zayhowski teaches that such lasers with a oscillator of only 0.75-1.5 mm length produce pulses of 0.2 ns duration and 141 &mgr;J pulse energy. The average output power was up to 120 mW with a maximum 1W laser diode pump power.
FIG. 1
illustrates a prior-art diode end-pumped miniature solid-state laser
100
including a Q-switched Nd:YAG solid state oscillator
110
that emits an output beam
150
at a wavelength of 1064 nm. In such a sold-state laser, a pump beam
121
from a discrete laser diode pump
120
operating at a pump wavelength of 808 nm is focused or imaged by lenses
131
,
132
onto an end face
112
of a Nd:YAG oscillator crystal
111
so that energy from the pump beam
121
is absorbed in the oscillator crystal
111
by exciting Nd ions. The crystal is typically a few millimetres in diameter and a few millimetres long. Stimulated emission of laser light occurs when the excited Nd ions are de-energised and the resultant light resonates in the oscillator by repeated reflections from the front face
112
of the crystal and a partially reflecting external mirror
115
to produce an oscillator beam
114
a proportion of which forms the output beam
150
. To promote the reflections the crystal may have first and second high-damage-threshold dielectric coatings (not shown) applied to the face
112
illuminated by the pump diode and also to an opposed face
113
respectively. The first coating on face
112
is designed to transmit with low loss the laser diode pump beam
121
and to reflect the Nd:YAG oscillator beam
114
, and the second coating on face
113
to transmit with low loss the oscillator beam
114
. One or other or both of the reflecting surfaces comprising face
112
and that of mirror
115
may be curved to provide a stable oscillator. As illustrated, the laser oscillator
110
comprises in optical alignment, in addition to the miniature Nd:YAG crystal
111
, and the partially reflecting output mirror
115
, a miniature Q-switch
116
to allow the generation of laser pulses. The function of the Q-switch
116
is alternately to prevent and allow the oscillator
110
to resonate, so that while not resonating, increased energy is stored and on resonating, pulses of laser light are emitted.
When a quasi-CW pulsed pump diode
120
is used, the control of the diode and the Q-switch are synchronised. The laser output beam
150
exits the laser in the same direction as the pump diode beam
121
ie away from the diode
120
. The principles of operation are well known to the art, and are, for example, described in ‘Solid-State Laser Engineering’ by Koechner W, Springer Verlag, N.Y., Fifth Edition 1999, p363-370.
As taught by Sipes, the pump beam
121
at the crystal face
112
must be of a size similar to the TEM
00
mode of the oscillator and have a power density of the order of 1-10 kWcm
−2
in the Nd:YAG crystal
111
to provide efficient operation. This restricts use of pump sources to discrete diodes. Since the work of Sipes, laser diodes have increased in CW power and diodes emitting 1-2W from a facet of approximately 100 &mgr;m×1 &mgr;m are commercially available. Thus the output beam
150
of this type of miniature laser is typically in the average power range up to several hundred milliwatts.
The above prior-art arrangements are now widely used fairly efficiently and controllably to produce microjoule pulses of some kilowatts power from solid state lasers. However, a significant disadvantage of their miniature design is that they are not scalable to much higher average power, or greater pulse energy or peak power. This is because:
i) the oscillator must be short (the shorter the better) to provide fast laser pulse dynamics and a short pulse,
ii) the pump power must be low to avoid induced thermal distortion degrading the oscillator beam quality, and
iii) for good beam quality, the pump beam must selectively pump the active crystal oscillator only in a small volume close to that of the TEM
00
mode.
As taught by Sipes, in a short oscillator, this last constraint requires high concentration of the pump beam for good performance. In particular, the constraints preclude use of higher power, larger area, laser diodes (or power diode arrays) because the output cannot be concentrated to a small spot corresponding to the TEM
00
mode of the laser oscillator.
Laser diode bars (or arrays of such bars) 10

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