Note: Descriptions are shown in the official language in which they were submitted.
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COMPACT MULTIPLE RESONATOR LASER SYSTEM
This is a division of copending Canadian Patent
Application Serial No. 2,217,055, filed March 29, 1996.
BACKGROUND OF THE INVENTION
The present invention relates to a laser system and more
particularly to a miniature pulsed laser system using coupled
resonator cavities and to a method of producing a beam of
coherent light which is eyesafe.
Increase in the use of lasers in recent years has produced
a requirement for lasers of higher power that are safe for the
human eye. The greater the power of the laser, the more risk
there is to people who may come into contact with the laser
beam when a coherent beam of light enters the eye cornea and
either passes through or is absorbed by the vitreous humor. The
portion of the beam that is not absorbed by the vitreous humor
is focused by the eye onto the retina. Under normal conditions,
the light energy is converted by the retina into chemical
energy to stimulate optical sensation. Injury can result to the
eye when the focused energy laser beam cannot be absorbed and
causes damage to the retina. This damage does not occur when
conventional sources of illumination are exposed to the eye
because the light is emitted in all directions and produces a
sizeable but not a focused image on the retina that can be
safely absorbed. Laser beams having wavelengths in the range of
1.5 ,um - 2.2 ,um are absorbed by the vitreous humor, thereby
alleviating damage to the retina. Laser systems used as optical
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radar and communication transmitters in populated locations
need to be operated so as to avoid eye damage.
Lasers operating in the 1.5 ,um - 2.2 ,um wavelength have
generally been of low efficiency and of larger size. Two
available eyesafe lasers are based on laser emissions in
erbium-doped solid state host materials pumped by pulsed gas
discharge lamps or frequency conversion of a neodymium laser
using stimulated raman scatter in a molecular gas, such as
methane. These devices, however, have shortcomings. The erbium
lasers typically have an efficiency of less than .1% owing to
the low stimulated emission coefficient of the laser transition
in erbium 3+ ion at a 1.54 ,um output and to the low efficiency
for optical pumping with a visible flashlamp. The erbium laser
can only be operated in a pulsed mode. Stimulated Kaman
conversion requires a cell containing a high pressure flammable
gas. This gas is excited by the neodymium pumped laser to emit
stimulated radiation in the eyesafe region. Kaman conversion
therefore is not amenable to continuous wave operation and the
Kaman process deposits energy in the conversion medium causing
thermal distortion so that the eyesafe Kaman laser cannot be
conveniently operated at high average power or repetition rate.
An article in Optics Communications, Volume 75, No. 3,4 of
March 1, 1990, entitled Generation of Tunable Mid-IR (1.8-2.4
,um) Laser From Optical Parametric Oscillation in KTP by J. T.
Lin and J. T. Montgomery, describes an optical laser system in
which an Nd:YAG (neodymium-doped yttrium aluminum garnet) laser
is used in an optical parametric oscillator setup where the
pumping beam of YAG (yttrium aluminum garnet) laser pumps an
optical parametric oscillator to produce an output in an
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eyesafe wavelength. Similarly, in U.S. Patent No. 5,181,211,
issued January 19, 1993, (Burnham et al.) for an Eye-Safe
Laser System, an Nd:YAG or Nd:YLF (neodymium-doped yttrium
lithium fluoride) solid state laser is used to produce a
polarized output beam which is passed through a non-linear
crystal in an optical parametric oscillator to convert the
wavelength of the pump laser to a wavelength that is absorbed
by the human eye.
An optical parametric oscillator or OPO places a
non-linear crystal within a resonant optical cavity in which
mirrors transmit the pump wavelength from a laser beam through
a non-linear crystal, such as potassium titanyl phosphate or
KTP. The non-linear crystal can be rotated to change the output
wavelength. The existence of a resonant optical cavity makes
the parametric oscillator superficially similar to lasers since
they also generate a coherent beam. However, since there is no
stimulated emission within the parametric oscillator cavity, it
does not act as a laser simply because the parametric
oscillator is in a resonant optical cavity. The oscillator can
be brought within the laser cavity. The use of a short pulse
(<10 ns) Nd:YAG laser to pump a non-critically phased matched
KTP optical parametric oscillator in the eyesafe region results
in unacceptably low conversion efficiencies, such as less than
ten percent. This low efficiency apparently was due to the
short pump pulses. When the OPO was placed intracavity to the
pump laser, the conversion efficiency increased but the output
consisted of multiple pulses rather than a clean single pulse
required for many applications.
The present laser system in contrast to the prior art uses
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coupled laser cavities to maintain the high efficiency of an
intracavity system while at the same time achieving a single
pulsed output to thereby overcome the problems of an extra
cavity optical parametric oscillator used in combination with
an Nd:YAG laser and also overcomes~the shortcomings of placing
the OPO intracavity to the pump laser . The present laser system
is very compact for placement in very small packages which
compactness has been accomplished using a single common
substrate mirror with four separately coated regions and a
single corner cube to form two primary laser resonators. A
second smaller corner cube is used to couple the resonators.
A typical optical parametric oscillator apparatus in which
the OPO is external of the laser may be seen in U.S. Patent No.
4,180,751, issued December 25, 1979, (Ammann)which has a laser
having a laser cavity mounted adj acent a second resonant cavity
of an optical parametric oscillator with the laser being
directed into the optical parametric oscillator . In U. S . Patent
No. 5,195,104, issued March 16, 1993, (Geiger et al.) an
internally stimulated optical parametric oscillator and laser
places the optical parametric oscillator within the laser
cavity to form a dual optical resonator containing a single
optical parametric oscillator and laser crystal intracavity. A
frequency modified laser which places a non-linear crystal
within the laser cavity can also be seen in the Anthon et al.
U.S. Patent No. 4,884,277, issued November 28, 1999.
SUMMARY OF THE INVENTION
A laser system includes an apparatus and method having a
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primary laser resonator having a laser medium therein for
producing a laser beam of a first wavelength and a second laser
resonator optically connected to the primary resonator to allow
a portion of the laser energy from the primary laser resonator
5 to pass into the secondary laser resonator. An optical
parametric oscillator is located intracavity of the secondary
laser resonator and includes a non-linear crystal for producing
a laser beam of a second wavelength therefrom. A coherent beam
output is coupled to the optical parametric oscillator for
producing an output beam of predetermined wavelength of the
second wavelength while blocking the output of the laser beam
of the first wavelength so that a dual resonator combines a
secondary laser cavity and an optical parametric oscillator to
produce a predetermined output wavelength. The compact multiple
resonator laser system has a substrate mirror system having
four mirror surfaces thereon positioned to form two laser
resonators. A multi-pass corner cube is mounted to fold the
light beams between a pair of substrate mirrored surfaces while
a transfer corner cube is positioned to transfer a laser beam
from one resonator to the second resonator to form a very
compact pair of laser resonators. One of the laser resonators
is a dual resonator forming both the laser resonator and an
optical parametric oscillator resonator. A method of producing
a coherent light beam of a predetermined output wavelength uses
the compact laser system apparatus.
In accordance with one aspect of the present invention,
there is provided a compact multiple resonator laser system
comprising: a substrate mirror having at least three mirror
surfaces thereon positioned to form reflection surfaces for at
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least two laser resonators, each of the mirror surfaces having
different degrees of reflectance for predetermined wavelengths;
a multi-pass corner cube is positioned to fold the laser beams
for at least the two laser resonators to thereby form at least
first and second laser resonators with the mirrored substrate;
a laser medium positioned in the first laser resonator; and
a transfer corner cube positioned to transfer a laser beam from
the first laser resonator to the second laser resonator thereby
producing a compact multiple resonator laser system.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention, taken in conjunction with the
invention disclosed in co-pending Canadian Patent Application
Serial No. 2,217,055, filed March 29, 1996, will be discussed
in detail hereinbelow with the aid of the accompanying
drawings, wherein:
Figure 1 is a optical schematic of a laser system in
accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is for a miniature pulsed laser
capable of producing 5-10 mJ at about 1.58 ,um and utilizes an
intracavity optical parametric oscillator in a unique coupled
cavity design to achieve high efficiency without multiple
pulsing in a compact mechanically and optically stable package.
The compact package is less than 75 cubic centimeters and may
have an eyesafe wavelength of 1.5 ,um - 1.6 ,um capable of
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generating 5-10 ns pulses for use in handheld rangefinders and
the like. The output of a miniature 1.064 ,um Nd:YAG laser is
shifted into the eyesafe region with a non-critical phased
matched potassium titanyl phosphate (KTP) optical parametric
oscillator (0P0). The coupled cavity design of Figure 1
maintains the high efficiency of an intracavity device while at
the same time achieving a single pulsed output. This coupled
cavity laser system meets the efficiency and output
requirements and allows the laser transmitter to be very
compact.
Referring to the schematic of Figure 1, the overall laser
transmitter 10 includes a primary 1.06 ,um resonator 11
containing a Nd:YAG rod 12 pumped by a flashlamp 19 along with
a pair of steering wedges 13 and a Q-switch 14, which is
illustrated as a chromium doped YAG saturable absorber. A
polarizing element 15 may be a brewster plate. These elements
are mounted inside a resonator formed by the 1.06 ,um decoupling
region of a common substrate mirror 16 having the substrate 17
having a 30% reflectance of 1.06 ,um mirror 18 mounted on the
substrate 17 and a 100% 1.06 ,um reflecting mirror 20 mounted on
the substrate to form a resonance cavity between the mirrored
surfaces 20 and 18. The laser rod 12 is in the beam path,
illustrated as 21, which is folded by the multi-pass corner
cube 22 providing folding surfaces to fold the beam 21. Thus,
the laser rod 12 is pumped by an optical flashlamp 19 to
produce the lasing action within the resonant cavity between
the mirrored surfaces 18 and 20. The beam passes through the
30% reflecting surface 18 where the beam is folded by the
transfer corner cube 25 and passes back through the substrate
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17 and through a mirrored surface portion 26 which has a zero
reflectance for the 1.06 ,um.
The 1.06 ,um laser beam passes into a secondary 1.06 ,um
resonator 27 where it is folded by the multi-pass corner cube
22 back to a fourth mirrored surface area 31 on the common
substrate 17. Mirror 31 has an 80% reflectance of 1.06 ,um
wavelength and allows 20% to pass therethrough into the 1.06 ~cm
dump 32. Since the mirror 31 is reflecting 80% of the 1.06 ~cm,
a portion of the energy is passed back through the secondary
resonant cavity 27 and back through the mirrored surface 26
through the corner cube 25 where a portion of the energy passes
through the mirrored surface 18 while a portion of it is
reflected back into the secondary cavity.
The secondary cavity 27 forms a dual optical resonator
which is both a secondary laser resonator and an optical
parametric oscillator resonator. The optical parametric
oscillator is formed by having the potassium titanyl
phosphate (or KTP) crystal 33 within the beam path within
the cavity 27. This OPO resonator is a 1.58 ,um resonator
containing the down scope 34 along with a pair of steering
wedges 35 and a KTP crystal 33. The resonator is formed by
the 1.58 ,um outcoupling region of the common substrate mirror
26 which is 50% reflective of the 1.58 ,um beam but transparent
to the 1.06 ,um beam. The 1.58 ,um mirror 31 reflects 100% of the
energy while reflecting only 80% of the 1.06 ,um beam.
Similarly, the 1.58 ,um resonator also uses the multi-pass
corner cube 22.
The 1.58 ,um output from the dual resonant cavity 27 passes
through the mirrored portion 26 and is coupled out of the
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system as the energy passes through the transfer corner cube 25
and impinges on the dichroic beam splitter 36. The dichroic
beam splitter reflects the entire 1.58 ~cm energy along the path
37 where it impinges against a second dichroic beam splitter 38
to produce a 1.58 ,um output 40. Thus, the output from the 1.58
,um resonator is produced from the output of the laser while the
1.06 ,um energy passes back through the dichroic beam splitter
36 and a portion of which passes through the mirrored surface
18 while a portion is reflected back through the beam splitter
36, transfer corner cube and back into the secondary laser
cavity 27.
The operation of the laser resonator is as follows : Firing
of the flashlamp causes the gain to begin to buildup in the
Nd:YAG rod 12. Initially, the single pass loss of the saturable
absorber 14 is high and this loss combined with the out
coupling mirror 18 losses prevents the buildup of laser
oscillation. Eventually, the round trip gain exceeds the round
trip losses and the 1.06 ,um field begins to grow in both
primary 1.06 ~m cavity 11 and the secondary 1.06 ,um cavity 27.
The feedback from the mirror in the secondary 1.06 ,um resonator
27 lowers the threshold at which the 1.06 ,um oscillation will
begin. As the 1.06 ~m field grows, the absorption loss due to
the saturable absorber 14 begins to saturate allowing yet more
growth of the 1.06 ,um field. The cycle continues until the
saturable absorber transmission has increased significantly and
a Q-switch 1.06 ~sm pulse has begun to develop. As the 1.06 ,um
field in the secondary resonator 27 grows, it eventually
reaches a level at which it begins to be converted by the KTP
crystal 33 into two longer wavelengths. The crystal 33 angle
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determines what two wavelengths are generated by the crystal 33
and the angle has been chosen in the present crystal, such as
to produce wavelengths of 1.58 ,um and 3.26 ~cm. In a pure
intracavity optical parametric oscillator, the nonlinear
5 conversion of the 1.06 ,um field to the longer wavelengths
causes the 1.06 ,um field to be depleted and to cease
oscillation before the stored energy in the Nd:YAG rod has been
fully extracted. This residual stored energy can result in one
or more secondary 1.06 ,um and 1.58 ,um pulses. By placing the
10 OPO in a secondary 1.06 ~cm cavity that does not contain the
1.06 ,um gain medium, the non-linear conversion process does not
directly interact with the 1.06 ,um field that extracts the
stored energy in the Nd:YAG rod 12. This allows the 1.06 ,um
oscillation to continue in the primary cavity 11 even as the
1.06 ,um field in the secondary cavity 27 begins to be depleted.
The net result is a suppression of premature termination of the
1.06 ,um oscillation. That leads to a significantly reduced
tendency for secondary pulsing and increased conversion of 1.06
,um pump to eyesafe 1.58 ,um output.
The key to the compactness of the laser 10 is
the use of a dual path corner cube 22 along with a
transfer corner cube 25 and a single common substrate
mirror 16 having the four mirrored surfaces thereon to
form all of the optical resonators, as shown in Figure 1. In
actual practices, it has found that the present laser
transmitter can be placed in a total volume of less than 75
cubic centimeters to produce an output in excess of 6 mJ of
output at 1.58 ,um. Thus, the output energy of greater than 5 mJ
per pulse at 1.58 ,um is achieved with the efficiency of
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intracavity OPO without the multiple pulsing problem
experienced with an intracavity laser OPO. In addition, the use
of a common substrate mirror 16 and the mufti-pass corner cube
22 along with the transfer corner cube 25 to form the coupled
resonator system results in an overall miniaturization of a
laser transmitter to a very small volume of space and also
allows for a high degree of alignment stability over extreme
temperature and vibration environments. The illustrated laser
transmitter of Figure 1, however, should not be considered as
limited to the schematic shown but should be considered
illustrative rather than restrictive.
