Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
2002-09-27
2004-02-03
IP, Paul (Department: 2828)
Coherent light generators
Particular temperature control
Heat sink
C372S006000, C372S107000
Reexamination Certificate
active
06687273
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laser oscillation apparatus using a semiconductor laser as an excitation source.
First, general features of a laser oscillation apparatus will be described referring to FIG.
11
.
FIG. 11
is a laser oscillation apparatus
1
, schematically showing a laser oscillation apparatus using a second harmonic, and it comprises a laser light source
2
, a condenser lens
3
, a laser crystal
4
, a nonlinear optical medium
5
, an output mirror
6
, and laser driving means
9
.
The laser light source
2
, the condenser lens
3
, the laser crystal
4
, the nonlinear optical medium
5
, and the output mirror
6
having a concave reflection surface are arranged on a same optical axis
10
. A first dielectric reflection film
7
is formed on a surface of the laser crystal
4
facing to the condenser lens
3
, and a second dielectric reflection film
8
is formed on a concave reflection surface of the output mirror
6
facing to the nonlinear optical medium
5
.
The laser light source
2
generates a laser beam. In the present embodiment, a semiconductor laser is used, and the laser light source
2
has a function as a pump light generator for generating a fundamental wave. The laser light source
2
is driven by the laser driving means
9
, and the laser driving means
9
can drive the laser light source
2
by pulse driving.
The laser crystal
4
is a medium of negative temperature and it is used for amplification of the light. As the laser crystal
4
, YAG (yttrium-aluminum-garnet), etc. doped with Nd
3+
ions is adopted. YAG has oscillation lines of 946 nm, 1064 nm, 1319 nm, etc. In addition to YAG, Nd with oscillation line at 1064 nm (YVO
4
), or Ti with oscillation lines at 700 to 900 nm (Sapphire), etc. is used as the laser crystal
4
.
The first dielectric reflection film
7
is highly transmissive to the laser light source
2
and is highly reflective to an oscillation wavelength of the laser crystal
4
. It is also highly reflective to a second harmonic. The second dielectric reflection film
8
is highly reflective to the oscillation wavelength of the laser crystal
4
and is highly transmissive to the second harmonic.
As described above, it may be designed in such manner that the laser crystal
4
is combined with the output mirror
6
, the laser beam from the laser light source
2
enters through the condenser lens
3
, and the entered laser beam is reflected between the first dielectric reflection film
7
and the second dielectric reflection film
8
and is pumped to the laser crystal
4
. The laser beam can be confined for a long time between the first dielectric reflection film
7
and the second dielectric reflection film
8
through the nonlinear optical medium
5
. As a result, the laser beam can be resonated and amplified, and a laser beam of the second harmonic can be projected through the output mirror
6
.
Brief description will be given on the nonlinear optical medium
5
.
When an electric field is applied on a substance, electrical polarization occurs. When the electric field is small, the polarization is proportional to the electric field. However, in case of strong coherent light such as the laser beam, proportional relationship between the electric field and the polarization is impaired, and nonlinear polarization components proportional to square or cube of the electric field become prominent.
Therefore, in the nonlinear optical medium
5
, the polarization generated by the laser beam contains a component proportional to square of the light wave electric field. By the nonlinear polarization, bonding occurs between light waves with different frequencies, and a second harmonic to double the light frequency is generated. The generation of the second harmonic is generally called “SHG (second harmonic generation)”.
In the conventional example as described above, the nonlinear optical medium
5
is placed in an optical resonance unit, which comprises the laser crystal
4
and the output mirror
6
, and this is called as internal type SHG. Conversion output is proportional to square of fundamental wave opto-electric power, and high light intensity in the optical resonance unit can be directly utilized.
As the nonlinear optical medium
5
, for instance, KTP (KTiOPO
4
; titanyl potassium phosphate), BBO (&bgr;-BaB
2
O
4
; &bgr;-lithium borate), LBO (LiB
3
O
5
; lithium triborate), etc. are used. Primarily, it is converted from 1064 nm to 532 nm.
KNbO
3
(potassium niobate), etc. is also adopted. Primarily, it is converted from 946 nm to 473 nm. In
FIG. 11
, &ohgr; is an angular frequency of the optical fundamental wave, and 2&ohgr; is an angular frequency of the second harmonic.
In the laser oscillation apparatus using the second harmonic, for the purpose of generating a higher harmonic from the optical fundamental wave oscillating in the optical resonance unit using a nonlinear crystal (KTP crystal), the following conditions are needed:
(1) Temperature control of the nonlinear crystal (phase coordination temperature at constant level of 25° C.)
(2) Phase coordinating conditions of the nonlinear crystal which is satisfied by adjusting a nonlinear crystal axis with respect to a fundamental wave oscillation axis in the optical resonance unit.
Therefore, the laser oscillation apparatus of conventional type has a cooling mechanism and an aligning mechanism of nonlinear crystal axis.
Referring to
FIG. 12
, description will be given now on the cooling mechanism and the aligning mechanism of the nonlinear crystal axis used in the past.
On an optical resonator block
11
made of a material with high heat transfer property, a recessed portion
12
for accommodating a nonlinear optical medium
5
is formed. An optical path hole
13
is provided, which passes through the recessed portion
12
and has an axis aligned with an optical axis
10
of the laser oscillation apparatus. The optical path hole
13
is cut by the recessed portion
12
. A laser crystal
4
is disposed on a part of the optical path hole
13
closer to an incident side, and an output mirror
6
is provided on an exit side end of the optical path hole
13
. On the lower surface of the optical resonator block
11
, a Peltier element
14
is fixed.
The nonlinear optical medium
5
is placed on and closely fitted to the bottom surface of the recessed portion
12
. The nonlinear optical medium
5
is held at the lower end of an angle adjusting jig
15
. The angle adjusting jig
15
has a knob
15
a
extended in a direction of &thgr; axis
16
running perpendicularly to the optical axis
10
. The knob
15
a
is protruded from the recessed portion
12
, and an angle &thgr; for the nonlinear optical medium
5
can be adjusted around the &thgr; axis
16
by the knob
15
a.
The nonlinear optical medium
5
is placed so that a nonlinear crystal axis of the nonlinear optical medium
5
is aligned with the optical axis
10
. Because the nonlinear optical medium
5
is cut out along the nonlinear crystal axis, when the nonlinear optical medium
5
is closely fitted to the bottom surface of the recessed portion
12
, the position of the nonlinear crystal axis is determined within a horizontal plane with respect to the optical axis
10
. The nonlinear optical medium
5
is rotated by turning the knob
15
a
while it is pressed against the bottom surface of the recessed portion
12
, and the angle &thgr; is adjusted so that the optical axis
10
and the nonlinear crystal axis run in parallel to each other within the same plane.
When the adjustment has been completed, the nonlinear optical medium
5
is fixed on the optical resonator block
11
by adequate means such as bonding or by a screw.
An excitation light passing through the first dielectric reflection film
7
is absorbed by the laser crystal
4
. A fundamental wave oscillated by the laser crystal
4
is reflected between the first dielectric reflection film
7
and the second dielectric reflection film
8
, and a second harmonic generated from the nonlinear optical medium
5
is projected through the o
Eno Taizo
Momiuchi Masayuki
Sagehashi Hideo
IP Paul
Kabushiki Kaisha Topcon
Nguyen Dung (Michael) T
Nields & Lemack
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