Stray light cutting structure for optical device

Details Stray light cutting structure for optical device Stray light cutting structure for optical device

2002-05-24

2004-03-16

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

C372S039000, C372S066000, C372S103000

Reexamination Certificate

active

06707838

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a stray light cutting structure for an optical device, and more particularly to a stray light cutting structure which cuts stray light which travels in such a direction as to enter an optical component in the optical device and to be reflected at a side surface thereof.
2. Description of the Related Art
In various optical devices, light reflected at a light transmitting surface of an optical component of the optical device sometimes travels through the device as stray light. For example, in a solid state laser in which an etalon is provided in a Fabry-Perot resonator at an angle to the axis of the resonator as disclosed, for instance, in Japanese Unexamined Patent Publication No. 7(1996)-263785, light reflected at a light transmitting surface of the etalon travels in a direction at an angle to the axis of the resonator.
In a Fabry-Perot resonator, oscillating light travels in parallel to optical axes of optical components such as mirrors forming the Fabry-Perot resonator, and a solid laser crystal and a wavelength convertor element disposed inside the resonator. When the stray light traveling in a direction at an angle to the axis of the resonator interferes with the oscillating light, great fluctuation in output power can occur. The problem in the solid state laser will be described in detail with reference to
FIGS. 14A
to
14
C, hereinbelow.
It is assumed that the solid state laser comprises, as shown in
FIG. 14A
, a semiconductor laser
10
as a pumping light source, a condenser lens
12
which condenses a laser beam
11
emitted from the semiconductor laser
10
, a solid laser crystal
13
which is pumped by the laser beam
11
, a resonator mirror
14
disposed forward of the solid laser crystal
13
, a wavelength convertor element
15
disposed inside a resonator formed by the resonator mirror
14
and the solid laser crystal
13
, a Brewster plate
16
for polarization control and an etalon
17
for oscillating wavelength selection.
In such a solid state laser, light emitted from the solid laser crystal
13
pumped by the laser beam
11
resonates between the rear end face
13
a
of the laser crystal
13
and the mirror face
14
a
of the resonator mirror
14
, whereby a solid laser beam
18
oscillates. The solid laser beam
18
is converted to its second harmonic
19
by the wavelength convertor element
15
and substantially only the second harmonic
19
emanates from the resonator mirror
14
. The Brewster plate
16
controls the direction of polarization of the solid laser beam
18
and the etalon
17
selects the oscillating wavelength (i.e., a longitudinal mode) of the solid laser beam
18
.
The reflectance of the etalon
17
has a strong wavelength-dependency and a wavelength to which the phases of reflection of opposite sides of the etalon are reverse to each other and accordingly, the reflectance of the etalon
17
to which is very low is selected, whereby the solid laser beam
18
oscillates at the selected wavelength. The reflectance of the etalon
17
to the solid laser beam
18
is minimized at this time and is evaluated to be about 2%. Since the etalon
17
is inclined by 1° to a direction normal to the optical axis of the resonator, stray light which is reflected at the surface of the etalon
17
in a direction at 2° to the optical axis of the resonator is generated as indicated
20
in FIG.
14
A.
The stray light
20
is reflected in total reflection at a side surface
15
a
of the wavelength convertor element
15
, which may be, for instance, a LiNbO3 crystal having periodic domain reversals, and is further reflected at the rear end face
13
a
of the solid laser crystal
13
provided with HR (high-reflection) coating as shown in FIG.
14
B.
Further, the stray light
20
can be reflected at the surface of the etalon
17
to travel in parallel to the optical axis of the resonator as shown in FIG.
14
C. That is, the stray light
20
can travel in the same direction as the solid laser beam
18
(abbreviated in
FIGS. 14B and 14C
) and can be sometimes superimposed on the solid laser beam
18
within the range thereof to interfere with the solid laser beam
18
.
When the stray light
20
interferes with the solid laser beam
18
, the state of interference varies depending on the phase state and/or the intensity of the stray light
20
, which increases and reduces loss in the resonator. Accordingly, slight stray light
20
can greatly change the output power of the solid state laser. Since the phase state of the stray light
20
changes according to change of strain of optical components and the components holding them, change of temperature of the optical components and the components holding them, and the like, the resonator becomes very instable and the output power of the solid state laser greatly fluctuates according to the state during assembly and/or the time from assembly.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object of the present invention is to provide a structure for cutting stray light which travels in such a direction as to enter an optical component in the optical device and to be reflected at a side surface thereof.
In accordance with a first aspect of the present invention, there is provided a stray light cutting structure for an optical device provided with an optical component through which a predetermined light beam travels in parallel to the optical axis of the optical device, the structure being for cutting stray light, which travels at an angle to the optical axis of the optical device to enter the optical component through one end face thereof, and comprising
at least one notch formed on one side face of the optical component.
In the stray light cutting structure of the first aspect, the stray light is cut after the stray light enters the optical component.
The notch may be formed only at one place in the longitudinal direction of the side face or at a plurality of places in the longitudinal direction of the side face.
When the notch is formed only at one place in the longitudinal direction of the side face of the optical component, it is preferred that the notch be formed at the middle of the optical component in the longitudinal direction thereof and the depth d1 of the notch satisfies formula d1>(L/2) tan &thgr; wherein &thgr; represents the angle between said one side face of the optical component and the direction of travel of the stray light in the optical component and L represents the length of the optical component.
When the notch is formed at a plurality of (N≧2) places in the longitudinal direction of the side face of the optical component, it is preferred that each of the notches be formed at the middle of each of the areas which are obtained by dividing the side face of the optical component into N equal parts in the longitudinal direction thereof and the depth d1 of each of the notches satisfies formula d1>(1/N)·(L/2) tan &thgr; wherein &thgr; represents the angle between said one side face of the optical component and the direction of travel of the stray light in the optical component and L represents the length of the optical component.
However the depth d1 of the notch should be limited so that the notch does not interfere with the predetermined light beam which travels in parallel to the optical axis of the optical device.
In the stray light cutting structure of the first aspect of the present invention, the stray light entering the optical component is cut by the notch when the stray light impinges upon the notch on the way to the side face of the optical component or after reflected (e.g., in total reflection) by the side face of the optical component.
In the case where a single notch is formed at the middle of the optical component in the longitudinal direction thereof, light which enters the optical component through one end thereof to be reflected at a part of the side face between said one end and the middle of the side face is all cut by the notch after reflected b

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