Optics: measuring and testing – Position or displacement
Reexamination Certificate
2001-12-17
2004-03-23
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Position or displacement
C356S004010
Reexamination Certificate
active
06710885
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a measuring system using a laser beam as a distance measuring light, and in particular, to an improvement to achieve a compact and lightweight driving system used for light mixing with the purpose of eliminating the problems in switching of optical paths, in adjustment of a light amount and in removal of a speckle pattern in the laser beam in a light wave distance measuring system.
Description will be given below on an optical system of a conventional type light wave distance measuring system referring to FIG.
5
.
The optical system primarily comprises a light projecting unit
1
, a distance measuring optical unit
2
, a photodetection unit
3
, and a distance measuring circuit
4
. Further, the light projecting unit
1
comprises a semiconductor laser
5
for emitting a laser beam as a distance measuring light, an optical expander
9
for entering the laser beam emitted from the semiconductor laser
5
to an optical fiber
8
via lenses
6
and
7
, gradient index lenses
12
and
13
for entering the laser beam projected from the optical fiber
8
to an optical fiber
11
, a phase plate
14
in a circular shape disposed between the lens
6
and the lens
7
, and a mixing motor
15
for rotating the phase plate
14
. The phase plate
14
, the mixing motor
15
, and the gradient index lenses
12
and
13
make up a mixing means
16
.
Now, description will be given on the distance measuring optical unit
2
.
A prism
17
and an objective lens
18
are arranged on an optical axis of the incoming and outgoing distance measuring light. The laser beam projected from the optical fiber
11
is reflected by the prism
17
toward an object to be measured (not shown). Being reflected by the prism
17
and having passed through the objective lens
18
, the distance measuring light
19
is projected to the object to be measured. The distance measuring light
19
is reflected by the object to be measured, passes through the objective lens
18
, and enters the light wave distance measuring system. Then, the distance measuring light
19
is reflected by the prism
17
and is entered to an optical fiber
26
of a photodetection unit
3
as described later.
A splitting prism
21
is disposed at a position opposite to the prism
17
. The splitting prism
21
has a light beam splitting surface
21
a
and a reflection surface
21
b
. The light beam splitting surface
21
a
splits a part of the distance measuring light
19
projected from the optical fiber
11
as an inner reference light
22
. A light amount adjusting plate
23
in the circular shape is rotatably mounted between the splitting prism
21
and the prism
17
. The light amount adjusting plate
23
is rotated by a light amount adjusting motor
24
and its position is determined. The light amount adjusting plate
23
and the light amount adjusting motor
24
make up a light amount adjusting means
25
.
A splitting prism
27
is arranged at a position opposite to the splitting prism
21
with the prism
17
between the splitting prism
27
and the splitting prism
21
. The splitting prism
27
has a light beam splitting surface
27
a
and a reflection surface
27
b
. The light beam splitting surface
27
a
allows a reflected distance measuring light
19
′ reflected by the prism
17
to pass, and the light
19
′ enters the optical fiber
26
. Lenses
28
and
29
are disposed between the reflection surface
21
b
and the reflection surface
27
b
. After passing through the lenses
28
and
29
, the inner reference light
22
is reflected by the reflection surface
27
b
and the light beam splitting surface
27
a
and enters the optical fiber
26
.
An optical path switching plate
31
in a circular shape is rotatably mounted between the prism
17
and the lens
29
on one side and the splitting prism
27
on the other side. Two rotating positions of the optical path switching plate
31
can be selected by an optical path switching motor
32
. The optical path switching plate
31
and the optical path switching motor
32
make up an optical path switching means
36
.
The photodetection unit
3
comprises condenser lenses
33
and
34
, and a photodetection element
35
. The reflected distance measuring light
19
′ projected from the optical fiber
26
is converged on the photodetection element
35
by the condenser lenses
33
and
34
and is received by the photodetection element
35
.
The distance measuring circuit
4
drives the semiconductor laser
5
to emit the light, and a distance to the object to be measured is calculated based on a photodetection signal from the photodetection element
35
.
Now, the mixing means
16
will be described.
When the semiconductor laser
5
emits the laser beam, it is unavoidable that speckle pattern occur. To eliminate the speckle pattern, the mixing means
16
is provided.
One of the causes to induce the light emission speckle pattern of the laser beam is the changes over time of a speckle pattern.
First, description will be given on the changes over time of the speckle pattern.
It is generally known that a light emission wavelength of the semiconductor laser
5
is changed depending on temperature. This is represented in FIG.
6
(A). That is, a light emission wavelength &lgr; of the semiconductor laser
5
is elongated as a tip temperature Tc is increased, and the light emission wavelength &lgr; is continuously changed with respect to the tip temperature Tc. This is because, when a refractive index n of an active layer is varied with the temperature change, the light emission wavelength &lgr; of the semiconductor laser
5
is changed accordingly. Here, it is supposed that the change of the refractive index of the active layer due to the temperature change is &Dgr;n. Then, when &Dgr;n L , i.e. the change of an optical length of the active layer, is smaller than ½ of a resonance wavelength of the semiconductor laser
5
, the resonance wavelength is continuously changed. When &Dgr;n L is greater than ½ of the resonance wavelength of the semiconductor laser
5
, a discontinuous phenomenon of the wavelength change called “mode jump” occurs.
In this phenomenon called “mode jump”, when the resonance wavelength of the semiconductor laser
5
is changed by more than one-half of the wavelength of the standing wave of light in the longitudinal direction of the resonator, it is jumped to longitudinal mode which is different from the oscillation mode up to that moment.
Now, referring to FIG.
6
(B), description will be given on the case where a rectangular modulated electric current is supplied to the semiconductor laser
5
.
It is assumed here that an electric current of a rectangular wave as shown in FIG.
6
(B)(
1
) is supplied to the semiconductor laser
5
from the time t0 to the time t1. Then, the speckle pattern as shown in
FIG. 7
is developed due to the coherence on the semiconductor laser
5
. On the other hand, the tip of the semiconductor laser
5
is heated up due to the driving current, and the temperature is increased up to a temperature value where a balance is kept between the heating value and a heat radiation value to a copper base where the semiconductor laser
5
is mounted or into the air. FIG.
6
(B)(
2
) shows such temperature rise of the tip.
When the temperature of the semiconductor laser
5
is increased, the light emission wavelength &lgr; is changed because of the cause as described above. When the light emission wavelength &lgr; is changed, the speckle pattern as shown in
FIG. 7
developed as an interference pattern is changed in association with such change. Light intensity wave-forms of the points A and B in
FIG. 7
are shown in FIG.
6
(B)(
3
) and FIG.
6
(B)(
4
) respectively. That is, at the point A of
FIG. 7
, the light intensity of the speckle pattern is at a high position at the time t0, and the light intensity is gradually decreased as the light emission wavelength &lgr; is changed (FIG.
6
(B)(
3
)). On the other hand, at the point B in
FIG. 7
, the light intensity of the s
Ohishi Masahiro
Waki Michio
Yabe Masaaki
Font Frank G.
Kabushiki Kaisha TOPCON
Nields & Lemack
Punnoose Roy M.
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