Optics: measuring and testing – By light interference
Reexamination Certificate
2002-02-15
2004-09-21
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Reexamination Certificate
active
06795196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wavelength measuring apparatus for measuring wavelength of light under measurement and in particular to wavelength measurement apparatus for measuring wavelength of sweep light that varies continuously.
2. Description of the Related Art
Conventionally, an interferometer is used to measure the wavelength of light under measurement.
FIG. 11
shows a Michelson interferometer. The Michelson interferometer
1100
comprises a reference light source
101
for emitting reference light having a known wavelength &lgr;0, a fixed mirror
1102
, a movable mirror
1103
provided slidably in parallel with the optical path, a half mirror
1104
provided at an angle of 1045 degrees from the optical path, a photo-detector for light under measurement
1105
, and a photo-detector for reference light
1106
.
In the Michelson interferometer
1100
, light under measurement having an unknown wavelength &lgr; is emitted toward Point B of the half mirror
1104
. Part of the outgoing light under measurement is reflected at the right angle at Point B of the half mirror
1104
, reversed by 180 degrees in direction by the fixed mirror
1102
, passes through Point A of the half mirror
1104
, and incident on the photo-detector for light under measurement
1105
. Other part of the light under measurement passes through Point B of the half mirror
1104
, reversed by 180 degrees in direction by the movable mirror
1103
, reflected at the right angle at Point A of the half mirror
1104
, and incident on the photo-detector for light under measurement
1105
.
Meanwhile, part of reference light emitted from the reference light source
1101
is reflected at the right angle at Point A of the half mirror
1104
, reversed by 180 degrees in direction by the fixed mirror
1102
, passes through Point B of the half mirror
1104
, and incident on the photo-detector for reference light
1106
. Other part of the light under measurement passes through Point A of the half mirror
1104
, reversed by 180 degrees in direction by the movable mirror
1103
, reflected at the right angle at Point B of the half mirror
1104
, and incident on the photo-detector for reference light
1106
.
In this way, on each photo-detector
1105
,
1106
are incident light that passed through the fixed mirror
1102
and light that passed through the movable mirror
1103
thus generating interference between these light beams. Thus, in case the movable mirror
1103
is slide in the direction of the arrow in the figure, output signals output from the photo-detectors include cyclic peaks caused by interference as shown in FIG.
12
.
The pitch length P of the photo-detector for light under measurement
1105
corresponds to the wavelength &lgr; of the light under measurement. In case the movable mirror
1103
is moved for a predetermined distance D, the wavelength &lgr; of the light under measurement is determined from the number of peaks n0 of the output signal from the photo-detector for light under measurement
1105
, the number of peaks n1 of the output signal from the photo-detector for reference light
1106
, and the wavelength &lgr;0 of the reference light, and represented by the following expression:
&lgr;=(n0
1)×&lgr;0 (1)
However, in a related art interferometer such as a Michelson interferometer, it is assumed that the wavelength of the light under measurement is fixed during measurement. Thus it was impossible to accurately measure the wavelength in case the wavelength of the light under measurement continuously varied. That is, the number of peaks n1 does not reflect local variation of the wavelength of the light under measurement so that the average value of varied wavelengths is measured in case the wavelength of the light under measurement varies while the movable mirror
1103
is slid.
SUMMARY OF THE INVENTION
An object of the invention is to provide wavelength measurement apparatus that can measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
In order to attain such an object, according to a first aspect of the invention, there is provided a wavelength measurement apparatus comprising:
an optical filter (such as a fiber-optic Etalon
3
in
FIG. 1
) to which a light beam is incident;
a photo-detector (for example a photodiode
4
in
FIG. 1
) for detecting the transmitted light of the optical filter, the photo-detector for outputting intensity of the transmitted light;
a counter (for example a counter
8
in
FIG. 1
) for counting the number of peaks of the output of the photo-detector to generate a count value; and
a controller (for example a CPU
14
in
FIG. 1
) for calculating the wavelength of the light beam based on the count value of the counter.
Here, the optical filter may be any optical filter that selectively transmits light having a predetermined length and may be composed of an interference optical filter where a multi-layered optical film is evaporated on Fabry-Perot Etalon, silica based glass, or silicon.
In the first aspect of the invention, the light under measurement is incident on the optical filter. The optical filter selectively transmits light having a predetermined waveform. The photo-detector detects the transmitted light that passed through the optical filter and outputs the light intensity of the transmitted light. In case the wavelength of the light under measurement is continuously swept, the measured is transmitted through the optical filter each time the wavelength of the light under measurement satisfies predetermined conditions that conform to the physical characteristics of the optical filter.
The predetermined wavelength interval (finesse) is a length determined according to the physical characteristics of the optical filter so that it is possible to know the correct value in advance based on a theoretical formulae such as the Airy's formulae or a measured value. Thus, the count value (number of peaks) currently counted by the counter represents a relative variation of the wavelength from the start of sweep to this point in time. The controller calculates the wavelength of the light under measurement based on the count value so that it can calculate the instantaneous wavelength value at this point in time. As a result, it is possible to measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
According to a second aspect of the invention, there is provided a wavelength measurement apparatus according to the first aspect of the invention, the controller resets the count value when light having a known reference wavelength is incident.
In the second aspect of the invention, an operator resets the count value via the controller when light having a known reference wavelength is incident. Accordingly, the count value of the counter while the wavelength of the light under measurement is being swept represents a relative variation from the reference wavelength. Thus, the controller can accurately calculate the wavelength of light under measurement under sweep. It is thus possible to measure the wavelength of the light under measurement under sweep process with high accuracy and in real time even in case the wavelength is continuously swept.
According to a third aspect of the invention, the wavelength measurement apparatus further comprises:
a synchronization signal output unit (for example a comparison register
13
in
FIG. 1
) for outputting a synchronization signal with a predetermined timing,
wherein the controller acquires the count value each time the synchronization signal from the synchronization signal output unit is detected.
In the third aspect of the invention, the synchronization signal output unit outputs a synchronization signal with a predetermine timing. The controller acquires the count value each time the synchronization signal from the synchronization signal output
Ando Electric Co. Ltd.
Fish & Richardson P.C.
Font Frank G.
Lee Andrew H.
LandOfFree
Wavelength measurement apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Wavelength measurement apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Wavelength measurement apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3188567