Optics: measuring and testing – By light interference – Using fiber or waveguide interferometer
Reexamination Certificate
1999-09-24
2002-11-05
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Using fiber or waveguide interferometer
C356S073100
Reexamination Certificate
active
06476919
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a reflectometry and a reflectometer for using low coherence light to measure a reflection intensity (distribution) in a measured optical circuit such as a optical waveguides or a optical module.
2. Description of the Related Art
FIG. 1
shows an example of a low-coherence reflectometer in a related art, here a low-coherence reflectometer based on a Michelson interferometer of optical fiber type. In the figure, numeral
1
denotes a light source made of a light-emitting diode for emitting low-coherence light having a polarization degree of 0.1, a spectral band width of 50 nm, and a center wavelength of 1.53 &mgr;n, numeral
2
denotes an optical fiber coupler having two input ports
2
-
1
and
2
-
2
and two output ports
2
-
3
and
2
-
4
, numeral
3
denotes a measured optical module comprising an optical fiber pigtail
3
a
, numeral
4
denotes a polarization controller, numeral
5
denotes an optical fiber delay line made of an optical fiber coil, numeral
6
denotes a reflection mirror, numeral
7
denotes a linear stage, numeral
8
denotes a photodetector, numeral
9
denotes a signal processing system, numerals
10
,
11
, and
12
denote optical connectors, and numerals
13
and
14
denote collimating lenses.
In the described configuration, light emitted from the light source
1
is incident on the optical fiber coupler
2
through the input port
2
-
1
and is made to branch to the output ports
2
-
3
and
2
-
4
. The light made to branch to the output port
2
-
3
is incident on the measured optical module
3
through the optical fiber pigtail
3
a
connected by the optical connector
10
as measurement light. The measurement light is reflected at each point responsive to the propagation distance of the measured optical module
3
and the reflected light signal propagates through the optical fiber pigtail
3
a
in the opposite direction and is incident on the output port
2
-
3
.
On the other hand, the light made to branch to the output port
2
-
4
of the optical fiber coupler
2
passes through the polarization controller
4
and the optical fiber delay line
5
, is made a collimated beam through the collimating lens
13
, is reflected on the total reflection mirror
6
, propagates through the path in the opposite direction, is incident on the output port
2
-
4
of the optical fiber coupler
2
, and is used as local light signal.
Here, the optical fiber delay line
5
is provided for balancing the lengths of both arms of the Michelson interferometer of optical fiber type in response to the length of the optical fiber pigtail
3
a
connected to the measured optical module
3
and is replaced by means of the optical connectors
11
and
12
at both ends of the optical fiber delay line
5
whenever necessary.
The reflected light signal from the measured optical module
3
and the local light signal reflected on the total reflection mirror
6
are combined by the optical fiber coupler
2
and the mixed lightwave signal is emitted from the input port
2
-
2
and is made a collimated beam through the collimating lens
14
, then the collimated beam is received at the photodetector
8
. The beat signals of the reflected light signal and the local light signal received at the photodetector
8
and subjected to photoelectric conversion are processed by the signal processing system
9
and the reflection intensity of the measured optical module
3
is measured from the length of the signal.
In the reflectometer, the input port
2
-
1
of the optical fiber coupler
2
is connected to the light source
1
, forming the light branch section, the input port
2
-
2
of the optical fiber coupler
2
is connected through the collimating lens
14
to the photodetector
8
, forming the lightwave combining section, the output port
2
-
3
of the optical fiber coupler
2
forms the optical measurement block, and the output port
2
-
4
of the optical fiber coupler
2
, the optical fiber delay line
5
, the collimating lens
13
, and the total reflection mirror
6
(containing the linear stage
7
), forming the variable optical delay circuit; they make up the interferometer.
The coherence length of the emitted light from the light source
1
is about 40 &mgr;m. Thus, for the reflected light signal to be able to interfere with the local light signal with respect to a specific position of the total reflection mirror
6
, the light path length of the reflected light signal needs to match the optical path length of the local light signal within the coherence length. Thus, if the total reflection mirror
6
is moved in the direction of light beams on the linear stage
7
, only the interference beat signals of the reflected light signal at the points of the measured optical module
3
corresponding to the total reflection mirror positions in a one-to-one correspondence can be provided and the intensity of each beat signal is measured and is multiplied by an appropriate constant, whereby the light power of the reflected light signal can be found. The spatial resolution of the reflectometry is given as &kgr;c
&dgr;&ngr; where &kgr; is a constant, c is a light velocity, n is a group index of measurement optical waveguide, and &dgr;&ngr; is the full width at half maximum of spectrum of emitted light from a light source. If the spectrum of the emitted light from the light source is of Gauss type, &kgr;=0.31.
Since the low-coherence reflectometer uses light interference to measure the light power of reflected light signal, the polarization controller
6
needs to be used to make the polarization state of local light signal and that of the reflected light signal to match. In many cases, the optical fiber pigtail
3
a
is connected to the measured optical module
3
as shown in FIG.
1
. If the measured optical module
3
is connected to one arm of the interferometer, the optical fiber delay line
5
needs to be connected to the other arm of the interferometer for balancing.
The polarization state of light propagating through the optical fiber changes according to bending of the fiber or the stress state. If the waveguide itself of the measured optical module has a double refraction property, the polarization state of light reflected at the points of the waveguide varies from one point to another. Therefore, to use different measured optical modules or optical fiber delay line or measure a optical waveguide having a double refraction property, the polarization controller needs to be used to adjust the polarization states of both; however, it is indispensable to eliminate the adjustment in order to save time and labor of measurement and realize fully automatic reflection measurement.
FIG. 2
shows an example of a low-coherence reflectometer in another related art, namely, a polarization-insensitive low-coherence reflectometer capable of measuring the reflection intensity of reflected light signal independently of the polarization state of the reflected light signal (namely, the light power of the reflected light signal). In the figure, numeral
15
denotes a polarizer, numeral
16
denotes a polarization beam splitter, numeral
17
denotes a photodetector, and numeral
18
denotes a signal processing system. Parts identical with those previously described with reference to
FIG. 1
are denoted by the same reference numerals in FIG.
2
.
In the example of the low-coherence reflectometer in the related art, a polarized wave diversity technology is adopted wherein local light signal and reflected light signal are separated into P wave and S wave by the polarization beam splitter
16
, the reflected light signal and the local light signal are made to interface with each other in their respective polarization states, and the interference intensities of the beat signals of the reflected light signal and the local light signal are detected by photodetectors
8
and
17
and the signal processing system
18
, and are added together.
Let the electric field elements of the P and S waves of the reflected light signal and
Horiguchi Masaharu
Mori Tohru
Takada Kazumasa
Ando Electric Co. Ltd.
Finnegan Henderson Farabow Garrett & Dunner LLP
Font Frank G.
Natividad Phil
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