Low-coherence reflectometer with polarization control

Details Low-coherence reflectometer with polarization control Low-coherence reflectometer with polarization control

2002-02-25

2004-08-10

Turner, Samuel A. (Department: 2877)

Optics: measuring and testing

By light interference

Using fiber or waveguide interferometer

C356S497000, C356S491000

Reexamination Certificate

active

06775005

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a low-coherence reflectometer which uses low-coherence light to measure the reflectance or its distribution in various optical circuits including light guides and optical modules.
2. Description of the Related Art
FIG. 5
is a block diagram showing an outline for the construction of a conventional low-coherence reflectometer. In
FIG. 5
, numeral
100
represents a low-coherence light source in the form of a light-emitting diode issuing low-coherence light. An end of an optical fiber
101
is connected to the exit end of the low-coherence light source
100
. Reference numeral
102
represents a photocoupler having four ports
102
a
-
102
d
and the other end of optical fiber
101
is connected to the port
102
a
. A photocoupler
102
receives low-coherence light as an input to the port
102
a
and splits it at a specified intensity ratio (say, 1:1) into two beams which exit from the ports
102
b
and
102
c
. One end of an optical fiber
103
is connected to the port
102
b
. Connected to the other end of the optical fiber
103
is an optical circuit
104
to be measured.
An optical fiber
105
is connected to the port
102
c
of the photocoupler
102
and a fiber-type optical isolator
106
is connected to the other end of the optical fiber
105
. A fiber-type optical isolator
106
has such characteristics that the input light from the optical fiber
105
is transmitted to an optical fiber
107
connected at the exit end but that the input light from optical fiber
107
is blocked against transmission to the optical fiber
105
. The other end of the optical fiber
107
is connected to a port
108
a
of a photocoupler
108
. An optical fiber
109
is connected to a port
108
b
of the photocoupler
108
. Numeral
110
represents a collimator lens preset to have a focal position at the end
109
a
of the optical fiber
109
; numeral
111
represents a reflector mirror for reflecting the light incident via the collimator lens
110
and it is mounted on a stage (not shown) for adjusting the distance to collimator lens
110
, An end of an optical fiber
112
is connected to port
108
c
of the photocoupler
108
.
An end of an optical fiber
113
is connected to the port
102
d
of the photocoupler
102
and the other end of optical fiber
113
is connected to a polarization controller
114
. Polarization controller
114
controls the state of polarization of the input light from optical fiber
113
. An optical fiber
115
is connected to the exit end of the polarization controller
114
. Numeral
116
represents a photocoupler having four ports
116
a
-
116
d
; connected to the port
116
a
is the other end of the optical fiber
112
of which an end is connected to the photocoupler
108
, and the optical fiber
115
is connected to the port
116
b
. The photocoupler
116
combines the input light to port the
116
a
with the input light to the port
116
b
and issues two beams that exit from the ports
116
c
and
116
d
in a specified intensity ratio (say, 1:1). Optical fibers
117
and
118
are connected to the ports
116
c
and
116
d
, respectively; the light travelling through optical fiber
117
is subjected to photoelectric conversion by a light-receiving device
119
and the light travelling through the optical fiber
118
is subjected to photoelectric conversion by a light-receiving device
120
. Numeral
121
represents a differential amplifier which amplifies the difference between the electrical signals output from the light-receiving devices
119
and
120
.
The conventional low-coherence reflectometer having the above-described construction operates as follows. First, the low-coherence light issuing from the low-coherence light source
100
is split by the photocoupler
102
and one branch of the coupler output is picked up as measuring light and launched into the optical circuit
104
via the optical fiber
103
, The reflected light produced in the optical circuit
104
is input to the port
102
b
of the photocoupler
102
via the optical fiber
103
and exits from the port
102
d
of the photocoupler
102
. The reflected light emerging from the photocoupler
102
passes through the polarization controller
114
and is input to the port
116
b
of the photocoupler
116
via the optical fiber
115
.
The other branch of the output light from the photocoupler
102
travels through the optical fiber
105
as local oscillator light and is transmitted through the fiber-type optical isolator
106
; thereafter, it is input to the port
108
a
of the photocoupler
108
via the optical fiber
107
. The local oscillator light passes through the photocoupler
108
and optical fiber
109
and exits from its end
109
a
; the emerging light is converted to parallel light by the collimator lens
110
and incident on the reflector mirror
111
. The local oscillator light is then reflected by the reflector mirror
111
, converged by the collimator lens
110
and launched into the optical fiber
109
at its end
109
a
. The local oscillator light entering the optical fiber
109
travels through the photocoupler
108
and optical fiber
112
in that order and is input to the port
116
a
of the photocoupler
116
.
The photocoupler
116
combines the reflected light input to the port
116
b
with the local oscillator light input to the port
116
a
. If the optical paths of the measuring light and the reflected light coincide with the optical path of the local oscillator light, interference occurs within the photocoupler
116
. The respective branches of the combined light are subjected to photoelectric conversion by the light-receiving devices
119
and
120
and the resulting electrical signals are processed by the differential amplifier
121
.
If the stage (not shown) is moved so that the reflector mirror
111
is moved along the optical axis at uniform speed to change the pathlength of the local oscillator light issuing from the photocoupler
108
, the amount of group retardation of the local oscillator light is changed. Hence, for each position of the reflector mirror
111
, the polarization controller
114
is operated to set the state of polarization of the reflected light to linear polarization at &thgr;=0° (as being parallel to the paper) and at &thgr;=90° (as being perpendicular to the paper) and the intensities of the corresponding beat signals I
0
and I
90
are measured with the differential amplifier
121
and their sum I
0
+I
90
is calculated; in this way, the optical power of the reflected light for each point in the optical circuit
104
can be measured independently of the state of polarization of the reflected light and the local oscillator light, thus making it possible to measure the reflectance distribution. For details of the technology outlined above, see Japanese Patent Laid-Open No. 97856/2000, for example.
In the conventional low-coherence reflectometer described above, the fiber-type optical isolator
106
is provided between the photocouplers
102
and
108
and this is in order to ensure that during measurement of the reflectance distribution in the optical circuit
104
, one branch of the local oscillator light emerging from photocoupler the
108
after reflection by the reflector mirror
111
and then travelling through the optical fiber
107
will not reach photocoupler the
102
to be combined there with the reflected light occurring within the optical circuit
104
. However, due to the provision of the fiber-type optical isolator
106
, the optical path of the local oscillator light starting with the issuance from the port
102
c
of the photocoupler
102
and ending at the photocoupler
116
where it is combined with the reflected light consists, in the order written, of the optical fiber
105
, fiber-type optical isolator
106
, optical fiber
107
, photocoupler
108
, optical fiber
109
, collimator lens
110
, reflector mirror
111
, collimator lens
110
, optical fiber
109
, photocoupler
108
and optical fiber
112
. This is quite a lon

Affiliated with

Also associated with

Rating

Low-coherence reflectometer with polarization control does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Low-coherence reflectometer with polarization control, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Low-coherence reflectometer with polarization control will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3361294