Low coherent reflectometer

Details Low coherent reflectometer Low coherent reflectometer

2001-10-16

2004-07-06

Gray, David (Department: 2851)

Optics: measuring and testing

Of light reflection

C356S479000, C398S182000, C398S142000, C398S146000, C398S148000, C398S149000, C398S150000

Reexamination Certificate

active

06760110

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to low coherent reflectometers that use low coherent light beams for measuring reflectance and reflecting positions in measured optical circuits such as optical waveguides, optical modules, and the like.
2. Description of the Related Art
FIG. 8
shows a simplified configuration of a conventional low coherent reflectometer. Herein, reference numeral
100
designates a low coherent light source such as a light emitting diode (LED) that radiates low coherent light beams (simply, referred to as low coherent beams). One end of an optical fiber
101
is connected to an outgoing terminal of the low coherent light source
100
. Reference numeral
102
designates an optical coupler having four ports, which are designated by reference numerals
102
a
to
102
d
respectively. The other end of the optical fiber
101
is connected to the port
102
a
of the optical coupler
102
. In the optical coupler
102
, low coherent beams incoming from the port
102
a
are subjected to branching in response to a prescribed intensity ratio (e.g., one-to-one ratio), so that branched beams are respectively output from the ports
102
b
and
102
c
. One end of an optical fiber
103
is connected to the port
102
b
of the optical fiber
102
. The other end of the optical fiber
103
is connected to a measured optical circuit
104
which is a measured subject having a reflecting point therein.
One end of an optical fiber
105
is connected to the port
102
c
of the optical coupler
102
. Reference numeral
106
designates a collimator lens whose focal point is set in advance and which is located at a terminal end
105
a
of the optical fiber
105
. Reference numeral
107
designates a reflecting mirror for reflecting incoming beams that are transmitted thereto by way of the collimator lens
106
. In addition, a stage (not shown) is provided to vary the distance between the collimator lens
106
and the reflecting mirror
107
. One end of an optical fiber
108
is connected to the port
102
d
of the optical coupler
102
, while the other end is connected to a received light signal processor
109
. The received light signal processor
109
provides two light receiving elements (not shown) that respectively receive light beams entering from the optical fiber
108
. The light receiving elements perform photoelectric conversion on the received light beams to produce electric signals. In addition, the light receiving elements also amplify differences between the electric signals.
Next, a description will be given with respect to the operations of the low coherent reflectometer shown in FIG.
8
. First, low coherent beams generated by the low coherent light source
100
are subjected to branching by the optical coupler
102
. The first of the branched beams are introduced into the measured optical circuit
104
as measurement beams by way of the optical fiber
103
. Then, the measured optical circuit
104
produces reflected beams, which are transmitted back to the port
102
b
of the optical fiber
102
by way of the optical fiber
103
.
The other of the branched beams output from the optical coupler
102
are introduced into the optical fiber
105
as local beams. Therefore, the local beams are output from the terminal end
105
a
of the optical fiber
105
and propagate towards the collimator lens
106
. The collimator lens
106
converts them to parallel beams, which are then subjected to reflection of the reflecting mirror
107
. The reflected beams are subjected to convergence by the collimator lens
106
. The converged beams are introduced into the optical fiber
105
from its terminal end
105
a
Then, they are transmitted to the optical coupler
102
via the port
102
c.
In the optical coupler
102
, the reflected measurement beams input from the port
102
b
and the reflected local beams input from the port
102
c
are combined. If the optical path for transmission of the measurement beams matches the optical path for transmission of the local beams, interference may occur in the optical coupler
102
. Of the combined beams produced inside of the optical coupler
102
, the beams output from the port
102
d
are subjected to photoelectric conversion and differential amplification by the light receiving elements, which are provided inside the received light signal processor
109
.
It is possible to vary the spatial optical path length by moving the reflecting mirror
107
on the stage along the optical axis direction at a constant velocity. Therefore, it is possible to vary the optical path length for propagation of the local beams leaving from the port
102
c
of the optical coupler
102
. The measurement beams travel from the port
102
b
of the optical coupler
102
to the measured optical circuit
104
via the optical fiber
103
, so that the reflected measurement beams travel backwards by way of the optical fiber
103
. Hence, the overall optical path length is established by the optical fiber
103
for transmission of the measurement beams. In addition, the local beams travel from the port
102
c
of the optical coupler
102
via the optical fiber
105
and also travel towards the reflecting mirror
107
via the collimator lens
106
, so that the reflected local beams travel backwards by way of the collimator lens
106
and the optical fiber
105
. Hence, the overall optical path length is established by the optical fiber
105
, collimator lens
106
, and reflecting mirror
107
f
or transmission and propagation of the local beams. When the overall optical path length of the measurement beams traveling between the port
102
b
of the optical coupler
102
and the measured optical circuit
104
is equal to the overall optical path length of the local beams traveling between the port
102
c
of the optical coupler
102
, collimator lens
106
and reflecting mirror
107
, interference occurs between these beams. Therefore, it is possible to measure the accurate position of the reflecting point in the measured optical circuit
104
. Incidentally, details of the aforementioned technique are described in various papers such as Japanese Unexamined Patent Publication No. 2000-97856, for example.
In the aforementioned low coherent reflectometer, the measurement beams are transmitted through the optical fiber
103
only. That is, only a single optical fiber is used to form an optical path for transmitting the reflected measurement beams, which are produced by the measured optical circuit
104
. As for the local beams, an overall optical path is composed of the optical fiber
105
and a spatial optical path which is formed across the terminal end
105
a
of the optical fiber
105
, collimator lens
106
, and reflecting mirror
107
, wherein the spatial optical path has a refractive index of approximately ‘1’.
As compared with the chromatic dispersions of the measurement beams and the reflected beams in the optical path formed by only the optical fiber
103
, the chromatic dispersion of the local beams in the optical path decreases because of the existence of the spatial optical path, in which the local beams leaving from the terminal end
105
a
of the optical fiber
105
propagate towards the reflecting mirror
107
via the collimator lens
106
so that the reflected local beams propagate backwards to reach the terminal end
105
a
of the optical fiber
105
. In other words, the spatial optical path causes a difference between the chromatic dispersions of the measurement beams and local beams. Such a difference adversely influences and deteriorates the spatial resolution in measurement of reflectance and the like.
In short, the reflecting point of the measured optical circuit
104
can be estimated by causing interference between the reflected measurement beams and the reflected local beams in the optical coupler
102
, which is adjusted by varying the spatial optical path of the local beams in response to the movement of the reflecting mirror
107
. As the spatial optical path becomes longer, the difference between the chromat

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