Optical delay circuit having optical loop for repeatedly...

Details Optical delay circuit having optical loop for repeatedly... Optical delay circuit having optical loop for repeatedly...

1999-08-12

2003-05-06

Mullen, Thomas (Department: 2632)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200

Reexamination Certificate

active

06559990

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an optical delay circuit and, more particularly, to an optical delay circuit available for an optical exchanger incorporated in an optical communication system and a method used in the optical delay circuit.
DESCRIPTION OF THE RELATED ART
A typical example of a prior art optical delay circuit is illustrated in
FIG. 1
of the drawings. The prior art optical delay unit is broken down into an optical switching element
100
and a controller
101
. The optical switching element
100
has a single input port and four output ports, i.e., the first output port a, the second output port b, the third output port c and the fourth output port d, and the input port is connected through four optical fibers (not shown) to the first, second, third and fourth output ports a/b/c/d. The optical delay element
100
is responsive to an instruction of the controller
101
so as to selectively transfer an optical signal from the input port through the optical fibers to the first, second, third and fourth output ports a/b/c/d.
An input optical signal line is connected to the input port, and output optical signal lines are respectively connected to the first, second, third and fourth output ports a/b/c/d. Time on the input optical signal line is divided into frames, and each frame is further divided into time slots. The first optical fiber does not introduce any time delay into the propagation to the first output port a. However, the other optical fibers introduce delay times different from one another. The second optical fiber introduces a time delay equal to a single time slot into the propagation from the input port to the second output port b, and the third optical fiber introduces a time delay twice as long as the time slot into the propagation from the input port to the third output port c. The fourth optical fiber introduces a time delay three times as long as the time slot into the propagation from the input port to the fourth output port d. The controller
101
instructs the optical delay element
100
to steer the optical signal to one of the first, second, third and fourth optical fibers.
An optical time switch is disclosed in Japanese Patent Publication of Unexamined Application No. 63-209395, and is illustrated in
FIG. 2
of the drawings. The prior art optical time switch has an input terminal
201
, and a wavelength converter
202
is connected to the input terminal
201
. The wavelength converter
202
converts the wavelength of an optical signal to one of the different wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, &lgr;
5
, &lgr;
6
and &lgr;
7
. When a multiple optical signal is supplied to the wavelength converter
202
as a frame divided into plural time slots, the wavelength converter
202
converts the wavelength of the pieces of multiple optical signal in the plural time slots to the wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, &lgr;
5
, &lgr;
6
and &lgr;
7
, respectively. Between the input terminal
201
and an output terminal
204
are serially connected optical delay circuits
231
,
232
and
233
which are similar in circuit configuration to one another.
An optical multiplexer
251
, an optical delay line
261
and an optical demultiplexer
271
form in combination the optical delay circuit
231
. The optical multiplexer
251
is of the type having a single input port and two output ports, and the optical demultiplexer
271
has two input ports and a single output port. The input port of the optical multiplexer
251
is connected to the wavelength converter
202
, and two output ports thereof are connected to the optical delay line
261
and one of the input ports of the optical demultiplexer
271
, respectively. The optical delay line
261
introduces a delay time 4T into the propagation from the optical multiplexer
251
to the optical demultiplexer
271
, and supplies the delayed optical signal to the other of the input ports of the optical demultiplexer
271
.
The optical delay circuit
232
also includes an optical multiplexer
252
, an optical delay line
262
and an optical demultiplexer
272
, which are arranged as similar to the optical multiplexer
251
, the optical delay line
261
and the optical demultiplexer
271
. An optical multiplexer
253
, an optical delay line
263
and an optical demultiplexer
273
also form in combination the optical delay circuit
233
, and are arranged as similar to those of the optical delay circuit
231
. Thus, the optical delay circuits
231
,
232
and
233
are similar in circuit configuration to one another. However, the optical delay lines
261
,
262
and
263
introduce different delay times into the propagation of the optical signal from the associated optical multiplexers
251
,
252
and
253
to the associated optical demultiplexers
271
,
272
and
273
.
A single bit of the optical signal occupies a time T, and each frame occupies a time period
8
T. The optical delay line
261
introduces the delay time equal to 4T. The optical delay line
262
introduces the delay time, which is equal to a half of the delay time introduced by the optical delay line
261
, i.e., 2T. The optical delay line
263
introduces the delay time, which is equal to a quarter of the delay time introduced by the optical delay line
261
, i.e., T.
A multiple optical signal contains pieces of data information on eight channels A, B, C, D, E, F, G and H, and the pieces of data information on the eight channels A, B, C, D, E, F, G and H are respectively assigned to the eight time slots. Assuming now that the multiple optical signal is supplied to the input terminal
201
, the wavelength converter
202
converts the eight pieces of the multiple optical signal respectively occupying the eight time slots to optical signals with the wavelengths &lgr;
0
, &lgr;
1
, &lgr;
2
, &lgr;
3
, &lgr;
4
, &lgr;
5
, &lgr;
6
and &lgr;
7
. The optical signals are supplied to the series of optical delay circuits
231
,
232
and
233
. The optical signals are suitably multiplexed, delayed and demultiplexed, and are rearranged in a different order. The multiple optical signal at the output terminal
204
carries the pieces of data information put in a different order H, F, C, G, A, D, E and B, by way of example. Thus, the prior art optical time switch achieves an exchange of time slots.
The prior art optical delay circuit shown in
FIG. 1
introduces the different delay times through the individual optical fibers. The prior art optical delay circuit shown in
FIG. 1
outputs three delayed optical signals at the output ports b, c and d, and, accordingly, three optical fibers are incorporated in the optical delay element
100
. When the prior art optical delay circuit is to be designed to produce n delayed optical signals, the optical delay element
100
requires n output ports and, accordingly, n optical fibers. The longer the delay time is, the longer the optical fiber becomes. If n is much greater than three, the optical delay element
100
becomes huge due to n optical fibers and, especially the nth optical fiber much longer than the first optical fiber. Thus, the problem inherent in the prior art optical delay circuit is the volume increased together with the number of delayed optical signals.
The prior art optical time switch differently retards the optical signals with different wavelengths. The pieces of data information are assigned to the different time slots, and are converted to the optical signals different in wavelength. Each of the optical delay circuits
231
,
232
and
233
generates two optical signals. If the multiple optical signal occupies n
1
time slots, the prior art optical time switch requires n
2
optical delay circuits satisfying the relation of n
1
=2
n2
. As a result, if the multiple optical signal occupies a large number of time slots, the prior art optical time switch also becomes huge.
SUMMARY OF THE INVENTION
It is therefore an important object of the present invention to provide an optical delay circuit, which is still small under the condition

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