Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
1999-07-14
2002-01-08
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C385S001000, C385S014000, C385S016000
Reexamination Certificate
active
06337762
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical switch and a wavelength converter used for optical communications or optical information processing, and more particularly to, an optical switch and a wavelength converter used for TDM (time division multiplexing) optical communications.
BACKGROUND OF THE INVENTION
A so-called all-optional switch and a wavelength converter are known generally. Using these optical switch and wavelength converter, very fast optical communications can be realized.
A known example of such an optical switch is a symmetrical Mach-Zehnder type all-optical switch (hereinafter referred to as ‘prior art 1’) disclosed in Japanese patent application laid-open No.7-20510 (1995) and Japanese Journal of Applied Physics, vol.32, pp.1746-1749, 1993. Although this optical switch is intended as a TDM demultiplexer, it can also generate optical pulse with a wavelength different from that of input pulse (Applied Physics Letters, vol.65, pp.283-285, 1994). Therefore, it can also function as a wavelength converter.
A further known example is a high-stability polarization separation type all-optical switch (hereinafter referred to as ‘prior art 2’), which is a modification of prior art 1 above, disclosed in Japanese patent application laid-open No.8-179385 (1996) and Applied Physics Letters, vol.67, pp.3709-3711, 1995. Also, another example of a polarization separation type all-optical switch, which operates in mechanism similar to that of prior art 2, is reported in IEEE Photonics Technology Letters, vol.8, pp.1695-1697, 1996. These polarization separation type optical switches can also function as a wavelength converter.
On the other hand, an all-optical switch (hereinafter referred to as ‘prior art 3’) that a Mach-Zehnder type interferometer is replaced by a combination of Sagnac type interferometer and semiconductor optical amplifier is reported in Electronics Letters, vol.30, pp.339-341, 1994. In this prior art 3, the operational principle is analogous to those in prior arts 1 and 2 and the operation can be conducted as fast as that in prior arts 1 and 2.
Further, a DISC type wavelength converter (hereinafter referred to as ‘prior art 4’) that the structure of the all-optical switch in prior art 2 is simplified is disclosed in Japanese patent application No.09-111633 (1997) and IEEE Photonics Technology Letters, vol.10, pp.346-348, 1998.
In prior arts 1 to 3, the all-optical switches extract optical signal from Return-to-Zero (RZ) optical signal sequence at intervals of certain time (TDM demultiplexer). The timing of extraction is controlled by control optical pulse that is input to the all-optical switch, with RZ optical signal sequence. By these all-optical switches, ultra-high-speed RZ optical signal sequence with a signal interval much shorter than carrier lifetime in the non-linear semiconductor waveguide or semiconductor optical amplifier can be demultiplexed. Meanwhile, the carrier lifetime in semiconductor is as long as 100 ps to 10 ns.
In converting the wavelength of RZ optical signal by using the all-optical switch described in prior arts 1 to 3, RZ optical signal sequence with a wavelength of &lgr;
1
is input to the input port of control optical pulse of the all-optical switch, and continuous light with a wavelength of &lgr;
2
is input to the input port of optical signal. Thereby, according to the existence of input RZ optical signal pulse, the all-optical switch opens and then shuts automatically after a given time. Thus, according to the existence of input RZ optical signal pulse, continuous light with a wavelength of &lgr;
2
turns on and then turns off automatically after a given time, being output as RZ optical signal with a wavelength of &lgr;
2
(these operations are explained in detail later). Meanwhile, the device in prior art 4 functions only an a wavelength converter of RZ optical signal. In using them as a wavelength converter, they can output optical pulse shorter than the carrier lifetime in the non-linear semiconductor waveguide or semiconductor optical amplifier.
Here, as one example of the conventional all-optical switches, the all-optical switch in prior art 1 will be explained referring to the drawings.
Referring to
FIG. 1
, the all-optical switch is provided with semiconductor waveguides
10
,
11
, a first input port
12
to which control optical pulse is input, a second input port to which signal optical pulse is input, and signal output ports
22
,
23
.
Control optical pulse (wavelength &lgr;
1
) input to the input port
12
is divided into 50:50, which correspond to first and second control optical pulses, at a branch point
13
. The first control optical pulse is led through a coupling point
16
to a semiconductor waveguide
10
. On the other hand, the second control optical pulse is led through a coupling point
17
to a semiconductor waveguide
11
. Here, the optical path length from the branch point
13
to the semiconductor waveguide
11
is longer than the optical path length from the branch point
13
to the semiconductor waveguide
10
. Therefore, the time when the second control optical pulse reaches the semiconductor waveguide
11
is later than the time when the first control optical pulse reaches the semiconductor waveguide
10
(here, the delay time is represented by &Dgr;t).
When the semiconductor waveguides
10
,
11
receive first and second control optical pulses, respectively, the refractive index of the semiconductor waveguides
10
,
11
changes transitionally (so-called non-linear change of refractive index occurs). Such non-linear change of refractive index occurs due to the change of carrier density inside the semiconductor waveguide. Namely, the refractive index reduces as the carrier density increases, and the refractive index increases as the carrier density reduces (band filling effect).
When the semiconductor waveguides
10
,
11
are semiconductor optical amplifiers, the refractive index increases for a certain period and then recovers. The period when the refractive index increases is nearly equal to the pulse width of control optical pulse. On the other hand, the time constant in recovery of refractive index is equal to the carrier lifetime in the semiconductor optical amplifier.
When the semiconductor waveguides are absorption-type semiconductor waveguides, the refractive index reduces for a certain period and then recovers. The period when the refractive index reduces is nearly equal to the pulse width of control optical pulse, and the time constant in recovery of refractive index is equal to the carrier lifetime in the semiconductor waveguide.
Signal optical pulse (wavelength &lgr;
2
) input to the input port
18
is divided into 50:50, which correspond to first and second signal optical pulses, at a branch point
19
. The first signal optical pulse is led through the coupling point
16
and the semiconductor waveguide
10
to a coupling point
20
. On the other hand, the second signal optical pulse is led through the coupling point
17
, the semiconductor waveguide
11
and a phase adjuster
26
to the coupling point
20
.
The first and second signal optical pulses are coupled at the coupling point
20
, where interference will occur. Namely, interference light occurs. This interference light is divided into 50:50, which correspond to first and second interference lights, at a branch point
21
. The first interference light is led though a wavelength filter
24
to the output port
22
, and the second interference light is led through a wavelength filter
25
to the output port
23
.
Meanwhile, the optical path extending from the branch point
19
through the semiconductor waveguide
10
to the coupling point
20
, and the optical path extending from the branch point
19
through the semiconductor waveguide
11
and the phase adjuster
26
to the coupling point
20
compose a so-called Mach-Zehnder type interferometer. Here, the optical paths are adjusted so that the optical path length of the arm extending from the branch point
19
through the semiconductor waveguide
10
to t
Lee John D.
McGinn & Gibb PLLC
NEC Corporation
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