Light-controlled light modulator

Details Light-controlled light modulator Light-controlled light modulator

2001-09-17

2004-06-22

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S237000, C359S238000, C398S053000, C385S005000

Reexamination Certificate

active

06753996

ABSTRACT:

This application is based on Patent Application Nos. 2000-286601 filed Sep. 21, 2000, 2001-249034 filed Aug. 20, 2001, 2001-249035 filed Aug. 20, 2001 and 2001-249037 filed Aug. 20, 2001 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light-controlled light modulator, and more particularly to a light-controlled modulation technique for modulating light with a wavelength identical to or different from input signal light with an arbitrary wavelength in response to the intensity of the input signal light in a wavelength division multiplexed optical network.
2. Description of the Related Art
Conventionally, as an optical transmission system for transmitting optical signals with different wavelengths, an optical transmission system using wavelength division multiplexing (WDM system) has been known which transmits the optical signals with different wavelengths by combining them into a single optical fiber. Recently, such WDM systems have been increasingly applied not only to one-to-one transmission, but also to network transmission.
In such WDM systems, a light-controlled light modulator is increasing its importance which carries out wavelength conversion, that is, which converts the wavelength of an optical signal traveling through an optical fiber into the same or different wavelengths.
FIG. 1
is a diagram showing a circuit of a conventional wavelength converter. The wavelength converter consists of a symmetric Mach-Zehnder interferometer that comprises SOAs (Semiconductor Optical Amplifiers)
105
and
106
, MMI (Multi-Mode-Interference) couplers
101
,
102
and
103
connected to the SOAs
105
and
106
, an MMI coupler
104
connected to the MMI couplers
102
and
103
, and optical waveguides interconnecting them. In
FIG. 1
, the reference numeral
107
designates signal light,
108
designates continuous light,
109
designates output light and
110
designates a port.
The operation of the wavelength converter with such a configuration will now be described.
The continuous light (CW light)
108
with a wavelength &lgr;j is launched into the MMI coupler
101
, and split into two optical waveguides. The two continuous light waves pass through the SOAs
105
and
106
and the MMI couplers
102
and
103
, and are coupled by the MMI coupler
104
to be emitted from the port
110
.
In this state of the wavelength converter, the optical signal &lgr;i(s)
107
with the wavelength &lgr;i is launched into the MMI coupler
102
, and then into the SOA
105
. Here, the optical signal
107
varies the refractive index of the SOA
105
.
As a result, the interference conditions change of the symmetric Mach-Zehnder interferometer comprising the MMI couplers
101
,
102
,
103
and
104
so that only when the signal light
107
is “1”, the output light with the wavelength &lgr;j is emitted from the port
109
. Thus, the optical signal with the wavelength &Dgr;i is transformed to light with the wavelength &Dgr;j to be emitted from the port
109
as the output light &Dgr;j(s).
In this method, the transmission rate of the input signal light is limited by the recovery time of carrier density changes of the SOAs
105
and
106
. Thus, the speed of the wavelength conversion of the optical signal is limited to about 20 Gbps at most.
FIG. 2
is a diagram showing another conventional wavelength converter. The wavelength converter comprises an SOA
201
, MMI couplers
202
and
203
connected to the SOA
201
, and a loop-type interferometer
209
connected between the MMI couplers
202
and
203
. In
FIG. 2
, the reference numeral
204
designates signal light,
205
designates continuous light,
206
designates counterclockwise traveling light,
207
designates clockwise traveling light,
208
designates output light and
210
and
211
each designate a port.
In the configuration, the continuous (CW) light
205
with the wavelength &Dgr;j is launched into the MMI coupler
203
through the port
211
, and is split into two parts by the MMI coupler
203
, which are delivered to the loop-type interferometer
209
. In the loop-type interferometer
209
, the two parts travel around the loop as the clockwise traveling light
207
and counterclockwise traveling light
206
, are recombined by the MMI coupler
203
to be emitted from the port
211
.
In this state, the signal light &Dgr;i(s)
204
with the wavelength &Dgr;i is launched into the MMI coupler
202
. The incident signal light
204
passes through the SOA
201
, which varies its refractive index. Thus, the light with the wavelength &Dgr;j traveling in the loop is affected by the change in the refractive index, resulting in phase variations as shown in FIG.
3
A.
The clockwise traveling light
207
brings about abrupt phase variations, followed by recovering of the phase at a rate corresponding to the recovery time of carrier density changes of the SOA
201
, and is launched into the MMI coupler
203
.
The counterclockwise traveling light
206
also undergoes similar phase variations. However, since it travels through the loop-type interferometer
209
longer than clockwise traveling light
207
by a distance &Dgr;L, it is launched into the MMI coupler
203
with a delay time &Dgr;&tgr;.
Accordingly, in the MMI coupler
203
, the time of the phase variations differs by an amount of &Dgr;&tgr;=&Dgr;L/(c

eq0
) between the clockwise traveling light
207
and the counterclockwise traveling light
206
, where c is the speed of light, and n
eq0
is the equivalent refractive index of the waveguide constituting the loop. The two continuous light waves with the same wavelength &Dgr;j interfere with each other in the MMI coupler
203
. In the course of this, their phases differ only during the time period &Dgr;&tgr; and nearly equal thereafter. As a result of the interference, the light is emitted from the port
210
only during the time slot &Dgr;&tgr; as illustrated in FIG.
3
B. In other words, the input optical signal with the wavelength &Dgr;i is transformed to the light with the wavelength &Dgr;j to be output to the port
210
as output light &Dgr;j(s)
208
.
In the wavelength converter with such a loop-type interferometer, the counterclockwise traveling light
206
and the clockwise traveling light
207
have the same phase variations during the time the light phase variations gradually recover in response to the carrier concentration in the SOA
201
, except for the time period &Dgr;&tgr;. Therefore, as a result of the interference, the effect of the variations in the refractive index in the SOA
201
are canceled out, and the light with the wavelength &Dgr;j is emitted from the port
211
except for the time period &Dgr;&tgr;. Thus, as illustrated in
FIG. 3B
, the waveform after the wavelength conversion output from the port
210
includes no low-rate components whose rate is limited by the recovery time due to the carrier density changes, enabling high-speed wavelength conversion with steep rising and falling edges.
In the wavelength converter with the loop-type interferometer
209
, however, the input signal light
204
is combined and output from the same port
210
as the output light
208
. Therefore, to separate the output light
208
from the input light
204
, a wavelength filter
212
must be connected to the output port
210
to extract only the output light
208
.
Furthermore, when the wavelength &Dgr;i of the signal light is the same as the wavelength &Dgr;j of the wavelength conversed light, the wavelength filter
212
cannot separate them. This means that the light before the wavelength conversion is mixed into the output light as noise. Thus, it has a problem of being unable to carry out the conversion of the same wavelength. In addition, since the 3 dB coupler
202
is used to split the continuous light
205
with the wavelength &Dgr;j, it has a problem of bringing about 3 dB additional loss in principle.
Moreover, when the wavelength converter with the loop-type interferometer is used, it is necessary that the

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