Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-07-02
2004-04-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S247000, C359S245000, C359S299000, C359S301000, C359S304000, C398S150000, C398S147000
Reexamination Certificate
active
06728019
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of Japanese patent Application No. 2000-205991, filed on Jul. 7, 2000, and Japanese patent Application No. 2001-132083, filed on Apr. 27, 2001.
FIELD OF THE INVENTION
This invention relates to an optical gate and an optical phase modulator, and more specifically relates to an optical gate applicable to optical transmission systems, optical network systems and optical switching systems and to an optical phase modulator useful for realizing such an optical gate.
BACKGROUND OF THE INVENTION
To realize an ultra large capacity and ultra high speed optical communication system, it is important to obtain an optical gate element capable of switching ON/OFF of an optical signal using the light. Especially, when a signal transmission rate per wavelength becomes as fast as 10 Gb/s or more, it is difficult to process a signal electrically in terms of the operation speed and the energy consumption. Accordingly, an optical gate or an optical switch to directly turn ON/OFF or to switch the optical signal using another optical signal has enthusiastically developed.
There are several types of conventional optical switches, for example, one using cross gain modulation (XGM) in a medium whose gain nonlinearly varies according to the intensity of input light such as a semiconductor optical amplifier (SOA), one using cross phase modulation (XPM) in a medium whose refractive index nonlinearly varies according to the intensity of input light such as a SOA, and the one using four wave mixing (FWM).
The response speed of the optical switch using the FWM is extremely high. However, it has a demerit to need high power for the ON/OFF of the light because its conversion efficiency is small and its wavelength dependency is large.
The optical switch using the XGM or XPM has a large switching efficiency because both XGM and XPM utilize the phenomena based on the process causing the pumping of actual carriers. It is reported that in the XPM, by switching ON/OFF of the optical signal by a control light pulse in an interference system, the operation speed of 10 Gb/s or more can be realized. However, since the standard XPM utilizes interferometers having two light paths such as Mach-Zehnder interferometers or Michelson interferometers, the circuit tends to be complicated. Furthermore, it is difficult to adjust the operation conditions of the two SOAs in a necessary perfect balance.
As means to solve the above problems in the XPM, having proposed is an optical switch to have a polarization division interferometer circuit configuration which physically has a single light path by dividing an optical signal into two orthogonal polarization components using a birefringent medium, and combining the two orthogonal polarization components again after passing them through the SOA of the nonlinear medium (for example, see N. S. Patel et al. Optics Letters, vol. 21, pp. 1466-1468, 1996). This optical switch is called as Ultrafast Nonlinear Interferometer (UNI).
FIG. 7
shows a schematic diagram of the optical switch disclosed in the above-mentioned paper. An optical signal
212
of wavelength 1550 nm enters an optical signal input port
210
, and a control light
216
of wavelength 1540 nm enters a control light input port
214
. The optical signal
212
contains, for example, a 40 Gbit/s optical clock signal of linear polarization, and the control light
216
contains a 40 Gbit/s optical RZ pulse train synchronized with the optical signal
212
.
The optical signal output
212
from the optical signal input port
210
enters a 7.5 m long polarization preserving fiber
218
at an angle of 45° of the polarization plane relative to the birefringent axis of the fiber. The polarization preserving fiber
218
functions as a birefringent medium to divide the input optical signal into two polarization components and to output them after separating them in the time base by the amount (12.5 ps) of the polarization mode dispersion of the polarization preserving fiber
218
. A WDM optical coupler
220
combines the output from the polarization preservation fiber
218
with the control light
216
from the control light input port
214
. The timing between the optical signal
212
and the control light
216
is adjusted so that at the output stage of the WDM optical coupler
220
, a control light pulse
226
is located between a preceding optical signal pulse
222
and a following optical signal pulse
224
output from the polarization preserving fiber
218
. The preceding optical signal pulse
222
, the control light pulse
226
, and the following optical signal pulse
224
enter a semiconductor optical amplifier (SOA)
228
in this order.
The SOA
228
is forward biased by a direct power source
230
. For example, the SOA
228
consists of a buried waveguide using the InGaAsP/InP system as an active layer material, and both ends are applied with antireflection coating. When the control light pulse
226
enters, the gain in the SOA
228
instantly decreases due to the stimulated emission, gain saturation occurs, and the carrier density in the SOA
228
decreases. Since the refractive index of the semiconductor depends on the carrier density of the inside (band filing effect), the refractive index variation (which results in XPM) occurs at this point. That is, the refractive index of the SOA
228
varies before and after the entry of the control light pulse
226
. Therefore, the following optical signal pulse
224
receives a phase shift different from that of the preceding optical signal pulse
222
while transmitting in the SOA
228
. Since the amount of the phase shift varies according to the optical intensity and wavelength of the control light pulse
226
and injected electric current of the SOA
228
, the optical intensity and wavelength of the control light pulse
226
and the injected current of the SOA
228
are set so that the amount of the phase variation of the following optical signal pulse
224
caused by the existence and the nonexistence of the control light pulse
226
becomes &pgr;. With this configuration, the phase of the following optical signal pulse
224
output from the SOA
228
differs by &pgr; according to whether or not the control light pulse
226
exists.
The optical signal pulses
222
and
224
passed through the SOA
228
enter a 7.5 m long polarization preserving fiber
232
in the direction that the polarization plane of the preceding pulse
222
coincides with the slow axis of the polarization preserving fiber
232
and the polarization plane of the following pulse
224
coincides with the fast axis of the fiber
232
. With this configuration, the time difference between the optical signal pulses
222
and
224
is almost disappeared after they passed through the polarization preserving fiber
232
. To cancel the individual difference of the polarization mode dispersion amount between the polarization preserving fibers
218
and
232
, a polarization phase adjuster
234
is disposed at the output of the polarization preserving fiber
232
. Reference numerals
224
a
and
226
a
denote the preceding optical signal pulse and the following optical signal pulse output from the polarization phase adjuster
234
respectively.
A polarizer
236
is disposed on the output side of the polarization phase adjuster
234
so that the polarizer
236
passes the light having the same polarization direction with that of the composite polarization of the preceding optical signal pulse
222
a
and the following optical signal pulse
224
a
when the optical signal pulse
226
exists. When the optical signal pulse
226
does not exist, the composite polarization direction of the preceding optical signal pulse
222
a
and the following optical signal pulse
224
a
becomes orthogonal to that of the polarizer
236
. Accordingly, the polarizer
236
passes only the optical signal in the condition that the control light pulse
226
exists out of the optical output from the polarization phase adjuster
234
. An optical bandpass filter
238
exclusively ex
Nishimura Kosuke
Tsurusawa Munefumi
Usami Masashi
Christie Parker & Hale LLP
Epps Georgia
KDD Submarine Cable Systems Inc.
Thompson Timothy J
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