Optical: systems and elements – Optical frequency converter
Reexamination Certificate
1999-02-26
2001-08-07
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Reexamination Certificate
active
06271960
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
In recent years, optical application technologies such as optical communication, optical information processing, and optical measurement have been rapidly developed, and a great need exists for the development of wavelength conversion technologies for an optical signal.
The present invention relates to a method and apparatus for wavelength conversion of signal light for converting a wavelength &lgr;s of signal light to an arbitrary wavelength of &lgr;1 to &lgr;n, and an optical transmission scheme using the apparatus.
2. Description of the Related Art
Technologies as mentioned above for converting the wavelength of an optical signal include, for example, a semiconductor optical device disclosed in Laid-open Japanese Patent Application No. 6-302903 (JP-302903/1994). The configuration of this semiconductor optical device is shown in FIG.
1
. Referring to
FIG. 1
, the semiconductor optical device is provided with slab optical waveguides
13
A,
13
B of fan shape at both ends of array waveguide
16
which comprises a plurality of waveguides having delay time difference. One slab optical waveguide
13
A has focal surface
14
A connected to a plurality of gain waveguides
17
. The other slab optical waveguide
13
B has focal surface
14
B connected to saturable absorption waveguide
20
having a saturable absorption area through passive waveguide
19
A and input/output gain waveguide
18
B.
The device operates in the following manner. First, gain waveguide
17
supporting a required output optical signal wavelength is injected with a current a little less than a threshold value current at which laser oscillation is started. At this point, since spontaneous emission light emitted from gain waveguide
17
is absorbed in the saturable absorption area, the laser oscillation is suppressed. In this state, when an input optical signal is incident on input/output waveguide
18
B, the light absorption is saturated in the saturable absorption area to generate the laser oscillation at a wavelength corresponding to the selected and conducted gain waveguide. This laser oscillation repeats on and off following the input optical signal, thereby causing the semiconductor optical device to serve as a wavelength conversion element. The outputted wavelength is instantaneously switched by switching the conducting gain waveguide
17
.
Also, in JP-102643/1997, an optical wavelength conversion circuit of oscillation suppression type as shown in FIG.
2
A and an optical wavelength conversion circuit of saturable absorption type as shown in
FIG. 2B
are disclosed.
In the optical wavelength conversion circuit of oscillation suppression type in
FIG. 2A
, semiconductor laser
11
in free oscillation at wavelength &lgr;2 by injecting a current at an oscillation threshold value or more is applied with intensity modulated signal light at wavelength &lgr;1. The oscillation of the semiconductor laser is suppressed when the applied signal light is on, thereby outputting wavelength converted signal light at wavelength &lgr;2 which forms a complementary signal train to the applied signal light at wavelength &lgr;1.
In
FIG. 2A
, the optical wavelength conversion circuit comprises DBR (Distributed Bragg Reflection) type semiconductor laser
11
in which optical amplifying portion
11
c
having no polarization dependency as a gain area and oscillation polarization selecting portion
11
d
are disposed in a resonator which has DBR area
11
a
serving as an oscillation wavelength selecting portion and end surface
11
b
as two reflecting surfaces. In the respective areas, currents are injected from corresponding electrodes
10
a
,
10
b
, and
10
c
respectively.
In the optical wavelength conversion circuit of saturable absorption type in
FIG. 2B
, semiconductor laser
12
, having a saturable absorption portion injected with a current at an oscillation threshold value or less and an optical amplifying portion injected with a current to the extent that it oscillates at a wavelength &lgr;2 when the absorption of the saturable absorption unit is reduced, is applied with intensity modulated signal light at a wavelength &lgr;1. The absorption of the saturable absorption portion is reduced to cause oscillation at wavelength &lgr;2 when the applied signal light is on, thereby outputting wavelength converted signal light at wavelength &lgr;2 which makes a signal train with the same sign as the applied signal light at wavelength &lgr;1.
Semiconductor laser
12
is a DBR type semiconductor laser in which saturable absorption portion
12
c
having no polarization dependency and optical amplifying portion
12
d
of polarization dependent type are disposed as a gain area in a resonator which has DBR area
12
a
and end surface
12
b
as two reflecting surfaces. Saturable absorption portion
12
c
and optical amplifying portion
12
d
have a property that the polarization dependency is low when a low current is injected and the polarization dependency is high when a high current is injected. Saturable absorption portion
12
c
serves as a saturable absorption area with no polarization dependency in a low current injected state. Optical amplifying portion
12
d
functions as a gain area, in which laser oscillation is started in a single mode by the mode gain difference dependent on the polarization when the absorption of saturable absorption portion
12
c
is reduced by the applied signal light being on in a high current injected state.
The operation of this circuit is as follows. The intensity modulated signal light at the wavelength &lgr;1 is applied to saturable absorption portion
12
c
with no polarization dependency. In saturable absorption unit
12
c
, carrier density is increased by stimulated absorption of the applied signal light, regardless of the polarization. With this, the loss in the laser resonator at the wavelength &lgr;2 is reduced, resulting in the oscillation at the wavelength &lgr;2 only when the applied signal light is on. In other words, the outputted level of the oscillation light at the wavelength &lgr;2 is modulated in the same sign in accordance with the on and off of the applied signal light at the wavelength &lgr;1, and the wavelength conversion is performed from the wavelength &lgr;1 to the wavelength &lgr;1.
An optical device having a saturable absorption area, hereinafter referred to as an SA (Saturable Absorber), has a saturable absorption effect caused by applying a semiconductor waveguide with a reverse bias, and is also referred to as an optical gate. Specifically, the SA has a nonlinear transmission characteristic dependent on the intensity of applied light, in which incident signal light with a low intensity is greatly absorbed and reduced, whereas incident signal light with a higher intensity causes a reduction in the absorption coefficient and exceeds the absorption capacity of the optical device to be transmitted. For realization of such SA, for example, Hashimoto et al. reported the realization thereof by applying a semiconductor laser amplifier with a reverse bias. (Y. Hashimoto et al. Technical Digest of CPT98, pp 215-216, Jan. 12-14, 1998).
SUMMARY OF THE INVENTION
The above-mentioned conventional signal conversion device has a disadvantage that an output level of converted signal light is low and unstable. In view of this disadvantage, it is an object of the present invention to provide a signal conversion device capable of converting a wavelength of signal light, specifically pulse-type signal light to an arbitrary wavelength and outputting stably the converted signal light.
In order to achieve the above-mentioned object, the signal conversion device of the present invention comprises:
a light source for emitting continuous wave (CW) light at a predetermined wavelength;
an optical element applied with signal light at a wavelength &lgr; and the CW light outputted from the light source and having a threshold value for a saturable absorption area such that the optical element absorbs the light at a light intensity level of either of t
Ishii Satoshi
Michishita Yukio
Lee John D.
NEC Corporation
Scully Scott Murphy & Presser
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