Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-05-22
2001-02-27
Negash, Kinfe-Michael (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S326000, C372S027000, C372S096000
Reexamination Certificate
active
06195188
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength division multiplex communication network in the technical field of optical communications and an optical wavelength conversion apparatus and a method usable in optical wavelength division multiplex communications and so forth, and particularly, to an optical wavelength conversion apparatus and a method for directly converting a wavelength of an optical signal into another as the optical signal is maintained.
2. Related Background Art
Conventionally, in a distributed Bragg reflector (DBR) type semiconductor with separate multi-electrodes, there has been proposed an optical wavelength conversion apparatus in which part of its active layer is used as a saturable absorber region (see Kondo et al., “Giga-bit Operation of Wavelength Conversion Laser”, 1990 International Topical Meeting on Photonic Switching, 13D-9, 1990).
FIG. 1A
shows the structure of that optical wavelength conversion apparatus. In
FIG. 1A
, reference numeral
521
designates a first active region, reference numeral
522
designates a saturable absorber region, reference numeral
523
designates a second active region, reference numeral
524
designates a phase adjusting region and reference numeral
525
designates a DBR region.
FIG. 1B
shows the operation of the optical wavelength conversion apparatus in FIG.
1
A. Its horizontal and vertical axes respectively indicate an injection current into the second active region
523
and the intensity of output light. It is seen therefrom that the current in its oscillation state obtained by increasing the injection current is larger than the current in its non-oscillation state obtained by decreasing the injection current from the oscillation state. As illustrated in
FIG. 1B
, when the injection current is set at a bias point A, no output light is emitted when no input light is input into the apparatus. When the input light is input into the apparatus, the light absorption coefficient of the saturable absorber region
522
is reduced and the laser hence reaches the oscillation state, emitting the output light. A wavelength of the output light can be varied by controlling currents injected into the phase adjusting region
524
and the DBR region
525
, so that the wavelength of the input light can be converted into a desired wavelength of the output.
In that optical wavelength conversion apparatus, since the required time within which the saturable absorber region
522
returns to its initial state after the wavelength conversion operation is dominated by the carrier injection time, the modulation speed of an optical signal for the wavelength conversion operation is normally limited to the order of nanoseconds and high-speed operation is thus impossible.
In order to solve the above problem, there has also been proposed another optical wavelength conversion apparatus which uses the oscillation of light in two mutually-perpendicular polarization modes in a semiconductor laser to convert the wavelength (see Japanese Patent Application Laid-open No. 6(1994)-120595).
FIG. 2
illustrates the structure of this optical wavelength conversion apparatus. In
FIG. 2
, a predetermined polarization mode (which corresponds to one polarization mode of light emitted from a semiconductor laser
601
) of input light
612
is selected by a polarization beam splitter device
608
, and the selected one is input into the semiconductor laser
601
through a lens
609
. The semiconductor laser
601
forms an external resonator cavity between its input facet and a mirror
652
and attains the laser oscillation at a desired wavelength with the selected polarization mode (this desired wavelength is picked up by a wavelength filter
662
with a voltage terminal
672
for controlling the wavelength). In contrast thereto, where the input light
612
does not contain the desired wavelength, the external cavity is formed between the input facet of the semiconductor laser
601
and another mirror
651
, and the laser oscillation occurs in the other polarization mode and at a given wavelength selected by a wavelength tunable filter
661
.
As discussed above, output light
613
can be obtained when the input light
612
does not contain the desired wavelength. Here, since the oscillation wavelength can be controlled by the wavelength tunable filter
661
, as a result, the wavelength of the input light
612
can be converted into the desired wavelength of the output light
613
. Though the signals of the input light
612
and the output light
613
are in an inverted relationship with each other, the output light
613
can be readily returned to an original signal of the input light
612
. Further, in
FIG. 2
, reference numeral
602
designates an anti-reflection coating provided on one output facet of the semiconductor laser
601
, reference numeral
603
designates an electrode for a current supply, reference numeral
604
designates a polarization beam splitter device for separating two mutually-perpendicular polarization modes emitted from the semiconductor laser
601
, reference numeral
610
designates a lens for guiding the light emitted from the semiconductor laser
601
to the polarization beam splitter device
604
, and reference numeral
611
designates an output lens for receiving output light
613
from mirror
651
.
In that optical wavelength conversion apparatus, however, wavelength tunable filters for each polarization mode are needed, as well as mirrors for constructing the respective external cavities. The large number of optical elements makes the structure complicated.
SUMMARY OF THE INVENTION
A first object of the present invention is therefore to provide an optical wavelength conversion apparatus in which high-speed operation is possible, the number of elements is small and the structure is simple. Another object of the invention is to provide an optical wavelength conversion method and a system using the optical wavelength conversion apparatus.
Another object of the present invention is to provide an optical wavelength conversion apparatus with a wide operation wavelength band which can perform wavelength conversion even when its input light receives insufficient gain from its semiconductor laser.
The present invention is generally directed to an optical wavelength conversion apparatus including a polarization switchable semiconductor laser whose oscillation polarization mode is switchable between two independent polarization modes of different wavelengths, a first mode selecting or control portion for selecting or producing light in the same polarization mode as one of the switchable polarization modes for inputting of the selected or produced light to the semiconductor laser, and a second mode selecting or control portion for selecting or producing light in the other polarization mode from light emitted from the semiconductor laser.
Specifically, the semiconductor laser may be a semiconductor laser which includes a periodical structure therein and an active layer with a nearly equal gain for the two independent polarization modes (such as a multi-electrode distributed feedback (DFB) semiconductor laser and a multi-electrode distributed Bragg reflector (DBR) semiconductor laser), and at least one of the first and second mode selecting or control portions may be a polarizer, a polarization maintaining fiber or the like. Further, the multi-electrode distributed feedback semiconductor laser may include a phase shift region.
In the above structure, when an injection current overs its threshold, the laser oscillates with one of the two polarization modes. Here, the oscillation polarization mode is determined by nonuniformity of the injection current, a slight difference between gains for the two polarization modes, polarization dependencies of the periodic structure or diffraction grating and the waveguide and so forth. The other polarization mode oscillation is suppressed and would not normally occur. However, if the input light has the same polarization as the suppressed
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Negash Kinfe-Michael
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