Optical waveguides – With optical coupler
Reexamination Certificate
1999-12-01
2001-02-20
Ngo, Hung N. (Department: 2874)
Optical waveguides
With optical coupler
C385S024000
Reexamination Certificate
active
06192170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multiple-wavelength light source for use as a light source for a wavelength-division-multiplexing (WDM) optical communication system, and a method of controlling oscillation frequencies of such a multiple-wavelength light source.
2. Description of the Related Art
Heretofore, one conventional multiple-wavelength light source for use as a light source for a WDM optical communication system comprises a multiple-wavelength semiconductor laser as shown in
FIG. 1
of the accompanying drawings.
The conventional multiple-wavelength semiconductor laser, which has been reported by Tanaka et al. in collected preprints for Electronic Communications Society General Conference, 1997, C-3-160, has four spot size convertor integrated lasers (hereinafter referred to as “SS-LD”), four optical waveguides (PLC: Planar Lightwave Circuits) of silica, and diffraction gratings produced by UV photolithography, all integrated on an Si substrate. It has been confirmed that the conventional multiple-wavelength semiconductor laser performs simultaneous oscillation at four wavelengths in a single mode.
More specifically, as shown in
FIG. 1
, the conventional multiple-wavelength semiconductor laser comprises four SS-LDs
102
for emitting laser beams having respective oscillation wavelengths, four optical waveguides
103
for leading the laser beams emitted by SS-LDs
102
to output end face
105
, and four diffraction gratings
104
disposed in the respective optical waveguides
103
and serving as external cavities for SS-LDs
102
. SS-LDs
102
, optical waveguides
103
, and diffraction gratings
104
are integrated on Si substrate
101
. Output end face
105
is coated with an antireflection coating for minimizing reflection of the laser beams. While the conventional multiple-wavelength semiconductor laser shown in
FIG. 1
emits laser beams having four oscillation wavelengths &lgr;1-&lgr;4, it can produce more oscillation wavelengths by adding one or more elemental structures in a parallel arrangement.
Each of SS-LDs
102
has a tapered waveguide for narrowing the radiation angle of the emitted laser beam thereby to reduce an optical coupling loss with optical waveguide
103
. Use of SS-LDs
102
is effective to lower the cost of the multiple-wavelength semiconductor laser because it does not require optical lenses for changing the radiation angles of the emitted laser beams.
Each of diffraction gratings
104
is fabricated by exposing an SiO
2
layer (which will become an upper cladding layer) above the core of optical waveguide
103
to interference UV radiation. Structural details, including a grating pitch and a shape, of diffraction gratings
104
determine the oscillation wavelengths of the laser beams output from output end face
105
.
However, the conventional multiple-wavelength semiconductor laser has been problematic in that since the oscillation wavelengths of the multiple-wavelength semiconductor laser are determined by the fabrication accuracy of the diffraction gratings, the yield of fabricated devices is poor if they are to be used as a light source for a WDM optical communication system because such a light source needs accurate wavelength settings.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a multiple-wavelength light source which allows oscillation wavelengths to be finely adjusted for an increased yield of fabricated devices.
To achieve the above object, a multiple-wavelength light source in accordance with the present invention has a plurality of heaters disposed near respective diffraction gratings for controlling the temperatures of the diffraction gratings. The temperatures of the respective heaters are controlled in a feedback loop in order to output maximum optical power levels at desired wavelengths. At this time, photodetectors output voltages depending on the optical power levels at the desired wavelengths, and a heater current control circuit supplies currents to the heaters in order to maximize the output voltages from the photodetectors.
Currents injected into the semiconductor laser devices may be modulated with different frequencies, and modulation frequency components are extracted from output signals from the photodetectors by electric filters. Currents are supplied to the heaters in order to maximize voltages of the extracted modulation frequency components.
In the multiple-wavelength light source, when the ambient temperature of the diffraction gratings is varied by the heaters disposed near the diffraction gratings, the oscillation frequencies of the semiconductor laser devices fluctuate because the refractive index of cores of optical waveguides beneath the diffraction gratings also varies. By supplying currents to the heaters in order to maximize the output voltages of the photodetectors, the oscillation frequencies of the semiconductor laser devices can accurately be controlled at desired frequencies.
Currents injected into the semiconductor laser devices may be modulated with different frequencies, and modulation frequency components are extracted from output signals from the photodetectors by electric filters. Currents are supplied to the heaters in order to maximize voltages of the extracted modulation frequency components for thereby controlling the oscillation frequencies of the semiconductor laser devices accurately at desired frequencies.
The above and other objects, features, and advantages of the present invention will become apparent from the following descriptions with reference to the accompanying drawings which illustrate examples of the present invention.
REFERENCES:
patent: 5396506 (1995-03-01), Ball
patent: 5550666 (1996-08-01), Zirngibl
patent: 5870512 (1999-02-01), Koch et al.
patent: 6061158 (2000-05-01), Delong
patent: 9-172429 (1997-06-01), None
patent: 10-48440 (1998-02-01), None
patent: 10-68833 (1998-03-01), None
T. Tanaka et al; Integrated External Cavity Laser COmposed of Spot-Size COnverted LD and UV Written Grating in Silica Waveguide on Si, Electron Letters; 1995; vol. 32; No. 13, pp. 1202-1203.
T. Tanaka et al, A Wavelength Hybrid Laser Array Composed of Spot-Size Converter Integrated LD and UV Written Grating in Silica Waveguide on Si, Electronic COmmunications Society General Conference; 1997; C-3-160; p. 345.
H. Kobayashi et al, Tapered Thickness MQW Waveguide BH MQW Lasers, Electronics Communications Society General Conference; 1995; SC-4-4, pp. 463-464.
T. Sugie et al, 1.3 um LD with Butt-Jointed Selectively Grown Spot-Size Converter, Electronics Communications Society General Conference; 1995; SC-4-5, pp. 465-466.
A. Takemoto et al, Title unreadable, Mitsubishi Denki Giho; vol. 71; No. 3; 1997, pp. 59-62.
S. Suzuki, Title unreadable, Japanese Journal of Optics; vol. 26; No. 8; 1997, pp. 418-423.
S. Suzuki, Silica-based Planar Lightwave Circuits, Ceramics Japan; vol. 32; No. 8; 1997, pp. 613-617.
Hayes, Soloway, Hennessey Grossman & Hage, P.C.
NEC Corporation
Ngo Hung N.
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