Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-12-21
2001-09-04
Pascal, Leslie (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06285477
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a multi-wavelength light source and a discrete-wavelength-variable light source, and more particularly, to a multi-wavelength light source for supplying one or more optical outputs with different wavelengths concurrently or selectively and a discrete-wavelength-variable light source capable of selecting one of a plural of wavelengths, which are suitable for transmission or tests of a wavelength division/multiplex transmission system.
BACKGROUND OF THE INVENTION
In wavelength division multiplex transmission systems, it is essential to reliably obtain laser lights with a number of close wavelengths. For transmission tests or tests of optical components used in wavelength division/multiplex transmission systems, there is the need for a laser light source highly stable in wavelengths and outputs.
ITU has recommended 0.8 nm (100 GHz) as the wavelength interval in wavelength division multiplex transmission systems. While temperature coefficients of wavelength changes of semiconductor lasers are approximately 0.1 nm/° C. That is, semiconductor lasers are very sensitive to temperature fluctuation. Therefore, it is difficult to maintain wavelength intervals of 0.8 nm in a number of semiconductor laser light sources over a long period. Moreover, in ordinary laser sources, injected current is used to stabilize optical outputs. Control current for stabilization of optical outputs causes changes in temperature, and it results in changes in wavelength. That is, control of optical outputs affects wavelengths, and makes it difficult to stabilize wavelengths.
A prior proposal to cope with the problem is to connect an optical filter and an optically amplifying element in a ring to form a multi-wavelength light source for collectively supplying multiple wavelengths.
FIG. 15
is a schematic block diagram showing a prior example A Fabry-Perot optical filter
210
, erbium-doped optical fiber amplifier
212
and optical fiber coupler
214
are connected to form a ring.
FIG. 16
show characteristic diagrams of the prior example of FIG.
15
. FIG.
16
(
1
) shows transparent wavelength characteristics of the Fabry-Perot optical filter
210
, FIG.
16
(
2
) shows amplifying characteristics of the optical fiber amplifier
212
, and FIG.
16
(
3
) shows the spectral waveform of output wavelength. The Fabry-Perot optical filter
210
is a kind of wavelength selecting optical filters having wavelength transparent characteristics which permit specific wavelengths in certain wavelength intervals called FSR (Free Spectral Range) to pass through as shown in FIG.
16
(
1
). Individual transparent wavelengths of the Fabry-Perot optical filter
210
are selected from the spontaneous emission light generated in the optical fiber amplifier
212
. The output spectral waveform coincides with that obtained by multiplying the transparent wavelength characteristics of the optical filter
210
by the amplifying characteristics of the optical fiber amplifier
212
. Theoretically, laser oscillation outputs are obtained in wavelengths where the gain of the optical fiber amplifier
212
surpasses the loss of the optical loop.
In the prior art example shown in
FIG. 15
, the output intensity is large near the gain center wavelength within the amplifying range of the optical fiber amplifier
212
, where oscillation is most liable to occur, and largely decreases in peripheral portions, as shown in FIG.
16
(
3
). That is, the prior art example cannot realize simultaneous oscillation in multiple wavelengths in substantially uniform output levels.
Moreover, wavelength interval in output light in the prior art example exclusively depends on transparent characteristics of the Fabry-Perot optical filter
210
. When the wavelength interval is 0.8 nm (100 GHz), the wavelength interval FSR of the transparent wavelength characteristics of the Fabry-Perot optical filter
210
is less than the uniform extension width of the erbium-doped optical fiber amplifier
212
. Therefore, even when a plurality of oscillation wavelengths are obtained near the gain center wavelength of the erbium-doped optical fiber amplifier
212
, mode competition occurs, and results in unstable output intensities and wavelength fluctuations of respective wavelengths.
A Fabry-Perot semiconductor lasers is a multi-wavelength light source, other than the fiber ring light source. However, it involves unacceptable fluctuations in oscillation wavelengths due to mode competition or mode hopping, and fails to uniform intensities of respective oscillated wavelength components.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a multi-wavelength light source and a discrete-wavelength-variable light source capable of simultaneously or selectively outputting one or more wavelengths with a uniform intensity.
Another object of the invention is to provide a multi-wavelength light source capable of selecting one or more wavelengths among a plurality of wavelengths.
Another object of the invention is to provide a multi-wavelength light source and a discrete-wavelength-variable light source immune to temperature fluctuations.
The invention uses a wavelength demultiplex/amplify/multiplexing unit for demultiplexing input light into a plurality of different predetermined wavelengths, optically amplifying individual wavelengths, and multiplexing the wavelengths, and connects its output to its input to form an optical loop. Since individual wavelengths are optically amplified by the wavelength demultiplex/amplify/multiplexing unit, laser oscillation in a plurality of wavelengths with substantially the same intensity is promised in the optical loop. Since the structure is simple and the most elements are passive ones, it is highly stable against temperature fluctuations.
When using wavelength demultiplexing means for demultiplexing input light into a plurality of predetermined wavelengths in predetermined wavelength intervals, the light containing multiple wavelengths in substantially constant wavelength intervals can be obtained. Usable as the wavelength demultiplexing means is a waveguide-type wavelength selecting filter, for example.
When using the optical band pass filter means which is transparent only to light within a predetermined wavelength band, the light source can prevent that light beyond the desired wavelength band circulates in the optical loop. This contributes not only to stabilization of laser oscillation but also to reliably preventing that the output contains undesirable wavelengths.
By using optical modulation means for intensity-modulating circulating light in the optical loop with a modulation signal having a frequency, which is an integer multiple of the circulation frequency (namely, c
L) in the optical loop, the light source can conjoin multiple-wavelength light into pulsating light synchronous with the modulation signal. Location of the optical modulation means may be either posterior to wavelength division or posterior to wavelength multiplexing. When it is located after wavelength division, fine adjustment of individual wavelengths is easier, but a plurality of optical modulating means for individual wavelengths must be used. When it is located after wavelength multiplexing, optical modulating means may be only one, but adjustment of individual wavelengths must be done in another portion. Polarization adjusting means may be provided in the input side of the optical modulating means to previously adjust polarization so as to ensure optimum operations of the optical modulating means. If, of course, necessary means is of a polarization holding type, polarization adjusting means may be omitted to reduce elements.
When individual optically amplifying means are capable of selectively supplying or blocking outputs to the wavelength multiplexing means, multiplex output light containing one or more selected wavelengths can be obtained. If each of the optically amplifying means comprises an optical amplifier for amplifying corresponding one of optical outputs from the wavelength demultiplex
Edagawa Noboru
Miyazaki Tetsuya
Yamamoto Shu
Christie Parker & Hale LLP
Kokusai Denshin Denwa Kabushiki Kaisha
Pascal Leslie
Phan Hanh
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