Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1998-07-21
2001-09-25
Leung, Quyen P. (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S019000, C372S023000
Reexamination Certificate
active
06295306
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser light source tunable in wavelength (hereinafter, a semiconductor laser may be simply referred to LD) for obtaining a specific reference wavelength which is used in the field of optical communication, in particular, for WDM (Wavelength Division Multiplex) transmission or the like, and relates to a tunable LD laser light source using a fiber grating as a wavelength selecting means.
2. Description of the Earlier Development
It is known that in a Fabry-Perot type of resonator which has one facet of an LD, having an antireflection coating through which a wavelength selected by using an element having a wavelength selection property is fed back into the LD and which has the other facet for forming an external resonator together with the one facet, a laser oscillation is carried out when the gain condition and the phase condition overcome a loss such as a scattering loss and the like.
As described above, a tunable LD light source is formed by selecting a laser beam having a desired wavelength from the output beam of the laser-oscillating LD, and thereafter by feeding it back into the LD.
Recently, such a tunable LD light source comes to be an indispensable one in the WDM transmission system which attracts public attention as a large capacity transmission technology in an optical communication network, which is required with tendency to use multimedia.
An earlier LD light source tunable in wavelength will be explained with reference to
FIG. 5
, as follows.
FIG. 5
shows an example of construction of an earlier tunable LD light source.
In the earlier example shown in
FIG. 5
, one of the facets of the LD
10
has a cloven surface
11
and the other of the facets has an antireflection coating
12
provided thereon.
The light beam outputting through the side of the antireflection coating
12
of the LD
10
is changed to a parallel beam through a lens
31
. In front of the lens
31
, a total reflection mirror
50
is arranged through a tunable bandpass filter
40
. The total reflection mirror
50
and the cloven surface
11
of the LD, which has a low reflection coating thereon form an external resonator.
In front of the cloven surface
11
of the LD
10
, a lens
33
is arranged. The laser beam transmitted through the lens
33
is transmitted through an optical fiber
60
and is output out of an output port
11
.
Next, the operation of the tunable LD light source shown in
FIG. 5
will be explained, as follows.
The light beam emitted from the side of the antireflection coating
12
of the LD
10
is changed to a parallel beam through the lens
31
, and then is directed to a tunable bandpass filter
40
to transmit only a light beam having a specific wavelength. Thereafter, the light beam is reflected by the total reflection mirror
50
to change the advancing direction thereof 180° and passes through the tunable band pass filter
40
and the lens
31
again, and directs back to (is fed back to) the LD
10
. The light beam directed to the LD
10
is reflected by the cloven surface
11
having a reflectance of several tens of % and is returned to the LD
10
again.
A laser oscillation is carried out in the external resonator which is constituted by the cloven surface
11
and the total reflection mirror
50
. The tunable band pass filter
40
of very narrow bandwidth, i.e., having a bandwidth (FWHM, i.e., Full width half maximum) of the transmission range of about 0.5 nm, is used.
Thus, the light beam obtained by laser oscillation is coupled to the optical fiber
60
through the lens
33
to enter the output port
61
. In such a construction, a tunable LD light source can be made by making the transmission wavelength of the tunable band pass filter
40
changeable.
However, because such an earlier LD light source tunable in wavelength shown in
FIG. 5
used only a tunable bandpass filter
40
as a wavelength selection means and required a narrow bandwidth (FWHM) of the transmission range of the filter. As a result, the cost for the tunable band pass filter
40
was very high and about 0.5 nm was the upper limit of the bandwidth (FWHM) of the transmission range of the tunable band pass filter
40
. According to an earlier tunable LD light source, it was difficult to obtain a laser oscillation having a narrow spectral width.
Because high-precision control for the tilt angle of the tunable band pass filter
40
to the incident light beam and for the temperature thereof were required in order to enhance the repeatability of selected wavelength, such an earlier tunable LD light source had the disadvantages in a productional aspect in that a complicated structure, a high optical axis adjustment technique and a high mounting technique were required.
SUMMARY OF THE INVENTION
The present invention was developed in view of these problems. It is therefore an object of the present invention to provide an LD light source tunable in wavelength, which has a simple structure and a high reliability, and which provides a stable laser oscillation of light having a narrow spectral width and gives a high productivity.
In order to solve the above problem, in accordance with an aspect of the invention, the semiconductor laser light source tunable in wavelength comprising: a semiconductor laser which has one facet having an antireflection coating; a tunable bandpass filter for transmitting a light having a specific wavelength from wavelength components of output light out of the one facet having the antireflection coating of the semiconductor laser; and a fiber grating member for receiving the transmitted light from the tunable band pass filter and for reflecting a light of the specific wavelength.
According to the tunable semiconductor laser light source having such a structure, after selecting a light beam having a specific wavelength from the beam emitted out of the facet having the antireflection coating of the LD, by the tunable band pass filter, the selected light beam is further narrowly selected in wavelength by the fiber grating member and the light beam of narrowed bandwidth progresses backward to be fed back into the LD, so that a laser oscillation is carried out.
Therefore, the tunable semiconductor laser light source of the present invention enables excellent wavelength selection and an oscillation of laser light beam having a narrow spectral width.
The fiber grating has a wavelength selection property of narrower bandwidth, in comparison with the tunable band pass filter in the two wavelength selection elements, and therefore has a high stability in wavelength to temperature change and enables relaxation of control accuracy for the incident angle to the tunable band pass filter and for the temperature thereof. According to the laser light source of the invention, it is possible to set the bandwidth (FWHM) of the transmission range of the tunable band pass filter larger than 0.5 nm, up to about 2 nm. As the result, the tunable semiconductor laser light source according to the invention does not require a complicated structure, a high optical axis adjustment technique, nor a high mounting technique.
In accordance with another aspect of the invention, the semiconductor laser light source tunable in wavelength comprising: a semiconductor laser which has one facet having an antireflection coating; a tunable bandpass filter for transmitting a light having a specific wavelength from wavelength components of output light out of the one facet having the antireflection coating of the semiconductor laser; and a plurality of fiber gratings connected in series, each of which receives transmitted light from the tunable band pass filter and reflects only an individual wavelength different from one another.
The plurality of fiber gratings may be a compound fiber bragg grating in which a plurality of gratings are arranged to connect to one another in series, each of which reflects only an individual wavelength different from one another, from the light transmitted through the band pass filter, and also may
Ando Electric Co. Ltd.
Leung Quyen P.
Pillsbury & Winthrop LLP
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