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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present Invention relates to a wavelength-variable semiconductor laser
light source for optical coherent communication, which varies the
oscillation wavelength of the semiconductor laser while continuing the
phase of oscillated light.
2. Background Art
FIG. 5 is a block diagram showing a conventional wavelength-variable
semiconductor laser light source. In FIG. 5, reference numeral 1 indicates
a semiconductor laser, reference numeral 1A indicates an antireflection
film, reference numerals 2A and 2B indicate lenses, reference numeral 5
indicates a diffraction grating, reference numeral 6 indicates a rotating
stage, reference numeral 7 indicates a parallel sliding stage, reference
numeral 8 indicates an arm, reference numeral 9 indicates a fixed plate,
reference numeral 11 indicates a wavelength setting part, reference
numeral 12 indicates a comparator, reference numeral 13 indicates a
parallel sliding driver, and reference numeral 16 indicates a displacement
gauge.
In the arrangement of FIG. 5, one end face of semiconductor laser 1 is
coated with antireflection film 1A. From the end face with the
antireflection film, outgoing light 3B is outputted. The outgoing light 3B
is transformed into collimated light by lens 2B and is incident on the
central part of diffraction grating 5. At this time, the central part of
the diffraction grating 5 and the other end face without an antireflection
film of the semiconductor laser 1 form an external resonator.
Semiconductor laser 1 oscillates at a single mode and outputs outgoing
light 3A from the other end Face.
Here, diffraction grating 5 is fixed on rotating stage 6 and the rotating
stage 6 is provided on parallel sliding stage 7 which moves in parallel
with the optical axis of semiconductor laser 1. Furthermore, the rotating
stage 6 is in contact with fixed plate 9 via arm 8. Therefore, arm 8
slides on the plane of fixed plate 9, whereby the parallel motion of
parallel sliding stage 7 is transformed into the rotational motion of
rotating stage 6; thus, the oscillation wavelength of semiconductor 1 is
varied under phase-continuous conditions by way of the parallel movement
of the parallel sliding stage 7.
Wavelength setting part 11 sets up the oscillation wavelength of
semiconductor 1. Displacement gauge 17 detects the amount of the parallel
displacement of parallel sliding stage 7 and outputs a displacement signal
which corresponds to the amount of the parallel displacement. Comparator
10 compares a set signal from wavelength setting part 9 and the
displacement signal from displacement gauge 16 and outputs a control
signal to parallel sliding driver 13 in accordance with the result of the
comparison. In this way, the oscillation wavelength of semiconductor laser
1 is set arbitrarily within the resolution of the displacement gauge under
phase-continuous conditions.
On the other hand, outgoing light 3A from the other end face without an
antireflection film of semiconductor laser 1 is transformed into
collimated light via lens 2A; and the collimated light becomes an output
light from the wavelength-variable semiconductor laser light source.
The set resolution of the oscillation wavelength of the semiconductor laser
1 is limited by the resolution of the rotation angle of the diffraction
grating which acts as an external mirror of the semiconductor laser; thus,
in order to raise the set resolution of the oscillation wavelength, it is
necessary to raise the resolution of the rotation angle of the diffraction
grating.
However, in the arrangement of FIG. 5, the rotation angle of the
diffraction grating 5 is calculated in accordance with the amount of the
parallel displacement of the parallel sliding stage 7; thus, the set
resolution of the oscillation wavelength of the semiconductor laser is
limited by the resolution of the displacement gauge 16 which detects the
amount of the parallel displacement of the parallel sliding stage.
The relationship between quantity .DELTA.X of the parallel displacement of
parallel sliding stage 7 and oscillation wavelength .lambda. of the
semiconductor laser 1 is represented by the following formula, in which d
is the interval of the grooves of the diffraction grating, .theta. is the
initial angle of tile diffraction grating, .DELTA..theta. is the angle of
the rotation of the diffraction grating, L is the initial length of the
external resonator, and m is the order of the oscillation mode.
.lambda.=2.multidot.d.multidot.sin(.theta.+.DELTA..theta.)=2(L+.DELTA.X)/m
FIG. 4 shows an example result of the calculation in accordance with the
above formula, in which interval d of the grooves of the diffraction
grating is 1/1200 mm, initial angle .theta. is 68.4.degree., initial
length L of the external resonator is 36.27 mm, order m of the oscillation
mode is 46800. According to FIG. 4, a displacement gauge which can detect
the amount of the parallel displacement of the parallel sliding stage 7
with the resolution of 10 nm or less is needed in order to make the set
resolution of the oscillation wavelength of semiconductor laser 1 to be
0.4 pm (picometer).
However, the resolution of the presently-obtainable displacement gauge
regarded as one having the highest resolution, which gauge is of a strain
gauge-type or a differential transformer-type, is 20 nm at best.
Therefore, a problem occurs in that the oscillation wavelength of the
semiconductor laser 1 cannot be set up with a resolution of 0.8 pm or
less, the value (0.8 pm) corresponding to the resolution (20 nm) of the
displacement gauge 16.
On the other hand, the displacement gauge based on an interference method
by using, for example, a Michelson interferometer can have a resolution of
20 nm or less. However, in this case, the system for constructing the
gauge is complicated; thus, it is not practical to use this type of gauge
for the wavelength-variable semiconductor laser light source in which the
oscillation wavelength of the semiconductor laser is set with high
resolution under phase-continuous conditions.
SUMMARY OF THE INVENTION
Therefore, the purpose of the present invention is to provide a
wavelength-variable semiconductor laser light source in which the
oscillation wavelength of the semiconductor laser can be set up at higher
resolution under phase-continuous conditions.
Therefore, the present invention provides a wavelength-variable
semiconductor laser light source comprising: a semiconductor laser whose
one end face is coated with an antireflection film; a parallel sliding
stage which moves in parallel with the optical axis of the semiconductor
laser by a parallel sliding driver; a beam splitter fixed on the parallel
sliding stage, for splitting outgoing light from the end face coated with
the antireflection film into transmitted light and reflected light; a
diffraction grating fixed on a rotating stage which is provided on the
parallel sliding stage, the diffraction grating for inputting the
transmitted light from the beam splitter so as to form an external
resonator with the other end face of the semiconductor laser; a
transformation mechanism for transforming the parallel motion of the
parallel sliding stage into the rotational motion of the rotating stage; a
lens for expanding the reflected light From the beam splitter; a PSD for
inputting the expanded light from the lens and outputting an electric
signal which is proportional to an amount of the displacement of the
expanded light the displacement being caused by the parallel movement of
the parallel sliding stage; a wavelength setting part for arbitrarily
setting the oscillation wavelength of the semiconductor laser; and a
comparator for comparing a set signal from the wavelength setting part
with the electric signal from the PSD and feeding the result of the
comparison back to the parallel sliding driver.
The wavelength-variable semiconductor light source described above may
further comprise a total reflection mirror for reflecting diffracted light
from the diffraction grating so as to return the light to the
semiconductor laser.
In addition, the wavelength-variable semiconductor light source may provide
a light source, fixed on the parallel sliding stage, for outputting light
to the PSD. In this case, the beam splitter is not provided.
In the case of providing the beam splitter, outgoing light from the end
face coated with an antireflection film of the semiconductor laser is
split into transmitted light and reflected light by the beam splitter
which is fixed on the parallel sliding stage, and the reflected light from
the beam splitter is expanded by the lens to be incident on the PSD. On
the other hand, in the case of providing the light source, outgoing light
from the light source is expanded by the lens to be incident on the PSD.
In the both cases, the oscillation wavelength of semiconductor laser 1 is
set arbitrarily with high resolution under phase-continuous conditions by
utilizing the detection characteristics with high resolution of the PSD.
BRIEF EXPLANATION OF THE DRAWINGS
FIG. 1 is a block diagram showing the wavelength-variable semiconductor
laser light source according to an embodiment of the present invention.
FIG. 2 is a block diagram showing the wavelength-variable semiconductor
laser light source according to another embodiment of the present
invention.
FIG. 3 is a block diagram showing the wavelength-variable semiconductor
laser light source according to the third embodiment of the present
invention.
FIG. 4 is a chart explaining the relationship between quantity .DELTA.X of
the parallel displacement and oscillation wavelength .lambda. of the
semiconductor laser.
FIG. 5 is a block diagram showing a conventional wavelength-variable
semiconductor laser light source.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 is a block diagram showing the wavelength-variable semiconductor
laser light source according to an embodiment of the present invention.
In FIG. 1, referencc numerals 2C and 2D indicate lenses, reference numeral
4 indicates a beam splitter which is fixed on parallel sliding stage 7,
and reference numeral 10 indicates a PSD, i.e., position sensitive device,
for example, B4243 type with long light-receptive plane, which is
available from Hamamatsu Photonics, Inc. Other parts in FIG. 1 are the
same as those shown in FIG. 5.
In FIG. 1, semiconductor laser 1 outputs outgoing light 3B from the end
face with antireflection film 1A. The output light 3B is transformed into
collimated light by lens 2B and is split into transmitted light 3C and
reflected light 3D by beam splitter 4. The transmitted light 3C from the
beam splitter 4 is input to the central part of diffraction grating 3. At
this time, an external resonator is formed by the central part of the
diffraction grating 5 and the other end face without an antireflection
film of semiconductor laser 1; thus, the semiconductor laser 1 oscillates
at a single mode and emits outgoing light 3A from the other end face
without an antireflection film.
Diffraction grating 5 is fixed on the rotating stage 6 and the rotating
stage 6 is provided on parallel sliding stage 7 which moves in parallel
with the optical axis of semiconductor laser 1. Furthermore, the rotating
stage 6 is in contact with fixed plate 9 via arm 8. The arm 8 slides on
the plane of fixed plate 9, whereby the parallel motion of parallel
sliding stage 7 is transformed to the rotational motion of rotating stage
6; thus, the oscillation wavelength of semiconductor 1 is varied under
phase-continuous conditions by way of the parallel movement of the
parallel sliding stage 7.
On the other hand, reflected light 3D from the beam splitter 4 is
transmitted through lenses 2C and 2D to be incident on PSD 10. At this
time, the reflected light 3D which forms an image on PSD 10 moves on the
PSD by an amount which is proportional to the amount of parallel movement
of parallel sliding stage 7; thus, PSD 10 outputs an electric signal which
is proportional to the amount of the parallel displacement of the parallel
sliding stage 7. Here, the relationship between quantity .DELTA.X of the
parallel displacement of parallel sliding stage 7 and quantity .DELTA.y of
the displacement of the reflected light 3D on the PSD is represented by
formula .DELTA.y=m.multidot..DELTA.X , in which m is the longitudinal
magnification (=b/a; see FIG. 1) of lens 2D.
According to the above formula, it is possible to enlarge the quantity
.DELTA.X of the parallel displacement of parallel sliding stage 7 by
setting a large value as the longitudinal magnification m. Therefore, by
enlarging the longitudinal magnification m within a range in which the
diameter of the beam spot of the reflected light 3D which forms an image
on PSD 10 does not protrude from the effective area for light reception of
PSD 10, the quantity .DELTA.X of the parallel displacement of parallel
sliding stage 7 can be detected with higher resolution. In addition, lens
2D may be substituted to a lens assembly constituted of 2 or more lenses.
Wavelength setting part 11 sets up the oscillation wavelength of
semiconductor 1. Comparator 12 compares a set signal from wavelength
setting part 11 and the electric signal from PSD 10 and outputs a control
signal to parallel sliding driver 13 in accordance with the result of the
comparison. In this way, the oscillation wavelength of semiconductor laser
1 is set arbitrarily with high resolution under phase-continuous
conditions.
On the other hand, outgoing light 3A from the other end face without an
antireflection film of semiconductor laser 1 is transformed into
collimated light via lens 2A; and the collimated light becomes an output
light from the wavelength-variable semiconductor laser light source.
Hereinbelow, a practical example of the operation of the
wavelength-variable semiconductor laser light source in FIG. 1 will be
explained.
In a case in which the diameter .phi. of the effective area for light
reception of PSD 10 is 3 mm, the minimum resolution of the PSD is 0.2
.mu.m, the emission range of semiconductor laser 1 is 3 .mu.m, and lenses
2B and 2C are of the same type, if the longitudinal magnification m of
lens 2D is set as "20", the resolution of the parallel displacement of
parallel sliding stage 7 is 10 nm. In this case, the set resolution of the
wavelength-variable semiconductor laser source of 0.4 pm can be obtained,
as shown in FIG. 4.
In addition, if the longitudinal magnification m of lens 2D is set as "40",
the resolution of the parallel displacement of parallel sliding stage 7
becomes 5 nm. In this case, the set resolution of the wavelength-variable
semiconductor laser source of 0.2 pm can be obtained, as shown in FIG. 4.
According to the emission range of semiconductor laser 1 of 3 .mu.m and the
diameter .phi. of the effective area for light reception of PSD 10 of 3
mm, it is possible to enlarge the longitudinal magnification m up to
"1000" at the most.
Next, FIG. 2 is a block diagram showing the wavelength-variable
semiconductor laser light source according to another embodiment of the
present invention.
In FIG. 2, reference numeral 14 indicates a total reflection mirror, and
other parts in FIG. 2 are the same as those shown in FIG. 1.
In FIG. 2, semiconductor laser 1 outputs outgoing light 3B from the end
face with antireflection film 1A. The output light 3B is transformed into
collimated light by lens 2B and is split into transmitted light 3C and
reflected light 3D by beam splitter 4. The transmitted light 3C from beam
splitter 4 is input to the central part of diffraction grating 5 and
diffracted light 3E is incident on total reflection mirror 14. At this
time, an external resonator with length L is formed by the total
reflection mirror 14 and the other end face without an antireflection film
of semiconductor laser 1; thus, the semiconductor laser 1 oscillates at a
single mode and emits outgoing light 3A from the other end face without an
antireflection film.
In the arrangement shown in FIG. 2, the-filtering characteristic of
diffraction grating 5 is improved by inputting the diffracted light 3E
from diffraction grating 5 to the diffraction grating 5 again. Other parts
in the arrangement are the same as those shown in FIG. 1, that is, the
oscillation wavelength of semiconductor laser 1 is set arbitrarily with
high resolution under phase-continuous conditions by expanding reflected
light 3D from beam splitter 4 via lens 2D and detecting the amount of
parallel displacement of parallel sliding stage 7 with high resolution.
Next, FIG. 3 is a block diagram showing the wavelength-variable
semiconductor laser light source according to the third embodiment of the
present invention.
In FIG. 3, reference numeral 15 indicates a light source and other parts in
FIG. 3 are the same as those shown in FIG. 1. That is, FIG. 3 shows an
arrangement in which special light source 15, light from which is incident
on PSD 10, is provided. Outputted light 3F from the light source 15 is
expanded by lens 2D and is incident on PSD 10, and the amount of the
parallel movement of parallel sliding stage 7 is detected with high
resolution, by which the oscillation wavelength of semiconductor laser 1
is set arbitrarily with high resolution under phase-continuous conditions.
In addition, a semiconductor laser or an LED light source can be used as
light source 15.
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