Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2001-03-27
2004-03-02
Ip, Paul (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S043010
Reexamination Certificate
active
06700904
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a light source for an external cavity laser to be used in the field of optical communication.
2. Description of the Related Art
A light source for an external cavity laser in the related art will be described below with reference to
FIGS. 8
to
11
.
As shown in
FIG. 8
, in the light source for an external cavity laser of the related art, an antireflection coating
15
A is applied onto one facet of a semiconductor laser
15
. A light beam emitted from the facet of the antireflection coating
15
A side is converted into a parallel light beam by a lens
5
. Wavelength selection of the parallel light beam is performed by a diffraction grating
2
. Subsequently, the light beam is returned to the diffraction grating
2
by a mirror
3
to perform wavelength selection again by the diffraction grating
2
and to feed back the light beam to the semiconductor laser
15
to perform laser oscillation.
With regard to the output light beam, the light beam emitted from the other facet of the semiconductor laser
15
is converted into a parallel light beam by lens
6
. After passage through an optical isolator
8
, the light beam is converged by lens
7
into an optical fiber
4
to be taken out.
The system illustrated in
FIG. 8
is called a Rittman type, and since wavelength selection is performed twice by the diffraction grating
2
in the round-trip, it is excellent in wavelength selectivity and is presently known as the most general type of system.
With the arrangement shown in
FIG. 9
, a beam splitter
9
, which takes out a part of the diffracted light beam that is fed back from the diffraction grating
2
to semiconductor laser
15
, is equipped between the semiconductor laser
15
and the diffraction grating
2
of the light source for an external cavity laser of
FIG. 8
, and the diffracted light beam
10
that is taken out by beam splitter
9
is converged and output into optical fiber
11
via an optical isolator
12
and a lens
13
. Since the diffracted light
10
is the light that has just undergone the wavelength selection twice by the diffraction grating
2
in the round-trip, though the optical output will be somewhat lower in comparison to the output of the optical fiber
4
, an extremely pure single-wavelength light beam, which does not contain any of the spontaneous emission components that emitted from semiconductor laser
15
, can be obtained (refer to Japanese Unexamined Patent Publication No.Hei.11-126943).
Meanwhile,
FIG. 10
shows an example where a band-pass filter
14
is used in place of the diffraction grating of the light source for an external cavity laser of FIG.
8
.
In a light source for an external cavity laser, such as those shown in
FIGS. 8
to
10
, though the antireflection coating
15
A that is applied to one facet of the semiconductor laser
15
is essential for forming the external resonator, the other facet is not provided in particular with a coating due to reasons of cost and is left in the form of a cleavage plane (the surface as it is when the element is cleaved; the reflectivity is approximately 32%), as in a general Fabry-Perot laser.
FIGS. 11 and 12
show examples where a total reflection coating
16
B, with a reflectivity of substantially 100%, is applied to one facet of a semiconductor laser
16
at the side on which an antireflection coating
16
A is not applied in order to increase the optical output of the optical fiber
4
or the optical fiber
11
of the examples of the related art in
FIG. 8
or
9
as much as possible.
In the example shown in
FIG. 12
, since the total reflection coating
16
B is used at one facet of the semiconductor laser
16
, the output light beam is obtained as the 0th-order light of the diffraction grating
2
.
Normally in a light source for an external cavity laser, when the efficiency of the resonator drops below that of a Fabry-Perot laser diode, with which both facets are cleavage planes, the optical resonance itself weakens and the laser emission conditions tend to become unstable.
As a most simple indicator for the efficiency of the resonator, the product of the reflectivities of both facets of the resonator may be compared (in the case of an external resonator, the product of the reflectivity of the facet at one side and the feedback efficiency of the external oscillator). In a Fabry-Perot laser diode, since both facets are cleavage planes and the reflectivity of each of the facets is approximately 32% in the case of an element with an emission wavelength of 1550 nm, the product will be:
0.32×0.32≈0.1
In the case of the external resonator type semiconductor laser light source, an antireflection coating is normally applied to one facet of the semiconductor laser, and the other facet of the semiconductor laser is a cleavage plane. In the case of an element for 1550 nm, this cleavage plane is a partially reflecting surface with a reflectivity of approximately 32%. Thus in an external resonator, the semiconductor laser will not undergo emission by itself and laser emission occurs as a result of the feedback of light. The feedback efficiency thus corresponds to the reflectivity of the other facet.
In case of the example shown in
FIG. 8
, if the coupling efficiency of the lens
5
is 50%, the diffraction efficiency of the diffraction grating
2
is 80%, and the reflectivity of the mirror
3
is 95%, thus:
Feedback efficiency≈0.5×(0.8×0.8)×0.95×100=30.4%
It is found that the feedback efficiency is thus lowered to approximately 30%.
Since the product of the reflectivities of both facets of the resonator is thus:
0.32×0.304≈0.097
It can be understood that the efficiency of the resonator is lowered and the emission condition tends to be unstable.
In the example of
FIG. 8
, since the light beam is made to undergo spectral separation twice by the diffraction grating in the round-trip in order to increase the wavelength selectivity, even if the diffraction efficiency is 80%, the efficiency is lowered to 64% in the round-trip. Accordingly, it is difficult to increase the feedback efficiency using the system of FIG.
8
.
Furthermore, in the light source for an external cavity laser of
FIG. 9
, since the beam splitter
9
is provided between the semiconductor laser
15
and the diffraction grating
2
, the light beam passes through the beam splitter
9
twice in addition to being diffracted by the diffraction grating
2
in the round-trip. Thus if the splitting efficiency of the beam splitter is given as 80% transmittance and 20% reflectance, the feedback efficiency is:
0.5×(0.8×0.8)×(0.8×0.8)×0.95×100=19.5%
Thus, the feed back efficiency is lowered to approximately 20%.
Likewise, the product of the reflectivities of the facets of the resonator is:
0.32×0.195≈0.06
Though it is desirable here to improve the coupling efficiency of the lens part, which is considered to be highest in loss, since the emission NA of the semiconductor laser takes on a large value of 0.4 to 0.5 and a cross-sectional area (emission area) of the active layer is only a few &mgr;m, that is extremely small, it is difficult to achieve significant improvements even when an aspherical lens having low aberration, etc. is used.
As shown in
FIG. 10
, the same applies in case where band-pass filter
14
is used. That is, since a band-pass filter
14
, that is narrow in half-width and excellent in wavelength selectivity, is generally high in transmission loss and the feedback efficiency thus tends to be low, it is also difficult to obtain stable oscillation conditions.
Also in the examples shown in
FIGS. 11 and 12
, though the optical output of the optical fiber
4
or the optical fiber
11
may be increased as much as possible by the application of the total-reflection coating
16
B, since oscillation tends to occur readily even with a slight reflection at the facet on which antireflection coating
16
A is applied, a composite resonator tends to be constructe
Ando Electric Co. Ltd.
Nguyen Tuan
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