Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2000-12-19
2003-07-15
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C250S226000
Reexamination Certificate
active
06594289
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a tunable laser source apparatus that can vary an oscillating wavelength used in the field of optical communications and precision measurements, and in particular, to a tunable laser source apparatus that can continuously sweep the oscillating wavelength over a wide band using an optical amplifying function as provided by a semiconductor laser, which covers a range of wavelengths over a wide band (hereafter referred to as an “LD”).
Various optical parts such as an optical fiber amplifier, an optical filter, and an optical isolator as well as each device constituting a transmission system which are all used for wavelength multiplexed communication must have their wavelength band characteristics measured.
Thus, a tunable laser source apparatus for a wide band is required which provides light of a predetermined wavelength.
In this case, the ability to continuously sweep the wavelength is desirable.
A tunable laser source apparatus called an “external cavity laser” has been spread which passes light from an optical amplifying element such as an LD which has a wide gain band, through a wavelength selecting element such as a diffraction grating which is arranged outside the element, to feed back light of a desired wavelength band, thereby causing laser oscillation within that wavelength band.
In this case, the most commonly used wavelength selecting element is a diffraction grating.
That is, a selected wavelength is varied by varying the angle of the diffraction grating relative to an incidence direction of light.
FIG. 8A
is a view useful in explaining the configuration of a tunable laser source apparatus of this kind, that is, a typical external cavity laser using a diffraction grating.
In addition,
FIGS. 8B
,
8
C,
8
D, and
8
E are views useful in explaining the principle of wavelength determination.
That is, an external cavity laser such as that shown in
FIG. 8A
comprises an LD
51
with an anti-reflection film (hereafter referred to as an “AR coat) applied to one end surface
51
a,
lasers
52
a
and
52
b,
and a diffraction grating
53
arranged on the AR-coated end surface
51
a.
The diffraction grating
53
is capable of rotation and translation.
The diffraction grating
53
and the other surface (the end surface that is not AR-coated) of the LD
51
b
constitute an external cavity.
Such an arrangement of the diffraction grating
53
that light from the LD
51
is diffracted directly to the LD
51
by the diffraction grating
53
, which receives the light, so that the light has a selected wavelength, is called a “Littrow mounting”.
Regardless of the use of the Littrow mounting, the oscillating wavelength of an external cavity laser including a wavelength selecting element is determined by two factors.
One of them is a wavelength that meets resonance conditions determined by the optical length of the entire resonator that causes laser oscillation.
In an optical resonator such as that shown in
FIG. 8B
, the optical length of the entire resonator (hereafter referred to as the “resonator length”) is denoted by L, the frequency of incident light is denoted by &ugr;, the power of incident light is denoted by P
0
, and the power of emitted light is denoted by P
1
.
As is well known, when the light speed is denoted by c, the free spectral range (hereafter referred to as the “FSR”) is expressed by:
(
FSR=c/
(2
L
).
As shown in
FIG. 8C
, for each FSR, there are a plurality of resonance frequencies at which transmittance (the power of emitted light P
1
/the power of incident light P
0
) is maximized.
When a resonance frequency is n times as large as the FSR, this frequency is called an “n order mode”.
Here, a wavelength corresponding to such a resonance frequency is called a “resonance wavelength”.
The other is the distribution of a gain with its band limited by a diffraction grating such as that shown in
FIG. 8D
or a general wavelength selecting element.
If the optical amplifying element such as the LD which has a gain over a wide band is used, the gain of the diffraction grating within a selected wavelength band is constant.
Thus, the distribution of the gain with its band limited may be considered to be identical to a selected wavelength spectrum of the diffraction grating.
Accordingly, a peak wavelength of the selected wavelength spectrum is hereafter simply called a “selected wavelength”.
Then, one of the modes which is located at a frequency having the highest gain starts to oscillate as shown in FIG.
8
E.
In general, the selected wavelength does not equal the oscillating wavelength.
FIGS. 9A
,
9
B,
9
C,
9
D, and
9
E shows variations in oscillating wavelength observed when the change rates of the resonance and selected wavelengths are not equal.
When a resonator length L and an incident angle &thgr; at which light is incident on the diffraction grating, both of which are schematically shown in
FIG. 9E
, are progressively reduced, the resonance and selected wavelengths shift toward a short wavelength side.
At this point, if a difference corresponding to the half of the FSR occurs between the resonance wavelength of the oscillating mode and the selected wavelength, the oscillating wavelength shifts from the oscillating mode to the adjacent one in such a manner that the state in
FIG. 9C
shifts to the state in FIG.
9
D.
This phenomenon is called a “mode hop” or “mode jump”.
Thus, to continuously vary the oscillating wavelength over a wide band, the oscillating resonance wavelength and the selected wavelength are linked together, that is, in the Littrow-mounting external-resonance laser, the resonator length and the angle of the diffraction grating are simultaneously varied while being maintained in an appropriate relationship, to restrain the mode hop.
Furthermore, in the tunable laser source apparatus having the external cavity structure such as the diffraction grating, the reflectivity on the external cavity side of the LD must be reduced to restrain internal modes of the LD.
Thus, a configuration with an AR coat comprising a dielectric film, that is, a configuration such as that shown in
FIG. 8A
which has the AR coat applied to the end surface
51
a
of the LD
51
is conventionally used.
The configuration with the AR coat simply applied to the end surface
51
a
of the LD
51
, however, provides an insufficient achievable reflectivity, and significant internal modes result from residual reflectivity.
Thus, such a configuration has the following adverse effects: the mode hop occurs as descried above, which may lead to multimode oscillation, and the variable wavelength band is insufficient, thereby increasing spontaneous emitted radiation.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a tunable laser source apparatus that is developed in view of the above problems, that can sufficiently restrain internal modes of an LD to reduce spontaneous emitted radiation in order to prevent multimode oscillation, and that can extend a wavelength varying range.
To attain the above object, according to one aspect of the present invention, there is provided a tunable laser source apparatus including an external cavity, the apparatus comprising:
a semiconductor laser including a reflection surface formed on one end, a surface with an anti-reflection film formed on the other end, and an active layer extending from the reflection surface toward the surface with the anti-reflection film; and
wavelength selecting means for selecting from laser light emitted from the semiconductor laser through the surface with the anti-reflection film and feeding laser light of a desired wavelength back to the semiconductor laser through the surface with the anti-reflection film,
wherein the semiconductor laser has a window region formed between a tip portion of the active layer extending toward the surface with the anti-reflection film and the surface with the anti-reflection film, the window region allowing the laser light of the desired wavelength fed back from the wavelength selecting means
Mattori Shigenori
Yamada Atsushi
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Ip Paul
Nguyen Tuan
LandOfFree
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