Coherent light generators – Particular resonant cavity – Specified output coupling device
Reexamination Certificate
2001-12-07
2004-02-10
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Specified output coupling device
C372S020000, C372S099000
Reexamination Certificate
active
06690709
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a device and a method for efficient outcoupling of optical power in an external cavity laser, such that the outcoupled light contains a reduced fraction of spontaneous emission compared with traditional devices and methods.
2. Background Information
An external cavity laser is a type of laser, which is often used when it is desirable to be able to vary the wavelength of the light emitted from the laser. An example of an external cavity laser is shown in
FIG. 1
a
. It comprises a light emitting and/or amplifying element
100
, for example a semiconductor laser die, a first reflecting external element
170
and a second reflecting external element
150
. The term “cavity” refers to an optical resonator cavity, which is the space between the end reflecting elements
150
,
170
in a laser. The term “external” refers to that the cavity is longer than the light emitting and/or amplifying element
100
. The first reflecting external element
170
can be replaced with the facet
102
of the light emitting and/or amplifying element
100
, if said facet
102
is at least partly reflecting. The second reflecting element is often arranged in combination with a wavelength selective element, for example a diffraction grating
140
. Such configuration with a reflecting element
150
and a diffraction grating
140
is often referred to as a Littman cavity. In the Littman cavity, the laser wavelength can be varied by changing the angle of the reflecting element
150
relative the diffraction grating
140
.
FIG. 1
b
shows a cavity configuration where the diffraction grating
141
itself is the second reflecting element. Such configuration is often referred to as a Littrow cavity. In the Littrow cavity, the laser wavelength can be varied by changing the angle of the grating relative the optical axis
199
in the cavity.
If, but not only if, the light emitting and/or amplifying element
100
is a semiconductor laser die, said light emitting and/or amplifying element includes an optical waveguide
106
. The optical waveguide
106
is narrower than the optical beam
181
and at least one converging optical element
110
is used for collimating the diverging beam
180
and focusing the collimated beam
189
. If a first reflecting external element
170
is used, at least one converging optical element
160
is used for collimating the diverging beam
191
and focusing the collimated beam
193
.
All interfaces, except for the first and second reflecting elements, should be arranged such that said interfaces do not reflect the light in the direction of the cavity optical axis
199
. Alternatively, said interfaces can be coated for anti-reflection. If, but not only if, the light emitting and/or amplifying element
100
is a semiconductor laser die, the facet
104
, facing the direction of the second reflecting element, is often coated for anti-reflection.
The optical power can be coupled out of the cavity, to the output beam or optical fiber, in several ways. For example, if a diffraction grating is used in a Littman or Littrow configuration, the light not diffracted but reflected from the diffraction grating, can be used as output optical power
184
. If the first reflecting element is a partly reflecting facet
102
of the light emitting and/or amplifying element
100
, the power
191
transmitted through the facet
102
can be used as the external cavity laser output. These, but not limited to these, examples of outcoupling methods will be referred to as traditional outcoupling methods.
The coherent emission from the external cavity laser is typically spectrally very narrow. However, the light emitting and/or amplifying element
100
also generates a broad spectrum of spontaneous emission. For a traditional external cavity laser emitting a total of, for example, 1 mW optical power into a single mode fiber, approximately 10 &mgr;W of the power is spontaneous emission. This power ratio of 20 dB is insufficient for many applications, for example, when the laser is used for characterization of optical filters. A laser source emitting a smaller fraction of spontaneous emission would be very attractive.
A method and device for reducing the fraction of spontaneous emission in the optical output from external cavity lasers has been demonstrated by Edgar Leckel et al. [Ref. 1]. The demonstrated device was used in a Littman external cavity laser as shown in
FIG. 2. A
beam-splitter
220
was placed between the wavelength selective element
240
, in this case a diffraction grating, and the light emitting and/or amplifying element
200
, in this case a semiconductor laser die. The beam-splitter
220
deflects a fraction of the incident lights in two opposite directions
224
226
corresponding to the two directions of propagating light
281
288
inside the cavity. The outcoupled beam
224
, originating from the light
281
propagating from the semiconductor laser die
200
towards the diffraction grating
240
, contains the same fraction spontaneous emission as for traditional outcoupling. The outcoupled beam
226
, originating from the light
288
propagating from the diffraction grating
240
towards the semiconductor laser die
200
, is spectrally filtered, such that the spontaneous emission has an angular distribution around the direction of propagation for the lasing wavelength. If the spectrally filtered outcoupled beam
226
is also spatially filtered, for example using a single mode optical fiber, the fraction of spontaneous emission of said beam is typically reduced by a factor of 1000. The main disadvantage with this method is that a large amount of the total outcoupled optical power is not spectrally filtered.
The optical power in the spectrally filtered beam
226
can be no more than equal to the optical power in the beam
224
that is not spectrally filtered. Therefore, no more than ½ of the optical power outcoupled by the beam-splitter can be used as a low spontaneous emission light source.
In U.S. Pat. No. 5,406,571 is a tunable laser oscillator is disclosed, which comprises a laser medium, an optical resonator, a wavelength selective element for adjusting the wavelength of a laser beam, and optical means for broadening the radiation in the resonator. The laser beam is decoupled from the resonator by means of an optical element after having passed the broadening means and prior to passing again through the laser medium. The laser beam is decoupled from the resonator such that its direction is independent of the beam wavelength.
The laser beam generated in an optical amplifying medium is divided into two beams by means of a prism. One of the beams comprises a reflection from the prism's first surface. No reduction of the spontaneous emission is obtained in this beam. The other beam consists of diffracted beam inside the prism. Thus, the beam is broadened and illuminates a larger area of the wavelength-detecting element. The beam is diffracted so that it propagates in same beam path but in opposed direction. The light is finally decoupled out of the laser cavity by means of the first prism. The first prism is realized in two geometries and a number of cavity configurations. However, the object of laser according to this document is:
to achieve high spectral purity, i.e. low spontaneous emission, for one of the beams decoupled from the cavity,
that high spectral purity at one of the decoupled beams is achieved without any major structural changes in the structural changes in the laser cavity,
that the direction of the decoupled spectrally pure beam is independent of the wavelength as well as the position of the wavelength selective element.
Moreover, this document does not mention or gives any hint of using a Faraday rotator.
However, a retardation plate is mentioned, which is a completely different element.
SUMMARY OF THE INVENTION
The present invention can couple part of a light beam propagating from a wavelength selective element, towards a light emitting and/or amplifying element, out
Berg Tomas
Hagberg Mats
Kleman Bengt
Lock Tomas
Nguyen Dung
Radians Innova AB
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