Continuously wavelength-tunable monomode laser source

Details Continuously wavelength-tunable monomode laser source Continuously wavelength-tunable monomode laser source

1999-02-19

2001-11-27

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular beam control device

Tuning

C372S064000, C372S032000, C372S026000, C372S009000, C372S102000

Reexamination Certificate

active

06324193

ABSTRACT:

This invention relates to a wavelength-tunable monomode laser source with external cavity.
We know that the resonant optic cavity of a laser source selects one or several wavelengths emitted by an amplifier laser medium. Most often, the system consists of two mirrors, whereas one of them is partially transparent, forming a so-called Fabry-Perot cavity. Such a Fabry-Perot cavity selects or resonates for semi-wavelengths equal to sub-multiples of the optic length L
op
of the cavity and hence generally quite close to one another. Several wavelengths can then be amplified by the wide spectrum amplifier medium, to provide a multimode laser.
For certain applications, monomode lasers are preferred. It is then necessary to implement a resonant optic cavity associating a selection means complimentary to the Fabry-Perot cavity, for instance by replacing one of its mirrors by a retroreflecting dispersing device.
Retroreflecting dispersing devices are commonly used in conventional optics. The best known device is probably the pitch p plane grating employed according to the Littrow configuration.
Generally speaking, a pitch p plane grating has a dispersion plane perpendicular to its lines. A collimated light beam of wavelength &lgr;, tilted by an angle &thgr;1 with respect to the normal axis of the grating which is parallel to the dispersion plane of the grating, generates a collimated beam which is also parallel to the dispersion plane and whose direction is tilted by an angle &thgr;2 with respect to the normal axis, whereas &thgr;1 and &thgr;2 are linked by the following relation:
p sin &thgr;
1
+p sin &thgr;2=&lgr;
In external cavity tunable laser sources operating with a so-called Littman-Metcalf configuration where the incident collimated beam forms an angle &thgr;1 with the normal axis of the grating, an additional mirror is placed with its normal axis forming an angle &thgr;2 on the grating. The wavelength &lgr; which respects the equation &lgr;=p sin &thgr;1+p sin &thgr;2, is dispersed by the grating at an angle &thgr;2, then is retroreflected onto the mirror which is then perpendicular, and is finally dispersed again into the grating in return and comes out at the input angle &thgr;1. This wavelength &lgr; is therefore selected in the cavity. Wavelength tunability is obtained by varying the orientation of the grating/mirror assembly, i.e. while varying &thgr;1, or while varying solely the orientation of the mirror, i.e. while varying &thgr;2 or finally while varying solely the orientation of the grating, i.e. while varying &thgr;1 and &thgr;2, whereby the relation &thgr;1-&thgr;2 should remain constant.
FIG. 1
represents a grating
11
implemented according to the Littman-Metcalf assembly in which the extremity
2
of a guided monomode amplifier medium
13
is placed at the focus of collimation optics
14
which generate a main collimated beam
15
of wavelength &lgr;.
This beam is parallel to the dispersion plane of the grating, i.e. to the plane perpendicular to the lines
16
of the grating
11
and forms an angle &thgr;1 with the normal axis
17
at the surface of the grating
11
. By diffraction onto the grating, the beam
15
produces a secondary collimated beam
18
, which lies in the dispersion plane and forms an angle &thgr;2 with the normal axis
17
. A plane mirror
19
is placed at right angle to the beam
18
and the beam is retroreflected through the whole system.
We know under these circumstances that, with p as the pitch of the grating, when the relation p sin &thgr;1+p sin &thgr;2=&lgr; is verified, the beam
15
comes back onto itself after first diffraction onto the grating
11
, retroreflection onto the mirror
19
and second diffraction onto the grating
11
. It therefore produces a point-image
8
superimposed with the extremity
2
.
The adjustment of such devices calls for accurate positioning of the grating round an axis perpendicular to the selection axis and parallel to the dispersion plane. This latter adjustment and its stability are quite difficult and will determine, in most cases, the quality of the result obtained.
For clarification purposes,
FIG. 2A
represents a view of the focal plane of the collimation optics
14
in which lies the extremity
2
of a guided amplifier medium and the spectrum generated in return by the assemblies shown on
FIG. 1
, when the amplifier produces a wide spectrum. We have therefore a spectrum ranging from a wavelength &lgr;1 to a wavelength &lgr;2 and for which the wavelength &lgr; is retroreflected onto the extremity
2
and is therefore selected in the cavity.
In practice, since the real rotation axis cannot be exactly parallel to the grating lines, displacement of the spectrum in the focal plane is accompanied by a movement perpendicular to the former and leads, when the wavelength &lgr;′ is retroreflected, to a configuration such as that represented on
FIG. 2B
, on which retroreflection is not provided exactly because of the offset of the spectrum, perpendicular to itself, at the same as parallel to itself in the focal plane of the collimation optics.
In the Littman-Metcalf configuration, this problem is raised when the grating
11
or the mirror
19
or both of them, revolve round an axis parallel to the lines
16
of the grating.
Such devices can also generate mode hopping. Indeed, rotation of the grating dispersing device changes the selected wavelength, but this wavelength must also respect the resonance condition of each optic cavity which indicates that the optic length Lop of the cavity (one way) is equal to an integer N of semi-wavelength:
L
op
=N·&lgr;/2
If the wavelength selected decreases, the cavity must be shortened at the same time, and vice versa when the wavelength increases, in order to keep the same integer N and prevent any mode hopping.
A continuous tunability device exempt from mode hopping has been suggested with a Littrow configuration, distinct from the Littman-Metcalf configuration (F. Favre and D. Le Guen, <<82 nm of continuous tunability for an external cavity semi-conductor laser>>, Electronics Letters, Vol. 27, 183-184, [1991]), but it involves a complex mechanical assembly using two translation movements and two rotation movements.
In an article of 1981, Liu and Littman (Optics Letters, Vol. 6, N°3, March 1981, pp. 117-118) describe a device comprising a mirror and a variable orientation mirror implemented for the execution of a variable wavelength monomode laser. The geometry suggested enables continuous wavelength scanning.
Besides, reflecting dihedra have been studied for a long time. In particular, the Japanese patent application JP-A-57.099793 of Jun. 21, 1981 suggests to use such a dihedron to build a retroreflecting dispersing device in a wavelength multiplexed optic fibre communication system, whereby the said lengths are fixed.
We also know such continuous tunable monomode laser source as described in the European patent application 0.702.438 which uses a Littman-Metcalf configuration defined with reference to
FIG. 3
of this title, comprising a plane grating
31
, an orthogonal reflecting dihedron
39
whose edge
391
is parallel to the dispersion plane of the grating
31
comprising the collimation axes.
The point A is the point of intersection of the collimation axes and of the grating, the point B′ is the optic extremity of the cavity located on dispersing system side, the point C′ is the intersection of the main collimation axis with the optic extremity of the cavity located on the amplifier medium side, the point D is the intersection of the plane containing the diffraction phase of the grating and of the parallel to the edge of the dihedron going through B′.
The angle AC′D is kept equal to 90° and the length AD is kept constant.
Such a device in which these conditions are met thanks to purely mechanical means, is satisfactory to a vast extent. However, it has proven useful to further enhance this device in order to facilitate its adjustment.
The purpose of the invention

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