This invention relates to tunable optical resonant devices and in particular but not exclusively to long external cavity lasers.

In this specification, the term optical is intended to include those parts of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region.

For coherent optical communication systems it is desirable to have narrow line width laser sources, and although these are obtainable from gas lasers it is generally necessary to filter the output of semiconductor lasers which typically have broad linewidths of a few megahertz. Such filtration is wasteful and it is preferable to stimulate the laser with feedback of a particular wavelength in order to narrow the output linewidth. One of the ways of introducing selected feedback is by a distributed feedback laser, but at present the linewidths achievable are not satisfactory. Narrower linewidths of the order of lOO Hz have been achieved with long external cavity lasers where the light output from one facet of a semiconductor laser is collimated and reflected from a grating back through the collimating lens onto the laser facet. This feedback stimulation optimises further emission of the same wavelength. An example of a tunable long external cavity laser is described in Electronics Letters 1983, 19, pp 110-112.

Typically the long external cavity laser consists of a semiconductor laser source, a collimating lens and a grating. Coarse tuning of the reflected wavelength is achieved by rotating the grating to alter the effective grating spacing and fine tuning is provided by adjustment of the path length between the laser and the grating. The path length may be altered by moving the grating closer to or further from the laser or by rotation of a tuning plate in the optical path. Rotation of such a tuning plate, which consists of a transparent plate of different optical density to air, causes the light to pass through at a greater or lesser angle and therefore to be in the plate for a d fferent path length. An arrangement of this type is shown in Figure 1. The disadvantage of known long external cavity lasers is that there are three or four separate components each requiring al gnment on the optical axis and a need to maintain that alignment during separate adjustments to the grating or grating and tuning plate. In the case of the three element system (no tuning plate) ft is necessary to provide a mechanism capable of both rotating and translating the grating or in the case of the four element arrangement two rotating mechanisms are required. One method of avoiding the need for a rotatable grating is described in a paper entitled 'A proposal for wavelength-tuning and stabilization of GaAs lasers with a graded index fibre segment in a dispersive cavity" by J.C. Vanderleeden Opto-electronics, vol 6, no. 6, 1974 PP443-449. A quarter pitch graded-index (GRIN) lens has deposited on one end a grating perpendicular to its central optical axis. A laser diode is positioned in contact with the other end of the lens and laterally offset from the optical axis so that light from the laser

diode impinges on the diffraction grating at an angle thereby providing the required selective feedback.

One disadvantage of this arrangement is that the laser diode must be offset a substantial distance from the optical axis of the lens. The acceptance sol d angle of lens decreases with increasing distance from the optical axis which for a lens with marginal f-number can increase the laser light losses, but also because a smaller area of the diffraction grating is then illuminated which adversely affects the resolution. Lens aberations also increase as the offset increases.

The present invention is directed towards minimising

_.. the number of components that require alignment and to simplifying the tuning adjustment mechanisms in optical resonant cavity devices while permitting the optical source to be positioned at or close to the optical axis. Accordingly the present invention provides a tunable optical resonant device having an external resonant cavity including an optical source, a collimator having a central optical axis, a grating positioned such that collimated light from the optical source is incident on the grating at a predetermined grating angle when the optical source is on the optical axis, and a support means for laterally offsetting the optical source relative to the optical axis. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a prior art device having a rotatable grating and a rotatable tuning plate; Figure 2 is a schematic representation of a prior art device using a GRIN lens and an offset laser diode; and Figure 3 is a schematic representation of a tunable laser resonant device according to the present invention.

Figure 1 shows a four element prior art arrangement of a external cavity laser comprising a laser, centred on the central optical axis A of a collimating lens 2, a tuning plate 3 for path length adjustment and a grating 4. The grating 4 and tuning plate 5 are each necessarily mounted independently on rotatable supports 5 and 6 respectively because of the requirement for independent rotation of these elements to effect fine and coarse tuning. Exemplary rays R, and R« illustrate marginal rays through the optical system.

Referring to Figure 2 there is shown another type of known external cavity laser comprising a laser diode 1 in contact with an end 7 of a GRIN lens 8 having a central optical axis B on whose other opposite end 9 is deposited a grating 10. The laser diode 1 is laterally offset a predetermined distance from the central optical axis B so that light from the laser diode 1 is collimated to impinge on the grating 9 at an angle α corresponding to the required tuned frequency as illustrated by marginal rays and R _* . The laser cavity can be tuned around this frequency to some degree by lateral displacement of the laser diode 1 about this design offset.

Referring now to Figure 3, there is shown an external cavity laser according to the present invention. A GRIN lens 11 constituting the collimator has ends 12 and 13 perpendicular to a central optical axis C. The lens 11 is short of a full quarter pitch.

A laser diode 14 is mounted near (ideally on) the central axis C a short distance from the end 12 on a support 15 which can move independently parallel and laterally relative to the axis C, as indicated by the arrows X and Y respectively.

A prism 16, glued to the end 13 of the lens 11 to form an assembly 18, has an internal angle β to present a

grating 17 formed on a prism surface at an angle β to collimated light from the lens 11. The angle β is chosen such that when the laser is diode 1 is on the central axis C the collimated light emerging from the lens 11 will be parallel to the axis C, and will provide feedback at a design frequency determined by the grating angle β, as shown by the exemplary marginal rays R _ς and R _g . The laser diode 14 can be coarse tuned by movement laterally about the central axis C which varies the angle at which the light impinges on the grating 17 without the need to rotate the grating 17 relative to the axis C. In contrast to the prior art arrangement of Figure 2, the laser diode 14 is located at or close to the central axis C thereby largely overcoming the disadvantages of such apparatus described above caused by necessarily having to locate the laser diode a significant distance away from the lens axis.

Because the need to rotate a grating has been el minated, the grating and collimating lens of an external cavity laser can be integrated into a single rigid assembly as shown in Figure 3 leaving only two elements to be initially maintained in alignment namely, the laser diode 1 and the lens grating assembly 18, which in the embodiment of Figure 3 comprises the GRIN lens 11 and the prism 16 although other rigid assemblies may be used to connect these elements.

The length of the lens 11 is arranged to be less than a 1/4 pitch to allow fine tuning of the laser cavity by relative longitudinal movement of the assembly 18 and laser 14 towards and away from the GRIN lens 15 to alter the cav ty length. The lens may be short of any odd multiple of a quarter pitch to permit fine tuning by movement parallel to the axis C. It is particularly preferred for the longitudinal and transverse translations to be provided by a piezoelectric devices mounted on a

single support, although they may be completely independent or be fixed to different elements relative to a common support. Mechanical, other electrical or thermal means for providing the relative movements of the components can be readily envisaged.

In some instances it may be desirable to mount the lens and grating in a jig or frame to provide interconnection, particularly if a spherical lens is used, or for the lens to be attached to the laser. A particularly compact arrangement is for the lens and grating to be manufactured integrally for example as a combined lens and prism grating.

A long external cavity laser as described above has been found to be able to tune the output of a semiconductor laser to a linewidth of the order of lOOKHz.