Tunable semiconductor laser

A tunable semiconductor laser comprises a propagation region in which a waveform can exist, the propagation region comprising sequential gain and control regions, the gain region comprising a light amplification region supplied by a source of excitation, and the control region comprising a periodic structure through which the waveform propagates. The control region can be linked to a source of current thereby to enable changes to be made to the refractive index thereof. It is preferred that the material of the propagation region is (Ga,In)(N,As). As a result, in the gain region the waveform will be less tightly confined and hence a higher gain can be produced without suffering from saturation of the gain material. Ideally, there will be tight confinement of the waveform in the control region to allow maximum advantage to be made of the change in refractive index. This can be achieved by controlling the physical configuration of the control region, such as by forming the propagation region with a lesser transverse width in the control region, and/or including non-semiconducting regions to confine the waveform. One way of achieving the latter is to include Al-containing layers in the propagation region; these can be oxidised to produce Al 2 O 3 .

DETAILED DESCRIPTION OF THE EMBODIMENTS A potential advantage of (Ga,In)(N,As) material system for 1.55&mgr;m lasers is the reduced refractive index step between active layers and cladding layers. The refractive index of a (In,Ga)(As,P) active region at 1.55&mgr;um is n&equals;3.58. A similar index of refraction exists for a 1.55&mgr;m (In,Ga)(N,As) active region. The cladding region in the materials systems are InP and GaAs respectively. GaAs has refractive index of n&equals;3.37 at 1.55&mgr;m and InP a refractive index of n&equals;3.16 at 1.55&mgr;m. The reduced index step in the (Ga,In)(N,As) system allows a less tightly confined mode. In combination with increased differential gain in this materials system, a higher output power can be expected. However, the band gap between the active and cladding layers remains similar, allowing similar electrical behaviour. A tightly confined mode is required in the grating section of the device. Here the highest possible phase change is required for the smallest change in carrier density. This is to avoid heating effects and excessive losses in the device. Whilst this apparently contradicts benefits of the loose confinement described above, this requirement can be met (for example) through the use of oxidation of Al-containing layers. A suitable layer is Al98Ga02As. In the following, AlAs will be referred to, meaning an Al-rich layer such as this, preferably one with an Al content above 80%. Therefore, the fabrication of the device can be considerably simplified, in that the loose and tight confinement can be achieved using only post process modifications to the same epitaxial layer structure. The grating may be formed in the conventional manner of etching a grating profile into the semiconductor in the desired locations, then overgrowing to complete the laser structure. Additionally the grating may be formed by the use of metal gratings, further simplifying the fabrication process. The grating may be formed by the oxidation through a mask (described in our copending application), further simplifying the process. The grating so formed may provide the lasing for the gain clamping mechanism in SOAs. An SOA may have an advantageous spot size owing the lower refractive index step. Referring to FIG. 1, a device 10 includes a ridge waveguide 12 in which a waveform 14 propagates. The ridge is divided into two portions; a gain portion 16 and a control portion 18 . The gain portion is supplied with a means of excitation by way of electrodes 20 above and below, visible in FIG. 2 , and thereby acts as a lasing means to amplify the waveform. The control portion 18 is formed with a periodic structure in order to act as a distributed Bragg reflector (DBR) and thereby select a desired wavelength for the lasing structure. Control electrodes 22 are placed above and below to permit a current to be established in the DBR region. The charge carrier density affects the refractive index, and therefore the current can be used to determine the periodicity “seen” by the waveform and hence the wavelength that is selected. FIG. 1 includes profiles 14 a and 14 b of the desired waveform. Profile 14 a is in the gain region and occupies a wide volume of material, whereas profile 14 b is in the control region and is limited more closely to that region. FIG. 2 shows a similar view in which a section on the ridge shows the periodic structure of the control region 18 . Similar profiles 14 a and 14 b of the desired waveform are also shown. As discussed above, it is an advantage of using the (Ga,In)(N,As) system that the refractive index step between that and the cladding layer is lesser and hence confinement in the laser region is looser. This means that the local maxima of the waveform intensity is lower and saturation is less likely. Accordingly a higher gain can be provided and hence a higher output power achieved. However, in the control region there is an apparently conflicting requirement, in that a looser confinement means a more widely spread waveform which “sees” a wider volume of semiconductor. Accordingly, the current density must be applied over a larger volume in order to obtain a variation of refractive index which achieves a specific variation in wavelength. This increases the heating effect of the current, the overall power consumption of the device, and the difficulty in control of currents in the two sections of the device to achieve a given output wavelength. FIGS. 3 a and 3 b show how tighter confinement of the waveform can be achieved in the control region. The propagation region is contained in a ridge 50 in which the layers of interest are, in order, a base layer 52 , a lower AlAs 54 layer covered with a number of (Ga,In)(N,As) layers 56 , an upper AlAs layer 58 , and a capping layer 60 of any suitable semiconductor material. The waveform 62 propagates mainly in the (Ga,In)(N,As) layers 56 but will extend into adjacent semiconducting layers. FIG. 3 a shows an arrangement for loose confinement, such as in the gain region. Only a brief (or no) exposure of the AlAs layers 54 , 58 is permitted and hence only a narrow part of the AlAs layers adjacent the sides of the ridge 50 oxidise to Al 2 O 3 . As a result, the AlAs layers immediately above and below the (Ga,In)(N,As) layer 56 remain available for propagation of the waveform 62 which can spread into the AlAs layers 54 , 58 above and below the (Ga,In)(N,As) layers 56 and also into the capping layer 60 and base layer 52 . FIG. 3 b shows a tighter confinement. More exposure of the AlAs layers 54 , 58 is permitted and accordingly the resulting Al 2 O 3 part thereof extends further into the ridge 50 . AlAs remains only in the central part of the layers 54 , 58 . The restricting effect of the Al 2 O 3 intrusions will limit its extent and reduce both its width and its height, as illustrated schematically. Confinement may also be achieved with further Al containing layers or different thicknesses. This allows greater control over the shape of the optical mode as it becomes more tightly confined. FIGS. 4 to 6 show an alternative means of confinement. The propagation region is again provided in a ridge 100 but this is of varying width. As with the embodiment of FIGS. 3 a to 3 c, in this embodiment the ridge comprises a base layer 102 , a lower AlAs layer 104 , (Ga,In)(N,As) layers 106 in which the waveform 112 principally exists, an upper AlAs layer 108 , and a capping layer 110 of any suitable semiconductor material. The AlAs layers 104 , 108 are again allowed to oxidise to form Al 2 O 3 denoted as 104 ′ and 108 ′ respectively, but in this case the extent of oxidation is constant along the length of the ridge 100 and hence provides a fine tuning of the confinement width. This need not be the case, and the approaches of both embodiments could be combined. The ridge is relatively narrower in the control region 114 than in the gain region 116 . Accordingly, the waveform 112 can occupy a wider space in the gain region 116 , as shown in FIG. 6 . In the control region, the physical constraints of the available semiconducting volume as limited further by the Al 2 O 3 layers 104 ′ and 108 ′ restrict the waveform to a tighter confinement, as desired. Waveform profiles 112 a and 112 b are shown in the gain region 116 and control region 114 respectively, illustrating this. Thus, the present invention provides a laser diode structure which allows good selectivity of wavelength and high gain. In this way, the advantages of the (Ga,In)(N,As) system can be employed more fully, although the principles of the invention can be applied in other material systems. It will be appreciated that many variations may be made to the above described embodiments without departing from the scope of the present invention. For example, the illustrated embodiments are two section devices whereas devices with three or more sections are common to overcome certain limitations of two section devices and to address other operating and fabrication issues. For example, a phase section without a grating and with a separate electrode can be included between the grating section and the gain section. Such multiple section devices which include the two sections of the present invention are encompassed.