WO2005124951A1

WO2005124951A1 - Dfb laser with lateral bragg gratings and facet bragg reflectors etches in one step

Info

Publication number: WO2005124951A1
Application number: PCT/GB2005/002395
Authority: WO
Inventors: Richard Hogg; Kristian Groom
Original assignee: The University Of Sheffield
Priority date: 2004-06-18
Filing date: 2005-06-17
Publication date: 2005-12-29
Also published as: WO2005124951A8; GB0413664D0; GB2416427A

Abstract

A distributed feedback laser comprises: a waveguide (17) extending along a longitudinal axis; a Bragg grating structure (21) arranged with respect to the waveguide to interact with electromagnetic radiation propagating along the waveguide so as to provide distributed feedback along the waveguide to enhance a mode of said radiation; and first facet means arranged at a first end of the waveguide to reflect propagating electromagnetic radiation back along the waveguide. The first facet means comprises a first Bragg reflector structure (3). In certain embodiments the first Bragg reflector structure and the Bragg grating structure are substantially different. For example, the Bragg grating may comprise transverse elements for lateral coupling with guided radiation, and the reflector may comprise a periodic structure of semiconductor material (41) and air gaps in the radiation path. The laser may have a second Bragg reflector structure (5) at the opposite end of the waveguide. Fabrication methods are described.

Description

DFB LASER WITH LATERAL BRAGG GRATINGS AND FACET BRAGG REFLECTORS ETCHES IN ONE STEP Field of the Invention The present invention relates to distributed feedback lasers, and in particular, although not exclusively, to distributed feedback semiconductor lasers.

Background to the Invention

Lasers are well known devices for generating intense beams of electromagnetic radiation for use in a variety of applications. Although the word "laser" was originally an acronym for light amplification by stimulated emission of radiation, it has come to be used to refer to any similar source producing a beam of any electromagnetic radiation, not necessarily visible light, such as infrared or microwave radiation. Thus, the term "laser" as used throughout this specification, including the claims, is not limited to devices producing beams of visible light, but instead encompasses any device which utilises the phenomenon of radiation amplification by stimulated emission of radiation. Similarly, the word "light" will be used generally to encompass both visible light and electromagnetic radiation outside the visible spectrum.

Fabry-Perot (F-P) type lasers are known, in which reflectors are arranged at opposite ends of a cavity. When the reflectors are broad-band, the F-P laser can operate simultaneously in several longitudinal modes, resulting in the output spectrum comprising a number of different wavelengths.

For certain applications however, for example in optical communications, a laser source providing a radiation beam at a single wavelength is desirable. It is known to use distributed feedback (DFB) lasers for such applications. A DFB laser is a laser in which a Bragg grating structure is arranged (i.e. distributed) along the waveguide portion (i.e. the portion of the laser in which the electromagnetic radiation propagates) so as to interact with propagating radiation to suppress multiple longitudinal modes and enhance a single longitudinal mode. Thus, the longitudinal grating interacts with electromagnetic radiation along the waveguide, rather than just at the ends as is the case with F-P devices. A known semiconductor DFB laser is shown highly schematically in fig. 1. The laser comprises a multiple-layer semiconductor waveguide structure, extending along a longitudinal axis A, that structure comprising lower cladding layer LC of semiconductor material, an active region AL (which may also be referred to as an active layer) of semiconductor material, and an upper cladding layer UC of semiconductor material formed over the active region. The active region AL material has a higher refractive index then the adjacent cladding layers, resulting in better confinement of electromagnetic radiation in the active layer. The active layer thus acts as a dielectric waveguide. Although not shown in figure, it is known for the lower cladding, upper cladding and active layers themselves to have multi-layer structure. A Bragg grating G is shown in highly schematic form, superimposed on the upper cladding layer UC. The Bragg grating G is arranged to provide a periodic variation in effective refractive index along the waveguide. Light propagating along the waveguide interacts with the periodic structure such that a single longitudinal mode is enhanced (that mode having a wavelength corresponding to the Bragg wavelength of the grating G) and other modes are suppressed.

The Bragg grating G may take a variety of forms. For example, it may be provided by a corrugated interface between two semiconductor layers with different refractive indices, or may comprise a spatially periodic refractive index variation written into a single layer. Laterally coupled DFB lasers are also known, where the grating G comprises longitudinal series of elements, arranged on either side of a longitudinal laser ridge in the upper cladding layer, where each element extends transversely with respect to the longitudinal axis, from the ridge. Other forms of Bragg grating structure, to provide distributed feedback, will be apparent to those skilled in the art, and may also be used in embodiments of the invention.

Returning to the device of fig. 1, facets FI and F2 are formed at opposite ends of the structure, the first facet FI being coated with a high reflectivity coating HR, and the second facet F2 being provided with an anti-reflection coating AR. Electrical power is supplied to the laser via electrodes, and a light beam is emitted from the active layer at the antireflection coating end.

In typical known semiconductor DFB devices, the facet FI is formed by cleaving the multi-layer semiconductor structure, in or on which the Bragg grating has already been formed. A problem with this technique, however, is that it is not possible to precisely control the cleaving position with respect to the grating; in effect, the position of cleaved facet with respect to the grating elements is random. Thus, there is no control of the facet phase, i.e. the phase of the reflected light at the laser facet with respect to the grating. The facet phase (determined by the position of the cleaved facet with respect to the grating elements) affects the performance of the laser device; it has an effect on how light photons and electrons interact, so affecting the output power, dynamic response and "chiφ" of the device. In this context, chiφ may be understood as follows. If one takes a laser and directly modulates it (modulates the current), the laser wavelength changes slightly during the modulation cycle. This dynamic change in the laser wavelength is termed chiφ and is due to carrier density changes resulting in changes to the refractive index of the waveguide.

The inability to control cleaved facet phase thus results in variation in performance from one device to the next, even when their Bragg grating structures are identical.

It is therefore an object of embodiments of the invention to provide DFB lasers, and methods of producing DFB lasers, which overcome, at least partially, the problem of variable facet phase.

Summary of the Invention According to a first aspect of the present invention there is provided a distributed feedback laser comprising: a waveguide extending along a longitudinal axis; a Bragg grating structure arranged with respect to the waveguide to interact with electromagnetic radiation propagating along the waveguide so as to provide distributed feedback along the waveguide to enhance a mode of the radiation; and first facet means arranged at a first end of the waveguide to reflect propagating electromagnetic radiation back along the waveguide, characterised in that the first facet means comprises a first Bragg reflector structure (which may also be described as a distributed Bragg reflector - DBR). It will be appreciated that the same, or at least similar techniques may be used to form the Bragg grating and Bragg reflector structures, and hence it is possible to achieve more precise positioning of the reflector with respect to the grating than is possible with the prior art technique of cleaving a facet. In other words, the techniques required to produce a Bragg reflector having the requisite structure are intrinsically suited to defining the position of the reflector precisely with respect to the grating. Lasers according to the present invention can thus be produced by methods which provide precise control over the facet phase, and can thus exhibit reduced variation in their operating characteristics. Another advantage of using a Bragg reflector structure rather than a coated, cleaved facet, is that the single mode of radiation enhanced by the distributed grating can be further enhanced, and other modes can be further suppressed. A DFB laser embodying the invention can thus more closely approach the ideal of a monochromatic output. In certain embodiments, the first Bragg reflector structure and the Bragg grating structure are substantially different. In other words, in such embodiments the Bragg reflector structure is not simply a continuation of the Bragg grating structure. This difference may be in one or more of the following features: etch depth; infill material; element refractive index; element thickness; element separation; phase; pitch; mark-space ratio. It will be appreciated that this list is not exhaustive; other differences are possible. In general, the differences in structure are such that the Bragg reflector structure has different properties to the Bragg grating structure. The grating structure is to interact with radiation to provide distributed feedback, whereas it is generally desirable for the Bragg reflector to have a high reflectivity to the wavelength of interest (i.e. the Bragg wavelength of the grating). The physical pitch of the first Bragg reflector structure may be different from that of the Bragg grating structure, but the first Bragg reflector structure and the Bragg grating structure may still have substantially the same Bragg wavelength. In certain embodiments, the first Bragg reflector structure and the Bragg grating structure are arranged in phase with one another. However, in alternative embodiments they are arranged a predetermined phase angle out of phase with one another. Thus, the facet phase may be tuned by appropriate location of the reflector structure with respect to the DFB structure. The Bragg grating structure may be arranged to couple laterally with electromagnetic radiation propagating along the waveguide. In contrast, the Bragg reflector structure may be arranged in the path of electromagnetic radiation propagating along the waveguide. In certain preferred embodiments the first Bragg reflector structure comprises a series of reflector elements (which may be described as Bragg mirrors) spaced apart along the longitudinal axis. Each element is preferably substantially planar and is arranged substantially peφendicularly with respect to the longitudinal axis, and the planar elements are arranged parallel to each other. Advantageously, the elements may be separated by air gaps, to provide a large contrast in refractive index between the elements and gaps. The resultant periodic variation in refractive index through the reflector structure can provide high reflectivity to the mode of interest. Alternatively, the gaps between the reflector elements may be filled with solid, or even liquid, material of suitable refractive index. In certain embodiments, the elements of the first Bragg reflector structure are separated by respective channels (which may also be described as slits or grooves) and these channels may typically extend in a direction generally transverse to the longitudinal axis. The channels may be gas-filled (such that they can be regarded as air gaps) or, in other embodiments, may be filled with solid or liquid material of a suitable refractive index. Preferred embodiments of the invention provide semiconductor DFB laser devices. For example, certain embodiments comprise a multi-layer semi-conductor structure extending along the longitudinal axis, the multi-layer structure comprising an active layer, a first (e.g. lower) cladding layer arranged on one side of (e.g. beneath) the active layer, and a second (e.g. an upper) cladding layer arranged on the opposite side of (e.g. above) the active layer. In such examples the waveguide comprises a first longitudinal portion of the multi-layer structure and the first Bragg reflector structure comprises a second (e.g. end) longitudinal portion of the multilayer structure, adjacent the first portion, and each channel of the first Bragg reflector structure extends through the second cladding layer and active layer and at least partially through the first cladding layer. It will be appreciated that the active and first and second cladding layers may be single layers, or alternatively may themselves have multi-layer structure. These deep channels, extending down through the active layer, may conveniently be produced by a process including etching, and the resultant Bragg reflector structure may be referred to as a deeply-etched Bragg mirror. In certain embodiments, the multi-layer semi-conductor structure further comprises a substrate, and the channels (slits, grooves, air gaps) of the first Bragg reflector structure extend through the second cladding, active, and first cladding layers to the substrate. Typically, the first Bragg reflector structure will have a constant effective pitch along the longitudinal axis (i.e. it will be unchiφed). Similarly, the Bragg grating structure will typically have a constant effective pitch, which may be equal to that of the first Bragg reflector. In certain preferred embodiments, the laser further comprises a second facet means arranged at a second end of the waveguide to reflect propagating electromagnetic radiation back towards the first end. This second facet means may comprises a second Bragg reflector structure, and this may have a structure as described above in relation to the first Bragg reflector. Use of a Bragg mirror at the second end can further improve the enhancement of a single mode of radiation. The first and second Bragg reflector structures may be periodic, each comprising substantially the same repeated sub-structure, the sub-structure being, for example, a thickness of semiconductor/dielectric material and an adjacent air-gap. To give different reflectivities the first and second Bragg reflector structures may comprise different numbers of the repeated sub-structure (e.g. different numbers of elements and air-gaps). Again, the pitch of the second Bragg reflector structure may be different from the pitch of the Bragg grating structure, although the second Bragg reflector structure and the Bragg grating structure may still have substantially the same Bragg wavelength. As with the first reflector structure, the second Bragg reflector structure and the Bragg grating structure may be arranged in phase with one another, or a desired angle out of phase. The first and second Bragg reflector structures may have substantially the same pitch. Preferrably, the first Bragg reflector structure is arranged to have a higher reflectivity than the second Bragg reflector structure, such that the second end is the output end, from which the laser emits radiation. It will be appreciated that this difference in reflectivity may conveniently be achieved by using reflection gratings having different orders (i.e. different numbers of reflecting elements). Certain preferred embodiments comprise a multi-layer semi-conductor structure extending along the longitudinal axis, the multi-layer structure comprising an active layer, a first (e.g. lower) cladding layer arranged on one side of (e.g. beneath) the active layer, and a second (e.g. an upper) cladding layer arranged on the opposite side of (e.g. above) the active layer. The waveguide comprises a first (e.g. central) longitudinal portion of the multi-layer structure, the first Bragg reflector structure comprises a second (e.g. end) longitudinal portion of the multi-layer structure, and the second Bragg reflector structure comprises a third (e.g. opposite end) longitudinal portion of the multi-layer structure, such that the first portion is arranged between the second and third portions. As an alternative to a second Bragg reflector (Bragg mirror), the second facet means may comprises a cleaved facet, which may be provided with an anti-reflection coating. In certain embodiments the waveguide comprises an active layer of semiconductor material extending along the longitudinal axis and a ridge (a longitudinal laser ridge) of semiconductor material extending along the longitudinal axis, over (above) the active layer. In such embodiments the Bragg grating structure preferably comprises a first plurality of grating elements, spaced apart along the longitudinal axis and extending transversely with respect to the longitudinal axis from one side of the ridge, and a second plurality of grating elements, spaced apart along the longitudinal axis and extending transversely with respect to the longitudinal axis from an opposite side of the ridge. These elements may comprise metallic material deposited on the active layer or on a layer of cladding material over the active layer, or alternatively may comprise lateral (transverse) ribs or ridges of semiconductor material. In the latter case, adjacent grating elements of the first plurality of grating elements and of the second plurality of grating elements may separated by etched slots, which may take the form of air-gaps. Alternatively, the gaps can be filled in with material such as dielectrics (e.g. SiN, SiO), or overgrown with semiconductor of different refractive index. Lasers embodying the invention may also comprise an electrode structure arranged to enable current to be injected into the laser.

According to a second aspect of the invention there is provided a method of manufacturing a distributed feedback laser, comprising the steps of: providing a multi-layer semi-conductor waveguide structure extending along a longitudinal axis, the multi-layer structure comprising an active layer, a first (e.g. lower) cladding layer arranged on one side of (e.g. beneath) the active layer, and a second (e.g. an upper) cladding layer arranged on the opposite side of (e.g. above) the active layer; forming a Bragg grating structure along a first longitudinal portion of the multi-layer semi-conductor waveguide structure so as to provide distributed feedback to electromagnetic radiation propagating along the first longitudinal portion; and forming a first Bragg reflector structure in a second longitudinal portion of the multi-layer semi-conductor waveguide structure, the second longitudinal portion being adjacent to a first end of the first longitudinal portion, and the first Bragg reflector being arranged to reflect electromagnetic radiation propagating to the first end back along the first longitudinal portion. Again, the techniques suitable for forming the Bragg grating and reflector structures are able to provide precise control of the reflector position with respect to the grating elements, and so enable precise control of facet phase at the first end to be achieved. Preferably, the method further comprises the step of: forming a second Bragg reflector structure in a third longitudinal portion of the multi-layer semi-conductor waveguide structure, the third longitudinal portion being adjacent to a second end of the first longitudinal portion, and the second Bragg reflector being arranged to reflect electromagnetic radiation propagating to the second end back along the first longitudinal portion towards the first end. In preferred embodiments, the method further comprises the step of forming a longitudinal ridge in the second cladding layer, along at least part of the first longitudinal portion of the multi-layer semi-conductor waveguide structure. Advantageously, this may be achieved by the steps of lithographically defining the position of the ridge on a surface of the second cladding layer and then selectively removing second cladding layer material (e.g. by etching) from either side of the ridge. The selective removal of second cladding layer material is preferably performed so as to leave a reduced-thickness layer of cladding material covering the active layer on either side of the ridge. The step of forming the Bragg grating structure then preferably comprises the steps of lithographically defining positions of grating elements on the reduced-thickness layer on either side of the ridge, and then forming the grating elements at said positions. Each grating element may extend from the ridge in a direction transverse to the longitudinal axis, and in certain preferred embodiments the step of forming the grating elements comprises depositing metallic material. Alternatively, the step of forming the Bragg grating structure may comprise the steps of lithographically defining the positions of grating elements on a surface of the second cladding layer and then selectively removing second cladding layer material from between the grating elements. The position of a longitudinal ridge and the positions of grating elements on either side of the ridge may, advantageously, be simultaneously defined in a single lithographic process, on a surface of the second cladding layer, and then second cladding layer material may simultaneously be removed from either side of the ridge and from between the grating elements to form the ridge and grating elements. The step of forming the, or each, Bragg reflector structure may comprise the steps of lithographically defining the positions of reflector elements on a surface of the second cladding layer and then selectively removing material from between the reflector elements. In embodiments comprising first and second Bragg reflectors, the elements of both reflectors may be defined in a single lithographic process. In methods which comprise the step of selectively removing material from between reflector elements, this step may, advantageously, comprise forming a respective gap (e.g. channel, slit, slot or groove) between each pair of adjacent reflector elements, the gap extending through the second cladding layer, and through the active layer. The gap may extend at least partially through the first cladding layer, and in embodiments where the multi-layer semi-conductor structure comprises a substrate (on which the first cladding layer, the active layer, and the second cladding layer are formed) each gap may extend down through the second cladding layer, active layer and the first cladding layer to the substrate. In such embodiments, the step of selectively removing material thus comprises first removing second cladding layer material, then removing active layer material, then removing first cladding layer material. In certain methods embodying the invention the positions of a longitudinal ridge, Bragg grating elements, and Bragg reflector elements on a surface of the second cladding layer are simultaneously lithographically defined (i.e. they are defined in a single lithographic step), and then material is removed from the waveguide structure to form the ridge and the grating and reflector elements. The step of removing material may comprise etching, and/or focussed ion beam milling. In certain embodiments, the step of providing a multi-layer semi-conductor waveguide structure comprises the steps of providing a substrate, forming the first cladding layer on the substrate, forming the active layer on the first cladding layer, and forming the second cladding layer on the active layer. At least one of the first cladding, active, and second cladding layers may itself be a multi-layer structure.

Brief Description of the Drawings Embodiments of the invention will now be described with reference to the accompanying drawings, of which:

Figure 1 is a schematic representation of a DFB laser in accordance with the prior art;

Figure 2 is a highly schematic representation of a DFB laser embodying the invention;

Figure 3 is a perspective view of another DFB laser embodying the invention;

Figure 4 is a schematic end view of the laser from Figure 3 (i.e. it is a representation of the front and back facets, which in this embodiment are the same);

Figure 5 is a schematic cross-section of the laser device shown in figures 3 & 4, along the line X-X shown in figure 4.

Figure 6 is a schematic cross-section of the laser device from figures 3, 4 & 5, taken along line A-A in figure 5;

Figure 7 is a schematic cross-section of the laser device from figures 3-6, taken along line B-B in figure 5;

Figure 8 is a schematic end view of the front facet of another DFB laser embodying the invention;

Figure 9 is a schematic horizontal cross-section of the DFB laser whose front facet is shown in figure 8; Figure 10 is a schematic cross-section of the laser device from figures 8 & 9, taken along line A-A in figure 9;

Figure 11 is a schematic cross-section of the laser device illustrated in figures 8-10 taken along line B-B in figure 9;

Figure 12 is a schematic end view of the front and back facets of another DFB laser embodying the invention;

Figure 13 is a schematic horizontal cross-section of the DFB laser whose end view is shown in figure 12;

Figure 14 is a schematic cross-section of the laser of figures 12 & 13, taken along line A-A in figure 13;

Figure 15 is a schematic cross-section of the laser device from figures 12-14, taken along line B-B in figure 13;

Figure 16 is a schematic horizontal cross-section of yet another DFB laser embodying the invention;

Figure 17 is schematic cross-section of the laser device of figure 16, taken along line

A-A;

Figure 18 is a schematic cross-section of the device from figures 16 & 17, taken along line B-B in figure 16;

Figure 19 is a highly schematic representation of a DFB laser embodying the invention;

Figure 20 is a schematic plan view of another DFB laser embodying the invention; and

Figures 21 - 24 are schematic plan views of parts of DFB lasers embodying the invention to illustrate different phase relationships between the Bragg Grating structures and the Bragg Reflector structures. Detailed Description of the Preferred Embodiments

Referring now to figure 2, a first embodiment of the invention is a DFB laser comprising a waveguide 1 which extends along a longitudinal axis A. The waveguide 1 has a first end 11 and a second, opposite end 12. A Bragg grating 2 is arranged with respect to the waveguide to provide distributed feedback along at least a portion of the waveguide's length. A first facet means 3 is arranged at the first end 11 of the waveguide to provide reflection to light in the waveguide. This first facet 3 comprises a Bragg reflector structure 4 (which may also be referred to as a distributed Bragg reflector, DBR). This reflector 4 comprises a series of reflector elements 41 spaced apart the longitudinal axis A, and separated by intermediate layers 42 of material having a different refractive index from the reflector elements 41 (which may also be referred to as mirror elements or simply Bragg mirrors). In this example, the second end 12 of the waveguide is a cleaved facet, which may be provided with an antireflection coating as described above in relation to the prior art. This laser emits a beam of radiation in the direction shown generally by arrow L.

Moving on to figures 3-7, these illustrate a DFB semiconductor laser embodying the invention. The laser is formed from a multi-layer semiconductor structure which comprises a substrate 13, a lower cladding layer 14 formed on the substrate, an active layer 15 formed on the lower cladding layer 14, and an upper cladding layer 16 formed over the active layer. A central longitudinal section of the device comprises a waveguide structure and a Bragg grating 2 arranged to provide distributed feedback. In this central section, material of the upper cladding layer 16 has been selectively removed to define a central longitudinal laser ridge 17. Thus, on either side of the ridge the upper cladding layer 16 covering the active layer 15 has reduced thickness. This layer of upper cladding material covering the active layer is denoted by reference numeral 161. Upper surfaces 162 of the layer 161 can thus be regarded as shoulders on either side of the central ridge 17. Metallic grating elements 21 are formed on these shoulder surfaces, the elements being regularly spaced apart, by air gaps, along the length of the laser. A first group of these elements 21 extends transversely from one side of the ridge or rib 17, and a corresponding group of elements 21 extends from the opposite of the ridge. Typically, an electrode will be attached to the upper surface of the ridge 17, and in use the laser light will be mostly confined in the portion of the active layer immediately beneath the ridge 17. The transverse, metallic grating elements 21 interact with the light to provide distributed feedback, and arrangement on either side of the ridge 17 results in a lateral coupling to the guided light.

At a first end of the central section there is provided a first facet means 3, which in this example is a deeply-etched Bragg mirror. This comprises a series of four reflector elements 41, separated by air gaps 42. The elements 41 have been produced by selective removal of upper cladding layer material, active layer material and lower cladding material, completely down to the substrate 13. In order words, channels, groves or slits have been formed in the multi-layer semiconductor structure to form the air gaps between the mirror elements 41. In this example, the gaps 42 are air- filled. This results in a large modulation of refractive index along the longitudinal axis A, through the Bragg reflector structure, which interacts with the light to give high reflectivity. In other embodiments, however, the gaps 42 may be filled with solid material of a suitable refractive index, as may the gaps between the metallic grating elements 21.

At the opposite end of the central section there is provided a second facet means 5. This is also a deeply-etched Bragg mirror (a Bragg reflector structure), comprising reflector elements 41 with air gaps 42 in-between. However, this second facet 5 has only two reflector elements and so the reflectivity of the second facet is lower than the first. Thus, the lower reflectivity facet defines the output end of laser structure, with an intense beam of radiation being emitted generally from the active layer, in line with the laser ridge 17.

A method suitable for producing the laser device shown in figures 3-7 is as follows:

1. Take a substrate 13 and form a laser structure 9, for example, by epitaxial growth (MBE, MOCVD, LPE) of lower doped cladding layers, active region and upper doped layers;

2. Photolithographically define the laser ridge 17 (i.e. define the position of the laser ridge on a surface of the upper doped layers (the upper cladding layer or layers)); 3. Form the laser ridge by etching (for example using wet chemical or dry RLE, or ICP techniques) the doped upper cladding layers to some depth (in this example the depth corresponds to a position just above the active region, leaving a thin layer of doped upper cladding material over the active region on either side of the ridge 17);

4. Form the DFB grating by firstly defining the positions of the grating elements on either side of the ridge 17 by a lithographic technique involving the patterning of an e-beam sensitive resist layer, and then depositing metallic material (e.g. gold), and then lifting off material to leave behind the metallic grating elements 21;

5. Mask all of the structure (e.g. wafer) with a mask such as a thin layer of SiO₂;

6. Pattern the mirror sections (this may comprise the simultaneous lithographic definition of the positions of the elements 41 of the Bragg reflectors at either end of the device, for example, using an e-beam, and then the gaps between the elements may be produced by an etching technique such as dry etching (for example, an ICP etch technique to give a desired vertical profile);

7. The laser device may then be completed with processing steps that will be familiar to those skilled in the art in connection with any other laser diode. This may involve the formation of an electrical contact only to the ridge section 17, and not to the DBR sections 3, 5. Thus, an upper electrode may be formed on the upper surface of the ridge 17, and a lower electrode may be formed on the lower surface of the substrate. Other electrode positions, depending on the device structure, will be apparent.

In alternative methods embodying the invention, the above steps 5 & 6 can be replaced with focused ion beam milling to form the Bragg mirror structures.

Referring now to figures 8-11, these illustrate a semiconductor DFB laser which is similar to that shown in figures 3-7 except that its front facet is formed with a cleaved edge rather than with a second Bragg reflector structure. Thus, this alternative embodiment has a Bragg reflector structure only at the first facet 3. Figure 8 shows the front facet, i.e. a view of the device looking along the longitudinal axis A. The rear facet of the device has an appearance identical to that shown in figure 4. Again, the waveguide structure provides a longitudinal ridge 17, with metallic grating elements 21 distributed along and on either side of ridge to provide distributed feedback by lateral coupling to the guided radiation.

Referring now to figures 12-15, these show an alternative embodiment. Again, the DFB laser is fabricated from a multi-layer semiconductor structure, but instead of arrays of transverse metallic elements, the grating providing distributed feedback is a periodic structure formed by selective etching of channels or slots 22 on either side of a central ridge 17. Each slot 22 has thus been formed by selective removal of a portion of the upper cladding layer or layers 16 and the remaining material thus defines a series of grating elements 21. Conveniently, the grating element 21 can be formed by a lithographic patterning of an upper surface of the upper cladding layer or layers 16, followed by subsequent etching. At a first end of the waveguide/distributed feedback grating structure there is provided a first facet means 3, and at the opposite, second end of the structure there is a second facet means 5. As with the embodiments of figures 3-7, the first and second facets of the present embodiment comprise deeply- etched Bragg mirror structures, the former comprising four reflective elements 41, and the latter comprising two reflective elements 41. The deep etching to produce these mirror elements has extended from an upper surface of the upper cladding layer or layers 16, down through the active layer 15 and the lower cladding layer or layers 14, terminating at the upper surface of the substrate 13. Figure 12 represents the appearance of both the front and rear facets, which in this example are the same.

A method suitable for producing the DFB laser of figures 12-15 is as follows:

1. Take a substrate 13 and form a laser structure on it (e.g. epitaxially grow lower doped cladding layers, the active region and upper doped layers, using MBE, MOCVD, or LPE techniques);

2. Define the DFB grating and simultaneously define the laser ridge. This may be achieved by patterning an e-beam sensitive resist on top of a mask of SiO₂ for example, followed by etching of the mask and subsequent etching of the underlying semiconductor (reactive ion etching, RIE, can be used for the SiO₂ and inductively coupled plasma, ICP, etching can be used for the semiconductor material);

3. Mask all of the wafer (i.e. structure) using a layer of SiO₂, for example; 4. Pattern the mirror sections (in the present case this may be done with e-beam lithography followed by dry etching, such as an ICP etch to give a desired vertical profile);

5. Process as with any other laser diode, for example by contacting to the ridge section, not to the DBR sections.

In alternative embodiments, steps 2 & 4 could be replaced by focused ion beam milling.

In other alternative embodiments the grating and mirror definition steps can be combined into one etch. A DFB laser produced by such a technique is illustrated in figures 16-18. The front and rear facets of the device of figures 16-18 correspond to the view illustrated in figure 12. In this embodiment the channels or slots 22 between the grating elements 21 and the slots or channels 42 between the mirror elements 41 all have the same depth, and extend completely through the upper cladding layer 17, the active layer 15, and the lower cladding layer 14 to the substrate 13 surface. The simultaneous lithographic definition of the positions of the grating and reflector elements enables the facet phases to be precisely controlled.

In the embodiments of figures 12-15 and 16-18, the grating elements 21 are separated by air gaps (i.e. the slots/channels are gas-filled), as are the reflector elements 41. However, in alternative embodiments these gaps may be filled with solid material, that material having a different refractive index from that of the cladding layers. The method described above in relation to the embodiment of figures 12-15 can thus be modified to include an additional step, namely that prior to step 3, one can fill in the grating with another material of different refractive index.

In embodiments of the invention it is possible to tune the distance between the DFB and mirror grating to engineer a desired reflected facet phase.

The DFB and Bragg mirror section pitches and mark-space ratios are not necessarily the same. They can be adjusted to achieve an optimum design, dictated by the desired wavelength of operation and the refractive index of the materials used in the laser's construction. The lower reflectivity second facet mirror may also be referred to as an output coupler. In forming the Bragg reflector structure, one is able to choose multiples of lambda/4n and the number of periods (i.e. the order of the Bragg reflector) to achieve the desired reflectivities for both the high reflector (first facet) and the output coupler (second facet).

An advantage of embodiments of the invention which utilise Bragg reflectors for both facets is that the requirement to cleave a facet is eliminated.

Methods embodying the invention enable a precise control of facet phase to be achieved. Facet phase is important in that it determines the output power and modulation response of the laser device. Thus, embodiments of the invention may be used in a wide range of applications, such as telecommunications, gas sensing and high power applications.

It will also be appreciated that certain embodiments of the invention take the form of a DFB laser with a deeply etched Bragg mirror positioned on a nanometre scale with respect to the DFB grating, allowing the facet phase to be engineered to suit particular applications of the laser diode. By using electron beam lithography, the DFB grating and Bragg mirror may be positioned with sufficient accuracy to each other so as to ensure that all lasers produced by the method may have the same facet phase.

It will also be appreciated that in the prior art, the facet phase of a DFB laser was not controlled during manufacture. As a result, for single mode DFB lasers of the prior art, a wide variation in operating characteristics was obtained as facet phase impacts upon output power, dynamic response, chiφ, etc. In contrast, in embodiments of the invention, facet phase may be controlled by positing etched mirrors at the facets with an accuracy of a few tens of nanometres, or even better, with respect to the DFB grating. Facet phase may therefore be engineered to obtain optimum device performance and increase manufacturing yields, which can also reduce the cost of manufacture.

Referring now to figure 19, this is a highly schematic side view of another laser embodying the invention. The laser comprises a semiconductor substrate 13 on which a waveguide 1 is arranged. The waveguide extends along a longitudinal axis A. A Bragg Grating structure 2 is arranged with respect to the waveguide 1 so as to provide distributed feedback and enhance a single longitudinal mode of radiation within the waveguide 1. The Bragg Grating structure is shown schematically as comprising a number of grating elements 21 spaced apart along the longitudinal axis of the device with uniform pitch. At a first end 11 of the waveguide 1 there is arranged a Bragg Reflector structure 3 comprising a plurality of spaced-apart semiconductor elements 41 with gaps 42 in between. These gaps may be filled with air, a different gas, or other materials, such as solids having a different refractive index from the semiconductor material of the elements 41. In this example, the gap 421 between the reflector element 41 closet to the first end 11 of the waveguide 1 is different from the gaps 42 between adjacent reflector elements 41. In this example gap 421 is larger than separation 42, although in alternative embodiments it may be smaller, or the same. The size of this gap 421 can be selected to provide a desired phase relationship between the Bragg Reflector (Bragg Mirror) structure and the Bragg Grating structure which extends along the waveguide 1. A second end 12 of the waveguide 1 is the end from which laser light is emitted, and this is indicated generally by L. An upper electrode 100 is contacted to the upper surface of the waveguide 1. A lower electrode 101 is contacted to the lower surface of the substrate 13. The portion of the device under the upper electrode 100 thus represents a gain region of the device, but as the Bragg Mirror section 3 is not contacted it has a gain less than zero (i.e. it is lossy).

Thus, it will be appreciated that the Bragg Grating of the device in figure 19, whose elements are indicated by reference 21, may be a metal or semiconductor structure which provides distributed feedback in the gain region of the device. The Bragg Mirror section 3 is a highly reflective structure (this structure is periodic and may comprise alternating regions of different refractive index such as dielectric/ semiconductor material and air gaps). The Bragg Mirror structure may have a different phase to the Bragg Grating structure. It may also have a different pitch and/or a different mark-space ratio of element thickness to inter-element separation. The Bragg Reflector (Mirror) portion of the device is not electrically contacted to, and so its gain is less than zero (hence the reflector portion 3 which may also be referred to as a facet means, is lossy). However, this does not cause a problem as the length of this Bragg Reflector structure is generally arranged so as to be short compared with the length of the waveguide 1. An advantage of the Bragg Reflector structure being contact free (i.e. no electrical contacts are made to the reflector elements 41 on their upper surfaces) is that the cost to manufacture the device is reduced.

It will be further appreciated that in the device shown in figure 19, the Bragg Mirror section 3 acts to reflect light back into the "cavity" (i.e. the wave-guide 1 portion of the device) with a controlled/tuned facet phase (achieved by controlling the positions of the Bragg Mirror elements 41 with respect to the Bragg Grating structure 2). It will also be appreciated that the Bragg Reflector structure 3 in an embodiment of the invention may be formed in a different processing step to that used for the formation of the Bragg Grating 2 for the waveguide 1. In the above-described embodiments it will also be appreciated that the Bragg Reflector structure at one end, or indeed both ends, of the waveguide is different from the Bragg Grating structure arranged to interact with radiation propagating along the waveguide.

Referring now to figure 20, this is a schematic plan view of another laser device embodying the invention. The device is arranged to emit radiation in a direction indicated generally by L. The device comprises a waveguide 1 extending along a longitudinal axis A, and this waveguide 1 may, for example, be provided by an active layer of semiconductor material beneath a laser ridge. Other forms of wave-guide are of course possible in alternative embodiments. Adjacent to the wave-guide 1 there is arranged a Bragg Grating structure 2 which comprises a plurality of transverse grating elements 21 extending laterally (i.e. in directions peφendicular to the longitudinal axis A) from either side of the waveguide 1. Each grating element 21 has substantially the same thickness t2 and the separation between adjacent elements 21 is a substantially constant d2. Thus, the pitch P2 of the grating structure 2 is given by P2 = t2 + d2. A first Bragg Reflector structure 3 is arranged next to a first end 11 of the waveguide 1. This first reflector structure comprises a series of 4 dielectric reflector elements 41 separated by air gaps 42. The reflector elements 41 have uniform thickness t3 and adjacent elements 41 have uniform separation d3 (in other words the air gaps 42 have uniform width d3). Thus, the physical pitch P3 of the first Bragg Reflector structure 3 is given by P3 = t3 + d3. There is an air gap d23 between the first end 11 of the wave-guide 1 and the first element 41 of the first Bragg Reflector structure 3. This size of this air gap d23 may be selected to give a desired phase relationship between the grating structure 2 and the reflector structure 3. Thus, d23 may be equal to d3 in certain embodiments, and in other embodiments may be larger or smaller than d3. In the present embodiment, although the physical pitches P2 and P3 of the Bragg Grating structure 2 and first reflector structure 3 are different, the structures are arranged to have the same Bragg wavelength λβ. This Bragg wavelength corresponds to the wavelength of peak reflection, and is given by λβ = 2 x P x n_e where P is the grating period and n_e is the effective refractive index of the structure to the fundamental mode of radiation propagating through it.

At a second end 12 of the waveguide 1 there is provided a second Bragg Reflector structure 5, which in this example comprises a series of three reflector elements 41 separated by air gaps 42. The reflector elements 41 of the second reflector structure 5 are substantially planar dielectric structures having uniform thickness t5 and constant separation d5. The physical pitch P5 of the second grating structure 5 is thus given by P5 = t5 + d5. In this particular example the structures of the first and second Bragg Reflectors 3 and 5 are substantially the same, thus t3 = t5 and d3 = d5. The Bragg wavelength of the second reflector structure 5 is that the same as that of the first reflector structure 3. The width of the air gap d52 between the second end 12 of the waveguide 1 and the adjacent element 41 of the second reflector structure 5 is again selected to give a desired phase relationship between the Bragg Grating structure 2 and the second Bragg Reflector structure 5. d52 may thus be the same as d5 or smaller or larger. d52 may be the same or different from d23.

It will thus be appreciated that the embodiment described with reference to figure 20 is an example of a laser device in which there is a difference in physical pitch between the Bragg Mirror sections 3, 5 and the Bragg Grating section 2 providing distributed feedback (i.e. there is a difference in pitch between the DBM's and the DFB). In certain embodiments, the Bragg Mirror sections may each be provided by a periodic structure comprising alternating gaps and semiconductor material. The gaps may be air filled, dielectric filled, or filled with some other material. The waveguide 1 may be a ridge waveguide, and as a result of the change in refractive index in passing from the ridge wave-guide/Bragg Grating structure to the mirror section (or sections) the physical pitch will typically change in order for the Bragg wavelengths of the different sections to remain the same.

Referring again to figure 20, as indicated above the waveguide portion 1 may, in certain examples, be the portion of an active layer of semiconductor material beneath a laser ridge. Thus, in propagating along the waveguide, radiation is largely confined to the portion (strip) of the active layer under the ridge or rib. Thus, this radiation propagates in solid semiconductor material having a certain refractive index. As radiation propagates, it interacts with the distributed Bragg Grating elements 21 which extend transversely from the rib. The arrangement is such that a particular wavelength, the Bragg wavelength of the waveguide/Bragg Grating structure, is enhanced. This Bragg wavelength is determined by a number of factors, including the refractive index of the active region, the structure and materials of the grating (including the physical pitch of the elements 21 and the material between the grating elements). The reflectors at each end of the waveguide have substantially the same structure as each other, but a different structure to that of the Bragg Grating 2. For example, in certain embodiments, each reflector structure 3, 5 is provided by a repeated sequence of solid planar elements and separating air gaps. In certain applications, it is desirable for the Bragg wavelength of the reflector structures to be the same as that of the grating structure. As the reflector structure (alternating semiconductor material and air gaps) is different from that of the waveguide and grating structure 1, 2 (confinement of propagating radiation substantially in an active layer of solid semiconductor, with lateral coupling to the grating elements) then, to give the same Bragg wavelength, in general the pitch of the reflector elements will be different from the pitch of the grating elements. In certain embodiments the gaps d23, d52 between the ends of the wave-guide and the reflector elements may be the same as the air gaps between the reflector elements themselves. Thus, the reflector structure may be arranged in phase with the grating structure 2 and this gives particularly good enhancement of the single Bragg wavelength and enables the device to provide an output which is more monochromatic. In other embodiments, however, the gaps between the reflector elements and the gap between the first reflector element and the waveguide end may be different. One reason for the difference in structure between the Bragg Reflectors 3, 5 and the grating 2 is that it is desirable for the reflectivity of each reflector to be as high as possible for a particular Bragg wavelength. One way to do this is to ensure that there is a large contrast in refractive index between the reflector element material and the material in between them. This can be achieved by constructing the reflector elements from solid semi-conductor material, and arranging for air gaps in between. In contrast, in the waveguide section light is confined to propagate in a continuous, solid active region of semiconductor material; in propagating along the waveguide 1 it does not pass through any air gaps. The interaction with the Bragg Grating 2 is by lateral coupling.

It will be appreciated from the above description that in embodiments of the invention the elements of the Bragg Reflector structures may be parallel to the elements of the Bragg Grating structure 2. In other words the "gratings" may be parallel to one another. There structures may each vary periodically along a common direction corresponding to the longitudinal axis A of the device.

Referring now to Figures 21 to 24, each is a schematic plan view of part of a respective laser device embodying the invention. Each device comprises a waveguide 1, a Bragg Grating structure 2 arranged with respect to the wave-guide 1 to provide distributed feedback to enhance a particular mode of radiation, and a Bragg Reflector structure 3 arranged next to an end 11 of the wave-guide 1. In practice, each device will extend in a direction to the left in the figures and this continuation of the waveguide and Bragg Grating structure 2 is indicated generally by dots D. At the bottom of each figure there is a representation P of the phase of the Bragg Grating structure 2, with a broken line at the end showing the position at which each waveguide 1 is arranged to terminate with respect to the phase of the Bragg Grating structure 2. In the embodiment illustrated in figure 21, the waveguide 1 is arranged to terminate at a nominal zero degrees position with respect to the phase of the Bragg Grating Structure 2. In other words, the first end 11 of the wave-guide 1 coincides with an edge of a grating element 21. The thickness of each grating element is t2, and the separation 22 between each pair of adjacent elements 21 is d2. Thus the pitch of the grating structure 2 is P2 = t2 + d2. The Bragg Reflector structure 3 has the same Bragg wavelength as the Bragg Grating structure 2, but has a different physical pitch P3 = t3 + d3. In the example shown in figure 21 the air gap d23 between the end 11 of the waveguide 1 and the first reflector element of the Bragg Reflector 3 is the same as the air gap d3 between adjacent pairs of reflector elements.

Moving on to figure 22, this shows part of a laser device having generally the same structure as that described above with reference to figure 21, except for the fact that the first end 11 of the wave-guide 1 occurs at a different position with respect to the phase of the grating structure 2. Compared with figure 21, in the embodiment shown in figure 22 the waveguide 1 terminates at a position 90° earlier with respect to the grating 2 phase. The separation d23 between the end 11 of the wave-guide 1 and the first element 41 of the reflector structure 3 is the same as in figure 21 and again corresponds to the substantially constant separation d3 between adjacent reflective elements 41. In the embodiment of figure 22, although the Bragg wavelength of the reflector structure 3 is still the same as that of the grating structure 2, the phase of the reflector structure 3 is different from that of the grating 2. Thus, by adjusting the position of the end of the wave-guide with respect to the adjacent grating structure 2, the "facet phase" has been adjusted.

Moving onto figure 23, this shows part of a similar embodiment in which the waveguide 1 terminates at a position 90° earlier still than that in figure 22. Thus, compared with the embodiment of figure 21, the waveguide of the embodiment of figure 23 terminates at a nominal -180° position. The gap d23 is again substantially equal to separation d3 and the Bragg wavelengths of the grating structure 2 and reflector structure 3 are substantially the same. Figure 23 therefore simply represents a further adjustment of facet phase.

Lastly, referring to figure 24, this embodiment is generally the same as that shown in figures 21 to 23, but the waveguide end 11 is arranged at the nominal -270° position. Otherwise, the structure of the device illustrated in figure 24 is the same as those shown in figures 21 to 23.

From the above description of figures 21 to 24 it will be appreciated that by keeping separation d23 constant, and using grating structures 2 and Bragg Reflectors structures 3 having the same Bragg wavelength, the position of the reflector 3 relative to the grating structure 2 can be adjusted to give a different facet phase. The embodiments of figures 21 to 24 differ only in the phase of laser light exciting the waveguide and entering the mirror section 3. This facet phase has a strong impact upon device parameters and an advantage of embodiments to the invention is that one can tune device performance by changing this phase. The mirror structure and the Bragg Grating structures may be formed by techniques such as etching, which enable precise control to be achieved over the relative positions of the grating and reflector elements. Thus, precise control over facet phase may be achieved.

Claims

1. A distributed feedback laser comprising: a waveguide extending along a longitudinal axis; a Bragg grating structure arranged with respect to the waveguide to interact with electromagnetic radiation propagating along the waveguide so as to provide distributed feedback along the waveguide to enhance a mode of said radiation; and first facet means arranged at a first end of the waveguide to reflect propagating electromagnetic radiation back along the waveguide, characterised in that said first facet means comprises a first Bragg reflector structure.

2. A laser in accordance with claim 1 , wherein the first Bragg reflector structure and the Bragg grating structure are substantially different.

3. A laser in accordance with claim 1 or claim 2, wherein the pitch of the first Bragg reflector structure is different from the pitch of the Bragg grating structure.

4. A laser in accordance with any preceding claim, wherein the first Bragg reflector structure and the Bragg grating structure have substantially the same Bragg wavelength.

5. A laser in accordance with any preceding claim, wherein the first Bragg reflector structure and the Bragg grating structure are arranged in phase with one another.

6. A laser in accordance with any one of claims 1 to 4, wherein the first Bragg reflector structure and the Bragg grating structure are arranged a predetermined phase angle out of phase with one another.

7. A laser in accordance with any preceding claim, wherein the Bragg grating structure is arranged to couple laterally with electromagnetic radiation propagating along the waveguide.

8. A laser in accordance with any preceding claim, wherein the Bragg reflector structure is arranged in the path of electromagnetic radiation propagating along the waveguide.

9. A laser in accordance with any preceding claim, wherein the first Bragg reflector structure comprises a series of reflector elements, said elements being spaced apart along the longitudinal axis.

10. A laser in accordance with claim 9, wherein each element is substantially planar and is arranged substantially peφendicularly with respect to the longitudinal axis, and the planar elements are arranged parallel to each other.

11. A laser in accordance with claim 9 or claim 10, wherein the elements of each pair of adjacent elements of the first Bragg reflector structure are separated by a respective air gap.

12. A laser in accordance with any one of claims 9 to 11, wherein the elements of each pair of adjacent elements of the first Bragg reflector structure are separated by a respective channel.

13. A laser in accordance with claim 12, comprising a multi-layer semi-conductor structure extending along the longitudinal axis, the multi-layer structure comprising an active layer, a first cladding layer arranged on one side of the active layer, and a second cladding layer arranged on the opposite side of the active layer, wherein the waveguide comprises a first longitudinal portion of the multi-layer structure and the first Bragg reflector structure comprises a second longitudinal portion of the multilayer structure, adjacent the first portion, and each channel of the first Bragg reflector structure extends through the second cladding layer and active layer and at least partially through the first cladding layer.

14. A laser in accordance with claim 13, wherein the multi-layer semi-conductor structure further comprises a substrate, the first cladding layer being formed on the substrate, the active laver being formed on the first cladding layer, and the second cladding layer being formed on the active layer, and wherein each channel of the first Bragg reflector structure extends through the second cladding layer, active layer and the first cladding layer to the substrate.

15. A laser in accordance with any one of claims 12 to 14, wherein each channel has been formed by a process comprising etching.

16. A laser in accordance with any one of claims 12 to 15, wherein each channel is filled with a gas.

17. A laser in accordance with any one of claims 12 to 15, wherein each channel is filled with solid material.

18. A laser in accordance with any preceding claim, further comprising a second facet means arranged at a second end of the waveguide to reflect propagating electromagnetic radiation back towards the first end.

19. A laser in accordance with claim 18, wherein the second facet means comprises a second Bragg reflector structure.

20. A laser in accordance with claim 19, wherein the first and second Bragg reflector structures are periodic, each comprising substantially the same repeated substructure.

21. A laser in accordance with claim 20, wherein the first and second Bragg reflector structures comprise different numbers of said repeated sub-structure.

22. A laser in accordance with any one of claims 19 to 21, wherein the pitch of the second Bragg reflector structure is different from the pitch of the Bragg grating structure.

23. A laser in accordance with any one of claims 19 to 22, wherein the second Bragg reflector structure and the Bragg grating structure have substantially the same Bragg wavelength.

24. A laser in accordance with any one of claims 19 to 23, wherein the second Bragg reflector structure and the Bragg grating structure are arranged in phase with one another.

25. A laser in accordance with any one of claims 19 to 23, wherein the second Bragg reflector structure and the Bragg grating structure are arranged a predetermined phase angle out of phase with one another.

26. A laser in accordance with any one of claims 19 to 25, wherein the first and second Bragg reflector structures have substantially the same pitch.

27. A laser in accordance with any one of claims 19 to 26, wherein the first Bragg reflector structure is arranged to have a higher reflectivity than the second Bragg reflector structure.

28. A laser in accordance with any one of claims 19 to 27, comprising a multilayer semi-conductor structure extending along the longitudinal axis, the multi-layer structure comprising an active layer, a first cladding layer arranged on one side of the active layer, and a second cladding layer arranged on the opposite side of the active layer, wherein the waveguide comprises a first longitudinal portion of the multi-layer structure, the first Bragg reflector structure comprises a second longitudinal portion of the multi-layer structure, and the second Bragg reflector structure comprises a third longitudinal portion of the multi-layer structure, the first portion being arranged between the second and third portions.

29. A laser in accordance with claim 18, wherein the second facet means comprises a cleaved facet.

30. A laser in accordance with claim 29, wherein the second facet means further comprises an anti-reflection coating arranged on the cleaved facet.

31. A laser in accordance with any preceding claim, wherein the waveguide comprises an active layer of semiconductor material extending along the longitudinal axis and a laser ridge of semiconductor material extending along the longitudinal axis, over the active layer.

32. A laser in accordance with claim 31, wherein the Bragg grating structure comprises a first plurality of grating elements, spaced apart along the longitudinal axis and extending transversely with respect to the longitudinal axis from one side of the ridge, and a second plurality of grating elements, spaced apart along the longitudinal axis and extending transversely with respect to the longitudinal axis from an opposite side of the ridge.

33. A laser in accordance with claim 32, wherein adjacent grating elements of the first plurality of grating elements and of the second plurality of grating elements are separated by etched slots.

34. A laser in accordance with claim 33, wherein the etched slots are gas-filled.

35. A laser in accordance with claim 33, wherein the etched slots are filled with solid material.

36. A laser in accordance with any preceding claim, and further comprising an electrode structure arranged to enable current to be injected into the laser.

37. A method of manufacturing a distributed feedback laser, comprising the steps of: providing a multi-layer semi-conductor waveguide structure extending along a longitudinal axis, the multi-layer structure comprising an active layer, a first cladding layer arranged on one side of the active layer, and a second cladding layer arranged on the opposite side of the active layer; forming a Bragg grating structure along a first longitudinal portion of the multi-layer semi-conductor waveguide structure so as to provide distributed feedback to electromagnetic radiation propagating along the first longitudinal portion; and forming a first Bragg reflector structure in a second longitudinal portion of the multi-layer semi-conductor waveguide structure, the second longitudinal portion being adjacent to a first end of the first longitudinal portion, and the first Bragg reflector being arranged to reflect electromagnetic radiation propagating to the first end back along the first longitudinal portion.

38. A method in accordance with claim 37, further comprising the step of: forming a second Bragg reflector structure in a third longitudinal portion of the multi-layer semi-conductor waveguide structure, the third longitudinal portion being adjacent to a second end of the first longitudinal portion, and the second Bragg reflector being arranged to reflect electromagnetic radiation propagating to the second end back along the first longitudinal portion towards the first end.

39. A method in accordance with claim 37 or claim 38, further comprising the step of forming a longitudinal ridge in the second cladding layer, along at least part of the first longitudinal portion of the multi-layer semi-conductor waveguide structure.

40. A method in accordance with claim 39, wherein the step of forming the longitudinal ridge comprises the steps of lithographically defining the position of the ridge on a surface of the second cladding layer and then selectively removing second cladding layer material from either side of the ridge.

41. A method in accordance with claim 40, wherein the step of selectively removing second cladding layer material comprises etching.

42. A method in accordance with claim 40 or claim 41, wherein the step of selectively removing second cladding layer material comprises removing second cladding layer material so as to leave a reduced-thickness layer of cladding material covering the active layer on either side of the ridge.

43. A method in accordance with claim 42, wherein the step of forming the Bragg grating structure comprises the steps of lithographically defining positions of grating elements on the reduced- thickness layer on either side of the ridge, and then forming the grating elements at said positions.

44. A method in accordance with claim 43, wherein each grating element extends from the ridge in a direction transverse to the longitudinal axis.

45. A method in accordance with claim 43 or claim 44, wherein the step of forming the grating elements comprises depositing metallic material.

46. A method in accordance with any one of claims 37 to 41, wherein the step of forming the Bragg grating structure comprises the steps of lithographically defining the positions of grating elements on a surface of the second cladding layer and then selectively removing second cladding layer material from between the grating elements.

47. A method in accordance with claim 46, comprising the step of simultaneously lithographically defining the position of a longitudinal ridge and the positions of grating elements on either side of the ridge on a surface of the second cladding layer, and the step of then selectively removing second cladding layer material from either side of the ridge and from between the grating elements to form the ridge and grating elements.

48. A method in accordance with any one of claims 37 to 47, wherein the step of forming the or each Bragg reflector structure comprises the steps of lithographically defining the positions of reflector elements on a surface of the second cladding layer and then selectively removing material from between the reflector elements.

49. A method in accordance with claim 48, as dependent on claim 38, comprising the steps of simultaneously lithographically defining the positions of reflector elements of the first and second Bragg reflectors on respective surface portions of the second cladding layer and then simultaneously selectively removing material from between the reflector elements of each reflector.

50. A method in accordance with claim 48 or 49, wherein the step of selectively removing material from between the reflector elements comprises forming a respective gap between each pair of adjacent reflector elements, said gap extending through the second cladding layer, and through the active layer.

51. A method in accordance with claim 50, wherein said gap extends at least partially through the first cladding layer.

52. A method in accordance with claim 51, wherein the multi-layer semiconductor structure further comprises a substrate, the first cladding layer being formed on the substrate, the active laver being formed on the first cladding layer, and the second cladding layer being formed on the active layer, and wherein each gap extends through the second cladding layer, active layer and the first cladding layer to the substrate.

53. A method in accordance with any one of claims 37 to 52, comprising the step of simultaneously lithographically defining the positions of a longitudinal ridge, Bragg grating elements, and Bragg reflector elements on a surface of the second cladding layer, and then simultaneously removing material from the waveguide structure to form the ridge and the grating and reflector elements.

54. A method in accordance with any one of claims 48 to 53, wherein the step of removing material comprises etching.

55. A method in accordance with any one of claims 48 to 53, wherein the step of removing material comprises focussed ion beam milling.

56. A method in accordance with any one of claims 37 to 55, wherein the step of providing a multi-layer semi-conductor waveguide structure comprises the steps of providing a substrate, forming the first cladding layer on the substrate, forming the active layer on the first cladding layer, and forming the second cladding layer on the active layer.

57. A method in accordance with any one of claims 37 to 56, wherein at least one of the first cladding, active, and second cladding layers is a multi-layer structure.