GB2416427A

GB2416427A - DFB laser

Info

Publication number: GB2416427A
Application number: GB0413664A
Authority: GB
Inventors: Richard Hogg; Kristian Groom
Original assignee: University of Sheffield
Current assignee: University of Sheffield
Priority date: 2004-06-18
Filing date: 2004-06-18
Publication date: 2006-01-25
Also published as: WO2005124951A8; GB0413664D0; WO2005124951A1

Abstract

The laser comprises Bragg reflectors 41 situated at the facets and a Bragg grating 21 along the longitudinal waveguide axis. The Bragg grating comprises transverse elements for lateral coupling with longitudinal radiation and the Bragg reflector comprises a periodic structure of semiconductor material and air gaps.

Description

24 1 6427 Distributed Feedback Laser

Field of the lavention

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 DEB 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. l, facets F1 and F2 are formed at opposite ends of the structure, the first facet F1 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 F1 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 "chirp" of the device. In this context, chirp 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 chirp 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 DEB lasers, and methods of producing DEB 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, characterized 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 mono- chromatic output.

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 perpendicularly 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 semiconductor structure extending along the longitudinal axis, the multilayer 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 multi- layer 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 unchirped). 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.

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.

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 multilayer 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.

Abain, 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 multilayer 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 reducedthickness 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 semiconductor 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 crosssection 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 crosssection 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 crosssection 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; and Figure l 8 is a schematic cross-section of the device from figures 16 & 17, taken along line B-B in figure 16.

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 DEB 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 re hector 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 reDector 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 RIE, 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 ofthe ridge 17); 4. Form the DEB 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 offmaterial 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 thelower 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 DEB 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 DEB 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 DEB and mirror grating to engineer a desired reflected facet phase.

The DEB 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/4p 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, chirp, 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.

Claims

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 comprises a series of reflector elements, said elements being spaced apart along the longitudinal axis.
3. A laser in accordance with claim 2, wherein each element is substantially planar and is arranged substantially perpendicularly with respect to the longitudinal axis, and the planar elements are arranged parallel to each other.
4. A laser in accordance with claim 3 or claim 4, wherein the elements of each pair of adjacent elements of the first Bragg reflector structure are separated by a respective air gap.
5. A laser in accordance with any one of claims 2 to 4, wherein the elements of each pair of adjacent elements of the first Bragg reflector structure are separated by a respective channel.
6. A laser in accordance with claim 5, comprising a multi-layer semiconductor structure extending along the longitudinal axis, the multilayer 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 multi layer 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.
7. A laser in accordance with claim 6, wherein the multi-layer semiconductor structure further comprises a substrate, the first cladding layer being formed on the substrate, the active layer 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.
8. A laser in accordance with any one of claims 5 to 7, wherein each channel has been formed by a process comprising etching.
9. A laser in accordance with any one of claims 5 to 7, wherein each channel is filled with a gas.
10. A laser in accordance with any one of claims 5 to 7, wherein each channel is filled with solid material.
11. 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.
12. A laser in accordance with claim 11, wherein the second facet means comprises a second Bragg reflector structure.
13. A laser in accordance with claim 12, wherein the first Bragg reflector structure is ananged to have a higher reflectivity than the second Bragg reflector structure.
14. A laser in accordance with claim 12 or claim 13, 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.
15. A laser in accordance with claim 11, wherein the second facet means comprises a cleaved facet.
16. A laser in accordance with claim 15, wherein the second facet means further comprises an anti-reflection coating arranged on the cleaved facet.
17. 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.
18. A laser in accordance with claim 17, 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.
19. A laser in accordance with claim 18, wherein adjacent grating elements of the first plurality of grating elements and of the second plurality of grating elements are separated by etched slots.
20. A laser in accordance with claim 19, wherein the etched slots are gasfilled.
21. A laser in accordance with claim 19, wherein the etched slots are filled with solid material.
22. 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.
23. A method in accordance with claim 22, 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.
24. A method in accordance with claim 22 or claim 23, 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.
25. A method in accordance with claim 24, 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.
26. A method in accordance with claim 25, wherein the step of selectively removing second cladding layer material comprises etching.
27. A method in accordance with claim 25 or claim 26, 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.
28. A method in accordance with claim 27, 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.
29. A method in accordance with claim 28, wherein each grating element extends from the ridge in a direction transverse to the longitudinal axis.
30. A method in accordance with claim 28 or claim 29, wherein the step of forming the grating elements comprises depositing metallic material.
31. A method in accordance with any one of claims 22 to 26, 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.
32. A method in accordance with claim 31, 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.
33. A method in accordance with any one of claims 22 to 32, 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.
34. A method in accordance with claim 33, as dependent on claim 23, 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.
35. A method in accordance with claim 33 or 34, 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.
36. A method in accordance with claim 35, wherein said gap extends at least partially through the first cladding layer.
37. A method in accordance with claim 36, 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.
38. A method in accordance with any one of claims 22 to 37, 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.
39. A method in accordance with any one of claims 33 to 38, wherein the step of removing material comprises etching.
40. A method in accordance with any one of claims 33 to 38, wherein the step of removing material comprises focussed ion beam milling.
41. A method in accordance with any one of claims 22 to 40, 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.
42. A method in accordance with any one of claims 22 to 41, wherein at least one of the first cladding, active, and second cladding layers is a multi-layer structure.