BACKGROUND This invention relates to lasers, for example to promoting single mode lasing and improving the ease of manufacture of distributed feedback lasers.

High-performance and low-cost laser modules are used in applications such as large-capacity and high-speed optical access networks.

A conventional laser diode generally comprises a semiconductor block which has a front face or facet, a rear face or facet opposite to the front facet and a laser cavity formed therebetween. The cavity traditionally comprises an active layer interposed between layers of p- or n-type semiconductor material. One or more coating layers, such as anti-reflection (AR) or high reflection (HR) coatings, may be applied to the front and the rear facets to provide a predetermined reflectivity.

In distributed feedback (DFB) lasers, a Bragg grating acts as the wavelength selective element for at least one of the faces and provides feedback, reflecting light back into the cavity to form the resonator. Traditionally, DFB lasers are AR coated on one side of the cavity and HR coated at the other side. The side with the AR coating is the front of the laser, through which light is to be emitted. The side with the HR coating is the back of the laser. The grating may act as a distributed mirror inboard of the AR coated side of the cavity. The HR coating acts as a mirror on the other side of the cavity. The HR coated side inhibits losses from the rear of the cavity.

It is desirable to confine the gain medium in an optical waveguide. A waveguide restricts the region in which light can propagate and comprises a region of increased refractive index relative to the surrounding material, such that total internal refection of light occurs within the waveguide. This makes it possible to direct the emitted light into a collimated beam, and allows a laser resonator to be built such that light can be coupled back into the gain medium.

It is preferable to break the cavity symmetry of a DFB laser to promote the operation of a single lasing mode. A tapered waveguide can be used to break the DFB structure symmetry. However, tapering the waveguide affects the refractive index of the waveguide, such that the grating must be chirped to result in a single wavelength output. Alternatively, a curved waveguide may be used to break the DFB structure symmetry, as described in EP 1677396 B1. However, this device uses a buried heterostructure (BH) waveguide. Manufacturing a laser with a BH waveguide is complex, as overgrowth of semiconductor material is required to bury the waveguide.

It is desirable to develop a laser that is easy to manufacture with a constant grating pitch that does not need to be chirped to promote a single lasing mode.

SUMMARY OF THE INVENTION

There is provided a laser having a rear reflector, a front reflector and a laser cavity having a length defined between the rear reflector and the front reflector, the laser cavity comprising: a substrate having an upper surface; and a waveguide projecting from the upper surface of the substrate; one of the rear reflector and the front reflector being constituted by a Bragg grating extending along the waveguide, the Bragg grating comprising a series of parallel elements of equal physical spacing, the elements being arranged across the width of the waveguide; wherein at least part of the waveguide has a varying width and in that part of the waveguide the waveguide is inclined relative to the length of the cavity such that the elements in the part are effective to promote lasing in the cavity by reflecting monochromatic light. Using a curved waveguide does not require the grating to be chirped in order to achieve a single lasing wavelength along the cavity. The waveguide may be arranged such that in the said part of the waveguide the product of the effective spacing of the elements for light travelling along the waveguide and the width of the waveguide is constant. This may result in a single lasing wavelength.

The width of the waveguide may be measured parallel to the elements.

The laser may be a distributed feedback laser. This may be a convenient operational format.

The waveguide may be inclined with respect to the elements. This configuration allows the effective pitch of the grating to be varied.

The width of the said part of the waveguide may decrease from the end of the waveguide part closest to the rear reflector to the end of the waveguide part closest to the front reflector. Another part of the waveguide other than the said part may have a constant width and may be aligned with the length of the cavity. The may allow flexibility in manufacturing the laser. The front reflector may be constituted by the Bragg grating. The rear reflector may be constituted by a cleaved facet. This is a convenient method for manufacturing the laser.

The rear reflector may be coated with a reflective coating. This may improve the performance of the laser.

The elements may be arranged perpendicular to the length of the cavity. This may be convenient for manufacturing the laser.

The waveguide may be asymmetric about any axis perpendicular to the length of the cavity. The waveguide may selectively promote a preferred lasing mode of the laser.

The waveguide may be a ridge waveguide. This allows convenient manufacturing of the laser.

The substrate may be a semiconductor substrate.

The laser may also comprise a pair of electrodes disposed on either side of the semiconductor substrate, the laser being configured such that light emission can be stimulated from the semiconductor substrate by applying a current across the electrodes. This is a convenient laser configuration.

The laser may be integrated with a modulator and/or semiconductor optical amplifier. This may allow the laser to be integrated with other optically functional structures.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example with reference to the accompanying drawings.

In the drawings: Figure 1 shows a vertical section through a laser.

Figures 2 illustrates a vertical section through the laser of Figure 1 at right angles to the section of Figure 1 .

Figure 3 illustrates a plan view of the cavity of the laser of Figure 1.

Figure 4 shows a detailed view of the waveguide of the laser of Figure 3. Figure 5 shows a schematic of the variation of waveguide parameters with distance along the waveguide.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in Figure 1 , one form of laser comprises a semiconductor block which has a front face 1 , a rear face 2 opposite to the front face and a laser cavity formed therebetween. The front and rear faces may be cleaved facets. It is preferable that the front and rear facets are aligned parallel to one another. A high reflection (HR) coating 3 may be applied to the rear facet. The back facet with the HR coating acts as a rear reflector. Light exits the laser cavity at the front face, shown at 4.

In the example shown in Figure 1 , the laser cavity comprises an active layer 5 interposed between layers of p- and n-type semiconductor material, shown at 6 and 7 respectively. In this example, the semiconductor layers are made from InP. However, other semiconductor materials, such as GaAs, may be used. The material forming the cavity may be selectively doped in the region of the p- and n-type layers 6, 7. Layers 5, 6 and 7 are defined in a substrate 13. A waveguide 8 projects from the upper surface of the substrate 13, as shown in Figure 2. The waveguide comprises a material with a refractive index greater than that of the substrate. Light is emitted from the end of the waveguide at the front face of the laser.

The waveguide may be a ridge waveguide. A ridge waveguide may be created by etching parallel trenches in the material either side of the waveguide to create an isolated projecting strip, typically less than 10 urn wide and several hundred μηη long. A material with a lower refractive index than the waveguide material can be deposited at the sides of the ridge to guide injected current into the ridge. Alternatively, the ridge may be surrounded by air on the three sides that are not in contact with the substrate beneath the waveguide. The ridge may also be coated with gold to provide electrical contact and to assist heat removal from the ridge when it is producing light.

The waveguide comprises a Bragg grating 9. The Bragg grating may be positioned between the waveguide ridge 8 and the p-lnP layer 6. The Bragg grating extending along the waveguide acts as at least one of the front or rear reflectors in the cavity.

The Bragg grating comprises a series of parallel elements 10 of regular physical spacing, A _P h _ys , as shown in the plan view of Figure 3. The parallel elements are arranged across the width of the waveguide, orthogonal to the length of the laser cavity. At least part of the waveguide has a varying width, w, along its length. The width of the waveguide is measured parallel to the elements 10. The width of the waveguide may vary along part of or along the entire length of the waveguide, as shown in Figure 3. The varying width of the waveguide gives rise to a variation in the effective refractive index of the waveguide, n _e ff, with respect to its length along the laser cavity. As the width of the waveguide decreases, the effective refractive index n _e ff of the waveguide decreases, the light being confined in a smaller volume.

In the example shown in Figures 3 and 4, the waveguide is tapered such that the width of the waveguide decreases with distance from the rear reflector to the front face.

For a straight waveguide, the physical grating pitch A _P h _ys measured parallel to the cavity length is proportional to A/n _e ff, where λ is the lasing wavelength. By varying the width of the waveguide (and hence varying n _e ff), in order to keep the lasing wavelength constant, the effective grating pitch as seen by light travelling along the waveguide must also be varied.

As shown in Figure 3, the waveguide is inclined relative to the length of the cavity such that the product of the effective spacing of the elements of the Bragg grating for light travelling along the waveguide, A _e ff, and the effective refractive index n _e ff of the waveguide is a constant. The waveguide may be inclined to the cavity length by an angle of 5D , 10 , 15D or 20 D . Preferably, one or both lateral faces of the waveguide is/are curved. Preferably the centreline of the waveguide as viewed on plan is curved. The curvature of a face may increase towards the front of the cavity. Figure 4 shows a region of the waveguide of Figure 3 in more detail. Curving the waveguide increases the path length of light travelling between the elements of the Bragg grating. Therefore, the effective pitch of the Bragg grating, A _e ff, is larger than the physical pitch measured parallel to the width of the waveguide, A _P h _ys . In Figure 4, a first position indicated at 1 1 is close to the rear face of the cavity. A second position indicated at 12 is closer to the front of the cavity. At position 1 1 , the width of the waveguide is wi, the effective refractive index is n _e ff,i and the effective pitch of the grating is Aeff,i . At position 12, the width of the waveguide is W2 and the effective refractive index is n _e ff,2 and the effective pitch of the grating is A _e ff,2. wi > W2, n _e ff,i > n, _e ff,2 and A _e ff,i < A _e ff,2. In both positions, the physical pitch of the waveguide measured along the cavity length (x direction) and parallel to the elements 10 of the Bragg grating has a constant value of A _P h _ys .

For optimum operation, at each position, the width and curvature of the waveguide are selected such that (wi ^* A _e ff,i) = (W2 * A _e ff,2).

Thus, the increase in A _e ff towards the front face of the laser cavity counteracts the decrease in effective refractive index due to the decreased width of the waveguide. As such, the Bragg grating elements in the curved part of the waveguide are effective to promote lasing in the cavity by reflecting monochromatic light. As a result, constant physical grating pitch, A _P h _ys , in the direction of the length of the cavity can be applied to the structure.

Using a curved waveguide can avoid the need for the grating to be chirped in order to achieve a single lasing wavelength along the cavity length. Instead the elements of the Bragg grating can all be equally spaced and parallel. Furthermore, using a ridge waveguide is a more manufacturable approach than using a buried heterostructure waveguide. This can result in a high yield, monochromatic DFB laser which can be conveniently manufactured.

In some embodiments, such as the configuration described above, the waveguide comprises a fully curved ridge waveguide. The ridge waveguide width changes to ensure that λ is a constant, as described above. The product of the waveguide width and the effective grating spacing is a constant. The physical grating pitch is constant. In other embodiments, the waveguide may have one part that is curved with varying width and one part that is straight (i.e. aligned parallel with the length of the cavity) and of uniform width in the direction perpendicular to the cavity length. The curved ridge waveguide width narrows towards the front face of the laser cavity to reduce the effective refractive index to ensure that the lasing wavelength, λ, in the curved part of the waveguide is the same as for the portion of the cavity that has a straight ridge waveguide. Thus, the physical grating pitch in the direction parallel to the length of the cavity can be constant across all parts of the waveguide.

The above-mentioned DFB laser can be integrated with an electroabsorption modulator (EAM). The EAM can have a straight waveguide or curved waveguide.

The front facet of the laser cavity may be coated with an anti reflection (AR) coating.

The Bragg grating may be fabricated by electron beam lithography. This allows the accuracy of the grating spacing to be controlled very accurately. The pitch of the grating may be approximately 300 nm, 200 nm, or 50nm.

The grating may be an index coupled grating, a gain coupled grating or a complex coupled grating. The layer comprising the waveguide and the grating may be fabricated from a p- doped or n-doped semiconductor material.

The laser structure may be integrated with another optically functional structure, for example an electroabsorption modulator, a Mach-Zehnder modulator, or an amplifier. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.