WO2023227189A1

WO2023227189A1 - Tilted semiconductor laser

Info

Publication number: WO2023227189A1
Application number: PCT/EP2022/063856
Authority: WO
Inventors: Xin Chen; Qi JIANG; Chaoyi WANG; Wang HAIBO; Bo Zhou
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2022-05-23
Filing date: 2022-05-23
Publication date: 2023-11-30

Abstract

Described is a distributed feedback laser (400, 500), the laser comprising: a light emissive structure (601); and a waveguide (401, 402, 501, 502, 503) located so as to receive light from the light emissive structure (601), the waveguide extending along a path between a front face (404, 505) and a rear face (403, 504), the rear face being more reflective than the front face; wherein the waveguide comprises a first portion (401, 501 ) adjacent the rear face and a second portion (402, 502, 503) adjacent the first portion (401, 501) and the front face (404, 505), the first portion comprising no Bragg grating, the second portion comprising a Bragg grating (405, 507); and wherein the path of the waveguide deviates at the meeting of the first portion (401, 501) and the second portion (402, 502, 503).

Description

TILTED SEMICONDUCTOR LASER

FIELD OF THE INVENTION

This invention relates to lasers, for example to distributed feedback lasers for use in applications such as telecommunications and data centres.

BACKGROUND

Single mode InP distributed feedback (DFB) lasers are widely used in telecommunication applications.

Figures 1 (a) and 1 (b) schematically illustrate an example of a conventional DFB laser. The DFB laser comprises a semiconductor structure which has a rear face or facet 101 , a front face or facet 102 opposite to the front face or facet and a laser cavity formed therebetween. The laser cavity comprises an active layer 103 interposed between layers of p- and n-type semiconductor material, shown at 104 and 105 respectively. A current can be applied across electrodes or contacts 106a, 106b on opposing top and bottom sides of the cavity respectively. The cavity, shown generally at 107, comprises a waveguide along which light may be guided. Light is emitted from the cavity at the front face 102, as shown at 108.

A Bragg grating 109 acts as the wavelength selective element and provides feedback, reflecting light back into the cavity to form the resonator. The grating may be constructed so as to reflect only a narrow band of wavelengths.

In a DFB laser, the rear facet 101 is normally coated with a high-reflection (HR) coating to enhance the output power. The rear facet 101 with the HR coating acts as a rear reflector. The front facet 102 is commonly coated with an anti-reflection (AR) coating to increase output power and reduce reflection. The grating 109 normally spans from the HR facet 101 to the AR facet 102.

To result in a single emission wavelength (A), the relationship between the grating pitch, A(x), and the waveguide refractive index profile n_efr (x) should meet the condition A = 2 n_eff(x) * A(x).

Single-mode DFB lasers may have straight waveguides or curved waveguides. A DFB laser usually has a constant waveguide width and a constant grating pitch, as this allows for efficient mass production. Normally, a DFB with a full grating has two competing lasing modes, due to symmetry. To break the symmetry, so that only one mode is lasing, various schemes have been proposed, including introducing a A/4 phase shift, along with HR/AR coating of the facets. However, the DFB yield is still sensitive to facet phases and external optical reflection.

For single-mode DFB lasers, the external optical feedback not only changes the threshold current, efficiency, linewidth, and intensity noise, but also destroys the modulated diagram and produces bit-error-rate (BER) saturation.

Normally, a DFB or electro-absorption modulated laser (EML) chip is soldered to an AIN submount. One conventional solution to minimise the refection from the light path back to the DFB or EML is to place an isolator close to the DFB or EML output facet, as schematically illustrated in Figure 2. The DFB or EML is shown at 201. A signal is supplied via slit 202 and flexible printed circuit board 203. A monitoring photodetector 204 is placed behind the device 201. Light exits the device 201 and passes through lens 205 and sapphire window 206 to isolator 207 and ferrule 208. A thermal electrical cooler 209 can be used to cool the structure.

Inserting an optical isolator in the DFB module can cut off the external optical feedback. However, the isolator is large in size. Extra space is needed on the AIN sub-mount or package and solder or epoxy is also required to attach the isolator in an additional processing step. An external isolator is also very expensive and, furthermore, it is difficult to achieve non-hermetic packages when using an isolator.

In an alternative solution, as shown in Figure 3, a monitoring photodetector (mPD) 301 is integrated on the back of the DFB chip 302 and the rear facet is generally AR coated. The mPD at the back of the chip absorbs the light from the DFB and thus the DFB laser effectively has AR coated facets at both the front and back faces of the cavity. Normally, to boost the output power of the DFB, a HR coating is applied to the back facet. As a result of the mPD at the back of the chip, the output power of the DFB is reduced by 2-3dB. This also results in a significantly larger chip size, making the chip more expensive.

In a further solution, a window structure can be used on the output facet. However, a window structure may involve an extra growth step and result in a bigger chip.

Using curved waveguide structure may also be used with a constant pitch grating, where the waveguide width is changed to match the Bragg reflection condition. However, a curved waveguide section with a grating requires very strict control on the waveguide width in relation to the curvature and is not generally suitable for mass production. In a DFB laser, the waveguide width should be compatible with the grating pitch to have stable emission wavelength, i.e. A=2n_effA, where n_efr is the effective refractive index of the waveguide and A is the grating pitch along the light propagation direction. A commercial DFB product should be easy to mass manufacture, with a high yield, which is why most DFB lasers have a constant grating period A, a straight waveguide and a constant waveguide width d, to give a uniform n_eff. Therefore, it is generally difficult to introduce a curved waveguide having a grating, unless using a chirped grating pitch, which is not convenient for mass production.

It is desirable to develop a laser that can overcome at least some of the above issues, with improved reflection tolerance and easy manufacturability.

SUMMARY OF THE INVENTION

According to one aspect, there is provided a distributed feedback laser, the laser comprising: a light emissive structure; and a waveguide located so as to receive light from the light emissive structure, the waveguide extending along a path between a front face and a rear face, the rear face being more reflective than the front face; wherein the waveguide comprises a first portion adjacent the rear face and a second portion adjacent the first portion and the front face, the first portion comprising no Bragg grating, the second portion comprising a Bragg grating; and wherein the path of the waveguide deviates at the meeting of the first portion and the second portion.

According to another aspect, there is provided a distributed feedback laser, the laser comprising: a light emissive structure; and a waveguide located so as to receive light from the light emissive structure, the waveguide extending along a path between a front face and a rear face, the rear face being more reflective than the front face; wherein the waveguide comprises a first portion adjacent the rear face and a second portion adjacent the first portion and the front face, the first portion comprising no Bragg grating, the second portion comprising a Bragg grating; and wherein the second portion comprises a region that extends along a straight path that is different from any path along which any region of the first portion extends.

The resulting DFB laser may have improved reflection tolerance and is straightforward to manufacture, as it does not require a chirped Bragg grating or very strict control on the waveguide width. The Bragg grating may extend along the entirety of the second region between its meeting with the first portion and the front face. This may be a possible configuration that provides improved tolerance to reflection with a structure that is easy to manufacture.

The second portion may comprise a first sub-portion adjacent the first portion and a second sub-portion adjacent the first sub-portion and the front face, the first sub-portion comprising no Bragg grating and the second sub-portion comprising the Bragg grating. This may be an alternative configuration that provides improved tolerance to reflection.

The first sub-portion and the second sub-portion may extend along respective paths. The respective paths may be collinear. The first-sub-portion and the second-sub portion may therefore follow a collinear path, but with the second sub-portion comprising a Bragg grating and the first sub-portion not comprising a Bragg grating.

The first sub-portion may extend along a curved path. A curved portion without a Bragg grating is more straightforward to manufacture than a curved portion with a Bragg grating. Therefore, this may allow for versatility in the structure of the first sub-portion of the waveguide, which may follow a straight or curved path.

The first portion may extend along a straight path. This may allow for easier manufacturing of the laser.

The first portion may extend along a curved path. A curved portion without a Bragg grating is more straightforward to manufacture than a curved portion with a Bragg grating. Therefore, this may allow for versatility in the structure of the waveguide.

The front and rear faces may be front and rear facets of a body of semiconductor material. The front and rear facets may be cleaved facets. This may be convenient for forming the faces during manufacture of the laser. The front and rear facets may alternatively be etched facets.

The Bragg grating may comprise a plurality of regularly spaced grating elements, each grating element extending parallel to the front face. The Bragg grating may be formed by techniques such as electron beam lithography. This can allow the accuracy of the grating spacing to be controlled very accurately. The grating may be an index coupled grating, a gain coupled grating or a complex coupled grating. This can allow for versatility in the structure and/or function of the Bragg grating. The greatest deviation between any part of the path of the first portion and any part of the path of the second portion may be 15 degrees. This may allow for efficient manufacture of the laser.

The greatest deviation between any part of the path of the first portion and a normal to the rear face may be 10 degrees. This may further allow for efficient manufacture of the laser.

The light emissive structure may comprise doped semiconductor layers. The doped semiconductor layers may be on either side of the waveguide. The light emissive structure may comprise electrodes on either side of the doped semiconductor layers. This may be a convenient laser configuration for stimulating the emission of light from the structure. Other configurations for providing a light emissive structure may alternatively be used.

The width of the first portion in a direction locally perpendicular to its path and parallel to the semiconductor layers may be constant. The width of the second portion in a direction locally perpendicular to its path and parallel to the semiconductor layers may be constant. This may allow the laser to be easily manufactured and the width of the waveguide portions may be chosen accordingly to result in a suitable effective refractive index of the waveguide portions. The width of the first portion in a direction locally perpendicular to its path and parallel to the semiconductor layers may alternatively be variable. As the first portion does not contain a Bragg grating, there is no need to meet the Bragg grating condition to give a stable emission wavelength and therefore this may allow for versatility in the structure of the first portion of the waveguide.

The length of the Bragg grating may be between 40% and 60% of the total length between the front and rear faces. This may break the symmetry of the waveguide and result in improved reflection tolerance of the laser.

The Bragg grating may have a constant pitch. This may allow for efficient production of the laser.

The path of the waveguide may abruptly deviate at the meeting of the first portion and the second portion. Tilting the second portion of the waveguide relative to the first portion may improve the reflection tolerance of the laser.

In some embodiments, the path of the first portion of the waveguide may gradually deviate before meeting the second portion. The front face may be coated with an anti-reflection coating and the rear reflector may be coated with a high-reflection coating. This may increase the power and improve the efficiency of the laser.

The waveguide may be a ridge waveguide or a buried heterostructure waveguide. This may allow for versatility of the laser structure.

The laser cavity may comprise a first semiconductor layer of a first doping type, a second semiconductor layer of a second doping type opposite to the first type, and an active region located between the first and second semiconductor layers, the first and second semiconductor layers being elongated in a direction extending between the rear face and the front face. This may be a convenient laser structure.

The Bragg grating may be located between the first and second semiconductor layers or within one of these layers in the second waveguide portion. This may be convenient for manufacturing the laser.

BRIEF DESCRIPTION OF THE FIGURES

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

In the drawings:

Figure 1 (a) schematically illustrates a side view of a conventional DFB laser.

Figure 1(b) schematically illustrates a top view of a conventional DFB laser.

Figure 2 schematically illustrates the use of an isolator with a DFB laser or EML.

Figure 3 shows an example of a device where a DFB laser chip and a photodetector are combined.

Figure 4 schematically illustrates a top view of one example of a DFB laser according to an embodiment of the present invention.

Figure 5 schematically illustrates a top view of another example of a DFB laser according to an embodiment of the present invention.

Figure 6(a) schematically illustrates a side view of the laser of Figure 4.

Figure 6(b) shows a cross-sectional view of the laser of Figure 4 along section A-A of Figure 6(a). DETAILED DESCRIPTION

Embodiments of the present invention use a partially tilted waveguide with a partial Bragg grating to form a reflection tolerant DFB laser. The section of the waveguide of the laser adjacent to the front face of the laser is tilted by an angle 0 relative to the normal to the plane of the front face and the waveguide width. The pitch of the partial Bragg grating (which is in the tiled section adjacent to the front face) can be constant. As a result, the product of the effective refractive index n_efr and the grating pitch A can be constant and the partial grating DFB laser with tilted grating section can emit light having a wavelength of A=2 n_effA/cos(0).

As illustrated in the top view of Figure 4, one example of a DFB laser comprises two cavity portions each comprising a part of the waveguide of the laser, which acts to guide light travelling along the cavity. The first portion of the waveguide 401 is optically coupled to the second portion 402.

The laser has a rear face 403 and a front face 404. The rear face and the front face a preferably both planar surfaces. The planes of the front and rear faces are preferably normal to the total length of the waveguide L, illustrated in Figure 4. Preferably, the length of the waveguide L is measured as the distance between the front and rear faces along a direction perpendicular to the plane of each of the faces. In this example, the rear face 403 and the front face 404 are both cleaved facets. The rear face and the front face may also be formed by other convenient methods. The rear face 403 is coated with a HR coating and the front face 404 is coated with an AR coating.

The laser 400 comprises a light emissive structure which produces light that is received by the waveguide. As will be described in more detail later, the light emissive structure may comprise doped semiconductor layers and electrodes on either side of the doped semiconductor layers.

Light exits the laser cavity from the waveguide at the front face 404 (i.e. the front face is the emissive face of the laser), as shown at 407. This may allow the laser 400 to provide an optical output or allow the laser 400 to be integrated with other optically functional structures.

The waveguide comprising portions 401 , 402 extends along a path between the rear face 403 and the front face 404. The waveguide of the laser comprises a material with a refractive index n greater than that of the surrounding material. The waveguide may be a ridge waveguide or a buried-heterostructure (BH) waveguide for manufacturing versatility. 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 pm wide and several hundred pm 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 sides that are not in contact with the substrate beneath the waveguide. A BH waveguide comprises a core made of a longer band-gap wavelength semiconductor material surrounded by cladding made of a shorter band-gap wavelength semiconductor material.

The first portion of the waveguide 401 is adjacent the rear face 403. The second portion of the waveguide 402 is adjacent the first portion 401 and the front face 404. The first portion 401 does not have a Bragg grating. The second portion 402 does have a Bragg grating, one element of which is shown at 405. The grating is therefore only at the front side (AR side) of the laser cavity.

The first and second portions of the waveguide extend along respective paths along which light can travel. In this example, both waveguide portions 401 , 402 have respective straight paths. The path of the second waveguide portion 402 is tilted relative to the path of the first waveguide portion 401 .

In this example, the length of the second portion 402 of the waveguide having the Bragg grating is equal to the length of the Bragg grating, L_g, which is approximately 40%-60% of the length of the whole laser cavity length, L. L_g is preferably measured along the same direction as the total length L of the cavity. Kappa*L is preferably between 0.8-1.2, preferably 0.8 to 1 (where Kappa is the coupling coefficient). In certain implementations, L_g may be equal to 40, 45, 50, 55, or 60% of L. The non-grating section of the waveguide (portion 401 in this example) therefore has total length L-L_g adjacent to the rear (HR-coated) facet.

The Bragg grating comprises a plurality of regularly spaced grating elements 405, each grating element 405 extending in a plane parallel to the front face 403.

The grating is vertical, each grating element lies in a plane parallel to the y axis, and is equally spaced along the x axis with grating pitch A, which corresponds to a Bragg grating wavelength Ao. The Bragg grating may be elongated along the length of the second waveguide portion 402. The elongation of the length is preferably orthogonal to the front face 404. The pitch A of the Bragg grating is preferably constant, allowing for easy manufacturing. This can avoid an expensive and complicated chirped grating. The first portion of the waveguide 401 meets the second portion of the waveguide 402 at 406. In the example shown in Figure 4, the second portion of the waveguide 402 comprising the grating section is tilted at a fixed angle, 0, relative to the path of the first portion of the waveguide (which in this example is straight and normal to the plane of the rear face 403). This angle is therefore relative to the normal to the front face 403 and preferably also normal to the rear face 404. As a result, the emission wavelength of the laser will be: A=2 n_effA/cos(0).

In the particular example shown in Figure 4, the first portion extends along a straight path. That is, paths of all regions of the first portion are along the same direction along the entirety of its length. In other implementations, the first portion may extend along a curved path. The second portion also extends along a straight path. That is, paths of all regions of the second portion are along the same direction along the entirety of its length.

The path of the waveguide deviates at the meeting of the first portion 401 and the second portion 402 (at 406). The deviation is preferably an abrupt deviation. In the example shown in Figure 4, the respective path of the second portion of the waveguide 402 deviates from the respective path of the first portion of the waveguide 401 by an angle 0. The angle 0 may be 10 degrees. The angle 0 may be 15 degrees. In other embodiments, angle 0 may be 5, 6, 7, 8 or 9 degrees. In the example shown in Figure 4, the first and second portions of the waveguides both extend along straight paths.

In general, the greatest deviation between any part of the path of the first portion (which may be straight or curved) and any part of the path of the second portion may be 15 degrees. In some embodiments, the greatest deviation between any part of the path of the first portion and any part of the path of the second portion may be 10 degrees. In some embodiments, the greatest deviation between any part of the path of the first portion and a normal to the rear face 403 may be 10 degrees. In some embodiments, the greatest deviation between any part of the path of the first portion and a normal to the rear face 403 may be 5 degrees.

In other words, the second portion 402 comprises a region that extends along a straight path that is different from any path along which any region of the first portion extends.

In this example, the width of the first portion 401 and the second portion 402 of the waveguide in a direction locally perpendicular to its respective path and parallel to the semiconductor layers is constant. For example, the width of the first portion and/or the second portion of the waveguide may be constant and may be between 1 .0 and 4.0 pm. This can help to ensure the operation of a single optical mode and may allow for easier manufacture of the laser. The waveguide width can also be chosen accordingly to achieve a desired effective refractive index. In some embodiments, the waveguide width of the first portion 401 may be different to the waveguide width of the second portion 402.

As the tilted grating section is along a certain crystal angle, the waveguide width is much easier to control, i.e. it is easier to meet the A=2n_eff A/cos(0) condition. Therefore, a laser having this structure is suitable for mass production.

In the example shown in Figure 4, the Bragg grating extends along the entirety of the second region between its meeting with the first portion and the front face.

Figure 5 shows an alternative embodiment of the present invention. As illustrated in the top view of Figure 5, DFB laser 500 comprises multiple cavity portions each comprising a part of the waveguide of the laser, which acts to guide light travelling along the cavity.

The waveguide comprises a first waveguide portion 501. The first waveguide portion 501 does not have a Bragg grating. In this example, the second portion of the waveguide, which comprises a Bragg grating, comprises a first sub-portion 502 and a second sub-portion 503. In this example, the first sub-portion 502 does not have a Bragg grating and the second subportion 503 has a Bragg grating.

The first portion 501 is optically coupled to the first sub-portion 502 of the second waveguide portion and the first sub-portion 502 is optically coupled to the second sub-portion 503 of the second waveguide portion.

The laser has a rear face 504 and a front face 505. The rear face and the front face a preferably both planar. The planes of the front and rear faces are preferably normal to the length of the waveguide L. In this example, the rear face 504 and the front face 505 are both cleaved facets. The rear face and the front face may also be formed by other convenient methods. The rear face 504 is coated with a HR coating and the front face 505 is coated with an AR coating.

The laser 500 comprises a light emissive structure which produces light that is received by the waveguide. As will be described in more detail later, the light emissive structure may comprise doped semiconductor layers on either side of the waveguide and electrodes on either side of the doped semiconductor layers. Light exits the laser cavity from the waveguide at the front face 505 (i.e. the front face is the emissive face of the laser), as shown at 506. This may allow the laser 500 to provide an optical output or allow the laser 500 to be integrated with other optically functional structures.

The portions of the waveguide 501 , 502, 503 extend along respective paths between the rear face 504 and the front face 505. The waveguide of the laser comprises a material with a refractive index n greater than that of the substrate. The waveguide may be a ridge waveguide or a BH waveguide for manufacturing versatility, as described above with reference to Figure 4.

The first portion of the waveguide 501 is adjacent the rear face 504. The first sub-portion 502 is adjacent the first portion 501. The second sub-part 503 of the second portion of the waveguide is adjacent the first sub-part 502 of the second portion and the front face 505. The first portion 501 and the first sub-portion 502 do not have a Bragg grating. The second subportion 503 does have a Bragg grating, one element of which is shown at 507. The grating is therefore only at the front side (AR side) of the laser cavity.

The first portion 501 , the first sub-portion 502 and the second sub-portion 503 extend along respective paths along which light can travel. In this example, the first portion 501 , the first sub-portion 502 and the second sub-portion 503 each have straight paths. That is, paths of all regions of a respective portion or sub-portion are along the same direction along the entirety of its respective length. The path of the first sub-portion 502 is tilted relative to the path of the first waveguide portion 401.

In this example, the length of the second sub-portion 503 of the second part of the waveguide having the Bragg grating, L_g, is approximately 40%-60% of the length of the whole laser cavity length, L. Kappa*L is preferably between 0.8-1.2, preferably 0.8 to 1. In certain implementations, L_g may be equal to 40, 45, 50, 55, or 60% of L. The non-grating section of the waveguide therefore has total length L-L_g adjacent to the rear (HR-coated) face.

Therefore, in embodiments of the present invention, the part of the waveguide that has a Bragg grating is approximately 40%-60% of the length of the whole laser cavity length L.

The Bragg grating comprises a plurality of regularly spaced grating elements 507, each grating element 507 extending in a plane parallel to the front face 505. The grating is vertical, each grating element lies in a plane parallel to the y axis, and is equally spaced along the x axis with grating pitch A, which corresponds to a Bragg grating wavelength Ao. The Bragg grating may be elongated along the length of the second sub-portion 503. The elongation of the length is preferably orthogonal to the front face 505. The pitch A of the Bragg grating is preferably constant, allowing for easy manufacturing. This can avoid an expensive and complicated chirped grating.

The first portion of the waveguide 501 meets the first sub-portion of the second portion of the waveguide at 508 and the first sub-portion 502 of the second portion of the waveguide meets the second sub-portion 503 of the second portion of the waveguide at 509.

In Figure 5, the first sub-portion 502 is tilted at a fixed angle, 0i, relative to the path of the first portion 501 of the waveguide (which in this example is straight and normal to the plane of the rear face 504). This angle is therefore relative to the normal to the front face 504 and preferably also normal to the rear face 505. As a result, the emission wavelength of the laser will be: A=2 n_effA/cos(0).

In some embodiments, the first sub-portion 502 and the second sub-portion 503 may extend along respective paths that are collinear. In other words, they both have straight paths and 0=01. In other embodiments, the first sub-portion 502 and the second sub-portion may both have straight paths, but 0 0i.

A part of the non-grating section may be curved. In the particular example shown in Figure 5, the first portion 501 extends along a straight path. In other implementations, the first portion may extend along a curved path and/or the first sub-portion of the second portion of the waveguide may extend along a curved path, and the second sub-portion 503 with the Bragg grating may extend along a straight path.

Therefore, in this example, a part of the non-grating section (the first sub-portion 502 of the second portion of the waveguide) adjacent to the grating part is tilted by an angle, 01, as a transition section (which is easy to manufacture), and the grating section (the second subportion 503 of the second portion of the waveguide) is tilted by a fixed angle, 0, relative to the normal to the front face 504. Therefore, the DFB emission wavelength of the laser is still A=2 n_effA/cos(0).

The rear part of the waveguide without the Bragg grating can therefore be straight (as shown in Figure 4) or can be partially tilted (as shown in Figure 5) or fully or partially curved. In the example shown in Figure 5, the first portion 501 , the first sub-portion 502 and second sub-portion 503 each extend along straight paths.

The path of the waveguide deviates at the meeting of the first portion 501 and the first subportion 502 of the second waveguide portion (at 508). The respective path of the first subportion of the second portion of the waveguide 502 deviates from the respective path of the first portion of the waveguide 501 by an angle 01. The deviation may be an abrupt deviation. For example, the angle 01 in Figure 5 may be 10 degrees. The angle 01 may be 15 degrees. In other embodiments, angle 01 may be 5, 6, 7, 8 or 9 degrees. The path of the waveguide may also deviate at the meeting of the first sub-portion 502 and the second sub-portion 502 (at 509) or, as mentioned above, the paths of the first sub-portion 502 and the second subportion 503 may extend along collinear paths. The respective path of the second sub-portion of the second portion of the waveguide 503 may deviate from the respective path of the first portion of the waveguide 501 by an angle 0. The deviation may be an abrupt deviation. For example, the angle 0 in Figure 5 may be 10 degrees. In other embodiments, angle 0 may be 5, 6, 7, 8 or 9 degrees. In general, the greatest deviation between any part of the path of the first portion 501 and any part of the path of the second portion (which in this example comprises sub-parts 502 and 503) may be 15 degrees or may be 10 degrees. The greatest deviation between any part of the path of the first portion 501 and a normal to the rear face 504 may be 10 degrees or may be 5 degrees.

In other words, the second portion comprises a region (second sub-portion 503) that extends along a straight path that is different from any path along which any region of the first portion 501 extends. In the particular example shown in Figure 5, the first sub-portion 502 also extends along a straight path that is different from any path along which any region of the first portion

501 extends.

In this example, the width of the first portion 501 , the first sub-portion 502 and the second subportion 503 of the waveguide in a direction locally perpendicular to its respective path and parallel to the semiconductor layers is constant. For example, the width of the portions 501 ,

502 and 503 may be constant and may be between 1.0 and 4.0 pm. This can help to ensure the operation of a single optical mode and may allow for easier manufacture of the laser. The waveguide width can also be chosen accordingly to achieve a desired effective refractive index. In some embodiments, the waveguide width of the first portion 501 may be different to the waveguide width of the first sub-portion 502 and/or the waveguide width of the second subportion 503. As the tilted grating section 503 is along a certain crystal angle, the waveguide width is much easier to control, i.e. it is easier to meet the A=2n_eff A/cos(0) condition. Therefore, a laser having this structure is suitable for mass production.

The general structure of the laser cavity in the above examples is schematically illustrated in Figures 6(a) and 6(b), which show side and cross-section views respectively. Figure 6(b) is a cross-sectional view (looking along the cavity, towards the front face) along section A-A in Figure 6(a), through the second portion of the waveguide. The parts are illustrated for the laser 400 in Figure 4. However, the laser 500 of Figure 5 may have the same or a similar structure of semiconductor layers comprising its respective portions.

As already mentioned above, the laser 400 generally comprises a semiconductor block which has a rear face or facet 403, a front face or facet 404 opposite to the front face or facet. The laser cavity is formed therebetween. As mentioned above, the total length of the laser cavity is L. The length L is preferably defined between the rear face 403 and the front face 404 and along a direction normal to these faces. The length of the second sub-portion 402 comprising the grating is L_g.

A HR coating is preferably applied to the rear face 403 and an AR coating is preferably applied to the front face 404. The back face with the HR coating acts as a rear reflector. It is preferable that the front and rear facets are aligned parallel to one another. Preferably, the front face is orthogonal to the length of the cavity and/or to the Bragg grating. Preferably, the rear face is orthogonal to the length of the cavity. The front and/or rear face(s) of the laser may be formed by cleaving. The width of the waveguide is preferably measured perpendicular to the length of the cavity.

The laser cavity comprises a first semiconductor layer of a first doping type, a second semiconductor layer of a second doping type opposite to the first type, and an active region located between the first and second semiconductor layers, the first and second semiconductor layers being elongated in a direction extending between the rear reflector and the front face. In the example shown in Figures 6(a) and 6(b), the laser cavity, shown generally at 601 , comprises an active layer 602 interposed between layers of p- and n-type semiconductor material, shown at 603 and 604 respectively. In one example, the semiconductor layers are made from InP. However, other semiconductor materials, such as GaAs and GaN, may be used. The material forming the cavity may be selectively doped in the region of the p- and n-type layers. The active layer of the laser may be a multiple quantum well (MQW) structure. The layers 602, 603, 604 are elongated in a direction extending between the rear face 403 and the front face 404.

A forward biased voltage can be applied across electrodes or contacts on opposing top and bottom sides of the cavity.

In the example shown in Figures 6(a) and 6(b), the waveguide is a ridge waveguide having a width of w, as shown in the cross-sectional view of Figure 6(b). Bragg grating 405 is positioned adjacent (i.e. immediately next to or close to) the front (AR-coated) face. In this example, the grating is located in the p-type semiconductor layer where the ridge waveguide forms. In other waveguide structures, the grating may alternatively be positioned in the n-type semiconductor layer 604. The n-doped layer 604 can be positioned on top of an n-doped substrate or semiinsulated substrate. Alternatively, the upper layer at 603 can be n-doped, and the lower layer at 604 can be p-doped and positioned on top of a p-doped substrate or semi-insulated substrate.

The Bragg grating is integral with the part of the second portion of the waveguide having the grating. The Bragg grating is elongated along the length of the cavity. The elongation of length of the grating is orthogonal to the front facet.

In the example shown in Figure 4, the grating extends along the complete length of the second waveguide portion 402. In the example shown in Figure 5, the grating extends along the complete length of the second sub-part 503 of the second waveguide portion.

The Bragg grating may be fabricated by electron beam lithography. This can allow the accuracy of the grating spacing to be controlled very accurately. The grating may be an index coupled grating, a gain coupled grating or a complex coupled grating. As mentioned above, the layer comprising the Bragg grating may be fabricated from a p-doped or n-doped semiconductor material.

In summary, the DFB laser described herein has a waveguide comprising a first portion adjacent the rear face of the laser and a second portion adjacent the first portion and the front face of the laser. The first portion comprises no Bragg grating and the second portion comprises a Bragg grating. The path of the waveguide deviates at the meeting of the first portion and the second portion and/or the second portion comprises a region that extends along a straight path that is different from any path along which any region of the first portion extends. Therefore, the second portion has a region that is tilted relative to the first portion. The resulting DFB laser may have improved reflection tolerance compared to other full, A/4 shifted and partial grating DFB lasers, reducing the impact of reflection on the DFB performance. It is straightforward to manufacture and does not require extra process steps, and it does not require a chirped Bragg grating. The tilted grating waveguide width is much easier to control than, for example, a curved waveguide with a grating, as it along a fixed crystal line. A laser having a structure as described above may therefore allow for a high manufacturing yield, whilst having a similar size to conventional DFBs and EMLs.

The laser described above with a tilted grating section does not suffer significantly from output power loss and is suitable for non-hermetic applications. The laser can work without an isolator, which can save on device space and cost.

The laser is preferably a single mode laser. The laser structure may be integrated with another optically functional structure, for example an electro-absorption modulator (for example to form an electro-absorption modulated laser), 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.

Claims

1. A distributed feedback laser (400, 500), the laser comprising: a light emissive structure (601); and a waveguide (401 , 402, 501 , 502, 503) located so as to receive light from the light emissive structure (601), the waveguide extending along a path between a front face (404, 505) and a rear face (403, 504), the rear face being more reflective than the front face; wherein the waveguide comprises a first portion (401 , 501) adjacent the rear face (403,

504) and a second portion (402, 502, 503) adjacent the first portion (401 , 501) and the front face (404, 505), the first portion comprising no Bragg grating, the second portion comprising a Bragg grating (405, 507); and wherein the path of the waveguide deviates at the meeting of the first portion (401 , 501) and the second portion (402, 502, 503).

2. A distributed feedback laser (400, 500), the laser comprising: a light emissive structure (601); and a waveguide (401 , 402, 501 , 502, 503) located so as to receive light from the light emissive structure (601), the waveguide extending along a path between a front face (404,

505) and a rear face (403, 504), the rear face being more reflective than the front face; wherein the waveguide comprises a first portion (401 , 501) adjacent the rear face (403, 504) and a second portion (402, 502, 503) adjacent the first portion (401 , 501) and the front face (404, 505), the first portion comprising no Bragg grating, the second portion comprising a Bragg grating (405, 507); and wherein the second portion (402, 502, 503) comprises a region (402, 503) that extends along a straight path that is different from any path along which any region of the first portion (401 , 501) extends.

3. The distributed feedback laser (400) as claimed in claim 1 or 2, wherein the Bragg grating extends (405, 507) along the entirety of the second region (402) between its meeting with the first portion (401) and the front face (403).

4. The distributed feedback laser (500) as claimed in claim 1 or 2, wherein the second portion comprises a first sub-portion (502) adjacent the first portion (501) and a second sub-portion (503) adjacent the first sub-portion (502) and the front face (505), the first sub-portion (502) comprising no Bragg grating and the second sub-portion (503) comprising the Bragg grating (405, 507).

5. The distributed feedback laser (500) as claimed in claim 4, wherein the first sub-portion (502) and the second sub-portion (503) extend along respective paths that are collinear.

6. The distributed feedback laser as claimed in claim 4, wherein the first sub-portion extends along a curved path.

7. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the first portion (401 , 501) extends along a straight path.

8. The distributed feedback laser as claimed in any preceding claim, wherein the first portion extends along a curved path.

9. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the front (404, 505) and rear (403, 504) faces are front and rear facets of a body of semiconductor material.

10. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the Bragg grating comprises a plurality of regularly spaced grating elements (405, 507), each grating element extending parallel to the front face (404, 505).

11. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the greatest deviation between any part of the path of the first portion (401 , 501) and any part of the path of the second portion (402, 502, 503) is 15 degrees.

12. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the greatest deviation between any part of the path of the first portion (401 , 501) and a normal to the rear face (403, 504) is 10 degrees.

13. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the light emissive structure comprises: doped semiconductor layers (602, 603, 604); and electrodes on either side of the doped semiconductor layers.

14. The distributed feedback laser (400, 500) as claimed in claim 13, wherein the width of the first portion (401 , 501) in a direction locally perpendicular to its path and parallel to the semiconductor layers is constant.

15. The distributed feedback laser as claimed in claim 13 or 14, wherein the width of the second portion in a direction locally perpendicular to its path and parallel to the semiconductor layers is constant.

16. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the length of the Bragg grating is between 40% and 60% of the total length between the front and rear faces.

17. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the Bragg grating has a constant pitch.

18. The distributed feedback laser (400, 500) as claimed in any preceding claim, wherein the path of the waveguide abruptly deviates at the meeting (406, 508) of the first portion and the second portion.