Tilted semiconductor laser

Abstract

A distributed feedback laser (400, 500) is described, the laser comprising: a light emitting structure (601); a waveguide (401, 402, 501, 502, 503) receiving light from the light emitting structure (601), the waveguide extending along a path between a front surface (404, 505) and a rear surface (403, 504), the rear surface being more reflective than the front surface; wherein the waveguide comprises a first portion (401, 501) adjacent to the rear surface and a second portion (402, 502, 503) adjacent to the first portion (401, 501) and the front surface (404, 505), the first portion not comprising a Bragg grating and the second portion comprising a Bragg grating (405, 507); and wherein the path of the waveguide is offset at an intersection of the first portion (401, 501) and the second portion (402, 502, 503).

Description

Technical Field

[0001] The present invention relates to lasers, for example to distributed feedback lasers used in applications such as telecommunications and data centers. Background Art

[0002] Single-mode InP distributed feedback (DFB) lasers are widely used in telecommunication applications.

[0003] Figures 1(a) and 1(b) schematically illustrate examples of conventional DFB lasers. The DFB laser includes a semiconductor structure having a rear surface or rear end face 101, a front surface or front end face 102 opposite to the front surface or front end face, and a laser cavity formed between the rear surface or rear end face 101 and the front surface or front end face 102. The laser cavity includes an active layer 103 inserted between a p-type semiconductor material layer and an n-type semiconductor material layer (shown as 104 and 105, respectively). Current can be applied to electrodes or contacts 106a, 106b on the opposite top and bottom sides of the cavity, respectively. The cavity (generally shown as 107) includes a waveguide along which light can be guided. Light is emitted from the front surface 102 of the cavity, as shown at 108.

[0004] The Bragg grating 109 acts as a wavelength selective element and provides feedback, reflecting light back into the cavity to form a resonator. The grating can be constructed to reflect only a narrow band of wavelengths.

[0005] In a DFB laser, the rear facet 101 is usually coated with a high-reflection (HR) coating to enhance the output power. The rear facet 101 with the HR coating acts as a back reflector. The front facet 102 is usually coated with an anti-reflection (AR) coating to increase the output power and reduce reflection. The grating 109 usually spans from the HR facet 101 to the AR facet 102.

[0006] To generate a single emission wavelength (λ), the grating pitch Λ(x) is proportional to the waveguide refractive index profile n eff The relationship between (x) should satisfy the condition λ=2n eff (x)*Λ(x).

[0007] Single-mode DFB lasers can have straight or curved waveguides. DFB lasers typically have a constant waveguide width and a constant grating pitch, as this allows for efficient mass production.

[0008] Typically, a DFB with a full grating has two competing lasing modes due to symmetry. In order to break the symmetry so that only one mode is lasing, various schemes have been proposed, including introducing a λ / 4 phase shift and applying HR / AR coating to the end face. However, the DFB yield is still greatly affected by the end face phase and external optical reflection.

[0009] For single-mode DFB lasers, external optical feedback not only changes the threshold current, efficiency, linewidth, and intensity noise, but also destroys the modulation pattern and produces bit-error-rate (BER) saturation.

[0010] Typically, a DFB or electro-absorption modulated laser (EML) chip is soldered onto an AlN substrate. A conventional approach to minimize reflections of the light path back to the DFB or EML is to place an isolator close to the DFB or EML output facet, such as Figure 2 Schematically shown. A DFB or EML is shown at 201. The signal is provided through a slit 202 and a flexible printed circuit board 203. A monitoring photodetector 204 is placed behind the device 201. The light leaves the device 201 and passes through a lens 205 and a sapphire window 206 to an isolator 207 and a ferrule 208. A thermoelectric cooler 209 may be used to cool the structure.

[0011] Inserting an optical isolator in the DFB module can cut off the external optical feedback. However, the isolator is large in size. Additional space is required on the AlN carrier or package, and solder or epoxy is required in an additional processing step to attach the isolator. External isolators are also very expensive, and in addition, it is difficult to achieve non-hermetic packaging when using isolators.

[0012] In another scheme, if Figure 3 As shown, a monitoring photodetector (mPD) 301 is integrated behind the DFB chip 302, and the rear facet is usually coated with an AR coating. The mPD on the back of the chip absorbs light from the DFB, so the DFB laser effectively has facets coated with an AR coating on both the front and rear surfaces of the cavity. Typically, in order to increase the output power of the DFB, an HR coating is applied to the back. Due to the mPD on the back of the chip, the output power of the DFB is reduced by 2dB to 3dB. This also results in a significantly larger chip size, making the chip more expensive.

[0013] In another approach, a window structure can be used on the output facet. However, a window structure may involve additional growth steps and make the chip larger.

[0014] Curved waveguide structures can also be used with constant pitch gratings, where the waveguide width is varied to match the Bragg reflection conditions. However, curved waveguide segments with gratings require very tight control of the curvature-dependent waveguide width and are generally not suitable for mass production. In DFB lasers, the waveguide width should be compatible with the grating pitch to have a stable emission wavelength, i.e., λ = 2n eff Λ, where n eff is the effective refractive index of the waveguide, and Λ is the grating spacing along the light propagation direction. Commercial DFB products should be easy to manufacture on a large scale with high yield, which is why most DFB lasers have a constant grating period Λ, a straight waveguide, and a constant waveguide width d to give a uniform n eff Therefore, it is usually difficult to introduce curved waveguides with gratings unless chirped grating pitches are used, which is not convenient for large-scale production.

[0015] It would be desirable to develop a laser that overcomes at least some of the above-mentioned problems, has improved resistance to reflection, and is easy to manufacture. Summary of the invention

[0016] According to one aspect, a distributed feedback laser is provided, the laser comprising: a light emitting structure; a waveguide receiving light from the light emitting structure, the waveguide extending along a path between a front surface and a rear surface, the rear surface being more reflective than the front surface; wherein the waveguide comprises a first portion adjacent to the rear surface and a second portion adjacent to the first portion and the front surface, the first portion not comprising a Bragg grating, the second portion comprising a Bragg grating; and wherein the path of the waveguide deviates at the intersection of the first portion and the second portion.

[0017] According to another aspect, a distributed feedback laser is provided, the laser comprising: a light emitting structure; a waveguide receiving light from the light emitting structure, the waveguide extending along a path between a front surface and a rear surface, the rear surface being more reflective than the front surface; wherein the waveguide comprises a first portion adjacent to the rear surface and a second portion adjacent to the first portion and the front surface, the first portion not including a Bragg grating, the second portion including a Bragg grating; and wherein the second portion comprises a region extending along a straight path, the straight path being different from any path along which any region of the first portion extends.

[0018] The resulting DFB laser can be more reflective and easier to manufacture because it does not require a chirped Bragg grating or very tight control of the waveguide width.

[0019] The Bragg grating may extend along the entire second region between the intersection of the second region and the first portion and the front surface. This may be a possible configuration that improves reflection resistance with an easily manufactured structure.

[0020] The second portion may include a first sub-portion adjacent to the first portion and a second sub-portion adjacent to the first sub-portion and the front surface, the first sub-portion not including a Bragg grating and the second sub-portion including a Bragg grating. This may be an alternative configuration to improve reflection resistance.

[0021] The first sub-portion and the second sub-portion may extend along respective paths. The respective paths may be co-linear. Thus, the first sub-portion and the second sub-portion may follow a co-linear path, but the second sub-portion comprises a Bragg grating and the first sub-portion does not comprise a Bragg grating.

[0022] The first subsection may extend along a curved path. A curved section without a Bragg grating is easier to manufacture than a curved section with a Bragg grating. Thus, this allows for a variety of structures of the first subsection of the waveguide, which may follow a straight path or a curved path.

[0023] The first section can extend along a straight path. This can make the laser easier to manufacture.

[0024] The first portion may extend along a curved path. A curved portion without a Bragg grating is easier to manufacture than a curved portion with a Bragg grating. Therefore, this may enable a variety of waveguide structures.

[0025] The front and rear surfaces may be front and rear facets of a body of semiconductor material. The front and rear facets may be cleaved facets. This may facilitate forming the surface during manufacture of the laser. The front and rear facets may also be etched facets.

[0026] The Bragg grating may comprise a plurality of regularly spaced grating elements, each extending parallel to the front surface. The Bragg grating may be formed by techniques such as electron beam lithography. This allows very precise control of the precision of the grating spacing. The grating may be an index coupled grating, a gain coupled grating or a complex coupled grating. This allows for a variety of structures and / or functions of the Bragg grating.

[0027] The maximum 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 enable efficient manufacture of the laser.

[0028] The maximum deviation between any part of the path of the first portion and the normal to the rear surface may be 10 degrees. This may further enable efficient manufacturing of the laser.

[0029] The light emitting structure may include a doped semiconductor layer. The doped semiconductor layer may be located on either side of the waveguide. The light emitting structure may include electrodes located on either side of the doped semiconductor layer. This may be a convenient laser configuration for simulating light emission from the structure. Other configurations for providing a light emitting structure may also be used.

[0030] The width of the first portion in a direction locally perpendicular to its path and parallel to the semiconductor layer may be constant. The width of the second portion in a direction locally perpendicular to its path and parallel to the semiconductor layer may be constant. This may allow the laser to be easily manufactured, and the width of the waveguide portion may be selected accordingly to produce a suitable effective refractive index of the waveguide portion. The width of the first portion in a direction locally perpendicular to its path and parallel to the semiconductor layer may also be variable. Since the first portion does not include a Bragg grating, it is not necessary to meet the Bragg grating condition to give a stable emission wavelength, so this may enable diversity in the structure of the first portion of the waveguide.

[0031] The length of the Bragg grating may be 40% to 60% of the total length between the front and back surfaces. This may disrupt the symmetry of the waveguide, thereby improving the laser's resistance to reflections.

[0032] The Bragg grating can have a constant pitch. This enables efficient production of lasers.

[0033] The path of the waveguide may diverge steeply at the intersection of the first and second portions. Tilting the second portion of the waveguide relative to the first portion may improve the laser's tolerance to reflections.

[0034] In some embodiments, the path of the first portion of the waveguide may gradually diverge before joining the second portion.

[0035] The front surface can be coated with an anti-reflective coating and the back reflector can be coated with a high-reflective coating. This can increase the power and improve the efficiency of the laser.

[0036] The waveguide can be a ridge waveguide or a buried heterostructure waveguide. This allows for a variety of laser structures.

[0037] The laser cavity may include 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 between the first semiconductor layer and the second semiconductor layer, the first semiconductor layer and the second semiconductor layer being elongated in a direction extending between the rear surface and the front surface. This may be a convenient laser structure.

[0038] 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 facilitate fabrication of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention is described below by way of examples with reference to the accompanying drawings.

[0040] In the attached picture:

[0041] FIG1( a ) schematically shows a side view of a conventional DFB laser.

[0042] FIG1( b ) schematically shows a top view of a conventional DFB laser.

[0043] Figure 2 The use of an isolator with a DFB laser or an EML is schematically shown.

[0044] Figure 3 A device example of a DFB laser chip and photodetector combination is shown.

[0045] Figure 4 A top view of an example of a DFB laser provided by an embodiment of the present invention is schematically shown.

[0046] Figure 5 A top view of another example of a DFB laser provided by an embodiment of the present invention is schematically shown.

[0047] Figure 6(a) schematically shows Figure 4 Side view of the laser.

[0048] Figure 6(b) shows Figure 4 A cross-sectional view of the laser along section AA in FIG6(a). DETAILED DESCRIPTION

[0049] Embodiments of the present invention use a partially tilted waveguide with a partial Bragg grating to form a reflection-resistant DFB laser. The segment of the laser waveguide adjacent to the front surface of the laser is tilted at an angle θ relative to the normal to the plane of the front surface and the waveguide width. The pitch of the partial Bragg grating (located in the tilted segment adjacent to the front surface) can be constant. Therefore, the effective refractive index n eff The product of λ and the grating pitch Λ can be constant, and a partial grating DFB laser with tilted grating segments can emit at a wavelength of λ = 2n eff Light of Λ / cos(θ).

[0050] like Figure 4 As shown in the top view of FIG. 4 , an example of a DFB laser includes two cavity sections, each cavity section including a portion of a waveguide of the laser for guiding light propagating along the cavity. A first portion of the waveguide 401 is optically coupled to a second portion 402.

[0051] The laser has a rear surface 403 and a front surface 404. Both the rear surface and the front surface are preferably planar surfaces. The planes of the front and rear surfaces are preferably perpendicular to the total length L of the waveguide, such as Figure 4 As shown. Preferably, the length L of the waveguide is measured as the distance between the front surface and the rear surface in a direction perpendicular to the plane of each surface. In this example, both the rear surface 403 and the front surface 404 are cleaved end surfaces. The rear surface and the front surface can also be formed by other convenient methods. The rear surface 403 is coated with an HR coating and the front surface 404 is coated with an AR coating.

[0052] The laser 400 includes a light emitting structure that produces light that is received by a waveguide. As will be described in more detail later, the light emitting structure may include a doped semiconductor layer and electrodes on either side of the doped semiconductor layer.

[0053] Light exits the laser cavity from the waveguide at the front surface 404 (ie, the front surface is the emitting surface of the laser), as shown at 407. This may allow the laser 400 to provide a light output or to be integrated with other optically functional structures.

[0054] The waveguide including portions 401, 402 extends along a path between the rear surface 403 and the front surface 404. The waveguide of the laser comprises a material having a refractive index n that is greater than the refractive index of the surrounding material. The waveguide may be a ridge waveguide or a buried-heterostructure (BH) waveguide to facilitate manufacturing diversity. A ridge waveguide may be produced by etching parallel grooves in the material on either side of the waveguide to produce isolated protruding bands that are typically less than 10 μm wide and several hundred μm long. A material having a lower refractive index than the waveguide material may be deposited on the sides of the ridge to direct the injected current into the ridge. Alternatively, the sides of the ridge may be surrounded by air that is not in contact with the substrate beneath the waveguide. The BH waveguide comprises a core made of a longer bandgap wavelength semiconductor material surrounded by a cladding made of a shorter bandgap wavelength semiconductor material.

[0055] A first portion of waveguide 401 is adjacent to rear surface 403. A second portion of waveguide 402 is adjacent to first portion 401 and front surface 404. First portion 401 does not have a Bragg grating. Second portion 402 does have a Bragg grating, one element of which is shown at 405. Thus, the grating is only on the front side (AR side) of the laser cavity.

[0056] The first and second parts of the waveguide extend along respective paths along which light can propagate. In this example, the two waveguide parts 401, 402 have respective straight paths. The path of the second waveguide part 402 is inclined relative to the path of the first waveguide part 401.

[0057] In this example, the length of the second portion 402 of the waveguide having the Bragg grating is equal to the length L of the Bragg grating. g , which is about 40% to 60% of the total laser cavity length L. Preferably, L is measured in the same direction as the total cavity length L. g Kappa*L is preferably between 0.8 and 1.2, preferably 0.8 to 1 (where Kappa is the coupling coefficient). In some implementations, L g can be equal to 40%, 45%, 50%, 55% or 60% of L. Thus, the non-grating section of the waveguide adjacent the rear (HR-coated) end face (portion 401 in this example) has a total length L - L g .

[0058] The Bragg grating comprises a plurality of regularly spaced grating elements 405 , each grating element 405 extending in a plane parallel to the front surface 403 .

[0059] 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 a grating pitch of Λ, which corresponds to the Bragg grating wavelength λ0. 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 surface 404. The pitch Λ of the Bragg grating is preferably constant to facilitate manufacturing. This can avoid expensive and complex chirped gratings.

[0060] The first portion of waveguide 401 meets the second portion of waveguide 402 at 406. Figure 4 In the example shown, the second part of the waveguide 402 including the grating portion is tilted at a fixed angle θ relative to the path of the first part of the waveguide (in this example straight and perpendicular to the plane of the rear surface 403). The angle is thus relative to the normal to the front surface 403 and preferably also to the normal to the rear surface 404. Thus, the λ of the laser is = 2n eff Λ / cos(θ).

[0061] exist Figure 4 In the particular example shown, the first portion extends along a straight path. That is, the paths of all regions of the first portion are in the same direction along its entire length. In other implementations, the first portion may extend along a curved path. And the second portion extends along a straight path. That is, the paths of all regions of the second portion are in the same direction along its entire length.

[0062] The path of the waveguide diverges at the intersection of the first portion 401 and the second portion 402 (at 406). The divergence is preferably a steep divergence. Figure 4In the example shown, the corresponding path of the second portion of waveguide 402 is offset from the corresponding path of the first portion of waveguide 401 by an angle θ. Angle θ may be 10 degrees. Angle θ may be 15 degrees. In other embodiments, angle θ may be 5, 6, 7, 8, or 9 degrees. Figure 4 In the example shown, both the first and second portions of the waveguide extend along a straight path.

[0063] Typically, the maximum deviation between any portion of the path of the first portion (which may be straight or curved) and any portion of the path of the second portion may be 15 degrees. In some embodiments, the maximum deviation between any portion of the path of the first portion and any portion of the path of the second portion may be 10 degrees. In some embodiments, the maximum deviation between any portion of the path of the first portion and the normal to the back surface 403 may be 10 degrees. In some embodiments, the maximum deviation between any portion of the path of the first portion and the normal to the back surface 403 may be 5 degrees.

[0064] In other words, the second portion 402 includes regions extending along a linear path that is different from any path along which any regions of the first portion extend.

[0065] In this example, the width of the first portion 401 and the second portion 402 of the waveguide in a direction locally perpendicular to their respective paths and parallel to the semiconductor layer is constant. For example, the width of the first portion and / or the second portion of the waveguide can be constant and can be between 1.0 μm and 4.0 μm. This helps to ensure operation of a single optical mode and makes the laser easier to manufacture. The waveguide width can also be selected accordingly to achieve a desired effective refractive index. In some embodiments, the waveguide width of the first portion 401 can be different from the waveguide width of the second portion 402.

[0066] Since the tilted grating segments are along a specific crystal angle, the waveguide width is easier to control, that is, it is easier to satisfy λ = 2n eff Λ / cos(θ) condition. Therefore, the laser with this structure is suitable for mass production.

[0067] exist Figure 4 In the example shown, the Bragg grating extends along the entire second region between the intersection of the second region and the first portion and the front surface.

[0068] Figure 5 An alternative embodiment of the present invention is shown. Figure 5 As shown in the top view of FIG. 5 , the DFB laser 500 includes a plurality of cavity sections, each of which includes a portion of a waveguide of the laser for guiding light to propagate along the cavity.

[0069] 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 comprising 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, while the second sub-portion 503 has a Bragg grating.

[0070] The first portion 501 is optically coupled to a first sub-portion 502 of the second waveguide portion, and the first sub-portion 502 is optically coupled to a second sub-portion 503 of the second waveguide portion.

[0071] The laser has a rear surface 504 and a front surface 505. The rear surface and the front surface are preferably planar surfaces. The planes of the front surface and the rear surface are preferably perpendicular to the length of the waveguide L. In this example, the rear surface 504 and the front surface 505 are both cleaved end surfaces. The rear surface and the front surface can also be formed by other convenient methods. The rear surface 504 is coated with a HR coating and the front surface 505 is coated with an AR coating.

[0072] The laser 500 includes a light emitting structure that produces light that is received by a waveguide. As will be described in more detail later, the light emitting structure may include a doped semiconductor layer on either side of the waveguide and electrodes on either side of the doped semiconductor layer.

[0073] Light exits the laser cavity from the waveguide at front surface 505 (ie, the front surface is the emitting surface of the laser), as shown at 506. This may allow the laser 500 to provide a light output or to be integrated with other optically functional structures.

[0074] Portions of waveguides 501, 502, 503 extend along respective paths between rear surface 504 and front surface 505. The waveguide of the laser comprises a material having a refractive index n which is greater than the refractive index of the substrate. The waveguide may be a ridge waveguide or a BH waveguide for ease of manufacturing versatility, as described above with reference to Figure 4 described.

[0075] The first portion of the waveguide 501 is adjacent to the back surface 504. The first sub-portion 502 is adjacent to the first portion 501. The second sub-portion 503 of the second portion of the waveguide is adjacent to the first sub-portion 502 of the second portion and the front surface 505. The first portion 501 and the first sub-portion 502 do not have a Bragg grating. The second sub-portion 503 has a Bragg grating, one element of which is shown at 507. Therefore, the grating is only on the front side (AR side) of the laser cavity.

[0076] The first portion 501, the first sub-portion 502, and the second sub-portion 503 extend along respective paths along which light can propagate. In this example, the first portion 501, the first sub-portion 502, and the second sub-portion 503 each have a straight path. That is, the paths of all regions of the respective portion or sub-portion are in the same direction along their entire respective lengths. The path of the first sub-portion 502 is inclined relative to the path of the first waveguide portion 401.

[0077] In this example, the length L of the second subsection 503 of the second portion of the waveguide having the Bragg grating is g is about 40% to 60% of the total laser cavity length L. Kappa*L is preferably between 0.8 and 1.2, preferably 0.8 to 1. In some implementations, L g can be equal to 40%, 45%, 50%, 55% or 60% of L. Thus, the non-grating section of the waveguide adjacent to the rear (HR-coated) surface has a total length L - L g .

[0078] Therefore, in an embodiment of the present invention, the portion of the waveguide having the Bragg grating is approximately 40% to 60% of the entire laser cavity length L.

[0079] The Bragg grating comprises a plurality of regularly spaced grating elements 507 , each grating element 507 extending in a plane parallel to the front surface 505 .

[0080] 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 a grating pitch of Λ, which corresponds to the Bragg grating wavelength λ0. The Bragg grating may be elongated along the length of the second subsection 503. The elongation of the length is preferably orthogonal to the front surface 505. The pitch Λ of the Bragg grating is preferably constant to facilitate manufacturing. This can avoid expensive and complex chirped gratings.

[0081] 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 .

[0082] exist Figure 5 In the example, the first subsection 502 is tilted at a fixed angle θ1 relative to the path of the first portion 501 of the waveguide (which in this example is straight and perpendicular to the plane of the rear surface 504). The angle is thus relative to the normal to the front surface 504 and preferably also to the normal to the rear surface 505. Thus, the laser has a λ = 2n eff Λ / cos(θ).

[0083] In some embodiments, the first sub-portion 502 and the second sub-portion 503 may extend along respective paths that are collinear with each other. In other words, they both have straight paths and θ=θ1. In other embodiments, the first sub-portion 502 and the second sub-portion may both have straight paths, but θ≠θ1.

[0084] A portion of the non-grating segment may be curved. Figure 5 In the particular example shown, the first portion 501 extends along a straight path. In other embodiments, 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 having the Bragg grating may extend along a straight path.

[0085] Therefore, in this example, a portion of the non-grating section adjacent to the grating section (the first subsection 502 of the second section of the waveguide) is tilted at an angle θ1 as a transition section (easy to manufacture), and the grating section (the second subsection 503 of the second section of the waveguide) is tilted at a fixed angle θ relative to the normal of the front surface 504. Therefore, the DFB emission wavelength of the laser is still λ=2n eff Λ / cos(θ).

[0086] Therefore, the rear part of the waveguide without the Bragg grating can be straight (eg Figure 4 ), or partially tilted (as shown Figure 5 ), and may also be fully or partially curved.

[0087] exist Figure 5 In the example shown, the first portion 501 , the first sub-portion 502 , and the second sub-portion 503 each extend along a linear path.

[0088] The path of the waveguide deviates (at 508) at the intersection of the first portion 501 and the first sub-portion 502 of the second waveguide portion. The corresponding path of the first sub-portion of the second portion of the waveguide 502 deviates from the corresponding path of the first portion of the waveguide 501 by an angle θ1. The deviation can be a steep deviation. For example, Figure 5 The angle θ1 in can be 10 degrees. The angle θ1 can be 15 degrees. In other embodiments, the angle θ1 can be 5, 6, 7, 8, or 9 degrees. The path of the waveguide can also deviate (at 509) at the intersection of the first subsection 502 and the second subsection 503, or as described above, the path of the first subsection 502 and the path of the second subsection 503 can extend along colinear paths with each other. The corresponding path of the second subsection 503 of the second portion of the waveguide can deviate from the corresponding path of the first portion of the waveguide 501 by an angle θ. The deviation can be a steep deviation. For example, Figure 5The angle θ in may be 10 degrees. In other embodiments, the angle θ may be 5, 6, 7, 8, or 9 degrees. Typically, the maximum deviation between any portion of the path of the first portion 501 and any portion of the path of the second portion (including sub-portions 502 and 503 in this example) may be 15 degrees or may be 10 degrees. The maximum deviation between any portion of the path of the first portion 501 and the normal to the back surface 504 may be 10 degrees or may be 5 degrees.

[0089] In other words, the second portion includes an area (second sub-portion 503) that extends along a linear path that is different from any path along which any area of ​​the first portion 501 extends. Figure 5 In the particular example shown, the first sub-portion 502 also extends along a linear path that is different from any path along which any area of ​​the first portion 501 extends.

[0090] In this example, the width of the first portion 501, the first sub-portion 502, and the second sub-portion 503 of the waveguide in a direction locally perpendicular to their respective paths and parallel to the semiconductor layer is constant. For example, the width of portions 501, 502, and 503 can be constant and can be between 1.0 μm and 4.0 μm. This helps to ensure operation of a single optical mode and makes the laser easier to manufacture. The waveguide width can also be selected accordingly to achieve a desired effective refractive index. In some embodiments, the waveguide width of the first portion 501 can be different from the waveguide width of the first sub-portion 502 and / or the waveguide width of the second sub-portion 503.

[0091] Since the inclined grating segment 503 is along a specific crystal angle, the waveguide width is easier to control, that is, it is easier to satisfy λ=2n eff Λ / cos(θ) condition. Therefore, the laser with this structure is suitable for mass production.

[0092] The overall structure of the laser cavity in the above example is schematically shown in Figures 6(a) and 6(b), which show a side view and a cross-sectional view, respectively. Figure 6(b) is a cross-sectional view (along the cavity, toward the front surface) through the second portion of the waveguide along section AA in Figure 6(a). Figure 4 Components for laser 400 are shown in FIG. Figure 5 The laser 500 may have the same or similar structure of semiconductor layers, including corresponding portions thereof.

[0093] As already mentioned above, the laser 400 generally comprises a semiconductor block having a rear surface or rear face 403, a front surface or front face 404 opposite to the front surface or front face. A laser cavity is formed between the rear surface or rear face and the front surface or front face. As mentioned above, the total length of the laser cavity is L. The length L is preferably defined between the rear surface 403 and the front surface 404 and in a direction perpendicular to these surfaces. The length of the second subsection 402 comprising the grating is L g .

[0094] The HR coating is preferably applied to the rear surface 403 and the AR coating is preferably applied to the front surface 404. The rear surface with the HR coating acts as a rear reflector. Preferably, the front facet and the rear facet are aligned parallel to each other. Preferably, the front surface is orthogonal to the length of the cavity and / or the Bragg grating. Preferably, the rear surface is orthogonal to the length of the cavity. The front surface and / or the rear surface of the laser can be formed by cleaving. The width of the waveguide is preferably measured perpendicular to the length of the cavity.

[0095] The laser cavity includes 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 between the first semiconductor layer and the second semiconductor layer, the first semiconductor layer and the second semiconductor layer being elongated in a direction extending between a rear reflector and a front surface. In the examples shown in FIGS. 6(a) and 6(b), the laser cavity (generally shown as 601) includes an active layer 602 inserted between a p-type semiconductor material layer and an n-type semiconductor material layer (shown as 603 and 604, respectively). In one example, the semiconductor layer is made of InP. However, other semiconductor materials, such as GaAs and GaN, may also be used. The material forming the cavity may be selectively doped in the regions of the p-type layer and the n-type layer. The active layer of the laser may be a multiple quantum well (MQW) structure. Layers 602, 603, 604 are elongated in a direction extending between the rear surface 403 and the front surface 404.

[0096] A forward bias voltage may be applied to electrodes or contacts on opposite top and bottom sides of the cavity.

[0097] In the examples shown in Figures 6(a) and 6(b), the waveguide is a ridge waveguide with a width of w, as shown in the cross-sectional view of Figure 6(b). The Bragg grating 405 is positioned adjacent to (i.e., next to or close to) the front (AR coated) surface. In this example, the grating is located in the p-type semiconductor layer in which the ridge waveguide is formed. In other waveguide structures, the grating can also be located in the n-type semiconductor layer 604. The n-type doped layer 604 can be located on top of an n-type doped substrate or a semi-insulating substrate. Alternatively, the upper layer at 603 can be n-doped, and the lower layer at 604 can be p-doped and located on top of a p-doped substrate or a semi-insulating substrate.

[0098] The Bragg grating is integral with the second portion of the waveguide having the grating. The Bragg grating is elongated along the length of the cavity. The length of the grating is elongated orthogonally to the front facet.

[0099] exist Figure 4 In the example shown, the grating extends along the entire length of the second waveguide portion 402. Figure 5 In the example shown, the grating extends along the entire length of the second sub-portion 503 of the second waveguide portion.

[0100] The Bragg gratings can be made by electron beam lithography. This allows the precision of the grating spacing to be controlled very precisely. The gratings can be index coupled gratings, gain coupled gratings or complex coupled gratings. As mentioned above, the layers comprising the Bragg gratings can be made of p-type doped semiconductor material or n-type doped semiconductor material.

[0101] In summary, the DFB laser described herein has a waveguide comprising a first portion adjacent to the rear surface of the laser and a second portion adjacent to the first portion and the front surface of the laser. The first portion does not include a Bragg grating and the second portion includes a Bragg grating. The path of the waveguide deviates at the intersection of the first portion and the second portion, and / or the second portion includes a region extending along a straight path that is different from any path along which any region of the first portion extends. Thus, the second portion has a region that is tilted relative to the first portion.

[0102] Compared to other full grating, λ / 4 shifted grating and partial grating DFB lasers, the reflection resistance of the resulting DFB laser can be improved, thereby reducing the impact of reflection on the DFB performance. This laser is simple to manufacture, does not require additional process steps, and 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 because it is along a fixed crystal line. Therefore, a laser with a structure as described above can have a high yield while having a size similar to conventional DFB and EML.

[0103] The laser described above with tilted grating segments does not suffer severely from output power loss and is suitable for non-hermetic applications. The laser can be operated without an isolator, which saves device space and cost.

[0104] The laser is preferably a single-mode laser. The laser structure can be integrated with other optical functional structures such as an electro-absorption modulator (eg, forming an electro-absorption modulated laser), a Mach-Zehnder modulator or an amplifier.

[0105] Applicants hereby disclose separately each individual feature described herein and any combination of two or more such features. With the common knowledge of those skilled in the art, such features or combinations can be implemented as a whole according to this specification, without considering whether such features or combinations of features can solve any problem disclosed herein; and without limiting the scope of the claims. This application shows that various aspects of the present invention can be composed of any such individual features or combinations of features. In view of the above description, it is obvious to those skilled in the art that various modifications can be made within the scope of the present invention.

Claims

1. A distributed feedback laser (400, 500), characterized in that: The laser comprises: A light emitting structure (601); a waveguide (401, 402, 501, 502, 503) receiving light from the light emitting structure (601), the waveguide extending along a path between a front surface (404, 505) and a rear surface (403, 504), the rear surface being more reflective than the front surface; wherein the waveguide comprises a first portion (401, 501) adjacent to the rear surface (403, 504) and a second portion (402, 502, 503) adjacent to the first portion (401, 501) and the front surface (404, 505), the first portion not comprising a Bragg grating, the second portion comprising a Bragg grating (405, 507); and wherein the path of the waveguide deviates at the intersection of the first portion (401, 501) and the second portion (402, 502, 503).

2. A distributed feedback laser (400, 500), characterized in that: The laser comprises: A light emitting structure (601); a waveguide (401, 402, 501, 502, 503) receiving light from the light emitting structure (601), the waveguide extending along a path between a front surface (404, 505) and a rear surface (403, 504), the rear surface being more reflective than the front surface; wherein the waveguide comprises a first portion (401, 501) adjacent to the rear surface (403, 504) and a second portion (402, 502, 503) adjacent to the first portion (401, 501) and the front surface (404, 505), the first portion not comprising a Bragg grating, the second portion comprising a Bragg grating (405, 507); and wherein the second portion (402, 502, 503) comprises a region (402, 503) extending along a straight path, the straight path being different from any path along which any region of the first portion (401, 501) extends.

3. The distributed feedback laser (400) according to claim 1 or 2, characterized in that: The Bragg grating (405, 507) extends along the entire second region (402) between the intersection of the second region (402) and the first portion (401) and the front surface (403).

4. The distributed feedback laser (500) according to claim 1 or 2, characterized in that: The second portion includes a first sub-portion (502) adjacent to the first portion (501) and a second sub-portion (503) adjacent to the first sub-portion (502) and the front surface (505), the first sub-portion (502) not including a Bragg grating, and the second sub-portion (503) including the Bragg grating (405, 507).

5. The distributed feedback laser (500) according to claim 4, characterized in that: The first sub-portion (502) and the second sub-portion (503) extend along respective paths that are co-linear with each other.

6. The distributed feedback laser according to claim 4, characterized in that: The first subsection extends along a curved path.

7. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The first portion (401, 501) extends along a straight path.

8. A distributed feedback laser according to any one of the preceding claims, characterized in that: The first portion extends along a curved path.

9. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The front surface (404, 505) and the rear surface (403, 504) are the front and rear faces of a body of semiconductor material.

10. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The Bragg grating comprises a plurality of regularly spaced grating elements (405, 507), each grating element extending parallel to the front surface (404, 505).

11. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The maximum deviation between any portion of the path of the first portion (401, 501) and any portion of the path of the second portion (402, 502, 503) is 15 degrees.

12. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The maximum deviation between any part of the path of the first portion (401, 501) and the normal to the rear surface (403, 504) is 10 degrees.

13. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The light emitting structure comprises: Doped semiconductor layers (602, 603, 604); Electrodes are located on either side of the doped semiconductor layer.

14. The distributed feedback laser (400, 500) according to claim 13, characterized in that: The width of the first portion (401, 501) is constant in a direction locally perpendicular to its path and parallel to the semiconductor layer.

15. The distributed feedback laser according to claim 13 or 14, characterized in that: The width of the second portion in a direction locally perpendicular to its path and parallel to the semiconductor layer is constant.

16. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The length of the Bragg grating is 40% to 60% of the total length between the front surface and the rear surface.

17. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The Bragg grating has a constant pitch.

18. The distributed feedback laser (400, 500) according to any one of the preceding claims, characterized in that: The path of the waveguide diverges steeply at a junction (406, 508) of the first portion and the second portion.

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