Flared DFB laser with partial grating

Abstract

A laser having a back reflector, a front surface spaced apart from the back reflector, and a cavity defined between the back reflector and the front surface, the cavity having a waveguide and comprising: a first cavity segment adjacent to the front surface, the first cavity segment including a Bragg grating and a first portion of the waveguide, the first portion having a first width, the length of the Bragg grating being in a range of 40% to 60% of the distance from the back reflector to the front surface; and a second cavity segment adjacent to the back reflector, the second cavity segment including a second portion of the waveguide, the second portion having a second width greater than the first width.

Description

Technical Field

[0001] The present invention relates to lasers and, for example, to improving the operation and manufacturability of distributed feedback (DFB) lasers. Background Art

[0002] like Figure 1(a) and 1(b) As shown, a standard DFB laser includes a semiconductor structure having a rear surface or back facet 10, a front surface or front facet 11 opposite the front surface or front facet, and a laser cavity formed between the rear surface or back facet 10 and the front surface or front facet 11. The laser cavity includes an active layer 13 interposed between a layer of p-type semiconductor material and a layer of n-type semiconductor material (shown at 14 and 15, respectively). A voltage can be applied to electrodes or contacts 17a and 17b on opposite top and bottom sides of the cavity. The cavity includes a waveguide (generally shown at 12) along which light can be guided. Light is emitted from the front surface 11 of the cavity.

[0003] The Bragg grating 16 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.

[0004] In a standard DFB laser, the rear facet is typically coated with a high-reflection (HR) coating to enhance output power. The HR-coated rear facet 10 acts as a back reflector. The front facet 11 is typically coated with an anti-reflection (AR) coating to increase output power and reduce reflections. The grating typically spans from the HR facet to the AR facet.

[0005] To generate a single emission wavelength (λ), the grating period Λ(x) is related to the local refractive index distribution n eff The relationship between (x) should satisfy the condition λ=2n eff (x)*Λ(x).

[0006] Single-mode DFB lasers can have straight or curved waveguides. Standard DFB lasers typically have a constant waveguide width and a constant grating pitch, as this allows for efficient mass production.

[0007] To achieve efficient single-mode operation, the waveguide width can be between 1.0 μm and 2.5 μm. However, when using narrow waveguide widths, the contact resistance is higher, and the current and optical densities along the cavity and HR facets are also higher, which may degrade performance.

[0008] Other approaches use a λ / 4 phase-shifted grating or use a chirped grating, a variable waveguide width, or an asymmetric corrugation-pitch-modulated (ACPM) grating.

[0009] However, it is difficult to further improve the performance of DFB lasers, such as increasing output power and reliability.

[0010] It is desirable to develop a laser with high output power, good reliability and manufacturability. Summary of the Invention

[0011] According to one aspect, a laser is provided, having a back reflector, a front surface spaced apart from the back reflector, and a cavity defined between the back reflector and the front surface, the cavity having a waveguide and comprising: a first cavity segment adjacent to the front surface, the first cavity segment comprising a Bragg grating and a first portion of the waveguide, the first portion having a first width, the length of the Bragg grating being in a range of 40% to 60% of the distance from the back reflector to the front surface (i.e., the total cavity length); and a second cavity segment adjacent to the back reflector, the second cavity segment comprising a second portion of the waveguide, the second portion having a second width greater than the first width.

[0012] A wider waveguide width at the rear facet of the laser reduces contact and series resistance, thereby lowering resistance and current density, and reducing the optical density at the facet while providing high output power and single-wavelength emission. This improves performance. The waveguide width in the non-grating section is wider than that in the grating section, so higher-order optical modes are not excited.

[0013] The cavity may further include a transition section, the transition section including a transition portion of the waveguide, the transition portion being located between and optically coupled to the first and second cavity segments. This may optically couple the first and second portions of the waveguide to guide light along the cavity for output at the front surface.

[0014] Each of the second cavity segment and the transition segment may not include a Bragg grating. This may stabilize the threshold current and increase the optical output power because the flared non-grating segment is more preferentially current pumped.

[0015] The width of the transition portion of the waveguide can vary between the first width and the second width. This can enable the first waveguide portion to be optically coupled to the second waveguide portion. The length of the transition segment can be equal to or shorter than the length of the second cavity segment.

[0016] The first width may be between 1.0 μm and 3.5 μm. This may enable single-mode operation of the laser.

[0017] The width of the first portion of the waveguide may be constant. This may make lasers easier to manufacture.

[0018] The period of the Bragg grating in the first cavity segment may be constant. This may further make the laser easier to manufacture.

[0019] The second width may be between 1.0 μm and 5.0 μm, which may enable the laser to operate in single mode with a more stable threshold current and higher optical output power.

[0020] The laser may be a distributed feedback laser. This may be a convenient operating format for single-mode laser oscillation.

[0021] The Bragg grating may be elongated along the length of the first cavity segment. The length extension may be orthogonal to the front surface. This may allow the grating to be positioned between semiconductor layers of the laser cavity.

[0022] The length of the Bragg grating may be in the range of 45% to 55% of the distance from the back reflector to the front surface. Values ​​in this narrower range may yield better performance than the wider range described above.

[0023] 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.

[0024] The waveguide can be a ridge waveguide or a buried heterostructure waveguide, which enables versatility in laser structure.

[0025] The front surface may be an emitting surface of the laser, which may enable the laser to provide light output or to be integrated with other optical functional structures.

[0026] The back reflector can be the back facet, and the front surface can be the front facet. The front and back facets can be cleaved surfaces. This is a convenient laser manufacturing method. The back reflector and front surface can also be formed by other convenient methods.

[0027] The product of grating intensity and length (Kappa*L g ) can be in the range of 0.6 to 1.5. This can achieve good performance.

[0028] The back reflector may be planar, and the distance from the back reflector to the front surface may be measured in a direction perpendicular to the back reflector.

[0029] 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 located between the first and second semiconductor layers, the first and second semiconductor layers being elongated in a direction extending between the back reflector and the front surface. This is a convenient laser configuration.

[0030] The Bragg grating may be located between the first and second semiconductor layers, or within one of these layers in the first cavity segment. This may facilitate fabrication of the laser. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will now be described by way of examples with reference to the accompanying drawings.

[0032] In the attached figure:

[0033] Figure 1(a) and Figure 1(b) show the top view and side view, respectively, of a standard DFB laser.

[0034] Figures 2(a) and 2(b) show a top view and a side view of a laser, respectively, wherein a first cavity segment adjacent to the front facet of the laser cavity has a Bragg grating and a waveguide with a narrower width than a second segment of the cavity adjacent to the rear facet.

[0035] Figure 3 Shown is a side view of an alternative configuration of a DFB laser with separate top contacts for the first and second cavity segments. DETAILED DESCRIPTION

[0036] As shown in Figures 2(a) and 2(b), an example of a DFB laser described herein includes three cavity segments 20, 21, 22, each of which includes a portion of a waveguide 20a, 21a, 22a for guiding light propagating along the cavity.

[0037] The laser typically includes a semiconductor block having a rear surface or rear end face 23 and a front surface or front end face 24 opposite to the front surface or front end face. The laser cavity is formed between the rear surface or rear end face 23 and the front surface or front end face 24. The total length of the laser cavity is L. This length is preferably defined between the rear surface and the front surface. A high-reflection (HR) coating is preferably applied to the rear end face 23, and an anti-reflection (AR) coating is preferably applied to the front end face 24. The rear end face with the HR coating acts as a rear reflector. Preferably, the front end face and the rear end face are aligned parallel to each other. Preferably, the front end face is orthogonal to the length of the cavity and / or the Bragg grating. Preferably, the rear end face is orthogonal to the length of the cavity. The front surface and / or rear surface of the laser can be formed by cleavage. The width of the waveguide is preferably perpendicular to the length of the cavity.

[0038] 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 located between the first and second semiconductor layers, wherein the first and second semiconductor layers are elongated in a direction extending between the rear reflector and the front surface. In the example shown in FIG2( b ), the laser cavity includes an active layer 25 interposed between a p-type semiconductor material layer and an n-type semiconductor material layer (shown as 26 and 27 , respectively). In this example, the semiconductor layers are made of InP. However, other semiconductor materials, such as GaAs, may also be used. The material forming the cavity may be selectively doped in regions of the p-type layer 26 and the n-type layer 27. The active layer 25 of the laser may be an unintentionally doped multiple quantum well (MQW) structure. Layers 25, 26, 27 are elongated in a direction extending between the rear surface 23 and the front surface 24.

[0039] A voltage may be applied to electrodes or contacts 29a and 29b on opposing top and bottom sides of the cavity.

[0040] The waveguide of the laser comprises a material with a refractive index n that is greater than the refractive index of the substrate. The waveguide can be a ridge waveguide or a buried-heterostructure (BH) waveguide for ease of manufacturing versatility. A ridge waveguide can be produced by etching parallel grooves in the material on either side of the waveguide to produce isolated protruding strips that are typically less than 10 μm wide and several hundred μm long. A material with a lower refractive index than the waveguide material can be placed on the sides of the ridge to guide the injected current into the ridge. Alternatively, the two sides of the ridge can be surrounded by air, which does not contact the substrate below the waveguide. A 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.

[0041] Light exits the laser cavity from the waveguide at the front surface 24 (ie, the front surface is the emitting face of the laser).

[0042] The cavity includes a first cavity segment 20 adjacent to a front surface 24. The first cavity segment 20 has a length L1 and includes a Bragg grating 28 and a first portion of a waveguide, generally shown as 20a in FIG2(a). The first portion 20a of the waveguide has a first width d1. d1 is preferably constant. For example, the width of the first portion of the waveguide can be constant and can be between 1.0 μm and 3.5 μm. This helps ensure single optical mode operation and makes it easier to manufacture the laser.

[0043] The Bragg grating 28 has a length L g The length of the Bragg grating 28 in the first section of the cavity 20 is in the range of 40% to 60% of the distance from the back reflector 23 to the front surface 24, i.e. 40% to 60% of L. The period of the Bragg grating in the first section is preferably constant to improve manufacturability. If the waveguide width d1 is variable, the grating period can also be variable to satisfy the condition λ = 2n eff (x)*Λ(x). The waveguide can be straight or curved, as long as the condition λ=2n is satisfied. eff (x)*Λ(x).

[0044] In the first cavity section 20, the grating strength (Kappa) and the length product Kappa*L g Preferably between 0.6 and 1.5.

[0045] In the example shown in Figure 2(b), the Bragg grating 28 is positioned near (i.e., immediately adjacent to or close to) the (AR-coated) front facet 24 between the active layer 25 and the p-type semiconductor layer 26. The grating can also be positioned between the active layer 25 and the n-type semiconductor layer 27. The Bragg grating is integral with the first cavity section of the laser. It extends along the length of the cavity, extending orthogonally to the front facet.

[0046] Preferably, the grating extends along the entire length L1 of the first cavity segment 20 (ie L1 = L g ). Thus, the first section of the cavity may have a length L1 that is in the range of 40% to 60% of the distance from the back reflector to the front surface. Alternatively, the grating may not extend all the way to the front face of the cavity, and there may be a subsection of the first section of the cavity 20 immediately adjacent to the front surface 24 that does not have a Bragg grating. In this case, the length of the Bragg grating is in the range of 40% to 60% of the distance from the back reflector to the front surface. Thus, the laser has a length L gThe grating is a portion of 40% to 60% of the total length of the waveguide. In both cases, preferably, L g In the range of 45% to 55% of the total laser cavity length, L g It may be equal to 40%, 45%, 50%, 55% or 60% of the distance from the back reflector to the front surface.

[0047] The cavity includes a second cavity segment 21 adjacent to the back reflector 23. The second cavity segment 21 has a length L2 and includes a second portion of the waveguide, as shown as 21a in Figure 2(a). The second portion 21a of the waveguide has a second width d2. The second width d2 is greater than the first width d1 so that higher-order optical modes are not excited. Preferably, d2 is between 1.0 μm and 5.0 μm. The waveguide width d2 can be constant or it can be variable along this portion of the waveguide. As a result, the laser has a wider waveguide segment that does not have a grating adjacent to the rear end face (coated with the HR coating). The second segment of the cavity serves as a gain segment.

[0048] Because the second portion 21a of the waveguide is wider than the first portion 20a, the surface area of ​​the rear waveguide is larger, resulting in a lower contact resistance than a conventional DFB laser. This may also improve thermal characteristics, as less heat may be generated due to improved heat dissipation. This also reduces series resistance, which can lower current density and reduce optical density at the end facet.

[0049] Optically coupling the first and second sections of the cavity is a transition section 22 located between the first section 20 and the second section 21. The transition section of cavity 22 has a length L3 and a width d3. The width of transition section d3 varies along the length of the transition section, between a first width d1 and a second width d2. In the example shown in FIG2(a), the width of the transition section varies linearly between the first width d1 and the second width d2. However, in other implementations, the width of the transition section can vary nonlinearly between the first width d1 and the second width d2.

[0050] As shown in Figures 2(a) and 2(b), each of the second cavity segment and the transition segment does not include a Bragg grating. Therefore, the total length of the non-grating segment of the cavity (composed of the second segment and the transition segment) is L2 + L3, adjacent to the rear surface (coated with the HR coating).

[0051] The length of the transition section 22 may be equal to or shorter than the length of the second section of the cavity 21 .

[0052] The first section 20 and the second section 21 may have a common top contact 29a, as shown in FIG2(b). Alternatively, Figure 3As schematically shown in FIG, the first and second sections of the waveguide can have separate top contacts 29a and 29c. Voltages can be applied to electrodes or contacts 29a and / or 29c and 29b on the opposite top and bottom sides of the cavity. This allows different sections of the laser to be controlled independently. This can be useful in DC or modulated laser applications.

[0053] Bragg gratings can be fabricated using electron beam lithography. This allows for very precise control of the grating pitch. The gratings can be index-coupled, gain-coupled, or complex-coupled gratings. The layers comprising the Bragg gratings can be made of either p-doped or n-doped semiconductor materials.

[0054] In summary, the DFB laser described herein preferably has a front surface coated with an anti-reflection (AR) coating and a rear surface coated with a high-reflection (HR) coating to achieve high output power. A flared partially grating laser has no grating in the rear section and a grating in the front section. The length of the non-grating section is between 40% and 60% of the total length of the laser cavity. The waveguide width in the non-grating section can be wider than the waveguide width in the grating section, so that higher-order optical modes are not excited.

[0055] Therefore, the DFB described in this paper has different waveguide widths along the length of the waveguide to improve device reliability and achieve high output power, but can still meet the single wavelength emission requirement (i.e., λ = 2n eff (x)*Λ(x)) to emit a single wavelength.

[0056] Flared partially grating DFB lasers maintain a stable, uniform distribution of optical longitudinal modes. This makes them more tolerant to external optical reflections. The front and rear facet output power ratios are more closely distributed, resulting in a higher laser yield. The electric field along the cavity is stable for different facet phases, potentially reducing spatial hole burning. The laser also achieves a more stable threshold current and higher optical output power because the flared non-grating segment is preferentially pumped. The wider waveguide near the rear surface, devoid of gratings, also results in lower contact resistance and better thermal properties.

[0057] The DFB design described in this paper can improve reliability, for example by avoiding HR-COMD, and optical output power while emitting light with a single wavelength at the front surface of the device. It is suitable for mass production, especially using steppers for better waveguide ridge control.

[0058] The laser structure can be integrated with another optically functional structure, such as an electroabsorption modulator, a Mach-Zehnder modulator, or an amplifier.

[0059] The laser is preferably a single-mode laser.

[0060] Applicants hereby disclose individually each individual feature described herein and any combination of two or more such features. It would be within the ordinary skill of those skilled in the art to be able to implement such features or combinations as a whole based on this specification, without regard to whether such features or combinations of features solve any of the problems disclosed herein, and without limiting the scope of the claims. Applicants indicate that aspects of the present invention may include any such individual features or combinations of features. In view of the foregoing description, it will be apparent to those skilled in the art that various modifications may be made within the scope of the present invention.

Claims

1. A laser, characterized in that: having a back reflector, a front surface spaced apart from the back reflector, and a cavity defined between the back reflector and the front surface, the cavity having a waveguide and comprising: a first cavity segment adjacent to the front surface, the first cavity segment comprising a Bragg grating and a first portion of the waveguide, the first portion having a first width, the Bragg grating having a length in a range of 40% to 60% of a distance from the back reflector to the front surface; a second cavity segment adjacent the back reflector, the second cavity segment comprising a second portion of the waveguide, the second portion having a second width greater than the first width; The laser is a distributed feedback laser; the second cavity segment does not include a Bragg grating, the cavity further includes a transition segment, the transition segment includes a transition portion of the waveguide, the transition portion is located between the first cavity segment and the second cavity segment and is optically coupled to the first cavity segment and the second cavity segment, the transition segment does not include a Bragg grating, and the length of the transition segment is equal to or shorter than the length of the second cavity segment; the width of the transition portion of the waveguide varies between the first width and the second width.

2. The laser according to claim 1, characterized in that The width of the transition portion of the waveguide varies between the first width and the second width.

3. The laser according to any one of the preceding claims, characterized in that The first width is between 1.0 μm and 3.5 μm.

4. The laser according to claim 1 or 2, characterized in that The width of the first portion of the waveguide is constant.

5. The laser according to claim 1 or 2, characterized in that: The period of the Bragg grating in the first cavity segment is constant.

6. The laser according to claim 1 or 2, characterized in that The second width is between 1.0 μm and 5.0 μm.

7. The laser according to claim 1 or 2, characterized in that The Bragg grating is elongated along a length of the first cavity segment.

8. The laser according to claim 1 or 2, characterized in that The length of the Bragg grating is in the range of 45% to 55% of the distance from the back reflector to the front surface.

9. The laser according to claim 1 or 2, characterized in that The front surface is coated with an anti-reflective coating and the back reflector is coated with a high-reflective coating.

10. The laser according to claim 1 or 2, characterized in that The waveguide is a ridge waveguide or a buried heterostructure waveguide.

11. The laser according to claim 1 or 2, characterized in that The front surface is the emitting surface of the laser.

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