Distributed feedback laser and method for manufacturing such a laser - Patents.com

Abstract

Distributed feedback laser and method for manufacturing such a laser - Patents.com According to a first aspect, the present disclosure relates to a distributed feedback (DFB) laser (100) comprising a laser section (110) including a planar substrate (101), a front surface (111), an active layer (102) substantially parallel to, but not coplanar with, the planar substrate (101) and configured to emit light through the front surface (111), and a first Bragg grating (115) disposed in a planar layer substantially parallel to, but not coplanar with, the active layer (102) on an opposite side of the active layer (102) to the planar substrate (101); and a mirror section (120) optically coupled to the laser section (110), the mirror section (120) including a second Bragg grating (125) configured to reflect light towards the front surface (111), the second Bragg grating (125) disposed in a planar layer coplanar with the active layer (102).

Description

[Technical field]

[0001] This disclosure relates to distributed feedback lasers. Additionally, this disclosure relates to methods of manufacturing such lasers. [Background technology]

[0002] Semiconductor devices configured to emit light are used in many technical fields, such as optical sensing and optical communications. Distributed feedback (DFB) lasers are a particular type of semiconductor device having a laser section including an active layer, such as a collection of quantum wells or multi-quantum-wells (MQW), configured to emit light, and a Bragg grating configured to select the central wavelength of the emitted light. The Bragg grating is disposed in a planar layer parallel to the active layer. Moreover, in such devices, the laser section is disposed between a front facet and a rear facet, and light is emitted out of the device through the front facet.

[0003] For most applications, single-mode operation of a laser is generally preferred, i.e., the emitted light has a bandwidth centered primarily around a single wavelength. However, in conventional DFB lasers, due to the presence of Bragg gratings in the laser section, it is generally advantageous to emit light at two different wavelengths (dual-mode operation), separated by a spectral range called the stop band of the laser.

[0004] The stop band is defined as the "reflectivity per unit length" of a DFB laser, cm -1 The coupling coefficient (κ), also called the coupling strength, of the laser section is given by:

[0005] To avoid dual-mode operation, an asymmetry is typically introduced into the DFB laser by adding, on the one hand, a mirror section consisting of a highly reflective (HR) coating on the rear facet and, on the other hand, an anti-reflection (AR) coating on the front facet. In said setup, called HR / AR DFB laser, one of the two modes of operation is favored over the other. Moreover, since almost no light is emitted through the rear facet, the optical power of the light emitted at the front facet is almost doubled compared to an uncoated DFB laser.

[0006] However, mainly due to manufacturing constraints, it is generally not possible to control the exact position of the rear facet relative to the Bragg grating. As a result, the light generated in the active layer acquires unpredictable phase shifts, also called "random phase states", when propagating back and forth in the laser section and reflecting on the HR coating. Such random phase states lead to variable performance between different DFB lasers, such as variations in output power and variations in the wavelength of the emitted light within the stop band. In particular, the wavelength of the emitted light may be less well controlled than within the stop band, and the single-mode yield, i.e. the probability that a DFB laser emits light in a single mode, is typically reduced to 80-90%, thereby imposing testing and selection of DFB lasers for applications such as wavelength division multiplexing (WDM) where a specific transmission channel is used.

[0007] To address such limitations, it is known to make DFB lasers in which an asymmetry is introduced by a laser section containing a phase shift equal to one-quarter of the period of the Bragg grating in the center of the Bragg grating and an anti-reflection coating on both the rear and front facets; see for example [Reference 1]. Such DFB lasers are called quarter-wave (λ / 4) phase-shifted DFB lasers and emit light with a central wavelength that is essentially in the center of the stop band. Such a solution therefore provides 100% single-mode yield and precise control of the emission wavelength. However, λ / 4 phase-shifted DFB lasers emit light at both the rear and front facets, causing a 50% reduction in the emitted power from one facet compared to HR / AR DFB lasers.

[0008] Alternatively, [Reference 2] proposes a DFB laser with a distributed Bragg reflector (DBR) in which a mirror section is arranged on one side of the laser section, the mirror section having a second Bragg grating arranged in a planar layer parallel to the active layer and coplanar with the planar layer of the first Bragg grating in the laser section.

[0009] In such a setup, during the fabrication of the DFB laser, it is possible to etch the first Bragg grating in the laser section and the second Bragg grating in the mirror section using the same etching mask. The spacing between the first and second gratings is thus precisely defined by said etching mask. It is therefore possible to precisely control the phase shift experienced by the light as it propagates from the first Bragg grating in the laser section to the second Bragg grating in the mirror section, thereby achieving both a high single mode yield and precise control of the wavelength of the emitted light.

[0010] However, in the mechanism of [Reference 2], the reflectivity of the mirror section depends on the product of the coupling strength of the mirror section and the length of the mirror section. In particular, the coupling strength of the mirror section seems to be low (approximately 200 cm-1 , long mirror sections, especially lengths between 300 and 480 micrometers, are necessary to obtain a satisfactory reflectivity of the mirror section. Moreover, such a low coupling strength leads to a narrow reflectivity spectrum of the mirror section, which imposes strict constraints on the difference between the refractive index of the laser section and that of the mirror section. In particular, such a low coupling strength imposes laser and mirror sections with very special vertical structures, including a single quantum well, which are not suitable for many applications.

[0011] Therefore, there is a need for alternative distributed feedback lasers having mirror sections that have higher coupling strength than the prior art while still maintaining short mirror sections. Summary of the Invention [Means for solving the problem]

[0012] Hereinafter, the term "comprises" is synonymous with "include" and "comprise", is inclusive and open, and does not exclude other non-cited elements. Furthermore, in this disclosure, when referring to a numerical value, the terms "about" and "approximately" are synonymous with a range included between 80% and 120%, preferably between 90% and 110% of the numerical value.

[0013] According to a first aspect, the present specification provides a distributed feedback (DFB) laser for emitting light having an emission spectrum centered around a predetermined central wavelength, comprising: a planar substrate, a laser section including a front surface, an active layer substantially parallel to but not coplanar with the planar substrate and configured to emit light through the front surface, and a first Bragg grating arranged in a planar layer substantially parallel to but not coplanar with the active layer, on the side of the active layer opposite the planar substrate; a mirror section optically coupled to said laser section, said mirror section including a second Bragg grating configured to reflect light towards said front surface, the second Bragg grating is disposed in a planar layer coplanar with the active layer, the second Bragg grating having a reflectance spectrum that includes the central wavelength of the emission spectrum. This relates to distributed feedback (DFB) lasers.

[0014] In this specification, the central wavelength of the emission spectrum is understood as the wavelength of the cavity mode of a DFB laser at which the emitted output power is maximum, e.g., having a side mode suppression ratio of more than about 40 dB compared to other cavity modes.

[0015] As used herein, a single-mode DFB laser is a DFB laser that emits light with an emission spectrum centered primarily around a single central wavelength. Such single-mode DFB lasers are therefore distinct from multimode DFB lasers, which emit light having an emission spectrum centered around several wavelengths.

[0016] As used herein, two coplanar planar layers means that the top surface of a first of the two planar layers is at a predetermined distance from the top surface of a second of the two planar layers that is less than a maximum distance, said maximum distance being less than about 50 nanometers, for example less than about 20 nanometers.

[0017] Conversely, when the predetermined distance exceeds the maximum distance, the two planar layers are not coplanar.

[0018] With such a configuration, the overlap between the guided optical light in the active layer and the second Bragg grating is improved compared to the prior art arrangement, so that the mirror section has a higher coupling strength than the mirror section disclosed in the prior art. Therefore, it is possible to obtain a satisfactory reflectivity with a shorter mirror section than the prior art. Subsequently, the length of the DFB laser is also reduced compared to the prior art, which is beneficial for reducing the installation area and cost.

[0019] Moreover, thanks to the high coupling strength of the mirror sections, such a configuration is particularly suitable for DFB lasers with high confinement of the optical mode in the active layer, in particular for DFB lasers configured to form high speed directly modulated lasers (DMLs) and for DFB lasers that are part of photonic integrated circuits configured to form high speed externally modulated lasers (EMLs).

[0020] In addition, due to the high coupling strength of the mirror section, the reflectivity spectrum of the mirror section is wider than that of the prior art, which overcomes the prior art limitation regarding the difference between the refractive index of the laser section and the refractive index of the mirror section. Therefore, such a mechanism is compatible with a wide variety of DFB laser structures, especially in terms of the materials used in the laser section and the mirror section.

[0021] Such compatibility is particularly advantageous in the case of DFB lasers in which the mirror section comprises a stack of layers of structures commonly used for purposes other than as a mirror section, such as the modulator section of an externally modulated laser, or structures used in passive waveguides, couplers, filters, or multiplexers.

[0022] Moreover, in such a configuration, the spacing between the first Bragg grating in the laser section and the second Bragg grating in the mirror section is deterministic, and light propagating from the first Bragg grating to the second Bragg grating experiences a predetermined phase shift, thus providing a DFB laser with deterministic performance.

[0023] In particular, it is possible to select the spacing between the first Bragg grating in the laser section and the second Bragg grating in the mirror section so as to obtain a suitable phase shift to increase the power of the light emitted at the front surface, which is beneficial for low linewidth and / or high power lasers.

[0024] According to one or further embodiments, the spacing between the first Bragg grating of the laser section and the second Bragg grating of the mirror section is introduced such that light propagating from the first Bragg grating to the second Bragg grating, i.e. when it propagates in transmission from one section to the other, acquires a phase shift approximately equal to π / 2, and thus when evaluated in reflection, i.e. when propagating back and forth between the two sections, said phase shift is approximately equal to π.

[0025] Such a phase shift allows the emitted light to have a wavelength spectrum centered within the stop band and the stop band.

[0026] According to one or further embodiments, at least one of the first and second Bragg gratings is a uniform Bragg grating.

[0027] In this specification, a uniform Bragg grating is a Bragg grating that includes a material with a uniform periodic variation of the optical refractive index.For example, a uniform Bragg grating includes alternating high and low refractive index regions and a constant distance (pitch) between the alternating high and low refractive index regions.Therefore, a uniform Bragg grating does not include several Bragg gratings with different pitches, or Bragg gratings in which the alternating high and low refractive index regions generate a phase shift, such as Bragg gratings in which the alternating high and low refractive index regions are locally missing high and low refractive index regions in the alternating high and low refractive index regions.

[0028] According to one or further embodiments, the first Bragg grating and the second Bragg grating have the same pitch.

[0029] According to one or further embodiments, the active layer extends into the mirror section and the second Bragg grating is etched through at least a portion of the active layer in the mirror section.

[0030] Such an arrangement provides a simple design of a DFB laser, in which the same active layer is used both to emit light in the laser section and to form the region of the second Bragg grating in the mirror section.

[0031] According to one or further embodiments, the mirror section includes an auxiliary waveguide comprising a transparent material, the auxiliary waveguide optically coupled to the active layer, and the second Bragg grating is etched through at least a portion of the auxiliary waveguide.

[0032] As used herein, a transparent material is a material that has an energy band gap above the energy corresponding to the wavelength of light that is configured to be emitted by the active layer of the DFB laser.

[0033] In such a configuration, the auxiliary waveguide is coupled to the laser section, for example using butt coupling techniques, evanescent coupling, selective area growth, or intermixing. Thus, a wide variety of auxiliary waveguide structures can be used in the mirror section, including structures commonly used for purposes other than in the mirror section, such as structures used in modulation sections, in couplers, or in multiplexers.

[0034] According to one or further embodiments, the auxiliary waveguide is a passive waveguide, which is a waveguide that does not include an active material configured to generate light.

[0035] Such an arrangement provides a mirror section that is passive and therefore requires no current or voltage power, and therefore consumes less power than an arrangement with an active mirror section.

[0036] According to one or further embodiments, the mirror section has a length along the direction of light propagation of less than about 200 micrometers.

[0037] According to one or further embodiments, the DFB laser is configured as a directly modulated laser.

[0038] According to a second aspect, the present disclosure relates to a photonic integrated circuit comprising at least a first DFB laser according to the first aspect and at least a first auxiliary section configured to receive light emitted by said DFB laser. Examples of such auxiliary sections include a phase or amplitude modulator, in which case the photonic integrated circuit is an externally modulated laser.

[0039] Due to the high coupling strength of the mirror sections, such DFB lasers can be integrated with a variety of auxiliary sections including a wide variety of structures.

[0040] According to one or further embodiments, the at least first auxiliary section is a modulation section configured to modulate at least one of the phase and amplitude of the light emitted by the at least first DFB laser.

[0041] According to one or further embodiments, the at least first auxiliary section is a transparent section configured to guide the light emitted by the at least first DFB laser, such as a coupler, multiplexer, demultiplexer, or filter.

[0042] According to a third aspect, the present disclosure relates to an integrated laser array comprising at least two DFB lasers according to the first aspect. According to one or further embodiments, the at least two DFB lasers are configured to emit light at at least two different wavelengths.

[0043] Such a configuration allows for control of the wavelength of emitted light while providing higher output power and smaller size than prior art integrated laser arrays.

[0044] According to a fourth aspect, the present disclosure relates to a method of manufacturing a distributed feedback laser as disclosed in any embodiment of the first aspect. More generally, the present disclosure provides a method of manufacturing a DFB laser, comprising the steps of: providing a planar substrate, an active layer substantially parallel to the planar substrate but not coplanar with the planar substrate, and a grating layer disposed on the planar layer substantially parallel to the active layer but not coplanar with the planar substrate, the grating layer being disposed on the side of the active layer opposite the planar substrate, - covering with a mask a first section of the assembly made of the planar substrate, the active layer and the grating layer, said first section intended to produce a laser section; - removing at least the grating layer in a second section of the assemblage made of the planar substrate, the active layer and the grating layer, said second section intended to produce a mirror section; - providing in a single step a grating mask at least partially covering the first section and at least partially covering the second section, said grating mask being configured to define a first Bragg grating in the first section and a second Bragg grating in the second section; using said grating mask to produce the first Bragg grating in the grating layer of the first section and the second Bragg grating in a planar layer of the second section, which is coplanar with the active layer.

[0045] According to one or further embodiments, the first Bragg grating and the second Bragg grating are produced in a single step using the grating mask.

[0046] The method according to the third aspect allows to make a DFB laser with a mirror section having a higher coupling strength than the prior art while keeping a simple process, and thus lifts the constraints on the types and structures of layers used in the laser section and the mirror section. In particular, such a method is compatible with different structures of distributed feedback lasers, directly modulated lasers, and externally modulated lasers.

[0047] Furthermore, the method according to the third aspect allows the first and second Bragg gratings to be defined in a single step and using a single grating mask, and therefore it is possible to precisely control the spacing between the first and second Bragg gratings in order to select the phase shift experienced by light on propagation between said first and second Bragg gratings.

[0048] Furthermore, the phase shift can be controlled to control the photon distribution along the laser section and the mirror section and to homogenize the power distribution by reducing the photon concentration in the mirror section, which among other things allows precise control of the wavelength of the emitted light, reduction of spatial hole burning effects, and increase of the power emitted at the front surface, which is beneficial for low linewidth and / or high power lasers.

[0049] According to one or further embodiments, a second Bragg grating is fabricated in at least a part of the active layer that extends to the mirror section.

[0050] Such an embodiment provides a simple design of a DFB laser, where the active layer is used both to emit light in the laser section and to form the region of the second Bragg grating in the mirror section.

[0051] According to one or further embodiments, the second Bragg grating is fabricated through only a portion of the thickness of the active layer, rather than throughout the entire thickness of the active layer.

[0052] According to one or more embodiments, the portion of the active layer has a thickness between about 50 nanometers and about 200 nanometers, preferably between about 50 nanometers and about 120 nanometers.

[0053] According to one or further embodiments, the second Bragg grating is fabricated in at least a part of the auxiliary waveguide that is coupled to the active layer of the first section.

[0054] According to one or further embodiments, after the step of removing at least the grating layer in the second section, the method further comprises the step of integrating a collection of semiconductor layers including the auxiliary waveguide layer in the mirror section such that the auxiliary waveguide layer is coupled to the active layer of the laser section, and the second Bragg grating is fabricated in at least a part of the auxiliary waveguide.

[0055] According to one or more embodiments, the active layer and the auxiliary waveguide are coupled through butt coupling, evanescent coupling, selective area growth, or intermixing.

[0056] According to one or further embodiments, the second Bragg grating is made only through a portion of the thickness of the auxiliary waveguide layer and not through the entire thickness of said auxiliary waveguide layer. [Brief description of the drawings]

[0057] Other advantages and characteristics of the invention will become apparent from the following description, illustrated by the figures. [Figure 1] 1 illustrates a cross-sectional view of a DFB laser along the direction of light propagation according to some embodiments. [Figure 2A] 5 illustrates steps of a method for manufacturing a DFB laser according to one embodiment. [Figure 2B] 5 illustrates steps of a method for manufacturing a DFB laser according to one embodiment. [Figure 2C] 5 illustrates steps of a method for manufacturing a DFB laser according to one embodiment. [Figure 3A] 5 illustrates steps of a method for manufacturing a DFB laser according to another embodiment. [Figure 3B] 5 illustrates steps of a method for manufacturing a DFB laser according to another embodiment. [Figure 3C] 5 illustrates steps of a method for manufacturing a DFB laser according to another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] 1 illustrates a cross-sectional view of an example of a DFB laser 100 according to the present disclosure along a direction of light propagation in the DFB laser 100. The DFB laser 100 includes a laser section 110 configured to emit light and a mirror section 120 configured to reflect the light emitted by the laser section 110.

[0059] Laser section 110 comprises a planar substrate 101, an active layer 102 on the planar substrate 101, and a first Bragg grating 115 etched through a grating layer 103 on the active layer 102. A top layer 105 is disposed on top of the first Bragg grating 103. In the example shown in FIG. 1, an optional contact layer 109 is disposed on top layer 105.

[0060] In operation, the active layer 102 is configured to emit light with an emission spectrum centered around a predetermined central wavelength and direct the light towards the mirror section 120. The active layer 102 may include a collection of semiconductor layers, for example multiple quantum wells based on InGaAsP or InGaAlAs or other ternary or quaternary materials.

[0061] The active layer 102 and the first Bragg grating 115 may be separated by a spacer layer 104, which is standard in the prior art for fabricating DFB lasers. Such a spacer layer 104 is configured to control the distance between the active layer and the first Bragg grating.

[0062] According to some embodiments, the spacer layer 104 and the top layer 105 comprise the same material such that the boundary between such layers is not visible in the DFB laser.

[0063] The planar substrate 101 may include an N-doped material, such as N-doped InP. The planar substrate 101 may also include semi-insulating InP.

[0064] The spacer layer 104, the grating layer 103, and the top layer 105 may include P-doped materials. For example, the spacer layer 104 and the top layer 105 may include P-doped InP, and the grating layer may include a P-doped quaternary material, such as P-doped InGaAs.

[0065] Alternatively, according to some embodiments, the P and N doping may be reversed such that the top layer 105, the spacer layer 104, and the grating layer 103 comprise N-doped material, while the planar substrate 101 comprises P-doped material.

[0066] The first Bragg grating 115 is etched through the grating layer 103 and comprises alternating high and low refractive index regions spaced apart at a predetermined first grating pitch. The first Bragg grating can be a uniform grating with a constant pitch. The first grating pitch depends on the wavelength of the emitted light and can be comprised, for example, between about 190 nanometers and about 250 nanometers for telecommunication wavelengths comprised between about 1270 nanometers and about 1580 nanometers. In the example shown in FIG. 1, the high refractive index regions of the first Bragg grating 115 are made from a portion of the grating layer 103, while the low refractive index regions of the first Bragg grating 115 comprise the same material as that comprised in the upper layer 105.

[0067] The laser section 110 can be configured to emit light when a voltage or current is applied between the top layer 105 and the planar substrate 101, or between materials electrically connected to the top layer 105 and the planar substrate 101.

[0068] The laser section 110 comprises a front surface 111 arranged perpendicular to the stacking direction of the layers 101, 102, 103, 104, 105. The front surface 111 is configured to transmit light out of the DFB laser 100.

[0069] According to some embodiments, the front surface 111 comprises an anti-reflective coating configured to facilitate transmission of light of a predetermined wavelength, such as, for example, a telecommunication wavelength. In particular, the front surface 111 may comprise an anti-reflective coating such that the reflectance of light of the predetermined wavelength on the front surface 111 is about 5% or less. According to some embodiments, the anti-reflective coating may comprise a layer of TiO 2 and one layer of SiO 2 or multiple such layers.

[0070] Mirror section 120 comprises a planar substrate 101 , a second Bragg grating 125 disposed on top of the planar substrate 101 , a top layer 105 and a rear surface 112 .

[0071] The rear surface 112 may include an anti-reflective coating so that possible reflections of light by the rear surface 112 do not interfere with the reflection of light in the mirror section 120 towards the laser section 110 .

[0072] The second Bragg grating 125 has a reflectivity spectrum that includes a central wavelength of the emission spectrum of the laser section. The second Bragg grating 125 comprises alternating high and low refractive index regions spaced apart by a second grating pitch. The second grating pitch is typically comprised between about 190 nanometers and about 250 nanometers at telecommunication wavelengths. The second Bragg grating can be a uniform grating of constant pitch. According to one or further embodiments, the second grating pitch is equal to the first grating pitch.

[0073] 1, the laser section 110 and the mirror section 120 may be at least partially covered by a contact layer 109 configured to provide an ohmic contact to the top layer 105, for example, to apply a voltage or current to the DFB laser so that it emits light. The contact layer 109 may include a metallic material such as, for example, Pt, Ti, or Au.

[0074] The second Bragg grating 125 is disposed in a planar layer coplanar with the active layer 102. Light generated in the active layer 102 of the laser section 110 propagates partially towards the mirror section 120 and is reflected back towards the laser section 110 upon propagating through the second Bragg grating 125.

[0075] When propagating from the laser section 110 to the mirror section 120 , the light experiences a certain phase shift that is related to the distance between the first Bragg grating 103 and the second Bragg grating 125 .

[0076] According to the first embodiment, the active layer 102 extends across the mirror section 120 and the second Bragg grating 125 is fabricated in at least a part of the active layer 102 that is within the mirror section 120 .

[0077] If the second Bragg grating 125 is fabricated in the active layer 102, a voltage or current may be injected to reduce absorption of light as it propagates through the second Bragg grating 125. Such injection may be facilitated, for example, by extending the contact layer 109 above the mirror section 120 as shown in FIG.

[0078] According to one or further embodiments of the DFB laser, the active layer 102 does not extend across the mirror section 120. In such embodiments, the second Bragg grating 125 is fabricated in an auxiliary waveguide layer that includes a transparent material, i.e., a material with an energy bandgap larger than the energy of the photons emitted by the laser section. Such a transparent material may include, for example, a stack of multiple quantum wells separated by barriers or a bulk material, where the energy bandgap of the quantum wells or bulk material is larger than the energy of the photons emitted by the laser section 110.

[0079] If the second Bragg grating 125 is fabricated in an auxiliary waveguide layer comprising a transparent material, it does not require electrical control and has a low absorption coefficient, e.g., about 15 cm -1 It inherently has an absorption coefficient less than

[0080] Applicants have discovered that in an embodiment as described with reference to FIG. 1, the reflectivity of mirror section 120 is approximately 200 cm due to the guided optical mode having improved overlap with second Bragg grating 125 as light propagates through mirror section 120 compared to prior art designs. -1 This shows that it is possible to exceed

[0081] 2A-2C represent steps of a method for manufacturing a DFB laser according to a first embodiment. In particular, 2A-2C show different steps (a)-(e) of the method for manufacturing a DFB laser in several cross-sectional views (A-A', B-B', CC'), respectively. Cross-sectional view A-A' shows the stack of layers in the laser section 110. Cross-sectional view B-B' shows the stack of layers in the mirror section 120. Cross-sectional view CC' shows the stack of layers along the direction of light propagation in the DFB laser, i.e. across the laser section 110 and the mirror section 120.

[0082] In step (a), an assembly of layers 101, 102, 103, 104, 105 is provided. Such an assembly comprises an N-doped planar substrate 101, an active layer 102 including an active material configured to emit light, a P-doped spacer layer 104, a P-doped lattice layer 103, and a P-doped top layer 105.

[0083] At the start of the fabrication process, in step (a), the top layer 105 has a very thin thickness, for example a thickness of about 20 nanometers, which is relatively thinner than the thickness of the top layer 105 of the DFB laser shown in FIG. 1, where epitaxial regrowth has been performed to increase the thickness of the top layer 105 to about 2-3 micrometers.

[0084] According to some embodiments, the active layer 102 may include, for example, multiple quantum wells configured to emit light.

[0085] According to some embodiments, the grating layer 103 may include an InGaAsP material. The grating layer 103 may have a thickness comprised between about 20 nanometers and about 80 nanometers.

[0086] According to some embodiments, the spacer layer 104 may have a thickness comprised between approximately 70 nanometers and 120 nanometers.

[0087] In step (b), a mask (not represented in Figures 2A-2C) is deposited on the first section 110 of the collection of layers 101, 102, 103, 104, 105, the remainder of the collection of layers defining the second section 120. Such a mask is used to completely etch the grating layer 103 of said second section 120 to expose the spacer layer 104. The first section 110 is intended to form a laser section, where light is generated, and the second section 120 is intended to form a mirror section, configured to reflect the light generated by the first section 110 towards said first section 110. Thus, in this specification, the first section 110 is referred to as the laser section 110 and the second section 120 is referred to as the mirror section 120.

[0088] 2C, in step (b), the etch may be further configured to remove a predetermined thickness of the spacer layer 104 to reach near the top surface of the active layer 102 and increase the etch depth into the active layer 102. Such increased etch depth improves the coupling strength in the mirror section of the DFB laser obtained by the method according to the present disclosure. In particular, according to an embodiment not shown herein, the etch is configured to remove the entire thickness of the spacer layer 104 in the mirror section to maximize the etch depth.

[0089] In step (c), a grating mask (not shown in Figures 2A-2C) is deposited on both the first section 110 and the second section 120. Then, on such grating mask, a first and a second Bragg grating (115, 125) are simultaneously defined, for example by structuring the grating mask by electron beam, holography or other techniques known to a person skilled in the art.

[0090] Furthermore, such first and second Bragg gratings (115, 125) are produced by partially etching the first and second sections, respectively, using the grating mask. The grating mask is configured to produce the first Bragg grating 115 and the second Bragg grating 125 separated by a predetermined distance after etching, for example a distance comprised between about 0 nanometers and about 10 micrometers.

[0091] According to some embodiments, the etching of the first Bragg grating 115 and the etching of the second Bragg grating 125 may be performed in a single step. According to other embodiments, the etching of the first Bragg grating 115 and the etching of the second Bragg grating 125 may be performed in two different steps.

[0092] A first Bragg grating 115 is created in the first section 110 by etching through the top layer 105 and the grating layer 103. Additionally, as shown in Figure 2C, a predetermined thickness of the spacer layer 104 can also be etched to ensure that the entire thickness of the grating layer 103 is etched in the laser section 110 and to reduce the sensitivity of the coupling strength of the laser section to the etch depth of the laser section.

[0093] The second Bragg grating 125 is created in the mirror section 120 by etching through the spacer layer 104 remaining after the etching of step (b) and through the active layer 102. Furthermore, unlike the example shown in Figure 2C, part of the planar substrate 101 may also be etched to ensure that the entire thickness of the active layer 102 is etched in the mirror section 120, to reach the highest possible grating strength and to reduce the sensitivity of the coupling strength of the mirror section 120 to the etching depth of the active layer 102.

[0094] In step (d), the grooves in the mirror section 120 caused by the etching are filled using epitaxial regrowth with a P-doped material, such as the same material as that contained in the top layer 105. Furthermore, such regrowth aims to planarize the top surfaces of the laser section 110 and the mirror section 120. This regrowth may include, for example, adding 200 nm to 500 nm of a P-doped material, such as P-doped InP.

[0095] In step (e), a final etch is performed to create the optical waveguide. In the example shown in Figures 2A-2C, the waveguide is a buried waveguide. The waveguide can also be further buried in a semi-insulating material such as P-doped or InP.

[0096] Alternatively, step (d) can include a thicker regrowth of the P-doped material, for example up to about 2 micrometers, or up to about 3 micrometers.

[0097] For example, in step (e), the final etch can be done only through the thick P-doped material (ie layers 105, 103, 104) to create a shallow ridge waveguide.

[0098] As another example, in step (e), a final etch can be performed through the P-doped material (ie layers 105, 103, 104) and through the active layer 102 to create a deep ridge waveguide.

[0099] After step (e), additional steps known to those skilled in the art may be performed to obtain a completed DFB laser.

[0100] In the method according to the present disclosure, the second Bragg grating 125 is etched primarily through the active layer 102, so that the coupling strength of the mirror section 120 exceeds that of the mirror section of a prior art DFB. In particular, the DFB laser obtained using the method according to the present disclosure has a coupling strength of 200 cm -1 The mirror section 120 has a diameter of 12 mm or more.

[0101] 3A-3C represent steps of a method for manufacturing a DFB laser with a second Bragg grating (125) fabricated in a transparent waveguide layer (132) coupled to the active layer (102). 3A-3C show different steps (a')-(f') of the method for manufacturing a DFB laser with several cross-sections (A-A', B-B', CC'), respectively. Cross-section A-A' shows the layer stack in the laser section 110. Cross-section B-B' shows the layer stack in the mirror section 120. Cross-section CC' shows the layer stack along the direction of light propagation in the DFB laser, i.e. across the laser section 110 and the mirror section 120.

[0102] As shown in Figures 3A-3C, step (a') is identical to step (a) already presented. In particular, an assembly is provided comprising layers 101, 102, 103, 104, 105. Such an assembly comprises an N-doped planar substrate 101, an active layer 102 comprising an active material configured to emit light, a P-doped spacer layer 104, a P-doped lattice layer 103 and a P-doped top layer 105.

[0103] In step (b'), a mask is deposited on the first section 110 of the assemblage of layers 101, 102, 103, 104, 105, the remainder of the assemblage of layers defining the second section 120. As in the first embodiment, in the second embodiment the first section 110 is intended to form a laser section, where light is generated, and the second section 120 is intended to form a mirror section, arranged to reflect the light generated by the first section 110 towards said first section 110.

[0104] After the mask is deposited, the top layer 105 , the grating layer 103 , the spacer layer 104 and the active layer 102 are etched from the second section 120 .

[0105] Furthermore, in step (b'), the etching may be configured to remove an additional thickness of the planar substrate 101 to ensure that the entire thickness of the active layer 102 of the mirror section 120 is etched away. This also allows for a subsequent step of regrowth starting from the planar substrate 101 which acts as a buffer layer to facilitate regrowth compared to regrowth starting from the remaining thickness of the active layer 102.

[0106] In step (c'), a collection 130 of semiconductor layers including an auxiliary waveguide layer 132 is integrated into the mirror section 120 so as to be coupled to the laser section 110. In particular, the auxiliary waveguide layer 132 of the collection 130 is coupled to the active layer 102 of the laser section 110.

[0107] 3A-3C, step (c') specifically illustrates integration based on the butt coupling technique, but other techniques such as evanescent coupling, selective area growth or intermixing can alternatively be used. A person skilled in the art will know how to adapt the method steps to the integration technique used.

[0108] The auxiliary waveguide layer 132 is configured to receive and guide light emitted by the laser section 110 when coupled to the laser section 110 .

[0109] 3A-3C, each semiconductor layer assembly 130 may include an auxiliary substrate layer 131 and an auxiliary top layer 133. The auxiliary substrate layer 131 may include the same material as the planar substrate 101. The auxiliary top layer 133 may include the same material as the spacer layer 104.

[0110] Each semiconductor layer collection 130 advantageously comprises a structure used in an element of a photonic integrated circuit, such as a structure used in a modulator, coupler or multiplexer.

[0111] 3A-3C, in step (c'), the top surface of the assemblage 130 may be much higher than the top surface of the layer 102, so an additional etching step may be performed to remove at least a part of the auxiliary top layer 133 and reach close to the auxiliary waveguide layer 132. For example, an etching step similar to step (b) of FIG.

[0112] Steps (d'), (e'), and (f') of the example shown in Figures 3A-3C are similar to steps (c), (d), and (e) of the example shown in Figures 2A-2C, except that in the example shown in Figures 3A-3C, the second Bragg grating 125 is produced by etching through the auxiliary waveguide layer 132 instead of by etching through the active layer 102.

[0113] 3A-3C, if the DFB laser belongs to a photonic integrated circuit, which also comprises sections such as couplers, multiplexers or modulators, the integration of the auxiliary waveguide layer 132 in the mirror section 120 can be advantageously performed without costs. Interestingly, in such a case, these sections are also made using integrated techniques, so that steps (b') and (c') are already part of the manufacturing process of the photonic integrated circuit. The method according to the present invention is therefore particularly simple to implement.

[0114] While the present invention has been described with respect to a limited number of embodiments, those skilled in the art having the benefit of this disclosure will appreciate that other embodiments may be devised which do not depart from the spirit of the invention disclosed herein. Accordingly, the scope of the present invention should be limited only by the appended claims.

[0115] [Reference 1] DEBREGEAS, H., FERRARI, C., PAPAZIAN, AR, et al. High-performance 100Gb / s DWDM transmitter through fully passive assembly of a single-chip array of directly modulated lasers with a SiO2 AWG. In : 2014 International Semiconductor Laser Conference. IEEE, 2014. p. 56-57 [Reference 2] SHIM, J.-I., KOMORI, K., ARAI, S., et al. Lasing characteristics of 1.5 mu m GaInAsP-InP SCH-BIG-DR lasers. IEEE journal of quantum electronics, 1991, vol. 27, no 6, p. 1736-1745)

Claims

1. A distributed feedback (DFB) laser (100) for emitting light having an emission spectrum centered around a predetermined center wavelength, comprising: A planar substrate (101), a laser section (110) including a front surface (111), an active layer (102) substantially parallel to but not coplanar with the planar substrate (101) and configured to emit light through the front surface (111), and a first Bragg grating (115) disposed in a planar layer substantially parallel to but not coplanar with the active layer (102) on an opposite side of the active layer (102) from the planar substrate (101); a mirror section (120) optically coupled to the laser section (110), the mirror section (120) including a second Bragg grating (125) configured to reflect light toward the front surface (111); Equipped with the second Bragg grating (125) is disposed in a planar layer coplanar with the active layer (102), the second Bragg grating having a reflectivity spectrum that includes the center wavelength of the emission spectrum; A distributed feedback (DFB) laser (100).

2. 2. The DFB laser according to claim 1, the first Bragg grating and the second Bragg grating are uniform Bragg gratings and have the same pitch; DFB laser.

3. 2. The DFB laser according to claim 1, the mirror section includes an auxiliary waveguide comprising a transparent material, the auxiliary waveguide optically coupled to the active layer (102); the second Bragg grating (125) is etched through at least a portion of the auxiliary waveguide; DFB laser.

4. 4. The DFB laser according to claim 3, the auxiliary waveguide is a passive waveguide; DFB laser.

5. 2. The DFB laser according to claim 1, a spacing between the first Bragg grating (115) of the laser section (110) and the second Bragg grating (125) of the mirror section (120) is introduced such that light propagating from the first Bragg grating (115) to the second Bragg grating (125) experiences a phase shift equal to approximately π / 2; DFB laser.

6. 2. The DFB laser according to claim 1, The mirror section (120) has a length along the direction of light propagation of less than about 200 micrometers. DFB laser.

7. 2. The DFB laser according to claim 1, Such a DFB laser is configured as a directly modulated laser. DFB laser.

8. At least a first DFB laser (100) according to claim 1; at least a first auxiliary section configured to receive light emitted by the DFB laser (100); 2. A photonic integrated circuit comprising:

9. 9. The photonic integrated circuit of claim 8, the at least first auxiliary section is a modulation section configured to modulate at least one of the phase and amplitude of the light emitted by the at least first DFB laser; Photonic integrated circuits.

10. 9. The photonic integrated circuit of claim 8, the at least first auxiliary section is a transparent section configured to guide the light emitted by the at least first DFB laser. Photonic integrated circuits.

11. 10. A laser system comprising at least two DFB lasers according to claim 1 configured to emit light at at least two different wavelengths. Integrated laser array.

12. 1. A method of manufacturing a DFB laser, comprising: providing a planar substrate (101), an active layer (102) that is substantially parallel to the planar substrate (101) but not coplanar with the planar substrate, and a grating layer (103) disposed on the planar layer that is substantially parallel to the active layer (102) but not coplanar with the active layer (102), the grating layer (103) being disposed on an opposite side of the active layer (102) from the planar substrate (101); - covering with a mask a first section of the assembly made of the planar substrate (101), the active layer (102) and the grating layer (103), said first section (110) intended to produce a laser section (110); - removing at least the grating layer (103) in a second section (120) of the assembly made of the planar substrate (101), the active layer (102) and the grating layer (103), said second section intended to produce a mirror section (120); providing, in a single step, a grating mask that at least partially covers the first section (110) and at least partially covers the second section (120), the grating mask being configured to define a first Bragg grating (115) in the first section (110) and a second Bragg grating (125) in the second section (120); using the grating mask to fabricate the first Bragg grating (115) in the grating layer (103) of the first section (110) and the second Bragg grating (125) in a planar layer of the second section (120) that is coplanar with the active layer (102); A method comprising:

13. 13. The method of claim 12, the first Bragg grating (115) and the second Bragg grating (125) are fabricated in a single step using the grating mask; method.

14. The method according to any one of claims 12 to 13, After removing at least the grating layer (103) in the second section, integrating the set of semiconductor layers (130) including the auxiliary waveguide layer (132) into the mirror section so that the auxiliary waveguide layer (132) is coupled to the active layer (102) of the laser section (110), wherein the second Bragg grating (125) is fabricated in at least a portion of the auxiliary waveguide (132). The method further comprises:

15. 15. The method of claim 14, The active layer (102) and the auxiliary waveguide (132) are coupled through butt coupling, evanescent coupling, selective area growth, or intermixing. method.

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