WO2012058742A1 - Laser dfb à réseau gravé en surface à couplage vertical - Google Patents

Laser dfb à réseau gravé en surface à couplage vertical Download PDF

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WO2012058742A1
WO2012058742A1 PCT/CA2010/001737 CA2010001737W WO2012058742A1 WO 2012058742 A1 WO2012058742 A1 WO 2012058742A1 CA 2010001737 W CA2010001737 W CA 2010001737W WO 2012058742 A1 WO2012058742 A1 WO 2012058742A1
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mesa
layer
grating
laser
optical
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PCT/CA2010/001737
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Christopher Watson
Kirill Pimenov
Valery Tolstikhin
Fang Wu
Yury Longvin
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Onechip Photonics Inc.
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Priority to CN2010800708123A priority Critical patent/CN103460527A/zh
Priority to EP10859123.1A priority patent/EP2636110A1/fr
Priority to PCT/CA2010/001737 priority patent/WO2012058742A1/fr
Publication of WO2012058742A1 publication Critical patent/WO2012058742A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • H01S5/1231—Grating growth or overgrowth details
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • H01S5/1237—Lateral grating, i.e. grating only adjacent ridge or mesa
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/204—Strongly index guided structures
    • H01S5/2045—Strongly index guided structures employing free standing waveguides or air gap confinement
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/22—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers having a ridge or stripe structure
    • H01S5/223—Buried stripe structure
    • H01S5/2232—Buried stripe structure with inner confining structure between the active layer and the lower electrode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/0206—Substrates, e.g. growth, shape, material, removal or bonding
    • H01S5/0208—Semi-insulating substrates
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
    • H01S5/042—Electrical excitation ; Circuits therefor
    • H01S5/0421—Electrical excitation ; Circuits therefor characterised by the semiconducting contacting layers
    • H01S5/0422—Electrical excitation ; Circuits therefor characterised by the semiconducting contacting layers with n- and p-contacts on the same side of the active layer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/2004—Confining in the direction perpendicular to the layer structure
    • H01S5/2018—Optical confinement, e.g. absorbing-, reflecting- or waveguide-layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/2054—Methods of obtaining the confinement
    • H01S5/2081—Methods of obtaining the confinement using special etching techniques

Definitions

  • the invention relates generally to the field of semiconductor lasers, and more particularly to the design of the distributed-feedback lasers for use in photonic integrated circuits.
  • III-V semiconductor laser diodes are the most efficient and compact sources of coherent radiation in near infrared spectral range. As such, they are an obvious choice for use in optical communication systems based on silica fibers which have windows of transparency falling into the emission range of these III-V semiconductor laser diodes. Whereas the basic principle of achieving an optical gain is the same for all laser diodes and relates to an inversion of free carrier population by electrical injection into a forward biased ⁇ junction, it is the optical cavity solution that mainly differentiates the semiconductor laser designs. One of the most common, and often most preferable, solutions is a distributed feedback (DFB) cavity.
  • DFB distributed feedback
  • DFB lasers semiconductor laser diodes featuring DFB cavities, hereafter referred to simply as DFB lasers, have become the laser sources of choice in advanced optical communication systems.
  • the DFB laser is an on-chip laser source solution for photonic integrated circuits (PICs), which combine different active and passive waveguide-based functional elements of the optical circuit by monolithically integrating them onto one semiconductor substrate.
  • PICs photonic integrated circuits
  • DFB lasers are amongst the most demanded and massively produced optical components for telecommunications markets.
  • the great majority of the DFB lasers addressing these markets are designed for operation in either the 1550nm or 1310nm wavelength windows, corresponding to the spectral ranges of minimal loss and minimal dispersion of silica fibers, respectively.
  • the material system of choice which naturally covers these wavelength windows, being indium phosphide (InP) based compound semiconductors from groups III and V, hereafter referred to as III-V semiconductors.
  • DFB lasers are an attractive design option that not only has the advantage of dynamically single-frequency operation, but also is suitable for a monolithic integration with other waveguide components, both active (e.g. electro-absorption modulator) and passive (e.g. mode converter), onto the same semiconductor substrate, which a laser with a cavity defined by cleaved-facets such as Fabry-Perot (FP) lasers are not.
  • active e.g. electro-absorption modulator
  • passive e.g. mode converter
  • the operating principle of the DFB laser cavity is based on multi-beam interference in an optical waveguide having periodic modulation of the effective index as seen by the guided waves, hereafter referred to as a waveguide Bragg grating. More specifically, a DFB cavity effect occurs when co-directional guided waves, which have experienced multiple reflections from the Bragg grating, are in-phase with each other and add through constructive interference. For any given grating order , this happens under conditions of a direct resonant coupling between the contra-directional guided waves occurring near the Bra wavelength given by Equation (1) below:
  • the DFB cavity is a wavelength selective cavity, with a selection defined by the grating pitch, an advantage that makes DFB lasers attractive for applications requiring emission wavelength control.
  • the basic operating principle of the DFB cavity is the same for any form and shape of the Bragg grating, its actual performance depends on many factors including the grating order, type of a spatial modulation of the effective index seen by the guided modes in the optical waveguide bearing the grating, e.g. whether this is a modulation of a real or an imaginary or both the real and imaginary parts of the effective index, and the coupling efficiency of the grating. From this prospective, the choices available to DFB laser designer are many, but in reality the most common solutions are compromises between performance and manufacturability.
  • the first order grating pitch should be only ⁇ 200nm so that for a duty cycle of 50% it requires -lOOnm lithography line resolution. This is not possible to achieve with a conventional (contact) optical lithography or even a stepper optical lithography usually available to laser fabrication and requires different and more expensive techniques to be used, such as direct electron beam writing, focused ion beam writing, or most commonly optical beam interference.
  • the waveguide Bragg grating is usually formed by fabricating an etched corrugation in a semiconductor layer that is close to the waveguide core as this leads to a strong coupling between the contra-directional guided waves in a spectral range near Bragg resonance and therein leads to favorable DFB cavity characteristics such as narrow resonance line and high rejection of wavelengths outside the resonance range.
  • This technique is well documented in the prior art, see for example S. Akiba et al in US Patent 4,506,367 entitled “Distributed Feedback Semiconductor Laser", Z-L Liau et al in US Patent 4,722,092 entitled “GalnAsP InP Distributed Feedback Laser” and G.L.
  • the fundamental feature of such techniques is that they all require an additional epitaxial growth, performed after the corrugated grating etch, to complete the laser waveguide structure and provide the semiconductor material suitable for forming the electric contact at the top of the laser structure.
  • An overgrowth is a process that not only complicates fabrication and reduces yield by generating defects at the interface of the etched corrugated grating in the proximity to (in a case of the index-coupled grating) or within (in a case of the gain-coupling grating) the laser gain region, but also restricts compatibility of so-processed DFB lasers with other functional elements and thereby limits their usability for monolithic PICs.
  • MGVI multi-guide vertical integration
  • the key element of the LCG-DFB laser is the surface etched grating (SEG) formed in the laser's top N-emitter layer by etching two parallel sets of periodic trenches separated by a strip of intact material between them.
  • SEG surface etched grating
  • This SEG provides both lateral confinement, as the average refractive index in each set of periodic trenches is lower than that in the intact material, and the DFB cavity which are processed in one fabrication step.
  • MGVI compatible LCG-DFB lasers featuring this design, as a generic building block of multi-functional PIC or a part of a bidirectional transceiver duplex PIC, have been reported by S.B. Kuntze et al in "Transmitter and Receiver Solutions for Regrowth Free Multi-Guide Vertical Integration” (Integrated Photonics Research 2010, Paper ITuC5) and V.
  • the effective ridge DFB cavity has inherent performance limitations associated with the way the SEG is defined and interacts with the guided waves in the laser cavity. More specifically, the coupling efficiency of such a DFB cavity is relatively low, as compared to its counterparts featuring corrugated grating defined within or in a close proximity to the optical waveguide. Since for a given length of the grating, the coupling efficiency defines the laser cavity quality and all performance features associated with it, less efficient coupling requires longer cavities and higher bias currents, which certainly is not a preferable choice in most laser designs and especially those intended for a high-speed direct modulation
  • Equation (3) A coefficient of direct resonance coupling between contra-propagating guided waves in a waveguide with m » order Bragg grating is given by Equation (3) below:
  • a c is the difference in optical permittivity for a wavelength close to the
  • the grating order is limited by the lithography line resolution. So, for a first order grating, which yields the highest coupling coefficient and does not produce radiation losses, in the 1310nm telecommunications wavelength window, the first order grating requires the line resolution be below lOOnm.
  • the first order grating is beyond diffraction limit of such a stepper, and the only available solutions for the stepper lithography are higher order gratings, presumably m ⁇ 3 .
  • increasing the grating order further reduces the resonance coupling coefficient, by a factor ofl/w , thereby making an evanescent-field coupled LCG-DFB even a more challenging task.
  • the coupling efficiency of the grating can be increased, but only if the effective ridges are narrow enough, as well as the LC-SEG areas that separate them - narrower than the wavelength in the semiconductor material, which is difficult to implement because of the lithography resolution limitations.
  • Another approach to enhancing the coupling efficiency of the LC-SEG which also exploits the idea of the lateral mode dilution, even though in a different way, has been described by C.
  • the LC-SEG DFB laser operates in the first order lateral mode, as opposed to the conventional fundamental zero order mode, thereby having the optical mode field mostly aside from the active effective ridge, where it is better overlapped with the LC-SEG and hence provides higher coupling with the LC-SEG.
  • This design has yet another advantage, in that it reduces the mode overlap with the metal contact atop the active effective ridge and impact of the SEG etch rounding / irregularities at the trench edges next to the active effective ridge, thereby reducing intracavity loss and further improving the coupling efficiency.
  • this laser needs to be complemented with a certain waveguide arrangement that transfers the first order lateral mode, in which the laser operates, into the zero order optical mode, in which optical signals can be coupled into a fiber or other parts of the PIC.
  • a certain waveguide arrangement that transfers the first order lateral mode, in which the laser operates, into the zero order optical mode, in which optical signals can be coupled into a fiber or other parts of the PIC.
  • the object of the invention is a vertically-coupled surface etched grating distributed feedback (VCSEG-DFB) laser, which, within the SEG-DFB design paradigm and MGVI design / fabrication platform, enables for a grating coupling efficiency enhancement at higher optical mode confinement with the gain region and lower sensitivity to the grating processing imperfectness as compared to its laterally- coupled surface etched grating distributed feedback (LCSEG-DF) counterpart.
  • VCSEG-DFB vertically-coupled surface etched grating distributed feedback
  • a VCSEG-DFB laser features a mesa structure with the SEG, in a form of a periodic set of trenches defined in the middle part of the mesa, etched down from the top surface of the mesa and perpendicular to the direction of propagation along the mesa, while mesa's top surface aside from the SEG is used to define top laser contact, in a form of two metal strips going along the mesa and disposed on both sides of the SEG.
  • Epitaxial structure of the mesa comprises the top and bottom emitters with a separate-confinement heterostructure (SCH) layers, including the gain region, presumably in a form of active multiple quantum well (MQW) layers, between them, such that, in use, the SEG is formed in the top emitter layer above the SCH layers.
  • SCH separate-confinement heterostructure
  • MQW active multiple quantum well
  • Vertical optical confinement is defined by the SCH, which is designed to provide a sufficiently high evanescent field coupling with the SEG in the middle part of the mesa.
  • Lateral optical confinement in such a structure opposite to the LCSEG-DFB design teachings of the prior art, is not provided by the SEG itself but can be arranged by profiling refractive index in the mesa layers below the SEG in a way suitable for a lateral guiding.
  • the fundamental optical guided mode shape in the VCSEG-DFB laser is vertically inverted as compared to that in the LCSEG-DFB laser: the former is laterally confined at the bottom of the mesa and overlaps with the VC-SEG by the bulk of the mode field, whereas the latter is laterally confined at the top of the mesa and overlaps with the LC-SEG only by the evanescent field at the lateral tails of the mode.
  • Increased overlap between the fundamental optical mode and the SEG in VCSEG-DFB laser is achieved without further compromising the mode confinement within the gain region, thereby enabling more design flexibility in a trade- off between higher coupling with the Bragg grating vs. higher modal gain.
  • the fundamental optical mode in the VCSEG-DFB laser is less subjected to metal loss than that in the LCSEG-DFB laser, since the metal strips atop the mesa can be disposed away from this mode, as well as rounding and etch irregularities at the grating trench edges can be made less critical by widening the grating trenches beyond the lateral mode size.
  • the grating coupling efficiency in such a VCSEG-DFB laser being increased, while the optical loss and manufacturing variations reduced against the prior art based on the LCSEG-DFB laser design.
  • a design of the VCSEG- DFB laser and regrowth-free method of manufacturing the same that comprises a laser PIN structure processed in a form of a mesa etched from the top surface of the upper, P- or N-, emitter layer, through the SCH, including the active, presumably MQW, gain region, down to the top surface of the lower, N- or P-, emitter, with the lower strip contacts disposed at the top surface of the lower emitter layer aside from the mesa and the upper strip contacts disposed at the top surface of the upper emitter layer, which also is the top surface of the mesa, and a SEG defined at and etched from this top surface, in the space between two upper strip contacts, through the upper emitter layer down to or into the SCH layers.
  • An optical waveguide is formed by the SCH that naturally provides a vertical confinement to the optical field and a certain special arrangement for the lateral profile of the refractive index in the bottom portion of the mesa to provide a lateral confinement to the optical field, such that, in use, the waveguide supports only one - fundamental - guided optical mode, which is laterally defined at the bottom of the mesa and interacts with the SEG at the top of the mesa, by the vertical tail of the mode penetrating the SEG and evanescent field coupling to the SEG.
  • An electrical injection under a forward bias of the PIN structure is achieved by applying a proper polarity voltage between the upper and lower strip contacts, with a lateral distribution of a majority carrier injection up from the lower emitter predetermined in a certain way by a relationship between the lower emitter and SCH layer sheet resistances, such that, in use, a lateral confinement - or a current aperture - can be provided in the bottom portion of the mesa in order to optimize the injection current overlap with the optical mode, thereby increasing the injection efficiency and enhancing the laser performance.
  • the design and regrowth-free manufacturing of the VCSEG-DFB laser of said species is provided that combines the lateral confinement of the optical field and that of the injection current at the bottom of the mesa in one lateral feature fabricated in the same processing step(s), which not only simplifies the device fabrication, but also ensures close to optimal overlap between the optical mode and the injection current.
  • a VCSEG-DFB laser is fully compatible with the MGVI design principles and manufacturing methodologies, thereby being suitable for a one-step growth monolithic integration with other passive and active functional elements into a multi-functional PIC, based on the MGVI platform.
  • Figures 1 A and IB depicts a schematic tree-dimensional view of an effective ridge LCSEG-DFB laser according to the prior art, using two parallel sets of periodic trenches separated by a strip of an intact semiconductor material to form both the effective ridge waveguide and the laterally-coupled waveguide Bragg grating within an etched mesa structure featuring one top - in the middle of the mesa - and two bottom - aside from the mesa - strip contacts;
  • Figures 1A and IB depict schematics of three-dimensional views of an effective ridge LCSEG-DFB lasers according to the prior art, using two parallel sets of periodic trenches separated by a strip of an intact semiconductor material to form both the effective ridge waveguide and the laterally-coupled waveguide Bragg grating within an etched mesa structure featuring one top - in the middle of the mesa - and two bottom - aside from the mesa - strip contacts;
  • Figure 1C depicts a schematic three-dimensional view of a LCSEG-DFB source structure employing a MGVI compatible design LCSEG-DFB laser as per Figure IB together with detector and vertical mode coupler.
  • Figure 2 A depicts schematic cross-sections of the exemplary VCSEG-DFB lasers according to embodiments of the invention related to lateral - optical field and electrical current - confinement arrangements
  • Figure 2B depicts a schematic three-dimension view of an exemplary VCSEG-DFB lasers according to an embodiments of the invention related to lateral - optical field and electrical current - confinement arrangements
  • Figure 3 depicts an exemplary layer structure of the VCSEG-DFB laser's mesa and two-dimensional profile of the fundamental guided mode therein, evanescent- field coupled to the SEG in the center top part of the mesa, and also provides similar features for a prior art LCSEG-DFB laser - for comparison;
  • Figure 4 depicts the calculated grating efficiency for LCSEG-DFB and VCSEG-DFB devices exploiting the exemplary layer structures presented in Figures 3 A and 3B as a function of grating duty cycle.
  • the invention is directed to providing, within a concept of a surface-etched grating distributed feedback (SEG-DFB) laser, an increased overlap between the guided optical mode propagating within a laser's waveguide and vertically-coupled to SEG, to arrange for an efficient DFB cavity and thereby enhance a laser performance while mitigating limitations of the prior art, using a laterally-coupled SEG, such as low coupling efficiency, high intracavity loss and strong impact of SEG fabrication imperfectness on both the coupling efficiency and intracavity loss.
  • SEG-DFB surface-etched grating distributed feedback
  • FIG. 1A Shown in Figure 1A is a schematic three-dimensional view 100A of a prior art embodiment of an effective-ridge LCSEG-DFB laser, as it has been originally proposed and demonstrated by L.M. Miller, et al in "A Distributed Feedback Ridge Quantum Well Heterostructure Laser", IEEE Photonics Technol. Lett., Vol. 3, pp. 6 (1991).
  • the device implemented in AlGaAs - GaAs material system, is grown on N + - substrate and features P-up / N-down contacts, the former being disposed at the back side of the (thinned and polished) substrate and the latter - atop the effective ridge, which is formed by the SEG shaped as two parallel but separated sets of periodic trenches.
  • it is not suitable for a monolithic integration with other functional elements onto the same substrate, least it is compatible with a regrowth-free MGVI platform.
  • Figure IB Illustrated by Figure IB is an adaptation of the LCSEG-DFB laser to this regrowth-free MGVI platform, shown as second schematic 100B, in a form of an integrated transmitter building block featuring N-up / P-down strip contacts, the former being disposed atop the effective ridge in the centre of the laser mesa and the latter aside from and at the bottom of the laser mesa, and also a back-side power monitor at laser's rear end and a passive waveguide at laser's front end, proposed and implemented by V. Tolstikhin et al in "Laterally-Coupled DFB Laser for One-Step Growth Photonic Integrated Circuits in InP", IEEE Photonics Techn. Lett., Vol. 21, P.
  • the device designed for and implemented in InP-based material system, is grown on semi-insulating (SI) substrate and can be monolithically integrated onto the same substrate, in one-step growth process, with other functional elements, to which it can be optically connected by a passive waveguide underneath the active - laser - waveguide.
  • SI semi-insulating
  • FIG. 1C there is shown a schematic three-dimensional view lOOC of the LCSEG-DFB source employing a MGVI compatible LCSEG-DFB design 1010, according to second schematic 100B in Figure IB, together with N-up rear facet power monitor 1020 and vertical mode transition 1030 to couple the laser output to the passive waveguide 1040 of the PIC. Also shown inset within Figure 1 C is the MGVI compatible LCSEG-DFB design which vertically represents a P-down / N-up laser PIN atop of passive (i.e.
  • the laser PIN structure 130 comprises N-InP 131 and P-InP 132 emitter layers at the top and the bottom, respectively, and also SCH layers 133 in between.
  • the SCH layers 133 are formed from AlGalnAs materials, with (compressively) strained MQW gain region 134 positioned in the middle of the SCH.
  • the laser mesa 140 is processed by etching from the top surface of N-InP emitter layer 131 , through this layer, the SCH layers 133, and the strained MQW gain region 134, down to the top surface of the P-InP emitter layer 132.
  • the laser effective ridge waveguide is formed by the SEG 150, which is etched from the top surface of the N-InP emitter layer 131, through this layer, down to the SCH layers 133.
  • the laser strip P- and N-contacts, 135 and 136 respectively, are disposed on the top surfaces of P-emitter layer 132 and N- emitter layer 131, aside from and atop of the mesa 140, respectively.
  • the SEG 150 being formed by a pair of periodic sequences of trenches 155 etched on either side at the edges of the N-InP emitter layer 131 leaving a central unetched region with periodic lateral ribs left remaining in the N-InP emitter layer 131.
  • such a grating should have a very narrow trench 155 to be etched from the mesa surface, which would limit the trench depth since the trench etch aspect ratio, defined as the trench depth to width ratio, usually cannot exceed few units.
  • the trench etch aspect ratio defined as the trench depth to width ratio
  • a high duty cycle SEG is limited in its etch depth, and the shallower it is, the closer to the laser guide core the top mesa surface needs to be in order to preserve the coupling efficiency at a level sufficient for a good DFB laser performance.
  • the top metal, laser-strip N-contact 136 is disposed on the same mesa surface, atop of the effective ridge formed by the two parallel sets of periodic trenches, also serving as the laterally-coupled waveguide Bragg grating, the shallower SEG means higher overlap between fundamental guided optical mode and the top contact metal, laser-strip N-contact 136, thereby resulting in a higher intracavity loss caused by this metal.
  • a LCSEG- DFB structure design faces a difficult trade-off between the demand for a high coupling efficiency, which suggests a high duty cycle, shallow SEG and thus thin top emitter layer, and a desire to reduce the intracavity optical loss, which, on the contrary, requires a thick top emitter layer.
  • a certain optimization within the LCSEG-DFB laser design concept that addresses the trade-off issue above indeed is possible, e.g. as reported by V.
  • exemplary layer structure 200 is a semiconductor layer vertical stack, grown in one growth step on a SI InP:Fe substrate 210, that has a laser PIN structure positioned atop of a passive optical waveguide 220, wherein the PIN structure comprises N-InP 231 and P-InP 232 emitter layers at the top and bottom, respectively, with SCH layers 233, including compressively strained MQW gain region layers 234, in between.
  • the device lateral structure design is different from that of the LCSEG-DFB laser design of the prior art, at least in the following two aspects.
  • the VCSEG-DFB design of the present invention has one set of the periodic trenches in the centre of the laser mesa, also defined at and etched from the mesa's top surface, which trenches form the vertically-coupled waveguide Bragg grating, but do not provide lateral guiding, while the upper contact metal still is disposed atop of the mesa but aside from the set of the periodic trenches in the mesa centre.
  • the lateral confinement for both the optical field and injection current is achieved by a lateral undercut of the semiconductor material at the bottom of the mesa.
  • a lateral undercut in InP-based material system is implementable by inserting a Ga x Ini. x As y Pi -y or Al x Ga y Ini -x-y As layer in between two InP layers and using highly selective chemical (wet) etcher that etches Ga x Ini. x As y Pi. y or Al x Ga y Ini.
  • Process-wise alternative but functionally similar solution is a lateral oxidation of Al-rich material, ternary Al x Ini -x As or quaternary Al x Ga y In 1-x- y As, sandwiched between two InP layers, instead of the lateral undercut, e.g. as disclosed by N. Iwai et al in the paper entitled "High-Performance 1 .3- m InAsP Strained-Layer Quantum-Well ACIS (Al-Oxide Confined Inner Stripe) Lasers", IEEE J. Selected Topics in Quantum Electron., Vol. 5, No 3, P. 694 ( 1999) or by M.H.M.
  • the resulted lateral diaphragm structure allows for a lateral confinement of both the optical field and the injection (hole) current, such that, in use, choice of material composition, thickness and width of the diaphragm layer allows for a design optimization towards highest possible overlap between the optical mode and the material gain in the active MQW region, without compromising other essential parts of the mesa design, e.g. the SCH layers and the active MQWs therein.
  • the lateral confinement for both the optical field and injection current is achieved by a lateral undercut of the semiconductor material of some of SCH layers, e.g. active MQW layers, in the middle of the mesa. Since these layers, by default, have to have narrower bandgap and hence higher refractive index that N- and P-InP emitter layers on both sides of the SCH, they must be made from the materials other than InP, most commonly Ga x Ini -x As y Pi. y and Al x Ga y Ini -x .
  • the lateral confinement for both the optical field and injection current is achieved by means of a selective anisotropic wet etching, first, the upper N-InP emitter layer down to the SCH layers and, second, the SCH layers, down to the lower P-InP emitter layer.
  • a selective anisotropic wet etching first, the upper N-InP emitter layer down to the SCH layers and, second, the SCH layers, down to the lower P-InP emitter layer.
  • FIG. 2B there is shown is a schematic three-dimensional view of the VCSEG-DFB laser structure 200. It is grown on SI Fe:InP substrate 210 and comprises, bottom to top: a lower P-InP emitter layer 215, an undercut AlGalnAs layer 220, upper and lower SCH layers 230B and 230A respectively with MQW gain region 231 in the middle, and an upper N-InP emitter layer 240 at the top, with SEG 260 etched from the top surface of this layer down to towards the upper SCH layer 230B, and in some instances to the upper SCH layer 230B.
  • a lower P-InP emitter layer 215 an undercut AlGalnAs layer 220, upper and lower SCH layers 230B and 230A respectively with MQW gain region 231 in the middle
  • an upper N-InP emitter layer 240 at the top, with SEG 260 etched from the top surface of this layer down to towards the upper SCH layer 230B, and in some instances to the
  • this structure may sit atop of the passive (i.e. transparent for radiation emitted by the laser) waveguide layers, which, in use, are inserted between the substrate and the lower P-InP emitter, which are not shown in Figure 2B for clarity.
  • the laser mesa 250 is processed by etching from the top surface of the upper N-InP emitter layer 240, through this layer and the upper and lower SCH layers 230, down to the top surface of the lower P-InP emitter layer 220.
  • SEG 260 may be for example a 3 rd order SEG, in a form of periodic trenches 265 separated by intact material 270 of the upper N-InP emitter layer 240, is defined in the central part of the mesa, with the grating etch depth, width and duty cycle being the design parameters available for the performance optimization, e.g. achieving the desirable high coupling efficiency between the lasing mode and the grating, without compromising the mode's loss due to scattering on grating edges or mode's gain under direct bias of the laser PIN.
  • the laser strip P- and N-contacts, 275A and 275B are disposed on the top surfaces of the upper N-InP emitter layer 240 and lower N-InP emitter layer 215, aside from and atop of the mesa 140, respectively, such that the SEG is positioned between two strip N-contacts 275B.
  • the DFB laser also has a mesa structure and the Bragg grating also is formed by etching the periodic trenches in the centre of the mesa, while the top metal contacts are disposed atop of the mesa, on both sides of the grating.
  • the Bragg grating also is formed by etching the periodic trenches in the centre of the mesa, while the top metal contacts are disposed atop of the mesa, on both sides of the grating.
  • Liau The manufacturing process flow disclosed by Liau includes what initially appears to be an undercut of a thin GalnAsP active layer within the mesa sandwiched between two InP layers to form the lateral guide and hence disclose an apparently comparable design to that of the recited invention.
  • Liau teaches to an approximate lateral etch of approximately 5.5 ⁇ to define the strip of the active layer from each side of the mesa.
  • Liau subsequently buries the active strip waveguide using mass transport techniques, for example the structure is taught as thermally annealed at a temperature between 640°C and 670°C in the presence of phosphine.
  • Liau then formed the SEG atop the mesa using holographic photolithography to form a first-order grating with a periodicity of approximately 2000A and depth 2500 ⁇ , although an alternate embodiment required etching ⁇ ⁇ deep for devices operating in the telecommunication windows of 1.3 ⁇ and 1.55 ⁇ .
  • the poor reproducibility of such deep, high aspect ratio trenches to form the SEG further impacted the performance of DFB devices fabricated by limiting factors such as reproducibility of linewidth, sidemode suppression etc.
  • embodiments of the invention adapted a VCSEG design to a MGVI platform that removes requirements for doped InP substrates, wafer thinning and back-side metallization contacts to one with an SI substrate, the emitter disposed above a passive waveguide(s) and utilizing lateral contacts beside the mesa.
  • embodiments of the invention exploit higher order SEG structures allowing the utilization of conventional optical stepper lithography for compatibility with standard semiconductor manufacturing, and restrict the grating etch depth to the upper emitter layer and do not extend it into the laser SCH and active layers in order to avoid damage / surface defects as well as recombination at the dry etch floor in the gain region.
  • embodiments of the invention exploit pure index coupling in the SEG whereas Liau tries to etch down into the active layer to achieve gain coupling that is advantageous from the point of view of side mode suppression.
  • Liau teaches to
  • embodiments of the invention establish either "gain-type” or "loss-type” complex coupling without the SEG penetrating the gain region by manipulating the sign of the imaginary part of the coupling coefficient by design, see K. Pimenov et al in "Analysis of High-Order Surface Etched Gratings for Longitudinal Mode Selection in DFB Lasers" (Proc. 10 th Int. Conf. Numerical Simulation of Optoelectronic Devices, TuC3, Sept. 2010). Accordingly the side mode suppression for high performance telecommunication DFB devices is achieved through a different coupling to that exploited by Liau.
  • embodiments of the invention specifically exploit undercut of the mesa or lateral shaping of the mesa to provide lateral optical confinement that otherwise does not exist within the MGVI.
  • embodiments of the invention exploit the high-contrast index step at the undercut which is buried in the prior art of Liau by the mass transport process thereby reducing the index step and diluting (laterally) the waveguide.
  • the undercut in Liau is for defining the active strip within the buried waveguide structure rather than providing the lateral optical confinement.
  • embodiments of the invention exploit the undercut in confining the injection current such that by appropriate design the mode-gain overlap can be optimized, which provides another degree of freedom in the design of the laser.
  • MGVI SEG-DFB approach of embodiments of the invention allows the use of either N-up / P-down and P-up / N-down designs, the former being more suitable for an efficient injection into SEG-DFB devices, either lateral or vertical, as well as a good ohmic contact on N-InP is easier to implement than one on P-InP as the P-lnGaAs cap usually added to improve the P-contact cannot be employed for a relatively thin upper emitter layer as it will absorb all the light.
  • a MGVI SEG- DFB allows regrowth-free fabrication unlike the multiple epitaxial growths of Liau, see US Patent 4,777, 148.
  • a key feature that differentiates such a MGVI VCSEG-DFB laser from a conventional LCSEG-DFB laser is the structure of the lasing optical mode.
  • FIG 3A is there is presented a contour plot of the optical mode of a LCSEG-DFB overlaid to the physical structure of the active region.
  • a semiconductor stack comprising lower SCH layer of thickness approximately 160nm, MQW active layer of thickness approximately 220nm, upper SCH layer of thickness 1 l Onm, and upper emitter layer of thickness approximately 550nm.
  • the mesa having a width of 4.4 ⁇ with the grating of width approximately 1.1 ⁇ etched through the upper emitter layer.
  • the optical mode is shown by contour plots with the centre of the optical mode towards the upper edge of the MQW active layer is fairly symmetric vertically.
  • FIG. 3B there is presented a contour plot of the optical mode of a MGVI VCSEG-DFB overlaid to the physical structure of the active region.
  • a semiconductor stack comprising undercut layer of thickness approximately 75nm, lower SCH layer of thickness approximately lOOnm, MQW active layer of thickness approximately 150nm, upper SCH layer of thickness 200nm, and upper emitter layer of thickness approximately 550nm.
  • the mesa having a width of 4.0 ⁇ with the grating of width approximately 0.95 ⁇ etched through the upper emitter layer.
  • the optical mode now has increased confinement within the MQW layer, resulting increased gain overlap for the optical mode thereby reducing the threshold current for the laser. Additionally the optical mode is now substantially confined within the structure defined by the upper and lower SCH layers and MQW active layer.
  • FIG. 4 there is depicted a graph for the calculated grating efficiency for LCSEG-DFB and VCSEG-DFB devices exploiting the exemplary layer structures presented in Figures 3A and 3B plotted as a function of grating duty cycle. Shown within the chart is first plot LCSEG 410 representing the prior art LCSEG structure, represented by Figure 3A, for implementing optical emitters within PIC platforms. As is evident the structure yields a periodic efficiency with grating duty cycle with peaks at approximately 10%, 47% and 88% duty cycle with efficiencies ( of approximately 12, 21 , and 57. respectively. An approximate 50% duty cycle being typical in prior art LCSEG devices due to the reduced fabrication complexity.
  • Second plot VCSEG 420 plots an embodiment of the invention, represented by Figure 3B, showing efficiencies ( l/cm) at the peaks of approximately 17.5, 33, and 104 respectively.
  • the VCSEG yields a 60% improvement in coupling efficiency and at approximately 90% duty cycle an improvement of over 80% is achieved.
  • an improvement in coupling efficiency can be beneficially exploited to shorten laser cavities, wavelength selective filters, and other devices exploiting gratings as well as enhance the performance of the resulting laser emitters.

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Abstract

L'invention porte sur un laser VCSEG-DFB, entièrement compatible avec des méthodologies de conception et de fabrication MGVI, pour une intégration monolithique à croissance unique dans des PIC multifonctionnels. Il comprend une structure PIN laser, sous forme mesa, gravée depuis une surface supérieure de couche d'émetteur supérieur à travers la région de gain active, vraisemblablement MQW, jusqu'à la surface supérieure de l'émetteur inférieur. Des contacts électriques inférieurs sont installés adjacents à la structure mesa disposée sur la couche d'émetteur inférieur avec des contacts à lame supérieurs disposés au-dessus de la couche d'émetteur supérieur sur le haut de la structure mesa. Un SEG est défini/gravé à la partir de la surface supérieure de la structure mesa, entre les contacts à lame supérieurs, à travers la couche d'émetteur supérieur en descendant jusqu'aux ou jusque dans les couches SCH. Un confinement vertical est assuré par la structure SCH et le profil latéral dans la partie inférieure de la structure mesa assure un confinement latéral. Le mode guidé interagit avec le SEG par la queue verticale pénétrant dans le SEG et un couplage par champ évanescent au SEG.
PCT/CA2010/001737 2010-11-02 2010-11-02 Laser dfb à réseau gravé en surface à couplage vertical WO2012058742A1 (fr)

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