WO2003103107A1 - Laser dbr monomode avec section de dephasage amelioree, et son procede de fabrication - Google Patents

Laser dbr monomode avec section de dephasage amelioree, et son procede de fabrication Download PDF

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Publication number
WO2003103107A1
WO2003103107A1 PCT/US2003/017166 US0317166W WO03103107A1 WO 2003103107 A1 WO2003103107 A1 WO 2003103107A1 US 0317166 W US0317166 W US 0317166W WO 03103107 A1 WO03103107 A1 WO 03103107A1
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Prior art keywords
grating
section
phase
laser
shift
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PCT/US2003/017166
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English (en)
Inventor
Wen-Yen Hwang
Klaus Alexander Anselm
Jun Zheng
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Applied Optoelectronics, Inc.
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Priority claimed from US10/159,347 external-priority patent/US6608855B1/en
Priority claimed from US10/159,361 external-priority patent/US6638773B1/en
Application filed by Applied Optoelectronics, Inc. filed Critical Applied Optoelectronics, Inc.
Priority to EP03731472A priority Critical patent/EP1509975A4/fr
Priority to AU2003240967A priority patent/AU2003240967A1/en
Publication of WO2003103107A1 publication Critical patent/WO2003103107A1/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
    • H01S5/1082—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 with a special facet structure, e.g. structured, non planar, oblique
    • 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
    • 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
    • H01S5/1203—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 over only a part of the length of the active region
    • 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
    • H01S5/124—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 incorporating phase shifts
    • 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
    • H01S5/124—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 incorporating phase shifts
    • H01S5/1243—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 incorporating phase shifts by other means than a jump in the grating period, e.g. bent waveguides

Definitions

  • This invention relates to distributed Bragg reflector (DBR) lasers and, in particular, to DBR lasers having a quarter-wavelength ( ⁇ /4) phase shift section for improved single- longitudinal mode operation thereof.
  • DBR distributed Bragg reflector
  • lasers There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic waves (light) in the cavity by stimulated emission, thereby providing optical gain.
  • Semiconductor lasers may be diode (bipolar) lasers or non-diode, unipolar lasers such as quantum cascade (QC) lasers. Semiconductor lasers are used for a variety of industrial and scientific applications and can be built with a variety of structures and semiconductor materials.
  • semiconductor lasers for forming a source of optical energy are attractive for a number of reasons.
  • Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices.
  • semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
  • the optical gain of a laser is a measure of how well a gain medium such as an active region amplifies photons by stimulated emission.
  • the primary function of the active region in a semiconductor laser is to provide sufficient laser gain to permit lasing to occur.
  • the active region may employ various materials and structures to provide a suitable collection of atoms or molecules capable of undergoing stimulated emission at a given lasing wavelength, so as to amplify light at this wavelength.
  • the active region may comprise, for example, a superlattice structure, or a single- or multiple quantum well (MQW) structure.
  • Amplification by stimulated emission in the active region of a semiconductor laser is described as follows.
  • the semiconductor active region contains some electrons at a higher, excited state or energy level, and some at a lower, resting (ground) state or energy level.
  • the number and percentage of excited electrons can be increased by pumping the active region with a pumping energy, from some energy source such as an electrical current or optical pump.
  • Excited electrons spontaneously fall to a lower state, "recombining" with a hole.
  • the recombination may be either radiative or non-radiative. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states.
  • Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon.
  • stimulated emission occurs when a photon with an energy equal to the difference between an electron's energy and a lower energy interacts with the electron.
  • the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon.
  • the second photon has the unique property that it has the same energy, frequency, and phase as the original photon.
  • the atom (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) I.e., one photon of a given energy, frequency, and phase produces a second photon of the same energy, frequency, and phase; and these two photons may each, if not absorbed, stimulate further photon emissions, some of which can themselves stimulate further emissions, and so on.
  • Amplification by stimulated emission requires that more photons are produced by stimulated emission than are absorbed by lower-state electrons.
  • This condition known as population inversion, occurs when there are more excited (upper lasing level) electrons than ground-state (lower lasing level) electrons. If there were more lower state than upper state electrons, then more photons would be absorbed by the lower energy electrons (causing upward excitations) than would be produced by stimulated emission.
  • population inversion however, enough electrons are in the excited state so as to prevent abso ⁇ tion by ground-state electrons from sabotaging the amplification process.
  • population inversion is achieved, stimulated emission predominates over stimulated absorption, thus producing amplication of light (optical gain). If there is population inversion, lasing is therefore possible, if other necessary conditions are also present.
  • Population inversion is achieved by applying a sufficient pumping energy to the active region, to raise a sufficient number of electrons to the excited state.
  • pumping energy may be utilized to excite electrons in the active region and to achieve population inversion and lasing.
  • semiconductor lasers of various types may be electrically pumped (EP), by a DC or alternating current.
  • Optical pumping (OP) or other pumping methods, such as electron beam pumping, may also be used.
  • EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region. This gives rise to an increase in spontaneous generation of photons, and also increases the number of excited state electrons so as to achieve population inversion.
  • an active region is sandwiched between the cavity mirrors, and pumped with a pumping energy to cause population inversion.
  • Photons are spontaneously emitted in the active region. Some of those photons travel in a direction perpendicular to the reflectors of the laser cavity. As a result of the ensuing reflections, the photons travel through the active region multiple times, being amplified by stimulated emission on each pass through the active region. Thus, photons reflecting in the cavity experience gain when they pass through the active region.
  • loss is also experienced in the cavity, for example by extraction of the output laser beam, which can be about 1% of the coherent cavity light, by absorption or scattering caused by less than perfect (100%) reflectance (reflectivity) of the cavity mirrors, and other causes of loss.
  • Gain is a function of wavelength.
  • a given wavelength is associated with a given threshold gain, and may be characterized by that threshold gain, for a given laser structure.
  • the lasing threshold current 95 current level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission.
  • the active region When the active region provides the threshold lasing gain over a given wavelength range, there will be a sufficient amount of radiative recombinations stimulated by photons, so that the number of photons traveling between the reflectors tends to increase, giving rise to amplification
  • the 105 cavity is a standing EM wave and the cavity of effective optical length L only resonates when the effective optical path difference between the reflected wavefronts is an integral number of whole wavelengths (the effective cavity length or optical path difference takes phase-shifting effects at the mirrors into account).
  • lasing is only possible at wavelengths for which the round-trip phase is a multiple of 2 ⁇ .
  • the 110 standing wave condition is termed the set of longitudinal modes of the cavity. Although there are an infinite number of such wavelengths, only a finite number of these fall within the wavelength range over which the gain spectrum of the active region exceeds the threshold lasing gain. The laser will lase only at one or more of the possible longitudinal (wavelength) modes which fit into this wavelength range.
  • Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs).
  • Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, in contrast to SELs, in which the radiation output is perpendicular to the wafer surface, as the name implies.
  • SELs in which the radiation output is perpendicular to the wafer surface
  • a cleaved facet mirror is used to obtain the feedback for laser oscillation.
  • FP laser the two facets of the diode, the rear and
  • the front (emitting) surface are cleaved to establish the dimensions of the structure such that a primary longitudinal mode of resonance will exist at the desired wavelength.
  • DFB distributed-feedback
  • DBR distributed-Bragg reflector
  • Such devices provide feedback for lasing as a result of backward Bragg scattering from periodic variations of refractive index, instead of using conventional facets or mirrors to provide reflection.
  • a DFB laser typically has a uniform diffraction grating constructed in the waveguide itself, adjacent to (i.e., above or below) the active region layer. In such a laser, optical feedback
  • the grating 130 is provided along the laser's cavity length by means of a diffraction grating whose stripes or teeth run perpendicular to the length (longitudinal direction) of the laser cavity.
  • the grating serves as an internal periodic feedback structure that establishes the wavelength of operation.
  • DBR laser typically denotes an edge-emitting laser comprising a grating that is not uniform, i.e. a grating that is not continuous over the entire longitudinal cavity.
  • the 135 DBR laser has one or two grating sections fabricated in a waveguide external to the active region, which grating sections provide part or all of the reflectance of the cavity mirrors.
  • the DBR laser typically is an edge-emitting laser having two separate DBRs or gratings, each at either end of, and outside, the active region layer. The DBRs reflect and thus provide feedback at the desired wavelength.
  • Diffraction gratings provide optical feedback by the period variation of the effective refractive index of the grating.
  • the period index variation causes a wavelength-selective feedback.
  • the diffraction gratings of DFB and DBR lasers have a reflectance that is a function of wavelength. Maximum reflectance of a diffraction grating occurs around its so-called Bragg wavelength ( ⁇ ), which is given by Eq. (1) below:
  • ⁇ is the period (pitch) of the grating and n e is the effective refractive index of the waveguide having the grating.
  • the Bragg wavelength ⁇ is determined by the grating pitch or period.
  • the period of DFB and DBR gratings is typically selected so that the Bragg wavelength ⁇ is equal to the desired operating wavelength of the laser. Diffraction gratings
  • Single-wavelength (single longitudinal mode) operation is desirable for many applications, such as in high-bit-rate optical fiber communication.
  • Single-mode operation refers to laser operation in which the intensity of the most-intense, or "primary,” mode lasing is
  • Single-mode operation may be said to occur when the side mode suppression is at least a minimum amount (e.g., 30dB), for example, the primary mode is at least 30dB greater than its side modes (and all other modes).
  • the laser emit at a single lasing wavelength at 1.31 ⁇ m (and other closely spaced
  • wavelengths 160 wavelengths
  • telecommunications wavelengths specified by the ITU grid such as lasing wavelengths of 1.55 ⁇ m (and other closely spaced wavelengths). These wavelength ranges are often used for telecommunications memeposes because the loss of silica fibers is comparatively low at these wavelengths.
  • the round-trip phase at the Bragg wavelength ⁇ is 7r+n27r, where n is an integer.
  • ⁇ & ⁇ 2 ⁇ is not a multiple of 2 ⁇ .
  • the phase condition for oscillation cannot be met at the Bragg wavelength ⁇ s (or corresponding Bragg frequency f B ), and no mode occurs at this wavelength.
  • there are two equally "primary" modes, at two frequencies slightly removed by some frequency f ⁇ from the Bragg frequency f B i.e. two optical modes
  • degenerate modes compete substantially equally for lasing as the dominant mode. Mode-hopping can also occur from one mode to the other. The two modes are thus problematic for wideband applications, since a single, narrow linewidth output is needed for wideband applications. Two modes having similar threshold gain are sometimes known as degenerate modes. The existence of two degenerate modes results in multi-mode operation, unpredictable
  • mode degeneracy 180 modes, or mode hopping, sometimes referred to as mode degeneracy. It is preferable to construct a laser in which there is only a single primary mode, i.e. in which there is no mode degeneracy.
  • the insertion of a 7r/2 optical phase shift section into the DFB structure is one way to "suppress” or “break” the mode degeneracy, so as to achieve single- mode operation. This phase shift region creates an extra ⁇ r/2 phase shift for each wave passing
  • the phase condition for oscillation can be met at the Bragg wavelength ⁇ B (or Bragg frequency f B ).
  • the ⁇ /2 phase shift technique provides for optical phase matching to adjust the main mode at the Bragg wavelength. This "reduces" the number of resonance modes to one, and its resonance frequency coincides with the Bragg frequency f B .
  • the 7r/2 phase shift section effectively provides lasing
  • ⁇ /4 is literally a length, not a phase shift, the terminology " ⁇ /4 phase shift” will be employed herein due to the conventional use of this term. It will be understood that a " ⁇ /4 phase shift", as the term is used in
  • Fig. 1 is a cross-sectional view of the layer structure of a DBR laser having a phase-shift section, in accordance with an embodiment of the present invention
  • Fig. 2 is a cross-sectional view of an alternative phase-shift section and grating layer, in 245 accordance with an embodiment of the present invention
  • Figs. 3A-H are cross-sectional views of the layer structure of the DBR laser of Fig. 1 at various stages of fabrication, in accordance with an embodiment of the present invention.
  • Figs. 4A-E are cross-sectional views of the layer structure of a DBR laser employing the alternative phase-shift section of Fig. 2 at various stages of fabrication, in accordance with an 250 embodiment of the present invention. '
  • the present invention provides a single-mode DBR laser with an improved phase-shift section and a method for fabricating a single-mode DBR laser with improved yield.
  • the present invention comprises an edge-emitting semiconductor DBR laser having two DBR gratings
  • phase shift section is a section where there is no grating, so that the phase shift section has an effective index of refraction different from that of the two DBR gratings.
  • phase-shift section and the grating sections are selected to achieve a phase shift approximately equal to ⁇ r/2, in a preferred embodiment, or enough of a phase shift to suppress mode degeneracy so that there is only one primary mode.
  • the DBR laser of the present invention may be utilized for various applications such as telecommunications applications.
  • the DBR laser may be designed to emit at 1.31
  • DBR laser 100 is an edge-emitting laser having
  • Laser 100 has anti-reflectance (AR) coatings 101 and 102 on its facets.
  • AR anti-reflectance
  • Laser 100 comprises layers and structures fabricated on substrate 112 by epitaxial or other deposition techniques.
  • the epitaxially grown layers of laser structure 100 may
  • epitaxial growth techniques such as molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), a vapor phase epitaxy (VPE) process such as or metalorganic chemical vapor deposition (MOCVD, also known as MOVPE), or other known crystal growth processes.
  • MBE molecular beam epitaxy
  • LPE liquid phase epitaxy
  • VPE vapor phase epitaxy
  • MOCVD metalorganic chemical vapor deposition
  • Other layers and structures that are not epitaxially grown may be deposited with non-epitaxial deposition techniques such as e-beam
  • Grating layer 140 has an upper cladding layer 132 (consisting of material 146), first ("exit,” or “front”) grating section 141, second ("back") grating section 143, and a phase-shift section 142 sandwiched between the first and second gratings 141, 143.
  • Gratings 141, 143 each comprise a plurality of diffraction grating stripes, rows or teeth 144 that run in the x direction,
  • First grating section 141 is approximately of length LI and comprises a grating having a grating pitch or period selected to have a Bragg wavelength ⁇ equal to the desired operating wavelength of the laser.
  • Second grating section 143 is approximately of length L3 and also comprises a grating having a grating pitch selected to have the same Bragg wavelength ⁇ .
  • gratings 141, 143 comprise a much larger number of grating periods than illustrated in the figures, which are not drawn to scale for simplicity of illustration.
  • each of grating sections 141, 143 covers hundreds of grating periods in various practical applications and embodiments.
  • Phase-shift section 142 is a grating-free phase-shift section interposed between the first and second gratings, and is of approximately length L2, and also
  • the gratings of sections 141, 143 are "in phase” with each other, and the gratingless phase-shift section 142 covers an length between the first and second grating sections 141, 143 which is "missing" grating periods that would otherwise be in phase with the grating periods of grating sections 141, 143.
  • Phase-shift section 142 may be referred to as a "distributed" phase-shift section because it extends over a substantial number of (missing) grating periods, between the first and second grating sections, and is of comparable length to these distributed grating reflectors 141, 143.
  • length L2 is no less than 1/10 the length of the smaller of lengths LI, L3.
  • none of lengths LI, L2, and L3 is more than
  • bottom and top contact layers 111, 133 consist primarily of a metal such as Au. Other materials may be employed in alternative embodiments.
  • Top contact layer 133 may also comprise a thin (e.g., about 25 ⁇ A to 50 ⁇ A thick) contact-facilitating layer (not shown) between the metal (Au) and cladding 132, comprised of a material such as doped InGaAs (with a
  • probes or wire bonds may be electrically coupled to top and bottom contacts 111, 133 to provide pumping current to laser 100, so as to give rise to population inversion.
  • substrate 112, buffer/cladding layer 113, spacer layer 145, and material 146 of grating layer 140 consist of InP, which is often used as a substrate material for 1.55 ⁇ m and 1.31 ⁇ m devices.
  • substrate 112 is about 120 ⁇ m thick; buffer/cladding layer 113 is about 0.8 ⁇ m thick; spacer layer 145 is about 40 nm thick; and cladding layer 132 is about 1.2 ⁇ m thick.
  • Other thicknesses are possible in alternative embodiments.
  • Active region 120 in an embodiment, comprises a structure such as multiple quantum wells (MQWs) which provide a gain spectrum suitable for emitting radiation at the desired wavelength, e.g. 1.31 ⁇ m.
  • Active region 120 may consist of a material such as (undoped) InGaAsP/InP (or InAlGaAs/InP) having, for example, seven quantum wells (QWs) (each QW having a QW layer and a corresponding QW barrier layer).
  • QWs quantum wells
  • active region 120 could have a single quantum well, superlattice, or other structure instead of an MQW structure.
  • active region 120 is about 0.1 to 0.2 ⁇ m thick.
  • Quaternary compounds or alloys composed of elements from the third group (Al, Ga, In) and the fifth group (P, As, Sb) of the periodic system are often employed in devices using InP substrates because they may be lattice- matched to InP.
  • Confinement layers 121, 122 have an index of refraction selected to be greater than that of the adjacent InP -based buffer/cladding layer 113 and spacer layer 145, so as to confine more of the optical field to the active region 120, which confinement layers 121, 122 surround.
  • the bandgap of a quaternary material, measured in eV, is typically specified in terms of the wavelength of light at which photons have the same energy as the bandgap.
  • confinement layers 121, 122 consist of a quaternary material, such as InGaAsP, lattice-matched to InP and having a bandgap 340 of about 1.1 ⁇ m, so as to provide an index of refraction of about 3.3, which is greater than the index of refraction of the adjacent InP layers (i.e., 3.2).
  • Confinement layers 121, 122 are about 100 nm thick in an embodiment.
  • AR coatings 101, 102 consist of a Ta 2 O 5 /SiO2 mixture and are about 1 ⁇ m thick, in an embodiment. Such AR coatings provide, in an embodiment, about 0.1% reflectance.
  • the grating material from which grating stripes 144 are made may consist of quaternary material lattice-matched to InP, such as InGaAsP, and having a bandgap selected to
  • the relative mole concentrations for the InGaAsP compound may be selected to achieve a bandgap of about 1.18
  • grating sections 141, 143 will have an effective index of refraction no (where "G” stands for “grating”).
  • G stands for "grating”
  • no is about 3.2333.
  • the effective index of refraction of phase-shift section 142, nps is about 3.23 (where "PS” stands for "phase-shift”).
  • Index II G is close to, but slightly greater than, nps.
  • phase-shift section 143 makes the effective index of refraction n G of grating sections 141, 143 slightly greater than that of phase-shift section 142, which does not contain these higher-index stripes. That is, the presence of a grating section having alternating index of refraction sections ensures that nG will be different than nps, which does not have a grating.
  • 365 section 142 being gratingless, it has an index of refraction nps necessarily different than the index of refraction n ⁇ of an adjacent grating section.
  • nc although slightly greater than nps, will be very close to nps. This is because index ni of grating stripes 144 is 3.4, close to 3.2, the index of refraction n 2 of the surrounding material 146 of cladding layer 132. 370
  • index ni of grating stripes 144 is 3.4, close to 3.2, the index of refraction n 2 of the surrounding material 146 of cladding layer 132.
  • the lengths LI, L3 of grating sections 141, 143, in an embodiment, are different (LI ⁇ L3), to provide an asymmetric reflectance. This causes more laser light to be output through the
  • a primary, exit beam 151 exiting through the side with shorter, lower-reflectance grating section 141 and AR coating 101
  • a weaker beam 152 exiting through the side having higher-reflectance, longer grating section 153 and AR coating 102
  • the stronger, primary beam 151 may be coupled to some external light-receiving device such as a lens, fiber, amplifier, modulator, and so on.
  • 380 beam 152 may be directed onto a monitor photodiode (not shown) for power monitoring or other purposes.
  • a given power ratio (the ratio of the power of the primary output beam 151 to the power of the secondary beam 152), such as 4:1 or some other ratio, may be desired.
  • the back-to-front reflectance ratio is a nonlinear function of the back-to-front grating section lengths L3, LI and their ratio L3:L1.
  • grating section lengths LI and L3 are selected, as described below, to achieve a desired reflectance ratio (e.g., about 2:1 or 3:1), so that output
  • phase-shift section 142 is determined as described below, in order to achieve the appropriate phase shift.
  • the pitch of grating sections 141, 143 is selected to provide a ⁇ equal to the operating wavelength, e.g. 1.31 ⁇ m.
  • a grating having a period length (pitch) is selected to provide a ⁇ equal to the operating wavelength, e.g. 1.31 ⁇ m.
  • each stripe is approximately 101 nm wide.
  • Each stripe is typically 1 to 5 times as high as it is wide.
  • 100 400 nm wide stripes may be about 100 to 500 nm in height.
  • stripes 144 are substantially rectangular, having a width of about 100 nm and a height of about 150 nm.
  • the "corners" of the grating profile may be somewhat rounded, in some applications, e.g. due to fabrication phenomenon such as mass transport.
  • phase-shift section 142 introduces an effective optical path length of ⁇ /4, which causes a phase shift of ⁇ /2 for light at ⁇ B .
  • phase-shift section 142 has a length L2 of about 100 ⁇ m. This section contains no grating but covers a length equal to about 500 grating periods of length 202.6 nm.
  • First and second DBR gratings 141, 143 serve as the mirrors for the laser cavity and must have sufficient reflectance for lasing to occur, and preferably have a desired asymmetric reflectance ratio, e.g. about 2.5:1.
  • the reflectance of a given grating section at ⁇ is a function of its length and its grating coupling factor K.
  • KL is a key dimensionless parameter determining the frequency selectivity and performance of a grating, with practical
  • the grating coupling factor K is a function primarily of factors such as the ⁇ n, the duty cycle, grating profile (i.e. stripe shape), height (thickness) of grating, distance from active region 120, degree of optical confinement in the confinement layers 121, 122, and other factors.
  • the lengths and other properties of gratings 141, 143 are selected so as to achieve a
  • phase-shift section contains no grating but covers a length L2 (100 ⁇ m) equal to about 500 grating periods.
  • lengths LI and L3 though different (in an asymmetric embodiment), will still include hundreds of grating periods in order to achieve the desired reflectance, and will be within the same order of magnitude.
  • Lengths LI, L3 must include hundreds of grating periods because grating sections 141, 143 are "distributed" reflectors that necessarily require many grating periods in order to achieve the desired reflectance necessary for lasing. For example,
  • the grating coupling factor K may be increased so as to increase cavity reflectance, to control for the loss in cavity reflectance caused by the asymmetric reflectance.
  • Length L2 about 100 ⁇ m in an embodiment (length of about 500 periods), is also of the same order of magnitude as lengths LI, L3.
  • Length L2 also has to cover a length equal to many grating periods (e.g., hundreds), because, as noted above, with stripes of index 3.4 present in cladding of index 3.2 in the grating sections, the effective index no is only slightly greater than nps (3.2333 is very close to 3.23). Therefore, in accordance with Eq. (3) above, where (no-nps) is
  • phase-shift section 142 may be referred to as a "distributed" phase-shift section because it extends over a substantial number of (missing) grating periods, between the first and
  • the back-to-front length ratio L3:L1 is about 2:1, and is related to, but not the identical to, the back-to-front reflectance ratio, because reflectance of a DBR grating is not a linear function of its length. Additionally, the front-to-back output power ratio
  • 470 4.2:1 is related to the back-to-front reflectance ratio 2.4:1, but is not identical to it.
  • column 1 indicates the cavity mode wavelength
  • the sixth entry (in bold) is the desired operating wavelength 1.31 ⁇ m.
  • Column 2 indicates the imaginary refractive index factor k (the imaginary component of the effective refractive index of the cavity) at various cavity mode wavelengths.
  • a lower threshold gain g is desired, all things being equal, and is preferably at a minimum at the operating (Bragg) wavelength ⁇ B .
  • a lower magnitude of k results in a lower threshold gain g, and therefore it is desirable for k to have a minimum magnitude at the operating wavelength ⁇ B .
  • Column 3 indicates the front-to-back power ratio (that is, the ratio of the power of main exit beam 151 to secondary beam 152) provided by the structure of laser
  • grating sections 141, 143 provide a back-to-front reflectance ratio sufficient to provide front-to-back output power ratio of about 4.2:1 (i.e., the inverse of 0.24). Also, it can be seen that the absolute value of the imaginary refractive index factor k and thus
  • 490 threshold gain g is approximately at a minimum at the desired operating wavelength 1.31 ⁇ m.
  • a current is applied via probes, wire bonds or other terminals electrically connected to metal contacts 133, 111, which pumps the active region 120 to cause population inversion and begin the lasing process.
  • Light emitted by spontaneous and stimulated emission reflects back and forth in the longitudinal (y axis) direction in the cavity, along the
  • the effective index of refraction no of grating sections 141, 143 is slightly greater than the effective index of refraction nps of phase-shift section 142. Given the difference between nps to nc phase-shift section 142 has a length L2 such that light traveling
  • This phase shift breaks the mode degeneracy that would otherwise occur and causes there to be only a single mode with the lowest threshold gain, i.e. the primary mode, at the Bragg wavelength ⁇ B .
  • the use of grating sections 141, 143 would result in mode degeneracy in which there
  • phase shift section 510 are two resonance modes having lowest, approximately equal threshold gains.
  • the phase shift section in effect, "shifts" both of these modes, one (the primary mode) toward ⁇ (thus decreasing its threshold gain) and the other, secondary mode, away from ⁇ (thus increasing its threshold gain). Because the secondary mode has its threshold gain increased relative to that of the primary mode, the secondary mode and mode degeneracy may be said to have been
  • phase-shift section 142 makes single-longitudinal mode operation possible, at ⁇ B .
  • Laser 100 emits a main exit laser beam 151 of wavelength ⁇ through the facet having AR coating 101 and a back beam 152 of smaller intensity from its back side, through AR coating
  • the side-mode suppression is at least a minimum amount, e.g. the primary mode is at least a certain amount (e.g., 30dB) greater than any other modes.
  • FIG. 2 there is shown a cross-sectional view of an alternative phase- shift section 242 and grating layer 240, in accordance with an embodiment of the present invention.
  • Grating layer 240 is similar to grating layer 140 of Fig. 1 except that it contains phase-
  • phase-shift section 142 is a section that does not contain a grating. Therefore, it has a different index of refraction than the two adjacent grating sections 141, 143, and has the proper length L2 so as to introduce the desired phase shift. Phase-shift section 242 also does not contain a grating. Phase-shift section 142 has no grating by virtue of having no
  • phase-shift section 242 contains grating stripe material section 249 but does not contain stripes, since the material of grating material section 249 does not contain intervening half-periods of material 146, i.e. it is not a grating, it is a gratingless slab of material equal in length (L2) to a large number of grating periods.
  • phase-shift section 242 also has a different index of refraction than the adjacent grating sections 141, 143.
  • the effective index of refraction of phase-shift section 242 is slightly greater than that of grating sections 141, 143, because it contains on average a greater degree of the higher-index grating stripe InGaAsP material (i.e., index 3.4) than the lower-index InP material 146 (i.e.,
  • the length L2 of alternative phase-shift section 242 is selected in accordance with Eq. (3) above to achieve a quarter-wavelength phase shift.
  • a phase-shift section 142 having no grating stripes may be formed by removing, from a continuous grating, the grating stripe material where the phase-shift section is to be formed.
  • a phase-shift section 242 having a grating stripe material section but with no grating or stripes may be formed by refraining from imparting a grating structure on a grating stripe material layer, over the region where the phase-shift section is to be formed, when forming the adjacent grating sections 141, 143.
  • FIG. 3A-H there are shown cross-sectional views of the layer structure
  • Fig. 3 A an initial set of layers are epitaxially grown, e.g. by MBE, onto substrate 112. These include buffer/cladding layer 113, bottom confinement layer 121, active region 120, top confinement layer 122, spacer layer 145, a grating stripe material layer 341, and a thin protective
  • PR layer 343 may be thin, about 50 to 100 nm in an embodiment, which is spun onto the wafer surface.
  • Grating stripe material layer 341 is the material out of which stripes 144 are to be
  • Protective layer 342 consists of InP and is used, in an embodiment, because the oxide is easier to remove from InP than from InGaAsP. In alternative embodiments, protective layer 342 is omitted.
  • the PR layer 343 is patterned to form a regular series of PR stripes which serve as a grating etch mask 345.
  • the grating etch mask 345 is formed by any suitable technique.
  • the grating etch mask is formed by lithographically defining the grating in the PR layer 343, by exposing the PR layer to two collimated, expanded beams from a blue or UV laser at an appropriate angle to form high contrast fringes with the
  • PR layer 343 is then developed and the developed portions removed with an appropriate solvent with grating etch mask 345 remaining.
  • Other techniques for writing a grating into the PR include using electron-beam (E-beam) lithography, a technique which uses special electron-sensitive resists.
  • each stripe 344 is covered by a respective stripe from grating etch mask 345 composed of PR.
  • PR stripes 345 may be removed with any suitable technique to result in the structure illustrated in Fig. 3D.
  • a suitable solvent, oxygen plasma, or other techniques can be used to remove the PR etch mask
  • a PR mask 351 is formed which covers and protects the grating stripes which are to form grating sections 141, 143, but which leaves exposed the grating stripes that are to be removed because they occur in the region where phase-shift section 142 is to be formed.
  • an etch is performed to remove the quaternary
  • the sample has two grating sections, separated by an intermediate, gratingless section. Because the two grating sections are formed from an initial, continuous grating, the gratings of the two sections are in phase with each other.
  • PR mask 351 is removed, e.g with solvent or oxygen plasma, to result in the
  • any oxide may then be removed from the sample by appropriate removal or cleaning, and then additional InP 146 is epitaxially grown to complete grating section 140 and cladding layer 132, as illustrated in Fig. 3H.
  • bottom contact 111 may be deposited onto the bottom of substrate 112, and top contact 133 may be formed on cladding layer 132, including a thin contact-facilitating layer between the metal
  • FIGS. 4A-E there are shown cross-sectional views of the layer structure of a DBR laser employing the alternative phase-shift section of Fig. 2 at various stages of fabrication. These views illustrate a method of fabricating a DBR laser 400, such as a modified DBR laser similar to laser 100 except for the different phase-shift section, in accordance with an
  • Fig. 4A illustrates a stage in fabrication similar to that described above with reference to Fig. 3A, except that the sample of laser 400, at this point in fabrication, contains a thin barrier layer 451 between PR layer 343 and protective layer 342. This layer extends over the section which is to contain the phase-shift section 242, and its purpose is to provide a barrier to prevent
  • Barrier layer 451 consists of a material such as SiO 2 , in an embodiment, although other suitable materials, such as or SiO, may also be employed in alternative embodiments.
  • a grating etch mask 345 is formed in PR layer 343, as described above with reference to Fig. 3B. However, an intermediate portion of grating etch mask 345
  • the resulting structure illustrated in Fig. 4C contains grating stripes in the regions where grating sections 141, 143 are to be formed, and a grating stripe material section 249 (having no grating
  • the PR stripes of PR mask 345 covering gratings 344 of grating sections 141, 143 and grating material section 249 is removed, as described above with respect to Figs. 3C-3D, to result in the structure illustrated in Fig. 4D. Any oxide may then be removed from the sample by appropriate removal or cleaning, and then additional InP 146 is epitaxially grown to complete
  • bottom contact 111 may be deposited onto the bottom of substrate 112, and top contact 133 may be formed on cladding layer 132, including a thin contact-facilitating layer between the metal (Au) of layer 133 and cladding layer 132, to form the laser structure 100 shown in Fig. 1.
  • HR (high reflectance) coatings are often employed, instead of AR coatings, on at least one of the laser facets, to provide additional reflectivity for the cavity mirror.
  • HR coatings One problem with the use of HR coatings is that the exact place of the cleave is unpredictable. Therefore, the reflectance provided by the HR coating occurs at an unpredictable place within the grating period, and therefore introduces a somewhat unpredictable, random
  • the DBR laser 100 of the present invention does not suffer this problem, because only AR coatings are employed, and the cavity mirrors' reflectance is supplied entirely by distributed grating sections 141, 143.
  • the facet cleave location within a particular grating period plays
  • grating-only (no HR coating) DBR laser 100 can be produced with more uniformity of performance (e.g., the operating wavelength, the front-to-back power ratio, etc.) and greater yield.
  • phase shift can also increase uniformity and thus yield. This is because the gratingless phase-shift section is distributed, i.e. spread out over the length of many (e.g. 500) grating periods. Therefore, slight imprecision in the exact length L2 from device to device causes almost no change in the phase shift effect. This is in contrast to gratings having a quarter- wavelength shift inserted directly into the grating, in which a very small variation in this phase-
  • phase-shift section can have a much more significant effect on the amount of phase shift. Further, fabrication of a direct phase shift section in a grating requires the fabrication of two or more out- of-phase gratings, which complicates the fabrication process. In contrast, the phase-shift section of the present invention may be fabricated (as described above with reference to Figs. 3 and 4) from a single, continuous grating by removing part of the grating during fabrication.
  • phase-shift section 142 of the present invention Another advantage of employing the phase-shift section 142 of the present invention is the achievement of more uniform optical intensity along the longitudinal (y axis) cavity, which also improves linearity; that is, the range over which a linear increase in output power can be attained is extended.
  • a laser having a directly-inserted quarter-wavelength shift section may lead to an optical intensity "spike" or peak at a given location in the longitudinal cavity,
  • phase-shift section of the present invention is spread out over hundreds of periods and may be of comparable length to the two DBR grating sections, there is no comparable optical intensity "spike" at a single location.
  • phase-shift section 660 advantageously gives rise to a more uniform distribution of optical power over the longitudinal cavity than is achieved by a directly-inserted phase-shift section which essentially exists at a single longitudinal location in the cavity.
  • the distributed phase-shift section of the present invention is not expected to interfere with dynamic wavelength stability during modulation to the degree that a directly-
  • stripes 144 are substantially rectangular in cross- section, i.e. the grating profile (the boundary z between the high and low refractive index 670 materials "seen” by light traveling in the cavity in the longitudinal, _ -axis direction) is a substantially rectangular periodic wave.
  • the grating profile is a substantially rectangular periodic wave.
  • other shapes than rectangular may be employed for the grating profile, such as square, trapezoidal, triangular/sawtooth, or sinusoidal, and duty cycles other than 50/50 may be employed.
  • the diffraction grating stripes 144 consist of a material 675 having an index of refraction ni which is less than the index of refraction n 2 of the surrounding material 146 of cladding layer 132 and spacer layer 145.
  • the light output is not symmetric.
  • the grating section having the first and second grating sections and the phase-shift section may be disposed below the active region (i.e., sandwiched between the active region and the substrate), instead of above the active region.
  • DBR grating sections are disposed both above and below the active region, i.e.
  • the gratingless phase-shift section of the present invention achieves a ⁇ /4 (i.e., 7r/2) phase shift at ⁇ B .
  • length L2 may be selected in accordance with Eq. (2) to achieve a phase shift less than 7r/2 but still significant enough to break mode degeneracy and achieve single-mode operation.
  • the gratingless phase-shift section cause a phase shift by some amount ⁇ s, where ⁇ MI N ⁇ ps ⁇ B /4, where ⁇ M ⁇ N is the minimum phase shift sufficient to provide desirable single-mode operation.
  • ⁇ MI N is about 7r/4 (corresponding to ⁇ /8).
  • ⁇ MIN is whatever phase shift is sufficient to achieve a side-mode suppression of at least 30dB.
  • side mode is any phase shift sufficient to achieve a side-mode suppression of at least 30dB.
  • ⁇ MI N is whatever phase shift is sufficient to cause there to be only one mode having the lowest threshold gain, which is significantly lower than the threshold gains of all other modes.
  • the phase shift section 700 present invention introduces an odd multiple of ⁇ /4 effective optical path length instead of a single ⁇ /4 optical path length, to achieve a 7r/2 phase shift.
  • the phase shift section may, in general, have a length L2 sufficient to introduce an effective optical path length of (2n + l) ⁇ /4, where n is an integer ⁇ 0, 1, 2, 3, ... ⁇ , which achives a ⁇ l2 phase shift.
  • materials, material systems, and layer thicknesses for the layers or elements of laser 100 other than those specified herein may be employed.
  • phase shift section in accordance with the present invention may have less than all of the layers and elements shown in the embodiments described above, and may have additional layers or structures not shown.
  • no optical confinement layers 121, 122, and/or no spacer layer 145, and/or no thin protective layer 342 employed during fabrication of grating layer 142 of structure 100.
  • thin protective layer 342 employed during fabrication of grating layer 142 of structure 100.
  • an etch stop layer composed of a quaternary material such as InGaAsP may be disposed in the cladding layer 132, between its boundary with top contact layer 133 and grating stripes 144.
  • non-section- 112(6) means for performing a specified function is not intended to be a means under 35 U.S.C. section 112, paragraph 6, and refers to
  • the present invention is well adapted to carry out the objects and attain the

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  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
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  • Semiconductor Lasers (AREA)

Abstract

L'invention se rapporte à un laser à émission par la tranche (100) destiné à générer une lumière laser à mode longitudinal quasi-unique. Une zone active à semi-conducteur (120) amplifie, par émission stimulée, la lumière présente dans la cavité du laser à une longueur d'onde d'émission de laser. Des première et seconde sections de réseau, (141, 143) adjacentes à la zone active (120), présentent respectivement des première et seconde réflectivités et un premier indice de réfraction effectif. Les première et seconde sections de réseau (141, 143) possèdent une longueur d'onde de Bragg sensiblement égale à la longueur d'onde d'émission du laser. Une section de déphasage exempte de réseau (142), qui est placée de manière adjacente à la zone active (120) et entre les première et seconde zones de réseaux (141, 143), possède un second indice de réfraction, différent du premier indice de réfraction, et une longueur suffisante pour imprimer à la lumière se trouvant à la longueur d'onde d'émission du laser un déphasage suffisant pour que l'on obtienne un fonctionnement en mode longitudinal.
PCT/US2003/017166 2002-05-31 2003-05-29 Laser dbr monomode avec section de dephasage amelioree, et son procede de fabrication WO2003103107A1 (fr)

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AU2003240967A AU2003240967A1 (en) 2002-05-31 2003-05-29 Single-mode dbr laser with improved phase-shift section and method for fabricating same

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US10/159,347 US6608855B1 (en) 2002-05-31 2002-05-31 Single-mode DBR laser with improved phase-shift section
US10/159,361 US6638773B1 (en) 2002-05-31 2002-05-31 Method for fabricating single-mode DBR laser with improved yield
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WO2018070432A1 (fr) * 2016-10-12 2018-04-19 古河電気工業株式会社 Élément laser à semi-conducteur
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CN112993751B (zh) * 2021-01-28 2022-08-19 湖北光安伦芯片有限公司 一种纳米柱vcsel光源结构及其制备方法

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