USRE38682E1 - Grating coupled vertical cavity optoelectronic devices - Google Patents
Grating coupled vertical cavity optoelectronic devices Download PDFInfo
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- USRE38682E1 USRE38682E1 US10/084,770 US8477002A USRE38682E US RE38682 E1 USRE38682 E1 US RE38682E1 US 8477002 A US8477002 A US 8477002A US RE38682 E USRE38682 E US RE38682E
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- 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
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- 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
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- 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
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
- H01S5/18369—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials
- H01S5/18372—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials by native oxidation
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- 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
- H01S5/227—Buried mesa structure ; Striped active layer
- H01S5/2275—Buried mesa structure ; Striped active layer mesa created by etching
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- 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
- H01S5/0424—Electrical excitation ; Circuits therefor characterised by the semiconducting contacting layers with n- and p-contacts on the same side of the active layer lateral current injection
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- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
- H01S5/06203—Transistor-type lasers
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- H01S5/00—Semiconductor lasers
- H01S5/06—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium
- H01S5/062—Arrangements for controlling the laser output parameters, e.g. by operating on the active medium by varying the potential of the electrodes
- H01S5/06226—Modulation at ultra-high frequencies
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- 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
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18308—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] having a special structure for lateral current or light confinement
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- 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
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18341—Intra-cavity contacts
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- 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
- H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
- H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
- H01S5/18361—Structure of the reflectors, e.g. hybrid mirrors
- H01S5/18369—Structure of the reflectors, e.g. hybrid mirrors based on dielectric materials
Definitions
- This invention relates to the field of semiconductor double heterostructure laser devices and, in particular, to those laser devices which use vertical cavities. It also relates to the field of corrugated optical waveguides and travelling wave optoelectronic devices.
- Next generation transmission systems are anticipating bit rates approaching 100 Gb/s in time division multiplexed architectures.
- the demand for such speeds is created by the growth of interactive multi-media services and is made possible by the terahertz bandwidth of optical fiber.
- Realizing optical sources with those modulation bandwidths remains a significant obstacle however.
- the state-of-the-art edge-emitting semiconductor lasers have 3 db bandwidths in the region of 30 GHz.
- the limitations on this bandwidth arise from the non-linear gain mechanism and from the maximum values of differential gain that can be realized.
- the non-linear gain effect is due to the presence of current transport in the SCH(separate confinement heterostructure) regions and the associated dynamic response.
- the SCH regions perform the functions of both carrier confinement and optical confinement.
- these regions can be reduced substantially. However, in the edge emitter these regions cannot be reduced to much less than about 1500 ⁇ in thickness and yet still maintain a reasonable value for I′, the optical confinement factor for the quantum well in the optical waveguide. Also, as the low index waveguide regions are placed closer to the quantum well, the large index difference interfaces produce larger waveguide loss.
- the laser bandwidth varies inversely with ⁇ p .
- L or R must be reduced.
- the laser length cannot be reduced below about 200 ⁇ m (by cleaving or by using dry etched mirrors). This limit is imposed by the rapidly rising threshold current and the reduced power capability.
- the output power is limited by the total device volume which is being reduced with the length. This tradeoff between speed and power with reduced length (constant power ⁇ bandwidth product) is common to all devices.
- the vertical cavity laser has emerged as an alternative to the edge emitting laser because it allows a different mode of operation.
- the mirror reflectivity is increased very close to unity and the cavity length is reduced to 1-3 wavelengths. Therefore the mirror loss term in the photon lifetime, i.e. In (1/R)/L may be designed to be essentially the same as in the edge emitter.
- the term ⁇ representing optical loss is no longer limited by waveguide loss as in the edge emitter but rather by scattering in the mirrors and at the edges of the vertical cavity. Therefore the widths of the SCH layers in the vertical cavity growth are no longer important for determining ⁇ and can be reduced for the purposes of reducing the non-linear gain effect and therefore of increasing the bandwidth.
- the vertical cavity device can be optimized to reduce the “non-linear gain” effect and the mirror can be flexibly designed to trade-off the number of reflector pairs for a lower ⁇ p .
- Lower ⁇ p implies lower differential stimulated lifetime ( ⁇ st *) and therefore larger bandwidth due to the increase in differential gain at the expense of a larger threshold current.
- the larger threshold results in lower maximum power due to the reduced current range for optical output and the increase in internal device heating.
- Vertical cavity devices also have an output which is randomly polarized. It would be very useful to have a means to predetermine and maintain the polarization.
- Another impediment to vertical cavity deployment is the coupling of the light to fibers. Current methods use polymer waveguides or mirrors to redirect the light from the vertical to the horizontal direction. This is not a cost effective approach.
- the limitations of the laser in terms of matching impedances between the optoelectronic device and the electronic interface are the same for a laser or for a detector. Therefore the solutions to eliminating undesirable reflections for the laser may also be used for a detector.
- the goal of the invention is to introduce a traveling wave concept for both a laser and a detector which can eliminate the reflections produced at mismatched interfaces.
- a semiconductor laser or detector which is a vertical cavity device constructed with a dual dielectric top Distributed Bragg Reflector (DBR) mirror wherein a diffraction grating in the second order is incorporated into the dielectric layer which is the first layer to be deposited upon the vertical cavity.
- the function of the grating is to change the direction of propagation of the light from vertical to horizontal so that the output of the vertical resonant cavity is a mode in a waveguide.
- the principle of operation is based on the diffraction of light produced by the vertical cavity laser into an optical mode which propagates in the guide.
- the polarization of the light is dictated by the polarization in the guide which itself is TE in general because of the high diffraction efficiency for TE light.
- the light propagating in the guide also diffracts into the vertical cavity and reinforces the polarization of the vertical cavity emission through the process of stimulated emission.
- the device is designed so that the vertical emission is essentially inhibited and the device output is totally diffracted waveguide output and therefore the power output can be increased by simply extending the laser in length within the constraint of chip size.
- the grating structure may be applied to realize modulators, detectors and amplifiers in addition to lasers.
- the device is described in one illustrative embodiment of the HFET inversion channel laser, which comprises one of the laser devices in the general family of inversion channel optoelectronic devices which are modulation doped devices.
- a refractory metal emitter provides a conduction path of hole flow into the laser active region by two-dimensional conduction.
- the two dimensional contour of the conduction with path is established by an N type implant under the metal emitter.
- the refractory metal emitter is constituted of two metal stripes one positioned on either side of an optical waveguide which utilizes the upper DBR mirror of the vertical cavity to provide its cladding.
- the device is a laterally injected laser and thus ion-implanted source contacts provide electron flow to the active laser channel, i.e. the inversion channel.
- the source contact metals stripes and the emitter metal strips form the electrodes for a coplanar transmission line.
- the device is designed for equal electrical and optical velocities.
- the structure can also be employed in other optoelectronic devices such as modulators, detectors and amplifiers. For example by the reversal of the electrode potentials, the performance of a grating coupled detector is obtained.
- FIG. 1 is a schematic view through the center of the grating control laser waveguide and parallel to the direction of propagation of the optical mode in the waveguide.
- the grating is blazed in the direction of propagation which is from left to right.
- the vertical cavity oscillation is shown and also the growth of the waveguide mode.
- FIG. 1.5 is a schematic view of the layer structure of the pn laser which may be inserted between the mirrors of the grating control laser in FIG. 1 .
- FIG. 2 is a cross-sectional view of the grating coupled laser implemented as the HFET Inversion Channel Laser at right angles to the waveguide.
- the optical mode 150 is confined by the N type implants 111 and 112 such that there is little loss by scattering from the metal emitter gate/emitter.
- the gate/emitters 110 , 109 provides holes and the sources 114 , 115 provides electrons.
- FIG. 3 is a schematic of the traveling wave three dimensional cross-section of the grating coupled HFET laser, detector, modulator or amplifier.
- the inversion channel technology is shown as the means of isolating the device and establishing contact to the source and gate electrodes.
- the source and gate electrodes establish a copolanar transmission line which is shown terminated in the characteristic impedance to obtain high speed.
- FIG. 4 shows the diffraction efficiency versus grating angle in the vertical cavity waveguide.
- FIG. 5 shows the calculated waveguide photon density versus device length for the grating coupled traveling wave laser with the parameters as shown, F cav is the vertically propagating photon density and F g is the laterally propagating photon density.
- FIG. 6 is a schematic of a top view of an integrated circuit chip which uses a single GCVCSEL as a master laser and several GC amplifiers with on-chip waveguide connections to create a very large single spatial mode and coherent output power.
- FIG. 7 shows the effect of the velocity matching on the optical response of the laser.
- FIG. 8 shows and compares the different current flow patterns in the HFET detector and the HFET laser.
- FIGS. 1 , 2 , 3 shows the layers of a structure in accordance with a preferred embodiment and from which all the devices of the invention can be made.
- the semiconductor layer structure and fabrication are in accordance with the Inversion Channel Technology and have been described in detail in application Ser. No. 60/028,576.
- a vertical cavity laser is constructed comprised of: a lower DBR mirror 106 consisting typically of alternating layers 108 of GaAs and 109 of AlAs which itself will be oxidized during the fabrication to form layers 109 of Al x O y , a active laser cavity 107 consisting of the standard SCH (separate confinement heterostructure) semiconductor laser structure containing say 3 quantum wells 120 which may be the laser structure of the inversion channel laser as discussed here or the conventional pn laser structure, a top DBR mirror consisting of alternating layers 101 , 102 , 103 , and 104 of two deposited dielectrics.
- SCH separate confinement heterostructure
- this structure consists of a layer 140 of P+ GaAs deposited on the lower DBR mirror, a layer 141 of P+ type AlGaAs (>40%) disposed on the P+ type GaAs layer; a layer 142 of P type of AlGaAs disposed on the P+ type AlGaAs layer, a PHEMT (Pseudomorphic High Electron Mobility Transistor) disposed on the P type AlGaAs layer, the PHEMT consisting of the sequence of layers marked in FIG.
- NID AlGaAs spacer layer 144 once to three NID quantum wells 145 of strained InGaAs separated by NID GaAs barriers 146 and collectively labeled 120 , a NID AlGaAs spacer layer 147 ( ⁇ 15% Al), an AlGaAs N+ type planar doped (very thin) layer 148 ( ⁇ 15% Al) which is typically referred to in the art as a modulation doped layer, and a NID AlGaAs gate capacitance layer 149 ( ⁇ 15% Al); a planar doped (very thin) P+ layer 150 151 of AlGaAs ( ⁇ 15% Al) disposed on the PHEMT structure; a P doped AlGaAs cladding or current blocking layer 151 152 disposed on the P+ planar doped layer; and a very thin layer 152 153 of about 100 A of
- the structure consists of a layer 160 of N+ GaAs deposited on the lower DBR mirror, a layer 161 of N+ doped AlGaAs (>40%) cladding, a layer 162 of ND GaAs, a layer 163 162 of NID AlGaAs (confinement region) with ⁇ 15% Al, a layer 163 of NID GaAs, a series of ND InGaAs quantum wells 164 separated by ND GaAs barriers 165 , a layer 166 of ND AlGaAs (confinement region) with ⁇ 15% Al, a layer 167 of P type doped AlGaAs (>40%) cladding and a very thin layer 168 of P+ type doped GaAs to serve as a top contact layer.
- the grating 101 is formed in the first layer of the mirror stack comprised of layers 101 , 102 , 103 and 104 . It is a blazed grating which diffracts light preferentially in one direction as shown by the arrows indicating the direction of light propagation.
- the DBR mirror layers 102 , 103 and 104 of the vertical cavity device are deposited on top of the cavity 105 107 and grating layer 101 , and the bottom mirror 106 is formed under the cavity 107 during the growth.
- This device is designated the Grating Coupled Vertical Cavity Surface Emitting Laser (GC VCSEL).
- GC VCSEL Grating Coupled Vertical Cavity Surface Emitting Laser
- the laser is constructed in the form of a waveguide as shown in FIG. 3 , wherein the refractory metal gate contacts which supply positive charge to the active region are separated into two stripes 109 and 110 and set back from the center of the guide to allow low loss propagation of the mode.
- the current is guided electrically into the active wells 113 by the N type implants 111 and 112 .
- This guiding action describes the two dimensional current flow indicated by the large arrows in FIG. 2 .
- These implants simultaneously provide the optical guiding function to maintain the optical mode in the center of the guide.
- the gate contacts become the masking feature which are used to define the ion implants 114 and 115 , which are sources used to supply positive charge to region 113 .
- N type gold alloy stripes are formed on implanted regions 114 and 115 to form a coplanar transmission line together with gates 110 and 109 .
- the device is isolated potentially by oxygen implantation 130 beneath regions 114 and 115 and formed with the same mask and also by the formation of Al x O y which penetrates from the side in regions 116 and 117 at the same time that the mirror layers 106 are formed beneath the device.
- the characteristic impedances Z o are of the transmission line are shown as terminations for high speed operation. For detector operation these are on the output of the device but for laser operation these are on the input to the device.
- the grating has a finite thickness and in addition the penetration of the light into the mirror is small for a high reflectivity mirror. Thus the intensity of the light traveling downward into the laser after reflection from the mirror decreases dramatically with penetration into the mirror.
- the amplitude of the light incident on the grating from within the cavity is substantially greater than the amplitude of the light incident on the grating by reflection from the mirror. Therefore the fraction of light diffracted to the right is substantially less greater than the fraction of light diffracted to the left.
- the design criterion that is followed is that the thickness of the diffraction grating should be approximately equal to the penetration depth of the mirror (this is also determined by the standing wave effects in the grating which are found from the diffraction analysis).
- the decay of intensity to about 10% occurs within the first 1 ⁇ 4 wavelength of the mirror stack and additionally, it is most practical to form the grating by etching through approximately the first layer of he mirror stack.
- the dielectrics chosen to form the mirrors are SiO 2 and undoped sputtered GaAs. This choice is dictated by the very large index difference which reduces the number of required pairs and the fact that our mirror must be deposited during the device processing. Given these layer components we have two choices to implement the grating 1)deposit a layer of GaAs and then pattern and etch this layer followed by a 1 ⁇ 4 wave of SiO 2 and then GaAs etc.
- the quantitative prediction of diffracted power has been a subject of much study over the years primarily because of its importance to the operation of the DFB laser.
- the gratings are usually designed to be first order since it is the first order forward and backward traveling waves which form the basis of coupled mode theory.
- second order has been used in the DFB to produce an optical loss mechanism in an effort to stabilize the mode position as described by Kazarinov and Henry (however the laser output is still via the first order wave). More generally the second order grating has been used as the output reflector for DBR lasers in order to obtain vertical emission.
- the second order grating when implemented as a waveguide corrugation produces both first order diffraction in the guide and second order diffraction normal to the guide then it can be used in the DBR to provide both reflection for the guided wave and output coupling of the laser light.
- the basis for the model is a three layer waveguide characterized by indices n 1 , n 2 , and n 3 in the regions above the guide, in the guide and below the guide. This analysis applies to diffraction from the guide to the direction normal to the guide.
- the waveguide cladding is formed by the bragg mirrors and thus the structure forms a multilayer waveguide with very complicated propagation coefficients.
- the wave propagating in a vertical cavity laser is characterized by a penetration depth.
- L ef L c +L pb +L ps
- L pb and L ps are the penetration depths of the bottom and top mirrors respectively which have been derived as L pt
- b tanh 2 ⁇ ( ⁇ ⁇ ⁇ ⁇ L M ⁇ t , b ) 2 ⁇ ⁇ ⁇ ⁇
- L Mt,b is the total front and back mirror thicknesses
- ⁇ n is the index difference between the two layers in a pair of the mirror.
- the TE mode is normally described by a function within the guide (asymmetric or symmetric) and by evanescent decay into the cladding on either side of the guide.
- the evanescent decay away from the guide is determined by the refractive index in the region.
- the mirror paris on the top are SiO 2 and GaAs corresponding to indices of 1.5 and 3.6.
- the bottom mirror is comprised of Al x O y and GaAs corresponding to indices of 1.6 and 3.6.
- the typical design will be 7 pairs on the bottom and 4-7 pairs on the top.
- the index of the core region (n 2 ), which is the vertical cavity itself, is determined by using a transmission matrix calculation for a slab waveguide.
- FIG. 4 we plot the efficiency of the diffraction process for a parallelogramic grating in the VCSEL waveguide and the results show that we can expect an efficiency of 3.5 ⁇ 10 ⁇ 4 1 . 5 ⁇ 10 ⁇ 3 for a blaze angle of 45° and a grating etch depth of about 0.15 ⁇ m.
- the simulation shows a maximum efficiency at an angle of 45°.
- Such large blaze angles may be difficult to achieve with ion beam etching and also, for a depth of 0.15 ⁇ m the bottom of the parallelogram may penetrate back to undercut the top of the parallelogram which is not desirable.
- the diffracted light propagates in the form of a guided wave.
- the guided wave will itself be diffracted back into the cavity at each position z and will be amplified.
- P out hvWdzv g t f F cav (3)
- t f the power transmissivity of the output port.
- P mir hv q ⁇ ⁇ e 1 + L cav ⁇ ⁇ vc ln ⁇ ( 1 / R ) ⁇ ( J - J TH ) ⁇ W ⁇ d z ( 4 )
- ⁇ vc the primary optical loss in the vertical cavity by design will be the second order diffraction by the grating of the vertically propagating wave.
- ⁇ diff ⁇ g ⁇ d eff ⁇ ⁇ 1 ⁇ 2 ⁇ A y ⁇ A y * C o ⁇ C o ′ ( 6 )
- ⁇ 1 and ⁇ 2 are the impedances of the guide and the incident medium (which in this case are the same)
- a y is the field intensity at the edge of the guide of the z propagating wave
- C o is the field intensity of the incident wave.
- the grating as a loss in the x direction for the wave C o and we can define a loss parameter ⁇ x by the statement.
- the power output is just the total output of the vertical cavity device with the efficiency of the diffraction as opposed to mirror transmission and is a linear function of the length of the laser.
- this power diffracts from the wave into the cavity and then back into the guide with the same efficiency as the cavity flux.
- this photon flux can amplify the cavity flux through an adjustment of F cav and therefore of (10).
- the amplification of waveguide power depends on the relative size of the mirror and diffractive loss. If the diffractive loss is very large, then the light realizes single pass gain, i.e. it passes through the cavity essentially once before it is diffracted back into the wave.
- the input photon term reduces the loss and therefore reduces the condition for threshold and therefore the threshold current.
- the K parameter which is used to determine threshold contains ⁇ ′ p and from (14), the photon lifetime is modified to ⁇ peff ′ ⁇ 1 m ⁇ p ′ ⁇ 1 (1 ⁇ ⁇ g F g v g ⁇ p ⁇ 1 /F cav L cav ) (15) i.e., the effective photon lifetime in the cavity increases with the input signal but decreases with the cavity flux itself.
- the efficiency was ⁇ e and the effective length was ⁇ diff / ⁇ g ⁇ ln (1/R)+L cav * ⁇ par ).
- This device may also be operated as an optical amplifier. As long as ⁇ diff is large enough to prevent lasing, then it may still be quite small, i.e. it may be easily be around 10 ⁇ 3 .
- FIG. 6 There is a particular benefit from using the device as an optical amplifier which is indicated in FIG. 6 .
- the top view of an integrated circuit chip is shown in which several grating coupled amplifiers 401 , 402 , 403 , 404 , 405 , 406 etc are shown connected by on-chip waveguides 410 , 411 , 412 413 etc to a single GCVCSEL 410 400 .
- the amplifiers take on identical phases if the optical path from the GCVCSEL is identical for each. This arrangement allows each amplifier to produce an amplified version of the GCVCSEL locked in frequency and phase.
- the outputs of the amplifiers are routed by on-chip waveguides 420 , 421 , 422 , 423 to a single output port.
- the output waveguide may emit from a cross-sectional area and may combine the outputs of all amplifiers so that a very high level of spatially coherent light is delivered to every small spot.
- This innovation provides a high power density and very high brightness
- the polarization of the vertical cavity wave is forced to coincide with that of the waveguide.
- the locking of the polarization occurs because the waveguide mode is injected into the cavity at each point and creates some level of stimulation emission.
- the emission in the cavity reproduces the same polarization.
- the random nature of the polarization of the vertical cavity laser output is therefore eliminated.
- the output of the waveguide will be TE 0 normally because it is most easily excited in the guide (earlier simulations of Lee and Streifer have shown in general that the diffraction efficiency of the TM 0 mode is at least 10 times smaller than the TE 0 mode, so TE 0 becomes the principle supported mode).
- the output is single mode until a certain level of power is achieved and then multimode behavior is observed.
- the onset of higher order VCSEL modes will require a much higher level of laser power to occur because of the stabilizing effect of the waveguide injected energy.
- the GCVCSEL will remain single mode until a higher order mode in the waveguide such as TE 1 and TE 2 matches to a higher order mode in the VCSEL.
- the mode suppression ration will be characteristic of the vertical cavity laser. Because of the single mode nature of the VCSEL, the tendency for mode partition noise will be much reduced. The RlN noise should be typical of the VCSEL.
- the photon response is proportional to the product of the injected carriers and the injected photons. Therefore in the laser cavity, at any position along the guide we need to consider a mixture of the dispersed pulse traveling on the transmission line and the dispersed pulse traveling in the waveguide and to represent their individual time dependencies.
- the identical structure performs the function of the traveling wave detector.
- Traditional detector geometries available to the designer include the classical vertically illuminated PIN structure or the waveguide structure which is edge illuminated.
- the inherent bandwidth-efficiency trade-off in the vertical device between absorption efficiency in the active layer and carrier transit time through it have been addressed significantly by the implementation of resonant cavity enhanced (RCE) structures wherein multiple optical passes are used to obtain almost total absorption in a very narrow active layer, i.e. in a quantum well.
- RCE resonant cavity enhanced
- the drawback of the RCE is the narrow optical bandwidth which results from the high finesse of the cavity.
- the wavelength selectivity offered by the cavity is exactly which is required to perform optical to electronic demultiplexing.
- the difficulty of course is to devise a means to inject the light into the cavity since the high reflectivity of the cavity will reject the optical input unless it is exactly positioned within the narrow bandwidth corresponding to the position of the cavity optical mode.
- the waveguide configuration it is typical to use a double heterostructure semiconductor structure with a quantum well active region.
- the thickness of the active layer can also be minimal since the absorption occurs along the length of the guide which can be designed for complete absorption.
- the transit time limitation and the RC limitation can be essentially the same as that of the vertically illumination RCE device.
- the efficiency of the device does suffer, however, from the problem of poor input coupling since the numerical aperture of the typical semiconductor waveguide is small and the mode is not well matched to the typical mode of the optical fiber.
- the TWPD still suffers from the problem of poor input coupling since the basic semi-conductor waveguide structure has remained unchanged. In fact the waveguide must be 1 ⁇ m or less in width to realize reasonable transmission line parameters and this exacerbates the coupling problem.
- traveling wave, waveguide or RCE designs Another limitation with the existing traveling wave, waveguide or RCE designs is the problem of integration with electronic devices.
- a traveling wave device intended for 100 GHZ operation must feed photocurrent to an FET or bipolar front end in a matched configuration to avoid reflections. It is not practical or cost effective to do this with hybrid connections except for specialized applications and it thus becomes essential to have an integrated approach.
- the integration should encompass the detector, the laser, and the electronic amplifier.
- the HFET resonant cavity enhanced detector is a novel means to address these problems since it combines the virtues of the vertical cavity RCE structure and the TW concept into a single device.
- This approach has several advantages which include 1) optimized mode matching to an optical fiber to achieve improved input coupling 2) optimized impedance matching to reduce reflections, 3) optimized velocity matching to reduce pulse dispersion, and 4) integrated circuit compatibility to achieve a low cost high performance package.
- FIGS. 1 , 2 and 3 show the traveling wave version of the HFET.
- FIG. 8 shows the appropriate current flows. This figure indicates that the collection of carriers in the devices is lateral for the electrons and vertical for the holes. If the collector is contacted with a negative bias then the holes are removed downward to the collector. On the other hand if the gate is contacted with a negative bias, the hole current flows upward to the gate electrode. It is noted that the collector operation can be achieved with zero collector to source bias since the barrier up which may be at a different frequency. Normally this energy will re-enter the cavity and may destabilize the laser.
- the asymmetry of the grating causes a dominant fraction of the power to be diffracted into the guide for light traveling towards the chip edge (direction of laser emission) and it also causes a dominant fraction of the light to be diffracted out of the waveguide for light approaching the laser from the edge of the chip.
- the grating asymmetry acts to reject the light re-entering the laser with a ratio of diffraction up to diffraction down which is between 50 and 100:1.
- the grating thus acts as an optical isolator.
- the GCVCSEL output is into a waveguide on the chip.
- the waveguide transports the light to the chip edge.
- Output coupling of the laser light to a fiber occurs from the waveguide to the five.
- One of the major problems in manufacturing involves attaching fiber to the chip and much effort has gone into the development of spot size transformers which are ways to match the waveguide mode size to the fiber aperture.
- the waveguide output is easily tapered in the lateral dimension to achieve this matching. In the vertical dimension the mode size can be enlarged considerably by the use of the ion implant to disorder the waveguide.
- the waveguide core is implanted with Si during the formation of the source and drain regions.
- This implant after RTA serves to disorder the bandgap of the quantum well layers which reduces the index. The net effect is to move the core index closer to the effective index of the upper and lower quantum well regions which causes the expansion of the mode into these regions. Judicious use of this implant and anneal step can be used to optimize the mode shape to increase the tolerances for the mode to fiber coupling problem.
- the mode may also be expanded downwards into the lower DBR mirror is by adjusting the width of the guide and therefore the extent of the lateral oxidation. As the lateral oxidation can no longer extend totally underneath the guide, the confinement will be reduced and the mode will expand downwards into an unoxidized mirror structure.
- the HFET laser is ideally suited to achieve very high bandwidths.
- the HFET laser is a laterally injected laser in which the gate/emitter and source injecting terminals lay next to each other along the surface and are therefore configured as adjacent electrodes as illustrated in FIG. 3 .
- the distance from the quantum wells to the semiconductor surface can be 1 ⁇ 2 wavelength as in most VCSEL designs and since the source drain implant is performed from approximately 1000 ⁇ above the quantum well to achieve minimum channel access resistance, then the positive and negative electrodes form an almost perfect coplanar transmission line.
- the vertical cavity of the HFET laser structure is formed by the deposition of the dielectric stack over the electrode structure. These dielectric layers form the top dielectric cover for the coplanar line and are therefore important in determining Z o .
- the HFET laser implements almost preferably a traveling wave laser. This traveling wave property extends the bandwidth of the laser to the limit imposed by the internal parasitic resistance and time constant because the coplanar electrical transmission line can be terminated on the chip by a transistor adjusted in impedance to match the Z o of the line.
- the photon pulse is a maximum height because the injected charge x photon density has been a maximum at every position and also the pulse width will be the narrowest because, assuming that the internal laser dynamics are sufficiently fast, the pulse width of the photons will be as narrow as the charge pulse. If there is negligible dispersion on the transmission line then the charge pulse and thus the photon pulse will retain its original delta function form, i.e. the photon pulse will remain essentially as fast as the input electrical pulse.
- the description of the technology changes to accommodate the detector are essentially the same as those of the laser above.
- the metal contact is opened, as in the conventional vertical cavity device, to allow the passage of the light in the vertical direction. However here the light is propagating both horizontally and vertically.
- the dielectric mirror of the vertical cavity device is used here in a multi-functional role.
- the semiconductor growth between the quantum well active region and the dielectric sack is very thin, i.e. approximately ⁇ /2n, where n is the average material index, in order to move the gate contact as close as possible to the inversion channel. This is necessary to form a high frequency transistor structure.
- the optical mode is still efficiently guided because the dielectric stack provides efficient guiding of the mode.
- the mode is centered in the quantum wells but extends well above the gate metal contact.
- the dielectric stack also forms a high finesse cavity in the vertical direction. Therefore the number of pairs in the stack and the index difference are selected to achieve the desired finesse for the resonant cavity.
- the multiple layer structure of the cavity mirror allows a large optical mode to be supported for propagation in the horizontal direction. The enlarged mode size increases the mode near-field pattern and decreases the far-field angles which facilitates easier coupling to a fiber.
- the input light is edge coupled to a waveguide at the chip edge with a cross-section designed to optimum coupling.
- the waveguide guides the light to the detector where it propagates along the device waveguide as shown in FIG. 3 .
- the key element to enable the combined waveguide, vertical cavity operation is a grating which is created in the first layer of the dielectric stack. This is a second order grating which diffracts a portion of the input wave into the vertical cavity at each position along the guide. Since the light propagating vertically in the cavity is absorbed essentially completely due to the resonant enhancement effect then the limitation on the detector efficiency, i.e. the length of detector required, is determined by the grating parameter ⁇ g . Because the grating index change can be made fairly large by suitable design of the dielectric stack then ⁇ g can be large and the waveguide length can be optimized for high speed operation.
- the HFET TW detector illustrated are FIG. 3 uses the gate contact to collect the hole current in order that it may form the signal line for the transmission line.
- the photo-charge is added synchronously to the electrical wave and there will be no distortion of the pulse due to velocity mismatch. If the transit time of holes to the gate from the channel and of electrons to the source contact from the channel are equal and if there is no dispersion on the transmission line or the waveguide then the input pulse shape will be preserved. Finally if the transmission line is exactly matched in its characteristic impedance, there will be no reflection and the pulse will be perfectly replicated, i.e. the impulse response of the detector would be infinite. These conditions can never be realized perfectly and it is the deviation of these criteria from the ideal which determines the actual detector bandwidth.
- the virtue of the traveling wave concept is that the impedance of the circuit following the detector does not combine with the detector impedance to produce delay if ideal line matching is achieved.
- parasitic RC delays which are intrinsic to the device are still an issue. These are the time constant associated with transferring the electron and hole from the absorption region to the source metal and gate metal respectively.
- the transmission line metal to metal spacing should be reduced and the channel doping increased as much as possible by aggressive technology scaling.
- the other fundamental delay is the transit time of electrons in the absorbing FET channel.
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Abstract
Description
where the parameters are R, the reflectivity of the cleaved facets and L, the length of the laser. The laser bandwidth varies inversely with τp. To increase speed either L or R must be reduced. With the semiconductor to air reflectivity fixed at about 0.3, the laser length cannot be reduced below about 200 μm (by cleaving or by using dry etched mirrors). This limit is imposed by the rapidly rising threshold current and the reduced power capability. The output power is limited by the total device volume which is being reduced with the length. This tradeoff between speed and power with reduced length (constant power×bandwidth product) is common to all devices.
Lef=Lc+Lpb+Lps
where Lpb and Lps are the penetration depths of the bottom and top mirrors respectively which have been derived as
where LMt,b is the total front and back mirror thicknesses, and κ is the coupling constant which is given by approximately
κ=2iΔn/λ
Here, Δn is the index difference between the two layers in a pair of the mirror. In the standard description of the three layer waveguide, the TE mode is normally described by a function within the guide (asymmetric or symmetric) and by evanescent decay into the cladding on either side of the guide. The evanescent decay away from the guide, is determined by the refractive index in the region. To predict the behavior of the vertical cavity guide we assume that the penetration depth(s) of the vertical cavity mirror(s) correspond to the evanescent decay of a wave if it were propagating in the waveguide formed by the vertical cavity as the core of the guide, and the top and bottom mirrors as the claddings. Therefore, by using Lpt,b we can determine the effective index of the top and bottom mirror regions from the point of view of a three layer waveguide. Using these indices we can then use the model developed for the waveguide diffraction into a three layer waveguide to determine the efficiency of the second order diffraction from the vertically propagating light to the guide wave.
Lps=0.16 μm
and therefore an effective index above the care of
n1eff=2.96
Similar calculations for the lower mirror yield n3cf=3.05. Then the diffraction efficiency is determined from the model by a three layer waveguide with indices of n1=2.96, n2=3.48 and n3=3.05. The index of the core region (n2), which is the vertical cavity itself, is determined by using a transmission matrix calculation for a slab waveguide. In
where Lx is the quantum well width, ηc is the electrical confinement factor, J and JTH are the current and current density and τp′ is the effective photon lifetime for the vertical cavity device given by
where αvc is the vertical cavity loss due to the diffraction grating. There will also be components of αvc which are due to free carrier absorption and parasitic diffraction but these will be ignored for the moment to focus on the desired effect. In the final formula we may simply replace ln(1/R) by Lcav*αpar to determine its effect since in all likelihood this term will dominate ln(1/R).
Pout=hvWdzvgtfFcav (3)
where tf is the power transmissivity of the output port. For the conventional transmission through the mirror (1) is substituted into (3) with tf=ln(1/R) to give the power increment
There is another power component which is the power diffracted into the guide. The transmissivity for this mechanism is the diffraction efficiency tf=ηdiff so we have
To determine αvc we note that the primary optical loss in the vertical cavity by design will be the second order diffraction by the grating of the vertically propagating wave. In the development of diffraction into the guide using the above model from a normally propagating wave we have determined the diffraction efficiency ηdiff as
where η1 and η2 are the impedances of the guide and the incident medium (which in this case are the same), Ay is the field intensity at the edge of the guide of the z propagating wave and Co is the field intensity of the incident wave. Thus we can say that the power diffracted into the guide from the incident wave is ηdiffCoCo*. However we can also regard the grating as a loss in the x direction for the wave Co and we can define a loss parameter αx by the statement.
CoCo*(1−e−α
so that for a grating thickness of x=t we have
α2=ηdiff/t (8)
Since this loss occurs only over the grating thickness then we multiply by confinement factor Γ=t/Lcav to obtain an effective value for the total cavity of
αvc=ηdiff/Lcav (9)
for use in (5) and (2). Therefore the waveguide power is
Now the design of the laser should be that αvc is the dominant loss in the cavity and that negligible power escapes through the two mirrors which may be stated, by using (9) in (4), as
ηdiff>>ln(1/R) (11)
where in this case (6) is modified to
and the threshold current has now become a function of the flux injected into the cavity. The determination of the function Jth(αFs/Fcav) requires a modification of the photon rate equation from
The equation on the right left is the conventional photon loss equation where the right hand side represents photon loss from the system. The equation on the right left is the photon loss equation when an optical input is fed to the device. It is clear that the input photon term reduces the loss and therefore reduces the condition for threshold and therefore the threshold current. Specifically, the K parameter which is used to determine threshold, contains τ′p and from (14), the photon lifetime is modified to
τpeff ′−1mτp ′−1(1−α
i.e., the effective photon lifetime in the cavity increases with the input signal but decreases with the cavity flux itself.
which is equivalent to the output of a vertical cavity device with a width of W, a length of L=1/αg and an efficiency of η=ηdiffηe/{ln(1/R)+Lcav*αpar). Equivalently, we could say the efficiency was ηe and the effective length was ηdiff/αg{ln (1/R)+Lcav*αpar). In the figure, the parameters are taken from a typical grating efficiency analysis and the parameters of ηdiff=5e−4 and αg=50 are used with a cavity designed for R=0.9999. Then the effective length is 330 um. Alternatively we could regard the device as having an efficiency of 63% for a length of L=530 um. Desirable parameters are therefore η=10−3 and αg=which can be achieved with the dielectric combination SiO2/GaAs and a grating etch depth (thickness) of 1000 Å. From our calculations for an asymmetrical grating the optimum blaze angle is about 30°. With these numbers, the laser output power for J−Jth=200 A/cm2 and a waveguide width of 10 μm is about 0.25 W.
Claims (40)
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