Patent Application
20060227823
This disclosure relates generally to electro-optic devices, and in particular but not exclusively, relates to a monolithically integrated surface emitting laser with dual resonant cavities.
Semiconductor lasers have a variety of applications including communication systems and consumer electronics. Generally, semiconductor lasers may be categorized as edge-emitting lasers or surface emitting lasers (“SELs”). An edge-emitting laser emits radiation parallel to a surface of the semiconductor wafer or die, while a SEL emits radiation substantially perpendicular to the surface. One common type of SEL is a vertical cavity SEL (“VCSEL”). A VCSEL includes a gain region within a resonant cavity having a surface aperture to emit light from the resonant cavity.
There are two main techniques for modulating a signal onto an optical carrier wave emitted from a semiconductor laser—direct modulation and external optical modulation. Direct modulation encodes the optical carrier wave with a signal by directly modulating the drive current applied to the gain region of the semiconductor laser. The bandwidths achieved by direct modulation are limited due to the finite relaxation oscillation time of an excited state electron within the gain region. This finite relaxation oscillation time can result in inter-symbol interference (“ISI”) between adjacent clock cycles. With external optical modulation, the semiconductor laser emits a continuous wave (“CW”) carrier, which is externally modulated by an external optical modulator (“EOM”). EOMs are typically distinct entities from the CW carrier source and therefore more expensive to manufacture than directly modulated lasers, but are capable of achieving higher modulation bandwidths.
Generally, EOMs may be categorized as electro-refraction modulators and electro-absorption modulators. Electro-refraction modulators rely on changes in the index of refraction of a material induced by an applied electric field to modulate the proportion of light through the modulator (for example Mach-Zehnder interferometer). Electro-absorption modulators achieve the desired light modulation by modifying the light absorbing properties of a material with an electric field.
Non-limiting and non-exhaustive embodiments of the invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Embodiments of an Electroabsorption VCSEL (vertical cavity surface emitting laser) Modulator (“EAVM”) and/or detector including dual resonant cavities are described herein. In the following description numerous specific details are set forth to provide a thorough understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
The illustrated embodiment of EAVM 100 includes a lower resonant cavity 105 (gain section) and an upper resonant cavity 110 (modulator section), a drive electrode 115, a ground electrode 120, signal electrodes 125A, B, C (collectively 125), a substrate layer 130, and a dielectric material 135. The illustrated embodiment of lower resonant cavity 105 includes a lower reflector 140, an oxide layer 145 having a confinement aperture 150 therein, barrier layers 155 and 160, a gain region 165 and a middle reflector 170. The illustrated embodiment of upper resonant cavity 110 includes middle reflector 170, barrier layers 175 and 180, an absorber region 185, upper reflector 190, and a surface aperture 195.
In one embodiment, during a optical source regime of EAVM 100, lower and upper resonant cavities 105 and 110 of EAVM 100 are weakly coupled micro-cavities, which together provide the functionality of an optical source and external optical modulator, respectively, but integrated into a single semiconductor die. Additionally, during an optical detecting regime of EAVM 100, gain region 165 may be disabled via appropriate biasing and absorber region 185 operated to detect an optical signal impinging upon surface aperture 195.
In one embodiment, substrate layer 130 is one layer of a semiconductor die, such as a gallium arsenide (GaAs) based semiconductor die, a silicon based semiconductor die, various other type III-V semiconductor materials, type IV semiconductor materials, or the like. In one embodiment, substrate layer 130 is a n-type doped GaAs substrate.
In the illustrated embodiment, lower, middle, and upper reflectors 140, 170, and 190 are distributed Bragg reflectors (“DBRs”) including alternating layers of GaAs and AlGaAs. In one embodiment, lower reflector 140 is fully reflective at the carrier wavelength of emitted optical signal 197, while middle and upper reflectors 170 and 190 are at least partially reflective to encourage lasing and partially transmissive to emit optical signal 197. The attributes of lower resonant cavity 105 may be selected for coarse resonance tuning of a carrier wavelength generated by gain region 165, while the attributes of upper resonant cavity 110 may be selected for fine resonance tuning of the carrier wavelength and to provide for adequate weak coupling between upper and lower resonant cavities 105 and 110. The thickness of each alternating layer within the reflector may be chosen to select a desired center resonance frequency and therefore nominal carrier wavelength of optical signal 197 emitted from EAVM 100. In one embodiment, where the carrier wavelength is selected to fall between 850 nm and 900 nm, the alternating layers of lower and middle reflectors 140 and 170 may have quarter, half, or full wavelength thickness to place the Bragg wavelength of lower and middle reflectors 140 and 170 at the desired carrier wavelength.
In one embodiment, lower, middle, and upper reflectors 140, 170, and 190 are doped to establish p-n junctions within upper and lower resonant cavities 105 and 110. For example, lower and upper reflectors 140 and 190 may be doped to have an n-type conductivity while middle reflector 170 may be doped to have a p-type conductivity, thereby creating an n-p-n structure. Of course, lower, middle, and upper reflectors 140, 170, and 190 may also be doped to create a p-n-p structure with a corresponding polarity change in the bias voltages/signals applied to electrodes 115, 125, and 130 (discussed below).
Returning to
Oxide layer 145 provides an electrical and optical barrier layer. Confinement aperture 150 defined in oxide layer 145 provides a sort of beam shaping function using both current and optical confinement. Oxide layer 145 has a lower index of refraction than confinement aperture 150 and therefore the optical intensity of the optical carrier wave is laterally confined to establish the optical mode along the center of EAVM 100 through confinement aperture 150 and beneath surface aperture 195. Furthermore, oxide layer 145 is an electrical insulator that restricts the DC drive current between drive electrode 115 and ground electrode 120 to flow through confinement aperture 150. By restricting the DC drive current to flow through confinement aperture 150, the stimulated emission is laterally concentrated (high carrier densities) in gain region 165 above confinement aperture 150 and below surface aperture 195. In one embodiment, oxide layer 145 is formed of Al(Ga) oxide and a wet selective oxidation technique is used to form confinement aperture 150. It should be appreciated that other electrical and optical barrier materials and fabrication techniques may be substituted. For example, another oxide layer with a confinement aperture may be placed above gain region 165, or two or more oxide layers with confinement apertures may be used above and/or below gain region 165 to increase the optical field or/and current confinements. In one embodiment, confinement aperture 150 is approximately 6 μm in diameter.
Barrier layers 155 and 160 surround gain region 165 and act to increase injection efficiency into gain region 165 from the surrounding material layers. In one embodiment, barrier layers 155 and 160 are formed of AlGaAs, while gain region 165 is a superlattice formed of InGaAs, GaAs, or other optically active materials. Similarly, other material constituents may be used to form barrier layers 155 and 160. In one embodiment, barrier layers 155 and 160 are approximately 50 nm thick. In one embodiment, the thickness of barrier layers 155 and 160 and gain region 165 are such that lower resonant cavity 105 is a half-wavelength cavity. The thicknesses of barrier layers 155 and 160 may be adjusted to adjust the resonant frequency of lower resonant cavity 105.
Gain region 165 acts as a gain medium to emit the optical carrier wave. Gain region 165 is driven by the DC current to create a charge carrier population inversion within gain region 165 and thereby establish conditions favorable for stimulated emission. The DC drive current is generated by applying an appropriate bias current between drive electrode 115 and ground electrode 120. In one embodiment, stimulated emission is created by forward biasing gain region 165 with drive electrode 115 and ground electrode 120.
Gain region 165 may be formed of a variety of optically active materials, including for example layers of InGaAs or GaAs with AlGaAs barriers. Gain region 165 may be constructed as a multi-layer quantum dot (“MQD”) structure or a multi-layer quantum well (“MQW”) structure. An MQD structure provides three dimensional carrier confinement, while the MQW structure provides one dimensional carrier confinement.
The MQD structure of
Returning to
Surface aperture 195 may be patterned in a variety of shapes, including a circle, as illustrated in
Finally, dielectric material 135 may be formed between the inner components of EAVM 100 and signal electrode 125 for planarization, mechanical protection, and electric isolation. In one embodiment, dielectric material 135 is a reflowable polymer material.
In a process block 605, the DC bias current is applied through drive electrode 115 and across gain region 165 to ground electrode 120. The DC bias current and associated DC bias voltage forward biases gain region 165 resulting in stimulated emission of an optical wave by gain region 165 (process block 610). In a process block 615, the optical wave resonates within lower and upper resonant cavities 105 and 110 resulting in lasing at the carrier wavelength (process block 615). In a process block 620, a DC reverse bias voltage is applied across absorber region 185 between signal electrode 125 and ground electrode 120. In a process block 625, a signal voltage containing the electrical signal to be modulated onto the optical carrier wave is superimposed on the DC reverse bias voltage. The signal voltage applied across absorber region 185 results in a corresponding modulation of the absorption coefficient of absorber region 185 due to the Quantum Confined Stark Effect (“QCSE”).
QCSE is a phenomenon which arises when an electric field is applied across the plane of heterostructure superlattices (e.g., the MQD and the MWQ described above). In a quantum well at zero electric field, the electron and hole quantized energy levels are defined by the well width (dimensions H and W in
In one embodiment, EAVM 100 is a tunable optical source capable of amplitude modulation at different optical wavelengths. As mentioned above, applying a voltage modulation across absorber region 185 not only modulates the optical absorption coefficient of absorber region 185 (amplitude modulation), but also modulates the index of refraction of absorber region 185 (or the absorption resonance wavelength). The absorption coefficient and the index of refraction are related by what is called the Kramers-Kronig relation. Accordingly, the nominal or center wavelength of absorption of absorber region 185 may be tuned by varying the DC reverse bias voltage applied across signal electrode 125 and ground electrode 120. Therefore, bias applied to absorber region 185 is used to control absorption losses in the mode and the value of coupling between the gain region 165 and absorber region 185. Additionally, at the time of fabrication, the geometry (e.g., Bragg wavelength and cavity length) of lower and upper resonant cavities 105 and 110 may be selected to select different wavelengths of operation.
EAVM 100 may be used as a general electro-optic building block, which may be tailored for a variety of electro-optic applications, such as an optical detector. An optical detector can be made to be tunable by placing the optical detector within a Fabry-Perot cavity. The Fabry-Perot cavity acts as a resonator to enhance the optical field intensity within the cavity at particular wavelength, or quarter, half, or full multiples thereof, via constructive interference. By placing the optical detector at peak E-field intensity locations within the Fabry-Perot cavity, as illustrated in
The active, passive, and DBR layers of EAVM 100 may fabricated using known molecular beam epitaxy (“MBE”) and metal-organic chemical vapor deposition (“MOCVD”) techniques, as well as others. Furthermore, EAVM 100 may be fabricated in a single epitaxial run to deposit both gain region 165 and absorber region 185 on a single semiconductor die, as a monolithically integrated device. Upper reflector 190 may be fabricated using a “quarter-wavelength thick” dielectric stack, which is deposited on top of signal electrode 125.
EAVM 100 may be used to optically interconnect a variety of different electronic circuits residing on the same semiconductor die, residing on different semiconductor dies (chip-to-chip), residing on different circuit boards (board-to-board and blade-to-blade), residing within different systems (system-to-system), or residing within different compute centers (rack-to-rack), as well as others.
The elements of electronic circuits 805 may be interconnected as follows. Processor(s) 815 are communicatively coupled to system memory 820, NV memory 825, DSU 830, and EAVM 100 to send and to receive instructions or data thereto/therefrom. In one embodiment, NV memory 825 is a flash memory device. In other embodiments, NV memory 825 includes any one of read only memory (“ROM”), programmable ROM, erasable programmable ROM, electrically erasable programmable ROM, or the like. In one embodiment, system memory 820 includes random access memory (“RAM”), such as dynamic RAM (“DRAM”), synchronous DRAM, (“SDRAM”), double data rate SDRAM (“DDR SDRAM”) static RAM (“SRAM”), and the like. DSU 830 represents any storage device for software data, applications, and/or operating systems, but will most typically be a nonvolatile storage device. DSU 830 may optionally include one or more of an integrated drive electronic (“IDE”) hard disk, an enhanced IDE (“EIDE”) hard disk, a redundant array of independent disks (“RAID”), a small computer system interface (“SCSI”) hard disk, and the like.
In one embodiment, EAVM 100 is electrically coupled to processor 815 via signal electrode 125 and optically coupled to waveguide 810, via a butt connection or the like with surface aperture 195, such that processors 815 of each electronic circuit 805 may communicate over waveguide 810 at high speed. EAVMs 100 may be used as optical transmitters only, optical receivers only, or optical transceivers. Embodiments of waveguide 810 may include free space, an optic fiber, a planar waveguide, an integrated waveguide (e.g., rib waveguide integrated within a semiconductor die including both electronic circuits 805), and the like.
The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.
These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
