Devices with optical gain in silicon

A photonic device includes a silicon semiconductor based superlattice. The superlattice has a plurality of layers that form a plurality of repeating units. At least one of the layers in the repeating unit is an optically active layer with at least one species of rare earth ion.

BACKGROUND OF THE INVENTION

1. Field of Use

This invention relates generally to optical switching methods and apparatus, and more particularly to optical switching methods and apparatus that achieve optical gain in silicon.

2. Description of the Related Art

Communication networks increasingly rely upon optical fiber for high-speed, low-cost transmission. Optical fibers were originally envisioned as an optical replacement for electronic transmission media, such as high-speed coaxial cable and lower-speed twisted-pair cable. However, even high-speed optical fibers are limited by the electronics at the transmitting and receiving ends, generally rated at a few gigabits per second, although 40 Gb/s systems have been prototyped. Such high-speed electronic systems are expensive and still do not fully exploit the inherent bandwidth of fiber-optic systems, measured in many terabits per second.

All-optical transmission systems offer many intrinsic advantages over systems that use electronics within any part of the principal transmission path. Wavelength-division multiplexing (WDM) electronically impresses different data signals upon different carrier frequencies, all of which are carried by a single optical fiber. The earliest WDM systems did not provide optical switching but only point-to-point WDM.

To achieve optical gain in a semiconductor metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy processes have been used to produce complex nanostructures of layered materials such as InGaAs, InGaAsP and InGaAsN. These direct band gap semiconductor materials belonging to group III and V columns of the periodic table of elements are well known sources for LEDs, Lasers, and optical amplifiers. However, these materials make inferior quality high sensitivity photon detectors at fiber communication wavelengths.

It would be desirable to have silicon based lasers, LEDs and optical amplifiers because such devices would help to resolve the difficulties of integrating optical and electronic functions on a single chip. The high thermal conductivity of silicon can result in operational advantages. However, up to now efforts to obtain silicon based LEDs, lasers and amplifiers, especially devices that operate at wavelengths from about 1.3 or 1.5 μm, have not been successful.

There are currently three common methods of doping rare-earth ions into a silicon lattice. These methods are, (i) doping by growth of amorphous material from a silicon/rare-earth compound, (ii) doping by chemical vapor deposition and (iii) ion-implantation and rapid thermal anneal. The potential of utilizing rare-earth ions in a semiconductor matrix for the development of LED's and lasers, has been reported by H. Ennen, et al.,Applied Physics Letters, Vol. 43, page 943 (1983). It has also been observed that the presence of oxygen in erbium-doped silicon can increased erbium photoluminescence, P. M. Favennec et al.,Japanese Journal of Applied Physics, Vol. 29, page L524, (1990)].

Attempts to produce optical gain by the use of rare earths or the inclusion of materials in silicon have also been disclosed in U.S. Pat. Nos. 5,646,425; 5,119,460; 4,618,381; 5,634,973; 5,473,174; and 5,039,190.

However, efforts to create a commercially viable silicon based rare-earth doped LED, laser or optical amplifier have not been successful due at least in part to the fact that the observed luminescence has been too weak to support such a device. Even when weak gain was observed, the radiative lifetimes measured were six orders of magnitude longer than those exhibited by InGaAs devices, making the doped or implanted silicon structures inadequate for telecommunication applications.

There is a need for silicon based, rare-earth containing, optical devices that have sufficient luminescence and strong gain. There is a further need for silicon based, rare-earth containing optical devices that can switch the separate WDM channels, carrier frequencies, in different directions without the necessity of converting optical signals to electronic signals. There is yet a further need for silicon based, rare-earth containing optical devices that are integrated on the same monolithic chip as associated support circuitry. Yet there is another need for silicon based, rare-earth containing optical devices that amplify and/or attenuate light in preferred telecommunications wavelengths, including but not limited to 1250 to 1650 nm. Still there is a further need for silicon based, rare-earth containing optical devices that use avalanche multiplication effects of silicon coupled with sufficient optical gain due to the presence of an optically active rare-earth ion.

SUMMARY OF INVENTION

Accordingly, an object of the present invention is to provide silicon based, rare-earth containing optical devices that have sufficient luminescence and strong gain.

Another object of the present invention is to provide silicon based, rare-earth containing optical devices that have high rare-earth containing ion densities.

Another object of the present invention is to provide silicon based, rare-earth containing optical devices that can switch the separate WDM channels, carrier frequencies, in different directions without the necessity of converting optical signals to electronic signals.

Still another object of the present invention is to provide silicon based, rare-earth containing optical devices with ion densities of at least 1020ions per cubic cm.

Yet another object of the present invention is to provide silicon based, rare-earth containing optical devices with high densities of optically activated tri-valent rare earth.

Yet another object of the present invention is to provide silicon based, rare-earth containing optical devices with silicon-based crystal field engineering to control the symmetry of the atoms comprising the superlattice.

Yet another object of the present invention is to provide silicon based, rare-earth containing optical devices with high densities of optically activated tri-valent rare earth.

A further object of the present invention is to provide silicon based, rare-earth containing, periodic superlattice optical devices.

Yet another object of the present invention is to provide silicon based, rare-earth containing optical devices that are integrated on the same monolithic chip as associated support circuitry.

A further object of the present invention is to provide silicon based, rare-earth containing optical devices that amplify and/or attenuate light in preferred telecommunications wavelengths, including but not limited to 1250 to 1650 nm.

Another object of the present invention is to provide silicon based, rare-earth containing optical devices that use avalanche multiplication effects of silicon coupled with sufficient optical gain due to the presence of an optically active rare-earth ion.

These and other objects of the present invention are achieved in a structure. The structure includes a silicon based superlattice with a plurality of layers that form a plurality of repeating units. At least one of the layers is an active region with at least one rare earth ion. At least a portion of the superlattice is made of substantially a Group III–V or II–VI material.

In another embodiment of the present invention, a structure is provided for efficient excitation or de-excitation mechanisms of a crystal field engineered rare-earth silicon-based superlattice. A silicon semiconductor based superlattice is provided that includes a plurality of layers which form a plurality of repeating units. At least one of the levers is an optically active layer with at least one species of rare earth ion. A first layer of semiconductor material is included. A second layer of semiconductor material is also provided. The superlattice is sandwiched between the first and second layers. The first and second layers each have a wider bandgap than the superlattice.

DETAILED DESCRIPTION

Referring now toFIGS. 1(a) through2(d), in one embodiment of the present invention a photonic device10is a silicon based superlattice11. Superlattice11includes a plurality of individual layers12that form a plurality of repeating units14. At least one of layer12is an active region layer16with at least one rare earth ion.FIG. 1(b) illustrates one embodiment of superlattice11with an erbium trivalent ion inside active region layer16that generates a crystal field.

Preferably repeating units14are periodic. At least a portion of active region layer16can be a narrow or a wide band gap semiconductor. This can be achieved when the rare-earth ion is grown as part of a narrow band gap silicon-based material or when the rare-earth ion is introduced into the material effectively to make the material a narrow band gap material. Photonic device10can be an LED, amplifier, laser, photodetector, AWG, modulator, phototransistor, quantum logic gate, photonic bandgap structure, FET, MOSFET, HFET, HBT, waveguide, and the like.

In one embodiment, superlattice11includes at least one amorphous layers12. In another embodiment, at least one of the layers in at least one repeating unit14has an amorphous layer12. At least one crystal growth modifier can be included in an individual layer12of each repeating unit14. Suitable growth modifiers include but are not limited to C, As, P, B, H, O, N, Sn, Pb and the like. Additionally, during the deposition of each layer12, additional iso-electronic centers can be added, including but not limited to oxygen, nitrogen and the like, to enhance or activate the rare-earth and control the deposition growth or surface structure of the atoms (i.e., the reconstruction).

Photonic device10can be a component in a telecommunication system. In various embodiments, photonic device10can produce optical gain to drive a laser emission at a preferred wavelength, for example 1500 to 1650 nm, amplify an incident optical signal, overcome optical losses of other system elements, or detect a optical signal at a preferred wavelength. Photonic device includes a superlattice that can be made of a variety of different layers and combinations thereof, 12 including but not limited to, silicon, silicon germanium, silicon oxide, oxygen-doped silicon, RE-doped silicon, rare earth silicates (RexSi1-x), rare earth silicon germanium (Rex(SiGe)1-x) and the like.

In one embodiment, superlattice11consists of dissimilar materials of type A, B and C in repeating units14, for example, of the type (ABA . . . ABA), or (ABCABC . . . ABC). The number and composition of repeating units14is determined by the rare-earth ion interaction cross-section or density required; i.e., to increase interaction cross-section, either the number of layers is increased, or the rare earth density in each active layer is increased, or a combination of both. The density of the rare-earth ion is determined by the stoichiometry of the layer12which includes the rare-earth ion. Another embodiment is an ABCABC layer sequence with dissimilar materials A, B, and C.

Oxygen is an electronegative atom and very efficient in bonding the rare-earth into a trivalent state. Therefore, oxygen doped silicon or silicon-based silicides are useful as component layers of superlattice11. In one embodiment of the ABA repeating unit14layer sequence, the A layer is an oxygen deficient erbium silicide, represented as ErxSi1-xmaterial, or an erbium containing silicon-germanium material, represented as Erx(SiyGe1-y)1-x. The B layer is an oxygen containing non-stoichiometric or stoichiometric silicon-based material such as SiOx, SiGeOx, or oxygen-doped silicon.

In one specific embodiment, layer A is an erbium containing silicon-based layer that is oxygen deficient; layer B is a silicon-rich erbium and oxygen deficient material layer; and Layer C is an oxygen containing silicon-based layer deficient of rare-earth ions. In this particular structure the erbium and oxygen silicon-based layers, respectively, are separated spatially by a predominately silicon transition layer which is deficient in both erbium and oxygen. Permutations of the preceding can be grown in order to gain the highest compromise between epitaxial structural ordering for low defect density and optical activation of the rare-earth ion.

Photonic device10can have at least one spacer layer18between two adjacent repeating units14. Additionally, a spacer layer18can be positioned between more than one pair of adjacent repeating units14, including all adjacent repeating units14. The spacer layer is used to improve the structural quality, symmetry, optical quality, or electronic quality of the superalattice. Additionally, superlattice11can be positioned or grown on a substrate20, including but not limited to a silicon substrate, or on a pseudo-substrate buffer layer that has a lattice constant which is different from a lattice constant of a bulk silicon substrate. Where a psuedo substrate is defined as thick layer with low defect surface density that is grown over the substrate.

In one specific embodiment, superlattice11is grown on silicon substrate20along (001)- and (111), (211), (311), (411) and the like growth directions of the silicon substrate20. The growth of the lattice matched and/or lattice mismatched layers12can be epitaxially grown on silicon substrate20or on a pseudo substrate that can be a bulk or superlattice strained, or relaxed buffer layer.FIGS. 2(b) illustrates superlattice11growth in the (111) direction.FIG. 2(c) illustrates an in-plane erbium/silicon active layer crystal structure without defects grown on a (111)-orientated surface.

In various embodiments, active region layer16has a lattice layer that is less than, the same as or equal to a lattice constant of silicon substrate20or pseudo-substrate buffer layer. It may be preferred for active region layer16containing the rare-earth atoms to be in a mechanically stressed state when grown epitaxially on silicon substrate20or pseudo substrate by either tension, lattice mismatching or compression. This reduces the defect density which in turn improves structural quality.

In certain embodiments, at least one layer in a repeating unit14has a lattice constant that is sufficiently different from, (i) a lattice constant of substrate20to have an opposite state of mechanical stress or (ii) a lattice constant of pseudo-substrate buffer layer to have an opposite state of mechanical stress. In one embodiment, at least two layers12of repeating units14have substantially equal and opposite mechanical strain states and, (i) each repeating unit14is substantially lattice matched to substrate20or (ii) each repeating unit14is substantially lattice matched to pseudo-substrate buffer layer. Additionally, the crystal field of the superlattice can be modified by a strain field induced by lattice mismatched layers in a repeating unit.

One example of strain balanced growth is an erbium containing silicon-based layer12that exhibits a lattice constant smaller than bulk silicon and is a lattice mismatched layer12. Pseudomorphic growth of the lattice mismatched layer12can occur if the critical thickness of an erbium silicide layer12is not exceeded. The erbium silicide layer12is elastically deformed and under tensile strain.

The tensile strain can be balanced by the growth of an equal but opposite strain using, for example, a silicon germanium alloy where the amount of germanium is selected so the lattice constant is larger than that of bulk silicon and the layer thickness is tuned to counteract the tensile force of the erbium silicide layer with an equal but opposite compressive force. If both of these layers12are below the critical layer thickness and are stable, then the tensile and compressively strained layer pairs can be repeated N times, where N can be a very large number, for example up to 10,000, such that the total thickness of the resulting superlattice is up to three orders of magnitude larger than the critical layer thickness of each of individual strained layers12. In this embodiment, the pseudomorphically grown strain balanced superlattice can be grown free of interfacial misfit dislocations and thus substantially free of surface states and trap level defects This type of strain balanced growth further reduces the segregation problem of epitaxial growth using impurities or rare-earth ions by periodically trapping the rare-earth ions below rare-earth deficient silicon-based layers. This method substantially solves the problem during epitaxial growth of layers involving segregating species—as they behave as a surfactant and are thus difficult to incorporate at high densities.

Repeating units14can have, (i) uniform layer constructions., (ii) non-uniform layer constructions, (iii) thickness that vary as a function of distance along a superlattice growth direction, (iv) layer chemical compositions and layer thickness that vary as a function of distance along a superlattice growth (v), at least two individual layers12, (vi) at least two individual layers12that have different thickness, (vii) at least two individual layers12that are made of different compositions, (viii) at least three individual layers12that are made of different compositions, (ix) a silicon layer12that includes rare earth ions, (x) a silicon germanium layer12, (xi) a silicon oxide layer12, (xii) an oxygen-doped silicon layer12, (xiii) a rare earth silicide layer12, (xiv) a rare earth silicon germanium layer, (xv) an electrically doped p- or n-type layer12or (xvi) hydrogenated silicon12, silicon oxynitride12, hydrogenated silicon oxynitride, the same thickness and a thickness of the individual layers of the repeating units varies as a function of distance along a superlattice growth direction.

At least a portion of repeating units14can have different thickness, such superlattice11may be varied adjusted to control optical or structural properties that vary in the growth process. Each repeating unit can be repeated N times, where N is a whole or partial integer. Repeating units14cab be made of layers12that are thin. In one embodiment, the thin layers12have a thickness of 1000 Å or less and in another embodiment they are thin enough be non-bulk material layers12.

The rare earth ion is preferably Er, Pr, Nd, Nd, Eu, Ho or Yb, and more preferably Er. In one embodiment, the rare earth ion has an energy level that is determined by a geometric symmetry of a crystal field produced by the constituent atomic arrangement and layer geometry of the superlattice11.

The crystal field of photonic device10is configured to be variable by altering the composition of the individual layers12. This is achieved by the growth of several superlattices on top on each other, with each superlattice being a constant composition of repeating units14but the number of repeating units14can vary in each superlattice. Alternatively, the crystal field of photonic device10can be configured to be variable by altering the thickness and chemical composition of individual layers12.

Active region layer16can include two or more different rare earth ions. This facilitates optical or electrical pumping of one ion by energy transfer from the other ion. Additionally, active region layer16can have a lattice constant substantially different to bulk silicon. Varying the lattice constant enables the electrical and optical properties to be adjusted and optimized.

Additionally, as illustrated inFIG. 2(d), the present invention provides intra-band transition of three-dimensionally configured quantum well structures, e.g., a rare earth atomic transition can behave as a true quantum-dot. This is contrasted with inter-band transitions between conduction and valence bands.

Referring toFIG. 3, optical device10has a core structure with alternating layers of silicon or silicon based compositions and rare earths. The rare earth can be capped with top and bottom layers26and28that can be pure silicon layers. Electrical contacts30are applied by implanting electrical dopants, including but not limited to rare earths or metals, to form low Schottky barrier suicides through top cap26into a core of superlattice11. Electrical contacts30can be implanted. This implantation method is described in U.S. Pat. No. 4,394,673, incorporated herein by reference. Erbium or other rare-earth silicide is a suitable material for electrical contacts30because it is ideal for high speed operation of optical gain/loss device24due to the superior ohmic contact resistance of the material. Such materials include di-silicides, refractory metals and aluminum.

Optical gain/loss24can be a waveguide24for optical propagation by mode confinement through a refractive index change. Planar electrical devices using monolithic approaches involving selective implantation can be utilized with waveguide24including but not limited to, planar, or lateral, p-i-n, n-i-n or p-i-p. The in-plane carrier mobility of superlattice11is generally higher than vertical transport in a direction perpendicular to layers12. The present invention can also be a hybrid HBT or HFET device that provides multi-electro-optic mode control. This can be achieved by incorporating in-plane carrier extraction and injection using electrical contacts30in conjunction with the electric fields that are produced above and below superlattice11.

In one embodiment, illustrated inFIGS. 4(a) and4(b), optical device10is a switch32. At least two optical gain devices10are combined on a substrate34. InFIG. 4, an input36is split and coupled to first and second gain/loss devices24, that can be fabricated on substrate34, and to two outputs38. The split can cause loss. First and second gain/loss devices38are biased for gain or loss to produce amplification or attenuation. The basis of optical switch32uses the simultaneous gain in one waveguide24and loss in the other to channel light through the former and not the latter. Splitting loss can be overcome in one or both waveguides24.

InFIG. 4(b), an all optical multiplexer40includes two inputs36input fibers that are optically routed to any of four outputs38. TheFIG. 4(b) architecture can be scaled to include any number of inputs36, outputs38and gain/loss devices24to form the fundamental fabric of an optical network router and replaces optical-electrical-optical switching with an all optical architecture.

TheFIG. 4(b) architecture can be scaled to an N×N optical cross-connect (OXC)42illustrated inFIG. 5. In this embodiment, optical gain/loss devices24are curved rather than linear. Light is coupled into and out of substrate34with a number of “N” conventional linear waveguides44that crisscross substrate34to form an N×N grid pattern. Linear waveguides44traverse in orthogonal directions where each direction is on a different level in the growth direction of substrate34. Each curved optical gain/loss device24joins the orthogonal linear waveguides44. Light is coupled into and out of the linear waveguides44and there is suppressed mode interference over the region where curved gain/loss waveguides24join with and linear waveguides44. Optical gain/loss devices24overcome the losses associated with the suppressed mode coupling, and light is switched between two orthogonal inputs when optical gain is initiated in curved optical loss/gain devices24.

Referring now toFIG. 6, another embodiment of the present invention is a wavelength router/selector46. In this embodiment, optical switches32form optical gates that are combined with passive wavelength routers of the type disclosed in U.S. Pat. No. 5,351,146, incorporated herein by reference. The passive wavelength router splits the various DWDM channels into separate waveguide gratings and then directs them into appropriate output fibers.

Selection of an output port is determined by the choice of wavelength used. Because multiple wavelengths may be used on each output fiber, multiple simultaneous wavelength paths exist from each input fiber. The wavelength routing properties of wavelength router/selector46are periodic two ways. First, the spacing between frequencies for each output selection are equal. Second, multiple free spectral ranges exist in the wavelength router/selector46so that the optical muting property also repeats. This is achieved without power splitting loss because for a given wavelength constructive interference occurs only at the waveguide or optical path designed for that wavelength, an all other waveguides cause destructive interference which prevents energy coupling to them.

The use of optical switches32enables wavelength router/selector46to be a dynamically re-configurable, all-optical wavelength router. This eliminates the need for optical frequency changers at the interface between the Level-1 and Level-2 networks or within the Level-2 networks. In current networks, this function is performed by multiple optical-electrical-optical conversion which are expensive and bulky.

Referring now toFIG. 7, the present invention is also an optical receiver48where photons are converted into electrons. Optical receiver includes superlattice11positioned between a p-doped layer50and an n-doped layer52which can both be made substantially of silicon. Electrodes54and56are coupled to p-doped layer50and n-doped layer52. Electrodes54and56, in combination with other circuit elements, provide biasing, small-signal amplification and noise filtering. Superlattice11can be integrally formed with substrate20or pseudo substrate. Additional elements can be integrated with substrate20or pseudo substrate. This level of integration enables optimum speed and minimal noise, giving the best signal to noise ratio and excellent detection characteristics. The present invention is also a tunable or non-laser such as an edge emitting laser58ofFIG. 8or a VCSEL60ofFIG. 9.

Edge emitting laser58includes superlattice11in the plane of substrate20or pseudo-substrate. Edge-emitting laser58includes electrodes to excite superlattice11, lower and upper optical waveguide cladding layers, a high mobility silicon layer used for electronic transistor construction, n-type well field effect transistors (FET), a field effect, gate oxide layer, a silicon oxide isolation layer, and lateral oxidation of silicon layers that are used for electronic and superlattice11isolation.

With VCSEL60, light travels in superlattice11orthogonal to the plane of substrate20or pseudo substrate. First and second mirrors62and64define a resonant cavity. VCSEL60includes electrodes to excite superlattice11, lower and upper optical waveguide cladding layers, a high mobility silicon layer used for electronic transistor construction, n-type well field effect transistors (FET), a field effect, gate oxide layer, a silicon oxide isolation layer, lateral oxidation of silicon layers that are used for electronic and superlattice11isolation, and a micromachines silicon micro-lens array in substrate20.

Reflectors62and64can be grown as Bragg gratings, or produced by cleaving facets on the ends of substrate20or pseudo-substrate. This cleaving process is described in U.S. Pat. No. 5,719,077, incorporated herein by reference. The output wavelength of the lasers58and60can be tuned by varying the repeating unit14of superlattice11, which changes the crystal field and hence the transition energy of the laser transition. Bragg elements Acting either as an integral cavity mirror or external feedback element, can be fabricated at the output end of lasers58and60to provide feedback which limits and controls linewidth. Tuning and bandwidth can be controlled by varying period of repeating unit14.

FIG. 8illustrates one embodiment of the integration of a rare-earth crystal field superlattice11grown epitaxially on substrate20and pseudo-substrate. Following the completion of superlattice11a spacer layer65is grown to isolate a high mobility silicon layer that is suitable for Si CMOS VLSI. This example of an MBE grown epitaxial compound silicon-based substrate20or pseudo-substrate can then be processed to form ion-implantation doped regions, electrical contacts to the doped ion-implanted regions, silicon oxide field effect gate regions and dielectric isolation regions.

The optical gain material is located beneath the final high mobility silicon layer. Optical gain regions can be electrically isolated using ion-implantation or deep trenches. The optical mode of planar waveguide photonic circuits can be designed as ridge-type, buried core, stripe or implant diffused geometries. Relative temperature stability of the emission wavelengths of the gain spectrum allow high frequency and power silicon electronics to be operated simultaneously.

The optical waveguide mode can be confined in the core region by appropriate growth of suitable lower cladding material. This can be achieved for the core and cladding layers by selectively altering the layer refractive index via impurity doping; or via the use of silicon germanium alloys or the use of silicon oxide buried layers. The latter example, can be implemented for the lower cladding oxide layers using epi-ready separation by oxygen implantation (SIMOX) silicon wafers, silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) starting substrates20or pseudo-substrates.

With VCSEL60, very few layers12are required to form mirrors62and64because the index contrast between layers12can be made very large. With the use of a silicon-based material, the refractive index difference between silicon-based DBR mirror62and buffer is much higher and the required number of layers12for mirrors62and64can be as low as two to ten.

VCSEL60uses superlattice11as the gain medium. The cavity is formed by a high reflectivity quarter wave pair Bragg mirror62and output coupling quarter wave Bragg reflector64. The multi period high reflectivity mirror64can be grown/fabricated preceding superlattice11or as part of the final processing steps of the CMOS compatible process. The backside of substrate20can be micro-machined into arrays of micro-lenses or optical fiber receptacles. This process would allow simple alignment of optical-interconnects from chip-to-chip or fiber-to-chip.

Additionally, a wavelength tuning member, sensor and control loop can each be coupled to lasers58and60. In response to a detected change in temperature or optical power the control loop sends an adjustment signal to the tuning member and the tuning member adjusts a voltage or current supplied to laser58and60to provide a controlled output beam of selected wavelength.

The period of repeating unit14can be chirped across substrate20or pseudo substrate as shown inFIG. 10. This provides a controlled variation of wavelength based on the position on substrate20or pseudo substrate. The crystal field varies with physical period and composition of the superlattice, thus varying the period in a continuous fashion (i.e., chirp) causes a continuous shift in crystal field and therefore laser output wavelength. Multiple lasers with different wavelengths, separated by discrete steps, can be produced on a single substrate20or pseudo substrate. This provides discrete step tuning from a single component with internal circuitry simply by electronic selection of the appropriate wavelength laser, that can be created on the same substrate20or pseudo substrate using standard VLSI techniques. In this manner, lasers58and60, and their electronic switching fabric, reside on the a single substrate20or pseudo substrate. The appropriate laser wavelength is then selected by electrical input signals which on-board chip components decode, or by simple external wiring which can be grown as selective area growth MOCVD. With this concept, superlattice11can be initially grown as a structure that changes its layer12thickness uniformly across a cross-sectional area. This is useful for a transmitter in a DWDM system.

As illustrated inFIG. 11, superlattice111can also be chirped by varying the thickness of the alternating rare-earth layers. In this embodiment, both the bandwidth and center wavelength are controlled.

Referring now toFIG. 12, optical receiver48can be combined with laser58or60on the same substrate20or pseudo substrate to form a monolithic transceiver66. Circuitry68is also fabricated on the same substrate20or pseudo substrate. Circuitry68can include an electrical amplifier, signal processor, diode laser driver and the like. Circuitry68can be used to, bias optical receiver48and lasers58,60, amplify the photons detected by optical receiver48, drive and modulate laser58and60, and the like. Circuitry68enables conversion of photons into electrons and enable electrons to drive and modulate laser58and60. Monolithic transceiver66can be used to replace the discrete elements in a standard telecommunications router.

In another embodiment, illustrated inFIG. 13, a monolithic optical router70includes a plurality of lasers58and60and a plurality of optical receivers48all combined on a single substrate20or pseudo substrate with circuitry72. Circuitry72biases the plurality of lasers58and60and optical receivers48to amplify the photons that are detected and then drive and modulate the plurality of lasers58and60. An additional set of circuit elements forms an electrical switching fabric74that enables signals generated by one or more of the optical receivers48to be routed to any laser58and60. Monolithic optical router70enables optical signals on any one of an input to be switched to any one of the outputs.

Another embodiment of the present invention, illustrated inFIG. 14, is a wavelength converter76with at least two optical loss/gain devices24, of Data is carried via modulated on an optical signal at a first wavelength, and input to the waveguide. A second wavelength is input to the waveguide and mixes with the first wavelength. The modulation from the first wavelength is transferred to the second wavelength by cross-gain or cross-phase modulation. The traveling wave (single pass) gain receives simultaneously the modulated optical signal at the first wavelength and the second optical signal at the desired wavelength. The first optical signal affects the gain of the traveling wave gain as seen by the second optical signal so as to impress a representation of variations in the envelope of the first optical signal onto the second optical signal. The basic structure is that of a Mach-Zehnder interferometer with cross-phase modulation.

FIG. 15illustrates a parametric nonlinear optical element78embodiment of the present invention. Superlattice11forms a first waveguide80, which can be optical loss/gain device24, that is optimized for optical gain an a predetermined pump wavelength, e.g. 1540 nm. Additional superlattice structures11form second and third waveguides82and84on opposite sides of first waveguide80. Second and third waveguides82and84are optimized for side-band frequencies adjacent to the pump wavelength, including but not limited to 1650 nm and 1450 nm.

Second and third waveguides82and84are positioned sufficiently close (i.e., close to the wavelength of the light; e.g. 1.5–3 um for 1500 nm light) as to achieve evanescent wave-coupling to first waveguide80in order to allow coupling. When the pump wavelength propagates through first waveguide80sideband wavelengths are driven in second and third waveguides82and84, and energy flows from the pump wavelength to the sideband wavelengths. This creates a passive wavelength converter through nonlinear optical coupling. Subsequent to the conversion, additional optical loss/gain devices24can be placed in order to suppress the residual pump and enhance either or both of the sideband wavelengths. In this embodiment, parametric nonlinear optical element78becomes an active element which switches between adjacent DWDM channels to allow wavelength routing.

The present invention is also a quasi-phase matched nonlinear element86, as shown inFIG. 16. In this embodiment, superlattice11is grown with a periodic variation in refractive index in order to induce gain at signal or idler frequencies of the input beam to quasi-phase matched nonlinear element86. The periodic variation is chosen to achieve quasi-phase-matching through periodic refractive index variation at the appropriate frequency. In this embodiment, the active region layers16act as a nonlinear optical crystal whose gain can be electrically enhanced. The signal and idler frequencies are those of an optical parametric oscillator (OPO) in the 1000–2000 nm telecommunications band. Acting as an OPO, the active region layers16can shift an input DWDM wavelength to another wavelength with the energy difference carried off by the idler wavelength.

In another embodiment, illustrated inFIG. 17, multiple optical loss/gain devices24gain/loss elements are used in an all optical add-drop multiplexer (OADM)88. OADM88includes an optical demultiplexer90that splits an input WDM signal into individual optical signals, leading to respective 2×2 switches. Each switch has another input that originates from a plurality of add lines and selects one of its inputs to be dropped and the other to continue along a main signal path.

The retained signals may be modulated and attenuated prior to being tapped and finally multiplexed together by a WDM multiplexer92. The tapped signals are opto-electronically converted and fed back to a controller, which can include controller software, that controls the switching, modulation and attenuation. This permits remote control of OADM88functions by encoding instructions for the controller into a low-frequency dither signal that is embedded within the individual optical signals. OADM88can, in real time, be instructed to reroute traffic, dynamically equalize or otherwise change optical channel power levels, and add or remove dither. A specific optical channel may be reserved for control purposes, allowing a network administrator to “log in” to OADM88and override the controller software algorithm. Optionally, the optical signals can be tapped upon entry to OADM88. A di-directional OADM can be constructed from two unidirectional OADM's88that can share the same controller. Additionally, a single, general multi-input multi-output switch can be used to provide an arbitrary mapping between individual input and output optical signals.

Alternatively, as shown inFIG. 18, the present invention is also a two-dimensional photonic bandgap (2D-PBG) structure94implanted in the output path of the input beam or waveguide Bandgap structure94includes superlattice11with periodic variation, and repeating units14of bandgap structure94are selected to optimize the diffraction of light. Bandgap structure94consists of an array of predominately cylindrical ion-implantation disordering doped or physically etched regions either within or external to the superlattice, orthogonal to the plane of substrate20or pseudo substrate which act as a diffraction grating.

Scatter radiation emitted form bandgap structure94fans out at angles with the angle of diffraction being determined by the wavelength of the radiation. Thus the photonic bandgap structure acts as a diffraction grating but with substantially higher efficiency and customized dispersion. Unlike a diffraction grating, bandgap structure94directs spatially resolved wavelengths in the forward direction. This is advantageous for highly dispersive, integrated wavelength selective opto-electronic structures. Bandgap structure94can be an active DWDM filter that separates various wavelengths. The separated wavelengths can then be coupled into their own waveguides. Each waveguide can then be coupled to a switch32.

In another embodiment, illustrated inFIG. 19, the present invention is a selectable wavelength add/drop multiplexer96that has a concentric ring waveguide98fabricated in substrate20or pseudo-substrate to form a “Light Coral” of the type described by Nanovation, “The Micro revolution”, Technology Review” July-August 2000, incorporated herein by reference, in which light of a frequency resonant with ring waveguide98is selectively coupled out of one vertical waveguide, and into the other vertical waveguide, via ring waveguide98which includes a superlattice11with optical gain/loss device24to enhances or suppress the wavelength coupled into ring waveguide98. The addition of optical gain/loss device24makes ring waveguide98act as selectable wavelength add/drop multiplexer96.

In one embodiment of the present invention, a structure is provided includes a silicon based superlattice with a plurality of layers that form a plurality of repeating units. At least one of the layers is an active region layer with at least one rare earth ion. At least a portion of the superlattice is made of substantially a Group III–V or II–VI material.

In another embodiment of the present invention, a structure is provided for efficient excitation or de-excitation mechanisms of a crystal field engineered rare-earth silicon-based superlattice. A silicon semiconductor based superlattice is provided that includes a plurality of layers which form a plurality of repeating units. At least one of the layers is an optically active layer with at least one species of rare earth ion. A first layer of semiconductor material is included. A second layer of semiconductor material is also provided. The superlattice is sandwiched between the first and second layers. The first and second layers each have a wider bandgap than the superlattice.

The present invention can also be an actively equalized array waveguide grating shown inFIG. 6. By combining the properties of optical gain and detection in devices that include superlattice11provides integrated optical monitoring devices and systems. Input wavelength division multiplexed signals are separated into the constituent individual wavelengths by the arrayed waveguide grating (AWG) and propagate through individual waveguides. Superlattice11can attenuate or amplify each of these signals independently, thereby providing dynamic spectral gain control and equalization. For example, an AWG can include superlattice11. Waveguides24of the AWG can have multiple electrodes that are configured to either provide control of the gain or a photodetection of the propagating optical signal. Active feedback of the gain sections can be controlled by monitoring the optical power in each of the waveguides and thus provide the capability of actively equalizing the AWG.

A further embodiment of the present invention is an optical integrated circuit100, illustrated inFIG. 20that includes many of the FIG.1–FIG. 19devices and embodiments. Such a circuit combines photons and electrons into a single substrate, fabricated by a single process, and enables both optical and electrical gain and control to be integrated together. Full VLSI functionality including all electrical functions currently employed in silicon VLSI such as memory, switching, gain, computation, fuzzy logic, error checking, and restoration. Likewise, all optical functions currently achieved by discrete passive and active components, such as optical switching, wavelength filtering, optical mixing, amplification, loss, MUX/DEMUX, detection, modulation, laser output, LED, and nonlinear effects, can also be integrated through silicon VLSI.