WDM signal detector

A detector includes a light detecting layer and a grating structure. The light detecting layer, which can be a photodiode, has an optical mode that resonates in the light detecting layer, and the grating structure is positioned to interact with the optical mode. The grating structure further couples incident light having a resonant frequency into the optical mode, and causes destructive interference to prevent light having the resonant frequency from escaping the detecting layer. The light detecting layer can be made transparent to light having other frequencies, so that a stack of such detectors, each having a different resonant frequency, can be integrated into a WDM detector that is compact and efficient.

BACKGROUND

Systems for detecting and demodulating wavelength division multiplexed (WDM) optical signals have conventionally deployed one of two approaches. One approach spatially separates the frequency components of the optical signal using an optical element such as a diffraction grating so that the frequency components traverse physically different paths to separate detectors. The separate detectors can then decode, demodulate, or otherwise convert the separated frequency components into respective electrical signals. A disadvantage of this approach is that the system must be relatively large to provide space for the separate optical paths and detectors. Another approach employs an array of detectors that are individually much smaller than the cross-section of the WDM signal. A different filter is positioned adjacent to each detector, so that each detector demodulates or converts only a single frequency component that the adjacent filter passes. A disadvantage of this approach is waste of optical power. In particular, the WDM signal must have a cross-section that is large enough to cover the area of multiple detectors, and each detector uses only a small fraction of the light incident on the area of the detector.

Resonant grating waveguide structures have been of interest for light separation and filtering. These structures employ gratings to couple specific wavelengths of incident light into waveguides. A simple configuration for a resonant grating waveguide includes a waveguide layer and a grating layer. The grating layer transmits a part of an incident light beam and diffracts a part of the incident light beam. The diffracted part enters the waveguide layer but through interaction with the grating layer can diffract out of the waveguide layer and interfere with the directly transmitted light. A resonant grating waveguide structure is designed to have a “resonance” such that incident light having a resonant frequency is coupled into the waveguide structure with high efficiently, while incident light at a non-resonant frequency passes through waveguide structure unaltered. The resonant frequency and the bandwidth of the resonance generally depend on the features of the grating and the waveguide layer. However, the bandwidth can be made sufficiently narrow for use in optical filters or separators.

A detector for WDM signals is desired that is compact and efficient in the use of optical power.

SUMMARY

In accordance with an aspect of the invention, a detector includes a light detecting layer and a grating structure. The light detecting layer has an optical mode that resonates in the light detecting layer, and the grating structure is positioned to interact with the optical mode. The grating structure also couples incident light having a resonant frequency into the optical mode and causes destructive interference to reduce or prevent transmission of light with the resonant frequency through the light detecting layer. The detector can be made transparent to non-resonant frequencies, so that a stack of such detectors, each having a different resonant frequency, can be integrated into a compact WDM detector.

DETAILED DESCRIPTION

In accordance with an aspect of the current invention, a wavelength division multiplexed (WDM) signal detector can include a stack of detector layers with each detector layer confining a different optical mode and measuring a WDM signal component corresponding to the confined mode. Each detector layer may contain a grating structure able to separate a target frequency component from incident light and confine the separated frequency component in an optical mode residing in or around a photodiode or other light detecting structure. In particular, the resonant light separated from a WDM signal can be trapped for a prolonged period of time (determined by the quality of the resonance) or equivalently can produce a greatly enhanced electric field in the vicinity of the detecting structure. Since only the resonant frequency of light is trapped, the efficiency of the detecting structure for detection of the light having the resonant frequency will be greatly enhanced compared to the efficiency at which non-resonant wavelengths are detected. Each detector layer in a stack can be further designed to transmit most non-resonant light to lower layers of the stack and to absorb most of the resonant light. As a result, different wavelength components of an incident WDM signal are detected at different depths in the stacked WDM detector. The WDM signal is not required to have a beam cross-section that is larger than the area of the WDM detector, and the WDM detector efficiently uses a high percentage of the incident light having each of the resonant frequencies.

FIG. 1Ashows a cross-section of a detector layer100in accordance with an embodiment of the invention. Detector layer100includes a grating structure110and a detecting structure120that may be constructed from multiple layers formed using integrated circuit processing techniques. In operation, a light beam I, which is incident on detector layer100, may contain light of multiple frequencies but particularly contains light having a frequency sometimes referred to herein as the resonant frequency of detector layer100. As described further below, detector layer100is designed to be highly efficient at detecting light that has the resonant frequency and at transmitting light having other frequencies. Light beam I is preferably a collimated beam but more generally has a beam divergence angle that is within the acceptance angle of detector layer100. Generally, structures having a broad resonance will have large acceptance angle. Beam I also has an angle of incidence selected according to design of the detector layer100, but in a typical configuration, light beam I would be incident normal to the surface of detector layer100Light beam I is illustrated inFIG. 1Aas being incident at an angle that helps to conceptually illustrate interfering rays.

Grating structure110is transparent but creates a refractive index variation in a pattern with dimensions selected to create interference effects at least for the resonant frequency of detector layer100.FIG. 1shows patterned structure110as being a separate layer overlying detecting structure120, but patterned structure110may alternatively be incorporated within or underlie detecting structure120. For example, grating structure110may incorporated in detecting structure120as a set of holes or doped or oxidized regions, so that an additional or separate layer is not required for grating structure110.

In an exemplary embodiment, grating structure110is a transparent diffraction grating.FIG. 1Billustrates a top view of detector layer100in an embodiment where grating structure110is a rectangular array of regions having a refractive index that differs from the refractive index of surrounding material. Other types of grating patterns could also be employed. For example, a grating with a hexagonal lattice could be used. A line grating might be employed in an embodiment where detector layer100measures light with a specific polarization and specific frequency since interference effects of a line grating differ depending on the orientation of a linear polarization of incident light relative to the direction of lines forming the grating. In general, any structure providing a periodic perturbation in refractive index could be used. The pattern required for grating structure110can be formed using conventional integrated lithographic masking techniques for patterned etching, doping, oxidation, or other treatment of detecting structure120or a separate layer formed above or below detecting structure120.

Grating structure110splits incident light I into directly transmitted light T and diffracted light D as shown inFIG. 1A. Detector layer100is generally made thinner than a wavelength of the light of interest, so that the amount of light reflected from detector layer100is small. Transmitted light T passes through detecting structure120, but detecting structure120is surrounded by material150and160having a lower refractive index so that diffracted light D can be trapped in a confined optical mode in and around detecting structure120. Grating structure110, which is located in detecting structure120or outside detecting structure120in an evanescent field of the confined optical mode, interacts with diffracted light D causing secondary diffraction. This secondary diffraction directs light S out of detecting structure120. Directly transmitted light T and secondary diffracted light S, which exit detecting structure120can interfere. For light of the resonant frequency of detector layer100, combination of transmitted light T and secondary light S results in complete destructive interference, so that the escape of light of the resonant frequency through the bottom surface detector layer100is limited or prevented. Detector layer100thus has a confined mode where light of the resonant frequency remains in detecting structure120for an extended period. Light having non-resonant frequencies are not confined in detecting structure120and are transmitted through detector layer100.

The dimensions, pattern, and refractive index of grating structure110and the thickness and refractive index of detecting structure120determine the relative amplitude and phase of transmitted light T and secondary diffracted light S and can be selected to create a resonance at a desired frequency. Rules for selecting the structural parameters need to produce a desired resonance are substantially the same as those known for resonant grating waveguide structures. For example, resonant grating structures such as described by Thuman et al., “Controlling the Spectral Response in Guided-Mode Resonance Filter Design,” Applied Optics, Vol. 42, No. 16, pp 3225-3233, (2003), which is hereby incorporated by reference in its entirety, could be altered to use detecting structures120in place of waveguides.

Detecting structure120in detector layer100is a photodiode including a layer122of P-type semiconductor material that forms a PN junction124with a layer126of N-type semiconductor material. Detecting structure120can alternatively employ other light detecting structures that provide similar optical characteristics, e.g., the same optical path length. The total thickness of detecting structure120is selected according to the desired resonant frequency of detector layer100as described above but typically will be a fraction of a wavelength or about 10 nm to 100 nm for visible light.

Trapping of the incident light of the resonant frequency in a confined mode in and around detecting structure120greatly enhances the efficiency with which detecting structure120absorbs light of the resonant frequency. The enhancement of detection efficiency of the resonant wavelength results because the confined light produces a strong electric field in detecting structure120or equivalently, because photons that are trapped in detecting structure120have more time to cause photoelectric effects in detecting structure120. Detector layer100is thus highly efficient at absorbing and detecting light having a frequency corresponding to the resonant frequency of detector layer100. Generally, a best condition for absorption comes at a critical coupling, where the rate of energy transfer between the incident light mode I and the confined mode is equal to the rate of energy dissipation, including scattering and absorption, of the confined mode. Under the critical coupling, the resonant component of the incident light beam will be completely dissipated in detecting structure120. If the rate of absorption is on the order of the scattering rate by grating structure110, almost 100% of the incident resonant light can be absorbed in detecting structure120to create electron hole pairs.

Electron-hole pairs created in detecting structure120are swept to respective electrodes by the biasing of junction124. Detector layer100when connected to a circuit (not shown) through contact structures130and132can thus generate a current having a magnitude that indicates the intensity of the component of incident light having the resonant frequency.

One fabrication process for detector layer100begins with growing or depositing a layer of N-type silicon to form semiconductor layer126. Semiconductor layer126would generally be grown on a substrate150that provides a base layer of a material such as silicon dioxide having a refractive index lower than that of layer126. Thin layers of semiconductor material such as Ge, SiGe, InxGa1-xAsyP1-y(where x, y depends on the desired resonant wavelength) with insitu doping can be grown or deposited with tightly controlled thicknesses in the required range (e.g., 10 nm to 100 nm) using known techniques such as LPCVD, MOCVD, MBE. The pattern of grating structure110or detecting structure120may be fabricated using techniques such as photolithography, electron-beam lithography, and nanoimprintlithography, but other techniques could alternatively be used. Layer126can uniformly cover an area for reception of light and may further extend laterally for contact structure130. P-type semiconductor is deposited on layer126to form semiconductor layer122and to create the PN junction124. Like layer126, layer124can uniformly cover the area for reception of light and may extend laterally for electrical contacts132. A layer of dielectric (such as Si3N4) about 5 nm to 500 nm thick can then be deposited on layer122and patterned using photolithography, electron-beam lithography, nanoimprintlithography, etching, or other suitable processes to form grating structure110. Alternatively, grating structure110can be formed by patterned, etching, oxidation, or other treatment of layer122or114. An insulating material160such as an oxide which has a refractive index that is lower than semiconductor layer122and different from grating structure110can then be deposited on grating structure110and semiconductor layer122. Conventional processing techniques can form electrical contacts130and132through insulating material160to provide electrical connections respectively to layers126and122.

FIG. 2shows a cross-sectional view of a detector layer200employing a grating structure210within a detecting structure220. In the embodiment ofFIG. 2, detecting structure220is a PIN photodiode including a P-type semiconductor layer222, an intrinsic semiconductor layer224, and an N-type semiconductor layer226. Grating structure210is formed in detecting structure220and particularly in intrinsic semiconductor layer224in the illustrated embodiment. Grating structure210could alternatively be in dope semiconductor layer222or226.

One fabrication process for detector layer200begins with depositing a layer of N-type silicon or other suitable semiconductor material to form semiconductor layer226on a substrate (not shown) that provides a base layer of a material such as silicon dioxide having a refractive index lower than that of layer226. Intrinsic layer224can then be deposited on layer226, and grating structure210can be formed in semiconductor layer226using a variety of alternative techniques. With one approach, a pattern of openings are etched in layer226and filled with a material having a refractive index that differs from that of layer226. Alternatively, areas of intrinsic semiconductor layer224can be oxidized, doped, or otherwise treated to alter the refractive index in the regions corresponding to grating structure210. For example, oxygen ion implantation, which is similar to SIMOX technology for SOI, can be use to create oxide regions. P-type silicon is then grown or deposited on intrinsic layer224to form layer222. Chemical mechanical polishing (CMP) can be applied during the fabrication process to improve planarity if necessary.

FIG. 3shows a cross-sectional view of a detector layer300. Detector layer300includes a grating structure210, a P-type semiconductor layer222, an intrinsic semiconductor layer224, and an N-type semiconductor layer226that are substantially as described above in regard toFIG. 2. Detector layer300differs from detector layer200in that detecting structure320of detector layer300includes quantum wells328in intrinsic layer224. As is known in the art, quantum wells328are regions having lower energy quantum states for electrons or holes, and the presence of quantum wells328can increase the detection efficiency of detecting structure320by providing regions with a high concentration of electron/hole states that are accessible at the energy of the photons to be detected.

FIG. 4illustrates an example of a WDM detector400in accordance with an embodiment of the invention containing multiple detector layers410-1to410-N, generically referred to herein as detector layers410. Each detector layer410may be structurally the same as a detector layer100,200, or300, as described above. Detector layers410are formed in a stack on a substrate420and separated from each other by layers of transparent insulating material430. Contact structures440are formed through insulator material430for electrical connection to detector layers410.

Detector layers410differ from each other in dimensions (e.g., thickness or grating structure) or composition so that each of detector layers410-1to410-N has a different resonant wavelength. In particular, if detector400is designed to detect or demodulate a WDM optical signal450containing components with frequencies f1to fN, detector layers410-1to420-N have resonances respectively corresponding to frequencies f1to fN. In operation, WDM signal450is directed onto to the surface of detector400and passes into a first detector layer410-1. Detector layer410-1captures the component of signal450having frequency f1. Detector layers410are all thin (e.g., less than the wavelengths of light in WDM signal450) and are nearly transparent to non-resonance frequencies. In particular, detector layer410-1is transparent the components of WDM signal450having frequencies f2to fN. Detector layer410-1when connected to an external circuit thus can produce a signal that is proportional to the intensity of the light component having frequency f1. The components of WDM optical signal450having the other frequencies f2to fNare absorbed at different depths in respective detector layers410-2to410-N, so that detectors410-2to410-N generate signals that respectively indicate the intensity of WDM signal components with frequencies f2to fN.

WDM detector400has several advantages. In particular, WDM detector400is compact and can be fabricated using integrated circuit manufacturing techniques, rather than requiring assembly of separate optical components such as filters or separators with electrical components such as photodiodes. WDM400also enables efficient signaling with a WDM optical signal having a beam profile that is smaller than the area of detector400, and a large portion of the optical energy is converted to electrical signals.

Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. For example, particular orders of layers and doping for specific embodiments of the invention have been described, but it will be understood by those skilled in the art that those configurations can be changed. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.