Optical coupler at interface between light sensor and waveguide

A method of operating an optical device includes inserting a light signal into a waveguide positioned on a base. A light sensor is positioned on the base and is configured to receive the light signal after the light signal exits from the waveguide. The method also includes elevating the light signal relative to the base before the light signal enters the light sensor. The light signal is elevated such that the light signal enters the light-absorbing medium in a position that is elevated above the position where the light signal would enter the light-absorbing medium if the light-absorbing medium received the entire light signal directly from the waveguide.

FIELD

The present invention relates to optical devices and more particularly to devices having an interface between a light sensor and an optical waveguide.

BACKGROUND

The use of optical and/or optoelectronic devices is increasing in communications applications. These device often include light sensors that receive light signals from a waveguide. These light sensors often employ a light absorbing material that absorbs light signals. During operation of the light sensor, an electrical field is present across the light absorbing material. When the light absorbing material absorbs a light signal, an electrical current flows through the light absorbing material. As a result, the level of electrical current through the light absorbing material indicates the intensity of light signals being received by the light absorbing material.

These waveguides that are present on optical and/or optoelectronic devices are often made of silicon. Because silicon does not absorb the light signals having the wavelengths that are used in communications applications, silicon is often not effective for use as the light absorbing medium in the light sensors for communications application. In contrast, germanium is a material that can absorb these light signals and is accordingly often used as the light absorbing medium in the light sensors for communications application. However, the use of germanium in combination with silicon waveguides can be associated with undesirably high levels of dark current. Dark current is the flow of electrical current through the light sensor when the light sensor is not receiving light signals. As a result, dark current is a source of noise for these light sensors.

For the above reasons, there is a need for an improved interface between waveguides and light sensors.

SUMMARY

An example of one embodiment of the optical device includes a waveguide on a base. The waveguide is configured to guide a light signal through a light-transmitting medium. The waveguide has a coupled portion and an uncoupled portion. An optical coupler is positioned on the coupled portion but not on the uncoupled portion of the waveguide. The coupled portion of the waveguide is between the optical coupler and the base. The optical coupler is configured such that at least a portion of the light signal enters the optical coupler from the waveguide. A light sensor is positioned on the base with the light-transmitting medium being between the light sensor and the base. The light sensor includes a light-absorbing medium configured to receive at least a portion of the light signal from the optical coupler. The light-absorbing medium is different from the light-transmitting medium.

A method of operating an optical device includes inserting a light signal into a waveguide positioned on a base. A light sensor is positioned on the base and is configured to receive the light signal after the light signal exits from the waveguide. The method also includes elevating the light signal relative to the base before the light signal enters the light sensor. The light signal is elevated such that the light signal enters the light-absorbing medium in a position that is elevated above the position where the light signal would enter the light-absorbing medium if the light-absorbing medium received the entire light signal directly from the waveguide.

DESCRIPTION

The optical device includes a light transmitting medium on a base. The device also includes a waveguide configured to guide a light signal through the light-transmitting medium. The optical device also includes a light sensor configured to receive the light signal from the waveguide. The light sensor includes a light-absorbing medium positioned such that a seed portion of the light-transmitting medium is between the light-absorbing medium and the base.

The waveguide includes a coupled region and uncoupled region. An optical coupler is positioned on the coupled region of the waveguide but not on the uncoupled region of the waveguide. The optical coupler is configured such that at least a portion of the light signal enters the optical coupler from the coupled portion of the waveguide. The portion of the light signal that enters the optical coupler is the coupled portion of the light signal. The entry of the coupled portion into the optical coupler elevates the light signal relative to the base.

The light-absorbing medium receives at least the coupled portion of the light signal from the optical coupler. Since the optical coupler elevates the light signal relative to the base before the light signal enters the light-absorbing medium, the light signal enters the light-absorbing medium in a position that is elevated above the position where the light signal would enter the light-absorbing medium if the coupler were not present. The elevation of the light signal in the light-absorbing medium moves the light signal away from the seed portion of the light-transmitting medium. Interaction of the light signal with the seed portion of the light-transmitting medium is a source of dark current. As a result, movement of the light signal away from the seed portion of the light-transmitting medium, reduces the dark current associated with the light sensor.

FIG. 1AthroughFIG. 1Dillustrate an optical device.FIG. 1Ais a topview of the device.FIG. 1Bis a cross-section of the device shown inFIG. 1Ataken along the line labeled B.FIG. 1Cis a cross-section of the device shown inFIG. 1Btaken along the line labeled C.FIG. 1Dis a cross-section of the optical device taken through the longitudinal axis of the waveguide on the device.

The device is within the class of optical devices known as planar optical devices. These devices typically include one or more waveguides immobilized relative to a substrate or a base. The direction of propagation of light signals along the waveguides is generally parallel to a plane of the device. Examples of the plane of the device include the top side of the base, the bottom side of the base, the top side of the substrate, and/or the bottom side of the substrate.

The illustrated device includes lateral sides10(or edges) extending from a top side12to a bottom side14. The propagation direction of light signals along the length of the waveguides on a planar optical device generally extend through the lateral sides10of the device. The top side12and the bottom side14of the device are non-lateral sides.

The device includes one or more waveguides16that carry light signals to and/or from optical components17. Examples of optical components17that can be included on the device include, but are not limited to, one or more components selected from a group consisting of facets through which light signals can enter and/or exit a waveguide, entry/exit ports through which light signals can enter and/or exit a waveguide from above or below the device, multiplexers for combining multiple light signals onto a single waveguide, demultiplexers for separating multiple light signals such that different light signals are received on different waveguides, optical couplers36, optical switches, lasers that act a source of a light signal, amplifiers for amplifying the intensity of a light signal, attenuators for attenuating the intensity of a light signal, modulators for modulating a signal onto a light signal, light sensors29that convert an light signal to an electrical signal, and vias that provide an optical pathway for a light signal traveling through the device from the bottom side14of the device to the top side12of the device. Additionally, the device can optionally, include electrical components. For instance, the device can include electrical connections for applying a potential or current to a waveguide and/or for controlling other components on the optical device.

The waveguide16is defined in a light-transmitting medium18positioned on a base20. The light-transmitting medium18includes a ridge22defined by trenches24extending partially into the light-transmitting medium18or through the light-transmitting medium18. Suitable light-transmitting media include, but are not limited to, silicon, polymers, silica, SiN, GaAs, InP and LiNbO3. A fourth light-transmitting medium26is optionally positioned on the light-light transmitting medium. The fourth light-transmitting medium26can serve as a cladding for the waveguide16and/or for the device. When the light-transmitting medium18is silicon, suitable fourth light-transmitting media26include, but are not limited to, silicon, polymers, silica, SiN, GaAs, InP and LiNbO3.

The portion of the base20adjacent to the light-transmitting medium18is configured to reflect light signals from the waveguide16back into the waveguide16in order to constrain light signals in the waveguide16. For instance, the portion of the base20adjacent to the light-transmitting medium18can be an optical insulator27with a lower index of refraction than the light-transmitting medium18. The drop in the index of refraction can cause reflection of a light signal from the light-transmitting medium18back into the light-transmitting medium18. The base20can include the optical insulator27positioned on a substrate28. As will become evident below, the substrate28can be configured to transmit light signals. For instance, the substrate28can be constructed of a light-transmitting medium18that is different from the light-transmitting medium18or the same as the light-transmitting medium18. In one example, the device is constructed on a silicon-on-insulator wafer. A silicon-on-insulator wafer includes a silicon layer that serves as the light-transmitting medium18. The silicon-on-insulator wafer also includes a layer of silica positioned on a silicon substrate. The layer of silica can serving as the optical insulator27and the silicon substrate can serve as the substrate28.

The optical device also includes a light sensor29configured to receive a light signal guided by the one or more waveguides16. The light sensor29is configured to convert the light signal to an electrical signal. Accordingly, the light signal can be employed to detect receipt of light signals. For instance, the light sensor29can be employed to measure the intensity of a light signal and/or power of a light signal. AlthoughFIG. 1illustrates a waveguide16carrying the light signal between the one or more components and the light sensor29, the device can be constructed such that the waveguide16carries the light signal directly from an optical fiber to the light sensor29.

A suitable light sensor29includes a light-absorbing medium32that absorbs light signals. During operation of the light sensor29, an electrical field is applied across the light-absorbing medium32. When the light-absorbing medium32absorbs a light signal, an electrical current flows through the light-absorbing medium32. As a result, the level of electrical current through the light-absorbing medium32indicates receipt of a light signal. Additionally, the magnitude of the current can indicate the power and/or intensity of the light signal. Different light-absorbing medium32can absorb different wavelengths and are accordingly suitable for use in a sensor29depending on the function of the sensor29. A light-absorbing medium32that is suitable for detection of light signals used in communications applications include, but are not limited to, germanium, silicon germanium, silicon germanium quantum well, GaAs, and InP. Germanium is suitable for detection of light signals having wavelengths in a range of 1300 nm to 1600 nm.

The light-absorbing medium32of the light sensor29is positioned on a seed portion34of the light-transmitting medium18. The seed portion34of the light-transmitting medium18can be continuous with the light-transmitting medium18included in the waveguide16or spaced apart from the waveguide16. During the fabrication of the device, the seed portion34of the light-transmitting medium18can be used to grow the light-absorbing medium32. For instance, when the light-transmitting medium18is silicon and the light-absorbing medium32is germanium, the germanium can be grown on the silicon. As a result, the use of the light-transmitting medium18in both the waveguides16and as a seed layer for growth of the light-absorbing medium32can simplify the process for fabricating the device.

The device includes an optical coupler36at the interface of the light-absorbing medium32and waveguide16. The optical coupler36is positioned on the waveguide16such that the waveguide16is positioned between the base20and the optical coupler36. The optical coupler36is positioned on a coupled portion38of the waveguide16but is not positioned on an uncoupled portion40of the waveguide16. The optical coupler36can be positioned such that the waveguide16is between the optical coupler36and the base20.

The optical coupler36can include or consist of a coupler medium through which the light signal travels. The coupler medium can be in contact with the light-transmitting medium18. For instance, the entry facet42of the optical coupler36can contact the light-transmitting medium18as illustrated inFIG. 1D. Alternately, there can be one or more layers of medium between the entry facet42of the optical coupler36and the light-transmitting medium18. The one or more layers can include or consist of one or more layers of anti-reflective coating and/or one or more layers of index matching medium.

The coupler medium can contact the light-absorbing medium32. For instance, an exit facet44of the optical coupler36can contact the light-absorbing medium32as illustrated inFIG. 1D. Alternately, there can be one or more layers of medium between the exit facet44of the optical coupler36and the light-absorbing medium32. The one or more layers can include or consist of one or more layers of anti-reflective coating and/or one or more layers of index matching medium.

The optical coupler36is configured such that a coupled portion38of a light signal enters the optical coupler36from the waveguide16. For instance, a coupled portion38of the light signal enters the optical coupler36through the entry facet42. The coupled portion38of the light signal can include all of the light signal traveling along the uncoupled portion40of the waveguide16or a portion of the light signal traveling along the uncoupled portion40of the waveguide16.

The optical coupler36is also configured such that the coupled portion38of the light signal exits from the optical coupler36. The optical coupler36is positioned such that the light-absorbing medium32receives at least the coupled portion38of the light signal from the optical coupler36. The optical coupler36can be configured such that an coupled portion38of the light signal remains in the waveguide16. As a result, the light signal can exit from both the exit facet44of the optical coupler36and a waveguide facet46. The light-absorbing medium32can be positioned to receive the coupled portion38of the light signal that exits from the exit face of the optical coupler36and the uncoupled portion38of the light signal that exits from the waveguide facet46.

The optical coupler36is configured such that the entry of the coupled portion38into the optical coupler36elevates the light signal relative to the base20as is evident from the arrow labeled E. For instance, the position of the optical coupler36over the waveguide16combined with the entry of the coupled portion38of the light signal into the optical coupler36elevates the light signal. Since the optical coupler36elevates the light signal relative to the base20before the light signal enters the light-absorbing medium32, the light signal enters the light-absorbing medium32in a position that is elevated above the position where the light signal would enter the light-absorbing medium32if the coupler were not present. As is evident from the arrow labeled E, the elevation of the light signal in the light-absorbing medium32reduces the interaction of the light signal with the seed portion34of the light-transmitting medium18.

The coupler medium includes a vertical taper. The vertical taper can be an adiabatic taper to reduce excitation of higher order modes before the light signal enters the light-absorbing medium32. The use of an adiabatic taper may be desirable when the waveguide16is a single mode waveguide16and may be less desirable when the waveguide16is a multi-mode waveguide16.

The elevated position of the light signal in the light-absorbing medium32moves the light signal away from the seed portion34of the light-transmitting medium18. The interaction of the light signal with the seed portion34of the light-transmitting medium18is a source of dark current in the light sensor29. As a result, the movement of the light signal away from the seed portion34of the light-transmitting medium18reduces the dark current associated with the light sensor29.

The coupler medium can include a horizontal taper as illustrated inFIG. 2A. The horizontal taper is also constructed to elevate the light signal relative to the base20as is evident from the arrow labeled E.

Although not illustrated, the coupler medium can include both a horizontal taper and a vertical taper.

The coupler medium can be a multi-mode interference device (MMI) as shown inFIG. 2B. The multi-mode interference device can cause excitation of higher order modes. When the waveguide16is a single mode waveguide, the portion of the light signal that is coupled into the multi-mode interference device increases as the length of the multi-mode interference device increases. In some instances, the multi-mode interference device (MMI) is long enough for the entire light signal to be coupled into the multi-mode interference device. As a result, the multi-mode interference device (MMI) can elevate the entire light signal relative to the base20. A suitable multi-mode interference device (MMI) can exclude tapers and can have six sides that are each substantially rectangular.

FIG. 1AthroughFIG. 2Billustrate the optical coupler36and the waveguide16as different pieces of the device. As a result, there is an interface between the light-transmitting medium18and the coupler medium. For instance, the interface can be an interface according to the growth of the coupler medium on the light-transmitting medium18or depositing the coupler medium on the light-transmitting medium18.

The coupler medium can be the same as the light-transmitting medium18or different from the light-transmitting medium18. In some instances, the coupler medium has an index of refraction that is greater than or equal to an index of refraction of the light-transmitting medium18. In an example, the optical coupler36includes or consists of crystal silicon or amorphous silicon and the light-transmitting medium18includes or consists of crystal silicon.

FIG. 3AandFIG. 3Billustrate an example construction of a light sensor29.FIG. 3Ais a perspective view of a light sensor29positioned on the optical device.FIG. 3Bis a cross-section of the light sensor29shown inFIG. 3Ataken along the line labeled C inFIG. 3A.

As discussed above, the light sensor29includes a light-absorbing medium32. The light-absorbing medium32includes doped regions50. Each of the doped regions50can be an N-type doped regions50or a P-type doped region50. For instance, each of the N-type doped regions50can include an N-type dopant and each of the P-type doped regions50can include a P-type dopant.

In some instances, the light-absorbing medium32includes a doped region50that is an N-type doped region50and the doped region50that is a P-type doped region50. The separation between the doped regions50in the light-absorbing medium32results in the formation of PIN (p-type region-insulator-n-type region) junction in the light sensor29.

Suitable dopants for N-type regions include, but are not limited to, phosphorus and/or arsenic. Suitable dopants for P-type regions include, but are not limited to, boron. A suitable concentration for the P-type dopant in a P-type doped region50includes, but is not limited to, concentrations greater than 1×1015cm−3, 1×1017cm−3, or 1×1019cm−3, and/or less than 1×1017cm−3, 1×1019cm−3, or 1×1021cm−3. A suitable concentration for the N-type dopant in an N-type doped region50includes, but is not limited to, concentrations greater than 1×1011cm−3, 1×1017cm−3, or 1×1019cm−3, and/or less than 1×1017cm−3, 1×1019cm−3, or 1×1021cm−3.

Electrical conductors52are each in contact with a different one of the doped regions50. Suitable electrical conductors52include, but are not limited to, metals. The device can be used in conjunction with electronics that are in electrical communication with the electrical conductors52. For instance, when the light-absorbing medium32includes a PIN junction, the electronics can apply a reverse bias across the PIN junction. When the light-absorbing medium32receives a light signal, an electrical current flows through the light-absorbing medium32indicating the receipt of the light signal.

The optical device can be constructed using fabrication technologies that are employing in the fabrication of integrated circuits, opto-electronic circuits, and/or optical devices. For instance, the ridge22for the waveguide16and/or the seed portion34can be formed in the light-transmitting medium18using etching technologies on a silicon-on-insulator wafer. Additionally or alternately, when the light-transmitting medium18is silicon and the coupler medium is amorphous or crystal silicon, the coupler medium can be deposited on the light-transmitting medium18. Horizontal tapers can be readily formed using masking and etching technologies. Suitable methods for forming vertical tapers are disclosed in U.S. patent application Ser. No. 10/345,709, filed on Jan. 15, 2003, entitled “Controlled Selectivity Etch for Use with Optical Component Fabrication,” and incorporated herein in its entirety. Additionally or alternately, when the light-transmitting medium18is silicon and the light-absorbing medium32is germanium, the germanium can be grown on the silicon.

AlthoughFIG. 2AthroughFIG. 3Billustrate the top of the seed portion34of the light-transmitting medium18being above the trench24, the trench24and the top of the seed portion34of the light-transmitting medium18can be at the same level.

There can be additional mediums over the optical devices illustrated inFIG. 1AthroughFIG. 3B. For instance, there can be one or more cladding layers over the waveguide16, optical coupler36and/or light-absorbing medium32. Examples of suitable cladding layers include, but are not limited to, silica.

AlthoughFIG. 1AthroughFIG. 3Billustrate the optical coupler36and waveguide16as different pieces of the device, they need not be different pieces of the device. For instance, the transition from the waveguide16into the optical coupler36can be continuous without an interface between the optical coupler36and the waveguide16. As an example, an the end of the waveguide16can be vertically tapered. The vertically tapered portion of the waveguide16can provide the end of the waveguide16with a shape that approximates the combined shape of the waveguide16and optical coupler36shown inFIG. 2A.