The present disclosure relates generally to optical devices and to an integrated variable optical attenuator using current injection on a multimode interferometer.
An optical attenuator is an electro-optical device that can be used to reduce a power level of an optical signal in an optical system. For example, an optical attenuator may be provided in an optical signal for equalizing powers in different channels, preventing saturation of photodetectors, or equalizing gains from different amplification sources, among other examples. An optical attenuator may be polarization independent and wavelength independent across a configured operational wavelength range and a configured operational temperature range to avoid introducing incorrect levels of attenuation or altering another characteristic, such as a polarization state or wavelength, of an optical beam.
In some implementations, a variable optical attenuator (VOA) includes a multi-mode interferometer (MMI), comprising: an optical waveguide, wherein a set of parameters of the optical waveguide is configured to cause a set of modes of an optical beam to interfere and self-image the optical beam; and a control component to apply a forward voltage across the VOA to control attenuation of the VOA, and wherein a change in a part of the refractive index of the optical waveguide is associated with carrier injection associated with the forward voltage.
In some implementations, an electro-optical device includes at least one MMI VOA, an MMI VOA, of the at least one MMI VOA, comprising: an input to receive an optical beam; an output to output the optical beam; and a multimode optical waveguide in between to couple the input to the output, a control component to apply a forward voltage across the MMI VOA to control attenuation of the MMI VOA,
In some implementations, an electro-optical device includes an MMI VOA, comprising: an input to receive an optical beam; an output to output the optical beam; and a multimode optical waveguide to couple the input to the output, wherein the optical waveguide is configured to self-imaging the optical beam within the optical waveguide; and a control component to apply a forward voltage across the MMI VOA to control attenuation of the MMI VOA.
The following detailed description of example implementations refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements.
Optical systems, such as optical communications systems, optical measurement systems, or optical testing systems, among other examples, may include an optical attenuator to apply attenuation to an optical beam. For example, an optical communications system may include an electro-optical device to apply attenuation to an optical beam for equalizing optical powers between different channels, avoiding saturation of a photodetector, or equalizing gains from different amplification sources, among other examples.
Variable optical attenuators (VOAs) may be used to apply different levels of attenuation to an optical beam at different times. For example, a Mach-Zehnder (MZ) interferometer (MZI) may be used for an MZ-VOA. For an integrated MZ-VOA, an optical beam is coupled into a chip (e.g., an indium phosphide (InP) receiver chip) and split into two arms of the MZI using a Y-branch splitter or a 1×2 multimode interferometer (MMI). As the split optical beam is passed through sections of the respective arms of the MZI, a phase shift is applied and, when the split optical beam is recombined, destructive interference between the two arms results in attenuation of the optical beam. In this example, the phase shift can be controlled in one arm of the MZI (e.g., in a single-ended drive scenario) or both arms of the MZI (e.g., in a differentially driven scenario) by changing dispersion properties of respective waveguides forming the arms of the MZI. The dispersion properties of the respective waveguides can be adjusted by applying heat to the respective waveguides (e.g., using a thermal phase electrode (TPE)) or by reverse biasing the MZI. However, using an MZ-VOA to attenuate an optical beam may result in excess optical loss as a result of the presence of couplers (e.g., at an input to the MZI) and combiners (e.g., at an output to the MZI). Additionally, MZIs may have relatively strong dependence on operating wavelength and chip temperature, which may result in optical loss and poorly calibrated performance. Additionally, physical asymmetries in arms of an MZI (e.g., as a result of fabrication tolerances) may result in further optical loss or poorly calibrated performance.
Another type of VOA that can be used is an electro-absorption (EA) VOA. An EA-VOA achieves a change in a refractive index (e.g., and a resulting attenuation) using an anisotropic electro-optical effect, which includes a linear electro-optic effect (e.g., the Pockels effect) and/or a non-linear electro-optic effect (e.g., the Kerr effect). The linear electro-optic effect occurs at relatively low magnitudes of applied electric field and in crystal structures without inversion symmetry. Accordingly, EA-VOAs are limited in waveguide material selection and orientation as well as in applied electric field strength. The non-linear electro-optic effect has a strong non-linear dependency on the electric field, which can result in distortion of an output optical signal. Accordingly, a strong dependency on applied bias, a large power dissipation, and a large temperature and wavelength dependency in EA-VOAs may result in difficult to configure feedback control loops.
Accordingly, it is desirable to provide an integrated VOA, such as for 100 Gigabit (Gb) or higher C or L-band receiver chips. For example, it is desirable for a VOA to have relatively strong temperature and wavelength independence, thereby enabling stable control of the VOA. Additionally, or alternatively, it is desirable for a VOA to use a relatively small range of control signals (e.g., a range of, for example, 0 Volts (V) to 3 V) to control attenuation in the VOA with linear increases in control signal currents providing a linear increase in optical attenuation (e.g., rather than a raised cosine transfer function relationship between current and optical attenuation in MZIs). Additionally, or alternatively, it is desirable to have a driving current of a VOA be relatively small (e.g., less than 30 milli-amps (mA)) to avoid excess power consumption requirements.
Some implementations described herein provide an efficient VOA using a multi-mode interferometer (MMI) with carrier injection. For example, a VOA may use carrier injection to cause a refractive index change within an MMI waveguide, which causes a destruction of the self-imaging phenomena within the MMI waveguide. This results in a reduced optical power coupled to the output waveguide. By controlling the refractive index change in the multimode waveguide, the degradation in the self-imaging phenomena can be controlled to achieve a desired optical attenuation of the optical beam in an output waveguide. Based on using carrier injection to cause the refractive index change in the MMI waveguide, the VOA achieves efficient attenuation over a wide range of optical wavelengths and chip temperatures, and achieves stable operation using a relatively small range of control signals and a relatively small driving current.
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The self-imaging is a property of electromagnetic field propagation in a multimode waveguide that has a constant lateral refractive index along the propagation direction, such as the MMI 110 of the MMI VOA. In self-imaging, the input optical beam replicates laterally at certain positions along the propagation direction. This replication may be single image, double image, triple image, or even more images depending on the design parameters of the MMI waveguide and optical signal wavelength. For example, as described in more detail with respect to
In some implementations, the MMI waveguide 110 may be coupled to a control component (not shown). For example, the MMI waveguide 110 may be associated with a bias source (e.g., an electrode or a set of electrodes) and a controller associated with controlling the bias source. In this case, the controller may generate and provide a control signal to cause the bias source to apply a biasing voltage, which controls attenuation of the MMI waveguide 110. As described in more detail below, applying the biasing voltage injects current into some regions of the MMI waveguide 110, which causes an asymmetrical change to the refractive index of the multimode MMI waveguide 110. Based on the asymmetrical change in the refractive index of the MMI waveguide 110, the self-imaging phenomena of the MMI waveguide 110 is disturbed resulting in a degradation of the power coupled to each image along propagation, and thus an attenuation in the optical power of the single image of the 1×1 MMI, as described in more detail below.
In some implementations, the MMI waveguide 110 may include a first section and a second section. For example, as shown in
In some implementations, the p-metal region 150 and the n-metal region 152 are a pair of ohmic contacts (e.g., aligned at a top and a bottom of an MMI) that cause a forward voltage to be applied across the MMI and cause a current to pass through the MMI. In some implementations, the pair of ohmic contacts cover only a portion of the MMI (e.g., less than an entirety of the optical core 154). In this case, as the current passes through the MMI (e.g., the optical core 154), carriers (e.g., electrons and holes) flow through the MMI and interact with an electromagnetic field propagating therethrough. The carriers cause a perturbation to a refractive index of the optical core 154 by changing absorption properties of the optical core 154. The change to the absorption properties and associated refractive index may be a result of a free carrier absorption (FCA) effect, a bandgap shrinkage effect, and/or a bandfilling effect. Based on changing the refractive index, a propagation of the electromagnetic field through the optical core 154 is perturbed, resulting in a change to the output optical beam relative to the input optical beam. For example, the output optical beam may be attenuated, split (into multiple beams), or combined (with another beam) relative to the input optical beam.
In some implementations, the p-metal region 150 and the n-metal region 152 may apply a forward voltage through only a portion of the optical core 154. For example, a geometry of the non-conductive region 140, and/or a position of the p-metal region 150 and the n-metal region 152 along a length of the MMI waveguide 110, may result in some portions of the optical core 154 being subject to different forward voltages than other portions (or a lack of forward voltage entirely). In this way, the refractive index of the optical core 154 can be selectively perturbed to control the attenuation (or other effect) on the optical beam passing through the MMI waveguide 110. In some implementations, geometric parameters (e.g., of the p-metal region 150, the n-metal region 152, or the non-conductive region 140, among other examples) may be controlled using an etching process, a cladding process, a masking process, or another type of layer formation or manufacturing process.
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By injecting current into multiple electro-optical devices 100 connected in series, the optical system 180 may achieve a higher level of attenuation relative to driving a single MMI VOA with the same total current. For example, the optical system 180 may achieve a higher level of attenuation (e.g., approximately twice the attenuation) at, for example, 10 mA, than is achieved by a single electro-optical device 100 driven with the same 10 mA. This may occur as a result of an attenuation slope of an electro-optical device 100 being greatest at lower currents. The filter 185 may be included in the optical system 180 to filter out higher order modes, which may be created by the disturbed self-imaging of an optical beam within each electro-optical device 100 that results from the current injection. For example, the filter 185 may be a passive 1×1 MMI (e.g., the first and second electro-optical devices 100 may be active MMIs) that is configured to filter higher order modes (e.g., modes higher than a first fundamental mode). In this way, the filter 185 ensures that only a fundamental mode is coupled into an output single mode waveguide of the optical system 180. Additionally, or alternatively, the filter 185 may improve attenuation efficiency.
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Process 300 may include additional implementations, such as any single implementation or any combination of implementations described herein.
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The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the implementations to the precise forms disclosed. Modifications and variations may be made in light of the above disclosure or may be acquired from practice of the implementations. Furthermore, any of the implementations described herein may be combined unless the foregoing disclosure expressly provides a reason that one or more implementations may not be combined.
As used herein, satisfying a threshold may, depending on the context, refer to a value being greater than the threshold, greater than or equal to the threshold, less than the threshold, less than or equal to the threshold, equal to the threshold, not equal to the threshold, or the like.
Even though particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure of various implementations. In fact, many of these features may be combined in ways not specifically recited in the claims and/or disclosed in the specification. Although each dependent claim listed below may directly depend on only one claim, the disclosure of various implementations includes each dependent claim in combination with every other claim in the claim set. As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover a, b, c, a-b, a-c, b-c, and a-b-c, as well as any combination with multiple of the same item.
When a component or one or more components (e.g., an MMI or one or more MMIs) is described or claimed (within a single claim or across multiple claims) as performing multiple operations or being configured to perform multiple operations, this language is intended to broadly cover a variety of architectures and environments. For example, unless explicitly claimed otherwise (e.g., via the use of “first component” and “second component” or other language that differentiates components in the claims), this language is intended to cover a single component performing or being configured to perform all of the operations, a group of components collectively performing or being configured to perform all of the operations, a first component performing or being configured to perform a first operation and a second component performing or being configured to perform a second operation, or any combination of components performing or being configured to perform the operations. For example, when a claim has the form “one or more components configured to: perform X; perform Y; and perform Z,” that claim should be interpreted to mean “one or more components configured to perform X; one or more (possibly different) components configured to perform Y; and one or more (also possibly different) components configured to perform Z.”
No element, act, or instruction used herein should be construed as critical or essential unless explicitly described as such. Also, as used herein, the articles “a” and “an” are intended to include one or more items, and may be used interchangeably with “one or more.” Further, as used herein, the article “the” is intended to include one or more items referenced in connection with the article “the” and may be used interchangeably with “the one or more.” Furthermore, as used herein, the term “set” is intended to include one or more items (e.g., related items, unrelated items, or a combination of related and unrelated items), and may be used interchangeably with “one or more.” Where only one item is intended, the phrase “only one” or similar language is used. Also, as used herein, the terms “has,” “have,” “having,” or the like are intended to be open-ended terms. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. Also, as used herein, the term “or” is intended to be inclusive when used in a series and may be used interchangeably with “and/or,” unless explicitly stated otherwise (e.g., if used in combination with “either” or “only one of”). Further, spatially relative terms, such as “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the apparatus, device, and/or element in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.
This patent application claims priority to U.S. Provisional Patent Application No. 63/607,962, filed on Dec. 8, 2023, and entitled “INTEGRATED VARIABLE OPTICAL ATTENUATOR.” The disclosure of the prior application is considered part of and is incorporated by reference into this patent application.
Number | Date | Country | |
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63607962 | Dec 2023 | US |