Patent Application
20180284493
The present application pertains to the field of optical devices and in particular to optical devices which employ phase shifting.
Optical devices such as optical switches based on the free-carrier effect are widely used in data communications and processing. In particular, optical switches have a number of advantages over electrical switches, including switching speed, power consumption, and stability. There is also a wider range of opportunities to integrate multiple functions into an optical switch assembly, as compared with microelectromechanical (MEMS) switches, for example. Interferometer-based optical switches use optical phase shifters to achieve the required switching function.
Typically, optical devices such as optical switches and modulators are built on various platforms such as silica-on-silicon, AlGaAs/GaAs, InP, and others known in the art. Silicon-on-Insulator (SOI) platform is often seen as advantageous as it allows for a compact form factor based on the large refractive index contrast inherent in that platform.
A problem that arises with carrier-effect based optical phase shifting based devices is that a temperature difference (ΔT) may be induced in different parts of the device, as a result of self-heating due to driving part of the optical device with an electrical voltage. As a result of self-heating in the driven part, and no heating in parts of the optical device which are not driven, a ΔT results, causing the phase shift to deviate from the value that was optimized during device design. For example, in Mach-Zehnder interferometer-based optical switches using this type of phase shifter, self-heating can cause a temperature difference which induces an unexpected phase shift (namely phase error) between the second arm and the first arm of the device during the switching operation. This phase error causes a deterioration in the switch output contrast ratio.
The ΔT is problematic as the temperature coefficient of refractive index (dn/dT) for each of the arms of an optical device is typically greater than zero for an inorganic material such as silicon and silicon dioxide. As a result, a ΔT between the second arm and the first arm results in a difference in the refractive index of the first arm relative to the second arm proportionate to ΔT and dn/dT.
A difference in the refractive index induced by self-heating between the first arm and the second arm results in a time-varying extra phase shift from the desired phase shift between the two arms, which may increase the cross talk of an optical switch or induce an error in a phase shifter. As a result, carrier effect based optical devices are commonly prone to optical phase drift, which arises from self-heating of the first arm of the optical device during operation. An external compensation circuit could be used as a solution to compensate the phase drift caused by self-heating. However, this complicates the required control circuit, which has to take into account the time dynamics of self-heating.
Therefore, there is a need for an optical device that is resistant to driving current induced crosstalk, and other limitations of the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present application. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present application.
In an embodiment, an optical phase shifter may be provided. The optical phase shifter may include an optical waveguide core and a carrier modification element to induce a phase shift in the optical waveguide core. A cladding is disposed proximate to the optical waveguide core, the cladding having a temperature coefficient of refractive index (dn/dT)cl opposite in sign to a temperature coefficient of refractive index (dn/dT)a of the optical waveguide core.
In an embodiment, an optical interferometer is provided. The optical interferometer may include: a first arm defining a first arm optical path; a second arm defining a second arm optical path; a beam splitter for splitting an input optical beam into first and second sub-beams for propagating in the first and second arm optical paths, respectively; a beam combiner for combining the first and second sub-beams propagated in the first and second arm optical paths, respectively, into an output optical beam; the first arm including a carrier modification element to induce a phase shift in the first arm optical path relative to the second arm optical path; and, a cladding proximate to the first arm, the cladding having a temperature coefficient of refractive index (dn/dT)cl opposite in sign to a temperature coefficient of refractive index (dn/dT)a of the first arm.
In an embodiment, a method of manufacturing an optical interferometer is provided. The method may include: defining a silicon structure on a substrate; providing a p-type region and an n-type region on opposing sides of the silicon structure; and, depositing a cladding proximate to the silicon structure, the cladding having a cladding temperature coefficient of refractive index (dn/dT)cl opposite in sign to the silicon structure. In some implementations, an effective temperature coefficient of refractive index (dn/dT)e of a waveguide structure formed by the silicon structure and the cladding is less than an effective temperature coefficient of refractive index (dn/dT)e of a waveguide structure formed by the silicon structure alone. In some implementations, an effective temperature coefficient of refractive index (dn/dT)e of a waveguide structure formed by the silicon structure and the cladding is about zero.
Further features and advantages will become apparent from the following detailed description, taken in combination with the appended drawings, in which:
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
In an embodiment, an optical phase shifter is provided. The optical phase shifter can reduce sensitivity of the effective index of refraction of the structure to temperature. In an implementation, an optical interferometer is provided, the optical interferometer including at least one such optical phase shifter. In an implementation, an optical switch or a modulator is provided including at least one such optical interferometer.
In an embodiment, the optical interferometer has a first arm defining a first arm optical path and a second arm defining a second arm optical path. A beam splitter is provided for splitting an input optical beam into first and second sub-beams for propagating in the first arm optical path and the second arm optical path, respectively. A beam combiner is provided for combining the first and second sub-beams propagated in the first and second arm optical paths, respectively, into an output optical beam. The first arm includes a carrier modification element to induce a phase shift in the first arm optical path relative to the second arm optical path. The optical interferometer further includes a cladding proximate to the first arm. The cladding has a cladding temperature coefficient of refractive index (dn/dT)cl opposite in sign to a temperature coefficient of refractive index (dn/dT)a of the first arm. A cladding having an opposite in sign (dn/dT)cl cladding reduces the performance deterioration (i.e. extinction ratio deterioration) of the optical device caused by temperature changes between the first arm and the second arm that are a result of driving the optical device.
In an implementation, the first arm comprises a waveguide core and the carrier modification element comprises a p-n junction, or a p-i-n junction. The cladding may be disposed proximate to at least one surface of the waveguide core. In an implementation, the first arm comprises a combination of a waveguide core supported by a silicon insulating substrate and the carrier modification element, and the cladding may be disposed around at least a portion of the waveguide core.
In an implementation, the first arm comprises a semiconductor junction, such as p-n, or a p-i-n, junction, and the cladding is proximate to at least a portion of the p-n, or p-i-n, junction as the case may be.
In an implementation, the cladding is made of a non-semiconductor material. In an implementation, the cladding is a polymer.
In an implementation, the carrier modification element comprises a carrier injection element. In an implementation, the carrier injection element comprises a p-i-n junction. In an implementation, the carrier modification element comprises a carrier depletion element. In an implementation, the carrier depletion element comprises a p-n junction.
In an implementation, due to the combination of the cladding and the first arm, the optical device is less sensitive to temperature differences between the first arm and the second arm than an optical device without the cladding. In the implementation, an absolute value of an effective temperature coefficient of refractive index (dn/dT)e for the combination of the cladding and the first arm is less than an absolute value of a first arm temperature coefficient of refractive index (dn/dT)a.
In an implementation, due to the combination of the cladding and the first arm, the optical device is relatively insensitive to temperature differences between the first arm and the second arm. In the implementation, an effective temperature coefficient of refractive index (dn/dT)e for the combination of the cladding and the first arm is about zero.
In an implementation, the cladding is proximate to the first arm and the second arm. In the implementation, an effective temperature coefficient of refractive index (dn/dT)e for the combination of the cladding and the first arm is about zero and an effective temperature coefficient of refractive index (dn/dT)e for the combination of the cladding and the second arm is about zero. In the implementation, the refractive index for each of the first arm and the second arm in combination with the cladding is insensitive to temperature.
In an implementation, an optical device is provided including the above optical interferometer. In an implementation, the optical device is a modulator. In an implementation, the optical device is an optical switch.
Referring to
Operation of the optical interferometer 100 will now be considered. In general, the carrier modification element may be operative as a carrier injection element to inject carriers into the first arm 120. Alternatively, the carrier modification element may be operative as a carrier depletion element to deplete carriers from the first arm 130. In both cases activation of the first arm 120 will induce a phase shift in the first arm optical beam relative to the second arm optical beam. For example, the first arm 120 may be operative to selectively phase shift the first arm beam to an inverse phase from the second arm beam to produce a null output beam at the output 127 as the second arm beam and the first arm beam cancel in phase due to destructive optical interference.
In general, with these types of optical interferometers 100, successful operation relies upon a predictable and stable difference in phase between the first arm 120 and the second arm 115. A problem that has been identified with optical interferometers 100, is that the carrier modification element 122 imparts a heating effect on the first arm 120, raising its temperature relative to the second arm 115. This difference in temperature between the first arm 120 and the second arm 115 affects the relative optical properties of the first arm 120 and the second arm 115. In particular, the refractive index of the silicon based optical devices typically varies with temperature.
The temperature coefficient of refractive index (dn/dT) provides a measure of how an optical structure's refractive index changes based on temperature. Each component of the optical structure, the silicon substrate, substrate SiO2 insulator, etc. has its own temperature coefficient of refractive index (dn/dT). The effective temperature coefficient of refractive index (dn/dT)e of an optical structure as a whole results from the combined effect of the temperature coefficient of refractive index (dn/dT) for each of the components of the optical structure. Generally for conventional silicon-based optical devices the effective temperature coefficient of refractive index (dn/dT)e is non-zero since both silicon's temperature coefficient of refractive index (dn/dT)Si˜2×10−4 and common insulators, such as the example of SiO2 with a temperature coefficient of refractive index (dn/dT)SiO2˜1×10−5, are positive values and as a result are additive.
When the value of the effective temperature coefficient of refractive index (dn/dT)e are non-zero, silicon-based optical devices are thermally sensitive. With a change in temperature, the effective index of refraction changes and, as a result, a relative phase shift will be introduced. In the case of optical devices with temperature differences between their second arm 115 and first arm 120, the phase shift will lead to a phase error when the first arm optical beam and second arm optical beam are combined.
Referring to
In another example, not illustrated, the carrier modification element 122 comprises a carrier depletion element and the first arm 120 comprises a p-n junction. In this example, a driving voltage Vds applied to the p-n junction depletes carriers, leading to a phase shift Δθ in the first arm optical beam.
In some implementations, the optical interferometer 200 may function as a modulator, where an input 105 is modulated by operation of the carrier modification element 122, where a phase difference is created by operation of the carrier modification element 122 based upon a measurement signal being applied as the driving voltage Vds. In some implementations, the optical device 200 may act as an optical switch, where an output 127 is switched on or off by selective activation of the carrier modification element 122.
Embodiments of the present invention are applicable to a variety of optical devices where there may be temperature differences between two or more parts of the optical device leading to a difference in the index of refraction between those parts. By way of example, the optical device could form a part of a 2×2 optical structure, such as a 2×2 modulator or 2×2 switch. The illustrations in the present application show a structure with a single first arm and a single second arm for simplicity, but the invention may be extended to cover multiple first arms and second arms, and multiple phase shifters, as may be required.
In all of these examples, activation of the carrier modification element heats the first arm 120 creating a temperature difference between the first arm 120 and the second arm 115. The optical interferometer 200 of
Referring to
The doped regions could be, for instance, a phosphorous-doped region for the n-type region 146 and a p-type boron-doped region for the p-doped region 144. These are, however, only examples and not intended to be limiting. Connections and contacts to apply a driving voltage Vds to the doped regions 144 and 146 to activate the carrier injection element are not included in this illustration.
Referring to
In a conventional optical phase shifter, the insulator cladding 150 extends over the entirety of the doped regions 144, 146 and the silicon waveguide core 148. An effective temperature coefficient of refractive index (dn/dT)e for such a first arm 120 would be a non-zero positive value, based on the combination of silicon's temperature coefficient of refractive index (dn/dT)Si and the insulator's, such as SiO2, temperature coefficient of refractive index (dn/dT)SiO2.
Unlike the conventional optical device, the present optical phase shifter 300 of
The cladding 130 may be selected to have a cladding temperature coefficient of refractive index (dn/dT)cl opposite in sign to a first arm temperature coefficient of refractive index (dn/dT)a of the other structures of the phase shifter 300 in the first arm 120. Preferably the cladding temperature coefficient of refractive index (dn/dT)cl is both opposite in sign and sized to reduce an effective temperature coefficient of refractive index (dn/dT)e for the first arm 120 such that an absolute value of the effective temperature coefficient of refractive index (dn/dT)e is less than the first arm temperature coefficient of refractive index (dn/dT)a of the remaining structures of the first arm 120 without the cladding 130. More preferably, the cladding temperature coefficient of refractive index (dn/dT)cl is both opposite in sign and sized to reduce an effective temperature coefficient of refractive index (dn/dT)e for the first arm 120 to about zero.
Determination of an effective temperature coefficient of refractive index (dn/dT) for a given structure may be determined through numerical techniques known in the art that model the effect of combining multiple structures of differing temperature coefficient of refractive index (dn/dT). Employing the techniques, a specific material may be selected for the cladding 130 to produce a desired effective temperature coefficient of refractive index (dn/dT)e for a given structure.
In some implementations, the cladding 130 may be a polymer cladding. In some implementations, the polymer may be silicone (i.e. a polysiloxane) based. Other cladding materials are contemplated, provided they are compatible with the materials and manufacturing processes of the optical device, and provide the required magnitude of cladding temperature coefficient of refractive index (dn/dT)cl opposite in sign from the first arm temperature coefficient of refractive index (dn/dT)a of the remaining structures of the first arm 120.
In another example, an optical switch employing only an insulator cladding 150 such as silica (SiO2) without the cladding 130, was modelled with L=1000 μm, w_p-i-n=2.0 μm, and the first arm 120 being sufficiently far from the second arm 115 such that self-heating only affects the first arm 120 leading to maximum temperature difference between the first arm 120 and the second arm 115. For an example where the insulator cladding 150 has a (dn/dT)i=2×10−4 K−1. The temperature change required for a π phase shift is ΔTπ=λ/2L×(dn/dT)−1 or 3.9K. Accordingly, the phase shift per degree K is 0.26π/K. Results for driving the prior art version of the optical switch at the examples of carrier surface combination velocity (SRV) of an ideal waveguide/cladding interface (SRV=10 cm/s) and a typically real waveguide/cladding interface (SRV=10000 cm/s) are provided below in Table 1.
In comparison with the prior art version, the optical interferometer 200 in the form of an optical switch was modelled. The optical switch employing a cladding 130 having a cladding temperature coefficient of refractive index (dn/dT)cl=−5×10−4 K−1 based on a typical polymer such as silicone (e.g. polysiloxane). This example is taken from scenario c illustrated in
Based on the results indicated in Tables 1 and 2, the selected cladding 130 in this example provides for about a 50% improvement in self-induced switch crosstalk.
By way of example only, and not intended to be limiting, Table 3 illustrates some examples of siloxane polymers and their corresponding refractive index (dn/dT) values. Other suitable cladding materials including other siloxane polymers are considered within the scope of the invention.
−5 × 10−4
Although the present application describes specific features and embodiments, it is evident that various modifications and combinations can be made thereto without departing from the invention. The specification and drawings are, accordingly, to be regarded simply as an illustration as defined by the appended claims, and are contemplated to cover any and all modifications, variations, combinations or equivalents that fall within the scope of those claims.
