Optical Filter and Method

Abstract

Methods and apparatuses for optical filtering are disclosed. According to an embodiment, an optical filter for an optical network is provided, the optical filter being configured to adaptively add or remove target wavelengths in a predetermined filtering range. The optical filter includes: a first resonator configured to have a first resonant wavelength outside a first sub-range of the predetermined filtering range when a first resonant control variable of the first resonator is set to a first value, and a second resonant wavelength within the first sub-range of the predetermined filtering range when the first resonant control variable of the first resonator is set to a second value; and a second resonator configured to have a third resonant wavelength outside a second sub-range of the predetermined filtering range when a second resonant control variable of the second resonator is set to a third value, and a fourth resonant wavelength within the second sub-range of the predetermined filtering range when the second resonant control variable of the second resonator is set to a fourth value.

Description

Technical Field

[0001] Embodiments of the present disclosure generally relate to optical filters, and more particularly, to optical filters for optical networks. Background Art

[0002] This section introduces aspects that contribute to a better understanding of the present disclosure. Therefore, the statements in this section should be read from this perspective and should not be construed as admitting the existence or non-existence of content in the prior art.

[0003] Tunable optical filters can play a key role in the deployment of wavelength division multiplexing (WDM) networks in order to select any received wavelength at any port. They can be used either to implement reconfigurable optical add-drop multiplexers (ROADMs) or to be embedded in WDM transceivers, in front of the photodetectors. In both cases, tunable optical filters bring flexibility to network planning and its upgrade, support software reconfiguration, and reduce inventory costs.

[0004] When used in ROADMs, tunable optical filters relieve the operator of the task of deploying and storing multiple variants of fixed optical add-drop multiplexers (OADMs), where each fixed OADM corresponds to a specific set of wavelengths, and replace the fixed OADMs with a single reconfigurable device. This brings the advantage of simplified network planning and saves the acquisition and maintenance costs of spare parts (spares) necessary to cope with possible failures, since a failure can be resolved with a single spare tunable device.

[0005] When a tunable optical filter is embedded in a WDM transceiver, the transceiver with such an embedded tunable optical filter can be used in scenarios where the WDM network utilizes an existing access network infrastructure configured with a passive splitter without wavelength selection capabilities. This would be the case where the WDM network covers an existing passive optical network (PON), such as for accessing 5G towers via a bidirectional connection.

[0006] Figure 1An example of a PON with WDM coverage implemented by a tunable optical filter is shown. As shown in this figure, a centralized unit (CU) or a distributed unit (DU) (or both) 101 is provided at the central office 103 and is configured to send signals including first, second, third, and fourth wavelengths (λ1, λ2, λ3, λ4) to an optical distribution node (ODN), including, for example, a wavelength division node based on an arrayed waveguide grating (AWG) 105. An optical line terminal (OLT) 107 is provided at the central office and is configured to transmit signals for the PON network. The OLT can be configured to convert, frame, and transmit signals for the PON network and to coordinate optical network terminal multiplexing for shared upstream transmission. The OLT sends signals to a coexisting optical filter in the ODN, which multiplexes the signals including the first, second, third, and fourth wavelengths with the upstream and downstream wavelengths of the PON.

[0007] The signal is sent from the AWG to a first splitter 109, which extracts the first and second wavelengths λ1, λ2 from the signal and sends these wavelengths to a first plug 113. The signal is sent from the first splitter to a second splitter 111, which extracts the third and fourth wavelengths λ3, λ4 and sends these wavelengths to a second plug 115 (such as a 5G tower). The signal is sent from the second splitter to an optical network terminal (ONT) 117, such as an end-user device.

[0008] In this scenario, the splitter or the tunable transceiver can include a tunable optical filter that can select WDM channels in the upstream (TX) or downstream (RX) band, with a typical channel spacing of 100 GHz and an isolation > 20 dB. In WDM transmission, separate bands are typically allocated for the upstream link spacing (US) and the downstream link spacing (DS), for example, 1528.77 - 1543.73 nm and 1547.72 - 1563.05 nm, respectively.

[0009] Currently, commercial tunable optical filters are based on microelectromechanical systems (MEMS), i.e., miniaturized electromechanical components that allow the selection of wavelengths by moving micromirrors.

[0010] Figure 2 The operating principle of a tunable optical filter based on a MEMS mirror 219 is shown. The tunable optical filter includes an optical system where light from an input fiber 221 is collimated on a fused silica grating 227, which diffracts light at different angles for each wavelength. The light is then reflected by the MEMS mirror 219 onto an output collimator 223, which couples a portion of the light into an output fiber 225. By modifying the tilt angle of the MEMS mirror, it is possible to tune the center wavelength of the optical filter.

[0011] However, the power consumption of MEMS-based optical filters may be too high for integration in pluggable modules. In addition, for application scales in scenarios such as 5G access networks and data centers, MEMS-based optical filters are costly. The high cost is due to their complex mechanical structures based on free-space optics and three-dimensional movement of micromirrors. In addition, there are few solutions that allow MEMS-based optical filters to be fabricated through CMOS-compatible processes (which can be obtained in standard electronic production lines). Even for high-volume manufacturing, this may prevent cost reduction.

[0012] The second solution available in commercial products is thin-film optical filters. These are stacks of dielectric layers with a thickness equal to one-quarter of the center passband wavelength. A cavity layer of one-quarter wavelength is added to form a resonator, where two sets of dielectric film stacks act as reflectors. Wavelength tuning is achieved by changing the angle of incidence of the incident light beam.

[0013] The characteristics of the optical filter are determined by the number and optical properties of the dielectric layers. Commonly used materials are quartz (SiO2) as the low-refractive-index layer and tantalum pentoxide (Ta2O5) as the high-refractive-index layer. These materials have a high refractive-index contrast, which reduces the number of layer pairs required for narrow passbands and low passband losses. The typical size is 2 square millimeters. Three main deposition techniques are used to achieve performance compatible with, for example, DWDM filtering applications: ion beam assisted deposition (IBAD), plasma assisted deposition (PAD), and ion beam sputtering (IBS). These techniques use an ion beam to bombard the target material while the target material accumulates on the substrate, aiming to prevent voids and defects in the material and improve yield.

[0014] However, the power consumption of thin-film optical filters may be too high. In addition, for 5G access networks and data centers, the cost of thin-film optical filters (associated with the manufacturing process and the control cost associated with the change in the angle of incidence) is relatively high. In addition, tunable thin-film optical filters cannot be integrated into silicon photonics chips using standard CMOS-compatible processes, and the footprint of the optical filter is large compared to the total area of the photonics chip. Summary of the Invention

[0015] This summary is provided to introduce in a simplified form some concepts that will be further described in the following detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0016] An object of the present disclosure is to provide an improved solution for reducing the cost and power consumption of optical filters.

[0017] According to a first aspect of the present disclosure, there is provided an optical filter for an optical network. The filter is configured to adaptively remove or add a target wavelength within a predetermined filtering range. For example, the optical filter can pass, demultiplex, or filter (remove or add) any target wavelength within the predetermined filtering range. The optical filter includes a first resonator configured to have a first resonant wavelength outside a first sub-range of the predetermined filtering range when a first resonant control variable of the first resonator is set to a first value. The first resonator is further configured to have a second resonant wavelength within the first sub-range of the predetermined filtering range when the first resonant control variable of the first resonator is set to a second value. The optical filter further includes a second resonator configured to have a third resonant wavelength outside a second sub-range of the predetermined filtering range when a second resonant control variable of the second resonator is set to a third value. The second resonator is further configured to have a fourth resonant wavelength within the second sub-range of the predetermined filtering range when the second resonant control variable of the second resonator is set to a fourth value. Each resonator can be independently controllable.

[0018] Accordingly, there is provided an optical filter that can use the first resonator to filter wavelengths in the first sub-range of the predetermined filtering range and can use the second resonator to filter wavelengths in the second sub-range of the predetermined filtering range. By using two resonators to separately filter a portion of the predetermined filtering range, the resonant wavelengths of each resonator do not need to change as much as when using only one resonator to filter wavelengths within the entire predetermined filtering range. Thus, less power may be required to move the resonator to the target wavelength.

[0019] The resonant control variable can be the electrical gate voltage of the resonator. The resonant control variable can be the temperature of the resonator.

[0020] The resonator can be configured such that when the resonator is in the "off" configuration (a non-operating configuration, a configuration that consumes a minimal amount of power, where no power or heat is intentionally supplied to the resonator), the resonant wavelength of the resonator is outside a predetermined filtering range. When the resonator is in the "on" configuration (an operating configuration, a configuration that consumes more power than the "off" configuration), where power or heat is (intentionally) supplied to the resonator, the resonant wavelength of the resonator can be changed to a wavelength within the predetermined filtering range. The predetermined filtering range can be the wavelength range that an optical filter should be able to filter. This can be determined by the channel wavelengths required in an optical system that uses the optical filter. The predetermined range can be set by the design of the resonator, where the resonator is designed (using a specific size, material, etc.) to allow them to have a resonant wavelength outside the predetermined filtering range when no heat or power is supplied to the resonator, and to be operable to have a resonant wavelength within the predetermined filtering range when heat or power is supplied to the resonator.

[0021] The optical filter can be configured to selectively change a first resonance control variable of a first resonator to a second value, which is the value at which a second resonant wavelength corresponds to a target wavelength (e.g., the value taken by the second resonant wavelength, or the value to which it is moved to the target wavelength). The optical filter can be configured to selectively change a second resonance control variable of a second resonator to a fourth value, which is the value at which a fourth resonant wavelength corresponds to a target wavelength (e.g., the value taken by the fourth resonant wavelength, or the value to which it is moved to the target wavelength).

[0022] When the target wavelength is closest to the first resonant wavelength, the first resonance control variable of the first resonator can be changed. When the target wavelength is closest to the third resonant wavelength, the second resonance control variable of the second resonator can be changed.

[0023] Therefore, less power can be used to move the resonant wavelength of the resonator to the target wavelength.

[0024] The optical filter can be configured to change the value of the first resonance control variable of the first resonator when the target wavelength is in a first sub-range. The optical filter can be configured to change the value of the second resonance control variable of the second resonator when the target wavelength is in a second sub-range. When the target wavelength is in the first sub-range, the second resonator can be configured to have a third resonant wavelength, and when the target wavelength is in the second sub-range, the first resonator can be configured to have a first resonant wavelength.

[0025] The second resonator can be configured such that if the first resonance control value cannot be changed from the first value to the second value, the second resonance control variable can be changed to a fifth value in order to generate a resonance wavelength in a first sub-range of a pre-determined filtering range. For example, in the presence of a fault associated with the first resonator, such as in a controller such as a heater where the controller changes the first resonance control value, the second resonator can also operate in the first sub-range (the second resonator operates over the entire pre-determined filtering range). A resonator fault can be considered when the resonance wavelengths of the resonators cannot be moved into the sub-bands they are intended to serve in normal use.

[0026] It should be understood that the reverse can also be true, where if the second resonance control value cannot be changed from the third value to the fourth value, the first resonance control variable can be changed to a sixth value in order to generate a resonance wavelength in a second sub-range of the pre-determined filtering range.

[0027] Thus, the lifespan of the optical filter can be extended because even in the presence of a fault associated with one resonator, the optical filter can continue to filter wavelengths in the pre-determined filtering range.

[0028] The first sub-range can extend substantially across half of the pre-determined filtering range. The second sub-range can substantially constitute the remainder of the pre-determined filtering range (or vice versa). The first sub-range and the second sub-range can correspondingly cover half of the pre-determined filtering range.

[0029] The first sub-range and the second sub-range can be separated by a guard range. The pre-determined filtering range can exclude the guard range. The guard range can be a set of wavelengths that are not used (e.g., by an optical system).

[0030] When the first resonance control variable of the first resonator is set to the first value, the first resonance wavelength can be in the guard range. When the second resonance control variable of the second resonator is set to the third value, the third resonance wavelength can be in the guard range.

[0031] When the first resonance control variable of the first resonator is set to the first value, the first resonance wavelength can be outside the pre-determined filtering range. When the second resonance control variable of the second resonator is set to the third value, the third resonance wavelength can be outside the pre-determined filtering range. It should be understood that outside the pre-determined filtering range can be respectively above or below the upper or lower boundary of the pre-determined filtering range, or in the guard band (which can be an area excluded from the pre-determined filtering range).

[0032] The first and / or third resonance wavelengths can be wavelengths shorter than the lower boundary of the pre-determined filtering range. The first and / or third resonance wavelengths can be wavelengths longer than the upper boundary of the pre-determined filtering range.

[0033] The first sub - range and the second sub - range may not overlap.

[0034] The optical filter may include a first heater and a second heater. The optical filter may be configured to heat the first resonator using the first heater and heat the second resonator using the second heater.

[0035] By using two separate heaters corresponding to the two resonators, if one heater fails, the other can continue to operate, so that the resonant wavelengths within the entire predetermined filtering range can be filtered.

[0036] The first value may be a first temperature, which is the temperature of the first resonator when the first resonator is not heated by the first heater. The third value may be a third temperature, which is the temperature of the second resonator when the second resonator is not heated by the second heater.

[0037] The first heater may include a first resistor. The second heater may include a second resistor. At least one of the first heater and the second heater may be formed of one of titanium and titanium nitride.

[0038] The first value and the third value may be the ambient temperature (e.g., the temperature that is substantially the same as the rest of the optical filter for the resonator).

[0039] At the first value, the first free spectral range of the first resonator may be greater than the predetermined filtering range. At the third value, the second free spectral range of the second resonator may be greater than the predetermined filtering range.

[0040] The optical filter may include no more than two resonators. For example, the optical filter may include one resonator for operating in the first sub - range and one resonator for operating in the second sub - range. However, it should be understood that each of these two resonators may include more than one resonator element, such as a ring resonator or a Bragg resonator. Thus, one of the two resonators may include multiple resonator elements, and the other of the two resonators may include multiple resonator elements.

[0041] The optical filter may include multiple resonators. When the respective resonant control values of the resonators are at the off (non - operating) values, each resonator has a resonant wavelength outside the predetermined filtering range, and when the respective resonant control values of the resonators are at the on (operating) values, each resonator has a resonant wavelength within the predetermined filtering range. In normal use, each resonator may operate on different sub - ranges of the predetermined filtering range.

[0042] The advantage of having multiple resonators is that each resonator can operate over a portion of a predetermined filtering range, but in the event of a failure, the resonators are operable to cover their portion as well as the portion of the failed resonator.

[0043] The first resonator may include a first ring resonator. The second resonator may include a second ring resonator. The first ring resonator and the second ring resonator may include different radii. The first resonator may include a first plurality of ring resonators. The second resonator may include a second plurality of ring resonators.

[0044] At least one of the first resonator and the second resonator may include a Bragg resonator (reflector).

[0045] At least one of the first resonator and the second resonator may include silicon.

[0046] The first resonator and the second resonator may be optically coupled to an input waveguide (e.g., flux, bus) for inputting light into the first resonator and the second resonator. The light input into the first resonator and the second resonator may include light corresponding to a target wavelength. The target wavelength may be removed from the light passing through the input waveguide.

[0047] At least one of the first resonator and the second resonator may be optically coupled to at least one output waveguide (e.g., tap off) for receiving the resonant wavelength of at least one of the first resonator and the second resonator (the resonant wavelength may be added to the output waveguide). At least one of the first resonator and the second resonator may be optically coupled to at least one output waveguide from which the resonant wavelength of at least one of the first resonator and the second resonator is removed (e.g., the wavelength may be removed from the flux). A waveguide that inputs the target wavelength and outputs a signal that does not contain the target wavelength may be considered to be both an input waveguide and an output waveguide simultaneously, or a flux waveguide. Thus, the target wavelength may be added to or removed from the output of the optical filter.

[0048] The target wavelength may be the wavelength of a channel to be added or dropped in an optical network. The optical network may be a wavelength division multiplexing network.

[0049] In another aspect of the present disclosure, an optical network including an optical filter is provided.

[0050] In another aspect of the present disclosure, a method of using an optical filter is provided. The method includes changing a first resonance control variable of a first resonator from a first value to a second value, where the first resonator includes a first resonance wavelength outside a first sub-range of a predetermined filtering range when the first resonance control variable of the first resonator is at the first value, and a second resonance wavelength within the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is at the second value, or changing a second resonance control variable of a second resonator from a third value to a fourth value, where the second resonator includes a third resonance wavelength outside a second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is at the third value, and a fourth resonance wavelength within the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is at the fourth value.

[0051] The method may further include changing the first resonance control variable of the first resonator to the second value, or changing the second resonance control variable of the second resonator to the fourth value, where the second value is the value at which the second resonance wavelength corresponds to the target wavelength, and the fourth value is the value at which the fourth resonance wavelength corresponds to the target wavelength.

[0052] When the target wavelength is closest to the first resonance wavelength, the first resonance control variable of the first resonator may be changed. When the target wavelength is closest to the third resonance wavelength, the second resonance control variable of the second resonator may be changed.

[0053] The method may further include changing the value of the first resonance control variable of the first resonator when the target wavelength is in the first sub-range. The method may further include changing the value of the second resonance control variable of the second resonator when the target wavelength is in the second sub-range.

[0054] The method may further include, if the first resonance control value cannot be changed from the first value to the second value, changing the second resonance control variable to a fifth value to generate a resonance wavelength in the first sub-range of the predetermined filtering range, and / or the first resonator (304) is configured such that if the second resonance control value cannot be changed from the third value to the fourth value, the first resonance control variable can be changed to a sixth value to generate a resonance wavelength in the second sub-range of the predetermined filtering range.

[0055] If a fault associated with the first resonator is detected (e.g., in the optical system or through the optical filter), the second resonance control variable may be changed to the fifth value to generate a resonance wavelength in the first sub-range of the predetermined filtering range. If a fault associated with the second resonator is detected, the first resonance control variable may be changed to the sixth value to generate a resonance wavelength in the second sub-range of the predetermined filtering range.

[0056] The method may further include receiving light input to an optical filter. The method may further include outputting light from the optical filter. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The above and other objects, features, and advantages of the present disclosure will become apparent from the following detailed description of its illustrative embodiments, which is to be read in conjunction with the accompanying drawings.

[0058] Figure 1 is a diagram showing the WDM covering the PON implemented by a tunable optical filter;

[0059] Figure 2 is a diagram showing a tunable optical filter based on a MEMS mirror;

[0060] Figure 3 is a diagram showing an optical filter according to an example;

[0061] Figure 4 is a diagram showing a method for an optical filter according to an example;

[0062] Figure 5 is a diagram showing shifting the resonant wavelengths of a first resonator and a second resonator within a predetermined filtering range according to an example;

[0063] Figure 6a is a graph showing the shift of the resonant wavelengths of the first and second resonators during normal use according to an example;

[0064] Figure 6b is a graph showing the shift of the resonant wavelength during normal use according to an example;

[0065] Figure 7 is a diagram showing the shift of the resonant wavelengths of the first and second resonators over the entire predetermined filtering range according to an example;

[0066] Figure 8 is a diagram showing an optical filter including a ring resonator according to an example;

[0067] FIG. 9 is a diagram showing an optical filter including two resonators, each resonator including a ring resonator, according to an example;

[0068] Figure 10a is a diagram showing an optical filter including two heaters according to an example;

[0069] Figure 10b is a diagram showing according to an example Figure 10a 3D diagram of the optical filter;

[0070] Figure 10cis a graph showing the correlation between the temperature increase and the resonant wavelength change of a ring resonator according to an example;

[0071] Figure 11a is a diagram showing a ring resonator according to an example;

[0072] Figure 11b is a diagram showing according to an example Figure 11a 3D diagram of the ring resonator;

[0073] Figure 12 is a diagram showing an optical filter including two resonators according to an example, each resonator including two ring resonators;

[0074] Figure 13 is a graph showing the optical filter curves of resonators including one ring resonator, two ring resonators, and three ring resonators according to an example; and

[0075] FIG. 14 shows an optical filter including two resonators according to an example, each resonator including a Bragg resonator. DETAILED DESCRIPTION

[0076] For purposes of explanation, details are set forth in the following description in order to provide a thorough understanding of the disclosed embodiments. However, it will be apparent to those skilled in the art that these embodiments can be practiced without these specific details or with equivalent arrangements.

[0077] Figure 3 An optical filter 302 including a first resonator 304 and a second resonator 306 is shown. The optical filter 302 can be implemented in an optical system or used in an optical network, such as a wavelength division multiplexing network (WDMN), coarse WDM (CWDM), dense WDM (DWDM), or any network topology within these categories, such as ring, point-to-point, star, etc. The optical filter 302 is configured to receive an input signal (light) (e.g., from an optical system or network) and output at least one output signal (e.g., to an optical system or network). The optical filter can be configured to receive (or can be configured to determine) an indication of a target wavelength (a wavelength to be added or dropped) to be filtered by the optical filter, e.g., from an optical system. The optical filter 302 is configured to adaptively add or remove (or drop) the target wavelength within a predetermined filtering range (the target wavelength can be a wavelength included in the input signal). This can be achieved using the first resonator 304 and the second resonator 306.

[0078] The predetermined filtering range can be the wavelength range that the optical filter can filter, and the range of values within the predetermined filtering range can be set by the design of the optical filter (e.g., by selecting specific materials, sizes, and / or types of various components, etc.). Each resonator can be configured to pass target wavelengths belonging to different sub-ranges within the predetermined filtering range. The first resonator 304 is configured to have a first resonance wavelength outside the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is set to a first value, and a second resonance wavelength within the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is set to a second value. Similarly, the second resonator 306 is configured to have a third resonance wavelength outside the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is set to a third value, and a fourth resonance wavelength within the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is set to a fourth value. The sub-range is a wavelength range smaller than the filtering range of the optical filter. In some examples, the wavelengths covered by the first and second sub-ranges do not overlap, i.e., different sets of wavelengths. In some examples, the wavelengths covered by the first and second sub-ranges are continuous. In some examples, the wavelengths covered by the first and second sub-ranges together provide the range of the optical filter. In some aspects, the first resonance wavelength is outside the first and second sub-ranges. In some aspects, the third resonance wavelength is outside the first and second sub-ranges. Thus, the first and second resonators can be configured to pass wavelengths within and outside the filtering range. Within the range of the optical filter, the first and second resonators can operate in different (non-overlapping) sub-ranges.

[0079] Figure 4 A corresponding method using the optical filter is shown. Specifically, Figure 4 A method is shown that includes changing the first resonance control variable of the first resonator from a first value to a second value, where the first resonator includes a first resonance wavelength outside the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is at the first value, and a second resonance wavelength within the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is at the second value (S408). The method further includes changing the second resonance control variable of the second resonator from a third value to a fourth value, where the second resonator includes a third resonance wavelength outside the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is at the third value, and a fourth resonance wavelength within the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is at the fourth value (S410).

[0080] Thus, each resonator can be operated such that the resonant wavelength of each resonator can be moved in and out of a predetermined filtering range. In normal use, the first resonator can be operated over a first sub-range and the second resonator can be operated over a second sub-range such that each resonator is used to filter a different part of the predetermined filtering range (e.g., normal use is the case when all resonators and corresponding components are operable such that the resonant wavelengths of the corresponding resonators can be moved into their respective sub-ranges).

[0081] The resonators can be configured such that in a "closed" or non-operating configuration, where no power or heat is intentionally provided to the resonators, the resonant wavelength of each resonator is outside the predetermined filtering range. If the input signal does not include the resonant wavelength of a resonator in the "closed" configuration, then no wavelengths will be filtered when the resonator is in the "closed" configuration. Power or heat can be provided to the resonators to change their resonant wavelengths to wavelengths within the predetermined filtering range (in which case the resonators will be in an "open" configuration). For example, the resonance control variable can be the electrical gate voltage of the resonator, and / or the resonance control variable can be the temperature of the resonator. It should be understood that one or both of these control variables can be used to control the resonant wavelength of either or both resonators.

[0082] This configuration of the optical filter is particularly advantageous because only one resonator needs to be operated to filter the target wavelength. Each resonator can only filter a part of the predetermined filtering range, and thus the resonators can be used to filter the target wavelength in their respective parts of the predetermined filtering range. In addition, each resonator can define half or substantially half of the predetermined filtering range. This means that the resonant wavelength of the resonator does not need to be changed too much because the resonator with the resonant wavelength closest to the target wavelength can be operated, so the resonant wavelength of any resonator is moved maximally over half of the predetermined filtering range (instead of one resonator being moved over the entire predetermined filtering range). Thus, power consumption is saved. For example, in the case of using the thermo-optic effect to change the effective refractive index of the resonator (e.g., using a local metal heater) to change the resonant wavelength of the resonator, less power is required to move each resonator over a part of the predetermined filtering range than to move one resonator over the entire predetermined filtering range.

[0083] The optical filter can be configured to selectively change a first resonance control variable of the first resonator to a second value, or change a second resonance control variable of the second resonator to a fourth value, where the second value is the value at which the second resonance wavelength corresponds to the target wavelength, and the fourth value is the value at which the fourth resonance wavelength corresponds to the target wavelength. Thus, either resonator can be selected based on the position of the target wavelength within the predetermined filtering range (e.g., when the target wavelength is closest to the first resonance wavelength, the first resonance control variable of the first resonator can be changed, and when the target wavelength is closest to the third resonance wavelength, the second resonance control variable of the second resonator can be changed. Thus, when the target wavelength is within the first sub-range, the first resonator can be operated, and when the target wavelength is within the second sub-range, the second resonator can be operated).

[0084] Although during normal use, the resonators can operate only over a portion of the predetermined filtering range, if one of the resonators fails to filter a target wavelength within the sub-range it serves during normal use (e.g., due to a failure of a heating element, power supply, resonator, etc., which can be detected by the optical system, optical filter, etc.), then the other resonator can be operated such that its resonance wavelength can correspond to any target wavelength within the entire predetermined filtering range, and thus filter target wavelengths anywhere within the predetermined filtering range (or can filter wavelengths within their sub-range and the sub-range within which the other resonator operates during normal use). For example, the second resonator can be configured such that if the first resonance control value cannot be changed from the first value to the second value, the second resonance control variable can be changed to a fifth value to generate a resonance wavelength within the first sub-range of the predetermined filtering range (and vice versa). Thus, the lifespan of the optical filter can be extended because even if one resonator is inoperable, the optical filter will still be operable over the entire predetermined filtering range.

[0085] Figure 5 Including two graphs, in the upper graph, the bandpasses of the two resonators are shown when the first resonator (but not the second resonator) is operated, and in the lower graph, the bandpasses of the two resonators are shown when the second resonator (but not the first resonator) is operated. Figure 5 The predetermined filtering range 512 is shown, which is divided into two sub-ranges, a first sub-range 514 and a second sub-range 516. In this example, each sub-range is essentially half of the predetermined filtering range, and they do not overlap. Dividing the predetermined filtering range into two halves or approximately two halves is advantageous because each resonator will operate over their portion of the predetermined filtering range using approximately the same amount of power. However, it should be understood that each resonator can operate over a different proportion of the predetermined filtering range depending on the design.

[0086] AsFigure 5 As shown by the upper curve in, the target wavelength 518 is within the first sub - range 514 of the predetermined filtering range 512. To filter the target wavelength 518, the first resonator is operated (in the "on" configuration) such that its resonant wavelength 520 is moved into the first sub - range of the predetermined filtering range of the target wavelength 518. The resonant wavelength 522 of the second resonator remains outside the predetermined filtering range 512 (the second resonator is in the "off" configuration). It should be understood that a resonator can include a pass - band containing the resonant wavelength, and wavelengths within the pass - band will be filtered. The movement of the resonant wavelength of the resonator mentioned herein can equally be interpreted as the movement of the pass - band of the resonator. Wavelengths within the first sub - range are denoted by λ i is represented.

[0087] An alternative case is shown in Figure 5 the lower curve in, where the target wavelength 518 is within the second sub - range 516 of the predetermined filtering range 512. To filter the target wavelength 518, the second resonator is operated such that its resonant wavelength 522 is moved into the second sub - range 516 of the predetermined filtering range 512 of the target wavelength 518 (the second resonator is in the "on" configuration). The resonant wavelength 520 of the first resonator remains outside the predetermined filtering range (the first resonator is in the "off" configuration). Wavelengths within the first sub - range are denoted by λ k is represented.

[0088] Figure 6a - Figure 6b shows the movement of the resonant wavelength from outside the predetermined filtering range 612 to within the predetermined filtering range 612. Figure 6a The upper curve in shows the movement of the resonant wavelength 620 of the first resonator from outside the predetermined filtering range over the first sub - range 614 of the predetermined filtering range. When the first resonator is in the "off" configuration, the resonant wavelength of the first resonator is outside the predetermined filtering range. In this example, when the first resonator is in the "off" configuration, the resonant wavelength of the first resonator is a wavelength shorter than the lower boundary of the predetermined filtering range 612. When the first resonator is operated, the resonant wavelength of the first resonator increases and moves through the first sub - range of the predetermined filtering range (the "on" configuration). Thus, the first resonator can be operated to have a resonant wavelength at any wavelength within the first sub - range. The free spectral range (FSR) of the first resonator can be greater than the size of the predetermined filtering range 612, where the free spectral range is the maximum interval of the wavelength (or equivalently, frequency) between two consecutive resonances of the resonator at a fixed control variable value. The fixed control variable value can be the control variable value when the resonator is in the "off" configuration. This can prevent more than one wavelength within the predetermined filtering range from being filtered simultaneously.

[0089] Figure 6aThe curve in the lower part shows the shift of the resonant wavelength 622 of the second resonator from outside the predetermined filtering range 612 onto the second sub-range 616 of the predetermined filtering range. In this example, when the second resonator is in the "off" configuration, the resonant wavelength 622 of the second resonator is a wavelength longer than the upper boundary of the predetermined filtering range 612. When the first resonator is in the "off" configuration, the resonant wavelength 622 of the second resonator is outside the predetermined filtering range 612. When the second resonator is operated (e.g., in the "on" configuration), the resonant wavelength of the second resonator decreases and can move through the second sub-range 616. The free spectral range of the second resonator is greater than the size of the predetermined filtering range 612, where the free spectral range is the maximum interval of wavelengths between two consecutive resonances of the resonator with a fixed control variable value. In Figure 6a The shift of the resonant wavelength of the second resonator described in the lower curve graph may apply to a situation where when the resonant wavelength 622 of the second resonator is a wavelength longer than the upper boundary of the predetermined filtering range 612, the power consumption associated with the resonator control variable is at its lowest value. Thus, when the power consumption increases, the resonant wavelength decreases and the resonant wavelength 622 of the second resonator can move through the second sub-range 616 of the predetermined filtering range 612.

[0090] Figure 6b A variant showing the shift of the resonant wavelength 622 of the second resonator is presented. In this example, a guard band 613 is provided within the predetermined filtering range 612 between the first sub-range 614 and the second sub-range 616, where when the corresponding resonator is in the "off" configuration, the resonant wavelength of the first or second resonator can be located within this guard band 613. The guard band can be considered as an area excluded from the predetermined filtering range 612. Figure 6b The curve in the upper part shows the shift of the resonant wavelength 620 of the first resonator and is the same as described with respect to Figure 6a where the first resonator is operable to shift its resonant wavelength 620 over the first sub-range 614. Figure 6b The curve in the lower part shows the shift of the resonant wavelength 622 of the second resonator from the guard band 613 onto the second sub-range 616 of the predetermined filtering range. In this example, when the second resonator is in the "off" configuration, the resonant wavelength 622 of the second resonator is a wavelength shorter than the lower boundary of the second sub-range 616. When the second resonator is in the "off" configuration, the resonant wavelength 622 of the second resonator is in the guard band 613 outside the second sub-range 612. When the second resonator is operated, the resonant wavelength of the second resonator increases. The free spectral range of the second resonator is greater than the size of the predetermined filtering range 612. In Figure 6bThe movement of the second resonator depicted in the lower curve can be applicable to a situation where, when the resonant wavelength 622 of the second resonator is a wavelength shorter than the lower boundary of the predetermined filtering range 612, the power consumption associated with the control variable is not at its minimum value. For example, when the resonant wavelength 622 of the second resonator is a wavelength within the guard band 613, the power consumption associated with the control variable can be at its minimum value.

[0091] Figure 7 A configuration is shown where the resonators are configured such that the resonant wavelengths of the first and second optical filters can operate over the entire predetermined filtering range 712. In this example, a guard band 713 is provided within the predetermined filtering range 712 between a first sub-range 714 and a second sub-range 716, where the resonant wavelength of the second resonator lies within this guard band when the second resonator is in the "off" configuration. When the first resonator is in the "off" configuration, the resonant wavelength of the first resonator lies below the lower boundary of the predetermined filtering range. Thus, in the "off" position, the resonant wavelength of the first resonator is less than the lower boundary of the predetermined filtering range 712. In the "off" position, the resonance of the second resonator is at a wavelength within the guard band 713. The guard band includes wavelengths or a set of wavelengths that are not required to be filtered or are not used. The predetermined filtering range can be considered to exclude the guard band. In this example, the free spectral range of the resonant wavelength of each of the first and second resonators is the same as or greater than the size of the predetermined filtering range 712. Thus, in this configuration, both the first resonator and the second resonator are configured to be operable over the entire predetermined filtering range.

[0092] In normal operation, the first resonator is configured to filter target wavelengths in the first sub-range 714 of the predetermined filtering range 712, and the second resonator is configured to filter target wavelengths in the second sub-range 716 of the predetermined filtering range 712. In this example, the first resonator is operated to filter the target wavelengths in the first sub-range by initially increasing the resonant wavelength 720 through the first sub-range 714 (e.g., increasing to the target wavelength). The second resonator is operated to filter the target wavelengths in the second sub-range 716 by increasing the resonant wavelength 722 such that the resonant wavelength of the second resonator moves through the second sub-range of the predetermined range (e.g., moving to the target wavelength).

[0093] If one of the first and second resonators fails, the other of the first resonator and the second resonator can operate to filter target wavelengths within the entire predetermined filtering range (target wavelengths within the first and second sub-ranges, for example, they can also operate in the sub-range belonging to the failed resonator). A resonator failure can be considered when the resonator is not able to shift the resonant wavelength into the sub-band it is intended to serve in normal use. In this example, the first resonator can be operated to increase its resonant wavelength within the entire predetermined filtering range 712 (from the first sub-band to the second sub-band). The second resonator can be operated to increase its resonant wavelength 722 through the second sub-band until periodicity causes its resonant wavelength to shift to the bottom of the first sub-band, and then the resonant wavelength can be increased through the first sub-band. Thus, the first and / or second resonator can filter any target wavelength within the predetermined filtering range. In such a configuration, in normal use each resonator only needs to operate on half of the range it can operate, so less power is required to operate the optical filter. However, if one of the resonators cannot operate in their designated sub-range, the other resonator is able to operate to filter wavelengths in both sub-ranges (e.g., over the entire predetermined range), which extends the life of the optical filter in the case where a part of the optical filter fails.

[0094] Note that for Figure 6a the configuration, in the case of a failure, as required, the first resonator can be operated to increase its resonant wavelength over the entire predetermined filtering range 712, and the second resonator can be operated to decrease its resonant wavelength over the entire filtering range.

[0095] Note that either or both of the resonators can be configured as described above. For example, when the resonator is in the "off" configuration, either or both of the resonators can have a resonant wavelength within the guard band, and / or either or both of the resonators can have a resonant wavelength outside the bands above and / or below the upper and / or lower parts of the predetermined filtering range. When the resonator consumes the least amount of power or heat, the resonator can have a resonant wavelength above or below the bands above and below the upper and lower parts of the predetermined filtering range, respectively, or a resonant wavelength within the guard band.

[0096] Various resonators can be used in the present invention defined by this claim. One such type of resonator is a ring resonator (e.g., a micro-ring resonator (MMR), an optical ring resonator).

[0097] The optical path difference (OPD) of a ring resonator can be given by the following formula:

[0098] OPD = 2πrn eff (1)

[0099] where r is the radius of the ring resonator, and neff is the effective refractive index of the waveguide material and depends on the optical properties of its guiding material. For resonance to occur, the following condition must be satisfied:

[0100] ODP = mλ res (2)

[0101] where λ res is the resonance wavelength and m is the number of modes of the ring resonator. For light to interfere constructively within the ring resonator, the circumference of the ring must be an integer multiple of the light wavelength. Thus, when the light incident on the ring resonator contains multiple wavelengths, only the resonance wavelength passes completely through the ring resonator.

[0102] Each ring resonator is characterized by a set of resonance frequencies λ separated by the free spectral range (FSR), the distance between two adjacent resonances res . For a ring resonator, the relationship between the resonance frequency value and the size (circumference) L of the ring is as follows:

[0103]

[0104] where n eff is the effective refractive index and m is the number of modes of the ring resonator. The free spectral range for a given λ is

[0105]

[0106] where n g is the group refractive index. Thus, a given wavelength resonance value can be achieved with different L values, and for a given λ value, the FSR strongly depends on the size of the ring and its material / design. The size and material or its design of the ring can be selected so that the ring resonator has an appropriate λ res value and FSR (especially considering the requirements of the above-mentioned predetermined filtering range). The predetermined range can be achieved by the design of the resonator, where the resonator is designed (using specific dimensions, materials, etc.) to allow them to have resonance wavelengths outside the predetermined filtering range when no heat or power is supplied to the resonator, and to be operable to have resonance wavelengths within the predetermined filtering range (over the entire range) when heat or power is supplied to the resonator.

[0107] Based on the operation range of the ring resonator corresponding to its spectral range. Two resonators constituting an optical filter can have almost the same FSR and can provide, for example, a bandwidth (BW) of at least 20 nanometers for applications in WDM networks (utilizing preselected DL, UL bands).

[0108] The FSR can have a minimum variation ΔFSR over the wavelength range of operation of the optical filter:

[0109]

[0110] (For a reference wavelength of 1530 nm and a variation of 20 nm, it is approximately 1 nm, assuming the optical properties are those of a standard silicon photonic waveguide (for this set of parameters, the radius of the ring is approximately 4.5 microns). This difference can be considered in the design, allowing for the necessary margin such that the FSR is greater than the predetermined filtering range.)

[0111] When the two resonators are not in operation, they can be tuned to have a resonant wavelength outside the predetermined filtering range. To this end, the FSR can be greater than the operating range (predetermined filtering range) to allow the resonators not to resonate at both the upper and lower boundaries of the predetermined filtering range.)

[0112] To allow operation in different parts of the spectrum, the radii of the two resonators may have a small difference (20 nm for the parameters considered above), which would imply that for the case considered above, one resonator (the resonator with the smaller radius) will have a larger FSR, i.e., 1 nm larger than the other resonator. However, taking into account the difference in FSR, the first and second resonators can still be configured such that the FSR of each resonator is greater than the predetermined filtering range, so that in the "off" configuration, the resonant wavelength of each resonator is outside the predetermined filtering range.)

[0113] An example of such a ring resonator is shown in Figure 8 .) Figure 8 A ring resonator 804 is shown, which is optically coupled to a first waveguide 826 (input or through waveguide, bus waveguide through which the signal propagates), and is also coupled to a second waveguide 828 (output or drop waveguide). A light beam (signal) passes through the first waveguide 826, where the light beam includes multiple wavelengths (λ1, λ2, λ3…λ i …λ n ). The light is coupled into the ring resonator 804, and the wavelengths of the light that are the resonant wavelengths λ i of the ring resonator interfere constructively in the ring resonator 804 (the signal includes the resonant wavelengths). In this example, the resonant wavelengths are coupled to the first waveguide 826 and cancel the wavelength λ i , such that the light flux does not include the resonant wavelength λ i . This can be used to filter a specific wavelength (e.g., channel) of light, where light of other wavelengths is allowed to pass through the first waveguide 826. The light of the resonant wavelength λ i of the resonator 804 is also coupled into the second waveguide 828. Thus, the resonant wavelength λ iLight can be coupled out of the resonator. This configuration provides the function of removing the resonant wavelength of the resonator 804 from the flux in the first waveguide 826, and provides the function of extracting the resonant wavelength of the resonator 804 in the second waveguide 828. It should be understood that depending on whether the wavelength is to be added or removed, the optical filter may include one or both outputs of the first and second waveguides.

[0114] FIG. 9 shows an optical filter including a first resonator 904 and a second resonator 1106, configured as described above with respect to Figure 8 that. In the case where both resonators are in the "off" state, the resonant wavelengths of the resonators are outside the predetermined filtering range (they may be in the guard band). Each resonator is configured to resonate at a different wavelength when in the "off" configuration. Thus, the resonators are designed (formed) to have specific resonant wavelengths that are different from each other in the "off" configuration. In this example, the radii of the first resonator and the second resonator are different, thereby giving each resonator a different resonant wavelength in the "off" configuration. The radius of the first resonator can be greater than or less than the radius of the second resonator, and vice versa.

[0115] As described with respect to Figure 5 - Figure 7 that, the optical filter is operable to control the shift of the resonant wavelengths of the first and second resonators, where the resonators are configured such that in normal use, the first resonator resonates at a resonant wavelength λ within a first sub-range of the predetermined filtering range of the optical filter i and is operable, while the second resonator resonates at a resonant wavelength λ within a second sub-range of the predetermined filtering range k and is operable.

[0116] In this example, the optical filter 902 further includes a first controller 930 for changing the control variable of the first resonator 904, and a second controller 932 for changing the control variable of the second resonator 906. The first controller 930 is operable to change the first resonance control variable of the first resonator 904. For example, the first controller is operable to change the first resonance control variable of the first resonator 904 from a first value to a second value. The first value can be a value at which the first resonant wavelength of the first resonator is outside the first sub-range of the predetermined filtering range. The second value can be a value at which the second resonant wavelength of the first resonator is within the first sub-range of the predetermined filtering range. The second controller 932 is operable to change the second resonance control variable of the second resonator 906 from a third value to a fourth value. The third value can be a value at which the third resonant wavelength of the second resonator is outside the second sub-range of the predetermined filtering range. The fourth value can be a value at which the fourth resonant wavelength of the second resonator is within the second sub-range of the predetermined filtering range.

[0117] Accordingly, the first controller 930 and the second controller 932 are operable to change the resonant wavelengths of the first resonator 904 and the second resonator 906, respectively, such that the resonant wavelengths of the resonators can be moved in and out of a predetermined filtering range as needed and can thus be used for wavelength filtering. The controller can receive an indication of the target wavelength to be filtered by the optical filter (e.g., a signal indicating the target wavelength to be filtered can be received from an optical system), and is operable to change the resonant wavelength of the appropriate resonator. Note that one controller can be used to change the resonant wavelengths of both the first resonator and the second resonator.

[0118] In FIG. 9(a), the figure shows a scenario where the first controller is in an “on” configuration and is operated such that the resonant wavelength of the first resonator is moved into a first sub-range of the predetermined filtering range. The second controller is in an “off” configuration, and thus the resonant wavelength of the second resonator is outside the second sub-range of the predetermined filtering range. Accordingly, the resonant wavelength λ i of the first resonator is removed from the first waveguide 926 and added to the second waveguide 928.

[0119] FIG. 9(b) shows an alternative scenario where the first controller is in an “off” configuration, and thus the resonant wavelength of the first resonator is outside the first sub-range of the predetermined filtering range. The second controller is in an “on” configuration and is operable to move the resonant wavelength of the second resonator into the second sub-range of the predetermined filtering range. Accordingly, the resonant wavelength λ k of the first resonator is removed from the first waveguide 926 and added to the second waveguide 928.

[0120] The corresponding controller is operable to make the first and second resonators have any resonant wavelength within the predetermined filtering range. Accordingly, the optical filter can be configured to select a single resonant frequency (wavelength) within the predetermined filtering range and only add / remove a specific channel. The controller can receive an indication of the target wavelength to be filtered (e.g., added or removed). The controller can receive an instruction as to whether the resonant wavelength of their corresponding resonator is to be changed (or the controller itself can determine based on the target wavelength whether their corresponding resonator is to be changed and whether it is within the relevant part of the predetermined filtering range). When the target wavelength is in the first sub-range, the first controller can change the first resonant control variable to change the resonant wavelength of the first resonator, and when the target wavelength is in the second sub-range, the second controller can change the second resonant control variable to change the resonant wavelength of the second resonator.

[0121] One way to change the resonant wavelength of an optical resonator is to change the effective refractive index of the material forming the resonator. This can be achieved by heating the resonator. For example, a heating element can be used to heat the resonator to a temperature at which the effective refractive index corresponds to the desired resonant wavelength.

[0122] Figure 10a An optical filter including a first heater 1039 and a second heater 1041 is shown. ( Figure 10b Shown is Figure 10a a 3D version of the optical filter). Figure 10a The arrangement of the optical filter is similar to that of FIG. 9, where a first ring resonator 1004 and a second ring resonator 1006 are provided and are close to a first waveguide 1026 and a second waveguide 1028. As shown in this figure, the first heater 1039 and the second heater 1041 are respectively located adjacent to the first resonator 1004 and the second resonator 1006. Each heater can be operated independently such that each of the first resonator 1004 and the second resonator 1006 can be heated separately. First and second controllers are not shown in this figure, however, it should be understood that the first controller and the second controller can correspondingly include (or be connected to) the corresponding heaters (or heating elements). Alternatively, in any of the examples described herein, a single controller can be connected to two heaters and be operable to control both heaters. The controllers can be operated such that their corresponding heaters are heated to (e.g., by providing current, power) a temperature at which the resonators associated with the corresponding controllers are also heated. As the resonator is heated, its effective refractive index also changes. This results in a change in the resonant wavelength. In Figure 10c the example shown outlines the effect of an increase in temperature of the resonator (ring resonator) on the resonant wavelength of the ring resonator. For example, Figure 10c the ring resonator has a diameter of 10 microns. As Figure 10c shown, there is a linear relationship between the increase in temperature and the increase in the resonant wavelength of the ring resonator. Thus, taking advantage of this correlation, it is possible to select the temperature to which the heating element is heated in order to heat the resonator to a temperature corresponding to the target wavelength in order to filter the target wavelength, as described with respect to the examples herein.

[0123] Materials such as silicon (Si) can be particularly advantageous when forming ring resonators with a specific resonant wavelength because they allow for high manufacturing precision and the ability to control the effective refractive index of the composite structure, which is determined by the manufacturing process. Using a material such as Si to form an optical filter, it is also possible to utilize the thermo - optic effect (change in optical properties due to temperature change (e.g., the thermo - optic coefficient of Si is (~300K)) to achieve fine tuning of the effective refractive index of the resonator. The tunability of the resonator is particularly relevant in WDM filtering applications, where the transmission channel carried by the selected wavelength (e.g., the target wavelength) must be added or removed at a given port.

[0124] Therefore, it is possible to reconfigure the add / drop scheme in a deployed network by changing the current fed to the heating element of the resonator. One way to heat a ring resonator in a silicon photonics circuit is through a resistor made of a thin film, which dissipates heat locally through Joule heating.

[0125] The optical filters described herein can extend the lifespan of the metal elements (such as heating elements) that perform tuning operations on the resonators of the optical filters. For tunable transceivers, this may be on the order of 10 years, but thermally induced stress can cause premature aging and failure of the materials. Therefore, according to embodiments, the lifespan of the heating element can be increased by a factor of 10 or more.

[0126] It is advantageous to have heating elements with high thermal stability so that they can withstand high temperatures. However, even very stable compounds, such as Ti / TiN thin films, have a characteristic resistance that varies with the operating temperature (the Ti / TiN film varies by 12% between 25 and 350 °C), and may experience premature failure if operated at temperatures up to 300 °C for a long time (the resistance change can be addressed through calibration). The advantage of this configuration is that each heating element does not need to be heated to such a high temperature because they do not need to operate over the entire pre-determined filtering range. Therefore, the lifespan of the heating element can be extended. The material used to form the heating element can be selected taking into account the required temperature increase in order to shift the resonant wavelength to the necessary wavelength.

[0127] For a tuning range of 10 nanometers in the resonant wavelength of the resonator, a temperature change of 100 °C may be necessary. However, due to the low thermal conductivity (1.38 w / m K) of the typical cladding material that can separate the heating element from the ring waveguide, during the tuning process in which the resonant wavelength of the resonator is changed, the temperature experienced by the resistor may be much higher than the temperature experienced by the resonator. In addition, there may be hot spots in the resistor that reach temperatures higher than the average. Therefore, calibration may be required to ensure the consistency of the temperature of the heating element and the shift of the resonant wavelength.

[0128] To predict the lifespan of the Ti / TN resistor, the following thermal model based on the Arrhenius equation can be used:

[0129]

[0130] where MTTF is the median time to failure, k is the Boltzmann constant, T is the temperature, Ea is the thermal activation energy, and A is a constant. Using this formula, obviously, when the temperature is reduced by a factor of 2, the lifetime of the resistor increases by a factor of 8.

[0131] Therefore, it is advantageous to limit the operating temperature of the metal heater formed from the above material to below 300 °C in order to provide a longer lifetime for the optical filter.

[0132] By providing an optical filter having two resonators, where the two resonators are configured such that the first resonator is tuned by a first set of heaters and the second resonator is tuned by a second set of heaters, the first resonant structure can be tuned to operate in the first half (or first sub-range) of a predetermined filtering range of the optical filter to add / remove channels having a carrier wavelength, and the second resonant structure can be tuned to operate in the second half (or second sub-range) of the predetermined filtering range to add / remove channels having a carrier wavelength. Another advantage is that by using resonators in a reduced portion of the predetermined filtering range (e.g., the operating range) of the optical filter, i.e., the portion containing the wavelengths to be added / removed, the power consumption is reduced. When heaters are used to heat the resonators, the power required decreases linearly with the resonant shift required for tuning. The resonant wavelength of the resonator is designed to be outside the predetermined filtering range, and the heater can shift the resonance of one resonator to a selected wavelength.

[0133] Therefore, a composite tunable integrated resonant element capable of operating wavelength filtering operations or add / remove operations from / to channel waveguides to bus waveguides can be provided in a manner that increases the efficiency of the tuning operation, saves power, and ensures its robustness against tuner device aging and performance loss, since it can operate at a lower temperature.

[0134] Any method of changing the resonant wavelength of a resonator (e.g., by changing the effective refractive index of the material forming the resonator) can be used to change the resonant wavelength of the resonator. For example, an alternative way to change the resonant wavelength of a resonator is to change the electrical gate voltage of the resonator (e.g., from no voltage to having a voltage). Thus, the resonance control variable can be voltage. This can be achieved, for example, by utilizing the carrier diffusion effect. The carrier diffusion effect can change the effective refractive index of the material forming the resonator by changing the carrier concentration in the material. This can be achieved, for example, in a ring resonator made of doped silicon, where P-type and N-type silicon form a PN junction; in such a configuration, the control variable can be the bias voltage applied between the P and N regions through metal contacts. Thus, the voltage can be changed to change the resonant wavelength of the resonator.

[0135] This configuration is in Figure 11aAs shown, the ring resonator 1104 is formed as a ring of P-type material, and the portion 1133 within the ring is formed of N-type material. The first metal contact 1135 is disposed on the portion 1133 inside the ring, and the second metal contact 1137 is disposed in the P-type region outside the ring. A bias voltage can be applied between the two regions through the metal contacts. The waveguide 11 is also formed of P-type material and can be used as any of the other waveguides described above. Figure 11b A 3D view of this configuration is shown.

[0136] Figure 12 Another configuration of an optical filter including ring resonators as the first resonator 1204 and the second resonator 1206 is shown. This example is similar to the optical filter shown in FIG. 9. However, in this example, the first resonator 1204 includes first and second ring resonators 1234, 1236, and the second resonator includes third and fourth ring resonators 1238, 1240. Light is coupled from the first waveguide 1226 to the first ring resonator 1234, then the light is coupled from the first ring resonator to the second ring resonator 1236, and then the light is coupled from the second ring resonator to the second waveguide 1228.

[0137] Similarly, in this example, the second resonator 1206 includes third and fourth ring resonators 1238, 1240, where light is coupled from the first waveguide 1226 to the third ring resonator 1238, then the light is coupled from the third ring resonator to the fourth ring resonator 1240, and then the light is coupled from the fourth ring resonator to the second waveguide 1228.

[0138] In this example, the first and second ring resonators have the same resonant wavelength when in the "off" configuration, and the first resonator is operated such that the shift in the resonant wavelength of the resonator includes heating the first and second ring resonators to the same temperature (or changing the resonant control values of the two ring resonators to the same value) such that they both have the same resonant wavelength. The third and fourth ring resonators are similarly configured to have the same resonant wavelength as each other, and the second resonator is operated such that the shift in the resonant wavelength of the resonator includes heating the first and second ring resonators to the same temperature (or changing the resonant control values of the two ring resonators to the same value) such that they both have the same resonant wavelength. The same controller or separate controllers can be used for the ring resonators in the same resonator. Similarly, the same heating element or separate heating elements can be used for the ring resonators in the same resonator. In the "off" configuration, the first and second resonators can have different resonant wavelengths or the same resonant wavelength.

[0139] It should be understood that this can be extended such that each resonator includes a plurality of ring resonators (e.g., 1, 2, 3, 4, etc.). The ring resonators within the same resonator can maintain the same resonant wavelength in their "off" and "on" configurations.

[0140] The effect of using multiple ring resonators in the Figure 12 configuration is shown in Figure 13 which shows the filtering curves of resonators including one ring, two rings, and three rings. The shape of the filtering curve and its optimum coupling can be determined by the separation width between the bus waveguide and the rings, the gap between two rings, and the waveguide characteristics. It can be seen from this figure that using two or more coupled ring resonators as the resonator provides a flatter filtering response with a sharper curve, which is particularly useful in reducing inter-channel crosstalk.

[0141] An alternative type of resonator that can be used in an optical filter to filter wavelengths is a Bragg resonator (Bragg reflector, distributed feedback Bragg reflector). A distributed feedback Bragg resonator is a multi-cavity optical filter using an integrated standing-wave resonator that uses a Bragg grating to reflect radiation at the resonant wavelength. The grating consists of a waveguide with periodic corrugations that exist in different shapes and have a pitch corresponding to a quarter wavelength of the resonant wavelength. These gratings are used as reflectors for an optical cavity or a set of coupled optical cavities, and the output is radiation with a spectrum characterized by a set of regularly spaced resonances, with the spacing A given by the reciprocal of the optical path of the radiation in the cavity.

[0142]

[0143] Thus, the Bragg resonator will reflect the resonant wavelength of the resonator and allow other wavelengths to pass through. Similar to the above ring resonators, the resonant wavelength of the Bragg resonator is changed by changing the effective refractive index of the cavity of the Bragg resonator. By heating the Bragg resonator, the effective refractive index can be changed, thereby changing the resonant wavelength. Thus, the Bragg resonator can be used similarly to the above ring resonators, where the optical filter can be configured to add or remove the resonant wavelength (or both, or either) in an optical system. In this example, heating of the resonator to change the resonant wavelength is described; however, any suitable method of changing the resonant wavelength can be implemented, such as by changing the effective refractive index.

[0144] Figure 14 shows an example of an optical filter including such a resonator. In FIG. 14, a first resonator 1404 (Bragg resonator) and a second resonator 1406 (Bragg resonator) are shown. The first resonator 1404 and the second resonator 1406 can be connected to a multimode interferometer (MMI) 1442 configured to send an input signal from an input waveguide to the resonators and configured to send the reflected radiation to a drop port 1446 (e.g., an output waveguide). Non-resonant wavelengths (e.g., non-reflected wavelengths) pass through the first resonator 1404 and the second resonator 1406 to the through port (output / through waveguide) 1444.

[0145] As described above with respect to other examples, the first resonator is configured to have a resonant wavelength outside of a first sub-range of a predetermined filtering range when a first resonance control variable of the first resonator is set at a first value, and a second resonant wavelength within the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is set at a second value. The second resonator is configured to have a third resonant wavelength outside of a second sub-range of the predetermined filtering range when a second resonance control variable of the second resonator is set at a third value, and a fourth resonant wavelength within the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is set at a fourth value.

[0146] Thus, the resonators can be used to filter target wavelengths in a sub-range of a predetermined filtering range, as described with respect to the examples above. As described above, the resonant wavelengths of the Bragg resonators can be changed by changing the temperature of the first and second resonators. The resonant wavelengths of the Bragg resonators can be changed by a first controller 1430 and a second controller 1432. The first and second controllers can include heating elements for changing the temperature of their respective resonators in the same manner as described with respect to the examples above.

[0147] Thus, as described above with respect to other examples, a signal can be input to the optical filter, where by changing the resonant wavelengths of the associated resonators, the target wavelength can be filtered by the first resonator or the second resonator, where the first resonator and the second resonator are normally used in the first sub-range or the second sub-range, respectively. The optical filter including the Bragg resonators can also be configured to operate over the entire predetermined filtering range, even if one resonator is inoperable in its sub-range, as described with respect to other examples herein.

[0148] Figure 14ashows a configuration where the first resonator 1404 is in an “on” configuration (e.g., heated), while the second resonator 1432 is in an “off” configuration. In particular, in the case where the target wavelength to be filtered is in the first sub-range of a predetermined filtering range, the first controller 1430 can be operated to shift the resonant wavelength of the first resonator to the target wavelength (e.g., by heating the first resonator). As shown in this example, the first resonator is configured to reflect the resonant wavelength λ i , which is output by the MMI to the output waveguide. The resonant wavelength does not pass through the first resonator, and thus, the signal transmitted through the through-port 1444 (e.g., the output waveguide) does not contain the resonant wavelength. The resonant wavelength is reflected by the resonator, and thus the resonant wavelength is transmitted through the drop-port 1446. Therefore, the resonant wavelength can be added or removed in an optical system connected to the optical filter.

[0149] Similarly, Figure 14b shows another case where the first resonator 1404 is in an “off” configuration and the second resonator 1406 is in an “on” configuration. In particular, in the case where the target wavelength to be filtered is in the second sub-range of a predetermined filtering range, the second controller 1432 can be operated to shift the resonant wavelength of the second resonator to the target wavelength (e.g., by heating the second resonator). As shown in this example, the second resonator is configured to reflect the resonant wavelength λ output by the MMI 1442 to the output waveguide k . The resonant wavelength does not pass through the second resonator, and thus, the signal transmitted through the through-port 1444 does not contain the resonant wavelength. The resonant wavelength is reflected by the resonator, and thus the resonant wavelength is transmitted through the drop-port 1446. Therefore, the resonant wavelength can be added or removed in an optical system connected to the optical filter.

[0150] The optical filter including the Bragg resonator can also be configured such that in the case where one of the resonators or controllers (e.g., heating element) fails, each Bragg resonator can operate in the sub-range of the other Bragg resonator, as described with respect to the above examples.

[0151] It will be appreciated that in any of the above examples, any number of resonators can be used, where each resonator can serve a different part of the predetermined filtering range. Therefore, multiple resonators can be used to filter different parts of the predetermined filtering range (e.g., N optical filters can filter 1 / N of the predetermined filtering range). Each resonator can be configured to operate in a different sub-range of the predetermined filtering range during normal use.

[0152] For example, three resonator configurations can be used, where a predetermined filtering range is divided into three parts. Two resonators operating on parts of the predetermined filtering range adjacent to the upper and lower boundaries of the predetermined filtering range can be tuned to a range that is one-third of the total operating range of the optical filter, while the resonator operating on the central part of the predetermined filtering range can be tuned over at least half of the predetermined filtering range.

[0153] The optical filter described in any of the above examples can include a processor configured to determine which resonator will be operated based on the received target wavelength, or can communicate with such a processor. The optical filter can receive a signal indicating a wavelength that will correspond to the target wavelength, and then the optical filter can select a relevant resonator for operation based on the position of the target wavelength within the predetermined filtering range, as described with respect to the above examples.

[0154] The optical filter can include or be connected to a processing circuit that can control the operation of the optical filter and can implement the methods described herein. The processing circuit can be configured or programmed to control the optical filter in the manner described herein. The processing circuit can include one or more hardware components, such as one or more processors, one or more processing units, one or more multi-core processors, and / or one or more modules. In a particular embodiment, each of the one or more hardware components can be configured to perform or be used to perform a single or multiple steps of the methods described herein with respect to the optical filter. In some embodiments, the processing circuit can be configured to run software to perform the methods described herein with respect to the optical filter. According to some embodiments, the software can be containerized. Thus, in some embodiments, the processing circuit can be configured to run a container to perform the methods described herein with respect to the optical filter.

[0155] In short, the processing circuit can be configured to instruct the controller to filter the target wavelength. The processing circuit can determine the target wavelength to be filtered and can send this information to the controller. Alternatively, the filter can include or be connected to a memory. The memory can include volatile memory or non-volatile memory. In some embodiments, the memory can include non-transitory media. Examples of memory include, but are not limited to, random access memory (RAM), read-only memory (ROM), mass storage media such as hard disks, removable storage media such as compact discs (CDs) or digital video discs (DVDs), and / or any other memory.

[0156] The processing circuit may be connected to a memory. In some embodiments, the memory may be used to store program code or instructions which, when executed by the processing circuit, cause the optical filter to operate in the manner described herein for the optical filter. For example, in some embodiments, the memory may be configured to store program code or instructions that may be executed by the processing circuit to cause the optical filter to operate according to the methods described herein. Alternatively or additionally, the memory may be configured to store any information, data, messages, requests, responses, indications, notifications, signals, etc. described herein. The processing circuit may be configured to control the memory to store the information, data, messages, requests, responses, indications, notifications, signals, etc. described herein.

[0157] In general, the various exemplary embodiments may be implemented using hardware or a special purpose circuit, software, logic, or any combination thereof. For example, some aspects may be implemented in hardware while other aspects may be implemented in firmware or software that may be executed by a controller, microprocessor, or other computing device, but the present disclosure is not limited thereto. Although the various aspects of the exemplary embodiments of the present disclosure may be illustrated and described as block diagrams, flowcharts, or using some other graphical representation, it will be readily understood that, as non-limiting examples, the blocks, devices, systems, techniques, or methods described herein may be implemented in hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controllers, or other computing devices, or some combination thereof.

[0158] Accordingly, it should be understood that at least some aspects of the exemplary embodiments of the present disclosure may be implemented in various components such as integrated circuit chips and modules. Accordingly, it should be understood that the exemplary embodiments of the present disclosure may be implemented in a device embodied as an integrated circuit, where the integrated circuit may include circuitry (and possibly firmware) for embodying at least one or more of a data processor, a digital signal processor, a baseband circuit, and a radio frequency circuit, which are configurable to operate in accordance with the exemplary embodiments of the present disclosure.

[0159] It should be understood that at least some aspects of the exemplary embodiments of the present disclosure may be embodied in computer-executable instructions executed by one or more computers or other devices, such as in one or more program modules. Generally, program modules include routines, programs, objects, components, data structures, etc., which perform specific tasks or implement specific abstract data types when executed by a processor in a computer or other device. The computer-executable instructions can be stored on a computer-readable medium, such as a hard disk, optical disk, removable storage medium, solid-state memory, RAM, etc. As those skilled in the art will understand, the functions of the program modules can be combined or distributed as needed in various embodiments. In addition, the functions can be fully or partially embodied in firmware or hardware equivalents, such as integrated circuits, field-programmable gate arrays (FPGAs), etc.

[0160] References to "an embodiment", "embodiment", etc. in the present disclosure mean that the described embodiment may include a particular feature, structure, or characteristic, but each embodiment is not necessarily to include the particular feature, structure, or characteristic. Moreover, these terms do not necessarily refer to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is considered within the knowledge of those skilled in the art to implement such feature, structure, or characteristic in connection with other embodiments, whether or not explicitly described.

[0161] It should be understood that although the terms "first", "second", etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element, without departing from the scope of the present disclosure. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed terms.

[0162] The terms used herein are only for the purpose of describing particular embodiments and are not intended to limit the present disclosure. As used herein, the singular forms "a" and "the" are also intended to include the plural forms, unless the context clearly indicates otherwise. It should also be understood that when the terms "comprises", "comprising", "has" are used herein, they specify the presence of the described features, elements, and / or components, but do not preclude the presence or addition of one or more other features, elements, components, and / or combinations thereof. As used herein, the term "connected" encompasses both direct and / or indirect connections between two elements.

[0163] This disclosure includes any novel feature or combination of features disclosed herein, or any generalization thereof. Various modifications and adaptations of the foregoing exemplary embodiments of this disclosure will become apparent to those skilled in the relevant art when read in conjunction with the accompanying drawings. However, any and all modifications will still fall within the scope of the non-limiting and exemplary embodiments of this disclosure.

Claims

1. An optical filter (302) for an optical network, the optical filter (302) being configured to adaptively add and / or remove target wavelengths in a predetermined filtering range (512), the optical filter comprising: A first resonator (304), configured to have a first resonance wavelength outside a first sub-range of the predetermined filtering range when a first resonance control variable of the first resonator is set to a first value, and a second resonance wavelength within the first sub-range (514) of the predetermined filtering range when the first resonance control variable of the first resonator is set to a second value; And A second resonator (306), configured to have a third resonance wavelength outside a second sub-range (516) of the predetermined filtering range when a second resonance control variable of the second resonator is set to a third value, and a fourth resonance wavelength within the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is set to a fourth value, Wherein the optical filter (302) is configured to selectively change the first resonance control variable of the first resonator (304) to the second value, or change the second resonance control variable of the second resonator (306) to the fourth value, the second value being the value at which the second resonance wavelength moves to the target wavelength, the fourth value being the value at which the fourth resonance wavelength moves to the target wavelength, and Wherein the second resonator (306) is configured such that if the first resonance control variable cannot be changed from the first value to the second value, the second resonance control variable can be changed to a fifth value to generate a resonance wavelength in the first sub-range of the predetermined filtering range, and / or the first resonator (304) is configured such that if the second resonance control variable cannot be changed from the third value to the fourth value, the first resonance control variable can be changed to a sixth value to generate a resonance wavelength in the second sub-range of the predetermined filtering range.

2. The optical filter according to claim 1, wherein, The resonance control variable is at least one of the following: the electrical gate voltage of the resonator; and the temperature of the resonator.

3. The optical filter according to claim 1, wherein Change the first resonance control variable of the first resonator (304) when the target wavelength is closest to the first resonance wavelength, and change the second resonance control variable of the second resonator (306) when the target wavelength is closest to the third resonance wavelength.

4. The optical filter according to claim 1, wherein The optical filter (302) is configured to change the value of the first resonance control variable of the first resonator when the target wavelength is in the first sub-range, and the optical filter is configured to change the value of the second resonance control variable of the second resonator when the target wavelength is in the second sub-range.

5. The optical filter according to claim 4, wherein, When the target wavelength is in the first sub-range, the second resonator is configured to have the third resonance wavelength, and when the target wavelength is in the second sub-range, the first resonator is configured to have the first resonance wavelength.

6. The optical filter according to claim 1, wherein, If a fault related to the first resonator is detected, a change of the second resonance control variable to a fifth value occurs to generate a resonance wavelength in the first sub-range of the predetermined filtering range, and if a fault related to the second resonator is detected, a change of the first resonance control variable to a sixth value occurs to generate a resonance wavelength in the second sub-range of the predetermined filtering range.

7. The optical filter according to any one of claims 1 to 6, wherein The first sub-range extends across half of the predetermined filtering range, and the second sub-range constitutes the remaining part of the predetermined filtering range.

8. The optical filter according to any one of claims 1 to 6, wherein, The first sub-range and the second sub-range are separated by a guard range (613).

9. The optical filter according to claim 8, wherein When the first resonance control variable of the first resonator is set at the first value, the first resonance wavelength is in the guard range, and when the second resonance control variable of the second resonator is set at the third value, the third resonance wavelength is in the guard range.

10. The optical filter according to any one of claims 1 to 6, wherein, The first resonance wavelength is outside the predetermined filtering range, and the third resonance wavelength is outside the predetermined filtering range.

11. The optical filter according to any one of claims 1 to 6, wherein, The first sub-range and the second sub-range do not overlap.

12. The optical filter according to any one of claims 1 to 6, wherein, At the first value of the first resonance control variable, the first free spectral range of the first resonator is greater than the predetermined filtering range, and at the third value of the second resonance control variable, the second free spectral range of the second resonator is greater than the predetermined filtering range.

13. The optical filter according to any one of claims 1 to 6, wherein, The optical filter includes no more than two resonators.

14. The optical filter according to any one of claims 1 to 6, wherein, The optical filter includes a plurality of resonators. When the respective resonance control values of the resonators are at the off value, each resonator has a resonance wavelength outside the predetermined filtering range, and when the respective resonance control values of the resonators are at the on value, each resonator has a resonance wavelength within the predetermined filtering range.

15. The optical filter according to any one of claims 1 to 6, wherein the target wavelength is the wavelength of a channel to be added or removed in the optical network.

16. An optical network comprising the optical filter according to any one of the preceding claims.

17. A method of using an optical filter configured to adaptively add and / or remove a target wavelength in a predetermined filtering range, the method comprising: changing the first resonance control variable of the first resonator from a first value to a second value, wherein the first resonator includes a first resonance wavelength outside the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is at the first value, and a second resonance wavelength within the first sub-range of the predetermined filtering range when the first resonance control variable of the first resonator is at the second value; or Changing a second resonance control variable of a second resonator from a third value to a fourth value, where the second resonator includes a third resonance wavelength outside a second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is at the third value, and a fourth resonance wavelength within the second sub-range of the predetermined filtering range when the second resonance control variable of the second resonator is at the fourth value, wherein the method further comprises: changing the first resonance control variable of the first resonator to the second value, or changing the second resonance control variable of the second resonator to the fourth value, the second value being the value at which the second resonance wavelength corresponds to the target wavelength, the fourth value being the value at which the fourth resonance wavelength corresponds to the target wavelength, and if the first resonance control variable cannot be changed from the first value to the second value, changing the second resonance control variable to a fifth value to generate a resonance wavelength in the first sub-range of the predetermined filtering range, or if the second resonance control variable cannot be changed from the third value to the fourth value, changing the first resonance control variable to a sixth value to generate a resonance wavelength in the second sub-range of the predetermined filtering range.

18. The method according to claim 17, wherein, Changing the first resonance control variable of the first resonator when the target wavelength is closest to the first resonance wavelength, and changing the second resonance control variable of the second resonator when the target wavelength is closest to the third resonance wavelength.

19. The method according to claim 17 or 18, wherein, The method further comprises: Changing the value of the first resonance control variable of the first resonator when the target wavelength is in the first sub-range, and changing the value of the second resonance control variable of the second resonator when the target wavelength is in the second sub-range.

20. The method according to claim 17, wherein If a fault related to the first resonator is detected, changing the second resonance control variable to a fifth value to generate a resonance wavelength in the first sub-range of the predetermined filtering range occurs, and if a fault related to the second resonator is detected, changing the first resonance control variable to a sixth value to generate a resonance wavelength in the second sub-range of the predetermined filtering range occurs.

21. The method according to claim 17 or 18, wherein The method further comprises receiving light input to the optical filter.

22. The method according to claim 17 or 18, wherein The method further comprises outputting light from the optical filter.

23. A computer-readable storage medium storing a computer program, the computer program comprising instructions that, when executed by a processor, cause the processor to perform the method according to any one of claims 17 to 22.

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