Optical System

Abstract

To provide an optical device improvement technology.SOLUTION: Configurations for an optical device used for light splitting and wavelength locking are disclosed. The optical device may be a two by three coupler with a first waveguide coupled to a second waveguide, and a third waveguide coupled to the second waveguide. The first and third waveguides may receive input light and optically couple light to the second waveguide. The output signals of the first, second, and third waveguides may have a constant phase difference from one another over a broadband wavelength range, which may allow for phase unwrapping. By phase unwrapping the output signals over an FSR and performing further phase unwrapping over the broadband wavelength range, a continuous signal may be produced and used to sequentially lock each wavelength of light emitted by light sources over the broadband wavelength range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates generally to optical couplers. More particularly, embodiments herein relate to optical systems having optical coupling waveguides that output signals that can be used for phase unwrapping for wavelength-locked light sources.

[0002] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 63 / 236,567, filed August 24, 2021, the contents of which are incorporated herein by reference in their entirety. [Background technology]

[0003] Optical systems typically use multiple light sources to measure various types of information. In some cases, it may be useful to monitor the optical properties of the light emitted by the light sources. For example, the optical properties of the light emitted by the light source may be measured and monitored to ensure that the light source has a certain degree of wavelength stability. Furthermore, wavelength locking may be achieved when working with a single wavelength or a small wavelength range, but the complexity of the wavelength range increases as the wavelengths expand to multiple wavelengths or a larger number of wavelengths.

[0004] Some of these optical systems can output light at multiple different wavelengths simultaneously and / or sequentially. However, as the number of wavelengths monitored increases, the size and complexity of the system also increase. Due to the scaling of the optical system's size according to the number of wavelength-locked wavelengths, optical systems used to monitor light may not be suitable for certain applications due to factors such as size and complexity. As an example, such optical systems may take up too much space to be reasonably incorporated into compact electronic devices such as mobile phones, tablet computing devices, laptops, and wearables. In addition, existing optical systems for monitoring light may function over a narrow wavelength range and may not operate over different or wider wavelength ranges. Therefore, it may be desirable to use a system for locking the wavelength of light over a large wavelength range while maintaining a compact form factor for incorporation into compact electronic devices. Summary of the Invention

[0005] Embodiments of systems, devices, methods, and apparatuses described herein are directed to a 2x3 coupler used for wavelength locking. Systems, devices, methods, and apparatuses are also described that are directed to phase unwrapping all wavelengths across a free spectral range and a broadband wavelength range. The 2x3 coupler can generate output signals having sinusoidal wavelength responses with a constant phase difference from one another across a broadband wavelength range, thereby enabling phase unwrapping across a broadband wavelength range. By phase unwrapping the output signal across the free spectral range and performing further phase unwrapping across a broadband wavelength range, a continuous signal can be generated and used to lock each wavelength across a broadband wavelength range. The continuous signal can create a one-to-one relationship between each wavelength and the phase-unwrapped signal.

[0006] In some examples, the present disclosure describes an optical device that may include a first waveguide configured to receive a first light and output a first output signal having a first wavelength response with a first phase shift, a second waveguide optically coupled to the first waveguide and configured to output a second output signal having a second wavelength response with a second phase shift, and a third waveguide optically coupled to the second waveguide and configured to receive the second light and output a third output signal having a third wavelength response with a third phase shift, where a phase difference between the first phase shift, the second phase shift, and the third phase shift is constant.

[0007] In some examples, the present disclosure describes an optical system for monitoring the wavelength of a light source. The optical system may include a light source configured to generate light, a splitter that receives light received from the light source and splits the light into a first split light and a second split light, a phase shifter that receives the first split light and is positioned to phase shift the first split light, and a 2×3 coupler configured to receive the first split light from the phase shifter and the second split light from the splitter, and output a first output signal, a second output signal, and a third output signal, each of the first output signal, the second output signal, and the third output signal having a respective intensity based on a respective interference between the first split light and the second split light. The optical system may also include a controller configured to monitor the wavelength of the light received by the splitter using the intensities of the first output signal, the second output signal, and the third output signal.

[0008] In some examples, this disclosure describes a method for phase unwrapping a signal. The method may include generating first, second, and third output signals, each having a wavelength response separated from one another by a constant phase difference, extracting unwrapped phase from the first, second, and third output signals over a wavelength range to generate an unwrapped phase signal, differentiating the unwrapped phase signal to generate a differentiated signal, comparing the differentiated signal to a threshold voltage to generate a compensated differentiated signal, and integrating the compensated differentiated signal to generate an integrated signal configured for use in generating a continuous signal for wavelength locking.

[0009] In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram of an exemplary wavelength locking system. [Figure 2] FIG. 1 is a block diagram of an exemplary wavelength locking system including a 2×3 coupler. [Figure 3A] A cross-sectional view of a 2x3 coupler. [Figure 3B] 3B is a cross-sectional view of an exemplary wavelength locking system with a 2×3 coupler shown in FIG. 3A. [Figure 4A] 3 is a graph illustrating an exemplary output signal from a wavelength locking system such as that shown in FIG. 2. [Figure 4B] 4B is a graph illustrating an example phase extracted from the example output signal of FIG. 4A. [Figure 5] FIG. 10 is a sample circuit diagram of an exemplary circuit for a phase unwrapped output signal. [Figure 6A] 6 is a graph of signals measured at various nodes of the circuit diagram of FIG. 5. [Figure 6B] 6 is a graph of signals measured at various nodes of the circuit diagram of FIG. 5. [Figure 6C] 6 is a graph of signals measured at various nodes of the circuit diagram of FIG. 5. [Figure 6D] 6 is a graph of signals measured at various nodes of the circuit diagram of FIG. 5. [Figure 7A] FIG. 10 is a cross-sectional view of another modified example of a 2×3 coupler. [Figure 7B] FIG. 7B is a schematic diagram of a wavelength locking system incorporating the 2×3 coupler of FIG. 7A. DETAILED DESCRIPTION OF THE INVENTION

[0011] The use of cross-hatching or shading in the accompanying figures is generally provided to clarify boundaries between adjacent elements and also to facilitate legibility of the figures. Thus, neither the presence nor absence of cross-hatching or shading is intended to convey or indicate any preference or requirement regarding particular materials, material properties, element proportions, element dimensions, commonalities of similarly illustrated elements, or any other characteristics, attributes, or properties with respect to any element shown in the accompanying figures.

[0012] It will be understood that the proportions and dimensions (whether relative or absolute) of the various features and elements (and collections and groups thereof), as well as the boundaries, separations and relationships presented therebetween, are provided in the accompanying figures solely to facilitate an understanding of the various embodiments described herein, and as such may not necessarily be presented or drawn to scale, and are not intended to imply any preference or requirement for the illustrated embodiment to the exclusion of the embodiment described with reference thereto.

[0013] Reference will now be made in detail to exemplary embodiments as illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit these embodiments to a single preferred embodiment. On the contrary, the following description is intended to cover alternatives, modifications, and equivalents, as may be included within the spirit and scope of the described embodiments as defined by the appended claims.

[0014] Two elements that are "coupled" to each other may be permanently or removably physically coupled to each other and / or operatively or functionally coupled to each other. Generally, physically coupled elements refer to a physical connection between two or more elements that at least partially defines or limits the relative positions of the elements. Furthermore, two or more elements that are operatively or functionally coupled may affect each other in that the operation of the first element may directly or indirectly affect or influence the operation of the second element. Additionally, two elements that are "optically coupled" to each other may allow light to pass and / or couple from one element to the other.

[0015] In the following description of the embodiments, reference is made to the accompanying drawings, which show, by way of illustration, specific embodiments that may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the various embodiments.

[0016] Disclosed herein is an integrated photonics system that uses one or more light sources that output multiple wavelengths that can be combined together using a 2x3 coupler with two inputs and three outputs. Specifically, the 2x3 coupler splits the light received at each of its inputs among its three outputs. When light is received simultaneously at both of its inputs, each of the three outputs will output a different combination of the input light. The 2x3 coupler can be used to combine light (simultaneously or sequentially) over a large wavelength range while maintaining a compact form factor. The output signal from the 2x3 coupler can be used by the integrated photonics system to control and stabilize the wavelength(s) of light emitted by the integrated photonics system via wavelength locking.

[0017] The integrated photonics system may further include an on-chip wavelength locking system for wavelength locking the light source to a target wavelength. In some embodiments, the integrated photonics system includes a wavelength locking system with multiple light sources, where the wavelength locking system can sequentially lock each of the multiple light sources to a respective target wavelength. At least some of the multiple light sources may each emit light at a different respective nominal wavelength, which collectively span a range of wavelengths (as determined by the overall specifications of the integrated photonics system), and therefore it may be desirable for the wavelength locking system to be able to effectively perform wavelength locking over the entire range of wavelengths. In general, the embodiments described herein may be capable of locking to wavelengths over a broadband wavelength range (e.g., a wavelength range spanning at least 1 micrometer). In other words, the wavelength locking system may be capable of locking the wavelengths of two light sources, where the wavelengths emitted by each light source are separated by at least 1 micrometer.

[0018] An on-chip wavelength locking system can utilize the 2x3 coupler described herein. Specifically, a 2x3 coupler can be configured to receive two input signals and output three output signals, each having an intensity based on the respective interference between the input signals when the two inputs contain phase-shifted light of specific wavelengths (collectively "input light") (as described in more detail herein). The intensity of each output signal is wavelength-dependent, so that as the wavelength of the input light changes, the intensity of each output signal also changes. Specifically, for each output, a sinusoidal wavelength relationship exists between the input light wavelength and the output intensity; in other words, the output intensity varies sinusoidally as a function of the input light wavelength. A 2x3 coupler can be configured so that the sinusoidal wavelength relationships for each output each have a different phase, such that there is no overlap between the maximum and minimum values ​​of the three output signals across a target wavelength range, thereby enabling the output signals to be used to lock the wavelength of the input light, as described further herein.

[0019] A wavelength locking system may include an optical splitter 2x3 coupler, which is used to split two inputs of light into three outputs of light. Wavelength locking devices can also be used to monitor and lock the wavelength of a corresponding light source to a target wavelength. Generally, each light source can sequentially emit a distinct wavelength of light to which it can be locked. The 2x3 coupler includes an intermediate waveguide between the two outer arms of the device, thus generating three output signals. As previously mentioned, the intensity of each output signal can be wavelength-dependent, such that the intensity of each output signal changes as the wavelength of the input light changes.

[0020] This wavelength locking device may be symmetrical about a horizontal axis, with the light in one arm having a phase shift relative to the light in the other arm (and / or the input light to the coupler), resulting in output signals each having an amplitude (e.g., intensity) and a sinusoidal wavelength relationship with respect to each other. The sinusoidal wavelength relationship is such that the maximum and minimum values ​​of at least one of the output signals do not align with the maximum and minimum values ​​of the other output signals. The 2x3 coupler may have a relatively small footprint while still outputting a signal used for locking the output to a desired wavelength. In some embodiments, this wavelength locking occurs for light having a broadband wavelength range of approximately 1 micrometer.

[0021] By coupling light into an intermediate waveguide located between the two outer waveguides of a 2x3 coupler, the two outer waveguides can generate an output signal with a sinusoidal wavelength relationship over a wide wavelength range. The output signal of a 2x3 coupler can be similar to that of three Mach-Zehnder interferometers ("MZIs"), but the 2x3 coupler can be smaller than this combination of MZIs because the size and complexity of the locking technique increase as the wavelength range increases.

[0022] As described herein, the constant phase difference is a sinusoidal wavelength relationship of the output signals resulting from a multi-mode 2x3 coupler, which can result in output signals with dead zones that are not aligned with each other. That is, each output signal can have an intensity based on the respective interference between the input signals. For each output, a sinusoidal wavelength relationship exists between the input optical wavelength and the output intensity, and the intensity of each output signal is wavelength-dependent such that the output intensity varies sinusoidally as a function of the input optical wavelength. Because each sinusoidal wavelength relationship among the output signals can have a different phase between the sinusoidal wavelength relationships and a constant phase difference, the output signals may not overlap between the maximum and minimum values ​​of the three output signals across a target wavelength range, allowing the output signals to be used to sequentially lock the wavelength of the input light.

[0023] While systems exist that can be used for wavelength locking, they may be limited to functioning over a narrow wavelength range (e.g., 50 nanometers), may introduce unaccounted phase shifts, may have high optical losses, may be too large for integration into electronic devices (e.g., mobile or wearable devices), etc. Wavelength locking can be used in a variety of photonics applications, including but not limited to telecommunications, medical devices, spectroscopy, etc. Maintaining wavelength precision in photonics applications can be important for accurate data collection, and wavelength locking can prevent undesired deviations in the wavelength of the emitted light. Wavelength locking and the method by which it is achieved are described in further detail herein with reference to FIG. 1.

[0024] As used herein, a "working zone" is a wavelength range of a single output signal where the single output signal has a non-zero or substantially near-zero slope. Additionally, a "dead zone" is a wavelength range of a single output signal where the single output signal has a zero or near-zero slope, such as at and around the peaks and troughs of the single output signal. In other words, a working zone is any portion of a single output signal that is not a dead zone. Thus, points within the working zone have higher slopes than points in the dead zone. In other embodiments having multiple output signals with sinusoidal wavelength relationships to one another, the multiple output signals may each have a coincident, different phase. While each of the multiple output signals may have a slope at or near zero, the dead zone for each output may fall at a different point within the wavelength range, and thus at least one output signal of the multiple output signals may have a slope that is not zero or is not a "dead zone."

[0025] As described herein, the dead zone of the output signal of other solutions can make it difficult to distinguish small changes in wavelength, which can then limit the accuracy of wavelength locking based on this output signal. In contrast, the wavelength locking device described herein can operate more effectively because, at any given wavelength, at least one output signal is in the corresponding working zone of that signal. Therefore, there can always be an output signal with a large slope and a large difference between the target wavelength and the measured wavelength.

[0026] Wavelength locking devices that sequentially generate these multiple output signals with a constant phase difference over the wavelength range of interest are typically small in size (at least compared to other previous devices that perform wavelength locking) and less complex than other structures with similar functionality, and can accurately determine the difference between the measured wavelength and the target wavelength of the signal. This determined difference can be used for wavelength locking. The use of sinusoidal output signals for wavelength locking is described in further detail with reference to FIG. 1.

[0027] As used throughout this specification, a reference number without an alphabetic letter following it can refer to the corresponding reference, the group of all references, or one or more of some of the references. For example, "209" can refer to any one of optical paths 209 (e.g., optical path 209a or optical path 209b, etc.), or it can refer to both optical paths 209, depending on the context in which it is used. The term optical path 209 may be used when discussing general properties of the optical paths.

[0028] Representative examples of applications of the methods and apparatus according to the present disclosure are described in this section. These examples are provided solely to add context and aid in understanding the described examples. Therefore, it will be apparent to one skilled in the art that the described examples can be practiced without some or all of the specific details. Other applications are possible, and therefore the following examples should not be construed as limiting.

[0029] These and other embodiments are described below with reference to Figures 1-7B. However, those skilled in the art will readily appreciate that the detailed description provided herein with respect to these figures is for illustrative purposes only and should not be construed as limiting.

[0030] (wavelength locking system) 1 shows a block diagram of an exemplary wavelength-locking system 100 including light source(s) 113, a splitter 102, and a wavelength-locking device 117. The light source(s) 113 emit input light along an optical path 103 to the splitter 102. In general, the optical path 103 may represent a waveguide, an optical fiber, a free-space optic, or other element or medium through which light travels. In the embodiment of FIG. 1, the optical path 103 is a waveguide passing through instead of a waveguide (e.g., light propagating through a medium such as air or polysilicon between devices) that may be used in a larger system to couple light into the wavelength-locking device 117.

[0031] The splitter splits the input light received from input optical path 103 and passes the split light to wavelength-locking device 117. Wavelength-locking device 117 typically includes three waveguides (e.g., in the form of a 2×3 coupler) that receive light (directly or indirectly) from splitter 102. In some embodiments, wavelength-locking device 117 may include an additional splitter that provides light to multiple waveguides optically coupled to each other. In other embodiments, wavelength-locking device 117 may include multiple waveguides and / or may not include an additional splitter. For example, two optical inputs may be passed to wavelength-locking device 117 via two optical paths, as opposed to one optical input that is split and passed to wavelength-locking device 117. Additionally, a phase shifter (not shown) may receive light from one output of splitter 102 and phase-shift the light passed to one of the waveguides. These embodiments are described in further detail with reference to FIGS. 2-7B.

[0032] In some embodiments, wavelength locking device 117 can generate output signals (e.g., output light) via optical paths 107a, 107b, and 107c used to lock the wavelength of light from light source 113. The output signals can be generated through interference between phase-shifted light passed to one of the waveguides of the 2x3 coupler. Wavelength locking device 117 can generate output signals, each of which has a sinusoidal relationship with the wavelength of the light and a constant phase difference between the sinusoidal relationships. Thus, the dead zones of the output signals may not align with any wavelength within the wavelength range of light input by light source(s) 113.

[0033] By analyzing the output of the wavelength locking device 117, the wavelength locking system 100 can identify shifts in the wavelength of the light output by the light source 113. Specifically, as the wavelength changes, the intensity of each output will change according to a sinusoidal wavelength relationship. A controller (not shown) can measure the change in one or more of the output signals and use one or more of these changes (e.g., the output signal with the largest magnitude change) as an indicator of the wavelength change. This change can be used by the controller as feedback in controlling the output of the light source 113.

[0034] Optionally, the output signal may be sent to a phase unlocking device (which may be an integrated circuit performing a specific function, digital logic, etc.) that extracts the phase from the output signal. This extracted phase information may be used to determine the difference between the measured wavelength and the target wavelength, so that the light source may be adjusted to emit the target wavelength. Wavelength locking systems and 2x3 couplers are described in further detail herein with reference to Figures 2-7B.

[0035] FIG. 2 shows a block diagram of an exemplary wavelength locking system 200, including a 2×3 coupler and a controller block that may include a set of photodetectors. The wavelength locking system 200 may be used to lock a light source to any target wavelength selected from a wavelength range. In some cases, the wavelength locking system 200 may be used to lock generated light to any of several different wavelengths over a wavelength range spanning one micrometer; however, it should be understood that the wavelength locking systems described herein may be used within any suitable wavelength range (e.g., spanning less than 100 nm, spanning at least 100 nm, spanning at least 500 nm, or spanning at least 1300 nm). Indeed, in some cases, the wavelength range is limited only by the range of wavelengths that can be emitted by the light source(s) and the underlying material used to form the coupler itself (e.g., the wavelengths at which light can be carried by a given waveguide material).

[0036] Wavelength locking system 200 includes splitter 205, phase shifter 210, 2×3 coupler 260, and controller 265. As shown in FIG. 2, splitter 205 is a 1×2 splitter, but any suitable component or combination of components can be used to achieve a similar optical splitting function (e.g., a 1×N splitter and an N×M splitter) to provide two inputs to 2×3 coupler 260. Splitter 205 is optically coupled to one or more light sources 213 and receives input light from the one or more light sources 213 via optical path 203. Splitter 205 can split the optical output of light source 213 between two output optical paths, passing a first split light to 2×3 coupler 260 via optical path 209a and a second split light to phase shifter 210 via optical path 209b. Phase shifter 210 can introduce a phase shift or delay into the light received via optical path 209b to create a phase difference between the light received via optical paths 290a and 290b. While 2×3 coupler 260 is illustrated in FIG. 2 in the context of wavelength-locking system 200, 2×3 coupler 260 can be used in any system for combining two inputs of light to output three signals. Controller 265 can receive first optical output signal 230, second optical output signal 235, and third optical output signal 240 from 2×3 coupler 260. Controller 265 can monitor the wavelengths of the light received by splitter 205 using the intensities of first output signal 230, second output signal 235, and third output signal 240.

[0037] The light source(s) 213 can emit light that is received by the splitter 205 along the optical path 203. The light source(s) 213 can include a single light source or multiple light sources. In some examples, the light source(s) 213 can be any coherent or quasi-coherent light source, or any combination thereof. Each light source 213 can emit light of a single wavelength or can be configured as a tunable light source that can emit over a wavelength range of light, e.g., 15 nanometers, although other tunable light sources can have different ranges greater than or less than 15 nanometers. Additionally, any number of light sources can be used in the wavelength locking system 200.

[0038] Continuing with FIG. 2 , splitter 205 passes the split light to 2×3 coupler 260 via optical paths 209a and 209b. 2×3 coupler 260 generally includes three waveguides. A first waveguide of 2×3 coupler 260 receives light via optical path 209a, and a third waveguide of 2×3 coupler 260 receives light via optical path 209b. A second waveguide is optically coupled to both the first and third waveguides and may be an intermediate waveguide disposed therebetween. All three waveguides of 2×3 coupler 260 may output light along unique ones of the output optical paths via corresponding outputs of 2×3 coupler 260. One example of a configuration of a 2x3 coupler 260 and its relative layout is discussed below with respect to Figures 3A and 3B, and another example is discussed below with respect to Figures 7A and 7B. Light couples from the first and third waveguides into a second intermediate waveguide, allowing the light between the waveguides to interfere with each other to generate three optical output signals 230, 235, 240.

[0039] Specifically, some of the light received by the first waveguide couples to the second waveguide and from there to the third waveguide. Similarly, some of the light received by the third waveguide couples to the second waveguide and from there to the first waveguide. As a result, each waveguide outputs light containing components received from both the first and third waveguides. The first waveguide can generate a first output signal 230, the second waveguide can generate a second output signal 235, and the third waveguide can generate a third output signal 240. As previously mentioned, each output signal can have an intensity based on the respective interference between the input signals with each other. For each output, a sinusoidal wavelength relationship exists between the input optical wavelength and the output intensity, and the intensity of each output signal is wavelength-dependent (also referred to herein as the "wavelength response" of the output signal) such that the output intensity varies sinusoidally as a function of the input optical wavelength. The construction and function of the three waveguides will be discussed in more detail with reference to Figures 3A-4B.

[0040] As mentioned above, controller 265 can receive first output signal 230, second output signal 235, and third output signal 249 from 2x3 coupler 260. Controller 265 can include a set of detectors 270 that converts first output signal 230, second output signal 235, and third output signal 240 into first digital output signal, second digital output signal, and third digital output signal. In some embodiments, first output signal 230, second output signal 235, and third output signal 240 can be light intensities, and first digital output signal 245, second digital output signal 250, and third digital output signal 255 can be digital signals equivalent to these light-based output signals that can be converted by the set of detectors 270.

[0041] The controller 265 can generate a feedback signal 275 based on the first, second, and third output signals 230, 235, and 240 to control the light source(s) 213. If the wavelength of light emitted by the light source(s) 213 deviates from the target wavelength, the feedback signal 275 adjusts the operation of the light source(s) to tune the wavelength of the light emitted by the light source(s) 213 back toward the target wavelength. For example, at a given wavelength, the first, second, and third output signals 230, 235, and 240 each have a corresponding target output value (e.g., a first output target, a second output target, and a third output target). Similarly, the first, second, and third digital output signals 245, 250, and 255 have corresponding target digital values.

[0042] The controller 265 can compare the first, second, and third output signals 230, 235, and 240 to their corresponding output target values ​​(e.g., by comparing the first, second, and third digital output signals 245, 250, and 255 to their corresponding digital target values) and can generate a feedback signal 275 as a function of deviation from these target output values. The feedback signal 275 then controls the light source(s) 213 to adjust one or more operating parameters that change the wavelength of the light source(s) 213. Thus, the controller 265 can provide closed-loop control of the light source(s) 213 to maintain the wavelength at the target wavelength by maintaining the first, second, and third output signals 230, 235, and 240 at their respective target output values.

[0043] As mentioned above, many existing optical couplers are large and difficult to reduce in size. A single 2×3 coupler 260 can be more compact and space-efficient than such existing couplers. In addition, wavelength locking system 200 can be temperature-insensitive, whereas many other optical couplers change their output with temperature. In addition, the output signal generated by wavelength locking system 200 may not have aligned dead zones (e.g., the dead zones of the output signal may be offset in time from one another), thus reliably generating output light that carries information about any mismatch between the actual wavelength or wavelength range of the output light and the target wavelength or wavelength range of the target light.

[0044] (2x3 coupler) Figure 3A is a cross-sectional view of a 2x3 coupler 301 that defines five regions. The first region (or "input region") is between lines S0 and S1 and generally represents the input region where the waveguides are spaced far enough apart so that coupling between them does not occur. The second region (or "first S-bend region") is between lines S1 and S2 and represents the first S-bend region, or the point where the sidewalls of the output waveguides begin to curve and, in some cases, light may begin to couple from the first and third waveguides (e.g., outer waveguides) to the second waveguide (e.g., intermediate waveguide). The third region (or "central region") is between S2 and S3 and represents the primary coupling region where light may couple between the first and third waveguides (e.g., outer waveguides) and the second waveguide (e.g., intermediate waveguide). The fourth region (or "second S-bend region") is between S3 and S4 and represents the second S-bend region, or the point where the curvature of the sidewalls of the first and third waveguides may end and light may terminate coupling from the first and third waveguides (e.g., outer waveguides) to the second waveguide (e.g., intermediate waveguide). The fifth region (or "output region") is between S4 and S5 and represents the region where light may be output from the first, second, and third waveguides without further coupling between the waveguides. These regions and their properties are discussed herein. The regions discussed herein are used for illustrative purposes and do not represent distinct regions within the device having different material compositions.

[0045] Additionally, the S-bend regions may be different shapes (e.g., 90 degrees, straight, 45 degrees, etc.), the waveguides in the central region may be tapered or non-tapered, and the waveguides in the input region may be curved, S-shaped, or any other shape. In still other embodiments, the first waveguide 315 and the third waveguide 325 may curve at different rates from each other, and there may not be any linear sections or S-bend shapes in the waveguides in the first region bounded by lines S0 and S1 and the second region bounded by lines S1 and S2.

[0046] The regions can have any configuration with waveguides of various shapes, as long as the waveguide configuration results in optical interaction in the desired manner within each region, as described below. In other words, the specific embodiments described herein are for illustrative purposes only and are not limiting; the waveguides can be straight or curved in various ways, as long as they have the desired optical coupling and wavelength response results. The desired optical coupling can include spacing of the waveguides in the input region (e.g., the region bounded by lines S0 and S1) so that little or no coupling occurs between the waveguides in the input region. In the input region, the waveguides can be positioned closer to each other so that optical coupling between the waveguides can begin to occur, while in the central region, the waveguides can be positioned relative to each other so that more optical coupling occurs between the waveguides. The waveguides in the second S-bend region can be positioned farther apart and separated so that light can no longer couple between the waveguides.

[0047] In some embodiments, the first waveguide 315, the second waveguide 320, and the third waveguide 325 may be strip waveguides, although in some instances, rib waveguides may be used with a rib-to-strip waveguide transition. Additionally, the first waveguide 315, the second waveguide 320, and the third waveguide 325 are shown with similar cross-hatching patterns, and are separate waveguides from one another (although they may be formed or constructed from similar or the same materials).

[0048] The 2×3 coupler 301 includes an input region (bounded in FIG. 3A by lines S0 and S1) that includes corresponding input regions of the first waveguide 315 and the third waveguide 325, and in the embodiment shown in FIG. 3A , includes the input region of the second waveguide 320. Specifically, when the 2×3 coupler 301 is incorporated into an optical system, the input regions of the first waveguide 315 and the third waveguide 325 can be optically connected to and receive light from other components of the optical system. Conversely, the second waveguide 320 can be incorporated into the optical system so that it resides within the input region, but does not receive light through the input region of the second waveguide 320. Thus, the second waveguide 320 in these examples can receive light only via coupling from the first waveguide 315 and the third waveguide 325. The second waveguide 320 is shown in FIG. 3A as having an input region located at the input region of the 2×3 coupler 301, but may alternatively begin in the second region (i.e., bounded by lines S1 and S2).

[0049] The second region (the first S-bend region, as shown bounded by S1 and S2) is a region where the distance between the first, second, and third waveguides 315, 320, 325 decreases between the first and third regions, enabling coupling between the waveguides in the third region. Thus, one or more of the first, second, and third waveguides 315, 320, 325 include one or more bends or curves (such as an S-bend or a C-bend). The terms S-bend and S-shape may be used interchangeably herein to describe the shape of the first and third waveguides 315, 325, as shown in the first and second S-bend regions of FIG. 3A . Each waveguide of a 2×3 coupler may have any suitable combination of curved and straight sections, and the curved sections may bend away from the trajectory of the previous segment at any suitable angle (e.g., 90 degrees, 45 degrees, 30 degrees, etc.). As mentioned above, although the second and fourth regions are referred to herein as first and second S-bend regions, this is for reference and explanation purposes only, and the waveguide bounded by S1 and S2 is not limited to an S-shaped bend.

[0050] In the variation shown in FIG. 3A , the first S-bend region has portions of the first waveguide 315 and the third waveguide 325 that are S-shaped, including the accompanying portion of the second waveguide 320 that is straight. In the first S-bend region, a portion of the first waveguide 315 is curved and has an S-bend shape, which positions the outer arm of the 2×3 coupler closer to a portion of the second (e.g., central) waveguide 320. Similarly, a portion of the third waveguide 325 has an S-bend shape that positions the other outer arm of the 2×3 coupler closer to a central portion of the second waveguide 320. While the second waveguide 320 is shown as straight in the input region and first S-bend region, these portions of the second waveguide 320 may be curved in other embodiments. In general, the shape of these portions of the second waveguide 320 depends primarily on the shape or configuration of the corresponding portions of the first waveguide 315 and the third waveguide 325 and the desired optical coupling efficiency. For example, these portions of the second waveguide 320 may be S-shaped to optically couple more light from one waveguide than the other.

[0051] Additionally, the third waveguide 325 is shown in the five regions of the wavelength locking system 300 with a shape that is approximately symmetrical or a mirror image of the corresponding portion of the first waveguide 315. In other embodiments, the third waveguide 325 may be symmetrical with respect to the corresponding portion of the first waveguide 315 in some regions but not in other regions. As yet another option, portions of the third waveguide 325 may not be symmetrical with respect to the corresponding portion of the first waveguide 315 in any of the five regions. As described herein, the first waveguide 315 may be understood to be a single waveguide extending between lines S0-S5, with various portions of the waveguide located in each of the regions indicated by adjacent lines (e.g., one portion in the region defined by S0 and S1, another portion in the region defined by S1 and S2, etc.). Similar logic may apply to the second waveguide 320 and the third waveguide 325.

[0052] The central region (e.g., extending from S2 to S3) is the region where light couples between the first, second, and third waveguides 315, 320, 325. Specifically, a portion of light traveling through the first waveguide 315 in the central region can couple to the second waveguide 320, and vice versa. Similarly, a portion of light traveling through the third waveguide 325 in the central region can couple to the second waveguide 320, and vice versa. In this manner, the central region of the 2×3 coupler 301 can be configured such that light received at the input region of the first waveguide 315 is at least partially coupled to the second waveguide 320, and at least a portion of that light is coupled from the second waveguide 320 to the third waveguide 325. Similarly, the central region of the 2×3 coupler 301 may be configured such that light received at the input region of the third waveguide 325 is at least partially coupled into the second waveguide 320, and at least a portion of that light is coupled from the second waveguide 320 into the first waveguide 315. As a result, when first and second input light are simultaneously introduced into the input regions of the first and third waveguides 315, 325, respectively, the first, second, and third waveguides 315, 320, 325 will each output a combination of the first and second input light.

[0053] In some variations, the central region is configured so that portions of the first, second, and third waveguides 315, 320, 325 can vary in width and increase or decrease in size within the central region. For example, as shown in FIG. 3A , the second waveguide 320 narrows from the second and fourth regions in the central region, while the first and third waveguides 315, 325 each widen from the second region to the fourth region in the central region. In some cases, the first, second, and third waveguides 315, 320, 325 are adiabatically tapered to provide adiabatic optical coupling between the first waveguide 315 and the second waveguide 320, and between the second waveguide 320 and the third waveguide 325. In some embodiments, portions of the first and second waveguides 315, 320 in the central region may remain adiabatically coupled and not be tapered.

[0054] As shown in the output region of FIG. 3A , the first and third waveguides 315, 325 may taper through the output region. That is, the first waveguide 315 may be wider around S4 than S5, and the third waveguide 325 may be similarly tapered. In addition, the second waveguide 320 has a wider cross-section, expanding from S4 to S5 in the output region. As shown, the second waveguide 320 may expand at a different location in the output region than where the first and third waveguides 315, 325 begin to expand. In still further embodiments, the locations of the constrictions in the first and third waveguides 315, 325 and the location of the second waveguide 320 may be at different locations in the output region.

[0055] The first waveguide 315 and the third waveguide 325 can be adiabatically optically coupled to the second waveguide 320 in the central region and to the first and second S-bend regions. In Figure 3A, the first gap 322 (e.g., the distance between the first waveguide 315 and the second waveguide 320) is approximately or exactly the same as the second gap 323 (e.g., the distance between the second waveguide 320 and the third waveguide 325), so that the second waveguide 320 can receive approximately equal amounts of light from both the first waveguide 315 and the third waveguide 325. As long as the first and second gaps 322, 323 are approximately equal, the optical coupling from the first and third waveguides 315, 325 to the second waveguide 320 will be approximately equal. For example, the first gap 322 may increase in width in the central region, and as long as the width of the second gap 323 is similarly changed, the optical coupling from the first and third waveguides 315, 325 to the second waveguide 320 will remain approximately the same.

[0056] 3A, the first waveguide 315 and the third waveguide 325 widen along the central region, while the second waveguide 320 decreases in width. In other embodiments, the central region may be omitted, and thus the waveguides may not change width when transitioning from the first S-bend region to the second S-bend region. In such embodiments, the waveguides may be optically coupled at the first and second S-bend regions.

[0057] The fourth region (the second S-bend region between S3 and S4) is a region where the distance between the first, second, and third waveguides 315, 320, 325 increases from the third region to the fifth region, terminating the coupling between these waveguides. Thus, one or more of the first, second, and third waveguides 315, 320, 325 include one or more bends or curves (such as an S-bend or a C-bend). For example, in the variation shown in FIG. 3A, the portions of the first waveguide 315 and the third waveguide 325 in the fourth region are S-shaped and are approximately symmetrical to the first S-bend region, and the corresponding portion of the second waveguide 320 has a constant cross-section. As described in further detail herein, the S-shaped portions of the first waveguide 315 and the third waveguide 325 can enhance optical coupling in the first S-bend region and reduce optical coupling in the second S-bend region between the first waveguide 315 and the second waveguide 320 and between the second waveguide 320 and the third waveguide 325.

[0058] In addition to the adiabatic optical coupling between the waveguides in the central region, optical coupling can also occur within the first and second S-bend regions. In these regions, the first and third waveguides 315 and 325 have S-bend-shaped cross sections and are symmetrical about the second waveguide. The S-bend cross section allows optical coupling between the first and second waveguides 315 and 320 because the gap width, as well as the gap width between the third and second waveguides, decreases between the two waveguides from the beginning to the end of the first S-bend region. That is, optical coupling between the waveguides can increase as the gap between the waveguides decreases. In other embodiments, optical coupling may not occur in these S-bend regions.

[0059] In other embodiments, portions of the first waveguide 315 and the third waveguide 325 may be asymmetric. Additionally, the second waveguide 320 may be curved (e.g., not straight) in other embodiments. While the widths of the first waveguide 315, the second waveguide 320, and the third waveguide 325 are shown as being approximately the same, they may differ in some embodiments.

[0060] In the output region (e.g., shown between S4 and S5), the first, second, and third waveguides are sufficiently separated so that there is no coupling between them. Thus, when light is introduced into the input regions of the first and / or third waveguides 315, 325, the first, second, and third waveguides will output first, second, and third output signals 330, 335, 340, respectively. When first and second input lights having the same wavelength but different phases are simultaneously introduced into the input regions of the first and third waveguides 315, 325, respectively, the first waveguide 315 outputs a first output signal 330 having a first wavelength response with a first phase shift, the second waveguide 320 outputs a second output signal 335 having a second wavelength response with a second phase shift, and the third waveguide 325 outputs a third output signal 340 having a third wavelength response and a third phase shift. Thus, the wavelength responses of the first, second, and third output signals 330, 335, 340 vary sinusoidally as a function of the wavelengths of the first and second input lights, but each output signal has a different phase compared to the other output signals (i.e., three different phases). The output signals and corresponding phases are described in further detail with reference to FIGS. 4A-6D.

[0061] 7A shows another variation of a 2×3 coupler 700 described herein. The 2×3 coupler 700 includes a first waveguide 705, a second waveguide 710, and a third waveguide 715, with the second waveguide 710 disposed between the first waveguide 705 and the third waveguide 715. Similar to the 2×3 coupler 301 of FIG. 3A, the 2×3 coupler 700 includes a first region (extending to line S1), a second region (disposed between lines S1 and S2), a third region (disposed between lines S2 and S3), a fourth region (disposed between lines S3 and S4), and a fifth region (extending from line S4).

[0062] The first region acts as a third region in which the first, second, and third waveguides 705, 710, 715 are not optically coupled to one another (thus, light is not transmitted between these waveguides within the first region). While the embodiment of the 2×3 coupler 700 shown in FIG. 7A shows the second waveguide 710 as extending into the input region, in other variations, the second waveguide 710 alternatively begins in the second region (thus, is not present within the first region). The second region acts as a first bend region in which the distance between the first, second, and third waveguides decreases, so that light can couple between the waveguides within the third region. It should be understood that some coupling may occur between the first waveguide 705 and the second waveguide 710 and / or between the second waveguide 710 and the third waveguide 715 within the first bend region. To bring the waveguides closer together, some or all of the first, second, and third waveguides 705, 710, 715 include one or more curved sections. For example, in the variation shown in Figure 7A, the first and third waveguides 705, 715 each curve toward the second waveguide 710, which is straight in the first bend region.

[0063] The third region acts as a coupling region where light couples between the first, second, and third waveguides 705, 710, 715. Specifically, a portion of light traveling through the first waveguide 705 in the coupling region can couple to the second waveguide 710, and vice versa. Similarly, a portion of light traveling through the third waveguide 715 in the central region can couple to the second waveguide 710, and vice versa. In this manner, the central region of the 2×3 coupler 700 can be configured such that the first waveguide 705 is at least partially coupled to the second waveguide 710 in the input region, and at least a portion of that light is further coupled from the second waveguide 710 to the third waveguide 715. Similarly, the central region of the 2×3 coupler 700 may be configured such that light received by the third waveguide 715 in the input region is at least partially coupled to the second waveguide 710, and at least a portion of that light is further coupled from the second waveguide 710 to the first waveguide 705. As a result, when first and second input light are simultaneously introduced into the first and third waveguides 705, 715, respectively, in the input region, the first, second, and third waveguides 705, 710, 715 will each output a combination of the first and second input light.

[0064] The fourth region acts as a second bend region where the distance between the first, second, and third waveguides increases, such that the first, second, and third waveguides 705, 710, and 715 are no longer optically coupled in the fifth region. For example, in the variation shown in FIG. 7A , the first and third waveguides 705, 715 each bend away from the second waveguide 710, which is straight in the second bend region. It should be understood that some coupling may occur between the first waveguide 705 and the second waveguide 710 and / or between the second waveguide 710 and the third waveguide 715 within the second bend region. The fifth region acts as an output region where each of the first, second, and third waveguides 705, 710, and 715 is optically isolated from the remaining waveguides. The fifth region may be used to provide outputs (eg, one from each waveguide) to other portions of an optical system incorporating the 2×3 coupler 700 .

[0065] In variations of the 2×3 coupler 700 shown in FIG. 7A , the widths of each of the first, second, and third waveguides 705, 710, 715 may be constant in the coupling region. In some of these variations, the second waveguide 710 has a width greater than the widths of the first and third waveguides 705, 715. In some of these variations, the width of the first waveguide 705 may be the same as the width of the third waveguide 715. When the 2×3 coupler 700 is configured as shown in FIG. 7A , the second waveguide 710 is wider than the first and third waveguides 705, 715 in the third region so as to have a constant width for each of the waveguides in this region, and the relative widths of the waveguides (and the spacing between them) may be selected to achieve a target phase difference between the wavelength responses of the outputs. In some of these variations, the first, second, and third waveguides 705, 710, 715 may have constant widths across some or all of the other regions (e.g., the first, second, fourth, and / or fifth regions). For example, in some variations, the first, second, and third waveguides have constant widths across the entire 2×3 coupler 700.

[0066] Specifically, the 2x3 coupler 700 is configured such that when the first and third waveguides 705, 715 each receive a first input light and a second input light having the same wavelength but a different phase, the first, second, and third waveguides each output light with a corresponding intensity having a sinusoidal wavelength relationship as described above. In some cases, the waveguides may be sized and positioned to achieve a 120-degree phase difference between the wavelength relationships for each output. In these examples, there is a 120-degree phase difference between the wavelength relationships of the output signals from the first and second waveguides 705, 710, the first and third waveguides 705, 715, and the second and third waveguides 710, 715. With a 120-degree phase difference between each of the wavelength relationships, at least one output of the 2x3 coupler 700 will be in the center of its working zone for all wavelengths in the range of input wavelengths.

[0067] The 2x3 coupler described above may be capable of operating over a range of input wavelengths such that the first, second, and third output signals each have their corresponding wavelength responses across this range. As a result, the 2x3 coupler may receive phase-shifted input light at any wavelength within the range, shifting the wavelength of the input light to induce a sinusoidal change in each output signal. Thus, as discussed herein, when the 2x3 coupler is used with a wavelength locking system, these output signals can be used to lock the wavelength of a light source to any target wavelength within the wavelength range. As described above, the wavelength of light received by the 2x3 coupler during operation of the optical system may span a wavelength range of at least 50 nm, at least 100 nm, at least 400 nm, at least 1000 nm, at least 1500 nm, etc.

[0068] (wavelength locking device) The 2×3 coupler described above can be incorporated into a wavelength locking system to lock the wavelength of a light source to a target wavelength as described above. For example, FIG. 3B shows an exemplary wavelength locking system 300 utilizing the 2×3 coupler 301 of FIG. 3A. The wavelength locking system 300 includes a splitter 305, a cladding 307, a phase shifter 310, and the 2×3 coupler 301 (which includes the first waveguide 315, the second waveguide 320, and the third waveguide 325 as described above). The cladding 307 surrounds the components of the wavelength locking system 300 to reduce optical loss and confine light to a propagation region, thus defining a waveguide. The components of the wavelength locking system 300 have similar functions and can be similarly configured as the corresponding components of the wavelength locking system 200. Wavelength locking system 300 is a structure that generates three output signals having wavelength responses with different phases, which are discussed below for purposes of wavelength locking and can be optionally unwrapped to cover a broadband wavelength range. Phase unwrapping is used to create a monotonic relationship between the extracted phase value and wavelength, which can aid in wavelength tracking as the wavelength shifts.

[0069] In the embodiment 300 of FIG. 3B, splitter 305 can receive input light and split the light across two outputs. Splitter 305 can be a 1×2 splitter that splits the light approximately (or exactly) equally between the two outputs. In other examples, splitter 305 can split the power asymmetrically, with 2×3 coupler 301 still outputting signals with different sinusoidal wavelength relationships. Any splitter or combination of splitters can be used as long as the two outputs provide light to corresponding inputs of 2×3 coupler 301; a 1×2 splitter is used for illustrative purposes only. A first output 304a of splitter 305 can be passed to a first input of 2×3 coupler 301, i.e., a first waveguide 315 within the input region of 2×3 coupler 301. The second output 304b may be passed to a second input of the 2x3 coupler 301 via a phase shifter 310 to introduce a phase shift into the light of the second output relative to the light of the first output. The phase shifter 310 may function as a delay line, similar to the delay introduced by different length arms of an MZI. In other words, the second output 304b may have a length that is sufficiently different from the first output 304a to introduce a phase delay. Alternatively, the phase shifter 310 may be an active component (e.g., an electro-optic phase shifter, a thermo-optic phase shifter, or an optomechanical phase shifter) that is actively controlled to generate a phase shift. At the input region of the 2x3 coupler 301, the light passes through the phase shifter 310 into the third waveguide 325.

[0070] As described above, the wavelength locking system 300 can be used to lock input light received by the wavelength locking system 300 to a target wavelength. To that end, the input light to the splitter 305 can be any wavelength from a range of wavelengths spanning a broadband wavelength range. Indeed, at different times, the wavelength locking system 300 can lock the input light to different wavelengths across a wide range of target wavelengths. In some examples, light of a first wavelength can be received as input light at a first time (locked to the first target wavelength), and light of a second wavelength can be received as input light at a second time (locked to the second target wavelength).

[0071] As described above, when input light of a given wavelength is received by splitter 305, 2x3 coupler 301 receives first and second input lights at first waveguide 315 and third waveguide 325, where the first and second input lights have the same wavelength but different phases. The first and second input lights couple between first, second, and third waveguides 315, 320, 325 to generate three output signals. A portion of the first light in each waveguide will interfere with a portion of the second light in that waveguide, resulting in an output signal with a constant intensity. As described above, this intensity is wavelength-dependent, so each output signal will have a sinusoidal wavelength relationship as described above, but different relative phases. That is, the three wavelength relationships may have a relative delay with each other (e.g., a constant phase difference with each other). This can result in dead zones from wavelength relationships that are offset from one another so that at any given wavelength, at least one output signal is present in its working zone (thus the overall wavelength locking system 300 effectively has no dead zones). This can provide the ability to precisely lock to any target wavelength within a wide wavelength range. Additionally, the output signal can be phase unwrapped so that all wavelengths can be locked. Phase unwrapping is described in further detail with reference to FIGS. 4A-6D.

[0072] 4A is a graph showing sample wavelength relationships of output signals from a wavelength locking system, and FIG. 4B is a graph showing an example of phase extracted from the output signals of FIG. 4A. Output signal graph 400 includes the intensity of each of first output signal 430, second output signal 435, and third output signal 440 as a function wavelength. These output signals may correspond to the first, second, and third output signals of FIG. 3. The output signals of FIG. 4A may be generated in the same manner as discussed with respect to the outputs of FIG. 3 output by the first, second, and third outputs 330, 335, and 340 of the first, second, and third waveguides 315, 320, and 325.

[0073] In the graph of FIG. 4A, the horizontal axis represents the wavelength range of light, and the vertical axis represents the signal amplitude. The horizontal axis of the graph has the arbitrarily assigned numbers 4 and 5, and the horizontal axis represents the wavelength range. FIGS. 4A, 4B, 6A, 6B, 6C, and 6D are numbered consistently with each other so that point 4 on each of the x-axes of these graphs is the same wavelength within the wavelength range. As shown in FIG. 4A, the phase difference of the wavelength responses of output signals 430, 435, and 440 is consistent, as is the relative offset between the output signals. The wavelength responses of output signals 430, 435, and 440 are generally sinusoidal signals.

[0074] The output signals can be used to measure any mismatch between the wavelength of light (i.e., generated by the light source and received by the wavelength locking system) and the target wavelength. Specifically, the target wavelength will have an expected intensity for each of output signals 430, 435, and 440. A difference between the measured intensity and these expected intensities indicates a mismatch between the measured wavelength and the target wavelength. Thus, the measured intensities can be used to determine the measured wavelength of the light. Additionally or alternatively, these measured intensities can be used to generate a feedback signal that is used to control the operation of the light source to alter the wavelength of the generated light and lock it to the target wavelength.

[0075] As described above, when an output signal is in a dead zone, a change in wavelength results in a relatively small change in the intensity of the output signal. As a result, the effectiveness of a single output signal in determining wavelength changes is limited to that output signal's dead zone. However, in the wavelength-locked systems described herein, multiple output signals may have a constant phase difference between their wavelength responses. This may result in a dead zone for each output signal that does not align with the dead zones of other output signals. As a result, useful information may be available from at least one output signal for every wavelength across the wavelength range.

[0076] Specifically, each of output signals 430, 435, and 440 has a different slope and inflection point. The maximum information for a given output signal is available for the wavelength when the slope is steepest or largest (i.e., within the working zone), which results in the largest change in signal intensity as a function of wavelength change. At every wavelength, at least one of output signals 430, 435, and 440 has a non-zero slope (e.g., there is no overlap between maxima or minima for all output signals), and therefore, each of output signals 430, 435, and 440 may have information available for wavelength locking at that particular wavelength. As discussed above, this may facilitate wavelength locking over a wide wavelength range (e.g., with a bandwidth such as 1000 nm).

[0077] 7B illustrates another variation of a wavelength-locking system 701, including the 2×3 splitter 700 depicted in FIG. 7A. As shown therein, the wavelength-locking system 701 includes a splitter 702 that receives input light at an input 755 (e.g., from a light source as described above) and splits the input light between a first output 704 a and a second output 704 b. The second output 704 b includes a phase shifter 706 that can introduce a phase shift at the second output 704 b relative to the first output 704 a, as described above. Thus, when light of a given wavelength is received at the input 755 of the splitter 702, the first and second outputs 704 a, 704 b of the splitter 702 will output light of that wavelength with two different phases.

[0078] The first and second outputs 704a, 704b optically couple the splitter 702 to the 2x3 splitter 700 at line S0. Specifically, the first and second outputs 704a, 704b are optically coupled to the first and third waveguides 705, 715, respectively, of the 2x3 coupler 700 at the input region of the 2x3 coupler 700. In this manner, when light of a given wavelength is introduced into the input 755 of the splitter 702, the first waveguide 705 receives the first input light having that wavelength and a first phase, and the third waveguide 715 receives the second input light having that wavelength and a second phase. Upon receiving these inputs, the first, second, and third waveguides 705, 710, 715 will output a first output signal 730, a second output signal 735, and a third output signal 740, respectively. These output signals will each have a constant intensity that varies sinusoidally as a function of the wavelength of the input light (i.e., as a result of interference between the phase-shifted first and second input lights).

[0079] These output signals may be used as feedback to determine the wavelength of the input light and / or to control the operation of the light source (e.g., to lock the input light to a target wavelength), as previously discussed. The wavelength responses of these output signals may be phase-shifted relative to one another so that, across a range of wavelengths, there is always at least one output signal that is not in a dead zone. These wavelength responses may be similar to those shown in FIG. 4A, but the relative phase between the wavelength responses of each output signal may be different. For example, the 2×3 coupler 700 may be configured such that there is a 120-degree phase shift between the wavelength responses for each of the first, second, and third output signals 730, 735, 740.

[0080] (Phase unwrap) As mentioned above, the systems described herein may optionally use phase unwrapping techniques when performing wavelength locking. As shown in FIG. 4B, extracted phase graph 450 shows the unwrapped phase of the free spectral range ("FSR") over a portion of the wavelength range of the input light, represented by the arbitrarily assigned numbers 4-5. The free spectral range is the interval between two consecutive reflected or transmitted light intensity maxima or minima in a single output signal 430, 435, 440 (i.e., the free spectral range of output signal 440 is shown in FIG. 4B). In other words, the free spectral range is the range of wavelengths between consecutive peaks or consecutive troughs in the output signal. By phase unwrapping the signal to reconstruct the signal's original phase, the discrepancy between the measured wavelength (or range of wavelengths) and the target wavelength, or target wavelength range of the ideal output signal(s), is obtained.

[0081] Phase unwrapping can be achieved by using the complex variable S to represent the operation of the 2 × 3 coupler. The general formula for the complex variable S may not be appropriate because it typically applies to narrowband devices and does not extract the unwrapped phase for broadband devices. The complex variable S derived for broadband devices may have an angle proportional to the phase shift between the first and third waveguides in Figure 3. The complex variable S can be expressed as follows:

number

number

number

number

[0082] where |a2|, |b2|, and |c2| are the amplitudes of the scattering parameters for the 2x3 coupler between the first input signal and the first, second, and third output signals 430, 435, and 440. I1, I2, and I3 are the corresponding FSR output signals 430, 435, and 440. Additionally, ΔΘ is the phase difference between output signals 430 and 440. Using these equations, to acquire data over a given wavelength range (e.g., a 1000 nm bandwidth or other bandwidths mentioned above), the unwrapped phase can be calculated as The wavelength range received by the 2x3 coupler included in the wavelength locking system 300 of Figure 3 can be sampled to generate an unwrapped phase signal.

[0083] As shown in Figures 4A and 4B, the phase has been extracted from the FSR of the output signal 430, but the unwrapped phase signal still includes the zero points of the output signal. Similar to the output signal graph 400, at the spikes or zero points 452 (e.g., vertical lines in the extracted phase graph 450, also referred to as "jumps") in the extracted phase graph 450, little or no information is available regarding wavelength locking at the particular wavelength where the zero points 452 occur. Additionally, as described above with reference to Figure 3, the phase may be shifted or delayed by a phase shifter. As the delay increases, the phase difference (e.g., free spectral range) may become smaller. Furthermore, as the delay increases, the wavelength locking efficiency increases.

[0084] FIG. 5 is an example circuit diagram for a phase-unwrapped output signal, and FIGS. 6A-6D show corresponding graphs of the output signal at points along the circuit diagram. Phase unwrapping circuit 500 may implement an algorithm for unwrapping the phase of the output signal. Phase unwrapping circuit 500 includes 2×3 coupler 560, photodetector 565, phase extraction block 570, differentiator 575, negative jump comparator 580 a, positive jump comparator 580 b, first summer 585, integrator 590, and second summer 595. Points A, B, C, and D are shown on phase unwrapping circuit 500, and the signals measured at each of these points are shown on the graphs of FIGS. 6A, 6B, 6C, and 6D, respectively.

[0085] Point A is the point where the signal shown on the graph of FIG. 6A is measured and is located between phase extraction block 570 and differentiator 575. Point B is the point where the signal shown on the graph of FIG. 6B is measured and is located between first summer 585 and integrator 590. Point C is the point where the signal shown on the graph of FIG. 6C is measured and is located between integrator 590 and second summer 595, and point D is the point where the signal shown on the graph of FIG. 6D is measured and is located after second summer 595. The signal measured at point D is the output signal of phase unwrap circuit 500.

[0086] The 2x3 coupler 560 outputs three output signals to the photodetector 565. The three output signals may be the optical output signals described with reference to Figures 3 and 4A-4B. The photodetector 565 receives the optical output signals and converts them into output signals as shown in the output signal graph 400 of Figure 4A. The photodetector 565 may be part of an array or may be a single unit. Furthermore, the photodetector 565 may be any suitable photodetector. The output signal from the photodetector 565 may be sent to a phase extraction block 570.

[0087] The phase extraction block 570 performs the operation described with respect to FIG. 4B, i.e., unwrapping the phase of the output signal. Thus, the phase extraction block can take a signal such as that shown in the graph of FIG. 4A and use it as an input to generate the signal of the graph shown in FIG. 4B. More specifically, the phase extraction block 570 can extract the unwrapped phase from the output signal (e.g., generate a signal corresponding to this unwrapped phase) by implementing the equation described with reference to FIG. 4B. Furthermore, the output of the phase extraction block 570 can be measured at point A, and this measurement is shown in the graph of FIG. 6A. It should be understood that the graph of FIG. 4B is a segment of the graph of FIG. 6A, specifically, an expanded view between points 4 and 5 on the horizontal axis of FIG. 6A.

[0088] As discussed above with reference to Figure 4B, the graph of Figure 6A (showing the extracted phase) may still include or have points corresponding to zeros in the output signal. At spikes or zeros in the extracted phase graph of Figure 6A (e.g., vertical lines in extracted phase graph 450), little or no information is available on wavelength locking for a particular wavelength, and therefore, a continuous curvature for wavelength locking may be desirable to create a one-to-one relationship between wavelength and output signal.

[0089] The spikes in the graph of FIG. 6A indicate that the signal is increasing or "jumping" by 2π. Typically, a phase jump occurs when the phase value entry exceeds a value of π or -π, such that the point jumps to the opposite value even if the phase is continuously increasing or decreasing. One way to detect and mitigate the sharp transitions or spikes in the graph of FIG. 6A is to differentiate the signal. The output of the differentiation block is a negative spike when the wavelength is swept in the positive direction (e.g., the phase jump occurs from π to -π). On the other hand, when the wavelength is swept in the negative direction (e.g., the phase jump occurs from -π to π), the output has a positive spike. Depending on whether the spike is detected from the positive or negative direction, 2π can be subtracted or added, respectively, until the phase is within -π to π.

[0090] Returning to FIG. 5, phase extraction block 570 can provide the extracted phase signal to differentiator 575, which differentiates the signal received from phase extraction block 570. Differentiator 575 can be used to detect spikes in the signal (e.g., jumps of 2π) and send this information in a differentiated signal to negative jump comparator 580a and positive jump comparator 580b. The differentiated signal is the output of differentiator 575, and is a negative spike when the wavelength is swept in the positive direction (e.g., a phase jump occurs from π to −π). Additionally, the output has a positive spike when the wavelength is swept in the negative direction (e.g., a phase jump occurs from −π to π).

[0091] Negative jump comparator 580a and positive jump comparator 580b are used to add or subtract 2π to the signal at the wavelength where the spike occurs. The signals from negative jump comparator 580a and positive jump comparator 580b can be summed together by first summer 585. The signal output of first summer 585 is measured at point B and shown in the graph of FIG. 6B. Negative jump comparator 580a and positive jump comparator 580b compare the differentiated signal to a specific threshold voltage. Negative jump comparator 580a and positive jump comparator 580b can differentiate between two cases: a first case where the wavelength increases and a second case where the wavelength decreases. The first case may result in a negative jump in the signal of FIG. 4B, and therefore 2π may be added to the signal by negative jump comparator 580a. Conversely, the second case may result in a positive jump, and 2π may be subtracted by positive jump comparator 580b. Negative and positive jumps are determined by comparing the differentiated signal to the negative and positive voltage thresholds of negative jump comparator 580a and positive jump comparator 580b.

[0092] The output of the first summer 585 may be passed to an integrator 590, which integrates the signal over 2π to generate a staircase signal, point C, corresponding to the graph shown in FIG. 6C. The staircase signal generated from the integrator 590 is a correction value that generates a perfectly linear and continuous signal. Because a continuous signal is desired to generate a one-to-one relationship between wavelength and signal, point C in FIG. 5 and the staircase signal of the graph in FIG. 6C may be summed in a second summer 595 so that the signal from the differentiator 575 generates a perfectly linear signal. This linear and continuous signal provides a one-to-one relationship between wavelength and signal, the graph shown in FIG. 6D. Point D, or the output of the summer 595, provides the amount of phase shift per wavelength required to wavelength lock the corresponding wavelength.

[0093] In practice, an optical device may include a number N of lasers operating at different wavelengths within the 1.4-2.4 μm wavelength range. The optical device may include drive electronics that allow time multiplexing between the lasers so that each can emit light at a specific time. The drive electronics may also include a feedback loop that controls the laser frequency with a controller. The input to this controller may be the output from a phase-unwrapped signal (the response signal), which the controller compares with a desired target value (e.g., a predetermined value determined during a calibration step described below). Due to environmental variations (e.g., temperature or other noise sources), laser frequency changes may affect the error signal (e.g., the difference between the setpoint and the response signal). The error signal may be used to change the laser drive current (e.g., control signal) value to set the laser wavelength to a desired operating value.

[0094] In some embodiments, a calibration step may be performed before operating in a closed-loop configuration to determine a set point equivalent to the target wavelength. The calibration step may be performed in an open-loop configuration, in which the response signal may be measured along with the laser wavelength while the drive current is swept. Since one laser emits light at a time, two or more lasers may not be locked simultaneously. However, in some embodiments, using one circuit for wavelength locking may be more efficient, thus allowing for scaling down at least one, more than two, or all of the weight, size, and power. Therefore, the calibration step may be performed for each laser individually.

[0095] Although process or method steps may be described in a sequential order, such processes and methods may be configured to work in any suitable order. In other words, any order or sequence of steps that may be described in this disclosure does not in itself imply a requirement that these steps be performed in that order. Furthermore, some steps may be performed simultaneously even though they are described or implied as not occurring simultaneously (e.g., by one step being described after another step). Furthermore, the description of a process by depiction in a drawing does not imply that the described process excludes other variations and modifications to the process, nor does it imply that the described process or any of its steps are required for one or more embodiments, or that the described process is preferred.

[0096] Representative examples of applications of the methods and apparatus according to the present disclosure are described in this section. These examples are provided solely to add context and aid in understanding the described examples. Therefore, it will be apparent to one skilled in the art that the described examples can be practiced without some or all of the specific details. Other applications are possible, and therefore the following examples should not be construed as limiting.

[0097] Although the embodiments of the present disclosure have been fully described with reference to the accompanying drawings, it should be noted that various changes and modifications will be apparent to those skilled in the art, and such changes and modifications are to be understood as being included within the scope of the embodiments of the present disclosure as defined by the appended claims.

Claims

1. 1. An optical system comprising: a light source configured to generate light; a splitter that receives light from the light source and splits the light received from the light source into a first light along a first optical path and a second light along a second optical path; a phase shifter positioned to receive the first split light along the first optical path and to phase-shift the first split light relative to the second split light; A 2×3 coupler, a first waveguide configured to receive the first light along the first optical path and to output a first output signal having a first wavelength response and a first phase shift; a second waveguide optically coupled to the first waveguide and configured to output a second output signal having a second wavelength response and a second phase shift; a third waveguide optically coupled to the second waveguide, the third waveguide configured to receive the second light along the second optical path and to output a third output signal having a third wavelength response and a third phase shift; a set of photodetectors positioned to receive the first output signal, the second output signal, and the third output signal; the first phase shift and the second phase shift are offset by a first phase difference; the second phase shift and the third phase shift are offset by a second phase difference; the first phase shift and the third phase shift are offset by a third phase difference; the first phase difference, the second phase difference, and the third phase difference are constant; the first waveguide, the second waveguide, and the third waveguide have constant widths in a coupling region; The optical system wherein the second waveguide has a width greater than the first and third waveguides at the coupling region.

2. a phase shifter operable to phase shift the second light; the phase shift of the second light controls wavelength locking efficiency of the first output signal, the second output signal, and the third output signal; The optical system of claim 1 , wherein the first phase shift, the second phase shift, and the third phase shift are constant over a wavelength range of about 1 micrometer.

3. The optical system of claim 1 , wherein the first and third waveguides are symmetrical with respect to the second waveguide.

4. 1. An optical system for monitoring the wavelength of a light source, comprising: the light source configured to generate light; a splitter that receives the light received from the light source and splits the light into a first split light along a first optical path and a second split light along a second optical path; a phase shifter positioned to receive the first split light along the first optical path and to phase-shift the first split light relative to the second split light; 1. A 2×3 coupler comprising a first waveguide, a second waveguide, and a third waveguide, receiving the first split light from the phase shifter in the first waveguide along the first optical path; receiving the second split light from the splitter in the third waveguide along the second optical path; a 2×3 coupler configured to output a first output signal from the first waveguide, a second output signal from the second waveguide, and a third output signal from the third waveguide, wherein the first output signal, the second output signal, and the third output signal each have a respective intensity based on a respective interference caused by a certain phase difference between the first split light and the second split light; a set of photodetectors positioned to receive the first output signal, the second output signal, and the third output signal; a controller configured to monitor the wavelength of the light received by the splitter using the intensities of the first output signal, the second output signal, and the third output signal; the first waveguide, the second waveguide, and the third waveguide have constant widths in a coupling region; The optical system wherein the second waveguide has a width greater than the first and third waveguides at the coupling region.

5. the controller includes the set of photodetectors; the set of photodetectors converting the first output signal, the second output signal, and the third output signal into a first digital output signal, a second digital output signal, and a third digital output signal; the controller comparing the first digital output signal, the second digital output signal, and the third digital output signal with a first target digital value, a second target digital value, and a third target digital value; the controller sends feedback signals to the light source to control the first output signal, the second output signal, and the third output signal with the first target digital value, the second target digital value, and the third target digital value.

5. The optical system of claim 4.

6. a phase extraction block that receives the first output signal, the second output signal, and the third output signal and extracts an unwrapped phase signal; a differentiator receiving the unwrapped phase signal, detecting zero points in the unwrapped phase signal; a differentiator configured to generate a differential signal indicative of the detected zero point; a set of comparators configured to adjust the zero point; an integrator configured to generate an integrated signal for use in generating a continuous signal for wavelength locking; A set of photodetectors, receiving the first output signal, the second output signal, and the third output signal; a set of photodetectors operable to send a corresponding sinusoidal signal for each of the first output signal, the second output signal, and the third output signal to the phase extraction block; a first summer that sums the first output signal, the second output signal, and the third output signal from the set of comparators; a second summer that sums the integrated signal with the differentiated signal to generate a sum signal that is used to determine a phase shift per wavelength that is used to lock the measured wavelength of light to a corresponding target wavelength of light; The optical system of claim 4 further comprising:

7. 7. The optical system of claim 6, wherein the integrated signal is a correction value used to create a one-to-one relationship between a particular wavelength of the first and second split light and the total signal.

8. the set of comparators a negative jump comparator; a positive jump comparator; the negative jump comparator adds 2π to the differential signal; the positive jump comparator subtracts 2π from the differential signal; the optical system further comprising a second summer configured to sum the integrated signal with the differentiated signal to generate a sum signal containing information used to sequentially lock onto each measured wavelength of light over a wavelength range of about 1 micrometer.

7. The optical system of claim 6.

9. the first output signal has a first sinusoidal wavelength response with a first phase shift; the second output signal has a second sinusoidal wavelength response with a second phase shift; the third output signal has a third sinusoidal wavelength response with a third phase shift; the first phase shift, the second phase shift, and the third phase shift are constant over a wavelength range of about 1 micrometer; 5. The optical system of claim 4.

10. The optical system of claim 4 , wherein the first output signal, the second output signal, and the third output signal are offset from one another by the same phase difference.

11. 5. The optical system of claim 4, wherein the phase shift of the first output signal, the second output signal, and the third output signal is constant over a wavelength range of about 1 micrometer.

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