Wavelength controller and system for tunable laser

Abstract

To provide a small-sized wavelength controller towards monolithic integration with a laser, and a laser system.SOLUTION: A wavelength controller (100) for receiving an input signal and in response thereto generating a control signal for tuning a wavelength of a laser includes: an interferometer-based circuit (110) that receives at least part of an output of the laser (10) as an input signal and generates phase-shifted intensity values based on the input signal; a vector-based signal synthesizer (120) configured to generate a complex vector-based signal based on the phase-shifted intensity values and to compute at least one vector-representing quantity substantially linearly correlated with the wavelength based on the complex vector-based signal; and a control signal generator (130) configured to generate the control signal at least partly based on the at least one vector-representing quantity.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present specification relates to a wavelength control device for a tunable laser and a laser system including a wavelength control device and a tunable laser. [Background technology]

[0002] Wavelength control devices and wavelength lockers (WLLs) are essential components in optical communications, particularly in stabilizing the wavelength output of lasers. Wavelength lockers essentially ensure the accuracy of optical communication systems, ensuring precision and consistency in laser performance, enabling, for example, efficient communication systems.

[0003] Current optical transceivers, commonly used in high-speed optical fiber communications in telecommunications and data centers, integrate all necessary components on a single chip. These components include lasers, modulators, amplifiers, and detectors. Communication applications such as dense wavelength division multiplexing (DWDM) have strict requirements for laser wavelength / frequency stability to avoid crosstalk between channels. For example, coherent transmission in ITU G.698.2 requires laser frequency stability of less than ±1.8 GHz.

[0004] In the prior art, multimode interference (MMI) devices, such as 1x2 MMI, 2x2 MMI, and 4x4 MMI configurations, are employed to split and combine optical signals. The input signal is split into two output signals, and the phase can be shifted by a 90° phase shifter. A photodiode (PD) is used to convert the optical signal to an electrical signal. Signal processing typically uses the outputs separately, selectively, or summed to determine the type of response.

[0005] Optical transceivers traditionally implement laser wavelength locking off-chip, and despite advances in wavelength locking technology, the trend is toward on-chip wavelength locking for further monolithic integration. However, miniaturization must not come at the expense of functionality and performance, so new and improved wavelength lockers are needed. Summary of the Invention

[0006] A general object is to provide an improved wavelength control device for a tunable laser and a corresponding laser system including such a wavelength control device.

[0007] In this specification, the wavelength control device may be referred to as a wavelength locker (WLL).

[0008] A specific objective of this specification is to provide a WLL (preferably on-chip) for arbitrary wavelength locking, i.e., locking the laser to any desired wavelength with minimal recalibration effort to improve operational flexibility.

[0009] Another specific object is to provide a high precision, wide range WLL with integrated lasers in a compact size and easy assembly process facilitated by wire bonding and requiring no optical alignment.

[0010] A further object is to provide a resource-efficient wavelength control device aimed at reducing resource requirements such as the number of photodiodes, contributing to cost-effectiveness and sustainability.

[0011] At least some of these and other objects are achieved at least in part by the present invention as defined in the independent claims. Preferred exemplary embodiments are set forth in the dependent claims.

[0012] According to one aspect, a wavelength control device for a wavelength tunable laser is provided, the wavelength control device being configured to receive an input signal and generate a control signal for adjusting the wavelength of the wavelength tunable laser in response thereto, the wavelength control device including: an interferometer-based circuit configured to generate phase-shifted intensity values ​​based on the input signal; a vector-based signal combiner configured to generate a complex vector-based signal based on the phase-shifted intensity values ​​and to calculate at least one vector-represented quantity based on the complex vector-based signal, wherein the at least one vector-represented quantity is approximately linearly correlated with wavelength; and a control signal generator configured to generate a control signal based at least in part on the at least one vector-represented quantity.

[0013] This provides a high performance wavelength control device and / or laser system.

[0014] For example, the wavelength control accuracy can be improved. In particular, the near-linear correlation between at least one vector representation quantity and wavelength allows the wavelength control device to accurately lock the tunable laser to more wavelengths.

[0015] Increased precision of wavelength control devices leads to improved performance of the optical or laser systems in which they are used (e.g., wavelength lockers), thus resulting in clearer and more reliable signal transmission.

[0016] Furthermore, improved performance and / or capacity can be achieved as wavelength control devices can accurately lock onto more wavelengths, e.g., allowing for the transmission of more data, which increases efficiency and means that systems incorporating wavelength control devices and wavelength lockers are more effective for high-speed data transmission applications.

[0017] Furthermore, precise wavelength locking can help reduce interference between different wavelengths, thus reducing signal degradation and information loss, ensuring that the transmitted data is received intact at the other end.

[0018] The wavelength control device may further be capable of handling any phase shift of the phase-shifted intensity values. Specifically, the phase shift may be any non-zero angle that is strictly greater than -180 degrees and strictly less than +180 degrees. Thus, greater flexibility in managing different signal conditions and improved robustness of the wavelength control device to, for example, variations in signal characteristics are provided.

[0019] Furthermore, it should be appreciated that one or more vector representation quantities can be considered vector defining properties.

[0020] In the context of this specification, the term "approximately linearly correlated" between two variables or quantities may mean that there is a strong degree of linear relationship between the variables or quantities. However, it does not necessarily mean a perfect linear relationship. When data is plotted on a graph, there may be some variation or deviation from a perfect straight line. The term "approximately" is used to account for these minor variations that do not significantly affect the overall linear correlation between the variables. Therefore, it is understood that the terms are clearly and sufficiently defined in light of the specific context provided in the specification, claims, and drawings of the patent application.

[0021] Alternatively, at least one vector representation may be linearly correlated with wavelength, such that any change in wavelength may result in a directly proportional change in the vector representation, without any deviation.

[0022] According to an exemplary embodiment, the at least one vector representation may include an angle and / or an amplitude of the complex vector-based signal. The angle and amplitude may also be referred to as the phase and magnitude / intensity of the complex signal, respectively. That is, the wavelength control device may convert wavelength information into values ​​of parameters such as intensity and angle.

[0023] According to another exemplary embodiment, the control signal generator may be configured to perform a control signal calculation based on at least one vector representation quantity. In other words, the control signal generator may be designed to perform calculations for generating a control signal, for example, at least one quantity representing a vector is used in these calculations. Essentially, the control signal generator can take a vector representation quantity as input for calculating the control signal.

[0024] According to yet another exemplary embodiment, the interferometer-based circuit may include a phase shifter for providing a phase shift when generating the phase-shifted intensity values. The phase shifter may be a thermal phase shifter that may be configured to induce a thermal phase shift. The thermal phase shifter may be, for example, a heater-based unipolar-to-bipolar converter.

[0025] The inclusion of a phase shifter in an interferometer-based circuit provides increased control over the interference pattern produced by the interferometer: by adjusting the phase shift, the circuit can manipulate the resulting interference pattern.

[0026] Therefore, by controlling the phase shift, digital locking to any wavelength can be easily achieved. In other words, the wavelength control device may provide the ability to easily tune and lock to any desired wavelength via the phase shifter using digital control (e.g., increasing the heat of a heater).

[0027] According to another exemplary embodiment, an interferometer-based circuit may comprise at least two different waveguides with different free spectral ranges (FSRs), including a first waveguide with a first FSR and a second waveguide with a second FSR, the second FSR being greater than the first FSR, allowing for higher wavelength resolution for signals passing through the first waveguide and a larger wavelength correction range for signals passing through the second waveguide.

[0028] In other words, an interferometer-based circuit can include at least two different waveguides, each with a different FSR, which can provide higher wavelength resolution for signals passing through the first waveguide and a larger wavelength correction range for signals passing through the second waveguide.

[0029] The combination of high resolution and a wide correction range makes interferometer-based circuits more robust. For example, a small FSR is required to increase accuracy (or wavelength resolution). However, because the wavelength correction range is within a period (2π), if the wavelength shifts beyond the FSR, the calculated vector-based quantity may be repeated, and such values ​​may be indistinguishable. Therefore, to enable both high accuracy and a wide range, two FSRs (a small FSR for high accuracy and a large FSR for a wide locking range) may be implemented. In other words, by designing an integrated large and small FSR structure, a wavelength control device can achieve both high frequency accuracy / resolution and a wide correction range.

[0030] According to yet another exemplary embodiment, the wavelength control device may further comprise an optical input switch for selectively providing one of at least two different signals as an input signal of the wavelength control device.

[0031] The wavelength control device may function as a wavemeter (typically capable of measuring a single wavelength at a time, with high accuracy), and may implement an optical switch so that two wavelengths can be compared efficiently.

[0032] Optical switches may alternatively or additionally be integrated into interferometer-based circuits.

[0033] According to an exemplary embodiment, the at least two different signals may include a transmitted (Tx) laser signal and a received (Rx) laser signal.

[0034] Thus, when coupled with an optical switch, the optical switch may allow the ability to select either the Rx or Tx wavelength to be measured (typical suppression of an optical switch may be around -20dB to -30dB).

[0035] In one example, the control signal calculation (performed by the control signal generator) may involve comparing a laser wavelength (Tx) with a wavelength from another (Rx) light source. A wavelength switch may be implemented such that the vector representation is first measured for Rx, then measured for Tx, and finally compared, and if necessary, the laser wavelength of Tx may be adjusted to match the wavelength of Rx.

[0036] According to another exemplary embodiment, the wavelength control device may be configured to provide time-interleaved tracking of the transmitted (Tx) and received (Rx) laser signals. In other words, if Tx and Rx are fed simultaneously into a single wavelength measurement structure, the wavelengths of Rx and Tx are indistinguishable and cannot be measured, so the wavelengths may instead be measured sequentially, i.e., time-interleaved.

[0037] This allows time-interleaved tracking of the transmitted (Tx) and received (Rx) laser signals to share the same optical detection unit, thus eliminating the need for separate detection units for the Tx and Rx signals, reducing complexity and cost (e.g., the total number of PDs and / or other components can be halved).

[0038] According to an exemplary embodiment, the wavelength control device may be configured to provide time-interleaved tracking to enable reduction of wavelength differences between a received (Rx) laser signal and a local oscillator signal (LO) and / or maintaining stability of a transmission laser of a transmitted (Tx) laser signal.

[0039] In other words, the wavelength difference between the received laser signal (Rx) and the local oscillator signal (LO) can be small, and the transmitted signal (Tx) can remain stable. Thus, a nearly coherent receiver can be provided, where the Rx signal and the Tx local oscillator are at substantially the same wavelength, thereby coherently amplifying the wavelengths.

[0040] Therefore, by reducing the wavelength difference between the received (Rx) signal and the local oscillator (LO) signal, the wavelength controller can reduce potential interference and signal degradation, resulting in a clearer, more reliable signal. Furthermore, maintaining the stability of the transmitted (Tx) signal can ensure consistent performance of the wavelength controller. This can be particularly beneficial in applications requiring high precision and reliability, such as telecommunications and data centers. In addition, time-interleaved tracking can potentially use available bandwidth more efficiently because signals are more precisely targeted to specific wavelengths.

[0041] The local oscillator may be tuned, i.e., it may comprise a laser with a tunable wavelength. Depending on the accuracy of the RX signal, the local oscillator (i.e., Tx laser) may need to be tunable over a range of, for example, about tens of GHz up to hundreds of GHz. Tuning may be achieved, for example, by external cavity tuning, thermal tuning, and / or current injection tuning.

[0042] According to another exemplary embodiment, an interferometer-based circuit may comprise: a first signal splitter configured to split an input signal into two different signal paths, a first signal path and a second signal path; a second signal splitter configured in the first signal path to split the signal of the first signal path into two portions having the same phase; a third signal splitter configured in the second signal path to split the signal of the second signal path into two portions having different phases and providing a non-zero relative phase shift; two signal combiners, each configured to perform signal combining based on an output of the second signal splitter and a respective output of the third signal splitter and to provide a combined signal; and two detectors, each configured to detect an intensity value of the combined signal of a respective signal combiner, wherein the intensity values ​​of the two detectors are phase-shifted with respect to each other and provided as inputs to a vector-based signal combiner.

[0043] In other words, only two detectors (e.g., single-ended photodetectors) may be required, thus requiring fewer resources, a smaller footprint, and therefore a lower technical challenge.

[0044] According to certain exemplary embodiments, the first signal splitter may be based on a 1x2 multimode interferometer (MMI), the second signal splitter may be based on a 1x2 MMI, the third signal splitter may be based on a 2x2 MMI, each signal combiner may be based on a 2x1 MMI, and each detection unit may be based on a photodetector.

[0045] MMIs, such as the 1x2 and 2x2 MMIs, may be thought of as simple branching components, with smaller footprints and reduced component phase errors compared to other MMIs, resulting in more compact and efficient designs. Additionally, 1x2 and 2x2 MMIs may offer wider bandwidths than larger MMIs.

[0046] As mentioned above, the wavelength control device may be capable of handling any phase shift of the phase-shifted intensity values. Specifically, the phase shift may be any non-zero angle that is strictly greater than -180 degrees and strictly less than +180 degrees. According to certain exemplary embodiments, the relative phase shift may be between 60 degrees and 120 degrees. This results in good performance, for example, in terms of signal-to-noise ratio (SNR). The relative phase shift may be close to 90 degrees, for example.

[0047] According to an exemplary embodiment, a phase-shift-based unipolar-to-bipolar converter may be incorporated into the first signal path or the second signal path to provide a thermal phase shift when generating the phase-shifted intensity values.

[0048] Thermal phase shifters (e.g., phase-shift-based unipolar-to-bipolar converters) can provide efficient phase changes and can achieve small on-chip size compared to other phase shifters (e.g., PN junctions).

[0049] According to an exemplary embodiment, the wavelength control device may be configured to adjust the wavelength to be within a limited wavelength range.

[0050] According to an exemplary embodiment, the wavelength control device may be a wavelength locker (WLL).

[0051] According to an exemplary embodiment, the wavelength control device may be integrated on the same integrated circuit as the laser, resulting in reduced component size and ease of manufacturing.

[0052] According to exemplary embodiments, the wavelength control device may be configured to receive at least a portion of the output of a tunable laser as an input signal, or may be configured to receive at least a portion of a received laser signal as an input signal, thus providing improved flexibility and ability to handle different types of input signals.

[0053] The portion of the tunable laser (or local oscillator) power received by the wavelength control device may be, for example, 0.1% to 10%. The flexibility of the optical power output allows for potential adaptation to different electronics and photodiode sensitivities. For example, switching the LO light from 0.1% to 10% may result in a photodiode power measurement that is two orders of magnitude higher. At least the portion of the power output may be on the order of, for example, sufficient light for proper measurement while preventing the photodiode from saturating due to excessive light.

[0054] According to another aspect, there is provided a laser system comprising a tunable laser and a wavelength control device according to any of the preceding claims.

[0055] For example, a laser system, especially when viewed as a closed loop, can be considered a wavelength locker, in other words, a wavelength locker can include a tunable laser and a wavelength controller.

[0056] This aspect may generally offer the same or corresponding advantages as the previous aspect. [Brief explanation of the drawings]

[0057] The above, as well as additional objects, features, and advantages of the present specification, will be better understood through the following illustrative and non-limiting detailed description, taken in conjunction with the accompanying drawings, in which like reference numerals are used for like elements unless otherwise noted.

[0058] [Figure 1] FIG. 1 is a schematic diagram illustrating an overview of a laser system having a tunable laser and an associated wavelength controller according to one embodiment. [Figure 2] FIG. 1 is a schematic diagram illustrating an example of an interferometer-based circuit according to an embodiment. [Figure 3] FIG. 10 is a schematic diagram illustrating another example of an interferometer-based circuit according to an embodiment. [Figure 4] FIG. 1 is a schematic diagram illustrating a particular example of an interferometer-based circuit according to one embodiment. [Figure 5] FIG. 1 is a schematic diagram illustrating an example of a wavelength control device according to an embodiment. [Figure 6] FIG. 1 is a schematic diagram illustrating an example of a laser system according to an embodiment. [Figure 7] FIG. 1 is a schematic diagram illustrating an example of a laser system according to an embodiment. [Figure 8A] FIG. 1 is a schematic diagram illustrating an example of an interferometer-based circuit according to an embodiment. [Figure 8B] FIG. 1 is a schematic diagram illustrating an example of angle versus wavelength relationships for a waveguide with a smaller FSR and a waveguide with a relatively larger FSR. [Figure 8C] FIG. 1 is a schematic diagram illustrating an example of a wavelength correction range for a given FSR. [Figure 8D] FIG. 10 schematically illustrates one example of how two different FSRs can be combined to provide a relatively large range of correction (large FSR) while maintaining a desirable accuracy (small FSR). [Figure 9] FIG. 10 is a schematic diagram illustrating another example of an interferometer-based circuit according to an embodiment. [Figure 10] FIG. 10 is a schematic diagram illustrating yet another example of an interferometer-based circuit according to an embodiment. [Figure 11A] FIG. 10 is a schematic diagram illustrating an example of a frequency response of the output of a photodetector of an interferometer-based circuit according to an embodiment. [Figure 11B] FIG. 11B is a schematic diagram illustrating the intensity versus frequency curve of a photodetector corresponding to FIG. 11A. [Figure 11C] FIG. 11C is a schematic diagram illustrating a substantially linear angle versus frequency curve corresponding to FIG. 11B when vector synthesis is performed. [Figure 12] FIG. 2 is a schematic diagram of an intensity versus frequency curve showing how a wavelength shift can be introduced by applying a particular voltage to a phase shifter. DETAILED DESCRIPTION OF THE INVENTION

[0059] The proposed technology will now be described with reference to various exemplary embodiments. As mentioned above, the proposed technology relates to a wavelength control device for a tunable laser and a corresponding laser system.

[0060] FIG. 1 is a schematic diagram illustrating an overview of a laser system 1000 having a tunable laser 10 and an associated wavelength control device 100 according to one embodiment.

[0061] Wavelength control device 100 is configured to receive an input signal and, in response thereto, generate a control signal for adjusting the wavelength of the tunable laser. In this particular example, wavelength control device 100 is configured to receive at least a portion of the output of laser 10 as the input signal.

[0062] The wavelength control device 100 includes an interferometer-based circuit 110 configured to generate phase-shifted intensity values ​​based on an input signal. The wavelength control device 100 further includes a vector-based signal combiner 120 configured to generate a complex vector-based signal based on the phase-shifted intensity values ​​and to calculate at least one vector-represented quantity based on the complex vector-based signal. The one or more vector-represented quantities are approximately linearly correlated with wavelength. The wavelength control device 100 also includes a control signal generator 130 configured to generate a control signal based at least in part on the at least one vector-represented quantity.

[0063] This provides a high performance wavelength control device and / or laser system, e.g., improved laser stability control with high precision over all wavelengths, or at least over a wider range of wavelengths, compared to conventional solutions.

[0064] In particular, the wavelength control device may include a wavelength-sensitive element that can convert wavelength to, for example, phase, and the phase information may be used to control the stability of the laser, where it will be appreciated that wavelength locking may be achieved without the need for an active optical control mechanism such as active optical phase shifting.

[0065] By way of example, the wavelength controller may be integrated onto the same integrated circuit as the laser, for example a photonic integrated chip (PIC).

[0066] In other words, a wavelength control device suitable for integrated WLL is provided that can create linear wavelength feedback for laser stability control with the same high precision for all wavelengths. The linear wavelength feedback can be created, for example, by constructing a complex vector-based signal (R + j * Q) with the help of two (cost-effective and / or slow) photodiodes and a relative photonic circuit.

[0067] After initialization / calibration, the tunable laser can be locked anywhere within a broad wavelength spectrum, which may include, but is not limited to, the C-band and L-band, where the C-band (conventional band) ranges from 1530 nm to 1565 nm and the L-band (long wavelength band) ranges from 1565 nm to 1625 nm.

[0068] The term "lock" in this context refers to the ability of a laser to maintain a stable output at a particular wavelength.

[0069] Furthermore, it should be appreciated that one or more vector representation quantities can be considered vector defining properties.

[0070] In the context of this specification, the term "approximately linearly correlated" between two variables or quantities may mean that there is a strong degree of linear relationship between the variables or quantities. However, it does not necessarily mean a perfect linear relationship. When data is plotted on a graph, there may be some variation or deviation from a perfect straight line. The term "approximately" is used to account for these minor variations that do not significantly affect the overall linear correlation between the variables. Therefore, it is understood that the terms are clearly and sufficiently defined in light of the specific context provided in the specification, claims, and drawings of the patent application.

[0071] Alternatively, at least one vector representation may be linearly correlated with wavelength, such that any change in wavelength may result in a directly proportional change in the vector representation, without any deviation.

[0072] According to an exemplary embodiment, the at least one vector representation may include an angle and / or an amplitude of the complex vector-based signal. The angle and amplitude may also be referred to as the phase and magnitude / intensity of the complex signal, respectively. That is, the wavelength control device may convert wavelength information into values ​​of parameters such as intensity and angle.

[0073] As mentioned above, linear wavelength feedback can be created by constructing a complex vector-based signal (R + j*Q) with the aid of, for example, two photodiodes and a relative photonic circuit. Therefore, for a given wavelength, the intensities of the two photodiodes can be used as part of the real (Re) and imaginary (Im) components of the complex vector-based signal, respectively. See Figures 11A-11C, which will be explained in more detail later. The combined complex value can be constructed as (Re - Calibration_average) + j*(Im - Calibration_average), where Calibration_average is the average value that can be calculated using WLL calibration. Two-signal vector synthesis can then be used, i.e., combining two signals to create a complex output. The angle of the complex vector-based signal can be calculated by sweeping different wavelengths to obtain a linear angle-wavelength curve. Either the angle or the amplitude of the complex vector-based signal, or both, can be linearly correlated with wavelength.

[0074] This provides a wavelength control device that has equal resolution for every wavelength of the tunable laser due to the linear correlation between angle (and / or amplitude) and wavelength. Furthermore, the linearity allows locking to any wavelength within the operating range of the tunable laser, with no deadlock range (i.e., no dead zone or range where the wavelength control device cannot lock to a particular wavelength).

[0075] The proposed circuit designs described herein may offer additional advantages compared to conventional circuit designs, such as smaller footprint, smaller component phase errors, simpler on-chip components, improved manufacturability, and easier phase error compensation.

[0076] According to another exemplary embodiment, the control signal generator may be configured to perform a control signal calculation based on at least one vector representation quantity. In other words, the control signal generator may be designed to perform calculations for generating a control signal, for example, at least one quantity representing a vector is used in these calculations. Essentially, the control signal generator can take a vector representation quantity as input for calculating the control signal.

[0077] The calculation of the control signal may be a calculation of a delta value. For example, for a given wavelength, there may be a corresponding value of a vector representation (e.g., angle), and a shift in wavelength may result in a different value of the vector representation. Thus, the calculated delta value may be the difference (e.g., absolute difference) between the values ​​of the vector representation for two different wavelengths. In other words, by simply measuring the wavelength of the laser itself, the delta value can be calculated by comparing the vector representation of the measured wavelength with the vector representation of a previously measured wavelength.

[0078] The delta value may be used as feedback information to adjust, control, and / or lock the wavelength of the laser, but may also be used to calculate a delta signal over time for further signal processing, including commonly accepted control procedures such as proportional-integral-derivative (PID) regulation.

[0079] According to yet another exemplary embodiment, the interferometer-based circuit may include a phase shifter for providing a phase shift when generating the phase-shifted intensity values. By way of example, the phase shifter may be a thermal phase shifter that may be configured to induce a thermal phase shift. The thermal phase shifter may be, for example, a heater-based unipolar-to-bipolar converter.

[0080] The inclusion of a phase shifter in an interferometer-based circuit provides increased control over the interference pattern produced by the interferometer: by adjusting the phase shift, the circuit can manipulate the resulting interference pattern.

[0081] Therefore, by controlling the phase shift, digital locking to any wavelength can be easily achieved. In other words, the wavelength control device may provide the ability to easily tune and lock to any desired wavelength via the phase shifter using digital control (e.g., increasing the heat of a heater).

[0082] The metal (i.e., heater) of the thermal phase shifter may be supplied with different voltages / currents such that the metal heats its waveguide, changing the index of the waveguide and thus the phase of the light (see, e.g., Figure 12).

[0083] Figure 12 is a schematic diagram of an intensity versus frequency curve showing how wavelength shifts can be introduced by applying specific voltages to a phase shifter, such that different wavelength shifts can be achieved by applying different voltages.

[0084] For example, during calibration, a fixed laser (i.e., no wavelength change) can be obtained by adjusting the phase shifter to extract the maximum power (peak power of the sine wave in Figure 12) and minimum power (bottom power of the sine wave). Furthermore, in the operating mode, the average value of the maximum and minimum power can be used to convert the unipolar photodiode current into a bipolar signal. Therefore, the average intensity of the detected maximum and minimum power can be obtained for unipolar-to-bipolar conversion.

[0085] Additionally, thermal phase shifters (on-chip) can be efficient for varying phase with low insertion loss, such as tuning at up to 22 kHz, however, for high speed tuning (e.g., greater than 1 MHz), non-thermal phase shifters (such as stress-optical and / or electro-optical phase shifters) can be used to provide the phase shift.

[0086] FIG. 2 is a schematic diagram illustrating an example of an interferometer-based circuit according to one embodiment.

[0087] In the example of FIG. 2 , the interferometer-based circuit includes a first signal splitter 112 configured to split an input signal into two different signal paths, a first signal path 115-1 and a second signal path 115-2; a second signal splitter 114 configured in the first signal path to split the signal of the first signal path into two portions having the same phase; a third signal splitter 116 configured in the second signal path to split the signal of the second signal path into two portions having different phases and providing a non-zero relative phase shift; and two signal combiners 117-1, 117-2, each of which combines a second The signal combiner 110 includes two signal combiners 117-1, 117-2 configured to perform signal combining based on the output of the signal divider 114 and the respective outputs of the third signal divider 116 to provide a combined signal, and two detectors 118-1, 118-2, each configured to detect an intensity value of the combined signal of the respective signal combiner 117-1, 117-2, the intensity values ​​of the two detectors 118-1, 118-2 being phase-shifted relative to each other and provided as inputs to a vector-based signal combiner (not shown in FIG. 2 ).

[0088] In other words, only two detectors (e.g., single-ended photodetectors) may be required, thus requiring fewer resources, a smaller footprint, and therefore a lower technical challenge.

[0089] FIG. 3 is a schematic diagram illustrating another example of an interferometer-based circuit according to one embodiment. The example of FIG. 3 is similar to the example of FIG. 2, except that the second signal path 115-2 includes a phase shifter 111 for providing a phase shift when generating the phase-shifted intensity values. As previously mentioned, the phase shifter may be a thermal phase shifter that can be configured to induce a thermal phase shift. The thermal phase shifter may be, for example, a heater-based unipolar-to-bipolar converter. It should be understood that a phase shifter may be incorporated into the first signal path 115-1 or the second signal path 115-2 to provide a thermal phase shift when generating the phase-shifted intensity values.

[0090] The phase shifter 111 allows for phase control by changing the optical path length. When the light is recombined in a coupler such as 117-2, interference occurs. This interference depends on the additional time the light spends on the signal path, specifically the second signal path 115-2 in FIG. 3. The change in optical path length can be, for example, about 1 μm (or up to 1 μm). As a result, a time shift can be equivalent to a phase shift.

[0091] 4 is a schematic diagram illustrating a particular example of an interferometer-based circuit according to one embodiment, in which the first signal splitter 112 is based on a 1×2 multimode interferometer (MMI), the second signal splitter 114 is based on a 1×2 MMI, the third signal splitter 116 is based on a 2×2 MMI, each of the signal combiners 117-1, 117-2 is based on a 2×1 MMI, and each of the detection units 118-1, 118-2 is based on a photodetector (PD).

[0092] Additionally, FIG. 4 illustrates the optional inclusion of a phase shifter 111 in the second signal path 115-2.

[0093] FIG. 5 is a schematic diagram illustrating an example of a wavelength control device according to an embodiment.

[0094] In this example, the wavelength control device may further comprise an optical input switch 90 for selectively providing one of at least two different signals as an input signal to the wavelength control device 100 .

[0095] The wavelength control device can function as a wavemeter (capable of measuring a single wavelength at a time with high accuracy), and therefore can implement an optical switch so that two wavelengths can be compared efficiently.

[0096] The optical switch may alternatively or additionally be integrated into the interferometer-based circuit, as shown schematically in dashed lines in FIG.

[0097] According to an exemplary embodiment, the at least two different signals may include a transmitted (Tx) laser signal and a received (Rx) laser signal.

[0098] Thus, when coupled with an optical switch, the optical switch may allow the ability to select either the Rx or Tx wavelength to be measured (typical suppression of an optical switch may be around -20dB to -30dB).

[0099] In one example, the control signal calculation (performed by the control signal generator) may involve comparing a laser wavelength (Tx) with a wavelength from another (Rx) light source. A wavelength switch may be implemented such that the vector representation is first measured for Rx, then measured for Tx, and finally compared, and if necessary, the laser wavelength of Tx may be adjusted to match the wavelength of Rx.

[0100] According to another exemplary embodiment, the wavelength control device may be configured to provide time-interleaved tracking of the transmitted (Tx) and received (Rx) laser signals. In other words, if Tx and Rx are fed simultaneously into a single wavelength measurement structure, the wavelengths of Rx and Tx are indistinguishable and cannot be measured, so the wavelengths may instead be measured sequentially, i.e., time-interleaved.

[0101] This allows time-interleaved tracking of the transmitted (Tx) and received (Rx) laser signals to share the same optical detection unit, thus eliminating the need for separate detection units for the Tx and Rx signals, reducing complexity and cost (e.g., the total number of PDs and / or other components can be halved).

[0102] According to an exemplary embodiment, the wavelength control device may be configured to provide time-interleaved tracking to enable reduction of wavelength differences between a received (Rx) laser signal and a local oscillator signal (LO) and / or maintaining stability of a transmission laser of a transmitted (Tx) laser signal.

[0103] In other words, the wavelength difference between the received laser signal (Rx) and the local oscillator signal (LO) can be small, and the transmitted signal (Tx) can remain stable. Thus, a nearly coherent receiver can be provided, where the Rx signal and the Tx local oscillator are at substantially the same wavelength, thereby coherently amplifying the wavelengths.

[0104] Therefore, by reducing the wavelength difference between the received (Rx) signal and the local oscillator (LO) signal, the wavelength controller can reduce potential interference and signal degradation, resulting in a clearer, more reliable signal. Furthermore, maintaining the stability of the transmitted (Tx) signal can ensure consistent performance of the wavelength controller. This can be particularly beneficial in applications requiring high precision and reliability, such as telecommunications and data centers. In addition, time-interleaved tracking can potentially use available bandwidth more efficiently because signals are more precisely targeted to specific wavelengths.

[0105] The local oscillator may be tuned, i.e., it may comprise a laser with a tunable wavelength. Depending on the accuracy of the RX signal, the local oscillator (i.e., Tx laser) may need to be tunable over a range of, for example, about tens of GHz up to hundreds of GHz. Tuning may be achieved, for example, by external cavity tuning, thermal tuning, and / or current injection tuning.

[0106] 6 is a schematic diagram illustrating an example of a laser system according to one embodiment. In this example, a laser is used as a local oscillator (LO) 70, and a splitter 80 is configured to split a small portion of the laser output into a separate branch for LO feedback to an optical input switch 90. This allows the possibility to match the LO and RX, and also change or tune the TX wavelength of the laser 70.

[0107] 7 is a schematic diagram illustrating an example of a laser system according to one embodiment. In this example, two separate lasers may be used: a transmit (Tx) laser 10, which is kept as stable as possible (without feedback to / from wavelength control device 100), and a local oscillator (LO) 70, which is essentially a separate laser that can be tuned using wavelength control device 100. For example, it would be possible to measure the output of the local oscillator 70 and tune the LO wavelength to match (or at least be relatively close to) the Rx wavelength of the received signal.

[0108] According to another exemplary embodiment, an interferometer-based circuit may comprise at least two different waveguides with different free spectral ranges (FSRs), including a first waveguide with a first FSR and a second waveguide with a second FSR, the second FSR being greater than the first FSR, allowing for higher wavelength resolution for signals passing through the first waveguide and a larger wavelength correction range for signals passing through the second waveguide.

[0109] In other words, an interferometer-based circuit can include at least two different waveguides, each with a specific FSR, where the difference in FSR allows for higher wavelength resolution for signals passing through the first waveguide and a larger wavelength correction range for signals passing through the second waveguide.

[0110] The combination of high resolution and a wide correction range makes interferometer-based circuits more robust. For example, a small FSR is required to increase accuracy (or wavelength resolution). However, because the wavelength correction range is typically within a period (2π), if the wavelength shifts beyond the FSR, the calculated vector-based quantity may repeat, and such values ​​may be indistinguishable. Therefore, to enable both high accuracy and a wide range, two FSRs (a small FSR for high accuracy and a large FSR for a wide locking range) may be implemented. In other words, by designing an integrated large and small FSR structure, a wavelength control device can achieve both high frequency accuracy / resolution and a wide correction range.

[0111] 8A is a schematic diagram illustrating an example of an interferometer-based circuit according to one embodiment. In the example of FIG. 8A, portions of the basic interferometer circuits of FIGS. 2-4 are replicated and integrated for use with different waveguides. As can be seen in the example of FIG. 8A, interferometer-based circuit 110 further includes two additional signal splitters 113-1 and 113-2, another signal splitter 114-2, and another signal splitter 116-2, as well as two additional signal combiners 117-3 and 117-4 and two more detectors 118-3 and 118-4.

[0112] Essentially, second signal path 115-2 includes an additional splitter 113-2 disposed upstream of signal splitters 116-1 and 116-2 for splitting the signal of second signal path 115-2 into a first waveguide 119-1 (corresponding to signal path 115-2A) having a first FSR and a second waveguide 119-2 (corresponding to signal path 115-2B) having a second FSR, the second FSR being greater than the first FSR, enabling higher wavelength resolution for the signal passing through first waveguide 119-1 and a larger wavelength correction range for the signal passing through second waveguide 119-2. Alternatively, the FSR of first waveguide 119-1 may be greater than the FSR of second waveguide 119-2.

[0113] 8, the second waveguide 119-2 with a large FSR is designed to measure the wavelength coarsely, while the first waveguide 119-1 with a small FSR is responsible for measuring the wavelength with high precision. However, the small FSR does not have a wide range, which means that the absolute wavelength may not be known.

[0114] For example, in a specific example, if the wavelength to be measured is exactly 1550 nm, two interferometers are used: one with an FSR of 100 nm and the other with an FSR of 1 nm. The interferometer with an FSR of 100 nm provides a rough estimate of the wavelength at 1550 + / - 0.5 nm. The second interferometer can then accurately determine whether the wavelength is within the 1550 + / - 0.5 nm interval, such as determining the wavelength as 1550.03 nm. Conversely, if only the second interferometer with an FSR of 1 nm were used, it might not be able to distinguish between wavelengths such as 1550.03 nm, 1551.03 nm, and 1552.03 nm. Thus, by measuring the coarse range, the exact wavelength can be pinpointed. Thus, both coarse and fine wavelength control are obtained.

[0115] FIG. 8B is a schematic diagram illustrating an example of angle versus wavelength relationships for a waveguide with a smaller FSR and a waveguide with a relatively larger FSR.

[0116] FIG. 8C is a schematic diagram illustrating an example of a wavelength correction range for a given FSR.

[0117] FIG. 8D is a diagram that schematically illustrates one example of how two different FSRs can be combined to provide a relatively large range of correction (large FSR) while maintaining desirable accuracy (small FSR).

[0118] FIG. 9 is a schematic diagram illustrating another example of an interferometer-based circuit according to one embodiment. FIG. 9 is similar to FIG. 8 except for the additional input switch present in the circuit solution shown in FIG. 9. To address coherent applications, an alternative design is proposed in which an additional optical switch is introduced to allow either the Tx or Rx signal to be fed into the WLL in time-division multiplexed mode, as previously described. This means that the Tx and Rx can share the same WLL structure, and individual feedback loops can be created separately using the switch 90 to maintain the stability of the Tx laser and keep the Rx-LO frequency small. As an example, the switch 90 can be a Mach-Zehnder (MZ)-based switch with an optional internal phase shifter 111 incorporated therein.

[0119] FIG. 10 is a schematic diagram illustrating yet another example of an interferometer-based circuit according to an embodiment. This represents an alternative embodiment for implementing different waveguides by using an optical switch 80. In this example, the second signal path 115-2 includes an optical switch 80 disposed upstream of the signal splitter 116 for switching between a first waveguide 119-1 having a first free spectral range (FSR) and a second waveguide 119-2 having a second FSR, the second FSR being greater than the first FSR, enabling higher wavelength resolution for signals passing through the first waveguide 119-1 and a larger wavelength correction range for signals passing through the second waveguide 119-2. For example, the switch 80 may be a MZ-based switch incorporating an optional internal phase shifter 111.

[0120] FIG. 11A is a schematic diagram illustrating an example of a frequency response of the output of a photodetector of an interferometer-based circuit according to one embodiment.

[0121] FIG. 11B is a schematic diagram illustrating the intensity versus frequency curve of a photodetector corresponding to FIG. 11A.

[0122] FIG. 11C is a schematic diagram illustrating a nearly linear angle versus frequency curve corresponding to FIG. 11B when vector synthesis is performed.

[0123] As an example, for a given wavelength, the intensity value of the photodetector can be determined and used as part of the real (Re) and imaginary (Im) components of a newly constructed complex signal. The resulting complex value is (Re - CAL_aver) + j * (Im - CAL_aver), where CAL_aver is the average value that can be calculated using WLL calibration. As shown in Figure 11C, sweeping the frequency / wavelength and calculating the angle of the complex signal yields a linear angle vs. frequency / wavelength curve.

[0124] Two-signal vector synthesis can then be used, i.e., combining two signals to create a complex output. The angle of the complex vector-based signal can be calculated by sweeping different wavelengths so that a linear angle-wavelength curve is obtained. The angle and / or amplitude of the complex vector-based signal can be linearly correlated with wavelength.

[0125] This provides a wavelength control device that has equal resolution for every wavelength of the tunable laser due to the linear correlation between angle (and / or amplitude) and wavelength. Furthermore, the linearity allows locking to any wavelength within the operating range of the tunable laser, with no deadlock range (i.e., no dead zone or range where the wavelength control device cannot lock to a particular wavelength).

[0126] The inventive concept has been primarily described above with reference to a limited number of examples. However, as will be readily understood by those skilled in the art, other examples than those disclosed above are equally possible within the scope of the inventive concept as defined by the appended claims.

Claims

1. A wavelength control device (100) for a tunable laser (10), comprising: The wavelength control device (100) is configured to receive an input signal and, in response thereto, generate a control signal for adjusting the wavelength of the tunable laser (10), the wavelength control device (100) comprising: an interferometer-based circuit (110) configured to generate phase-shifted intensity values ​​based on the input signal; a vector-based signal combiner (120) configured to generate a complex vector-based signal based on the phase-shifted intensity values ​​and to calculate at least one vector-represented quantity based on the complex vector-based signal, wherein the at least one vector-represented quantity is approximately linearly correlated with the wavelength; and a control signal generator (130) configured to generate the control signal based at least in part on the at least one vector representation quantity; A wavelength control device (100) comprising:

2. The wavelength control device (100) of claim 1, wherein the at least one vector representation quantity comprises an angle and / or an amplitude of the complex vector-based signal.

3. 3. The wavelength control device (100) of claim 1 or 2, wherein the control signal generator (130) is configured to perform a control signal calculation based on the at least one vector representation quantity.

4. 4. The wavelength control device (100) of claim 1, wherein the interferometer-based circuit (110) comprises a phase shifter (111) for providing a phase shift when generating the phase-shifted intensity values.

5. 5. The wavelength control device of claim 1, wherein the interferometer-based circuit comprises at least two different waveguides having different free spectral ranges (FSRs), including a first waveguide having a first FSR and a second waveguide having a second FSR, the second FSR being greater than the first FSR, enabling higher wavelength resolution for signals passing through the first waveguide and a larger wavelength correction range for signals passing through the second waveguide.

6. 6. The wavelength control device (100) of claim 1, further comprising an optical input switch (90) for selectively providing one of at least two different signals as the input signal of the wavelength control device (100).

7. The wavelength control device (100) of claim 6, wherein the at least two different signals include a transmitted (Tx) laser signal and a received (Rx) laser signal.

8. The wavelength control device (100) of claim 7, wherein the wavelength control device (100) is configured to provide time-interleaved tracking of the transmitted (Tx) laser signal and the received (Rx) laser signal.

9. 9. The wavelength control device (100) of claim 8, wherein the wavelength control device (100) is configured to provide the time-interleaved tracking to enable reduction of wavelength differences between the received (Rx) laser signal and a local oscillator (70) signal and / or maintaining stability of a transmission laser of the transmitted (Tx) laser signal.

10. The interferometer-based circuit (110) comprises: a first signal splitter 112 configured to split the input signal into two different signal paths (115-1, 115-2), a first signal path (115-1) and a second signal path (115-2); a second signal divider (114) in the first signal path (115-1) configured to divide the signal of the first signal path (115-1) into two parts having the same phase; a third signal divider (116) in the second signal path (115-2) configured to divide the signal of the second signal path (115-2) into two parts having different phases and providing a non-zero relative phase shift; two signal combiners (117-1, 117-2), each configured to perform signal combining based on an output of the second signal divider (114) and a respective output of the third signal divider (116) and provide a combined signal; two detectors (118-1, 118-2), each detector (118-1, 118-2) configured to detect an intensity value of the combined signal of a respective signal combiner (117-1, 117-2), the intensity values ​​of the two detectors (118-1, 118-2) being phase shifted relative to each other and provided as inputs to the vector-based signal combiner (120); The wavelength control device (100) of any one of claims 1 to 9, comprising:

11. 11. The wavelength control device (100) of claim 10, wherein the first signal splitter (112) is based on a 1x2 multimode interferometer (MMI), the second signal splitter (114) is based on a 1x2 MMI, the third signal splitter (116) is based on a 2x2 MMI, each of the signal combiners (117-1, 117-2) is based on a 2x1 MMI, and each of the detection units (118-1, 118-2) is based on a photodetector.

12. 12. A wavelength control device (100) according to claim 10 or 11, wherein the relative phase shift is between 60 and 120 degrees.

13. 13. The wavelength control device (100) of claim 10, wherein a phase-shift-based unipolar-to-bipolar converter (111) is incorporated in the first signal path (115-1) or the second signal path (115-2) to provide a thermal phase shift when generating the phase-shifted intensity values.

14. The wavelength control device (100) of any one of claims 1 to 13, wherein the wavelength control device (100) is configured to tune the wavelength to be within a restricted wavelength range.

15. The wavelength control device (100) according to any one of claims 1 to 14, wherein said wavelength control device (100) is a wavelength locker (WLL).

16. The wavelength control device (100) of any one of claims 1 to 15, wherein the wavelength control device (100) is integrated on the same integrated circuit as the laser (10).

17. The wavelength control device (100) of any one of claims 1 to 16, wherein the wavelength control device (100) is configured to receive at least a portion of the output of the wavelength tunable laser (10) as the input signal, or is configured to receive at least a portion of a received laser signal as the input signal.

18. A laser system (1000) comprising a tunable laser (10) and a wavelength control device (100) according to any one of claims 1 to 17.

Images