Integrated wavelength locker

Abstract

The utility model discloses an integrated wavelength locker, which comprises an asymmetric Mach-Zehnder interference device, the asymmetric Mach-Zehnder interference device comprises a first beam splitter, a beam combiner, a first waveguide and a second waveguide, the length of the first waveguide is larger than that of the second waveguide, and the beam combiner comprises a second beam splitter and a third beam splitter. The first waveguide and the second waveguide are arranged between the first beam splitter and the beam combiner; the first waveguide and the second waveguide are both thin-film lithium niobate waveguides, the first waveguide is provided with a length difference region, the length difference region is arranged in the e-light direction of the first waveguide, and the part, outside the length difference region, of the first waveguide is the same as that of the second waveguide.

Description

Technical Field

[0001] This utility model relates to the field of laser technology, and in particular to an integrated wavelength locker. Background Technology

[0002] Tunable lasers are widely used in optical inspection, optical sensing, and optical communication. To accurately output a specified wavelength, tunable lasers require wavelength calibration and wavelength locking, which is costly.

[0003] In recent years, with the development of emerging applications such as artificial intelligence and cloud computing, data center traffic has exploded. To address the ever-increasing traffic problem, digital coherent communication is gradually being extended to short-range communication. Although short-range digital coherent communication does not require large-scale tunable lasers, it does require single-wavelength lasers with wavelength locking capabilities. Because short-range communication involves large deployments, low-cost and small-size optical modules are needed, thus requiring low-cost and small-size wavelength locking. Conventional wavelength locking is achieved using free-space optics, including FP etalons, beam splitters, and photodetectors for monitoring. Wavelength locking schemes based on FP etalons are large and complexly packaged, making it impossible to achieve low cost and small size, and thus difficult to meet the application requirements of short-range digital coherent communication.

[0004] To overcome the problems of large size and high packaging cost of wavelength locking schemes based on FP etalons, on-chip integrated wavelength lockers can be used. Furukawa Electric Corporation of Japan proposed a planar optical waveguide integrated wavelength locker based on SiO2-ZrO2 with high refractive index difference (Nishita M, Higa Y, Matsubara N, et al. Compact tunable DBR / Ring laser module integrated with extremely-high-ΔPLC wavelength locker[C] / / Optical Fiber Communication Conference.Optica Publishing Group,2020:M2A.6.). Mitsubishi Electric Corporation of Japan proposed a compact integrated wavelength locker based on silicon photonics (Suzuki J, Hasegawa K, Masuyama K, et al. ASi Photonics-Based Compact Wavelength Locker for SmallTunable Laser Modules[J].IEEE Photonics Technology Letters,2023,36(1):20-22.).

[0005] Silicon photonics-based wavelength lockers can integrate photodetectors on-chip, resulting in a very compact structure. However, silicon has a relatively large thermo-optic coefficient, making it highly susceptible to environmental stability, and the locked wavelength is greatly affected by temperature and power variations.

[0006] In addition, there is currently another type of wavelength locker, see [link to relevant documentation] Figure 9 The diagram illustrates the structure of an asymmetric Mach-Zehnder interferometer. Arrows indicate the direction of light. The input light is split into two beams of equal power by a 1x2 beam splitter 100'. These beams then pass through two arms of different lengths (the length difference between the two arms is ΔL), and finally are combined by a 2x1 beam combiner 200' before being output. The two beams travel different optical path lengths in the two arms, resulting in wavelength-dependent periodic interference enhancement and destructive interference. The free spectral range (FSR) of the asymmetric Mach-Zehnder interferometer can be calculated using the following formula:

[0007]

[0008] Where λ is the wavelength, n g Let be the group refractive index of light, and ΔL be the length difference between the two arms. A typical transmission spectrum of an asymmetric Mach-Zehnder interferometer is shown below. Figure 10 As shown, the FSR is 100 GHz. The characteristic of the transmission of an asymmetric Mach-Zehnder interferometer changing periodically with wavelength can be used for wavelength locking. Similarly, as disclosed in Chinese Patent Application No. 202210961961.3, temperature drift has a significant impact on the function of the wavelength locker, but currently, no method has been proposed to improve the temperature stability of this wavelength locker, therefore further improvement is needed. Utility Model Content

[0009] The technical problem to be solved by this utility model is to address the shortcomings of the existing technology by providing an integrated wavelength locker with low temperature drift coefficient and good temperature stability.

[0010] The technical solution adopted by this utility model to solve the above-mentioned technical problem is as follows: an integrated wavelength locker, comprising an asymmetric Mach-Zehnder interferometer, wherein the asymmetric Mach-Zehnder interferometer includes a first beam splitter, a beam combiner, a first waveguide, and a second waveguide, wherein the length of the first waveguide is greater than that of the second waveguide, and both the first waveguide and the second waveguide are disposed between the first beam splitter and the beam combiner; characterized in that:

[0011] Both the first waveguide and the second waveguide are thin-film lithium niobate waveguides. The first waveguide has a length difference region, which is located in the e-beam direction of the first waveguide. The portion of the first waveguide outside the length difference region is the same as that of the second waveguide.

[0012] By taking advantage of the small thermo-optic coefficient of thin-film lithium niobate waveguides in the e-light direction, the length difference region of the two arms of an asymmetric Mach-Zehnder interferometer is set in the e-light direction of the waveguide, thereby providing a wavelength locker with a smaller temperature drift coefficient and better temperature stability.

[0013] Preferably, the first beam splitter is a 1x2 beam splitter, and the first waveguide and the second waveguide are respectively connected to the two outputs of the first beam splitter. The beam combiner is a 2x1 beam combiner, and the first waveguide and the second waveguide are also respectively connected to the two inputs of the beam combiner.

[0014] According to one aspect of the present invention, the integrated wavelength locker further includes a second beam splitter, which is a 1x2 beam splitter. The input end of the second beam splitter is used for optical input, one of the output ends of the second beam splitter is connected to the input end of the first beam splitter of the asymmetric Mach-Zehnder interferometer, and the other output end of the second beam splitter directly outputs light for optical power monitoring. The output end of the beam combiner outputs light for wavelength monitoring.

[0015] According to another aspect of the present invention, the integrated wavelength locker further includes a second beam splitter and a third beam splitter, both of which are 1x2 beam splitters. The asymmetric Mach-Zehnder interferometer has two identical structures, referred to as the first asymmetric Mach-Zehnder interferometer and the second asymmetric Mach-Zehnder interferometer, respectively.

[0016] The input of the second beam splitter is used for optical input. One of the outputs of the second beam splitter is connected to the input of the third beam splitter. The other output of the second beam splitter directly outputs light for optical power monitoring. One of the outputs of the third beam splitter is connected to the input of the first beam splitter of the first asymmetric Mach-Zehnder interferometer. The other output of the third beam splitter is connected to the input of the first beam splitter of the second asymmetric Mach-Zehnder interferometer. The outputs of the beam combiners of the two asymmetric Mach-Zehnder interferometers output light for wavelength monitoring.

[0017] To achieve arbitrary wavelength locking, the phase difference between the two asymmetric Mach-Zehnder interferometers is 90°.

[0018] Compared with the prior art, the advantages of this utility model are: by utilizing the characteristic that the thin-film lithium niobate waveguide has a small thermo-optic coefficient in the e-light direction, the length difference region of the two arms of the asymmetric Mach-Zehnder interferometer is set in the e-light direction of the waveguide, thereby providing a wavelength locker with a smaller temperature drift coefficient and better temperature stability. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of the thin-film lithium niobate waveguide according to an embodiment of the present invention;

[0020] Figure 2 This is a schematic diagram of the asymmetric Mach-Zehnder interferometer according to an embodiment of the present invention;

[0021] Figure 3 The transmission spectrum of an asymmetric Mach-Zehnder interferometer with an actual FSR of 100 GHz is shown.

[0022] Figure 4 This is a schematic diagram of a comparative example of an asymmetric Mach-Zehnder interferometer according to an embodiment of the present invention;

[0023] Figure 5 The temperature drift coefficients of the asymmetric Mach-Zehnder interferometer along the z-direction for the waveguide with the length difference are given at different frequencies.

[0024] Figure 6 The temperature drift at different frequencies of the asymmetric Mach-Zehnder interferometer with the length difference portion of the waveguide along the y-direction;

[0025] Figure 7 This is a schematic diagram of one embodiment of the integrated wavelength locker of this utility model;

[0026] Figure 8 This is a schematic diagram of another embodiment of the integrated wavelength locker of this utility model;

[0027] Figure 9 This is a schematic diagram of an asymmetric Mach-Zehnder interferometer.

[0028] Figure 10 This is the transmission spectrum of a typical asymmetric Mach-Zehnder interferometer. Detailed Implementation

[0029] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions.

[0030] In the description of this utility model, it should be understood that the terms "center," "longitudinal," "transverse," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are only for the convenience of describing this utility model and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Since the embodiments disclosed in this utility model can be arranged in different directions, these terms indicating direction are only for illustration and should not be regarded as limitations. For example, "upper" and "lower" are not necessarily limited to directions opposite to or consistent with the direction of gravity. In addition, features defined with "first" and "second" may explicitly or implicitly include one or more of such features.

[0031] Thin-film lithium niobate possesses a high electro-optic coefficient and a wide transparency window, making it suitable for fabricating high-speed electro-optic modulators with wide applications in optical communications. See also... Figure 1 The diagram shows a cross-section of a thin-film lithium niobate waveguide. From bottom to top, the layers are: substrate material 1 (commonly silicon substrate), silicon oxide layer 2, and lithium niobate layer 3. Optionally, a capping layer 4, made of silicon oxide, can be placed on top of the lithium niobate layer 3. Asymmetric Mach-Zehnder interferometers based on thin-film lithium niobate have also been developed. In this invention, thin-film lithium niobate is used to fabricate an integrated wavelength locker based on an asymmetric Mach-Zehnder interferometer, enabling integration with devices such as modulators.

[0032] Because lithium niobate is an anisotropic material, the refractive index of the waveguide in the curved section varies with the angle and differs from that along the e-ray (z-axis) and o-ray (x-axis and y-axis) waveguides. Therefore, to achieve precise FSR control, bending the waveguide (e.g., ...) is not feasible. Figure 9 The length difference specified by the asymmetric Mach-Zehnder interferometer (as shown) can only be achieved through waveguides in either the e- or o-direction. Lithium niobate exhibits different thermo-optical coefficients in the e- and o-direction directions, in addition to the different refractive indices. The thermo-optical coefficient in the e-direction is approximately 10. -6 The order of magnitude is greater than that of the o-ray, while the thermo-optic coefficient in the o-ray direction is approximately 10. -5 The magnitude is approximately 10 times that of the o-ray direction.

[0033] For the reasons stated above, see Figure 2This diagram illustrates a specific embodiment of the asymmetric Mach-Zehnder interferometer based on thin-film lithium niobate of this invention, where arrows indicate the direction of light. The interferometer includes a first beamsplitter 100, a beam combiner 200, a first waveguide 11, and a second waveguide 12. Both the first waveguide 11 and the second waveguide 12 are thin-film lithium niobate waveguides, serving as the two arms of the asymmetric Mach-Zehnder interferometer, and are both positioned between the first beamsplitter 100 and the beam combiner 200. The first waveguide 11 is longer than the second waveguide 12. The input terminal of the first beamsplitter 100 receives light input, and the two output terminals of the first beamsplitter 100 are the first waveguide 11 and the second waveguide 12, respectively. The ends of the two waveguides furthest from the first beamsplitter 100 are connected to the two input terminals of the beam combiner 200, respectively. Thus, the input light is split into two beams of equal power after passing through the first beam splitter 100. These beams are then transmitted through the first waveguide 11 and the second waveguide 12, respectively, and then combined by the beam combiner 200 before being output. The two beams travel different optical paths in the two waveguides, which will produce periodic interference enhancement and interference cancellation related to the wavelength.

[0034] Since the length of the first waveguide 11 is greater than that of the second waveguide 12, the first waveguide 11 has a length difference region 111, as shown in the dashed box in the figure. The first waveguide 11 achieves the length difference with the second waveguide 12 through two length difference regions 111, while the rest of the two waveguides remain consistent. The length difference region 111 is located in the e-beam direction, i.e., the z-axis direction, thereby reducing the influence of external temperature on the asymmetric Mach-Zehnder interferometer. The common parts of the first waveguide 11 and the second waveguide 12 can be connected using different waveguide forms, including but not limited to the connection method shown in the figure, as long as the connection form of the two waveguides is consistent, thereby achieving precise FSR control. The measured transmission spectrum of an asymmetric Mach-Zehnder interferometer with an FSR of 100 GHz is shown in the figure. Figure 3 As shown.

[0035] For comparison, see Figure 4 The length difference region of the two waveguides is set in the y direction, and the temperature drift coefficient of the integrated wavelength locker is tested together with the embodiment of this utility model.

[0036] By varying the chip's operating temperature, the temperature drift coefficient of the asymmetric Mach-Zehnder interferometer along the z and y directions in the length difference region was tested. The test results are as follows: Figure 5 and Figure 6 As shown, the temperature drift coefficient of the asymmetric Mach-Zehnder interferometer with the length difference region along the y-direction is approximately nine times that of the asymmetric Mach-Zehnder interferometer with the length difference region along the z-direction. The temperature drift coefficient of commonly used fused silica-based FP etalons is approximately -1 GHz / ℃. The temperature drift coefficient of the thin-film lithium niobate-based asymmetric Mach-Zehnder interferometer with the length difference region along the z-direction is smaller than that of the fused silica-based FP standard, indicating better temperature stability.

[0037] Therefore, the asymmetric Mach-Zehnder interferometer based on thin-film niobate waveguide of this invention can be used to realize a high-performance integrated wavelength locker.

[0038] See Figure 7 An embodiment of an integrated wavelength locker employing the aforementioned asymmetric Mach-Zehnder interferometer is shown, where arrows indicate the direction of light. It includes a second beamsplitter 300 (1x2 beamsplitter) and the aforementioned asymmetric Mach-Zehnder interferometer. The input of the second beamsplitter 300 is used for light input, one of its outputs is connected to the input of the first beamsplitter 100 of the asymmetric Mach-Zehnder interferometer, and the other output is directly output. Thus, the input light is split into two beams after passing through the second beamsplitter 300 (1x2 beamsplitter). One beam is directly output for optical power monitoring (optical output 1), and the other beam, after passing through the aforementioned asymmetric Mach-Zehnder interferometer, is output by the beam combiner 200 for wavelength monitoring (optical output 2).

[0039] See Figure 8 Another embodiment of an integrated wavelength locker employing the aforementioned asymmetric Mach-Zehnder interferometer is shown, where arrows indicate the direction of light. A single asymmetric Mach-Zehnder interferometer can only be used to lock wavelengths at fixed intervals; therefore, in this embodiment, integrating two asymmetric Mach-Zehnder interferometers allows for meshless wavelength locking. The two asymmetric Mach-Zehnder interferometers have identical structures and are designated as the first asymmetric Mach-Zehnder interferometer A and the second asymmetric Mach-Zehnder interferometer B, respectively.

[0040] and Figure 7 The difference in the illustrated embodiment is that, in this embodiment, the integrated wavelength locker further includes a third beamsplitter 400 (1x2 beamsplitter), which is connected between the second beamsplitter 300 and the first beamsplitter 100 of the first asymmetric Mach-Zehnder interferometer A. One output of the second beamsplitter 300 is connected to the input of the third beamsplitter 400, while the other output of the second beamsplitter 300 remains a direct output. One output of the third beamsplitter 400 is connected to the input of the first beamsplitter 100 of the first asymmetric Mach-Zehnder interferometer A, while the other output of the third beamsplitter 400 is connected to the input of the first beamsplitter 100 of the second asymmetric Mach-Zehnder interferometer B.

[0041] The input light is split into two beams after passing through the second beam splitter 300. One beam is used for optical power monitoring (output beam 1). The other beam is split into two beams again after passing through the third beam splitter 400. These two beams then pass through two asymmetric Mach-Zehnder interferometers for wavelength monitoring (the outputs of the two asymmetric Mach-Zehnder interferometers are output beam 2 and output beam 3, respectively). The two asymmetric Mach-Zehnder interferometers have the same FSR, but their outputs have a 90° phase difference, meaning the transmission spectrum is offset by 1 / 4 of the FSR, allowing for arbitrary wavelength locking. This phase difference can be achieved by changing the length or width of the two waveguides, a method already in use and will not be elaborated upon here.

Claims

1. An integrated wavelength locker, comprising an asymmetric Mach-Zehnder interferometer, the asymmetric Mach-Zehnder interferometer comprising a first beam splitter (100), a beam combiner (200), a first waveguide (11), and a second waveguide (12), wherein the length of the first waveguide (11) is greater than that of the second waveguide (12), and both the first waveguide (11) and the second waveguide (12) are disposed between the first beam splitter (100) and the beam combiner (200); characterized in that: The first waveguide (11) and the second waveguide (12) are both thin-film lithium niobate waveguides. The first waveguide (11) has a length difference region (111), which is located in the e-light direction of the first waveguide (11). The part of the first waveguide (11) outside the length difference region (111) is the same as the second waveguide (12).

2. The integrated wavelength locker of claim 1, wherein: The first beam splitter (100) is a 1x2 beam splitter, and the first waveguide (11) and the second waveguide (12) are respectively connected to the two output terminals of the first beam splitter (100). The beam combiner (200) is a 2x1 beam combiner, and the first waveguide (11) and the second waveguide (12) are also respectively connected to the two input terminals of the beam combiner (200).

3. The integrated wavelength locker of claim 1 or 2, wherein: The integrated wavelength locker also includes a second beam splitter (300), which is a 1x2 beam splitter. The input of the second beam splitter (300) is used for optical input. One of the outputs of the second beam splitter (300) is connected to the input of the first beam splitter (100) of the asymmetric Mach-Zehnder interferometer. The other output of the second beam splitter (300) directly outputs light for optical power monitoring. The output of the beam combiner (200) outputs light for wavelength monitoring.

4. The integrated wavelength locker of claim 1 or 2, wherein: The integrated wavelength locker also includes a second beam splitter (300) and a third beam splitter (400), both of which are 1x2 beam splitters. The asymmetric Mach-Zehnder interferometer has two identical structures, referred to as the first asymmetric Mach-Zehnder interferometer (A) and the second asymmetric Mach-Zehnder interferometer (B). The input of the second beam splitter (300) is used for optical input. One of the outputs of the second beam splitter (300) is connected to the input of the third beam splitter (400). The other output of the second beam splitter (300) directly outputs light for optical power monitoring. One of the outputs of the third beam splitter (400) is connected to the input of the first beam splitter (100) of the first asymmetric Mach-Zehnder interferometer (A). The other output of the third beam splitter (400) is connected to the input of the first beam splitter (100) of the second asymmetric Mach-Zehnder interferometer (B). The outputs of the beam combiners (200) of the two asymmetric Mach-Zehnder interferometers output light for wavelength monitoring.

5. The integrated wavelength locker of claim 4, wherein: The phase difference between the two asymmetric Mach-Zehnder interferometers is 90°.

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