Polarization-insensitive cascade digital Fourier transform spectrometer

Abstract

The invention relates to a polarization-insensitive cascade digital Fourier transform spectrometer, which comprises a beam splitter used for dividing an input optical signal into a first optical signal and a second optical signal; the Mach-Zehnder interferometer comprises a first arm, a second arm and a plurality of optical switch units, the first arm is used for transmitting a first optical signal, and the second arm is used for transmitting a second optical signal; the plurality of optical switch units are sequentially cascaded on the first arm and the second arm, and the optical switch units on the first arm are in one-to-one correspondence with the optical switch units on the second arm; the waveguide structures of the first arm and the second arm are micron-sized integrated optical waveguide structures, and the micron-sized integrated optical waveguide structures have geometric symmetry in width and height; the beam combiner is used for combining the first optical signal transmitted by the first arm and the second optical signal transmitted by the second arm into an interference signal; and the detector is used for converting the interference signal into an electric signal and outputting the electric signal. According to the invention, high-resolution spectrum reconstruction can be realized.

Description

Technical Field

[0001] This invention relates to an on-chip Fourier transform spectrometer, and more particularly to a polarization-insensitive cascaded digital Fourier transform spectrometer. Background Technology

[0002] On-chip spectrometers offer unparalleled advantages in size, weight, and power consumption compared to traditional benchtop instruments in fields such as spectral sensing, optical network performance monitoring, hyperspectral imaging, and RF spectrum analysis, making them a key research focus in optoelectronic integration. Currently, there are three main types of on-chip spectrometers. The first type is a scaled-down version of a traditional benchtop spectrometer, based on wavelength multiplexing principles and relying on dispersive components such as gratings and microring resonators. The second type is a narrowband filtering on-chip spectrometer, which selectively transmits specific wavelengths through a series of narrowband filters (such as Fabry-Perot filters and electrochromic filters), and then measures the transmitted light intensity using a detector. The third type uses a Mach-Zehnder interferometer (MZI) array to achieve Fourier transform spectrometry. By measuring interference signals with different optical path differences and reconstructing the spectrum through Fourier transform, its core competitiveness lies in its combination of high resolution and wide bandwidth, as well as strong stability, making it the preferred solution for high-precision applications.

[0003] The current traditional third-class spectrometers use the strong birefringence effect of 220nm SOI waveguides, which makes the spectral response dependent on the polarization state of the incident light, thus limiting its practicality. In order to improve the resolution of the Fourier spectrometer, it is necessary to increase the number of delay line orders (i.e., increase the number of optical switches). However, under the traditional high-loss waveguide technology, the series connection of multiple switches will cause the optical signal to decay sharply and the signal-to-noise ratio to deteriorate, thus limiting the number of switches that can be integrated and the final spectral resolution. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a polarization-insensitive cascaded digital Fourier transform spectrometer that can achieve high-resolution spectral reconstruction.

[0005] The technical solution adopted by this invention to solve its technical problem is: to provide a polarization-insensitive cascaded digital Fourier transform spectrometer, comprising: A beam splitter is used to split an input optical signal into a first optical signal and a second optical signal. The Mach-Zehnder interferometer includes a first arm, a second arm, and multiple optical switch units. The first arm is used to transmit a first optical signal, and the second arm is used to transmit a second optical signal. The multiple optical switch units are cascaded on the first arm and the second arm, and there is a one-to-one correspondence between the optical switch units on the first arm and the optical switch units on the second arm. The waveguide structure of the first arm and the second arm is a micrometer-scale integrated optical waveguide structure, and the micrometer-scale integrated optical waveguide structure has geometric symmetry in width and height. A beam combiner is used to combine the first optical signal transmitted by the first arm and the second optical signal transmitted by the second arm into an interference signal. A detector is used to convert the interference signal into an electrical signal output.

[0006] There are N optical switch units on both the first arm and the second arm; the optical path difference of the nth optical switch unit on the first arm along the first optical signal transmission direction is... The optical path difference of the delay of the nth optical switch unit on the second arm along the second optical signal transmission direction is ,in, It is the unit length of the delay optical path difference.

[0007] The optical switch unit is a deep trench isolated silicon-doped thermo-optical switch.

[0008] The optical switch unit is formed by etching deep grooves on both sides of the waveguide structure.

[0009] Both the first and second arms have integrated on-chip optical attenuators, which are located before each optical switch unit from the fourth stage onwards.

[0010] The integrated optical waveguide structure is either a ridge waveguide structure or a strip waveguide structure.

[0011] The polarization-insensitive cascaded digital Fourier transform spectrometer is fabricated using an SOI wafer with a top silicon layer that is polarization-insensitive.

[0012] Beneficial effects Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art: The present invention utilizes the ultra-low loss characteristics of polarization-insensitive technology to break through the limitation of the number of optical switches cascaded, and successfully achieves that even when more than 10 optical switches are cascaded, the optical signal will not be drastically attenuated. This means that 1024 sampling points can be obtained in a single scan. Compared with the traditional design with fewer stages, the spectral resolution is improved by several orders of magnitude, and no complex external mechanical parts are required. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a polarization-insensitive cascaded digital Fourier transform spectrometer according to an embodiment of the present invention. Figure 2 This is a simulation diagram of the waveguide structure and its TE / TM mode field distribution in an embodiment of the present invention; Figure 3 This is a cross-sectional schematic diagram of the optical switch unit in an embodiment of the present invention. Detailed Implementation

[0014] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0015] Embodiments of the present invention relate to a polarization-insensitive cascaded digital Fourier transform spectrometer, such as... Figure 1 As shown, it includes: Beam splitter 1 is used to split the input optical signal into a first optical signal and a second optical signal; The Mach-Zehnder interferometer 2 includes a first arm, a second arm, and multiple optical switch units 5. The first arm is used to transmit a first optical signal, and the second arm is used to transmit a second optical signal. The multiple optical switch units 5 are cascaded on the first arm and the second arm, and there is a one-to-one correspondence between the optical switch units 5 on the first arm and the optical switch units 5 on the second arm. The waveguide structure of the first arm and the second arm is a micrometer-scale integrated optical waveguide structure, and the micrometer-scale integrated optical waveguide structure has geometric symmetry in width and height. Beam combiner 3 is used to combine the first optical signal transmitted by the first arm and the second optical signal transmitted by the second arm into an interference signal; Detector 4 is used to convert the interference signal into an electrical signal output.

[0016] The cascaded digital Fourier transform spectrometer of this embodiment is fabricated using an SOI wafer with a top silicon layer thickness that is polarization insensitive. The waveguide structures of the first and second arms of the Mach-Zehnder interferometer 2 are designed as either micrometer-scale large-section ridge waveguides (fabricated via shallow etching) or strip waveguides (fabricated via deep etching). This waveguide design based on a thick silicon layer endows the device with two key physical characteristics. First, it possesses inherent polarization insensitivity due to the high geometric symmetry of the large-section waveguide in both width and height, such as... Figure 2 As shown, the effective refractive index difference between TE and TM modes is extremely small (Δneff≈0), which decouples the optical path difference generated by light propagation in the waveguide from the polarization state of the incident light, eliminating the polarization sensitivity problem that is difficult to avoid in traditional silicon photonic devices from the physical level. Secondly, it has excellent ultra-low transmission loss characteristics. The large mode field distribution significantly reduces the overlap integral between the optical field and the rough surface of the waveguide sidewall, thereby greatly suppressing scattering loss and reducing the transmission loss to below 0.2 dB / cm.

[0017] Therefore, this embodiment utilizes the high symmetry of the polarization-insensitive large-section waveguide to achieve an isotropic response to incident light of any polarization state. This not only simplifies the optical path design but also eliminates interference fringe blurring caused by polarization mode dispersion (PMD), significantly improving the accuracy of spectral reconstruction.

[0018] The aforementioned low-loss characteristic is a key prerequisite for the successful integration of up to 10 or more cascaded optical switches in this embodiment, ensuring that the optical signal maintains an extremely high signal-to-noise ratio when it reaches the detector after long-distance delay line transmission and the superposition of insertion losses from multiple switching nodes. Based on the aforementioned low-loss and polarization-insensitive waveguide structure, this embodiment can construct a digital delay line network consisting of 10 cascaded optical switch units on the interferometer arm of a Mach-Zehnder interferometer (MZI). This digital delay line network adopts a binary cascaded architecture, that is, the delay path difference of the nth optical switch unit on the first arm along the first optical signal transmission direction is... The optical path difference of the delay of the nth optical switch unit on the second arm along the second optical signal transmission direction is ,in, The unit length of the optical path difference is the delay. The delay waveguide is divided into 10 independent delay bits by using 10 optical switches. The physical delay waveguide length corresponding to each delay bit strictly follows a multiple of 2n. By controlling the on / off state combination of these 10 optical switch units, the optical path difference can be digitally reconstructed with high throughput, thereby generating 1024 discrete optical path differences (OPD).

[0019] To further improve the system's performance and stability, such as Figure 3 As shown, the optical switch unit in this embodiment can be a doped silicon thermo-optical switch with deep trench isolation, which can be obtained by etching deep trenches on both sides of the waveguide. This can block heat diffusion, reduce power consumption and reduce thermal crosstalk.

[0020] To optimize the optical power balance of the cascaded links, this embodiment also integrates an on-chip optical attenuator 6 on both the first and second arms. The on-chip optical attenuator 6 is located before each optical switch unit 5 after the fourth stage, and is used to dynamically adjust the link loss and suppress stray light interference, thereby ensuring a high extinction ratio and a high signal-to-noise ratio of the optical switch network in complex cascaded states.

[0021] In this embodiment, the incident light enters the beam splitter 1 via the input waveguide and is split into two beams, which then enter the reference arm and modulation arm of the Mach-Zehnder interferometer 2, respectively. After the two beams pass through the aforementioned reconfigurable optical switch network and receive a specific phase delay, they interfere at the beam combiner 3 at the output end. The final interference signal is received by the on-chip integrated germanium (Ge) photodetector 4 and converted into an electrical signal. Subsequently, a high-resolution target spectrum is reconstructed through digital signal processing.

[0022] It is easy to see that the effective mode area of ​​the polarization-insensitive waveguide is much larger than that of the traditional 220nm waveguide, which greatly reduces nonlinear effects (such as two-photon absorption) and enables it to withstand higher incident light power. Combined with the optimized switch and attenuator design mentioned above, the cumulative insertion loss after the optical signal passes through more than 10 switch units and corresponding delay lines is still controlled within the high response linear region of the detector. As a result, the signal-to-noise ratio (SNR) of the output interference signal is significantly better than that of the traditional silicon photonics spectrometer, effectively overcoming the technical bottleneck of the traditional 220nm process, which cannot cascade multiple stages of switches due to excessive loss.

Claims

1. A polarization-insensitive cascaded digital Fourier transform spectrometer, characterized in that, include: A beam splitter is used to split an input optical signal into a first optical signal and a second optical signal. The Mach-Zehnder interferometer includes a first arm, a second arm, and multiple optical switch units. The first arm is used to transmit a first optical signal, and the second arm is used to transmit a second optical signal. The multiple optical switch units are cascaded on the first arm and the second arm, and there is a one-to-one correspondence between the optical switch units on the first arm and the optical switch units on the second arm. The waveguide structure of the first arm and the second arm is a micrometer-scale integrated optical waveguide structure, and the micrometer-scale integrated optical waveguide structure has geometric symmetry in width and height. A beam combiner is used to combine the first optical signal transmitted by the first arm and the second optical signal transmitted by the second arm into an interference signal. A detector is used to convert the interference signal into an electrical signal output.

2. The polarization-insensitive cascaded digital Fourier transform spectrometer according to claim 1, characterized in that, There are N optical switch units on both the first arm and the second arm; the optical path difference of the nth optical switch unit on the first arm along the first optical signal transmission direction is... The optical path difference of the delay of the nth optical switch unit on the second arm along the second optical signal transmission direction is ,in, It is the unit length of the delay optical path difference.

3. The polarization-insensitive cascaded digital Fourier transform spectrometer according to claim 1, characterized in that, The optical switch unit is a deep trench isolated silicon-doped thermo-optical switch.

4. The polarization-insensitive cascaded digital Fourier transform spectrometer according to claim 3, characterized in that, The optical switch unit is formed by etching deep grooves on both sides of the waveguide structure.

5. The polarization-insensitive cascaded digital Fourier transform spectrometer according to claim 1, characterized in that, Both the first and second arms have integrated on-chip optical attenuators, which are located before each optical switch unit from the fourth stage onwards.

6. The polarization-insensitive cascaded digital Fourier transform spectrometer according to claim 1, characterized in that, The integrated optical waveguide structure is either a ridge waveguide structure or a strip waveguide structure.

7. The polarization-insensitive cascaded digital Fourier transform spectrometer according to claim 1, characterized in that, It is manufactured using SOI wafers with a top silicon thickness that is insensitive to polarization.

Images