Time multiplexing spectrum analysis chip based on modulator array and spectrum reconstruction method

Abstract

The invention discloses a time multiplexing spectrum analysis chip based on a modulator array and a spectrum reconstruction method. The chip comprises an input end main waveguide; the micro-ring array is composed of micro-ring resonant cavities with different FSRs; the modulator array is composed of independent Mach-Zehnder intensity modulation units, and optical signals are coupled and input through a micro-ring drop; the spatial light structure beam combiner is used for converging light signals at the output end of the modulator array in a free space; the optical detector is used for receiving the converged optical signal and outputting a time sequence electric signal; the electrode array is used for applying discrete switch control signals to the modulation units in the time dimension. According to the scheme, channels with different transmission spectrums are constructed through the micro-ring array, the on-off state of the modulator array is coded, specific currents are applied to all modulators according to codes, the sum of all paths of light intensity in different coding states is collected, spectrum reconstruction is carried out on the collected light intensity sequence, and the spectrum of the light intensity sequence is obtained. And the spectral information of the incident end can be efficiently restored.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of spectrometer, in particular to a time multiplexing spectrum analysis chip based on modulator array and a spectrum reconstruction method. BACKGROUND

[0002] Spectrometer is one of the important tools for analyzing the composition of matter. However, the traditional device is difficult to be used in the field due to its poor stability, long optical path and large volume of light splitting device, which limits its application scenarios. In recent years, with the development of optical integration technology, on-chip micro spectrometer system has gradually achieved higher resolution, smaller volume, lower power consumption and other performance breakthroughs. In the future, on-chip spectrometer is expected to replace the traditional spectrometer system and be widely used in medical, industrial, environmental protection and even wearable devices. Among the many on-chip spectrometer schemes, micro ring is an important component. The micro ring is used as a filter, and its resonance peak traverses a range of wavelengths, and the light intensity at the drop end reflects the intensity of each wavelength to restore the spectrum.

[0003] This kind of scheme or adjusts the micro ring through the heater to traverse the wavelength, which usually limits the detection range of the spectrometer to the FSR of the ring. In addition, due to the dependence of high-resolution spectrometer on micro ring with high quality factor, the light intensity at the drop end is weak, which makes the signal-to-noise ratio low. Or adjust the micro ring size through the micro ring array, but at this time the light intensity is distributed in multiple detectors, and the signal-to-noise ratio is also poor in weak light spectrum detection. Therefore, we propose a time multiplexing spectrum analysis chip based on modulator array. SUMMARY

[0004] The present application is proposed to overcome the shortcomings of the prior art, and provides a time multiplexing spectrum analysis chip based on modulator array and a spectrum reconstruction method, which has the advantages of high light energy utilization, single detector and high resolution.

[0005] The present application is achieved by the following technology: A time multiplexing spectrum analysis chip based on modulator array, comprising: an input end main waveguide; a micro ring array composed of a plurality of micro ring resonant cavities with different free spectral ranges, each micro ring being coupled with the main waveguide and outputting an optical signal of a corresponding wavelength at the drop end; a modulator array composed of N independent Mach-Zehnder intensity modulation units, each modulation unit being coupled with the drop end of a micro ring and receiving an optical signal from the corresponding micro ring at the input end; each modulation unit comprising: a 1x2 beam splitter, two waveguide arms, a 1x2 beam combiner and a thermo-optic phase shift structure realized by a heater located above the waveguide arm; a spatial light structure combiner for converging the light signals of all the output ends of the modulator array in free space; a light detector for receiving the light signals converged by the combiner and outputting time-series electrical signals; an electrode array corresponding to each heater in the modulator array, for applying discrete switch control signals to each modulation unit in the time dimension, so that each modulator is in the "on" or "off state in a set time period; wherein the electrode array generates time-series control signals according to a preset modulation coding matrix A(N, M), N is the number of micro-rings, and M is the number of time sampling points; the switch state of each modulation unit in M time-series cycles constitutes a binary sequence.

[0006] The time-multiplexed spectral analysis chip based on the modulator array, the spatial light structure combiner comprises: a grating coupler array arranged at the output end of each modulation unit, for vertically coupling the light signals in the waveguide to free space; a lens for focusing the light beams from each grating coupler to the light-sensitive surface of the detector.

[0007] The time-multiplexed spectral analysis chip based on the modulator array, The radius of each micro-ring in the micro-ring array is nonlinearly graded, so that the free spectral range (FSR) of adjacent micro-rings differs by more than 5 times the 3dB line width, to ensure that the orthogonality of each channel transmission peak in the spectral domain is significant.

[0008] The time-multiplexed spectral analysis chip based on the modulator array, A heater is arranged above the waveguide of each Mach-Zehnder modulation unit, and the electrode array controls the temperature change of each heater by applying a constant current or a pulsed current, thereby changing the effective refractive index of the waveguide through the thermo-optic effect, and realizing on / off modulation of the output light intensity; wherein the "on" state corresponds to the heater applying a current to make the phase difference of the two arms an odd multiple of π, and the "off state corresponds to the phase difference being an integer multiple of 0 or 2π.

[0009] A spectral reconstruction method based on the spectrometer, comprising the following steps: S1. Construct a modulation coding matrix A(N, M) ∈ {0, 1}^(N×M), wherein each column corresponds to a time sampling point, and each row corresponds to the switch state of a modulator; the number of "1"s in each column of the matrix A is not less than N / 2; S2. According to the matrix A, generate time-series current signal sequences {i 11 ,i 12 ,…,i1m},i 21 ,…,i 2m}, …, {i n1 ,…,i nm}; S3. The to-be-tested optical signal is input into a main waveguide, the time sequence current signal is applied through an electrode array, the modulator array is controlled to switch according to the A matrix, and an M-dimensional time sequence optical intensity signal I = [I1, I2, …, IM] is collected on a detector. m ] ⊤ ; S4. The system is calibrated in advance by using a calibration light source: the system is irradiated by using a known spectrum S0 (lambda), and the output I0 at M time points is recorded; by measuring the transmission response of each modulation state i to each wavelength lambda j , an M*V-dimensional system response matrix T (M, V) is constructed, wherein T ij represents the normalized transmission rate of the i-th modulation state to the j-th wavelength point. S5. An optimization problem is solved: min ||T*S - I||2+ lambda1||D*S||2+ lambda2||S||1, wherein S is a to-be-reconstructed spectrum vector V-dimensional, D is a first-order difference operator, lambda1 and lambda2 are regularization coefficients, and an optimal spectrum estimation .

[0010] Compared with the prior art, the present application has the beneficial effects: 1. The embodiment of the present application adopts a silicon light platform, which has many advantages such as low loss, high integration density and CMOS compatibility.

[0011] 2. The present application constructs a transmission spectrum family through a micro-ring array, randomly combines the transmission spectra of multiple channels through a switch, constructs a low-correlation transmission matrix, and thus obtains high-efficiency spectrum reconstruction capability.

[0012] 3. The present application greatly improves the light energy utilization rate of incident light through a multi-channel combination mode, uses a single detector, improves the signal-to-noise ratio of the device, and optimizes the spectrum detection capability of weak light. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 is a structural schematic diagram of a spectrum instrument. Among them: main waveguide 1, micro-ring array 2, modulator array 3, spatial light structure combiner 4, detector 11, electrode array 6.

[0014] Figure 2 is a schematic diagram of a Mach-Zehnder modulator array (upper part) and a waveguide cross-section diagram (lower part). Among them: beam splitter 7, two-arm waveguide 10, beam combiner 8, heater 9.

[0015] Figure 3is a schematic diagram of the total beam combiner. Among them: grating coupler array 13, lens 12, detector 11.

[0016] Figure 4 is a flowchart of the present application, including: input light → micro-ring branching → modulator coding → spatial beam combining → detector acquisition → matrix calibration → regularization reconstruction.

[0017] Figure 5 is a transmittance matrix diagram (a) and spectral reconstruction results (b).

[0018] Figure 5 a is a heat map of the system transmittance matrix T(200,1000), with the horizontal axis representing wavelength (1500-1600nm) and the vertical axis representing 200 modulation states.

[0019] Figure 5 b is a comparison of the reconstructed spectrum with the true broadband light + discrete double-peak spectrum.

[0020] In the figure, the main waveguide 1, the micro-ring array (micro-ring resonator array) 2, the modulator array 3, the spatial light structure beam combiner 4, the electrode array 6, the beam splitter 7, the beam combiner 8, the heater 9, the two-arm waveguide 10, the detector 11, the lens 12, the grating coupler array 13. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with reference to specific examples and the accompanying drawings. EMBODIMENT

[0022] As Figure 1 shown, the on-chip spectrometer of the present embodiment is prepared on an SOI platform, with a waveguide height of 220nm and a width of 450nm. The micro-ring array 2 includes 128 silicon-based micro-rings, with the micro-ring circumference increasing non-linearly from 40um to 436um, C i =40+3× i 1.01 ; According to the formula FSR = λ 2 / ( n g L ), the FSR of adjacent micro-rings decreases from 20nm to 2nm, and the degree of resonance peak overlap between channels is low.

[0023] The drop end of each micro-ring is connected to a Mach-Zehnder modulator unit 3. As Figure 2 shown, each modulator is composed of a beam splitter 7, a two-arm waveguide 10 (width 300nm), and a beam combiner 8. A TiN heater 9 (width 2um, length 50um) is deposited 1um above the upper arm.

[0024] When the heater applies a constant current, the local temperature rise, waveguide refractive index changes, so that the phase difference of the two arms is regulated to π (on state) or 0 (off state). When "on", the output light intensity is maximum; when "off", the output is attenuated to nearly 0 due to interference cancellation.

[0025] As shown in Figure 3 , each modulator output is connected to the center wavelength 1550nm, grating coupler array 13, which vertically couples the waveguide light to free space, and after focusing through the convex lens 12, it converges on the spatial light detector 11.

[0026] As shown in Figure 4 , the spectral reconstruction process is as follows: S1. Construct A(128,200) encoding matrix: use pseudo-random m sequence to generate binary matrix, the number of "1" in each column is greater than 64 (the proportion is greater than 50%), and the Pearson correlation coefficient between most two column vectors in the matrix is less than 0.1. S2. Generate timing current: map each column of A to 128 heater instructions, each instruction lasts for 50μs, and the total period is 10ms; S3. Collect signals: input the light signal (1500-1600nm) of the measured spectrum, and the detector outputs 200 sampling point light intensity signals I=[I1,I2,…,I 200 ] T ; S4. System calibration: use wavelength tunable laser (step 0.1nm) to irradiate the system, and measure each wavelength λ j under 200 modulation states, and construct T(200,1000) matrix. Each row corresponds to a modulation state, and each column corresponds to a wavelength point. After normalization, it is used as a reconstruction base; S5. Use CVX toolbox to solve the optimization problem: min ||T·S - I||2+ λ1||D·S||2+λ2||S||1, where D is a first-order difference operator.

[0027] Figure 5a The T matrix shows sparse and low correlation characteristics.

[0028] Figure 5b In the middle, the spectral signal containing wideband spectrum and 1540nm and 1540.2nm discrete double peaks (intensity ratio 1:1) is reconstructed, and two wave peaks (FWHM=0.18nm) are distinguished.

[0029] The embodiments in the above description can be further combined or replaced, and the embodiments are merely used to describe the preferred embodiments of the present application, and are not intended to limit the concept and scope of the present application. Without departing from the design idea of the present application, various changes and improvements made by those skilled in the art to the technical solutions of the present application shall fall within the protection scope of the present application. The protection scope of the present application is given by the appended claims and any equivalent technical solutions thereof.

Claims

1. A time-multiplexed spectral analysis chip based on a modulator array, characterized in that, include: Input waveguide (1); The micro-ring array (2) is composed of multiple micro-ring resonant cavities with different free spectral ranges. Each micro-ring is coupled to the main waveguide (1) and outputs an optical signal of the corresponding wavelength at the drop end. The modulator array (3) consists of N independent Mach-Zehnder intensity modulation units. Each modulation unit is coupled to the drop end of a microring, and its input end receives the optical signal from the corresponding microring. Each modulation unit includes: a 1×2 beam splitter (7), two waveguide arms (10), a 1×2 beam combiner (8), and a thermo-optical phase-shifting structure, which is implemented by a heater (9) located above the waveguide arms; A spatial optical structure beam combiner (4) is used to converge the optical signals from all outputs of the modulator array (3) in free space; A photodetector (11) is used to receive the optical signal converged by the beam combiner (4) and output a timing electrical signal; The electrode array (6) is connected one-to-one with each heater (9) in the modulator array (3) to apply discrete switching control signals to each modulation unit in the time dimension, so that each modulator is in the "on" or "off" state within a set time period. The electrode array (6) generates timing control signals according to the preset modulation coding matrix A(N,M), where N is the number of micro-rings and M is the number of time sampling points; the switching state of each modulation unit within M timing cycles constitutes a binary sequence.

2. The time-multiplexed spectral analysis chip based on a modulator array according to claim 1, characterized in that: The spatial optical structure beam combiner (4) includes: The grating coupler array (13) set at the output end of each modulation unit is used to vertically couple the optical signal in the waveguide to free space. Lens (12) is used to focus the outgoing light beams from each grating coupler onto the photosensitive surface of the detector (11).

3. The time-multiplexed spectral analysis chip based on a modulator array according to claim 1, characterized in that: The radius of each microring in the microring array (2) is nonlinearly varied, so that the difference in free spectral range (FSR) between adjacent microrings is greater than 5 times their 3dB linewidth, so as to ensure that the transmission peaks of each channel are significantly orthogonal in the spectral domain.

4. The time-multiplexed spectral analysis chip based on a modulator array according to claim 1, characterized in that: Heaters (9) are provided above the two-arm waveguides (10) of each Mach-Zehnder modulation unit. The electrode array (6) controls the temperature change of each heater (9) by applying a constant current or pulse current, thereby changing the effective refractive index of the waveguide through the thermo-optic effect and realizing the on / off modulation of the output light intensity. The "conduction" state corresponds to the heater applying current so that the phase difference between the two arms is an odd multiple of π, while the "cut-off" state corresponds to a phase difference of 0 or an integer multiple of 2π.

5. A spectral reconstruction method based on the spectrometer described in claims 1-4, characterized in that, Includes the following steps: S1. Construct a modulation coding matrix A(N,M) ∈ {0,1}^(N×M), where each column corresponds to a time sampling point and each row corresponds to the on / off state of a modulator; the number of "1"s in each column of matrix A is no less than N / 2; S2. Based on matrix A, generate the time-series current signal sequence {i} corresponding to each heater (9). 11 i 12 ,…,i 1m },{i 21 ,…,i 2m }, …, {i n1 ,…,i nm }; S3. Input the optical signal to be measured into the main waveguide (1), apply the timing current signal through the electrode array (6), control the modulator array (3) to switch according to matrix A, and collect the M-dimensional timing optical intensity signal I=[I1,I2,…,I] on the detector (11). m ] ⊤ ; S4. Pre-calibrate the system using a calibration light source: Illuminate the system with a known spectrum S0(λ) and record the output I0 at M time points; measure the output I0 at each wavelength λ under each modulation state i. j The transmission response is obtained, and an M×V dimensional system response matrix T(M,V) is constructed, where T ij This represents the normalized transmittance of the i-th modulation state at the j-th wavelength point; S5. Solve the optimization problem: min ||T·S - I||² + λ₁||D·S||² + λ₂||S||¹, where S is the V-dimensional spectral vector to be reconstructed, D is the first-order difference operator, and λ₁ and λ₂ are regularization coefficients, to obtain the optimal spectral estimate. .

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