Optical feedback type tunable Fabry-Perot filter

Abstract

The invention belongs to the technical field of optical filters, and discloses an optical feedback type tunable Fabry-Perot filter, which comprises a supporting structure formed by connecting a fixed lens cone and a load lens cone through a mortise and tenon joint structure; the resonant cavity assembly comprises a first reflecting mirror and a second reflecting mirror which are parallel to each other; the tuning driving module comprises piezoelectric ceramics, and the cavity length of the resonant cavity assembly is regulated and controlled by applying voltage to the piezoelectric ceramics; and the optical feedback module is used for calculating the cavity length and the cavity length deviation according to a diffraction pattern generated by the detection light in a diffraction gap between the first reflector and the second reflector so as to adjust the driving voltage of the piezoelectric ceramic. By introducing an optical feedback mechanism based on the diffraction effect, the cavity length change is monitored and compensated in real time, the problem of output peak drift caused by piezoelectric ceramic creep and hysteresis is solved, and meanwhile, the small-size characteristic and high stability are kept.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of optical filter, and particularly relates to an optical feedback type tunable Fabry-Perot filter. BACKGROUND

[0002] The Fabry-Perot filter is based on the principle of parallel plate multi-beam interference, and selective transmission of specific wavelengths is achieved by adjusting the cavity length of the resonant cavity, which has the advantages of high spectral resolution and wide free spectral range. However, the traditional Fabry-Perot filter and the existing improved scheme still have the following defects:

[0003] The existing tunable Fabry-Perot filter relies on piezoelectric ceramic to drive and adjust the cavity length, but piezoelectric ceramic has inherent creep and hysteresis, which will cause uncontrollable changes in the cavity length during long-term operation, and further cause the output wavelength to drift, which seriously affects the stability of the filter; the existing capacitive feedback type scheme can monitor the cavity length, but the preparation process of the metal electrode is complex, which easily introduces additional loss, and the compatibility of the electrode and the cavity mirror limits the integration of small size structure.

[0004] Therefore, there is an urgent need for a feedback adjustment scheme that is suitable for small size structure, can effectively suppress the output drift caused by the characteristics of piezoelectric ceramic, and is easy to integrate, in order to improve the stability and reliability of the tunable Fabry-Perot filter. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides an optical feedback type tunable Fabry-Perot filter, which introduces an optical feedback mechanism based on diffraction effect to monitor and compensate the cavity length change in real time, solves the problem of output peak drift caused by piezoelectric ceramic creep and hysteresis, and maintains small size characteristics and high stability. The present application provides the following technical solutions:

[0006] An optical feedback type tunable Fabry-Perot filter, comprising a support structure, a resonant cavity assembly, a tuning drive module and an optical feedback module;

[0007] The support structure comprises a fixed mirror tube and a load mirror tube, and the fixed mirror tube and the load mirror tube are connected through a mortise and tenon structure;

[0008] The resonant cavity assembly comprises a first mirror and a second mirror which are parallel to each other;

[0009] The tuning drive module comprises a piezoelectric ceramic, and the cavity length of the resonant cavity assembly is adjusted by applying voltage to the piezoelectric ceramic;

[0010] The optical feedback module is used to calculate the cavity length and the cavity length deviation according to the diffraction pattern generated by the detection light in the diffraction gap between the first mirror and the second mirror, so as to adjust the driving voltage of the piezoelectric ceramic.

[0011] Preferably, the first mirror and the second mirror are both dielectric film mirrors.

[0012] Preferably, the first quartz gasket, the second quartz gasket and the third quartz gasket are further included;

[0013] The first quartz gasket is located between the first mirror and the fixed lens barrel;

[0014] The second quartz gasket is located between the second mirror and the piezoelectric ceramic;

[0015] The third quartz gasket is located between the piezoelectric ceramic and the load lens barrel.

[0016] Preferably, the driving voltage range of the piezoelectric ceramic is 0-60V, and the displacement voltage ratio is 0.055μm / V.

[0017] Preferably, the optical feedback module comprises a detection light source, a diffraction detection unit, a signal processing circuit and a feedback control circuit;

[0018] The detection light source is used to emit detection light to irradiate the diffraction slit between the first mirror and the second mirror;

[0019] The diffraction detection unit is used to receive the diffraction fringe pattern generated by the diffraction slit;

[0020] The signal processing circuit is used to calculate the cavity length d and the cavity length deviation Δd according to the diffraction fringe pattern;

[0021] The feedback control circuit is used to adjust the driving voltage of the piezoelectric ceramic according to the cavity length variation.

[0022] Preferably, the detection light source adopts a helium-neon laser;

[0023] The laser emitted by the helium-neon laser is collimated into parallel light by a collimating lens with a focal length of 10mm, and then vertically incident to the diffraction slit between the first mirror and the second mirror.

[0024] Preferably, a narrow-band filter is arranged in front of the diffraction detection unit to filter ambient stray light.

[0025] Preferably, the signal processing circuit extracts the bright fringe center in the diffraction fringe pattern by threshold segmentation algorithm and edge detection algorithm, and calculates the adjacent bright fringe spacing Δx, and calculates the cavity length d by the formula d=kλ0L / Δx, wherein k is the diffraction order, λ0 is the wavelength of the detection light, and L is the distance from the diffraction slit to the detector.

[0026] Preferably, the feedback control circuit calculates a voltage compensation amount AV according to a piezoelectric ceramic displacement voltage ratio of the cavity length deviation Ad, and drives the piezoelectric ceramic through a high-voltage operational amplifier.

[0027] The beneficial effects of the present application are as follows:

[0028] The present application provides an optical feedback type tunable Fabry-Perot filter, which introduces an optical feedback mechanism based on diffraction effect, monitors and compensates the cavity length change in real time, solves the output peak drift problem caused by piezoelectric ceramic creep and hysteresis, and maintains small size characteristics and high stability. BRIEF DESCRIPTION OF DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the present application, the following briefly introduces the drawings needed in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor:

[0030] Figure 1 It is a longitudinal sectional view of the embodiment one of the present application;

[0031] Figure 2 It is a working schematic block diagram of the optical feedback system of the present application.

[0032] Explanation of reference signs:

[0033] 1, fixed lens barrel; 2, first quartz gasket; 3, first mirror; 4, second mirror; 5, second quartz gasket; 6, piezoelectric ceramic; 7, glue injection hole; 8, third quartz gasket; 9, load lens barrel; 10, diffraction slit; 11, light transmission hole. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0035] Embodiment one

[0036] The present application provides an optical feedback type tunable Fabry-Perot filter, which includes a support structure, a resonant cavity assembly, a tuning drive module and an optical feedback module. Next, combined with the drawings shown in Figure 1 , Figure 2 The various components of the optical feedback type tunable Fabry-Perot filter of the present application are described in detail:

[0037] The support structure comprises a fixed barrel 1 and a load barrel 9, the fixed barrel 1 and the load barrel 9 are connected through a mortise and tenon structure, both are made of aluminum alloy, and the diameter of the fixed barrel 1 and the load barrel 9 is 20mm and the width is 10mm. The fixed barrel 1 is provided with a protrusion at the end, and the load barrel 9 is provided with a groove at the corresponding position, the protrusion is embedded in the groove during assembly to realize positioning, without additional fasteners, simplifying the structure and ensuring coaxiality.

[0038] The resonant cavity assembly comprises a first mirror 3 and a second mirror 4 which are parallel to each other. Both mirrors are multilayer dielectric film mirrors with a diameter of 12mm and a thickness of 5mm, and the reflectivity in the wavelength range of 1500-1600nm is not less than 90%. The two mirrors are arranged in parallel to form a resonant cavity, and the cavity length is adjusted by a tuning drive module.

[0039] The tuning drive module uses a piezoelectric ceramic 6 as a driving element, the piezoelectric ceramic 6 has an outer diameter of 12mm, an inner diameter of 6mm and a thickness of 2mm. A small deformation is generated by applying a direct current voltage to the piezoelectric ceramic 6, thereby changing the cavity length of the resonant cavity assembly. The driving voltage range of the piezoelectric ceramic 6 is 0-60V, and the displacement voltage ratio is 0.055μm / V. The cavity length can be adjusted in the range of 0-3.3μm, which meets the wide range of wavelength tuning requirements.

[0040] The filter also has an auxiliary structure, including a first quartz gasket 2, a second quartz gasket 5 and a third quartz gasket 8. The first quartz gasket 2 is located between the first mirror 3 and the fixed barrel 1, with an outer diameter of 12mm, an inner diameter of 6mm and a thickness of 1mm. The second quartz gasket 5 is located between the second mirror 4 and the piezoelectric ceramic 6, with an outer diameter of 12mm, an inner diameter of 6mm and a thickness of 1mm. The third quartz gasket 8 is located between the piezoelectric ceramic 6 and the load barrel 9, with an outer diameter of 16mm, an inner diameter of 6mm and a thickness of 1mm. Each quartz gasket is made of quartz material, which plays a role in isolation, uniform force transmission and pollution prevention.

[0041] The optical feedback module is used to calculate the cavity length d and the cavity length deviation Δd according to the diffraction pattern generated by the detection light in the diffraction slit 10 between the first mirror 3 and the second mirror 4, so as to adjust the driving voltage of the piezoelectric ceramic 6. Specifically, the optical feedback module comprises a detection light source, a diffraction detection unit, a signal processing circuit and a feedback control circuit.

[0042] The detection light source adopts a helium-neon laser with a wavelength of 632.8nm, a power of 5mW and a spot diameter of ≤0.5mm. After the laser emitted by the helium-neon laser is collimated into parallel light by a collimating lens with a focal length of 10mm, it is vertically incident into the micron-level diffraction slit 10 between the first mirror 3 and the second mirror 4.

[0043] The diffraction detection unit adopts a 1280x960 pixel CMOS photoelectric detector array, is installed at a position about 100 mm±5 mm away from the diffraction slit 10, and is used to collect the diffraction fringe pattern generated by the diffraction slit 10. A narrow-band filter with a center wavelength of 632.8 nm and a bandwidth of 10 nm is arranged in front of the diffraction detection unit, and is used to filter ambient stray light.

[0044] The signal processing circuit is used to calculate the cavity length d and the cavity length deviation Δd according to the diffraction fringe pattern, and includes a preamplifier, a 16-bit A / D converter, and a FPGA signal processing chip. Specifically, the center of the bright fringe in the diffraction fringe pattern is extracted through a threshold segmentation algorithm and an edge detection algorithm. The FPGA signal processing chip first controls the 16-bit A / D converter through an SPI interface to convert the analog signal of the diffraction light amplified by the preamplifier into a 16-bit digital signal, and uses a FIFO buffer to store the image data of 1280x960 pixels. First, the diffraction fringe image collected by the CMOS detector is denoised through 5x5 Gaussian filtering, and then the image grayscale range is mapped to 0-255 through grayscale stretching to improve the contrast between the bright fringe and the background. Subsequently, the optimal threshold is solved by counting the image grayscale histogram, and the image is binarized to separate the bright fringe from the background. Then, the image grayscale gradient is calculated through an edge detection algorithm, non-maximum suppression is performed to refine the edges, and the left edge coordinate X left and the right edge coordinate X right of the bright fringe are extracted row by row. The single-row bright fringe center coordinates are calculated, and the effective single-row center coordinates in the bright fringe coverage range are averaged to obtain the final center X k of the bright fringe corresponding to the diffraction order. The distance Δx between adjacent bright fringes is calculated using the center coordinates of adjacent diffraction order fringes, and the cavity length d is calculated through the formula d=kλ0L / Δx, where k is the diffraction order, λ0 is the detection light wavelength, and L is the distance from the diffraction slit 10 to the detector. In addition, the cavity length deviation Δd=d-d0 is calculated, where d0 is the target cavity length.

[0045] The feedback control circuit is used to adjust the driving voltage of the piezoelectric ceramic 6 according to the cavity length change. Specifically, the feedback control circuit compares the real-time cavity length with the target value, outputs a voltage compensation signal to the piezoelectric ceramic driver matched with the piezoelectric ceramic 6, thereby driving the piezoelectric ceramic 6 to realize the stability of the cavity length. The feedback control circuit adopts an STM32 microcontroller and a PID algorithm.

[0046] In the feedback control circuit, the STM32 microcontroller takes the cavity length deviation Δd from the FPGA as the input deviation e of the PID algorithm, and calculates the proportional term , the integral term , and the differential term The voltage compensation amount AV is obtained by superimposing the three, combined with the piezoelectric ceramic 6 displacement voltage ratio, amplified by the high voltage operational amplifier, and then driving the piezoelectric ceramic 6. The cavity length is adjusted in real time with a 1 ms period, so that Ad≤±0.01 μm. The PID algorithm is a mature technology, and stable control can be achieved by trial and error method to set the parameters.

[0047] The voltage compensation amount AV is calculated according to the cavity length deviation Ad and the piezoelectric ceramic 6 displacement voltage ratio, and then the piezoelectric ceramic 6 is driven by the high voltage operational amplifier to make the cavity length deviation Ad≤±0.01 μm.

[0048] In this embodiment, AB glue (A liquid: B liquid = 1:2) is used to sequentially bond the first mirror 3, the second mirror 4, the first quartz gasket 2, the second quartz gasket 5, the third quartz gasket 8 and the piezoelectric ceramic 6. The first mirror 3 is glued on the non-coated surface and then attached to the first quartz gasket 2, and the second mirror 4 is glued on the non-coated surface and then attached to the second quartz gasket 5, with a pressure of 0.1-0.3 MPa. The other side of the second quartz gasket 5 is glued and then attached to the piezoelectric ceramic 6, ensuring that there is no residual glue on the side wall of the piezoelectric ceramic 6. The bonded assembly is placed in a 135°C oven for 2 hours or under pressure for 24 hours to complete the curing.

[0049] The above-mentioned bonded assembly is installed in the fixed lens barrel 1 and the load lens barrel 9, and then positioned by the mortise and tenon structure. The joint is preliminarily fixed by using α-cyano acrylate adhesive, and then 3 times of viscous AB glue is injected into the glue injection hole 7 after 1-2 hours of standing, and then applied to the joint gap for secondary curing to enhance stability.

[0050] The filter is placed on the three-axis cage adjustment frame and the two-dimensional displacement table. The laser is imaged on the observation screen after passing through the light transmission hole 11. The parallelism of the mirror is coarsely adjusted by the laser pen, and the observation screen shows only one bright spot to complete the coarse adjustment of the parallelism. Fine adjustment is performed in combination with the spectrum monitor output spectrum.

[0051] The helium-neon laser is fixed on the three-dimensional adjustment frame. After the light source is turned on, the laser direction is finely adjusted by the adjustment frame, so that the center of the laser beam coincides with the center of the diffraction gap 10 between the two mirrors. A white paper is placed behind the diffraction gap 10 to observe whether the light spot falls in the middle of the diffraction gap 10. The position of the collimating lens is adjusted to ensure that the laser is incident as parallel light.

[0052] An initial voltage V0=10V is applied to the piezoelectric ceramic 6, the CMOS detector is started, and the current diffraction fringe image is collected. The center distance between the 1st bright fringe and the 0th bright fringe is identified by the FPGA algorithm, and recorded as the target fringe spacing Δx. Different voltages are applied to the piezoelectric ceramic 6, and the fringe spacing is repeatedly collected to establish a "cavity length d-adjacent bright fringe spacing Δx" calibration curve, and verify the accuracy of the formula d=kλ0L / Δx.

[0053] The target cavity length d0=1.65 μm is set, and the system automatically outputs an initial voltage of 10 V; a slight disturbance is artificially applied (for example, the load mirror barrel 9 is lightly touched with tweezers to simulate the cavity length change caused by vibration), and the response of the feedback system is observed: the detector collects the change in fringe spacing in real time, the FPGA calculates the cavity length deviation Δd within 10 ms, the STM32 outputs a compensation voltage, and the piezoelectric ceramic 6 restores the cavity length to d0±0.01 μm within 50 ms; the output peak drift is recorded by the spectrometer for 24 hours of continuous monitoring, and the drift is ensured to be stable within ±0.1 nm.

[0054] When the input optical signal is vertically incident on the resonant cavity through the light transmission hole 11, the light is reflected multiple times between the first mirror 3 and the second mirror 4 to form multiple-beam interference, only satisfying “2nd=kλ1” (n is the refractive index of the medium in the cavity, d is the cavity length, k is the diffraction order, which is an integer, and λ1 is the wavelength of the incident light. The light is transmissively output. If the piezoelectric ceramic 6 changes in length due to creep or hysteresis, the diffraction fringe spacing will change, the diffraction detection unit collects the change signal and transmits it to the signal processing circuit, and the cavity length deviation is calculated; the feedback control circuit outputs a voltage compensation to the piezoelectric ceramic 6 according to the deviation, adjusts the thickness to restore the cavity length to the target value. The spectral resolution of the filter is ≤2 nm, the free spectral range is ≥33 nm, and the output peak drift is controlled within ±0.1 nm after optical feedback adjustment.

[0055] The above-described embodiments are only descriptions of preferred modes of the present application and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art should fall within the protection scope determined by the claims of the present application.

Claims

1. An optically feedback tunable Fabry-Perot filter, characterized in that, The support structure, the resonant cavity assembly, the tuning driving module and the optical feedback module are included. The support structure includes a fixed mirror tube and a load mirror tube, and the fixed mirror tube is connected with the load mirror tube through a mortise and tenon structure. The resonant cavity assembly includes a first mirror and a second mirror which are parallel to each other. The tuning driving module includes a piezoelectric ceramic, and the cavity length of the resonant cavity assembly is adjusted by applying a voltage to the piezoelectric ceramic. The optical feedback module is used to calculate the cavity length and the cavity length deviation according to the diffraction pattern generated by the detection light in the diffraction gap between the first mirror and the second mirror, so as to adjust the driving voltage of the piezoelectric ceramic.

2. The filter of claim 1, wherein, The first mirror and the second mirror are both dielectric film mirrors.

3. The filter of claim 1, wherein, A first quartz gasket, a second quartz gasket and a third quartz gasket are further included. The first quartz gasket is located between the first mirror and the fixed mirror tube. The second quartz gasket is located between the second mirror and the piezoelectric ceramic. The third quartz gasket is located between the piezoelectric ceramic and the load mirror tube.

4. The filter of claim 1, wherein, The driving voltage range of the piezoelectric ceramic is 0-60V, and the displacement voltage ratio is 0.055μm / V.

5. The filter of claim 1, wherein, The optical feedback module includes a detection light source, a diffraction detection unit, a signal processing circuit and a feedback control circuit. The detection light source is used to emit detection light to irradiate the diffraction gap between the first mirror and the second mirror. The diffraction detection unit is used to receive the diffraction fringe pattern generated by the diffraction gap. The signal processing circuit is used to calculate the cavity length d and the cavity length deviation Δd according to the diffraction fringe pattern. The feedback control circuit is used to adjust the driving voltage of the piezoelectric ceramic according to the cavity length change.

6. The filter of claim 5, wherein, The detection light source adopts a helium-neon laser. The laser emitted by the helium-neon laser is collimated into parallel light by a collimating lens with a focal length of 10mm, and then vertically incident to the diffraction gap between the first mirror and the second mirror.

7. The filter of claim 5, wherein, A narrow-band optical filter is arranged in front of the diffraction detection unit to filter ambient stray light.

8. The filter of claim 5, wherein, The signal processing circuit extracts the bright fringe center in the diffraction fringe pattern through a threshold segmentation algorithm and an edge detection algorithm, and calculates the adjacent bright fringe spacing Δx. The cavity length d is calculated through the formula d=kλ0L / Δx, wherein k is the diffraction order, λ0 is the wavelength of the detection light, and L is the distance from the diffraction gap to the detector.

9. The filter of claim 8, wherein, The feedback control circuit calculates the voltage compensation ΔV according to the cavity length deviation Δd and the displacement voltage ratio of the piezoelectric ceramic, and then drives the piezoelectric ceramic through a high-voltage operational amplifier.

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