Patent Publication Number: US-2023152153-A1

Title: Device for measuring transmittance curve of fabry-perot using frequency comb light source and method using the same

Description:
FIELD 
     This application relates to the technical field of detection of transmittance of Fabry-Perot (FP), and in particular to a device and method for measuring a transmittance curve of an FP using a frequency comb light source. 
     BACKGROUND 
     Fabry-Perot (FP) etalon is a kind of interferometer, mainly composed of two flat glass or quartz plates. It can be used as a high-resolution filter or a precision wavelength meter for high-resolution spectral analysis. In the laser system, it is usually used to shrink the line in the cavity or make the laser system work in a single mode. It may be used as a medium bandwidth control and tuning device for broadband picosecond lasers. It may also be widely used as a frequency discriminator in Doppler wind measurement or aerosol detection of lidar. Due to the wide application of FP etalon, the standard transmittance curve of FP etalon is a very important parameter. 
     The standard transmittance curve of FP etalon is generally measured by adjusting the wavelength of the incident light source, or adjusting the cavity length of the FP or the angle of the incident light. However, laser sources with wavelength tuning function are very expensive, and it is difficult to find laser sources with a corresponding wide wavelength tuning range for some FP etalons with a wide free spectral range, and FP etalons with adjustable cavity length are also very expensive. Moreover, it is difficult to ensure that the change of the tuning of the wavelength or the angle of the incident light is linear, and each change of the step size will introduce a new error, which ultimately leads to insufficient measurement accuracy. 
     Specifically, an ordinary etalon consists of two parallel reflecting surfaces. 
     When plane beam U 0  is incident on the etalon, it will be continuously reflected and transmitted on the two reflecting surfaces. As shown in  FIG.  1   , incident angle of U 0  is θ. Amplitudes of the transmitted beams are: 
         U   1   ′=U   0 (1− R   1 )(1− R   2 );
 
         U   2   ′=U   0 (1− R   1 )(1− R   2 ) R   1   R   2   e   iδ ;
 
         U   3   ′=U   0 (1− R   1 )(1− R   2 ) R   1   2   R   2   2   e   2iδ ;
 
         U   4   ′=U   0 (1− R   1 )(1− R   2 ) R   1   3   R   2   3   e   3iδ ;
 
       . . . 
       where, 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       4 
                       ⁢ 
                       π 
                       ⁢ 
                       nh 
                       ⁢ 
                       cos 
                       ⁢ 
                       θ 
                     
                     λ 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     total amplitude of transmitted beam is 
     
       
         
           
             
               
                 
                   
                     U 
                     ′ 
                   
                   = 
                     
                   
                     
                       
                         U 
                         0 
                       
                       ( 
                       
                         1 
                         - 
                         
                           R 
                           1 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ( 
                       
                         1 
                         - 
                         
                           R 
                           2 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             R 
                             1 
                           
                           ⁢ 
                           
                             R 
                             2 
                           
                           ⁢ 
                           
                             e 
                             
                               i 
                               ⁢ 
                               δ 
                             
                           
                         
                         + 
                         
                           
                             R 
                             1 
                             2 
                           
                           ⁢ 
                           
                             R 
                             2 
                             2 
                           
                           ⁢ 
                           
                             e 
                             
                               2 
                               ⁢ 
                               i 
                               ⁢ 
                               δ 
                             
                           
                         
                         + 
                         
                           
                             R 
                             1 
                             3 
                           
                           ⁢ 
                           
                             R 
                             2 
                             3 
                           
                           ⁢ 
                           
                             e 
                             
                               3 
                               ⁢ 
                               i 
                               ⁢ 
                               δ 
                             
                           
                         
                         + 
                         … 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   
                     
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               R 
                               1 
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           ( 
                           
                             1 
                             - 
                             
                               R 
                               2 
                             
                           
                           ) 
                         
                       
                       
                         1 
                         + 
                         
                           
                             R 
                             1 
                           
                           ⁢ 
                           
                             R 
                             2 
                           
                           ⁢ 
                           
                             e 
                             
                               i 
                               ⁢ 
                               δ 
                             
                           
                         
                       
                     
                     ⁢ 
                     
                       U 
                       0 
                     
                   
                 
               
             
           
         
       
     
     transmittance of etalon is 
     
       
         
           
             T 
             = 
             
               
                 
                   
                     U 
                     ′ 
                   
                   ⁢ 
                   
                     U 
                     
                       ′ 
                         
                       * 
                     
                   
                 
                 
                   
                     U 
                     0 
                   
                   ⁢ 
                   
                     U 
                     0 
                     * 
                   
                 
               
               = 
               
                 
                   
                     ( 
                     
                       1 
                       - 
                       
                         R 
                         1 
                       
                     
                     ) 
                   
                   ⁢ 
                   
                     ( 
                     
                       1 
                       - 
                       
                         R 
                         2 
                       
                     
                     ) 
                   
                 
                 
                   
                     
                       ( 
                       
                         1 
                         - 
                         
                           
                             R 
                             1 
                           
                           ⁢ 
                           
                             R 
                             2 
                           
                         
                       
                       ) 
                     
                     2 
                   
                   + 
                   
                     4 
                     ⁢ 
                     
                       
                         
                           R 
                           1 
                         
                         ⁢ 
                         
                           R 
                           2 
                         
                       
                     
                     ⁢ 
                     
                       
                         sin 
                         2 
                       
                       ( 
                       
                         δ 
                         2 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
     where U′* expresses conjugate function of U′, and U 0 * expresses conjugate function of U 0 , when amplitude reflectivity of the two surfaces of the etalon are equal, that is, R 1 =R 2 , intensity reflectance of each surface is R=R 1   2 . transmittance of etalon may be simplified to the following form, 
     
       
         
           
             
               
                 
                   T 
                   = 
                   
                     1 
                     
                       1 
                       + 
                       
                         
                           
                             4 
                             ⁢ 
                             R 
                           
                           
                             
                               ( 
                               
                                 1 
                                 - 
                                 R 
                               
                               ) 
                             
                             2 
                           
                         
                         ⁢ 
                         
                           
                             sin 
                             2 
                           
                           ( 
                           
                             δ 
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     It can be seen from the above expression that the transmittance T of FP is related to cavity length, wavelength and angle of incident light. We can either directly obtain the transmittance curve of FP by changing the wavelength, or measure the transmittance curve by the equivalent wavelength change caused by changes in the incident angle and cavity length. However, no matter if the incident angle is changed, the cavity length is changed, or the wavelength of the laser source is changed, the change cannot be guaranteed to be linear and accurate, and multiple measurements of parameters with different parameter values will introduce random relative errors. 
     SUMMARY 
     In order to overcome the above-mentioned shortcomings in the prior art, this application provides a device for measuring a transmittance curve of an FP by using a frequency comb light source and method using the same. 
     In order to achieve the above objective, this application adopts the technical solution as follows. 
     A device for measuring a transmittance curve of an FP using a frequency comb light source, including the following components sequentially arranged in an optical path: 
     a single frequency pulse laser generating single frequency pulse laser; 
     a frequency comb laser converting received single frequency pulse laser into frequency comb laser; and 
     an FP to be detected receiving laser output from the frequency comb laser; 
     where the device further includes a first receiving unit receiving laser from an output end of the frequency comb laser and performing component and spectrum analysis, and a second receiving unit receiving laser from an output end of the FP to be detected and performing component and spectrum analysis. 
     Specifically, the device further includes a first beam splitter splitting the frequency comb laser into a first output of laser and a second output of laser, where the first output of laser is emitted into the first receiving unit and the second output of laser is emitted into the FP to be detected. 
     Specifically, the first receiving unit includes a second beam splitter receiving laser output from the first beam splitter, and two laser beams split by the second beam splitter are respectively emitted into a first detector of the first receiving unit and a first spectrometer of the first receiving unit. 
     Specifically, the second receiving unit includes a third beam splitter, a second detector, and a first spectrometer, and two laser beams split by the third beam splitter are respectively emitted to the second detector and the first spectrometer. 
     Specifically, the device further includes a collimator provided with an optical aperture and arranged between the first beam splitter and the FP to be detected. 
     Specifically, the FP to be detected is an air gap etalon. 
     In addition, the FP to be detected may also be a solid etalon. 
     Specifically, the device further includes a computer, where a signal input end of the computer is connected to an output end of the first receiving unit and an output end of the second receiving unit, respectively, and a control end of the single frequency pulse laser is connected to a signal output end of the computer. 
     A method using the above device for measuring the transmittance curve of the FP by using the frequency comb light source, including the following operations: 
     S 1 , obtaining the required single frequency pulse laser and the frequency comb laser with a set type, and assembling the device; 
     S 2 , transforming, by the frequency comb laser, the single frequency pulse laser output from the single frequency pulse laser into lasers with different frequency components, passing, by the lasers with different frequency components, through the FP, and measuring, by the detector and the spectrometer, transmittances corresponding to the different frequency components at one time; and 
     S 3 , performing, by the computer, polynomial fitting on transmittances corresponding to all frequency components to obtain the transmittance curve. 
     Specifically, specific operations of measuring transmittances corresponding to the different frequency components in operation S 2  are: 
     S 21 , removing the FP in the device, obtaining energy in the first detector of the first receiving unit and the second detector of the second receiving unit, and obtaining an energy ratio N 1 =energy value of the first detector/energy value of the second detector, where N 1  is configured as a calibration coefficient; and 
     S 22 , after calibration, comparing a relative energy change of a frequency component measured by the first spectrometer of the first receiving unit and a frequency component measured by the second spectrometer of the second receiving unit, and multiplying the relative energy change by the calibration coefficient to obtain a transmittance of a corresponding frequency. 
     The advantage of this application is that since the frequency comb light source has multiple frequency components with equal frequency intervals, there is no nonlinearity, so it may measure the transmittance curve of the FP etalon at one time. And the frequency interval and spectral range are adjustable. This method greatly reduces the cost of measuring the transmittance curve, improves the measurement accuracy and effectiveness, and has good theoretical and practical value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram for calculating a transmittance of a common etalon in background art. 
         FIG.  2    is a diagram of working principle of a frequency comb laser, where FIG. a shows a time domain diagram and FIG. b shows a frequency domain diagram. 
         FIG.  3    is a schematic structural diagram of this application. 
         FIG.  4    is a schematic diagram of obtaining the transmittance using this application. 
         FIG.  5    shows a transmittance curve T obtained by theoretical calculation and a transmittance curve T 1  obtained by using the device and method of this application. 
     
    
    
     The meanings of the reference numerals in the figures are as follows: 
       1 —single frequency pulse laser  2 —frequency comb laser  3 —FP 
       41 —first beam splitter  42 —second beam splitter  43 —third beam splitter  5 —collimator 
       61 —first detector  62 —first spectrometer  71 —second detector  72 —second spectrometer 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
     As shown in  FIG.  3   , a device for measuring a transmittance curve of an FP using a frequency comb light source, including the following components sequentially arranged in an optical path: 
     a single frequency pulse laser  1  generating single frequency pulse laser; 
     a frequency comb laser  2  converting received single frequency pulse laser into frequency comb laser; and 
     an FP  3  to be detected receiving laser output from the frequency comb laser; 
     the device further includes a first beam splitter  41 , a first receiving unit, and a second receiving unit. The first receiving unit is configured to receive laser from an output end of the frequency comb laser  2  and perform component and spectrum analysis. The second receiving unit is configured to receive laser from an output end of the FP  3  to be detected and perform component and spectrum analysis. The first beam splitter  41  splits the frequency comb laser into a first output of laser and a second output of laser. The first output of laser is emitted into the first receiving unit and the second output of laser is emitted into the FP  3  to be detected. The first receiving unit includes a second beam splitter  42 , a first detector  61 , and a first spectrometer  62 . The second beam splitter  42  splits the light emitted into the first receiving unit into two beams and transmits them to the first detector respectively  61  and the first spectrometer  62 , respectively. The second receiving unit includes a third beam splitter  43 , a second detector  71 , and a second spectrometer  72 . The third beam splitter  43  splits the light emitted into the second receiving unit into two beams and transmits them to the second detector  71  and the second spectrometer  72 , respectively. 
     The single frequency pulse laser  1  outputs multiple frequency components at equal intervals through the frequency comb laser  2 , and the multiple frequency components pass through the first beam splitter  41  to be split into a beam A 1  and a beam A 2 . The second beam splitter  42  splits the beam A 2  into a beam B 1  and a beam B 2 , and then detects a frequency component of the beam B 2  by the first spectrometer  62 , and the first detector  61  receives energy of the beam B 1 . The beam A 1  passes through the FP  3  etalon, and the beam is split into a beam C 1  and a beam C 2  by the third beam splitter  43 , and then a frequency component of the beam C 2  is detected by the second spectrometer  72 , and the beam C 1  is received by the second detector  71  to measure its energy. The energy changes of the first detector  61  and the second detector  71  are used for calibration, and then by comparing the relative energy changes of the corresponding frequency components before and after the FP  3 , the transmittance of the corresponding frequency may be obtained. The transmission curve may be obtained by fitting each frequency component by a polynomial. 
     The device further includes a collimator  5  provided with an optical aperture and arranged between the first beam splitter  41  and the FP  3  to be detected, so as to adjust the light beam emitted to the FP  3  to be detected. 
     Specifically, the FP  3  to be detected is an air gap etalon or a solid etalon. 
     The device further includes a computer (not shown), where a signal input end of the computer is connected to an output end of the first receiving unit and an output end of the second receiving unit and a control end of the single frequency pulse laser  1  is connected to a signal output end of the computer. The computer controls and processes the data. 
     Embodiment 2 
     A method using the above device for measuring the transmittance curve of the FP by using the frequency comb light source as described in Embodiment 1, including the following operations: 
     S 1 , obtaining the required single frequency pulse laser  1  and the frequency comb laser  2  with a set type, and assembling the device; 
     S 2 . according to the working principle of the frequency comb laser  2  in  FIG.  2   , transforming, by the frequency comb laser  2 , the single frequency pulse laser output from the single frequency pulse laser into lasers with different frequency components, passing, by the lasers with different frequency components, through the FP  3 , and measuring, by the detector and the spectrometer, transmittances corresponding to the different frequency components at one time; and 
     Specific operations of measuring transmittances corresponding to the different frequency components are: 
     S 21 , removing the FP  3  in the device, obtaining energy in the first detector  61  of the first receiving unit and the second detector  71  of the second receiving unit, and obtaining an energy ratio N 1 =energy value of the first detector  61 /energy value of the second detector  71 , where N 1  is configured as a calibration coefficient; and 
     S 22 , after calibration, comparing a relative energy change of a frequency component measured by the first spectrometer  62  of the first receiving unit and a frequency component measured by the second spectrometer  72  of the second receiving unit, and multiplying the relative energy change by the calibration coefficient to obtain a transmittance of a corresponding frequency. 
     S 3 , performing, by the computer, polynomial fitting on transmittances corresponding to all frequency components to obtain the transmittance curve. 
     In this embodiment, taking a solid etalon as an example, the main parameters are shown in the following table. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Technical 
                 Require- 
                 Technical 
                 Require- 
               
               
                 parameter 
                 ment 
                 parameter 
                 ment 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Center wavelength (nm) 
                 532 
                 Surface accuracy 
                 λ/100 
               
               
                   
                   
                 @633 nm 
               
               
                 Material refractive index 
                 1.461 
                 Fine number 
                 17.9 
               
               
                 Beam divergence full angle 
                 8 
                 Peak transmittance 
                 0.860 
               
               
                 (mrad) 
               
               
                 Thickness of etalon (mm) 
                 0.1311 
                 FWHM(pm) 
                 41.3 
               
               
                 Optical aperture (mm) 
                 40 
                 Center wavelength 
                 532.12 
               
               
                   
                   
                 (nm) 
               
               
                 Effective aperture (mm) 
                 30 
                 Resource spectral 
                 739 
               
               
                   
                   
                 range (pm) 
               
               
                 Reflectivity 
                 86% 
               
               
                   
               
            
           
         
       
     
     Combining the parameters in the table with the calculation methods in the background technology, the numerical simulation results of the etalon transmittance curve may be obtained. 
     It can be seen from the above numerical simulation results that when the thickness of the etalon is 131 μm and the two optical surfaces are coated with 86% reflective film, the center wavelength of the etalon is 532.37 nm and the peak transmittance is 86%. 
     According to this solution, that is, according to the principle shown in  FIG.  4   , according to the different frequency components in  FIG.  4   , when different frequency components pass the FP  3  etalon, the transmittance will change with the change of frequency, and the overall change trend will be consistent with the actual transmittance curve of FP  3 , so the transmittances corresponding to different frequency components may be measured at one time by this method, and then perform polynomial fitting on the transmittances corresponding to all frequency components to get the transmittance curve needed, as shown in  FIG.  5   . 
     In  FIG.  5   , T represents the theoretical transmittance curve, and T 1  represents the transmittance curve obtained by the method described herein. As can be seen from the figure, the two agree very well. This method of obtaining the FP  3  transmittance curve through one measurement greatly improves the measurement accuracy and effectiveness, and has great theoretical and application value. 
     The above are only preferred embodiments of this application and are not intended to limit this application. Any modification, equivalent replacement and improvement made within the spirit and principle of this application should be included in the scope of protection of this application.