Rotary optical delay line fast calibration system and delay time fast calibration method

Abstract

The application discloses a kind of rotating optical delay line fast calibration system and delay time fast calibration method, the method is by respectively being placed in the detection light path and the pumping light path of terahertz time-domain spectroscopy system to the rotating optical delay line to be calibrated and straight line delay line, builds rotating optical delay line fast calibration system, by obtaining the rotating optical delay line to be calibrated, the position of the phase difference of terahertz signal when zero is located at the both ends of light step respectively straight line delay line encoder feedback position indicator number, can realize the fast calibration of rotating optical delay line window total delay time.This calibration method is simple to operate, and can provide basis for the wide application of rotating optical delay line.

Description

Technical Field

[0001] This invention relates to a rapid calibration system and method for rotating optical delay lines, specifically a rapid calibration method for the delay time of rotating optical delay lines in a terahertz time-domain spectroscopy system. Background Technology

[0002] In optical coherent detection systems, optical delay lines can detect sample information by changing the relative delay between the reference light and the probe light. They are widely used in terahertz time-domain spectroscopy, optical coherent tomography, ultrafast time-resolution spectroscopy, and pump-probe technologies. They are also a key subsystem that affects the accuracy of the acquired signal, the signal-to-noise ratio, and the spectral resolution.

[0003] In terahertz time-domain spectroscopy systems, high-scan-rate rotating optical delay lines are required for efficient terahertz pulse sampling in order to achieve rapid imaging and real-time monitoring of terahertz detection. However, during the commissioning of the rotating optical delay line, variations in the incident position and angle of the laser beam, as well as early-stage processing errors in the delay line model, all affect the actual delay time of the delay line, causing it to deviate from the theoretical delay time. This, in turn, impacts the accuracy and consistency of the sampled signal.

[0004] Therefore, there is an urgent need for a method to quickly calibrate the delay time of a rotating optical delay line, so as to facilitate the rapid calibration of the total window delay time of a rotating optical delay line in a terahertz time-domain spectroscopy system. Summary of the Invention

[0005] This invention provides a rapid calibration system and method for rotating optical delay lines. The method involves placing the rotating optical delay line to be calibrated and a linear delay line in the probe and pump optical paths of a terahertz time-domain spectroscopy system, respectively, to construct the rapid calibration system. By acquiring the encoder feedback position readings of the linear delay line when the terahertz signal phase difference is zero at both ends of the optical step, the total delay time of the rotating optical delay line window can be rapidly calibrated. The system is simple to operate and applicable to the rapid calibration of different types of rotating optical delay lines, providing a foundation for the widespread application of rotating optical delay lines.

[0006] This invention is achieved through the following technical solution:

[0007] This invention provides a rapid calibration system for the delay time of a rotating optical delay line, comprising a femtosecond laser, a first beam splitter prism, a second beam splitter prism, a linear delay line, a photoconductive antenna excitation end, a bias voltage, a third beam splitter prism, the rotating optical delay line to be calibrated, a mirror, a photoconductive antenna detection end, a lock-in amplifier, and an oscilloscope. The first beam splitter prism, the second beam splitter prism, the linear delay line, and the photoconductive antenna excitation end are arranged sequentially according to the optical path direction to form a pump optical path; the first beam splitter prism, the third beam splitter prism, the mirror, the rotating optical delay line to be calibrated, and the photoconductive antenna detection end are arranged sequentially according to the optical path direction to form a detection optical path. One path of the laser emitted by the femtosecond laser passes through the pump optical path, is biased by the bias voltage, and enters the lock-in amplifier; the other path passes through the detection optical path and also enters the lock-in amplifier. Finally, the signal is displayed on the oscilloscope.

[0008] This invention also provides a method for rapid calibration of the delay time of a rotating optical delay line, using the rapid calibration system described in this invention; the rapid calibration method includes the following steps:

[0009] Step 1: Select the appropriate linear delay line for calibration;

[0010] Step 2: Build a rapid calibration system for rotating optical delay lines. By adjusting the relative optical paths of the pump optical path and the probe optical path, the position where the phase difference of the terahertz signal is zero is located at the middle position C of the optical step.

[0011] Step 3: Adjust the linear delay line so that the positions where the phase difference of the terahertz signal is zero are located at both ends of the optical step, and record the position readings d1 and d2 of the linear delay line;

[0012] Step 4: Repeat Step 3 multiple times to calculate the average reading difference between the positions of the corresponding straight delay lines at both ends of the optical step, and obtain the actual delay time of the working cycle of the rotating optical delay line to be calibrated.

[0013] Step 5: Based on the working cycle of one rotation of the rotating optical delay line to be calibrated, repeat steps 2 to 4 to achieve delay calibration for all working cycles of one rotation of the rotating optical delay line to be calibrated.

[0014] Furthermore, step one specifically includes:

[0015] 1.1) Based on the theoretical delay time of the rotating optical delay line to be calibrated, its actual delay time is estimated;

[0016] 1.2) Determine the delay time of the linear delay line to be calibrated based on the theoretical delay time of the rotating optical delay line to be calibrated;

[0017] 1.3) Based on the required sampling interval of the terahertz time-domain spectroscopy system, determine the delay accuracy ΔT of the calibration linear delay line, and obtain the minimum step length Δl of the calibration linear delay line:

[0018]

[0019] Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

[0020] Furthermore, step two specifically includes the following steps:

[0021] 2.1) Establish a rapid calibration system for rotating optical delay lines;

[0022] 2.2) Place the rotating optical delay line to be calibrated in the detection optical path of the terahertz time-domain spectroscopy system and put it into operation. Place the linear delay line in the pump optical path of the system and keep it stationary at the midpoint. Record the position reading of the linear delay line at this time as d.

[0023] 2.3) The photocurrent generated at the detection end of the photoconductive antenna is input into the oscilloscope via a lock-in amplifier;

[0024] 2.4) The rotating optical delay line to be calibrated has multiple working cycles in one rotation. The remaining working cycles of the rotating optical delay line to be calibrated are blocked to ensure that only one working cycle of light is transmitted during the calibration process of the rotating optical delay line to be calibrated. At this time, the photocurrent generated at the detection end of the photoconductive antenna has only one optical step.

[0025] 2.5) Adjust the trigger voltage and trigger mode of the signal in the oscilloscope so that the optical step generated by the working range of the rotating optical delay line to be calibrated appears stably on the oscilloscope screen. Record the positions of the two ends of the optical step as end A and end B, respectively.

[0026] 2.6) Adjust the optical path difference between the pump optical path and the probe optical path in the system so that the position C where the phase difference of the terahertz signal is zero is in the middle of the optical step.

[0027] Furthermore, step three specifically includes the following steps:

[0028] 3.1) A stepper motor is driven by a pulse signal to change the delay distance of the linear delay line, so that the position C of the terahertz signal phase difference in the oscilloscope moves forward and coincides with the A end of the optical step. The position reading d1 of the linear delay line at this time is obtained by recording the number of pulse signals received by the encoder during the movement of the linear delay line.

[0029] 3.2) Apply the pulse signal again to drive the stepper motor, changing the delay distance of the linear delay line so that the position C where the terahertz signal phase difference is zero in the oscilloscope moves backward and eventually coincides with end B of the optical step. By recording the number of pulse signals m received by the encoder during the movement of the linear delay line, the position reading d2 of the linear delay line at this time is obtained:

[0030] d2=d1+m·Δl

[0031] Where Δl is the minimum step distance of the stepper motor.

[0032] Furthermore, step four specifically includes the following steps:

[0033] 4.1) Repeat step three multiple times to obtain the average reading difference Δd between the positions of the corresponding straight delay lines at both ends of the optical step generated by the rotating optical delay line to be calibrated;

[0034]

[0035] Where, d 2i Let d be the linear delay line reading position when the terahertz signal recorded for the i-th time is located at the last end of the optical step. 1i The linear delay line reading position is where the terahertz signal recorded in the i-th time is located at the front end of the optical step, and N is the number of repeated measurements;

[0036] 4.2) Using the relationship between the delay distance and delay time of a linear delay line, the actual delay time Δt of the rotating optical delay line to be calibrated is obtained:

[0037]

[0038] Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

[0039] Furthermore, step five specifically includes the following steps:

[0040] 5.1) Based on the working cycle of one rotation of the rotating optical delay line to be calibrated, the remaining working cycles of the rotating optical delay line to be calibrated are blocked in turn to ensure that only one working cycle is open for light during each calibration.

[0041] 5.2) Repeat steps two through four until the delay calibration of all working cycles of the rotating optical delay line to be calibrated is achieved.

[0042] The present invention has the following advantages:

[0043] The method of this invention can quickly calibrate the total window delay time of a coherent detection system. Its calibration operation is simple and can provide a foundation for the widespread application of rotating optical delay lines. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the rapid calibration system for rotating optical delay lines according to Embodiment 1 of the present invention.

[0045] Figure 2 This is a flowchart of the rapid calibration method for the delay time of the rotating optical delay line according to Embodiment 2 of the present invention;

[0046] Figure 3 This is a schematic diagram of the optical step generated by the rotating optical delay line according to an embodiment of the present invention;

[0047] Figure 4 This is a schematic diagram showing the terahertz signal located at the middle position of the optical step in an embodiment of the present invention;

[0048] Figure 5 This is a schematic diagram showing the terahertz signal located at the very front of the optical step in an embodiment of the present invention.

[0049] Figure 6 This is a schematic diagram showing the position of the terahertz signal at the last end of the optical step in an embodiment of the present invention. Detailed Implementation

[0050] The present invention will be further described in detail below through specific embodiments. The following embodiments are merely descriptive and not limiting, and should not be used to limit the scope of protection of the present invention.

[0051] Example 1

[0052] A rapid calibration system for the delay time of a rotating optical delay line, such as Figure 1 As shown, the system comprises the following components: a femtosecond laser 1, a first beam splitter 2, a second beam splitter 3, a linear delay line 4, a photoconductive antenna excitation terminal 9, a bias voltage 10, a third beam splitter 5, a rotating optical delay line to be calibrated 6, a reflector 7, a photoconductive antenna detection terminal 8, a lock-in amplifier 11, and an oscilloscope 12. The first beam splitter 2, the second beam splitter 3, the linear delay line 4, and the photoconductive antenna excitation terminal 9 are arranged sequentially according to the optical path direction to form the pump optical path. The first beam splitter 2, the third beam splitter 5, the reflector 7, the rotating optical delay line to be calibrated 6, and the photoconductive antenna detection terminal 8 are arranged sequentially according to the optical path direction to form the detection optical path. One path of the laser emitted by the femtosecond laser 1 passes through the pump optical path, is biased by the bias voltage 10, and then amplified by the lock-in amplifier 11. The other path passes through the detection optical path and is also amplified by the lock-in amplifier 11. Finally, the signal is displayed on the oscilloscope 12.

[0053] Example 2

[0054] like Figure 2As shown, this embodiment is a rapid calibration method for the delay time of a rotating optical delay line. The rotating optical delay line to be calibrated is taken as an example of the multi-reflection rotating optical delay line based on the involute principle in patent CN104166233A. The method includes the following steps:

[0055] Step 1: Select the appropriate linear delay line (4) for calibration;

[0056] Step 2, build as follows Figure 1 The rotating optical delay line rapid calibration system shown adjusts the relative optical path lengths of the pump and probe optical paths so that the position where the phase difference of the terahertz signal is zero is located at the middle position C of the optical step.

[0057] Step 3: Adjust the linear delay line 4 so that the positions where the phase difference of the terahertz signal is zero are located at both ends of the optical step, and record the position readings d1 and d2 of the linear delay line 4.

[0058] Step 4: Repeat step 3 multiple times to calculate the average reading difference between the positions of the linear delay line 4 at both ends of the optical step, and obtain the actual delay time of the working cycle of the rotating optical delay line 6 to be calibrated.

[0059] Step 5: Based on the working cycle of one rotation of the rotating optical delay line 6 to be calibrated, repeat steps 2 to 4 to achieve delay calibration for all working cycles of one rotation of the rotating optical delay line 6 to be calibrated.

[0060] Furthermore, the selection of the linear delay line 4 for calibration in step one specifically includes:

[0061] 1.2) Based on the theoretical delay time of the rotating optical delay line 6 to be calibrated, its actual delay time is estimated;

[0062] 1.2) Determine the delay time of the calibration linear delay line 4 based on the theoretical delay time of the rotating optical delay line 6 to be calibrated;

[0063] 1.3) Based on the required sampling interval of the terahertz time-domain spectroscopy system, determine the delay accuracy ΔT of the calibration linear delay line 4. Then, the minimum step length Δl of the calibration linear delay line 4 is:

[0064]

[0065] Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

[0066] Preferably, the delay time of the calibration linear delay line 4 is greater than the theoretical delay time of the rotating optical delay line 6 to be calibrated. It is generally recommended that the delay time of the calibration linear delay line 4 be twice the theoretical delay time of the rotating optical delay line 6 to be calibrated.

[0067] Preferably, the calibration linear delay line 4 is driven by a stepper motor by a pulse signal. The displacement that a single pulse can produce on the stepper motor is the minimum step length of the linear delay line 4. The position reading of the linear delay line 4 after movement can be obtained by feedback from the pulse signal received by the photoelectric encoder.

[0068] Preferably, the delay accuracy of the calibration linear delay line 4 is less than the sampling interval of the terahertz time-domain spectroscopy system, and the smaller the minimum step length of the linear delay line 4, the higher the accuracy of the system calibration result.

[0069] Furthermore, step two, establishing a rapid calibration system for a rotating optical delay line, involves adjusting the relative optical paths of the pump and probe optical paths to ensure that the position where the terahertz signal phase difference is zero is located at the middle position C of the optical step. This includes the following steps:

[0070] 2.1) Construct the rapid calibration system for the rotating optical delay line described in Example 1, such as... Figure 1 As shown, the system includes: a femtosecond laser 1, a beam splitter 2, a beam splitter 3, a linear delay line 4, a photoconductive antenna excitation terminal 9, a bias voltage 10, a beam splitter 5, a rotating optical delay line to be calibrated 6, a reflector 7, a photoconductive antenna detection terminal 8, a lock-in amplifier 11, and an oscilloscope 12. Among them, beam splitter 2, beam splitter 3, linear delay line 4, and photoconductive antenna excitation terminal 9 form the pump optical path, while beam splitter 2, beam splitter 5, reflector 7, rotating optical delay line to be calibrated 6, and photoconductive antenna detection terminal 8 form the detection optical path.

[0071] 2.2) The working principle of the rapid calibration system for rotating optical delay lines is as follows: The rotating optical delay line 6 to be calibrated is placed in the detection optical path of the terahertz time-domain spectroscopy system and is put into operation. The linear delay line 4 is placed in the pump optical path of the system and is kept stationary at the midpoint. The position reading of the linear delay line 4 at this time is d.

[0072] 2.3) The photocurrent generated by the photoconductive antenna detection terminal 8 is input into the oscilloscope 12 via the lock-in amplifier 11.

[0073] 2.4) Since the rotating optical delay line 6 to be calibrated has 6 working cycles in one rotation, and the light transmission duty cycle in a single working cycle is less than 100%, the femtosecond laser received by the photoconductive antenna detection end 8 is a square wave signal modulated by the rotating optical delay line 6 to be calibrated with the same duty cycle. Therefore, one rotation of the rotating optical delay line 6 to be calibrated will generate 6 photocurrent steps at the photoconductive antenna detection end 8. The remaining 5 working cycles of the rotating optical delay line 6 to be calibrated are blocked to ensure that only one working cycle of the rotating optical delay line 6 is light-transmitting during the calibration process. Figure 3 As shown, at this time, the photocurrent generated at the photoconductive antenna detection end 8 has only one optical step.

[0074] 2.5) Adjust the trigger voltage and trigger mode of the signal in the oscilloscope 12 so that the optical step generated by the working range of the rotating optical delay line 6 to be calibrated appears stably on the screen of the oscilloscope 12. Record the positions of the two ends of the optical step as A and B respectively.

[0075] 2.6) Adjust the optical path difference between the pump optical path and the probe optical path in the system, such as... Figure 4 As shown, position C, where the phase difference of the terahertz signal is zero, is located in the middle of the optical step.

[0076] Further, step three, adjusting the linear delay line 4 so that the positions where the terahertz signal phase difference is zero are located at both ends of the optical step, and recording the position readings d1 and d2 of the linear delay line 4, specifically includes:

[0077] 3.1) A stepper motor is driven by a pulse signal to change the delay distance of the linear delay line 4, causing the position C of the terahertz signal with zero phase difference in the oscilloscope 12 to move forward, as shown in the image. Figure 5 As shown, the A end of the linear delay line 4 coincides with the optical step. By recording the number of pulse signals received by the encoder during the movement of the linear delay line 4, the position reading d1 of the linear delay line 4 at this time is obtained.

[0078] 3.2) Apply the pulse signal again to drive the stepper motor, changing the delay distance of the linear delay line 4, so that the position C where the terahertz signal phase difference is zero in the oscilloscope 12 moves backward, and finally as shown... Figure 6 As shown, the position of the linear delay line 4 coincides with end B of the optical step. By recording the number of pulse signals m received by the encoder during the movement of the linear delay line, the position reading d2 of the linear delay line 4 at this time is obtained:

[0079] d2=d1+m·Δl (2)

[0080] Where Δl is the minimum step distance of the stepper motor.

[0081] Further, step four repeats step three multiple times to calculate the average reading difference between the positions of the corresponding straight delay line 4 at both ends of the optical step, thus obtaining the actual delay time of the working cycle of the rotating optical delay line 6 to be calibrated. Specifically, this includes:

[0082] 4.1) Repeat step three N times to obtain the average reading difference Δd between the positions of the linear delay lines 4 at both ends of the optical step generated by the rotating optical delay line 6 to be calibrated;

[0083]

[0084] Where, d 2i Let d be the position of the linear delay line 4 when the terahertz signal recorded for the i-th time is located at the last end of the optical step. 1iThe position of the linear delay line 4 is the position of the terahertz signal recorded for the i-th time when it is at the front end of the optical step, and N is the number of repeated measurements.

[0085] 4.2) Using the relationship between the delay distance and delay time of the linear delay line 4, the actual delay time Δt of the rotating optical delay line 6 to be calibrated is obtained:

[0086]

[0087] Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

[0088] Furthermore, step five, based on the working cycle of one rotation of the rotating optical delay line 6 to be calibrated, repeats steps two to four to achieve delay calibration for all working cycles of one rotation of the rotating optical delay line 6 to be calibrated. Specifically, this includes:

[0089] 5.1) By Figure 1 It can be seen that the working cycle of the rotating optical delay line 6 to be calibrated is determined by the number of plane mirrors in the delay line. There are 6 working cycles for one rotation of the rotating optical delay line 6 to be calibrated. The remaining 5 working cycles of the rotating optical delay line to be calibrated are blocked in turn to ensure that the rotating optical delay line 6 to be calibrated only has one working cycle for light transmission during each calibration.

[0090] 5.2) Repeat steps two through four six times until the delay calibration of all working cycles of the rotating optical delay line 6 to be calibrated is achieved.

Claims

1. A method for rapid calibration of the delay time of a rotating optical delay line, characterized in that, Includes the following steps: Step 1: Select the appropriate linear delay line for calibration; Step 2: Construct a rapid calibration system for a rotating optical delay line. By adjusting the relative optical path lengths of the pump and probe optical paths, the position where the terahertz signal phase difference is zero is located at position C, the middle of the optical step. Step 2 specifically includes the following steps: 2.1) Construct a rapid calibration system for a rotating optical delay line; the rapid calibration system includes a femtosecond laser, a first beam splitter prism, a second beam splitter prism, a linear delay line, a photoconductive antenna excitation end, a bias voltage, a third beam splitter prism, the rotating optical delay line to be calibrated, a mirror, a photoconductive antenna detection end, a lock-in amplifier, and an oscilloscope; wherein, the first beam splitter prism, the second beam splitter prism, the linear delay line, and the photoconductive antenna excitation end are arranged sequentially according to the optical path direction to form a pump optical path; the first beam splitter prism, the third beam splitter prism, the mirror, the rotating optical delay line to be calibrated, and the photoconductive antenna detection end are arranged sequentially according to the optical path direction to form a detection optical path; the laser emitted by the femtosecond laser passes through the pump optical path and is biased by the bias voltage before entering the lock-in amplifier, and the other path passes through the detection optical path and also enters the lock-in amplifier, and finally the signal is displayed on the oscilloscope; 2.2) Place the rotating optical delay line to be calibrated in the detection optical path of the terahertz time-domain spectroscopy system and put it into operation. Place the linear delay line in the pump optical path of the system and keep it stationary at the midpoint. Record the position reading of the linear delay line at this time as d. 2.3) The photocurrent generated at the detection end of the photoconductive antenna is input into the oscilloscope via a lock-in amplifier; 2.4) The rotating optical delay line to be calibrated has multiple working cycles in one rotation. The remaining working cycles of the rotating optical delay line to be calibrated are blocked to ensure that only one working cycle of light is transmitted during the calibration process of the rotating optical delay line to be calibrated. At this time, the photocurrent generated at the detection end of the photoconductive antenna has only one optical step. 2.5) Adjust the trigger voltage and trigger mode of the signal in the oscilloscope so that the optical step generated by the working range of the rotating optical delay line to be calibrated appears stably on the oscilloscope screen. Record the positions of the two ends of the optical step as end A and end B, respectively. 2.6) Adjust the optical path difference between the pump optical path and the probe optical path in the system so that the position C where the phase difference of the terahertz signal is zero is located in the middle of the optical step; Step 3: Adjust the linear delay line so that the positions where the phase difference of the terahertz signal is zero are located at both ends of the optical step, and record the position readings d1 and d2 of the linear delay line; Step 4: Repeat step 3 multiple times to calculate the average reading difference between the positions of the corresponding straight delay lines at both ends of the optical step, and obtain the actual delay time of the working cycle of the rotating optical delay line to be calibrated. Step 5: Based on the working cycle of one rotation of the rotating optical delay line to be calibrated, repeat steps 2 to 4 to achieve delay calibration for all working cycles of one rotation of the rotating optical delay line to be calibrated.

2. The method for rapid calibration of the delay time of a rotating optical delay line as described in claim 1, characterized in that, Step one specifically includes: 1.1) Based on the theoretical delay time of the rotating optical delay line to be calibrated, its actual delay time is estimated; 1.2) Determine the delay time of the linear delay line to be calibrated based on the theoretical delay time of the rotating optical delay line to be calibrated; 1.3) Based on the required sampling interval of the terahertz time-domain spectroscopy system, determine the delay accuracy ΔT of the calibration linear delay line, and obtain the minimum step length Δl of the calibration linear delay line: Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

3. The method for rapid calibration of the delay time of a rotating optical delay line as described in claim 1, characterized in that, Step three specifically includes the following steps: 3.1) A stepper motor is driven by a pulse signal to change the delay distance of the linear delay line, so that the position C of the terahertz signal phase difference in the oscilloscope moves forward and coincides with the A end of the optical step. The position reading d1 of the linear delay line at this time is obtained by recording the number of pulse signals received by the encoder during the movement of the linear delay line. 3.2) Apply the pulse signal again to drive the stepper motor, changing the delay distance of the linear delay line so that the position C where the terahertz signal phase difference is zero in the oscilloscope moves backward and eventually coincides with end B of the optical step. By recording the number of pulse signals m received by the encoder during the movement of the linear delay line, the position reading d2 of the linear delay line at this time is obtained: d2=d1+mΔl Where Δl is the minimum step distance of the stepper motor.

4. The method for rapid calibration of the delay time of a rotating optical delay line as described in claim 3, characterized in that, Step four specifically includes the following steps: 4.1) Repeat step three multiple times to obtain the average reading difference Δd between the positions of the corresponding straight delay lines at both ends of the optical step generated by the rotating optical delay line to be calibrated; Where, d 2i Let d be the linear delay line reading position when the terahertz signal recorded for the i-th time is located at the last end of the optical step. 1i The linear delay line reading position is where the terahertz signal recorded in the i-th time is located at the front end of the optical step, and N is the number of repeated measurements; 4.2) Using the relationship between the delay distance and delay time of a linear delay line, the actual delay time Δt of the rotating optical delay line to be calibrated is obtained: Where c is the speed of light in a vacuum, and n0 is the refractive index of air.

5. The method for rapid calibration of the delay time of a rotating optical delay line as described in claim 4, characterized in that, Step five specifically includes the following steps: 5.1) Based on the working cycle of one rotation of the rotating optical delay line to be calibrated, the remaining working cycles of the rotating optical delay line to be calibrated are blocked in turn to ensure that only one working cycle is open for light during each calibration. 5.2) Repeat steps two through four until the delay calibration of all working cycles of the rotating optical delay line to be calibrated is achieved.

Images