Saturable absorber test system and test method

Abstract

The invention relates to the technical field of laser optical devices, in particular to a saturable absorber testing system and method, and the system comprises a laser which is used for transmitting a pulse optical signal with a specific wavelength; the optical amplifier is used for receiving and amplifying the pulse optical signal transmitted by the laser; the adjustable attenuator comprises a half slide and a polarization beam splitter; wherein the half slide is used for changing the polarization direction of incident ray polarized light, so that an amplified optical signal input into the adjustable attenuator is subjected to adjustable attenuation of a specific degree when passing through the polarizer and the beam splitter; the to-be-tested sample placing device is used for placing a to-be-tested saturable absorber sample; the optical power meter is used for measuring the optical power intensity before and after the optical signal passes through the to-be-measured sample in the path; the signal processing unit is used for processing data to obtain a nonlinear transmission curve of the saturable absorber sample, the optical path structure of a traditional test system is optimized, and the influence of dispersion in a test optical path on a measurement result is effectively reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser optics, in particular to a saturable absorber testing system and testing method. BACKGROUND

[0002] A saturable absorber is an optical material with nonlinear absorption characteristics, whose absorption intensity changes with the change of light intensity. Specifically, it exhibits high absorption capacity at low light intensity, while at high light intensity, the absorption capacity decreases and even tends to saturation. This nonlinear absorption characteristic makes the saturable absorber have a wide range of applications in the fields of laser technology, optical communication and ultrafast optics.

[0003] As the most commonly used passive mode-locking element in lasers, the saturable absorber helps to achieve longitudinal mode phase synchronization in the laser cavity through nonlinear absorption during normal operation of the laser, producing ultra-short pulse laser. This nonlinear absorption characteristic directly determines the output pulse characteristics of the mode-locked laser, so it is extremely important to select a saturable absorber material with appropriate performance parameters in order to achieve stable mode-locking of the laser.

[0004] Currently, the most commonly used method for measuring the nonlinear transmission curve of a saturable absorber by those skilled in the art is the double-channel balance method, also known as the double-channel intensity scanning method. The widely used double-channel balance method requires the light signal emitted by the laser source to be divided into two paths through a coupling device, and then passed through the saturable absorber sample to be tested and the reference blank control group, as disclosed in CN109861062A. However, the signal light power emitted by the laser cannot be fully applied to excite the nonlinear characteristics of the sample to be tested, and the key parameters of the saturable absorber cannot be accurately characterized. In addition, in the traditional double-channel balance method, the total length of the optical fiber is relatively large, and when the continuous de-modulation attenuation is performed, the coupling ratio of the coupling optical path will fluctuate with the change of the incident light power, ultimately affecting the accuracy of the measurement results. SUMMARY

[0005] The purpose of the present application is to provide a reliable saturable absorber testing system and testing method, which improves the problems of poor measurement accuracy caused by the long total length of the optical fiber and the complex optical path structure in the prior art method.

[0006] To achieve the above-mentioned purpose, the present application provides a saturable absorber testing system, comprising: a laser for emitting a pulse light signal of a specific wavelength; an optical amplifier for receiving and amplifying the pulse light signal emitted by the laser, so that the optical power of the amplified light signal output by the optical amplifier meets the preset power standard; The adjustable attenuator comprises a half-wave plate and a polarization beam splitter, wherein the half-wave plate is used to change the polarization direction of the incident linearly polarized light, so that the amplified optical signal input into the adjustable attenuator is attenuated to a certain degree when passing through the polarization beam splitter, and the polarization beam splitter is used to split the incident laser into probe light and reference light, wherein the splitting ratio is dynamically adjusted by rotating the half-wave plate; The sample placement device is used to place the saturable absorber sample to be tested. The optical power meter is used to measure the optical power intensity of the optical signal before and after passing through the sample to be tested. The signal processing unit is used to process the two sets of optical power value data measured by the optical power meter to obtain the power intensity of the probe light and the transmittance of the probe light passing through the saturable absorber sample, and draw a nonlinear transmittance curve of the saturable absorber sample.

[0007] In some embodiments of the present application, the laser uses a femtosecond laser source, wherein the wavelength of the femtosecond laser source is one of 1550 nm, 1300 nm, and 1060 nm. The laser is a polarization maintaining output, and the output beam is a highly linearly polarized laser.

[0008] In some embodiments of the present application, the laser is connected to the sample placement device through an optical amplifier and an adjustable attenuator.

[0009] In some embodiments of the present application, the half-wave plate is a achromatic plate and is installed in a high-precision rotating mount using a threaded adapter. The polarization beam splitter is a coated with an antireflection film grating laser polarizer and is installed in a high-precision rotating mount.

[0010] In some embodiments of the present application, the optical power meter comprises a first optical power meter and a second optical power meter, wherein the first optical power meter is used to measure the optical power of the laser reflected by the side window of the polarization beam splitter, and the second optical power meter is used to measure the optical power of the laser after the probe light transmits through the saturable absorber.

[0011] In some embodiments of the present application, a saturable absorber testing method of a saturable absorber testing system is also disclosed, comprising: Step one, rotate the half-wave plate to change the direction of its optical axis, so that the incident laser signal is attenuated to different degrees when passing through the polarization beam splitter, and the polarization beam splitter is used to split the incident laser into reflected light and transmitted light. Step two, the transmitted light as the detection light of the subsequent test, the reflected light as the reference light of the subsequent test, complete a complete saturable absorber performance evaluation, the specific process is: in the process of the adjustable attenuator from high to low attenuation change, the first light power meter is used to record the reflected laser power through the polarizing beam splitter in real time, a set of reference light power data is obtained; at the same time, the second light power meter is used to measure the light power of the detection light after passing through the saturable absorber sample, a set of detection received light power data is formed, based on the linear change relationship between the reflected light and the detection light in the power intensity, the detection input light power value under the corresponding attenuation condition can be calculated according to the reference light power data, at the same time, the detection input light power value calculated and the detection received light power ratio are defined as the transmittance of the saturable absorber sample under the same attenuation degree, based on different attenuation conditions, the corresponding detection received light power and the corresponding sample transmittance are calculated one by one, and finally the nonlinear transmission curve of the sample to be tested is drawn.

[0012] The advantages and beneficial effects of the present application are: 1. The light signal emitted by the laser adopted in the present application is no longer divided into two paths by a coupler, but a more accurate and simple control method is adopted: a combination optical system of a half-wave plate and a polarizing beam splitter is integrated in the adjustable attenuator part. The present application integrates the "light splitting" process with the adjustable attenuator, not only optimizes the overall structure of the optical path, but also solves the problem that the coupling ratio of the coupling device in the traditional test method fluctuates with the increase of the incident light intensity, which finally affects the test results. 2. The adjustable attenuator part adopts a spatial optical path structure, which greatly shortens the total length of the optical fiber in the saturable absorber test system, and effectively reduces the influence of the dispersion effect of the optical fiber medium on the measurement results of the saturable absorption characteristics.

[0013] The technical solutions of the present application will be further described in detail below through the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 It is a structure block diagram of a saturable absorber test system in an embodiment of the present application. Figure 2 It is a device connection schematic diagram of a saturable absorber test system in an embodiment of the present application. Figure 3 It is a working principle schematic diagram of the adjustable attenuator composed of a half-wave plate and a Glan polarizer in an embodiment of the present application. DETAILED DESCRIPTION

[0015] In the description of the present application, it should be noted that the terms "upper", "lower", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the present application is usually placed, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In the description of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "arrangement", "installation", "connection" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the communication between the two elements inside. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0016] The embodiments of the present application will be described in detail below with reference to the drawings.

[0017] As Figure 1 shown, the present application provides a saturable absorber test system, comprising: a laser for emitting a pulsed light signal of a specific wavelength; an optical amplifier for receiving and amplifying the pulsed light signal emitted by the laser, so that the optical power of the amplified light signal output by the optical amplifier meets a preset power standard; an adjustable attenuator comprising a half-wave plate and a polarization beam splitter; wherein the half-wave plate is used to change the polarization direction of the incident linearly polarized light, so that the amplified light signal input into the adjustable attenuator undergoes a specific degree of adjustable attenuation when passing through the polarization beam splitter, and the polarization beam splitter is used to split the incident laser into probe light and reference light, wherein the splitting ratio is dynamically adjusted by rotating the half-wave plate; a sample placement device for placing a saturable absorber sample to be tested; an optical power meter for measuring the optical power intensity of the light signal before and after passing through the saturable absorber sample in the path; a signal processing unit for processing the two sets of optical power value data measured by the optical power meter to obtain the power intensity of the probe light and its transmittance through the saturable absorber sample, and drawing a nonlinear transmittance curve of the saturable absorber sample accordingly.

[0018] It needs to be understood that the coupling device is no longer needed to divide one light signal into two light signals behind the adjustable attenuator in the optical path transmission channel, and the half-wave plate and the polarization beam splitter are used in the adjustable attenuator part to accurately adjust the light signal intensity through the sample to be tested, and in the process of the adjustable attenuator from high to low attenuation change, the first optical power meter is used to record the laser light power reflected by the polarization beam splitter in real time to obtain a set of reference light power data; at the same time, the second optical power meter is used to synchronously measure the light power of the probe light after passing through the sample to be tested to form a set of probe receiving light power data. Based on the linear change relationship between the reflected light and the probe light in the power intensity, the probe input light power value under the corresponding attenuation condition can be calculated according to the reference light power data.

[0019] The advantages and beneficial effects of the present application are: 1. The light signal emitted by the laser is no longer divided into two paths by the coupler, but a more accurate and simple control method is adopted: a combined optical system of half-wave plate and polarization beam splitter is integrated in the adjustable attenuator part. This single-channel optical path structure integrates the "light splitting" process with the adjustable attenuator, not only optimizing the overall structure of the optical path, but also solving the problem that the coupling ratio of the coupling device in the traditional test method fluctuates with the increase of the incident light intensity, which ultimately affects the test results. 2. The adjustable attenuator part adopts a spatial optical path structure, which greatly shortens the total length of the optical fiber in the saturable absorber test system, and effectively reduces the influence of the dispersion effect of the optical fiber medium on the measurement results of the saturable absorption characteristics.

[0020] In some embodiments of the present application, the laser adopts a femtosecond laser source, wherein the wavelength of the femtosecond laser source is one of 1550nm, 1300nm and 1060nm. The laser is a polarization maintaining output, and the output beam is a highly linearly polarized laser.

[0021] In some embodiments of the present application, the laser is connected with the sample to be tested through an optical amplifier, an adjustable attenuator and a sample placement device.

[0022] In some embodiments of the present application, the half-wave plate adopts an achromatic glass plate and is installed in a high-precision rotary mounting seat through a threaded adapter; The polarization beam splitter adopts a coated antireflection film grating laser polarizer and is installed in a high-precision rotary mounting seat.

[0023] Specifically, by accurately rotating the half-wave plate, the polarization direction of the incident linearly polarized light can be changed, so that the incident light beam is attenuated to different degrees through the grating polarizer, and accurate adjustment of the subsequent output power is realized.

[0024] As Figure 2As shown, in some embodiments of the present application, the optical power meter comprises: a first optical power meter, a second optical power meter; wherein the first optical power meter is used to measure the optical power of the laser reflected by the polarization beam splitter side window, and the second optical power meter is used to measure the optical power of the light beam after the probe light transmits through the saturable absorber.

[0025] It needs to be understood that in the embodiments of the present application, the adjustable attenuator adopts a spatial optical path form, so when connected with the front and rear devices, if the optical fiber channel adopted by the connected device needs to use a collimator to align the optical path, the optical energy emitted by the femtosecond laser is ensured to be received by the two power meters (referring to the first optical power meter and the second optical power meter) as much as possible.

[0026] In some embodiments of the present application, a saturable absorber testing method is also disclosed, comprising the following steps: Step one, rotate the half-wave plate to change the direction of its optical axis, so that the incident laser signal undergoes different degrees of adjustable attenuation when passing through the polarization beam splitter. The polarization beam splitter is used to divide the incident laser into reflected light and transmitted light; Step two, the transmitted light is used as the probe light for subsequent testing, and the reflected light is used as the reference light for subsequent testing, to complete a complete performance evaluation of the saturable absorber, the specific process is as follows: during the attenuation change process from high to low of the adjustable attenuator, the first optical power meter is used to record the laser power reflected by the polarization beam splitter in real time, and a group of reference light power data is obtained; at the same time, the second optical power meter is used to measure the light power after the probe light passes through the saturable absorber sample, and a group of probe receiving light power data is formed. Based on the linear variation relationship between the reflected light and the probe light in power intensity, the probe input light power value under the corresponding attenuation condition can be calculated according to the reference light power data. At the same time, the ratio of the calculated probe input light power value to the probe receiving light power is defined as the transmittance of the saturable absorber sample under the same attenuation degree. Based on different attenuation conditions, the corresponding probe receiving light power and the corresponding sample transmittance are calculated one by one, and finally the nonlinear transmission curve of the sample to be tested is drawn.

[0027] The beneficial effects of the present application are: 1. In the embodiments of the present application, another form of testing method and system is provided, and the adjustable attenuator adopts a spatial optical path form. This design significantly simplifies the complexity of the optical path structure, effectively shortens the total length of the optical fiber in the saturable absorber testing system, and reduces the influence of the dispersion effect of the optical fiber medium on the measurement results of the saturable absorption characteristics.

[0028] 2. In the present application, the optical signal emitted by the laser is no longer divided into two paths by the coupler, but a more accurate and simple control method is adopted: a combination of half-wave plate and polarization beam splitter is integrated in the adjustable attenuator. This design adjusts the polarization state of the incident light by precisely controlling the rotation angle of the half-wave plate, and then uses the polarization beam splitter to divide the incident laser into probe light and reference light, where the ratio of the light is dynamically adjusted by rotating the half-wave plate; During the attenuation change from high to low in the adjustable attenuator, the first optical power meter is used to record the laser power reflected by the polarization beam splitter in real time to obtain a set of reference light power data; at the same time, the second optical power meter is used to measure the light power of the probe light after passing through the sample of the saturable absorber to form a set of probe light power data. Based on the linear change relationship between the reflected light and the probe light in power intensity, the probe input light power value under the corresponding attenuation condition can be calculated according to the reference light power data. This method optimizes the optical path structure, integrates the functions of attenuation and light splitting, shortens the total length of the optical fiber in the saturable absorber test system, effectively reduces the influence of the dispersion effect of the optical fiber medium on the measurement results of the saturable absorption characteristics, and provides a more reliable data basis for subsequent optical property measurement of the sample.

[0029] It is also understood by those skilled in the art that the dispersion of the optical fiber has a multi-faceted impact on the measurement of the nonlinear transmission curve of the saturable absorber, which is specifically manifested as follows: first, the dispersion of the optical fiber will cause the pulse of the incident light to broaden, thereby reducing the peak power of the light pulse passing through the saturable absorber. For example, assuming that the peak power of the incident light pulse is 1kW, but due to the influence of the dispersion effect, the actual peak power reaching the saturable absorber may be reduced to 600W. This reduction in power will cause the saturable absorber to fail to reach a fully saturated state, which will adversely affect the accurate depiction of the nonlinear transmission curve. Second, the pulse broadening caused by the dispersion effect of the optical fiber will also cause the actual measured value of the horizontal coordinate (such as the peak power of the incident light) of the nonlinear transmission curve to deviate from the expected value, thereby causing the measured curve to shift in the horizontal coordinate direction. The nonlinear transmission curve is usually characterized in that at low incident light intensity, the absorption loss is large; as the incident light intensity increases, the absorption loss gradually decreases and eventually tends to be saturated. However, if the saturable absorber fails to be fully saturated, the saturated part of the curve will not be accurately presented, resulting in a deviation in the curve shape from the actual situation, thereby affecting the accuracy of the measurement results.

[0030] The embodiments of the present application will be described in detail below with reference to specific examples.

[0031] As Figure 2As shown, in this embodiment, the laser source is selected from a free-space femtosecond laser seed source (ZIMAO, TCR-FS-1560-0.1), the pulse width and repetition frequency of the laser are 200 fs and 80 MHz respectively, the output beam is linearly polarized light, and the average output power can reach 40 mW. In this embodiment, a polarization-dependent attenuation device is selected, which requires that the light source used in the test system must be a polarization-maintaining output. In addition, the test system also has a minimum requirement for the output power of the light source. If the output power of the light source is too low, it may not be able to achieve a saturated absorption state for the to-be-tested saturable absorber material, ultimately affecting the measurement results.

[0032] The adjustable attenuator part adopts a combination structure of a half-wave plate and a polarization beam splitter. The half-wave plate is selected from an achromatic glass plate (Thorlabs, AHWP05M-1600) and is installed in a high-precision rotary mount (PRM1 / M) using a threaded adapter (Thorlabs, AD12F). The polarization beam splitter is selected from a coated glass-laser polarizer (Thorlabs, GL10-C) and is also installed in a high-precision rotary mount (Thorlabs, PRM1GL10 / M). The working principle is as follows: by precisely rotating the half-wave plate, the polarization direction of the incident linearly polarized light can be changed, so that the incident light beam undergoes different degrees of attenuation when passing through the Glan polarizer, achieving precise adjustment of the subsequent output power. In this embodiment, the attenuator part adopts a spatial light path. After passing through the half-wave plate, the light beam reaches the Glan polarizer. The working principle of the Glan polarizer is as follows: Figure 3 As shown, the light in the incident light beam that is in the same direction as the optical axis of the Glan polarizer is allowed to pass through, and the other components are reflected. Therefore, with the adjustment of the front half-wave plate, the components of the output light and the reflected light will change continuously. In this embodiment, the manual adjustment range of the attenuator is 3 dB-35 dB, i.e. the insertion loss of the spatial attenuator part is 3 dB.

[0033] In this embodiment, a thin film of carbon nanotube material is used as the to-be-tested saturable absorber. During testing, the carbon nanotube film needs to be placed at the end face of the optical fiber to ensure that the light beam passes through the carbon nanotube material to undergo saturation absorption.

[0034] Two power meters are used to measure the reflected light and output light through the Glan polarizer in this embodiment, wherein the first light power meter is a digital handheld light power meter (Thorlabs, PM100D) matched with a standard diode power probe to measure the light power intensity reflected from the Glan polarizer, and the laser light power reflected by the polarizing beam splitter is recorded in real time by the first light power meter during the attenuation change from high to low of the adjustable attenuator to obtain a set of reference light power data; at the same time, the second light power meter is used to measure the light power of the probe light after passing through the to-be-measured saturable absorber sample to form a set of probe received light power data. Based on the linear variation relationship between the reflected light and the probe light in terms of power intensity, the probe input light power value under the corresponding attenuation condition can be calculated according to the reference light power data, and the second light power meter is a handheld power meter with a fiber connector, which is used to measure the light beam intensity after the probe light transmits through the saturable absorber.

[0035] It should be noted that in the embodiments of the present application, the adjustable attenuator adopts a spatial light path, so when connected with the front and rear devices, if the fiber channel of the connected device needs to use a collimator to align the light path, the light energy emitted by the femtosecond laser is ensured to be received by the two power meters (i.e., the first light power meter and the second light power meter) as much as possible, that is, the insertion loss of the attenuator part is reduced.

[0036] The specific measurement steps adopted in the embodiments of the present application are as follows: After the laser is turned on, the rotation angle of the half-wave plate is adjusted to ensure that the incident light beam gradually increases from small to large, the angle of the Glan polarizer is kept unchanged, and the light signal intensities measured by the first light power meter and the second light power meter under the half-wave plate at different angles are recorded in turn. It should be noted that the data recorded under the condition of relatively low-power incident light is more intensive. Based on the linear variation relationship between the reflected light and the probe light in terms of power intensity, the probe input light power value under the corresponding attenuation condition can be calculated according to the reference light power data measured by the first light power meter. At the same time, the ratio of the calculated probe input light power value to the probe received light power measured by the second light power meter is defined as the transmittance of the saturable absorber sample under the same attenuation degree. Based on different attenuation conditions, the corresponding probe received light power and the corresponding sample transmittance are calculated one by one, and finally the nonlinear transmission curve of the to-be-measured sample is drawn, and the curve is fitted according to the following formula: ; wherein, T is the intensity-dependent transmittance of the to-be-measured sample, I is the probe light intensity, is the key parameter modulation depth, is the key parameter saturation intensity, is the background loss of light transmission in the sample.

[0037] In the present application, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. If there is a conflict between the definitions in the specification and those in the patent specification, the definitions in the specification are intended to prevail. In addition, the terms used herein are merely for the purpose of describing the embodiments of the present application and are not intended to limit the present application.

[0038] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, but not to limit the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can still be modified or equivalently replaced, and these modifications or equivalent replacements should not make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A saturable absorber testing system, characterized in that, include: A laser is used to emit pulsed light signals of a specific wavelength. An optical amplifier is used to receive and amplify the pulsed optical signal emitted by a laser, so that the optical power of the amplified optical signal output by the optical amplifier meets the preset power standard. An adjustable attenuator includes a half-wave plate and a polarizing beam splitter. The half-wave plate is used to change the polarization direction of the incident ray polarized light, so that the amplified light signal input to the adjustable attenuator undergoes a specific degree of adjustable attenuation when it passes through the polarizing beam splitter. The polarizing beam splitter is used to split the incident laser into a probe beam and a reference beam, and the splitting ratio is dynamically adjusted by rotating the half-wave plate. A sample placement device for placing a saturable absorber sample to be tested; An optical power meter is used to measure the optical power intensity of a light signal before and after it passes through the sample under test in a path. The signal processing unit is used to process the two sets of optical power values ​​measured by the optical power meter to obtain the power intensity of the probe light and its transmittance through the saturable absorber sample, and to plot the nonlinear transmission curve of the saturable absorber sample accordingly.

2. The saturable absorber testing system according to claim 1, characterized in that, The laser is a femtosecond laser source, wherein the wavelength of the femtosecond laser source is one of 1550nm, 1300nm, and 1060nm; The laser is a polarization-maintaining output, and the output beam is a highly linearly polarized laser.

3. The saturable absorber testing system according to claim 2, characterized in that, The laser is connected to the sample placement device via an optical amplifier and an adjustable attenuator.

4. The saturable absorber testing system according to claim 3, characterized in that, The half-wave plate is an achromatic glass plate and is mounted in a high-precision rotary mounting base using a threaded adapter. The polarization beam splitter uses a Glan laser polarizer with an antireflection coating and is mounted in a high-precision rotary mount.

5. The saturable absorber testing system according to claim 4, characterized in that, The optical power meter includes: a first optical power meter and a second optical power meter; wherein, the first optical power meter is used to measure the optical power of the laser reflected from the side window of the polarization beam splitter, and the second optical power meter is used to measure the optical power of the laser after the probe light passes through the saturable absorber.

6. A method for testing a saturable absorber based on the saturable absorber testing system according to any one of claims 1 to 5, characterized in that, include: Step 1: Rotate the half-wave plate to change its optical axis direction, so that the incident laser signal undergoes adjustable attenuation to different degrees when it passes through the polarization beam splitter. The polarization beam splitter is used to split the incident laser into two parts: reflected light and transmitted light. Step 2: The transmitted light is used as the probe light for subsequent tests, and the reflected light is used as the reference light for subsequent tests, completing a full performance evaluation of the saturable absorber. The specific process is as follows: During the attenuation change of the adjustable attenuator from high to low, the optical power of the laser reflected by the polarization beam splitter is recorded in real time using a first optical power meter to obtain a set of reference optical power data. At the same time, the optical power of the probe light after passing through the saturable absorber sample under test is measured synchronously using a second optical power meter to form a set of probe-received optical power data. Based on the linear change relationship between the power intensity of the reflected light and the probe light, the probe input optical power value under the corresponding attenuation condition can be calculated based on the reference optical power data. At the same time, the ratio of the calculated probe input optical power value to the probe-received optical power is defined as the transmittance of the saturable absorber sample under the same attenuation level. Based on different attenuation conditions, the corresponding probe-received optical power and its corresponding sample transmittance are calculated one by one, and finally, the nonlinear transmission curve of the sample under test is plotted.

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