A method for measuring color center density in a BBO crystal

Abstract

The application discloses a method for measuring color center density in a BBO crystal, comprising the following steps: building a measurement system containing the measured BBO crystal; calculating the total loss of the optical system corresponding to different input pump powers of the measurement system at different temperatures based on the optical characteristic parameters of the optical system in the measurement system; and calculating the color center density of the measured BBO crystal corresponding to different pump powers at different temperatures according to the total loss and the optical characteristic parameters of the optical system. The application calculates the color center density of the BBO crystal based on the optical characteristic parameters and the total loss of the measurement system by constructing a simple measurement system, and the measurement equipment and method are simple and fast, the measurement cost is greatly reduced, and the influence factors are less, so that a good foundation is provided for the accuracy of the measurement result.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical technology, and more particularly, to a method for measuring color center density in a BBO crystal. BACKGROUND

[0002] The development of nonlinear frequency conversion technology breaks through the limitation of laser gain medium, and greatly expands the available waveband of laser. Among numerous nonlinear optical materials, β-BBO crystal is widely used due to its high nonlinear coefficient, wide transparent waveband, high damage threshold and good anti-hydration performance, etc. Especially in the ultraviolet light (such as 355 nm laser) pumped optical parametric oscillator, only one BBO crystal can cover the visible and near-infrared waveband (410 nm-2600 nm).

[0003] However, in the process of improving the average power and repetition frequency of the optical parametric oscillator, the optical performance of the BBO crystal will gradually decrease. This is because under the irradiation of high repetition frequency and high average power ultraviolet light, a large number of photo-induced reversible defects, i.e. color centers, are generated in the BBO crystal through the process of two-photon effect, etc. The color centers in the BBO crystal cause the increase of light loss of the crystal, and convert the absorbed light into heat and interfere with the phase matching in the crystal, which seriously limits the application of the BBO crystal and the development of the field of nonlinear optics. Therefore, in order to extend the application of the BBO crystal in the field of high-power ultraviolet optics, it is necessary to study the properties of the color centers in the BBO crystal, and the density of the color centers in the BBO crystal (indicating the number of color centers per unit volume) is particularly important.

[0004] At present, the commonly used methods for measuring the density of color centers in the crystal mainly include absorption spectrum analysis method, photoluminescence method, electron microscope method and laser scattering method, etc. Among them, the absorption spectrum analysis method identifies the absorption peak of a specific color center by measuring the absorption of light at different wavelengths, and calculates the color center density by absorbance. However, the accuracy of this method is limited, and it is difficult to accurately distinguish the color centers caused by light irradiation.

[0005] The photoluminescence method evaluates the density and energy level intensity of the color centers by exciting the color centers in the crystal and measuring the luminescence intensity. However, this method requires high-performance lasers and sensitive detectors, and the cost of experimental equipment is high.

[0006] The electron microscope method directly observes the color centers and other defect structures in the crystal by using transmission electron microscope or scanning electron microscope. However, this scheme needs to slice the sample, and the electron microscope is very expensive.

[0007] The laser scattering method evaluates the density and characteristics of the color centers by analyzing the scattering spectrum of the laser in the crystal through Raman spectroscopy or Brillouin scattering method, but the color center scattering signal is weak, and a high-sensitivity detection system is required, and a complex theoretical model and data processing are required.

[0008] In summary, there is currently a lack of a simple and accurate color center density measurement method. SUMMARY

[0009] The application provides a method for measuring color center density in a BBO crystal, which calculates the color center density of the BBO crystal based on the optical characteristic parameters and total loss of the measurement system by constructing a simple measurement system. The measurement equipment and method are simple and fast, greatly reducing the measurement cost, and having fewer influencing factors, thereby providing a good foundation for the accuracy of the measurement results.

[0010] The application provides a method for measuring color center density in a BBO crystal, which includes:

[0011] A measurement system containing the measured BBO crystal is built;

[0012] Based on the optical characteristic parameters of the optical system in the measurement system, the total loss of the optical system corresponding to different input pump powers at different temperatures is calculated;

[0013] The color center density of the measured BBO crystal corresponding to different pump powers at different temperatures is calculated according to the total loss and the optical characteristic parameters of the optical system.

[0014] Preferably, the measurement system includes a half-wave plate corresponding to the wavelength of the pump beam, an optical system, a first power meter and a second power meter, the first power meter is arranged between the half-wave plate and the incident end of the optical system, and the second power meter is arranged downstream of the exit end of the optical system.

[0015] Preferably, the optical system is an optical parametric oscillator.

[0016] Preferably, the optical parametric oscillator includes an input coupling mirror and an output coupling mirror, and the measured BBO crystal is arranged between the input coupling mirror and the output coupling mirror.

[0017] Preferably, based on the optical characteristic parameters of the measurement system, the total loss of the measurement system corresponding to different input pump powers at different temperatures is calculated, specifically including:

[0018] In the working state of the optical parametric oscillator, the half-wave plate is continuously rotated to change the readings of the first power meter and the second power meter;

[0019] The difference between the total pump power and the reading of the first power meter is taken as the input pump power;

[0020] The corresponding total loss is calculated using the optical characteristic parameters, the input pump power and the reading of the second power meter:

[0021]

[0022] wherein, l is total loss, P signal is the reading of the second power meter, representing the output signal light power of the optical parametric oscillator in working state; P threshold is the pump threshold power of the optical parametric oscillator, which is one of the optical characteristic parameters; R is the reflectivity of the output coupling mirror, which is one of the optical characteristic parameters; P pump is the input pump power of the optical parametric oscillator in working state.

[0023] Preferably, the corresponding color center density is calculated by using the following formula:

[0024]

[0025] wherein, N is the color center density of the measured BBO crystal; l0 is the inherent loss of the intracavity element of the optical parametric oscillator, which is one of the optical characteristic parameters; σ is the absorption cross section of the color center to the signal light, which is one of the optical characteristic parameters; l c is the effective length of the measured BBO crystal, which is one of the optical characteristic parameters.

[0026] Preferably, the pump power when the reading of the first power meter is the largest during the rotation of the half-wave plate is the total pump power.

[0027] Preferably, when the reading of the second power meter is not 0 during the rotation of the half-wave plate, the difference between the total pump power and the reading of the first power meter at this time is the pump threshold power of the optical parametric oscillator.

[0028] Other features and advantages of the present application will become apparent from the following detailed description of exemplary embodiments thereof, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0029] The accompanying drawings incorporated in and forming a part of the specification, illustrate embodiments of the present application and, together with the description, serve to explain the principles of the application.

[0030] Figure 1 is the flow chart of the method for measuring the color center density in the BBO crystal provided by the present application;

[0031] Figure 2 is one embodiment of the measuring system provided by the present application;

[0032] Figure 3 is the corresponding relationship between the input pump power and the output signal light power at 185℃ and 135℃ obtained by using the measuring system shown in Figure 2 DETAILED DESCRIPTION

[0033] ​Various exemplary embodiments of the present application will now be described in detail with reference to the accompanying drawings. Note that the relative arrangement, numerical expressions, and numerical values of components and steps set forth in these embodiments are not limiting to the scope of the present application unless specifically stated otherwise.

[0034] The following description of at least one exemplary embodiment is merely exemplary in nature and is in no way intended to limit the scope of the application or its application or uses.

[0035] Techniques, methods, and apparatus known to those of ordinary skill in the relevant art can not be discussed in detail herein, but should be considered as part of the specification in appropriate circumstances.

[0036] In all examples shown and discussed herein, any specific values should be interpreted as merely exemplary, and not as a limitation. Thus, other examples of the exemplary embodiments can have different values.

[0037] The present application provides a method for measuring the color center density in BBO crystal, by constructing a simple measurement system, based on the optical characteristics parameters and total loss of the measurement system to calculate the color center density of BBO crystal, the measurement equipment and method are simple and fast, greatly reduce the measurement cost, and the influencing factors are less, which provides a good foundation for the accuracy of the measurement results.

[0038] When strong light irradiates on the BBO crystal, the multi-photon absorption effect in the BBO crystal will cause the bound state electrons in the lattice to ionize, forming free moving electrons and holes. At the same time, the disordered thermal motion of the electrons and holes in the BBO crystal makes them recombine with a certain probability. Under the combination of the two effects, the density of free electrons and holes in the BBO crystal will gradually tend to a dynamic equilibrium state. The free electrons and holes in the BBO crystal will cause the optical performance of the crystal to change, which is equivalent to the reversible defects in the BBO crystal, that is, the color center. The specific form of the color center includes but is not limited to F center, gray trace, etc.

[0039] The color center in the BBO crystal will cause it to increase the absorption loss of light, so by measuring the size of the absorption loss, the color center density in the crystal can be known. Therefore, a measurement system with an optical system can be built, the total loss of the optical system in the measurement system is calculated, and then the color center density is calculated.

[0040] Based on the above, as Figure 1 shown, the present application provides a method for measuring the color center density in BBO crystal, which includes:

[0041] S110: Build a measurement system containing the measured BBO crystal, the measurement system includes an optical system and a measuring instrument.

[0042] S120: calculating total loss of the measurement system corresponding to different input pump powers of the measurement system at different temperatures based on the optical characteristic parameter of the optical system in the measurement system.

[0043] S130: calculating the color center density of the measured BBO crystal corresponding to different pump powers at different temperatures according to the total loss and the optical characteristic parameter of the optical system.

[0044] Specifically, in S110, the measurement system includes a half-wave plate corresponding to the pump beam band, an optical system, a first power meter and a second power meter. The half-wave plate is used to adjust the pump power input into the optical system, the first power meter is arranged between the half-wave plate and the incident end of the optical system, and the second power meter is arranged downstream of the exit end of the optical system.

[0045] As an embodiment, the optical system is an optical parametric oscillator. In the case of meeting the phase matching condition, one high-energy pump photon will be converted into one signal photon and one idler photon in the nonlinear crystal through the optical parametric process in the optical parametric oscillator. By building the optical parametric oscillator to make the signal light or idler light resonate in the resonant cavity, efficient energy conversion can be realized. Understandably, the optical system can also be other optical systems containing nonlinear optical processes.

[0046] As an embodiment, as shown in Figure 2 The optical parametric oscillator includes an input coupling mirror 4 and an output coupling mirror 6, and the measured BBO crystal 5 is arranged between the input coupling mirror 4 and the output coupling mirror 6. A polarization beam splitter prism 3 corresponding to the pump beam band is arranged between the half-wave plate 2 and the incident end of the optical system. After the pump light 1 passes through the half-wave plate 2, it enters the polarization beam splitter prism 3. Part of the pump light is transmitted from the polarization beam splitter prism 3 and injected into the optical parametric oscillator, and part of the pump light is reflected from the polarization beam splitter prism 3 and enters the first power meter 9. A beam splitter 7 is arranged on the output light path of the optical parametric oscillator. The signal light 8 is reflected by the beam splitter 7 and enters the second power meter 10, which is used to measure the power of the signal light 8 output by the optical parametric oscillator.

[0047] On the basis of the above, in S120, the total loss of the measurement system corresponding to different input pump powers of the measurement system at different temperatures is calculated based on the optical characteristic parameter of the measurement system, and specifically includes:

[0048] S1201: continuously rotating the half-wave plate under the working state of the optical parametric oscillator to change the readings of the first power meter and the second power meter.

[0049] S1202: taking the difference between the total pump power and the reading of the first power meter as the input pump power.

[0050] The pump power at which the reading of the first power meter is at its maximum during the rotation of the half-wave plate is the total pump power.

[0051] S1203: Calculate the corresponding total loss l using optical characteristic parameters, input pump power, and the reading of the second power meter.

[0052] Optical characteristic parameters include the pump threshold power P of the optical parametric oscillator. threshold And the reflectivity R of the output coupling mirror.

[0053] Among them, combined Figure 2 When the reading of the second power meter 10 is non-zero during the rotation of the half-wave plate 2, the difference between the total pump power and the reading of the first power meter 9 at this time is the pump threshold power P of the optical parametric oscillator. threshold .

[0054] Total loss l and pump threshold power P in an optical parametric oscillator threshold Input pump power P of the optical parametric oscillator under operating conditions pump and output signal optical power P signal The relationship between them is:

[0055]

[0056] Where η is the power conversion efficiency between the pump light and the signal light, expressed as: η = P signal / P pump γ is the ratio of the input pump power to the pump threshold power, expressed as γ = P pump / P threshold .

[0057] Therefore, we can conclude that:

[0058]

[0059] The total loss l of the measurement system consists of the inherent loss l0 of the cavity components of the optical parametric oscillator and the loss caused by the color center. The above relationship can be expressed as:

[0060] l=Nσl c +l0 (3)

[0061] Where N is the color center density of the BBO crystal under test; l0 is the inherent loss of the cavity components of the optical parametric oscillator, which is one of its optical characteristic parameters; σ is the absorption cross section of the color center for the signal light, which is also one of its optical characteristic parameters; l c The effective length of the BBO crystal being tested is one of its optical characteristic parameters.

[0062] Therefore, we can conclude that:

[0063]

[0064] In S130, the corresponding color center density can be calculated by using formula (4).

[0065] As an example, the effective length of the measured BBO crystal is 36 mm, the reflectivity of the output coupling mirror is 0.7, and the absorption cross section of the color center to the signal light is 8 x 10 -15 mm 2 The intrinsic loss of the intracavity element of the optical parametric oscillator is 0.0474867.

[0066] During the experiment, at the preset temperature, the half-wave plate 2 is rotated to adjust the pump power injected into the optical parametric oscillator. When the reading of the first power meter 9 is the maximum, the total pump power is 17.85 W. Continue to rotate the half-wave plate 2, when the second power meter 10 starts to produce a non-zero reading, the pump threshold power P threshold of the optical parametric oscillator can be obtained by subtracting the power shown by the first power meter 9 from the total pump power. Then gradually increase the pump power to make the optical parametric oscillator enter the working state.

[0067] After entering the working state, gradually rotate the half-wave plate 2 to continuously reduce the reading of the first power meter 9, so as to obtain the input pump power P pump of the optical parametric oscillator by calculating the difference between the total pump power and the reading of the first power meter. At the same time, the output signal light power P signal is obtained by reading the second power meter 10.

[0068] Figure 3 The corresponding relationship between the input pump power and the output signal light power at 185°C and 135°C obtained by using the measurement system shown in Figure 2 .

[0069] Then, the total loss of the measurement system is calculated according to formula (2), and the corresponding color center density of the measured BBO crystal is calculated according to formula (4).

[0070] Based on the above method, the color center density corresponding to different input pump powers at different temperatures can be measured. Table 1 shows the measurement results at 185°C and 135°C in the embodiment of Figure 2 .

[0071] Table 1: Measurement results

[0072]

[0073] While certain embodiments of the application have been described by way of example, it should be appreciated that those skilled in the art can certainly make modifications to the described embodiments without departing from the scope and spirit of the application. The scope of the application is defined in the accompanying claims.

Claims

1. A method for measuring color center density in a BBO crystal, characterized in that, The application relates to a method for measuring the color center density of a BBO (Beta-Barium Borate) crystal. The method comprises the following steps: building a measuring system containing the BBO crystal to be measured; calculating the total loss of the measuring system corresponding to different input pump powers of the measuring system at different temperatures based on the optical characteristic parameters of an optical system in the measuring system; calculating the color center density of the BBO crystal to be measured corresponding to different pump powers at different temperatures according to the total loss and the optical characteristic parameters of the optical system; the measuring system comprises a half-wave plate corresponding to the wave band of a pump light beam, an optical system, a first power meter and a second power meter, the half-wave plate is used for adjusting the pump power input into the optical system, the first power meter is arranged between the half-wave plate and the incident end of the optical system, and the second power meter is arranged downstream of the exit end of the optical system; the optical system is an optical parametric oscillator; the optical parametric oscillator comprises an input coupling mirror and an output coupling mirror, and the BBO crystal to be measured is arranged between the input coupling mirror and the output coupling mirror; the calculation of the total loss of the measuring system corresponding to different input pump powers of the measuring system at different temperatures based on the optical characteristic parameters of the measuring system comprises the following steps: continuously rotating the half-wave plate under the working state of the optical parametric oscillator so that the readings of the first power meter and the second power meter change; taking the difference between the total pump power and the reading of the first power meter as the input pump power; where l is the total loss, P signal is the reading of the second power meter, indicating the output signal light power of the optical parametric oscillator in the working state; P threshold is the pump threshold power of the optical parametric oscillator, which is one of the optical characteristic parameters; R is the reflectivity of the output coupling mirror, which is one of the optical characteristic parameters; P pump is the input pump power of the optical parametric oscillator in the working state; calculating the corresponding total loss by using the optical characteristic parameters, the input pump power and the reading of the second power meter; Wherein, N is the color center density of the measured BBO crystal; lo is the inherent loss of the intracavity element of the optical parametric oscillator, which is one of the optical characteristic parameters; σ is the absorption cross section of the color center to the signal light, which is one of the optical characteristic parameters; l c is the effective length of the measured BBO crystal, which is one of the optical characteristic parameters.

2. The method of measuring color center density in a BBO crystal according to claim 1, characterized in that, calculating the corresponding color center density by using the following formula:

3. The method of measuring color center density in a BBO crystal according to claim 1, characterized in that, the pump power corresponding to the maximum reading of the first power meter in the process of rotating the half-wave plate is the total pump power. when the reading of the second power meter is not 0 in the process of rotating the half-wave plate, the difference between the total pump power and the reading of the first power meter at this time is the pump threshold power of the optical parametric oscillator.

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