Method for measuring color center density in BBO crystal

Abstract

The invention discloses a method for measuring the color center density in a BBO crystal. The method comprises the steps that a measuring system containing the measured BBO crystal is built; based on the optical characteristic parameters of the optical system in the measurement system, calculating the total loss of the optical system corresponding to different input pumping powers of the measurement system at different temperatures; and calculating the color center density of the measured BBO crystal corresponding to different pumping powers at different temperatures according to the total loss and optical characteristic parameters of the optical system. According to the method, the simple measurement system is constructed, the color center density of the BBO crystal is calculated based on the optical characteristic parameters and the total loss of the measurement system, the measurement equipment and the measurement method are simple and rapid, the measurement cost is greatly reduced, influence factors are few, and a good basis is provided for the accuracy of a measurement result.

Description

Technical Field

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

[0002] The development of nonlinear frequency conversion technology has broken through the limitations of laser gain media and greatly expanded the available wavelength of lasers. Among many nonlinear optical materials, β-BBO crystals are widely used due to their high nonlinear coefficient, wide transparent band, high damage threshold and good anti-deliquescent performance, especially in optical parametric oscillators pumped by ultraviolet light (such as 355nm laser). Only one BBO crystal can cover the visible light and near-infrared bands (410nm-2600nm).

[0003] However, in the process of increasing 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, the BBO crystal produces a large number of photoreversible defects, namely color centers, through processes such as the two-photon effect. The color centers in the BBO crystal increase the loss of light in the crystal, convert the absorbed light into heat and interfere with the phase matching in the crystal, which seriously limits the application of BBO crystals and the development of the field of nonlinear optics. Therefore, in order to extend the application of BBO crystals in the field of high-power ultraviolet optics, it is necessary to study the properties of the color centers in the BBO crystal, among which 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 crystals mainly include absorption spectroscopy, photoluminescence, electron microscopy, and laser scattering. Among them, absorption spectroscopy measures the absorption of light at different wavelengths, identifies the absorption peak of a specific color center, and calculates the color center density by absorbance. However, this method has limited accuracy and it is difficult to accurately distinguish the color centers caused by light irradiation.

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

[0006] Electron microscopy uses a transmission electron microscope or a scanning electron microscope to directly observe color centers and other defect structures in crystals. However, this method requires thin slices of samples, and electron microscopes are very expensive.

[0007] The laser scattering method evaluates the density and characteristics of color centers by analyzing the scattering spectrum of laser in the crystal through Raman spectroscopy or Brillouin scattering. However, the color center scattering signal is weak, requiring a highly sensitive detection system, and complex theoretical models and data processing.

[0008] In summary, there is currently a lack of a simple and accurate method for measuring color center density. Summary of the invention

[0009] The present application provides a method for measuring the color center density in a BBO crystal. By constructing a simple measurement system, the color center density of the BBO crystal is calculated based on the optical characteristic parameters and total loss of the measurement system. The measurement equipment and the measurement method are simple and fast, which greatly reduces the measurement cost, and there are fewer influencing factors, which provides a good foundation for the accuracy of the measurement results.

[0010] The present application provides a method for measuring the color center density in a BBO crystal, comprising:

[0011] Build a measurement system including the BBO crystal to be measured;

[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 of the measurement system at different temperatures is calculated;

[0013] The color center density of the measured BBO crystal corresponding to different temperatures and different pump powers is calculated based on 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 pump beam band, 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 output end of the optical system.

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

[0016] Preferably, 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.

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

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

[0019] taking the difference between the total pump power and the indication of the first power meter as the input pump power;

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

[0021]

[0022] Where l is the total loss, P signal is the reading of the second power meter, indicating the output signal optical 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 when the optical parametric oscillator is in working state.

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

[0024]

[0025] Where N is the color center density of the measured BBO crystal; l0 is the intrinsic 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 for 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 maximum during the rotation of the half-wave plate is the total pump power.

[0027] Preferably, when the indication of the second power meter is non-zero during the process of rotating the half-wave plate, the difference between the total pump power and the indication 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 of the present application with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0031] Figure 2 An embodiment of the measurement system provided by the present application;

[0032] Figure 3 To use Figure 2 The corresponding relationship between the input pump power and the output signal light power at 185°C and 135°C obtained by the measurement system shown. DETAILED DESCRIPTION

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

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

[0035] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered part of the specification.

[0036] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not limiting. Therefore, other examples of the exemplary embodiments may have different values.

[0037] The present application provides a method for measuring the color center density in a BBO crystal. By constructing a simple measurement system, the color center density of the BBO crystal is calculated based on the optical characteristic parameters and total loss of the measurement system. The measurement equipment and the measurement method are simple and fast, which greatly reduces the measurement cost, and there are fewer influencing factors, which provides a good foundation for the accuracy of the measurement results.

[0038] When strong light is irradiated on the BBO crystal, the multi-photon absorption effect in the BBO crystal will cause the bound electrons in the lattice to be ionized, forming freely moving electrons and holes. At the same time, the disordered thermal motion of electrons and holes in the BBO crystal causes them to recombine with a certain probability. With the combination of these two effects, the density of free electrons and holes in the BBO crystal will gradually tend to a state of dynamic equilibrium. The free electrons and holes in the BBO crystal will cause the optical properties of the crystal to change, which is equivalent to reversible defects in the BBO crystal, that is, color centers. The specific forms of color centers include but are not limited to F centers, gray traces, etc.

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

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

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

[0042] S120: 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.

[0043] S130: Calculate the color center density of the measured BBO crystal corresponding to different temperatures and different pump powers according to the total loss and the optical characteristic parameters 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 to 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 output end of the optical system.

[0045] As an embodiment, the optical system is an optical parametric oscillator. When the phase matching condition is met, in the optical parametric oscillator, a high-energy pump photon will be converted into a signal photon and an idler photon through an optical parametric process in a nonlinear crystal. By building an optical parametric oscillator to make the signal light or the idler light resonate in the resonant cavity, efficient energy conversion can be achieved. It can be understood that the optical system can also be other optical systems that include nonlinear optical processes.

[0046] As an example, Figure 2 As shown, 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. The pump light 1 enters the polarization beam splitter prism 3 after passing through the half-wave plate 2, and a part of the pump light is injected into the optical parametric oscillator after being transmitted through the polarization beam splitter prism 3, and a 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, and the signal light 8 enters the second power meter 10 after being reflected by the beam splitter 7, 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, based on the optical characteristic parameters of the measurement system, the total loss of the measurement system corresponding to different input pump powers of the measurement system at different temperatures is calculated, specifically including:

[0048] S1201: When the optical parametric oscillator is in working state, the half-wave plate is continuously rotated 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 when the reading of the first power meter is the maximum during the rotation of the half-wave plate is the total pump power.

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

[0052] The 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 in an optical parametric oscillator and pump threshold power P threshold , input pump power P of the optical parametric oscillator in working state pump And the output signal optical power P signal The relationship between them is:

[0055]

[0056] Where η is the power conversion efficiency from pump light to 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] From this we can get:

[0058]

[0059] The total loss l of the measurement system consists of the intrinsic loss l0 of the intracavity 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 measured BBO crystal; l0 is the intrinsic 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 for 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.

[0062] From this we can get:

[0063]

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

[0065] As an example, the effective length l of the BBO crystal under test is 36 mm, the reflectivity R of the output coupling mirror is 0.7, and the absorption cross section σ of the color center for the signal light is 8×10 -15 mm 2 , the intrinsic loss l0 of the intra-cavity components of the optical parametric oscillator is 0.0474867.

[0066] During the experiment, at a 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 can be obtained to be 17.85 W. The half-wave plate 2 is rotated continuously. When the second power meter 10 starts to produce a non-zero reading, the pump threshold power P of the optical parametric oscillator can be obtained by subtracting the power shown by the first power meter 9 from the total pump power. threshold Then the pump power is gradually increased to put the optical parametric oscillator into working state.

[0067] After entering the working state, the half-wave plate 2 is gradually rotated to make the reading of the first power meter 9 decrease continuously. Then, the input pump power P of the optical parametric oscillator can be obtained by calculating the difference between the total pump power and the reading of the first power meter. pump At the same time, the output signal optical power P is obtained by reading the second power meter 10 signal of power.

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

[0069] Then, the total loss of the measurement system is calculated according to formula (2), and the color center density of the corresponding BBO crystal under test 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 Figure 2 The measurement results at 185°C and 135°C in the examples are:

[0071] Table 1: Measurement results

[0072]

[0073] Although some specific embodiments of the present application have been described in detail by way of example, it should be understood by those skilled in the art that the above examples are only for illustration, not for limiting the scope of the present application. It should be understood by those skilled in the art that the above embodiments may be modified without departing from the scope and spirit of the present application. The scope of the present application is defined by the appended claims.

Claims

1. A method for measuring the color center density in a BBO crystal, characterized in that: include: Build a measurement system including the BBO crystal to be measured; 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; 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.

2. The method for measuring the color center density in a BBO crystal according to claim 1, characterized in that: 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, wherein 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 output end of the optical system.

3. The method for measuring the color center density in a BBO crystal according to claim 2, characterized in that: The optical system is an optical parametric oscillator.

4. The method for measuring the color center density in a BBO crystal according to claim 3, characterized in that: 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.

5. The method for measuring the color center density in a BBO crystal according to claim 4, characterized in that: Calculating the total loss of the measurement system corresponding to different input pump powers of the measurement system at different temperatures based on the optical characteristic parameters of the measurement system, specifically including: 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; taking the difference between the total pump power and the indication of the first power meter as the input pump power; The corresponding total loss is calculated using the optical characteristic parameters, the input pump power, and the reading of the second power meter: Where l is the total loss, P signal is the reading of the second power meter, indicating the output signal optical 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 when the optical parametric oscillator is in working state.

6. The method for measuring the color center density in a BBO crystal according to claim 5, characterized in that: The corresponding color center density is calculated using the following formula: Where N is the color center density of the measured BBO crystal; l0 is the intrinsic 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 for 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.

7. The method for measuring the color center density in a BBO crystal according to claim 5, characterized in that: The pump power when the reading of the first power meter is maximum during the rotation of the half-wave plate is the total pump power.

8. The method for measuring the color center density in a BBO crystal according to claim 5, characterized in that: When the indication of the second power meter is non-zero during the process of rotating the half-wave plate, the difference between the total pump power and the indication of the first power meter at this time is the pump threshold power of the optical parametric oscillator.

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