Laser resonator and method for controlling laser resonator

Abstract

To provide a laser resonator which is capable of generating laser light with which the degree of influence of two primary modes is high.SOLUTION: A laser resonator 1 comprises a medium container 11, a first optical element 17, a second optical element 20, actuators (first actuator 18, second actuator 21), a light measuring instrument 22, and a processing device 30. The medium container 11 generates light that includes at least a first mode and a second mode. The actuators move the first optical element 17 and / or the second optical element 20 along the resonance optical path, causing the resonator length to change. The light measuring instrument 22 measures the output of light that resonates by a resonance circuit. The processing device 30 converts a relationship between the output measured by the light measuring instrument 22 and the measurement time to a relationship between a power spectrum and a frequency by Fourier transformation, and actuates the actuators based on the value of the power spectrum of a frequency calculated on the basis of the distance between the medium container 11 and the first optical element 17.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This application relates primarily to laser resonators. [Background technology]

[0002] Patent Document 1 discloses a laser resonator with an adjustable resonator length. Patent Document 1 describes that by appropriately adjusting the resonator length, it is possible to have two longitudinal modes while suppressing another mode, thereby improving noise characteristics. Specifically, the resonator of Patent Document 1 includes a resonator designed to have two modes and a resonator for frequency doubling, and changes the resonator length designed to have two modes based on the output of the laser light output from the resonator for frequency doubling so that the intensity of the output of the laser light is maximized. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Special Publication No. 2007-515765 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Document 1, when controlling the cavity length, only the intensity of the output laser light is considered, and as a result, there is a case where a laser in which the influence of the two modes is high cannot be generated, so there is room for improvement.

[0005] The present application has been made in view of the above circumstances, and a main object of the application is to provide a laser resonator capable of generating laser light in which the influence of the two main modes is high. [Means for solving the problem]

[0006] The problem to be solved by the present application is as described above. Next, the means for solving this problem and the effects thereof will be described.

[0007] According to a first aspect of the present application, a laser resonator having the following configuration is provided. That is, the laser resonator includes a medium container, a first optical element, a second optical element, an actuator, an optical measuring instrument, and a processing device. The medium container has a laser medium therein, and generates light including at least a first mode and a second mode by stimulated emission when irradiated with excitation light. The first optical element is located at a first end of a resonant optical path that resonates the light generated in the medium container, and reflects the light. The second optical element is located at a second end of the resonant optical path, and reflects the light. The actuator moves at least one of the first optical element and the second optical element along the resonant optical path to change the resonator length. The optical measuring instrument measures the output of light resonating through the resonant optical path. The processing device converts the relationship between the output measured by the optical measuring instrument and the measurement time into a relationship between a power spectrum and a frequency by a Fourier transform, and operates the actuator based on the value of the power spectrum of the frequency calculated based on the distance between the medium container and the first optical element.

[0008] According to a second aspect of the present application, there is provided a method for controlling a laser resonator, as follows. That is, light including at least a first mode and a second mode is generated by stimulated emission by irradiating an excitation light to a medium container having a laser medium therein. The output of light generated in the medium container and resonated in a resonant optical path is measured. The relationship between the measured output and the measurement time is converted to a relationship between a power spectrum and a frequency by a Fourier transform. At least one of the first optical element and the second optical element located at the second end of the resonant optical path is moved along the resonant optical path based on a value of a power spectrum of a frequency calculated based on a distance between the medium container and a first optical element located at a first end of the resonant optical path that resonates the light generated in the medium container. Effect of the Invention

[0009] According to the present application, it is possible to realize a laser resonator capable of generating laser light in which the influence of the two main modes is high. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of a laser resonator. [Diagram 2] FIG. 2 is an explanatory diagram showing interference between different modes of laser light in a laser resonator. [Diagram 3] 6 is a graph showing the results of numerical analysis of each mode of laser light. [Figure 4] 1 is a graph showing the relationship between phase difference and S / N ratio. [Diagram 5] 1 is a graph showing the relationship between phase difference and S / N ratio. [Figure 6] 13 is a flowchart of a position adjustment process. [Figure 7] 11 is a graph showing the internal output, the pulse output, and the Fourier power spectrum before and after optimization by the position adjustment process. [Figure 8] 11 is a table showing the results of calculating the occurrence frequency of the main mode for a number of cases. [Figure 9] Graph showing the relationship between the pressure of the laser medium and the signal-to-noise ratio. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Next, an embodiment of the present application will be described with reference to the drawings. First, the configuration of a laser resonator 1 will be described with reference to FIG.

[0012] The laser resonator 1 generates an alkali laser by stimulated emission and outputs it as a pulsed laser. However, the laser resonator 1 is not limited to use for an alkali laser. For example, if a double longitudinal mode, which will be described later, can be realized, the present technology can also be applied to metal vapor lasers other than alkali metals, gas lasers, or solid-state lasers. In addition, the present technology can also be applied to continuous wave lasers, not limited to pulsed lasers.

[0013] As shown in FIG. 1, the laser resonator 1 includes a light source 10. The light source 10 is, for example, a laser diode, and generates excitation light 101. The excitation light is light that excites a laser medium. In this specification, light is not limited to visible light, but means light in a broad sense including infrared light and ultraviolet light. A medium container 11, a first dichroic mirror 12, a second dichroic mirror 13, and an optical damper 14 are arranged on the optical path of the excitation light 101 generated by the light source 10. The excitation light 101 generated by the light source 10 passes through the second dichroic mirror 13, the medium container 11, and the first dichroic mirror 12 in this order, and is absorbed by the optical damper 14.

[0014] The medium container 11 is a container that contains a laser medium. In this embodiment, a vaporous alkali metal, specifically cesium vapor, is sealed in the medium container 11. As described above, the laser resonator 1 can be applied to various lasers, and in that case, a laser medium according to the type of laser to be applied is contained in the medium container 11. The excitation light 101 passes through the inside of the medium container 11. As a result, the laser medium in the medium container 11 is excited and the energy level becomes high. Then, the energy level of the laser medium decreases, and light corresponding to the difference in energy is emitted. The light emitted from the medium container 11 resonates with the laser resonator 1. Hereinafter, the light generated in the medium container 11 by stimulated emission and the light that resonates the light are collectively referred to as laser light 102.

[0015] The first dichroic mirror 12 and the second dichroic mirror 13 reflect light in a first wavelength range and transmit light in a second wavelength range. In this embodiment, the first dichroic mirror 12 and the second dichroic mirror 13 are selected so that the first wavelength range includes the wavelength of the laser light 102 and the second wavelength range includes the wavelength of the excitation light 101. As a result, the first dichroic mirror 12 and the second dichroic mirror 13 transmit the excitation light 101.

[0016] The first dichroic mirror 12 is disposed between the medium container 11 and the polarizing beam splitter 15 so that the reflecting surface faces the medium container 11 and the laser light 102 reflected by the first dichroic mirror 12 is irradiated onto the polarizing beam splitter 15. The second dichroic mirror 13 is disposed between the medium container 11 and the mirror 19 so that the reflecting surface faces the medium container 11 and the laser light 102 reflected by the second dichroic mirror 13 is irradiated onto the mirror 19. Therefore, the laser light 102 is reflected by the first dichroic mirror 12 and the second dichroic mirror 13. In the optical path of the laser light 102 reflected by the first dichroic mirror 12, the polarizing beam splitter 15, the Pockels cell 16, and the first optical element 17 are disposed in the order in which the laser light 102 passes.

[0017] The polarizing beam splitter 15 is an optical element that switches between transmitting and reflecting the laser beam 102 depending on the polarization of the laser beam 102. For example, the polarizing beam splitter 15 transmits the S-polarized laser beam 102 and reflects the P-polarized laser beam 102. The properties of the polarizing beam splitter 15 for S-polarized light and P-polarized light may be reversed. As will be described in detail later, the polarizing beam splitter 15 reflects the laser beam 102, causing the laser beam 102 to travel back and forth along the resonant optical path, and the polarizing beam splitter 15 transmits the laser beam 102, causing the laser beam 102 to be output to the outside. The polarizing beam splitter 15 is disposed between the first dichroic mirror 12 and the Pockels cell 16 in such an orientation that the laser beam 102 reflected by the polarizing beam splitter 15 passes through the Pockels cell 16 and is irradiated onto the first optical element 17.

[0018] The Pockels cell 16 can be switched by applying a voltage between a normal state in which the laser light 102 is transmitted without being polarized and a polarization state in which the laser light 102 is polarized and transmitted. By transmitting the laser light 102 through the Pockels cell 16 in a polarized state, the laser light 102 is polarized by 45 degrees.

[0019] The first optical element 17 is a mirror or a prism, and reflects the laser light 102. When the first optical element 17 is a mirror, it may be a plane mirror or a concave mirror. The first optical element 17 is a first end of the resonant optical path. The first optical element 17 is disposed in such a position that the laser light 102 reflected by the first optical element 17 returns along the same path and proceeds toward the medium container 11. The laser light 102 reflected by the first optical element 17 returns along the same path and proceeds toward the medium container 11.

[0020] A first actuator 18 is attached to the first optical element 17. The first actuator 18 is driven in response to an input drive signal. The first actuator 18 moves linearly by a mechanism such as a ball screw, a linear motor, or a solenoid. This allows the first optical element 17 to move along the resonant optical path, that is, along the optical axis direction of the laser light 102 incident on the first optical element 17. The resonator length changes as the first optical element 17 moves along the resonant optical path. The timing and amount of movement of the first actuator 18 will be described later.

[0021] The polarizing beam splitter 15 and the Pockels cell 16 function as an optical switching element. Specifically, by applying a voltage to the Pockels cell 16 to polarize it, the polarization of the laser light 102 changes by 90 degrees when the laser light 102 goes back and forth through the Pockels cell 16. As a result, the laser light 102 passes through the polarizing beam splitter 15. The laser light 102 that passes through the polarizing beam splitter 15 is guided and output by an output mirror 23 that is disposed on the opposite side of the polarizing beam splitter 15 to the Pockels cell 16. That is, the laser light 102 that passes through the Pockels cell 16 while a voltage is applied to the Pockels cell 16 is output to the outside. Then, by inputting a pulse signal whose voltage value changes at high speed to the Pockels cell 16, the pulsed laser light 102 is output to the outside.

[0022] In this way, the cavity dumping method is a method of providing an optical switching element outside the medium container 11, changing the direction of the laser light 102 by the optical switching element, and outputting it to the outside. By using the cavity dumping method, it is possible to output a pulsed laser light 102. Note that a known Q switching method is also available as a method of outputting the pulsed laser light 102. However, since the radiation life of the laser transition is short in an alkali laser, sufficient energy cannot be accumulated by the Q switching method. Therefore, it is preferable to output the pulsed laser light 102 to the outside by the cavity dumping method as in this embodiment.

[0023] The optical switching element shown in this embodiment is merely an example, and the optical switching element may be configured with other optical elements or a combination thereof. When a laser other than an alkali laser is used, the pulsed laser light 102 may be output to the outside by a Q switching method or the like.

[0024] In the optical path of the laser light 102 reflected by the second dichroic mirror 13, a mirror 19 and a second optical element 20 are arranged in the order that the laser light 102 passes through.

[0025] The mirror 19 is disposed between the second dichroic mirror 13 and the second optical element 20. The mirror 19 is disposed in such a position that the mirror 19 reflects the laser light 102 irradiated thereto toward the second optical element 20.

[0026] The second optical element 20 is the second end of the resonant optical path. That is, the resonant optical path is the optical path of the laser light 102 between the first optical element 17 and the second optical element 20. Therefore, the resonator length is the length of the optical path between the first optical element 17 and the second optical element 20. The second optical element 20 has the same configuration as the first optical element 17, so a description thereof will be omitted. In addition, a second actuator 21 is attached to the second optical element 20. The second actuator 21 has the same configuration as the first actuator 18, so a description thereof will be omitted. In this embodiment, actuators are attached to both the first optical element 17 and the second optical element 20, but an actuator may be attached to only one of them.

[0027] A part of the laser light 102 irradiated to the mirror 19 is transmitted through the mirror 19. That is, the laser light 102 irradiated to the mirror 19 is branched into the laser light 102 that transmits through the mirror 19 and the laser light 102 that is reflected by the mirror 19. The branched light of the laser light 102 that transmits through the mirror 19 is measured by the optical measuring instrument 22. The optical measuring instrument 22 outputs a current signal according to the output of the laser light 102 or a voltage signal converted from the current signal. In other words, the optical measuring instrument 22 measures and outputs the change in the output of the laser light 102 over time. Note that, in FIG. 1, the light irradiated from the second dichroic mirror 13 to the mirror 19 and transmitted through it is measured by the optical measuring instrument 22, but the laser light 102 may be measured at any location inside the laser resonator 1. For example, an optical measuring instrument 22 may be disposed on the opposite side of the mirror 19 from the second optical element 20, and the light irradiated from the second optical element 20 to the mirror 19 and transmitted through the mirror 19 may be measured by the optical measuring instrument 22.

[0028] The resonant optical path of the laser resonator 1 of this embodiment is an example, and optical elements may be added, changed, or omitted.

[0029] The laser resonator 1 includes a processing device 30. The processing device 30 performs calculations based on the measurement results of the optical measuring device 22, and operates at least one of the first actuator 18 and the second actuator 21 based on the calculation results, thereby performing control to change the resonator length to an appropriate value.

[0030] Next, the mode of the laser light 102, particularly the double longitudinal mode, will be described.

[0031] In the laser light 102, there exists one or more standing waves determined by the resonator length, and these standing waves are called longitudinal modes. Among the longitudinal modes, the mode that is the main source of energy or output is called the main mode, and the other secondary modes are called sub-modes. The sub-modes can also be considered as noise relative to the main mode.

[0032] When there are two main modes, it is called a double longitudinal mode, and the main modes are called the first and second modes, respectively. The first and second modes are concepts different from the academically defined primary and secondary modes. All secondary modes in the case of a double longitudinal mode are called the third mode. The third mode is a general term for multiple modes. In the case of a double longitudinal mode, for example, the first and second modes have a higher internal intensity (kW / cm) than the secondary modes. 2 ) will increase by more than 20 times.

[0033] The case where there is one main mode is called a single longitudinal mode. When the laser medium is uniformly spread, the single longitudinal mode is likely to oscillate, but due to a phenomenon called spatial hole burning, the dual longitudinal mode may oscillate. The dual longitudinal mode tends to produce a larger output of the laser light 102 than the single longitudinal mode, but the oscillation may not be stable. The laser resonator 1 of this embodiment oscillates the laser in a stable dual longitudinal mode by changing the resonator length and other parameters to appropriate values.

[0034] Hereinafter, interference between multiple modes will be described with reference to FIG.

[0035] 2, the first optical element 17 is designated as M1, the second optical element 20 is designated as M2, the distance from the center of the medium container 11 to the first optical element 17 is designated as b, the distance from the center of the medium container 11 to the second optical element 20 is designated as a, and the resonator length is designated as L. In the resonator optical path, the laser light 102 exists as a standing wave between the first optical element 17 and the second optical element 20.

[0036] FIG. 2 shows the change in the electric field and the output of three modes, β, β+1, and β+2. β is a rational integer. Since the phase of the light differs in each mode, there is a phase difference between each mode. Therefore, there are parts where the outputs reinforce each other and parts where they cancel each other out. Also, the upper left of FIG. 2 shows a graph showing the phase difference at the center of the medium container when b / L=1 / 3. The upper right of FIG. 2 shows a graph showing the phase difference at the center of the medium container 11 when b / L=1 / 2.

[0037] Here, the spatial electric field distribution of the vibration mode of the laser light 102 resonating in the laser resonator 1 at a certain moment is described by the following equation (1). E β (z)=2E 0 sin(βπz / L) (1) However, E 0 is the electric field amplitude, and z is the distance from the first optical element 17 to the center of the medium container 11. Furthermore, at the center position of the medium container 11, the β+jth mode has an optical phase advancement by the amount shown in the following formula (2) compared to the βth mode. Note that in the following formulas (2) and (3), the unit of Δφ is radian. Δφ=jπb / L (2) Δφ is the phase difference between the two modes at the center of the medium container 11. As shown in FIG. 2, when the phase difference between the two modes satisfies the following formula (3), the interference between the two longitudinal modes is minimized. Δφ=mπ / 2 (m=1,3,5...) ···(3) From equations (2) and (3), the following equation (4) can be obtained, which shows the relationship between the distance b from the center of the medium container 11 to the first optical element 17 and the cavity length L when the interference between the two longitudinal modes is minimized. b / L=m / 2n (n=1,2,3... m=1,3,5...)...(4)

[0038] Next, the conditions such as the phase difference when a stable dual longitudinal mode is generated will be described with reference to Fig. 3 to Fig. 5. In the following description, when the "phase difference" is referred to as the mode, it refers to the "phase difference at the center of the medium container 11".

[0039] FIG. 3 shows the results of calculating the laser output and the phase difference of each mode by numerical analysis. In the graph of FIG. 3, the mode marked with a circle is the main mode, and the other modes are the secondary modes. In both graphs, the distance b from the center of the medium container 11 to the first optical element 17 is set to 1 / 3 m. The graph on the left is for the case where the resonator length L is 2.0 m, and since b / L=1 / 6, the above formula (4) is satisfied. In other words, the interference between the two longitudinal modes is minimized. On the other hand, the graph on the right is for the case where the resonator length L is 5.0 m, and since b / L=1 / 15, the above formula (4) is not satisfied. In other words, the interference between the two longitudinal modes is not minimized. Odd is a state in which a mode is present at a phase difference of 0 degrees, and Even is a state in which no mode is present at a phase difference of 0 degrees. Under the conditions of this numerical analysis, when the resonator length changes by 200 nm, Odd and Even are interchanged.

[0040] Only the left graph Even in FIG. 3 shows the double longitudinal mode, and the left graph Odd in FIG. 3 and the right graph Odd and Even in FIG. 3 show the state where the double longitudinal mode is not generated. As shown in the left graph Even in FIG. 3 where the double longitudinal mode is generated, it can be seen that the laser output becomes large when the double longitudinal mode is generated and the output of the sub-mode other than the main mode is small. It can also be seen that when the double longitudinal mode is generated, the phase difference between the first mode and the second mode, which are the main modes, is about 90 degrees, and that the even state is necessary to generate the double longitudinal mode. In order to always keep the resonator length in the even state, it is necessary that the minimum control distance of the first actuator 18 or the second actuator 21 is 200 nm or less. The minimum control distance is the minimum value of the distance that the processing device 30 can specify as the movement amount of the first actuator 18 or the second actuator 21 in the control of the first actuator 18 or the second actuator 21 by the processing device 30.

[0041] 4 is a graph showing the relationship between the phase difference and the S / N ratio. The phase difference indicates the phase difference between adjacent longitudinal modes, such as β and β+1. The S / N ratio is the amplitude of the third mode relative to the amplitudes of the first and second modes. In other words, the lower the S / N ratio, the lower the influence of the third mode, in other words, the higher the possibility of oscillation of the dual longitudinal mode laser light 102.

[0042] As shown in FIG. 4, when the phase difference between adjacent longitudinal modes is 90 degrees, the signal-to-noise ratio is lower than when the phase difference is 15 degrees and 30 degrees, and it is found that there is a higher possibility that the laser light 102 in the dual longitudinal modes will oscillate.

[0043] Fig. 5 is a graph showing the SN ratio when the phase difference between the first mode and the second mode is around 90 degrees. As shown in Fig. 5, the SN ratio is minimum when the phase difference between the first mode and the second mode is 90 degrees. It is also found that when the phase difference is in the range of 88 degrees or more and 92 degrees or less, the SN ratio is significantly lower than when the phase difference is outside this range. For the above reasons, the phase difference between the first mode and the second mode is preferably 88 degrees or more and 92 degrees or less, and more preferably 90 degrees.

[0044] Next, a process for controlling the actuators 18 and 21 to adjust the resonator length and oscillating a stable dual longitudinal mode laser beam 102 will be described with reference to Fig. 6 to Fig. 8. Fig. 6 is a flow chart of the position adjustment process. Fig. 7 is a graph showing the internal output, pulse output, and Fourier power spectrum before and after optimization by the position adjustment process. Fig. 8 is a table showing the results of calculating the appearance frequency of the main mode for a number of cases.

[0045] Hereinafter, the process of adjusting the cavity length by controlling the actuators 18, 21 is referred to as a position adjustment process. First, the processing device 30 acquires the measurement result of the laser light 102 by the optical measuring instrument 22 (S101 in FIG. 6). As described above, this measurement result indicates the relationship between the output of the laser light 102 and the measurement time. The measurement time is the time when the laser light 102 is measured, in other words, the measurement time. The internal output 91 shown in FIG. 7 corresponds to the measurement result of the optical measuring instrument 22.

[0046] Next, the processing device 30 performs a Fourier transform on the measurement result of the optical measuring device 22 (S102). This Fourier transform obtains the relationship between the power spectrum and frequency of the laser light 102. The Fourier power spectrum 93 shown in Fig. 7 corresponds to the relationship between the power spectrum and frequency of the laser light 102 obtained by the Fourier transform.

[0047] The frequency obtained by the Fourier transform is not the frequency at which the longitudinal mode occurs, but the frequency of the composite wave of the many standing waves occurring in the laser resonator 1. Since these standing waves have different frequencies, the standing waves interfere with each other, causing beats. In addition, the peak with a high power spectrum in the Fourier power spectrum 93 is due to the main mode. In the Fourier power spectrum 93 in FIG. 7, the mode marked with a circle is the main mode. In the case of double longitudinal modes, the peak with a high power spectrum corresponds to the composite wave of the first and second modes. For example, the peak that appears at the multiple of the frequency of the main mode in the Fourier power spectrum 93 in the upper left of FIG. 7 is a peak that appears due to the Fourier transform, and is a peak related to the main mode. The smallest frequency among the peaks related to the main mode is called the "appearance frequency of the main mode." In addition, small peaks other than the peak related to the main mode indicate the sub-mode, in other words, the third mode.

[0048] Next, the processing device 30 reads out the occurrence frequency of the main mode from the storage device of the processing device 30 (S103). The occurrence frequency of the main mode is calculated in advance by the following method and stored in the storage device of the processing device 30. Note that instead of calculating and storing the occurrence frequency of the main mode in advance, the occurrence frequency of the main mode may be calculated at the stage of performing the position adjustment process. When the phase difference between the first and second modes in the Fourier power spectrum 93 is 90 degrees, that is, in the case of dual longitudinal modes, the occurrence frequency of the main mode can be calculated based on the following formulas (5), (6), and (7). Δφ=180 b / L (5) Δv = c / 2L (6) v dual =Δv×90÷Δφ=(c / 2L)×90÷Δφ=(c / 2L) · L / 2b=c / 4b ···(7) Equation (5) is an equation showing the phase difference between the βth mode and the β+1th mode, specifically, the phase difference when j=1 in equation (2). In equations (5) and (7), the unit of Δφ is degrees. Equation (6) is an equation showing the mode interval. The mode interval is the difference in frequency between adjacent longitudinal modes or standing waves. In equation (6), c is the speed of light. Equation (7) is an equation showing the appearance frequency of the main mode in the composite wave of multiple standing waves generated in the laser resonator 1. As shown in equation (7), when the phase difference between the first mode and the second mode is 90 degrees, the appearance frequency of the main mode can be calculated from the distance b from the center of the medium container 11 to the first optical element 17 and the speed of light c.

[0049] For example, if the speed of light c = 300,000 km / s, in cases 1 to 4 shown in Figure 8, the distance b from the center of the medium container 11 to the first optical element 17 is 1 / 3 m, so the appearance frequency of the main mode is calculated as the speed of light c / (2 × 1 / 3) = approximately 225 MHz.

[0050] Next, the processing device 30 sets the value of the power spectrum of the main mode and the value of the power spectrum of the third mode (S104). Specifically, the processing device 30 extracts the highest power spectrum in the vicinity of the occurrence frequency of the main mode calculated in advance, and sets the value of the power spectrum as the value of the power spectrum of the main mode. For example, an approximation curve may be obtained by curve fitting for a data group obtained by Fourier transform, and the value of the power spectrum of the highest peak in the vicinity of the occurrence frequency of the main mode may be set as the value of the power spectrum of the main mode. Next, the processing device 30 calculates the value of the power spectrum of the third mode based on the peak caused by the third mode. The value of the power spectrum of the third mode relative to the value of the power spectrum of the main mode is called the power spectrum ratio. The power spectrum ratio is the same as the above-mentioned SN ratio, and corresponds to the third mode influence degree. The power spectrum ratio indicates the influence of the third mode in the laser light 102, and the smaller the value, the smaller the influence of the third mode and the higher the influence of the two main modes (the first mode and the second mode). As described above, the third mode included in the laser beam 102 has a much smaller output than the main mode. Therefore, the third mode in the laser beam 102 can be regarded as noise. Similarly, the power spectrum caused by the third mode in the Fourier power spectrum 93 can also be regarded as noise.

[0051] Next, the processing device 30 controls the actuators 18 and 21 so that the power spectral ratio becomes minimum (S105). Specifically, the processing device 30 controls the first actuator 18 or the second actuator 21 to move the first optical element 17 or the second optical element 20, while calculating and storing the power spectral ratio intermittently. Thereafter, the processing device 30 aligns the first optical element 17 and the second optical element 20 to the position where the power spectral ratio becomes minimum. By performing the above position adjustment process, the influence of the third mode can be reduced, so that the laser resonator 1 can oscillate a stable dual longitudinal mode laser light 102.

[0052] Next, an experiment is described in which the output of the laser light 102 is measured when the resonator length is changed. The top four graphs in Fig. 7 show the results of measuring the internal output 91, which is the output of the laser light 102 resonated in the laser resonator 1, and the pulse output 92, which is the output of the pulsed laser light 102 output to the outside by being guided by the output mirror 23, when laser oscillation is performed under the conditions of the resonator length being 0.69 m, 1.0 m, 2.0 m, and 5.0 m, and the distance b from the center of the medium container 11 to the first optical element 17 being 1 / 3 m. Furthermore, the Fourier power spectrum 93 calculated by performing a Fourier transform on the internal output 91 is shown in the upper right of each graph.

[0053] The lower part of Fig. 7 shows the experimental results when a position adjustment process is performed to optimize the positions of the first optical element 17 and the second optical element 20. As shown in Fig. 7, it can be seen that a high pulse output 92 can be achieved by performing the position adjustment process.

[0054] In this embodiment, either the first optical element 17 or the second optical element 20 is moved, but both the first optical element 17 and the second optical element 20 may be moved.

[0055] Furthermore, the positions of the first optical element 17 and the second optical element 20 when the power spectral ratio is minimized may be obtained in advance by experiment or numerical calculation. In this case, the obtained positions become the initial positions of the first optical element 17 and the second optical element 20. In this case, the processing device 30 can search for the positions of the first optical element 17 and the second optical element 20 when the power spectral ratio is minimized simply by finely adjusting the positions of the first optical element 17 and the second optical element 20.

[0056] The positions of the first optical element 17 and the second optical element 20 when the power spectrum ratio is minimized, i.e., the resonator length, may change depending on the environment. Examples of the environment include the temperature around the resonator, the air pressure, or the attitude of the laser resonator 1 itself. Therefore, even if the alignment is performed when the laser resonator 1 is shipped, it is preferable to perform the position adjustment process again after the laser resonator 1 is installed. It is also preferable to perform the position adjustment process at a predetermined timing thereafter.

[0057] Next, the relationship between the pressure of the laser medium and the S / N ratio will be described. Figure 9 shows the results of an experiment measuring the relationship between the pressure of the laser medium in the medium container 11, the output of the laser light 102, and the S / N ratio when the laser medium is cesium. As shown in Figure 9, the S / N ratio increases in the order of the pressure of the laser medium of 360 Torr, 760 Torr (=1 kPa), and 1520 Torr. Although it depends on the use of the laser light 102 and other conditions, this experiment shows that when the laser medium is cesium, it is preferable that the pressure of the medium container 11 is 760 Torr (=1 kPa) or less.

[0058] It should be noted that the functions of each element, including the processing device 30, disclosed in this disclosure can be performed using circuits or processing circuits, including general-purpose processors, special-purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, and / or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuits. In this disclosure, a circuit, unit, or means is hardware that performs the recited functions or hardware that is programmed to perform the recited functions. The hardware may be hardware disclosed herein or other known hardware that is programmed or configured to perform the recited functions. In the case where the hardware is a processor, which is considered a type of circuit, the circuit, means, or unit is a combination of hardware and software, and the software is used to configure the hardware and / or the processor.

[0059] As described above, the laser resonator 1 of this embodiment includes the medium container 11, the first optical element 17, the second optical element 20, actuators (the first actuator 18 and the second actuator 21), the optical measuring instrument 22, and the processing device 30. The medium container 11 has a laser medium therein, and generates light including at least a first mode and a second mode by stimulated emission when irradiated with the excitation light 101. The first optical element 17 is located at a first end of a resonant optical path that resonates the light generated in the medium container 11, and reflects the light. The second optical element 20 is located at a second end of the resonant optical path, and reflects the light. The actuators 18 and 21 move at least one of the first optical element 17 and the second optical element 20 along the resonant optical path to change the resonator length. The optical measuring instrument 22 measures the output of the light resonating through the resonant optical path. The processing device 30 converts the relationship between the output measured by the optical measuring device 22 and the measurement time into the relationship between the power spectrum and the frequency by a Fourier transform, and operates the actuators 18, 21 based on the value of the power spectrum of the frequency calculated based on the distance between the medium container 11 and the first optical element 17.

[0060] This makes it possible to generate laser light 102 in which the first and second modes are dominant. In particular, by calculating the relationship between the power spectrum and the frequency by Fourier transform, the influence of the third mode on the first and second modes can be easily obtained.

[0061] In the laser resonator 1 of this embodiment, the processing device 30 operates the actuators 18, 21 so as to reduce the ratio of the power spectrum value of the frequency caused by the third mode to the power spectrum value of the frequency calculated based on the distance between the medium container 11 and the first optical element 17.

[0062] This makes it possible to specifically estimate the effect of the third mode on the first and second modes, and to align the first optical element 17 and the second optical element 20 to appropriate positions.

[0063] In the laser resonator 1 of this embodiment, the processing device 30 calculates the third mode influence, which is the ratio of the power spectrum value of the frequency attributable to the third mode to the power spectrum value of the frequency calculated based on the distance between the medium container 11 and the first optical element 17, and operates the actuators 18, 21 so that the third mode influence is minimized.

[0064] This makes it possible to generate laser light 102 in which the first mode and the second mode are more predominant.

[0065] In the laser resonator 1 of this embodiment, the processing device 30 intermittently calculates the third mode influence degree while operating the actuators 18, 21 to move at least one of the first optical element 17 and the second optical element 20, and aligns the first optical element 17 and the second optical element 20 to a position where the third mode influence degree is minimum.

[0066] A simple process can generate laser light 102 in which the first and second modes are dominant.

[0067] In the laser resonator 1 of this embodiment, the optical measuring instrument 22 measures branched light obtained by branching the light traveling from the medium container 11 toward the first optical element 17 or the second optical element 20.

[0068] Compared with a configuration in which light passing through the medium container 11 is measured, light can be measured with a simpler configuration.

[0069] In the laser resonator 1 of this embodiment, the phase difference between the first mode and the second mode at the center of the medium container 11 in the optical path direction is 88 degrees or more and 92 degrees or less.

[0070] By setting the phase difference at or near 90 degrees, the first and second modes tend to become dominant.

[0071] In the laser resonator 1 of this embodiment, the laser medium is cesium. The pressure of the laser medium inside the medium container 11 is 100 kPa or less.

[0072] By setting the pressure within the above range, the first and second modes tend to predominate.

[0073] Although the preferred embodiment of the present application has been described above, the above configuration can be modified, for example, as follows.

[0074] The optical measuring instrument 22 in the above embodiment measures the laser light 102 traveling from the medium container 11 to the second optical element 20, but it may also measure the laser light 102 traveling from the medium container 11 to the first optical element 17. Alternatively, the light passing through the medium container 11 may be extracted and measured.

[0075] Alternatively, the processing device 30 does not necessarily need to search for the minimum value of the power spectral ratio as long as the power spectral ratio is smaller than that before the position adjustment process.

[0076] In the above embodiment, the frequency interval is used as the mode interval, but the wavelength interval may be used. [Explanation of symbols]

[0077] 1 Laser resonator 11 Media Container 17 First Optical Element 18 First Actuator 20 Second optical element 21 Second actuator 22 Optical Measuring Instruments 30 Processing equipment

Claims

1. a medium container having a laser medium therein and generating light including at least a first mode and a second mode by stimulated emission when irradiated with excitation light; a first optical element that is located at a first end of a resonant optical path that resonates the light generated in the medium container and reflects the light; a second optical element located at a second end of the resonant optical path and configured to reflect the light; an actuator that moves at least one of the first optical element and the second optical element along the resonator optical path to change a resonator length; an optical measuring instrument for measuring the output of light resonating through the resonant optical path; a processing device that converts the relationship between the output measured by the optical measuring device and the measurement time into a relationship between a power spectrum and a frequency by a Fourier transform, and operates the actuator based on the value of the power spectrum of the frequency calculated based on the distance between the medium container and the first optical element; A laser resonator comprising:

2. 2. The laser resonator according to claim 1, The processing device operates the actuator so as to reduce the ratio of the power spectrum value of the frequency due to the third mode to the power spectrum value of the frequency calculated based on the distance between the medium container and the first optical element.

3. 3. The laser resonator according to claim 2, the processing device calculates a third mode influence, which is a ratio of a power spectrum value of a frequency attributable to the third mode to a power spectrum value of a frequency calculated based on a distance between the medium container and the first optical element, and operates the actuator so that the third mode influence is minimized.

4. 4. The laser resonator according to claim 3, The processing device intermittently calculates the third mode influence degree while operating the actuator to move at least one of the first optical element and the second optical element, and aligns the first optical element and the second optical element to a position where the third mode influence degree is minimum.

5. 5. A laser resonator according to claim 1, The optical measuring instrument is a laser resonator that measures branched light obtained by branching light traveling from the medium container toward the first optical element or the second optical element.

6. 6. A laser resonator according to claim 1, A laser resonator in which a phase difference between the first mode and the second mode at the center of the medium container in the optical path direction is 88 degrees or more and 92 degrees or less.

7. 7. A laser resonator according to claim 1, the laser medium is cesium; A laser resonator in which the pressure of the laser medium inside the medium container is 100 kPa or less.

8. Irradiating a medium vessel having a laser medium therein with excitation light to generate light including at least a first mode and a second mode by stimulated emission; measuring the output of light generated in the medium container and resonating in a resonant optical path; The relationship between the measured output and the measurement time is converted to the relationship between the power spectrum and frequency by Fourier transform. A method for controlling a laser resonator, comprising: moving at least one of a first optical element and a second optical element located at a second end of a resonant optical path along the resonant optical path based on a value of a power spectrum of a frequency calculated based on a distance between the medium container and a first optical element located at a first end of the resonant optical path that resonates light generated in the medium container.

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