JP2015038922A - Laser device, control method and optoacoustic measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress damage of an optical component within a resonator with high efficiency and at low cost in a laser device containing alexandrite as a laser medium.SOLUTION: The resonator is formed from a pair of mirrors 53 and 54 which are provided while interposing a laser rod 51 therebetween. A Q switch 55 changes a Q factor of the resonator. When an application voltage to the Q switch 55 is a first voltage corresponding to OFF of the Q switch, oscillation control means 57 irradiates the laser rod 51 with excitation light and after the irradiation with the excitation light, the application voltage to the Q switch 55 is changed from the first voltage to a second voltage corresponding to ON of the Q switch, thereby emitting pulse laser light. OFF voltage regulation means 58 lights on a flash lamp 52 in the state where a regulation voltage for making the Q factor of the resonator into a value exceeding a laser oscillation threshold is applied to the Q switch 55, and regulates the first voltage on the basis of the intensity of light, measured by a photodetector 59, outputted from the resonator caused by the irradiation with the excitation light.

Description

  The present invention relates to a laser device and a control method thereof, and more particularly to a laser device that emits pulsed laser light using a Q switch and a control method thereof. The present invention also relates to a photoacoustic measurement device including such a laser device.

  An ultrasonic inspection method is known as a kind of image inspection method capable of non-invasively examining the state inside a living body. In the ultrasonic inspection, an ultrasonic probe capable of transmitting and receiving ultrasonic waves is used. When ultrasonic waves are transmitted from the ultrasonic probe to the subject (living body), the ultrasonic waves travel inside the living body and are reflected at the tissue interface. The reflected ultrasound is received by the ultrasound probe, and the internal state can be imaged by calculating the distance based on the time it takes for the reflected ultrasound to return to the ultrasound probe. .

  In addition, photoacoustic imaging is known in which the inside of a living body is imaged using a photoacoustic effect. In general, in photoacoustic imaging, a living body is irradiated with pulsed laser light such as a laser pulse. Inside the living body, the living tissue absorbs the energy of the pulsed laser light, and ultrasonic waves (photoacoustic waves) are generated by adiabatic expansion due to the energy. By detecting this photoacoustic wave with an ultrasonic probe or the like and constructing a photoacoustic image based on the detection signal, in vivo visualization based on the photoacoustic wave is possible.

  Here, in the photoacoustic measurement, a laser device that emits pulsed laser light using a Q switch is often used. Patent Document 1 describes a laser device that uses an alexandrite crystal as a laser medium and can emit pulsed laser light having a wavelength of 800 nm and a wavelength of 755 nm.

JP 2013-89680 A

Alexandrite Laser Technology, M.L.Shand, SPIE Vol. 610 Scientific and Engineering Applications of Commercial Laser Devices (1986) Characteristics of alexandrite lasers in Q-switched and tuned operation, C.L Sam et al, SPIE Vol. 247 Advances in Laser Engineering and Applications (1980) Tunable Alexandrite lasers, JOHN C. WALLING et al, IEEE JOURNAL OF QUANTUM ELECTRONICS VOL.QE-16 NO.12, DECEMBER 1980

  By the way, it is known that the alexandrite laser has a high probability of damage to optical components in the resonator. Regarding this cause, Non-Patent Document 1 analyzes that the cause is spatial and temporal energy concentration by multimode. To deal with this problem, Non-Patent Document 2 describes that damage to optical components is reduced by approaching the TEM00 mode. However, since alexandrite has a small stimulated emission cross section at room temperature, a large energy loss occurs when it is limited to the TEM00 mode using an opening or the like. As another attempt, Non-Patent Document 3 describes that oscillation is performed with an energy density sufficiently weaker than a damage threshold of an optical component. However, in order to lower the energy density, the diameter of the alexandrite rod must be increased. Alexandrite has a problem that the manufacturing cost is higher and the cost is higher than YAG (yttrium, aluminum, garnet). In Patent Document 1, no means for reducing damage to optical components in the resonator is taken.

  In view of the above, the present invention provides a laser device using alexandrite as a laser medium, and a laser device capable of suppressing damage to optical components in a resonator with high efficiency and low cost, and a control method thereof. Objective.

  Moreover, this invention provides the photoacoustic measuring device containing the said laser apparatus.

  To achieve the above object, the present invention provides a laser medium including alexandrite, an excitation light source for irradiating the laser medium with excitation light, a resonator including a pair of mirrors provided with the laser medium interposed therebetween, and a resonator A Q switch that changes the Q value of the resonator according to the applied voltage, and applies the Q value of the resonator when the applied voltage is a first voltage corresponding to Q switch off. The voltage is lower than the first voltage, the Q switch is lower than the Q value of the resonator when the second voltage corresponds to the Q switch on, the voltage is applied to the Q switch, and the Q switch is driven. When the applied voltage to the Q switch driving means and the Q switch is the first voltage, the laser medium is irradiated with the excitation light, and after the excitation light is irradiated, the applied voltage to the Q switch is changed from the first voltage to the second voltage. By changing the voltage An oscillation control means for emitting laser light, a photodetector for measuring light output from the resonator, and an adjustment voltage for causing the Q switch to have a Q value of the resonator exceeding a laser oscillation threshold value. OFF to adjust the first voltage based on the intensity of the light output from the resonator caused by the excitation light irradiation, which is measured by the photodetector, by irradiating the laser medium with the excitation light in the applied state. Provided is a laser device comprising voltage adjusting means.

  The off-voltage adjusting means preferably adjusts the first voltage so that the Q value of the optical resonator when the first voltage is applied to the Q switch is equal to or less than the laser oscillation threshold value.

  The adjustment voltage may include a first adjustment voltage that is higher than the reference voltage by ΔV1 and a second adjustment voltage that is higher than the reference voltage by ΔV2. ΔV1 and ΔV2 may be the same value or different.

  In the above, the reference voltage may be the first voltage before adjustment, or may be a voltage determined according to the temperature in the resonator.

  ΔV1 and ΔV2 may be determined depending on the energy of the excitation light applied to the laser medium.

  The off-voltage adjusting means applies the second adjustment voltage to the Q switch and the intensity of the light output from the resonator measured by the photodetector when the first adjustment voltage is applied to the Q switch. The first voltage may be adjusted based on the difference from the intensity of the light output from the resonator measured by the photodetector.

  The off-voltage adjusting unit may periodically adjust the first voltage.

  The off-voltage adjusting means may adjust the first voltage when the temperature change from the temperature at the previous adjustment of the first voltage exceeds a threshold value.

  Alternatively, the off-voltage adjusting means may change the applied voltage to the Q switch to the second voltage after the excitation light is applied to the laser medium with the applied voltage to the Q switch being the first voltage. Before the light detector detects that light is output from the resonator, the first voltage may be adjusted.

  The photodetector may measure scattered light from an optical element outside the resonator.

  Alternatively, the photodetector may measure light divided from the output light of the resonator.

  The light detector may measure light output from a mirror that is not an output mirror among a pair of mirrors constituting the resonator.

  The present invention also includes a laser medium including alexandrite, an excitation light source for irradiating the laser medium with excitation light, a resonator including a pair of mirrors provided across the laser medium, and an optical path of the resonator. A Q switch for changing the Q value of the resonator according to the applied voltage, wherein the Q value of the resonator when the applied voltage is the first voltage corresponding to the Q switch off, the applied voltage is the first A Q switch that is lower than the voltage and lower than the Q value of the resonator at the time of the second voltage corresponding to the Q switch ON, a photodetector that measures light output from the resonator, and a voltage to the Q switch The Q switch driving means for driving the Q switch, and the laser medium is irradiated with the excitation light when the voltage applied to the Q switch is the first voltage, and the voltage applied to the Q switch after the excitation light irradiation From the first voltage to the second Excitation to the laser medium with oscillation control means that emits pulsed laser light by changing the pressure, and an adjustment voltage that causes the Q value of the resonator to exceed the laser oscillation threshold is applied to the Q switch A laser apparatus comprising: an off-voltage adjusting unit that irradiates light and measures a first voltage based on the intensity of light output from the resonator due to irradiation of excitation light, measured by a photodetector And a detection means for detecting a photoacoustic wave generated in the subject after emitting the laser beam toward the subject, and a signal processing means for performing signal processing based on the detected photoacoustic wave An acoustic measurement device is provided.

  In the photoacoustic measurement apparatus of the present invention, the signal processing means may be an image generation means for generating a photoacoustic image based on the detected photoacoustic wave.

  Furthermore, the present invention is a Q switch that is disposed on an optical path of a resonator including a pair of mirrors provided across a laser medium including alexandrite and changes a Q value of the resonator according to an applied voltage, The Q value of the resonator when the applied voltage is the first voltage corresponding to the Q switch off, and the resonator when the applied voltage is lower than the first voltage and the second voltage corresponding to the Q switch on. Applying a voltage for adjusting the Q value of the resonator to a value exceeding the laser oscillation threshold value, irradiating the laser medium with excitation light, Q switch-off comprising measuring laser light output from the resonator due to irradiation, and adjusting a first voltage corresponding to the Q switch-off based on the intensity of the measured laser light Electric To provide an adjustment method.

  The laser apparatus and the control method thereof according to the present invention irradiates a laser medium with excitation light in a state where an adjustment voltage that causes the Q value of the resonator to exceed the laser oscillation threshold is applied to the Q switch. The intensity of the laser beam generated due to the light irradiation is measured, and the first voltage corresponding to the Q switch-off is adjusted based on the measured intensity of the laser beam. Since the birefringence of the Q switch changes as the temperature changes, when the Q switch is turned off by applying the first voltage, the Q value of the resonator can be below the laser oscillation threshold at a certain temperature. However, the Q value of the resonator may exceed the laser oscillation threshold at different temperatures. If the Q value of the resonator exceeds the laser oscillation threshold when the Q switch is turned off, the Q switch may not be turned off, and laser oscillation may occur locally in the resonator when the pump medium is irradiated with excitation light. If the Q switch is turned on in a state in which laser oscillation has occurred locally, the light concentrates on the locally laser-oscillated portion, causing damage to the optical components in the resonator. In the present invention, by adjusting the first voltage, it is possible to maintain the Q switch off even when the temperature changes, and a local laser generated when the Q switch does not completely turn off when the Q switch is off. Oscillation can be suppressed and damage to optical components in the resonator can be suppressed. In the present invention, it is possible to suppress damage to optical components in the resonator without limiting to the TEM00 mode using an aperture or the like, and energy loss can be reduced. In addition, it is not necessary to increase the diameter of the laser medium in order to reduce the energy density, and damage to optical components in the resonator can be suppressed at a low cost.

1 is a block diagram showing a laser device according to an embodiment of the present invention. The graph which shows the relationship between the applied voltage to Q switch, and the intensity | strength of the light radiate | emitted from a resonator. The graph which shows the relationship between the applied voltage to Q switch, and the intensity | strength of the light radiate | emitted from a resonator. The graph which shows the relationship between temperature and Q switch-off voltage. The graph which shows the relationship between the applied voltage to Q switch, and the light intensity radiate | emitted from a resonator. The timing chart which shows the operation waveform of each part. The block diagram which shows the 1st aspect of optical measurement. The block diagram which shows the modification of the 1st aspect of optical measurement. The block diagram which shows the 2nd aspect of optical measurement. The block diagram which shows the modification of the 2nd aspect of optical measurement. The block diagram which shows the 3rd aspect of optical measurement. The block diagram which shows the example which combined the 1st aspect and 2nd aspect of optical measurement. The flowchart which shows the operation | movement procedure of a 1st voltage adjustment. The block diagram which shows the photoacoustic measuring device containing the laser apparatus of one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a laser apparatus according to an embodiment of the present invention. The laser light source unit 13 includes a laser rod 51, a flash lamp 52, mirrors 53 and 54, a Q switch 55, a drive unit 56, an oscillation control unit 57, an off voltage adjustment unit 58, and a photodetector 59. The laser rod 51 is a laser medium. Alexandrite crystal is used for the laser rod 51. The light emitted from the laser rod 51 has a predetermined polarization axis. For example, the light emitted from the laser rod 51 is P-polarized light.

  The flash lamp 52 is an excitation light source and irradiates the laser rod 51 with excitation light. A light source other than the flash lamp 52 may be used as the excitation light source. The mirrors 53 and 54 are opposed to each other with the laser rod 51 interposed therebetween, and the mirrors 53 and 54 constitute a resonator. The optical path in the optical resonator is not necessarily linear, and a prism or the like may be provided on the optical path between the mirrors 53 and 54 to bend the optical axis. The mirror 54 is an output coupler (OC), and laser light is emitted from the mirror 54. A Q switch 55 is inserted in the resonator. The Q switch 55 changes the Q value of the resonator by changing the polarization state of the light passing therethrough according to the applied voltage. The Q switch 55 can be an electro-optic element that changes the polarization state of the light passing therethrough according to the applied voltage. A wavelength selection means may be further inserted in the resonator to form a wavelength tunable laser.

  The Q switch 55 sets the resonator in a low Q state when the applied voltage is the first voltage corresponding to the Q switch off. The low Q state refers to a state where the Q value of the resonator is lower than the laser oscillation threshold value. The first voltage is, for example, a voltage that causes the Q switch 55 to function as a quarter wavelength plate. The Q switch 55 sets the resonator in a high Q state when the applied voltage is the second voltage corresponding to the Q switch being turned on. The high Q state refers to a state where the Q value of the resonator is higher than the laser oscillation threshold value. The second voltage is lower than the first voltage. The second voltage is, for example, 0 V (no voltage applied), and at this time, the polarization state of the light transmitted through the Q switch 55 does not change.

  When the first voltage is applied to the Q switch 55, the Q switch 55 functions as a quarter wavelength plate, and the P-polarized light incident on the Q switch 55 from the laser rod 51 passes through the Q switch 55 and is circular. Polarized light is reflected by the mirror 53 and enters the Q switch 55 in the reverse direction. The circularly polarized light incident on the Q switch 55 in the reverse direction passes through the Q switch 55 to become S polarized light, and returns to the laser rod 51 as S polarized light. The gain of S-polarized light of the laser rod 51 is low and laser oscillation does not occur. On the other hand, when the applied voltage to the Q switch 55 is 0 V (second voltage), the P-polarized light incident on the Q switch 55 from the laser rod 51 is transmitted through the Q switch 55 as P-polarized light and reflected by the mirror 53. To do. The P-polarized light reflected by the mirror 53 passes through the Q switch 55 without changing the polarization state, and returns to the laser rod 51 as P-polarized light. The gain of the P-polarized light of the laser rod 51 is high and laser oscillation occurs.

  The driving unit 56 applies a voltage to the Q switch 55 to drive the Q switch 55. The oscillation control unit 57 controls the voltage applied to the Q switch 55 through the driving unit 56. Further, the lighting of the flash lamp 52 is controlled. The oscillation control unit 57 turns on the flash lamp 52 when the voltage applied to the Q switch 55 is the first voltage, and irradiates the laser rod 51 with excitation light. After excitation of the laser rod 51, for example, at a timing when the inversion distribution density in the alexandrite crystal becomes sufficiently high, the voltage applied to the Q switch 55 is changed from the first voltage to the second voltage. Giant pulses can be obtained by rapidly changing the Q value of the resonator from low Q to high Q.

  Here, the birefringence of the electro-optic element used in the Q switch 55 has temperature dependency, and the voltage that causes the Q switch 55 to function as a quarter-wave plate varies depending on the temperature. The temperature in the resonator may change over time. At one point in time, the Q switch 55 can function as a quarter-wave plate when the first voltage is applied, but at another point the first voltage When Q is applied, the Q switch 55 may not function as a quarter wavelength plate. If the Q switch 55 does not function as a ¼ wavelength plate when the Q switch is off, the Q switch 55 cannot be turned off, that is, the Q value of the resonator cannot be sufficiently lowered. Laser oscillation may occur locally. The present inventors thought that such local oscillation could cause damage to optical components in the resonator. Therefore, in the present embodiment, the first voltage corresponding to the Q switch off is adjusted so that laser oscillation does not occur when the Q switch is off even when the temperature changes.

  The off voltage adjusting means 58 adjusts the first voltage corresponding to the Q switch off. The photodetector 59 measures the light emitted from the resonator. As the photodetector 59, a photodiode, a power monitor, or the like can be used. The off-voltage adjusting means 58 irradiates the laser rod 51 with excitation light in a state where an adjustment voltage that causes the Q switch 55 to make the Q value of the resonator exceed the laser oscillation threshold is applied. Since the Q value of the resonator exceeds the laser oscillation threshold value, laser light is emitted from the output-side mirror 54 due to excitation light irradiation. The off-voltage adjusting unit 58 adjusts the first voltage based on the light intensity detected by the photodetector 59. The off-voltage adjusting unit 58 adjusts the first voltage so that the Q value of the resonator is equal to or lower than the laser oscillation threshold when the first voltage is applied to the Q switch 55. Ideally, the first voltage is adjusted to a voltage that causes the Q switch 55 to act as a quarter-wave plate at the temperature during adjustment.

  FIG. 2 shows the relationship between the voltage applied to the Q switch 55 and the intensity of light emitted from the resonator. When the flash lamp 52 was turned on while changing the voltage applied to the Q switch 55 and the intensity of light emitted from the resonator at each voltage was measured, the result shown in FIG. 2 was obtained. The excitation energy is 12.6J. In this example, the voltage that causes the Q switch 55 to function as a quarter wavelength plate is 2.2 kV. When the applied voltage to the Q switch 55 is in the range of 0.8 kV centered on 2.2 kV (2.2 kV ± 0.4 kV), the Q value of the resonator is sufficiently low and laser oscillation does not occur.

  In the range where the applied voltage to the Q switch 55 is lower than 1.8 kV and higher than 2.6 kV, the Q value of the resonator is not sufficiently low, and laser oscillation occurs and light from the mirror 54 (leakage light). Is emitted. The intensity of light emitted from the mirror 54 increases as the difference from the voltage that causes the Q switch 55 to function as a quarter-wave plate increases. The graph showing the relationship between the applied voltage of the Q switch 55 and the intensity of light emitted from the resonator is symmetrical with respect to the voltage change around 2.2 kV, which is the voltage that causes the Q switch 55 to function as a quarter wavelength plate. It has sex.

  When the temperature of the Q switch 55 changes and the voltage that causes the Q switch 55 to function as a quarter wavelength plate changes from 2.2 kV, the relationship between the applied voltage and the intensity of light emitted from the resonator is The voltage is shifted to the higher side or the lower side by the amount of change in the voltage that acts as a quarter wave plate. When the first voltage applied to the Q switch 55 at the time of excitation deviates from the range of 0.8 kV centered on the voltage that causes the Q switch 55 to function as a quarter wavelength plate, the Q switch 55 is not completely turned off. Undesirable laser oscillation occurs when the Q switch is off.

  FIG. 3 shows the relationship between the voltage applied to the Q switch 55 and the intensity of light emitted from the resonator when the excitation energy is 13.5 J. In FIG. 2, the voltage range in which laser oscillation does not occur is 0.8 kV centered on 2.2 kV, whereas in FIG. 3, the voltage range in which laser oscillation does not occur is about 0.2 kV centered on 2.2 kV. It is 4 kV. Thus, as the excitation energy increases, the voltage range in which laser oscillation does not occur becomes narrower.

FIG. 4 shows the relationship between temperature and Q switch-off voltage. The solid line indicates the voltage that causes the Q switch 55 to act as a quarter wave plate. The voltage that causes the Q switch 55 to function as a quarter-wave plate has a negative slope with respect to temperature changes. The inclination is −35 V / ° C. at a wavelength of 750 nm when the Q switch 55 is a Pockels cell using a KD ^* P crystal. Dashed lines indicate the upper and lower limits of the voltage at which laser oscillation does not occur. If the voltage applied to the Q switch 55 when the flash lamp 52 is lit is in the range between the upper limit and the lower limit, laser oscillation does not occur when the Q switch is off. The difference between the upper limit and the lower limit corresponds to the adjustment margin.

  Here, the adjustment voltage applied to the Q switch 55 when the off-voltage adjustment unit 58 adjusts the first voltage is, for example, a first adjustment voltage that is higher than a reference voltage by a predetermined voltage (ΔV1), and A second adjustment voltage that is lower than the reference voltage by a predetermined voltage (ΔV2) may be included. The reference voltage is, for example, the first voltage before adjustment. The reference voltage may be a voltage determined according to the temperature in the resonator. For example, a reference voltage corresponding to the temperature at the time of adjustment may be acquired from a table by referring to a table that stores the temperature and the reference voltage in association with each other.

  The values of ΔV1 and ΔV2 may be changed according to the energy of the excitation light applied to the laser rod 51. For example, referring to a table that stores the excitation energy and the values of ΔV1 and ΔV2 in association with each other, the values of ΔV1 and ΔV2 corresponding to the excitation energy from the table to the laser rod 51 may be acquired. The values of ΔV1 and ΔV2 may be equal to each other or different from each other. The off-voltage adjusting means 58 is configured such that the light intensity measured by the photodetector 59 when the first adjustment voltage is applied to the Q switch 55 and the second adjustment voltage is applied to the Q switch 55. The first voltage is adjusted based on the difference from the light intensity measured by the photodetector 59 when the light is on.

  FIG. 5 shows the relationship between the voltage applied to the Q switch 55 and the light intensity emitted from the resonator. The first voltage adjustment will be described with reference to FIG. When the voltage applied to the Q switch 55 is in a predetermined range centered on the voltage that causes the Q switch 55 to function as a quarter wavelength plate, laser oscillation does not occur and no light is emitted outside the resonator. A (mJ / V) represents a change (gradient) in light intensity with respect to a voltage change in a range where the applied voltage to the Q switch 55 is higher than a predetermined range centered on a voltage that causes the Q switch 55 to function as a quarter wavelength plate. And Further, the change (slope) of the light intensity with respect to the voltage change in the range where the applied voltage to the Q switch 55 is lower than a predetermined range centered on the voltage that causes the Q switch 55 to function as a quarter wavelength plate is −a (mJ / V). The values of the inclinations a and -a are set in advance according to the excitation energy.

  The first adjustment voltage V1 is a voltage (Voff + α) higher by α than the first voltage (Voff) before adjustment, and the second adjustment voltage V2 is the first voltage (Voff) before adjustment. Is a voltage (Voff−α) lower than α. The magnitude of α is appropriately determined according to the excitation energy of the laser rod 51, the resonator loss, and the like. For example, when the excitation energy is 13.5 J, α is set to 0.6 kV, which is higher than 0.4 kV (see FIG. 3), which is the width of the range in which the Q value of the resonator is equal to or less than the laser oscillation threshold. . The light intensity measured by the photodetector 59 when the flash lamp 52 is lit while the first adjustment voltage V1 is applied to the Q switch 55 is A1, and the second adjustment voltage is applied to the Q switch 55. The light intensity measured by the photodetector 59 when the flash lamp 52 is turned on with V2 applied is A2.

  Light intensity measured by the photodetector 59 when the first adjustment voltage V1 is applied to the Q switch 55, and light detection when the second adjustment voltage V2 is applied to the Q switch 55 The difference from the light intensity measured by the instrument 59 is defined as A1-A2. If A1-A2> 0, the first voltage Voff before adjustment is higher than the voltage that causes the Q switch 55 to function as a quarter-wave plate at the temperature during adjustment. In this case, the off-voltage adjusting unit 58 may adjust the first voltage in a decreasing direction. On the contrary, if A1-A2 <0, the first voltage Voff before adjustment is lower than the voltage that causes the Q switch 55 to function as a quarter-wave plate at the temperature during adjustment. Yes. In this case, the off-voltage adjusting unit 58 may adjust in the direction of increasing the first voltage.

  The voltage that causes the Q switch 55 to act as a quarter-wave plate under the temperature at the time of adjustment is a voltage at which the measured light intensity is the same on the high voltage side and the low voltage side, for example, the measured light intensity is A1 or A2. Can be obtained by calculating the voltage between and the voltage between the two voltages. For example, the voltage at which the measured light intensity is the same intensity A2 as when the second adjustment voltage V2 is applied on the higher voltage side than the voltage that causes the Q switch 55 to function as a quarter wavelength plate is V1- (A1-A2) / a can be calculated from the difference between the inclination and the light intensity. The intermediate voltage between this voltage and the second adjustment voltage V2 is {V1− (A1−A2) / a + V2} / 2 = {Voff + α− (A1−A2) / a + Voff−α} / 2 = Voff− ( A1-A2) / 2a. The voltage obtained in this manner may be the adjusted first voltage.

  FIG. 6 shows an operation waveform of each part. At the time of laser emission, a first voltage, for example, a voltage of 2.2 kV is applied to the Q switch 55 (b). The flash lamp 52 is turned on with a voltage of 2.2 kV applied to the Q switch 55 (a), and then the voltage applied to the Q switch 55 is temporarily lowered to 0 V (b). When the Q value in the resonator changes, Q switch pulse oscillation occurs, and pulse laser light is emitted from the mirror 54 (c).

  When adjusting the first voltage, first, a first adjustment voltage higher than the first voltage, for example, 2.8 kV, is applied to the Q switch 55 (b). The flash lamp 52 is turned on with a voltage of 2.8 kV applied to the Q switch 55 (a). Since the Q value of the resonator exceeds the oscillation threshold value, after the flash lamp 52 is turned on, laser oscillation occurs in the resonator, and long pulse light is emitted from the mirror 54 (c). The intensity of light emitted from the resonator is measured by the photodetector 59.

  Next, a second adjustment voltage lower than the first voltage, for example, 2.2 kV is applied to the Q switch 55 (b), and the flash lamp 52 is applied with a voltage of 2.2 kV applied to the Q switch 55. Is lit (a). Since the Q value of the resonator exceeds the oscillation threshold value, after the flash lamp 52 is turned on, laser oscillation occurs in the resonator, and long pulse light is emitted from the mirror 54 (c). The intensity of light emitted from the resonator is measured by the photodetector 59. The first voltage is adjusted based on the light intensity when the first adjustment voltage is applied and the light intensity when the second adjustment voltage is applied.

  Next, measurement of the intensity of light emitted from the resonator will be described. The photodetector 59 may divide part of the light emitted from the resonator, receive the divided light, and measure its intensity. Or it is good also as receiving the scattered light from the optical element outside a resonator, and measuring the intensity | strength. Furthermore, instead of the output-side mirror 54, the light emitted from the rear-side mirror 53 may be received and the intensity measured.

  FIG. 7 shows a first mode of optical measurement. In this example, a beam splitter 61 is provided on the optical path of the light emitted from the mirror 54. The beam splitter 61 reflects a part of the light emitted from the mirror 54 and transmits the rest. The beam splitter 61 transmits most of the light emitted from the mirror 54 and reflects only a part of the light toward the photodetector 59. The photodetector 59 receives the light reflected by the beam splitter 61 and measures its intensity.

  FIG. 8 shows a modification of the first mode of optical measurement. In this example, a half-wave plate 64 and a polarizer 65 are provided on the optical path of the light emitted from the mirror 54. The half-wave plate 64 is disposed so as to be rotatable around the optical axis. The polarizer 65 transmits linearly polarized light in a specific direction. For the polarizer 65, for example, a polarization beam splitter that transmits linearly polarized light in a specific direction and reflects linearly polarized light in a direction orthogonal thereto can be used. By rotating the half-wave plate 64 around the optical axis, it is possible to change the ratio of the amount of light transmitted and reflected by the subsequent polarizer 65. The photodetector 59 is disposed on the unused optical path and measures the intensity of the light divided by the polarizer 65.

  FIG. 9 shows a second mode of optical measurement. In this example, a shutter 62 is provided on the optical path of the light emitted from the mirror 54. The shutter 62 is inserted on the optical path of the light emitted from the mirror 54 when adjusting the first voltage. Long pulse light emitted from the mirror 54 during the adjustment of the first voltage is blocked by the shutter. The photodetector 59 receives the light scattered by the shutter 62 and measures its intensity.

  The optical element that generates the scattered light is not limited to the shutter, and the photodetector 59 may be configured to receive the scattered light from optical elements other than the shutter 62. FIG. 10 shows a modification of the second mode of optical measurement. In this modification, an optical element 63 such as a mirror, a lens, or a diffusion plate is provided on the optical path of the light emitted from the mirror 54. The optical element 63 is used when, for example, light emitted from the mirror 54 enters a light guide member such as an optical fiber. When light enters the optical element 63, a part of the light is scattered and scattered light is generated. The scattered light may be measured by the photodetector 59.

  FIG. 11 shows a third mode of optical measurement. When laser oscillation occurs in the resonator, most of the light is emitted from the mirror 54 which is an output coupler, but part of the light is also emitted from the rear-side mirror 53. The photodetector 59 receives the light emitted from the rear side mirror 53 and measures its intensity.

  Each aspect of the optical measurement described above can be used in appropriate combination. FIG. 12 shows an example in which the first mode and the second mode are combined. A half-wave plate 64 and a polarizer 65 are provided, and a part of the light emitted from the mirror 54 is divided by the polarizer 65 as in the first embodiment. In FIG. 12, a beam damper 66 for absorbing light is provided at the destination of the light divided by the polarizer 65. The photodetector 59 receives the light scattered by the beam damper 66 and measures its intensity.

  Next, an operation procedure for adjusting the first voltage will be described. FIG. 13 shows an operation procedure for adjusting the first voltage. The off-voltage adjusting unit 58 applies the first adjustment voltage to the Q switch 55 through the driving unit 56 (step S1). The off-voltage adjusting means 58 turns on the flash lamp 52 and irradiates the laser rod 51 with excitation light (step S2). When the laser rod 51 is irradiated with excitation light, laser oscillation occurs in the resonator, and long pulse light is emitted from the output-side mirror 54. The photodetector 59 measures the intensity of light emitted from the mirror 54 (step S3).

  The off-voltage adjusting unit 58 applies the second adjustment voltage to the Q switch 55 through the driving unit 56 (step S4). The off-voltage adjusting means 58 turns on the flash lamp 52 and irradiates the laser rod 51 with excitation light (step S5). When the laser rod 51 is irradiated with excitation light, laser oscillation occurs in the resonator, and long pulse light is emitted from the output-side mirror 54. The photodetector 59 measures the intensity of light emitted from the mirror 54 (step S6).

  The off-voltage adjusting unit 58 adjusts the first voltage based on the light intensity measured in step S3 and the light intensity measured in step 6 (step S7). The off-voltage adjusting unit 58 determines the adjustment amount of the first voltage based on the difference between the light intensity measured in step S3 and the light intensity measured in step 6, for example. The off-voltage adjusting unit 58 sets the adjusted first voltage for the driving unit 56. The driving unit 56 applies the adjusted first voltage to the Q switch 55.

  The off-voltage adjusting unit 58 may periodically adjust the first voltage. The off-voltage adjusting unit 58 periodically adjusts the first voltage every 10 minutes or every 30 minutes, for example. Instead of or in addition to periodically adjusting the first voltage, the temperature in the resonator is monitored with a temperature sensor or the like, and the temperature change from the previous adjustment exceeded the threshold value. Sometimes the first voltage may be adjusted. Alternatively, after the excitation light is applied to the laser rod 51 in a state where the applied voltage to the Q switch 55 is the first voltage, and before the applied voltage to the Q switch 55 is changed to the second voltage, The photodetector 59 may detect whether or not light is emitted from the resonator, and the first voltage may be adjusted when the light is emitted.

  Note that either the first adjustment voltage application or the second adjustment voltage application may be performed first. Further, the application of the first adjustment voltage and the application of the second adjustment voltage are not necessarily performed continuously, and the normal Q is applied after the first adjustment voltage is applied and the long pulse light is emitted. It is also possible to emit pulsed laser light by performing switch pulse oscillation, and then emit long pulsed light by applying a second adjustment voltage.

  Subsequently, a photoacoustic measurement apparatus including the laser apparatus according to an embodiment of the present invention will be described. FIG. 14 shows a photoacoustic measuring apparatus including the laser light source unit 13. The photoacoustic measurement apparatus 10 includes an ultrasonic probe (probe) 11, an ultrasonic unit 12, and a laser light source unit 13. In the embodiment of the present invention, an ultrasonic wave is used as an acoustic wave. However, the ultrasonic wave is not limited to an ultrasonic wave, and is audible as long as an appropriate frequency is selected in accordance with an object to be examined and measurement conditions. An acoustic wave having a frequency may be used.

  The laser light emitted from the laser light source unit 13 is guided to the probe 11 using light guide means such as an optical fiber, and is irradiated from the probe 11 toward the subject. The irradiation position of the laser beam is not particularly limited, and the laser beam may be irradiated from a place other than the probe 11. When the first voltage is adjusted in the laser light source unit 13, for example, a shutter provided between the laser light source unit 13 and the light irradiation unit to the subject is closed, and the Q switch 55 (FIG. 1) is closed. It is preferable to prevent the subject or the like from being irradiated with the long pulse light generated by turning on the flash lamp 52 by applying the adjustment voltage.

  In the subject, ultrasonic waves (acoustic waves) are generated by absorbing the energy of the laser beam irradiated by the light absorber. The probe 11 is acoustic wave detection means, and has, for example, a plurality of ultrasonic transducers arranged one-dimensionally. The probe 11 detects an acoustic wave (photoacoustic wave) from within the subject by using a plurality of one-dimensionally arranged ultrasonic transducers. The probe 11 transmits acoustic waves (ultrasound) to the subject and receives reflected acoustic waves (reflected ultrasound) from the subject with respect to the transmitted ultrasound.

  The ultrasonic unit 12 includes a reception circuit 21, an AD conversion unit 22, a reception memory 23, a data separation unit 24, a photoacoustic image generation unit 25, an ultrasonic image generation unit 26, an image synthesis unit 27, a control unit 28, and transmission control. A circuit 29 is included. The receiving circuit 21 receives a photoacoustic wave detection signal detected by the probe 11. Further, the detection signal of the reflected ultrasonic wave detected by the probe 11 is received. The AD conversion means 22 converts the photoacoustic wave and reflected ultrasonic detection signals received by the receiving circuit 21 into digital signals. The AD conversion means 22 samples the photoacoustic wave and reflected ultrasonic detection signals at a predetermined sampling period based on, for example, a sampling clock signal having a predetermined period. The AD conversion means 22 stores the sampled photoacoustic wave and reflected ultrasonic detection signals (sampling data) in the reception memory 23.

  The data separation unit 24 separates the sampling data of the detection signal of the photoacoustic wave and the sampling data of the detection signal of the reflected ultrasonic wave stored in the reception memory 23. The data separation unit 24 inputs the sampling data of the photoacoustic wave detection signal to the photoacoustic image generation unit 25. The separated reflected ultrasound sampling data is input to the ultrasound image generating means (reflected acoustic wave image generating means) 26.

  The photoacoustic image generation means 25 generates a photoacoustic image based on a photoacoustic wave detection signal detected by the probe 11. The generation of the photoacoustic image includes, for example, image reconstruction such as phase matching addition, detection, logarithmic conversion, and the like. The ultrasonic image generation unit 26 generates an ultrasonic image (reflected acoustic wave image) based on the detection signal of the reflected ultrasonic wave detected by the probe 11. The generation of an ultrasonic image also includes image reconstruction such as phase matching addition, detection, logarithmic conversion, and the like.

  The image synthesizing unit 27 synthesizes the photoacoustic image and the ultrasonic image. The image composition unit 27 performs image composition by superimposing a photoacoustic image and an ultrasonic image, for example. The synthesized image is displayed on image display means 14 such as a display. It is also possible to display the photoacoustic image and the ultrasonic image side by side on the image display means 14 without performing image synthesis, or to switch between the photoacoustic image and the ultrasonic image.

  The control means 28 controls each part in the ultrasonic unit 12. The control means 28 sends a trigger signal to the laser light source unit 13, for example. When the oscillation control means 57 (FIG. 1) of the laser light source unit 13 receives the trigger signal, it turns on the flash lamp 52, and then switches the applied voltage to the Q switch 55 from the first voltage to the second voltage. A pulse laser beam is emitted. The control means 28 sends a sampling trigger signal to the AD conversion means 22 in accordance with the laser light irradiation, and controls the photoacoustic wave sampling start timing.

  When the ultrasonic image is generated, the control unit 28 sends an ultrasonic transmission trigger signal to the transmission control circuit 29 to instruct ultrasonic transmission. When receiving the ultrasonic transmission trigger signal, the transmission control circuit 29 causes the probe 11 to transmit ultrasonic waves. The control means 28 sends a sampling trigger signal to the AD conversion means 22 in synchronization with the timing of ultrasonic transmission, and starts sampling of reflected ultrasonic waves.

  In the present embodiment, when the first voltage is applied to the Q switch 55, the Q switch is turned off, and when the second voltage lower than the first voltage is applied to the Q switch 55, the Q switch is turned on. It corresponds to. In the first voltage adjustment, light that is emitted from the resonator by irradiating the laser rod 51 with excitation light while applying an adjustment voltage to the Q switch 55 in which the Q value of the resonator exceeds the laser oscillation threshold value. The first voltage corresponding to the Q switch off is adjusted based on the measured light intensity. By adjusting the first voltage, even when the voltage that causes the Q switch 55 to function as a quarter-wave plate changes due to a temperature change, the first voltage is laser-oscillated with the Q value of the resonator. The voltage can be kept below the threshold.

  Since the birefringence of the Q switch 55 changes depending on the temperature, when the first voltage is continuously applied to the Q switch 55 in a fixed manner, if the temperature of the Q switch 55 changes with time, the resonance occurs at a certain point. In some cases, the Q value of the device exceeds the laser oscillation threshold, and laser oscillation occurs locally without the Q switch being fully turned off. If the Q switch is switched on while the laser oscillation is occurring locally when the Q switch is off, the light may concentrate on the part that was locally lasing during the Q switch pulse oscillation, possibly damaging the optical components. There is. In the present embodiment, by adjusting the first voltage, it is possible to maintain the Q switch off even when the temperature changes, suppress local laser oscillation when the Q switch is off, Damage to the optical component can be suppressed.

  Here, in Non-Patent Document 2, damage to optical components in the resonator is reduced by limiting to the TEM00 mode using an aperture or the like. However, since alexandrite has a small stimulated emission cross section at room temperature, a large energy loss occurs when it is limited to the TEM00 mode using an opening or the like. On the other hand, in this embodiment, since it is not necessary to provide an opening or the like, energy use efficiency can be improved as compared with Non-Patent Document 2. In Non-Patent Document 3, the damage to the optical components in the resonator is reduced by oscillating at a sufficiently low energy density with respect to the damage threshold of the optical components. However, in order to lower the energy density, the diameter of the alexandrite rod must be increased, resulting in higher costs. Unlike the nonpatent literature 3, this embodiment does not need to make the diameter of the laser rod 51 thick, and can suppress the damage of the optical components in the resonator at a low cost.

  In the above-described embodiment, it has been described that two adjustment voltages are applied at the time of adjusting the first voltage and the intensity of light emitted from the resonator is measured in each case. However, the application of the adjustment voltage is not necessarily 2. It is not necessary to apply it once, and it may be applied at least once. For example, when Q switch pulse oscillation is performed and light is already emitted from the resonator when the Q switch is off, that is, before the voltage applied to the Q switch 55 is switched from the first voltage to the second voltage. If light is emitted from the resonator, the applied voltage to the Q switch 55 at that time is regarded as one of the adjustment voltages, and the adjustment voltage is applied once more to adjust the first voltage. Good.

  When light is already emitted from the resonator when the Q switch is off and the applied voltage (first voltage before adjustment) of the Q switch 55 at that time is regarded as one of the adjustment voltages, the first voltage before adjustment Is higher than the voltage that causes the Q switch 55 to act as a quarter-wave plate at the temperature at the time of adjustment, and the resonator has a lower side than the voltage that causes the Q switch 55 to act as a quarter-wave plate at the temperature during adjustment. A voltage that makes the Q value higher than the laser oscillation threshold may be applied as the adjustment voltage. On the contrary, if the first voltage before adjustment is lower than the voltage that causes the Q switch 55 to act as a quarter wavelength plate under the temperature at the time of adjustment, the Q switch 55 is set to 1/4 at the temperature at the time of adjustment. A voltage that makes the Q value of the resonator higher than the laser oscillation threshold on the side higher than the voltage that acts as the wave plate may be applied as the adjustment voltage. Whether the first voltage before adjustment is higher or lower than the voltage that causes the Q switch 55 to function as a quarter-wave plate under the temperature at the time of adjustment, is the temperature in the resonator changing in the increasing direction, Judgment can be made based on whether or not the direction has changed.

  In the above-described embodiment, the probe 11 has been described as detecting the photoacoustic wave and the reflected ultrasonic wave in the photoacoustic measurement device 10, but the probe used for generating the ultrasonic image and the probe used for generating the photoacoustic image Need not be identical. Photoacoustic waves and reflected ultrasonic waves may be detected by separate probes. Moreover, although the laser apparatus demonstrated the example which comprises a part of photoacoustic measuring device in the said embodiment, it is not limited to this. The laser device of the present invention can also be used for a device different from the photoacoustic measuring device.

  As mentioned above, although this invention was demonstrated based on the suitable embodiment, the laser apparatus of this invention, its control method, and a photoacoustic measuring device are not limited only to the said embodiment, The said embodiment is not limited. Those in which various modifications and changes have been made to the configuration are also included in the scope of the present invention.

10: Photoacoustic measuring device 11: Probe 12: Ultrasonic unit 13: Laser light source unit 14: Image display means 21: Reception circuit 22: AD conversion means 23: Reception memory 24: Data separation means 25: Photoacoustic image generation means 26 : Ultrasonic image generating means 27: Image synthesizing means 28: Control means 29: Transmission control circuit 51: Laser rod 52: Flash lamp 53, 54: Mirror 55: Q switch 56: Driving means 57: Oscillation control means 58: Off voltage Adjustment means 59: photodetector 61: beam splitter 62: shutter 63: optical element 64: half-wave plate 65: polarizer 66: beam damper

Claims (16)

A laser medium containing alexandrite;
An excitation light source for irradiating the laser medium with excitation light;
A resonator including a pair of mirrors provided across the laser medium;
A Q switch disposed on the optical path of the resonator and changing a Q value of the resonator according to an applied voltage, wherein the applied voltage is a first voltage corresponding to Q switch off. A Q switch that lowers the Q value below the Q value of the resonator when the applied voltage is lower than the first voltage and is a second voltage corresponding to Q switch on;
Q switch driving means for applying a voltage to the Q switch and driving the Q switch;
When the voltage applied to the Q switch is the first voltage, the laser medium is irradiated with excitation light, and after the excitation light irradiation, the voltage applied to the Q switch is changed from the first voltage to the second voltage. Oscillation control means for emitting pulsed laser light by changing the voltage to
A photodetector for measuring light output from the resonator;
The laser switch is irradiated with excitation light in a state in which an adjustment voltage for causing the Q switch to exceed the laser oscillation threshold value is applied to the Q switch, and the photodetector measures, A laser apparatus comprising: an off-voltage adjusting unit that adjusts the first voltage based on an intensity of light output from the resonator due to irradiation of excitation light.   The off-voltage adjusting means adjusts the first voltage so that a Q value of the optical resonator when the first voltage is applied to the Q switch is equal to or lower than a laser oscillation threshold value. The laser device according to claim 1.   3. The adjustment voltage according to claim 1, wherein the adjustment voltage includes a first adjustment voltage that is higher by ΔV1 than a reference voltage and a second adjustment voltage that is higher by ΔV2 than the reference voltage. Laser device.   The laser apparatus according to claim 3, wherein the reference voltage is a first voltage before adjustment.   The laser apparatus according to claim 3, wherein the reference voltage is a voltage determined according to a temperature in the resonator.   6. The laser device according to claim 3, wherein the ΔV <b> 1 and the ΔV <b> 2 are determined depending on energy of excitation light irradiated on the laser medium.   The off-voltage adjusting means includes: the intensity of light output from the resonator measured by the photodetector when the first adjustment voltage is applied to the Q switch; and 7. The first voltage is adjusted based on a difference from the intensity of light output from the resonator measured by the photodetector when the adjustment voltage of 2 is applied. 2. The laser device according to item 1.   The laser apparatus according to claim 1, wherein the off-voltage adjusting unit periodically adjusts the first voltage.   The said off voltage adjustment means adjusts the said 1st voltage, if the temperature change from the temperature at the time of the adjustment of the 1st voltage of the last time exceeds a threshold value. Laser equipment.   The off-voltage adjusting means changes the voltage applied to the Q switch to the second voltage after the laser medium is irradiated with the excitation light while the voltage applied to the Q switch is the first voltage. 8. The laser device according to claim 1, wherein the first voltage is adjusted when the photodetector detects that light is output from the resonator before being performed. 9.   The laser device according to claim 1, wherein the photodetector measures scattered light from an optical element outside the resonator.   The laser device according to claim 1, wherein the photodetector measures light divided from output light of the resonator.   11. The laser device according to claim 1, wherein the photodetector measures light output from a mirror that is not an output mirror among a pair of mirrors constituting the resonator. A laser medium containing alexandrite;
An excitation light source for irradiating the laser medium with excitation light;
A resonator including a pair of mirrors provided across the laser medium;
A Q switch disposed on the optical path of the resonator and changing a Q value of the resonator according to an applied voltage, wherein the applied voltage is a first voltage corresponding to Q switch off. A Q switch that lowers the Q value below the Q value of the resonator when the applied voltage is lower than the first voltage and is a second voltage corresponding to Q switch on;
A photodetector for measuring light output from the resonator;
Q switch driving means for applying a voltage to the Q switch and driving the Q switch;
When the voltage applied to the Q switch is the first voltage, the laser medium is irradiated with excitation light, and after the excitation light irradiation, the voltage applied to the Q switch is changed from the first voltage to the second voltage. Oscillation control means for emitting pulsed laser light by changing the voltage to
The laser switch is irradiated with excitation light in a state in which an adjustment voltage for causing the Q switch to exceed the laser oscillation threshold value is applied to the Q switch, and the photodetector measures, A laser device comprising off-voltage adjusting means for adjusting the first voltage based on the intensity of light output from the resonator due to irradiation of excitation light;
Detecting means for detecting a photoacoustic wave generated in the subject after emitting the laser light toward the subject;
A photoacoustic measuring device comprising signal processing means for performing signal processing based on the detected photoacoustic wave.   The photoacoustic measurement device according to claim 14, wherein the signal processing unit is an image generation unit that generates a photoacoustic image based on the detected photoacoustic wave. A Q switch disposed on the optical path of a resonator including a pair of mirrors provided with a laser medium including alexandrite and changing the Q value of the resonator according to an applied voltage, the applied voltage being a Q switch The Q value of the resonator when the first voltage corresponds to OFF is the Q value of the resonator when the applied voltage is lower than the first voltage and the second voltage corresponds to Q switch ON. Applying a voltage for adjustment that causes a Q value of the resonator to exceed a laser oscillation threshold value to a Q switch that is lower than the value;
Irradiating the laser medium with excitation light;
Measuring laser light output from the resonator due to irradiation of the excitation light; and
Adjusting a first voltage corresponding to Q switch-off based on the measured intensity of the laser beam.

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