JP6583894B2 - Q-switch laser device - Google Patents

Description

  The present specification discloses a Q-switched laser device that oscillates pulsed laser light when optically excited. In particular, a Q-switch laser apparatus using a yttrium, aluminum, garnet crystal containing a rare earth element as a laser medium is disclosed. In this specification, yttrium, aluminum, and garnet crystals are referred to as YAG.

  There is known a laser apparatus that utilizes a phenomenon in which laser light is oscillated when excitation light is input to a resonance system in which YAG containing a rare earth element is inserted. A technique for oscillating pulsed laser light by inserting a Q switch in the resonance system is also known. Non-Patent Document 1 discloses a Q-switched laser device using YAG containing a rare earth element as a laser medium.

There is a demand to increase the peak power of pulsed laser light. For that purpose, it is estimated that it is promising to increase the spatial density of the excitation light input to the laser medium. That is, it is expected that the peak power of the pulsed laser beam is increased by collecting the excitation light and inputting it into the laser medium. Actually, in the case of a continuous wave laser device, the power of the laser light is increased by condensing the excitation light.
However, in the case of a Q-switch laser apparatus, even if the degree of concentration of excitation light is increased, only the repetition frequency of the pulse laser light is increased, and the peak power of the pulse laser light is not increased. In order to deal with this problem, the techniques described in Patent Documents 1 and 2 have been developed. This technique does not increase the concentration of the excitation light, but widens the beam diameter of the excitation light input to the laser medium. That is, the total amount of inversion distribution accumulated when the optical switch is opened is increased. Specifically, instead of condensing the excitation light, the beam area of the excitation light is expanded and incident on the laser medium. When the beam diameter is enlarged, the total number of inversion distributions is increased, so that the output energy of the pulse laser beam oscillated from the Q switch laser device, that is, the peak power is increased.

When excitation light is incident on the laser medium, the laser medium is heated by quantum defects. Specifically, the laser medium is locally heated within and near the excitation light irradiation range on the excitation light incident surface. When the laser medium is locally heated, the laser medium is thermally expanded locally, the incident surface of the excitation light is deformed, and adversely affects the oscillation of the laser light (hereinafter referred to as a heat problem).
According to the techniques described in Patent Documents 1 and 2, that is, a technique for expanding and inputting the beam area of excitation light (hereinafter referred to as an excitation volume increasing technique), excitation per unit area on the excitation light incident surface of the laser medium The optical power is reduced, and the amount of heat generated per unit volume of the laser medium is reduced.

When a solid plate transparent to the excitation light is joined to the excitation light incident surface of the laser medium (hereinafter referred to as an end cap technology), deformation of the excitation light incident surface can be suppressed. End cap technology is also advantageous in solving thermal problems.
As excitation light for exciting the Q-switch laser device, continuous light or pulsed light can be used. When pulsed light is used as the pumping light, the time average value of pumping light power incident on the pumping light incident surface of the laser medium is reduced, and the thermal problem can be suppressed. On the other hand, the inversion distribution can be increased because instantaneous strong excitation is possible. The more the excitation is performed with pulse light having a lower repetition frequency, the lower the time average value of the excitation light power, and the characteristics of the Q-switched laser device can be enhanced while strongly suppressing thermal problems (hereinafter referred to as low frequency excitation technology).
In the techniques of Patent Documents 1 and 2, the thermal problem is suppressed by using a combination of an excitation volume increasing technique, an end cap technique, and a low frequency excitation technique.
The disclosure contents of Non-Patent Document 2 and Patent Document 3 will be described later.

High Average Power Diode End-Pumped Composite Nd: YAG Laser Passively Q-switched by Cr4 +: YAG Saturable Absorber, Nicolaie Pavel, Jiro Saikawa, Sunao Kurimura and Takunori Taira, Jpn. J. Appl. Phys. Vol. 40 (2001) pp 1253-1259 Intrinsic reduction of the depolarization loss in solid-state lasers by use of a (110) -cut Y3Al5O12 crystal, Ichiro Shoji and Takunori Taira, Applied Physics letters.Vol. 80 Number 17, (2002) pp 3048-3050

JP 2003-158325 A US Pat. No. 6,950,449 Japanese Patent No. 3585891

  The techniques of Patent Documents 1 and 2 using a combination of an excitation volume increasing technique, an end cap technique, and a low frequency excitation technique are excellent techniques for suppressing the thermal problem and increasing the peak power of the pulsed laser beam. The problem is that the repetition frequency must be lowered (specifically, about 100 Hz).

  For example, there is a demand to measure the distribution of substances in fine particles such as PM2.5, and development of a mass imaging apparatus has been promoted. The analyzer requires pulsed laser light having a repetition frequency of about 1 kHz, for example. In the techniques of Patent Documents 1 and 2, since the low frequency excitation technique is adopted, the above request cannot be met.

  In the techniques of Patent Documents 1 and 2, the repetition frequency of the excitation light was increased to about 1 kHz. Then, it was found that the effect of removing the heat problem is high by the excitation volume increasing technique and the end cap technique, and the peak power does not decrease so much even if the repetition frequency of the excitation light is increased to about 1 kHz. However, it has been found that when laser light is input to a wavelength conversion device and wavelength conversion is performed, the peak power after wavelength conversion is extremely reduced. In order to oscillate the pulsed laser light that is used after wavelength conversion, it is necessary to use a combination of the excitation volume increasing technology, the end cap technology, and the low frequency excitation technology. I found out that When a pulsed laser beam for wavelength conversion is oscillated by a Q switch laser device using YAG, it is necessary to suppress the repetition frequency to about 100 Hz.

  In the present specification, a technique for oscillating a pulse laser beam for wavelength conversion without depending on a low frequency excitation technique is disclosed. If the pulsed laser beam is linearly polarized, the peak power after wavelength conversion is maintained high. In this specification, a technique for oscillating a pulse laser beam linearly polarized from YAG without depending on a low frequency excitation technique is disclosed.

Excluding the low-frequency excitation technology, although the intensity of the pulsed laser beam could be secured, the cause of the deterioration of the linear polarization degree of the pulsed laser beam was studied. Since the strength is ensured, the thermal problem should be suppressed, and the cause of the deterioration of the linear polarization degree was never known. As a result, using the excitation volume increase technology and end cap technology, thermal problems in the region along the optical axis of the laser medium can be suppressed without using the low frequency excitation technology, and the intensity of the pulsed laser light On the other hand, it was confirmed that a local heating region was formed at the joint surface between the laser medium and the end cap, and the degree of linear polarization of the laser beam deteriorated due to the birefringence phenomenon occurring in the local heating region. Specifically, YAG containing a rare earth element tends to generate heat and expand due to quantum defects due to excitation, but YAG itself containing no rare earth element does not generate heat, and stress is generated between the two. The more they are bonded at the atomic level, the stronger the stress at the interface. On the other hand, if the bonding of these interfaces is not strict, the stress of both is relieved. However, in that case, the exhaust heat effect by YAG not containing rare earth elements is reduced.
From the above knowledge, the idea has been obtained that these problems can be solved all at once if there is a new crystal axis that is less susceptible to polarization degree degradation due to locally generated stress even if it is firmly bonded. As a result of research, it was found that if the YAG is arranged in the direction in which the <100> axis extends along the optical axis of the resonance system of the laser device, the above phenomenon can be suppressed and the laser light can be prevented from deteriorating from linearly polarized light. .

The Q-switch laser device disclosed in the present specification includes an end cap, a laser medium, and a Q switch, which are arranged on a straight line in the order. In the Q-switch laser device disclosed in this specification, pulse laser light emitted from the Q switch is input to a wavelength converter, and the wavelength-converted pulse laser light is output. The end cap is made of YAG not containing a rare earth element, and the laser medium is made of YAG containing a rare earth element. The YAG that forms the end cap and the YAG that forms the laser medium are arranged such that the <100> axis extends along the optical axis of the laser device. YAG forming the YAG and the laser medium to form an end cap is engaged against each other.

Usually, YAG is arranged in the direction in which the <111> axis extends along the optical axis of the laser device. This is because YAG grows along the <111> axis, so that rod-shaped crystals extending in the <111> axis direction are easily available. Even in the laser devices disclosed in Non-Patent Document 1, Patent Document 1 and Patent Document 2, YAGs are arranged in the direction in which the <111> axis extends along the optical axis.
<111> A rod-shaped YAG extending in the axial direction (which contains a rare earth and absorbs the excitation light and oscillates the laser light) has an excitation light incident surface on which YAG (the rare earth is contained) is aligned. 1), it is possible to prevent the excitation light incident surface from being deformed by the influence of heat due to the end cap effect. When used in combination with the excitation volume increasing technique, pulse laser light having a high peak power can be oscillated without using a low frequency excitation technique. However, if the low frequency excitation technique is not used together, it will deteriorate from linearly polarized light.
On the other hand, when YAG is arranged in the direction in which the <100> axis extends along the optical axis of the laser device, linearly polarized light is maintained without using a low-frequency excitation technique. The same can be obtained when the YAG is arranged in the direction in which the <110> axis extends along the optical axis of the laser device. According to the laser device described in this specification, pulse laser light having a high peak power while oscillating linearly polarized light is oscillated. Laser light with high peak power can be obtained after wavelength conversion.

  Non-Patent Document 2 and Patent Document 3 disclose a technique in which YAG used for a laser medium is arranged in a direction in which the <100> axis extends along the optical axis of a resonance system. This technique relates to a technique for oscillating continuous wave laser light, and is a technique in the case where the range along the optical axis of YAG used for the laser medium is heated over the entire length of the laser medium. It has been reported that when the range along the optical axis of the laser medium is heated over the entire length, linear polarization is maintained by arranging the <100> axis in a direction extending along the optical axis. . Compared to the technology disclosed in this specification, although the <100> axis and the optical axis are arranged in parallel, the cause of deterioration from linearly polarized light is different, and the process of this technology is simplified. It is not what you do. Countermeasures for phenomena that occur in the range extending along the optical axis of the laser medium, and countermeasures for phenomena that occur on the incident surface of the excitation light (junction interface between the additive-free YAG that is the end cap and the rare-earth element-added YAG that is the laser medium) Are of course different. Non-Patent Document 2 and Patent Document 3 have been reported by the present inventors, but the energy level of the input excitation light differs between the oscillation device for continuous laser light and the oscillation device for pulse laser light, and the laser medium. The degree of heating differs, and the heating area of the laser medium is different. It has not been expected that the continuous laser light oscillation technique reported in Non-Patent Document 2 and Patent Document 3 can also be used for the Q-switch laser light oscillation technique.

  According to the laser device disclosed in this specification, it is possible to oscillate pulsed laser light that provides light having a high degree of linear polarization and high peak power even after wavelength conversion without adopting low-frequency excitation technology. . As a result, it becomes possible to provide pulse laser light (pulse laser light having a high repetition frequency and linearly polarized light) necessary for realizing a next-generation mass imaging apparatus.

The figure which shows typically the structure of the Q switch laser apparatus of an Example. The figure which shows the relationship between the repetition frequency of excitation light, and the energy of a pulse laser beam. The figure which shows the relationship between the repetition frequency of excitation light, and the linear polarization degree of a pulse laser beam. The figure which shows typically the structure of the Q switch laser apparatus of another Example. The figure which shows typically the structure of the Q switch laser apparatus of another Example. The figure which shows typically the structure of the Q switch laser apparatus of another Example. The figure which shows typically the structure of the Q switch laser apparatus of another Example. The figure which shows typically the structure of the Q switch laser apparatus of another Example. The figure which shows typically the structure of the Q switch laser apparatus of another Example.

The features of the embodiments described below are listed first.
(Mode 1) YAG containing Nd is used as a laser medium.
(Mode 2) YAG containing no rare earth element is used for the end cap.
(Mode 3) YAG including Cr ⁴⁺ is used for the Q switch. Operates as a passive Q switch using a saturable absorber.
(Mobile 4)
The end cap, the laser medium, and the Q switch are arranged in a straight line according to the order thereof, the pulse laser light emitted from the Q switch is input to the wavelength converter, and the wavelength converted pulse laser light is input to the mass imaging apparatus. Q-switched laser device that outputs,
The end cap is made of yttrium / aluminum / garnet crystal (YAG) containing no rare earth element,
The laser medium is made of YAG containing a rare earth element,
The <100> axis or the <110> axis of YAG forming the end cap and YAG forming the laser medium extends along the straight line,
YAG forming the end cap and YAG forming the laser medium are joined,
A Q-switched laser device, wherein a pulsed laser beam emitted from the Q-switch ensures a peak power of 2.4 MW when the repetition frequency is 1 kHz.

In FIG. 1, reference numeral 2 is a semiconductor laser device that emits excitation light 4. In the experiment, a pulse current indicated by reference number 2a was applied to obtain pulsed excitation light indicated by reference number 4a. The wavelength of the excitation light 4 was 808 nm, its power was 100 W, and the pulse width was 120 μs. The experiment was performed by changing the repetition frequency between 100 Hz and 1 kHz.
Reference numeral 6 is a YAG that does not contain a rare earth element and is an end cap. It has a disk shape with a diameter of 5 mm and a thickness of 1 mm. YAG6 has its <100> axis oriented in the thickness direction.
Reference numeral 8 is a YAG containing 1.1 at% Nd, which is a laser medium. It has a disk shape with a diameter of 5 mm and a thickness of 4 mm. YAG8 has its <100> axis facing the thickness direction.
YAG6 is joined to the end face of Nd: YAG8 on the semiconductor laser device 2 side. YAG6 containing no rare earth element is transparent to excitation light having a wavelength of 808 nm. The excitation light 4 is input to the end surface of the Nd: YAG 8 on the semiconductor laser device 2 side. Excitation volume increasing technology is used for the excitation light 4.
Reference numeral 10 is YAG containing Cr ⁴⁺ and has an initial transmittance of 40%. Cr ⁴⁺ : YAG10 is a saturable absorber and operates as a passive Q switch.
Reference numeral 12 is an output coupler, on which a mirror film is formed. A mirror film is also formed on YAG6. A resonance system is formed by the mirror film of the output coupler 12 and the mirror film of YAG6. Inside the resonance system, an end cap (YAG6), a laser medium (Nd: YAG8), and a passive Q switch (Cr4 ⁺ : YAG10). ) And are arranged. The end cap, the laser medium, and the passive Q switch are arranged on a straight line orthogonal to the two mirror films. YAG6 and Nd: YAG8 are arranged such that the <100> axis extends along the straight line. Regarding Cr ⁴⁺ : YAG10, the relationship between the crystal axis and the optical axis is not particularly limited. The <111> axis may be parallel to the optical axis, and the <100> or <110> axis may be parallel to the optical axis.

  When the excitation light 4 was input to the laser device, the pulse laser light 14 was oscillated from the output coupler 12. The wavelength of the pulse laser beam 14 is 1064 nm, and the repetition frequency is equal to the repetition frequency of the excitation light 4. The pulse width (half width) of the pulse laser beam 14 was 600 ps.

  FIG. 2 shows the relationship between the repetition frequency of the excitation light (equal to the repetition frequency of the pulse laser light) and the energy (mJ) of the pulse laser light. The energy of the pulse laser beam does not decrease so much even if the repetition frequency of the excitation light is increased. Even if the low-frequency excitation technology was released and excitation was performed at 1 kHz, energy of about 1.42 mJ (2.4 MW at peak power) could be secured.

  FIG. 3 shows the relationship between the repetition frequency of the excitation light and the linear polarization degree of the pulsed laser light 14. The upper part of the vertical axis indicates that the degree of deterioration from linearly polarized light is larger. A curve 32 shows the relationship when the <111> axis and the optical axis are parallel. It shows that the pulse laser beam 14 deteriorates from linearly polarized light when the repetition frequency of the excitation light is increased. On the other hand, the curve 34 shows the relationship when the <100> axis and the optical axis are parallel. This shows that the pulse laser beam 14 maintains linearly polarized light even when the repetition frequency of the excitation light is increased. When the <100> axis and the optical axis are parallel, a pulsed laser beam capable of obtaining light with high intensity after wavelength conversion is obtained. Although not shown, a curve similar to the curve 34 can be obtained even if the <110> axis of the YAG is parallel to the optical axis. Even if the <110> axis of the YAG is parallel to the optical axis, a pulsed laser beam can be obtained that can obtain light with high intensity after wavelength conversion.

In addition, this invention is not limited to the said Example, A various deformation | transformation is possible based on the meaning of this invention, and these are not excluded from the scope of the present invention.
In the above embodiment, the end cap, the laser medium, and the Q switch are arranged in this order. However, the end cap, the laser medium, the end cap, and the Q switch may be arranged in this order. You may arrange | position in the order of an end cap. 4 to 9 schematically show the configuration of a Q-switched laser apparatus according to another embodiment. In this embodiment, the passive Q switch 10 is used, but a Q switch controlled from the outside may be used. Nd is used as the rare earth element added to the laser medium, but other rare earth elements may be provided.
The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology illustrated in the present specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

2: Semiconductor laser device 4: Excitation light 6: End cap (YAG not containing rare earth elements)
8: Laser medium (1.1 at% Nd: YAG)
10: Q switch (Cr ⁴⁺ : YAG)
12: Output coupler 14: Pulsed laser beam

Claims (1)

An end cap, a laser medium, and a Q switch are arranged in a straight line according to the order thereof, and is a Q switch laser device that inputs pulsed laser light emitted from the Q switch to a wavelength converter,
The end cap is made of yttrium / aluminum / garnet crystal (YAG) containing no rare earth element,
The laser medium is made of YAG containing a rare earth element,
The <100> axis or the <110> axis of YAG forming the end cap and YAG forming the laser medium extends along the straight line,
Q-switched laser apparatus characterized by YAG forming the laser medium and YAG forming the end cap is engaged against each other.