Patent Grant
11482831
This application is a U.S. National Stage application of International Patent Application No. PCT/JP2018/031387, filed on Aug. 24, 2018, which claims priority to Japanese Patent Application No. 2017-170416, filed on Sep. 5, 2017, the entire content of all of which is incorporated by reference herein.
The present invention relates to a configuration of a laser device that generates pulsed laser light by passive Q-switching, and relates to a method of driving the laser device.
Oscillation of laser light is caused by introducing excitation light into an optical resonator that is configured such that a laser medium is disposed in the optical path between an incident mirror and an exit mirror. One technique to generate pulsed laser light is Q-switching. Q-switching is a technique to control oscillation by a Q-switch provided inside the optical resonator, and is roughly categorized into active Q-switching and passive Q-switching. In cases of active Q-switching, a Pockels cell or the like serving as the Q-switch needs to be externally controlled actively in order to cause laser light to oscillate. On the other hand, in cases of passive Q-switching, a saturable absorber is used instead of a Pockels cell or the like, and light absorption and light transmission in the saturable absorber are automatically controlled, so that the saturable absorber serves as the Q-switch. As such, passive Q-switching eliminates the need for active control, and thus makes it possible to simplify the device structure, reduce the size of the device, and offer the device for a reasonable price. Thus, resonators which employ passive Q-switching are particularly preferably used in small-size laser devices.
Widely-used small-size laser oscillators (laser devices) employing passive Q-switching are those which include Nd:YAG as a laser medium. In cases where Nd:YAG is used, oscillation wavelength is 1064 nm and laser light emitted by a laser diode can be used as excitation light, and thus the laser device as a whole can be reduced in size easily. In such cases, the times of occurrence of pulses of output laser light can be controlled by controlling the times of occurrence of pulses of excitation light. Such a small-size laser device has been developed mainly for the purpose of igniting an engine of an automobile and the like. In cases where laser light is used to ignite an engine, the laser light preferably has high peak intensity and short pulse width (output pulse width), because such laser light achieves good energy efficiency in engine ignition. A passively Q-switched laser including Nd:YAG as a laser medium is capable of easily generating short pulses on the order of a picosecond (10−12 second), which is shorter than a pulse width of 1 ns (nanosecond: 10−9 second), and thus is capable of providing particularly preferred characteristics when used as a laser for engine ignition. Non-patent Literature 1 discloses an arrangement in which Yb:YAG is used as a laser medium and a laser diode is used as a source of excitation light and thereby laser light with such short pulse widths (output pulse width) is generated. Furthermore, relationships between the output pulse width of light generated using passively Q-switched laser and various parameters are disclosed in, for example, Non-patent Literature 2. As such, conventionally, many passively Q-switched lasers have employed Nd:YAG as a laser medium, which easily achieves high output.
On the other hand, also for purposes other than engine ignition, use of such a small-size laser device makes it possible to reduce the size of an apparatus as a whole. For example, in a blood glucose meter like that disclosed in Patent Literature 1, infrared light (laser light) directly used in measurement is obtained by optical parametric oscillation (OPO). In OPO, by introducing pump light into a non-linear crystal, light having a different wavelength from the pump light is caused to oscillate, and the light obtained through the oscillation is directly used in measurement of blood glucose level. The pump light used here is mid-infrared laser light, and therefore the aforementioned small-size laser device employing passive Q-switching is preferred also in order to generate pump light in such a blood glucose meter.
[Patent Literature 1]
[Non-Patent Literature 1]
When the foregoing small-size laser device employing passive Q-switching is used, different pulse widths are necessary for the case of the foregoing engine ignition and for the case of a blood glucose meter (pump light for OPO). In particular, in cases where mid-infrared light with a wavelength not less than 3 μm is to be caused to oscillate in OPO, the gain is small because of a small optical constant of the non-linear crystal used in OPO, and also the non-linear crystal is damaged by irradiation energy of 10 MW/cm2 or greater. Therefore, in such cases, pump light with a relatively low intensity and a long pulse of not less than 4 ns is used. For example, it is most preferable that the length of the non-linear crystal in the direction of the optical axis of incident/output light is, for example, about 20 mm, and that the pulse width of pump light is, for example, about 10 ns. This pulse width is significantly longer than the pulse width for the case of engine ignition disclosed in Non-Patent Literature 1. That is, the pump light for OPO is required to have a longer output pulse width than that for the cases disclosed in Non-patent Literature 1 and the like.
Non-patent Literature 2 discloses that the output pulse width tp of light generated using passively Q-switched laser and cavity length lc (optical length between the incident mirror and the exit mirror in consideration of refractive index) are in linear relation to each other. Specifically, Non-patent Literature 2 discloses that the output pulse width tp is represented by the following equations (1) to (4):
where: c is the speed of light, To is the initial transmittance of the saturable absorber, δf is the ratio between the final and initial population inversion densities in Q-switching, and δt is the ratio of population inversion density when the photon number is maximum in Q-switching; and R is the reflectivity of the exit mirror of the resonator, Lg is the percentage of round-trip optical loss at the resonator, σSA and σg represent the stimulated-emission cross-sections of the saturable absorber and the laser medium, respectively, ASA and Ag represent effective areas during laser oscillation of the saturable absorber and the laser medium, respectively, σESA is the excited absorption cross section of the saturable absorber, and a is a constant near 1. It is apparent from the equation (1) that, in order to increase the output pulse width tp, it is only necessary to increase the cavity length lc. That is, a passively Q-switched laser in which the cavity length lc has a large value can be used as a source of pump light for OPO.
However, the output pulse width tp depends not only on the cavity length lc but also on other parameters. Note here that the stimulated-emission cross-section σg of the laser medium differs greatly depending on the type of laser medium. In cases where Nd:YAG, which is widely used as the laser medium as described earlier, is used, the stimulated-emission cross-section σg is large; therefore, in order to obtain a large pulse output through Q-switching, it is necessary to have oscillation strongly limited by the saturable absorber, and thus necessary to reduce the initial transmittance T0 of the saturable absorber (equal to or less than 0.3). This works such that the output pulse width tp becomes shorter, and therefore is advantageous for generation of picosecond short pulses for use in engine ignition as described earlier. However, this is disadvantageous when used as the foregoing pump light for OPO.
Another known material that can be used as a laser medium in a passively Q-switched laser device similarly to Nd:YAG is Yb:YAG, which is doped with Yb instead of Nd. In cases of Yb:YAG, the oscillation wavelength is 1030 nm, and laser light emitted by a laser diode can be used as excitation light as with the case of Nd:YAG. Therefore, it is possible to obtain a small-size laser device by using Yb:YAG, as with the case of Nd:YAG. Furthermore, since the stimulated-emission cross-section σg is small when Yb:YAG is used as the laser medium, it is possible to employ a large initial transmittance T0 (equal to or less than 0.7). As such, the output pulse width tp can be made longer more easily than when Nd:YAG is used as the laser medium.
The inventors conducted diligent studies, and found that, in order to configure a small-size passively Q-switched laser device whose laser medium contains Yb such that the output pulse width is long, it is preferable to bring a transverse oscillation mode close to single mode. The present invention was made in view of this object.
A laser device in accordance with the present invention is a laser device including an optical resonator that includes a saturable absorber and a laser medium that is arranged to emit light upon absorption of excitation light, the laser medium being arranged to emit the light upon an input of the excitation light, which is pulsed light, into the optical resonator, the optical resonator being arranged to amplify the light emitted by the laser medium to obtain laser light and output the laser light as output light, the laser medium being doped with ytterbium (Yb), a pulse width of the excitation light, a cavity length of the optical resonator, and the saturable absorber being set such that the excitation light is single-pulsed light and that the output light is composed of a plurality of pulses, the laser device comprising a spatial filter that is disposed in an optical path of the light inside the optical resonator or that is disposed in an optical path of the output light outside the optical resonator, the spatial filter being configured to filter out a portion of the light or of the output light around an optical axis.
The present invention is arranged as described above, and therefore makes it possible, when generating high-energy nanosecond pulses in a small-sized passively Q-switched laser, to configure the laser such that the output pulse is long, and makes it possible to bring a transverse oscillation mode close to single mode.
The following description will discuss a laser device in accordance with Embodiment 1 of the present invention. Laser light generated by this laser device is especially preferably used as pump light for OPO that emits mid-infrared laser light.
The laser device 1 is arranged such that the excitation light 100, which is generated by the laser diode (excitation light source) 110, has a wavelength of 940±1.5 nm, which falls within the absorption wavelength range of Yb:YAG. An optical resonator 20 includes an incident mirror 21 and an exit mirror 22 arranged such that the cavity length is lc, and the excitation light 100 enters the optical resonator 20 via the incident mirror 21. The incident mirror 21 transmits the excitation light 100 but reflects the output light 200, whereas the exit mirror 22 is a half mirror; therefore, the optical resonator 20 is capable of (i) confining laser light that will eventually become the output light 200 between the incident mirror 21 and the exit mirror 22 and amplifying the light and (ii) receiving the excitation light 100 via the incident mirror 21 from left in
In cases where the above laser device 1 is used in a blood glucose meter such as that disclosed in Patent Literature 1, the blood glucose meter as a whole, including the laser device serving as a source of pump light, is particularly required to be small in size. As is clear from the results of study by the inventors shown in
Note that the optical resonator 20 in accordance with Embodiment 1 actually employs a sintered optical element 30, which is an integrated, sintered body made up of the laser medium 31 and the saturable absorber 32. Furthermore, the incident mirror 21 is in the form of a thin film disposed on the surface of the sintered optical element 30 on the left side of
The heat dissipator 87, in cases of air cooling, is preferably in the form of a fin as illustrated in
Furthermore, in cases where the pulse width of the excitation light 100 is long and the duration of a continuous input of the excitation light 100 is long, the output light 200 is outputted a plurality of times during one continuous input of the excitation light 100. The Q-switching action of the saturable absorber 32 in such a case is such that, when excitation levels involved in light absorption are sufficiently occupied by electrons, light transmits and therefore the Q-switch turns on, and thereby the output light 200 is outputted. If light absorption and light transmission are carried out in a similar manner immediately after that, since the excitation levels of the saturable absorber 32 are occupied to some extent, the Q-switch requires only a smaller amount of energy to turn on again, as compared to the energy required for the first output.
For the reason as set forth above, subsequent pulses are easier to be outputted, have higher pulse energy, and have higher efficiency, and, in a case of four outputs, an efficiency of 10% is obtained.
However, when the laser output light 200 that has been actually generated was used as pump light for OPO, the non-linear crystal in OPO was damaged even by low intensity. The cause of this is that, as a mode size within the optical resonator increased for the output pulse width tp to be longer, components other than a component TEM00, which is the fundamental mode in the transverse oscillation mode, became large. That is, the transverse oscillation mode became likely to be multimodal, and, when such multimodal light entered the saturable absorber, the timing of the switching action of the saturable absorber became non-uniform, resulting in generation of components not functioning as pump light for OPO. Such useless components of the pump light do not contribute to the oscillation in OPO, and are absorbed by the non-linear crystal and cause heat generation. This may lead to damage to the non-linear crystal.
Also with a configuration in which the output pulse width tp of the foregoing output light 200 is long and in which the output light 200 is outputted in the form of a plurality of pulses during one continuous input of the excitation light 100, the mode size in the optical resonator becomes large, and the transverse oscillation mode becomes multimodal (the modes other than the component TEM00, which is fundamental mode, make greater contribution). Such situations, in which the transverse oscillation mode becomes multimodal, were especially noticeable when the output light 200 with a pulse width suitable for pump light for OPO, for example, a pulse width of not less than 10 ns, is to be obtained. In order to solve such an issue, it is demanded that, when a laser device is configured such that the output pulse width is long, the transverse oscillation mode be close to single mode.
To this end, the laser device 1 includes, as illustrated in
(a) of
The path 431 is a small hole that allows passage of only the light at and near the optical axis of the output light 200 at the beam waist of the output light 200. The opening 431A is in the form of preferably a perfect circle or a near-perfect circle. The diameter da of the opening 431A is set with reference to the diffraction limit at the beam waist of the output light 200. Specifically, when the diameter da of the opening 431A is slightly greater than the size of the diffraction limit at the beam waist, only the component TEM00 can easily pass through the opening 431A.
The length la of the path 431, for the purpose of allowing passage of only the component TEM00, is preferably shorter than the length (Rayleigh length) of the beam waist. Note, however, that the length la of the path 431 may be any length provided that it is not greater than 1.2 mm, in consideration of processing technique and the durability of the opening 431A.
The tapered portion 432 may be positioned upstream (closer to the laser diode 110) of the path 431 in the optical path of the output light 200 or may be positioned downstream (closer to the exit of the output light 200) of the path 431.
In cases where the tapered portion 432 is positioned upstream of the path 431, the diaphragm 43 is highly effective in filtering out or blocking light from the downstream side. On the other hand, in cases where the tapered portion 432 is positioned downstream of the path 431, the diaphragm 43 is highly effective in spatial filtering of light coming from the upstream side. The tapered portion 432 is a structure that is necessary to make the path 431 shorter than the Rayleigh length while keeping the mechanical strength of the diaphragm 43; however, light reflection at the tapered surface is not zero. Therefore, the tapered portion 432 is usually positioned on the opposite side of the path 431 from the source of light that is to be subjected to filtering (that is, positioned downstream of the path 431), in many cases.
Furthermore, the angle θa between the tapered portion 432 and the optical axis of laser light is preferably not less than 20° and less than 90°. If the angle is θa too small, long-distance light propagation is necessary for convergence of laser light associated with the spatial filtering, resulting in an increase in size of the device. This is not practical. Thus, the angle θa is preferably not less than 20°.
As described earlier, the output light 200 is emitted in a pulsed form. In this regard, the removal of the components other than the component TEM00 results in a change in pulse shape of the output light 200.
In cases where mid-infrared laser light is caused to stably oscillate, there is a threshold for the intensity of pump light. In cases where mid-infrared laser light is caused to oscillate by OPO, pump light with an intensity equal to or greater than the threshold is required. In cases where the output light 200 that has been generated in the same conditions as shown in
As the opening size of the diaphragm 43 decreases, the diaphragm 43 becomes more effective as the spatial filter; however, also a greater proportion of the component TEM00 is blocked at this opening and the intensity of the output light 200 decreases. Therefore, the size of the opening is preferably slightly greater than the diffraction limit size at the beam waist, particularly preferably about 1.0 to 1.4 times the diffraction limit size. Although the spatial filter 40 employed here is arranged such that there is an optical system (first lens 41, second lens 42) which forms a beam waist in the optical path and that the diaphragm 43 is situated at the beam waist, the configuration of the spatial filter 40 may be any configuration, provided that similar effects are obtained. As used herein, the term “diffraction limit size” is intended to mean the diameter of laser light at the beam waist formed by the spatial filter, and the “opening size” of the diaphragm 43 is intended to mean the diameter da of the opening in the diaphragm 43.
In the structure illustrated in
The wavelength at which oscillation occurs depends on the type of laser medium, and a material constituting the saturable absorber is selected in consideration of the type of laser medium and the characteristics of laser light to be caused to oscillate. In the foregoing laser device 1, Cr:YAG is used as the saturable absorber 32; however, some other material can alternatively be used as the saturable absorber. Generally, an optical material doped with Cr (such as Cr:ZnSe) can be used as a material for such a saturable absorber. In cases where the saturable absorber 27 and the laser medium 26 are provided separately as illustrated in
Furthermore, in the arrangement of
As described earlier, when light from the optical resonator shown in
In the above examples, Yb:YAG is used as the laser medium and Cr:YAG or Cr:ZnSe is used as the saturable absorber. Note, however, that in cases where the cavity length is long similarly to the above arrangement, providing the spatial filter like that described above is also effective even in cases where some other laser medium and saturable absorber are used. Furthermore, the output light from this laser device is used in OPO in the above examples; however, also for use for some other purpose, the earlier-described arrangement is also effective, if the pulse width of the output light is preferably broad and the transverse oscillation mode is preferably made monomodal.
As illustrated in
The OPO 311 is a device that includes a non-linear crystal which wavelength-converts laser light coming from the laser device 1, and includes an incident side half mirror 312, an exit side half mirror 314, and a non-linear crystal 313 disposed between the incident side half mirror 312 and the exit side half mirror 314. Output light 200 that passes through the incident side half mirror 312 enters on the non-linear crystal 313, and is converted to light having a wavelength longer than that of the output light 200, and when the light is reflected and confined between the incident side half mirror 312 and exit side half mirror 314, the light is amplified by optically parametric amplification. The amplified light passes through the exit side half mirror 314, becomes wavelength-converted light 201 and is outputted.
As the non-linear crystal 313, AgGaS that is suitable for this kind of wavelength conversion is used under the condition of phase matching. By adjusting the type and matching conditions of the non-linear crystal 313, it is possible to adjust the wavelength (oscillation wavelength) of the emitted wavelength-converted light 201. As the non-linear crystal, it is also possible to use GaSe, ZnGeP2, CdSiP2, LiInS2, LiGaSe2, LiInSe2, LiGaTe2 and the like. The wavelength-converted light 201 that is emitted from the OPO 311 has a repetition frequency and a pulse width that correspond to the output light 200.
The optical system 315 is a member that outputs the light that has been wavelength-converted by the OPO 311. The optical system 315 may include a condenser lens and/or a beam splitter. For example, by including a condenser lens as the optical system 315, it is possible to reduce the beam spot size.
The light detector 330 receives the reflected light 201A from the to-be-measured object 320, and outputs the intensity of the reflected light 201A as an electrical signal.
Note that, although Embodiment 2 deals with an example in which light reflected at the to-be-measured object 320 is detected by the light detector 330, the measurement apparatus may alternatively be arranged such that light that has passed through the to-be-measured object 320 is detected by the light detector 330.
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