JP2023177027A - Laser oscillator and laser oscillation method - Google Patents

Abstract

The present invention provides a laser oscillator that emits fluorescence with a broad spectral bandwidth in the mid-infrared region and oscillates signal light at a specific wavelength in the spectral bandwidth. [Solution] Equipped with a crystal doped with a dopant and having second-order optical nonlinearity, and an optical system that causes the crystal into which excitation light is incident to laser oscillate, the crystal has birefringent phase matching, periodic polarization inversion, It also has either quasi phase matching using a structure or random phase matching using a polycrystalline structure, and excitation light excites fluorescence with a broad spectral bandwidth in the mid-infrared region. The present invention provides a laser oscillator that outputs idler light of a difference frequency between excitation light and signal light lased at a specific wavelength or a specific wavelength width in a spectral bandwidth by an optical system due to phase matching. [Selection diagram] Figure 1

Description

The present invention relates to a laser oscillator and a laser oscillation method.

Non-Patent Document 1 states, "We exploited Nd ³⁺ laser emission at 1061.9 nm ... in a self-x ⁽²⁾ active GdAl ₃ (BO ₃ ) ₄ :Nd ³⁺ laser crystal."
[Prior art documents]
[Non-patent literature]
[Non-patent Document 1] A. Brenier, C. Tu, J. Li, Z. Zhu, and B. Wu, "Self-sum- and -differencefrequency mixing in GdAl ₃ (BO ₃ ) ₄ :Nd ³⁺ for generation of tunable ultraviolet and infrared radiation," Opt. Lett. 27, 240-242 (2002)

In a first aspect of the invention, a laser oscillator is provided. The laser oscillator includes a crystal doped with a dopant and having second-order optical nonlinearity, and an optical system that causes the crystal into which excitation light is incident to oscillate a laser, and the crystal has birefringent phase matching, periodicity, and It further has phase matching of either quasi phase matching using a polarization inversion structure or random phase matching using a polycrystalline structure, and the excitation light produces a broad spectral bandwidth in the mid-infrared region. is excited by the fluorescence of do.

In the above laser oscillator, the spectral bandwidth may be 0.1 μm or more.

In any of the above laser oscillators, the dopant may be a transition metal ion, and the excitation light may cause vibrational transition of the transition metal ion to excite the fluorescence.

In any of the above laser oscillators, the dopant may be a rare earth ion selected from Yb, Tm, Er, and Ho.

In any of the above laser oscillators, the crystal may be CdSe _1-x S _x doped with Cr ²⁺ or Fe ²⁺ (x=0.0 to 0.4).

In any of the above laser oscillators, the crystal may be ZnSe _1-x S _x (where x=0.0 to 1.0) doped with Cr ²⁺ or Fe ²⁺ as a dopant.

In any of the above laser oscillators, the crystal may be oriented to satisfy a non-critical phase matching condition with respect to changes in the beam propagation direction.

In any of the above laser oscillators, the crystal is arranged in a direction that satisfies a non-critical phase matching condition with respect to a change in the wavelength of at least one of the excitation light, the signal light, and the idler light. may have been done.

In any of the above laser oscillators, the crystal may be oriented to satisfy a non-critical phase matching condition with respect to a change in temperature of the crystal.

In any of the above laser oscillators, the optical system may include a first mirror and a second mirror that are diagonally opposed to each other. In any of the above laser oscillators, the optical system is arranged on the optical path of the light reflected by the second mirror, and when the light reflected by the second mirror is incident, the optical system directs the light to the second mirror. It may have a third mirror that reflects the light toward the second mirror. In any of the above laser oscillators, the optical system is arranged on the optical path of the light reflected by the first mirror, and when the light reflected by the first mirror is incident, the optical system The optical device may include an optical device that selects the signal light having the specific wavelength or the specific wavelength width.

In any of the above laser oscillators, the optical device may include any one of a diffraction grating, an acousto-optic wavelength tunable filter, a birefringence filter, and a prism.

Any of the above laser oscillators may further include a wavelength meter that measures the wavelength of the light that passes through the third mirror. Any of the above laser oscillators may further include a controller that selects the specific wavelength or the specific wavelength width of the signal light by controlling the optical device based on the measurement value of the wavelength meter. good.

In a second aspect of the invention, a laser oscillation method is provided. The laser oscillation method involves emitting fluorescence with a broad spectral bandwidth in the mid-infrared region by irradiating excitation light onto a crystal doped with a dopant and having second-order optical nonlinearity; Laser oscillation of signal light at a specific wavelength or the specific wavelength width, birefringent phase matching, quasi phase matching using a periodically poled structure, and a polycrystalline structure possessed by the crystal. outputting idler light having a difference frequency between the excitation light and the signal light by utilizing the phase matching of the random phase matching used.

In the above laser oscillation method, the crystal may be CdSe _1-x S _x doped with Cr ²⁺ or Fe ²⁺ (where x=0.0 to 0.4).

In any of the above laser oscillation methods, the crystal may be ZnSe _1-x S _x doped with Cr ²⁺ or Fe ²⁺ (where x=0.0 to 1.0).

Any of the above laser oscillation methods may further include selecting a wavelength range of the idler light by selecting a plurality of the crystals having mutually different values of x.

Any of the above laser oscillation methods may further include arranging the crystal in an orientation that satisfies a non-critical phase matching condition with respect to changes in the beam propagation direction.

In any of the above laser oscillation methods, the crystal is oriented in a direction that satisfies a non-critical phase matching condition with respect to a change in wavelength of at least one of the excitation light, the signal light, and the idler light. The method may further include arranging.

Any of the above laser oscillation methods may further include arranging the crystal in an orientation that satisfies a non-critical phase matching condition with respect to a change in temperature of the crystal.

Note that the above summary of the invention does not list all the features of the invention. Furthermore, subcombinations of these features may also constitute inventions.

1 is a schematic diagram of a lasing system 10 according to one embodiment. FIG. FIG. 2 is a schematic diagram of an oscillator that performs only laser oscillation, according to a comparative example. FIG. 3 is a schematic diagram of a difference frequency generator that only generates a difference frequency according to a comparative example. It is a graph showing an example of theoretical phase matching characteristics of the laser oscillator 100 according to one embodiment. It is a graph showing an example of theoretical phase matching characteristics of the laser oscillator 100 according to one embodiment. It is a graph showing an example of theoretical phase matching characteristics of the laser oscillator 100 according to one embodiment. It is a graph showing an example of output characteristics of signal light and idler light actually measured by the laser oscillator 100 according to one embodiment. It is a graph showing an example of output characteristics of pump light and idler light actually measured by the laser oscillator 100 according to one embodiment. It is a graph showing an example of output characteristics of signal light and idler light actually measured by the laser oscillator 100 according to one embodiment. It is a graph showing an example of time waveforms of excitation light, signal light, and idler light actually measured by the laser oscillator 100 according to one embodiment. 7 is a graph for explaining the relationship between the sulfide (S) concentration x, the wavelength λ _s of the signal light, and the wavelength λ _i of the idler light in the laser oscillator 100 according to one embodiment.

Hereinafter, the present invention will be described through embodiments of the invention, but the following embodiments do not limit the invention according to the claims. Furthermore, not all combinations of features described in the embodiments are essential to the solution of the invention.

FIG. 1 is a schematic diagram of a lasing system 10 according to one embodiment. In FIG. 1, the optical path is shown by a solid line, and small arrows are shown on the optical path where light travels in only one direction. Further, in FIG. 1, the direction of signal flow is indicated by a large arrow. Further, in FIG. 1, the crystal axis orientation of the crystal 140 is indicated by the c-axis, and the beam propagation direction is indicated by the a-axis. Note that when using another material as the crystal 140, the relationship between the c-axis and the a-axis may be other than that shown in FIG. 1.

The laser oscillation system 10 according to this embodiment oscillates signal light and generates idler light at a difference frequency using a crystal doped with a dopant and having second-order optical nonlinearity. In other words, in the laser oscillation system 10, such a crystal has a function not only as a laser oscillator but also as a difference frequency generator. Specifically, the laser oscillation system 10 irradiates such a crystal with excitation light to oscillate signal light, converts the wavelength of the excitation light and signal light with the crystal, and converts the excitation light and signal light into wavelengths longer than the excitation light and signal light. Outputs idler light of the same wavelength.

The laser oscillation system 10 includes a laser oscillator 100, an energy meter 201, an energy meter 203, an oscilloscope 205, and a user interface 207. In FIG. 1, a laser oscillator 100 is shown as a dashed area.

The laser oscillator 100 includes a crystal 140, which is an example of the crystal described above, and an optical system 130. The laser oscillator 100 according to this embodiment may further include an excitation light source 110 and a half-wave plate 120 in order to input excitation light into the optical system 130. The laser oscillator 100 according to this embodiment may further include a half mirror 150, a wavelength meter 160, and a filter 170 into which the light from the optical system 130 is input. The laser oscillator 100 according to this embodiment may further include a controller 180 and a drive unit 190 to select the wavelength of the signal light to be oscillated by the optical system 130. In FIG. 1, an optical system 130 is shown as a dotted line area inside the laser oscillator 100, which is shown as a dotted line area.

The excitation light source 110 outputs pulsed excitation light. The excitation light source 110 may be a laser light source such as a solid-state laser, a semiconductor laser, or a gas laser. For the excitation light source 110, a crystal material doped with rare earth ions, such as Tm:YAG, Nd:YAG, Yb:YAG, Ho:YLF, may be used. As an example, the excitation light source 110 may be a Tm:YAG laser that outputs excitation light with a wavelength of 2.01 μm, a pulse width of 300 ns, a pulse repetition rate of 10 Hz, and a pulse energy of 0 to 16 mJ. In the following description, the wavelength of excitation light may be expressed as λ _p , and similarly, the wavelengths of signal light and idler light may be expressed as λ _s and λ _i , respectively.

The half-wave plate 120 changes the polarization state of the excitation light output from the excitation light source 110 and makes it enter the optical system 130 . As an example, the half-wave plate 120 may convert the excitation light into linearly polarized light that is polarized in the depth direction of the paper in FIG. 1 and input it to the optical system 130. In FIG. 1, the polarization direction of the excitation light is indicated by ◎.

Crystal 140 has second-order optical nonlinearity doped with a dopant, as described above. Crystal 140 also has birefringent phase matching.

The crystal 140 emits fluorescence with a broad spectral bandwidth in the mid-infrared region when excited by the excitation light. The crystal 140 further outputs idler light having a difference frequency between the excitation light and the signal light due to birefringence phase matching.

As a specific example, the above-mentioned dopant may be a transition metal ion, such as Cr ²⁺ or Fe ²⁺ . In this case, the crystal 140 emits fluorescence due to vibrational transition, that is, vibronic transition, of the transition metal ion caused by the excitation light. The dopant concentration in the crystal 140 is, for example, 40 ppm. Other examples of dopants in crystal 140 may be rare earth ions that have a broad spectrum in the mid-infrared region. Specifically, the dopant may be any rare earth ion of Yb, Tm, Er, and Ho.

The spectral bandwidth of the fluorescence emitted by the crystal 140 by the excitation light is 0.1 μm or more.

The host material to which a dopant is added in the crystal 140 may be, for example, CdSe or ZnSe. The crystal 140 according to the present embodiment is CdSe _1-x S _x (where x=0.0 to 0.4) doped with Cr ²⁺ or Fe ²⁺ as a dopant; for example, Cr:CdSe (the dopant is Cr ²⁺ and x=0.0). For example, the crystal 140 is irradiated with excitation light having a wavelength of 2.01 μm under a temperature condition of 20° C., thereby exciting fluorescence in the vicinity of 2.2 to 2.7 μm.

For example, the crystal 140 is coated with anti-reflection (AR) coating for excitation light and signal light. The AR coating may have an average reflectance of less than 1.5% for light with a wavelength of 1.9 to 3.3 μm, for example.

The optical system 130 causes the crystal 140, into which the excitation light is incident, to oscillate as a laser. The optical system 130 causes the crystal 140 to laser-oscillate signal light at a specific wavelength or a specific wavelength width in the spectral bandwidth of fluorescence emitted by the excitation light. Note that the specific wavelength or specific wavelength width referred to here is, for example, a design wavelength, and may have a wavelength width according to a Gaussian distribution. In the following description, a specific wavelength or a specific wavelength width may be simply referred to as a specific wavelength.

The optical system 130 according to this embodiment includes a first mirror 131, a second mirror 133, a Brewster plate 135, a third mirror 137, and a diffraction grating 139. As shown in FIG. 1, the optical system 130 includes a plurality of optical elements arranged in an "X" shape. Note that in the optical system 130, arranging the plurality of optical elements in an "X" shape is merely an example, and other arrangement configurations of the plurality of optical elements may be adopted. In this case, some of these optical elements may be omitted, and other optical elements may be added in addition to or in place of them.

The first mirror 131 and the second mirror 133 are arranged obliquely facing each other, and a crystal 140 is arranged between them. The first mirror 131 and the second mirror 133 are made of ZnSe, for example.

The first mirror 131 and the second mirror 133 have a high reflectivity for the spectral bandwidth of the fluorescence emitted by the crystal 140 by the excitation light. Excitation light enters the first mirror 131 from the opposite side to the second mirror 133 side. For example, the first mirror 131 transmits 90% of the excitation light output from the excitation light source 110.

The idler light output from the crystal 140 enters the second mirror 133 from the first mirror 131 side. The second mirror 133 transmits the idler light to the side opposite to the first mirror 131 side. As an example, the second mirror 133 transmits 73% of the idler light output from the crystal 140. Note that the second mirror 133 may transmit part of the excitation light and the signal light together with the idler light.

Brewster plate 135 is arranged on the optical path between second mirror 133 and third mirror 137. Brewster plate 135 is made of ZnSe, for example. Here, depending on the host material of the crystal 140, the crystal 140 having birefringence phase matching described above may have the polarization directions of two input lights the same or orthogonal to each other for generating idler light with a difference frequency. It is necessary to do so. When the host material of the crystal 140 is CdSe, the polarization directions of the two input lights, that is, the excitation light and the signal light, must be orthogonal to each other. Therefore, the Brewster plate 135 is configured to polarize the light incident on the Brewster plate 135 in a direction perpendicular to the polarization direction of the excitation light and transmit the polarized light. Note that Brewster plate 135 may not be disposed within optical system 130 depending on the host material of crystal 140.

The third mirror 137 is placed on the optical path of the light reflected by the second mirror 133. When the light reflected by the second mirror 133 is incident, the third mirror 137 reflects the light toward the second mirror 133 . The third mirror 137 has a reflectance of 98%, for example. The light that enters the third mirror 137 from the second mirror 133 via the Brewster plate 135, for example, the above-mentioned signal light with the wavelength λ _s , is linearly polarized light that is polarized in a direction parallel to the plane of the paper in FIG. It is. In FIG. 1, the polarization direction of signal light is indicated by an arrow.

The diffraction grating 139 is placed on the optical path of the light reflected by the first mirror 131. When the light reflected by the first mirror 131 is incident on the diffraction grating 139, the diffraction grating 139 diffracts only the signal light having the above-mentioned specific wavelength toward the first mirror 131. Light with wavelengths other than the specific wavelength is not diffracted toward the first mirror 131. In other words, the diffraction grating 139 selects the wavelength of light to be round-tripped between the first mirror 131, the second mirror 133, the third mirror 137, and the diffraction grating 139 in the optical system 130. In other words, the diffraction grating 139 causes only light of a specific wavelength to round-trip in the optical system 130. As an example, the diffraction grating 139 has a groove count of 600/mm and a diffraction efficiency η>90%. Note that the diffraction grating 139 is arranged on the optical path of the light reflected by the first mirror 131, and when the light reflected by the first mirror 131 is incident, the diffraction grating 139 is an optical element that selects the signal light having the above-mentioned specific wavelength. This is an example of a device. The optical device may include an acousto-optic wavelength tunable filter, a birefringence filter, a prism, etc. instead of or in addition to the diffraction grating 139.

Half mirror 150 reflects part of the light that passes through third mirror 137 toward wavelength meter 160 and transmits the remaining light to enter energy meter 201 . Wavemeter 160 measures the wavelength of light that passes through third mirror 137. More specifically, the wavelength meter 160 measures the wavelength λ _s of the light transmitted through the third mirror 137 and reflected by the half mirror 150, for example, the signal light described above. Wavemeter 160 outputs the measured value to controller 180.

The filter 170 is placed on the optical path of the light that passes through the second mirror 133. Among the lights transmitted through the second mirror 133, the filter 170 filters the excitation light with a wavelength λ _p and the signal light with a wavelength λ _s while transmitting the idler light with a wavelength λ _i . The filter 170 is, for example, a plate made of Ge and coated with an AR coating for idler light. The AR coating may have an average reflectance of less than 0.75% for light with a wavelength of 8 to 10 μm, for example. In FIG. 1, the polarization direction of the idler light transmitted through the filter 170 is indicated by ◎. Note that in place of the filter 170, an optical element capable of wavelength separation, such as a mirror, a prism, or a diffraction grating, may be used.

The controller 180 controls the diffraction grating 139 based on the measurement value of the wavelength meter 160 to select the wavelength of the fluorescence to be round-tripped in the optical system 130, that is, the wavelength of the signal light to be excited in the crystal 140. More specifically, the controller 180 may select the wavelength of the signal light by controlling the drive unit 190 and adjusting the rotation angle of the diffraction grating 139 based on the measurement value of the wavelength meter 160.

The drive unit 190 rotates the diffraction grating 139 under the control of the controller 180. For example, the drive unit 190 may be able to be driven to rotate around an axis extending in the depth direction of the paper in FIG. 1 with the diffraction grating 139 mounted thereon. In FIG. 1, an example of the rotation direction of the drive unit 190 is shown by a black arrow.

The energy meter 201 receives light, for example, signal light, that has passed through the half mirror 150, and detects its intensity. Energy meter 203 receives the idler light that has passed through filter 170 and detects its intensity. Each of the energy meters 201 and 203 is electrically connected to an oscilloscope 205, and outputs a voltage value to the oscilloscope 205 according to the intensity of the detected light. The oscilloscope 205 displays on the screen temporal changes in the voltage values input from each of the energy meters 201 and 203. The oscilloscope 205 is also electrically connected to the user interface 207 and outputs to the user interface 207 an analog waveform indicating a change in the voltage value over time.

The user interface 207 may display the analog waveform input from the oscilloscope 205 on the screen. User interface 207 may be electrically connected to controller 180. The user interface 207 may instruct the controller 180 on the wavelength of the idler light output from the laser oscillator 100 based on user input.

In the laser oscillator 100 according to the present embodiment described above, the controller 180 provides a function or a correspondence table that indicates the correspondence between the rotation angle of the diffraction grating 139 and the wavelength of light round-tripped in the optical system 130 by the diffraction grating 139. Reference data such as the following may be stored in advance and the driving unit 190 may be controlled based on the reference data. In this case, the laser oscillator 100 may not include the wavelength meter 160 and the half mirror 150, and the reflectance of the third mirror 137 may be set to 100%.

FIG. 2 is a schematic diagram of an oscillator that performs only laser oscillation, according to a comparative example. In the oscillator according to the comparative example, excitation light with wavelength λ _p is transmitted through mirror 301 and is incident on crystal 303, and fluorescence excited in crystal 303 by the excitation light travels back and forth between mirror 305 and mirror 301 repeatedly. As a result, a signal light having a wavelength λ _s is oscillated from the mirror 305 . As an example, the crystal 303 is Cr:CdSe and the wavelength λ _p is ~2 μm, in which case the wavelength λ _s is 2.2-2.7 μm.

FIG. 3 is a schematic diagram of a difference frequency generator that only generates a difference frequency according to a comparative example. In the difference frequency generator according to the comparative example, when the excitation light with the wavelength λ _p and the signal light with the wavelength λ _s are input to the crystal 311, the idler light with the wavelength λ _i becomes the difference frequency due to the phase matching of the crystal 311. generated. The angular frequency ω _i of the difference frequency idler light is defined by the following equation 1 using the angular frequency ω _p of the excitation light and the angular frequency ω _s of the signal light.
[Number 1]

数２の定義に従い、位相整合条件（Δｋ）は、アイドラ光の波数ベクトルｋ_ｉ、シグナル光の波数ベクトルｋ_ｓ、励起光の波数ベクトルｋ_ｐを用いて下記の数３で定義される。
［数３］

ここで、波数ベクトルと屈折率の関係は下記の数４、数５および数６で定義される。
［数４］

［数５］

［数６］

Therefore, the wavelength λ _i of the idler light of the difference frequency is defined by the following equation 2 using the wavelength λ _p of the excitation light and the wavelength λ _s of the signal light.
[Number 2]

According to the definition of Equation 2, the phase matching condition (Δk) is defined by Equation 3 below using wave number vector k _i of idler light, wave number vector k _s of signal light, and wave number vector k _p of excitation light.
[Number 3]

Here, the relationship between the wave number vector and the refractive index is defined by Equation 4, Equation 5, and Equation 6 below.
[Number 4]

[Number 5]

[Number 6]

As an example, the crystal 311 is CdSe, the wavelength λ _p is ~2 μm, the wavelength λ _s is 2.5 ~ 2.7 μm, and the wavelength λ _i in this case is 8~2 μm based on the above equation 2. It becomes 10 μm.

The inventor of the present application discovered the property that a difference frequency can be generated even if the crystal used for generating a difference frequency is doped with a dopant. We found that it is possible to do both. That is, compared to the comparative examples shown in FIGS. 2 and 3, the laser oscillator 100 according to the present embodiment not only functions as a laser oscillator but also functions as a difference frequency generator. Specifically, the laser oscillator 100 irradiates the crystal 140 with excitation light to oscillate the signal light, and also oscillates the difference frequency between the excitation light and the signal light under the condition that the birefringence phase matching of the crystal 140 is satisfied. Can output idler light.

FIG. 4 is a graph showing an example of theoretical phase matching characteristics of the laser oscillator 100 according to one embodiment. The horizontal axis of the graph in FIG. 4 indicates the phase matching angle θ [degrees], and the vertical axis indicates the wavelengths λ _s and λ _i [μm].

［数８］

［数９］

［数１０］

According to the laser oscillator 100 of the present embodiment, calculation is theoretically made from the Sellmeyer dispersion formula for the ordinary ray (subscript o) and extraordinary ray (subscript e) of CdSe shown in Equations 7 to 8 below. By using the refractive index n, the phase matching characteristics shown in the graph of FIG. 4 can be calculated from Equations 3 to 6 above. In the case of CdSe, the polarization directions are selected so that the idler light is an ordinary ray, the signal light is an extraordinary ray, and the excitation light is an ordinary ray. In other words, the polarization directions of the idler light and the signal light are orthogonal to each other. A certain Type-2 phase matching is used. From Equations 3 to 6 above, the Type-2 phase matching condition in CdSe is expressed by Equation 10 below. Here, the superscripts o and e of the refractive index n indicate ordinary rays and extraordinary rays, respectively. The refractive index of each of the ordinary ray and the extraordinary ray can be accurately calculated in the wavelength range of the following equation 9 by using the following equations 7 and 8.
[Number 7]

[Number 8]

[Number 9]

[Number 10]

In FIG. 4, a graph of the change in the wavelength λ _s of the signal light when the wavelength λ _p of the excitation light is fixed at 2.015 μm is shown at the bottom of the paper, and a graph of the change in the wavelength λ _i of the idler light is shown at the bottom of the paper. Shown at the top of the paper. As an example, when the wavelength λ _s of the signal light is the length of the position indicated by a circle, the wavelength λ _i of the idler light is the length of the position indicated by a circle.

As shown in FIG. 4, it is understood that for the phase matching angle θ from 90 degrees to around 64 degrees, the wavelength λ _s of the signal light and the wavelength λ _i of the idler light can each take two different values. . Specifically, when the phase matching angle θ is continuously changed from 90 degrees to around 64 degrees, the wavelength λ _s of the signal light changes continuously from 2.595 μm to around 2.300 μm, and the idler light The wavelength λ _i of the signal light continuously changes from around 9.015 μm to around 16.00 μm, or the wavelength λ _s of the signal light changes continuously from around 2.175 μm to around 2.300 μm, and the wavelength of the idler light changes continuously from around 2.175 μm to around 2.300 μm. The wavelength λ _i continuously changes from around 27.00 μm to 9.015 μm.

That is, according to the laser oscillator 100 according to the present embodiment, the wavelength λ _s of the signal light can be changed from 2.595 μm to 2.5 μm by theoretically changing the phase matching angle θ within a range from 90 degrees to around 64 degrees. The wavelength λ _i of the idler light can be varied within a range from 9.015 μm to around 27.00 μm.

According to the laser oscillator 100 of this embodiment, for example, when the center wavelength λ _p of the excitation light is 2.015 μm and the center wavelength λ _s of the signal light measured by the wavelength meter 160 is 2.595 μm, Based on the above equation 2, it can be theoretically determined that the center wavelength λ _i of the idler light is 9.015 μm. Further, from the graph of FIG. 4 or Equation 3, it can be determined that the phase matching angle θ in this case is theoretically 90 degrees.

FIG. 5 is a graph showing an example of theoretical phase matching characteristics of the laser oscillator 100 according to one embodiment. The horizontal axis of the graph in FIG. 5 indicates the phase matching angle θ [degrees], and the vertical axis indicates the wavelength λ _s [μm] of the signal light. The right side of the graph in FIG. 5 shows the relationship between the shading of the line shown in the graph and the wavelength λ _p [μm] of the excitation light.

In the graph of FIG. 5, a circle is shown at a position corresponding to the circle shown on the graph of the wavelength λ _s of the signal light in FIG. Comparing FIGS. 4 and 5, the graph of the change in the wavelength λ _s of the signal light shown in FIG. 4, that is, the graph of the change in the wavelength λ _s of the signal light marked with a circle in FIG. It is understood that changing the length of the wavelength λ _p causes continuous variation.

That is, according to the laser oscillator 100 according to the present embodiment, theoretically, the wavelength λ _p of the pumping light can be changed within the range of around 1.70 μm to around 2.13 μm, and the phase matching angle θ can be changed from 90 degrees to 64 degrees. By changing the wavelength λ _s of the signal light within a range of about 1.80 μm to about 2.86 μm, the wavelength λ s of the signal light can be changed within a range of about 1.80 μm to about 2.86 μm.

FIG. 6 is a graph showing an example of theoretical phase matching characteristics of the laser oscillator 100 according to one embodiment. The horizontal axis of the graph in FIG. 6 indicates the phase matching angle θ [degrees], and the vertical axis indicates the wavelength λ _i [μm] of the idler light. The right side of the graph in FIG. 6 shows the relationship between the shading of the line shown in the graph and the wavelength λ _p [μm] of the excitation light.

In the graph of FIG. 6, a circle is shown at a position corresponding to the circle shown on the graph of the wavelength λ _i of the idler light in FIG. Comparing FIG. 4 and FIG. 6, the graph of the transition of the wavelength λ _i of the idler light shown in FIG. 4, that is, the graph of the transition of the wavelength λ _i of the idler light marked with a circle in FIG. It is understood that changing the length of the wavelength λ _p causes continuous variation.

That is, according to the laser oscillator 100 according to the present embodiment, theoretically, the wavelength λ _p of the pumping light can be changed within the range of around 1.70 μm to around 2.13 μm, and the phase matching angle θ can be changed from 90 degrees to 64 degrees. By changing the wavelength λ _i of the idler light within a range of about 8.5 μm to about 29 μm, the wavelength λ i of the idler light can be changed within a range of about 8.5 μm to about 29 μm.

FIG. 7 is a graph showing an example of output characteristics of signal light and idler light actually measured by the laser oscillator 100 according to one embodiment. The lower horizontal axis of the graph in FIG. 7 indicates the wavelength λ _s [μm] of the signal light, and the upper horizontal axis indicates the wavelength λ _i [μm] of the idler light. The left vertical axis of the graph in FIG. 7 indicates the energy of the signal light [mJ], and the right vertical axis indicates the energy of the idler light [arb. units].

The graph in FIG. 7 shows the results when the wavelength λ _s of the signal light is sequentially changed from around 2.2 μm to around 2.7 μm by sequentially rotating the diffraction grating 139 based on the measurement results of the wavelength meter 160. , plot data of the intensity of signal light sequentially detected by the energy meter 201 is shown by a black circle. The graph of FIG. 7 also shows the position of the energy meter 203 when the phase matching angle θ of the crystal 140 is 90 degrees and the wavelength λ _s of the signal light is sequentially changed from around 2.2 μm to around 2.7 μm. Plot data of the wavelength λ _i of the idler light sequentially measured by the energy meter 203 and the intensity of the idler light detected by the energy meter 203 is shown by circles.

Here, FIG. 4 and FIG. 7 will be compared. When the phase matching angle θ of the crystal 140 is 90 degrees, according to the graph in FIG. 4, theoretically, if the signal light wavelength λ _s is 2.595 μm, the idler light wavelength λ _i is 9.015 μm. . On the other hand, when the phase matching angle θ of the crystal 140 is 90 degrees, the graph in FIG. 7 shows that the idler light has the highest energy at a wavelength λ _i around 9.015 μm, The signal light has a measured wavelength λ _s around 2.595 μm.

That is, according to the laser oscillator 100 according to the present embodiment, the wavelength λ _i of the idler light output from the laser oscillator 100 is set as follows under the same conditions as in FIG. It is possible for the user to set the desired wavelength.

FIG. 8 is a graph showing an example of output characteristics of pump light and idler light actually measured by the laser oscillator 100 according to one embodiment. The horizontal axis of the graph in FIG. 8 indicates the energy of excitation light [mJ], and the vertical axis indicates the energy of idler light [μJ]. In FIG. 8, the measured energy of each light is shown as a circle when the reflectance of the third mirror 137 is set to 60%, and the energy of each light is shown as a circle when the reflectance of the third mirror 137 is set to 98%. Measured values are shown as black circles. The graph in FIG. 8 shows the case where the excitation light wavelength λ _p is set to 2.01 μm, the signal light measurement wavelength λ _s is 2.58 μm, and the idler light measurement wavelength λ _i is 9.10 μm. Figure 2 shows the results of measuring the energy of each light.

As shown in the graph of FIG. 8, it is understood that increasing the reflectance of the third mirror 137 increases the energy of the idler light. It is also understood that when the energy of the excitation light is linearly increased, the energy of the idler light is also linearly increased.

FIG. 9 is a graph showing an example of output characteristics of signal light and idler light actually measured by the laser oscillator 100 according to one embodiment. The horizontal axis of the graph in FIG. 9 indicates the angle [degrees] of the half-wave plate 120, and the vertical axis indicates the pulse energy [arb. units]. In FIG. 9, the energy measurement values of the signal light are shown in black when the angle of the half-wave plate 120 is rotated from 0 degrees to 180 degrees to change the polarization direction of the excitation light incident on the optical system 130. The energy measurements of the idler light are shown as circles. In the graph of FIG. 9, the wavelength λ _p of the excitation light is set to 2.01 μm, the energy of the excitation light is set to 10 mJ, the pulse width of the excitation light is set to 400 ns, and the measurement wavelength λ _s of the signal light is 2.58 μm. The results of measuring the energy of each light when the measurement wavelength λ _i of the idler light is 9.10 μm are shown.

As shown in the graph of FIG. 9, it is understood that the energy of each of the signal light and idler light is maximized when the half-wave plate 120 is set at 45 degrees and 135 degrees, respectively.

FIG. 10 is a graph showing an example of time waveforms of excitation light, signal light, and idler light actually measured by the laser oscillator 100 according to one embodiment. The horizontal axis of the graph in FIG. 10 is time [100ns/div. ], and the vertical axis indicates the intensity [arb. units]. FIG. 10 shows the respective time waveforms of excitation light, signal light, and idler light. Similar to FIG. 9, the graph in FIG. 10 shows that the wavelength λ _p of the excitation light is set to 2.01 μm, the energy of the excitation light is set to 10 mJ, and the pulse width of the excitation light is set to 400 ns, so that the measurement wavelength λ _s of the signal light is 2.58 μm, and the measurement wavelength λ _i of the idler light is 9.10 μm. The results of measuring the intensity of each light are shown below.

As shown in the graph of FIG. 10, it is understood that a time difference occurs after the excitation light is incident on the optical system 130 until the signal light is oscillated in the crystal 140 and the idler light is generated at a difference frequency.

FIG. 11 is a graph for explaining the relationship between the sulfide (S) concentration x, the wavelength λ _s of the signal light, and the wavelength λ _i of the idler light in the laser oscillator 100 according to one embodiment. The lower horizontal axis of each of graphs 11a, 11b, 11c, and 11d indicates the wavelength λ _s [μm] of the signal light, and the upper horizontal axis of each indicates the wavelength λ _i [μm] of the idler light. . The vertical axis of the graph 11a indicates the phase matching angle θ [degrees]. The vertical axis of the graph 11b indicates the walk-off angle ρ [degrees]. The vertical axis of the graph 11c indicates the allowable width Δθ·L [degrees·cm] of the phase matching angle θ. The vertical axis of the graph 11d indicates the allowable width Δλ _s ·L [nm·cm] of the wavelength λ _s of the signal light. The bottom part of FIG. 11 shows the relationship between the shading of the lines shown in each of the graphs 11a, 11b, 11c, and 11d and the sulfide (S) concentration x.

The graph shown in FIG. 11 is calculated by changing the sulfide (S) concentration x in 0.025 increments, and more specifically, x=0, 0.025, 0.05, 0.075, 0.1, Calculations were made for each of 0.125, 0.15, 0.175, 0.2, 0.225, 0.25, 0.275, 0.3, 0.325, and 0.35. In the embodiment of FIG. 11, as an example, the excitation light source 110 is a Ho:YLF laser that outputs excitation light with a wavelength of 2.05 μm.

As described above, the crystal 140 according to this embodiment is Cr:CdSe _1-x S _x (where x=0.0 to 0.4). The crystal 140 is in at least one of the first phase matching state, the second phase matching state, and the third phase matching state at any of x=0.0 to 0.4. It can be.

グラフ１１ａにおいて、第１の位相整合状態となる箇所をｄ（Δｋ）／ｄθ＝０で示す。なお、Δｋは、位相不整合を意図しており、位相不整合は波長変換の効率を妨げるファクタである。よって、Δｋが０になることが望ましい。 The first phase matching state is a state in which the first order approximation of the phase matching condition (Δk) with respect to the change in the angle θ formed between the crystal axis direction of the crystal 140 and the beam propagation direction satisfies the condition of Equation 11 below. This is intended to be a state that satisfies non-critical phase matching conditions with respect to changes in the propagation direction.
[Number 11]

In the graph 11a, the location where the first phase matching state occurs is indicated by d(Δk)/dθ=0. Note that Δk is intended for phase mismatching, and phase mismatching is a factor that impedes wavelength conversion efficiency. Therefore, it is desirable that Δk be 0.

グラフ１１ａ、グラフ１１ｂおよびグラフ１１ｃのそれぞれにおいて、励起光の波長を固定した状態でシグナル光およびアイドラ光の波長の変化に対して上記の数１２の条件を満足する第２の位相整合状態となる箇所を破線と共にｄ（Δｋ）／ｄλ_ｓ、ｉ＝０で示す。 The second phase matching state is a state in which the first order approximation of the phase matching condition (Δk) with respect to a change in the wavelength of at least one of the excitation light, signal light, and idler light satisfies the condition of Equation 12 below. This is intended to be a state that satisfies non-critical phase matching conditions with respect to changes in wavelength.
[Number 12]

In each of graphs 11a, 11b, and 11c, when the wavelength of the excitation light is fixed, a second phase matching state that satisfies the condition of Equation 12 is achieved with respect to changes in the wavelengths of the signal light and idler light. The location is indicated by d(Δk)/dλ _{s, i} =0 along with a broken line.

The third phase matching state is a state in which the first-order approximation of the phase matching condition (Δk) with respect to a change in crystal temperature T satisfies the condition of Equation 13 below, that is, it is non-critical with respect to a change in crystal temperature. The state is intended to satisfy the phase matching condition.
[Number 13]

In the laser oscillator 100 according to this embodiment, the crystal 140 may be arranged in an orientation that satisfies the non-critical phase matching condition with respect to changes in the beam propagation direction. Alternatively, the crystal 140 may be arranged in an orientation that satisfies the non-critical phase matching condition with respect to changes in the wavelength of at least any of the excitation light, signal light, and idler light. That is, even if the crystal axis orientation of the crystal 140 is set with respect to the beam propagation direction so that the crystal 140 is in one of the first phase matching state, the second phase matching state, and the third phase matching state, good.

According to the laser oscillator 100 according to the present embodiment, by arranging the crystal 140 in an orientation that satisfies the non-critical phase matching condition with respect to changes in the beam propagation direction, the crystal axis orientation of the crystal 140 is not set correctly and the phase When an error occurs in the matching angle θ, or when the crystal axis direction of the crystal 140 changes due to vibration of the table on which the laser oscillator 100 is installed, an error occurs in the phase matching angle θ. , it is possible to maintain phase matching without substantially changing the wavelength λ _i of the idler light. In the state that satisfies the non-critical phase matching condition, that is, the first phase matching state, as shown in graph 11c, the allowable width Δθ·L of the phase matching angle θ is extremely large, that is, the state is resistant to changes in the phase matching angle θ. You can say that. Further, the phase matching in the first phase matching state is sometimes referred to as angular non-critical phase matching or angular non-critical phase matching. Note that in the case of the first phase matching state, the walk-off angle ρ is 0.0 degree, as shown in the graph 11b.

According to the laser oscillator 100 according to the present embodiment, when the crystal 140 is arranged in a direction that satisfies the non-critical phase matching condition with respect to a change in the wavelength of at least one of the excitation light, the signal light, and the idler light, the crystal 140 By slightly tuning the crystal axis orientation, that is, by slightly tuning the phase matching angle θ, it is possible to significantly tune the wavelength λ _i of the idler light to about several μm. In the state that satisfies the non-critical phase matching condition, that is, the second phase matching state, as shown in graph 11d, the allowable width Δλ _s ·L of the wavelength λ _s of the signal light is extremely large, that is, the phase matching state is maintained. It can also be said that this is a state in which it is easy to tune the wavelength λ _i of the idler light. Further, the phase matching in the second phase matching state is sometimes referred to as spectral non-critical phase matching or spectral non-critical phase matching. Note that even in the case of the second phase matching state, the walk-off angle ρ between the excitation light and the signal light is less than 0.35 degrees, as shown in the graph 11b. If the walk-off angle ρ, which is the angle that occurs between multiple input lights, is so small, it is difficult to make the interaction length of the multiple input lights, that is, the distance and time that the lights can stay together long enough. can.

According to the laser oscillator 100 according to the present embodiment, when the crystal 140 is arranged in an orientation that satisfies the non-critical phase matching condition with respect to temperature changes, a temperature increase due to self-absorption of the crystal 140 that can occur when using a high-power laser It is possible to maintain phase matching even when the temperature changes or the environmental temperature changes. It can be said that the state that satisfies the non-critical phase matching condition, that is, the third phase matching state, has an extremely large temperature tolerance range ΔT·L for phase matching, that is, is resistant to changes in the phase matching temperature T. Further, the phase matching in the third phase matching state is sometimes referred to as temperature non-critical phase matching or temperature non-critical phase matching.

According to the laser oscillator 100 according to the present embodiment, for example, when it is desired to set the wavelength λ _i of the output idler light to a necessary value within the range of around 9 μm to around 25 μm, the sulfide concentration x is set to x=0. It is also understood from FIG. 11 that the adjustment can be made within the range of .0 to 0.4. It is also understood from FIG. 11 that in this case, if it is desired to further reduce the walk-off angle ρ, the phase matching angle θ may be set to 90 degrees or close to 90 degrees.

As explained above, according to the laser oscillator 100 according to the present embodiment, by irradiating the crystal 140 doped with a dopant and having second-order optical nonlinearity with excitation light, a broad spectrum in the mid-infrared region can be obtained. Bandwidth fluorescence is emitted. Further, the signal light is lased at a specific wavelength in the spectral bandwidth. Then, by utilizing the birefringence phase matching of the crystal 140, idler light having a difference frequency between the excitation light and the signal light is output. Thereby, according to the laser oscillator 100 according to this embodiment, the effects described using FIGS. 1 to 11 can be achieved.

As described above, the crystal 140 according to this embodiment may be Cr:CdSe _1-x S _x (where x=0.0 to 0.4). In this case, the laser oscillator 100 according to this embodiment may further select the wavelength range of the idler light by selecting a plurality of crystals 140 having mutually different x values. Alternatively or in addition to this, the laser oscillator 100 according to the present embodiment may further arrange the crystal 140 in an orientation that satisfies the non-critical phase matching condition with respect to changes in the beam propagation direction, or It may be arranged in a direction that satisfies the non-critical phase matching condition with respect to changes in at least one of the wavelengths of the idler light.

In the above embodiments, the crystal 140 may be Fe:CdSe, and as in the embodiments, sulfide (S) may be added to Fe:CdSe depending on the purpose of the idler light and the required wavelength. It may be added as appropriate, and specifically, it may be Fe:CdSe _1-x S _x (where x=0.0 to 0.4).

In the above embodiments, the crystal 140 has been described as having birefringence phase matching. Alternatively, the crystal 140 may have quasi-phase matching using a periodically poled structure or random phase matching using a polycrystalline structure. The crystal 140 having quasi-phase matching using a periodically poled structure or random phase matching using a polycrystalline structure is, for example, ZnSe _1-x S _x doped with Cr ²⁺ or Fe ²⁺ as a dopant. (However, x may be between 0.0 and 1.0).

Although the present invention has been described above using the embodiments, the technical scope of the present invention is not limited to the range described in the above embodiments. It will be apparent to those skilled in the art that various changes or improvements can be made to the embodiments described above. It is clear from the claims that such modifications or improvements may be included within the technical scope of the present invention.

The order of execution of each process, such as the operation, procedure, step, and stage in the apparatus, system, program, and method shown in the claims, specification, and drawings, is specifically defined as "before" or "before". It should be noted that they can be implemented in any order unless the output of the previous process is used in the subsequent process. Even if the claims, specifications, and operational flows in the drawings are explained using "first," "next," etc. for convenience, this does not mean that it is essential to carry out the operations in this order. It's not a thing.

10 Laser oscillation system 100 Laser oscillator 110 Excitation light source 120 Half-wave plate 130 Optical system 131 First mirror 133 Second mirror 135 Brewster plate 137 Third mirror 139 Diffraction grating 140 Crystal 150 Half mirror 160 Wavemeter 170 Filter 180 Controller 190 Drive unit 201, 203 Energy meter 205 Oscilloscope 207 User interface 301, 305 Mirror 303 Crystal 311 Crystal

Claims (19)

a crystal doped with a dopant and having second-order optical nonlinearity;
an optical system that causes the crystal into which the excitation light is incident to oscillate as a laser;
The crystal is
It further has phase matching of any one of birefringence phase matching, quasi phase matching using a periodic polarization inversion structure, and random phase matching using a polycrystalline structure,
Emitting fluorescence with a broad spectral bandwidth in the mid-infrared region by the excitation light,
Due to the phase matching, an idler light having a difference frequency between the excitation light and a signal light lased at a specific wavelength or a specific wavelength width in the spectral bandwidth by the optical system is output.
laser oscillator. The spectral bandwidth is 0.1 μm or more,
A laser oscillator according to claim 1. The dopant is a transition metal ion, and the fluorescence is emitted when the excitation light causes a vibrational transition of the transition metal ion.
A laser oscillator according to claim 2. The dopant is a rare earth ion selected from Yb, Tm, Er, and Ho.
A laser oscillator according to claim 2. The crystal is CdSe _1-x S _x (where x=0.0 to 0.4) doped with Cr ²⁺ or Fe ²⁺ as a dopant.
A laser oscillator according to claim 1. The crystal is ZnSe _1-x S _x (where x = 0.0 to 1.0) doped with Cr ²⁺ or Fe ²⁺ as a dopant.
A laser oscillator according to claim 1. The crystal is arranged in an orientation that satisfies a non-critical phase matching condition with respect to changes in the beam propagation direction.
A laser oscillator according to claim 5 or 6. The crystal is arranged in an orientation that satisfies a non-critical phase matching condition with respect to a change in the wavelength of at least one of the excitation light, the signal light, and the idler light.
A laser oscillator according to claim 5 or 6. The crystal is arranged in an orientation that satisfies a non-critical phase matching condition with respect to a change in temperature of the crystal,
A laser oscillator according to claim 5 or 6. The optical system is
a first mirror and a second mirror arranged diagonally opposite each other;
a third mirror that is disposed on the optical path of the light reflected by the second mirror and reflects the light toward the second mirror when the light reflected by the second mirror is incident;
The signal light is placed on the optical path of the light reflected by the first mirror, and when the light reflected by the first mirror is incident, the signal light has the specific wavelength in the spectral bandwidth or the specific wavelength width. having an optical device to select,
has,
A laser oscillator according to claim 1. The optical device includes any one of a diffraction grating, an acousto-optic wavelength tunable filter, a birefringence filter, and a prism.
The laser oscillator according to claim 10. a wavelength meter that measures the wavelength of light transmitted through the third mirror;
The laser according to claim 10 or 11, further comprising a controller that selects the specific wavelength or the specific wavelength width of the signal light by controlling the optical device based on the measured value of the wavelength meter. oscillator. emitting fluorescence with a broad spectral bandwidth in the mid-infrared region by irradiating excitation light to a crystal doped with a dopant and having second-order optical nonlinearity;
lasing signal light at a specific wavelength or specific wavelength width in the spectral bandwidth;
The excitation light is produced by utilizing any of the phase matching properties of the crystal, including birefringence phase matching, quasi-phase matching using a periodic polarization inversion structure, and random phase matching using a polycrystalline structure. and outputting an idler light having a difference frequency between the signal light and the signal light. The crystal is CdSe _1-x S _x (where x=0.0 to 0.4) doped with Cr ²⁺ or Fe ²⁺ as a dopant.
The laser oscillation method according to claim 13. The crystal is ZnSe _1-x S _x (where x = 0.0 to 1.0) doped with Cr ²⁺ or Fe ²⁺ as a dopant.
The laser oscillation method according to claim 13. further comprising selecting a wavelength range of the idler light by selecting a plurality of the crystals having mutually different values of x;
The laser oscillation method according to claim 14 or 15. further comprising arranging the crystal in an orientation that satisfies a non-critical phase matching condition with respect to changes in the beam propagation direction;
The laser oscillation method according to claim 14 or 15. further comprising arranging the crystal in a direction that satisfies a non-critical phase matching condition with respect to a change in wavelength of at least one of the signal light and the idler light;
The laser oscillation method according to claim 14 or 15. further comprising arranging the crystal in an orientation that satisfies a non-critical phase matching condition with respect to a change in temperature of the crystal;
The laser oscillation method according to claim 14 or 15.

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