JP4142179B2 - Multilayer mirror - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a multilayer mirror that can be used for pulse compression of laser light.
[0002]
[Prior art]
Ultrashort pulse light having a pulse width on the order of femtoseconds (fs) is expected to be applied to various measurement techniques and processing techniques. In addition, if the band of ultrashort pulse light is wide, its application range is also widened.
[0003]
Broadband light can be generated by making high-intensity light incident on an optical nonlinear medium. That is, the refractive index of the optical nonlinear medium changes with the light intensity. When high-intensity light is incident on an optical nonlinear medium, self-phase modulation (SPM), that is, a change in the phase velocity of the light accompanying a change in the refractive index of the medium occurs, and the emitted light from the nonlinear medium is white light having a plurality of wavelength components. It becomes. There is optical parametric amplification (OPA) as means for amplifying the white light thus obtained. In non-parallel optical parametric amplification in which signal light and pump light are phase-matched non-parallelly in a nonlinear optical crystal in OPA, for example, if the second harmonic of a titanium sapphire laser is used as pump light, the visible range is 4000 cm ⁻¹ to 6000 cm at maximum ^-1 broadband ultra-short pulse light can be generated.
[0004]
Such white ultra-short pulse light can be regarded as superposition of single-wavelength pulse light having individual wavelength components. This white ultra-short pulse light is subjected to group velocity dispersion (GVD), that is, frequency chirp by a dispersion medium such as nonlinear optical crystal or air due to the difference in group velocity for each wavelength component, and as the light travels The pulse width widens. In order to compress the spread pulse width, it is only necessary to advance the slow wavelength component and delay the fast wavelength component. As a result, the rate at which pulses having individual wavelength components overlap within the same time increases, resulting in a narrow pulse width.
[0005]
Various optical systems that perform the above-described group velocity dispersion compensation, in other words, frequency chirp compensation, are considered. Among them, the one using a dielectric multilayer mirror is more than the other in terms of loss and size. Is also excellent. Such dielectric multilayer mirrors are described in US Pat. No. 2,754,214 and US Pat. No. 5,734,503.
[0006]
[Problems to be solved by the invention]
However, the mirror described in Japanese Patent No. 2754214 has a narrower band used for frequency chirp compensation than the reflection band, and the mirror described in US Pat. No. 5,734,503 can perform frequency chirp compensation at a wavelength of 720 nm to 890 nm. The frequency band is as narrow as about 2700 cm ⁻¹ . In other words, the performance of any mirror is still not sufficient, and these mirrors compress optical pulses having a frequency band exceeding 4000 cm ⁻¹ of OPA to a pulse width of the order of several to several tens of fs with high reflection efficiency. It is difficult.
[0007]
The present invention has been made in view of such problems, and provides a multilayer mirror capable of compressing the width of an optical pulse having a visible wavelength range, particularly a wavelength band of 500 nm to 770 nm, on the order of several to several tens of fs. For the purpose.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, a multilayer mirror according to the present invention is a multilayer mirror formed by laminating a plurality of dielectric films having different refractive indexes on a substrate, and has a reflectance of 95% or more in the visible light band. The refractive index of the outermost film constituting the exposed surface is lower than the refractive index of the film immediately below the outermost film, and the optical film thickness of the film immediately below the outermost film is smaller than the optical film thickness of the outermost film. The dielectric film from the outermost film side is thin and the optical film thickness of each dielectric film is such that the group delay of the long wavelength component in the band is larger than the group delay of the short wavelength component Increase in a direction from the outermost film side toward the substrate side along a curve increasing with a quadratic function with respect to the number of layers , and the group delay with respect to the wavelength is at least at a wavelength of 500 nm to 750 nm with a slope of 0. Monotonically increasing from 05 to 0.15 (fs / nm) or wave In 500Nm～770nm, it characterized in that it is set to monotonically increasing slope 0.15~0.19 (fs / nm).
[0009]
According to the multilayer film mirror of the present invention, since the outermost film has a low refractive index, the electric field concentration of the outermost film due to irradiation is reduced even when it is irradiated with high-intensity laser light. Destruction can be suppressed. In this case, in order to set the group delay of the long wavelength component in the band to be larger than the group delay of the short wavelength component, at least the refractive index of the film immediately below the outermost film is higher than this, and The optical film thickness is preferably thinner than this. Since the optical film thickness is set as described above, the multilayer mirror can have a reflected light frequency band of 500 nm to 770 nm, in other words, a reflected light frequency band exceeding 7000 cm ⁻¹ at the maximum. Since this band is significantly wider than that of the conventional multilayer mirror, it is possible to perform chirp compensation of broadband light.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the multilayer mirror and the broadband laser according to the embodiment will be described. The same reference numerals are used for the same elements or elements having the same function, and redundant description is omitted.
[0011]
FIG. 1 is a cross-sectional configuration diagram of a multilayer mirror according to the first embodiment. This multilayer mirror includes a glass substrate SB and a multilayer film ML formed on the substrate SB. The multilayer film ML is formed by alternately stacking two types of dielectric films having different refractive indexes. In the present embodiment, the number of layers N of the multilayer film ML is 40. The odd-numbered dielectric films 1, 3, 5, 7,... 39 from the substrate SB side are high refractive index films (H), and the even-numbered dielectric films 2, 4, 6, 8,. , A low refractive index film (L). The high refractive index film (H) is TiO ₂ and the low refractive index film (L) is SiO ₂ . The refractive indexes n ₀ , n ₁ , and n ₂ of the glass substrate SB, TiO ₂ , and SiO ₂ are 1.52, 2.35, and 1.46, respectively. The outermost film 40 is exposed to air, and the exposed surface forms an interface between the air having a refractive index n _AIR = 1.0 and the outermost film 40.
[0012]
FIG. 2 is a table showing the optical film thickness (nm) and the preferred range (nm) of each film according to the first embodiment. The table shows the first, second,... Nth layers in order from the bottom. The optical film thickness is a product (n × d) of the refractive index n and the thickness d of each film.
[0013]
FIG. 3 is a graph showing the relationship between the numbers 1, 2,... 40 of the multilayer films shown in FIG. 2 and the optical film thickness (nm). The solid line in the graph indicates a film thickness approximation curve, and when the direction from the air side to the substrate side is a positive direction, this curve generally indicates a quadratic function.
[0014]
FIG. 4 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG. In this mirror, the group delay monotonously increases with a slope of 0.05 to 0.15 (fs / nm) at a wavelength of 500 nm to 750 nm. The group delay at a wavelength of 500 nm is in the range of 0 to 5 fs, the group delay at a wavelength of 600 nm is in the range of 10 to 15 fs, and the group delay at the wavelength of 700 nm is in the range of 23 to 28 fs.
[0015]
In this multilayer mirror, since the group velocity of the long wavelength component of the incident light is delayed relative to the group velocity of the short wavelength component, the pulse width of the laser beam whose pulse width is expanded by the dispersion medium is compressed. Can do.
[0016]
FIG. 5 is a table showing the optical film thickness (nm) and the preferred range (nm) of each film according to the second embodiment. The table shows the first, second,... Nth layers in order from the bottom, and the number of layers is N = 54.
[0017]
FIG. 6 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG. In this mirror, the group delay monotonously increases with a slope of 0.05 to 0.15 (fs / nm) at wavelengths of 500 nm to 800 nm. The group delay at a wavelength of 500 nm is in the range of 0 to 5 fs, the group delay at a wavelength of 600 nm is in the range of 5 to 10 fs, and the group delay at the wavelength of 700 nm is in the range of 20 to 25 fs.
[0018]
Also in this multilayer film mirror, the group velocity of the long wavelength component of the incident light is delayed relative to the group velocity of the short wavelength component, so the pulse width of the laser beam whose pulse width is expanded by the dispersion medium is compressed. Can do.
[0019]
FIG. 7 is a table showing the optical film thickness (nm) and the preferred range (nm) of each film according to the third embodiment. The table shows the first, second,... Nth layers in order from the bottom, and the number of layers is N = 56.
[0020]
FIG. 8 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG. In this mirror, the group delay monotonously increases with a slope of 0.05 to 0.15 (fs / nm) at wavelengths of 500 nm to 800 nm. The group delay at a wavelength of 500 nm is in the range of 0 to 5 fs, the group delay at a wavelength of 600 nm is in the range of 13 to 18 fs, and the group delay at the wavelength of 700 nm is in the range of 20 to 25 fs.
[0021]
Also in this multilayer film mirror, the group velocity of the long wavelength component of the incident light is delayed relative to the group velocity of the short wavelength component, so the pulse width of the laser beam whose pulse width is expanded by the dispersion medium is compressed. Can do.
[0022]
FIG. 9 is a table showing the optical film thickness (nm) and the preferred range (nm) of each film according to the fourth embodiment. The table shows the first, second,... Nth layers in order from the bottom, where the number of layers is N = 40.
[0023]
FIG. 10 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG. In the present mirror, the group delay monotonously increases with a slope of 0.15 to 0.19 (fs / nm) at wavelengths of 500 nm to 770 nm. The group delay at a wavelength of 500 nm is in the range of 0 to 5 fs, the group delay at a wavelength of 600 nm is in the range of 5 to 10 fs, and the group delay at the wavelength of 700 nm is in the range of 23 to 27 fs.
[0024]
Also in this multilayer film mirror, the group velocity of the long wavelength component of the incident light is delayed relative to the group velocity of the short wavelength component, so the pulse width of the laser beam whose pulse width is expanded by the dispersion medium is compressed. Can do.
[0025]
FIG. 11 is a table showing the optical film thickness (nm) and the preferred range (nm) of each film according to the fifth embodiment. The table shows the first, second,... Nth layers in order from the bottom, where the number of layers is N = 40.
[0026]
FIG. 12 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG. In the present mirror, the group delay monotonously increases with a slope of 0.15 to 0.19 (fs / nm) at wavelengths of 500 nm to 770 nm. The group delay at a wavelength of 500 nm is in the range of 0 to 5 fs, the group delay at a wavelength of 600 nm is in the range of 5 to 10 fs, and the group delay at the wavelength of 700 nm is in the range of 23 to 27 fs.
[0027]
Also in this multilayer film mirror, the group velocity of the long wavelength component of the incident light is delayed relative to the group velocity of the short wavelength component, so the pulse width of the laser beam whose pulse width is expanded by the dispersion medium is compressed. Can do.
[0028]
The multilayer mirror according to the first to fifth embodiments is a multilayer mirror formed by laminating a plurality of dielectric films having different refractive indexes on the glass substrate SB, and has a visible light band, particularly a wavelength band of 500 nm to 750 nm. And the refractive index of the outermost film constituting the exposed surface is lower than the refractive index of the film immediately below the outermost film, and the group delay of the long wavelength component in this band The optical film thickness of each dielectric film including the outermost film is set so that is larger than the group delay of the short wavelength component.
[0029]
According to these multilayer mirrors, since the outermost film has a low refractive index, the electric field concentration of the outermost film due to irradiation is reduced and the destruction is caused even when the laser beam is irradiated with high intensity laser light. Can be suppressed. In this case, in order to set the group delay of the long wavelength component in the band to be larger than the group delay of the short wavelength component, at least the refractive index of the film immediately below the outermost film is higher than this, and The optical film thickness is preferably thinner than this. Since the optical film thickness is set as described above, the multilayer mirror can have a reflected light frequency band of 500 nm to 770 nm, in other words, a reflected light frequency band exceeding 7000 cm ⁻¹ at the maximum. Since this band is significantly wider than that of the conventional multilayer mirror, it is possible to perform chirp compensation of broadband reflected light.
[0030]
In addition, FIG. 14 is a table | surface which shows Formula (1)-(9) for using in order to set the said optical film thickness. The setting procedure is as follows.
[0031]
The reflectance R shown in Expression (8) is set to be 95% or more for each light within the wavelength band of 500 nm to 770 nm.
[0032]
Φ shown in Equation (9) is a variable whose first-order frequency derivative indicates group delay (GD) and whose second-order frequency derivative indicates group delay dispersion (GDD), in other words, the argument of the complex amplitude reflectance R. Yes, in the wavelength band of 500 nm to 750 nm, the group delay GD is set to monotonously increase with respect to the wavelength. Since R and Φ (GD) have wavelength dependence, calculation is performed for each wavelength in the band.
[0033]
R and Φ can be calculated if the k-th layer complex amplitude reflectance γ _{k + 1 k} is obtained. The complex amplitude reflectance γ ₂₁ of the first layer and the complex amplitude reflectance γ ₃₂ of the second layer are calculated from equations (4) and (5), respectively. Note that the complex amplitude reflectance γ _{k + 1 k} of the k-th layer is calculated from Equation (6). Note that the Fresnel coefficient r in these equations is calculated from the refractive index equation (2) where n _j is the refractive index of the j-th film, and δ in the equations is the angle of incidence of the j-th light θ _j , j-th layer of the film thickness of the d _j of the incident light wavelength is lambda, is given by equation (1). The complex amplitude reflectance γ ₁₀ is given by Expression (3), and cos θ _j that gives θ _j is given by Expression (7). Note that n _air a refractive index of the incident medium, in this example a n _air ≒ 1.0 for air, the theta _air incident angle of the light incident surface, i.e., at an angle of incidence of light on the mirror is there. When calculation is performed so as to satisfy the above settings, the multilayer mirrors according to the above three embodiments can be obtained.
[0034]
FIG. 13 is a system configuration diagram of a broadband laser using any one of the multilayer mirrors described above. This broadband laser includes a pair of multilayer mirrors 100a and 100b, and performs chirp compensation.
[0035]
This broadband laser includes a laser light source 101 made of a titanium sapphire laser that emits laser light having a wavelength of 790 nm. Laser light emitted from the laser light source 101 has a pulse width of 130 fs, a light intensity of 400 μJ, and a repetition frequency of 1 kHz.
[0036]
This laser light is divided into two by a beam splitter BS, and one of the divided lights is a variable ND filter (attenuator) 102, a lens 103, and a sapphire substrate 104 that widens the wavelength band of the laser light passing therethrough to at least 500 nm to 800 nm. , The mirror 105, the mirrors 106 and 107, the optical filter 108, the mirror 109, and the mirror 111, and enters the nonlinear optical crystal (BBO) 112 as signal light. The optical filter 108 is an optical filter 108 that selects laser light having a wavelength of 750 nm or less from laser light having a wavelength of 500 nm to 800 nm, and signal light having a wavelength of 500 nm to 750 nm is incident on the nonlinear optical crystal 112.
[0037]
The other of the divided light beams is a mirror 201, a lens 202, a non-linear optical crystal (LiB ₃ O ₄ ) 203, a lens 204, and a second harmonic wave having a wavelength of 395 nm, and a fundamental wave having a wavelength of 790 nm. The light enters the nonlinear optical crystal 112 as excitation light having a wavelength of 395 nm through the transmitting harmonic separator 205, the prism 206, and the lenses 207 and 208. Note that the polarization directions of the signal light and the excitation light are orthogonal.
[0038]
When the signal light is incident on the nonlinear optical crystal 112 while the nonlinear optical crystal 112 is excited by the irradiation of the excitation light, the signal light is amplified by the optical parametric effect. The optical path length adjusting mirror 107 is positioned so that the amplified signal light has a wavelength band of 550 nm to 700 nm. The amplified signal light is reflected by the mirror 113 a and enters the nonlinear optical crystal 112 again. At this time, the excitation light that has passed through the nonlinear optical crystal 112 is also reflected by the mirror 113b and enters the nonlinear optical crystal 112 again. As a result, the signal light is amplified again, is reflected by the mirror 111, and enters the mirror 110. At this time, since the mirror 113a is slightly inclined in the vertical direction, the optical path is spatially separated, the mirror 110 reflects the amplified signal light, and the reflected signal light reflects the mirrors 114 and 115, the periscope PS, and the mirror 117. Through the prism pair 300a and 300b.
[0039]
The amplified signal light (wavelength 550 nm to 700 nm) has group delay dispersion due to passage through the path up to this point. The prism pair 300a, 300b slightly compensates for this dispersion, and the compensated signal light passes through the side portion without passing through the mirror 117 and enters the multilayer mirror pair 100a, 100b. Here, since the band of the incident laser light (wavelength 550 nm to 700 nm) is narrower than the reflection band (wavelength 500 nm to 770 nm) of the multilayer mirrors 100a and 100b, the reflected light is efficiently chirp-compensated.
[0040]
The multilayer mirrors 100a and 100b are arranged in parallel to each other, and the incident angle of light to the multilayer mirror 100a is set within 45 degrees. The signal light is reflected several times by the multilayer mirror pair 100a, 100b and then emitted from it. The wavelength of the emitted light is 550 nm to 700 nm, the pulse width is 5 fs or more, and the light intensity is 5 μJ or more.
[0041]
As described above, the broadband laser according to the present embodiment is based on the reflected light band (500 nm to 770 nm) of the multilayer mirrors 100a and 100b from the laser beam having a predetermined wavelength (790 nm) output from the laser light source 101. Are also provided with optical systems BS, 102 to 116, 201 to 208, 300a, 300b, and 300c that generate laser light in a narrow band (wavelength 550 nm to 700 nm) and enter the multilayer mirror. Since the band of the laser beam by this optical system is narrower than the reflection band of the multilayer mirrors 100a and 100b, the reflected light is efficiently chirp-compensated.
[0042]
【The invention's effect】
As described above, the multilayer mirror according to the present invention can compress the width of an optical pulse having a wavelength band of 500 nm to 770 nm to the order of several to several tens of fs by performing the above setting. A broadband laser using this mirror can emit a broadband laser beam with an ultrashort pulse.
[Brief description of the drawings]
FIG. 1 is a cross-sectional configuration diagram of a multilayer mirror according to a first embodiment.
FIG. 2 is a table showing optical film thicknesses (nm) and preferred ranges (nm) of the respective films according to the first embodiment.
3 is a graph showing the relationship between numbers 1, 2,... 40 of each multilayer film shown in FIG. 2 and the optical film thickness (nm).
4 is a graph showing a relationship between a reflected light wavelength (nm) and a group delay (fs) by the multilayer mirror shown in FIG. 2;
FIG. 5 is a table showing optical film thicknesses (nm) and preferred ranges (nm) of respective films according to the second embodiment.
6 is a graph showing a relationship between a reflected light wavelength (nm) and a group delay (fs) by the multilayer mirror shown in FIG.
FIG. 7 is a table showing an optical film thickness (nm) and a preferred range (nm) of each film according to the third embodiment.
8 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG.
FIG. 9 is a table showing an optical film thickness (nm) and a preferred range (nm) of each film according to the fourth embodiment.
10 is a graph showing the relationship between the reflected light wavelength (nm) and the group delay (fs) by the multilayer mirror shown in FIG. 9;
FIG. 11 is a table showing optical film thicknesses (nm) and preferred ranges (nm) of respective films according to the fifth embodiment.
12 is a graph showing a relationship between a reflected light wavelength (nm) and a group delay (fs) by the multilayer mirror shown in FIG.
FIG. 13 is a system configuration diagram of a broadband laser using a multilayer mirror.
FIG. 14 is a table summarizing equations (1) to (9) for use in setting the optical film thickness.
[Explanation of symbols]
SB ... Glass substrate, ML ... Multilayer film.

Claims (2)

In a multilayer mirror formed by laminating a plurality of dielectric films having different refractive indexes on a substrate,
The reflectance in the visible light band is 95% or more,
The refractive index of the outermost film constituting the exposed surface is lower than the refractive index of the film immediately below the outermost film,
The optical film thickness immediately below the outermost film is thinner than the optical film thickness of the outermost film, and
The optical film thickness of each film of the dielectric film is set to the number of layers of the dielectric film from the outermost film side so that the group delay of the long wavelength component in the band is larger than the group delay of the short wavelength component. On the other hand, it increases in a direction from the outermost film side to the substrate side along a curve increasing with a quadratic function , and the group delay with respect to the wavelength is at least 0.05 to 0. 5 at a wavelength of 500 nm to 750 nm. Multilayer characterized by monotonically increasing at 15 (fs / nm) or monotonically increasing at a slope of 0.15 to 0.19 (fs / nm) at a wavelength of 500 nm to 770 nm. Membrane mirror.   The multilayer mirror according to claim 1, wherein the visible light band has a wavelength of 500 nm to 770 nm.

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