JP7195022B2 - Chirp mirrors and chirp mirror units - Google Patents

Description

The present invention relates to a chirped mirror capable of compensating for dispersion of light pulses with extremely short pulse widths (ultrashort light pulses) including femtosecond light pulses, and a chirped mirror unit using the same.

As described in Japanese Patent No. 3569777 (Patent Document 1), recently, femtosecond light pulses are used in control of molecular states and electronic states of solids, chemical reaction control, material processing, and the like. A femtosecond light pulse is light having a high peak intensity of, for example, about 10 ¹² W (watts) or more in a very short pulse width of about femtosecond (10 ⁻¹⁵ ) seconds.
A femtosecond light pulse is formed by superimposing light beams of various wavelengths in phases.
When a femtosecond light pulse propagates through a medium in which the speed of light differs for each wavelength, i.e., in a medium in which the group velocity of light has wavelength dependence, light of a certain wavelength propagates in the direction of propagation of light of another wavelength. , the pulse width is broadened and the peak intensity is lowered due to the relatively fast progress at or superimposition thereof. Chirp is a phenomenon in which the group velocity of light depends on the wavelength and the velocity of light varies with wavelength.

Since the characteristics of the femtosecond optical pulse are impaired by the amount of chirping that causes the pulse width to widen and the peak intensity to decrease, a prism or dielectric multilayer mirror can be used to generate a chirped femtosecond optical pulse whose velocity is shifted according to the wavelength. to femtosecond light pulses without deviation (dispersion compensation).
For example, a dielectric multilayer having a plurality of Bragg films with different film thicknesses can generate a femtosecond optical pulse in which light with wavelengths in the visible range is superimposed and chirped faster than light of other colors in the propagation direction due to propagation. If black is reflected by a film mirror (an example of a chirped mirror) and the optical path length of the red light in the reflection is longer than the optical path lengths of other colors of light, the reflection on the chirped mirror will cause the red light that has shifted quickly to the difference in the optical path length. , and if the optical path length difference is adequate, the chirp of the red light will be eliminated. Also, for other colors, if the number of layers and the film thickness of the chirped mirror are adjusted to adjust the difference in the optical path length, the same compensation can be achieved.

Since the number of layers and film thickness of the Bragg film in the chirped mirror are fixed by the formation of each black film (dielectric multilayer film), the difference in the optical path length in one reflection is fixed, and the light is relatively slow. The degree of return (dispersion compensation amount) or its wavelength distribution is fixed. Only femtosecond light pulses can be adequately compensated. This is extremely cumbersome, as chirped mirrors having multiple Bragg films designed for each type of propagation path and femtosecond optical pulse set must be prepared.
Therefore, in the chirp amount variable device of Japanese Patent No. 3569777 (Patent Document 1), two chirp mirrors are arranged facing each other, and a movable mirror or a fixed mirror for changing the number of times of reflection of the femtosecond light pulse between them is arranged. A mirror is also placed. According to this apparatus, by changing the number of times of reflection on the chirped mirror, it is possible to perform dispersion compensation of a femtosecond optical pulse with a dispersion compensation amount that is a natural number multiple of the dispersion compensation amount per reflection.

Japanese Patent No. 3569777

In the apparatus described above, femtosecond optical pulses are dispersion-compensated at discrete dispersion-compensation amounts that are natural-number multiples of a predetermined dispersion-compensation amount, and fine adjustment of the dispersion-compensation amount is difficult.
In addition, if the number of reflections is changed to a relatively large number such as 10 or more in order to increase the types of dispersion compensation amount, the apparatus becomes large-scaled.
Furthermore, the reflectivity in chirped mirrors is practically less than 100%, and the more reflections, the lower the final (total) reflectivity and the greater the loss of femtosecond light pulses.
SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a chirped mirror capable of finely compensating dispersion of ultrashort optical pulses such as femtosecond optical pulses.
Another main object of the present invention is to provide a chirped mirror capable of suppressing the number of reflections and suppressing loss of ultrashort optical pulses due to dispersion compensation.
In addition, a main object of the present invention is to provide a compact chirped mirror unit that can easily perform dispersion compensation by having the chirped mirror described above.

In order to achieve the above object, the invention according to claim 1 provides a chirped mirror comprising a substrate, a high refractive index layer made of a high refractive index material and a low refractive index layer made of a low refractive index material formed on the substrate. and a dielectric multilayer film that is alternating films with layers, wherein each of the high refractive index layers and each of the low refractive index layers as a whole has a respective predetermined physical thickness, and the dielectric At any wavelength within a predetermined wavelength range related to the body multilayer film, the value of the group velocity delay dispersion GDD is a function of the incident angle θ, and monotonically increases or decreases within the predetermined incident angle range, The size of the predetermined wavelength range is 10 nm or more, and the size of the predetermined incident angle range is 15° or more.
In the invention according to claim 2, in the above invention, the fitting straight line of the average value GDD _ave of the group velocity delay dispersion GDD in the predetermined wavelength range is the incident angle θ within the predetermined incident angle range and the incident angle θ within the predetermined incident angle range GDD _ave =a(θ−θ ₀ )+b, −200≦a≦200, −6000≦b≦6000 for the basic incident angle θ ₀ , which is a predetermined value of It is.
The invention according to claim 3 is characterized in that, in the above invention, the predetermined wavelength range includes 1025 nm or more and 1035 nm or less, and the center wavelength is 1030 nm.
According to a fourth aspect of the invention, in the above invention, the predetermined wavelength range includes 780 nm or more and 820 nm or less, and the center wavelength is 800 nm.

In order to achieve the above object, the invention according to claim 5 provides a chirped mirror unit, comprising: the chirped mirror; a chirped mirror rotating mechanism for rotating the chirped mirror relative to the incident light path; an optical path adjustment mirror that adjusts a reflected light path; and an optical path adjustment mirror moving mechanism that allows the optical path adjustment mirror to be rotatable with respect to the reflected light path from the chirped mirror and parallel to the incident optical path. the optical path adjusting mirror moving mechanism rotates the optical path adjusting mirror so as to be parallel to the chirped mirror and moves the optical path adjusting mirror so as to reflect light pulses from the chirped mirror; and adjusting the reflected light path to a constant light path parallel to the incident light path.

The main effect of the present invention is to provide a chirped mirror capable of finely performing dispersion compensation for ultrashort optical pulses including femtosecond optical pulses.
Further, the main effect of the present invention is to provide a chirped mirror capable of suppressing the number of reflections and suppressing loss of ultrashort optical pulses due to dispersion compensation.
Further, the main effect of the present invention is to provide a compact chirped mirror unit that can easily perform dispersion compensation by having the above-described chirped mirror.

(a) is a schematic diagram showing a conventional chirped mirror unit (dispersion compensation unit) or its operation, and (b) is a graph showing the relationship between wavelength and GVD in (a). 4 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 1. FIG. 7 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 2. FIG. 7 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 3. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 4. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 5. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 6. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 7. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 8. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 9. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 10. FIG. 11 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 11. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 12. FIG. 14 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 13. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 14. FIG. 15 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 15. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 16. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 17. FIG. 10 is a graph showing the physical film thickness of each layer in the dielectric multilayer film of Example 18. FIG. 4 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 1. FIG. 7 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 2. FIG. 10 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 3. FIG. 10 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 4. FIG. 10 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 5. FIG. 10 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 6. FIG. 10 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 7. FIG. 10 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 8. FIG. 10 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 9. FIG. 10 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 10. FIG. 11 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 11. FIG. 14 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 12. FIG. 14 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 13. FIG. 14 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 14. FIG. 19 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 15. FIG. 19 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 16. FIG. 19 is a graph showing the incident angle dependency of GDD on the reflecting surface of Example 17. FIG. 14 is a graph showing the incident angle dependence of GDD on the reflecting surface of Example 18. FIG. 4 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 1 in a predetermined incident angle range. 4 is a graph showing GDD (wavelength dependence) of Example 1 in a predetermined wavelength range. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 2 in a predetermined incident angle range. 4 is a graph showing GDD (wavelength dependence) of Example 2 in a predetermined wavelength range. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 3 in a predetermined incident angle range. 10 is a graph showing GDD (wavelength dependence) of Example 3 in a predetermined wavelength range. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 4 in a predetermined incident angle range. FIG. 11 is a graph showing GDD (wavelength dependence) of Example 4 in a predetermined wavelength range; FIG. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 5 in a predetermined incident angle range. 10 is a graph showing GDD (wavelength dependence) of Example 5 in a predetermined wavelength range. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 6 in a predetermined incident angle range. 10 is a graph showing GDD (wavelength dependence) of Example 6 in a predetermined wavelength range. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 7 in a predetermined incident angle range. FIG. 11 is a graph showing GDD (wavelength dependence) of Example 7 in a predetermined wavelength range; FIG. 10 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 8 in a predetermined incident angle range. FIG. 11 is a graph showing GDD (wavelength dependence) of Example 8 in a predetermined wavelength range; FIG. 10 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 9 in a predetermined incident angle range. 10 is a graph showing the GDD (wavelength dependence) of Example 9 in a given wavelength range; 10 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 10 in a predetermined incident angle range. 10 is a graph showing the GDD (wavelength dependence) of Example 10 in a given wavelength range. 11 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 11 in a predetermined incident angle range. 11 is a graph showing GDD (wavelength dependence) of Example 11 in a predetermined wavelength range. 7 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 12 in a predetermined incident angle range. FIG. 13 is a graph showing GDD (wavelength dependence) of Example 12 in a predetermined wavelength range; FIG. 13 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 13 in a predetermined incident angle range. FIG. 13 is a graph showing GDD (wavelength dependence) of Example 13 in a predetermined wavelength range; FIG. 14 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 14 in a predetermined incident angle range. FIG. 11 is a graph showing GDD (wavelength dependence) of Example 14 in a predetermined wavelength range; FIG. 10 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 15 in a predetermined incident angle range. FIG. 11 is a graph showing GDD (wavelength dependence) of Example 15 in a predetermined wavelength range; FIG. 16 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 16 in a predetermined incident angle range. FIG. 12 is a graph showing GDD (wavelength dependence) of Example 16 in a predetermined wavelength range; FIG. 10 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 17 in a predetermined incident angle range. FIG. 12 is a graph showing GDD (wavelength dependence) of Example 17 in a given wavelength range; FIG. 10 is a graph showing (a) reflectance and (b) GDD (incidence angle dependence) of Example 18 in a predetermined incident angle range. FIG. 10 is a graph showing the GDD (wavelength dependence) of Example 18 in a given wavelength range; FIG. 1 is a schematic diagram of a chirped mirror unit (dispersion compensation unit) according to the present invention; FIG.

Hereinafter, examples of embodiments according to the present invention will be described as appropriate with reference to the drawings. In addition, the form of this invention is not limited to these examples.

A chirped mirror according to the present invention has a substrate with a reflective surface, and performs dispersion compensation for ultrashort optical pulses by reflecting the ultrashort optical pulses on the reflective surface.
A case where the ultrashort optical pulse is a femtosecond optical pulse will be described below, but in the present invention, the ultrashort optical pulse is not limited to the femtosecond optical pulse. Also, the present invention can be applied to light pulses other than ultrashort light pulses and light of various periods including femtoseconds. Hereinafter, femtosecond light pulses are simply referred to as light pulses unless otherwise specified.

When an optical pulse propagates through a medium, it chirps due to a deviation in velocity for each wavelength depending on the type of medium, widening the pulse width of the optical pulse and decreasing the peak intensity of the optical pulse.
For example, when the medium is quartz, the group velocity of light at a wavelength of 400 nm (blue light) is about 198 nm/fs (nanometers per femtosecond), and the group velocity at 550 nm (green light) is about 202 nm/fs. The group velocity at 700 nm (red light) is about 204 nm/fs, and in the wavelength range of 400 nm to 1200 nm, the group velocity (nm/fs) increases slowly and monotonically as the wavelength (nm) increases. .
Therefore, in the optical pulse propagating through quartz, the red light is relatively faster than the green light and the blue light, and is shifted so that the red light leads in the propagation direction.
Note that the group velocity V _g (nm/fs) is a function of the wavelength λ (nm), the refractive index of the medium is n (λ), and the speed of light is c (nm/fs). 1].

Then, as an index of the deviation of the group velocity, the group velocity dispersion GVD (Group Velocity Dispersion, fs ² /cm, femtosecond femtosecond per centimeter) represented by the following [Equation 2] is used. The group velocity dispersion GVD corresponds to the gradient of the group velocity. If GVD=0, the group velocity has no wavelength dependence, and the optical pulse propagating in the medium does not chirp. On the other hand, if GVD.noteq.0, there is wavelength dependence in the group velocity, and an optical pulse propagating in a medium with GVD.noteq.0 chirps.

The peak intensity _Imax or the pulse width Δτ of the optical pulse that has undergone chirp is expressed by the following [Equation 3] and [Equation 4] in order using the group velocity dispersion GVD. Here, x is the thickness of the medium (cm), C ₁ and C ₂ are predetermined constants, and I _max,0 and Δτ ₀ are the initial peak intensity (W) and the initial pulse width (fs), respectively. be.

Such a chirped light pulse is reflected on the reflecting surface of the chirped mirror with the light of each wavelength having a difference in optical path length, thereby being dispersion-compensated.
For example, if an optical multilayer film having a plurality of Bragg films with different film thicknesses is formed on the reflecting surface of the chirped mirror, the Bragg reflection causes the light pulse to have a difference in the optical path length of each wavelength. is reflected by It is also possible for the diffraction grating to reflect light pulses with different optical path lengths for each wavelength.
The light pulse undergoes a phase shift, or reflection phase φ(ω) of the chirped mirror, which is a function of the angular velocity ω due to Bragg reflection. The reflection phase φ(ω) is expressed by the following [Equation 5], where t (seconds) is time and C is a predetermined constant.

In [Equation 5] relating to the reflection phase, the lowest order term of the time delay of ω, _-∂φ /∂ω| Delay), and is a value corresponding to the staying time of the chirped mirror (within the optical multilayer film). The central wavelength is a wavelength included in a predetermined wavelength range (target wavelength range), and is grasped corresponding to the peak wavelength (the wavelength at which the intensity is maximized) in the optical pulse targeted for dispersion compensation. Note that the center wavelength may not be the median value of the target wavelength range. Also, the intensity of the optical pulse need not be maximum at the center wavelength.
In addition, −∂ ² φ/∂ω ² | ₀ , which is the lowest order term of the nonlinear term of ω, is substituted with the value of the center wavelength as φ, and corresponds to the group velocity delay dispersion GDD (Group Delay Dispersion). , which is an index of the phase shift. And GDD= ^-∂2φ / ^∂ω2 .
The peak intensity _Imax or the pulse width Δτ of the optical pulse that has undergone chirp can also be expressed by the following [Equation 6] and [Equation 7] using the group velocity delay dispersion GDD in the same manner as the group velocity dispersion GVD. .

Dispersion compensation performed by reflecting a light pulse that has passed through a predetermined propagation path on a chirped mirror is maximized if the following [Equation 8] is satisfied.
Here, i is a number assigned to each type of medium in the propagation path. be. Also, GVD ₁ is the GVD of quartz and GVD ₂ is the GVD of air. In addition, medium thickness ₁ is the quartz thickness (pathlength in quartz) and medium thickness GVD ₂ is the air thickness (pathlength in air).
That is, if the chirped mirror has a GDD that cancels out the GVD in the entire propagation path, dispersion compensation is achieved.
In the wavelength region of 400 nm to 1200 nm, both the GVD of quartz and the GVD of air monotonically decrease, and the GVD of quartz is 363.49 fs ² /cm and the GVD of air is 0.21 fs ² /cm for light with a wavelength of 800 nm. The GVD of quartz is about 1000 times the GVD of air. ) the GVD in the case of progression will be approximately the same.

Conventionally, as shown in FIG. 1( a ), from the viewpoint of ensuring the ease of forming an optical multilayer film and the stability of operation (realization of operation as designed), the angle of incidence and reflection of a light pulse on a chirped mirror Mp The corner was fixed.
Therefore, for example, a light pulse P2 is incident on the opposing chirped mirror Mp so as to be reflected at each of the reflection points R1 to R10 a total of 10 times at a predetermined angle of incidence or reflection. The GVD is reduced each time due to the reduction in GDD due to each reflection (10 solid lines in the same figure), and dispersion is compensated in the wavelength range from 760 nm to 850 nm (optical pulse P3). . Incidentally, the solid lines in FIG.
The chirped mirror Mp here has a GDD corresponding to 1/10 of the GVD in the wavelength range at the incident angle, and is about −60 fs ² ( 1/10 of 600) and flat. A chirped mirror Mp with a negative GDD is also called a negative dispersion mirror.
The optical pulse P1 is emitted from an oscillation device, and has, for example, seven wavelength components (wavelength components W1 to W7 in order from the short wavelength side in FIG. 1A) in phase with each other. By the time the wavelength components W7, W6, . Chirp to lead. ^The GVD of the chirped optical pulse P2 becomes like the one-dot chain line G in FIG. 1(b). The GVD is indicated by a solid line R1 that is closest (uppermost) to the dashed-dotted line G. Also, at the second reflection, the GVD becomes the next closest (second from the top) solid line R2 in FIG. Thus, a light pulse P3 is obtained which has been compensated to be equivalent to the light pulse P1.
In such a conventional facing chirped mirror Mp, when the thickness of the optical glass BK7 changes, the ^number of reflections is set to Although it is possible to compensate for dispersion by changing the thickness, another chirped mirror Mp separately designed for the optical multilayer film (Bragg film structure) is required for thicknesses other than the discrete thickness.

On the other hand, in the present invention, the optical multilayer film (Bragg film structure) is designed such that the GDD monotonously increases or decreases within a predetermined incident angle range according to the change in the incident angle of the light pulse. That is, the GDD of the chirped mirror is a function of the incident angle and monotonically increases or decreases within a predetermined incident angle range.
Therefore, by adjusting the incident angle of the light pulse to the chirped mirror reflecting surface by rotating the chirped mirror relative to the light pulse incident path, the GDD can be continuously changed, and the thickness of the optical glass BK7 can be varied. Even if the propagation path of the optical pulse changes variously, such as changing to , dispersion compensation can be performed on the optical pulse.
In the present invention, a chirped mirror that monotonically increases or decreases in GDD according to changes in the incident angle of the light pulse may be incorporated in an optical system that reflects about 10 times. , the GDD can be easily fine-tuned by relative rotation of the chirped mirrors.

In such a change of GDD, it is sufficient that GDD monotonically increases or decreases in accordance with the magnitude of the incident angle at the center wavelength of the target wavelength band.
Further, if the width Δθ of change in the incident angle θ (the size of the predetermined incident angle region) is 15° or more, it is possible to sufficiently secure the adjustment width and adjustment accuracy of the GDD. If the basic incident angle θ ₀ defined as the most frequently used angle (at which the GDD takes a value aimed at in design) is included within the range Δθ of the change in the incident angle, the basic incident angle θ ₀ is used as the standard. By adjusting the incident angle θ within the range of change Δθ, the GDD can be more easily adjusted. The basic incident angle θ ₀ may be any value as long as it is within the predetermined incident angle range, and can be said to be a constant (predetermined value) within the predetermined incident angle range. Also, the width Δθ of the change in the incident angle can be 5° or more, 10° or more, 20° or more, or any other value or more. The larger the value, the greater the GDD adjustment range, or the finer the GDD can be adjusted.
Furthermore, the target wavelength range may be limited according to the application, the ease of manufacturing the chirped mirror, etc. If it is 1025 nm or more and 1035 nm or less or includes this, a Yb: YAG laser with a center wavelength of 1030 nm If the wavelength is from 780 nm to 820 nm or includes this, it will be suitable for optical pulse dispersion compensation for a Ti:Sapphire laser with a central wavelength of 800 nm.
In addition, if the average value GDD _ave of GDD in the wavelength range of interest has the relationship shown in the following [Equation 9] using the incident angle θ and the basic incident angle θ ₀ , the chirped mirror is It becomes easier to adjust GDD. Here, a is a constant −200≦a≦200 [fs ² /°] and b is a constant −6000≦b≦6000 [fs ² ].

Further, a chirped mirror unit (dispersion compensation unit) capable of continuous adjustment of GDD is formed by the chirped mirror of the present invention and a rotating mechanism capable of rotationally moving the chirped mirror so as to adjust the rotation angle. can be
Further, in such a chirped mirror unit, a plurality of chirped mirrors may be provided, a plurality of rotatably movable and parallelly movable chirped mirrors may be provided, or one or more rotating chirped mirrors may be provided. and one or more ordinary mirrors (mirrors intended only for reflection, ie, propagation direction conversion, without dispersion compensation or GDD change) may be provided.

An embodiment of the invention will now be presented.
However, the examples do not limit the scope of the present invention. In particular, although the central wavelength is 800 nm or 1030 nm in the examples, the central wavelength in the present invention is not limited to these wavelengths.
Also, depending on how the present invention is perceived, the example may become a substantial comparative example outside the scope of the present invention, or the comparative example may become a substantial example within the scope of the present invention. .

As an embodiment of the present invention, formation of a chirped mirror having dielectric multilayer films with different film structures on one side (reflective surface) of the same plate-like substrate was simulated.
The substrate has a circular plate shape with a diameter of 30 mm and is made of optical glass BK7.
Incidentally, the dielectric multilayer film on the reflective surface can actually be formed by alternately vapor-depositing film materials in a state in which each film thickness is controlled by vacuum vapor deposition.

In the dielectric multilayer films in Examples 1 to 6, odd-numbered layers are Ta ₂ O ₅ (high-refractive-index layers made of a high-refractive-index material) and even-numbered layers are SiO ₂ (low-refractive-index layers made of a low-refractive-index material). Each layer in turn has a physical thickness as shown in FIGS. The total number of layers of the dielectric multilayer films in Examples 1 to 6 is 72, 72, 72, 72, 40, 72 in order.
In the dielectric multilayer film in Example 7, the odd layers are Nb ₂ O ₅ (high refractive index layers), the even layers are SiO ₂ , and each layer has a physical film thickness as shown in FIG. The total number of layers of the dielectric multilayer film in Example 7 is 44.
In the dielectric multilayer films of Examples 8 and 9, odd-numbered layers are Ta ₂ O ₅ and even-numbered layers are SiO ₂ , and each layer has a physical film thickness as shown in FIGS. The total number of dielectric multilayer films in Examples 8 and 9 is 58 and 70, respectively.
In the dielectric multilayer films of Examples 10 and 11, odd-numbered layers are Nb ₂ O ₅ and even-numbered layers are SiO ₂ , and each layer has a physical film thickness as shown in FIGS. The total number of layers of the dielectric multilayer films in Examples 10 and 11 is 62 and 66, respectively.
In the dielectric multilayer films of Examples 12-16, odd-numbered layers are Ta ₂ O ₅ and even-numbered layers are SiO ₂ , and each layer has a physical film thickness as shown in FIGS. The total number of layers of the dielectric multilayer films in Examples 12 to 16 is 62, 62, 72, 50 and 50 in order.
In the dielectric multilayer films of Examples 17 and 18, odd-numbered layers are Nb ₂ O ₅ and even-numbered layers are SiO ₂ , and each layer has a physical film thickness as shown in FIGS. The total number of layers of the dielectric multilayer films in Examples 17 and 18 is 44, 44, respectively.

On each reflective surface of Examples 1 to 18, the GDD distribution (0° of GDD An incident angle dependence of ∼90°) was measured by simulation.
The wavelength of the light pulse was 1030 nm or around 1025 nm and 1035 nm (Yb:YAG laser) in Examples 1 to 6, 8 to 9, and 12 to 16, and in Examples 7, 11, 17 to 18. The wavelength is 800 nm or around 780 nm and 820 nm (Ti: Sapphire laser).
Incidence angle dependencies from 0° to 90° for Examples 1-18 are shown in FIGS. 20-37, respectively.

In Example 1 (FIG. 20), when 5≦θ≦30 (Δθ=25), the GDD related to the central wavelength (1030 nm) or the wavelengths around it (1025, 1035 nm), that is, the target wavelength range of 1025 to 1035 nm monotonically increases. A fitting straight line obtained by fitting the mean of the GDD distribution at 1025 nm to 1035 nm by the method of least squares (the above [Equation 9]) has a=37.2 and b=−1060.9. At the basic incident angle θ ₀ =5 (s-polarized light), the GDD of the center wavelength is −1000. FIG. 38 shows the reflectance and GDD (partially enlarged view of FIG. 20) of Example 1 in this incident angle range (incidence angle range). In addition, Example 1 when the wavelength is gradually changed for each predetermined incident angle (θ = 5, 10, 15, 20, 25, 30) in the target wavelength region and the region before and after it, that is, 1020 to 1040 nm. , and the GDD (wavelength dependence) are shown in FIG. The fitting straight line is determined on a plane where the horizontal axis is θ and the vertical axis is GDD, and may be fitted by a method other than the least squares method.
On the other hand, in 45≦θ≦68 (Δθ=23), the GDD related to the target wavelength range monotonously decreases.

In Example 2 (FIG. 21), the GDD in the target wavelength range of 1025 to 1035 nm monotonously increases when 5≦θ≦30 (Δθ=25), and a=19.9, b=−988. 2. At θ ₀ =5 (s-polarized light), GDD of the center wavelength is −1000. FIG. 40 shows the reflectance and GDD of Example 2 in this incident angle range. 41.
On the other hand, in 45≦θ≦52 (Δθ=7), the GDD related to the target wavelength band monotonously decreases.
Furthermore, in 75≦θ<90 (Δθ=15), the GDD related to the target wavelength band monotonously decreases.

In Example 3 (FIG. 22), when 5≦θ≦30 (Δθ=25), the GDD in the target wavelength range of 1025 to 1035 nm increases monotonically, and the fitting straight line a=0.003 and b=−999. .7. At θ ₀ =5 (s-polarized light), GDD of the center wavelength is −1000. FIG. 42 shows the reflectance and GDD of Example 3 in this incident angle range, and FIG. shown in
On the other hand, in 40≦θ≦57 (Δθ=17), the GDD related to the target wavelength range monotonously decreases.
Furthermore, in 70≦θ<75 (Δθ=5), the GDD related to the target wavelength band monotonously decreases.

In Example 4 (FIG. 23), when 5≦θ≦30 (Δθ=25), the GDD in the target wavelength range of 1025 to 1035 nm monotonically decreases, and the fitting straight line a=−19.2, b=− 1013.4. At θ ₀ =5 (s-polarized light), the GDD of the fitting straight line (linear (Ave)) is −1000. FIG. 44 shows the reflectance and GDD of Example 4 in this incident angle range, and FIG. shown in
On the other hand, in 38≦θ≦62 (Δθ=24), the GDD related to the target wavelength region monotonously decreases.

In Example 5 (FIG. 24), when 5≦θ≦30 (Δθ=25), the GDD in the target wavelength range of 1025 to 1035 nm increases monotonically, and the fitting straight line a=12.0, b=−301. .9. At θ ₀ =5 (s-polarized light), GDD of the fitting straight line is −300. FIG. 46 shows the reflectance and GDD of Example 5 in this incident angle range, and FIG. shown in
On the other hand, in 38≦θ≦62 (Δθ=24), the GDD related to the target wavelength region monotonously decreases.

In Example 6 (FIG. 25), when 12≦θ≦30 (Δθ=18), the GDD related to the target wavelength range of 1025 to 1035 nm increases monotonically, and the fitting straight line a=154.9, b=−2946. .0. The GDD at the center wavelength of 1030 nm monotonically increases in 5≦θ≦30 (Δθ=25). At θ ₀ =5 (s-polarized light), the GDD of the fitting straight line is approximately −3000. FIG. 48 shows the reflectance and GDD of Example 6 in this incident angle range, and FIG. shown in
On the other hand, in 38≦θ≦43 (Δθ=5), the GDD related to the target wavelength band monotonously decreases.
Furthermore, in 50≦θ≦55 (Δθ=5), the GDD related to the target wavelength region monotonously decreases.
Also, in 62≦θ≦72 (Δθ=10), the GDD related to the target wavelength range monotonously decreases.

In Example 7 (FIG. 26), when 5≦θ≦30 (Δθ=25), the GDD related to the target wavelength range of 780 to 820 nm increases monotonously, and the fitting straight line a=1.3 and b=−44. .3. At θ ₀ =5 (s-polarized light), the GDD of the target wavelength range average (Ave) is −40. FIG. 50 shows the reflectance and GDD of Example 7 in this incident angle range. , and the GDD are shown in FIG.
On the other hand, in 70≦θ≦77 (Δθ=7), the GDD related to the target wavelength band monotonously decreases.

In Example 8 (FIG. 27), when 37≦θ≦53 (Δθ=16), the GDD in the target wavelength range of 1025 to 1035 nm increases monotonically, and the fitting straight line a=33.3, b=−666. .6. At θ ₀ =45 (s-polarized light), GDD of the fitting straight line is −650. FIG. 52 shows the reflectance and GDD of Example 8 in this incident angle range, and FIG. shown in
On the other hand, in 0≦θ≦33 (Δθ=33), the GDD related to the target wavelength band monotonously decreases.
Furthermore, in 55≦θ≦58 (Δθ=3), the GDD related to the target wavelength region monotonously decreases.
In addition, in 70≦θ<90 (Δθ=20), the GDD related to the target wavelength range monotonously decreases.

In Example 9 (FIG. 28), when 37≦θ≦54 (Δθ=17), the GDD related to the target wavelength range of 1025 to 1035 nm increases monotonically, and the fitting straight line a=33.6, b=−664. .9. At θ ₀ =45 (s-polarized light), GDD of the fitting straight line is −650. FIG. 54 shows the reflectance and GDD of Example 9 in this incident angle range, and FIG. shown in
On the other hand, in 23≦θ≦33 (Δθ=10), the GDD related to the target wavelength region monotonously decreases.
Also, in 85≦θ<90 (Δθ=5), the GDD related to the target wavelength region monotonically increases.

In Example 10 (FIG. 29), when 38≦θ≦53 (Δθ=15), the GDD related to the target wavelength range of 780 to 820 nm increases monotonically, and the fitting straight line a=1.8, b=−34. .1. The GDD of the center wavelength monotonously increases in 36≦θ≦55 (Δθ=19). At θ ₀ =45 (s-polarized light), the average GDD of the target wavelength range is −33. The reflectance and GDD of Example 10 in this incident angle range are shown in FIG. , and the GDD are shown in FIG.
On the other hand, in 0≦θ≦10 (Δθ=7), the GDD related to the target wavelength band monotonously decreases.

In Example 11 (FIG. 30), when 40≦θ≦52 (Δθ=12), the GDD related to the target wavelength range of 780 to 820 nm increases monotonously, and the fitting straight line a=1.8, b=−34. .1. The GDD of the center wavelength monotonously increases in 35≦θ≦55 (Δθ=20). At θ ₀ =45 (s-polarized light), the average GDD of the target wavelength range is −30. FIG. 58 shows the reflectance and GDD of Example 11 in this incident angle range. , and the GDD are shown in FIG.
On the other hand, in 0≦θ≦30 (Δθ=30), the GDD related to the target wavelength range monotonously decreases.

In Example 12 (FIG. 31), the GDD in the target wavelength range of 1025 to 1035 nm monotonically decreases when 25≦θ≦64 (Δθ=39).
FIG. 60 shows the reflectance and GDD of Example 12 in the incident angle range of 5≦θ≦30. Rate and GDD are shown in FIG. According to FIGS. 31 and 60, the GDD related to the target wavelength range monotonically decreases at 0≦θ≦13 (Δθ=13), and the GDD related to the target wavelength range decreases at 19≦θ≦22 (Δθ=3). monotonically increasing. The GDD of the center wavelength monotonically decreases when 0≦θ<15 (Δθ=15), and monotonically increases when 15≦θ≦22 (Δθ=7).

In Example 13 (FIG. 32), the GDD in the target wavelength range of 1025 to 1035 nm monotonously decreases when 19≦θ≦67 (Δθ=48).
FIG. 62 shows the reflectance and GDD of Example 13 in the incident angle range of 5≦θ≦30. Rate and GDD are shown in FIG. According to FIGS. 32 and 62, the GDD related to the target wavelength range monotonously decreases at 0≦θ≦7 (Δθ=7), and the GDD related to the target wavelength range decreases at 15≦θ≦17 (Δθ=2). monotonically increasing. The GDD of the center wavelength monotonously decreases when 0≦θ<12 (Δθ=12), and monotonically increases when 12≦θ≦17 (Δθ=5).

In Example 14 (FIG. 33), the GDD related to the target wavelength range of 1025 to 1035 nm monotonically decreases at 30 ≤ θ ≤ 36 (Δθ = 6), and at 40 ≤ θ ≤ 43 (Δθ = 3), the target The GDD related to the wavelength range monotonously increases, and the GDD related to the target wavelength range monotonously decreases at 46 ≤ θ ≤ 52 (Δθ = 6), and at 62 ≤ θ ≤ 82 (Δθ = 20), the target The GDD related to the wavelength band is monotonically decreasing.
FIG. 64 shows the reflectance and GDD of Example 14 in the incident angle range of 5≦θ≦30. Rate and GDD are shown in FIG. According to FIGS. 33 and 64, the GDD related to the target wavelength range increases monotonously in the range of 9≦θ≦16 (Δθ=7). Note that the GDD of the center wavelength monotonically increases in 5≦θ≦22 (Δθ=17).

In Example 15 (FIG. 34), the GDD in the target wavelength range of 1025 to 1035 nm monotonously decreases at 0≦θ≦22 (Δθ=22), and at 57≦θ<90 (Δθ=33), the target The GDD related to the wavelength band is monotonically decreasing.
FIG. 66 shows the reflectance and GDD of Example 15 in the incident angle range of 35≦θ≦55. Rate and GDD are shown in FIG. According to FIGS. 34 and 66, the GDD for the target wavelength range monotonically decreases at 35≦θ≦42 (Δθ=7), and the GDD for the target wavelength region at 50≦θ≦53 (Δθ=3) is monotonically increasing. The GDD of the center wavelength monotonically decreases when 30≦θ≦42 (Δθ=12), and monotonically increases when 45≦θ≦55 (Δθ=10).

In Example 16 (FIG. 35), the GDD in the target wavelength range of 1025 to 1035 nm monotonically decreases when 0≦θ≦42 (Δθ=42).
FIG. 68 shows the reflectance and GDD of Example 16 in the incident angle range of 35≦θ≦55. Rate and GDD are shown in FIG. Note that the data for θ=55 is omitted in FIG. 69(b).
According to FIGS. 35 and 68, the GDD related to the target wavelength range monotonically increases in 51≦θ≦53 (Δθ=2). The GDD of the center wavelength monotonically decreases when 0≦θ≦43 (Δθ=43), and monotonously increases when 50≦θ≦54 (Δθ=4).

In Example 17 (FIG. 36), the GDD at the center wavelength of 800 nm monotonously decreased at 0 ≤ θ ≤ 32 (Δθ = 32), monotonically increased at 33 ≤ θ ≤ 42 (Δθ = 9), and 43 ≤ θ ≤ monotonically decreasing at 53 (Δθ=10), monotonically increasing at 53≦θ<61 (Δθ=8), monotonically increasing at 33≦θ≦42 (Δθ=9), and monotonically increasing at 62≦θ≦67 (Δθ=5 ) monotonically decreases.
FIG. 70 shows the reflectance and GDD of Example 17 in the incident angle range of 35≦θ≦55. Rate and GDD are shown in FIG. According to FIGS. 36 and 70, there is no portion in which the GDD of the target wavelength range of 1025 to 1035 nm monotonically increases or decreases in this incident angle range.

In Example 18 (FIG. 37), the GDD related to the target wavelength range of 780 to 820 nm monotonously decreases in 0≦θ≦32 (Δθ=32). Note that the GDD at a center wavelength of 800 nm monotonically decreases at 0 ≤ θ < 37 (Δθ = 35), monotonically increases at 37 ≤ θ ≤ 49 (Δθ = 12), and at 49 < θ ≤ 70 (Δθ = 21) It monotonically decreases and monotonically increases when 70<θ≦80 (Δθ=10).
FIG. 72 shows the reflectance and GDD of Example 16 in the incident angle range of 35≦θ≦55. Rate and GDD are shown in FIG. According to FIGS. 37 and 72, the GDD in the target wavelength range monotonously increases in 42≦θ≦44 (Δθ=2).

According to these embodiments, the GDD can be changed continuously by changing the incident angle of the light pulse with respect to the reflective surface of the chirped mirror.
The incident angle θ of the light pulse can be easily adjusted by a chirped mirror unit (dispersion compensation unit) as shown in FIG.
That is, the chirped mirror unit U includes a case 2 having an entrance IN and an exit OUT for a light pulse P (propagation path, that is, the optical path is indicated by a dashed line), and an optical multilayer film E (dielectric multilayer film). a chirped mirror M having a reflecting surface F; a chirped mirror rotating mechanism 3 capable of rotating the chirped mirror M and holding it at a desired rotational position; and an optical path adjusting mirror moving mechanism 5 that can be translated and held at a desired rotational position or translational position.
A chirped mirror M (such as one of Examples 1 to 18) is provided inside the case 2 via a chirped mirror rotating mechanism 3 . The chirped mirror rotating mechanism 3 has a vertical axis 6 connected to the chirped mirror M, rotates the chirped mirror M about a vertical line including one point (for example, the center point) of its reflecting surface F, and Hold in the desired rotational position. Here, one point on the reflecting surface F is a point that receives (reflects) the light pulse P entering from the entrance IN.
The optical path adjusting mirror 4 is provided inside the case 2 via an optical path adjusting mirror moving mechanism 5 . The optical path adjustment mirror moving mechanism 5 has a vertical axis (not shown) connected to the optical path adjustment mirror 4 and a rail 7 into which the axis is inserted. It is rotatable around and translatable along a rail 7 parallel to the propagation path of the light pulse P entering from the entrance IN, and is held at a desired rotational or translational position. . The optical path adjusting mirror 4 here is a low-dispersion mirror in which the GDD is approximately 0 at any incident angle (at least over the entire target incident angle range) within the target wavelength range. The optical path adjustment mirror 4 may be a metal mirror without an optical multilayer film, a mirror with an optical multilayer film, or a chirped mirror.
The output port OUT is arranged on a virtual extension line of the rail 7 or adjacent thereto, and is arranged on the opposite side to the input port IN. The optical path adjustment mirror 4 changes the propagation direction of the light pulse P from the chirped mirror M, that is, the direction of the reflected optical path, to a direction parallel to the rail 7 toward the exit OUT. Incidentally, the exit port OUT may be arranged on the same side as the entrance port IN. Also, the optical path adjustment mirror 4 may change the propagation direction of the light pulse P to a direction that is not parallel to the rail 7 or to a direction that is not parallel to the light pulse P entering from the entrance IN. Also, a plurality of optical path adjustment mirrors 4 may be provided to change the reflected optical path in various ways.

In the chirped mirror unit U, the chirped mirror M is rotated by the chirped mirror rotating mechanism 3 so that the light pulse P from the entrance IN has an incident angle θ that is GDD corresponding to the GVD of the light pulse P up to the entrance IN. rotated. Since the optical path adjustment mirror 4 is a low-dispersion mirror, there is no need to consider its GDD, and the adjustment of the incident angle .theta. by rotating the chirped mirror M is easy. In the chirped mirror unit U, the chirped mirror M may be replaceable for the purpose of changing the changeable GDD width (area size), the target wavelength range, and the like. Alternatively, the incident angle θ may be adjusted by moving the optical path of the light pulse P incident on the chirped mirror M instead of rotating the chirped mirror M, or along with the rotation thereof.
In addition, if the optical path adjustment mirror 4 is rotated so as to be parallel to the chirped mirror M and further translated so as to reflect the light pulse P from the chirped mirror M, the light dispersion-compensated by the chirped mirror M is The pulse P is emitted from the output port OUT along a constant optical path parallel to the light pulse P from the input port IN, and there is no need to readjust the optical system outside the chirped mirror unit U each time the chirped mirror M is rotated. ,It does not take time and effort.
If the optical path adjusting mirror 4 is a chirped mirror, the chirped mirror M is angularly adjusted (GDD adjusted) so that the reflected light pulse has a GDD equivalent to that compensated by the optical path adjusting chirped mirror. .

In Examples 1 to 18, the value of GDD related to the center wavelength has a predetermined range of incident angles θ in which the values monotonically increase or decrease with respect to the incident angle θ.
For example, in Example 1, in the range of 5≦θ≦30, the GDD associated with the center wavelength of 1030 nm monotonically increases from about −1000 to about −180 with respect to the incident angle θ.
By changing the incident angle θ within such a range of incident angles θ, the GDD can be changed continuously, and by monotonically increasing or monotonically decreasing, the change in GDD always increases as the incident angle θ increases. Alternatively, the GDD can be easily adjusted by adjusting the incident angle θ.

Further, if the range of the incident angle θ in which the GDD monotonously increases or monotonously decreases (the width of change in the incident angle θ) is 15° or more, the adjustment range of the incident angle θ becomes sufficient, and the GDD can be adjusted. It can be sufficiently secured (for example, in the range of 5 ≤ θ ≤ 30 in Example 1, GDD can be adjusted from -1000 to -180), or GDD can be finely adjusted (for example, 5 ≤ θ ≤ 30 in Example 3 range). The width Δθ of the change in the incident angle can be 5° or more, 10° or more, 20° or more, or any other value or more. The larger the value, the greater the GDD adjustment range, or the finer the GDD can be adjusted.

Furthermore, if the target wavelength range is 1025 to 1035 nm, it is suitable for dispersion compensation of optical pulses related to Yb: YAG laser with a center wavelength of 1030 nm, and if it is 780 to 820 nm or less, it is suitable for Ti: Sapphire laser with a center wavelength of 800 nm. It is suitable for dispersion compensation of such optical pulses.

In addition, if the GDD monotonically increases or decreases with respect to the incident angle θ within the target incident angle change range at any wavelength within the target wavelength range, the GDD can be similarly adjusted over the entire target wavelength range. A chirped mirror M is provided in which the GDD is adjusted appropriately. When dispersion-compensating an optical pulse of a Yb:YAG laser with a central wavelength of 1030 nm, light with wavelengths on both sides of the central wavelength is also included as a component, so the GDD of the regions on both sides is the same as the GDD of the central wavelength. If it changes, it will contribute to more appropriate dispersion compensation.
For example, in Example 1, as shown in FIG. 39B, the GDD of 10 is larger than the GDD of the incident angle θ [°] = 5 for any wavelength in the target wavelength range of 1025 to 1035 nm, Similarly, the GDD is larger in the order of 15, 20, 25, and 30, and the GDD is monotonic with respect to the incident angle θ within the target incident angle change region (5 ≤ θ ≤ 30) at any wavelength within the target wavelength range. As the incident angle increases, the GDD monotonically increases over the entire wavelength range of interest.

In addition, if each distribution of GDD versus incident angle θ is similar (within the incident angle region of interest) for the center wavelength and the wavelengths at both ends of the wavelength band of interest, then for most of the wavelength band of interest, the GDD vs. incident angle θ Chirped mirrors M are provided that are similar in value and have GDD adjusted appropriately over most or all of the wavelength range of interest.
For example, in Example 1, as shown in FIG. The GDD distribution of the maximum end of the target wavelength range) is along a straight line rising to the right from GDD=−1000 to −200, and overlaps with each other. Therefore, the value of GDD with respect to the angle of incidence θ is similar over most or all of the wavelength range of interest.
In Example 1, with respect to the distribution of the average value GDD _ave of GDD in the target wavelength range with respect to the incident angle θ, the GDD distribution at the minimum end of the target wavelength range, the GDD distribution at the center wavelength, and the GDD distribution at the maximum end of the target wavelength range are all within 15% above and below the change width of GDD _ave . Where the GDD value (−40) at the maximum edge of the target wavelength range (1035 nm) is farthest from the GDD _ave value (−160) at θ=30 in Example 1, the GDD _ave value in the target wavelength range is The change width is 840 (absolute value of -1000-(-160)), and 15% is -160 + 840 x 0.15 = -34, and -34≧-40, so the value of GDD _ave The value of GDD at the maximum end of the target wavelength range is within 15% above the change width of GDD _ave , and the GDD distribution at the minimum end of the target wavelength range, the GDD distribution at the center wavelength, and the maximum target wavelength range High similarity with edge GDD distribution. In addition, it is not within 15% of the change width of GDD _ave , but it can be within 5%, 10%, 20%, or within another arbitrary value, and the smaller the GDD distribution in the target wavelength range. becomes higher. Also, instead of GDD _ave , at least one of the reference line and the width of change for which the fitting straight line is within a predetermined percentage above and below can be grasped.
Considering such a point of view, in Example 17, the GDD distribution at the minimum end of the target wavelength range, the GDD distribution at the center wavelength, and the GDD distribution at the maximum end of the target wavelength range at any incident angle of 0 ≤ θ < 90 There is less uniformity with the distribution, and there are less incident angle ranges that both monotonically increase or both monotonically decrease, so that the other embodiments are easier to adjust GDD.

Also, for the center wavelength of the target wavelength range and other wavelengths, when each distribution of GDD with respect to the incident angle θ (within the target incident angle range) is parallel except for a constant, GDD is performed while compensating for high-order dispersion. can also be adjusted.
For example, in Example 5, the value of GDD increases as a function of wavelength, as shown in FIG. 47, so this mirror has a negative Third Order Dispersion (TOD) ( TOD<0). Here, TOD is the second nonlinear term of ω in [Equation 5] above, and TOD=−∂ ³ φ/∂ω ³ | ₀ . As the angle of incidence increases, the GDD monotonically increases without changing the magnitude relationship between the wavelengths. That is, the GDD, which is the second-order dispersion, can be adjusted while the third-order dispersion is fixed.

Furthermore, the fitting straight line of GDD _ave is -200≦a≦200 [fs ² /°] in the above [Equation 9], b is a constant, and −6000≦b≦6000 [fs ² ]. If so, the GDD will change more gently in the target incident angle range, and the change in GDD based on the change in the incident angle θ will be easier to handle, and the GDD will be easier to control.
Examples 1 to 3, 5 to 7 (5≦θ≦30), Example 4 (5≦θ≦29), and Example 8 (37≦θ≦ 53), Example 9 (37≤θ≤54), Example 10 (36≤θ≤55), and Example 11 (35≤θ≤55). In Example 14, a=226.3, and from this point of view, the change is too rapid.

Also, if the GDD changes from negative to positive in the target incident angle range, two types of characteristics, a positive dispersion mirror and a negative dispersion mirror, can be realized by one chirped mirror, and GDD can be adjusted in various ways. This is convenient, and the dispersion compensating unit becomes compact because it is not necessary to arrange both the positive dispersion mirror and the negative dispersion mirror.
Example 6 (5≦θ≦30, GDD=−2800 to 1000) and Examples 12 to 15 have such characteristics.

3 Chirp mirror rotating mechanism 4 Optical path adjusting mirror 5 Optical path adjusting mirror moving mechanism E Optical multilayer film (dielectric multilayer film) F Reflecting surface M Chirp mirror U・・Chirp mirror unit.

Claims (5)

a substrate;
a dielectric multilayer film, which is an alternating film of high refractive index layers made of a high refractive index material and low refractive index layers made of a low refractive index material, formed on the substrate;
and
Each of the high refractive index layers and each of the low refractive index layers as a whole has a predetermined physical thickness,
At any wavelength within a predetermined wavelength range of the dielectric multilayer film, the value of the group velocity delay dispersion GDD is a function of the incident angle θ, and monotonically increases or monotonically decreases within the predetermined incident angle range. death,
The size of the predetermined wavelength range is 10 nm or more,
A chirped mirror, wherein the size of the predetermined incident angle region is 15° or more. The fitting straight line of the average value GDD _ave of the group velocity delay dispersion GDD in the predetermined wavelength range is the angle of incidence θ within the predetermined incident angle range and the basic incident angle θ ₀ , which is a predetermined value within the predetermined incident angle range,
GDD _ave =a(θ−θ ₀ )+b,
-200≤a≤200,
-6000≤b≤6000,
2. The chirped mirror according to claim 1 , which satisfies the following condition. The predetermined wavelength range includes 1025 nm or more and 1035 nm or less,
3. The chirped mirror according to claim 1, wherein said central wavelength is 1030 nm. The predetermined wavelength range includes 780 nm or more and 820 nm or less,
3. The chirped mirror according to claim 1, wherein said central wavelength is 800 nm. a chirped mirror according to any one of claims 1 to 4;
a chirped mirror rotating mechanism that rotates the chirped mirror relative to the incident optical path;
an optical path adjustment mirror that adjusts the reflected optical path of the chirped mirror;
an optical path adjustment mirror moving mechanism that allows the optical path adjustment mirror to be rotatable with respect to the reflected optical path from the chirped mirror and movable in parallel with the incident optical path;
and
the optical path adjustment mirror moving mechanism rotates the optical path adjustment mirror so as to be parallel to the chirped mirror and moves the optical path adjustment mirror so as to reflect light pulses from the chirped mirror;
The chirped mirror unit, wherein the optical path adjustment mirror adjusts the reflected optical path to a constant optical path parallel to the incident optical path.

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