JP2002082220A - Dielectric multilayer film compensation mirror - Google Patents

Abstract

(57) [Summary] An object of the present invention is to reduce vibration of dispersion. According to the present invention, a dielectric multilayer film dispersion-compensating mirror conventionally manufactured using two kinds of dielectrics, a high refractive index dielectric and a low refractive index dielectric, is replaced with a conventional high refractive index dielectric. And two types of low-refractive-index dielectrics and a dielectric having an intermediate refractive index between the two types of dielectrics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dielectric multilayer film dispersion-compensating mirror, and more particularly to dispersion compensation, that is, in the field of manufacturing and utilizing equipment for oscillation, amplification and control of short pulse lasers. The present invention relates to compensation or control of wavelength dependence of delay time during generation and propagation of short optical pulses.

[0002]

2. Description of the Related Art A conventional dielectric multilayer film dispersion compensating mirror will be described. In recent years, lasers capable of generating ultrashort (several to 100 femtosecond) light pulses are rapidly developing. Measurement of ultrafast phenomena in materials utilizing the ultrafast nature of ultrashort light pulses, multivalent ion generation using the fact that the electric field strength becomes equivalent to the Coulomb field in atoms due to high energy and short pulse width , X-ray generation and the like are performed by using such light pulses. When such an ultrashort optical pulse is generated and propagated, the wavelength dependence (dispersion) of the group delay time becomes a problem. If the group delay time differs depending on the wavelength, an ultrashort optical pulse having a spectrum with a wide wavelength width will have different wavelength components having different propagation times, and as a result, the pulse width will increase,
The high speed and high strength cannot be fully utilized. Dispersion exists in all materials, and in this case, it is particularly problematic for the dispersion in the cavity of the mode-locked laser oscillator used to generate ultrashort light pulses and the reflection of ultrashort light pulses. This is dispersion by the dielectric multilayer mirror used. Evaluation of variance is generally
Evaluation is performed using a value obtained by differentiating the group delay time with an angular frequency at a certain wavelength. Usually, the first derivative of the group delay time is defined as a second dispersion, the second differentiation is defined as a third dispersion, and the nth differentiation is defined as an (n + 1) th dispersion. Call. If the secondary dispersion is zero irrespective of the wavelength, the group delay time is constant irrespective of the wavelength, so that the pulse width does not increase due to the dispersion. Further, by controlling the dispersion, it is possible to shape the time waveform of the ultrashort light pulse into a desired shape to some extent.

[0003] The dielectric multilayer film dispersion compensating mirror cancels the dispersion in the resonator of such a mode-locked laser oscillator, the dispersion of a substance that transmits an ultrashort optical pulse, or the time waveform of an ultrashort optical pulse. Is made in order to shape into a desired shape. This reflecting mirror is based on a dielectric multilayer film reflecting mirror that can be used in the far-infrared to X-ray region. The dielectric multilayer mirror is a dielectric material that is transparent to light in the predetermined wavelength range on a smooth glass substrate surface for reflecting light. The higher index is referred to as the high index dielectric and the lower index is referred to as the low index dielectric, which are relative to each other and not distinguishable by absolute index values.) Are alternately stacked in several tens of layers. FIG. 4 shows an example of the structure. When the optical thickness of each film is reduced to 1/4 of a certain wavelength, a high reflectance is obtained in the vicinity of the wavelength. The larger the difference between the refractive indices of the two types of dielectrics, the greater the reflectance with a smaller number of layers. Can be
It is generally known that the reflection wavelength band can be widened. However, its dispersion characteristics, reflection band,
Since it is uniquely determined by the refractive index, the film thickness and the number of layers of the dielectric used, it is difficult to obtain any desired dispersion characteristics. Also, in the combination of the above dielectrics, for example, the bandwidth is about 200 nm around the center wavelength of 800 nm, and the spectrum width of the ultrashort light pulse having a pulse width of less than 10 femtoseconds is 200 nm. Therefore, if such a reflecting mirror reflects such an optical pulse, the spectrum of the pulse will be narrowed, resulting in an increased pulse width. It is extremely difficult to handle without increasing the pulse width. To simply widen the band, the optical thickness of the preceding dielectric is not all constant.For example, a plurality of dielectric multilayers having different high reflection wavelength ranges are superimposed on each other, or the thickness is increased from the substrate to the surface layer. It can be realized by gradually increasing or decreasing. However, in these methods, the dispersion greatly fluctuates within the high reflection band, and it is extremely difficult to handle ultrashort light pulses with these film configurations.

[0004] A dielectric multilayer film dispersion compensating mirror uses each of the dielectric multilayer film reflecting mirror designs as an initial value and uses an optimization method such as a quasi-Newton method or an annealing method to make each layer to have a desired dispersion. The film thickness was optimized, and based on the result. FIG. 5 shows an example of the film configuration of a conventional dielectric multilayer film dispersion compensating mirror. In this figure, the film number on the horizontal axis indicates the film on the surface of the reflecting mirror as 1
2, 3, 4,... As approaching the glass substrate. The vertical axis is the optical thickness of each film. This is an example of a reflecting mirror designed to compensate for intra-cavity dispersion of a mode-locked titanium sapphire laser oscillator used for generating ultrashort (several to 100 femtosecond) optical pulses in recent years. When the pulse makes one round trip, one having dispersion in the resonator, such as titanium sapphire as an amplification medium, a dielectric multilayer film output mirror for extracting output, and a synthetic quartz prism for compensating dispersion in a wide band The sum of the dispersion and the second-order dispersion when the light is reflected five times by the reflecting mirror is set to be zero from 650 nm to 1000 nm. The design is performed by optimizing each film thickness by the quasi-Newton method so as to obtain a desired dispersion, with a film configuration in which the film thickness is gradually reduced from the substrate to the surface layer as an initial value.

In order to obtain good dispersion characteristics in such a dielectric multilayer film dispersion compensating mirror, the error of each film thickness must generally be about 1/1000 of the center wavelength in a predetermined wavelength range. Are known. Normally, in such an apparatus for producing a dielectric multilayer film, the optical thickness of the film can be controlled with such precision as described above because the optical thickness of the film is usually approximately
The optical thickness of each film in the above-mentioned example of the dielectric multilayer dispersion compensation mirror is 100 nm or more.
Optimization has been performed to be 100 nm or more. However, if the film thickness is limited first when performing optimization, it is not really the best solution if the number of parameters to be optimized is several tens like this reflector. It is a good solution from around, but it may converge to a sub-optimal solution. In order to prevent such a situation, optimization is first performed without limiting the film thickness, and thereafter, the thickness of the layer having an optical thickness of 100 nm or less is reduced to 100 nm, and the thickness of all the layers is reduced. The optimization was performed again with a thickness limit of 100 nm or more, and the film structure shown in FIG. 5 was obtained. FIG. 6 shows the reflectance, the wavelength characteristic of the secondary dispersion, and the target value of the secondary dispersion of the structure shown in FIG. The reflectivity in this figure is calculated from the film structure shown in FIG. 5 earlier. For the second-order dispersion, the phase calculated from the film structure is differentiated twice by the angular frequency at each wavelength. Calculated. Looking at this figure, 2
It can be seen that the secondary dispersion is oscillating. This oscillation increases the time width of the optical pulse during dispersion compensation, and when used in a mode-locked oscillator, modulates the spectral intensity of the output optical pulse according to the oscillation of this dispersion. Because it causes problems, how to reduce it is an important issue.

[0006]

In the above-mentioned conventional example, there is dispersion oscillation, which increases the time width of an optical pulse during dispersion compensation, or produces an output light when used in a mode-locked oscillator. Since the spectrum intensity of the pulse causes a problem such as modulation in accordance with the oscillation of the dispersion, it is important to reduce the intensity. An object of the present invention is to solve such a problem and reduce the vibration of this dispersion.

[0007]

SUMMARY OF THE INVENTION In the present invention, a dielectric multilayer film dispersion compensating mirror which has conventionally been manufactured using two types of dielectrics, a high refractive index dielectric and a low refractive index dielectric, is now described. Three types of dielectrics (high refractive index dielectrics) are added to the two types of high refractive index dielectrics and low refractive index dielectrics, plus dielectrics having intermediate refractive indices (referred to as intermediate refractive index dielectrics). Body, low-refractive-index dielectric, and intermediate-refractive-index dielectric are relative to each other and are not distinguished by absolute refractive index values.) Can be measured.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As shown in FIG. 4, in the case of a structure in which low-refractive-index dielectrics and high-refractive-index dielectrics are alternately overlapped,
When the optical thickness of each layer is 1/4 of a certain wavelength, the reflectance at that wavelength is the largest, and when the optical thickness is smaller than 1/4, the reflectance decreases. Also, the larger the difference between the refractive indices of the low refractive index dielectric and the high refractive index dielectric, the greater the reflectance.
In FIG. 5, optimization without initial film thickness limitation
1/4 of 650 nm, the shortest wavelength of interest 16
The presence of a layer thinner than 2.5 nm indicates that some non-zero reflection near the film is required. If strong reflection is required in the vicinity of the film, the optical thickness of the film is 650 nm, which is the target wavelength of the mirror, １ ／ of 1000 nm, ie, 162.5 nm.
Should be between ~ 250 nm, and if not needed,
This is because the thickness of the layer should be zero as a result of the optimization. If the film thickness is made larger than the optimum film thickness due to the film thickness limitation, the reflection near the film becomes unnecessarily large, deviating from the optimum condition. Therefore, if the film is made of a dielectric material having an intermediate refractive index between the high refractive index dielectric material and the low refractive index dielectric material (intermediate refractive index dielectric material),
Even with the same film thickness, the reflection becomes small, so that it is possible to approach the optimum condition.

The design of a reflecting mirror using three types of dielectrics as described above is performed in the following procedure. First, a multilayer film in which two types of dielectric materials, a high refractive index dielectric material and a low refractive index dielectric material, are alternately laminated, and a multilayer film whose optical thickness is all １ ／ of a predetermined wavelength, or a high reflection wavelength region. The initial value is a multilayer film in which a plurality of different dielectric multilayer films are stacked, or a multilayer film having a structure in which the film thickness is gradually increased or decreased as going from the substrate to the surface layer, but without limiting the thickness, The optimization of the film thickness of each layer is performed by using an optimization method such as a quasi-Newton method or an annealing method so as to obtain a desired dispersion. This is not really the best solution if the film thickness is limited at the time of optimization when the number of parameters to be optimized is several tens like this reflector. Although it is a good solution when viewed from the vicinity, it may converge to a solution that is not the best, and in order to prevent such a situation, optimization is first performed without limiting the film thickness. In the resulting film configuration, the optical thickness is reduced to 100 nm by replacing the high refractive index dielectric film having an optical thickness of 100 nm or less with an intermediate refractive index dielectric film. For a low refractive index dielectric film having an optical thickness of 100 nm or less, the optical thickness is set to 100 nm,
This time, optimization is performed again under the condition that the optical thickness of each layer is 100 nm or more to obtain a desired film configuration. FIG. 1 shows a configuration diagram of a dielectric multilayer film dispersion compensating mirror using three types of dielectrics obtained as a result. In FIG. 1, the expression “high-refractive index or intermediate-refractive-index dielectric” indicates that it is determined to be a high-refractive-index dielectric or an intermediate-refractive-index dielectric depending on the result of optimization.

FIG. 2 shows a film structure of an embodiment of a dielectric multi-layer dispersion compensation mirror using three kinds of dielectrics according to the present invention. 5 is an example of a film structure of a reflector designed to compensate for intra-cavity dispersion of a mode-locked titanium sapphire laser oscillator. In this figure, the film number on the horizontal axis is 1, with the film on the surface of the reflecting mirror being 1, 2, 3, 4,... As approaching the glass substrate. The vertical axis is the optical thickness of each film. This means that when a pulse makes one round trip in the resonator, it has dispersion in the resonator,
For example, the dispersion of titanium sapphire as an amplification medium, a dielectric multilayer output mirror for extracting output, the dispersion of a synthetic quartz prism for compensating dispersion over a wide band, and the secondary dispersion when reflecting this mirror five times are combined. From 650 nm to 1000
It is set to zero over nm. In this embodiment, first, two types of conventional dielectrics shown in FIG. 5 (in this embodiment, titanium dioxide (Ti
O ₂ ) and silicon dioxide (SiO ₂ ) as a low refractive index dielectric. The initial value is a film configuration in which the film thickness is gradually reduced from the substrate to the surface layer, which is used in the design of the dielectric multilayer film dispersion-compensating mirror using the above method. Based on this, optimization of each film thickness is performed by the quasi-Newton method so as to obtain a desired dispersion. In the resulting film configuration, the optical thickness is
The high-refractive-index dielectric film of 100 nm or less is replaced with an intermediate-refractive-index dielectric film (alumina (Al ₂ O ₃ ) is used in this embodiment) to make the optical thickness 100 nm. Optical thickness of 100
For low refractive index dielectric films below nm, the optical thickness is 10
The thickness is set to 0 nm, and optimization is performed again under the condition that the optical thickness of each layer is 100 nm or more to obtain the film configuration in FIG.
FIG. 6 shows the reflectance, the wavelength dependence of the secondary dispersion, and the target values of the secondary dispersion in the case of the film configuration of FIG. In this case as well as with conventional two kinds of dielectric with dielectric multilayer film dispersion compensation reflector, the vibration of the dispersion is present, the average value of the variance of the deviation from the target value is 7.1 fs ^2, FIG. 5 is smaller than the value of 9.4 fs ² in the case of the conventional film structure shown in FIG. This means that the dielectric multilayer dispersion compensation mirror using three kinds of dielectrics has less dispersion oscillation than the conventional dielectric multilayer dispersion compensation mirror using two kinds of dielectrics. Show.

FIG. 3 shows another embodiment of a dielectric material for canceling the dispersion of synthetic quartz, which is often used as a component of optical parts, in a thickness of 0.5 mm in the visible to near infrared region of 525 nm to 675 nm. 1 shows a film structure of an embodiment of a multi-layer dispersion compensation mirror. Also in this embodiment, first, the conventional 2 shown in FIG.
Type of dielectric (in this example, a titanium dioxide (TiO ₂₎ as the high refractive index dielectric, using silicon dioxide as the low refractive index dielectric (SiO _2). Other high refractive index dielectric as tantalum oxide ( V) (Ta ₂ O ₅ ) etc. can also be used.) A film configuration that is used in the design of a dielectric multi-layer dispersion-compensating mirror using Is the initial value. Based on this, optimization of each film thickness is performed by the quasi-Newton method so as to obtain a desired dispersion. The resulting film configuration has an optical thickness of 10
An intermediate refractive index dielectric film (alumina (Al ₂ O ₃ ) in this embodiment is used as a high refractive index dielectric film of 0 nm or less. Yttrium oxide (Y ₂ O ₃ ) or the like is used as another intermediate refractive index dielectric. Can be used.) And the optical thickness is 100
to nm. For the low-refractive-index dielectric film with an optical thickness of 100 nm or less, the optical thickness was set to 100 nm, and this time, optimization was performed again under the condition that the optical thickness of each layer was 100 nm or more. 3 is obtained. FIG. 7 shows the relationship between the reflectance and the wavelength dependence of the second-order dispersion in the case of the film configuration of FIG. 3 and the target value of the second-order dispersion by using a conventional dielectric multilayer film dispersion-compensating mirror composed of two types of dielectrics. And the second order dispersion. In this case as well as with conventional two kinds of dielectric with dielectric multilayer film dispersion compensation reflector, the vibration of the dispersion is present, the average value of the variance of the deviation from the target value at 1.9Fs ^2, prior Is smaller than the value of 4.6 fs ² for the film structure of FIG. Also in this embodiment, the dielectric multilayer dispersion compensation reflector using three types of dielectrics has less dispersion vibration than the conventional dielectric multilayer dispersion compensation reflector using two types of dielectrics. Is shown.

[0012]

As described above, according to the present invention, it is possible to design a dielectric multilayer film dispersion compensating mirror having less dispersion oscillation, and to reduce the time width extension of the light pulse during dispersion compensation. Also, a remarkable effect of reducing the modulation of the output light pulse spectrum intensity that occurs in response to the oscillation of the dispersion when used in a mode-locked oscillator can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a structure of a dielectric multilayer film dispersion compensating mirror using three kinds of dielectrics of the present invention.

FIG. 2 is a diagram showing a film configuration of a dielectric multilayer film dispersion compensating mirror using three types of dielectrics, which is one of the embodiments of the present invention.

FIG. 3 is a diagram showing a film configuration of a dielectric multilayer dispersion compensation mirror using three types of dielectrics according to another embodiment of the present invention.

FIG. 4 is an explanatory view showing a structure of a conventional dielectric multilayer mirror.

FIG. 5 is a diagram showing a film configuration of a conventional dielectric multilayer film dispersion-compensating reflector using two types of dielectrics.

FIG. 6 shows the reflectivity and second-order dispersion of a dielectric multilayer film dispersion-compensating mirror using three types of dielectrics, which is one of the embodiments of the present invention, and a conventional dielectric using two types of dielectrics. It is a figure which shows the reflectance of a multilayer dispersion compensation reflection mirror, and a comparison of a secondary dispersion.

FIG. 7 shows the reflectivity and second order dispersion of a dielectric multilayer film dispersion compensating mirror using three types of dielectrics according to another embodiment of the present invention, and a conventional dielectric using two types of dielectrics. It is a figure which shows the reflectance of a multilayer dispersion compensation reflection mirror, and comparison of a secondary dispersion.

Claims (1)

[Claims] 1. A smooth glass surface for reflecting light, which is made of three types of dielectrics, a high refractive index dielectric, an intermediate refractive index dielectric, and a low refractive index dielectric, which are transparent to light in a predetermined wavelength range. A composed dielectric multilayer film dispersion compensating mirror.