CN112666641B

CN112666641B - Design method of broadband low-dispersion chirped mirror

Info

Publication number: CN112666641B
Application number: CN202110062472.XA
Authority: CN
Inventors: 王胭脂; 陈瑞溢; 张宇晖; 王志皓; 许贝贝; 朱美萍; 赵娇玲; 朱晔新; 易葵; 邵建达
Original assignee: Shanghai Institute of Optics and Fine Mechanics of CAS
Current assignee: Shanghai Institute of Optics and Fine Mechanics of CAS
Priority date: 2021-01-18
Filing date: 2021-01-18
Publication date: 2022-06-28
Anticipated expiration: 2041-01-18
Also published as: CN112666641A

Abstract

A design method of a broadband low-dispersion chirped mirror structure comprises the following steps:

wherein S represents a substrate, H and L represent high and low refractive index materials having an optical thickness of lambda/4, respectively,

is a bottom highly reflective film layer, m₁Is the number of cycles of the high reflection film layer,

is a symmetrical periodically chirped layer, a_nIs the coefficient of chirp layer, m₂The number of the periodic chirp layer periods is A, and air is A. The invention adds the period chirp layer on the low dispersion mirror with the high reflection film structure, so that different wavelengths are reflected at the same time after penetrating through the same optical thickness in the film, namely, the Group Delay Dispersion (GDD) is zero by giving the same group delay time (GD) to all the wavelengths, and the low dispersion effect is also realized. By adjusting the parameter m₁，a_nAnd m₂Group delay time and reflectivity within different bandwidths can be regulated. The broadband low-dispersion chirped mirror effectively improves the low-dispersion bandwidth of the dielectric film, and has the most important meaning for the development of the ultrafast laser technologyAs defined above.

Description

Design method of broadband low-dispersion chirped mirror

Technical Field

The invention belongs to an ultrafast laser film, in particular to a design method of a broadband low-dispersion chirped mirror.

Background

With the development of ultrashort laser technology, laser pulses are compressed to several femtoseconds, peak power can reach the magnitude of kilowatts, a low dispersion mirror is one of the most common optical elements in an ultrashort pulse laser system, and the ultrashort laser technology provides new requirements for optical films: wider operating bandwidth and efficient dispersion control. The low dispersion mirror ensures that the ultrashort pulse only changes the transmission direction after being reflected by the low dispersion mirror and does not generate extra dispersion by providing zero Group delay dispersion (Group delay dispersion) in the reflection bandwidth. However, since the reflection bandwidth, the dispersion and the damage threshold of the low dispersion mirror affect and restrict each other, designing and preparing the low dispersion mirror with wider reflection bandwidth and higher damage threshold is a research focus of the high-power ultrashort pulse laser. The traditional high-reflection mirror is formed by stacking high-refractive index materials with the optical thickness of one quarter wavelength, and the ratio of the high-reflection low-dispersion bandwidth to the high-refractive index is in positive correlation. Such as HfO₂/SiO₂The reflection bandwidth of the structured dielectric film is about 90nm, and TiO₂/SiO₂The reflection bandwidth of the structured medium film system is about 150 nm.

With the development of ultra-strong ultra-short laser towards higher energy and pulse width pulses, the spectrum of the laser pulse exceeds 200nm, and the low dispersion mirror of the traditional structured film system cannot meet the requirement. Therefore, increasing the high-inverse low-dispersion bandwidth of low-dispersion mirrors is crucial for the development of ultrashort lasers. The conventional quarter-wave film with a regular film structure is a low dispersion mirror that reflects all wavelengths at the surface layer simultaneously to realize low dispersion, and the bandwidth of the low dispersion is limited by the ratio of the refractive indexes of the high and low refractive index materials.

Disclosure of Invention

The invention aims to provide a low-dispersion mirror initial structure and a design method based on a period chirp structure, wherein the low-dispersion mirror initial structure combines a period chirp layer and a high-reflectivity layer, namely, the period chirp layer is added on a regular film with the thickness of a quarter-wavelength film, and the low-dispersion effect can be realized by giving the same group delay time to all wavelengths. The required low-dispersion mirror can be obtained by utilizing a low-dispersion mirror initial structure with a high-reflection layer at the bottom and a periodic chirp layer at the top and selecting proper parameters through optimization of film system design software.

The technical scheme of the invention is as follows:

a design method of broadband low-dispersion chirped mirror is characterized in that a low-dispersion mirror initial structure based on a period chirped layer, and an initial film system structure is as follows:

wherein S represents a substrate, H and L represent a high refractive index material and a low refractive index material having an optical thickness of lambda/4, respectively,

is a bottom highly reflective film layer, m₁Is the number of cycles of the bottom highly reflective film layer,

is a symmetrical periodically chirped layer, a_nCoefficients of an array which monotonically increases or decreases, m₂The number of periods of the periodic chirp layer is shown, A represents air;

the structure enables all the wavelengths to be reflected simultaneously after propagating the same optical path in the film layer by giving the same group delay time to all the wavelengths, and achieves the effect of lower dispersion with wider bandwidth. After selecting proper parameters and optimizing, the required final structure of the low-dispersion mirror can be obtained.

The initial structure and the design method of the low dispersion mirror are characterized by comprising the following steps:

1) selecting appropriate high refractive index material and low refractive index material according to group delay time, reflectivity, polarization, incident angle and bandwidth of the dispersion mirror to be prepared, wherein the commonly used high refractive index material is Nb₂O₅、Ta₂O₅、HfO₂Etc., the low refractive index material is SiO₂The refractive index n of the high refractive index material_HAnd the refractive index n of the low refractive index material_LIs obtained by inversion in an actual coating experiment;

2) the initial structure of the low dispersion mirror based on the period chirp layer is as follows:

is a symmetrical periodically chirped layer, a_nFor monotonically increasing or decreasing arrays, m₂The number of periods of the periodically chirped layer, a, represents air. Selecting appropriate parameters: period number m of high reflection film layer₁The chirp coefficient a of the periodic chirp layer is generally selected to be 10-30_nIs selected in the range of 0.5 to 1.5, period number m₂Is selected in the range of 1-20, m₁And m₂Are all positive integers;

3) determining basic parameters of the low dispersion mirror, including reflectivity, polarization state, incident angle, working bandwidth, target group delay time and used high-low refractive index material, determining parameters of the initial structure of the low dispersion mirror, including m₁、a_nAnd m₂Then, optimizing the membrane system by utilizing membrane system design software (TFCalc, Essential Macleod, Optilayer and the like) and corresponding algorithms (variable, gradualevaluation, needleoptimization and the like) and corresponding algorithms to obtain a final required result;

4) and observing whether the final result meets the index requirement required by the low-dispersion mirror. If the group delay time requirement of the required low dispersion mirror can not be met, the period number m of the period chirp layer cavity is adjusted₂Modifying initial structural parameters of the low-dispersion mirror, and repeating the steps 2 and 3 for multiple times of optimization until the requirements of the low-dispersion mirror are finally met; if the reflectivity requirement of the low dispersion mirror is not met, the number m of the film layer periods with high reflectivity is increased₁Repeating the step 2 and the step 3 for a plurality of times of optimization until the requirement of the low dispersion mirror is finally met; if the bandwidth requirement of the low dispersion mirror is not met, the coefficient a of the period chirp layer cavity is adjusted_nAnd repeating the steps 2 and 3 until the requirement of the low-dispersion mirror is finally met.

Compared with the prior art, the invention has the technical effects

1. The initial design of the low dispersion mirror is provided, a periodic chirp layer is added on a regular film with the thickness of a quarter-wavelength film, and the effect of low dispersion is realized by giving the same group delay time to all wavelengths.

2. Based on the initial design, the dispersion curve can be further optimized on the premise of ensuring the low dispersion of the broadband, so that the dispersion compensation is better.

Drawings

FIG. 1 is a diagram of an initial film structure of a first embodiment of a broadband low-dispersion chirped mirror according to the present invention.

FIG. 2 is a diagram of a final film structure of an embodiment of the broadband low-dispersion chirped mirror according to the invention.

FIG. 3 is a graph of group delay time and reflectivity for a broadband low-dispersion chirped mirror according to a first embodiment of the present invention.

FIG. 4 is a graph of group delay dispersion of a first embodiment of the broadband low-dispersion chirped mirror according to the present invention.

FIG. 5 is a diagram of an initial film system of a second embodiment of the broadband low-dispersion chirped mirror according to the invention.

FIG. 6 shows the final film structure of the broadband low-dispersion chirped mirror according to the second embodiment of the present invention.

FIG. 7 is a graph of group delay time and reflectivity for a second embodiment of the broadband low-dispersion chirped mirror according to the present invention.

FIG. 8 is a graph of group delay dispersion of a second embodiment of the broadband low-dispersion chirped mirror according to the present invention.

Detailed Description

The following detailed description of the embodiments of the present invention is provided in conjunction with the accompanying drawings, but should not be construed as limiting the scope of the invention.

Referring to fig. 1, fig. 1 is a schematic diagram of a low dispersion mirror structure based on a period chirped structure of the present invention, as shown, the low dispersion mirror structure is composed of a period chirped layer and a high reflective film layer, wherein the period chirped layer is on top of a quarter-wavelength regular film system high reflective layer. The high-reflection film layer and the periodic chirp layer are alternately composed of high-refractive-index materials and low-refractive-index materials.

Example one

The low dispersion mirror index for this embodiment is: group delay time 120fs, group delay dispersion 0fs²The reflectivity is more than 98.5 percent, the P polarized light has the incident angle of 45 degrees, the corresponding bandwidth is 200nm, and the central wavelength is 800 nm.

The design steps are as follows:

1. according to the group delay time, group delay dispersion and bandwidth requirement, the group delay time is larger and the bandwidth is wider, so that the high-refractive-index material Ta with higher refractive index is selected₂O₅The low refractive index material is SiO₂The refractive index parameter of the high and low refractive index materials is represented by the Cauchy formula

Determined as shown in table 1.

	A₀	A₁	A₂
				SiO₂	1.44293	1.1622618e-2	-3.705533e-4
Ta₂O₅	2.01486	3.01116301e-2	-7.635062e-4

TABLE 1

2. Selecting proper parameters according to the target group delay time of the low-dispersion mirror, the group delay dispersion value and the reflectivity requirement: m is₁＝20，m₂＝11，a_nThe expression of the low dispersion mirror based on the periodic chirp structure is obtained by taking an arithmetic progression with a leader a of 0.75, a tolerance of 0.05 and a term n of 4 as follows: S/(HL)²⁰[(0.75H0.80L0.85H0.9L)(0.9H0.85L0.8H0.75L)]¹¹A, wherein S is a fused silica substrate, H and L each represent Ta having an optical thickness of lambda/4₂O₅And SiO₂The material(s) of the material(s),a represents air. The structure of the film system is shown in FIG. 1.

3. Based on S/(HL)²⁰[(0.75H0.80L0.85H0.9L)(0.9H0.85L0.8H0.75L)]¹¹The initial design of/A, the reference wavelength is 865nm, P polarized light under the incident angle of 45 degrees is selected, and an optimized target value is set: group delay time of 120fs and target group delay dispersion of 0fs²And optimizing by using membrane system design software TFCalc to obtain a final membrane system structure as shown in FIG. 2.

4. FIG. 3 is a graph of reflectivity and group delay time of the low dispersion mirror meeting the requirement, and FIG. 4 is a graph of group delay dispersion of the low dispersion mirror meeting the requirement, in which the reflectivity is greater than 98.5% at 725-925nm, the group delay time is 120fs at 725-925nm, and the group delay dispersion is 0 + -25 fs at 725-925nm²And finally obtaining the film system structure meeting the requirements of the low-dispersion mirror.

Example two

The low dispersion mirror index for this embodiment is: group delay time 140fs and group delay dispersion 0fs²The reflectivity is more than 99.5 percent, the P polarized light has an incident angle of 45 degrees, the corresponding bandwidth is 240nm, and the central wavelength is 850 nm.

The design steps are as follows:

1. according to the group delay time, group delay dispersion and bandwidth requirement, the group delay time is larger, the bandwidth is wider, so the high refractive index material Nb with higher refractive index is selected₂O₅The low refractive index material is SiO₂The refractive index parameter of the high and low refractive index materials is represented by the Cauchy formula

Determined as shown in table 2.

	A₀	A₁	A₂
				SiO₂	1.44293	1.1622618e-2	-3.705533e-4
Nb₂O₅	2.15786	3.61226445e-2	2.024012e-3

TABLE 2

2. Selecting proper parameters according to the target group delay time of the low-dispersion mirror, the group delay dispersion value and the reflectivity requirement: m is₁＝15，m₂＝16，a_nThe expression of the low dispersion mirror based on the periodic chirp structure is obtained by taking an arithmetic progression with a leader a of 0.8, a tolerance of 0.05 and a term n of 3 as follows: S/(HL)¹⁵[(0.80H0.85L0.9H)(0.9L0.85H0.8L)]¹⁶A, wherein S is a fused silica substrate, and H and L respectively represent Nb having an optical thickness of lambda/4₂O₅And SiO₂Material, a represents air. The structure of the film system is shown in FIG. 1.

3. Based on S/(HL)¹⁵[(0.80H0.85L0.9H)(0.9L0.85H0.8L)]¹⁶The initial design of/A, the reference wavelength is 900nm, P polarized light under the incident angle of 45 degrees is selected, and an optimized target value is set: group delay time of 140fs and target group delay dispersion of 0fs²And optimizing by using a film system design software TFCalc to obtain a final film system structure as shown in figure 2.

4. FIG. 3 is a graph of the reflectivity and group delay time of the satisfactory low-dispersion mirror, and FIG. 4 is a graph of the satisfactory group delay of the satisfactory low-dispersion mirrorA late dispersion curve, wherein the reflectivity is greater than 99.5% at 730-970nm, the group delay time is 140fs at 730-970nm, and the group delay dispersion is 0fs at 730-970nm²And finally obtaining the film system structure meeting the requirements of the low-dispersion mirror.

The invention has important significance for the design of the low-dispersion mirror and is beneficial to promoting the application of the low-dispersion mirror in an ultrafast laser system.

Claims

1. A method of designing a broadband low dispersion chirped mirror, the method comprising the steps of:

1) the method is characterized in that a periodic chirp layer is added on a high-reflection layer, and an initial film system structure is designed as follows:

is a symmetrical periodically chirped layer, a_nFor monotonically increasing or decreasing arrays, m₂Selecting proper number m of periods of high reflection film layer for different designed bandwidth, dispersion amount and reflectivity₁Chirp coefficient of periodic chirp layer a_nAnd the number m of repetition periods₂；

2) Selecting a substrate layer material, a high refractive index material, a low refractive index material and the number m of cycles of a reflectivity film layer according to the requirements of group delay time, reflectivity, polarization, incidence angle and bandwidth of the designed broadband low-dispersion mirror₁Periodic chirp layer chirp coefficient a_nAnd the number m of cycles₂；

3) Optimizing the film system by using film system design software on the basis of the initial structure, and observing whether the result meets the indexes required by the low-dispersion mirror:

if the group delay time of the needed low dispersion mirror can not be reached, the period number m of the period chirp layer is adjusted₂Then, returning to the step 2) for further optimization, and entering the step 4) if the index requirement required by the low-dispersion mirror is met;

if the bandwidth of the low dispersion mirror can not be achieved, the chirp coefficient a of the period chirp layer is adjusted_nThen, returning to the step 2) for further optimization, and entering the step 4) if the index requirement required by the low-dispersion mirror is met;

thirdly, if the reflectivity requirement of the low dispersion mirror can not be met, the number m of cycles of the high-reflectivity film layer is increased₁Returning to the step 2) for further optimization, and entering the step 4) if the index requirement required by the low-dispersion mirror is met;

4) and finishing the design of the broadband low-dispersion mirror meeting the required indexes.

2. The method of designing a broadband low-dispersion chirped mirror according to claim 1, characterized in that: the material of the substrate layer is quartz glass or CaF₂(ii) a The high refractive index material comprises TiO₂、Nb₂O₅、Ta₂O₅、HfO₂、ZrO₂Fluoride, sulfide or Si; the low refractive index material is SiO₂、Al₂O₃Or MgF₂。

3. The method of designing a broadband low-dispersion chirped mirror according to any one of claims 1 to 2, wherein the number m of periods of the high reflectivity film layer is₁Is selected from the range of 10-30, and the chirp coefficient a of the periodic chirp layer_nIs selected in the range of 0.5-1.5, number of cycles m₂Is selected in the range of 1-20.