JP2990266B1 - Sinusoidal wavelength scanning interferometer and sinusoidal wavelength scanning light source device - Google Patents

Abstract

A sinusoidal wavelength scanning interferometer enables highly accurate measurement of an optical path difference over a wavelength. Kind Code: A1 A parallel light having a sine wave wavelength scanned from a SWS light source device is split via a beam splitter, and one light is incident on a mirror which is oscillated by a piezoelectric element. And the other light is incident on the measurement surface of the object to be measured. Reflecting surface of mirror 22 (reference surface)
The light reflected by the measurement surface of the device under test is combined by the beam splitter 20 and interferes. Based on the interference signal S (t) thus obtained, a signal indicating a phase change that changes sinusoidally is obtained, and the amplitude and phase of the signal indicating the phase change are obtained. Subsequently, an optical path difference of a wavelength or more is obtained based on the obtained amplitude, an optical path difference of a wavelength or less is obtained based on the obtained phase, and an optical path difference of a wavelength or more is calculated with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sinusoidal wavelength scanning interferometer and a sinusoidal wavelength scanning light source device, and more particularly, to a sinusoidal wavelength scanning interferometer for measuring an optical path difference equal to or longer than an optical wavelength, and to use the interferometer. The present invention relates to a sinusoidal wavelength scanning light source device suitable as a light source.

[0002]

2. Description of the Related Art Interferometry using a plurality of light wavelengths for measuring an optical path difference longer than a light wavelength has been actively studied recently. These interferometry include two-wavelength interferometry, wavelength scanning interferometry, white light dispersion interferometry, and white light interferometry. In the two-wavelength interferometry, an optical path difference that is equal to or longer than the light wavelength is measured using a composite wavelength created from the two wavelengths. In the wavelength scanning interferometry, an optical path difference equal to or longer than an optical wavelength is measured by detecting a temporal change in the phase of an interference signal obtained by temporally scanning the wavelength of a light source. In the white light dispersion interferometry, interference for each wavelength is spatially separated by a diffraction grating or the like, and the optical path difference equal to or longer than the light wavelength is measured by detecting the phase of the interference signal for each wavelength.

In the white light interferometry, utilizing the fact that the visibility of interference fringes is maximized at a position where the optical path difference becomes zero, the optical path difference of the light wavelength or more is measured by mechanically displacing the reference surface. .

[0004]

By the way, a semiconductor laser is used as a light source of a wavelength scanning interferometer, and the wavelength of light from the semiconductor laser can be continuously scanned by changing an injection current to the semiconductor laser. Wavelength scanning has been widely used, but its scanning width is 0.1 n at the maximum.
m, and the continuity of the scanned wavelength was not good, and it was insufficient to measure the optical path difference of the wavelength or more with high accuracy.

An object of the present invention is to provide a sinusoidal wavelength scanning interferometer capable of measuring an optical path difference of a wavelength or more with high accuracy. Another object of the present invention is to provide a sinusoidal wavelength scanning light source device capable of generating light having a wide scanning width when scanning the wavelength continuously in a sinusoidal shape and having good continuity of the scanned wavelength. It is in.

[0006]

According to a first aspect of the present invention, there is provided a sinusoidal wavelength scanning interferometer according to the present invention, comprising: a light source for generating light having a sinusoidally wavelength-scanned light; Is divided and reflected on the measurement surface and the reference surface of the device under test, and then an interference optical system that combines and reflects each reflected light, and a sine that vibrates the reference surface or the measurement surface of the device under test with a sine wave. Wave vibration means, photoelectric conversion means for converting the interference light obtained by the interference optical system into an electrical interference signal, and obtaining a signal indicating a sinusoidally changing phase change of the interference signal based on the interference signal Means for determining the amplitude and phase of the signal indicating the phase change, and the optical path difference between the light reflected on the measurement surface of the device under test and the light reflected on the reference surface based on the determined amplitude and phase. Optical path difference calculating means for calculating It is characterized in that.

[0007] The signal indicating the phase change is a signal according to claim 2 of the present application.
Assuming that the signal indicating the phase change is φ (t), the amplitude is Z _b , the phase is α, and the angular frequency of the sinusoidally scanned light is ω _b , the signal φ (t) can be expressed by the following _{equation, φ (t) = Z b} cosω b t + α. Further, the optical path difference calculating means, the optical path difference L _b on the basis of the amplitude Z _b as shown in the claims 3, the following _{equation, Z b = (2πb / λ} 0 2) L b ( where, lambda ₀ : the central wavelength of the sine wave to wavelength scanning light b: the calculated by sinusoidally wavelength scanning light scanning width), the optical path difference L _a on the basis of the phase alpha, equation, alpha = - (2π / λ ₀₎ is calculated by L _a, the optical path of the light reflected by the reference surface and light reflected by the measurement surface of the calculation to the optical path difference L _b, the object to be measured based on L _a the difference L, the following _{equation, L = mλ 0 + L a} ( _{where, m c = (L b -L} a) / λ 0 m: m positive integer obtained by rounding off the decimal point _c) as characterized by calculating by I have.

That is, the phase of the interference signal obtained via the interference optical system and the photoelectric conversion means changes in a sinusoidal manner,
The amplitude Z _b is proportional to the optical path difference L _b and the scanning width b of the light whose wavelength is scanned sinusoidally. From this characteristic, a signal indicating a phase change is obtained by calculating the amplitude Z _b of φ (t),
The optical path difference Lb _equal to or longer than the wavelength can be measured. Also,
The phase α, which is the time average of the phase of the interference signal, is the phase of the conventional interference signal when the sinusoidal wavelength scanning is not performed,
Due to this phase α, the optical path difference L _a of the portion equal to or smaller than the wavelength of the optical path difference
Can be obtained with high accuracy. Then, these optical path differences L _b , L _b
By combining _a , the optical path difference L equal to or greater than the wavelength
The measurement can be performed with the same accuracy as the conventional measurement accuracy of the optical path difference below the wavelength. The optical path difference L _b, in order to perform a combination of L _a is the measurement accuracy of the optical path difference L _b may be higher than one-half of the central wavelength lambda ₀ of the light wavelength scanning, the optical path difference measurement accuracy of L _b is proportional to the scanning width b of the light wavelength scanning, the desired measurement accuracy by increasing the scanning width b is obtained.

According to a fourth aspect of the present invention, there is provided a sinusoidal wavelength scanning light source that generates light having a predetermined optical spectrum width;
A first diffraction grating that receives parallel light from the light source and spatially distributes the first-order diffracted light according to each wavelength by diffraction, and converts the first-order diffracted light diffracted by the first diffraction grating according to each wavelength. A first lens that forms an image at the position of the focal plane, and a movable lens disposed at the position of the focal plane of the first lens,
A slit for extracting a specific wavelength component of the light of each wavelength distributed on the focal plane, and a slit driving unit for oscillating the slit in a sinusoidal manner and scanning the wavelength of the light to be extracted by the slit in a sinusoidal manner. A second lens that converts the light extracted by the slit into parallel light, and light that has been converted into parallel light by the second lens is incident, and the propagation direction of the first-order diffracted light of all wavelengths is diffracted in the same direction. And a second diffraction grating. The light source can be a super luminescent diode as described in claim 5 of the present application.

According to the invention of claim 4 of the present application, the parallel light having a predetermined light spectrum width from the light source is diffracted by the first diffraction grating, and the first-order diffracted light is transmitted through the first lens. An image is formed on the focal plane of the first lens, and an optical spectrum distribution of the light source is obtained. A specific wavelength component is extracted by the slit located at the focal plane. By vibrating the slit in a sinusoidal manner, light whose wavelength is scanned in a sinusoidal manner is extracted. The light whose wavelength has been scanned in this manner becomes parallel light by the second lens, and the first diffraction light of all wavelengths is diffracted by the second diffraction grating so as to have the same propagation direction.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a sinusoidal wavelength scanning interferometer and a sinusoidal wavelength scanning light source device according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows a sinusoidal wavelength scan (SWS) according to the present invention.
FIG. 1 is a schematic diagram showing an embodiment of an interferometer. As shown in FIG. 1, the SWS interferometer is a Michelson-type interferometer, which mainly includes an SWS light source device 10, a beam splitter 20, a mirror 22, and a photoelectric conversion element 2.
8, an A / D converter 30, and a personal computer 32. Incidentally, reference numeral 24 denotes a piezoelectric element for causing the mirror 22 to vibrate in a sine wave, and reference numeral 26 denotes an object to be measured.

The SWS light source device 10 generates light whose wavelength has been scanned in a sine wave shape.
(t) is

[0013]

[Number 1] represented by _{λ (t) = λ 0 +} Δλ = λ 0 + bcosω b t ... (1). That is, the light from the SWS light source device 10 is expressed by the formula
As shown in (1), center wavelength λ ₀ , scanning width b, angular frequency ω
It is scanned sinusoidally at _b . This SWS light source device 1
The parallel light from 0 is split by the beam splitter 20, and one of the split lights is incident on the reflection surface (reference surface) of the mirror 22, where it is reflected and the other split light is the object to be measured 26. And is reflected here.

The light reflected on the reference surface of the mirror 22 and the light reflected on the measurement surface of the device under test 26 are combined by the beam splitter 20 and interfere with each other, and the interference light is converted into a photoelectric conversion element such as a photodiode. 28, where it is converted into an electrical signal (interference signal). The interference signal is converted into a digital signal by the A / D converter 30 and then taken into the personal computer 32, where it is reflected on the reference surface of the mirror 22 and on the measurement surface of the device under test 26. Processing for obtaining an optical path difference from light is performed.

When the optical path difference is L, and the center wavelength λ ₀ and the scanning width b of the wavelength-scanned light are b≪λ ₀ , the phase change of the interference signal caused by the wavelength scanning is expressed by the following equation. Given by

[0016]

２ (t) = 2πL / λ (t) = − 2π [Δλ (t) / λ ₀ ² ] L + (2π / λ ₀ ) L (2) Further, the amplitude a of the mirror 22 is controlled by a piezoelectric element. When the sinusoidal oscillation is performed at the angular frequency ω _c and the sinusoidal phase modulation is further applied to the interference signal, the interference signal S (t) is expressed by the following equation.

[0017]

[Number 3] S (t) = M (t ) + M (t) Vcos (Z c cosω c t + Z b cos ω b t + α) ... (3)

[0018]

Where Z _c = (4π / λ ₀ ) a, Z _b = (2πb / λ ₀ ² ) L _b , α = − (2π / λ ₀ ) L _a (4) In (), M (t) indicates a change in the intensity of light from the SWS light source device 10, and V is a visibility indicating the contrast of the interference light.

Now, the personal computer 32 transmits the interference signal S
By taking (t) at a predetermined sampling frequency (8 × ω _c / 2π) and processing the interference signal S (t) using Fourier transform according to the double sine wave phase modulation interferometry, the following equation is obtained. A signal φ indicating a phase change of the interference signal S (t)
Find (t).

[0020]

Equation 5] _{φ (t) = Z b cosω} b t + α ... (5) Then, determining the amplitude Z _b and phase alpha of the signal phi (t).
The value of Z _b can be obtained from the amplitude of the component of ω _b by Fourier transforming the signal φ (t), and the value of α can be obtained by the time average of the signal φ (t).

The proportional constant (2πb / λ ₀ ² ) shown in equation (4)
If (2π / λ ₀₎ is known, it is possible to determine the optical path difference L _b from the value of the obtained Z _b, it is possible to obtain the optical path difference L _a from the value of alpha. Note that the value of α is from −π to π
Since determined in the range of the values of L _a is 1-? _0/2 from lambda ₀ /
2 range. Now, the wavelength λ ₀ included in the optical path difference L _b
Is m, the optical path difference L is

[0022]

L = mλ ₀ + L _a (6) Further, the order m is, the higher the measurement accuracy of the optical path difference L _b than lambda _0/2, can be obtained by rounding off the decimal point numbers m _c obtained by the following equation.

[0023]

By Equation 7] _{m c = (L b -L a} ) / λ 0 ... (7) the value of the thus alpha, the value L _a wavelength following part of the optical path difference L can be determined by several nm accuracy Since the order m of the wavelength λ ₀ included in the optical path difference L can also be obtained, the optical path difference L having a wavelength or more can be measured with high accuracy of several nm by using equation (6).

By the way, the measurement accuracy of the optical path difference L _b has the formula
(4) is proportional to the sine wave in the scanning width b of the wavelength scanning light as is apparent from, the measurement accuracy of the optical path difference L _b lambda
_To make it higher than 0/2, it is necessary to increase the scanning width b. Next, the details of the SWS light source device 10 that can increase the scanning width b of the wavelength scanning and have good continuity will be described.

FIG. 2 is a device configuration diagram showing an embodiment of the SWS light source device 10. The SWS light source device 10 mainly includes a super luminescent diode (SLD) 1
1, lenses 12, 14, 17 and diffraction gratings 13, 18
, A slit 15 and a slit driving means 16. The SLD 11 can generate light having a relatively wide optical spectrum width. The light generated from the SLD 11 is collimated by the lens 12 and is incident on the diffraction grating 13.

The parallel light is diffracted by the diffraction grating 13 and the first-order diffracted light is imaged by the lens 14 at the position of the lens focal plane. Since the direction of the first-order diffracted light differs according to each wavelength, an optical spectrum distribution of the SLD 11 is obtained on the lens focal plane as shown in FIG. Slit 1
Reference numeral 5 is movably disposed at the position of the focal plane of the lens 14, and extracts a specific wavelength component from the light of each wavelength distributed on the focal plane according to the position of the slit 15. The light of the specific wavelength component extracted in this manner is
After being collimated by 7, it is incident on the diffraction grating 18. The first-order diffracted light of all wavelengths is diffracted by the diffraction grating 18 so as to have the same propagation direction, and becomes incident light to the interferometer.

Here, when performing wavelength scanning, the slit 15 is vibrated in a sine wave shape by slit driving means 16 such as a voice coil of a speaker. This allows
As shown in FIG. 3, light of a wavelength component corresponding to the position of the slit 15 is extracted, and wavelength scanning is performed. S of the above configuration
The central wavelength λ ₀ of the SLD 11 of the WS light source device 10 is 789.
3 nm, and the optical spectrum width was about 20 nm. The diffraction gratings 13 and 18 use 1200 lines / mm, and the values of ω _b / 2π and ω _c / 2π are each 30 Hz.
And 480 Hz.

The object 26 is moved by a known amount to
The path difference is changed, and Z_bAnd the result is
The wavelength scanning width 2b obtained from the above was 17.3 nm. Ma
Z _bOptical path difference L obtained from the value of_bThe measurement error of
It was found to be 0.26 μm. Thereby, the optical path difference L
_bMeasurement accuracy of the center wavelength λ₀(= 789.3 nm) for 2 minutes
L is higher than 1_bAnd L_aCan be combined with
It turned out to be.

Next, another embodiment of the SWS light source device will be described with reference to FIG. This SWS light source device 40
Is mainly composed of a semiconductor laser 41, a lens 42, a diffraction grating 43, and a mirror 44, as shown in FIG. The light from the semiconductor laser 41 becomes parallel light via the lens 42 and enters the diffraction grating 43. The first-order diffracted light from the diffraction grating 43 is vertically reflected by the mirror 44 and returns to the semiconductor laser 41. Thus, an external resonator is formed between the reflection surface (not shown) behind the semiconductor laser 41 and the mirror 44.

The 0th-order diffracted light from the diffraction grating 43 becomes output light from the SWS light source device 40, and is output to a subsequent interferometer. Here, wavelength scanning is carried out by vibrating rotating the mirror 44 with a sine wave of the angular frequency omega _b. The diffraction angle of the first-order diffracted light at this time is given by the following equation:

[0031]

[Equation 8] becomes _{ψ d (t) = ψ d0} + Δψ d (t) = ψ d0 + acosω b t ... (8). Note that ψ _d0 is an initial value, and a is the amplitude of the vibration rotation. The above equation is the equation of the diffraction grating,

[0032]

[Equation 9] substituted into λ (t) = d _{[sin ψ 1 -sin ψ d (t} ) ] _{= λ 0 + Δλ (t)} ... (9), and _{cos Δψ d (t) ≒ 1} , sin Δψ d (t) ≒ Δ
Using the approximation of ψ _d (t),

[0033]

[Number 10] _{Δλ (t) = dacosψ d0 cosω} b t ... (10) is obtained. Here, d is the grating interval of the diffraction grating. Using a semiconductor laser 41 with a wavelength of 790 nm and an output of 50 mW,
A WS interferometer was configured. Diffraction grating is a holographic diffraction grating of 1200 / mm, the incident angle [psi ₁ is 75 degrees, the initial value [psi _d0 diffraction angle was 5 °. The mirror 44 is driven by a laser scanner, a = 7.4 mrad, ω _b / 2π = 40
Hz, ω _c / 2π = 1280 Hz. The central wavelength λ ₀ of the SWS light source device was 783.3 nm.

The object 26 is moved by a known amount to
The path difference is changed, and Z_bAnd the result is
The wavelength scanning width 2b obtained from the above was 6.15 nm. Ma
Z _bOptical path difference L obtained from the value of_bThe measurement error of
It was found to be 0.17 μm. Thus, L_bAnd L
_aIt turned out that the combination with is possible. Book
The interferometer optical system of the SWS interferometer according to the present invention has the form
It is not limited to the state. In addition, a mirror to be a reference surface
Not only when vibrating, but also when
You may make it do. Furthermore, the SWS light source device is
Not only for the light source of the SWS interferometer according to
Can be used.

[0035]

As described above, the SWS according to the present invention is described.
According to the interferometer, an optical path difference of a wavelength or more can be measured with high accuracy. Further, according to the SWS light source device according to the present invention, it is possible to generate light having a wide scanning width when continuously scanning the wavelength in a sine wave shape and having good continuity of the scanned wavelength.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an embodiment of a SWS interferometer according to the present invention.

FIG. 2 is a device configuration diagram showing an embodiment of the SWS light source device shown in FIG.

FIG. 3 is a diagram showing an optical spectrum distribution of an SLD obtained at a lens focal plane in FIG. 1;

FIG. 4 is a device configuration diagram showing another embodiment of the SWS light source device according to the present invention.

[Explanation of symbols]

 10, 40 SWS light source device 11 Super luminescent diode (SLD) 12, 14, 17, 42 Lens 13, 18, 43 Diffraction grating 15 Slit 16 Slit driving means 20 Beam splitters 22, 44 Mirror 26 ... DUT 28 ... Photoelectric conversion element 30 ... A / D converter 32 ... Personal computer

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01B 9/00-11/30 102 G01F 1/00-1/11

Claims (5)

(57) [Claims] A light source for generating light having a wavelength scanned in a sine wave shape; and dividing the light from the light source and reflecting the light on a measurement surface and a reference surface of an object to be measured. Interference optical system for causing interference, sinusoidal vibration means for sinusoidally oscillating the reference surface or the measurement surface of the device under test, and photoelectric conversion means for converting interference light obtained by the interference optical system into an electrical interference signal Means for obtaining a signal indicating a phase change that changes in a sinusoidal manner of the interference signal based on the interference signal, means for obtaining an amplitude and a phase of the signal indicating the phase change, and based on the obtained amplitude and phase, A sinusoidal wavelength scanning interferometer, comprising: optical path difference calculating means for calculating an optical path difference between light reflected on the measurement surface of the device under test and light reflected on the reference surface. 2. Assuming that the signal indicating the phase change is φ (t), the amplitude is Z _b , the phase is α, and the angular frequency of the sinusoidally scanned light is ω _b , the signal φ (t) ) Is represented by the following equation: φ (t) = Z _b cos ω _b t + α. Wherein the optical path difference calculating means, the optical path difference L _b on the basis of the amplitude Z _b, the following _{equation, Z b = (2πb / λ} 0 2) L b ( where, lambda _0: the sinusoidal the center wavelength of the wavelength scanning light b: the calculated by sinusoidally wavelength scanning light scanning width), the optical path difference L _a on the basis of the phase alpha, equation, α = - (2π / λ 0 ) is calculated by L _a, the calculated and the optical path difference L _b, the optical path difference L between the light reflected by the reference surface and light reflected by the measurement surface of the object to be measured based on L _a, the following _{wherein, L = mλ 0 + L a} ( _{where, m c = (L b -L} a) / λ 0 m: positive obtained by rounding off the decimal point m _c integer) of claim 2, characterized in that calculated by Sinusoidal wavelength scanning interferometer. 4. A light source that generates light having a predetermined optical spectrum width, and a first diffraction grating that receives parallel light from the light source and spatially distributes first-order diffracted light according to each wavelength by diffraction. A first lens for forming an image of the first-order diffracted light diffracted by the first diffraction grating at a focal plane position corresponding to each wavelength; and a movable lens disposed at the focal plane position of the first lens. A slit for extracting a specific wavelength component of the light of each wavelength distributed on the focal plane; and a slit drive for oscillating the slit in a sine wave shape and scanning the wavelength of the light to be extracted by the slit in a sine wave shape. Means, a second lens that converts the light extracted by the slit into parallel light, and light that has been converted into parallel light by the second lens is incident, and changes the propagation direction of the first-order diffracted light of all wavelengths by diffraction. A second diffraction grating having the same direction, and a sinusoidal wavelength scanning light source device, comprising: 5. The sinusoidal wavelength scanning light source device according to claim 4, wherein said light source is a super luminescent diode.