JPH04168330A - Light wavelength meter - Google Patents

Abstract

PURPOSE:To achieve a higher stability by performing a wavelength measurement using a fixed Fabry Perot interferometer based on the results of the wavelength measurment using a two flux interferometer to facilitate a highly accurate measurement of an unknown wavelength. CONSTITUTION:Light 1 to be measured is branched in two with a halfmirror 2 and one part thereof is impinged into a two-flux interferometer 8 to detect an interference fringe thereof with a photodetecting section 10. The other part of the light is impinged into a fixed Fabry Perot interferometer 5 through a lens system 3. The lens system 3 works to expand a luminous flux or bring an angle of expanse to the flux so that four or six interference fringes with the interferometer 5 appear. Reference light with a known wavelength lambdatau emitted from a semiconductor laser light source 7 is branched in two with a halfmirror 4 and after impinged into the interferometers 8 and 5 respectively, the respective parts of the light are detected with a photodetecting section 9 and a line sensor 6. Output signals of the photodetecting sections 9 and 10 and the sensor 6 are processed with a signal processing section 11 to calculate a wavelength lambdax of the light 1 to be measured. The wavelength lambdatau is stabilized with a light source control section 12 so as to keep the interference fringe of the reference light with the interferometer 5 from changing.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical wavelength meter used for high-precision interferometric measurement, for example, measuring the wavelength of a coherent light source such as a semiconductor laser light source.

[Conventional technology]

Some conventional wavelength meters utilize two-beam interference measurement. An example is shown in FIG. A reference light with a known wavelength λ emitted from the reference laser light source 35 and a measured light with an unknown wavelength λ are superimposed and enter the two-beam interferometer. The superimposed lights are separated into two directions by a beam splitter 36, one of which is reflected by a movable mirror 37 and the other by a fixed mirror 38, and then superimposed by the beam splitter 36 to form a reference beam and a measured beam. The light is separated into two and enters separate photodetectors 39 and 40. Here, movable mirror 3
7 is moved by L, the change in the optical path length difference of the three beams superimposed by the beam splitter 36 is 2nL (n is the refractive index of the medium), and the number of changes in the interference fringes of the measured light at this time is N8 There is the following relationship between N and the number of changes N in interference fringes of the reference destination.

2nL=λ, N, =λ, N, −−−−−−−(1
) Therefore, from now on, the wavelength λ of the light to be measured is λ8=λ,・
N, /N, (2), which can be determined from the ratio of the number of changes in interference fringes.

Figure 6 shows an overview of a wavelength meter using multiple fixed Fabry-Perot interferometers as another conventional example (References A, Fisher, R, Kullmer and W.
,Demtroder: Opt,Commun,3
9 (1981) 277, ).

The measured light having an unknown wavelength λ and the reference target having a known wavelength λ pass through the same optical path. A part of the light incident on the half mirror 41 is incident on the monochromator 42, and the rest is transmitted through the half mirrors 43, 44, and 45 to three fixed Fabry-Perot interferometers 46, 47, and 48 with different cavity lengths.
incident on . Although not shown in this figure, the incident light of the fixed Fabry-Perot interferometers 46, 47, and 48 has a slight divergence angle so that 4 to 6 interference fringes appear. The outputs of monochromator 42 and three fixed Fabry-Perot interferometers 46, 47, 48 are detected by diode arrays 49.51, 52.53, respectively.

Next, a method of wavelength measurement using a fixed Fabry-Perot interferometer will be explained. The resonator length of the fixed Fabry-Perot interferometer is d, the refractive index inside the resonator is n, the wavelength of the measured light is λ8, and the wavelength of the reference light is λ.

Then, if λ ignores the wavelength dependence of phase change □Φ due to reflection π, then λ 2nd+□Φ=m0. λ+ ”matλ, ---(3
) π holds true. Here, m 6 Hg m a * is the interference order due to each wavelength, and is generally not an integer, so m
,,=m,+e,,m□=m t + e x・・φ
・Set as (4). m19m is an integer that means the interference order of the innermost bright line of the ring-shaped interference fringe, and e.

, e is the decimal part (fraction) of the order at the center of the interference fringe. The fraction e r r e x can be determined by measuring the diameters of several ring-shaped interference fringes, and is an integer m,
, m, are determined by a method known as the integer matching method. In order to accurately determine the integer part m99m,
It is necessary to know the approximate value of the wavelength with a certain degree of accuracy, and the larger the order, and therefore the measurement resolution, the better.
A highly accurate approximation is required. For this reason, as shown in FIG.
By using the interference fringes of 6.47.48 and increasing the accuracy of wavelength measurement one by one, highly accurate wavelength measurement becomes possible.

[Problem to be solved by the invention]

However, in the case shown in FIG. 5, in order to improve accuracy, it is sufficient to increase the movable distance of the movable mirror 37;
This poses a problem in that mechanical instability increases and accuracy decreases. Further, in order to move the movable mirror 37 smoothly, a sophisticated technique is required, resulting in an increase in cost.

Furthermore, the system shown in FIG. 6 requires three fixed Fabry-Perot interferometers with a certain level of precision, and therefore has the problem of being expensive.

In addition, there is a high-precision measurement method using a scanning Fabry-Perot interferometer, but methods using piezo elements are not only mechanically unstable but also suffer from the nonlinearity of piezo elements. may affect the accuracy. However, it is not practical to use a vacuum pump to sweep the pressure in order to eliminate the effects of atmospheric pressure, temperature, and humidity and improve the linearity of the piezo element, as it requires a very large device and takes a long time to measure. .

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a highly accurate and inexpensive optical wavelength meter.

[Means and effects for solving the problem] The optical wavelength meter according to the present invention inputs a reference light having a known wavelength and a measured light having an unknown wavelength into a two-beam interferometer, and calculates the difference between each light beam. The wavelength of the light to be measured is measured by comparing the results of interferometric length measurement using
Furthermore, by using a fixed Fabry-Perot interferometer with a known resonator length, wavelength measurement can be performed with higher precision. Furthermore, since only one fixed Fabry-Perot interferometer is required, it is inexpensive.

Furthermore, in the optical wavelength meter according to the present invention, a semiconductor laser light source whose wavelength is compared in advance with another standard wavelength using the fixed Fabry-Perot interferometer and whose wavelength is stabilized by feedback control is used as a reference light source. There is. Therefore, it can be constructed at a lower cost. A more detailed explanation will be given below based on the conceptual diagram shown in FIG.

In the figure, l is the light to be measured with an unknown wavelength λ, 2 and 4 are half mirrors, 13 is a lens system, 5 is a fixed Fabry-Perot interferometer, 6 is a line sensor, 7 is a semiconductor laser light source, and 8 is a dual beam Interferometer, 9. IO is a photodetector, and 11 is a signal processor.

The light to be measured 1 is split into two by a half mirror 2. One of them enters the two-beam interferometer 8, and its interference fringes are detected by the photodetector lO. The other beam passes through lens system 3 and enters fixed Fabry-Perot interferometer 5 . The lens system 3 functions to spread the light beam or give it a divergence angle so that four to six interference fringes appear due to the fixed Fabry-Perot interferometer. A reference beam with a known wavelength λ emitted from a semiconductor laser light source 7 is split into two by a half mirror 4, and each beam enters a two-beam interference system 8 and a fixed Fabry-Perot interferometer 5, and then is optically detected. 9 and the line sensor 6. Light detection section 9. The output signals of lO and line sensor 6 are processed by signal processing section 11,
The wavelength λ of the light to be measured is calculated. In addition, in order to prevent the interference fringes of the reference light from the fixed Fabry-Perot interferometer 5 from changing, the light source controller 12 controls the wavelength λ of the semiconductor laser light source 7;
control to stabilize. The absolute value of the wavelength λ of the semiconductor laser light source 7 is measured in advance by comparing it with the wavelength of a standard light source, for example, a laser light source whose wavelength has been stabilized with very high precision. A fixed Fabry-Perot interferometer 5 is used for this comparative measurement, and the relationship between the cavity length and λ8 and the interference fringes is also measured there.

By applying feedback control to the semiconductor laser light source 7 based on this measurement result, a stable reference light source of wavelength λ7 can be obtained.

To calculate the wavelength λ8 of the light to be measured 1, an approximate value λ□' is first obtained from the ratio of the number of changes in the interference fringes of the two-beam interferometer 8, as in the conventional example shown in FIG. Next, the interference order of the fixed Fabry-Perot interferometer 5 is determined. Similar to the conventional example shown in FIG. 6, the fixed Fabry-Perot interferometer 5 has a cavity length of d, a refractive index of n, and a wavelength of λ1. λ, the order by m
, 10e.

When m x + e x , =2nd+□ ・・・・・・・・・・・・(5) π e, ) λ is the effective resonator length including the phase change due to reflection, Already measured. However, in situations where temperature is a factor, the temperature should also be measured. If λ8 is determined with high accuracy for the order, the integer m8 can be determined, and as a result, the measurement accuracy is determined by the measurement accuracy of the fraction e, so that the measurement accuracy is improved.

〔Example〕

Hereinafter, the present invention will be described in detail based on the illustrated embodiments.

FIG. 2 shows the optical system of the first embodiment, in which reference numeral 13 denotes the light to be measured, and 14 denotes a polarizing beam splitter 115 that splits the measured light 13 into two measured beams 13a and 13b by polarization. This is a beam expander that expands the light to be measured 13a. Reference numeral 16 denotes a polarizing beam splitter that splits a reference light 22 (described later) into two reference lights 22a and 22b, and also splits the optical axes of the measured light 13a and the reference light 22a.
It is designed to allow 17 is a spherical Fabry-Perot interferometer, 18 is a temperature stabilizer, 19 is a line sensor, 20 is a polarizing filter that passes only S-polarized light, 21 is a semiconductor laser light source that serves as a reference light source, 22 is a reference light, and 23 is a signal processing section , 24 is a semiconductor λ laser light source control unit, 25.31 is a square plate, 26 is a mirror,
27 is a polarizing beam splitter - 128° 32 is a mirror,
29.33 is a movable corner cube that moves as one. 30 is an interference fringe detection unit that detects interference fringes caused by the measured beams 13a and 13b, and 34 is a reference beam 22a and 22b.
This is an interference fringe detection section that detects interference fringes caused by the interference fringe, and although the details are omitted, each is composed of a polarizing element, a photodetector, and the like. The polarizing beam splitter 27, mirrors 28, 32, and movable corner cube 29.33 constitute a two-beam interferometer.

Next, the effect will be explained.

The light to be measured 13 is split by a polarizing beam splitter 14 into light to be measured 13a having a P polarization component and light to be measured 13b having an S polarization component. The light to be measured 13b passes through the λ□ plate 25, the direction of its polarization is rotated by 456 rotations, is reflected by the mirror 26, and enters the polarizing beam splitter 27. The polarized beam splitter 27 then splits the light into a P polarized component and an S polarized component. The former is reflected by a mirror 28 and a corner cube 29, and the latter is reflected by a mirror 32 and a corner cube 33.
are superimposed on each other to generate interference fringes, and the interference fringe detection unit 30
incident on . The output of the interference fringe detection section 30 is sent to the signal processing section 23.
The changes in the interference fringes caused by the movement of the corner cubes 29 and 33 are counted. On the other hand, the beam diameter of the light to be measured 13a is expanded by the beam expander 15 and enters the spherical Fabry-Perot interferometer 17, and interference fringes are detected by the line sensor 19 as shown in FIG. Further, the reference light 22b, which is the P-polarized component of the reference light 22, passes through the λ polarization beam splitter 16, passes through the □ plate 3■, rotates its polarization direction by 45 degrees, and enters the polarization beam splitter 27. . Then, like the measured light tab, it is divided into two polarized components, and the corner cube 2
9.33 and enters the interference fringe detection section 34, and changes in the interference fringes are counted. In this way, the measured light 13b and the reference light 22b pass through the same optical path after being separated by the polarizing beam splitter 27 until they are combined again by the polarizing beam splitter 27. on the other hand,
The reference light 22a, which is the S-polarized component of the reference light 22, is reflected by the polarizing beam splitter 16, enters the spherical Fabry-Perot interferometer 17, and is transmitted through the polarizing filter 20 as shown in FIG. 3 to the line sensor 19. Detected in That is, four or five interference fringes of the measured light 13a that has been expanded by the beam expander 15 and entered the spherical Fabry-Perot interferometer 17 appear in a ring shape, and the positions of the interference fringes are detected by the line sensor 19. Further, the reference light 22a enters the vicinity of the center of the line sensor 19 as a beam.

Also, when the wavelength λ changes, it changes to the line sensor 19.
is detected as an intensity change by the signal processing section 23, and is fed back to the light source control section 24, which controls the semiconductor laser light source 21 to change the wavelength λ.
, are removed. Note that since the polarizing filter 20 is provided, the light to be measured 13a is not detected at the same time.

On the other hand, since the measurement target light 13a only needs to be determined by the diameter of the ring, there is no need to detect it near the center.

The signal processing section 23 calculates the wavelength of the light to be measured 13 from the outputs of the interference fringe detection section 30.34 and the line sensor 19. The amount of movement of the movable corner cube 29°33 is ΔL
Then, the change in interference fringes due to each light is N r '= 4 nΔL/λ7. N, = 4nΔL/λ
, . . . (6), and λ1 is determined by equation (2). Since it is sufficient to obtain an accuracy of about lo-6, N,, N, should be about 10', and if N,, N, can be read up to □, ΔL should be about 50 mm. Further, since the stability of the wavelength of the reference light 22 may be about lo-7, it can be controlled relatively easily.

Next, if the interference order m −+ e x of the spherical Fabry-Perot interferometer 17 by the light to be measured 13 is about 4×10′ or less, mx can be determined from equation (5).

For example, if λ = 780 nm, the cavity length is approximately 80 mm.
If e8 is found to the order of 0.01, then λ
, a measurement accuracy of 5×10−” is obtained.

In this way, this practical example enables high-precision measurement of the unknown wavelength λ, and has excellent stability because the high-precision measurement section (spherical Fabry-Perot interferometer 17) has no mechanically moving parts. There is. Furthermore, since only one spherical Fabry-Perot interferometer 17 is required, the cost is low.

In addition, a wavelength-stabilized semiconductor laser light source 21 is used as a reference light source for the low-precision measurement section (two-beam interferometer), and a fixed Fabry-Perot interference device used in the high-precision measurement section is used to compare the wavelength with another standard wavelength. Since a total of 17 units are used, it can be constructed at a lower cost.

In this embodiment, a spherical Fabry-Perot interferometer 17 is used, but a planar Fabry-Perot interferometer may also be used. In this case, instead of the beam expander 15, a lens system is used to give a slight spread angle to the light to be measured 13a.

FIG. 4 is a schematic diagram of the optical system of the second embodiment, in which a half mirror that splits the light to be measured l and a half mirror that splits the reference light from the semiconductor laser light source 7 are combined into one beam splitter. In addition, even after the measured light and the reference light overlap, the Fresnel lens 3-, which has high wavelength dependence, magnifies only one of the light beams, so that the line sensor 6 can perform appropriate measurements. This is what I did.

The other configurations are basically the same as the first embodiment.

In addition to the effects of the first embodiment, this embodiment has a half mirror.
It has the advantage that it can be configured more compactly by reducing the number of .

〔Effect of the invention〕

As described above, the optical wavelength meter according to the present invention performs wavelength measurement using a fixed Fabry-Perot interferometer based on the results of wavelength measurement using a two-beam interferometer, so it is possible to measure unknown wavelengths with high precision. can be done relatively easily, and fixed fabrication/
The Perot interferometer has no mechanically moving parts, so it has excellent stability.

Furthermore, since only one fixed Fabry-Perot interferometer is required, the cost is low. In addition, a wavelength-stabilized semiconductor laser light source is used as the reference light source, and a fixed Fabry-Perot interferometer for wavelength measurement is used to compare the wavelength with another standard wavelength, so other wavelength standards are used. It is not necessary to provide a layer, and it can be constructed at low cost.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of the optical system of the optical wavelength meter according to the present invention;
The figure shows the optical system of the first embodiment, FIG. 3 shows the projection state of the measured light and reference light on the line sensor of the first embodiment, and FIG. 4 shows the optical system of the second embodiment. Schematic diagrams of the system, FIGS. 5 and 6, each show a conventional optical system. 1. 13... Light to be measured, 2,4... Half mirror 13... Lens system, 3'φ medium Fresnel lens, 5... Fixed Fabry-Perot interferometer, 6,19...
... Line sensor, 7, 21 ... Semiconductor laser light source, 8 ... Two-beam interferometer, 9. lO...light detection section, 1
1.23...Signal processing section, 12...Light source control section, 1
4,16.27...Polarizing beam splitter-115.
・Beam expander, 17... Spherical Fabry・
Perot interferometer, 18... temperature stabilizer, 20...
Polarizing filter, 22... Reference light, 24... Semiconductor laser light source control unit, 25.31... λ/2 plate, 26
．． 28.32... Mirror, 27'... Beam splitter-129, 33... Movable corner cube, 30
．． 34...Interference fringe detection section. Figure 2 F Figure 3 Figure 1-4

Claims (2)

[Claims] (1) A two-beam interferometer into which a first light with a known wavelength and a second light with an unknown wavelength are simultaneously incident, and interference fringes caused by the first light that has passed through the two-beam interferometer are detected. a first photodetection section; a second photodetection section that detects interference fringes due to the second light that has passed through the two-beam interferometer; and a fixed Fabry-Perot device into which the first light and the second light are incident. an interferometer, a third light detection unit that detects the output of the fixed Fabry-Perot interferometer, and optics that guide first and second lights to both the two-beam interferometer and the fixed Fabry-Perot interferometer. Motoko and
Input the output signals of the first, second and third photodetectors.
a signal processing unit that processes a signal, and a light source control unit that controls a light source of the first light based on the output signal of the third light detection unit, and includes a two-beam interference signal of the first light and a second light The wavelength of the second light is measured by comparing the two-beam interference signal by
An optical wavelength meter that measures the wavelength with higher precision from the result and the interference signal from the fixed Fabry-Perot interferometer. (2) The light source of the first light is a semiconductor laser light source whose wavelength has been compared in advance with another standard using the fixed Fabry-Perot interferometer and stabilized by feedback control via the light source controller. The optical wavelength meter according to claim 1, characterized in that: