JPS6131807B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for determining the wavelength of light, and more particularly, the wavelength of continuous or pulsed laser light is determined by using at least two optical interferometers [for example, Fabry-Perot Interferometers with different free spectral ranges]. )] for sequential determination with high accuracy. BACKGROUND ART In recent years, as research and development on spectroscopy of dye lasers has been actively conducted, it is desired to measure the wavelength of laser light with high precision. The conventionally known optical wavelength measurement method is the Michelson type (Michelson type moving carriage).
interferometer), Fizeau type
interferometer, polarization-sensitive interferometer with wavenumber readout
with wavenumber readout), Fabry-Perot type interferometer, etc. Although this method can measure the wavelength of continuous wave lasers with high accuracy, it is extremely difficult to measure the wavelength of pulsed lasers. Although the method described above is capable of measuring the wavelength of a pulsed laser, it is difficult to match the incident light to the interferometer over a wide wavelength range. Although this method can measure the wavelength of both continuous wave and pulsed lasers, it has some drawbacks in terms of accuracy. As described above, conventional wavelength measurement methods have advantages and disadvantages. The present invention has been made in view of the above, and it is an object of the present invention to provide a method and apparatus that can measure the wavelength of both continuous wave lasers and pulsed lasers with high accuracy by a measurement method that successively approximates the above method. . The purpose of this is to prepare at least two optical interferometers whose interference lengths increase successively, and to observe the position of the singular point of the light to be measured relative to a known standard position through the first optical interferometer. The first approximation value of the wavelength of the measured light is calculated from the interference order of the first optical interferometer and the first optical interferometer. The position of the singular point of the measurement light is observed, and the second order of the wavelength of the measurement light is determined from the position of this singular point and the interference order of the second optical interferometer determined using the first approximation value. This is achieved by an iterative determination method of optical wavelengths that calculates approximate values. Furthermore, this sequential determination method includes a beam splitter that splits the light to be measured into at least two optical paths, an etalon and a photodiode array placed in the first optical path, and a digital converter connected to the photodiode array. Singularity position observation device including
and a singularity position observation device including an etalon having an interference length larger than the interference length of the etalon disposed in the second optical path, a photodiode array, and a digital converter connected to the photodiode array. , is achieved by a device that sequentially determines the wavelength of the light to be measured from the output of the digital converter. Here, the singular point refers to a bright spot or dark spot of interference light created through an optical interferometer.
Furthermore, the standard position mentioned above means a position determined from the interference length specific to the optical interferometer or a position of the singular point determined by passing a reference light of a known wavelength through the optical interferometer. The present invention will be explained in detail below. Prepare at least two interferometers with successive interference lengths, such as Fabry-Perot etalons, and first create an interference ring by passing the light to be measured through the first optical interferometer with the smaller interference length. Known standard position D ₁
Observe the position δ ₁ of the singular point (bright point or dark point of the interference ring) of the light to be measured (wavelength λ) with respect to the measured light (wavelength λ). D ₁ −n ₁ λ=δ ₁ (1) Here, the standard position D ₁ is the position determined from the interference length specific to the optical interferometer, or the singular point determined by passing a reference light of a known wavelength through the optical interferometer. , and is previously determined by the method described below. The first approximation value λ ₁ of the wavelength of the measured light is calculated from the position δ 1 of the singular point and the interference order n ₁ of the interference ring of the first optical interferometer (rounded off to the nearest whole number, determined as an integer value ₎ . Calculated from formula (2). λ ₁ = D ₁ - δ ₁ /n ₁ (2) Next, the measured light is passed through a second optical interferometer and the position δ ₂ of the singular point of the measured light with respect to the second known standard position D ₂ is observed. . D ₂ −n ₂ λ=δ ₂ (3) Interference order n of the second optical interferometer determined using the position δ ₂ of this singular point and the first approximation value λ ₁ obtained by equation (2) ₂ (rounded off to the nearest whole number, determined as an integer value), the second approximation value λ ₂ of the wavelength of the light to be measured is calculated from equation (4). λ ₂ = D ₂ − δ ₂ /n ₂ (4) Therefore, by using the third, fourth, etc. optical interferometers and repeating the improvement of the approximate value by the above method, the The wavelength of the light to be measured can be determined with high accuracy. FIG. 1 shows an example of the configuration of an optical interferometer when a standard position is determined from the interference length specific to the optical interferometer. The standard position can be determined by attaching a piezoelectric element PZI to the optical interferometer I and measuring the fringe of a reference light of a known wavelength with respect to the voltage applied to this element. FIG. 2 shows an example of an optical system of an apparatus for carrying out the method for sequentially determining optical wavelengths of the present invention. _M0 ～ _M4
is a beam splitter that splits the measured light L ₀ or the reference light L _s . I ₁ to _{I 4} are first to fourth optical interferometers whose interference lengths increase successively, such as Fabry-Perot etalons, and an example of their specific configuration is shown in FIG. 3. R ₁ to _{R 4} are photo diode arrays (RETICON CCPD) that convert interference rings from each optical interferometer projected thereon into electrical signals. These signal patterns are each digitized by A/D converters AD ₁ to _{AD 4} and input to computer MC for wavelength calculation. Example In the apparatus shown in FIG. 2, each etalon I ₁ to I ₄ has mirror spacing of 15 μm, 0.2 mm, and 2 mm as shown in FIG.
mm, 20 mm, and the fringes from each etalon are captured as a spatial image by a lens using a photo diode array (256 elements, spatial resolution 17 μm) R ₁
~Created an interference ring on _R4 . Etalons I ₂ to _{I 4} were configured so that the opposite side to the mirror surface was polished into a curved surface to provide a lens effect and to reduce the number of optical components. For etalons I ₃ and I ₄ , the inside of the mirror surface was vacuumed to avoid the dispersion effect of air. In addition, each etalon's reflective surface has flat high reflection characteristics (reflectance of 80 to 90
%) was applied. A He-Ne laser (633 nm) was used as a reference light. As shown in Table 1, first, the fringe order for each etalon with respect to the reference light was determined. etalon
For I ₁ to _{I 3} , Ar ion lasers with known wavelengths (SP-166 type, oscillation line: 514.5 nm, 501.7 nm,
496.5nm, 488.0nm, 476.5nm, 472.7nm,
For etalon _I4 , a dye laser (CW) tuned to the absorption line of iodine was used.
-599-21 type) to create those fringes, and determined Table 1 from the relative positions of them and the fringes created by the reference beam.

[Table] Using the fringe order of the reference light He-Ne laser determined in Table 1, the wavelengths of various dye lasers as the light to be measured were determined by the above-mentioned successive approximation method. As a result, we were able to determine the wavelength of both continuous and pulsed lasers with an accuracy of ±2×10 ^-7 . Since the photo diode array used in this example is of an optical integration type, there is no restriction on the irradiation time of laser light, and the wavelength can be measured even for short pulse lasers. The energy of the laser beam to be measured was about 2 to 5 μJ due to the sensitivity limit of the photo diode array. Also, the calculation processing time required by the computer CP to determine the wavelength is approximately 25ms.
It was hot.

[Brief explanation of the drawing]

FIG. 1 is a front view showing a cross section of an example of an optical interference device used in the present invention, and FIG. 2 is a front view showing an example of an apparatus for carrying out the invention as an optical system. FIG. 3 is a diagram showing the configuration of each etalon used in the embodiment of the present invention. Symbols in the figure: Lo...Measurement target, _Ls ...Reference light,
_M0 to _M4 ...beam splitter, _I1 to _I4 ...optical interferometer or etalon, _R1 to _R4 ...photo diode array, _AD1 to _AD4 ...A/D converter,
MC...Calculator.

Claims (1)

[Claims] 1. At least two optical interferometers whose interference lengths increase successively are prepared, and the measured light is passed through the first optical interferometer to determine the singularity of the measured light relative to the first known standard position. Observe the position, calculate the first approximation value of the wavelength of the measured light from the position of the singular point and the interference order of the first optical interferometer, and pass the measured light to the second optical interferometer. Observe the position of the singular point of the light to be measured with respect to the known standard position of A method for sequentially determining a light wavelength, characterized by calculating a second-order approximate value of the wavelength of light. 2. The method for sequentially determining a light wavelength according to claim 1, wherein the singular point is a bright point (nλ) or a dark point ((n+1/2)λ) of the interference light. 3. Claim 1, wherein the standard position is a position determined from an interference length specific to an optical interferometer or a position of a singular point determined by passing a reference light of a known wavelength through an optical interferometer. The method for sequentially determining the optical wavelength according to item 1 or 2. 4. The method for sequentially determining a light wavelength according to claim 3, wherein the singular point is a bright spot or a dark spot of interference light produced by the reference light. 5 Singularity position observation including a beam splitter that splits the light to be measured into at least two optical paths, an etalon placed in the first optical path, a photodiode array, and a digital converter connected to the photodiode array. and a singularity position observation device including an etalon with a coherence length larger than the coherence length of the etalon disposed in a second optical path, a photodiode array, and a digital converter connected to the photodiode array. A device for sequentially determining the wavelength of the light to be measured from the output of the digital converter.