JPS61126403A - Measuring method of interference - Google Patents

Abstract

PURPOSE:To suppress phase variation of interference light to within a practical range by setting the difference in overall optical path length between reference light and object light in specific relation with the wavelength of a coherent light source and its wavelength variation. CONSTITUTION:Luminous flux from a semiconductor laser 1 is passed through a collimator lens 2 and split by a beam splitter 3 into two; one forms an image on an image pickup tube 9 by being reflected by a reference plane mirror 4, and the other forms an image after being transmitted through an object lens, reflected by a reference plane mirror 6, and passed through an image forming lens 8, so that their images are observed through a monitor television 10. In this case, positions of the mirrors 4 and 6 are so set that the difference L in optical length between the reference light and object light bear a specific relation, shown by an inequality, with the wavelength lambda0 of the light source and its variation quantity DELTAlambda.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an interference measurement method for measuring the flatness of an object, spherical dust, lens performance, etc. from interference fringes produced by interference of two light beams.

Problems to be solved by the two-vane invention The oscillation wavelength of a semiconductor laser changes depending on temperature and driving current. Figure 2 shows the temperature dependence of the oscillation wavelength when the drive current is constant, and the wavelength change rate is 0.3 to 1.
It is about 0 person/℃. The jump in wavelength is due to the jump in the axial mode, and the change in the oscillation wavelength differs depending on whether the temperature rises or falls, and there is hysteresis. Also, the third
The figure shows the current dependence of the oscillation wavelength, and the wavelength change rate is about 0.03 to 0.1 person/mA. In this case too, mode jumps and hysteresis exist.

As described above, semiconductor lasers are rarely used as light sources for interferometric measurements due to their unstable oscillation wavelength and other reasons. This is because fluctuations in the wavelength of the light source cause fluctuations in the phase of the interference light. This means that the interference fringes do not stand still and are unstable, which is a fatal problem in interferometric measurements.

FIG. 4 shows a block diagram of lens aberration measurement using a Twyman-Green interferometer. Semiconductor laser 3be.

The light beam emitted from -1 is made into a parallel light beam by a collimator lens 2, and then split into two beams by a beam splitter 3. One of them passes through a reference plane mirror 4, and the other passes through a test lens 5, and then is reflected by a reference spherical mirror 6 having a spherical center at the focal point of the test lens 5, and is guided again to the same optical path by a beam splitter 3. He interferes. The interference light beam 7 is imaged by the imaging lens 8 on the imaging surface of the imaging tube 9, and is displayed on the monitor television 10.
observed in

The complex amplitude distributions on the imaging plane of the reflected light (reference light) WR from the reference plane mirror 4 and the reflected light (test light) WI from the reference spherical mirror 6 are respectively WR=A-exp(i k Ll) WI=B- exp(i k L2+φ)A, B: Amplitude of reference light, test light; Wave number (=2π/λ) λ: Wavelength; Optical optical path length L2 of reference light; Optical path length of test light If φ is the wavefront aberration of the tested lens, the light intensity of the interference light is I = l WR + WI l ” = A2 + B2 + 2ABscos (-2-・(Ll-L2
)+φ), and the interference light intensity is the optical path length difference (Ll−
L2) or changes cosineally with respect to 1/λ.

Therefore, if the wavelength of the light source fluctuates, the intensity of the interference light will fluctuate, resulting in an error in obtaining the interference fringes as two-dimensional information and measuring the wavefront aberration φ of the lens.

For example, it is said that for optical discs, it is necessary to suppress the residual allowable aberration of the optical system to a wavefront aberration of λ/10 or less (Akira Arimoto, "Study of the allowable aberration of the optical system for optical video discs", Optics; Volume 12, No. 6, P, 491.1
(December 1983), measurement accuracy of 2710 or higher is required to measure the wavefront aberration of an objective lens whose aberrations have been removed to λ/10 or less.

When trying to measure with an accuracy of 2710 or more, there are various error factors, but if the reading error due to fluctuations in interference fringes is λ/40, then in a 5-page Twyman-Green interferometer as shown in Figure 4, The phase fluctuation of the interference light is π
It is necessary to suppress it to /1o or less.゛If we dare to use a semiconductor laser as an interference light source, the temperature,
One solution is to suppress fluctuations in the oscillation wavelength by controlling the drive current to a constant level, or to stabilize the oscillation wavelength by feedback controlling the oscillation wavelength.

In the field of optical communications, research is underway on frequency stabilization of semiconductor lasers (frequency = speed of light in vacuum/wavelength), and the former method described above reduces temperature fluctuations to 1/1 soooot3eq.
, (wavelength variation of about 6X10 -2X10 nm), and in the latter method, the oscillation frequency variation is about 1MHz (λ =
830 nm) 3X10-6n as a semiconductor laser
wavelength fluctuation of about m) has been realized.

If an interferometer is configured using a wavelength-stabilized semiconductor laser as a light source, the phase change of the Nara interference light will be 2π・(L, -L2)/(1Cr8~1O-7
)・λ, it becomes so small that it can be ignored.

However, such a wavelength-stabilized light source is an optical system.

6 pages The control system configuration becomes complicated.

The present invention has been made in view of these points, and an object of the present invention is to propose an interference measurement method that suppresses phase fluctuations of interference light within a practical range with a simple configuration.

Means for Solving the Problems The present invention eliminates phase fluctuations in interference fringes due to wavelength fluctuations of the light source by setting (Ll-L2) so as to satisfy the optical path length difference between the reference light and the test light. This is to keep the amount within a practical range.

Working wavelength λ. Let the phase of the interference light at 2π(Ll-L2)/
λ. , the phase of the interference light when the wavelength changes by Δλ is δ
When -δ and - are constant, the optical path length difference (Ll-L2) and the amount of wavelength change Δλ have a relationship as shown in FIG. Therefore, in order to suppress the amount of wavelength change to δ1 or less, (Ll-L2) and Δλ may be set in the shaded area in FIG.

Assuming that the permissible variation in interference fringes in order to read interference fringes with an accuracy of 2710 or higher is λ/40, when the light beam travels back and forth along the optical path as in a Twyman-Green interferometer, it is λ/2.
Since this corresponds to the phase of 2π, this is a good order.

Even if the wavelength fluctuation of the semiconductor laser is not suppressed, the wavelength fluctuation is minute enough to read the interference fringes as two-dimensional information (approximately 1/6o seconds with an image pickup tube), for example, the wavelength λ. -8300 people, wavelength variation Δλ ・−〇. Assuming one person, in order to suppress the amount of phase change to π/10 or less, the optical path length difference (Ll-L2) should be set to 3.41 MI or less based on curve formation.

Embodiment FIG. 1 is a block diagram showing an embodiment of the interference measurement method of the present invention. A light beam emitted from a semiconductor laser 1 is collimated into a parallel light beam by a collimator lens 2, and is split into two light beams by a beam splitter 3. One of the light beams passes through the reference plane mirror 4,
The other beam passes through the test lens 5 and then passes through the reference spherical mirror 6.
The beams are reflected by the beam splitter 3, and are overlapped again by the beam splitter 3 to cause interference. The interference light beam 7 is imaged on an image pickup tube 9 by an imaging lens 8, and interference fringes are observed on a monitor television 1o. At this time, taking into account the amount of wavelength fluctuation Δλ, the optical path length difference (Ll−L2) between the reference light and the test light is Δλ・(L
l−L2)/λ.・Reference plane mirror 4. so as to satisfy (λ.+Δλ)<0.05. The present invention can be realized by setting the position of the reference spherical mirror 6.

Effects of the Invention As described above, according to the present invention, it is possible to suppress phase fluctuations of interference light due to wavelength fluctuations of a light source with an extremely simple configuration, and it is extremely useful in practice.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an interference measurement method in an embodiment of the present invention, Fig. 2 is a diagram showing the temperature dependence of the oscillation nine-vane wavelength of a semiconductor laser, and Fig. 3 is a diagram showing the drive current of the oscillation wavelength of the semiconductor laser. FIG. 4 is a diagram showing the dependence, FIG. 4 is a configuration diagram of a Twyman-Green interferometer, and FIG. 6 is a diagram showing the amount of phase change of interference light with respect to the optical path length difference. 1... Semiconductor laser, 2... Collimator lens, 3... Beam splitter, 4...
...Reference plane mirror, 5...Plane mirror. Name of agent: Patent attorney Toshio Nakao and one other person Formerly) V Maro & Mamoru (J') Ylv Otsujo, Yingko-ku

Claims (1)

[Claims] The light emitted from a coherent light source with wavelength variation characteristics such as a semiconductor laser is divided into two parts, one part is irradiated onto a test object, and the test light that is transmitted or reflected by this test object is separated into two parts, and the other part is a reference light. When guiding light to the same optical path and obtaining interference fringes caused by the two light beams as two-dimensional information, let L be the total optical path length difference between the reference light and the test light, and let Δλ be the wavelength fluctuation of the light source with wavelength λ_0. When Δλ・L/λ_0(λ_0+Δ
An interference measurement method characterized in that phase fluctuations in interference fringes due to wavelength fluctuations of a light source are suppressed to a practical range by setting L so as to satisfy the relationship: λ) < 0.05.