JPS60119428A - Wavelength scanning type semiconductor laser interferometer - Google Patents

Abstract

PURPOSE:To scan an interference fringe in a field of view, and to raise the read accuracy of the interference fringe by using a semiconductor laser for a light source, modulating its driving current, and scanning a wavelength of the semiconductor laser. CONSTITUTION:For instance, in case of a Mach-Zehnder interferometer, a semiconductor laser 1 is used, and a luminous flux from this semiconductor laser becomes parallel light by a condenser lens 2, and it is divided into two luminous fluxes by a beam splitter 3-1. One luminous flux is used for an inspecting luminous flux, and in this luminous flux, a transmitting object 10 to be measured is placed, and this luminous flux passes through the second beam splitter 3-2 by a reflecting mirror 4-2 and forms an interference fringe on a photodetector 6 together with the other luminous flux (reference luminous flux) from a reflecting mirror 4-1. An optical path difference between both the luminous fluxes can be adjusted by rotating the beam splitter 3-1 and moving the reflecting mirror 4-1.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical interferometer, and in particular to a wavelength scanning semiconductor suitable for use in inspection of optical components that require high accuracy in reading interference fringes. This relates to laser interferometers.

[Background of the invention]

In conventional optical interferometers, the wavelength of the light source is fixed;
In order to improve the accuracy of reading interference fringes, methods such as installing a frequency shifter on one side of the light beam or modulating the phase of the light with a piezoelectric vibrator have been used, but these devices are expensive. In addition, there were drawbacks such as the need for many adjustment points and the need for a high-voltage drive power source.

[Purpose of the invention]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor laser interferometer that eliminates the above-mentioned drawbacks, is inexpensive, simple, and easy to operate, and is capable of electrically reading interference fringes with high accuracy.

[Summary of the invention]

That is, the present invention uses a semiconductor laser as a light source for a Twyman Grin interferometer, a Matsuhatsu-ender interferometer, etc.
By modulating the drive current and scanning the wavelength of the semiconductor laser, the interference fringes are scanned within the field of view, thereby improving the accuracy of reading the interference fringes.

[Embodiments of the invention]

First, the present invention will be explained in detail.

Generally, in a semiconductor laser, when the drive current is changed by ΔI (mA), the improvement shifts. For example, if we take a base-type semiconductor laser as an example, the wavelength shift amount Δλ is Δλ=0.006・ΔI (nm) ・・・・・・・・・・・・
...It is given by (1).

On the other hand, the complex amplitude of light at any point (x, y) on the interference batten is time-averaged and becomes: V(x, y) = uo(x, y) exp(1k-
r(, (x, y)) ur (x+ y)exp(
ik-r, (x, y) given by 1. Here uo,
u is the amplitude of the object light and reference light, ro and r are the optical path lengths of the object light and reference light from the reference plane, and k is the wave number. From this, the intensity distribution is I (x, y) = lVlx, y) 12 = uo2(
X, y) 10 u, 2 (X, y) Therefore, an interference batt is obtained that has the maximum intensity at the point (x, y) that satisfies the following, and the minimum intensity at the point that satisfies the following.

Now, consider scanning the interference batten by one wavelength (2π phase). In other words, the optical path difference between the three beams t= rQ (X
r y) If there are n waves in rr(x+y), then n・λ=t...Old・Gan・(2) holds true. Next, according to equation (1), if the driving current I of the semiconductor laser is increased and the wavelength is shifted by Δλ in the long wavelength direction, there are n −1 waves within the optical path difference (n − 1) (λ+Δλ)-4・Old River...(3) holds true. If n is eliminated from equations (2) and (3), λ
2 t=-, old...Use...(4) Δλ is obtained.

From equations (1) and (4), ΔI = 10/4 (mA)...--(5) Equation (5) shifts the interference wave by one wavelength in an interferometer with an optical path difference t. This gives the change in current ∆factor. Figure 1 is a graph of this relationship. For example, when the optical path difference is 10 m, ΔI = 1 mA, and normally the median operating value of the semiconductor laser is 6 mA as shown in Figure 2.
Since it is 0 to 100 mA, it can be seen that wavelength scanning is possible with extremely small current modulation. In contrast to such a large current change, the change in optical output is extremely small.

FIG. 3 is a block diagram showing one embodiment of the present invention, and shows the case of a Matsuhatsu Ender type interferometer. This interferometer uses a semiconductor laser 1 as a light source, converts the light beam from the semiconductor laser into parallel light using a condenser lens 2, splits it into two light beams using a beam splitter 3-1, uses one of the light beams as a light beam for inspection, and Transparent object 1 to be measured in the beam of light
0 and forms interference fringes on the photodetector 6 with the other beam (reference beam) from the reflecting mirror 4-1 through the second beam splitter 3-2 by the reflecting mirror 4-2. It is. The optical path difference between these three beams can be adjusted by rotating the beam splitter 3-1 and moving the reflecting mirror 4-1.

FIG. 4 is a diagram showing an example of an interference pattern obtained from this interferometer. In FIG. 4, the interval between interference fringes L2 (indicated by a solid line) and L2 (indicated by a dotted line) corresponds to a wavefront shift of one wavelength. Change the wavelength of semiconductor laser 1 from λ1 to λ2
When the wavelength is 2 ri, interference fringes Lt are generated, and when the wavelength is λ2, interference fringes L2 are generated, and the interference fringes move. When the current of the semiconductor laser 1 is modulated with a frequency ω and an amplitude Δ■, and data is acquired through photoelectric conversion by the photoelectric detector 6, only that frequency component can be extracted by passing it through a filter with a frequency ω. This results in a high S/N signal. In other words, it is possible to eliminate vibration components caused by disturbances to the interferometer and the influence of temperature, etc., and reading accuracy can be improved by irregularly repeating measurements many times and taking the average. Furthermore, it is possible to display the phase distribution after subtracting errors of the interferometer itself, inclination, focusing errors, etc. in a contour diagram or the like.

FIG. 5 shows another embodiment of the present invention (h factor,
This invention is applied to a Twyman Green interferometer. A beam from a light source consisting of a semiconductor laser 1 is turned into a semi-linear beam by a condenser lens 2, and divided into two beams by a beam splitter 3. One beam, which becomes a reference beam, is reflected by a reflection mirror 4, The light passes through and is irradiated onto the photodetector 6. The beam transmitted through the beam splitter 3 is used as an inspection light flux, and this light flux is reflected by the object to be inspected 10 and further passed through the beam splitter.
3 and forms interference fringes on the photodetector 6 together with the reference beam. At this time, the optical path difference of the three beams is
can be adjusted by moving in the optical axis direction.

FIG. 6(a) is a circuit diagram showing an embodiment of the semiconductor laser drive circuit used in the present invention, and FIG. 6(b) is a signal waveform diagram thereof.

This circuit is for modulating the semiconductor laser LD with the amplitude ΔI with the bias current Io as the center value. Now, apply a voltage of -VE volts to point A in the diagram, and adjust the volume R.
1, if the potential at point B (base of transistor Q) is set to -■B volts, a voltage of V=(VB 0.7) (Via) will be applied across the semiconductor laser LD. Therefore, if the resistance of the semiconductor laser is r, then l0=V/(Rs+r
) can flow, and the semiconductor laser LD oscillates. This value is defined as the bias current.

Current modulation with an amplitude of ΔIo is applied externally to point C in the figure.
Input a sine wave or a rectangular wave with a frequency ω as shown in FIG. 6(b), and drive the semiconductor laser to
It is driven by a modulation current of 6 sin ωt.

〔Effect of the invention〕

As described above, according to the VC and the present invention, it is possible to read interference fringes with high precision just by slightly modulating the driving current of the semiconductor laser light source.

Furthermore, it is also possible to measure the surface of the spot of an optical head used for recording and reproducing an optical disk and to measure the aberration inherent in the semiconductor laser itself with high precision.

[Brief explanation of the drawing]

Figure 1 is a graph showing the current change required to shift the wavelength of the semiconductor laser by one wavelength with respect to the optical path difference between the two light beams of the interferometer. Figure 2 shows the current and optical output characteristics of the semiconductor laser. Graph, FIG. 3 is a configuration diagram showing one embodiment of the present invention, FIG. 4 is a diagram showing an example of an interference pattern according to the present invention, FIG. 5 is a diagram showing another embodiment of the present invention, and FIG. Figure (a
) is a circuit diagram showing an example of a semiconductor laser drive circuit used in the present invention, and FIG. 6(b) is a signal waveform diagram thereof. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 3.3-1.3-2... Beam splitter-14, 4-1, 4-2... Reflection mirror, 6... Photodetector, 10... Object to be inspected. Agent Patent attorney Akira Takahashi f'') (9) ゝ\Nino■ 1 Fig. 2 d 6g tt) t2 尤flr-difference (ru~ 2nd Fig. 2θ 4ρ 〆ρ 3ρ 1ρρ, familiarization 1 ninme 屓-(mt -A〕 Tonzu Zutomi 4th item y N Omori”)

Claims (1)

[Claims] In an interferometer that splits a light beam from a light source into two, one as a reference beam and the other as a test beam, and irradiates the three beams onto the same surface again to produce interference fringes, a semiconductor laser is used as the light source. A wavelength scanning semiconductor characterized in that the wavelength of the semiconductor laser is scanned and the intensity distribution of interference fringes is changed in an alternating current manner by slightly modulating the operating current of the semiconductor laser from its central operating value. Laser interferometer.