JPH0444939B2 - - Google Patents

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, and in order to improve the accuracy of reading interference fringes,
For example, methods such as providing 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 both of these devices are expensive and require many adjustment points. Moreover, it had drawbacks such as the need for a high-voltage driving 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 drawbacks, is inexpensive, simple, easy to scan, and can electrically read interference fringes with high accuracy.

[Summary of the invention]

That is, the present invention uses a semiconductor laser as a light source in a Twyman Grin interferometer, a Matsuhatsu-Ender interferometer, etc., modulates its drive current, and scans the wavelength of the semiconductor laser to create interference fringes within the field of view. The purpose is to improve the accuracy of reading interference fringes by scanning.

[Embodiments of the invention]

First, the principle of the present invention will be explained.

In general, in semiconductor lasers, the drive current ΔI (mA)
If you change just that, the wavelength will shift. for example,
Taking a grooved substrate type semiconductor laser as an example, the wavelength shift amount Δλ is given by Δλ=0.006·ΔI (nm) (1).

On the other hand, the complex amplitude of light at any point (x, y) on the interference pattern is time-averaged and is given by V(x, y)=u ₀ (x, y) exp{ik・r ₀ (x ,y)}
+u _r (x, y) exp {ik・r _r (x, y)}. Here, u ₀ and u _r are the amplitudes of the object beam and the reference beam, respectively, r ₀ and r _r are the optical path lengths of the object beam and the reference beam from the reference plane, and k is the waveform, which is given by k = 2π/λ. . From this, the intensity distribution is I (x, y) = | V (x, y) | ² = u ₀ ² (x, y) + u _r ² (x, y) + 2u ₀・u _r・cos ₀ − r _r )}, the maximum intensity is at the point (x, y) that satisfies 2π/λ{r ₀ (x, y) − r _r (x, y)} = m (m: integer), 2π/ λ{r ₀ (x, y)−r _r (x, y)}=π/2・m
(m: integer) An interference pattern is obtained that has the minimum intensity at points that satisfy (m: integer).

Now, consider scanning the interference pattern by one wavelength (2π phase). That is, if there are n waves within the path difference between the two light beams l=r ₀ (x, y)−r _r (x, y), n·λ=l (2) holds true. Next, according to equation (1), if the driving current of the semiconductor laser is increased and the wavelength is shifted in the longer wavelength direction by Δλ, there will be n-1 waves within the optical path difference l, so (n-1 )(λ+Δλ)=l...(3) holds true. By eliminating n from equations (2) and (3), l=λ ₂ /Δλ (4) is obtained. From equations (1) and (4), ΔI = 10/l (mA) ...(5) Equation (5) expresses the change in current ΔI to shift the interference pattern by one wavelength in an interferometer with an optical path difference of l. It is something to give. Figure 1 is a graph of this relationship. For example, when the optical path difference is 10 mm,
Since ΔI=1 mA and the average operating value of a semiconductor laser is normally 60 to 100 mA as shown in FIG. 2, it is understood that wavelength scanning is possible with extremely small current modulation. Furthermore, with respect to such a change in current, 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, and a light beam from this semiconductor laser is passed through a condensing lens 2.
The beam splitter 3-1 divides the beam into two beams, one beam is used as the inspection beam, the transmitting object 10 to be measured is placed in this beam, and the second beam is parallelized by the reflecting mirror 4-2. The light beam passes through the beam splitter 3-2 and forms interference fringes on the photodetector 6 together with the other beam (reference beam) from the reflecting mirror 4-1. The optical path difference between these two 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 Figure 4, the interference fringes
The interval between L ₁ (indicated by a solid line) and L ₂ (indicated by a dotted line) is 1
This corresponds to a shift in the wavefront by a wavelength. Semiconductor laser 1
_{If the wavelength of _}
L ₂ occurs and the interference fringes move. Semiconductor laser 1
When the current is modulated with frequency ω and amplitude ΔI, and data is acquired by photoelectric conversion using the photoelectric detector 6, only that frequency component can be extracted by passing it through a filter with frequency ω, resulting in a high S/N ratio. It becomes a signal. That is, it is possible to remove vibration components caused by disturbance 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, inclinations, focusing errors, etc. of the interferometer itself in a contour diagram or the like.

FIG. 5 is a block diagram showing another embodiment of the present invention, in which the present invention is applied to a Twyman Green interferometer. A beam from a light source consisting of a semiconductor laser 1 is made into parallel light 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, and is sent to the beam splitter. 3 to the photodetector 6
is irradiated. The beam transmitted through the beam splitter 3 is used as an inspection light beam, and this light beam is reflected by the object to be inspected 10 and further reflected by the beam splitter 3 to form interference fringes together with the reference beam on the detector 6. . At this time, the optical path difference between the two beams can be adjusted by moving the reflecting mirror 4 in the optical axis direction.

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

This circuit uses a semiconductor laser LD with a bias current
This is for modulating with amplitude ΔI with I ₀ as the center value. Now, if a voltage of -VE volts is applied to point A in the figure, and the potential at point B (base of transistor Q) is set to -V _B volts using polyurethane _R1 , the voltage across the semiconductor laser LD will be as follows. A voltage of V = (-V _B -0.7) - (V _E ) is applied. Therefore, if _{the resistance of the semiconductor laser is r, I 0 =} V/
A current of (R ₃ +r) can flow, and the semiconductor laser LD oscillates. This value is taken as the bias current.

Current modulation with an amplitude of ΔI ₀ is performed by inputting a sine wave or a rectangular wave with a frequency ω as shown in Figure 6b from the outside to point C in the figure, and changing the semiconductor laser so that I = I ₀ + ΔI ₀ sinωt. It is driven by modulated current.

〔Effect of the invention〕

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

Furthermore, it is also possible to measure the wave circle of a spot on 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 drawings]

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, 3rd
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. 6 a is a diagram showing an example of the interference pattern according to the present invention. FIG. 6B is a circuit diagram showing an example of a semiconductor laser drive circuit used in the present invention, and FIG. 6B is a signal waveform diagram thereof. 1... Semiconductor laser, 3, 3-1, 3-2...
Beam splitter, 4, 4-1, 4-2...Reflection mirror, 6...Photodetector, 10...Object to be inspected.

Claims (1)

[Claims] 1. 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 then irradiates the two beams onto the same surface again to produce interference fringes, a semiconductor is used as the light source. A wavelength scanning semiconductor laser interferometer, characterized in that a laser is used to scan the wavelength of the semiconductor laser by modulating the drive current of the semiconductor laser, thereby changing the intensity distribution of the interference fringes.