JPH0237972B2 - KUTSUSETSURITSUBUNPUGATARENZUNOSHUSASOKUTEIHOHO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for accurately measuring spherical aberration of a gradient index lens.

Lenses generally have various aberrations, but spherical aberration is the most basic aberration. This spherical aberration is defined as follows. That is, as shown in FIG. 1, the intersection of the Gaussian image plane (focal plane of the paraxial ray 2) 3 of the lens 1 and the lens optical axis 4 is generally set to 0,
If the intersection point of the far-axis ray 5 that is incident parallel to the optical axis and passes through the lens 1 and crosses the Gaussian image plane 3 at a sufficiently large distance a from the optical axis is B, then the distance between OB and ΔSt is the transverse aberration ( transverse aberration), and the intersection C between the far axis ray 5 and the optical axis 4 and the above O
The distance ΔSl between the points represents the axial longitudinal aberration.

In the example shown in FIG. 1, the signs are Δst<O and ΔSl<O.

By graphing the relationship between △Sl and a obtained by varying the distance a from the optical axis as described above as shown in Figure 1A, it is possible to calculate the axial longitudinal aberration (LSA) among the spherical aberrations. In addition, transverse aberration (TSA) among spherical aberrations can be shown by graphing the relationship between tanU and △St as shown in Figure 2B, with the angle formed by paraxial ray 2 and distal ray 5 being U. It will be done.

The Hartmann test is known as a general method for measuring the above-mentioned spherical aberration in lenses such as photographic lenses and telescope objective lenses.

In this Hartmann test, a parallel beam of light emitted from a collimator is received by a test lens directly facing the collimator. If a diaphragm with equally spaced pinholes is placed between the collimator and the test lens, each ray of light emitted from the pinholes will pass through the lens like a ray of light and converge at its focal point. , when sequentially photographing with two dry plates placed in front and behind the focal point, the light ray intersects the optical axis based on the incident height h of the ray on the lens, the heights h ₁ and h ₂ of both plates, and the distance L between both plates. The distance l of the point from the front dry plate is determined by l(h)=L×h ₁ /h ₁ +h ₂ .

Next, with h on the vertical axis and l on the horizontal axis, draw a curve of l(h) and find the value lp of l for h = O.
−lp is the spherical aberration.

However, when the above measurement method is applied to the measurement of spherical aberration of a gradient index lens, the following problems unique to gradient index lenses arise.

In other words, even if the diameter of the pinhole in the diaphragm placed between the collimator and the test lens is made sufficiently small, the diameter of the beam exiting the lens will expand by a considerable amount compared to when it enters the lens.
For this reason, the diameter of the light spot on the dry plate described above becomes quite large, and the peripheral part is blurred, so there is a problem that the distance h ₁ or h ₂ from the optical axis cannot be measured with high accuracy.

An object of the present invention is to solve the above-mentioned problems and provide a method that can measure the spherical aberration of a gradient index lens with high precision.

The present invention will be explained in detail below.

A gradient index lens has a refractive index on the optical axis of
Refractive index n(r) at a distance r from the optical axis where n ₀
However, n ² (r) n ² ₀ [1-(gr) ² ] ......(1) g: A lens made of transparent glass or plastic that has a refractive index distribution expressed by a distribution constant, and is generally is a cylindrical body whose both end faces are parallel planes and which has the above-mentioned refractive index distribution in the radial direction.

In the case of such a gradient index lens, the third
As shown in the figure, a light beam 11 is incident on one end surface of a gradient index lens 10 while being deviated from the optical axis, travels inside the lens in a sine curve, and then exits from the other end surface.

And the beam spot radius W on the incident end surface
However, if the spot size W ₀ is larger than that inherent to the gradient index lens 10, the beam will be focused by the focusing action of the lens 10 and then diffused as shown in Figure 3A, making it impossible to measure with high precision. It disappears.

Conversely, if the beam spot radius W is smaller than the above-mentioned characteristic spot size _W0 , the beam having a divergence angle due to diffraction is collimated and emitted as shown in Fig. 3 (b), so the beam diameter d becomes large, and as shown in the above-mentioned Similarly, measurement accuracy deteriorates.

In order to avoid the above problem, in the method of the present invention, a light beam is deviated from the optical axis to one end surface of a gradient index lens whose spherical aberration is to be measured, and the beam spot radius W on the lens end surface is The light is controlled to be incident so that it almost matches the specific spot size W ₀ .

Here, the unique spot size W ₀ can be determined as W ₀ =λ/2π·n ₀ ·g (2).

In equation (2), λ is the wavelength of the light beam used, n ₀ is the refractive index on the optical axis of the lens, and g is the refractive index distribution constant in equation (1), and generally W ₀ =10 to 25 μm is used. In this way, if the beam spot radius on the entrance end face of the lens is chosen to be the above-mentioned characteristic spot size, the incident beam will proceed with a constant spot size, and in each cross section perpendicular to the central axis, there will be a gap between the rays of the beam. There is no shift in phase velocity.

For this reason, the beam emitted from the lens has almost no divergence angle and is very narrow, so the distance between the beam center and the optical axis on a virtual measurement surface far from the lens end face can be measured by photoelectric measurement, for example, as shown in the example below. By using these methods in combination, measurements can be made with extremely high precision.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to embodiments shown in the drawings.

FIG. 4 shows a plan view of the entire apparatus for carrying out the method of the present invention, and FIG. 5 schematically shows a plan view of the main parts. Reference numeral 10 denotes a gradient index lens for measuring spherical aberration, and this lens 10 has a V-groove on a fine movement stage 12 that can move in a total of six axes consisting of three parallel axes of X-Y-Z and three tilt angle axes. This fine movement stage 12 is placed on a base stage 13 that can be moved and adjusted in the X-Z axis directions.

Further, another six-axis fine movement stage 14 is placed on this base stage 13, and a slit plate 15 is attached to this fine movement stage 14. They are placed at right angles to the optical axis, generally within a few millimeters apart.

This slit plate 15 is a transparent glass dry plate provided with a blocking coating having an opening width of about 10 μm.

Further, a photodetector 16 is arranged behind the slit plate 15 so as to be perpendicular to the optical axis, and the amount of light received by the photodetector 16 is measured by a power meter 17.

A laser light source 1 is located in front of the lens 10 to be measured.
8. A polarizing beam splitter 19, a quarter wavelength plate 20, and a matching lens 21 are sequentially arranged with their central axes aligned.

In the above device, as the light source 18, for example, λ=
A 6328 Å linearly polarized helium neon laser is used, and the beam 22 from the light source 18 is passed through a polarizing beam splitter 19 and a quarter wave plate 20. As a result, the reflected light is removed in the direction of arrow 23 because its plane of polarization is perpendicular to the light ray immediately emitted from the laser. Next, quarter wave plate 2
The beam passing through the lens 10 is passed through the matching lens 21, and the spot radius W on the end face of the gradient index lens 10 is determined by the above-mentioned equation (2).
The beam is narrowed down to match the characteristic spot size W ₀ of , and is made incident on the lens 10 to be measured deviated from the optical axis and parallel to the optical axis.

As the matching lens 21, it is preferable to use a lens whose focal length f is equal to: f= _2W0.2Wl /0.82λ λ=wavelength Wl=beam radius of the laser beam incident on the lens. As an example, f=
Use a microscope objective lens of about 50 mm. Note that instead of using the matching lens 21, the laser beam may be passed through a pinhole having a hole diameter equal to the above-mentioned specific spot size W ₀ and then made incident on the lens to be measured.

However, it is advantageous to condense the light into a specific spot size using the lens 21 as in the embodiment because the beam power is large and the detection sensitivity is high.

When measuring the spherical aberration of the gradient index lens 10 using the above device, initially the slit plate 15 is
is placed on the first virtual measurement plane P1.

Next, while measuring the amount of light received by the photodetector 16 through the aperture 15A of the slit plate 15 with the power meter 17, the slit plate 15 is moved in the direction perpendicular to the optical axis by the fine movement stage 14 until the amount of light received is the maximum. The distance the slit moves from the optical axis to the above position is measured. This moving distance is determined by the fine movement stage 1 supporting the slit plate 15.
It is read with a dial gauge 24 placed in contact with 4.

Through the above operation, the beam incident on the end face of the gradient index lens 10 at a distance h from the optical axis is bent by the lens 10 and emitted, and then
The distance h ₁ between the point intersecting P ₁ and the optical axis can be measured with high precision.

Next, the slit plate 15 is translated in the Z-axis direction in FIG. 4, and the same operation as above is performed on the second virtual measurement surface P2 to determine the distance from the intersection of the output beam and the measurement surface P2 to the optical axis. Measure _h2 .

The positions of the measurement planes P1 and P2 may be arbitrary, but if they are too far away from the lens end face, the spread of the beam will increase and the measurement accuracy will deteriorate, so it is desirable to select them at a position of about 3 m/m or less from the lens end face. This process is performed for incident rays at various distances h to obtain h ₁ .

Next, the slit plate 15 is moved in parallel in _the Z-axis direction in _FIG . Measure the distance _h2 . The beam deviation amounts h ₁ and h ₂ from the optical axis at both measurement surfaces P1 and P2 are measured in the same manner as above.

This process is repeated to obtain h ₁ and h ₂ for incident rays at various distances h (plus side and minus side).

A specific numerical example is shown in FIG.

In this example, the lens to be measured 10 has a diameter of 2 m/m,
Distribution constant g=0.31m/m ^-1 , lens length 0.22 period length,
Using a gradient index lens with a specific spot size W ₀ = 15μφ, the amount of deviation of the incident beam from the optical axis is sequentially varied at a pitch of 1/10 of the lens radius (9 points to the positive side from the optical axis, 9 points to the negative side) This is a graph of h ₁ and h ₂ obtained by adding 9 points to the side.

The distance from the end surface of the lens to be measured to the first measurement surface P1 is 200μm, and the distance to the second measurement surface is 600μm.
I have selected m.

The left and right vertical axes of the graph in FIG. 6 represent beam deviation amounts h ₁ and h ₂ from the optical axis at measurement surfaces P1 and P2, respectively. The dashed line in the center indicates the paraxial focal position.

In this way, by connecting both points h ₁ and h ₂ corresponding to each h with a straight line and finding the distance between the intersection of the +h and -h straight lines and the central axis, this becomes the spherical aberration.

FIG. 7 shows a graph of spherical aberration (LSA) obtained from the graph of FIG. 6.

In the graph in Figure 7, the vertical axis is the lens radius ro.
The distance h between the incident beam and the optical axis is shown when is set to 1, and the horizontal axis represents the amount of spherical aberration (unit: μm).

As described above, according to the method of the present invention, there is almost no spread of the beam within the lens to be measured and after exiting the lens, so it is not affected by coma or astigmatism, and from the end surface of the lens to be measured to the measurement planes P1, P
2 can be made relatively large with almost no increase in beam diameter, the amount of beam deviation from the optical axis at both measurement surfaces P1 and P2 can be easily adjusted using a simple device. Can be measured with high precision.

In addition, when measuring the amount of deviation mentioned above, as shown in the example shown in the figure, the amount of light received by the emitted beam is measured through a light shielding body with a minute aperture width such as a slit or pinhole, and the position of the light shielding body where the maximum amount of light can be obtained is determined. If this is done, the center position of the beam can be determined very easily and with high precision.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the definition of spherical aberration of a lens, Figure 2 (a) and (b) are graphs showing axial longitudinal aberration (LSA) and transverse aberration (TSA), respectively, among spherical aberrations, and Figure 3 (a) and (b) are graphs showing the lateral aberration (TSA), respectively. 4 is a side sectional view showing a problem when the incident beam diameter is inappropriate in measuring the spherical aberration of a gradient index lens, FIG. 4 is a plan view showing an example of an apparatus for carrying out the method of the present invention, and FIG.
The figure is a plan view showing the main parts of the apparatus shown in Fig. 4, Fig. 6 is a graph showing the amount of beam deviation from the optical axis at both measurement planes P1 and P2 for specific numerical examples, and Fig. 7 is the graph shown in Fig. 6. 1 is a graph of a gradient index lens obtained from the graph of FIG. 10... Gradient index lens, 15... Slit plate, 16... Photo detector, 18... Laser beam, 19... Polarizing beam splitter, 2
0... Quarter wavelength plate, P1, P2... Measurement surface.

Claims (1)

[Claims] 1. A light beam is deviated from the optical axis onto one end surface of a gradient index lens, and the beam spot radius W ₀ on the lens end surface is W ₀ =λ/2π・n ₀・g Where, λ: Wavelength of light used n ₀ : Lens center refractive index g : Output beam on a virtual measurement surface that is a certain distance from the other end surface of the lens and is incident so as to almost satisfy the relationship of refractive index distribution constant of the lens. A method for measuring aberrations of a gradient index lens, the method comprising measuring the distance between the optical axis and the optical axis. 2. Arrange a light shielding body with an aperture of minute width on the measurement surface, and place a photodetector behind the light shielding body, and while measuring the amount of light received by the photodetector, move the light shielding body in a direction perpendicular to the optical axis. The distance between the center of the emitted beam and the optical axis on the measurement surface is determined by moving the light shielding body from a reference point to a position where the amount of received light is maximum. 2. A method for measuring aberrations of a gradient index lens according to item 1.