JPH03225250A - Measuring instrument for refractive index distribution - Google Patents

Abstract

PURPOSE:To measure the refractive index distribution of a preform with high accuracy by controlling the temp. of circular cylindrical glass and matching liquid always constant. CONSTITUTION:The measuring instrument has an incident optical system 6 which makes rays incident from the direction perpendicular from the central axis of the circular cylindrical glass 1 and an image pickup means 10 which receives exit light 9 entering from the incident optical system 6 and past the circular cylindrical glass 1 and delivers the electric signal of the photodetected image. The circular cylindrical glass 1 is mounted in a cell 2 filled with the matching liquid 3 having the refractive index approximately equal to the refractive index in the outermost peripheral part thereof. A quartz glass window 21 perpendicular to the incident light is provided in the incident position and exit position of the ray of the cell 2 and a jacket 22 i which a liquid medium 23 for controlling the temp. of the matching liquid 3 circulates is provided on the outer side of the cell 2. The exit light 9 is received by the image pickup means 10 and the exit angle phi thereof is measured. Since the temp. of the matching liquid 3 is kept constant by the liquid medium 23, the fluctuation in the refractive angle of the matching liquid 3 an the circular cylindrical glass 1 or the quartz glass window 21 is suppressed.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an apparatus for measuring the refractive index distribution of cylindrical glass used, for example, in optical fiber preforms or rod lenses.

[Conventional technology]

Cylindrical glass used for optical fiber preforms (base materials) and rod lenses has a refractive index in the radial direction that has a substantially square distribution, and a refractive index in the axial direction that is uniform. An optical fiber is formed by drawing this. Accurately measuring the refractive index distribution of the preform before drawing is necessary to obtain a good product. As a method for measuring refractive index distribution, for example, Japanese Patent Application Laid-Open No. 1983-1995
Japanese Patent Application No. 336 discloses a method in which a light beam is made incident in a direction perpendicular to the central axis of a preform for an optical fiber, and its exit angle is determined to measure the refractive index distribution of the preform. FIG. 1O shows a refractive index distribution measuring device disclosed in the same publication. As shown in the figure, the output light from an incident optical system consisting of a light source 6 and a lens 7 enters a preform l installed in a matching oil 3 in a cell 2, and is emitted through the preform l. The light passes through an output optical system having a lens 51 and is projected onto the viewing surface of the TV camera 52. As shown in FIG.
The 60th-order diffracted light spot 4 is among the 0th-order diffracted light spot 40, the 1st-order diffracted light spot 41, the 2nd-order diffracted light spot 42, and the 60th-order diffracted light spot 42 of the output light projected onto the observation surface 43 of the camera 52.
0 is extracted by two-dimensional analysis, and the output angle φ is determined from the X coordinate Xr of this 0th order diffracted light spot 40 and the focal length f of the output optical system as follows: φ=tan-'fxr/f) . Then, the refractive index distribution n (rl) of the preform 1 is calculated from this exit angle φ using the following formula. By providing a slit and extracting only the zero-order diffracted light spot 40 of the emitted light, the refractive index distribution is measured without being influenced by higher-order diffracted light spots.

[Problems to be Solved by the Invention] However, when the refractive index is determined using such a measuring device, the temperature of the matching liquid 3 changes due to the temperature change of the matching liquid 3.
The refraction angle of the preform L and preform L with respect to the quartz glass window that is the exit port of the cell 2 changes, and an error may occur in the obtained output angle φ. Furthermore, if a temperature change occurs during measurement, there is a problem in that the refractive index distribution becomes asymmetric even when measuring the preform 1 whose refractive index distribution is point symmetrical. The present invention was made to solve the above problems,
It is an object of the present invention to provide a refractive index distribution measuring device that can obtain a more accurate refractive index distribution of a preform. [Means for Solving the Problems] A refractive index distribution measuring apparatus to which the present invention is applied to solve the above problems will be described with reference to drawings corresponding to embodiments. As shown in FIGS. 1 and 2, the refractive index distribution measuring device of the present invention includes an entrance optical system 6 that receives light from a direction perpendicular to the central axis 1a of a cylindrical glass 1; and an imaging means lO for receiving the emitted light 9 passing through the cylindrical glass l and sending out an electric signal of the received light image.
A cylindrical glass l is mounted in a cell 2 filled with a matching liquid 3 having a refractive index approximately equal to the refractive index of the outermost portion of the cylindrical glass l. As shown in FIG. 3, a quartz glass window 21 perpendicular to the incident light is provided at the incident and exit positions of the light beam of the cell 2, and a liquid medium 23 for controlling the temperature of the matching liquid 3 is provided outside the cell 2. A jacket 22 is provided which allows the crystallization.

[Effect]

In the measuring device of the present invention, the emitted light 9 that enters the cylindrical glass in a direction perpendicular to the central axis la and exits is received by the imaging means 10 and its emitted angle φ is measured. Matching liquid 3
Since the temperature of is kept constant by the liquid medium 23 which circulates through the jacket 22 of the cell 2 and controls the temperature, fluctuations in the refraction angle between the matching liquid 3 and the cylindrical glass l or the quartz glass window 21 are suppressed. . [Examples] Examples of the present invention will be described in detail below. FIG. 1 shows a schematic configuration of an embodiment of the refractive index distribution measuring device of the present invention, and FIG. 2 shows the optical path in the measuring device. In these figures, 2 is a cell equipped with a preform l, and a machining oil 3 is installed in the cell 2 to eliminate sudden changes in refractive index on the surface of the preform l.
is fulfilled. The preform 1 can be moved along with the cells 3 by a moving table 4. 5 is a light source consisting of, for example, a He-N e laser oscillator; 6 is an input optical system; the input optical system 6 converges the incident light from the light source 5 so as to be minimized at the center of the preform l. 7a and 7b, the mirror 8 is a screen that forms a projected image of the emitted light 9 emitted from the preform 1;
1 is a TV camera for observing the projected image projected on the screen 8, and 11 is a control unit that determines the exit angle from the position of the projected image obtained by the TV camera IO and calculates the refractive index distribution. The entire apparatus is installed in a constant temperature room set to the temperature at which the preform I is measured. FIG. 3 shows an enlarged view of the cell 2. The cell 2 is a rectangular parallelepiped-shaped container, and its M2a and bottom surface are provided with mechanically polished quartz glass 2.21. The preform 1 is mounted through its side. The inside of the cell 2 is filled with a matching oil 3 having a refractive index substantially equal to that of the outermost periphery of the preform 1, and a jacket 22 containing a liquid medium 23 is attached to the outside of the cell 2. As shown in FIG.
A circulation circuit for the liquid medium 23 is formed. 36 is a pump. A sensor 32 of a temperature detection section 33 for measuring the temperature of the matching oil 3 is attached inside the cell 2. A temperature control unit 35 is connected to the temperature detection unit 33 and controls the temperature of the liquid medium 23 based on the signal.
1 houses the temperature regulator 34 thereof. FIG. 5 is a block diagram showing the configuration of the control section 11. As shown in FIG. In the figure, 12 is a frame memory that stores data of projected images 40 to 42 (see FIG. 6) on the screen 8 observed with the TV camera 1O, and 13 is a frame memory that binarizes the data stored in the frame memory 12 and performs linear approximation. calculation means for performing 1
4 is a position calculation means for calculating the intersection of the straight line obtained by the calculation means 13 and a plane perpendicular to the central axis 1a of the preform 1 and through which the incident light passes; 15 is the output angle φ from the coordinates of the intersection obtained by the position calculation means 14. This is an exit angle calculation means that calculates. 16 is the preform l based on the output angle φ calculated by the output angle calculation means.
17 is an output means consisting of a display section and a recording section. The refractive index profile measurement of the preform I is carried out by installing the entire measuring device in a constant temperature room set within ±1°C of the measurement temperature, and by driving the pump 36 of the temperature control device to pump the liquid medium 23. The temperature of the matching liquid 3 is kept within ±0. lT: Adjust within. As shown in FIG. 2, the light sent from the light source 5 passes through a plane P perpendicular to the central axis 1a of the optical fiber preform l, is reflected by a mirror 7a, enters the preform l, and is refracted. and emit light. The emitted light 9 passing through the preform 1 is reflected by the mirror 7b, and reaches the screen 8 which is perpendicular to the light rays emitted when the preform 1 is not present, that is, through the cell 2 and the matching oil 3. Points a, b, and c of the incident light and the outgoing light 9 exist on the plane P if there is no change in the refractive index in the axial direction of the preform 1, but in reality they are scattered by the preform 1 and Projected as shown. Observing each of the projected images 40 to 42 with the TV camera 10,
The sixth step is to take the output signal into the frame memory 12.
The data of each projection image stored in the frame memory 12 is binarized by the calculating means 13 and linear approximation is performed by the method of least squares to obtain an approximate straight line 20 shown in the figure. After that, the first position calculating means 14 calculates the approximate straight line 20 and the plane perpendicular to the central axis 2 of the preform l and through which the incident light passes, that is, y as shown in FIG.
Find the intersection 'jc with the axis, and if there is no preform l,
That is, the output angle calculation means 15 calculates the output angle φ from the coordinate values of the intersection Yc with the projected image of the output light passing through the cell 28 and the matching oil 3 as the reference point O, and the calculated output angle φ is stored in the memory L8. . This operation is performed at each position in the radial direction of the beam 1 while moving the moving table 4, and the output angle φ at each position is determined and stored in the memory 18. This memory 18
The respective exit angles φ stored in are sent to the refractive index distribution calculation means 16 to calculate the refractive index distribution n tr> and sent to the output means 17. Even if the temperature of the matching liquid 3 fluctuates due to heat generated from the measuring device during measurement, the temperature of the matching liquid 3 is constantly measured by the sensor 32, and the temperature controller 35
is kept constant by adjusting the temperature of the circulating liquid medium 23. Therefore, fluctuations in the refraction angle between the matching liquid 3 and the preform 1 and the quartz glass window 21 are suppressed, and the output angle φ does not vary, making it possible to obtain an accurate refractive index distribution. As described above, for example, as shown in FIG.
Figure 8 shows the results of measuring the relationship between the incident position r and the output angle φ, and when the refractive index distribution nfrl was determined from the output angle φ shown in Figure 8, the characteristics shown in Figure 9 were obtained. . This measurement was repeated 30 times to obtain the relative refractive index difference Δ Δln+−nalX100/n expressed by the following formula, and the standard deviation 0 of this relative refractive index difference Δ was calculated and normalized by the relative refractive index difference Δ. As a result, ( o/Δ) = 0.001. In the above embodiment, the screen 8 and the TV camera l
Although a case has been described in which the projected image of the emitted light is observed using an image sensor, the same effect as in the above embodiment can be achieved even when the projected image is observed using an image sensor.

【Effect of the invention】

As explained above, the refractive index distribution measuring device of the present invention has the following features:
The temperatures of the cylindrical glass and the matching liquid are always controlled to be constant. Therefore, the refraction angle between the matching liquid and the cylindrical glass or the quartz glass window that serves as the cell's exit port will not change, causing errors in the exit angle, making it possible to measure the refractive index distribution of the preform with high precision. can.

[Brief explanation of drawings]

FIG. 1 is a schematic side view of an embodiment of an apparatus to which the present invention is applied;
FIG. 2 is an explanatory diagram showing the principle of refractive index distribution measurement in the above embodiment, FIG. 3 is an enlarged perspective view showing the cell of the above embodiment, and FIG. 4 is a schematic explanatory diagram of the temperature control device of the above embodiment. FIG. 5 is a block diagram showing the control section of the above embodiment, FIG. 6 is a diagram showing a projected image of the above embodiment, FIG. 7 is a characteristic diagram showing the refractive index distribution of the preform, and FIG. 8 is a diagram showing the above embodiment. FIG. 9 is a refractive index distribution characteristic diagram obtained from the output angle characteristics measured according to the example, FIG. 10 is a schematic plan view of the conventional example, and FIG. 11 is a diagram showing a projected image of the conventional example. be. l...Preform 2...Senoshi 3...Matching liquid 4...Moving table 5...Light source
6... Incoming optical system 7a, 7b... Mirror 9... Outgoing light 11... Control section 13... Calculating means 15... Outgoing angle calculating means 17... Output means 20... Approximate straight line 22...Jacket 24...Medium inflow pipe 31...Liquid temperature adjustment container 33...Temperature detection section 35...Temperature control section 8...Screen lO...TV camera 12... - Frame memory 14...Position calculating means 16...Calculating means 18...Memory 21...Quartz glass window 23...Liquid medium 25...Medium outflow pipe 32...Sensor 34... Temperature control body 36... Pump shop 1,000 and 1: Figure 8 Figure 9

Claims (1)

[Claims] 1. An entrance optical system that enters a light beam from a direction perpendicular to the central axis of the cylindrical glass; and an imaging means that receives the light that enters from the entrance optical system and passes through the cylindrical glass and sends out an electrical signal of the received light image. In a refractive index distribution measuring device installed in a cell in which the cylindrical glass is filled with a matching liquid whose refractive index is approximately equal to the refractive index of the outermost portion of the cylindrical glass, the incident and exit positions of the light beam of the cell are perpendicular to the incident light. 1. An apparatus for measuring refractive index distribution, characterized in that the cell is provided with a quartz glass window, and a jacket in which a liquid medium for controlling the temperature of a matching liquid circulates outside the cell.