Method for controlling diameter of GRIN lens fiber and fiber drawing equipment

Abstract

When a GRIN lens fiber is drawn from a preform, control of a fiber diameter is improved in order to increase a production yield of the GRIN lens fiber having a fiber diameter within a desired range. The problem is solved by controlling the drawing speed using a fiber diameter c, which is obtained by correcting a fiber diameter a using the fiber diameter b and a fiber diameter . The fiber diameter a is measured using a diameter measuring instrument A that measures an outer diameter of the GRIN lens fiber, which is being elongated inside a heating furnace, the fiber diameter b is measured using a diameter measuring instrument B that measures an outer diameter of the GRIN lens fiber outside the heating furnace, and the fiber diameter is a value of the fiber diameter a measured a specified period of time T earlier.

Claims

1. A method of controlling an outer diameter of a GRIN lens fiber, the method comprising: (I) drawing said GRIN lens fiber from a preform, said drawing including: (i) increasing a temperature of a furnace, wherein said furnace surrounds and encloses a region where said preform of said GRIN lens fiber is softened, and a heater is disposed inside said furnace and surrounds said region; (ii) welding said preform to a silica rod, lowering said welded preform inside said heater, and heating and softening said welded preform; (iii) elongating said softened preform, as said GRIN lens fiber; and (iv) winding said GRIN lens fiber onto a winding drum disposed below and outside said furnace; (II) measuring said GRIN lens fiber, said measuring including: (i) measuring an outer diameter a of said GRIN lens fiber during said drawing said GRIN lens fiber from said preform, at a first measurement position located below and outside said heater and inside said furnace; and (ii) measuring an outer diameter b of said GRIN lens fiber during said drawing said GRIN lens fiber from said preform, at a second measurement position located below and outside said furnace and forward of said winding drum; and (III) correcting and controlling said GRIN lens fiber, said correcting and controlling including: using a value b and a value a from a memory, included in a correction device, that stores values of said outer diameter a and said outer diameter b of said GRIN lens fiber obtained from said measuring said GRIN lens fiber, and obtaining a difference D between said value a and said value b where D=b, wherein said value b is said outer diameter of said GRIN lens fiber measured at said second measurement position at a certain time, and said value is said outer diameter of said GRIN lens fiber measured at said first measurement position at a time that is a period T before said certain time; (ii) correcting a measurement error of said measured outer diameter a at said first measurement position at said time by said difference D, obtaining a corrected outer diameter c where c=a+D=a+b, and transmitting said corrected outer diameter c to an automatic controller; and (iii) controlling a drawing speed by controlling a rotation speed of said winding drum by said automatic controller, so as to obtain a corrected outer diameter c which is closer to a target value than said corrected outer diameter c.

2. The method of claim 1, wherein: said period T (sec) is within a range of (d20)/VT(d+20)/V, where d (mm) is a distance between said first measurement position and said second measurement position, and V (mm/sec) is said drawing speed.

3. The method of claim 1, wherein: said first measurement position is positioned below and away from a center of a heating portion of said heater by 70 mm or more and inside said furnace.

4. The method of claim 1, wherein: a distance between said first measurement position and said second measurement position is 200 mm.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory view of equipment for drawing a GRIN lens according to an embodiment (when a drawing speed is controlled).

(2) FIG. 2 is an explanatory view of the equipment for drawing a GRIN lens according to the embodiment (when a preform is set).

(3) FIG. 3 is an explanatory view of the equipment for drawing a GRIN lens according to the embodiment (when a timer starts).

(4) FIG. 4 is a flowchart of a correction device.

REFERENCE NUMERALS

(5) 1 heating furnace 2 heater 3 ascending/descending device 4 winding drum 5 diameter measuring instrument A 6 diameter measuring instrument B 7 sensor 8 correction device 9 automatic controller 10 preform 11 GRIN lens fiber 12 weight 13 silica rod

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment

(6) FIGS. 1 to 3 illustrate equipment for drawing a GRIN lens fiber according to an embodiment. This drawing equipment includes a heating furnace 1, a heater 2, an ascending/descending device 3, a winding drum 4, a diameter measuring instrument A (5), a diameter measuring instrument B (6), a sensor 7, a correction device 8, and an automatic controller 9.

(7) The heating furnace 1 is a very high temperature vertical pipe furnace, the temperature of which can be increased up to 2100 C.

(8) The winding drum 4 includes a horizontal movement mechanism that prevents a turn of a fiber from overlapping another turn of the fiber. In the present embodiment, a drawn GRIN lens is directly wound using the winding drum 4. By controlling a rotation speed of the winding drum 4, a drawing speed is controlled. A capstan roller may be provided before the winding drum in order to control the drawing speed by controlling a rotation speed of the capstan roller.

(9) The diameter measuring instrument A (5) uses a commercial image processing device using a camera. The camera needs to be calibrated in advance using an object such as an optical fiber having a known dimension.

(10) A fiber diameter a, which is measured using the diameter measuring instrument A, is transmitted to the correction device 8.

(11) The diameter measuring instrument A (5) is provided at a position in which elongation of a GRIN lens fiber is substantially complete. In the present embodiment, the position is 80 mm below the center of a heating portion of the heater 2.

(12) The diameter measuring instrument B (6) uses a commercial outer diameter measuring device of a light-blocking type. A fiber diameter b, which is measured using the diameter measuring instrument B, is transmitted to the correction device 8.

(13) The measurement position of the diameter measuring instrument B (6) is preferably a position close to the exit of the heating furnace as much as possible. In the present embodiment, the measurement position of the diameter measuring instrument B (6) is positioned about 200 mm below the measurement position of the diameter measuring instrument A. Since the path of the GRIN lens fiber is vertical, the distance d in the GRIN lens fiber path between the measurement positions of the diameter measuring instrument A and the diameter measuring instrument B is about 200 mm.

(14) As illustrated in FIG. 3, the sensor 7 detects that a lower end of a weight 12 has passed the sensor 7, and transmits a detection signal to the correction device 8.

(15) The correction device 8 outputs to the automatic controller 9 a fiber diameter c in accordance with the fiber diameter a from the diameter measuring instrument A and the fiber diameter b from the diameter measuring instrument B.

(16) The automatic controller 9 controls the rotation speed (the drawing speed) of the winding drum such that the fiber diameter c becomes c which is closer to a specified target value. By controlling the rotation speed of the winding drum, the drawing speed at which the GRIN lens fiber is drawn is controlled.

(17) A trigger of fiber diameter correction can be not only transmitted from the sensor 7 but also manually transmitted.

(18) Next, the embodiment of a method for controlling a diameter of a GRIN lens according to the present invention will be described.

(19) After the temperature of the heating furnace 1 has been increased up to a specified temperature, the winding drum 4 is caused to start rotation and a horizontal movement. A preform 10 with a silica rod 13 welded thereto is disposed in the ascending/descending device 3.

(20) The ascending/descending device 3 is started to descend so as to descend the preform 10 to a specified position. This state is illustrated in FIG. 2. When the preform 10 is softened and the weight 12 is descended by a certain distance to a lower side of the heating furnace, the ascending/descending device 3 is started to descend at a constant speed. The descending speed is a constant speed that corresponds to the drawing speed of the GRIN lens.

(21) As illustrated in FIG. 3, the moment when the lower end of the weight 12 passes the sensor 7, the detection signal of the sensor 7 is transmitted to the correction device 8. This turns on a timer of the correction device 8. The timer is set to count an appropriate time (85 seconds in the present embodiment) for the fiber diameter of the GRIN lens fiber having been drawn to stabilize. When the set time has elapsed (when the timer reaches 0), the correction device 8 performs correction of the fiber diameter a, and transmits the corrected value, the fiber diameter c, to the automatic controller 9. Before the timer reaches 0, the difference between the fiber diameter a and the fiber diameter b is not calculated, and a+0 is set to the fiber diameter c.

(22) When the weight 12 is descended near the winding drum 4, the weight 12 is detached, an end of an elongated part is attached to the winding drum 4 using adhesive tape, and winding and drawing begin. Also at this time, automatic controller 9 is started.

(23) An average rotation speed of the winding drum 4 for a preform having a particular diameter is set so as to correspond to a ratio between a feeding speed and a target diameter. However, before the timer reaches 0, the difference between the fiber diameter a and the fiber diameter b is not calculated. Therefore, the automatic controller 9 performs automatic control in accordance with the fiber diameter c to which a+0 is set. Thus, the average rotation speed of the winding drum 4 constantly changes.

(24) After 54 seconds from the start of drawing, the fiber diameter was stabilized by the automatic control. When 85 seconds had elapsed and the timer reached 0, the correction device 8 started correction. At this time, the fiber diameter a, which was a measured value using the diameter measuring instrument a, was 125.3 m. The fiber diameter b, which was a measured value using the diameter measuring instrument B, was 121.7 m, a value that was smaller than the fiber diameter a by 3.6 m. Thus, it is observed that the measured value using the diameter measuring instrument A has an error.

(25) The correction is performed by calculating the fiber diameter c that is obtained by adding a difference (fiber diameter bfiber diameter ) to the fiber diameter a, where is a fiber diameter measured using the diameter measuring instrument A before T seconds, a specified period of time. That is, fiber diameter c=fiber diameter a+fiber diameter bfiber diameter . The fiber diameter c is transmitted from the correction device 8 to the automatic controller 9, where the automatic correction is performed such that the fiber diameter c becomes closer to the target value of 124.5 m. In the present embodiment, the specified period of time T varies between the 1.9 to 1.99 seconds in 10 ms. Since the drawing speed V is about 106.7 mm/second, and the distance d between the measurement position of the diameter measuring instrument A and the measurement position of the diameter measuring instrument B is about 200 mm, T varies substantially in the following range:
(d12.3)VT(d2.7)/V

(26) That is, T falls within (d20)/V, and the distance in the GRIN lens between the measurement positions of the fiber diameter b and the fiber diameter is within 20 mm. Thus, fiber diameter control can be performed substantially without problems.

(27) In several seconds after the correction has started, the fiber diameter b (an actual fiber diameter of the GRIN lens fiber) substantially falls within a range of 124.51 m. Thus, a fiber having a desirable fiber diameter can be obtained.

(28) Referring to FIG. 4, a program of the correction device 8 will be described below.

(29) After the program is started, whether or not the lower end of the weight 12 reaches the sensor 7 as illustrated in FIG. 3 is determined in step 101, WEIGHT 12 PASSED SENSOR 7? If a NO is returned, step 101 is repeated and a waiting state is entered until the lower end of the weight 12 reaches the sensor 7. If a YES is returned, the program proceeds to step 102.

(30) In step 102, a START TIMER operation is performed in order to turn ON the timer included in the correction device 8. A period of time suitable to stabilizing the fiber diameter of the drawn GRIN lens fiber (85 seconds in the present embodiment) has been set to the timer.

(31) In step 103, TIMER REACHES 0?, whether or not the set period of time (85 seconds) has elapsed is determined. If NO is returned, the program proceeds to step 104. If YES is returned, the program proceeds to a correction operation beginning in step 105.

(32) In step 104, an operation of OBTAIN FIBER DIAMETER c (FIBER DIAMETER a+0) FROM CORRECTION DEVICE 8 AND TRANSMIT FIBER DIAMETER c TO AUTOMATIC CONTROLLER 9 is performed. At this time, since the correction operation (difference D=fiber diameter bfiber diameter ) has not been performed, the difference D is 0, and the fiber diameter a obtained from the diameter measuring instrument A is transmitted to the automatic controller 9 as the fiber diameter c. This operation is repeated at intervals of 10 ms until the timer reaches 0. The automatic controller performs automatic control such that the fiber diameter a transmitted from the correction device becomes closer to a preset target value of 124.5 mm.

(33) After it has been determined that the timer reaches 0 in step 103, an OBTAIN FIBER DIAMETER FROM MEMORY AND FIBER DIAMETERS a and b FROM DIAMETER MEASURING INSTRUMENTS A and B operation is performed in step 105.

(34) The memory is included in the correction device. In the memory, 20 fiber diameters a, which have been transmitted from the diameter measuring instrument A at intervals of 100 ms, are sequentially stored. When the number of the transmitted fiber diameters a exceeds 20, the fiber diameters a are sequentially discarded from the top (first stored). That is, the fiber diameter A stored in the top location is measured 1.9 seconds before the last stored fiber diameter a. Since the fiber diameter (fiber diameter a stored in the top location) is obtained in every 10 ms (0.01 seconds), the fiber diameter to be obtained is a fiber diameter measured 1.9 to 1.99 seconds before. At the same time, the present fiber diameters a and b are obtained from the diameter measuring instruments A and B.

(35) In step 106, COMPUTE DIFFERENCE D=FIBER DIAMETER bFIBER DIAMETER , the difference D is computed.

(36) The fiber diameter b is a measured value presently measured using the diameter measuring instrument B. The fiber diameter is a measured value of a part of the GRIN lens near a part where the fiber diameter b is measured using the diameter measuring instrument A 1.9 to 1.99 seconds earlier. The fiber diameter b is correctly measured. Since the fiber diameter is measured in the furnace, it is highly probable that the measured value is incorrect. DIFFERENCE D=FIBER DIAMETER bFIBER DIAMETER equals to a measurement error of the diameter measuring instrument A.

(37) In step 107, a COMPUTE FIBER DIAMETER c=FIBER DIAMETER a+DIFFERENCE D AND TRANSMIT FIBER DIAMETER c TO AUTOMATIC CONTROLLER operation is performed.

(38) The fiber diameter c is obtained by correcting the fiber diameter presently measured using the diameter measuring instrument A so as to eliminate (or reduce) the measurement error. Thus, the fiber diameter c represents an almost correct fiber diameter at the measurement position of the diameter measuring instrument A. The fiber diameter c is transmitted to the automatic controller 9, which then performs automatic control such that the fiber diameter c transmitted from the correction device becomes closer to a preset target value of 124.5 mm.

(39) Steps 105 to 107 are repeatedly performed at intervals of 10 ms, and the fiber diameter c is transmitted to the automatic controller at intervals of 10 ms.

(40) As described above, according to the present invention, the fiber diameter of the preform in the heating furnace at a part that has been almost completely elongated (fiber diameter c) can be correctly measured. By performing automatic control in accordance with this fiber diameter, the acceptable product ratio (yield) is significantly improved compared to the related art method.

(41) Table 1 shows the results of drawing performed five times in accordance with the above-described embodiment. Table 2 shows the results of comparative examples in which drawing is performed five times using a related-art automatic control method (other conditions are the same as those of the embodiment). In the related-art method, automatic control is performed only on the basis of the fiber diameter b measured using the diameter measuring instrument B installed outside the heating furnace.

(42) TABLE-US-00001 TABLE 1 Acceptable product ratio (%) Example 1 53.20 Example 2 59.19 Example 3 71.89 Example 4 78.61 Example 5 54.77 Average 63.53

(43) TABLE-US-00002 TABLE 2 Acceptable product ratio (%) Comparative example 1 42.81 Comparative example 2 24.94 Comparative example 3 43.32 Comparative example 4 31.81 Comparative example 5 48.14 Average 38.20

(44) In Tables 1 and 2, Acceptable product ratio represents the ratio of a length that is compliant with the specification (target diameter1.0 m) to the entire length in percentage. As clearly seen from Tables 1 and 2, the acceptable product ratio increases by 25% on average according to the present invention compared to that with the related art method.