JPH06324223A - Image fiber - Google Patents

Abstract

PURPOSE:To provide the image fiber which is inexpensive and has decreased crosstalks by providing the image fiber with clads common with plural cores and specifying the crosstalk parameter thereof. CONSTITUTION:The image fiber having the clads common with the plural cores has the crosstalk parameter B satisfying conditions 200>B>0.7, where B is expressed by formula. In the formula, beta1 is the parameter by the structure of the fibers; u01, w01 are the intrinsic values of an LP01 mode, Cd) is the pitch of the core, (a) is the radius of the core, Z is the length of the image fiber, betais the propagation constant of the LP01 mode and Km is the vessel function of a second kind of deformation of(m) order. The smaller value of the crosstalk parameter B is more preferable for the purpose of decreasing the crosstalks but the brightness is lower as the value of B is smaller and, therefore, the lower limit thereof is limited from the reasons of practicability. Then, images are well transmitted by confining the range of the crosstalk parameters B within the range of the conditions.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to image fibers for transmitting optical images.

[0002]

2. Description of the Related Art Image fibers are known as a technique for transmitting an optical image formed by an objective lens or the like to another space. In general, an image fiber is composed of a common clad and many cores. Such an image fiber is manufactured as follows. A large number of relatively thick optical fibers called strands are bundled, heated, and spun. At this time, the cladding of each fiber is melted and integrated.

[0003]

Such an image fiber is desired to have a small diameter, especially in the medical field. However, when the image fiber is made thinner,
When the core pitch (the distance between the centers of two adjacent cores) is 10 μm or less, crosstalk occurs due to the wave nature of light, and the transmitted image is significantly deteriorated.

A so-called random image fiber having several kinds of cores is known as an image fiber for reducing such crosstalk, that is, image deterioration, but it is manufactured by using plural kinds of optical fibers. It is expensive because of it. An object of the present invention is to provide an image fiber which is inexpensive and has less crosstalk.

[0005]

The present invention provides an image fiber having a plurality of cores and a common cladding,
The crosstalk parameter B satisfies the following equation. 200>B> 0.7 (1)

[0006]

[Equation 2] Where β ₁ is the parameter of the fiber structure,
u ₀₁ and w ₀₁ are the eigenvalues of the LP ₀₁ mode, d is the pitch of the core, a is the radius of the core, Z is the length of the image fiber,
β is the propagation constant of the LP ₀₁ mode, and K _m is the modified Bessel function of the second kind of the mth order.

[0007]

In the image fiber, it is generally known that crosstalk occurs when the core pitch becomes smaller. As a parameter expressing this crosstalk,
The crosstalk parameter B defined by the above equation is known. For this, see "Transmission Characteristics of Image Fibers", The Institute of Electronics and Communication Engineers, '83 .11, vol.J66-CN.
o.11 ”for more details.

For the purpose of reducing crosstalk, the smaller the value of the crosstalk parameter B is, the more desirable it is.
Since the brightness decreases as the value of B decreases, its lower limit is limited for practical reasons. In the present invention, the range of the crosstalk parameter B is within the range of the above equation. The upper and lower limits were experimentally obtained. Therefore, if the image fiber satisfies this condition, the image is transmitted well.

The actual crosstalk of the image fiber is smaller than the crosstalk obtained by calculation from the crosstalk parameter of the above equation. This is different from the design value due to manufacturing errors (dispersion of wire diameters, fluctuations in outer diameter when spinning the image fiber, etc.), fluctuations in the refractive index, and residual stress inside the actual image fiber. It is thought to be because of this.

[0010]

EXAMPLES Next, experiments conducted by the present inventors will be described. Four types of image fiber (Samples I to I
V) was prepared and crosstalk was measured for these samples I to IV. Three types of light of red (R), green (G), and blue (B) were used as the measurement light.

Each sample has a large number of cores C in a common clad as shown in FIG. These cores are arranged so that their centers form an equilateral triangle when three adjacent cores are arbitrarily selected.

Table 1 shows the specifications of the four types of samples I to IV. The core diameter, core pitch, and clad thickness in the table are as shown in FIG. Further, n ₁ is the refractive index of the core, and n ₂ is the refractive index of the clad.

[0013]

[Table 1] The crosstalk parameter B of these samples I to IV for red light is shown in Table 2.

[0014]

[Table 2]

Of the three types of crosstalk parameters B for red, green and blue, the crosstalk parameter B for red is the largest, and the crosstalk parameter B for green and blue is always smaller than the crosstalk parameter B for red. ing. Therefore, when evaluating the deterioration of the image, only the crosstalk parameter B for the red light needs to be evaluated. For this reason, Table 2 shows only the crosstalk parameter B for red light.

The crosstalk was measured using the optical system having the structure shown in FIG. A pinhole plate 14 is arranged in front of the lamp 12. The pinhole is regarded as a point light source, and the light emitted from the pinhole is condensed by the lens 18 having the aperture stop 16 having an F number of 1.4 and one of the many cores in the image fiber 20 is condensed. Make the core full.

On the other hand, the image forming lens 22 is arranged on the exit end side of the image fiber 20, and the image of the exit end face of the image fiber 20 is taken by the TV camera 26 via the image forming lens 22 and the filter 24. The image information obtained by the TV camera 26 is input to the computer 30 to analyze the magnitude of crosstalk, and
It is input to the monitor 28 and the image of the exit end face is displayed.

Samples I to IV each have a length of 1.5.
m and 3.5 m in length were prepared. Lamp 12
Xenon lamp is used for red light (wavelength 600n
m), green light (wavelength 516 nm), blue light (wavelength 476)
24) in the figure by preparing three types of filters that transmit (nm)
Was placed at the position indicated by and the measurement was performed for each.

As shown in FIGS. 4 to 7, the crosstalk increases as the wavelength becomes longer, and the ratio of the light intensity remaining in the central core (incident core) decreases as the B value increases. I understand.

For example, in the red wavelength (600 nm), B is the most
In sample IV (1.5 m) having a small value, the light intensity ratio of the central core is approximately 0.6, while in sample I (3.5 m) having the largest B value, it is approximately 0.2. In the image fiber having a larger B value than these samples, the crosstalk is further increased and the light intensity ratio of the central core is decreased. Conversely, when the B value is decreased, the crosstalk is decreased and the optical power of the central core is decreased. It can be easily predicted that the strength ratio will be improved.

The measurement results of Samples I, II, III, and IV are shown in FIGS. 4, 5, 6, and 7, respectively. In FIG. 2, the number of pitches is 0 for the core C0 on which light is incident, 1 for the closest core C1, and 2 for the next closest core C2.
Is counted as 2.

When an image was observed using a 3.5 m image fiber of Sample I, crosstalk was at the practically lower limit level, but the brightness was sufficient. In addition, the 3.5m image fiber of Sample IV is
The brightness was slightly insufficient, but there was little crosstalk and a high-contrast image was obtained. From these experimental results, it is concluded that the practically acceptable range of the fiberscope is that the crosstalk parameter B for light with a wavelength of 600 nm (red light) satisfies the following formula.

200>B> 0.7 (3) The upper limit of the above expression indicates the limit of crosstalk, and if it exceeds this value, the resolution is significantly deteriorated. The lower limit indicates the limit of brightness. For example, I and IV in Table 1 have different core occupancy ratios of 1.6 times. If B is 0.7 or less, sufficient brightness cannot be obtained.

Further, in consideration of the balance between brightness and crosstalk, it is very desirable for practical use that the crosstalk parameter B satisfies the following expression. 100>B> 5 (4) By the way, in order to obtain a high-resolution image fiber, it is necessary to reduce the core pitch, and inevitably the core diameter is also reduced. For example, an existing image fiber for angioscope has an outer diameter of about 0.3 mm and a pixel number of 30.
The number is about 00 and the core diameter is 2 to 3 μm. This type of image fiber is required to have a higher number of pixels, and a core diameter of 1 μm is required to realize about 10,000 pixels.
It needs to be made as small as possible. When the core diameter is reduced in this way, the V value is reduced, and the limitation of the propagation mode becomes a problem. Here, the V value is a standardized frequency defined by the following equation.

[0025]

[Equation 3] Here, λ is the wavelength of light, a is the core radius, and n ₁ and n ₂ are the refractive indices of the core and the clad, respectively.

FIG. 8 is a graph showing the energy ratio of excited modes when light with an incident F-number of 1.4 is incident on an optical fiber having an NA (numerical aperture) of about 0.5. In this graph, the horizontal axis is the core diameter and the vertical axis is the energy (arbitrary scale). As can be seen from this graph, as the core diameter becomes smaller, the LP ₁₁ mode and LP ₂₁ mode light with large energy is not excited and becomes dark.

Therefore, in order to obtain brightness, at least LP
It is desirable that the V value satisfies the following equation so that ₁₁ modes are excited. V> 2.405 (6) If the V value satisfies the following equation, the LP ₂₁ mode is excited, which is more desirable.

V> 3.83 (7) In order to satisfy the equation (7) when the core diameter is 3 μm, the NA needs to satisfy the following equation.

[0029]

[Equation 4] Further, to satisfy the equation (7) when the core diameter is about 1 μm,
NA must satisfy the following equation.

[0030]

[Equation 5]

However, when the length of the image fiber is about several tens of cm, the loss of the transmittance is small, so that the optical fiber can be realized by ordinary optical glass. However, when the length is several meters, a high transmittance is required. In general glass, increasing the refractive index lowers the transmittance, making it difficult to satisfy the expression (9). Therefore, when the expression (6) is applied as the condition, the condition becomes the following expression, which can be realized by the multi-component glass.

[0032]

[Equation 6]

By satisfying these conditions and the expression (1), it is possible to realize a bright image guide with high pixel resolution and little crosstalk. The refractive index of the multi-component glass is a low refractive index, and is generally about 1.47 to 1.53. Therefore, the refractive index of the core material is at least 1.
54 or more is required.

Further, in the image fiber, the thicker the clad, the less the crosstalk and the image with high contrast can be obtained, but on the other hand, the core occupancy rate becomes low, so that the brightness decreases. The core occupancy ratio is required to be at least about 20% in order to secure practically sufficient brightness, and it is more preferable to be about 30 to 40%. The image fiber used for an endoscope or the like is generally about several tens of cm to 5 m, and in order to satisfy the above occupancy in this range and to suppress crosstalk to a level where there is no practical problem, the cladding thickness is It is desirable to satisfy the formula.

1.8 μm>t> 1 μm (11) It was found from experiments that it is particularly desirable that the range of the following equation be satisfied when the core diameter is about 2 μm.

1.7 μm>t> 1.4 μm (12) A method for further reducing the crosstalk of the image fiber will be described. When crosstalk occurs (for example, the light incident on one core is emitted from another core at the emission end), the intensity pattern at the emission end is observed. The mode pattern can be observed. From this, it can be seen that crosstalk is reduced by cutting off higher-order modes. For example, as shown in FIG. 9, if an image fiber whose core diameter is partially thin is used, higher-order modes are blocked in a portion where the core diameter is small, and only low-order modes with little crosstalk are propagated. Is possible.

As another means, even if residual stress is applied to the inside by, for example, partially quenching when spinning the image fiber, the mode pattern changes, so that crosstalk is reduced.

[0038]

According to the present invention, an image fiber with less crosstalk and a lower cost can be obtained.

[Brief description of drawings]

FIG. 1 shows a configuration of an apparatus for measuring crosstalk of an image fiber.

FIG. 2 shows an end face of an image fiber.

FIG. 3 shows core diameter, core pitch, and clad thickness.

FIG. 4 is a graph showing the measurement results of Sample I.

FIG. 5 is a graph showing the measurement results of Sample II.

FIG. 6 is a graph showing the measurement results of Sample III.

FIG. 7 is a graph showing the measurement results of Sample IV.

FIG. 8 is a graph showing a ratio of energy of modes excited when light is incident on an optical fiber.

FIG. 9 shows an image fiber with a partially reduced core diameter.

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[Procedure amendment]

[Submission date] July 9, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction content]

The measurement results of Samples I, II, III, and IV are shown in FIGS. 4, 5, 6, and 7, respectively. Also,
The specific numerical values of are shown in Table 3, Table 4, Table 5, and Table 6, respectively.
You The number of pitches in the figure and table is 0 for the core C0 on which light is incident and the closest core C in FIG.
1 is counted as 1, and the next closest core C2 is counted as 2.

[Table 3]

[Table 4]

[Table 5]

[Table 6]

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 7

[Correction method] Change

[Correction content]

[Figure 7]

Claims (1)

[Claims] 1. An image fiber having a plurality of cores and a common cladding and having a crosstalk parameter B satisfying the following expression. 200>B> 0.7 where, Where u ₀₁ and w ₀₁ are eigenvalues of the LP ₀₁ mode, d is the pitch of the core, a is the radius of the core, Z is the length of the image fiber, β is the propagation constant of the LP ₀₁ mode, and K _m is the m-th order. This is a modified Bessel function of the second kind.