JPH0815535A - Image fiber - Google Patents

Abstract

PURPOSE:To provide an image fiber which consists of plural cores commonly having a clad and decreases crosstalks by specifying the core diameter, inter- core distance and numerical aperture of this image fiber. CONSTITUTION:The core diameter of the image fiber consisting of the plural cores commonly having the clad and having the inter-core distance <=4.0mum and the length <=10m, defined as amum, the inter-core distance, defined as dmum, and the numerical aperture, defined as the number expressed by equation I, are so set as to satisfy the numerical values expressed by equation II. In the equations, n1 denotes the refractive index of the cores, n2 denotes the refractive index of the clad. In such a case, the numerical aperture of >=0.56 is attained by applying a glass material of a multicomponent system to the extremely fine fiber of the core diameter <=1.5mum, by which the loss of the color balance of the images to be transmitted is lessened. Further, the length of the image fiber having NA >=0.4 is preferably <=10mum in order to lessen coloring of the images to yellow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image fiber having a high image quality and a small diameter, which is applied to endoscopes, micromachines and the like used in medical and industrial fields.

[0002]

2. Description of the Related Art Generally, as a medical endoscope using an image fiber of this type, one having a device configuration shown in FIG. 1 has been conventionally used.

That is, in the configuration of the endoscope apparatus 11 shown in FIG. 1, the image (T) of the object T to be observed illuminated by the illumination optical system (not shown) is divided into a plurality of images through the objective lens group 12. An image is formed on the incident end face 13a of the image fiber 13 formed by bundling the optical fibers, and the formed observation image (T) is the image fiber 1
3 is transmitted, and is emitted from the emission end face 13b and observed through the eyepiece lens 14.

In such an endoscope apparatus, the outer diameter has been reduced in recent years, and recently, for example, the outer diameter is reduced to 1 mm or less so that the inside of a blood vessel can be observed. What has been made is being made. Along with this, it is desired to reduce the diameter of the image fiber applied to the endoscope apparatus. Further, it is desired to increase the density of pixels, and for this reason, there is a tendency that the core diameter of each optical fiber forming the image fiber becomes smaller and the clad thickness becomes thinner.

Further, in recent years, an image fiber having a further reduced core diameter is expected for image input or transmission of a micromachine or the like. Such an ultra-fine diameter image fiber is not usually constructed by bundling individual optical fibers constituting the image fiber, but a so-called so-called core in which these cores share a clad. The one with a structure called fiber conduit is mainly used.

FIG. 2 schematically shows a cross-sectional structure of the fiber conduit, and the fiber conduit 15
Is configured to include a plurality of cores 17 arranged inside the common clad 16 so as to be spaced apart from each other.

[0007]

However, in these image fibers, it is desired to increase the number of pixels as described above, and it is necessary to reduce the core diameter and the clad thickness between the cores. . Therefore, since the core diameter of the fiber and the thickness of the cladding are several times or less than the wavelength of light, crosstalk is likely to occur due to mode coupling between the cores, resulting in a significant deterioration in the quality of the transmitted image. .

Therefore, in order to reduce the crosstalk and ensure good image quality, there is known a so-called random image fiber technology in which the shapes and arrangements of the cores are randomly configured. In such a structure, the brightness of pixels and the crosstalk between cores become non-uniform, which makes it difficult to view an image.

On the other hand, in an ordinary image fiber in which cores having the same diameter are arranged at equal intervals, it is necessary to increase the thickness of the cladding to some extent in order to reduce crosstalk and ensure good image quality. Then, this time, the occupancy of each core per unit area in the cross section of the fiber bundle decreases, so that it is not possible to secure a sufficient amount of transmitted light, and at the same time, the resolution becomes low. It was

Although quartz-based glass materials have hitherto been used as the glass material applied to these fibers, since the numerical aperture of the silica-based image fiber cannot be increased due to the characteristics of the material, the above-mentioned cloth is used. The amount of talk generated becomes large.

That is, in the quartz type image fiber,
Since the numerical aperture cannot be increased as described above, when the core diameter becomes extremely small, the number of propagation modes propagating in the core decreases, so that the cutoff wavelength of the propagation mode propagating a relatively large amount of light is It exists in the observation wavelength range, and radiation of that mode and crosstalk increase. The radiation mode light and the light exuding from the core are absorbed or radiated by the jacket layer surrounding the outer peripheral portion of the fiber bundle, so that the color balance of the transmitted image is lost,
The image quality deteriorates significantly.

The present invention has been made in order to solve the above-mentioned problems of the prior art. The object of the present invention is to prevent crosstalk, which is a problem in the high pixel and ultra-fine image guide as described above. It is to provide less image fiber.

[0013]

In order to achieve the above object, the present invention provides a plurality of cores sharing a clad, an inter-core distance of 4.0 μm or less and a length of 10 m.
In the image fiber below, the core diameter is aμ
m, distance between cores dμm, numerical aperture Where (n ₁ is the refractive index of the core and n ₂ is the refractive index of the clad), the following equation (1) It is characterized by satisfying.

In the present invention, the maximum wavelength used is λ
When μm, the following equation (2) It is characterized by satisfying.

Further, according to the present invention, in a multi-component glass fiber having a core-to-core distance of 4.0 μm or less and a core diameter of 2.0 μm or less in which a plurality of cores share a clad, the core diameter is a μm. , Core distance dμm, numerical aperture Where (n ₁ is the refractive index of the core and n ₂ is the refractive index of the clad), the following equation (3) And satisfies the following equation (4) It is characterized by satisfying.

Regarding the phenomenon of crosstalk in the image fiber described above, the paper “Transmission characteristics of image fiber” (Journal of the Institute of Electrical Communication '83 .11 vol.J66-C
No.11). Then, the state of crosstalk can be inferred by using the mathematical formulas shown in the paper.

The quality of the transmitted image, that is, the magnitude of the crosstalk, refers to the crosstalk parameter (referred to as the crosstalk parameter in the paper. This crosstalk parameter will be referred to as B hereinafter). ) Can be represented. The larger the B value, the larger the crosstalk and the poorer the image quality. On the other hand, the smaller the B value, the smaller the crosstalk and the better the image quality.

However, the B value of the image fiber when the propagation mode is the LP ₀₁ mode can be specifically expressed by the following equation (5). K ₀ and K ₁ are the modified Bessel functions of the second kind of the 0th and 1st orders, u ₀₁ and w ₀₁ are the eigenvalues of the LP ₀₁ mode, a is the fiber core diameter, d is the fiber spacing, and z is the fiber. , Β ₀₁ is the propagation constant of the LP ₀₁ mode. v is a normalized frequency and is a value represented by the following equation (6). Here, n ₁ and n ₂ are the refractive indices of the core and the clad, respectively, and λ is the wavelength of light propagating in the fiber.

Also, for higher-order propagation modes other than the LP ₀₁ mode, the B value can be calculated based on the concept described in the above paper. The graph of FIG. 4 (hereinafter referred to as graph 1) shows the calculation result of the change in the B value when the wavelength is changed by the above calculation formula. It can be seen from Graph 1 that the logarithm of the B value is proportional to the reciprocal of the wavelength.

By the way, the crosstalk of the image fiber is a linear combination of the crosstalk of each mode. This is based on the idea that the light transmitted through the fiber is a linear combination of each mode, and if the light amount transmitted through the fiber is E _total , this value is expressed by the following equation (7).

Here, A _ml represents the amplitude ratio of the electromagnetic fields of each mode excited by the incident light, and E _ml is the electric field distribution function (mode function) of each mode. This A _ml depends on the electrolytic distribution of the light incident on the end face of the fiber, and can be calculated by the following equation (8). Here, O (r, θ) is the electric field distribution of the light incident on the end face of the fiber, and r, θ is the polar coordinate of the point on the incident face of the core with the center of the core as the origin.

In an endoscope, a converging lens system is generally used as an objective optical system to form an image on the end face of the fiber. Therefore, the electric field distribution O (r,
As θ), a point image amplitude distribution when a point light source is imaged by a lens can be considered. The point image amplitude distribution is generally given by the following equation (9). λ is the wavelength, Fno. Is the aperture of the incident optical system, and J ₁ is the first-order Bessel function of the first kind.

Further, the mode function is specifically expressed by the following equation (1
0). J _m and K _m are the Bessel function of the first kind of the m-th order, the modified Bessel function of the second kind of the m-th order, u _ml and w _ml are the eigenvalues of the LP _ml mode, and a is the diameter of the core.

The square of A _ml representing the amplitude ratio of each mode represents the intensity ratio of each mode. This is called the weight of each mode.

The case where the subject is a point light source has been described above, but the actual subject has a certain spread. In this case, the distribution of the formed image is considered to be a set of point images. The light source used for observation is considered to be incoherent, so the total weight (= Weight)
_m1 ) is expressed by the following equation (11). Here, A _ml is the amplitude ratio of each mode represented by the equation (8), and s is the range of image spread.

Equations (8), (9), (10) and (11) are
The power of each mode excited in the fiber
It is shown in the graph of FIG. 5 (hereinafter referred to as graph 2). This
Rough 2 is for a fiber with a numerical aperture (NA) of 0.5
Then, the Fno. When the light of 1.4 is incident
Each mode excited in the fiber [LP₀₁Mo
LP, LP₁₁Mode (b), LP_{twenty one}mode
(C), LP₀₂Mode (d), LP₁₂Mode (e)]
Constant energy ratio and pixel pitch (distance between cores)
(3.68 μm) and calculated by changing the core diameter.
The result is shown. In graph 2, the vertical axis is the fiber
The ratio of the energy of each mode excited in the
It is expressed in kale. The horizontal axis represents the diameter of the core.
There is. As is clear from Graph 2, the basic mode [LP
₀₁LP close to mode (a)] _m1Mode [LP₁₁mode
(B) and LP_{twenty one}Mode (C)] is LP_ml(L> 1)
A relatively large amount of energy is excited compared to the mode.
I understand.

Table 1 shows measured values of the energy ratio of each mode transmitted by a fiber having a core diameter of 2.22 μm, an inter-core distance of 3.73 μm and an NA of 0.5.

Table 1 shows the ratio of the amount of light propagating in each mode when the amount of light propagating in the entire fiber is normalized to 1.0 when the core diameter is 2.2 μm and the numerical aperture is 0.5. However, the total energy is standardized to 1. It can be read from this Table 1 that the calculated values shown in Graph 2 are qualitatively correct.

As described above, in the LP _m1 mode close to the fundamental mode, a relatively large energy is excited. However, such a relationship is somewhat fluctuated depending on the electric field distribution shape of the incident light, and generally the energy excited in the fiber. The quantity is the Fno. It has been found from the study that the higher the LP _m1 mode becomes, the larger becomes.

As described above, L close to the basic mode
Since the P _m1 mode has relatively large energy,
Considering that the crosstalk of the image fiber is a linear combination of the crosstalk of each mode with the above weight ratio, the image fiber with a smaller B value in the LP _m1 mode has less crosstalk and has a better image quality. It can be said that there is.

Therefore, in the graph of FIG. 6 (hereinafter referred to as graph 3), the pixel pitch of the image fiber is made constant,
The B value at an arbitrary wavelength of the above LP ₀₁ mode when the core diameter is changed is shown. The horizontal axis of the graph 3 represents the core diameter, and the vertical axis represents the logarithm of the absolute value of the B value.

The graph of FIG. 7 (hereinafter referred to as graph 4)
LP) with the same calculation as above ₁₁Arbitrary wavelength of mode
Shows the B value in. The horizontal axis of graph 4 shows the core diameter
However, the vertical axis represents the logarithm of the absolute value of the B value.

As can be seen from the graphs 3 and 4, there is a core diameter that minimizes the B value in the LP _m1 mode when the pixel pitch is constant. That is, when the resolution is constant, there is a core diameter that minimizes crosstalk.

Therefore, in order to practically minimize the crosstalk, it suffices to set the core diameter that minimizes the B value in the LP _m1 mode (this core diameter is hereinafter referred to as the optimum value of the core diameter).

Further, as is clear from the graphs 3 and 4,
It can be seen that the optimum value of the core diameter shifts to the larger value as the wavelength of the propagation mode becomes longer. Similarly, it can be seen that the higher the order of the propagation mode (the larger m), the larger the optimum value of the core diameter shifts.

Further, as shown in the graph 1 and as described above, the B value exponentially increases as the wavelength becomes longer. That is, in the long wavelength region, the crosstalk is greatly increased and the image quality is significantly deteriorated. Therefore, it is desirable that the optimum value of the core diameter is set to be optimum on the long wavelength side.

Considering these points, the optimum value of the above core diameter was examined for the image fiber with a core distance of 4.0 μm or less, which causes crosstalk, and the optimum value of the core diameter was examined. It has been found that the relationship between the optimum value range of, the pixel pitch, and the numerical aperture is expressed by the following equation (1).

Where a is the core diameter (μm), d is the inter-core distance (μm), (N is the refractive index of the core, and n ₂ is the refractive index of the clad). Therefore, by satisfying the expression (1), it is possible to realize a high quality image fiber with less crosstalk.

This relationship holds even in an image fiber in which cores having different core diameters or cores having different refractive indices share a clad, and in that case, d or a in Expression (1), or NA. The value of may be calculated using the average value of the fiber. In such a fiber, the effect of setting the optimum value is very large as compared with an image fiber in which the same cores are aligned. However, as described above, when the structure or arrangement of the individual cores is greatly changed, the brightness of the individual cores and the crosstalk between the cores become non-uniform, making it difficult to see the image.

Next, let us consider the color balance of the transmitted image in the image fiber. Generally, when the core diameter becomes very small, the number of propagation modes propagating in the core decreases. In particular, in a silica-based image fiber, the relative refractive index difference cannot be increased. Therefore, when the core diameter becomes extremely small, the number of propagation modes propagating in the core decreases to several.

As described above, the LP _m1 mode close to the fundamental mode is excited with a relatively large amount of energy. Therefore, if the number of propagation modes is very small, the cut-off wavelength of the LP _m1 mode close to the fundamental mode exists in the observation wavelength range, and the emission of that mode and crosstalk increase. Since these are absorbed or radiated by the jacket layer around the fiber bundle, the color balance of the transmitted image is disturbed and the image quality is significantly deteriorated. Therefore, in order to secure the color balance, it is necessary to set the normalized frequency of the fiber at the maximum wavelength of the used wavelength to a relatively large value with respect to the cutoff frequency of the LP _m1 mode close to the fundamental mode.

Further, in the graph of FIG. 8 (hereinafter referred to as graph 5), in an image fiber of a certain spec,
The conceptual diagram shows how the light quantity is lost with respect to the wavelength used. The vertical axis of graph 5 is the loss, and the horizontal axis is the used wavelength. The arrows in graph 5 and _.. indicate the cutoff wavelengths of the LP _m1 mode, respectively.

As shown in the graph 5, the loss of light amount is maximum on the short wavelength side of several tens of nm from the cutoff wavelength and minimum on the short wavelength side slightly longer than the cutoff wavelength.

As described above, the LP _m1 mode, which is close to the fundamental mode, is excited with a relatively large amount of energy, but the Fno. In this case, as can be seen from Graph 2, the energy amount is relatively large up to the LP ₂₁ mode. Also, as described above, each L
Near the cutoff wavelength of P _m1 mode (several tens of nm on the short wavelength side)
Since the loss of the light amount of the LP _m1 mode is the maximum, the color balance of the transmitted image is improved if the LP ₂₁ mode cutoff wavelength exists near the maximum wavelength of the observation wavelength range where the absolute amount of crosstalk is very large. The image quality is degraded and the image quality is significantly deteriorated.

Therefore, if the fiber structure is determined so that the cutoff wavelength of the LP ₂₁ mode is more than several tens of nm away from the maximum wavelength used, the color balance of the transmitted image due to the loss of the light amount of the LP ₂₁ mode can be improved. Collapse is reduced.

When the maximum wavelength used is λ μm, the core diameter is a μm, and the numerical aperture is NA, the above conditions can be expressed by the following formula (2).

Therefore, by adopting the structure satisfying the expressions (1) and (2), the color balance of the transmitted image is good,
In addition, it is possible to realize a high-quality image fiber with less crosstalk.

Now, let us consider the ratio of the total amount of energy excited in the fiber to the total amount of energy of the image formed on the fiber entrance end face in the incident optical system. As is clear from Graph 2 above, the smaller the core diameter, the smaller the value. Especially L
Since the P ₁₁ and LP ₂₁ modes are excited with relatively large energy, the conversion efficiency becomes very small in a fiber having a core smaller than the core diameter that causes the LP ₂₁ mode to be in the cutoff state. Therefore, in such a state, the transmitted image becomes dark, which is not desirable.

Considering this, if the condition that the cutoff wavelength of LP ₂₁ mode is longer than the maximum wavelength of the observation wavelength range by several tens nm or more is used, the transmitted image is bright and the color balance is good. An image is obtained. Specifically, the condition is expressed by the following equation (12).

In equation (12), λ is about 0.7 μm, assuming that the wavelength used is in the visible range. For example, if the core diameter is considered to be 3 μm or less at this time, it can be seen from Expression (12) that the numerical aperture (NA) must be 0.28 or more.
Further, in the case of a very thin fiber having a core diameter of 2 μm or less, it is desirable that the numerical aperture (NA) is 0.42 or more. Therefore, the equation (1) can be rewritten as the following equation (4) under the condition that the core diameter is 2.0 μm or less in the visible region.

Furthermore, in recent years, thinner image fibers have been expected for image input or transmission of micromachines and the like, and in such a case, the core diameter is very small. ), The numerical aperture of the fiber must be 0.56 or more. Therefore, the equation (1) can be rewritten as the following equation (13) under the condition that the NA is 0.56 or more and the core diameter is 1.5 μm or less.

Therefore, NA 0.56 or more, core diameter 1.5
By satisfying the expression (13) with μm or less, it is possible to realize an ultrafine fiber having a high image quality in which the color balance of the transmitted image is good and the crosstalk is small. However,
When the core diameter is 0.85 μm or less, the NA satisfying the expression (12) becomes about 1.0, which makes it difficult to realize such a fiber.

Conventionally, a fiber having a large numerical aperture has been produced in the fiber to which the silica glass material has been used, which has been used for the small diameter fiber, but 0.1 to 0.
It is about 45. In the image fiber to which the multi-component glass material is applied, the numerical aperture is about 0.2 to 0.8.
Something is made to the extent. Therefore, the core diameter is 1.5
In the case of an ultrafine fiber of μm or less, by applying a multi-component glass material to a numerical aperture of 0.56 or more, collapse of the color balance of an image to be transmitted can be reduced.

However, in the multi-component glass material, if the numerical aperture is increased and the refractive index is increased, the transmittance on the short wavelength side is lowered. Therefore, if the propagation distance becomes very long, the transmitted image will be colored yellow.

In order to reduce these, it is desirable that the image fiber having an NA of 0.4 or more has a length of 10 m or less, and the image fiber having an NA of 0.6 or more has a length of 8.5 m or less. Is desirable. Further, in a high NA image fiber having an NA of 0.8 or more, it is desirable that the length is 7 m or less.

As described above, by adopting the structure according to the present invention, it is possible to provide an image fiber having good image quality with less crosstalk.

[0056]

EXAMPLES Examples of image fibers to which the present invention is applied will be described below with reference to Tables 2, 3 and 9 to 13. Table 2 shows the specifications of the image fibers of Examples 1 to 6 of the present invention and the values of the formula (1).

[Table 2] Each example will be described below.

In Example 1, the bundle diameter was 0.3 mm and the length was 3.
It is an image fiber with 5 m and NA of 0.5. From the fiber specification table, it can be seen that the formulas (1) and (2) are satisfied. Fig. 9 shows the state of crosstalk when light is introduced into any one core of this image fiber.
Is shown in the graph (hereinafter, referred to as graph 6). The numerical value on the horizontal axis of the graph 6 represents the core number, and the core containing light is set to 0. FIG. 3 shows details of the core numbers. The vertical axis of the graph is the light quantity ratio of the core at the exit end face of the fiber when the quantity of incident light is set to 1 (hereinafter, referred to as Example 2).
The graphs used in ~ 5 are shown in the same notation). As is clear from the graph 6, it can be seen that the fiber of Example 1 has little crosstalk and has good image quality.

Example 2 is an image fiber having a bundle diameter of 0.24 mm and a thin fiber having a length of 3 m and an NA of 0.624. As in Example 1, the above formulas (1), (2)
Meets Figure 1 shows the state of crosstalk when light is introduced into any one core of this image fiber.
A graph of 0 (hereinafter referred to as graph 7) is shown. Graph 7
As is apparent from the above, the fiber of Example 2 is a very thin fiber, but has a small amount of crosstalk and a good image quality.

Example 3 is an image fiber having a bundle diameter of 0.6 mm, which is slightly large, but has a small core-to-core distance and excellent resolving power, and has a length of 3 m and an NA of 0.718. Similar to the first embodiment, the formulas (1) and (2) are satisfied. FIG. 11 shows the state of crosstalk when light is introduced into any one core of this image fiber.
Is shown in the graph (hereinafter referred to as graph 8). As is clear from the graph 8, it can be seen that the fiber of Example 3 has a small amount of crosstalk, a good image quality, and a very high resolution.

Example 4 has a bundle diameter of 0.8 mm and a length of 5 m.
The NA is 0.45, which is a relatively low NA and is a long image fiber. As in Example 1, the above formula (1),
(2) is satisfied. The state of crosstalk when light is introduced into any one core of this image fiber is shown in the graph of FIG. 12 (hereinafter referred to as graph 9). As is clear from the graph 9, it can be seen that the fiber of Example 4 has less crosstalk and good image quality. Further, because of the low NA, it is possible to suppress a decrease in transmittance on the short wavelength side, which is a problem in a multi-component long fiber. Therefore, although the fiber length is as long as 5 m, there is no particular problem with the transmittance characteristic of the transmitted image.

Example 5 has a bundle diameter of 0.2 mm and a length of 0.
It is a very thin image fiber with a high NA of 0.806 at 5 m. As in Example 1, the above formula (1),
(2) is satisfied. A graph of FIG. 13 (hereinafter referred to as graph 10) shows the state of crosstalk when light is introduced into any one core of this image fiber.
As is clear from Graph 10, the fiber of Example 5 has a small amount of crosstalk and a good image quality, although it is a very fine fiber. In addition, since the fiber length is as short as 0.5 m, it is possible to suppress a decrease in the transmittance on the short wavelength side, which is a problem in a multi-component high NA fiber. Therefore, there is no particular problem with the transmittance characteristic of the transmitted image.

[0062]

As described above, according to the present invention, it is possible to provide a high quality image fiber having high resolution and less crosstalk.

[0063]

[Brief description of drawings]

FIG. 1 is a layout configuration diagram schematically showing an outline of an endoscope using a general image fiber.

FIG. 2 is a perspective view schematically showing a cross-sectional structure of a fiber conduit of an image fiber applied to the endoscope.

FIG. 3 is a diagram showing details of the numbers of the respective cores when light is incident on any one core of the image fiber.

FIG. 4 is a graph (graph 1) showing a result of calculation of change in crosstalk parameter when wavelength is changed.

FIG. 5 is a graph (graph 2) showing the power of each mode excited in the fiber.

FIG. 6: The pixel pitch of the image fiber is made constant,
LP when changing the core system ₀₁At any wavelength of the mode
Graph showing the crosstalk parameter (graph 3)
Is.

FIG. 7: The pixel pitch of the image fiber is made constant,
LP when changing the core system ₁₁At any wavelength of the mode
Graph showing the crosstalk parameter (graph 4)
Is.

[Fig. 8] In an image fiber of a certain spec,
It is a graph (graph 5) which shows the state of the light amount loss with respect to a used wavelength by a conceptual diagram.

9 is a graph (graph 6) showing the state of crosstalk when light is incident on any one core of the image fiber of Example 1. FIG.

FIG. 10 is a graph (graph 7) showing a state of crosstalk when light is incident on any one core of the image fiber of Example 2.

FIG. 11 is a graph (graph 8) showing a state of crosstalk when light is incident on any one core of the image fiber of Example 3.

FIG. 12 is a graph (graph 9) showing the state of crosstalk when light is incident on any one core of the image fiber of Example 4.

FIG. 13 is a graph (graph 10) showing a state of crosstalk when light is incident on any one core of the image fiber of Example 5.

[Explanation of symbols]

 11 Endoscope device 13 Image fiber 16 Clad 17 Core

Claims (3)

[Claims] 1. A plurality of cores share a clad,
In an image fiber with a core distance of 4.0 μm or less and a length of 10 m or less, the core diameter is a μm, the core distance is d μm, and the numerical aperture is (N ₁ is the refractive index of the core and n ₂ is the refractive index of the clad) A multi-mode image fiber characterized by satisfying: 2. When the maximum wavelength used is λ μm, the formula (2) The image fiber according to claim 1, characterized in that: 3. A plurality of cores share a clad,
In a multi-component glass fiber having a core distance of 4.0 μm or less and a core diameter of 2.0 μm or less, the core diameter is a μm, the core distance is d μm, and the numerical aperture is (N ₁ is the refractive index of the core, and n ₂ is the refractive index of the clad), equation (3) And satisfies the equation (4) Image fiber characterized by satisfying.