JPH06324222A - Image fiber - Google Patents

Abstract

PURPOSE:To provide the image fiber which prevents the deterioration in image quality by suppressing crosstalks, is free from unequal brightness, is small in diameter and has a high resolution. CONSTITUTION:This image fiber is constituted by randomly mingling respectively 600 pieces each of fibers having the specified outside diameter of clads 9 and five kinds of different core diameters randomly bundled and are then softened by heating and thereafter, the fibers are stretched in this state to a smaller diameter. Consequently, the image fiber fused with cores 8 to a conduit form is formed. The core diameters (after spinning) of the image fiber formed by such method have 5 kinds of the different values but the inter-fiber spacings are fixed. Values V attain five kinds of the different values when prescribed light (NA=0.495, lambda=650) is made incident on such image fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-quality, small-diameter image fiber.

[0002]

2. Description of the Related Art As an endoscope using an image fiber, an endoscope 1 as shown in FIG. 6 has been conventionally known. As shown in FIG. 6, the image (T) of the object T illuminated by the illumination optical system (not shown) is the objective lens group 2
An image is formed on the incident end face 3a of the image fiber 3 which is formed by bundling a plurality of fibers via the. The image (T) formed on the entrance end face 3 a is emitted from the exit end face 3 b of the image fiber 3 and then observed through the eyepiece lens 4. The diameter of the endoscope 1 has been reduced in recent years, and an endoscope having an outer diameter of 1 mm or less has been researched and developed so that the inside of a blood vessel can be observed.

[0003]

The image fiber applied to such an endoscope has an extremely small outer diameter of about several hundred μm, a core diameter of several μm, and a pixel number of about 2 to 3,000. However, the image quality is not always satisfactory.

As such a small-diameter image fiber, one having a structure called a fiber conduit in which a clad is shared without using separate fibers is mainly used. FIG. 7 is a diagram schematically showing a cross section of the fiber conduit 5. The fiber conduit 5 includes a plurality of cores 7 arranged in a common cladding 6 and spaced from each other. .

In such a structure, since the clad 6 is continuous, a light component leaking into the clad 6 from the boundary surface between the clad 6 and the core 7 is mixed into another core 7, that is, a so-called cross. The talk phenomenon easily occurs.

[0006] This phenomenon is caused especially by the interval between the cores 7 of 10
This becomes remarkable when the diameter is reduced to about μm or less. Therefore, it is necessary to provide the clad 6 having a sufficient thickness so that the crosstalk phenomenon does not occur between the fibers.

However, in recent years, it has been desired to increase the number of pixels in these ultra-fine diameter image fibers as well. Therefore, it is necessary to reduce the diameter of the core 7 and thin the clad 6 between the cores 7 to increase the pixel density (density of the core 7).

However, when the distance between the cores becomes small and the cladding thickness becomes several times the wavelength of light or less, crosstalk occurs due to mode coupling between the fibers, resulting in a marked deterioration in image quality.

In order to prevent the deterioration of the image quality due to such crosstalk, it is necessary to increase the thickness of the clad 6, but then, the occupancy rate of the core per unit area in the cross section of the fiber bundle becomes small. And
As a result, it is not possible to obtain a bright image and it is not possible to achieve a high pixel count.

Therefore, in the conventional technique, it is difficult to achieve a high resolution of the image fiber because it is necessary to prevent crosstalk. The present invention has been made to eliminate such an adverse effect, and an object of the present invention is to provide a small-diameter and high-resolution image fiber that can suppress crosstalk and prevent deterioration of image quality and that has no uneven brightness. And

[0011]

In view of wave theory, a fiber can be regarded as a kind of waveguide, and when light is incident on the fiber, various wave modes are excited in the core, Light is transmitted by propagating each mode in the fiber. Then, the image fiber can analyze the light transfer characteristics as a large number of such optical waveguides integrated.

The crosstalk phenomenon in the image fiber is analyzed in the paper “Transmission characteristics of the image fiber” (IEICE Transactions '83 .11 vol.J66-C No.11), and the mathematical formulas shown in the paper are used. Can infer the state of crosstalk.

Crosstalk parameters shown in this paper (called crosstalk parameters in this paper)
The value (hereinafter, referred to as B value) is a value indicating the magnitude relation of crosstalk, and the larger the B value, the more remarkable the crosstalk. For example, the B value in the LP ₀₁ mode is given by the following formula 1.

[0014]

[Equation 1]

Here, u ₀₁ and w ₀₁ are eigenvalues of the LP ₀₁ mode, a is the core radius of the fiber, d is the pitch of the fiber,
z is the length of the fiber, β is the propagation constant of the LP ₀₁ mode, K
_m is a modified Bessel function of the second kind of the m-th order. Further, v is a normalized frequency determined by the specifications of the fiber, and is represented by the following mathematical formula 2.

[0016]

[Equation 2]

However, k = 2π / λ (λ is the wavelength of light propagating in the fiber), and n ₁ and n ₂ are the refractive indices of the core and the clad, respectively. The following equation 3 is a parameter called the numerical aperture (NA) of the fiber.

[0018]

[Equation 3]

The B values of the other high-order modes are calculated according to the concept shown in the above-mentioned paper. By the way, based on the idea of the above-mentioned paper, the wavelength of light is 500 nm, and the core diameter of the fiber is 1.95 μ.
When the B value is calculated for an image fiber having m, a pixel interval of 3.68 μm, NA = 0.49, and a fiber length of 1.5 m, the values shown in Table 1 below are obtained.

[0020]

[Table 1]

It can be seen from Table 1 that the B value in the LP ₀₁ mode is the smallest value. Therefore, it is presumed that the _LP01 mode has the least crosstalk. Here, assuming that the light transmitted through the fiber is a linear combination of each mode, and Etotaling these, this Etotal is expressed by the following formula 4.

[0022]

[Equation 4]

Here, A _ml is a weight of each mode, and E _ml is an electric field distribution function (mode function) of each mode. The weight A _ml depends on the electric field distribution function of the light incident on the end face of the fiber, and can be calculated by the following formula 5.

[0024]

[Equation 5]

Here, O (r, θ) is the electric field distribution of the incident light, r and θ are polar coordinates with the origin at the center of the fiber core, r is the distance, and θ is the angle. As described above, since the _LP01 mode has the smallest crosstalk magnitude, the weight of the _LP01 mode is the largest and the weight of other modes is small. From the equation, it can be seen that the total crosstalk transmitted in the fiber is the smallest.

The above description is for the case where the propagation constants (β) of the fibers constituting the image fiber are equal to each other, and crosstalk between fibers having different propagation constants (β) is not considered.

In an actual fiber, mode conversion occurs due to various factors such as fluctuation of the refractive index, nonuniformity of the interface between the core and the clad, scattering by impurities mixed in the fiber, and the propagation constant (β) The crosstalk phenomenon also occurs between different fibers.

However, when the propagation constants (β) are different, the crosstalk is smaller than when the propagation constants are the same. In order to make the propagation constant (β) of the fiber different, the value of the normalized frequency (V) may be made different.

From the equation (2) shown above, it can be seen that the V value depends on the radius (a) of the core. A so-called random image fiber is known as a measure against crosstalk that focuses on this point.

Therefore, as shown in FIG. 1, in the random image fiber, a plurality of cores 8 having different sizes and shapes are randomly dispersed in the clad 9, and the propagation constant (β ) Are made different to reduce crosstalk.

In the random image fiber, when considering only the crosstalk between the adjacent cores, the order of the fibers is determined so that the closest cores 8 always have different V values. Even if they are regularly arranged, at least three kinds of fibers having different V values are required.

Therefore, as a conventional random image fiber, an example using three types of fibers is known.
However, since it is practically difficult to regularly arrange thousands of fibers, they are mixed appropriately. At this time, in n types of fibers, the probability that other adjacent fibers are not of the same type is calculated by the following equation (6).

[0033]

[Equation 6]

In this equation, when n = 3, the probability is 0.27%, which is probabilistically very low. Therefore, in the present invention, the probability is improved and the image quality is improved by increasing the types of fibers. For example, when 5 types (n = 5) of fibers are used, the probability is 4.7%, and the probability is about 17
A double improvement can be achieved. Similarly, considering the probability that one or two identical cores are adjacent to each other, it is understood that the image quality can be dramatically improved by using five or more kinds of fibers.

In this case, the greater the difference in V value between the fibers forming the image fiber, the smaller the crosstalk. On the other hand, when the difference is too large, the difference in the amount of transmitted light between the fibers due to the difference in core diameter. Since the size becomes large, adverse effects such as uneven brightness occur.

Therefore, if there is at least one fiber having a different number of propagation modes, the highest-order mode does not depend on the fiber having a small V value, which is desirable for reducing crosstalk. In this case, since the difference in V value can be small, the effect of uneven brightness due to the difference in core diameter can be suppressed to a level at which there is no problem.

As described above, using the principle that the lower order mode has less crosstalk, for example, 2
It can be seen that when the number of modes that can be propagated in one fiber is made different, the light of the highest order mode and higher modes that can propagate in one fiber does not propagate into the other fiber, and thus it is understood that crosstalk does not occur.

In order to reduce the crosstalk only by the difference in V value, the difference in V value must be made as large as possible. However, by making the number of modes different, the same level can be obtained even if the difference in V value is small. Crosstalk is realized.

FIG. 2 is a diagram showing the dispersion curve of a uniform core optical fiber, and the normalized frequency (V value) in each mode.
FIG. 4 is a diagram showing a relationship between the transmission constant and the propagation constant (β) (source, “optical fiber”: Ohmsha, author; Takanori Ogoshi and 2 others). In the figure, the horizontal axis is the V value and the vertical axis is β / k (k is the wave number). In addition, the following formula 7

[0040]

[Equation 7]

The scale in which the normalization variable (b) calculated in (3) is taken as the vertical axis is also shown. As is apparent from FIG. 2, for example, the dispersion curve (A) showing the LP ₀₁ mode shows the lowest order mode, and regardless of the V value,
Always propagates in the fiber (no cutoff),
In higher modes (B, C, D, E, F, G), there is a cutoff.

For example, the dispersion curve (B) showing the LP ₁₁ mode is broken at V = 2.405, which is
The LP ₁₁ mode is not excited in a fiber with V <2.405 even if the propagation constant (β) is arbitrarily set, which means that the light of this mode cannot propagate in the fiber. .

Similarly, the LP ₂₁ mode (C) and the LP ₀₂ mode (D) are cut off at V = 3.83. Therefore, if the V value of the first fiber is set to "3" and the V value of the second fiber is set to "4", LP ₀₁ , LP ₁₁ and LP ₂₁ are contained in the second fiber. ,
Although each mode of LP ₀₂ propagates, only the LP ₀₁ and LP ₁₁ modes propagate in the first fiber.

As a result, in the image fiber formed by mixing these two types of fibers, LP ₂₁ ,
Since each mode of LP ₀₂ is propagated only in the second fiber, there is no crosstalk between the first fiber and the second fiber for these modes, reducing the overall crosstalk. Will be done.

Next, the transmission efficiency of the light transmitted by the image fiber will be described. In order to reduce the diameter of the image fiber and maintain high resolution, it is inevitable that the fiber spacing and the core diameter are also reduced.
Furthermore, if the core diameter is made smaller, the propagation modes decrease,
When the V value is 2.405 or less, the single mode is set.

FIG. 3 is a graph of the energy ratio of each mode (wavelength is 600 nm) excited when light with an F number of 1.4 is incident on an optical fiber having a numerical aperture (NA) of 0.5. It is shown.

In the figure, the horizontal axis is the core diameter, and the vertical axis is the energy ratio on an arbitrary scale. As is clear from this graph, it is understood that not only the LP ₀₁ mode (A) but also the LP ₁₁ mode (B) and the LP ₂₁ mode (C) transmit a large proportion of energy. Also, LP ₁₁ mode (B)
To transmit LP ₂₁ mode (C), the core diameter is about 1 μm or more.
The above size is required, and if it is less than this, it is predicted that the brightness will be significantly deteriorated. Further, the same can be said for the F number of the incident light flux in the range of 2-1.

Therefore, in order to secure the brightness, the LP ₁₁ mode (B) is used in the visible range (about 400 nm to 650 nm).
Is required to be propagated, it is necessary to satisfy the relationship of V> 2.405.

In order to increase the transmission efficiency sufficiently,
It is more desirable if the LP ₂₁ mode (C) can also be propagated. At this time, the propagation condition of the LP ₂₁ mode (C) needs to satisfy the relationship of V> 3.83.

Since the random image fiber is formed by bundling a plurality of fibers having different V values, a fiber having a thickness in the middle of the fibers having a practical center value is "V>3.8".
It is sufficient if the 3 ”relationship is satisfied.

Furthermore, all types of fiber have a "V>
It is ideal if the relationship of 2.405 and V> 3.83 ″ is satisfied. For example, an image fiber used in an angioscope that has been practically used has an outer diameter of 0.3.
mm, and the core of 2-3 μm has a fiber interval of 4
About 3000 lines are filled in a size of about μm. In this specification, in order to satisfy the relationship of “V> 3.83”, the following equation 8

[0052]

[Equation 8] Need to satisfy the relationship.

However, since the image quality of about 3000 pixels is poor, an image fiber of about 10,000 pixels is desired. In order to realize this specification, it is necessary to set the core diameter to 1 μm and the fiber interval to about 2 μm, which makes it difficult to satisfy the relationship of “V> 3.83”. For this reason, the brightness is slightly lowered to "V> 2.
The condition for satisfying the relation of “405” is calculated by the following formula 9

[0054]

[Equation 9] Becomes

Therefore, when the core diameter is 2-3 μm or less, the fiber NA is 0.4 or more, and the core diameter is 1 μm.
When it is m or less, NA is required to be 0.498 or more.
Regarding the thickness (d) of the clad, the crosstalk increases as the thickness (d) becomes thinner, while on the other hand, if it is too thick, the brightness becomes insufficient. In order to make these both compatible, the thickness (d)
Is preferably defined by the following relationship, that is, the relationship of 1.8 μm>d> 0.8 μm.

Here, in order to define the NA in the relationship of the above equations 8 or 9, the glass material applied to the fiber is preferably a multi-component system rather than a silica system. However, in the case of a multi-component system, it is difficult to set the refractive index of the cladding to about 1.5 or less, so that the refractive index (n ₁ ) of the core needs to be defined as n ₁ > 1.56.

When the refractive index (n ₁ ) of the core exceeds 1.7, the fiber having a length of 0.5 to 5 m is colored yellow or green depending on the nature of the glass material. .

Therefore, the refractive index (n ₁ ) of the core is 1.7.
It is desirable to satisfy the relation of> n ₁ . If the refractive index (n ₂ ) of the clad satisfies the relationship of 1.53> n ₂ > 1.48, it is possible to form the clad with a multi-component glass material.

All the above relationships can be applied to the embodiments of the present invention described later. Further, when bundling fiber strands having different core diameters, V of each fiber is bundled.
Due to the different values, the cutoff frequency of each mode is different.

For this reason, the transmission efficiency of a fiber having a small core diameter may be reduced, which may be disadvantageous in terms of brightness. In such a case, for example, as shown in a third embodiment to be described later, the number of fibers having a small core diameter and the number of fibers having a large core diameter are respectively reduced, and the number of intermediate fibers is increased to cut the fiber. It is possible to reduce the decrease in transmission efficiency due to the off phenomenon.

As is clear from the above description,
The configuration for improving the transmission efficiency and removing the uneven brightness may be in conflict with the configuration for reducing the crosstalk.

Especially when an extremely thin fiber is applied, since the number of modes propagating in the core is originally small, it may be difficult to reduce the crosstalk by making the number of propagation modes different for each fiber. .

Therefore, it is necessary to consider the balance between the crosstalk and the light amount and brightness when determining the actual configuration parameters (core diameter, refractive index, etc.) of the image fiber.

[0064]

EXAMPLE A high-diameter and high-resolution image fiber capable of suppressing crosstalk and preventing deterioration of image quality by satisfying the above relationships will be described below with reference to specific examples.

The units of the clad outer diameter, core diameter (strand), core diameter (after spinning) and fiber spacing shown in Tables 2 to 5 described later are all "μm". The first embodiment will be described.

[0066]

[Table 2]

As shown in Table 2 above, in the image fiber applied to this example, 600 fibers each having a constant cladding outer diameter and five different core diameters (strands) were randomly selected. It is made by mixing and spinning.

Specifically, the fibers are randomly bundled and then heated and softened to be stretched and thinned. As a result, the core is fused to form the conduit-shaped image fiber.

The core diameter (after spinning) of the image fiber formed by such a method had five different values, but the fiber spacing was constant at 3.8 μm. As a result, it became possible to eliminate uneven brightness.

When a predetermined light (NA = 0.495, λ = 650) was made incident on the image fiber, the V value became five different values. As a result,
It became possible to reduce crosstalk.

As described above, in the present embodiment, it is possible to provide a small-diameter and high-resolution image fiber which can suppress the crosstalk and prevent the deterioration of the image quality and have no uneven brightness. The second embodiment will be described.

[0072]

[Table 3]

As shown in Table 3 above, the image fiber applied to this example was prepared by randomly mixing five kinds of fibers (element wires) having different clad outer diameters and core diameters, 600 fibers each. It is made by spinning.

The image fibers formed by such a method had five different core diameters (after spinning), but the fiber spacing was 3.8 μm on average. As a result,
It became possible to eliminate uneven brightness.

When a predetermined light (NA = 0.495, λ = 650) was made incident on the image fiber, the V value became five different values. As a result,
It became possible to reduce crosstalk.

As described above, in the present embodiment, it is possible to suppress the crosstalk and prevent the deterioration of the image quality, and it is possible to provide an image fiber having a small diameter and a high resolution with no uneven brightness. A third embodiment will be described.

[0077]

[Table 4]

Table 4 above shows the structure of the fiber after spinning. In the image fiber applied to the present embodiment, the fiber having the smallest core diameter (after spinning) becomes the single mode (λ = 650), so the largest core (core diameter = 1.13) and the smallest core (core diameter). = 0.87)
15% of the total number of cores, the second largest core (core diameter = 1.07) and the second smallest core (core diameter = 0.93) are 25%, and the remaining cores (core diameter = 1) are 20%. It is configured as a percentage.

If the composition ratio of one fiber is increased, the significance of mixing different fibers is diminished. Therefore, the difference in the composition ratio of each fiber can be randomized within a range of about 10%. preferable.

With this configuration, also in the case of the present embodiment, similarly to the above-described embodiments, it is possible to suppress crosstalk and prevent deterioration of image quality, and to have a small diameter and high resolution image fiber without uneven brightness. Can be provided. A fourth embodiment will be described.

[0081]

[Table 5]

Table 5 above shows the structure of the fiber after spinning. Core applied to this example (after spinning)
Are all configured to be capable of propagating the LP ₁₁ mode,
By randomly mixing the numbers of the respective cores at the same ratio, it is possible to achieve the same effect as that of the above-described embodiment.

In this embodiment, the number of propagation modes that each fiber can propagate is defined, and crosstalk is reduced by the combination of the difference in V value and the difference in the number of propagation modes.

The fifth embodiment will be described. The image fiber of the present example comprises five types of cores shown in Table 2 above. Specifically, as shown in FIG. 5, each core (A, B, C, D, E) is regularly arranged.

The core (A) has the largest diameter and the core (E) has the smallest diameter. Each core (A, B, C,
By arranging (D, E) as shown in FIG. 5, different kinds of cores are arranged between the adjacent cores, and as a result, the same effect as that of the above-described embodiment can be achieved. It will be possible.

The present embodiment is not limited to the above-mentioned structure, and the same effect can be obtained even if the number of cores is four, for example. Further, it is needless to say that the present invention is not limited to the above-mentioned configuration and can be variously modified.

For example, when the light intensity pattern at the exit end of light is observed in the state where crosstalk occurs, a higher-order mode pattern is observed in the core separated from the incident core. Therefore, it can be seen that the crosstalk can be reduced by cutting off the higher-order modes. For example, as shown in FIG. 4, it is possible to reduce crosstalk by bending a part of the image fiber to reduce the outer diameter of the fiber.

Specifically, when an image fiber having a small core diameter is applied, high-order modes are blocked in a portion having a small core diameter, and as a result, only low-order modes with little crosstalk can be propagated. Becomes

As another method, when the image fiber is drawn, the mode pattern of each fiber can be changed by applying a residual stress to the inside by, for example, partially quenching. Therefore, crosstalk can be reduced.

When a transmission image is observed by applying an image fiber in which fibers having different core diameters are bundled, the brightness per unit area varies depending on the difference in core diameter.
May cause light spots. In this case, for example, TV
By performing image processing in an observation system (not shown), it is possible to remove the brightness unevenness. For example, when a TV camera for an endoscope is used, it is common to take a white object serving as a reference before observing and take a white balance of the image. At this time, it is preferable to simultaneously perform white balance and brightness correction via a brightness correction circuit (not shown).

[0091]

According to the present invention, it is possible to provide an image fiber having a small diameter and a high resolution, which can suppress the crosstalk to prevent the deterioration of the image quality and have no uneven brightness.

[Brief description of drawings]

FIG. 1 is an end view showing a configuration of a random image fiber having a plurality of cores having different sizes and shapes.

FIG. 2 is a diagram showing a dispersion curve of a uniform core optical fiber.

FIG. 3 is a graph showing the energy ratio of each excited mode to the core diameter.

FIG. 4 is an external view showing a configuration of an image fiber according to a modified example of the present invention, in which only a part is bent to reduce the outer diameter of the fiber.

FIG. 5 is a diagram schematically showing a configuration of an image fiber according to a fifth embodiment of the present invention.

FIG. 6 is a diagram schematically showing a configuration of a conventional endoscope.

FIG. 7 is a diagram schematically showing a cross section of a fiber conduit.

Claims (3)

[Claims] 1. An image fiber comprising a plurality of fibers having different V values, wherein the image fiber includes at least two types of fibers having different numbers of propagation modes. 2. An image fiber comprising at least five types of fibers having different V values from each other. 3. The image fiber according to claim 1, wherein the number of fibers having different V values is different.