JP3084126B2 - Light transmission optical system using fine image fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a light transmitting optical system using a small-diameter image <br/> Fiber high quality.

[0002]

2. Description of the Related Art As an endoscope using an image fiber, there is a conventional endoscope as shown in FIG. The endoscope 1 forms an image of an object illuminated by an illumination optical system (not shown) on an incident end face of an image fiber 3 in which a plurality of fibers are bundled by an objective lens 2, and this image is formed by the image fiber 3. This is transmitted and observed from the exit end face by the eyepiece lens 4 or the like. In recent years, such endoscopes have become smaller in diameter, so that the inside of blood vessels can be observed.
A fiber using an image fiber having an outer shape of 1 mm or less has been manufactured. As described above, an endoscope having a small external shape has a small number of pixels and a low resolution because an extremely thin image fiber is used.

[0003]

As such a small-diameter image fiber, those having a structure in which a clad is shared without using each fiber separately are mainly used. Therefore, to prevent crosstalk between adjacent fibers,
It was necessary to provide a sufficiently thick cladding. However, in recent years, it has been desired to increase the number of pixels in these ultrafine image fibers as well. Therefore, it is necessary to make the cladding thinner by reducing the core diameter.However, when the core diameter becomes smaller and the cladding thickness becomes several times the wavelength of light or less, the light modes are mixed between adjacent image fibers. This causes crosstalk due to mode coupling between fibers, which has not been a problem in the related art, and the image quality is significantly reduced. In order to reduce the crosstalk, a technique of randomly arranging the shapes and arrangements of the cores is conventionally known, but the random structure has a drawback that the screen is difficult to see. In order to prevent the image quality from deteriorating due to such crosstalk, it is necessary to increase the thickness of the clad. However, in this case, the occupation ratio of the core in the fiber bundle cross-sectional area is reduced, and a bright image is obtained. And the disadvantage that the number of pixels cannot be increased remains. As described above, in the related art, the image fiber cannot improve the resolving power so much to avoid the influence of the crosstalk.

[0004] In view of the above problems, the present invention uses an image fiber having a small diameter and an array, and suppresses the deterioration of image quality due to crosstalk. and to provide a light transmitting optical system using a radial image Fiber.

[0005]

SUMMARY OF THE INVENTION
Optical transmission system using diameter image fiber is continuous
Multiple cores with a diameter of 5μm or less are arranged in the cladding
The incident optical system
Light and converge it to the exit end of the image fiber.
A light transmission optical system for transmitting the image fiber;
The mode function of the LP ₀₁ mode of each core and the
The point image amplitude distribution must satisfy the following condition:
It is characterized by. However, Δ (r) ≡E ₀₁ (r) −F (r), and E ₀₁ (r) is LP ₀₁
The mode function of the mode, F (r) is the point image amplitude of the incident optical system.
It is cloth. Further, the light transmission optical system according to the present invention has the following
It is preferable to satisfy the conditional expression. FNo. ≧ a / (λu) (10 ′) where FNo. Is the F-number determined by the incident optical system
Bar, λ is the wavelength of light transmitted through the image fiber, u
Eigenvalues of the core LP ₀₁ mode, a is the radius Der core
You. In the light transmission optical system according to the present invention,
The incident optical system uses a fiber LP ₀₁
Generates aberrations so that they almost match the mode function of the mode
It is preferred that The phenomenon of crosstalk in the image fiber described above is described in the paper "Transmission Characteristics of Image Fiber" (Transactions of the Institute of Electronics, Information and Communication Engineers, '83 .11 vol.J66-CN)
o.11), and the crosstalk can be inferred from the equations shown in the paper. The value of a crosstalk parameter (referred to as a crosstalk parameter in the paper) shown in this paper (hereinafter referred to as a B value) is a value indicating a magnitude relationship between crosstalks. The talk becomes noticeable. The B value in the _LP01 mode is specifically given by the following equation. B = [{-2u ₀₁ ² K ₀ (ω ₀₁ d / a)} / {v ² K
₁ ² (ω ₀₁ )｝] · z / β (1)

Here, u ₀₁ and ω 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 an m-th order modified Bessel function of the second kind. Further, v is a normalized frequency determined by the specifications of the fiber, and is represented by the following equation (2). v = ka√ (n ₁ ² −n ₂ ² ) (2) where k = 2π / λ (λ is the wavelength of light propagating in the fiber), and n ₁ and n ₂ are the refractive indexes of the core and the cladding, respectively. is there. It should be noted that the B value can be determined for the other higher-order modes according to the concept described in the above-mentioned paper.

[0007] By the way, based on the idea of the above-mentioned paper, the same specifications as in the above case (the wavelength of light is 500 nm,
The core diameter of the fiber is 1.95 μm, and the pixel interval is 3.6.
When the B value is calculated for an image fiber of 8 μm, NA = 0.49, and fiber length is 1.5 m), the values shown in Table 1 below are obtained. Table 1 shows that the B value in the _LP01 mode is the smallest value. Therefore, it is inferred that the _LP01 mode has the least crosstalk.

If the light transmitted through the fiber is a linear combination of each mode, and this is Etotal, this Etotal is expressed by the following equation (3). Here, A _ml is the weight applied to 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 fiber end face, and can be obtained by the following equation. Here, O (r, θ) is the electric field distribution of the incident light, and r,
θ indicates polar coordinates with the center of the fiber as the origin, where r is the distance and θ is the angle.

[0009] As described above, since the size of the crosstalk LP smallest ₀₁ mode, LP ₀₁ mode Wait for the largest, if and weight of the other mode is such give incident light small, (3 Equation (1) shows that the total crosstalk transmitted by the fiber is minimized. Next, a method for providing the above-described incident light will be described. FIG. 1 shows an enlarged view of light incident on one fiber. The light emitted from the point light source is condensed by the objective lens at the incident end of the fiber, and enters the core. In FIG. 1, the optical axis of the objective lens is drawn so as to coincide with the central axis of the core.
Since the image of the point light source is spread by diffraction in the objective lens, it has an intensity distribution as shown in the figure. The electric field distribution function of this image is called point image amplitude distribution, Given by Note that J _m (x) is an m-order Bessel function,
λ is the wavelength of light propagating in the fiber, FNo. Denotes an aperture (F number) of the incident optical system. When the image of the point image amplitude distribution is formed on the end face of the fiber and a plurality of modes are excited in the fiber, the weight of each mode is obtained by converting O (r, θ) of the equation (4) into the equation (5). Will be replaced. (4) shows that by expanding the function of the incident light mode function of each mode, considering the case that focused on the center of the core, the point image form of the amplitude distribution is LP ₀₁ mode wait LP ₀₁ mode when substantially equal to the shape of the mode function is greatest, the weight of the other modes is reduced.

[0010] At this time, for either it is sufficient to what extent match point image amplitude distribution of the incident optical system mode function of LP ₀₁ mode, it is desirable to satisfy the following equation. Here, Δr≡E ₀₁ (r) −F (r), and Er
₀₁ (r) is a mode function of the _LP01 mode, and F (r) is a point image amplitude distribution of the incident optical system.

[0011] Next, in order to maximize the weight of the LP ₀₁ mode, FNo of the input optical system. The method for determining is described. Specifically, the mode function of LP ₀₁ mode is It is expressed as The mode function is given by the formulas (7-1) and (7-2).
As can be seen from the equation, the power is concentrated in the core. Therefore, the shape of the mode function is dominated by the expression (7-1) which is a function in the core. Therefore, in order to increase the weight of the LP ₀₁ mode, the incident light function (5)
The equation and the mode function (7-1) in the core may be made substantially equal (see FIG. 1).

An FNo. For making the function (5) of the incident light and the mode function (7-1) in the core substantially equal. Can be considered as follows. That is, equation (5), (7
Equation -1) is a pseudo-periodic function including the Bessel function of the first kind. If the points where these two functions become 0 first are almost equal, the point image amplitude distribution and the mode function have almost the same form. The point where each of them is initially 0 is determined by the equations (5) and (7−
The values in parentheses in 1) are 3.8317 and 2.4048, respectively.
This is the point. Therefore, the coefficients in parentheses are 3.83 respectively.
17 and 2.4048 so that the FNo. Is determined, LP ₀₁
The mode is excited relatively large, and an image with little crosstalk is obtained. When the points where the values of the two functions (5) and (7-1) become zero are matched, the FNo. Is FNo. ≒ 1.97a / (λu ₀₁ ) (8)

However, when the weight is actually calculated, as shown in FIG. 2, the point r _{1 at} which the equation (5) first becomes 0 is obtained.
Is (7-1) equation towards first greater than the point r ₂ becomes zero is will be the locus of the two curves are more closely in the core radius in Fig. 2, the weight of the LP ₀₁ mode for other modes Is preferred. Coefficient when the (8) thereof, when changing the coefficients in Table 2 in FIG. 3 (where the core radius is 1 [mu] m, lambda is 600 nm, u ₀₁ is 2.00657) As the weight of 1. It is about 2.2 instead of 97. Therefore, FNo. Values 1.97a / (λu ₀₁₎ as long as it is within the range of ± 20% of the value indicated by the LP ₀₁ mode Waits is sufficiently large, the weight of the other modes is almost negligible, minimizing crosstalk Can be fastened.

Further, according to FIG. 2 described above, FNo. ≒ 2.2a / (λu₀₁Equation (9) obtained by (9) is LP₀₁Maximize mode weight
FNo. Condition, but the weight of other modes
When calculated, LP is within a certain range around equation (9). ₀₁Mo
The value will be a sufficiently small value for the weight of C.
Therefore, within this range, LP₀₁Other than mode
Since the influence of Stoke is small, there is little deterioration in image quality.
In addition, as can be understood from FIG.
If the same, the larger the NA, the lower the LP₀₁Mode electric field component
The fabric is well confined within the core. That is, the core diameter is the same.
If it is 1, the larger the NA of the image fiber
Advantageous in terms of Stoke, quartz-based image fiber
(Generally NA = about 0.3)
Fiber (in general, NA = 0.4 or more) increases NA
This is advantageous.

So far, only the center of the core has been discussed with respect to the relationship between the mode function and the point image amplitude distribution, but it is sufficient to consider only the center of the core for the entire core.
Next, a description will be given. Tables 3 (A) and (B) in FIG.
Is changed, the coefficient (F) on the right side of equation (8) or (9) is changed.
No. × λu ₀₁ / a) and the weight of each mode.
Table (A) shows the weight of each mode at the center of the core as an amplitude, and Table (B) shows the weight of each mode when integrated over the entire core as an intensity ratio. Table 3 (A), (B), the case of considering the entire core also thought about only the core center, the weight occupied by the LP ₀₁ mode is found to exhibit a similar trend. Further, when the square of the weight of the LP ₀₁ mode is W, if the ratio within this value is squared weights of all modes than about 50%, the influence of crosstalk than LP ₀₁ mode is reduced. FIG. 6 shows the value of W with the coefficient (FNo. · Λu ₀₁ / a) plotted on the horizontal axis. Fiber specification is core diameter a =
Calculated as 1.95 mm, NA = 0.5, λ = 600 nm.

As can be understood from this figure, W is about 50
%, The coefficient on the right side of the equation (9) is 1.4.
It may be within the range of ~ 8. This can be expressed by the following equation (10). 8 · a / (λu ₀₁ ) ≧ FNo. ≧ 1.4 · a / (λu ₀₁ ) (10) Also, if the thickness of the cladding can be secured to some extent, (1
Even if the value is out of the range of the expression (0), if the following expression (10 ') is satisfied, the crosstalk is sufficiently small, and there is no practical problem. FNo. ≧ a / (λu ₀₁ ) (10 ′) As described above, the FNo. The point image amplitude distribution and the mode function of the _LP01 mode can be made close to the same shape by using the appropriate FNo. The crosstalk of the image fiber can be minimized by selecting.

By the way, in order to achieve the same object,
Changing the shape of the point image amplitude distribution by the aberration of the optical system will be described next. That is, when the spread of the mode function of LP ₀₁ mode is large, the aforementioned FNo. In the method according to the above, the emission NA (= 1 / [2FNo.]) Of the optical system satisfying the expression (8) may be considerably smaller than the NA of the fiber. In this case, only a significantly smaller amount of light than the amount of light that can be transmitted by the fiber is made to enter the fiber, so that crosstalk is small but sufficient brightness cannot be obtained. To avoid this, the FNo. By generating an aberration while reducing the may be brought close to the shape of the point image amplitude distribution in the shape of a LP ₀₁ mode. In that case,
As means for generating aberration, a normal spherical or aspherical lens may be used, or a phase filter or a quartz filter may be used. Alternatively, a hologram optical element (HOE) or the like may be used because the optical system or the like is fine.

The shape of the mode function is described above (7-
As shown in the expressions 1) and (7-2), it depends on the eigenvalue. Since the eigenvalue changes with the wavelength, the shape of the mode function also changes with the wavelength. Therefore, even if possible to increase the weight of the LP ₀₁ mode only certain wavelengths, in other wavelengths is contemplated that the wait mode other than LP ₀₁ mode is increased. In order to prevent this, chromatic aberration is generated in the incident optical system so that the point image amplitude distribution and the mode function of the _LP01 mode become almost equal in a wide wavelength range (for example, 400 nm to 600 nm which is a visible range). What should I do? In particular, B indicating the magnitude of crosstalk
As can be seen from Equations (1) and (2), the value increases rapidly as the wavelength is longer. FIG. 7 shows this state in a graph. Therefore, regarding the generation of chromatic aberration, it is preferable that the mode function and the point image amplitude distribution have substantially the same shape, giving importance to the long wavelength side rather than the short wavelength side.
Further, when the observation system has a sensitivity distribution with respect to wavelength, it is desirable that the mode function and the point image amplitude distribution have substantially the same shape, giving importance to a wavelength region having high sensitivity according to the spectral sensitivity.

Similarly, the fiber core and the cladding element
The refractive index does not change with wavelength as a core material
Low dispersion glass, high refractive index at wavelength as clad material
Long wavelength by using high-dispersion glass
The difference in the refractive index between the core and the cladding on the side
Wear. Thus, √ (n) in equation (2)₁ ^Two-N _Two
^Two) (Equivalent to the NA of the fiber)
Function on the long wavelength side₀₁Mode function of mode
Can be reduced.

By the way, the description so far includes the ideal condition that the propagation constant β of each fiber constituting the image fiber is equal to each other, including the contents analyzed in the above-mentioned paper “Transmission characteristics of image fiber”. We examined the occurrence of crosstalk only for the subject. Therefore, it is considered that crosstalk does not occur between fibers having different propagation constants β. However, crosstalk actually occurs between fibers having different propagation constants. It is generally known that the smaller the difference between the propagation constants of the fibers, the greater the amount of this phenomenon. Therefore, it is desirable to minimize this effect. For that purpose, it is necessary to make the difference in the propagation constant between adjacent fibers constituting the image fiber as large as possible.

In order to make the propagation constant different between the fibers, it is sufficient to change the value of the normalized frequency v between adjacent fibers. However, in an image fiber in which each fiber is a multimode fiber, there are a plurality of modes, and it is necessary to make the propagation constants of each mode not approximate to each other. Generally, image fibers in which crosstalk is a problem, each fiber is very thin,
For example, it is an image fiber having a core diameter of about 5 μm or less. Each fiber of such an image fiber has a small normalized frequency v value, and the number of modes in the fiber is relatively small (from several to at most ten). Therefore, only the propagation constant of the lower-order mode needs to be considered, and LP
It is sufficient if there is no overlap between the ₀₁ mode and the LP ₁₁ mode. That is, the LP of the fiber with the largest v value
_Since the propagation constant of the LP ₀₁ mode of the fiber having the smallest v value should be larger than the propagation constant of the ₁₁ mode,
What is necessary is to satisfy the following expression (11). _b 11max ≦ b 01min (11) ^{_{where, b ml = (β ml 2}} -k 2 n 2 2) / (k 2 n 1 2 -
k ² n ₂ ² ), k = 2π / λ, β _ml are LP _ml mode propagation constants. (For the b value, see "Optical Fiber" (published in 1983) and the like.) The relationship between the b value and the v value in each mode is as shown in FIG.

By the way, as described above, LP₀₁mode
If the weight of is large, ignore higher-order modes altogether
Good. Image fiber with N kinds of v-value fibers
, The b value of the fiber having the nth smallest v value is b
_01nAs Δb_01nIs defined by the following equation (12).
And Δb_01nIf equation (13) is satisfied, the crosstalk is
Less likely. Δb_01n= B_01n-B_{01 (n-1)} (12) Δb = (b_01max-B_01min) / (N−1) (13) where N ≧ 3 and n ≧ 2. Where the b value is the propagation constant
β is normalized by the angular frequency k, and b_01nIs the nth
Fiber with the smallest v value and the n-1st smallest v value
LP with fiber ₀₁Difference of b value of mode, N is different
It is the number of fiber types having a v-value.

Even if the value of the expression (13) varies to some extent, the effect of reducing the crosstalk can be expected, and its width is about ± 20%. When Δb exceeds ± 20% with respect to the expression (13), the propagation constant of the adjacent fiber becomes a close value, and the crosstalk increases. Therefore, when the equation (13) is rewritten in consideration of this condition, the following equation is obtained. 0.8 ( _{b01max− b01min} ) / (N−1) ≦ _Δb01n ≦ 1.2 ( _{b01max− b01min} ) / (N−1) (13 ′) Here, changing the v value is expressed by equation (2). As is evident from the above, it is to change the core diameter or the refractive index of the core (or clad). FIG. 8 shows the relationship between the b value and the normalized frequency v value.
Is shown in LP ₀₁ mode b values and the normalized frequency v
The relational expression of the value is (14) when the v value is in the range of about 2 to 12.
It can be approximated by an equation. b ≒ 1.04 × 1 / (0.047v 2 + 0.73v) (14) the rate of change with respect to v values of b value, Δb / Δv = (0.094v + 0.73) / (0.047v 2 + 0.73v ² ) (15)

Δv is given by Δv = [(0.04v + 0.73) ² v ² /(0.094 v + 0.73)] Δb ≒ 0.73v ² Δb (16) Considering the condition for setting Δb to a constant value , And N kinds of core diameters, the respective b values are b ₁ , b ₂ , ‥
If ‥, b _n , ‥‥ b _N , v that equally divides b _{1 to} b _N
n can be expressed approximately _{v n = v n-1 +} Kv n-1 2 (b N -b 1) / (N-1) (17). The coefficient K in the above equation (17) is about 0.73 from the above equation, but may have a certain width, and the range is preferably about the equation (18). 0.5 ≦ K ≦ 1 (18)

Further, particularly in an endoscope or the like, since the distal end portion has a complicated structure and a sufficient space for the image fiber cannot be secured, the image fiber becomes extremely thin, and the LP becomes low.
_{Even in 01} mode, it may not be possible to sufficiently confine it in the core. In such a case, fibers having only a thin end portion are bundled as shown in FIG.
As shown in the figure, if an image fiber with only a thin tip and a thick middle part is used, crosstalk is likely to occur in the details, but if the length of the details is short enough, the crosstalk will not be a problem. LP ₀₁ due to large core diameter
Since the mode is sufficiently confined in the core and the cladding is thick, an image fiber with less crosstalk can be provided.

[0026]

The first embodiment of the present invention will be described below.
You. In this embodiment, the core diameter of the fiber is 2 μm.
m, pitch between fibers is 4μm, length of fiber
Is 1.5 m and NA is 0.5 to construct the image fiber.
The objective located at the entrance end face of this image fiber
The lens is FNo. Combine both as 1.81
Was. Point image of the objective optical system obtained by the configuration of the present embodiment
Amplitude distribution and LP in core ₀₁The relation between the mode and the mode function
FIG. 10 shows the relationship. In the figure, the broken line indicates the point image
Width distribution, solid line is LP₀₁The mode function of each mode is also shown
It is. The wavelength of the light at this time is 600 nm. or,
The coefficient relating to the expression (10) for this embodiment is represented by FNo. λu₀₁/A≒2.23. Also, the weight value calculated by equation (4) at this time
Table 4 below shows the relationship between and the magnitude of the B value.
You.

[0027] As can be seen from Table 4, when placed in the point image in the center of the core, the weight of the non-LP ₀₁ mode is almost 0. The modes other than the _LP01 mode are modes in which the B value is large and the amount of crosstalk is very large. However, in this embodiment, since these weights are almost zero, it can be seen that the crosstalk is reduced. As described above, in this embodiment, since the image fibers are arranged in a line and the image fibers having a small diameter are used, and the crosstalk can be reduced, an image fiber which is finer and has excellent resolution can be obtained.

Next, a second embodiment of the present invention will be described. In the second embodiment, the image fiber of the first embodiment has an FNo. Is a combination of a 2.05 objective lens. When the wavelength of light indicates a mode function point image amplitude distribution and LP ₀₁ mode when the 500nm graphically, Figure 1
Becomes 1 like, the point image amplitude distribution in broken lines, also Mode function of LP ₀₁ mode is shown respectively in solid lines. Table 5 below shows the relationship between the weight of each mode and the magnitude of the B value at this time. In the case of this embodiment, the weight is almost 0 in modes other than the _LP01 mode at a wavelength of 500 nm. However, for example, when observing visually, the sensitivity of the human eye is about 500
It has a peak near nm (dark adaptation), and then the sensitivity decreases as the wavelength increases. Therefore, in this case, even if the crosstalk slightly increases on the long wavelength side, the image quality is not particularly adversely affected. Thus, combined with the wavelength of the weight of the LP ₀₁ mode in the maximum peak wavelength used, the effect of reducing crosstalk is high.

Next , a third embodiment will be described. Although the above-described embodiments relate to fibers having the same core diameter, this embodiment uses an image fiber in which six types of fibers having different core diameters are bundled. Table 6 shows the core diameter, v value, and propagation constant β of each fiber.
However, these numerical values are values when the refractive index of the fiber core is 1.595, the refractive index of the cladding is 1.516, and the wavelength λ of light propagating in the fiber is 600 nm. Table 6

In this embodiment, the difference Δv between the v values of the six types of fibers is calculated by using the equation (17) to set the value of the coefficient K to 0.8.
, And the respective v values were determined. As can be understood from Table 6, the values of the propagation constant β of each fiber are different at substantially equal intervals. By forming an image fiber by bundling six types of fibers as shown in Table 6, even when fibers having similar propagation constants are arranged adjacently, the difference in the propagation constants has a certain magnitude. Therefore, an image fiber with less crosstalk can be obtained. When bundling fibers having different core diameters, if three or more types of fibers are provided, fibers having the same core diameter can be arranged without being adjacent to each other. However, it is extremely difficult to arrange thousands of fibers regularly. On the other hand, by increasing the types of core diameters as in the present embodiment, even if fibers are mixed at random, the probability that fibers having the same core diameter are arranged adjacently decreases. Further, if the types of fibers having different core diameters are excessively increased, the difference in the propagation constant β becomes small and crosstalk increases. Therefore, it is desirable that the types of fibers having different core diameters be about 3 to 7 types.

Next, a fourth embodiment of the present invention will be described. In this embodiment, an image fiber having a relatively high NA is used. The specifications of this image fiber are
Core diameter 2 μm, pitch 4 μm, NA 0.68, wavelength 58
7.56 nm, FNo. Is 1.78. Point image amplitude distribution and LP of the incident optical system in the present embodiment
FIG. 12 shows the relationship between the ₀₁ mode and the mode function.
become that way. In addition, when the weight of each mode function is shown,
Table 7 below. In the case of this embodiment, since the NA of the image fiber is large, the low-order mode function is well confined in the core, which is desirable for reducing crosstalk.

Next, in the following embodiment, another method for reducing the crosstalk of the image fiber will be described. 13 and 14 relate to the fifth embodiment. FIG. 13 shows one fiber constituting the image fiber.
(The high refractive index region is indicated only as high and the low refractive index region is indicated only as low in the figure.) . That is, the refractive index changes along the traveling direction of the light in the fiber, and the v value in the above equation (2) is changed. In this case, the change in the refractive index may be periodic or aperiodic. Such fibers are bundled so that, for example, regions having different refractive indices are opposed to each other between adjacent fibers (by shifting the axial position of each fiber or the period of change in refractive index). Figure 1
4 is an image fiber shown in FIG. By adopting such a structure, since the modes of the adjacent fibers of the image fiber are different, the effect of reducing the crosstalk can be obtained.

FIGS. 15 and 16 show a modification of the fifth embodiment. One fiber shown in FIG.
The inner refractive index changes in the axial direction between a high refractive index and a low refractive index. An image fiber shown in FIG. 16 is obtained by bundling such fibers so that, for example, regions having different refractive indices are opposed to each other between adjacent fibers (by shifting the axial position of each fiber). It is.

FIGS. 17 and 18 correspond to FIGS. 14 and 16, respectively.
FIG. 17 shows another example of the configuration. In FIG. 17, the image fiber changes in such a manner that the high refractive index region and the low refractive index region of the core 7 of each fiber 6 are aligned in the direction orthogonal to the axial direction. are doing. In FIG. 18, the high refractive index region and the low refractive index region of the clad 8 of each fiber 6 are aligned and changed in the direction orthogonal to the axial direction. In the case of the configuration example of the image fiber shown in FIGS. 14, 16, 17, and 18, the values of the high refractive index regions and the low refractive index regions of each fiber 6 shown in the drawings are not necessarily equal to each other. Need not be.

FIG. 19 shows a sixth embodiment. In this embodiment, the image fibers are arranged such that the birefringent fibers 10 have different birefringence directions. In FIG. 19, the refractive index in the direction indicated by the arrow is higher than the refractive index in the direction orthogonal to the direction indicated by the arrow. The arrangement of the fibers is preferably a hexagonal close-packed structure in consideration of the visibility of the image, but may be a random array structure. In this embodiment, since the mode functions of adjacent fibers are different, crosstalk is small. Further, as a modification of the image fiber of FIG. 19, as in a fiber 11 shown in FIG. 20, the vibration direction of light having a high refractive index is directed to a fiber having a longer distance among adjacent fibers (this embodiment). (In the example, the vertical direction). With such a structure, light having a low refractive index in an oscillating direction has an advantage that crosstalk is small because the direction of crosstalk is a fiber that is farther among adjacent fibers.

[0036] Further, FIGS. 21 and 22 are intended to show the seventh embodiment, twisting about the central axis of the axial direction base material of single fiber 13 are added. Such fibers 13 are bundled to form an image fiber as shown in FIG. In the figure, the arrow indicates the direction of the twist, and the twist is applied in the same direction for each fiber. Therefore, each fiber 13
Since the torsional stress and the like remain, the refractive index of the fiber with respect to the two circular deflections differs, and the same operation and effect as the embodiment of FIG. 20 can be obtained. As shown in FIG. 23, an image fiber may be formed by mixing fibers 14 whose twisting direction is opposite to that of the fibers 13.

FIG. 24A shows an embodiment of an image fiber in which the distal end portion is reduced in diameter. This image fiber 1
Reference numeral 6 indicates that the tip portion is formed to have a smaller die diameter 17 as compared with a conventional image fiber (see FIG. 1B),
8, the fiber diameter is widened. With this configuration, it is possible to obtain an image fiber with little space and little crosstalk.

In each of the above embodiments, to increase the amount of light incident on each core of the image fiber,
A multi-lens may be provided on the incident end face of the image fiber. For example, in the example shown in FIG. 25, a hologram optical element (HOE) 21 is provided as a multi-lens on the incident end face of the image fiber 20. Since the hologram optical element 21 is advantageous for producing a lens having a fine shape, there is an advantage that it is high-performance and inexpensive.

[0039]

As described above, the light transmission optics according to the present invention
According to the system, the fiber diameter can be made smaller, the deterioration of image quality due to crosstalk is reduced, and excellent resolution is obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram when a point image amplitude distribution by an incident optical system and a mode function in a fiber are almost matched.

FIG. 2 is a diagram showing a relationship between a point image amplitude distribution with respect to a fiber core diameter and a mode function.

FIG. 3 is a table showing a change in weight in each mode when the coefficient on the right side of Expressions (8) and (9) is changed.

FIG. 4 is a diagram showing a power distribution when the NA of the image fiber is changed.

FIG. 5 (A) in Table 3 shows FNo. Table showing the weight of each mode at the center of the core by amplitude when
(B) is FNo. 7 is a table showing the weight of each mode in the entire core in the case of changing.

[6] LP ₀₁ mode for the square W of the weight (8) and (9) is a diagram showing the relationship of the coefficients of the right-hand side of the equation.

FIG. 7 is a graph showing a relationship between a B value which is a parameter of crosstalk and a wavelength λ of light propagating in a fiber.

FIG. 8 is a diagram showing a relationship between a b value and a normalized frequency v value for each mode.

FIG. 9A is a cross-sectional view of a main part of a fiber whose distal end is reduced in diameter, and FIG. 9B is a front view of a main part of an image fiber whose distal end is reduced in diameter.

FIG. 10 is a diagram showing a relationship between a point image amplitude distribution of an incident optical system and a mode function in a fiber in the first embodiment of the present invention.

FIG. 11 is a view similar to FIG. 10 for a second embodiment.

FIG. 12 is a view similar to FIG. 10 for a fourth embodiment.

FIG. 13 is a diagram showing a refractive index of a fiber core according to a fifth embodiment.

14 is a configuration diagram of an image fiber obtained by bundling the fibers shown in FIG.

FIG. 15 is a diagram showing a refractive index of a cladding of a fiber according to a modification of the fifth embodiment.

16 is a configuration diagram of an image fiber obtained by bundling the fibers shown in FIG.

FIG. 17 is a diagram illustrating another configuration example of the image fiber according to the fifth embodiment.

FIG. 18 is a view similar to FIG. 17, showing another configuration example of a modification of the fifth embodiment.

FIG. 19 is a diagram showing, with arrows, directions in which the refractive index in each fiber is high in the sixth embodiment.

FIG. 20 is a modification example of FIG.

FIG. 21 is a schematic perspective view showing a state where a fiber preform is twisted in the seventh embodiment.

FIG. 22 is an end view of an image fiber obtained by bundling the fibers shown in FIG. 21;

FIG. 23 is an end view of an image fiber showing a modification of FIG. 22;

FIG. 24 is a diagram showing a tip of an image fiber;
(A) is a diagram showing the one of the present embodiment, and (B) is a diagram showing the one of the prior art.

FIG. 25 is a cross-sectional view of a main part in which a hologram optical element is provided on an incident end face of an image fiber.

FIG. 26 is a diagram showing a general optical system of an endoscope using an image fiber.

[Explanation of symbols]

1 endoscope, 3, 16 image fiber, 2 ...
Objective lens, 6, 10, 11, 13, 14 ... fiber, 7 ... core, 8 ... clad.

Claims (3)

(57) [Claims] 1. A light condensed by an incident optical system on an incident end face of an image fiber constituted by arranging a plurality of cores having a diameter of 5 μm or less in a continuous clad and transmitted to an exit end face of the image fiber. In the transmission optical system, the mode function of the _LP01 mode of each core of the image fiber and the point image amplitude distribution of the incident optical system satisfy the following conditional expression.
A light transmission optical system characterized by satisfying . However, Δ (r) ≡E ₀₁ (r) −F (r), and E ₀₁ (r) is LP ₀₁
The mode function of the mode, F (r) is the point image amplitude of the incident optical system.
It is cloth. 2. The light transmission optical system according to claim 1, wherein the following conditional expression is satisfied. FNo. ≧ a / (λu) (10 ′) where FNo. Is the F number determined by the incident optical system, λ is the wavelength of light transmitted through the image fiber, u
The eigenvalues of the core LP ₀₁ mode, a is the radius of the core. 3. An optical transmission system according to claim 1, wherein said incident optical system generates an aberration such that a point image amplitude distribution substantially coincides with a mode function of an _LP01 mode of the fiber. Optical system.