JPH0766085B2 - Fiber type optical filter - Google Patents

Description

The present invention relates to an optical filter that is small in size, high in performance, flexible, and easy to connect with an optical fiber.

Conventionally, a fiber-free Perot plate has been known as an optical filter. Fuaburipero plate is put silver on both surfaces of a thin layer of M _g F ₂ (magnesium Hutu reduction), energy reflectance of 95%
When the thickness of M _g F ₂ is set to about 1/4 of the desired wavelength λ _o , a filter having a wavelength width of transmitted light of several Å to several hundred Å can be obtained. In addition to the above Fabry-Perot plate, a filter using a birefringent crystal (crystal or mica) is known.

However, since these conventional filters are bulk type,
When connecting with an optical fiber, an optical device such as a lens or a fine movement table is required, so it is weak against vibration and the device is small.
It has the drawback that it cannot be made lighter.

SUMMARY OF THE INVENTION In order to eliminate such drawbacks of the related art, the present invention provides a small-sized and high-performance optical filter by using a single polarization fiber and a fiber type polarizer. Less than,
The present invention will be described in detail with reference to the drawings.

FIG. 1 is an embodiment of the present invention, wherein 1 and 2 are fiber type polarizers and 3 to 7 are single polarization fibers. 8 ~
14 core, 15 and 16 metals (e.g. Al), 17 to 21 is a stress applying section B ₂ O ₃ or B ₂ O ₃ -G _e O ₂ was added to the SiO _2.
Regarding the single polarization fiber, Hosaka et al.
s single polarization fibers with asymmetric strai
n birefringence "(Electron.Lett., vol.17, no.15, P.53
0, 1981). As for the fiber type polarizer, Japanese Patent Application No. 56-208048 by Hosaka et al.
This is described in detail in "Method for producing fiber type polarizer".

The performance of the fiber type polarizer is 1 cm long and 10 dB extinction ratio. Since the extinction ratio of the polarizer is required to be about 30 dB, the length of the fiber type polarizer is 3 cm.

The principle of the optical fiber of the present invention will be described below. As shown in Fig. 2, the electric field components seen in the (X _n-1 , Y _n-1 ) coordinate system. In the coordinate system (X _n , Y _n ) rotated only clockwise, the electric field component Is expressed by the following equation.

However Is.

If the phase difference between the X-axis and Y-axis polarization modes caused by passing through the single polarization fiber (length d) is 2δ, the single polarization fiber as a phase shifter is It is represented by the matrix.

However Where λ is the wavelength of light and n _f and n _s are equivalent refractive indices of both polarization modes, respectively.

Now, as shown in FIG. 1, the direction of the minimum absorption loss in the fiber type polarizer (the direction parallel to the metal surface) is defined as the x-axis, and N
As shown in FIG. 3 (showing the case of N-6), the main axes of the single polarization fibers of the book are arranged so as to form a fan shape between 0 and π / 2 (rad), and the n-th ( The angle between the principal axis (Xn axis) of the single polarization fiber (n-1 to N) and the X axis is θ _n (rad)
When Arrange so that the following conditions are satisfied.

In this case, the electric field component {E _xN, E _yN} emitted from the fiber-type polarizer 2, {E _xO, E _yO} field component entering the fiber type polarizer 1 is given by the following equation when the.

However, Is a matrix representing a fiber type polarizer, Is represented. Equation (6), after some calculation, Is rewritten as However, cosx = cosδcosα (9). From the equation (8), the transmittance of the optical filter of the present invention is Becomes

[References for Formulas (6) to (10): JWEva
ns, "Solc Birefringent Filter", J.Opt.Soc.Am, vol.4
8, no.3, pp.142-145,1958] Fig. 4 shows the number of single polarization fibers N = 64, the length of each single polarization fiber d = 6 cm, and the mode birefringence B = (n _f −n _s )
The transmittance T _s for 1.3 × 10 ⁻⁴ is plotted against the wavelength λ.

As can be seen from FIG. 4, the transmission wavelength is 1 μm to 2 μm.
, Λ ₀ = 1.3 μm and λ ₀ = 1.56 μm. That is, at λ ₀ = 1.3 μm and λ ₀ = 1.56 μm, The following conditions are met. If the wavelength width at which the transmittance is 0.5 is the half width Δλ (Half Width), the relationship between Δλ and N is as shown in FIG. The half-width Δλ changes depending on N, but when N = 175, Δλ = 10Å, and when N = 17, Δλ = 100Å.

In the above embodiment, the transmission wavelength λ ₀ is 1.3 μm and 1.56 μm.
However, the transmission wavelength can be selected as a desired wavelength by appropriately changing the mode birefringence index B, the length d of the single polarization fiber and the number N of the single polarization fibers. It is also clear that the half width can be set to a desired value.

As is clear from the above description, according to the present invention, it is possible to realize a high-performance and small-sized optical filter. Further, if the mode birefringence B of the single polarization fiber is a value of about 1 × 10 ⁻⁴ or more, its characteristics do not change even if it is bent up to a bending radius of about 5 cm. It has the great feature of being highly flexible.

Furthermore, since the fiber type optical filter of the present invention can be easily connected to an optical fiber, if it is used for wavelength division multiplexing optical communication or the like,
There are enormous advantages.

[Brief description of drawings]

FIG. 1 is an embodiment of the present invention, FIG. 2 is a diagram showing coordinate exchange, and FIG. 3 is a diagram showing a state in which the main axes of a single polarization fiber are arranged in a fan shape when N = 6. FIG. 4 is a diagram showing the wavelength characteristics of the transmittance of the fiber type optical filter of the present invention, and FIG. 5 shows the relationship between the half width Δλ and the number N of single polarization fibers of the fiber type optical filter of the present invention. FIG. 1,2 ...... Fiber type polarizer, 3 to 7 ...... Single polarization fiber, 8 to 14 ...... Core, 15, 16 ...... Metal, 17 to 21 ...... Stress applying part.

Claims (1)

[Claims] 1. A single-polarized fiber and a fiber-shaped polarized light, which have the same propagation axis of light and satisfy the following equation, in an optical filter that allows only light of a desired wavelength λ _o to pass from light of various wavelengths. The child is placed, n _f , n _s ; Equivalent refractive index of two polarization modes of a single polarization fiber d; Length of each single polarization fiber m; Positive integer Minimum loss of light in fiber type polarizer Direction x
As the axis, the principal axis of the n-th single polarization fiber is arranged at an angle θ _n satisfying the following equation and sequentially connected, n; an integer between 1 and N; the number of single-polarization fibers: a fiber-type optical filter in which the fiber-type polarizers are respectively connected to both ends of N single-polarization fibers.