JPH0766086B2 - Fiber type optical filter - Google Patents

Description

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

Conventionally, a fiber-free Perot plate has been known as an optical filter. The Fabry Perot plate is silvered on both sides of a thin layer of MgF ₂ (magnesium fluoride), and has an energy reflectance of 95%.
When the thickness of MgF ₂ is set to about 1/4 of the desired wavelength λ ₀ , a filter having a wavelength width of transmitted light of about 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. 1 to 5 are fiber type polarizers, 6 to 9 are single polarization fibers, 10 to 18 are cores, and 19 to 22 are B ₂ O ₃ or B. ₂ O ₃ —GeO ₂ added with SiO _{2 is} a stress applying portion, and 23 to 27 are metals (for example, Al). Regarding the single polarization fiber, Hosaka et al.
olarization fibers with asymmetric strain birefrin
gence "(Electron Lett., vol.17, No.15, P.530, 1981). Further, regarding a fiber type polarizer, Japanese Patent Application No. 56-208048" Faiba type "by Hosaka et al. A method for manufacturing a polarizer ”is described in detail.

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 filter 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 a 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 in which the absorption loss in the fiber type polarizer is the minimum (the direction parallel to the metal surface) is the x-axis, and the main axis X and the x-axis of the N single polarization fibers The angle is π / 4 (ra
In d), the fiber-type polarizer and the single-polarization fiber are connected alternately, and the fiber-type polarizer is placed at both ends. The length l of the n-th single-polarization fiber of N single-polarization fibers is given by l = 2 ^n-1 d (n = 1 to N) (5) (d; Length). At this time, the electric field component {E _xN , E _yN } emitted from the fiber type polarizer 5 is given by the following equation, where the electric field component incident on the fiber type polarizer 1 is {E _x0 , E _y0 }.

However, Is. Equation (6) is after some calculation Is rewritten as From the equation (9), the transmittance of the optical filter of the present invention is Becomes

Equation (10) can be further simplified, Becomes [References for Formulas (6) to (11) above:
BHBillings, “A Tunable Narrow−Band Optical Filt
er ", J.Opt.Soc.Am., vol.37, No.10, pp.738-746,1947] Fig. 3 shows the number of single polarization fibers N = 6, the minimum single polarization fiber. Length d = 6 cm, mode birefringence B = (n _f −n
_s ) = 1.3 × 10 ⁻⁴ , where T _L is the plot for wavelength λ.

As can be seen from FIG. 3, 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 = 11, Δλ <1Å, when N = 8, Δλ = 7.5Å, and when N = 5, Δλ = 60Å.

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, the characteristic does not change even when bent up to a bending radius of about 5 cm. It has the great feature of being highly flexible.

Further, since the fiber type optical filter of the present invention can be easily connected to the optical fiber, it has a great advantage when used for wavelength division multiplexing optical communication.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing coordinate conversion, FIG. 3 is a diagram showing wavelength characteristics of transmittance of a fiber type optical filter of the present invention, and FIG. 4 is a diagram showing the present invention. It is a figure which shows the relationship between the half value width (DELTA) (lambda) of a fiber type optical filter, and the number N of the single polarization fiber. 1 to 5 ... fiber type polarizer, 6 to 9 ... single polarization fiber, 10 to 18 ... core, 19 to 22 ... stress applying part, 23 to 27
……metal.

Claims (1)

[Claims] 1. A single-polarization fiber and a fiber-type polarization in which the propagation axes of the light are made coincident and satisfy the following expression in an optical filter that allows only the light of a desired wavelength λ ₀ to pass through among lights of various wavelengths. The children are staggered, n _f , n _s ; Equivalent refractive index of two polarization modes of a single polarization fiber d; Shortest single polarization fiber length m; Positive integer Most loss of light in fiber type polarizer X in small direction
The axis of each single polarization fiber is π with respect to the x-axis.
It is inclined by / 4 (rad), the length l of the n-th single polarization fiber satisfies the following equation, and l = 2 ^n-1 dn; an integer N between 1 and N; single polarization Number of fibers A fiber type optical filter characterized in that a single polarization fiber and a fiber type polarizer are alternately connected to each other to form a fiber type polarizer.