JP2001033638A - Optical fiber type wavelength filter and wavelength period adjustment method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low-cost optical fiber type wavelength filter capable of realizing a low insertion loss, and to provide a wavelength period adjustment method of the optical fiber type wavelength filter capable of easily adjusting the wavelength period. SOLUTION: A twin core optical fiber 4 is provided with a first core and second cores having refractive index distribution allowing propagation of single- mode light in a nearly central part and its peripheral part in a sectional view of a section perpendicular to the longitudinal direction. In the twin core optical fiber 4, two extension parts 14, 16, (i.e., optical coupling parts) generating optical coupling between the first core and the second cores apart from each other in the longitudinal direction are formed. The twin core fiber 4 is bent in the longitudinal direction between the extension parts 14, 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber type wavelength filter and a wavelength period adjusting method thereof, and more particularly, to an inexpensive optical fiber type wavelength filter which can realize low insertion loss and easy adjustment of the wavelength period, and its wavelength. It relates to a period adjustment method.

[0002]

2. Description of the Related Art In recent years, a wavelength division multiplexing transmission system has been used in the field of optical communication, and accordingly, optical components for various uses have been developed. Above all, optical filter elements are used for blocking light of unnecessary wavelengths, for flattening the wavelength of amplification characteristics of optical fiber amplifiers, and the like, and their applications are further expanding. A Fabry-Perot interference filter using an etalon has been conventionally used as an optical filter element. This is to adjust the spacing between the reflectors and the reflectance so that a specific wavelength does not pass through, by repeatedly reflecting the light to be split between the reflectors with a plurality of reflectors facing each other, causing interference fringes. Thus, the characteristics as a filter are obtained.

[0003]

However, this Fabry-Perot interference filter requires a plurality of reflectors to be manufactured and the intervals and reflectance of these reflectors must be adjusted, so that the manufacturing process is complicated and the price is high. Have a problem. In addition, since the propagation light is once taken out of the optical fiber and transmitted through the Fabry-Perot interference filter, and then coupled back to the optical fiber, the insertion loss tends to increase.

[0004] An optical fiber type wavelength filter and a wavelength period adjusting method of the present invention have been made in view of the above circumstances, and have the following objects. That is, an object of the present invention is to provide an inexpensive optical fiber type wavelength filter capable of realizing low insertion loss. Still another object of the present invention is to provide a method of adjusting the wavelength cycle of an optical fiber type wavelength filter that can easily adjust the wavelength cycle.

[0005]

An optical fiber type wavelength filter and a wavelength period adjusting method of the present invention employ the following means in order to solve the above-mentioned problems. That is, in the optical fiber type wavelength filter according to the first aspect, when a plurality of cores having a refractive index distribution capable of transmitting single-mode light are viewed in a cross section perpendicular to the longitudinal direction, a substantially central portion and a periphery thereof. In the optical fiber provided in the portion, the optical coupling portion that generates light coupling between the respective cores, a plurality in the length direction of the optical fiber, is an optical fiber type wavelength filter formed intermittently, A bend is provided between the optical coupling portions in a length direction of the optical fiber.

According to the optical fiber type wavelength filter of the first aspect, since it can be constituted by one member (optical fiber), the number of parts can be reduced. In addition, since the filter is an optical fiber type filter, it can be directly connected to an optical fiber for transmission. Further, since the optical path length difference can be generated between the cores only by adjusting the bending direction and the curvature between the optical coupling portions, the wavelength period can be easily adjusted.

According to a second aspect of the present invention, in the optical fiber type wavelength filter according to the first aspect, a bent portion between the optical coupling portions is fixed so as to maintain the bending direction and the curvature. It is characterized by having.

According to the optical fiber type wavelength filter according to the second aspect of the present invention, a configuration is employed in which the bent portion between the optical coupling portions is fixed so as to maintain the bending direction and the curvature constant. The period is kept constant.

According to a third aspect of the present invention, there is provided a method for adjusting a wavelength period of an optical fiber type wavelength filter, wherein a plurality of cores having a refractive index distribution capable of transmitting single mode light are viewed in a cross section perpendicular to the length direction. In an optical fiber provided at a substantially central portion and a peripheral portion thereof, a plurality of optical coupling portions for generating light coupling between the respective cores are provided in the length direction of the optical fiber,
In the wavelength cycle adjusting method for an optical fiber type wavelength filter for adjusting the wavelength cycle of an optical fiber type wavelength filter formed intermittently, a bending in a length direction of the optical fiber is added between the respective optical coupling portions. The wavelength period is adjusted by adjusting a bending direction and a curvature.

According to the wavelength period adjusting method of the optical fiber type wavelength filter according to the third aspect, an arbitrary optical path length difference is generated between the cores only by adjusting the bending direction and the curvature between the optical coupling portions. Therefore, the wavelength period can be easily adjusted.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION As described below, an optical fiber type wavelength filter according to the present invention has a plurality of cores having a refractive index distribution capable of transmitting single mode light and having a cross section perpendicular to the longitudinal direction. A plurality of optical coupling portions for generating light coupling between the cores are formed intermittently in the length direction of the optical fiber in the optical fibers provided in the substantially central portion and the peripheral portion when viewed. In addition, the optical fiber is characterized in that a bend in the length direction of the optical fiber is provided between the optical coupling portions. Hereinafter, an embodiment of an optical fiber type wavelength filter of the present invention will be described with reference to the drawings.

As shown in the perspective view of FIG. 1 and the cross-sectional view of FIG. 2, the optical fiber type wavelength filter of the present embodiment has a structure of a communication single mode optical fiber, that is, a cladding 1 having a flat refractive index distribution. A twin-core optical fiber in which a first core 2 having an increased refractive index is disposed in a central portion, and a second core 3 having an increased refractive index is also disposed in a peripheral portion thereof. It is. A plurality of second cores 3 may be arranged in the peripheral region, but for convenience of explanation, the twin-core optical fiber 4 in which only one second core 3 is arranged is referred to as an optical fiber wavelength type. The filter will be described.

The refractive index distribution of the second core 3 may be the same as or different from the refractive index distribution of the first core 2. However, the second core 3 is substantially affected by light propagating through the first core 2 in a length (for example, several meters or less, at most several tens of meters) used as an optical fiber component. It is important to place them at such a distance. That is, if the distance between the first core 2 and the second core 3 is too short, part of the light propagating through the first core 2 will be coupled to the second core 3. However, if the difference between the propagation constants of the first core 2 and the second core 3 is large, light coupling hardly occurs.
The distance between the second core 3 and the second core 3 may be relatively small.

A description will be given below in consideration of a case where a normal single mode optical fiber (not shown) is connected to both ends of the twin core optical fiber 4 described above, and light is incident from one of the single mode optical fibers. to continue. The light incident on one single-mode optical fiber propagates through a central core (not shown) of the single-mode optical fiber and is incident on the first core 2 of the twin-core optical fiber 4. The light passes through the twin-core optical fiber 4 as it is and is emitted from the opposite end face to the other single-mode optical fiber (not shown). The twin-core optical fiber 4 in this state is the same as a normal single-mode optical fiber, and the second core does not function at all and is in the same state as not existing.

When a part of the twin-core optical fiber 4 is heated to a temperature of a few hundred degrees above the softening point and a tensile stress is applied, the twin-core optical fiber 4 is elongated in its length direction and its outer diameter increases in the length direction. Squeezed shape (tapered shape) that gradually becomes smaller
Deform to. As shown in FIG. 2, the central portion of the tapered shape has a smaller outer diameter (the outer diameter d1 of the clad 1).
The outer diameter d2 of the first core 2 and the outer diameter d3 of the second core 3 also become smaller, and the distance L1 between the first core 2 and the second core 3 becomes smaller. The light becomes narrower, and the light propagating through the first core 2 is coupled to the second core 3. After the optical coupling section (downstream in the light propagation direction), the propagation light is divided into both the first core 2 and the second core 3 and propagates independently.

As shown in FIG. 1, when two such optical coupling portions 5 and 6 are formed in succession, the light split by the first optical coupling portion 5 is regenerated by the second optical coupling portion 6. Because it combines
Light interference occurs. This structure is the same as the Mach-Zehnder interference system. FIG. 3 schematically shows the structure of the Mach-Zehnder interference system. Two waveguides 7, 8 are a first 3dB coupler 9 and a second 3dB
Coupled by a coupler 10. The two wavelengths λ1 and λ2 propagating in the waveguides 7 and 8 are branched by the first 3 dB coupler 9 and travel through the two waveguides 7 and 8 separately. The light propagating through the respective waveguides 7 and 8 is converted into the second 3
The amount of light that interferes with the dB coupler 10 and is branched into each of the first core 2 and the second core 3 after coupling is different due to the phase difference of the interference light. By adjusting the phase difference of each interference light for the wavelengths λ1 and λ2,
It can be separated as shown in FIG.

FIG. 4 shows the phase difference (horizontal axis) at the time of coupling at the second optical coupling unit 6 when the electric field intensity is split at the first optical coupling unit 5 so that the electric field intensity is exactly half, and (The intensity on the vertical axis). When the phase difference is an even multiple of π, the power is increased to double the power, but when the phase difference is an odd multiple, the power becomes zero due to cancellation. First
By using the fact that the phase difference due to the wavelength changes in proportion to the difference between the propagation constants of the core 2 and the second core 3, the wavelength to be removed is coupled to the second core 3, whereby the first The light propagating through the core 2 can be provided with filter characteristics.

Adjustment of the propagation constant difference between the first core 2 and the second core 3 and the distance between the two melt-stretched optical coupling portions 5 and 6 to adjust the period of the phase change. Alternatively, as a method of changing the propagation constant without changing the profile of the twin-core optical fiber 4, a method of bending the twin-core optical fiber 4 to give an optical path length difference to light propagating through the two cores is known. is there. When this method is used, the period of the phase change can be obtained by changing the bending state (bending direction and curvature) of the twin-core optical fiber 4. If the twin-core optical fiber 4 has no difference in the propagation constant between the first core 2 and the second core 3, the phase change period is the distance between the two melt-stretched optical coupling parts 5, 6. And is determined only by the bending state of the twin-core optical fiber 4.

In the case of a twin-core optical fiber 4 having a propagation constant difference between the first core 2 and the second core 3, the propagation constant difference between the first core 2 and the second core 3 and the melt-stretched 2 The phase difference is determined by the sum of the inherent phase difference determined by the distance between the two optical coupling portions 5 and 6 and the phase difference caused by the optical path length difference between the first core 2 and the second core 3 due to the bending of the twin-core optical fiber 4. The period of the change is determined. Even in this case, the period of the arbitrary phase change can be obtained by changing the bending state of the twin-core optical fiber 4.

Here, the period of the phase change depending on the wavelength when the twin core optical fiber 4 having the same propagation constant of the first core 2 and the second core 3 is bent is calculated. First
The optical path length difference between the second core 3 and the second core 3 is the second core 3
Is the difference between the actual optical path length caused by passing through the inside or outside of the bent twin-core optical fiber 4 and the effect of the change in the refractive index of the second core 3 due to the photoelastic effect caused by the bending distortion of the twin-core optical fiber 4. It will be combined. The phase change period △ λ at this time is calculated by the following equation (1) when the twin-core optical fiber 4 is bent by making a circular loop R times.

[0021]

(Equation 1)

As shown in FIGS. 5 and 6, this equation (1)
Is a straight line 1 included in a circle formed by the bent twin-core optical fiber 4 and passing through the axis of the first core 2 in a cross section 4a perpendicular to the length direction of the twin-core optical fiber 4.
1a and an angle formed by a straight line 11b drawn from the axis of the first core 2 to the axis of the second core 3 (provided that the inner peripheral side of the bent twin-core optical fiber 4 is a starting point). The angle is 0 degree when the second core 3 is inside the circumference formed by the circle with respect to the first core 2 and 180 degrees when it is outside.

FIG. 7 shows, for example, Δr = 34 μm, A = −
The relationship between Δλ and θ in the case of 0.32, n = 1.46, λ = 1.5 μm, and R = 1 is shown. Δλ is 6n depending on θ
It is possible to take an arbitrary value of m or more. Δλ of 6 nm or less can be easily obtained by increasing the number of times of R. After changing the bending state of the twin-core optical fiber 4 (curvature, ie, the number of times of R, and bending direction, ie, θ), a desired wavelength characteristic is obtained, the outer peripheral portion of the twin-core optical fiber 4 (the outer peripheral portion of the clad 4) is bonded. By fixing with the agent and fixing the bending state,
A wavelength filter that is stable with respect to the surrounding environment can be manufactured. The loss peak value of the filter becomes maximum (ideally infinity) when the coupling degree of the optical coupling units 5 and 6 is 3 dB. However, by shifting the coupling degree from 3 dB, the loss peak value becomes an arbitrary value. It is possible to adjust.

According to the optical fiber type wavelength filter of the present embodiment, the twin-core optical fiber 4 in which the first core 2 is provided substantially in the center in cross section and the second core 3 is provided in the peripheral portion thereof, A plurality of optical coupling portions 5 and 6 in the length direction
Can be constituted by one member (only the twin-core optical fiber 4), so that the number of parts is small and the manufacturing cost can be reduced as compared with the conventional case. Furthermore, since it is an optical fiber type filter, it can be directly connected to a transmission optical fiber, and it is also possible to realize low insertion loss of propagation light.
Further, a twin-core optical fiber 4 is provided between each of the optical coupling portions 5 and 6.
By adopting a configuration in which bending is performed in the longitudinal direction, it is possible to easily adjust the wavelength period simply by adjusting the curvature of the bending and the bending direction.

Further, according to the optical fiber type wavelength filter of the present embodiment, the bent portion between the optical coupling portions 5 and 6 is fixed with an adhesive or the like so as to keep the curvature and the bending direction constant. By adopting, the curvature and the bending direction that affect the wavelength period can be kept constant without changing,
It is possible to maintain a stable wavelength cycle that is hardly affected by the surrounding environment.

Further, according to the method of adjusting the wavelength period of the optical fiber type wavelength filter of the present embodiment, since the curvature and the bending direction of the bending between the optical coupling portions 5 and 6 are merely adjusted, the wavelength period can be easily adjusted. Can be performed.

In the above embodiment, the twin core optical fiber 4 having a single second core 3 has been described as an example. However, the present invention is not limited to this, and two or more second cores 3 are provided. A configuration may be adopted.

[Example] A twin-core optical fiber 4 was prepared in which the first core 2 and the second core 3 were arranged at the positions shown in FIG. The characteristics of the twin-core optical fiber 4 are as shown in Table 1 below. That is, the outer diameter d1 of the clad 1 is 125.5 μm, the outer diameter d2 of the first core 2 is 9.7 μm, the relative refractive index of the first core is 0.33%, and the second core is Outer diameter d3 of core
Is 9.6 μm, and the relative refractive index of the second core is 0.35
%. The distance L2 from the axis of the first core 2 to the outer edge of the clad 1 is 62.6 μm, and the center-to-center distance L1 between the first core 2 and the second core 3 is 34.1 μm.

[0029]

[Table 1]

The twin-core optical fiber 4 is cut into a length of 2 m, and an acrylic coating resin near the center is cut to a length of about 20 m.
m. While the center of the exposed clad 4 was heated until it was melted using a gas burner, the twin core optical fiber 4 was gently stretched in the longitudinal direction. At this time, as shown in FIG. 8, general single-mode optical fibers 12 and 13 are fusion-spliced to both ends of the twin-core optical fiber 4 by 2 m each (connection points a,
b) For two open ends, a wavelength of 1.55 from one
A superluminescent laser beam emitting near μm was incident, and the other side was connected to an optical spectrum analyzer (not shown) to monitor the spectrum of the amount of light passing therethrough. Does not form a loop and remains in a straight line).

At a wavelength of 1.55 μm at a wavelength of 1.55 μm, the stretching was terminated when the value decreased by 3 dB from the initial value. The extending part 14 was fixed to a quartz reinforcing device 15 by bonding. Then, at a position 50 cm away from the stretching section 14 toward the injection side, the coating resin is removed in the same manner and stretching is performed.
6 was formed. That is, the spectrum of the amount of transmitted light changes with stretching, and the stretching is terminated when the amount of transmitted light at a wavelength of 1.55 μm becomes about 4 dB. Thereafter, similarly to the extending section 14, the extending section 16 was fixed to a quartz reinforcing device 17 by bonding.

Next, as shown in FIG.
Was formed, and the wavelength dependence of the transmission loss was measured with a spectrum analyzer (see FIGS. 9 to 11). When a trial of loop creation for the same sample was repeated several times to create various bending states of the twin-core optical fiber 4, one was obtained in a wavelength range around 1.5 μm.
A wavelength period in the range of about 0 nm to 100 nm could be obtained. Further, by increasing the number of loops to two, a shorter wavelength period of about 4 nm could be obtained.

[0033]

According to the optical fiber type wavelength filter according to the first aspect of the present invention, an optical fiber having a core provided at a substantially central portion and a peripheral portion thereof in a sectional view is provided in a longitudinal direction thereof. Since a plurality of optical coupling portions are formed intermittently, it can be constituted by one member (optical fiber).
Since the number of components is small, the manufacturing cost can be reduced as compared with the conventional case. Furthermore, since it is an optical fiber type filter, it can be directly connected to a transmission optical fiber, and it is also possible to realize low insertion loss of propagation light. In addition, by adopting a configuration that bends the optical fiber in the length direction between the optical coupling portions, the wavelength period can be easily adjusted simply by adjusting the curvature and the bending direction of the bend. Becomes possible.

Further, according to the optical fiber type wavelength filter according to the second aspect of the present invention, a configuration is employed in which the bent portion between the optical coupling portions is fixed so as to maintain the curvature and the bending direction constant. Since the curvature and the bending direction which influence the wavelength cycle are kept constant without changing, it is possible to maintain a stable wavelength cycle which is hardly affected by the surrounding environment.

According to the method of adjusting the wavelength period of the optical fiber type wavelength filter according to the third aspect of the present invention, since only the curvature and the bending direction of the bending between the optical coupling portions are adjusted, the adjustment of the wavelength period can be easily performed. It is possible to do.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a twin-core optical fiber which is an optical fiber type wavelength filter of the present invention.

FIG. 2 is a diagram showing the twin-core optical fiber,
It is sectional drawing in the cross section perpendicular | vertical to a length direction.

FIG. 3 is a diagram schematically illustrating a Mach-Zehnder interference system, and is a schematic diagram.

FIG. 4 is a graph showing a spectrum of a transmitted light amount obtained at the time of manufacturing the twin-core optical fiber of the present invention.

FIG. 5 is a view showing the twin-core optical fiber,
It is sectional drawing in the cross section perpendicular | vertical to a length direction.

FIG. 6 is a view showing the twin-core optical fiber,
It is sectional drawing in the cross section perpendicular | vertical to a length direction.

FIG. 7: Δr = 3 in the twin-core optical fiber
4 μm, A = −0.32, n = 1.46, λ = 1.5 μ
9 is a graph showing the calculation results of △ λ and θ when m and R = 1.

FIG. 8 is a diagram showing a connection state between the twin-core optical fiber and a single-mode optical fiber in the embodiment.

FIG. 9 is a graph showing measurement results obtained by forming a loop in a twin-core optical fiber and measuring the wavelength dependence of transmission loss using a spectrum analyzer.

FIG. 10: A loop is formed in a twin-core optical fiber,
6 is a graph showing measurement results obtained by measuring the wavelength dependence of transmission loss with a spectrum analyzer.

FIG. 11 A loop is formed in a twin-core optical fiber,
6 is a graph showing measurement results obtained by measuring the wavelength dependence of transmission loss with a spectrum analyzer.

[Explanation of symbols]

2, 3, 1st core, 2nd core (core), 4 ... twin core optical fiber (optical fiber, optical fiber type wavelength filter), 5, 6 ... optical coupling section

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Tetsuya Sakai 1440 Mutsuzaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Office (72) Inventor Kenji Nishiide 1440 Mukurosaki, Sakura City, Chiba Prefecture Inside Fujikura Sakura Office (72) Inventor Ryozo Yamauchi 1440 Mutsuzaki, Sakura City, Chiba Prefecture F-term in Fujikura Sakura Works (reference) 2H050 AA07 AC69 AC84 AD16

Claims (3)

[Claims] A plurality of cores (2, 3) having a refractive index distribution capable of transmitting single mode light are provided in a substantially central portion and a peripheral portion thereof when viewed in a cross section perpendicular to the length direction. The optical fiber (4) is provided with a plurality of optical coupling portions (5, 6) for generating light coupling between the respective cores, the plurality of optical coupling portions being intermittently formed in the length direction of the optical fiber. An optical fiber type wavelength filter, wherein the filter (4) is provided with a bend between the optical coupling portions in a length direction of the optical fiber. 2. The optical fiber type wavelength filter according to claim 1, wherein a bent portion between the respective optical coupling portions is fixed so as to maintain a bending direction and a curvature. Wavelength filter. 3. A plurality of cores having a refractive index distribution capable of propagating single mode light are provided on an optical fiber provided at a substantially central portion and a peripheral portion thereof when viewed in a cross section perpendicular to the length direction. An optical coupling unit that generates light coupling between the cores, a plurality of optical coupling units in the length direction of the optical fiber, an optical fiber type wavelength filter that adjusts the wavelength period of the optical fiber type wavelength filter formed intermittently; In the wavelength period adjusting method, the wavelength period is adjusted by bending the optical fiber in the longitudinal direction between the optical coupling portions and adjusting the bending direction or the curvature or both. A wavelength period adjusting method for an optical fiber type wavelength filter characterized by the above-mentioned.