JPS604161Y2 - optical filter - Google Patents

Description

[Detailed Description of the Invention] This invention relates to an optical filter for selecting light of a specific wavelength in optical fiber communication.

Optical fiber communication systems have several superior features compared to conventional communication systems using electric cables, and are therefore expected to be a communication system that can be applied to a variety of fields.

Among them, the optical wavelength division multiplexing communication method, which takes advantage of the fact that optical fiber has low loss over a wide range of light wavelengths and propagates multiple light beams with different wavelengths in a single optical fiber, increases transmission capacity. It is expected that this method will increase the usefulness of optical fiber communication systems in terms of making it easier to install new lines and adding new lines, and research and development is progressing toward practical application.

In order to implement an optical wavelength division multiplexing communication system, several basic optical circuits and optical components are required, including an optical filter for selectively extracting light of a specific wavelength.

This is primarily used in receivers to prevent light from one wavelength channel from leaking into another wavelength channel.

The optical properties of this optical filter include high light transmittance (or reflectance) for wavelengths that are desired to be passed, and a high degree of removal of light at other wavelengths.

Conventionally, as such optical filters, interference filters made of dielectric multilayer films, filters that combine this with colored glass filters that have large absorption at wavelengths other than the wavelength of light that is desired to pass, and the like are known.

In such conventional filters, in order to effectively remove light of wavelengths outside the desired optical wind, that is, to sharpen the filter characteristics, the number of dielectric layers must be increased. It is inevitable that losses due to absorption and scattering will increase, and prices will rise due to increased manufacturing man-hours.

In addition, in principle, it should be possible to make the filter characteristics considerably steeper by increasing the number of layers, but in reality, deviations from the design values such as the thickness of each layer and its refractive index are unavoidable, and the filter characteristics become steeper than a certain point. The problem is that it is difficult to obtain a filter with such characteristics.

Furthermore, when the number of layers is large, the deviation of the transmission wavelength from the set value becomes large due to the above-mentioned deviation, etc., which causes considerable difficulty in manufacturing the filter.

Of course, it is also possible to obtain steep filter characteristics by cascading multiple filters with less steep characteristics, which are relatively easy to manufacture, but in that case, an increase in price due to an increase in the number of parts can be avoided. do not have.

The purpose of this invention is to provide an optical filter that has steep filter characteristics, is easy to manufacture, and is small and economical.

According to this invention, the first and second
has a refractive index distribution that gradually decreases from the central axis toward the periphery in a plane perpendicular to the central axis, and the length in the central axis direction is relative to the meandering pitch P of the light beam. hand(
1/4+172n) P (n is a positive integer including zero), and first and second convergent optical transmission bodies are installed in cascade with their central axes aligned substantially on a straight line; a wavelength-dependent reflective film inserted between the second end face of the first convergent light transmitting body and the first end face of the second convergent light transmitting body; A center point located near at least one of the first end surface of the convergent light transmission body and the second end surface of the second convergence light transmission body, and on the end surface, the central axis intersects with the end surface. An optical waveguide means optically connects two points selected such that they are approximately equidistant from each other and are not located on a straight line passing through the center point. .

The optical filter according to this invention will be described in detail below with reference to the drawings.

First, FIG. 1 shows a perspective view of the first embodiment of this invention.

In the figure, 1 and 2 indicate that the refractive index n (r) in a cross section perpendicular to the central axis A is the distance from the central axis A, r is the refractive index on the central axis A, and a is a positive constant. They are first and second convergent light transmitting bodies, respectively, which have a property that is approximated when the light beams are in contact with each other, and have end surfaces 10, 11 and 12, 13, respectively, which are substantially perpendicular to the central axis A.

Both lengths in the direction of the central axis A are set to about 174, which is the optical beam meandering pitch P determined by the following equation.

In this embodiment, the first and second focusing optical transmitters 1 and 2 are glass round rods with a diameter of 1.8 ribs, which are provided with a refractive index distribution approximately expressed by the formula (1) by ion exchange. was used.

In this case, the refractive index and constant a on the central axis are each n
0=1.63. a=0.12 mm-'', and the meandering pitch of the light beam was about 181 m.

Therefore, the lengths of the first and second convergent light transmitters are set to 4.
It was set to 5 mm.

Between the end face 11 of the first convergent optical transmitter 1 and the end face 12 of the second convergent optical transmitter 2, an interference filter film 3 made of a dielectric multilayer film consisting of alternating layers of ZnS and Mf2 is deposited by vapor deposition. Formed.

As shown in the figure, the direction of the central axis A is set to the Z axis, and the direction X is placed on the end surface 10 of the first convergent optical transmission body 1.

The y-axis is xi on the end surface 13 of the second focusing optical transmission body 2,
Determine the yl axis.

The X axis and the X1 axis, and the y axis and the y1 axis are parallel to each other.

(Xly) on the end surface 10 of the first convergent optical transmission body 1
= (Xly) = (o)
The output optical fibers 22 were respectively installed parallel to the z-axis with their ends close to the positions t-0, 5).

The unit of dimensions on the coordinate axes is kaku, and the same shall apply hereinafter.

Furthermore, (xl
, yl) = (+0.5. 0) and (x"t y')
As an optical waveguide for optically connecting the positions of = (ot +0.5), a coupling optical fiber 23 was installed so that its both ends were parallel to the two axes.

FIG. 2 shows the optical filter configured as described above at X,
A cross-sectional view taken along a plane including l and two axes, Figure 3 is the same <
yty' and a cross-sectional view taken along a plane containing the two axes.

To explain the movement of a light beam using these figures together, first, when a propagating light beam is made to enter the first convergent light transmission body 1 through the input optical fiber 21, it will undulate as it enters the first convergent light transmission body 1.
becomes a meandering light beam C, which enters the interference filter film 3 obliquely, passes therethrough, travels in the second convergent light transmitter 2 while also undulating, and enters the coupling optical fiber 23, becoming a coupled light beam. d.

The combined light beam d enters the second convergent light transmission body 2 through the combined optical fiber 23 at an angle rotated by 90 degrees when viewed from the emitted position, and travels as a second meandering light beam e,
The light enters the interference filter film 3 obliquely, and after passing through there, it travels in the first convergent light transmission body 1 while also winding, and is coupled to the output optical fiber 22, becoming propagating light f.

In this way, since the light beam passes through the interference filter film 3 twice, the spectrum of the light beam emitted from the output optical fiber 22 has components equivalent to those obtained when the light beam passes through two filters with similar characteristics. I understand.

As is well known, in a convergent optical transmission medium, the diameter of the light beam changes periodically at a period of 172 times the meandering pitch P of the light beam.

Therefore, in this embodiment, the sum of the lengths of the first and second convergent optical transmission bodies 1.2 between the input optical fiber 21, the output optical fiber 22, and the coupling optical fiber 23 is approximately the meandering pitch of the optical beam. 172, the beam diameters at the end face 10 of the first convergent light transmitter 1 and the end face 13 of the second convergent light transmitter 2 are approximately equal.

Therefore, the light beam is efficiently coupled from the input optical fiber 21 to the coupling optical fiber 23 and the output optical fiber 22, and an optical filter with a small minimum insertion loss can be obtained.

The interference filter film 3 used here is formed directly on the end face 11 of the first focusing light transmitting body 1 by vapor deposition, but since it is not required to have very steep filter characteristics as a single film, it is not necessary to use normal control. The number of layers may be as long as it does not impair properties, and production is easy and inexpensive.

In the above embodiment, the end of the input optical fiber 21 and one end of the coupling optical fiber 23 are installed in a plane including the y-axis, the X1 axis, and the second axis, and the other end of the coupling optical fiber 23 is and the end of the output optical fiber 22 were placed in a plane including the y-axis, y1-axis, and y-axis.

As a result, the first meandering light beam C and the second meandering light beam e are transmitted to the first and second convergent light transmitters 1 and 2.
Proceed in orthogonal planes inside.

Light with a wavelength other than the wavelength transmitted by the interference filter film 3 is reflected by the interference filter film 3 and becomes a reflected light beam g, but this is not coupled to the output optical fiber 22, which impairs the filter characteristics. Never.

However, the angle formed by the plane through which each of the first and second meandering light beams C and e passes is not limited to 90° as in this embodiment, and the reflected light beam g is directed to the output optical fiber 22. It may be small or, conversely, large, as long as it does not combine.

FIG. 4 is a perspective view of a second embodiment of this invention.

In this embodiment, instead of the coupling optical fiber 23 of the first embodiment, a third convergent optical transmission body 4 having a length of approximately 174 times the meandering pitch P of the light beam in the direction of the central axis A1 is used.
A reflective film 5 is combined on one end surface 15 of the reflector.

To explain this in comparison with the case of the first embodiment, on the end surface 13 of the second convergent optical transmission body 2, the center on the line connecting the positions where both ends of the coupling optical fiber 23 were previously installed. Point o1 ie (Xl.

3/') = (0, 25, 0, 25) point as central axis A1
The third converging light transmitting body 4 has its end face 14 so that
is fixed in close proximity to the end surface 13 of the second convergent optical transmission body 2.

The positional relationship of the first convergent light transmission body 1, the input optical fiber 21, the output optical fiber 22, etc. is almost the same as in the first embodiment.

According to this structure, the first meandering light beam C has a central axis A
Since the light enters the third convergent light transmitter 4 from a position shifted from that of the light beam 1, it also meanderes through it as an incident coupled light beam d□, is further reflected by the reflective film 5, and becomes a reflected light beam. It becomes a combined beam d2 and enters the second convergent light transmission body 2, and becomes a second meandering light beam e.

The length of the third convergent optical transmission body 4 is the optical beam meandering pitch P
174, the length in the central axis N direction through which the incident coupled light beam d1 and the reflected coupled light beams pass back and forth is 1/2 of the light beam meandering pitch P, and the third focusing property The diameter of the light beam at the end face 14 of the optical transmission body 4 is kept the same as that of the incident light.

Therefore, as mentioned before, the first convergent light transmission body 1
Since the diameter of the light beam is maintained between the end face 10 of the second convergent optical transmission body 2 and the end face 13 of the second convergent optical transmission body 2, the light beam is efficiently coupled from the input optical fiber 21 to the output optical fiber 22, and the minimum insertion An optical filter with low loss can be obtained.

This invention can be modified in several ways in addition to the embodiments described above.

For example, in the above embodiment, the light beam passes through the interference filter film 3 twice, but in order to obtain even steeper filter characteristics, the light beam may pass through the filter film many times. It's okay.

To do this, move the other end of the output optical fiber 22 at the end surface 10 of the first convergent optical transmission body 1 to a position other than the y-axis and the y-axis of the end surface 10 and 0.5 mm away from the central axis. By coupling the end of another optical fiber to the corresponding position on the end face 13 of the second converging light transmission body 2, the light beam can pass through the interference filter film 3 three times. can.

Furthermore, in the above embodiments, an optical fiber, a convergent optical transmission body, etc. were used as the optical waveguide means for folding back the light beam, but the invention is not limited to these, and an optical waveguide using a dielectric layer, etc. A small prism that utilizes total reflection can also be used.

Furthermore, as the interference filter film, in addition to the central transmission type, a short wavelength transmission type or a long wavelength transmission type filter film can be used.

The input optical fiber 21 and the output optical fiber 22 are not necessarily required, and a direct light source and a light receiver may be installed in close proximity.

The length of the convergent optical transmission body is 174 of the optical beam meandering pitch P.
The length is not limited to , but may be an odd multiple of the length, or even if the length is slightly shorter than these lengths, it can be used without any problem by adjusting the optical fiber position.

As is clear from the above description, according to this invention, since the main component is a small converging light transmission body, the entire filter is small and can be manufactured at low cost.

In addition, since the light beam can be passed through the filter film inserted between the focusing light transmitters via the optical waveguide over and over again by changing the angle, even if the filter film itself does not have very good characteristics, the results can be improved. In other words, it has a great effect in that steep filter characteristics can be easily obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the first embodiment of this invention, FIG. 2 is a cross-sectional view of a plane including x, xi, and two axes in FIG. 1, and FIG. , yl and z axes, and FIG. 4 is a perspective view showing a second embodiment of this invention. In the figure, 1.2.4 is a focusing optical transmitter, 3 is an interference filter film, 5 is a reflective film, 10 to 15 are end faces, and 21 to 23
is an optical fiber.

Claims (1)

[Scope of utility model registration request] has first and second end surfaces substantially perpendicular to the central axis, has a refractive index distribution that gradually decreases from the central axis toward the periphery in a plane perpendicular to the central axis, and has a refractive index distribution that gradually decreases from the central axis toward the periphery; The length is (1/4+172n) with respect to the meandering pitch P of the light beam.
P (n is a positive integer including zero) and are installed in series with their central axes aligned substantially in a straight line; a wavelength-dependent reflective film inserted between the second end face of the convergent light transmission body and the first end face of the second convergence light transmission body; and the first convergent light transmission body. installed in the vicinity of at least one of the first end surface of the body and the second end surface of the second convergent light transmission body, and approximately relative to the center point on the end surface where the central axis intersects with the end surface. 1. An optical waveguide that optically connects two points that are equidistant and that are selected so that they are not located on a straight line passing through their center points.