JPH09178970A - Interference filter module - Google Patents

Abstract

(57) Abstract: It is intended to prevent the insertion loss polarization dependence and the polarization dispersion from increasing while improving the light blocking characteristics. SOLUTION: A plurality of interference filters are arranged in series inclining with respect to an optical axis, and as many pairs of the interference filters as possible are arranged so that two interference filters forming a pair are arranged orthogonally. It is a combined interference filter module. As a typical example, two interference filters 10a and 10b inclined by an angle θ with respect to the optical axis are arranged in series, and both interference filters are orthogonal to each other when the inclination direction with respect to the optical axis is viewed along the optical axis. There is a configuration that is combined in such a positional relationship. Fiber collimators 12a and 12b are provided before and after both interference filters.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference filter module in which a plurality of interference filters are arranged in series,
More specifically, the present invention relates to an interference filter module having a good cutoff characteristic and capable of reducing insertion loss polarization dependency and polarization dispersion by arranging two interference filters forming a pair at right angles. This interference filter module is useful as a filter device used in the field of, for example, wavelength division multiplexing optical communication.

[0002]

2. Description of the Related Art As is well known, a filter is an optical component having a characteristic of transmitting light of a certain wavelength but blocking light of another wavelength, and is widely used in the field of optical communication.
There are many types of filters, one of which is an interference filter. The interference filter is a filter that uses the interference of light generated by an interference film (thin film layer), such as the thickness of the film formed on the substrate, the refractive index of the film forming substance, the number of layers of the film, and the thickness of each layer. By combining them, a band pass filter (BPF), a short wavelength band pass filter (SWPF), a long wavelength band pass filter (LWPF), etc. can be manufactured. Generally, a dielectric multilayer film or the like is used as the interference film, and a thin film having a high refractive index (for example, TiO ₂ ) and a thin film having a low refractive index (for example, S
For example, a structure in which iO ₂ ) is alternately deposited on a glass substrate over several tens of layers is adopted.

The interference filter module has such a structure that such an interference filter is tilted by several degrees with respect to the optical axis, and a fiber collimator is arranged before and after the interference filter. The interference filter is installed so as to be tilted in order to prevent reflected light from the filter surface or the like from returning to the input side.

[0004]

Due to technological advances such as improvement of the amplification factor of optical amplifiers, there are multiple wavelengths used for transmission in optical communication. Along with this, while transmitting light of a certain wavelength, transmission wavelengths of other wavelengths are transmitted. It is important to improve the light blocking property. However, with the conventional configuration using one interference filter as described above, sufficient cutoff characteristics cannot be obtained.

In order to improve the cutoff characteristic of the filter, it is possible to arrange a plurality of interference filters in series. However, the interference filter tilted with respect to the optical axis
It has an insertion loss polarization dependency (difference in light intensity between P wave and S wave), and causes polarization dispersion (difference in propagation velocity between P wave and S wave).
Therefore, if a plurality of interference filters are simply arranged in parallel, the cutoff characteristic is improved, but the insertion loss polarization dependency is increased and the polarization dispersion is also increased by the number of arranged interference filters.

An object of the present invention is to provide an interference filter module which improves the cutoff characteristic of light and does not increase the insertion loss polarization dependence and polarization dispersion.

[0007]

SUMMARY OF THE INVENTION The present invention is an interference filter module in which a plurality of interference filters are arranged in series at an angle with respect to the optical axis. Here, a plurality of interference filters are combined as many pairs as possible, and the two interference filters forming a pair are combined so as to be orthogonally arranged. The orthogonal arrangement means a positional relationship in which the inclination directions of the interference filter with respect to the optical axis are orthogonal to each other when viewed in the direction along the optical axis.

A typical example is a case where two interference filters of the same type are arranged in series. In that case, both interference filters, when the tilt direction with respect to the optical axis is viewed along the optical axis,
They are combined in a positional relationship such that they are orthogonal to each other. Specifically, a casing having a through hole along the longitudinal direction and having a filter holder mounting hole so as to be orthogonal to the through hole, and a first interference filter fixed to the inclined end face, the through hole of the casing. A first filter holder mounted inside and a second filter holder fixed to a side surface of the second interference filter and mounted in a filter holder mounting hole of the housing are provided at both ends of the through hole. Each can be manufactured by mounting a fiber collimator.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION When a plurality of interference filters are arranged in series, the cutoff characteristic is improved in proportion to the number of used filters.
However, when light is incident on the interference filter tilted with respect to the optical axis, the transmission loss and the transmission propagation speed of the P-polarized light and the S-polarized light are slightly different from each other, resulting in insertion loss polarization dependence and polarization dispersion.
Therefore, for example, if two interference filters are arranged in parallel,
The difference in transmission loss between P-polarized light and S-polarized light and the difference in transmission propagation velocity are doubled, and the insertion loss polarization dependency and polarization dispersion are increased. However, when two interference filters forming a pair are arranged orthogonally, P-polarized light that passes through one interference filter becomes S-polarized light at the other interference filter, and light that passes through one polarization filter as S-polarized light occurs at the other interference filter. Since it becomes P-polarized light, the insertion loss polarization dependency and the polarization dispersion are canceled by the pair of interference filters, and only the cut-off characteristic is improved.

From this, if an even number of interference filters are used, the insertion loss polarization dependence and the polarization dispersion can be almost completely canceled by arranging two filters in pairs and orthogonally arranging them. Further, even when an odd number of interference filters are used, only insertion loss polarization dependence and polarization dispersion that are the same as when one interference filter is used are generated, and the cutoff characteristic is significantly improved. .

[0011]

1 is a conceptual diagram showing an embodiment of an interference filter module according to the present invention, which is the simplest example in which two interference filters are arranged orthogonally. For convenience of explanation, coordinate axes are assumed as shown. That is, the z-axis is taken in the optical axis direction, the horizontal x-axis is perpendicular to it, and the vertical y is y-axis.
Take the axis. Two interference filters 10a and 10b inclined by an angle θ with respect to the optical axis (z axis) are arranged in series on the optical axis.
The first interference filter 10a uses the x axis as the center of rotation for x
The y-plane is tilted by an angle θ (the tilt direction is indicated by arrow A). The second interference filter 10b is
It is in a state in which the xy plane is tilted by an angle θ with the y axis as the center of rotation (the tilt direction is indicated by arrow B). Thus, the interference filters 10a and 10b are combined so as to be orthogonal to each other when the tilt directions (arrow A and arrow B) with respect to the optical axis (z axis) are viewed along the optical axis (z axis). It will be. This is the orthogonal arrangement. Then, the fiber collimators 12a and 12b are arranged before and after the both interference filters 10a and 10b.

Both interference filters 10a, 1 used here
0b has the same structure, for example, TiO ₂ on a glass substrate.
The thin film and the SiO ₂ thin film are alternately deposited over 30 to 40 layers. Each interference filter 10a, 1
The inclination angle θ of 0b with respect to the optical axis (z axis) is several degrees (for example, 4 °
~ 8 degrees).

FIG. 2 shows a comparison result of insertion loss characteristics between the prior art using only one interference filter and the module of the present invention in which two interference filters are combined as described above. The used center wavelength of the interference filter used is 1537
It is a band pass filter (BPF) having a half width of 3 nm in nm.
In the prior art, the cutoff characteristic at a wavelength deviated from the central wavelength by 2 nm is only about 7 dB. On the other hand, in the configuration of the present invention, the cutoff characteristic at the wavelength deviated from the central wavelength by 2 nm is 14
It is about dB, and it can be seen that the cutoff characteristic is improved to about double in proportion to the number of used interference filters being doubled.

In the case of the conventional one interference filter structure, the insertion loss polarization dependence is about 0.05 dB, and the polarization dispersion is about 0.04 ps (picosecond). On the other hand, in the configuration of the above-described embodiment, the insertion loss polarization dependency and the polarization dispersion are almost eliminated even though the two interference filters are used, and the adverse effect of the one interference filter arranged at an angle is reduced. Could be canceled by the interference filter.
If two interference filters are simply arranged in parallel,
The insertion loss polarization dependence is about 0.1 dB, and the polarization dispersion is about 0.
It was 08 ps. From this, it can be seen that the interference filter module of the present invention has improved cut-off characteristics, yet can minimize the adverse effects of insertion loss polarization dependence and polarization dispersion.

FIG. 3 is an assembly explanatory view showing an embodiment of the interference filter module according to the present invention, and FIG. 4 is a vertical sectional view showing a state after assembly. Here, the housing 2 has a through hole 20 in the longitudinal direction (optical axis direction) and a filter holder mounting hole 22 provided so as to be orthogonal to the through hole 20.
4 is used. The filter holder attachment hole 22 may have a penetrating structure or a non-penetrating structure. The housing 24 may be, for example, a processed stainless steel block or the like. A first filter holder 26 having a cylindrical shape with an outer diameter to be fitted into the through hole 20 of the housing 24 and having one end surface cut obliquely at an angle θ is prepared. Then, the first interference filter 10a is attached to the oblique end face. Also, the filter holder mounting hole 22
A second filter holder 32 having an outer diameter that fits into the inner surface of the second filter holder and formed with a cutout surface 28 for mounting a filter on the side surface and a light transmission hole 30 formed in the direction perpendicular to the cutout surface 28. prepare. Then, the second interference filter 10b is attached to the cutout surface 28 on the side surface.

Then, the first filter holder 26 is fitted into the through hole 20 and fixed at a predetermined position, and the second filter holder 32 is fitted into the filter holder mounting hole 22 and fixed at a predetermined position. Each filter holder 2
6, 32 has a groove 34 for rotational alignment on the end surface,
After the fitting, the adjusting jig is inserted and rotated so that the jigs can be installed in respective predetermined directions, and fixed at the position by welding, soldering, or the like. For example, in the case of soldering, a solderable layer (for example, a gold-plated layer) is provided at the joint between the housing 24 and the filter holders 26 and 32, and a hole into which solder can be inserted (shown by reference numeral 36 in FIG. 4) is formed. It may be provided and solder may be inserted and fixed.

A first fiber collimator 12a and a second fiber collimator 12b are inserted and fixed at both ends of the through hole 20 of the housing 24. that time,
When the outer diameter of the fiber collimator is small, as shown in the drawing, cylindrical bushes 30 are fitted into both ends of the through hole 20, and the fiber collimator 12a,
Insert 12b and assemble. This allows easy assembly so that the optical axes coincide.

The above embodiment is an example in which two interference filters of the same kind are combined, but the present invention is not limited to such a configuration. In order to completely cancel the adverse effects such as insertion loss polarization dependence and polarization dispersion, it is desirable to combine the same kind of interference filters as described above.
In the case of a configuration using an odd number (2n + 1: where n is an integer) of interference filters, the interference filters are arranged in as many pairs as possible (that is, n pairs), and 2 pairs are formed.
The interference filters are combined so that the tilt angle with respect to the optical axis is orthogonal to each other when viewed in the direction along the optical axis. As a result, even when an odd number of interference filters are combined, the insertion loss polarization dependence and polarization dispersion do not increase by the number of sheets, and can be suppressed to the same level as at most one.

[0019]

As described above, according to the present invention, a plurality of interference filters tilted with respect to the optical axis are arranged in series, and two interference filters forming a pair are arranged orthogonally. Also, it is possible to significantly improve only the cutoff characteristic without increasing the polarization dispersion. by this,
For example, it can greatly contribute to performance improvement in the field of wavelength division multiplexing optical communication.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an embodiment of an interference filter module according to the present invention.

FIG. 2 is a graph showing the insertion loss characteristics of the present invention and the prior art.

FIG. 3 is an assembly explanatory view showing an embodiment of an interference filter module according to the present invention.

FIG. 4 is a vertical cross-sectional view showing a state after the assembly.

[Explanation of symbols]

 10a, 10b interference filter 12a, 12b fiber collimator

Claims (3)

[Claims] 1. A plurality of interference filters are arranged in series inclining with respect to an optical axis, and the interference filters are formed as many pairs as possible, and two interference filters forming a pair are inclined with respect to the optical axis. An interference filter module, wherein the interference filter modules are combined so as to be orthogonal to each other when viewed in a direction along the optical axis. 2. Two interference filters inclined with respect to the optical axis are arranged in series, and both interference filters are combined so as to be orthogonal to each other when the inclination direction with respect to the optical axis is viewed along the optical axis. An interference filter module characterized in that 3. A housing having a through hole extending in the longitudinal direction and having a filter holder mounting hole so as to be orthogonal to the through hole, and a first interference filter fixed to an inclined end surface of the housing. A first filter holder mounted on the housing, a second filter holder fixed on the side surface of the second interference filter and mounted in the filter holder mounting hole of the housing, and mounted on both ends of the through hole. The interference filter module according to claim 2, further comprising a fiber collimator.