JPS60222815A - Variable optical filter - Google Patents

Abstract

PURPOSE:To make variable central path wavelength and pass wavelength band width by having movable shielding plates which shield part of the reflecting surface of a movable/stationary corner reflector moving within the main plane of an angle dispersing element. CONSTITUTION:Incident light from an incident fiber 6 is conerted to parallel luminous fluxes by a distributed index collimating lens 7 and said fluxes are made incident on a diffraction graing 9 set at a prescribed angle by a glass block 8. The fluxes are converged to the inside of the reflecting surface of the corner reflector 10 through the lens 7 and are then made into the divergent light advancing backward. Said light passes through the lens 7, by which the light is again converted to the parallel luminous fluxes. The paralllel luminous are again conducted to the grating 9, by which the fluxes are diffracted again and pass through the lens 7 so as to converge at the end face of an exit fiber 16. The 1st and 2nd shielding plates 11, 12 are moved using levers 14, 15 for moving the shielding plates interlocked to said plates. The position of a slit 13 is thereby changed and the central pass wavelength and pass wavelength band width are made variable.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical filter whose center passage wavelength and passage wavelength bandwidth are variable.

[Background of the invention]

In a wavelength division multiplexing optical communication system, in order to extract and measure a desired signal from received light, an optical filter whose center passage wavelength and passage wavelength bandwidth are variable is required. Conventionally, spectrometers using diffraction gratings have been used for optical filters with variable center passing wavelengths, but if the input and output fiber parameters match, a wide passing wavelength band as shown below can be obtained. It is difficult to do so. FIG. 1 shows an example of a diffraction grating type variable optical filter. In FIG. 1, light incident from an input fiber 1 becomes a parallel beam at a lens 2, becomes a diffracted beam at a diffraction grating 3, passes through the lens 2 again, and is focused onto an output fiber 5. At this time, when the diffraction grating 3 is rotated by the drive unit 4, the wavelength of the light focused on the output fiber 5 changes, forming a variable optical filter. However, the end face of the input fiber 1 having the same optical parameters as the output fiber 5 is mapped onto the end face of the output fiber 5 only at a specific single wavelength, and it is impossible to obtain a wide passing wavelength bandwidth. However, it was impossible to make the passing wavelength bandwidth variable.

[Purpose of the invention]

The present invention makes it possible to obtain a variable optical filter in which the center passing wavelength and the passing wavelength bandwidth can be made variable.

[Summary of the invention]

In order to achieve the above object, the variable optical filter according to the present invention guides incident light from an input fiber to an angular dispersion element to form diffracted light, and focuses the diffracted light onto an output fiber end face installed near the end face of the input fiber. In the variable optical filter, a corner reflector having a reflection surface larger than the real image of the input fiber over a desired pass wavelength band and returning the incident diffracted light to antiparallel is arranged near the imaging position of the diffracted light. Then, the diffracted light is guided to the angle dispersion element again, and the obtained re-diffracted light is focused on the end face of the output fiber, and the reflecting surface of a movable corner reflector or a fixed corner reflector moves within the main plane of the angle dispersion element. By having a movable shielding plate that shields a part of the wavelength, the center passing wavelength and the passing wavelength bandwidth can be made variable.

[Embodiments of the invention]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 2 is a perspective view showing an embodiment of the tunable optical filter according to the present invention, FIG. 3 is a diagram showing the operating principle of the above embodiment, and FIG.
The figure is a diagram showing the characteristics of the variable optical filter obtained in the above example. In the embodiment shown in FIG. 2, an incident light beam entering from an input fiber 6 is converted into a parallel beam by a distributed index collimating lens 7, and is incident on a diffraction grating 9 set at a predetermined angle by a glass block 8. The light converted into diffracted light by the diffraction grating 9 is converged within the reflection surface of the corner reflector 10 that passes through the distributed index collimating lens 7, and then becomes a retrograde diverging light that passes through the distributed index collimating lens 7. The light is converted into a parallel beam and guided to the diffraction grating 9 again. The parallel light beam diffracted again by the diffraction grating 9 passes through the distributed refractive index collimating lens 7 to the output fiber 16.
converges on the end face of Here, the first shielding plate 11 and the second shielding plate 12.

By moving the shielding plates using shielding plate moving levers 14 and I5 provided on the shielding plates, respectively, and changing the position of the slit 13, the center passing wavelength and the width of the passing wavelength band region can be made variable.

FIG. 3 shows the relative positions of the input/output fiber, the corner reflector, and the shielding plate on the input/output fiber side end face of the distributed index collimating lens in FIG. 2 above. In FIG. 3, reference numeral 17 is a reflection surface on the incident side of the corner reflector 10, 18 is a reflection surface on the output side of the corner reflector 10, and 19 and 20 are the reflection surfaces on the input side of the corner reflector 10.
7 and the area reflected by the output side reflective surface 18, and 21 and 22 indicate areas where the diffracted light of wavelength λ2 is reflected by the input side reflective surface 17 and the output side reflective surface 18. The light beam emitted from the end face of the input fiber 6 is diffracted by the diffraction grating 9, and is converged and reflected in the vicinity of the region 19 to 21 of the incident side reflective surface 17 of the corner reflector IO in the wavelength range of λ1 to λ2. The wavelength range Δλ=λ2−λ□ is approximately given by the following equation.

Here, S is the width of the slit 13 sandwiched between the first shielding plate 11 and the second shielding plate 12, D is the core diameter of the input fiber 6, f is the focal length of the distributed index collimating lens, (dθ/dλ
) is the angular dispersion of the diffraction grating. If a plane reflecting mirror with the same side length S is provided in the same place instead of the corner reflector 10, the diffracted light in the wavelength range λ1 to λ2 will go backwards, and the diffraction grating 9
It is clear that the light is diffracted again at the point where the light is diffracted again and an image is formed on the core end face of the input fiber 6. In this embodiment using the corner reflector 10, the light beam incident on the incident-side reflective surface 17 is reflected by the areas 19 to 21, guided to the output-side reflective surface 18, reflected by the areas 20 to 22, and then reflected by the diffraction grating. Guided by 9. That is, while the diffracted light travels backwards via the corner reflector 10,
The position of the image moves by a distance T in a direction perpendicular to the optical axis of the distributed index collimating lens 7. The amount of movement T is determined by the position of the diffracted light incident on the corner reflector 10.

The light flux that has gone backward from the output-side reflective surface 18 is re-diffracted by the diffraction grating 9 and forms an image on the core end surface of the output fiber 16 provided at a predetermined position moved by a distance T from the core end surface of the input fiber 6. Therefore, the first shielding plate 1 shields a part of the reflective surface of the corner reflector 10, which has a reflective surface larger than the real image of the core end face of the input fiber 6 over the entire desired wavelength band.
If the positions of the first and second shielding plates 12 are moved without changing the slit width S sandwiched between them, the center passing wavelength can be made variable without changing the passing wavelength bandwidth Δλ in equation (1). can. In addition, the slit 1 can be removed without changing the center position of the slit 13.
~If the shielding plates 11 and 12 are moved to change the width S,
It is possible to change only the passing wavelength bandwidth.

In the embodiment shown in FIG. 2, the corner reflector 10 is fixed and the shielding plates 11 and 12 are movable, but if the corner reflector of a predetermined size is moved approximately along the Rowland circle of the diffraction grating, the pass band can be adjusted. It is obvious that the width can be constant and the center passing wavelength can be made variable.

It is also obvious that if both the corner reflector 10 and the shielding plates 11, 12 are movable, the same effect as the embodiment shown in FIG. 2 can be produced. FIG. 4 shows the characteristics of the variable optical filter obtained in the above example, and curves a and b indicate that the input fiber 6 has a core diameter of 50t1m and the distributed refractive index collimating lens 7
The focal length f is 4.2 mn, and the angular dispersion d of the diffraction grating 9 is
These are experimental values when O/dλ is 0.26 (//l11)-' and the slit width S is 160 and 260 IM.

〔Effect of the invention〕

As shown in the embodiments, the tunable optical filter of the present invention guides incident light from an input fiber to an angular dispersion element to become diffracted light, and focuses the diffracted light on the end face of an output fiber installed near the end face of the input fiber. In the filter, a corner reflector is disposed near the imaging position of the diffracted light, and has a reflecting surface larger than the real image of the input fiber over a desired pass wavelength band, and returns the incident diffracted light to antiparallel; The diffracted light is guided to the angle dispersion element again, and the obtained re-diffracted light is focused on the end face of the output fiber, and one of the reflecting surfaces of a movable corner reflector or a fixed corner reflector moves within the principal plane of the angle dispersion element. Since it has a movable shielding plate that shields the
By moving the movable shielding plate, it is possible to obtain an optical filter in which one or both of the center passing wavelength and the passing wavelength bandwidth can be made variable.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional diffraction grating type tunable optical filter, FIG. 2 is a perspective view showing an embodiment of the tunable optical filter according to the present invention, and FIG. 3 is a diagram showing the operating principle of the above embodiment. FIG. 4 is a characteristic diagram of the variable optical filter obtained in the above example. 6... Input fiber 9... Angular dispersion element (diffraction grating) 10... Corner reflector 11.12... Movable shielding plate 16... Output fiber Patent applicant Nakamura, patent attorney representing Nippon Telegraph and Telephone Public Corporation Junnosuke 1-1 Figure 1'F2

Claims (1)

[Claims] In a variable optical filter that guides incident light from an input fiber to an angular dispersion element to form diffracted light and focuses the diffracted light on an output fiber end face installed near the end face of the input fiber, near the imaging position of the diffracted light, A corner reflector having a reflection surface larger than the real image of the input fiber over the desired passing wavelength band and returning the incident diffracted light to antiparallel is arranged, and the diffracted light is guided to the angle dispersion element again. The invention further includes a movable shielding plate that focuses the re-diffracted light onto the end face of the output fiber and shields a movable corner reflector that moves within the principal plane of the angular dispersion element or a part of the reflective surface of the fixed corner reflector. Features a variable optical filter.