JP2003066270A - Phased array space optical filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a phased array space optical filter which can extend the pass band while maintaining sharp rectangular rise and fall characteristics. SOLUTION: The phased array space optical filter is provided with at least one optical fiber for input, a first convex lens, an optical medium provided on the side opposite to the side of the optical fiber for input of the first convex lens, a second convex lens provided on the side opposite to the first convex lens side of the optical medium, and at least one optical fiber for output provided on the side opposite to the optical medium side of the second convex lens. The thickness or the refractive index of the optical medium in the transmission direction of a luminous flux made incident on the optical medium is changed in N steps (N is an integer equal to or larger than one) in the direction perpendicular to the optical axis of the luminous flux made incident on the optical medium and at unequal intervals in the direction of the optical axis so that the pass band width may be extended.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phased array spatial optical filter, and more particularly to a method for selecting an optical wavelength in the field of an optical communication system, an optical switching system or an optical measurement system. The present invention relates to a technique that is effective when applied to an optical filter used for (tuning) or splitting. 2. Description of the Related Art In the field of optical communication systems, optical switching systems, and optical measurement systems, when selecting (tuning) or demultiplexing an optical wavelength, an optical filter, for example, a phased array spatial optical filter is used. You. FIG. 8 is a block diagram showing a schematic configuration of a conventional phased array spatial light filter. In the figure, 2 is an input optical fiber, 3 is a first convex lens, 4 is a second convex lens, 5 is an output optical fiber, and 9 is a glass plate. In the phased array spatial optical filter shown in FIG. 8, the input light from the input optical fiber 2 is converted into a parallel light beam by the first convex lens 3, one of which is air and the other is a glass plate 9. And propagates in two optical paths. This 2
Two light beams having a phase difference due to the optical path length difference between the two optical paths are condensed by the second lens 4 and interfere with each other. in this way,
The phased array spatial optical filter shown in FIG. 8 constitutes a kind of Mach-Zehnder interferometer utilizing the difference in optical path length. In addition, such a technology is described in `` RABetts, S.
J.Frisken.and D.Wong: "Split-beam fourier filter
and its application in a gain-flattened EDFA ", Pro
ceedings of OFC'95 paper TuP4 ". [0003] However, the conventional phased array spatial light filter shown in FIG. 8 has the following problems. (1) The pass characteristic (transmission characteristic) of the conventional phased array spatial optical filter becomes a cosine characteristic with respect to the wavelength,
It was impossible in principle to realize a narrow band filter characteristic. (2) The number of optical input / output ports is not one and plural. (3) In the conventional phased array spatial optical filter, the pass wavelength can be varied, but the pass bandwidth cannot be varied. (4) In the conventional phased array spatial optical filter, it has not been possible to realize a chirping characteristic that widens the bandwidth while maintaining a sharp rectangular pass band characteristic. SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a phased array spatial optical filter which causes multi-beam interference to achieve a narrow band pass characteristic. An object of the present invention is to provide a technology that can be easily realized. Another object of the present invention is to increase the passband of a phased array spatial optical filter while maintaining a rectangular sharp rising and falling characteristic, which was impossible in principle in the conventional example. It is an object of the present invention to provide a technique capable of realizing a so-called chirping characteristic. The above and other objects and novel features of the present invention will become apparent from the description of the present specification and the accompanying drawings. [0005] Of the inventions disclosed in the present application, the outline of a representative one will be briefly described.
It is as follows. That is, the present invention provides at least one input optical fiber, a first convex lens, an optical medium provided on a side of the first convex lens opposite to a side on which the input optical fiber is provided, and A second convex lens provided on the side of the medium opposite to the side on which the first convex lens is provided, and at least one output lens provided on the side of the second convex lens opposite to the side on which the optical medium is provided; A phased array spatial optical filter including an optical fiber, wherein the optical medium has a thickness or a refractive index in a propagation direction of a light beam incident on the optical medium with respect to an optical axis of the light beam incident on the optical medium. N (N
Is an integer, and changes in steps of N ≧ 1) at irregular intervals in the optical axis direction so as to increase the pass bandwidth. Embodiments of the present invention will be described below in detail with reference to the drawings. In all the drawings for describing the embodiments, components having the same function are denoted by the same reference numerals, and repeated description thereof will be omitted. [First Embodiment] FIG. 1 is a block diagram showing a schematic configuration of a phased array spatial optical filter which is a premise of a phased array spatial optical filter according to a first embodiment of the present invention. The phased array spatial optical filter shown in FIG. 1 is such that an optical attenuator 8 and a phased array medium 1 are provided between a first convex lens 3 and a second convex lens 4.
This is different from the conventional phased array spatial light filter shown in FIG. The phased array medium 1 is obtained by, for example, subjecting a glass substrate of quartz or the like to chemical etching, reactive ion etching, optical polishing, machining, or the like.
It is processed in N steps (where N is an integer and N ≧ 1) so that the thickness differs by ΔL and the width is evenly spaced. However, anti-reflection coating is applied to both end faces of the phased array medium 1 to prevent reflection, or the input / output end faces are obliquely polished. The first convex lens 3 and the second convex lens 4 are spherical or aspherical lenses, and are made of optical glass such as quartz glass, BK7 glass, TAF3 glass, sapphire glass, plastic, or a polymer material. . As the input optical fiber 2 and the output optical fiber 5, a single mode optical fiber, a dispersion shift fiber, a polarization maintaining fiber, or the like is used. Optical attenuator 8
Is the ND (ne
utral density) A filter or a slit is used. Hereinafter, the operation of the phased array spatial light filter shown in FIG. 1 will be described in detail. The input light from one input optical fiber 2 is converted into a parallel light flux by the first convex lens 3, and after propagating in free space, has a thickness N different by ΔL.
The light passes through the phased array medium 1 (stepped quartz glass substrate 101 with steps) processed into a plurality of steps. At this time, the parallel light beam is split into N light beams according to the N steps. At this time, each parallel light beam passing through the quartz glass substrate 101 having a thickness different by ΔL
The optical path length difference n determined by the product of the thickness difference ΔL and the refractive index n
A constant phase difference is added to each other according to ΔL. These parallel light beams are converged by the second convex lens 4 and coupled to one output optical fiber 5. At this time, each light beam causes optical multi-beam interference, and this multi-beam interference has an effect of extremely narrowing the pass bandwidth. That is, the interference light waves are strengthened at the focal point of the second convex lens 4 when the optical path length difference is an integral multiple of the wavelength, and are weakened when the optical path length difference is an integral multiple of the half wavelength. Here, the optical path length difference n △ L is selected to be an integral multiple of the wavelength. Therefore, the phase relation of each light beam is the same.
Therefore, the phased array spatial optical filter shown in FIG. 1 can realize a phased array spatial optical filter having a narrow pass band. Further, by providing an ND filter having different transmittance depending on the position where the parallel light beam is incident in the parallel light beam as the optical attenuator 8, it becomes possible to adjust the electric field intensity distribution of the parallel light beam. Generally, input optical fiber 2
When the output optical fiber 5 is a single mode optical fiber, the electric field intensity distribution in the cross section of the parallel beam is Gaussian, reflecting the electric field intensity distribution in the core of the single mode fiber. By using the detector 8, for example, it is possible to make the electric field strength distribution a | Sinc | function. Further, a structure in which the phase of the light propagating through the step portion of the phased array medium 1 is provided with a region displaced by π in the section of the step position x = π ＝ 2π, 3π〜4π, ‥‥, that is,
By changing the optical path length difference n △ L by λ / 2, the electric field intensity distribution of the parallel light beam propagating through the phased array medium 1 can be made a Sinc function. Mathematically, it is known that the Fourier transform of the Sinc function becomes a rectangular function, and it can be said that this optical system performs the Fourier transform. As a result, the pass band shape of the light spectrum of the phased array spatial light filter becomes rectangular, and the pass band can be flattened. The pass band can be flattened without using the optical attenuator 8. For example, the width of the staircase near the optical axis is wide at first, and gradually becomes narrower as the distance from the optical axis increases.
The electric field intensity distribution is set to | Sinc | so that it becomes zero at a certain point and becomes gradually wider as the distance from the optical axis increases.
It is also possible to make it functional. As a result, it is possible to realize a flat pass band characteristic which is not available in the related art as the pass band characteristic of the phased array spatial optical filter. As described above, the phased-array spatial optical filter shown in FIG. 1 can realize a bandpass characteristic having a much narrower band than that of the conventional example. In addition, since the interference position changes depending on the wavelength, as shown in FIG. 1, if a multi-core fiber array is arranged at the focal position in advance, the output port can be changed according to the wavelength. The function can be realized. further,
If the phased array medium 1 is rotated in the direction perpendicular to the parallel light beam, that is, if the phased array medium 1 is tilted in the direction perpendicular to the parallel light beam, the optical path length changes. It can be changed. FIG. 2 is a block diagram showing a schematic configuration of a phased array spatial optical filter according to an embodiment of the present invention. The phased array spatial optical filter of the present embodiment is different from the phased array spatial optical filter shown in FIG. 1 in that the thickness of the phased array medium 1 is changed at irregular intervals in the optical axis direction. That is, the phased array spatial optical filter of the present embodiment and the phased array spatial optical filter shown in FIG.
N (where N is an integer and N ≧ 1)
It is. 1) are common in that they are formed in steps.
In the phased array spatial optical filter shown in FIG. 4A, the thickness of the step portion of the phased array medium 1 changes at equal intervals (the difference in the thickness (△ L) of the adjacent step portions is constant), whereas In the phased array spatial light filter according to this embodiment, the thickness of the stepped portion of the phased array medium 1 changes at irregular intervals (the difference in thickness (△ L) between adjacent stepped portions is different). The phased array medium 1 includes, for example,
A glass substrate made of quartz or the like is subjected to chemical etching, reactive ion etching, optical polishing, machining, or the like, so that the difference in thickness (△ L) between adjacent stairs is unequally spaced, and the width is equal. It is formed by processing into N steps so as to have an interval. By changing the thickness of the phased array medium 1 at unequal intervals in the optical axis direction as in the present embodiment, a phased array spatial optical filter could not be realized in principle in a conventional example. In other words, it is possible to realize a so-called chirping characteristic in which the pass band is increased while maintaining the rectangular sharp rising and falling characteristics. In the phased array spatial light filter shown in FIG. 1, a quartz glass substrate was used as the phased array medium 1, but the present invention is not limited to this. Other optical glass materials, plastics, Alternatively, a polymer material or the like can be used. [Embodiment 2] FIG. 3 is a block diagram showing a schematic configuration of a phased array spatial optical filter which is a premise of a phased array spatial optical filter according to Embodiment 2 of the present invention. The phased array spatial optical filter shown in FIG. 3 differs from the phased array spatial optical filter shown in FIG. 1 in that a refractive index changing glass substrate 104 whose refractive index changes stepwise is used as the phased array medium 1. The refractive index change glass substrate 104 is made of, for example, germanium oxide (GeO ₂ ), phosphorus pentoxide (P ₂ O ₅ ),
Alternatively, a quartz glass substrate doped with any of boron oxide (B ₂ O ₃ ) or a quartz glass substrate subjected to high-pressure hydrogen treatment is externally irradiated with ultraviolet (UV) rays generated by, for example, an excimer laser. The refractive index of the glass substrate is changed in N steps by causing a change in the refractive index due to the light-induced effect. In the phased array spatial optical filter shown in FIG. 3, the same effect as in the phased array spatial optical filter shown in FIG. 1 can be obtained. In the phased array spatial optical filter of the present embodiment, the refractive index of the refractive index changing glass substrate 104 is changed at irregular intervals in the optical axis direction.
It differs from the phased array spatial light filter shown in FIG. 1 in that a refractive index changing glass substrate 104 that changes stepwise is used. That is, the phased array spatial optical filter of the present embodiment and the phased array spatial optical filter shown in FIG. 3 are common in that the refractive index of the phased array medium 1 changes stepwise, but is shown in FIG. In the phased array spatial optical filter, the refractive index of the phased array medium 1 changes at equal intervals in the optical axis direction, whereas in the phased array spatial optical filter of the present embodiment, the refractive index of the phased array medium 1 is It changes at irregular intervals in the optical axis direction. The phased array spatial light filter of the present embodiment also has a so-called chirping characteristic, which increases the pass band while maintaining the rectangular steep rising and falling characteristics, which was impossible in principle in the conventional example. Can be realized. In addition, as the phased array medium 1 of the present embodiment, a material in which a polymer material such as fluorinated polyimide is irradiated with an electron beam, ultraviolet light, or SOR light from the outside to change the refractive index stepwise is used. Is also possible. Embodiment 1 FIG. 4 is a block diagram showing a schematic configuration of a phased array spatial optical filter according to Embodiment 1 of the present invention. The phased array spatial optical filter shown in FIG.
Embodiment 2 is different from Embodiment 1 in that an optical fiber aggregate 102 in which a number of short single mode optical fibers as small as about μm is bundled. The optical fiber aggregate 102 is bent in an arc shape with a certain curvature so that the optical length differs from a neighboring optical fiber by a certain length ΔL. However, both end faces of the optical fiber assembly 102 are coated with a non-reflection coating to prevent reflection, or an input / output end face is obliquely polished. Also in the phased array spatial optical filter shown in FIG. 4, each light emitted from each optical fiber of the optical fiber assembly 102 causes multi-beam interference at the focal point of the second convex lens 4. As a result, when the optical path length difference is an integral multiple of the wavelength, they reinforce each other, and when the optical path length difference is an integral multiple of a half wavelength, they reinforce each other. Therefore, an output optical fiber (for example, a single mode optical fiber) 5 is provided at that point. If
Narrow band optical filter characteristics can be realized.
As described above, according to the phased array spatial optical filter shown in FIG. 4, it is possible to easily realize a narrower band filter characteristic as compared with the conventional example, as in the first embodiment. Further, by changing the radius of curvature of the optical fiber aggregate 102, the optical path length difference ΔL between adjacent optical fibers of the optical fiber aggregate 102 can be increased, so that the selectable wavelength interval of the phased array spatial optical filter can be set arbitrarily. It becomes. Thereby, the degree of freedom in designing a phased array spatial light filter can be greatly improved. Further, since the optical fiber aggregate 102 is used, insertion loss can be reduced. [Embodiment 2] FIG. 5 is a block diagram showing a schematic configuration of a phased array spatial light filter according to Embodiment 2 of the present invention. The phased array spatial optical filter shown in FIG. 5 includes an optical circulator 6 in the input optical fiber 2.
This is different from the phased array spatial optical filter shown in FIG. 1 in that a plane reflecting mirror 7 is provided after the phased array medium 1. In the phased array spatial optical filter shown in FIG. 5, the third terminal 6c of the optical circulator 6
Is input to the input optical fiber 2 connected to the first terminal 6a. The input light input to the input optical fiber 2 is converted into a parallel light flux by the first convex lens 3, propagates in free space, and is then processed into N steps of thickness N different in thickness by ΔL. (Quartz glass substrate 101). This quartz glass substrate 101
Are reflected by the plane reflecting mirror 7, propagate through the quartz glass substrate 101 and free space again, are condensed by the first convex lens 3, and are coupled to the input optical fiber 2.
The light coupled to the input optical fiber 2 passes through the second terminal 6 b of the optical circulator 6 and is output from the output optical fiber 5. Also in the phased array spatial light filter shown in FIG. 5, a parallel light beam reciprocating in the phased array medium 1 causes multi-beam interference at the focal point of the first convex lens 3. As a result, when the optical path length difference is an integral multiple of the wavelength, they are strengthened, and when the optical path length difference is an integral multiple of a half wavelength, they are weakened, so that a narrow band optical filter characteristic can be realized. According to the phased array spatial light filter shown in FIG. 5, the second convex lens 4 becomes unnecessary, and the phased array medium 1
It is not necessary to provide the output optical fiber 5 on the side from which the parallel light beam is emitted, so that the optical axis adjustment becomes easier as compared with the related art. Further, in the phased array spatial optical filter shown in FIG. 5, since the parallel light flux is reflected by the plane reflecting mirror 7 and reciprocates in the phased array medium 1, the step for obtaining the optical path length difference ΔL is only ΔL / 2. Therefore, there is an advantage that the thickness can be reduced. In the phased array spatial optical filter shown in FIG. 5, the phased array medium 1 and the plane reflecting mirror 7 can be integrated. [Embodiment 3] FIG. 6 is a block diagram showing a schematic configuration of a phased array spatial optical filter according to Embodiment 3 of the present invention. The phased array spatial optical filter shown in FIG. 6 applies heat to the phased array medium 1 using a split-sleeve-shaped thick film heater 10 made of tungsten (W) or nichrome (NiCr) or the like. The difference from the phased array spatial light filter shown in FIG. 1 is that the refractive index of the array medium 1 is changed continuously. As shown in FIG. 6, when the phased array medium 1 is uniformly heated from the side surface using the thick film heater 10 or the like, the effective refractive index based on the thermo-optic effect changes, and the amount of the change becomes equal for each step. This corresponds to a small increase in the optical length by the same amount of δn △ L (where δn is the amount of change in the refractive index) in each step. As a result, the interference condition of the multi-beams passing through the phased array medium 1 shifts to another wavelength, so that the passing wavelength of the phased array spatial optical filter can be changed. In the phased array spatial light filter shown in FIG. 6, the phased array medium 1 may be made of an optical glass material other than quartz glass, or a polymer material such as fluorinated polyimide. In the phased array spatial optical filter shown in FIG. 6, for example, lithium niobate (LiNbO ₂ ) or the like is used as the phased array medium 1, and the effective refractive index is changed based on the electro-optic effect by applying an electric field. Then, the passing wavelength of the phased array spatial light filter may be changed. Alternatively, for example, by using tellurium dioxide (TeO ₂ ) or the like and applying ultrasonic waves, the effective refractive index is changed based on the acousto-optic effect, and the passing wavelength of the phased array spatial optical filter is changed. Good. As described above, in the phased array spatial optical filter shown in FIG. 6, by applying an electric field, heat, or ultrasonic energy to the phased array medium 1, the refractive index of the phased array medium 1 is continuously changed. Therefore, it is possible to change the pass wavelength or pass band of the phased array spatial optical filter, which is impossible in the conventional example. Embodiment 4 FIG. 7 is a perspective view showing a schematic configuration of a phased array medium 1 of a phased array spatial light filter according to Embodiment 4 of the present invention. The phased array spatial optical filter of the fourth embodiment is different from the phased array spatial optical filter shown in FIG. 1 in that a quartz glass substrate 103 with a spiral staircase is used as the phased array medium 1. This quartz glass substrate 103 with a spiral staircase is
In a plane perpendicular to the optical axis of the parallel light beam converted by the convex lens 3, the thickness of the parallel light flux changes spirally in a radial direction around the optical axis. Thus, in the phased array spatial optical filter of the fourth embodiment, the steps are formed in the radial direction, so that the entire phased array spatial optical filter can be made compact. In the phased array spatial light filter of the fourth embodiment, as the phased array medium 1, the refractive index of the phased array medium 1 falls within a plane perpendicular to the optical axis of the parallel light beam converted by the first convex lens 3. The refractive index may be changed in a spiral step shape in the radial direction about the axis. As described above, the invention made by the inventor has been specifically described based on the embodiment. However, the present invention is not limited to the embodiment, and can be variously modified without departing from the gist of the invention. Needless to say, The effects obtained by typical ones of the inventions disclosed in the present application will be briefly described as follows. According to the present invention, it is possible to realize a so-called chirping characteristic in which the pass band is increased while maintaining a sharp rising and falling characteristic of a rectangular shape, which was impossible in principle in the conventional example. It becomes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a schematic configuration of a phased array spatial optical filter which is a premise of the phased array spatial optical filter according to the first embodiment of the present invention. FIG. 2 is a perspective view showing a schematic configuration of a phased array spatial optical filter according to Embodiment 1 of the present invention. FIG. 3 is a block diagram showing a schematic configuration of a phased array spatial optical filter which is a premise of the phased array spatial optical filter according to Embodiment 2 of the present invention. FIG. 4 is a block diagram showing a schematic configuration of a phased array spatial light filter according to Embodiment 1 of the present invention; FIG. 5 is a block diagram showing a schematic configuration of a phased array spatial optical filter according to Embodiment 2 of the present invention. FIG. 6 is a block diagram showing a schematic configuration of a phased array spatial optical filter according to Embodiment 3 of the present invention. FIG. 7 is a perspective view showing a schematic configuration of a phased array medium of a phased array spatial light filter according to Embodiment 4 of the present invention. FIG. 8 is a block diagram showing a schematic configuration of a conventional phased array spatial light filter. [Description of Signs] 1 ... Phased array medium, 2 ... Input optical fiber, 3
.. A first convex lens, 4 a second convex lens, 5 an output optical fiber, 6 an optical circulator, 7 a plane reflecting mirror, 8
... optical attenuator, 9 ... glass plate, 10 ... heater, 101 ... glass substrate with steps, 102 ... optical fiber assembly, 103 ...
Glass substrate with spiral staircase, 104: glass substrate with variable refractive index.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Yoshio Suzuki             2-3-1 Otemachi, Chiyoda-ku, Tokyo Sun             Within the Telegraph and Telephone Corporation F term (reference) 2H079 AA06 AA12 BA01 CA07 DA17                       EA05 EB27

Claims (1)

Claims: 1. An optical fiber provided on at least one input optical fiber, a first convex lens, and a side of the first convex lens opposite to a side on which the input optical fiber is provided. A medium, a second convex lens provided on a side of the optical medium opposite to the side where the first convex lens is provided, and at least a second convex lens provided on a side of the second convex lens opposite to the side provided with the optical medium. A phased array spatial optical filter including one output optical fiber, wherein the optical medium has a thickness or a refractive index in a propagation direction of a light beam incident on the optical medium, and a light beam incident on the optical medium. N (N is an integer, N
.Gtoreq.1) stepwise array optical filters characterized by changing at unequal intervals in the optical axis direction so as to increase the pass bandwidth.