Optical filter and optical amplifier employing said optical filter

Abstract: The disclosure relates to an optical filter to be used for signal selection, removal of noise light, wavelength equalization of signal light, etc. in an optical communication, optical measurement or the like, and an optical amplifier employing such optical filter. Signal light is dispersed in different angles according to each wavelength by a diffraction grating and the dispersed light is selectively transmitted or reflected by a transmission type spatial filter or reflection type spatial filter, and is then coupled with an output fiber bundle as non-dispersed light by the diffraction grating. Thus, an optical filter capable of varying wavelength pass band characteristics and an optical amplifier uniform in wavelength amplification factor can be provided.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an optical filter and to an optical amplifier including such a filter. Generally, in the optical communication as referred to above, optical amplifiers which amplify light as it is are employed. Through employment of the optical amplifier as a repeater or relay unit, an economical system may be constructed, since it is not required to convert light into an electrical signal. Particularly, for transmission of wavelength division multiplexed signal light, it is possible to transmit signals in large capacities for servicing, since the wavelength division multiplexed light may be amplified at one time by the optical amplifier. For such an optical amplifier, erbium-doped optical fibers have been employed. The erbium-doped optical fiber amplifier as referred to above has such advantages that it has a high amplification factor, with a superior noise characteristic (as described on Micro-optics news Vol. 8, No. 1, page 23, 1990). However, it is known that, in the optical amplifier as described above, amplification factor thereof varies with respect to the wavelength of its incident signal light, and such wavelength dependency is shown by a dotted line in Fig. 4. Therefore, when the wavelength division multiplexed light is collectively amplified for transmission by connecting the optical amplifiers in many stages, intensities of the signal light for respective wavelengths markedly differ at the receiving point. Thus, depending on channels, received signal power level may vary, with generation of cross talk, thus resulting in deterioration in the sensitivity for the reception. Accordingly, in order to transmit the wavelength division multiplexed signal in the equal power level, an equalizer or filter is required for smoothing wavelength depending characteristic. As the optical filters currently used, those employing dielectric multi-layered films are widely utilized. Since the optical characteristics of such an optical filter are determined by the construction of the dielectric multi-layered film, it is difficult to obtain a sharp characteristic or an arbitrary passing characteristic. Moreover, part of the optical fiber amplifier is held between a flat plate and a refraction grating, thereby constituting the filter (IEEE PHOTONICS TECHNOLOGY LETTERS VOL. 3 NO.2 February 1991 P. 118-120). However, by the conventional optical filter as described above, any desired wavelength characteristics can not be obtained. Furthermore, by the construction to be held between two plates, the size of the entire optical filter becomes large at 40cm, and in the optical filter of this type, the filter characteristic is unstable, since the filter characteristics are adjusted by the pressure of the two plates. Thus, there have been various problems to be solved in the known optical filter. From GB-A-2 240 190, it is known a reflector filter which has a transmission grating and a microbead retroreflector screen located behind a movable screen having a slit. The radiation of the appropriate wavelength strikes the screen through the slit and is retroreflected back through the grating. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical filter having small wavelength depending characteristics only and in particular a small polarization dependency with respect to the incident light and a high diffraction efficiency. Another object of the present invention is to provide an optical filter which is simple in construction and stable in functioning at high reliability, and can be readily manufactured at low cost. In accomplishing these objects, according to one aspect of the present invention, there is provided an optical filter which comprises an input fiber bundle for emitting light and an output fiber bundle for receiving the light, first and second diffraction grating means disposed in the light path between the input and output fiber bundles, a spatial filter provided in the light path between said diffraction grating means, and lens means each provided in the light path between said input fiber bundle, said first diffraction grating means said spatial filter, said second diffraction grating means and said output fiber bundle, said optical filter changing wavelength pass-band characteristics of light by altering characteristics of said spatial filter, wherein said diffraction grating means is a Fourier diffraction grating which satisfies conditions as represented by 0.5λ < d < 1.5λ, and 0.2d < h < 0.5d wherein λ represents the wavelength employed, d is a diffraction grating groove pitch, and h is a depth of the grating groove. According to another aspect of the present invention, there is provided an optical filter which comprises an input fiber bundle for emitting light and an output fiber bundle for receiving the light, a diffraction grating means disposed in the light path between said input and output fiber bundles, a first lens means disposed in the light path between said input and output fiber bundles and said diffraction grating means, a reflecting mirror reflecting light back to said diffraction grating means (13) and a second lens means disposed in the light path between said diffraction grating means and said reflecting mirror (16); said optical filter changing wavelength pass-band characteristics of light by altering spatial reflectance of said reflecting mirror, characterized in that said diffraction grating means is a Fourier diffraction grating which satisfies conditions as represented by 0.5λ < d < 1.5λ, and 0.2d < h < 0.5d wherein λ represents the wavelength employed, d is a diffraction grating groove pitch, and h is a depth of the grating groove. Optical amplifiers including such optical filters are defined in claims 6,7,12 and 13. With the present invention, any wavelength pass-band characteristics as desired can be obtained not only by providing the spatial limitation such as the slit at the optical section for reflection or transmission, but also by imparting certain characteristics to the reflectance or transmittance. The Fourier diffraction grating which meets the conditions as given above can be made small in polarization dependency with the respect to the incident light and high in diffraction efficiency. So, in the optical filter according to the present invention, favourable characteristics with small loss and noise are obtained. Accordingly, the arrangement of the invention provides a remarkable effect particulary when an optical amplifier is employed. Also, by the arrangement according to the present invention as described above, the optical filter which is capable of transmitting wavelength multiple signal light at equal intensity over a long distance or through multiple distributions can be advantageously provided at low cost. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which; Fig. 1 is a schematic perspective diagram showing general construction of an optical filter according to one preferred embodiment of the present invention, Figs. 2(a) and 2(b) are diagrams showing relation between spatial transmittance of a spatial filter and wavelengths for explaining the embodiment of Fig. 1, Fig. 3(a) and 3(b) are also diagrams showing another relation between spatial transmittance and wavelengths for further explaining the embodiment of Fig. 1, Fig. 4 is a diagram showing wavelength pass-band characteristics of the optical filter of Fig. 1, Fig. 5 is a diagram representing wavelength amplification characteristics of an optical amplifier according to one preferred embodiment of the present invention, Fig. 6 is a schematic block diagram showing construction of the optical amplifier according to the embodiment of the present invention, Figs. 7(a) and 7(b) are wavelength pass-band characteristic diagrams for the optical filter of the embodiment, Fig. 8 is a diagram similar to Fig. 1, which particularly relates to another embodiment of the present invention, Figs. 9(a) and 9(b) are diagrams showing relation between spatial reflectance characteristic and the pass-band characteristic of a reflecting mirror means in the embodiment of Fig. 8, Figs. 10(a) to 10(b) are diagrams similar to Figs. 9(a) and 9(b), which particularly relate to another reflecting mirror means, Fig. 11 is a schematic block diagram similar to Fig. 6, which particularly relates to an optical amplifier of another embodiment, Fig. 12 is a characteristic diagram showing relation between grating or groove interval and diffraction efficiency in the diffraction grating, and Fig. 13 is a characteristic diagram showing relation between groove depth and diffraction efficiency in the diffraction grating. DETAILED DESCRIPTION OF THE INVENTION Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring now to the drawings, there is shown in Fig. 1, a construction of an optical filter F1 according to one preferred embodiment of the present invention which generally includes an input fiber bundle 1 and an output fiber bundle 9 for guiding light, first and second diffraction gratings 3 and 8 disposed beside the input and output fiber bundles 1 and 9, with a transmission type space filter 7 provided beside the first and second gratings 3 and 8, a first lens 2 disposed between the input fiber bundle 1 and the first diffraction grating 3, a second lens 5 disposed between the first diffraction grating 3 and the space filter 7, a third lens 6 disposed between the space filter 7 and the diffraction grating 8, and a fourth lens 10 disposed between the second diffraction grating 8 and the output fiber bundle 9. The optical filter F1 as described above is arranged to be capable of changing wavelength pass-band characteristics of light by altering characteristics of said spatial filter 7 as described in detail hereinbelow. In the arrangement of Fig. 1, light emitted from the input fiber bundle 1 is collimated by the lens 2 so as to be incident upon the diffraction grating 3. The incident light is dispersed as at 4 according to each wavelength by the diffraction grating 3. The dispersed light 4 incident upon the lens 5 forms an image on a focal plane of the lens 5. The transmission type spatial filter 7 disposed on the focal plane of the lens 5 is adapted to selectively transmit the dispersed light, and the dispersed light partially transmitted by the spatial filter 7 is collimated by the lens 6 so as to be incident upon the diffraction grating 8. The dispersed light is converted into non-dispersed light by the diffraction grating 8, and is coupled with the output fiber bundle 9 by the lens 10. Accordingly, by altering the transmittance with respect to a specific wavelength position X on the focal plane of the lens 5, any desired wavelength pass-band characteristics may be imparted. The pass-band wavelength region is represented by a following equation, δλ = d·cos()·δx/f wherein f is the focal length of the lenses 5 and 6, d is the grating interval of the diffraction gratings 3 and 8,  is the incident angle to the grating, λ is the working wavelength, δx is the pass-band width of the spatial filter, and δλ is the pass-band wavelength region. The relation between spatial transmittance and wavelength of the spatial filter 7 in the case where the focal length of the lenses 5 and 6 is set to 10mm, and the groove pitch of the diffraction gratings 3 and 8, to 1.5µm, and the working wavelength, to 1.5µm is shown in Figs. 2 and 3. Figs. 2(a) and 2(b) and 3(a) and 3(b) show the relations between the spatial transmittance of the spatial filter 7 and the wavelength. Figs. 2(a) and 2(b) relate to the case where a slit-like aperture having a transmittance as shown in Fig. 2(a) is employed for the spatial filter 7, and at this time, the spatial filter 7 has the wavelength transmission characteristics as shown in Fig. 2(b) for the optical band-pass filter which allows only a specific band to pass therethrough. By altering the slit width thereof, the pass wavelength band can be changed, while the pass wavelength is determined by the slit position thereof. Figs. 3(a) and 3(b) show another relation between the spatial transmittance and wavelength. From such examples, it is seen that in the present invention, the spatial transmission characteristics correspond to the wavelength characteristics, and any desired wavelength pass characteristics can be readily obtained. Accordingly, if the pass-band which becomes an inverse characteristic represented by a solid line with respect to the wavelength amplification factor characteristic of an erbium-doped optical fiber amplifier as shown by a dotted line in Fig. 4 has been given to the spatial filter 7, the wavelength amplification characteristics of the optical fiber amplifier may be made uniform as shown in Fig. 5. Thus, even when the optical amplifiers are connected in multiple stages, uniform wavelength division multiplexed light signals can be transmitted, with constant level maintained. For the spatial filter 7, a spatial modulation element such as a liquid crystal panel or the like may be employed besides the slit, dry plate, etc. Meanwhile, for the diffraction grating 3, a Fourier diffraction grating satisfying the conditions as follows is used. 0.5λ < d < 1.5λ, and 0.2d < h < 0.5d where λ is the working wavelength, d is the groove pitch of the diffraction grating, and h is the depth of the grating groove. It should be noted here that the Fourier diffraction grating is the grating having a shape whose cross section is represented by superposition of a fundamental sine wave and its higher infinite harmonic. A graphical diagram in Fig. 12 shows relation in characteristics between the groove pitch and the efficiency of a primary diffraction light in the Fourier diffraction grating, while that in Fig. 13 represents relation in characteristics between the groove depth and the efficiency of the primary diffraction light also in the Fourier diffraction grating. The solid line relates to a case where polarization of the incident light is perpendicular with respect to the groove of the grating, the one-dotted chain line, to the case where polarization of the incident light is parallel with respect to the groove of the grating, and the dotted line represents the average value. When the grating interval d becomes smaller than 1.5λ, diffraction light higher than the secondary diffraction light is not produced, and therefore, the primary diffraction efficiency is high. Meanwhile, when the grating interval d becomes smaller than 0.5λ, the diffraction efficiency is reduced to half. Similarly, if the grating groove depth h becomes deeper than 0.5d, the diffraction grating is also reduced to half. When the grating depth h is smaller than 0.2d, the diffraction efficiency is lowered to half. The Fourier diffraction grating which meets the above conditions is small in a polarization dependency with respect to the incident light, and is high in the diffraction efficiency. Therefore, in the optical filter, favorable characteristics with small loss and noise may be obtained. More preferably, if the end faces of the input fiber bundle 1 and the output fiber bundle 9 are cut slantwise or polished as shown at 1a and 9a in Fig. 1, the reflection attenuation amount is lowered for reduction of noises which may be generated in the optical amplifier. Fig. 6 is a block diagram showing a general construction of an optical amplifier employing the optical filter according to the present invention. In Fig. 6, the signal light 23 is combined, by an optical combiner 24, with exciting light 25 from an exciting light source 26 so as to be incident upon an optical amplification fiber portion 22, in which the optical filter 21 as described earlier with reference to Fig. 1 is inserted, and thus, amplified light 27 is emitted from the end of the optical amplification fiber portion 22. It is to be noted that the optical filter 21 having the construction as shown in Fig. 1 is inserted in the fiber portion 22 having the optical amplification function. Fig. 7(a) shows the pass-band characteristics of the optical filter 21 according to the present invention. In the case where the erbium-doped fibers are employed for the fibers, the amplification wavelength band region of 1.5µm, and exciting wavelengths of 0.98 to 1.48µm, shorter than the above may be obtained, and therefore, the exciting wavelength region is also allowed to pass in the pass-band characteristics. Meanwhile, the optical filter 21 may be disposed at the emitting end of the light amplification fiber section 22. In this case, the pass-band region may be narrowed as shown in Fig. 7(b) in order to eliminate the exciting light 25. By such arrangement, an optical amplifier having a flat pass wavelength region can be provided. Fig. 8 shows an optical filter F2 according to another embodiment of the present invention, which generally includes an input fiber bundle 11 and an output fiber bundle 14 disposed in a parallel relation to each other, a diffraction grating 13, a reflecting mirror 16, a lens 12 disposed between the ends of the input and output fiber bundles 11 and 14 and the diffraction grating 13, and another lens 15 disposed between said diffraction grating 13 and said reflecting mirror 16 as illustrated. In Fig. 8, light emitted from the input fiber 11 is collimated by the lens 12, and incident upon the diffraction grating 13. The incident light 19 is dispersed according to each wavelength by the diffraction grating 13. The dispersed light forms an image on the focal plane of the lens 15 according to the wavelength. The reflecting mirror 16 is disposed on the focal plane of the lens 15 for selectively reflecting the dispersed light. The dispersed light thus reflected is again incident upon the diffraction grating 13 through the lens 15. An Y-axis of the reflecting mirror 16 is slightly inclined about an X axis thereof with respect to the direction of grooves of the diffraction grating 13. By the diffraction grating 13, the dispersed light becomes non-dispersed light so as to be coupled with the output fiber bundle 14. Accordingly, by altering the reflectance with respect to the specific wavelength position of the reflecting mirror 16, any desired wavelength pass-band characteristics may be obtained. The pass-band wavelength region is represented by a following equation, δλ = d·cos()·δx/f wherein f is the focal length of the lens 15, d is the groove pitch of the diffraction grating 13,  is the incident angle to the grating, λ is the working wavelength, δx is the pass-band width of the reflecting mirror 16, and δλ is the pass-band wavelength region. The relation between spatial reflectance and pass wavelength of the reflecting mirror 16 in the case where the focal length of the lens 15 is set to 10mm, and the grating interval of the diffraction grating 13, to 1.5µm, and the working wavelength, to 1.5µm is shown in Figs. 9(a) and 9(b). Fig. 9(a) shows the relation between the spatial reflectance of the reflecting mirror 16 and the wavelength, with the dispersing direction of the diffraction grating 13 being represented by X. Figs. 9(a) and 9(b) relate to the case where a slit-like mirror is employed for the reflecting mirror 16 and the wavelength transmission characteristics thereof is as shown in Fig. 9(b) for the optical band-pass filter which allows only a specific band to pass therethrough. By altering the slit width δx thereof, the pass wavelength band δλ can be changed, while the pass wavelength λ  is determined by the slit position X  thereof. Figs. 10(a) and 10(b) show relation between the spatial reflectance of another reflecting mirror 16 and wavelength. As is seen from these examples, in the present invention, the spatial reflecting characteristics correspond to the wavelength characteristics, and any desired wavelength pass characteristics can be readily obtained. Accordingly, if the pass-band having the inverse characteristic of the wavelength amplification factor of an erbium-doped optical fiber amplifier as shown in Fig. 4 has been given to the reflecting mirror 16, the wavelength amplification characteristics of the optical fiber amplifier may be made uniform. Thus, even when the optical amplifiers are connected in multiple stages, uniform wavelength multiple light signals can be transmitted. Moreover, by the employment of the reflecting optical system, the number of parts may be reduced for compact size. For the reflecting mirror 16, a spatial modulation element such as a liquid crystal panel or the like may be employed besides the slit. Meanwhile, for the diffraction grating 19, a Fourier diffraction grating satisfying the conditions as follows is used. 0.5λ < d < 1.5λ, and 0.2d < h < 0.5d where λ is the working wavelength, d is the groove pitch of the diffraction grating, and h is the depth of the grating groove. The Fourier diffraction grating which meets the above conditions is small in a polarization dependency with respect to the incident light, and is high in the diffraction efficiency in the same reason as explained earlier with respect to the first embodiment. Therefore, in the optical filter, favorable characteristics with small loss and noise may be obtained. More preferably, if the end faces of the input fiber bundle 11 and the output fiber bundle 14 are cut slantwise or polished as shown at 11a and 14a in Fig.8, the reflection attenuation amount is lowered for reduction of noises. Moreover, if the diffraction grating 13 and the input and output fiber bundles 11 and 14 are disposed in a retro-optical arrangement, the lens 12 and the lens 15 may be replaced by one lens for a common use. Fig. 11 is a block diagram showing a general construction of an optical amplifier employing the optical filter according to the present invention. In Fig. 11, the signal light 33 is combined by an optical combiner 34, with exciting light 35 from an exciting light source 36 so as to be incident upon an optical amplification fiber portion 32, in which the optical filter 31 as described earlier with reference to Fig. 8 is inserted, and thus, amplified light 37 is emitted from the end of the optical amplification fiber portion 32. Fig. 7(a) also shows the pass-band characteristics of the optical filter 31 according to the above embodiment. In the case where the erbium-doped fibers are employed for the fibers, the amplification wavelength band region of 1.5µm, and exciting wavelengths of 0.98 to 1.48µm, shorter than the above may be obtained, and therefore, the exciting wavelength region must also be allowed to pass in the pass-band characteristics of the optical filter. Meanwhile, the optical filter 31 may be disposed at the emitting end of the light amplification fiber portion. In this case, the pass-band region may be narrowed as shown in Fig. 7(b) in order to eliminate the exciting light 35. By such arrangement, an optical amplifying device having a flat pass wavelength region can be provided. In the present invention, the amplification fibers need not necessarily be limited to those to which erbium is added, but fibers for amplification of 1.3µm band employing, for example, praseodymium, neodymium or the like may also be employed as well. Similarly, a semi-conductor type optical amplifier may also be used. Moreover, according to the present invention, a concave diffraction grating or no-equal interval curve grating can also be used therefor, in which case the lenses may be dispensed with for further reduction in the size of the optical amplifying device.

Claims: An optical filter which comprises an input fiber bundle (1) for emitting light and an output fiber bundle (9) for receiving the light, first and second diffraction grating means (3, 8) disposed in the light path between the input and output fiber bundles (1, 9), a spatial filter (7) provided in the light path between said diffraction grating means (3, 8), and lens means (2, 5, 6, 10) each provided in the light path between said input fiber bundle (1), said first diffraction grating means (3), said spatial filter (7), said second diffraction grating means (8) and said output fiber bundle (9), said optical filter changing wavelength pass-band characteristics of light by altering characteristics of said spatial filter (7), wherein said diffraction grating means (3, 8) is a Fourier diffraction grating which satisfies conditions as represented by 0.5λ < d < 1.5λ, and 0.2d < h < 0.5d wherein λ represents the wavelength employed, d is a diffraction grating groove pitch, and h is a depth of the grating groove. An optical filter as claimed in claim 1, wherein said spatial filter (7) is a transmission type spatial filter, and said optical filter is arranged to be capable of changing wavelength pass-band characteristics of light by altering spatial transmittance of said transmission type spacial filter. An optical filter as claimed in claim 1 or 2, wherein end faces (1a, 9a) of said input and output fiber bundles (1, 9) are formed to be slantwise. An optical filter as claimed in claim 1 or 2, wherein said diffraction grating means (3, 8) is a concave diffraction grating. An optical filter as claimed in claim 1 or 2, wherein said diffraction grating means is a non-equal interval curve grating. An optical amplifier including an optical filter as claimed in anyone of claims 1 to 5, optical fibers added with a rare earth element, and light source for optically exciting said rare earth element, wherein said spatial filter (7) has characteristics of flattening a wavelength-depending gain of said optical amplifier. An optical amplifier including an optical filter as claimed in anyone of claims 1 to 5, and an optical semi-conductor amplifier, wherein said spatial filter has characteristics of flattening a wavelength-depending gain of said optical semi-conductor amplifier. An optical filter which comprises an input fiber bundle (11) for emitting light (19) and an output fiber bundle (14) for receiving the light, a diffraction grating means (13) disposed in the light path between said input and output fiber bundles (11, 14), a first lens means (12) disposed in the light path between said input and output fiber bundles (11, 14) and said diffraction grating means (13), a reflecting mirror (16) reflecting light back to said diffraction grating means (13) and a second lens means (15) disposed in the light path between said diffraction grating means (13) and said reflecting mirror (16), said optical filter changing wavelength pass-band characteristics of light by altering spatial reflectance of said reflecting mirror (16), characterized in that said diffraction grating means (13) is a Fourier diffraction grating which satisfies conditions as represented by 0.5λ < d < 1.5λ, and 0.2d < h < 0.5d wherein λ represents the wavelength employed, d is a diffraction grating groove pitch, and h is a depth of the grating groove. An optical filter as claimed in claim 8, wherein end faces of said input and output fiber bundles are formed to be slantwise. An optical filter as claimed in claim 8, wherein said diffraction grating means is a concave diffraction grating. An optical filter as claimed in claim 8, wherein said diffraction grating means is a non-equal interval curve grating. An optical amplifier including an optical filter as claimed in anyone of claims 8 to 11, optical fibers added with a rare earth element and a light source for optically exciting said rare earth element, wherein said reflecting mirror (16) has characteristics of flattening a wavelength-depending gain of said optical amplifier. An optical amplifier including an optical filter as claimed in anyone of claims 8 to 11, and an optical semi-conductor amplifier, wherein said reflecting mirror (16) has characteristics of flattening a wavelength-depending gain of said optical semi-conductor amplifier.

Inventors: ASAKURA HIROYUKI, IIDA MASANORI, MIWA TETSUJI, ASAKURA, HIROYUKI, IIDA, MASANORI, MIWA, TETSUJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: H01S3
IPC codes: H01S3, G02B27, G02B6, G01J3