An optical amplifier

Abstract: An optical waveguide (11) having a first cross section is thinned to constitute a thinned portion having a second cross section, as long as a predetermined length. Surface of the thinned portion (11a) is contacted with a laser glass member (12) doped, with a rare earth element, such as Er. Due to the thinned diameter, the optical fiber (11a) is optically coupled with the laser glass member (12). A pumping light (λ₂) is input to either into the optical waveguide (11) together with a signal light (λ₁) or directly into the laser glass member (12). Signal light (λ₁) coupled from the thinned portion (11a) to the laser glass member (12) is amplified by stimulated emission of the laser glass. Thus amplified signal light is coupled back to the thinned portion (11a) so as to propagate along the optical waveguide (11). The thinned portion (11a) can be as short as several centi-meters compared with ten meters to more than one hundred meters of conventional optical amplifier. In the optical waveguide (11) where the pumping light (λ₂) is directly input to the laser glass member (12), the complicated multiplexer is not necessary, resulting in a reliable operation of the amplifier.

Description: Field of the Invention This invention relates to an optical amplifier which amplifies a signal light propagating in an optical waveguide by means of laser glass doped with a rare earth. Description of the Related Art A prior art optical amplifier employing a glass fiber doped with a rare earths is constituted as typically shown in FIG. 1. In the figure, the numeral 40 denotes an amplifier substrate; the numerals 41 and 42 denote an optical fiber for signal light transmission; the numeral 43 denotes an optical fiber doped with a rare earth element; the numeral 44 denotes a light source typically formed with a laser, for emitting a pumping light; the numeral 45 denotes a multiplexer; and the numeral 46 denotes a demultiplexer. A signal light of 1.536 µm wavelength incident to optical fiber 41 is input to optical fiber 43 doped with Er (erbium). A pumping light of, for example, 1.49 µm or 0.98 µm wavelength is introduced by multiplexer 45 so as to transmit into optical fiber 43 together with the signal light. During the transmission in optical fiber 43 doped with rare earth, electrons in the rare earth element are pumped up by the pumping light so as to amplify the signal light due to a stimulated emission effect. The amplified light is transmitted via demultiplexer 46 into optical amplifier 42. Pumping light which is unnecessary now is removed by demultiplexer 46 so as to be abandoned. Direction of the pumping light propagation in the rare-earth-doped optical fiber 43 may be chosen opposite, i.e. backward, to the signal light propagation. Moreover, demultiplexer 46 may be replaced with an isolator of the pumping light. The above-described prior art optical amplifier employing the optical fiber doped with the rare earth requires relatively large number of components and requires as long as several meters or more than one hundred meters of the rare-earth doped optical fiber 43. Moreover, the multiplexer requires its precise alignment; therefore, it has been difficult to satisfy the requirements for compactness as well as high reliability for the use in, for example, a submarine cable system. SUMMARY OF THE INVENTION It is a general object of the invention to provide a compact optical amplifier. It is another object of the invention to provide a reliable optical amplifier requiring no complicated components nor complicated alignment work thereof. This object is solved according to claim 1 by providing an optical waveguide amplifier for amplifying a signal light propagating therein, comprising: an optical waveguide having a first cross section, a thinned portion of said optical waveguide having a second cross section substantially smaller than said first cross section, said optical waveguide having an input end for receiving said signal light; a glass member formed of a laser glass material doped with a rare earth element, said laser glass member contacting said thinned portion; and a light source for emitting a pumping light to be introduced into said glass member. Preferred embodiments are listed in dependent claims 2 to 24. The thinned portion can be as short as several centi-meters compared with several tens of meters of conventional optical amplifier. The above-mentioned features and advantages of the present invention, together with other objects and advantages, which will become apparent, will be more fully described hereinafter, with reference being made to the accompanying drawings which form a part hereof, wherein like numerals refer to like parts throughout. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a prior art optical amplifier; FIG. 2(a) schematically illustrates a first preferred embodiment of the present invention; FIG. 2(b) shows a cross-sectional side view cut orthogonally to an axis of an amplifier portion of the FIG. 2(a) first preferred embodiment; FIG. 3(a) shows a cross-sectional side view cut along axis of thinned portion of a second preferred embodiment of the present invention; FIG. 3(b) shows a cross-sectional view cut orthogonal to axis of the FIG. 3(a) preferred embodiment; FIG. 4 schematically illustrates radial distribution of light power changing along a gradually thinned optical fiber; FIG. 5 shows amplification gain of the first preferred embodiment versus diameter of the thinned portion of the optical fiber; FIG. 6 schematically illustrates a third preferred embodiment of the present invention; FIGs. 7 schematically illustrate a fourth preferred embodiment of the present invention; FIG. 8 schematically illustrate a cross-sectional view of the thinned portion of FIG. 8 preferred embodiment; FIG. 9 schematically illustrated a fifth preferred embodiment of the present invention; and FIGs. 10 schematically illustrate longitudinal cross-sectional views of thinned portions of sixth preferred embodiments of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to drawings, preferred embodiments of the present invention are hereinafter described in detail. FIGs. 2 show a first preferred embodiment of the present invention. An optical fiber 11 for transmitting a signal light λ₁(referred to hereinafter as a signal fiber) is formed of quartz consisting of a core 1a and a cladding 1b, and has a 125 µm diameter d₁ to allow propagation of a 1.5 µm band signal light. Signal fiber 11, as an optical guide, is thinned as a portion 11a having 10 to 50 µm diameter d₂, typically 30 µm, for approximately 40 mm long by being heated and elongated. Next, glass soot doped with Er is coated on the thinned portion 11a, and is heated so as to melt and fuse thereon, so as to provide a glass member 12 doped with a rare earth (referred to hereinafter as a laser glass member 12). Diameter or thickness d₃ of laser glass member 12 is generally chosen equal to or larger than d₁ of signal fiber 11, that is, chosen 125 µm or larger, so that its mechanical strength is secured. Refractive index of the laser glass member 12 is approximately equal to that of cladding 1b of the signal fiber 11. Material of laser glass member 12 will be further described later in detail. A pumping light λ₂ having wavelength 1.49 µm or 0.98 µm emitted from a laser light source formed of, for example, a popularly employed semiconductor laser is superposed onto a signal light λ₁ via a popularly employed multiplexer 14 which is formed of a half mirror selectively reflecting only the pumping light λ₂, so that the pumping light λ₂ propagates into thinned portion 11a. At thinned portion 11a, both the signal light λ₁ and pumping light λ₂ leak out into laser glass member 12 via outer surface of thinned portion 11a. This is because, as well known, the thinner the diameter becomes, the more extends the radial distribution of light power of a light propagating in an optical fiber as schematically illustrated in FIG. 4; accordingly, a part of the light power leaks out of the optical fiber. This also means that an external light can come into the thinned portion of the optical fiber via its outer surface; in other words, the outside and the inside of the optical fiber are optically coupled at the thinned portion of the optical fiber. Laser glass member 12 is formed of quartz containing ten to several thousands ppm, preferably several hundred ppm, of Er (erbium) doped therein. As widely known, when both of an approximately 1.536 µm signal light λ₁ and an approximately 1.49 or 0.98 µm pumping light λ₂ are input into the laser glass the signal light is amplified. That is, when electrons in the Er doped in the laser glass is pumped by the 1.49 µm pumping light λ₂, there is caused a stimulated emission so as to emit an approximately 1.536 µm wavelength light. Consequently, the signal light λ₁ is amplified. Thus amplified signal light λ₁ in laser glass member 12 is returned via the thinned portion 11a back to signal fiber 11. Pumping light λ 2 ' unnecessary now in signal fiber 11 is removed therefrom by a demulitiplexer 15, which is widely employed and formed typically in the same way as multiplexer 14. Thus, only the amplified light signal λ 1 ' is transmitted out into signal fiber 11. In order to achieve an adequate light amplification, it is preferable that slope 11a' of the transition from diameter d₁ of signal fiber 1 to diameter d₂ of thinned portion 11a is symmetric to slope 11a'' of the transition from diameter d₂ of thinned portion 11a to diameter d₁ of signal fiber 11, i.e. two slopes 11a' and 11a'' in FIG. 2(a) are symmetric with respect to the mid point of thinned portion 11a. Laser glass member 12, which can be as short as several centi-meters as described above compared with the FIG. 1 prior art amplifier requiring as long as several meters to more than one hundred meter long rare-earth-doped glass fiber, allows to achieve a compact optical amplifier. Though the FIG. 2(a) preferred embodiment shows a forward pumping where the pumping light propagates in the same direction as the signal light, it is apparent that the present invention can be embodied in a backward pumping where the pumping light is input via demultiplexer 15, which is used as a multiplexer, so as to propagate into thinned portion 11a in the opposite direction to the signal light. FIG. 3 shows a second preferred embodiment of the present invention. Thinned portion 21a of signal fiber 21 and laser glass member 22 are constituted with substantially identical material and in substantially same size to those of the first preferred embodiment, except that laser glass member 22 comprises a flat surface 22a in addition to curved surface 22b as shown in FIG. 3(b), which is a cross-sectional view cut orthogonally to the axis. Flat surface 22b contacts a surface of a supporting substrate 25. Shape of the cross-sectional views is preferably chosen semicircular, parabolic, elliptic or others. Laser glass member 22 is produced in such way that soot of the laser glass material is coated around outside of thinned portion 21a, and thus coated glass fiber is heated to melt in the same way as that of the first preferred embodiment; next, the fiber glass having the melt glass therearound is put into a mold, and is cooled therein; next, the cooled glass is taken out of the mold. Thus, the glass is formed according to the shape of the mold. Supporting substrate 25 is provided with a light source 23 of a pumping light λ₂ having 1.49 µm or 1.98 µm wavelength formed of typically a popularly employed semiconductor laser, etc., so that the pumping light λ₂ is injected directly into laser glass member 22 via a lens 24 or a transparent plate. Lens 24 focuses and longitudinally distributes the pumping light along the vicinity of the thinned portion 21a. When the input signal light λ₁ is propagating in thinned portion 21a, the signal light leaks out to laser glass member 22 in the same was as the first preferred embodiment. Thus leaked out signal light in laser glass member 22 is amplified by the pumping light in the same principle as the first preferred embodiment. Thus amplified signal light is optically coupled back to thinned portion 22a so as to be transmitted, as indicated λ 2 ', into signal fiber 21. In this preferred embodiment, too, in order to remove the unnecessary pumping light, a demultiplexer may or may not be provided at the output of the amplifier in the same way as the first preferred embodiment. In the second preferred embodiment, approximately 1 db amplification gain was achieved. Moreover, the second preferred embodiment having the light source 23 installed in the substrate 25 as well as being able to omit the multiplexer allows to achieve a compact light amplifier constitution and allows deletion of troublesome alignment work of the multiplexer. Accordingly, the production cost can be low, the production process can be simple as well as the reliability of the operation can be enhanced. Though in the above second preferred embodiment light source 23 is installed in supporting substrate 25, the light source may be located over or on the side of laser glass member 22. As a third preferred embodiment of the invention, the FIG. 2(a) preferred embodiment can be added with light isolators 16-1 and 16-2, respectively between multiplexer 14 and thinned portion 11a, and between thinned portion 11a and demultiplexer 15, as shown in FIG. 6. In FIG. 6, the arrows in the multiplexer and demultiplexer indicate the direction along which the signal light, the pumping light and any other stimulated emission light (i.e. a noise light) can propagate. Isolators 16-1 and 16-2 prevent the lights sent out into signal fiber 11 from returning back into glass member 12. The isolators contribute to an increase in the light amplification gain by several decibels. FIG. 7(a) shows a plan view of a fourth preferred embodiment of the present invention, where the optical transmission line is formed of an optical thin base material waveguides 52. FIG. 7(b) shows a side view looking at input/output side of the FIG. 7(a) preferred embodiment. FIG. 8 schematically illustrates a cross-sectional view cut along ling A-A′ of FIG. 7(a). In a substrate 51 formed of quartz glass, there is formed an optical waveguide 52 having a refraction index higher than that of substrate 51, according to a widely employed technique. A signal light λ₁ and a pumping light λ₂ carried by an optical fiber transmission line 53 are input into an end of optical wave guide 52-1. Optical waveguide 52 is narrowed for a predetermined length so as to form a narrowed portion 54 while the thickness is kept same. In to the vicinity of narrowed portion 54, a dopant, such as Er, is diffused so as to form a laser glass portion 55. Signal light λ₁ and pumping light λ₂ go to and come from laser glass portion 55 via the sides of narrowed portion 54. Signal light λ₁′ amplified in laser glass portion 55 in the same way as the previous preferred embodiments is output via optical waveguide 52-2 to an optical fiber transmission line (not shown in the figure). FIG. 9 schematically illustrates a plan view of a fifth preferred embodiment of the present invention, where are additionally provided a multiplexer 58 and demultiplexer 59, each formed of an optical thin base material waveguide in the same way as transmission line 52, according to a widely known technique. A pumping light λ₂ is input to an optical thin base material waveguide 56, and unnecessary pumping light λ₂′ is removed to an optical thin film waveguide 57. FIGs. 10 schematically illustrate cross-sectional views cut along the thinned portion 54 of optical waveguide of sixth preferred embodiments of the present invention. Optical waveguide 52′ is thinned for a predetermined length so as to form a thinned portion 60 or 60′. Laser glass member 61 or 61′ is covering over thinned portion 60 or 60′, respectively. Surface of thinned portion 60 is coplanar with surfaces of optical waveguides 52′, while bottom of thinned portion is not coplanar with bottoms of optical waveguides 52′, as shown in FIG. 10(a). Bottom of thinned portion 60′ is coplanar with bottoms of optical waveguides 52″, while the surface of thinned portion 60″ is not coplanar with bottoms of optical waveguides 52″, as shown in FIG. 10(b). Though an optical waveguide formed in a substrate has been referred to in the fourth to sixth preferred embodiments, it is apparent that the optical waveguide may be replaced with a ridge-type optical thin base material waveguide formed over a glass substrate. Though in the above preferred embodiments it is explained that the diameter of the optical fiber at the normal diameter portion 11 and 21 is 125 µm; the amplifier portion 11a and 21a is typically 30 µm (10 to 50 µm); wavelength of the pumping light is 1.49 or 0.98 µm; and the laser glass is doped with Er, these are preferable conditions for amplifying 1.536 µm signal light. For other-wavelength light amplifier, the dimensions may be different, and a different rare-earth material, such as neodymium (Nd); yttrium (Yb); samarium (Sm) or praseodymium (Pr) may be doped approximately several tens to several tens thousands ppm, preferably 100 to 200 ppm, and a pumping light having a wavelength appropriate for the employed dopant may be employed. Moreover, aluminum may be additionally doped thereto as well known in order to enhance the activity of the rare earth dopant so as to expand the gain bandwidth of the amplification. Amount of the aluminum addition is preferably several hundreds to several tens of thousands ppm. Though in the above-described preferred embodiments, the thinned portion 11a or 21a was referred to as the present invention 40 mm long, the length may be arbitrarily changed according to the requirement. Moreover, the light source 23 in the second preferred embodiment may be provided in a plural quantity along the thinned portion 21a according to the requirement.

Claims: An optical waveguide amplifier for amplifying a signal light propagating therein, comprising: an optical waveguide (11,21,52,52',52'') having a first cross section, a thinned portion (11a,21a,54,60,60') of said optical waveguide having a second cross section substantially smaller than said first cross section, said optical waveguide having an input end for receiving said signal light (λ₁); a glass member (12,22,55,61,61') formed of a laser glass material doped with a rare earth element, said laser glass member contacting said thinned portion; and a light source (13,23) for emitting a pumping light (λ₂) to be introduced into said glass member, whereby said light output from said thinned portion has been amplified. An optical waveguide amplifier according to claim 1, wherein said optical waveguide is an optical fibre (11,21). An optical waveguide amplifier according to claim 1 or 2, wherein said second cross section is approximately 8 to 40% of said first cross section. An optical waveguide amplifier according to one of the preceding claims, wherein said second cross section has approximately 10 to 50 µm diameter for amplifying an approximately 1.536 µm wavelength light. An optical waveguide amplifier according to claim 1, wherein said optical waveguide is an optical thin base material waveguide (52) formed in a glass substrate (51), said optical thin base material waveguide having a width and thickness. An optical waveguide amplifier according to claim 5, wherein said second cross section is thinner than said first cross section while they have the same width. An optical waveguide amplifier according to claim 5, wherein said second cross section is narrower than said first cross section while they have the same thickness. An optical waveguide amplifier according to one of claims 5 to 7, wherein said glass member (55) is formed in an adjacent portion of said glass substrate (51) with respect to said thinned portion (54). An optical waveguide amplifier according to claim 1, wherein said optical waveguide (52',52'') is a ridge-type optical thin base material waveguide formed on a glass substrate (51), said ridge-type optical thin base material waveguide having a width and thickness. An optical waveguide amplifier according to claim 9, wherein said glass member (61,61') is formed over and on sides of said ridge. An optical waveguide amplifier according to one of the preceding claims, wherein a slope of transition from said first cross section to said second cross section is symmetrical to a slope of transition from said second cross section to said first cross section with respect to a midpoint of said thinned portion (11a,21a,54,60,60'). An optical waveguide amplifier according to one of the preceding claims, wherein said glass member (12,22,55,61,61') is doped with a dopant selected from a group of erbium, neodymium, ytterbium, samarium and praseodymium, wherein an amount of said dopant is approximately ten ppm to several thousands ppm. An optical waveguide amplifier according to claim 12, wherein said laser glass member (12,22,55,61,61') is further doped with aluminum, wherein an amount of said aluminum is several hundreds ppm to several tens of thousands ppm. An optical waveguide amplifier according to one of the preceding claims, wherein a multiplexer (14,58) is provided on said optical waveguide (11,52,52',52'') for introducing said pumping light into said optical waveguide (11,52,52',52'') so as to propagate into said thinned portion (11a,54,60,60'). An optical waveguide amplifier according to claim 14, wherein said multiplexer (14,58) is located at an input side of said signal light to said thinned portion (11a,54,60,60'). An optical waveguide amplifier according to claim 14, wherein said multiplexer (14,58) is located at an output side of said signal light from said thinned portion (11a,54,60,60'). An optical waveguide amplifier according to one of claims 14 to 16 further comprising a demultiplexer (15,59) provided on said optical waveguide (11,52,52',52''), wherein said pumping light propagating from said thinned portion (11a,54,60,60') is removed from said optical waveguide. An optical waveguide amplifier according to claim 17, wherein said demultiplexer (15,59) is located opposite from said multiplexer (14,58) with respect to said thinned portion (11a,54,60,60'). An optical waveguide amplifier according to claim 17 or 18, wherein said optical waveguide (52), said thinned portion (54), said multiplexer (58), and said demultiplexer (59) are formed in or on a glass substrate (51). An optical waveguide amplifier according to claim 1, wherein said light source (23) emits said pumping light via a surface of said glass member (22). An optical waveguide amplifier according to claim 20, wherein said glass member (12,22,55,61,61') has a curved surface and a flat surface for receiving said pumping light. An optical waveguide amplifier according to claim 21, wherein said curved surface reflects said received pumping light to a vicinity of said thinned portion (11a,21a,54,60,60'). An optical waveguide amplifier according to one of claims 20 to 22 further comprising a substrate (25) on which said thinned portion (21a) and said light source (23) are installed, said pumping light being introduced via a hole provided on said substrate to said glass member (22). An optical waveguide amplifier according to claim 23 further comprising a lens (24) or a transparent plate between said glass member (22) and said light source (23).

Inventors: INAGAKI SHINYA, SASAKI KAZUYA, TAKEDA KEIKO, YOSHIZAWA SAKAE, INAGAKI, SHINYA, SASAKI, KAZUYA, TAKEDA, KEIKO, YOSHIZAWA, SAKAE
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: H01S3
IPC codes: H01S3, G02B6, G02F1