Optical amplifiers, and amplification methods

Abstract: An optical amplification medium (4) doped with Er 3+ ions is selected from the group of a fluoride glass, a chalcogenide glass, a telluride glass, a halide crystal, and a lead oxide based glass. The Er 3+ ions are excited by light of at least one wavelength in the range of 0.96 µm to 0.98 µm. A laser or an optical amplifier includes this optical amplification medium (4) doped with Er 3+ ions. Furthermore, an optical amplification method performs an optical amplification using the optical amplifier having the optical amplification medium doped with Er 3+ ions. Thus, the laser to be applied in the field of optical communication, the optical amplifier having the characteristics of low noise and high gain, and the optical amplification method can be provided.

Description: The present invention relates to optical amplifiers with the properties of low noise and high gain, and amplification methods. In recent years, the development of an optical amplifier, in which an optical fiber having a core doped with a rare earth element is provided as an amplification medium, has been worked on for the applications to the field of optical communication. Particularly, an erbium (Er 3+ )-doped fiber amplifier (EDFA) has been developed, and also the development efforts are being made to increase applications of the EDFA to an optical communication system. Recently, a wavelength division multiplexing (WDM) technique has been studied extensively to cope with the diversification of communication service to be expected in coming years. The WDM technique is an optical communication technique that uses a system of multiplexing wavelengths for the sake of an effective use of available transmission medium leading to enlarge a transmission volume. One of the characteristics to be required to the EDFA applied in the WDM technique is a small variation in an amplification gain with respect to a signal wavelength. Because, there are power differentials among optical signals which are transitionally amplified by passing through a multi-stage arrangement of the EDFAs, so that it is difficult to perform the signal transmission with uniform characteristics maintained across all of the wavelengths being used. Presently, therefore, the EDFA showing a flat gain region with respect to the predetermined wavelengths has been investigated by persons skilled in the art. Attention is being given to an erbium(E 3+ )-doped fluoride fiber amplifier (F-EDFA) as a most promising candidate as the EDFA, in which a fluoride-based fiber is used as a host of Er 3+ . The F-EDFA is characterized by its emission spectrum caused by a transition from the 4 I 13/2 level to the 4 I 15/2 level of Er 3+ ions in the fluoride glass at a wavelength band of 1.55 µm. Fig. 1 shows a typical amplitude spontaneous emission (ASE) spectrum of the F-EDFA. This figure also shows the ASE spectrum of an Er 3+ -doped silica glass fiber (S-EDFA). As shown in the figure, the emission spectrum (a full line in the figure) of the F-EDFA is broader than the emission spectrum (a dashed line in the figure) of the S-EDFA. In addition, the response curve of the F-EDFA is smoother than that of the S-EDFA and is flat on top without any steep portion depended on a wavelength in the predetermined wavelength region (M. Yamada et al., IEEE Photon. Technol. Lett., vol. 8, pp882-884, 1996). Furthermore, experiments of wavelength division multiplexing have been carried out using multi-staged F-EDFAs, for Embodiment a cascade configuration with a 980 nm pumped S-EDFA and a 1480 nm pumped F-EDFA (M. Yamada et al., IEEE Photon. Technol. Lett., vol. 8, pp620-622, 1996). In spite of the above development efforts, the F-EDFA has a problem that it cannot reduce a noise figure (NF) as much as that observed in the S-EDFA because of the following reasons. Fig. 2 is an energy diagram of Er 3+ . A phonon energy takes a value of the order of 1,100 cm -1 when the EDFA uses a silica optical fiber as an amplification medium (i.e., in the case of the S-FDFA), so that a favorable population inversion between the 4 I 13/2 level and the 4 I 15/2 level can be formed by an efficient excitation to the 4 I 13/2 level as a result of a phonon emitted relaxation from higher energy levels to the 4 I 13/2 level after exciting to the 4 I 13/2 level by 0.98 µm pump light (Fig. 2 (A)). Consequently, the S-EDFA enables a reduction in the NF to about 4 dB close to a quantum limit (3 dB). On the other hand, the F-EDFA cannot perform an excitation to the 4 I 13/2 level using a transmission from the 4 I 15/2 level to the 4 I 11/2 level because of its low phonon energy. That is, the F-EDFA has a phonon energy of about 500 cm -1 which is almost half of the S-EDFA's phonon energy, so that it is difficult to cause a phonon emitted relaxation from the 4 I 11/2 level to the 4 I 13/2 level and to obtain an amplification gain by 0.98 µm pump light. In this case, therefore, an amplification gain at a wavelength of 1.55 µm is obtained by directly exciting from the 4 I 15/2 level to the 4 I 13/2 level using light at a pump wavelength of about 1.48 µm (Fig. 2 (B)). However, this kind of the excitation is an initial excitation of the gland energy level to the higher energy level, so that it is difficult to make a favorable population inversion in which the number of Er 3+ ions at higher energy levels exceed those at lower energy levels, resulting in the high NF (i.e., 6 to 7 dB). Therefore, the conventional F-EFDA with favorable noise characteristics has not been realized, compared with that of the S-EFDA. Furthermore, document US-5 247 529 discloses an excited state absorption (ESA) by a wavelength λ 3 = 850 nm, and the energy levels utilized in this ESA are 4 I 13/2 and 4 S 3/2 . The function of this ESA is to decrease the degree of optical amplification. In addition, documents JP-A-04 149 525 and US-5 134 517 disclose amplification structures which are similar to what is utilized according to the present invention except for the fluoride glasses as amplification medium. It is an object of the invention to solve the above problem (i.e., high noise figure) of the conventional F-EDF and to provide an optical amplifier with the properties of low noise, and high and flat gain, and an amplification method. According to the present invention, this object is solved according to the appended independent claims. Advantageous modifications according to the present invention are as defined by the appended dependent claims. Fig. 1 is a typical amplitude spontaneous emission (ASE) spectrum of the F-EDFA; Fig. 2 is an energy diagram of Er 3+ for the conventional S-EDFA (A) and the conventional F-EDFA (B); Fig. 3 is a graphical representation of the relationship between the wavelengths and the absorption or emission cross section with respect to the energy state between the 4 I 15/2 level and 4 I 11/2 level; Fig. 4 is an energy diagram of Er 3+ for the F-EDFA of the present invention; Fig. 5 is a schematic block diagram of an optical amplifier using a Er 3+ -doped ZrF 4 -based fluoride fiber in accordance with the present invention; Fig. 6 is a graphical representation of the relationship between the pump wavelengths and the signal gains with respect to the optical amplifier using the Er 3+ -doped ZrF 4 -based fluoride fiber of Fig. 5; Fig. 7 is a schematic block diagram of an optical amplifier as one of the preferred embodiments of the present invention; Each of optical amplifiers and amplification methods of the present invention is mainly characterized by employing at least one pump light at a wavelength in the range of 0.96 µm to 0.98 µm for the excitation of Er 3+ from the ground level to the 4 I 11/2 level because of the following reasons. Fig. 3 is a graph representing the relationship between the pump wavelengths and the cross-sectional areas (in the figure, a full line shows a absorption cross section and a dashed line shows an induced emission cross section) with respect to an energy state between the 4 I 15/2 level to 4 I 11/2 level. In the wavelength region of over about 980 nm, as shown in the figure, the induced emission cross section area (dashed line) becomes greater than the absorption cross section area(full line). Therefore an induced emission transition from the 4 I 11/2 level to the 4 I 15/2 level tends to occur more strongly compared with an absorption transition from the 4 I 15/2 level to the 4 I 11/2 level in that wavelength region, so that the excitation to the 4 I 11/2 level cannot be occurred effectively. Alternatively, as shown in the figure clearly, the excitation to the 4 I 11/2 level can be effectively occurred by pumping at a wavelength shorter than 980 nm. In this case, on the other hand, the pump ESA (Excited State Absorption) from the 4 I 11/2 level to the 4 F 7/2 level is more likely to take place. As shown in Fig. 4, however, the excitation to the 4 I 13/2 level can be eventually attained because of the step of relaxing the 4 F 7/2 level to the 4 I 13/2 level. (Embodiment 1) Referring now to Fig. 5, there is shown a basic configuration of an optical amplifier having an Er 3+ -doped ZrF 4 -baed fluoride fiber as one of the preferred embodiments of the present invention. For more detailed explanation, an excitation spectrum (the pump wavelength dependency of the signal gain) of the above fiber is shown in Fig. 6. The optical amplifier comprises two optical isolators 1, 2, an pump light source 3, and an Er 3+ -doped ZrF 4 -based fluoride fiber sandwiched between the optical isolators 1, 2. In this embodiment, the fiber 4 is of 25 m in length with the cut-off wavelength of 1 µm, and also the doping concentration of Er 3+ in its core is 200 ppm. In this embodiment, furthermore, a signal wavelength is 1530 nm, an input signal power is -30 dBm, and a pump light power is 60 mW. Depending on the above configuration of the optical amplifier, the maximum gain can be obtained at a pump wavelength of 970 nm. As shown in Fig. 6, however, a negative gain is observed at a pump wavelength of 980 nm. This wavelength is conventionally used for exciting to the 4 I 11/2 level, and thus we recognize that we cannot obtain the gain at the pump wavelength of 980 nm. Therefore, any wavelength in the range of 960 nm to 980 nm, preferably in proximity to 970 nm is effective to obtain a gain by exciting to the 4 I 11/2 level. Then the amplification characteristics of the above fiber 4 is investigated by a forward excitation using pump light at a wavelength of 970 nm (i.e., the pump light is launched into the fiber 4 from the upstream side of. the fiber 4 by the light source 3). In this case, the input signal power launched into the fiber 4 is -30 dBm. When the pump light power is 132 mW, an obtained gain at a wavelength of 1.53 µm is 30 dB and a noise figure (NF) is 4.5 dB. Also, the NF is 3.5 dB when the wavelength is 1.55 µm. When the above fiber 4 is excited by an pump light with the wavelength of 1.48 µm, an improvement degree of the NF is 1.5 dB or over with reference to the NF at 1.55 µm of 5 dB or over. In addition, we confirmed that the NF was improved (decreased) when the pump light was of any wavelength in the range of 960 nm to 980 nm, compared with that of exciting at 1.48 µm. Furthermore, the NF is improved by the excitation using two or more wavelengths in the range of 960 nm and 980 nm. (Embodiment 2) The same optical amplifier as that of Embodiment 1 is used in this embodiment to measure the NF by introducing WDM signals at eight different wavelengths in the range of 1530 to 1560 nm. The input signal power launched into the optical amplifier is -20 dBm per one wavelength. When the excitation is performed with a total pump light power of 150 mW using the pump wavelength of 970 nm, the observed NF is 5 dB or less by introducing the WDM signals at the wavelengths in the range of 1530 to 1560 nm. (Embodiment 3) In this Embodiment, the amplification characteristics of an optical amplifier are estimated using the same WDM signals as those of Embodiment 2 except of the follows. In this Embodiment, an optical amplifier is the same one as that of Embodiment 1 or 2 except that a bidirectional pump method is used for launching different pump light into the fiber 4. The method comprises the steps of applying pump light at wavelengths in the range of 960 to 980 nm from the front (i.e., the upstream side of the fiber 4 in the same direction as that of the signal light) and simultaneously applying pump light at a wavelength of 1480 nm from the rear (i.e., the downstream side of the fiber 4). Fig. 7 shows a configuration of the optical amplifier. Comparing with a configuration of the optical amplifier shown in Fig. 5, an additional light source 5 for the excitation to the 4 I 13/2 level is further installed in the optical amplifier and positioned at the downstream side of the Er 3+ -doped fluoride fiber 4. The pump light power for the front is 50 mW, while the pump light power for the rear is in the range of 100 mW to 150 mW. In addition, the optical amplifier shows the NF of 5 dB or less for the wavelengths of 1530 nm to 1560 nm, allowing the gain excursion of 2 dB or less for the signal wavelength. (Embodiment 4) In each of Embodiments 1 to 3 described above, the amplification characteristics of the optical amplifier using the Er 3+ -doped ZrF 4 -based fluoride fiber as its amplification medium are evaluated. In this Embodiment, an amplification medium as a host of Er 3+ is selected from the group of an InF 3 -based fluoride fiber, a chalcogenide glass-based fiber, a TeO 2 -based fiber, and a PbO-based fiber, instead of the ZrF 4 -based fluoride fiber to prepare an optical amplifier. Then the optical amplifier having any one of the above fibers is subjected to the same experiments as those of Embodiments 1 to 3 to evaluate its amplification characteristics. As a result, the optical amplifier having any one of the fibers listed above as the amplification medium shows the NF of 5 dB or less. Consequently, as explained above, Embodiments 1 to 4 allow the amplification of 1.55 µm band by the excitation to the 4 I 11/2 level which enables to achieve a low noise amplification whether an infrared-transparent fiber such as a fluoride one (which is regarded as an improper medium by persons skilled in the art) is used as a host of Er 3+ . Hence, the optical amplifier having the characteristics of a flat gain with a wide amplification bandwidth and a low noise is obtained. The optical amplifier thus obtained can be applied in a communication system to increase a transmission volume thereof and to provide a diversification of the system configuration to achieve the wide dispersion of an optical communication, the substantial reduction in a manufacturing cost thereof, and so on.

Claims: A method for amplifying an optical signal at a wavelength band of 1.5 µm, which uses an optical amplification medium ( 4 ) doped with Er 3+ ions, characterized by comprising a step of exciting said Er 3+ ions to 4 F 7/2 energy level by two-stage absorption of a first pump light from a first light source ( 3 ) of at least one wavelength in a range of 0.96 µm sip to but not including 0.98 µm, and the Er 3+ ions excited to 4 F 7/2 energy level being relaxed to 4 I 13/2 energy level and where said optical amplification medium ( 4 ) is selected from a group of a fluoride glass, a chalcogenide glass, a telluride glass, a halide crystal, and a lead oxide based glass. An optical amplification method as claimed in claim 1, characterized in that said optical amplification medium ( 4 ) is in a shape of a fiber. An optical amplifier for amplifying an optical signal at a wavelength band of 1.5 µm, having an optical amplification medium ( 4 ) doped with Er 3+ ions, and a first light source ( 3 ), characterized in that said optical amplification medium ( 4 ) is selected from a group of a fluoride glass, a chalcogenide glass, a telluride glass, a halide crystal, and a lead oxide based glass, and said Er 3+ ions are excited to 4 F 7/2 energy level by two-stage absorption of a pump light of at least one wavelength in a range of 0.96 µm up to but not including 0.98 µm from said light source ( 3 ), the Er 3+ ions excited to 4 F 7/2 energy level is relaxed to 4 I 13/2 energy level. An optical amplifier as claimed in claim 3, characterized in that said optical amplification medium (4) is in a shape of a fiber. An optical amplifier as claimed in claim 3, further having a second light source (5), characterized in that said Er 3+ ions are excited to 4 I 13/2 energy level by a second pump light at a wavelength corresponding to an energy difference between 4 I 15/2 level and 4 I 13/2 level of said Er 3+ ions from said second light source (5), wherein said first pump light is launched from the same direction of said optical signal, and said second pump light is launched from the opposite direction of said optical signal. An optical amplifier as claimed in claim 5, characterized in that said optical amplification medium (4) is in a shape of a fiber. An optical amplification method as claimed in claim 1, characterized by further comprising a step of launching a second pump light, which is emitted in an opposite direction of said first pump light, from a second light source ( 5 ) for exciting said Er 3+ ions from 4 I 15/2 energy level to 4 I 13/2 level into said optical amplification medium ( 4 ). An optical amplification method as claimed in Claim 7, characterized in that said optical amplification medium (4) is in a shape of a fiber.

Inventors: KANAMORI TERUTOSHI, OHISHI YASUTAKE, SUDO SHOICHI, YAMADA MAKOTO, KANAMORI, TERUTOSHI, OHISHI, YASUTAKE, SUDO, SHOICHI, YAMADA, MAKOTO
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1997
Main classification: H01S3
IPC codes: H01S3