The present invention relates to a wide-band optical amplification device. In particular, the present invention relates to a wide-band optical amplification device by means of bismuth fluorescence relevant to an optical communication, an optical fiber amplifier, a high optical power amplifier, a high power laser, and a laser oscillator.
In recent years, luminescence in the infrared region was found from a Bi (Bismuth) doped silica glass. By use of this new type of fluorescence, it is expected to realize a wide-band amplifier and a wide-band laser oscillator including an optical fiber amplifier operating in the 1.3 μm optical information communication wavelength range.
On the other hand, an Er (erbium) doped fiber amplifier used in the optical communication has its amplification bandwidth in 1.55 μm region.
The zero-dispersion wavelength of the commonly used single mode silica fiber exists, however, in 1310 nm region, and an optical amplifier suitable to such wavelength region is limited only to a fluoride fiber such as Pr(praseodymium):ZBLAN. A problem of this fluoride is its sensitivity to environment conditions such as humidity. It is therefore desired to realize an amplifier in the band from 1000 nm to 1600 nm which is insensitive to environmental changes.
Furthermore, as for a high power laser, the output power of the laser using Nd (neodymium) as a fluorescent center is limited by the influence of the ESA (Excited-State Absorption).
[Patent document 1] Japanese Patent Publication No. 11-029334.
[Patent document 2] Japanese Patent Publication No. 2002-252397.
[Non-Patent document 1] K. Murata, Y. Fujimoto, M. Nakatsuka, T. Kanabe and H. Fujita, “Bi and SiO2 as a new laser material for an intense laser”, Fusion Engineering and Design, 44 (1999), pp. 437-439.
[Non-Patent document 2] Y. Fujimoto, M. Nakatsuka, T. Omae, M. Yoshida and Y. Sudo “New luminescent properties of Bi doped silica glass in 1.3 μm range”, Journal of The Institute of Electronics, Information and Communication Engineering, C-I, Vol. J83-C, No. 4, (2000), pp. 354-355.
[Non-Patent document 3] Y. Fujimoto and M. Nakatsuka, “Luminescent properties in 1.3 μm range of Bi-doped silica glass by 0.8 μm range excitation and their application to optical communication”, Journal of The Institute of Electronics, Information and Communication Engineering, C-I, Vol. J84-C, No. 1, (2001), pp. 52-53.
[Non-Patent document 4] Y. Fujimoto and M. Nakatsuka, “Infrared fluorescence from bismuth doped silica glass”, Jpn. J. Appl. Phys., Vol. 40 (2001), No. 3B, pp. L279-L281.
[Non-Patent document 5] Y. Fujimoto and M. Nakatsuka, “Optical amplification in bismuth-doped silica glass”, Appl. Phys. Lett., 82 (2003), pp. 3325-3326.
[Non-Patent document 6] Y. Fujimoto, H. Matsubara and M. Nakatsuka, “A Fluorescence Spectrum at 1.3 μm of Bismuth-Doped Silica Glass with 0.8 μm Excitation”, CLEO/QELS'01, CWJ1, Baltimore Convention Center, USA, May 9, 2001, Technical Digest Series.
[Non-Patent document 7] Y. Fujimoto, H. Matsubara and M. Nakatsuka, “New Fluorescence from Bi-Doped Silica Glass and its 1.3-μm Emission with 0.8-μm Excitation for Fiber Amplifier”, CLEO/Pacific Rim 2001, Nippon Convention Center, Chiba, JAPAN, Jul. 15-19, 2001, Technical Digest Series.
[Non-Patent document 8] Y. Fujimoto and M. Nakatsuka, “New fluorescence at 1.3-μm with 0.8-μm excitation from Bi-doped silica glass”, CLEO/Europe-EQEC, 2003, CG8-2-FR1, 23-27 Jun., 2003, International Congress Center (ICM) Munich, Germany.
[Non-Patent document 9] Yasushi FUJIMOTO and Masahiro NAKATSUKA, “New fluorescence at 1.3-μm with 0.8-μm excitation from Bi-doped silica glass and its optical amplification”, XX International Congress on Glass, 0-07-077, Sep. 27-Oct. 1, 2004, Kyoto International Conference Hall, Kyoto, JAPAN.
[Non-Patent document 10] Shoichi KISHIMOTO, Masahiro TSUDA & Koichi SAKAGUCHI, Yasushi FUJIMOTO and Masahiro NAKATSUKA, “Novel bismuth-doped optical amplifier for 1.3-micron telecommunication band”, XX International Congress on Glass, 0-14-029, Sep. 27-Oct. 1, 2004, Kyoto International Conference Hall, Kyoto, JAPAN.
Bi-doped silica glass has a silica glass as a main composition, but it exhibits a very broad fluorescence in the region from 1000 nm to 1600 nm. Making use of the characteristics, the present invention provides a wideband amplifier by configuring an optical amplifier in which this fluorescent material (including the optical fiber) is used. Since the main composition of this optical fiber is a silica glass, it withstands environmental changes. An optical amplification of this fiber only at a single wavelength of 1.3 μm has been confirmed as described in said non-patent document 5, and no demonstrations of the amplification have been made at any other wavelengths.
More specifically, this novel fluorescent material is processed into a bulk form or a fiber form in the present invention, and by superimposing an excitation light in the visible range with a wavelength tunable infrared probe light to be amplified in the sample, a wide band amplifying device in the infrared region is realized.
In view of the situation described above, the purpose of the present invention is to provide a wideband optical amplifying device capable of amplifying wideband signals in the infrared region.
In order to achieve the above objects, the present invention provides:
[2] the wideband optical amplifying device described above in [1], characterized in that the optical amplification is realized in a wavelength range from 1000 nm to 1600 nm by using the glass or the crystal including bismuth as a fluorescent center and by an optical excitation.
[3] the wideband optical amplifying device described above in [1], characterized in that the wavelength range for amplification is from 1000 nm to 1600 nm and a plurality of wavelengths within this range can be simultaneously amplified.
[4] the wideband optical amplifying device described above in [1], characterized in that the wavelength range for amplification is from 1000 nm to 1600 nm, and a chirped light (a light pulse whose spectral wavelength is arranged in a time sequence) with ultra short pulses can be amplified.
[5] the wideband optical amplifying device described above in [1], characterized in that the wavelength range for amplification is from 1000 nm to 1600 nm, and a light from a source with a continuous wideband spectrum can be amplified.
[6] the wideband optical amplifying device described above in any one of [2] to [5], characterized in that the wavelength of the excitation light is from 400 nm to 1000 nm.
[7] the wideband optical amplifying device described above in any one of [2] to [5], characterized in that the wavelength of the excitation light lies in any one of the wavelength ranges of 500±100 nm, 700±100 nm, 850±100 nm, and 950±100 nm.
[8] the wideband optical amplifying device described above in any one of [2] to [5], characterized in that the excitation light has at least two or more wavelengths within the excitation wavelength range.
[9] the wideband optical amplifying device using the bismuth fluorescent material described above in [8], characterized in that the equalizing property of the amplification characteristics is at most 25% over a wavelength region from 1000 nm to 1400 nm.
[10] the wideband optical amplifying device described above in any one of [2] to [9], characterized in that the amplifying device is uses as a laser oscillator.
A wideband optical amplifying device using bismuth fluorescent material in accordance with the present invention is characterized in that an optical amplification is realized by optical excitation of a glass or a crystal having bismuth as a fluorescent center, and that a wavelength range for amplification is from 1000 nm to 1600 nm. Therefore, wideband amplification is realized, which makes a large capacity optical communication system feasible.
Embodiments in accordance with the present invention are described in detail.
In this figure, 1 is an excitation LD light source (0.8 μm), 2 is a box of an optical system, 2A is its first input connector, 2B is its second input connector, 2C is its output connector, 3 is an optical fiber cable, 4, 6 and 10 are adaptors, 5 is a bismuth fiber (sample) with connectors attached, 7 is an optical spectrum analyzer, 8 is a wavelength tunable LD light source (1260 nm to 1360 nm) as a probe LD light source, 9 and 11 are FC type connectors with vertically polished faces, 12 is an optical isolator, and 13 is a single mode fiber.
Table 1 shows definitions of various measured values.
Table 1 shows definitions of measured values in the amplification signal measurement system.
Here, a case where the excitation LD light source 1 is off and the probe (signal) LD light source 8 is off is taken as a measured value of the background signal A, a case where the excitation LD light source 1 is off and the probe LD light source 8 is on is taken as a measured value of the signal light (1.3 μm) B, a case where the excitation LD light source 1 is on and the probe LD light source 8 is on is taken as an amplified output value C (measured value of signal light plus excitation light), and a case where the excitation LD light source 1 is on and the probe LD light source 8 is off is taken as an excitation light (0.8 μm) D passed through a sample (bismuth fiber with connectors attached) 5.
An optical amplification coefficient Gain is a ratio between an output light power and an incident light power, and is expressed by the following equation.
Gain=(C−D)/(B−A)=I/I0 (1)
Here, I is an output light power, and I0 is an incident light power. In addition, a gain coefficient g of a sample having thickness t is defined as follows.
g=(1/t)ln(I/I0) (2)
A sample under test is a Bi-doped silica fiber 5 with connectors attached, which is made of a Bi-doped silica glass with Bi concentration of 0.5 mol %.
Here, the wavelength of the probe (signal) LD light source (wavelength tunable probe light source for amplification) 8 includes the zero dispersion wavelength 1310 nm in its tunable range, and is increased by a step of 20 nm from 1260 nm to 1360 nm. The amplified output light from the Bi-doped silica fiber with connectors attached 5 is measured. The result is shown in
Subsequently, an amplification experiment was performed by using the Bi-doped silica optical fiber in accordance with the present invention.
In this figure, 21 is an excitation LD light source (0.8 μm), 22 is a box of an optical system, 22A is its first input connector, 22B is its second input connector, 22C is its output connector, 23 is an optical fiber cable, 24 is a measuring system for a fiber or a bulk, 25 is an optical fiber (Bi-doped silica glass), 26 is an OFR focuser, 27 is an optical spectrum analyzer, 27A, 33A, 34A, 34B, 46A, 48A, 48B are connectors, 28 and 36 are LD drivers, 29, 37, 39 to 43 are device cables with connectors attached, 30 is a probe LD light source (1.3 μm), 31, 45, 50, 52, 54, and 56 are connectors (SC/PC), 32 and 46 are FC-SC transforming adaptors, 33 and 47 are isolators, 34 and 48 are FC-FC transforming adaptors, 35 is a fiber coupler, 38 is a device change-over box, 38A is its input terminal, 38B is its output terminal, 44 is a 1.272 μm LD light source, 49 is a 1.297 μm LD light source, 51 is a 1.307 μm LD light source, 53 is a 1.323 μm LD light source, 55 is a 1.347 μm LD light source, and 57 is a (FC/APC) connector for monitoring signals.
An optical fiber (Bi-doped silica glass) 25 used in this experiment was very fragile and easy to break. Therefore, as shown in
The core diameter of the Bi-doped silica fiber used in this case was 13 μm, and the core diameter of the fiber of the excitation light source used was 50 μm. Therefore, since light focusing of 50 μm or less can not be realized in principle, a coupling loss due to this must be taken into account. By reducing the coupling loss by means of fusion splicing for example, more efficient amplification system can be expected in the near future. In addition, since the power level of the excitation light source is reduced to a level of 100 mW, availability of a single mode excitation semiconductor laser, which is now used as an excitation light source of an amplifier for an optical communication and provides output power of about 100 mW in typical cases, can be positively considered. This will promote prospect in fabricating an amplifier for the optical communication significantly.
As an experiment of simultaneous amplification with multiple wavelengths, multiple amplification characteristics was measured with five wavelengths 1272 nm, 1297 nm, 1307 nm, 1323 nm, and 1347 nm with an anchor wavelength of 1308 nm. The results are shown in
a) shows a result of simultaneous amplification with two wavelengths 1272 nm and 1308 nm,
As is clear from the previous results, it is understood that the simultaneous amplification with two wavelengths can be achieved by using the Bi-doped silica fiber. Fluctuations in optical gain between wavelengths is considered to come from the difference in coupling efficiencies (both on the incidence side and exit side) between different wavelengths in the case where the free space light is coupled. For example, by adjusting the coupling, the respective amplification factor changes. In any event, an improvement is expected, for example, by fusion splicing of the objective fibers.
The results described above show that fabrication of an amplifier with high efficiency is possible by an amplifier arrangement in the form of a fiber. Therefore, by reducing the loss by fusion splicing of the objective fibers using a fusion splicer, the development of higher efficient amplifier is expected. In addition, since the possibility to use an excitation light source with output power in a class of 100 mW is raised, the progress toward realization of a practical device has been made. As for the wavelength multiplexing amplification, two wavelength amplification was confirmed with bandwidth of 75 nm or more.
Next, a result of two wavelength amplification using a bulk glass is described. A sample used in the measurement is Bi2O3 (1.0 mol %), Al2O3 (7 mol %), SiO2 (91.9 mol %), Tm2O3 (0.1 mol %). The sample surfaces are both polished to be perpendicular to the incident beam. The measurement system differs from that used in the case of a fiber amplification only in that the optical fiber (Bi-doped silica glass) 25 shown in
As can be seen from the table, simultaneous amplification at two wavelengths is demonstrated even when the bulk type glass is used. Amplification at multiple wavelengths is possible regardless of the fiber structure or the bulk structure.
Next experiment was carried out by using a Bi-doped optical fiber with fusion splicing. The main part of the experimental apparatus is the same as the one shown in
The Bi-doped silica fiber used here has a core-clad structure, and a core diameter is 9 μm. Since the LD light source for excitation and signal has an output form with a multimode (MM) fiber of 50 μm in core diameter, a silica MM fiber is used to splice the Bi-doped silica fiber. The measured result of the dependence of the amplification factor on the length of the optical fiber is shown in
The excitation LD power derived into the optical fiber measured by the cut back method was 520 mW. It was 353 mW at a point 1 cm from the splicing point. After that, an attenuation of the excitation light of 15 mW per 1 cm each was observed. From this, a loss at the splicing point is estimated to be about 30%, or about 150 mW. Furthermore, the loss coefficient of the optical fiber at 1.3 μm wavelength band measured by the cut back method was 0.0977 cm−1 (−42.4 dB/m). As shown in
Subsequently, the following experiment was performed by arranging the Bi-doped silica fiber which was fusion spliced with single mode fibers at the both ends. Main part of the experimental apparatus is shown in
The Bi-doped silica fiber used here has a core-clad structure, and the core diameter is 9 μm. The fusion spliced fiber length was 5.5 cm. Since the LD light source for excitation (845 nm) has an output form with a single mode (SM) fiber, a silica SM fiber is used to splice with the Bi-doped silica fiber. The splicing part was shown in a photograph in
Power of the excitation LD derivered into the optical fiber was measured 81.4 mW, which is about ⅙ of the excitation power when 9.7 dB of gain was obtained. From
As described above, the basic property with respect to a wideband amplifier using the Bi-doped silica glass has been measured, thereby leading to an expectation to realize a wideband amplifier in a wavelength range around 1.3 μm.
The basic configurations of a wideband amplifier based on the experimental results described above are shown in
A feasibility to equalize the amplification characteristics is shown in the following. The Bi-doped silica glass has excitation wavelength regions of 500±100 nm, 700±100 nm, 850±100 nm and 950±100 nm, each having different fluorescence spectrum shape. By making use of at least two excitation wavelengths, a gain equalization can be expected.
As shown in
The equalization characteristics described above may be changed by a composition of the Bi-doped silica glass. Therefore, the excitation wavelength might be different for new composition, but it can be considered to lie within ±50 nm around 850 nm.
The two wavelength amplification was confirmed over a band width of 75 nm or more. This demonstrates that the Bi-doped silica fiber in accordance with the present invention can be operated as a wideband amplifier, has a function of simultaneous amplification at multiple wavelengths, and can realize a gain equalization by simultaneous excitation at two or more excitation wavelengths.
According to the present invention, an optical amplification over most of the band which is exhibited by the fluorescence spectrum of the Bi-doped silica glass is realized, and a wideband amplifier which will give the optical communication a larger capacity is realized. Furthermore the simultaneous optical amplification over a wideband will realize a function as an optical amplifier to amplify a chirped ultra short optical pulse. Due to this, it is possible to apply to various uses including a laser for processing, and a THz wave generation.
The present invention is not limited to the above-described embodiments, various modifications are possible based on the spirit of the invention. These modifications are not excluded from the scope of the invention.
The wideband optical amplifying device in accordance with the present invention can be applied to optical communication, an optical fiber amplifier, a high power optical amplifier, a high peak power laser, and a laser oscillator.
Number | Date | Country | Kind |
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2005-059908 | Mar 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2006/303739 | 2/28/2006 | WO | 00 | 8/30/2007 |