Abstract:
An optical amplifier includes an optical fiber having a core doped with transition metal ions, and at least one glass cladding enclosing the core. By using the fiber, the optical amplifier of the invention has a gain bandwidth of more than 300 nm including 1300-1600 nm band in low-loss optical communication.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to optical fiber amplifiers, and more particularly to a fiber amplifier having a core doped with transition metal. 
   2. Description of the Related Art 
   Many types of optical fiber amplifiers are known in the art and are suitable for various optical communication bands. 
   U.S. Pat. No. 6,646,796 disclosed a wide band erbium-doped fiber amplifier. It connected two erbium-doped fiber amplifiers to achieve a super wide band amplifier. The first erbium-doped fiber amplifier pumped by a 980 nm laser diode was used to amplify the input optical signal of the C-band, which has a wavelength ranging from 1530 nm to 1560 nm. The second erbium-doped fiber amplifier pumped by a 1480 nm laser diode was used to amplify the input optical signal of the L-band, which has a wavelength ranging from 1570 nm to 1600 nm. 
   The disadvantage of the above-mentioned U.S. patent is that the band of the optical signal that can be amplified falls only in the C-band and the L-band, which has a bandwidth shorter than 80 nm. This bandwidth is only a part of a 300 nm bandwidth used in the optical communication. Besides, it needs two erbium-doped fiber amplifiers and two pumping lasers of different wavelength, which increase the cost of the system. 
   U.S. Pat. No. 6,515,795 disclosed a thulium-doped fiber amplifier. The fiber has a borosilicate glass material cladding and a core comprising a germinate glass material doped with Tm 3+ . It can amplify the optical signal of 1400-1540 nm with a pumping source. The pumping source used in the amplifier is a semiconductor laser, having essentially a wavelength of 780-800 nm. 
   The disadvantage of the above-mentioned amplifier is that the melting point of the fiber with borosilicate glass material cladding is about 800° C. such that the fiber cannot be connected by fusion to the present fused silica fiber that has a melting point of 1600° C. It increases the system cost by using additional optical elements or mechanical structures as optical couplers. In addition, the bandwidth of the optical signal that can be amplified is about 160 nm, which does not cover the whole 300 nm bandwidth of the optical communication. 
   U.S. Pat. No. 5,805,332 disclosed a praseodymium-doped fiber amplifier. The fiber has a fused silica cladding and a core comprising zirconium dioxide doped with praseodymium ion. The fiber amplifier is well suited for use in the 1300 nm band when pumped with an optical source having a wavelength of about 1000 nm. However, the 1300 nm band does not cover the whole band in the optical communication. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to provide a transition metal doped fiber amplifier, which has a gain bandwidth of more than 300 nm including the 1300-1600 nm band used in optical communication. 
   To achieve the objective of the present invention, an optical fiber amplifier provided by the present invention comprises an optical fiber having a core doped with transition metal ions, and at least one glass cladding enclosing the core. By using the fiber, the optical fiber amplifier of the invention has a gain bandwidth of more than 300 nm including the 1300-1600 nm band used in low-loss optical communication. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic drawing of an apparatus for fabricating a Cr 4+ :YAG crystal fiber according to the laser-heated pedestal growth apparatus of the present invention; 
       FIG. 2  is a schematic drawing showing the grown procedure of the crystal fiber according to the present invention; 
       FIG. 3  shows a schematic drawing of a molten zone during growth; 
       FIG. 4  is a cross-sectional view of the grown double-clad Cr 4+ :YAG fiber; 
       FIG. 5  is a plot showing the relationship between the gross gain and the pumping power subject to various input signal powers; 
       FIG. 6  is a plot showing the gross gain and the ASE spectrum at different signal wavelength. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A transition metal doped optical fiber amplifier in accordance with the present invention essentially comprises a fiber doped transition metal ion such as Cr 4+ , V 3+ , Ni 2+ , or etc. The fiber provided in the preferred embodiment of the present invention is a double-clad crystal fiber doped with chromium. The method of making the fiber according to the present invention comprises a first step of growing a Cr 4+ :YAG crystal fiber having a diameter of 68 μm by using the laser-heated pedestal growth (LHPG) method, and a second step of obtaining a double-clad crystal fiber enclosed by fused silica by using codrawing laser-heated pedestal growth (CDLHPG) method. 
     FIG. 1  is a schematic drawing showing an apparatus for fabricating a crystal fiber according to the LHPG method. CO 2  laser beam  11  provided by a CO 2  laser generator (not shown) is emitted to a beam splitter  12  such that the CO 2  laser beam is split into a generally annular beam. A bending mirror  13  is used to reflect the annular beam from the beam splitter  12  and to project it to a paraboloidal mirror  14 . The paraboloidal mirror  14  is used to reflect the annular beam from the bending mirror  13  and to focus the annular beam on one end of a source crystal rod  20 , which is YAG crystal doped with Cr 4+  ions. Referring to  FIG. 2 , the end of the source crystal rod  20  can be melted to form a molten zone  21  by the laser beam  11 . An oriented seed  30  is fed downward to dip in the molten zone  21 , and then the oriented seed  30  is pulled upward slowly, while the source crystal rod  20  is fed toward the molten zone  21  so that a new grown crystal core  40  is formed at the end of the oriented seed  30 , wherein the crystal orientation of the new grown crystal core is the same as that of the oriented seed  30 . Different ratio of the pull speed of the oriented seed  30  to the feed speed of the source crystal rod  20  determines the diameter of the new grown crystal fiber  40 . For example, if the ratio of the pull speed of the oriented seed  30  to the feed speed of the source crystal rod is 16:1, the diameter ratio of the new crystal fiber  40  to the source crystal rod is 1:4. 
     FIG. 3  is a schematic drawing showing the cladding step according to the CDLHPG method. A Cr 4+ :YAG crystal fiber having a diameter of 68 μm made by the above-mentioned LHPG method is inserted into a fused silica tube  50  having an inner diameter of 76 μm. The fused silica tube  50  thus obtained is heated by the laser beam of the LHPG system to downwardly grow a double-clad crystal fiber  60 . The method is called codrawing laser-heated pedestal growth (CDLHPG) method. By controlling suitable growth factor of the CDLHPG method, a double-clad crystal fiber enclosed by fused silica can be achieved. As shown in  FIG. 4 , the double-clad crystal fiber thus obtain has a Cr 4+ :YAG crystal core  61 , an inner cladding  62  and an outer cladding  63 . 
   The diameters of the core  61 , the inner cladding  62  and the outer cladding  63  are 25 μm, 100 μm and 320 μm respectively. In the measurement of gross gain, a 1064 nm Yb-fiber laser is used as a pump source. The pump beam is guided into a signal mode fiber by a coupled lens and combined with 1520 nm signal beam through an optical coupler, such as a wavelength-division-multiplexing (WDM) coupler, and the optical coupler output is spliced to the double-clad crystal fiber by a fusion splicer. The signal light from the output of the double-clad crystal fiber was concentrated by a 10 mm-focus lens. The unabsorbed pumping light was filtered by a filter at a wavelength of 1064 nm, and than the gross gain is determined under different signal input power. The definition of the gross gain is the gain of the output signal power with pumping to the output signal power without pumping, and the contribution of the amplified spontaneous emission (ASE) has to be of course subtracted. The result of the measurement is shown in  FIG. 5 . As shown in  FIG. 5 , the gross gain of 10 dB is obtained at the pump power of 0.83 W under the input signal power of −20 dBm. 
   On the application of the amplifier, the propagation directions of the pumping light and the outputted amplified signal light can be controlled to be same or reverse. In order to transit a signal light or pumping light in a single direction, or to prevent the reflected light caused by connection of optical elements from interfering with the signal light or pumping light, an optical isolator can be provided at the output end of the transition metal doped fiber or at the input end of the optical coupler for signal light passing. The optical isolator is but not limited to a Faraday isolator. 
   Besides, the gross gains to various wavelengths of the input signal lights under the pump power of about 1 W are measured. As shown in  FIG. 6 , the gross gain of 16 dB is obtained at the signal wavelength of 1.47 μm. The bandwidth of the optical amplifier of the present invention that is calculated by comparing the measured gross gain and the ASE spectrum is about 270 nm. 
   Cr 4+ :YAG crystal fiber used in the above-mentioned embodiment has a strong spontaneous emission that just covers the low-loss-window of the optical fiber. The crystal fiber has a 270 nm bandwidth under a 3-dB gain condition and a 400 nm bandwidth under a 6-dB gain condition. This covers the O-band, E-band, S-band, C-band, and L-band of the optical communication. The bandwidth of the transition metal doped fiber amplifier of the invention is far widely than that of the traditional erbium-doped fiber amplifier. In addition, the absorption spectrum of the fiber amplifier of the present invention ranges from about 0.9 μm to about 1.2 μm so that the fiber amplifier of the invention is compatible with the pumping light of 0.98 μm used by the well-known erbium-doped fiber amplifier (EDFA). 
   Presently it needs to connect plural amplifiers to widen band, and no one single amplifier can fully cover the whole 1.3-1.6 μm of optical communication. Under limited bandwidth the dense wavelength division multiplex has chock point about increasing channels and results in fabricating difficultly. The gain bandwidth of the transition metal doped optical fiber amplifier of the present invention is about ten times than that of a traditional erbium-doped fiber amplifier. One single transition metal doped optical fiber amplifier of the present invention can cover the whole 1.3-1.6 μm of optical communication, including the low loss band around 1.4 μm. The present invention improves the disadvantage of connecting plural amplifiers for covering the whole optical communication band in the art. In addition, since the main material of the double-clad fiber is fused silica, the fiber of the present invention can be connected to the fibers currently used by fusion directly, eliminating the inconvenience of using optical coupling devices. The optical amplifier of the present invention can replace the currently existing system and has a very high useful value.