Abstract:
The present invention relates to an optically active device comprising a plurality of stages of optical amplifying sections cascaded on an input light propagation path, and a structure for effectively preventing an upstream pumping light source from being destroyed by ASE light propagating in a direction opposite to the input light. The optically active device comprises, at least, a front-stage optical amplifying section and a rear-stage optical amplifying section which are adjacent to each other on the input light propagation path. Each of the front-stage optical amplifying section and rear-stage optical amplifying section includes an amplification fiber doped with ytterbium as an optically active material and a pumping light source for supplying the amplification optical fiber with pumping light in the band of 0.98 μm for pumping the optically active material. In particular, the optically active device comprises a deterioration preventing structure for preventing the pumping light source in the front-stage optical amplifying section from deteriorating a performance. An embodiment of the deterioration preventing structure is realized by a wavelength-multiplexing fiber coupler of a wavelength division type arranged between the front-stage optical amplifying section and rear-stage optical amplifying section. The wavelength-multiplexing coupler has a port for selectively eliminating from the input light propagation path at least an ASE component in the band of 0.98 μm from backward ASE light from the rear-stage optical amplifying section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optically active device which comprises an amplification optical fiber doped with an optically active material. 
         [0003]    2. Related Background Art 
         [0004]    Examples of optically active devices which comprise amplification optical fibers doped with optically active materials include fiber laser light sources and optical fiber amplifiers, which have been widespread in various uses such as processing in electronic/mechanical fields, medicine, measurement, and optical communications. In uses requiring high-power laser light such as laser processing in particular, unnecessary energy becomes heat by the photon energy difference between pumping light and output laser light. From the viewpoint of lowering this thermal energy, it has been desired that the pumping light and output laser light have respective wavelengths close to each other. Therefore, the use of ytterbium (Yb), which is one of rare-earth elements, as an optically active material added to amplification optical fibers has been increasing. 
         [0005]    As shown in  FIG. 8 , a Yb-doped fiber (YbDF) usually has an output laser light wavelength in the wavelength region of 1030 nm to 1100 nm where Yb ions have a gain because of absorption and induced emission characteristics of Yb. On the other hand, the band of 0.92 μm (0.92 to 0.93 μm) and the band of 0.98 μm (0.97 to 0.98 μm) have typically been in use as the pumping light wavelength. From the above-mentioned viewpoint of making the respective wavelengths of output laser light and pumping light closer to each other, the band of 0.98 μm is desirable as the pumping light wavelength in particular.  FIG. 8  is a graph showing the wavelength dependency of normalized unsaturated absorption coefficient concerning various samples of Yb-doped fiber (YbDF  41  to  43 ). The samples YbDF  41  to  43  have various characteristics as shown in  FIG. 3 . Namely, the sample YbDF  41  has an unsaturated absorption peak of 250 dB/m, a core diameter of 2.4 μm, and a cladding diameter of 125 μm. The sample YbDF  42  has an unsaturated absorption peak of 180 dB/m, a core diameter of 4.0 μm, and a cladding diameter of 125 μm. The sample YbDF  43  has a double cladding structure with an unsaturated absorption peak of 9 dB/m, a core diameter of 15.0 μm, and an inner cladding diameter of 125 μm. 
         [0006]    However, a multimode-pumping laser diode (LD) is used as a pumping light source in the cladding pumping scheme often employed in uses requiring high-power laser light. In this case, the pumping power reaches several watts, whereby the temperature of an LD chip is hard to adjust. As a result, there is a possibility of the pumping light wavelength changing because of fluctuations in temperature of the LD chip. In the absorption spectrum of Yb, the band of 0.98 μm has a sharp peak, whereby the pumping light wavelength may deviate from this absorption peak depending on changes therein. This means the occurrence of a state where the pumping light is hardly absorbed by Yb but is transmitted through the Yb-doped optical fiber (YbDF). From such a viewpoint, it is desirable that the band of 0.92 μm be used as the pumping light wavelength in cladding pumping. 
         [0007]    In the core pumping scheme employing a single-mode pumping LD, on the other hand, it is hard to neglect the present state where 0.98-μm-band-pumping LD modules incorporating temperature-adjusting devices are widely available in the market for communications so that there is no obstacle for pumping in the band of 0.98 μm. 
       SUMMARY OF THE INVENTION 
       [0008]    The present inventors have examined the above prior art, and as a result, have discovered the following problems. That is, when the pumping light wavelength and output laser light wavelength are close to each other as in Yb pumping, the occurrence of an ASE component having the same wavelength as the pumping light wavelength becomes remarkable within the YbDF. In this case, there is a risk of the pumping light source itself being destroyed by the ASE component generated on the downstream side of a signal propagation path returning to the pumping light source positioned on the upstream side. 
         [0009]    The present invention has been developed to eliminate the problems described above. It is an object of the present invention to provide an optically active device having a structure for effectively preventing a pumping light source on the upstream side from being destroyed by the backward propagation of the ASE component having the same wavelength as the pumping light wavelength generated on the downstream side in the multistage amplification of input light in the propagation path thereof. 
         [0010]    The optically active device according to the present invention comprises a plurality of optical amplifying sections cascaded on a path for propagating input light having a predetermined wavelength. Specifically, the optically active device comprises a front-stage optical amplifying section, positioned on the upstream side as seen in a propagating direction of input light in optical amplifying sections adjacent to each other selected from the plurality of optical amplifying sections, a rear-stage optical amplifying section positioned on the downstream side as seen in the propagating direction of input light in the optical amplifying sections adjacent to each other, and a deterioration preventing structure for preventing the front-stage pumping light source constituting a part of the optical amplifying sections from deteriorating a performance. 
         [0011]    The front-stage optical amplifying section includes, at least, a front-stage amplification optical fiber and a front-stage pumping light source. The front-stage amplification optical fiber is doped with ytterbium as an optically active material. By way of a multiplexer, the front-stage pumping light source supplies the front-stage amplification optical fiber with pumping light for pumping ytterbium, which is pumping light containing at least a wavelength component in the band of 0.98 μm. On the other hand, the rear-stage optical amplifying section includes, at least, a rear-stage amplification optical fiber doped with ytterbium as an optically active material and a rear-stage pumping light source, whereas the rear-stage pumping light source supplies the rear-stage amplification optical fiber with pumping light for pumping ytterbium by way of a multiplexer. 
         [0012]    In particular, the deterioration preventing structure in the optically active device according to the present invention includes any of a structure preventing, in the backward ASE (Amplified Spontaneous Emission) light generated in the rear-stage amplification optical fiber and directed from the rear-stage optical amplifying section to the front-stage optical amplifying section, the ASE component in the band of 0.98 μm (i.e., the ASE component having the same wavelength as the pumping light outputted from the front-stage pumping light source) from being directly incident on the front-stage pumping light source by way of the multiplexer in the front-stage optical amplifying section and a structure attenuating the power of the ASE component in the band of 0.98 μm until the power is lower than a level uninfluential to the deterioration in a performance of the front-stage pumping light source before the backward ASE light is incident on the multiplexer in the front-stage optical amplifying section. 
         [0013]    In the optically active device according to the present invention, the front-stage amplification optical fiber may include at least one amplification optical fiber cascaded onto a path propagating the input light. 
         [0014]    The optically active device according to the present invention has a structure for amplifying the light to be amplified fed into the rear-stage optical amplifying section in the front-stage optical amplifying section beforehand in order to obtain high-power laser light from the rear-stage optical amplifying section. In the rear-stage optical amplifying section, the pumping light from the rear-stage pumping light source is fed into the rear-stage amplification optical fiber containing an optically active material (Yb), whereby the optically active material is pumped. At this time, the light to be amplified is fed into the rear-stage amplification optical fiber and thus is amplified. Also, wideband ASE light is generated within the rear-stage amplification optical fiber. Even when the pumping light wavelength is included in the wavelength region of this ASE light, the ASE component having the same wavelength as that of the pumping light is kept from returning to the front-stage pumping light source within the front-stage optical amplifying section by the deterioration preventing structure or its power is fully lowered thereby, which prevents the front-stage pumping light source from being destroyed. 
         [0015]    In the optically active device according to the present invention, ytterbium (Yb) is employed as the optically active material added to each of the amplification optical fibers in the front- and rear-stage optical amplifying sections. The optically active material is pumped with pumping light containing a pumping light component in the band of 0.98 μm at least in the front-stage optical amplifying section. The amplification optical fiber doped with Yb as the optically active material generates high-power ASE light, while the pumping light wavelength is included in the wavelength region of the ASE light. Therefore, if the ASE light generated in the rear-stage optical amplifying section returns to the front-stage optical amplifying section positioned on the upstream side, there will be a high risk of the front-stage pumping light source included in the front-stage optical amplifying section being destroyed. However, this optically active device is provided with the deterioration preventing structure for preventing or restraining the backward ASE light from returning to the front-stage pumping light source, whereby the front-stage pumping light source is kept from being destroyed. 
         [0016]    Preferably, in the optically active device according to the present invention, the deterioration preventing structure includes a wavelength-multiplexing fiber coupler of a wavelength division type provided between the front-stage optical amplifying section and the rear-stage optical amplifying section. This wavelength-multiplexing fiber coupler has, at least, a first port optically connected to a light entrance end of the rear-stage optical amplifying section, a second port which is optically connected to a light exit end of the front-stage optical amplifying section and transmits therethrough an ASE component in a wavelength band excluding the pumping light wavelength in the backward ASE light inputted through the first port, and a third port for selectively outputting an ASE component having the same wavelength as the pumping light wavelength in the backward ASE light inputted through the first port. In this case, the wavelength-multiplexing fiber coupler as the deterioration preventing structure yields a low insertion loss, and is also advantageous in terms of safety. 
         [0017]    Preferably, in the optically active device according to the present invention, an end part of the third port in the wavelength-multiplexing fiber coupler is terminated without reflection while having a heat-dissipating mechanism. This structure is suitable for eliminating stray light. 
         [0018]    Preferably, the rear-stage amplification optical fiber included in the rear-stage optical amplifying section in the optically active device according to the present invention has a double cladding structure for realizing cladding pumping. At this time, it will be preferred if the rear-stage pumping light source included in the rear-stage optical amplifying section outputs pumping light in a single transverse mode. The pumping light in the single transverse mode outputted from the rear-stage pumping light source propagates through the cladding of the rear-stage amplification optical fiber, thereby pumping the optically active material. 
         [0019]    In the optically active device according to the present invention, the deterioration preventing mechanism may provide the backward ASE light (ASE component having at least the same wavelength as the pumping wavelength) with a transmission loss equal to or lower than that of means for coupling the pumping light outputted from the front-stage pumping light source with the front-stage amplification optical fiber. Such a structure can also effectively lower the incidence level of the unnecessary ASE component reaching the front-stage pumping light source. 
         [0020]    The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
         [0021]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a view showing the structure of a first embodiment of the optically active device according to the present invention; 
           [0023]      FIG. 2A  shows a cross-sectional structure of an amplification optical fiber employed in a rear-stage optical amplifier in the optically active device according to the first embodiment, whereas  FIG. 2B  is a refractive index profile of this amplification optical fiber; 
           [0024]      FIG. 3  is a table listing properties of various samples (YbDF  41  to  43 ) of amplification optical fibers employable in the optically active device according to the first embodiment; 
           [0025]      FIG. 4  is a graph showing the wavelength dependency of ASE light power in each part of the optically active device according to the first embodiment; 
           [0026]      FIGS. 5A and 5B  are transmission loss spectra of an optical isolator; 
           [0027]      FIG. 6  is a transmission spectrum of an optical coupler (preventing means) included in the optically active device according to the first embodiment; 
           [0028]      FIG. 7  is a view showing the structure of a second embodiment of the optically active device according to the present invention; and 
           [0029]      FIG. 8  is a graph showing the wavelength dependency of normalized unsaturated absorption coefficient concerning various samples of Yb-doped fiber (YbDF  41  to  43 ). 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    In the following, embodiments of the optically active device according to the present invention will be explained in detail with reference to  FIGS. 1 ,  2 A,  2 B,  3 ,  4 ,  5 A,  5 B, and  6  to  8 . In the explanation of the drawings, the same constituents and the same parts will be referred to with the same numerals while omitting their overlapping descriptions. 
       First Embodiment 
       [0031]    To begin with, a first embodiment of the optically active device according to the present invention will be explained.  FIG. 1  is a view showing the structure of the first embodiment of the optically active device according to the present invention. The optically active device  1  according to the first embodiment shown in  FIG. 1  is an optical amplifier which amplifies light fed to an input connector  11  and outputs collimated light obtained by collimating the amplified light with an output collimator  12 . In particular, the optically active device  1  comprises a front-stage optical amplifying section  1 A and a rear-stage optical amplifying section  1 B which are successively arranged along a propagating direction of the input light directed from the input connector  11  to the output collimator  12 . The front-stage optical amplifying section  1 A includes an optical isolator  21 , an optical coupler  31 , a YbDF  41 , an optical isolator  22 , a bandpass filter  50 , an optical coupler  32 , a YbDF  42 , and an optical isolator  23  which are successively arranged along the propagating direction of input light, while the rear-stage optical amplifying section  1 B includes a combiner  60  and a YbDF  43 . An optical coupler  33  is arranged between the front-stage optical amplifying section  1 A and the rear-stage optical amplifying section  1 B. The front-stage optical amplifying section  1 A further includes an optical coupler  34  and a front-stage pumping light source  71  which are optically connected to the optical couplers  31 ,  32 . On the other hand, the rear-stage optical amplifying section  1 B includes a plurality of rear-stage pumping light sources  72  optically connected to the combiner  60 . 
         [0032]    Each of the YbDFs  41  to  43  is an amplification optical fiber containing silica glass as its host glass, in which an optical waveguide region is doped with elementary Yb as an optically active material. In particular, the YbDF  43  contained in the rear-stage optical amplifying section  1 B comprises a core  43   a  (having a maximum refractive index n 1 ) which is doped with elementary Yb and propagates light to be amplified, and a cladding region  43   b  surrounding the core  43   a  as shown in  FIG. 2A . The cladding region  43   b  is constituted by an inner cladding  43   b   1  (having a refractive index n 2  (&lt;n 1 )) which propagates pumping light components from the plurality of pumping light sources  72 , and an outer cladding  43   b   2  (having a refractive index n 3  (&lt;n 2 )) surrounding the inner cladding  43   b   1 . The front-stage pumping light source  71  is a single-mode pumping LD which outputs pumping light (in the wavelength band of 975 nm) to be supplied to the YbDFs  41 ,  42 . On the other hand, the plurality of rear-stage pumping light sources  72  are multimode pumping LDs which output pumping light (in the wavelength band of 915 nm) to be supplied to the YbDF  43 . Each of the YbDFs  41  to  43  amplifies light in the wavelength band of 1064 nm.  FIG. 2B  shows a refractive index profile  430  of the YbDF  43 , in which areas  431 ,  432 ,  433  indicate respective refractive indexes of parts in the diagonal direction of the core  43   a , inner cladding  43   b   1 , and outer cladding  43   b   2 . 
         [0033]    Each of the optical isolators  21  to  23  transmits light therethrough forward from the input connector  11  to the output collimator  12 . The optical coupler  34  divides the pumping light outputted from the front-stage pumping light source  71  into two so that they are fed to the optical couplers  31 ,  32 , respectively. The optical coupler  31  outputs the pumping light arriving from the optical coupler  34  to the YbDF  41 , and light (light to be amplified) arriving from the optical isolator  21  to the YbDF  41 . The optical coupler  32  outputs the pumping light arriving from the optical coupler  34  to the YbDF  42 , and light (light to be amplified) arriving from the optical isolator  21  to the YbDF  42 . 
         [0034]    The optical coupler  33  is a wavelength-multiplexing fiber coupler and is provided between the front-stage pumping light source  71  and amplification optical fiber  43  as a deterioration preventing structure for preventing deteriorations in performances of the front-stage pumping light source  71  (e.g., destruction of the front-stage pumping light source  71 ) from occurring. The optical coupler  33  has a first port  33   d  optically connected to a light entrance end of the combiner  60  (the light entrance end of the rear-stage optical amplifying section  1 B), a second port  33   b  optically connected to the light output end of the optical isolator  23  (the light output end of the front-stage optical amplifying section  1 A), and a third port  33   a  (ASE transmission output port) which selectively outputs an ASE component having the same wavelength as the pumping light wavelength (in the wavelength band of 975 nm) from the wideband ASE light inputted from the combiner  60  through the first port  33   d . Namely, the optical coupler  33  outputs the light arriving from the optical isolator  23  through the second port  33   b  to the combiner  60  through the first port  33   d , and the ASE component having the same wavelength as the pumping light wavelength (in the wavelength band of 975 nm) from the wideband ASE light inputted from the combiner  60  through the first port  33   d  to the ASE transmission output port  33   a . The combiner  60  outputs the light arriving from the optical coupler  33  through the first port  33   d  to the YbDF  43 , and the pumping light (in the wavelength band of 915 nm) arriving from the plurality of rear-stage pumping light sources  72  to the YbDF  43 . The bandpass filter  50  selectively transmits therethrough the wavelength to be amplified in the light arriving from the optical isolator  22  and outputs it to the optical coupler  32 . 
         [0035]    The optically active device  1  according to the first embodiment operates as follows. The pumping light (in the wavelength band of 975 nm) outputted from the front-stage pumping light source  71  is divided into two, which are outputted from the optical coupler  34  to the optical couplers  31 ,  32 , respectively. The pumping light inputted to the optical coupler  31  is supplied forward to the YbDF  41 . The pumping light inputted to the optical coupler  32  is supplied forward to the YbDF  42 . The pumping light (in the wavelength band of 915 nm) outputted from the plurality of rear-stage pumping light sources  72  is supplied forward to the YbDF  43  through the combiner  60 . 
         [0036]    The light inputted from the input connector  11  is fed into the YbDF  41  through the optical isolator  21  and optical coupler  31 , and is amplified in the YbDF  41 . The first-order amplified light outputted from the YbDF  41  is fed into the YbDF  42  through the optical isolator  22 , bandpass filter  50 , and optical coupler  32 , and is amplified in the YbDF  42  as well. The second-order amplified light outputted from the YbDF  42  is fed into the YbDF  43  through the optical isolator  23 , optical coupler  33 , and combiner  60 , and is further amplified in the YbDF  43 . The final amplified light outputted from the YbDF  43  is outputted as collimated light by the output collimator  12  to the outside of the optically active device  1 . 
         [0037]    Namely, the optically active device  1  according to the first embodiment causes the YbDFs  41  to  43  to successively amplify the light fed to the input connector  11 , and outputs the resulting amplified light as collimated light from the output collimator  12  to the outside of the optically active device  1 . For example, a pulse-modulated YAG laser, LD, or the like is connected to the input connector  11 , and the collimated light outputted from the output collimator  12  is used for processing and measurement. CW light may be fed into the input connector  11 . 
         [0038]      FIG. 3  is a table listing properties of the above-mentioned YbDFs  41  to  43  as samples of amplification optical fibers included in the optically active device  1  according to the first embodiment. The YbDF  41  has an unsaturated absorption peak of 250 dB/m, a core diameter of 2.4 μm, and a cladding diameter of 125 μm. The YbDF  42  has an unsaturated absorption peak of 180 dB/m, a core diameter of 4.0 μm, and a cladding diameter of 125 μm. The YbDF  43  has a double cladding structure with an unsaturated absorption peak of 9 dB/m, a core diameter of 15.0 μm, and a cladding diameter (inner cladding diameter) of 125 μm. Only the YbDF  43  has a low unsaturated absorption peak, since it assumes cladding pumping instead of core pumping. The cladding diameter in the YbDF  43  indicates the inner cladding diameter. In the YbDF  43 , a coating (outer cladding  43   b   2 ) having a lower refractive index is provided on the outside of the inner cladding  43   b   1 , so as to enable cladding pumping. The full width at half maximum of the bandpass filter  50  is 3 nm. 
         [0039]      FIG. 4  is a graph showing the wavelength dependency of ASE light power in each part of the optically active device  1  according to the first embodiment. Here, a case where no optical coupler  33  as the deterioration preventing structure is provided will be explained. In  FIG. 4 , curve A indicates the power spectrum of forward ASE light at the input end of the YbDF  43 , curve B indicates the power spectrum of backward ASE light at the input end of the YbDF  43 , and curve C indicates the power spectrum of backward ASE light at the input end of the YbDF  42 . 
         [0040]    When the average input power to the input connector  11  is −5 dBm, ASE light having the power spectra indicated by the curves A, B in  FIG. 4  is seen at the input end of the YbDF  43 . The backward ASE light (curve B in  FIG. 4 ) at the input end of the YbDF  43  exhibits a time-averaged total power as high as about 300 mW and also has a peak near a wavelength of 975 nm. The isolator  23  is supposed to prevent the backward ASE light from being incident on the front-stage pumping light source  71 . However, as shown in  FIGS. 5A and 5B , an isolator having a backward isolation peak at a wavelength of light to be amplified (in the band of 1064 nm) shows only a backward isolation of less than 20 dB at a pumping light wavelength (in the band of 975 nm). In  FIG. 5B , acute backward isolation striae in the wavelength region shorter than 1064 nm seem to be noises due to the stability of the light source which do not exist in the actual isolator. 
         [0041]    As a result, as shown in  FIG. 4 , the backward ASE light (curve C in  FIG. 4 ) at the input end of the YbDF  42  has a spectrum widened to the vicinity of 975 nm as well. The backward ASE light component in this wavelength region (near 975 nm) reaches the front-stage pumping light source  71  through the optical couplers  32 ,  34 . The average total power of the backward ASE component is about 1 mW, which is not problematic in general but yields temporal fluctuations in the case of pulse oscillation and the like. Further, there is a possibility of an instantaneously large power being injected into the front-stage pumping light source  71  in the case where an influence of self-pulsation or the like within the YbDF exists or in a transitional phase at the time of turning on the power or the like. As a result, there is a possibility of causing deteriorations in performances of the front-stage pumping light source  71  such as destruction thereof. 
         [0042]    Therefore, in the first embodiment, the optical coupler  33  is provided between the optical isolator  23  and combiner  60  in order to prevent the deteriorations in performances of the front-stage pumping light source  71  such as destruction from occurring. Among the pigtails of the optical coupler  33 , one (third port  33   a ) connected to none of the optical isolator  23  and combiner  60  preferably has a reflection preventing structure  33   c . This reflection preventing structure  33   c  may be a structure in which the leading end of the third port  33   a  is terminated without reflection by fusion with a coreless fiber, a structure in which the leading end is sufficiently distanced from a shiny metal or the like so as not to generate diffuse reflection and the like, or a structure having a heat-dissipating mechanism. 
         [0043]    Preferably, the optical coupler  33  has a transmission characteristic substantially equal to that of the optical coupler  32  at the pumping light wavelength of the front-stage pumping light source  71 .  FIG. 6  is a graph showing examples of transmission characteristics of the optical coupler  33  included in the optically active device  1  according to the first embodiment. In  FIG. 6 , curve A indicates the transmission characteristic of the optical coupler  32 , curve B indicates a bad example of the transmission characteristic of the optical coupler  33 , and curve C indicates another bad example of the transmission characteristic of the optical coupler  33 . When the transmission characteristic of the optical coupler  33  deviates from that of the optical coupler  32  as shown in  FIG. 6 , the ASE component centered at a wavelength of 975 nm is not completely eliminated by the optical coupler  33  but reaches the front-stage pumping light source  71  through the optical coupler  32 . Therefore, the optical coupler  33  is desired to have a transmission characteristic substantially equal to that of the optical coupler  32 . 
       Second Embodiment 
       [0044]    A second embodiment of the optically active device according to the present invention will now be explained.  FIG. 7  is a view showing the structure of the second embodiment of the optically active device according to the present invention. The optically active device  2  according to the second embodiment is also an optical amplifier which amplifies light inputted from an input connector  11  and outputs thus amplified light as collimated light from an output collimator  12 . As with the optically active device  1  according to the first embodiment, the optically active device  2  comprises a front-stage optical amplifying section  2 A and a rear-stage optical amplifying section  2 B which are arranged successively from the input connector  11  to the output collimator  12 . The front-stage optical amplifying section  2 A includes an optical isolator  21 , an optical coupler  31 , a YbDF  41 , an optical isolator  22 , a bandpass filter  50 , a YbDF  42 , an optical coupler  32 , and an optical isolator  23  which are successively arranged along a propagating direction of input light, while the rear-stage optical amplifying section  2 B comprises a combiner  60  and a YbDF  43 . The front-stage optical amplifying section  2 A includes an optical coupler  34  and a front-stage pumping light source  71  which are optically connected to the optical couplers  31 ,  32 . The rear-stage optical amplifying section  2 B includes a plurality of rear-stage pumping light sources  72  optically connected to the combiner  60 . 
         [0045]    The optically active device  2  according to the second embodiment shown in  FIG. 7  differs from the optically active device  1  according to the first embodiment shown in  FIG. 1  in that pumping light is supplied backward from the optical coupler  32  disposed downstream of the YbDF  42  to the YbDF  42  and that no optical coupler  33  is provided. 
         [0046]    The optically active device  2  according to the second embodiment operates as follows. The pumping light (in the wavelength band of 975 nm) outputted from the front-stage pumping light source  71  is divided into two, which are outputted from the optical coupler  34  to the optical couplers  31 ,  32 , respectively. The pumping light inputted to the optical coupler  31  is supplied forward to the YbDF  41 . The pumping light inputted to the optical coupler  32  is supplied backward to the YbDF  42 . The pumping light (in the wavelength band of 915 nm) outputted from the plurality of rear-stage pumping light sources  72  is supplied forward to the YbDF  43  through the combiner  60 . 
         [0047]    The light to be amplified inputted from the input connector  11  is fed into the YbDF  41  through the optical isolator  21  and optical coupler  31 , and is amplified in the YbDF  41 . The first-order amplified light outputted from the YbDF  41  is fed into the YbDF  42  through the optical isolator  22 , bandpass filter  50 , and optical coupler  32 , and is amplified in the YbDF  42  as well. The second-order amplified light outputted from the YbDF  42  is fed into the YbDF  43  through the optical isolator  23  and combiner  60 , and is further amplified in the YbDF  43 . The final amplified light outputted from the YbDF  43  is outputted as collimated light by the output collimator  12  to the outside of the optically active device  2 . 
         [0048]    Thus, the second embodiment has a structure for backward-pumping the YbDF  42  without providing the optical coupler  33  of the first embodiment. Because of this structure, the backward ASE component in the pumping wavelength region from the YbDF  43  and output components such as self-pulsation are transmitted through the optical coupler  32  and made incident on the YbDF  42  without being incident on the optical coupler  34  from the optical coupler  32 . 
         [0049]    In the structure mentioned above, however, a commercially available 0.98-μm-band-pumping LD yields a reflectance of about 10 dB when seen from the optical coupler  34 . Therefore, the parts of YbDF  41  and  42  resonate by themselves, whereby the structure fails to function as an optical fiber amplifier. Such a problem does not occur in a structure using a pumping LD module with a low reflectance or inserting a 0.98-μm-band isolator immediately downstream of the front-stage pumping light source  71  as a matter of course. 
       Other Embodiments 
       [0050]    Without being restricted to the embodiments mentioned above, the present invention can be modified in various ways. For example, YbDF is represented as an optical amplification medium in the above-mentioned first and second embodiments. However, the optically active device according to the present invention can also be employed in the case of 1.53-μm-band pumping using an Er-doped optical fiber (EDF). 
         [0051]    The deterioration preventing structure for preventing unnecessary ASE components from being incident on the front-stage pumping light source  71  may be a long-period fiber grating having a loss peak in the band of 0.98 μm while being able to eliminate light having the same wavelength as that of the pumping light of the front-stage pumping light source  71  by coupling to a cladding mode, and yielding a low loss at the wavelength of light to be amplified instead of the molten WDM fiber coupler. In any case, a fiber-type device is desirable from the viewpoint of suppressing the insertion loss and preventing the device itself from being optically damaged. A coupler of dielectric multilayer film filter type is also employable as the molten WDM fiber coupler. 
         [0052]    As in the foregoing, in the structure of amplifying input light in a plurality of stages in a propagating path thereof, the optically active device according to the present invention can effectively prevent deteriorations in performances such as destruction of a pumping light source on the upstream side from being caused by the backward propagation of ASE light generated on the downstream side. 
         [0053]    From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.