Abstract:
The present invention relates to a fiber optic amplifier comprising a structure enabling favorable signal amplification characteristics to be maintained even in an environment exposed to radiation. The fiber optic amplifier according to the present invention comprises a bleaching system for restoring glass defects generated within an optical fiber for amplification upon exposure to ultraviolet rays. This bleaching system carries out at least one of optical bleaching and thermal bleaching for the optical fiber for amplification. The optical bleaching is effected when bleaching light having a shorter wavelength shorter and a higher power than pumping light is supplied into the optical fiber for amplification, whereas the fiber optic amplifier has a structure which can introduce and eliminate the bleaching light. On the other hand, the thermal bleaching is effected by heating the optical fiber for amplification, whereas the fiber optic amplifier comprises a structure for maintaining the surface temperature of the optical fiber for amplification within a predetermined range.

Description:
RELATED APPLICATIONS 
     This is a Continuation-In-Part application of International Patent Application serial No. PCT/JP00/00467 filed on Jan. 28, 2000, now pending which in turn claims the benefit of Japanese Patent Application No. 1999-028976, filed Feb. 5, 1999, the disclosures of which Applications are incorporated by reference herein. The benefit of the filing and priority dates of the International and Japanese Applications is respectfully requested. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fiber optic amplifier for amplifying signal light including one or more wavelengths within a predetermined wavelength band. 
     2. Related Background Art 
     Demands for optical amplifiers have rapidly been enhancing in order to respond to needs for increasing communications capacity, elongating repeater distances, and the like in optical communications. Among others, fiber optic amplifiers have a great merit from the viewpoint of reducing cost and enhancing functions in optical communications systems, since they can amplify signal light including one or more wavelengths within a predetermined wavelength band without converting them into electricity. 
     In general, a fiber optic amplifier comprises an optical fiber for amplification, doped with a rare-earth element, and a pumping light supply system for supplying pumping light to the optical fiber for amplification. The pumping light supply system usually includes a semiconductor laser, which is a pumping light source, and an optical coupler for guiding the pumping light into the optical fiber for amplification. In such a configuration, signal light is amplified within the optical fiber for amplification due to the stimulated emission light emitted from the rare-earth element excited by the pumping light. For example, the erbium-doped fiber optic amplifier whose optical fiber for amplification is doped with erbium, as a rare-earth element, collectively amplifies the signal light in a 1.55 μm wavelength band when pumping light having a wavelength of 1.48 μm or 0.98 μm is supplied thereto. 
     SUMMARY OF THE INVENTION 
     The inventors have studied the prior art mentioned above and, as a result, found problems as follows. Namely, it has been known that, when fiber optic amplifiers are utilized in an environment in which they are inevitably exposed to radiation, e.g., in the vicinity of an atomic furnace, in outer space, or the like, glass defects occur upon exposure to the radiation (see Chie Fukuda et,al., “Hydrogen and radiation resistance of erbium-doped fibers,” OFC&#39;94 Technical Digest, pp.304-305, FF3). The occurrence of such glass defects would increase transmission loss of the signal light or pumping light, thereby deteriorating signal amplification characteristics of the fiber optic amplifiers. FIG. 1 is a graph showing a measured relationship between radiation dose (rad) and transmission loss (dB/m) with respect to light having a wavelength of 1.3 μm concerning an erbium-doped optical fiber (optical fiber for amplification). The erbium-doped optical fiber used for this measurement has a relative refractive index difference of 2% with respect to a cladding, and an outside diameter of 3 μm, while being co-doped with 1.0% by weight of aluminum and 0.1% by weight of erbium. From FIG. 1, it can be seen that the transmission loss of the optical fiber for amplification installed in a radiation environment increases as the radiation dose enhances. On the other hand, FIG. 2 is a graph showing a relationship between radiation dose and gain concerning a counter-propagating pumping type optical fiber amplifier equipped with an erbium-doped optical fiber. Here, the wavelength of pumping light is 1.48 μm, the length of the erbium-doped optical fiber is 30 μm, the wavelength of signal light is 1.55 μm, and the input power of signal light is 35 mW. From FIG. 2, it can be seen that the gain of the optical fiber decreases as the radiation dose increases. 
     In order to overcome problems such as those mentioned above, it is an object of the present invention to provide a fiber optic amplifier comprising a structure capable of maintaining favorable signal amplification characteristics even in an environment exposed to radiation. 
     The fiber optic amplifier according to the present invention is an optical apparatus for amplifying signal light including one or more wavelengths within a predetermined wavelength band, and comprises, at least, an optical fiber for amplification having an entrance end for taking in signal light from a transmission line and an exit end for sending out amplified signal light onto the transmission line, the optical fiber for amplification constituting a part of the transmission line and being doped with a rare-earth element, and a pumping light source for supplying pumping light into the optical fiber for amplification. Here, optical isolators for inhibiting unnecessary light from propagating are disposed between the entrance end of the fiber optic amplifier and the entrance end of the optical fiber for amplification, and between the exit end of the fiber optic amplifier and the exit end of the optical fiber for amplification, respectively. Also, between these optical isolators, an optical coupler for guiding the pumping light emitted from the pumping light source into the optical fiber for amplification is disposed on at least one of the entrance end side and exit end side of the optical fiber for amplification. 
     In particular, the fiber optic amplifier according to the present invention is characterized in that it comprises a bleaching system for carrying out bleaching for the optical fiber for amplification. Here, bleaching refers to eliminating the defects generated in the glass constituting the optical fiber for amplification upon exposure to radiation, thereby restoring its original glass structure. This bleaching system reduces the transmission loss of signal light or pumping light caused by glass defects, whereby favorable signal amplification characteristics can be maintained. 
     Bleaching for the optical fiber for amplification includes optical bleaching and thermal bleaching, whereas the above-mentioned bleaching system carries out at least one of the optical bleaching and thermal bleaching. Here, it is not necessary for such bleaching to be carried out continuously while the fiber optic amplifier is operating, and similar effects can also be obtained when it is carried out at predetermined intervals. 
     In the case where the glass defects generated within the optical fiber for amplification are to be restored by optical bleaching, it is preferred that the bleaching system include a bleaching light source for supplying bleaching light having a wavelength shorter than that of the pumping light and a power higher than that of the pumping light into the optical fiber for amplification. In this case, between the above-mentioned optical isolators, an optical coupler for guiding the bleaching light from the bleaching light source into the optical fiber for amplification is disposed on one of the entrance end side and exit end side of the optical fiber for amplification. 
     Here, the wavelength of the above-mentioned bleaching light lies within the range of 0.2 μm to 1.0 μm, more preferably within the range of 0.2 μm to 0.9 μm. The bleaching light is not always required to be continuous light, and may be pulse light. 
     Further, for carrying out optical bleaching for selectively eliminating from the transmission line the bleaching light supplied into the optical fiber for amplification, the bleaching system preferably comprises a bleaching-light-eliminating optical coupler which is disposed between the above-mentioned optical isolators so as to hold, in cooperation with the bleaching-light-introducing optical coupler, the optical fiber for amplification. Due to this configuration, the bleaching light is prevented from being emitted outside the fiber optic amplifier together with the signal light (thus reducing noise). 
     In the case where the glass defects generated within the optical fiber for amplification are to be restored by thermal bleaching, on the other hand, the above-mentioned bleaching system preferably includes at least a heater for heating the optical fiber for amplification. Also, in order for the surface temperature of the optical fiber for amplification, which is an object to be heated, to be maintained within a predetermined range, this bleaching system comprises a temperature sensor for detecting the surface temperature of the optical fiber for amplification and a controller for carrying out temperature control of the heater according to measurement information from the temperature sensor. 
     The surface temperature of a specific optical fiber for amplification lies within the range of 50° C. to 200° C., more preferably within the range of 100° C. to 150° C. 
     In the fiber optic amplifier according to the present invention, the bleaching system can be configured so as to be able to carry out one or both of the above-mentioned optical bleaching and thermal bleaching. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention. 
     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 
     FIG. 1 is a graph showing a relationship between the transmission loss and radiation dose concerning an optical fiber for amplification; 
     FIG. 2 is a graph showing a relationship between the gain of a fiber optic amplifier and the radiation dose to which the optical fiber for amplification is exposed; 
     FIG. 3 is a view showing a schematic configuration of an embodiment of the fiber optic amplifier according to the present invention; and 
     FIGS. 4A to  4 C are views showing respective configurations of fiber optic amplifiers prepared for experiments for measuring bleaching effects; namely, FIGS. 4A to  4 C are views showing respective configurations of a fiber optic amplifier which is an example of the fiber optic amplifier according to the present invention and comprises a bleaching system for carrying out optical bleaching, a fiber optic amplifier which is another example of the fiber optic amplifier according to the present invention and comprises a bleaching system for carrying out thermal bleaching, and a fiber optic amplifier which is prepared as a comparative example and comprises no bleaching system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the fiber optic amplifier according to the present invention will be explained with reference to FIGS. 3,  4 A,  4 B,and  4 C. Here, in the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions. 
     FIG. 3 is a view showing a schematic configuration of an embodiment of the fiber optic amplifier according to the present invention. In FIG. 3, the fiber optic amplifier  10  has an input end  10   a  and an output end  10   b , between which an optical fiber for amplification  18  constituting a part of a transmission line is disposed. Between the entrance end  10   a  of the amplifier  10  and the entrance end of the optical fiber for amplification  18 , an optical isolator  12 , an optical coupler  14 , and an optical coupler  16  are successively disposed in the advancing direction of signal light. Between the exit end  10   b  of the amplifier  10  and the exit end of the optical fiber for amplification  18 , on the other hand, an optical coupler  20 , an optical coupler  22 , and an optical isolator  24  are successively disposed in the advancing direction of signal light. The optical coupler  14  functions so as to guide the bleaching light emitted from a bleaching light source (semiconductor laser)  32  into the optical fiber for amplification  18 , whereas the optical couplers  16 ,  20  function to guide the respective pumping light emitted from pumping light sources (semiconductor lasers)  28 ,  30  into the optical fiber for amplification  18 . 
     The optical fiber for amplification  18  is a silica-based optical fiber, whose core is doped with a rare-earth element. The doped rare-earth element is excited by the pumping light outputted from the semiconductor lasers  28 ,  30 ; and, as the signal light taken in via the input end  10   a  is incident thereon, stimulated emission light having a wavelength substantially identical to the signal light is released therefrom. Examples of such a rare-earth element include erbium (Er), neodymium (Nd), praseodymium (Pr), thulium (Tm), and the like. In the case where erbium is utilized as a doping element, erbium element is pumped by pumping light having a wavelength of 1.48 μm or pumping light having a wavelength of 0.98 μm. When signal light in a 1.55 μm wavelength band is incident on erbium element while the latter is in a pumped state, stimulated emission light in the 1.55 μm wavelength band is released within the optical fiber for amplification  18 , whereby the signal light is amplified. Here, since the 1.55 μm wavelength band is the wavelength band with the lowest loss in silica-based optical fibers, signal light in the 1.55 μm wavelength band is often utilized in optical communications. Therefore, fiber optic amplifiers employing an erbium-doped optical fiber as the optical fiber for amplification  18  have a very wide range of uses. Also, the optical fiber for amplification  18  has a predetermined length (e.g., 30 m) in order to secure a desired amplification factor, and is accommodated within a package while in a state wound like a coil. 
     The optical coupler  16  disposed between the entrance end of the optical fiber for amplification  18  and the input end  10   a  of the amplifier  10  functions so as to multiplex the incident signal light and bleaching light and the pumping light emitted from the optical semiconductor laser  28  (hereinafter referred to as co-propagating pumping light), and guide thus multiplexed light into the optical fiber for amplification  18 . On the other hand, the optical coupler  20  disposed between the exit end of the optical fiber for amplification  18  and the output end  10   b  of the amplifier  10  functions so as to transmit therethrough the signal light and bleaching light propagated through the optical fiber for amplification  18  and guide the pumping light emitted from the semiconductor laser  30  (hereinafter referred to as counter-propagating pumping light) into the optical fiber for amplification  18 . 
     Here, an optical coupler in which a dielectric multilayer filter made of alternately stacked two kinds of materials having different refractive indices is inserted, for example, can be utilized as the optical couplers  16 ,  20 . The semiconductor lasers  28 ,  30  output predetermined wavelengths of pumping light which are substantially identical to each other. For example, in the case where an erbium-doped optical fiber having a core doped with erbium element is employed as the optical fiber for amplification  18 , doped erbium element will be pumped if pumping light having a wavelength of 1.48 μm or a wavelength of 0.98 μm is supplied into the erbium-doped optical fiber. Also, due to the above-mentioned configuration, the co-propagating, pumping light emitted from the semiconductor laser  28  is separated from the main line (the line through which the signal light propagates) by the optical coupler  20  and advances toward the semiconductor laser  30 , whereby it would not be sent out into the transmission line by way of the exit end  10   b . Similarly, the counter propagating pumping light emitted from the semiconductor laser  30  is separated from the main line by the optical coupler  16  and advances toward the semiconductor laser  28 , whereby it would not be sent out onto the transmission line by way of the entrance end  10   a.    
     The optical coupler  14  disposed between the entrance end  10   a  of the amplifier  10  and the optical coupler  16  functions so as to multiplex the signal light taken in via the input end  10   a  and the bleaching light emitted from the semiconductor laser  32  and guide thus multiplexed light to the optical coupler  16 . On the other hand, the optical coupler  22  disposed between the optical coupler  20  and the exit end  10   b  of the amplifier  10  functions so as to selectively separate the bleaching light from the multiplexed light of signal light and bleaching light having passed through the optical coupler  20 , and guide only the signal light to the isolator  24 . Namely, the optical coupler  14 , the semiconductor laser  32 , and the optical coupler  22  constitute a bleaching system for carrying out optical bleaching for the optical fiber for amplification  18 . For example, an optical coupler in which a dielectric multilayer filter made of alternately stacked two kinds of materials having different refractive indices is inserted can be utilized as the optical couplers  14 ,  22 . The semiconductor laser  32  emits, as continuous light or pulse light, bleaching light for carrying out the bleaching of the optical fiber for amplification  18 . For example, in the case where a silica-based optical fiber is employed as the optical fiber for amplification  18 , very efficient optical bleaching can be carried out if the semiconductor laser  32  emitting bleaching light within a wavelength range of 0.2 to 1.0 μm, preferably at a wavelength of 0.5 μm, is employed. When the use of pumping light having a wavelength of 9.8 μm is taken into consideration, the wavelength of bleaching light more preferably lies within a wavelength range of 0.2 to 9.0 μm. 
     The optical isolators  12 ,  24 , which transmit therethrough the light advancing from the entrance end  10   a  toward the exit end  10   b  but block the light advancing from the exit end  10   b  toward the entrance end  10   a , are disposed between the entrance end  10   a  of the amplifier  10  and the optical coupler  14 , and between the exit end  10   b  and the optical coupler  22 , respectively. Namely, the optical isolator  12  transmits therethrough the signal light taken in via the input end  10   a  toward the optical fiber for amplification  18 , and blocks the light advancing from the exit end  10   b  toward the entrance end  10   a  such as the counter-propagating pumping light emitted from the semiconductor laser  30  or the signal light and the like reflected by other optical components. On the other hand, the optical isolator  24  transmits therethrough the signal light propagated through the optical fiber for amplification  18  toward the exit end  10   b , and blocks the light advancing from the exit end  10   b  toward the entrance end  10   a  such as the noise entering from the exit end  10   b.    
     Disposed in the vicinity of the optical fiber for amplification  18  is a heater  26  for heating the optical fiber for amplification  18  so as to carry out thermal bleaching. This heater  26  may be arranged so as to heat the optical fiber for amplification  18  from outside the package accommodating the optical fiber for amplification  18 . For example, in the case where a silica-based optical fiber is employed as the optical fiber for amplification  18 , efficient thermal bleaching can be effected if the optical fiber for amplification  18  is heated to such an extent that its surface temperature lies within the range of 50° C. to 200° C., preferably within the range of 100° C. to 150° C. Here, the surface temperature of the optical fiber for amplification  18  or the temperature of the atmosphere in the vicinity of the optical fiber for amplification  18  is monitored by a temperature sensor  29 , and temperature management is carried out by a controller  27  according to a result of measurement from the temperature sensor  29 . The heater  26 , controller  27 , and temperature sensor  29  constitute a thermal bleaching system. 
     Operations and effects of the fiber optic amplifier according to this embodiment will now be explained. In the case where a fiber optic amplifier in which an optical fiber (optical fiber for amplification) doped with a rare-earth element is employed as optical amplification means is utilized in an environment exposed to radiation, glass defects occur within the optical fiber for amplification upon exposure to the radiation, whereby signal amplification characteristics of the fiber optic amplifier may deteriorate. 
     For this matter, the fiber optic amplifier  10  according to this embodiment comprises the optical bleaching system constituted by the semiconductor laser  32 , optical coupler  14 , and optical coupler  22 ; or the thermal bleaching system constituted by the heater  26 , controller  27 , and temperature sensor  29 . 
     In the optical-bleaching system in this embodiment, the bleaching light emitted from the semiconductor laser  32  is made incident on the optical fiber for amplification  18  via the optical coupler  16 . When the bleaching light enters the optical fiber for amplification  18 , a part of the glass defects generated upon exposure to radiation is restored. Therefore, such optical bleaching reduces the transmission loss in signal light or pumping light caused by the glass defects, thereby making it possible to maintain favorable signal amplification characteristics even in an environment exposed to radiation. Also, efficient bleaching is possible if the bleaching of the optical fiber for amplification  18  is effected by optical bleaching. 
     In particular, the optical bleaching system in this embodiment can be constructed easily since it is constituted by the semiconductor laser  32 , the optical coupler  14 , and the optical coupler  22 . Also, since the bleaching light from the semiconductor laser  32  is guided to the optical fiber for amplification  18  by way of the optical coupler  14 , bleaching can be carried out in parallel with the operation of amplifying the signal light without separating the optical fiber for amplification  18  from the transmission line, whereby the maintenance of the fiber optic amplifier  10  can be carried out quite easily and quickly. 
     Further, the optical bleaching system in this embodiment comprises the optical coupler  20 , which selectively eliminates the bleaching light from the combined wave of signal light and bleaching light propagated through the optical fiber for amplification  18  (whereby only the signal light passes therethrough). Therefore, the bleaching light is prevented from being sent out to the transmission line via the exit end  10   b  together with the signal light, whereby signals can be outputted with less noise. 
     On the other hand, the thermal bleaching system in this embodiment is constituted by the heater  26  and its control system (the temperature sensor  29  and the controller  27 ). In this thermal bleaching, the optical fiber for amplification  18  is heated with the heater  26 , whereby a part of the glass defects generated upon exposure to radiation is restored as in the above-mentioned optical bleaching. As a result, the transmission loss in signal light or pumping light caused by the glass defects is reduced by the thermal bleaching, whereby favorable signal amplification characteristics can be maintained. Such thermal bleaching also enables the bleaching of the optical fiber for amplification  18  in a simple apparatus configuration. 
     Though the fiber optic amplifier  10  according to the above-mentioned embodiment comprises both of the optical bleaching system and thermal bleaching system, it may comprise only one of them. 
     Also, while the fiber optic amplifier  10  according to the above-mentioned embodiment comprises both of the co-propagating pumping light supply system, constituted by the semiconductor laser  28  and the optical coupler  16 , for supplying the co-propagating pumping light to the optical fiber for amplification  18  constituting a part of the transmission line, and the counter-propagating pumping light supply system, constituted by the semiconductor laser  30  and the optical coupler  20 , for supplying the counter-propagating pumping light to the optical fiber for amplification  18  constituting a part of the transmission line, it may comprise only one of these pumping light supply systems. 
     Though the optical bleaching system in the fiber optic amplifier  10  according to the above-mentioned embodiment is configured so as to guide the bleaching light into the optical fiber for amplification  18  from the entrance end side thereof by using the semiconductor laser  32  and the optical coupler  14 , it may be configured so as to guide the bleaching light into the optical fiber for amplification  18  from the exit end side thereof. Also, it may be configured such that the bleaching light can be made incident on the optical fiber for amplification  18  from both of the entrance end side and exit end side thereof. 
     Further, the entry of bleaching light or heating of the optical fiber for amplification  18  may be carried out intermittently or periodically. 
     Results of measurement of signal amplification characteristics of the fiber optic amplifier according to the present invention in a radiation environment will now be explained. 
     Prepared are three kinds of fiber optic amplifiers, i.e., a fiber optic amplifier  100  comprising an optical bleaching system (including a semiconductor laser  32 , an optical coupler  14 , and an optical coupler  22 ) as shown in FIG. 4A, a fiber optic amplifier  200  comprising a thermal bleaching system (including a heater  26 , a temperature sensor  29 , and a controller  27 ) as shown in FIG. 4B, and a fiber optic amplifier  300  as a comparative example comprising neither an optical bleaching system nor a thermal bleaching system as shown in FIG.  4 C. 
     Each of thus prepared fiber optic amplifiers  100 ,  200 ,  300  is a counter-propagating pumping type optical fiber amplifier comprising, a semiconductor laser  30  and an optical coupler  20 . 
     In each of the prepared fiber optic amplifiers  100 ,  200 ,  300 , the optical fiber for amplification  18  is an erbium-doped optical fiber which is co-doped with 1.0% by weight of aluminum and 0.1% by weight of erbium. The optical fiber for amplification  18  has a total length of 30 m and is wound like a coil. Emitted from the semiconductor laser  30  is counter-propagating pumping light having a wavelength of 1.48 μm and an output of 35 mW. From the input end  10   a , signal light having a wavelength of 1.55 μm and an input power of −35 dB is taken in. 
     In the prepared optical fiber amplifier  100 , bleaching light having a wavelength of 0.5 μm and an output of 50 mW is continuously emitted from the semiconductor laser  32 . In the prepared optical fiber amplifier  200 , the surface temperature of the optical fiber for amplification  18  is set to about 100° C. with the heater  26 . The fiber optic amplifier  300  prepared as the comparative example comprises neither an optical bleaching system nor a thermal bleaching system. 
     The measurement of signal amplification characteristics was carried out by measuring respective gains of the fiber optic amplifiers  100 ,  200 ,  200  having the configurations mentioned above with respect to light having a wavelength of 1.55 μm after each of the fiber optic amplifiers  100 ,  200 ,  300  had been used for 200 hours in a radiation environment of 1000 rad/h. As a result, the gain of the fiber optic amplifier  100  was 33 dB, the gain of the fiber optic amplifier  200  was 29 dB, and the gain of the fiber optic amplifier  300  was 24 dB. From these results of measurement, it was confirmed that the fiber optic amplifiers  100 ,  200  according to these examples were able to maintain favorable signal amplification characteristics even in the radiation environment, as compared with the fiber optic amplifier  300  of the comparative example. Here, the transmission loss of the optical fiber for amplification in the fiber optic amplifier  300  of the comparative example was 810 dB at a wavelength of 1.3 μm. 
     In accordance with the present invention, as explained in the foregoing, the bleaching system enabling optical bleaching or thermal bleaching for an optical fiber for amplification can restore a part of glass defects generated in the optical fiber for amplification upon exposure to radiation. As a result, the transmission loss in signal light or pumping light caused by the glass defects can be reduced, whereby favorable signal amplification characteristics can be maintained even in a radiation environment. 
     Also, since the optical bleaching supplies bleaching light into the optical fiber for amplification by way of an optical coupler in the present invention, the bleaching of the optical fiber for amplification can efficiently be carried out even during an operation of amplifying the signal light. Further, since there is provided an optical coupler for separating the bleaching light propagated through the optical fiber for amplification from the main line through which the signal light propagates, the bleaching light is prevented from mingling with the signal light and being sent out onto the transmission line, whereby a fiber optic amplifier with less noise is realized. 
     On the other hand, the thermal bleaching in the present invention can be realized when the optical fiber for amplification is heated directly or indirectly by way of a fiber-accommodating package, whereby bleaching of the optical fiber for amplification can be carried out in a simple configuration. 
     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.