Abstract:
A system is provided for partially or completely offsetting optical fiber amplifier gain spectrum variations due to temperature by setting the optical pump signal leading into the amplifier to a desired wavelength value. Optical fiber amplifiers have gain spectrum variations associated with temperature and gain spectrum variations associated with the optical pump signal wavelength. By controlling the optical pump signal wavelength supplying the amplifier, the optical fiber amplifier gain variations due to temperature can be partially or completely offset, which helps minimize total amplifier spectral gain variations. The invention is applicable to dense wavelength division multiplexing (DWDM) systems.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to optical communications. Specifically, this invention relates to controlling the variation of optical fiber amplifier gain due to changes in temperature. 
     2. Description of the Related Art 
     Optical fiber communications systems provide for low loss and very high information carrying capacity. Most advanced optical fiber communication systems now in place owe success and operating characteristics to optical fiber amplifiers such as the erbium-doped fiber amplifier (EDFA). The gain bandwidth of this amplifier is sufficient to permit simultaneous amplification of multiple channels and, for this reason may be used for dense wavelength division multiplexing (DVVDM). 
     In optical fiber amplifiers, like EDFAs, for DWDM systems the gain spectrum is required to be approximately uniform. However, as the temperature of the optical fiber varies, the shape of the gain spectrum changes significantly. It would be useful to control this variation of gain or to compensate within the amplifier for the variation. 
     Several solutions have been proposed or attempted. One of the attempted solutions deals with controlling the temperature of the optical fiber with a heater. This solution though causes high heat in the optical fiber which reduces the overall life and reliability of the optical fiber. 
     Another proposed solution involves gain flattening filters which have a transmission spectrum that varies with temperature in an appropriate manner related to the gain spectrum. However, this option has not been shown to be commercially viable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for controlling optical fiber amplifier gain due to temperature, and hence the output signal of the optical fiber amplifier. 
     According to one aspect of the present invention, an optical pump signal which has a certain wavelength is received by an optical fiber amplifier. The amplifier has a gain variation caused by temperature and a gain variation caused by the wavelength of the optical pump signal received. The gain variation due to temperature may be counteracted by the gain variation due to changes in optical pump wavelength. Moreover, an optical pump signal with a desired wavelength may be set by the present invention, and the set pump signal may be used to cause a gain which acts to partially or completely offset the optical fiber amplifier gain variation caused by temperature. 
     An optical fiber amplifier has an associated gain spectrum which is dependent upon temperature and upon optical pump signal wavelength. A change in either the temperature or the wavelength of the optical pump signal may cause a change in the gain spectrum. FIG. 1 illustrates, in graph form, the difference in gain variation of the same optical fiber amplifier between the amplifier gain at a temperature of −5° C. and the amplifier gain at a temperature of 70° C. 
     FIG. 2 graphically illustrates the gain variation of an optical amplifier due to changes in the optical pump signal wavelength. Specifically, FIG. 2 graphically illustrates the gain of an optical fiber amplifier at a wavelength of 972.5 nm and compares that with the gain of the same optical fiber amplifier at a wavelength of 980 nm. 
     The present invention may be used to minimize gain variation due to temperature (FIG. 1) by offsetting this change in gain with changes in optical amplifier gain from pump wavelength (FIG.  2 ). For example, referring to FIG. 1, where the fiber temperature is 70° C., at the amplifier&#39;s spectrum range value of 1540 nm, there is a positive dB factor. The same optical fiber amplifier energized by a 980 nm pump laser, as seen in FIG. 2, at the amplifier&#39;s spectrum range value of 1540 nm, shows a negative dB factor. Accordingly, the positive gain at 1540 nm (FIG. 1) can be partially or completely offset by the negative gain of a 980 nm pump laser wavelength (FIG.  2 ). 
     Various kinds of optical fiber amplifiers are available and each one has an associated gain variation for temperature and an associated gain variation for optical pump signal wavelength. Each different amplifier will have a unique gain curve which is different from the gain curves represented in FIGS. 1 and 2. Specifically, FIGS. 1 and 2 represent the gain curves of a typical 1725-CBJA2 optical fiber amplifier. The concept of partially or completely offsetting the amplifier gain due to temperature would be the same for other optical fiber amplifiers. 
     Various methods and devices may be used to modify the optical pump signal wavelength to the extent required. These methods and devices include using a fiber grating device, control systems, and a ring reflector cavity system. 
     The fiber grating device can make use of high thermal variation glass or the use of an external Bragg reflector. According to another aspect of the invention, a Bragg reflector may be packaged such that any change in temperature applies stress to the Bragg reflector which changes the optical pump signal wavelength. 
     According to another aspect of the invention, a Fabry-Perot cavity may be used to create a phase shift. The phase shift can be used to set the optical pump wavelength characteristics as required to partially or completely offset optical amplifier gain due to temperature. The Fabry-Perot cavity can be a bulk Fabry-Perot cavity or make use of two Bragg reflectors as applied in a compound Bragg reflector Fabry-Perot cavity. 
     According to another aspect of the invention, a controller may be used to adjust the temperature of the optical pump signal source or to proportion the amount of optical pump signal portions combined to form an optical pumps signal with the desired wavelength characteristics. 
     According to another aspect of the invention, a ring reflector cavity system may be used to control amplifier gain due to temperature by altering the optical pump wavelength. The ring-reflector cavity systems may include the use of a long period grating filter, a dielectric filter, or an optical isolator. 
    
    
     These and other advantages and features of the invention will become apparent from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the difference in gain of an optical fiber amplifier at −5 degrees Celsius and the same optical fiber amplifier at 70 degrees Celsius; 
     FIG. 2 is a graph showing the difference in gain of an optical fiber amplifier at a wavelength of 972.5 nm and the same optical fiber amplifier at a wavelength of 980 nm; 
     FIG. 3 is a diagram of a first embodiment of the present invention, which uses a fiber grating device to select an optical signal wavelength which partially or completely offsets the amplifier gain due to temperature; 
     FIG. 4 is a graph showing obtainable wavelengths in a Bragg reflector for a single mode fiber and a high thermal variation glass fiber; 
     FIG. 5 is an illustration of an optical communication system with a Bragg reflector device for setting a desired optical signal wavelength; 
     FIG. 6 is an illustration of an optical communication system with a bulk Fabry-Perot cavity; 
     FIG. 7 is a diagram showing a second embodiment of the invention, which has a compound Bragg reflector for selecting an optical signal wavelength which partially or completely offsets the amplifier gain due to temperature; 
     FIG. 8 is an illustration of a system that includes a temperature varying dielectric filter for use in the present invention; 
     FIG. 9 is a diagram of a third embodiment of the invention which employs a controller and gain schedule look-up table; 
     FIG. 10 is a diagram of a fourth embodiment of the invention which uses a ring-reflector cavity to select an optical signal wavelength which partially or completely offsets the amplifier gain due to temperature; 
     FIG. 11 is an illustration of a system that includes a long period fiber grating device for use in the present invention; 
     FIG. 12 is an illustration of a system that includes a dielectric filter used within a narrow pass band filter; 
     FIG. 13 is an illustration of a system that includes a unidirectional ring reflector cavity and an optical isolator; 
     FIG. 14 is a diagram of a fifth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described with reference to the embodiments illustrated in FIGS. 3-14. Other embodiments may be utilized and structural changes may be made without departing from the spirit or scope of the present invention. 
     FIG. 3 illustrates a pump laser package  1  that contains a laser chip  3  where the laser chip  3  has a front facet  9 . An optical fiber  7  is connected to the pump package  1  by a fiber pigtail  8 . Along the optical fiber  7  is a fiber grating device  5 , such as a Bragg reflector. The optical signal travels from the pump laser package  1  through the fiber grating device  5  and into an optical fiber amplifier  2 . An input transmission line  4  leads into the amplifier  2  and is connected to the erbium doped fiber (not illustrated). An output transmission fine  6  leads out of the erbium doped fiber. 
     The fiber grating device  5  is formed by exposing the fiber  7  to ultraviolet light. The ultraviolet light causes a disruption in the glass core of the optical fiber  7 . What this disruption causes at-a very slight level is a change of the refractive index of the glass in the fiber  7 . The change in refractive index allows only signals within a very narrow set of wavelength ranges to pass through the fiber grating  5 . Therefore, the fiber grating  5  can be used to set the wavelength of the optical signal incident on the amplifier  2 . In an alternative embodiment of the invention, the fiber grating device  5  may employ reflectors, such as mirrors or dielectric filters, to limit the spectrum of the input signal at the amplifier  2  to a certain wavelength range. 
     In operation, the pump laser package  1  produces an optical signal, with a certain wavelength, which travels through the optic fiber  7 . The pump laser package  1  operates in a range of from about 950 nm to about 1000 nm with the typical optical signal in a range of from about 970 nm to about 990 nm. The optical signal is propagated through the grating device  5 . The grating device  5  affects the wavelength characteristic of the pump laser signal by reflecting a portion of the signal back into the pump laser as a feedback signal. This feedback signal causes the pump laser signal to have the same wavelength as the reflection of the grating device  5 . The wavelength of the reflection changes with changes in temperature in a manner that partially or completely offsets the gain change due to temperature in the optical fiber amplifier  2 . Finally, an amplified optical signal is transmitted from the amplifier  2  along the output optical fiber  6 . The embodiment shown in FIG. 3 does not require a temperature sensor because the fiber grating device  5  may be arranged to change in an appropriate and corresponding manner with the temperature gain characteristics of the optical fiber amplifier  2 . 
     The use of a fiber grating device to set the wavelength of the optical signal may require the use of high thermal variation glass. FIG. 4 illustrates the broad spectrum of wavelengths available when using high thermal variation glass as compared with standard single mode fiber. For systems where the range of pump laser wavelength variation is large, a high thermal variation glass fiber may be used. 
     Referring now to FIG. 5, a Bragg reflector  61  may be located in the optical fiber  7 . The fiber  7  transmits signals to the optical fiber amplifier  2 . In the FIG. 5 embodiment, the optical fiber  7  is fixed to supports  64  by epoxy or other structure. Mechanical stress applied to the Bragg reflector  61  is adjusted by support mounts  65  and thermal expansive material  66 . The optical signal travels through the optical fiber  7  and into the Bragg reflector  61 . As the temperature changes, the thermal expansive material  66  expands or contracts. This expansion or contraction applies or relieves stress on the Bragg reflector  61 , thereby changing the wavelength of the reflection from the Bragg reflector  61  and hence the wavelength of the optical pump signal applied to the amplifier  2 . 
     By knowing the gain characteristics of the optical fiber amplifier  2 , an appropriate Bragg reflector and thermal expansive material  66  can be selected. The selected wavelength of the optical signal may be confined to a very narrow range, if desired. This set optical signal wavelength may be used to partially or completely offset the gain due to temperature in the amplifier  2 . 
     FIG. 6 illustrates an apparatus that has two partially reflective mirrors  74 ,  75  which form a Fabry-Perot cavity  76  along the optical fiber  7 . The cavity  76  circulates reflected optical pump signals which also overlap the incoming optical pump signal. Also, a temperature sensitive material  77  is used between the mirrors  74 ,  75 . With changes in temperature, the temperature sensitive material  77  changes the length of the cavity  76  to alter the wavelength of the output pump signal on line  73 . By knowing the gain characteristics of the optical fiber amplifier  2 , the appropriate partially reflective mirrors  74 ,  75  and temperature sensitive material  77  can be selected to form a cavity  76  which sets the optical pump signal wavelength transmitted to the amplifier  2  to partially or completely offset the amplifier  2  gain due to temperature. 
     Another form of a Fabry-Perot cavity uses multiple Bragg reflectors as seen in FIG.  7 . In the apparatus shown in FIG. 7, an optical pump signal travels through the optical fiber  7 , through a first fiber optic grating device  23  (a Bragg reflector), through a suitable separation distance of optical fiber  27 , through a second fiber grating device  25  (A Bragg reflector) and ultimately into the optical fiber amplifier  2 . The wavelength characteristics of the optical pump signal are changed as a result of optical feedback from the Fabry-Perot cavity  29 . The Fabry-Perot cavity  29  causes a phase shift that is correlated to temperature. With suitable placement and selection of the fiber grating devices  23 , 25 , an accurate phase shift can be applied to the optical signal to set the optical pump signal wavelength which partially or completely offsets gain due to temperature in the amplifier  2 . 
     Referring now to FIG. 8, a diode  81  emits an optical pump signal with a broad spectrum which travels through coupling lenses  84 , a temperature varying dielectric filter  85 , and through lenses  86 . Portions of the optical pump signal are reflected by the dielectric filter  85  and focussed back into the laser diode  81 . Use of the dielectric filter  85  limits the optical pump signal, incident on the amplifier  2 , to a narrow wavelength range. The optical pump signal set by the filter  85  may be used to partially or completely offset the optical fiber amplifier gain changes due to temperature. Finally, an amplified signal with reduced amplifier gain variation is transmitted along an optical output fiber  6 . 
     Referring now to FIG. 9, the wavelength characteristics of the pump signal incident on the amplifier  2  may be controlled by heating the pump  37  which generates the optical pump signal. The pump signal generated by the pump  37  is transmitted through the optical fiber  7  and ultimately into the optical fiber amplifier  2 . A sensor  33  measures the temperature of the erbiumdoped fiber. The signals from the sensor  33  are transmitted to a controller  38 . 
     Based upon the sensed temperature, the controller  38  determines a desired optical pump signal wavelength value based upon a gain schedule table  36 . When a desired wavelength is determined, the controller  38  sends a corresponding signal to control a thermo-electric cooler (TEC)  39 . The TEC  39  heats the pump  37 , thereby causing the pump  37  to emit an optical pump signal with the set wavelength which will partially or completely offset gain changes due to temperature of the optical fiber amplifier  2  signal. The TEC  39  may also adjust the temperature of the pump  37  by adjusting a chip  3 , see FIG. 3, located within the pump laser package  1 . 
     A still further embodiment of the present invention is illustrated in FIG.  10 . The FIG. 10 embodiment has a pump laser  41  which generates an optical signal transmitted through the optical fiber  7  to a first splitter  45 . The optical signal is split by the splitter  45 . One of the split signals  451  is sent to a second splitter  47 . The other split signal  452  is sent first to a narrow pass band filter  49 , and then on to the second splitter  47 . At second splitter  47 , the split signal  452  is coupled onto optical fiber  7 , to become the first feedback signal  453  travelling towards the pump laser  41  in the direction opposite to the direction of split signal  451 . The second splitter  47  receives the first split signal  451 , and splits it again, sending one of the split signals  471  on to the optical fiber amplifier  2 . The other split signal  472  travels to the narrow band pass filter  49  and is then coupled by first splitter  45  onto the optical fiber  7  as second feedback signal  474  travelling toward the pump laser  41 . First and second feedback signals  453  and  474  are used to set the pump laser wavelength by locking or affecting the optical signal on line  7  to partially or completely offset gain due to temperature of the optical fiber amplifier  2 . 
     The narrow pass band filter  49 , see FIG. 10, may be a long period grating filter as seen in FIG.  11 . The long period grating filter receives incoming optical signal  91  which is fed through an optical fiber  7 . The core  94  of the optical fiber  7  contains a long grating  95  which has reflective characteristics causing scattered optical signals  96 . The long period grating device scatters all optical signals but the optical signal with the required wavelength to partially or completely offset the optical fiber amplifier gain due to temperature. The optical signal with the set wavelength is then transmitted into the optical fiber amplifier  2 . 
     The narrow pass band filter  49 , see FIG. 10, may also be a dielectric filter as depicted in FIG.  12 . In this system an optical pump signal is transmitted through an optical fiber  7 , into a lens or group of lenses  104 , a dielectric filter  105 , a second lens or set of lenses, and ultimately through an optical fiber amplifier  2 . The optical pump signal is filtered by the dielectric filter  105  to set the optical signal wavelength to partially or completely offset the optical fiber amplifier  2  gain due to temperature. 
     As seen in FIG. 13, an optical pump  111  generates an optical signal which is transmitted through an optical fiber  7  and into a first splitter  115 . The first splitter  115  sends a portion of the optical pump signal to a second splitter  117  and a portion into a ring reflector cavity which contains an optical isolator  118  and a filter  119 . The second splitter  117  transmits the feedback signal  116  from the ring reflector cavity back towards the pump  111  and transmits the forward optical pump signal to the optical fiber amplifier  2 . The ring reflector cavity, which includes the optical isolator  1   18  and filter  119 , and splitters  115 ,  117  works to set the optical pump signal wavelength transmitted to the optical fiber amplifier  2  by providing optical feedback to the pump  111 . By knowing the gain characteristics of the optical fiber amplifier  2  an appropriate optical isolator  118  and filter  119  can be selected which will work to set the optical pump signal wavelength. The set wavelength will partially or completely offset the optical fiber amplifier  2  gain due to temperature. 
     The apparatus shown in FIG. 14 controls two pumps  57 ,  58  and their proportional input to create a combined optical signal. Specifically, an optical signal is generated through the pumps  57 ,  58 . The first pump  57  generates an optical pump signal portion which is combined with an optical pump signal portion generated by the second pump  58  to form the optical pump signal for the system. The optical pump signal is transmitted through the optical fiber  7  and ultimately into the optical fiber amplifier  2 . 
     The temperature of the erbium-doped fiber in the amplifier is measured by a suitable temperature sensor  55 . The pumps  57 ,  58  are driven by respective current sources  59 ,  60 . A controller  61  receives the temperature reading from the temperature sensor  55 . Based upon this temperature reading the controller  61  uses a gain schedule look up table  63  to determine the desired optical pump signal wavelength and the proper proportions of the optical pump signal portions required from the pumps  57 ,  58 . The controller  61  then controls the amount of current applied by the sources  59 ,  60  to control the proportion of the optical pump signal portions from the pumps  57 ,  58  and thereby set the optical pump signal wavelength to a value which will partially or completely offset gain due to temperature of the optical fiber amplifier  2 . The pumps  57 ,  58  may be a pump laser, for example, such as a 980 nm pump laser. 
     Reference has been made to embodiments in describing the invention. However, additions, deletions, substitutions, or other modifications which would fall within the scope of the invention defined in the claims may be implemented by those skilled in the art and familiar with the disclosure of the invention without departing from the spirit or scope of the invention. Accordingly, the invention is not to be considered as limited by the foregoing description, but is only limited by the scope of the appended claims