Abstract:
An all fiber optical filter is formed by stretching an optical fiber. The all fiber filter includes a core, an inner cladding, and an outer cladding. A core index of refraction is greater than an outer cladding index of refraction. The outer cladding index of refraction is greater than an inner cladding index of refraction. The all fiber optical filter attenuates a portion of an optical signal by transferring optical energy from the core to the outer cladding by evanescent coupling. The all fiber optical filter has a compact structure, which prevents bending and provides stable temperature performance. The all fiber optical filter is preferably used in Wavelength Division Multiplexing (WDM) systems for gain flattening of gain responses from Erbium Doped Fiber Amplifiers (EDFAs). Alternatively, the all fiber optical filter is used in other applications where optical filtering or attenuation is needed. The all fiber optical filter is manufactured by holding a length of an appropriate optical fiber between two clamps, heating the optical fiber, and stretching the optical fiber until a predetermined characteristic of the all fiber optical filter is achieved.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) of the co-pending U.S. provisional application Ser. No. 60/101,853 filed on Sep. 25, 1998 and entitled “ALL-FIBER EDFA GAIN FLATTENING FILTER” The provisional application Ser. No. 60/101,853 filed on Sep. 25, 1998 and entitled “ALLFIBER EDFA GAIN FLATTENING FILTER” is also hereby incorporated by reference. 
     RELATED APPLICATIONS 
     This is a divisional of U.S. patent application Ser. No. 09/246,985, filed Feb. 9, 1999, now U.S. Pat. No. 6,362,916 the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of fiber optic communications. More particularly, the present invention relates to the field of filtering of amplified signals used in fiber optic communications systems. 
     BACKGROUND OF THE INVENTION 
     Fiber optic communication systems use optical fibers to carry a modulated lightwave signal between a transmitter and a receiver. A cross-section of a typical optical fiber is illustrated in FIG.  1 . The optical fiber  2  includes a core  4  and a cladding  6 . Optionally, the optical fiber  2  includes a jacket  8 . In a typical optical fiber, the core  4  has an index of refraction greater than the cladding  6 , thereby forming an optical waveguide. By maintaining the core diameter within an allowed range, light traveling within the core  4  is limited to a single mode. If included, the jacket  8  protects the outer surface of the cladding  6  and absorbs stray light traveling within the cladding  6 . A typical single mode optical fiber intended for use in communication systems operating at a 1300 mn wavelength band or a 1550 nm wavelength band has a core diameter of about 8 μm and a cladding outside diameter of 125 μm. If the jacket  8  is included, the jacket  8  typically has an outside diameter of 250 μm. 
     In Wavelength Division Multiplexing (WDM) systems, multiple signals are carried by various wavelengths of light through a single optical fiber. A typical WDM system is shown in FIG.  2 . The WDM system  10  includes a transmission system  11 , which includes a series of transmitters  12 ,  14 , and  16 , each coupled to a multiplexer  18 . The multiplexer  18  provides an output, which is coupled to an optical fiber  20 . Over long distances amplifiers  22  are included along the optical fiber  20 . The optical fiber  20  is then also coupled to a receiving system  23 , which includes a demultiplexer  24  and a series of receivers  26 ,  28 , and  30 . The optical fiber  20  is coupled to an input of the demultiplexer  24  of the receiving system  23 . Outputs of the demultiplexer  24  are coupled to the series of receivers  26 ,  28 , and  30 . 
     In the WDM system  10 , a first transmitter  12  transmits a light signal at a first wavelength (λ 1 ), a second transmitter  14  transmits a light signal at a second wavelength (λ 2 ) and so forth until an nth transmitter  16  transmits a light signal at an nth wavelength (λ n ). The shortest wavelength signal and the longest wavelength signal form a wavelength band. The signals are combined by the multiplexer  18 , which then transmits the light signals along the optical fiber  20 . Over distance the power of the light signals decrease due to attenuation. The light signals are typically amplified about every 50-100 km. For the 1550 nm wavelength band, this amplification is generally performed by an Erbium Doped Fiber Amplifier (EDFA)  22 . When the light signals reach their destination they are separated by the demultiplexer  24 . The light signals are then received by the receivers  26 ,  28 , and  30 . Light signal intensity versus wavelength for a typical wavelength band of WDM light signals is illustrated in FIG.  3 . 
     Flat gain response for EDFAs is crucial to the performance of the WDM system  10 , since small variations in gain for various wavelengths will grow exponentially over a series of in-line EDFAs  22 . Agrawal in “Fiber Optic Communication Systems,” (Wiley, 2nd ed., 1997, pp 414-415) teaches that numerous methods can be used to flatten the gain response of these amplifiers. One method of flattening this gain response is to use channel filters to equalize the gain for various wavelengths. Another method is to adjust the input powers of different wavelengths so that amplification results in uniform intensity for various wavelengths. A third method is to use inhomogeneous broadening of the EDFA gain spectrum to equalize wavelength intensity. A fourth method is to use multiple EDFAs tuned to different wavelength ranges and configured with feedback loops. A final method is to use a filter or series of filters to selectively attenuate the gain response of an EDFA. 
     A typical gain versus wavelength response for an EDFA is shown in FIG.  4 A. When utilizing a filter or series of filters to flatten gain response, an optical filter, with an attenuation curve as shown in FIG. 4B, can be used to selectively attenuate the gain response. The resulting attenuated EDFA gain is shown in FIG.  4 C. As shown in FIG. 4C, this attenuated EDFA gain is substantially flat over a range of wavelengths including 1530 nm to 1560 nm. Without a substantially flat gain, the quality of the signal received by the receivers  26 ,  28 , and  30  will be poor. 
     There are many different known methods for selectively attenuating the gain response of an EDFA in order to improve the signal quality of the signals received by the receivers  26 ,  28 , and  30 . U.S. Pat. No. 5,260,823 to Payne et al. entitled, “Erbium-Doped Fibre Amplifier with Shaped Spectral Gain,” teaches that a wavelength-selective resonant coupling between a propagating core mode to a cladding leaky mode can be used for filtering a wavelength band for EDFA gain flattening. A periodic perturbation of the core forms a grating and the selected wavelength is attenuated by the resonant coupling between the core and the cladding. By varying the perturbation length, various selected wavelengths can be attenuated. Payne et al. also teach that multilayered dielectric coatings can be used for making an optical filter for EDFA gain flattening. A multilayered filtering apparatus includes two coupling lenses and a multilayered dielectric filter. The two coupling lenses connect to an optical fiber and sandwich the multilayered dielectric filter. The multilayered dielectric filter is designed to cancel out the larger gain around the peak wavelength and to be transparent elsewhere. 
     U.S. Pat. No. 5,473,714 to Vengsarkar entitled, “Optical Fiber System Using Tapered Fiber Devices,” teaches that tapered fiber devices can be used for filtering in an optical telecommunications system. Vengsarkar teaches that by tapering an optical fiber, light can be attenuated by wavelength cutoff and direct coupling from a core to a cladding. The tapered fiber device is formed from the optical fiber by heating the optical fiber and stretching it. The taper reduces the diameter of the core to a value close to the cutoff wavelength. Light with wavelengths near and above the cutoff wavelength are coupled directly to the cladding. 
     U.S. Pat. No. 5,583,689 to Cassidy et al. entitled “Filter With Preselected Attenuation/Wavelength Characteristic,” teaches that a fiber grating, with spatially separated parts having different attenuation characteristics, can perform filtering for EDFA gain flattening. The fiber grating is preferably a side-tap Bragg fiber grating. By varying the pitch along the fiber grating an appropriate attenuation profile can be provided for flattening the EDFA gain response. 
     U.S. Pat. No. 5,067,789 to Hall et al. entitled, “Fiber Optical Coupling Filter and Amplifier,” teaches that a light-attenuating light path adjacent to a first core within a cladding can be used to filter wavelengths about a specific wavelength for EDFA gain flattening. The light attenuating light path is preferably one or more lossy cores that are evanescently coupled to the first core. The evanescent coupling between the first core and the light attenuating light path is greatest where the effective index of refraction of the first core equals the effective index of refraction of the light attenuating light path. By choosing a single mode or a higher multimode optical waveguide structure for the light attenuating light path, the effective index of refraction for the light attenuating light path can be varied. Hall et al. teach that the index of refraction for the material for the light attenuating light path should be greater than the index of refraction for the material for the first core. Hall et al. further teach that as an alternative embodiment the lossy core could be a lossy annular region located concentrically about the first core and within the cladding. A necessary feature of this filter is that the lossy core or the lossy annular region has specific light absorption characteristics. Since the lossy core or the lossy annular region is contained completely within the cladding, the specific light absorption characteristics dissipates light energy that has been filtered from the first core to the lossy core or the lossy annular region. The absorption characteristics of the lossy core or the lossy annular region determine an amount of attenuation of the filtered wavelengths. 
     Each of these known methods for filtering an amplified signal from an EDFA can be inefficient, unreliable, and expensive. There is currently a lack of efficient filters for gain flattening in fiber optic systems, which are easy to manufacture and use within a WDM system. 
     SUMMARY OF THE INVENTION 
     An all fiber optical filter is formed by stretching an optical fiber. The all fiber filter includes a core, an inner cladding, and an outer cladding. A core index of refraction is greater than an outer cladding index of refraction. The outer cladding index of refraction is greater than an inner cladding index of refraction. The all fiber optical filter attenuates a portion of an optical signal by transferring optical energy from the core to the outer cladding by evanescent coupling. The all fiber optical filter has a compact structure, which prevents bending and provides stable temperature performance. 
     The all fiber optical filter is preferably used in Wavelength Division Multiplexing (WDM) systems for gain flattening of gain responses from Erbium Doped Fiber Amplifiers (EDFAs). Alternatively, the all fiber optical filter is used in other applications where optical filtering or attenuation is needed. 
     The all fiber optical filter is manufactured by holding a length of an appropriate optical fiber between two clamps, heating the optical fiber, and stretching the optical fiber until a predetermined characteristic of the optical fiber is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a cross section of an optical fiber of the prior art. 
     FIG. 2 illustrates a block diagram of a WDM system of the prior art. 
     FIG. 3 illustrates a graph of intensity versus wavelength for a wavelength band of WDM light signals of the prior art 
     FIG. 4A illustrates an EDFA gain curve over a range of wavelengths of the prior art 
     FIG. 4B illustrates a filter attenuation curve over a range of wavelengths for gain band flattening of the prior art. 
     FIG. 4C illustrates an attenuated EDFA gain curve over a range of wavelengths using a filter of the prior art 
     FIG. 5 illustrates a linear cross section of an all fiber optical filter of the present invention. 
     FIG. 6 illustrates a cross-section of the all fiber optical filter of the present invention. 
     FIG. 7 illustrates the all fiber optical filter and additional structure of the present invention. 
     FIGS. 8A,  8 B, and  8 C illustrate configurations including an EDFA, a first all fiber optical filter, and a second all fiber optical filter of the present invention. 
     FIGS. 9A and 9B illustrate intensity versus wavelength for an EDFA gain response and a filtered EDFA gain response of the present invention. 
     FIG. 10 illustrates an EDFA and an all fiber optical filter of the present invention. 
     FIG. 11 illustrates a WDM system including the all fiber optical filter of the present invention. 
     FIG. 12 illustrates a first apparatus for making the all fiber optical filter of the present invention. 
     FIG. 13 illustrates a second apparatus for making the all fiber optical filter of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A linear cross section of an all fiber optical filter of the present invention is illustrated in FIG.  5 . The all fiber optical filter  32  has a core  34 , an inner cladding  36 , an outer cladding  38 , and a filter length  40 . A cross-section of the all fiber optical filter  32  showing the core  34 , the inner cladding  36 , and the outer cladding  38  is illustrated in FIG.  6 . The core  34  has a core diameter. The inner cladding  36  has an inner cladding thickness. The outer cladding  38  has an outside diameter. Indexes of refraction for the core  34 , the inner cladding  36 , and the outer cladding  38  are referred to as a core index of refraction, an inner index of refraction, and an outer index of refraction, respectively. The core index of refraction is preferably greater than the outer index of refraction. The outer index of refraction is preferably greater than the inner index of refraction. By an appropriate selection of the core index of refraction, the inner index of refraction, and the outer index of refraction as well as selecting the core diameter and the inner cladding thickness, optical energy from an optical signal within a wavelength range is transferred from the core  34  to the outer cladding  38  by evanescent coupling. 
     The core  34  of the all fiber optical filter  32  is a single mode waveguide. A convention used when discussing optical waveguides is to refer to an effective index of refraction, which is defined as a waveguide propagation constant β divided by a free space wave number k o . The effective index of refraction is both wavelength dependent and mode dependent. A core effective index of refraction for the core  34  has a value between the inner index of refraction and the core index of refraction. Reducing the core diameter reduces the core effective index of refraction provided that the single mode continues to propagate. The outer cladding  38  is a multimode waveguide. The outer cladding is sufficiently large that an outer effective index of refraction for a first mode is equal to the outer index of refraction. The inner cladding  36  forms a barrier between the core  34  and the outer cladding  38 . Optical energy will transfer from the core  34  to the cladding  38  by evanescent coupling if the core effective index of refraction is near to the outer index of refraction and the barrier is sufficiently narrow. Since the core effective index of refraction depends upon the core diameter, the core diameter determines a wavelength range that could couple from the core  34  to the outer cladding  38 . 
     The core diameter, the core effective index of refraction, and the outer index of refraction determine a peak attenuation wavelength and a wavelength band about the peak attenuation wavelength that couples from the core  34  to the outer cladding  38 . Optical energy that couples from the core  34  to the outer cladding  38  and is propagating in the first mode can couple back to the core  34 . Accordingly, the outer diameter of the outer cladding and the filter length  40  adjust the peak attenuation wavelength and the wavelength band about the peak wavelength. Depending upon a variation of the core effective index of refraction with wavelength, other peak attenuation wavelengths and wavelength bands could couple from the core  34  to the outer cladding  38 . 
     The all fiber optical filter  32  and additional structure is illustrated in FIG.  7 . The additional structure includes an input length  42 , an output length  44 , a first transition  46 , and a second transition  48 . The input length  42  connects to the first transition  46 , which connects to the all fiber optical filter  32 . The all fiber optical filter  32  connects to the second transition  48 , which connects to the output length  44 . The core  34 , the inner cladding  36 , and the outer cladding  38  of the all fiber optical filter  32  extend through the input length  42 , the first transition  46 , the second transition  48 , and the output length  44 . The thickness of the inner cladding  36 , within the input length  42  and the output length  44 , is greater than an evanescent coupling thickness that allows evanescent coupling between the core  34  and the outer cladding  38  within the input length  42  and the output length  44 . The input length  42  and the output length  44  are coupled to an optical fiber system by appropriate means available for coupling optical fiber components. 
     An exemplary configuration including an EDFA and a cascaded series of all fiber optic filters used to flatten the EDFA gain over wavelength ranges of 1529 nm to 1562 nm and 1580 nm to 1620 nm is illustrated in FIG.  8 A. The EDFA  52  is coupled to a first all fiber optical filter  54 . The first all fiber optical filter  54  is coupled to a second all fiber optical filter  56 . An input optical signal  58  is provided to the EDFA  52 , which amplifies the input optical signal  58  and provides an amplified optical signal. The amplified optical signal is then provided to the first all fiber optical filter  54 , which filters the amplified optical signal and provides a first filtered optical signal. The first filtered optical signal is then provided to the second all fiber optical filter  56 , which filters the first filtered optical signal and provides an output optical signal  60 . 
     Other configurations for the EDFA  52 , first all fiber optical filter  54 , and the second all fiber optical filter  56  are illustrated in FIGS. 8B and 8C. In FIG. 8B, the first all fiber optical filter  54  is coupled to the EDFA  52 , which is coupled to the second all fiber optical filter  56 . In FIG. 8C, the first all fiber optical filter  54  is coupled to the second all fiber optical filter, which is coupled to the EDFA  52 . 
     In the preferred embodiment of the present invention, intended to operate in the wavelength ranges of 1529 nm to 1562 mn and 1580 nm to 1620 nm, the core  34 , the inner cladding  36 , and the outer cladding  38  are silica glasses. The indexes of refraction are preferably 1.467 for the core index of refraction, 1.411 for the inner index of refraction, and 1.424 for the outer index of refraction. The core diameter is preferably within the range and including 3 μm and 6 μm. An outer diameter for the inner cladding  36  is preferably within the range and including 12 μm and 30 μm. The outside diameter of the outer cladding  38  is preferably within the range and including 50 μm and 85 μm. The filter length  40  is preferably within the range and including 10 mm and 20 mm. Specific dimensions for the preferred embodiment are a result of a forming process, which preferably uses an optical spectrum response for the all fiber optical filter  32  as a critical parameter. 
     The preferred embodiment for the all fiber optical filter  32  is formed by identifying a preferred peak EDFA gain response and a preferred wavelength band about the preferred peak gain response that is to be flattened. An inverse of the gain response for the preferred wavelength band becomes a preferred target response for the all fiber optical filter  32  such that the all fiber optical filter  32  provides a preferred attenuation response that is near to the preferred target response after the forming process. 
     Referring to FIG. 8A, the EDFA  52  provides the amplified optical signal, which is used to determine the first peak EDFA gain response and the first wavelength band. For a test EDFA used in testing an all fiber optical filter of the present invention, the preferred peak EDFA gain response was found to be at 1533 nm with a preferred relative gain response of 6.0 dB. The relative gain response is defined as the difference between a specific gain response for a specific wavelength and a minimum gain response for the wavelength range. The preferred target response about 1533 nm was used in the forming process so that after the forming process, the all fiber optical filter  32  provided the preferred attenuation curve. 
     An alternative embodiment is formed by identifying an alternate target response for an alternate peak EDFA gain response and an alternate wavelength band about the alternate peak EDFA gain response. For the test EDFA, the alternate peak wavelength was found to be at 1552 nm with an alternate relative gain response of 3.83 dB. 
     Referring to FIG. 8A, tests were performed in which the EDFA  52  was the test EDFA, the first all fiber optical filter  54  was the preferred embodiment of the all fiber optical filter described above and having the preferred attenuation response, and the second all fiber optical filter  56  was the alternative embodiment of the all fiber optical filter described above and having an alternate attenuation response. Test results using this configuration for the wavelength range from 1529 nm to 1562 nm are illustrated in FIG.  9 A. The EDFA gain response is shown as the curve A. The first target response is the inverse of the EDFA gain response from 1529 nm to 1540 nm. The second target response is the inverse of the EDFA gain response from 1540 nm to 1562 nm. The output optical signal  60  is shown as the curve B, which shows a substantially flat attenuated EDFA gain curve over the wavelength range from 1529 nm to 1562 nm. 
     Test results using this configuration for the wavelength range from 1580 nm to 1620 are illustrated in FIG.  9 B. The EDFA gain response is shown as the curve C. The output optical signal  60  is shown as the curve D, which shows a substantially flat attenuated gain curve over the wavelength range from 1580 nm to 1620 nm. 
     An alternative embodiment comprising the EDFA  52  and a single all fiber optical filter is illustrated in FIG.  10 . Depending upon the gain response of the EDFA  52 , the single all fiber optical filter  62  will suffice to flatten the gain response of the EDFA  52 . The EDFA  52  is coupled to the single all fiber optical filter  62 . The input optical signal  58  is provided to the EDFA  52 , which amplifies the input optical signal  58  and provides an amplified optical signal. The amplified optical signal is then provided to the single all fiber optical filter  62 , which filters the amplified optical signal and provides the output optical signal  60 . 
     A WDM system with EDFA gain flattening including one or more all fiber optical filters according to the present invention is illustrated in FIG.  11 . The WDM system  66  includes a transmission system  11 , which includes a series of transmitters  12 ,  14 , and  16  each coupled to a multiplexer  18 . The multiplexer  18  provides an output, which is coupled to an optical fiber  20 . Over long distances EDFAs  22  and the one or more all fiber optical filters  68  are included along the optical fiber  20 . The optical fiber  20  is then also coupled to a receiving system  23 , which includes a demultiplexer  24  and a series of receivers  26 ,  28 , and  30 . The optical fiber  20  is coupled to an input of the demultiplexer  24  of the receiving system  23 . Outputs of the demultiplexer  24  are coupled to the series of receivers  26 ,  28 , and  30 . 
     In the WDM system  66 , a first transmitter  12  transmits a light signal at a first wavelength (λ 1 ), a second transmitter  14  transmits a light signal at a second wavelength (λ 2 ), and so forth until an nth transmitter  16  transmits a light signal at an nth wavelength (λ n ). The light signals are combined by the multiplexer  18 , which then transmits the light signals along the optical fiber  20 . Over distance the power of the light signals decrease due to attenuation. The light signals are amplified approximately every 50-100 km by the EDFAs  22 . The one or more all fiber optical filters  68  flatten the EDFA gain for the light signals, as discussed above. When the light signals reach their destination they are separated by the demultiplexer  24 . The light signals are then received by the receivers  26 ,  28 , and  30 . 
     A first apparatus for manufacturing the all fiber optical filter of the present invention is illustrated in FIG.  12 . The first apparatus  70  includes a heating source  72 , a first clamp  74 , a second clamp  76 , a first stepper motor  78 , a second stepper motor  79 , a first drive means  80 , and a second drive means  81 . The first clamp  74  is placed to one side of the heating source  72 . The second clamp  76  is placed adjacent to the heating source  72  on the side opposite to the first clamp  74 . The first clamp  74  is connected to the first stepper motor  78  by the first drive means  80 . The second clamp  76  is connected to the second stepper motor  79  by the second drive means  81 . 
     A first method of manufacture uses the first apparatus  70 . An initial length of optical fiber  82  is held between the first clamp  74  and the second clamp  76 . The heating source  72  heats the optical fiber  82  to within an allowed temperature range. The first stepper motor  78  actuates the first drive means  80 . The second stepper motor  79  actuates the second drive means  81 . Consequently, the first clamp  74  and the second clamp  76  are further separated. This further separation stretches the optical fiber  82 . When a predetermined stretch distance has been reached the first and second stepper motor  78  and  79  are stopped, which stops the first and second clamp  74  and  76 . Finally, the heating source  72  is removed, the heating source  72  is turned off, or the optical fiber  82  is removed from the heating source  72 . This results in an all fiber optical filter, according to the present invention, having a predetermined filter length 
     A second and preferred apparatus for manufacturing the all fiber optical filter of the present invention is illustrated in FIG.  13 . The second apparatus  84  includes the first apparatus  70 , a process control unit  86 , a light source  88 , and an optical spectrum analyzer  90 . The light source  88  is located at one end of the optical fiber  82 . The optical spectrum analyzer  90  is located at the end of the optical fiber  82  opposite to the light source  88 . The process control unit  86  controls and monitors the heating source  72  through a first control link  92 . The process control unit  86  controls the first stepper motor  78  through a second control link  94 . The process control unit  86  controls the second stepper motor  79  through a third control link  95 . The process control unit  86  controls the light source  88  through a fourth control link  96 . The process control unit  86  controls and monitors the optical spectrum analyzer  90  through a fifth control link  98 . 
     A second and preferred method of manufacture uses the second apparatus  84 . The initial length of optical fiber  82  is held between the first clamp  74  and the second clamp  76 . The process control unit  86  signals and monitors the heating source  72 . The heating source  72  heats the optical fiber to within the allowed temperature range. The process control unit  86  turns on the light source  88 . The light source  88  couples light to the optical fiber  82 . Preferably, the light source  88  is a white light source. The optical fiber  82  transmits the light to the end of the optical fiber  82  opposite the light source  88 . The light exits the optical fiber  82 . The process control unit turns on the optical spectrum analyzer  90 . The optical spectrum analyzer  90  detects the light that exits from the optical fiber  82 . The process control unit  86  signals the first and second stepper motors  78  and  79 . The first and second stepper motors  78  and  79  further separate the first and second clamps  74  and  76 . This further separation stretches the optical fiber  82 . As the optical fiber  82  is stretched, the light signal at the end of the optical fiber adjacent to the optical spectrum is monitored for a predetermined optical spectrum response that is based on the target response, as described above. When the optical spectrum analyzer  90  detects the predetermined optical spectrum response, the process control unit  86  stops the first and second stepper motors  78  and  79 , thereby stopping the first and second clamp  74  and  76 . Finally, the process control unit signals the heating source  72  to stop heating. This results in an all fiber optical filter, according to the present invention, having a desired attenuation response. 
     Preferably, the optical fiber  82  used to form the all fiber optical filter  32  of the present invention has a core with an initial diameter of 8.3 μm, an inner cladding with an initial outside diameter of 45 μm, and an outer cladding with an initial outside diameter of 125 μm. Preferably, a length of 6 mm is heated by the heating source  72  to a temperature within the range between 900° C. and 1100° C. The optical fiber  82  is stretched to a length of about 15 mm. Preferably, the specific stretch length and other dimensions of the all fiber optical filter are determined by the predetermined optical spectrum response. 
     It will be readily apparent to one skilled in the art that other various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined by the appended claims. Specifically, the all fiber optical filter of the present invention could be used to flatten the gain of other rare earth doped fiber amplifiers or the all fiber optical filter of the present invention could be used to filter or attenuate any optical signal.