Abstract:
A method and apparatus for controlling gain tilt in an optical amplifier that adjusts the tilt based on the optical signals entering the optical amplifier and in particular, on a “feed-forward”, predictive methodology that monitors optical signals entering an optical amplifier as opposed to prior art, spectral monitoring techniques that monitor optical signals emanating from an optical amplifier and adjust tilt accordingly.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to the field of optical communications and in particular to an optical amplifier having fast Raman tilt control.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical communication systems typically use wavelength-division multiplexing to increase transmission capacity. More specifically, a plurality of optical signals each having a different wavelength are multiplexed together into a wavelength division multiplexed (WDM) signal. The WDM signal is transmitted over a transmission line, and then subsequently demultiplexed so that individual optical signals may be individually received.  
           [0003]    An optical amplifier is typically used in such optical communication system to amplify the WDM signal. Since such optical amplifiers have a relatively broad band, the optical amplifier permits each individual optical signal in the WDM signal to be amplified at the same time.  
           [0004]    Generally, an optical amplifier includes an optical amplifying medium, such as an erbium-doped fiber (EDF). The WDM signals travel through the optical amplifying medium. The optical amplifier also includes a light source, such as a laser diode, which provides an optical “pump” to the optical signals traveling through the optical amplifying medium. In a common application, repeating devices, each having an optical amplifier, are interposed into the transmission line to facilitate the transmission of optical signals over great distances.  
           [0005]    Moreover, the gain of an optical amplifier is dependent on the wavelength of the amplified signal. This dependence is defined as the “gain tilt” of the optical amplifier. Therefore, when a WDM signal is amplified by an optical amplifier, each of the individual optical signals multiplexed together may be amplified with a different gain. Accordingly, the gain tilt of an optical amplifier must be considered when using an optical amplifier to amplify WDM signals and a continuing need exists in the art for methods and apparatus which adjust for gain tilt in optical amplifiers.  
         SUMMARY OF THE INVENTION  
         [0006]    I have developed a method and apparatus for controlling gain tilt in an optical amplifier. Unlike prior art methods which provide adjustment based on signals leaving the optical amplifier, my inventive method and apparatus adjusts the tilt based on the optical signals entering the optical amplifier.  
           [0007]    Viewed from a first aspect, my invention is directed to a method of tilt control which is based on a “feed-forward”, predictive methodology that monitors the total optical signal power entering an optical amplifier as opposed to prior art, spectral monitoring techniques that monitor optical signals emanating from an optical amplifier and adjust tilt accordingly.  
           [0008]    Viewed from another aspect, my invention is directed to an optical amplifier apparatus that adjusts the gain tilt based upon the optical signals entering the optical amplifier.  
           [0009]    Additional objects and advantages of my invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]    [0010]FIG. 1 is a schematic representation of an optical amplifier constructed according to the teachings of the present invention;  
         [0011]    [0011]FIG. 2 is a schematic representation of an alternative embodiment of an optical amplifier constructed according to the teachings of the present invention;  
         [0012]    [0012]FIG. 3 is a further schematic representation of an optical amplifier according to the teachings of the present invention and;  
         [0013]    [0013]FIG. 4 is a schematic representation of variations to the optical amplifier according to the teachings of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    With reference now to FIG. 1, there is shown in schematic form a two stage optical amplifier  100 , which exhibits my inventive teachings. More specifically, optical amplifier  100  includes two stages, stage- 1  (shown in the FIG. 1 as  110 , and stage- 2  (shown in the FIG. 1 as  120 ).  
         [0015]    Each of the two individual stages  110  and  120  includes a pump and a monitor diode. In particular, stage- 1   110  includes pump  112  for pumping stage- 1  and monitor diode- 1   114  for monitoring the input of stage- 1 , while stage- 2   120  includes pump  122  for pumping stage- 2  and monitor diode- 2   124  for monitoring the output of stage- 2 . Shown further in FIG. 1, the two stages  110  and  120  are optically connected by variable optical attenuator (VOA)  130 , interposed between stage- 1   110  and stage- 2   120 .  
         [0016]    As can be appreciated by those skilled in the art, the overall gain of optical amplifier  100  is the ratio between the output and the input power or the difference in optical power as measured by monitor diode- 1   114  and monitor diode- 2   124  where the gain is expressed in dB and the power measured in dBm. Stated precisely,  
         GAIN= MPD 2 −MPD 1  
         [0017]    Further, the gain of the amplifier is produced by doped-fiber, for example, erbium doped fiber. The net gain of such an optical amplifier is the difference between the gain produced by the erbium doped fiber and loss introduced from components such as isolators, couplers, VOA&#39;s or gain flattening filters.  
         [0018]    In my inventive method and apparatus, the tilt of the optical amplifier  100  is adjusted by adjusting the VOA  130 . In this inventive manner, virtually any adjustment to tilt is possible.  
         [0019]    Further, it is possible with my inventive apparatus and method to provide an optical amplifier with tilt control over a broad range of response times. While sub-millisecond adjustments are oftentimes desirable to prevent loss of signal in a WDM transmission system, longer times may be suitable for different applications.  
         [0020]    Still further, several different implementations of my inventive optical amplifier are possible. More specifically, and depending upon amplifier construction including erbium doped fiber type(s) used, the number of VOA&#39;s and their positioning depends upon the specific application environment.  
         [0021]    With reference now to FIG. 2, there is shown an alternative optical amplifier  200 , having multiple VOA&#39;s ( 230 - 1 ,  230 - 2 )  
         [0022]    With continued reference now to FIG. 2, a two stage optical amplifier  200  includes two stages, stage- 1  (shown in the FIG. 2 as  210 , and stage- 2  (shown in the FIG. 2 as  220 ).  
         [0023]    Each of the two individual stages  210  and  220  includes a pump and a monitor diode. In particular, stage- 1   210  includes pump  212  for pumping stage- 1  and monitor diode- 1   214  for monitoring the input of stage- 1 , while stage- 2   220  includes pump  222  for pumping stage- 2  and monitor diode- 2   224  for monitoring the output of stage- 2 . Shown further in FIG. 2, the two stages  210  and  220  are optically connected by multiple variable optical attenuators (VOA)  230 - 1 ,  230 - 2  . . .  230 -N, interposed between stage- 1   210  and stage- 2   220 .  
         [0024]    As was shown prior, for mid-stage access amplifiers, a single VOA may be used to adjust for the gain, the mid stage loss padding and the tilt control. If such an amplifier needs to provide a large gain range, then multiple VOA&#39;s, such as that shown in FIG. 2 may advantageously be used.  
         [0025]    Turning our attention now to FIG. 3, there is shown schematic form a two stage optical amplifier  300 , which exhibits my inventive teachings. More specifically, optical amplifier  300  includes two stages, stage- 1  (shown in the FIG. 3 as  310 , and stage- 2  (shown in the FIG. 1 as  320 ).  
         [0026]    Each of the two individual stages  310  and  320  includes a pump and a monitor diode. In particular, stage- 1   310  includes pump- 1   312  for pumping stage- 1  and monitor diode- 1   314  for monitoring the input of stage- 1 , while stage- 2   320  includes pump- 2   322  for pumping stage- 2  and monitor diode- 2   324  for monitoring the output of stage- 2 . As was shown in a similar manner during our discussion of the optical amplifier  100  of FIG. 1, the two stages  310  and  320  of optical amplifier  300  depicted in this FIG. 3 are optically connected by variable optical attenuator (VOA)  330 , interposed between stage- 1   310  and stage- 2   320 .  
         [0027]    Shown further in FIG. 3, output monitor (OMON)  350  and amplifier control unit  360 . Output monitor (OMON)  350  monitors the overall output of the amplifier  300  and provides feedback input to amplifier control unit  360 , which in turn, controls the pumps  312 ,  322 , and VOA  330 .  
         [0028]    At this point, one can appreciate the distinctions between our inventive feed-forward, fast tilt control optical amplifier and a slow tilt control optical amplifier.  
         [0029]    In particular, in a slow tilt control optical amplifier, the spectral response would be measured at the overall output of the amplifier  300 , and the VOA  330  would be adjusted by the action of amplifier control unit  360  to achieve a target gain tilt.  
         [0030]    In sharp contrast, and according to my inventive teachings of fast tilt control, the VOA is adjusted dynamically based on input power. Stated more precisely:  
         Δ VOA f (input power, gain).  
         [0031]    In particular, where “Att” is the attenuation or loss,  
           VOA  value= Att (input power,  FA  gain)+ Att ( OMON )+ Att ( FA  gain);  
         [0032]    Where  
         [0033]    i. Att(FA gain) is the attenuation needed to achieve substantially flat gain—which may be an attenuation value determined during manufacturing calibration of the fiber amplifier, and in particular an erbium doped fiber amplifier;  
         [0034]    ii. Att(OMON) is an attenuation correction based on a target tilt and OMON value; and  
         [0035]    iii. Att(input power, FA gain) is an attenuation value based on monitor diode- 1   314  and fiber amplifier gain setting. This function is dependant of the transmission fiber used, signal band used (C-band, L-band, extended L-band, etc.) as well as other parameters from the system (channel spacing, channel loading scheme.)  
         [0036]    In this inventive manner, virtually any adjustment to tilt is possible. More particularly, the attenuation of the input power and fiber amplifier gain advantageously may be continuously adjusted or adjusted when input power change reaches a threshold.  
         [0037]    Additionally, it should be apparent to those skilled in the art that any of a variety of devices may be used in place of the VOA&#39;s shown herein. In particular, VOA&#39;s used as tilt devices may advantageously be replaced by any device having an adjustable wavelength loss function.  
         [0038]    In particular, devices that exhibit loss functions for a given voltage V1, wherein a short wavelength is more attenuated than a long wavelength, would exhibit a different loss at a different voltage, V2.  
         [0039]    Additionally, other devices which would prove suitable replacements for the VOA&#39;s shown and described herein include: Micro-Electro-Mechanical Systems (MEMS) devices, electromagnetic VOA&#39;s, and liquid crystal devices—as well as other devices exhibiting a controllable loss.  
         [0040]    Clearly, such devices could exhibit rapid response times, in the micro-second range. Longer responding times, such as millisecond response times, may be satisfactory for particular applications.  
         [0041]    Finally, and with reference now to FIG. 4, there is shown in schematic form various different configurations of optical amplifiers constructed according to my present teachings, both without a mid-stage erbium-doped fiber amplifier (EDFA) (FIG. 4 a - c ) and with a mid-stage EDFA.  
         [0042]    More specifically, in FIG. 4 a , there is shown a configuration including pumps  411 ,  413  in communication with VOA  412  which is interposed between the two pumps, such as the configurations shown previously. An alternative of this arrangement is shown in FIG. 4 d , in which a mid-stage EDFA  433  is interposed between the two pumps  441 ,  444  as well. FIG. 4 b - f  show additional arrangements and even hybrid arrangements including both “fast” tilt control and normal, slow tilt control as depicted in FIG. 4 e  having fast tilt VOA  4512  and slow tilt VOA  452 .  
         [0043]    Of course, it will be understood by those skilled in the art that the foregoing is merely illustrative of the principles of this invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.