Abstract:
A method of determining the gain characteristic of a Raman amplifier includes the steps of launching a pump light signal into a fiber; preliminary adjusting the power of the pump light signal to a value that lies in a range where the amplified spontaneous emission noise originating from the pump light signal is substantially proportional to the on/off gain provided by the power of the pump light signal; monitoring the power of the amplified spontaneous emission noise signal; varying the power of the pump light signal; measuring a variation in the power of the amplified spontaneous emission noise signal corresponding to the pump power variation; and determining the gain characteristic of the amplifier from the relative variation in pump power and the measured variation in noise power.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to the field of optical fiber communication systems and more particularly to a method and apparatus for determining gain characteristic of a distributed Raman amplifier. The invention is based on a priority application EP 03 292 825.1 which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     In optical fiber communication systems, optical signals propagating along an optical fiber undergo signal attenuation due to absorption and scattering in optical fibers. Therefore, optical signals require periodic amplification over long distances, which can be performed either by electrical repeaters or by optical amplifiers. Known optical amplifier types include Erbium-doped fiber amplifiers (EDFAs), semiconductor optical amplifiers and Raman amplifiers. Due to its flat gain over a wide signal wavelength band, the Raman amplifier has gained increasing attention in the recent past as ideal amplifier candidate for wavelength division multiplex (WDM) signal transmission.  
         [0003]     The Raman amplification process is based on the Raman effect, which is a non-linear optical process that occurs only at high optical intensities and involves coupling of light propagating through the non-linear medium to vibrational modes of the medium, and re-radiation of the light at a different wavelength. Re-radiated light upshifted in wavelength is commonly referred to as a Stokes line, whereas light downshifted in wavelength is referred to as an Anti-Stokes line. The Raman effect is described by quantum mechanics as scattering of photons at molecules which thereby undergo a transition of their vibrational state. Raman amplification involves stimulated Raman scattering, where the incident beam, having a higher optical frequency, often referred to as the pump beam, is used to amplify the lower frequency beam often referred to as the Stokes beam or the signal beam through the Raman effect.  
         [0004]     In contrast to EDFAs, where the amplification properties are dependent only on the EDFA module, the transmission line itself is used as the gain medium of a distributed Raman amplifier and thus, amplification properties such as gain and gain equalization are closely related to the type, properties and characteristics of the fiber used and the fiber condition. In a silica fiber for example, the strongest Raman scattering, i.e. the maximum Raman gain occurs at a frequency shift of about 13.2 THz, which corresponds to a wavelength shift of about 50-100 nm for pump wavelengths between about 1 and 1.5 μm. It is impossible to accurately predict the performance of a Raman amplifier, including gain, gain equalization and noise spectrum, without thorough knowledge of the fiber types, properties characteristics, and condition along the fiber optic transmission line.  
         [0005]     Distributed Raman amplification is typically characterized by the on-off gain, i.e., the ratio of the signal power measured at the output end of the fiber when the pumps are on relative to the signal power when the pumps are off. In order to operate a particular Raman amplifier, the gain characteristic of the corresponding fiber link must be known to allow proper adjustment of the pump power. It would be possible to do a preliminary calibration of the Raman efficiency of various fiber types versus the pump power in the laboratory and to use these values for actual installations in the field. But in fact, the Raman gain strongly depends on the loss of the fiber at the pump wavelength and the actual attenuation characteristic of an installed fiber link is not known in advance but must be determined in the field. In addition, local losses due to for example fiber splices cannot be predicted accurately and the installed fiber may not have exactly the same Raman efficiency as the fiber of that type that had been preliminary calibrated in the laboratory. Methods that trust in nominal fiber characteristics only, can therefore achieve an accuracy not better than about ±20%. It is therefore necessary to determine the Raman characteristics of an actual transmission line in the field. A measurement of the Raman on/off gain in the field is, however, difficult because it may on the one hand not be possible at system installation to dispose a signal source for the measurement and on the one hand, the two fiber ends where the measurement has to be performed are typically 50 to 100 km apart.  
         [0006]     It is thus an object of the present invention to provide a method and related apparatus for determining the gain characteristic of a distributed Raman amplifier at system installation, which can be performed from one side of an already installed fiber link.  
       SUMMARY OF THE INVENTION  
       [0007]     These and other objects that appear below are achieved by launching a pump power into an installed fiber link and measuring backward amplified spontaneous emission (ASE) noise. The pump power is adjusted to a regime where the power of the ASE noise is substantially proportional to the on/off gain delivered by the pump. The pump power is then varied and the on/off gain deduced from an observed variation in the ASE power.  
         [0008]     In particular, the method includes the steps of launching a pump light signal into the fiber; preliminary adjusting the power of the pump light signal to a value that lies in a range where the amplified spontaneous emission noise (linear scale)originating from the pump light signal is substantially proportional to the on/off gain (linear scale) delivered by the pump; monitoring the power of the amplified spontaneous noise signal; varying the power of the pump light signal; measuring a variation in the power of the amplified spontaneous noise signal corresponding to the pump power variation; and determining the gain characteristic of the amplifier from the relative variation in pump power and the measured variation in noise power.  
         [0009]     The invention thus provides a very accurate and simple method to determine the Raman gain characteristics in the field without requiring special signal sources and other dedicated measurement equipment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which  
         [0011]      FIG. 1  shows a Roman amplifier and  
         [0012]      FIGS. 2   a - 2   d  show in four diagrams the ASE noise figure as a function of the Raman gain for different fiber types. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0013]     A Roman amplifier which is to be characterized by the measurement according to the invention is shown in  FIG. 1 . The Raman amplifier RA uses as gain medium a preceding fiber link F, the Raman gain characteristics of which need to be determined. The Raman amplifier RA as such contains a pump source LD and a Raman multiplexer RM which couples the pump light from the pump source LD in backwards direction into the fiber link F. The pump light causes stimulated Raman scattering in the fiber span length S when a signal traverses the link and thus amplifies the signal. A tap coupler TC is provided at the output of the Raman amplifier which serves to extract a small fraction of the amplified signal light from the fiber and feed it via a band-pass filter BP to a photodiode PD which determines an output power value of the signal.  
         [0014]     In operation, the pump light power must be adjusted to the required Roman gain. Such adjustment, however, requires thorough knowledge of a parameter characteristic of the present Raman gain such as the on-off Raman gain. It would however be equally possible to describe the Raman gain by any other suitable parameter.  
         [0015]     The basic principle of the present invention is to make use of amplified spontaneous emission (ASE) that occurs in reverse direction as compared to the pump light. ASE occurs in the fiber in presence of the pump signal even without a data signal and is due to Raman spontaneous scattering as opposed to Raman stimulated scattering that occurs only when a signal traverses through the fiber. ASE thus appears at the wavelength of the Stokes line. In presence of a data signal, ASE is an optical noise that perturbs the data detection. The aim of the invention is to predict the pump power that will be necessary to perform the gain planned on future data signals with the specific fiber and local losses that are encountered in the particular installation. The invention thus provides a method for characterization of the Roman gain efficiency in the absence of a data signal since it may not be possible to light on a data signals at the time when the amplifier is installed.  
         [0016]     The graphs in  FIGS. 2   a  to  2   d  show the noise figure as a function of the Raman on/off gain and thus as a function of the pump power. The four graphs show curves for four different optical fibers having different attenuation and Raman efficiency and for different fiber lengths. In particular,  FIG. 2   a  shows a graph for the following parameters: 
        signal attenuation=0.21 dB/km,     pump attenuation=0.25 dB/km, and     Raman efficiency=0.4 W −1 km −1 .        
 
         [0020]     The graph is  FIG. 2   b  is taken for a fiber with the parameters: 
        signal attenuation=0.20 dB/km,     pump attenuation=0.35 dB/km, and     Raman efficiency=0.4 W −1 km −1 .        
 
         [0024]      FIG. 2   c  shows a graph of a fiber with the parameters: 
        signal attenuation=0.25 dB/km,     pump attenuation=0.35 dB/km, and     Raman efficiency=0.4 W −1 km −1 .          
         [0028]     And finally,  FIG. 2   d  shows a graph for a fiber with these parameters: 
        signal attenuation=0.25 dB/km,     pump attenuation=0.35 dB/km, and     Raman efficiency=0.7 W −1 km −1 .        
 
         [0032]     We found that the noise figure is almost constant with the pump power In a range of the on/off gain of about 9.5 dB whatever the Raman efficiency of the fiber, the length of the link fiber (from 60 to 120 km), the attenuation at signal wavelength and the attenuation at pump wavelength is NF is in this context the noise figure due to ASE, only. It is defines in this context as: 
 
 NF   dB   =P   AsEdBm   −G   dB −10*LOG( hνΔν ), 
 
 where P ASEdBm  is the measured ASE power, G dB  is the on/off Raman gain expressed in dB and LOG(hνΔν) is a constant term with respect to pump power. 
 
         [0033]     In a small signal regime i.e. when Raman pump depletion is negligible, which is the case for ASE, the following relation for the Raman gain G dB  holds true:  
             Δ   ⁢           ⁢     G     d   ⁢           ⁢   B           G     d   ⁢           ⁢   B         =       Δ   ⁢           ⁢   P     P       ,       
 
 where P is the pump power in mW, ΔP is a variation of (i.e., a difference in) the pump power and ΔG dB  is the corresponding variation of the Raman gain. 
 
         [0034]     In the range where the noise figure is substantially constant with the pump power, any variation of the Raman gain G dB  with the pump power P must be directly seen as a variation of the ASE power P ASEdBm , i.e. 
 
ΔG dB =ΔP AEdBm , 
 
         [0035]     In this range, we can therefore write:  
         Δ   ⁢           ⁢     P     ASE   ⁢           ⁢   dbm         =       G     d   ⁢           ⁢   B       ⁢         Δ   ⁢           ⁢   P     P     .           
 
         [0036]     The invention makes use of this range where the noise figure is constant with the Raman gain and where the ASE noise power (linear scale) generated by the pump is therefore proportional to the on/off Raman gain (linear scale) delivered by this pump. According to the invention, a pump light signal is launched into the fiber at a pump power P p  that is in the range for which the noise figure is constant with P p . Then a variation ΔP p  of P p  is applied. In the preferred embodiment, we apply a variation in the order of 10% to 20%. The ASE power is then measured at the Stokes line, i.e., about 13.2 THz from the pump wavelength, which is the maximum ASE power. We use the band-pass filter BP, which has a filter width of a few nanometers. We thus obtain ΔP ASEdBm . Knowing ΔP p /P p  and ΔP ASEdBm , we can determine G dB  at the given pump power P p  from the relation given above.  
         [0037]     This Raman gain G dB  that corresponds to a particular pump power P p  can simply be extrapolated to any other G dB  produced by a corresponding pump power P p  (simple proportionality provided that there is no pump depletion taking place due to high power signals traversing the fiber). The gain characteristics of the fiber is thus found.  
         [0038]     It has to be noted that the invention characterizes the Raman gain efficiency of a fiber link at a certain pump power that corresponds to about 9.5 dB on/off gain. Later in operation of the system, other values for the pump power will be applied to amplify signals but making use of the proportionality measured earlier.  
         [0039]     Coming back to  FIGS. 2   a  to  2   d , we found that we must operate for our measurement at a pump power P p  that results in a Raman gain G dB  of approximately 9.5 dB. For this first step, i.e. to find G dB  of approximately 9.5 dB (±20%), the data sheets from a preliminary calibration in a laboratory for the actual fiber type are used. As explained before, such preliminary fiber calibration will just provide the required accuracy of ±20%. Then we apply the method described above. As an alternative, the range where the ASE is proportional to the on/off gain can be determined by scanning the NF curve as in  FIGS. 2   a - d.    
         [0040]     The Raman amplifier as shown in  FIG. 1  has a controller CTR which tunes the power of the Raman pump LD to the required value and measures the ASE power detected by photodiode PD. In an initialization procedure, the amplifier performs the measurements described above, i.e., adjusting the pump power to the range where the noise figure is constant with the Raman gain, varying the pump power and measuring the corresponding variation in the ASE power. The controller then stores the gain characteristic derived from this initialization and uses it during subsequent “normal” operation. The controller thus has a storage for storing the gain value derived from the initialization. While the initialization can in principle be executed manually, it is preferable that the controller is programmed to perform the initialization automatically.  
         [0041]     While the invention has been explained in a preferred embodiment, it should be clear to those skilled in the art that the invention is not restricted to the particular details and figures given there. Conversely, those skilled in the art will appreciate that various modifications and substitutions would be possible without departing from the concepts of the invention. For example, those skilled in the art would know, that the Ramon pump can have more than one loser diodes or other pump light sources operating at different wavelengths to obtain a smooth and broad gain curve. The invention has been explained as an initialization procedure that is performed upon system installation. Although this is indeed an important application of the invention, it is however not restricted thereto. Conversely, the method according to the invention can be equally applied at any later stage during the lifetime of the system. We have shown in an embodiment with counter-propagating pump signal. The invention can, however, equally be applied with co-propagating pump signal.