Abstract:
For providing an optical measuring signal to an optical component to be measured, the spectral density of the optical signal is broadened until relevant non-linear effects in the optical component occur, at most, by combining a plurality of initial optical signals to create the optical signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to providing an optical measuring signal to an optical component ( 10 ) to be measured.  
         [0002]     When performing optical measurement methods or using optical measurement equipment, e.g. OTDRs, high power probing signals are desirable since the response signals from a device under test are proportional to the level of the stimulus signal. However, if the device under test is for example a fiber, then it is possible that non-linear effects in the fiber limit the maximum power level of the optical probing signals depending on fiber and signal properties. Such adverse effects of high power levels of the optical probing signal can be 4-wave mixing, cross-modulation, Raman scattering, or Brillouin scattering.  
         [0003]     A state-of-the-art light source used in optical test equipment is for example a semi-conductor laser diode that exhibits a narrow optical spectrum. The demand for higher optical power can&#39;t be simply satisfied with a stronger laser diode because such a device is most likely not available if one is already working with high powered devices and because non-linear effects in fibers start to arise.  
         [0004]     UK-A-2359684 discloses a reduction of stimulated Brillouin backscattering in optical transmission systems by broadening the frequency spectrum of transmitted signals utilizing the non-linear effects of self phase modulation or cross-phase modulation to counteract the Brillouin scattering. Modifying the spectral characteristics of optical signals is further known e.g. from EP-A-767395.  
       SUMMARY OF THE INVENTION  
       [0005]     Therefore, it is an object of the invention to provide improved intensification of an optical signal power. The object is solved by the independent claims. Other preferred embodiments are shown by the dependent claims.  
         [0006]     In the context of the present invention the term relevant non-linear effects can be defined, e.g. as the loss of the optical power to frequencies newly generated by the non-linear effects.  
         [0007]     An advantage of embodiments of the present invention is the possibility to use high-power probing signals having a higher maximum power level without showing non-linear effects, compared to the high-power probing signals known from the prior art. This possibility is enabled by the present invention since the invention increases the maximum power level of an optical probing signal by broadening the spectral density of the signal. The amount of the broadening, i.e. the spectral distribution and spectral width of the probing signal that can be tolerated depends on the type of measurement the probing signals are used for.  
         [0008]     In a preferred embodiment of the invention the broadening of the spectral density of the optical signal is performed by using at least two initial optical signals to create the optical signal, the initial optical signals having different center wavelengths. This embodiment implements the invention in an easy way. In the respective apparatus of the invention it is preferred to combine two or more laser diodes with a preferably low-loss combiner to produce a high-power output signal with a spectral distribution that can be preferably set by proper selection of the laser diodes. The individual laser diodes have preferably approximately the same optical power. More preferably, the spacing of the center wavelengths of each laser diode is not equal between at least two of the center wavelengths.  
         [0009]     In order to enhance the effect of the invention, it is preferred to use five to ten laser diodes within a total spectral width of approximately 5 to 20 nanometers. This can preferably be done by using an N-port combiner which preferably shows coupling efficiencies C that are greater than 1/N and are preferably as close as possible to 1. When using such a combiner the total output power increases considerably and the total output P tot  can reach P tot =N×P 0 ×C.  
         [0010]     It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Other objects and many of the attendant advantages of the present invention will be readily appreciated and become better understood by reference to the following detailed description when considering in connection with the accompanied drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Features that are substantially or functionally equal or similar will be referred to with the same reference sign(s).  
         [0012]      FIG. 1   a  shows an example of a laser diode of the prior art together with a optical fiber connected to the laser diode;  
         [0013]      FIG. 1   b  shows an optical spectrum emitted by the laser diode of  FIG. 1   a;    
         [0014]      FIG. 2   a  laser diodes combined according to an embodiment of the present invention; and  
         [0015]      FIG. 2   b  shows a combined spectrum generated by the laser diodes of  FIG. 2   a.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Referring now in greater detail to the drawings,  FIG. 2   a  shows a schematic illustration of an embodiment 1 of an apparatus according to the present invention. Embodiment 1 comprises as laser sources four laser diodes  2   a ,  2   b ,  2   c  and  2   d . The laser diodes  2   a ,  2   b ,  2   c , 2   d  emit initial optical signals  4   a ,  4   b ,  4   c ,  4   d , respectively, into four optical fibers  6   a ,  6   b ,  6   c ,  6   d , respectively. All laser diodes  2   a ,  2   b ,  2   c ,  2   d  emit approximately the same optical power.  
         [0017]     The four initial optical signals  4   a ,  4   b ,  4   c ,  4   d  in the optical fiber  6   a ,  6   b ,  6   c ,  6   d  are combined with the help of a low-loss combiner  8  to an optical signal  10 .  
         [0018]     Combining the four laser diodes  2   a ,  2   b ,  2   c ,  2   d  with the combiner  8  produces the optical signal  10  with a high output power and with a spectral distribution that can be set by selection of the center wavelength of the initial optical signals  4   a ,  4   b ,  4   c ,  4   d  of the laser diodes  2   a ,  2   b ,  2   c ,  2   d.    
         [0019]     As shown in  FIG. 2   b  all initial optical signals  4   a ,  4   b ,  4   c ,  4   d  have approximately the same initial optical power P ini . However, the spacing  12   a  between the center wavelength λ of the initial optical signals  4   a  and  4   b  preferably is not the same as the spacing  12   b  between the center wavelength λ of the initial optical signals  4   c  and  4   d  and is different, e.g. bigger than the spacing  14  between the center wavelength λ of the initial optical signals  4   b  and  4   c.    
         [0020]     The 4-port combiner  8  shows a coupling efficiency C that is greater than ¼ and is close to 1. The total output power P tot  can be calculated as follows: P tot =4×P 0 ×C, P 0  being the power of a single laser diode  2   a ,  2   b ,  2   c ,  2   d , assuming all diodes  2   a ,  2   b ,  2   c ,  2   d  emit the same optical power. With a proper selection of the wavelength λ of the individual laser diode  2   a ,  2   b ,  2   c ,  2   d , the resulting spectrum according to  FIG. 2   b  can minimize non-linear effects in the optical fiber  10  yielding a much higher response signal in an optical measurement, e.g. an OTDR measurement, and thus a gain in a signal to noise ratio, in measurement speed, or in measurement accuracy etc. In embodiment 1 the added spacings  12   a ,  14  and  12   b  between the center wavelength of initial optical signal  4   a  and initial optical signal  4   d  amount to approximately 5 nm. In embodiment 1 the center wavelengths of the initial optical signals  4   a ,  4   b ,  4   c ,  4   d  have been chosen to be 1310 nm, 1312 nm, 1313 nm, 1315 nm, respectively.