Abstract:
The invention comprises a multilevel optical signal system comprising two or more light source branches and an optical power-combiner, wherein each branch comprising a light source, an optical modulator and an electrical driver for the modulator, wherein each electrical driver is configured for being driven by electrical signals to drive the modulator to modulate the light generated by the light source into a corresponding 2-level data signalsuch that the respective 2-level data signals differs in power level.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a multilevel optical signal system and in particular a multilevel optical signal system suitable for converting electrical signal to optical signal. 
       BACKGROUND ART 
       [0002]    The need for fast transmission of signals in optical fibers is general increasing. Whereas today&#39;s optical interconnects are based on optical NRZ (non return to zero) modulation formats with two optical power levels, future interconnects might be based on other modulation schemes, multi-level optical links being one proposal, and several methods of converting electrical signals to multilevel signals have been suggested. 
         [0003]    U.S. Pat. No. 8,380,085 (NEC LABORATORIES AMERICA INC) describes a method of processing data that includes receiving a plurality of binary electronic signals and generating an optical signal by a number of lasers that is equal to or greater than the number of binary electronic signals. The optical signal is generated at one of a plurality of intensity levels, and each intensity level represents a particular combination of bit values for the plurality of binary electronic signals. The optical signal is converted into an electronic signal having the plurality of intensity levels. An apparatus for processing data is provided that includes a plurality of lasers configured to emit light at a plurality of frequencies, and a plurality of modulators configured to receive a plurality of binary electronic signals and to modulate the light 
       DESCRIPTION OF THE INVENTION 
       [0004]    Whereas the method disclosed in U.S. Pat. No. 8,380,085 may increase the transmission rate in optical fiber, there is still a need for an effective multilevel optical signal where the resulting optical signal can be decoded with a relatively low bit error rate. 
         [0005]    In an embodiment of the invention it is an object to provide a fast multilevel optical signal system suitable for converting electrical signal to optical signal and where the multilevel optical signal can be converted to electrical signals with a low bit error rate. 
         [0006]    These and other objects have been solved by the invention or embodiments thereof as defined in the claims and as described herein below. 
         [0007]    It has been found that the invention or embodiments thereof have a number of additional advantages which will be clear to the skilled person from the following description. 
         [0008]    The term “substantially” should herein be taken to mean that ordinary product variances and tolerances are comprised. 
         [0009]    All features of the inventions and embodiments of the invention as described above including ranges and preferred ranges can be combined in various ways within the scope of the invention, unless there are specific reasons not to combine such features. 
         [0010]    The inventors of the present invention has found that by providing the multilevel optical signal system such that the data signals are in phase and are synchronized in frequency to have fully timely overlying bit rate a very effective multilevel optical signal system is provided which can be reconverted to electrical signals in a relatively simple and effective way. 
         [0011]    In an embodiment of the invention modulated light from a number of branches are combined in a passive, optical power-combiner into one optical signal. Each branch comprises a CW (continuous wave) light source, L j , an optical modulator and an electrical driver for the modulator. Each electrical driver takes the two-level data signal, D j , and all data signals are synchronous in phase and frequency with a bit rate of B. The combined output will have the same baud-rate as the data signals bit rate, B, now carrying all N signals, with 2 n different symbols and a total bit-rate of N*B. 
         [0012]    In an embodiment the optical power level out of each branch is adjusted to different levels, or weights. A preferred design is binary stepped weights so that the output of branch n+1 is the double of branch n. For N=2 the suggested 4-level signal generated by branch 1 having a power of P 0 and branch 2 a power of 2P 0 . 
         [0013]    In an embodiment the branches of the multilevel optical signal system are configured to generate the respective 2-level data signals such that the power from a laser source of a first branch is P0 and the power of from a laser source of an N&#39;th branch is 2̂N times P0. 
         [0014]    In the general case, the transmitted power is P out =ΣD j *P j =ΣD j *W j *P 0 ; j=1 . . . N. 
         [0015]    In the binary weighted 4-level transmitter P out =P 0 ΣD j *2 j-1 =P 0 {D 1 2D 2 }, i.e., the levels are 0, P 0 , 2P 0 , 3P 0 . Or by different scaling, 0, ½P 0 , P 0 , 1½P 0 . 
         [0016]    The light sources L 1  . . . L N  in each branch will advantageously generate light having different wavelengths and may also be designed with deliberately different wavelengths. In an embodiment the integrating photo receiver on the receive side will not distinguish the wavelengths, only integrate the power of light. Applying different wavelengths may add to distinguish the signals e.g. for splitting the signals e.g. by optical filters at the receiver. 
         [0017]    In an embodiment the light sources must not correlate in optical phase as interference may occur in the combiner and constructive or destructive interference is undesirable. The combiner advantageously add optical power and it is therefore desired that the coherence length between the sources to be relatively short. There is no reason to believe that separate lasers would not be uncorrelated and have very short and insignificant coherence length. 
         [0018]    In an embodiment it is desired that the lasers are not narrow band, but relatively broad, so stochastically destructive interference will become a relatively small part of the combined power. In other words, the phases of the optical carriers must preferably be misaligned. 
         [0019]    A “dithering” signal may be applied to the forward current to the lasers so that the phase changes over time and/or to some extend decreases the phase alignment between lasers. The frequencies of the dithering signals may be prime factors to avoid common frequencies. 
         [0020]    In an embodiment Fabry-Perot lasers are used and in another narrow line width lasers are used. In both cases, difference in wavelength and the above described dithering signal may be applied. Other continuous wave light sources could also be applied. 
         [0021]    Requirements on the linearity of the electrical drivers are avoided as each drivers will have only two levels. All drivers can be equal and operate equally. Also the light modulators need no strict linearity. This can be achieved with today&#39;s technology. 
         [0022]    The weight of the light of each branch (same as the optical power level of each branch) can be maintained by monitoring the CW output of the laser before the modulator, or even after the modulator if a fixed modulation index is maintained, e.g., 50% modulation over a period of time. 
         [0023]    On the receive side, a multi-level-capable receiver is needed to detect and decode the optical signal. The suggested transmitter does not impose special requirements to the receiver over other multi-level-capable receivers. 
         [0024]    Compared to a 2-level transmitter that can provide optical power up to P 0 , a 4-level binary weighted transmitter of an embodiment of the invention requires 3 dB of the optical link budget, because the power levels of the two branches would have to be ½P 0  and P 0  with available technology. The loss in the optical power combiner is considered negligible. Other multi-level transmitters may impair the link budget even further by imposing complicated linearity requirements to the drivers and optical modulators which this proposal avoids. It means at an unchanged baud-rate, the link bit-rate can be doubled at a cost of 3 dB optical penalty. There may be other inevitable penalties but the solution provided by the present invention is close to ideal. 
     
    
     
         [0025]      FIG. 1  shows a preferred embodiment of the multilevel optical signal system comprising an optical power combiner and a number of branches here represented by branches J and N, where each branch comprises a light source, a modulator and a driver for the modulator. 
       
    
    
       [0026]    In use the electrical signals D j  and D N  are transmitted to the drivers or the respective branches where they are concerted to respective 2-level optical data signals by the light sources L J ,L N  and the modulators of the respective branches. The 2-level optical data signals are transmitted to the optical power combiner where they are combined to a multilevel optical signal P out .