Abstract:
A virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers. The present invention employs a fixed laser oscillator in combination with a signal controlled microwave oscillator. A single sideband mixer processes the signals output by the laser oscillator and microwave oscillator to generate a single controlled optical frequency signal.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to optical phase locked coherent receivers, and more particularly, to a virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers.  
           [0002]    Conventional optical phase locked coherent receivers have used laser signal controlled oscillators. It has been determined that the FM response of available laser signal controlled oscillators is undesirable and is unsuitable for phase locking of the optical phase locked coherent receiver in which it is used.  
           [0003]    It is therefore an objective of the present invention to provide for a virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention provides for a virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers. The present invention is intended for use in coherent optical transmitter and receiver systems. In a receiver, the signal controlled laser oscillator may be used as a phase locked laser oscillator, a key element of a coherent optical detector. The shortcoming of currently available signal controlled laser oscillators prevent their use in this application since their response to AC control signals are unsuitable for operation of the phase lock loop. The present invention circumvents this problem by using an implementation that is a combination of a signal controlled microwave oscillator and a fixed laser oscillator.  
           [0005]    The present invention has suitable signal controlled (FM) response for use in the optical phase locked coherent receiver, whereas a conventional laser signal controlled oscillator FM response is unsuitable and prevents phase locking.  
           [0006]    An exemplary signal controlled laser oscillator comprises a signal controlled laser oscillator that receives a fixed bias input signal and outputs an optical frequency signal f 0 . A signal controlled microwave oscillator receives a frequency control input signal and outputs a microwave frequency signal, f m . A single sideband mixer processes the signals output by the laser oscillator and microwave oscillator to generate a single controlled optical frequency signal.  
           [0007]    The single sideband mixer may comprise an optical single sideband mixer that includes a zero degree power splitter coupled to an output of the signal controlled laser oscillator, and a ninety degree hybrid coupler coupled to an output of the signal controlled microwave oscillator. First and second optical modulators having first inputs are coupled to first outputs of the power splitter and hybrid coupler, and second inputs are coupled to second outputs of the power splitter and hybrid coupler. An output ninety degree hybrid coupler is coupled to outputs of the first and second optical modulators for outputting upper sideband and lower sideband output signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:  
         [0009]    [0009]FIG. 1 illustrates a first exemplary embodiment of a virtually coherent signal controlled laser oscillator in accordance with the principles of the present invention; and  
         [0010]    [0010]FIG. 2 illustrates a second exemplary embodiment of a virtually coherent signal controlled laser oscillator employing an optical single sideband mixer. 
     
    
     DETAILED DESCRIPTION  
       [0011]    Referring to the drawing figures, FIG. 1 illustrates a first exemplary embodiment of a virtually coherent signal controlled laser oscillator  10  in accordance with the principles of the present invention. The virtually coherent signal controlled laser oscillator  10  comprises a signal controlled laser oscillator  11  that receives a fixed bias input signal. The output of the signal controlled laser oscillator  11  is coupled to a single sideband mixer  12 . A signal controlled microwave oscillator  13  having a frequency control input signal outputs a microwave frequency, f m  that is input to the single sideband mixer  12 . The output of the single sideband mixer  12  is a single controlled optical frequency signal.  
         [0012]    If a carrier frequency to which the signal controlled laser oscillator  11  is to be phase locked is defined as f o , the signal controlled laser oscillator  11  is tuned to a frequency offset from f o  by the microwave frequency, f m  generated by the signal controlled microwave oscillator  13 . The single sideband mixer  12  combines the outputs of the signal controlled microwave oscillator  13  and signal controlled laser oscillator  11  and generates a controlled output signal at f o . Since the signal controlled laser oscillator  11  is not subject to active control of its frequency within the phase locked control loop, its dynamic response does not prevent phase lock to the signal input to the phase locked loop.  
         [0013]    Care must be taken in the selection of fm since several criteria must be met: Where optical filtering is used to suppress the signal controlled laser oscillator output frequency from the single sideband mixer  12 , the offset frequency from the desired f o  must be large enough so that the signal controlled laser oscillator output signal at the single sideband mixer  12  is suppressed to the required degree. Further, the signal displaced by 2xf m  from f o  must also be suppressed. Moreover, the tunability of the signal controlled microwave oscillator  13  must be large enough to compensate all of the optical frequency uncertainties in the transmission system so that the virtually coherent signal controlled laser oscillator  10  functions as required. This includes uncertainties associated with the laser oscillator  11  as well as the signal controlled microwave oscillator  13 .  
         [0014]    The single sideband mixing function provided by the single sideband mixer  12  shown in FIG. 1 may also be implemented by means of an optical single sideband mixer  12   a  having an exemplary configuration as shown in FIG. 2. More particularly, FIG. 2 illustrates an exemplary embodiment of a virtually coherent signal controlled laser oscillator  10   a  employing the optical single sideband mixer  12   a.    
         [0015]    The exemplary embodiment of a virtually coherent signal controlled laser oscillator  10   a  comprises a signal controlled laser oscillator  11  that receives a fixed bias input signal, and a signal controlled microwave oscillator  13  having a frequency control input signal. The signal controlled laser oscillator  11  outputs an optical frequency, f 0 . The signal controlled microwave oscillator  13  outputs a microwave frequency, f 0 +f m .  
         [0016]    Outputs of the signal controlled laser oscillator  11  and signal controlled microwave oscillator  13  are input to the optical single sideband mixer  12   a . The radio frequency single sideband mixer  12   a  comprises a zero (0) degree power splitter  14 , a ninety (90) degree hybrid coupler  15 , two optical modulators  16   a ,  16   b , and an output ninety (90) degree hybrid coupler  17 .  
         [0017]    The zero (0) degree power splitter  14  receives the output from the signal controlled laser oscillator  11 , and the ninety (90) degree hybrid coupler  15  receives the output from the signal controlled microwave oscillator  13 . Respective first outputs of the zero (0) degree power splitter  14  and ninety (90) degree hybrid coupler  15  are input to first inputs of two optical modulators  16   a ,  16   b . Respective second outputs of the zero (0) degree power splitter  14  and ninety (90) degree hybrid coupler  15  are input to second inputs of the two optical modulators  16   a ,  16   b.    
         [0018]    Respective outputs of the two optical modulators  16   a ,  16   b  are coupled to inputs of the output ninety (90) degree hybrid coupler  17 . The output ninety (90) degree hybrid coupler  17  outputs upper sideband and lower sideband output signals. The upper sideband output signal is at frequency f 0 +2xf m  and the lower sideband output signal is at frequency f 0 .  
         [0019]    The hybrid optical-microwave single sideband mixer  11   a  illustrated in FIG. 2 is new art in the context of the present invention along with that of the virtually coherent signal controlled laser oscillator  10 . Either desired sideband may be derived from this mixer  11  a by selection of the appropriate signal phasing at the optical mixer inputs or outputs.  
         [0020]    Thus, improved virtually coherent signal controlled laser oscillators have been disclosed. It is to be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.