Abstract:
A radio frequency stabilization system (e.g., for use with a laser transmitter) in an optical communication system is provided. The present invention relates to maintaining a stable RF level in an optical link despite temperature fluctuations, including a transmitter section, a receiver section, a plurality of feedback loops connected to each of the transmitter section and the receiver section. The present invention further discloses a method of stabilizing an RF level in an optical link which method include providing an optical signal transmitter, an optical signal receiver, and a plurality of feedback loops to the optical transmitter section and the optical receiver section.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to an optical link and more specifically to stabilization of the radio frequency (RF) level of the optical link. A method is also disclosed for utilizing the disclosed system for stabilization of the radio frequency (RF) level of the optical link.  
           [0003]    2. Related Art  
           [0004]    It is well known that the efficiency of a laser drops over temperature. It is also known that there exists a tracking error that prevents an exact track of the output levels of the laser over temperature. Due to a combination of these two factors, it is observable that the radio frequency (RF) level of an optical level changes significantly over temperature. The above issue affects the modulation depth that is perceived by the laser, which results in the same RF power affecting the laser and inducing certain extra noise conditions such as Rayleigh noise, etc., under certain conditions.  
           [0005]    In a typical optical link, such as, inter alia, that found in a cable television (CATV) transmission system having a transmitter section and a receiver section, the RF level in the transmitted signal will vary with four parameters, namely, the laser temperature; the transmission link distance; the RF input changes to the transmitter section; and the overall transmission system gain.  
           [0006]    The RF level at the receiver section will depend mainly on only two parameters, the optical modulation index (OMI) of the laser; and the optical power at the receiver section. However, to a lesser extent, the RF level also depends on the receiver section sensitivity, which changes from wavelength to wavelength; and the matching gain, which is an RF design issue.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore a feature of the present invention to overcome the above shortcomings related to optical links by providing a novel apparatus and method for maintaining the RF level stability over an optical link regardless of the temperature variation.  
           [0008]    The present invention seeks to reduce the RF level stabilization over temperature utilizing two concepts. First, on the transmitter section end, the invention seeks to maintain the product of the optical modulation depth and the output power constant. On the receiver section end, appropriate circuitry is utilized to pick up and restore any RF level that was not maintained at the transmitter section.  
           [0009]    The invention relies on the realization that the product of the laser OMI and the output power impacts the RF level. Accordingly, on the transmitter section side, a sub-5 Mhz carrier is impressed prior to the laser. This modulation is then simultaneously picked up by the laser rear face photo diode monitor and the receiver photo diode. The invention then maintains the RF level of the optical link constant by a combination of control of both the transmitter section front end and the receiver section front end. The advantage of the above procedure is that it monitors the RF level independent of loading on the transmitter, significantly reduces Rayleigh noise in sparsely loaded return systems, and provides mechanics to control the RF level on both the receiver and transmitter ends which is essential to maintaining the optical link below clipping behavior.  
           [0010]    The present invention uses three feedback loops in the transmitter section and two feedback loops in the receiver section to both change, when necessary, and maintain at a particular level, RF levels related to an optically transmitted signal. The present invention further allows the following six functions may be performed simultaneously:  
           [0011]    1. Preserve or change OMI;  
           [0012]    2. Maintain output power;  
           [0013]    3. Compensate for temperature changes;  
           [0014]    4. Compensate for laser or system tracking errors  
           [0015]    5. Provide gain at the right place, that is, in the transmitter section or the receiver section, as needed; and  
           [0016]    6. Provide for RF input changes.  
           [0017]    Finally, the apparatus and method of the present invention may also be used for any laser system, and in particular for Dynamic Noise-power-ratio (NPR) Boost (DNB) or enhancement as disclosed in related US patent application no.(later), filed July, 2001, and incorporated herein by reference.  
           [0018]    In a first general aspect, the present invention provides an apparatus for maintaining a stable RF level in an optical link, said apparatus comprising: a transmitter section; a receiver section; a plurality of feedback loops operationally connected to said transmitter section; and a plurality of feedback loops operationally connected to said receiver section.  
           [0019]    In a second general aspect, the present invention provides a method of stabilizing an RF level in an optical link, said method comprising: providing an optical signal transmitter section; providing an optical signal receiver section; providing a plurality of feedback loops to said optical signal transmitter section; and providing a plurality of feedback loops to said optical signal receiver section.  
           [0020]    In a third general aspect, the present invention provides an optical transmission system comprising: an optical signal transmitter section; an optical signal receiver section; an RF stabilization system operationally connected to said optical signal transmitter section; and an RF stabilization system operationally connected to said optical signal receiver section.  
           [0021]    The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description.  
         [0023]    [0023]FIG. 1 is a block diagram representation of a current stabilization system according to an embodiment of the related art.  
         [0024]    [0024]FIG. 2 is a block diagram representation of a basic RF level stabilization system according to an embodiment of the present invention.  
         [0025]    [0025]FIG. 3 is a block diagram representation of an enhanced RF level stabilization system according to an embodiment of the present invention.  
         [0026]    [0026]FIG. 4 is a block diagram representation of a full RF level stabilization system according to an embodiment of the present invention.  
         [0027]    [0027]FIG. 5 is a block diagram representation of a functional RF level stabilization system according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0028]    The following is a detailed explanation, with reference to the attached drawings, of the apparatus for RF level stabilization of an optical link over temperature variations, and a method for using the apparatus for RF level stabilization of an optical link over temperature variations, in embodiments of the present invention. It should be noted that the same reference numbers are assigned to components having approximately the same functions and structural features in the following explanation and the attached drawings to preclude the necessity for repeated explanation thereof.  
         [0029]    [0029]FIG. 1 shows a block diagram representation of a current stabilization system  100  for an optical link according to an embodiment of the related art. Current stabilization system  100  includes a transmitter section  102 , an optical link  150 , and a receiver section  105 . Transmitter section  102  includes an RF path comprising RF IN  amplifier circuit  110  and RF IN  attenuator circuit  120 , which are coupled to a laser transmitter  140 . Laser transmitter  140  is powered by supply voltage V cc , and includes a laser device (e.g., a laser diode)  142 , a bias input circuit  141 , and a back facet (BF) monitor circuit  143 . A first transmitter feedback loop  135  provides a bias current feedback signal  130  from BF monitor circuit  143  signal to the bias input circuit  141 . The bias current feedback bias signal  130  derived from the back facet monitor circuit  143  represents the DC current range of the laser device  142 .  
         [0030]    The output signal from the laser device  142  is carried over optical link  150 , and is received by receiver section  105 . Receiver section  105  includes photo diode circuit  160 , which is controlled by optical modulation voltage (OMV) control circuit  161 , and powered by supply voltage V cc . The output of photo diode circuit  160  is coupled to an RF path comprising RF OUT  amplifier circuit  170  and RF OUT  attenuator circuit  180 .  
         [0031]    Optical link  150  may be comprised of, inter alia, erbium doped fiber amplifiers (EDFAS), semiconductor optical amplifiers (SOA&#39;s), and various kinds of optical fiber, such as, inter alia, single mode fiber (SMF) or dispersion compensating fiber (DCF).  
         [0032]    An inherent drawback in the related art current stabilization system  100 , is that this current stabilization system  100  cannot compensate for two well known phenomenon, namely that the efficiency of a laser drops as temperature rises, and that tracking errors exist which prevent an exact tracking of the output level of the laser as the temperature varies.  
         [0033]    Referring now to FIG. 2, a block diagram representation of a basic RF IN  level stabilization system  200  according to an embodiment of the present invention. Basic RF IN  level stabilization system  200  includes a transmitter section  202 , an optical link  150 , and a receiver section  205 . Transmitter section  202  is similar to transmitter section  102  of the current stabilization system  100  of FIG. 1. Receiver section  205  is similar to receiver section  105  of the current stabilization system  100  of FIG. 1, except that receiver section  205  includes an added first receiver feedback loop  266 .  
         [0034]    First receiver feedback loop  266  provides a feedback signal from the optical modulation voltage (OMV) control circuit  161  to the RF OUT  attenuator circuit  180 . The optical modulation voltage (OMV) control circuit  161  produces an feedback signal which is proportional to changes in the optical level of the photo diode  160 . First receiver feedback loop  266  includes a feedback attenuation circuit  265  to scale the feedback signal to a level compatible with RF OUT  attenuator circuit  180 .  
         [0035]    This embodiment (FIG. 2) compensates for changes in the optical power, including the tracking error. However, this embodiment cannot accommodate changes in the optical modulation index caused by changes in laser efficiency.  
         [0036]    [0036]FIG. 3 is a block diagram representation of an enhanced RF level stabilization system according to an embodiment of the present invention. Enhanced RF level stabilization system  300  includes a transmitter section  302 , an optical link  150 , and a receiver section  305 . Transmitter section  302  is similar to transmitter section  102  of the basic RF level stabilization system  200  of FIG. 2, except that transmitter section  302  includes an added second transmitter feedback loop  336 . Receiver section  305  is similar to receiver section  205  of the basic RF level stabilization system  200  of FIG. 2.  
         [0037]    Second transmitter feedback loop  336  provides a feedback attenuation circuit  332  from the back facet monitor  143  to the RF IN  attenuator circuit  120 . The feedback attenuation circuit  332  produces an attenuated feedback signal which is proportional to the bias current from the laser. Second transmitter feedback loop  336  includes a feedback attenuation circuit  332  to scale the feedback signal to a level compatible with RF OUT  attenuator circuit  180 . Feedback attenuation circuit  332  may include a diode, transistor, or other attenuation circuit (e.g., a PIN transistor circuit).  
         [0038]    [0038]FIG. 4 is a block diagram representation of a full RF level stabilization system according to an embodiment of the present invention. Full RF level stabilization system  400  includes a transmitter section  402 , an optical link  150 , and a receiver section  405 . Transmitter section  402  is similar to transmitter section  302  of the enhanced RF level stabilization system  300  of FIG. 3, except that two additions have been made. First, an oscillator circuit  445  has been added to the output of the RF attenuator  120 . Oscillator circuit  445  introduces a signal tone of e.g., 100 kHz or thereabouts. Second, a third transmitter feedback circuit  430  has been added which provides a feedback loop from the back facet monitor  430  to the RF attenuator  120 .  
         [0039]    Third transmitter feedback loop  455  provides a feedback signal from the back facet monitor  143  to the RF IN  attenuator  120 . Third transmitter feedback loop  455  includes a filter oscillator circuit, an RF detector circuit, and a feedback attenuator circuit. The RF in the transmitted signal is sensed by the RF detector circuit of feedback circuit  430  and is then fed to the feedback attenuator circuit which reduces or increases attenuation as needed.  
         [0040]    Receiver section  405  is similar to receiver section  305  of the enhanced RF level stabilization system  300  of FIG. 3, except that receiver section  405  includes an added second receiver feedback circuit  440  which provides a feedback loop from the photo diode  160  to the RF OUT  attenuator  180 . A signal from photo diode circuit  160  is fed to second receiver feedback circuit  440  which includes a filter oscillator circuit, an RF IN  sensor circuit, and a feedback attenuator circuit. The RF in the received signal is sensed by the RF detector circuit of second receiver feedback circuit  440  and is then fed to the feedback attenuator circuit which reduces or increases attenuation as needed.  
         [0041]    [0041]FIG. 5 is a block diagram representation of a functional RF level stabilization system according to an embodiment of the present invention. Functional RF level stabilization system  500  includes a transmitter section  502 , an optical link  150 , and a receiver section  505 . Transmitter section  502  is similar to transmitter section  402  of the full RF level stabilization system  400  of FIG. 4, except that modulation and demodulation circuits have been added to the transmitter section  502  and the receiver section  505 , respectively.  
         [0042]    The modulation circuit  520  coupled to the oscillator circuit  445  of the transmitter section  502  enables the output signal from oscillator circuit  445  to be selectively varied, which in turn will selectively vary the output at node  410  of the RF IN  attenuator circuit  120 . This operation effectively tunes the output of the RF IN  attenuator circuit  120 , and hence the laser device  142 .  
         [0043]    Similarly, the demodulation circuit  540  in receiver section  505  provides demodulation of the feedback circuit  440 .  
         [0044]    Embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.