Abstract:
A method for operating an optical transmitter for transmission of an optical signal over a dispersive fiber optic media to a remote receiver. The method includes the steps of providing a respective bias level of a first RF signal and a second RF signal input to an optical modulator that modulates the optical signal; determining an output level of the optical modulator in response to the provided bias levels and adjusting a bias level of at least one of the first and second RF input signals based upon the determined output level and an expected output level at a configuration set point for the provided respective bias levels.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is related to U.S. patent application Ser. No. 11/258,732 filed Oct. 25, 2005, and assigned to the common assignee. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to optical transmitters for analog RF signals, and in particular to externally modulated solid state lasers. More particularly, the invention relates to the use of a programmed processor coupled to the external modulator of the laser for continuously adjusting the bias of the two RF inputs to minimize composite second order beat (CSO) distortion.  
         [0004]     2. Description of the Related Art  
         [0005]     Modulating the analog intensity of the optical signal from a light-emitting diode (LED) or semiconductor laser with an electrical signal is known in the art for transmitting analog signals such as sound and video signals, on optical fibers. Although such analog techniques have the advantage of significantly smaller bandwidth requirements than digital pulse code modulation, or analog or pulse frequency modulation, amplitude modulation puts stringent requirements on the noise and nonlinearity associated with the optical source.  
         [0006]     For that reason, direct modulation of the laser has been used in connection with 1310 nm lasers where the application is to short transmission links that employ fiber optic links with zero dispersion. For applications in metro and long haul fiber transmission links the low loss of the link requires that externally modulated 1550 nm lasers be used, but such external modulation techniques are more complex mixture of the number and type RF channels, with modulation types ranging from analog to QAM. The present invention is therefore addressed to the problem of providing a software algorithm for simultaneously adjusting the bias of the two RF inputs of the external modulator so that the optical output signal can be used in single mode fiber used in metro and long haul optical networks.  
       SUMMARY OF THE INVENTION  
       [0007]     It is an object of the present invention to provide an improved optical transmission system using externally modulated lasers.  
         [0008]     It is another object of the present invention to provide a method for biasing the laser in a 1550 nm analog optical transmission system utilizing two cascade or series connected modulators.  
         [0009]     It is also another object of the present invention to provide a software algorithm in a microcontroller to independently and simultaneously adjust the bias of an external modulator used in a 1550 nm analog or QAM optical transmission system for broadband RF.  
         [0010]     It is still another object of the present invention to provide an adaptive system for adjusting the DC bias and pilot tones of linear analog optical transmission systems suitable for long haul dispersive optical fiber media.  
         [0011]     It is still another object of the present invention to provide a real time digital signal processor control circuit for controlling the optical characteristics of the optical signal from an externally modulated laser.  
         [0012]     Briefly, and in general terms, the present invention provides an optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising a semiconductor laser for reproducing an optical signal; an external modulator for modulating the optical signal with a broadband analog radio frequency (RF) signal; and bias adjustment means connected to the input of the external modulator for adapting the modulation characteristics of the external modulator to minimize distortion in the received signal at the remote receiver.  
         [0013]     In another aspect, the present invention provides an optical transmitter for generating a modulated optical signal for transmission over a dispersive fiber optic link to a remote receiver having an input for receiving a broadband radio frequency signal input; a semiconductor laser for producing an optical signal to be transmitted over an optical fiber; and an external modulator for modulating the optical signal with the analog signal including first and second RF inputs. A predistortion circuit is coupled to the second RF input for reducing the distortion in the signal present at the receiver end of the fiber optic link.  
         [0014]     In another aspect, the present invention provides an optical signal output from the modulator which causes the received signal at the other end of the transmission system to compensate for the effect of composite second order (CSO) distortion generated in the dispersive optical fiber link, which results in noise in the received signal and unacceptable quality in the demodulated RF signal for standard AM modulated broadcast CATV channels. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a block diagram of a portion of an optical transmitter for generating a modulated optical signal in accordance with an illustrated embodiment of the invention; and  
         [0016]      FIGS. 2A and 2B  are flowcharts of a process for operating the modulator bias controller in the transmitter of  FIG. 1  according to the coarse tuning aspect of the present invention; and  
         [0017]      FIG. 3  is a flowchart of a process for operating the bias controller of  FIG. 1  according to the fine tuning aspect of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Details of the present invention will now be described, including exemplary aspects and embodiments thereof. Referring to the drawing and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic manner. Moreover, the drawings are not intended to depict every feature of actual embodiment nor the relative dimension of the depicted elements, and are not drawn to scale.  
         [0019]     The present invention is directed to a software algorithm optical transmitter for generating a modulated optical signal for transmission over dispersive fiber optic links in which a broadband radio frequency signal input is applied to first and second RF inputs of an external modulator for modulating the output of a semiconductor laser. The transmitter includes a digital signal processor coupled to the output of the modulator for continuously sampling and monitoring the output and independently and simultaneously adjusting the DC bias of the first and second RF inputs to minimize an error signal characteristic of the optical signal, that is characteristic of or an indicator of the noise associated with composite second order (CSO) distortion of the received signal at a remote receiver.  
         [0020]     Turning to  FIG. 1 , there is shown a simplified block diagram of a portion of an optical transmitter in an embodiment of the invention. The transmitter includes a laser assembly (e.g., a DFB laser diode)  12  and an external modulator  14 . The external modulator  14  modulates the CW output of the laser  12  with an information-containing pair of radio frequency signals (RF 1 , RF 2 ), which are applied from a CSO demodulation and bias controller (modulation controller)  18 . A typical transmitter is depicted in U.S. patent application Ser. No. 11/258,732, incorporated herein by reference.  
         [0021]     The modulation controller  18  includes a programmed digital signal processor  93  that adjusts the bias of the RF signal inputs so that the composite triple beat (CTB) and composite second order beat (CSO) performance over a frequency range of 40 to 800 MHz is optimized, which is important for the application of the transmitter as a central office transmitter for the transmission of CATV carriers and QAM signals in the frequency range of up to 870 MHz. The transmitter may typically be used to transmit any combination of up to 112 carriers (e.g. 6 MHz channels) and QAM signals up to 870 MHz.  
         [0022]     Turning now to the optical signal, the laser  12  provides optical signals at a wavelength of either 1545+/−1 nm,  1555 +/−5 nm, or odd ITU channels  21  through  29 , depending upon the application. The optical output of the laser  12  is coupled to the input of the modulator.  
         [0023]     The external modulator  14  in the preferred embodiment consists of two series connected stages, each with a distinct RF input, labeled RF 1  and RF 2  respectively. The output of the second stage of the modulator is coupled to an optical fiber, which is coupled to a pigtail or optical connector which permits the transmission fiber optic link to be connected. A tap  42  is provided on the output to allow the output signal to be sampled. The sampled signal is coupled into a photodetector, which converts the optical signal into an electrical signal for processing. The sampled signal is used to set an operating point of the modulator  14  through output RF 2  for purposes of controlling CSO performance.  
         [0024]     The RF input from the post amplifier of the optical transmitter is applied to a signal splitter  50  which creates two RF channels  51  and  52 . A first pilot tone is applied to the RF channel  51  from the pilot tone line  100 . The signal on the first RF channel  51  is then applied to a CTB electrical predistortion circuit  54 , for the purpose of reducing the CTB distortion at the receiver end of the optical fiber link. The DC level on the first RF channel  51  is controlled by a bias control unit  60 , which sends an analog bias level to bias isolator  55  which couples the bias level to the RF channel  51 , which is then applied to the first RF input, RFI, on the external modulator  14 .  
         [0025]     The signal on the second RF channel  52  is applied to an attenuator  53 , which is controlled from the digital signal processor  94 . A second pilot tone is then applied to the output of the attenuator  53  from the pilot tone line  101 . The combined signal is then applied to a delay line (DL)  56 .  
         [0026]     The DC level on the second RF channel  52  is controlled by a bias control unit  60 , which sends an analog bias level to bias isolator  57  which couples the bias level to the RF channel  52 , which is then applied to the second RF input, RF 2 , on the external modulator.  
         [0027]     The pilot tones to be applied to the modulator are generated by a pilot processor  90 , which produces a digital signal that is applied to a digital to analog converter and filter  91 . The output of the pilot D/A and filter  91  is then applied to a pilot level control unit  92 , which sets the analog level. The pilot signal is then applied to a switch  93 , which is controlled by the bias digital signal processor (DSP)  93 . The switch  93  then switches the pilot tone to either line  100  or line  101 , or both.  
         [0028]     The bias DSP  93  functions to adjust the modulator bias based upon measurements from the output optical signal.  
         [0029]     The digital signal processor  83  is coupled to the output of the modulator for independently adjusting the DC bias of the first and second RF inputs to control a characteristic of the optical signal, such as noise associated with composite second order (CSO) distortion at a remote receiver.  
         [0030]     As shown in  FIG. 1 , the output of the external modulator is tapped and coupled to a photo detector  95  which converts the optical signal into an electric signal. The electric signal is applied to a demodulator  96  along with a pilot clock signal. The demodulated analog RF signal is then applied to an analog to digital converter  97 , which provides a digital representation of the RF signal to the bias DSP  94 . A memory  98  is also associated with the bias DSP  94  for storing data.  
         [0031]     The electrical signal is converted by an analog-to-digital converter into a digital signal, which is applied to a digital signal processor or microcontroller to allow the output to be continuously sampled and adjustments made on a real time basis.  
         [0032]     The output of the digital signal processor is applied to control the DC bias component of the respective RF signals applied to the first and second RF inputs of the modulator RF 1  and RF 2 .  
         [0033]     The applied electrical signals have three components—a DC bias level, a pilot tone, and the applied RF information signal which modulates the laser beam and conveys the data or video signal to the remote receiver. The digital signal processor uses an algorithm, to be subsequently described, to set the appropriate DC bias level as a result of measurements on the optical signal will vary with time and temperatures, so the output signal must be continuously monitored during operation and adjustments made to the DC bias levels.  
         [0034]     Turning to  FIGS. 2A and 2B , there are depicted flowcharts for the process for operating the digital signal processor  93  during the coarse tuning aspect of the present invention.