Directly modulated laser optical transmission system with phase modulation

Abstract

An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver including a laser; an input coupled to the laser for directly amplitude modulating the laser with an analog RF signal to produce an output optical signal including an amplitude modulated information-containing component; and a phase modulator coupled to the output of the laser for reducing the distortion present in the received optical signal at the remote receiver.

What is claimed is: 
     
       1. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising:
 a laser; 
 an input coupled to the laser for directly amplitude modulating the laser with an analog RF signal to produce an output optical signal including an amplitude modulated information-containing component and phase noise; 
 a phase modulator coupled to an output of the laser for separately modulating the output optical signal output from the laser in such a manner that distortion present in a received optical signal, received at the remote receiver, is reduced; and 
 a delay circuit having an input coupled to the analog RF signal and an output coupled to the phase modulator to generate a modulating electrical signal synchronized with the output optical signal to cause the phase modulator to reduce the phase noise in the output optical signal. 
 
     
     
       2. A transmitter as defined in  claim 1 , wherein the phase modulator increases a stimulated Brillouin scattering threshold experienced by the received optical signal at the remote receiver. 
     
     
       3. A transmitter as defined in  claim 1 , wherein the laser is a semiconductor external cavity laser. 
     
     
       4. A transmitter as defined in  claim 1  wherein a wavelength of the output optical signal is in the 1530 to 1570 nm range. 
     
     
       5. A transmitter as defined in  claim 1 , wherein the analog RF signal has a bandwidth greater than one octave and includes a plurality of distinct information carrying channels. 
     
     
       6. A transmitter as defined in  claim 1 , further comprising a pre-distortion circuit for modifying the analog RF signal applied to the input coupled to the laser to compensate for a nonlinear response of the laser affecting the received optical signal at the remote receiver. 
     
     
       7. A transmitter as defined in  claim 1 , wherein the modulation of the output of the laser is selectively adjustable to compensate for the distortion present in the received optical signal at the remote receiver. 
     
     
       8. A transmitter as defined in  claim 1 , wherein the delay circuit further includes a low frequency signal generator to increase the stimulated Brillouin scattering threshold experienced by the received optical signal at the remote receiver. 
     
     
       9. A transmitter as defined in  claim 1 , further comprising a tone generator circuit, wherein the delay circuit and the tone generator circuit are coupled to the phase modulator for adjusting a compensation applied by the phase modulator to the output of the laser to cancel the distortion in the received optical signal. 
     
     
       10. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver, the transmitter comprising:
 an input for providing an RF signal; 
 a laser coupled to receive the RF signal on the input and being directly modulated by the RF signal to produce an output optical signal, the output optical signal including an amplitude modulated information-containing component and a phase modulated component; 
 an electro-optical element coupled to an output of the laser for reducing noise in the output optical signal and producing a modified optical signal coupled to the fiber optical link; and 
 a biasing circuit coupled to the input to receive the RF signal and coupled to the electro-optical element to cause the electro-optical element to compensate for distortion in the received optical signal at the remote receiver, 
 wherein the biasing circuit comprises a conditioning circuit comprising:
 an attenuator to adjust an amplitude of the RF signal propagating within the biasing circuit; 
 a tilt circuit to adjust a frequency-dependent gain applied to the RF signal propagating within the biasing circuit; and 
 a phase delay circuit. 
 
 
     
     
       11. The transmitter of  claim 10 , wherein the distortion in the received optical signal results from the phase modulated component from chirp produced by the laser. 
     
     
       12. The transmitter of  claim 10 , wherein the distortion in the received optical signal results from distortion introduced to the output optical signal by the fiber optic link and depends upon a length of the fiber optic link. 
     
     
       13. The transmitter of  claim 12 , wherein the biasing circuit comprises a tone generator coupled to the electro-optical element to compensate for the distortion that would be introduced by the fiber optic link on the output optical signal. 
     
     
       14. The transmitter of  claim 10 , wherein the electro-optical element comprises a phase modulator coupled to receive the output optical signal from the laser and to selectively adjust the output optical signal. 
     
     
       15. The transmitter of  claim 10 , wherein the conditioning circuit is selectively adjustable to compensate for distortion in the received optical signal at the remote receiver. 
     
     
       16. The transmitter of  claim 10 , wherein the electro-optical element is selectively adjustable to compensate for distortion in the received optical signal at the remote receiver. 
     
     
       17. A method of modulating an optical signal for transmission over a fiber optic link to a remote receiver, the method comprising:
 applying an analog RF signal to a laser for directly amplitude modulating the laser to produce an output optical signal including an amplitude modulated information-containing component and a phase modulated component; 
 phase modulating the output optical signal with an electro-optical element in such a manner that distortion present in the receive optical signal, at the remote receiver, is reduced; 
 coupling a phase shifting circuit to an input providing the analog RF signal and in the phase shifting circuit adjusting an amplitude and/or a phase of a received portion of the analog RF signal to generate an adjusted received portion of the analog RF signal; and 
 applying the adjusted received portion of the analog RF signal to the electro-optical element to compensate for distortion in the received optical signal at the remote receiver resulting from the output optical signal propagating through the fiber optic link, 
 wherein adjusting the amplitude and/or the phase of the received portion of the analog RF signal in the phase shifting circuit comprises:
 adjusting the amplitude of the received portion of the analog RF signal with an attenuator; 
 adjusting a frequency-dependent gain applied to the received portion of the analog RF signal with a tilt circuit; and 
 adjusting a phase of the received portion of the analog RF signal with a phase delay circuit. 
 
 
     
     
       18. The method of  claim 17 , further comprising adjusting the amplitude and/or phase of the received portion of the analog RF signal, such that the electro-optical element compensates for distortion by canceling the phase modulated component. 
     
     
       19. The method of  claim 17 , further comprising:
 in the phase shifting circuit, producing a low frequency signal; and 
 combining the low frequency signal with the adjusted received portion of the analog RF signal and applying a combination to the electro-optical element to compensate for the distortion. 
 
     
     
       20. The method of  claim 17 , wherein the electro-optical element is a phase modulator. 
     
     
       21. The method of  claim 17 , wherein compensating for the distortion in the received optical signal at the remote receiver comprises selectively adjusting an amount of compensation applied by the electro-optical element to the output optical signal. 
     
     
       22. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising:
 a laser;   an input coupled to the laser for directly amplitude modulating the laser with an analog RF signal to produce an output optical signal including an amplitude modulated information-containing component and a second component resulting from non-linearity of the laser;   a phase modulator coupled to an output of the laser for separately modulating the output optical signal output from the laser in such a manner that distortion present in a received optical signal, received at the remote receiver, is reduced; and   a delay circuit having an input coupled to the analog RF signal and an output coupled to the phase modulator to generate a modulating electrical signal synchronized with the output optical signal to cause the phase modulator to reduce the second component in the output optical signal.    
     
     
       23. A transmitter as defined in claim 22, wherein the phase modulator increases a stimulated Brillouin scattering threshold experienced by the received optical signal at the remote receiver.  
     
     
       24. A transmitter as defined in claim 22, wherein the laser is a semiconductor external cavity laser.  
     
     
       25. A transmitter as defined in claim 22 wherein a wavelength of the output optical signal is in the 1530 to 1570 nm range.  
     
     
       26. A transmitter as defined in claim 22, wherein the analog RF signal has a bandwidth greater than one octave and includes a plurality of distinct information carrying channels.  
     
     
       27. A transmitter as defined in claim 22, further comprising a pre-distortion circuit for modifying the analog RF signal applied to the input coupled to the laser to compensate for a nonlinear response of the laser affecting the received optical signal at the remote receiver.  
     
     
       28. A transmitter as defined in claim 22, wherein the modulation of the output of the laser is selectively adjustable to compensate for the distortion present in the received optical signal at the remote receiver.  
     
     
       29. A transmitter as defined in claim 22, wherein the delay circuit further includes a low frequency signal generator to increase the stimulated Brillouin scattering threshold experienced by the received optical signal at the remote receiver.  
     
     
       30. A transmitter as defined in claim 22, further comprising a tone generator circuit, wherein the delay circuit and the tone generator circuit are coupled to the phase modulator for adjusting a compensation applied by the phase modulator to the output of the laser to cancel the distortion in the received optical signal.  
     
     
       31. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising:
 a laser;   an input coupled to the laser for directly amplitude modulating the laser with an analog RF signal to produce an output optical signal including an amplitude modulated information-containing component and a second component resulting from chirp in the laser;   a phase modulator coupled to an output of the laser for separately modulating the output optical signal output from the laser in such a manner that distortion present in a received optical signal, received at the remote receiver, is reduced; and   a delay circuit having an input coupled to the analog RF signal and an output coupled to the phase modulator to generate a modulating electrical signal synchronized with the output optical signal to cause the phase modulator to reduce the second component in the output optical signal.    
     
     
       32. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising:
 a laser;   an input coupled to the laser for directly amplitude modulating the laser with an analog RF signal to produce an output optical signal including an amplitude modulated information-containing component and an optical phase distortion component resulting from chirp in the laser;   a phase modulator coupled to an output of the laser for separately modulating the output optical signal output from the laser in such a manner that distortion present in a received optical signal, received at the remote receiver, is reduced; and   a delay circuit having an input coupled to the analog RF signal and an output coupled to the phase modulator to generate a modulating electrical signal synchronized with the output optical signal to cause the phase modulator to reduce the optical phase distortion component in the output optical signal.    
     
     
       33. An optical transmitter for generating a modulated optical signal for transmission over a fiber optic link to a remote receiver comprising:
 a laser;   an input coupled to the laser for directly amplitude modulating the laser with an analog RF signal to produce an output optical signal including an amplitude modulated information-containing component and an optical phase distortion component;   a phase modulator coupled to an output of the laser for separately modulating the output optical signal output from the laser in such a manner that distortion present in a received optical signal, received at the remote receiver, is reduced; and   a delay circuit having an input coupled to the analog RF signal and an output coupled to the phase modulator to generate a modulating electrical signal synchronized with the output optical signal to cause the phase modulator to reduce the optical phase distortion component in the output optical signal.