Technique for accurate carrier frequency generation in of DM system

• US 5,239,400 A
• Filed: 07/10/1991
• Issued: 08/24/1993
• Est. Priority Date: 07/10/1991
• Status: Expired due to Fees

• Current Assignee: Arizona Board of Regents (University of Arizona)
• Original Assignee: Arizona Board of Regents (University of Arizona)
• Inventors: Liu, Ming-Kang
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): PASCAL, LESLIE C
• Application Number:
  US07/727,972
• Time in Patent Office: 776 Days
• Field of Search: 359/124, 359/125, 359/133, 359/132, 359/121, 359/187
• US Class Current: 398/95
• CPC Class Codes: H04B 10/50 Transmitters

Abstract

An Optical Frequency Division Multiplexing system achieves close channel spacings and thus high density of communications channels in a particular frequency band by providing a very precise carrier frequency stabilization technique. Each laser-generated local carrier signal is locked to a corresponding reference signal. All of the reference signals are generated by a common tunable laser circuit that produces a sequence of bursts of successively higher frequencies determined by resonant points of a single Fabry-Perot filter. Each transceiver of the Optical Frequency Division Multiplexing system includes a frequency tracking circuit that converts received signals from the optical domain to the electrical domain and utilizes an intermediate frequency filter circuit and a servo control circuit to adjust the tunable laser frequency and lock it to the present reference frequency.

Claims

1. In an optical frequency division multiplexing system, circuitry comprising in combination:

(a) a first tunable laser responsive to an electrical data input signal and also responsive to an electrical frequency control signal and transmitting a data-modulated optical output signal via a first optical fiber to a passive coupler;

(b) a second optical fiber carrying a plurality of laser-produced signals coupled by the passive coupler;

(c) a reference frequency generating circuit operating to generate a plurality of optical reference signals each having a different reference frequency during a plurality of successive time intervals, respectively, and supplying the optical reference signals to the passive coupler;

(d) a frequency tracking circuit including

(i) a first photodetector receiving various optical signals from the second optical fiber and producing corresponding electrical signals including cross terms between the reference frequency signals and the data-modulated optical output signal;

(ii) an intermediate frequency filter circuit receiving the electrical signals from the first photodetector including the cross terms therein, and producing an output signal if a frequency difference between a present reference frequency signal and a data-modulated signals in the electrical signal produced by the first photodetector is within the pass band of the intermediate frequency filter;

(iii) a peak detector circuit receiving the output signal of the intermediate frequency filter and producing a signal representative of a peak of the output of the intermediate frequency filter circuit,

(iv) a servo circuit responsive to the signal produced by the peak detector circuit for producing the electrical frequency control signal,

whereby the optical output signal produced by the first tunable laser becomes locked to the present reference signal.

2. The circuit of claim 1 including a low pass filter circuit coupling the output of the peak detector circuit to an input of the servo circuit.

3. The circuit of claim 1 including a polarizer coupling an optical output of the tunable laser to the first optical fiber, the polarizer having a control input receiving an electrical control signal produced by the servo circuit.

4. The circuit of claim 1 including a mixer circuit coupling an output of the peak detector circuit to the input of the servo circuit, the mixer circuit multiplying the electrical data input signal by the output signal produced by the peak detector circuit.

5. The circuit of claim 1 wherein the reference frequency generating circuit includes:

1) a second tunable laser producing an optical output and having a frequency control input;

2) a Fabry-Perot filter receiving the optical output of the second tunable laser and producing the optical reference signals in response to the optical output of the second tunable laser;

3) a second photodetector receiving the optical reference signals and producing corresponding electrical reference signals, respectively;

4) a control circuit producing a stepped output waveform and applying it to the frequency control input of the second tunable laser.

6. The circuit of claim 5 wherein the control circuit includes a comparator circuit responsive to the electrical reference signals, to set a monostable circuit if the frequency of the optical output of the second tunable laser is within one of the pass bands of the Fabry-Perot filter, a counter connected to count when the monostable circuit is not set and stop counting when the monostable circuit is set, and a digital-to-analog convertor coupled to produce a frequency control signal on the frequency control input of the second tunable laser.

7. A circuit generating a plurality of reference frequencies during a plurality of corresponding successive time intervals, comprising in combination:

(a) a tunable laser producing an optical output and having a frequency control input;

6 (b) a Fabry-Perot filter receiving the optical output and producing the optical reference signals in pass bands of the Fabry-Perot filter in response to the optical output;

(c) a photodetector receiving the optical reference signals and producing a corresponding electrical signal; and

(d) a control circuit receiving the electrical signal, producing a multi-level waveform, and applying it to the frequency control input of the tunable laser, the control circuit including

i. a threshold detection circuit responsive to the electrical signal and generating a digital signal,

ii. a monostable circuit receiving the digital signal and generating a monostable pulse at a transition of the digital signal,

iii. a counter circuit receiving the digital pulse and disabled to count during the presence of the digital pulse, and

iv. a digital-to-analog converter circuit receiving an output of the counter circuit and generating the stepped output waveform proportional to the counter value.

8. A circuit in every transceiver of an optical frequency division multiplexing system that locks an optical frequency division multiplexing channel frequency with respect to a plurality of the reference frequencies, the circuit comprising in combination:

(a) a tunable laser responsive to an electrical data input signal, and also responsive to a frequency control signal, and transmitting a data-modulated optical output signal via a first optical fiber to a star passive coupler;

(b) a second optical fiber carrying a plurality of laser-produced signals in the optical frequency division multiplexing system coupled by the star passive coupler;

(c) a polarizer adjusting the polarization of the output optical signal produced by the tunable laser;

(d) a 2-to-1 coupler;

(e) a 1-to-2 photonic switch directing the optical output signal to the first optical fiber after the frequency of the tunable laser has been locked to a reference frequency, and directing a resulting optical output signal to a 2-to-1 coupler during an initial frequency tracking stage, the 2-to-1 coupler combining the resulting optical output signal from the laser-produced signals via the second optical fiber to produce a combined optical signal;

(f) a photodetector receiving the combined optical signal and producing an electrical signal proportional to the square of the combined optical signal, the electrical signal consisting of terms whose central frequencies are equal to a frequency difference of two optical signals in the combined signal;

(g) an intermediate frequency filter circuit receiving the electrical signal from the photodetector, the intermediate frequency filter producing an output which is nonzero only if the electrical signal from the photodetector includes terms whose central frequencies are close to the intermediate frequency of the filter, the intermediate frequency of the intermediate frequency filter being equal to the frequency difference of an optical reference signal and a corresponding optical frequency division multiplexing channel frequency;

(h) a peak detector circuit receiving the output of the intermediate frequency filter and producing an output signal representative of a peak of the output signal of the intermediate filter circuit;

(i) a multiplication circuit generating an output proportional to a product of the peak detector output signal and the data input signal;

(j) a first low pass filter circuit receiving the output of the peak detector circuit and generating an averaged value of the output of the intermediate frequency filter circuit to activate the servo circuit during the initial tracking stage;

(k) a second low pass filter circuit receiving the output of the multiplication circuit and generating an averaged value of the output of the intermediate frequency filter circuit; and

(1) a servo circuit adjusting the frequency control signal and adjusting a polarization control signal of the polarizer to maximize the output of the second low pass filter, the servo circuit being inactivated during an initial tracking operation during which the output of the first low pass filter is at a first level, the servo circuit being activated when the output of second low pass filter is at a second level after the initial tracking stage.

9. A method of operating an optical frequency division multiplexing system, comprising the steps of:

(a) applying an electrical data input signal to a first tunable laser and transmitting a data-modulated optical output signal produced by the first tunable laser via a first optical fiber to a passive coupler;

(b) generating a plurality of optical reference signals each having a different reference frequency during a plurality of successive time intervals, respectively;

(c) transmitting a plurality of laser-produced signals from the passive coupler to a first photodetector by means of a second optical fiber to produce electrical signals including cross terms between the reference frequency signals and the data-modulated optical output signals;

(d) applying the electrical signals from the first photodetector to an intermediate frequency filter circuit and producing an electrical output signal if a present reference frequency signal and a component of the data-modulated optical output signal are within a pass band of the intermediate frequency filter;

(e) applying the electrical output signal produced by the intermediate frequency filter to a peak detector circuit and producing a signal at an output of the peak detector circuit representative of a peak of the output signal produced by the intermediate frequency filter circuit; and

(f) operating a servo circuit in response to the signal produced by the peak detector circuit to produce an electrical frequency control signal, and applying the electrical frequency control signal to the first tunable laser to control the frequency of the data-modulated optical output signal;

whereby the optical output signal produced by the first tunable laser becomes locked to the present reference signal.

10. The method of claim 9 including filtering the signal produced by the peak detector circuit to produce a disable signal and disabling the servo circuit from controlling the first tunable laser during an initial time period, and mixing the signal produced by the peak detector circuit with the electrical data input signal to produce a mixed signal, and filtering the mixed signal to produce the electrical frequency control signal when the servo circuit is not disabled by the disable signal.

11. The method of claim 9 wherein step (c) includes the steps of:

(1) operating a second tunable laser to produce an optical output signal;

(2) applying the optical output signal produced by the second tunable laser to a Fabry-Perot filter to produce the optical reference signals in pass bands of the Fabry-Perot filter;

(3) applying the optical reference signals to a second photodetector to produce corresponding electrical signals;

(4) applying the corresponding electrical signals to a threshold detection circuit to generate a digital signal if one of the corresponding electrical signals is within a pass band of the Fabry-Perot filter;

(5) applying the digital signal to a monostable circuit to generate a monostable pulse at a transition of the digital signal;

(6) applying the monostable pulse to a counter circuit to disable its counting during the presence of the digital pulse;

(7) applying the outputs of the counter to inputs of a digital-to-analog converter circuit to generate a stepped output waveform signal proportional to contents of the counter; and

(8) using the stepped output waveform signal as the electrical frequency control signal.

12. A method for generating a plurality of optical reference signals during a plurality of corresponding successive time intervals, the method comprising the steps of:

(a) operating a tunable laser to produce an optical output signal;

(b) applying the optical output signal to a Fabry-Perot filter to produce one of the optical reference signals if the optical output signal is within a pass band of the Fabry-Perot filter;

(c) applying the optical reference signal to a photodetector to produce a corresponding electrical signal;

(d) applying the corresponding electrical signal to a threshold detection circuit to generate a digital signal;

(e) applying the digital signal to a monostable circuit to generate an output pulse of the monostable circuit at a transition of the digital signal;

(f) applying the monostable pulse to a counter circuit to disable its counting during the presence of the digital pulse;

(g) applying the outputs of the counter to inputs of a digital-to-analog converter circuit to generate part of a multi-level output waveform signal proportional to the counter value;

(h) applying the output waveform signal to a frequency control input of the tunable laser; and

(i) enabling the counter to count after the end of the output pulse of the monostable circuit.

13. A method of operating an optical frequency division multiplexing system, comprising the steps of:

(a) applying an electrical data input signal to a first tunable laser and transmitting a data-modulated optical output signal produced by the first tunable laser via a first optical fiber to a passive coupler;

(b) generating a plurality of optical reference signals each having a different reference frequency during a plurality of successive time intervals, respectively;

(c) transmitting a plurality of laser-produced signals from the passive coupler to a first photodetector by means of a second optical fiber to produce electrical signals including cross terms between the reference frequency signals and the data-modulated optical output signals;

(d) producing an electrical output signal if a present reference frequency signal and a component of the data-modulated output signal are within a pass band of a filtering device;

(e) producing a signal representative of a peak of the electrical output signal; and

(f) producing an electrical frequency control signal in response to the signal representative of the peak, and applying the electrical frequency control signal to the first tunable laser to control a carrier frequency of the optical output signal;

whereby the optical output signal produced by the first tunable laser becomes locked to the present reference signal.

14. The method of claim 13 wherein step (c) includes the steps of:

(1) operating a second tunable laser to produce an optical output signal;

(2) applying the optical output signal produced by the second tunable laser to a Fabry-Perot filter to produce the optical reference signals;

(3) applying the optical reference signals to a second photodetector to produce corresponding electrical signals;

(4) generating a digital signal if one of the corresponding electrical signals is within a pass band of the Fabry-Perot filter;

(5) generating a pulse at a transition of the digital signal;

(6) producing a stepped output waveform signal having steps various edges of which correspond in time to times of occurrence of various corresponding edges of the pulse; and

(7) using the stepped output waveform signal as the electrical frequency control signal.

15. In an optical frequency division multiplexing system, circuitry comprising in combination:

(a) a tunable laser responsive to an electrical data input signal and also responsive to a frequency control signal and transmitting a data-modulated optical output signal to a passive coupler;

(b) means for carrying a plurality of laser-produced signals produced by remotely located lasers and coupled by the passive coupler;

(c) a reference frequency generating circuit operating to generate a plurality of optical reference signals from a single light source, each of the optical reference signals having a different reference frequency;

(d) a frequency tracking circuit including

(i) a photodetector receiving various optical signals from the carrying means and producing corresponding electrical signals including cross terms between the reference frequency signals and the data-modulated optical output signal;

(ii) a filter circuit receiving the electrical signals from the photdetector and the cross terms and producing an output signal if a frequency difference between a present reference frequency signal and a data-modulated optical output signal component in the electrical signals produced by the photodetector is within the pass band of the filter;

(iii) a detector circuit receiving the output signal of the filter and producing a signal representative of predetermined feature of the output signal of the filter circuit, and

(iv) a servo circuit responsive to the signal produced by the detector circuit for producing the frequency

whereby the optical output signal produced by the first tunable laser becomes locked to the present reference signal.

16. A method of operating an optical frequency division multiplexing system, comprising the steps of:

(a) applying an electrical data input signal to a tunable laser and transmitting a data-modulated optical output signal produced by the tunable laser to a passive coupler;

(b) generating a plurality of optical reference signals by means of a single light source, each of the optical reference signals having a different reference frequency;

(c) transmitting a plurality of laser-produced signals produced by remotely located lasers, respectively, from the passive coupler to a photodetector to produce electrical signals including cross terms between the reference frequency signals and the data-modulated optical output signals;

(d) producing an electrical output signal if a present reference frequency signal and a component of the data-modulated output signal are within a pass band of a filtering device;

(e) producing a signal representative of a predetermined feature of the electrical output signal; and

(f) producing a frequency control signal in response to the signal representative of the predetermined feature, and applying the frequency control signal to the tunable laser to control a carrier frequency of the data-modulated optical output signal;

whereby the optical output signal produced by the tunable laser becomes locked to the present reference signal.

17. A circuit generating a plurality of reference signals during a plurality of corresponding successive time intervals, comprising in combination:

(a) a tunable laser producing an optical output signal and having a frequency control input;

(b) an optical filter receiving the optical output signal and producing the optical reference signals in pass bands of the filter in response to the optical output signal;

(c) a photodetector receiving the optical reference signals and producing a corresponding electrical signal; and

(d) a control circuit receiving the electrical signal, producing a multi-level waveform, and applying it to the frequency control input of the tunable laser.