Bias signal generation for a laser transmitter in a passive optical network
View Patent ↗The teachings presented herein disclose a method and apparatus for controlling the optical power of a laser in a passive optical network transmitter that outputs a modulated optical signal responsive to a modulated input signal. In one or more embodiments, such a control method comprises detecting the voltage of the modulated input signal, and setting the DC bias level of the laser as a function of the detected voltage. These teachings may be implemented, for example, by a laser control circuit in the transceiver module of an optical network unit (“ONU”). Such an ONU may be advantageously used in a hybrid coaxial cable-optical fiber network, such as used in DPONs which interface cable system subscriber equipment to cable system head-end equipment.
1. A method of controlling the optical power of a laser in an optical network unit, used in a passive optical network, that outputs a modulated optical signal responsive to a RF input signal, the method comprising the steps of:
detecting the voltage of said RF input signal,
setting the DC bias level of the laser as a function of the detected RF input signal voltage,
wherein said step of setting the DC bias level comprises:
in a first period of time, increasing with a first time constant the DC bias level from an initial value to a first value configured to set operating point of the laser out of clipping point of the laser,
in a subsequent second period of time, adjusting with a second time constant the DC bias level from said first value to a second value depending on the average power of said RF input signal during a predefined time period so as to optimize a figure of merit of said optical network unit.
2. The method of claim 1 , wherein, if the power of said RF input signal is lower than a predefined value, than said second value is settled to a DC bias level lower than said first value, said second value being configured to set operating point of the laser out of clipping point of the laser.
3. The method of claim 1 , wherein, if the power of said RF input signal is higher than a predefined value, than said second value DC bias level is settled to a DC bias level higher than said first value, said second value being configured to set operating point of the laser out of clipping point of the laser.
4. The method of claim 2 , wherein the predefined value is configured to turn on said laser.
5. The method of claim 1 , wherein said first time constant is lower than said second time constant.
6. The method of claim 5 , wherein said first time constant is at least one order of magnitude faster than said second time constant.
7. The method of claim 1 , wherein said figure of merit is noise power ratio.
8. The method of claim 1 , wherein, in said second period of time, the output optical signal of the laser is not responsive to the RF input signal.
9. A laser control circuit for controlling the optical power of a laser in an optical transmitter, used in a passive optical network, that outputs a modulated optical signal responsive to a RF input signal, the laser control circuit comprising:
a circuit configured to detect the voltage of the RF input signal; and
a bias control circuit configured to set the DC bias level of the laser as a function of the detected voltage,
wherein said detector circuit comprises:
a first amplifier to amplify said RF input signal and generate a first amplified RF input signal,
a detector to receive said first RF amplified input signal and generate a detected output signal and a reference output signal,
a second amplifier with:
a positive terminal to receive said detected output signal,
a negative terminal to receive said reference output signal,
an output terminal to output a second amplified signal,
a feedback line coupling said negative terminal to said output terminal,
a diode clamping circuit provided in said feedback line to prevent saturation of said second amplifier,
a level shift diode having a first terminal coupled to said output terminal of the second amplifier and a second terminal coupled to said bias control circuit and configured to shift level of said second amplified signal,
a ground line,
a variable clamping circuit coupling the second terminal of said level shift diode to said ground line.
10. The laser control circuit of claim 9 , wherein said variable clamping circuit comprises:
a clamping diode having a first terminal coupled to the second terminal of said level shift diode and a second terminal,
an RC circuit coupling the second terminal of said clamping diode to said ground line.
11. The laser control circuit of claim 9 , wherein said detector comprises Schottky diodes.
12. The laser control circuit of claim 9 , wherein said second amplifier is a fast operational amplifier.
13. The laser control circuit of claim 9 , further comprising a pre-amplifier to amplify the modulated input signal to generate a pre-amplified modulated input signal and a splitter to divert the pre-amplified modulated input signal to the first amplifier and to the laser to modulate the optical output signal of the laser.
14. The laser control circuit of claim 9 , further comprising an RF impedance matching circuit coupled to the pre-amplifier to receive the pre-amplified modulated input signal and to the laser to modulate the optical output signal of the laser.
15. The method of claim 3 , wherein the predefined value is configured to turn on said laser.