TUNING A MULTI-CHANNEL OPTICAL TRANSMISSION SYSTEM

• US 20200136350A1
• Filed: 10/25/2018
• Published: 04/30/2020
• Est. Priority Date: 10/25/2018
• Status: Active Grant

• Current Assignee: ADTRAN Incorporated (ADTRAN Holdings, Inc.)
• Original Assignee: ADTRAN Incorporated (ADTRAN Holdings, Inc.)
• Inventors: Joffe, Daniel M., Sandstrom, Leif
• Granted Patent: US 10,797,467 B2
• Time in Patent Office: Days
• CPC Class Codes: H01S 5/0078 for frequency filtering
• CPC Class Codes: H01S 5/0612 controlled by temperature
• CPC Class Codes: H01S 5/06213 Amplitude modulation
• CPC Class Codes: H01S 5/06837 Stabilising otherwise than by an applied electric field or current, e.g. by controlling the temperature
• CPC Class Codes: H01S 5/1092 Multi-wavelength lasing
• CPC Class Codes: H04B 10/504 using direct modulation
• CPC Class Codes: H04B 10/516 Details of coding or modulation
• CPC Class Codes: H04B 10/541 Digital intensity or amplitude modulation
• CPC Class Codes: H04B 10/572 Wavelength control

Abstract

An optical transmission system includes a laser module generating a modulated optical waveform, including both amplitude and frequency modulation, at center frequencies corresponding to different operating temperatures; and an optical shaping filter, with passbands corresponding to the different center frequencies, that converts at least part of the frequency modulation to additional amplitude modulation. The optical transmission system is tuned by: determining a range of temperatures at which the laser module center frequencies are within a passband of the optical shaping filter; setting the laser module to a temperature, within the range of temperatures, at which the modulated optical waveform is within the passband; measuring an average output power of the optical shaping filter; and adjusting the temperature of the laser module to a target temperature, within the range of temperatures, at which an output condition is achieved, based on the average output power and/or extinction ratio of the filtered waveform.

Claims

1. An optical transmission system comprising:

a laser module configured to generate a modulated optical waveform at any one of a plurality of center frequencies corresponding to a plurality of operating temperatures of the laser module, the modulated optical waveform comprising both amplitude modulation and frequency modulation;

an optical shaping filter that is configured to implement a plurality of passbands corresponding to the center frequencies of the modulated optical waveform, and that is configured to convert at least part of the frequency modulation in the modulated optical waveform to additional amplitude modulation for the modulated optical waveform; and

a controller comprising at least one processor configured to tune the optical transmission system by performing operations that comprise;

for a first passband among the plurality of passbands of the optical shaping filter, determining a first range of temperatures at which the laser module generates the modulated optical waveform at center frequencies that are within the first passband;

setting the laser module to a first temperature, within the first range of temperatures, at which the laser module generates the modulated optical waveform at a first center frequency that is within the first passband;

measuring, based on an amplitude of a first filtered optical waveform that is output from the optical shaping filter, an average output power of the optical shaping filter; and

adjusting the first temperature of the laser module to a target temperature, within the first range of temperatures, at which a target output condition is achieved, comprising;

adjusting the first temperature of the laser module to a first target temperature, within the first range of temperatures, at which the average output power of the first filtered optical waveform that is output from the optical shaping filter satisfies a first output criteria, or

adjusting the first temperature of the laser module to a second target temperature, within the first range of temperatures, at which the output power and extinction ratio of the first filtered optical waveform that is output from the optical shaping filter satisfy a second output criteria.

2. The optical transmission system of claim 1, wherein the operations further comprise:

for a second passband among the plurality of passbands of the optical shaping filter, determining a second range of temperatures at which the laser module generates the modulated optical waveform at center frequencies that are within the second passband;

adjusting the laser module from the first temperature to a second temperature, within the second range of temperatures, at which the laser module generates the modulated optical waveform at a second center frequency that is within the second passband;

measuring, based on an amplitude of a second filtered optical waveform that is output from the optical shaping filter, an average output power of the optical shaping filter; and

adjusting the second temperature of the laser module to a target temperature, within the second range of temperatures, at which the average output power of the second filtered optical waveform that is output from the optical shaping filter satisfies the target output condition.

3. The optical transmission system of claim 1, wherein the laser module comprises a directly modulated laser that is tunable over a range of frequencies that comprises the plurality of passbands of the optical shaping filter, and

wherein the optical shaping filter is a bandpass filter configured to limit an operating bandwidth of the modulated optical waveform.

4. The optical transmission system of claim 3, wherein the optical transmission system further comprises an optical transmitter that is configured to modulate an electrical signal that drives the laser module to generate the modulated optical waveform.

5. The optical transmission system of claim 1, wherein adjusting the first temperature of the laser module to the first target temperature or to the second target temperature comprises:

determining a rate of change of the output power with respect to the first center frequency or with respect to the first temperature of the laser module; and

adjusting the first temperature of the laser module to the first target temperature or to the second target temperature at which the rate of change of the output power satisfies a first condition or a second condition, respectively.

6. The optical transmission system of claim 1, wherein adjusting the first temperature of the laser module to the target temperature, within the first range of temperatures, at which the target output condition is achieved comprises:

determining a reference temperature for the laser module, within the first range of temperatures, at which (i) the average output power of the optical shaping filter is maximized at a maximum average output power, or (ii) a specified extinction ratio is achieved and the average output power is within a specified amount of the maximum average output power.

7. The optical transmission system of claim 6, wherein determining the reference temperature for the laser module, within the first range of temperatures, at which the average output power of the optical shaping filter is maximized comprises:

determining, for a plurality of temperatures of the laser module within the first range of temperatures, a corresponding plurality of average output powers from the optical shaping filter; and

selecting, from among the plurality of temperatures, the reference temperature as corresponding to (i) a maximum average output power among the plurality of average output powers, or (ii) an average output power among the plurality of average output powers that is within the specified amount of the maximum average output power, and at which the specified extinction ratio is achieved.

8. The optical transmission system of claim 6, wherein the operations further comprise:

setting the first temperature of the laser module to be offset from the reference temperature such that a modulation frequency of a first modulation level in the modulated optical waveform is less than 5 GHz away from a center of the first passband of the optical shaping filter.

9. The optical transmission system of claim 1, wherein the operations further comprise:

based on adjusting the first temperature of the laser module to the target temperature, within the first range of temperatures, at which the target output condition is achieved, further applying a temperature dither signal to the laser module to offset the first temperature of the laser module;

measuring, based on an updated first filtered optical waveform that is output from the optical shaping filter, an updated average output power of the optical shaping filter; and

adjusting the first temperature of the laser module to an updated first temperature at which the updated average output power of the optical shaping filter satisfies the target output condition.

10. The optical transmission system of claim 1, wherein the laser module is further configured to generate the modulated optical waveform having a first modulation frequency and a second modulation frequency, and

wherein the operations further comprise:

initializing the temperature of the laser module to an initial temperature at which the first modulation frequency of the laser module is aligned with the optical shaping filter, based on a center frequency of the first passband of the optical shaping filter.

11. A method of tuning an optical transmission system that comprises a laser module and an optical shaping filter, the method comprising:

driving the laser module to generate a modulated optical waveform at any one of a plurality of center frequencies corresponding to a plurality of operating temperatures of the laser module, the modulated optical waveform comprising both amplitude modulation and frequency modulation;

filtering the modulated optical waveform with the optical shaping filter to convert at least part of the frequency modulation in the modulated optical waveform to additional amplitude modulation for the modulated optical waveform, the optical shaping filter configured to implement a plurality of passbands corresponding to the plurality of center frequencies of the modulated optical waveform;

for a first passband among the plurality of passbands of the optical shaping filter, determining a first range of temperatures at which the laser module generates the modulated optical waveform at center frequencies that are within the first passband;

setting the laser module to a first temperature, within the first range of temperatures, at which the laser module generates the modulated optical waveform at a first center frequency that is within the first passband;

measuring, based on an amplitude of a first filtered optical waveform that is output from the optical shaping filter, an average output power of the optical shaping filter; and

adjusting the first temperature of the laser module to a target temperature, within the first range of temperatures, at which a target output condition is achieved, comprising;

adjusting the first temperature of the laser module to a first target temperature, within the first range of temperatures, at which the average output power of the first filtered optical waveform that is output from the optical shaping filter satisfies a first output criteria, or

adjusting the first temperature of the laser module to a second target temperature, within the first range of temperatures, at which the output power and extinction ratio of the first filtered optical waveform that is output from the optical shaping filter satisfy a second output criteria.

12. The method of claim 11, further comprising:

for a second passband among the plurality of passbands of the optical shaping filter, determining a second range of temperatures at which the laser module generates the modulated optical waveform at center frequencies that are within the second passband;

adjusting the laser module from the first temperature to a second temperature, within the second range of temperatures, at which the laser module generates the modulated optical waveform at a second center frequency that is within the second passband;

measuring, based on an amplitude of a second filtered optical waveform that is output from the optical shaping filter, an average output power of the optical shaping filter; and

adjusting the second temperature of the laser module to a target temperature, within the second range of temperatures, at which the average output power of the second filtered optical waveform that is output from the optical shaping filter satisfies the target output condition.

13. The method of claim 11, wherein the laser module comprises a directly modulated laser that is tunable over a range of frequencies that comprises the plurality of passbands of the optical shaping filter, and

wherein the optical shaping filter is a bandpass filter configured to limit an operating bandwidth of the modulated optical waveform.

14. The method of claim 13, wherein the optical transmission system further comprises an optical transmitter that is configured to modulate an electrical signal that drives the laser module to generate the modulated optical waveform.

15. The method of claim 11, wherein adjusting the first temperature of the laser module to the first target temperature or to the second target temperature comprises:

determining a rate of change of the output power with respect to the first center frequency or with respect to the first temperature of the laser module; and

adjusting the first temperature of the laser module to the first target temperature or to the second target temperature at which the rate of change of the output power satisfies a first condition or a second condition, respectively.

16. The method of claim 11, wherein adjusting the first temperature of the laser module to the target temperature, within the first range of temperatures, at which the target output condition is achieved comprises:

determining a reference temperature for the laser module, within the first range of temperatures, at which (i) the average output power of the optical shaping filter is maximized at a maximum average output power, or (ii) a specified extinction ratio is achieved and the average output power is within a specified amount of the maximum average output power.

17. The method of claim 16, wherein determining the reference temperature for the laser module, within the first range of temperatures, at which the average output power of the optical shaping filter is maximized comprises:

determining, for a plurality of temperatures of the laser module within the first range of temperatures, a corresponding plurality of average output powers from the optical shaping filter; and

selecting, from among the plurality of temperatures, the reference temperature as corresponding to (i) a maximum average output power among the plurality of average output powers, or (ii) an average output power among the plurality of average output powers that is within the specified amount of the maximum average output power, and at which the specified extinction ratio is achieved.

18. The method of claim 16, further comprising:

setting the first temperature of the laser module to be offset from the reference temperature such that a modulation frequency of a first modulation level in the modulated optical waveform is less than 5 GHz away from a center of the first passband of the optical shaping filter.

19. The method of claim 11, further comprising:

based on adjusting the first temperature of the laser module to the target temperature, within the first range of temperatures, at which the target output condition is achieved, further applying a temperature dither signal to the laser module to offset the first temperature of the laser module;

measuring, based on an updated first filtered optical waveform that is output from the optical shaping filter, an updated average output power of the optical shaping filter; and

adjusting the first temperature of the laser module to an updated first temperature at which the udated average output power of the optical shaping filter satisfies the target output condition.

20. The method of claim 11, wherein the laser module is further configured to generate the modulated optical waveform having a first modulation frequency and a second modulation frequency, and

wherein the operations further comprise:

initializing the temperature of the laser module to an initial temperature at which the first modulation frequency of the laser module is aligned with the optical shaping filter, based on a center frequency of the first passband of the optical shaping filter.