Intensity modulated direct detection optical transceiver

Patent metadata

Field	Value
Publication number	US-9735883-B2
Application number	US-201615060318-A
Country	US
Kind code	B2
Filing date	Mar 3, 2016
Priority date	Oct 9, 2015
Publication date	Aug 15, 2017
Grant date	Aug 15, 2017

Abstract

An optical transceiver using a modulator biased to a non-linear region of the modulator's power transfer function can extend the possible transmission distance of intensity modulated direct detection (IMDD) n-level pulse amplitude modulated (PAM-n) signals. A non-linear look-up-table may compensate for non-linear characteristics of the modulator. An adaptive decision threshold look-up-table may be used to provide adaptive decision levels at the receiver.

First claim

What is claimed is:

1. An optical transmitter system comprising:
a transmission digital processor for outputting a Pulse Amplitude Modulated (PAM) digital transmission signal;
a driving component for converting the digital transmission signal into an analog transmission driving signal; and
an optical modulator for producing an optical transmission signal based on the analog transmission driving signal by modulating the optical transmission signal in an approximately linear region of an amplitude transfer function of the optical modulator corresponding to a non-linear region of a power transfer function of the optical modulator.

2. The optical transmitter system of claim 1 , wherein the transmission digital processor is configured for receiving a PAM symbol stream and generating the digital transmission signal by applying pre-compensation to the PAM symbol stream to compensate for chromatic dispersion of an optical fiber used in transmitting the optical transmission signal.

3. The optical transmitter system of claim 2 , wherein the pre-compensation comprises one or more pre-compensation functions combined together, the pre-compensation functions comprising one or more of:
a non-linear look-up-table (NL-LUT) for adjusting amplitude levels of PAM symbols of the PAM symbol stream to compensate for non-linear distortions resulting from operating the optical modulator in the non-linear region of the power transfer function;
pulse shaping for adjusting a resulting spectral shape of the optical transmission signal;
dispersion pre-compensation for adjusting the resulting optical transmission signal to compensate for chromatic dispersion; and
DAC and driver pre-compensation for adjusting the resulting optical transmission signal to compensate for characteristics of the driving component.

4. The optical transmitter system of claim 3 , wherein one or more of the pulse shaping functionality, dispersion pre-compensation functionality and DAC and driver pre-compensation functionality are combined together into a single linear pre-compensation function.

5. The optical transmitter system of claim 4 , wherein the transmission processor up-samples the adjusted PAM symbol stream output from the NL-LUT prior to the linear pre-compensation function.

6. The optical transmitter system of claim 3 , wherein the NL-LUT is generated during an initialization stage of the optical transmitter system.

7. The optical transmitter system of claim 6 , wherein the NL-LUT compensates for an average symbol offset between desired PAM symbol levels and actual PAM symbol levels measured at an output of the optical modulator during the initialization stage.

8. The optical transmitter system of claim 3 , wherein the NL-LUT comprises a loop gain compensation component to control an amount of compensation applied.

9. The optical transmitter system of claim 8 , wherein the loop gain compensation component is set based on a bit error ratio at a receiver of the optical transmission signal.

10. The optical transmitter system of claim 1 , wherein the optical modulator is biased to operate around a bias voltage of between about 0.6 V π and about 0.9 V π , where V π is a voltage at which the optical modulator modulates an optical signal to a lowest intensity level.

11. The optical transmitter system of claim 1 , wherein the bias voltage of the optical modulator is set based on a bit error ratio at a receiver of the optical transmission signal.

12. The optical transmitter system of claim 1 , wherein the digital transmission signal encodes the data using n levels of pulse amplitudes, wherein n=2, 4, 8 or 16.

13. The optical transmitter system of claim 1 , further comprising:
an optical receiver coupled to the optical transmitter through a length of fiber optic cable, the optical receiver comprising:
an optical detector for producing an electrical output corresponding to a detected optical signal; and
a receiver processor for processing the electrical output of the optical detector to generate a symbol stream transmitted in the detected optical signal including a decision threshold look-up-table (DT-LUT) associating a range of values with a symbol, the DT-LUT being adaptively updateable based on measured average symbol amplitudes.

14. The optical transmitter system of claim 13 , wherein the optical transmitter system is capable of transmitting data at rates of greater than 50 Gbps with a bit error ratio of less than 10 −3 over the length of fiber optic cable for lengths up 250 km.

15. A method of transmitting data over an optical fibre, the method comprising:
encoding the data using Pulse Amplitude Modulation (PAM) to provide a symbol stream;
processing the symbol stream to generate a modulator driving signal; and
modulating an optical signal according to the modulator driving signal using an optical modulator operating in an approximately linear region of an amplitude transfer function of the optical modulator corresponding to a non-linear region of a power transfer function of the optical modulator.

16. The method of claim 15 , further comprising:
transmitting the modulated optical signal over a length of fiber optic cable;
detecting the modulated optical signal at a receiver to generate an electrical signal corresponding to the detected modulated optical signal; and
determining symbols present in the electrical signal to re-construct the transmitted symbol stream.

17. The method of claim 16 , wherein determining the symbols present in the electrical signal comprises using an adaptive decision threshold look-up-table providing a range of values associated with different symbols.

18. The method of claim 16 , wherein the processing of the symbol stream comprises adjusting the symbol stream to compensate for chromatic dispersion of the modulated optical signal transmitted over the length of the fiber optic cable.

19. The method of claim 16 , wherein the processing of the symbol stream comprises adjusting the symbol stream according to a non-linear look-up-table compensating for non-linear distortions resulting from operating the optical modulator in the non-linear region of the power transfer function.

20. The method of claim 16 , wherein the method is capable of transmitting data at rates of greater than 50 Gbps with a bit error ratio of less than 10 −3 over the length of fiber optic cable for lengths up 250 km.

21. The method of claim 15 , wherein the modulating comprises biasing the optical modulator to operate around a bias voltage of between about 0.6 V π and about 0.9 V π , where V π is a voltage at which the optical modulator modulates an optical signal to a lowest intensity level.

Assignees

• Chung Sung
• Tang Xuefeng
• Li Chuandong
• Zhang Zhuhong
• Huawei Tech Co Ltd

Inventors

• Chung Sung
• Tang Xuefeng
• Li Chuandong
• Zhang Zhuhong

Classifications

• H04B10/541 Primary

  Digital intensity or amplitude modulation · CPC title

• H04B10/6161

  Compensation of chromatic dispersion · CPC title

• H04B10/25133

  including a lumped electrical or optical dispersion compensator (H04B10/2519, H04B10/2525 takes precedence) · CPC title

• H04B10/588

  in external modulation systems · CPC title