Optical source, communications network optical apparatus and method of providing an optical signal

• US 10,461,863 B2
• Filed: 04/25/2018
• Issued: 10/29/2019
• Est. Priority Date: 03/27/2014
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Testa, Francesco, D'Errico, Antonio, Ciaramella, Ernesto, Giorgi, Luca, Huang, Wei-Ping, Presi, Marco
• Primary Examiner(s): Li, Shi K
• Application Number:
  US15/962,687
• Publication Number:
  US 20180248625A1
• Time in Patent Office: 552 Days
• CPC Class Codes: H04B 10/2569 due to polarisation mode dispersion [PMD]
• CPC Class Codes: H04B 10/50 Transmitters
• CPC Class Codes: H04B 10/503 Laser transmitters

Abstract

An optical source comprises a first laser arranged to generate a first optical signal having a first state of polarization (SOP) and a first optical frequency; a second laser arranged to generate a second optical signal having a second SOP, substantially orthogonal to the first SOP, and having a second optical frequency, different from the first optical frequency by a preselected frequency difference; a polarisation beam coupler arranged to combine the first optical signal and the second optical signal into a composite optical signal comprising both the first optical signal and the second optical signal having said substantially orthogonal SOPs; and an output arranged to output the composite optical signal.

Claims

1. An optical source comprising:

a first laser arranged to generate a first optical signal having a first state of polarization and a first optical frequency;

a second laser arranged to generate a second optical signal having a second state of polarization, substantially orthogonal to the first state of polarization, and having a second optical frequency, different to the first optical frequency by a preselected frequency difference, Δ

ν

;

a polarization beam coupler arranged to combine the first optical signal and the second optical signal into a composite optical signal comprising both the first optical signal and the second optical signal having said substantially orthogonal states of polarization;

an output arranged to output the composite optical signal; and

a feeder optical fiber coupled at one end to the output and having a polarization mode dispersion coefficient and a length,

wherein the preselected frequency difference, Δ

ν

, is inversely proportional to a differential group delay, τ

, which is proportional to the polarization mode dispersion coefficient and the length of the feeder optical fiber.

2. The optical source of claim 1, wherein the preselected frequency difference Δ

ν

, has a value of up to

3. The optical source of claim 2, wherein the Maxwell adjustment factor has a value of at least 4.

4. The optical source of claim 1, wherein the composite optical signal has an optical power and τ

is the differential group delay that causes a 1.5 dB variation in the optical power.

5. The optical source of claim 1, wherein the polarization beam coupler has a principal state of polarization and the first state of polarization and the second state of polarization are each rotated by 45 degrees relative to the principal state of polarization, and wherein the polarization beam coupler is arranged to combine the first optical signal and the second optical signal into two said composite optical signals each comprising a respective component of the first optical signal and a respective component of the second optical signal, and wherein the optical source comprises a further output, each said output arranged to output a respective one of the composite optical signals.

6. A communications network optical apparatus comprising:

the optical source of claim 1;

a polarization selective optical filter arranged to receive a said composite optical signal generated by the optical source and arranged to transmit a portion of the composite optical signal having a preselected state of polarization; and

an optical device arranged to operate at the preselected state of polarization and arranged to receive said portion of the composite optical signal transmitted by the polarization selective optical filter.

7. A communications network base station comprising:

the optical source of claim 1; and

an optical receiver having an electrical bandwidth and arranged to receive at least a portion of a said composite optical signal,

wherein the preselected frequency difference is greater than twice the electrical bandwidth of the optical receiver.

8. The communications network base station of claim 7, the optical source further comprising:

a polarization selective optical filter arranged to receive a said composite optical signal generated by the optical source and arranged to transmit a portion of a said composite optical signal having a preselected state of polarization; and

an optical device arranged to operate at the preselected state of polarization and arranged to receive said portion of the composite optical signal transmitted by the polarization selective optical filter and arranged to modify said portion of the composite optical signal and to transmit the modified portion of the composite optical signal,

and wherein the optical receiver is arranged to receive the modified portion of the composite optical signal.

9. A method of providing an optical signal to a delivery location, the method comprising:

providing a first optical signal having a first state of polarization and a first optical frequency;

providing a second optical signal having a second state of polarization, substantially orthogonal to the first state of polarization, and having a second optical frequency, different to the first optical frequency by a preselected frequency difference, Δ

ν

;

combining the first optical signal and the second optical signal while maintaining the states of polarization of the first and second optical signals, to thereby form a composite optical signal comprising both the first optical signal and the second optical signal having said substantially orthogonal states of polarization; and

transmitting the composite optical signal to the delivery location through a feeder optical fiber having a polarization mode dispersion coefficient and a length,

wherein the preselected frequency difference, Δ

ν

, is inversely proportional to a differential group delay, τ

, which is proportional to the polarization mode dispersion coefficient and the length of the feeder optical fiber.

10. The method of claim 9, wherein the preselected frequency difference, Δ

ν

, has a value of up to

11. The method of claim 10, wherein the preselected frequency difference Δ

ν

, has a value of up to

12. The method of claim 11, wherein the adjustment factor has a value of at least 4.

13. The method of claim 10, wherein the composite optical signal has an optical power and τ

is the differential group delay that causes a 1.5 dB variation in the optical power.