Wavelength locker with liquid crystal tunable filter generating transmitted and reflected outputs

• US 7,221,429 B1
• Filed: 11/21/2003
• Issued: 05/22/2007
• Est. Priority Date: 06/02/2003
• Status: Expired due to Term

• Current Assignee: Lumentum Technology UK Limited (Lumentum Holdings Inc.)
• Original Assignee: Xtellus Inc. (Lumentum Holdings Inc.)
• Inventors: Cavanaugh, Shanti A., Hood, James M., Molinari, Louis J.
• Primary Examiner(s): Schechter; Andrew
• Assistant Examiner(s): Vu; Phu
• Application Number:
  US10/719,078
• Time in Patent Office: 1,278 Days
• Field of Search: 349/198, 372/29.011
• US Class Current: 349/198
• CPC Class Codes: G02F 1/216 using liquid crystals, e.g. liquid crystal Fabry-Perot filters
• CPC Class Codes: G02F 2201/307 Reflective grating, i.e. Bragg grating
• CPC Class Codes: G02F 2201/58 Arrangements comprising a monitoring photodetector
• CPC Class Codes: G02F 2203/055 wavelength filtering
• CPC Class Codes: G02F 2203/06 Polarisation independent

Abstract

A wavelength locker operates with a reference tunable liquid crystal filter having integrated photodetectors deposited on the front and backside of the liquid crystal cell to track power intensity of the accepted passband signal and its rejected signal compliment at a 50% power point where the two signals cross on the side of the filter transmission peak. The tunable filter is tuned by an offset wavelength from the laser wavelength such that 50% power is transmitted through the filter at the center wavelength of the laser. The wavelength locker may be configured with a liquid crystal tunable etalon or a liquid crystal tunable bandpass filter. A method for locking an optical signal to a desired frequency is also included and provides a substantially linear feedback signal computed by dividing the rejected signal power by the accepted signal power and passing it to laser transmitter to enable it to correct for frequency drift. Optional features of the present invention include an integrated thermal sensor and heater and temperature compensation control schemes.

Claims

1. A wavelength locker system, comprising:

an optical tap placed in the optical path of a laser transmitter for splitting a laser signal into a tapped signal and a laser output signal,

a free space tunable filter accepting the tapped signal and producing two signals that add to form a representation of the tapped signal,

photodetectors coupled to the tunable filter, said photodetectors being configured to capture each of said signals output from said tunable filter and to produce two electrical signals that represent the power intensity of each of said two signals output from the tunable filter, and

an electronics unit configured to accept the two electrical signals produced by the photodetectors and to generate a feedback signal in response thereto.

2. The wavelength locker system of claim 1, wherein the photodetector means are integrated onto the tunable filter.

3. The wavelength locker system of claim 1, wherein the tunable filter is a liquid crystal tunable filter.

4. The wavelength locker of claim 3, wherein the tunable filter is a bandpass filter and where the two signals produced thereby include a reflected signal and a transmitted signal.

5. The wavelength locker of claim 3, wherein the liquid crystal tunable filter is a tunable etalon.

6. The wavelength locker of claim 1, wherein the electronics unit is a microcontroller.

7. The wavelength locker of claim 6, wherein the microcontroller also monitors the thermal environment of the tunable filter.

8. The wavelength locker of claim 7, wherein the microcontroller controls tuning of the tunable filter.

9. The wavelength locker of claim 1, wherein the tunable filter is tuned to an offset of a target lasing frequency at a 50% power transmission.

10. A wavelength locker system, comprising;

an optical tap placed in the optical path of a laser transmitter for splitting a laser signal into a tapped signal and laser output signal,

a tunable filter accepting the tapped signal and producing two electrical signals that add to form a representation of the tapped signal, said filter tuned at an offset from a target lasing frequency at a point in which the two electrical signals output from the tunable filter intersect to form a lock point, and

an electronics unit configured to generate a feedback signal in response to the two electrical signals output from the liquid crystal.

11. The wavelength locker of claim 10, wherein the feedback signal is coupled to the laser to form a feedback loop.

12. The wavelength locker of claim 10, wherein the feedback signal describes the direction and magnitude difference between the lock point and laser output signal.

13. The wavelength locker of claim 10, wherein the tunable filter is a liquid crystal device.

14. The wavelength locker of claim 13, wherein the liquid crystal device includes an integrated temperature sensor.

15. The wavelength locker of claim 13, wherein the liquid crystal device includes a metal seal.

16. The wavelength locker of claim 13, wherein the liquid crystal device includes an epoxy seal.

17. The wavelength locker of claim 15, wherein the metal gasket surrounds an aperture of the liquid crystal device.

18. The wavelength locker of claim 10, wherein the liquid crystal device includes a control system for maintaining the device operation at various temperatures.

19. A method of wavelength locking, including the steps of:

a. Providing a tunable filter that produces reflected and complimentary transmitted signal outputs,

b. Reading the temperature of the tunable filter,

c. Adjusting the tunable filter,

d. Measuring the outputs of the tunable filter, and

e. Generating a wavelength locker feedback signal response to the information collected in step d.

20. The method of claim 19, wherein step e includes a division operation.