Method of characterizing fiber bragg gratings using iterative processing

• US 20070025432A1
• Filed: 05/16/2005
• Published: 02/01/2007
• Est. Priority Date: 05/15/2004
• Status: Active Grant

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford Management Co.)
• Inventors: Ozcan, Aydogan, Kino, Gordon, Digonnet, Michel
• Granted Patent: US 7,385,683 B2
• Time in Patent Office: Days
• US Class Current: 375/224
• CPC Class Codes: G01M 11/3145 Details of the optoelectronics or data analysis
• CPC Class Codes: G01M 11/3172 Reflectometers detecting the back-scattered light in the frequency-domain, e.g. OFDR, FMCW, heterodyne detection
• CPC Class Codes: G02B 2006/02166 Methods of designing the gratings, i.e. calculating the structure, e.g. algorithms, numerical methods
• CPC Class Codes: G02B 6/0208 characterised by their structure, wavelength response G02B6/02114, G02B6/02171, G02B6/02195 take precedence; multiple layer cores or claddings G02B6/036, protective coverings G02B6/4429

Abstract

A method determines a complex reflection impulse response of a fiber Bragg grating. The method includes providing a measured amplitude of a complex reflection spectrum of the fiber Bragg grating. The method further includes providing an estimated phase term of the complex reflection spectrum. The method further includes multiplying the measured amplitude and the estimated phase term to generate an estimated complex reflection spectrum. The method further includes calculating an inverse Fourier transform of the estimated complex reflection spectrum, wherein the inverse Fourier transform is a function of time. The method further includes calculating an estimated complex reflection impulse response by applying at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum.

Claims

1. A method of determining a complex reflection impulse response of a fiber Bragg grating, the method comprising:

(a) providing a measured amplitude of a complex reflection spectrum of the fiber Bragg grating;

(b) providing an estimated phase term of the complex reflection spectrum;

(c) multiplying the measured amplitude and the estimated phase term to generate an estimated complex reflection spectrum;

(d) calculating an inverse Fourier transform of the estimated complex reflection spectrum, wherein the inverse Fourier transform is a function of time; and

(e) calculating an estimated complex reflection impulse response by applying at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum.

2. The method of claim 1, further comprising:

(f) calculating a Fourier transform of the estimated complex reflection impulse response; and

(g) calculating a calculated phase term of the Fourier transform of the estimated complex reflection impulse response.

3. The method of claim 2, wherein calculating the calculated phase term of the Fourier transform comprises using a Hilbert transformation of the Fourier transform of the estimated complex reflection impulse response.

4. The method of claim 2, further comprising:

(h) using the calculated phase term of (g) as the estimated phase term of (c) and repeating (c)-(e).

5. The method of claim 4, wherein (c)-(h) are iteratively repeated until the estimated complex reflection impulse response reaches convergence.

6. The method of claim 5, wherein convergence is reached when an average difference between estimated complex reflection impulse responses obtained after two consecutive iterations is less than a predetermined value.

7. The method of claim 6, wherein the predetermined value is 0.1% of the estimated complex reflection impulse response of an iteration.

8. The method of claim 4, wherein (c)-(h) are iteratively repeated a predetermined number of times.

9. The method of claim 1, wherein providing the measured reflection spectrum amplitude comprises measuring a reflection power spectrum and taking the square root of the reflection power spectrum.

10. The method of claim 1, wherein providing the measured reflection spectrum amplitude comprises providing a previously-measured reflection spectrum amplitude.

11. The method of claim 1, wherein providing the estimated phase term of the complex reflection spectrum comprises providing an initial estimated phase term equal to a real or complex constant.

12. The method of claim 1, wherein providing the estimated phase term of the complex reflection spectrum comprises providing an initial estimated phase term equal to a measured phase term of the complex reflection spectrum of a second fiber Bragg grating substantially similar to the fiber Bragg grating from which the measured amplitude of the complex reflection spectrum is measured.

13. The method of claim 1, wherein applying the at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum comprises setting the inverse Fourier transform to zero for times less than zero.

14. The method of claim 1, wherein the complex reflection impulse response has a known temporal duration and applying the at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum comprises setting the inverse Fourier transform to zero for times greater than the known temporal duration of the complex reflection impulse response.

15. The method of claim 1, wherein the fiber Bragg grating has a known bandwidth and applying the at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum comprises adjusting the inverse Fourier transform to provide the known bandwidth.

16. The method of claim 1, wherein the reflection impulse response is a real function and applying the at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum comprises using only the real portion of the inverse Fourier transform.

17. A computer-readable medium having instructions stored thereon which cause a general-purpose computer to perform the method of claim 1.

18. A computer system comprising:

means for estimating an estimated phase term of a complex reflection spectrum of a fiber Bragg grating;

means for multiplying a measured amplitude of the complex reflection spectrum of the fiber Bragg grating and the estimated phase term to generate an estimated complex reflection spectrum;

means for calculating an inverse Fourier transform of the estimated complex reflection spectrum, wherein the inverse Fourier transform is a function of time; and

means for calculating an estimated complex reflection impulse response by applying at least one constraint to the inverse Fourier transform of the estimated complex reflection spectrum.

19. A method of determining a complex transmission impulse response of a fiber Bragg grating, the method comprising:

(a) providing a measured amplitude of a complex transmission spectrum of the fiber Bragg grating;

(b) providing an estimated phase term of the complex transmission spectrum;

(c) multiplying the measured amplitude and the estimated phase term to generate an estimated complex transmission spectrum;

(d) calculating an inverse Fourier transform of the estimated complex transmission spectrum, wherein the inverse Fourier transform is a function of time; and

(e) calculating an estimated complex transmission impulse response by applying at least one constraint to the inverse Fourier transform of the estimated complex transmission spectrum.

20-32. -32. (canceled)

33. A method of characterizing a fiber Bragg grating, the method comprising:

(a) providing a measured amplitude of a Fourier transform of a complex electric field envelope of an impulse response of the fiber Bragg grating;

(b) providing an estimated phase term of the Fourier transform of the complex electric field envelope;

(c) multiplying the measured amplitude and the estimated phase term to generate an estimated Fourier transform of the complex electric field envelope;

(d) calculating an inverse Fourier transform of the estimated Fourier transform of the complex electric field envelope, wherein the inverse Fourier transform is a function of time; and

(e) calculating an estimated electric field envelope of the impulse response by applying at least one constraint to the inverse Fourier transform of the estimated Fourier transform of the complex electric field envelope.

34-45. -45. (canceled)

46. An apparatus for characterizing at least one fiber Bragg grating, the apparatus comprising:

a laser pulse source, the laser pulse source generating at least one input laser pulse;

an optical spectrum analyzer;

a first optical path optically coupled to the laser pulse source, the first optical path comprising a pulse stretcher and an attenuator, wherein a first portion of the input laser pulse propagates from the laser pulse source and is stretched by the pulse stretcher and is attenuated by the attenuator;

a second optical path optically coupled to the first optical path and comprising a mirror, wherein a first portion of the stretched and attenuated laser pulse from the first optical path is reflected from the mirror;

a third optical path optically coupled to the first optical path and comprising a first fiber Bragg grating, wherein a second portion of the stretched and attenuated laser pulse from the first optical path is reflected from the first fiber Bragg grating;

a fourth optical path optically coupled to the second optical path, the third optical path, and the optical spectrum analyzer, wherein the reflected pulse from the mirror and the reflected pulse from the first fiber Bragg grating propagate to the optical spectrum analyzer; and

a fifth optical path optically coupled to the laser pulse source and the optical spectrum analyzer, the fifth optical path comprising a delay line, wherein a second portion of the input laser pulse propagates from the laser pulse source along the fifth optical path to the optical spectrum analyzer.

47. The apparatus of claim 46, further comprising a sixth optical path optically coupled to the first optical path and the fourth optical path, the sixth optical path comprising a second fiber Bragg grating, wherein a third portion of the stretched and attenuated laser pulse from the first optical path is reflected from the second fiber Bragg grating, the reflected pulse from the second fiber Bragg grating propagating to the optical spectrum analyzer.

48. The apparatus of claim 46, wherein the laser pulse source comprises a mode-locked laser with a temporal width between approximately 2 picoseconds and approximately 4 picoseconds.

49. The apparatus of claim 46, wherein the optical spectrum analyzer has a resolution less than or equal to 10 picometers.

50. The apparatus of claim 46, wherein the pulse stretcher comprises a single-mode fiber optic cable which broadens the temporal width of a pulse propagating therethrough by a factor of approximately 2 to 5.

51. The apparatus of claim 46, wherein the attenuator is greater than approximately 20 dB.

52. The apparatus of claim 46, wherein the mirror comprises a bare fiber end.

53. The apparatus of claim 46, wherein the mirror comprises a mirrored fiber end.