Optical system and method

• US 6,654,516 B2
• Filed: 08/20/2002
• Issued: 11/25/2003
• Est. Priority Date: 08/20/2001
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: So, John Ling Wing
• Primary Examiner(s): ULLAH, AKM E
• Application Number:
  US10/224,202
• Publication Number:
  US 20030081893A1
• Time in Patent Office: 462 Days
• Field of Search: 385/1, 385/4, 385/11, 385/12, 385/13, 356/477, 356/73.1, 359/337.5, 372/22
• US Class Current: 385/27
• CPC Class Codes: G02B 6/2706 as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
• CPC Class Codes: G02B 6/278 Controlling polarisation mode dispersion [PMD], e.g. PMD compensation or emulation PMD minimised transmission systems H04B10/2569
• CPC Class Codes: G02B 6/2793 Controlling polarisation dependent loss, e.g. polarisation insensitivity, reducing the change in polarisation degree of the output light even if the input polarisation state fluctuates
• CPC Class Codes: G02B 6/2931 Diffractive element operating in reflection
• CPC Class Codes: G02B 6/29313 characterised by means for controlling the position or direction of light incident to or leaving the diffractive element, e.g. for varying the wavelength response switching G02B6/35
• CPC Class Codes: G02B 6/29391 Power equalisation of different channels, e.g. power flattening
• CPC Class Codes: G02B 6/29394 Compensating wavelength dispersion G02B6/29376 takes precedence; dispersion compensated optical transmission systems H04B10/2507
• CPC Class Codes: G02B 6/29395 configurable, e.g. tunable or reconfigurable switching G02B6/35
• CPC Class Codes: G02B 6/3512 the optical element being reflective, e.g. mirror
• CPC Class Codes: G02B 6/356 in an optical cross-connect device, e.g. routing and switching aspects of interconnecting different paths propagating different wavelengths to (re)configure the various input and output links switching polarized beams G02B6/3594; power equalizers G02B6/356 and G02B6/3594; path selecting means H04Q11/0001
• CPC Class Codes: G02B 6/3588 of the processed beams, i.e. controlling during switching of orientation, alignment, or beam propagation properties such as intensity, size or shape
• CPC Class Codes: H04B 10/2941 using an equalising unit, e.g. a filter H04B10/296 takes precedence
• CPC Class Codes: H04J 14/0221 Power control, e.g. to keep the total optical power constant
• CPC Class Codes: H04J 14/02216 by gain equalization

Abstract

Optical network dispersion compensation with adaptive dynamic optical filters which relate magnitude and phase of multichannel optical signals.

Claims

1. A method of optical link dispersion compensation, comprising:

(a) providing a relative phase measurement for an optical link, said phase measurement determined for each wavelength of a set of wavelengths;

(b) using the results of step (a) in computing a first transfer function phase for each of said wavelengths, said first transfer function corresponding to said optical link;

(c) using the results of step (b) in computing an amplitude for said first transfer function; and

(d) applying an optical filter to said optical link, said optical filter with a second transfer function, said second transfer function corresponding to said first transfer function.

2. The method of claim 1, wherein:

(a) said optical filter includes a dispersive element, a plurality of attenuation elements, and a recombining element;

(b) whereby an input signal including said set of wavelengths may be split by said dispersive element into separate signals, and each of said separate signals may be attenuated by a corresponding one of said plurality of attenuation elements, and these attenuated separate signals may be recombined into an output signal by said recombining element.

3. The method of claim 1, wherein:

(a) said providing a phase measurement of step (a) of claim 1 includes detecting a relative group delay for each of said wavelengths.

4. The method of claim 1, wherein:

(a) said set of wavelengths are cover wavelengths in a multichannel optical network.

7. The method of claim 5, wherein:

(a) said providing a phase measurement of step (a) of claim 1 includes detecting a relative group delay for each of said wavelengths.

12. The procedure of claim 11, further comprising:

(a) doubling the size of the transform;

(b) inserting zero frequencies in the middle of the transform spectrum to expand the size of the transform; and

(c) repeating procedure of claim 1 for recovering phase, group delay and chromatic dispersion parameters of a system transfer function.

5. A method of adaptive optical network dispersion compensation, comprising:

(a) providing a relative phase measurement for each wavelength of a set of wavelengths used in an optical network;

(b) using the results of step (a) in computing a first transfer function phase for each of said wavelengths;

(c) using the results of step (b) in computing an amplitude for said first transfer function;

(d) applying an optical filter in said optical network, said optical filter with a second transfer function, said second transfer function corresponding to said first transfer function; and

(e) repeating steps (a)-(d) to adapt to optical network conditions.

6. The method of claim 5, wherein:

(a) said optical filter includes a dispersive element, a plurality of attenuation elements, and a recombining element;

(b) whereby an input signal including said set of wavelengths may be split by said dispersive element into separate signals, and each of said separate signals may be attenuated by a corresponding one of said plurality of attenuation elements, and these attenuated separate signals may be recombined into an output signal by said recombining element.

8. A method of optical filter synthesis, comprising:

(a) providing an amplitude for each of a plurality of wavelengths;

(b) using the results of step (a) in computing a transfer function with magnitude corresponding to said amplitude for said plurality of wavelengths, said computing including complex cepstrum analysis of said magnitude.

9. The method of claim 8, further comprising:

(a) zero-padding said magnitude prior to said complex cepstrum analysis.

10. The method of claim 9, further comprising:

(a) windowing said complex cepstrum prior to discrete Fourier transforming.

11. A procedure for measuring the chromatic dispersion parameter of a multi-wavelength system transfer function with at least one wavelength from the group delay parameter that is extracted from the phase parameter recovered from measured Intensity data, comprising the following steps:

(a) measure intensity data to generate magnitude data, said intensity data can be spectral reflectance or transmittance data;

(b) compute logarithmic magnitude data;

(c) perform Inverse Fourier Transform on logarithmic magnitude data;

(d) generate complex cepstrum signal using even function signal property;

(e) optionally perform windowing on the complex cepstrum signal;

(f) reconstruct logarithmic system transfer function by performing a Fourier Transform on (optionally windowed) complex cepstrum signal;

(g) recover magnitude of system transfer function from the Real Part of the Fourier Transform results using the anti-log or exponent function;

(h) recover phase of system transfer function from the Imaginary Part of the Fourier Transform results;

(i) compute the group delay parameter from the recover phase parameter; and

(j) compute the chromatic dispersion parameter from the group delay parameter.

13. The procedure of claim 12, further comprising a methodology of designing the magnitude function of a system transfer function to meet a given specification of a multi-wavelength system transfer function with at least one wavelength such that the design error falls within a given tolerance band, e(w)=|H(w)|−

|H′

(w)|, where H(w) is the ideal system transfer function and H′

(w) is the designed or approximation of the ideal transfer function, by iterating the said steps in claims 11 and 12, the magnitude function of a desired system transfer function can be approximated as closely as possible in order to meet a given tolerance band.

14. A procedure for specifying the magnitude function of a system transfer function of an optical component for a multi-wavelength system with at least one wavelength given the Chromatic Dispersion parameter of a system transfer function or the Group Delay parameter or the Phase parameter, said procedure for a given or desired Group Delay comprising the following steps to determine both log|H(k)| and φ

(k) given τ

(k);

(a) compute τ

(k) given the desired group delay τ

_d(k);

(b) compute pseudo-Cepstral coefficients p(n) via an N-point IDFT of τ

(k);

(c) compute ĥ

_p(n) from p(n) as follows;

${\stackrel{⋒<br><br>}{h}}_p<br><br>\left(n\right)=[\begin{array}{cc}\frac{p<br><br>\left(n\right)}{n},& 1\le <br><br>n<<br><br>\frac{N}{2}\\ 0,& n=0,\frac{N}{2}\\ -{\stackrel{⋒<br><br>}{h}}_p<br><br>\left(N-n\right),& \frac{N}{2}<<br><br>n\le <br><br>N-1;<br><br>\end{array}$

(d) compute Ĥ

(k) via DFT of ĥ

_p(n), the imaginary part of Ĥ

(k) is the unwrapped phase spectrum φ

(k) of the recovered transfer function;

(e) compute log|H(k)| by forming a sequence g(n)=sgn(n).ĥ

(n) for 0≦

n<

N and taking the DFT of g(n);

(f) compute the recovered magnitude function |H(k)| by taking exp[log|H(k)|] where log is the natural logarithm function;

(g) finding the recovered transfer function of the optical filter as;