Wavelocker for Improving Laser Wavelength Accuracy in WDM Networks
The present invention includes novel techniques, apparatus, and systems for optical WDM communications. Various wavelocker apparatus and methods are disclosed that measure the frequency offsets between signal lasers and reference lasers. The measured offsets are used to adjust the signal laser frequencies to meet their target frequencies. The absolute accuracy of the reference laser frequency is improved by measuring the absorption of the reference laser by a gas cell with known fixed absorption lines versus the reference laser frequency. Apparatus and methods are disclosed to cover scenarios in which the reference laser polarization is aligned with the signal lasers, as well as those in which the reference laser polarization is not aligned with the signal lasers. The wavelocker apparatus may or may not be located at the same network site as the signal lasers.
1 . A method for improving the accuracy of WDM laser frequencies, the method comprising the following steps:
(a) defining a desired target frequency to which a signal laser is to be tuned, within a predefined acceptable range of system tolerance;
(b) tuning a signal laser to an initial frequency which may or may not fall within the predefined acceptable range of the target frequency;
(c) beating the signal laser with a reference laser by combining and transmitting onto an optical detector, the light emitted by the signal laser at the initial frequency with the light emitted by the reference laser at a reference frequency, thereby creating a beat signal having a beat frequency equal to the difference between the signal laser frequency and the reference laser frequency;
(d) scanning the reference laser across a range of frequencies offset from the target frequency by predetermined amounts, and measuring the amplitude of the beat signal at a plurality of points within the range;
(e) processing the measured amplitudes of the beat signal to determine the actual frequency of the signal laser, and calculating the difference between the actual frequency and the target frequency;
(f) adjusting the frequency of the signal laser to account for the calculated difference; and
(g) repeating steps (c), (d), (e) and (f) to keep the signal laser within the predefined acceptable range of the target frequency.
2 . The method of claim 1 wherein the frequency of the reference laser is calibrated by measuring the absorption of the reference laser light through a gas cell with known absorption peaks at precise absolute frequencies.
3 . The method of claim 1 wherein the light emitted by the reference laser and the light emitted by the signal laser are linearly polarized along the same axis with polarization-maintaining optics.
4 . The method of claim 3 wherein a polarization controller is used to align the polarization of the light emitted by the reference laser with the polarization of the light emitted by the signal laser.
5 . The method of claim 1 wherein the polarization of the signal laser and the polarization of the reference laser are not constrained to be aligned, and the polarization of either the signal laser or reference laser is scrambled by a polarization scrambler.
6 . The method of claim 5 wherein the beat signal is split by a polarizing beam splitter into two orthogonally polarized signals that are detected by two separate optical detectors, and wherein processing circuitry extracts a corresponding beat signal.
7 . The method of claim 1 wherein the desired target frequency is a subchannel frequency offset from an ITU channel frequency.
8 . The method of claim 1 wherein the reference laser is used to monitor and control the frequencies of a plurality of signal lasers.
9 . The method of claim 8 wherein the reference laser is at a different location from at least one of the plurality of signal lasers, and wherein the reference laser communicates with any such signal laser via a network overhead channel.
10 . A system for improving the accuracy of WDM laser frequencies, the system comprising:
(a) a signal laser to be tuned to a desired target frequency, within a predefined acceptable range of system tolerance, wherein the signal laser is tuned to an initial frequency which may or may not fall within the predefined acceptable range of the target frequency;
(b) a coupler that can combine, and transmit onto an optical detector, the light emitted by the signal laser at the initial frequency with the light emitted by a reference laser at a reference frequency, thereby creating a beat signal having a beat frequency equal to the difference between the signal laser frequency and the reference laser frequency; and
(c) control circuitry that can:
(i) scan the reference laser across a range of frequencies offset from the target frequency by predetermined amounts;
(ii) measure the amplitude of the beat signal at a plurality of points within the range;
(iii) process the measured amplitudes of the beat signal to determine the actual frequency of the signal laser;
(iv) calculate the difference between the actual frequency and the target frequency;
(v) adjust the frequency of the signal laser to account for the calculated difference; and
(vi) repeat steps (i)-(v) to keep the signal laser within the predefined acceptable range of the target frequency.
11 . The system of claim 10 wherein a coupler directs a fraction of the reference laser light to pass through a gas cell, the gas cell having known absorption peaks at precise absolute frequencies, and wherein an optical detector measures the absorption of the reference laser light by the gas in the gas cell.
12 . The system of claim 10 wherein a polarization-maintaining coupler and polarization-maintaining fibers are used to keep the polarization of the light from the reference laser aligned with the polarization of the light from the signal laser.
13 . The system of claim 10 wherein a polarization controller with feedback circuitry is used to align the polarization of the light emitted by the reference laser with the polarization of the light emitted by the signal laser.
14 . The system of claim 10 wherein the polarization of either the reference laser or the signal laser is scrambled by a polarization scrambler.
15 . The system of claim 14 wherein the polarization of the light from the reference laser is linear, the polarization of the light from the signal laser changes randomly, and the beat signal is split by a polarizing beam splitter into two orthogonally polarized signals that are detected by two separate optical detectors, and wherein processing circuitry extracts a corresponding beat signal.
16 . The system of claim 10 wherein the desired target frequency is a subchannel frequency offset from an ITU channel frequency.
17 . The system of claim 10 wherein the reference laser is used to monitor and control the frequencies of a plurality of signal lasers.
18 . The system of claim 17 wherein the reference laser is at a different location from at least one of the plurality of signal lasers, and wherein the reference laser communicates with any such signal laser via a network overhead channel.