Higher rate channels are making their way into networks initially designed for 10 Gbps. These higher rate services of 40 Giga bits per seconds (Gbps) and someday 100 Gbps have large bandwidths and, as such, are much more susceptible to inter-channel crosstalk effects.
Filtering techniques may be used to overcome the undesired crosstalk. However, insertion of a device such as an optical filter may result in a decrease of the transmitted signal power, which in telecommunication is referred to as “insertion loss.”
Optical tunable filters may be used to overcome undesired crosstalk effects. Unfortunately, tunable filters do not maintain their central wavelength well over a long period of time or a wide temperature range. Tunable filter shift off the channel center results in significant signal power degradation (i.e., insertion loss) through off-center filtering or complete signal extinction. Given their wavelength instability, tunable filters have not been commonly used to deal with crosstalk effects.
A method or corresponding apparatus in an example embodiment of the present invention employs a tunable filter to shape a channel signal within a dense wavelength division multiplexing (DWDM) signal. An input optical power detector measures a modulated source channel signal at an input of the tunable filter. An output optical power detector measures a filtered modulated source channel signal at an output of the tunable filter. A controller is configured to adjust a center wavelength of the tunable filter as a function of a difference between measurements of optical power by the input optical power detector and the output optical power detector.
Another example embodiment of the present invention measures an optical power of a modulated source channel signal. The example embodiment filters the modulated source channel signal to generate a filtered modulated source channel signal and measures an optical power of the filtered modulated source channel signal. A center wavelength used in the filtering to filter the modulated source channel signal is adjusted as a function of a difference between measurements of optical powers.
Yet another example embodiment of the present invention shapes a channel signal within a dense wavelength division multiplexing (DWDM) signal using a tunable filter. A method or corresponding apparatus in this example embodiment adjusts a center wavelength of the tunable filter as a function of a difference between measurements of optical powers of a modulated source channel signal input to the tunable filter and a filtered modulated source channel signal output by the tunable filter.
Another example embodiment of the present invention adjusts a center bandwidth of a tunable filter. A method or corresponding apparatus in this example embodiment filters a modulated source channel signal to produce a filtered modulated source channel signal. The example embodiment measures an optical power of the modulated source channel signal and the filtered modulated source channel signal. The example embodiment adjusts the center bandwidth of the tunable filter as a function of a difference between measurements of the optical power of the modulated source channel signal and the filtered modulated source channel signal.
Yet another example embodiment of the present invention shapes a channel signal within a dense wavelength division multiplexing (DWDM) signal. The example embodiment adjusts a center wavelength of a tunable filter as a function of a pilot tone control of a modulated source channel signal and a filtered modulated source channel signal.
A description of example embodiments of the invention follows.
In order to compensate for the insertion loss, a module for reshaping the channel signal 140 according to an example embodiment of the present invention is employed to reduce or minimize the signal insertion loss. The insertion loss is minimal when the modulated source channel signal 130 and the tunable filter 150 are perfectly aligned (i.e., the modulated source channel signal 130 and the tunable filter 150 have the same center wavelength). The example embodiment 100 monitors the modulated source channel signal 130 before entering and after exiting the tunable filter 150, and continually or continuously adjusts the tunable filter 150 to ensure that the modulated source channel signal 130 and the tunable filter 150 are perfectly aligned.
In the view of the foregoing, the following description illustrates example embodiments and features that may be incorporated into a system for shaping a channel signal, where the term “system” may be interpreted as a system, subsystem, device, apparatus, method, or any combination thereof.
An example embodiment of the present invention relates to shaping a channel signal within a dense wavelength division multiplexing (DWDM) signal. A method or corresponding apparatus in this example embodiment employs a tunable filter. An input optical power detector measures a modulated source channel signal at an input of the tunable filter. An optical power detector measures a filtered modulated source channel signal at an output of the tunable filter. A controller is configured to adjust a center wavelength of the tunable filter as a function of a difference between measurements of optical power by the optical power detectors.
The modulated source channel signal may be a laser modulated source channel signal.
The system may filter the modulated source channel signal at a bandwidth narrower than a bandwidth of the modulated source channel signal. The system may adjust a bandwidth for filtering to have a narrower bandwidth than the modulated source channel signal to minimize crosstalk between the modulated source channel signal and a neighboring DWDM signal. The modulated source channel signal may be filtered using Gaussian filtering.
The system may adjust the center wavelength used for filtering to reduce optical wavelength drift. The center wavelength used for the filtering may be adjusted as a function of minimizing the difference between measurements of optical power by the optical power detectors. The center wavelength used for the filtering may also be adjusted as a function of maximizing the measurements of optical power by the output optical power detector. The system may apply dither control to the filtering and adjust the center wavelength used for the filtering as a function of the dither control.
The system may obtain the measurements of the optical power during predetermined time intervals. The system may measure at least one signal modulation parameter selected from a group consisting of amplitude modulation, frequency modulation, and phase modulation.
Another example embodiment of the present invention measures an optical power of a modulated source channel signal. The example embodiment filters the modulated source channel signal to generate a filtered modulated source channel signal, and measures an optical power of the filtered modulated source channel signal. A center wavelength used for the filtering of the modulated source channel signal is adjusted as a function of a difference between measurements of optical powers.
The modulated source channel signal may be a laser modulated source channel signal.
The system may filter the modulated source channel signal at a bandwidth narrower than a bandwidth of the modulated source channel signal. The system may adjust a bandwidth for filtering to have a narrower bandwidth than the modulated source channel signal to minimize crosstalk between the modulated source channel signal and a neighboring DWDM signal. The modulated source channel signal may be filtered using Gaussian filtering.
The system may obtain the measurements of the optical power during predetermined time intervals. The system may measure at least one signal modulation parameter selected from a group consisting of amplitude modulation, frequency modulation, and phase modulation.
The system may adjust the center wavelength used for filtering to reduce optical wavelength drift. The center wavelength used for the filtering may be adjusted as a function of minimizing the difference between measurements of optical power by the optical power detectors. The center wavelength used for the filtering may also be adjusted as a function of maximizing the measurements of optical power by the output optical power detector. The system may apply dither control to the filtering and adjust the center wavelength used for the filtering as a function of the dither control.
Yet another example embodiment of the present invention relates to a tunable filter for shaping a channel signal within a dense wavelength division multiplexing (DWDM) signal. A method or corresponding apparatus in this example embodiment adjust a center wavelength of the tunable filter as a function of a difference between measurements of optical powers of a modulated source channel signal input to the tunable filter and a filtered modulated source channel signal output by the tunable filter.
Another example embodiment of the present invention relates to adjusting a center bandwidth of a tunable filter. A method or corresponding apparatus in this example embodiment filters a modulated source channel signal to produce a filtered modulated source channel signal. The example embodiment measures an optical power of the modulated source channel signal and the filtered modulated source channel signal, and adjusts the center bandwidth of the tunable filter as a function of a difference between measurements of the optical power of the modulated source channel signal and the filtered modulated source channel signal.
Yet another example embodiment of the present invention relates to shaping a channel signal within a DWDM signal. The example embodiment adjusts a center wavelength of the tunable filter as a function of a pilot tone control of a modulated source channel signal and a filtered modulated source channel signal.
A pilot tone is typically a slow modulation of the modulated source signal by one or more of the following: amplitude modulation, frequency modulation, or phase modulation. The center wavelength used for filtering may be adjusted as a function of minimizing the difference between the pilot tone modulation parameters of the modulated source channel and the pilot tone modulation parameters of the filtered modulated source channel.
In this example embodiment, an optical source node 210 (i.e., optical transmitter node) transmits a modulated source channel optical signal 230 (e.g., 40 Gbps modulated optical signal) through a transmission path 220, which may include an optical fiber. The optical signal 230 propagates through the fiber 220.
The module for reshaping channel signal 240 of this example embodiment 200 may include an input optical power detector 260 that measures an optical power of the input modulated source channel signal 230 into the tunable filter. Similarly, an output optical power detector 270 may measure the optical power of the filtered modulated source channel signal 255 exiting the tunable filter 250. In order to minimize the insertion loss caused by the tunable filter 250, the example embodiment 200 may employ a controller 280 that measures the difference between the optical power measurements 265, 275 of the modulated source channel signal 230 and the filtered modulated source channel signal 255, and continuously adjusts a center bandwidth of the tunable filter 250 to minimize the difference between the measurements of the optical powers 265, 275. The controller 280 may also adjust the center bandwidth of the tunable filter 250 by maximizing the measurement 275 of the optical power of the filtered modulated source channel signal 255. The controller may adjust the center bandwidth of the tunable filter 250 both to minimize the difference between the measurements of the optical powers and to maximize the optical power of the filtered modulated source channel signal 255.
The example embodiment 200 may employ one or more photodiodes to implement the input optical power detector 260 and/or the output optical power detector 270. By employing more than one photodiode, the example embodiment 200 eliminates any errors or unnecessary tuning that may be associated with any regular signal power fluctuations.
In order to overcome large crosstalk effects caused by the large bandwidth of 40 Gbps signals, the tunable filter 250 may be configured have a narrower bandwidth than the bandwidth of the modulated source channel signal. By narrowing the bandwidth of the source channel signal, the example embodiment 200 may overcome the crosstalk effects. By employing a tunable filter, the example embodiment may achieve as much as 3 Decibels (i.e., 3 dB) of improvement in the quality of the modulated source signal.
The tunable filter 250 may be configured as a Gaussian-shaped filter. When employing a Gaussian-shaped filter, the example embodiment may employ an optical differential phase-shift keying (DPSK) modulated signal. The example embodiment may employ a tunable filter 250 implemented with other filter types or shapes. The type or shape of the tunable filter may vary based on the application used. The tunable filter employed by the example embodiment 200 may be configured to be a colorless filter.
An input power detector 360 (e.g., an input tap photodiode) measures the power of the incoming modulated source signal 330. An analog-to-digital converter (A/D) 381 may be used to convert the measurements of power 365 by the input power detector 360 from analog to digital so they can be used by the controller 380.
Similarly, an output power detector 370 (e.g., an output tap photodiode) measures the power of the filtered modulated source signal 330. An analog to digital converter 381 may be used to convert the measurements of power 365 by the output power detector 370 from analog to digital so the measurements can be used by the controller 380.
The example embodiment 300 may obtain the power measurements during predetermined time intervals.
A controller 380, which may include a microcontroller, a processor, and/or a microprocessor, receives the measurements of power 365, 375 from the input power detector and the output power detector, and generates a control signal 385. The example embodiment 300 employs the control signal 385 to adjust a center wavelength of the tunable filter 350. Additionally, the example embodiment may employ the control signal 385 to adjust a bandwidth of the tunable filter 350.
The controller 380 may generate the control signal based on minimizing the difference between the measurements of the power 365 of the modulated source channel signal 330 and the power 375 of the filtered modulated source channel signal 355. The example embodiment may generate the control signal based on maximizing the power 375 of the filtered modulated source channel signal 355.
The controller 380 may employ dither control to adjust the center bandwidth of the tunable filter 350. To employ dither control, a dither signal having a dither frequency much lower (i.e., slower) than the frequency range of the tunable filter 350 is applied to the system to cause small deviations in performance that the controller 380 detects and then corrects. Alternatively, the dither control may be employed using amplitude modulation, where the dither amplitude modulation applied is much smaller than the amplitude sensitivity of the tunable filter 350 to cause small deviations in performance that the controller 380 detects and corrects.
The dither control may be employed using phase or frequency modulation, where the dither phase or frequency modulation applied is much smaller than the phase or frequency sensitivity of the tunable filter 350 to cause small deviations in performance that the controller 380 detects and corrects.
The example embodiment 300 may apply the control signal 385 to the tunable filter 350 to adjust a center wavelength of the tunable filter to control insertion loss. The example embodiment may also apply the control signal 385 to the tunable filter 350 to adjust the bandwidth of the tunable filter 350.
The example embodiment may monitor the power measurements during predetermined time intervals. For instance, the example embodiment may monitor the insertion loss 410 in one hour long time intervals to find the minimum insertion loss 440 and the center wavelength 420 corresponding to the minimum insertion loss 440. Once a minimum point 460 is obtained, the example embodiment may continue to monitor the power measurements during predetermined time intervals. For example, the example embodiment 400 may search locations positioned at 20 Pico-meters (or any other predetermined distance) to the left and 20 Pico-meters (or any other predetermined distance) to the right of the previously determined minimum 460 to look for a new minimum insertion loss. If a minimum insertion loss is not obtained the example embodiment may continue to search until it obtains a minimum insertion loss.
In
The values on the vertical axis 441 demonstrate the insertion loss 410 (
The tunable filter used to generate the data used in creating the curve 441 in the plot 401 of
The example embodiment 500 measures an optical power 565 of a modulated source channel signal 530, and filters the modulated source channel signal 530 to produce a filtered modulated source channel signal 555. The example embodiment 500 also measures the optical power 575 of the filtered modulated source channel signal 555. Using the measured optical powers 555, 575, the example embodiment determines the difference 585 between these measurements 530, 555. A center bandwidth of the filter 550 is adjusted as a function of the difference between the measurements of optical powers 530, 555.
In order to minimize cross talk between the modulated source channel signal and a neighboring dense wavelength division multiplexing (DWDM) signal, the example embodiment may adjust the center wavelength of the filter to have a bandwidth narrower than the modulated source channel signal.
The example embodiment 600 includes an adjustment module 695 that adjusts a center wavelength of a tunable filter 650 as a function of a difference 670 between the power measurements 665, 675 of the modulated source channel signal 630 input to the tunable filter 650, and the filtered modulated source channel signal 655 output by the tunable filter 650.
The example embodiment 700 employs a filtering module 750 to filter a modulated source channel signal 730 to produce a filtered modulated source channel signal 755. The example embodiment 700 employs measurement module 701 to measure optical powers 765, 775 of the modulated source channel signal and the filtered modulated source channel signal. The example embodiment also obtains the difference between the optical power of the modulated source channel signal and the filtered modulated source channel signal 770. The example embodiment also includes an adjustment module 795 that adjusts the center bandwidth of the tunable filter based on the difference 770 between measurements 765, 775 of the optical power of the modulated source channel signal and the filtered modulated source channel signal.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.