US 20100232789 A1
An optical device includes an interferometer for a received optical differential phase shift keying DPSK signal, and an equalizer integrated with the interferometer in a manner for reducing from optical filtering effects an interference by signal bits of the DPSK signal with signal bits of a contiguous DPSK signal. The interferometer is a Michelson delay interferometer type, but can also be a Mach-Zehnder delay interferometer type on fiber, waveguide or other optical structure. The equalizer is a Fabry-Perot type equalizer, but can be a ring resonator type or a fiber based equalizer.
1. An optical device comprising:
an interferometer for a received optical differential phase shift keying DPSK signal, and
an equalizer integrated with said interferometer for reducing from optical filtering effects an interference by signal bits of the DPSK signal with signal bits of a contiguous DPSK signal, said Interferometer including dual reflecting elements for its constructive and destructive path reflectors and said equalizer being integrated by optically placing said equalizer in both a constructive path and a destructive path of said interferometer with a reflecting element in an optical path of said constructive path before said equalizer.
2. The optical device of
3. The optical device of
4. The optical device of
5. The optical device of
6. A method comprising the steps of:
providing constructive and destructive optical paths for a received optical differential phase shift keying DPSK signal, and
integrating an equalizer with said providing for reducing from optical filtering effects an interference by signal bits of the DPSK signal with signal bits of a contiguous DPSK signal, said providing including an interferometer with dual reflective elements for its constructive and destructive path reflectors and said integrating including optically placing said equalizer in both said constructive and destructive paths of said interferometer with a reflective element in an optical path of said constructive path before said equalizer.
7. The method of
8. The method of
9. The method of
10. The method of
This application is a divisional of co-pending parent U.S. application Ser. No. 11/692,432, entitled “INTER-SYMBOL INTERFERENCE-SUPPORESSED COLORLESS DPSK DEMODULATION”, filed on Mar. 28, 2007, which in turn is related to U.S. application Ser. No. 11/279,767, entitled “COLORLESS DIFFERENTIAL PHASE SHIFT KEYED AND LOW CROSSTALK DEMODULATORS”, filed on Apr. 14, 2006, and U.S. application Ser. No. 11/619,499, entitled “OPTICAL EQUALIZATION FILTERING OF DWDM CHANNELS”, filed on Jan. 3, 2007, both of which are incorporated by reference herein.
The present invention relates generally to optical communications, and, more particularly, to a colorless differential phase shift keying demodulator for suppressing inter-symbol interference in dense wave division multiplexing communication systems.
Internet-based traffic has been growing exponentially due to the rapid increase of the microelectronic processing power, expansion of communication networks towards ubiquity, and emerging of modern bandwidth-thirsty business and personal applications such as video-on-demand (VoD) and storage area network (SAN). As the backbone to provide the transportation pipelines for such traffic volumes, the optical network has received demands for larger bandwidth capacity.
As a result, the 10 Gb/s bandwidth per channel in dense wavelength division multiplexing (DWDM) optical system is becoming inadequate. DWDM system with higher transmission rate of 40 Gb/s has begun to be deployed in long haul and metro optical networks.
In these new DWDM systems, the conventional on-off-keying (00K)-based intensity modulation scheme, also called non-return-to-zero (NRZ), has very limited performance in long-haul optical transmission. New modulation formats such as differential phase-shift-keying (DPSK), differential quadrature phase-shift-keying (DQPSK) and duobinary have been developed to mitigate the fiber detrimental effects, achieve higher ONSR tolerance and/or deliver better spectral efficiency.
Among these new modulation formats, optical DPSK has become a popular candidate for 40 Gb/s DWDM transmission due to its tolerance to fiber nonlinearities and higher receiver sensitivity. It offers 3 dB OSNR improvement with a balanced receiver and a decrease of self-phase modulation (SPM) and cross-phase modulation (XPM) due to the constant envelope modulation. Although DPSK has superior transmission performance, its relatively broad spectrum limits the spectral efficiency of DPSK-based DWDM systems. Under 50 GHz ITU grids, 40β DPSK signal suffers from inter-symbol interference (ISI), where optical pulse width broadening due to narrow filtering leads to the interference between neighboring bits. The optical filtering effect in 50 GHz spaced systems leads to interference between neighboring signal bits. This phenomenon is known as the inter-symbol interference (ISI). The ISI effect can cause a dramatic increase in signal bit error rate.
This schematic of
Proposed methods to mitigate ISI effect or to reduce the ISI problem caused by the strong filtering effect include use of spectral efficient modulation schemes; coding; side band pre-filtering methods; electronic equalization and optical equalization.
Use of spectral efficient modulation schemes such as optical duobinary and DQPSK modulation schemes can achieve 33 GHz at 90% spectral width. Therefore the signals are more tolerable to the filtering effect caused by the optical elements. However the duobinary signal has poor tolerance to nonlinear effect and therefore cannot has limited transmission span. The DQPSK modulation requires more complex and expensive transmitters and receivers.
An advanced coding scheme can be used to introduce correlation of the signal and control the power spectral density, and even lead to a reduction of signal spectral width. The downside is that the implementations are still technically challenging or very expensive for applications at a high speed such as 40 Gb/s.
Side band pre-filtering methods such as single-side-band (SSB) filtering and vestigial-side-band (VSB) filtering reduce the optical signal spectral width (to as much as half) to better fit into the passband width of the optical channel. The disadvantage is the increased complexity and compromised signal performance.
Electronic equalization such as electronic post-detection processing is used to improve system performance. The operation is typically based on feed-forward equalizers (FFE), decision feedback equalizers (DFE), maximum likelihood sequence estimation (MLSE), etc. It is shown that electronic equalization can partially cancel ISI and lead to an opening of the receiving signal eye. However, the performance of EDC is limited because the phase information of the incoming optical signals is lost due to OE conversion. Optical equalizers can be applied together with EDC.
An optical equalizer technology developed by applicants previously, an intra-channel optical equalizer, is a special optical filter. It is known that for a signal pulse not to have ISI it must satisfy the Nyquist criteria, and some popular Nyquist pulses have raised-cosine profile for their Fourier transforms. Therefore, it is desirable to set the transfer function of the band limited channel to a raised-cosine shape. Based on the given profiles of the passive optical filtering elements in the optical link, a corresponding optical equalizer is designed to complement them and produce an overall raised-cosine profile as shown in
The filter has a periodic profile with free spectral range (FSR) equal to the channel spacing of the DWDM signal and center frequency locked to the ITU-T channel grid, therefore it works on all the DWDM channels within the band.
Simulation results show about a 6 dB Q factor improvement for the back-to-back signals and 3 dB improvement after transmission over about 500 km fiber.
An optical equalizer with such a scheme can be designed based on Fabry-Perot (FP) interferometer theory and fabricated using dielectric thin-film technology. Comparison of ISI suppression without optical equalization, see eye diagrams 4A, 4B, and with optical equalization, see eye diagrams 4C, 4D, for a 40 Gb/s DPSK signal shows an improvement in the receiving signal particularly for the constructive port
A disadvantage of an optical equalizer is the requirement of an additional optical element in the transmission link. Also, as an athermal device without a temperature control mechanism, it might have temperature drift and have center frequency offset to the DPSK demodulator.
Accordingly, there is a need for an optical solution that integrates the functions of a DPSK demodulator and optical equalizer to reduce the inter-symbol interference ISI from the filtering effect on the optical path.
In accordance with the invention, an optical device includes an interferometer for a received optical differential phase shift keying DPSK signal, and an equalizer integrated with the interferometer in a manner for reducing from optical filtering effects interference by signal bits of the DPSK signal with signal bits of a contiguous DPSK signal. In an exemplary embodiment, the equalizer is integrated with the interferometer by optically placing a first of the equalizer 811 at an input and constructive path of the interferometer and a second of the equalizer 813 at a destructive path of the interferometer, the first and second of the equalizer having half of a filter ripple depth. The interferometer is a Michelson delay interferometer type, but can also be a Mach-Zehnder delay interferometer type on fiber, waveguide or other optical structure. The equalizer is a Fabry-Perot type equalizer, but can be a ring resonator type or a fiber based equalizer. Alternatively, a layer of glass in at least one optical path of the integrated interferometer or equalizer can be used for varying temperature of the glass to change the index of refraction of the glass, thereby varying an optical path through the glass.
In another aspect of the invention, a method includes the steps of providing constructive and destructive optical paths for a received optical differential phase shift keying DPSK signal, and integrating an equalizer with the providing for reducing from optical filtering effects an interference by signal bits of the DPSK signal with signal bits of a contiguous DPSK signal. In an exemplary embodiment, the integrating includes optically placing a first of the equalizer 811 at an input and the constructive path of the providing and optically placing a second of the equalizer 813 at the destructive path, the first and second of the equalizer having half of a filter ripple depth. The interferometer can be a Michelson Interferometer type and the equalizer can be a Fabry-Perot type equalizer. Alternatively, a layer of glass in at least one optical path of the integrated interferometer or equalizer can be used for varying temperature of the glass to change the index of refraction of the glass, thereby varying an optical path through the glass.
These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings.
The inventive inter-symbol interference ISI suppression integrates the functions of a DPSK demodulator and optical equalizer. It contains a delay interferometer to separate the constructive and destructive ports or paths of the received DPSK signal, and an intra-channel equalizer to mitigate the filtering effect from the optical path and thus suppress the ISI. The ISI-suppressed DPSK demodulator is a combination of a Michelson interferometer (MI)-based DPSK demodulator, see
For the DPSK demodulator part, a free-space Michelson Interferometer MI structure is used. The input is through a beam splitter/combiner 501. The mirrors 503, 505 at both arms within the MI have reflectivity of 100% or close to 100%, and the optical path length difference between the two interfering arms is equal to the delay, which is 20 ps to achieve colorless operation at 50 GHz FSR system
where vc is the 50 GHz FSR.
The FP interferometer (or FP etalon) consists of two partially reflective mirror surfaces 601 and a FP cavity 602, as shown in
Here R is the reflectivity of the mirrors. For the optical equalizer application, the reflectivity value is between 10% and 20%.
The data of the optical devices used in the simulation (including the AWG multiplexer and demultiplexer, optical interleaver) are taken from measurement data of actual devices used in the field, which makes the simulation result closer to an actual value. Standard Mach-Zehnder interferometer (MZI)-based DPSK demodulator from a simulation software library is used. Its delay time is set to be 20 ps to reflect the colorless feature and it exhibits the same performance as an MI-based DPSK demodulator in simulations. For the FP equalizing filter, theoretical device data are used based on the model described for
In the simulation, the FP equalizing filter and the colorless DPSK demodulator are represented separately so that their relative positions in the integrated device can be varied, and the performance of different configurations can be compared. Also, the equalizer element can be singled out to compare the transmission performance with and without the inventive ISI suppression mechanism.
There are several possible configurations to integrate the colorless DPSK demodulator and the FP optical equalizer into the inventive ISI-suppressed DPSK demodulator. Each of them is described for their fabrication, feasibility and performance are compared. The configurations are grouped into three main types. The first type is the basic MI type, which uses standard MI-based DPSK demodulator with integrated optical equalizing elements outside its two interfering arms. The second type is the modified MI type, which modifies the MI structure to achieve certain features or benefits. The third type is non-MI type, which does not use MI structure for DPSK demodulation.
Configurations for the basic MI type, which uses a standard MI-based DPSK demodulator with integrated optical equalizing elements outside its two interfering arms, are shown in
In the second configuration shown in
The third configuration, shown in
The fourth configuration of the invention,
In the Modified MI type structures, shown in
The first modified configuration, shown in
The second modified configuration,
The third and fourth modified, configurations, see
The fifth modified configuration, shown in
The configuration of
An exemplary non-Michelson Interferometer MI structure for practicing the inventive DPSK modulation is shown in
Diagrams showing the ISI suppression performance of embodiments of the invention are shown in
In a DPSK demodulator without any FP equalizing filter in the optical path, the simulated Q factor of the received 40 Gb/s DPSK signal is 19.6 dB, as shown by the simulated eye diagram of
However, the eye diagram of
An advantage of the device configuration according to
Turning now to the performance considerations of an ISI suppressed DPSK demodulator device. Based on the configuration of
The optical performance of the ISI-suppressed DSPK demodulator sample was measured with an analyzer, consisting of a fast sweep laser source, an optical test head, three precision power meters, and a PC controller. The measurement was performed from 1520 nm to 1570 nm to over the entire C-band and beyond. Measurement step size was 5 pm. Averaging was taken to reduce the measurement noise, particularly the phase measurement which is very sensitive to environmental variation such as temperature and vibration.
The plots of
For the integrated ISI suppression DPSK demodulator, because the slopes of the DPSK demodulator element are sharper than the slopes of the equalizing filter element, the resultant passband of the constructive port does not show dips at the ITU-T grid center, but still has peaks. However, the effect of the equalizer dip is still obvious. Because unlike the regular demodulator whose two output ports have similar insertion loss peaks (about 0.45 dB), the constructive port and destructive port of the integrated device have different insertion loss peaks at around 3.75 dB and 1.45 dB respectively. The 2.3 dB difference is the result of additional equalizer dips. This figure is slightly smaller than the 2.5 dB from the design. The equalizer dip occurs at the valleys of the destructive port.
Compared to a regular colorless DPSK demodulator without ISI suppression, there is an extra 1 dB loss experienced by the inventive device embodiment of
Referring now to
The measured results show similar PDL behaviors between the two output ports, see
The trends of CD curves in the passband for both ports are different, see
The behaviors of the differential group delay DGD profiles between the two output ports are also quite different, see
Turning now to the schematic of
Three demodulation configurations 1413 1, 1413 2 or 1415 are tested and compared. The first demodulation configuration 1413 1 is with a regular 50 GHz colorless DPSK demodulator only, the second demodulation configuration 1413 2 is with separate optical equalizer and colorless DPSK demodulator devices, and the third configuration 1415 is with the inventive ISI-suppress colorless DPSK demodulator containing integrated equalizing.
The receiving eye diagrams of these three demodulation configurations 1413 1, 1413 2 or 1415, both constructive and destructive ports, are shown in
These eye diagrams clearly show that the corresponding two latter configurations 1413 2 or 1415, with optical equalization, produce better eye opening at the receiver compared to the DPSK demodulation without equalization. The improvement can be observed on both the constructive and destructive ports. The eye opening improvement between the external optical equalizer and integrated equalizer is small and cannot be judged by eye. However the destructive port output power level of the integrated device seems to be a bit lower. Similar results are obtained when the laser wavelength is tuned to other ITU-T channels. These results confirm that an ISI-suppressed colorless DPSK demodulator with an integrated optical equalizing element can reduce the ISI effect and improve the receiving signal quality after transmission.
The inventive DPSK modulator with optical equalizing provides features and advantages unavailable with current optical transmission techniques. The inventive ISI-suppressed DPSK demodulator device can mitigate the ISI caused by optical filtering effects during the transmission for 40 Gb/s DPSK signal. It has colorless operation which allows the same device to operate on any ITU-T 50 GHz DWDM channel, thus reducing the inventory requirement. It does the basic function of demodulating the DPSK signal.
Compared to prior separate optical equalizer technology, which has similar ISI suppression performance, the integrated DPSK demodulator has the following advantages: compact, eliminates additional fiber, smaller insertion loss, a greater tolerance to temperature variation and lower manufacturing cost.
Compact: The size of the integrated ISI-suppressed DPSK demodulator is only slightly larger than the regular DPSK demodulator. This is smaller than having two separate devices, and will reduce the footprint on the transponder line cards.
Eliminating additional fiber: With the integration, the equalizing and demodulation elements are placed together. Therefore the fiber between two devices is eliminated, so is the fusion splice or connectors and adapter in between. This will make the line cards more tidy and reduce fiber management task.
Smaller insertion loss: Even though it seems that the integrated device has larger insertion loss than regular DPSK demodulator (about 1 dB, based on the actual measurement), the overall insertion loss is actually smaller than the combined loss of separate optical equalizer and DPSK demodulator. Without considering the 2.5 dB dip, the spec insertion loss value of the separate optical equalizer and DPSK demodulator is 3.4 dB (1.2 for equalizer and 2.2 for demodulator), while for integrated device the spec value is 2.2 dB. This is because the integrated design eliminated a pair of fiber collimators that couple the light between free space and fiber inside the device, and the fiber collimators are the main contributor to insertion loss.
More tolerant to temperature variation: When the two elements, interferometer and equalizer, are integrated inside a small hermetically sealed package, they experience similar impact of environmental temperature change. Therefore, if there is a wavelength drift caused by the temperature change, these two components will more likely to have the same amount of drift (albeit small) and maintain good relative position. Separate equalizer and demodulator devices are likely to experience larger relative temperature-induced wavelength drift.
Lower manufacturing cost: Because of the elimination of components such as some fiber collimators and reducing the number of packaging from two to one, the manufacturing cost is reduced.
The inventive ISI-suppressed DPSK demodulator can be used in a 100 GHz spaced system. The inventive ISI-suppressed DPSK demodulator is designed for a 50 GHz-spaced system, and its equalizing element imposes dips on every 50 GHz ITU-T grid frequency. This is also optimized for a 100 GHz-spaced system where the 40 Gb/s signal experiences very different filtering effect (much less).
The eye diagrams of
Even though the inventive device is athermal and can operate without any temperature control, it can experience slight frequency shift due to temperature. A tuning mechanism may be added to not only compensate for the frequency shift caused by temperature variation, but can also compensate for laser drift. Tuning can be added to the device by inserting a layer of special glass in the optical paths. This glass material would have a larger thermal coefficient. By varying its temperature, the refractive index of the glass material will change and lead to the variation of optical path length, allowing the spectrum of the DPSK demodulator to be tuned.
The inventive optical device, the ISI-suppressed DPSK demodulator, integrates an optical equalizer and colorless DPSK demodulator to mitigate the filtering effect-induced ISI for a 40 Gb/s DPSK signal in 50 GHz-spaced optical DWDM system. One exemplary embodiment of the invention includes two colorless FP-based optical equalizing filters with half of the filter ripple dip depth, one at the dual fiber port with Input and Output A, the other at the Output B port. The experimentally measured optical characteristics and ISI suppression performance demonstrate that the invention improves the transmission performance of a 40 Gb/s DPSK signal by reducing the ISI. Comparing to prior designs incorporating separate optical equalizer and demodulator devices, the inventive integrated device is more compact, has better optical performance, e.g., smaller insertion loss and better tolerance to temperature variation, and has lower cost. Therefore, the inventive integrated device is useful for improving transmission in DWDM networks.
The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. It is anticipated, however, that departures may be made therefrom and that obvious modifications will be implemented by those skilled in the art. It will be appreciated that those skilled in the art will be able to devise numerous arrangements and variations which, although not explicitly shown or described herein, embody the principles of the invention and are within their spirit and scope.