Inter-Symbol Interference-Suppressed Colorless DPSK Demodulation

Abstract

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.

Description

[0001]This application 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.BACKGROUND OF THE INVENTION[0002]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.[0003]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 trans...

AI Technical Summary

Benefits of technology

[0020]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.

Problems solved by technology

As a result, the 10 Gb / s bandwidth per channel in dense wavelength division multiplexing (DWDM) optical system is becoming inadequate.

In these new DWDM systems, the conventional on-off-keying (OOK)-based intensity modulation scheme, also called non-return-to-zero (NRZ), has very limited performance in long-haul optical transmission.

Although DPSK has superior transmission performance, its relatively broad spectrum limits the spectral efficiency of DPSK-based DWDM systems.

Under 50 GHz ITU grids, 40 G 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.

The ISI effect can cause a dramatic increase in signal bit error rate.

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.

The downside is that the implementations are still technically challenging or very expensive for applications at a high speed such as 40 Gb / s.

The disadvantage is the increased complexity and compromised signal performance.

However, the performance of EDC is limited because the phase information of the incoming optical signals is lost due to OE conversion.

A disadvantage of an optical equalizer is the requirement of an additional optical element in the transmission link.

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