High-Sensitivity Demodulation of Multiple-Data-Rate WDM-DPSK Signals using a Single Interferometer

Abstract

We demonstrate simultaneous reception of multiple-rate wavelength-division-multiplexed optical-DPSK signals using a single interferometer. The demodulation approach provides rate-flexibility and scalability, enabling penalty-free performance and compliance with existing channel-rate and channel-spacing standards. 1Introduction Optical differential-phase-shift-keying (DPSK) modulation has received considerable attention by the free-space optical (FSO) communications community and the telecom industry for its increased sensitivity over commonly used on-off-keying [1-6], and reduced peak-to-average power ratio which mitigates nonlinear effects in fiber-optic applications. These benefits have led to many Tbit/s long-haul fiber-optic experiments using hundreds of WDM-DPSK channels [7-10]. The advantages of DPSK come at the expense of increased complexity, requiring a phase modulator in the transmitter (TX), and an optical delay-line interferometer (DI) and balanced detection in the receiver (RX) to derive maximum benefit. Of these elements, the DI is the most technically challenging and the least mature. For good performance, the arms of the DI must be precisely fabricated to within a small fraction of a bit-period, and they must be stable to a small fraction of a wavelength, requiring careful thermo-mechanical packaging and/or stabilization electronics [11][12][13][14], adding to size, weight, power, and cost.Miyamoto and coauthors [15] demonstrated interferometer sharing by converting 42.7 Gbit/s WDM-DPSK channels to WDM-duobinary channels on the 100 GHz ITU grid using a 50 GHz free-spectral-range (FSR) DI. Their innovative DPSK RX has advantages in dispersion-limited fiber links, but has inherent sensitivity penalties of 3 dB due to single-ended DPSK reception and an additional penalty due to the rate-to-FSR mismatch. While their simplified approach enables ITU compatibility, the ~14% mismatch leads to a waveform-dependent penalty of ~1.2 dB for 33% RZ pulse-shapes, further reducing the sensitivity benefit of DPSK [16]. To avoid mismatch penalty, the interferometer delay (τ d = 1/FSR) must be an integer multiple n of the bit-period τ bit , i.e., τ d = nτ bit (see e.g., [17]). Here, we use this relation and the DI's periodic transfer function to demonstrate simultaneous demodulation of 2.5 Gbit/s and 5 Gbit/s WDM-DPSK signals using a single DI. This technique can be scaled to higher rates and channel count, and operate within existing standards without waveform-dependent penalties. Description of the multi-rate multi-channel DPSK approachPenalty-free multi-channel DPSK reception can be achieved by leveraging the DI's periodic transfer function (Fig. 1a), and constraining the received wavelength spacing (Δν ch ), which enables simultaneous demodulation of all channels (λ's). WDM channel separation is achieved via optical demultiplexing after the DI [15,18,19] (Fig. 1b). dB Normalized Frequency Offset, Δf/FSR cos 2 () sin 2 ()

Laser communication transmitter and receiver design

“…Typically, adjacent bits are differentially encoded with a time-separation τ d of one symbol duration or a bit period, τ bit , but this can generally be extended to an integer number n of symbol periods, i.e., τ d = nτ bit . This can provide some flexibility in implementing simplified multi-rate and multi-channel receivers (see, e.g., [134][135][136]), a subject discussed further in section 5.2.…”

“…Alternatively, for harmonically related data rates, a fixed DI sized to the lowest rate can be used [134] along with appropriate differential precoding. In this case, the same DI can be used to simultaneously demodulate multiple-rate WDM-DPSK signals (see section 5.2.3), providing both rate-flexibility and WDM scalability [136]. …”

“…This relaxes signal coherence requirements [264], minimizes frequencyalignment penalties, and simplifies TX precoding, which can be implemented with an OR gate to logically combine DATA and CLOCK inputs to drive a T-flip-flop [140,[434][435][436], causing the transmitted phase to change whenever the DATA is a '1'. However, for multi-rate and multi-channel DPSK applications discussed later, it can be desirable to use a multiple-bit delay to accommodate simplified implementations that provide rate-flexibility and scalability with penalty-free performance that is compatible with existing channel-rate and channel-spacing standards [135,136].…”

“…Thus, for near-harmonically related data rates, a single interferometer can be used to simultaneously demodulate multiple-rate WDM-DPSK signals [136]. To further diminish deviation from the ITU grid and to provide additional rate/alignment flexibility, a 2.68 GHz DI could be used to simultaneously demodulate 2.5, 2.67, 10.7, 40, 42.7, and 43.02 Gbps SONET and G.709-compliant WDM-DPSK signals within 700MHz of the ITU grid and with less than 7% delay-error.…”

Performance of Stereo Multiplexing in systems using direct detection with optimum dispersion maps

“…This supposes a big challenge concerning both transmission performance and hardware requirements. Lately, much interest has been put in schemes that reduce the symbol rate by sharing the transmitted data in two optical wavelengths [1][2][3][4]. By doing so, not only the bandwidth requirements of the components are relaxed, but it is also expected a gain in robustness against impairments that scale with the symbol rate, e.g., dispersion.…”