We propose a novel coherent self-heterodyne receiver structure based on phase modulation detection that potentially simplifies the front-end of a coherent optical receiver. The scheme has been demonstrated via simulations and experimentally for a 10 Gb/s DQPSK transmission system.
© 2012 OSA
In recent years the exponential growth of Internet data traffics has resulted in a steady increase of the transmission capacity of Dense Wavelength Division Multiplexed (DWDM) optical networks. Advanced optical modulation formats that offer high spectral efficiency are being increasingly deployed to cope with this trend [1–3]. The state-of-the-art optical coherent receiver employs a 90° hybrid front end and a local oscillator (LO) laser for homodyne demodulation [1, 4]. The integration of the hybrid remains a technical challenge and the laser phase noise requirements on the LO can be difficult to achieve, especially with higher order modulation formats. Differential modulation techniques, such as DQPSK, that employ self-homodyne direct detection using delay line interferometers (DLIs) [3, 5] could potentially reduce the requirements on the laser linewidth. However, the DLI approach would require significantly more complex hardware implementation to migrate to higher order modulation formats such as differential m-PSK and differential m-QAM. The authors in  have reported the realization of a direct-detection multilevel transmission for 16-QAM incorporating phase pre-integration and advanced digital signal processing (DSP).
In this paper, we propose a novel coherent receiver scheme based on the phase modulation (PM) detection method  that can recover the differential E-field from the harmonics of the modulating carrier without an LO laser. In previous work, we employed PM detection method for characterizing the laser phase noise . Similar approaches to harmonic demodulations have been employed in near-field scanning optical microscopy  and Fourier-domain optical coherence tomography , but to our knowledge this is the first demonstration in coherent optical communications. In this work we demonstrate a self-coherent DQPSK system operating at 10 Gb/s by implementing our PM detection method at the receiver. The performance of the proposed technique is compared with other widely employed DQPSK receiver structures through simulations and experimental results. The proposed receiver configuration provides a cost effective solution for the front-end of optical coherent receivers, and has the potential of migrating to higher order modulation formats without a significant increase in complexity.
2. Analytical model
In the following, we develop the analytical model for the proposed receiver scheme in Fig. 1(a) . The E-field of the modulated optical output from an IQ modulator can be expressed in complex notation (for brevity we have omitted the laser phase and intensity noises) asFig. 1(a) has a one-symbol delay and the lower arm is phase modulated by a sinusoidal waveform. The incident E-field falling on the photo-detector can be written as
Ignoring the first two terms in Eq. (3) (which are constant for PSK and can be cancelled using balanced photo-detectors, or a single photo-detector with a DC-block) and expanding the third term:
Using the Bessel coefficient expansions:Eqs. (5), (6) and (7) that the I and Q components of the complex differential optical modulation envelope,, can be found at the even and odd harmonics of i(t). For bandwidth efficient demodulation, we can also obtain the I and Q components from the baseband and the first harmonic, respectively. The output current of the photo-detector in terms of baseband and first harmonic could be expressed as:Figure 2(a) shows the block diagram of the off-line DSP for the captured photo-detector current. A simple carrier phase recovery algorithm for was applied prior to demodulation. The least mean-square (LMS) algorithm compensates for the differential drift in the interferometer arms due to the random fluctuation, for example, in temperature. For balanced I/Q outputs the PM index b is adjusted so that Jo(b) = 2J1(b) ~0.81, or b ~0.90, as verified on the electrical spectrum analyzer.
3. Experiment setup
Figure 1(a) shows the experimental setup for a back-to-back 5 Gbaud/s DQPSK link using the coherent PM detection method. The transmitter employed a sample grating distributed Bragg reflector (SGDBR) laser  that has a linewidth of 5 MHz at 1547.6 nm when biasing only the gain section at 100 mA. The I and Q arms of a dual parallel Mach-Zehnder modulator (biased close to the null points) was driven at 5 Gb/s by the complementary PRBS outputs of an Anritsu pulsed pattern generator (PPG), which were decorrelated by 59 bits using an electrical delay. The received power of the DQPSK signal was monitored after the first variable optical attenuator (VOA). The first low noise EDFA served as the pre-amplifier for the receiver front-end. The output signal was further amplified by a second EDFA. Each EDFA was followed by 2 nm optical filters to reduce the ASE noise, and the second VOA maintained the optical power falling on the photo-detector.
The one-symbol delay in one arm of the interferometer was set to Ts = 200 ps at 5 Gbaud/s. An EOspace Z-cut lithium niobate phase modulator in the other arm was driven by a 5 GHz signal generator. The signal generator output was amplified to provide sufficient drive (modulation index b) to the phase modulator in order to achieve balanced power at baseband and the first harmonic as observed on an RF spectrum analyzer. The light from the delayed and phase modulated arms were recombined via a coupler whose output was detected by an 11 GHz photo-detector with an integrated transimpedance amplifier (TIA). Two polarization controllers (one on each arm of the interferometer) were employed to manually optimize the beat signal at the TIA output and minimize the polarization mode dispersion effect of the modulator. An Agilent real-time scope captured the output signal at 40 GSamples/s that was then fed into a computer for off-line DSP processing as illustrated in Fig. 2(a). Matched filtering  was applied to the baseband I-component and the demodulated Q-component.
In order to evaluate the PM detection technique, we also implemented a self-homodyne, optical coherent receiver  as shown in Fig. 1(b). The two arms of the delay interferometer were now fed into an optical 90° hybrid with the output signals detected by a pair of 43 Gb/s photo-detectors with integrated TIAs. The I and Q output signals from the photo-detectors were also captured by the Agilent real-time scope at 40 GSamples/s for post-processing as shown in Fig. 2(b).
4. Simulation and experimental results
The first VOA in the experimental setup adjusted the back-to-back optical power in order to compare the receiver sensitivity of the PM detection method with the self-homodyne optical receiver. We also performed simulations of both experimental systems in Fig. 1, and of a direct detection DQPSK receiver with two DLIs [3, 5], using VPIphotonics software. The results were compared in terms of the error vector magnitudes (EVM).
Figures 2(c) and 2(d) show the example experimental symbol constellations at −30 dBm received power for the self-heterodyne receiver with PM detection and the self-homodyne receiver with 90° hybrid, respectively. The experimental results show that the recovered DQPSK symbols have an EVM of 18% with PM detection and 14.7% with the 90° hybrid. Figure 3(a) compares the receiver sensitivity of the two methods. The plots show that PM detection has a power penalty that is about 3 dB (at 21% EVM) relative to the self-homodyne receiver with 90° hybrid. The simulation results in Fig. 3(b) are in general agreement with the experimental results and confirmed the sensitivity reduction between PM detection method and self-homodyne receivers.
The 3dB penalty of PM detection method was primarily caused by the additional ASE beat noise from the EDFAs due to heterodyne demodulation. The accumulated number of noise photons after the EDFAs chain was (where is the spontaneous emission factor, is the number of EDFAs, and is the amplifier gain), which is much greater than the number of photons from shot-noise (1 photon) .
The carrier-to-noise ratio after the EDFAs chain was largely determined by ASE noise, as the shot-noise-limit is negligible in such a system :Fig. (4) . Consequently, the in-band ASE noise of the self-heterodyne receiver was two times that of the self-homodyne receiver. This leads to a 3 dB penalty in the receiver sensitivity for PM detection.
We have proposed and demonstrated for the first time a coherent self-heterodyne receiver based on PM detection for DQPSK modulation through analytical model, simulations and experiments. The DQPSK signal from a back-to-back 10 Gb/s link was successfully demodulated using the proposed method. Although there is a 3dB penalty associated with heterodyne demodulation under ASE-limited condition, the coherent PM detection receiver not only offers the potential for a low-cost integrated solution but can also significantly simplify the front end of optical coherent receivers. Moreover, this receiver configuration could be feasibly migrated toward higher order modulation formats such as differential m-PSK and differential m-QAM without a significant increase in hardware requirements.
This work was supported in part by the Irish Research Council for Science, Engineering and Technology, by Science Foundation Ireland through the Investigator Programme, by the Fulbright Commission and by the Faculty Development Fellowship of the University of Nebraska-Lincoln.
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