OW4C.2.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Investigation of black-box phase regeneration using single bi-directional PPLN waveguide André A. C. Albuquerque(1), Benjamin J. Puttnam(2), Miguel V. Drummond(1) Áron Szabó(3), Dániel Mazroa(3), Satoshi Shinada(2), Naoya Wada(2) and Rogério N. Nogueira(1) (1) Instituto de Telecomunicações, Campus Universitário de Santiago, Aveiro 3810-193, Portugal, (2) Photonic Network System Laboratory, NICT, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8759, Japan (3) Dept. of Telecoms. & Media Informatics, Budapest University of Tech. & Economics, Magyar tudósok körútja 2, Budapest 1117, Hungary E-mail:
[email protected]
Abstract: We investigate a novel in-line phase regeneration set-up using a single bi-directional PPLN waveguide for both generation of phase correlated signals and phase-sensitive regeneration and injection-locking for carrier phase recovery. OCIS codes: (130.3730) Lithium niobate; (190.4970) Parametric oscillators and amplifiers;
1. Introduction In addition to noiseless amplification [1], the property of phase-sensitive amplifiers (PSAs) to amplify or attenuate signals according to their phase makes them viable candidates for all-optical regeneration of phase modulated data formats [2-5]. The most common technique of implementing a PSA is to use four-wave mixing (FWM) in a fiberoptic parametric amplifier (FOPA) to generate phase correlated signal, idler and pump waves that are used as the input to a second phase sensitive (PS) FOPA. However, recently, second-order non-linear processes in periodicallypoled lithium-niobate (PPLN) waveguides have generated increasing interest since high non-linear coefficients may be achieved in crystals of only a few centimeters in length with low spontaneous noise emission, low crosstalk and immunity to stimulated Brillouin scattering (SBS) [3,4]. Furthermore, PPLNs may be potentially integrated with other optical components, being attractive for numerous optical signal processing applications and in the nondegenerate configuration are naturally more suited to multi-channel operation than FOPAs [5]. Although lower gain, high pump power requirement and thermal instability are disadvantages in PSA applications, compared to FOPAs [4], inline or black-box phase regeneration has been achieved [6,7] with good performance. Here, we show for the first time, to our knowledge, that phase-sensitive optical processing can be achieved in a single PPLN waveguide. We investigate the performance of novel phase regeneration set-up where a bi-directional PPNL is combined with a semiconductor laser, which performs carrier recovery/modulation stripping in the C-band, and used for both generation of the phase correlated pump and signal waves in one direction and for the phase sensitive phase regeneration of a 10 Gb/s BPSK signal in the other. We see that a trade-off between optimum phase and amplitude squeezing, and also the stability of the regenerator exists. We observe that the optimum performance occurs with moderate pump powers since thermal instability and green-light induced refractive index change causes limitations at high powers. 2. Experimental Description The setup, wavelength (λ) and phase (ϕ) relationships of the waves for a dual pump degenerate PSA based on a single PPLN waveguide are shown in Fig. 1, where the indices s, p1, p2, sN and p2N stand for signal, pump 1, pump 2, signal noise and pump 2 noise, respectively. The PPLN was a free-space device of 6 cm length doped with magnesium oxide to prevent photorefractive damage with an insertion loss depending on free-space coupling of
Fig. 1. Experimental set-up for phase regeneration measurements.
978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America
OW4C.2.pdf
OFC/NFOEC Technical Digest © 2013 OSA
around 3.5 dB. The quasi-phase matching wavelength (λQPM) was 1549.3 nm at 21oC and the grating period of 19.1 µm. The signal for regeneration (λs) was generated in an external cavity tunable laser (ECTL) with a wavelength of 1549.32 nm and modulated with a 10 Gbit/s BPSK signal with pseudo random bit sequence of length 2 15–1. To emulate a signal with amplitude and phase noise the optical output was combined with an ASE source consisting of low-power erbium-doped fiber amplifiers (LP-EDFAs) and a 3 nm band-pass filter (BPF) centered on the signal wavelength on the high loss arm of a 90/10 coupler. Before regeneration, the signal was amplified in a high power EDFA (HP-EDFA) before a variable optical attenuator (VOA) used to control the regenerator input power, and a 1 nm optical band-pass filter (BPF) to reduce the ASE outside of the signal band. Next, part of the modulated signal was removed in a 90/10% optical tap to be combined with the two phase correlated pumps required for degenerate phase sensitive regeneration. The signal path also contained a polarization controller (PC) and an optical delay line (ODL) used to match the optical path length. After another PC, the remainder of the modulated signal was then combined with the pump 1 signal (λp1 = 1546.9 nm) in a WDM coupler producing less than 1 dB loss on both arms. A signal at the second pump wavelength was then generated in the PPLN in a phase insensitive interaction through cascaded SHG/DFG (copier stage [1]) with the instantaneous phase of pump 2 determined by the pump 1 and signal phase according to the relation ϕp2+ϕp2N =2ϕs+2ϕsN –ϕp1. The input power going into the PPLN waveguide in the copier direction was about 27.3 dBm. As in [2], the phase erasure process stripped the BPSK modulation from the signal, but transferred the excess phase noise to the generated pump 2 signal according to ϕp2N =2ϕsN. This phase noise was then removed in the semiconductor slave laser (SL) located after two optical circulators (Circ. I and II) and a WDM multiplexer/demultiplexer (MUX/DEMUX), used to select only the pump 2 wavelength at the input to the SL and to combine it with the other pump tapped off from the pump 1 path. Since injection locking is a narrow band filtering mechanism with a sub-GHz bandwidth [8], the SL removes any higher frequency fluctuations produced by the ultrafast processes of SHG/DFG. The input power of the SL was set to a value that allowed the SL to track the frequency of the injected wave up to a few hundred MHz. The combined pump 1 and pump 2 signals were then spectrally symmetrical and phase-locked to the modulated signal carrier wave which was then recombined with both pumps in a 90/10% coupler. The 3 waves were then passed through a liquid crystal on silicon based optical processor (OP) which was used to adjust the relative power of each signal and selectively block one or both pumps to allow a comparison of the regenerator output signal with and without phase sensitive regeneration. After the OP, the polarization of the signals was set to the optimum axis with an additional PC before the signals were re-amplified, filtered and passed back through the PPLN waveguide in the opposite direction (PSA stage direction) via the optical circulator (Circ. I) Hence the natural squeezing transfer function of the PS PPLN was used to regenerate BPSK symbols on the real axis [2]. The input power entering the PPLN in the PSA stage direction was varied during the investigation with a maximum value of 28.9 dBm. A phase error-tracking mechanism was applied to correct for any path length deviations caused by sub-kilohertz thermal and acoustic fluctuations after the 90/10% coupler in the pump path. The deviations of the two equalized path lengths were compensated by an electrical phase-locked loop (PLL) shown in the phase locking control box in Fig. 1 and a lead zirconate titanate (PZT) fiber stretcher in the pump path. The feedback error signal for the PLL was generated by detecting a fraction of the regenerated signal after the PSA stage containing a weak dithering tone in the kHz range with a low-noise photodiode (PD). The detected photocurrent was then fed into a lock-in amplifier, and the feedback signal was used to drive the PZT stretcher after being amplified by an inverter (comparator) and a high voltage amplifier, as described in [1]. The signals were detected using a single polarization coherent receiver with a 100 kHz linewidth ECTL used as the local oscillator and the outputs of the 90º optical hybrid received with DCcoupled PDs at the input to a 40 GSample.s−1 real-time sampling oscilloscope. The scope had a 13 GHz bandwidth and a variable bandwidth BPF was used to filter the regenerated signal before the PD. 3. Results The regenerative properties of the black-box phase regeneration setup using a single bi-directional PPLN waveguide were evaluated by measuring the standard deviations of the amplitude and phase angle of the detected symbols in both PS and phase insensitive (PI) modes (σPSA and σPIA) using 4096 symbols. Phase or amplitude squeezing are characterized by the σPIA/σPSA ratios, with higher ratios corresponding to better regenerative properties. In PS operation mode the power of the two pumps was set to the same value at the OP. Fig. 2-(a) shows the variation of the ratio between the standard deviations PI and PS modes, as a function of the ratio between the signal and pump 1 powers (Psignal/Ppump1). The total input power going into the PPLN waveguide in the PSA direction, PPSA, was set to 28.9 dBm. Phase squeezing (σPIA/σPSA>1 for ϕ) of the noisy signal was observed for all the Psignal/Ppump1 ratios, but with worst performances for higher Psignal/Ppump1 ratios. This is believed to be caused by operation of the regenerator closer to the gain saturation regime where phase-to-phase transfer functions
OW4C.2.pdf
OFC/NFOEC Technical Digest © 2013 OSA
Fig. 2. Phase and amplitude squeezing as a function of : (a) Psignal/Ppump1; (b) PPSA. Constellation diagram of the received signal in: (c) PIA mode; (d) PSA mode with Psignal/Ppump1 = -2 dB; (e) PSA mode with Psignal/Ppump1 = -16 dB.
start to become less uniform as it was reported in [9] for a non-degenerate PSA. In this gain saturation regime, amplitude squeezing should also be expected but no clear evidence of this behavior was observed. The variation of phase and amplitude squeezing properties of the regenerator with PPSA are also shown in Fig 2-(b) with Psignal/Ppump1 = -10 dB. According to these results, better phase regeneration is achieved for higher values of PPSA because of higher PS gain. However, this also means higher phase-to-amplitude noise conversion leading to lower σ PIA/σPSA for the amplitude. For higher power levels of light propagating within the PPLN waveguides the effects of green-light induced refractive index change are more intense [4]. Refractive index changes caused by this effect not only change the optical path length of the signal + pump 1 arm, but also the pump 2 generation in the copier stage, introducing higher instability on the PLL and injection locking and affecting the performance of the regenerator. 4. Summary We report an experimental investigation of the performance of a novel regeneration set-up based on using a single bi-direction PPLN and carrier phase recovery in an injection locked semiconductor laser. The PPLN waveguide was used to both generate the phase correlated waves required from phase-sensitive interaction in one directions and performing the phase-sensitive regeneration in the other. We observed that best performance in terms of phase regeneration was achieved for lower signal to pump power ratios and the maximum input power in the PSA direction. However, a trade-off between high phase sensitive gain provided by high pumping power and amplitude noise and system instability is created. These results show that PSAs can be implemented in a single PPLN waveguide with bi-directional propagation allowing savings of devices, temperature control and complexity, particularly for high-order phase modulated signals where multiple BPSK phase regenerators can be used as elementary building blocks for more complex applications [2]. However, the increased optical power within a single waveguide was observed to cause more severe green-light induced refractive index change and thermal instability. A. A. C. Albuquerque’s Ph.D. is funded by “Fundação para a Ciência e Tecnologia” (SFRH/BD/78425/2011) and also projects CONTACT (PTDC/EEA-TEL/114144/2009) and POFCOM (PTDC/EEA-TEL/122792/2010). 5. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
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