Multichannel phase squeezing in a PPLN-PPLN PSA

OFC/NFOEC Technical Digest © 2012 OSA

Multi-channel phase squeezing in a PPLN-PPLN PSA Benjamin J. Puttnam, Áron Szabó*, Dániel Mazroa*, Satoshi Shinada, and Naoya Wada National Institute of Information and Communications Technology (NICT), 4-2-1, Nukui-kita, Koganei, Tokyo 184-8795, Japan. *Dept. of Telecoms. & Media Informatics, Budapest University of Tech. & Economics, Magyar tudósok körútja 2, Budapest 1117, Hungary E-mail:[email protected].

Abstract: We investigate phase squeezing of up to four signal channels in a non-degenerate phase-sensitive amplifier constructed from periodically-poled-lithium-niobate waveguides and observe that additional channels have little impact the phase squeezing performance. OCIS codes: (130.3730) Lithium niobate; (190.4970) Parametric oscillators and amplifiers;

1. Introduction The property of phase-sensitive amplifiers (PSAs) to amplify or attenuate signals according to their phase leads to several useful applications, including noiseless amplification and regeneration of phase modulated signals [1, 2]. The most common technique of implementing a PSA is to use four-wave mixing (FWM) in a fiber-optic parametric amplifier (FOPA) to generate phase correlated signal, idler and pump signals that are used as the input to a second phase sensitive (PS) FOPA. Recently, cascaded second-order non-linear processes of second harmonic and difference-frequency generation (SHG/DFG) in periodically-poled lithium-niobate (PPLN) waveguides has 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) [3-6]. Furthermore, they may be potentially integrated with other optical components, such as modulators for transmitter applications [7], making them attractive for a wide range of optical signal processing application. Previously, C-band, PS behaviour has been demonstrated in PPLN based PSAs both in the WDM compatible, nondegenerate idler configuration using a FOPA for idler generation [3, 4] and with degenerate signal, idler and pump in a ridge waveguides [5] and phase-squeezing, increasing with the phase sensitive dynamic range (PSDR), has been demonstrated in a PPLN-PSA which also used a FOPA for idler generation [6]. Here, we investigate phase squeezing in a PSA constructed from two PPLN waveguides in the non-degenerate idler configuration for the first time and demonstrate regeneration of up to four 10Gb/s binary-phase shift keyed (BPSK) channels. We use a coherent receiver for constellation analysis of BPSK signals, distorted by broadband noise, and observe that adding additional channels has minimal impact on the phase squeezing characteristics. 2. Experimental description The experimental set-up, shown in Fig. 1, comprised two PPLN waveguides, PPLN A and PPLN B which were doped with magnesium oxide to prevent photorefractive damage and packaged in temperature controlled, fiber pigtailed modules with insertion losses, including fiber coupling, of 5.5dB and 7.5dB respectively. The quasi-phase matching wavelength (λQPM) of both devices was 1549.9nm at 40ºC and grating period of 18.9µm. The first stage of the set-up was to generate the pump (λp), with phase correlated signal (λs1,2,3 & 4), and idler (λ i1,2,3 & 4) pairs in the first PPLN. λp was generated by an external cavity tunable laser (ECTL) amplified by a high-power erbium-doped fiber amplifier (HP-EDFA) and filtered with a 1.4nm optical band pass filter (OBPF) to remove excess noise. Four further p =1549.4nm S1 =1554.7nm

S4 =1557.7nm

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Figure 1. Experimental set-up and signal phase relationships for phase squeezing measurements including the transmitter (Tx), interferometer, phase-insensitive amplifier (PIA) and phase sensitive amplifier (PSA) and receiver (Rx)

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ECTLs were used to generate the four signal channels that were coupled together and combined with the pump on the high loss arm of 90/10 optical tap with variable optical attenuators (VOAs) and polarization controllers (PCs) used to control the relative power and polarization of each channel. λp was set to match the λQPM of the PPLNs and the λs wavelengths were chosen to achieve the high consistent gain from the HP-EDFAs required for amplification. λs2,3 & 4 were spaced at 100GHz intervals at wavelengths of 1557.7nm, 1556.9.xnm and 1556.2 while λs1 was chosen to be 1554.7nm, approximately 200Ghz from λs2 to remove penalties caused by 2 nd order SHG/DFG products. All the signals were then transmitted through PPLN A, acting as the PI amplifier (PIA), where the phasecorrelated idler waves were generated, where φ iN=2φp-φsN and φrel=0, and each idler typically possessed 10dB lower power than the corresponding signal. The signal/idler pairs were then separated from λp in different arms of a WDM coupler. The signal arm contained a programmable filter, which allowed for channel power equalization to compensate for the weaker idlers and allow selective idler blocking to switch between PS and PI operation. Also included on the signal arm was an optical delay to precisely align the pump and signal path lengths and a phase modulator where the 10Gb/s BPSK signal, based on a 2 15-1 length pseudo random bit sequence was modulated onto each of the signal/idler pairs. The pump arm contained a 1nm OBPF to remove all λs and λi signals, a lead-zirconatetitanate (PZT) Fiber stretcher to compensate for variations in the optical path length of the two arms caused by thermal or acoustic fluctuations and an additional PC. The pump was then re-combined with the λs and λi signals on the high power arm of a 4-port 90/10 power coupler with the data modulation on each signal/idler pair (φdata) then becoming the relative phase shift of the recombined waves, enabling the natural squeezing transfer function of the PSA [2] to act as a regenerator for the BPSK symbols on the real axis. The low power output of the coupler was used as the input to a feedback circuit used to the PZT, based on interference of λ p and small λp component propagating in the signal arm. The high power output, containing the pump and correlated signal/idler pairs was then passed in to PPLN B in phase-sensitive operation with a VOA used to control the input power. The input power into both PPLNs was 30dBm corresponding to PSDR, estimated from previous measurements of 7dB. The signals were received using a single polarization coherent receiver with an ECTL used for the local oscillator (LO) and the outputs of the 90º optical hybrid received with DC-coupled photodiodes (PDs) at the input to a 50Gs•s-1 real-time sampling oscilloscope. The scope had a 20GHz bandwidth and 0.6nm OBPF was used to select the channel for reception before the PD. To investigate the regeneration performance, the BPSK driving signal was combined with the noise output of a function generator and the level of distortion was optimized by monitoring with a coherent signal analyzer which also allowed fast BER measurement to ensure zero bit errors, ensuring all data points were pushed to the correct symbol phase. Offline signal analysis was performed for various signal/idler combinations with the PS and PI operation compared by suppressing the idlers in the programmable filter. 3. Results

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Firstly, with all signal and idlers present each of signal channels was received in turn and PI and PS operation compared by blocking or passing the appropriate idler in the programmable filter. The received constellations were than analyzed and the phase squeezing performance compared by considering the ratio of the standard deviation (SD) of the phase angle (σφ) and signal amplitude (σ A) in both cases. The phase angle SD ratio (σφ-PIA/σφ-PSA) was measured to be positive, indicating phase squeezing, in all cases with values of 1.3, 1.28, 1.27 and 1.28 for signal channels λs1,2,3 & 4 respectively. The small reduction in the level of phase squeezing for λs2,3 & 4 attributed to the crosstalk from neighboring channels, since the OBPF was not sufficiently narrow to isolate the closely packed channels. As expected in the linear regime of PS gain, some amplitude noise was added to the signals being squeezed with the ratio of amplitude noise from PI to PS operation (σ A-PIA/σ A-PSA) showing little variation between signal channels with values of 0.9, 0.89, 0.91 and 0.9 for λs1,2,3 & 4 respectively. Figure 2 (a)-(d) show the constellation plots of each of the received channels in PI and PS operation for signals at λs1,2,3 & 4 respectively. P

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Figure 2. (a) to (d) Constellation plots of λs1,2,3 & 4 for PI and PS operation and (e) optical spectrum at PSA output

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In each case the small amount of phase squeezing measured is evident in both a visible reduction in the distribution of bit centres and a squeezing of the circular trajectory, typical of phase modulators, towards the real axis. Figure 2(e) shows the optical spectrum at the output of the PS PPLN (B), and shows a number of interesting features. The non-negligible crosstalk products are centered on the QPM band, with the most powerful crosstalk component 22dB below the signal and idler pairs, showing that crosstalk is not prohibitive for up to 4 signal channels. Hence, although the weaker efficiency of the 2nd order non-linear process in PPLNs currently limits the achievable phase sensitive response, the 2-stage SHG/DFG process is less susceptible to unwanted crosstalk that occurs from additional FWM products in FOPAs that hampers multi-channel operation. Figure 2(e) also shows how the bandwidth of this PSA set-up is limited by the spectra of the EDFAs used to amplify signals at the waveguide inputs with the amplifier gain peak at 1544nm clearly visible in the noise spectrum. Next, we focused on the phase squeezing on λs1 and measured the σφ-PIA/σφ-PSA ratio as function of the number of other channels present to investigate if additional channels affect the phase squeezing properties of the PSA. For each number of channels (1, 2, or 4) measurements of phase squeezing were taken with and without the idlers corresponding to the non-measured signal channels. Hence, phase squeezing was quantified with both phasesensitive (idlers present) and phase-insensitive (idlers blocked) neighboring channels. Figure 3 shows the summary of these measurements displaying the ratio of the σφ-PIA/σ φ-PSA and σA-PIA/σ A-PSA ratios in Figure 3(a) and Figure (b) respectively. Additionally, to investigate stronger phase squeezing regimes, the noise power level used to distort the BPSK modulation was increased for additional measurements denoted as „high noise‟ in Figure 3.

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Figure 3. PSA induced change of (a) phase angle (σφ-PIA/σφ-PSA) and (b) amplitude ( σA-PIA/σA-PSA) as a function of the number of signal channels

Figure 3(a) shows a σφ-PIA/σφ-PSA ratio greater than 1, indicating phase squeezing, and shows the magnitude of phase squeezing is broadly constant as a function of the number of channels and there is very little difference between PS and PI neighbouring channels. Despite the additional noise, the squeezed constellations were very similar to those in Figure 2, hence the high noise level enabled measurement of a larger σφ-PIA/σφ-PSA ratio. For the 4channel case at both noise levels, the σφ-PIA/σφ-PSA ratio drops slightly for the PS case compared to PI mode, so perhaps the presence of 4 pairs of coupled signal and idlers has some saturation effect but the difference is small and further investigation is required before conclusion can be drawn on the saturation properties of these PSAs. Figure 3(b) shows that the high noise case, which allowed measurement of stronger phase squeezing, also leads to stronger amplitude noise addition with a σ A-PIA/σ A-PSA value of 0.8 compared to 0.9 for the lower noise case, but there is no variation observed as a function of signal channels for PI or PS neighbouring channels. 4. Summary We have investigated the phase squeezing of multiple signal channels in a non-degenerate phase-sensitive amplifier constructed from periodically-poled-lithium-niobate waveguides for the first time. We demonstrate regeneration of up to four 10Gb/s binary-phase shift keyed channels see that the presence of additional channels and subsequent crosstalk from higher order non-linear products has small impact on the magnitude of phase squeezing achieved. These results show that PPLN waveguides may be useful regeneration of phase modulated data formats. 5. References [1] Z.Tong et al, “Towards ultrasensitive optical links enabled by low-noise phase-sensitive amplifiers”, Nature Photonics 5, pp.430-436 (2011) [2] R.Slavík et al“All-optical phase and amplitude regenerator for next-generation telecoms. systems” Nature Photonics 4, pp.690-695 (2010) [3] K.J.Lee et al,”Phase-sensitive amplification based on quadratic cascading in a PPLN waveguide” Optics Exp. 17 (22), pp.20393-20400 (2009) [4] B.J.Puttnam et al,“Large Phase Sensitive Gain in PPLN waveguides with High Pump Power” PTL 23(7), pp. 426−428 (2011) [5] T.Umeki et al “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides” Optics Express, 19 (7), pp.6326-6332 (2011) [6] B.J.Puttnam et al, “Experimental Investigation of Phase Squeezing in a Non-Degenerate PSA Based on a PPLN Waveguide‟ Proc. ECOC, paper Tu.5.Le Saleve.2 (2011) [7] C.Lundström et al “Optical Modulation Signal Enhancement Using a Phase Sensitive Amplifier” Proc. OFC, paper OWL6 (2011)

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