We investigate the phase squeezing characteristics of non-degenerate phase-sensitive-amplifiers (PSAs) based on periodically-poled-lithium-niobate (PPLN) waveguides. We implement two PSA configurations with phase insensitive idler generation performed in both highly-non-linear-fiber (HNLF) and PPLN waveguides. In both cases we demonstrate regeneration of a noisy BPSK signal, despite net signal attenuation in the phase sensitive PPLN, and show that the level of phase squeezing varies with the phase sensitive dynamic range (PSDR). We observe that weak idler generation in the PPLN limits the achievable PSDR and that use of HNLF for idler generation leads to the largest PSDR. However, in phase regeneration measurements we observe that the pump phase modulation, required to overcome stimulated Brillouin scattering, adds significant amplitude noise, which increases with the PSDR.
©2011 Optical Society of America
The property of phase-sensitive amplifiers (PSAs) to amplify or attenuate signals according to their phase leads to several intriguing properties. In addition to the possibility of noiseless amplification [1, 2], dispersion compensation , and optical signal enhancement , PSAs are attractive devices for performing regeneration of phase modulated signals [5–7]. 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 waves that are used as the input to a second FOPA operated as a PSA [2–7]. However, the use of cascaded second-order non-linearity in periodically-poled lithium-niobate (PPLN) waveguides offers a number of potential advantages prompting recent interest in their use for phase sensitive applications [8–14]. High non-linear coefficients may be achieved in crystals of only a few centimeters in length together with low spontaneous noise emission, low crosstalk and no intrinsic frequency chirp, offering the prospect of compact, low latency, broadband devices. Additionally, PPLNs are relatively immune to stimulated Brillouin scattering (SBS), which limits pump power in FOPA-based PSAs and requires techniques such as pump phase modulation or use of specialist strained fibers to increase the SBS threshold. Furthermore, due to their size, potential exists for integration of PPLN waveguides with other optical components, such as modulators for transmitter applications , for use in a wide range of optical signal processing application, particularly if the second harmonic maybe directly manipulated, as in [8,9].
Previously, PPLNs have been used in a variety of optical signal processing applications , but have only recently been explored as PSAs, particularly at telecom wavelengths. Phase-sensitive (PS) operation was demonstrated by direct difference frequency generation (DFG) using a 1.054nm pump in a bulk lithium-niobate optical parametric amplifier  and degenerate parametric amplification was demonstrated at telecom wavelength in a PPLN waveguide in  for generation of squeezed states for continuous-variable quantum communication experiments. More recently, degenerate phase sensitive operation with using a 10Gb/s binary-phase-shift-keyed (BPSK) signal was also demonstrated in a PPLN ridge waveguide . A non-degenerate idler PSA based on cascaded sum frequency generation (cSFG) and DFG in a PPLN waveguide was proposed for C-band PS operation  with a small phase sensitive dynamic range (PSDR) measured. Subsequently, large PSDR was demonstrated in [13, 14] with idler generation performed in a FOPA.
Here, we focus on the regenerative properties of PSAs. We expand on the results in  and characterize the phase squeezing properties of PPLN-PSAs in the non-degenerate case. In addition to a PSA based on the combination of a FOPA and PPLN , we explore and demonstrate regeneration of 10Gb/s BPSK signal in a PSA comprising two PPLN waveguides for the first time. For both FOPA-PPLN PSA (FP-PSA) and PPLN-PPLN PSAs (PP-PSAs), we use a coherent receiver to measure regenerative effects on signals, distorted by broadband noise, through phase squeezing in packaged PPLN waveguides. In both cases we demonstrate regeneration, despite net signal attenuation, and show that the level of phase squeezing varies with the PSDR. We observe that idler generation in the PPLN limits the achievable phase-sensitive response of the PP-PSA, whilst when using the FOPA for idler generation significant amplitude noise, arising from the required pump phase modulation, and increasing with the PSDR, becomes the performance limit.
A theoretical model of phase sensitive (PS) operation in a PPLN waveguide is described in  with the regenerative capability arising from the discriminating amplification of components is orthogonal quadrature as shown in Fig. 1(a) .
Since the real component of the signal has higher gain, its phase is squeezed to 0 and π phase states and a non-linear transfer function, calculated for 10dB PSDR in Fig. 1(b), maybe derived as follows in Eq. (1):
3. Characterization of phase-sensitive response
Characterization of phase-sensitive operation and measurement of phase squeezing for a 10Gb/s BPSK signal were performed for two PSA configurations with the idler generation or phase insensitive amplifier (PIA) stage implemented using both a FOPA and a second PPLN with matching quasi phase matching wavelength (λQPM). FOPA idler generation was initially investigated to take advantage of the high efficiency of FWM for idler generation and allow use of the most efficient PPLN (PPLN A) available, as the phase sensitive component. PPLN A had a grating period of 19.1µm and a λQPM of 1557.7nm at a temperature of 40°C. However, to take advantage of potential benefits of integration and direct manipulation of the second harmonic, it is necessary to investigate PP-PSAs and this was done using two PPLN devices (PPLN B and PPLN C) with matching λQPM of 1549.9nm at 40°C and grating period of 18.9µm. All devices were doped with magnesium oxide to prevent photorefractive damage at high pump powers and reduce the operating temperature . The 4.5cm long waveguides were packaged in temperature controlled fiber pigtailed modules with total insertion losses, including fiber coupling, of 5.5dB for PPLN A and PPLN B, and 7.5dB for PPLN C. For all measurements, the non-degenerate idler configuration was used with the pump located at the λQPM of the PS-PPLN and the signal and generated idler spaced symmetrical around it. As defined in , this may also be considered the copier in a copier + PSA configuration.
3.1 PSA characterization set-up
Measurements of phase-sensitive gains were obtained for the two PSA configurations as a function of the input phase state and power to the PS-PPLN. The experimental setup used to characterize is shown in Fig. 2 , together with phase relations where λp,s,i denotes the wavelength and φp,s i denotes the phase of the pump, signal and idler respectively and φrel denotes the relative phase (2φp-φs-φi) between them. The first stage was to generate phase-correlated signal (λs), pump (λp) and idler (λp) waves. This was done using a FOPA based on highly-non-linear-fiber (HNLF) for the FP-PSA, and using PPLN B for the PP-PSA. An external cavity tunable laser was used to generate λp, with the wavelength set by the λQPM of the device under test. For the FP-PSA only, λp was phase modulated with 3 RF tones to suppress SBS in the fiber but this was not required for the PP-PSA. λp was then amplified by a high-power erbium-doped fiber amplifier (HP-EDFA) and filtered with a 1.4nm optical filter to remove excess noise. A second tunable laser was used to generate λs and this was combined with the pump on the high loss arm of 90/10 optical tap. λs was chosen as trade-off between the gain spectrum of the FOPA and the HP-EDFA with values of 1549.9nm for FP-PSA and 1554.9nm for the PP-PSA. Polarization controllers (PCs) were used to align the signal and pump light and optical taps were used for monitoring of power and back-reflected light. Both signals were then passed into either PPLN B for the PP-PSA or 150m of HNLF for the FP-PSA where λi was generated (according to the relation φi=2φp-φs) and φrel=0. The HNLF had a zero dispersion wavelength (λ0) of 1545nm and nonlinear coefficient (γ) of 10 (W•km)−1.
The pump power was 27dBm at the HNLF input and 30dBm for the phase insensitive PPLN. In both cases, λs power was adjusted to prevent saturation in the PIA and to compensate for signal gain variation and differences in idler generation efficiency. The signal gain was 18dB for the HNLF-PIA and −3dB or the PPLN-PIA. After the PIA, a variable optical attenuator (VOA) was used to reduce the optical power at the input to a liquid crystal-on-silicon based optical processor (OP). The OP was used to equalize the λs and λi powers and adjust the relative phase (φrel) of the signals by adding a static phase shift (φstatic) to λp. Hence, φstatic controlled φrel and subsequently the coupling between λs, λi, λp and the induced second harmonic (λSH), allowing measurement of PS characteristics. λSH was measured at 779nm for PPLN A and 775nm for PPLN B and C with the precise λQPM optimized by temperature tuning in each case. The OP also allowed λs or λi to be selectively blocked and convert the PS-PPLN to phase insensitive (PI) operation. All 3 waves were then amplified in a second HP-EDFA and a VOA and PC were used to control the power and polarization at the input of the PS-PPLN. An optical spectrum analyzer (OSA) and power meter were used to evaluate the PS gain and attenuation.
3.2 PSA characterization results
Figure 3 shows the characterization of the phase sensitive response as a function of the static phase shift (φstatic) applied to the pump and the input pump power at the PS-PPLN. For all plots, the phase sensitive gain and attenuation is plotted relative to PI operation achieved by suppressing the idler in the OP. Figures 3(a) and 3(b) show the PS response as a function of φstatic (also φrel) for fixed input power of 27dBm for the FP- PSA, [Fig. 3(a)] and 30dBm for the PP- PSA, [Fig. 3(b)]. For the FP-PSA, the plot of 27dBm was used due to issues of thermal instability at the highest pump powers which made reliable measurement of PS-response away from minima and maxima difficult. Since, varying φrel alters the pump and signal coupling, over the measurement timescale, for Ppump<24dBm, it was necessary to optimize the temperature for thermal variations induced by phase shifts applied in the OP. It was observed that matching λQPM to the maximum signal attenuation allowed measurement of the largest PSDR but meant that phase matching was not necessarily ideal for measuring the optimum PSA gain. Hence, for the measurements in Figs. 3(c) and 3(d), the temperature of the waveguide was optimized separately for the minima and maxima of the PS-response.
In all waveguides investigated, this region of thermal instability was also accompanied by the generation of an intense visible green light at the third harmonic wavelength (λTH), believed to originate from a SFG process between the second harmonic and input signals. The presence of the λTH is believed to reduce the coupling between input signals and λSH depletion occurs both as a result of power transfer to λTH and also through green light induced infrared absorption (GRIIRA) [16–18]. These issues were investigated more thoroughly in  where it was believed that they contribute to large discrepancy between observed PS response and that predicted from a theoretical model based on , but that did not include these effects.
Figure 3 shows that despite the problems arising from thermal instability, a significant PS gain maybe achieved in both PSA configurations, although it also shows that up to 10dB higher PSDR is achieved with the FP-PSA compared to the PP-PSA. With idler generation in the FOPA and using the PPLN A, which was selected due to its strong non-linear effect in previous measurements , a PSDR of 16.5dB was achieved at maximum pump power, the largest PSDR for a degenerate PPLN based PSA reported to date. However, only a 6.2dB PSDR was measured for the PP-PSA. This is surprising since it was envisaged that removing the requirement for pump-phase modulation to increase the SBS threshold, would improve the achievable PSDR. However, In the PP-PSA, the generated idler was at least 10dB below the signal channel, compared to 1.5dB for the FP-PSA. Whilst the signal to noise ratio of the idler was still sufficient at 25dB, the requirement to attenuate all signals to below 10dBm at the input to the optical processor meant that the signal and idler were attenuated relative to the noise floor, resulting in a signal-to-noise ratio (SNR) degradation at the EDFA. The input SNR was approximately 15dB, up to 30dB below the pump power and combined with the requirement to equalize the signal and idler therefore meant that significant noise was added to both in the HP-EDFA. Furthermore the effect of the dynamic thermal instability observed in the devices at higher powers manifests itself as uncertainty or jitter in the λQPM, reducing the strength of the non-linear coupling.
Despite the reduced PSDR of the PP-PSA the relationship with increasing pump power into the PS-PPLN is similar in both configurations and consistent with the theoretical plots shown in . The discrepancy between the signal and idler power for PS suppression, shown in Fig. 3(a), is believed to originate from noise added by the pump-phase modulation, which is not evident for the PP-PSA, where pump phase modulation was not required. As observed in FOPA based PSAs  and an FP-PSA , such effects are more detrimental to PS attenuation, since the parametric attenuation minima of the PSA response is significantly narrower than the gain peak when plotted on a logarithmic scale, reducing the tolerance to the resulting phase uncertainty.
4. Phase squeezing and regeneration of 10Gb/s BPSK signals
4.1 Regeneration experiment description
The experimental set-up to investigate phase squeezing of a 10Gb/s BPSK signal is shown in Fig. 4 together with the signal phase relationships at various points. An identical transmitter set-up to that described in section 3.1 was used and the same HNLF and PPLN devices were used to perform non-degenerate idler generation. However, after the PIA, this time λs and λi were separated from λp in different arms of a wavelength-division-multiplexing (WDM) coupler. The signal arm contained a programmable filter, which allowed equalization of λs and λi powers and selective blocking of λi 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 215-1 pseudo random bit sequence was modulated onto both λs and λi. The pump arm contained an OBPF to remove λs and λi, a lead-zirconate-titanate (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 λs and λi on the high power arm of a 4-port 90/10 power coupler with the data modulation, φdata, then becoming the relative phase shift, φrel, of the recombined waves, enabling the natural squeezing transfer function of the PSA [Fig. 1(b)] 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 drive the PZT, based on interference of λp and the small λp component propagating in the signal arm. The signals were received using a single polarization coherent receiver with an additional external cavity tunable laser used for the local oscillator (LO) and the outputs of the 90° optical hybrid received with DC-coupled photodiodes (PDs) at the input to a 50Gs•s−1 real-time sampling oscilloscope with 20GHz bandwidth. To investigate the regeneration performance, the BPSK driving signal was combined with the artificial 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 as a function of the pump and second harmonic (λSH) power with the PS and PI operation compared by suppressing the idler in the programmable filter.
4.2. Regeneration results
Figure 5 shows the constellation and phase angle histograms for all samples at the highest achievable input pump power at the maximum PSDR and noise power for both the FP and PP-PSAs.
Figure 5 shows that despite an overall signal loss in the PS-PPLN, in both cases, the PSDR (11dB for FP, 6dB for PP-PSA) provides sufficient phase squeezing to provide regeneration of the BPSK signal impaired by broadband optical noise. The angular standard deviation (σφ) for the π and 0-level respectively was reduced by a factor of 2.3 and 1.95 for the FP-PSA and 1.45 and 1.35 for the PP-PSA. In addition to the squeezing of the decision symbols, Fig. 5 also shows the transition trajectory is also squeezed to the real axis in both cases. However, some phase-to-amplitude conversion also occurs with the standard deviation of the signal amplitude (σA) for the π and 0-level respectively, increased by a factor 0.4 and 0.2 for FP-PSA and 0.15 and 0.1 for the PS-PSA. Hence, these figures show that the increased PSDR of the FP-PSA allows for a stronger phase squeezing effect at the expense of additional amplitude noise originating from the pump phase modulation. In both cases, the asymmetry of the 0 and π phase states, evident in the histograms and the slightly elliptical distribution of the phase constellation is believed to originate from imperfections in the modulator and driving circuitry.
Next, signal analysis was performed as a function of PSDR. For the PP-PSA, the PSDR was varied by changing the input pump power into the PS-PPLN. For the FP-PSA, It was observed that varying λp directly, affected both the stability of the PLL and PPLN temperature. In general, the stability of the phase-locking, which used the interference between pump components in interferometer arm as the reference signal, was affected by the pump phase modulation. Additionally, this particular PPLN waveguide, chosen for its strong phase-sensitive response and because the higher λQPM was better matched to the λ0 of the HNLF, had additional instability at high pump powers, as observed in . In those measurements, it was also observed that issues of 3rd harmonic generation and resulting infra-red absorption lead to fluctuations of λSH power at high pump powers. Hence, the maximum pump power was reduced slightly to 27dBm to improve accuracy at the cost of lower maximum PSDR (10dB) and phase squeezing capacity. Furthermore, for the FP-PSA, it was preferred to tune the temperature of the PS-PPLN to reduce the efficiency of the SHG process and control the PSDR which could then be estimated from measurement of the λSH power and the device characterization in . For both PSA configurations the power of the noise signal combined with the modulator drive signal was reduced to improve stability and repeatability across all measurements.
Figure 6(a) shows a σφ-PIA/σφ-PSA ratio greater than 1, indicating phase squeezing can be measured for both PSAs in all cases and that the magnitude of the phase squeezing increases with the PSDR. As shown in section 2, this relationship is expected and although the change of standard deviation appears to be linear for both configurations, further analysis of noise distribution is required to verify this. Figure 6(b) also shows that σA-PIA/σA-PSA is below 1 in all cases showing that some phase-to-amplitude noise conversion occurs in PS operation that is not present when the 2nd PPLN is operated in PI-mode. The interesting result occurs when looking at the amount of amplitude noise as the PSDR increases. For the PP-PSA, the ratio of the standard deviation between PI and PS operation does not increase with PSDR with a fixed value of approximately 0.92. For the FP-PSA, the σA-PIA/σA-PSA was measured to be 0.78 at the lowest PSDR and reducing to 0.68 at the highest. Although some additional amplitude noise is expected, and evident in the PP-PSA case, the large amount of phase-to-amplitude noise conversion observed with the FP-PSA is thought to originate from imperfect phase-locking and noise originating from the pump phase dithering. These factors, in combination with limitation of pump power, proved to limit the effectiveness of the phase regeneration achieved with the FP-PSA. Conversely, the absence of pump phase modulation, removes some of these instabilities in the PP-PSA, where the performance limit appears to be the low PSDR achieved, resulting from the weak idler generation. These results show that although considerable room for improvement in the phase sensitive response exist, PP-PSAs can achieve good regeneration performance even with the moderate PSDR measured here. It is hoped that improved devices will allow stronger phase sensitive response. Additionally, the results of the FP-PSA, further highlight the importance of embracing more sophisticated techniques to overcome SBS for PSAs utilizing FOPAs such as those described in [5, 6].
We investigate the phase squeezing characteristics of a C-band, PPLN based non-degenerate phase sensitive amplifier using both a Fiber-Optic-Parametric-Amplifier (FP-PSA) and a second PPLN (PP-PSA) for phase insensitive idler-generation. In both cases, we demonstrate regeneration of a BPSK signal impaired by broadband phase noise, despite net signal attenuation, with the standard deviation of phase angle reduced by up to 2.3 times. In both PSA configurations, phase squeezing was observed to increase with phase sensitive dynamic range (PSDR), as expected by theory. For the FP-PSA, phase-sensitive operation caused significant addition of amplitude noise (≤40% increase of σAmplitude) that also increased with PSDR. Although phase-to amplitude noise is expected, it is believed that the measured value is partially a result of instability originating from the pump-phase modulation. For the PP-PSA, where pump-phase modulation was not required, less amplitude noise ((≤0.15% increase of σAmplitude) was added and remained broadly constant with increasing PSDR. However, the phase regenerative qualities of the PP-PSA were limited by the low PSDR originating from the weak idler generation. Overall the results confirm that PPLN waveguides merit investigation as candidates for all-optical regeneration in fiber-optic communication systems, but reveal the need to improve the device control and characteristics to exploit the advantages of using non-linear waveguides to realize phase sensitive amplifiers.
We thank Y.Tomiyama, and the NICT technical staff for assistance with experiments and acknowledge support from NICT-BME collaborative project.
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