We demonstrate a χ(2)-based in-line PSA with a carrier-recovery and phase-locking system for a phase shift keying (PSK) signal. By doubling the signal phase using a wavelength conversion technique, the carrier was recovered from a PSK signal. The carrier phase was synchronized to a local oscillator using optical injection locking. Phase sensitive amplification with a wide phase sensitive dynamic range of 20 dB was achieved using degenerate parametric amplification in a periodically poled LiNbO3 (PPLN) waveguide. The phase regeneration effect was examined for a degraded signal by means of constellation analyses and bit-error rate measurements. The in-line PSA also operated successfully as a repeater amplifier in a 160 km fiber link without a power penalty. Finally, we demonstrate the regeneration of non-linear impairments induced by fiber non-linearity.
©2013 Optical Society of America
Improvement of the signal-to-noise ratio (SNR) in photonic networks is a key requirement for achieving higher transmission capacity because, according to Shannon's theory, the SNR is an essential quantity that determines the maximum spectral efficiency . Low noise optical amplifiers are required for further high-capacity communication. However, the noise figure (NF) of conventional phase insensitive amplifiers (PIA) such as the erbium doped fiber amplifier (EDFA) and the semiconductor optical amplifier (SOA), which are based on stimulated emission, cannot be improved to below the 3 dB quantum limit . A simple way to achieve a high SNR is to increase the signal intensity. However, the transmission reach will ultimately be limited by the interplay between the Kerr effect in the fiber and amplified spontaneous emission (ASE) from the amplifiers, such as the Gordon-Mollenauer effect . Phase sensitive amplifiers (PSA) have been attracting a lot of attention because of their potential for low noise amplification and the phase regeneration of phase modulated signals. The essential difference between a PSA and a PIA is that a PSA amplifies only one of the two quadrature phase components in the signal light, while deamplifying the other. This unique property makes it possible to realize low noise amplification with a quantum limited noise figure (NF) of 0 dB  and phase regeneration using phase squeezing .
The most common way of implementing a PSA is to use four-wave mixing (FWM) in a fiber-optic parametric amplifier (FOPA). The FOPA can provide a high gain by using a pump laser emitting at the 1.5-μm telecom-band wavelength. The compatibility of fiber with optical communication is also an advantage of FOPA. An improved SNR was demonstrated in an optical link using a fiber-based non-degenerate PSA . A phase and amplitude regenerator has been demonstrated in a fiber-based PSA . Moreover, phase and amplitude regeneration was demonstrated for a differential phase-shift keying (DPSK) signal in a field link with the regenerator placed in-line or at the receiver . However, in such fiber based PSAs the signal power should be smaller than the pump power to avoid unwanted crosstalk induced by a secondary FWM process. In addition, phase modulation is required to suppress the stimulated Brillouin scattering (SBS) of a high power pump. Furthermore, in a PSA utilizing FWM, the pump and signal are in the same wavelength range thus a very high contrast optical filter is required to separate them. Extrinsic spontaneous emission generated by an EDFA may degrade the NF of the PSA.
On the other hand, recent advances on periodically poled LiNbO3 (PPLN) have enabled us to construct a χ(2)-based PSA in waveguides with a length of only a few centimeters. The precise phase matching characteristics and high nonlinearity of PPLN waveguides mean that they may achieve a low crosstalk and immunity to SBS. They may also potentially be integrated with several functions on the chip. And more importantly, a χ(2)-based PSA utilizes the second harmonic of the signal as a pump. Therefore, the signal and pump should be separated in a simple configuration. Despite these advantages, the χ(2)-based PSA was difficult to adapt for telecom applications because it was difficult to obtain sufficient gain without pulsed pumping . χ(2)-based PSAs for optical communication were explored through progress on PPLN waveguide technology. A non-degenerate PSA was demonstrated that employed cascaded second-harmonic generation (SHG) and optical parametric amplification (OPA) in a single PPLN waveguide , and cascaded SHG and difference frequency generation (DFG) and cascaded SHG and OPA in two PPLN waveguides . This cascaded SHG/OPA approach can simplify the device structure, because SH waves are generated and used only in a PPLN waveguide. However, the drawbacks of the approach are a small gain, the difficulty of separating the pump, signal, and idler, and its inability to perform a degenerate operation.
Recently, we demonstrated a CW pumped PSA with a high gain of over + 10 dB that employs two individual highly efficient PPLN ridge waveguides for SHG and OPA . We also confirmed the low noise characteristics of the PSA with an NF below 3 dB . In these experiments, both the pump and signal were generated from a master laser. This configuration interferes with the application to in-line amplification. In-line operation is especially important for utilizing the PSA as a repeater amplifier in transmission line. To achieve in-line operation, we must generate a local pump that synchronizes with the phase of the signal.
In this work, we experimentally demonstrate a χ(2)-based in-line PSA using PPLN ridge waveguides with a carrier-recovery and phase-locking system utilizing wavelength conversion and optical injection locking. The configuration has no extra EDFA in the signal line. The performance is investigated experimentally using DPSK signals. We examined the phase regeneration effect for a signal degraded by phase noise. The in-line PSA operates successfully as a repeater amplifier in a 160 km dispersion managed link. We also demonstrated the regeneration of an impairment of the non-linear phase noise, which was induced by the interplay between the Kerr effect in the fiber and ASE.
2. Configuration of in-line PSA and principle of carrier recovery
Figure 1 shows the configuration of the in-line PSA. It has three PPLN waveguides with one for carrier recovery, one for pump generation, and one for optical parametric amplification (OPA). The optical devices in the in-line PSA consisted solely of polarization maintaining components. The polarization of the input signal was adjusted and 10% of the power tapped off for the carrier-recovery/phase-locking stage. The tapped signal and a free-running local CW laser as Local 1 were mixed and amplified by an EDFA. For carrier recovery, we used a cascaded second harmonic generation (SHG) and difference frequency generation (DFG) scheme in a PPLN waveguide. The signal generates an SH wave, which simultaneously mixes with Local 1 to generate an idler wave. Then, the idler phase satisfies the following relation.
By doubling the signal phase, the carrier is recovered from a PSK signal without a carrier component. The carrier phase is copied to the 1.5-μm-band idler wave by the DFG process. The idler wave was filtered using a band-pass filter (BPF) and an arrayed waveguide grating (AWG). Then, the idler wave was injected into a semiconductor slave laser as the second local wave (Local 2) for phase locking. Local 1 and Local 2 were ± 200 GHz from the carrier frequency of the signal. At the PSA, we used two PPLN ridge waveguides with a high conversion efficiency of over 2000%/W . The PPLN waveguides for pump generation and OPA were assembled into fiber-pigtail modules with four input/output ports to allow SH pumping. Local 1 and Local 2 were combined in the AWG, amplified with the EDFA, and injected into PPLN module 2 to generate a sum frequency (SF) wave around 770 nm, which corresponds to the SH wavelength of the signal. Then, we obtained an SF pump power of over 300 mW. The generated SF pump and data signal were injected into PPLN module 3 for OPA. There was no additional PIA booster, such as an EDFA, in the data signal path. The dichromatic mirrors in the two modules effectively suppressed the unwanted output of the 1.54-μm-band pumps and ASE generated by the EDFA. To achieve a stable PSA output, an optical phase-locking loop (PLL) based on a piezoelectric transducer (PZT) was used to compensate for the slow relative phase drifts between the signal and SF-pump lights induced by temperature variations and acoustic vibrations.
3. Experimental results and discussion
First, we examined the performance of the in-line PSA using a CW signal and a 40-Gbit/s non-return-to-zero (NRZ)-DPSK signal at a wavelength of 1535.8 nm. Both the in-phase (amplification) and quadrature-phase (deamplification) conditions of the relative phase between the signal and SF-pump lights were individually obtainable by changing the setting of the PLL. Figure 2(a) shows the in-phase and quadrature-phase PSA output spectra for the CW signal as well as the output signal without pumping. The input power of the CW signal was about 0 dBm. For the in-phase signal, we obtained an internal gain of + 11 dB. While for the quadrature-phase signal, we obtained deamplification of −9 dB. These results clearly show the phase sensitive property. The highly efficient PPLN ridge waveguide yielded a high phase sensitive gain of over 10 dB, which was achieved with a CW pump and a single pass through the PPLN waveguide. Figure 2(b) shows the PSA output spectra for the DPSK signal with an input power of −10 dBm. Even with the modulated signal, we obtained an internal gain of + 11 dB and deamplification of −9 dB, which were the same as for the CW signal. This result shows that the carrier phase recovery scheme was working properly for the DPSK signal. Thanks to the carrier phase recovery system, we obtained a high phase-sensitive dynamic range (PSDR) of 20 dB for both the CW and DPSK signals.
Figure 2 also shows that the PSA output spectra have high SNRs of over 60 and 45 dB (at 0.01 nm resolution) for the CW signal and the DPSK signal, respectively. These noise levels were the intrinsic ASE levels from the parametric amplification generated by the PPLN waveguide for OPA . As mentioned above, we used no booster amplifier in the signal path. So, the in-line PSA provides a high SNR without excess ASE noise.
We examined the phase regeneration effect for a signal with phase noise. Figure 3 shows the experimental setup. A CW light from an external cavity laser diode (ECLD) was modulated by 10 or 40 Gbit/s NRZ-DPSK data using a Mach-Zehnder modulator (MZM). To emulate the effects of phase noise, the DPSK signal was modulated by an additional phase modulator at a specified frequency. The magnitudes of the phase perturbation were controlled by varying the modulation depth to emulate different noise levels. The phase regeneration performance was characterized using constellation analyses and bit-error rate (BER) measurements. The BER was measured with a differential receiver, which consisted of a delay interferometer (DI), a balance PD, and an error detector (ED). The signal was detected by a digital coherent receiver to obtain a constellation diagram. The real and imaginary parts of the transmitted signal were detected by balanced photo detectors and digitized at 50 GS/s by a digital storage oscilloscope.
The signal rate was 10 Gbit/s for the constellation measurement. The signal light ran through the phase noise modulator, which was driven at 2.1 GHz. Figure 4 shows the constellation diagrams for the signal light at different phase noise levels and the signal after applying in-line PSA. We adjusted the intensities of these signals so that they were the same by using the EDFA and variable optical attenuator (VOA) before measuring the constellation. The phase deviations of the input signals were ± 20°, ± 30°, and ± 45° per symbol for the different noise levels. These phase deviations were squeezed down to about ± 15° for the PSA outputs. These results clearly show that the in-line PSA operates precisely even for a signal with phase noise. The quadrature phase components of the noise decrease greatly due to the phase regeneration of the in-line PSA.
Next, we examined the BER characteristics of a DPSK signal. The signal rate was 40 Gbit/s. The phase modulator that added the phase noise was driven at 10.3 GHz. The signal power after modulation was −10 dBm. Figure 5(a) shows the BER characteristics of the DPSK signal with and without phase noise and after phase regeneration using the in-line PSA. There are no power penalties if we apply the in-line PSA to a signal without phase noise. Phase noise produces a power penalty. The power penalty of the signal was greatly improved after the signal had passed through the in-line PSA. The degree of improvement depended on the amount of phase noise added. In this experiment, a 5.5 dB improvement was obtained at a BER of 10−9 compared with the degraded signal. We also observed the improvement in the eye diagram. Figure 5(b) shows the eye diagrams for the signals with phase noise and after PSA at a received power of −32 dBm. It is shown that a clear eye diagram is achieved after PSA.
We then employed the in-line PSA as a repeater in a 160 km transmission fiber link with two 80 km spans. The experimental setup is shown in Fig. 6. A CW light from an ECLD was modulated by 40-Gbit/s NRZ-DPSK data with a 29-1 pseudo-random bit sequence (PRBS). The DPSK signal was amplified to + 10 dBm with an EDFA, and then launched into a transmission fiber. Each fiber of the 80 km span was dispersion managed, and consisted of a single mode fiber (SMF) and a reverse dispersion fiber (RDF). The total dispersion and loss were 0.6 ps/nm and −18 dB for span1, −1.5 ps/nm and −19.5 dB for span2.
Figure 7 shows the bit-error rate characteristics of a 160 km fiber link with or without the in-line PSA as a function of the attenuation, which include the fiber loss (−37.5 dB) and linear loss caused by the attenuator before the pre-amplifier of the receiver. The margin of the received power is increased at the PSA during use. The obtained 3-dB power margin agrees with the net gain of the in-line PSA. This result clearly shows the in-line operation of the PSA without a power penalty.
One large problem in transmission systems is the phase noise induced by nonlinear effects in the fiber. So, we tried the phase regeneration of a signal degraded by nonlinear effects in a fiber. To induce phase noise caused by the interplay between the Kerr effect in the fiber and ASE, we positioned an attenuator in front of the EDFA to change the OSNR. Then, the signal was amplified to + 19 dBm and launched into an 80 km transmission fiber consisting of an SMF and an RDF. Figure 8 shows the constellation diagrams for the input signals to the PSA after fiber transmission and the PSA output signals for various OSNR levels. These results clearly show that the PSA works as a regenerator for phase noise induced by nonlinear effects in fiber.
We demonstrated an in-line PSA based on highly efficient PPLN waveguides, for the first time. The carrier-recovery and phase-locking system consisted of a wavelength converter and an optical injection-locking scheme. The configuration had no extra EDFA in the signal line. Phase regenerative amplification was achieved with a wide phase sensitive dynamic range of 20 dB for DPSK signals. The phase regeneration effect was examined using constellation analyses and bit-error rate measurements. We also successfully implemented the in-line PSA as a repeater amplifier in a 160 km fiber link. Finally, we demonstrated the regeneration of non-linear impairments induced by the interplay between the Kerr effect in the fiber and ASE.
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