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We present a new method to perform phase regeneration of phase encoded signals. In our concept called “hybrid optical phase squeezer (HOPS),” a multilevel phase-quantized signal is synthesized through the coherent addition of a phase-conjugate copy of the signal and a phase harmonic of the signal with a frequency shifter. Unlike the conventional method by phase sensitive amplification, HOPS does not use any optical parametric gain such that only optical elements with low optical nonlinearity are necessary for optical phase quantization. In the proof-of-concept experiment, it is confirmed that a 2-level HOPS can perform quadrature squeezing with an extinction ratio of 40 dB. Simultaneous phase regeneration of two coherent wavelength-division-multiplexed 10.75-Gb/s binary phase-shift keyed signals is successfully demonstrated using a 2-level HOPS based on a semiconductor optical amplifier.
© 2014 Optical Society of America
1. Introduction
Recent progress in nonlinear optics such as optical parametric processes promises the realization of future low-energy networks based on all-optical path switching [1]. In particular, all-optical quantization of optical phase is a versatile technique to be used in optical telecommunications in addition to the photonic analog-to-digital conversion [2]. Optical phase quantization has been demonstrated using phase sensitive amplification (PSA) [3], being this technique the most efficient so far to realize 2-level phase quantization, historically known as “quadrature squeezing” [4]. It enables the regeneration [5–9], format conversion [10, 11] and phase comparison [12] of phase encoded signals. Quadrature squeezing provides two important functions for the future optical node. The first one is phase regeneration of binary phase-coded signals. Dual-pump PSA (DP-PSA) operating in the gain saturation regime enables simultaneous regeneration of amplitude and phase [5–7]. The second one is de-multiplexing of the two orthogonal phase components of highly spectral-efficient multilevel signals [13]. This operation, for instance de-multiplexing of a 16 quadrature-amplitude modulation (16QAM) signal into two 4-level amplitude-shift keyed signals, is considered as the first step for an all-optical format conversion of multilevel signals. In contrast to simultaneous phase and amplitude regeneration, the phase de-multiplexing operation does not require gain saturation; in fact it causes signal distortion problems. Instead, high gain extinction ratio (GER) needs to be achieved to fully suppress the unwanted phase component; typically a GER greater than ~25 dB is required. To achieve a high GER in the conventional PSA, where in-phase component is amplified while quadrature phase component is de-amplified by the same factor [5, 14], high pump power is needed to create a certain level of optical parametric gain. The high power requirement is relieved by the use of a new mechanism called “sideband-assisted PSA”, where a high GER can be obtained since the quadrature component is drastically attenuated with respect to the in-phase component in the presence of low optical nonlinearity [10, 15].
So far, regeneration of binary phase-shift-keyed (BPSK) and quadrature phase-shift-keyed (QPSK) signals have been successfully demonstrated by DP-PSA employing either highly nonlinear fibers (HNLF) with suppressed Stimulated Brillouin Scattering [6, 8] or periodically poled lithium niobate (PPLN) based components [7, 9]. The effect of in-line BPSK regenerator realized by these technologies was verified in the 38 channel dense wavelength-division multiplexing (WDM) transmission field-trial [16] and in the 5460-km long-haul multi-span transmission [17]. Also, phase de-multiplexing of a QPSK signal into BPSK signals has been demonstrated using sideband-assisted PSA, which employed state-of-the-art low dispersion HNLF [13]. Although these experiments demonstrate excellent performance, their applicability is limited due to its building components, which are bulky and require high pump power levels, typically more than 150 mW. For the future practical applications, it is desirable that optical phase quantization can be assessed using various kinds of elements, especially the ones which are compact and low power consuming, suitable for photonic integration. This is hard to achieve in the all-optical method using PSA-based technologies, whose mechanism relies on substantially high level of optical nonlinearity.
On the other hand, quadrature squeezing is realized by the coherent addition of the signal and its phase conjugate copy. If the frequency separation between the two signals is small enough, coherent addition can be performed by a frequency shifter equipped with a phase-locked loop (PLL) for the phase control. Compared to the methods based on PSA, this approach may be considered as a hybrid method combining an optical parametric process and high frequency electronics. As no optical parametric gain is utilized, this method is free from the issues mentioned above and is suitable for practical application.
Furthermore, to support the future low-energy optical network, the ability to handle WDM signals efficiently is an important requirement that must be met by optical signal processing technologies. This continues to be a challenge and there are only a few demonstrations available [18–21]. To perform phase regeneration of WDM signals through optical parametric processes, phase-locked pumps need to be prepared for all the channels. However, if all the signal carriers have a fixed phase relationship, which is the case of the so called coherent WDM (CoWDM) [22], the ensemble of phase-locked pumps can be easily obtained from an optical frequency comb that is phase-locked to the signal. In this sense, the hybrid method that we addressed above is advantageous because the PLL, which is necessary for phase locking, is already included in the system.
Hence, in this paper, we propose a new concept based on the hybrid method to quantize optical phase with arbitrarily high GER using a standard nonlinear optical material. We call it hybrid optical phase squeezer (HOPS). In section 2, we explain the operation principle of HOPS. In section 3, we present proof-of-concept experiments. In section 4, we demonstrate simultaneous phase regeneration of two CoWDM BPSK signals using 2-level HOPS. Finally, we discuss the feature of HOPS in section 5.
2. Principle of HOPS
For an optical signal bearing a phase ϕ, M-level phase quantization is achieved by coherently adding a phase conjugate with a phase of -(M-1)ϕ to the signal. The response of the synthesized signal, A, is expressed as
where ϕs is the phase of the synthesized signal and m is the mixing ratio [2]. As indicated by Eq. (1), the amplitude of the synthesized signal becomes a sinusoidal function of the phase with a periodicity of 2π/M. The optimum phase quantization is achieved when m ≈1/(M-1), while high GER is achieved when m ≈ 1. The procedure of phase quantization involves three processes; phase conjugation, multiplication and coherent addition. In DP-PSA, all the processes take place simultaneously in a single element and m becomes a parameter that cannot be controlled directly. In general, m ≈1 is attained when quite high level of optical nonlinearity is induced, which is hardly met in the standard nonlinear optical elements. The main idea of HOPS is to circumvent this difficulty by separating the process of coherent addition from other nonlinear processes.Figure 1 illustrates the concept of HOPS in the case of M = 2 (2-level HOPS). The signal at νs is first mixed with a pump at νp to generate a phase conjugate (idler) by four-wave mixing (FWM) in a nonlinear medium. Then, the signal and idler are filtered out and equalized in amplitude. After amplification by a standard optical amplifier such as an erbium-doped fiber amplifier (EDFA), the signal-idler pair is guided to an amplitude modulator (AM), which is driven sinusoidally as when generating carrier-suppressed return-to-zero pulses. The AM splits the input signal into two components whose frequencies are up- and down-shifted by the modulation frequency, Ω. If the modulation frequency is equal to the frequency difference between the signal and the pump, e.g. Ω = νp - νs, the up- (or down-) converted signal and the down (or up-) converted idler are located in the same frequency and interfere. The amplitude of the interference signal, which is generated at νp, becomes a sinusoidal function of the “relative phase” with a period of π. Here, the “relative phase” is defined as the phase difference between the two signals of the same frequency Ω; one is the modulation signal (electrical) applied to the AM and the other is the beat signal (optical) between the signal and the pump. This phase-to-amplitude response ensures that the interference signal becomes the phase regenerated version for the input of the BPSK signal. In this scheme, high GER can be easily achieved by precisely adjusting the power levels of the signal-idler pair. We should clarify however, that in this process, phase noise is converted to amplitude noise via the cosine transfer function. Moreover, if the FWM efficiency is very low optical-signal-to-noise-ratio (OSNR) degradation observed by the idler, will be transferred to the final phase regenerated signal. Therefore, to achieve a full signal regeneration (i.e., both amplitude and phase regeneration), HOPS needs to be used with an amplitude regenerator that does not increase the phase noise. As such an amplitude regenerator, semiconductor optical amplifiers (SOA) [23] or injection-locked diode laser [24] are attractive candidates in terms of compact size and low power consumption. Still, compared to PSA, this approach allows for design flexibility because the performance of each regenerator (i.e., amplitude and phase regenerator) can be optimized separately.
For M > 2, M-level HOPS should be arranged similarly to 2-level HOPS, with the phase conjugate signal interfering with the (M - 1)th phase harmonic signal. This can be achieved in several ways. Figure 2 presents two examples of 4-level HOPS; single-pump configuration and dual-pump configuration. The two concepts are slightly different regarding the way of using the AM. The AM is used for the signals after the optical parametric process in the single-pump configuration, while it is used for the pump laser before the optical parametric process in the double-pump configuration.
Fig. 2 Principle of 4-level HOPS. (a) Single-pump configuration and (b) dual-pump configuration. AM: Amplitude modulator, OCG: Optical comb generator.
Figure 2(a) depicts the single-pump configuration, where the signal at νs is mixed with a single pump at νp; their frequency difference is denoted as f (≡ νp - νs). The phase conjugate (-ϕ) generated at νi(−1) = νs + 2f and the third phase harmonics (3ϕ) generated at νi(3) = νs - 2f are coherently added by an AM which is driven by 2f, producing an interference signal at νs. Alternatively, the AM may be replaced by two cascaded AMs driven by f .
Figure 2(b) depicts the dual-pump configuration. In this scheme, a frequency comb is generated from a pump laser using an AM (or a similar device such as phase modulator). Two modes at νp1 = νp - 2Ω (pump-1) and νp2 = νp + 2Ω (pump-2) are selected as pumps, where Ω is the modulation frequency. The two pumps are mixed with the signal at νs and launched into a nonlinear optical element; the frequency difference between signal and pump-1 is denoted as Δ (≡ νs - νp1). In the nonlinear element, the third phase harmonics (3ϕ) is generated at νi(3) = νp1 + 3Δ through the cascaded degenerate FWM with pump-1 and phase conjugate (-ϕ) is generated at νi(−1) = νp2 - Δ through the non-degenerate FWM with pump-1 and pump-2. The frequency difference between the phase conjugate and the third phase harmonics, δ (≡ νi(−1) - νi(3)), is given by
If the modulation frequency is equal to the frequency separation between signal and pump, e.g. Ω = f, the phase conjugate and the third phase harmonics are located in the same frequency, which is νp + Ω, generating an interference signal. The power of the third phase harmonics is proportional to the power of pump-1, while the power of the phase conjugate is proportional to the power of the two pumps. Therefore, by adjusting the power ratio between pump-1 and pump-2, the two idlers can be coherently added with the desired mixing ratio, m.When δ ≠ 0, δ gives the beat frequency between the two idlers. If we define reference frequency as νref ≡ νp - Ω, Eq. (2) is written as
Equation (3) implies that if the signal frequency deviates from νref, the optical power generated at νp + Ω oscillates four times the frequency offset. This phase comparison function holds not only for the CW signal but also for the QPSK signal owing to the π/2 phase period of the power transfer function. Thus, 4-level HOPS provides a phase discrimination signal for the QPSK signal, which is useful to stabilize νs to νref. Likewise, by using M-level HOPS, it is possible to phase lock a local CW laser to the carrier phase of a M-level phase-shift keyed (PSK) signal. For stable operation, the interference signal should preferably have a high contrast (e.g., GER>10dB).3. Experimental proof-of-concept
This section presents the experimental verification of the proposed concept. First, the static characteristics of the 2- and 4-level HOPS are confirmed through the measurement of the power transfer functions. Then, the basic performance of 2-level HOPS is confirmed observing the effect of quadrature squeezing over a phase-noise loaded BPSK signal. In all the experiments, SOA are used as the nonlinear material because of its attractive characteristics; it requires low pump power levels and allows for photonic integration.
3.1 Power transfer function of 2- and 4-level HOPS
We measured the power transfer function of HOPS using the setup shown in Fig. 3, where an optical frequency comb was used as the light source. The frequency comb was generated from a CW laser (frequency: ν0) using an optical comb generator (OCG) that was driven by a stable RF signal (frequency: Ω = 43 GHz) from a synthesizer. In the evaluation of 2-level HOPS, two modes at ν0 - Ω and ν0, used as the signal and the pump, respectively, were selected from the comb using an optical processor (OP-1) and launched into an SOA. At the SOA output, the signal at ν0 - Ω and the idler generated at ν0 + Ω were selected using a second optical processor (OP-2). The two OPs had the function of controlling the phase and the amplitude of each mode independently. Using OP-2, the signal power was reduced to the same level as the idler within an error of ± 4%. The signal-idler pair was then guided to a Mach-Zehnder amplitude modulator (MZM), which was driven by the same RF signal used for driving the OCG. The output spectrum of the MZM was monitored by an optical spectrum analyzer (OSA) while changing the signal phase through OP-1. Figure 4(a) plots the power of the interference signal generated at ν0 as a function of the signal phase, θ, introduced by OP-1. The curve in the figure is a theoretical fit to the data obtained from Eq. (1) with M = 2 and m = 0.99. We verified that the phase dependence had a period of π as shown in Fig. 4(a), which is a clear indication of a 2-level phase quantization. Figure 4(b) shows the optical spectra observed at θ = 0.6 rad and θ = 2.2 rad, from which it is confirmed that a large GER of 40 dB is achieved.
Fig. 3 Experimental setup used for measuring the amplitude transfer function of 2- and 4-level HOPS. OCG: Optical comb generator, Syn: Synthesizer, OP: Optical processor, MZM: Mach-Zehnder amplitude modulator, OSA: Optical spectrum analyzer.
Fig. 4 (a) Power transfer function of 2-level HOPS. (b) Output spectra of MZM in 2-level HOPS. (c) Power transfer function of 4-level HOPS obtained with m = 0.6 (bold; red) and m = 0.75 (dash; blue).
The performance of 4-level HOPS in the dual-pump configuration was also studied using the same setup, except that the output was observed after the SOA. Also here, we selected the mode at ν0 - Ω as the signal and two other modes at ν0 ± 2Ω as the pumps. In this measurement, the idler generated at ν0 + Ω at the SOA output was monitored as the output signal, and the mixing ratio could be changed to a desired value by adjusting the pump power at ν0 + 2Ω through OP-1. Figure 4(c) reports the power transfer functions measured for two different mixing ratios (m = 0.6 and m = 0.75). The phase period of π/2 indicates that the idler generated at ν0 + Ω is the 4-level phase quantized signal. The power transfer functions show GERs of 12 dB and 17 dB, for m = 0.6 and m = 0.75, respectively. The measured GERs, which are well above the optimum value (≈7 dB) for phase regeneration of a QPSK signal, are also sufficiently high for use as a phase comparator. In our recent work of de-multiplexing of a QPSK signal into two BPSK signals, SOA-based 4-level HOPS was used to prepare two phase-locked pumps symmetrically arranged around the QPSK signal [13].
In HOPS, it is essential that the modulation frequency of the MZM is equal to the frequency difference between signal and pump. This ideal condition was fulfilled in the proof-of-concept experiment described above, where the frequency difference was predetermined by the synthesizer. However, in a practical scenario, the frequency difference is unknown. Even if it could be detected and used to drive the MZM, a stable interference signal could not be obtained unless the modulation signal is applied to the MZM at the precise right timing. This approach, which belongs to feed-forward control, requires a highly sophisticated implementation technique. However, when the MZM is driven in the ideal condition the interference signal becomes stable. This suggests that the condition required for driving the MZM is satisfied by stabilizing the interference signal through feedback control. This method does not need any information about the frequency difference and is easy to implement. Therefore, PLL is employed in HOPS as the main component and the MZM, which is introduced for the purpose of coherent addition, is also used as an element of PLL as it will be shown in the following section.
3.2 Quadrature squeezing by 2-level HOPS
In the I-Q plane, 2-level HOPS projects the signal constellation onto an axis defined by the pump phase; here we call it “gain axis” after the manner of PSA. Figure 5(a) illustrates the effect of 2-level HOPS for the input of phase-noise loaded BPSK signal. We confirmed this noise squeezing property using the setup shown in Fig. 5(b). The 10.75-Gbit/s BPSK signal (pseudo-random binary sequence: 215 - 1) was generated from an external cavity laser at 1550 nm (frequency: ν0 = 195.4 THz). After amplification by an EDFA, another phase modulation of 672 MHz was applied to the signal to introduce phase distortion of ± 12°. The pump beam was obtained from the same laser, whose frequency was up-shifted from the signal by Ω0 = 43 GHz using a combination of a phase modulator and an optical bandpass filter. After amplification by an EDFA, the pump beam was mixed with the signal and launched into the SOA. The power of the signal and the pump launched into SOA were −3 dBm and 3 dBm, respectively and the total output power of the SOA was 13 dBm. Figure 6(a) shows the input and output spectra of the SOA. The signal at ν0 and the idler generated at ν0 + 2Ω0 were filtered out at the SOA output and their power was equalized using an OP. The signal-idler pair was amplified to 10 dBm by an EDFA and guided to an MZM, which was driven by a voltage-controlled oscillator (VCO) with frequency Ω (≈43 GHz). Figure 6(b) shows the signal spectra observed at the input and output of the MZM. At the MZM output, the interference signal generated at ν0 + Ω0 was filtered out and monitored. We found that its power changed by more than 20 dB, as confirmed by the spectra shown in Fig. 6(c), which were recorded with the 672-MHz phase distortion switched off. A part of the output signal was guided to a photo detector (PD) for monitoring the power and providing a control signal to the VCO. The VCO frequency was controlled so that the power of the interference signal remained constant.
Fig. 5 (a) Transformation of a phase-noise loaded BPSK signal by quadrature squeezing. (b) Experimental setup for phase regeneration of the BPSK signal by 2-level HOPS. MOD: BPSK modulator, PPG: Pulse pattern generator, Syn: Synthesizer, PM: Phase modulator, F: Optical bandpass filter, OP: Optical processor, MZM: Mach-Zehnder amplitude modulator, PD: Photo detector, OMA: Optical modulation analyzer.
Fig. 6 (a) Signal spectra at the input (dash) and output (bold) of SOA. (b) Signal spectra at the input (dash) and output (bold) of MZM. (c) Output signal spectra with maximum (bold) and minimum (dash) power.
Figure 7(a) shows the oscilloscope trace of the output voltage of the PD when the VCO was in free running condition. For comparison, the trace recorded without the pump is also shown in the figure. We note that the minimum power of the output signal did not become zero because the input signal was phase noise loaded. Figure 7(b) shows the constellations of the input signal and the output signal with the VCO in the locked condition at different output power levels (A ~C). As shown in Fig. 7(b), the signal constellation changed as the locking point is shifted. This is a clear evidence of quadrature squeezing and shows that the orientation of gain axis can be changed by the adjustment of locking point. The phase regenerated BPSK signal is obtained when the output power is near the maximum. The phase deviation, which was measured by an optical modulation analyzer (OMA), was reduced from ± 12% to ± 4%, while the amplitude noise was increased from 4.8%rms to 9.8%rms. This performance is similar to that we observed with the sideband-assisted PSA [15].
Fig. 7 (a) Oscilloscope trace of the PD output voltage recorded when the VCO was in free running condition. (b) The constellations for the input and the output signals obtained at different locking points (A~C).
4. Simultaneous phase regeneration of two CoWDM BPSK signals by 2-level HOPS
In the case of CoWDM signals, optical signal processing can be performed efficiently by using an optical frequency comb as the light source of the pumps. In this section, we demonstrate simultaneous phase regeneration of two 10.75-Gb/s BPSK CoWDM signals using 2-level HOPS equipped with an optical frequency comb. The experimental setup is shown in Fig. 8. From an external cavity laser at 1550 nm (ν0 = 193.4 THz), an optical frequency comb with a mode spacing of Ω0 = 43 GHz was generated using an OCG. From the frequency comb, two pairs of modes at {ν0 - 3Ω0, ν0 + Ω0} and {ν0 - 2Ω0, ν0 + 2Ω0} were obtained at the two output ports of OP-1. The first pair was used to generate two phase coherent 10.75-GHz BPSK signals (PRBS: 215 - 1) and the second pair was used as the pumps. To emulate the effect of nonlinear phase noise, 672-MHz phase modulation of ± 24° was applied to the BPSK signals using a phase modulator driven sinusoidally. The CoWDM signals and the pumps were mixed by a 3-dB (2 × 2) optical coupler (OC). In each output arm of the OC (labeled as ch.1 and ch.2), the signal at νs ( = ν0 - 3Ω0 in ch.1 and ν0 + Ω0 in ch.2) was combined with the pump at νs + Ω0 and launched into an SOA for phase conjugation. The input power to the SOA was −6 dBm and 0 dBm for the signal and the pump, respectively. The two outputs from the SOA were combined by OP-2, where the signal-idler pairs from the two channels were filtered out and equalized in power. The output of OP-2, whose spectrum is shown in Fig. 9(a), was amplified to 10 dBm by an EDFA and guided to an MZM. The output of the MZM was divided by a 3-dB OC and the interference signals generated at ν's = νs + Ω0 were filtered out. A part of the interference signal of ch. 1 was detected to control the VCO driving the MZM and hence a stable condition for both interference signals at ch. 1 and ch. 2 was reached simultaneously. Figure 9(b) shows the output spectra of the MZM observed at two different locking points. It was observed that the interference signal of ch. 1 and ch. 2 changed synchronously when the locking point was changed. The pump phase of ch. 2 was adjusted through OP-1 so that phase regeneration was achieved simultaneously for the two channels.
Fig. 8 Experimental setup for simultaneous phase regeneration of two BPSK CoWDM signals. OCG: Optical comb generator, OP: Optical processor, Mod: BPSK modulator, F: Filter, PPG: Pulse pattern generator, PM: Phase modulator, PC: Polarization controller, OC: Optical coupler, SOA: Semiconductor optical amplifier, MZM: Mach-Zehnder amplitude modulator, Syn: Synthesizer, PD: Photo detector, OMA: Optical modulation analyzer.
Fig. 9 (a) Output spectra of OP-2. (b) Output spectra of the MZM observed at two different locking points.
The phase regenerated signals were assessed using an OMA, which consisted of a coherent receiver and a real-time oscilloscope. Figure 10 plots the bit error rates (BERs) curves as a function of OSNR of the input and output signals of each channel. The BER curve of the noise-free (B-to-B) BPSK signal is also included as a reference. Each inset show a sample constellation diagram of the input and the output signals. The phase deviation (peak-to-peak) was reduced from ± 24° to ± 7°. The phase degraded signal (input curve) showed a ~1.2 dB OSNR penalty, which was effectively mitigated by the 2-level HOPS (output curve). While the BER improvement proves the effect of HOPS under the condition of single tone phase noise, assessment using white noise would further clarify actual performance in the inline operation.
Fig. 10 BER curves measured for (a) ch.1 and (b) ch. 2. The insets show the constellation diagrams of the input and output signals.
In the present scheme, the optical parametric process is performed separately for the two channels. In PSA, it is possible to regenerate two BPSK signals simultaneously in a single nonlinear element, provided a proper system optimization to minimize inter-channel crosstalk [19]. However, for larger number of signals, parallel signal processing employing same number of PSA-PLL units seems necessary. In the case of CoWDM signals, however, HOPS constitutes an efficient and simple solution because the coherent addition can be performed in a single MZM-PLL unit as demonstrated here. However, care should be taken in the allocation of the WDM signals to avoid cross-talk in the MZM. In the present experiment with the 172GHz channel separation, which was equal to four times the MZM modulation frequency, the 3rd order tone generated in one channel overlapped with the output signal of the neighboring channel. Since the power of the 3rd order tone was 32 dB below that of the 1st order tone, the effect of the leakage field was almost negligible. It should be mentioned that the present experiment was conducted in a way that the impact of cross-talk was reduced because the two WDM signals were perfectly correlated.
Although our experiments were conducted using the same laser for the signal and the pumps, it fairly simulates actual inline operation in the future optical communication system. Conventionally, the pumps phase-locked to the signal are provided from a local laser using an optical PLL [25]. For this purpose, the local laser needs to have a wide control bandwidth; typically, a bandwidth which is ten times the laser linewidth is necessary to suppress the phase noise of a diode laser [26]. In HOPS, the local laser is free from this constraint because the error signal is fed-back to an internal VCO. The VCO used in the experiments has a bandwidth of 10 MHz, which should be sufficient for suppressing the phase noise of a diode laser with submegahertz linewidth. In the present experiment, the PLL worked mainly to suppress the phase noise of the VCO. The use of narrow linewidth lasers is becoming popular in coherent transmission systems and is hence expected to allow inline operation of HOPS, where PLL would work to suppress the phase fluctuation between the signal and the pump.
In the phase regeneration of BPSK or QPSK signals by DP-PSA, two phase-locked pumps can be generated from the incoming signal using a modulation stripping technique [27]. Even in this scheme, slow feed-back loop using a piezoelectric transducer (PZT) needs to be employed to compensate for the slow drift of the phase and long-term operation is hampered by the limited dynamic range of the PZT. In the phase de-multiplexing operation, the modulation stripping technique is not able to generate two phase-locked pumps symmetrically arranged around the signal. In any case, a practical system strongly requires the employment of a PLL and therefore HOPS becomes a powerful candidate because of its already included PLL.
5. Discussion and concluding remarks
In this paper we have presented a new concept called hybrid optical phase squeezer (HOPS) to enable optical phase multi-level quantization using any type of nonlinear optical element with arbitrarily high GER. While optical phase quantization involves three processes: phase conjugation, multiplication and coherent addition, our approach relies on realizing the coherent addition separately from the other nonlinear processes. This is the key element that sets HOPS apart from other technologies (e.g. PSA), allowing the use of any low optical nonlinear device (as long as it can perform FWM), and facilitating the achievement of a high GER.
Using an SOA as a nonlinear element, we experimentally demonstrated the concept. First, we showed the basic characteristics in the static regime. GERs as high as 40 dB and 17 dB were observed for 2-level and 4-levels of quantization, respectively. Next, we studied the performance of 2-level HOPS employing phase-noise loaded input signals. The signal and its phase conjugate were coherently added with a MZM whose modulation frequency was actively controlled to stabilize the output signal power. The effect of quadrature squeezing over a phase-noise loaded BPSK signal was then exposed. Finally, using an SOA-based 2-level HOPS, we successfully regenerated two channels of phase-noise loaded 10.75-Gb/s CoWDM BPSK signals. This implies that all-optical regeneration is no longer limited to single channel operation but it can be applied to WDM signals as long as they are coherently related (i.e. CoWDM).
We remark that in HOPS, as the signal gain is provided by a standard optical amplifier rather than by the nonlinear element, the signal OSNR is degraded by the addition of amplified spontaneous emission (ASE). However, in 2-level HOPS, the quadrature component of ASE is diminished by the coherent addition with the phase conjugate, leading to a 3dB amplifier noise reduction. Thus, including the noise squeezing property, the performance of 2-level HOPS is very similar to PSA-based processing. Furthermore, phase regeneration of BPSK signals by 2-level HOPS is accompanied by a frequency shift, while phase regeneration of QPSK signals by single-pump 4-level HOPS is not. If desired, another MZM may be added to the 2-level HOPS for restoring the original frequency.
The signal baud-rate that can be handled by HOPS is fundamentally limited by the speed of MZM. If we assume the modulation frequency of MZM to be at least twice the signal bandwidth and the availability of 50-GHz MZM, it is expected that phase regeneration of up to 25-Gb/s BPSK signal can be achieved. In the realization of M-level HOPS, for M > 4, generation of high order phase harmonics with high signal-to-noise ratio becomes the key. Using two nonlinear optical elements is effective for this. Although it was not presented in this paper, we have arranged a dual-pump 8-level HOPS that utilize two SOAs and confirmed GER of more than 12 dB in the static mode operation.
Regarding the nonlinear element choice, we recognize that SOA can perform phase-sensitive frequency conversion over a wide wavelength range at baud rates as high as 40 Gbaud [11]. While it is widely recognized as an attractive element suitable for photonic integration, silicon optical waveguides have generated considerable recent research interest because of their fast relaxation time and the compatibility with the CMOS process [28, 29]. Unfortunately, the optical nonlinearity that emerges in those elements is not sufficiently large to achieve high GER in PSA. However, HOPS may provide a compelling opportunity to perform silicone photonics-based optical phase quantization.
Acknowledgments
This work was supported by Project for Developing Innovation Systems of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
References and links
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