1. Introduction

Phase sensitive amplification (PSA) [1–3] is known to be superior over conventional, phase insensitive amplification (PIA) [4]. The main advantage is the possibility to reach noise figures lower than 3 dB [5], which is important for advanced communication [6], imaging [7] and signal processing [8] applications as well as for multi-wavelength oscillators [9] which make use of the cyclical gain spectrum exhibited by PSAs under a specific configuration. Nonlinear PSAs can also serve as phase noise regenerators in noise coherent communication channels.

Several PSA configurations employing either two [10] or one pump [11] were investigated and reported in the literature. In most reported single-pump configurations, the pump propagates in the anomalous dispersion region, thereby inducing PSA in the spectral vicinity of the pump [12]. A narrow band (NB) PSA was demonstrated recently, where the pump experiences normal dispersion, enabling a widely tunable PSA whose longest operational wavelength was 1781 nm [13].

The 2 μm wavelength region is very important for gas sensing, imaging and spectroscopy. Tunable coherent sources based on conventional PIA were reported [14] for this spectral region and can serve as a convenient source for a variety of applications. However, detection sensitivity is a major problem in this regime and therefore, a high quality low noise amplifier, such as a PSA is desirable. However, no PSA for the 2 μm region was demonstrated to date.

In this work we demonstrate a PSA for the 2 μm wavelength region utilizing a three-fiber-stage NB parametric process that exhibits a cyclical gain spectrum [9, 13]. In order to reach gain at the long wavelength, we employ the principle of NB FWM where phase matching conditions are satisfied in two narrow spectral regions located far from the pump, one being near and even beyond 2 μm. A pulsed pump employing a few ns pulses at a duty cycle of 1:250, having peak powers of about 70 W was used. The wavelength of the pump was tuned from 1565 nm to 1540 yielding a shift in the spectral location of the phase sensitive gain spectrum (with a width of about 0.1 nm) from 1952 nm to 2098 nm. The gain bandwidth is controllable by the fiber lengths and their dispersion properties. The amplifier gain and its variation due to phase sensitivity was determined in two ways. First, from amplified spontaneous emission (ASE) spectra with no input signal. Second, the conversion efficiency, to the 2 μm region, of a tunable narrow linewidth input signal with a wavelength near 1281 nm was measured. The gain is easily determined then from the conversion efficiency. A gain variation of more than 20 dB was observed in the latter case for a signal wavelength change of less than 30 pm. The expected process of deamplification, which should yield negative gain values was all but diminished and was observed only for a small part of the gain spectrum. The reason is the fact that the amplifier operated in saturation due to the high peak pump power.

2. Experimental setup

The experimental setup of the tunable NB PSA is shown schematically in Fig. 1. A cw tunable laser operating between 1535 and 1565 nm is modulated by a Mach-Zehnder (MZ) interferometer using 4 ns long pulses at a 1 MHz repetition rate to form the pump. The pump pulses undergo two stages of amplification by an Erbium doped fiber amplifiers (EDFA) with the second amplifier operating in deep saturation, such that the pump peak power scales with the duty cycle and reaches a value of 70 W. This corresponds to an average power of 600-700mW at output of the second EDFA.

 

Fig. 1 Experimental schematic.

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High pump powers are required in order to ensure that the observed idler is larger than the background noise. Since the pump is pulsed with a low duty cycle, the observed signals are 24 dB smaller than their actual level and hence would not be observable with lower pump powers. The input pump power to the third fiber is essentially the same since propagation losses in the short fibers are negligible. Also, neither the parametric process in the first fiber nor Raman scattering (which takes place in the first fiber stage) cause measurable attenuation of the pump power. Hence the input pump power to the PSA stage is basically the same as that at the input to the first fiber. The probe signal is provided by a quantum-dot distributed-feedback (QD DFB) laser [15], emitting at 1281 nm. The pump and the probe signal are combined in a 90:10 coupler and fed into a 30-m-long dispersion shifted fiber (DSF) with a zero dispersion wavelength of 1590 nm and nonlinear interaction coefficient of γ = 2.4 (W·km)⁻¹. Conventional PIA takes place in this first fiber whose output spectrum comprises three phase matched waves: the pump, the signal and an idler. In the absence of a signal, the fiber output exhibits a broad band ASE spectrum. A second stage comprising a 5-m-long conventional (dispersive) single mode fiber (SMF-28) follows and is used to modify the relative phases of the three waves, before they enter a third fiber having dispersion properties similar to those of the first fiber. NB PSA occurs in the third fiber. Tuning the frequency of the signal changes the accumulated mutual phase differences among the three lines due to the propagation in the second (dispersive) fiber, yielding in turn, in the third fiber, a cyclical gain spectrum, whose periodicity is determined by the second fiber. The gain peaks correspond to the frequencies where maximum phase matching is achieved in the third fiber and conversely, gain valleys occur at maximum phase mismatch.

3. Results

Figure 2 shows output spectra, with the signal turned off, for a pump wavelength of 1557.6 nm.

 

Fig. 2 PSA output for pump placed at 1557.6 nm a) entire spectra and zoom on b) short and c) long wavelength ASE spectrum.

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The entire output spectrum which is presented in Fig. 2(a) shows also a peak at 1680 nm which is due to Raman gain induced by the pump. Enlarged views of the short and long wavelength ASE spectra are shown in Fig. 2(b) and Fig. 2(c), respectively. Each ASE region is 0.2 to 0.3 nm wide and comprises three gain cycles. The spectral widths in the frequency domain of the two spectra in Fig. 2 are equal, 30 GHz. The gain spectra patterns in the short and long wavelength regions are similar, but inverted with respect to each other, showing a phase sensitivity of approximately 10 dB. However the resolution of 0.05 nm of this spectral measurement is close to the gain period of 0.1 nm and therefore this measurement is only a qualitative description of the NB PSA. A quantitative characterization is obtained by injection of a short-wavelength signal and determining the conversion efficiency to the long-wavelength region from which the gain can be easily obtained. These measurements are presented in Fig. 3.

 

Fig. 3 Gain measurements for a PSA using a 5 m long SMF: (a) Output signal with the pump off, (b) long-wavelength ASE without an applied signal, (c) idler for different signal wavelengths at PSA input, (d) idler for different signal wavelengths, at PSA output (e) calculated gain in the third fiber (PSA), (f) calculated gain in first fiber (PIA), (g) calculated on/off gain in the three fiber system, obtained from the conversion efficiency between the long wavelength idler and the short wavelength signal.

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The signal at 1286.4 nm was generated by a QD DFB laser [15] whose wavelength can be tuned somewhat by slight changes of the drive current which modify its output power by only at fraction of 1 dB. Figure 3(a) shows a measurement of the signal at the amplifier output when the pump is turned off. The long-wavelength ASE spectrum (with no signal applied) is depicted in Fig. 3(b) and shows two unresolved gain periods with a gain variation of less than 10 dB. Figures 3(c) and 3(d) present a series of long-wavelength idler amplitudes generated by varying the DFB signal wavelength across the short-wavelength ASE spectrum at the input and output of the third fiber, respectively. The signal and idler powers at the input to the third fiber are estimated to have approximately the same powers, 8-10 μW. The ratio between output and input amplitudes is the PS gain presented in Fig. 3(e) with highest value reaching 30 dB and with a maximum gain variation of 20 dB. Negative gain (deamplification) is only seen in the first cycle. The large pump power saturates the gain and diminishes deamplification in all other parts of the gain spectrum.

The idler conversion efficiency η, defined as the ratio of idler power to signal input power, yields the actual signal gain$G=\eta +1$. This PIA gain spectra (of the first fiber stage) and that of the complete (three fibers) system are shown in Fig. 3(f) and 3(g), respectively. The limited spectral resolution plays no role in this case, since each measurement is narrow band and only the peak of the observed spectrum has to be determined. For each measurement, we calculate the on/off gain of the entire three fiber system by comparing the 1280 nm output signal with the pump off [Fig. 3(a)] with the long wavelength output when the pump is on. Since the signal is CW, and the PSA is pumped by pulses, the measured output has to be increased by the pump duty cycle namely, 24 dB.

Five clear gain periods are observed in Fig. 3(g) with a peak value of 50 dB and a phase sensitive gain variation of 20 dB, 10 dB larger than found in the ASE spectrum. The width of each gain period is about 0.11 nm; it matches the ASE pattern seen in Fig. 3(b).

The gain pattern of the PSA can be determined by calculating the gain properties in the third fiber. The evolution of the signal follows [16]

In principle, large spectral ranges as used here require the use of many high order dispersion parameters in order to achieve good quantitative modeling. In the present case however, the simple model (Eqs. 1 and 2) serve only as a qualitative guide and therefore it is sufficient to only use dispersion parameters up to β₄. For θ = π/2, the signal shows maximum gain, while for θ = -π/2, the rate of change of P_s is negative and de-amplification takes place. The width of the gain periods depend on the acquired phase mismatch in the second fiber. This mismatch depends on the length of the dispersive fiber and on the spectral detuning. While for fixed wavelengths the detuning is constant, the periodicity of the gain can be controlled by the length of the dispersive fiber. Using a 10 m long fiber yielded a gain periodicity of 0.06 nm while the gain variations remained high at a level of 20 dB.

Since we demonstrated that the gain spectrum follows the ASE pattern, it is now possible to characterize the amplifier properties for wavelengths, where no tunable signal is easily available. Figures 4 and 5 show ASE spectra in the vicinity of the shortest and longest operational wavelengths of the amplifier, respectively. Three pump wavelengths were used in each case. Near 1950 nm in Fig. 4, the ASE level is well above the background and the enlarged view shows clearly three cycles. A small peak appears on the short wavelength side of the pump, it is a result of some residual ASE of the EDFA. For the long wavelength in Fig. 5, the ASE peak is near 2100 nm and shows two or three unresolved cycles. A peak due to the Raman gain is observable once more in both Figs. 4 and 5. The parametric process efficiency reduces as pump-signal detuning increases. The pump power had to be adjusted therefore in order to always obtain NB-ASE above the background noise. The required average EDFA power for the shortest pump signal detuning was 27.8 dBm while for the largest detuning, it increased to 28.5 dBm. Consequently, the observable SRS peak increases as seen in Fig. 5b.

 

Fig. 4 PSA output for a pump at 1565.5 nm: (a) entire spectra and (b) zoom on long-wavelength ASE region, for three ASE measurements with a pump detuning of 0.02 nm.

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Fig. 5 PSA output for a pump at 1540 nm: (a) entire spectra and (b) zoom on long-wavelength ASE region, for three ASE measurements with a pump detuning of 0.02 nm.

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The gain spectral width of each cycle in Fig. 4 is wider than that in Fig. 5 since the gain narrows as the wavelength lengthens. This is easily explainable from Eq. (2).

4. Conclusion

To conclude, we have demonstrated the first phase-sensitive parametric fiber amplifier operating in the 2 μm wavelength range using a pulsed pump. The extremely long operating wavelength is reached by operating the amplifier in the NB regime, namely with the pump propagating in the normal dispersion regime. The amplifier structure is based on three fiber stages which together lead to a cyclical gain. This gain was determined by characterizing the ASE spectra and also by injection of a tunable CW signal in the short-wavelength gain regime at 1281 nm and determining the parametric conversion efficiency to the long wavelengths at 1991 nm and consequently obtaining the gain. The maximum achievable gain level was 50 dB and the phase sensitive gain variation was as large as 20 dB. Operation of the PSA was found to be extremely stable over many hours.