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A fiber optical parametric amplified optical orthogonal frequency division multiplexing (OFDM) signal with intensity modulation transfer (IMT) blocking is proposed. A novel blank carrier blocking method is adopted to suppress pump to signal intensity interference. Compared with regular optical signal, the IMT induced interference can be well blocked in the experiment. The error vector magnitude (EVM) and signal gain are also investigated in the experiment.
© 2014 Optical Society of America
1. Introduction
Recently, the fiber optical parametric amplifier (FOPA) which depends on the four-wave mixing nonlinear processing, has got lots of attention due to its wide gain bandwidth, flat gain spectrum and low noise Fig [1–3]. By correctly setting the parameters of FOPA, it can achieve performance of 70 dB gain or 100 nm gain range with 3 dB ripple [4, 5]. It offers the prospect of amplification within large bandwidth. The FOPA has been applied in signal amplification such as differential phase shift keying (DPSK) or quadrature amplitude modulation (QAM) based optical signal [6, 7]. In order to suppress the stimulated Brillouin scattering (SBS) effect, the phase modulated pump is generally adopted in FOPA [8, 9]. In this case, several radio frequency (RF) tones are induced for suppression. Due to the ultrafast Kerr nonlinear response in parametric process, the intensity modulation transfer (IMT) would present and turn pump intensity noise into signal power fluctuation [10, 11]. The phase modulation with RF tones eliminates the SBS effect while induces frequency variation for the pump, which would cause intensity fluctuation of gain. This fluctuation of the pump would lead to significant high intensity variation on the signal by exponential gain as well as frequency dependence of the gain spectrum. The amplified signal would suffer from the fluctuation due to IMT and get worse performance. Unfortunately, the effect of IMT has not been considered in the previous signal amplification.
In this paper, to our best knowledge, we firstly demonstrate a fiber optical parametric amplified optical orthogonal frequency division multiplexing (OFDM) signal transmission with IMT blocking. Due to flexible subcarrier operation, the OFDM modulation can easily realize spectrum processing [12–15]. Thus we propose a novel blank carrier blocking method to suppress the IMT during amplification. It can eliminate the fluctuation on the frequencies of affected subcarriers caused by IMT. In the experiment, a FOPA amplified 17.36 Gb/s direct-detected (DD) optical 4QAM-OFDM signal is successfully demonstrated in single pump case. Compared with the regular system, the 4QAM-OFDM signal with IMT blocking can reach a bit rate error (BER) of 1 × 10−3 at optical signal to noise ratio (OSNR) of 13.2 dB while the on-off keying (OOK) signal is beyond forward error correction (FEC) limit.
2. Principle
The schematic of proposed blank carrier blocking method is illustrated in Fig. 1.Due to the IMT effect, comb-like interference components across the whole signal spectrum would be observed, which can be expressed as
where An is the amplitude of the interference component and fk is the central frequency of RF tones. The interference is shown in Fig. 1(a). In practice, the amplitudes of higher frequencies components would become smaller due to conservation of energy. For the regular multi-level signal, the interference components are blended with the signal, which are impossible to separate as Fig. 1(a) shows. Due to the convenient processing of spectrum, we propose a novel blank carrier blocking method with OFDM modulation. The interference components are equidistant RF tones in frequency domain, and we can adopt equidistant blank subcarriers to block the interference. The OFDM signal can be expressed aswhere Cl is the data symbol, fl is the index of OFDM subcarriers, N is the total number of subcarriers and NTS is the time duration of OFDM symbol. There we define the bandwidth of OFDM signal as BOFDM. By adjusting the value of N, the RF interference components can satisfy withAmong all the OFDM subcarriers, if the subcarriers satisfy with fl∈{f’k}, Cl would be set to zero. It means that the subcarriers are blank carriers. Thus the interference components from the pump would drop into the blank carriers after FOPA amplification, which is shown in Fig. 1(b). This interference can be removed by a digital comb band-stop filter (BSF) at the receiver. Due to the flexibility of OFDM signal, if the positions of RF tones are changed, we can shape the signal spectrum by adjusting the number of OFDM subcarriers and the interval of blank subcarriers. It can ensure that the RF tones fall on the blank subcarriers of OFDM signal at the receiver. Besides, the subcarrier has a certain bandwidth, which could guarantee the RF tones dropping in the bandwidth range of blank subcarriers.
3. Experimental setup
We have performed experiment to verify the proposed scheme in single-pump FOPA case and the experimental setup is depicted in Fig. 2.A CW tunable laser source (Santec TSL-510) is employed as pump source. Two combined radio frequencies (RF1 and RF2) at 100 MHz and 300 MHz are phased modulated to increase the SBS threshold. The two frequencies are produced by same digital-to-analog convertor (DAC) in an arbitrary wave generator and they are exactly phase-locked during experiment. We adopt modulator with Vpi of 2.2V. Due to the low half-wave voltage of the phase modulator, more sideband frequencies would be inspired during modulation, which are sufficient to suppress the SBS. A tunable optical filter with 3 dB bandwidth of 1 nm is used to suppress the out-of-band ASE noise caused by the commercial erbium-doped fiber amplifier (EDFA). The OFDM signal is generated by Matlab program and then uploaded to the commercial 50 GS/s arbitrary waveform generator (AWG70001A) with 10 bits digital-analog converter (DAC), and its spurious-free dynamic range (SFDR) value is below −55 dBc within 10 GHz bandwidth. The binary data streams are mapped with 4QAM format after serial to parallel transform. Then the data symbols are sent for subcarrier modulation by inverse fast Fourier transform (IFFT), where the blank carrier blocking is performed simultaneously. Due to the IMT, the pump would induce RF interference spaced at 100 MHz. Within 10 GHz signal bandwidth, we adopt total 500 subcarriers and each subcarrier occupies a bandwidth of 20 MHz. In order to block the IMT, the blank carrier is added every five subcarriers. It means that the RF interference spaced at 100 MHz is blocked within the signal bandwidth. A cyclic prefix of 1/16 symbol length is added to maintain the correct FFT window at the receiver. In the experiment, the RF interference beyond 2.5 GHz is rather small and we mainly consider the blocking less than 2.5 GHz. The measured OFDM spectrum is shown in Fig. 3(a).Only 1/4 of the bandwidth adopts blank carrier blocking, which results in a data rate of 17.36 Gb/s. If no blank carrier blocking is adopted, the unaltered data rate would be 18.32 Gb/s. The loss of bandwidth is 5.2% after employing IMT blocking. The detailed spectrum is shown in Fig. 3(b). We adopt Hermitian symmetry for IFFT and the generated OFDM signal can be directly modulated onto the signal light. A Mach-Zehnder modulator (MZM) with Vπ of 5.5 V is used to realize the optical signal modulation. A DFB laser at 1535 nm is used as the signal light source and the optical OFDM signal power is set to be −12 dBm. The combined signal and pump are then sent into a 400 m highly nonlinear dispersion shifted fiber (HNL-DSF). The zero dispersion wavelength (ZDW), nonlinear coefficient, dispersion slope and attenuation factor of the HNL-DSF are 1550 nm, γ = 13.5 W−1km−1, 0.02 ps/nm2/km and α = 0.9 dB/km, respectively. The optical spectra with and without pump are shown in Fig. 3(c), where the pump wavelength is set to 1551.92 nm. An ON-OFF gain of 23.8 dB is observed at a pump power of 33 dBm. Generally, the zero dispersion wavelength of the HNL-DSF can be changed by imposing temperature distribution to the fiber, which would result in the fluctuation of the gain spectrum of FOPA (e. g. the peak location of the gain spectrum). During the experiment, the temperature is stable for the optical fiber and we haven’t observed any fluctuation of the FOPA. In practical application, temperature control device might be needed if the environmental temperature varies in large range. In DWDM case, the phase difference between the pump and each wavelength light differs from each other, which would result in variety of IMT value on each wavelength channel. However, the frequencies that IMT occurs are same as single channel case. The proposed method remains suitable for DWDM case. The detected signal spectrum is shown in Fig. 3(d), where the RF interference drops into the blank carriers. At the receiver, we adopt another tunable optical filter to suppress the out-of-band noise before signal detection. The detected signal is sampled by a digital signal oscillator (DSO-X93204A) with 80 GS/s and 32 GHz bandwidth. The offline processing is adopted to demodulate the OFDM signal with BSF. In the experiment, we adopt direct detection to verify the signal with IMT blocking. The blocking method is realized by operating the OFDM subcarriers in digital domain. For coherent or fast OFDM, the signal spectrum is also consisting of subcarriers, which can be shaped by DSP technology. So the IMT blocking method can also be applied in them. For the fast OFDM, the positions of the blank carriers should be well calculated because the subcarrier spacing of fast OFDM is halved compared with conventional OFDM signal [16].
Fig. 2 Experimental setup (PM: phase modulator; AWG: arbitrary waveform generator; MZM: Mach-Zenhder modulator; PC: polarization controller; HNL-DSF: highly nonlinear dispersion shifted fiber; ISO: isolator; OSA: optical spectrum analyzer; DSO: digital signal oscillator).
Fig. 3 The measured spectra (a) of OFDM signal before FOPA; (b) of detailed OFDM signal within IMT region; (c) of optical signal with and without amplification; (d) of the OFDM signal after FOPA.
The measured BER performance of optical 4QAM-OFDM signal with IMT blocking is reported in Fig. 4.In the experiment, the optical OFDM signal is combined with an ASE source with variable optical attenuation, which can obtain different OSNR values for measurement. The OSNR is measured by the optical spectrum analyzer and the corresponding resolution is 0.1 nm. It can be observed that the required OSNR at BER of 1 × 10−3 (FEC limit) is about 13.2 dB. We also measure the performance of interfered carriers without IMT blocking, which is also plotted in Fig. 4. Due to the IMT effect, the BER of these carriers is far beyond 1 × 10−3, which cannot be recovered by FEC coding.
Fig. 4 BER curves for proposed 4QAM-OFDM signal and interfered carriers without IMT blocking (OSNR resolution: 0.1nm).
We also compare the system performance with different modulation formats. In the experiment, we adopt OOK modulation format for comparison. Both OOK and OFDM signals are within a bandwidth of 5 GHz. The measured BER curves are shown in Fig. 5.For OFDM signal, the OSNR penalty at BER of 1 × 10−3 is about 0.86 dB and the required OSNR for the same BER is about 13 dB after amplification. For OOK signal, the required OSNR is about 11.5 dB before amplification. However, the BER is beyond 3 × 10−2 at same OSNR value after FOPA. It can be seen that the OFDM signal maintains good performance after FOPA while the performance of OOK signal deteriorates a lot after FOPA. It is because that OFDM modulation offers the possibility of adjusting subcarriers to shape the spectrum, whereas OOK does not.
We also measure the signal performance under different pump powers. Figure 6(a) and (b) depict the error vector magnitude (EVM) and gain spectrum of 4QAM-OFDM signal at different pump powers respectively. During measurement, the pump power is set to be 34 dBm and 32 dBm. Figure 6(a) represents the EVM performance as a function of the average signal power PS. The EVM can reach an optimum value at around −12 dBm of signal power. The EVM begins to deteriorate when the signal power becomes larger or smaller. Due to the gain saturation, the EVM of larger signal power deteriorates faster than that of lower power. The constellation diagrams are shown as insets in Fig. 6(a) when the FOPA is operated in both the normal and saturated regime. The constellation distortion becomes evident in saturation case. The gain spectrum is plotted in Fig. 6(b). The signal gain is improved by 4.8 dB when the pump power increases from 32 dBm to 34 dBm. The gain saturation appears at about −11 dBm of signal power.
Fig. 6 EVM and Gain Measurements. (a) The measured EVM of signal at different pump powers; (b) the gain spectrum at different pump powers (Ps: optical power of signal).
4. Conclusion
In conclusion, we have proposed a novel FOPA amplified optical OFDM signal with IMT blocking in this paper. A blank carrier blocking method is adopted to eliminate the IMT induced interference. This blocking can well match the interference bandwidth by adjusting the carrier number and bandwidth. In the experiment, the 17.36 Gb/s optical OFDM signal with IMT blocking is successfully demonstrated in the FOPA amplified system. The experiment results indicate the prospect in future fiber optical parametric amplified system.
Acknowledgments
The financial supports from National High Technology 863 Program of China (No. 2013AA013403/2013AA013303), Beijing Nova Program (No. Z141101001814048), National NSFC (No. 61307086/61205066/61275074/61475024) and Beijing Excellent Ph.D. Thesis Guidance Foundation (No. 20121001302) are gratefully acknowledged. The project is also supported by Fund of State Key Laboratory of IPOC (BUPT) and Key Lab of OFS&C (UESTC), Ministry of Education.
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