We have proposed and experimentally investigated polarization insensitive all-optical up-conversion for ROF system based on FWM in a semiconductor optical amplifier (SOA). The parallel pump is generated based on odd-order optical sidebands and carrier suppression using an external intensity modulator and a cascaded optical filter. Therefore, the two pumps are always parallel and phase locked, which makes system polarization insensitive. After FWM in a SOA and optical filtering, similar to single sideband (SSB) 40GHz optical millimeter-wave is generated only using 10GHz RF as local oscillator (LO). The receiver sensitivity at a BER of 10-9 for the up-converted signals is -28.4dBm. The power penalty for the up-converted downstream signals is smaller than 1dBm after 20km SSMF-28 transmission.
© 2009 Optical Society of America
Radio-over-fiber (ROF) is a promising technique in providing broadband wireless access services in the emerging optical-wireless networks [1–20]. It provides many advantages and benefits compared with electrical signal distribution such as low attenuation loss, large bandwidth, immunity to radio frequency interference, easy installation and maintenance, low power consumption, and multi-service operation. The all-optical up-conversion technique in ROF system provides an effective way to simplify the architecture of the ROF network [5–8]. In order to realize seamless integration of WDM-PON and ROF system, the baseband signals need to be up-converted to RF frequency. Many schemes have been proposed to realize all optical up-conversion signals based on external modulator [9–10]. In , quadruple frequency millimeter wave (mm-wave) is generated by odd-sidebands suppression modulation technique. Receiver sensitivity is reduced due to the DC-component is removed by an optical filter at receiver. Therefore, the maximum transmission distance of optical mm-wave over fiber will be limited due to the low receiver sensitivity. Fiber dispersion also limits the transmission distance of the optical mm-wave signal. To overcome fiber dispersion, single sideband (SSB) modulation is a good choice . In , double frequency mm-wave is generated by SSB modulation. Moreover, these schemes [7–10] based on the external modulator are polarization sensitive. Another scheme to accomplish all-optical up-conversion for ROF system is to use four-wave mixing (FWM) in nonlinear medium because of its positive conversion efficiency and wide LO frequency bandwidth . However, only double frequency mm-wave is generated and polarization sensitivity of this FWM system is not discussed in . Recently, polarization insensitive FWM in nonlinear optical fiber based on co-polarized pump scheme has been demonstrated in [12–13], which is an effective way to increase the system stability. In [12–13], two pumps are generated from different laser source; therefore, their phase is not locked. Moreover, two polarization controllers (PC) are used to keep the two lightwaves to have the same polarization direction. FWM in fiber has large conversion ranges, but due to phase matching requirements, the optical frequency of the pump must be matched closely to the dispersion of the nonlinear waveguide to achieve acceptable efficiency. Although response times of highly detuned FWM in semiconductor optical amplifier (SOA) are less than hundreds of femto-seconds, strongly polarization sensitivity is a significant challenge in wavelength conversion techniques including FWM in fiber or SOA.
In this paper, we have proposed and experimentally demonstrated a novel polarization insensitive scheme to realize all-optical up-conversion for ROF system with parallel pumps based on FWM in a SOA. Two pumps are generated by an intensity modulator driven by 10 GHz RF signal based on odd-sidebands suppression modulation technique. The central carrier is removed by one cascaded optical filter . Because the two pumps come from the same laser, the pumps always have the same polarization direction and phase locked. Therefore, the operation of all-optical up-conversion is polarization insensitive. After FWM in a SOA, the up-converted signal at 80GHz repetitive frequency, which has the same characteristic of an 80GHz double-sideband (DSB) signal, is generated. An optical filter is used to remove one sideband to obtain quadruple frequency millimeter wave (mm-wave), which is similar to a SSB mm-wave signal . The scheme can also be employed to realize all-optical up-conversion of multi-channel (WDM) signal.
Figure 1 shows the principle of polarization-insensitive all-optical up-conversion for ROF systems based on parallel pump FWM in a SOA. In the central station, an intensity modulator (IM) and a cascaded optical filter are employed to generate quadruple frequency optical (4f) mm-wave, which the odd-order sidebands and the optical carrier are suppression. Obviously, the generated two second-order sidebands have the parallel polarization direction and phase locked. Then the two pumps are combined with the signal lightwave by using an optical coupler (OC). The two converted new signals can be obtained after FWM process in the SOA. A tunable optical filter (TOF) is used to suppress the pump signal. In this scheme, the optical signal similar to DSB signal is generated, which includes two converted signals and original signal after FWM in SOA. However, when one sideband is removed by an optical filter or optical interleaver (IL), the remaining signal is SSB-like signal, which includes one up-converted sideband and original signals. As we know that SSB signal can realize dispersion free long distance transmission. In the base station, the optical quadruple repetitive frequency mm-wave will be generated when they are detected by O/E converter after transmission. The electrical mm-wave signal after O/E converter will be down-converted by a mixer to retrieve the baseband data.
3. Experimental setup and results
Figure 2 shows the experimental setup for all-optical up-conversion. The lightwave generated from the DFB laser at 1543.8nm is modulated by the IM1 driven by 10GHz sinusoidal wave. The IM1 is DC-biased at the top peak output power when the LO signal is removed. 10GHz RF microwave signal with a peak-to-peak voltage of 12V. The half-wave voltage of the IM is 6V; by this way, the odd-order modes are suppressed. The optical spectrum after IM1 is shown in Fig. 2 as inset (i). We can see that the first-order modes are suppressed and the frequency spacing between the second-order modes is equal to 40GHz. The carrier is removed by using a fiber Bragg grating (FBG). The output optical spectrum of FBG is shown in Fig. 2 as inset (ii). The two second-order sidebands are used as two parallel pumps. Because two pumps come from one laser, the pumps always have the same polarization direction and phase locked. The CW lightwave from another DFB laser at 1537.9nm is modulated by the second IM2 driven by 2.5Gbit/s electrical signal with a PRBS length of 231 -1 to generate regular non-return-to-zero (NRZ) optical signal. The corresponding launched power measured at point (a) and (b) in Fig. 2 is 6.02dBm and 11.2dBm, respectively. The 2.5Gbit/s NRZ optical signals and two pump signals are combined by an optical coupler (OC) before the EDFA. The coupled signal power injected into the SOA is 9.1dBm. The optical spectra before and after the SOA are shown in Fig. 2 as inset (iii) and (iv), respectively. The SOA has 3-dB gain bandwidth of 68-nm, small signal fiber-to-fiber gain of 28-dB at 1552nm, polarization sensitivity smaller than 1dB, and noise figure of 6-dB at 1553nm. After the SOA, new up-converted signals are generated due to FWM, which is shown in Fig. 2 as inset (iv). Then a tunable optical filter (TOF) with a bandwidth of 0.5nm is used to suppress the pump signals. The optical spectrum after the TOF is shown in Fig. 2 as inset (v). We can see that only the converted and original signals are kept. The DSB signals with 80GHz frequency spacing between two converted sidebands are generated. In order to obtain the SSB signals, a 50/100 optical interleaver is used to remove one sideband. The optical spectrum after optical interleaver is shown in Fig. 2 as inset (vi). We can see that 2.5Gbit/s OOK signals are carried by the SSB-like signals with 40GHz frequency spacing between the converted signals and carrier, namely, the optical quadruple frequency mm-wave carried 2.5GHz signals is obtained. The power delivered to the fiber is 2dBm. After transmission over 20km SMF-28, the optical mm-wave is detected by O/E conversion via a PIN photo-diode (PD) with a 3-dB bandwidth of 50 GHz.
Figures 3(a) and 3(b) show the eye diagrams of the optical mm-wave signals before and after transmission over 20km SMF-28 measured at point (c) in Fig. 2, respectively. Figure 3(b) shows that the eye diagram is still clear after the optical mm-wave signals are transmitted over 20km fiber. Yu in Ref.  has shown that different carrier-to-sideband ratio (CSR) can affect the performance of the ROF system. In our experiment, the eye diagrams in Figs. 3(a) and 3(b) have some difference because the CSR is not optimized. The converted electrical signals are amplified by an electrical amplifier (EA) with a bandwidth of 10GHz centered at 40GHz. An electrical LO signal at 40GHz is generated by using a frequency multiplier from 10 to 40GHz. We use the electrical LO signal and a mixer to down-convert the electrical mm-wave signal to retrieve the downlink baseband signals. We use a bit error ratio (BER) tester to measure the downstream signals after down-conversion. The measured BER curve is shown in Fig. 4. The corresponding eye diagrams after transmission over 20km SMF-28 before and after down-conversion, measured at point (d) in Fig. 2, are also displayed in Figs. 4(a) and 4(b), respectively. The power penalty is smaller than 1dB after 20 km SSMF-28 transmission.
We have proposed and experimentally demonstrated a novel polarization insensitive all-optical up-conversion for ROF system using parallel pumps technique based on FWM in SOA. This scheme has some unique advantages such as polarization insensitive, high wavelength stability, and low-frequency bandwidth requirement for RF signal and optical components. 40GHz optical mm-wave SSB-like signal is generated by using 10GHz LO. 40GHz polarization insensitive all-optical up-conversion has been realized and the receiver sensitivity at a BER of 10-9 for the converted signals is -28.4dBm. The power penalty for the converted downstream signals is smaller than 1dB after 20 km SMF-28 transmission.
This work is partially supported by National “863” High-tech Research and Development Program of China (Grant No.2007AA01Z263, 2008AA01Z4473291), the Hunan provincial Natural Science Foundation of China (Grant No. 06JJ50108) and the Open Fund of Key Laboratory of Optical Communication and Lightwave Technologies (Beijing University of Posts and Telecommunications, Ministry of Education, P. R. China)
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