Broadband Radio frequency (RF) photonic front-ends are one of the vital applications of the microwave photonics. A tunable and broadband RF photonic front-end integrating with the optoelectronic oscillator (OEO) based local oscillator has been proposed and experimentally demonstrated, in which only one phase modulator (PM) is employed thanks to the characteristic of the PM. The silicon-on-insulator based narrow-bandwidth band-pass filter is introduced for signal processing. The application condition of the proposed RF photonic front-end has been discussed and the performance of the front-end has also been measured. The SFDR at a frequency of about 7.02 GHz is measured to be 88.6 dB-Hz2/3.
© 2014 Optical Society of America
Microwave photonics has attracted a lot of research interest in the past decades for the applications in antenna remote, phased array and so on, owing to the inherently large processing bandwidth, low loss, attractive size and immunity to electro-magnetic interference [1,2]. RF photonic front-ends as one of the key applications of microwave photonics have been extensively investigated [3–8]. Typical microwave systems incorporate mixing process at the modulator, RF oscillator, signal processor and down-converter in the RF photonic front-ends area  as shown in Fig. 1.The photonic-assisted technology accompanying the large dynamic range, fast-tunability (~μs) and broadband features can be highlighted  relative to the electro-method [10,11] which would be limited by the generation and re-radiation of the local oscillator (LO) and the reduced dynamic range due to multi stages down-conversion. For most of the RF photonic front-ends reported, the photonic tunable down-conversion is realized by the tunable LOs . The LOs can be settled by several types, such as one more modulator and a microwave source to generate the LO  or directly introduce another tunable laser  to serve as the LO for mixing. The optoelectronic oscillator (OEO) with a large frequency range and ultralow phase noise can be employed as the LO for mixing in the front-end to achieve a large processing range and fast tunability, which can convert the energy from continuous wave light to the microwave even millimeter wave signals [14,15]. However, to avoid the impact on the oscillation of the OEO, some methods have been proposed, such as utilizing the polarization splitter to divide the RF signal and the LO at different polarization  or employing some more modulators to up-convert the RF signal to unique wavelength band and filtering out the RF input signal with a wavelength filter in the OEO loop .
In this paper, a large processing range photonic front-end is proposed and experimentally demonstrated. A feedback loop is introduced to the proposed RF photonic front-end to generate the LO for mixing as the LO loop in Fig. 1 shown. The characteristic of the phase modulator (PM) has been made full use of in the proposed RF photonic front-end to integrate the LO to the up-conversion system with the same PM. Moreover, an SOI-based narrow bandwidth band-pass filter is applied to the front-end for signal processing. The processing range of the proposed RF photonic front-end can cover X- to Ka-band thanks to the large frequency range of the OEO based LO and the processing ability of the SOI-based signal processor. The spurious free dynamic range (SFDR) of the front-end is measured to be 88.6 dB-Hz2/3 at a frequency of about 7.02 GHz. The application condition of this front-end has also been discussed. This proposed RF photonic front-end would be a potential design for the monolithic chip integration of the front-ends.
The schematic diagram of the proposed RF photonic front-end integrating with the OEO based LO is shown in Fig. 2(a).The RF signal received by the broadband antenna is up-converted to optical-domain by a PM and processed by the tunable band-pass filter (TBPF2) and then mixed and down-converted to get the RF output signal which can be applied for post-processing as the blue-dashed frame shown in Fig. 2(a). There are two TBPFs in the whole RF photonic front-end: the broad bandwidth one (TBPF1) is applied for the LO loop and the narrow bandwidth one (TBPF2) is served as the signal processor for photonic-based microwave signal processing. These two TBPFs in this front-end could be controlled by the control signal given by the Post-Processing module. The same PM is utilized to construct the OEO based LO as the LO loop circuit contains a PM, TBPF1, photo-detector (PD1) and power amplifier (PA) . The principle of the proposed RF photonic front-end integrating with the OEO based LO with only one modulator will be explained below by Fig. 2(b).
A continuous wave (CW) laser is used as the optical carrier and modulated by the combination of a RF input signal and the OEO based LO with the frequency of separately fs and fLO, and the modulated signal is filtered by TBPF1. The center of the filter is set subtly to keep all the first order of the modulated RF input signal in the filter band with a bandwidth of about Bw and only either the −1 or + 1 order of the LO modulated signal would exist after the filter with a frequency relationship of fs<Bw/2, fs < fLO and fLO ≈Bw/2 as shown in Fig. 2(b). The filtered signal is separated to two parts by a 1:1 coupler and one way is for high-resolution photonic signal processing and the other way is sent back to the LO loop and detected by PD1. After PD1, there are mainly two frequency components would exist as shown in Fig. 2(b): the new generated intermediate frequency (IF) signal with a frequency of (fLO-fs) and the LO as presented in Round1. The RF input signal is nearly vanished after PD1 as the beat-frequency signal between the ± 1 order of the RF input phase-modulated signal and the optical carrier have the same frequency and amplitude but a π phase difference. If the beat frequency (fLO-fs) signal would exist all along from now and be amplified by the PA, there would be a badly impact on the LO, but fortunately, after the modulation and PD1 during Round2, the beat frequency (fLO-fs) signal would be vanished as the same reason of the RF input signal as shown in Round2 and also the Round3. The characteristic of the PM has been made full use of in this RF photonic front-end to eliminate the impact of the RF input signal and the beat frequency (fLO-fs) signal on the OEO based LO. So, only one modulator is required in the proposed OEO-based RF photonic front-end for both the up-converting process and OEO loop, without the impact on the oscillation of the OEO.
Then a stable LO is achieved for mixing. The mixing signal of the LO and RF input signal is processed by the narrow bandwidth band-pass filter (TBPF2) and down-converted to electric-domain by the PD2 to achieve the RF output signal. The direct-current (DC)-block and low-pass filter are introduced to remove the DC-component and high-frequency component such as the LO and RF signal. The frequency of the LO can be tuned to achieve a large frequency range which is determined by the bandwidth tunability of TBPF1 and limited by the response bandwidth of the PM and PD1 .
3. Experimental setup and results
In the experiment, a CW with 1550.024 nm wavelength, 1 kHz line-width and 23.2 mW optical power is sent into the PM with a modulated bandwidth and half-wave voltage of separately 40 GHz and 7V and modulated by the combination of the two-tone signal and the OEO based LO. A wave-shaper (Finisar 1000s) with a tunable bandwidth is utilized as the TBPF1 and the PD1 with a bandwidth of 10 GHz is applied to detecting and down-converting the signal. Two-stage PAs are used to make the gain of the OEO based LO loop larger than 1 to obtain and keep the LO.
An erbium doped fiber amplifier (EDFA) with a gain of about 20 dB is introduced to overcome the loss of the processing link and set before the SOI 3-stage cascaded MZIs based signal processor. The bandwidth of the SOI-filter based signal processor is about 1.536 GHz and the performance is analyzed in . As the device is polarization sensitive (TE mode with low loss), a polarization controller (PC) is put before it to control the polarization of light before sending into the device. The silicon-based narrow bandwidth band-pass filter is used to filter out the desire part of the mixing signal for down-converting. Then the processed signal is detected by PD2 with a bandwidth of 40 GHz and responsivity rate of 0.62W/A. The RF output signal is filtered by a DC-block and a LPF to remove the dc component and the higher frequency component. The signal will be measured by the electrical spectrum analyzer (ESA).
The tunability of the LO with 3 different frequencies is measured by tuning the bandwidth of the TBPF1 as shown in Fig. 3(a).The smallest bandwidth (Bwmin) of TBPF1 and the bandwidth of the PD are all 10 GHz, so in this experiment limited by the device, the LO frequency can be not lower than Bwmin/2 = 5 GHz and larger than 10 GHz. A single mode fiber with a length of about 200 meters is introduced to the LO loop to obtain the delay and lower the phase noise of the LO. The phase noise picture is shown in Fig. 3(b) and the phase noise (measured by the Agilent E5052B) at an offset of 10 kHz relative to the center frequency of the oscillation of the OEO are all below −102.5 dBc/Hz.
In order to investigate the performance of the RF photonic front-end integrated with an OEO based LO and the SOI-based signal processor, two-tone signal centered at frequency of 7.02 GHz with a separation of about 10 MHz and generated by the Agilent 8267D is injected to the PM. The bandwidth of the TBPF1 we used in this experiment is 15 GHz and the center frequency is set properly near the carrier to get a LO at 7.227 GHz. The experimental result of the RF photonic front-end is also measured as shown in Fig. 4..The down-converted signal measured by the ESA is shown in Fig. 4(a). The SFDR of the front-end is measured to be 88.6 dB-Hz2/3 which is not so desirable, with a noise floor of about −132.9 dBm/Hz, as shown in Fig. 4(b). There are mainly three reasons for the low SFDR: first, the high noise floor due to the ASE noise from EDFA which has been discussed in ; second, the low link gain which is mainly because of the large loss of the microwave cables, the low optical power of the carrier limited by the low input optical power of the PD and the low responsivity of PD; third, the intrinsic drawbacks in linearity induced by the commercial modulator as described in . So these limitations can be improved by adopting high performance PD, low noise PA, low loss microwave cables, and the new domain-inversion directional coupler modulator .
For application in reality, the separation between the received RF signal and the LO can’t be too small and the input power of the RF signal is also limited. Figure 5 shows the application condition on the separation △f between the RF signal and the LO, and the input RF power. As can be seen from Fig. 5, there is a step response of the LO output power relative to the separation at different input RF power. At a certain input RF power, the LO output power is not changed as the separation is decreased, until the critical point, the LO output is vanished. As the RF input power increases, the critical point of the separation would be enlarged as show in the Fig. 5. So there should be a tradeoff between the RF input power and the separation. But fortunately, the separation can be tuned by tuning the frequency of the LO and the RF input power received by the antenna is not so large. The influence of the low energy signal received by the antenna on the LO can be negligible.
In this paper, a simple broadband and tunable RF photonic front-end integrating an OEO-based LO has been proposed and experimentally demonstrated, with only one PM for both the up-converting of the RF input signal and also the LO loop. SOI-based narrow bandwidth filter with a bandwidth of about 1.536 GHz is utilized for signal processing in this front-end. The dynamic range of the proposed RF photonic front-end at frequency of about 7.02 GHz is measured to be 88.6 dB-Hz2/3 and the processing range of the proposed photonic front-end can cover from X- to Ka-band which is only decided by the processing ability of the signal processor and the frequency range of the LO. The application condition of the front-end has also been discussed, that there would be a tradeoff between the input power of the RF signal and the separation between the RF signal and LO. The proposed simple design of the RF photonic front-end integrating with the LO loop would be a potential setup for monolithic integration of the front-end.
This work was partially supported by National Program on Key Basic Research Project (973) under Contract 2012CB315703, NSFC under Contract 61120106001, 61271134, 61090391 and the Program for New Century Excellent Talents in University (NCET-10-0520).
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