1 Introduction
Microwave photonics (MWP) is a discipline that brings together the worlds of radiofrequency engineering and optoelectronics. Initially focused towards defense applications, has recently expanded to address a considerable number of civil applications: cellular, wireless and satellite communications, distributed antenna systems, sensing, medical imaging, etc [
1-
3]. Very recently, 5G mobile communications are targeted for 2020 and will need to meet very high demands: 1000-fold gain in capacity, connections from 100 billion devices, up to 10 Gb/s per user. MWP is considered one of the key enabling technology, which will be used to break the operation bandwidth limitation of pure microwave techniques.
MWP devices and systems have been used to realize different functionalities with unique performance that could not been realized using pure electronics techniques [
4-
6]. The general functionalities that have been realized and kept in investigation include microwave photonics filters (MPFs), optoelectronic oscillators (OEOs), arbitrary waveform generation (AWG), frequency mixing (FMIX), optically assisted analog to digital conversion (ADC), instantaneous frequency multiplication (IFM), tunable true time delay (TTD), tunable phase shifting (TPS), optical phase locked looping (OPLL), MWP front-ends (MWP-FE) and optical beamforming networks (OBFN).
MWP has been rapidly developed for more than 20 years, MWP systems and links have relied almost exclusively on discrete optoelectronic devices and standard optical fibers and fiber-based components. These configurations are bulky, expensive, power-consuming and lack flexibility. Emerging applications scenarios which will be a major driving force will require size, weight and power consumption (SWaP) reduction, reconfigurability and flexibility. Meeting these stringent requirements is only possible by developing integrated microwave photonics (IMWP) technology [
7] and designing generic architectures that can be reconfigured via software-defined networking (SDN) techniques. There are several platforms that have been frequently used to demonstrate IMWP functionalities: indium phosphide (InP), silica planar lightwave circuits (PLCs), silicon-on-insulator (SOI), Si
3N
4/SiO
2, (TriPleX
TM) waveguide technology and LiNbO
3. Each technology has specific strengths but integration in single platform without sacrificing an overall system performance has not been achieved. In the near future, we have to identify the most suitable material platform which is compatible with generic fabrication models and the achievable performance should be compatible with present and future radio standards, such as the linearity, noise figure and dynamic range. In addition, another big challenge is that radio frequency (RF)/electronic/photonic components and subsystems needed to be integrated in a single platform.
Fig.1 Publications and citations records in the MWP topic of China, USA and Japan in the past 20 years (Data: from Web of Sciences) |
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As shown in Fig. 1, in the past 20 years, MWP technology has been widely investigated and the number of publications is kept increasing rapidly. The publication number on the topic of MWP from China has become the top one since 2009. Moreover, the increasing rate is kept much higher than the other countries. The total publication number in the past 20 years is approaching to the one of USA, as shown in Fig. 2. However, the average citation of each publication is only about 1.67, which is much lower than the other countries. It means that the quality of the publications from China should be largely improved. From 10 years ago, China started to support the development of MWP devices and system, as shown in Table 1. Based on the supporting of these funding, we have developed many key enabling technologies, in the meantime, the total publication number is increased rapidly year by year.
Fig.2 Total publications and average citation of each publication of the active countries (Data: from Web of Sciences) |
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Tab.1 Selected funded major projects on MWP in the past few years |
| year | type | research topics | principle investigator (PI) |
| 2004 | 863 | key technology of 10 Gb/s laser module for optical communications | Ninghua Zhu
(IOS-CAS) |
| 2007 | 863 | key technology of integrated microwave photonic phase shifter based on SOI/PDLC | Weiyou Chen
(Jilin Univ.) |
| 2007 | 863 | study on high speed MWP electro-optic modulator based on novel polymer materials | Xiaogong Wang
(Tsinghua Univ.) |
| 2009 | 863 | high-speed linear modulated laser and transceiver module for radio over fiber | Liang Xie
(IOS-CAS) |
| 2009 | 863 | research on the key technology of microwave photonic detection based on optical and wireless convergence | Xiaoxia Zhang
(UESTC) |
| 2011 | 863 | photonic integration technology and system application | Ninghua Zhu
(IOS-CAS) |
| 2011 | 973 | ultra high speed and low power photonic integrated circuits technology for information processing | Jianping Chen
(SJTU) |
| 2012 | 973 | basic research on MWP for broadband and large dynamic-range millimeter wave devices and application | Xiaoping Zheng
(Tsinghua Univ.) |
| 2012 | 973 | basic research on MWP devices and integrated systems for broadband ubiquitous access | Yuefeng Ji
(BUPT) |
As shown in Table 1, two major “973” projects were funded by the Chinese Ministry of Science and Technology in 2012, simultaneously. One of the two “973” projects is entitled with basic research on MWP devices and integrated systems for broadband ubiquitous access, which is also named by another simplified name of Intelligent Radio Over Fiber (IROF). This “973” project’s principal investigator is Prof. Yuefeng Ji from Beijing University of Posts and Telecommunications (BUPT), and totally 10 teams from different research organizations have been involved into this project such as Institute of Semiconductors, Chinese Academy of Sciences (IOS-CAS) and Tsinghua University etc. The main goals of this project are to reveal the mechanism behind efficient conversion between lightwave and microwave, to develop the fine MWP signal processing techniques, and to derive the coordinated system model in a distributed network architecture. Several impressive results have been demonstrated experimentally, such as 24 GHz directly modulation laser [
8] and 60 GHz integrated microwave photonic modulation chip.
The other “973” project is entitled with basic research on MWP for broadband and large dynamic-range millimeter wave devices and applications. The PI is Prof. Xiaoping Zheng from Tsinghua University, totally 7 universities in China have been involved into this project, such as Peking University, Shanghai Jiao Tong University (SJTU) and Beihang University. The main goals of this project are to investigate the fundamental theory and key enabling technologies for millimeter wave devices and applications based on MWP technology, in which the time-bandwidth product is targeted to be improved with three orders. Up to now, some impressive results have been achieved, such as the spectrum manipulating technique in the optical domain for its applications in MWP links and devices [
9,
10], and the phase error detection and phase noise (PN) compensation techniques in photonic microwave dissemination system [
11].
In this review paper, we will first introduce the recent advances in MWP devices in Section 2, and then system-based works will be described in Section 3. Finally, future prospective will be discussed and conclusion will be made in Section 4.
2 Key MWP devices
MWP devices is related to those devices that is specially used in MWP system, as shown in Table 2. Some of them are similar to microwave and optical devices, such as broadband linear modulators and photodetectors (PDs). However, there still exist some devices that are designed and fabricated based on MWP techniques, such as directly modulation laser, OEOs and MPFs. In this section, we will introduce some exciting progresses on the MWP devices.
Tab.2 Comparison of microwave, optical and MWP devices |
| functionality | microwave devices | optical devices | MWP devices |
| source | oscillator | laser | direct modulation laser/OEO |
| modulation | modulator | electric absorber/LiNO3 | broadband linear modulator |
| waveguide | RF cable | optical fiber | RF cable and optical fiber |
| detection | detector | PD | broadband linear PD |
| amplification | RF amplifier | EDFA/SOA | RF and optical amplifier |
| filter | RF filter | optical filter | MWP filter |
IOS-CAS is one of the leading optoelectronics research organizations in China with a State Key Lab on Integrated Optoelectronics, a national engineering research center for optoelectronic devices and an advanced optoelectronic devices packaging platform. In IOS-CAS, several groups are working on MWP from integrated chips, modules to application system. Among them, the microwave optoelectronics research lab is working on not only IMWP devices and modules such as directly modulation laser and optical analog signal processor, but also application systems such as phase stable MWP link and true time-delay beamforming system.
In 2012, Prof. Zhu and Prof. Wang’s group from IOS-CAS reported a distributed feedback (DFB) laser with 24 GHz bandwidth. The packaged prototype devices and bandwidth characteristics of the 24 GHz analog direct modulation laser is shown in Fig. 3 [
8]. In addition, based on a distributed feedback semiconductor optical amplifier (DFB-SOA), a widely tunable single passband MPF is proposed and experimentally demonstrated. By tuning the central wavelength of DFB-SOA base filter, a single-channel MPF with frequency tuning range from 5 to 35 GHz is obtained [
12].
Fig.3 Prototype devices and bandwidth characteristics of the 24 GHz analog direct modulation laser [8] |
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Figure 4 gives the experimental setup for microwave generation using an electro-absorption modulator (EAM) integrated in between two DFB lasers [
13]. In this scheme, the wavelengths of the DFB lasers are tuned by adjusting their bias currents. The light beams from both DFB lasers are injected into the EAM and mixed with each other to generate microwave signal. Figure 5 shows the optical and electrical spectra. In Fig. 5(a), four-wave mixing effect can still be observed when the optical wavelength difference is over 30 GHz due to strong optical coupling between the two lasers. From Fig. 5(b), it can be seen that a sharp peak at the beat frequency has a 24-dB signal noise ratio. The frequency of the generated microwave signal can be tuned by changing the bias currents of the DFB lasers.
Fig.4 Experimental setup for microwave signal generation using an EAM integrated in between two DFB lasers [13] |
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Fig.5 (a) Optical spectrum and (b) corresponding electrical spectrum (dashed line). The electrical spectrum after adjusting the bias current of the DFB laser 2 is also included [13] |
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As shown in Fig. 6, Prof. Zhao from IOS-CAS fabricated three-section amplified feedback lasers (AFL) on InP substrate. By controlling the currents injected into the three sections of the AFL, various working states including single mode, dual mode, periodic oscillation and chaotic states have been demonstrated [
14]. Dual-mode AFL has also been employed in OEO structures, simultaneously functioning as a laser source, active photonic filter, and modulator [
15].
Fig.6 AFL (upper) consisting of a DFB, a phase and an amplifier section; dual-mode state of an AFL (lower left); tunable output from an AFL-based OEO (lower right). SCH: separate confinement heterostructure; MQW: multiple quantum well [14,15] |
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Prof. Huang’s group in IOS-CAS proposed to use integrated semiconductor twin-microdisk laser for microwave signal generation under mutually optical injection through a connected optical waveguide with the mode wavelength intervals adjusted by injection currents [
16]. Dynamical characteristics are also investigated for square resonator microlasers subject to optical injection, the microwave signals are obtained from the electrode of the microlasers subject to optical injection related to the oscillation of carrier density in the active region as shown in Fig. 7 [
17].
Fig.7 (a) Schematic of the experiment system setup. ML: master laser; SL: slave laser, i.e., microsquare laser; PA: power amplifier; PSA: PSA series spectrum analyzer; SMF: single-mode fiber; OSA: optical spectrum analyzer; (b) principle schematic of photonic generated microwave inside the microcavity laser. T: time; EFn – EFp: the difference of the Fermi levels [16,17] |
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There are several groups from Tsinghua University actively working in the field of IMWP devices. Prof. Chen proposed a full-band (from L-band to W-band) RF photonic frontend, which is based on the integrated high
Q, large processing range tunable signal processor as presented in Fig. 8(a). Based on this RF photonic frontend, the RF signal would be up-converted to optical domain and processed by the full-band signal processor to obtain the desired signal band. As shown in Fig. 8(b), the spurious free dynamic range (SFDR) of the frontend measured has been presented, at frequency from 1 to 65 GHz. This is the first proposed RF photonic frontend, which could work from L-band to W-band and this frontend will likely have extensive applications in either the wireless communications or radar communications field [
18,
19].
Fig.8 Full-band RF photonic frontend. (a) Schematic diagram of the full-band RF photonic frontend based on the integrated full-band tunable signal processor; (b) measured SFDR of the frontend from L-band to U-band, covering from 1 to 65 GHz [18, 19] |
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Fig.9 High responsivity, high speed and high power integrated photodiodes based on back-to-back stacked UTC structure. CPW: coplanar waveguide [20] |
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Prof. Luo’s group from Tsinghua University realized an uni-travelling-carrier photodiodes (UTC-PDs), in which the photon-absorption is mainly in the p-layers and only electrons diffusion and drift are needed, have exhibited high speed and saturation optical power performance which are important for high performance microwave photonic links. As shown in Fig. 9, the 3-dB bandwidth reaches over 40 GHz for the first time in such high responsivity mesa structure PDs, with a measured photocurrent over 47 mA at 30 GHz at 1-dB compression point [
20].
Fig.10 High spectral purity mm-wave carrier generation by modulation sideband injection locking of integrated dual wavelength laser diode (LD) [21]. ML: master laser; SL: slave laser |
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In addition, as shown in Fig. 10, an integrated dual wavelength laser diode (LD) is fabricated for optical generation of millimeter-wave (MMW) carrier with high spectral purity [
21]. The integrated device consists of two DFB lasers and one Y-branch coupler monolithically integrated on the same AlGaInAs multiple quantum well (MQW) active layer. A 42 GHz mm-wave carrier is generated from a 5.25 GHz modulation signal, corresponding to eight-fold frequency multiplication. The measured PN performance of the 42 GHz mm-wave carrier is as low as -94.6 dBc/Hz at 10 kHz offset.
Several groups from Huazhong University of Science and Technology (HUST) are also working on IMWP devices mainly based on passive/silicon photonics devices. Prof. Wang’s group at Wuhan National Laboratory for Optoelectronics (WNLO) realized an ultra-high peak rejection notch MPF using silicon waveguides, silicon microring resonators, and silicon photonic crystal nanocavities as shown in Fig. 11 [
22].
Fig.11 Concept and principle of the notch MPF with ultra-high peak rejection and the measured RF responses of tunable ultra-high peak rejection MPF under different optical carrier wavelengths. LSB: lower sideband; USB: upper sideband [22] |
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Fig.12 (a) Scanning electron microscope (SEM) images of the cascaded microring resonators (CMRRs); (b) and (c) tunability of central frequency of the MPF; (d) and (e) tunability of bandwidth of the MPF. FSR: free spectral range [23] |
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In 2010, Prof. Zhang from WNLO in HUST declared to implement an MPF with the highest
Q factor of 3338. In 2013, this research group has demonstrated a notch MPF based on cascaded SOI microring resonators (MRRs) with different radii [
23]. As shown in Fig. 12, the tuning of input laser wavelength is not necessary and the frequency tunability is continuous. They demonstrated a central frequency tuning range from 19 to 40 GHz, and a wide bandwidth tuning range from 5.5 to 17.5 GHz.
A few researchers from SJTU are working on silicon photonics based IMWP devices such as tunable delay line and optical filter for microwave signal generation. From Prof. Chen’s group, as shown in Fig. 13(a), they proposed an N-bit reconfigurable optical true time delay line (RTTDL) using cascaded optical switches and waveguide delay lines [
24]. The optical delay can be varied by reconfiguring the switches to provide different optical paths for the light to pass through. The switch is made of a 2×2 Mach-Zehnder interferometer (MZI) with a PIN diode integrated in one arm to enable fast electrical tuning. Figure 13(b) shows the microscope image of the fabricated 7-bit RTTDL. The chip size is 7 mm × 1.5 mm. Figure 13(c) presents the images of the chip after home package. The chip is wire-bonded to a print circuit board and coupled with a fiber array. Figure 13(d) shows the output pulses after various delays. The delay is relative to the reference pulse which passes through the shortest path. The delay increases from 10 to 640 ps when each stage is switched to the longer waveguide. The delay reaches to the maximum of 1.27 ns when all longer waveguides are selected. Therefore, the RTTDL can provide 128 delays with a resolution of 10 ps. The RTTDL has time delay invariance to RF frequency. Figure 13(e) shows the phase of the received RF signal as a function of frequency when the carrier light passes through the shortest and longest paths. The phases changes linearly with the RF frequency. The time delay is given by the phase derivative to the angular frequency. The relative delay between the longest and shortest path is 1.3 ps, agreed with the pulse transmission measurement.
Fig.13 (a) Schematic structure of the RTTDL. The switch is based on a 2×2 multimode interference (MMI); (b) optical microscope image of the fabricated chip; (c) optical photo of the package chip; (d) measured output pulses with 10 ps to 1.27 ns optical delays with respect to the reference pulse; (e) measured frequency responses of the unwrapped transmission phase for the minimum and the maximum delays [24] |
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Fig.14 (a) Micrograph of the fabricated SCMR and measured microwave frequency responses for varied applied DC voltages; (b) micrograph of the fabricated SCMR with its drop port as the output, and temporal waveforms of the generated 29-GHz / 39-GHz MMW signals [25] |
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Researchers from Prof. Su’s group in SJTU demonstrated tunable microwave photonic filtering and MMW signal generation with a silicon photonic device termed as self-coupled micro-resonator (SCMR) [
25]. By tuning the resonance notch interval of an SCMR with an interferometric central coupler and using the through port as the output, they have demonstrated a continuously tunable microwave photonic notch filter with a high rejection ratio over 25 dB and a wide tuning range over 20 GHz. The micrograph of the fabricated device and the measured frequency responses are shown in Fig. 14(a). While if using the drop port of the device as shown in Fig. 14(b), with an input of optical frequency comb (OFC) spaced by ~9.7 GHz, photonic generations of ~39-GHz and ~29-GHz MMW signals can be achieved by thermally tuning the passband spacing.
Fig.15 Schematic diagram of the frequency-shifted heterodyne method within one setup. In the case of the Mach-Zehnder modulator (MZM) as DUT, the microwave frequency of MZM is set close to twice of that of PM. In the case of PM as DUT, the microwave frequency of PM is set close to twice of that of MZM. In the case of PD as DUT, the microwave frequency of MZM is set close to that of PM [29]. ESA: E series spectrum analyzer |
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Besides of the fabrication of IMWP devices, high-speed optical modulators and PDs are basic components in MWP links. High-frequency responses of these optoelectronic devices are critical to the wideband electrical-to-optical or optical-to-electrical signal conversion. The conventional swept frequency method needs a pair of modulator and PD, and it relies on de-embedding the frequency response of the assisted devices except the device under test (DUT) [
26], since the measured results are always contributed by both the modulator and the PD in the setup. Recently, Zhang and Liu at University of Electronic Science and Technology of China (UESTC) proposed a novel frequency-shifted heterodyne method and demonstrated for the self-calibrated high-frequency response measurements of Mach-Zehnder modulators (MZMs) [
27,
28], phase modulators (PMs) [
29,
30] and PDs [
31]. The method is illustrated in Fig. 15 within one setup. The MZM, PM and PD can be independently self-calibrated measured at different modulation frequencies through carefully setting the frequency relationship between the driving microwave signals. The method enables high-resolution self-calibrated frequency response measurement of MZMs, PMs and PDs, and eliminates the need to correct the influence from other assistant devices in the measurement, which largely simplifies the high-frequency characterization of high-speed optoelectronic devices.
3 MWP system
In the past few years, MWP subsystems towards practical applications have been demonstrated in China, such as programmable MPF, stable RF phase distribution, indoor building radio access network, OBFN and optical vector spectrum analyzer, which will be introduced simply as following.
Prof. Zheng from Tsinghua University has been working on the spectrum manipulating in the optical domain and its applications in MWP links and devices. Spectrum structures of microwave photonic links with dispersion [
32] or nonlinearity from MZM [
33,
34] or EA [
35], and their evolutions during the photo detection have been analyzed, and compensation techniques for fiber dispersion and nonlinearity for modulators have been proposed by manipulating optical spectrum with DPMZM [
32] or spatial light modulator [
33,
37]. And FSDR of radio over fiber transmission links with commercial optical devices can be easily improved using these techniques to more than 122 dB·Hz
2/3 [
32,
34] for MZM based links and 115 dB·Hz
2/3[
36] for EA based link with a frequency lower than 20 GHz, and to 107 dB·Hz
2/3[
36] for MZM based link with 60 GHz frequency band. The technique for dispersion compensation is widely applied to enhance performances of other MWP subsystem. A high resolution time delay control is realized by the technique to add a quadratic optical phase shift and compensates the group velocity dispersion (GVD) of the fiber simultaneously. A coarse tuning range of 1717 ps and a high resolution of 0.67 ps is achieved [
37]. It is employed to suppress the dispersion induced PN in optically generated MMW and fiber transmission. An MMW PN amelioration of 12.63 dB at 10 kHz offset and 11.23 dB at 100 kHz offset is achieved in a 40 GHz, 25 km optical MMW link thanks to this technique [
38]. Also, we realized dispersion free transmission of ultrawideband (UWB) over fiber under a different length of fiber [
39].
In addition, Prof. Zheng is also working on the highly reconfigurable microwave photonic single-bandpass filter with complex continuous time impulse responses [
40,
41]. The filter is based on the so-called equivalent electrical slicing together with programmable optical spectrum processor [
40]. The schematic diagram of the filter is shown in the figure [
41]. The incoherent light from a incoherent broadband optical source (BOS) (e.g. erbium-doped fiber amplifier (EDFA) or light-emitting diode (LED)) is polarized and split into two branches via a 1 × 2 coupler (C1). One branch is modulated by the microwave input via a single-drive MZM biased at the minimum transport point, as shown in Fig. 16. The other branch is spectrum tailored via a WaveShaper which is a commercial programmable optical processor, and time delayed via a variable optical delay line (VDL). The two branches are then combined again in the same polarization via a 2 × 2 coupler (C2). The two outputs of C2 go through a dispersive element which is a length of dispersion-compensating fiber (DCF) in opposite directions via two circulators and differentially detected. WaveShaper acts as a programmable optical filter with arbitrary amplitude and phase responses. VDL is for tuning the central frequency of the filter. As shown in Figs. 17 and 18, the shape of MWP filter keeps unchanged when tuned from DC-20 GHz, and the highest
Q value is 634. Additionally, a flat-top shape with lower
Q value when tuned from DC-20 GHz has also been realized, as shown in Fig. 18.
Fig.16 Incoherent-BOS-based MPF features single-bandpass, widely tunable (0-20 GHz), arbitrary shape with Q value controllable. Followings show some figures to prove the abilities of the filter [41]. PC: polarization controller; BPD: balanced photodetector |
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Fig.17 Shape keeps unchanged when tuned from DC-20 GHz, and the highest Q value is 634 [41] |
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Fig.18 Unchanged flat-top shape with lower Q value when tuned from DC-20 GHz [41] |
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Prof. Li and Prof. Feng from Jinan University are also working on MPFs. Prof. Feng’s research is focused on the investigation of new microwave photonic signal processing techniques by using a Fourier-domain optical processor (FD-OP). She reported a series of FD-OP based microwave photonic signal processors that exploit the gain-bandwidth and reconfigurability advantages of photonics to overcome the limitations of conventional processors [
42]. Prof. Li realized a MPF with an ultra-high
Q value of 4895 utilizing cascaded optical-electrical feedback loops. As shown in Fig. 19, an infinite impulse response (IIR) filter with both optical and electronic signals in a feedback loop is proposed to overcome the problem of optical coherent interference. In addition, we improve the
Q value of an IIR filter with two cascaded optical-electrical feedback loops, in which the Vernier effect can help to select the frequency components passing through both cascaded loops and therefore significantly increase the FSR of the filter. As a result, an highest reported
Q value of 4895.31 is achieved for the proposed IIR filter [
43]. In addition, the ability to vary the
Q value of the proposed microwave photonic filter can be achieved by carefully adjusting loop lengths of the two cascaded filters, and the frequency response of the filter with a fixed
Q can also be changed by tuning the bias of the MZMs in the two loops.
Fig.19 (a) Experimental setup; (b) frequency response of the IIR filters with one loop; (c) frequency response of the IIR filter with two cascaded loops [43]. DML: directly modulated laser; VNA: vector network analyzer; EA: electrical amplifier; PD: photo detector; ODL: optical delay line |
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In order to meet the requirements for the development of three networks (telecommunication network, radio and television network, Internet), the radio and television network not only can provide voice, data and high-definition television and other cable services for the users, but also will provide Internet, mobile TV, mobile communications and other wireless services. In view of the above requirements, our research group mainly focuses on studying the architecture, system components and key technologies of the RF access system. Prof. Xin from BUPT has reported several significant works [
44-
47]. They realize a flexible scheduling and dynamic bandwidth allocation is realized by using orthogonal frequency division multiplexing (OFDM) subcarrier to carry multi service. In addition, the wireless/optical fiber integrated system with multi-users, long distance, and large capacity is realized via combining radio-over-fibre (ROF) with passive optical network (PON) technology, as shown in Fig. 20. Moreover, as shown in Fig. 21, a real-time video transmission platform based on 60 GHz technology is built, which can realize the 1 Gb/s high definition (HD) video transmission with free bit error rate, and realize the seamless integration of the optical fiber 25 km and wireless 10 m access. The radio passive optical network (RPON) access system, whose index has reached the international advanced level, has been considered as the promotion project by Shanxi Province Radio and Television Information Network Co., Ltd.
Fig.20 Structure diagram for integrated access network. OLT: optical line terminal; ODN: optical distribution network [45–47] |
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Fig.21 60 GHz HD video real-time optical access platform |
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In photonic microwave dissemination system, a novel phase error detection and PN compensation scheme have been proposed by Prof. Dong from SJTU [
48-
51]. As shown in Fig. 22, the former contains an optic heterodyne beating and an electrical heterodyne beating, while the latter includes an electrical phase locked loop (PLL) and an acousto-optic frequency shifter (AOFS). Based on the dual-heterodyne phase error transfer (DHPT) and fast phase corrections scheme, we have achieved highly stable remote distribution microwave signal of 10 GHz over 100 km, 20 GHz over 100 km, 100 GHz over 60 km spooled optic fiber, and 1 THz over 42 km dark optic fiber respectively. Long-term stability of microwave signal dissemination is achieved and shown in Fig. 23.
Fig.22 Experimental setup for stable dissemination millimeter-wave signal system [48–51]. OFCG: optical frequency comb generator; PMC: polarization maintaining coupler; AWG: arrayed waveguide grating |
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Fig.23 Allan deviation of remote distribution system [48–51] |
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Prof. Sun from HUST published some impressive works on the generation, modulation and measurement of microwave signal. They demonstrate a photonic approach to simultaneously realize a frequency-multiplied and phase-shifted microwave signal based on the birefringence effects in the high nonlinear fiber (HNLF) [
52]. The phase shift caused by asymmetric variations in refractive indexes of fiber between two orthogonal polarization states is introduced into two coherent harmonics which will be beaten in the PD. A microwave signal at doubled- or quadrupled-frequency with a full 2π phase shift is obtained over a frequency range from 10 to 30 GHz. In addition, they realized a tunable optical carrier-to-sideband ratio (OCSR) by introducing two RF signals with phase difference to drive an MZM. Stimulated Brillouin scattering (SBS) is adopted to remove the undesirable sideband, converting double sideband (DSB) to single sideband (SSB) [
53]. Wide ranges of OCSR from -23 to 44 dB and -21 to 44 dB are obtained for the DSB and SSB modulation. Moreover, they experimentally demonstrate a novel approach for microwave frequency measurement utilizing birefringence effect in the HNLF [
54]. As shown in Fig. 24, a frequency measurement range of 2.5–30 GHz with a measurement error within 0.5 GHz is achieved except 1–2.5 GHz and 16-17.5 GHz.
Fig.24 (a) Measured and theoretical (solid line) amplitude comparison function (ACF) when Pm=270, 285 and 300 mW; (b) measured microwave frequencies when the input microwave frequency is tuned from 0 to 30 GHz; (c) distribution of the measurement errors [54] |
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Fig.25 Schematic illustration of the sub-Nyquist sampled photonic analog-to-digital converter based on of the techniques of photonic time stretch and compressive sensing.LPF: low pass filter; DSP: digital signal processing; PRBS: pseudorandom binary sequence [55–61] |
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Prof. Chi and his colleagues in Zhejiang University, Hangzhou focus on the photonic realization of compressive sensing (CS) for the acquisition of wideband sparse signals in recent years [
55-
61]. They have proposed and investigated some approaches to optically realize the functions which are necessary for the CS, such as random mixing and integration [
55-
59]. To largely decrease the required sampling rate, they have proposed a sub-Nyquist sampled photonic analog-to-digital converter based on the techniques of photonic time stretch and CS [
60,
61], which is shown in Fig. 25.
Prof. Xu from BUPT has proposed a satellite repeater architecture which consists of multiple paths to support repeating signals among the C, Ku, K and Ka bands [
62,
63], as shown in Fig. 26. Note that all the paths share a common local oscillators (LOs) generation module and an optical switch matrix that dramatically saves LOs and space on the satellite. The whole system can be divided into three sections, the multi-band LOs generation section, the band to band conversion section and the optical cross-connect section. In particular, these three sections can be compatible with each other since all their functions are implemented in optical domain. Consequently, this concept of microwave photonic repeater is very suitable for transparent and broadband telecom missions. Besides, such microwave photonic system with parallel structure offers high scalability, thus it can be upgraded to large scale with the number of channels. In the future, once integrated into a single chip, this concept will bring the current all-microwave repeaters to a new era with unprecedented performance improvement.
Fig.26 Conceptual architecture of multi-band satellite repeater based on optical frequency combs. LNA: low noise amplifier; WDM: wavelength-division multiplexing [62,63] |
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The MWP group from Beihang University is at a unique position to investigate and pursue applications of the microwave photonic technologies to bridge the gap between the microwave and photonic worlds. In the past few years, their focus has been on microwave photonic technologies based on OFCs, as shown in Fig. 27. Through generating versatile and tunable optical combs by electrical-to-optical (EO) modulation [
64], RF waveform generation and spectrum measurement over a wide frequency range are demonstrated [
65]. Based on innovative cavity structures that enable wavelength-, polarization-multiplexing in ultrafast fiber lasers, multiple OFCs could be generated from a single optical cavity. Due to the correlation between such generated comb pairs that suppresses the common-mode noises, asynchronous optical sampling with high bandwidths could be realized [
66], which would enable fast, high-resolution characterization and manipulations of, up-to-THz, microwave signals with many potential applications in sight.
Fig.27 MWP research activities in Beihang University based on OFCs [64] |
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Recently, Prof. Yi et al. in SJTU have solved the problems and demonstrated a rectangular MPF based on SBS effect in optical fiber [
67-
71] and offering tunability on bandwidth, central frequency and selectivity as shown in Fig. 28. A sweeping-pump multi-stage configuration with feedback control is implemented to achieve the rectangular MPF with high selectivity. The obtained 20 dB shape factor is as low as 1.056, which is the best reported result for MPF in ~GHz bandwidth. Furthermore, they solved the polarization-dependent SBS gain issue and realized a polarization-independent MPF. The SBS noise is reduced by adopting a multi-stage configuration to limit the gain at each stage. Finally, the filter selectivity for a four-stage configuration is as high as 57 dB for a 2.1 GHz bandwidth. In this case, the signal-to-noise ratio penalty is only 2.6 dB for a 4 Gbit/s OFDM signal in quadrature phase shifted keying (QPSK) format.
Fig.28 Filter bandwidth, central frequency and selectivity tuning of SBS based MPF [67–71] |
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Emergency services, when large-capacity long-distance optical cables are cut during natural disasters such as earthquake and tsunami, as well as mobile backhauling between wireless macro stations, require wireless links with both large transmission capacity and long transmission distance. Broadband mm-wave generation based on photonic techniques can overcome the insufficient bandwidth of commercially available electrical components and promote the seamless integration of wireless and fiber-optic networks. The research group from Fudan University has accomplished a series of record-breaking experimental demonstrations on large-capacity/long-distance wireless mm-wave signal delivery based on photonic mm-wave generation techniques as well as the favorable coordination of multiple multi-dimensional multiplexing techniques introduced in Fig. 29 [
72]. For the large-capacity wireless mm-wave signal delivery, they first experimentally demonstrated up to 108-Gb/s polarization-division multiplexed quaternary phase shift keying (PDM-QPSK) wireless mm-wave signal delivery at W-band [
73], up to 400-Gb/s hybrid PDM-QPSK and polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) wireless mm-wave signal delivery at both Q-band and W-band [
74], and up to 432-Gb/s PDM-16QAM wireless mm-wave signal delivery at W-band [
75]. For both large-capacity and long-distance wireless mm-wave signal delivery, they successfully achieved field trial demonstrations of 20-Gb/s PDM-QPSK signal delivery over 1.7-km wireless distance at W-band [
76], 80-Gb/s PDM-QPSK signal delivery over 300-m wireless distance at W-band [
77], and 100-Gb/s QPSK signal delivery over 100-m wireless distance at W-band [
78].
Fig.29 Different approaches for the realization of large-capacity/long-distance wireless mm-wave signal transmission [72]. MIMO: multiple-input multiple-output |
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Fig.30 Schematic diagram of the high-resolution optical vector analyzer based on optical single-sideband modulation [80]. OSSB: optical single-sideband; ODUT: optical device-under-test |
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To characterize the spectral responses of optical devices with ultra-high resolution [
79], optical vector analyzers (OVAs) based on optical single-sideband (OSSB) modulation were developed by Prof. Pan from Nanjing University of Aeronautics and Astronautics (NUAA) [
80], which sweep the frequency and analyze the responses using the mature and high-resolution microwave technologies. Figure 30 shows the schematic diagram of the OSSB-based OVA. Although the reported resolution has been better than 0.8 fm [
80], its application is restricted by the limited measurement range and measurement errors due to the O/E and E/O conversions.
In their recent work, the measurement range of the OSSB-OVA was boosted based on an OFC. With each carrier in the OFC to achieve the magnitude and phase responses in a certain range, the measurement range of the OSSB-based OVA is extended by about n times if an n-comb-line OFC is applied. In the experiment, a 105-GHz measurement range is achieved by employing a 5-comb-line OFC with 20-GHz frequency spacing. In the OSSB-based OVA, the frequency responses are only carried by the beat note of the optical carrier and one of the first-order sideband. By suppressing the optical carrier, the desired beat note is removed, leaving only the measurement errors. By simultaneously detecting the OSSB with and without carrier adopting balanced PD, the accurate frequency responses can be obtained with a large dynamic range. Thus, an OSSB-based OVA with wide measurement range, ultrahigh-resolution, and high accuracy can be achieved.
Fig.31 Block diagram scheme of a conceptual software-defined satellite payload based on MWP; experimental demonstration of 2 ´2 microwave photonic switch for HD video [81] |
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A conceptual software-defined satellite payload based on MWP has been proposed and investigated by Prof. Pan from NUAA [
81], as shown in Fig. 2 [
31]. By introducing microwave photonic components and subsystems to realize multiple LO generation and distribution, frequency conversion, analog-to-digital conversion (ADC), digital to analog conversion (DAC), switching, and multi-beamforming functions, the proposed satellite payload can implement high-efficiency on-board parallel signal processing with enhanced capacity and significantly reduced mass, volume, and power consumption.
In their recent work, the potential to apply the photonic technologies to the satellite payloads is studied. Technologies such as the optical LO sources generation based on OEO, parallel photonic microwave FMIX for signal transportation and photonic signal switching based on high speed optical switch have been demonstrated. A 2×2 photonic satellite repeater experimental demonstration for switching of HD video signals with 1.5-Gb/s data rate is shown in Fig. 31. Two HD video signals are up-converted to the Ku-band (16 GHz) and transmitted to the free space through two antennas. In the receive site of the microwave photonic repeater, the received signals from the two antennas are modulated onto two different optical carriers. The two optical signals are simultaneously down-converted to the IF-band by multiply with an OEO-based LO signal, thanks to the parallel FMIX by a photonic mixer. The downconverted IF signals are then separated by a wavelength-division de-multiplexer. A high-speed optical micro-electro-mechanical system (MEMS) switch controlled by computer is used for the switching of the two channels. After photodetection and further downconversion, the switched HD video signals are sent to separate displayers. The HD video signals are successfully transmitted to the receiver site through the photonic-based payload, and the HD signal can be flexibly exchanged in the analog domain by the optical MEMS switch without cross interference.
Fig.32 Schematic diagram of the proposed distributed MIMO chaotic radar based on WDM technology, geometric model of two-dimensional localization with two transmitters and two receivers, and the geometric locations of six samples of the estimated positions and their corresponding actual positions [82] |
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Construction of distributed radar systems based on microwave photonic technologies has been researched [
82]. A distributed multiple-input multiple-output chaotic radar using wavelength-division multiplexing (WDM) technology is proposed and demonstrated [
82], as shown in Fig. 32. The wideband quasi-orthogonal chaotic signals are generated by different OEOs, and emitted by separated antennas to gain spatial diversity against the fluctuation of a target’s radar cross section. In this way, the detection capability will be enhanced. The signals received by the receive antennas and the reference signals from the OEOs are delivered to the central station for joint processing by exploiting WDM technology. The centralized signal processing avoids precise time synchronization of the distributed system and greatly simplifies the remote units, which improves the localization accuracy of the entire system. A proof of concept experiment for two-dimensional localization of a metal target is demonstrated, with a maximum position error of less than 6.5 cm.
Compared to many other techniques for photonic UWB signal generation based on impulse radio, Wang et al. proposed novel methods to generate noise-like UWB signal in optical domain, which was called chaotic UWB signal [
83,
84]. Based on the chaotic dynamics of an optically injected semiconductor laser with optical feedback, the chaotic UWB signal with a fractional bandwidth of 116% and the central frequency of 6.88 GHz was experimentally generated [
84]. The experimental setup was shown in Fig. 33, and the generated chaotic UWB signal was shown in Fig. 34. The spectrum of the generated chaotic UWB signal was in full compliance with the Federal Communications Commission (FCC) spectral mask. Moreover, by controlling the injection strength and frequency detuning of the chaotic laser, the generated UWB signals’ spectrum bandwidth and central frequency could be flexibly adjusted in a certain range. Furthermore, based on the photonic chaotic UWB signal, they proposed a UWB radar system for remote ranging [
85] and a microwave-photonic sensor for remote water-level monitoring [
86]. This remote radar system not only realizes the remote control of the radar, but also has a high range resolution with antijamming ability, which can be used for the military radar in hilltops under bad conditions and the radar systems of islands, dangerous areas, and harsh industrial environments. The microwave-photonic sensor has advantages in remote real-time water-level measurements and high range resolution, which can provide early warning of events for flood control projects, coal mines production, the flood control stations of alpine valleys.
Fig.33 Experimental setup for photonic chaotic UWB signal generation [83]. VA: variable attenuator; EAM: electro-absorption modulator; OOK: on-off keying |
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Fig.34 Waveforms (a) and RF spectra (b) of the experimentally generated chaotic UWB signal |
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Radio over fiber technology has great potential applications in many fields, especially for the high frequency microwave signal transmission over a long fiber distance. However, the dispersion induced RF power fading is the main factor deteriorating the performance of radio over fiber system with the conventional DSB. The tunable optical filter based on the integrated waveguide microring resonator was designed and fabricated with the UV-soft selective imprint technique, which has the tunable notch depth and the tunable central frequency. As shown in Fig. 35, it was utilized to filter out one sideband of the DSB signal. The chromatic dispersion induced RF power fading was overcome effectively by the tunable ring resonator with the demonstration of the successful transmission of 7.2 GHz microwave signal over 70 km single-mode fiber (SMF) [
87].
Fig.35 Single sideband (SSB) radio over fiber by using the integrated waveguide tunable microring resonator [87] |
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Prof. Li from IOS-CAS proposed a simple method to realize stable radio-frequency phase distribution over optical fiber by phase-drift auto-cancellation without using any assistant LO source, as shown in Fig. 36 [
88]. Besides, neither active phase discrimination nor dynamic phase tracking/compensation is required in this open-loop design. The phase-drifts of the RF signal induced by fiber-length variations are eliminated automatically via RF mixing. Since no assistant LO signal is involved in our scheme, the complicated frequency-estimation of the received RF signal as well as the dynamic frequency-locking between the LO and the received RF signal is not required anymore, which greatly simplifies the system. Experimentally, phase-drift-free RF signal is successfully achieved. The root-mean-square (RMS) timing jitter is 0.76 ps when a tunable optical delay line (TODL) inserted between the remote antenna unit (RAU) and local station is changed from 0 to 300 ps.
Fig.36 Schematic diagram of the stable RF phase distribution scheme [88]. AWG: array waveguide grating; PS: power splitter; OC: optical circulator |
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To characterize a device-under-test (DUT) with ultra-fine transmission response, optical vector network analyzer (OVNA) is a promising device. The OVNA based on single-sideband modulation usually suffers from huge measurement errors due to the nonlinearity of the modulator. Prof. Li proposed an OVNA with improved accuracy based on dual-parallel Mach-Zehnder modulator (DPMZM). By properly setting the bias voltage of the DPMZM, the measurement error is significantly reduced, as shown in Fig. 37 [
89].
Fig.37 Proposed OVNA with improved accuracy [89]. TLS: tunable laser source; TOBPF: tunable optical bandpass filter; EVNA: electrical vector network analyzer; ASE: amplified spontaneous emission |
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IFM based on photonic techniques has attracted great attention recently. An IFM system is usually used in radars and electronic warfare systems for detecting and classifying unknown signals. Prof. Li proposed an IFM system with adjustable measurement range and resolution based on SBS in optical fiber, see Fig. 38 [
90]. The proposed technique can be switchable between a wideband tunable narrow measurement range (~2 GHz) with high resolution (±0.05 GHz) and a fixed wide measurement range (12 GHz) with moderate resolution (±0.25 GHz).
Fig.38 Proposed IFM system.ISO: isolator [90] |
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The implementation of all-optical circuits for computing, information processing, generation and networking could overcome the severe speed limitations currently imposed by electronic-based systems [
91-
123]. A promising approach toward the implementation of ultrafast all-optical circuits is to emulate the developments in the electronic domain, i.e., to follow similar component and design strategies, using photonic technologies [
91-
94]. For this purpose, high-speed optical signal processors, such as optical differentiator [
95-
101], integrator [
102-
105], Fourier transformer [
106,
107] and Hilbert transformer [
108,
109], have recently attracted an increasing interest in optical communications, pulse shaping or sensing applications that use optical signals [
110-
114].
Prof. Li from IOS-CAS has focused his research topics on all-optical signal processor [
124,
125]. Recently, he proposed and experimentally demonstrated an analog optical signal processor based on a phase-shifted DFB-SOA and an optical filter [
125]. The proposed analog optical signal processor can be reconfigured to perform signal processing functions including ordinary differential equation solving and temporal intensity differentiation. The reconfigurability is achieved by controlling the injection currents. Our demonstration provides a simple and effective solution for all-optical signal processing and computing.
Fig.39 Phase-shifted DFB-SOA. (a) Schematic diagram and (b) the physical image of the packaged phase-shifted DFB-SOA [125] |
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4 Conclusion
Recent progresses of MWP have been reviewed including the main projects and representative results from almost all the active research teams in China. In the past five years, many impressive progresses in MWP devices and subsystem have been made to support the development of national major assignments. Compare to the rapid development of MWP subsystem, the IMWP devices should be further largely improved in the near future. In the next five years, the national major research and development program has considered MWP, in particular IMWP devices, as one of the key enabling technologies for improving the performance of information system.
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Fig.1 Publications and citations records in the MWP topic of China, USA and Japan in the past 20 years (Data: from Web of Sciences)
Tab.1 Selected funded major projects on MWP in the past few years
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