Progress in microwave photonics research
Abstract: Microwave photonics focuses on the combination of microwaves and photons in concepts, devices, and systems. Typical studies include the generation, processing, and conversion of microwave signals, and the distribution and transmission of microwave signals in optical links. The research results have promoted the emergence of new technologies, such as optical wireless (RoF) communication, cable television (CATV) subcarrier multiplexing and fiber transmission, phased array radar optical beamforming network and microwave frequency domain measurement technology. Wait.
Abstract microwave includes: photonic microwave generation, photonic signal processing andconversion, distribution of microwave signals in optical links, andso on. The research results promote new technologies such AsRadio over Fiber (RoF) communications, the subcarrier multiplex and fiber transmission of Cable T el evision (CATV), optical control beamforming network in phased array radar, test technologies inmicrowave frequency, and so on.
Fund Project:Project supported by the National Natural Science Foundation of China (60736002, 60807026) 1 The emergence of background optical wavelength division multiplexing technology generated by microwave photonics and the invention of erbium-doped fiber amplifiers have led to the rapid development of optical communication. Optical fiber communication has many advantages such as low loss, anti-electromagnetic interference, ultra-wideband, easy to be multiplexed in wavelength, space and polarization. At present, single-channel 40~160Gb/s and single fiber 10 Tb/s transmission have been realized.
As capacity transmission rates continue to increase, fiber optic systems require the use of microwave technology in optical transmissions and receivers.
At the same time, microwave technology is rapidly evolving as demand for wireless communication capacity increases. Microwave communication can be transmitted in any direction, easy to construct and reconstruct, and realize interconnection with mobile devices; the emergence of cellular systems enables microwave communication to have high spectrum utilization. However, the limited bandwidth of the microwave band has become a serious problem, and people are beginning to consider the use of the new 30-70 GHz band. The 60 GHz optical-to-air (ROF) system is becoming a popular technology for broadband access due to its high access rate and the need to apply for additional licenses. The transmission loss of the 60 GHz signal in the atmosphere is as high as 14 dB/km, which means that the channel frequency can be reused more frequently in cellular mobile communication. However, the traditional microwave transmission medium has a large loss in long-distance transmission, and the fiber system has low loss and high bandwidth characteristics, which is attractive for microwave transmission and processing.
The integration of fiber technology and microwave technology has become an important new direction. Theoretically speaking, the theoretical basis of microwave technology and fiber technology is the theory of electromagnetic wave fluctuation. In optoelectronic devices, when the wavelength is small enough to consider the wave effect, electromagnetic wave theory is used to design and study optoelectronic devices, such as waveguide or traveling wave devices. The unification of the theoretical basis allows microwave devices and optoelectronic devices to be integrated on the same chip using the same materials and technologies, which greatly promotes the combination of the two disciplines and promotes the birth of a new interdisciplinary, microwave photonics.
The concept of microwave photonics was first proposed in 1993 [1]. Its research content covers various fields related to microwave technology and fiber technology [2]. It mainly focuses on two aspects: one is to solve the traditional optical fiber communication technology to the microwave frequency band. Developmental issues, including lasers, optical modulators, amplifiers, detectors, and fiber-optic transmission links; second, the use of optoelectronic devices to solve microwave signal generation and control problems, mainly photo-generated microwave sources, microwave photon filters, light Domain microwave amplifier, synthesis and control of photo-induced microwave electrical signals.
2 Key Technologies in Microwave Photonics 2.1 Using Optical Methods to Generate Microwave Signals The development of microwave communication to high frequencies of 30 to 70 GHz is a great challenge for traditional microwave devices. At this time, the use of optical technology to generate microwave signals has shown great appeal. There are many ways to generate microwaves using optical techniques, and the simplest principle is the optical heterodyne method. What is the frequency, phase and power of the two light waves? Brown 1,? Brown 2, ? Quasi 2 and P1, P2. When two beams of similar frequency and the same wave of polarization are simultaneously incident on the high-frequency photodetector for beat frequency, the available output current is:
Where R is the photoelectric conversion efficiency of the detector. It is not difficult to see that the microwave signal with the frequency of |? Brown 1-? Brown 2| can be generated by the beat frequency, and the frequency and phase of the generated signal are determined not only by the frequency difference of the two beams but also by the phase difference. In order to ensure the low phase noise and stability of the microwave signal, the two beams are required to have high coherence. To this end, many new methods for eliminating phase noise generated by lasers have been reported in recent years. There are mainly light injection locking method [3], optical phase-locked loop method [4]. However, the light injection phase-locking method has a small locking range, typically a few hundred megahertz. The optical phase-locked loop method requires that the laser should keep up with the phase change of the master laser, which requires a small loop delay. Both methods require an externally stable microwave signal source, which increases the cost and is not conducive to practical use. Commercialized.
With integrated technology, two lasers can be combined. Thus, the two beams are generated in the same gain medium, and the coherence is good, so that the locking technique can be avoided. In 1995, David Wake of the British Telecommunications Research Institute used two longitudinal modes in a multi-longitudinal mode DFB laser to capture the frequency and obtained the output of the 42 GHz signal.
Recently, technologies utilizing dual-wavelength fiber lasers are being developed. Fiber lasers are lightweight and low cost. In the general fiber laser, the gain medium is mostly made of erbium-doped fiber and has uniform widening characteristics. Various methods have been used to suppress the mode competition caused by uniform widening to realize a dual-wavelength fiber laser and to generate microwave signals ranging from 3 to 60 GHz. Such as the use of low temperature suppression uniform widening [5], distributed dispersion cavity, polarization hole burning, space burning hole, partially separated structure dual-wavelength DFB fiber laser [6] and so on.
Another photo-generated microwave method utilizes an optical modulation technique [7], as shown in Figure 1. The external modulator is an intensity or phase modulator. In the case of linear modulation, a difference frequency signal of twice the modulation frequency can be generated. If deep modulation technology is used, a microwave signal with a modulation frequency of 4 times can be generated. The advantage of using the out-of-optical modulation method is that the frequency can be tuned by changing the frequency of the microwave modulated signal. Compared to the former method, the stability and phase noise of the microwave signal generated by this method depends on the microwave modulation signal and the modulator, and the device requirements are relatively low. In 2005, the research team of Yao Jianping of Canada proposed to use a large microwave input power to drive a lithium niobate modulator and then use a fiber grating filter to filter out the optical carrier component to obtain two optical sidebands. After the beat frequency, 32~50GHz broadband is obtained. Adjusted millimeter wave signal. In recent years, China has reported a lot in this respect. Combined with the frequency doubling effect of nonlinear photonic devices, it can generate microwave signals with frequencies ranging from 6 to 60 GHz [8].
It should be noted that due to the advancement of high-frequency electronic devices, commercial microwave source modules of 60 GHz or less are currently on the market, and the method of photo-generated microwave should be developed to a higher frequency to reflect its own advantages. The highest frequency report is produced. The beat frequency output [9] of 1 000 GHz and 25 GHz has entered the field of terahertz technology. In addition, it is still attractive to generate and transmit ultra-wideband pulse signals in the optical domain using the gain saturation recovery characteristics of semiconductor optical amplifiers, optical polarization modulation, dispersion effects, and the like. It can provide UWB pulse light source with good compatibility with fiber optic system for optical ultra-wideband (UWBOF) communication [10].
2.2 Optical Modulators The use of optical fibers to transmit microwave subcarrier signals presents new requirements for adaptive modulation of optical modulators. The direct modulation technique is simple in that it directly loads the microwave subcarrier signal onto the light wave by changing the injection current of the semiconductor laser. The direct modulation bandwidth is limited by the resonant frequency of the laser. The use of a quantum structure can reduce the threshold current of the semiconductor laser, increase the differential gain, and increase the bandwidth. In order to further increase the bandwidth, it is necessary to reduce the photon lifetime and the gain compression coefficient. However, due to the limitation of the gain compression factor, the direct modulation bandwidth is hard to exceed 30 GHz at room temperature.
External modulation techniques are required to modulate microwave signals around 60 GHz or higher into optical carriers. The traveling wave structure of LiNbO3 modulator can realize the bandwidth of 70 GHz [11]. Electro-absorption modulator can also be used. Due to its low volume driving voltage, it is easy to integrate with laser and photodetector. A light modulating device for the foreground.
There are some flexible methods in the modulation technology, such as frequency up-conversion and optical heterodyne. The frequency up-conversion method modulates the lower frequency microwave signal to the optical transmission, and converts the frequency at the base station to obtain the high-frequency microwave signal, which reduces the requirement of the optical modulator, but increases the complexity of the base station; The difference method transmits two optical signals with a certain frequency difference, and the baseband signal is modulated on the optical. The two optical waves are beaten at the base station to obtain a microwave signal, but this method will be affected by the dispersion of the optical fiber.
2.3 Photodetectors Photodetectors that are practical in microwave photonics must have different performance than conventional optical communication systems: one is high speed; the other is high power output, ie high saturation operating point; the third is directly on the device Converted to microwave power and transmitted from the microwave antenna. Devices that currently meet the above requirements are referred to as single transit carrier photodiodes (UTC-PD). In this device, only electrons are utilized to activate carriers, and holes are confined to a certain area. Utilizing the high mobility of electrons, the response rate of the device is greatly improved. The waveguide structure is adopted to increase the length of the light absorption; the optimal transmission line impedance is designed to obtain a high response rate and a high saturation power. It has been reported that 1.5 RHz 1.5-Hz signals have been detected and there have been reports of UTC-PDs with transmit antennas or modulators as monolithic integrated devices.
2.4 Microwave Photonic Filter Microwave photonic filter is an important part of photon signal processing technology. The processing of signals in the electrical domain is limited by the frequency band and sampling frequency, and the processing speed and accuracy are affected. It is called the electronic “bottleneckâ€. The microwave photonic filter provides a new method to solve the traditional “bottleneck†problem. The input radio frequency (RF) signal is modulated onto the optical signal by the modulator, the processing of the RF signal is performed in the optical domain, and finally the filtered microwave signal is output through the optical receiver. The advantages of this method are: low loss, high bandwidth, no electromagnetic interference, light weight and support for high sampling frequency. The use of wavelength division multiplexing technology also provides the feasibility of parallel processing of space and wavelength.
Microwave photonic filters were originally applied to radar systems and aerospace applications that require high-speed signal processing. With the deepening of ROF system research, microwave photonic filters have been applied in communication systems, especially in millimeter-wave ROF systems. At present, international research focuses on designing new filter structures to achieve higher frequency response, negative tap coefficients, adjustability, reconfigurability, and larger dynamic range. There are two traditional methods: the first method is to use an electrical differential structure. This structure was implemented as early as 1995, but this method has poor adjustability and reconfigurability, and the bandwidth of the power receiving device is limited. The second method is to implement a full-coefficient filter using complex optoelectronic devices, but this method is costly. Recently, many new low cost structures have been reported to implement microwave photonic filters with negative coefficients. Among them, the method of using polarization state and external modulator is the most attractive [12]. On the other hand, in the ROF system, the combination of microwave photon filtering function and other signal processing functions will greatly reduce system cost and enhance functional centralization.
2.5 Analog-to-Digital Converters In some analog systems, such as radar and broadband communication systems, digital signal processing methods are used for better performance and faster reconfiguration. The weak point of the analog-to-digital converter in the electric domain gradually increases with the increase of frequency, because the CMOS digital converter is limited by factors such as sampling clock jitter, sample-and-hold circuit stabilization time, and comparator processing speed. A 100 GHz sampled analog to digital converter available in digital signal processing is difficult to implement. The method proposed by microwave photonics is called optical time stretching, sampling frequency up to 480 GHz, and 96 GHz bandwidth [13]. The basic principle of optical time stretching is to use photon processing to slow down the electrical signal speed to improve the electrical domain. The analog to digital converter. The light processing process has three steps: wavelength-time conversion, wavelength domain processing, and wavelength-time mapping. The converted slow electrical signal can be transformed by a constant-amplitude converter (A/D).
2.6 Optical Domain Microwave Amplifiers Amplify the microwave signal in the optical domain using the gain of a common erbium-doped fiber amplifier and the interaction of light and microwave, as shown in Figure 2. The direct current light output by the external cavity laser is modulated by the microwave signal input in the intensity modulator. The DC bias point of the modulator is stabilized near the half-wave voltage, and the output optical signal is amplified by the erbium-doped fiber amplifier and filtered by the optical band-pass filter to filter out the spontaneous radiation noise. Finally, the input optical receiver recovers the amplified microwave signal. The experimental results show that with the microwave frequency constant at 4 GHz, the microwave gain is always stable at around 17 dB as the input microwave signal increases, showing good stability, and the signal-to-noise ratio of the output microwave signal will follow. Improve.
2.7 Overcoming the influence of microwave subcarriers on fiber transmission links The transmission characteristics of microwaves in optical fibers are important research contents of microwave photonics. As early as when hybrid audio-fiber systems are used to transmit analog cable television (CATV) signals, The link transmission characteristics are the focus of attention, and the corresponding theoretical model has been used to analyze the transmission characteristics of the ROF link. In a higher-speed ROF link than CATV, fiber dispersion becomes the main factor limiting the transmission distance, and PMD and various nonlinear effects are also more obvious. For chromatic dispersion, it is generally considered to be solved by performing single sideband modulation techniques in the optical domain. The most straightforward method is to use fiber grating filtering to obtain a single sideband signal, but the filter itself introduces dispersion into the system. Studies have shown that the nonlinearity of the external modulator severely limits the dynamic range of the entire microwave link, and the nonlinear effects such as cross-phase modulation caused by a large transmission power will further aggravate the deterioration of system performance [14]. Signals of different digital modulation formats have very different requirements for the millimeter-wave optical fiber transmission link. Therefore, various modulation formats such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM) are transmitted in the microwave optical fiber transmission system. And the link characteristics when using Orthogonal Frequency Division Multiplexing (OFDM) technology for baseband digital signals and intermediate frequency signals are hot topics in recent research.
3 System Application Microwave Photonics The earliest system application was the “deep space network†in the Mojave Desert in the north of Los Angeles in the late 1970s. The Deep Space Network is a large-scale dish distributed over a few tens of kilometers. A cluster of antennas with a maximum antenna diameter of 70 m. A fiber optic transmission system is established between these antennas to deliver an ultra-stable microwave reference signal of 1.420405752 GHz. All antenna elements are synchronized by this frequency, using the concept of phased arrays to make them work like a huge antenna, enabling communication and tracking with spacecraft in outer space. Later in the 1990s, hybrid coaxial cable-fiber CATV systems with microwave photonics technology also achieved commercial success.
In recent years, an important application target of microwave photonics is the transmission of microwave carrier signals for wireless communication using optical fibers. That is to study the intra-fiber RF transmission system, such as the optical-borne wireless (ROF) communication system. ROF combines the advantages of microwave and fiber optic communication, enabling microwaves to achieve low loss transmission in fiber. The ROF can be used to implement signal transmission and distribution between the central office and each microcellular antenna. The utility model has the advantages that the complex microwave processing unit can be placed in the central office, and the base station portion only has two parts of the photoelectric conversion unit and the microwave transmitting antenna, and the simple structure of the base station can greatly reduce the cost, and is beneficial to improving the frequency reuse degree and the cell density. The ROF technology is completely transparent to the frequency and modulation format. When the frequency and modulation format change, there is no need to change the base station. It only needs to upgrade the central station, which is very beneficial to the upgrading of the wireless communication network.
The British Telecom D.Wake team established an early 60 GHz ROF system in 1997 that can simultaneously carry analog satellite television signals and digital signals. The 60 GHz millimeter wave signal is generated based on the optical beat frequency of the master-slave structure laser frequency-locked frequency. Subsequently, in 2006, Sejong University of Korea built a 60 GHz ROF system with IF transmission and far-end mixing. The scheme uses IF signals in the optical fiber and mixes at the remote station to avoid the influence of fiber link dispersion. Recently, Professor GKChang of Georgia Institute of Technology in the United States has built a 2.5Gb/s 40GHz WDM-ROF system by combining with WDM-PON technology [15]. The biggest feature in this structure is not There is an intermediate frequency (IF) signal, so the baseband signal that can be transmitted is no longer affected by the intermediate frequency. In the design of the base station, the base station is not light-sourced, which simplifies the design of the base station. In the field of ROF system research, Japanese research institutes have strong capabilities, mainly in the development of new high-performance LiNbO3 modulators and UTC-PD and other optoelectronic devices. In 2007, NTT Corporation of Japan reported the implementation of error-free transmission of 10Gb/s digital baseband signals in a 125GHz ROF system. It can be seen that the ROF system design will develop towards full-duplex, wavelength division multiplexing, functional integration, low cost and high speed. Chinese researchers have made great progress in the past two years, completing the system experiment of 60GHz millimeter wave wired and wireless hybrid optical transmission [16]; the establishment of 32 GHz ROF HDTV service transmission platform; research on optical OFDM signal ROF system [17], etc. .
Military applications are a major area of ​​research in microwave photonics. It has obvious advantages in applications such as phased array radar and radar antenna fiber remote system [18]. For example, the light control microwave beamforming network uses the light control real-time delay device to power the multi-channel microwave signal with the feeder network distribution structure. Processing such as distribution, phase shifting, power synthesis, etc., to achieve control of the spatial distribution of microwave signals. The light-controlled broadband phased array radar has high scanning speed, high resolution, strong anti-interference ability, and can greatly reduce the volume and weight. It is very suitable for airborne and shipborne radar systems. The application of the modified technology in communication is a light-controlled smart antenna. A smart antenna is a multi-antenna technology that uses an antenna array to form a controllable beam that points and tracks the user at any time. It has the advantages of increasing communication capacity and speed, reducing electromagnetic interference, reducing the transmission power of mobile phones and base stations, and having a positioning function; it can reduce the influence of multipath fading, and obtain more users or higher data rates.
The research results of microwave photonics are also widely used in intelligent transportation, highway traffic communication systems [19] and ultra-high-speed train communication systems. The ROF-based traffic communication system can support fast handover management and dynamic bandwidth allocation, and has strong competitiveness in the fields of mobile communication and vehicle communication.
4 Conclusion In the past 30 years, microwave photonics has developed in theory, devices, key technologies and system applications, and some applications have been practical [20]. Microwave technology and optoelectronic technology are two of the advances in information technology. Major subject. Since the development of microwave technology, it has achieved remarkable achievements in many fields such as communication and national defense. Optoelectronic technology, especially optical communication, has a dynamic new technology growth point in the past 30 years, and has increased the speed and capacity of communication systems to an unprecedented degree. The mutual integration of the two will have a profound impact on modern information technology.
As an emerging interdisciplinary subject, microwave photonics has broad application prospects. In addition to applications in cable TV, ROF communications and radar, possible future applications of microwave photonics include broadcasting, wireless multimedia services, high-definition video streaming, Gigabit wireless LAN, personal area networks, optical detection and measurement, and radio astronomy. And can expect further research and development in the fields of terahertz technology, high sensitivity sensing and quantum key distribution.
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Abstract microwave includes: photonic microwave generation, photonic signal processing andconversion, distribution of microwave signals in optical links, andso on. The research results promote new technologies such AsRadio over Fiber (RoF) communications, the subcarrier multiplex and fiber transmission of Cable T el evision (CATV), optical control beamforming network in phased array radar, test technologies inmicrowave frequency, and so on.
Fund Project:Project supported by the National Natural Science Foundation of China (60736002, 60807026) 1 The emergence of background optical wavelength division multiplexing technology generated by microwave photonics and the invention of erbium-doped fiber amplifiers have led to the rapid development of optical communication. Optical fiber communication has many advantages such as low loss, anti-electromagnetic interference, ultra-wideband, easy to be multiplexed in wavelength, space and polarization. At present, single-channel 40~160Gb/s and single fiber 10 Tb/s transmission have been realized.
As capacity transmission rates continue to increase, fiber optic systems require the use of microwave technology in optical transmissions and receivers.
At the same time, microwave technology is rapidly evolving as demand for wireless communication capacity increases. Microwave communication can be transmitted in any direction, easy to construct and reconstruct, and realize interconnection with mobile devices; the emergence of cellular systems enables microwave communication to have high spectrum utilization. However, the limited bandwidth of the microwave band has become a serious problem, and people are beginning to consider the use of the new 30-70 GHz band. The 60 GHz optical-to-air (ROF) system is becoming a popular technology for broadband access due to its high access rate and the need to apply for additional licenses. The transmission loss of the 60 GHz signal in the atmosphere is as high as 14 dB/km, which means that the channel frequency can be reused more frequently in cellular mobile communication. However, the traditional microwave transmission medium has a large loss in long-distance transmission, and the fiber system has low loss and high bandwidth characteristics, which is attractive for microwave transmission and processing.
The integration of fiber technology and microwave technology has become an important new direction. Theoretically speaking, the theoretical basis of microwave technology and fiber technology is the theory of electromagnetic wave fluctuation. In optoelectronic devices, when the wavelength is small enough to consider the wave effect, electromagnetic wave theory is used to design and study optoelectronic devices, such as waveguide or traveling wave devices. The unification of the theoretical basis allows microwave devices and optoelectronic devices to be integrated on the same chip using the same materials and technologies, which greatly promotes the combination of the two disciplines and promotes the birth of a new interdisciplinary, microwave photonics.
The concept of microwave photonics was first proposed in 1993 [1]. Its research content covers various fields related to microwave technology and fiber technology [2]. It mainly focuses on two aspects: one is to solve the traditional optical fiber communication technology to the microwave frequency band. Developmental issues, including lasers, optical modulators, amplifiers, detectors, and fiber-optic transmission links; second, the use of optoelectronic devices to solve microwave signal generation and control problems, mainly photo-generated microwave sources, microwave photon filters, light Domain microwave amplifier, synthesis and control of photo-induced microwave electrical signals.
2 Key Technologies in Microwave Photonics 2.1 Using Optical Methods to Generate Microwave Signals The development of microwave communication to high frequencies of 30 to 70 GHz is a great challenge for traditional microwave devices. At this time, the use of optical technology to generate microwave signals has shown great appeal. There are many ways to generate microwaves using optical techniques, and the simplest principle is the optical heterodyne method. What is the frequency, phase and power of the two light waves? Brown 1,? Brown 2, ? Quasi 2 and P1, P2. When two beams of similar frequency and the same wave of polarization are simultaneously incident on the high-frequency photodetector for beat frequency, the available output current is:
Where R is the photoelectric conversion efficiency of the detector. It is not difficult to see that the microwave signal with the frequency of |? Brown 1-? Brown 2| can be generated by the beat frequency, and the frequency and phase of the generated signal are determined not only by the frequency difference of the two beams but also by the phase difference. In order to ensure the low phase noise and stability of the microwave signal, the two beams are required to have high coherence. To this end, many new methods for eliminating phase noise generated by lasers have been reported in recent years. There are mainly light injection locking method [3], optical phase-locked loop method [4]. However, the light injection phase-locking method has a small locking range, typically a few hundred megahertz. The optical phase-locked loop method requires that the laser should keep up with the phase change of the master laser, which requires a small loop delay. Both methods require an externally stable microwave signal source, which increases the cost and is not conducive to practical use. Commercialized.
With integrated technology, two lasers can be combined. Thus, the two beams are generated in the same gain medium, and the coherence is good, so that the locking technique can be avoided. In 1995, David Wake of the British Telecommunications Research Institute used two longitudinal modes in a multi-longitudinal mode DFB laser to capture the frequency and obtained the output of the 42 GHz signal.
Recently, technologies utilizing dual-wavelength fiber lasers are being developed. Fiber lasers are lightweight and low cost. In the general fiber laser, the gain medium is mostly made of erbium-doped fiber and has uniform widening characteristics. Various methods have been used to suppress the mode competition caused by uniform widening to realize a dual-wavelength fiber laser and to generate microwave signals ranging from 3 to 60 GHz. Such as the use of low temperature suppression uniform widening [5], distributed dispersion cavity, polarization hole burning, space burning hole, partially separated structure dual-wavelength DFB fiber laser [6] and so on.
Another photo-generated microwave method utilizes an optical modulation technique [7], as shown in Figure 1. The external modulator is an intensity or phase modulator. In the case of linear modulation, a difference frequency signal of twice the modulation frequency can be generated. If deep modulation technology is used, a microwave signal with a modulation frequency of 4 times can be generated. The advantage of using the out-of-optical modulation method is that the frequency can be tuned by changing the frequency of the microwave modulated signal. Compared to the former method, the stability and phase noise of the microwave signal generated by this method depends on the microwave modulation signal and the modulator, and the device requirements are relatively low. In 2005, the research team of Yao Jianping of Canada proposed to use a large microwave input power to drive a lithium niobate modulator and then use a fiber grating filter to filter out the optical carrier component to obtain two optical sidebands. After the beat frequency, 32~50GHz broadband is obtained. Adjusted millimeter wave signal. In recent years, China has reported a lot in this respect. Combined with the frequency doubling effect of nonlinear photonic devices, it can generate microwave signals with frequencies ranging from 6 to 60 GHz [8].
It should be noted that due to the advancement of high-frequency electronic devices, commercial microwave source modules of 60 GHz or less are currently on the market, and the method of photo-generated microwave should be developed to a higher frequency to reflect its own advantages. The highest frequency report is produced. The beat frequency output [9] of 1 000 GHz and 25 GHz has entered the field of terahertz technology. In addition, it is still attractive to generate and transmit ultra-wideband pulse signals in the optical domain using the gain saturation recovery characteristics of semiconductor optical amplifiers, optical polarization modulation, dispersion effects, and the like. It can provide UWB pulse light source with good compatibility with fiber optic system for optical ultra-wideband (UWBOF) communication [10].
2.2 Optical Modulators The use of optical fibers to transmit microwave subcarrier signals presents new requirements for adaptive modulation of optical modulators. The direct modulation technique is simple in that it directly loads the microwave subcarrier signal onto the light wave by changing the injection current of the semiconductor laser. The direct modulation bandwidth is limited by the resonant frequency of the laser. The use of a quantum structure can reduce the threshold current of the semiconductor laser, increase the differential gain, and increase the bandwidth. In order to further increase the bandwidth, it is necessary to reduce the photon lifetime and the gain compression coefficient. However, due to the limitation of the gain compression factor, the direct modulation bandwidth is hard to exceed 30 GHz at room temperature.
External modulation techniques are required to modulate microwave signals around 60 GHz or higher into optical carriers. The traveling wave structure of LiNbO3 modulator can realize the bandwidth of 70 GHz [11]. Electro-absorption modulator can also be used. Due to its low volume driving voltage, it is easy to integrate with laser and photodetector. A light modulating device for the foreground.
There are some flexible methods in the modulation technology, such as frequency up-conversion and optical heterodyne. The frequency up-conversion method modulates the lower frequency microwave signal to the optical transmission, and converts the frequency at the base station to obtain the high-frequency microwave signal, which reduces the requirement of the optical modulator, but increases the complexity of the base station; The difference method transmits two optical signals with a certain frequency difference, and the baseband signal is modulated on the optical. The two optical waves are beaten at the base station to obtain a microwave signal, but this method will be affected by the dispersion of the optical fiber.
2.3 Photodetectors Photodetectors that are practical in microwave photonics must have different performance than conventional optical communication systems: one is high speed; the other is high power output, ie high saturation operating point; the third is directly on the device Converted to microwave power and transmitted from the microwave antenna. Devices that currently meet the above requirements are referred to as single transit carrier photodiodes (UTC-PD). In this device, only electrons are utilized to activate carriers, and holes are confined to a certain area. Utilizing the high mobility of electrons, the response rate of the device is greatly improved. The waveguide structure is adopted to increase the length of the light absorption; the optimal transmission line impedance is designed to obtain a high response rate and a high saturation power. It has been reported that 1.5 RHz 1.5-Hz signals have been detected and there have been reports of UTC-PDs with transmit antennas or modulators as monolithic integrated devices.
2.4 Microwave Photonic Filter Microwave photonic filter is an important part of photon signal processing technology. The processing of signals in the electrical domain is limited by the frequency band and sampling frequency, and the processing speed and accuracy are affected. It is called the electronic “bottleneckâ€. The microwave photonic filter provides a new method to solve the traditional “bottleneck†problem. The input radio frequency (RF) signal is modulated onto the optical signal by the modulator, the processing of the RF signal is performed in the optical domain, and finally the filtered microwave signal is output through the optical receiver. The advantages of this method are: low loss, high bandwidth, no electromagnetic interference, light weight and support for high sampling frequency. The use of wavelength division multiplexing technology also provides the feasibility of parallel processing of space and wavelength.
Microwave photonic filters were originally applied to radar systems and aerospace applications that require high-speed signal processing. With the deepening of ROF system research, microwave photonic filters have been applied in communication systems, especially in millimeter-wave ROF systems. At present, international research focuses on designing new filter structures to achieve higher frequency response, negative tap coefficients, adjustability, reconfigurability, and larger dynamic range. There are two traditional methods: the first method is to use an electrical differential structure. This structure was implemented as early as 1995, but this method has poor adjustability and reconfigurability, and the bandwidth of the power receiving device is limited. The second method is to implement a full-coefficient filter using complex optoelectronic devices, but this method is costly. Recently, many new low cost structures have been reported to implement microwave photonic filters with negative coefficients. Among them, the method of using polarization state and external modulator is the most attractive [12]. On the other hand, in the ROF system, the combination of microwave photon filtering function and other signal processing functions will greatly reduce system cost and enhance functional centralization.
2.5 Analog-to-Digital Converters In some analog systems, such as radar and broadband communication systems, digital signal processing methods are used for better performance and faster reconfiguration. The weak point of the analog-to-digital converter in the electric domain gradually increases with the increase of frequency, because the CMOS digital converter is limited by factors such as sampling clock jitter, sample-and-hold circuit stabilization time, and comparator processing speed. A 100 GHz sampled analog to digital converter available in digital signal processing is difficult to implement. The method proposed by microwave photonics is called optical time stretching, sampling frequency up to 480 GHz, and 96 GHz bandwidth [13]. The basic principle of optical time stretching is to use photon processing to slow down the electrical signal speed to improve the electrical domain. The analog to digital converter. The light processing process has three steps: wavelength-time conversion, wavelength domain processing, and wavelength-time mapping. The converted slow electrical signal can be transformed by a constant-amplitude converter (A/D).
2.6 Optical Domain Microwave Amplifiers Amplify the microwave signal in the optical domain using the gain of a common erbium-doped fiber amplifier and the interaction of light and microwave, as shown in Figure 2. The direct current light output by the external cavity laser is modulated by the microwave signal input in the intensity modulator. The DC bias point of the modulator is stabilized near the half-wave voltage, and the output optical signal is amplified by the erbium-doped fiber amplifier and filtered by the optical band-pass filter to filter out the spontaneous radiation noise. Finally, the input optical receiver recovers the amplified microwave signal. The experimental results show that with the microwave frequency constant at 4 GHz, the microwave gain is always stable at around 17 dB as the input microwave signal increases, showing good stability, and the signal-to-noise ratio of the output microwave signal will follow. Improve.
2.7 Overcoming the influence of microwave subcarriers on fiber transmission links The transmission characteristics of microwaves in optical fibers are important research contents of microwave photonics. As early as when hybrid audio-fiber systems are used to transmit analog cable television (CATV) signals, The link transmission characteristics are the focus of attention, and the corresponding theoretical model has been used to analyze the transmission characteristics of the ROF link. In a higher-speed ROF link than CATV, fiber dispersion becomes the main factor limiting the transmission distance, and PMD and various nonlinear effects are also more obvious. For chromatic dispersion, it is generally considered to be solved by performing single sideband modulation techniques in the optical domain. The most straightforward method is to use fiber grating filtering to obtain a single sideband signal, but the filter itself introduces dispersion into the system. Studies have shown that the nonlinearity of the external modulator severely limits the dynamic range of the entire microwave link, and the nonlinear effects such as cross-phase modulation caused by a large transmission power will further aggravate the deterioration of system performance [14]. Signals of different digital modulation formats have very different requirements for the millimeter-wave optical fiber transmission link. Therefore, various modulation formats such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM) are transmitted in the microwave optical fiber transmission system. And the link characteristics when using Orthogonal Frequency Division Multiplexing (OFDM) technology for baseband digital signals and intermediate frequency signals are hot topics in recent research.
3 System Application Microwave Photonics The earliest system application was the “deep space network†in the Mojave Desert in the north of Los Angeles in the late 1970s. The Deep Space Network is a large-scale dish distributed over a few tens of kilometers. A cluster of antennas with a maximum antenna diameter of 70 m. A fiber optic transmission system is established between these antennas to deliver an ultra-stable microwave reference signal of 1.420405752 GHz. All antenna elements are synchronized by this frequency, using the concept of phased arrays to make them work like a huge antenna, enabling communication and tracking with spacecraft in outer space. Later in the 1990s, hybrid coaxial cable-fiber CATV systems with microwave photonics technology also achieved commercial success.
In recent years, an important application target of microwave photonics is the transmission of microwave carrier signals for wireless communication using optical fibers. That is to study the intra-fiber RF transmission system, such as the optical-borne wireless (ROF) communication system. ROF combines the advantages of microwave and fiber optic communication, enabling microwaves to achieve low loss transmission in fiber. The ROF can be used to implement signal transmission and distribution between the central office and each microcellular antenna. The utility model has the advantages that the complex microwave processing unit can be placed in the central office, and the base station portion only has two parts of the photoelectric conversion unit and the microwave transmitting antenna, and the simple structure of the base station can greatly reduce the cost, and is beneficial to improving the frequency reuse degree and the cell density. The ROF technology is completely transparent to the frequency and modulation format. When the frequency and modulation format change, there is no need to change the base station. It only needs to upgrade the central station, which is very beneficial to the upgrading of the wireless communication network.
The British Telecom D.Wake team established an early 60 GHz ROF system in 1997 that can simultaneously carry analog satellite television signals and digital signals. The 60 GHz millimeter wave signal is generated based on the optical beat frequency of the master-slave structure laser frequency-locked frequency. Subsequently, in 2006, Sejong University of Korea built a 60 GHz ROF system with IF transmission and far-end mixing. The scheme uses IF signals in the optical fiber and mixes at the remote station to avoid the influence of fiber link dispersion. Recently, Professor GKChang of Georgia Institute of Technology in the United States has built a 2.5Gb/s 40GHz WDM-ROF system by combining with WDM-PON technology [15]. The biggest feature in this structure is not There is an intermediate frequency (IF) signal, so the baseband signal that can be transmitted is no longer affected by the intermediate frequency. In the design of the base station, the base station is not light-sourced, which simplifies the design of the base station. In the field of ROF system research, Japanese research institutes have strong capabilities, mainly in the development of new high-performance LiNbO3 modulators and UTC-PD and other optoelectronic devices. In 2007, NTT Corporation of Japan reported the implementation of error-free transmission of 10Gb/s digital baseband signals in a 125GHz ROF system. It can be seen that the ROF system design will develop towards full-duplex, wavelength division multiplexing, functional integration, low cost and high speed. Chinese researchers have made great progress in the past two years, completing the system experiment of 60GHz millimeter wave wired and wireless hybrid optical transmission [16]; the establishment of 32 GHz ROF HDTV service transmission platform; research on optical OFDM signal ROF system [17], etc. .
Military applications are a major area of ​​research in microwave photonics. It has obvious advantages in applications such as phased array radar and radar antenna fiber remote system [18]. For example, the light control microwave beamforming network uses the light control real-time delay device to power the multi-channel microwave signal with the feeder network distribution structure. Processing such as distribution, phase shifting, power synthesis, etc., to achieve control of the spatial distribution of microwave signals. The light-controlled broadband phased array radar has high scanning speed, high resolution, strong anti-interference ability, and can greatly reduce the volume and weight. It is very suitable for airborne and shipborne radar systems. The application of the modified technology in communication is a light-controlled smart antenna. A smart antenna is a multi-antenna technology that uses an antenna array to form a controllable beam that points and tracks the user at any time. It has the advantages of increasing communication capacity and speed, reducing electromagnetic interference, reducing the transmission power of mobile phones and base stations, and having a positioning function; it can reduce the influence of multipath fading, and obtain more users or higher data rates.
The research results of microwave photonics are also widely used in intelligent transportation, highway traffic communication systems [19] and ultra-high-speed train communication systems. The ROF-based traffic communication system can support fast handover management and dynamic bandwidth allocation, and has strong competitiveness in the fields of mobile communication and vehicle communication.
4 Conclusion In the past 30 years, microwave photonics has developed in theory, devices, key technologies and system applications, and some applications have been practical [20]. Microwave technology and optoelectronic technology are two of the advances in information technology. Major subject. Since the development of microwave technology, it has achieved remarkable achievements in many fields such as communication and national defense. Optoelectronic technology, especially optical communication, has a dynamic new technology growth point in the past 30 years, and has increased the speed and capacity of communication systems to an unprecedented degree. The mutual integration of the two will have a profound impact on modern information technology.
As an emerging interdisciplinary subject, microwave photonics has broad application prospects. In addition to applications in cable TV, ROF communications and radar, possible future applications of microwave photonics include broadcasting, wireless multimedia services, high-definition video streaming, Gigabit wireless LAN, personal area networks, optical detection and measurement, and radio astronomy. And can expect further research and development in the fields of terahertz technology, high sensitivity sensing and quantum key distribution.
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