All-optical instantaneous RF signal frequency measurement system based on a linear ACF with a steep slope.

A new microwave photonic structure for measuring the frequency of an RF signal, to the best of our knowledge, is presented. The frequency of an unknown RF signal can be determined by simply measuring the system output optical powers. The proposed frequency measurement system can be designed so that the ratio of the two system output optical powers as a function of the RF signal frequency or the amplitude comparison function (ACF) has a steep linear slope over a wide frequency range. This enables the RF signal frequency to be measured in high resolution and high accuracy. The proposed frequency measurement system has a simple and compact structure, and is free of high-speed photodetectors as well as RF components and instruments. It also has a fast response time compared to many reported photonics-based frequency measurement systems. A proof-of-concept experiment is carried out. Experimental results show a linear ACF with a slope of more than 4.4 dB/GHz over a frequency measurement range of 5-26 GHz and a frequency measurement accuracy of better than ±0.1G H z.

All-optical high-order subharmonic mixer with a high signal-to-noise ratio and stable performance.

A photonics-based high-order subharmonic mixer, which enables a low-frequency LO source to be used for high-frequency RF signal frequency downconversion, is presented. It is based on an optically injected semiconductor laser, which is oscillated in the period-one state, sandwiched between two optical phase modulators. It has the advantages of a simple and compact structure, wide bandwidth, absence of electrical components, reconfigurable subharmonic mixing operation, stable output IF signal performance, high signal-to-noise ratio, infinite LO-to-RF port isolation, and high LO-to-IF port isolation. Furthermore, it is suitable for use in remote antenna applications. We set up a proof-of-concept experiment that demonstrates a reconfigurable second-, fourth-, sixth-, and eighth-order subharmonic mixing operation for different input RF signal frequencies and powers. The experimental results also demonstrate that the proposed structure exhibits a stable output IF signal performance, which overcomes the IF signal phase stability problem in the reported high-order subharmonic mixers.

Wavelength switching technique for phase interrogation of Mach Zehnder interferometer-based optical sensors.

A method for determining the phase shift of a Mach Zehnder interferometer (MZI) is presented. It is based on switching the wavelength of continuous wave (CW) laser light illuminating the MZI and measuring the interferometer output amplitudes at DC and switching frequency. The method can measure the MZI phase shift unambiguously over the entire phase shift range of 2π. A practical proof of concept demonstration shows that the method can perform real-time measurement with high repeatability and accuracy limited by the optical frequency drift and power fluctuation of the lasers. The method does not require modifications of the sensor or accessing to the laser electronics and also uses simple detection. It is, therefore, suitable for bio and medical sensing applications.

Photonic microwave frequency divider with a tunable division ratio and harmonic suppression capability.

A photonic microwave frequency divider that is capable to realise tunable high order frequency division, is presented. It is based on injecting an RF phase modulated optical signal into an off-the-shelf DFB laser operating at period-N state. Optical frequency components with a frequency separation of 1/N times the input RF signal frequency are generated by the DFB laser. An optical bandpass filter can be employed to select two optical frequency components to be detected by a photodetector to obtain a divide-by-N RF signal without harmonic components. The proposed frequency divider can be operated over a wide frequency range and has high reconfigurability as it is free of electrical components. Experimental results demonstrate the realisation of frequency division operation with a tunable 1/2 to 1/5 division ratio for different input RF signal frequencies of 8 to 20 GHz by adjusting the DFB laser forward bias current. Over 35 dB harmonic component suppression is demonstrated. A proof-of-concept experiment is also set up to show the frequency divider based on an optically injected semiconductor laser is capable to operate at a high input RF signal frequency of 50 GHz and has a tunable high order division ratio of 1/2 to 1/8.

Parity-time-symmetric optoelectronic oscillator based on higher-order optical modulation.

An optoelectronic oscillator (OEO) for single-frequency microwave generation, enabled by broken parity time (PT) symmetry based on higher-order modulation using a Mach-Zehnder modulator, is proposed and demonstrated. Instead of using two physically separated mutually coupled loops with balanced gain and loss, the PT symmetry is realized using a single physical loop to implement two equivalent loops with the gain loop formed by the beating between the optical carrier and the ±1st-order sidebands and the loss loop formed by the beating between the ±1st-order sidebands and the ±2nd-order sidebands at a photodetector. The gain and loss coefficients are made identical in magnitude by controlling the incident light power to the modulator and the modulator bias voltage. Once the gain/loss coefficient is greater than the coupling coefficient, the PT symmetry is broken, and a single-frequency oscillation without using an ultra-narrow passband filter is achieved. The approach is evaluated experimentally. For an OEO with a loop length of 10.1 km, a single-frequency microwave signal at 9.997 GHz with a 55-dB sidemode suppression ratio and -142-dBc/Hz phase noise at a 10-kHz offset frequency is generated. No mode hopping is observed during a 5-hour measurement period.

Microwave frequency measurement based on a frequency-to-phase mapping technique.

A new frequency-to-phase mapping technique for measuring a radio-frequency (RF) signal frequency is presented. The concept is based on generating two low-frequency signals where their phase difference is dependent on the input RF signal frequency. Hence, the input RF signal frequency can be determined by using a low-cost low-frequency electronic phase detector to measure the phase difference between the two low-frequency signals. The technique can measure the frequency of an RF signal instantaneously and has a wide frequency measurement range. The proposed frequency-to-phase-mapping-based instantaneous frequency measurement system is experimentally verified over the 5 to 20 GHz frequency measurement range with errors of less than ±0.2 GHz.

Broadband microwave photonic frequency translator with a wide frequency shifting range.

A microwave photonic topology for shifting the frequency of an input microwave signal is presented. It operates based on a single sideband frequency mixing approach. The amount of microwave signal frequency shift is determined by a local oscillator frequency. The proposed frequency translator (FT) has a large bandwidth and a wide frequency shifting range. It can be designed to obtain a large spurious signal suppression ratio and a low frequency translation loss. Results are presented for the novel structure, which demonstrates the realization of a 2-18 GHz FT with a 10 kHz to 100 MHz frequency shifting range. The results also show the spurious signals are more than 31 dB below the frequencyshifted signal, and a low loss of only around 4 dB for frequency shifting a 10 GHz microwave signal.

Multichannel microwave photonic based direction finding system.

A new microwave photonic topology for RF signal direction finding is presented. It is based on a dual-parallel Mach Zehnder modulator (DPMZM) in series with an optical phase modulator (PM). The direction of an RF signal received by the antennas connected to an RF port of the DPMZM and the PM can be determined from the power ratio of two system output low frequency components, without the need to know the incoming RF signal amplitude in advance. The proposed structure is suitable for implementing a long baseline technique for direction finding and can be extended to have multiple antenna elements in remote locations. In addition to direction finding, the system also has the ability to measure an RF signal Doppler frequency shift to determine an object speed and moving direction when it is used in a radar receiver. Results obtained using the proposed structure demonstrate less than ±2.5° errors over a 3.2° to 81.5° angle of arrival measurement range for different RF signal modulation indexes of 0.02, 0.08 and 0.16. Doppler frequency shift measurement with less than 0.8 Hz errors is also demonstrated.

Simple photonics-based system for Doppler frequency shift and angle of arrival measurement.

A novel photonic approach for simultaneously measuring both the Doppler frequency shift (DFS) and the angle of arrival (AOA) of a microwave signal in a radar system is presented. It has the same structure as a fiber optic link consisting of a laser, an optical modulator and a photodetector. The incoming microwave signal and a reference signal are applied to the optical modulator. Beating of the echo and reference signal sidebands at the photodetector generates a low-frequency electrical signal. The DFS and the AOA can be determined from the frequency and the power of the low-frequency electrical signal measured on an electrical spectrum analyzer. The system has a very simple structure and is low-cost. It has a wide operating frequency range and a robust performance. Experimental results demonstrate a DFS measurement at around 15 GHz with errors of less than ±0.2 Hz, and a 0° to 90° AOA measurement with less than ±1° errors.

Linearized fiber-optic link with a high spurious free dynamic range over a wide frequency range.

An all-optical linearized fiber-optic link is presented. It solves the problem in most reported structures where a high spurious free dynamic range (SFDR) can only be obtained in a limited frequency range. The link only involves a laser, an optical modulator, and a photodetector. The novel design in the modulator bias setting enables the third-order intermodulation distortion to be suppressed by controlling only one bias voltage of a dual-parallel Mach-Zehnder modulator (DPMZM) while the other two bias voltages are set to bias the sub-MZM and the main MZM of the DPMZM at the standard peak and null point, respectively. The link has a very simple structure and does not require any electrical component, and hence a high SFDR can be obtained over a wide frequency range. Techniques are proposed using off-the-shelf components for stabilizing the modulator bias setting to maintain high SFDR performance when the link is operated in practice. Measured results demonstrate a high SFDR of 120.5 dB⋅Hz4/5±1.6 dB over a 2-20 GHz frequency range in an unamplified linearized fiber-optic link. To our knowledge, this is the first report of a linearized fiber-optic link with around 120 dB⋅Hz4/5 over such a wide frequency range.

Cascaded modulator topology for frequency conversion in antenna remoting applications.

A new cascaded modulator structure that has the ability to realize high conversion efficiency microwave frequency downconversion, while at the same time able to overcome two fundamental limitations in the dual-parallel modulator approach, is presented. It is based on utilizing the polarization-dependent modulation efficiency property in LiNbO3 electro-optic modulators. The new structure allows the modulators for the RF signal and local oscillator (LO) modulation to be placed in different locations suitable for antenna remoting applications, and it has infinite isolation between the LO and RF signal ports. We present experimental results demonstrating that the proposed structure can be used to realize high conversion efficiency frequency downconversion over wide RF and intermediate frequency (IF) signal frequency ranges as the reported dual-parallel-modulator-based microwave photonic frequency downconverter. Very high isolation of more than 70 dB between the LO and RF signal ports is also demonstrated.

Broadband high dynamic range fiber optic link based on a dual-polarization modulator.

This paper presents a simple, linearized fiber-optic link that is capable of realizing a high spurious free dynamic range (SFDR) at different input RF signal frequencies without the need of readjusting system parameters. The link is based on a commercial dual-polarization modulator followed by a linear polarizer. The third-order nonlinearity at the third-order intermodulation distortion frequency can be cancelled by designing the angle of the linear polarizer. No electrical component is involved in the linearization process. The high SFDR performance is theoretically analyzed, simulated using photonic simulation software, and experimentally verified. Experimental verification of the dual-polarization modulator-based linearized fiber-optic link shows that a high SFDR of more than 124 dB⋅Hz4/5 is obtained at different input RF signal frequencies over a 2-18 GHz bandwidth. An SFDR of 127.3 dB⋅Hz4/5 is also demonstrated with the use of an optical amplifier to increase the link output average optical power, which is among the highest reported SFDRs measured in a fiber-optic link.

Ultra-wide bandwidth photonic microwave phase shifter with amplitude control function.

This paper presents a new technique for realizing continuous 0°-360° RF signal phase shift over a very wide bandwidth. It is based on using single-sideband modulation together with optical filtering to largely suppress one of the RF modulation sidebands over a wide input RF frequency range, and controlling the phase of the optical carrier to shift an RF signal phase. The technique does not require expensive electrical or optical components to realize an RF signal phase shift over 2-40 GHz frequency range with a flat amplitude and phase response performance. This overcomes the current technology limitation in which no reported phase shifter structure has demonstrated the capability of operating in such a wide bandwidth. Experimental results demonstrate only ± 1 dB amplitude variation and ± 5° phase deviation from the desired RF signal phase shift over 2-40 GHz bandwidth and the RF signal amplitude control function. The phase shifter wavelength insensitive performance is also demonstrated experimentally.

Photonic microwave quadrature filter with low phase imbalance and high signal-to-noise ratio performance.

A photonic microwave quadrature filter is presented. It has a very simple structure, very low phase imbalance, and high signal-to-noise ratio performance. Experimental results are presented that demonstrate a photonic microwave quadrature filter with a 3 dB operating frequency range of 10.5-26.5 GHz, an amplitude and phase imbalance of less than ±0.3 dB and ±0.15°, and a signal-to-noise ratio of more than 121 dB in a 1 Hz noise bandwidth.

Photonic microwave phase shifter based on dual-sideband phase-control technique.

An all-optical photonic microwave phase shifter that can realize a continuous 0°-360° phase shift is presented. The phase-shifting operation is implemented by controlling the phase of the two RF phase-modulation sidebands while keeping the optical carrier phase fixed. The use of two RF modulation sidebands, instead of a single sideband used in most conventional phase shifters, has the advantage of high-output RF signal power, and consequently high signal-to-noise ratio performance. Experimental results demonstrate the full -180° to +180° phase shift over a wide microwave frequency range from 11 to 26.5 GHz, and 14 dB increase in the output RF signal power compared to a conventional phase shifter.

Instantaneous high-resolution multiple-frequency measurement system based on frequency-to-time mapping technique.

A new microwave photonic instantaneous frequency measurement system that can simultaneously measure multiple-frequency signals while achieving very high resolution and wide frequency measurement range is presented. It is based on the frequency-to-time mapping technique implemented using a frequency shifting recirculating delay line loop and a narrowband optical filter realized by the in-fiber stimulated Brillouin scattering effect. Experimental results demonstrate the realization of a multiple-frequency measurement capability over a frequency range of 0.1-20 GHz that can be extended to 90 GHz, and with a measurement resolution of 250 MHz.

Microwave photonic mixer with high spurious-free dynamic range.

A new linearized photonic mixer structure, which can fully eliminate the third-order intermodulation distortion, is presented. It is based on an integrated dual-parallel Mach-Zehnder modulator to which an optimized RF split and an optimized optical phase shift are applied, in series with a Mach-Zehnder modulator driven by the LO. The mixer achieves a very high spurious-free dynamic range performance, it enables essentially infinite isolation between the RF and LO ports, and it has the ability to function over a multioctave frequency range. Experimental results demonstrate a record measured spurious free dynamic range performance of 127 dB·Hz(4/5), which is over 22 dB higher than that of the conventional dual-series Mach-Zehnder modulator-based microwave photonic mixer.

Microwave photonic mixer based on a single bidirectional Mach-Zehnder modulator.

A microwave photonic mixer based on a single electro-optic Mach-Zehnder intensity modulator operating in both directions is presented. In this mixer structure, the light from the optical source travels in opposite directions inside the modulator and is modulated by both the RF signal and the local oscillator (LO). The output optical spectrum comprises the RF signal and LO sidebands without the optical carrier. This enables a high conversion efficiency mixing operation to be obtained. The mixer has a simple structure, and its performance is insensitive to the modulator bias voltage; hence no DC bias voltage and no modulator bias controller are required to obtain robust high conversion efficiency mixing operation. Experimental results are presented showing large conversion efficiency improvement of 25.7 dB compared to the conventional dual Mach-Zehnder modulator-based microwave photonic mixer and a modulator bias insensitive mixing performance.

Optical-to-RF phase shift conversion-based microwave photonic phase shifter using a fiber Bragg grating.

A novel microwave photonic phase shifter structure is presented. It is based on the conversion of the optical carrier phase shift into an RF signal phase shift via controlling the carrier wavelength of a single-sideband RF-modulated optical signal into a fiber Bragg grating. The new microwave photonic phase shifter has a simple structure and only requires a single control to shift the RF signal phase. It also has the ability to realize multiple phase shifts. Experimental results demonstrate a continuous 0°-360° phase shift with low amplitude variation of <2 dB and low phase deviation of <5° over a wideband microwave range.

High conversion efficiency microwave photonic mixer based on stimulated Brillouin scattering carrier suppression technique.

A new microwave photonic mixer that can achieve a high conversion efficiency is presented. It is based on using the stimulated Brillouin scattering loss spectrum to suppress the optical carrier at the output of two optical phase modulators driven by the RF signal and the LO, respectively. Experimental results are presented, which demonstrate a high conversion efficiency of 11.3 dB corresponding to over 26 dB improvement compared to the conventional dual-series Mach-Zehnder modulator based microwave photonic mixer, and wide bandwidth of 0.2 to 20 GHz mixing operation.