Search

Flexibility and tunability are critical for frequency conversion in antenna, radar and modern communication systems. In this study we propose and experimentally demonstrate a wideband tunable frequency converter by using an optoelectronic oscillator (OEO) to generate high-purity local oscillator (LO) signal for flexible frequency conversion. In the proposed frequency converter, a dual-polarization quadrature phase shift-keying (DP-QPSK) modulator is utilized to receive the signal to be converted and also to form a tunable OEO based on a dispersion-induced microwave photonic filter. By adjusting the bias voltage of the DP-QPSK modulator, an LO signal with accurate adjustable frequency is generated by the OEO. The measured phase noise of generated LO signal can reach -110 dBc/Hz at 10 kHz offset. An experiment is performed to verify the feasibility of this proposed frequency converter. The 7.6-GHz radio frequency (RF) signal is freely converted to different intermediate frequency (IF) signals, respectively. Moreover, the proposed microwave photonic converter is applied into wireless transmission systems. A 1-Gb/s NRZ wireless signal centered at 6.5 GHz is down-converted to baseband successfully.

We propose and demonstrate a simple method to accomplish arbitrary free spectral range (FSR) control of optical frequency combs (OFC). The approach is based on the combination of an optical tapped delay line (OTDL) structure and a phase modulator (PM). The OTDL structure is utilized to multiply the FSR of input OFC, and the PM is used to divide the FSR of the consequent OFC. We illustrate that the proposed method allows one to divide or multiply the FSR of original OFC by any anticipated integer or fractional factor. Numerical simulation results match well with our theoretical analysis.

A CS2-filled Er-doped fiber, which can provide high nonlinearity and gain, low and flat all-normal dispersion profile, low loss and single-mode operation in the wavelength range of 1500-2500 nm, is proposed and analyzed by numerical simulation. When the 1900-nm 0.1-ps initial pulse with a peak power of 30 kW is injected into the active fiber, the generated supercontinuum (SC) spectrum has a double-hump structure in the 1550-nm window and covers about 800-nm wavelength range spanning from 1400 to 2200 nm at -20 dB level. Furthermore, the 1550-nm pulse can be obtained synchronously with the 1900-nm initial pulse when the amplified pulse with a temporal double-peak shape is passed through a band-pass filter with its center wavelength at 1550 nm.

Optoelectronic oscillator (OEO) has received growing attentions from researchers in the field of demodulations for optical sensors because of its excellent performance of combining optics and electronics in signal processing. In this article, we present a wideband-tunable OEO for angular velocity measurement. The OEO consists of two cascaded microwave photonic filters (MPFs), namely, a dispersion-induced MPF for mainly determining the OEO’s oscillating frequency and a two-tap MPF for the fine selection of the oscillating frequency. With proper control of polarization of the optical signal, the angular velocity is first mapped into the central frequency shift of the dispersion-induced MPF, which will then cause an oscillating frequency shift of the OEO. This approach ensures a large-scale factor for the angular velocity measurement due to the fact that a small phase change in the optical domain causes a large frequency shift in the electrical domain. An experiment is conducted to validate the scheme. The experimental results show that the oscillating frequency shift can reach nearly 700 MHz when the system rotates at about 1 rad/s by using a 560-m Sagnac loop. In addition, the measurement range can be achieved to 5.24 rad/s with a 2.5-GHz oscillating frequency tuning range and the maximum measurement error is 0.06 rad/s.

Mode selection is crucial for a stable single-mode oscillation of an optoelectronic oscillator (OEO). In this article a parity-time (PT)-symmetric frequency-tunable OEO is proposed and experimentally demonstrated, to realize the precise frequency tuning without using any narrowband filter. The proposed PT-symmetric OEO is implemented based on two identical optoelectronic hybrid loops mainly consisting of a shared dual-parallel Mach-Zehnder modulator (DPMZM) and a photodetector. The PT-symmetry condition can be achieved by enabling antisymmetric gain/loss in the two loops through manipulating the bias voltages of the two child-MZMs of DPMZM, and continuous frequency tunability is realized by adjusting the bias voltage of the parent-MZM. A theoretical model is developed for the proposed PT-symmetric OEO to analyze the dynamics of the PT-supermodes as the gain/loss coefficient changing. Furthermore, an experiment is performed. Without high-Q filter, stable single-mode oscillation of the PT-broken supermode at 4.2 GHz with side-mode suppression ratios of 60 and 50 dB is demonstrated in the OEO with 6 and 2000 m fiber loop lengths, respectively. The frequency tuning ranges are 1.4 MHz and 1.5 kHz for two OEOs with different fiber loop lengths.

An image compression-encryption scheme utilizing the combination of fiber specklegram-based compressive sensing (CS) and double random phase encoding (DRPE) was demonstrated. The original image is compressed and encrypted by CS and then re-encrypted by DRPE. The measurement matrices for CS are constructed from multimode fiber specklegrams resulting from modes excitation and interference, which show better performance compared with those based on Gaussian random variables. Since fiber specklegrams have strong dependence on configuration, such measurement matrix can be constructed either by one specklegram or by specklegrams from different launched wavelengths. The double-encrypted image demonstrated good safety and reliability in several common attacks and presented high key sensitivity, which indicates good performance of the proposed scheme.

A temperature-insensitive magnetic field sensor with high interrogation performance and large measurement range based on an optoelectronic oscillator (OEO) merging a Mach–Zehnder interferometer (MZI) is proposed and experimentally demonstrated. In the OEO loop, a finite impulse response (FIR)-microwave photonic filter (MPF) and an infinite impulse response-MPF are cascaded to select the oscillating frequency and further suppress the side mode. The center frequency of the FIR-MPF, which is composed of a non-coherent broadband optical source, a MZI, a dispersion compensating fiber (DCF), and a photodetector, is determined by dispersion of DCF and the free spectral range (FSR) of the MZI jointly. In one arm of the MZI, a section of optical fiber is bonded with a giant magnetostrictive material (GMM) to achieve the magnetic field sensing, where the GMM is acted as a magnetic field-strain transducer. When the magnetic field varies, the FSR of the MZI will be changed, resulting in an oscillating frequency shift of the OEO. Therefore, a high interrogation performance magnetic field measurement can be achieved by simply monitoring frequency change of microwave signal generated by OEO in the microwave domain. Moreover, by placing a section of unprocessed fiber with appropriate length in the other arm of the MZI, the temperature cross-talk for the magnetic field can be alleviated greatly. In the experiment, the magnetic field sensitivity of 1.33 MHz/mT with a large range of 20.9 mT to 58 mT is obtained and the measurement error induced by temperature is as low as 0.09 mT.

Mode excitations in D-shaped multimode fiber are analyzed in detail in terms of the effect of launch offset, polishing depth, and length of the D-shaped section. Based on the analysis, specklegrams from D-shaped multimode fiber structures are proposed to be applied in refractive index sensing. Glycerol solutions with different concentrations were used to study the relationship between the zero-mean normalized cross-correlation coefficient (ZNCC) of fiber output specklegrams and the surrounding refractive index. Responses of different D-shaped fiber structures are investigated and compared. For all of the structures, regular gradual changes of the ZNCC for each specklegram with the refractive index are found in a wide refractive index range of 1.352-1.444, which shows the feasibility of refractive index sensing. A high sensitivity being 10.47 RIU−1 is obtained by the structure with the polishing depth being $18~\mu \text{m}$ and length being 5cm. The proposed refractive sensing scheme is easy to realize with wide measurement range and high flexibility.

The active mode-locking (AML) technique has been widely used in erbium-doped fiber lasers to generate picosecond pulse trains. Here we propose a novel active mode-locking dual-loop optoelectronic oscillator (AML-DL-OEO), which can generate microwave frequency comb (MFC) signals with adjustable comb spacings. Based on this scheme, the order of harmonic mode-locking is dramatically decreased for a certain AML driving frequency compared with a single-loop AML-OEO. Thus, the supermode noise caused by harmonic mode-locking can be efficiently suppressed. In addition, the sidemodes are well suppressed by the dual-loop architecture. An experiment is performed. MFC signals with different comb spacings are generated under fundamental or harmonic mode-locking states. AML-DL-OEO systems with different length differences between two loops are implemented to evaluate supermode noise suppression capability. The performance of the generated MFC signals is recorded and analyzed.

An optoelectronic oscillator (OEO)-based fiber optic current sensor (FOCS) with greatly improved sensitivity is proposed and experimentally demonstrated. A microwave photonic filter (MPF) induced by the dispersion effect of a linearly chirped fiber Bragg grating (LCFBG) is used to select the frequency of the OEO oscillating signal. A two-tap MPF formed by a polarization multiplexed composite cavity is cascaded to achieve a stable single mode oscillation. When the current changes, the magneto-optic phase shift induced by Faraday effect will be introduced between the left and right circularly polarized lights transmitted in the reflective sensing unit. The magneto-optic phase shift is converted to the phase difference between the optical carrier and sidebands through a LiNbO3 Mach–Zehnder modulator. This phase difference is the decisive factor for the center frequency of the cascaded MPF as well as the oscillating frequency. Therefore, the current can be measured in the microwave frequency domain, which can improve the interrogation speed and accuracy to a large extent. The experimental results show that the oscillating frequency shifts up to 407.9 MHz as the current increases by 1 A.

High-accuracy and high-speed optoelectronic oscillator (OEO)-based magnetic field and electric current fiber measurement is presented in this paper. Thanks to the Giant-Magnetoresistance Effect and Faraday Effect, the magnetic field and electric current can be transformed to the changes in characteristics of light propagating in optical fiber. Then, the use of OEO can translate the magnetic field and electric current dependent changes in optical domain to the oscillating frequency changes in microwave domain in order to increase the interrogation speed and resolution. The theoretical analysis and principle are investigated systematically. Experimental results are then provided to validate the OEO-enabled magnetic field and electric current measurement techniques.

This paper introduces a planarisation SiO2 layer into the configuration of ZnO‐silicon‐on‐insulator for constructing an acoustical resonant cavity to enhance the acoustic density on a silicon‐on‐insulator waveguide. The improved configuration with a planarisation SiO2 layer can increase the acousto‐optical interaction by about three times and 20% compared with acousto‐optical configuration of ZnO pads only in an interdigital transducer region and all over a silicon on insulator. Moreover, an introduction of a planarisation SiO2 layer can reduce acoustic reflection on optical waveguide and optical waveguide loss for an acousto‐optical device on silicon‐on‐insulator optical waveguides.

An analysis model for an optical waveguide microcantilever sensor is developed combining the optical and mechanical models. An improved optical waveguide microcantilever sensor with a buffer is provided and taken as an example to explore using the analysis model. A systematic and detailed discussion about the couplers for the input waveguide to optical waveguide cantilever and the optical waveguide cantilever to the output waveguide of the improved waveguide cantilever sensor is presented. The sensitivity of an improved optical waveguide cantilever sensor is evaluated by analyzing the input/output waveguide, buffer, microcantilever, and gap. An improved optical waveguide microcantilever sensor by adding a buffer shows a sensitivity of $5.7\times 10^{-4}nm^{-1} $ , which is improved by 51.3%, compared with a conventional optical waveguide microcantilever sensor. The design of an optical waveguide cantilever sensor is a trade-off of different design parameters. These will be helpful for the study of an optical waveguide cantilever sensor.

The three-core photonic crystal fiber (TC-PCF) is proposed to obtain large group-velocity dispersion (GVD) because of the coupling effects between the fundamental modes (FMs) of the central core and higher-order modes (HOMs) of the two side cores. The supermodes of the TC-PCF can provide concave anomalous- and normal-GVD profiles with large peak values at the maximum dispersion wavelengths (MDWs), which can be shifted in three wavelength windows of 1030 nm, 1550 nm and 1900 nm by properly tuning the refractive indexes of the cores or the air-hole diameters. Furthermore, the numerical results show that two segments of the TC-PCFs with large anomalous GVDs and opposite values of the third-order dispersion (TOD) can provide much higher efficiency of pulse compression than one segment of the TC-PCF. Additionally, the TC-PCFs with large normal GVDs can be used to stretch pulses due to their low nonlinearities and short fiber lengths.

We report a dual-wavelength fiber laser sensor based on a uniform fiber Bragg grating (UFBG) and a Polyvinyl alcohol (PVA) film-coated long-period grating (LPG) as sensor probe for simultaneous detection of relative humidity (RH) and temperature. Two UFBGs and the LPG were inscribed in three single mode fibers utilizing phase/amplitude mask technology and mobile scanning technique via the 248 nm UV laser. In our experiment, the RH function based on the differential intensity measurement at dual-wavelength output from 30% to 85% is in a quadratic equation, and the RH sensitivity coefficient of 0.358 dB/% shows good linear relationship and stability from 55% to 85%. Meanwhile, the temperature sensitivity coefficients based on wavelength detection and differential intensity detection are 9.1 pm/°C and 0.21 dB/°C, respectively. The structure of dual-wavelength fiber laser sensor with high signal to noise ratio (SNR), narrow spectral width and good stability enables simultaneous measurement of the RH and temperature with high accuracy and good repeatability.

A novel scheme to generate power-efficient FCC-compliant UWB has been proposed using polarization-maintaining FBG-based spectral shaper and incoherent wavelength-to-time mapping. The target UWB pulse is designed based on the linear combination of four Gaussian pulses with the same pulsewidth, which can be considered as a Gaussian pulse that is bandpass-filtered by a four-tap nonuniformly-spaced filter. FCC-compliant UWB with high spectral power efficiency (46.2–57.2%) can be realized using optimized parameters. Temporal gate and spectral shaper are two key components of an incoherent spectral shaping and wavelength-to-time mapping (SS-WTTM) system. Owing to strong nonlinear absorption characteristic of electro-absorption modulator (EAM), lowpass-filtered electrical pulse is compressed for the implementation of narrow optical gate. 45-ps optical gate is achieved experimentally using 10-Gb/s EAM driven by 10-Gb/s bit pattern generator. This cost-effective gate can be extended to general incoherent SS-WTTM method to substitute commonly-used high-speed gate. Furthermore, a simple spectral shaper is designed based on a uniform polarization-maintaining FBG. To the best of our knowledge, it is the first time to achieve power-efficient FCC-compliant UWB using all-fiber spectral shaper, which has the advantages of small size, low insertion loss, and feasibility of integration. Proposed UWB generator is demonstrated by both simulation and experiment. FCC-compliant UWB with 10-dB bandwidth of 6.25-GHz, and spectral power efficiency of 55.9% is realized successfully.

In this paper, we propose and experimentally demonstrate a dual-parameter sensor based on no-core fiber (NCF) cascaded with elliptical-core spun fiber (ECSF) in a Sagnac loop. Both NCF and ECSF are sealed in a capillary with the magnetic fluid to detect the magnetic field and temperature simultaneously. By introducing a section of NCF inside an elliptical-core spun fiber loop mirror (E-FLM) with optimizing fiber length, two extinct dips can be observed from 1480 to 1680 nm in the transmission spectrum. One dip is the result of multimode interference (MMI), while the other is induced by Sagnac interference of E-FLM. Hence, the simultaneous measurement of magnetic field and temperature can be realized by monitoring the shift of different wavelengths of two dips. Experimental results show that the dip of MMI is sensitive to the magnetic field (713.07 pm/mT), while the E-FLM has almost no response to magnetic field. For the two dips monitored in the temperature measurement, the sensitivities of proposed sensor are −34.8 pm/°C and 304.55 pm/°C, respectively.

This Letter presents a novel, to the best of our knowledge, linearized analog photonic link (APL) based on a phase-coherent orthogonal light wave generator that consists of a polarization-dependent Mach–Zehnder modulator (MZM) and a polarization controller (PC). By adjusting the PC and bias voltage of MZM, the third-order intermodulation (IMD3) terms can be suppressed while retaining a high gain for the fundamental terms, which indicates that the spurious free dynamic range (SFDR) of the proposed APL can be much improved. To further verify the feasibility of the proposed APL, a proof-of-concept experiment is performed, and the performances are compared with conventional APL. The experimental results demonstrate that a 14 dB improvement in the fundamental to IMD3 power ratio and an SFDR of 100.2 d B ⋅ H z 2 / 3 or 119.1 d B ⋅ H z 2 / 3 for a noise floor of − 139 d B m / H z or − 163.9 d B m / H z are achieved. In addition, an orthogonal frequency division multiplexing signal with 30 MHz bandwidth centered at 2.5 GHz is delivered by our proposed APL, whose signal-to-noise ratio is increased by 10 dB, compared to conventional APL.

A novel demodulation method for the sensing system based on an identical weak fiber Bragg grating (IWFBG) array is proposed in this paper. With the help of a wavelength-swept laser, the reflection spectrum and transmission spectrum of an FBG can be mapped into two pulse signals with opposite polarity in time domain, both of which contain the wavelength information of the FBG. Therefore, improved signal intensity and sensing performance can be obtained when the reflection and transmission spectra of the FBG are both adopted for demodulation. A simulation has been done to a quasi-distributed sensing system based on the IWFBG array. The results show that the improvement of the signal intensity is related to the number of the FBGs with the same center Bragg wavelength. The location of the event is realized through the intensity comparison of the pulse signals, and the wavelength change of the FBG can be directly obtained by monitoring the time delay difference between the reference pulse and the sensing pulse.

A photonics-assisted microwave frequency comb (MFC) generator based on a polarization-multiplexed dual loop optoelectronic oscillator (OEO) is proposed. Experimental results demonstrate the generated MFC has adjustable comb interval and low phase noise.

A novel, to the best of our knowledge, interrogation scheme based on an optoelectronic oscillator (OEO) with high sensitivity and high speed response for a fiber optical current sensor utilizing a reflective interferometer is proposed and experimentally demonstrated. Due to the Faraday effect, a magneto-optic phase shift induced by current variation is generated between two orthogonal light waves. The polarization-dependent properties of the Mach–Zehnder modulator are used to convert the magneto-optic phase shift into the phase difference between the optical carrier and sideband, which is then mapped to the oscillating frequency shift by closing an OEO loop. A high current sensitivity of 152.5 kHz/A with a range of 0–2.5 A is obtained in the experiment.

In this paper, a relative humidity (RH) and temperature sensing scheme with high sensitivity based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. A polyvinyl alcohol-Fiber Bragg grating-Fabry Perot (PVA-FBG-FP) filter, in which the PVA film acts as an RH-strain transducer, is employed in the OEO to implement a single passband microwave photonic filter (MPF). In the OEO loop, the oscillating frequency is determined by the center frequency of the PVA-FBG-FP filter and the laser source. When the external RH or temperature changes, the central wavelength of PVA-FBG-FP filter is shifted, leading to the change in the center frequency of the MPF. By simply monitoring the variation of oscillating frequency using an electrical spectrum analyzer or a digital signal processor, the measurements for the RH and temperature can be achieved. In the experiment, the sensitivities of the RH and temperature are measured to be as high as 508.3 MHz/%RH and 1.8118 GHz/°C, respectively.

An image processing algorithm named gray level co-occurrence matrix (GLCM) is introduced to improve the fiber specklegram sensors applied to displacement sensing. The GLCM algorithm is used to analyze the Fourier phase spectra of step-index multimode fiber (MMF) specklegrams. Approximately linear relations between the GLCM features and the lateral displacement between the launching SMF and the MMF have been obtained, which indicate the capability of lateral displacement sensing by GLCM analysis of the specklegrams. Experimental results show that lateral displacement detection can be realized by MMFs with a wide range of parameters and the method exhibits better performance than the conventional NIPC (normalized intensity inner product coefficient) method.

A novel bidirectional broadcast radio-over-fiber-wavelength division multiplexing-passive optical network (RoF-WDM-PON) based on self-coherent detection and photonic frequency up and down-conversion is presented. In the proposed scheme, a single sideband (SSB) modulated pilot-carrier is transmitted to the optical network unit (ONU), together with the downstream signal by polarization multiplexing on the same fiber link. The SSB pilot-carrier serves as a local oscillator laser to realize coherent detection and an optical carrier of the upstream signal simultaneously. In this way, the optical carrier phase noise and frequency offset in coherent detection have been efficiently eliminated in ONU. Moreover, the detection of the downstream baseband signal and millimeter wave (MMW) signal by homodyne and heterodyne detection can be simultaneously implemented in the proposed scheme. Due to the reuse of SSB light wave, the upstream radio-frequency signal is down-converted at the optical line terminal (OLT) directly by photonic-mixing technique. The performance of proposed RoF-WDM-PON system is evaluated for 20 km signal-mode fiber full-duplex transmission of a 40 Gb/s 16-QAM baseband signal and 10 Gb/s DPSK wireless signal.

A novel optical fiber magnetic field sensor based on D-shaped fiber modal interferometer and magnetic fluid is proposed and experimentally investigated. Thanks to the etched cladding of D-shaped fiber, the resulting interference is strongly influenced by the surrounding magnetic fluid, which leads to a high magnetic field sensitivity. Simultaneous measurement of the magnetic field and temperature was realized by monitoring the wavelength shift of the two resonance dips at the same time. In the experiment, the magnetic field and temperature sensitivity can reach 99.68 pm/Oe and −77.49 pm/°C, respectively. The proposed magnetic field sensor based on D-shaped fiber is featured with high sensitivity, low cost and simple configuration.

A novel approach to generating background-free microwave pulse based on both bright and dark temporal gate and a single photodetector is proposed and demonstrated. Incoherent wavelength-to-time conversion (WTTC) using only bright or dark gate renders the generation of positive or negative pulse separately. By using both gates simultaneously, background-free pulse is realized with enhanced amplitude compared with that obtained using only bright or dark gate. A dual-output intensity modulator functions as both the bright and dark gate. Narrow drive pulse is provided by the combination of two polarity-opposite electrical pulses with a relative time-delay, which releases the requirement of expensive narrow-pulse generator. Incoherent WTTC using dark gate is demonstrated experimentally. The system resolution remains the same as that using bright gate. Furthermore, UWB pulse with 10-dB bandwidth of 3.8-GHz is generated based on 1-Gb/s gate driven by 1-Gb/s data sequence. Several other background-free pulses are generated to verify the tunability of central frequency, pulse envelope, duty cycle, and repetition rate. In addition, the effect of gate width on WTTC is explored experimentally. With narrower gate width, higher system solution and maximum achievable frequency can be implemented. 10.4-GHz and 14.5-GHz background-free pulses are generated when using 1-Gb/s and 10-Gb/s gate respectively.

A novel scheme for the generation of FCC-compliant UWB pulse is proposed based on modified Gaussian quadruplet and incoherent wavelength-to-time conversion. The modified Gaussian quadruplet is synthesized based on linear sum of a broad Gaussian pulse and two narrow Gaussian pulses with the same pulse-width and amplitude peak. Within specific parameter range, FCC-compliant UWB with spectral power efficiency of higher than 39.9% can be achieved. In order to realize the designed waveform, a UWB generator based on spectral shaping and incoherent wavelength-to-time mapping is proposed. The spectral shaper is composed of a Gaussian filter and a programmable filter. Single-mode fiber functions as both dispersion device and transmission medium. Balanced photodetection is employed to combine linearly the broad Gaussian pulse and two narrow Gaussian pulses, and at same time to suppress pulse pedestals that result in low-frequency components. The proposed UWB generator can be reconfigured for UWB doublet by operating the programmable filter as a single-band Gaussian filter. The feasibility of proposed UWB generator is demonstrated experimentally. Measured UWB pulses match well with simulation results. FCC-compliant quadruplet with 10-dB bandwidth of 6.88-GHz, fractional bandwidth of 106.8% and power efficiency of 51% is achieved.

The detection of liquid level and temperature based on a fiber ring cavity laser sensing configuration is presented and demonstrated experimentally. The sensing head contains a fiber Bragg grating (FBG) and a single-mode–cladding-less–single-mode multimode interferometer, which also functions as wavelength-selective components of the fiber laser. When the liquid level or temperature is applied on the sensing head, the pass-band peaks of both multimode interference (MMI) filter and FBG filter vary and the two output wavelengths of the laser shift correspondingly. In the experiment, the corresponding sensitivities of the liquid level with four different refractive indices (RI) in the deep range from 0 mm to 40 mm are obtained and the sensitivity enhances with the RI of the liquid being measured. The maximum sensitivity of interferometer is 106.3 pm/mm with the RI of 1.391. For the temperature measurement, a sensitivity of 10.3 pm/°C and 13.8 pm/°C are achieved with the temperature ranging from 0 °C to 90 °C corresponding to the two lasing wavelengths selective by the MMI filter and FBG, respectively. In addition, the average RI sensitivity of 155.77 pm/mm/RIU is also obtained in the RI range of 1.333–1.391.

An ultrafast high-sensitivity refractive index (RI) and temperature-sensing system based on an optoelectronic oscillator (OEO) is proposed and demonstrated in this paper. A Fabry-Perot fiber Bragg grating (FP-FBG), which combines a gap with two FBGs in a silica V-shaped slot and characterizes a narrow notch in the reflection spectrum, is incorporated in the OEO to implement a microwave photonic filter and perform oscillating frequency selection. A microwave signal is generated by the OEO, whose oscillating frequency is determined by the center frequency of the FP-FBG notch, which varies with the surrounding environments. The RI or the temperature change can be accordingly measured by monitoring the frequency shift of the microwave signal using an electrical spectrum analyzer or a digital signal processor. An experiment is performed to verify the proposal. An RI sensitivity of 413.8 MHz/0.001RIU and a temperature sensitivity of 2516 MHz/°C are successively demonstrated.

With the development of nanoscale transducers, surface acoustic wave (SAW) with frequencies above 10 GHz can now be excited on the optical waveguides. However, the fundamental mode is very difficult to be excited for SAW at ultrahigh frequencies. Moreover, as the wavelength of SAW is reduced to below the optical wavelength, the acoustic field across the optical waveguide can no longer be approximated as uniform but is spatially periodic. Here, we develop a discussion about integrated acousto-optic (AO) interaction at ultrahigh frequencies. The electromechanical coupling coefficients of different modes are evaluated for SAW at ultrahigh frequencies. The acousto-optic overlap is analyzed to evaluate integrated AO interaction for each given acoustic wavelength and optical waveguide size. An analytic model is presented for integrated AO interaction at ultrahigh frequencies. The research shows that it would be a trade-off for the design of an integrated AO device with ultrahigh frequencies.

An approach to generate the linearly chirped microwave waveform with simultaneous frequency and bandwidth doubling by using cascaded phase modulator and dual-output dual-parallel Mach-Zehnder modulator (DO-DPMZM) is proposed. The lightwave emitted from a laser diode is firstly modulated by a splitting parabolic signal to realize a symmetric-triangle linear frequency sweeping light. The frequency sweeping light is then sent to a DO-DPMZM driving by a radio frequency (RF) signal. By properly setting the direct current biases of two sub-modulators, single sideband modulation is realized at each output of the DO-DPMZM. By introducing a time delay in one branch of the modulator’s outputs, a linearly chirped microwave waveform with both frequency and bandwidth doubling can be generated after optical-to-electrical conversion. The central frequency and bandwidth of the generated waveform can be easily adjusted by tuning the frequency of the RF signal and electrical coding signal of the arbitrary waveform generator. The linearly chirped microwave waveforms with bandwidth of 10 GHz and central frequency of 30 GHz or bandwidth of 12 GHz and central frequency of 40 GHz are numerically generated. Due to the simple structure and flexible tunability, the proposal may find applications in future high resolution radar systems.

A multiband radio-over-fiber system for a fifth-generation (5G) mobile communication technology mobile fronthaul network is proposed, which can transmit radio frequency (RF) signals in four different frequency bands of 700 MHz, 1.8 GHz, 3.5 GHz, and 26 GHz with different data rates simultaneously. The proposed system can satisfy the multiscenario demand of 5G and realize 4G/5G coexistence. A dual-polarization binary phase-shift keying modulator is utilized to alleviate the interference between multiple-frequency bands. The system is analyzed theoretically and verified through simulation. The variations of error vector magnitudes (EVMs) of four transmitted RF signals in function of the received optical power (ROP) are investigated. The simulation results show that the system has good performance after 10 km standard single-mode fiber (SMMF) transmission. When the ROP is above − 3.3 d B m , the EVM of the system conforms to the 3GPP specification. The power penalty of the system is within 1.9 dB at the 3GPP EVM performance specification after transmitting over a 10 km SSMF.

By incorporating a phase-shifted fiber Bragg grating (PS-FBG) inscribed in few-mode fiber (FMF) named FM-PS-FBG for the first time, a novel frequency-switchable optoelectronic oscillator (OEO) to perform simultaneous strain and temperature measurement is proposed and experimentally demonstrated. In the OEO loop, two switchable oscillation frequencies are determined by the reflected bands of FM-PS-FBG based on different mode couplings and corresponding notches. Due to the characteristics of modes, two OEO-generated microwave signals exhibit different strain and temperature sensitivities. Therefore, through monitoring the frequency shifts of microwave signals, the simultaneous strain and temperature sensing can be realized by the proposed OEO sensor with high-speed and high-resolution interrogation.

A novel multifold clock recovery structure with demultiplexing function based on a frequency-doubling optoelectronic oscillator (FD-OEO) incorporating a common commercial phase modulator (PM) is proposed and experimentally demonstrated. A special phase modulation is obtained thanks to polarization-sensitive feature of LiNbO 3 crystal of the PM. The special phase modulated signal is then split two parts connected by two pairs of polarization controllers (PCs) and polarizers to perform polarization interference and phase modulation to intensity modulation conversion. By controlling the PCs, the joint use of PC and polarizer in the loop is to guarantee an OEO with fundamental frequency self-oscillation, while the other outside of the OEO obtains frequency-doubled microwave signal at the output of the photodetector. Additionally, an optical filter can be inserted into the outside branch to only allow ± 2nd-order sidebands pass through and realize frequency quadrupling after beating at the PD. Therefore, a prescaled clock at 10 GHz, a line-rate clock at 20 GHz and a 40 GHz frequency-doubled clock can be extracted from the injection-locked OEO respectively when a 2 × 10 Gb ∕ s optical time division multiplexing (OTDM) data signal is injected. A twofold time division demultiplexing is demonstrated. Theoretical analysis is developed, which is validated by an experiment.

A novel Erbium-doped fiber ring cavity laser sensor for refractive index (RI) measurement based on a special designed few-mode-singlemode-few-mode structure is proposed and experimentally demonstrated. The few-mode fiber is a home-made concentric ring core fiber (CRCF) which can only support two scalar modes. Thus a stable mode interference occurs which functions as a sensing head and band-pass filter to select the lasing wavelength simultaneously. A sensitivity of −45.429 nm/RIU is obtained in the range of 1.333–1.363. High optical signal to noise ratio (OSNR) of ∼45 dB and narrow 3-dB bandwidth of ∼0.1 nm indicate that the fiber ring laser sensing system has a high resolution and accuracy RI measurement.

We propose and experimentally demonstrate a cost-effective approach to achieve optical single sideband (OSSB) modulation with tunable optical carrier-to-sideband ratio (OCSR) based on a bi-directional use of an OSSB modulator. The key concept of the proposed scheme is to use an OSSB modulator which is incorporated in a Sagnac loop where a light wave is modulated in the form of SSB modulation along the clock-wise direction while the orthogonal light is not modulated along the counter clock-wise direction due to the velocity mismatch. The signals along two directions are combined together at the output of the Sagnac loop via a polarization beam splitter (PBS). Thus a partial orthogonal SSB (PO-SSB) modulated signal is obtained. The generated signal is then sent into a polarization controller (PC) followed by a polarizer. An OSSB-modulated signal with tunable OCSR from −10 to 27 dB is achieved by adjusting the PC in the experiment. The proposed scheme is wavelength independent, which can support low microwave bandwidth and achieve flexible wavelength tunability.

A temperature self-calibrated potential of hydrogen (pH) sensor based on the single mode fiber-tapered dual core photonic crystal fiber-single mode fiber (SMF-TDCPCF-SMF) structure cascaded with a fiber Bragg grating (FBG) is proposed and demonstrated. The TDCPCF structure formed Mach-Zehnder interferometer (MZI) is modified with a coating of graphene oxide/polyvinyl alcohol (GO/PVA) hybrid hydrogel to realize the measurement of pH, and the uncoated FBG is used to calibrate temperature. In our experiment, the sensitivity coefficient of 0.69 nm/pH with R2=0.99 and the hysteresis loss of less than 0.007 are achieved within the pH range from pH 4.00 to pH 9.85. The measured response time from pH 7.00 to pH 4.00, 6.00 and 9.85 are no higher than 10s. Moreover, the resonant wavelengths of MZI and FBG also exhibit good linear relationship with the temperature sensitivity coefficient of 0.15 nm/°C (R2=0.99) and 0.09 nm/°C (R2=0.97) respectively. It is demonstrated successfully that the proposed sensor has broad application prospects in the field of environmental monitoring, biological sensing and chemical analysis, due to the good performance of the temperature self-calibrated pH monitoring, repeatability, linearity, response time and reversibility.

Monolayer WS2 electro-absorption modulator (EAM) on GaN waveguide is investigated. By covering monolayer WS2 on top of the optical waveguide and applying different voltages to change the refractive index of WS2, a strong interaction between monolayer WS2 and light is obtained, which leads to a significant change of the imaginary part of effective mode index (EMI) in the waveguide. A systematic and detailed discussion has been presented to describe and design the EAM. Results show that the EAM can achieve 153 GHz 3-dB modulation bandwidth, around 70% modulation depth and lower insertion loss with 10 μm-long active region. These will be helpful for the study of two-dimensional material electro-optic modulator.

A photonic-assisted microwave mixer using a Sagnac-loop-based modulator and polarization-dependent modulation is proposed and experimentally demonstrated. In this scheme, a dual-parallel Mach-Zehnder modulator (DPMZM) is incorporated in a Sagnac loop, where light is single-sideband (SSB) modulated along one direction with no modulation along the opposite direction due to the traveling-wave velocity mismatch in DPMZM. The SSB-modulated light and unmodulated optical carrier (OC) with opposite propagation directions and orthogonal polarizations are combined via a polarization beam combiner, and thus, a partial orthogonal SSB-modulated signal is obtained, which is then injected into a Mach-Zehnder modulator (MZM) via a polarization controller to ensure the polarization of the unmodulated OC paralleling with the transverse electric (TE) principal axis of MZM. Thus, only the OC is modulated by the data, and the first-order sidebands pass through the MZM directly without modulation based on the polarization dependence of MZM. After a polarizer for polarization interference and a photodetector for optical-to-electrical conversion, a phase-stable microwave signal is obtained. By adjusting the bias point of MZM, clear eye diagrams, and waveforms of 1 Gb/s on-off keying (OOK), amplitude-shift keying (ASK), and binary-phase-shift keying (BPSK) signals at 8 GHz are experimentally achieved.

A novel technology to obtain binary phase-coded ultrawideband (UWB) signals for direct-sequence spread-spectrum communication systems is investigated by using a cost-effective incoherent source. The bipolar encoding is performed based on an all-fiber spectrum shaper composed of two FBG arrays to tailor the optical spectrum, and a section of single-mode fiber to achieve incoherent frequency-to-time conversion. We demonstrate a 1.325-Gb/s UWB encoding system by the use of binary spreading codes of 4-chip length via computer simulations. The proposed bipolar UWB encoding technology can be applied to high-speed UWB-over-fiber communication systems.

Fiber ring laser sensor for curvature measurement based on a single-mode–no-core–single-mode structure is proposed and experimentally demonstrated. Compared with the conventional curvature system, the proposed laser sensor system has a high optical signal-to-noise ratio and a narrow 3-dB bandwidth, which enables the system to monitor the variation of micro-bending more distinctly. In the experiment, changes in peak wavelength show a blue shift when the curvature varies from 0.1225 to 0.4062 $\text{m}^{-1}$ . The sensitivity of the curvature is about −22.33 nm/ $\text{m}^{-1}$ in the range of 0.2121–0.3463 $\text{m}^{-1}$ . The response to the temperature of the fiber laser sensor is also discussed, where temperature sensitivity of 9.23 pm/°C within the range from 10 °C to 100 °C is obtained.

A novel approach to the optoelectronic oscillator (OEO) for the generation of a microwave signal with simultaneously frequency and phase tunability is proposed and experimentally demonstrated. In the proposed OEO, a carrier phase-shifted double-sideband modulated optical signal generated using a dual-parallel Mach–Zehnder modulator (DPMZM) is split into two parts. For one part, the signal is injected into a photodetector (PD) after passing through a section of high nonlinear fiber and then fed back into the DPMZM to form an OEO loop. The stimulated Brillouin scattering effect is used to select the oscillation frequency which equals to the Brillouin frequency shift. For the other part, a carrier phase-shifted single-sideband modulated signal is achieved by a tunable filter, and then converts to a microwave signal by a second PD. The tunable phase of the generated microwave signal is achieved by controlling the bias voltage of the DPMZM to change the phase difference between the optical carrier and the sideband. Thanks to the wavelength-dependent effect of the Brillouin frequency shift, the tunable frequency is realized by tuning the frequency of the laser source. Microwave signals with a tunable frequency from 8.950 to 9.351 GHz and a tunable phase from 0° to 360° are experimentally generated.

An approach to generate high-speed and wideband frequency shift keying (FSK) signals based on carrier phase-shifted double sideband (CPS-DSB) modulation is proposed and experimentally validated. The core part of the scheme is a pair of cascaded polarization-sensitive LiNbO3 Mach–Zehnder modulators and phase modulators, whose polarization directions of the principal axes are mutually orthogonal to each other. A proof-of-concept experiment is carried out, where a 0.5 Gb/s FSK signal with the carrier frequencies of 4 and 8 GHz and a 1 Gb/s FSK signal with the carrier frequencies of 8 and 16 GHz are generated successfully.

A dual-frequency optoelectronic oscillator (OEO) incorporating cascaded fiber-Bragg-grating Fabry–Perot (FBG-FP) cavities designed for both curvature and temperature sensing is proposed and experimentally validated. The OEO is composed of two tunable laser diodes (TLDs), a phase modulator (PM), two FBG-FP cavities, a spool of fiber, a photodiode (PD) and an electrical amplifier (EA). One FBG-FP cavity is curved on a twin-core fiber (TCF) and the other is curved on a single mode fiber (SMF), with one core of the TCF aligned and spliced with the core of the SMF. The oscillating frequencies of the OEO are determined by the wavelength differences of the TLDs and the notches of the FBG-FP cavities. The oscillating frequency related to the twin-core-fiber-Bragg-grating Fabry–Perot cavity is both curvature and temperature sensitive thanks to the off-set core of the TCF, while the oscillating frequency corresponding to the single-mode-fiber-Bragg-grating Fabry–Perot cavity is only temperature sensitive. By monitoring the oscillating frequencies of the OEO, high-sensitivity curvature and temperature sensing are realized. The oscillating frequencies are linearly proportional to the curvature and temperature, with sensitivities as high as −1.19 GHz/m−1 and 1.14 GHz/oC, respectively.

Compressive sensing (CS) is a recently developed theory which allows reconstruction of sparse signals with the number of measurements much lower than that required by the Nyquist sampling, and sensing the signals with a measurement matrix (MM) is one of the indispensable key procedures. Based on the specklegrams of multimode fiber (MMF) with offset-launching and coreless multimode fiber (CMF) with center-launching, MM was constructed and applied in CS. The constructed MMs provide improved performances compared with the conventional Gaussian MM in CS when applied to image reconstruction. Specklegrams of MMF with different launching offsets and CMF with different wavelengths were respectively used to construct the MMs by different construction methods, which demonstrated the flexibility of the combination of fiber specklegrams to get a MM. By careful comparisons, it can be concluded that MMs based on the fiber specklegrams perform better in providing a good Peak Signal to Noise Ratio. Moreover, the construction methods can be easily implemented, which is highly promising for compressive sensing.

A dual-wavelength fiber ring laser based on multimode fiber-polarization maintaining fiber Bragg grating-multimode fiber (MMF-PMFBG-MMF) filter for simultaneously axial strain, temperature and refractive index (RI) sensing is proposed and experimentally demonstrated. In the ring laser, stable dual-wavelength lasing is determined by the MMF-PMFBG-MMF filter with two polarization states. The fiber birefringence affected by axial strain is far less than the effect of the temperature. Through monitoring the variations of each wavelength shift and output power, the simultaneous measurement for the axial strain, temperature and RI is realized. In our experiment, the proposed fiber laser sensor exhibits an axial strain sensitivity of 1.16 × 10-3nm/μe and an RI sensitivity of 81.2dB/RIU. Meanwhile, the temperature sensitivities of two wavelengths are experimentally measured to be 9.74 × 10-3nm/°C and 9.2 × 10-3nm /°C, respectively.

In this paper, a novel quasi-fan Solc structure filter based on elliptical-core spun fiber for twist sensing has been experimentally investigated and theoretically analyzed. The discrete model of spun fiber has been built to analyze the transmission characteristics of proposed sensor. Both experimental and simulated results indicate that the extinction ratio of the comb spectrum based on quasi-fan Solc birefringent fiber filter varies with twist angle and agrees well with each other. Based on the intensity modulation, the proposed twist sensor exhibits a high sensitivity of 0.02219 dB/(°/m). Moreover, thanks to the invariability of the fiber birefringence and the state of polarization of the input light, the proposed twist sensor has a very low temperature and strain sensitivity, which can avoid the cross-sensitivity problem existing in most twist sensors.

We propose and experimentally validate a compact and flexible structure of optical signal processing employing a polarization-dependent phase modulator (PM) based optoelectronic oscillator (OEO). The key point of the proposed design is to make good use of the polarization-dependent feature of the LiNbO 3 crystal of the PM to realize special phase modulation to intensity modulation conversion. The phase modulation indexes along the principal direction and orthogonal direction of the PM are different. Thus an intensity signal can be obtained after the lights in the two orthogonal directions are combined at a polarization (Pol) after the PM. A polarization controller (PC) is inserted between the PM and Pol to introduce a static phase shift. One part of the special phase modulated data signal is introduced to the OEO loop to form a stable injected oscillation. While the other part is sent to a branch to do serial-to-parallel conversion or modulation format conversion. Non return-to-zero (NRZ) to return-to-zero (RZ) format conversion with tunable duty cycle which can be adjusted by tuning the phase shift introduced by the PC continuously is achieved in the branch. The receiver sensitivity of the converted RZ signal at the bit error rate (BER) of 10−9 has an improvement of about 2 dB compared to the original NRZ signal.