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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.

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.

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.

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.

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.

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.

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 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.

Since the curvature induced variations of mode interference in multimode fiber (MMF) can be well represented by the fiber specklegrams, a bending recognition scheme based on the analysis of MMF specklegrams is proposed and verified. Amounts of specklegrams from the facet of MMF under different bendings were detected and used for training and testing in an image recognition algorithm based on deep learning. Good recognition results are provided by the specklegrams from MMF with diameter being 105 and 200 μ m , for which the average accuracies of bending status recognition are respectively 92.8% and 96.6%. Because specklegram can represent the status of the whole section of MMF, such scheme indicates the capability to distinguish the specklegrams even when the MMF is under complicated bending. In this sense, the scheme presents the potential of a single MMF being used as a bending indicator or status monitor independently, which may find applications in distinguishing the status of certain structures, such as robotic arms, mechanical fingers and some disabled auxiliary equipments.

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.

An optoelectronic oscillator (OEO) for high-speed and high-sensitivity displacement sensing with extended measurement range is proposed and experimentally demonstrated. The key parts of the OEO are a broadband optical source (BOS), a Mach-Zehnder interferometer (MZI) with unbalanced arms, a spool of dispersion compensating fiber (DCF) and a photodetector (PD) which jointly act as a single-bandpass microwave photonic filter (MPF) to choose the oscillating frequency of OEO. The change of displacement between the two collimating lenses embedded in one arm of the MZI will induce the change of free spectral range (FSR) of the MZI, leading to a frequency shift of the single-bandpass MPF as well as the oscillating signal of the OEO. Thanks to the pair of collimating lenses in the MZI, a displacement sensing with a large range from 0 mm to 12 mm is carried out, and a displacement resolution of 0.07 μm with a sensitivity of 0.285 MHz/μm is obtained.

A cost-effective ultra-dense wavelength-division-multiplexed passive optical network (UD-WDM PON) with speed of 12.5 Gbit/s and channel spacing of 12.5 GHz is proposed and demonstrated. The distributed feedback (DFB) lasers modulated in 4-level pulse amplitude modulation (4-PAM) format are used for downstream links, and the reflective semiconductor optical amplifiers (RSOAs) together with an optical frequency comb modulated in quadrature phase shift keying (QPSK) format are used for upstream links. We can achieve the error-free transmission of the upstream signals with speed of 12.5 Gbit/s even after 20 km single-mode fiber (SMF). The power penalty obtained by using the frequency comb generator instead of a tunable laser is around 0.5 dB. By using 11 DFB lasers and a set of intensity and phase modulators, it is possible to provide the seed light for 297 optical network units (ONUs) within the C-band.

A simultaneous two-channel optical time division multiplexing (OTDM) demultiplexing is proposed and experimentally demonstrated by using a bidirectionally operated highly nonlinear fiber (HNLF) followed by a narrow-band offset filter. The performance of the proposed demultiplexer is evaluated in a 40 Gb/s OTDM 100 km transmission system. A maximum power penalty of 2.4 dB is obtained for the worst demultiplexed channel. The proposal might be interesting since it offers a powerful tool for developing ultrafast photonic networks.

A novel bandwidth-enhanced polarization division multiplexed intensity modulation-direct detection (IM-DD) short reach fiber optical communication system is proposed and experimentally demonstrated by utilizing a dual polarization-dual parallel Mach Zehnder modulator (DP-DPMZM). By adjusting bias voltages of the DP-DPMZM, the chromatic dispersion induced frequency notches can be shifted to higher frequencies, which indicates the transmission 3 dB-bandwidth of proposed system can be much more enhanced comparing with the conventional IM-DD system. A proof-of-concept experiment is performed for dual parallel Mach Zehnder modulator-direct detection (DPMZM-DD) system and DP-DPMZM-direct detection (DP-DPMZM-DD) system, whose performances are compared with the conventional Mach Zehnder modulator-direct detection (MZM-DD) system over 100 km standard single-mode fiber transmission. The experimental results show that the DPMZM-DD system and DP-DPMZM-DD system have a significant improvement of the bit error rate performance, compared to its counterpart MZM-DD system.

A tunable optoelectronic oscillator (OEO) based on polarization controlled frequency response shifting of a dispersion-induced microwave photonic filter (MPF) is proposed and experimentally demonstrated. The MPF consists of a phase modulator (PM), a linear chirped fiber Bragg grating (LCFBG), and polarization multiplexed dual loops. Thanks to the polarization properties of the LiNbO3 crystal, the center frequency of the MPF can be tuned by simply adjusting the polarization state of the optical signal, and a tunable optoelectronic oscillator is established. Compared to the previous structure, the expensive wavelength-tunable laser is not needed in the proposed structure. In addition, the instability of oscillation frequency caused by the bias drifts of the modulator is avoided. A pure oscillation signal with a tunable frequency range from 3.3 to 7.3 GHz is generated. The single-sideband phase noise of the oscillation signal is about −110 dBc/Hz at 10-kHz offset. During more than 30 min observation, the oscillation frequency drift is about 7 kHz.

A novel approach to the generation of an optical frequency comb with a widely tunable center wavelength and comb spacing based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The OEO is implemented using a polarization modulator (PolM), a phase-shifted fiber Bragg grating (PS-FBG), and a photodetector (PD). The PolM is a special phase modulator that supports phase modulation along the two principal axes with opposite modulation indexes. The joint operation of the PolM, the PS-FBG, and the PD corresponds to a frequency-tunable microwave photonic bandpass filter. When the output from the PD is fed back to the PolM, the OEO starts to oscillate and the oscillation frequency can be tuned by tuning the center frequency of the microwave photonic bandpass filter through tuning the optical wavelength. The optical comb is then generated by tapping part of the optical signal from the PolM and sending it to a second PolM. The joint operation of the two PolMs generates an optical comb with the comb spacing tunable by tuning the center frequency of the microwave photonic filter. Through introducing a second wavelength into the OEO, a duplicated optical comb at the second wavelength is generated. An experiment is performed. An optical frequency comb with tunable frequency spacing from 6.6 to 15.3 GHz and a tunable center wavelength from 1500 to 1580 nm is generated.

A novel approach for the implementation of microwave photonic filter with positive and negative coefficients based on an unbalanced Mach-Zehnder modulator (MZM) is proposed. In the proposed filter, a microwave signal to be filtered is applied to an unbalanced MZM. Thanks to the unbalance between the two arms of the modulator, a π phase shift is obtained by adjusting the wavelength interval between the adjacent wavelengths, which leads to the generation of the positive and negative coefficients. The theoretical fundamentals of the design are described, which show that the required unbalanced MZM in the scheme can be well fabricated by the current technology, and the required other components, including the wavelength multiplexer, are also commercially available devices. An eight-tap microwave photonic filter with positive and negative coefficients is demonstrated. The tunability and reconfigurability of the eight-tap microwave photonic filter are also investigated to verify our approach.

An improved multiplier-free feed-forward carrier phase estimation algorithm is proposed for dual-polarization quadrature-phase-shift-keying (DP-QPSK) with coherent detection. The bit error rate (BER) performance, block length effect and linewidth tolerance of the proposed algorithm are evaluated for a 112 Gbit/s DP-QPSK system. A linewidth symbol duration product of 2.9×10−4 is demonstrated for 1 dB optical signal-to-noise-ratio (OSNR) penalty at BER of 10−3 for the proposed algorithm. The hardware complexity of the proposed multiplier-free algorithm is demonstrated to be much lower than that of the 4th power algorithm.

An optical vector network analyzer (OVNA) based on unbalanced double-sideband (UB-DSB) modulation with improved measurement accuracy is proposed and experimentally demonstrated. It is different from an OVNA based on optical single-sideband (OSSB) modulation in which one-to-one mapping between the optical and radio frequency responses is employed to measure the magnitude and phase responses of an optical component, the proposed technique measures the magnitude and phase responses by taking into consideration of the power of the other sideband through solving two equations that are associated with the UB-DSB modulation, thus the errors due to the residual power of the other sideband in an OSSB modulation based approach are completely eliminated. A mathematical model providing the transfer function of an optical component is derived. The measurement of a phase-shifted fiber Bragg grating and a linearly chirped fiber Bragg grating is performed. Comparing with the measured results based on OSSB modulation, obvious improvement in measurement accuracy is demonstrated.

A temperature independent 80-Gb/s 100-km transmission system is demonstrated with the use of spectral phase modulation-based tunable dispersion compensator (TDC). The principle of dispersion compensation based on spectral phase modulation as well as the relationship between spectral phase modulation function and group velocity dispersion (GVD) are theoretically studied. TDC based on spectral phase modulation is implemented. The performance of 80-Gb/s transmission system is experimentally evaluated. The non-linear relationship between temperature and temperature-induced dispersion fluctuations is demonstrated through the asymmetric temperature-induced power penalty without dispersion compensation. With respect to the low temperature area, the temperature-induced dispersion fluctuations are smaller than those in the high temperature area. By using the proposed TDC, temperature independent 80-Gb/s transmission is successfully demonstrated under a temperature range of –20 – 60 ?C with a power penalty of less than 0.8 dB.

An approach for chromatic dispersion measurement is proposed and experimentally demonstrated based on an optoelectronic oscillator (OEO) incorporating a single bandpass microwave photonic filter (MPF), which is implemented by cascading a finite impulse response (FIR) MPF and an infinite impulse response (IIR) MPF jointly. The key concept of the proposed design is mapping the chromatic dispersion of a device under test (DUT) to the oscillating frequency of the OEO by embedding the DUT into the OEO loop. The oscillating frequency is mainly determined by the central frequency of the MPF, which depends on the length difference of the two arms of the Mach-Zehnder interferometer (MZI) and the chromatic dispersion value of the fiber cooperatively. The bandwidth of the MPF is further narrowed by an IIR filter based on a recirculating fiber loop for stabilizing OEO oscillation. The chromatic dispersion of fibers with lengths ranging from 20 km to 100 km are measured in the experiment.

A 10-GHz clock recovery from a 16 × 10-Gbit/s optical time-division-multiplexed (OTDM) data stream is experimentally demonstrated using an electro-absorption modulator and 40-Gbit/s electric time-division-multiplexed (ETDM) demultiplexer. The recovered clock signal exhibits excellent stability, with root square (RMS) jitter of 328 and 345 fs corresponding to back-to-back and transmission over 100 km, respectively.

A 16 × 10-Gb/s optical time-division-multiplexing (OTDM) 100-km transmission system with home-made multiplexer is demonstrated experimentally. A demultiplexer based on two cascaded electro-absorption modulator (EAM) and a clock recovery unit form a feedback loop. Error-free demultiplexing from 160-Gb/s to 10-Gb/s after over 100-km transmission is achieved. The power penalty is about 3.5 dB with the bit error rate of 10 −9 .

In this paper, a theoretical model is proposed for the analysis of timing jitter induced by intrachannel interactions in optical fiber communication systems using chirped fiber grating (CFG) as dispersion compensator. And for the first time, the impact of group-delay ripple (GDR) on timing jitter is taken into account in detail, theoretically and numerically. Results show that the GDR may increase or decrease the timing jitter of certain pair of adjacent pulses, deciding by the ripple parameters. The maximum influence will be obtained when ripple period is about 2/5 of the input pulse width. In practical systems, the GDR enhance the timing jitter for some adjacent pulses while reduce it for some other adjacent pulses, depending on the pulse pattern and the GDR parameters. In our simulation, the GDR guarantees a 3.04 ps increase of the standard deviation of the timing jitter.

An optoelectronic oscillator (OEO) for the generation of a microwave signal with both tunable frequency and phase is proposed and experimentally demonstrated. In the proposed OEO, a single-sideband (SSB) polarization-modulated signal is generated by a polarization modulator (PolM) and a phase-shifted fiber Bragg grating (PS-FBG). The SSB polarization-modulated signal is then split into two parts. One part is sent to a photodetector (PD) and then fed back to the PolM to form the OEO loop. The other part is sent to an electronically tunable polarizer (ETP) and then applied to a second PD. The frequency tunability is achieved by tuning the wavelength of the optical carrier. The tunable phase is introduced by tuning the principal axis of the ETP. An experiment is performed, and a microwave signal with a tunable frequency from 6.6 to 13.1 GHz and a tunable phase from -180° to 180° is generated. The phase noise performance of the generated microwave signal is also evaluated, which is -101.9 dBc/Hz at an offset of 10 kHz.

A novel solution for polarization mode dispersion (PMD) compensation is proposed by applying tension on a tapered high-birefringence linearly chirped fiber Bragg grating. The compensated differential group delay changes with the applied tension, for its cross-sectional area varies nonlinearly along the grating. Its performance as PMD compensator for short pulses fiber-optical transmission system and 10 Gb/s NRZ fiber-optical transmission system is demonstrated experimentally.

We propose a novel method to analyze Raman amplifiers (RAs) by solving difference equations. This method is highly convenient for analysis of the transient effects of Raman amplifiers under dynamic conditions. Some properties of time-division multiplexing (TDM)-pumped RAs are analyzed theoretically. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 407–410, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20154