Baofu Zhang

Generation of Low Phase-Noise Frequency-Sextupled Signals Based on Multimode Optoelectronic Oscillator and Cascaded Mach–Zehnder Modulators

We have demonstrated a scheme to realize the generation of frequency-sextupled signals with low phase noise (PN). The structure is based on a multimode optoelectronic oscillator (OEO) and two cascaded Mach-Zehnder modulators (MZMs). A drive source signal is modulated by the MZM1, which is biased at the minimum transmission point (MITP); the generated first-order sidebands are injected into the multimode OEO, which is contributed by the MZM2, the single-mode fiber, the photodetector, and the amplifier. The third-order sidebands are generated by the MZM2, which is also biased at the MITP. The modulated signal is then divided into reflection components and transmission components by a fiber Bragg grating (FBG). The reflection components, including the first-order sidebands, are sent to the photodetector and fed back to the MZM2 to form an optoelectronic hybrid loop. The transmission components, including the third-order sidebands, are heterodyned by another photodetector to generate a frequency-sextupled signal. Compared with the drive signal, the generated signal has a lower PN due to the selection of the oscillation mode. In the experiment, a frequency-sextupled signal from 15 to 24 GHz is generated by a drive signal, which is tuned from 2.5 to 4 GHz. The PN of the generated signal at 24 GHz is -103.6 dBc/Hz@10 kHz; 14.6 dB PN reduction is realized.

Real-time transverse force sensing using fiber Bragg grating through direct Stokes parameters measurement

Characteristics of a fiber Bragg grating (FBG) transverse force sensor based on Stokes parameters are presented. Real-time force measurement is achieved through direct measurement of the Stokes parameters at single wavelength. A proportional relationship and linear fit are found between Stokes parameters and applied force. The sensitivity and dynamic range dependence on the state of polarization (SOP) of the incident light is investigated theoretically and experimentally. A maximum sensitivity of 0.037/N is experimentally achieved and it can be improved further by adjusting the incident SOP. This design significantly reduces system complexity and improves data processing speed, which has great practical value in real-time FBG sensing applications.

Tunable optoelectronic oscillator with an embedded delay-line oscillator for fine steps

Abstract. Tunable optoelectronic oscillator (OEO) with an embedded delay-line oscillator (EDO) for fine steps is presented. A delay-line oscillator consisting of an amplifier, phase shifter, and electrical delay line is embedded in the general single loop OEO. The oscillation frequency is controlled by the EDO loop phase, which is adjusted by the phase shifter. Without any narrow radio frequency bandpass filter, about 340 MHz tunable range with a fine step of ∼100  kHz is realized by directly tuning the direct current voltage of the phase shifter for 8 GHz oscillation. We also experimentally demonstrate κ-band tunable microwave signal generation within 22.18 to 23.32 GHz. The tunable accuracy and tunable range have a better performance compared to other OEO schemes with a fine frequency tunable step. The phase noise of the generated 22.25 GHz microwave signal is −108.7  dBc/Hz at 10 kHz offset.

Large-dynamic-range time pre-compensation scheme for fiber optic time transfer

In this paper we experimentally demonstrate a transmission delay compensation scheme for precise fiber-optic time transfer. The scheme is based on a clock counter and an electronic variable delay line, which theoretically can provide unlimited compensation range. We perform successive tests in three optical fiber links of different lengths in which both continuous drifts and abrupt hop of the transmission delay are effectively compensated. The total transmission delay variation induced in the experiments is much larger than most of the reported cases. This large-dynamic compensation scheme is quite suitable for time transfer links whose transmission delay varies a lot.

Representing expectation values of projectors as series for evolution reconstruction

We propose a Hamiltonian-independent approach for evolution reconstruction, which reconstructs the density operator of an evolving quantum system on the basis of recovering its time-varying expectation values of projectors. We represent band-limited expectation values of projectors as series, in which the coefficient of each term is a sum of an infinite set of measurement results. Both of the series for multiple measurement records and for a single record are given. We demonstrate using them to recover an expectation value and reconstruct the density operator of an evolving two-dimensional quantum system. The theoretical and simulative results prove that the two series are effective when performing a small number of measurements. Our approach is applicable to any quantum system of countable dimensions with arbitrary Hamiltonian.

Use of the Stokes Parameters of FBG for Transverse Strain Sensing and the Optimization of the Grating Structure

Abstract In this paper, the birefringence effects on fiber Bragg grating (FBG) written into standard single mode fiber under transverse load are analyzed. The evolutions of states of polarization (SOPs) of transmitted light in linear birefringence gratings are studied. We demonstrate that the first normalized Stokes parameter (s1) of the transmitted light in FBG has monotonous relationship with linear birefringence induced by transverse load and can be advantageously used for real-time transverse strain sensor, which is different from the basic sensing theory of the FBG sensors based on direct spectral analysis. The evolution of s1 of FBG in transmission with respect to the transverse load is studied experimentally, and a sensitivity of about 0.089/N is obtained. We also design a special gratings structure to reduce the effect of cross-sensitivity which is typically a limitation of most FBG sensors. The simulations show a great dependence of measurement performances with respect to the shape of transmission spectrum.

Novel Polarization Splitter Based on Highly Birefringent Dual-core Photonic Crystal Fibers with Hollow Ring Defects

Abstract A novel polarization splitter based on photonic crystal fibers (PCFs) is proposed in this paper. It is a new kind of highly birefringent dual-core PCFs which the orthogonal polarization modes with dissimilar coupling lengths is separated from each other easily. Compared to the traditional methods achieving high birefringence, the designed highly birefringent PCFs uses hollow ring defects in its one direction and then breaks the structural symmetry of PCFs. And also the design or fabrication limitations for the PCF-based polarization splitter can be partly overcome by this new type of PCFs. The transmission properties, the coupling lengths for the two orthogonal polarizations and the performance of the proposed polarization splitter are investigated respectively and analyzed numerically. The results show that it is possible to obtain a 0.50-mm long polarization splitter. Its bandwidth is about 30.6 nm or 16.0 nm as the splitting ratio is better than −10 dB or −20 dB.

Online evolution reconstruction from a single measurement record with random time intervals for quantum communication

Online reconstruction of a time-variant quantum state from the encoding/decoding results of quantum communication is addressed by developing a method of evolution reconstruction from a single measurement record with random time intervals. A time-variant two-dimensional state is reconstructed on the basis of recovering its expectation value functions of three nonorthogonal projectors from a random single measurement record, which is composed from the discarded qubits of the six-state protocol. The simulated results prove that our method is robust to typical metro quantum channels. Our work extends the Fourier-based method of evolution reconstruction from the version for a regular single measurement record with equal time intervals to a unified one, which can be applied to arbitrary single measurement records. The proposed protocol of evolution reconstruction runs concurrently with the one of quantum communication, which can facilitate the online quantum tomography.

A Quick Method of Measuring the Transmission Time of Optical Fiber

It is difficult to determine the peak of optical signal transmission time in the time transfer system based on single photon detector. In order to solve the problem, a novel scanning measurement method is proposed to quickly measure the optical signal transmission time, taking the advantages of the remainder theorem and the peak scanning means. The experimental results demonstrate that the new method can not only achieve high accuracy of less than 10 ns between the computed result and the real peak, but also save time in the peak determination.

Fiber-based multipoint dissemination of radio frequency in tree topology

In this paper, a scheme is proposed to achieve multipoint dissemination of radio frequency in fiber optic network with tree topology. The phase changes of the frequency signal are canceled at the remote end by a passive phase conjugation scheme, which makes it feasible to build a tree-topology dissemination network with high cost effectiveness and flexibility. In the proof-of-concept experiment, a 50 km point-to-point 1 GHz frequency delivery link is first established with fractional instability of 5.96×10-17 for 20000 s averaging time. Then another branch is connected to the local end by fiber spool of 38.5 km to test multipoint dissemination performance. Respective fractional instability of 7.41×10-17 and 4×10-17 at 20000 s averaging time shows that this simple and effective scheme is free of mutual interference on multipoint-accessed condition and able to construct a flexible radio frequency dissemination network.