Light propagation in random media is a subject of interest to the optics community at large, with applications ranging from imaging to communication and sensing. However, real-time characterization of wavefront distortion in random media remains a major challenge. Compounding the difficulties, for many applications such as imaging (e.g., endoscopy) and focusing through random media, we only have single-ended access. In this work, we propose to represent wavefronts as superpositions of spatial modes. Within this framework, random media can be represented as a coupled multimode transmission channel. Once the distributed coherent transfer matrix of the channel is characterized, wavefront distortions along the path can be obtained. Fortunately, backreflections almost always accompany mode coupling and wavefront distortions. Therefore, we further propose to utilize backreflections to perform single-ended characterization of the coherent transfer matrix. We first develop the general framework for single-ended characterization of the coherent transfer matrix of coupled multimode transmission channels. Then, we apply this framework to the case of a two-mode channel, a single-mode fiber, which supports two randomly coupled polarization modes, to provide a proof-of-concept demonstration. Furthermore, as one of the main applications of coherent channel estimation, a polarization imaging system through single-mode fibers is implemented. We envision that the proposed method can be applied to both guided and free-space channels with a multitude of applications.

Mode purity measurement is crucial for various applications utilizing few-mode fibers and related devices. In this paper, we propose a simple and accurate method for measuring the mode purity of the output optical field in few-mode ring-core fibers (RCFs). Mode purity can be calculated solely from the outgoing intensity distribution with high precision. This method is theoretically capable of measuring the mode purity of RCFs that support orbital angular momentum modes with an infinite number of azimuthal orders and has strong applicability to various RCF types and image qualities simultaneously. We demonstrate our approach numerically and verify it experimentally in a few-mode RCF supporting four (five) mode groups at 1550 (1310) nm. A polarization test method is proposed to verify its accuracy. We believe that this straightforward and cost-effective characterization method for RCFs and RCF-based devices can promote the development of mode-division multiplexing technology and its applications.

We propose an encryption technique for underwater visible light communication (UVLC) based on chaotic phase scrambling (PS) and conjugate frequency hopping (CFH). The technique is experimentally tested using an 8-level pulse amplitude modulation (PAM-8) and a 1.2 m underwater link. The security key of the phase scrambling code is generated according to a logistic map, and the frequency hopping is achieved by adding the same zero frequency points to the signal spectrum. The maximum transmission rate of 2.1 Gbit/s is measured with bit-error-rate (BER) below 7% the hard-decision forward error correction (HD-FEC) threshold of 3.8×10-3.

Optical fiber distributed acoustic sensing (DAS) based on phase-sensitive optical time domain reflectometry (φ-OTDR) is in great demand in many long-distance application fields, such as railway and pipeline safety monitoring. However, the DAS measurement distance is limited by the transmission loss of optical fiber and ultralow backscattering power. In this paper, a DAS system based on multispan relay amplification is proposed, where the bidirectional erbium-doped fiber amplifier (EDFA) is designed as a relay module to amplify both the probe light and the backscattering light. In the theoretical noise model, the parameters of our system are carefully analyzed and optimized for a longer sensing distance, including the extinction ratio (ER), span number, span length, and gain of erbium-doped fiber amplifiers. The numerical simulation shows that a bidirectional EDFA relay DAS system can detect signals over 2500 km, as long as the span number is set to be more than 100. To verify the effectiveness of the scheme, a six-span coherent-detection-based DAS system with an optimal design was established, where the cascaded acoustic-optic modulators (AOMs) were used for a high ER of 104 dB. The results demonstrate that the signal at the far end of 300.2 km can be detected and recovered, achieving a high signal-to-noise ratio of 59.6 dB and a high strain resolution of 51.8pε/Hz at 50 Hz with a 20 m spatial resolution. This is, to the best of our knowledge, a superior DAS sensing distance with such a high strain resolution.

Optical chaos communication and key distribution have been extensively demonstrated with high-speed advantage but only within the metropolitan-area network range of which the transmission distance is restricted to around 300 km. For secure-transmission requirement of the backbone fiber link, the critical threshold is to realize long-reach chaos synchronization. Here, we propose and demonstrate a scheme of long-reach chaos synchronization using fiber relay transmission with hybrid amplification of an erbium-doped fiber amplifier (EDFA) and a distributed fiber Raman amplifier (DFRA). Experiments and simulations show that the hybrid amplification extends the chaos-fidelity transmission distance thanks to that the low-noise DFRA suppresses the amplified spontaneous emission noise and self-phase modulation. Optimizations of the hybrid-relay conditions are studied, including launching power, gain ratio of DFRA to EDFA, single-span fiber length, and number of fiber span. A 1040-km chaos synchronization with a synchronization coefficient beyond 0.90 is experimentally achieved, which underlies the backbone network-oriented optical chaos communication and key distribution.

Vector bending sensing has been consistently growing in many fields. A low-cost and high sensitivity vector bending sensor based on a chirped long-period fiber grating (LPFG) with an off-axis micro helix taper is proposed and experimentally demonstrated. The grating is composed of several sections of single-mode fiber with gradually larger lengths, and the off-axis micro helix tapers with fixed lengths when they are fabricated by using the arc discharge technology. The large refractive index modulation in the micro-helix taper greatly reduces the sensor size. The total length of the sensor is only 4.67 mm. The micro-helix taper-based LPFG can identify the bending direction due to the asymmetric structure introduced by the micro helix. The experimental results show that the transmission spectra of the sensor have distinct responses for different bending directions, and the maximum bending sensitivity is 14.08 nm/m^-1 in the range from 0.128 m^-1 to 1.28 m^-1. The proposed bending sensor possesses pronounced advantages, such as high sensitivity, small size, low cost, and orientation identification, and offers a very promising method for bend measurement.

An optical scrambler using a whispering-gallery-mode (WGM) micro-bottle cavity to scramble a complex optical signal to generate an uncorrelated output is proposed. We experimentally demonstrated this micro-cavity scrambler by using chaotic laser light as the incident signal and studied the influence of the coupling state. Experiments achieved full scrambling with a low cross correlation of 0.028 between the output and the input. Results indicate that the scrambling effect originates from the interference among numerous WGMs in the bottle cavity. It is believed that the micro-bottle cavity with an efficient scrambling function can become a promising candidate for encryption.

With energy–time entangled biphoton sources as the optical carrier and time-correlated single-photon detection for high-speed radio frequency (RF) signal recovery, the method of quantum microwave photonics (QMWP) has presented the unprecedented potential of nonlocal RF signal encoding and efficient RF signal distilling from the dispersion interference associated with ultrashort pulse carriers. In this paper, its capability in microwave signal processing and prospective superiority are further demonstrated. Both QMWP RF phase shifting and transversal filtering functionality, which are the fundamental building blocks of microwave signal processing, are realized. Besides good immunity to the dispersion-induced frequency fading effect associated with the broadband carrier in classical MWP, a native two-dimensional parallel microwave signal processor is provided. These results well demonstrate the superiority of QMWP over classical MWP and open the door to new application fields of MWP involving encrypted processing.

In this Letter, the optical amplification characteristics of the home-made Bi/P co-doped silica fiber were systematically explored in the range of 1270–1360 nm. The maximum gain of 24.6 dB was obtained in the single-pass amplification device, and then improved to 38.3 dB in the double-pass amplification device for -30dBm signal power. In addition, we simultaneously investigated the laser performance of the fiber with the linear cavity. A slope efficiency of 16.4% at ∼1313nm was obtained with a maximum output power of about 133 mW under the input pump power of 869 mW at 1240 nm. As far as we know, it is the first laser reported based on the bismuth-doped fiber in China.

In the fields of light manipulation and localization, quasiperiodic photonic crystals, or photonic quasicrystals (PQs), are causing an upsurge in research because of their rotational symmetry and long-range orientation of transverse lattice arrays, as they lack translational symmetry. It allows for the optimization of well-established light propagation properties and has introduced new guiding features. Therefore, as a class, quasiperiodic photonic crystal fibers, or photonic quasicrystal fibers (PQFs), are considered to add flexibility and richness to the optical properties of fibers and are expected to offer significant potential applications to optical fiber fields. In this review, the fundamental concept, working mechanisms, and invention history of PQFs are explained. Recent progress in optical property improvement and its novel applications in fields such as dispersion control, polarization-maintenance, supercontinuum generation, orbital angular momentum transmission, plasmon-based sensors and filters, and high nonlinearity and topological mode transmission, are then reviewed in detail. Bandgap-type air-guiding PQFs supporting low attenuation propagation and regulation of photonic density states of quasiperiodic cladding and in which light guidance is achieved by coherent Bragg scattering are also summarized. Finally, current challenges encountered in the guiding mechanisms and practical preparation techniques, as well as the prospects and research trends of PQFs, are also presented.