用于内窥成像的多芯光纤在弯曲条件下传输的光场相位容易出现复杂的随机扰动，为相干成像中的相位恢复带来极大挑战。本文提出了一种可以用于相干成像的螺旋多芯光纤设计，通过调控纤芯尺寸、纤芯间距和螺距来抑制弯曲等外界扰动对纤芯间群时延差和功率串扰的影响。本文建立了弯曲条件下螺旋多芯光纤纤芯光程的数学模型；根据变换光学基本原理，利用有限元仿真软件对螺旋多芯光纤的模式特性进行数值仿真计算。设计的螺旋多芯光纤具有20 μm的芯间距和20 π/m的扭转率，共有6层91个纤芯，不同层的纤芯尺寸不同。无弯曲时芯间群时延差小于6 fs/m；当弯曲半径大于5 cm时，芯间群时延差的变化小于32 fs/m，100 m长度上纤芯间串扰的仿真计算结果低于-550 dB。螺旋多芯光纤的芯间群时延差对弯曲不敏感，在相干成像中代替普通光纤束传递光场，有助于降低相干图像恢复方法的复杂度，可以广泛应用于光纤显微成像、超快激光成像等领域。

Recently, transmitting diverse signals in different cores of a multicore fiber (MCF) has greatly improved the communication capacity of a single fiber. In such an MCF-based communication system, mux/demux devices with broad bandwidth are of great significance. In this work, we design and fabricate a 19-channel mux/demux device based on femtosecond laser direct writing. The fabricated mux/demux device possesses an average insertion loss of 0.88 dB and intercore crosstalk of no more than - 29.1 dB. Moreover, the fabricated mux/demux device features a broad bandwidth across the C+L band. Such a mux/demux device enables low-loss 19-core fiber (de)multiplexing over the whole C+L band, showing a convincing potential value in wavelength-space division multiplexing applications. In addition, a 19-core fiber fan-in/fan-out system is also established based on a pair of mux/demux devices in this work.

We propose a high-sensitivity bidirectional torsion sensor using a helical seven-core fiber taper embedded in multimode fiber (MHSTM). Sensors with different taper waists and helical pitches are fabricated, and their transmission spectra are obtained and analyzed. The waist and length of the sandwiched seven-core fiber are finally determined to be 68 µm and 3 mm, respectively. The experimental results show that the clockwise and counterclockwise torsion sensitivities of the proposed sensor are 2.253 nm/(rad/m) and -1.123 nm/(rad/m), respectively. When tapered waist diameter reduces to 48 µm, a superior torsion sensitivity of 5.391 nm/(rad/m) in the range of 0–4.24 nm/(rad/m) is obtained, which is 46 times as large as the traditional helical seven-core fiber structure. In addition, the MHSTM structure is also relatively stable to temperature variations.

Multicore fiber (MCF) which contains more than one core in a single fiber cladding has attracted ever increasing attention for application in optical sensing systems owing to its unique capability of independent light transmission in multiple spatial channels. Different from the situation in standard single mode fiber (SMF), the fiber bending gives rise to tangential strain in off-center cores, and this unique feature has been employed for directional bending and shape sensing, where strain measurement is achieved by using either fiber Bragg gratings (FBGs), optical frequency-domain reflectometry (OFDR) or Brillouin distributed sensing technique. On the other hand, the parallel spatial cores enable space-division multiplexed (SDM) system configuration that allows for the multiplexing of multiple distributed sensing techniques. As a result, multi-parameter sensing or performance enhanced sensing can be achieved by using MCF. In this paper, we review the research progress in MCF based distributed fiber sensors. Brief introductions of MCF and the multiplexing/de-multiplexing methods are presented. The bending sensitivity of off-center cores is analyzed. Curvature and shape sensing, as well as various SDM distributed sensing using MCF are summarized, and the working principles of diverse MCF sensors are discussed. Finally, we present the challenges and prospects of MCF for distributed sensing applications.