Photoacoustic imaging (PAI) is a noninvasive biomedical imaging technology capable of multiscale imaging of biological samples from organs down to cells. Multiscale PAI requires different ultrasound transducers that are flat or focused because the current widely-used piezoelectric transducers are rigid and lack the flexibility to tune their spatial ultrasound responses. Inspired by the rapidly-developing flexible photonics, we exploited the inherent flexibility and low-loss features of optical fibers to develop a flexible fiber-laser ultrasound sensor (FUS) for multiscale PAI. By simply bending the fiber laser from straight to curved geometry, the spatial ultrasound response of the FUS can be tuned for both wide-view optical-resolution photoacoustic microscopy at optical diffraction-limited depth (~1 mm) and photoacoustic computed tomography at optical dissipation-limited depth of several centimeters. A radio-frequency demodulation was employed to get the readout of the beat frequency variation of two orthogonal polarization modes in the FUS output, which ensures low-noise and stable ultrasound detection. Compared to traditional piezoelectrical transducers with fixed ultrasound responses once manufactured, the flexible FUS provides the freedom to design multiscale PAI modalities including wearable microscope, intravascular endoscopy, and portable tomography system, which is attractive to fundamental biological/medical studies and clinical applications.

提出并实现了一种基于色散调谐主动锁模光纤激光器技术的多普勒频移测量方法，该方法利用大色散谐振腔主动锁模激光器的输出波长与调制频率呈线性这一特性，通过测量锁模激光器输出波长来标定输入系统的微波频率，进而获得多普勒频移.通过模拟仿真，分析了激光器腔长、色散量以及锁模阶数等参量对系统频率测量特性的影响，在理论指导下，针对不同应用场景设计了分别基于长光纤及线性啁啾布拉格光纤光栅的大色散谐振腔结构，实现了载波中心频率分别为12.2 MHz、265 MHz、682 MHz和1.118 7 GHz情况下的频率偏移测量.所提出的测量方法具有结构简单、设计灵活的优点，在多普勒频移测量方面具有应用价值.

Interaction of acoustic waves and microbubbles occurs in numerous biomedical applications including ultrasound imaging, drug delivery, lithotripsy treatment, and cell manipulation, wherein the acoustically driven microbubbles routinely act as active microscale oscillators or actuators. In contrast, microbubbles were utilized here as passive receivers to detect broadband ultrasound waves in aqueous environments. The microbubble was photothermally generated on a microstructured optical fiber (MOF) tip, forming a flexible Fabry–Pérot cavity whose gas–water interface was sensitive to ultrasound waves. The MOF severed as both a low-loss waveguide and a compact light condenser, allowing high-efficiency generation and stabilization of ultrasmall microbubbles. Integrated with all-fiber interferometry, a 10 μm diameter microbubble exhibited a low noise-equivalent pressure level of ～3.4mPa/Hz1/2 and a broad bandwidth of ～0.8MHz, capable of detecting weak ultrasounds emitted from red blood cells irradiated by pulsed laser light. With advantages of high sensitivity, compact size, and low cost, the microbubble-based ultrasound sensor has great potential in biomedical imaging and sensing applications.

A wavelength-swept fiber laser is proposed and successfully demonstrated based on a bidirectional used linear chirped fiber Bragg grating (LC-FBG). The wavelength-swept operation principle is based on intracavity pulse stretching and compression. The LC-FBG can introduce equivalent positive and negative dispersion simultaneously, which enables a perfect dispersion matching to obtain wide-bandwidth mode-locking. Experimental results demonstrate a wavelength-swept fiber laser that exhibits a sweep rate of about 5.4 MHz over a 2.1 nm range at a center wavelength of 1550 nm. It has the advantages of simple configuration and perfect dispersion matching in the laser cavity.

Birefringence is critical in dual-polarization fiber-laser-based fiber-optic sensing systems, as it directly determines the beat frequency between the two polarizations. A study of pump induced birefringence in dual-polarization fiber lasers is presented here, which shows that the pump induced birefringence is a result of the interplay among pump induced refractive index change, laser dynamics, and anisotropy inside fiber lasers. For erbium-doped fiber lasers, pumping at 1480 nm is better than pumping at 980 nm in lower pump induced birefringence. Moreover, injection at 532 nm for an adequately long enough time can permanently reduce anisotropy and, hence, reduce pump induced birefringence.

Photothermal/photoacoustic (PT/PA) spectroscopy provides useful knowledge about optical absorption, as well as the thermal and acoustical properties of a liquid sample. For microfluidic biosensing and bioanalysis where an extremely small volume of liquid sample is encapsulated, simultaneous PT/PA detection remains a challenge. In this work, we present a new optofluidic device based on a liquid-core optical ring resonator (LCORR) for the investigation of PT and PA effects in fluid samples. A focused 532 nm pulsed light optically heats the absorptive fluid in a capillary to locally create a transient temperature rise, as well as acoustic waves. A 1550 nm CW laser light is quadrature-locked to detect the resonance spectrum shift of the LCORR and study thermal diffusion and acoustic wave propagation in the capillary. This modality provides an optofluidic investigative platform for biological/biochemical sensing and spectroscopy.

We present a theoretical analysis and an experimental study of the impacts of external optical feedback on dual-frequency fiber lasers. The external optical feedback can effectively suppress the phase noise of the beat notes of dual-frequency fiber lasers, provided some requirements are satisfied. The polarization of the optical feedback is important for the fiber laser’s stability, and it can also tune the beat note frequency. A side effect of external optical feedback, as demonstrated in the experiments, is lowering the sensitivity of the dual-frequency fiber laser-based sensors, although such degradation is not obvious.