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.

All-optical ultrasound probes that contain a photoacoustically-based ultrasound generator paired with a photonic acoustic sensor provide a promising imaging modality for diagnostic and MRI-compatible applications. Here, we demonstrate the fabrication of a fiber-based all-optical ultrasound probe and its applications in pulse-echo ultrasound imaging. The ultrasound generator is fabricated on a 125 μm multimode optical fiber by forming a light-absorbing multiwalled carbon nanotube (MWCNT)-polydimethylsiloxane (PDMS) composite coating on its distal end. A peak-to-peak acoustic pressure of 0.95 MPa was achieved with laser irradiation at 2.46 μJ by chemically functionalizing the fiber surface to enable a strong adsorption. Ultrasound reception was performed by a fiber-laser ultrasound sensor that translates ultrasound pressure into differential lasing-frequency changes. By linearly scanning the probe, ex vivo two- and three-dimensional imaging of a segment of swine trachea was demonstrated by detecting the echo ultrasound signals and reconstructing the acoustic scatterers. The probe presents axial and lateral resolutions at 150 and 62 μm, respectively. The small-sized, side-looking all-fiber ultrasound probe presents a promising approach for assembling an interventional endoscopy.

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.

We report on temperature compensation for beat-frequency-encoded dual-polarization fiber laser sensors based on a cleave-rotate-splice method. By cleaving the laser cavity into two segments with comparable lengths, aligning them with a rotated angle of 90°, and then fusion splicing the two halves, the temperature sensitivity in terms of beat-frequency variation can be greatly reduced from 1.99 to 0.30 MHz/°C (or by 84.9%). In contrast, the sensitivity to point loaded mass hardly changes. We also find that the beat-frequency fluctuation decreases from ±30 to ±25 kHz as a result of the temperature compensation.

A dual-frequency laser Doppler velocimeter implemented by a dual-polarization fiber grating laser is proposed, with the two laser frequencies produced by the two orthogonally polarized laser outputs of the fiber grating laser. The reflected laser outputs from a moving target experience the Doppler frequency shift, which is shown to be linearly related to the velocity and the beat note frequency difference between the laser outputs and the reflected light. With a digital frequency demodulation scheme, a high sensitivity of 0.64 MHz/(m/s) and a velocity resolution of less than 0.5% of the velocity for velocity measurement are demonstrated, which shows that the proposed laser Doppler velocimeter is capable of measurement of wide range of velocity.