Yizhi Liang

Type IIa Bragg grating based ultra-short DBR fiber laser with high temperature resistance.

We report on the fabrication of a thermally resistant ultra-short distributed Bragg reflector (DBR) fiber laser based on the photo inscription of two wavelength-matched type IIa gratings in a thin-core Er-doped fiber. With continuous UV exposure, each Bragg reflector initially grows as a type I grating, followed by decay in strength, and then re-grows as a type IIa grating with enhanced thermal resistance. The DBR laser, with an entire length of 13 mm, can stably operate at 600°C with single longitude mode, which provides potential applications in high temperature environments.

Stabilization of microwave signal generated by a dual-polarization DBR fiber laser via optical feedback.

Microwave signals can be generated by beating the two orthogonal polarization modes from a dual-frequency fiber grating laser. In this paper, we present that the phase noise of the microwave signal can be significantly reduced via optical feedback by cascading an external cavity. This is achieved as a result of the bandwidth narrowing of each polarization laser mode when introducing phase-matched feedbacks into the laser cavity. By optimizing the external cavity length and the feedback ratio, the noise level over low frequencies has been reduced by up to 30 dB, from -42 to -72 dBc/Hz at 1 kHz, and from -72 to -102 dBc/Hz at 10 kHz. Meanwhile the relaxation resonant peaks can be eliminated. Compared with the existing techniques, the present method can offer a cost-effective, low-noise microwave signal, without the requirement for complex electrical feedback system.

Detection of an extremely small mass with a dual-polarization fiber grating laser

We demonstrate the ability of a fiber grating laser with dual-polarization, single-longitudinal-mode output to measure an extremely small mass (or transverse load). The minimum detectable mass is 0.28 milligram by reducing the noise level of the output beat signal.

Spatial Sensitivity Characterization of Dual-Polarization Fiber Grating Laser Sensors

In this paper, the spatial sensitivity of a dual-polarization fiber grating laser sensor is characterized by measuring the response in terms of beat-frequency change to localized transverse loading at different longitudinal positions. The induced frequency shift is proportional to the induced local birefringence change and the normalized light intensity, as a result of the resonant nature of the cavity. Measured result suggests that the sensitivity profile is mainly determined by the cavity length, as well as the grating reflectivities and the gain. We found that higher mass (or force) sensitivity can be obtained by shortening the laser cavities. Sensitivity as high as 51.1 MHz/g (or 5.192 GHz/N) has been achieved with a laser which has an effective cavity length close to 1 mm. The corresponding mass resolution reaches

Effect of Pump Light Polarization and Beat Note Stabilization for Dual-Polarization Fiber Grating Laser Sensors

9.78 \times 10^{-5}

Fiber-Laser-Based Ultrasound Sensor for Photoacoustic Imaging.

g. The result offers useful guidance for the transducer design for fiber laser sensors toward the detection of extremely small mass or weak perturbations.

Highly sensitive fiber laser ultrasound hydrophones for sensing and imaging applications

Dual-polarization fiber grating lasers have been exploited as high-sensitivity photonic sensors for a variety of measurands by monitoring the frequency of the output beat signal. In this paper, we demonstrate that the environmentally induced perturbations on polarization orientation of the pump light can cause considerable changes in output beat frequency, which can significantly affect the resolution of the sensor especially for static or quasi-static measurements. We found that the beat frequency change with the pump orientation is mainly caused by the resonantly enhanced nonlinearity associated with the generation of green fluorescent light, rather than thermal effect or Kerr electro-optic effect. In order to obtain stable beat signal and improve the accuracy of the fiber laser sensor, the fiber lasers are annealed in advance and then connected to polarization-maintaining fibers and devices to avoid the influence of external perturbations. The frequency fluctuation has been reduced from 1.5 to about 0.1 MHz, and the corresponding resolution of the sensor to static transverse load is improved from 1.76 × 10 -4 to 1.17 × 10 -5 N/mm.

A 16-Element Multiplexed Heterodyning Fiber Grating Laser Sensor Array

Photoacoustic imaging, especially for intravascular and endoscopic applications, requires ultrasound probes with miniature size and high sensitivity. In this paper, we present a new photoacoustic sensor based on a small-sized fiber laser. Incident ultrasound waves exert pressures on the optical fiber laser and induce harmonic vibrations of the fiber, which is detected by frequency shift of the beating signal between the two orthogonal polarization modes in the fiber laser. This ultrasound sensor presents a noise equivalent pressure of tens of Pa over a ∼50 MHz bandwidth. We demonstrate this new ultrasound sensor on an optical-resolution photoacoustic microscope. The axial and lateral resolutions are measured. The field of view is on the order of 1 millimeter in lateral directions. We further demonstrate its application in photoacoustic microscopy. The present fiber laser ultrasound sensor offers a new tool for all-optical photoacoustic imaging.

2 MHz multi-wavelength pulsed laser for functional photoacoustic microscopy

We have developed highly sensitive photonic ultrasound hydrophones based on polymer-packaged dual-polarization-mode fiber lasers. The incident ultrasound wave is scattered by the polymer cylinder due to the difference in elastic property. The scattered wave can drive harmonic vibration of the cylinder and result in optical response in terms of beat-frequency variation of the laser output. Experimental results exhibit a broadband ultrasound response at frequencies below 1 MHz. The individual vibration modes are excited by the ultrasound waves with different efficiencies, yielding a frequency-dependent response. A hydrophone with a diameter of 5 mm presents a detection limit of 2  mPa/Hz1/2 at 200 kHz. We further demonstrate its capability of ultrasound imaging for applications in underwater acoustics and sonar systems.

Acoustic and Ultrasonic Detection With Radio-Frequency Encoded Fiber Laser Sensors

In this paper, we demonstrate the implementation of a 16-element array of heterodyning fiber grating laser sensors cascaded in a single optical fiber. Each laser consists of a Bragg grating pair inscribed in a short section of Er-doped fiber. The laser offers a RF-domain beat signal between orthogonally polarization modes and can act as a photonic sensor for the measurement of external perturbations, by monitoring the beat frequency. The laser sensors are wavelength multiplexed by inscribing fiber gratings with different pitches, and frequency multiplexed in RF domain by controlling the intracavity birefringence via CO2 laser side irradiation. The pump power depletion as a result of mode mismatch between the active and passive fibers is a main limitation factor for the multiplexing scale. We can multiplex ten laser sensors at most with a single pump laser with an output power of 130 mW and six more sensors can be multiplexed with an additional pump laser at the other end. The measurement ability of an individual laser sensor can be affected when multiplexed in array, in terms of abrupt frequency jumps and the raised noise level. As a result, the force resolution at 1 kHz is changed from 3 to 6 nN/√Hz, and the minimal detectable static force is changed from 68 to 120 μN. This is attributed to the off-resonance grating couplings and the reflections at the splicing points. This effect can be weakened by selecting mode-matching fibers and optimizing grating profiles, or simply using additional optical isolators.