Photoacoustic imaging (PAI) has been a fast-growing biomedical imaging modality in recent years. Absorbers are imaged in biological tissue by detecting laser-induced ultrasound waves via PAI. This provides hemodynamic information for the diagnosis of tumors, strokes, and other critical diseases. A typical photoacoustic microscope uses piezoelectric ultrasonic transducers to detect the photoacoustic signals. However, owing to the tradeoff between sensitivity and sensor size, building a miniaturized photoacoustic microscope with superior imaging capability is highly challenging. Therefore, the applications of PAI in wearable and endoscopic applications are limited. Our research group has developed a new optical ultrasonic sensor using a self-heterodyne fiber laser as the acoustically sensitive element. However, the laser, optical amplifier, photodetector, and signal demodulator may all cause noise and limit the detection capability. This study analyzes the noise characteristics and constructs a high-performance fiber-based photoacoustic microscope. This imaging probe can be used as a gastrointestinal endoscope for tumor screening or as a head-mounted microscope for brain imaging in a free-moving state.

We exploited a compact fiber laser as the ultrasound sensor. Ultrasound waves can deform the sensor and induce a change in the lasing frequency. To measure the acoustic response, we used both x- and y-polarized light and beat them at the photodetector to generate a radio-frequency. The variation in lasing frequency was then measured at radio frequency using modern electronics with high resolution. Here, we theoretically analyzed the noise of the fiber sensor, optical amplifier, photodetector, and signal demodulation acquisition module in the ultrasonic detection system. By measuring the noise n_0,beat signal power P_rf,and frequency noise Δf_noise with different input optical powers, we examined the dependence of the noise on the input power. Further, we implemented a photoacoustic microscope using an optical sensor for ultrasound detection and imaged the blood vessels in a biological sample. The signal-to-noise ratios (SNRs) were also measured while varying the input power of the sensing light.

First, we calculated the noise levels of the fiber laser and optical amplifier, shot and thermal noises of the photodetector, and the noise of the signal demodulation acquisition module (Fig. 2). We found that when the input optical power is less than 8.5 mW, the noise from the data-acquisition system accounts for a large proportion of the total system noise and has a main contribution to the noise; when the input optical power exceeds 8.5 mW, the noise of laser and optical amplifier dominates. We then measured the system noise n_0,beat signal power P_rf,and image frequency noise Δf_noise as the functions of input optical power (Fig 3). When the input optical power increases to more than 10 mW, the frequency noise Δf_noise approaches its minimum and the root-mean-square of Δf_noise is ~44 kHz. In photoacoustic microscopy, the optical ultrasound sensor was used to detect laser-induced ultrasound waves. We imaged a mouse ear in vivo with different input powers (Fig 5). When the input optical power is 1.7 mW, the peak-to-peak frequency noise is 185 kHz. When the input optical power increases to 15.7 mW, the noise is reduced to 110 kHz and the imaging SNR is enhanced by 4.5 dB.

This paper studies the noise characteristics of a laser-based optical ultrasound sensor. We determine the dependence of noise n₀ of the optical fiber ultrasonic sensor system and that of the beat signal frequency noise Δf_noise on the input optical power P_rf of the signal light. By increasing P_rf,the frequency noise can be considerably reduced, yielding an enhancement in the SNR. The frequency fluctuation is reduced from 185 to 110 kHz when the input power is increased to 15.7 mW. The corresponding noise equivalent pressure (NEP) is reduced from 32.9 to 19.5 Pa, and the imaging SNR is enhanced by 4.5 dB.

Optical fiber is thin, flexible, and suitable for both endoscopy and wearable instrumentation. This study demonstrates that optical fiber technology opens new possibilities to implement small high-performance photoacoustic imaging modalities. Here, we have considerably improved the sensitivity of the optical ultrasound sensor, thus providing better imaging results. With the improved sensor, we aim to implement a photoacoustic endoscope for gastrointestinal cancer diagnosis and a head-mounted photoacoustic microscope for free-state neuroimaging.