In this paper, a CW multimode blue LD (L450G1, Thorlabs, Inc.) is used as the excitation source. The light beam emitting from the LD is focused onto the sample by a set of lenses to excite PA signals. Different from solid-state lasers, which are usually considered as ideal light sources, the emitter size of LD has to be considered when designing a high-resolution LD-based PAM system. For the blue LD we employed, the emitter size has a dimension of 1 × 35 μm2 (vertical × horizontal). Therefore, to achieve high lateral resolution, the horizontal dimension is demagnified to prevent an elliptical focal spot. The optical system that we used to demagnify the size of the emitter on the image plane is shown in Fig. 1a. According to geometrical optics, the size of the image can be expressed as: h′ = (f4/f3) ∙ (f2/f1) ∙ h, where fi is the focal length of the lens Li (i = 1, 2, 3, 4), and h is the size of the object. Thus, by determining the ratio of (f4/f3) and (f2/f1), the size of the object on the image plane can be demagnified accordingly, leading to high-resolution imaging with the LD-based PAM system.
Based on the above analysis, we set up a high-speed and high-resolution LD-based PAM system as shown in Fig. 1b. The laser beam emitting from the LD is first collimated by an aspheric lens (A240TM-A, f1 = 8 mm, Thorlabs, Inc.). Since the LD has different beam divergence angle (30o and 6o in the vertical and horizontal directions, respectively), the laser beam is then expanded along the horizontal direction by a pair of cylindrical lenses (GCL-110114, f2 = 25 mm, Daheng Optics; LJ1267RM-A, f3 = 250 mm, Thorlabs, Inc.). An iris (~ 8 mm in diameter) is placed after the second cylindrical lens to control the light beam to a circular shape. The laser beam is then reflected by a 1D GM before it is focused by an objective lens (LMU-20X-UVB, f4 = 9.9 mm, Thorlabs, Inc.) onto the sample for PA signals excitation, achieving rapid scanning with a line-scanning interval of 50 μm on the sample. A hybrid scanning that synchronizes 1D optical and two-dimensional mechanical scanning is applied to achieve fast imaging for the whole sample [7, 15]. The laser-induced PA signals will be detected by a focused ultrasonic transducer (V324-SU, 25 MHz central frequency, Olympus NDT, Inc.), and amplified by two amplifiers (56 dB, two ZFL-500LN-BNC+, Minicircuit, Inc.). The signals are then filtered by a low-pass filter (BLP-70+, DC-60 MHz, Minicircuit, Inc.) before being collected by a data acquisition card (ATS9350, Alazar Technologies, Inc.). Finally, the signals are processed to reconstruct LD-based PAM images on a computer display.
The details about achieving high imaging speed using 1D GM can be found in our previous work [7]. In brief, as shown in the scanning trajectory (Fig. 1c), the 1D GM repeatedly reflects the incident laser pulses to scan samples along the x-axis with a line-scanning interval of ~ 50 μm, which is smaller than the acoustic focal spot of the ultrasonic transducer to maintain high detection sensitivity. The Y motorized stage [L-509.10SD00, PI (Physik Instrumente) Singapore LLP] is synchronized with the 1D GM and moves along the y-axis when the 1D GM finishes a line scan. After the Y motorized stage travels the preset distance, the X motorized stage [L-509.10SD00, PI (Physik Instrumente) Singapore LLP] moves along the x-axis with a step size of 50 μm. The Y motorized stage and the 1D GM system are then synchronized and scan again. The scanning process will stop when the entire sample is scanned completely.
Based on the above parameters of the optical lenses, the theoretical resolution can be estimated as h′ = (f4/f3) ∙ (f2/f1) ∙ h = (9.9/250) ∙ (25/8) ∙ 35 μm ≈ 4.3 μm. To generate light pulses, the blue CW LD is driven by a commercially-available pulse driver (PCO-7121, Directed Energy, Inc.) at a repetition rate of 30 kHz. The waveform of an emitted light pulse measured with a photodiode detector (PDA10A2, Thorlabs, Inc.) is shown in Fig. 1d, showing a pulse duration of ~ 25 ns. Figure 1e shows the spectrum of the emitted light measured with an optical spectrometer (USB 2000+, Ocean Optics, Inc.). The center wavelength of the laser is ~ 439 nm, which is shorter than that in the specification datasheet (center wavelength ~ 446 nm). The spectrum in the datasheet is measured in a CW operating condition with a power of ~ 3 W, while the power of the pulse-driven LD in this paper is < 5 mW. The observed spectral shift can be accounted for the differences in LD chip temperature as power differs between the CW operating mode and the pulse-driven mode.