In this study, a beat signal recalibration method was developed for optical coherence tomography (OCT) with a high-speed wavelength-swept source. By adopting a voltage-controlled oscillator (VCO) modulated by a sinusoidal waveform as a trigger for the OCT system, the broadening of the beat frequency due to laser tuning rate variations was recalibrated. For this study, OCT based on a Fourier domain mode-locked (FDML) laser at a sweep rate of 60.9 kHz was demonstrated. OCT images of 1500×409 pixels were obtained with the sensitivity of 100 dB. Temporal frequency variations in the FDML laser and OCT images obtained with the proposed technique were also described. When compared to a conventional recalibration method using optical components, swept source optical coherence tomography operated with a VCO clock exhibited superior performance and high stability. From the experimental results, it was demonstrated that the proposed method is sufficient to recalibrate the time-frequency variations in interferometry with a high-speed wavelength-swept source.
We report on a high-speed time-domain optical coherence tomography (TD-OCT) technology that is capable of imaging 10 fps with 2000 A-scan. A duplex scanning optical delay line that is driven by a 1-kHz sinusoidal wave function was implemented and characterized. The reference and sample arms in a Michelson interferometer were simultaneously scanned using two identical piezoelectric transducers (PZTs). We adopted a sinusoidal wave function with constant voltages as the input functions for the PZTs. A recalibration of the measurement depth was necessary for the sinusoidal wave function operation. A recalibration with a spatial transformation was performed on the A-scan. We achieved an A-scan speed of 6 m/s. To the best our knowledge, this is the highest speed achieved in conventional TD-OCT systems to date.
A high power flashlamp-pumped Nd:YAG laser at 1123 nm with a repetition rate of 40 Hz is investigated. Simple but specially coated resonator mirrors are adopted to suppress the parasitic oscillation at gain transition lines higher than 1123 nm. A maximum average output power of 40 W with an electrical pumping energy of 45.7 J and pumping pulse duration of 150 µs is achieved. To the best of our knowledge, the output power of 40 W at a wavelength of 1123 nm is the highest record up to now.
In this study, a duplex scanning optical delay line is implemented and characterized for optical coherence tomography. The reference and sample arms in Michelson interferometer are simultaneously scanned by use of two identical piezoelectric transducers (PZT). It is obtained that scanning range, thermal drift, repeatability, and axial scanning velocity of duplex optical scanning delay line are distinctly superior to those of single scanning optical delay line.
The Nd3+, Cr4+ co-doped GGG epitaxial thin films for self Q-switched waveguide laser has been fabricated by a two-target pulsed laser deposition (PLD) method. The concentrations of Nd and Cr ion in the co-doped GGG thin films are well controlled by changing respective KrF laser ablation fluence and repetition rate for Nd:GGG and Cr,Ca:GGG sintered targets. The structure of Nd,Cr:GGG thin films on YAG substrate shows a planar waveguide structure with high numerical aperture. It is confirmed that Nd3+ and Cr4+ ions are optically active as laser active ions and saturable absorbers respectively at 1.06 micrometer to be used for monolithic self-Q-switched waveguide laser to generate high peak power and short pulse output.
The new material processing characteristic of aluminum nitride (AlN) ceramic is compared with microsecond, nanosecond and femtosecond laser ablation. The conventional laser material processing technology with longer pulsewidth laser such as TEA CO2 laser, Q-switched YAG laser, and excimer lasers leads to the thermal shock or lateral damage on target material, and those thermal effect causes the surface modification of AlN ceramic target. The comparative study of the laser ablation with microsecond TEA CO2 laser pulse, nanosecond KrF excimer laser pulse, and femtosecond Ti:sapphire laser pulse is performed in time domain. Using intense ultrashort titanium sapphire laser two-photon laser ablation of TiO2 photo-catalyst was also investigated experimentally and theoretically aiming at the enhancement of photo-catalyst reaction. The black-surfacing of the TiO2 photo- catalyst crystal was successfully achieved by drilling a large number of conical micro-holes with two-photon laser ablation. The ablated surface has a roughness of sub micrometer order, and no heat-affected zone was observed. The simple equation is developed to explain two-photon ablation process of the TiO2 photo- catalyst and the dependence of the ablation characteristic on the pulsewidth.
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