We present here a novel full field pump-probe photothermal dynamics microscopy (PTDM) which uses a numerical lockin mechanism for capturing full field photothermal responses and is capable of imaging 2D thermal dissipation dynamics by varying the time delay between the probing and pump nano-second pulses. PTDM may find interesting applications in biology and medicine. As one example, we report PTDM imaging nuclei contained on intact hematoxylin and eosin (H&E) stained prostate cancer specimens may potentially be used to distinguish low grade and high grade prostate cancer.
Convergence of light towards a desired location in optically diffusive and aberrative media is highly relevant to
optical methods of biomedical imaging. In this study, we demonstrated the feasibility of employing photoacoustic
signals originating from an optically absorptive target as feedback for shaping the incident wavefront to increase
optical energy density at the absorptive target. The wavefront of a collimated laser beam was shaped by an array of
two-dimensional MEMS deformable mirrors and then transmitted through optically scattering paraffin. The phase of
light reflected by each mirror was varied (0-2π) iteratively to maximize the amplitude of the photoacoustic signal.
The photoacoustic signal potentially provides a non-invasive and reliable feedback for manipulating spatial phase
distribution of light to achieve focusing in diffusive media and may facilitate optical imaging at greater depths.
Purpose: Ultrasound and optical coherence tomography (OCT) are widely used techniques for diagnostic imaging
of the eye. OCT provides excellent resolution, but limited penetration. Ultrasound provides better penetration, but an
order-of-magnitude poorer resolution than OCT. Photoacoustic imaging is relatively insensitive to scattering, and so
offers a potential means to image deeper than OCT. Furthermore, photoacoustic imaging detects optical absorption,
a parameter that is independent of that detected by conventional ultrasound or OCT. Our aim was to develop a
photoacoustic system suitable for imaging the eye.
Methods: We developed a prototype system utilizing a focused 20 MHz ultrasound probe with a central aperture
through which optics were introduced. The prototype system produced 1-μJ, 5-nsec pulses at 532 or 1064 nm with a
20-μm spot size at a 500 Hz repetition rate. The photoacoustic probe was mounted onto computer-controlled linear
stages and pulse-echo ultrasound and photoacoustic images obtained on ex vivo pig eyes and in vivo mouse eyes.
Results: Lateral resolution was significantly improved by use of a laser spot size much smaller than the acoustic
beamwidth. Photoacoustic signals were obtained primarily from melanin in ex vivo tissues and from melanin and
hemoglobin in vivo. Image fusion allowed superposition of photoacoustic signals upon the anatomic features
detected by conventional ultrasound.
Conclusion: Photoacoustic imaging detects the presence of clinically relevant pigments, such as melanin and oxyand
deoxy-hemoglobin, and, potentially, from other pathologic pigments occurring in disease conditions (tumors,
nevii, macular degeneration). Fine-resolution photoacoustic data provides information not detected in current
ophthalmic imaging modalities.
In this paper, we discussed different mechanisms which are proved to ensure phase locking in a laser array. We also reported the phase locking in solid-state (crystal) laser array and fiber laser array by using the self-imaging confocal resonator which provides high feedback efficiency, quite insensitive to power variations among the pump beams, simply modal profile and also can achieve phase locking passively. A passive approach is to utilize the process of self-adjustment in lasing frequency to adapt to changes in the optical path lengths. The phase-locked mode is highly stable despite the phase variations in the individual elements caused by thermal and mechanical effects.
This paper presents results of a program to develop compact, independently-addressable, pulsed infrared laser arrays useful for applications such as laser topographic mapping. Our approach is based on combining the technology of high-power diode lasers with the recent development of self-Q-switching via saturable absorption in co-doped solid-state laser crystals. Proof-of-concept was demonstrated by the design, construction, and characterization of an independently-addressable, three-element monolithic diode laser array end-pumping a single Cr,Nd:YAG solid-state crystal. Pulse durations of approximately 5 ns at 1.06 micrometer and pulse energies of 8 microjoules at repetition rates up to 400 Hz were obtained with individual pulses exhibiting both excellent repeatability and outstanding spatial mode characteristics.
AlGaInP-based, high power laser diodes operating at wavelengths of 630 to 645 nm have been designed, fabricated, and characterized. Cw output powers approaching 1 Watt and thresholds below 400 A/cm2 have been achieved. Measurement of internal laser parameters indicates low internal loss and transparency current, high gain, and moderate internal quantum efficiency. Characteristic temperature data suggest that the lowest practical operating wavelength of similar diodes is close to 620 nm. Examination of performance as a function of cavity length indicates an optimum length in the range of 1200 micrometers .
The dynamics of a two-element nonlinear-coupled laser array are briefly reviewed. It is pointed out that if the pump of the laser array reduces to zero for a time period shorter than the population decay time, then the system reduces to two coupled equations for the relative intensity and relative phase. It is well-known from the Poincare-Bendixson theorem that such an autonomous system of dimension two (without detuning) cannot exhibit chaos. By creating two evanescent-coupled lasers in a Nd:YAG etalon using diode end-pumping, the coupling strength between the two laser elements in the array can be continuously varied by adjusting the position of the pumping beams. This allows the observation of the phase locking process over a wide range of coupling strength. We have found that the locking is as fast as the on-set of lasing without undergoing a transition state as along as the coupling is strong enough to ensure such locking. The instantaneous locking is also independent of the coupling strength once the coupling is strong enough to ensure phase locking. New interpretation of phase locking process for a phase locked laser array is provided.
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