Simultaneous Measurement of Group Refractive Index Dispersion and Thickness of Fused Silica Using a Scanning White Light Interferometer.

In this study, we simultaneously measured the group refractive index dispersion and thickness of fused silica using a scanning white light interferometer on a spectral range from 800 to 1050 nm. A delay error correction was performed using a He-Ne laser. The accuracy of the measured group refractive index dispersion of fused silica, when compared to the temperature-dependent Sellmeier equation, is within 4 × 10-5.

Thermal suppression of high-repetition rate SBS pulse compression in liquid media.

Thermal problems of high-repetition-rate stimulated Brillouin scattering (SBS) pulse compression in liquid media are theoretically and experimentally analyzed in detail. A wedge lens with less coma-aberration was designed using the ray tracing method and the thermally induced beam-pattern distortion was compensated by inhibiting thermal convection. The heat transfer form and fluid state were quantitatively analyzed for different SBS liquid media. For a 74-W pump power, 3-kHz pulse-compressed phase-conjugation mirror with an energy efficiency of 36.2% is achieved. A potential optimization method of continuously adjusting SBS output characteristics using a mixed medium is proposed and theoretically demonstrated, to improve energy efficiency.

Minimizing cross sectional pulse width difference between central and edge parts of SBS compressed beam.

For minimizing the spatial cross-sectional pulse width difference in the reflected SBS compressed beam, two new methods, blocking beam edge and parameter optimization, are proposed and compared experimentally. Results show that the sub-nanosecond compressed pulse width at the beam edge can be obtained by using both two methods in this paper. The pulse width difference between the beam center and the edge is minimized through selecting a proper medium and the optimized structural parameters in a single-cell SBS compressor. Its energy efficiency reaches up to 81.5% by using the medium HT110 and is two times higher than that of the blocking beam edge method. The compressed pulse width's stability improved greatly by using these two methods.

Rotating off-centered lens in SBS phase conjugation mirror for high-repetition-rate operation.

A new method using a rotating off-centered lens is proposed to reduce the heat accumulation at the focal spot of a stimulated Brillouin scattering phase conjugation mirror at high-repetition-rate operation. Theoretical simulation of the beam intensity pattern at the focal point indicates there is less coma-aberration using a rotating off-centered focusing lens than with a rotating wedge and a conventional lens. The resultant SBS output parameters using this new method are substantially improved comparable to those of a non-rotating conventional method for high-repetition-rate operation, while the former operates quite well for higher power and the latter operates only for lower input power. High reflected energy and a good beam pattern are demonstrated using the proposed method in the present experimental conditions of 50 mJ at 1 kHz.

Sub-nanosecond stimulated Brillouin scattering pulse compression using HT270 for kHz repetition rate operation.

With mitigation of thermal effects in a generator cell based on a rotating off-centered lens, the effects of thermal blooming, self-defocusing, and thermal convection in the amplifier cell have experimentally proven to be the main factors limiting high-repetition-rate stimulated Brillouin scattering (SBS) pulse compression. To alleviate these effects, Galden HT270, which has a large viscosity coefficient, is used and compared experimentally. The operating repetition rate using HT270 was improved from 200 Hz to 1,000 Hz, comparable to the values in the literature. With a pump energy of 50 mJ at 1,000 Hz, the pump pulse was compressed down to 820 ps using HT270 with an energy efficiency of 52.2%. If the injection energy is further increased, the SBS energy efficiency can be increased beyond this value.

High resolution three-dimensional flash LIDAR system using a polarization modulating Pockels cell and a micro-polarizer CCD camera.

An innovative flash LIDAR (light detection and ranging) system with high spatial resolution and high range precision is proposed in this paper. The proposed system consists of a polarization modulating Pockels cell (PMPC) and a micro-polarizer CCD camera (MCCD). The Pockels cell changes its polarization state with respect to time after a laser pulse is emitted from the system. The polarization state of the laser-return pulse depends on the arrival time. The MCCD measures the intensity of the returning laser pulse to calculate the polarization state, which gives the range. A spatial resolution and range precision of 0.12 mrad and 5.2 mm at 16 m were obtained, respectively, in this experiment.

Coherent beam combination using self-phase locked stimulated Brillouin scattering phase conjugate mirrors with a rotating wedge for high power laser generation.

The self-phase locking of a stimulated Brillouin scattering-phase conjugate mirror (SBS-PCM) allows a simple and scalable coherent beam combination of existing lasers. We propose a simple optical system composed of a rotating wedge and a concave mirror to overcome the power limit of the SBS-PCM. Its phase locking ability and the usefulness on the beam-combination laser are demonstrated experimentally. A four-beam combination is demonstrated using this SBS-PCM scheme. The relative phases between the beams were measured to be less than λ/24.7.

Feasibility of cascaded multi-dithering technique for coherent addition of a large number of beam elements.

The capacity of cascaded multi-dithering technique in terms of scalability is analyzed by developing equations and performing simulations whose results are found to be in agreement with the earlier experimental result of a sixteen fiber beam combination using the cascaded multi-dithering technique.

Cascaded multi-dithering theory for coherent beam combining of multiplexed beam elements.

The Cascaded Multi-Dithering theory, which allows coherent beam combining of M-by-N beam elements, is presented in this paper. The theory of Cascaded Multi-Dithering is briefly introduced, and demonstrated experimentally by combining sixteen-beams to verify its feasibility as an active phase control for scaling up the power of fiber lasers.

Precise autofocus method employing normalized fluorescence image size in a two-photon polymerization nanofabrication system.

The autofocus method has been investigated to improve the precise positioning of a substrate surface at the center of the laser focal spot in two-photon polymerization (TPP) nanofabrication. For this purpose, we developed a novel autofocus method using normalized image size, which was calculated with the second momentum radius (SMR) of two-photon induced fluorescence (TPIF). The SMR of TPIF was theoretically analyzed and experimentally compared with the average intensity of TPIF for various input laser powers. The results show that the proposed method enhanced the precision and robustness of autofocus in TPP. Specifically, the experimental creation of ascending voxel arrays demonstrated both the method's immunity to input laser power change, and a high precision of ±0.045 µm. To test the practical feasibility of the proposed autofocus method, 300 µm×260 µm single-layer honey-comb structures were successfully fabricated with precompensation and dynamic compensation using the proposed autofocus method.

Smart three-dimensional imaging LADAR using two Geiger-mode avalanche photodiodes.

In this paper, we propose a new method that is capable of obtaining a clear 3D image by the reduction of false alarms caused by noise in the stage of acquisition of raw time of flight (TOF) data. This method is implemented by intensity dividing a laser-return pulse into two Geiger-mode avalanche photodiodes (GmAPDs); an AND gate compares the arrival time of the electrical signals from the GmAPDs. Despite the fact that the energy of a laser-return pulse is decreased by half, the false alarm probability is drastically decreased because the noise distributed randomly in the time domain is filtered out. The experimental measurement is in agreement with the theoretical analysis. As a result, we can obtain a clear 3D image despite the high noise.

Autofocusing method using fluorescence detection for precise two-photon nanofabrication.

We propose a method capable of focusing a laser beam on a substrate automatically via fluorescence detection from the resin of a two-photon nanofabrication system. When two-photon absorption (TPA) occurs by focusing the laser beam in the resin, fluorescence is emitted from the focusing region in the visible range. The total pixel number above the threshold value of the fluorescence images obtained by a CCD camera is plotted on a graph in accordance with the focus position. By searching for the position when the total pixel number undergoes an abrupt change in the pre-TPA region, the correct configuration of the focused laser beam can be found. Through focusing tests conducted at four vertices of a 500 µm x 500 µm square placed arbitrarily inside SCR500 resin, the errors of the autofocusing method were found to range from -100 nm to + 200 nm. Moreover, this method does not leave any polymerized marks. To verify the usefulness of the autofocusing method, the fabrication of a pyramid structure consisting of 20 layers was attempted on a coverglass. It was completely fabricated without losing a layer.

Development and analysis of a photon-counting three-dimensional imaging laser detection and ranging (LADAR) system.

In this paper, a photon-counting three-dimensional imaging laser detection and ranging (LADAR) system that uses a Geiger-mode avalanche photodiode (GAPD) of relatively short dead time (45 ns) is described. A passively Q-switched microchip laser is used as a laser source and a compact peripheral component interconnect system, which includes a time-to-digital converter (TDC), is set up for fast signal processing. The combination of a GAPD with short dead time and a TDC with a multistop function enables the system to operate in a single-hit or a multihit mode during the acquisition of time-of-flight data. The software for the three-dimensional visualization and an algorithm for the removal of noise are developed. For the photon-counting LADAR system, we establish a theoretical model of target-detection and false-alarm probabilities in both the single-hit and multihit modes with a Poisson statistic; this model provides the prediction of the performance of the system and a technique for the acquisition of a noise image with a GAPD. Both the noise image and the three-dimensional image of a scene acquired by the photon-counting LADAR system during the day are presented.

Proportional enlargement of movement by using an optically driven multi-link system with an elastic joint.

Diverse movements using optical manipulation have been introduced. These are generally performed in the focal region of the laser beam. To achieve a wider range of movements based on precise motion transformation, an effective method for optical manipulation that overcomes the important obstacles such as small optical trapping forces, friction, and the viscosity of fluids is required. A multi-link system with an elastic joint is introduced that provides precise motion transformation and amplification. By considering the physical properties of the structure and the optical trapping force, an elastic micron-scale joint with the simple shape of a thin plate was designed. As a further example of a multi-link system with an elastic joint, a double 4-link system for motion enlargement was designed and fabricated. By performing experimental evaluations of the fabricated structures, it was confirmed that multi-link systems with an elastic joint were effective tools for precise motion transformation through optical manipulation.

Autofocus technique for three-dimensional imaging, direct-detection laser radar using Geiger-mode avalanche photodiode focal-plane array.

An autofocus technique is proposed for a three-dimensional imaging, direct-detection laser radar system that uses a Geiger-mode avalanche photodiode focal plane array (GmAPD-FPA). This technique is implemented by pointing laser pulses on a target of interest and observing its scattered photon distribution on a GmAPD-FPA. Measuring the standard deviation of the photon distribution on a GmAPD-FPA enables the best focus condition to be found. The feasibility of this technique is demonstrated experimentally by employing a 1 × 8 pixel GmAPD-FPA. It is shown that the spatial resolution improves when the GmAPD-FPA is located in the best focus position found by the autofocus technique.

Development of the 2.8 microm emission doubly shifted Raman laser using stimulated Brillouin scattering in a cascaded cavity.

A doubly shifted Raman laser using CH(4) gas has been developed for 2.8 microm generation, pumped by a Nd:YAG laser with 65.5 mJ at 17 ns. A dichroically coated meniscus-type lens is modified to utilize the backward stimulated Brillouin scattering and backward Stokes beams from a previous laser design [Appl. Opt.46, 5516-5521 (2007)APOPAI0003-693510.1364/AO.46.005516]. A maximum output energy of 4.76 mJ at 2.80 microm wavelength has been achieved in the cascaded resonator. A maximum conversion efficiency of 8.9% has been achieved at a CH(4) gas pressure of 600 psi. The obtained spatial beam profile is quite smooth, and its output pulse width is 10 ns.

A compact system for simultaneous measurement of linear and angular displacements of nano-stages.

We report on a novel compact interferometery system for measuring parasitic motions of a precision stage. It is a combination of a Michelson interferometer with an auto-collimator, of which full physical dimension is mere 70 mm x80 mm x35 mm (WxLxH) including optical components, photo-detectors, and electronic circuits. Since the beams, which measure displacement and angle, can be directed at the same position on the moving mirror, the system is applicable for testing small nano-stages where commercial interferometers are not able to be used. And thus, errors from nano-scale deformation of the moving mirror can be minimized. We find that the residual errors of linear and angular motion measurements are 2.5 nm in peak-to-peak and 0.2'', respectively.

Design and construction of an Offner spectrometer based on geometrical analysis of ring fields.

A method to obtain an aberration-corrected Offner spectrometer without ray obstruction is proposed. A new, more efficient spectrometer optics design is suggested in order to increase its spectral resolution. The derivation of a new ring equation to eliminate ray obstruction is based on geometrical analysis of the ring fields for various numerical apertures. The analytical design applying this equation was demonstrated using the optical design software Code V in order to manufacture a spectrometer working in wavelengths of 900-1700 nm. The simulation results show that the new concept offers an analytical initial design taking the least time of calculation. The simulated spectrometer exhibited a modulation transfer function over 80% at Nyquist frequency, root-mean-square spot diameters under 8.6 µm, and a spectral resolution of 3.2 nm. The final design and its realization of a high resolution Offner spectrometer was demonstrated based on the simulation result. The equation and analytical design procedure shown here can be applied to most Offner systems regardless of the wavelength range.

Improvement of range precision in laser detection and ranging system by using two Geiger mode avalanche photodiodes.

In this paper, the improvement of range precision in a laser detection and ranging (LADAR) system by using two Geiger mode avalanche photodiodes (GmAPDs) is described. The LADAR system is implemented by using two GmAPDs with a beam splitter and applying comparative process to their ends. Then, the timing circuit receives the electrical signals only if each GmAPDs generates electrical signals simultaneously. Though this system decreases the energy of a laser-return pulse scattered from the target, it is effective in reducing the range precision. The experimental results showed that the average value of standard deviation of time of flights was improved from 61 mm to 37 mm when the pulse energy is 0.6 µJ. When the time bin width is 0.5 ns, the single-shot precision error of the LADAR system was also improved from 280 mm to 67 mm.

Three-dimensionally crossing manifold micro-mixer for fast mixing in a short channel length.

In this study, we report a neo-conceptive three-dimensionally (3D) crossing manifold micromixer (CMM) embedded in microchannel. Fabricated by sequential processes of photolithography and two photon absorption stereolithography, this leads to a microfluidic system with a built-in micromixer in a site controlled manner. The effectiveness of CMM is investigated numerically and experimentally. Through the numerical simulation, it is estimated that a high mixing ratio of 90% can be obtained even in a channel length shorter than five times the channel width. This compares well with the conventional passive type of micromixers that have a gradual increase in mixing efficiency with the length of the channel. Furthermore, the mixing performance of the realized CMM built-in microchannel is observed by confocal microscopy.