The nematode C. elegans, a millimeter-long roundworm, is a well-established model organism for studies of neural development and behavior, however physiological methods to manipulate and monitor the activity of its neural network have lagged behind the development of powerful methods in genetics and molecular biology. The small size and transparency of C. elegans make the worm an ideal test-bed for the development of physiological methods derived from optics and microscopy. We present the development and application of a new physiological tool: femtosecond laser dissection, which allows us to selectively ablate segments of individual neural fibers within live C. elegans. Femtosecond laser dissection provides a scalpel with submicrometer resolution, and we discuss its application in studies of neural growth, regenerative growth, and the neural basis of behavior.

Femtosecond laser pulses in the near-infrared region have potential applications in nanosurgery in cell biology. Femtosecond laser pulses can be used to selectively disrupt and dissect intracellular organelles in living cells. We report on disruption of fluorescence-labeled nucleus and mitochondria in living HeLa cells using a femtosecond Ti:sapphire laser oscillator with a repetition rate of 76 MHz. We examined the effectiveness of the restaining method in combination with fluorescence recovery after photobleaching (FRAP) analysis to discern disruption or bleaching.

Micro-patterning of proteins has been attracted much attention as a potential technique to realize bio-microdevice. In this work, as a new method to realize non-destructive micro-patterning of proteins, laser transfer printing for a um-sized protein crystal was developed by utilizing focused femtosecond laser. The micro-patterning was performed to transfer the protein crystal which was adhered on a source substrate to a target substrate which was underlaid on the source substrate. An 800-nm femtosecond laser was focused in a water between the source and target substrates on an inverted microscope with a 100x objective lens. When the laser focal point was scanned at the position with distance of a few um far from the source substrate, the protein crystals were detached by a shockwave and cavitation bubble generation at the circumstance of the focal point and transferred to the target substrate forming a line pattern. The line width of the protein crystal was a few tens um with the scanning speed of 90 μm/sec. Furthermore, multi-patterning of several kinds of protein crystals was realized by this method. The pattering resolution is comparable or better than that by another multi-material transfer printing, such as ink jet printing, micro-printing, and laser direct writing.

Recently ultrashort laser pulses became most important for micro structuring and biomedical applications such as refractive surgery. Ultrashort laser pulses tightly focused to a small spot easily provide intensity sufficient to induce nonlinear ionization. A plasma is generated and heated in the focus resulting in optical breakdown. The energy deposited in the plasma and the mechanical effects subsequent to optical breakdown are utilized by modern applications of ultrashort laser pulses to induce controlled highly reproducible material alteration. A model including both nonlinear pulse propagation and plasma generation is introduced to numerically investigate the interaction of ultrashort laser pulses with the self-induced plasma in the vicinity of the focus. The numerical code is based on a (3+1)-dimensional nonlinear Schroedinger equation describing the pulse propagation. A multi rate equation model recently published by B. Rethfeld is used to simultaneously calculate the generation of free electrons. It is the first numerically simple approach to describe nonlinear ionization that allows a non static energy distribution of free electrons in the conduction band. The code is applicable to any transparent Kerr medium, whose linear and nonlinear optical parameters are known. Numerical calculations based on this model are used to understand the dependence between the size, the geometry and the free electron density of ultrashort laser pulse induced optical breakdown plasmas in various focusing geometries. The code enables to use arbitrary initial conditions for the laser field in the focus. More realistic focusing scenarios than the simple assumption of focused gaussian beams can be taken into account. Nonlinear side effects, such as streak formation occurring in addition to optical breakdown during ultrashort pulse refractive eye surgeries can be numerically investigated.

The recently developed technique of ultrafast third harmonic generation (THG) micro-spectroscopy is discussed. The approach is easily adapted to a standard laser scanning microscope and allows for two and three photon resonances to be identified in non-fluorescent unlabeled samples. This work provides nonlinear microscopists with a tool for further understanding the contrast and damage mechanisms they will encounter under nonlinear excitation. Here, we use THG micro-spectroscopy to investigate the nonlinear optical properties of hemoglobin over the spectral range of 770 -1000 nm with 100-fs duration, ~1-nJ energy laser pulses. We demonstrate the ability of this approach to distinguish different ligand binding states in physiological solutions of human hemoglobin.

We review the state of the art of ultrashort laser pulse characterization techniques. Two main methods will be mentioned: frequency-resolved optical gating (FROG) and spectral-phase interferometry for direct electric-field reconstruction (SPIDER). Basics of the techniques are introduced, and a comparison will be made on the pros and cons of both methods. We will then present some recent developments in the field of pulse characterization, including the development of an ultracompact and robust pulse characterization device-GRENOUILLE, the extension of pulse-measurement techniques into both time and space, and the measurement of extremely complex and extremely weak pulses.

Spectral Interferometry for Direct Electric Field Reconstruction (SPIDER) is one of several methods for characterizing ultrashort optical pulses. SPIDER allows for the measure of the pulse duration, but also allows the extraction of the spectral phase from a femtosecond pulse. Advances in femtosecond laser development in recent years has led to commercially available femtosecond pulsed laser systems with pulse lengths of less than 10~fs. New lasing materials and advances in fiber laser technology has allowed manufacturers to produce femtosecond pulsed lasers that operate at wavelengths outside the traditional 800~nm range of Ti:sapphire. The need for commercially available pulse characterization instruments is clear. SPIDER based spectral phase measurement systems have been adapted to facilitate the advances in femtosecond laser technology.

A number of nonlinear imaging modalities, such as two-photon excitation and second harmonic generation, have gained popularity during the last decade. These, and related methods, have in common the use of a femtosecond laser in the near infrared, with the short pulse duration making the nonlinear excitation highly efficient. Efforts toward the use of pulses with pulse duration at or below 10 fs, however, have been a great challenge, in part due to the fact that shorter pulses have been found to cause greater sample damage. Here we provide a brief review of the MIIPS method for correction of phase distortions introduced by high numerical aperture objectives and the introduction of simple phase functions capable of preventing three-photon induced damage, reducing autofluorescence, and providing selective probe excitation.

I demonstrate the closed-loop control of a programmable liquid-crystal spatial light modulator using real-time SHG FROG. Control theory states that proportional only controllers are almost never suitable for very tight control because any calibration errors can result in offsets between the input (desired result) and output. In this work, the pulse output from the real-time FROG software is sent to proportional-integral controller for maintaining the spectral phase of a pulse. The additional integral control term removes the constant offset that plagues proportional only controllers. The response time of pulse shaping system is a few seconds, limited only by the speed of the pulse shaper.

Ultrashort laser pulses are usually expressed in terms of the temporal and spectral dependences of their electric field. This approach disregards any couplings between the spatial coordinates and time and/or frequency. This assumption, however, often fails, as the generation and manipulation of ultrashort pulses require the introduction of spatio-temporal couplings. Furthermore, disregarding these couplings in ultrashort pulses also greatly limits the potential applications that could only be possible by exploiting the spatio-temporal behaviors. For these reasons, spatio-temporal couplings are receiving increased attention from researchers in recent years. Most of the work presented to date, however, focuses on a few particular couplings, lacking a general and rigorous analysis. We present a rigorous and mathematically elegant theory of first-order spatio-temporal distortions of Gaussian pulses and beams. We write pulses in four possible domains, xt, xω, kω, and kt, including the couplings. We identify couplings in intensity profiles as: pulse-front tilt, spatial dispersion, angular dispersion, and time vs. angle. We identify four new couplings that occur in phase: "wave-front rotation," "wave-front-tilt dispersion," "angular temporal chirp," and "angular frequency chirp." In addition, we provide normalized, dimensionless definitions for them, which range from -1 to 1. Finally, we show that for such parameters as pulse length, bandwidth, beam spot size and divergence angle, two separate definitions are required as "local" and "global" quantities, in presence of the couplings. Our approach completely determines the explicit relations between various spatio-temporal couplings in Gaussian pulses and beams. It can be generalized to arbitrary profiles by using computational analysis.

The couplings between the spatial coordinates and time and/or frequency are very common in ultrashort laser pulses. We previously showed that, the ultrashort pulse intensity and phase measurement devices, single-shot FROG and GRENOUILLE also measure some of the very common spatio-temporal distortions. Specifically, GRENOUILLE yields a sheared trace in frequency if the input pulse has spatial chirp. It also yields a trace shifted in delay, if the input pulse has pulse-front tilt. The shear and shift can also be used to measure the distortions. While this approach holds valid for relatively simple pulse, as the pulse gets more complicated, so does the effect of the spatio-temporal distortions. Therefore, we develop methods to extract the spatio-temporal distortions from GRENOUILLE traces, even for fairly complex pulses and distortions. First, we have developed a general model of GRENOUILLE for arbitrary spatio-temporal input beams. We then develop two algorithms to be run on distorted GRENOUILLE traces. The first perturbative algorithm is approximate, but is adequate for most cases where the spatio-temporal distortions are relatively small. The advantage of this perturbative approach is that it requires little modification to the existing FROG program, which is fast, reliable and robust. The second rigorous algorithm is numerically more complicated but is capable of accurately measuring the pulse intensity and phase and the spatio-temporal distortion parameters in more general cases. We tested this algorithm with several pulses that have various complexities and showed that this new algorithm retrieves the intensity and phase and the spatio-temporal distortions very accurately.

This paper reports the imaging of a silicon flip-chip with high resolution by detection of the photocurrent generated by the two-photon absorption of 1530nm light from a femtosecond Er:fiber laser. High resolution imaging was made possible by the inclusion of a silicon solid immersion lens, which increased the numerical aperture of the microscope. Using this technique, features on a sub-micron scale are clearly resolvable with excellent contrast, and the resolution of the system was found to be 325nm.

We report the use of electric field induced second harmonic generation to probe electrical signals in a CMOS chip. The second harmonic of incident 2.3μm illumination provided by a femtosecond optical parametric oscillator was measured and shown to depend quadratically on both optical intensity and on the applied DC electric field. By using a near infrared photomultiplier tube it was possible to monitor directly the electrical waveform in the chip on the oscilloscope.

Optical coherence tomography (OCT) is a contactless and non-invasive technique nearly exclusively applied for bio-medical imaging of tissues. Besides the internal structure, additionally strains within the sample can be mapped when OCT is performed in a polarization sensitive (PS) way. In this work, we demonstrate the benefits of PS-OCT imaging for non-biological applications. We have developed the OCT technique beyond the state-of-the-art: based on transversal ultra-high resolution (UHR-)OCT, where an axial resolution below 2 μm within materials is obtained using a femtosecond laser as light source, we have modified the setup for polarization sensitive measurements (transversal UHR-PS-OCT). We perform structural analysis and strain mapping for different types of samples: for a highly strained elastomer specimen we demonstrate the necessity of UHR-imaging. Furthermore, we investigate epoxy waveguide structures, photoresist moulds for the fabrication of micro-electromechanical parts (MEMS), and the glass-fibre composite outer shell of helicopter rotor blades where cracks are present. For these examples, transversal scanning UHR-PS-OCT is shown to provide important information about the structural properties and the strain distribution within the samples.

Femtosecond laser irradiation has various noticeable effects on fused silica. It can locally increase the index of refraction and modify the material chemical selectivity. Regions that have been exposed to the laser are etched hundred fold faster than unexposed regions. These effects are of practical importance from an application point-of-view and open new opportunities for the development of integrated photonics devices that combine structural and optical functions. Various observations reported in the literature indicate that those effects are potentially related to a combination of both structural changes and the presence of internal stress. In this paper, we present further investigations on the effect of femtosecond laser irradiation on fused silica substrate (a-SiO₂). In particular, we use nanoindentation and holography-based birefringence measurements, coupled with direct SEM observations on chemically etched specimens to characterize the effect of various laser parameters such as power, scanning speed and irradiation pattern. We show evidence of an interface between two different etching regimes that may be related to the presence of two different material phases induced by the laser irradiation.

Ionization dynamics based on various ionization and carrier-loss mechanisms is theoretically studied for bulk crystalline and amorphous solid dielectrics irradiated by ultrashort (femtosecond) laser pulses. Multi-photon ionization is found to be predominant at low laser intensities (I_las ∼ 10 TW/cm²) with small contribution of avalanche process, which is more significant for amorphous, rather than crystalline materials. At higher intensities - I_las ~ 10² TW/cm² - ionization is considerably enhanced by transient bandgap renormalization due to ultrafast ac-Stark effect and multi-particle interactions in electron-hole plasma, but is strongly damped by Auger recombination for electron-hole plasma densities N_e > 10²⁰ cm^-3 and accompanied by microscopic damage of corresponding dielectric materials.

Fiber lasers offer an excellent technology base for production of an industrial-quality tool for precision microfabrication, answering the need to expand the capabilities of laser material processing beyond traditional welding, cutting, and other industrial processes. IMRA's FCPA μJewel^TM femtosecond fiber laser has been developed to address the particular need for direct-write lasers for creation of clean and high-quality micron and sub-micron features in materials of commercial interest. This flexible Yb:fiber chirped-pulse amplification architecture, capable of operating at rep-rates between 100 kHz and 5 MHz, balances the need for higher-repetition rate with that of sufficient pulse energy to work at or near ablation threshold, while meeting industrial standards for temperature, shock and vibration. Demonstration of the need for higher-repetition rates for direct write processes will be provided in this paper. Further, results of laser-processing of materials typically used in flat panel displays, photomasks, and waveguide production using the FCPA μJewel^TM laser will be presented.

By use of femtosecond laser pulses, waveguide writing in transparent materials has attracted much interest. We present the fabrication of waveguide devices inside bulk PMMA. Symmetric waveguides can be fabricated by using a slit beam shaping method.

We describe the use of a crossed-beam irradiation system in three-dimensional femtosecond laser microprocessing to obtain three-dimensionally isotropic spatial resolution. In the crossed-beam geometry, two orthogonal objective lenses are arranged to share a common focal point. The synthesized focal spot produces an isotropic illumination volume. We demonstrate that microfluidic channels with substantially circular cross-sectional shapes can be directly fabricated inside glass by using the crossed-beam irradiation system.

We present a high-accuracy force or displacement sensor made only of fused silica. This device merges integrated optics and micro-mechanics in a monolithic substrate. It differs from previous micro force sensor works in that the measured variable is acquired optically, rather than electrically. The device was manufactured using a combination of femtosecond laser pulses and chemical etching. A single manufacturing step was used to define both the optical and the mechanical features. This approach dramatically simplifies the overall fabrication and eliminates alignment issues associated with sequential fabrication processes. Our displacement sensor is composed of a mobile platform and a fixed frame. These components are linked together through sixteen high-aspect ratio double-compound flexures. This design firmly restrains the motion of the platform along a single axis. The range of motion exceeds 1-millimeter. Integrated waveguides are used to measure the displacement of the displacement with accuracy better than 50-nm.

We report on the observation of a two-dimensional discrete soliton in a cubic 5 × 5 fs laser written waveguide array for the first time. In addition to the localization the sharp defined edges of the array allow to study the influence of the array's boundaries. The results are in excellent agreement with the theoretical predictions. These results provide the basis for a variety of future applications for nonlinear two-dimensional integrated optical devices.

Optical waveguide writing with femtosecond laser pulses represents a good alternative to traditional fabrication methods thanks to its simplicity, flexibility and possibility to realize 3D structures. The direct use of a laser oscillator allows a simpler setup, without amplification stages, greater processing speed, up to 1 cm/s, and intrinsically symmetric waveguide cross-sections due to isotropic heat diffusion. In this work we report on the fabrication and optical characterization of waveguides at telecom wavelengths by a stretched-cavity (26 MHz repetition rate) Ti:Sapphire oscillator. The best results have been obtained on Corning 0211 and the previously unexplored Schott IOG10. Operation at 1.55-micron is demonstrated and a comparison between optical properties of the waveguides on the two glasses is made. The refractive index profiles have been measured with two different techniques: the innovative Digital Holography Microscopy (DHM), applied for the first time to optical waveguides, and near-field refractive index profilometry (RNF). The shape of the refractive index profile was found to depend strongly on the glass type. We demonstrate passive photonic devices at 1.55-micron, exploiting the unique 3D capabilities of the technique. These devices include: (i) a 1x2 splitter, obtained by writing two straight waveguides at an angle and separated by a depth displacement; (ii) a 1x4 splitter, realized by combining 1x2 splitters on different planes in the depth; (iii) a WDM coupler, with a good rejection of the 980-nm signal with respect to the 1550-nm one. Perspectives of the technique will also be addressed.

In this paper, a simple method of pulse narrowing by double Q-switching is presented. In energy transfer distributed feedback dye laser (ETDFDL) when acceptor dye acts as a saturable absorber, pulse narrowing is observed in both donor and acceptor emission regions. In prism-dye cell configuration using second harmonic of Nd-YAG laser as pump source, the ETDFDL output is obtained from dyes Rhodamine 6G (R6G) as donor and Acid Blue 7 (Ab7)/Thionine (Th) as acceptors. In Rhodamine 6G and Acid Blue 7 combination pulse narrowing is observed only in the acceptor region whereas in the other combination namely Rhodamine 6G and Thionine, it is observed in both donor and acceptor regions. This is because of good overlap of the emission spectra of donor with the absorption spectra of acceptor. The detailed study is presented in this paper.

We report on the development of diode-pumped high power continuous-wave (CW) and ultrashort pulse Yb:KGW lasers for applications in nonlinear multimodal microscopy. In the CW regime we generated 5W of average output power from a simple three-mirror cavity, with 22% of optical-to-optical conversion efficiency. The CW laser was used as a platform for development of a high power mode-locked oscillator. We achieved 2.3W of average output power in the mode-locked regime with ~210 fs duration pulses centered around 1047 nm at a repetition rate of 97 MHz. This corresponds to 24 nJ of energy per pulse and 114 kW of peak power. The demonstrated laser will be used in second- and third-harmonic generation microscopy of biological samples.

A novel new design for an 8-pass multipass Titanium doped sapphire femtosecond amplifier (MPA) is studied. Ultrafast amplifiers based on the chirped pulse amplification (CPA) technique have been widely used to amplify the output pulses of Kerr lens mode locked (KLM) Ti:sapphire lasers from the nanojoule to the microjoule level. The system presented here also takes advantage of CPA to reduce the peak power and thus the potential damage to optical components from self-focusing. The amplifier scheme is based on a single curved mirror and a Brewster cut Ti:sapphire laser rod. Optical excitation of the Ti:sapphire gain medium is achieved by pumping with a Q-switched and frequency doubled Nd:YLF laser at 527 nm. The rear face of the gain crystal is coated to form a high reflector for both the pumping wavelength (490-550 nm) and the amplified seed pulse (740-860 nm). In this configuration the gain crystal itself acts as a second mirror, reducing the size of the amplifier and allowing for the most effective use of the pumping energy. By employing a Brewster cut lasing crystal the amount of active gain material can be adjusted for maximum gain. The advantages of this approach, compared to traditional two curved mirror MPA designs, are the reduced foot print and the ability to easily adjust the amount of gain material. At the same time the system retains the low amplified spontaneous emission (ASE) and temporally clean output pulse characteristic of MPA systems.

Recently, ultra-fast pulse lasers have been attractive in the field of fine processing applications, such as three-dimensional optical waveguides, photonic crystals and ablation. Because femotosecond lasers have the superior characteristics of a short pulse width and high peak power, we can reduce thermal influence that causes splatters, roll up of the edge and damages of glass substrate in the thin films ablation. Due to this non-thermal ablation process, a control of the stable and fine process can be available. In this letter, we LFT (Laserfront Technologies, Inc., former NEC Laser Solution Division) have developed a femtosecond laser system. It consists of a mode-locked fiber oscillator, a regenerative amplifier, a pulse compressor and a third harmonic generator. The gain media of the regenerative amplifier is Nd:glass. The output energy of the regenerative amplifier is 3mJ at 7 Hz repetition rate. The final THG (351 nm) output energy is 300μJ, 10% conversion efficiency was obtained. Using above femtosecond laser, we conducted ablation processing of thin films such as aluminum on the glass substrate. The results of fine processing are reported.

This paper presents the characteristics of energy transfer distributed feedback dye laser (ETDFDL) in a mixture of Rhodamine B (RhB) and Thionine (Th) dyes pumped by 532 nm Nd:YAG laser both theoretically and experimentally. The characteristics of donor DFDL, the acceptor DFDL, the dependence of their pulse widths and output powers on donor-acceptor concentrations and pump power are studied. Experimentally the output energy of DFDL is measured at the emission peaks of donor and acceptor dyes for different pump powers, donor-acceptor concentrations and the tunability is observed from 565 nm to 665 nm using prism dye cell arrangement.

Novel solid-state picosecond lasers provide a strong benefit for high precision micro-machining. Pulse repetition rates as high as > 500 kHz with pulse energies of > 4 μJ enable fast machining with the precision of low fluence ablation. In addition, new potentials for these lasers are given by advanced modulators with digital timing control that allow the user to generate sequences or groups of pulses: E.g. a sequence of two pulses can be generated and repeated up to 300 kHz. The amplitude of these two pulses can be adjusted independently and the delay is selectable in 20 ns steps. This kind of pulse-strategies with picosecond lasers can support higher ablation rates, similar to the machining results that were demonstrated with double ns-pulses, recently. In another application, groups of > 20 pulses were repeated with > 50 kHz for ultra-precise machining. The distribution of the energy yields a few hundred nJ per pulse and results in an ablation depth per pulse in the range of several nm. Therefore the ablation depth formed by a group can be digitally controlled by the number of pulses in group. Samples for high quality drilling, cutting and structuring of several materials will be presented and the new potentials of this kind of picosecond laser processing with improved precision and speed will be discussed.

Generation of InAs-surface-emitted terahertz radiation by application of an ultrashort pulse 1064 nm parabolic fiber amplifier source is reported for the first time. The fiber amplifier delivers 100 fs pulses at a repetition rate of 75 MHz and an average power of maximum 12 W. This new excitation laser for surface-emitters generates high brightness broadband THz radiation ranging from 100 GHz to over 2.5 THz. THz detection is demonstrated based on two-photon absorption at low-temperature-grown GaAs dipole receivers.

We present a method for controlling the reflection amplitude and phase of uniform fiber Bragg gratings (FBGs) during their fabrication process. It is done by measuring the spectral interference between the reflections from the FBGs and the fiber end by an optical spectrum analyzer and performing a fast Fourier transform. The method allows correction of the FBGs until obtaining the needed parameters during the writing process, as well as at any time after that. We also demonstrate the use of cascaded uniform FBGs for the generation of periodic optical pulses with arbitrary waveform. It is a significantly simplified structure compared to complex fiber Bragg grating shapes. The pulse shaping is based on splitting of the input pulses by low reflecting FBGs into a number of replicas and their superposition with proper amplitude, time delay and phase shift that depend on the FBG parameters. The reflection amplitude and phase of each grating are unambiguously determined by the needed pulse shape. This method was experimentally verified for converting sinusoidally phase-modulated radiation of CW laser diode into a Gaussian pulse train with a pulse width of 30 ps.

We have demonstrated femtosecond laser fabrication of submicrometer-sized voids in fused silica. Femtosecond laser pulses of 100 fs were focused into fused silica with a 0.9 numerical aperture (NA) objective lens under various incident conditions. The void shape is linearly drawn in the direction of the laser irradiation, when a single pulse is irradiated. The irradiation of multiple pulses induces multiple spherical voids which make a void array. The void shape also depended on the depth of the focus point beneath the fused silica surface, because the amount of self-focusing has a significant effect on the generation of the voids. The void shape was narrower and longer when the laser pulse was focused into the deeper position (up to 70 μm) in the sample. In addition, a 90 degree bend waveguide was fabricated in combination with a void array reflector. Since both reflector and optical waveguides were fabricated by femtosecond laser only, this technique would be useful to develop 3-dimensional optical devices.

We describe two novel, practical ultrashort laser pulse measurement devices, which are also experimentally very simple. The first one is an "ultra-broadband" pulse characterization device that is based on FROG, but uses transient grating (TG) process. TG FROG involves forming an induced grating in a piece of glass by crossing two pulses in space and time and then diffracting a third pulse off it to create a fourth diffracted pulse. The TG process is inherently very broadband and automatically phasematched. We have implemented an ultrasimple TG FROG device, which can also operate single-shot. First, three beams are created using a simple mask. Then, a cylindrical beams line-focuses the beams horizontally, where the induced grating is generated. The variation of the relative delay is achieved by crossing the two grating-creation beams at an angle using a Fresnel biprism. Then, by detecting the diffracted pulse with spatial resolution, the TG FROG trace is captured. The second device that we present aims to measure ultrashort pulses with complex spectral and temporal structure. Spectral interferometry (SI) works perfectly for this purpose. SI simply involves measuring the spectrum of the sum of the unknown (shaped) and known (reference) light waves. Unfortunately, SI is very difficult to align and maintain aligned, as it requires that the two beams be nearly perfectly collinear. We solved this problem by utilizing optical fibers. Spectral resolution is also significantly improved by using spatial fringes, avoiding time-domain filtering.

A noble measurement method by using a homodyne interferometer and Hilbert transform has been proposed for characterizing frequency sweeping light sources used in traditional optical frequency domain reflectometer (OFDR) and optical frequency domain imaging (OFDI). A Michelson interferometer with a tunable laser generates a sinusoidal beating signal. A phase of measured beating signal as a function of time is approximately proportional to optical frequency of the swept light source during frequency tuning and can be obtained by the Hilbert transformation. Thus, optical frequency chirp can be determined by a simple equation related with the phase of the beating signal from the interferometer. We have demonstrated the effectiveness and the simplicity of our proposed method by testing a temperature-tuned frequency sweeping DFB-LD and a commercial external cavity tunable laser source as practical examples. In the case of DFB-LD, the frequency sweep becomes more linear while the amount of frequency sweep saturates as the amplitude of the control voltage applied to a TEC driver increases, and the frequency-tuning rate increases as the repetition rate decreases. We also found that a commercial frequency-sweeping laser has a feed back control to adjust its frequency-sweeping rate such that the tuning rate oscillates around an intended value as a function of time. We have demonstrated the possibility of using a self-homodyne interferometer as a powerful tool for characterizing frequency sweeping laser sources. We expect this method will be useful for improving the performance of many optical frequency domain measurement techniques such as OFDR, FD-OCT or OFDI.

Multiphoton microscopy is a very promising method for 3D imaging of living cells. The fluorochromes are solely excited at the laser focus by multiphoton absorption using near-infrared femtosecond laser pulses. The arising fluorescence serves for a pixel-to-pixel imaging with a resolution in the submicron range. At higher laser powers, the multiphoton absorption creates a micro plasma which induces an outwardly propagating shock wave. The rapidly expanding cavitation bubble causes disruption of the material, with hardly any interaction with the surrounding tissue as the optical breakdown proceeds faster than the thermal conduction. This combination offers the possibility of simultaneous manipulation and analysis of living cells or cell organelles. Manipulation is achieved using laser pulses with an energy of a few nanojoules while imaging is done at less than 1 nJ. The obtained resolution allows the precise cutting of single cell organelles without compromising the cells` viability. Thus, the implementation is excellently suited for cell surgery. We conducted ablation of different subcellular structures, like mitochondria, at different pulse energies within living cells while studying cell viability.

We report on the investigation of the nonlinear refractive index in femtosecond laser written waveguide arrays in fused silica. The nonlinear refractive index is significantly reduced compared to the unmodified material. Due to the dependence of the processing parameters the effective nonlinearity in such waveguide structures can be tuned. This offers additional flexibility in the design of nonlinear devices.

We "see" light only when some material detectors (dipoles) respond to the incident EM field. EM fields do not operate on each other to make themselves visible to us. Superposition of multiple fields becomes manifest only when the intrinsic properties of these dipoles allow them to respond to all the superposed fields simultaneously and thereby summing the effects of all the fields. Accordingly, depending upon the different intrinsic properties of the detectors and the physical conditions of measurements (integration times, etc.) the manifestation of the "coherence" properties for the same set of superposed fields could be different. It is then prudent to represent the autocorrelation function for superposed fields in terms of the dipole undulation of the detectors rather than the fields themselves. Then the physics of the detectors and the measurement conditions automatically becomes an inherent part of the discussion on coherence. We illustrate our premise by presenting the analysis to understand the behavior of beam splitters, two-beam interferometers and an N-beam grating "interferometer" in terms of the autocorrelation functions due to a short pulse as would be experienced by the material dipoles of the beam splitters and detectors. Our approach reveals that superposition effects to become manifest the multiple fields must be physically superposed simultaneously on the detecting dipoles and hence the process is causal.