Nd:YAG has favorable material properties to minimize the thermally induced lenses in lasers at cryogenic temperatures. In the present work we measured a significant reduction of the thermal lens in a Nd:YAG rod cooled with liquid nitrogen. The power range for stable fundamental-mode laser oscillation was demonstrated to be significantly enlarged at low temperatures. The experimental results were compared to analytical models and finite-element simulations.

We have developed a stable 7 terawatt (TW) (168 mJ per pulse, 24 fs pulse duration) Ti: sapphire laser system operating at 50 Hz for a generation of femtosecond X-ray pulses by inverse Compton scattering. We corrected the wavefront distortion of these high intensity laser pulses with adaptive optics using a Shack-Hartmann type wavefront sensor and a deformable mirror. We have also started developing a compact all-solid-state Yb: Sr₅(PO₄)₃F (Yb: S-FAP) laser system to realize a practical X-ray pulse generation system. We measured thermal lensing induced in Yb: S-FAP crystal for design of a high-energy regenerative amplifier. In addition, we measured wavefront of the amplified pulses in the Yb: S-FAP regenerative amplifier with the wavefront sensor.

CO-lasers on the basis of sealed-off active elements (SAE) with an average operational life of ~3000 hours nave a simple design and are cost-effective. They don't need gas blow off and cryogenic equipment, as well as the system of toxic gas evacuation etc. Generally, sealed-off active elements are operated at the temperatures of the cooling agent of 10°C<t<15°C. Cryogenic cooling is impossible for them, so it is interesting to perform optimization of the laser parameters at the tolerable for SAE negative temperatures. There has been performed the research on the basic output parameter of the automated modified CO-laser, which is applicable for research in medicine, ecology, active media and material study and which we described earlier, at negative temperatures of SAE up to t ~-40 °C. These measurements were made using an inexpensive standalone cooling system ("Haake Circulators"). The measurements have shown that both the width of area of spectrum modification and the power of generation at separate lines go up as the degree of cooling grows.

We have developed a computer model calculating bare cavity transverse eigenmodes for super-gaussian unstable resonators, including aperture diffraction in the gain medium. This generalized simulation, based on the Fox and Li Power Method, reduces the input parameters to five: rod longitudinal position, cavity magnification, super-gaussian order of the output coupler reflectivity, and Fresnel numbers for the cavity and rod apertures. Using two-dimensional FFT's to discretize the Huygen-Fresnel numbers, the output fields at the plane of the rod aperture and exiting the output coupler were subjected to beam quality (M²) and extraction efficiency (Xeff) analysis. Beam quality was found to be the most sensitive to cavity magnification, with M² values varying as much as 30% or more with 3% shifts in magnification, which can occur during rod lensing. Avoiding peaking M² values is demonstrated with design curves for two different cavity Fresnel numbers, and super-gaussian orders. The cavity Fresnel number and the super-gaussian order are shown to only weakly affect beam quality, although extraction efficiency varies strongly with the latter. Finally, optimized rod longitudinal position was explored for promising combinations of the other four parameters, and it was found to be near the high reflector (HR) end of the cavity, in terms of M² analysis.

A theory is presented that explains the observed spontaneous transition to the high-brightness synchronous array-state of a multicore fiber laser array. The mechanism, based on nonlinear refraction, is shown to be robust. Results, so far of a 19-core array, indicate possible scalability toward much larger array size.

In this paper, the linear propagation of x rays in laser produced plasma is studied theoretically, with a quantum mechanical technique and the so-called V representation, where the representation transformation is made by using the potential Hamiltonian V. A modified expression relating the phase difference between the probe and the reference x ray light to the plasma electron density is derived, in which the electron density gradient and its higher-order effects are taken into account. The coupling relation between phase and amplitude of x-ray is derived, and the solutions with higher-order corrections are given. An important parameter is given, which is related to the errors of the electron density measurement using x-ray interferometry. It is depicted that providing the parameter is less than one, the x-ray interferometry can be used for the measurement of the electron density, while, the greater value of the parameter, the higher order modifications need to make.

Continuous monitoring of high power (above 2 kW) CO₂ laser beams with camera based systems has not been effective because beam sampling optics have not been available. Camera based systems allow real-time imaging of the entire beam profile which in turn enables real-time tuning and alignment of the laser, as well as enabling instantaneous recognition of beam misalignment in the optical delivery train. Spiricon and II-VI have jointly developed a new method for in-line, passive sampling and beam profiling of high power, multi-kilowatt CO₂ lasers. The system uses conventional optics in a novel sampling arrangement, coupled to a Spiricon Pyroelectric IR Camera and Laser Beam Analysis software.

The qualitative and quantitative description of the laser tuning effect during a pulse forming has been described. The mechanism of the line hopping in the CO laser has been presented. A simple arrangement enabled observing the pulse evolution of all oscillating spectral transitions contributing in the total shape of the output pulse. As shown in the experiment, the jumps from line to line over the pulse duration give the sequence of lines, which is the subsequence of the static laser signature. Thus, the changes of the refractive index can be estimated by setting the static signature against the line hopping during the pulse operation of the laser. On the other hand, the changes of the refractive index during the pulse evolution have been measured directly with a Mach-Zehnder interferometer. The experiment has been performed on the slab-waveguide laser with an unstable positive branch optical resonator. The experiment allowed observing the dynamic evolution of the pulse in time and spectrum. The method of the investigations allows estimating the changes of a pressure and temperature during the pulse evolution, and can explain the effect of bulges observed at the output pulse shape.

This paper discusses the novel adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time, the frequency of corrected aberrations is less then 25 (30) Hz. The amplitude of such aberrations, about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations - up to 15th Zernike polynomial (excluding tip-tilt).

We present the results of measurements of thermal fluctuations in microspheres. Experimental noise spectra are in good agreement with the theoretical model of recently predicted thermorefractive noise.

We demonstrate novel techniques to manipulate spectral properties of high quality factor whispering-gallery modes (WGM) in optical dielectric microresonators. These include permanent frequency trimming of WGM frequencies by means of UV photosensitivity of germanium doped silica resonators; electro-optical tuning of WGM in lithium niobate resonators, and cascading of microresonators for obtaining second-order filtering function. We present theoretical interpretation of experimental results, and examples of applications of these techniques for photonic microwave filtering.

We measured the coupling of the florescence from semiconductor nano-crystals and sub-micron sized dye-doped beads to high-Q whispering gallery modes (WGM) of a microsphere resonator. With Q-factors as high as 109 the florescence could be extracted in a controlled way via a prism coupler. We observed nearly 100 % modulation in the spectrum which reflects the coupling to the WGMs. With the help of a beam scanning confocal microscope we were able to address a single 500 nm sized dye-doped bead on the sphere’s surface and to collect and analyze its florescence in a well defined manner via the prism coupler.

Dielectric spheres and cylinders can support high-Q modes due to internal reflection of the light. However, small deviations of the shape of these resonators away from symmetry lead to the onset of chaotic ray dynamics and the resulting suppression of the mode lifetimes. We demonstrate, that in such conditions even the regular modes are strongly affected by small deformations, due to the phenomenon of chaos-assisted tunneling, and develop a quantitative theory of this effect.

A combination of laser-induced ultrasound generation and ultrasonic holography for spatial control of the generated ultrasonic pulse is presented. Ultrasound is produced by absorption of laser pulses at an absorbing layer in a water tank via the optoacoustic effect. In order to produce a defined ultrasonic frequency in the MHz range, the laser pulses are harmonically time-modulated using an acousto-optic modulator (AOM). Additionally, the laser intensity is spatially controlled. This is realized with a high resolution liquid crystal spatial light modulator (LCD). A computer generated pattern is displayed at the LCD and projected by the expanded laser beam to an absorptive layer in the water tank. As a result, the emitted ultrasonic wave emerges in a predetermined way, which is an acoustical analogue to the effect of a "diffractive optical element" in laser optics. The flexible method of optical ultrasound generation and diffractive steering promises new applications in medical and technical ultrasound diagnostics.

Conventionally, lasers are designed to operate at the middle of thermally stable zones, where the fundamental mode size is insensitive to thermal perturbation, but is inversely proportional to width of stability zone, which will give rise to inconvenience or even difficulty in practice when large mode size is required. We propose a new simple approach, namely thermally-near-unstable resonator. The laser is designed to operate at the border of stability zone instead, where it has large fundamental mode size at gain media. With increase of pump power, mode size would grow up automatically to a value suitable for monomode operation. Stability of cavity on whole pump range can also be easily guaranteed. And there is a point where the laser power is insensitive to driving perturbations. However, the laser beam quality is sensitive to driving and thermal perturbations for the mode size depends severely on thermal focusing. Large-scale improvement in beam quality is demonstrated experimentally.

Morphology dependent resonances of dielectric microspheres are used for polarization insensitive optical channel dropping from an optical fiber half coupler to a silicon photodetector in the M-band. The dropped channels are observed in the elastic scattering and the transmission spectra. The highest quality factor morphology dependent resonances have a repetitive channel separation of 0.14 nm and a linewidth of 0.06 nm. The filter drops approximately 10% (0.5 dB) of the power at the resonance wavelength. The power detected by the photodiode is estimated to be approximately 3.5% of the power in the fiber.

In this paper, an integral solutions for the transverse and longitudinal components of the laser field for ultrashort pulsed-beam propagation in free space are derived, applying Fourier transform and the paraxial approximation in frequency domain. Furthermore, using the complex analytical signal (CAS) theory, the solutions for the components of a pulsed Gaussian-like beam in rectangular coordinate are derived and contrasted with those derived with slowly-varying-envelop-approximations (SVEA). Meanwhile, the results show that if the pulses as short as sub-cycle, SVEA will cause the spatial singularity for both transverse and longitudinal components of the laser field and CAS must be applied to eliminate the singularity.

In the study of the propagation of light in an inhomogeneous isotropic medium such as the plasma, the customary approximation of neglecting the effect of electron-density gradient is not accurate. In this paper, we are concerned with continuous variable linear index of effective plasma refraction and the effect of plasma electron-density gradient on the beam parameters. To understand the effect of the electron-density gradient more clearly, the numerical simulation will be made. We find that the larger the propagation distance and the higher the distribution of the plasma electron density is, the greater the effective becomes.

The authors have successfully demonstrated continuous-wave oscillation in a Nd-doped tellurite glass microsphere laser at 1.06 μm for the first time. Microspheres with diameters of 50 to a few hundred micrometres are fabricated by melting using an electric heater. Emission spectra reveal that the devices exhibit resonances corresponding to whispering gallery modes. Lasing threshold of pump power was about 5 mW.

It is needed to divert the unwanted harmonic waves out of high power laser system to perform Inertial Confinement Fusion (ICF) and several color separation gratings (CSGs) have been put forward. For a general multilevel grating, the Fraunhofer diffraction patterns of harmonics in ICF laser system have been derived taking no account of optical dispersion. According to the wavelength relationship of the harmonics, the multilevel CSG adaptive for ICF must have special structure features to meet the requests. It is found that a CSG can be used in ICF when its steps number being common multiple of 3 and at the same time the optical path difference for 3ω harmonics between neighboring steps being integral multiple of its wavelength. However, the optical dispersion practically exists for actual laser system and the functions of CSG will be affected. Though by optimization method, the step width and step height of CSG can be changed slightly to compensate the influences, but its performance will be played down. For the CSGs with general structure, the influences of chromatic dispersion have been analyzed when the number of steps and phase delay difference between two neighboring steps varied. By comparison, it is found that the CSG with smaller steps and step height difference is more preferable for color separation in ICF.

We propose to fabricate a dielectric cavity sustaining high-Q whispering gallery modes from a periodically poled material possessing a quadratic nonlinearity to achieve an efficient interaction among the modes. We show that the periodical poling allows for compensation of both the material and the cavity dispersions that prohibits the nonlinear interaction otherwise. Such a cavity might be a basic element of a family of efficient nonlinear devices operating at a broad range of optical wavelengths.

There are numerous applications that require stabilization of mode-locked lasers. Mode-locked ring laser sensors have been demonstrated to have a sensitivity to rotation of the order of the rotation of the earth, and sensitivity to optical path changes of less than 0.01 Angstrom. These performances could be enhanced by several order of magnitude through stabilization. Stabilized and accurate femtosecond pulse trains have also applications in ultrafast communication, where there is a need to synchronize with subfemtosecond jitter independent sources. We have demonstrated stabilization on a short time scale of both the repetition rate and the average frequency of a mode-locked laser, using an ultra-low expansion quartz reference cavity. We will discuss techniques to extend the short term stabilization to long time scales, by locking the laser to atomic lines.

There is currently a significant amount of interest in the development of integrated optical sensors for a variety of applications. Attractive features of these types of sensors include miniature size and low operating powers. We have demonstrated a novel integrated optical sensor platform based on high-Q silica microsphere resonators and stripline-pedestal anti-resonant reflecting optical waveguide (SPARROW) optical couplers. Advantages of the planar SPARROW coupler include high light coupling efficiency, robust planar structure, and standard optical chip processing/fabrication techniques. High light coupling efficiencies (approaching 100%) have been observed using this sensor platform. Several sensor configurations are presented here, including one designed to reduce input optical power requirements. High resolution flexured-mass-based sensors which have been demonstrated include acceleration and seismic sensors. Proposed sensor concepts based on thermo-optic effects are also discussed.

We describe the development of Nd:YVO₄ lasers with continuous wave emission at 1.342 μm; theycon tain either one or two crystals which are longitudinally pumped by fiber-coupled diode laser arrays. The two systems deliver respectively 7.3 and 12.1 W of output power. The best slope efficiency is 40% and the output beam divergence is about two times the diffraction limit. The influence of the Nd concentration, of the pump spot-size, and of the resonator configuration on the lasing performances have been experimentally studied. We also present and discuss data on spectroscopic properties of the crystals and on the thermally induced lens.

Different N-folded multiple-pass resonator schemes were realized and tested in high power fast-transverse-flow CO₂ lasers. In all cases beam ellipticity occur due to gain saturation followed by beam self-action in active medium with fast transverse gas flow. To avoid this effect new resonator structure was proposed where N-folded passes are arranged in different crossed planes declined to the gas flow direction. Fast-transverse-flow industrial CO₂ laser "Lantan" with this resonator scheme demonstrates high beam quality up to 2.5 kW CW output power with differential efficiency of 16%.

We describe an adaptive optical closed loop system with bimorph mirror as a wavefront corrector and CCD camera at the focal plane of the lens as a sensor to obtain a good focal spot. Adaptive system can correct for the low-order aberrations with the frequency of corrected aberrations about 5 Hz. These parameters are mostly determined by the deformable mirror properties and multi-dither algorithm.

We present experimental and theoretical results on aberration control in solid state laser amplifiers and resonators. In lasers with diffraction-limited beam quality, aberrations cause diffraction losses that reduce the output power. In laser amplifiers, aberrations in the active medium degrade the beam quality of the amplified beam. Adaptive optics can be used to correct for the aberrations and thus increase output power and beam quality, respectively. The required precision of the adaptive aberration correction can be estimated with a simple mode expansion model in which an aberrated TEM₀₀ mode is expanded into Gauss-Hermite modes. Apart from these theoretical results we will present experimental results for a MOPA (Master-Oscillator-Power-Amplifier) laser system consisting of a Nd:YVO₄ master oscillator and two Nd:YAG power amplifiers. A micromachined deformable mirror was used in a closed-loop system to correct for the thermo-optic aberrations of the amplifiers. A beam with a beam quality of M²= 2.5 at an out power of 80 W was obtained. The deformable mirror was controlled by a genetic algorithm.

Coherence properties of real laser beams can be crucial for many applications, e.g. in lithographic processes or for Bragg grating writing. Knowledge of the coherence distribution together with the amplitude and phase distribution allows for a complete beam characterization, which enables the numerical simulation of beam propagation through virtually any relevant optical system. Classical measurement methods of coherence properties are Young’s double hole interferometry and Shear interferometry. But due to their interferometric nature experimental realization of both methods is quite difficult and obtainable accuracies are usually not satisfying. The reconstruction of the Wigner distribution from a couple of measured intensity distributions in the waist region of a beam provides a simple measurement setup delivering fairly accurate results. This is demonstrated by an experimental comparison of these three methods.

A single-photon device based on a semiconductor quantum dot embedded in an optical microcavity is described. The spontaneous emission lifetime, multi-photon suppression, and spectral linewidth are measured. It is then shown that consecutively emitted photons possess a large degree of quantum-mechanical indistinguishability, with a mean wave-packet overlap as large as 0.8. This demonstration is accomplished through a Hong-Ou-Mandel-type two-photon interference experiment.

We describe the dispersive and nonlinear optical properties of microresonator-modified waveguides. While many applications of microresonators demand ultra-high quality factors and as a result impose strict fabrication tolerances, we examine a variety of useful devices that may be may be constructed using small numbers of only moderately-high Q resonators using the current state of the art in fabrication technology.

Optical micro-manipulation has seen a resurgence of interest in recent years which has been due in part to new application areas and the use of tailored forms of light beam. We look at the use of Laguerre-Gaussian and Bessel light beams in this paper and also at the creation of optically bound matter using counter-propagating Gaussian beams.

We present a technique for the absolute measurement of very low-level scattering. The method is absolute in that it relies on fundamental physics and does not require calibration against standards maintained by the National Institute of Standards. In a bi-directional mode-locked ring laser, a difference in longitudinal mode frequency between the two senses of circulation of the intracavity pulses can be measured as a beat note between the corresponding outputs. A very small amount of light backscattered by a sample (located at the pulse crossing) from one sense of circulation into the other, causes the beat note to vanish. The threshold in mode frequency difference that causes locking is a measure of the scattering amplitude.