Alexander M. Sergeev

Effect of terbium gallium garnet crystal orientation on the isolation ratio of a Faraday isolator at high average power

We present a comprehensive and systematic investigation of the fundamental physical limitations of Faraday isolation performance at high average powers that are due to thermally induced birefringence. First, the operation of various Faraday isolator designs by use of arbitrary orientation of cubic magneto-optic crystals is studied theoretically. It is shown that, for different Faraday isolator designs, different crystal orientations can optimize the isolation ratio. Second, thermo-optic and photoelastic constants for terbium gallium garnet crystals grown by different manufacturers were measured. Measurements of self-induced depolarization are made for various orientations of crystallographic axes. The measurements are in good agreement with our theoretical predictions. Based on our results, it is possible to select a crystal orientation that optimizes isolation performance at high average powers, resulting in a 5-dB enhancement over nonoptimized orientations.

Femtosecond energy transfer within the LH2 peripheral antenna of the photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas palustris LL

Abstract The ultrafast energy transfer among the pigments of the B800–850 membrane antenna pigment—protein complex of the photosynthetic purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas palustris LL has been studied with ≈ 100 fs tunable infrared pulses at room temperature and 77 K. It is shown that the B800→B850 transfer time is similar in both species and occurs with a characteristic time constant of 0.6–0.8 ps at room temperature and 2.4–2.6 ps at 77 K. Measurements of absorption anisotropy in the 800 nm band shows that the depolarizing energy transfer among B800 molecules is slower than the B800→B850 transfer. At room temperature such a transfer occurs with a time constant of 0.8–1.6 ps and at 77 K with a time constant much longer than the excited state lifetime. A fast ≈ 300 fs non-depolarizing, almost temperature-independent, relaxation process is also observed within the B800 band. Energy transfer between a pair of almost parallel B800 molecules and possibly also vibrational relaxation are discussed as the possible origins of this process.

In situ monitoring of laser modification process in human cataractous lens and porcine cornea using coherence tomography

We demonstrate that optical coherence tomography (OCT) is a convenient diagnostic tool to monitor pulse-to-pulse kinetics in laser interactions with biological tissue. In experiments on laser modification and ablation of the cataractous human lens and the porcine cornea we have applied this technique in situ to investigate different modes of preablation tissue swelling, crater formation and thermally affected zone development. The cataractous lens is an example of highly scattering media whereas the cornea is initially low scattering. The radiation with different wavelengths has been employed including that of a YAG:Er laser (λ=2.94 μm), a glass:Er laser (λ=1.54 μm), YAG:Nd lasers (λ=1.32 μm and λ=1.44 μm), as well as of the fifth harmonic of a Nd:YAP laser (λ=0.216 μm). Pulse-to-pulse OCT monitoring has been accompanied by the probe beam shielding diagnostics to provide the time-resolved observation of the interaction dynamics.

In vivo optical coherence tomography of human skin microstructure

A compact effective optical coherence tomography (OCT) system is presented. It contains approximately equals 0.3 mW superluminescent diode with spectral width 30 nm FWHM (providing approximately equals 15 micrometers longitudinal resolution) and fiber interferometer with integrated longitudinal scanning. The dynamic range 60 dB allows to observe structure of human skin in vivo up to 1.5 mm in depth. A comparison of obtained tomographs with data of histologic analysis of the same samples of the skin have been carried out to identify the observed structures and determine their optical properties. This technique allows one to perform noncontact, noninvasive diagnostic of early stages of different pathological state of the skin, to measure the burn depth and to observe the process of the recovery. Unlike scanning confocal microscopy, OCT is more suitable for an endoscopic investigation of the mucous membranes of hollow organs. Possible diagnostic applications include dermatology, gastroenterology, gynecology, urology, oncology, othorinolaryngology, transplantology. The most promising features are the potential possibility of differential diagnosis of precancer and various types of cancer, estimation of the invasion depth, differential diagnosis of inflammation and dystrophic processes, control of radical operative treatment.

Parametric amplification of chirped laser pulses at 911-nm and 1250-nm wavelengths

The analysis of tuning characteristics for parametric amplification in KD*P has shown that the application of KD*P crystals may considerably enhance the possibilities of certain optical parametric amplifiers of both terawatt and petawatt level. For instance, at pumping with a wavelength of (lambda) 3 equals 0.527 micrometers , which is most promising for the creation of such systems, the KD*P-based amplifiers may work far from the degenerate mode, e.g., at (lambda) s- 0.911 micrometers and (lambda) i-1.25 micrometers . For operation at these wavelengths there are currently master oscillator of femtosecond pulses with pulse duration of up to 30 fs. In this paper elements of the system are discussed, and their parameters are optimized.

Monitoring and animation of laser ablation process in cataracted eye lens using coherence tomography

Optical coherent tomography (OCT) enables one to follow the pulse-to-pulse kinetics of laser interactions with turbid biological tissues. In experiments we investigate the effect of free running mid infrared laser radiation of different wavelengths on a cataract-suffered human lens in vitro. Different regimes of laser ablation and preablation surface transformations are monitored in situ.

Capabilities of optical coherence tomography in laryngology

We present first result of using the optical coherence tomography (OCT) in complex clinical studies in laryngology. Mucosa of the upper and middle portions of larynx is of special interest for OCT applications: it is clinically important, easily accessed by an endoscopic OCT probe, and possesses a well defined and rich tomographic structure. We have examined several tens of patients with abnormalities in vocal folds. The diagnosis was made based on clinical data including laryngoscopy and finally confirmed morphologically. When examining larynx mucosa, an endoscopic OCT probe has been introduced through a standard laryngoscope lumen, so that OCT imaging has been performed in parallel with visual observation. The OCT studies have demonstrated that in comparison with stratified healthy mucosa, carcinomatous regions have no tomographically differentiated structure, thus allowing one to exactly define the border of a tumor. Vocal nodules are imaged as poorly scattering regions without clear boundaries under preserved epithelium. Cysts of gland mucosa are seen with OCT as sharply delineated shadows at the depth of several hundred micrometers. We have also examined several patients with carcinoma after a course of radiation therapy and observed different changes in OCT images of adjoining epithelium corresponding to metaplasia, hyperplasia, and sclerosis.

Second-Harmonic Generation of Super Powerful Femtosecond Pulses Under Strong Influence of Cubic Nonlinearity

A theoretical model of second-harmonic generation (SHG) under strong influence of cubic nonlinearity was verified in experiment. Effective energy conversion in thin potassium dihydrogen phosphate crystals at peak intensity up to 5 TW/cm2 (B-integral equaled 6.4) was demonstrated and no crystal damage was observed. Comparative analysis of SHG of radiation at the fundamental wavelengths of 910 and 800 nm showed the major advantages of the first one. The double-pass geometry of SHG in an ultrathin crystal on a substrate is discussed in detail. Additional correction of parabolic spectral phase of the SH radiation allows pulse duration to be shortened from 20 to 9 fs for 910 nm fundamental wavelength and from 20 to 12 fs for 800 nm.

Multicascade boradband optical parametric chirped pulse amplifier based on KD*P crystals

We have experimentally demonstrated the existence of super-broadband non-degenerated phase matching for a signal with a wavelength of 911 nm in KD*P crystal pumped with wavelength of 527nm. Parametric amplification coefficient of more than 107 in three cascades is achieved. This resulted in pulse energy 10mJ at the output of third cascade. It is shown that in the KD*P crystal chirped pulses of conventional femtosecond sources (a Ti:Sa laser at 911 nm and a Cr:forsterite laser at 1250 nm) can be amplified up to the level that ensures multipetawatt power after compression.

Intense laser pulse dynamics in dense gases

The results of a theoretical investigation on the interaction between a strong ionizing laser pulse and an initially neutral gas are presented, based on a 3-dimensional numerical simulation of the radiation-gas system. The electromagnetic radiation is described, in the frame of a quasi-optical model, in terms of the complex amplitude of the electric field envelope, while the free-electron density, which determines the laser propagation characteristics, is treated by means of a fluid model, considering different ionization mechanisms. Ions are assumed to be at rest and the electron motion is nonrelativistic. The macroscopic manifestations of the ionization induced scattering of the laser radiation are discussed in detail with reference to specific experimental conditions.