This work is devoted to the study of the generation regime of an ultrashort pulse bunch in an erbium-doped fiber laser with hybrid mode-locking in the region of a low positive total intra-cavity group velocity dispersion. The pulse bunch has an optical spectrum 33 nm wide with a central wavelength of 1530 nm and has 2 pulses inside with a duration of ~143 fs at the full-width-half-maximum. The radio frequency spectrum in the region of the main pulse repetition rate, equal to ~8.5 MHz, demonstrates, depending on the pump power, the presence of 4 to 8 side peaks associated with the second pulse in the bunch. A comparison of the single-pulse and multi-pulse regimes demonstrates a twofold increase in the average output optical power of a pulse bunch for equal pump power and cavity parameters.
The work is devoted to the optimal wavelengths selection for the task of methane monitoring using an acousto-optical spectrometer installed on board a satellite. The results of mathematical modeling are given. It is shown that the spectral width of the instrumental function has a significant impact on the selection result, while the atmosphere model used does not have a significant effect.
In this paper, we have studied ML features in a sub-200-fs erbium-doped all-fiber laser based on a saturable absorber obtained by high-pressure-high-temperature treatment of commercially available single-walled carbon nanotubes. We have shown that there is a significant effect on the ultrafast optical properties of saturable absorber due to the high-density well-aligned structure of newly-developed single-walled carbon nanotubes and related it to the mode-locked laser performance. We have obtained a low-intensity-noise ultra-short stretched pulse generation with a repetition rate of 42.22 MHz, a spectral pulse width of 30 nm, and average output power of 11.3 mW with long-term stability of ~ 0.06% during 3 h measurement time
We present a device using Shack-Hartmann wavefront sensor for measuring concave optical parts. A technique based on the device makes it easy to measure the main parameters of the aspherical equation of concave aspherical mirrors, radius of closest sphere Rv and eccentricity ε. The described method allows you to reconstruct the shape of the controlled surface in the form of an equation specified during its manufacture.
In this paper, we have developed a chirped-pulse Er-doped all-fiber ultrashort pulse (USP) laser suitable for highresolution Raman distributed temperature sensor (RDTS) system application. Chirped-pulse regime which is determined by the positive net-cavity dispersion of +0.12 ps2 allows us to increase the energy of the pulses for effective signal-to-noise ratio in receiving system. It also helps to avoid the influence most of nonlinear effects due to relatively long pulses duration of ~24.6 ps. The average power of the pulses is estimated to be ~1 mW from master oscillator, and currently increased up to ~15 mW by power amplifier. A relatively low repetition rate of ~ 7.925 MHz with signal-to-noise ratio ~ 69 dB was achieved using the resonator length of ~25.6 m. To characterize short-term stability of the obtained regime we have also measured the relative intensity noise of the laser, which is < -107 dBc/Hz in the range of 3 Hz - 1 000 kHz.
We study the operating speed of high-speed photodetector based on GaSb/GaInAsSb/GaAlAsSb heterostructure with frontal bridge contact depending on the wavelength, radiation power, and bias voltage. The ultra-fast fiber lasers and high-speed oscilloscope were used for measurements.
We simulate supercontinuum generation in a fiber pumped by noise-like laser pulses for possible use in mid-IR spectroscopic breath analysis. The study uses a commercially available InF3 ZBLAN fiber and a Tm-doped all-fiber passively mode-locked laser. The supercontinua are modeled with input laser parameters in two different generation regimes with an average power of 560 mW and pulse durations of 300 and 800 fs. The maximum numerically achieved spectrum extends from 1 to 8.4 μm.
We have studied generation of stable and low-noise de-chirped ultrashort solitons in bound states and we have experimentally demonstrated the formation multi-bound solitons with the controllable number of bound states 7 < N < 17 by pump power variation. A numerical simulation of the influence of various types of fluctuations on the generation mode was also carried out.
Nowadays fiber optic Raman distributed temperature sensors (RDTS) are broadly used for e.g. fire detection, gas leaks detection in pipelines and aircraft icing monitoring. The most common sources of probe pulses for RDTS are CW lasers with external intensity modulation or Q-switched lasers with a pulse duration of several tens of nanoseconds which limits the RDTS effective spatial resolution to a few meters. In this paper we have developed an Er-doped ultrashort pulse (USP) mode-locked fiber laser and implemented it to Raman distributed temperature sensor.This feature allow us to achieve high spatial resolution (down to several centimeters) and high signal-to-noise ratio in the receiving system. An all-fiber erbium-doped laser of 180 fs pulse duration and an average output power of 30 mW was used as a source of probing pulses. We have studied limiting factors of fiber sensor effective length such as high pulse repetition rate of 12.2 MHz and intensity noise of the USP laser (relative intensity noise RIN ~ 6.3 ∙ 10-4). Furthermore, the peak power of the USP laser has to be at kW level to get high signal-to-noise ratio in the receiving system and pulse duration < 100 ps at full-width-half-maximum for desirable spatial resolution of ~2 cm. Moreover, we have developed an experimental prototype of RDTS with the spatial resolution of ~ 0.1 m limited by the receiving system according to the detector bandwidth of 2 GHz, the effective sensor length of 3 m and ±1.5 °C temperature measurement error. As a result, a list of requirements for a new laser source for distributed temperature sensor was formulated.
A high-quality flat wave front is usually used to calibrate the Shach-Hartmann wave-front sensors. The article discusses the possibility of calibrating sensors with spherical wave fronts. Special attention is paid to the consideration of calibration in standard laboratory conditions. The mathematical apparatus and the scheme of the experiment are considered.
We applied terahertz (THz)-pulsed spectroscopy to study ex vivo the refractive index and absorption coefficient of human brain gliomas featuring different grades, as well as perifocal regions containing both intact and edematous tissues. Glioma samples from 26 patients were considered and analyzed according to further histological examination. In order to fix tissues for the THz measurements, we applied gelatin embedding, which allows for sustaining their THz response unaltered, as compared to that of the freshly excised tissues. We observed a statistical difference between the THz optical constants of intact tissues and gliomas of grades I to IV, while the response of edema was similar to that of tumor. The results of this paper justify a potential of THz technology in the intraoperative label-free diagnosis of human brain gliomas for ensuring the gross-total resection.
We have developed a method of the terahertz (THz) solid immersion microscopy for the reflection-mode imaging of soft biological tissues. It relies on the use of the solid immersion lens (SIL), which employs the electromagnetic wave focusing into the evanescent-field volume (i.e. at a small distance behind the medium possessing high refractive index) and yields reduction in the dimensions of the THz beam caustic. We have assembled an experimental setup using a backward-wave oscillator, as a source of the continuous-wave THz radiation featuring λ= 500 μm, a Golay cell, as a detector of the THz wave intensity, and a THz SIL comprised of a wide-aperture aspherical singlet, a truncated sphere and a thin scanning windows. The truncated sphere and the scanning window are made of high-resistivity float-zone silicon and form a unitary optical element mounted in front of the object plane for the resolution enhancement. The truncated sphere is rigidly fixed, while the scanning window moves in lateral directions, allowing for handling and visualizing the soft tissues. We have applied the experimental setup for imaging of a razor blade to demonstrate the advanced 0:2λ resolution of the proposed imaging arrangement. Finally, we have performed imaging of sub-wavelength-scale tissue spheroids to highlight potential of the THz solid immersion microscopy in biology and medicine.
We have performed the in vitro terahertz (THz) spectroscopy of human brain tumors. In order to fix tissues for the THz measurements, we have applied the gelatin embedding. It allows for preserving tissues from hydration/dehydration and sustaining their THz response similar to that of the freshly-excised tissues for a long time after resection. We have assembled an experimental setup for the reflection-mode measurements of human brain tissues based on the THz pulsed spectrometer. We have used this setup to study in vitro the refractive index and the amplitude absorption coefficient of 2 samples of malignant glioma (grade IV), 1 sample of meningioma (grade I), and samples of intact tissues. We have observed significant differences between the THz responses of normal and pathological tissues of the brain. The results of this paper highlight the potential of the THz technology in the intraoperative neurodiagnosis of tumors relying on the endogenous labels of tumorous tissues.
Currently, lasers are widely used for surgery, medical diagnostics and oncology research. Unfortunately, most of the used laser sources have a significant drawback - the lack of operating wavelength tuning possibility, which imposes significant limitations on the investigation of biological tissues spectral properties and searching for the optimal mode of their treatment. Comparison between different promising mid-IR sources was made. We report on development of mid-infrared (mid-IR) tunable lasers based on the Cr2+:CdSe single-crystals. These lasers operate in CW mode with the maximum output power of up to 2 W and possible tuning range from 2.2 to 3.6 μm.
Ultrashort pulse (USP) fiber lasers have found applications in such various fields as frequency metrology and spectroscopy, telecommunication systems, etc. For the last decade, mode-locking (ML) fiber lasers have been under carefully investigations for scientific, medical and industrial applications. Also, USP fiber sources can be treated as an ideal platform to expand future applications due to the complex ML nonlinear dynamics with a presence of high value of group velocity dispersion (GVD) and the third order dispersion in the resonator. For more reliable and robust launching of passive mode-locking based on a nonlinear polarization evolution, we used a highly nonlinear germanosilicate fiber (with germanium oxides concentration in the core ~ 50 mol. %) inside the cavity and we have obtained ultrashort stretched pulses with a high peak power and energy. In this work relative intensity noise and frequency repetition stability is improved by applying isolator-polarizer (ISO-PM) with increased extinction ratio Pext and by compensation of intracavity group-velocity dispersion from the value β2 ~ - 0.021 ps2 to ~ - 0.0053 ps2 at 1550 nm. As a result, we have obtained the low-noise stretched pulse generation with duration ~ 180 fs at a repetition rate ~ 11.3 MHz (with signal-tonoise ratio at fundamental frequency ~ 59 dB) with Allan deviation of a pulse repetition frequency for 1 s interval ~ 5,7 * 10-9 and a relative intensity noise < -101 dBc / Hz.
In this paper we consider two approaches widely used in testing of spherical optical surfaces: Fizeau interferometer and Shack-Hartmann wavefront sensor. Fizeau interferometer that is widely used in optical testing can be transformed to a device using Shack-Hartmann wavefront sensor, the alternative technique to check spherical optical components. We call this device Hartmannometer, and compare its features to those of Fizeau interferometer.
Today ultrashort pulse (USP) fiber lasers are in great demand in a frequency metrology field, THz pulse spectroscopy,
optical communication, quantum optics application, etc. Therefore mode-locked (ML) fiber lasers have been extensively
investigated over the last decade due the number of scientific, medical and industrial applications. It should be noted,
that USP fiber lasers can be treated as an ideal platform to expand future applications due to the complex ML nonlinear
dynamics in a laser resonator. Up to now a series of novel ML regimes have been investigated e.g. self-similar pulses,
noise-like pulses, multi-bound solitons and soliton rain generation. Recently, we have used a highly nonlinear
germanosilicate fiber (with germanium oxides concentration in the core ~ 50 mol. %) inside the resonator for more
reliable and robust launching of passive mode-locking based on the nonlinear polarization evolution effect in fibers. In
this work we have measured promising and stable ML regimes such as stretched pulses, soliton rain and multi-bound
solitons formed in a highly-nonlinear ring laser and obtained by intracavity group velocity dispersion (GVD) variation in
slightly negative region. As a result, we have obtained the low noise ultrashort pulse generation with duration < 250 fs
(more than 20 bound pulses when obtained multi-bound soliton generation with intertemporal width ~ 5 ps) at a
repetition rate ~ 11.3 MHz (with signal-to-noise ratio at fundamental frequency > 59 dB) and relative intensity noise
<-101 dBc / Hz.
The effect of strong localization of electromagnetic field in colloidal photonic crystals (PCs) is considered in present paper. It is shown theoretically that due to lateral modulation of dielectric permittivity of PC the sharp peaks of light’s intensity arise at the band-gap pumping, and light field decays parametrically with depth. The light itself localises at the near-surface volume of the PC and enhances nonlinear light conversion. Theoretical model to explain generic physical picture is presented for two-dimensional PC, and the analytical results are compared with numerical simulations by finite-difference time-domain method for solving the Maxwell’s equations. The fingerprints of enhanced high harmonic generation, which are observed in our experimental studies with globular quartz PCs, justify the main theoretical predictions.
Fiber Bragg grating (FBG) strain sensors are powerful tools for structural health monitoring applications. However, FBG sensor fabrication and packaging processes can lead to a non-linear behavior, that affects the accuracy of the strain measurements. Here we present a novel nondestructive calibration technique for FBG strain sensors that use a mechanical nanomotion transducer. A customized calibration setup was designed based on dovetail-type slideways that were mechanized using a stepping motor. The performance of the FBG strain sensor was investigated through analysis of experimental data, and the calibration curves for the FBG strain sensor are presented.
In this paper, an advantage of sapphire shaped crystal use for highly efficient terahertz (THz) waveguiding is discussed. The THz photonic crystal waveguide has been manufactured using the edge-defined film-fed growth (EFG) or Stepanov technique of shaped crystal growth. The effective mode index and extinction coefficient of the waveguide have been experimentally studied using the THz pulsed spectroscopy. The observed results have shown that the multichannel sapphire crystal allows guiding the THz waves with minimal dispersion in frequency range of 1:0 to 1:55 THz and minimal loss of 2 dB/m at 1:45 THz. The waveguides based on sapphire shaped crystals can be employed in wide range of THz technology applications, including non-destructive evaluation of materials, medical diagnostics, and sensing in aggressive environment.
In this paper we consider two approaches widely used in testing of wide aperture optics: Fizeau interferometer and Shack-Hartmann wavefront sensor. Fizeau interferometer that is common instrument in optical testing can be transformed to a device using Shack-Hartmann wavefront sensor, the alternative technique to check wide aperture optical components. We call this device Hartmannometer, and compare its features to those of Fizeau interferometer.
In this paper we consider two approaches widely used in optical testing: Shack-Hartmann wavefront sensor and Fizeau
interferometer technique. Fizeau interferometer that is widely used in optical testing can be easily transformed to a device
using Shack-Hartmann wavefront sensor, the alternative technique to check optical components. We call this device
Hartmannometer, and compare its features to those of Fizeau interferometer.
Recently similariton (or self-similar pulse) fiber lasers have attracted great attention due to their capabilities of highenergy
pulse generation that could find different applications in science and industry. Moreover it is very important to
reach stable pulse generation for the application as a frequency divider in optical frequency standard. Hybrid modelocking
mechanism was used for obtaining stable similariton generation at 38 MHz pulse repetition frequency. It
involves two types of mode-locking mechanisms in the cavity - saturation of carbon nanostructures absorber (recovery
time Trt ~ 500 fs) and nonlinear polarization evolution based on the nonlinear Kerr-effect (Trt ~ 10 fs). It was shown that
total intracavity dispersion should be slightly positive for generating stable similaritons with duration of less than 90 fs
and spectral bandwidth of more than 50 nm at 11.2 mW output average power that could be further applied in an all-fiber
MOPA setup.
KEYWORDS: Wavelets, Terahertz radiation, Skin, Signal processing, Spectroscopy, Signal to noise ratio, Fast wavelet transforms, Signal detection, Interference (communication), Terahertz spectroscopy
De-noising of terahertz (THz) pulsed spectroscopy (TPS) data is an essential problem, since a noise in the TPS system data prevents correct reconstruction of the sample spectral dielectric properties and to perform the sample internal structure studying. There are certain regions in TPS signal Fourier spectrum, where Fourier-domain signal-to-noise ratio is relatively small. Effective de-noising might potentially expand the range of spectrometer spectral sensitivity and reduce the time of waveform registration, which is an essential problem for biomedical applications of TPS. In this work, it is shown how the recent progress in signal processing in wavelet-domain could be used for TPS waveforms de-noising. It demonstrates the ability to perform effective de-noising of TPS data using the algorithm of the Fast Wavelet Transform (FWT). The results of the optimal wavelet basis selection and wavelet-domain thresholding technique selection are reported. Developed technique is implemented for reconstruction of in vivo healthy and deseased skin samplesspectral characteristics at THz frequency range.
This summer BMSTU SPIE Student Chapter have decided to visit Kabardino-Balkaria Republic of Caucasus (Russia) and spent there a week with children in a camp. It was called Summer school. We decided to organize it in order to engage talented and curious children in Optics and to show them how science could be funny. Education and entertainment program included such activities as lectures, optical demonstrations, laser games, hiking in the forest, and others. As a result children had a good time outdoors, learned interesting facts about optics and lasers, and of course found new friends who are keen to know more too. Four Chapter members and about 70 children of age 10-16 took part in this event.
Scattering of THz radiation in inhomogeneous media is an important reason of reducing spectral information in received THz signal for different applications. Scattering of THz radiation in thin layers of dielectric material was studied, particularly, in fabric materials. A structure of different types of cloth fabric was studied, and mathematical model of fabric was suggested, which is a system of dielectric cylinders. Scattering of THz radiation was modeled numerically. Scattering cross-section and indicatrix were calculated, and then spectral transmittance was computed. During a set of experiments THz spectral transmittance of various fabric materials were obtained and numerical results were approved. The developed method for receiving scattering characteristics of different materials can be used for extracting useful information from received THz signal.
Reconstructing the dielectric permittivity profile (depth dependence of sample dielectric permittivity) is an important inverse problem. We present a new method for permittivity profile reconstruction based on terahertz time-domain spectroscopy signal processing. Reconstruction is accomplished in two steps. First, the sample pulse function is reconstructed using sample time-domain reflection data. Low- and high-frequency noise filtering and the interpolation of the pulse function at low frequencies are then applied. Second, an invariant embedding technique is used to calculate the dielectric permittivity profile based on the sample pulse function. Samples with known permittivity profiles have been studied experimentally using this procedure in order to verify this algorithm. This algorithm is stable to additive Gaussian white noise as shown using mathematical modeling based on the finite-difference time-domain technique. Possible applications of this permittivity profile reconstruction technique are discussed.
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