Absorption spectra of sixteen polymers have been examined using terahertz (THz) time-domain spectroscopy system in
THz frequency range (0.1 - 3 THz). All the experimental data showed a characteristic wide absorption peak at the
frequencies around 2.5 THz accompanied by smoothly decreasing of refraction (except "transparent" polymers). Such
behavior could be described by a modified Debye model. We also tried to clarify the impact of flexibility,
polymerization, crystalline degree and number of monomer end groups on polymer dielectric properties. We suggest that
the absorption mechanism in spectral range lower than 3 THz is related with relaxation and amorphous state of polymers.
We report about preliminary results of using upconversion luminophores (UCLs) for tissue imaging. We manufactured
luminophores particles of different sizes and tissue-mimicking phantoms for this study. Results of our experiments on
imaging correspond well with our Monte Carlo simulations of luminescence detection from an imbedded vessel filled
with UCLs.
The advent of the Dense Wavelength Division Multiplexing (DWDM) technology in Optical Fiber Networks (OFNs) has resulted in the necessity of developing advanced Optical Add/Drop Multiplexers (OADMs) on the basis of submicron Bragg gratings. The OADMs for dense multichannel OFNs with bit rates 10-40 Gbits/s per channel and channel spacing 200, 100 and 50 GHz must possess rectangular-shaped reflection/transmission spectra and linear phase characteristic within the stop/passband. These features can not be achieved using uniform Bragg gratings and therefore nonuniform gratings with space-modulated coupling coefficient and/or phase shifts π should be used. We present the recent advances in the design and fabrication of narrowband wavelength-selective filters for DWDM applications using submicron nonuniform Bragg gratings. The peculiarities of propagation, interaction and diffraction of electromagnetic waves in nonuniform grating structures are considered. Narrowband reflection filters using single-mode quartz fibers with side-polishing and relief gratings on dielectric and polymeric materials are designed and fabricated. The filters have nearly rectangular shape of the stopband with 0.4-1.6 nm width and peak reflectivity R > 99% in the 1.55 μm wavelength region. The architecture of multichannel OADMs for high-speed OFNs using planar polymer Integrated Photonic Circuits (IPCs) with biharmonic Bragg gratings, possessing phase shifts π, is discussed.
The advent of the technology of Dense Wavelength Division Multiplexing (DWDM) in Optical Fiber Networks (OFNs) has resulted in the necessity of developing advanced Optical Add/Drop Multiplexers (OADMs) on the basis of submicron Bragg gratings. The OADMs for dense multichannel OFNs with bit rates 10 – 40 Gbits/s per channel and channel spacing 200, 100 and 50 GHz must possess rectangular-shaped reflection/transmission spectra and linear phase characteristic within the stop/passband. These features can not be achieved with uniform periodic Bragg gratings and therefore nonuniform gratings with space-modulated coupling coefficient should be used. We present the recent advances in the design and fabrication of narrowband wavelength-selective optical filters for DWDM applications on the basis of single-mode fibers with side-polishing and periodic relief Bragg gratings with apodized coupling coefficient. The peculiarities of propagation, interaction and diffraction of electromagnetic waves in nonuniform Bragg grating structures are considered. Narrowband reflection filters based on side-polished fibers and submicron relief gratings on SiO2 and SiO materials are designed and fabricated. The filters have stopband width 0.4 – 0.8 nm and peak reflectivity R > 98% in the 1.55 ?m wavelength communication region. Narrowband flat-top reflection filters for DWDM applications based on side-polished fibers and periodic relief Bragg gratings are designed. The schemes for multichannel integration of Bragg grating filters into OFNs are presented.
The precision technique for side polishing of single-mode quartz fibers is developed. The technique comprises cutting curved groove in silica block, gluing a section of bare fiber into the groove, and subsequent grinding and polishing of the silica block/fiber assembly. We succeeded in fabricating up to six side-polished fibers in one block with effective interaction length 2-4 mm. The accuracy of polishing depth was achieved at 1 micrometers using in-situ monitoring of transmission of 1.3 micrometers laser light through the fiber. The developed technique combines high accuracy, reproducibility and low cost in commercial production. Side- polished single-mode fibers fabricated with this technique can find application as elements of Bragg grating transmission filters, narrowband reflectors, optical add/drop multiplexers, couplers, polarizers, sensors, etc.
The technology of dense wavelength division multiplexing (DWDM), which is widely used in modern optical fiber communication networks for increasing bit rates, requires development of advanced Bragg grating filters with improved rectangular-shaped reflection/transmission spectra and linear phase characteristics within the stop/passband. Such spectra cannot be achieved with uniform Bragg gratings, and therefore nonuniform gratings with space-modulated amplitude should be developed. We present the new approach to the design of nonuniform grating profiles, which is based on utilization of superimposed biharmonic gratings with sinusoidally modulated amplitude and periodically located distributed phase shifts (pi) . With this approach Bragg grating profiles possessing narrowband rectangular-shaped transmission spectra are designed using coupled-wave theory and computer simulations.
An approach is proposed to phase conjugation (PC) of broadband light that is a problem for traditional PC at stimulated Brillouin scattering. Experimental and simulation results of a work aimed to obtain a complex conjugated wave of a broadband laser beam in three-wave mixing in a nonlinear crystal are presented. It is demonstrated by direct wavefront measurements that in mixing of the broadband first harmonic signal wave with the single- frequency second harmonic pump wave having quasi-plane wavefront the arising broadband first harmonic idler wave has a wavefront complex conjugated to the one of the signal light.
Transmission and reflection of ultrashort (100 fs - 100 ps) transform-limited Gaussian light pulses through/from biharmonic Bragg grating with sinusoidally - modulated amplitude and periodically located phase shifts ? is considered theoretically using numerical simulations. The method of investigation comprises coupled-wave theory and spectral approach. The effect of input pulse duration upon the shape of transmitted and reflected pulses is examined. It is shown that 100 Ps input pulses are transmitted through biharmonic grating without distortions if centered at Bragg wavelength (inside grating passband) and are reflected from the grating without distortions if centered at wavelengths inside the stopband. The results of this study indicate that biharmonic gratings can find application for demultiplexing and multiplexing optical signals in multichannel high-bit-rate fiber communication networks with bit rates up to 10 Gbits/s per channel.
Zig-Zag-Slab-Amplifier had active element 4.5 X 40 X 43 cm3 in dimensions, six-bounce geometry of beam pass, pump cavity with transverse lamp orientation, and diffuse reflectors. Tests showed: the gain is equal to 8 per single pass at 77 kJ of pumping; its non-uniformity over about half of amplifier aperture is less than +/- 2.5 percent; the depolarization distortion over operating area is less than 0.5 percent.
R. Bikmatov, Charles Boley, I. Burdonsky, V. Chernyak, A. Fedorov, A. Goltsov, V. Kondrashov, S. Koptyaev, N. Kovalsky, V. Kuznetsov, David Milam, James Murray, Michael Pergament, V. Petryakov, Ruslan Smirnov, Victor Sokolov, E. Zhuzhukalo
Pinhole plasma effects on parameters of the laser beam passing through the spatial filter in conditions of interest for large scale ICF laser facilities were investigated. The experiments on pinhole irradiation were conducted at power density range 1010-1011 W/cm2 with approximately 15 ns laser pulses. Al, Fe, and Ta pinholes were used. The diagnostic approach was chosen based on probing the pinhole region with frequency doubled 3-ns-long laser pulse. Ablative-plasma dynamics was studied with shadowgraphy and interferometry. Also measured were the parameters of transmitted probing beam in the near- and far-fields. The rate of pinhole 'closure' is found to decrease with the increase in the atomic number of pinhole material. The rate o pinhole closure ranges from approximately 5*106 cm/s for aluminum pinhole down to approximately 2*106 cm/s for tantalum pinhole in experiments with power density at the pinhole edge of approximately 50 GW/cm2. For aluminum and steel pinholes the parameters of the transmitted probing beam deteriorate to unacceptable level for approximately 15-20 ns after the irradiation start. In the same experimental conditions the pinholes of tantalum exhibits acceptable performance till the end of the irradiation process. Fast plasma jets converging to the pinhole axis with velocities up to approximately 107 cm/s and significantly deteriorating transmitted probing beam quality are observed. Reasonable agreement was found between the data obtained in experiments with circular pinholes and linear edge experiments.
Propagation and interaction of electromagnetic waves in nonuniform Bragg grating structures with space-modulated coupling coefficient and phase shifts are investigated theoretically on the base of coupled-wave model. For gratings with antisymmetically-modulated coupling coefficient the simple analytical expression for reflectivity and transmitivity are obtained. They involve the infinite set of Born approximations and are valid not only for 'shallow' gratings with (kappa) L<<1 (kappa is coupling strength, L is grating length), but also for 'deep' gratings with appaL>=1. One the base of these expressions the method for designing the shape of Bragg grating structures, which possess the required transmission spectra, is proposed. The gratings with space- modulated amplitude and phase shifts, possessing the nearly rectangular transmission spectrum, are designed with their potential application for channel separation in high-bit- rate optical fiber communication networks.
This paper summarizes the results of theoretical and experimental investigations concerning the designing and fabrication of submicron nonuniform Bragg gratings with phase shifts on semiconductors, which were obtained in the Laser Research Center of Russian Academy of Sciences during the last two years. The applicability of phase-shifted gratings for wavelength-selective filtering and channel separation in high-bit-rate optical fiber networks is discussed.
The concept to use a slab as active element, working in zig- zag geometry, and also as Fresnel rhomb, seems to be rather attractive. However, in this case different depolarization effects in active element are of crucial importance. We have carrier out the estimation of depolarization effects arising both due to mechanical loading of an active element at its fastening and due to thermooptical distortions. To check up these rigid requirements to depolarization (0.1% - 0.01%) careful measurements of depolarization effects and their sources are being carried out. Mechanical loading gives one of the main contributions in depolarization at fastening of active element. Using model experiments with glass Fresnel rhomb under mechanical loading we have measured depolarization effects. It is proposed to use additional glass plate to compensate beam depolarization in zig-zag slab. The received results allow to expect successful use of the slab amplifier as a Fresnel rhomb providing rather high quality of optical materials of active element.
The processes of propagation, interaction and diffraction of electromagnetic waves in periodic mediums with sinusoidally modulated coupling coefficient, which is provided by biharmonic multiple (pi) -phase shifted Bragg grating, are investigated theoretically. The applicability of biharmonic grating for improving the transmission characteristics of narrowband wavelength-selective optical filters is discussed.
Beam anomalies in transmission of Gaussian light beams through the dielectric waveguide with incorporated periodic relief grating are investigated theoretically. The anomalies take place near the critical incidence, determined by Bragg condition, when the divergency of the incident beam is of the order or greater than the angular width of Bragg resonance. They manifest themselves as macroscopic transformation of transmitted beam shape and can be accompanied by multiple oscillations of intensity in the beam spot. The transformation of the beam shape is stipulated by interference effect due to the resonant excitation of guided modes in the waveguide by incident bounded light beam and subsequent rescattering of these modes in the direction of transmitted beam propagation.
The analysis of light propagation, diffraction and amplification in active multilayer corrugated waveguides with incorporated nonharmonic relief gratings, carried out by Rayleigh-Fourier formalism, is presented. On the base of this theoretical analysis the new possibilities of biharmonic grating for the design of single-frequency DFB lasers are discussed. The lasing characteristics of DFB heterolasers with biharmonic resonator are determined. It is shown that the biharmonic resonator with phase shift pi and space-modulated coupling coefficiency is capable of providing stable single-frequency oscillation at Bragg wavelength. The three-beam maskless holographic method for the fabrication of submicron biharmonic relief structures on the surface of n- AIIIBV semiconductors is developed. The biharmonic structure of period d equals 0.5 micrometer, possessing phase shift pi, is fabricated on n-InP in the process of laser-induced maskless wet photochemical etching under action of three Ar+ laser beams.
The main goal of this report is to analyze the feasibility to use phase conjugation (PC) in megajoule class lasers for inertial confinement fusion (ICF). Phase conjugation has the potential for relieving the fabrication requirements to any optical elements and for compensation of residual thermo-optical distortions. The key problems for phase conjugation are the dynamic range over which phase conjugation operates efficiently, the reflected energy limit, the effect of the laser light bandwidth, and the possibility to mount the PC subsystem into a real megajoule laser. Analytical results of the possibility of use of a classical stimulated Brillouin scattering (SBS) mirror and based on a nonlinear crystal phase conjugator in ICF- laser are presented.
This report presents results of a study of reabsorbed spontaneous emission (RSE) influence on the spatial inversion distribution in an active medium of large phosphate slab. Experimental results are also presented for a model case of active medium pumping by monopulse second harmonic of Nd-glass laser having specific profile of spatial distribution. RSE is observed for this case. A model is proposed for RSE simulation in solid bodies of arbitrary shape. It is shown that RSE influence is taken into account completely by first approximation, considering the rest fluorescence part of active medium to be an additional source of pump in order to define inversion in the arbitrary point of active medium. Fluorescence kinetics coincides with numerical results of a model experiment. Calculations are carried out of inversion redistribution due to RSE in a large aperture slab of phosphate glass demonstrating that spatial distribution distortions of inversion are small by the moment when inversion maximum is achieved, however, 10 - 12% of stored energy may relate to RSE effect at the absence of amplified spontaneous emission (ASE). Fluorescence kinetics measurements are carried out over slab cross-section demonstrating good agreement with model results.
The new concept of single-mode resonator for single-frequency distributed feedback (DFB) lasers is presented. This concept is based on embedded nonharmonic distributed Bragg structure with sinusoidally modulated coupling coefficient, which is a combination of two superimposed sinusoidal Bragg gratings of equal heights and slightly different periods. Resonant frequencies and corresponding threshold gains of such distributed resonator are calculated theoretically by using the coupled-mode theory. The designed resonator provides stable single-frequency oscillation and has lasing characteristics (frequencies and thresholds) very similar to those of well-known distributed resonator with quarter-wavelength shift. The developed concept of resonator with sinusoidally modulated coupling coefficient can be applied both to semiconductor laser diodes with incorporated Bragg relief grating and to DFB fiber lasers with refractive index grating. The important advantage of designed new single-mode Bragg structure, as compared to quarter-wavelength-shifted structure, is that it can be fabricated very easily on semiconductor surfaces and in photosensitive fibers by direct three-beam holographic writing.
Interaction of electromagnetic guided modes with index/gain Bragg gratings in step-index round optical fibers is considered theoretically. The analysis includes the construction of exact solution of Maxwell equations in core and cladding regions, which strictly obeys the electromagnetic boundary conditions on core/cladding interface and on Bragg grating boundaries. The grating profiles, for which the exact solutions of Maxwell equations can be expressed in the closed analytical form, are determined and the corresponding expressions for scattered electromagnetic fields are presented. The constructed solution enlarges the class of exactly solvable diffraction problems. It can be used to examine and to control numerical theories of light propagation and scattering optical fibers with intracore Bragg gratings.
The new concept of a single-mode resonator for single-frequency distributed feedback (DFB) lasers is presented. This concept is based on an embedded nonharmonic distributed Bragg structure with a sinusoidally modulated coupling coefficient, which is a combination of two superimposed sinusoidal Bragg gratings of equal heights and slightly different periods. Resonant frequencies and corresponding threshold gains of such a distributed resonator are calculated theoretically by using the coupled-mode theory. The designed resonator provides stable single-frequency oscillation and has lasing characteristics (frequencies and thresholds) very similar to those of a well-known distributed resonator with quarter-wavelength shift. The developed concept of a resonator with a sinusoidally modulated coupling coefficient can be applied both to semiconductor laser diodes with incorporated Bragg relief grating and to DFB fiber lasers with refractive index grating. The important advantage of designed new single- mode Bragg structure, as compared to quarter-wavelength-shifted structure, is that it can be fabricated very easily on semiconductor surfaces and in photosensitive fibers by direct three- beam holographic writing.
We report a new method for the highly efficient transformation of a linear polarized laser beam into a beam with any required polarization. The physics of polarization conversion is based on resonant excitation of surface electromagnetic waves (SEWs) on metallic gratings with period d ~ λ (where λ is wavelength of light) and subsequent SEW rescattering into the direction of specular reflection. The parameters of such gratings (period, depth, and groove profile) are calculated by means of the developed simple analytical theory. The grating phase retarders with reflectivity R = 92 to 97% for transformation of linearly polarized CO2 and CO laser beams into a circular polarized beam are fabricated. The efficiency of these phase converters included in a 1.5-kW CO2 laser technology system for metal cutting is demonstrated. The converters essentially enhance the quality and velocity of cutting and their high strength to resist optical damage is shown.
We report the new method of highly efficient transformation of linear polarized laser beam into another beam, with any required polarization performed by reflection grating. The parameters of this grating were calculated theoretically. The grating converters with a reflectivity of R >= 96% for transformation of linearly polarized CO2 laser beam into a circular polarized beam have been fabricated. The efficiency of these converters included in a 1.5 kW CO2 laser technological system for metal cutting is demonstrated.
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