Thermal lensing in silver gallium selenide parametric oscillator crystals

We performed an experimental investigation of thermal lensing in silver gallium selenide (AgGaSe(2)) optical parametric oscillator crystals pumped by a 2-μm laser at ambient temperature. We determined an empirical expression for the effective thermal focusing power in terms of the pump power, beam diameter, crystal length, and absorption coefficient. This relation may be used to estimate average power limitations in designing AgGaSe(2) optical parametric oscillators. We also demonstrated an 18% slope efficiency from a 2-μm pumped AgGaSe(2) optical parametric oscillator operated at 77 K, at which temperature thermal lensing is substantially reduced because of an increase in the thermal conductivity and a decrease in the thermal index gradient dn/dT. Cryogenic cooling may provide an additional option for scaling up the average power capability of a 2-μm pumped AgGaSe(2) optical parametric oscillator.     

Efficient continuous-wave frequency doubling of a tunable CO(2) laser in AgGaSe(2)

Second-harmonic output at 4.6-5.5 mum of the order of 6 mW with a 0.12% external conversion efficiency has been obtained by pumping a AgGaSe(2) crystal with a low-power cw CO(2) laser. The surface damage threshold of AgGaSe(2) for cw radiation was found to be inside the limit of 33-45 kW/cm(2) in the 9.2-10.8-mum wavelength region. Another important limitation of the pump power connected with a thermal lensing effect in crystal was determined experimentally. A comparison was made of AgGaSe(2) and ZnGeP(2) crystals as materials suitable for the efficient generation of the second harmonic of cw CO(2) laser radiation.     

Nonlinear absorption of laser light in Bi12GeO20 single crystals

Two-photon absorption (TPA) at 532 nm was investigated in BGO (Bi₁₂GeO₂₀) single crystals using a 16 ps pulse width Nd: YAG laser. The TPA coefficient was measured to be 2.2–2.6 cm/GW for undoped BGO. This suggests that nonlinear absorption plays a significant role in the carrier generation processes induced by short laser pulses. The TPA coefficient for heavily Al-doped BGO is only slightly smaller (1.9–2.3 cm/GW) than that of the undoped BGO. This result has specific importance since linear absorption in the visible wavelength region is strongly suppressed by Al doping.     

Rigorous theory of the diffraction of Gaussian beams by finite gratings: TM polarization

Diffraction of TM-polarized Gaussian beams by N equally spaced slits (finite grating) in a planar perfectly conducting thick screen is treated. We extend to the TM polarization case the results of a previous paper where the TE polarization was considered. The far-field diffraction patterns, the transmission coefficient tau, and the normally diffracted energy E as a function of several optogeometrical parameters are analyzed within the so-called vectorial region. The existence of constant-intensity angles in the far field when the incident beam wave is scanned along the N slits is shown. In addition, the property E=Ntau/lambda, valid in the scalar region, is extended to the TM polarization case in the vectorial region, lambda being the wavelength. The coupling between slits is analyzed, giving an oscillating amplitude-decreasing function as the separation between slits increases, where the period for these oscillations is the wavelength lambda. Finally, the extraordinary optical transmission phenomena that appear when the wavelength is larger than the slit width (subwavelength regime) are analyzed.     

Photoinduced non-linear optical effects in lanthanum calcium borate single crystals

We have revealed that lanthanum calcium borate (La₂CaB₁₀O₁₉) crystals show two-photon absorption (TPA) favored by UV laser field. UV-induced TPA measurements were performed under illumination of a Xe-F laser (λ = 217nm) as a photoinducing (pumping) beam. This pumping laser beam created a thin surface layer (about 85 nm) that was a source of the observed photoinduced TPA. Use of the longer photoinducing wavelength leads to a substantial decrease of the photoinduced TPA. The performed DFT calculations of the band energy dispersion clearly show that the effect observed is due to specific band energy dispersion between the sub-bands. The Raman shifted Nd-YAG laser radiation (λ = 1.9 μm) as well as its second and fourth harmonics (λ = 950 and λ = 475 nm, respectively) were used as fundamental (probing) beams of the TPA. The highest values of the TPA β coefficient were achieved for polarization of the pumping light directed along the second-order crystallographic axis. The obtained values of TPA coefficients are higher than for BiBO crystals that were also investigated by us.     

Difference Frequency Generation of 5-18 mum in a AgGaSe(2) Crystal

Mid-infrared radiation in the 5-18-mum range has been obtained by difference frequency generation in a AgGaSe(2) crystal by pumping with the output of a type I LiNbO(3) optical parametric oscillator (OPO). Here we suggest the use of a LiTaO(3) retarder to achieve an orthogonal state of polarization between OPO outputs that are necessary for efficient pumping of a AgGaSe(2) crystal. Several tens of kilowatts of peak power near 8 mum and continuously tunable operation in the above range have been obtained.     

Upconversion fluorescence and optical power limiting effects based on the two- and three-photon absorption process of a new organic dye BPAS

A new organic dye, trans-4, 4'-bis(pyrrolidingl) stilbene (BPAS), with large two-photon absorption (TPA) and three-photon absorption (3PA) has been synthesized. The molecular TPA cross section sigma2' at 550-670 nm and the 3PA cross section sigma3' at 720-1000 nm have been measured. The biggest sigma2' and sigma3' was 5.77 x 10(-47) cm4s/photon and 27 x 10(-75) cm6 s2 at 600 and 980 nm, respectively. In the experiment process we found that the strongest TPA wavelength is not at two times of the strongest linear absorption wavelength, but there is some blue shift. The 3PA-induced optical power-limiting properties have also been illustrated at 980 nm.     

Determination of optical coefficients and fractal dimensional parameters of cancerous and normal prostate tissues

Optical extinction and diffuse reflection spectra of cancerous and normal prostate tissues in the 750 to 860 nm spectral range were measured. Optical extinction measurements using thin ex vivo prostate tissue samples were used to determine the scattering coefficient (μ(s)), while diffuse reflection measurements using thick prostate tissue samples were used to extract the absorption coefficient (μ(a)) and the reduced scattering coefficient (μ'(s)). The anisotropy factor (g) was obtained using the extracted values of μ(s) and μ'(s). The values of fractal dimension (D(f)) of cancerous and normal prostate tissues were obtained by fitting to the wavelength dependence of μ'(s). The number of scattering particles contributing to μ(s) as a function of particle size and the cutoff diameter d(max) as a function of g were investigated using the fractal soft tissue model and Mie theory. Results show that d(max) of the normal tissue is larger than that of the cancerous tissue. The cutoff diameter d(max) is observed to agree with the nuclear size for the normal tissues and the nucleolar size for the cancerous tissues. Transmission spectral polarization imaging measurements were performed that could distinguish the cancerous prostate tissue samples from the normal tissue samples based on the differences between their absorption and scattering parameters.     

Measuring cutoff wavelength of HE21, TE01, and TM01 modes: a novel method using single-polarization launching

A new method for measuring the cutoff wavelength of HE21, TE01, and TM01 modes in single-mode fibers is reported. The method is based on the difference in polarization between the HE11 and second-order modes launched in the offset condition. A single-polarization fiber several meters in length is used as a launching fiber to eliminate cladding mode disturbance and realize linear polarization launching into a test fiber. Good agreement between theoretical and measured cutoff wavelengths for fibers with various refractive-index profiles confirms the high accuracy of the method. A resolution of +/-0.005 microm is attained with the method.     

Polarization separation in titanium-diffused waveguides on lithium niobate substrates

The influence of technological parameters of the process of thermal diffusion of titanium in lithium niobate substrates on the polarization-dependent losses of obtained optical waveguide has been theoretically studied. It is established that the anisotropy of refractive index variation leads to different conditions of polarization eigenmode cutoff that can be used for separating extraordinary and quenching ordinary polarization modes. Experiments with titanium-diffused waveguides showed the possibility of extraordinary polarization mode separation above 40 dB in the C-range (1530–1565 nm) of telecommunication wavelengths.     

Thermo-Optic Dispersion Formula of AgGaSe(2) and Its Practical Applications

The thermo-optic constants of AgGaSe(2) have been measured at 2.052, 3.3913, 5.2955, and 10.5910 mum. These results combined with values reported in the literature were used to construct the thermo-optic dispersion formula that correctly reproduces the temperature-dependent phase-matching conditions for second-harmonic generation and sum-frequency generation of a CO(2) laser as well as the thermally induced lensing effects for the 2.05-mum pumped parametric oscillators.     

Wavelength and temperature performance of polarization-transforming fiber

We have theoretically and experimentally investigated an optical fiber with circular polarization modes on one end and linear polarization modes on the other end. We call this fiber a polarization-transforming fiber because the local modes, or polarization states they represent, are converted from linear to circular, and visa versa, in the fiber. We have developed and implemented a postdraw process for making polarization-transforming fiber samples 30 mm long with losses less than 1 dB and a polarization-mode conversion from circular to linear greater than 20 dB. Also, we have modeled and measured the dependence on wavelength and temperature of polarization-transforming fiber samples. The measured normalized wavelength dependence of a sample fiber 30 mm long was approximately 1.4 x 10(-4) nm(-1), and the measured normalized temperature dependence was approximately 6 x 10(-4) degrees C(-1). These values are better in some cases than values for conventional high-birefringent fiber quarter-wave plates.     

Generation of high-repetition-rate femtosecond pulses from 8 to 18 microm

We generated subpicosecond pulses from 8 to 18 mum by difference-frequency mixing in a 1-mm-thick AgGaSe(2) crystal, the 130- and 180-fs output pulses (1.45 < lambda < 1.85 mum) from an 84-MHz-repetition-rate optical parametric oscillator. Numerical simulations show that intrapulse and interpulse group velocity dispersion determine minimum pulse duration above and below 15 mum, respectively. By cross correlation (upconversion) of 10.5-mum pulses with 90-fs, 810-nm pulses in AgGaS(2), the pulse length was measured to be 310 fs in good agreement with simulations.     

Experimental and theoretical study of skylight polarization transmitted through Snell's window of a flat water surface

The celestial polarization pattern may be scrambled by refraction at the air-water interface. This polarization pattern was examined in shallow waters with a submersible polarimeter, and it was calculated by using land measurements ('semiempirical predictions') and models of the skylight polarization. Semiempirically predicted and measured e-vector orientations were significantly similar. Conversely, predicted percent polarization was correlated but lower than measurements. Percent polarization depended on wavelength, where at high sun altitudes maximal percent polarization generally appeared in the UV and red spectral regions. The wavelength dependency of polarization may lead to differential spectral sensitivity in polarization-sensitive animals according to time and type of activity.     

Polarization sensitivity of the SUMER instrument on SOHO

The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory (SOHO) satellite is sensitive to the state of linear polarization of the incident radiation primarily owing to two optical elements, the holographic grating and the wavelength scan mirror. The large angle of incidence of light striking the scan mirror, which varies from roughly 73.3 degrees to 81.6 degrees (with respect to the mirror normal), causes the mirror to act as a linear polarizer. Similarly, the spectrometer grating operates at incidence angles between 16.7 degrees and 35.0 degrees , adding to the polarization effect at some wavelengths. Measurement and characterization of this polarization sensitivity as a function of wavelength were performed with the engineering model optics (scan mirror and grating) and synchrotron radiation, which is nearly 100% linearly polarized, from the Super Anneau de Collisions d'Orsay (SUPERACO) positron storage ring in Orsay. The polarization sensitivity or modulation factor of the SUMER instrument was found to be between 0.4 and 0.6, depending on the wavelength and the angle of incidence of light striking the scan mirror; this agrees with the calculated polarization properties based on the measured optical constants for the silicon carbide mirror and grating.     

Femtosecond measurements of two-photon absorption coefficients at lambda = 264 nm in glasses,crystals, and liquids

Using ultraviolet femtosecond pulses with high irradiance stability, we measured the two-photon absorption (TPA) coefficients in a number of substances with a total accuracy of approximately 10%. Six commercial fused-silica samples (KU-1, Coming 7940, SQ, Suprasil, Herasil, and Infrasil) possess TPA coefficients (beta values) of approximately 2 x 10(-11) cm/W. For crystalline quartz and sapphire, the following beta values were obtained: (1.2 +/- 0.2) x 10(-11) and (9.4 +/- 1.2) x 10(-11) cm/W, respectively. In beta-barium borate crystal the TPA coefficient depends on crystal cut, beam polarization, or both and varies from (47 +/- 5) x 10(-11) to (68 +/- 6) x 10(-11) cm/W. For eight liquids that were studied (water, heavy water, ethanol, methanol, hexane, cyclohexane, 1,2-dichloroethane, and chloroform) the beta value lies from (34 +/- 3) x 10(-11) to (95 +/- 11) x 10(-11) cm/W.     

Liquid-crystal blazed grating with azimuthally distributed liquid-crystal directors

We propose a novel formation method of arbitrary phase profiles of circular light by controlling azimuthal angles of liquid-crystal directors; its principle is described theoretically. A new liquid-crystal blazed grating is demonstrated by use of the proposed method. It is revealed that the first-order diffraction efficiency reaches the maximum value (theoretically 100%, experimentally approximately 90%) at an optimum applied voltage when the phase difference between the extraordinary and ordinary rays agrees with one-half the wavelength. Furthermore, the polarization states of the diffracted light beams are analyzed by Stokes parameter measurements, and unique polarization-splitting properties are revealed.     

Nonlinear Defocusing of Laser Radiation in a Nonlinear Absorbing Medium

Abstract

Nonlinear thermal defocusing of laser radiation in water due to two-photon absorption is detected. A theoretical interpretation of the results obtained is carried out. The method developed permits the definition of two-photon absorption (TPA) coefficient of a substance. The TPA coefficient β of water at λ = 216 nm is measured to be (2 ± 0·5) × 10^?10 cm W^?1.     

Large-area Fabry-Perot modulator based on electro-optic polymers

We present a large-area electro-optic Fabry-Perot modulator utilizing a photoaddressable bis-azo polymer placed between two dielectric mirrors with an open aperture of 2 cm. A modulation efficiency of 1% at an effective modulation voltage of 20 V for a wavelength of 1.55 microm is demonstrated. By comparing distance tuning of the cavity with wavelength tuning, an effective electro-optic coefficient of -7 pm/V is measured.     

Polarization of holographic grating diffraction. II. Experiment

The transmittance, ellipsometric parameters, and depolarization of transmission, diffraction, and reflection of two volume holographic gratings (VHGs) are measured at a wavelength of 632.8 nm. The measured data are in good agreement with the theoretical simulated results, which demonstrated the correlation between the diffraction strength and the polarization properties of a VHG. Vector electromagnetic theory and polarization characterization are necessary for complete interpretation of the diffraction property of a VHG. The diffraction efficiency is measured at 532 nm in a polarization-sensing experiment. The measured data and theoretical simulation have demonstrated the potential application of the holographic beam splitter for polarization-sensor technology.