Disappearance of relaxation oscillation frequencies in a multimode solid-state laser

We study analytically the Tang, Statz and deMars rate equations describing a solid-state Fabry-Perot laser. When the modes have equal gains, there is a critical number of lasing modes, above which the low-frequency relaxation oscillations responsible for antiphase dynamics disappear. These results are generalized to include unequal modal gains resulting from a Lorentzian gain profile.     

Multimode pumped continuous-wave solid-state Raman laser

We demonstrate the continuous-wave operation of a solid-state Raman laser containing a barium nitrate crystal as the Raman medium. The Raman laser, which has a singly resonant cavity, is pumped by multimode radiation. The Raman oscillation threshold is reached at approximately 2 W of pump power. As much as 500 kW/cm2 of Stokes power density at 60-kW/cm2 pump power density is obtained in the cavity.     

Multimode semiconductor laser with selective optical feedback

We study a multimode semiconductor laser subject to moderate selective optical feedback. The steady state of the laser is destabilized by a Hopf bifurcation and exhibits a period-doubling route to chaos. We also show the existence of a heteroclinic connection between a saddle node and an unstable focus that can be associated with experimentally observed multimode low-frequency fluctuations. This heteroclinic connection coexists with a chaotic attractor resulting from the period-doubling process.     

Coupling laser radiation to a fabry-perot cavity with a single-mode optical fiber

Three aspects of coupling to Fabry-Perot cavities used in optical frequency standards are discussed: the use of a single-mode optical fiber to maintain coupling stability while improving vibration isolation of the cavity, the required stability of the coupling geometry, and the phase and polarization variations resulting from fiber movement. Optical fiber coupling should be useful when laser linewidths and stabilities at the Hertz level are desired.     

Wavelength tuning characteristics of a fabry-perot semiconductor laser with an external short cavity

Wavelength tuning characteristics of a Fabry-Perot semiconductor laser with an external short cavity are analyzed, in which the oscillation wavelength can be changed by slightly altering the external cavity length. Analysis is based on rate equations for an optical power and carrier density. It is shown that the wavelength tuning range is dominated by a change of carrier density through the effect of carrier-induced refractive index change in the active layer of a laser diode. This depends on effective coupling coefficients of the optical field iteratively reflected back to the laser diode by the external mirror, and on reflection coefficients of an anti-reflection coated laser facet and the external mirror which compose the external cavity. The effective coupling coefficient is also derived using the waveguide theory and Kirchhoff ’s theory. Finally, an unstable condition which may limit a stable wavelength tuning range is shown by results of a linear stability analysis of rate equations.     

Dielectric relaxation in benzotrihalides at microwave frequencies

The permittivity and dielectric loss of benzotrifluoride and benzotrichloride in benzene and decalin solution have been measured at four microwave frequencies viz., 30.25, 24.50, 18.26 and 9.83 GHz and also at 1 MHz and optical frequency at 35°C. The permittivity and dielectric loss at different frequencies have been plotted against concentration in wt. fraction. The slopes of these straight lines have been used for complex plane plots (a″ vs a′. The dispersion locus for the main dispersion region for both compounds in benzene and decalin are Debye semicircles. The values of relaxation times for overall rotation (τ ₁) and group rotation (τ₂) have been found out and reported. Whereas τ₁ is found to be viscosity dependent, τ₂ is practically independent of viscosity. The relaxation time for group rotation has been found to vary with the frame to which it is attached. Dipole moments of the compounds in both benzene and decalin have been reported.     

Multimode laser emission from dye-doped hollow polymer optical fibre

Well-resolved multimode laser emission was observed for the first time from a freestanding microring cavity based on Rhodamine B dye-doped hollow polymer optical fibre by transverse pumping. Fibres with different diameters such as 180, 460, 640 and 800 μm were fabricated from a dye-doped hollow polymer preform. A blueshift in the mode structure was observed with decrease in fibre diameter leading to wide range tunability of the laser emission.     

Amplitudes and shapes of fabry-perot resonances in laser interference thermometry of semiconductors and dielectrics

The possibilities of the most sensitive method of laser thermometry of transparent and semitransparent plates have been determined for the varying optical properties of the surface and the conditions of the surface probing. The contrast of light interference in the plates was studied as a function of the diameter of the probing laser beam, the angle of the light incidence onto the surface and its roughness, and the presence of antireflecting and reflecting films on the surface. It is established that the contrast also depends on the transverse temperature gradient in the plate. The measurements were made on silicon single crystals and the optical K-8 glass.     

Bulk viscosity and relaxation frequencies in molten alkali nitrates

Measurements of ultrasonic absorption and speed were carried out in molten LiNO₃, NaNO₃, RbNO₃, KNO₃ and CsNO₃ by the pulse method. The results indicate that the bulk viscosity is due to a structural mechanism. The hole theory provides a model for this mechanism. Comparing the results of measurements at ultrasonic and Brillouin frequencies, the values of relaxation frequencies were deduced. These results were compared to those provided by the hole theory.     

Multimode interference filter for tuning of a mode-locked all-fiber erbium laser

We present a mode-locked all-fiber erbium laser that can be tuned in wavelength while in pulsed operation. A low-cost multimode interference bandpass filter based solely on standard fibers was employed in a sigma-shaped cavity design. By bending the fiber filter, the spectrum could be continuously shifted by up to 11.6 nm without interrupting pulsed operation, and output powers of more than 3.3 mW at pulse durations below 350 fs were achieved.     

Hydrogen plasma absorption coefficients at laser frequencies

Detailed numerical calculations of hydrogen-plasma absorption coefficients are performed for a number of laser wavelengths between 0·3371 and 10·6 μ, in a wide range of plasma densities (10¹⁶-10²¹ cm^-3) and temperatures (0·5–10⁵ eV). The numerical data are then utilized to determine plasma temperatures in pulsed linear discharge experiments which measure CO₂-laser i.r. absorption. Some features of dense plasma laser-heating are also discussed.     

Picosecond time-resolved laser emission of coumarin 102: Solvent relaxation

The time-resolved laser emission of coumarin 102 was investigated in various aprotic and protic solvents at picosecond resolution by frequency upconversion technique. The spectral shift of the transient emission spectrum is attributed to solvent reorientation and the results are discussed.     

Critical dielectric relaxation of highly deuterated betaine arsenate at microwave frequencies

The dielectric behavior of highly deuterated betaine arsenate in the frequency range up to 11 GHz is described by a Cole-Davidson relaxation. The characteristic dynamics show a critical slowing down near the para-antiferroelectric phase transition. The temperature dependence of the dielectric constant is analyzed in terms of a pseudo-one-dimensional Ising model. Evidence is given for a dependence of antiferroelectric coupling on deuteration which can explain the change from a ferro- to an antiferroelectric ordering on deuteration. Measurements of the dielectric relaxation under a dc bias field parallel to the critical axis elucidate the unusual phase transition behavior at low temperatures. A multicritical point appears in theE-T-phase diagram. Frustration effects due to competing ferro- and antiferroelectric interactions are not seen.     

Methods for fine-tuning frequencies of absorption of actinide complexes to frequencies of laser generation lines

On the basis of analysis of relationships relating, among themselves, the positions of bands in the spectra of electron and IR absorption of uranyl complexes in solutions and the donor ability of neutral ligands, the possibility is shown for fine-tuning frequencies of absorption of complexes to frequencies of argon- and CO₂-laser generation lines and using them in the development of methods of control over chemical reactions involving actinides. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 420–422, May–June, 1999.     

Absolute frequencies of CO2 laser transitions by multiplication of CO2 laser difference frequencies

Frequencies of CO₂ laser transitions have been compared to the Cs standard by a four-step frequency chain using difference-frequencies from five CO₂ isotope lasers. A tungsten-nickel diode generates the differences and their harmonics. Measurement of the 10.71 μm R(6) transition of ¹³C¹⁸O₂ is described. The absolute frequency was found to be 27 979 469.512(65) MHz. Simultaneously, the four other CO₂ frequencies in the chain were also determined.     

Multimode fiber laser beam cleanup based on stochastic parallel gradient descent algorithm

We present experimental research on multimode fiber laser beam cleanup based on a stochastic parallel gradient descent (SPGD) algorithm. The multimode laser is obtained by injecting a 1064 nm central wavelength single mode fiber laser into a multimode fiber and the system is setup by using phase only liquid crystal spatial light modulators (LC-SLM). The quality evaluation function is increased by a factor of 10.5 and 65% of the laser energy is encircled in the central lobe when the system evolves from open-loop into close-loop state. Experimental results indicate the feasibility of the multimode fiber laser beam cleanup by adaptive optics (AO).     

Multimode diode laser correlation spectroscopy using gas-filled porous materials for pathlength enhancement

A novel pathlength enhancement approach has been applied in multimode diode laser correlation spectroscopy measurements of gas concentrations. Reference and sample gas cells made of porous polystyrene foam and alumina ceramic, respectively, were employed in proof-of-principle measurements of molecular oxygen. Equivalent pathlengths of 164 and 52?cm were obtained for the reference and sample cells with physical pathlengths of 4 and 1?cm, respectively. With a measurement time of 60?s, a physical-pathlength-integrated sensitivity of 15?ppm?m was achieved with an enhancement of more than one order of magnitude compared with an open-air setup. Practical application aspects were investigated including gas cell size and light alignment tolerance. This generic approach for pathlength enhancement provides advantages in terms of compactness and robustness.     

Electro-optical detection of ZnO-Based unbalanced fabry-perot resonators

Zinc oxide films prepared by magnetron sputtering are introduced to the electro-optical detection to improve the voltage sensitivity. With the help of large piezoelectric effect, ZnO films make up for the deficiency of the low electro-optical coefficient. In this paper, we illustrated the mechanism of piezo-induced enhancements, and measured the electric signals on the line of coplanar waveguides. This method breaks through the limit of material species for optoelectronically responsive sensing, and shows promising applications in optical detection.