Pump Radiation Power Research Articles

Method for determination of effective coupling coefficients in (1 + 1)GTWave fibers

Abstract This paper proposes a theoretical and experimental method to determine the effective coupling coefficients for the pump radiation in (1 + 1)GTWave fibers and also gives mathematical expressions derived from analytical solutions for the pump radiation power distribution to estimate values thereof. The conditions for carrying out experiments to measure effective coupling coefficients, as well as results of these measurements are presented.

Energy and spectral characteristics of a parametric generator based on a nonlinear ZnGeP2 crystal pumped by a Ho : YAG laser

We have experimentally realised the average generation power of a parametric generator based on a ZnGeP2 single crystal, which constituted ∼1.5 W at an efficiency of ∼28.6 % in the wavelength range of 3.5 – 4.8 μm with an average pump radiation power of ∼5.5 W and its energy density of ∼ 0.47 J cm−2. It is shown that there are threshold conditions for the average pump power, providing the realisation of parametric generation. In this case, an increase in the generation efficiency with an increase in the energy density of pump radiation to ∼ 0.4 J cm−2 is observed; with its further increase, the output power increases due to a decrease in the parametric generation threshold. Parametric generators (PGs) based on ZnGeP2 single crystals with such characteristics are promising for solving many applied problems, including generation of terahertz radiation when nonlinear crystals are pumped by radiation from these PGs at difference frequency.

Feedback-controlled and digitally processed coherent population trapping resonance conversion in 87Rb vapour to high-contrast resonant peak

This work reports on the study of a new approach to achievement of high-contrast resonant signals from coherent population trapping (CPT) resonances in 87Rb vapour based on feedback control and real-time digital processing of several measured parameters. This method consists in stabilisation of the value of a function depending on several system parameters measured as the frequency difference of the bichromatic pump radiation is scanned through adjustment of the pumping radiation power with a feedback loop. The present work made use of two such parameters: the pumping radiation power incident on and exiting from the optical cell. Exploration of the proposed method has shown that stabilisation of a linear combination of these two parameters results in a resonant peak whose contrast exceeds that of regular CPT resonance by more than two orders of magnitude at relatively slow CPT resonance scan rates, (scanning frequency of the frequency difference of the bichromatic field ∼1 Hz). When dynamically exciting the CPT resonance (the scan frequency of the frequency difference of the bichromatic field equal to 2 kHz), the resonant peak contrast was enhanced by over an order of magnitude.

Coherent population trapping on 87Rb atoms in small-size absorption cells with buffer gas

Coherent population trapping (CPT) on 87Rb atoms in neon atmosphere has been studied in small-size glass absorption cells under conditions of pumping with narrow-band laser radiation at the D2 line of the main doublet. Parameters of the absorption signal have been measured in 3-mm-diameter cells at buffer gas (Ne) pressures varied within 200–400 Torr, cell temperatures within 65–120°C, and pumping radiation power densities within 30–400 μW/cm2. Optimum values of the buffer gas pressures, cell temperature, and pumping power are determined at which the short-term instability of the resonance line is at minimum. Orientational shifts of the CPT resonance signal in gas-filled cells and small-size cells with antirelaxation coating have been compared.

Generation of optical-terahertz biphoton pairs via spontaneous parametric down-conversion

We examine the features of parametric down-conversion for generating non-classical optical-terahertz biphoton fields. The calculations were performed for a second-order correlation function and noise reduction factor (NRF), which were selected as measures of non-classicality of the field states. NRF measurement schemes were found to be less advantageous. Overall system temperature at which non-classical effects are still observable has been estimated as about 10 K. The priority of the main complicating factors for the quantum-optical measurements in the terahertz range has been determined. Thus, the most important parameter is the temperature of the system, after that come the absorption losses at terahertz frequencies, and the parametric conversion efficiency has the lowest impact. Increasing the pump radiation power has been found to be inefficient for "noise" suppression in this case. The calculations performed were based on lithium niobate dispersion properties due to its outstanding nonlinear optical properties and because it is frequently used for optical-to-terahertz frequency conversion.

Spectral luminescence and information characteristics of transparent lead fluoride nano-glass-ceramics doped with erbium ions

Spectral luminescence properties of synthesized transparent nano-glass-ceramics doped with erbium ions are studied. It is shown that, as a result of the secondary heat treatment of nano-glass-ceramics, the width of the luminescence spectrum at half maximum increases more than by 15 nm. Luminescence life-time dependences of erbium ions in studied samples are measured, their absorption cross-sections are determined, and their emission cross-sections are calculated by the McCumber method. Spectral and ultimate information characteristics of erbium-doped fiber amplifiers based on synthesized nano-glass-ceramics are numerically simulated. It is shown that, in the unsaturated gain regime, a substantial ripple of the absorption and emission cross-section spectra and shorter lifetime of the metastable level of the erbium ion reduce the pumping radiation power that is required for obtaining the given peak gain, narrow the gain spectra, and lower the transmission capacity of devices based on nano-glass-ceramics compared to the initial glass. Conversely, fiber amplifiers based on calcium fluoride glass-ceramics, for which the lifetime of the metastable level increases with increasing annealing temperature, have an advantage in the transmission capacity over devices based on the primary glass.

Ultra-low repetition rate mode-locked fiber laser with high-energy pulses

This paper reports on the results of research into passively modelocked fiber laser with a record-setting optical length of the resonant cavity amounting to 3.8 km. Significant elongation of the laser resonator led to more than two orders of magnitude increase in the output pulse energy at the same pump radiation power. At ultra-low (for mode-locked lasers) pulse repetition rate (77 kHz) and pulse duration of 3 ns the energy per pulse reached 3.9 microJ. At this moment this is the highest pulse energy on record generated directly from a mode-locked laser without Q-switching, cavity dumping techniques, or additional optical amplifiers.

Photolytical XeF(C–A) laser amplifier of femtosecond optical pulses: gain measurements and pump efficiency

Gain characteristics of a photolytically driven XeF(C–A) laser amplifier are studied experimentally in the unsaturated amplification regime. The gaseous active medium is optically pumped by vacuum-ultraviolet (VUV) radiation from two large-area multichannel surface discharges initiated along opposite walls of the amplifier chamber. A total gain factor of 102 is obtained for the ultrashort optical pulses under multipass amplification in the active volume of 40 × 18 × 4 cm3 dimensions with a spatially homogeneous gain distribution. Spectral measurements reveal a good conservation of the seed pulse spectrum. Small-signal gain reaching 2×10-3 cm-1 is observed for the blue-green seed pulses of 150 fs duration, as well as for the continuous seed radiation at 488 nm. The obtained gain values, being compared with the gain calculated for the measured pumping radiation power, indicate that the quantum yield of the XeF(B) formation as a result of the XeF2 photodissociation is high and approaches unity within the spectral band of the XeF2 VUV photodissociation continuum.

Spectral-angular characteristics of scattered radiation under pumping near the D 1 and D 2 lines of the Na atom

The dependence of the form of the scattering spectra on different detunings of the pump wavelength from the sodium doublet D-lines has been investigated. The spectral-angular characteristics of three-photon scattered radiation have been obtained. It has been shown that the composition of the scattered-radiation spectrum strongly depends on the pump radiation power density and the concentration of sodium atoms. Our investigations of the spectral-angular characteristics of the radiation scattered by sodium vapors have revealed new regularities in the generated radiation spectrum.

Conditions of phase matching for the second harmonic generation with ZnGeP2 crystals

The reasons for the discrepancy between the available experimental data on the phase-matching conditions for the second harmonic generation with nonlinear ZnGeP2 crystals are elucidated. The calculated and experimental phase-matching angles agree for crystals at room temperature and low pumping radiation power. Simultaneous solution of the consistent shortened equations for the complex amplitudes and of the heat conductivity equation demonstrates that pumping by intense laser beams causes the angles phase-matching to decrease by 5–25° and the long-wavelength edge of phase matching for doubled frequency of CO2-laser radiation to increase from 10.32 to 10.8 μm. An excess decrease in the angles phase-matching by 1–3° is due to the asymmetry of the thermal processes caused by the birefringence.

Cooperative emission of dye molecules at high dephasing rates

The dependence of the emission of a concentrated (∼1019 cm−3 solution of Rhodamine C on the power density Φ of exciting laser radiation was studied. The emission intensity for the power density of exciting radiation above ∼1025 cm−2 s−1 was found to have a nearly quadratic dependence on the power density Φ, and this emission was interpreted as the cooperative spontaneous emission of a Dicke type. For Φ≲1025 cm−2 s−1, the emission intensity increased with increasing Φ according to the exponential law. This emission was interpreted as the amplified spontaneous emission. The spectra of cooperative emission depended on the pump radiation power only weakly. The absence of lasing in dye solutions at high concentrations, which is a well-known phenomenon, was shown to be caused by the development of the cooperative spontaneous emission and not by the concentration quenching, and the former process is more rapid than the latter.

Efficient tunable LiF:F2crystal lasers containingF2andF2+color centers

Optimization of the technology of fabrication of active LiF (F2→F2+) laser elements operating at room temperature ensured high parameters of broad-band emission of light by the F2 and F2+ color centers in a nonselective resonator pumped by the second harmonic of a pulse-periodic YAG:Nd3+ laser (λ = 0.532 μm). The efficiency of the LiF (F2→F2+) laser, expressed in terms of the pumping, was 38%, whereas in terms of the absorbed energy it was 41%. The service life of one pump channel of the LiF (F2→F2+) laser exceeded 5 × 105 pulses when the pump radiation power density was 150 MW/cm2 and the diameter of the active channel was d = 1.5 mm. Broad-band lasing of the F2 color centers in the same system, but without focusing of the pump radiation, was observed for a period of 15 min at a pulse repetition frequency 12.5 Hz when the pump power density was 40 MW/cm2.

Optical Kerr effect in glass fiber waveguides with weak and strong birefringence

A birefringence induced by the optical Kerr effect in single-mode glass fiber waveguides was investigated. A study was made of the dependences of the process of inducing the birefringence on the natural birefringence, on the orientation of the polarization of the radiation, and on the pump radiation power. Oscillations of the intensity of a probe wave were observed under certain conditions of excitation of these fiber waveguides with pump radiation.

Operation of large-diameter pulse-discharge lamps emitting short pulses

An analysis was made of the load characteristics of pulse-discharge lamps of 1.6 mm diameter producing flashes of 10−6–10−3 sec duration. It was found that the nominal flash energy of commercially available lamps could be increased considerably by altering the filling. This made it possible to increase the pump radiation power by a factor of 3–5 and to raise correspondingly the laser output power shortening at the same time the pulse duration to (1–2)×10−4 sec. Equations are given for estimating the maximum loads of lamps under hard operation conditions.