Performance of internal-mirror frequency-stabilized He-Ne lasers emitting green,yellow or orange light

A comprehensive control system has been developed for the stabilization of internalmirror He-Ne lasers to their power envelopes. It is designed particularly for use with lasers emitting at 552, 505, or 490 THz (543, 594, 612 nm). The aim has been to combine convenience in routine operation with good day-to-day resettability and minimal frequency drift during a day. The lasers are observed to emit in up to four modes simultaneously and their tuning behaviour is highly alignment-sensitive. They also tend to flip between modes having orthogonal linear planes of polarization. Reliable control of polarization and mode frequency are possible using permanent magnets and active length control through thermal expansion. The stabilized single-frequency outputs have frequency drifts of about 2×10^–8 per year, and powers of between 150 W and 850 W.     

Theoretical calculations and Raman spectrum of intercalation modes in LixInSe

By using an extended linear-chain model which includes the interlayer forces, we have calculated the new vibrational modes, of Li intercalated InSe. The dispersion curves along thek _z wavevector perpendicular to the layers for the -polytype are determined in the first Brillouin zone. Assuming that the interlayer interaction is not modified upon intercalation and the interaction between lithium atom and adjacent layers in the van der Waals plane has the same value than the interlayer one, the new modes are determined with the force constant given by the rigid layer mode of the, -polytype at 18 cm^–1. This model gives the variation of the acoustic branches and the appearance of two optical intercalation modes at higher frequencies. The Brillouin zone boundary modes of the acoustic branches at 18 and 41 cm^–1 in the pure material are calculated to be 22 and 50 cm^–1 respectively forx=1/2. The dispersion of the new optical branches is flat along thez-direction and frequencies are obtained at 96 cm^–1 for the Li mode perpendicular tok _z and at 218 cm^–1 for the Li mode parallel tok _z. We compare also our results with the Li mode frequencies obtained in a total energy calculation. Raman scattering experiments have been performed in intercalated sample in order to verify the proposed model.     

Quantum dot photonic devices for lightwave communication

For InAs-GaAs based quantum dot lasers emitting at 1300 nm digital modulation showing an open eye pattern up to 12 Gb/s at room temperature is demonstrated, at 10 Gb/s the bit error rate is below 10^-12 at -2 dBm receiver power. Cut-off frequencies up to 20 GHz are realised for lasers emitting at 1.1 m. Passively mode-locked QD lasers generate optical pulses with repetition frequencies between 5 and 50 GHz, with a minimum Fourier limited pulse length of 3 ps. The uncorrelated jitter is below 1 ps. We use here deeply etched narrow ridge waveguide structures which show excellent performance similar to shallow mesa structures, but a circular far field at a ridge width of 1 m, improving coupling efficiency into fibers. No beam filamentation of the fundamental mode, low -factors and strongly reduced sensitivity to optical feedback is observed. QD lasers are thus superior to QW lasers for any system or network.Quantum dot semiconductor optical amplifiers (QD SOAs) demonstrate gain recovery times of 120–140 fs, 4–7 times faster than bulk/QW SOAs, and a net gain larger than 0.4 dB/(mm*QD layer) providing us with novel types of booster amplifiers and Mach–Zehnder interferometers.These breakthroughs became possible due to systematic development of self-organized growth technologies. PACS 81.07.Ta; 81.16.Dn; 42.55.Px; 42.60.-v     

Photoacoustic soot sensor for in-situ black carbon monitoring

The PhotoAcoustic Soot Sensor (PASS) for in situ black carbon mass monitoring is presented. The sensor combines a high-power laser diode ( = 802 nm;P = 450 mW) and a novel spectrophone setup to achieve a portable sensor system for black carbon measurements. The acoustic resonator with aQ-factor of 300 is operated in its 2nd azimuthal mode at 6670 Hz. To estimate the effects of the window position, laser beam collimation, and different loss mechanisms on spectrophone sensitivity a model treating these effects with respect to the signal strength of the azimuthal modes is described. It gives a cell constant of 5.8 V/(W m^–1) in good agreement with 5.9 V/(W m^–1) obtained from measurements with particulate carbon. Additionally, this model permits a method for an absolute calibration of the spectrophone. To improve the signal-to-noise ratio, the photoacoustic signal is estimated by a weighted least-squares fit to an averaged line profile of the excited normal mode instead of a direct measurement of peak height. Finally, the application of this data processing algorithm yields a detection limit of 1.5 × 10^–6 m^–1 or 0.5 g black carbon per m³     

Modulation-free acetylene-stabilized lasers at 1542 nm using modulation transfer spectroscopy

We report frequency stabilization of diode lasers using modulation transfer spectroscopy of an acetylene transition (¹³C₂H₂, ₁+₃, P(16)) at 1542 nm. We realize modulation-free acetylene-stabilized lasers with a frequency stability of about 10^–11 and an absolute frequency accuracy of about 20 kHz.This revised version was published online in March 2005. In the previous version, the published online date was missing     

1.55-μm non-anti-reflection-coated fiber grating laser for single-longitudinal mode operation

A 1.55-m fiber grating laser (FGL) was fabricated by optically packaging a non-anti-reflection (AR) coated Fabry–Perot (FP) laser to a lensed fiber grating. The FGL demonstrates a single-longitudinal mode operation with a side-mode suppression ratio (SMSR) of up to 40 dB. The SMSR oscillates and diminishes to <30 dB as the increase of injection current above 38 mA, and the tilt of the fiber approaches 5° away from the facet normal of the FP laser. We have performed numerical simulations on the single-longitudinal mode operation for the FGL. The SMSR for the FGL increases over 40 dB as the increase of the fiber grating reflectivity (R _g) above 0.7 with non-AR-coated FP laser facet. Our calculations also show that the strong current-dependent SMSR oscillation is from the mode selection by the fiber grating external cavity and the heating effect in the FP laser.     

Simple dispersion measurement of a long-span,single-mode fibre using the longitudinal mode spacing of a semiconductor laser

A simple method for measuring dispersion in long-span single-mode fibre, by using directly modulated multi-longitudinal mode semiconductor lasers, is proposed. Experimental values for single-mode fibre dispersion over a 44.3km span at 1.1 and 1.5m are 19 and 17 ps km^–1 nm^–1, respectively, and are in good agreement with Gloge's theory. The zero-dispersion wavelength estimated from the experimental results is about 1.28m, 8 nm shorter than the theoretical value.     

Investigation of the active media in pulsed gas discharge lasers by the Rozhdestvenskii hook method

In this review we generalize the results of our experimental investigations in which the Rozhdestvenskii hook method was first used to look for an inverted population in new spectral transitions. An inverted population in new 3p–3s transitions of the neon atom with =588.1, 594.4, 650.6, and 653.2 nm was discovered in the active medium of a Penning plasma laser emitting the =585.3 nm neon line in Ne-H₂ and Ne-H₂-Ar mixtures. Results of investigations of a number of active media are given. An analysis is made of the spatial and temporal distributions of copper atoms in the ground state and of their influence on the lasing efficiency of a copper-vapor laser. Using the rhenium atom as an example, it was shown to be possible to obtain a sufficiently high concentration of refractory metal atoms (>10¹⁴ cm^–3) in an active medium to form active media of lasers utilizing self-terminating transitions and also plasma lasers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–27, December, 1994.     

A pulsed laser employing Ar+ ions at high currents and low pressures

There are several papers [1–5] on pulsed Ar⁺ lasers, but all the tests have been done at low current densities (100 A/cm²). We have used densities up to 15–20 kA/cm², the best pressure range being 8×10^–3 to 2×10^–2 mm Hg. The generation under these conditions has some features not seen under ordinary conditions.     

Generation of picosecond pulses in solid-state lasers using new active media

Results are reported of investigations aimed at generating nanosecond radiation pulses in solid-state lasers using new active media having broad gain lines. Passive mode locking is accomplished for the first time in a BeLa:Nd³⁺ laser at a wavelength 1.354 m, and in a YAG:Nd³⁺ laser on a 1.32–m transition. The free lasing and mode-locking regimes were investigated in an alexandrite (BeAl²O₄:Cr³⁺) laser in the 0.72–0.78–m range and in a synchronously pumped laser on F 2- centers in LiF in the 1.12–1.24–m region. The features of nonlinear perception of IR radiation by the eye, using a developed picosecond laser on F₂ ^– centers, are investigated for the first time.Translated from Lazernye Sistemy, pp. 67–86, 1982.