Karel Hamal

Active–passive mode-locked Nd:YAG laser with passive negative feedback

Stationary ultrashort light pulses of 10-psec duration and with an energy of 10 μJ per pulse were obtained from an active-passive mode-locked Nd:YAG laser by using a two-photon absorption limiter (GaAs) inserted into the resonator. The stability of energy in the steady-state part of the pulse train, containing approximately 90 pulses, is better than ±1.5%. The dynamics of the pulse-shortening mechanism are described, and it is shown that maximum pulse compression is reached after only approximately 10 round trips.

Optical signal path delay fluctuations caused by atmospheric turbulence

We report the first direct measurements, to our knowledge, of optical signal path delay fluctuations caused by optical turbulence in the atmosphere. The experiments were based on satellite laser ranging. Our initial motivation was to identify all the random error contributors in satellite laser ranging. We measured and identified the random path fluctuations caused by the atmosphere in the range of units of picoseconds. An appropriate fluctuation model was developed.

Single photon counting module for space applications

The paper reports the results of research and development of a single photon avalanche detector (SPAD) for use in the harsh and hostile conditions of outer space. The photon counting detector was developed for space projects related to the synchronization of timescales via a space clock using optical pulses. The detector is based on a SPAD manufactured on silicon using the K14 process, and operated in an active quenching mode. Its operation over an extreme temperature range and under high optical overload has been tested together with its sensitivity to radiation in space. The technology demonstrator of the detectors for the China Laser Time Transfer mission was developed and tested. The mission launch is expected in the year 2008.

Time-walk-compensated SPAD: multiple-photon versus single-photon operation

The SPAD has proven already its capability of timing single- photon events with picosecond accuracy; it does that also for multi-photon events, but introduces here a time walk effect: with received energies of 1000 photons and more, the measured epoch time is shifted 200 ps or more towards earlier times; although the specific SPAD type used shows the lowest time walk effect of all measured silicon avalanche diodes, this effect still might introduce range errors of up to 30 mm, when measuring distances to satellites. It has been shown that this time walk effect is connected with a very small change of the avalanche rise time; this effect has been successfully used to develop an electronic circuit which measures this rise time difference, and uses it to compensate automatically almost all of the time walk effect. Some prototypes have been built and tested successfully in the satellite laser ranging station Graz; improved versions of the circuit are operated or tested now successfully in other SLR stations. It has been shown that the time walk effect can be reduced to more or less zero, for a dynamical range from single photon up to more than 1000 photons. For best time walk compensation, the circuit is adjusted for a specific laser pulse length; it has been shown however, that this adjustment also gives good time walk compensation for other laser pulse lengths.

Mode locking of a Nd:YAlO 3 laser at the 1.34-μm transition by a second-harmonic nonlinear mirror

A second-harmonic nonlinear mirror formed by a LiIO(3) frequency doubler and a dichroic output mirror is employed to mode lock the 1.34-microm transition of a pulsed Nd:YAlO(3) laser. Pulses of 15-ps duration are reliably generated simultaneously at 1.34 and 0.67 microm.

Mode locking of a Nd:YAlO/sub 3/ laser at 1.08 and 1.34 mu m wavelengths using a single LiIO/sub 3/ crystal

Passive mode locking of a Nd:YAlO/sub 3/ laser at 1.08 and 1.34 mu m using a nonlinear mirror based on second harmonic generation is described. A single 30 degrees cut frequency-doubling LiIO/sub 3/ crystal was used to mode lock both transitions, demonstrating the superiority of this mode-locking technique over that using saturable absorbers. Pulses as short as 50 ps at 1.08 mu m and 15 ps at 1.34 mu m were obtained. A comparative analysis of the mode-locking performance at the two fundamental wavelengths is presented, indicating that the longer pulse duration at 1.08 mu m is due to the higher gain and an insufficient number of round-trips in the pulse train development. >

Luminescence sensitization in Nd-Cr-Ce doped yttrium aluminates

Energy transfer from Ce to Cr in YAG is radiative and relatively inefficient. If excited below ∼500 nm, the opposite transfer is dominant. YAP: Ce, Cr showed relatively efficient Ce→Cr transfer only. The sensitization of Nd3+ luminescence by Ce3+ ions depends on the overlap of the Ce3+ emission band with Nd3+ absorption lines. Cr3+→Nd3+ transfer is characterized by an expressive non-radiative portion. It is inefficient in YAG but very efficient in YAP. No UV-induced colour centres were found in YAG: Nd, Cr grown under Ar-H2 atmosphere and doped with 10−3 wt. % Cr, but at a higher Cr concentration anomalous absorption between UV absorption edge and ∼ 650 nm was stabilized. Ce3+ admixture in YAG:Nd, Cr and/or reducing treatment of the crystals facilitate the decomposition of the centres. The decomposition is accompanied with a strong Nd3+ luminescence. Therefore, YAG: Nd, Ce, Cr is an advisable active laser material. On the other hand the same centre in YAP: Nd, Cr seemed to be more stable even in the presence of cerium ions.

Centimetre precision eye-safe satellite laser ranging using a Raman-shifted Nd :YAG laser and germanium photon counter

Centimetre precision eye-safe satellite laser ranging has been applied, for the first time, to a wide range of satellites using a Raman-shifted picosecond Nd:YAG laser operating at 1543 nm and a germanium photon-counting detector. The ranging has been accomplished for retro-reflector equipped satellites up to a distance of 30 000 km. The experiment paves the way for the deployment of eye-safe lasers in space applications.

Comparison of preparation speed of Er:YAG laser and conventional drilling machine

Clinical tests of the caries treatment with a Er:YAG laser system have been very promising. The problem of the Er:YAG laser drilling machine is the speed of preparation. It is not possible to increase the laser energy and repetition rate because this process is directly connected with temperature elevation. Therefore attention has been paid to define the differences between the classical and the laser drilling effect with a safe but effective laser energy and repetition rate. For the experiment, an Er:YAG laser drilling machine with an articulated arm was designed. Thirty samples of extracted human teeth were cut by both this system and classical drilling machine. The circumference, area and structure of surface sections were observed and analyzed. From the comparison of the measured time of preparation significant differences followed from the time of preparation. The time of preparation related to 1 mm2 of the cut surface was 5.42 sec for the Er:YAG laser machine and 0.66 sec for the classical method. From this comparison it follows that the laser system is 8 times slower than the classical preparation technique. In both cases the roughness of the surface analyzed by SCAN was found similar. Also, there were no differences between the distribution of the elements, especially calcium and phosphorus.

Er:YAG laser ablation: evaluation after a two-year clinical treatment

The aim of the clinical study is to evaluate Er:YAG laser ablation after two year-long clinical treatment. One hundred fifty cavities were volunteered for checking. Three restorative materials were used following manufacturers directions. For the experiment, an Er:YAG laser drilling machine was applied. The laser delivered energy from 100 to 450 mJ, repetition rate from 1 to 4 Hz. The length of the generated pulses was 200 microseconds. During our experiments cooling of the teeth was achieved by fine water mist. The number of pulses was from 16 to 489. Caries of enamel and dentin were treated. Old insufficient fillings were also removed (not amalgam or metal alloys). The experiments followed the guidelines of the Declaration of Helsinki (1964), Tokyo (1975), Venice (1983) and Hong-Kong (1989). Clinical evaluation of fillings after 6, 12, 18 and 24 months based on ADA recommendation was used. Eight criteria were applied for the restoration control. Composite resins and glassionomers could be used as filling materials.