Margarita Ang Deneva

Development of a tunable, competition less flash-lamp pumped Nd:YAG laser generated of a chosen pair of two lines

On the base of our previous experience in the dual-color lasers we present our actual development - experimental realization, theory of a flash-lamp pumped Nd3+:YAG laser, that produces output at two desired (tunable) lines in the spectral range of 1μm -1.44 μm in free lasing and Q-switching operation. The laser operation avoids the wave’s competition effect for the two generations operating in two different parts of the active volume of an active single crystal and two closely disposed prism-selected resonators. Also, as we have shown, varying the partial laser volumes and the losses at each resonator in combination, it is possible to produce the generation at the two desired wavelengths with near equal output energy, peak power simultaneously or at desired time interval. We have found and treated some peculiarity of the formation of the generation, such as the actually used parts of the partial volumes.

Study the penetration of IR laser radiation in human teeth: determination of the absorbed and scattered parts

By using the developed by us approaches and instrumentation, we have obtained and presented series of systematized data, which are important for the use of the laser light in infrared (IR) spectral region. The obtained data include: 1) reflectivity of the human tooth dentin; 2) the spatial intensity distribution in the cross-section of the light beam penetrating the tooth’s dentin; 3) the absorbed and the diffused parts of the laser light that have been determined separately through combination of optical and calorimetric techniques. The last result is the most important because it permits to calculate the dentin absorption and scattering coefficients. The study is performed for the laser light at two easily generated wavelengths – 1.06 μm and 1.36 μm, emitted by the Nd:YAG laser that is well known, commercially available, economical and widely used in many laboratories and medical institutions. The study is made on the basis of fresh in-vitro teeth samples from the persons of Bulgaria, Sofia region.

New devices for applications in lasers and optical communications based on the wedged interference structures

We present two novel applications of optical properties of the interference wedge (compact realization of the Fizeau Interferometer) for the case of illumination with a small diameter laser beam. The first one is a new and competitive wavelength division multiplexing structure. The main advantages of the device are the possibility for separate frequency tuning of each input-output channel as well as its compactness. The made computer simulation proves that the structure can work with very short laser pulse duration (~ 0.1 ns), which corresponds to pulse repetition rate of order of ten GHz and more, thus fulfilling the requirements of modern digital communication systems. The theoretical analysis and the experimental check with laboratory model of a free-optical communication system show a sufficient resonant narrowline transmission up to 75% with the linewidth of emission from 0.05 nm to few nm. The second proposal is a device which allows for distant laser measurement of small linear translation of a rigid object for distances from a few meters up to hundred meters.

All-optical laser spectral narrowing and line fixing at atomic absorption transition by injection competition and gain knock-down techniques

We present two original, all optical techniques, to produce a narrowline laser light, fixed at the frequency of a chosen reference atomic absorption transition. The first type of systems is an essential improvement of our method 3,4 for laser spectral locking using a control by two frequency scanned, competitive injections with disturbed power ratio by the absorption at the reference line. The new development eliminates the narrowing limiting problem, related with the fixed laser longitudinal mode structure. We have proposed an original new technique for continuously tunable single mode laser operation in combination with synchronously and equal continuous tuning of the modes of the amplifier. By adapting the laser differential rate equations, the system is analyzed theoretically in details and is shown its feasibility. The results are in agreement with previous our experiments. The essential advantage, except simplicity of realization, is that the laser line can be of order of magnitude and more narrowed than the absorption linewidth. The second system is based of the laser amplifier arrangement with a gain knock-down from the competitive frequency scanned pulse, except at the wavelength of the desired absorption reference line. The essential advantages of the last system are that the problem of fixing laser mode presence is naturally avoided. The theoretical modeling and the numerical investigations show the peculiarity and advantages of the system proposed. The developed approaches are of interest for applications in spectroscopy, in DIAL monitoring of the atmospheric pollutants, in isotope separation system and potentially - for creation of simple, all optical, frequency standards for optical communications. Also, the continuously tunable single mode laser (and the combination with the simultaneously tunable amplifier) presents itself the interest for many practical applications in spectroscopy, metrology, and holography. We compare the action and the advantages of the two systems proposed.

A new approach for tunable subnanosecond pulse generation using an active mirror: theoretical and experimental study

We report a simple technique for generation of tunable subnanosecond pulses (0.1-0.3 ns) based on single spike selection from the relaxation oscillations in a dye laser by use of an active mirror. The selection is in original two-wavelength cavity with competitive generations at two wavelengths, the second of which is forced in appropriate moment by the quickly included amplification of the active mirror to suppress the initially started lasing in the first channel. To produce such switching we form the pump pulse by temporal division of a 15-50 ns standard pump laser pulse using a Pockels cell with a 1-2 ns switching time and an optical delay line. We have shown that in addition, such formation increases essentially the selected spike power. The proposed technique widens the choice of suitable lasers in which the single spike selection technique may be applied to include dye lasers excited by a Q-switched solid-state laser or a Cu-vapor laser. The other advantages in comparison with the given in the literature single-spike selection techniques is the improved reproducibility of the selection for high (~100-200%) pump pulse power fluctuations, shorter pulse duration and pulse shape improvement. Both theoretical consideration and experimental verification are carried out.

Development of original injection-locked linear laser amplifiers: noise characteristics

Recently, using an aopposite injection we have introduced a novel laser injection-locking approach for high gain (~104-108) and linear amplification of modulated low power (~μW-mW) laser radiation. In this work we report the investigation of the noise characteristics of such amplifier at the example of high repetition rate pulse pumped linear dye lasers. The background emission of the laser at the laser modes outside the signal and counter injection wavelengths is considered as a main source of the noise. By modeling the laser action with adapted system of differential equations and numerical analysis, solving this system for the cavity modes, we have obtained the total noise and its spectral distribution. The results show that for suitably chosen conditions the intracavity noise is very low or acceptable (~10-2 with respect to the signal output). The essential dependencies of noise characteristics from the system parameters are carried out.

Development of pulsed dual-wavelength lasers: improvement of the tuning characteristics using an active mirror

We present two simple and effective approaches for widening of the tuning range of the two-wavelength generation in two-channel laser cavity. The first is based on the use of combination of interference wedge and linear neutral filter and the second on the use of active mirror. This widening is combined with near equal intensities in both emissions. The use of the active mirror assures a real simultaneous two-wavelength operation. We consider a dye laser.

Development of high-spectral-purity tunable lasers using an auxiliary selected generation

We have developed and investigated a simple solution of tunable laser with strongly decreased background emission (max pick-to-pick power ratio ≤ 10-8) by generation at additional wavelengths, except the selected. The cavity scheme permits to obtain easily laser output only at the selected wavelength, using a simple system with interference wedges. The spectral position and the losses at both generations are easily and independently controlled to needed optimal values. The theoretical treatment based on the rate equation system, shows that for the appropriate chosen conditions the technique developed permits to reduce by approximately two orders of magnitude the intensity of the background emission about the selected line (pick power to be less than 10-8). The investigation is on the example of pulsed dye lasers. The results are applicable for other tunable lasers with homogeneously broadened active medium such as Ti:sapphire, diode, etc. The lasers of considered type (with high spectral purity) are of essential interest for application in biomedical investigations of tissue luminescence spectrum, in Raman spectroscopic investigation, in isotope separation.

Development of diode-pumped Yb3+:doped crystal lasers for application in spectroscopy and metrology: two-wavelength and single-mode operation

We demonstrate and investigate (theory, experiment) a peculiar mode of cw Yb3+:doped crystal laser operation when two emissions, at two independently tunable wavelengths, are simultaneously produced. Both emissions are generated from a single pumped volume and take place either in a single beam or in spatially separated beams. The laser employs original two-channel cavities that use a Passively Self-Injection Locking (PSIL) control to reduce the intracavity losses or a very simple for practical realization two near collinear channels cavity. A two-wavelength, single, longitudinal mode generation is also obtained. The results reported are based on the example of Yb3+:GGG laser and Yb3+:YAH laser. The lasers operate in the range 1023-1033 nm and 1030-1040 nm, respectively with a total output power of ~0.4 W.

Rectangular laser pulses formation by an interferometer-type device that uses Cr4+:YAG plate

We propose a simple interferometer-type-device that permits to produce a variable length (1 - 15 ns) rectangular laser pulses, starting from a long (approximately 10 - 50 ns) conventional pulse that is emitted by Q-switched Nd3+-doped laser. The system uses an one time switched electro-optical pulse division at the input laser pulse and cutting all-optically a rectangular part of one of the formed partial pulses by appropriate bleaching of interferometer (Fabry-Perot, Fizeau) which gap is an Cr4+:YAG plate. The precisely controlled bleaching is produced in reliable manner using the second partial pulses and an optical delay line. The length of the obtained rectangular pulse can be precisely varied between 1 and 15 ns. The technique proposed can be applied also for the other lasers.