Файл: Antsiferov V.V., Smirnov G.I. Physics of solid-state lasers (ISBN 1898326177) (CISP, 2005)(179s).pdf

Physics of Solid-State Lasers

Fig. 4.4 Diagram of a solid state laser with stable supershort radiation pulses: 1) convex lense resonator mirror with R = 30 cm, T = 99.7%; 2) cuvettr with a saturating absorbent; 3) spherical lens (f = 10 cm); 4) diaphragm, separating longitudinal modes; 5) active medium; 6) Archard–Taylor polarisation prism with ends under the Brewster angle; 7) electro-optical gate; 8) flat output mirror of the resonator; 9) high-reflection mirror; 10) high-current photomultiplier 14ELU-FK; 11) electronic unit.

This was achieved by moving the cuvette with the dye solution along the resonator axis. The dye cryptocyanine was used for the passive synchronisation of the modes of the resonator of the ruby laser, and N3247y polymethine dye was used for the Nd-doped laser. The longitudinal modes of the resonator were separated by the diaphragms (4) with a diameter of 1.5 mm in the ruby laser and 2.0 mm in the laser on Nd ions. The lasing of stable supershort radiation pulses was examined in active media (5): a ruby crystal with a diameter of 7 mm, length 120/180 mm with the ends cut under the Brewster angle; Nd:YAG, diameter 5 mm, length 100 mm, and Nd in phosphate glass of type GLS-22, diameter 6 mm, length 130 mm, with the bevelled and clarified ends. The pumping of the active rods was carried out with an IFP-800 lamp in a quartz single unit illuminator, pumping pulse time was 0.25 ms. The ruby crystal was cooled in distilled water, and the Nd rods were cooled with a liquid filter, cutting off the ultraviolet radiation of pumping.

The negative feedback was produced using an electro-optical amplitude modulator of the ML-102A type (7), operating on the basis of the transverse Pockels effect. To reduce the controlling voltage, the investigators used the quarter-wave circuit of modulation of the Q-factor of the laser resonator in which the modulator (7) was placed between the solid mirror of the resonator and the polariser (6). The part of intra-resonator radiation (~5%), reflected from the polariser (6), was transferred to the photoreceiver of the system of the inertia negative feedback. The voltage of complete closure of the ML-102A gate at a wavelength of 1.06 µm was approximately 170 V.

For the effective stabilisation of the regime of stationary SRP, the time to establishment of negative feedback was shorter than the characteristic

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time of the variation of the radiation intensity averaged over the axial period (~10–6 s) and longer than the time required by the light beam to pass around the laser resonator (~10–8 s). The depth of the inertia feedback was sufficient to compensate the decrease of the losses in the laser resonator formed as a result of clarification of the saturating absorber and leading, in the normal conditions, to scintillation of the giant pulse. In the experiments, the time to establishment of the circuit of the inertia negative feedback was 20 ns. The circuit of the negative feedback consisted of a solid mirror (9) of ELU-FK high-current photomultiplier (7) and the electronic unit (11).

The time parameters of lasing were recorded with coaxial photographic elements with an S1-75 oscilloscope with a time resolution of ~1.5 ns and Agat-SF1 electronic-optical camera with a resolution of the order of 10–12 s. The lasing spectrum was controlled with a Fabry–Perot interferometer and a photographic camera with a long-focus lens. The energy of the lasing pulse was measured with an IMO-2 device.

4.3.2 Parameters of supershort radiation pulses

Ruby lasers

Free lasing in a ruby laser in the normal condition always takes place in the regime of non-attenuating pulsations of radiation intensity. In contrast to the Nd-doped lasers, in the ruby laser it is far more difficult to obtain stable supershort radiation pulses throughout the entire free lasing pulse. This requires a more careful selection of the parameters of the inertia negative feedback system.

At the start of the transition lasing process, radiation of the ruby laser consisted of a large number of intensity fluctuation transients in accordance with the static nature of formation of the SRP [6]. The lasing spectrum was approximately 0.008 nm wide and had a random structure. At the end of the transition process with a duration of approximately 0.1 ms, the structure of the lasing spectrum was ordered and became angular. Subsequently, in the stage of formation of stationary SRP (~0.1 ms) the process of selection of the most intensive fluctuation pulse was completed and the laser resonator contained one pulse with the duration, shape and power determined by the parameters of the laser system.

The width of the lasing spectrum in the regime of lasing of stationary supershort radiation pulses in a ruby laser (Fig. 4.5) was ~0.07 nm. This width of the spectrum corresponds to the duration of the supershort radiation pulses of approximately 8 ps. When evaluating the duration of the SRP, no account was made of phase modulation because its contribution to the width of the spectrum of the stationary SRP was very small [46], and it was assumed that the SRP pulses have the form

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Fig. 4.5 Oscillograms of the intensity of supershort radiation pulses of a ruby laser in the regime of passive synchronisation of modes in introduction of intra-resonator losses.

of the square of the hyperbolic secant [47]. The duration of the produced SRP was an order magnitude shorter than the duration of the pulses produced in solid-state lasers with active synchronisation of the modes [7].

At the transverse size of the beam in the cuvette of approximately 0.3 mm and the mean radiation power in the resonator of approximately 500 W, the peak intensity in the saturating absorber was ~8×10 7 W/cm2. This is an order magnitude higher than the saturation intensity of cryptocyanine (6×10 6 W/cm2) and corresponds to the condition of the highest rate of compression of the SRP in the saturating absorber [48].

The lasing regime of the stationary SRP in the ruby laser was interrupted slightly earlier than the end of the pumping pulse. This interruption of lasing, according to additional investigations, is caused by the formation of a transverse instability of radiation inside the laser resonator.

Nd:YAG lasers

Like the ruby crystal, the Nd:YAG crystal is characterised by a uniformly broadened gain line with the half width slightly smaller than in the ruby crystal. However, the lifetime of the upper working level of Nd:YAG is an order of magnitude shorter. This results in the situation in which the time to establishment of the stationary lasing regime in the Nd:YAG laser is reduced by almost an order magnitude, in comparison with the ruby laser, and is ~10 µs (Fig. 4.6). In the stationary lasing regime of SRP, the radiation of the Nd:YAG laser was also represented by a continuous sequence of single pulses, followed by an axial period of 13 ns. The envelope of the lasing spectrum was bell-shaped without a fine structure. This indicated the absence of the time structure of the supershort radiation pulses. Throughout the entire lasing process, the structure of the spectrum was almost constant. This indicated that no

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Fig. 4.6 Oscillograms of the intensity of radiation of an Nd:YAG laser in the regime of passive synchronisation of modes with addition of intra-resonator losses.

phase modulation took place. The duration of the SRP, calculated from the width of the generated spectrum, was 10 ps.

It should be noted that in the Nd:YAG laser, the transition to the regime of stable stationary supershort radiation pulses was obtained in a wider range of the radiation of the laser parameters (pumping, the extent of the feedback, technical perturbations) in comparison with the ruby laser.

Nd doped phosphate glass lasers

Nd glass is characterised by a wide gain band and can be used to produce light pulses of subpicosecond duration. Peak power may reach the values at which the non-linear-optical effects start to operate, such as the phase and self-modulation of the produced pulses associated with the non-linearity of the refractive indices of the resonator elements. At a high value of phase self-modulation, an instability forms in the process of passive synchronisation of the modes in the lasing regime of the stationary SRP.

In Nd doped lasers on phosphate glass GLS-22, the duration of the transition process was approximately 30–50 µs (Fig.4.7a). During this period, the carrier frequency radiation was shifted to the short-wave region of the spectrum by 0.6 nm. The width of the lasing spectrum of the laser with a passive gate on N 3274y dye, whose relaxation time is ~13 ps, in the steady stationary regime of lasing of SRP (Fig. 4.7b) was 2.3 nm, and the shape of the spectrum was described with sufficient accuracy by the square of the hyperbolic secant [47]. This width of the lasing spectrum corresponds to the duration of the supershort radiation pulse of 0.5 ps.

In passive synchronisation of the modes with a gate based on N 3955 dye, with a relaxation time of 22 ps, we obtained stationary SRP with the duration of the order of 2 ps. To obtain the same duration of the SRP in the giant pulse regime, investigations were carried out using a saturating absorber with a considerably shorter relaxation time of 4.6 ps [49] and 2.5 ps [50].

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The larger shift of the carrier frequency of lasing was recorded in the Nd doped laser on silicate glass in which the width of the gain band was slightly larger than in the phosphate glass laser. In addition, in the case of silicate glass, the non-linearity of the refractive index is higher and silicate glass shows effects of phase modulation with a high degree of probability leading to the formation of an instability in the regime of stationary supershort radiation pulses.

The larger deviation of the carrier frequency from the maximum of the gain band leads to a decrease of the efficiency of amplification of the produced pulse, and the gain factor for the noise with no phase modulation, was higher than for the phase-modulated pulse. The starter with a more efficient gain displaces the produced pulse from the lasing and the increase of the lasing intensity results in the phase modulation and shift of the carrier frequency.

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Chapter 5

Increasing the lasing efficiency of solid-state lasers

In the solid-state lasers on ions in the dielectrics (crystals and glasses) the matrix of the dielectrics plays a significant role in the lasing processes; it is therefore necessary to ensure that the active impurity ions are added uniformly to the matrix in a sufficiently high concentration, without disrupting its optical and mechanical properties.

The matrix should be optically homogeneous and transparent in the spectral range of pumping and lasing radiation and should have high heat conductivity, heat resistance, mechanical and optical stability. In addition, the matrix should satisfy the technological requirements on optical treatment. At present, there are hundreds of solid-state matrixes of active media used for lasing. In most cases, activators in the solidstate lasers are represented by the ions of elements of transition groups. The atoms of these groups are characterised by the presence of internal partially filled electron shells. The transitions in which these non-filled shells take part also determine the spectral-luminescence properties of the active medium. The activators are implanted in the matrix and exist there in the form of ions.

In the widely used solid-state lasers on active media with trivalent ions of the iron transition group, these ions lose the screening shell and their spectra in different matrixes differ from the spectra of the free ions and depend on the forces of the crystal field of the matrix. The ions of other transition elements of the group of rare earths change only slightly their spectral-luminescence properties with changes of the solid-state matrix, because of the presence of screening electron shells in these ions. The working electron shells of rare-earth ions are screened to such an extent that the crystal fields of the matrixes have only a slight effect on their state and configuration.

At present, the solid-state lasers are used most widely in practice, regardless of strong competition with other types of lasers. The solid-

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state lasers are characterised by high lasing energy parameters combined with the capacity of working in the greatly differing conditions, from the regime of lasing of supershort radiation pulses to the regime of continuous high-power lasing. They are also exceptionally reliable, durable, and compact in comparison with other types of lasers.

One of the main problems facing the developers of solid-state lasers is how to increase the efficiency factor (EF) of solid-state lasers which in the measurement of lasers is on the level of several percent. It is also important to widen the spectral range of lasing of the solidstate lasers. The problems of increasing the efficiency of lasing of the solid-state lasers have been examined by the authors of this book in Ref. 1.

5.1 INCREASING PUMPING EFFICIENCY

The absorption factor of pumping radiation by the active medium is determined by the efficiency of the illumination system, the efficiency of radiation of the pumping pulse and the matching of the radiation spectrum of the pumping pulse with the spectral bands of absorption of the active media. The illumination system is improved by optimising the focusing of pumping radiation in the area of position of the active element and by increasing the efficiency of the illuminator.

To increase the pumping efficiency, it is necessary to minimise the possible losses of light energy in the elements of the pumping system and non-active absorption in the active medium. The presence of the narrow absorption bands of the solid-state active media in comparison with the radiation spectrum of the pumping lamps causes that a large part of the energy of the pumping lamps is not utilised by the active medium or is used with insufficient efficiency. In addition, the spectral components of the active medium with low absorption require multiple passes.

To improve pumping efficiency, it is important to ensure the spectral transformation of part of the pumping radiation not absorbed in the active medium, in the region of its active absorption. Under specific conditions, the plasma of the pumping lamps may be such an effective re-emitter. The return to the lamps of the light energy not absorbed in the active medium and its subsequent repeated radiation with the losses not exceeding 20% is efficient from the energy viewpoint (‘light boiler’) [2]. The ‘light boiler’ is realised with the highest efficiency when the active medium is situated between the reflector and the plasma layer, i.e. with the coaxial active element. Using such an element made of Nd glass, the authors of Ref. 3 obtained the efficiency factor in the regime of free lasing of 9% in relation to electric energy. In the pumping system of the ‘light boiler’ type, with the Nd:YAG active

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elements positioned symmetrically around the pumping lamps with close packing in the quartz single unit, the efficiency factor of laser was ~3% in the giant pulsed regime [3].

The efficiency of transformation of the pumping energy may decrease using, as the material of the reflector in the illuminator, zirconium oxide (ZrO2) or silicate ceramics (kersil), characterised by a high degree of diffusion reflection in the spectral range from 400 to 1000 nm and by the possibility of alloying with different materials for the conversion of the reflective light spectrum. The application of reflectors made of kersil in the optical pumping systems of the lasers increases the efficiency factor of lasers by 30%, improves the energy and spatial characteristics of radiation, increases endurance and durability of the laser systems. Reflectors have been produced from kersil for absorbing ultraviolet pumping radiation and converting this radiation in the region of the absorption bands of solid-state media [4]. The ultraviolet pumping radiation is also used for quartz glass lamps with a titanium silicate coating [5].

In Ref. 6 and 7, the ultraviolet radiation of pumping lamps was converted to the green range spectrum in one of the absorption bands of chromium ions in the alexandrite crystal using a solution of KN120 dye.

5.1.1 Selective pumping of the active medium

The selective pumping of solid-state media is realised using both gasdischarge sources of narrow-band radiation and laser semiconductor emitters. The main contradiction, formed in the development of selective pulsed pumping lamps is based on the requirement for the high brightness of the plasma that can be achieved only at high plasma temperature and density. In this case, the plasma of the pumping lamps becomes almost completely non-transparent and its spectrum discontinuous. In continuous arc pumping lamps it is possible to produce a narrow-band radiation spectrum; this was also realised in krypton gas discharge lamps which have been used efficiently for the pumping of continuous solid-state lasers.

The sapphire lamps with the additions of alkali metals are an efficient means of the pumping of low-threshold solid-state media at low levels of pumping power. When pumping an Nd:YAG laser with a potassiumrubidium sapphire lamp, the pumping efficiency was twice that in the case of the krypton lamp [8], as a result of better matching of the radiation spectrum of the resonance lines of K and Rb with the absorption spectrum of the garnet crystal.

The application of hybrid conditions of supplying power to the gasdischarge lamps with a constant and pulsed component of the discharge

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makes it possible to change the radiation spectrum of the pumping lamp and increased its efficiency [5].

5.1.2 Laser pumping

In comparison with lamp pumping, laser pumping increases the efficiency with respect to the energy contribution by almost two orders of magnitude. This greatly extends the possibilities of application of rare-earth active media. The increase of the energy contribution makes it possible to reduce greatly the requirements on the upper working level, its lifetime and the laser transition section. Consequently, the gain factor of the active media greatly increases, together with increase of lasing energy.

Laser pumping makes it possible to greatly reduce the level of technical perturbations of the active medium and the resonator, shorten the length of the active medium without any loss of its general gain, increase the threshold density of the laser radiation flux, and solve the problem of self-focusing of radiation in optical amplifiers

5.1.3 Laser diode pumping

Recently, the extent of application of semiconductor lasers for the laser pumping of solid state lasers has greatly increased [10,11]. This is caused by the relatively rapid development of the technology of production of powerful light-emitting diodes, matrixes and semiconductor lasers.

The main advantage of solid-state lasers with diode pumping in comparison with lamp pumping is the higher efficiency factor. In the group of the solid-state lasers, Nd lasers have been used most extensively. The efficiency factor of these lasers in lamp pumping does not exceed 1–3%. This is caused by poor matching of the absorption bands of the Nd ions with a wide radiation spectrum of the pumping lamps. In contrast to lamps, the laser diodes have a narrow spectral emission line enabling good matching of the radiation spectrum with the absorption bands of solid-state active media. The development of high efficiency laser diode gratings on GaAlAs emitting at a wavelength of 810 nm with a power higher than 0.2 W has resulted in the efficiency factor of 8% in the Nd:YAG laser in the lasing regime of the TEMoo mode (a single zero transverse mode) [12]. Increase of the power of laser pumping to 1 W resulted in the development of continuous single-mode Nd:YAG lasers with the total efficiency higher than 10% [13,14]. When using yttrium vanadate with Nd ions as the active medium, it is possible to obtain the efficiency factor of a laser of 19% [15].

The solid-state lasers with diode pumping are characterised by higher stability of the frequency of laser radiation in comparison with lasers with lamp pumping. The extremely higher stability of the frequency

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of solid-state lasers with lamp pumping is not better than 17 kHz because of the presence of technical noise, characteristic of the pumping pulse [16]. The application of diode pumping and a monolithic ring-shaped resonator with active stabilisation has improved the stability of the lasing frequency of Nd:GGG laser to 30 Hz [17].

Diode pumping has simplified the design of solid-state lasers, greatly reduced their weight and dimensions. Diode pumping sources have wider functional possibilities of controlling the pumping regime in comparison with the lamp sources. They make it possible to regulate in a wide range the duration and shape of the pumping pulse, stabilise the pumping energy and greatly improve the lasing parameters of the solid-state lasers. The application of diode pumping increases by more than an order of magnitude the service life of solid-state lasers operating in both the continuous and pulsed regimes.

In addition to the above mentioned advantages, it is also important to mention the current shortcomings of the solid-state lasers with diode pumping in comparison with lamp pumping: considerably lower energy and power of emitted radiation, and a considerably higher price of a diode pumping source in comparison with the lamps.

In the solid-state lasers with diode pumping there are two configurations of the pumping system: longitudinal and transverse. Longitudinal pumping (from the end of the active element) makes it possible to obtain better mode matching and a longer length of the absorption part of radiation which weakens the requirement on the pumping wavelength, i.e. the degree of control of the temperature of the laser diode. However, longitudinal pumping requires more powerful diodes with a low divergence of radiation.

The application of transverse pumping in the solid-state lasers enables a large number of diode gratings to be used. These gratings are positioned parallel to the axis of the active element and normal to the direction of propagation of laser radiation in the resonator. This greatly increases the pumping power.

In most cases, to obtain the maximum mode matching of pumping radiation, the diameter of the active element should be small. This restriction leads to a short length (~3 mm) of absorption of pumping radiation and lower efficiency in comparison with longitudinal pumping. Thus, to obtain the corresponding level of the output power in the solid-state laser with transverse pumping, it is necessary to use a considerably larger number of laser diodes in comparison with the laser with longitudinal pumping. Regardless of these shortcomings, transverse pumping makes it possible to increase almost without bounds the radiation power (to the kilowatt level) of the solid-state lasers with diode pumping.

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