Nanosecond laser sources

4.2.1 Nd:YAG laser system

In experiments, we employed a single-mode Nd³⁺:YAG laser with passive Q-switching using a LiF:F2– crystal [110]. The active Nd³⁺:YAG crystal has a diameter of 6.3 mm and length of 65 mm. It was mounted in a single-flashlamp monoblock laser head. An INP-5/60A-1 flashlamp with a BPL 75/33U power supply was used to pump the active element. It was possible to vary the pump energy in discrete steps by changing the capacitance of the storage capacitors in the power supply. The optical scheme of the laser is shown in Figure 4.6. The laser cavity was formed by two mirrors and included a quarter-wave plate, two diaphragms, a birefringent crystal, a saturable absorber and an active element, all located on the same optic axis. The birefringent prism, made of crystalline quartz, provided 3 mm spatial separation between the ordinary and extraordinary rays. Saturable absorber was performed by an LiF:F² crystal whose initial transmission coefficient was 14%.

The diameters of the diaphragms were 2.6 mm and the resonator was 104 cm long.

An amount of longitudinal laser modes is determined by three resonators formed

71 by the birefringent prism with the mirrors, only by the mirrors, and by the birefringent prism with the mirror on right in Figure 4.6. The diaphragms ensured that one transverse mode was selected. Passing the birefringent crystal the beam was divided in ordinary and extraordinary beams, one of which directed out of laser. The laser output beam passes the saturable absorber and an undepleted region of the active element, and this increased intensity of the output beam.

Figure 4.6. The scheme of the optical cavity of a single-mode YAG: Nd³⁺ laser [110].

Since the ratio of ordinary and extraordinary beams inside birefringent crystal is determined by polarization of incident light, by rotating the quarter-wave plate around the optical axis of the resonator, a smooth change in output beam is achieved from minimum to maximum value. This means that there is a fairly wide range of changes in the active element pump energy, within which laser operation near the generation threshold is possible due to a smooth change in the quality factor of the resonator. Therefore, the laser can operate in a single-frequency mode with a significant change in the active element pump energy [110]. During the Nd³⁺:YAG experiments, the laser generated pulses with a duration of τ^las = 19.1 ns.

4.2.2 The second, third and fourth harmonic generation

Phenomenon of the second harmonic generation (SHG) manifests itself as the frequency doubling of the intense light wave propagating in the non-centrosymmetric medium. On the microscopic level, the lack of inversion symmetry implies the presence of the intrinsic electric field that results in the asymmetry of the oscillations of the optical electrons around equilibrium positions. That is the amplitude of the displacement of the electron depends on the how the electric filed of the fundamental wave oriented with respect to the intrinsic filed. On the macroscopic level, such an asymmetry can be described in terms of the quadratic dependence of the polarization of the medium on the amplitude of the electric field.

Therefore, when an intense light wave at frequency ω propagates in such a medium, the electric polarization and displacement of the media have a component oscillating at frequency 2ω, which gives rise to the SHG. In order to maximize the amplitude of the second harmonic wave, one need to achieve phase matching

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condition when the SHG wave generated along the fundamental beam have the same phase (see, for example, Ref. [111]).

In the experiment, we employed a crystal of potassium titanyl phosphate (KTiOPO⁴, abbreviated as KTP) to generate the second harmonic of the Nd³⁺:YAG radiation. Radiation at the wavelength of 532 nm at the pulse duration of τlas = 15.1 ns was obtained.

The phenomenon of the third harmonic generation (THG) manifests itself as tripling frequency of the light wave propagating in the medium. In contrast to the SHG, the THG is permitted in a medium of any symmetry. However in practice, using the non-centrosymmetic media is more convenient because they enable so-called cascade mechanism of the THG.

Specifically, in the non-centrosymmetric medium, the high intensity fundamental wave at frequency ω produces the nonlinear polarization oscillating at frequency 2ω, i. e. the SHG process take place. However, if the second-order nonlinearity is strong, the amplitude of the light wave at frequency 2ω generated in the medium is high enough to participate in sum-frequency generation process ω + 2ω = 3ω, i.e. to the THG. Such a “cascade” process also requires phase matching to maximize the efficiency of the frequency conversion. Since it is difficult to achieve phase synchronism conditions for the both SHG and THG, in practice, it is to use separate crystals for these processes. The part of the fundamental wave is converted into the second harmonic in the first crystal and then the fundamental and second harmonic waves produce the third harmonic in the second nonlinear crystal.

In this work, laser radiation with a wavelength of 1064 nm was partially converted into radiation with a wavelength of 532 nm using a KTP crystal. Then, the fundamental beam interacts with SHG beam in the potassium dihydrogen phosphate crystal (KH²PO⁴, abbreviated KDP) producing the THG wave at wavelength of 355 nm having the pulse duration of τ^las = 12.5 ns.

The fourth harmonic generation (FHG) is possible in the non-centrosymmetric crystals, however similar to the THG, in practice, the quadrupling frequency of the fundamental wave in cascade regime is much more efficient than using direct fourth-order nonlinear process [112]. In in this work, the second harmonic of the Nd:YAG laser beam at the wavelength of 532 nm was generated in KTP crystal and then converted in to the fourth harmonic at the wavelength of 266 nm at pulse duration of τ^las= 10.8 nm in the barium betaborate crystal (β-BaB²O⁴, abbreviated BBO).

4.2.3 Frequency tunable IR laser source

In this work, we used optical parametric generator (OPG) pumped by Nd³⁺:YAG laser to generate nanosecond light pulses tunable in the wavelength range of 1350 - 4000 nm.

73 OPG is based on the process of the difference frequency generation in the crystal with the second-order optical nonlinearity. In this process, the interaction of the light waves at frequencies ω¹ and ω³ results in the generation of the wave at the difference frequency ω²=ω³-ω¹. Importantly, at the phase matching conditions, the amplitudes of the signal and idler waves at frequencies ω² and ω¹, respectively, grow exponentially in the nonlinear crystal having the gain coefficient proportional to the amplitude of the pump wave at frequency ω3. It is worth noting that in contrast to the difference frequency generation, the amplitude of the second harmonic wave is a linear function of the crystal length.

The exponential growth of the signal wave amplitudes implies that if the second-order susceptibility is high and/or nonlinear crystal is long enough, the signal and idler waves can grow to measurable amplitude from the noise level. That is similar to conventional lasers, introducing a positive feedback by placing the nonlinear crystal in the optical cavity may allow one to arrive at generation of the signal and idler waves. Importantly, however that the frequencies of these waves can be tuned in a wide range provided that the energy conservation ω³ = ω¹ + ω² and phase matching conditions are satisfied.

In the generic OPG, a nonlinear crystal is placed in an optical cavity (Figure 4.7).

Both mirrors are transparent for the pump wave (ω3) and have a high reflection coefficient either only for signal (ω1) or for both signal (ω1) and idler (ω2) waves.

Similar to the conventional laser, the generation occurs when parametric gain exceeds the loss in the cavity, i.e. at some threshold pump intensity. Above the threshold, the intensity of the signal and/or idler beams is determined by the pump intensity and nonlinear susceptibility, while frequencies ω1 and ω2 are given by the phase matching condition. Since the synchronism is determined by the orientation of the nonlinear crystal; therefore, the relationship between ω¹ and, correspondingly, ω²=ω³-ω¹ can be tuned by rotating crystal and/or by changing its temperature [111].

Figure 4.7. Schematic illustration of an optical parametric generator.

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Figure 4.8. Optical scheme of a parametric generator.

In this work, we employed OPG (LaserVision Inc) pumping by the YAG: Nd³⁺

laser, operating at the repetition rate of 10 Hz. The OPG setup is shown in Figure 4.8.

A linearly polarized laser beam with a wavelength of 1064 nm incidents on a beam splitter, which divides the IR beam into two. One of the beams passed through the KTP crystal to generate the second harmonic at a wavelength of 532 nm. A half-wave plate was installed in front of the generator, which made possible to smoothly control the SHG efficiency by changing the orientation of the plane of polarization. The second beam at fundamental frequency passed through a delay line and then combined with the first beam. The optical attenuator in the delay line allows one to control the pump energy.

Figure 4.9. Illustration of the relative positions of nonlinear crystals in an optical resonator during frequency conversion.

Then SHG beam is used for pumping the optical parametric oscillator (OPO), in which two KTP crystals placed in the cavity in such that they compensate the pump beam displacement from the cavity axis due to refraction (see Figure 4.9) ensuring multiple roundtrips. After the OPO the radiation passes through a half-wave plate capable of operating in a wide wavelength range. The half-wave plate rotates the plane of polarization at 90 degrees. The output beam of the OPO is combined with the delayed beam with a wavelength of 1064 nm and enters the optical parametric

75 amplifier (OPA) containing four nonlinear crystals of potassium titanyl arsenate (KTiOAsO4, abbreviated KTA), arranged in such a way to compensate the beam deflection (see Figure 4.9). After the OPO, the signal and idler beams are orthogonally linear polarized, i.e. we can choose one at the by introducing the plates oriented at the Brewster angle. This parametric system can generate radiation in the wavelength ranges 710 - 885 nm and 1350 - 5000 nm with a pulse duration of τlas = 6 - 8 ns. Management and configuration of the system were carried out using a computer equipped with special software.