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

Generation of powerful single-frequency giant radiation pulses

Fig.3.2 Interferograms of the lasing spectra of the master and high-power lasers in the absence (a) and in capture (b) of the radiation wavelength of the external signal, the range of dispersion of the interferometer is 0.4 nm (a) and 8 pm (b); c) oscillogram of the intensity of radiation of the giant pulse in capture of the wavelength of the external signal.

Fig.3.3 Diagram of a four-mirror powerful laser with electro-optical modulation of the Q-factor: I – master single-frequency quasi-continuous laser; II – powerful laser with electrical modulation of the Q-factor: 2–4) electro-optical decoupling of the lasers; 5) ruby; 6,7) electro-optical gate; 6) Pockels cell; 7) polarisation prisms.

the circuit of injection of the external signal with a ring-shaped resonator of a powerful laser (Fig. 3.4). In this circuit, the effect of the giant radiation pulse on the master laser was eliminated without using electro-optical decoupling owing to the fact that the capture of radiation of the external signal took place not only with respect to the spectrum but also direction. This greatly simplified the experimental system. In the ring-shaped laser, the input and output of radiation was ensured through the prism 4 with a disrupted total internal reflection. The length of the resonator of the ring-shaped laser was 1.5 m, and the longitudinal modes were separated by a diaphragm with a diameter of 1.7 mm. The Q-modulation of the resonator was carried out by the electro-optical gate (9) which

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Fig.3.4 Diagram of a powerful single-frequency ruby laser with a ring resonator: I – master single-frequency quasi-continuous laser; II – powerful laser with electrooptical modulation of the Q-factor: 4–7) rotating prisms of the resonator; 9) electrooptical gate consisting of the Pockels cell and polarisation trace; 10) ruby.

Fig.3.5 Parameters of lasing of a powerful single-frequency ruby laser with electrooptical modulation of the Q-factor of the resonator: a) interferrogram of the radiation spectrum of the giant pulse, the range of dispersion of the interferometer 1.6 pm; b) oscillogram of radiation intensity; c) sequence of interferograms of the radiation spectrum of the giant pulse, illustrating the range of rearrangement of the radiation wavelength, region of dispersion of the interferometer 240 pm.

consisted of a Pockels cell on a DKDP crystal with ends under the Brewster angle and of a polarisation trace.

In these experiments, the giant radiation pulse was generated on a single longitudinal mode (Fig. 3.5a) with a power of 50 MW and with smooth retuning of the radiation wavelength in the range 250 pm (Fig. 3.5c).

The application of a more powerful single-frequency, quasi-continuous master laser with spherical mirrors in the regime of inertia of the spectrum with the parameters, shown in Fig. 1.16, greatly simplified the circuit

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Generation of powerful single-frequency giant radiation pulses

of experimental equipment. In this case, experiments were carried out with a linear four-mirror variant of the injection circuit without electrooptical decoupling between the lasers. The higher radiation intensity of the master laser made it possible to inject its radiation into the resonator of the powerful laser through its dense mirror. In this case, the reverse effect of the giant pulse on the master laser was very small. The spherical front of the master laser was matched with the flat front of the powerful laser using a telescope. When using, in the powerful laser, a ruby crystal with sapphire tips with a length of 240/320 mm and a diameter of 40 mm, the single-frequency lasing of the giant pulse with a power of 200 MW was recorded. The retuning of the radiation wavelength was carried out by misaligning one of the mirrors of the master laser, and its range was 1.20 pm.

For the ruby laser, the experimentally measured minimum value of the intensity of the external signal resulting in the stable capture of its radiation wavelength in the centre of the gain line, was approximately 0.1 W/cm2.

3.3 POWERFUL SINGLE-FREQUENCY TUNABLE Nd-DOPED LASERS WITH INJECTION OF THE EXTERNAL SIGNAL

The method of injection of the external signal in Nd lasers with electrooptical modulation of the Q-factor was investigated by the authors of this work in Ref. 28, 34–38. In the Nd lasers, the injection circuits for the external signal included relatively powerful single-frequency master lasers whose radiation was injected into the resonator of the powerful laser through a dense resonator mirror. In this case, it is not necessary to carry out optical decoupling of the lasers. To develop a powerful singlefrequency Nd:YAG laser, the circuit for injection of the external signal included a master Nd:YAG laser in the regime of the single-frequency quasistationary lasing of the TEMmnq modes, whose parameters are shown in Fig. 2.7. Using a garnet crystal 100 mm long and 6 mm in diameter in a powerful laser it is possible to obtain (in the regime of capture of the radiation wavelength of the external signal) singlefrequency lasing of the giant pulse of the Nd:YAG laser with a power of 40 MW with retuning of the radiation wavelength in the range 0.35 nm.

In order to increase the width of the range of retuning of the radiation wavelength in the vicinity of 1.06 µm, tests are carried out with the circuit of injection of the external signal with a Nd laser in phosphate glass GLS-22 (Fig. 3.6). The master laser was represented by a sin- gle-frequency quasistationary laser on Nd in glass with the lasing parameters shown in Fig. 2.3. The increase of the energy of quasistationary lasing of the TEMooq modes in the master laser was achieved by increasing

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Fig.3.6 Diagram of a powerful single-frequency adjustable tuneable neodymium laser in glass with electronic modulation of the Q-factor: I – master single-frequency quasi-continuous neodymium laser in glass; II – powerful laser: 10) electro-optical gate; 11) neodymium rod GLS-22 diameter 15 mm, length 302 mm.

the length of the Nd rod to 300 mm and the diameter of the diaphragm to 3 mm, with the resonator length being increased at 230 cm. Consequently, the energy of single-frequency lasing of the master laser on glass was increased to 0.5 J.

In the region of capture of the radiation wavelength of the external signal, the radiation spectrum of the giant pulse was almost completely identical with the radiation spectrum of the external signal (Fig. 3.7a). The absence of ‘beating’ of the intensity of radiation of the giant pulse (Fig. 3.7b) shows that lasing in this case takes place on a single longitudinal mode. The power of the giant radiation pulse was 180 MW, the width of the range of retuning of the radiation wavelength was 5.6 nm (Fig. 3.7c).

The experimental measured intensity of the radiation of the external signal, required for stable capture of its radiation wavelength by the giant pulse of the Nd laser, was approximately 0.5 W/cm2. With increase of the pumping energy, the width of the band of capture of the radiation wavelength slowly increased. The rapid increase of the radiation intensity of the external signal did not result in any significant broadening of the capture band of its length.

Evaluation of the band of capture of the frequency of radiation of the external signal (ω s – ω 0) by the giant pulse was carried out assuming that the inverse population in the active medium during the period of linear development of the giant pulse (td ~ 10–7 s) remained almost unchanged (∂ n(ω ,t)/∂ t = 0). Consequently, the spectral density of radiation I(ω , t) is governed by the equation

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Generation of powerful single-frequency giant radiation pulses

Fig.3.7 Parameters of lasing of a powerful single-frequency neodymium laser in glass with electro-optical modulation of the Q-factor of the resonator: a) interferogram of the radiation spectrum of the giant pulse, the rate of dispersion of the interferometer 8 pm; b) oscillogram of radiation intensity; c) sequence of spectrograms of radiation of the giant pulse, illustrating the tuning range of the radiation wavelength.

the photon in the resonator; S(ω ) is the intensity of spontaneous radiation; U(ω s) is the intensity of the external signal; n(ω ) is the inverse population. Since the circuit of the non-uniformly broadened line has the Gaussian shape with the width Γ , we have the following equation for the inverse population and the intensity of spontaneous noise:

Assuming that I(ω , 0) = 0, we obtain the solution of the equation (3.1) in the form:

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where Id is the spectral density of radiation at the end of the linear stage of development of the giant pulse.

The criterion of capture of the wavelength of external radiation may be the condition:

Equation (2.5) shows that the band of capture of the radiation wavelength of the external signal slowly increases with increasing intensity of the external signal U(ω s). At same time, the duration of linear development of the giant pulse slowly decreases with increasing intensity of the external signal.

3.4 SINGLE-FREQUENCY TUNABLE Nd LASERS WITH PASSIVE Q- MODULATION

The energy and spectral characteristics of radiation of Nd lasers in crystals YAG, BLN, Cr:GSGG and LNA with passive Q-modulation with gates based on lithium fluoride crystals with F2– dye centres have been examined in detail by the authors of this book in Ref. 29–38.

Because of their simple and easy operation, phototropic gates are used widely for the passive modulation of the resonators of solid-state lasers. However, passive gates on dyes have high losses and are not stable. The crystals with the dye centres in strontium fluoride were used for

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