Compact Nd:YAP/V:YAG nanosecond pulse generator at 1342 nm

A compact Q-switched laser with a separate Nd:YAP gain part and V:YAG saturable absorber was designed and constructed. The use of a Nd:YAP gain crystal may have certain advantages, such as polarization and wavelength stability, over the more common Nd:YAG active medium. Constructed Nd:YAP/V:YAG compact laser provides linearly polarized radiation at a wavelength of 1342 nm. Stable pulses 12 ns long with energy up to almost 0.2 mJ were generated. This laser system could serve as an eye-safe radiation source of long-range high-resolution LIDAR for autonomous vehicle control.


Introduction
The aim of this work was to design and construct a compact longitudinally diode-pumped laser system Nd:YAP/V:YAG in a regime of passive Q-switching generating stable nanosecond pulses in the 1.34 μm spectral region. This compact and stable laser system could serve as a laser source of longrange high-resolution LIDAR for autonomous vehicle control [1,2]. The 1.34 μm radiation plays a vital role in eye safety due to its higher absorption in water and three-order higher exposure limit in comparison with the 1.06 μm radiation [3]. Such a compact laser could be also a more flexible and economical alternative to the Nd:YAG/V:YAG microchip laser constructed previously [4,5].
In the past, a Nd:YAG/V:YAG compact laser generating at a wavelength of 1318 nm has already been constructed [6]. In terms of polarization and wavelength stability, it might be interesting to use Nd:YAP crystal as an active medium that enables generation at a wavelength of 1342 nm [7]. This wavelength may also be more suitable for LIDAR applications compared to 1318 nm and 1.06 μm radiation due to lower background radiation intensity based on the higher absorption of solar radiation in atmospheric water vapor [1,8].
This article describes the designed and constructed longitudinally diode-pumped compact Q-switched laser with a separate Nd:YAP gain part and V:YAG saturable absorber. Furthermore, the results of the measured output radiation parameters are presented here. Up to 193 μJ of pulse energy, 16.2 kW of pulse peak power, and pulse length ∼ 12 ns at a wavelength of 1342 nm were achieved. pumping pulse repetition rate. The following subsections briefly describe the components of this laser. Furthermore, the methods by which its output radiation characteristics were obtained are presented here.

Laser head components
A cylindrical-shaped Nd:YAP crystal with active ions concentration 0.5 at% Nd/Y served as an active medium for laser generation at a wavelength of 1342 nm. This crystal with a longitudinal axis parallel to the b axis (Pbnm) was 16 mm long and had a diameter of 5 mm. It was designed to absorb most of the pumping radiation. Because of Nd:YAP crystal anisotropy, a linearly polarized output radiation of the compact laser was expected. The emission spectra of the Nd:YAP crystal in the spectral region of 1.3 μm for both possible orthogonal polarizations perpendicular to the longitudinal axis of the crystal are depicted in Fig. 2a. It is obvious, that polarization direction E ∥ c corresponds to the higher gain at a wavelength of 1342 nm. The spectral dependence of Nd:YAP crystal absorption cross-section for unpolarized radiation in the supposed pumping region is given in Fig. 2b. Also, the pumping spectral area used for compact laser pumping is marked here. The thermal effect on the absorption cross-section was not investigated. However, this influence could be assumed to be small, as measured for a Nd:YAG sample with similar optical properties to the Nd:YAP crystal [9].
To achieve laser generation in the regime of passive Q-switching, the V:YAG saturable absorber in a cylindrical shape with a diameter of 5 mm, initial transmission of 85%, and longitudinal-cut orientation [100] was used. V 3+ ions in the V:YAG crystal exhibit anisotropy in excite state absorption and therefore some output parameters of constructed Nd:YAP/V:YAG compact laser may vary depending on the rotation of the V:YAG crystal around the longitudinal axis relative to the Nd:YAP crystal [10,11]. Both active crystal Nd:YAP and saturable absorber V:YAG were fabricated by Crytur Ltd.
Layers of pumping and output mirror of the plan-plan laser resonator were deposited on the face of Nd:YAP active crystal and V:YAG saturable absorber, respectively. The pumping mirror had high reflectivity ( R > 99.5 ) for radiation at 1.34 μm and high transmission ( T > 85% @ 0.8 nm) for pumping radiation. The output mirror on the V:YAG face with a reflectivity of 90% @ 1.34 μm was adjustable to optimize resonator stability and output laser parameters. Since the Nd:YAP crystal exhibits also strong gain at a wavelength of 1.06 μm [12], resonator layers were designed to prevent this radiation generation, and their reflectivity at this wavelength was lower than 5%. Some parameters summary of presented Nd:YAP/V:YAG compact laser is given in Table. 1.

Diode-pumping system
Compact laser longitudinally pumping was realized with the fiber-coupled laser diode (DILAS, core diameter 400 μm , numerical aperture 0.22) in a pulse regime with a repetition rate of 10 − 500 Hz and a pulse duration of 1 ms. The electric current and repetition rate of the laser diode were ranged during the measurement. Therefore, wavelength and energy at the output of the fiber varied within a range of 17 − 20 mJ and 804.9 − 807.1 nm, respectively. The dependence of pumping laser diode pulse energy and wavelength on pulse repetition rate at several values of electric current is shown in Fig. 3. The laser diode was energized to the source Laser Electronics LDC1000. Radiation from the laser diode was focused into the Nd:YAP crystal by two aspheric lenses with the focal lengths f 1 = 8 mm and f 2 = 11 mm. The radius of the pumping beam waist measured by the knife-edge method was 283 μm and its position inside the active crystal was optimized to obtain the minimal laser generation threshold.

Measurement devices
The InGaAs spectrometer StellarNet DWARF-Star NIR-25 was used to measure the spectrum of the output laser radiation. The mean power of the compact laser output was measured by power meter Molectron EPM-2000e with the probe Coherent PowerMax PS19Q ( 0.1 mW − 1 W ). The fast InGaAs PIN photodiode (EOT, model ET-3000, rise time < 175 ps) connected to the oscilloscope Tektronix TDS 3052B (500 MHz, 5 GS/s) was used to scan output temporal characteristics. The laser beam spatial structure was detected based on two-photon absorption by the silicon camera Data-Ray WinCamD-UCD23.

Results and discussion
The Nd:YAP/V:YAG compact laser was put into operation under pulsed laser diode-pumping with a repetition rate from 10 to 500 Hz. The generation of a single Q-switched pulse was ensured by adjusting the diode current amplitude, since the source for pumping the laser diode did not allow the generation of pulses shorter than 1 ms. Obtained results of output parameters are presented below followed by a short discussion.

Laser emission wavelength and polarization
For the constructed Nd:YAP/V:YAG compact laser a central wavelength of 1342 nm was measured, as shown in Fig. 4. This corresponds to the 4 F 3∕2 → 4 I 13∕2 transition of Nd 3+ ions in YAP [12]. The radiation of the laser was linearly polarized in the tested range of pumping repetition rates even though the pumping radiation was unpolarized. This was due to Nd:YAP anisotropy. The polarization was in the E ∥ c direction which corresponds to the much higher gain against the E ∥ a direction at a wavelength of 1342 nm, see Fig. 2a. Furthermore, it was observed that the rotation of the V:YAG crystal around the longitudinal axis relative to the Nd:YAP did not affect the output polarization direction. During this rotation, however, there was a change of the generated pulse energy and duration due to V:YAG crystal anisotropy of excited state absorption for a certain polarization direction. During laser construction, the aim was to adjust the optimal relative position of both crystals which corresponded to the highest output pulse energy and the shortest pulse length.  Fig. 3 Dependence of a pumping laser diode pulse energy, b pumping laser diode wavelength on pulse repetition rate at several values of electric current

Laser power, energetic, and time characteristics
The dependence of the output mean power, pulse energy, duration, and peak power on the pumping pulses repetition rate is shown in Fig. 5. The mean power was increasing with the pumping repetition rate and the maximum value of 46 mW was reached, see Fig. 5a. Its slope was decreasing with an increasing repetition rate. Therefore, the pulse energy was also decreasing at a higher repetition rate, as can be seen from Fig. 5b. The explanation may be as follows. Thermal load at high frequencies leads to the induction of a thermal lens and subsequent to the mode volume decreases inside the gain medium. The energy values obtained were in the range of 92 − 193 μJ. The output pulse duration which is depicted in Fig. 5c hardly varied with the pumping repetition rate, because it primarily depends on saturable absorber losses. Its average value was equal to 11.8 ± 0.1 ns (FWHM). The pulse peak power is given as the fraction of the falling pulse energy and the constant pulse length. This implies the decreasing character of the pulse peak power dependence on the repetition rate with a maximum of 16.2 kW and a minimum of 7.9 kW, as shown in Fig. 5d. An example of the temporal structure of generated pulses at a pumping repetition rate of 500 Hz is given in Fig. 6, 7.

Laser beam spatial structure and divergence
In the entire range of pumping pulse repetition frequencies, higher spatial transverse modes were probably not observed. The laser beam spatial structure for the lowest and the highest pumping frequency is given in Fig. 7a-b. As plotted in Fig. 8c, the divergence angle was increasing with respect to the pumping pulse frequency. This could probably also be explained by the thermal lens effects inside the gain medium. In addition, the beam spatial structure could have been influenced by two-photon absorption that occurred during detection.

Output parameters stability
The emission central wavelength and polarization did not fluctuate within a range of the used pumping repetition rates. A decrease in generated pulse energy and peak power as a function of the pumping repetition rate was observed. This decrease was steeper compared to the results with the Nd:YAG/V:YAG compact laser obtained in earlier investigations [6]. This can be explained on the basis of thermal conductivity which is better in the case of the Nd:YAG crystal compared to the Nd:YAP crystal [13].
For output pulse energy, pulse length, and peak power, the stability in terms of relative error fluctuations at a given pumping pulse repetition frequency was investigated. The relative error of these parameters was very small and did not exceed 2%, as shown in Fig. 9. The deviation of the measured pulse length, which also affected the deviation of the peak power, was primarily influenced by the resolution of the measurement method which was at a level of 1 ns.

Summary and conclusion
A compact laser diode-pumped Q-switched laser with independently adjustable resonator mirrors, consisting of Nd:YAP and V:YAG crystals, was designed and realized. This laser generates linearly polarized radiation at a wavelength of 1342 nm. The fiber-coupled laser diode in pulsed-regime was used for pumping. Pumping radiation was focused into the Nd:YAP gain crystal using two smallsized aspherical lenses.
A summary of the Nd:YAP/V:YAG compact laser parameters obtained is given in Table 2. The highest output pulse energy 193 μJ and peak power 16.2 kW were obtained at a pumping pulse frequency of 20 Hz. These parameters were decreasing for higher frequencies due to thermal stress. In addition, the divergence of the output beam was increasing with this thermal effects. The pulse length with a mean value of 11.8 ± 0.1 ns, polarization, and emission wavelength did not change depending on the pumping pulse frequency. The relative error of the pulse energy, pulse length, and peak power at a given frequency did not exceed 2%. The advantage of a laser system designed in this way is the possibility of easy modification. It would also be possible to increase generation efficiency by using a pumping source with shorter pulses. Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.