1 Introduction

Optical pumped solid-state lasers are a grate class of under progress lasers that are applicable in different types (Wang and Groves 1992; Boulon 2012; Davarcioglu 2010; Kunda et al. 2001). Selection of a suitable pump source in laser designing is important and affects the total optical efficiency and beam quality (Byer 1988). The need for laser systems with a high power and high efficiency has led to the development of solid-state lasers pumped by laser-diodes. High efficiency and small size of the system compared to solid-state laser pumped by flash lamp make it applicable in different areas (Koechner 1999). Solid-state lasers, which are side-pumped by laser-diodes, possess higher power and efficiency, improved beam quality, and longer functionality compared to those pumped by flash lamp (Garrec et al. 1996; Konno and Yasui 1998; Leger and Goltsos 1992). Laser-diodes consist of linearly juxtaposed tiny emitters, which may emit at various wavelengths. Laser-diode 808 nm wavelength lies at the absorption peaks of Nd atoms and is a suitable source for pumping the Nd:YAG. More spectral coincidence in diode pumped lasers is compared to flash lamp pumped ones reduces unuseful wavelengths absorbed in the gain medium. This causes the reduction of the thermal effects (Panahy et al. 2008).

Laser diode beam represents a high divergence and low coherence, which reduces the feasibility of their concentration into an active medium. Moreover, provision of optical components is costly for this purpose. In the near-coupling method, laser-diode is positioned as close to the active medium as possible. However, due to the existence of cooling system surrounding the active medium, in high power system designs, laser-diodes could not be placed too close to the active medium. Thus, appropriate interference optics is utilized in such cases (Eryilmaz 2004; Sipes  1985). In this paper, a simple and low-cost design method is proposed for this aim. The design and simulation of the system are discussed in Sect. 2 of the present paper. The experimental results are given in Sect. 3 and finally a brief conclusion is reported in Sect. 4.

2 Design and Simulation

The following equation is used for a new and different laser head design,

$$\frac{1}{p} + \frac{1}{q} = \frac{1}{f},$$
(1)

where “p” and “q” are the distances of laser-diode and active medium from the reflector, respectively. “f” is the focal length of the reflector. The reflector was gold-plated, which had a good reflection (98 %) at 808 nm wavelength. Cross-section of the designed air-cooled laser head is shown in Fig. 1.

Fig. 1
figure 1

Cross-section of the designed laser head. Pump beam reflects from gold-plated surface and focuses into the active medium

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For an optimal design and accomplishment of research objectives (achieving maximum efficiency), laser system was simulated using LASCAD software. In this simulation, thermal analysis of the active medium was done by studying the volume under pumping. To acheive a high beam quality, we attempted to reduce cavity mode volume. Figure 2 shows thermal stresses of laser rod. In this figure, maximum heat rate is clearly observable at the input of laser-diode beam. To decrease this thermal stress a cooling fan is used above the laser head.

Fig. 2
figure 2

Heat distribution in Nd:YAG laser rod from top view

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Figure 3 shows the output beam profile of the laser, which indicates a high quality beam. In Fig. 4, diagram of the laser quality factor is presented. A laser beam with the beam quality factor of M 2 ≤ 1.08 can be observed in this figure.

Fig. 3
figure 3

Output beam profile of Nd:YAG laser

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Fig. 4
figure 4

Diagram of laser beam quality factor simulated by LASCAD software

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3 Experimental Results

The solid-state gain medium was 0.8 % Nd:YAG cylindrical crystal rod, which was 4 mm in diameter and 50 mm long. To employ a suitable laser-diode for pumping, the absorption spectra were obtained using UV–VIS deviceFootnote 1 as shown in Fig. 5. As it can be observed in the absorption spectrum of Nd:YAG crystal, the most intensive absorption peak lies on 808 nm wavelength.

Fig. 5
figure 5

Absorption spectrum of Nd:YAG

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The selected pumping source was GaA1As laser-diode with an emitting wavelength of 808 nm. The emitted spectrum was recorded by Spectral Star S100 spectrometer (a CCD array detector with a grating as diffraction element). The results are shown in Fig. 6.

Fig. 6
figure 6

Output spectrum of laser-diode

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Laser-diode consists of a radiative line with the width of 960 µm (12 rods of 80 µm) and length of 13.1 mm. Its vertical divergence is 45° and horizontal divergence is 12°. Output power of the laser is 1200 W at pulse duration of 230 µs.

Figure 7 illustrates laser-diode output energy versus input current. As the figure shows, there is a linear relation between the input current and output energy from laser-diode. To prevent the variation of laser-diode wavelength caused by temperature rise, laser-diode is cooled by a heat sink.

Fig. 7
figure 7

Output energy diagram of laser-diode versus input current

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The laser resonator was 100 mm in physical length, with two flat mirrors, a high reflector with the reflecting coefficient of 100 % as back mirror and the other one with the reflecting coefficient of 90 % as output coupler. The experimental setup is shown in Fig. 8.

Fig. 8
figure 8

Experimental setup of the fabricated laser

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The energy measurement results are shown in Fig. 9. Based on this figure the maximum output pulse energy is 14 mJ which corresponds to the optical efficiency of 6 %.

Fig. 9
figure 9

Experimental measured output energy versus input energy

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The temporal shape of the output pulses of laser, which is recorded by a Techtroniks oscilloscope, is shown in Fig. 10. Its width is about 200 µs at half maximum. Utilizing a Z-scan setup with a (Gentech) beam profiler, the beam waist and divergence of laser output were measured. Then M 2 factor was calculated using Eq. 2. The experimental value of beam quality factor was M 2 = 1.1. Simulation by LASCAD software confirmed this value for M 2 which was about 1.08.

Fig. 10
figure 10

The temporal shape of the output pulses of laser

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$$M^{2} = \, \pi d\theta /\lambda^{4} .$$
(2)

4 Conclusion

A new, simple laser head was designed, simulated, and fabricated for a diode side-pumped Nd:YAG laser. By the proposed system, the inadvisable effects caused by flash lamp were eliminated due to good consistency between the output of the laser-diode and absorption spectrum of the active medium. Considering the simulation and experimental results, it was shown that using uniform diode pump of laser active medium can prevent thermal stresses. These stresses reduce the beam quality factor (M 2) in laser oscillators. Furthermore, reduction of mode volume and efficient mirror surfaces accompanied by uniform pump of laser active medium using laser-diodes can serve as a suitable method to obtain high beam quality factor.