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Experimental investigation of single and dual pulses in a passively Q-switched Nd:YAG microchip laser with a Cr4+:YAG saturable absorber has been reported. The dual pulses consist of a main and a satellite pulse with respective spectra, intensities, and durations. It is found that the preponderant oscillating mode gives birth to the main pulse, and the other oscillating mode corresponds to the satellite pulse. Our results demonstrate that the dual-pulse emission results from double-longitudinal-mode oscillation in high pump regime.
©2011 Optical Society of America
1. Introduction
Q-switched and mode locked lasers have attracted much attention due to their ability of generating high peak power pulse with short durations [1–6]. Different from actively Q-switched lasers, passively Q-switched lasers do not require any external switching electronics and exhibit features of simplicity, compactness, low cost, and high efficiency [7–9]. In solid-state lasers, saturable absorbers (SAs), such as Cr4+:YAG and semiconductor saturable-absorber mirror (SESAM), have been widely utilized for passively Q-switched operation. In contrast with other SAs, Cr4+:YAG crystal possesses a number of desirable properties, including better photochemical and thermal stabilities, larger thermal conductivity, higher damage threshold, as well as broader absorption band [10–12]. Neodymium doped (Nd3+-doped) materials are usually used as gain medium in passively Q-switched solid-state lasers for their high gain, good thermal and mechanical properties [13–15]. Zayhowski et al. firstly reported passively Q-switched operation in a laser diode (LD) pumped solid-state Nd:YAG microchip laser [16]. Additionally, other Nd3+-doped materials, such as Nd:YVO4 and Nd:GAGG, have achieved Q-switching operation by using a Cr4+:YAG SA [17–19]. The peak power of such Q-switched pulses approaches to tens hundreds and thousands of kilowatts [20, 21].
However, in the high-power regime of the passively Q-switched Nd3+-doped lasers, it is demonstrated that Q-switched lasers can easily operate at multi-pulse emission states [22]. Recently, Dong et al. observed multi-pulse oscillation in a LD pumped microchip Cr,Nd:YAG self-Q-switched laser [23], and the results could be explained as different transverse mode coupling and competition. Moreover, an additional smaller pulse (i.e., satellite pulse) accompanying with main pulse was also reported. This operation might be attributed to the presence of saturation behavior when the initial transmission of the output coupling was lower enough [24]. Apart from the mechanism described in Refs [23, 24], a slow ion relaxation from the lower laser energy level to the ground state of gain medium could also generate the satellite pulse [25].
In this paper, we focus on the experimental observation of single- and dual-pulse oscillation as a function of pump power in a LD pumped passively Q-switched Nd:YAG microchip laser with a Cr4+:YAG SA. Single pulse at the nearly TEM00-mode operation is obtained due to single-longitudinal-mode oscillation of laser. The corresponding bandwidth of the Q-switched pulse is estimated to be ~0.03 nm. The dual pulses are comprised of a main pulse and a satellite pulse with respective intensities and durations. The corresponding spectrum of the dual-pulse waveform exhibits two peaks with a separation of ~0.15 nm. Further investigations show that the main (satellite) pulse corresponds to the stronger (weaker) spectrum component. Experimental results indicate that the dual-pulse operation results from the coexistence of double longitudinal modes, which is different from that of Refs [23–25]. In addition, the repetition rate of the passively Q-switched Nd:YAG/Cr4+:YAG microchip laser increases almost linearly with the pump power, which is ~3.3 kHz at threshold pump of ~3.34 W and increases to ~8.8 kHz at the maximum incident pump of ~4.36 W.
2. Experimental setup
The experimental setup for passively Q-switched microchip laser is schematically shown in Fig. 1 . The microchip laser consists of a short piece of Nd:YAG bonded directly to a thin Cr4+:YAG SA. The Nd:YAG with a 1.0 at.% Nd has a dimension of Φ3×1.5 mm, and the Cr4+:YAG SA exhibits an initial transmission of 75% at 1064 nm. Surface 1 of the composite crystal is coated with an antireflection coating at 808 nm and a total reflection coating at 1064 nm. Surface 2 is coated for antireflection at 808 nm and reflection of 85% at 1064 nm. This antireflection film could increase the transmission of pump light and reduce the rebound pump light to protect the pump source. Surface 1 works as input coupler and Surface 2 acts as output coupler. The temperature is mastered at 25 °C with thermoelectric cooler to improve the absorbable characteristics of the laser crystal. The crystal is end pumped at Surface 1 by a CW LD bar at 808 nm. The maximum pump power reaches ~5 W at a drive current of ~5 A. The pump light from the LD bar is launched into the Nd:YAG by a GRIN Lens with a diameter of ~1.5 mm. The spot of the pump focus at the laser crystal is approximately 115 µm × 433 µm. An optical spectrum analyzer (OSA, ~3dB band width, 0.02 nm spectral resolution, 1.0 nm spectral span) and a 6-GHz digital storage oscilloscope (DSO) together with a 45-GHz photodiode detector (PD) are employed to monitor the laser output simultaneously.
3. Experimental results and discussions
The Cr4+:YAG works as SA and is used to realize passive Q-switching in Nd:YAG laser. Q-switched operation can be achieved when the pump power is beyond the threshold pump of ~3.1 W. Figure 2 shows the single-pulse emission and single laser spectrum of the passively Q-switched laser in the nearly TEM00-mode operation at pump power of ~3.5 W. The pulse energy and pulse repetition rate are ~32.5 µJ and ~4.4 kHz, respectively. The corresponding peak power and duration are estimated as ~45.5 kW, and ~715 ps, respectively. The output spectral width of the Q-switched pulse is estimated to be ~0.03 nm. According to the laser resonator theory [26], the separation between adjacent longitudinal modes in laser cavities is determined by Δλc = λc2/2nLc, where Lc is the length of resonator and λc is the laser wavelength. For a 2-mm laser resonator, Δλc is calculated to be ~0.15 nm at 1064 nm. Obviously, the spectral width is narrower than the separation of the longitudinal modes, indicating that the Q-switched laser operates at single-longitudinal-mode oscillation state in this regime. The spatial profile of output beam is shown in Figs. 2(c) and 2(d), and suggests the TEM00 Gaussian mode.
Fig. 2 Pulse characteristics in temporal (a) and spectral (b) domain when the pump power is ~3.52 W. The corresponding repetition rate is ~4.4 kHz. Two- (c) and three- (d) dimensional output-beam qualities, which suggests the TEM00 Gaussian mode.
When the pump power increases to ~3.93 W, dual-pulse oscillation can be observed. Figure 3(a) shows the typical dual-pulse emission at pump power of ~4.36 W. The corresponding repetition rate is determined to be ~8.8 kHz. One can observe that a main pulse and a satellite pulse with respective intensities coexist in the passively Q-switched laser. The pulse-pulse separation in the dual-pulse emission is ~6 ns. That is, the main and satellite pulses are generated in a single Q-switched process as they emit at the same repetition rate. Figure 3(b) shows the corresponding optical spectrum of the Q-switched pulses. The output spectrum exhibits dual-peak structure with a separation of ~0.15 nm. According to the theory prediction mentioned above, the separation of adjacent longitudinal modes is ~0.15 nm. It is clear that there are two adjacent longitudinal modes coexisting in the proposed laser. Based on the experimental results, we conclude that the output dual pulses arise from the double longitudinal modes oscillation. Moreover, the preponderant oscillating mode gives birth to the main pulse, and the other oscillating mode corresponds to the satellite pulse. By using a Glan prism, the polarizations of output laser are measured and we can find that the main and satellite pulses share the same polarization states in passively Q-switched Nd:YAG/Cr:YAG lasers. The spatial profile of output beam is shown in Figs. 3(c) and 3(d).
Fig. 3 Pulse characteristics in temporal (a) and spectral (b) domain when the pump power is ~4.36 W. The corresponding repetition rate is ~8.8 kHz. Two- (c) and three- (d) dimensional output-beam qualities.
We further investigate pulse evolution as a function of pump power. As shown in Fig. 4 , the temporal evolution of dual-pulse waveform is coincided with that of spectral domain, which confirms again that the main (satellite) pulse corresponds to the stronger (weaker) optical spectrum component. As shown in Fig. 4(a), the laser changes from single pulse to dual pulses when the pump power exceeds ~3.93 W. Under the strong pump level, multi-pulse oscillation can be generated in Q-switched lasers [22]. We note that, in the LD pump passively Q-switched lasers, the laser cannot be generated until the cavity gain reaches a value exceeding the overall optical losses inside the cavity, which makes the SA rapidly bleach into a high transmission state and consequently Q-switched pulses are achieved. Therefore, the main pulse is prior to the weaker pulse, the pulse-pulse separation decreases and the strength of the satellite pulse increases with the enhancement of the pump power. Figure 4(b) shows the corresponding spectral evolution versus pump power. We can observe that the central wavelengths of the two independent pulses shift to longer wavelength, and the strength of the dual-peak structure both increases, while the wavelength separation maintain a given constant with the increase of pump power. It is well known that gain media temperature increases with the pump strength. As a result, the emission line width of the stimulated transition broadens and the spectral line shifts toward the longer wavelength [27, 28]. Moreover, the length-variation of cavity caused by the increase of gain media temperature can also result in red-shift in spectral domain. Consequently, it is reasonable that the central wavelength in the laser shifts to longer wavelength slightly.
In our experiments, by slightly moving the Nd:YAG/Cr4+:YAG crystal along the pump beam direction, we found that the spectrum of main pulse (satellite pulse) can locate at either in the short- or long-wavelength side. However, the main pulse was always generated before the satellite pulse in temporal domain, as shown in Fig. 5 . It is worth noting that the wavelength of the preponderant mode is determined by the mode competition. The experimental results indicate that the space between two central wavelengths of main pulse and satellite pulse are coincident with the separation of adjacent longitudinal modes quite well. The Nd:YAG lasers have a gain bandwidth of ~0.45 nm [26], which is approximately three times larger than the separation of the adjacent longitudinal modes (~0.15 nm) in the laser cavity. As a result, two adjacent modes at most may be generated simultaneously above the threshold of the laser. Moreover, the repetition rate of the passively Q-switched Nd:YAG/Cr4+:YAG microchip laser as a function of pump power is shown in Fig. 6 . We can observe that the repetition rate of this passively Q-switched laser increases almost linearly with the pump power. And the repetition rate is ~3.3 kHz at threshold pump of ~3.34 W and increases to ~8.8 kHz at the maximum incident pump of ~4.36 W.
Fig. 5 Pulse characteristics in temporal (a) and spectral domain (b) under different positionsof pump light spot in the Nd:YAG/Cr4+:YAG crystal.
4. Conclusion
In conclusion, single- and dual-pulse oscillation delivered from a LD pumped passively Q-switched Nd:YAG microchip laser with a Cr4+:YAG SA has been investigated under different pump levels. Single Q-switching pulse with pulse energy of ~32.5µJ and duration of ~715 ps is emitted at repetition rate of ~4.4 kHz. The dual-pulse waveform consists of a main pulse and a satellite pulse with respective intensities and optical spectra. Experimental results indicate that the dual-pulse emission results from double-longitudinal-mode oscillation. The preponderant oscillating mode gives birth to the main pulse, and the other oscillating mode corresponds to the satellite pulse. Moreover, the pulse-pulse separation of dual pulses decreases slowly with the increase of pump power. The experimental result is in good agreement with the theory prediction in a concrete laser cavity. In addition, the repetition rate of the passively Q-switched Nd:YAG/Cr4+:YAG microchip laser increases almost linearly with the pump power.
Acknowledgments
This work was supported by the ‘light of western’ project of the Chinese Academy of Sciences and by the National Natural Science Foundation of China under Grants 61107034. Corresponding author (S. Zhu). Tel.: + 862988887611; fax: + 862988887603; electronic mail: slzhu@opt.ac.cn.
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