Transversal parasitic oscillation suppression in high gain pulsed Fe2+:ZnSe laser at room temperature
Introduction
Fe2+:ZnSe laser operating over 4–5 μm is promising for mid-IR tunable solid-state lasers, which development is of considerable interest for great potential in applications of industry, medical and defense [1], [2], [3], [4], [5]. Many significant results have been presented since an optically pumped Fe2+:ZnSe laser was reported by Adams [6]. In 2015, Martyshkin reported a repetition rate Fe2+:ZnSe laser with maximum average power of 35 W at repetition rate of 100 Hz under 77 K [7]. In 2016, Kozlovsky reported a Fe2+:ZnSe laser with highest output energy of 10.6 J at single-shot operation, which was pumped by a free-running Er:YAG laser with pulse duration of hundreds of microseconds [8]. In both cases, Fe2+:ZnSe crystal was cooled by liquid nitrogen (LN). In 2018, Frolov presented a thermoelectrically cooled Fe2+:ZnSe laser tunable over 3.75–4.82 μm, and the output energy reached 7.5 J in single-shot operation at 220 K when pumped by a 2.94 μm Er:YAG laser [9]. The lifetime of the upper laser level of Fe2+ decreased with temperature from τ = 60 μs at 80 K to 0.37 μs at room temperature [10], so it is necessary to cool Fe2+:ZnSe crystal to low temperatures or use short pulsed pump sources with pulse duration consistent with the lifetime of the upper level. In 2017, a room temperature repetitively pulsed Fe2+: ZnSe laser with average output power up to 20 W at 20 Hz repetition rate pumped by HF laser was reported by Velikanov [11]. This laser used a big Fe2+:ZnSe crystal with diameter D = 64 mm, resulting a longer dissipating heat time for each pulse. Because of the significant influence of crystal overheating on laser characteristics, no higher repetition rate Fe2+: ZnSe laser working at room temperature has been presented.
Above results show that Fe2+:ZnSe laser is a high gain laser, which has great potential in high energy output, especially pumped by short pulsed lasers. Just as many other high gain solid lasers [12], [13], Transversal Parasitic Oscillation (TPO) suppression is one of the most important scientific problems. One way to suppress TPO is decreasing the pump spot diameter (d) under a fixed Fe2+:ZnSe crystal diameter(D) [9], [11], [14]. Here, we define ξ = d2/D2 as the effective utilization rate of Fe2+:ZnSe crystal. And the value of ξ in reference [9], [11] was 10.24% and 5.47%, respectively. To increase ξ without the development of transverse parasitic oscillation, a new Fe2+:ZnSe active element with an inner doped layer in the form of a meniscus was manufactured by Balabanov [15], in which ξ was up to 25%. In order to obtain high output energy and avoid crystal damage, the crystal diameter D should be increased, meaning a lower ξ. However, lower ξ has many negative consequences. For example, need larger Fe2+:ZnSe element and higher requirement for heat dissipation, which are problems facing by high energy Fe2+:ZnSe laser, especially repetitively pulsed laser at room temperature.
A Fe2+:ZnSe laser experimental setup end-pumped by a non-chain pulsed HF laser was established in our laboratory at room temperature. Fe2+:ZnSe crystal was grown by Vertical Bridgman method from melt, which Fe dopant was added to the raw materials before growth process. Homogeneous doping by Fe dopant was achieved straightforwardly during the crystal growth. The purpose of this paper was to analyze the causes of TPO and find an effective way to suppress TPO effect, which is helpful for improving the performance of Fe2+:ZnSe laser at room temperature.
Section snippets
TPO suppression
TPO will deplete inverted population in high-gain Fe2+:ZnSe laser, which goes against the performance improvement of such laser. The main reason of TPO is that the laser parasitic gain Gp in the direction perpendicular to the optical axis is larger than the loss of radiation Γp in this direction. The threshold condition can be written as [14]:where, Gp、Γp can be given by the following equations:
Here, D is diameter of the active element, d-diameter of pump
Experimental equipment
The experimental equipment of the Fe2+:ZnSe laser has been established in our laboratory. Fig. 2(a) and Fig. 2(b) are optical scheme and experimental layout, respectively.
A non-chain pulsed HF laser similar to that reported in [17] was utilized as the pump source. The maximum output energy of the pump laser was greater than 4 J with spot size of 48 mm and full width at half maximum (FWHM) of 100 ns. The FWHM well matches the upper level lifetime of Fe2+:ZnSe laser at room temperature. Fe2+:ZnSe
Experimental results
The present work aims at exploring a method to efficiently suppress TPO so as to improve the output performance of the Fe2+:ZnSe laser. The output energy of Fe2+:ZnSe laser could be scaled up by increasing the pump energy. Limited by low damage threshold of Fe2+:ZnSe crystal, the pump energy density can’t be increased freely. Increase the diameter of pump spot d at safe pump energy density can improve the output energy, but TPO problem also occurs under large pump spot. So, it is significant to
Conclusion
We have successfully suppressed TPO in a high gain pulsed Fe2+:ZnSe laser at room temperature by reducing pump spot size and coating graphite around the active element. Usually, it is costly and difficult to obtain a big size active element. While coating graphite could increase the effective utilization rate of the active element without increasing the size of the element, which is a good way to improve the output performance of the Fe2+:ZnSe laser. In our experiments, ξ increased from 25% to
CRediT authorship contribution statement
Qikun Pan: Conceptualization, Methodology, Writing - original draft. Jijiang Xie: Supervision, Investigation. Fei Chen: Supervision, Data curation, Writing - review & editing. Kuo Zhang: Visualization. Deyang Yu: Data curation. Yang He: Formal analysis. Junjie Sun: Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This study was supported by National Key R & D Program of China (2016YFE0120200), Natural National Science Foundation of China (NSFC) (61705219), Jilin Province Science and Technology Development Plan Project (20190103133JH), Youth Innovation Promotion Association of CAS (No.2017259).
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