Instability and Ultra-high frequency spectrum pulse train in passively Q-switched Yb: LuAG ceramic laser

doi:10.21203/rs.3.rs-3125652/v1

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Instability and Ultra-high frequency spectrum pulse train in passively Q-switched Yb: LuAG ceramic laser

https://doi.org/10.21203/rs.3.rs-3125652/v1

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Passive Q-switched pulse lasers equipped with high quality gain media are expected to be the preferred UHF pulse sources containing THz pulse train. Based on a Yb: LuAG passively Q-switched laser pumped by a laser diode with a Yb ratio of 15at.% in Yb: LuAG, stable quasi-single longitudinal mode laser pulse trains and quasi-double longitudinal mode oscillation unstable pulse trains were observed experimentally, and the ultra-high-frequency (UHF) pulses excited by the high power unstable pulse trains were studied. The spectrum of the pulse train includes the low frequency spectrum with the base frequency below 500 KHz and the ultra-high frequency spectrum with the base frequency above 10⁴ KHz. The base frequency change law of UHF spectrum with the increase of pump power conforms to the frequency doubling frequency principle of spectrum changing to higher frequency. With the increase of pump power, the fundamental frequency increases by 2ⁿ, reaching 0.32 THz in the experimental range of this paper. After the UHF pulse train is filtered at 1.25*10⁵ KHz, a single UHF pulse train and its corresponding spectrum are obtained. The average peak power of UHF spectrum pulse increases with the increase of pump power.

Instability-pulse train

Ultra-high frequency spectrum

passively Q-switched

Yb: LuAG ceramic laser

Passively Q-switched lasers are widely used in dangerous object ignition, target recognition, Radar, differential analysis, nonlinear frequency conversion, terahertz generation and so on [1–6]. Laser gain material is one of the important components in the design of passive Q-switched laser. Using high quality laser gain material is an effective way to obtain high power laser beam. Among many laser materials, Yb3 + doped laser gain materials are favored, and high quality Yb3 + doped laser gain materials are often synthesized by Czochralski (CZ) method. The optical emission cross section area of Yb3 + doped laser gain material is similar to the base transverse mode size of passive Q-switched laser, which can avoid the mode interference of complex transverse mode at high power, and has a long excitation photon lifetime (951µs), which is more suitable for pulsed high energy and peak operation [7, 8]. The energy level radiation transition of Yb3 + ion is mainly between the same electronic level 2F7/2 and 2F5/2 [7, 9], and the cross relaxation is almost negligible [10–12], which can be used as a gain material for laser design with dense particles and high photon efficiency. Yb: YAG is the more studied Yb3 + ion doped gain laser material. Yb: LuAG laser gain material research is early, but in recent years has attracted wide attention. The molar mass of Lu (175 g/mol) is not much different from that of Yb (173 g/mol), and Yb: LuAG laser gain medium is more conducive to temperature management [13, 14]. Yb: LuAG laser gain materials are two kinds of mainly single crystal and crystalline ceramics. Yb: LuAG single crystal laser gain materials are subject to many limitations, such as: due to the limitation of crystal structure, production is difficult, and the increase of Yb content will reduce the optical quantum efficiency [9]. Yb: LuAG ceramics are simpler to synthesize than single crystal materials, and the size is not strictly controlled. In terms of optical quantum efficiency, Yb: LuAG ceramics with highly doped Yb3 + have higher optical quantum efficiency and are not easily saturated with light quantum [15]. Laser diode-pumped passively Q-switched lasers have various designs and extensive research, and can output high peak power pulse trains at high repetition frequency. Another important research direction of passive Q-switched lasers is peak unstable multi-pulse. A passively Q-switched laser with Nd-doped laser gain material can obtain a passive Q-switched multi-pulse train by placing dispersive elements such as plasma and diffraction grating in the laser cavity [16–19]. Effectively debug the laser into multi-mode oscillation, forming peak instability passively Q-switched multi-pulse trains that dispersive elements can not be placed in the laser cavity [20, 21]. The overall structure of LuAG ceramics includes single center, multi-center, rare ions (mainly solid solutions), and partially disordered crystal field garnets [22]. The laser gain material synthesized by Yb particles embedded in the LuAG substrate retains the LuAG polycenter structure, which is reflected in the absorption spectrum showing multiple absorption peaks [15, 23]. However, in order to observe the multi-longitudinal mode caused by the material structure in the Q-switched laser, the size design of the Q-switched laser resonator is strict. The multi-longitudinal mode Q-switched lasers reported in recent years are mostly caused by various effects in the laser field. Laser mode coupling, laser hole burning and Stark energy level splitting effect of laser material in strong light field [24] are the main effects of laser multi-mode oscillation formation. Passively Q-switched Yb: LuAG laser microchips as the gain material, where the stable pulse train is formed by coupling the main mode with its side mode [25, 26]. The Cr, Yb: YAG laser gain material forms a self-Q-switched laser in a plano-concave resonator, and the nonlinear coupling between the main mode and side mode oscillations forms a stable pulse train [21]. In chip Cr4+, Nd3+ : YAG gain material self-Q-switched laser, stable single-longitudinal mode laser pulse was observed experimentally, and unstable pulse train caused by double-longitudinal mode oscillation of space hole burning effect [27]. The transverse multi-mode competition and coupling of Cr and Nd: YAG self-Q-switched lasers can cause periodic pulsation of peak power unstable pulse train [28, 29].

The unique and superior performance of ultra-high frequency (UHF) pulse (including terahertz pulse) has become a research hotspot. Unstable pulse laser is the preferred laser source for high power THz pulse generation. The ultra-high frequency (UHF) pulse is caused by the high power pulse exciting the medium into plasma in the laser cavity under the condition of resonance wave energy and phase asymmetry. In this paper, Yb: LuAG ceramic material is both a gain material and a medium for high power excitation of high frequency pulse. which are not described here. Researchers have different views on the formation of ultra-high frequency pulses (particle flow), and there are many views [30, 31], which are not listed here. Our experimental group thinks that the electric vector harmonic asymmetric excitation of laser UHF pulse is more suitable for the experimental phenomenon in this paper. Because of the phase delay and the asymmetry of the electric vector of high power laser harmonic wave, the corresponding vector of the inverse symmetry of the electric vector of the same frequency (or frequency doubling) is not zero. In this case, the electric vector of the high-power laser will excite photocurrent to generate UHF electromagnetic wave [31], and UHF pulses will be generated in pulsed lasers with saturable absorption media. This UHF pulse contains the THz band. Generally, two-beam laser interference produces THz waves, and the incomplete symmetry of the electric vector of the strong laser comes from the phase and energy differences between the two laser beams [32–34]. It is difficult to debug dual light speed interference. There are also incomplete phase and energy symmetries between fundamental and high order oscillations in the same laser [31]. Under the same conditions, the energy transmission and harmonic phase wave symmetry of the unstable pulse are worse than that of the stable pulse sequence, and the power of the excited UHF pulse is relatively higher. The UHF pulses and THz waves generated in the gain medium by the incomplete symmetry of laser interference light vectors are the same generation principle. THz waves are generated mostly by femtosecond lasers, which are expensive then limit the application range of THz waves. The THz waves with varying frequency spectrum can be detected in passively Q-switched lasers by selecting excellent laser medium. In our experiments, a UHF pulse train varying with 2n fundamental frequency (n is a natural number) was observed in the Yb: LuAG laser gain ceramic passively Q-switched laser, and the power reached 0.32 THz (THz order: 0.1–10THz). In the experiment, Yb: LuAG ceramic material is the gain material of passively Q-switched laser also the medium excited by high power laser pulse to generate UHF spectrum. Our experimental group selected a passive Q-switched pulse laser designed with Yd laser gain medium, and the UHF produced by it does not have the rule that the fundamental frequency changes 2n with the increase of pump power. The "FFT" display spectrum of oscilloscope can make the spectrum reflect the frequency oscillation of the pulse train in real time. The study of UHF pulse of Yb: LuAG laser gain ceramic passive Q-switched laser has an incentive effect on simplifying the design and universalizing the use of THz pulse laser.

The structure of the article is as follows:

The incomplete symmetry of laser energy transmission and the quasi-double longitudinal mode oscillation of Yb binary degenerate energy level splitting are the basis of the formation of unstable pulse trains. The characteristic range of the unstable pulse in the experiment is the same order of magnitude as that of the stable pulse. In order to illustrate this range, the basic characteristic test of the unstable pulse train is arranged under the same experimental conditions. The ultrahigh frequency pulse is generated by the asymmetric excitation of the medium into plasma in the optical path of the coherent laser vector. Compared with stable pulse, the energy of UHF pulse generated by unstable pulse resonance frequency is higher under the same conditions. In this paper, the unstable pulse and the corresponding UHF pulse variation are described in detail. Then the change of UHF pulse frequency and single harmonic pulse train and spectrum are introduced.

The first half of Fig. 1 shows the schematic diagram of the Yb: LuAG(Yb: Lu3Al5O12) ceramic gain material laser of continuous light in this experiment. The continuous light output cavity is shaped in the shape of "V" (the red light part in the figure), and the cavity length is 565 mm. Yb in the experiment: LuAG ceramics, Yb proportion is 15 at.%, there are strong absorption peaks near 940 nm and 970 nm [15]. The wavelength is 970 nm ± 2 nm fiber coupled diode laser LD as the pump source, the laser output continuous light, and the beam diameter is 104 µm, numerical aperture 0.22. Yb: LuAG ceramic gain material is 4 mmx4 mmx3 mm, wrapped with indium foil around, fixed on the surface of the copper block through the water cooling. The water cooling temperature is maintained at 17 ° C, the temperature difference between the ambient temperature and the gain medium cannot exceed 5 ° C, and the laboratory temperature is controlled at 20 ° C by air conditioning. The LD pump light is focused onto the Yb: LuAG laser gain material by a 1:1 focusing coupling system. A dichroic mirror S is placed between the focusing system and the Yb: LuAG ceramic gain material. The dichroic mirror S near the pump source is coated with a high transmittable film of 800–1000 nm near-infrared light, and the other side is coated with a high reflective film of 1020–1200 nm infrared light. A1 is a concave mirror with a curvature radius of 300 mm, and its reflection is highly reflective of infrared light at 900–1100 nm. The transmittance of the coupled output mirror OC at 1000–1090 nm infrared band is 5%. The laser output of the coupling mirror passes through the 5:95 spectroscope, 5% of the laser beam is fed into the spectral analyzer (spectrometer model Ando AQ6317C, ranges: 600–1750 nm), and 95% of the laser beam is connected to the optical power meter (Thorlabs-PM100D, Thorlabs-CAL). The infrared sensor Card (THORLABS Detector Card VRC2 400–640 nm 800–1700 nm) assisted the observation.

Figure 1 Schematic diagram of Yb: LuAG passive Q-switched laser experimental device. Based on the schematic diagram of Yb: LuAG continuous laser, it is composed of a concave mirror A2 and a saturable absorption mirror SESAM. The laser resonator is "M" shape and the cavity length is 980 mm. The saturable absorber SESAM (BATOP, Gmbh) helps to form passive Q-switched pulses with a modulation depth of 0.7%, saturation of 130µJ/cm2 and relaxation time of 1 ps. The concave mirror A2 is highly reflective at 900–1100 nm near infrared wavelengths with a radius of curvature of 200 mm. Oscilloscope (Agilent Technologies DS09254A) is used to display waveform and collect data. The Tektronix MDO4024C Mixed Domain Oscilloscope displays pulse changes on multiple screens. The oscilloscope has a fast Fourier transform (FFT) spectrum display. The newly added beam D3 is linked to Oscilloscope (Agilent Technologies DS09254A or Tektronix MDO4024C Mixed Domain Oscilloscope). The pulse frequency spectrum display is obtained by pressing the oscilloscope built-in function key "fft" when the pulse is displayed.

Figure 1 Experimental schematic diagram. Yb: LuAG Continuous Laser (part of red light) and Passively Q-switched Yb: LuAG/SESAM laser (the whole set). S: Dichroic mirror, A1: Concave with 300 mm radius of curvature, OC: 95% output coupler, D: Beam splitter mirror of 5% reflectance and 95% transmittance, D1: Connecting to an optical power meter, D2: connecting to the optical spectrum analyzer, D3: Connecting to the oscilloscope, A2: Concave with 200 mm radius of curvature.

3.1 Output power and pumping absorbed power

The design of Yb: LuAG gain material continuous laser is shown in Fig. 1. Continuous laser output is observed when the pump absorption power is greater than Yb: LuAG gain medium laser threshold value 1.65 W. The average output power is plotted as a function of the pump absorbed power as shown in Fig. 2. It can be seen from Fig. 2 that the pump absorbed power is between 1.65 W and 8.0 W, and the increase of the laser output continuous laser power is linear with the increase of the pump absorbed power, and the central wavelength is near 1046 nm.

The peak-unstable multipulse trains observed in this experiment oscillate in fundamental transverse mode, which is consistent with the longitudinal mode oscillation of Yb3 + particle gain materials reported previously [26]. High order transverse mode is observed when the output power of Yb: LuAG CW laser is less than 50 mW, and no high order transverse mode is observed when the output power is greater than 50 mW.

3.2 Quasi-longitudinal mode and quasi double mode

The mutual conversion of Yb: LuAG ceramic laser near the central wavelength of 1046nm continuous laser output of quasi-single longitudinal mode (multiple longitudinal modes are tightly formed into a group, the distance between adjacent longitudinal modes is less than 1 nm) and quasi-double longitudinal mode (multiple longitudinal modes are tightly formed into two groups with a center distance of a few nanometers) is shown in Fig. 3 (1). In the process of passively Q-switched Yb: LuAG ceramic laser, one case is quasi-single longitudinal mode oscillation as shown in Fig. 3 (2). The other case is Q-switched quasi-double longitudinal mode oscillation. Figure 3(3) (a) shows the quasi-double longitudinal mode of the Yb: LuAG ceramic laser in the continuous optical output state. When the output power is 160 mW, the quasi-double longitudinal mode spacing is 2.42 nm. Figure 3(3) (b) shows the quasi-double longitudinal mode under the Q-adjusted state of Yb: LuAG ceramic. When the output power is 190 mW, the quasi-double longitudinal mode spacing is 2.80nm.

The conversion phenomenon shown in Fig. 3 (1) will occur in the passively Q-switched longitudinal mode, but it is different from the whole quasi-single longitudinal mode in the quasi-single longitudinal mode, which is reflected in the unstable multi-pulse train when the quasi-single longitudinal mode shown in Fig. 3 (1) occurs in the pulse coupling process, and the output pulse is a stable pulse train when the quasi-single longitudinal mode shown in Fig. 3 (2) occurs. According to the dissipative structure energy symmetry theory, the quasi-single longitudinal mode in the laser debugging process is half of the two degeneracy level is suppressed, the energy of the whole laser is missing, and the output is unstable pulse train. While the quasi-single longitudinal mode oscillation of two degeneracies in the laser energy transmission is symmetrical, which basically includes the excitation particles of the whole laser, the energy transmission is stable and symmetrical, and the output is stable pulse train. When the pumping power of the laser increases, the energy level will split [35]. However, the stable pulse is still stable, and the repetition rate increases to reach the pulse splitting. The unstable pulse is still unstable, and the increase of repetition rate is mainly achieved by the continuous occurrence of low power pulse. This is what we observed in our experiments.

3.3 Unstable pulse train of high power

Yb: LuAG Q-switched laser unstable pulse oscillation formation threshold absorption power is 1.80W. The average repetition frequency increases approximately linearly with the increase of laser pump absorption power (63 KHz-161 KHz), showing as Fig. 4. In the absorption power range of 1.80W-8.39W, the average peak power of Yb: LuAG Q-switched laser pulse increases approximately linearly with the increase of pump absorption power (1.5mW-660.3mW). The Yb: LuAG passively Q-switched laser in a light-to-light conversion efficiency is 7.80% and the slope efficiency 10.03%. In the absorption power range of 1.80W to 8.39W, the average energy of the pulse increases approximately linearly with the increase in the absorption power of the laser pump (0.28µJ-7.68µJ). In order to protect the material and film, the pumping power of the laser was not increased in this experiment.

The Yb: LuAG passively Q-switched laser has no saturation phenomenon in the range of pump power in this experiment. The average output power of unstable laser Yb: LuAG, the width of single pulse, the average peak power of pulse and the pulse repetition frequency with frequency less than 500 KHz are all in the same order of magnitude as the stable pulse under the same conditions (due to discussion the unstable pulse train is the main task in the paper, the stable pulse train is discussed in other text). In addition, if there is only one mode left when the laser oscillates in the quasi-double longitudinal mode state, it is an oscillation with nearly half of the energy transmission missing, and the pulse is also an unstable pulse. The spectrum of unstable pulse is analyzed and its excited UHF pulse is analyzed.

3.4 Low frequency and ultra high frequency pulse train

In order to compare the spectrum of stable pulse and unstable pulse, the low frequency spectrum of stable pulse train is obtained by Fourier transform in Tektronix MDO4024C Mixed Domain Oscilloscope, which is shown in Fig. 5 (1) when the absorbed power is 2.5W(output power: 260 mW). The high frequency part is not displayed due to the screen limitation. The first three frequency lines of the spectrum are calculated to be 56.82KHz, 113.61KHz and 170.45KHz respectively, and the relation is harmonic frequency (Note: the pulse train in Fig. 5(1) is a composite screen display, and the spectrum is consistent with the most obvious pulse train).

Both the low frequency (less than 500 KHz) and the ultra-high frequency (UHF) (more than 10⁴ KHz) of the peak unstable pulse train are listed in Fig. 5 (2). The low frequency energy of the pulse train is much higher than the UHF energy. Figure 5(2)(blf) and 5(2)(dlf) are the low frequency spectra corresponding to the pulse sequences 5(2)(a)(output power 260 mW) and 5(2)(c)(output power 280 mW). The spectrum in Fig. 5(2)(blf) has at least two groups of spectrum with independent harmonic frequency relationship, one of which has slightly higher energy, and the values of the first three frequencies of its spectrum are 28.13 KHz, 56.15 KHz, and 84.28 KHz respectively. The pulse train in Fig. 5(2)(a) is assumed to be a single period (stable pulse train, as shown in Fig. 5(1)), and the estimated frequency is 56.2 KHz, with a difference of 0.74 KHz from 56.82 KHz and a difference of 0.05 KHz from 56.15 KHz. The low frequency spectrum of the unstable pulse train is of the same order of magnitude as that of the stable pulse train at the same output power. In Fig. 5(2)(blf), the corresponding frequency values of the other group of spectrum have little difference with the above frequency values, and the energy difference is small. The first three frequency values of the spectrum are respectively 27.42 KHz, 54.72 KHz and 82.25 KHz. The difference of subsequent frequency line values gradually increased, then the two spectral peaks gradually separated, and the two groups of spectral peaks could be clearly seen. The fundamental frequency values of the two groups of spectrum differ by 0.71 KHz. In Fig. 5(2)(a), the pulse train is relatively stable. The frequency spectrum and energy difference of the two pulse trains formed by quasi-double longitudinal mode are relatively small (0.71 KHz), and the total pulse train is relatively stable. Figure 5(2)(dlf) is the low-frequency portion of the pulse train spectrum in Fig. 5(2)(c). One group of frequency spectrum that can be measured the frequency value, the first three frequency values are: 60.08 KHz, 121.1 KHz, 181.2 KHz. The pulse train in Fig. 5(2)(c) is assumed to be a single period, and the estimated frequency value is 61.4 KHz, which differs 0.06 KHz from the fundamental frequency of Fig. 5(2)(dlf) of 60.08 KHz. Figure 5(2)(c) The low frequency part of the pulse train can only measure a set of spectral values. It shows that the other longitudinal modes in the quasi-double longitudinal mode have low energy and no obvious spectral peak. The spectrum of Fig. 5(2)(dlf) has many more spectrum lines behind the spectrum of stable pulse train 5(1), and the whole pulse train is unstable.

The low frequency part of the unstable pulse train is discussed above, and the corresponding ultra-high frequency part is discussed below. According to the harmonic symmetry breaking principle, the ultra-high frequency pulse is formed by the harmonic high-power pulse exciting the medium into plasma in the optical path [34]. The high-power pulse is gone, and the UHF pulse is gone.

Figure 5(2)(a) UHF part of Fourier spectrum transformation of the pulse train as shown in 5(2)(b). The two sets of ultra-high frequency spectrum values are 1.25*10⁵ KHz, 2.5*10⁵ KHz, 3.75*10⁵ KHz and 7.773*10⁴ KHz, 1.552*10⁵ KHz, 2.333*10⁵ KHz, respectively. Figure 5(2) (d) is the UHF spectrum diagram of the 5(2) (c) pulse train, and the small figure is the low frequency spectrum. The two sets of ultra-high frequency spectrum are: 2.5*10⁵ KHz, 3.779*10⁵ KHz and 1.555*10⁵ KHz, 3.104*10⁵ KHz, 4.667*10⁵ KHz. Compare 5(2)(b), 5(2)(d), and the spectrum of peak instability pulse train is increasing with the increase of output power. The spectrum of 2.5*10⁵ KHz and 3.779*10⁵ KHz at 280 mW corresponds to the spectrum of 1.25*10⁵ KHz, 2.5*10⁵ KHz and 3.75*10⁵ KHz at 260 mW. The spectral line does not have a distinct 1.25*10⁵ KHz frequency line, the latter two spectral lines exist, and the spectrum is in the process of conversion to higher frequency spectral spectrum. At 280 mW, the spectral line energy of 1.555*10⁵ KHz, 3.104*10⁵ KHz and 4.667*10⁵ KHz is significantly enhanced than that of 260 mW output power. The fundamental frequency of the spectrum changes from the fundamental frequency 7.773*10⁴ KHz at 260 mW to the fundamental frequency 1.555*10⁵ KHz at 280 mW. This group harmonic frequency is success conversion to the higher harmonic frequency spectrum, which is consistent with the frequency doubling principle generated by higher spectrum [5]. In the process of frequency spectrum conversion to higher spectrum with the increase of pump absorption power, the fundamental spectrum energy of the spectrum decreases, and the energy of the second spectral line increases obviously, and the second spectral line will become the next fundamental frequency. For easier of viewing, the first three spectral lines of the above spectrum are listed in Table 1. There are clustered pulse lines next to the 4.667*10⁵ KHz spectral line, which is the new UHF pulse frequency line that appears later. There is also a later 1.0*10⁵ KHz based spectral line in the spectrum, which should be the frequency oscillation of another pulse train in the laser crystal excited in the plasma.

Table 1

Frequency spectrum varying
Output power (mW)	Low frequency (KHz)			Ultra high frequency (KHz)
260	28.13	56.15	84.28	1.25*10⁵	2.5*10⁵	3.75*10⁵
260	27.42	54.72	82.25	7.773*10⁴	1.552*10⁵	2.333*10⁵
280	60.08	121.1	181.2		2.5*10⁵	3.779*10⁵
280				1.555*10⁵	3.104*10⁵	4.667*10⁵

The UHF in Figure (6) (1) is the graph of the spectrum increasing with the pump power recorded by the oscilloscope by clicking "fft" on the oscilloscope in the pulse display state. With the increase of pump power, the base frequency of UHF pulse spectrum increases by 2n. We trace a set of harmonic frequency spectrum, and the experimental measurement shows that the fundamental frequency of 7.8W is 3.20*1011Hz. With the increase of pump power, the average power of unstable pulse train increases, and the group of frequency spectrum of UHF pulse also increases, as shown in Fig. 5(2)(b),(d). The average peak value of UHF pulse train excited by unstable high power pulse train of passive Q-switched lasers increases with the increase of pump power. High pumping power is easy to damage the material, and the pump power was not continued to increase in the experiment. In UHF pulse train obtaining the log₂(frequency) as shown in Figure (6) (1), it has a linear relationship with the pump power of the laser. After high-pass filtering and Fourier transform of the pulse train in Fig. 5(2)(a), the UHF pulse train and corresponding spectrum as shown in Fig. 6 (2) are obtained, and the base frequency of the spectrum is 1.25*10⁵ KHz. It is calculated in the experiment that the peak pulse in Fig. 5(2)(a) is 9.04 times of the average value of the peak pulse in Fig. 6(2)(a), and the low frequency pulse in 5(2)(c) is 4.2 times of the UHF peak pulse train, but the frequency spectral line after filtering is more complex than that in Fig. 6(2)(b). With the increase of laser pumping power, the average peak power of UHF pulse increases. There are also ultra-high frequency lines whose frequencies are not harmonized with each other, and it is difficult to obtain a single harmonized frequency spectrum by filtering.

A tuned high-power pulse train can produce a tuned ultra-high-power pulse train. Based on the theory of asymmetric energy transmission of unstable multi-pulse resonant spectrum excited high energy UHF pulses, the UHF pulse generation, pulse train and spectrum variation of unstable pulse lasing are experimentally studied. Unstable pulse trains induced by asymmetry of energy transmission in laser dissipative systems (UHF pulses are also excited by the harmonic electric vector asymmetry of stable laser pulses, but the energy is slightly lower) can be excited by higher energy UHF pulses. High power pulses (frequencies less than 500 KHz) disappear, and the corresponding UHF pulses (frequencies above 10⁴ KHz) disappear. In this experiment, the UHF pulse base frequency increases exponentially by 2ⁿ (n is a natural number) with the increase of pumping power, and UHF pulse train is generated by the passively Q-switched Yb: LuAG ceramic gain medium laser with a Yb ratio of 15at.% near the wavelength of 1046 nm. This variation is not consistent with all laser gain materials. In the experimental range of this paper, the fundamental frequency of UHF pulse reaches 0.32 THz. After high-pass filtering, a single ultra-high frequency pulse train and spectrum with a spectral base frequency of 1.25*10⁵ KHz are obtained, and a single spectral time train of ultra-high frequency pulses is clearly displayed. With the increase of laser pumping power, the average peak power of UHF pulse increases. There are also ultra-high frequency lines whose frequencies are not harmonized with each other, and it is difficult to obtain a single harmonized frequency spectrum by filtering.

Data availability Data underlying the results presented in this paper are not publicly available before published.

Conflict of interest： The authors declare no competing interests.

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Instability and Ultra-high frequency spectrum pulse train in passively Q-switched Yb: LuAG ceramic laser

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Abstract

Figures

1 Introduction

2 Experiment

3 Results and discussion

3.1 Output power and pumping absorbed power

3.2 Quasi-longitudinal mode and quasi double mode

3.3 Unstable pulse train of high power

3.4 Low frequency and ultra high frequency pulse train

4 Conclusion

Declarations

References

Additional Declarations

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