An LED-Side-Pumped Intracavity Frequency-Doubled Nd,Ce:YAG Laser Producing a 2\,W Q-Switched Red Beam

Jianping Shen(沈建平)$^{1\hyperlink{s}{*}}$, Shaocong Xu(徐少聪)$^{1\hyperlink{s}{*}}$, Peng LU(芦鹏)$^{1}$, Rongrong Jiang(江容容)$^{1}$,\\ Wei Wang(王巍)$^{2}$, Siwei Zhang(张四维)$^{2}$, Fengyang Xing(邢凤阳)$^{2}$,\\ Yang Chen(陈阳)$^{1}$, and Liang Chen(陈亮)$^{1}$

doi:10.1088/0256-307X/41/3/034201

⇦Chinese Physics Letters, 2024, Vol. 41, No. 3, Article code 034201⇨ An LED-Side-Pumped Intracavity Frequency-Doubled Nd,Ce:YAG Laser Producing a 2 W Q-Switched Red Beam Jianping Shen (沈建平)1*, Shaocong Xu (徐少聪)1*, Peng LU (芦鹏)1, Rongrong Jiang (江容容)1, Wei Wang (王巍)2, Siwei Zhang (张四维)2, Fengyang Xing (邢凤阳)2, Yang Chen (陈阳)1, and Liang Chen (陈亮)1 Affiliations 1College of Electronic and Optical Engineering & College of Flexible Electronics, Nanjing University of Posts and Telecommunications, Nanjing 210046, China 2Laser Institute, Qilu University of Technology (Shandong Academy of Sciences), Qingdao 266000, China Received 29 December 2023; accepted manuscript online 26 February 2024; published online 25 March 2024 ^*Corresponding authors. Email: jianpingshen@njupt.edu.cn; xv8483055@163.com Citation Text: Shen J P, Xu S C, Lu P et al. 2024 Chin. Phys. Lett. 41 034201 Abstract We report a high-average-power acousto-optic (AO) Q-switched intracavity frequency-doubled red laser based on a high-efficiency light-emitting-diode (LED) pumped two-rod Nd,Ce:YAG laser module. Under quasi-continuous wave operation conditions, a maximum output power of 1319.08 nm wavelength was achieved at 11.26 W at a repetition rate of 100 Hz, corresponding to a maximum optical efficiency of 13.9% and a slope efficiency of 17.9%. In the active Q-switched regime, the pulse energy of the laser was as high as 800 µJ at a repetition rate of 10 kHz with a pulse width of 1.5 µs. Under non-critical phase-matched KTP crystal conditions, an average power of 2.03 W of 658.66 nm through intracavity frequency-doubling was obtained at a repetition frequency of 10 kHz with a duration of 1.3 µs, and the $M^{2}$ factor was measured to be about 5.8. To the best of our knowledge, this is the highest average power of an LED-pumped AO Q-switched 1319 nm laser and intracavity frequency-doubled red laser reported to date.

DOI:10.1088/0256-307X/41/3/034201 © 2024 Chinese Physics Society Article Text The 1.3 µm laser is currently valuable in research as a pump source in oscillator power amplifiers and fibre Raman amplifiers, and was also utilized to obtain high-power red laser outputs by frequency doubling.[1-5] In recent years, in virtue of the low-cost and miniaturization of light-emitting diodes (LEDs), and the significant enhancement of power, the medium bandwidth pumping generated by LEDs was expected to be an option for cost-effective lasers. We envisaged a scheme for a novel LED-pumped all-solid-state laser to generate 1.3 µm lasers. Nevertheless, the majority of technological solutions have concentrated on all-solid-state LED-pumped 1.0 µm lasers with Nd$^{3+}$ ion-doped crystals so far.[6-9] Until now, no reports on LED-pumped 1.3 µm high-power all-solid-state lasers have been published, meaning that the red laser as a result of frequency doubling could not achieve laser output. This was attributed to the fact that the Nd$^{3+}$ emission cross section at 1.3 µm was approximately 1/5 of the 1.0 µm emission cross section, which demanded a substantially higher pump photon density and made it difficult to collect all the luminous flux emitted by the LED Lambertian emission. The results of the power density in the crystal was low, and the overlap between TEM$_{00}$ and LED pump modes was deviated.[10-15] Thus, to capture more emission from the LED to enhance the coupling efficiency and enable higher mode matching, it was necessary to modify the LED emission mechanism, optimize the pump structure and increase the area fill factor to achieve a higher-efficiency LED pump laser. In this work, we analyze the emission mechanism of LEDs and address the coupling efficiency of LED-pumped lasers using non-diffuse scattering LEDs to rationalize the five-way pumping structure of LEDs.[16-18] To further enhance the intracavity photon number density, we adopt a high-efficiency LED-pumped two-rod Nd,Ce:YAG laser-cavity structure. Under 80 W pump energy, a maximum average power of 11 W at 1319 nm is achieved at a repetition rate of 100 Hz in quasi-continuous wave (QCW) operation. Afterwards, a red laser output with a pulse width of 1.2 µs and a maximum average power of 2 W is obtained by intracavity doubling at a repetition frequency of 10 kHz with acousto-optic (AO) Q-switching. In addition, a red laser with a duration of 292 ns was realized at a repetition frequency of 500 Hz, corresponding to a maximum peak power of 7 kW. Optimization of the LED Pump Structure. To achieve a highly efficient LED pumping system and to enhance the mode overlap between LED pump mode and laser cavity oscillation mode, the LED model product and pumping structure are needed to be optimized for selection and design. Firstly, the LED (Luminus-SST-10-IRD-B90H-810nm) was selected via multi-model LED comparison experiments and luminescence characteristics research experiments. Figure 1 illustrates the radiation per unit polarity at different angles as a proportion of the total radiation. Additionally, the inset of Fig. 1 shows the LED Lambertian emission two-dimensional (2D) beam intensity profile by the laser beam analyzer (Cinogy, CMOS-1201). To better understand the experimental results, we performed some theoretical simulations on the LED Lambertian emission. From the perspective of light intensity distribution characteristics, the radiation model of LED Lambertian emission can be regarded as the Gaussian-like beam distribution characteristics. The Gaussian-like equation is simulated by Eq. (1) for the LED luminous occupation ratio: \begin{align} \beta(\theta)=\frac{I(\theta)}{I_{\rm LED}}=y_{0}+A\cdot \exp{\Big[-0.5 \Big(\frac{\theta -\theta_{\rm c}}{W}\Big)^{2}\Big]}, \tag {1} \end{align} where $\theta$ denotes the dispersion angle of the LED; $I (\theta$) and $I_{\rm LED}$ are the radiance per unit of polarity and the total radiance of the LED, respectively; $\beta (\theta)$ represents the percentage of unit radiation at the corresponding $\theta$ angle; $y_0$, $\theta_{\rm c}$, and $W$ indicate the Gaussian coupling constants.

Fig. 1. LED single-lamp luminous Gaussian distribution graph. Inset: cross-film light intensity distribution.

Fig. 2. (a) Beam divergence characteristics of a single lamp, and (b) cross section of the LEDside five-way pump structure. $D$ indicates the distance from the LED light-emitting chip to the core of the crystal.

Secondly, the compatible pump structure was designed on the basis of the simulated LED light-emitting mechanism. Figure 2(a) shows the cross section of light transmission of a single-row LED pump crystal. To enable the light intensity to be uniformly coupled to the crystal, five-row LED arrays are arranged in pentagons to improve the pump coupling efficiency and reduce thermal effects, as shown in Fig. 2(b). With regard to non-axial light intensity distribution and approximate LED pump circumferential symmetry, the transmission efficiency of the LED can be calculated using the equation \begin{align} \eta(\varphi)=\frac{\int_0^{\varphi} \beta (\theta)d\theta}{\int_0^{180^\circ} \beta(\theta)d\theta }, \tag {2} \end{align} where $\varphi$ indicates the angle of use of the LED pump structure. Figure 3 further displays the fitted curves for the transmission efficiency $\eta$. In consideration of the pump power density, the divergence angle optimization design adopted in the LED pump laser system is around 44$^{\circ}$. The corresponding optical transmission efficiency is 43%, which could significantly improve the coupling efficiency.

Fig. 3. Theoretical analysis of the relationship between the angle of LED transmission to the crystal and the transmission efficiency of LED arrays.

Thirdly, the LED pump structure could effectively reduce the temperature gradient in the crystal and result in less crystal damage due to the broad divergence angle of the LED pumping system's suppression of aspheric aberrations and homogenization of the light intensity distribution. The thermal lens focal length could be obtained by measuring the focus point position of the He–Ne laser. The experimentally measured thermal focal length values were greater than 2.5 m, indicating that the thermal effect of the laser system was very insignificant. Moreover, based on the optimized design of high-pump-intensity LED pump modules, LED-pumped 1.3 µm high-power all-solid-state lasers could be realized. Experimental Setup. Figure 4(a) shows the construction of the 1319 nm all-solid-state laser for QCW operation, in which the system consists of two LED-side-pumped Nd,Ce:YAG modules connected in series and the resonant cavity. The side pump module consists of 200 LEDs soldered in 5 rows of stacked arrays in pentagonal symmetry around a crystal bar. To optimize pumping uniformity and to further decrease the thermal effect of the Nd,Ce:YAG crystal, the side-pumping head module consisted of five rows of LED stacked arrays, which were devised in a pentagonal arrangement around the crystal rod. Meanwhile, to maximize the efficiency of the LED pump, the LEDs were operating at a frequency of 100 Hz, with a pulse width of 2 ms, and the temperature was controlled at 5 ℃. Two Nd,Ce:YAG crystal rods (diameter 5 mm, length 120 mm, containing 1.0 at.% Nd$^{3+}$ and 0.05–0.1 at.% Ce$^{3+})$ were tandem-mounted in a stable cooling water circulation structure, while the water temperature was maintained at around 20 ℃ and the crystal was side-sanded to further homogenize the pumping distribution. Afterwards, to suppress the mode competition between the 1338 nm and 1319 nm lasers, M1 was coated with a high-transmission near the 1338 nm wavelength. The QCW 1.3 µm fundamental cavity was formed by a rear mirror M1 ($R>99.9{\%}$ @1319 nm & $T>90{\%}$ @1064 nm & $T>70{\%}$ @1338 nm) and an output plane coupler M2 ($T=1$, 3, 8% @1310–1330 nm). Figure 4(b) depicts the intracavity frequency-doubled red pulsed laser. Firstly, the Q-switched 1.3 µm fundamental cavity consisted of a rear reflector M1, LED-side-pumped dual-rod Nd,Ce:YAG tandem modules, an AO Q-switch, and a dichroic mirror M4. A 1319 nm AO Q-switched module (model: QSG27-13-50W, RF signal frequency: 27 MHz) was utilized to generate a pulsed laser. Secondly, the red pulsed laser resonator consisted of M3, a KTP crystal as the second harmonic generator (SHG), and the laser output mirror M4 (HR@1320 nm&&HT@660 nm). M3 was used as a harmonic separator between the fundamental-frequency light and second-harmonic light with HR-coated at 660 nm and HT-coated at 1320 nm to prevent interference of the doubled-frequency laser for fundamental-frequency modules. The KTP crystal ($5\times 5\times 15$ mm$^{3}$) was cut to match critical type II ($\theta =58.9^{\circ}$, $\varphi=0^{\circ}$) and was HT coated at both 1319 nm and 660 nm to adapt to the fundamental-frequency optical wavelength of 1319 nm. In addition, the KTP crystal was wrapped in indium foil and placed in a water-cooled copper oven that was precisely controlled at 20 ℃ to ensure maximum frequency-doubling efficiency. The overall Q-switched 1.3 µm fundamental cavity length was approximately 470 mm, and the red pulsed laser cavity length was close to 100 mm.

Fig. 4. Schematic diagram of the LED-pumped QCW Nd,Ce:YAG 1319 nm laser (a), and the intracavity SHG red laser (b).

Results and Discussion. Figure 5 depicts the spectrum of the fundamental laser and red laser under maximum output power. An infrared laser peak wavelength of 1319.08 nm corresponding to a full-width-half-maximum (FWHM) of 0.28 nm was obtained at the observation point using a spectrometer (YOKOGAWA-AQ6375). A visible-band spectrometer (Ocean, HR4000) was selected to measure the peak wavelength of the red laser at 658.66 nm, corresponding to an FWHM of 0.95 nm. Figure 6 provides a detailed representation of the output power of the 1319 nm laser under various coupling transmissivity values in the QCW regime. Under the maximum pump power of 80 W, the maximum output power obtained at $T=8$% @1310–1330 nm is 11.36 W, corresponding to an optical conversion efficiency of 13.9% and a slope efficiency of nearly 17.9%. Meanwhile, at $T=1$% @1319 nm, the pumping threshold was only 7.4 W and the 1319 nm laser output power did not reach saturation level at different transmittance. Based on the input–output power characteristics, it is concluded that the two LED-side-pumped Nd,Ce:YAG laser structures could effectively increase the cavity pump power density, and the output power had comparatively huge rise space.

Fig. 5. Spectrum of the fundamental laser and red laser.

Fig. 6. The 1319 nm output power versus LED incident power in the QCW regime. Inset: maximum output power of 1319 nm.

Figure 7 expresses the laser output performance of the fundamental-frequency laser and the red laser Q-switching operating at different frequencies. Firstly, a Q-switched 1.3 µm fundamental laser system was utilized to achieve a maximum average power of 7.8 W at a repetition frequency of 10 kHz. Secondly, as a result of intra-cavity frequency doubling, at the highest pumping power, a pulse width of 1.3 µs and a maximum average power of 2 W red laser were achieved at a repetition frequency of 10 kHz. At a repetition frequency of 500 Hz, the pulse width could be reduced to 292 ns and the corresponding peak power could reach 7 kW. Finally, the average output power of the Q-switched 1319 nm and 660 nm laser was monotonically increased with the pumping power from 27 W to 81 W, while the corresponding red laser pulse width could decrease from 1.3 µs to 292 ns with the pumping power. Moreover, the pulse width and pulse train of the 660 nm laser at the highest pump power was measured by a fast photodiode (Thorlabs. PDA10A2) at repetition frequencies of 500 Hz, as shown in Fig. 8.

Fig. 7. Output power of the Q-switched laser and pulse duration of 660 nm as a function of incident pump power.

Fig. 8. Temporal profile of (a) a single pulse at 660 nm at a repetition rate of 500 Hz, and (b) the 660 nm laser pulse trains at a repetition rate of 500 Hz.

Fig. 9. Quality of the 660 nm beam measured near the beam waist. Top inset: the maximum output power at 660 nm. Bottom right illustration: transverse mode patterns of 660 nm for inner-cavity frequency doubling.

The $M^{2}$ factors of $M_{x}^{2}=6.5$ and $M_{y}^{2}=5.0$ for the red laser near the beam waist are fitted by least-square curves, as shown in Fig. 9. The upper inset of Fig. 9 shows the power meter at the maximum power output of the red laser and the lower right inset shows the far-field laser cross-film mode light distribution taken by the CCD, where the red laser is operating in multi-mode. In addition, the beam quality of the red laser was degraded with the increase in pump power due to the non-uniform temperature control of the nonlinear crystal, and could be further optimized by subsequent use of a mode-selecting technique. In summary, we report a watt-class AO Q-switched intracavity frequency-doubling red laser based on a highly efficient LED-pumped Nd,Ce:YAG laser module with high-average and high-peak-power operation. For QCW operation, the highest average output power of the 1319 nm fundamental laser is 11.3 W at a repetition frequency of 100 Hz, corresponding to a maximum optical conversion efficiency of 12.9% and a slope efficiency of 17.9%. In addition, a maximally averaged red laser output of 2 W is achieved by intracavity frequency doubling at the repetition frequency of 10 kHz, corresponding to the red laser $M^{2}$ factors $M_{x}^{2}=6.5$ and $M_{y}^{2}=5.0$. Moreover, by further compressing the output pulse width, a red laser with a duration of 292 ns is realized at the repetition frequency of 500 Hz, corresponding to a maximum peak power of 7 kW. As far as our capability is concerned, this is the highest average power of an LED-pumped AO Q-switched 1319 nm laser and intracavity frequency-doubled red laser reported to date. Acknowledgments. Nanjing University of Posts and Telecommunications Foundation (Grant Nos. JUH219002 and JUH219007), and Key Laboratory of Functional Crystals and Laser Technology, TIPC, CAS Foundation (Grant No. FCLT 202201). References High-Power All-Fiber Supercontinuum Laser Based on Germania-Doped FiberUltrafast Fiber Laser Based on Tungsten Sulphoselenide MaterialsFemtosecond Fiber Laser Based on BiSbTeSe₂ Quaternary Material Saturable Absorber28W red light output at 659.5nm by intracavity frequency doubling of a Nd:YAG laser using LBOA 658-W VCSEL-pumped rod laser module with 52.6% optical efficiencySPIE ProceedingsDesign and development of a high-power LED-pumped Ce:Nd:YAG laserAnalytical model for thermal lensing and spherical aberration in diode side-pumped Nd:YAG laser rod having Gaussian pump profileFirst demonstration of green and amber LED-pumped Nd:YAG laserLiNdP₄ O₁₂ laser pumped with an Al_x Ga_{1− x} As electroluminescent diodeHigh-power and high optical conversion efficiency diode-end-pumped laser with multi-segmented Nd:YAG/Nd:YVO₄Effect of thermally induced birefringence on high power picosecond azimuthal polarization Nd:YAG laser systemHigh Stability LED-Pumped Nd:YVO4 Laser with a Cr:YAG for Passive Q-SwitchingDesign and Development of a High-Performance LED-Side-Pumped Nd:YAG Rod LaserHigh Average Power Green Laser Based on LED-Side-Pumped Q-Switched Nd:YAG LaserLED-side-pumped Nd:YAG laser with >20% optical efficiency and the demonstration of an efficient passively Q-switched LED-pumped solid-state laserPump pulse characteristics of quasi-continuous-wave diode-side-pumped Nd:YAG laserHigh-sensitive terahertz detection by parametric up-conversion using nanosecond pulsed laser

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