Direct generation of 3.17 mJ green pulses in a cavity-dumped Ho3+-doped fiber laser at 543 nm

Tianran Li; Ziyu Wang; Jinhai Zou; Jinfen Hong; Qiujun Ruan; Hang Wang; Zhipeng Dong; Zhengqian Luo

doi:10.1364/PRJ.474977

Abstract

High-energy pulsed lasers in the green spectral region are of tremendous interest for applications in space laser ranging, underwater detection, precise processing, and scientific research. Semiconductor pulsed lasers currently are difficult to access to the so-called “green gap,” and high-energy green pulsed lasers still heavily rely on the nonlinear frequency conversion of near-IR lasers, precluding compact and low-cost green laser systems. Here, we address this challenge by demonstrating, for the first time to the best of our knowledge, millijoule-level green pulses generated directly from a fiber laser. The green pulsed fiber laser consists of a 450 nm pump laser diode, a

{Ho}^{3 +}

-doped ZBLAN fiber, and a cavity-dumping module based on a visible wavelength acousto-optic modulator. Stable pulse operation in the cavity-dumping regime at 543 nm is observed with a tunable repetition rate in a large range of 100 Hz–3 MHz and a pulse duration of 72–116 ns. The maximum pulse energy of 3.17 mJ at 100 Hz is successfully achieved, which is three orders of magnitude higher than those of the rare-earth-doped fiber green lasers previously reported. This work provides a model for compact, high-efficiency, and high-energy visible fiber pulsed lasers.

1. INTRODUCTION

High-energy, short-pulse visible lasers have important applications in microscopy, medical diagnosis, laser displays, and underwater detection [1 –3]. Visible semiconductor lasers have gained a lot of attention as extremely efficient and compact light sources, but are still under developed in the “green gap” wavelength of 500–550 nm. So far, the main technology for green pulsed lasers is the nonlinear frequency conversion (e.g., frequency doubling [4], sum frequency [5], and parametric process [6]) of near-IR fiber or solid-state lasers [7]. Although this technique has been capable of 1 kW-average-power, 1 J green pulse generation by frequency doubling [8], it usually suffers from system complexity, vibration sensitivity, and high cost. In contrast, the direct generation of green pulses from rare-earth-doped fiber lasers is preferred, because a green pulsed laser source can be compact, user friendly, with low cost, and maintenance free. Therefore, there is always motivation to develop green rare-earth-doped fiber pulsed lasers. Some special application scenarios, such as earth–moon ranging [9], urgently need this kind of high-energy green pulsed fiber laser with a relatively compact structure as the preferred light source.

The rare-earth-doped ZBLAN glass fiber is considered the ideal gain medium to directly generate a high-efficiency visible laser due to its low phonon energy in the absence of multiphoton relaxation [10 –15]. Several kinds of rare-earth ions ( ${Er}^{3 +}$ [16], ${Ho}^{3 +}$ [17 –20], $\Pr^{3 +}$ [21 –23], and ${Tb}^{3 +}$ [24]) in fluoride fibers can be used for green emission. In recent years, with the rapid development of high-power blue GaN laser diodes (LDs), the frequency down-conversion green laser using $\Pr^{3 +}$ -doped or ${Ho}^{3 +}$ -doped ZBLAN fibers has reached watt-level output power in CW operation [25,26]. Nakanishi et al. reported a 598 mW, 522.2 nm $\Pr^{3 +}$ -doped fluoride single-clad fiber laser with a slope efficiency of 43% [23]. Most recently, Ji et al. used a 0.5% (molar fraction) ${Ho}^{3 +}$ -doped fluoroaluminate single-clad multimode fiber (30 cm long) to demonstrate watt-level laser emission in the green band [26]. Although a CW rare-earth-doped fiber laser operating at a green wavelength has been widely investigated, to the best of our knowledge, few reports on green pulsed fiber lasers have previously been published. Kojou et al. presented a 536–542 nm actively Q-switched $\Pr^{3 +}$ -doped fiber laser with 4 μJ green pulse energy [27]. Luo et al. demonstrated a compact self-Q-switched up-conversion Er:ZBLAN all-fiber laser operating at 543.4 nm [28]. In 2019, Li et al. reported a self-Q-switched ${Ho}^{3 +}$ :ZBLAN fiber to achieve 264 nJ green pulse at 550 nm [29]. However, one should notice that the green pulse energy from these Q-switched fiber lasers [27 –29] is still limited to a relatively low level (less than a few microjoule), which could restrict such green pulsed fiber lasers to certain applications (e.g., earth–moon laser ranging [9]). Interestingly, it is well-known that cavity dumping is an excellent technique to achieve a laser pulse with the advantages of large pulse energy, a wide repetition-rate tuning range, and high efficiency [30,31]; however, until now, to the best of our knowledge, a cavity-dumped fiber laser in the visible region has not yet been reported.

In this paper, a high-energy cavity-dumped ${Ho}^{3 +}$ -doped ZBLAN fiber (Le Verre Fluoré) pulsed laser operating at $\sim 543 nm$ is experimentally demonstrated for what we believe is the first time. A 450 nm LD was used to pump a high-concentration ${Ho}^{3 +}$ :ZBLAN fiber to provide green down-conversion gain, and a highly reflective mirror and a homemade fiber end-facet mirror at green wavelengths were employed to form the laser resonant cavity. Then, the green cavity dumping module is constructed by a visible-wavelength spatial modulator and a Z-type light feedback, which will support the completed oscillation of the first-order diffraction light and fully extract out the zeroth-order light. The laser directly generates a 317 mW, 543 nm green pulsed output with a repetition rate tuning from 100 Hz to 3 MHz and a slope efficiency of 55.3%. Stable cavity-dumped green pulses with a 116 ns duration could obtain a green pulse energy as high as 3.17 mJ at 100 Hz. Such a high-energy pulsed fiber green laser source could provide a new, promising light source for many applications in long-distance laser ranging, underwater detection, and microscopy.

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Figure 1(a) shows the photograph and Fig. 1(b) shows the corresponding schematic of our proposed green cavity-dumped ${Ho}^{3 +}$ :ZBLAN fiber pulsed laser. The green laser consists of a 450 nm LD, an active fiber ( ${Ho}^{3 +}$ :ZBLAN fiber), three highly reflective mirrors ( $M_{1}$ , $M_{3}$ , and $M_{4}$ ), a fiber end-facet mirror ( $M_{2}$ ), and a green spatial modulator. The 18 cm long ${Ho}^{3 +}$ : ZBLAN fiber (3000 parts per million by weight, a numerical aperture of 0.23, 7.5/125 μm core/cladding diameters) is used as gain medium and its absorption coefficient is about 2.9 dB/cm at 450 nm [32]. The 12 W, 450 nm LD as pump source is coupled to the gain fiber through a 7.5 mm focal-length aspherical lens ( $L_{1}$ ) with a coupling efficiency of 11%. The green resonant cavity is constructed by the homemade fiber end-facet mirror ( $M_{2}$ ) and the plane mirror ( $M_{3}$ ). The fiber mirror $M_{2}$ is directly connected to the gain fiber by a standard FC/PC fiber adaptor. The mirror $M_{4}$ was used to extract the cavity-dumped light. All of them ( $M_{2}$ , $M_{3}$ , and $M_{4}$ ) were coated with the same multilayer dielectric ${SiO}_{2} / {Ta}_{2} O_{5}$ films. The insets of Fig. 1(c) give the microscopic images of the fiber end-facet mirror $M_{2}$ , indicating the good and uniform deposition of the multilayer films. The transmission spectrum of the three mirrors shown in Fig. 1(c) shows a high transmittance of $> 95 %$ at 450 nm pump wavelength and a high reflectivity of $> 99.5 %$ at a $\sim 540 nm$ green oscillation wavelength. The highly reflective dichroic mirror ( $M_{1}$ ) is rotated at a $\sim 30 °$ angle to input the pump light into the gain fiber and separate the green light into the spatial cavity-dumped module.

(a) Photograph of green cavity-dumped Ho3+-doped fiber laser. (b) Schematic of green cavity-dumped Ho3+-doped fiber laser. (c) Optical transmission spectra of the visible-reflection mirror (M1) at 30° deflection, fiber pigtail mirror (M2), and visible-reflection mirrors (M3, M4), respectively.

Figure 1.(a) Photograph of green cavity-dumped ${Ho}^{3 +}$ -doped fiber laser. (b) Schematic of green cavity-dumped ${Ho}^{3 +}$ -doped fiber laser. (c) Optical transmission spectra of the visible-reflection mirror ( $M_{1}$ ) at 30° deflection, fiber pigtail mirror ( $M_{2}$ ), and visible-reflection mirrors ( $M_{3}$ , $M_{4}$ ), respectively.

2. EXPERIMENT SETUP AND OPERATION PRINCIPLE

The customized visible-wavelength spatial modulator is a key element for green cavity dumping. The visible spatial acousto-optic modulator with an RF operation frequency of 100 MHz can work in the wide visible range of 530–640 nm. The first-order diffraction efficiency at 540 nm is $\sim 53 %$ , and the insertion loss is 0.5 dB. A pair of plane-convex lenses ( $L_{2}$ and $L_{3}$ ) is used to compress the green light beam into the modulator, and the switching rise and fall time can be as short as $\sim 50 ns$ . In our experiment, the resonant cavity formed by $M_{3}$ and $M_{2}$ establishes a complete feedback of the first-order diffracted light, and the cavity-dumped pulses are extracted from the zeroth-order light by $M_{4}$ . The operational principle of the green acousto-optic cavity dumping is described in Fig. 2. When a periodical electric signal, shown in Fig. 2(a), is used to modulate the cavity dumping system, the formation of cavity-dumped green laser pulses will experience the three stages shown in Fig. 2(b). (1) With the high-level electric signal, the first-order diffraction light is with high loss and the green laser cannot oscillate, so this stage is the energy storage phase. (2) With the low-level electric signal, the first-order diffraction light becomes low loss, and the green resonator completely oscillates without power output (owing to the full reflector $M_{3}$ ); therefore, in this stage the green laser energy is fully stored in the cavity. (3) Finally, once the electric signal is quickly switched to a high level again, the intracavity green laser is instantaneously dumped by the zeroth-order light path, and therefore a high-energy green pulse can be achieved. After that, a new cycle can begin. In Fig. 2(c), to understand the mechanism of acousto-optic cavity dumping technology, we experimentally investigated the formation process of an acousto-optic cavity-dumped pulse by controlling the duty ratio of the electric signal. When the duty ratio is large (i.e., the low-voltage window is longer), the output pulse in the low-voltage window is a Q-switched pulse with low pulse energy. In contrast, when the duty ratio is small (i.e., the low-voltage level window is less than 1 μs), a very strong cavity-dumped pulse arising in the next high-voltage window can be clearly observed. This means that the device has a longer energy storage phase to get higher energy pulses at low repetition rates, as well as a wide range of repetition rate tunability. At the same time, considering that the pulse width is limited by the rise/fall time of the modulator, the duty cycle of the electrical signal is properly adjusted to ensure that the oscillation time in the cavity is several times longer than the rise/fall time to obtain a stable pulse output. The most important advantage is that this high-energy pulse can be extracted in a round trip time, independent of the time required to build up the power in the resonator cavity. Thus, the pulse width mainly depends on the length of the resonator when the switching time of the modulator does not exceed the round-trip time of the resonator.

$(a) Periodical electric signal. (b) Formation of cavity-dumped green laser pulses and diffraction efficiency measurement at 543 nm of visible wavelength spatial modulator. (c) Process of generating cavity-dumping pulses by controlling the duty cycle of trigger signals.$

Figure 2.(a) Periodical electric signal. (b) Formation of cavity-dumped green laser pulses and diffraction efficiency measurement at 543 nm of visible wavelength spatial modulator. (c) Process of generating cavity-dumping pulses by controlling the duty cycle of trigger signals.

In our experiment, the output performance of the green pulsed laser in the spectral and time domains is characterized by a 350–1750 nm optical spectrum analyzer (AQ-6315E, Ando) with a 0.05 nm resolution, an integrated-sphere power meter (S142C, Thorlabs), a photodetector (DET10A, Thorlabs), a digital oscilloscope (TDS1012C, Tektronix), and a radio frequency (RF) spectrum analyzer (GSP-930, Gwinstek).

3. EXPERIMENTAL RESULTS

A. High-Energy Green Pulse at 100 Hz

To verify the feasibility of high-energy green pulse generation, we output the green cavity-dumped fiber laser at a 100 Hz repetition rate. Figure 3 gives the output characteristics. Under a $\sim 20 cm {Ho}^{3 +}$ :ZBLAN gain fiber, the green laser threshold is about 330 mW, and the green cavity-dumped pulses were clearly observed. The typical oscilloscope trace in the inset of Fig. 3(a) shows the stable pulse trains with a 10 ms period (i.e., 100 Hz repetition rate) and a pulse-to-pulse fluctuation as low as 5%. The pulse duration is 116 ns, and the single pulse envelope has a symmetric Gaussian-like intensity profile. As shown in Fig. 3(b), we also measured the output RF spectra of cavity-dumped pulses. The RF SNR is greater than 42 dB, and the wideband output RF spectrum [the inset of Fig. 3(b)] has no other frequency components or modulation, implying the good stability of the green cavity-dumped laser.

Figure 3.Characteristics of the green cavity-dumped pulse operation at the millijoule energy level. (a) Single pulse. Inset: typical oscilloscope trace. (b) RF output spectra at 100 Hz. Inset: corresponding broadband RF output. (c) Output optical spectrum. Inset: close look at 543.02 nm. (d) Average output power as a function of the pump power envelope with gain fiber at different lengths. Inset: beam quality parameter and near-filed intensity distribution.

The green output spectra at a 1 mJ pulsed energy are shown in Fig. 3(c). The central wavelength locates at 543.02 nm and the 3 dB linewidth is as narrow as 0.12 nm. Furthermore, we optimized the length of the ${Ho}^{3 +}$ :ZBLAN fiber to improve the output efficiency of the green pulsed laser. As plotted in Fig. 3(d), under the different lengths of 16, 18, and 21 cm, the slope efficiency is 45.0%, 55.4%, and 42.8%, respectively, and the highest output power is 317 mW at the 100 Hz repetition rate, corresponding to pulse energy as high as 3.17 mJ. Although the output power is not a roll-off from Fig. 3(d), the increase of the output power is limited due to the thermal damage of the ${Ho}^{3 +}$ :ZBLAN fiber end facet (behind the $L_{1}$ lens). Under the low fiber coupling efficiency of the 450 nm pump laser, the residual pump power is high to accumulate the abundant thermal damage to the fluoride fiber end facet. The laser intensity distribution [see the inset of Fig. 3(d)] and $M^{2}$ factors of the generated green fiber laser beam were measured by a beam quality analyzer (WinCamD-UCD12, DataRay). The measured $M_{x, y}^{2}$ parameters are 4.41 and 4.31, respectively, indicating that the green fiber laser is in multimode operation. The multi-mode operation originates from the 9.9 V parameter of 7.5 μm core diameter of the ${Ho}^{3 +}$ :ZBLAN fiber at 543 nm.

Figure 4 presents the evolution of the optical spectrum and single pulse envelope as the output energy at the 100 Hz repetition rate varies. When the output power increased from 6 to 200 mW, the output spectrum of the green fiber laser was measured with a 0.05 nm resolution, as shown in Fig. 4(a). Although the cavity mirrors ( $M_{1} − M_{3}$ ) for a high-Q cavity cover a wide reflection band from 500 to 590 nm, as shown in Fig. 1(c), the lasing occurs only at a single wavelength of $\sim 543 nm$ without oscillating in multiple wavelengths. The central wavelength is always clamped at 543 nm and the 3 dB linewidth is less than 0.15 nm due to the filtering effect of acousto-optic Bragg diffraction in the process of the cavity-dumping operation.

Figure 4.(a) Output optical spectrum and (b) single pulse of the green cavity-dumped pulse under different pulsed energy.

Figure 4(b) shows the pulse profiles under the different pulse energies. It is clearly seen that the pulse profile remains almost unchanged (i.e., a Gaussian-like profile and 116 ns pulse duration). It should be attributed to the fact that the pulse duration in the cavity-dumped fiber laser is decoupled from the laser gain, resulting in the pulse duration at the same repetition rate being determined only by the resonator length. This means that in such green cavity-dumped ${Ho}^{3 +}$ -doped ZBLAN fiber lasers, even higher pulse repetition rates (such as a few MHz), narrower pulse duration (a few nanoseconds), and higher pulse energy are possible.

B. Repetition-Rate Tuning

A cavity-dumped laser is exceptionally steady for high pulse repetition rates (far above the inverse upper-state lifetime), because some light is always left in the laser resonator after pulse extraction and can serve as a seed for the next pulse. This means it is possible to obtain a very large pulse repetition rate tuning range based on the cavity-dumping technique. As shown in Fig. 5, the repetition rate tuning range can be from kilohertz to 3 MHz, and the pulse trains are stable with an intensity fluctuation as low as 5%. In our experiment, different repetition rates must satisfy the time conditions of energy storage and oscillation, so the electrical duty cycle must be fine-tuned slightly at different repetition rates.

Figure 5.Pulse trains based on cavity-dumping technology at different repetition rates.

Figure 6(a) gives the function of the green output power and pulse energy as the repetition rate at the fiber-coupling pump power of 0.85 W varies. Under the optimized 18 cm long gain fiber, the average output power ( $\sim 310 mW$ ) is almost constant in the case of a $< 7.1 kHz$ repetition rate, but the output power decreases when the repetition rate is over 7.1 kHz. The possible reason is that the time interval between two adjacent pulses (repetition rate $> 7.1 kHz$ ) is less than the upper energy-level lifetime ( $\sim 140 μs$ ) of ${Ho}^{3 +}$ :ZBLAN fiber, so it does not get the maximum utilization of the population inversion. The corresponding pulse width and peak power at different repetition rates are shown in Fig. 6(b). As the repetition rate increased, the pulse width became short, and the $\sim 70 ns$ shortest width is limited by the rise/fall time of the acousto-optic device. At the 3.17 mJ energy, we calculate the peak power precisely by integrating the pulse profile. The peak power is 24.6 kW at 100 Hz. To further enlarge the green pulse energy and compress the pulse width, a high-speed green acousto-optic spatial modulator with a smaller light beam size should be combined with a large-mode-area gain fiber.

Figure 6.Output characteristics of green cavity-dumped pulse at different repetition rate. (a) Corresponding average power and pulse energy at different repetition rates. (b) Corresponding pulse width and peak power at different repetition rates.

To better illustrate the advantages of our green cavity-dumped fiber laser over other green fiber pulsed lasers previously reported, Table 1 compares the output performance of the typical rare-earth-doped pulsed fiber lasers operating at green wavelengths. Note that the previous techniques mostly used active Q-switching or passive Q-switching [17,27 –29,34], but usually suffer from the restrictions, such as low pulse energy (only a few nJ or μJ), wide pulse duration (even μs), and narrow range (kHz) repetition rate. In this work, the cavity-dumping technique in a green fiber laser is exploited for the first time to significantly improve the output performance of green pulses. From Table 1 we can see that: (1) the green pulse energy is as high as 3.17 mJ at 543 nm, which is three orders of magnitude higher than previous reports [27 –29], and (2) the repetition rate of our cavity-dumped green fiber laser can be tuned in a wide range of 0.1–3000 kHz, which is also two orders of magnitude larger than previous reports [27 –29]. Moreover, the cavity-dumped green pulse has the shortest duration of 73 ns, which is also difficult to achieve by the nanosecond pulses previously reported based on Q-switching technology.Table 1.

Performance Comparison of Rare-Earth-Doped Pulsed Fiber Lasers at Green Wavelengths

Rare-Earth Ion	Operation Regime	Wavelength (nm)	Maximum Pulse Energy (μJ)	Repetition Rate (kHz)	Pulse Duration (ns)	Reference
$\Pr^{3 +}$	AO-Q-switch	520–526 536–542	1.99 4.16	8.3	160 298	[27]
${Er}^{3 +}$	Self-Q-switch	543.4	0.138	25.9–50.8	1950	[28]
${Er}^{3 +}$	Passive Q-switch	543	0.0252	42.61–181.2	490–1990	[33]
${Ho}^{3 +}$	Self-pulsing	539–550	/	100	1000	[17]
	Self-Q-switch	550	0.264	58.61–70.59	889	[29]
	Cavity-dumping	543	3170	0.1–3000	73–116	This work

4. CONCLUSION

In summary, we have proposed and demonstrated a high-energy green cavity-dumped fiber laser at 543 nm. The green down-conversion gain was provided by a section of 18 cm high-concentration ${Ho}^{3 +}$ :ZBLAN fiber pumped by a 450 nm LD. The cavity dumping operation is accomplished by employing a visible-wavelength spatial modulator and designing the resonant cavity. The 3.17 mJ green pulses are directly generated, which is typically three orders of magnitude higher than that of a conventional Q-switched fiber laser. Moreover, the pulse repetition rate of the green cavity-dumped fiber laser can be widely tuned in a range of 100 Hz–3 MHz. This cavity-dumped green fiber laser is an important step toward compact, large pulse energy, and high-efficiency pulse laser systems in the visible spectrum.

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