Mid-infrared (MIR) lasers, which emit in the 3 µm wavelength range, have been an active area for their application potential in modern health care, materials processing, and nonlinear optics^[1–6]. The development of high-quality fluoride fibers, specifically ${\mathrm{ZrF}}_4\text{-}{\mathrm{BaF}}_2\text{-}{\mathrm{LaF}}_3\text{-}{\mathrm{AlF}}_3\text{-}\mathrm{NaF}$ (ZBLAN), has significantly advanced the research on MIR fiber lasers due to their inherent advantages, including exceptional heat dissipation, excellent beam quality, compact structure, and efficient laser output^[7–9]. Among ZBLAN fibers doped with rare-earth elements, ${\mathrm{Er}}^{3+}$-doped ZBLAN fiber can produce lasing in the 2.8 µm wavelength range, achieved through the energy level transition from ${{}^4\mathrm{I}}_{11/2}$ to ${{}^4\mathrm{I}}_{13/2}$, utilizing a well-established $\sim 976\mathrm{nm}$ laser diode pump source. Therefore, ${\mathrm{Er}}^{3+}$-doped ZBLAN fibers demonstrate significant advantages like large stimulated emission cross sections, achieving a high doping concentration of the active ${\mathrm{Er}}^{3+}$ dopant. These qualities make them well-suited for lasing at 2.8 µm with high efficiency^[10,11]. Consequently, using ${\mathrm{Er}}^{3+}$-doped ZBLAN fiber for 2.8 µm lasing has attracted considerable interest recently, with most research efforts directed toward both continuous-wave (CW) and pulsed operation modes^[12–14].

Compared to CW lasers, pulsed lasers have high peak power, huge pulse energy, and fast response speed, making them the preferred choice for some specific applications^[15,16]. Techniques like $Q$-switching, mode locking, and gain switching are used to generate pulsed lasing^[2,11,17]. Among these techniques, $Q$-switching, which includes both passive and active $Q$-switching, is frequently used to generate pulses with durations ranging from nanoseconds to microseconds^[18,19]. The passive $Q$-switching technique mainly involves the use of saturable absorbers (SAs), such as low-dimensional materials and semiconductor saturable absorber mirrors (SESAMs)^[20,21]. Mainly, the SAs fabricated using low-dimensional materials offer benefits like low cost, broadband absorption, and compact structures, making them a research hotspot in recent years^[22–25]. However, in terms of meeting the requirements for high single-pulse energy, long-term stability, and adjustable pulse repetition rates, active $Q$-switching is more competent than passive methods. Therefore, active $Q$-switching techniques using acousto-optic modulators (AOMs), electro-optic modulators (EOMs), or mechanical switches within the laser cavity have been employed to attain pulses with shorter durations and higher peak powers^[26–29]. Among these $Q$-switching techniques, the one with EOMs offers advantages such as faster switching speeds, higher conversion efficiency, and flexible control over switching times, rendering them employed widely in the visible and near-infrared wavelength ranges. In solid-state laser operation, high peak power $Q$-switched MIR lasers have been reported using electro-optic (EO) crystals such as ${\mathrm{KTiOPO}}_4$ (KTP) or ${\mathrm{LiNbO}}_3$^[30,31]. However, EOMs have rarely been realized for MIR fiber lasers with higher conversion efficiencies and higher operating repetition rates. In 2021, Shen et al. achieved a milestone by demonstrating the first EO $Q$-switched fiber laser operating around 2.7 µm using rubidium titanyl phosphate (RTP) as an EOM. This achievement resulted in the generation of $Q$-switched pulses with a duration reduced to 13.1 ns and a conversion efficiency of approximately 0.6%^[26].

In this work, the ${\mathrm{La}}_3{\mathrm{Ga}}_5{\mathrm{SiO}}_{14}$ (LGS) crystal was used as an EOM within the MIR fiber laser cavity to generate robust actively $Q$-switched pulses with high efficiency, adjustable repetition rate, and long-term stability of around 2.8 µm. As a novel electro-optical crystal, the LGS crystal is a uniaxial optical active crystal with advantages such as a high optical damage threshold, wide transparency range, moderate EO coefficient, and small piezoelectric ringing effect^[32]. The LGS crystal exhibits a wide transmittance range from 0.5 to 4.5 µm, with an impressive damage threshold exceeding 900 MW/cm²^[33,34]. As a result, the effectiveness of the LGS crystal as an EOM has been successfully demonstrated in solid-state lasers^[35,36]. In this study, the LGS crystal is used within a laser cavity constituted by an ${\mathrm{Er}}^{3+}$-doped ZBLAN fiber, generating $Q$-switched pulses at 2.79 µm. At an 18 kHz repetition rate, the high slope efficiency of 11.4% can be observed. Furthermore, the maximum output power reached 451 mW with the pulse width measured at 64 ns under a pump power of 4.4 W. The pulse energy and peak power were calculated to be 25 µJ and 391 W corresponding to the measurement. The robust and high-efficiency output results affirm the reliability of LGS for MIR actively $Q$-switched fiber lasers, presenting new possibilities for future high-power MIR applications, including biomedical, defense, and nonlinear MIR photonics.

The schematic diagram of the actively $Q$-switched ${\mathrm{Er}}^{3+}$-doped ZBLAN fiber laser using LGS is shown in Fig. 1. The pump source is a commercial 976 nm laser diode, which had a pigtail fiber with a core diameter of 200 µm and a numerical aperture of 0.22. A double-clad ZBLAN fiber (Le Verre Fluoré) with a core diameter of 15 µm and a numerical aperture of 0.12 was selected as the gain fiber. The fiber had a length of 3.3 m and a heavily doped concentration of erbium ions with a mole fraction of 7% ensured a high pump absorption efficiency surpassing 90%. The inner cladding of the double-clad fiber was a double-truncated circular shape with a size of $240\mathrm{\mu m}\times 260\mathrm{\mu m}$ to ensure a high pump efficiency. The cross section of the fiber is shown in the inset of Fig. 1, where the bright circle in the center shows the fiber core surrounded by the inner cladding. The pump laser beam was introduced into the inner cladding of the gain fiber after collimation and focused through a pair of ${\mathrm{CaF}}_2$ lenses (L1 and L2) with focal lengths of 50 mm. A dichroic mirror (DM) was positioned between lenses L1 and L2 at a 45° angle, featuring reflectivity greater than 95% at around 2.79 µm and transmission exceeding 95% at 976 nm. This configuration can facilitate the extraction of the 2.79 µm laser output. Another ${\mathrm{CaF}}_2$ lens (L3) with a 25 mm focal length was placed to collimate the emitted 2.79 µm laser. After passing through the EOM unit consisting of a polarizer, an LGS crystal, and a $\lambda /4$ wave plate, the laser was reflected by a gold-coated mirror with high reflectivity to achieve the cavity feedback.

The EO $Q$-switched laser was operated in the pulse-on mode. The LGS crystal, with a clear aperture of $4\mathrm{mm}\times 4\mathrm{mm}$ and a length of 42 mm, served as the EO crystal in this laser configuration. Two surfaces through which light passes in the crystal were treated with antireflection coatings designed for the 2.79 µm wavelength. The insertion loss of the LGS is less than 0.23 dB. The LGS crystal can operate a $Q$-switching in the transverse field configuration, with an electric field applied in the $x$ axis; the light signal propagates along the $z$ axis simultaneously. High-speed EO $Q$-switching was achieved through a periodic electric field at both ends of the LGS crystal using a high-voltage driver. The voltage was set as 4.3 kV herein to match the size of the applied LGS crystal. The extinction ratio was tested at approximately 28 dB. The repetition rate of the driver could be adjusted from 1 to 18 kHz, with a rise time of $\sim 20\mathrm{ns}$, enabling the modulation of the EOM on and off. The polarizer (Thorlabs, LPMIR050-MP2) had an extinction ratio higher than ${10}^6\mathrm{:}1$ and a transmission efficiency exceeding 80% at around 2.8 µm. Two ${\mathrm{MgF}}_2$ plates with their fast axes crossed and a designed wavelength of 2.79 µm were used to construct the $\lambda /4$ wave plate.

Initially, the fiber laser cavity was configured without an EOM unit to fine-tune the CW laser output. Under these conditions, the slope efficiency was measured at approximately 12.2%, and the average energy exhibited a linear increase with the applied pump power. Subsequently, the polarizer, LGS crystal, and $\lambda /4$ plate were sequentially inserted into the cavity and carefully adjusted to their optimal positions. In this configuration, the ${\mathrm{Er}}^{3+}$ ions could effectively absorb pump energy, promoting the transition of populations to the upper energy level when the $Q$-switch was deactivated. The long lifetime of 6.9 ms for the upper energy level ${{}^4\mathrm{I}}_{1/2}$ and the energy transfer upconversion process prove advantageous for storing a substantial amount of energy^[37]. Consequently, a giant pulse could be instantly generated when the $Q$-switch is activated. Consequently, actively $Q$-switched pulses could be produced with a low pump threshold by applying a periodic electric field to the LGS crystal at a specific frequency and an appropriate duty cycle.

For checking the performance of the output pulses, measurements of output power, pulse width, and calculated peak power were conducted with respect to the launched pump power under an 18 kHz repetition rate, as illustrated in Fig. 2. The $Q$-switch was activated by applying a pulsed high-voltage signal lasting 5 µs, resulting in a corresponding duty cycle of 9%. A pyroelectric optical power meter was used to obtain the average power under different conditions. Consequently, stable pulses were obtainable with a low pump power threshold of 0.6 W. With the increase of pump power, the output average power exhibits a linear growth and reaches 451 mW under 4.4 W pump power, as depicted in Fig. 2. The corresponding light-to-light conversion efficiency was approximately 10.3%. The efficiency of the laser output with respect to the absorbed pump power was estimated to be approximately 17.9%. The slope efficiency, as determined from the measured and linearly fitted data, was approximately 11.4%. These values represent the highest average power and slope efficiency in electro-optical $Q$-switched fiber lasers operating at 2.8 µm, to the best of our knowledge. The exceptional output efficiency can be primarily attributed to the well-constructed fiber laser cavity and the utilization of the LGS crystal, which exhibits high transmission and modulation depth at 2.8 µm.

Thanks to the high output power, the pulse energy reached 25 µJ at 18 kHz under a 4.4 W pump power. Simultaneously, the waveform of the pulses was measured using an HgCdTe photodetector (Vigo System, PCI-2TE-12) with a digital oscilloscope (RIGOL, MSO8204), as shown in Fig. 2. There is a noticeable decrease in pulse width from 910 to 64 ns with the increase in pump power. This phenomenon can be attributed to the augmented intracavity gain and the higher concentration of upper-level populations. The calculated peak power significantly increases with the elevated pump power due to increased pulse energy and decreased pulse width, as depicted in Fig. 2, with the maximum value reaching 391 W. The pump power had not been further increased to avoid possible damage to the optical components within the cavity.

The inset of Fig. 2 displays single pulse envelopes obtained under various pump powers. Shorter pulse widths can be observed under higher pump powers, which can surpass the pulse widths of most passively $Q$-switched pulses in the MIR region. However, increasing pump power alone presents challenges in further pulse width compression, as Fig. 2 displays. Theoretically, the $Q$-switched pulse width is proportional to the cavity round-trip time (TR)^[38]. The TR is directly proportional to the length of the laser cavity, according to the following relation. $\mathrm{TR}=2nL/c$^[29], where $n$, $L$, and $c$ denote the refractive index of the active fiber, cavity length, and light speed in vacuum, respectively. For our fiber laser structure, the calculated TR was approximately 34 ns. Based on this analysis, extending the laser cavity leads to longer TRs and wider pulse widths. However, an elongated fiber length enhances pump absorption and increases slope efficiency lasing. Consequently, reducing the fiber length and increasing the core diameter or ${\mathrm{Er}}^{3+}$ dopant concentration can further reduce the width of $Q$-switched laser pulses. Additionally, previous studies have shown that replacing the mirror in the laser cavity with a grating as cavity feedback can narrow the linewidth by suppressing amplified spontaneous emission^[26]. Therefore, there is significant potential for further shortening the pulse width of the output lasers.

The spectral information of the actively $Q$-switched pulses was captured by a Fourier transform spectrometer (Arcoptix, FTMIR-L1-120-4TE-HR). The spectrum was recorded under an 18 kHz repetition rate and 2.8 W pump power, as Fig. 3 illustrates. The spectrum exhibits a full width at half-maximum (FWHM) of 2.71 nm centered at 2785.90 nm. The central wavelength remained stable with the variation in repetition rate and pump power. Nevertheless, due to the limitations of the spectrometer, more detailed information about the output spectrum, such as the possible water absorption lines, has not been obtained. It is worth noting that the linewidth can be further narrowed by exploiting the wavelength-selecting effect of the grating.

Furthermore, the radio-frequency (RF) spectrum was measured by an RF spectrum analyzer (Keysight, N9000B) with a bandwidth of 26 GHz. A signal-to-noise ratio (SNR) of 52 dB was captured, as shown in Fig. 3(b), indicating stability and excellent output performance as a $Q$-switched fiber laser. Then, the output power stability of the fiber laser was recorded over a 1-h period under the same laser parameters, as shown in the inset of Fig. 3(b). The RMS fluctuation was calculated to be approximately 0.36%. Furthermore, no degradation was observed over a 1-h operating period as well as throughout the experimental period, indicating the long-term stability of the laser.

One of the key advantages of this actively $Q$-switched scheme is its adjustability of the repetition rate, which the high-voltage driver controls. Measurements were carried out across a range from 1 to 18 kHz to examine the influence of the repetition rate on the pulses. The pump power was maintained at 1.0 W to suppress the double pulses. The $Q$-switch was activated for approximately 5 µs duration, with slight adjustments made to optimize the output pulses. The results are presented in Fig. 4. The average power initially exhibited a significant increase with the rising repetition rate and eventually reached a stable level, as illustrated in Fig. 4(a). Generally, the average power remained relatively consistent under different repetition rates^[26]. The duration for which the $Q$-switch was active remained nearly the same for all repetition rates. In this scenario, the duty cycle decreased with the decrease in repetition rate, going from 9% at 18 kHz to 0.5% at 1 kHz, resulting in lower average power at lower repetition rates. The slight variations in average power at high repetition rates are attributed to minor instabilities. Furthermore, the pulse width experienced a considerable increase with the rising repetition rate. This behavior can be attributed to the reduction in cavity gain, which, in turn, diminishes the population density accumulation at the ${{}^4\mathrm{I}}_{11/2}$ level. Consequently, as the repetition rate increases from 1 to 18 kHz, the pulse width increases sharply from 94 to 602 ns, as shown in Fig. 4(a).

The pulse energy and peak power were calculated according to the pulse width and average power, as shown in Fig. 4(b), which exhibit remarkable reduction with increment in repetition rate, consistent with the trend observed in pulse width. This behavior is characteristic of a typical $Q$-switched laser, where the maximum pulse energy is stored at the lowest repetition rate. Consequently, the largest pulse energy of 45.7 µJ and peak power of 485.7 W could be obtainable at 1 kHz repetition rate and 1.0 W pump power. The pulse energy and peak power can be further enhanced by directly reducing the repetition rate. However, as the pump power was further elevated, the increment of pulse energy and peak power gradually slowed down due to the rise of pulse spikes at higher pump powers, particularly under low repetition rates. Several potential solutions could be explored to address this issue in future research, such as introducing higher cavity feedback and enhancing the modulation depth to minimize the occurrence of pulse spikes.

Pulse-to-pulse stability is a crucial characteristic of an excellent fiber laser. Therefore, normalized pulse trains were tested under different repetition rates and are presented in Fig. 5. The pulse train was measured at different repetition rates within a 5 ms sweep range. The temporal pulse trains exhibit minimal time jitter at 1, 10, and 18 kHz, with no other satellite pulses observed. The RMS between the pulses was calculated to be 1.52%, 3.20%, and 2.72% under 1, 10, and 18 kHz repetition rates, respectively, revealing stability between pulses. The unstable pump laser output and the surrounding environmental disturbances may be the primary cause of the fluctuations of the pulses. Moreover, the output laser pulses remain stable throughout the testing process, indicating the long-term stability of the laser cavity and LGS crystal.

This work presents the application of an LGS crystal with a high damage threshold and wide transparency range as an EOM in the 2.8 µm waveband. A robust, actively $Q$-switched laser was realized in the ${\mathrm{Er}}^{3+}$-doped ZBLAN fiber laser cavity. The developed fiber laser can generate MIR laser pulses at 2785.90 nm with a low pump threshold and high output power. Furthermore, the slope efficiency of 11.4% surpasses that of other existing electro-optical $Q$-switched fiber lasers in the 2.8 µm waveband so far. The results manifest the stability and high efficiency of the LGS crystal as an EOM acting in the MIR fiber laser cavity. Taking advantage of adjustable repetition rates, high efficiency, and long-term stability, the findings in this study exhibit significant potential for the generation of actively $Q$-switched MIR lasers of around 2.8 µm. It, in turn, broadens the application scope of short-pulse, high-energy laser devices. Furthermore, it can potentially advance developments in infrared countermeasures, remote sensing, and biomedical applications.