1. Introduction

Lasers in the mid-infrared (mid-IR) spectral region are of great interest for a wide range of scientific and technological applications including spectroscopy, medical surgery, frequency metrology, missile countermeasures, remote sensing and materials processing [1–3]. Mid-IR coherent emission can generally be produced by optically-pumped ion-doped crystal (e.g. Er³⁺:YAG, Pr³⁺:BIGGSe, Cr²⁺:CdS_0.8Se_0.2) and glass (e.g. Er³⁺, Ho³⁺, Dy³⁺-doped fluoride and chalcogenide) lasers, electrically and optically-pumped semiconductor lasers (e.g. antimonide, IV-VI semiconductors, quantum cascade lasers), solid-state laser-pumped optical parametric oscillators (OPOs), difference frequency generation (DFG) sources, and optically or electrically-pumped gas (e.g. CO₂, CO, C₂H₂, HCN) lasers [4]. Compared to these laser sources, mid-IR fiber lasers have advantages of high efficiency, inherent simplicity, compactness, outstanding heat-dissipating capability, and excellent beam quality. Although gas-filled photonic crystal fiber [4] and transition-metal doped single crystal fiber [5] have shown their potential for mid-IR lasers, ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) fiber that has an extended infrared edge (> 6 μm), low maximum phonon energy (< 600 cm⁻¹), and low loss (< 0.1 dB/m) has always been considered as ideal host for lasing materials at wavelengths where silica fiber lasers are absent [6]. Mid-IR fiber lasers at 3 µm have been successfully demonstrated with erbium (Er³⁺), holmium (Ho³⁺), and dysprosium (Dy³⁺) ions doped ZBLAN fiber lasers [7–18]. Over the last decade, great efforts have been made to escalate the power of continuous-wave (cw) rare-earth-doped mid-IR fiber lasers and several ten-watt-level cw fiber lasers at 3 μm have been achieved recently by using heavily Er³⁺-doped ZBLAN fibers [7–10]. However, constrained by the fragility and the susceptibility to opto-mechanic effects of ZBLAN glass, the output power of cw ZBLAN fiber lasers cannot be scaled up dramatically like kW-level silica fiber lasers by launching more pump power into the gain fiber. Although coherent beam combining techniques can be used to further power scale mid-IR fiber lasers, the unavailability of corresponding fiber devices in the mid-IR range blocks their implementation in the near future. A pulsed mid-IR fiber laser is an alternative approach to achieve high peak power mid-IR lasers for specific applications such as mid-IR nonlinear wavelength conversion and medical surgery where high power is essential and shorter pulses are preferred since they can significantly reduce collateral damage. Pulsed lasers can generally be produced by modulating the laser cavity with various Q-switching and mode-locking techniques. Q-switched and mode-locked rare-earth doped ZBLAN fiber lasers have attracted increasing interest in the last few years.

Q-switched fiber lasers capable of producing microsecond or nanosecond pulses with peak powers several orders of magnitude higher than in the cw mode are highly desired for cascaded Raman fiber lasers at longer wavelength for a number of applications [19,20]. Q-switching is a widely used laser technique in which the Q-factor of the laser cavity is modulated actively by a modulator or passively by a saturable absorber. The first actively Q-switched mid-IR fiber laser near 3 μm was demonstrated in 1994 [11]. An acousto-optic modulator (AOM) and a rotating mirror were used as switching elements in an Er³⁺-doped ZBLAN fiber laser. A pulse duration of 100 ns and peak power exceeding 2W were achieved. With the significant progress of high power Er³⁺-doped ZBLAN fiber lasers in these years, an actively Q-switched mid-IR laser with a pulse energy of 100 μJ, pulse duration of 90 ns, average power of 12 W, and peak power of 0.9 kW, were recently achieved [9]. Compared to bulky and complicated active Q-switching approaches, passive Q-switching has advantages of simplicity and compactness with no need of additional electric equipment. The first passively Q-switched Er³⁺-doped ZBLAN fiber laser was demonstrated by using InAs epilayers as the saturable absorber [12]. 1.2 μs pulses with an energy of 1.25 μJ and peak power of 1.04 W at repetition rate of 1.1 kHz were achieved. However, the damage threshold of InAs epilayers is low thereby limiting their application in high power mid-IR laser. Most recently we reported an Er³⁺-doped ZBLAN fiber laser passively Q-switched by a Fe²⁺:ZnSe crystal [13]. 370 ns pulses at 2.78 µm with a pulse energy of 2.0 μJ and peak power of 5.34 W were achieved at a repetition rate of 161 kHz. Since the primary water absorption peak is at 3 µm, pulsed lasers with wavelengths closer to 3 μm are more efficient for laser surgery and some other particular applications. Because of this, Ho³⁺-doped ZBLAN fiber laser, which has an operation wavelength longer than Er³⁺-doped ZBLAN fiber laser [14–17], is attracting more interest. An actively Q-switched Ho³⁺/Pr³⁺-doped ZBLAN fiber laser at 2.87 µm producing 78 ns pulses with a peak power of 77 W at a repetition rate up to 300 kHz was reported very recently [15]. In this paper we report our investigations on Ho³⁺-doped ZBLAN fiber lasers passively Q-switched by a Fe²⁺:ZnSe crystal and graphene, respectively. Because free-running Ho³⁺-doped ZBLAN fiber lasers generally operate at longer wavelengths than Ho³⁺/Pr³⁺-codoped ZBLAN fiber lasers, Q-switched pulses at 2.93 µm were obtained using both saturable absorbers.

2. Ho³⁺-doped ZBLAN fiber laser at 3 µm

Because of the low maximum phonon energy of ZBLAN glass, Ho³⁺-doped ZBLAN can lase at 3 µm through a radiative transition from ⁵I₆ to ⁵I₇ as shown in Fig. 1. In low concentration Ho³⁺-doped ZBLAN, the lifetime of the upper laser level ⁵I₆ (3.5 ms) is usually shorter than that of the lower laser level ⁵I₇ (12 ms). The 3 μm laser emission is generally self-terminated due to population accumulation in the ⁵I₇ state. The population accumulation in the ⁵I₇ state can be solved by codoping with Pr³⁺ ions, through the energy transfer process between Ho³⁺ and Pr³⁺ ions [14]. Population accumulation can also be eliminated through excited state absorption processes in high concentration Ho³⁺-doped ZBLAN [21]. In a free-running configuration, the operating wavelength of a Ho³⁺-doped ZBLAN laser is usually longer than that of a Ho³⁺/Pr³⁺ codoped ZBLAN laser, due to the relatively large population remaining in the ⁵I₇ state. Consequently, Ho³⁺-doped ZBLAN fiber was used to achieve a passively Q-switched fiber laser at a wavelength closer to 3 µm.

 

Fig. 1 Energy-level diagram of Ho³⁺-doped ZBLAN and transitions related to the laser emission at 2.9 µm. ESA1 and ESA2 represent exited state absorptions and ETU1 and ETU2 represent energy transfer upconversions.

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The absorption spectrum of Ho³⁺-doped ZBLAN glass was measured and is shown in Fig. 2(a). In addition to strong absorption in the visible, Ho³⁺-doped ZBLAN has absorption in the 1.1 μm - 1.2 μm range with a peak at 1150 nm, where semiconductor lasers and Raman fiber lasers pumped by Yb³⁺-doped silica fiber lasers are readily available. The fluorescence of Ho³⁺-doped ZBLAN glass was measured and is shown in Fig. 2(b). The strong emission at 1.2 µm corresponds to the transition ⁵I₆→⁵I₈. Although the branch ratio of the transition ⁵I₆→⁵I₈ to the transition ⁵I₆→⁵I₇ is about 9:1, the 2.9 μm laser can be easily obtained in a high concentration Ho³⁺-doped ZBLAN fiber because the 1.2 µm quasi-three-level laser is overwhelmed by the 2.9 µm quasi-four-level laser, in which population inversion can be obtained with a small excited state population [22]. The inset of Fig. 2(b) shows the fluorescence of Ho³⁺-doped ZBLAN in the 3 µm region. The transition ⁵I₆→⁵I₇ peaks at 2.85 μm and extends up to 3 μm, making Ho³⁺ a favorite active element for 3 μm laser systems. Actively Q-switched singly Ho³⁺-doped and Ho³⁺/Pr³⁺ codoped ZBLAN fiber lasers were reported recently [15,16]. However, an acousto-optic modulator (AOM) was used in both demonstrations, which make the laser expensive and complicated. Below, we discuss our demonstration of a passively Q-switched singly Ho³⁺-doped ZBLAN fiber laser operating at 2.93 μm, in which both Fe²⁺:ZnSe crystal and a graphene coated fiber mirror were respectively used as the saturable absorber.

 

Fig. 2 (a) The absorption and (b) fluorescence of 3 mol% Ho³⁺-doped ZBLAN. Inset of (a) shows the absorption of Ho³⁺-doped ZBLAN in the 1.1-1.24 µm range; inset of (b) shows the fluorescence of Ho³⁺-doped ZBLAN in the 3 µm region.

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3. Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser

Fe²⁺:ZnSe has been used for high power mid-IR laser pulse generation because of its large absorption cross-section and small saturation fluence (60 mJ/cm²) together with the excellent opto-mechanical properties (damage threshold ~2 J/cm²) [13,23,24]. The Fe²⁺-doped ZnSe crystal (IPG Photonics) used in our experiment was fabricated by post-growth thermal diffusion of iron in polycrystalline ZnSe. The thickness of the Fe²⁺: ZnSe crystal was 2 mm. The absorption of the Fe²⁺:ZnSe crystal in the 2-8 µm range was measured with an FT-IR spectrometer (Spectrum One, Perkin Elmer) and is shown in Fig. 3. Clearly, the Fe²⁺:ZnSe has absorption between 2.5 and 4.5 µm with a peak around 3 µm. Therefore, Fe²⁺:ZnSe crystal is an excellent saturable absorber for mid-IR lasers at 3 µm. Because of its high damage threshold, Fe²⁺:ZnSe crystal has been used in a passively Q-switched mid-IR Er³⁺:YAG laser to produce 6-mJ, 50-ns giant pulses at 2.936 μm [23]. Most recently, we demonstrated a Fe²⁺:ZnSe passively Q-switched Er³⁺-doped ZBLAN fiber laser with more than 300 mW average output power, only limited by the available pump power [13].

 

Fig. 3 Absorption of the Fe²⁺:ZnSe crystal used in our experiment. Inset shows the Fe²⁺-doped ZnSe crystal.

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The schematic of the experiment setup is shown in Fig. 4. A 1150 nm Raman fiber laser pumped by a 1100 nm Yb³⁺-doped silica fiber laser was used as the pump. Two sapphire lenses with 25.4 mm focal length were used to collimate and focus the pump light to the flat cleaved end of the Ho³⁺-doped ZBLAN fiber. A dichroic mirror with a transmission of 89% at 1150 nm and a reflectivity of > 99% at 2.9 μm was placed between the two lenses to couple the laser beam out. The Ho³⁺-doped ZBLAN fiber has a length of about 2.5 m, a core dopant concentration of 20000 ppm, core NA of 0.16 and core diameter of 10 μm. The flat cleaved end of the Ho³⁺-doped ZBLAN fiber works as the output coupler of the laser cavity to provide 4% feedback by Fresnel reflection. The other end of the Ho³⁺-doped ZBLAN fiber was angle cleaved to eliminate Fresnel reflection. The laser beam coming from the angle cleaved end was collimated and focused onto the Fe²⁺:ZnSe saturable absorber by two CaF₂ lenses. The transmitted light was then collimated by another CaF₂ lens and reflected by a highly reflective mirror. A Ge filter was used to block the pump and background noise below ~2 μm. An InSb detector with rise time of 7 ns was used to measure the time domain performance of the Q-switched fiber laser. The pulse trains were recorded by an oscilloscope with a bandwidth of 100 MHz (Tektronix TDS 1012). The average power was measured by a thermal power meter (Thorlabs, S310C).

 

Fig. 4 Schematic of experiment setup for Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser.

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The threshold of this laser was about 0.6 W, but at this power the pulsed operation was not stable. Stable Q-switched pulses were obtained when the launched pump power was 1 W. Figure 5(a) shows a typical pulse train under the launched pump power of 1.8 W. The pulse amplitude fluctuation is less than 3%. The pulse envelop of the Q-switched pulses at the same launched pump power is shown in Fig. 5(b); pulses with a duration of 910 ns were obtained. The measured average output power, repetition rate, pulse duration and calculated pulse energy as a function of the launched pump power are shown in Fig. 6. The measured repetition rate increases and the pulse width decreases as the launched pump power increases. The repetition rate increases from 88 KHz to 105 KHz and the pulse duration decreases from 1.15 μs to 0.82 μs in the stable Q-switched regime. When the laser was pumped with the maximum available launched pump power of ~2.2 W, the average output power was 47.2 mW and the pulse energy was 450 nJ, corresponding to a peak power of 0.55 W. The slope efficiency of this laser is 2.5%, which is lower than the 12.3% of the tunable Q-switched Ho³⁺-doped ZBLAN fiber laser [17] due to the large loss of the cavity caused by the Fresnel reflection of the non-anti-reflection coated optics (two sapphire lenses, three CaF₂ lenses, and the Fe²⁺:ZnSe crystal) and the un-optimized free space optics. Much higher efficiency can be obtained if the various optical components are optimized for maximum coupling and minimum reflection. The spectrum of the Q-switched Ho³⁺-doped ZBLAN fiber laser was measured with a monochromator (SPEX 270). As shown in Fig. 7, the center wavelength is at about 2935 nm, which is a typical wavelength for high concentration Ho³⁺-doped ZBLAN fiber lasers. Clearly, the wavelength of this passively Q-switched single Ho³⁺-doped ZBLAN fiber laser is longer than that of the Ho³⁺/Pr³⁺-codoped fiber laser. Figure 8 shows the radio frequency (RF) spectrum of the fiber laser measured at a launched pump power of 1.8 W with a spectrum analyzer (Advantest R3267). The peak is at 104 KHz which corresponds to the repetition rate of the pulse train shown in Fig. 5(a). The signal-to-noise ratio (SNR) is about 40 dB, which indicates a fairly stable pulse operation of this fiber laser.

 

Fig. 5 (a) Pulse train and (b) pulse envelop of Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser at launched pump power of 1.8 W.

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Fig. 6 (a) The average output power (red squares) and the pulse energy (blue dots), (b) the repetition rate (red squares) and the pulse duration (blue dots) of the Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser as a function of the launched pump power.

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Fig. 7 Spectrum of the passively Q-switched Ho³⁺-doped ZBLAN fiber laser at a launched pump power of 1 W.

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Fig. 8 RF spectrum of the Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser measured at a launched pump power of 1.8 W

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4. Graphene Q-switched Ho³⁺-doped ZBLAN fiber laser

Graphene, a two-dimensional lattice of carbon atoms in a honeycomb structure, was found to possess extraordinary nonlinearities and ultrafast recovery times of photo-excited electrons at picosecond and femtosecond timescales [25,26]. Moreover, graphene is a zero-bandgap material and its absorption is only determined by the optical conductivity constant and is independent of optical frequency. It has already been verified that graphene has an ultrabroad absorption ranging from the visible to the THz waveband [27]. Thus, graphene is a promising ultra-broadband saturable absorber. Q-switched and mode-locked operation of Yb³⁺-, Er³⁺-, and Tm³⁺-doped silica fiber lasers based on graphene saturable absorbers have been successfully demonstrated in the 1, 1.5 and 2 μm wavelength regions, respectively [28–32]. Therefore, it is of high interest for us to investigate the applicability of graphene as a saturable absorber to generate mid-IR pulses in the 3 μm region. Moreover, nanometer-sized graphene flakes can be deposited on fiber end facets or side-polished/tapered fibers to make fiber-based saturable absorbers, which can be used to make compact and reliable Q-switched and mode-locked all-fiber lasers. Next, we discuss a Ho³⁺-doped ZBLAN fiber laser Q-switched by a graphene deposited fiber mirror.

In our experiment, graphene was first obtained by liquid exfoliation of graphite (Bay Carbon) in 1-methyl-2-pyrrolidinone (NMP). In a typical process, 0.1 mg/mL solution was sonicated for 24 hours and allowed to settle overnight. The top 75% of the supernatant was then removed and centrifuged at 1500 rpm for 40 min. This yielded few-layer graphene flakes. The solution containing the graphene flakes were found to be stable for months and were used for the preparation of the saturable absorber. The wavelength-independent absorption of graphene in the infrared range was confirmed by measuring the transmission of few layers of graphene, which was drop cast on silicon substrate. Figure 9 shows the flat infrared transmission of graphene from 2 µm to 12 µm. The absorption coefficient is almost wavelength independent, so it is expected that graphene can be used as a saturable absorber for passively Q-switching over a very wide IR wavelength range. The Raman spectroscopy of the sample with a few layers of graphene was performed using a Renishaw Raman system with a pump laser at 514 nm and is shown in the inset; the observed Raman spectrum is consistent with previous reports [33,34]. To make a fiber-based saturable absorber for the Ho³⁺-doped ZBLAN fiber laser, a fiber mirror was first fabricated by ion-beam deposition of a dichroic thin film (highly reflective @ 2.9 µm and highly transparent @ 1150 nm) onto the end facet of a striped and cleaved optical fiber (Thorlabs AFS105/125). Subsequently, the graphene coated fiber mirror was prepared by the method of optically driven deposition [35]. A 975 nm laser diode was spliced to one end of the fiber mirror (the end without coating) and the other coated end was immersed in the graphene solution. After turning on the 975 nm laser diode for 25 minutes at 30 mW, graphene was deposited on the fiber end facet by optical and thermal gradient forces. The microscopic photographs of the fiber mirror before and after graphene deposition are shown in Figs. 10(a) and 10(b), respectively. Obviously, a neat graphene thin film was deposited onto the fiber mirror. It is very surprising to us that the graphene thin film is very smooth and homogenous over the entire fiber end facet, not only over the core area. This may manifest that thermal gradient forces contributes more significantly to the thin film deposition than optical gradient force. The dichroic thin film may also facilitate the homogeneous deposition. Further investigation into the mechanics of the graphene thin film deposition and how to control the thin film thickness is currently under way. The absorption loss of the deposited graphene thin film was measured to be ~33% (−1.74 dB). Considering that the absorption loss of each graphene layer is 2.3% (−0.10 dB) [36], we estimated that about 17 layers of graphene were deposited on the fiber mirror.

 

Fig. 9 The transmission of a 5-layer graphene thin film deposited on a silicon substrate. Inset: Raman spectrum of the 5-layer graphene.

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Fig. 10 The fiber mirror end before (a) and after (b) graphene was deposited.

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The experimental setup for the graphene Q-switched Ho³⁺-doped ZBLAN fiber laser is schematically shown in Fig. 11. It is very similar to that of the Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser except that a graphene coated fiber mirror was used to butt-couple to the back end of the Ho³⁺-doped ZBLAN fiber. The fiber mirror itself is highly reflective at ~3 μm and highly transparent at the pump wavelength; hence the graphene deposited on the fiber mirror acts as a saturable absorber.

 

Fig. 11 Schematic of experimental setup for graphene Q-switched Ho³⁺-doped ZBLAN fiber laser.

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The threshold of this laser is about 0.4 W which is lower than that of the Fe²⁺:ZnSe Q-switched laser due to the significantly reduced cavity loss. Stable Q-switched pulses start at 0.6 W and are maintained until the launched pump power reaches about 1.5 W. Q-switched operation was no longer stable when the pump power was further increased, most likely due to thermal effects in the graphene thin film. A typical Q-switched pulse train is shown in Fig. 12. The pulse amplitude fluctuation is about 1.7%, which is smaller than that of the Fe²⁺:ZnSe Q-switched laser. As observed in a general Q-switched fiber laser, the repetition rate increases while the pulse width decreases with the increased pump power. The repetition rate increases from 64 KHz to 92 KHz and the pulse duration decreases from 1.40 μs to 1.18 μs in the stable Q-switched regime. The measured average output power, repetition rate, pulse duration and calculated pulse energy as a function of the launched pump power are shown in Fig. 13. When the laser was pumped with 1.5 W, the average output power was measured to be 102 mW and the pulse duration was 1.18 μs. The pulse energy and the peak power were calculated to be 1.1 μJ and 0.95 W, respectively. The slope efficiency of this fiber laser is 8.9%, which is much higher than that of the Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser due to the reduced cavity loss. The efficiency is relatively lower than the 12.3% of the tunable Q-switched Ho³⁺-doped ZBLAN fiber laser [17]. However, by optimizing fiber length and dopant concentration of Ho³⁺-doped ZBLAN fiber, higher efficiency is expected. The spectrum of this laser was similar to that of the Fe²⁺:ZnSe Q-switched fiber laser. The peak wavelength, however, was found to be little longer than that of the Fe²⁺:ZnSe Q-switched fiber laser at the same pump power. Figure 14 shows the radio frequency (RF) spectrum of the graphene Q-switched fiber laser measured at a launched pump power of 1.5 W. The peak is at 92 KHz which corresponds to the repetition rate of the pulse train shown in Fig. 12(a). The SNR is about 53 dB, which is 13 dB larger than that of the Fe²⁺:ZnSe Q-switched laser. This consists with the low pulse amplitude fluctuation of the graphene Q-switched fiber laser.

 

Fig. 12 (a) Pulse train and (b) pulse envelop of graphene Q-switched Ho³⁺-doped ZBLAN fiber laser at launched pump power of 1.5 W.

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Fig. 13 (a) The average output power (red squares) and the pulse energy (blue dots), (b) the repetition rate (red squares) and the pulse duration (blue dots) of graphene Q-switched singly Ho³⁺-doped ZBLAN fiber laser as a function of the launched pump power.

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Fig. 14 RF spectrum of the graphene Q-switched Ho³⁺-doped ZBLAN fiber laser measured at a launched pump power of 1.5 W

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5. Discussion

Q-switched Ho³⁺-doped ZBLAN fiber lasing has been demonstrated by using Fe²⁺:ZnSe crystal and graphene saturable absorbers, respectively. Our experiments demonstrated that both Fe²⁺:ZnSe crystal and graphene are promising saturable absorbers for mid-IR fiber lasers at 3 μm. Because Fe²⁺:ZnSe crystal is a bulk component, a collimating and focusing setup has to be employed and consequently the laser cavity becomes complicated and needs careful alignment. However, Fe²⁺:ZnSe crystal can be used to generate very high energy mid-IR pulses because of its small saturation energy and large damage threshold. Since several 10-watt-level Er³⁺-doped ZBLAN fiber lasers and watt-level Ho³⁺-doped ZBLAN fiber lasers have been demonstrated, watt-level or 10-watt-level Q-switched mid-IR fiber lasers can be realized by using Fe²⁺:ZnSe crystal as the saturable absorber. The damage threshold of the graphene deposited fiber saturable absorber was measured to be 6.5 kW/cm², which is much smaller than that of Fe²⁺:ZnSe crystal. Therefore, graphene saturable absorber cannot be used for high power Q-switched mid-IR fiber lasers. However, graphene is generally in the form of nano-sized flakes that can be deposited on optical fiber mirror or deformed optical fiber [37] and a fiber optic saturable absorber can be fabricated. Thus compact, rugged, and reliable pulsed mid-IR fiber lasers can be developed with graphene deposited fiber devices.

In our experiments, the Fe²⁺:ZnSe Q-switched Ho³⁺-doped ZBLAN fiber laser has lower efficiency than the graphene Q-switched Ho³⁺-doped ZBLAN fiber laser. This is caused by the high cavity loss of the Fe²⁺:ZnSe Q-switched laser due to the Fresnel reflection of uncoated lenses and the crystal, as well as the aberrations in the sub-optimal free space collimating and focusing setup. The performance of the Fe²⁺:ZnSe Q-switched fiber laser can be improved significantly by using anti-reflection (AR) coated aspheric lenses and AR-coated Fe²⁺:ZnSe crystals. On the other hand, the flat cleaved end of the Ho³⁺-doped ZBLAN fiber acting as the output coupler has a reflectivity of only 4%, so significant improvement in efficiency is expected if a dichroic mirror that has proper reflectivity at the laser wavelength and high transmission at pump wavelength is used as the output coupler. Because Fe²⁺:ZnSe crystal has a high damage threshold and 30 mJ pulses at 2.94 μm has been obtained in a Q-switched Er:YAG laser using Fe²⁺:ZnSe saturable absorber [38], a Q-switched Ho³⁺-doped ZBLAN fiber laser with 10-W-level average power is possible due to the advantages of fiber lasers such as outstanding heat-dissipating capability and high efficiency. Because the pulse energy increases and the pulse width decreases with the increased pump power, much higher energy and shorter pulses are expected in Fe²⁺:ZnSe Q-switched Ho³⁺-doped or other rare-earth doped ZBLAN fiber lasers.

Since no free space optics was used in the laser cavity, the graphene Q-switched Ho³⁺-doped ZBLAN fiber laser has higher efficiency and better stability than the Fe²⁺:ZnSe Q-switched laser. Over 50 dB SNR of the RF spectrum indicates a highly stable operation of the graphene Q-switched fiber laser. Although this laser was found to be stable only at a range of pump powers, a graphene Q-switched fiber laser can be used as a compact and stable pulsed seed laser for a power amplification laser system. Most importantly, since graphene based mode-locked Yb³⁺-, Er³⁺-, and Tm³⁺-doped fiber lasers have been demonstrated at ~1 μm, 1.5 μm, and 2 μm, respectively [28,30,32], it is expected that graphene mode-locked fiber lasers at 3 μm can be achieved by using a few-layer graphene deposited fiber mirror and optimizing the fiber length and the output coupler.

6. Conclusion

In conclusion, we have demonstrated a passively Q-switched singly Ho³⁺-doped ZBLAN fiber laser using Fe²⁺-doped ZnSe crystal and graphene deposited fiber mirror as the saturable absorber, respectively. For Fe²⁺:ZnSe crystal Q-switched laser, 460 nJ pulses with a repetition rate of 104 kHz were obtained at the maximum available launched pump power of 2.2 W. Since the damage threshold of Fe²⁺:ZnSe is as large as 2 J/cm², much higher energy pulses at 3 μm can be achieved by optimizing the laser cavity and using more powerful pumps. For the graphene Q-switched laser, stable pulses were obtained only for launched pump power ranges of 0.6-1.5 W and noticeable time jitter was observed as the pump power was increased to the maximum available pump power of 2.2 W. Nevertheless, 1.1 μJ pulses with repetition rate of 92 kHz, corresponding to an average output power of 102 mW were obtained. Since graphene has been used to mode lock rare-earth doped silica fiber lasers in the near IR, it is expected that mode-locked RE-doped ZBLAN fiber laser at 3 μm can be developed in the near future by using optimizing the graphene deposited fiber mirror and the fiber laser cavity.