1. Introduction

Mid-infrared (mid-IR) lasers have attracted a great deal of attention owing to their wide range of applications in such as medical diagnosis, chemical bond spectroscopy, gas sensing, missile countermeasures, and nonlinear mid-infrared photonics [1]. Various technologies have been exploited to produce mid-IR laser including quantum cascade lasers (QCLs), solid state lasers, difference frequency generation (DFG), sum frequency generation (SFG), optical parametric oscillator (OPO), etc [2]. With the development of infrared glass fiber, mid-IR fiber lasers has become a promising area of research, because of their unique advantages such as excellent beam quality, good heat dissipation, high efficiency and flexible design. Until recently the research in mid-IR fiber lasers has been mainly focused at waveband of 2~4 μm by employing Tm³⁺ (producing gain around 2.0 and 2.3 μm) [3, 4 ], Ho³⁺ (producing gain at about 2.1, 2.8, 3.0, 3.2, and 3.9 μm) [5–9 ], Er³⁺ (producing gain at 2.9 and 3.5 μm) [10, 11 ] and Dy³⁺ (producing gain at 2.9 µm) [12] doped ZBLAN fibers.

Compared to CW and mode-locked fiber lasers, Q-switched fiber lasers especially with tunable wavelength at 3 μm waveband can offer energetic and flexible pulses with ns or ms pulse duration that are desirable in some special applications such as gas sensing, plastic and polymer processing, laser scalpel, non-invasive medical diagnosis, infrared countermeasures and laser radar. Currently, the reported ~3 µm mid-IR Q-switched fiber lasers can be roughly divided into two major categories. The first one is active Q-switching where the acousto-optic modulator (AOM) is employed inside the cavity [13–17 ]. A 2.8 µm actively Q-switched Er³⁺-doped ZBLAN fiber laser with pulse duration of 90 ns and power of 10 W has been recently demonstrated [13]. Using a piece of singly Ho³⁺-doped ZBLAN fiber, we have achieved active Q-switching at 3.002 µm with pulse duration of 260 ns [14]. Also T. Hu et.al demonstrated an actively Q-switched Ho³⁺/Pr³⁺ co-doped ZBLAN fiber laser operating at 2.86 µm with pulse duration of 70 ns [16]. More recently, we have presented the wavelength tunability of actively Q-switched Ho³⁺-doped ZBLAN fiber laser by using an Au-coated plane ruled grating [17]. The second one is passive Q-switching in which the compact saturable absorber (SA) such as graphene, Fe²⁺:ZnSe crystal and semiconductor saturable absorber mirror (SESAM) is introduced into the cavity instead of the externally driven modulators [18–21 ]. This technique exhibits the advantages of compact structure and low cost compared to the active Q-switching. Graphene, as a novel kind of two-dimensional (2D) Dirac material, possesses a prominent broadband saturable absorption property owing to its zero bandgap structure and has been widely applied in Q-switching fiber lasers at 1 µm [22], 1.55 µm [23] and 2 µm [24] bands. Recently, a 2.78 µm graphene-based Q-switched pulsed Er³⁺-doped ZBLAN fiber laser with pulse duration of 2.9 µs and pulse energy of 1.67 µJ has been demonstrated [18]. However, relatively low damage threshold and modulation depth of graphene are not suited to achieving high energy Q-switched pulses. SESAM, as a type of mature commercial saturable absorber, has not only a remarkably excellent performance but also the ability of customizing some of its parameters e.g., modulation depth, non-saturable loss, recovery time, etc. However, the distributed Bragg reflector (DBR) in SESAM requires increased reflection layers for the long operation wavelength hence improving the cost and complicating the fabrication. For example, the DBR operating at 3 μm needs 25 layer AlAs/GaAs pairs with a thickness of 12 μm and 24 hours fabrication process at a growth rate of 0.5 μm/h. Very recently, we have presented a passively Q-switched Ho³⁺-doped ZBLAN fiber laser at 2.97 μm using a specifically designed broadband SESAM [19]. In this SESAM, an InAs absorber layer was sandwiched between an Au-coated mirror and a GaAs wafer to improve the operation band and damage threshold [19]. Furthermore, Fe²⁺:ZnSe crystal as a result of its high damage threshold (~2 J/cm²), large absorption cross-section and small saturation fluence has been also widely applied in high energy mid-IR Q-switching of especially solid-state lasers [25, 26 ]. As the transmission type SA, it is also very suitable for the pulsed fiber lasers with the ring cavity or linear cavity involving some specific cavity terminators such as FBG, CFBG, bulk grating. Recently, based on Fe²⁺:ZnSe crystal SA, a passively Q-switched 2.8 µm Er³⁺-doped ZBLAN fiber laser with pulse duration of 370 ns and pulse energy of 2.0 µJ [20] and 2.93 µm Ho³⁺-doped ZBLAN fiber laser with pulse duration of 0.82 µs and pulse energy of 460 nJ have been demonstrated [21].

In this paper, we report for the first time a tunable passively Q-switched ZBLAN fiber laser with the tuning range from 2919.1 nm to 3004.2 nm using a Fe²⁺:ZnSe crystal as the SA and a plane ruled grating as the wavelength tuning element. The performances e.g., pulse duration, repetition rate, output power, pulse energy and slope efficiency of this fiber laser were also investigated when tuning the wavelength and varying the initial transmission (IT) of the Fe²⁺:ZnSe crystal. The revealed rules are quite critical for designing the passively Q-switched fiber lasers in this wavelength region.

2. Experiment setup and results

The schematic diagram of the tunable passively Q-switched Ho³⁺-doped ZBLAN fiber laser using Fe²⁺:ZnSe crystal as the SA is shown in Fig. 1 . Here, the laser from two commercially available high power 1150 nm diode lasers (Eagleyard Photonics) was coupled into the gain fiber by polarization multiplexing via a polarized beam splitter (PBS) (Thorlabs) and focusing using an anti-reflection coated ZnSe objective lens (Innovation Photonics, LFO-5-6-0.975/3 µm, 0.25 NA) with a 6.0 mm focal length acting as the collimator for light out-coupled from the fiber core as well. A dichroic mirror (Thorlabs, 96%T@1150 nm, 95%T@ ~3 µm) was placed between the PBS and the ZnSe objective lens at an angle of 45° with respect to pump beam to output the laser emission. Afterwards a 3 µm filter with FWHM of 500 nm (Throlabs) was used to block the residual pump. The gain fiber was a piece of double-clad ZBLAN fiber (Fiberlabs) whose pump cladding is D-shaped with a diameter of 125 µm and a numerical aperture (NA), core has a diameter of 10 µm and a NA of 0.2. Herein, the Ho³⁺ ions dopant concentration was 1.5 mol.%. Employing the typical cutback method, the launch efficiency and fiber absorption coefficient were measured to be 80% and 0.36 m⁻¹, respectively. Thus the selected 7.0 m fiber could provide 92% pump absorption efficiency. One fiber end was perpendicularly cleaved acting as output coupler with the aid of 4% Fresnel reflection. The other fiber end was cleaved at an angle of 8° to avoid the parasitic lasing. The light from the angle cleaved end was collimated via an anti-reflection coated ZnSe objective lens and finally terminated by a Au-coated plane ruled grating (Thorlabs, 300 lines per mm, blaze wavelength λ_B = 3 µm, blaze angle θ _B = 26.7°) with a distance of 50 cm from the angle cleaved fiber end, which was used to provide the feedback while tuning the laser transition. A confocal scheme consisting of two same plano-covex CaF₂ lens (Thorlabs LA5370) with focal length of 40 mm was inserted between the ZnSe objective lens and the grating along the light path. A cube polycrystalline Fe²⁺:ZnSe crystal with 6.7 mm length, 6.7 mm width and 1.6 mm thickness (IPG photonics) as shown in the inset of Fig. 1 acting as the SA was placed at the focus of the confocal scheme to maximize the influence. Three Fe²⁺:ZnSe crystals with ITs of ~69%, ~79% and ~89% at 3.0 µm waveband were used respectively in the experiment. Note that they were all coated with anti-reflection films at 1~5 µm. An InAs detector with a response time of 2 ns driven by an in-house designed circuit was employed and connected with a 500 MHz bandwidth digital oscilloscope to record the pulse temporal waveform. An RF spectrum analyzer (Advantest R3267) with resolution bandwidth of 10 Hz to 100 MHz was utilized to measure the signal-to-noise ratio (SNR) of pulses. A monochromator with scanning resolution of 0.1 nm (Princeton instrument Acton SP2300) was employed to measure the laser spectrum.

 

Fig. 1 Experimental setup of tunable passively Q-switched Ho³⁺-doped ZBLAN fiber laser based on Fe²⁺:ZnSe crystal SA. Inset: Photograph of Fe²⁺:ZnSe crystal sample.

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Firstly, the Fe²⁺:ZnSe crystal with IT of 69% was employed and the grating was adjusted to maximize the output power. The oscillator started to operate at continuous wave (CW) regime at the launched pump power of 0.506 W. Then, stable Q-switched pulses were observed as the launched pump power was increased to 0.664 W and the Q-switching could be maintained to maximum pump power of 3.32 W. The typical pulse train and envelop of single pulse were shown in Fig. 2(a) . The pulse duration and repetition rate were measured to be 1.92 µs and 62.74 kHz, respectively. The right inset of Fig. 2(a) shows the RF spectrum of the output at a scanning range of 45 kHz with a resolution of 100 Hz. The high SNR of 51 dB indicates the laser operated at stable Q-switching regime. Figure 2(b) shows the output power, pulse energy, repetition rate, duration and peak power as a function of the launched pump power. Here, the pulse energy and peak power were estimated according to their corresponding output power, repetition rate and pulse duration owing to the high SNR. It is observed that the repetition rate increases from 24.83 kHz to 62.74 kHz and the pulse duration decrease from 5.05 µs to 1.92 µs, respectively as the pump power is increased from 0.664 W to 3.32 W, resulting from the faster population built-up and higher energy storage on ⁵I₆ level. No pulse duration saturation even at the maximum pump power was observed indicating the SA at this pump level was not fully bleached. Maximum output power of 266 mW was achieved at a slope efficiency of 9.02% corresponding to the pulse energy of 4.24 μJ, which were only limited by the available pump power.

 

Fig. 2 (a) Temporal domain pulse train at the launched pump power of 3.32 W using 69% IT Fe²⁺:ZnSe crystal SA. Inset: single pulse profile at a 26 µs scanning range and RF spectrum at a scanning range of 45 kHz. (b) Measured output power, repetition rate, pulse duration and calculated pulse energy and peak power as a function of the launched pump power.

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Then the laser transition was tuned by rotating the platform holding the plane ruled grating from 45′ in clockwise direction to 43′ in anticlockwise direction with respect to the position of delivering maximum output power. Figure 3(a) shows the optical spectra of Q-switched pulses at the maximum launched pump power of 3.32 W. Tuning range of ~85 nm spanning from 2919.1 nm to 3004.3 nm with a linewidth of ~1.0 nm over the entire tuning range was gained. In our experiment, the wavelength tuning resolution was mainly limited by the used rotating platform whose adjustment resolution was 2′. Therefore, the average tuning resolution under full-power operation was calculated to be ~2 nm. Note that the spectrum profiles of tuned wavelengths shown in a linear scale were all normalized for their visibilities. Further rotating the grating, once beyond the tuning edges, the stable Q-switching was ceased and changed to the CW operation with a suddenly varied wavelength of 2976.0 nm and substantially broadened FWHM bandwidth of 11.5 nm, as shown in Fig. 3(b). This is due to the sharply reduced gain outside of the tuning range leading to the failed competition with CW oscillation. In this case, the back feedback of CW oscillation came from the residual Fresnel reflection of the plano-convex CaF₂ lens close to the perpendicularly cleaved fiber end.

 

Fig. 3 (a) Measured output spectrum as tuning the ⁵I₆→⁵I₇ transition at the maximum launched pump power of 3.32 W. (b) Measured output power once the grating beyond tuning edge.

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Afterwards, either 79% or 89% IT Fe²⁺:ZnSe crystal was employed to replace the previous 69% IT one. Similar stable Q-switched pulses were observed for both 79% and 89% IT Fe²⁺:ZnSe crystals only with slightly shortened tuning ranges of 2920.1~3003.9 nm and 2919.9~3001.2 nm, respectively. It was because that higher intra-cavity gain was required to maintain the Q-switching for higher IT. Figure 4(a) shows the repetition rate and pulse duration as a function of the tuning wavelength using the Fe²⁺:ZnSe SAs with ITs of 69%, 79% and 89%, respectively, at the maximum launched pump power of 3.32 W. It is observed that the repetition rate decreases and the pulse duration increases near-linearly for all the three IT SAs as tuning towards long wavelength direction. This is resulted from the combined function of changes of gain, threshold, dispersion and modulation depth of SA with the varied wavelength. Moreover, slightly higher repetition rate is observed for higher IT as a result of the decreased threshold thus shortening the time for inversion population to arrive at the threshold level. On the other hand, the pulse duration also slightly increases with IT considering the fact that lower threshold leads to the decrease of accumulated population density on upper laser level thus longer time for generating a Q-switched pulse is required. Finally, the shortest pulse duration of 1.23 µs at the tuning edge of 2919.1 nm was achieved using 69% IT SA, while the maximum repetition rate of 98.5 kHz was obtained using the 89% IT SA at its tuning edge of 2919.9 nm. Figure 4(b) shows the output powers and pulse energies with respect to ITs of 69%, 79%, 89% as a function of tuning wavelength at the maximum pump power. It can be seen that for the three SAs with different ITs, both output power and pulse energy increase initially and then decrease with wavelength, well matching with the intra-cavity gain profile. However, we can see an about 20 nm gap between the peak positions of output power and pulse energy resulting from the decreased pulse repetition rate. It can be also seen that the output power and pulse energy are higher for SA with larger IT as it has a lower insertion loss. The maximum output power of 337.38 mW and pulse energy of 5.64 µJ were obtained at 2970.5 nm and 2991.5 nm by employing the Fe²⁺:ZnSe SA with IT of 89%. Figure 4(c) shows the relative slope efficiencies as a function of tuning wavelength. Similar to the variations of output powers, their slope efficiencies coherently increase initially to the maximum at ~2970.5 nm and then decrease with tuning wavelength. At ~2970.5 nm, the maximum slope efficiencies (optical-to-optical efficiencies) were calculated to be 9.02% (8.01%), 10.8% (9.58%) and 11.44% (10.16%) with respect to the ITs of 69%, 79% and 89%, respectively, indicating increasing IT benefits to the improvement of the slope efficiency. Note that the efficiencies are higher than the level in previous report [21] mainly owing to the use of anti-reflection coating of the lens and Fe²⁺:ZnSe crystal and enough pump absorption.

 

Fig. 4 (a) Repetition rates and pulse durations, (b) output powers and pulse energies and (c) slope efficiencies as a function of tuning wavelength when employing 69%, 79%, 89% IT Fe²⁺:ZnSe crystals as the SA, respectively at the maximum launched pump power of 3.32 W.

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3. Discussion

In this section, further performance optimizations on the passively Q-switched pulses in terms of pulse duration, repetition rate and tunable range were discussed. Also, the relative analysis on how to achieve mode locking was given.

In our experiment, the shortest pulse duration of 1.23 µs was achieved utilizing 69% IT Fe²⁺:ZnSe crystal at the short tuning edge of 2919.1 nm. Note that this pulse duration was longer than those demonstrated in Ref [20, 21 ]. mainly due to the use of longer gain fiber as a result of lower ions dopant concentration. It is well known that the limited pulse duration of a fixed Q-switched laser could be roughly estimated according to the following formula [27]:

On the other hand, it should be noticed that the mode-locking cannot be generated just by changing the position of the Fe²⁺:ZnSe crystal with respect to two plano-covex CaF₂ lens. In order to obtain CW mode locking, the following formula must be satisfied [29]:

4. Conclusion

We have demonstrated a wavelength tunable passively Q-switched Ho³⁺-doped ZBLAN fiber laser by using a Fe²⁺:ZnSe crystal as SA and a plane-ruled grating as the wavelength selector. The maximum wavelength tuning range of stable Q-switching was over 85 nm and the output linewidth was ~1 nm. It is concluded that the pulse duration increases and the repetition rate decreases with wavelength, and lower IT benefits to narrowing pulse duration while higher IT will improve the repetition rate, pulse energy and average power. The shortest pulse duration of 1.23 µs was achieved by using Fe²⁺:ZnSe crystal with IT of 69% at the short wavelength edge of available tuning range. The maximum output power of 337.38 mW and pulse energy of 5.64 µJ were achieved at 2970.5 nm and 2991.5 nm, respectively using 89% IT Fe²⁺:ZnSe crystal. Moreover, it is worth noting that further broadening of tuning range within the gain bandwidth (typical 2.75~3.05 µm) is expected if the residual Fresnel reflections from all intra-cavity components can be further decreased. However, larger scale wavelength tuning should explore new transitions, e.g., ⁴F_9/2→⁴I_9/2 transition of Er³⁺-doped ZBLAN fiber with ~700 nm emission bandwidth ranging from ~3200 nm to ~3900 nm [11].