1. Introduction

The availability of radiation at mid-infrared (mid-IR) wavelengths in the 2.5–5 μm range is crucial for various applications as the corresponding photon energies overlap with the strong vibrational molecular resonances of most common constituents of atmospheric gases as well as those of liquid water [1, 2].

Fiber lasers are an efficient way to generate light at near and mid-infrared wavelengths. Due to their numerous advantages including high beam quality, flexible beam delivery and compact size, fiber-based laser systems have become widely used sources in the near-IR [3]. In contrast, mid-IR fiber laser technology is still in its infancy, mainly due to the non-existence of fiber coupled optical components to create all-fiber cavities, which severely limits their applicability.

The availability of an efficient wavelength selective component is crucial for the development of narrow-band tunable mid-IR laser sources which are for example required in spectroscopy. Using a bulk diffraction grating, Zhu et al. reported a 130 nm tunable CW Er:ZBLAN mid-IR fiber laser [4], Crawford et al. achieved a 150 nm tunablile CW Ho:ZBLAN fiber laser [5], while Li et al. demonstrated a 80 nm tunable Ho:ZBLAN pulsed laser in a similar setup [6]. However, the incorporation of a bulk grating into the laser cavity adds extra complexity and loss which makes the system less robust and more expensive. Consequently, the preferred method is to integrate FBGs into the laser cavity to realize true all-fiber lasers without bulk-optical components.

The feasibility of utilizing a femtosecond laser to directly inscribe FBGs into the core of an active optical fiber provides an elegant avenue for the development of future mid-IR all-fiber laser systems. In 1996, Davis et al. demonstrated that it was possible to directly inscribe a pathway of refractive index change into various glasses by translating the sample relative to the tight focus of a femtosecond laser beam [7]. The first FBG inscribed with femtosecond laser pulses was later demonstrated by Mihailov et al. by irradiating a standard Ge-doped silica fiber through a phase mask, which resulted in a strong Bragg resonance (−45 dB transmission dip) caused by an index modulation of Δn = 1.9×10⁻³ [8]. More recently, it was shown that the femtosecond laser phase mask technique could also be used to inscribe FBGs into active ZBLAN fibers to realize all-fiber single wavelength mid-IR laser systems around 3 μm [9, 10]. However, in all these reports, chemical or mechanical stripping of the polymer coating of the fiber was required before the inscription process, which reduces the mechanical strength of the fiber and/or requires complex recoating methods.

Consequently, Mihailov et al. inscribed Bragg gratings through the polymer coating of hydrogen loaded SMF-28 fiber [11], and Bernier et al. demonstrated the inscription of mechanically strong FBGs through the coating of various optical fibers with a diameter ranging from 50 to 250 μm [12]. While these demonstrations showed that it was possible to use ultrafast lasers for the fabrication of high mechanical strength FBGs, the reliance on specially designed phase masks still limits the flexibility and costs of this inscription method. In addition to the more conventional techniques for producing a modulated refractive index perturbation within the core of the optical fiber, which include the use of phase masks as well as interferometric techniques, femtosecond laser-inscription of FBGs has also successfully been demonstrated using a highly flexible point-by-point (PbP) approach, initially in a standard telecommunication fiber [13]. The major advantage of this technique is that it does not require a phase mask, yet an arbitrary longitudinal refractive index modulation profile can be inscribed into the core of an optical fiber simply by programming the desired pattern into the movement of a translation stage that moves the fiber through the focus of a femtosecond laser. Using the PbP technique, the gratings are typically written using relatively high laser pulse energies which result in the formation of an array of Type-II-IR microvoids within the core of the fiber and a correspondingly high refractive index contrast [14, 15]. In 2013, Hudson et al. demonstrated a single longitudinal mode fiber laser operating at 2914 nm in which, the narrow linewidth feedback was provided by a FBG that was directly inscribed into the Ho³⁺-Pr³⁺ co-doped fluoride fiber using femtosecond laser PbP inscription technique [16]. In this work it was also shown that in contrast to silica-glass fibers, the induced modifications in ZBLAN fibers do not constitute microvoids, but relatively strong Type-I-IR modifications.

An alternative direct write technique that overcomes some limitations of the PbP method is the continuous or modified core-scanning inscription technique which, in general, can lead to the formation of stronger gratings as the induced index modification can be extended through the entire cross section of the fiber core, even in large-mode area (LMA) fibers [17]. Moreover, the fabrication of more complex grating geometries like chirped FBGs becomes feasible as has been demonstrated in the case of silica-fibers [18]. However, the need to strip off the polymer coating of the fiber before the actual FBG fabrication remains an issue.

In this paper, we report on the inscription of a mechanically robust FBG into a 480 μm wide double-clad active ZBLAN fiber in a simple one-step process that neither requires a phase mask nor the removal of the polymer coating of the fiber. Based on this grating, we demonstrate a widely tunable narrow-linewidth CW mid-IR Ho³⁺-Pr³⁺ co-doped ZBLAN all-fiber laser with an output power of 0.29 W. By applying mechanical tension and compression to the extremely robust FBG, an overall tunability of 37 nm, spanning from 2850 nm to 2887 nm, was achieved.

2. Fiber Bragg grating fabrication

A Ti:Sapphire femtosecond laser (Spectra-Physics Hurricane) was used to inscribe the uniform FBG. The laser emits pulses at a repetition rate of 1 kHz at a wavelength of 800 nm with a duration of 115 fs. The fiber used in the experiments was a Ho³⁺-Pr³⁺ co-doped double clad ZBLAN fiber with a core diameter of 13 μm and a numerical aperture (NA) of 0.13 to ensure a single-mode operation at 2880 nm. The octagonal shaped first cladding had a diagonal length of 125 μm and the outer cladding diameter of 210 μm was surrounded by an acrylic coating of 480 μm as schematically shown in Fig. 1(a). To ensure that the fiber could be held perfectly straight during the inscription process, a custom designed V-groove was machined into a glass substrate using a picosecond laser. The fiber was then immersed in index matching oil, placed into the V-groove and subsequently covered with a 100 μm thick glass coverslip to eliminate aberrations caused by curved air-glass interfaces. This assembly was then mounted onto a programmable three-axis air-bearing translation stage to move the fiber in a rectangular pattern transversely through the focus of the laser beam. The amplitude of this rectangular modulation was adjusted to ensure that the induced index modification extends through the entire core of the fiber during the fabrication process.

 

Fig. 1 (a) Cross-sectional view of the active fiber and schematic representation of the process of focusing the inscription laser into its core (b) Femtosecond laser direct write setup and (c) a microscopic image of the uniform FBG (top-view).

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Femtosecond laser pulses with an energy of 270 nJ were focused inside the core of the optical fiber using a 40x dry-objective with an NA of 0.6 (Olympus). The diameter of the laser beam at the entrance of the objective (which had a focal length of 4.5 mm) was 4.8 mm, corresponding to an effective NA of 0.53. The FBG was written at a translation speed of 80 μm/s corresponding to a spatial overlap between consecutive pulses of approximately 92%. Under these fabrication conditions, uniform planes of Type-I modifications were inscribed into the fiber that covered the entire cross-section of the 13 μm core. Fig. 1 shows a schematic representation of the fabrication process as well as a differential interference contrast (DIC) microscopic image (top-view) of the femtosecond laser inscribed highly uniform pattern within the core. The physical length of the inscribed second-order grating was 15 mm (grating period = 1.97 μm). Note that a second-order grating was chosen as to avoid overlap between adjacent grating planes while maximizing the grating strength for a given length. The total time required to inscribe the gratings was approximately 2 hours.

3. Tunable fiber laser setup and results

The tunable fiber laser consisted of a 1 m long section of double-clad ZBLAN fiber, co-doped with Ho³⁺ and Pr³⁺ rare-earth ions (molar concentration of 35000 : 2500 ppm) and is depicted schematically in Fig. 2. A high-power multi-mode 1150 nm laser diode was used to pump the active fiber and a CaF₂ lens with a focal length of 20 mm (anti-reflection coated for 2900 nm) focused the pump beam into the fiber. In-between the laser diode and the pump coupling lens, a dichroic mirror with high reflectivity (98%) at the laser wavelength and high transmission (96%) at the pump wavelength was used to split the pump and signal wavelengths. The input end of the fiber was perpendicularly cleaved, and the 4% Fresnel reflection acted as a low reflectivity broadband mirror while feedback from the high reflectivity FBG (~50%) at the other end completed the Fabry-Perot laser cavity. This reflectivity value suggests that the induced refractive index contrast is approximately 1×10⁻⁴ [23]. Previous studies demonstrate that the reflectivity of the FBG could be increased further if required, by optimizing the parameter of the direct write procedure [17, 24]. Without applying any tension or compression to the FBG, the laser spectrum was initially centered at 2880 nm and had a linewidth of 105 pm. The output spectrum of the laser was captured using an optical spectrum analyzer with 100 pm resolution. In Fig. 3, the output power of the laser with respect to absorbed pump power is shown, indicating a slope efficiency of 17% and a threshold of 66 mW. The efficiency could be improved further by using a longer length of the active fiber and/or by inscribing a broadband (chirped) grating at the input end of the fiber with a reflectivity larger than 4% provided by Fresnel reflection. Fig. 3. inset shows an image of the laser beam profile having a mode-field diameter (MFD) of 17.8 μm, which indicates the absence of any transverse modes other than the fundamental LP₀₁ mode.

 

Fig. 2 Experimental setup for the tension and compression tuning of the FBG.

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Fig. 3 Laser output power with respect to absorbed pump power. The inset shows the laser beam profile with an MFD of 17.8 μm.

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Applying tension or compression to the FBG changes the effective period Λ of the grating and thus its Bragg wavelength (mλ_B = 2Λn_eff, where n_eff is the effective index of refraction of the core mode and m is the order of the grating). The deformation in the grating period is proportional to the amplitude of the perturbation force and this results in tuning of the lasing wavelength. However, the tuning of lasing wavelength by applying tension or compression is limited by the mechanical strength of the fiber since it is susceptible to fracture under even moderate tensile forces if any microvoids, scratches and cracks are present. To mechanically stretch the FBG for wavelength tuning, we fixed one end of the grating while the other end was glued onto a linear translation stage using a fast-drying epoxy as shown in Fig. 2. When axial tension was applied to the FBG by adjusting the micrometer screw, the associated mechanical stretching of the fiber produced a linear red-shift of the laser wavelength of 1.29 nm/millistrain $(m\mathit{ϵ}=\frac{\mathrm{\Delta }z}{L}\times {10}^3)$. The tensile tuning range was limited to 7 nm as the optical fiber experienced structural failure beyond 31.29 N. This applied tension was estimated by considering Young’s modulus of ZBLAN glass as 53 GPa [19]. For comparison, we also performed tensile tuning tests under identical conditions but using a fiber that was stripped of its polymer coating before FBG inscription. In this case, the observation reveals the fiber stress fracture limit was enhanced by 62% when the inscription was formed through the polymer coating compared to that of a stripped fiber, making this inscription method more robust and promising for practical devices.

The fiber strength limitations related to tension tuning are considerably relaxed if a compression force is applied as the compressive strength for materials is generally higher than their tensile strength [20]. For compression measurements, it is essential to compress the fiber strictly along its axis to avoid buckling. The fiber section containing the FBG was therefore fixed to an elastic steel beam of length L = 16.5 cm, width W = 1.2 cm and thickness H = 0.1 cm. This steel beam was placed in-between a fixed stage and a movable translation stage that was controlled by a micrometer screw [21]. To prevent vertical movement and buckling of the fiber during the tuning, the FBG was secured within a flexible substrate (thickness D ~ 4 mm) having a low Young’s modulus of 1.03 GPa. Upon curing, the substrate adheres to the elastic steel beam which has a high Young’s modulus of 180 GPa. Note that utmost care was taken to avoid micro-bubbles inside the flexible substrate during the curing process as adverse effects could otherwise result in the micro-bending of the fiber while tuning. The inward translation of the movable block deformed the beam into an arc shape, where the displacement of the movable block Δz, normalized by the length of the beam L, is related to the arc angle θ by [22],

As shown in Fig. 4. compression measurements indicate a maximum blue-shift of the laser wavelength of 30 nm. This is equivalent to a tuning range that is more than 4 times larger compared to the tuning range achieved by stretch-tuning, because of the higher compression strength of the ZBLAN fiber over its tensile strength. The laser linewidth across the entire tuning range was less than 112 pm. Fig. 5. illustrates the wavelength shift of the CW laser with respect to the displacement of the micrometer screw, normalized to the length of the beam, Δz/L demonstrating continuous tunability from 2850 nm to 2887 nm.

 

Fig. 4 Combined spectra of the shifted laser output peaks.

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Fig. 5 Measured tuning range of the laser source.

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4. Conclusion

In conclusion, to the best of our knowledge, this is the first demonstration of the inscription of a mechanically strong FBG through the polymer coating of a doped double-clad fluoride fiber without a phase mask. A stable, 37 nm wavelength tunable FBG-based narrow-linewidth continuous wave fiber laser was demonstrated by applying tension and compression to the FBG. This demonstration paves way to the development of robust, narrow-linewidth, all-fiber mid-IR lasers for practical applications outside the laboratory.