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Abstract
We report the femtosecond laser inscription of fiber Bragg gratings (FBGs) in an Er-doped fluoride glass fiber used for lasing at a mid-infrared wavelength of 2.8 µm. The lasing evolution is discussed in terms of the FBG reflectivity, wavelength transition to the Bragg wavelength, and output power of the mid-infrared fiber laser. A first-order and short (2.5-mm-long) Bragg grating showed a reflectivity of 97%, because of a laser-induced index modulation of 1.1 × 10−3. This modulation was sufficient to saturate this system’s output power. The laser oscillator is designed to lase in the atmospheric window of 2799–2800 nm slope. Further, this oscillator’s efficiency is as high as 29.1% for the launched pump power over the range of 0.4–4.6 W and at a lasing wavelength of 2799.7 nm. This oscillator also exhibited a FWHM bandwidth of 0.12 nm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Plane-by-plane femtosecond machining has been used to fabricate fiber Bragg gratings (FBGs) in various types of fibers due to its ability to control the grating parameters, such as the Bragg pitch, grating length, grating width, and tilt angle over a wide range of values [1–5]. This technique is considered to be a strong candidate for application in an all-fiber mid-infrared (MIR) laser system [6–11]. The series of studies by Bernier and Faucher demonstrates an efficient 2.8-µm MIR Er-doped fluoride glass fiber with a highly reflective FBG inscribed using a femtosecond laser [9–15]. For an 8-mm-long FBG, the reflectivity reached 95–99% at a wavelength of around 2825 nm, and the slope efficiency of the passively cooled laser reached 35.4% with respect to the absorbed pump power [9,10], which is the highest efficiency reported to date for heavily doped fibers, to the best of our knowledge. However, the (polymer) second cladding had to be removed before the inscription of the FBGs, which reduces the mechanical strength of the fiber and requires recoating the jacket and additional treatment after the inscription process. To reduce the deterioration in mechanical strength, Paradis et al. and Bharathan et al. demonstrated a tunable MIR laser with FBGs inscribed through the second cladding [8,15,16]. The inscription was performed using a 800-nm femtosecond laser and a dry objective lens of NA = 0.53, where the refractive index change of the laser-irradiated area was estimated to be approximately 1 × 10−4 [8]. When using a low-NA lens, the first cladding will be modified along the path of the femtosecond laser beam [2,17], especially near the laser focal point, and the volume of the modified region will increase with decreasing NA. Due to the low-NA objective, only a second-order grating (grating pitch Λ = 1.97 µm) was fabricated to avoid overlap between adjacent planes [8]. For micromachining with a femtosecond laser beam, since a shorter-wavelength laser can more easily achieve energy transfer to electrons through efficient multi-photon absorption [18–21], shorter wavelengths are known to be more efficient for fabricating microstructures [21]. In addition, taking into account the diffraction limit, shorter-wavelength lasers are superior to longer-wavelength lasers. On the other hand, a near ultraviolet (NUV) laser should be absolutely avoided because almost all polymers strongly absorb NUV light. The fabrication of a Bragg grating structure using a visible (517 nm) femtosecond laser has been demonstrated for a range of grating parameters in various optical fibers such as silica fiber [1,2] photosensitive fiber [3], CYTOP polymer fiber [4] and fluoride fiber [5]. Owing to the flexible plane-by-plane inscription method, the lasing evolution can be monitored by varying the FBG reflectivity during the grating fabrication. This strategy was reported in [11], where the output power of the 3.55-µm laser was monitored by varying the FBG reflectivity of the output coupler in a forward-pump configuration. It is poorly understood how the lasing evolution progresses, which is necessary for the development of a practical MIR fiber laser.
In this paper, the second harmonic (513 nm) of a femtosecond laser is used to inscribe first-order Bragg gratings into an Er-doped fluoride glass fiber used for a 2.8-µm CW fiber laser. The FBGs are inscribed through a polymer second cladding with a 1.4 NA oil-immersion objective lens to avoid deteriorating the mechanical strength of the fiber. The lasing evolution process is discussed in terms of the FBG reflectivity, wavelength transition to the Bragg wavelength, and output power during FBG inscription to investigate the lasing evolution and to optimize the FBG fabrication. To the best of our knowledge, this is the first comprehensive report of in situ measurements of lasing evolution for fiber lasers.
2. Experimental setup and FBG fabrication
The fiber used in the present study was purchased from FiberLabs, Inc. The double-clad fiber has a 6 mol.% ErF3-doped ZBLAN core, an undoped square cladding, and a polyacrylate coating, which functions as the second cladding. In consideration of the pump absorption and effective pump inner-cladding mode mixing, the square cladding was employed to efficiently absorb the pump light [22]. The core/first/second cladding are 15/240/400 µm in diameter and 0.12/0.52 in NA. Figure 1 shows the normalized intensity of measured and estimated pump absorption for the fiber.
Fig. 1 Intensity of fluorescence from Er-doped fluoride fiber (open circles) and light transmitted through the fiber (closed circles). Laser diode at a wavelength of 975.7 nm was used as the light source.
The absorption was evaluated by measuring the fluorescence intensity and transmitted pump light through fluorescence measurements and the cut-back technique [23], using a laser diode (LD) source locked at 975.7 nm. The absorption curves obtained by the two methods were approximated for fiber lengths between 3.0 and 7.0 m. When the fiber length was 7.0 m, the fluorescence and transmitted light intensities were a few percent. Based on the results, we used a 7-m-long fiber in the subsequent experiments.
Figure 2 shows the experimental setup used for FBG inscription.
The pump laser was used for backward pumping to monitor the laser output power during FBG inscription. The residual pump and MIR laser beam reflected from the FBG were separated by a dichroic mirror (DM). A grating structure was inscribed with a laser pulse train (513 nm, 10 kHz, 400 fs, 150 nJ) focused in the vicinity of the fiber core using an oil-immersion objective lens with an NA of 1.4 (60 × with a working distance of 170 µm). The objective lens focuses the beam to a spot size of 0.68 µm at the focal point with a Rayleigh length of 0.60 µm, as calculated based on the beam diameter of 2.3 mm at the lens incident plane and the beam quality factor of 1.2. A fiber sample was mounted on a three-dimensional motor-controlled stage, and the laser focus position was adjusted by moving the stage in the X, Y and Z directions. Grating planes were inscribed by scanning the fiber sample in the Y and Z directions across the fiber core at constant velocity with a frequency of 90 Hz and 1 Hz, respectively, such that the entire core was irradiated. The fiber was translated downwards with respect to the laser focal point in the Z direction. The grating pitch Λ was set at 0.947 µm to give a Bragg wavelength λB of 2799.5 nm, based on the Bragg condition [24].
3. FBG reflectivity
FBG reflectivity was evaluated by recording transmission spectra at a grating length of 0.5–3.0 mm. The transmission spectra were obtained using a super-continuum light source (Leukos, STM-200-MIR) with the bandwidth of 800–4000 nm and an optical spectrum analyzer (OSA, Thorlabs OSA205). The OSA resolution was set to 350 pm for this measurement.
Figure 3 shows transmission spectra for a series of grating lengths.
The peak wavelengths λ were found to be 2799.7 nm (Λ = 0.947 µm). As can be seen from the graph, the FBG reflectivity increases with increasing grating length. The reflectivity increases to 15.2 dB at L = 2.5 mm, including an optical loss by gratings. According to the measurements, the FBG fabricated in our experiment is shorter than FBGs reported by other researchers (R = 19.9 dB at L = 6.6 mm [5], R = 13–15.2 dB at L = 8 mm [9], L = 15 mm [8]). The optical loss induced by FBG inscription was estimated to be approximately 0.22–0.46 dB from the spectrum (L = 2.5 mm), which is higher than previously reported losses [9], because of the higher laser fluence [25]. Since such an optical loss could lead to undesirable scattering of pump light in the case of a forward-pump configuration, the index change should be optimized so that more efficient cavities can be designed. The FBGs with a wavelength of around 2799.7 nm were fabricated in subsequent experiments. Figure 4 shows measured and calculated FBG reflectivity as a function of grating length L.
The FBG reflectivity RB was simply modeled using the following equation:
where δn is the refractive index change, RF is the Fresnel reflectivity, and T is the FBG transmittance. Light reflected by the Fresnel reflection at the fiber end (right side of Fig. 2) is expressed as TRF. The calculation is in good agreement with the experimental result when the calculation is performed assuming δn = 1.1 × 10−3 at λ = 2799.7 nm. The index change is comparable to the value reported in [12]. The slight difference in the range of 2.0–3.0 mm could be explained by the remaining optical power distribution of light transmitted in cladding modes, which is not reflected by the grating planes. According to the experimental FBG reflectivity, an FBG with a grating length of 2.0–2.5 mm will be large enough to be used as a high reflector. Even with a short grating length of 2.5 mm, the FBG reflectivity is found to be 97% in the MIR range centered at 2799.7 nm due to the high index change of the grating planes.4. Lasing evolution process in terms of wavelength and output power
Two fiber samples with a Bragg grating structure were prepared to monitor laser wavelength and output power, measured respectively by the OSA and power meter (PM, Gentec, XLP12-3S-H2-D0), during FBG inscription. Figure 5 shows peak wavelength as a function of the grating length, where the peaks were obtained using the peak tracking mode of the OSA.
Fig. 5 Wavelength transition during FBG inscription. The figure insets show emission spectra at a grating length of 0.0, 1.5 and 2.5 mm. Resolution: 200 pm.
Note that only the largest peak is displayed on the graph. At the beginning of FBG inscription (0 ≤ L ≤ 0.3 mm), the wavelength is unstable as it depends on the amplified spontaneous emission (ASE; see ASE spectrum in Fig. 5, L = 0.0 mm) because Fresnel reflection by the grating planes is dominant over Bragg reflection. Some ASE remains at L = 1.5 mm (RB = 91%), but it completely disappears at L = 2.5 mm (RB = 99%). From the ASE spectra of L = 0.0 mm and 1.5 mm, we confirmed that the depression spikes were caused by atmospheric absorption. The peak wavelength begins to shift stepwise toward the Bragg wavelength λB after L = 0.3. Beyond L = 0.62 mm (RB = 42%), the peak wavelength becomes stable at 2799.7 nm, which approximately corresponds to λB. It was experimentally found that the laser wavelength at 2799.7 nm (within the atmospheric window) was designed and obtained reproducibly as numerically predicted. The irradiation time and total time required to inscribe a 2.4-mm FBG were approximately 45 and 90 min, respectively.
To confirm the growth and saturation of the output power, the laser power was measured during FBG inscription, where the pump power was set at 2.08 W. Figure 6 shows the evolution of the output power and the calculated reflectivity RB as a function of grating length.
Fig. 6 Output power at a pump power of 2 W and the calculated reflectivity RB as a function of grating length. The inset shows the output power at the beginning of FBG inscription.
It was assumed that the peak wavelength and reflectivity of the FBG prepared for this measurement approximately correspond to λB = 2799.7 nm and RB, respectively. Note that the spikes in the solid line at shorter grating lengths were caused by automatic switching of the dynamic range of the power meter used. Saturation of the output power was defined by the variation of the average power (at 100 µm intervals) becoming approximately zero. The FBG inscription was complete at L = 2.4 mm, after which the output power was saturated. As can be seen from Fig. 6, the output power becomes greater with increasing grating length, where the saturation power was found to be 0.46 W at L = 2.0 mm, corresponding to an FBG reflectivity of RB = 97%. When the wavelength peak reached λB = 2799.7 nm (L = 0.62 mm), the power was as high as 0.42 W, which is approximately equal to 90% saturation. Thus, it was experimentally confirmed that the growth and saturation of output power can be observed by monitoring the output power during FBG inscription.
To evaluate the laser characteristics, the slope efficiency and wavelength were obtained with the fiber sample used in the last experiment. The laser setup was arranged in the backward-pumping configuration shown in Fig. 2. The laser threshold was linearly extrapolated to approximately 400 mW. The laser slope efficiency reached 29% with respect to the launched pump power in the range of 0.4–4.6 W, as shown in Fig. 7.
The pump power was restricted to 5 W to avoid damage to the fiber end, which had no protective treatment [26,27]. By accounting for the residual pump power of 2.2% and Fresnel reflection of 3.8% of the launched pump power, the slope efficiency with respect to the absorbed pump power was estimated to be 30.9%, which is less than the Stokes efficiency of 34.8% [28]. We also confirmed that the power of 2.8-µm laser light transmitted through the 2.4-mm FBG was 5.5% of the output power. The emission spectrum of the laser was measured for an output power of 0.50 W, as presented in Fig. 8, showing the wavelength centered at 2799.7 nm with a FWHM of less than 0.12 nm.
Fig. 8 Laser emission spectrum for an output power of 0.5 W. The figure inset shows the narrowband spectrum measured with a resolution of 200 pm.
From a practical perspective, the thermal stability was also investigated using another fiber sample with a FBG prepared with same method. Figure 9 shows the thermal characteristics of the FBG with a reflection peak at 2799.3 nm, showing the stability of Fig. 9(a) output power and Fig. 9(b) wavelength as a function of temperature.
Fig. 9 Thermal characteristics of (a) laser output power and (b) wavelength as a function of temperature. The fiber was heated until the wavelength was stable.
The thermal characteristics were obtained during laser emission, where the fiber was heated until the wavelength was stable at each temperature. It was confirmed that the output power did not change before 90 °C. After the temperature exceeded 100 °C, the power began to drop and became unstable because the induced index modification gradually decreased [12]. Note that laser emission was unstable after 270 °C, which can be seen from Figs. 9(a) and (b), and this could be due to heat deterioration of the polymer coating. The temperature dependence of gratings is therefore determined at 15.9 pm/°C in the temperature range of 25–260 °C. There were no significant changes in the lasing wavelength and reflectivity of the FBG prepared using our method after 2000 hours of operating at an output power of 5 W, the fiber ends were protected by a CaF2 endcap and a water-cooled fiber mount was used to reduce thermal effects [27]. The protective CaF2 endcap was used for extended operation to protect the fiber ends from heat damage and deliquescence. During laser emission, the temperature at the FBG was found to be less than 30 °C, as monitored through a thermal imaging camera (FLIR E4).
5. Conclusions
We presented in situ measurements of lasing evolution in terms of FBG reflectivity, laser wavelength, and output power during FBG inscription for CW laser operation at 2.8 µm. Using the second harmonic (513 nm) of a femtosecond laser and a high NA objective, we demonstrated the fabrication of first-order FBGs for the grating plane in a fluoride fiber, through the polymer second cladding. The comprehensive measurements of lasing evolution provided here will contribute to more efficient FBG fabrications.
Funding
Amada Foundation for Metal Work Technology; JST A-STEP stage III type B (NexTEP B); Japan Science and Technology Agency “Adaptable and Seamless Technology Transfer Program through Target-driven R&D”.
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