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Abstract
Monolithic distributed Bragg reflector (DBR) cavity which directly integrates fiber Bragg gratings (FBGs) into the photosensitive RE-doped fibers is a promising configuration in constructing compact and efficient single frequency fiber lasers (SFFLs). Yet, the doping level of rare-earth (RE) ions has generally to be sacrificed in the classical Ge-photosensitized RE-doped silica fibers because of the dramatic refractive index increase caused by the introduction of Ge. Here, we demonstrate an approach to realize the trade-off between photosensitivity and RE doping concentration. We validate that the addition of a small amount of cerium (0.37wt.%) instead of Ge could photosensitize Yb3+-doped silica fiber (YDF), while maintaining fiber numerical aperture (NA) at 0.12 under a high 2.5-wt.% Yb doping level. Based on the short monolithic DBR cavity constructed by this germanium-free photosensitive highly YDF, a 1064 nm fiber laser with a 48.6% slope efficiency and an over 200 mW power on two orthogonally polarized modes could be realized. Further stable and linear-polarized 1064 nm SFFL is also demonstrated in a designed monolithic polarization maintaining cavity with an output power of 119 mW and an efficiency of 26.4%. Our results provide an alternative way to develop photosensitive highly RE-doped fibers towards monolithic laser cavity application.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Over the past decades, single frequency fiber lasers (SFFLs) opened widespread applications in high-precision spectroscopy, coherent beam combining and lidar because of their compact all-fiber configuration, narrow spectral linewidth and ultra-low noise [1–5]. Among these schemes of SFFLs, the distributed Bragg reflector (DBR) cavity is a widely adopted configuration for its simplicity and has been utilized in the wavelength regions from 0.9 to 2.9 µm [6–9]. The basic structure of DBR cavity includes a short section of rare earth (RE) doped fiber and a pair of passive narrow-band fiber Bragg gratings (FBGs). To achieve stable single-longitudinal-mode (SLM) operation, the length of DBR cavity has to be scaled down to centimeter-scale so that the longitudinal mode interval is larger enough—up to several gigahertz (GHz)—to exceeds the bandwidth of output FBG [1]. As a result, the length of the active fiber is limited to several centimeters or even millimeters and these high-gain active fibers are highly desired for SFFLs.
RE doped soft glass fibers have been recognized as suitable gain media in SFFLs because of the high unquenching doping levels of RE ions [10–13]. Yet, their low-loss and high-strength fusion splicing with silica fibers still remains an obstacle towards all-fiber configuration. Special asymmetric fusion splicing technique rather than common splicer has to be used because the inherent softening temperature and thermal expansion are quite different between soft glasses and silica glasses [14]. As a comparison, RE-doped silica fibers are irreplaceable gain media due to their advantages of easy fusion splicing, environmental adaptability and resistance, waveguide controllability. However, it is generally recognized that the efficiency and power of SFFLs by RE-doped silica fibers are lower than that of soft fibers due to the limited RE ions solubility. To solve this problem, different methods have been proposed in recent years to propel their SFFLs application. A straightforward one is to improve the RE doping concentration as much as possible via optimizing fiber fabrication technique. For instance, using a 1.2-cm Yb3+-doped silica fiber with absorption coefficient as high as 2400 dB/m at 976 nm, Li et al. have recently achieved a 642-mW 1064 nm DBR SFFL with a remarkable slope efficiency of 66.4% [7]. Furthermore, another promising strategy is inscribing FBGs direct into RE doped silica fiber to form a monolithic DBR cavity [15–19]. Such architecture allows a cavity constructed totally by active fibers and also avoids the fusion-loss and mode-field mismatch between active fibers and passive FBGs. Via femtosecond laser direct inscription (FLDI), Lv et al. have demonstrated a 1080-nm DBR SFFL by integrating FBGs into Yb3+-doped silica fiber, providing an important reference for monolithic single-frequency DBR fiber laser construction [19].
Apart from FLDI, establishing monolithic DBR cavity in RE-doped photosensitive fibers by mature UV laser phase mask technique have also obtained extensive research focus, as this approach can produce well-confined FBGs [20–23]. The classical approach to achieve fiber photosensitivity is co-doping several molar percentages germanium (Ge) into fiber core, which would lead to the dramatic increase of refractive index and fiber NA [23]. Consequently, the doping level of RE ions and the fiber absorption coefficient have to be sacrificed in these Ge-photosensitized RE-doped silica fibers for achieving single-mode transmission. In refer. [24], the fiber NA still reaches 0.174 even though the absorption of the photosensitive YDF is reduced to 60 dB/m, leading to a low laser output power below 1 mW. How to address the trade-off between photosensitivity and doping level of RE ions still remains to be the unsolved problem for the development of photosensitive RE-doped silica fiber towards monolithic SFFL application.
In this letter, we demonstrate that a small amount of cerium (0.37wt.%) instead of germanium could be used to achieve fiber photosensitivity while maintaining numerical aperture (NA) at 0.12 under a 2.5-wt.% Yb doping level. The UV induced index modulation reaches 0.8 × 10−4, which allows to inscribe FBG into this YDF with reflectivity over 99%. Furthermore, the fiber absorption of 1400 dB/m at 976 nm permits us to boost the laser output power and efficiency in a short fiber length. By integrating HR-FBG and LR-FBG into a short 3-cm YDF, we obtain a 1064 nm fiber laser with an output power over 200 mW and a conversion efficiency of 49%. Monolithic linear-polarized DBR SFFL is realized by a polarization-maintaining cavity design. Our results provide an alternative way to develop photosensitive highly RE-doped fibers towards monolithic laser cavity application.
2. Fiber fabrication and FBG inscription
All the Ce-doped fibers and YDF were prepared by the modified chemical vapor deposition (MCVD) method combined with the solution doping technique. For YDF, P and Al are also co-doped to sustain the doping homogeneity of Yb and adjust NA. The diameter of YDF is 125 µm with a 6-µm core size. The pump absorption coefficient at 976 nm is elevated to 1400 dB/m due to high Yb doping level, allowing efficient pump absorption under short fiber length. Detailed fiber fabrication process can be found in our previous work [25]. The inscription of FBGs was conducted by phase mask technique and a 248-nm UV laser [26]. A 10-mm silica phase mask was used to define the fringe patten. The phase mask has a period of 733 nm, corresponding to a fringe pattern period of (Λ) 366.5 nm in the fiber. The center wavelength (λ) of the inscribed FBGs thus can be defined around 1064 nm by the λ=2*Λ*neff, where the neff is the core refractive index. The 248-nm UV light source is a KrF excimer laser (Coherent, Excistar) with a pulse energy of 10 mJ, a repetition rate of 200 Hz and a pulse duration of 10 ns. The beam size of the 248-nm UV laser is 3 mm × 5 mm with uniform energy distribution, it was then expanded to 6 mm × 10 mm and focused onto the fiber. The reflectivity of the grating was monitored in situ during the irradiation by launching broadband light and measuring the reflected or transmitted light spectra with an optical spectrum analyzer (Yokogawa, AQ6370C).
3. Photosensitivity of Ce co-doped fiber
It is generally accepted that the refractive index changes of Ce co-doped fibers on UV exposure, i.e., photosensitivity, originate from the valency states variation of Ce, specifically, the transformation of Ce3+ to Ce4+ [27]. The 4f1-4f05d1 transitions of Ce3+ in silica glass have a broad UV absorption band peaking around 300 nm [28]. Under the exposure of UV light, e.g., 248 nm, the Ce3+ ions are inclined to lose one electron (e-) and transform into Ce4+. At the same time, the hole (h+) centers in the glass matrix would trap these released electrons. This process can be denoted as follow:
This mechanism enables FBGs formation in Ce photosensitized fibers when combined with UV laser phase mask technique. Especially, different to the Ge-photosensitized fibers, hydrogen loading process could be avoid which simplifies the FBG inscription process to a large extent. In order to explore the influence of Ce concentration on refractive index modulation, we first conduct the FBGs inscription by the fibers containing 3700, 6000, 10000 and 12000ppm Ce respectively, without loading hydrogen.
Figure 1(a) is the recorded reflectivity evolution of FBGs. It turns out the reflectivity of the imprinted FBGs in these Ce doped fibers could over 95%, and the grating reflectivity increase speed improves rapidly with increasing Ce concentration. Moreover, a turning point occurs once the Ce concentration exceeds 10000 ppm. According to the mode coupling theory [20], the UV induced refractive index modulation amplitude (Δn) of the gratings can be derived by reflectivity (R) via R = tanh2(ΔnπL/λ), where L is the grating length, λ is the center wavelength and equals to 1064 nm. As shown in Fig. 1(b), we calculated the maximum refractive index changes of the FBGs under different Ce concentrations, which increase from 0.8 × 10−4 to 2.0 × 10−4 and present a little decrease to 1.9 × 10−4 when Ce concentration reaches 12000ppm. Furthermore, we also tried to imprint FBG in the fiber without Ce doping, no any observable FBG formed confirms that the refractive index change stems from the Ce introduction. Regarding to the slight decrease of refractive index modulation in the fiber with 12000ppm Ce concentration, we speculate that the reason is the reduction of effective Ce3+ concentration because the Ce3+ ions tend to cluster at high concentration [27].
Fig. 1. (a) Reflectivity evolution of FBGs under different Ce concentrations. (b) Derived UV-induced refractive index modulation amplitude.
As pointed out in our previous study, the glass refractive index would increase linearly with Ce concentration [28]. Considering 3700 ppm (0.37wt.%) Ce doping enables FBG inscription, we fixed this Ce level in YDF preparation. Benefiting from the small amount of photosensitizer of Ce, 2.5-wt.% Yb, 4.6-wt.% P and 2.2-wt.% Al were doped into this YDF while maintaining the fiber NA at 0.12 [25]. To figure out the photosensitivity difference between this YDF and Ge doped photosensitive fiber, we selected a commercial Ge doped fiber (SMF-28) for comparison, the Ge concentration in this fiber is 3.53 mol% and equals to 6.0wt% [29]. These fibers were also loaded with hydrogen for 100 h under 4 MPa pressure. Both the hydrogen loaded and unloaded fibers are used for FBG inscribing. The hydrogenation mechanism for the Ge doped photosensitive fibers has been ascribed to the germanium defect related absorption-band around 240 nm and the photosensitivity presents an approximately linear relationship with Ge concentration [30,31]. As can be seen from Fig. 2(a), the grating reflectivity by hydrogen loaded commercial fiber increases rapidly, reaching over 99% within 1 minute, while saturable reflectivity for the unloaded commercial fiber is below 20%. Although the increase of grating reflectivity inscribed in the YDF is lower than the hydrogen loaded commercial fiber, the saturable reflectivity still exceeds 95%. Furthermore, we also compared the photosensitivity of the 1.0-wt% Ce doped fiber (without hydrogenation) with SMF-28 (with hydrogenation) in detail, as shown in Figure S1, the grating reflectivity presents nearly same increase tendency, indicating the Ce doped fiber could reach the same photosensitivity as SMF-28 (with hydrogenation) under a much lower doping concentration.
Fig. 2. (a) Reflectivity evolution of FBGs by commercial Ge-doped fiber (SMF-28) and YDF with loading hydrogen and without loading hydrogen. Inset in a photograph of FBG written in YDF. (b) Derived UV-induced refractive index modulation amplitude of the YDF. (c) Transmission spectra of FBGs by YDF with loading hydrogen and (d) without loading before and after annealing treatment.
Additionally, the photosensitivity of YDF was not significantly improved by hydrogenation, which is because, as mention above, the photosensitivity of Ce doped fibers originates from its valency states variation. Inset is the photograph of FBG written in YDF with 1-cm grating length. The derived Δn for YDF, as shown in Fig. 2(b), presents a the maximum Δn of 0.8 × 10−4 and guarantees the FBG inscription in this photosensitive YDF. One of the main concerns with photoinduced refractive index changes is their thermal stability [32]. In order to investigate the thermal reliability of FBGs inscribed in YDF, we annealed several samples in a vacuum oven at 120 °C for 10 hours. As shown in Fig. 2(c-d), it turns out that the grating reflectivity, especially the grating by the YDF without loading hydrogen, exhibits a nearly negligible decrease after annealing treatment, showing the high thermal reliability of the Ce-photosensitized fibers.
4. 1064 nm laser by monolithic non-PM DBR cavity
This photosensitivity allows us further integrating FBGs into this YDF to form a monolithic short DBR cavity. The experimental setup is shown in Fig. 3(a). To achieve SLM operation, we used a short 3-cm long YDF to build the monolithic cavity, where the HR-FBG and LR-FBG were written to its ends with a 1-cm interval. Both the gratings length of HR and LR is 1 cm. A cooling system controls the temperature of the laser cavity, which is housed in a copper tube. The cavity is forward-pumped by a single-mode 976-nm LD via a wavelength division multiplexer (WDM). Inset is the photography of cavity, where green up-conversion fluorescence in YDF can be observed when pumped by 976 nm LD. Figure 3(b) is the transmission spectra of HR an LR gratings. The dip of HR grating transmission is 20.23 dB, corresponding to a reflectivity over 99%. The inscribed LR grating has a reflectivity of 60.4% with a narrow 3-dB bandwidth of 0.08 nm. The output power characteristics is shown in Fig. 3(c). The maximum laser output power of 210 mW was achieved at the launched pump power of 457 mW, corresponding to a laser efficiency of 48.6%. The linear increase tendency shows the output power could be further elevated by higher pump power. It is worth noting that the conversion efficiency is still lower than that of DBR cavity with a pair of passive FBGs by the commercial YDF with 2400 dB/m absorption coefficient [7]. By using photosensitive YDF with higher absorption coefficient, the conversion efficiency of monolithic cavity would be further elevated.
Fig. 3. (a) Experimental setup of the monolithic DBR laser by photosensitive YDF. Inset is the photograph of the cavity. (b) Transmission spectra of HR-FBG and LR-FBG written in YDF. (c) The laser output power versus the launched pump power.
As shown in Fig. 4(a), the laser wavelength is 1064 nm with an optical signal-noise-ratio (OSNR) larger than 60 dB. The longitudinal mode characteristics of this monolithic DBR fiber laser was analyzed by a scanning Fabry–Perot (FP) interferometer (Thorlabs, SA200-8B) with a free spectral range (FSR) of 1.5 GHz and a resolution of 7.5 MHz. As illustrated in Fig. 4(b), we found that each main resonance is accompanied by a weak resonance with a frequency spacing of 64 MHz. A similar phenomenon was observed in Tm-doped DFB laser [33]. By applying uneven pressure to the cavity, these weak resonances could be removed temporarily under some conditions (Fig. 4(c)). Considering the calculated grating effective lengths of HR-FBG and LR-FBG are 0.37 cm and 0.15 cm [34], respectively, the effective cavity length is around 1.5 cm and corresponding longitudinal mode interval is 6.8 GHz. We, thus, infer that the weak and main resonances belong to the two orthogonal polarization modes in LR-FBG, respectively. From the 64-MHz frequency spacing, we can calculate the peak wavelength difference (Δλ) of the two reflection bands is around 2.4 × 10−4 nm. Because the resolution of the used optical spectrum analyzer is 0.02 nm and much larger than the peak wavelength difference, these two reflection peaks are not distinguished in the transmission spectrum of FBG. Additionally, based on the value ofΔλ, the birefringence in LR-FBG can be determined to be approximately 3.3 × 10−7 [35], which is at the same order of passive single-mode fiber. We speculate that the two peaks in LR-FBG is formed in inscription process and may be caused by the mechanical stress or heat accumulation.
Fig. 4. (a) 1064 nm laser output spectrum of monolithic DBR cavity. (b) Longitudinal mode characteristics measured by a scanning Fabry–Perot interferometer. (c) Longitudinal mode characteristics when applying uneven pressure to the monolithic cavity.
5. 1064 nm laser by monolithic PM DBR cavity
Aiming to eliminate these weak resonances, we further fabricated a polarization-maintaining YDF (PM YDF) to construct a PM DBR cavity and enable only one polarization oscillation. This PM YDF was fabricated starting from the same preform as YDF to give identical core composition and refractive index profile. The birefringence of this PM YDF is measured to be 4.2 × 10−4. The upgraded experimental setup is shown in Fig. 5(a). Insets are the photographs of the cross section of YDF and PM YDF. We inscribed the LR-FBG into PM YDF and HR-FBG into YDF, respectively, which then were fusion spliced together with a 1-cm PM YDF between the gratings. A PM-WDM was used to separate the pump and signal laser. To obtain narrower linewidth of LR-FBG, we further optimized focusing situation of UV beam in inscription process. The 3-dB bandwidth of LR-FBG inscribed in PM YDF is 0.063 nm. As shown in Fig. 5(b), we controlled the transmission peak of the fast axis of LR-FBG into the bandwidth of HR-FBG to make sure only one polarization oscillated. Adjusting the transmission peak of slow axis into the bandwidth of HR-FBG is also feasible. The scanning result of FPI is shown in Fig. 5(c), no any weak resonances exist and only two main resonances occur in one scanning period, indicating SLM operation from this PM cavity. Furthermore, the 1-h measurement results by FPI also show that the SLM operation could be maintained stably. The polarization extinction ratio was measured to be greater than 20 dB across all the output power levels. And these results also confirm the weak and main resonances in Fig. 4(b) indeed from two orthogonally polarized modes. As shown in Fig. 6(a), we measured the output power from this monolithic PM cavity with controlling the cavity temperature at 20 °C, the maximum output power is 119 mW with an efficiency of 26.4%. The output power is stable with a fluctuation of 1.2% (rms), as shown in the inset of Fig. 6(a). Compared to the monolithic DBR cavity in Fig. 3(a), we ascribe the decreased output power and efficiency to the elevated cavity loss, which might be caused by the following two aspects. First, the core of PM YDF is elliptical due to the introduction of stress rods, as shown in the inset of Fig. 5(a), leading to the mode-field mismatch between YDF and PM YDF. The second aspect is the transmission loss of PM YDF is 0.7 dB/m, which is much higher than the 0.02 dB/m of non-PM YDF. We infer that the 0.7 dB/m loss of the PM YDF is caused by partial crystallization of core glass, because the fiber drawing temperature of PM YDF is lower than that of YDF. It is believed that the laser slope efficiency of the monolithic PM cavity can be further improved by decreasing the loss of PM YDF and optimizing its core shape. To further demonstrate the laser stability, we measured a 2-hours RF beating spectra using an electrical spectrum analyzer (ESA, Keysight, N9020A). A 150-MHz acoustic optical modulator (AOM) was used to generate a frequency shift to avoid low-frequency interference. As shown in Fig. 6(b), no other longitudinal mode signal was observed, except for the zero frequency and the beating signal at 150 MHz, manifesting the long-term laser stability without any mode-hopping.
Fig. 5. (a) Experimental setup of the monolithic polarization-maintaining DBR SFFL. Insets are the fiber cross section of YDF and PM-YDF, respectively. (b) Transmission spectra of HR and LR FBGs. (c) Longitudinal mode characteristics measured by the FPI and its stability in 1 h.
Fig. 6. (a) Output power of PM DBR SFFL cavity, inset is the power stability. (b) RF beating spectra in two-hours span. (c) Measured laser linewidth by the self-heterodyne method. (d) RIN spectrum of the SFFL in a frequency range of 0–10 MHz.
Using a delayed self-heterodyne system with a 150-MHz AOM and 20-km delay line, we measured the heterodyne signal (Fig. 6(c)) under the maximum laser output power, which can be well fitted by a Lorentzian line shape. A 20-dB linewidth is 140 kHz identified by the Lorentzian fitting, which indicates that the laser linewidth of ∼7 kHz or less. The relative intensity noise (RIN) is measured by the ESA with a resolution bandwidth of 51 Hz. The laser power is attenuated from 119 mW to 0.15 mW to protect photodetector. The output RIN in the frequency range 0 of 10 MHz is shown in Fig. 6(d). The RIN spectrum is dominated by peaks at the relaxation oscillation frequency (ROF) in about 0.95 MHz with the RIN level of around -100 dB/Hz. After ROF, the intensity amplitude of output RIN decreases constantly to around -130 dB/Hz until frequency reaches to around 4 MHz.
6. Conclusions
In conclusion, we have demonstrated a germanium-free photosensitive highly Yb3+-doped silica fiber, which can achieve the trade-off between photosensitivity and Yb3+ doping concentration. A small amount introduction of Ce into this YDF permits adequate UV induced index modulation of 0.8 × 10−4 for FBG inscription. The reflectivity of these FBGs can exceed 95% and also present high thermal stability. Benefiting from the low Ce doping, the NA of YDF maintains at 0.12 under a 2.5wt.% Yb doping level, enabling a high fiber absorption coefficient of 1400 dB/m at 976 nm and efficient pump absorption. A 1064 nm DBR fiber laser with a 48.6% efficiency and an output power over 200 mW was achieved from a short 3-cm monolithic DBR cavity. Two orthogonally polarized modes were oscillated in this non-PM cavity. Linearly-polarized 1064 nm SFFL was realized by inscribing the LR-FBG into a PM YDF and adjusting its transmission peak of the fast axis into the bandwidth of HR-FBG. The output power and efficiency of linearly-polarized 1064 nm SFFL were decreased to 119 mW and 26.4%, respectively, because of the increased fiber loss (0.7 dB/m) and elliptical fiber core of the PM YDF. It is believed that the laser slope efficiency of the monolithic PM cavity can be further improved by decreasing the loss of PM YDF and optimizing its core shape. Our results provide a promising alternative way to develop photosensitive highly RE-doped fibers towards monolithic laser cavity application.
Funding
Science and Technology Commission of Shanghai Municipality (STCSM, SKLSFO2022-02); National Natural Science Foundation of China (62205356, 61975216); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0650000).
Disclosures
The authors declare no competing interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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