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Abstract
We report on a red-diode-clad-pumped continuous-wave (CW) and mode-locked Er:ZBLAN fiber laser at 3.5 µm for the first time. Numerical simulation shows that a heavily-doped Er:ZBLAN fiber is favorable for effective generation of 3.5 µm laser through 658 nm laser diode pumping. Using a 7.0 mol.% Er:ZBLAN fiber, CW output power of 203 mW was experimentally obtained at 3462 nm. By incorporating a home-made semiconductor saturable absorber mirror into the cavity, diode-pumped CW mode-locked 3.5 µm Er:ZBLAN fiber laser was first demonstrated with an average power of 19 mW, a pulse duration of 18.1 ps, and a repetition rate of 46 MHz. The research results show that red-diode-clad-pumping provides a simple and potential scheme for 3.5 µm CW and mode-locked Er:ZBLAN fiber laser.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-infrared (Mid-IR) lasers in the 3-5 µm spectral region are of tremendous interest in applications including molecule spectroscopy, medicine, and sensing because many gas and organic molecules have strong characteristic absorption peaks in this spectral region [1–4]. Rare-earth-doped fluoride fiber laser is an effective route to generating 3-5 µm laser [5–12]. Based on the transition of 4F9/2→4I9/2 of Er3+ ion, mid-IR 3.5 µm Er:ZBLAN fiber lasers have been demonstrated in various operation modes such as continuous-wave (CW) or wavelength-tuning [13–15], Q-switching [16], gain-switching [17], and mode-locking [18–20].
Currently 3.5 µm Er:ZBLAN fiber lasers necessitate the dual-wavelength pumping (DWP) with 976 nm and 1973nm pump sources [21]. In the DWP scheme, a 976 nm light first pumps the electrons to the metastable state 4I11/2 as virtual ground state, and then the electrons are pumped to laser upper level 4F9/2 by subsequent 1973nm pump and circulate among the laser upper level, laser lower level 4I9/2 and virtual ground state. For the DWP scheme, the 1973nm pump light needs to be coupled into the fiber core for efficient absorption. The core pumping inevitably increases the coupling difficulty. Prior to the DWP, direct pumping from real ground state 4I15/2 to the laser upper level using a 655-nm dye laser was tried in 1992 [22], in which the core pumping was required and only ∼2 mW output power was obtained. Later, direct pumping scheme has never been reported due to the lack of suitable pump source. Benefitting from the development of high-power red laser diode, direct pumping scheme deserves to be reinvestigated and is expected to become an alternative approach to excite the 3.5 µm Er:ZBLAN fiber laser. Compared to the existing DWP, diode-clad-pumping will not only simplify the pump source but also reduce the coupling difficulty, which will promote the application of 3.5 µm Er:ZBLAN fiber laser. This simple pumping scheme can be also used for Q-switched and mode-locked Er:ZBLAN fiber lasers, facilitating the development of compact and robust 3.5 µm pulsed laser sources.
In this Letter, we numerically and experimentally demonstrated the feasibility of red-diode-clad-pumped Er:ZBLAN fiber laser at 3.5 µm. Numerical simulation shows that a heavily-doped Er:ZBLAN fiber is favorable for 658 nm diode-clad-pumped 3.5 µm laser. Using a 7.0 mol.% Er:ZBLAN fiber as gain medium, a maximum output power of 203 mW was obtained by optimizing the output coupling. Based on a home-made superlattice-based SESAM, a red-diode-clad-pumped mode-locked Er:ZBLAN fiber laser at 3.5 µm was also realized, delivering an average power of 19 mW and pulse duration of 18.1 ps at 46 MHz. Our results show that red-diode-clad pumping is a simple and effective pump scheme for 3.5 µm CW and mode-locked Er:ZBLAN fiber laser.
2. Absorption spectrum of Er:ZBLAN bulk glass
We first measured the transmission spectrum of Er:ZBLAN bulk glass that was fabricated from fluoride fiber preform. The measured sample had a size of 4 × 4×8 mm3 with an Er-doping concentration of 1.0 mol.%. The absorption cross section, shown in Fig. 1, was calculated from the measured transmission spectrum with the following formula:
where σabs was the absorption cross section, L the length of measured sample (L = 8 mm), T(λ) the transmission spectrum of sample, and Nc the amount of Er3+ ion contained per unit volume (Nc = 1.6 × 1020 /cm3). In the range from 300 to 1200 nm, there are twelve absorption bands, corresponding to the transitions from the ground state 4I15/2 to the different excited states marked in Fig. 1. To achieve population inversion for the 3.5 µm laser, one can excite the electrons directly from ground state 4I15/2 to the upper laser energy level 4F9/2 by a 658 nm pump source. Nowadays, high-power red laser diodes are commercially available, well matching with the absorption band around 658 nm. The absorption cross section of Er:ZBLAN bulk glass was calculated to be 2.3 × 10−21 cm2 at 658 nm with a full width at half maximum (FWHM) of 19.5 nm. The large FWHM eliminates the temperature control requirement of laser diode. We also calculated the emission cross section using McCumber theory [23], which is 3.8 × 10−21 cm2 at 658 nm. Both the absorption cross section and emission cross section were used for the following numerical simulation.3. Numerical simulation
To optimize the design of cavity parameters such as the output coupling and fiber length, we carried out the numerical simulation based on the model presented in Ref. [24]. Spectroscopic parameters were kept identical to those reported in Ref. [24] except for the pump parameters. We chose three pieces of Er:ZBLAN fibers with different Er-doping concentrations and lengths in numerical simulation. Fluoride fiber possesses an absorption coefficient of 0.035 m−1 at 3.5 µm. Besides, each aspheric lens introduced a single-pass loss of 2.5%. To guarantee roughly equal pump absorption (∼76%), the 1.7 mol.% Er:ZBLAN fiber should absorb the pump light in a round trip while the 4.0 and 7.0 mol.% Er:ZBLAN fibers only need a single pass to absorb the pump light. Under a launched pump power of 8.6 W, we simulated the output power variation with the output coupling for the three pieces of Er:ZBLAN fibers, as shown in Fig. 2. Heavily-doped Er:ZBLAN fiber laser had the maximum output power and broadest tolerable range of output coupling. This could be attributed to the highest gain in a heavily-doped Er:ZBLAN fiber laser. In addition to a highest gain, the heavily-doped Er:ZBLAN fiber laser also had a lowest cost owing to a shortest gain fiber required. The optimal output coupling was ∼25% for efficient power extraction in the 7 mol.% Er:ZBLAN fiber laser.
Fig. 2. Dependence of the output power on the transmittance of output coupler for three pieces of Er:ZBLAN fibers with different doping concentrations.
4. Experimental setup and results
Guided by numerical simulation, we built the diode-clad-pumped 3.5 µm Er:ZBLAN fiber laser, as shown in Fig. 3. Two multimode laser diodes (Huaguang Optoelectronics, Ltd.) were used as pump source, which were combined by a polarized beam splitter. The pump source could deliver a total output power of 12 W at 658 nm and directly excite the electrons from the ground state to laser upper level (Inset of Fig. 3). Different from the DWP scheme where the 1973nm pump light is required to be coupled into fiber core, here the 658 nm pump light only needs to be coupled into the fiber cladding. Diode-clad-pumping scheme not only reduces the coupling difficulty but also simplifies the laser complexity. According to numerical simulation, we chose the 7.0 mol.% Er:ZBLAN fiber (Le Verre Fluoré) as the gain medium. It had a length of 2.15 m with a core of 15 µm (NA = 0.12) that was surrounded by a 260 µm cladding (NA > 0.46). The circular cladding was cut by two parallel flats separated by 240 µm. Using the cut-back method, the coupling efficiency and absorption coefficient were measured to be 87% and 2.7 dB/m at 658 nm for 7.0 mol.% Er:ZBLAN fiber, respectively. The front fiber tip was perpendicularly-cleaved and butted-against the output coupler (OC). The rear fiber tip was cleaved with an angle of ∼8°to reduce Fresnel reflection and avoid parasitic oscillation. Both ends of the fiber were mounted in an aluminum V-groove for efficient heat dissipation. In order to filter out the residual pump light and the laser emission at 2.8 µm, we adopted a dielectric mirror M2 with a high reflectivity at 3.5 µm (>99.5%) and high transmittance at 2.8 µm (T > 80%) and 658 nm (T > 90%). The employed semiconductor saturable absorber mirror (SESAM) for mode-locking operation had a modulation depth of 14%, recovery time of 7 ps, saturation fluence of 50 µJ/cm2, and non-saturation loss of 23.6% at 3.5 µm [25].
Fig. 3. Schematic of red-diode-clad-pumped CW and mode-locked Er:ZBLAN fiber laser at 3.5 µm. Inset: energy level diagram and the lifetimes of different energy levels. LD, 658 nm laser diode; PBS, polarized beam splitter; OC, output coupler; L, aspheric lens with a focal length of 12.7 mm; M1∼2, dielectric mirrors; SESAM, semiconductor saturable absorber mirror.
At first, we substituted the SESAM with an Au-coated mirror for CW laser test. Figure 4 shows CW output power of 3.5 µm laser as a function of the launched 658 nm pump power. We compared the output powers of 3.5 µm laser with three different OCs (TOC = 8%, 15%, and 26.5%). Experimental results showed that the optimal transmittance of OC was 26.5% in the CW regime, which agreed well with numerical simulation results. Using the OC with a transmittance of 26.5%, we obtained a maximum CW output power of 203 mW at 3462 nm under a pump power of 8.6 W. Compared to the Dye laser core-pumping, the laser diode clad-pumping increased the output power by two orders of magnitude. However, due to the large quantum defect, both the diode-pumped and the Dye laser-pumped 3.5 µm fluoride fiber lasers showed a low slope efficiency [22]. For achieving long-term stable operation, it is necessary to splice a protective endcap to the fiber tip because the fiber tip degradation was observed in current power level. The diode-clad-pumped fiber laser operated in the single transverse mode with a Gaussian beam profile, as shown in the inset of Fig. 4.
Fig. 4. Dependence of CW output power on the launched pump power for different OCs. Inset: CW output beam recorded by mid-IR CCD.
Fig. 5. (a) CW mode-locked pulse train in nanosecond time scale. (b) RF spectrum of the mode-locked pulse train with a resolution bandwidth (RBW) of 1 kHz.
A home-made SESAM was used to initiate and sustain the mode-locking of the Er:ZBLAN fiber laser. Considering an additional non-saturation loss (23.6%) of SESAM, we chose an OC with a transmittance of 15% for mode-locking experiment. Figure 5(a) shows a typical mode-locked pulse train with pulse interval of 21.7 ns, corresponding to a repetition rate of 46 MHz. It was confirmed by radio frequency (RF) spectrum with fundamental frequency of 46 MHz, as shown in Fig. 5(b). The signal-to-noise ratio (SNR) in RF spectrum reached 66.5 dB and no other sideband component was observed, indicating that stable mode-locking operation was obtained. Under the launched pump power of 8.6 W, the mode-locked fiber laser delivered an average output power of 19 mW. The low output power was attributed to the non-saturable loss of SESAM. Limited by the low output power and sensitivity of commercial autocorrelator at 3.5 µm, we failed to directly measure the pulse duration. In order to obtain the pulse duration, we amplified the output power via a dual-wavelength-pumped Er:ZBLAN fiber amplifier adapted from an existing mode-locked fiber laser [25], and built a high-sensitivity autocorrelator based on two-photon absorption and lock-in amplifier. Seeded by the mode-locked pulses, the average output power was amplified to 35 mW under the pump power of 2.2 W at 976 nm and pump power of 2.0 W at 1973nm. Assuming a Gaussian pulse profile, the measured pulse duration was 18.1 ps, as shown in Fig. 6(a). Because the amplified pulse had a peak power of only 42 W, the accumulated nonlinear phase shift in the 2.4 m Er:ZBLAN fiber amplifier was less than 0.02 rad and we could assume that the pulse duration barely changed in the process of amplification. The optical spectrum from amplifier had a FWHM of 2.9 nm centered at 3462 nm, as shown in Fig. 6(b). In the future, the mode-locked output power can be improved by a higher pump power and a lower-loss SESAM.
Fig. 6. (a) Autocorrelation trace of the mode-locked pulses. (b) The corresponding mode-locked pulse spectrum.
5. Conclusion
In conclusion, we report on a red-diode-clad-pumped CW and mode-locked Er:ZBLAN fiber laser at 3.5 µm for the first time. Effect of Er-doping concentration on the output power was investigated by numerical simulation, indicating that heavily-doped Er:ZBLAN fiber is more suitable to generate 3.5 µm laser when clad-pumped by 658 nm laser diode. Guided by numerical simulation, we built a red-diode-clad-pumped 3.5 µm Er:ZBLAN fiber laser. An output power of 203 mW was obtained in CW operation. Passive mode-locking was also realized by a mid-IR SESAM, with an average power of 19 mW and pulse duration of 18.1 ps at 46 MHz. The research results show that direct red-diode-cladding-pumping is a potential scheme for developing 3.5µm CW and ultrafast Er:ZBLAN fiber lasers.
Funding
National Natural Science Foundation of China (62005161, 61975120, 62075126, 91850203); Shanghai Municipal Education Commission (19CG12).
Disclosures
The authors declare no conflicts of interest.
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.
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