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Abstract
An all-fiber mid-infrared supercontinuum (MIR-SC) laser source with a power of over 2 W beyond 3.8 µm is demonstrated. The SC laser involves a silica-fiber-based SC laser as a pump source and a piece of fluoroindate (InF3) fiber as a nonlinear medium. The influence of pump pulse repetition rate on the SC characteristics is carefully studied. In the InF3 fiber, the pump pulse with spectral coverage of 1.9-2.6 µm is converted into MIR-SC with a broadest spectral coverage of 1.9-4.9 µm and a maximal average power of 11.8 W. Up to 2.18 W is measured in the spectral region beyond 3.8 µm, which, to the best of the authors’ knowledge, demonstrates the record SC power in this waveband to date.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-power mid-infrared supercontinuum (MIR-SC) laser sources covering the 3-5 µm atmospheric window have gained more and more attention. Soft-glass fibers including fluoride (such as ZrF4-based [1,2] and InF3-based [3]) and chalcogenide (such as As2S3-based [4] and As2Se3-based [5]) glass fibers have been developed for efficient MIR-SC generation with spectrum beyond 3 µm. Among these fibers, ZrF4 fibers have been used to generate high-power SC with average power up to 10.5 W [6], 15.2 W [7], 21.8 W [8] and 30.0 W [9], but the attenuation-induced loss limited the spectrum of those SCs to <4.2 µm. Recently, fluotellurite fiber was proposed as a robust nonlinear fiber and a maximal SC output power of 22.7 W was obtained [10], but the generated SC spectrum was limited to 3.95 µm in the long-wavelength side for such high-power SC. Chalcogenide fibers exhibit much lower phonon energy compared to fluoride fibers [11], therefore the chalcogenide-fiber-based SC lasers show much longer spectral edge, for instance, 6.4 µm [12] and 11 µm [5]. Nevertheless, the damage threshold of the chalcogenide fibers is much lower than those of fluoride fibers [11], which indicates the unfitness of chalcogenide fibers for high-power SC generation. In fact, the average power of the chalcogenide-fiber-based SC laser sources have been limited to hundreds of milliwatts [5,12,13]. Moreover, it has been the researchers’ continuing pursuit to improve the power ratio at long-wavelength region (e.g., >3.8 µm) under high-power operation. It is hard to achieve such goal using the frequently-used ZrF4-based fiber due to the sharply increased attenuation loss beyond 4 µm [7]. InF3 fibers have a medium phonon energy compared to ZrF4 fibers and chalcogenide fibers [11], and a much higher damage threshold compared to chalcogenide fibers [11], making itself an ideal candidate for high-power SC generation covering the waveband of 3-5 µm. Furthermore, InF3 fibers could be used to obtain high-power MIR-SC with high power ratio located at long-wavelength region owing to its transmission window extending over 5 µm.
Recent progress in soft-glass-fiber fabrication and postprocessing technique (mainly referring to the introduction of low-loss InF3 fibers, fusion splicing technique and endcapping technique) facilitates the broadband and watt-level SC generation with spectrum extending to 5 µm [3,14,15]. Very recently, a 11.3-W SC with 20-dB spectral coverage of 1.85-4.53 µm was achieved in a piece of InF3 fiber pumped by a picosecond laser system operating at 1956 nm [16]. However, the spectral intensity at long-wavelength side was limited and the power ratio beyond 3.8 µm was calculated to be 10.3% via a spectral integral, corresponding to power of 1.16 W. Such result indicates that the merits of the InF3 fibers over ZBLAN fibers have not been fully exploited yet. Hence, more work should be done to improve the long-wavelength power as well as the long-wavelength power ratio of the InF3-fiber-based SC laser sources.
In this letter, efforts are concentrated on the power enhancement of long-wavelength range (specially referring to the waveband of 3.8-5 µm) of fluoride-fiber based MIR-SC laser sources. To enable high-power and efficient MIR-SC generation, we use a piece of InF3 fiber as nonlinear medium and a single-mode thulium-doped SC laser operating at 1.9-2.6 µm waveband as pump source. Spectral and power characteristics of the obtained SC are investigated carefully under a fixed signal pulse duration of 1 ns and different signal pulse repetition rate (PRR) of 3/2/1.5 MHz. Owing to the low-loss fusion splicing technique and all-fiber-integrated configuration, up to 2.18 W out of the total SC power (11.8 W) is measured in the spectral region beyond 3.8 µm.
2. Experimental setup
The experimental setup is depicted in Fig. 1. The whole SC laser source is comprised of a 1550-nm seed laser, an amplification and frequency-shift module with output spectrum spanning the 1.9-2.6 µm region, a piece of single-mode transition fiber, and a piece of InF3 fiber. The seed laser is an electrically modulated pulsed distributed feedback laser operating at 1550 nm with variable PRR of 1.0-3.0 MHz and a pulse duration of 1 ns. The amplification and frequency-shift module is composed of an erbium-ytterbium-doped fiber amplifier (EYDFA), a piece of single-mode fiber (SMF1), and a single-mode thulium-doped fiber amplifier (TDFA). The specifications for the used optical fibers are shown in Table 1. The core/cladding diameter for the used EYDF and TDF is 10/125 and 10/130, respectively. The corresponding core/cladding numerical aperture (NA) is 0.12/0.46 and 0.15/0.46. The nominal cladding absorption coefficient of the used EYDF and TDF is 2.9 dB/m @915 nm and 4.5 dB/m @793 nm, respectively.
Fig. 1. Experimental setup of the SC laser. EYDFA, erbium-ytterbium-doped fiber amplifier; SMF, single mode fiber; TDFA, thulium-doped fiber amplifier.
Table 1. Specifications for the used optical fibers.
The used InF3 fiber has a core diameter of 7.5 µm and a core NA of 0.3. The single-mode cut-off wavelength is 2.9 µm and therefore the spectral components beyond 2.9 µm propagating in this fiber is in fundamental mode. We numerically modeled the dispersion characteristics of such fiber and the result is shown in Fig. 2(a). At the spectral range from 2.0 to 5.5 µm, the dispersion curve is ultra-flat (with dispersion among 7.0-9.6 ps·nm−1km−1), which is favorable for broadband and spectrally-flat MIR-SC generation. Furthermore, the calculated zero-dispersion wavelength (ZDW) is located at 1.64 µm. Since the pump wavelength of 2-µm waveband is in the anomalous regime and is far away from the ZDW, asymmetrical spectral extension towards the long-wavelength region is expected. To reduce the coupling loss as well as to ease the heat load at the splicing joint, a piece of SMF (SMF2) was introduced to bridge the TDF and the InF3 fiber. The core diameter and core NA of SMF2 are 7 µm and 0.2, respectively. The 11-meter-long InF3 fiber and the SMF2 were jointed by fusion splicing technique described in Ref. [17]. In order to test the coupling loss of the splicing joint, a continuous-wave fiber laser operating at 2000 nm was fusion-spliced to SMF2. The output power of the InF3 fiber with respect to that of the test light source was plotted in Fig. 2(b). The inset of Fig. 2(b) showed the microscopic image of the fusion splice joint. The slope of the power curve in Fig. 2(b) is 92.14%, indicating that the overall transmission loss of the splicing joint and the InF3 fiber is 0.36 dB. Considering the attenuation-induced loss of the used InF3 fiber of 0.33 dB, the achieved coupling loss of the splicing joint is 0.03 dB@2000 nm. The achieved fusion splicing joint was proof-tested with a total weight of 70 g, corresponding to a high strength of 8.1 kpsi. No damage was observed and no extra loss was introduced after the proof test. To protect the fiber tip from photodegradation during the high-power operation, an endcap made of multimode AlF3 fiber was fabricated. The used AlF3 fiber has a core diameter of 70 µm and a core NA of 0.2. In the experiment, both the TDFA and the InF3 fiber were placed on an aluminium plate water-cooled at 20 °C for efficient heat dissipation. The SC spectra were recorded by a grating-based monochromator with a liquid-nitrogen-cooled InSb detector. A thermopile power meter was used to measure the SC power. In order to analyze the power evolution of the interested waveband, SC power were filtered by long-pass filters with cut-off wavelength of 2.4 µm or 3.8 µm and measured by thermopile power meter.
Fig. 2. (a) Calculated dispersion of the used InF3 fiber and (b) transmission test result of the silica-InF3 fiber fusion splicing joint. Inset showed the fusion splice joint.
3. Results and discussions
As the laser pulses propagate in the SC laser source, a series of linear and nonlinear effects are involved. The seed pulse on nanosecond scale is firstly amplified in the EYDFA. Afterwards, in the SMF1, the nanosecond pulse is broken into ultrafast sub-pulses under mechanism of modulation instability, then red-shifted via soliton dynamics such as soliton fission and Raman soliton self-frequency shift (SSFS). Herein, a broadband SC spectrum of 1.5-2.3 µm is obtained. Next, the obtained SC spectrum is reshaped in the TDFA, where the spectral components at 1.9-2.1 µm range are amplified and those at 1.5-1.8 µm range are absorbed by the Tm3+ ions. Besides, the amplified signal at 1.9-2.1 µm range enhances the SSFS process and a spectrum covering the 1.9-2.6 µm waveband is obtained at the end of the TDF. As is shown in Fig. 1, the seed pulse operating at 1.55 µm is converted into broadband SC pulse ranging from 1.9 to 2.6 µm after the amplification and frequency-shift module. The detailed power and spectral characteristics of the TDFA was similar to that reported in our previous paper [9]. In SMF2, the nonlinear effects continue to play a role but no apparent spectral broadening are observed since the SC spectrum has been extended into the high-attenuation region of silica fiber. In the following InF3 fiber, drastic spectral extension into the MIR region occurs mainly under the mechanism of SSFS, and a broadband MIR-SC was obtained.
In the experiment, the signal PRR was set to 3 MHz at first. The spectral evolution of the SC laser is depicted in Fig. 3(a). Corresponding SC power and pump power (i.e., the output power of SMF2, labelled in parentheses) are shown in the legend. Since the output laser pulse of SMF2 is composed of numerous soliton pulses, the dominant nonlinear effect responsible for the spectral extension in InF3 fiber is soliton dynamics such as SSFS. Continual nonlinear spectral extension towards the MIR region was observed as the pump power increased. For pump power of 0.77 W, most SC power was located around 2.2 µm and the long wavelength edge (LWE, defined as the wavelength with spectral intensity equivalent to the noise floor in the long-wavelength side) was ∼3.0 µm. As the pump power was increased to 1.12 W, the LWE of the generated SC was extended to ∼3.2 µm, and the spectral intensity around 3 µm was enhanced. For the pump power higher than 4.92 W, the LWE of the generated SC exceeded 3.8 µm. Further increase in pump power brought further enhancement in spectral intensity in the long wavelength side. For pump power higher than 12.5 W, a spectral dip around 4.2 µm was clearly observed, which was caused by the absorption of the CO2 molecules in the monochromator and in the light path outside of the monochromator. For the highest pump power of 17.5 W, the LWE of the generated SC was expanded to 4.6 µm, and the corresponding output power was 11.7 W. To avoid possible endcap damage under high-power operation, no pump power higher than 17.5 W was tried under signal PRR of 3 MHz in the experiment. The SC power lying in the spectral region above 2.4 µm and above 3.8 µm was measured and shown in Fig. 3(b). Under maximal output power of 11.7 W, the power above 2.4 µm and 3.8 µm was 7.26 W and 0.75 W, i.e., corresponding to 62.1% and 6.4% of the total power. At this time, the spectral intensity corresponding to the wavelength above 3.8 µm is limited, which is mainly due to the inefficient accumulation of nonlinearity in the InF3 fiber.
Fig. 3. (a) Spectral evolution of the obtained SC laser for PRR of 3 MHz. Corresponding output powers of the SMF2 are labelled in parentheses. (b) Evolution of SC power lying in the spectral region above 2.4 µm and 3.8 µm.
In order to improve the power and the power ratio located in the long wavelength region (especially referring to the spectral components above 3.8 µm), optimization of increasing peak pump power was adopted. The signal PRR was reduced to 2 MHz and 1.5 MHz. The maximal pump power used in the experiment is 15.6 W and 18.3 W for the occasions with signal PRR of 2 MHz and 1.5 MHz. For these two occasions, the spectral evolution is essentially the same to that of 3 MHz; the main difference in spectral intensity is the obvious enhancement at long wavelength side. The spectra under maximal output power for the three signal PRRs are plotted in Fig. 4(a). As the signal PRR decreases, one could easily observe the intensity enhancement for the spectral range beyond 3 µm. For these three occasions, the LWE of the generated SC under maximal pump power in the experiment is 4.6 µm, 4.7 µm, and 4.9 µm, respectively. When the signal PRR was set to 1.5 MHz, the obtained SC under maximal pump power showed a 10-dB and 20-dB spectral coverage of the generated SC were 2.05-4.60 µm and 1.96-4.77 µm, respectively, which is labelled in Fig. 4(b).
Fig. 4. (a) Spectra comparison for different PRR of 3 MHz, 2 MHz, and 1.5 MHz, under pump power of 17.5 W, 15.6 W and 18.3 W, respectively. (b) Spectral details of the generated SC with signal PRR of 1.5 MHz and SC power of 11.8 W.
The SC power as well as the corresponding power conversion efficiency (defined as the ratio of SC power with respect to the output power of the SMF2) is plotted in Figs. 5(a) and 5(b), respectively. For every signal PRR, SC power goes up steadily as the output power of the SMF2 increases, but with a decreasing slope. Such decreasing slope indicates the dropping of power conversion efficiency shown in Fig. 5(b). The dropping of the power conversion efficiency mainly results from the peak-power-dependent nonlinear effects. On one hand, SSFS-based spectral extension is a lossy process, and higher peak pump power would result in stronger SSFS and thus more loss. On the other hand, the wavelength-dependent mode-field mismatch between the SMF2 and the InF3 fiber also plays a role on the drop of power conversion efficiency. In the experiment, the lowest power conversion efficiency is 64.4% (11.8 W out of 18.3 W), corresponding to the broadest SC spectrum shown in Fig. 4(b).
Fig. 5. (a) SC power and (b) power conversion efficiency with respect to the output power of the SMF2 for different PRRs.
Figures 6(a) and 6(b) plot the measured SC power and corresponding power ratio evolution beyond 2.4 µm and 3.8 µm for different signal PRRs. One could see from Fig. 6(a) that the average power located in such two wavebands grow steadily all the way through. In Fig. 6(b), both power ratios are improved when the peak pump power is increased. Herein, two methods were used to improve the peak pump power: (a) by increasing the average power while keeping signal PRR constant, and (b) by reducing the signal PRR while keeping the average pump power stable. A maximal output power of 9.03 W (76.7%) above 2400 nm and 2.18 W (18.5%) above 3800 nm is achieved when the InF3 fiber was pumped by maximal pump power of 18.3 W with signal PRR of 1.5 MHz. The SC power beyond 3.8 µm under signal PRR of 1.5 MHz (2.18 W) are improved approximately by a factor of 3 compared with that of 3 MHz (0.75 W). We are confident that the power ratio beyond 3.8 µm would be further improved for a lower signal PRR, but the power in such waveband would drop drastically. In the case of lower signal PRR, the limited output power of the TDFA would set a limit to the SC power beyond 3.8 µm.
Fig. 6. (a) Average power and (b) power ratio of spectral components beyond 2400 and 3800 nm with respect to the SC power for different PRRs.
4. Conclusion
In conclusion, an all-fiber mid-infrared supercontinuum laser source with 11.8 W output power and an overall spectral coverage of 1.9-4.9 µm is demonstrated. Up to 2.18 W (18.5%) out of the total SC power is measured in the spectral region beyond 3.8 µm, which is hard to achieve in the frequently-used fluorozircornate based fiber. MIR-SC lasers with higher output power and better durability would benefit from novel fiber endcaps such as those made of water resistance materials or equipped with humidity-resistant coatings. This work, to the best of the authors’ knowledge, not only extends the spectrum of 10-watt-level InF3-fiber-based SC lasers to the 5-µm waveband, but also reports the record output SC power beyond 3.8 µm generated in soft-glass fibers to date.
Funding
China Postdoctoral Science Foundation (2016M593022, 2017T100793); National Natural Science Foundation of China (61235008, 61405254, 61435009).
Disclosures
The authors declare no conflicts of interest.
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