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Abstract
We demonstrate a watt-level mid-infrared supercontinuum source, with the spectrum covering the infrared region from 2 to 6.5 µm, in an all-fiber structured laser transmission system. To further improve the SC spectral bandwidth, power and system compactness in the follow-up As2S3 fiber, we theoretically and experimentally explored some knotty problems that would potentially result in the As2S3 fiber end-facet failure and low SC output power during the high-power butt-coupling process and proposed an optimal coupling distance on the premise of the safety of As2S3 fiber end face. In addition, we also built a multi-pulse pumping model for the first time to more precisely estimate the SC spectral evolution in As2S3 fiber. This work will give an important reference to someone who is working on the all-fiber structured, high-power mid- and far-infrared supercontinuum source.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mid-infrared supercontinuum (MIR-SC) laser source have attracted much attention in diversified applications including molecular fingerprinting [1], bio-medicine [2], hyper-spectral microscopy [3], and national defense [4]. Fiber-based MIR-SC offer both superior brightness and good beam quality [5]. In recent years, thanks to the rapid development of the high-power fiber lasers, the various characteristics of fiber-based MIR-SC lasers, such as spectral bandwidth, flatness and output power have been well improved, laying a good foundation for their practical applications. Recently, the reported high-power mid-infrared SC are mainly based on fluorite fibers [6], with corresponding SC spectral range below 4 µm, due to the multi-phonon absorption edge above 4 µm of fluorite fibers [7]. However, the spectrum above 4 µm covers many important molecules footprint wavelength, such as CO, NO and so on, so it’s key in some poisonous gas monitoring applications. When SC lasers are used as active illumination sources such as hyper spectral imaging and remote sensing [8], wide spectral range (above 4.5 µm) and high SC power are indicators that cannot be ignored. Tapered or micro-structured nonlinear fibers exhibit a limitation on power capacity though the output SC spectrum is extremely wide [9–13]. In recent years, many impressive results (listed in Table 1) on all-fiber structured high-power MIR-SC laser are mainly achieved in high-nonlinear fluoride or chalcogenide glass (ChG) fiber pumped by high-power Er/Tm-doped fiber amplifiers seeded by distributed feedback laser (DFBL). In 2012, Gattass et al. reported a MIR-SC laser with spectral long-wavelength edge extended to 4.8 µm, corresponding to an average power of 565 mW in 2 m-long step-index As2S3 fiber with core diameter of 10 µm [14]. In 2017, Yin et al. extended the MIR-SC spectrum beyond 5 µm in cascaded ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) and step-index As2S3 fibers, corresponding to an output power of 97.1 mW [15]. In the next year, Francis et al. demonstrated a MIR-SC source with high spectral flatness spanning from 1.9 to 6.4 µm in concatenated step-index InF3 and As2Se3 fibers [16]; in the same year, Ramon et al. optimized the each section of the SC system, boosting the output power to 1.39 W in a 4 m-long As2S3 step index fiber, and 417 mW in the follow-up As2Se3 fiber, extending the far-infrared edge to 11 µm [17]. Since then, there have been rare reports on watt-level all-fiber structured MIR-SC generation in ChG fibers. In addition, it is worth mentioning that Robichaud et al. used a piece of As2Se3 fiber with an anti-reflection coating on the fiber tips, boosting the SC laser output power to 825 mW [18], which gives us a new way on far-infrared SC power promotion in As2Se3 fiber. However, it’s a pity that none of the above-mentioned results give readers a detailed analysis on some key issues of coupling between the heterogeneous material soft glass fibers in high-power condition and the mid-, far-infrared SC spectral evolution in multi-pulse or multi-soliton pump condition.
Table 1. Characteristics of ChG All-fiber Structured MIR-SC Laser Sourcesa
In this paper, we demonstrate a watt-level MIR-SC laser source with spectrum spanning from 2 to 6.5 µm in a tandem structured nonlinear fiber transmission system. This system consists of a home-made ZBLAN fiber-based high-power SC source with ultra-high spectral flatness and a piece of commercial step-index As2S3 fiber. The detailed information on the pump source scheme and related parameters are presented in Ref. [19]. For better spectral optimization in the follow-up fiber, we made some improvements on our previous reported ZBLAN fiber-based SC laser source, including long wavelength components power ratio and total average output power, which proved of great importance on the further spectral broadening in the following stage. In addition, we also explored the damage mechanism and coupling efficiency between As2S3 and ZBLAN fiber under different coupling distances theoretically and experimentally. Furthermore, for a better estimation on pulse evolution and spectral broadening in soft-glass fiber, we built a multi-pulse pumping model with different pulse durations and pulse intervals for the first time, and the simulated curve shows a well agreement with the experimental results. Thanks to the above-mentioned works, we experimentally achieved a MIR-SC system with an output power of 1.13 W, and the corresponding spectrum was broadened to 6.5 µm (at 30 dB level).
2. Experimental setup
Figure 1 depicts the experimental setup of the all-fiber structured 2-6 µm MIR-SC generation in cascaded ZBLAN and As2S3 step-index fiber. Similar to our previous reported scheme, we adopted a 1550 nm pulsed DFBL (Connet, VENUS-M) with 1 ns pulse duration as the seed source. An effective wavelength converting process was adopted to transfer 1.55 µm pulses to 2 µm by modulation instability and Raman soliton self-frequency shift (SSFS) effect in 25 m-long SM-28 fiber. And the transferring efficiency from 1.55 µm to 2 µm was improved by self-absorption amplification in 4 m-long single-mode thulium-doped fiber; a two-stage double-clad single-mode thulium-doped fiber amplifier (SM-TDFA) was used as the pre-amplifier and the near-infrared pre-spectral shaper; the following stage 25/400 large-mode-area thulium-doped fiber amplifier (LMA-TDF) was adopted to scale the power to several tens of watts; in the MIR-SC generation stage, a 7 m-long ZBLAN fiber is fusion spliced to the pigtail of mode-field adaptor (MFA) with loss of 0.15 dB, the obtained ultra-flattened spectrum covering from 2-4 µm. To dissipate the excess heat in high-power pumping condition and work at a suitable temperature, we well-coiled the gain fiber of each stage amplifier and ZBLAN fiber on an aluminum water-cooled plate (temperature set to 15 °C). All the fusion spliced points were fixed on it with high refractive index UV glue. The ZBLAN fiber-based SC source was then acted as the pump light, injected the SC light into a 4 m-long As2S3 fiber with core/cladding diameter of 9/170 µm. The end of ZBLAN fiber was angle cleaved to 8 degrees using a fiber cleaver (Vytran, LDC401A) so as to reduce laser optical reflection while As2S3 was flat cleaved to improve the coupling efficiency. We carefully adjusted the position of the fibers using a pair of 5-axis stages in combination with a microscope, which was used to observe the coupling position of the two fibers and real-time monitor whether any damage to fiber tips occurred or not. Furthermore, a MIR Laser Beam Profiler (Ophir-Spiricon, SP90405) was adopted to ensure the beam profile in a fundamental operation. During the infrared spectral measurement, we adopted a liquid-nitrogen-cooled HgCdTe detector (Zolix Inc.) with the aid of a grating-based monochromator (Zolix Inc.), a lock-in amplifier (Standford Inc.) and a chopper (Zolix Inc.). The spectral resolution of this optical spectrum analyzer is 0.4 nm in 0.8-5 µm, 0.8 nm in 5-10 µm and 1.6 nm in 10-22 µm; its available spectral range covering 0.8-22 µm. A set of long-pass filters were properly adopted to avoid any overlap of the diffraction light of short wavelength with longer wavelength light in spectral measurement.
Fig. 1. Schematic diagram of the all-fiber structured 2-6 µm MIR-SC laser source. (SMF: single-mode fiber; SM-TDF: single-mode Tm-doped fiber; MFA: mode-field adapter; LMA-TDF: large-mode-area thulium-doped fiber.)
3. Results and discussion
In our previous work, the generated ZBLAN fiber-based SC showed high spectral flatness and high-power stability, but the power ratio at long wavelength region has been experimentally proved to be inefficient for further spectral broadening in chalcogenide glass fiber. In the latest scheme, we lowered the repetition rate to 600 kHz to improve peak power, lengthened the ZBLAN fiber to 7 m to intensify the nonlinearity and optimized the silica-ZBLAN fusion splicing point loss to 0.12 dB to get a better wavelength conversion efficiency. Benefiting from the above-mentioned measures, as can be seen in Fig. 2, the SC spectrum gradually broadens to 4.2 µm, the intrinsic high attenuation edge of ZBLAN fiber, with a maximum output power of 5.03 W.
The ZBLAN fiber-based SC laser is then acts as the pump source injected into a 4 m-long As2S3 step-index fiber. To ensure an optimal butt-coupling position between the two fiber end faces, we adopted the MIR Laser Beam Profiler to observe the real-time beam profile in the process of fiber coupling, finding that any slight axial offset will lead to a deterioration of the beam quality. As for SC power enhancement, in principle, the smaller the coupling distance, the higher the coupling efficiency. However, we found that it would be easier to result in the damage of the fiber tip under the watt-level pump power when the coupling distance was set to several micrometers. The inset in Fig. 3 shows the damaged end face of the As2S3 fiber.
Fig. 3. The calculated power density at different coupling distances, measured efficiency (ME), calculated efficiency (CE) and the damaged distances at different coupling powers (red crosses) (the inset shows the damaged fiber end face of the As2S3 fiber).
To further analyze this problem, we referenced the Maxwell electromagnetic field theory and the predecessors’ analysis on single mode fiber coupling [20–24]. On these bases, considering the interference between the incident and reflected light field at the fiber end facet, we did some modifications on these theories and further optimized the single-mode fiber-to-fiber optical propagation model to achieve a more precise estimation on the coupling efficiency at different fiber coupling distances.
The modified light transmission coefficient T(δ) is given by Eq. (1):
The relationship between optical power density on the end face of the receiving fiber and fiber coupling distance is also investigated. Since the receiving fiber core size is small enough, we assumed that the energy of the incident laser is evenly distributed on the fiber core. Then the optical power density is given by Eq. (2):
Where P, NA and w0 are the average power, numerical aperture and the beam waist of the source fiber, respectively.Moreover, we carried out a series of experiments to verify the above theoretical analyzed results. The curves between coupling efficiency, optical power density and coupling distances are depicted in Fig. 3, showing that a minor adjustment of coupling distance has little effect on the transmission efficiency, yet results in significant variation of power density. Here, since only single reflection between fiber tips was considered in the theoretical model, the measured coupling efficiency decreased slightly faster than the theoretically calculated one with the increase of coupling distance. As shown in Fig. 3, we could also see that the power density of zero coupling distance is more than 10 times that of 40 microns of coupling distance. Red crosses in this figure indicates the damaged distances with the increase of the input power, which seems a downward trend of power handling capacity. In theory, the fiber damage threshold doesn’t change with the variation of coupling distance. But when the fiber tips get closer and closer to each other, the reflected energy from the As2S3 fiber (high Fresnel reflectivity of 17%) is more easily to interfere with the incident light and then form a stronger energy field with the increase of the pump power, which will considerably increase the optical power density on fiber end faces and then accelerate the breakdown mechanism of the As2S3 fiber to a certain extent [25]. Thus, the possibility of As2S3 fiber tip failure can be considerably reduced by increasing the butt-coupling distance between two fiber end faces. By a mass of experiments, we find that while the coupling distance is adjusted to near 40 µm, the As2S3 fiber could withstand up to 4.8 W of input power with a slightly sacrifice of coupling efficiency. Moreover, a butt-coupling device is well designed to achieve a long-term stability of coupling between the ZBLAN and As2S3 fiber (see Fig. 4(b)). In this device, the two fibers were fixed onto a pair of aluminum V-grooves by UV adhesive with high robustness and then aligned on a pair of 5-axis stages with high precision (see Fig. 4(a)). For the realization of long-term connection stability and high mechanical strength, we fix the aligned V-grooves onto two quartz rods using UV adhesive (see Fig. 4(b)). Besides, the overall device will be protected and cooled by the high-purity nitrogen gas in the process of experiment.
Fig. 4. (a) Micro-structure picture of butt coupling between ZBLAN and As2S3 fiber. (b) Integrated device for butt-coupling.
Figure 5(a) depicts the spectral output with different length of the As2S3 fiber at repetition rate of 600 kHz and pump power of 4.8 W. As can be seen in the figure, with the increase of the As2S3 fiber length, result from the high-peak-power soliton red-shift effect, the fiber length extension can provide longer distance for effective pulses and material interaction, and will accumulate more nonlinearity, so the corresponding spectral broadening is more intensive; meanwhile, the spectral flatness has been considerably improved. The red dashed line represents the wavelength dependent attenuation of the As2S3 fiber. The dips at 2.7 µm and 4 µm in the As2S3 SC spectrum are caused by the absorption of O-H and S-H bonds, respectively [26].
Fig. 5. (a) Output spectra from As2S3 fiber with different fiber length (solid lines) and loss of the As2S3 fiber (short dash line). (b) Output spectra with the variation of input power in 4 m-long As2S3 fiber. Legend in (b) shows the SC output power versus the input power.
As for the SC power improvement, Fig. 5(b) shows the output spectra and the corresponding output power from 4 m-long As2S3 fiber with different pump power. Since the As2S3 fiber exhibits all-normal dispersion up to the wavelength of 6.5 µm, the SC spectra from ZBLAN fiber was mainly broadened by self-phase modulation and stimulated Raman scattering effects. In this figure, a remarkable spectral broadening could be observed in the 4 m-long As2S3 fiber as the pump power was increased to 4.85 W. We can see that even at a relatively low pump power of 1.21 W, spectrum at long wavelength edge could extend above 4 µm; with the further increase of the pump power, more and more spectral energy was transferred to the As2S3 fiber high attenuation region (around 4 µm) with the aid of stimulated Raman scattering effect and was then weakened. Besides, the nonlinear coefficient of the As2S3 fiber is decreasing when the spectrum gradually broadened towards the long wavelength region. Eventually, the broadening of the spectrum stopped at 6.5 µm. We believe that the above-mentioned two factors are the main restrictions on the further extension of the MIR-SC spectrum.
Figure 6(a) shows the power distribution of different spectral components and the transmission efficiency of the overall spectrum in As2S3 fiber with the increase of the incident power. When the pump power was increased to 4.85 W, the total output power from As2S3 fiber exceeds 1.13 W, more than half of the total power extends beyond 4 µm, and power beyond 5 µm was also scaled up to 156 mW. The red curve shows a decline of the transmission efficiency of overall spectrum, from 47% to 23%, with the increase of the incident power. Figure 6(b) depicts the corresponding beam profile at the maximum output power, which shows a nearly Gaussian distribution, indicating a good beam quality.
Fig. 6. (a) Power distribution of different spectral components and transmission efficiency of the overall spectrum in As2S3 fiber vary with the incident power. (b) SC laser beam profile with output power of 1.13 W.
4. Theoretical analysis of SC generation
To explain the nonlinear effect in the optical fiber, we used a time-domain generalized nonlinear Schrodinger equation [27] for the evolution of A(z,T):
In the simulation work of spectral evolution in ZBLAN fiber, the central wavelength of the input pulse was set at 2 µm, since in the TDFA system the pulses with high peak power are mainly located at 2 µm wavelength region. Figure 7(a) shows the comparison between the experimental spectrum and the simulated result in the 7 m-long ZBLAN fiber. Except for a slightly narrower spectral bandwidth, the simulated result is well agreed with the experimental one.
Fig. 7. (a) Comparison between the simulated (red line) and the measured (blue line) spectrum from 7 m-long ZBLAN fiber. (b) Comparison between the simulated (red line) and the measured (blue line) spectrum output from 4 m-long As2S3 fiber, the inset shows the input pulses set in the simulation.
For SC generation in As2S3 fiber, considering the complexity of the pulse evolution in this system including the soliton fission and the SSFS effects in anomalous dispersion region, which will lead to the multi-pulse effect, we use a multi-pulse pumping mechanism for the first time to approximately define the ZBLAN fiber-based SC that incident into the arsenic sulfide fiber. Specifically, considering the high computational complexity of multi-pulse pump condition, after a mass of optimization work, we defined the input laser pulse as a bunch of three pulse trains, which contains three pulses in total, with pulse durations of 1.5 ps, 150 fs and 15 fs, respectively; the corresponding pulse peak power of each pulse is 8 kW, 20 kW and 16 kW. In addition, we also found that the intervals among these pulses also have some effects on the output SC spectral characteristics. So, for further precision promotion, in this calculation work, we set the optimal time delays between the pulses of 15 fs to 150 fs as 2.5 ps and 150 fs to 1.5 ps as 10 ps. (see the inset in Fig. 7(b)). Figure 7(b) shows the comparison between the experimental output spectrum and the simulated spectrum in a 4 m-long As2S3 fiber, indicating a fine consistency. In addition, since the multi-pulse transmission in the As2S3 fiber, our imprecise assumption of the SC laser featured parameter in the simulation may lead to some modulation structure in the gained SC spectrum when compared with the measured one.
When compared with the reported results on all-fiber structured high-power mid-, far-infrared supercontinuum, our work is quite competitive in spectral width as well as in total output power, the details were listed in Table 1.
5. Conclusions
In conclusion, a watt-level SC source covering 2-6.5 µm band is achieved in cascaded ZBLAN and As2S3 step-index fiber nonlinear transmission system. To boost the SC power in the As2S3 fiber and explore a feasible solution on ZBLAN and ChG fiber butt-coupling in high-power condition, we analyzed the damage mechanism and coupling efficiency of As2S3 fiber at different coupling distances theoretically and experimentally. Finally, we proposed a safe fiber coupling distance of 40 µm, under which watt-level SC output power in As2S3 fiber could be achieved. Moreover, we, for the first time, built a multi-pulse pumping model and achieved a fine simulation on multi-pulse or multi-soliton pumped nonlinear fiber SC generation process. And the simulated spectrum is well agreed with the experimental one. Thanks to the above-mentioned efforts, at last, we improved the SC output power up to 1.13 W and the corresponding spectral long wavelength edge to 6.5 µm at 30 dB level in a segment of 4 m-long As2S3 fiber. However, our works did not radically resolve the low coupling efficiency and damage issue of the ChG fiber butt coupling in high-power condition. Thus, the ChG fiber end face with anti-reflection structure or coating is of great necessary in our further high-power mid-, far-infrared SC generation work.
Funding
National Natural Science Foundation of China (61627815, 61875094, 61905126); Natural Science Foundation of Zhejiang Province (LY19F050006); China Postdoctoral Science Foundation (2018M642386).
Disclosures
The author declares no conflicts of interest.
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