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Abstract
We demonstrated an ultra-broadband supercontinuum (SC) laser source with a wavelength range spanning the near-infrared (NIR) to mid-infrared (MIR) region. The SC spectrum was generated in a very short piece of highly nonlinear silica fiber (HNLF) which has a zero-dispersion wavelength (ZDW) of 1.55 µm. The pump source used has a spectral coverage of 1.5∼2.4 µm which covers the ZDW of HNLF, resulting in a dramatic blue and red shift of the spectrum through strong non-linear effects. As the pump laser pulse launched into HNLF, a SC spectrum with broadband range of 0.92∼2.92 µm and maximum average power of 5.09 W was achieved, which sets record coverage of HNLF-based watts magnitude SC laser sources for now, to the best of the authors’ knowledge. The setup consists of silica fiber that can be considered easy-to-implement and with a cost-effectiveness scheme for ultra-broadband SC generation that could be easily applied to optical fiber sensing and spectral imaging technology.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber-based supercontinuum (SC) laser sources with wide range of wavelengths, good beam quality and compact size have attracted much attention for its importance in numerous applications such as bioimaging, spectroscopy-based pollution monitoring, medical treatments, chemical detection, as well as infrared illumination, and so on [1–7]. With the development of thulium-doped fiber-optic amplifier in SC technology, the range of silica-based SC has been covered to 2∼2.7 µm [8]. However, it cannot be further extended due to high phonon energy and low non-linear coefficient of many commonly used silica fibers [9]. As a result, various glass fibers (e.g., fluoride, fluorotellurite, chalcogenide and germania fibers) have been developed to broaden the spectral coverage of SC laser sources. For fluoride fibers, in 2018, Yang et al. demonstrated a 1.9∼5.1 µm SC source with a maximum average power of 4.06 W which based on a InF3 fiber and pumped by a thulium-doped fiber amplifier (TDFA) with a spectral range of 2∼2.7 µm [10]. For fluorotellurite fibers, recently, Guo et al. reported 22.7 W SC generation from 0.93 to 3.95 µm in a piece of all-solid fluorotellurite fiber pumped by a high-power 1.93∼2.5 µm SC fiber laser [11]. For chalcogenide fibers, very recently, Yan et al. demonstrated a watt-level SC source based on a 4 m-long As2S3 fiber with a spectral coverage of 2∼6.5 µm which pumped by a 2∼4.2 µm SC fiber laser with a maximum average power of 5.03 W [12]. Although various excellent SC sources were obtained based on different types of soft glass fibers, doped silica fibers-based SC source still have obvious advantages in terms of operational stability and potentially compact.
In recent years, germanium-doped silica fiber (GDF) is attracting a lot of attention due to their numerous advantages of such as high nonlinearity, strong mechanical strength, high damage threshold, and longer MIR transparency etc [13]. According to this, a variety of GDF-based SC laser sources have been developed by researchers. In 2019, Jain et al. demonstrate a SC laser source with highest output power of 6 W and broadest spectral range of 1∼3 µm, which was generated in a 70 mol.% GDF and pumped by a broadband four-stage erbium fiber-based MOPA [14]. In 2021, Wang et al. obtained a high power of 33.6 W SC laser source with a spectral range of 1.8∼3.0 µm in a 16 cm-long 64 mol.% GDF, which was pumped by a 39 W TDFA with picosecond pulse [15]. In 2022, Yang et al. demonstrated a high powers SC source with a wavelength of 1.9∼3.5 µm based on a very short piece of highly GDF, which was pumped by a broadband spectrum covering 1.9∼2.6 µm [16]. Recently, we demonstrated an ultra-broadband 1.85∼3.57 µm SC source with maximum average power of 1.9 W which based on a 0.9-m long 94 mol.% GDF and pumped by a flat SC laser with range of 1.9 µm to 2.7 µm [17]. In summary, these SC sources are all based on highly GDFs, however, these fibers are usually difficult and expensive to obtain. Compared with highly GDFs, silica-based HNLF with low GeO2 doping is also worth noting on account of its high nonlinear coefficient, low cost and easily acquired. In 2016, Saldaña-Díaz et al. reported a milliwatt level SC laser source with a spectral range of 1.1∼2.1 µm which based on 63 m long HNLF and pumped by a commercial EYDFA with an Er-doped mode-locked fiber laser as seed [18]. In 2019, they further boosted the spectral coverage to 1.1∼2.3 µm by shifting the pumping to the ZDW of HNLF based on a similar structure [19]. However, limited by the power and coverage of the pump source, the widths of these SC spectrums were only ∼1.2 µm and the average output power was much less than 1 W, thus there is a lot of room for improvement. Very recently, Wang et al. demonstrated a MIR SC laser source from 1.5 to 3.1 µm with 4.12 W average power in a 7 cm-long HNLF, which was pumped by a TDFA with a spectral range of 1.9∼2.7 µm [20]. Although the width of the spectrum was up to 1.6 µm, the pump source was positioned away from the ZWD of HNLF, resulting in an absence of the NIR band of SC spectrum.
In this work, we report an ultra-broadband SC laser source based on a very short piece of HNLF, which was pumped by a high powers SC laser with a spectral coverage of 1.5∼2.4 µm. Thanks to the pumping spectrum covered the ZDW of HNLF, a broadband SC spectrum with a width of ∼2 µm ranging from 0.92 µm to 2.92 µm was achieved. At the maximum pump power of 8.01 W, the output power of SC laser was 5.09 W corresponding to an optical conversion efficiency of 63.5%. Moreover, the stability of SC output power and effects of different seed pulse width on spectral scope were also studied.
2. Experimental setup
The schematic of the all-silica fiber structured 0.92∼2.92 µm SC source is shown in Fig. 1. It consisted of a 1.55 µm seed source, two-stage erbium-ytterbium-codoped fiber amplifier (EYDFAs), single-mode fiber (SMF) and a 12 cm-long HNLF. The 1.55 µm seed source was an electrically modulated pulsed semiconductor laser with both variable pulse repetition rate of 100-1000 kHz and pulse duration of 1-10 ns. The optical isolator was used between seed source and EYDFAs to prevent harmful feedback light. Subsequently, the 1.55 µm seed pulse was amplified by EYDFAs. The first stage EYDFA was fabricated by a 10 W multi-mode LD at 976 nm to provide the pump light, a 2 + 1 × 1 signal, and a pump light combiner, a 2.5 m length of SM-EYDF as the gain fiber. The combiner was used to couple both the seed pulse and pump light (with ∼95% efficiency) into the gain fiber. The SM-EYDF had a core/cladding diameter of 6/130 µm, a core/cladding numerical aperture (NA) of 0.20/0.45, and a cladding absorption coefficient of ∼1.1 dB/m at 915 nm. The structure of second stage EYDFA was similar to the first stage EYDFA, while the length of SM-EYDF was 3 m and maximal output power of the 976 nm LD was 40 W. The SM-EYDF used here had a core/cladding diameter and a core/cladding NA of 10/128 µm and 0.22/0.46, the cladding absorption coefficient was ∼2.4 dB/m at 915 nm. The optical isolator was used between each EYDFA to prevent harmful feedback light. A 10-m-long of single mode fiber (SMF) was attached behind the second EYDFA to initial broadening of the spectrum. Lastly, only 12-cm-long HNLF was fused to SMF for the generation of SC, and the output end of the HNLF was cleaved to 7° to avoid any back-reflections. The whole EYDFAs and HNLF were water-cooled down to 15 °C on an aluminum plate for heat dissipation, as well as promote efficiency. The seed pulses were monitored by using 500-MHz digital oscilloscope and an InGaAs photodetector (EOT ET-5000F, USA, with response time of approximately 28 ps). The seed spectrum and pre-extend spectrum were recorded by an optical spectrum analyzer with a measurement range of 1200∼2400 nm (Yokogawa, AQ6375). The SC spectrum characterizing system comprised a power meter (Thorlabs) to examine the power, an optical spectrum analyzer with a measurement range of 600∼1700nm (Yokogawa, AQ630D) to acquire the SC spectral at a short wavelength (<1.6 µm), a Mid-IR/IR spinning grating spectrometer (A.P.E, waveScan) with a measurement range of 1500∼6500 nm to record the spectra distribution at a long wavelength (>1.6 µm).
Fig. 1. Experimental setup of the all-silica fiber structured SC source (ISO: isolator; EYDFA: erbium-ytterbium-codoped fiber amplifier; SMF: single-mode fiber; HNLF: highly nonlinear fiber).
The used of HNLF was produced by a commercial company (Yangtze Optical Fibre and Cable Company Ltd., NL-1550-Zero type). The nonlinear coefficient data at 1.55 µm from the manufacturer was greater than 10 W−1/km. Figure 2(a) shows the dispersion characteristics of such fiber. As can be seen, the ZDW was located at 1.55 µm, indicating that a strong nonlinear effect can be generated by pumping in this band. The inset of Fig. 2(a) was optical microscope image of the fiber cross section. The core NA of HNLF and SMF were 0.35 and 0.2, respectively. The HNLF was fusion spliced to the SMF by using the standard single-mode fusion-spliced procedure. A continuous-wave fiber laser operating at 1.55 µm was used to test the coupling loss of the splicing joint under very low power. The output power of the HNLF with respect to that of the 1.55 µm laser source was plotted in Fig. 2(b). The inset of Fig. 2(b) was the microscopic image of fusion splice joint. The slope of the power curve in Fig. 2(b) was 88%, indicating that the splicing loss between HNLF and SMF was ∼0.55 dB.
Fig. 2. (a) Dispersion curve of the HNLF from the data provided by the manufacturer, inset showed the optical microscope image of the fiber cross section. (b) Transmission test result of the SMF-HNLF fiber fusion splicing joint. Inset showed the fusion splice joint.
3. Results and discussions
In the experiments, the seed source’s pulse repetition rate and pulse width were set to 400 kHz and 5 ns, respectively. To verify the accuracy of the seed parameters, we have measured the spectral characteristics and temporal of the seed pulses, as shown in Fig. 3. As can be seen in Fig. 3(a), the seed laser had a center wavelength of 1.55 µm with a bandwidth of 0.5 nm. As can be seen in Fig. 3(b), the pulse train of seed laser had a pulse period of ∼2.52 µs which was equivalent to the repetition rate of ∼397 kHz. The measured pulse width was ∼5.2 ns as shown in Fig. 3(c). Both repetition rate and pulse width approximated the set value manifesting that the parameters of the seed source were comparatively accurate.
Fig. 3. (a) The optical spectrum of the seed laser. (b) The oscilloscope trace of the seed laser. (c) The pulse width of the seed pulse.
The 1.55 µm seed pulse with average power of ∼100 mW was first injected into the EYDFAs for amplifications, and then a 10-m-long SMF for pre-broadening. The 1.55 µm laser characteristics after EYDFAs amplification were shown as black curve in Fig. 4. The spectrum after EYDFA2 under max pump power was shown in Fig. 4(a). Although the spectrum has broadened spanning from 1.5 to 2.0 µm, the most of the pulse energy was located at the around of 1.55 µm. As can be seen in Fig. 4(b), the output power of 1.55 µm laser increased with the pump power. With the pump power was increased from 0 to 24 W, the average output power after EYDFA2 was increased to 6.98 W corresponding the slope efficiency was ∼26%. When the pump power was increased to 40 W, the EYDFA2’s output power reached a maximum of 9.97 W, corresponding the slope efficiency was ∼18.8%. The characteristics of SC spectrum after SMF were shown as red curve in Fig. 4. As can be seen in Fig. 4(a), the spectrum extended up to ∼2.4 µm. We conjecture that the expansion of the spectrum was caused by the nonlinear effects of self-phase modulation (SPM), modulation instability (MI) and the stimulated Raman scattering (SRS) mechanism in SMF [8,21]. It could be seen that the pulse energy was evenly distributed over the coverage of the SC spectrum, which facilitates the continued expansion of the SC spectrum in HNLF. As marked in Fig. 4(b), the output power gradually saturated since the launched pump power increased from 0 W to 40 W, a maximum output power of 8.01 W was obtained finally. And the slope efficiency of the output power decreased from ∼23% to ∼9.7% with the increase of 976 nm pump power.
Fig. 4. (a) The optical spectrums after EYDFA2 and SMF. (b) The output power with the pump power after EYDFA2 and SMF.
Figure 5(a) have exhibited the spectral evolution of SC spectrum after HNLF as a function of the pump power (i.e., the output power after SMF), corresponding to the output powers of 1.75, 3.04, 4.16, 4.72 and 5.09 W with the pump powers of 2.66, 4.67, 6.43, 7.30 and 8.01 W, respectively. Obviously, as a result of increasing pump power, the spectral coverage of SC was gradually extended. Due to the pump spectrum completely covered the ZDW of the HNLF, the spectrum expanded sharply towards short and long waves. We conjecture that the spectral red shifted can be attributed to a combination of SPM, soliton fission (SF), Raman soliton self-frequency shift (SSFS) and the blue shifted can be attributed to formation of dispersive waves (DW) [22,23]. The SC spectrum was broadened to 1.1∼2.5 µm under the pump power of 2.66 W, whereas most of the laser energy was still located in the around of 1.55 µm. With the pump power increased, the short wave length edge of SC spectrum continued expansion into the NIR region, and the long wavelength edge was expanded to farther MIR region. Broadband SC generation was obtained in the HNLF when the pump power was increased to the maximum of 8.01 W, the SC spectrum covered the entire 0.92∼2.92 µm wavelength region with the maximum output power of 5.09 W. Spectral drops around 2.7 µm matched the water vapor’s absorption due to the long propagation distance in the monochromator’s free-space. Figure 5(b) presents the evolutions of the slope efficiency and conversion efficiency (defined as the ratio of SC spectrum power to SMF output power) changing with the pump power at 976 nm. As shown by the black curve in Fig. 5(b), the slope efficiency of HNLF output power reached 15.7% when the pump power was less than 16 W. It dropped to 4.6% with the increase of the pumping power to 40 W. The reason maybe that the wavelength extension caused by non-linear effects are energy lossy process, the higher the peak pump power, the stronger the non-linear effect and the more the energy loss will be [24]. The red curve in Fig. 5(b) indicates that the conversion efficiency also decreased with increasing pump power. But thanks to the non-linear fiber’s length of only 12 cm, all the conversion efficiencies were maintained at over 60%. The lowest conversion efficiency was ∼63.5% when the output power reaches a maximum of 5.09 W, corresponding to the red spectrum curve presented in Fig. 5(a). The dropping of the conversion efficiency mainly results from the peak-power-dependent nonlinear effects. Because the SSFS-based spectral extension is a lossy process, and higher peak pump power would result in stronger SSFS and thus more loss [24].
Fig. 5. (a) Spectral evolution of the SC laser source after HNLF by increasing the output power of SMF. (b) Evolution of slope efficiency and power conversion efficiency with 976 nm pump power.
Further, the characteristics of the output SC spectrum after SMF and HNLF depending on the pulse widths of 1, 5, and 10 ns were investigated. It is worth noting that the maximum pump power of 976 nm LD was kept constant. Figure 6(a) shows the spectrum after SMF at different pulse widths. As can be seen, both output SCs spanned from 1.5 to 2.4 µm under the pulse widths of 1 and 5 ns. It should be noted that there was a significant residual pumping spike at 1.55 µm in the output spectrum when the pulse width was 1 ns. As the pulse width increased to 10 ns, the long wave edge of the output spectrum dropped to ∼2.2 µm, and the spectrum covered a range of 1.5∼2.2 µm. Subsequently, with these spectrums injected into the HNLF, varies considerably of the ultra-broadband SC spectrum were obtained, as is shown in Fig. 6(b). It could be seen that the shape of the pump spectrum had a significant effect on the ultra-broadband SC spectrum’s shape. When the pulse width was 1 ns, the ultra-broadband SC spectrum spanned from 0.92 to 2.92 µm. However, the residual pumping at 1.55 µm significant increase which was similar as the 1.5∼2.4 µm pump spectrum. The increase in residual pumping may be caused by a higher peak power at a smaller pulse width. When the pulse width was increased to 10 ns, the ultra-broadband SC spectrum’s long wave edge also dropped to ∼2.8 µm. It was believed that the peak power was declined when the pulse width was increased, thus weakening the Raman-assisted SSFS and resulting in a smaller range of SC spectrum. In summary, the generation of ultra-broadband SC spectrum is most favorable when the pulse width is 5 ns, the obtained spectrum has the widest coverage and the smallest residual pumping.
Fig. 6. (a) Spectral characteristics of the SMF with the 976 nm LD pump power of 40 W under different pulse widths of 1 ns, 5 ns, and 10 ns. (b) SC comparison for different pulse widths under the maximal launched pump power.
Moreover, to investigate the long-term stability of the all-silica fibers-based SC laser, the output power data under pulse width of 5 ns was record for one hour, which was plotted in Fig. 7. To quantify the power stability, the root mean square (RMS) value was calculated to be ∼0.05 W, corresponding to an RMS power variation of ∼0.98%. Indicating that the output power was almost constant during the test, showing the excellent power stability of the all-silica fiber-based SC laser.
4. Conclusion
In brief, we demonstrated an ultra-broadband SC generation with an all-fiber configuration in a HNLF. The SC spectrum covered from 0.92 µm to 2.92 µm due to the high non-linearity coefficient of the HNLF and the rational choice of pumping wavelength. The SC’s maximum output power was 5.09 W corresponding to an optical conversion efficiency of 63.5%. The measured power fluctuation of 0.98% (RMS) at continuous 60 minutes indicated excellent power stability. To the best of the authors’ knowledge, demonstrates the widest spectrum among HNLF based watts level SC sources to date. This SC laser source has many advantages such as compact, easy attainability and cost-effectiveness, which has a promising application as the ideal light source for optical fiber sensing and spectral imaging technology. In further research, the SC spectrum’s width can be further boosted by cascading fluoride fibers.
Funding
National Natural Science Foundation of China (61421002, 62005040, U20A20210); Fundamental Research Funds for the Central Universities (ZYGX2019Z012, ZYGX2020KYQD003, ZYGX2021YGCX014); Sichuan Province Science and Technology Support Program (21YYJC2977).
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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