Mid-Infrared Ultraflat Broadband Supercontinuum Generation with 10 dB Bandwidth of 2340 nm in a Tapered Fluorotellurite Fiber

Abstract

We demonstrate mid-infrared ultraflat broadband supercontinuum (SC) generation in a 40 cm long tapered fluorotellurite fiber pumped by a Raman soliton source. By tapering the end of the large-core-diameter fluorotellurite fiber, the dispersion is regulated and the nonlinear effect is enhanced, which effectively extends the mid-infrared SC spectral range and increases the spectral flatness. Finally, we obtained an SC light source with a spectral range from 1.8 to 4.7 μm; the 10 dB bandwidth of the source completely covers 1.88–4.22 μm, which has the farthest flat spectral edge in fluorotellurite fibers. The output power of the SC laser is about 1.04 W, and the power ratio of those above 3 μm in the spectrum to the total SC is ~24%. The optical-to-optical conversion efficiency is about 75%. Our results show that tapering of fluorotellurite fiber is an effective method to further extend and flatten the mid-infrared SC.

1. Introduction

Recently, 2–5 μm mid-infrared (MIR) supercontinuum (SC) laser sources have attracted much attention and found significant application in frequency metrology, long-distance remote sensing, defense, and security [1,2,3,4,5]. Recently, many efforts have been made to improve the output power [6,7,8,9], extend the spectral range [10,11,12], and enhance the spectral flatness of MIR SC laser sources [13,14]. In particular, some fields such as medical diagnostics [15] and hyperspectral imaging [16] benefit from a broader and flatter bandwidth, which extends the spectrum in the MIR range, enhancing both detection resolution and sensitivity. In previous studies, fluoride fibers were often used to generate flat SC with 10 dB bandwidths covering 2–4 μm. Wu et al. reported an 11.3 W all-fiber mid-infrared SC laser source with a 10 dB bandwidth spectral range of 1.9–4.3 µm using fluoroindate fibers as the nonlinear media [17]. Yin et al. utilized fluoride fiber as nonlinear medium to generate an SC source with an output power of 15.2 W and a 10 dB bandwidth of 2090 nm (1960~4050 nm) [18]. Yang et al. used a 1.9–2.6 µm SC laser to pump a fluoride fiber, obtaining a MIR SC source with an output power of 20.6 W and a 10 dB bandwidth spectral range of 1.92–4.08 µm [19]. Although the high-power broadband SC is widely realized, fluoride fibers have poor chemical and thermal stability, and it is difficult to achieve long-term stable work in real applications [20].

In recent years, fluorotellurite fibers (TBY) with a broadband transmission window of 0.4–6 µm and excellent chemical and thermal stability have been researched extensively in MIR SC generation [21]. The fluorotellurite fiber has large nonlinear refraction, allowing the use of fibers as short as tens of centimeters in length, which is beneficial for thermal management. Yao et al. reported on an SC light source using a 6.8 μm core diameter fluorotellurite fiber as a nonlinear medium; the spectral range covers 0.95–3.93 μm with 10.4 W output power. The superior long-term stability of fluorotellurite fibers was verified [22]. Li et al. obtained a 22.7 W MIR SC source with a 10 dB spectral bandwidth ranging from 1890 to 3523 nm in 11 μm core diameter fluorotellurite fiber [13]. Guo et al. reported a 25.8 W SC light source with a 10 dB spectral bandwidth ranging from 2000 to 3500 nm in a fluorotellurite fiber [14]. To significantly increase the output power of the SC laser source, Jiao et al. use a large-core-diameter fluorotellurite fiber of 30 μm to generate a high-power SC. A 50 W all-fiber MIR SC light source with a 10 dB spectral range of 1400~3250 nm was obtained [23]. Although large-core-diameter fibers effectively increase SC power, the reduction in nonlinear coefficients results in the limitation of spectral broadening. As with the 2 μm light source or broad-spectrum light source used by the above researchers to pump the fluorotellurite fiber, the energy of the pump pulse is mainly concentrated at ~2 μm, resulting in low energy proportions at the long wavelength of the SC spectrum. Recently, Yang et al. demonstrated a 10.4 W flat SC light source with a 10 dB spectral range of 1.9–4.1 μm in a 34 μm core diameter fluorotellurite fiber [24]. The broadband and flat SC are caused by the excitation of 2180 nm as well as 2350 nm dual Raman soliton lasers. Although the SC spectrum has been effectively extended by improving the light source, due to the long-wavelength absorption loss of the fluorotellurite fiber, the spectrum is limited to within ~4 μm. Thus, it is still necessary to explore other ways to extend the spectral range and enhance the spectral flatness of MIR SC light sources.

In this paper, we experimentally demonstrate an MIR ultraflat broadband SC with a long-wavelength edge beyond 4700 nm, with a 10 dB bandwidth of about 2340 nm, generated in tapered fluorotellurite fibers pumped by a Raman soliton source. By introducing a tapered fiber part at the end of the LMA fluorotellurite fiber, the dispersion is regulated and the nonlinear effect is enhanced. In comparison with previous works on fluorotellurite fiber-based SC laser sources, the 10 dB bandwidth of the generated SC light in the tapered fiber is improved very much.

2. Experiments and Results

In our experiments, the core, the inner cladding, and the outer cladding materials for fluorotellurite fibers we selected were 70TeO₂–20BaF₂–10Y₂O₃ (70TBY), and 65TeO₂-25BaF₂-10Y₂O₃ (65TBY) and 60TeO₂-30BaF₂-10Y₂O₃ (60TBY), respectively; the refractive indexes are shown in Figure 1a. The three fluorotellurite glasses have similar glass transition temperatures (T_g) (~424 °C for 70TBY, ~422 °C for 65TBY, ~421 °C for 60TBY) and have high stability. Thus, the materials are suitable for fiber drawing and further heated tapering. All-solid double-clad fluorotellurite fibers (38/76/250 μm) were prepared using the secondary suction method [25], and the numerical apertures (NAs) of the fiber’s core and inner cladding at 2 µm are all about 0.3. The background loss value of the fiber at 1.5 μm was about 0.2 dB/m by using a cut-back method. Subsequently, a tapered fiber was fabricated by using a melt-drawn fiber taper system. The homemade fluorotellurite fiber has a 34 cm long untapered region with a 38 μm core diameter, a 2 cm long tapered transition region, and a 4 cm long taper waist with a 14 μm core diameter, as shown in Figure 1b. The dispersion curves of this fiber with different core diameters were calculated using the commercial software MODE solutions 7.11.1584 for x64 (Lumerical Solutions, Inc., Vancouver, BC, Canada) with the full vectorial finite difference method; the results are shown in Figure 1c. The first zero-dispersion wavelengths (ZDWs) of the fiber are all located at ~2.1 μm, and with the fiber core diameter reduced, the second ZDWs are generated and move to the shorter wavelength. The nonlinear coefficient (γ) curves of fibers with different core diameters were calculated by using a nonlinear refractive index of 3.5 × 10⁻¹⁹ m²W⁻¹ for fluorotellurite glasses [13], as shown in Figure 1d.

Figure 2 shows the experimental setup for MIR SC generation in TBY fiber, including a 1960 nm femtosecond laser source, a lens-coupling system, a high-peak power Raman soliton generation stage, and an MIR SC generation stage, utilizing the tapered fluorotellurite fiber. Similarly to our previous work, the seed source of the 1960 nm femtosecond laser is a 1.95 μm Raman soliton obtained by a 1.5 μm femtosecond laser pumping a 1.6 m highly nonlinear germanate fiber (germanium concentration 98 mol.%) [24]. A commercially available chirped fiber Bragg grating (CFBG, TERAXION) and a pair of transmission gratings are used as the broadener and compressor, respectively. The compressed pulse laser is coupled into a large-mode-area (LMA) silica fiber through an aspherical mirror-coupling system for self-compression and Raman soliton self-frequency shift (SSFS). To avoid a large loss of Raman solitons moving to the long-wavelength region in the silica fiber, we chose a 23 cm long 32/250 LMA silica fiber, whose smaller NA (0.075) ensures high beam quality (M² = 1.32) of output laser. At the MIR SC generation stage, a tapered 40 cm long TBY double-clad fiber, mentioned earlier, was fusion-spliced to the silica fiber to broaden the spectrum. The output signals of the MIR-SC light source were monitored using an optical spectrum analyzer and a power meter.

In the experiment, by using an acousto-optic modulator (AOM), the repetition frequency of the 1960 nm femtosecond pump source is reduced to 980 kHz to increase the peak power of the pulse. The center wavelength of the source is located at about 1960 nm, and the corresponding pulse width is about 500 fs. Initially, we couple 1960 nm ultrashort pulses into a 32/250 fiber with a coupling efficiency of ~73% using a precise fundamental-mode-matching technique. As shown in Figure 3a, with the increase in pump power, the split Raman soliton in the LMA silica fiber gradually shifts to the long-wavelength region. Since the ZDW of the LMA silica fiber is located at ~1.3 µm, the pump pulse operates in the anomalous dispersion regime, and soliton-related dynamics cause spectral broadening [26]. Continuing to increase the pump power to 20 W, the Raman soliton is frequency-shifted to ~2.3 µm, and then stops shifting, constrained by high losses of the silica fiber at longer wavelengths. As shown in Figure 3b, the output power of the LMA silica fiber increases to 1.38 W as the pump power rises from 2 W to 20 W. We calculated the power of the split Raman soliton by integrating the spectral area, finding that it constitutes approximately 29% of the total power, with a pulse energy of about 405 nJ. Figure 3c gives the corresponding pulse width of about 95 fs at 20 W pump power, and the peak power of the Raman soliton is calculated to be about 4.2 MW.

Figure 4a shows the spectrum evolution of the generated SC laser under different output powers from the above tapered fluorotellurite fiber. When the pump power is ≥8 W (the corresponding average output power is ≥540 mW), the Raman soliton generated in LMA silica fiber is located in the anomalous dispersion region of the above fluorotellurite fiber; large spectral broadening was caused by self-phase modulation, higher-order soliton compression, soliton fission, and the Raman soliton self-frequency shift. As the launched average pump power was increased to 20 W, the long-wavelength edge was extended to 4.7 µm. The 10 dB bandwidth of the generated SC light was about 2340 nm, excluding the pump light, and the corresponding spectral range was from 1880 to 4220 nm with excellent flatness. Figure 4b shows the dependence of the output power of the generated SC light in the above tapered fluorotellurite fiber on the power of the pump laser. The measured average output power of the generated SC laser was gradually increased to 1.04 W with the pump power being increased to 20 W, and the corresponding optical-to-optical conversion efficiency is about 75% (1.38 W to 1.04 W) in the fluorotellurite fiber.

By integrating the spectral area, we calculated the ratios of the power above 3 μm in the spectrum to the total spectral power. Figure 4c shows the power of above 3 μm in the spectrum and its percentage relative to different pump powers. The power ratio above 3 μm first increases with the increase in pump power, and then when the pump power exceeds 12 W, it almost holds at ~24%, before slightly decreasing. The low power percentage of above 3 μm in the spectrum at <12 W pump power is mainly because the Raman soliton has not yet fully shifted into the abnormal dispersion region of the TBY fiber to form a large spectral broadening. When the pump power exceeds 12 W, the decrease in the >3 μm power percentage in the SC spectrum is due to increased fiber loss as the spectrum extends towards longer wavelengths, resulting in energy loss in the long-wavelength portion. Corresponding to this process is a slower increase for >3 μm of power in the SC spectrum.

To verify the effect of fiber tapering on spectral broadening, we conducted experiments using a 38 μm core diameter untapered TBY fiber with the same length of 40 cm. Similarly to the above experimental process, we tested the output spectrum of the untapered TBY fiber at different pump powers, and the results are shown in Figure 5. As the pump power increases, the spectral long-wavelength edge slowly extends to about 4 μm and then the spectrum almost stops broadening. At a maximum pump power of 20 W, the long-wavelength edge was extended to about 4.1 µm. The 10 dB bandwidth of the generated SC light source was about 2080 nm, excluding the pump light, and the corresponding spectral range was from 1900 to 3980 nm. The output power of the generated SC light in the untapered fluorotellurite fiber is almost the same as in Figure 4b. Compared to the spectrum obtained in the above tapered TBY fiber, the SC spectrum obtained in the untapered fiber is slightly narrower, and, more notably, the spectral flatness is poor. In addition, due to the significant loss of the TBY fiber at long wavelengths, the intensity of the SC spectrum sharply decreases in wavelengths above 4 μm. This can be explained by combining the dispersion curve in Figure 1c and the nonlinear coefficient in Figure 1d. The tapered part at the end of the TBY fiber enhances nonlinear effects and adjusts dispersion, achieving further spectral broadening in long-wavelength regions and flattening the SC spectrum. The experimental results demonstrate that fluorotellurite fiber tapering is an effective method for overcoming fiber loss limitation and achieving further spectral broadening.

Figure 6 presents a comparison of TBY-fiber-based SC laser sources. In addition, we have summarized various parameters of TBY-fiber-based SC laser sources in

Table 1. Parameters of TBY-fiber-based SC laser sources.

Spectral Range (μm)	10 dB Bandwidth Range (μm)	Power (W)	Optical-to-Optical Conversion Efficiency	Reference
0.93–3.99	2–3.5	25.8	60.6%	[14]
1.8–4.2	1.9–4.1	10.4	74%	[24]
1.22–3.74	1.4–3.25	50	68.47%	[23]
0.93–3.95	1.89–3.52	22.7	57.2%	[13]
0.95–3.93	1.9–3	10.4	65%	[22]
1.02–3.44	1.8–3.3	4.5	42.9%	[21]
1.8–4.75	1.88–4.22	1.04	75%	This work

. In our study, the long-wavelength edge (LWE) of the SC spectrum extends 550 nm compared to previous results in a TBY fiber. Note that although the power of the whole SC only reaches the watt level, the power ratio of the long-wavelength spectrum is greatly increased compared to previous reports. To the best of our knowledge, our result has the farthest flat spectral edge in fluorotellurite fibers.

3. Simulation and Discussions

To verify the spectral broadening mechanism of SC lasers in the tapered TBY fiber (pump power 20 W), we have simulated the SC spectral domain evolution processes in the LMA silica fiber and the LMA TBY fiber pumped by femtosecond Raman soliton by solving the generalized nonlinear Schrodinger equation (GNLSE) [27,28]:

$${\displaystyle \frac{\mathit{\partial }A\left(z,t\right)}{\mathit{\partial }Z}}=-{\displaystyle \frac{\alpha }{2}}A\left(z,t\right)+{\sum }_{n=1}^{\infty }{\displaystyle \frac{i^{n+1}{\beta }_n}{n!}}{\displaystyle \frac{{\mathit{\partial }}^{n}A\left(z,t\right)}{\mathit{\partial }{T}^n}}+i\gamma \left(1+{\displaystyle \frac{i}{{\omega }_0}}{\displaystyle \frac{\mathit{\partial }}{\mathit{\partial }t}}\right)(A(z,t){\int }_{-\infty }^{+\infty }R\left(t^{\prime }\right){\left|A\right(z,t-t^{\prime }\left)\right|}^{2}d{t}^{\prime })$$

(1)

where A(z,t) is the complex temporal profile, α is the fiber loss, β_n is the dispersion coefficient associated with the Taylor expansion of the propagation constant at the reference frequency, γ is the nonlinear coefficient, and τ_shock is the additional shock time. R(t) is the Raman response function that includes both instantaneous electronic and delayed Raman contributions.

The initial pump pulses possessed a hyperbolic secant field profile:

$$A\left(0,T\right)=\sqrt{P_0}sech\left(T/T_0\right)$$

(2)

where P₀ and T₀ are the peak power and input pulse width, respectively. The Raman soliton order of the pump pulse was calculated as:

$$N^2={\displaystyle \frac{L_D}{L_{NL}}}={\displaystyle \frac{\gamma P_0{T}_0^2}{\left|{\beta }_2\right|}}$$

(3)

The split-step Fourier method was used to solve the GNLSE, and 2¹⁷ time and frequency discretization points and a longitudinal step size <5 µm were used to ensure the accuracy of the numerical simulations.

In our numerical simulation, the pulse propagation is divided into four stages: a 23 cm long LMA silica fiber, a 0.34 m long LMA TBY fiber, a 2 cm long taper region of the TBY fiber, and a 4 cm long small-core-diameter TBY fiber, which corresponds to the fibers used in the experiment. The initial pulse parameters include the operating wavelength (1.95 μm), pulse width (500 fs), input power (1.38 W), and repetition frequency (980 kHz). In addition, the nonlinear coefficients, group velocity dispersion curves, and loss curves of the TBY fibers are also imported into the MATLAB simulation program. The Raman response function of the silica fiber is described in paper [29] and the Raman response function of the TBY fiber is derived from the Raman gain spectrum of fluorotellurite glass [22].

The simulated spectral and temporal evolution process of the pulse in the LMA silica fiber as well as in the LMA TBY fiber are shown in Figure 7 and Figure 8, respectively. For the pump pulse propagation inside the silica fiber segment from 0 to 0.05 m, the spectral broadening was caused by self-phase modulation (SPM). The femtosecond pulse is compressed to its narrowest point at 0.05 m, and then the first Raman soliton is split out. Next, the split Raman soliton in the LMA silica fiber is gradually shifted to the long-wavelength region (~2.4 μm) and enters the TBY fiber. The large spectral broadening inside the fluorotellurite fiber was mainly caused by self-phase modulation, soliton fission, and the Raman soliton self-frequency shift. Noting that the SC spectrum was further extended to 4.7 μm and flattened at the long-wavelength region due to the enhancement of the nonlinear effect caused by the tapered fiber part. According to Figure 1c, as the fiber core diameter decreases to 14 μm, the wavelength above 4.5 μm is located in the anomalous dispersion region of the fiber. However, in the simulations and experiments, we did not observe the formation of dispersion waves at about 5 μm. This can be explained by the higher transmission loss of TBY fiber at longer wavelengths. Figure 7 shows the results of the experimental spectrum and the simulated spectrum at the 20 W pump power, which confirms the above interpretation. Engineering of the nonlinearity and dispersion of a nonlinear fiber could be a more promising way to further improve the performance of the SC spectrum.

4. Conclusions

In conclusion, we obtained an ultraflat SC spectrum with a 10 dB bandwidth of 2340 nm in a home-made tapered TBY fiber. A high-peak-power Raman soliton femtosecond laser was used as the pump source, which was obtained by SSFS technique in a 0.23 m-long LMA silica fiber. A tapered TBY fiber is efficiently fused to the LMA silica fiber to enhance the stability of the system. The tapered part at the end of the fiber increases the nonlinear coefficient and regulates the dispersion, and then the SC spectrum was further broadened and flattened at the long-wavelength region. Our results show that tapering of a fluorotellurite fiber is an effective method to obtain an MIR ultraflat broadband supercontinuum.