There is great interest in sources of coherent radiation in the mid-wave infrared (3–5 μm), and instruments based on fiber can offer major practical advantages. This range, and much broader, can be covered easily by supercontinuum generation in soft glass fibers, but with low power spectral density. For applications that require intense ultrashort pulses, fiber sources are quite limited. In this Letter, we report a fiber-based system that generates 100 fs pulses with 5 nJ energy, continuously wavelength-tunable over 2–4.3 μm through the soliton self-frequency shift (SSFS) in fluoride fibers. The pulse energies are 2 orders of magnitude higher than those previously achieved by SSFS, around 3 μm, and the range of wavelengths is extended by 1000 nm. Peak power ranges from 20 to 75 kW are achieved across the tuning range. Numerical simulations are in good agreement with the experimental results, and indicate the potential for few-cycle soliton generation out to 5.6 μm. Fiber-integrated sources of femtosecond pulses tunable across this region should be valuable for mid-infrared applications.
© 2016 Optical Society of America
The mid-infrared (MIR) spectral region is of great importance to both fundamental research and applications. MIR spectroscopy  offers unique capabilities to detect and identify various molecules in different environments, including real-time breath analysis , trace gas detection, and early cancer diagnostics . The 3–5 μm region is a transparent window in the atmosphere, and is suitable for LIDAR and remote sensing. These technologies can be greatly improved through the development of intense coherent MIR sources, which will also facilitate studies in strong-field physics, high-harmonic generation, and attosecond science .
New scientific discoveries and practical applications in the MIR region will require versatile light sources with high spectral density and continuous tunability over a wide spectral range. Currently, MIR light sources are based mainly on solid-state laser systems. Quantum cascade lasers  emit from 3 to 25 μm. They are usually operated continuous wave, and reach watt-level powers . Demonstrations of short pulse operation are rare . Kerr-lens mode-locked lasers cover a broad range around 2.4 μm, with impressive few-cycle pulses . Difference frequency generation (DFG) using near-infrared lasers can provide pulsed sources over a broad MIR range . Microjoule-level pulses, tunable over 3–20 μm, were obtained from a solid-state optical parametric oscillator (OPO) pumped by a Ti:sapphire amplifier system . The output from the DFG process can be boosted to higher pulse energy by optical parametric chirped pulse amplification . However, all these solid-state systems require bulky and complex optical systems.
Fiber-based light sources compete with solid-state systems due to their compactness, low cost, and maintenance-free operation. MIR supercontinuum spanning more than 3 octaves (1.4–14.3 μm) was generated in a chalcogenide fiber . The power spectral density is naturally low, and many applications would benefit from intense short pulses. Mode-locked erbium (Er) ZBLAN () fiber lasers have recently been demonstrated at 2.8 μm but are not tunable [13,14]. Fiber-based light sources employed DFG to generate femtosecond pulses with 10 pJ energy tunable over the MIR [15,16]. MIR frequency combs serve as increasingly important tools for massively parallel spectroscopy with unprecedented speed, precision, sensitivity, and spectral coverage . They have been realized by degenerate OPO systems with fiber-based seed lasers and bulk crystals [18,19], which motivates the development of fiber-based high-energy MIR femtosecond pulses.
The soliton self-frequency shift (SSFS) in optical fibers provides high-quality femtosecond pulses with wide spectral tunability [20,21]. Recently, SSFS has been explored to reach the MIR region . Standard silica fibers are limited to about 2.5 μm, while germanium-doped silica fibers reach almost 3 μm. To obtain soliton shifting further into the MIR, soft-glass fibers with low phonon energies are desirable. Koptev et al. achieved spectral coverage up to 2.65 μm in microstructured tellurite fibers with 100 fs soliton pulses . However, sub-50-pJ pulse energies were obtained, limited by the high nonlinearity of the fiber. Impressive results with soliton wavelengths of up to 3.42 μm were realized in a chalcogenide fiber pumped at 2.8 μm . The picojoule-level energy was limited by the high nonlinearity of the chalcogenide fiber and the low pump pulse energy.
Fluoride glass fibers have drawn attention due to their wide MIR transparency window, relatively high strength, and low background loss . Compared to tellurite and chalcogenide fibers, they have much smaller nonlinear indices and shorter zero-dispersion wavelengths (ZDWs). Based on these properties, 100 fs solitons with roughly 100 times the energy (10 nJ level) or peak power of solitons in tellurite or chalcogenide fibers are expected. Approximately 0.5 nJ pulses of unspecified duration at 2.5 μm were obtained in an early experiment on SSFS in fluoride fiber . Recently, Salem et al. demonstrated supercontinuum spanning 1.8 octaves in this kind of fiber, pumped by a thulium (Tm) fiber amplifier, which was in turn seeded by a Raman-shifted erbium fiber laser . In that work, a dispersion-flattened fiber was pumped with 10 nJ and 100 fs pulses at 2.1 μm, which is near the ZDW.
Here we demonstrate a fiber-based system that generates 100 fs, nanojoule-energy soliton pulses via SSFS in fluoride fibers. Relative to experiments on continuum generation , we enhance Raman soliton formation by pumping the fluoride fiber in a region with strong anomalous dispersion, using shorter pulses with about 15 times higher peak power. The generated Raman solitons are continuously tunable from 2 to 4.3 μm. The pulse energy is a 2 orders of magnitude improvement over previous Raman solitons at wavelengths beyond 2.5 μm. Numerical simulations of the soliton propagation are in good agreement with the experimental results, and indicate the potential for few-cycle soliton generation above 5 μm. The work reported here demonstrates that SSFS can serve as an important source of coherent ultrashort pulses with broad tunability for applications in the 3–5 μm range.
We employed two kinds of fluoride fiber for the study of SSFS (Thorlabs models P3-32F-FC-2 and P3-23Z-FC-1). The indium fluoride () fiber has a core diameter of 9 μm and numerical aperture of 0.26. The single-mode cutoff wavelength is 3.2 μm, and the propagation loss is specified as lower than over 2.0–4.5 μm. The ZDW for the fiber is . Properties of zirconium fluoride () fiber are included in Supplement 1. The fibers have benefits over the fibers in their capability to transmit wavelengths beyond 5.4 μm . The fibers are protected in patch cables, with both ends terminated by angled ferrule connectors to reduce reflections. Here we will focus on results obtained with the fiber, while results from the fiber are presented in Supplement 1.
To achieve a tunable light source with a spectrally separated shifted soliton, pumping deep in the anomalous dispersion regime of fiber with low nonlinearity is desirable. In this way, a smaller number of fundamental solitons is excited, and energy conversion to the most redshifted soliton is maximized . Our experiments are based on pumping at 1.9 μm, which is the longest wavelength we can reach with stable pulses.
Numerical simulations were performed to assess the feasibility of reaching the MIR region through SSFS in these fibers (see Supplement 1 for details of the simulations). Considering the single-mode cutoff wavelength of the fibers and our focus on the 3–5 μm region, simulations were conducted assuming only fundamental-mode propagation. We employed the generalized nonlinear Schrödinger equation, including dispersion terms up to the 8th order, stimulated Raman scattering, self-steepening, and propagation loss increasing exponentially with wavelength beyond 4 μm . Due to the large variation of the effective area of the mode over this wide wavelength range, the frequency dependence of the effective area is accounted for by introducing a first-order correction to the timescale associated with self-steepening and optical shock formation at the pump wavelength .
Numerical results of launching 40 nJ and 70 fs -shaped pulses at 1.9 μm into the fundamental mode of a 2 m long fiber are shown in Fig. 1. The soliton number N for the input pulse is . The spectral and temporal evolution of the pulses along the fiber exhibit the main features expected for fiber pumped in the anomalous dispersion regime with femtosecond pulses. Higher-order temporal solitons are initially formed, which then undergo fission under the perturbation of the higher order dispersion and Raman scattering to break up into a series of constituent fundamental solitons. These experience frequency shifts, shed energy into dispersive waves in the normal dispersion regime, and eventually separate spectrally and temporally. The most shifted soliton (labeled RS in Fig. 1) is at 4.3 μm and has 80 fs duration and 5 nJ energy.
The experimental setup is shown in Fig. 2. An Er fiber chirped pulse amplification system delivers 550 fs pulses with 1.3 μJ energy at 1.55 μm and 1.5 MHz repetition rate. These are shifted to longer wavelengths by SSFS in a polarization-maintaining rod-type photonic crystal fiber (PCF)  with a mode-field diameter of 55 μm. The output pulses are collimated by a lens with a focal length of 50 mm, and a long-pass filter (LPF) that cuts on at 1.8 μm is used to select the most redshifted soliton centered at 1.9 μm, which has 120 nJ pulse energy and 70 fs duration (see Supplement 1). A half-wave plate is used to suppress residual unshifted light and other shifted solitons due to the polarization dependence of the blocking range of the LPF. These pulses are coupled into the fluoride fiber using an aspheric lens and collimated at the output using a protected silver reflective collimator with a 7 mm focal length. They then pass through a LPF which cuts on at 2.85 or 3.75 μm for selecting the most redshifted soliton. Several Fourier-transform optical spectrum analyzers (OSAs), each covering a different wavelength range, are required to measure the output spectra. The pulse duration is measured using an interferometric autocorrelator based on two-photon absorption in an InGaAs photodetector.
Experiments were conducted with a 2 m fiber. A 6 mm focal length lens was used to couple light into the fiber. By varying the incident pulse energy from 3 to 120 nJ using neutral density filters, we are able to continuously tune the center wavelength of the output pulses from 2 to 4.3 μm. The fiber is specified to be single mode for wavelengths beyond 3.2 μm. The excitation of higher-order modes at the pump wavelength does not prevent the generation of Raman solitons in the fundamental mode. Any shifted energy in the higher-order modes experiences high loss upon passing the cut-off wavelength. This ensures a single-mode output beam after the LPF (for wavelengths beyond 3.2 μm). Soliton pulses with few-nanojoule energies and durations are obtained at each wavelength. The representative examples shown in Fig. 3 correspond to peak powers between 45 and 75 kW. The LPF introduces some chirp on the pulses, which leads to a 10% deviation from the transform limit. Further shifting to longer wavelengths should be possible with higher pump energy. However, at higher pulse energy we observe damage to the end-face connector.
It is worth mentioning that even with the excitation conditions chosen to enhance Raman-soliton formation, broad supercontinuua are generated, as expected from the numerical results above. When the most shifted soliton is near 4 μm, a continuum that spans more than 2 octaves is produced, with the short-wavelength edge (see Supplement 1).
We estimate that about 35% of the incident light is coupled into the fundamental mode. Over the range of wavelengths generated, 10%–20% of the energy coupled into the fundamental mode of the fiber is converted to the most shifted soliton. The overall practical energy conversion efficiency is therefore 3.5%–7%.
The experimental results agree well with the numerical simulations that assume the measured coupling efficiency, for both the soliton wavelengths and energies. The comparison for the most redshifted solitons is shown in Fig. 4. The nonlinear coefficient of the fiber exhibits strong wavelength dependence because the mode area of the fiber increases with wavelength. Incorporating this dependence into the simulation is crucial to accurate modeling of the long-wavelength edges of the spectra (see Supplement 1).
For wavelengths below 3.2 μm, the output is not specified to be single mode. For the fiber, the V number is 3.6 at 2 μm and there are three guided modes. The resulting might be adequate for many applications. To obtain a strictly single-mode beam below 3.2 μm, which is the cut-off wavelength of the fiber, experiments were conducted using a 1 m long fiber. The fiber provides similar pulse performance as the fiber for wavelengths around 3 μm, but it extends the wavelength region with single-mode output down to 2.3 μm. The use of the two fluoride fibers allows the generation of pulses with spectral coverage over 2–4.3 μm and a diffraction-limited output beam for wavelengths above 2.3 μm.
It should be possible to achieve significantly better performance than the results reported here by optimizing the conditions of the SSFS. For example, simulations show that launching 40 nJ in the fundamental mode of a 5 m long fiber yields solitons that reach 5 μm. This would correspond to 120 nJ incident on the fiber. In the experiments described above, only 40 nJ out of the total 120 nJ pulse energy is effectively utilized. With better coupling design and the use of bare fibers to avoid damage, it should be possible to couple 80 nJ in the fiber. With that energy coupled into a 2 m long fiber, simulations predict shifting out to 5.6 μm (Fig. 4). Shifting to even longer wavelengths will be limited by the large anomalous dispersion and high loss of the fiber.
While the source described here is an important step toward a practical, high-performance instrument, several improvements should be considered. Our approach uses cascaded SSFS stages, so it is reasonable to expect high noise due to nonlinear noise amplification and the coupling of spectral, temporal, and intensity fluctuations in the SSFS process. Our measurements show that the long-term stability of the source is similar to that of the 1.9 μm source (see Supplement 1). This implies that a stable MIR source can be obtained provided the pump source is stable. The short input pulses used here ensure that noise has a minimal influence on the soliton fission and spectral-shifting processes. Further optimization of the SSFS process for high-power pulses, for shifts to greater wavelengths, and for better noise performance should be possible. As is well known, the soliton frequency shift is inversely proportional to the fourth power of the pulse duration . A source of even shorter pulses at 2 μm will improve the performance in all respects. More studies will be needed to optimize the design for specific parameters, but there is considerable room for improvement from our current results: in the same fiber used here, 50 fs, solitons at 5 μm are consistent with the soliton area theorem.
In summary, 100 fs pulses with nanojoule energies, wavelength-tunable from 2–4.3 μm, are generated by the SSFS in fluoride fibers. With peak powers around 50 kW, these are the most intense MIR pulses generated by a fiber source to date. A fiber-based, broadly tunable source of short pulses should facilitate numerous applications in the important 3–5 μm region.
National Science Foundation (NSF) (ECCS-1306035); Office of Naval Research (ONR) (N00014-13-1-0649); National Institutes of Health (NIH); National Institute of Biomedical Imaging and Bioengineering (NIBIB) (R01 EB017274); Defense Sciences Office, DARPA (DSO, DARPA) (W911NF-14-1-0012); Natural Sciences and Engineering Research Council of Canada (NSERC) (CGSD3-438422-2013).
L. G. W. acknowledges support from NSERC. The authors thank Thorlabs for the loan of the OSA used in the experiments.
See Supplement 1 for supporting content.
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