Multi-watt, multi-octave, mid-infrared femtosecond source

Abstract

Spectroscopy in the wavelength range from 2 to 11 μm (900 to 5000 cm^-1) implies a multitude of applications in fundamental physics, chemistry, as well as environmental and life sciences. The related vibrational transitions, which all infrared-active small molecules, the most common functional groups, as well as biomolecules like proteins, lipids, nucleic acids, and carbohydrates exhibit, reveal information about molecular structure and composition. However, light sources and detectors in the mid-infrared have been inferior to those in the visible or near-infrared, in terms of power, bandwidth, and sensitivity, severely limiting the performance of infrared experimental techniques. This article demonstrates the generation of femtosecond radiation with up to 5 W at 4.1 μm and 1.3 W at 8.5 μm, corresponding to an order-of-magnitude average power increase for ultrafast light sources operating at wavelengths longer than 5 μm. The presented concept is based on power-scalable near-infrared lasers emitting at a wavelength near 1 μm, which pump optical parametric amplifiers. In addition, both wavelength tunability and supercontinuum generation are reported, resulting in spectral coverage from 1.6 to 10.2 μm with power densities exceeding state-of-the-art synchrotron sources over the entire range. The flexible frequency conversion scheme is highly attractive for both up-conversion and frequency comb spectroscopy, as well as for a variety of time-domain applications.

Fig. 1. OPA setup.

The Kerr-lens mode-locked Yb:YAG thin-disc oscillator delivers 1.3-μJ pulses of 230-fs duration with a 37.5-MHz repetition rate. A dielectric mirror (OC) transmits either 3% or 15% of the oscillator power. The reflected light can be attenuated by a half-wave plate (λ/2) and thin-film polarizer (TFP) sequence and is directly used to pump the nonlinear crystal (Xtal), which is either PPLN or LGS. The seed generation is accomplished in a fiber with either a 2.7-μm (for PPLN) or 10.3-μm (for LGS) mode-field diameter. To suppress cross-phase modulation, the polarization is cleaned in front of the fiber. Aspheric lenses were used for coupling and collimation. The long wavelengths of the continuum and the pump are overlapped in space with a dichroic mirror (DM) and in time with a translation stage in the pump arm (τ). The polarizations of seed and pump are optimized for maximal conversion to the MIR. After the nonlinear crystal, the beams are collimated. The MIR is separated by means of a beam splitter (BS). The optics in the box with the dashed blue outline were only set up for supercontinuum generation (SCG) after the PPLN OPA. All lenses in the box and the Si plate for dispersion compensation were made from silicon and anti-reflection–coated from 3 to 5 μm. The lenses were plano-convex. The first has a focal length of 75 mm, and the second and third have focal lengths of 25 mm. The supercontinuum was roughly collimated with a parabolic mirror of 2-inch focal length. The ZGP crystals are described in detail in the main text. For dispersion compensation after the first ZGP crystal, a 2-mm sapphire (Sa) and a 5-mm CaF₂ plate were inserted at Brewster’s angle.

Fig. 2. Seed spectra.

The dashed lines show the full fiber continua, and the solid lines show the seed spectra after the dichroic mirrors. The seed for the PPLN OPA is generated in a 20-cm polarization-maintaining fiber with normal dispersion and a 2.7-μm mode-field diameter (red lines). It theoretically allows the amplification of MIR wavelengths down to 3.3 μm. For amplifying wavelengths longer than 6 μm, it is beneficial to use the continuum generated in an 8-cm-long fiber with a 10.3-μm mode-field diameter (blue lines), which provides up to an order of magnitude more seed power. The filtered powers (solid lines) are 220 mW (PPLN OPA) and 800 mW (LGS OPA), respectively. The spectra are measured with an optical spectrum analyzer. The difference wavelength (λ_i) on the top axis refers to the central pump wavelength (λ_p = 1030 nm) and the seed wavelength on the bottom axis (λ_s), that is, ${\mathrm{\lambda }}_{\mathrm{i}}^{-1}=|{\mathrm{\lambda }}_{\mathrm{p}}^{-1}-{\mathrm{\lambda }}_{\mathrm{s}}^{-1}|$.

Fig. 3. MIR power.

Simulation and experimental results for PPLN with a 28-μm poling period and LGS with a tuning angle of about φ = 38.6° (type II phase matching). For tight focusing into the PPLN (spot diameter d of the pump is 120 μm, wine red squares), up to 0.9 W of MIR was generated, which is in excellent agreement with the simulation (triangles with wine color filling; slope efficiency, 23%). If the full pump power was used (d = 300 μm, red squares), then 5.1 W of MIR at 4.1-μm wavelength was generated. The agreement with the simulations (triangles with red color filling) is good, although they predict a slightly steeper slope (slope efficiencies, 15 and 17.5%). For LGS (blue squares), a maximal power of 1.3 W was reached. This is also in good agreement with simulations (triangles with blue color filling; slope efficiencies, 5.4 and 6.0%). The x axis is scaled by the square root of the pump power, whereas the y axis is logarithmic, which represents the characteristic power relation for OPAs in the low depletion regime (8). The displayed lines show linear fits for extracting the slope efficiencies. All experimental results have been corrected for reflection losses.

Fig. 4. Tuning curves.

(A) Generated MIR power for maximal pump power and tuning periods from 28 to 25.5 μm (from left to right in −0.5-μm steps) of the PPLN. The spectrum centered at 4.2 μm is shaped through CO₂ absorption. The power was measured 25 cm behind the nonlinear crystal. (B) Tuning curve measured with a type I phase-matched LGS crystal. The OPA operates most powerfully around 8.2 μm (slightly blue-shifted from type II). Upon detuning from this central point, the phase-matched wavelengths split, allowing the generation of very broadband spectra (vine red line). The spectra below 6 μm (black line) and above 10 μm (light blue line) needed different delays to be generated because of the uncompressed seed pulse. With type I phase matching, a maximal MIR power of 1.0 W could be generated. Type I, however, allows the generation of slightly more broadband spectra than type II. More information is provided in section S5. Power spectral density is provided in units of mW/cm⁻¹ and μW/f_rep, where f_rep = 37.5 MHz is the oscillator repetition rate. This unit is particularly interesting for frequency comb applications (14).

Fig. 5. Coverage of the MIR spectral region from 1.6 to 10.2 μm through two OPA channels.

The supercontinuum spanning from 1.6 to 7.1 μm (at −30 dB) was generated in a 2-mm-thick ZGP crystal by means of cascaded quadratic nonlinearities. The continuum generation stage was pumped by 67-fs pulses emerging from the PPLN OPA and a precompression stage. The spectrum was recorded about 1.5 m away from the nonlinear crystal and partly shows strong attenuation due to atmospheric absorptions. The total average power of 1.8 W was measured behind the parabolic collimation mirror. A short-pass filter (blue area) and a long-pass filter (orange area) were used to determine the powers of the individual components. The gray area additionally shows the spectrum generated from a separate LGS OPA with a measured output power of 300 mW. Whereas both OPA channels could not be operated in parallel with the used pump laser, they can be readily combined at the presented power levels if a state-of-the-art mode-locked thin-disc oscillator (34) is used.