High-intensity mid-infrared pulses at 1 kHz repetition rate

In this section we present a laser source for high-intensity mid-infrared pulses at 1 kHz repetition rate. The mid-infrared pulses are generated in a three-stage scheme. First, high-power, ultrashort near-infrared pulses are generated in a 1 kHz regenerative Ti:sapphire laser system. In a second stage, these pulses are used to generate tunable, near-infrared pulses in an optical parametric amplifier. Finally, mid-infrared pulses are generated via difference frequency mixing. We restrict the presentation to the most important points. A detailed description can be found in [41, 42]. This laser system is used for the time-resolved experiments on quantum cascade lasers.

2.2.2.1 Regenerative amplification and optical parametric amplification

The scheme for the generation of ultrafast, high-power near-infrared pulses is shown in Fig.

2.8. It is based on chirped pulse amplification (CPA) [43]. Here, weak 50 fs near-infrared

800 nm, 2nJ, 85 MHz

Figure 2.8: Chirped pulse amplification laser system for the generation of amplified fem-tosecond pulses (T.F.P.=thin film polarizer, HR=high reflector, OC=output coupler) [41].

signal and

Figure 2.9: Two-stage optical parametric amplifier for the generation of tunable femtosecond signal and idler pulses in the near-infrared spectral range (λ=1-2 µm) [41].

pulses centered at 800 nm are generated in a Ti:sapphire oscillator with a repetition rate of 85 MHz and a pulse energy of 2 nJ. In a next stage, these pulses are temporally stretched to a pulse length of approximately 300 ps with a grating pulse stretcher to decrease significantly the peak power below the damage threshold of the following elements. These stretched pulses are then amplified in several round trips in a regenerative amplifier containing a 20 mm long Ti:sapphire crystal in the cavity, which is pumped by 7 mJ pulses from a synchronized frequency-doubled Nd:YLF laser. A Pockel’s cell couples the pulses in and out with a repetition rate of 1 kHz. Finally the pulses are re-compressed to a pulse length of 80 to 100 fs in a grating compressor. The pulse energy now is typically 500µJ.

In a further stage (see Fig. 2.9), near-infrared pulses (λ = 1−2µm) are generated in an optical parametric amplifier (OPA). Only half of the output power of the regenerative amplifier is needed. A small fraction (≈1 %) of the amplified 800 nm pulses is focused into a sapphire plate generating a white-light continuum, which is used for seeding. A fraction of 10 % is spatially and temporally overlapped with the seed pulses in a BBO crystal to generate in a first stage signal and idler pulses with pulse energies of together 200 nJ in the range between 1.2 and 2.2µm via type-II phasematching. In a second stage these pulses are spatially and temporally overlapped with the resulting 90 % of the pump pulses at a different position of the BBO crystal and thereby amplified to pulse energies of together 70−80µJ.

2.2.2.2 Difference frequency mixing with signal and idler pulses

The mid-infrared pulses are generated via difference frequency mixing of signal and idler pulses in a 1 mm thick, type-I oriented GaSe crystal. For this, the setup shown in Fig.

2.10 was constructed. The temporal overlap is adjusted with a delay stage that separates signal and idler pulses using broadband dichroic mirrors. A telescope generates an effective focal length of 700 mm which leads to signal and idler foci of 500µm diameter at the GaSe crystal. By adjusting the GaSe and BBO crystal angles appropriately, type-I phasematching occurs. Behind the GaSe crystal, residual signal and idler components are suppressed using suitable long-pass interference filters with onset wavelengths in the mid-infrared. A reference

L1

Figure 2.10: Mid-infrared generation via difference frequency mixing of signal and idler pulses in GaSe (L₁: f = 150 mm,L₂: f =−100 mm). The filter behind the GaSe crystal is needed to block residual signal and idler pulses. A HeNe laser beam is overlapped with the mid-infrared beam [41].

100 200 300 0

1

-400 0 400 0

1

Intensity (normalized)

Photon energy (meV)

15 10 5

MIR Wavelength (µm)

(b) (a)

τ_p=120 fs

Two-photon absorption signal (norm.)

t in fs

Figure 2.11: (a) Spectra of the mid-infrared femtosecond pulses generated with phasematched difference frequency mixing of signal and idler pulses for several phasematching angles [42].

(b)Autocorrelation trace of a mid-infrared pulse centered at λ = 10µm measured via two-photon absorption in InSb. The measured autocorrelation width corresponds to a pulse length of τ_p = 120 fs.

beam is split off using a KBr plate. It is overlapped with the beam of a HeNe adjustment laser. In this way, steerability of the mid-infrared beam throughout the rest of the setup is accomplished.

Mid-infrared pulse parameters As in the high-frequency laser system, the pulses are tunable by adjusting the phasematching angle of the GaSe crystal. In addition to this the wavelengths of the signal and idler pulses have to be adjusted. The tuning range is wider than for the high-frequency system, it is in the spectral range between 3−20µm. The pulse length is τ_p = 120 fs for λ = 10.0µm [Fig. 2.11 (b)]. It varies strongly with the center wavelength as depicted in table 2.1. For wavelengths longer than 10µm the pulse lengths increase rapidly. For a wavelength of 12.5µm the pulse duration is 360 fs, which is far from the bandwidth limit. For these longer wavelengths pulse lengthening occurs as a result of dispersion in the GaSe crystal and subsequent optics. This can partly be compensated with

Table 2.1: Pulse lengths τ_p for different center wavelengthsλ [42].

λ / µm τ_p / fs

5.5 54

9.6 108

10.0 120 12.5 360

a pulse compressor as has been shown in [42].

The intensity of the mid-infrared output is measured directly using a sensitive ther-mopile detector. Smaller average powers for the experiments are determined with a cali-brated HgCdTe detector. The pulse energy is 1 µJ around λ = 5µm and falls off inversely proportional to the photon energy at longer wavelengths (approximately constant number of photons per pulse).