Supercontinuum amplification by Kerr instability

The versatility of optical parametric amplifiers makes them excellent sources for ultrashort visible and infrared pulses that drive strong-field physics experiments. We extend four-wave optical parametric amplification to the strong-field regime, known as Kerr instability amplification, and experimentally verify the modified noncollinear conditions for optimum amplification. We confirm that Kerr instability amplification can be used to amplify spectra that span nearly an octave in a single beam. We also amplify the near-infrared portion of the spectrum to generate the third harmonic downstream.

Figure 1

(a) The setup for supercontinuum amplification. See text for details. Spectral amplification (log colors) measured in the (b) visible and (c) near IR as a function of seed-pump angle. (d) Kerr instability amplification calculation of the gain as a function of relative seed-pump angle and wavelength for a 785-nm pump at $I_0=1.5\times {10}^{13}$ W/${\mathrm{cm}}^2$. The pink line is the phase-matching condition for FWM, while the black dashed line is maximum gain in KIA. The two green boxes are for the experimentally measured regions, shown in (b) and (c).

Figure 2

(a) Temporal evolution of simulated supercontinuum generation matching experimental conditions. (b) Spectral amplification measurement of the supercontinuum seed at $4.9^{\circ }$ relative seed-pump angle as a function of pump delay. The amplification process maintains the seed temporal character. Black dashed lines are the regions used to generate the $4.9^{\circ }$ contributions in Figs. 1 and 1.

Figure 3

Spectral amplification measurement of the supercontinuum seed at $6.5^{\circ }$ relative seed-pump angle. (a) Amplification occurs only for the IR side of the seed. Black dashed lines are lineouts to generate the spectrum in (b). (b) Supercontinuum seed (blue) and amplified (black dashed) spectra. The wavelength region $>900$ nm was taken with an IR spectrometer.

Figure 4

Spectral amplification measurement of the supercontinuum seed from $4.1^{\circ }$ to $5.3^{\circ }$ as a function of delay. As the pump-seed angles (given in figures) increased, the amplified bandwidth increases.

Figure 5

Spectral amplification measurement of the supercontinuum seed from $5.7^{\circ }$ to $6.5^{\circ }$ as a function of delay. For these larger angles, there was a discontinuity in the amplification. At $6.5^{\circ }$, there was no amplification in the visible.

Figure 6

Simulation and experimental amplification at $4.9^{\circ }$. The KIA calculation (black solid lline) agrees well with the two-dimensional (2D) simulation (orange dashed line); the amplification difference comes from KIA not accounting for the creation of an idler field. The spatial profile reduced the amplification of the transform-limited supercontinuum seed pulse when the seed and pump beam waists were the same (purple dotted-dashed line); the large gain in the blue end of the spectrum is because there was a negligible seed spectrum in this region. The bandwidth of the experimentally measured amplified spectrum (blue dotted line) is in good agreement with theory.

Figure 7

Characterizing the NIR portion of the amplified spectra. (a) The measured and (b) reconstructed spectrogram after 500 iterations, with an error of 1.01. (c) The pulse intensity measured duration was 24 fs; compensating for the dispersion (e) predicts a 20 fs pulse. (d) Spectral amplitude and (f) phase are also presented.

Figure 8

Third-harmonic generation from amplified NIR spectrum. (a) The third harmonic, centered at 287 nm, was only created when the NIR portion is strongly amplified. The short coherence length, $L_{\text{coh}}\approx 3 \mu \text{m}$, limited the third-harmonic efficiency. (b) The FWHM bandwidth of the NIR spectrum was 83 nm, with a transform-limited pulse duration of 15 fs.