Lez #31+32 Laser Principles II

 We first finalized the laser lecture introducing the solid state laser principle. Then mode locking for pulsed laser generation, active and passive methods, gain and losses compensation.  

Lez #29+30 Laser Principles I

Recap on Absorption, Spontaneous and stimulated emission. Laser principles, treshold condition. Passive resonator. Is a laser superior to a conventional sources? Trading spatial and temporal coherence with flux. Statistical properties of light. How to make a laser: strategies for 2/3/4 level systems, complete equation with lasing dependent inversion.

Lez #27+28 ESONERO 1

Lez #25+26 NOPA II

 We started again from the Phase Matching (PM) condition and the necessity of using birefringent nonlinear crystals to satisfy it. Specifically, we saw in detail the use of BBO crystals (negative uniaxial) to achieve Type I PM (ooe), in collinear and non-collinear configurations. We saw how the “magic angle” can be determined by means of a numerical script, by drawing the phase-matching curves as a function of the signal wavelength and crystal angle Ɵ for different values of the noncollinearity angle α. We saw that different pumping wavelengths (515 and 400 nm) feature different magic angles, corresponding to distinct values of Ɵ. We also evaluated the wavevector mismatch as a function of ʎ and Ɵ, at the magic angle, and quantified the deviation from perfect phase-matching caused by fixing a specific value of Ɵ.

Finally, we explored the NOPA architecture's technical layout, detailing the various optical components and the pivotal role of the delay line in tuning the pump-signal delay inside the nonlinear crystal. We noted that with a chirped seed, amplification is restricted entirely to the spectral components that temporally overlap with the pump. As a result, simply moving the delay line enables a tunable spectral shift of the amplified pulse across the phase-matching region.

Lez #23+24 NOPA I

 We started by revising the derivation of the phase-matching bandwidth in three-wave mixing, which is inversely proportional to the group velocity mismatch (GVM) between the signal and idler.

We discussed the twofold importance of Optical Parametric Amplification (OPA), which is characterized by frequency tunability and its potential for broadband amplification. We examined the positive feedback loop between the signal and idler that gives rise to the exponential scaling of the OPA gain with the crystal length L, as well as the various factors that limit this gain in the short-pulse regime.

We then discussed the Phase Matching (PM) condition and the necessity of using birefringent nonlinear crystals to satisfy it. Specifically, we explored the use of BBO crystals (which are negative uniaxial) to achieve Type I PM. In this configuration, the signal and idler experience the ordinary refractive index (n_o ), while the pump experiences an effective refractive index -a combination of n_e and n_o- determined by the angle θ. We saw how non-collinear geometry introduces an additional degree of freedom, the pump-signal angle α, to fulfill PM. For a specific value of α (the "magic angle"), this allows us to simultaneously satisfy the PM condition—and hence obtain efficient amplification—over a broad spectral interval of the seed.

Next, we discussed the temporal walk-off between interacting pulses, defining the pulse-splitting length. We studied the optimal crystal length for OPA processes, noting that it varies critically depending on whether the GVM of the signal and idler with respect to the pump have the same or opposite signs.

Finally, we saw the result of the generalization of broadband PM to non-collinear geometries (NOPA). In this case, optimal broadband PM is achieved when the projection of the idler's group velocity onto the signal's propagation direction is exactly equal to the signal's group velocity. Consequently, while collinear broadband amplification requires a degenerate configuration (ω_1 =ω_2), the non-collinear geometry relaxes this constraint.

Lez #21+22 Pulse duration measurements, Autocorrelator

 We discussed different type of autocorrelation techniques for measuring the duration of ultrashort pulses, namely linear autocorrelator, non-collinear SHG autocorrelator and collinear SHG autocorrelator, discussing the different advantages/disadvantages. We also introduced FROG (frequency-resolved optical gating) as a convenient approach to characterize an optical pulse.

Lez #19+20 Esercitazione 1 esonero