Harmonic conversion efficiency

Figure 6.15. Refractive index dispersion and harmonic conversion efficiency versus interaction kngthfor various degrees of phase mismatch Ak.

Figure 6.17. Harmonic conversion efficiency in a waveguide under phase-matched conditions versus path length for various values of ot a 10-mW input power and for the optimum value of the overlap integral as explained...

Figure 27 Third harmonic conversion efficiency as a functionn of fundamental beam intensity calculated for a 1 pm thick polymer thin film with X<3 (-3m w,w,w) 5x10 e.s.u.

With commercially available YDFL as pumps, powers > 40 W at 1178 nm are feasible. This sets an upper limit to the conversion efficiency needed in the subsequent second harmonic generation. Numerical simulations for the amplifier and resonator Raman laser configuration indicate feasibility of the system with sufficient SBS suppression. ESO has assembled the amplifier configuration, and has demonstrated up to 4 W CW at 1178 nm. ESO s goal is to have compact and turnkey commercial fiber lasers for LGS/AO within 3 years. 

In the case of SHG in waveguide nonlinear crystals, we describe a theoretical model which accounts for the temporal behavior of the interacting pulses and the possible z-dependence of the phasematching condition. The model also describes the observed saturation and subsequent decrease in SHG conversion efficiency in the waveguide samples, as a result of two-photon absorption (TPA) of the second harmonic (SH) wave. The results of this model are later compared with experimental data from SHG experiments using femtosecond pulses in the waveguide nonlinear crystals of periodically-poled potassium titanyl phosphate (ppKTP) and appKTP. This model is presented in section 2.3. 

Second harmonic generation (SHG) was first observed in single crystal quartz by Franken and coworkers (1) in 1961. These early workers frequency doubled the output of a ruby laser (694.3 nm) into the ultraviolet (347.15 nm) with a conversion efficiency of only about 10 % in their best experiments, but the ground had been broken. 

A typical dependence of the conversion efficiency on loss is shown in Fig. 6. The strong dependence on the fundamental and harmonic loss is clear. It limits the usable length of the waveguide to Lopt. It is clear that if 2 cm long waveguides are to be useful, losses below 1 dB cnr1 are needed at both frequencies ... 

A normalized conversion efficiency of 7% W xcm 2 was achieved in a first experiment. Although very high propagation loss of about 100 dB/cm at the 800 nm second harmonic wavelength resulted from the repoling, these figure of merit values were further increased to 14% W xcm 2 for a 1 mm device [78]. It is probable that the interface between the two oppositely poled regions is the source of the loss. Another theoretical route that requires special materials... 

By simultaneously injecting both beams into a DR1 PMMA film, within hours a self-written QPM nonlinearity of 90 pm V1 was permanently inscribed, allowing further use of the film as a second harmonic generator. The limitation is that the harmonic is generated near its absorption peak and hence only very thin films with a small net conversion efficiency can be used [116]. Detuned from resonance, the nonlinearity is naturally significantly decreased. Nonetheless this method remains potentially very attractive. 

Fig. 11. (a) FOM t 0, in % W-1cm-2 and (b) conversion efficiency FOM rj0 in % W-1cm-2 as a function of core thickness with the same nonlinearity din each case. Here the dashed line identifies the perfectly phasematched case, the solid lines identify the and the dotted lines the +/0 QPM structures respectively. The individual symbols relate to the MDPM structures with (solid) and +/0 (open). , MDPM, TM0(co)—>TM1(2co) , O -MDPM, TM0(co) T, V QPM maximum and , A QPM optimum. Calculation done for DANS d=6 pm V"1 with a second harmonic loss of 40 dB cm4 and a fundamental loss of 5 dB cnr1 for a sample length of 2 mm. The MDPM structures are calculated assuming two layers perfectly index matched to the DANS parameters... 

In the general case of second harmonic generation, it can be shown that the conversion efficiency Izo/lm is strongly dependent on the phase synchronization factor 1/2) where Ak = i 2o) respectively,... 

The irradiation of atoms and molecules with two lasers of different frequency and known relative phase has been shown not only to throw light on the ATI process in atoms [46] but has produced a dramatic enhancement in the conversion efficiency in high harmonic generation [47]. In addition, there is theoretical evidence that molecular dynamics may be controlled using two-colour excitation [48]. 

The conversion efficiency is directly proportional to the overlap integral between fundamental and harmonic optical fields of given modes... 

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