*Towards Enhancing the Efficiency of Nonlinear Optical Generation DOI: http://dx.doi.org/10.5772/intechopen.80816*

*Nonlinear Optics - Novel Results in Theory and Applications*

surface (7 m ROC) dielectric coated for 70% R @ 10.6 μm] and a plane ZnSe dichroic mirror M2 (R > 90%@10.74 μm, T > 90%@5.37 μm) (refer to **Figure 2b**). The length of this external cavity (~1.21 m) was such as to push the g1 × g2 value viz., 1.17 beyond the region of stability. There was a remarkable enhancement in the generation of SH output when M2 was fine tuned to ascertain its parallelism with the convex face of mirror 'M1'. Performance of this multi-pass cavity with respect to the generation of SH was characterised by varying the pump energy incident on the crystal and measuring the corresponding energy of the SH beam emerging through 'M2' (**Figure 5**). When the cavity is perfectly aligned, the pump photons coming through the Mirror M1 are in phase, at every instant, with the fraction of the unconverted pump that is reflected off it. This increases the effective energy input to the crystal and that, in turn, results in a correspondingly increased SH output. This fact is amply clear from **Figure 4** in conjunction with **Figure 5**. It is apparent that for a maximum input pump energy of ~6.5 mJ, the single pass SH output is ~0.55 mJ (**Figure 4**) while according to **Figure 5**, the same input of 6.5 mJ gets enhanced to ~9.2 mJ due to cavity effect. The corresponding SH multi pass output is ~1.625 mJ, almost a three-fold increase when compared to the single pass case. Considering 9.2 mJ as the input energy, the SHG efficiency can be estimated to be ~17.66% - a clear ~209% improvement as against the single pass case. To be noted here that the pump energy has actually been maintained at ~6.5 mJ and therefore the conversion efficiency has risen by ~295% as a matter of fact. In these experiments, both pump and SH beams suffered significant Fresnel reflection losses during their repeated back and forth passage through the crystal that was not anti-reflection coated. Further, as the pump laser output coupler M1 is only 23% reflective at 5.35 μm, a major part of the SH generated in the reverse direction escapes through this mirror. The dramatic improvement in the SH conversion efficiency that has been obtained in the multi-pass case is thus by no means an optimised one. Increasing the reflectivity of the rear mirror at the SH wavelength in addition to employing a crystal with broadband anti reflection coating on both its entrance and exit faces should be able to fully exploit the decided advantage of a multi-pass case. We also note here that this scheme does not suffer from the conventional single pass walk off [16] between the pump and the SH beams as mirror M2 is almost transparent to the SH beam thereby providing feedback only to the pump beam. As the second

*Dependence of multi-pass second harmonic output on the effective input pump energy following cavity effect.*

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**Figure 5.**

harmonic beam generated in the forward direction alone is extracted in this configuration, its spatial quality is practically same as that of a single pass case. Therefore no special effort was expended to monitor the spatial quality of the SH beam. However, the visual observation of a clear well defined spot when the generated beam was focussed by a 10 cm focal length CaF2 lens on a graphite plate bore testimony to its satisfactory spatial character.

Towards comparing the SH power conversion efficiency in the single and multipass cases, we monitored the temporal profiles of the pump and the corresponding SH pulses with the external cavity in aligned and misaligned conditions. In order to obtain smooth temporal profiles devoid of mode beating, we captured the power profiles in all the four cases with oscilloscope set in bandwidth limited mode and the same are displayed in the traces of **Figure 6** from where the single pass internal peak power SH conversion efficiency can be readily estimated to be ~10.48%. The power conversion efficiency is thus greater than the energy conversion efficiency (8.45%) of the SHG process. This is because the peak power always exceeds the average intra-pulse power of the pump beam and higher is the intensity at the pump wavelength, better is the SH conversion. This observation is in general concurrence with the finding of several researchers [13, 17, 18]. When the cavity is perfectly aligned, the photon flux at the entrance face of the crystal comprises of two components at any point of time; (i) the photons constituting the output of the pump laser and (ii) the photons constituting the fraction of the unconverted pump beam that is reflected off the convex surface of the output coupler of the pump laser. When the cavity is aligned, these two components fall in step and an overall rise in the power level of the input pulse is thus the end result. A comparison of the input power profile traces for aligned and misaligned conditions as recorded in **Figure 6** clearly substantiates this fact. The rise in the input power level, in turn, leads to an enhanced SH conversion yielding a peak power conversion efficiency of ~22.36%, more than twice that is possible by single-pass conversion. Actually though, since the pump laser output has remained the same for both the aligned and misaligned cases, the effective SH peak power conversion efficiency stands at 35.8%, a neat enhancement of 341% due to the cavity effect.

In the next set of experiments, we captured the temporal profiles of the pump and the corresponding cavity enhanced second harmonic pulses by setting the oscilloscope at its highest bandwidth (Tektronix MSO 3054) and the same are depicted in **Figure 7a**. That the emission of the pump laser is on multimode is evidenced by the rich modulation present in the temporal profile of the pump as well as the corresponding SH pulses. The lower trace of **Figure 7b** depicts the time expanded temporal profile of the pump pulse where an oscillation of period ~7 ns arising out of the beating of two longitudinal modes, matching with the round trip

## **Figure 6.**

*Temporal profiles of the fundamental (bottom trace) and the corresponding second harmonic (top trace) captured in bandwidth limited mode; a: Single pass conversion, b: Multi-pass conversion.*
