Based on a study of the newly developed nonlinear optical m aterials, potassium tit any 1 phosphate and lithium trib orate, we have dem onstrated two highly efficient tunable frequency- up and -down conversion schemes, i.e.,
- The low-threshold, high-efhciency all-solid-state K TP OPO tunable from 1.54 to 1.84 ////n (signal) and 3.3 to 2.6 fiin (idler).
- The all-solid-state NCPM type I LBO sum-frequency generation scheme tu n able from 0.623 to 0.65 fim.
To our knowledge, the ~40% external conversion efficiency from the pum p to the signal of the CPM K TP O PO , and the tunable sum-frequency generation based on the non-critical tem perature phase-m atched LBO are the first dem onstrations of such devices.
The principal conclusions of this work are as follows;
G \iha’s model provides a practical and useful m ethod for pum p threshold calculation for OPOs. In this work, we have m ade two contributions to further extend this model. The first is the extension to make it suitable for any type of phase-m atching of a, SRO. The second is the extension to make it suitable for the double-pass-pum p SRO configuration. The m atching of the pum p and signal beams to enable the greatest power transfer from the pum p to the signal and idler waves is an im portant factor in a param etric interaction
process, and it is m easurable by the introduced param eter The comparison
169
C H A P T E RS . CONCLUSIONS 170
of operating characteristics between the AO Q-switched laser pnrnped K TP OPOs and the EO Q-switched laser pum ped K T P OPOs presented in our work has experim entally proved how im portant the hgrn value of an OPO system is. Therefore, the calculation of the hsm param eter is shown to be a useful tool in OPO design.
At high pum p intensity levels for an O PO, a high value of signal outp u t coupling T is needed to avoid back-conversion. For the same pum p intensity, the intracavity signal intensity will also be lower for large values of T, which should reduce the chance of optical dam age to the crystal.
The double-pass-pum p and the intracavity OPO have been dem onstrated to be the m ost effective configurations for low-threshold, high-efficiency and TEMuu operation of singly- resonant OPOs, and the two configurations will be particularly im portant in a critical- phase-m atching geom etry where only the non-resonant wave is an extraordinary wave. A useful set of therm o-optical coefficients for LBO has been deduced experim entally in this work, and they can be used to predict tem perature phase-m atching conditions in the range of 20-200"C.
Our study has shown th a t LBO is non-critical tem perature ph ase- m at ch able for SFG over a large wavelength range. In particular, type I NCPM LBO SFG is suitable for generating tunable light in visible, and near infrared spectral regions; and type II NCPM LBO SFG is suitable for generating tunable light from the visible to the deep uv spectral regions. The tunable red light generation device based on the highly efficient K TP OPO and NCPM LBO SFG shows great promise as a novel tunable frequency up-conversion scheme, with much higher overall conversion efficiency than standard frequency up-conversion schemes (SFG-t-OPO).
The successful development of K T P OPOs and the study of the LBO NCPM SFG have opened the door for future work with several o])portunities. As we have m entioned in chapter 3, KTA and other isom orphs of K T P are good alternative m aterials for the OPO when tunable light in the range 2-5 fim is required. The similar phase-m atching properties and the relatively higher nonlinear effective coefficient of these m aterials should provide similar operating characteristics to the K TP based OPO. The essential phase-m atching condition of the NCPM LBO based SFG is tha t one input wa.ve has to be infrared, and the other wave can be any wavelength from the near infrared to as short as the cut-ofi wavelength of LBO, namely 0.16 fim. Therefore tunable light in. the spectral regions from
CHAPTER. 8. CONCLUSIONS 171
the visil)le to deep uv can be generated effectively in the scheme infrared OPO 4- NCPM LBO SFG, in which the pum p laser is separated into two legs, one is up-converted into its second, third, ..or fifth harm onic frequency, and the other one is down converted into tunable infrared light by the O PO, and then both lights are mixed in the NCPM LBO for the tunable up-conversion generation. This frequency up-conversion scheme, we believe, is particular im portant for the uv and the deep uv s])ectral regions.