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Introduction to field
Optics is the study of interaction of the electromagnetic radiation with matter. The word ‘Nonlinear Optics’ was first coined by E. Schrodinger in 1943. Nonlinear optics is the study of nonlinear response of material to applied optical fields. When an electromagnetic wave interacts with a material, it induces a polarization i.e., dipole moment per unit volume, it arises due to the displacement of the centre of positive charge with respect to the centre of negative charge (as shown in Fig. 1). Fig. 2 shows the graphical representation of the linear and nonlinear optics regime.
Fig. 1: Polarisation of Atom
Fig.2: Polarization vs applied electric field for linear and non-linear regime
Research topic
The development of optical parametric oscillator (OPO) has revolutionised the field of nonlinear optics due to their highly tunable, coherent and high-power output from ultraviolet to mid-IR region, operating in temporal domain from continuous wave (cw) to femtosecond (fs) time scale. To generate output in the picosecond and femtosecond time-scale, OPOs are synchronously pumped using mode-locked lasers. However, given that synchronously pumped OPOs require mode locked ultrafast laser as pump source which results in high cost, large size and high complexity, over the recent years the approach of mode-locking of OPOs pumped by cw lasers has been explored and successfully demonstrated. Fig. 3 shows the different configuration of OPOs.
Singly resonant oscillator (SRO)
Triply resonant oscillator (TRO)
Doubly resonant oscillator (DRO)
Pump enhanced Singly resonant oscillator (PE-SRO)
Fig.3: Different Configurations of OPO
The generated pulse width in all the demonstrated cw OPOs till date is far from the transform limited duration for the available gain bandwidth, it is highly desirable to explore approaches which could result into broadband multi-axial cw-pumped OPO, that could be phase-locked efficiently leading to the generation of stable ultrashort pulses. Dispersion compensation is one of the approaches to generate broadband multi-axial output. Fig. 4 shows the frequency selection in cw pumped conventional DRO and dispersion compensated DRO. We are working on the theoretical and practical realization of a universal passive technique for broadband phase-coherent spectral generation based on OPOs pumped by low cost cw laser and incorporation of intracavity mode-locking element to generate stable tunable short pulses with cavity dispersion control.
Conventional CW-Pumped DRO (CDRO)
CW-Pumped Dispersion-compensated (DC) DRO (DCDRO)
Fig.4: Frequency selection in CDRO and DCDRO
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