Optical square wave sources are particularly important for applications in high speed signal processing and optical communications. In most realizations, optical square waves are generated by electro-optic modulation, dispersion engineering of mode-locked lasers, polarization switching, or by exploiting optical bi-stability and/or optical delayed feedback in semiconductor diode lasers, as well as vertical-cavity surface-emitting lasers (VCSELs). All such configurations are bulky and cause significant timing jitters. Here we demonstrate the direct generation of optical square waves from a polarization-maintaining figure-eight nonlinear amplifying loop mirror (NALM) configuration that uses an embedded high index glass micro-cavity as the nonlinear element. Such a NALM mimics the behavior of a saturable absorber and has been used to reach passive mode-locking of pico- and even nano-second pulses. In our method, the NALM, including a high-Q micro-ring resonator, acts as an ultra-narrowband spectral filter and at the same time provides a large nonlinear phase-shift. Previously we have demonstrated that such a configuration enables sufficient nonlinear phase-shifts for low-power narrow-bandwidth (~100 MHz FWHM) passive mode-locked laser operation. Here we demonstrate the switching of stable optical square wave pulses from conventional mode-locked pulses by adjusting the cavity properties. In addition, the square wave signal characteristics, such as repetition rate and pulse duration, can be also modified in a similar fashion. The source typically produces nanosecond optical square wave pulses with a repetition rate of ~ 120 MHz at 1550nm. In order to verify the reach of our approach, we compare our experimental results with numerical simulations using a delay differential equation model tailored for a figure-eight laser.
Structured coherent optical beams including optical vortices, hollow Gaussian beam, and Airy beam have found wide range of applications in variety of fields in science and technology. All existing techniques used to date to generate such beams suffer from different limitations including lower output power and restricted wavelength coverage. On the other hand, interactions of such beams with nonlinear media are mostly unexplored. We have been involved in the study of nonlinear interaction of optical beams with different spatial structures. Here, we discuss our recent results on nonlinear generation of various structured laser beams including optical vortices, perfect vortices, Airy beam, and hollow Gaussian beam and some of their applications
We report on development of tunable, cw, single frequency ultraviolet (UV) source based on intra-cavity frequency
doubling of an optical parametric oscillator (OPO). The singly resonant OPO (SRO) is realized using 25 mm long MgOdoped
periodically poled stoichiometric lithium tantalite (MgO:sPPLT). The SRO, pumped in the green, is intra-cavity
frequency doubled using two different crystals β-BaB2O4 (BIBO) and periodically poled KTiOPO4 (PPKTP) to generate
tunable UV radiation. The BIBO based source produces UV radiation with output power as much as 770 mW at 398.24
nm in elliptic (0.66) spatial beam while pumping at 8W of green power. The UV source can be tuned across 355.2-418
nm with a continuous wavelength tunability of 62.8 nm in single frequency radiation with instantaneous line-width of
14.5 MHz. On the other hand, the PPKTP based UV source generates maximum UV power of 336 mW at 398.28 nm and
wavelength tunability of 18.1 nm in a TEMOO spatial mode with ellipticity of 0.93 for 5W of green pump power. The
line-width of the UV radiation is measured to be 18.5 MHZ. In comparison, PPKTP based UV source provides better
performance than BIBO in terms of beam quality and power stability.