Meenu Ahlawat

Accurate in-situ gas temperature measurements in dielectric barrier discharges at atmospheric pressure

Atmospheric pressure (AP) dielectric barrier discharges are frequently of interest for treating delicate substrates such as polymers or biological materials. In spite of its capital importance, thermometry in AP plasmas is subject to much uncertainty. We report temperature measurements in noble gases, nitrogen, and air using sensitive, accurate fibre-optic instrumentation that is a priori immune towards high voltages and high-frequency electromagnetic fields generally encountered in plasma environments.

Agile multicasting based on cascaded χ (2) nonlinearities in a step-chirped periodically poled lithium niobate

We experimentally demonstrate the possibility of agile multicasting for wavelength division multiplexing (WDM) networks, of a single-channel to two and seven channels over the C band, also extendable to S and L bands. This is based on cascaded χ(2) nonlinear mixing processes, namely, second-harmonic generation (SHG)-sum-frequency generation (SFG) and difference-frequency generation (DFG) in a 20-mm-long step-chirped periodically poled lithium niobate crystal, specially designed and fabricated for a 28-nm-wide SH-SF bandwidth centered at around 1.55 μm. The multiple idlers are simultaneously tuned by detuning the pump wavelengths within the broad SH-SF bandwidth. By selectively tuning the pump wavelengths over less than 10 and 6 nm, respectively, multicasting into two and seven idlers is successfully achieved across ~70 WDM channels within the 50 GHz International Telecommunication Union grid spacing.

Tailoring and tuning of the broadband spectrum of a step-chirped grating based frequency doubler using tightly-focused Gaussian beams

We demonstrate theoretically and experimentally, that the non-uniform spectra of second harmonic generation (SHG) from an unapodized step-chirped periodically poled nonlinear optical grating can be apodized utilizing tightly-focused Gaussian beams to suppress the ripple in its wideband response. In our example, by increasing focusing, a ripple-free response is progressively achieved over a 6-dB bandwidth of >5 nm, with a beam waist of 20 µm. With this tight focusing arrangement, a continuous tuning of 11-nm is also demonstrated by simply changing the focal point by 5.8 mm within the step-chirped grating based APPLN.

Tunable single-to-dual channel wavelength conversion in an ultra-wideband SC-PPLN

We experimentally demonstrate tunable dual channel broadcasting of a signal over the C-band for wavelength division multiplexed (WDM) optical networks. This is based on cascaded χ(2) nonlinear mixing processes in a specially engineered, 20-mm-long step-chirped periodically poled lithium niobate with a broad 28-nm second harmonic (SH) bandwidth in the 1.55-μm spectral range. A 10-GHz picosecond mode-locked laser was used as a signal along with a CW pump to generate two pulsed idlers, which are simultaneously tuned across the C-band by detuning of the pump wavelength within the broad SH bandwidth. Variable-input, variable-output scheme of tuned idlers is successfully achieved by tuning the signal wavelength. Pump or signal wavelength tuning of ~10 nm results in the idlers spreading across 30 nm in the C-band.

Design, fabrication and characterization of a specially apodized chirped grating for reciprocal second harmonic generation

A specially-designed apodized chirped PPLN based on particular positioning of poled regions within the periods has been realized theoretically and experimentally to demonstrate the reciprocal response in the SHG spectra over a 30-nm bandwidth, for up-chirp and down-chirp directions. The simulation results are compared with another apodized chirped PPLN for which the placement of poled regions is deviated from optimum positions. The average power difference is less than 0.75 dB and the standard deviations of extrema on second harmonic power responses are 1.34 dB and 1.64 dB for two up-chirp and down-chirp directions respectively.

Ultrabroadband flattop wavelength conversion based on cascaded sum frequency generation and difference frequency generation using pump detuning in quasi-phase-matched lithium niobate waveguides

Flattop wavelength conversion over a 70 nm bandwidth in a 45-mm-long periodically poled lithium niobate (PPLN) waveguide based on nonlinear cascaded sum frequency generation (SFG) and difference frequency generation processes using two separated wavelengths, is realized. A 2 dB peak-to-peak ripple over the conversion bandwidth in the C band is observed when the two pumps are fixed for perfectly phase-matched SFG conversion; this is significantly reduced to 0.5 dB by slight detuning of one of the pumps to a longer wavelength. We also demonstrate multichannel broadcasting using two pumps to convert one signal wavelength to three idlers. Using the aperiodically poled lithium niobate waveguide increases the idler separation.

Flexible All-Optical Wavelength Shifters Using Strong Focusing in a Wideband Engineered PPLN

Wavelength shift of modified second-harmonic bandwidth in a 20-mm-long engineered chirped periodically poled lithium niobate is experimentally demonstrated by focalpoint displacement of a strongly focused pump beam of a 30-μm beam waist. Utilizing this non-confocal arrangement in a cascaded second harmonic generation and difference frequency generation process, a CW probe as a signal wavelength is variably shifted within an 8-nm span by pump tuning using a ~7-nm second-harmonic bandwidth.

Tunable wavelength shifters using tight focusing in a wideband engineered PPLN crystal

Enhanced tuning of pump bandwidth of an efficient broadband engineered PPLN is realized with a non-confocal focusing to variably shift signal wavelengths in the C-band based on cascaded second-harmonic and difference-frequency generation.

Effect of a tightly focused Gaussian beam on the broadband SHG response of chirped poled lithium niobate

It is demonstrated, both theoretically and experimentally, that employing tightly focused Gaussian beam is possible to engineer the efficiency profiles and suppresses ripples in the second-harmonic response of a 30-nm-bandwidth chirped aperiodically poled lithium niobate.