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Journal of Lightwave Technology

Generation of Nearly Flattop Ultrabroadband Response in a QPM Device Using Phase Shifter

Narayanan Balaji, Toijam Sunder Meetei, Meerasha Mubarak Ali, Shanmugam Boomadevi, M. Senthilkumar, Krishnamoorthy Pandiyan

Published Nov 16, 2018• 9 citations

Abstract

We have theoretically demonstrated the efficient generation of nearly flattop ultrabroadband response in a periodic quasiphase matching (QPM) device using a single phase shifter domain. The flat broadband response is generated using optical parametric generation (OPG) with a QPM device of grating period Λ = 27 μm. The choice of grating period and pump source depends on the group velocities of signal and idler wavelengths to match close to the degeneracy point. We have identified the location of phase shifter domain, which generates a nearly flattop OPG gain bandwidth of Δλ = 1.41 μm. The variations in broadband response are studied when the location and width of phase shifter domains are altered. When compared to a periodic device, the phase shifter domain included QPM structure exhibits higher tolerance to temperature in retaining the smoothness of the broadband response. Furthermore, we have generated the same ultrabroadband response by introducing multiple phase shifter domains at suitable locations in a periodic QPM device.

DOI: 10.1109/jlt.2018.2881796

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Bandwidth enhancement and response flattening of wavelength conversion based on single-pass and double-pass cascaded second harmonic generation and difference frequency generation were investigated in segmented quasi-phase matched (QPM) gratings. It is shown that the signal and pump bandwidths are both efficiently widened by increasing the segment number of the QPM grating and optimising the poling period of each segment. The ripple on the matching response is also very small. The conversion bandwidth in a 3-cm-long three-segment waveguide reaches 150–160 nm, which is over the whole conventional band and long-wavelength band. Larger signal bandwidth can be obtained with a little response flatness penalty and conversion efficiency penalty, which can be compensated by increasing the input pump power. Compared with a sinusoidally chirped optical superlattice device, a wavelength converter based on the segmented gratings has higher conversion efficiency, broader bandwidth and better pump-wavelength tolerance, and is easier to fabricate in practice.

An incomplete period of the last quasi-phase-matching (QPM) segment in a periodically poled nonlinear crystal represents a key problem in implementing a nonlinear wavelength-conversion device with round-trip or oscillating waves of interaction. Such a segment at the crystal end implies broken QPM periodicity for the reflected waves of interaction. We demonstrate numerically that with a grating structure at the crystal end for the reflecting signal the wavelength-dependent phase shift at Bragg reflection can compensate for the phase mismatch of the reflected signal, idler, and pump of an optical parametric process. Therefore, by slightly shifting the signal wavelength we can maintain the phase-matching condition with any length of the last QPM segment. It is shown that with a grating structure the conversion efficiency of the round-trip optical parametric process can always be optimized.

A configuration for wavelength conversion and optical amplification by parametric interaction using a nonlinear optical device is proposed. It enables pump generation through second harmonic generation (SHG), difference frequency generation (DFG), and optical parametric amplification (OPA) using a multiple-quasi-phase-matched (M-QPM) LiNbO3 waveguide in a bidirectional manner. Wavelength conversion for the 1.4–1.6 µm band is experimentally demonstrated. In addition, it is demonstrated that the parametric gain band can be changed using various detunings between the pump and QPM wavelengths used for the DFG/OPA process. The proposed method would be useful for enabling high-capacity optical transmission outside the 1550-nm band.