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Physical Review E

Nonlinear frequency conversion with quasi-phase-mismatch effect

Hongchen Guo, Sing Hai Tang, Yi-qiang Qin, Yong Zhu

Published Jun 29, 2005• 13 citations
Publisher

Abstract

In conventional frequency conversion based on chi(2)-induced nonlinear optical effects, a number of phase-matching techniques have been deployed to resolve the phase mismatch between the interacting waves. In this work, we report that in the quasi-phase-matching scheme, the accumulation of the phase mismatch can induce a quasi-phase-mismatch effect. And on the contrary, instead of eliminating the phase mismatch, modulating the quasi-phase-mismatch effect can lead to bandwidth enhancement and multiple-wavelength conversion. Both of these are essential factors for applications in nonlinear frequency conversion. As examples, designs with reset-periodic and cascaded-periodic optical superlattices based on such modulation are presented.

DOI
10.1103/physreve.71.066615

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In conventional frequency conversion based on chi(2)-induced nonlinear optical effects, a number of phase-matching techniques have been deployed to resolve the phase mismatch between the interacting waves. In this work, we report that in the quasi-phase-matching scheme, the accumulation of the phase mismatch can induce a quasi-phase-mismatch effect. And on the contrary, instead of eliminating the phase mismatch, modulating the quasi-phase-mismatch effect can lead to bandwidth enhancement and multiple-wavelength conversion. Both of these are essential factors for applications in nonlinear frequency conversion. As examples, designs with reset-periodic and cascaded-periodic optical superlattices based on such modulation are presented.
Cascaded nonlinear phase shifts via second-harmonic generation are theoretically studied for quasi-phase-matched (QPM) structures. Both the effective phase mismatch and the structure itself contribute to the resultant nonlinear phase shift. Depending on the sign of the effective phase mismatch, structural contribution may enhance or depress the nonlinear phase shift. The fundamental wave will experience a nonlinear phase shift even if the effective phase matching is satisfied. By properly designing the effective phase mismatch, QPM structures with large material-based phase mismatch may provide large nonlinear phase shifts before saturation.
Analysis of Novel Cascaded chi(2)chi^(2)chi(2) (SFG +++ DFG) Wavelength Conversions in Quasi-Phase-Matched Waveguides
A novel cascaded second-order nonlinear interaction (/spl chi//sup (2)/), which is simultaneously based on sum frequency generation (SFG) and difference frequency generation (DFG) processes, is proposed and analyzed in quasi-phase-matched wavelength converters. Analytical expressions with clear physical insights are derived for the converted light. It is shown that the same conversion efficiency can be achieved by employing two pump sources with lower output power (P/sub p1/,P/sub p2/) in this novel scheme as compared with the conventional cascaded wavelength conversion technique based on second-harmonic generation and difference frequency generation (SHG+DFG) with a single higher power pump beam (P/sub p/=P/sub p1/+P/sub p2/). The theoretical results are consistent with the experimental ones. It is found that the pump wavelength difference can be separated by a span as large as 75 nm, while large 3-dB signal conversion bandwidth is retained. The results show that the novel cascaded /spl chi//sup (2)/ wavelength conversion scheme is very attractive for practical applications.
Cascaded second-order nonlinear interaction (&chi(2));based wavelength conversion technique has attracted much attention due to its unique characteristics such as low noise and broadband, which are critical in fiber communication networks. In this report, wavelength conversions based on the newly proposed SFG-DFG (sum frequency generation - difference frequency generation) and conventional SHG-DFG (second harmonic generation - difference frequency generation) are studied and compared both experimentally and theoretically in a LiNbO3 quasi-phase matched (QPM) waveguide. It is shown that the same conversion efficiency can be achieved by employing two pump sources with only half power each (P1, P2) in the SFG-DFG scheme as compared with the SHG-DFG scheme with a single higher power pump beam (P=P1+P2). It is shown that the cascaded SFG-DFG based wavelength conversion has a larger 3-dB pump tolerance bandwidth. The theoretical results are consistent well with the experimental ones. It is found that the pump wavelength difference can be separated by a span as large as 75 nm, while 3-dB signal conversion efficiency is retained in a 45 mm-long device. It is also exhibited that tolerance of temperature for the cascaded SFG-DFG remains the same as that of the cascaded SHG-DFG based devices. The results show that the SFG-DFG wavelength conversion scheme is very attractive for practical applications.
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.
Quasi-phase-matched parametric downconversion via cascaded optical nonlinearities in an aperiodically poled MgO:LiNbO3 superlattice was studied in theory and experiment. Due to the cascading effect and the abundant reciprocal structure in an aperiodic optical superlattice, multiple-wavelength parametric downconversion in a wide spectrum range can be obtained. Enhancement of the conversion efficiency and output stability through coupling of two nonlinear processes is demonstrated. The result also reveals that the cascaded parametric downconversion process can be used to efficiently downconvert the fundamental wavelength to a longer wavelength of the infrared region. The process can be additionally served as an efficient mechanism to enhance THz wave propagation in a nonlinear optical medium.