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Chinese Physics Letters

Wavelength Converter Based on Linearly Chirped Gratings in Lithium Niobate Through Cascaded Second-Order Processes

Shiming Gao, Changxi Yang, Jin Guo-Fan

Published Jul 30, 2003• 6 citations
Publisher

Abstract

We report a wavelength converter based on linearly chirped gratings in a lithium niobate crystal through cascaded second-order nonlinear processes. The influence of the chirp coefficient on the conversion bandwidth and the efficiency is analysed. The exact phase matching point is optimized at the 0.33 times of the waveguide total length. When the waveguide length is 3 cm, the bandwidth of the linearly chirped grating increases 42% with the efficiency penalty of 1.7 dB compared with the periodic grating. At the same mean conversion efficiency of -1.4 dB via adjusting the waveguide total lengths, the bandwidth of the linearly chirped grating is 36% broader than the periodic grating.

DOI
10.1088/0256-307x/20/8/326

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 24, 2010•1 citation
We report a wavelength converter based on linearly chirped gratings in a lithium niobate crystal through cascaded second-order nonlinear processes. The influence of the chirp coefficient on the conversion bandwidth and the efficiency is analysed. The exact phase matching point is optimized at the 0.33 times of the waveguide total length. When the waveguide length is 3 cm, the bandwidth of the linearly chirped grating increases 42% with the efficiency penalty of 1.7 dB compared with the periodic grating. At the same mean conversion efficiency of -1.4 dB via adjusting the waveguide total lengths, the bandwidth of the linearly chirped grating is 36% broader than the periodic grating.
High-efficiency ultra-broadband wavelength converters based on double-pass quasi-phase-matched cascaded sum and difference frequency generation including engineered chirped gratings in lossy lithium niobate waveguides are numerically investigated and compared to the single-pass counterparts, assuming a large twin-pump wavelength difference of 75 nm. Instead of uniform gratings, few-section chirped gratings with the same length, but with a small constant period change among sections with uniform gratings, are proposed to flatten the response and increase the mean efficiency by finding the common critical period shift and minimum number of sections for both single-pass and double-pass schemes whilst for the latter the efficiency is remarkably higher in a low-loss waveguide. It is also verified that for the same waveguide length and power, the efficiency enhancement expected due to the use of the double-pass scheme instead of the single-pass one, is finally lost if the waveguide loss increases above a certain value. For the double-pass scheme, the criteria for the design of the low-loss waveguide length, and the assignment of power in the pumps to achieve the desired efficiency, bandwidth and ripple are presented for the optimum 3-section chirped-gratings-based devices. Efficient conversions with flattop bandwidths > 84 nm for lengths < 3 cm can be obtained.
For the single-pass and double-pass wavelength converters based on cascaded sum and difference frequency generation in quasi-phase matched lithium niobate waveguide, we present the criteria for a choice of the waveguide length and the assignment of pump powers to achieve the desired efficiency, ripple and bandwidth with a large pump wavelength difference of 75 nm. It is found that using the double-pass structure with a 95% reflectivity for the sum frequency wave, the mean efficiency is increased at least 5 dB compared to that of the single-pass one with the same length and input pump powers and the bandwidth is large enough to cover the entire conventional telecommunications C-band and long-wavelength L-band. To flatten the excessive response fluctuation of the double-pass scheme, we propose tuning one of the pumps to longer wavelengths incurring a small penalty of a slight reduction in the mean efficiency which can be compensated by increasing the input pump powers. We also show that to achieve the same amount of flatness, double-pass converters have a smaller efficiency penalty compared to the single-pass converters.
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.
Broadband wavelength converters based on difference frequency generation (DFG), single-pass and double-pass cascaded second harmonic generation and difference frequency generation (SHG+DFG), single-pass and double-pass cascaded sum and difference frequency generation (SFG+DFG) are fulfilled by utilizing segmented grating structure in lithium niobate waveguide. Under the small-signal approximation, the effects of the waveguide length and the response flatness on DFG-based wavelength conversion are investigated by use of matrix operator, and then a feasible scheme to enhance the bandwidth and stability of signal and pump wave is proposed. For single-pass/double-pass SHG+DFG wavelength conversion scheme and single-pass/double-pass SFG+DFG wavelength conversion scheme, the effect of segmented grating structure on conversion bandwidth is studied theoretically and numerically, the results show that the signal conversion bandwidth can be broadened by optimizing the aperiodic grating. Moreover, the influence of "balance condition" on double-pass SFG+DFG-based wavelength conversion is analyzed, one can achieve enhanced conversion efficiency and conversion bandwidth by adjusting the power and wavelengths of pump sources according to the "balance condition". In the end, a reasonable suggestion is presented to achieve broadband wavelength conversion under different requirements.