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Applied Optics

Improved cascaded sum and difference frequency generation-based wavelength converters in low-loss quasi-phase-matched lithium niobate waveguides

Amirhossein Tehranchi, Raman Kashyap

Published Oct 22, 2009• 10 citations
Publisher

Abstract

We numerically evaluate the wavelength converters based on cascaded sum and difference frequency generation in quasi-phase-matched lithium niobate waveguides with and without loss. A technique is also proposed to flatten the response by increased detuning of the pump wavelength. We present the criteria for the design of waveguide length and the assignment of pump power to achieve the desired efficiency, ripple and bandwidth, assuming a large pump wavelength difference of 75 nm.

DOI
10.1364/ao.48.00g143

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We numerically evaluate the wavelength converters based on cascaded sum and difference frequency generation in quasi-phase-matched lithium niobate waveguides with and without loss. A technique is also proposed to flatten the response by increased detuning of the pump wavelength. We present the criteria for the design of waveguide length and the assignment of pump power to achieve the desired efficiency, ripple and bandwidth, assuming a large pump wavelength difference of 75 nm.
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.
We investigate response flatting of cascaded sum- and difference- frequency generation (SFG/DFG)-based wavelength conversion in quasi-phase-matched (QPM) periodically poled lithium niobate waveguides for wavelength division multiplexing optical communication systems. The cascaded SFG/DFG-based configuration shows more robustly combined properties than conventional DFG-based and cascaded second-harmonic generation and DFG (SHG/DFG)-based configurations. The response fluctuation, generated while enhancing the conversion bandwidth, can be efficiently reduced by the method of pump detuning or QPM period detuning.
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.
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.
The principle of all-optical wavelength conversion based on the cascaded sum and difference frequency generation (SFG+DFG) in quasi-phase-matched (QPM) periodically poled LiNbO3 (PPLN) waveguides is introduced. The conversion efficiency of the wavelength converter is calculated. The influences of the powers and span of two pumps on the conversion efficiency are analyzed. Bandwidth of one pump wavelength is discussed. The tuning is about 0.5 nm. The signal pulse width is compressed about 0.68 times.