Photorefraction Management in Lithium Niobate Waveguides: High-Temperature vs. Cryogenic Solutions

TL;DR

Photorefraction in lithium niobate waveguides perturbs phase-matching and limits power-handling in nonlinear and quantum devices. The authors compare photorefraction effects in Ti:PPLN waveguides at high temperature and cryogenic temperatures, using two samples to study SFG phase-matching spectra and demonstrating a cryogenic-compatible suppression method via an auxiliary 532 nm light. They find that high-temperature operation substantially suppresses photorefractive distortions, while cryogenic operation reveals frozen-in charge effects that can be partly mobilized by green illumination to restore near-$\mathrm{sinc}^2$ phase-matching and boost SFG power, albeit with residual shifts and incomplete reversibility. These results offer a practical route to managing photorefraction in cryogenic quantum photonics and potentially in space- and low-energy-budget platforms.

Abstract

Lithium niobate sees widespread use in nonlinear and quantum optical devices, such as for sum- and difference-frequency generation or spontaneous parametric down-conversion. In lithium niobate waveguides, nonlinear optical processes are often limited by the so-called photorefractive effect, which limits the maximum input or output powers and impacts the nonlinear spectral response. Therefore, strategies for the management of photorefractive damage are a key consideration in device design. Usually, the photorefractive damage threshold, i.e. the maximal permissible operating power, can be increased by high temperature operation of devices. This approach, however, is not applicable in cryogenic environments, which may be required for specialized applications. To better understand the impact of photorefraction in nonlinear optical applications, we study the impact of photorefraction on the phase-matching spectra of two nonlinear-optical sum-frequency generation experiments at 1) high temperatures and 2) cryogenic temperatures. Furthermore, we present an approach to reduce the impact of photorefraction which is compatible with cryogenic operation. This comprises an auxiliary light source, propagating in the same waveguide, which is used to restore phase-matching spectra impacted by photorefraction, as well as reduce pyroelectric effects. Our work provides an alternative route to photorefraction management applicable to cryogenic environments, as well as in situations with tight energy budgets like space applications.

Figures (6)

• Figure 1: Experimental setup to investigate the phase-matching of a Ti:PPLN waveguide (a) at elevated temperatures, and (b) under cryogenic conditions. The heated waveguide is pumped with one pulsed and one continuous-wave (CW) laser to enable sum-frequency generation (SFG) and the generated beam is detected with a spectrometer. The cryogenic waveguide is pumped with two CW lasers to enable SFG. The generated beam is spectrally filtered and detected with a power meter. This process is combined with a CW auxiliary laser source at 532nm to suppress photorefractive damage. HWP: half-wave plate, PBS: polarizing beam splitter, DM: dichroic mirror, L: aspheric lens, BPF: band-pass filter.
• Figure 2: Repeated measurement of the phase-matched SFG spectrum of the high temperature waveguide. At low power (first row), the waveguide shows an unperturbed spectral shape and only minuscule changes between repeated measurements (different colored lines). At low temperature (left column), as the power is increased, the spectrum shifts towards shorter wavelengths, gets perturbed and shifts between measurements. With increased temperature, the onset of both effects shifts to higher pump powers.
• Figure 3: Relative position of the SFG maximum over the pump power for the high temperature waveguide. With increasing pump power (blue), the center of the phase-matched SFG shifts towards shorter wavelengths. As the temperature is increased, this shift decreases in magnitude. At temperatures over 380 K the phase-matching returns to its original position within the runtime of the measurement as the pump power is decreased (orange). At the lower temperatures the spectrum stays disturbed, showing significant hysteresis.
• Figure 4: Phase-matched SFG spectrum of the cryogenic waveguide. (a) The waveguide shows a perturbed spectrum which differs from the optimal shape due to the build-up of electrical charges in the waveguide region. This spectral shape does not change when the waveguide is kept cold and non-illuminated for one week, indicating that the charges are not moving under cryogenic conditions. (b) As soon as the light from a green laser diode is simultaneously coupled to the waveguide, the spectrum starts to change. The dark blue data points correspond to the same measurement as shown in (a), when the green light is still turned off. The spectral shape approaches the ideal $\mathrm{sinc}^2$ shape and the SFG power increases when illuminating the waveguide for 12 minutes with the green light.
• Figure 5: Increasing the pump power of the SFG process results in photorefractive damage to the cryogenic waveguide. Any initial damage was compensated before with the light from a green laser diode (see Fig. \ref{['fig:LT_PerturbedSpectrum']}). (a) The green light is turned off and the pump power is gradually increased. The peak power of the SFG process first follows the trend of the increasing pump power and then drops again. (b) - (e) Individual spectra measured for different pump powers. Increasing the power results in broadening of the spectrum and the evolution of side peaks.