Optical parametric oscillators (OPOs) serve as versatile devices for producing tunable coherent radiation by employing nonlinear frequency conversion. They are capable of covering spectral ranges not adequately addressed by direct laser emission.
OPOs function as light sources, resembling lasers, but they utilize optical gain from parametric amplification in a nonlinear crystal instead of stimulated emission. Similar to lasers, OPOs exhibit a threshold for pump power, below which the output power is negligible.
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A Brief Introduction to the Parametric Process
Parametric interaction is a three-wave mixing phenomenon that occurs in a non-centrosymmetric medium, such as a crystal, as explained by the experts at Photoniques. The pump, characterized by the highest angular frequency ωp, is the most intense beam. When the signal wave, with a frequency of ωs, is incident alongside the pump on the nonlinear crystal, the signal undergoes amplification while the pump is depleted. Simultaneously, an idler wave is generated at the difference frequency ωi = ωp – ωs.
Optical parametric processes are multiwave interactions that take place in nonlinear media. In these processes, several optical waves excite a medium, and its nonlinear response generates new radiation at a frequency that is a simple combination, such as the sum or difference, of the incoming frequencies. The evolution equations governing these interactions generally require fewer approximations compared to those used for lasers.
Working of Optical Parametric Oscillators
Similar to a laser oscillator, it is possible to create an optical parametric oscillator (OPO) by placing the nonlinear crystal within an optical cavity. In the OPO, parametric generation occurs and builds up on quantum noise, with the signal and idler waves getting amplified during each round trip in the optical cavity. Because of the simultaneous presence of three interacting waves, the configurations of optical resonators for OPOs are more varied than in the case of a laser oscillator.
In an optical parametric oscillator (OPO), the pump light's electric fields drive some of the crystal's electrons to oscillate. Through careful design choices, the crystal's electrons respond not only by oscillating at the frequency of the pump light but also by exhibiting components of their oscillation at the signal and idler frequencies.
This leads to the emission and amplification of signal and idler light. As the signal and idler waves amplify, the pump wave weakens. This process is effective when mirrors are placed around the crystal to enhance the light fields within it. The presence of mirrors forces the reflected light to repeatedly pass through the crystal, turning the system into an oscillator. Without mirrors, it would function as an optical parametric amplifier (OPA).
What are Photonic Crystal OPOs?
The miniaturization of devices has long been a key goal in microelectronics and photonics, driven by the desire for denser integration, improved functionalities, and a significant reduction in power consumption. Semiconductor photonic crystals (PhCs) hold a notable position among various nanostructures, offering the capability to create quasi-ultimate optical cavities.
Researchers have reported the implementation of PhCs in an important class of optical sources known as optical parametric oscillators (OPOs), aiming to develop a new category of OPOs with enhanced capabilities, as explained in Nature Photonics.
The OPO is developed using a 20-μm-long semiconductor photonic crystal cavity and operates within the telecom wavelengths. Parametric oscillation is achieved by thermally tuning high-quality-factor modes into a triply resonant configuration while other parametric interactions are effectively suppressed.
The estimated lowest pump power threshold for this source is in the range of 50–70 μW. This OPO demonstrates characteristics of an ideal degenerate optical parametric oscillator, making it suitable for applications in quantum optical circuits and contributing to the development of densely integrated, highly efficient nonlinear sources for squeezed light or entangled photon pairs.
Applications of Octave-Spinning Tunable Infrared Parametric Oscillators
A group of researchers have successfully demonstrated ultra-widely tunable doubly resonant OPOs in lithium niobate nanophotonics in the article published in Science Advances, extending their applicability to the near-infrared and visible ranges.
These nanophotonic OPOs, with both cubic and quadratic non-linearities, provide widely tunable coherent sources essential for various applications, including multichannel optical communications and LiDAR. The advancement in lithium niobate nanophotonics opens up new possibilities for achieving highly tunable and compact OPO devices across different spectral ranges.
The research team achieved remarkable progress in dispersion-engineered periodically poled lithium niobate nanophotonics, showcasing ultrawide tunable optical parametric oscillators (OPOs). Utilizing 100 ns pulses near 1 μm, the team generated output wavelengths tunable from 1.53 μm to 3.25 μm within a single chip, achieving output powers reaching tens of milliwatts.
This groundbreaking achievement marks the first instance of an octave-spanning tunable source in nanophotonics that extends into the mid-infrared, opening up possibilities for numerous integrated photonic applications.
A Novel Backward Wave OPO
Backward wave optical parametric oscillators (BWOPO) signify a shift in the paradigm of optical parametric oscillation. Unlike conventional optical parametric oscillators, where the generated signal and idler propagate in the direction of the pump, a BWOPO produces signal and idler waves that counter-propagate.
This unique anti-parallel interaction enables distributed feedback without the need for a cavity, rendering the BWOPO a robust, easily aligned, and highly efficient single-pass device.
In this method, an ion exchange grating is initially created in KTiOPO4 (KTP), acting as both the waveguide and the coercive field grating. The challenges in fabrication are addressed through coercive field engineering. Following this, the waveguide undergoes periodic poling, eliminating the necessity for further heat treatment. The oscillation threshold is achieved at a pump energy of 325 nJ, showcasing a significant 19-fold reduction compared to a recent bulk BWOPO experiment using the same laser for pumping.
Integrated lasers on chips, especially in the visible and near-infrared spectrum, play a crucial role in advancing various quantum technologies such as quantum sensors, clocks, and sources for single- and entangled-photon pairs.
On-chip microresonator optical parametric oscillation (OPO) has emerged as a promising approach for generating lasers across a wide range of wavelengths. Recent advancements in 3D printing and manufacturing techniques have facilitated the fabrication of efficient nanoscale OPOs, further enhancing their importance in modern optics and quantum technology.
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References and Further Reading
Gamdan Optics, (2023). OPO Process and How They Work. [Online]
Available at: https://www.gamdan.com/blog/optical-parametric-oscillators
Melkonian, J. et. al. (2021). Optical parametric oscillators. [Online]
Available at: https://www.photoniques.com/articles/photon/abs/2021/05/photon2021110p53/photon2021110p53.html
Mutter, P. et. al. (2023). The First Backward Wave Optical Parametric Oscillator Waveguide. [Online]
Available at: https://www.diva-portal.org/smash/get/diva2:1806981/FULLTEXT01.pdf
Marty, G. et al. (2021). Photonic crystal optical parametric oscillator. Nat. Photonics 15, 53–58. Available at: https://doi.org/10.1038/s41566-020-00737-z
Ledezma, L. et. al. (2023). Octave-spanning tunable infrared parametric oscillators in nanophotonics. Science Advances, 9(30), eadf9711. Available at: https://doi.org/10.1126/sciadv.adf971