Methods to generate coherent light on-chip are particularly needed in frequency ranges for which common laser gain media do not exist or do not perform well. Our invention represents the first nanophotonic optical parametric oscillator (OPO) whose outputs are widely spectrally separated and which operates with ultra-low pump power using the technologically mature platform of silicon nanophotonics. Our nanophotonic OPO has distinct advantages in power efficiency (record-low sub-mW threshold power), device scalability, and access to a broad range of output wavelengths. Furthermore, it can be directly integrated with compact chip-based lasers, without need of an intermediate amplifier.
The invention is a chip-integrated optical parametric oscillator whose signal and idler output fields are widely separated in frequency, and which is created using the technologically mature platform of silicon nanophotonics. The optical parametric oscillator device consists of a microring resonator with integrated waveguides on-chip for coupling pump light into the resonator and extracting generated light out of the resonator. The resonator-waveguide system is defined in a thin film (sub-micrometer) of silicon nitride (Si3N4) that sits on top of a lower cladding of silicon dioxide (SiO2) and is grown using standard chemical vapor deposition techniques on a silicon substrate.
The optical parametric oscillation mechanism is based on cavity-enhanced degenerate four-wave mixing, a nonlinear optical process in which light that is injected at a pump wavelength gets converted to signal and idler wavelengths, subject to energy and momentum conservation. While this process is well-known in the literature, our specific approach encompasses a few new techniques that lead to important improvements on the existing state-of-the-art. In particular, we engineer the dispersion of the resonator (cavity and resonator are used interchangeably here) to support only one pair of widely separated signal and idler wavelengths. The specific choice of wavelengths can be broadly tuned by adjusting the device geometry (cross-section of the resonator) and the pump wavelength. The nanophotonic resonators we use have high optical quality factors (very low losses) and very small modal volumes (strong optical field confinement), which together result in a threshold power at or below the one milliwatt level.
The invention is novel in several aspects. First, it uses scalable manufacturing processes based on silicon photonics to realize the first chip-integrated OPO whose outputs are widely separated. For example, devices we have demonstrated in the lab cover visible and telecommunications wavelengths using a near-infrared pump, and both coarse tuning and fine tuning of the wavelengths comprising the OPO can be realized through adjustments to the resonator geometry. Next, the device is very power efficient. The threshold power is at the sub-mW level, which is 50 times smaller than other works (that are not even based silicon photonics). With such small power consumption, the device can be directly integrated with on-chip/compact pump lasers, which are readily available in the near-infrared. Avoiding the need for an intermediate amplifier is of considerable practical benefit.
The invention solves the problem of on-chip generation of coherent laser light at frequencies for which suitable laser gain media do not exist or do not perform very well. It has particular relevance in the context of integrated photonics, where a great amount of technology is being built to enable the routing and manipulation of light on a chip, including at visible wavelengths of relevance to atomic systems used as wavelength references and clocks. Spectroscopy and sensing of biochemical systems on a chip is also a major area of development, particularly considering the ability to combine integrated photonics with microfluidics. However, laser sources required for many applications are currently more mature in the near-infrared and telecom wavelengths. Our device essentially converts this laser light to the visible, and in doing so retains the coherence properties associated with lasers while operating at a wavelength relevant to the aforementioned applications.
In comparison to direct laser generation on-chip, our invention is not limited by the availability and spectral bandwidth of laser gain media. For example, even though various types of chipintegrated visible wavelength lasers have been demonstrated, they are not as broadly tunable as our OPO.
There are certainly other types of OPO technologies, but none which are comparable to ours in terms of wide spectral separation between signal and idler are based on silicon photonics, a technologically mature fabrication platform that can be accessed through commercial foundry facilities. This access has significant commercial implications, in decreasing the cost of production through the use of scalable fabrication processes, and the potential for integration into systems. Technically, our invention only requires sub-mW pump power, which makes the onchip/compact integration with the requisite near-infrared pump laser possible.