We demonstrate injection locking of Fabry–Perot lasers to electro-optic (EO) frequency combs using comb powers as low as 1 nW, more than two orders of magnitude below the levels required for semiconductor optical amplification. This approach extends EO comb spectral translation from the telecom band to 780 nm via second-harmonic generation in silicon nitride microring resonators. We further generate EO combs at 420 nm by translating 780 nm combs through a four-wave-mixing process in rubidium vapor. Integrating these techniques paves the way toward chip-scale cascaded frequency conversion from the telecom band into the blue, providing compact and scalable light sources for quantum technologies.
We demonstrate injection locking of Fabry–Perot lasers to electro-optic (EO) frequency combs using comb powers as low as 1 nW, more than two orders of magnitude below the levels required for semiconductor optical amplification. This approach extends EO comb spectral translation from the telecom band to 780 nm via second-harmonic generation in silicon nitride microring resonators. We further generate EO combs at 420 nm by translating 780 nm combs through a four-wave-mixing process in rubidium vapor. Integrating these techniques paves the way toward chip-scale cascaded frequency conversion from the telecom band into the blue, providing compact and scalable light sources for quantum technologies.
Electro-optic frequency combs (EO combs) enable high spectral and temporal resolution in atomic, molecular, and cavity-based spectroscopy that is at the heart of many quantum sensing applications. However, producing EO combs in the visible range remains challenging due to the limited availability of key components such as narrow-linewidth lasers and electro-optic phase modulators. Rubidium (Rb) is a highly nonlinear material that supports various four-wave mixing (4WM) processes, which are utilized for spectral translation and the generation of squeezed light. Here, we show the spectral translation of a 780 nm EO comb to 420 nm (a spectral separation of 330 THz) via a 4WM process in Rb vapor. We demonstrate the capability to extend the output comb bandwidth and signal-to-noise ratio by varying input power levels. Through this coherent process, we can generate frequency-agile combs in the blue, including a 1 kHz spaced comb with over one million individual comb teeth, of relevance for spectroscopic measurement of narrow-linewidth features such as electromagnetically induced transparency. Leveraging this approach across different 4WM interactions in Rb should allow for further wavelength conversion and the generation of correlated frequency combs, paving the way for sub-shot-noise (squeezed-light) measurements.
To transfer and expand in the optical telecommunication range the accurate 5S1/2(F = 3) – 5D5/2 (F = 5) rubidium frequency reference at 778 nm (385.285 THz), we employ a second harmonic generation (SHG) process coupled to an optical frequency comb (OFC) both based on the use of a low power and narrow linewidth laser diode operating around 1556 nm. The Rb optical frequency reference is known with an accuracy of 2.6 × 10−12 and exhibits a frequency stability of 4 × 10−13τ−1/2, with a Flicker plateau of 2 × 10−14 for τ > 200 s. With 20 mW of laser power at 1556 nm, we perform a SHG process in a periodically poled lithium niobate (PPLN) crystal and we obtain 6.5 μW of harmonic radiation at 778 nm, used to phase lock the IR laser against a Rb optical frequency standard (OFS). This Rb-referenced fundamental radiation at 1556 nm is duplicated as 34 lines spaced by 10 GHz using an electro-optic phase modulator (EOM). This preliminary result is mainly limited by unexpected high optical losses in the Fabry Perot cavity, which comprises the EOM.
Photonic structures such as photonic bandgap fibers and high confinement nanowaveguides have proven to be excellent platforms for studying nonlinear optical interactions tailored towards applications in spectroscopy, quantum communication, quantum computation protocols, optical clockwork and precision frequency metrology. This thesis discusses our approach towards exploiting these high confinement media for demonstrating novel few photon nonlinear optical interactions such as two-photon absorption and cross-phase modulation in hot atomic vapors (Rubidium) confined inside hollow-core photonic bandgap fibers (PBGF), generating ultra-broadband optical frequency combs that utilize cascaded parametric four-wave mixing and mode-locked femtosecond pulses in silicon nitride nanowaveguides. We generate large optical depths in such a Rb-PBGF system, and the tight light confinement, high vapor density and long interaction length allow us to perform nonlinear optics at ultralow power. We observe 25% all-optical modulation with <20 photons, i.e., a few attojoules of energy, and large cross-phase shifts of 0.3 mrad per photon with a response time <5 ns in Rb-filled PBGF. This result takes us to within an order of magnitude of single-photon switching, and improves upon previous experiments for freely propagating optical fields, including those in cold-atoms. Using high quality factor silicon nitride optical microcavities we show that the gain from the four-wave mixing process can lead to optical parametric os- cillation, allowing for the generation of multiple new wavelengths as wide as an octave of wavelengths. Next we show that by dispersion engineering the waveguide dimensions, we can generate combs by pumping at 1064 nm. The advantage of this platform is that we can independently tune the free spectral range (FSR) and the dispersion. We exploit this property of the silicon nitride microresonator platform to generate microcombs with various FSRs such as 20, 40-, 80-,100-GHz. Next we go on to characterize the spectral and temporal dynamics of the microresonator based combs and demonstrate that such parametric frequency combs can generate modelocked ultra-short pulses.
The on-chip generation of large and complex optical quantum states will enable low-cost and accessible advances for quantum technologies, such as secure communications and quantum computation. Integrated frequency combs are on-chip light sources with a broad spectrum of evenly-spaced frequency modes, commonly generated by four-wave mixing in optically-excited nonlinear micro-cavities, whose recent use for quantum state generation has provided a solution for scalable and multi-mode quantum light sources. Pulsed quantum frequency combs are of particular interest, since they allow the generation of single-frequency-mode photons, required for scaling state complexity towards, e.g., multi-photon states, and for quantum information applications. However, generation schemes for such pulsed combs have, to date, relied on micro-cavity excitation via lasers external to the sources, being neither versatile nor power-efficient, and impractical for scalable realizations of quantum technologies. Here, we introduce an actively-modulated, nested-cavity configuration that exploits the resonance pass-band characteristic of the micro-cavity to enable a mode-locked and energy-efficient excitation. We demonstrate that the scheme allows the generation of high-purity photons at large coincidence-to-accidental ratios (CAR). Furthermore, by increasing the repetition rate of the excitation field via harmonic mode-locking (i.e. driving the cavity modulation at harmonics of the fundamental repetition rate), we managed to increase the pair production rates (i.e. source efficiency), while maintaining a high CAR and photon purity. Our approach represents a significant step towards the realization of fully on-chip, stable, and versatile sources of pulsed quantum frequency combs, crucial for the development of accessible quantum technologies.
Silicon photonics has drawn a lot of attention over the last decades, mainly in telecom-related application fields where the nonlinear optical properties of silicon are ignored or minimized. However, silicon’s high χ(3) Kerr optical nonlinearity in sub-micron-scale high-confinement waveguides can enable significant improvements in traditional nonlinear devices, such as for wavelength conversion, and also enable some device applications in quantum optics or for quantum key distribution. In order to establish the viability of silicon photonics in practical applications, some big challenges are to improve the optical performance (e.g., optimize nonlinearity or minimize loss) and integration of optics with microelectronics. In this context, we discuss how electronic PIN diodes improve the performance of wavelength conversion in a microring resonator based four-wave mixing device, which achieves a continuous-wave four-wave mixing conversion efficiency of −21.3 dB at 100 mW pump power, with enough bandwidth for the wavelength conversion of a 10 Gbps signal. In the regime of quantum optics, we describe a coupled microring device that can serve as a tunable source of entangled photon pairs at telecommunications wavelengths, operating at room temperature with a low pump power requirement. By controlling either the optical pump wavelength, or the chip temperature, we show that the output bi-photon spectrum can be varied, with implications on the degree of frequency correlation of the generated quantum state.
Atomic frequency combs memories that coherently store optical signals are a key building block for optical quantum computers and quantum networks. Integrating such memories into compact and chip-scale devices is essential for scalable quantum technology, but to date most demonstrations have been in bulk materials or waveguides with large cross-sections, or using fabrication techniques not easily adaptable to wafer scale processing. We demonstrate compact chip-integrated atomic frequency comb storage in rare earth doped thin-film lithium niobate. Our optical memory exhibits a broad storage bandwidth exceeding 100 MHz, and optical storage time of over 250 ns. The enhanced optical confinement in this device structure enables three orders of magnitude reduction in optical power as compared to large ion-diffused waveguides for the same Rabi frequency. These compact atomic frequency comb memories pave the way towards scalable, highly efficient, electro-optically tunable quantum photonic systems that can store and manipulate light on a compact chip.