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Register a new userResonant Electro-Optic Frequency Comb
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Added by
Harald G. L. Schwefel
Publication date
2018
fields
Physics
and research's language is
English
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High speed optical telecommunication is enabled by wavelength division multiplexing, whereby hundreds of individually stabilized lasers encode the information within a single mode optical fiber. In the seek for larger bandwidth the optical power sent into the fiber is limited by optical non-linearities within the fiber and energy consumption of the light sources starts to become a significant cost factor. Optical frequency combs have been suggested to remedy this problem by generating multiple laser lines within a monolithic device, their current stability and coherence lets them operate only in small parameter ranges. Here we show that a broadband frequency comb realized through the electro-optic effect within a high quality whispering gallery mode resonator can operate at low microwave and optical powers. Contrary to the usual third order Kerr non-linear optical frequency combs we rely on the second order non-linear effect which is much more efficient. Our result uses a fixed microwave signal which is mixed with an optical pump signal to generate a coherent frequency comb with a precisely determined carrier separation. The resonant enhancement enables us to operate with microwave powers three order magnitude smaller than in commercially available devices. We can expect the implementation into the next generation long distance telecommunication which relies on coherent emission and detection schemes to allow for operation with higher optical powers and at reduced cost.
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Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications and for precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultra-low optical loss and highly co-localized microwave and optical felds, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ~ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.
Beginning with a continuous wave laser at 1064 nm, we generate a 30 GHz electro-optic frequency comb which contains 100 lines spanning 3 THz. The initial comb is subsequently amplified, spectrally broadened in normal dispersion photonic crystal fiber, and then temporally compressed to provide 74 fs pulses with average power of up to 2.6 W. When launched into a second photonic crystal fiber with anomalous dispersion, a supercontinuum spanning 800-1350 nm is generated. Second harmonic generation allows for extension of the 30 GHz comb into the visible, yielding greater than 300 THz of total spectral bandwidth. Such a broad bandwidth, high repetition rate comb is a compelling source for astronomical spectrograph calibration.
Electro-optic modulators from non-linear $chi^{(2)}$ materials are essential for sensing, metrology and telecommunications because they link the optical domain with the microwave domain. At present, most geometries are suited for fiber applications. In contrast, architectures that modulate directly free-space light at gigahertz (GHz) speeds have remained very challenging, despite their dire need for active free-space optics, in diffractive computing or for optoelectronic feedback to free-space emitters. They are typically bulky or suffer from much reduced interaction lengths. Here, we employ an ultrathin array of sub-wavelength Mie resonators that support quasi bound states in the continuum (BIC) as a key mechanism to demonstrate electro-optic modulation of free-space light with high efficiency at GHz speeds. Our geometry relies on hybrid silicon-organic nanostructures that feature low loss ($Q = $ 550 at $lambda_{res} = 1594$ nm) while being integrated with GHz-compatible coplanar waveguides. We maximize the electro-optic effect by using high-performance electro-optic molecules (whose electro-optic tensor we engineer in-device to exploit $r_{33} = 100$ pm/V) and by nanoscale optimization of the optical modes. We demonstrate both DC tuning and high speed modulation up to 5~GHz ($f_{EO,-3 dB} =3$ GHz) and shift the resonant frequency of the quasi-BIC by $Deltalambda_{res}=$11 nm, surpassing its linewidth. We contrast the properties of quasi-BIC modulators by studying also guided mode resonances that we tune by $Deltalambda_{res}=$20 nm. Our approach showcases the potential for ultrathin GHz-speed free-space electro-optic modulators.
Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with the characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of the frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems and the measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate the feedback with the feedforward. With the additional gain of > 40 dB by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500 $mu$s, 50 GHz in 125 $mu$s, and 25 GHz in 50 $mu$s (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach-Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at the two different wavelengths enables the estimation of the frequency shift of all comb modes.
Superconducting cavity electro-optics (EO) presents a promising route to coherently convert microwave and optical photons and distribute quantum entanglement between superconducting circuits over long-distance through an optical network. High EO conversion efficiency demands transduction materials with strong Pockels effect and excellent optical transparency. Thin-film Lithium Niobate (TFLN) offers these desired characteristics however so far has only delivered unidirectional conversion with efficiencies on the order of $10^{-5}$, largely impacted by its prominent photorefractive (PR) effect at cryogenic temperatures. Here we show that, by mitigating the PR effect and associated charge-screening effect, the devices conversion efficiency can be enhanced by orders of magnitude while maintaining stable cryogenic operation, thus allowing a demonstration of conversion bidirectionality and accurate quantification of on-chip efficiency. With the optimized monolithic integrated superconducting EO device based on TFLN-on-sapphire substrate, an on-chip conversion efficiency of 1.02% (internal efficiency, 15.2%) is realized. Our demonstration indicates that with further device improvement, it is feasible for TFLN to approach unitary internal conversion efficiency.
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