Frequency-comb-assisted broadband precision spectroscopy with cascaded diode lasers

Abstract

Frequency comb assisted diode laser spectroscopy, employing both the accuracy of an optical frequency comb and the broad wavelength tuning range of a tunable diode laser, has been widely used in many applications. In this letter we present a novel method using cascaded frequency agile diode lasers, which allows extending the measurement bandwidth to 37.4 THz (1355 -1630 nm) at MHz resolution with scanning speeds above 1 THz/s. It is demonstrated as a useful tool to characterize a broadband spectrum for molecular spectroscopy and in particular it enables to characterize the dispersion of integrated microresonators up to the fourth order.Frequency combs [1,2], providing an equidistant grid of lines with precisely known frequencies over a broad spectral range, have substantially advanced precision spectroscopy over the past decades. To date, diverse spectroscopic methods employing frequency combs have been invented, such as direct frequency comb spectroscopy [3], Fourier transform spectroscopy [4] and dual-comb spectroscopy [5]. Among these methods, frequency comb assisted diode laser spectroscopy [6], enabling broadband spectral characterization with fast measurement speed (> 1 THz/s) and simple implementation, has been successfully applied for distance measurement [7,8], dynamic waveform detection [9], plasma diagnostics [10] and molecular spectroscopy [11,12]. One application benefiting from these advantages is the dispersion characterization of high-Q microresonators [13][14][15][16][17], while alternative methods using direct frequency comb [18,19], white light source [20] or sideband spectroscopy [21] have several limitations including system complexity, low measurement speed, narrow bandwidth and inability to measure microresonators with free spectral ranges (FSR) exceeding 100 GHz.The dispersion characterization is important for the dispersion engineering of integrated high-Q microresonators for Kerr frequency comb generation [22,23] and bright dissipative Kerr soliton formation [24][25][26][27]. In addition, properly engineered higher order dispersion can lead to the emission of a dispersive wave via the process of soliton Cherenkov radiation [27][28][29][30]. Several techniques based on geometry variation [31] and additional material layers [13,32] have been demonstrated to tailor the dispersion. However, to measure the higher order dispersion of microresonators, frequency comb assisted diode laser spectroscopy is currently limited by its measurement bandwidth, which is mainly determined by the wavelength tuning range of the used laser. Therefore, using more than one laser to cover different spectral ranges is desired to overcome the bandwidth limitation thus en-OS

Terahertz metamaterials and systems based on rolled-up 3D elements: designs, technological approaches, and properties

“…The interplay of resonances in those systems leads to an emergence of extremely sharp and strong peaks in polarization spectra: one of such systems has demonstrated a maximum polarization azimuth rotation of α > 85° and a maximum peak-to-peak amplitude Δα > 150° for relative frequency shift Δf/f = 0.4%. Such sharpness of peaks can be used for continuous dynamic control of the polarization of transmitted radiation by small variations of laser frequency (see a review and a paper on frequency-tunable lasers9899). Polarization manipulation at fixed frequency can be implemented by shifting the half-wave resonance of helices.…”

Reconfigurable Radiofrequency Photonic Filters Based on Soliton Microcombs

“…The rapidly maturing integrated Kerr microcombs show significant potential for microwave photonics. Yet, state-of-the-art microcomb based radiofrequency (RF) filters have required programmable pulse shapers [1][2][3] , which inevitably increase the system cost, footprint, and complexity. Here, by leveraging the smooth spectral envelope of single solitons, we demonstrate for the first time microcomb based RF filters free from any additional pulse shaping.…”

“…Thanks to the ever-maturing photonic integration, RF photonic systems and subsystems have been brought to new height 6,7 , in terms of footprint, scalability, and potentially cost-effectiveness. Particularly, RF filtering towards chip-scale is a key enabling function [1][2][3][8][9][10][11][12][13] . Paradigm demonstrations include the integration of the basic filtering blocks, such as delay lines 8 , optical spectral shaper 9 , programmable mesh topologies 10 , ring resonators 11 , as well as the use of stimulated Brillouin scattering (SBS) in waveguides 12 .…”

“…Among others, RF filters constructed from tapped delay line (TDL) structures attract great attention. They can be classified into two types 14 , depending on whether the filtering profile is given by the physical path delays 10,13 or the light source spectra [1][2][3]8,9,15,16 . While the former approach is straightforward, a multiwavelength source combined with dispersive propagation also functions as a TDL filter.…”

“…The large comb spacing of microcombs also enhances RF filters with broader Nyquist zone (spur free range), lower latency 1 , less dispersion induced fading, as well as larger number counts of equivalent delay lines 2 , unparalleled by other approaches. However, so far all these microcomb based RF filters have been implemented on either dark pulses 1,21 or complex soliton crystal states 2,3 . Additional programmable pulse shaping modules are inevitably required to equalize or smooth the comb spectral shape.…”