arXiv subjects

Scott A. Diddams

Publications and source records attributed to Scott A. Diddams.

At least 127 records · Page 7Linked to original sources

arXiv2017

Self-referenced frequency combs using high-efficiency silicon-nitride waveguides

We utilize silicon-nitride waveguides to self-reference a telecom-wavelength fiber frequency comb through supercontinuum generation, using 11.3 mW of optical power incident on the chip. This is approximately ten times lower than conventional approaches using nonlinear fibers and is enabled by low-loss (<2 dB) input coupling and the high nonlinearity of silicon nitride, which can provide two octaves of spectral broadening with incident energies of only 110 pJ. Following supercontinuum generation, self-referencing is accomplished by mixing 780-nm dispersive-wave light with the frequency-doubled output of the fiber laser. In addition, at higher optical powers, we demonstrate f-to-3f self-referencing directly from the waveguide output by the interference of simultaneous supercontinuum and third harmonic generation, without the use of an external doubling crystal or interferometer. These hybrid comb systems combine the performance of fiber-laser frequency combs with the high nonlinearity and compactness of photonic waveguides, and should lead to low-cost, fully stabilized frequency combs for portable and space-borne applications.

physics.optics↗

arXiv2017

Soliton crystals in Kerr resonators

Strongly interacting solitons confined to an optical resonator would offer unique capabilities for experiments in communication, computation, and sensing with light. Here we report on the discovery of soliton crystals in monolithic Kerr microresonators-spontaneously and collectively ordered ensembles of co-propagating solitons whose interactions discretize their allowed temporal separations. We unambiguously identify and characterize soliton crystals through analysis of their 'fingerprint' optical spectra, which arise from spectral interference between the solitons. We identify a rich space of soliton crystals exhibiting crystallographic defects, and time-domain measurements directly confirm our inference of their crystal structure. The crystallization we observe is explained by long-range soliton interactions mediated by resonator mode degeneracies, and we probe the qualitative difference between soliton crystals and a soliton liquid that forms in the absence of these interactions. Our work explores the rich physics of monolithic Kerr resonators in a new regime of dense soliton occupation and offers a way to greatly increase the efficiency of Kerr combs; further, the extreme degeneracy of the configuration space of soliton crystals suggests an implementation for a robust on-chip optical buffer.

physics.optics↗

arXiv2016

Stably accessing octave-spanning microresonator frequency combs in the soliton regime

Microresonator frequency combs can be an enabling technology for optical frequency synthesis and timekeeping in low size, weight, and power architectures. Such systems require comb operation in low-noise, phase-coherent states such as solitons, with broad spectral bandwidths (e.g., octave-spanning) for self-referencing to detect the carrier-envelope offset frequency. However, stably accessing such states is complicated by thermo-optic dispersion. For example, in the Si3N4 platform, precisely dispersion-engineered structures can support broadband operation, but microsecond thermal time constants have necessitated fast pump power or frequency control to stabilize the solitons. In contrast, here we consider how broadband soliton states can be accessed with simple pump laser frequency tuning, at a rate much slower than the thermal dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4 microresonators, including the generation of a multi-soliton state with a pump power near 40 mW and a single-soliton state with a pump power near 120 mW. We also develop a simplified two-step analysis to explain how these states are accessed in a thermally stable way without fast control of the pump laser, and outline the required thermal properties for such operation. Our model agrees with experimental results as well as numerical simulations based on a Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover, it also explains an experimental observation that a member of an adjacent mode family on the red-detuned side of the pump mode can mitigate the thermal requirements for accessing soliton states.

physics.optics↗

arXiv2016

Microresonator Brillouin Laser Stabilization Using a Microfabricated Rubidium Cell

We frequency stabilize the output of a miniature stimulated Brillouin scattering (SBS) laser to rubidium atoms in a microfabricated cell to realize a laser system with frequency stability at the $10^{-11}$ level over seven decades in averaging time. In addition, our system has the advantages of robustness, low cost and the potential for integration that would lead to still further miniaturization. The SBS laser operating at 1560 nm exhibits a spectral linewidth of 820 Hz, but its frequency drifts over a few MHz on the 1 hour timescale. By locking the second harmonic of the SBS laser to the Rb reference, we reduce this drift by a factor of $10^3$ to the level of a few kHz over the course of an hour. For our combined SBS and Rb laser system, we measure a frequency noise of $4\times10^4$ $Hz^2/Hz$ at 10 Hz offset frequency which rapidly rolls off to a level of 0.2 $Hz^2/Hz$ at 100 kHz offset. The corresponding Allan deviation is $\leq2\times10^{-11}$ for averaging times spanning $10^{-4}$ to $10^3$ s. By optically dividing the signal of the laser down to microwave frequencies, we generate an RF signal at 2 GHz with phase noise at the level of -76 dBc/Hz and -140 dBc/Hz at offset frequencies of 10 Hz and 10 kHz, respectively.

physics.optics↗

arXiv2016

State of the Field: Extreme Precision Radial Velocities

The Second Workshop on Extreme Precision Radial Velocities defined circa 2015 the state of the art Doppler precision and identified the critical path challenges for reaching 10 cm/s measurement precision. The presentations and discussion of key issues for instrumentation and data analysis and the workshop recommendations for achieving this precision are summarized here. Beginning with the HARPS spectrograph, technological advances for precision radial velocity measurements have focused on building extremely stable instruments. To reach still higher precision, future spectrometers will need to produce even higher fidelity spectra. This should be possible with improved environmental control, greater stability in the illumination of the spectrometer optics, better detectors, more precise wavelength calibration, and broader bandwidth spectra. Key data analysis challenges for the precision radial velocity community include distinguishing center of mass Keplerian motion from photospheric velocities, and the proper treatment of telluric contamination. Success here is coupled to the instrument design, but also requires the implementation of robust statistical and modeling techniques. Center of mass velocities produce Doppler shifts that affect every line identically, while photospheric velocities produce line profile asymmetries with wavelength and temporal dependencies that are different from Keplerian signals. Exoplanets are an important subfield of astronomy and there has been an impressive rate of discovery over the past two decades. Higher precision radial velocity measurements are required to serve as a discovery technique for potentially habitable worlds and to characterize detections from transit missions. The future of exoplanet science has very different trajectories depending on the precision that can ultimately be achieved with Doppler measurements.

astro-ph.IM↗

arXiv2016

Rapid, broadband spectroscopic temperature measurement of CO2 using VIPA spectroscopy

Time-resolved spectroscopic temperature measurements of a sealed carbon dioxide sample cell were realized with an optical frequency comb combined with a two-dimensional dispersive spectrometer. A supercontinuum laser source based on an erbium fiber mode-locked laser was employed to generate coherent light around 2000 nm (5000 cm-1). The laser was passed through a 12-cm long cell containing CO2, and the transmitted light was analyzed in a virtually imaged phased array- (VIPA-) based spectrometer. Broadband spectra spanning more than 100 cm-1 with a spectral resolution of roughly 0.075 cm-1 (2.2 GHz) were acquired with an integration period of 2 ms. The temperature of the CO2 sample was deduced from fitting a modeled spectrum to the line intensities of the experimentally acquired spectrum. Temperature dynamics on the time scale of milliseconds were observed with a temperature resolution of 2.6 K. The spectroscopically-deduced temperatures agreed with temperatures of the sample cell measured with a thermistor. Potential applications of this technique include quantitative measurement of carbon dioxide concentration and temperature dynamics in gas-phase chemical reactions (e.g., combustion), and plasma diagnostics.

physics.optics↗

arXiv2015

Phase Coherent Link of an Atomic Clock to a Self-Referenced Microresonator Frequency Comb

The counting and control of optical cycles of light has become common with modelocked laser frequency combs. But even with advances in laser technology, modelocked laser combs remain bulk-component devices that are hand-assembled. In contrast, a frequency comb based on the Kerr-nonlinearity in a dielectric microresonator will enable frequency comb functionality in a micro-fabricated and chip-integrated package suitable for use in a wide-range of environments. Such an advance will significantly impact fields ranging from spectroscopy and trace gas sensing, to astronomy, communications, atomic time keeping and photonic data processing. Yet in spite of the remarkable progress shown over the past years, microresonator frequency combs ("microcombs") have still been without the key function of direct f-2f self-referencing and phase-coherent frequency control that will be critical for enabling their full potential. Here we realize these missing elements using a low-noise 16.4 GHz silicon chip microcomb that is coherently broadened from its initial 1550 nm wavelength and subsequently f-2f self-referenced and phase-stabilized to an atomic clock. With this advance, we not only realize the highest repetition rate octave-span frequency comb ever achieved, but we highlight the low-noise microcomb properties that support highest atomic clock limited frequency stability.

physics.optics↗

arXiv2015

A microrod-resonator Brillouin laser with 240 Hz absolute linewidth

We demonstrate an ultralow-noise microrod-resonator based laser that oscillates on the gain supplied by the stimulated Brillouin scattering optical nonlinearity. Microresonator Brillouin lasers are known to offer an outstanding frequency noise floor, which is limited by fundamental thermal fluctuations. Here, we show experimental evidence that thermal effects also dominate the close-to-carrier frequency fluctuations. The 6-mm diameter microrod resonator used in our experiments has a large optical mode area of ~100 μm$^2$, and hence its 10 ms thermal time constant filters the close-to-carrier optical frequency noise. The result is an absolute laser linewidth of 240 Hz with a corresponding white-frequency noise floor of 0.1 Hz$^2$/Hz. We explain the steady-state performance of this laser by measurements of its operation state and of its mode detuning and lineshape. Our results highlight a mechanism for noise that is common to many microresonator devices due to the inherent coupling between intracavity power and mode frequency. We demonstrate the ability to reduce this noise through a feedback loop that stabilizes the intracavity power.

physics.optics↗

arXiv2015

Mid-Infrared Optical Frequency Combs based on Difference Frequency Generation for Molecular Spectroscopy

Mid-infrared femtosecond optical frequency combs were produced by difference frequency generation of the spectral components of a near-infrared comb in a 3-mm-long MgO:PPLN crystal. We observe strong pump depletion and 9.3 dB parametric gain in the 1.5 μm signal, which yields powers above 500 mW (3 μW/mode) in the idler with spectra covering 2.8 μm to 3.5 μm. Potential for broadband, high-resolution molecular spectroscopy is demonstrated by absorption spectra and interferograms obtained by heterodyning two combs.

physics.optics↗

arXiv2015

Self-referencing a continuous-wave laser with electro-optic modulation

We phase-coherently measure the frequency of continuous-wave (CW) laser light by use of optical-phase modulation and f-2f nonlinear interferometry. Periodic electro-optic modulation (EOM) transforms the CW laser into a continuous train of picosecond optical pulses. Subsequent nonlinear-fiber broadening of this EOM frequency comb produces a supercontinuum with 160 THz of bandwidth. A critical intermediate step is optical filtering of the EOM comb to reduce electronic-noise-induced decoherence of the supercontinuum. Applying f-2f self-referencing with the supercontinuum yields the carrier-envelope offset frequency of the EOM comb, which is precisely the difference of the CW laser frequency and an exact integer multiple of the EOM pulse repetition rate. Here we demonstrate absolute optical frequency metrology and synthesis applications of the self-referenced CW laser with <5E-14 fractional accuracy and stability.

physics.optics↗

arXiv2015

Broadband dispersion engineered microresonator on-a-chip

Control of dispersion in fibre optical waveguides is of critical importance to optical fibre communications systems and more recently for continuum generation from the ultraviolet to the mid-infrared. The wavelength at which the group velocity dispersion crosses zero can be set by varying fibre core diameter or index step. Moreover, sophisticated methods to manipulate higher-order dispersion so as shape and even flatten dispersion over wide bandwidths are possible using multi-cladding fibre. Here we introduce design and fabrication techniques that allow analogous dispersion control in chip-integrated optical microresonators, and thereby demonstrate higher-order, wide-bandwidth dispersion control over an octave of spectrum. Importantly, the fabrication method we employ for dispersion control simultaneously permits optical Q factors above 100 million, which is critical for efficient operation of nonlinear optical oscillators. Dispersion control in high Q systems has taken on greater importance in recent years with increased interest in chip-integrable optical frequency combs.

physics.optics↗

arXiv2015

Attosecond Timing in Optical-to-Electrical Conversion

The most frequency-stable sources of electromagnetic radiation are produced optically, and optical frequency combs provide the means for high fidelity frequency transfer across hundreds of terahertz and into the microwave domain. A critical step in this photonic-based synthesis of microwave signals is the optical-to-electrical conversion process. Here we show that attosecond (as) timing stability can be preserved across the opto-electronic interface of a photodiode, despite an intrinsic temporal response that is more than six orders of magnitude slower. The excess timing noise in the photodetection of a periodic train of ultrashort optical pulses behaves as flicker noise (1/f) with amplitude of 4 as/Sqrt(Hz) at 1 Hz offset. The corresponding fractional frequency fluctuations are 1.4x10-17 at 1 second and 5.5x10-20 at 1000 seconds. These results demonstrate that direct photodetection, as part of frequency-comb-based microwave synthesis, can support the timing performance of the best optical frequency standards, and thereby opens the possibility for generating microwave signals with significantly better stability than any existing source.

physics.optics↗

arXiv2014

Open-Air, Broad-Bandwidth Trace-Gas Sensing with a Mid-Infrared Optical Frequency Comb

A mid-Infrared frequency comb is produced via an optical parametric oscillator (OPO) pumped by an amplified 100 MHz Yb:fiber mode-locked laser. We use this source to make measurements of the concentration of the atmospherically-relevant species of CH4 and H2O over a bandwidth of 100 nm centered at 3.25 um. Multiple absorption lines for each species are detected with millisecond acquisition time using a virtually-imaged phased array (VIPA) spectrometer. The measured wavelength-dependent absorption profile is compared to and fitted by a model, yielding quantitative values of the atmospheric concentration of both CH4 and H2O in a controlled indoor environment, as well as over a 26 m open air outdoor path.

physics.optics↗

arXiv2014

Noise and dynamics of stimulated Brillouin scattering microresonator lasers

We use theoretical analysis and numerical simulation to investigate the operation of a laser oscillating from gain supplied by stimulated Brillouin scattering (SBS) in a microresonator. The interaction of the forward, backward, and density waves within the microresonator results in a set of coupled-mode equations describing both the laser's phase and amplitude evolution over time. Using this coupled-mode formalism, we investigate the performance of the SBS laser under noise perturbation and identify the fundamental parameters and their optimization to enable low-noise SBS operation. The intrinsic laser linewidth, which is primarily limited by incoherent thermal occupation of the density wave, can be of order hertz or below. Our analysis also determines the SBS laser's relaxation oscillation, which results from the coupling between the optical and density waves, and appears as a resonance in both the phase and amplitude quadratures. We further explore contributions of the pump noise to the SBS laser's performance, which we find under most circumstances to increase the SBS laser noise beyond its fundamental limits. By tightly stabilizing the pump laser onto the microcavity resonance, the transfer of pump noise is significantly reduced. Our analysis is both supported and extended through numerical simulations of the SBS laser.

physics.optics↗

arXiv2014

Tunable, Stable Source of Femtosecond Pulses Near 2 μm via Supercontinuum of an Erbium Mode-Locked Laser

A source of ultrashort pulses of light in the 2 μm region was constructed using supercontinuum broadening from an erbium mode-locked laser. The output spectrum spanned 1000 nm to 2200 nm with an average power of 250 mW. A pulse width of 39 fs for part of the spectrum in the 2000 nm region, corresponding to less than six optical cycles, was achieved. A heterodyne measurement of the free-running mode-locked laser with a narrow-linewidth continuous wave laser resulted in a near shot noise-limited beat note with a signal-to-noise ratio of 45 dB in a 10 kHz resolution bandwidth. The relative intensity noise of the broadband system was investigated over the entire supercontinuum, and the integrated relative intensity noise of the 2000 nm portion of the spectrum was 0.0017. The long-term stability of the system was characterized, and intensity fluctuations in the spectrum were found to be highly correlated throughout the supercontinuum. Spectroscopic limitations due to the laser noise characteristics are discussed.

physics.optics↗

arXiv2014

Phase Steps and Hot Resonator Detuning in Microresonator Frequency Combs

Experiments and theoretical modeling yielded significant progress towards understanding of Kerr-effect induced optical frequency comb generation in microresonators. However, the simultaneous interaction of hundreds or thousands of optical comb frequencies with the same number of resonator modes leads to complicated nonlinear dynamics that are far from fully understood. An important prerequisite for modeling the comb formation process is the knowledge of phase and amplitude of the comb modes as well as the detuning from their respective microresonator modes. Here, we present comprehensive measurements that fully characterize optical microcomb states. We introduce a way of measuring resonator dispersion and detuning of comb modes in a hot resonator while generating an optical frequency comb. The presented phase measurements show unpredicted comb states with discrete π and π/2 steps in the comb phases that are not observed in conventional optical frequency combs.

physics.optics↗

arXiv2013

Optical amplification and pulse interleaving for low noise photonic microwave generation

We investigate the impact of pulse interleaving and optical amplification on the spectral purity of microwave signals generated by photodetecting the pulsed output of an Er:fiber-based optical frequency comb. It is shown that the microwave phase noise floor can be extremely sensitive to delay length errors in the interleaver, and the contribution of the quantum noise from optical amplification to the phase noise can be reduced ~10 dB for short pulse detection. We exploit optical amplification, in conjunction with high power handling modified uni-traveling carrier photodetectors, to generate a phase noise floor on a 10 GHz carrier of -175 dBc/Hz, the lowest ever demonstrated in the photodetection of a mode-locked fiber laser. At all offset frequencies, the photodetected 10 GHz phase noise performance is comparable to or better than the lowest phase noise results yet demonstrated with stabilized Ti:sapphire frequency combs.

physics.optics↗

arXiv2013

Absolute and relative stability of an optical frequency reference based on spectral hole burning in Eu$^{3+}$:Y$_2$SiO$_5$

We present and analyze four frequency measurements designed to characterize the performance of an optical frequency reference based on spectral hole burning in \EuYSO. The first frequency comparison, between a single unperturbed spectral hole and a hydrogen maser, demonstrates a fractional frequency drift rate of $5 \times 10^{-18}$ s$^{-1}$. Optical-frequency comparisons between a pattern of spectral holes, a Fabry-Pérot cavity, and an Al$^+$ optical atomic clock show a short-term fractional frequency stability of $1 \times10^{-15} τ^{-1/2}$ that averages down to $2.5^{+1.1}_{-0.5} \times 10^{-16}$ at $τ= 540~s$ (with linear frequency drift removed). Finally, spectral hole patterns in two different \EuYSO crystals located in the same cryogenic vessel are compared, yielding a short-term stability of $7 \times10^{-16} τ^{-1/2}$ that averages down to $5.5^{+1.8}_{-0.9} \times 10^{-17}$ at $τ= 204$~s (with quadratic frequency drift removed).

physics.atom-ph↗