Optical clocks have achieved accuracies better than 1 part in 1018 and are now some of the best measurement devices ever made, significantly surpassing previous-generation microwave clocks in terms of stability. A significant challenge is to transition optical clocks to field environments, which requires the ruggedization and miniaturization of the atomic reference and clock laser along with their supporting lasers and electronics. In this talk, I discuss the use of a stimulated-Brillouin-scattering (SBS) laser based on a compact fiber resonator to run an optical clock, demonstrating a potential portable replacement for the bulk-cavity-stabilized lasers typically used as the stable oscillator in these systems. We achieve a short-term stability of 3.9 x 10-14 in 1 s, outperforming the best microwave clocks. I also discuss our development of integrated photonics and detectors for chip-based ion traps as a pathway towards miniaturizing the clock’s atomic reference through elimination of free-space optics for light delivery and collection.
We demonstrate an integrated photonic platform for control of complex atomic systems. The platform includes multiple waveguide layers and a suite of passive photonic circuit components supporting a wavelength range from 370-1100 nm. In particular, we demonstrate a novel dual-layer vertical grating coupler used for efficiently directing visible light to precise positions above the chip surface. These circuits are compatible with traditional CMOS fabrication techniques and are well suited for improving the scalability of quantum information processing systems based on trapped-ion technology. A chip-scale waveguide platform at visible wavelengths could also prove useful in a variety of bio-photonic and sensing applications requiring precise light delivery or readout in a compact footprint.
We demonstrate a scalable multi-layer integrated photonics platform that operates over a multi-octave wavelength range, from the near-ultraviolet (NUV) to the near-infrared (NIR). The platform is CMOS compatible and consists of silicon nitride (Si3N4) and alumina (Al2O3) optical waveguides cladded with silicon dioxide (SiO2). We demonstrate low-loss waveguides and passive components including diffractive vertical grating couplers for input/output (I/O). The multilayer nature of the platform enables complex routing of multiple wavelengths, making it useful for a variety of applications including integrated atomic-molecular-optical (AMO) and biophotonic systems.
An optical comb source based on a slab-coupled optical waveguide amplifier (SCOWA) is presented. The laser is
harmonically mode-locked at 10.287 GHz repetition rate and stabilized to an intra-cavity Fabry-Pérot etalon via Pound-
Drever-Hall locking. The Fabry-Pérot etalon serves as a reference for the optical frequency of the comb-lines and
suppresses the fiber cavity modes to allow only a single longitudinal mode-set to oscillate, generating a frequency comb
spaced by the repetition rate. The pulse-to-pulse timing jitter and energy fluctuations are < 2 fs and < 0.03%,
respectively (integrated from 1Hz to 100 MHz). Fundamental to this result is the incorporation of the SCOW amplifier
as the gain medium and the use of an ultra-low noise sapphire-loaded cavity oscillator to mode-lock the laser. The
SCOWA has higher saturation power than commercially available gain media, permitting higher intra-cavity power as
well as available power at the output, increasing the power of the photodetected RF tones which increases their signal-to-noise
ratio. A high visibility optical frequency comb is observed spanning ~3 nm (at -10 dB), with optical SNR > 60 dB
for a cavity with no dispersion compensation. Initial results of a dispersion compensated cavity are presented. A spectral
width of ~7.6 nm (-10 dB) was obtained for this case and the pulses can be compressed to near the transform limit at
For the past several years, we have been developing a new class of high-power, low-noise semiconductor optical gain
medium based on the slab-coupled optical waveguide (SCOW) concept. The key characteristics of the SCOW design are
(i) large (> 5 x 5 μm), symmetric, fundamental-transverse-mode operation attained through a combination of coupledmode
filtering and low index-contrast, (ii) very low optical confinement factor (Γ ~ 0.3-0.5%), and (iii) low excessoptical
loss (αi ~ 0.5 cm-1). The large transverse mode and low confinement factor enables SCOW lasers (SCOWLs) and
amplifiers (SCOWAs) having Watt-class output power. The low confinement factor also dictates that the waveguide
length be very large (0.5-1 cm) to achieve useful gain, which provides the benefits of small ohmic and thermal
resistance. In this paper, we review the operating principles and performance of the SCOW gain medium, and detail its
use in 1550-nm single-frequency SCOW external cavity lasers (SCOWECLs). The SCOWECL consists of a doublepass,
curved-channel InGaAlAs quantum-well SCOWA and a narrowband (2.5 GHz) fiber Bragg grating (FBG) external
cavity. We investigate the impact of the cavity Q on SCOWECL performance by varying the FBG reflectivity. We
show that a bench-top SCOWECL having a FBG reflectivity of R = 10% (R = 20%) has a maximum output power of
450 mW (400 mW), linewidth of 52 kHz (28 kHz), and RIN at 2-MHz offset frequency of -155 dB/Hz (-165 dB/Hz).
In semiconductor lasers, key parameters such as threshold current, efficiency, wavelength, and lifetime are closely related to temperature. These dependencies are especially important for high-power lasers, in which device heating is the main cause of decreased performance and failure. Heat sources such as non-radiative recombination in the active region typically cause the temperature to be highly peaked within the device, potentially leading to large refractive index variation with bias. Here we apply high-resolution charge-coupled device (CCD) thermoreflectance to generate two dimensional (2D) maps of the facet temperatures of a high power laser with 500 nm spatial resolution. The device under test is a slab-coupled optical waveguide laser (SCOWL) which has a large single mode and high power output. These characteristics favor direct butt-coupling the light generated from the laser diode into a single mode optical fiber. From the high spatial resolution temperature map, we can calculate the non-radiative recombination power and the optical mode size by thermal circuit and finite-element model (FEM) respectively. Due to the thermal lensing effect at high bias, the size of the optical mode will decrease and hence the coupling efficiency between the laser diode and the single mode fiber increases. At I=10Ith, we found that the optical mode size has 20% decrease and the coupling efficiency has 10% increase when comparing to I=2Ith. This suggests SCOWL is very suitable fr optical communication system.
This paper discusses use of optical frequency combs generated by modelocked semiconductor lasers for coherent photonic signal processing applications. Key in our approach is a high Q cavity, supermode suppression and low spontaneous emission. Targeted applications of the stabilized optical frequency combs lie in areas of metrology, optical sampling, arbitrary waveform generation and communications using coherent detection.
Advanced analog-optical sensor, signal processing and communication systems could benefit significantly from wideband (DC to > 50 GHz) optical modulators having both low half-wave voltage (Vpi) and low optical insertion loss. An important figure-of-merit for modulators used in analog applications is Tmax/Vpi, where Tmax is the optical transmission of the modulator when biased for maximum transmission. Candidate electro-optic materials for realizing these modulators include lithium niobate (LiNbO3), polymers, and semiconductors, each of which has its own set of advantages and disadvantages. In this paper, we report the development of 1.5-um-wavelength Mach-Zehnder modulators utilizing the electrorefractive effect in InGaAsP/InP symmetric, uncoupled semiconductor quantum-wells. Modulators with 1-cm-long, lumped-element electrodes are found to have a push-pull Vpi of 0.9V (Vpi*L = 9 V-mm) and 18-dB fiber-to-fiber insertion loss (Tmax/Vpi = 0.018). Fabry-Perot cutback measurements reveal a waveguide propagation loss of 7 dB/cm and a waveguide-to-fiber coupling loss of 5 dB/facet. The relatively high propagation loss results from a combination of below-bandedge absorption and scattering due to waveguide-sidewall roughness. Analyses show that most of the coupling loss can be eliminated though the use of monolithically integrated inverted-taper optical-mode converters, thereby allowing these modulators to exceed the performance of commercial LiNbO3 modulators (Tmax/Vpi ~ 0.1). We also report the analog modulation characteristics of these modulators.
The development of photonic devices for the next generation of optical networks is dependent on advances in ultrafast materials and in the success of waveguide devices comprised of these materials. This includes new methods of producing integrated-optical devices by innovative growth techniques or novel hybridization schemes. We describe aspects of the ultra-fast optical communications program at Georgia Tech that involve the development of hybridized and integrated- optical devices and devices for use in ultrafast optical data links. Two major components are under development: (1) a tapered rib electro-absorption modulator that includes an integrated spot-size converter for hybridization with a passive silica-waveguide tapped delay line. This unique hybridized semiconductor/glass waveguide provides the basic building block of the transmitter multiplexer. (2) a quasi- phase matched multilayer AlGaAs waveguide designed for surface-emitted second-harmonic generation. This device provides an all-optical serial-to-parallel converter and thereby demultiplexes ultrafast optical data streams. We describe our recent advances in materials growth and waveguide design and the impact on the performance of these devices.
The ability to engineer the free carrier lifetime of epitaxially grown semiconductors without significantly affecting the desirable nonlinear optical properties would allow the development of an entire new class of high-speed photonic devices. The primary method of achieving this is the controlled introduction of mid-gap defects via a variety of techniques including low temperature growth. We report on a systematic investigation of low-temperature-grown materials including bulk GaAs and Be-doped In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells. Using both wavelength-dependent time-resolved nonlinear bandedge absorption spectroscopy and far infrared Terahertz spectroscopy, we unambiguously discriminate between recombination and trapping events and determine the carrier lifetime and mobility in a contactless fashion. We correlate the far infrared response and the bandedge response and thereby explain the apparent discrepancies with previous measurements and clarify the physical origin of the optical nonlinearity as well as the defect densities, carrier lifetimes and mobility.