Darwin K. Serkland

Two-element phased array of antiguided vertical-cavity lasers

We demonstrate antiguided coupling of two adjacent vertical-cavity surface-emitting lasers (VCSELs), obtaining a 1×2 phase-locked array at 869 nm. The lateral index modification required for antiguiding is achieved by a patterned 3 nm etch performed between two epitaxial growths. In contrast with prior coupled VCSELs, adjacent antiguided VCSELs can emit in phase and produce a single on-axis lobe in the far field. Greater than 2 mW of in-phase output power is demonstrated with two VCSELs separated by 8 μm. Moreover, phase locking of two VCSELs separated by 20 μm is observed, indicating the possibility of a promising class of optical circuits based upon VCSELs that interact horizontally and emit vertically.

The MAC - a miniature atomic clock

The authors are developing a chip-scale atomic clock (CSAC), more than two orders of magnitude smaller and lower power than any existing technology. As an intermediate milestone, en route to the ultimate CSAC objectives, we have developed a miniature atomic clock (MAC), combining the low-power CSAC physics package with a low-parts count, low-power digital control and microwave system. The MAC is a complete packaged atomic clock, with overall size of 10 cm/sup 3/, power consumption >200 mW, and short-term stability /spl sigma//sub y/(/spl tau/)/spl sim/4/spl times/10/sup -10//spl tau//sup - 1/2 /. The MAC provides a valuable testbed for the further development and refinement of the CSAC physics package as well as for the development of the CSAC control electronics prior to undertaking the costly and time-consuming size-reduction effort which will be necessary to meet the ultimate CSAC objectives. The MAC itself may find applications in commercial and military timing systems which require the relatively small size and power consumption of the MAC now, rather than wait for the evolution of the 1 cm/sup 3/, 30 mW CSAC.

High single-mode power observed from a coupled-resonator vertical-cavity laser diode

We report a monolithic coupled-resonator vertical-cavity laser with an ion-implanted top cavity and a selectively oxidized bottom cavity which exhibits single fundamental-mode operation. The output powers are as high as 6.1 mW with side mode suppression ratios greater than 30 dB. The sizes of the implant and oxide current apertures are shown to be important for demonstrating the required selectivity for the fundamental lasing mode. With a fixed bias current on the implant cavity and increasing oxide cavity current, mode switching from single-mode operation to multimode operation and back to single-mode operation was observed. The intensities of the fundamental and first transverse modes were calculated by solving a set of multimode rate equations. The calculation indicates that the observed mode switching can be identified with changes in the optical length of the oxide cavity with increasing pump current. The observed mode dynamics are unique to coupled-resonator vertical-cavity lasers.

Micromachined Accelerometers With Optical Interferometric Read-Out and Integrated Electrostatic Actuation

A micromachined accelerometer device structure with diffraction-based optical detection and integrated electrostatic actuation is introduced. The sensor consists of a bulk silicon proof mass electrode that moves vertically with respect to a rigid diffraction grating backplate electrode to provide interferometric detection resolution of the proof-mass displacement when illuminated with coherent light. The sensor architecture includes a monolithically integrated electrostatic actuation port that enables the application of precisely controlled broadband forces to the proof mass while the displacement is simultaneously and independently measured optically. This enables several useful features such as dynamic self-characterization and a variety of force-feedback modalities, including alteration of device dynamics in situ. These features are experimentally demonstrated with sensors that have been optoelectronically integrated into sub-cubic-millimeter volumes using an entirely surface-normal, rigid, and robust embodiment incorporating vertical cavity surface emitting lasers and integrated photodetector arrays. In addition to small form factor and high acceleration resolution, the ability to self-characterize and alter device dynamics in situ may be advantageous. This allows periodic calibration and in situ matching of sensor dynamics among an array of accelerometers or seismometers configured in a network.

VCSELs for atomic clocks

The spectroscopic technique of coherent population trapping (CPT) enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards. All-optical interrogation enables the reduction of the size and power consumption of an atomic clock by two orders of magnitude, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources due to their low power consumption and circular output beam. Several research teams are currently using VCSELs for DARPAs chip-scale atomic clock (CSAC) program with the goal of producing an atomic clock having a volume < 1 cm^3, a power consumption < 30 mW, and an instability (Allan deviation) < 1x10^-11 during a 1-hour averaging interval. This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth will be discussed in particular, since atomic clock performance is especially sensitive to this parameter.

Direct Modulation Characteristics of Composite Resonator Vertical-Cavity Lasers

We report the small-signal modulation characteristics of a monolithic dual resonator vertical cavity surface emitting laser. The modulation response is described by a system of rate equations with two independent carrier populations and a single longitudinal optical mode. The independent optical overlaps and differential gains of the two active regions can each be adjusted to maximize the output response. We show that under certain conditions, the composite resonator may achieve a higher bandwidth than a single cavity laser with the same photon density. We find the relaxation oscillation frequency to depend mainly on the total photon density and not the individual currents in the two cavities. With appropriate current injection, the composite resonator laser achieves a maximum -3-dB bandwidth of 12.5 GHz and a maximum modulation current efficiency factor of approximately 5GHz/ma1/2

The Miniature Atomic Clock - Pre-Production Results

The authors have developed a miniature atomic clock (MAC) for applications requiring atomic timing accuracy in portable battery-powered applications. Recently, we have completed a pre-production build of 10 devices in order to evaluate unit-to-unit performance variations and to gain statistical confidence in the performance specifications, environmental sensitivity, and manufacturability.

VCSELs for atomic sensors

A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies. These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells. The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium. The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam. This paper describes the required VCSEL characteristics for atomic clocks and magnetometers. The fundamental VCSEL requirement is single-frequency output with tunability to the particular spectroscopic line of interest. Single-polarization and single-transverse-mode operation are implicit requirements. VCSEL amplitude noise and frequency noise are also important because they contribute significantly to the sensor signal-to-noise ratio. Additional desired VCSEL attributes are low cost, low power consumption, and several years of continuous operating lifetime. This paper also describes the 894-nm VCSELs that we have developed for cesium-based atomic sensors. In particular, we discuss VCSEL noise measurements and accelerated lifetime testing. Finally, we report the performance of prototype atomic clocks employing VCSELs.

Fabrication and performance of two-dimensional matrix addressable arrays of integrated vertical-cavity lasers and resonant cavity photodetectors

Massively parallel interconnects and scannerless imaging are applications that would benefit from high-density two-dimensional arrays of lasers. Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for these applications due to their small size and high efficiency. We have successfully fabricated 64 /spl times/ 64 element arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) monolithically integrated on semi-insulating GaAs substrates. In order to reduce the input and output connections to the array, we employ a matrix addressable architecture, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The pitch of devices in the array is 55 /spl mu/m, and the total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 /spl Omega/. The design, fabrication, and performance of these arrays are discussed.

Single transverse mode operation of electrically pumped vertical-external-cavity surface-emitting lasers with micromirrors

We report an electrically pumped vertical-external-cavity surface-emitting laser (VECSEL) that is designed for wafer-scale fabrication. Single-mode continuous-wave operation is demonstrated at a wavelength of 970 nm. The device structure incorporates a curved micromirror output coupler that is produced using a micromolding process. In addition to outlining the VECSEL fabrication process, we quantify its spatial and spectral modal characteristics.