After many years of directed investment, integrated photonics technologies have begun to enable sophisticated solutions for fiber-optic communications. The increasing maturity and availability of photonic integrated circuits will ultimately enable a variety of revolutionary chip-scale applications including LIDAR, chemical and biological sensing, precision metrology, quantum information processing, and free-space laser communications. However, providing impact in applications beyond datacom has proved challenging because the toolset developed for digital communications often fails to meet the broader needs of new applications. This talk describes DARPA efforts to improve passive and active integrated photonics components, along with relevant programs that are driving technology innovation.
The Modular Optical Aperture Building Blocks (MOABB) program is an effort to develop optical LIDAR using planar photonics components in the place of bulky optics and slow, costly mechanical beam steering elements. MOABB creates an optical phased array which combines coherent light from individual emitter elements placed on a wavelength-scale pitch. Electrically-addressed phase modulators control directionality for transmit and receive functions. Key MOABB device challenges include the dense integration of small phase modulators, high-power tunable lasers, and high-speed drive electronics.
The Direct On-chip Digital Optical Synthesizer (DODOS) program seeks to develop a chip-scale optical frequency synthesizer using a self-referenced optical frequency comb to precisely control the output of a narrowband tunable laser. DODOS has driven the development of high Q microresonators, chip-scale modelocked lasers, efficient frequency doublers, and wideband passive elements that operate with low loss across an octave of spectrum.
We report on mode selection and tuning properties of vertical-external-cavity surface-emitting lasers (VECSELs) containing coupled semiconductor and external cavities of total length less than 1 mm. Our goal is to create narrowlinewidth (<1MHz) single-frequency VECSELs that operate near 850 nm on a single longitudinal cavity resonance and tune versus temperature without mode hops. We have designed, fabricated, and measured VECSELs with external-cavity lengths ranging from 25 to 800 μm. We compare simulated and measured coupled-cavity mode frequencies and discuss criteria for single mode selection.
We report on the development of single-frequency VCSELs (vertical-cavity surface-emitting lasers) for sensing the position of a moving MEMS (micro-electro-mechanical system) object with resolution much less than 1nm. Position measurement is the basis of many different types of MEMS sensors, including accelerometers, gyroscopes, and pressure sensors. Typically, by switching from a traditional capacitive electronic readout to an interferometric optical readout, the resolution can be improved by an order of magnitude with a corresponding improvement in MEMS sensor performance. Because the VCSEL wavelength determines the scale of the position measurement, laser wavelength (frequency) stability is desirable. This paper discusses the impact of VCSEL amplitude and frequency noise on the position measurement.
We report on the development of 850-nm high-speed VCSELs optimized for low-power data transmission at cryogenic
temperatures near 100 K. These VCSELs operate on the n=1 quantum well transition at cryogenic temperatures (near
100 K) and on the n=2 transition at room temperature (near 300 K) such that cryogenic cooling is not required for initial
testing of the optical interconnects at room temperature. Relative to previous work at 950 nm, the shorter 850-nm
wavelength of these VCSELs makes them compatible with high-speed receivers that employ GaAs photodiodes.
We report the demonstration of a fully micro-fabricated vertical-external-cavity surface-emitting laser (VECSEL)
operating at wavelengths near 850 nm. The external-cavity length is on the order of 25 microns, and the external mirror
is a dielectric distributed Bragg reflector with a radius of curvature of 130 microns that is micro-fabricated on top of the
active semiconductor portion of the device. The additional cavity length, relative to a VCSEL, enables higher output
power and narrower laser linewidth, and micro-fabrication of the external mirror preserves the manufacturing cost
advantages of parallel lithographic alignment.
We designed and fabricated an optical system containing high efficiency diffractive optical elements (DOEs)
with large numerical apertures (NA) for an all-optical gate, based on a Symmetric Self-Electro-Optic Effect
Device (S-SEED) technology. The S-SEEDs are the active elements that perform the optical switching in the
optical interconnect. Multiple, off-axis DOEs are used to collect and focus light onto the S-SEEDs and the
Input/Output optical fibers. Each S-SEED has at least seven input signals, two alignment signals, and two
output signals. Each signal uses a DOE. DOE fabrication is relatively mature and utilizes the precise lateral
alignment inherent in photolithography to produce arrays compatible with dense optical interconnects.
Losses across the system have a negative impact on the S-SEED switching speeds. The primary challenge of
DOEs is the diffractive optic efficiency that corresponds to high NAs. Lower efficiencies, due to
requirements for large deflection angles, lead to extremely small feature sizes in the outer zones of the DOEs.
We optimize DOE efficiency with modifications to the blaze geometry and by selecting the appropriate
number of levels for specific deflection angles. The system layout is modified to reduce complexity by
working in collimated space between the S-SEEDs instead of imaging onto relay mirrors. This reduces the
spatial frequency of the DOEs and increases system tolerance by not imaging mirror defects. Finally, we
quantify the effects of lithographic masks misalignment and look at the step geometry deviations and their
effects on DOE efficiency.
A future generation of high-performance low-power atomic systems is expected to require VCSEL linewidths below 10
MHz for compatibility with the natural atomic linewidth (5 MHz for cesium) that is realized with atomic beams, trapped
atoms, and trapped ions. This paper describes initial efforts at Sandia to reduce VCSEL linewidth by increasing the
effective cavity length of an 850-nm monolithic VCSEL. In particular, two aspects of VCSEL design will be discussed:
the Q of the VCSEL cavity, and the linewidth enhancement factor of the active region material. We report a factor of
two linewidth reduction, from 50 MHz for our standard oxide-aperture VCSEL to 23 MHz for an extended-cavity
This paper describes technologies developed at Sandia National Laboratories to support a joint DoD/DoE initiative to create a compact, robust, and affordable photonic proximity sensor for munitions fuzing. The proximity fuze employs high-power vertical-cavity surface-emitting laser (VCSEL) arrays, resonant-cavity photodetectors (RCPDs), and refractive micro-optics that are integrated within a microsensor whose volume is approximately 0.01 cm3. Successful development and integration of these custom photonic components should enable a g-hard photonic proximity fuze that replaces costly assemblies of discrete lasers, photodetectors, and bulk optics. Additional applications of this technology include void sensing, ladar and short-range 3-D imaging.
Optical time-domain reflectometry (OTDR) is an effective technique for locating faults in fiber communication links.
The fact that most OTDR measurements are performed manually is a significant drawback, because it makes them too
costly for use in many short-distance networks and too slow for use in military avionic platforms. Here we describe and
demonstrate an automated, low-cost, real-time approach to fault monitoring that can be achieved by integrating OTDR
functionality directly into VCSEL-based transceivers. This built-in test capability is straightforward to implement and
relevant to both multimode and single mode networks.
In-situ OTDR uses the transmitter VCSEL already present in data transceivers. Fault monitoring is performed by
emitting a brief optical pulse into the fiber and then turning the VCSEL off. If a fault exists, a portion of the optical
pulse returns to the transceiver after a time equal to the round-trip delay through the fiber. In multimode OTDR, the
signal is detected by an integrated photodetector, while in single mode OTDR the VCSEL itself can be used as a
detector. Modified driver electronics perform the measurement and analysis.
We demonstrate that VCSEL-based OTDR has sufficient sensitivity to determine the location of most faults commonly
seen in short-haul networks (i.e., the Fresnel reflections from improperly terminated fibers and scattering from
raggedly-broken fibers). Results are described for single mode and multimode experiments, at both 850 nm and 1.3 μm.
We discuss the resolution and sensitivity that have been achieved, as well as expected limitations for this novel
approach to network monitoring.
This paper describes the photonic component development taking place at Sandia National Laboratories, ARDEC and the Army Research Laboratory in support of an effort to develop a robust, compact, and affordable photonic proximity sensor for munitions fuzing applications. Successful implementation of this sensor will provide a new capability for direct fire applications. The technologies under investigation for the optical fuze design covered in this paper are vertical-cavity surface-emitting lasers (VCSELs), vertical-external-cavity surface-emitting lasers (VECSELs), integrated resonant-cavity photodetectors (RCPDs), and refractive micro-optics. The culmination of this work will be low cost, robust, fully integrated, g-hardened components suitable for proximity fuzing applications. The use of advanced photonic components will enable replacement of costly assemblies that employ discrete lasers, photodetectors, and bulk optics. The integrated devices will be mass produced and impart huge savings for a variety of Army applications. The specific application under investigation is for gun-fired munitions. Nevertheless, numerous civilian uses exist for this proximity sensor in automotive, robotics and aerospace applications. This technology is also applicable to robotic ladar and short-range 3-D imaging.
Vertical-external-cavity surface-emitting lasers (VECSELs) combine high optical power and good beam quality in a device with surface-normal output. In this paper, we describe the design and operating characteristics of an electrically-pumped VECSEL that employs a wafer-scale fabrication process and operates at 850 nm. A curved micromirror output coupler is heterogeneously integrated with AlGaAs-based semiconductor material to form a compact and robust device. The structure relies on flip-chip bonding the processed epitaxial material to an aluminum nitride mount; this heatsink both dissipates thermal energy and permits high frequency modulation using coplanar traces that lead to the VECSEL mesa. Backside emission is employed, and laser operation at 850 nm is made possible by removing the entire GaAs substrate through selective wet etching. While substrate removal eliminates absorptive losses, it simultaneously compromises laser performance by increasing series resistance and degrading the spatial uniformity of current injection. Several aspects of the VECSEL design help to mitigate these issues, including the use of a novel current-spreading n type distributed Bragg reflector (DBR). Additionally, VECSEL performance is improved through the use of a p-type DBR that is modified for low thermal resistance.