We report on the design and characterization of multi-mirror vertical-cavity surface-emitting lasers (VCSELs) that achieve linewidths less than 2 MHz. We have fabricated all-semiconductor multi-mirror VCSELs at 850 nm that operate in a single mode and are suitable for high-resolution spectroscopy. Cold-cavity linewidth measurements confirm increased quality factors relative to standard VCSEL resonators. Frequency noise power spectral density measurements exhibit 1/f noise and white-noise floors consistent with Lorentzian linewidths less than 2 MHz.
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 have been investigating the use of coaxial multimode VCSEL/PD (vertical cavity surface emitting laser/photodiode)
pairs for positional sensing with emitter to target mirror distances on the order of 1mm. We have observed large
variations in signal levels due to the strong optical feedback in these close-coupled systems, employing either
heterogeneously integrated commercial components or our own monolithically integrated devices. The feedback effect
is larger than anticipated due to the annular geometry of the photodetector. Even though there is very little change in
the measured VCSEL total output power, the optical feedback induces variations in the transverse mode distributions in
these multimode VCSELs. The higher order modes have a larger divergence angle resulting in changes in the reflected
light power incident upon the active detector area for a large range of emitter/mirror separations. We will review the
experimental details and provide strategies for avoiding these variations in detected power.
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.
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.
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
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.
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.
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 DARPA's 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.
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.
A fluidic cavity vertical-cavity surface-emitting laser (VCSEL) is presented for the detection of biological agents via introducing the analytic biofluid into the high finesse laser cavity. The optical properties of the fluid as modified by the biological cells they contain are sensed by monitoring the output optical intensity and wavelength of the laser. As a preliminary study, our first generation electrically pumped GaAs/AlGaAs based fluidic cavity VCSEL is described, with emphasis on the system design and techniques for the system construction. The device shows a strong spontaneous emission and a considerable wavelength shift when DI water is capillarily fed into the fluidic cavity.
Significant advancements have been made in the characterization and understanding of the degradation behavior of the III-V semiconductor materials employed in Vertical Cavity Surface Emitting Laser (VCSEL) diodes. Briefly, for the first time a technique has been developed whereby it is possible to view the entire active region of a solid state laser in a Transmission Electron Microscope (TEM) using a novel Focussed Ion Beam (FIB) prepared plan-view sample geometry. This technique, in conjunction with TEM cross-section imaging has enabled a three-dimensional characterization of several of the degradation mechanisms that lead to laser failure. It is found that there may occur an initial drop in laser power output due to the development of cracks in the upper mirror layers. In later stages of degradation, dislocations are punched out at stress-concentrating sites (e.g. oxide aperture tips) and these dislocations can then extend over the active region in a manner consistent with recombination enhanced dislocation motion. Alternatively, complex three-dimensional dislocation arrays which exhibited dendritic-like growth and which cover the entire active region can nucleate on a single defect.
Several thousand glass optical fibers fused together is routinely used as fiber image guides for medical and other image remoting applications. Fiber image guides also offer possibility for flexible optical interconnect links with potentially thousands of bi-directional parallel channels with data rates as high as 10 Gbps per channel, leading to more than Tera bits per second aggregate data transfer rates. A fair number of fiber image guide based link demonstrations using vertical cavity surface emitting lasers have been reported. However, little is known about designable parameters and optimization paradigms for applications to massively parallel optical interconnects. This paper discusses critical optical parameters that characterize a massively parallel link. Experimental characterizations were carried out to explore some of the fundamental interactions between single-mode 850 nm VCSELs and fiber image guides having different numerical apertures, 0.25, 0.55 and 1.00. Preliminary optical simulation results are given. Finally, potential directions for further experimental and analytical explorations, and for applicability into designable link systems are suggested.
Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for massively parallel interconnects and scannerless imaging applications due to their small size, high efficiency and amiability to formation of high-density 2-dimensional arrays. We have successfully fabricated 4096 element arrays (64×64) containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on a 55 micron pitch monolithically integrated on semi-insulating GaAs substrates. We employ a matrix addressable architecture to reduce the input and output electrical connections to the array, 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 design, fabrication and performance of these arrays is discussed.
Improvements in the performance of InGaAsN quantum well VCSELs operating near 1300 nm are reported. The effects of alloy composition on the photoluminescence intensity, linewidth, and anneal-induced wavelength blueshift of molecular beam epitaxial InGaAsN quantum wells are detailed. VCSELs employing a conventional p-n diode structure are demonstrated and compared to devices using two n-type DBR mirrors and an internal tunnel diode. Room-temperature differential efficiencies as high as 0.24 W/A, output powers of 2.1 mW, and a maximum CW operating temperature as high as 105 degree(s)C have all been demonstrated in these devices.
Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for applications requiring high-density 2-dimensional arrays of lasers, such as massively parallel interconnects or imaging applications. We have successfully fabricated 64x64 arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) on semi-insulating GaAs. In order to reduce the input/output pin count, we employed 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 microns, and total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 ohms. The epitaxial design, fabrication and performance of these arrays is discussed.
Massively parallel optical interconnects are appropriate to ease the data bandwidth bottleneck that will occur in future computing applications. Vertical cavity surface emitting lasers (VCSELs) are promising sources for emerging 2D optical systems such as free space and guided wave optical interconnects. We discuss the development of high performance VCSEL arrays, including individually addressable and matrix addressable arrays. We also show the characteristics of GaAs microelectronic driver and photoreceiver chips that have been designed to interface with Si-based CMOS circuitry. Finally, the potential of these source and receiver modules for use in free space or guided wave parallel channel optical interconnect architectures will be described.
The impressive performance improvements of laterally oxidized VCSELs come at the expense of increased fabrication complexity for 2-dimensional arrays. Since the epitaxial layers to be wet-thermally oxidized must be exposed, non-planarity can be an issue. This is particularly important in that electrical contact to both the anode and cathode of the diode must be brought out to a package. We have investigated four fabrication sequences suitable for the fabrication of 2- dimensional VCSEL arrays. These techniques include: mesa etched polymer planarized, mesa etched bridge contacted, mesa etched oxide isolated (where the electrical trace is isolated from the substrate during the oxidation) and oxide/implant isolation (oxidation through small via holes) all of which result in VCSELs with outstanding performance. The suitability of these processes for manufacturing are assessed relative to oxidation uniformity, device capacitance, and structural ruggedness for packaging.
Vertical cavity surface emitting lasers (VCSELs) which operate in multiple transverse optical modes have been rapidly adopted into present data communication applications which rely on multi-mode optical fiber. However, operation only in the fundamental mode is required for free space interconnects and numerous other emerging VCSEL applications. Two device design strategies for obtaining single mode lasing in VCSELs based on mode selective loss or mode selective gain are reviewed and compared. Mode discrimination is attained with the use of a thick tapered oxide aperture positioned at a longitudinal field null. Mode selective gain is achieved by defining a gain aperture within the VCSEL active region to preferentially support the fundamental mode. VCSELs which exhibit greater than 3 mW of single mode output power at 850 nm with mode suppression ratio greater than 30 dB are reported.
After successfully bonding VCSEL arrays to GaAs dummy chips and CMOS chips with three different bonding techniques, the thermal resistance and crosstalk of the bonded VCSEL arrays were measured. The thermal resistance of the VCSELs bonded to a GaAs substrate was found to be as low as 1100 K/W, indicating a high quality contact. Less than 100 K/W thermal crosstalk was also observed in the VCSEL arrays with a pitch of 250 micrometers . The thermal resistance of the VCSEL bonded to a CMOS chip with a standard bonding pad design has also been measured, which is 2490 K/W. The high thermal resistance is due to the dielectric layers underneath the bonding pads.
The fabrication and performance of selectively oxidized 850 nm vertical cavity surface emitting laser (VCSEL) diodes which emit through transparent GaP substrates is reported. Emission through the substrate is advantageous for many VCSEL configurations, such as for the incorporation of optical elements in the substrate or flip-chip integration to microelectronic circuitry. The short wavelength bottom- emitting VCSELs are fabricated by wafer fusion using an inert gas low temperature annealing process. The electrical characteristics of n- and p-type GaAs/GaAs and GaAs/GaP wafer bonded interfaces have been examined to optimize the annealing temperature. A significant reduction of the current-voltage characteristics of the VCSELs bonded to GaP substrates has been achieved whereby the bottom-emitting VCSELs show similar threshold voltage as compared to top- emitting lasers.
Optocouplers are used for a variety of applications aboard spacecraft including electrical isolation, switching and power transfer. Commercially available light emitting diode- based optocouplers have experienced severe degradation of light output due to extensive displacement during damage occurring in the semiconductor lattice caused by energetic proton bombardment. A new optocoupler has been designed and fabricated which utilizes vertical cavity surface emitting laser (VCSEL) and resonant cavity photodetector (RCPD) technologies for the optocoupler emitter and detector, respectively. Linear arrays of selectively oxidized GaAs/AlGaAs VCSELs and RCPDs, each designed to operate at a wavelength of 850 nm, were fabricated using an airbridge contacting scheme. The airbridged contacts were designed to improve packaging yields and device reliability by eliminating the use of a polyimide planarizing layer which provided poor adhesion to the bond pad metallization. Details of the airbridged optocoupler fabrication process are reported. Discrete VCSEL and RCPD devices were characterized at temperatures between -100 degree(s)C to 100 degree(s)C. Devices were packaged in a face-to-face configuration to form a single channel optocoupler and its performance was evaluated under conditions of high-energy proton bombardment.
We report the uniformity characteristics of low threshold of 1060 nm and high power 850 nm 8 X 8 individually addressable oxide-confined VCSEL arrays. Uniformity of lasing thresholds and operating characteristics are described, as well as thermal issues for 2D laser arrays.
The monolithic integration of coupled resonators within a vertical cavity laser opens up new possibilities due to the unique ability to tailor the interaction between the cavities. We report the first electrically injected coupled resonator vertical-cavity laser diode and demonstrate novel characteristics arising form the cavity coupling, including methods for external modulation of the laser. A coupled mode theory is used model the output modulation of the coupled resonator vertical cavity laser.
This paper presents the construction of the smart pixel arrays which perform AND and XOR functions with three-input and one-output optical signals for the application of an optical database filter. The device is based on oxide confined VCSELs bump bonded to GaAs MESFET pixels. The MSM photodetectors are monolithically integrated with MESFETs.
To insert high performance oxide-confined vertical-cavity surface-emitting lasers (VCSELs) into the manufacturing arena, we have examined the critical parameters that must be controlled to establish a repeatable and uniform wet thermal oxidation process for AlGaAs. These parameters include the AlAs mole fraction, the sample temperature, the carrier gas flow and the bubbler water temperature. Knowledge of these critical parameters has enabled the compilation of oxidation rate data for AlGaAs which exhibits an Arrhenius rate dependence. The compositionally dependent activation energies for AlxGa1-xAs layers of x equals 1.00, 0.98 and 0.92 are found to be 1.24, 1.75, and 1.88 eV, respectively.
We examine the threshold characteristics of selectively oxidized VCSELs as a function of the number, thickness, and placement of the buried oxide apertures. The threshold current density for small area VCSELs is shown to increase with the number of oxide apertures in the cavity due to increased optical loss, while the threshold current density for broad area VCSELs decreases with increasing number of apertures due to more uniform current injection. Reductions of the threshold gain and optical loss are achieved for small area VCSELs using thin oxide apertures which are displaced longitudinally away from the optical cavity. We show that the optical loss can be sufficiency reduced to allow lasing in VCSELs with aperture area as small as 0.25 micrometer2.