Development of VCSELs for Optical Nerve Stimulation

Matthew Dummera, Klein Johnsona, Mary Hibbs-Brennera, Matthew Kellerb,Tim Gongb, Jonathon Wellsb, and Mark Bendettb

aVixar, 15350 25th Ave. N, Plymouth MN, USAbLockheed Martin Aculight, 22121 20th Ave. SE, Bothell WA, USA

ABSTRACT

Neural stimulation using infrared optical pulses has numerous potential advantages over traditionalelectrical stimulation, including improved spatial precision and no stimulation artifact. However,realization of optical stimulation in neural prostheses will require a compact and efficient opticalsource. One attractive candidate is the vertical cavity surface emitting laser. This paper presentsthe first report of VCSELs developed specifically for neurostimulation applications. The targetemission wavelength is 1860 nm, a favorable wavelength for stimulating neural tissues. Continuouswave operation is achieved at room temperature, with maximum output power of 2.9 mW. Themaximum lasing temperature observed is 60 C. Further development is underway to achievepower levels necessary to trigger activation thresholds.

Keywords: Nerve Stimulation, Infrared, VCSEL, Semiconductor Laser

1. INTRODUCTION

Artificial stimulation of neural tissue has been an important tool for identifying nerve connectivity and func-tionality, as well as development of neural prostheses. Historically, the most widely used methods for nervestimulation have been electrical. However the electrode-tissue interface has many limitations including damageto neural tissue by high current or mechanical contact, susceptibility to environmental interference, and introduc-tion of high-frequency artifacts to the stimulation signal.13 In addition, conductivity of surrounding tissue leadsto undesired current spreading and poor spatial specificity.1 These shortcomings have prompted exploration intoalternative means of stimulation.46 Recently, it has been discovered that relatively low levels of pulsed infraredlaser light are capable of triggering neural activity in both motor and sensory systems.7 This approach hasbeen determined to have many advantages over direct electrical stimulation. For example, no contact is requiredbetween the tissue and the source, and optical activation appears not to produce any stimulation artifact.1 Fur-thermore, a focused laser beam can be used to pinpoint small numbers of neurons, thereby improving the spatialresolution.7

Optical neurostimulation could be instrumental in advancing neurophysiological research and expanding itsuse in clinical applications. Neural prostheses, devices which aim to restore sensory or motor function by directlyinterfacing with the nervous system, might benefit from this new technology. One such device is the cochlearimplant, which restores hearing in deaf patients by stimulating auditory nerves. These devices require multiplestimulation sites that activate the spiral ganglion neurons lining the cochlea, each site corresponding to a specificauditory frequency. The spectral resolution of the implant depends on the total number of stimulation channels aswell as the physical spacing between them. Currently, electrical implants utilize up to 22 intra-cochlear electrodes.However, studies have shown that beyond 4-8 channels, speech comprehension does not improve due to crosstalkbetween electrodes.8,9 (Some improvement in resolution has been demonstrated by using specialized codingand multiple electrodes simultaneously to induce current steering.10) On the other hand, mid-infrared lighthas been shown to exhibit very minimal scattering, and therefore does not spread laterally upon incidence.11,12

Consequently the spatial resolution achievable by optical stimulation could greatly improve the performance ofcochlear implants beyond what is capable from even the best electrical devices.13

Further author information: (Send correspondence to Matthew Dummer)E-mail: mdummer@vixarinc.com, Telephone: (763) 746-8045

Figure 1. Reflectivity spectrum of the as-grown VCSEL wafer compared with theoretical calculation

In vivo optical experiments thus far have been conducted in laboratory animals using external infrared lasersto deliver the stimulation signal through surgically implanted optical fibers.1,14 However for clinical applicationsin humans, such methods are not practical. Therefore, implementing optical stimulation in neural prostheses willrequire development of a miniaturized implantable source. One especially suitable candidate is the vertical cavitysurface emitting laser (VCSEL). VCSELs are type of diode laser designed such that the optical beam is emittedorthogonal to the wafer surface. These lasers offer small footprint, low power consumption, high efficiency, andsimple packaging, all of which are desirable for an implantable device. Also, their unique geometry offers theability to be fabricated in 2-dimensional arrays for increased output power, or addressing multiple locationsindependently with a single chip.15

The goal of this work is to develop VCSELs specifically for neurostimulation applications. The requiredspecifications for these devices have been identified and a first round of VCSELs have been fabricated andtested. Preliminary results demonstrate continuous wave and pulsed lasing at the desired wavelength (1860 nm),with up to 3 mW of output power. Power levels necessary for neural activation threshold have not yet beendemonstrated, but the initial measurements are an encouraging starting point for the future role of VCSELsin neural prosthetics. This paper details the design and characterization of these first VCSELs, and discusseschallenges still ahead for this application.

2. REQUIREMENTS OF VCSELS FOR NEURAL PROSTHETICS

The physiologic mechanism responsible for optical activation of the neuron is most likely attributed to directheating of the tissue, rather than electric field interaction or photochemical effects.14 That being the case,neural tissue activation does not necessitate a precise wavelength. Rather, the initiation of the action potentialrelies on the local temperature rise, and hence total optical energy absorbed is the critical factor. Since theabsorption coefficient of tissue varies as a function of wavelength, selection of the wavelength can be used tospecify the penetration depth of the optical signal. Wavelengths between 1840 nm and 1880 nm, correspondingto penetration depths from 1129 to 308 m, respectively, provide practical working distances for stimulation.16

For the VCSEL development, we have chosen to target the center of this range ( = 1860 nm, dp = 819 m).

The long wavelength of 1860 nm poses a significant challenge for VCSELs due to fundamental materiallimitations. Furthermore, the material composition of the VCSEL affects many other aspects of the deviceperformance such as output power, temperature range, and modulation rate. Besides wavelength, the greatestchallenge is achieving high output power, since long-wavelength VCSELs have traditionally been limited to afew milliwatts. Recently large-aperture devices and multi-aperture arrays have been used to achieve powers oftens to hundreds of milliwatts, respectively, at 1550 nm.17 Very little data has been reported on VCSELs near1860 nm,18 but similar results should be achievable.

Figure 2. Photograph of wafer with fabricated VCSEL die

3. VCSEL DESIGN AND FABRICATION

VCSELs are fabricated using wafer scale processes similar to other optoelectronic devices. The material designis significantly more complex than other diode lasers because all of the structures comprising the laser mustbe integrated vertically. Though widely commercially available at wavelengths between 800 and 1000 nm, longwavelength VCSELs have required different materials platforms that have taken much longer to develop. The keychallenge has been finding semiconductor materials with the proper active-region bandgap that are compatiblewith high index-contrast mirrors. For our target wavelength of 1860nm, both indium phosphide and galliumantimonide are possible substrate choices. Indium phosphide is lattice-matched to various alloys emitting be-tween 1.0-1.7 m, and strained materials can be incorporated to extend the emission range beyond 2.0 m.19

Gallium antimonide has a wider range of alloy compositions, although growth and fabrication techniques forthese materials are not well established. Fabrication techniques for InP-based devices are comparatively moremature, and commonly used for edge-emitting lasers and high speed transistors. Significant efforts have alsobeen made to develop InP-based VCSELs for telecommunications and chemical sensing.20 Given the maturityof InP-based fabrication, we have chosen to pursue this material platform for the 1860 nm VCSEL development.However demonstrations of VCSELs emitting over 2.0 m on GaSb have also recently been reported, suggestingthat the antimonide materials could be investigated for our application in the future.21

Fabrication of the VCSELs begins with growth of the epitaxial base structure by metal organic chemicalvapor deposition (MOCVD). The layer stack consists of a gain region between two highly reflective mirrorsto form a resonant optical cavity. Gain is achieved at the target wavelength by incorporating compressivelystrained InGaAs quantum wells in the active region to lower the bandgap of the material. Tensile strainedInGaAsP barriers compensate the wells to prevent relaxation of the lattice. Like most VCSELs, the smalloverlap between the optical mode and the active region necessitates very high mirror reflectivity (>99%) toachieve lasing threshold. The lower mirror consists of an epitaxially grown distributed Bragg reflector (DBR)with more than 30 periods of alternating high- and low-index InGaAsP. The upper mirror is only partially grownepitaxially; an additional dielectric coating is later deposited to increase the reflectivity. The layer structure alsoincludes a tunnel junction above the active region to improve lateral current spreading and reduce optical lossesdue to p-doping. Reflectivity measurements of the wafers after growth compare the theoretical design to theactual layer structure. Fig. 1 shows the wafer reflectivity compared with the simulated spectrum obtained fromtransfer matrix calculations. The DBR stopband is clearly shown from 1780nm to 1920nm with a reflectivity>99%. The dip near the center of the stopband signifies the Fabry-Perot resonance of the cavity, indicating thepreferential lasing wavelength of the device (1842 nm). Photoluminescence experiments were also performed toconfirm that the active region gain peak was well aligned with the cavity resonance.

Post-growth device processing utilizes standard microelectronic fabrication techniques. An optical photographof the fabricated devices on the wafer is shown in Fig. 2. Annular metal contacts surrounding each VCSELare defined to inject carriers into the laser active region. The ring contacts are interconnected to correspondingbondpads for flip-chip die bonding or wirebonding to custom packages. Lateral current confinement within the

Figure 3. Schematic of VCSEL die containing a 2x2 VCSEL array. Die dimensions are 350 x 350 x 250 m3 (LxWxH).

VCSEL is created by ion implantation, which reduces the conductivity of the material outside the desired activeregion. Deeply etched trenches are also used to electrically isolate between adjacent VCSELs in an array. Thedielectric mirrors that comprise part of the upper DBR are defined above the ring contacts, and are terminatedwith a top metal reflector to boost reflectivity. This necessitates that the VCSELs are bottom emitting, andtherefore windows in bottom-side cathode metal are required for emission from the subsrate. A schematic of thefinal die containing four individually addressable VCSELs is depicted in Fig. 3. The die size is 350 m per side.

Figure 4. VCSEL light output and operating voltage versus applied current for implant aperture diameters between 10and 50 m. Measurements were performed CW at 20 C

4. EXPERIMENTAL RESULTS

Initial device testing has been conducted at the wafer level. Samples were placed on a copper stage to allowthermal and electrical contact to the backside of the wafer. Anode contacts were directly probed, and a hole in thestage allowed the emitted light to be incident on a large area photodetector. Measurements of light output versusapplied current and voltage (LIV) for VCSELs with various active diameters are shown in Fig. 4. Measurementswere taken continuous wave, with stage temperature controlled at 20 C. Electrical characterization shows adiode turn-on voltage of 0.7 V and the series resistance is inversely proportional to the current aperture area.Continuous wave lasing was observed for all device diameters between 10 and 100 m. Lasing threshold alsovaried as a function of area, with the smallest devices exhibiting thresholds as low as 1.4 mA. Figure 5(a)illustrates the trade-off between series resistance and threshold current over the range of aperture sizes. Thedifferential quantum efficiency at threshold was between 15-22% for all designs, although self heating resulted in

(a) (b)

Figure 5. (a) Measured series resistance and laser threshold current versus aperture diameter. (b) Comparison of LIVcharacteristic under CW and pulsed excitation for a 20 m diameter VCSEL.

Figure 6. Continuous wave L-I measurements at various stage temperatures for 12m and 20m diameter VCSELs.

decreased efficiency at higher currents. The 50 m aperture devices exhibited the highest peak power, 2.9 mW at70 mA. Greater than 1 mW of power was achievable from a 30 m device when biased at 15 mA. This operatingpoint corresponded to a peak wall plug efficiency of 6%.

To isolate the effects of self-heating, pulsed measurements were performed with 1 s pulses and 1% duty cycles,which is faster modulation than necessary for neural stimulation. Figure 5(b) shows pulsed versus continuouswave performance for a 20 m aperture VCSEL. Under pulsed operation, peak power up to 4.0 mW at 50 mA wasachieved (Fig. 4(b)). Slope efficiency is the same for CW and pulsed operation. However the reduced thermalrollover increases the maximum wall plug efficiency to 10% when operating in pulsed mode. For longer pulselengths (>10 s), negligible increase in output power was observed compared with CW due to the VCSELs shortthermal time constant. The continuous wave output power is therefore a more relevant measurement for ourtarget modulation rates. Effects of external heating on the VCSEL output power have also been examined. Fig.6 shows the LIV characteristic as a function of temperature for 12 and 20 m VCSEL designs. Both device sizesexhibit similar temperture performance. Cooling the stage to 13 C results in a significant increase in outputpower compared with room temperature operation. Similarly, by heating the stage to 37 C, the peak poweris reduced by about 50% compared with 20 C. An increase in threshold current is also observed, owing to thereduction in gain as the active region is heated. The maximum observed lasing temperature was 60 C for bothdevices.

Wavelength measurements have been conducted by coupling the output of the VCSEL into a multimode fiberand analyzing the signal with a long-wavelength optical spectrum analyzer (OSA). The output spectrum of theVCSEL is shown in Fig. 7(a). The device exhibits single spectral mode operation with a peak wavelength of

(a) (b)

Figure 7. (a) Optical output spectrum of a 20 m VCSEL (b) Output wavelength versus stage temperature for constantoperating current

1859.6 nm. The spectral width is less than 0.1nm, limited by the resolution of the OSA. Measurement of thewavelength at various stage temperatures shows very stable operation over a wide temperature range. The laserexhibits a linear red shift in wavelength at a rate of 0.13 nm/C (Fig. 7(b)). The thermal tuning rate is similarto the rate reported for other long wavelength VCSELs.18

5. CONCLUSION

The recent discovery of optical neural stimulation could enable new prosthetic devices for sensory impairedpatients. VCSELs look especially promising as optical sources, and the long-wavelength devices presented abovedemonstrate a first step toward implantable devices. We have achieved CW and pulsed operation at the desiredwavelength, and temperature operation up to 60 C. The maximum CW power measured was 2.9 mW at 20 C.To our knowledge, this is the highest continuous wave output power demonstrated for a VCSEL at 1860 nm.A wall plug efficiency of 6% has also been demonstrated at 1 mW of output power. Although power levels toachieve neural activation have not yet been met, there do not appear to be any fundamental roadblocks. Futurework will focus on increased optical power, improvement in efficiency, and optimizing the thermal performance.

ACKNOWLEDGMENTS

This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) underSPAWAR Systems Center, Pacific (SSC PAC) Contract No. N66001-09-C-2008. The views, opinions, and/orfindings contained in this article/presentation are those of the author/presenter and should not be interpreted asrepresenting the official views or policies, either expressed or implied, of the Defense Advanced Research ProjectsAgency or the Department of Defense

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