Progress in High-Power, High-Speed VCSEL Arrays

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Progress in high-power high-speed VCSEL arrays

Richard F. Carson, Mial E. Warren, Preethi Dacha, Thomas Wilcox, John G. Maynard, et al.

Richard F. Carson, Mial E. Warren, Preethi Dacha, Thomas Wilcox, John G. Maynard, David J. Abell, Kirk J. Otis, James A. Lott, "Progress in high-power high-speed VCSEL arrays," Proc. SPIE 9766, Vertical-Cavity Surface-Emitting Lasers XX, 97660B (18 March 2016); doi: 10.1117/12.2215009 Event: SPIE OPTO, 2016, San Francisco, California, United States Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

Invited Paper

Progress in High-Power, High-Speed VCSEL Arrays Richard F. Carson, Mial E. Warren, Preethi Dacha, Thomas Wilcox, John G. Maynard, David J. Abell, and Kirk J. Otis TriLumina Corp. 800 Bradbury Dr. SE, Suite 116, Albuquerque, NM 87123 (United States of America)

James A. Lott Technische Universität Berlin Zentrum für Nanophotonik, and Institut für Festkörperphysik , Berlin, D-10623 (Federal Republic of Germany)

ABSTRACT Flip-chip bonding enables a unique architecture for two-dimensional arrays of VCSELs. Such arrays feature scalable power outputs and the capability to separately address sub-array regions while maintaining fast turn-on and turn-off response times. These substrate-emitting VCSEL arrays can also make use of integrated micro-lenses for beam shaping and directional control. Advances in the performance of these laser arrays will be reviewed and emerging applications are discussed. Keywords: VCSEL, laser array, laser illumination, integrated micro-lens, LIDAR, beam-shaping, NIR illumination, automotive VCSEL applications

1. VCSEL ARRAY TECHNOLOGY Vertical Cavity Surface Emitting Lasers (VCSELs) have become commonly used in communications systems over the last 20 years of development and are common in sensor applications such as laser mice.1-4 More recently, VCSELs have been used in two-dimensional parallel arrays.5-8 We have reported previously on a back-emitting VCSEL array approach for high power with relatively high modulation speed (up to 10 Gb/s).9 This approach to VCSEL arrays was extended to pulsed power and scanning illumination applications.10 In the present work, we report on recent progress that extends these results and show the use of back emitting VCSEL arrays in associated automotive applications, such as illumination for driver monitoring systems and flash LIDAR.

Vertical-Cavity Surface-Emitting Lasers XX, edited by Kent D. Choquette, James K. Guenter, Proc. of SPIE Vol. 9766, 97660B · © 2016 SPIE CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2215009 Proc. of SPIE Vol. 9766 97660B-1 Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 7/23/2018 Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

1.1

High power, back-emitting VCSEL arrays

Flip-chip bonding is an effective way to make electrical connections to a back emitting VCSEL array. In such an approach, the gallium arsenide VCSEL die is flip-chip bonded to a dielectric ceramic sub-mount with metal patterned on its surface.11 An example of that technique appears in Figure 1, where the die is bonded to a ceramic sub-mount. This die contains 169 separate VCSEL elements on the front side of the chip (bonded to the sub-mount). It features a micro-lens array contained within the gold frame on the perimeter of the emitting surface. Figure 1. Flip-chip laser assembly with micro-lenses on the VCSEL chip.

Each VCSEL element within Shorted solder-bumped this array is a separate mesa cathode contact Cathode sub-mount structure. The anode side of metal each element is connected to a common sub-mount Anode sub-mount contact as shown in the metal cross-sectional diagram of Figure 2. The bonding is accomplished by the use of Combined Solder-bumped mesaarray solder bumps that are formed defined VCSEL laser output on the VCSEL surface and elements (anodes) beam are reflowed to attach the VCSEL to the sub-mount metal. Here, the anodes of Silicon or GaAs the individual VCSEL ceramic VCSEL elements are connected to the subcommon sub-mount anode chip mount pad via solder bumps that are reflowed to the metallization Figure 2. Cross-section of VCSEL array structure. of the sub-mount. The VCSEL elements are designed to emit light through the back of the substrate. Cathode contact is made by intentionally shorting a similar mesa structure to a current return layer in the epitaxial structure of the VCSEL. The cathode contacts are similarly reflowed to a common metal layer on the sub-mount. The cathode side of the array is connected to the corresponding contact on the sub-mount by a set of common “shorted” solder bumps.12 One option afforded by this approach is that micro-lenses can be fabricated on the emission side of the substrate. Such micro-lenses can be offset with respect to the elements of the

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VCSEL array in various patterns within the array to collimate and/or manipulate the combined array light beam.10,13 This results in addressable emission zones that can be used to advantage in illumination applications. 1.2

Preferred back-emission VCSEL wavelengths

Back emitting VCSELs are constrained to wavelength ranges where the GaAs substrate is transparent. At wavelengths shorter than about 900 nm, the substrate exhibits too much absorption loss. At wavelengths greater than approximately 1200 nm, epitaxial structures for high power VCSELs become somewhat impractical for the GaAs/GaAlAs based material stack.14 In order for the VCSELs to operate effectively as illuminators for sensor systems, they must match reasonably well with available detectors. We thus are building back emitting VCSEL arrays at several wavelengths. A wavelength of 905 nm provides best match with the shallow absorption depth of CMOS and CCD cameras, including gated imaging arrays for time-of-flight (TOF) LIDAR sensors.15,16 It is also matches well with Silicon Avalanche Photodiodes (Si-APDs) and Si-APD arrays.16 When the VCSEL operates at 940 nm, its output corresponds to the maximum sensitivity of Silicon P-I-N detectors and arrays.17 A wavelength of 980 nm enables operation with InGaAs detectors and arrays.19

The optical power and required current associated with a VCSEL array scales with the number of apertures and size of each aperture. An example appears for a seven element 905 nm back emitting array where each element in the array is a VCSEL having a 14µm aperture. Here, the threshold current of the array is 40 mA and the peak optical power at the thermal roll-over point is 73 mW. Multiple arrays such as this one can be combined to form individual emission zones in a single-chip NIR illuminator, as will be shown.

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Figure 3. Light output and voltage vs. current for a 905 nm backemitting VCSEL array having seven elements with 14 µm apertures.

Laser Safety Considerations

VCSEL arrays are more complex sources for purposes of laser safety calculations than single VCSEL emitters. The individual emitters may be imaged on the retina and, depending on the optical system, may need to be treated as point sources. In addition, the emitters are densely packed within the array, so there is a cumulative effect from the whole array that needs to be considered. Both the individual point source and the combined array extended source properties need to be calculated for a VCSEL array and the MPE (Maximum Permissible Exposure) values for both cases need to be considered. Fortunately, the ANSI Z136.1-2014 standard has a relevant example calculation, based on closely spaced fibers in a parallel connector.20 The extended source character of the array and the relatively low power for each element of the VCSEL array allow for higher MPE levels than an equivalent total power single aperture laser. A diffuser can be used in the applications described here, which will allow the resulting systems to be treated as extended sources only. While there are still limitations, this allows large overall illumination levels while maintaining an eyesafe condition.

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2. VCSEL V ARR RAYS FOR MULTI-ZON M NE NIR ILLUMINATIO ON

2.1

Usiing VCSEL Arrrays for Illum mination

NIR illumination is needed d to maximize the utility of imaging sensoors. Image-bassed user interfaaces in the automotive n particular, as direct NIR illuumination can overcome o the limitations l of changing c ambieent light environmentt can benefit in conditions. In addition to compensating for the variations in lightingg, NIR illuminaation can be useed to highlightt regions mbient lighted background foor the benefit of o image processsing algorithm ms. Manufactuurers are of interest annd filter out am now offeringg color image sensors s with NIR N capability added to the buuilt-in pixel-leevel filters.19 Because LEDs have h the advantage off low cost and freedom from speckle or cohherence noise, they tend to bee the most com mmon light souurces for NIR illuminnation. The diisadvantages of o LEDs incluude the very broad emissioon profile thaat can be diffficult to concentrate to a smaller field fi and a lim mited optical coonversion efficciency at higheer powers.21 VCSEL V arrays that use m fo or beam shapinng can providee a much moree usable emisssion profile annd the combinnation of integrated micro-lenses many incoheerent emitters within each arrray greatly reeduces coherennce noise comppared to conveentional edge-eemitting laser diodes or individual VCSEL V devicees.

2.2

“Sm mart Illumination” as appliied to driver monitoring m sysstems

The flexibiliity inherent in flip-chip bondded VCSEL arrrays on sub-m mounts enabless designs that can c implementt a wide variety of illlumination opttions. For exaample, a numbber of separateely addressed sub-arrays s can be implementted on a single chip. These sub-arrrays can be coombined with the integratedd micro-lensess to produce beams b that em manate at ultiple zones of o NIR illuminnation. This reesults in an illuuminator that iss tailored to match m the various anglles to create mu field of view w of an imaging g sensor. Addditionally, the zones z of illumiination can be actively controolled. Using feedback f from the imaage processing algorithms of such a system,, the illuminatiion pattern cann be made to “trrack” an area or o object of interest by b adjusting illlumination levvels of the variouus zones. Fig gure 4 shows the t case of a driver d monitorring system thhat tracks head position, eye opening, o and eye e nation zones are a gaze. The NIR illumin b the beams in i the figure. By represented by 'V' the use off zoned illum mination, such a r improveed efficiency and a system can realize signal-to-noiise ratio for im mage processiing applications.. In this example, the t _ illumination would concen ntrate on the eyyes d by th he light colorred (the zone defined arrow) to help the image processing p systeem determine iff an unsafe drriving practicee is taking placee, thus enabliing warnings or automated drriving function ns. Figure 4. 4 Smart illuminnation concept as a applied to drivver monitoring.

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2.4 Compact, wide field-of-view NIR illuminator example Our illuminator designs are arranged to match the 4:3 and 16:9 aspect ratios that are commonly used in video camera applications. The number of illumination zones in a given design depends on the application that the illuminator will be used for. Figure 5 shows the emission side of a 13-zone emitter in a QFN package, along with its simulation result. The micro-lens pattern is clearly visible on the emission surface of the array, and wire bonds allow electrical connection to each of 13 zones in the array. Within this design, all the beam shaping is done by the micro-lenses, so there is no need

13-Zone Ray Trace

13-Zone Scanning Emitter

Figure 5. Packaged implementation and ray trace simulation of illumination pattern from a 13-zone VCSEL array. Each of the zones is from a single sub-array with offset micro-lenses to control the beam direction and divergence. This simulation does not include a diffuser in the light path.

for an external lens. The entire pattern covers approximately a 45° X 25° field of view. The ray trace simulation shows all the sub-arrays or zones on at one time. It shows clear definition of the individual sub-fields that are addressed by each of the 13 sub-arrays in the device, but a larger overlap of the sub-fields may be desired in practice. In such a case, a holographic diffuser can be added for additional smoothing and for eye safety benefits. The actual generated illumination pattern from our 13-zone design appears in the images of Figure 6. Here, individual zones and combinations of zones are turned on to demonstrate the patterned illumination. The illumination patterns are

Multi-Zone

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Figure 6. Illumination patterns from the 13-zone illuminator of Figure 5.

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projected onto an opal glass diffuser, so the bright spot that consistently appears in the center of each picture is an image of the array itself through the opal glass and appears due to the camera’s depth of field. VCSEL arrays are a compact and efficient alternative to LEDs for NIR illumination applications. The flip-chip packaging approach used by TriLumina allows for combinations of addressable sub-arrays on a single die. The integrated micro-lenses enable each sub-array to illuminate a portion of the camera field of view. Software control of the illumination can be used to select zones to illuminate and the intensity of illumination, based on feedback from the imaging system. The combination of multiple, mutually incoherent sources reduces speckle significantly.

3. VCSEL ARRAYS FOR LIDAR

3.1

Flash LIDAR Types

Flash LIDAR systems based on Time-Of-Flight (TOF) depth sensing combine a detector array with the measured return times from pulses of light to provide range information. Unlike more traditional scanned LIDAR approaches, these systems do not require the use of a scanner or high brightness light source. They require high overall power, due to the fact that an entire field of view is illuminated at once.22 Additionally, the uniformity of the illumination is important to the overall system performance. In the case of a full field of view flash LIDAR, the detector is a two-dimensional focal plan array and the TOF information is added to provide the third dimension via a depth map, as each pixel of the focal plane array measures a return time from its part of the field of view. The extent of the field of view is determined by the detector array and its associated optics. In most cases, the illuminator provides a single pulse per frame, resulting in a low duty cycle, but a very high peak optical power requirement. This approach is often called “Staring 3D” or “3D Flash” LIDAR. Pulse width requirements are based on range resolution requirements and the detector array properties. Typical pulse widths are in the ~1-25 ns range. A variation on this approach is a segmented flash LIDAR. In a typical case, this approach may use a one-dimensional linear detector array, and include integration over a number of pulses. This drives requirements for an illuminator that operates at higher duty cycle, but correspondingly lower peak power. In both of these cases, VCSEL arrays can be scaled to meet the requirements of a high power, low-brightness pulsed illumination source. 3.2

Laser Driver Requirements for Flash LIDAR

In order to fulfill the requirements of high power and short pulse width, flash LIDAR systems usually have used laser sources such as diode-pumped solid state lasers, operated in short-pulse mode by the use of a Q-switch. A directly driven semiconductor laser may be used to realize the requirements of compactness, cost savings, and higher repetition rates. In such a case, the energy to drive the laser must be stored electrically in a capacitor bank and discharged into a short, low rise time, high-current pulse. For full field of view LIDAR applications, the output light pulse width should be on the order of 1 to 25 ns and the pulse repetition rate should be between 30 and 100 Hz to match camera frame rates. The peak current may be in the hundreds to low thousands of Amperes. For segmented flash LIDAR, the repetition rate may be up to 100 kHz, with peak currents in the low hundreds of Amperes.

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Current sources for short-pulse high-current semiconductor laser driver applications are now commonly available. For maximum efficiency of coupling into a semiconductor laser, the output impedance of the pulsed current source should be low and must be closely matched to the input impedance of the laser. This requires minimal series inductance, as any amount of series inductive voltage drop represents lost energy from a given current pulse. Figure 7 shows these effects schematically for the case of a VCSEL array. Because the VCSEL elements themselves are in parallel, the overall inductance and resistance values go down as the number of elements in the array goes up. When larger elements are used, the series impedance values are smaller for each individual element. Capacitance for each element is usually

Current Pulse In

… Sub-Mount With Parasitic Paths

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Figure 7. A simplified schematic of the equivalent circuit elements in a flip-chip VCSEL array.

very low, so has less effect on laser output pulse shapes in the ten nanosecond range, even as the laser array scales to several hundred elements on a chip. The result is that VCSEL arrays show very low overall differential impedance, often considerably lower than edge-emitting semiconductor laser configurations. The flip-chip bonding arrangement of Figures 1 and 2 allows for low inductance, low series resistance, and low parallel capacitance in the sub-mount. The sub-mount design for the VCSEL arrays of Figure 8 is specifically designed to provide a low-inductance connection by the use of a strip-line that connects with commercially available pulsed current circuits. VCSEL arrays can thus be very effective in providing efficient energy transfer from a pulsed current drive circuit. Because the impedances of individual array chips are so low, it is possible to Figure 8. A strip-line connected suboptimize matching to a given circuit by tiling chips in series connections onto mount and VCSEL array designed for a single sub-mount. This has been demonstrated to optimize the overall low impedance. match to a given pulse generator circuit and results in an effective multiplication of the slope efficiency, relative to a single die. Peak current into the array is reduced by an increased load potential due to the series diode drops, but pulse integrity is shown to be maintained.10 Two examples of this tiling approach are shown in Figure 9, where five and twelve VCSEL array chips are connected in series. In both cases, each of the VCSEL chips has 150 individual elements in parallel, though the chip size and aspect ratio are different for each of the two cases.

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Five-in-series (1 mm x 2 mm die with strip-line)

Twelve-in-series arrangement with 2 mm x 2 mm die

Figure 9. Two different series-connected VCSEL array realizations.

3.3 Test Results for Single High Peak Power VCSEL Arrays

Peak Power (W)

Amplitude (A.U.)

We have tested VCSEL arrays at 905 nm and 940 nm in the high pulsed power, low duty cycle mode of operation. These tests have been performed on devices that are flip-chip bonded onto low impedance sub-mounts as in Figures 8 and 9. The result for the single 905 nm die appears in Figure 10. This is a 150 element array where each aperture in the array is 16 µm in diameter. The pulsed current source is a 100 Ampere source with a fixed 7 ns pulse width. The test is conducted at a 10 kHz repetition rate. The output pulse was measured with a high speed detector and shown to have a 7.2 ns Full-Width Half-Maximum (FWHM) width at the maximum current of 92 Amperes. The peak power was 39 Watts as shown.

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Figure 10. Power and pulse shape for a 150 element, 905 nm VCSEL array having 16 µm aperture diameters. Pulse shape is shown for a 92 Ampere peak input current

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A single 980 nm VCSEL die with the same element count and aperture size was tested with the same 100 Ampere, 7 ns pulsed current supply as in Figure 10. The result for the 10 kHz repetition rate appears in Figure 11. Here, the peak power was 45 W and the pulse width was 5.6 ns (FWHM) at the peak current of 104 Amperes.

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Figure 11. Power and pulse shape for a 150 element, 980 nm VCSEL array having 16 µm aperture diameters. Pulse shape is shown for a 104 Ampere peak input current.

Peak Power (W)

Amplitude (A.U.)

The same type of 980 nm VCSEL with 150 elements and 16 µm aperture diameters was operated with a custom pulsed current source. This source is designed for an approximate 10 ns operational pulse width and can supply a peak current of approximately 300 Amperes. The result appears in Figure 12 for the 10 kHz repetition rate. Note that the power saturates at 110 Watts, though the pulse shape remains relatively well behaved as shown for the 255 Ampere peak current point and has a FWHM width of 5.8 ns.

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Figure 12. Power and pulse shape for a 150 element, 980 nm VCSEL array having 16 µm aperture diameters. Pulse shape is shown at approximately a 255 Ampere input current.

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Peak Power (W)

Amplitude (A.U.)

We have previously shown that for the low duty cycle values inherent in this type of operation, these saturation effects are due to gain saturation, rather than thermal loading.10 Such a conclusion is supported by the results of Figure 13. Here, we substitute the array from Figure 12 with one that has the same 150 element layout but where the aperture size for each element is 20 µm. In this test, done with the same pulsed current source at a 10 kHz repetition rate, the saturation current goes from 225 Amperes to 350 Amperes as in Figure 13. In both cases, the saturation current density is 7.3 mA/µm2. The pulse width is 5.9 ns (FWHM) at a peak current of 420 Amperes.

Time (ns) Approx. Peak Current (A)

Figure 13. Power and pulse shape for a 150 element, 980 nm VCSEL array having 20 µm aperture diameters. Pulse shape is shown at approximately a 420 Ampere input current.

Peak Power (W)

Amplitude (A.U.)

When we conduct the same test with the same pulsed current source on a 150 element array having 26 µm aperture diameters, we see that the saturation effect pushes out beyond the available current. This result appears in Figure 14. Here, we drive the VCSEL array to 420 Amperes and the peak power extends to 250 Watts, still showing a pulse width of 5.9 ns (FWHM). As before, the repetition rate is 10 kHz.

Time (ns) Approx. Peak Current (A)

Figure 14. Power and pulse shape for a 150 element, 980 nm VCSEL array having 26 µm aperture diameters. Pulse shape is shown at approximately a 420 Ampere input current point.

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O p C G G G G O G C® G C

C

Normalized Normalized Power Power

A further result that supports the conclusion that thermal effects are not at work in the curves of Figures 12 and 13 is the fact that samples from this same 980 nm VCSEL run show a very stable over-temperature result. This is illustrated in Figure 15, where the array with 26 µm elements is operated with the 100 Ampere, 7 ns current source that was used in the tests of Figures 10 and 11 and the peak current is 96 Amperes. We see that the normalized peak power only varies about 6% from room temperature to 120C.

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Figure 15. Over-temperature performance of a 150 element array with 26 µm diameter apertures, operated at 10 kHz with a 7 ns pulse width and a peak current of 96 Amperes.

3.4 Test Results for Series-Connected High Peak Power VCSEL Arrays

Amplitude (A.U.)

Peak Power (W)

As stated, a series connection of the VCSEL arrays can be advantageous due to the low series impedance associated with each of the individual arrays. This is shown to be the case for both the 905 nm and 980 nm arrays discussed. The first example of this is the five-in-series connection of 905 nm VCSEL arrays shown in Figure 9. Each of the die in this arrangement has 150 elements with 20 µm aperture diameters, configured within a 1 mm by 2 mm die size. The assembly was tested on the same high current 10 ns pulsed current source used to obtain the results of Figures 12 through 14 for the single die. The results, shown in Figure 16, indicate that a peak power of 750 Watts can be obtained at a peak current of 380 Amperes. The peak current has been reduced from the 420 Amperes seen for the single devices

Approx. Peak Current (A)

Time (ns)

Figure 16. Power and pulse shape for a five-in-series assembly of 150 element, 905 nm VCSEL arrays having 20 µm apertures. Pulse shape is shown at approximately a 380 Ampere peak input current.

due to the additional diode drops for each of the series connections on the assembly. At this 380 Ampere peak input current value, the pulse width is 11.2 ns (FWHM). The large change in slope at 300 Amperes is associated with a mode shift in the VCSEL devices.

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A 12-in-series assembly of 980 nm die was tested in the configuration of Figure 17. Here, the 2 mm x 2 mm die have 150 elements per array, with a 16 µm aperture for each element. The peak power for this array was 1200 Watts at a peak current of approximately 300 Amperes. The pulse width was measured at 7.2 ns (FWHM).

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Figure 17. Power and pulse shape for a twelve-in-series assembly of 150 element, 980 nm VCSEL arrays having 16 µm diameter apertures. Pulse shape is shown at approximately a 300 Ampere peak input current.

We have shown that back emitting, flip-chip-mounted VCSEL arrays and assemblies can provide light pulses having the attributes of high peak power and short pulse width in the low duty cycle mode of operation needed for flash LIDAR applications. They exhibit low series impedance, enabling them to be assembled into series or parallel configurations to optimize impedance matching with pulsed current driver circuits.

4.

SUMMARY

In this work, we have reported on our progress toward realizing back-emitting VCSEL arrays for use with illumination and flash LIDAR systems. We have shown how the back emitting VCSEL architecture, when combined with flip-chip mounting on sub-mounts and etched lenses on the emission surface of the GaAs, can be used to realize versatile illumination approaches for camera systems and for flash LIDAR.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the work of David Robinson and Kevin Toledo for their help with device and optical characterization. We also acknowledge the BIRD Foundation for their support of part of this work.

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