Red VCSELs offer the benefits of improved performance and lower power consumption for medical and industrial sensing, faster printing and scanning, and lower cost, higher speed interconnects based upon plastic optical fiber (POF). However, materials challenges make it more difficult to achieve the desired performance than at the well-developed wavelength of 850 nm. This paper will describe the state of the art of red VCSEL performance and the results of development efforts to achieve improved output power and a broader temperature range of operation. It will also provide examples of the applications of red VCSELs and the benefits they offer. In addition, the packaging flexibility offered by VCSELs, and some examples of non-Hermetic package demonstrations will be discussed. Some of the red VCSEL performance demonstrations include output power of 14 mW CW at room temperature, a record maximum temperature of
Multimode 850 nm VCSELs based upon the AlGaAs materials system have been the standard optical source for glass fiber optic-based data communication links since the mid-1990s. Although the first demonstration of red VCSELs followed fairly quickly after the demonstration of the industry standard “all-semiconductor” 850 nm VCSEL, the commercialization of red VCSEL technology has proceeded much more slowly due to the materials limitations that have made the development more challenging.
The AlGaAs materials system which is used for 850 nm VCSELs provides good lattice matching over the full range of compositions, a reasonably good refractive index contrast between the high index (AlGaAs with approximately 15–20% mole fraction AlAs) and low index (AlAs) materials used for the mirrors, and a high (approximately 0.35 eV) conduction band offset between the GaAs quantum wells and the AlGaAs compositions normally used as quantum well barriers. However, the 650–700 nm emission wavelength range requires use of GaInP quantum wells with AlGaInP barrier layers, with the compositions limited to those which are nearly lattice matched to a GaAs substrate. The AlGaAs materials system is usually used for the mirrors. Several limitations for these shorter wavelength VCSELs exist: (1) the available conduction band offset is smaller and ranges from approximately 0.17 eV at 650 nm to 0.23 eV at 700 nm [
The earliest reports of red VCSEL demonstrations were in 1993 from Sandia National Labs and Chiao Tung University in Taiwan [
Due to the wavelength dependence of the conduction band offset available in the AlGaInP materials system, the peak output power achieved and the temperature range of operation is a strong function of wavelength. A fairly early paper [
The temperature range of operation is also a strong function of wavelength. The first demonstrations of red VCSELs in 1993 required pulsed operation to lase [
Efforts have been made to extend the feasible wavelength range of red VCSELs, both to shorter wavelengths (<650 nm) and to longer wavelengths (>700 nm). The first red VCSELs reported below 699 nm actually included wavelengths as short as 639 nm, although they only operated under pulsed conditions [
Reliability data has been fairly limited. An early report on aging and failure analysis performed the testing under fairly unrealistic conditions, that is, current drive that was 3x past the rollover point, resulting in a junction temperature of around 250°C [
Since one of the main applications for red VCSELs is for data communication over plastic optical fiber, the achievable modulation rate is a key parameter of interest. An early measurement [
Novel approaches for dealing with the limited electrical and thermal conductivity have included the incorporation of transparent indium tin oxide (ITO) contacts that extend across the entire VCSEL aperture [
Mode control, that is, for achieving single transverse mode VCSELs, is also a challenge and is typically achieved by reducing the aperture size. Kasten et al. used a photonic crystal approach to achieve single-mode performance [
The goals of the work reported here were to increase the output power, temperature range of operation, achievable wavelength range, and reliability of red VCSELs. Specifically, the targets were a minimum of 1 mW single-mode power from 0–60°C, 10 mW multimode power up to 40°C, and at least 1 mW of useable multimode power at 80°C. Another goal was to extend performance to 720 nm with >1 mW of useable output power.
The red VCSEL structure is illustrated schematically in Figure
Schematic of VCSEL structure (From Proceedings of the SPIE, Vol. 7952, paper 795208).
Current and index confinement is provided by an oxide confinement layer located 2 periods above the quantum well active region. The devices are top-emitting with a ring contact patterned around the current aperture on the front side of the device. The substrate was thinned to 200
Wafers were probed on an automated probe station with wafer temperature control. 100% probe testing of the light output and voltage versus drive current (
Reliability measurements under pulsed conditions were carried out on devices in hermetic TO-46 packages. Resistance to humidity was evaluated at 50°C, 85% humidity on devices packaged in TO-46 headers but with the glass window removed from the lid. In both cases devices are biased during life testing at the accelerated environmental conditions. However, the devices are removed from the oven at each test point and tested at room temperature and room humidity, which was typically 20–25°C and 40% relative humidity.
One of the most challenging aspects of designing red VCSELs has been achieving useable output power over the temperature ranges required by the applications of interest. Figure
Light output and voltage versus current (
Figure
Performance of single-mode 680 nm VCSELs at 25°C. (a) Overlaid
One of the key questions of interest in the production of devices is the uniformity across a wafer. The wavelength of a VCSEL is approximately proportional to thickness of the layers, so a 1% variation of thickness can result in approximately a 7 nm variation in wavelength. In addition, the oxidation diameter can also vary across a wafer due to small differences in layer thickness, doping, or composition. Both of these effects can impact performance of a VCSEL. For instance, the temperature characteristics of a VCSEL depend upon the offset between the gain peak and the Fabry-Perot resonance. Since the gain peak wavelength is less sensitive to thickness and therefore nearly constant across the wafer, while the Fabry-Perot resonance may have a range of 5–10 nm, this offset varies across the wafer. The ability to do automated wafer scale testing allows us to gather statistics on uniformity.
Figure
(a) Histogram showing the wavelength distribution of 60,000 VCSELs tested at 40°C on a 4′′ wafer. (b) Threshold current versus wavelength. (c) Peak output power versus wavelength. The shaded regions in (b) and (c) indicate the wavelength range corresponding to the vast majority of devices on the wafer (from Proceedings of the SPIE, Vol. 7952, paper 795208).
As one might expect, the threshold current is U-shaped and rises at the longer wavelengths, due to a larger offset between the gain peak and the Fabry-Perot cavity, but devices are still lasing at 709 nm, where the offset is approximately 40 nm. Peak output power at 40°C versus wavelength for several aperture sizes is shown in Figure
Red VCSELs have typically been limited in the maximum output power that can be achieved in part because the larger aperture devices are more sensitive to temperature. Improved design has allowed larger devices to be built. Improvements include the use of quantum well barrier layers with tensile strain to improve the conduction band offset, tailoring of the doping profile in the mirrors to reduce series resistance, and the use of a slightly thicker high aluminum-containing mirror layers, and thinner low aluminum-containing mirror layers (while keeping the sum of the two equal to
High output power devices at 25°C. (a)
We have fabricated devices with wavelengths in the range from 700 to 720 nm, but unlike previous reports [
Performance at 25°C of an AlGaInP QW-based VCSEL at (a) 716 nm, and (b) at 719 nm (from Proceedings of the SPIE, Vol. 7952, paper 795208).
There are some applications where lasers are typically pulsed at a low duty cycle, such as industrial sensors, or the computed radiography application described in the applications section below. Pulse widths in the range of 1
Figure
Output power versus current for multimode 680 nm VCSELs operated in pulsed mode with a 1
Pulsing can also increase the peak output power of the arrays described above. Figure
An output power of nearly 120 mW is achieved from a
In pulsed mode the device self-heating is reduced, and therefore the device rollover point (where increasing the current actually results in a reduction of output power) is extended to significantly higher drive current. However, this leads to a question: if device lifetime is reduced by higher current (or, more accurately, current density) can one operate a device in pulsed mode at these higher current ranges for useful periods of time? Furthermore, are there any transient effects, such as stress created by repeated junction temperature cycling resulting from the current cycling that might actually accelerate the degradation of the devices beyond what is normally expected from the current drive alone? For instance, the VCSEL lifetime is commonly found to be reduced proportionally to the square of the current density. An increase of drive current from 8 mA to 30 mA might be expected to reduce the lifetime by a factor of 14 due to current density alone. Under CW conditions, the increase in junction temperature from the higher current adds to the acceleration of failure. Using the empirical model for acceleration of failure, we estimate that a 30 mA CW drive current would reduce lifetime by a factor of 500 under CW conditions.
To experimentally evaluate the effect of pulsing on reliability we developed a capability for testing the VCSELs in pulsed mode. Both single-mode and multimode devices were packaged in TO-46 headers and mounted on boards that were placed in ovens. The devices were pulsed with a pulse width of 1
Peak output power versus test time for 670 nm VCSELS tested in pulsed mode. The output power testing was performed at room temperature. The lower curves correspond to a smaller diameter single-mode device, while the upper curves correspond to a multimode device (from Proceedings of the SPIE, Vol. 7952, paper 795208).
The multimode devices have a CW peak output power around 5 mW and were pulsed to one of two different current levels, 18 mA or 30 mA. The single-mode devices have a CW peak output power of approximately 2.5 to 3 mW and were pulsed to 7 mA. A burn-in effect can be seen in the first 100–200 hours, where the output power increases, but after the burn-in period, the output power has been stable during the 6596 hours of test at 50°C, corresponding to 824 hours of actual pulsed on-time.
Table
Calculation of acceleration factors assuming an acceleration temperature of 50°C and a use condition of 10 mA and 25°C.
DC or pulsed | Acceleration current | Acceleration factor |
---|---|---|
DC | 10 | 4.5 |
18 | 88 | |
30 | 2438 | |
Pulsed | 10 | 6 |
18 | 20 | |
30 | 55 |
While we do not yet have sufficient failures to project a lifetime, this table predicts a very significant improvement in lifetime under pulsed conditions, assuming no transient effects, which is consistent with our observations. We would certainly expect devices operated CW at 30 mA at 50°C for the equivalent of 824 hours (6596 test hours times the 12.5% duty cycle) to have failed. The lack of failures also prevents us from completely ruling out acceleration due to thermal transients when operated under pulsed conditions, but the lack of degradation observed in Figure
More conventional reliability testing is carried out under conditions of constant current drive. Temperature and current are the most commonly assumed acceleration factors, with humidity being an acceleration factor for devices in non-Hermetic packages. We performed evaluation of VCSELs under dry conditions by placing 186 multimode devices on test at three different temperatures and currents, and periodically removed the devices from the ovens to test output power at room temperature. Failure was defined as a 3 dB reduction in output power as compared to the output power at time zero. Devices were aged at 50°C, 85°C, and 105°C, and 7, 11, and 15 mA of drive current. By assuming the same failure acceleration model described above, we calculated the acceleration factor for each of the test conditions relative to a use condition of 40°C and a current drive of 8 mA. We then created a “meta-analysis” of the failures by translating the time to each failure at its test condition to the equivalent time at 40°C and 8 mA. The results are shown in the plot in Figure
A plot of the failures times of the devices from our reliability evaluation of 670 nm multimode devices translated to their equivalent failure times at the use condition of 40°C and 8 mA. The
Since some applications require a nonhermetic package, and most active optical devices are sensitive to a humid environment, it is important to understand the acceleration of failures due to exposure to humidity. We have performed environmental testing on our chips under accelerated conditions of temperature and humidity. Five chips were packaged in a TO can with the window removed and placed on boards in a chamber held at 50°C and 85% humidity. During aging they were driven continuously with 5 mA of drive current. The parts were periodically removed from the environmental chamber, and
Output power versus hours on accelerated life testing for 690 nm VCSELs. Devices were maintained at 50°C and 85% humidity and driven with 5 mA during aging.
The target applications for red VCSELs fall outside of the existing data communication market where the vast majority of VCSELs have been applied. VCSELs can bring unique value to medical sensor and diagnostic devices, office equipment such as laser printing, industrial sensors, and low cost communications based upon plastic optical fiber. Design and packaging solutions to address some of these uses are described below.
As a first example, an important optically based noninvasive medical sensor application is oximetry. Pulse oximetry, which measures the oxygen content of arterial blood, is well-established using LEDs, while tissue or regional oximetry, which measures venous or capillary blood, is an emerging application. Near-infrared spectroscopic-based imaging, which relies on differing absorption and scattering as a function of quantity and oxygenation of blood for image contrast, is an active area of research. All versions of oximetry take advantage of the varying absorption coefficient as a function of wavelength for different types of hemoglobins, that is, oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, or methemoglobin as is illustrated in Figure
The absorption spectra versus wavelength for four different components of hemoglobin (from
These sensors are often disposable, body worn sensors requiring a low cost, very compact package that can accommodate multiple chips of various wavelengths. Figure
(a) A PLCC package incorporating 3 VCSEL chips. The package dimensions are 2.8 mm × 3.2 mm. (b) Room temperature
A second example application is plastic optical fiber (POF) links based upon PMMA fiber materials which have been implemented for sensor and data links in automobiles, and are being considered for home networks. PMMA-based fiber has secondary absorption minima in the red. Absorption at 850 nm is too high for links more than a few meters. While the potential for high speed data rates and the packaging simplicity of VCSELs makes them ideal for this application, wavelengths in the range of 650–680 nm are a necessity for low loss links. POF links based upon LEDs have been implemented in automobile sensors and entertainment networks. POF links for home networking are being developed and will require data rates in the 1 Gbps range and above at low cost, making VCSELs an attractive solution. Figure
Eye diagrams of a pseudorandom bit sequence at 1.25 Gbps (left) and 3 Gbps (right) measured with a Vixar red VCSEL emitting at 670 nm.
A third application example takes advantage of the ease with which VCSELs can be fabricated in multilaser arrays on a single chip. Vixar has been developing a laser scanner with no moving parts for computed radiography, a form of X-ray imaging that results in a digitized image by storing the X-ray image in a storage phosphor screen, and then reading out the phosphor with a red laser. The red laser stimulates the emission of blue light which is detected and digitized. However, the width of the standard screen, 14 inches, requires a fairly long optical path for scanning with a single laser. A linear laser array could reduce the size of the scanning mechanism and make the equipment more robust. This application requires a wavelength in the 650–700 nm range. However, creating a linear array of lasers 14 inches long requires tiling array chips in a chip on board configuration.
An early report on this product was published by Dummer et al. [
Photos of the 2′′ solid-state scanner. (a) The circuit board including the electronics for controlling the scanner function. (b) A closeup of the board with the VCSEL chips attached but without wire bonding. One full VCSEL array is visible, and the edges of two others. The intersections between chips are between the alignment crosses. (c) A picture of the scanner in operation.
The results reported in this paper describe improvements in the temperature range of operation, the magnitude of output power and the range of wavelengths that can be achieved in red VCSELs. The improved performance is the result of attention to many details of the design including quantum well active layer design, mirror design, mask layout, proper choice of gain peak resonance cavity offset, and epitaxial materials quality. There is no silver bullet, but the improvement is the result of the accumulation of many incremental steps of optimization.
We have demonstrated red VCSELs lasing at 689 nm up to 115°C for smaller aperture single mode devices. Of more importance is the temperature range of “useable” power. Single mode devices have produced 1 mW of output power up to 60°C, and multimode devices provide up to 1.5 mW of power at 80°C. 14 mW of output power at room temperature has been achieved from a single VCSEL aperture, and as much as 44 mW of power from a chip containing multiple apertures within a small area.
The range of wavelengths achievable from this materials system has been extended out to 719 nm, with 2 mW of output power at room temperature at that wavelength. While VCSELs in this wavelength range have been demonstrated in the AlGaAs materials system, the results demonstrate improved output power as compared to the previously reported results. We have not explored the wavelength region below 670 nm to any substantial degree, but the improvements we have seen at 670 nm and above, combined with previous reports of devices operating at 650 nm, suggest that operation over a useful power and temperature range at 650 nm should be feasible. However, operation at wavelengths substantially below 650 nm with useful power or temperature ranges remains questionable in this materials system.
The benefits of pulsing the VCSEL have been investigated. Peak output power of 35 mW from one multimode aperture has been demonstrated for a 10% duty cycle and 1
A 4′′ wafer diameter process and automated wafer probe testing that allows the gathering of statistics on uniformity have been developed. Wavelength uniformity across the wafer is approximately 8 nm, and average threshold current and output power uniformity do not vary significantly within that wavelength range.
The feasibility of using low cost non-Hermetic packages was demonstrated by 3500 hours of continuous operation at 50°C and 85% humidity in a package open to the environment.
Red VCSEL technology has struggled to reach the marketplace due to performance limitations caused by the materials challenges in overcoming thermal and environmental demands. We believe that the results reported here illustrate devices that are ready for use in a wide variety of applications.
This material is based upon work supported by the National Science Foundation under Grant no. IIP-0823022. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The laser scanner work was funded by the National Institutes of Health under Award no. R44RR025874 from the National Center for Research Resources. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Center for Research Resources or the National Institutes of Health.