VCSEL Based Optical Sensors Jim Guenter and Jim Tatum Honeywell VCSEL Products 830 E. Arapaho Road, Richardson, TX 75081 (972) 470 4271 (972) 470 4504 (FAX) Jim.Guenter@Honeywell.com Jim.Tatum@Honeywell.com Up until perhaps 150 years ago, all available forms of artificial illumination involved lighting a fire. Today, sources of light and the technologies that produce them are almost too numerous to count, and selecting the best for a particular application can be daunting. Even if we limit consideration to compound semiconductor pn-junction devices (LEDs and junction lasers), choices abound. Encoding Strip Optical Input (c) (b) (a) λ specific absorber Signal Wavelength Optical Output Strip movement Reference Wavelength Scattering Medium Figure 1 (a) Turbidity sensor (b) Chemical sensor (c) Optical encoder. Assume that small size, high intensity, and reliability of LEDs and junction lasers have led us to choose a base technology. Assume further that we will rely on typical, economical types of LEDs and lasers, ignoring the more exotic and much more expensive variants. We still have some work to do to make a final device selection. For many applications, the best solution is a device that has been commercially available for only five years, the vertical cavity surface-emitting laser (VCSEL). (VCSELs emerged from the laboratory into commercial reality in 1996 when Honeywell introduced the first VCSEL to the market. Since then, VCSELs have essentially replaced all Fabry-Perot type edge emitting lasers [EELs] in applications such as high-speed data communication on multimode optical fiber [1].) How to know if VCSELs are the best choice for a sensor application? Walking through a generalized example will illustrate the process. Optical sensors can be classified generically as reflective, coherence-based, or transmissive. Reflective sensors include proximity, range finding, and bar coding; they often have the source and the detector in the same housing. (Reflective sensors can often be thought of as a folded transmissive
sensor) Coherence sensors may be speckle-based or interferometric. These sensors take advantage of the interference of laser light with itself to create dark and light spots that can be sensed on a detector, which is also generally in the same housing. In order to illustrate the use of VCSELs in sensor applications, however, this paper will now focus on the design considerations of a transmissive optical sensor. Many applications require non-contact sensing, implemented by the detection of light transmitted and somehow modulated between emitter and detector, with the emitter and detector at opposite ends of the optical path. There are many types of these transmissive sensors, but they can be grouped into three general categories: amplitude (such as turbidity, or muddiness ); spectral (such as chemical sensors based on absorption peaks); and opaque/transparent detection (such as in encoders or other beambreak systems). These are illustrated schematically in Figure 1. Let s compare three possible light sources for these applications: LEDs; EELs; and VCSELs (of which there are two types, single mode and multimode). Table 1 shows typical values for several characteristics of each device type. Some characteristics are strongly modified by the packaging, so the table assumes devices with no external optics. The optics, after all, are a significant part of the total sensor design, and one of the chief reasons one source type might be preferred over another. Attribute Symbol Units SM VCSEL MM VCSEL EE Laser LED Electrical Power P elec mw 5 20 60 60 Optical Power P opt mw 1 5 10 1 Efficiency at Popt=1mW h % 20 10 5 2* Wavelength l nm 760-860 670-870 630-1300 400-1300 Spectral Width λ nm 0.01 0.5 2 50 Spectral Tuning (Temperature) λ/ Τ nm/ C 0.06 0.06 0.3 0.3 Spectral Tuning (Current) λ/ Ι nm/ma 0.25 0.09 Beam Angle (full width at half of maximum value) <15 ~15 15 par. 35 perp 120 Table 1 Comparison of device characteristics for various semiconductor optical sources. *Assumes emission from a single surface of the chip, a requirement in high-radiance designs. Optical efficiency is important, because it translates directly into cost and size. Before we do the comparison, however, some definitions are in order. Since only the light that ultimately strikes the detector provides useful signal, it is generally necessary to have a relatively narrow beam angle, or divergence, of the emitted light. If the source is not itself of sufficiently narrow divergence, optics are used to reduce the angle. How
large these optics must be is itself a function of the source divergence. Figure 2 shows the illumination patterns for the three sources at a constant distance. It is obvious that the VCSEL would require the smallest lens, followed by the EEL, and the LED would run a distant third. (Additionally, more complex optics may be required to correct the ellipticity and astigmatism of the beam emitted from an EEL.) LED VCSEL EEL Figure 2 Beam patterns of various optical emitters If we limit our consideration to lensless designs, we can compute the electrical efficiency, and thus the potential battery lifetime in a particular illumination application. Figure 3 shows the result of such a computation. The difference in lifetime can be more than two orders of magnitude, enough to make or break some handheld applications and the values shown ignore the power used by the rest of the circuit! The extremely low power required by VCSELs can enable applications where electrical power is at a premium, such as when using miniature batteries or even remote power (think tiny solar cells). CW Operating Time (Hrs) 10000 1000 100 10 0.1 0.01 1 Miniature Batteries AAA AA C 10 100 1000 10000 Battery Rating (ma-hrs) Figure 3 Battery lifetime for a 1mW optical output power into a 30 degree cone angle for a single mode VCSEL (Blue), multimode VCSEL (Black), EEL (Green), and LED (Red).
High CW output power, whether at high or low efficiency, is much less important for the kinds of applications discussed here, because the regulatory eye-safety standards impose maximum limits on the total accessible laser power, usually at a level of 1 mw or less. For some transmissive sensors a specific wavelength of light is required. Examples of this type of sensor are chemical detectors where the spectral absorption signature of the chemical is interrogated. In this case, it is necessary to have the laser wavelength precisely tuned to the absorption cross section of the chemical. Examples include Carbon Monoxide, Methane, and Oxygen sensing. The laser wavelength must be controlled over the environmental conditions, or scanned continuously. Traditionally, distributed feedback (DFB) lasers, exotic variants of the EEL, have been used in this application because of their single lasing mode characteristics, but they suffer from significant cost disadvantages. Single mode VCSELs offer many of the advantages of a DFB at significantly lower price points. LEDs, or even standard EELs, are not very useful when a single narrow wavelength is required, as Figure 4 makes clear. In addition to controlled spectral width, it is preferable for the spectral peak to change as little as possible with temperature, to avoid the addition of costly temperature control hardware. Among the sources, VCSELs have the lowest temperature coefficient by a factor of five. 0 Intensity (dbm) -10-20 -30-40 -50 1nm Lasers 10nm LED -60-70 Wavelength (nm) Figure 4 Optical spectrum of a single mode VCSEL (Blue), EEL (Green) and LED (Red). Note the wavelength scale for the LED is 10x larger than the laser sources. Another consideration is the possible need for coherence. Coherence is needed when the optical sensor is based on interference of the optical signal to generate light and dark regions. Examples of these are extremely high-resolution optical encoders, some fiber sensors, and speckle based statistical encoders. While this is related to the specific wavelength requirement in chemical sensors, it imposes a much more stringent requirement on the spectral width of the source. A final consideration for a transmissive optical sensor is the packaging. Traditional packaging of EELs has been in a TO style header, though other configurations are also available. LEDs have a variety of packaging techniques but cannot be used where a laser source is required. VCSELs combine the attributes of lasers
and LED style packaging to open up a wide variety of options, including chip on board, surface mount, and plastic encapsulation, which are not generally available with EELs [2]. Combined with the optics miniaturization afforded by their narrow divergence, VCSELs enable the smallest total system outline. One other aspect of VCSEL manufacture enables some more unusual sensor configurations. The low power dissipation and top-emitting structure allows arrays of multiple elements on a single chip to be fabricated easily, making multi-beam systems nearly as miniature as those relying on a single beam. This has been a somewhat artificial exercise, because the selected performance requirements inevitably led to VCSELs. While they are the best choices in many circumstances, there are some cases where LEDs or EELs might be more suitable. A few examples: General area illumination, where LEDs are not only sufficient, they are superior; Yellow, green, or blue light applications, where inexpensive laser sources are not yet available; Applications not requiring eye-safety, but requiring very high optical powers, where EELs are the only economical choice. As VCSELs continue to mature, however, they become the choice for more and more applications. The best of an LED and a laser, in a single tiny chip, no matches required. References [1] J. A. Tatum and K. P. Jackson, VCSELs Enable High Speed Data Communications, Lightwave, April 2000. Available at www.honeywell.com/vcsel. [2] J. A. Tatum, Packaging Flexibility Propels VCSELs beyond Telecommunications, Laser Focus World, June 2000. Available at www.honeywell.com/vcsel