Vertical Cavity Surface Emitting Lasers (VCSELs) were commercialized in 1996 primarily for use in data communication networks using fiber optic cables. Since then, many millions of devices have been deployed with arguably the highest reliability of any optoelectronic component ever produced. Other applications for VCSELs have since emerged, principally as the light source replacement for Light Emitting Diodes (LEDs) in optical mice. Further expansion of VCSELs into other optical components is just beginning as more OEMs realize the inherent benefits of using a VCSEL-based optoelectronic component.

VCSEL sensor

VCSELs can be thought of as a marriage of two key optoelectronic devices, the LED and the Edge Emitting Laser (EEL). They combine into one device many of the advantages of the LED, such as surface emission, wafer-level testing and diverse packaging options, with the advantages of EELs, such as coherence, beam quality, and efficiency [1]. Some of the key physical properties of VCSELs, EELs, and LEDs are summarized in Table 1. The performance and packaging advantages of VCSELs has enabled them to be used in applications where LEDs were not the optimal choice and where the EELs could not penetrate because of packaging limitations. A prime example of this situation is the optical mouse.

VCSEL sensor

The use of optoelectronic components in consumer appliances is now mainstream, with sensors for proximity, turbidity, velocity, and ambient light among some of the more commonly deployed [2]. To date, these sensors have generally utilized LEDs as the light source primarily because of the low cost of deployment. With the advent of ever more “green” initiatives, the usefulness of these sensors is being challenged. One reason regards total power consumption. For an equivalent amount of useful optical power, the VCSEL will provide about a 10x reduction of required electrical power. Another reason is the examination of how to better utilize sensors to improve the entire system. To understand this point more clearly, it is instructive to examine more closely the function of a turbidity sensor.

Enlarge this picture Fig. 1. Schematic of a turbidity sensor.

In its simplest form, a turbidity sensor is comprised of a light source impinging on a medium, and a detector placed normal to the incoming light beam. Other detectors can be used to normalize the optical power as shown in Fig. 1. This type of turbidity sensor has been used in a large number of clothes washing machines, dishwashers, water purifiers, etc. The output of the sensor is proportional to the clarity or the turbidity of the sample, which is typically aqueous, but could be any other solution, or even a gas-based suspension.

While some useful information can be gleaned from amplitude data alone, the ability to further probe the nature of the suspension would be beneficial. When a coherent (single wavelength) light and polarized source is used as the illuminator, it is possible to further analyze the suspension, extracting additional information that can be used as the basis for improved system control.

In a typical washing process, the solubilization of debris such as oil, dirt, and grease is accomplished by a surfactant in the detergent. The surfactant essentially allows for these materials to be dissolved into the water. Once in the water, the surfactant forms a shell around the outside of the debris, causing it to remain in solution. The size of a typical surfactant molecule is on the order of a few nanometers, while the size of the debris, once surrounded by the surfactant, can be a thousand times larger. Therefore, one piece of potential information that can be used to enhance the washing process is knowledge of the size of the particles in the solution.

Enlarge this picture Fig. 2. Normalized intensity as a function of scattering angle and particle size

When illuminated with coherent light, the angle of scattering is dependent on the size of the particles in the suspension. The theoretical basis for scattering of light from particles of an arbitrary size is well developed, generally known as the Mie theory, and is fully described in reference [3]. In general, the effects of particle size on scattering are most dramatic when the size of the particles is near to or smaller than the wavelength of the probe light, but useful information can be obtained for particles up to 100 mm in diameter.

Fig. 2 is a plot of the normalized intensity as a function of the scattering angle for particle sizes ranging from 5 mm to 100 mm. The analysis here is done assuming non-absorbing spherical particle cross section. When the particles are non-spherical, then the scattering angle will depend on the shape of the particles. For example, a particle that is cylindrical will scatter differently in the two dimensions of the cylinder. This information can be used to determine the size and shape of the particles in the suspension. To fully utilize the capability of the VCSEL-based turbidity sensor, the detectors shown in Fig. 1 could be placed at specific angles to determine the state of a washing cycle, or could be replaced with a low cost CCD camera to fully analyze the washing cycle.

Another benefit to using a VCSEL light source is the known state of polarization of the light source. Polarization can also be used to distinguish characteristics of the solution [4]. In this case, Mie theory is further applied to include the polarizability of the surfactant in the solution.

Enlarge this picture Table 1. Comparison of the electro-optic properties of VCSELs, EELs, and LEDs. *Efficiency is defined with the optical energy emitted a 30 degree cone.

When the detergent is added to the water in the wash cycle, Micelles are formed due to the polar environment of the water. When probed with polarized light, the scattered light will tend to maintain its initial state of polarization. However, when the surfactant has acted on the debris, the result is an emulsion that contains much larger particles and results in light scattering that is not preferentially polarized.

Using this knowledge, the turbidity sensor in Fig. 1 can be further enhanced to include a polarization analysis by placing a polarizer at 90 degrees to the incident laser light. When the fluid contains only detergent, there will be minimal scattering that changes polarization, but as the wash progresses, more light will enter the polarized detector.

Turbidity sensing can also be beneficial in applications beyond washing. Similar concepts can be utilized in water purification systems in both the consumer and industrial environments. The ability to measure very low turbidity levels (0 to 10 Nephelometric Turbidity Units or NTUs) is beneficial to drinking water purification systems, and the ability to determine the particle size would be beneficial in determining water cleanliness in applications such as swimming pools and spas where the chemical agents used react in a similar manner as the surfactants in detergents.

While the degree of complexity of the two methods above is certainly much higher than in traditional turbidity sensors, the amount of useful information obtained from the sensor will allow the machine to accurately control the amount of detergent, the amount of waste water, and ultimately the amount of energy used, all of which are becoming precious commodities, as well as differentiators in appliance design.

The ability to measure the speed of particular objects in a consumer appliance is also of interest. Self-mixing is a technology that can be deployed to measure velocity of a mechanical part, fluid, or aerosol without any physical contact. The technology is based on a small amount of reflection from the object in motion being scattered back into the laser cavity, which modifies the lasing characteristics. This interruption causes measurable changes in the average optical power, and has a frequency component that is directly related to the speed of the moving object [5].

This approach can also be used to measure the vibration of a target object. Applications to the appliance industry include water flow measurements, motor speed, drum speed, motor and drum vibration, compressor vibration, all of which could be used to predict failure and warn the consumer. Self-mixing requires a coherent optical source, such as a laser, and VCSELs can be optimized to enhance the self-mixing effects.

Another useful application of a VCSEL based sensor is in detecting the proximity of an object. The most ubiquitous example of a proximity sensor is the automated flush systems incorporated in many public restrooms. Today’s devices utilize LEDs as the transmitter. This seemingly benign sensor consumes several megawatts of electricity each year and is the cause for several tons of CO2 emission each year. (That calculation  assumes 100 M sensors, with each LED consuming 200 mW of power, operated with a duty cycle of 10 percent, and an average CO2 production of 1.35 lbs. per kW/hr).

Simply replacing the LED with a VCSEL would reduce the amount by more than a factor of 10, and would result in a better sensor. Some of the advantages of the VCSEL-based proximity sensor include the ability to sense objects further away, sense smaller objects, increased ambient light rejection, and more. The very same sensor can be used to measure either the displacement or the amount of movement of an object by simply measuring the relative amounts of signal before and after the displacement. Displacement sensors are useful to determine if a mechanical motion has occurred, such as a piston movement.

Optoelectronic sensors represent one of several technical solutions available to the appliance design engineer today. The advent of VCSELs as a new source within optoelectronics has opened up a new set of useful sensors that can be utilized to make more efficient and smarter appliances.

For more information, email: Jim.tatum@finisar.com

References

1. 1. J. A. Tatum, “Vertical Cavity Surface Emitting Lasers: Packaging Propels VCSELs beyond Communications,” Laser Focus World, 2000.
2. 2. J. A. Tatum and A. Lalonde, “VCSELs in Various Sensor Applications,” Sensors Expo Conference, 2007
3. 3. Light Scattering by Small Particles, H. C. Van de Hulst, Dover Press, 1981.
4. 4. A. Boscolo and S. Stibelli, “A New Sensing Device for Washing Machines,” IEEE Transactions on Industry Applications, vol. 24, no. 3, pp 499-502 (1988).
5. 5. L. Scalise, Y. Yu, G. Guiliani, G. Plantier, and T. Bosch, “Self-Mixing Laser Diode Velocimetry: Application to Vibration and Velocity Measurement,” IEEE Transactions on Instrumentation and Measurement, vol. 53, No. 1, pp. 223-232, 2004.