In industrial process monitoring environments, it is important to maximize plant productivity by having distributed sensing components of high reliability. Fiber lasers for moisture sensing leverage high reliability platforms developed for the telecommunications and optical data storage industries in order to project mean times to failure (MTTF’s) in excess of 100,000 hours.

Test Conditions

Fiber laser operation is enabled by a high power “pump” semiconductor diode laser. The reliability of this component is key to the reliability of the fiber laser system. Accelerated aging studies have been performed to evaluate the reliability of the pump laser component. A total of 28 pump lasers were evaluated in this exercise. The lasers were operated at 70^oC at a constant power which resulted in an approximate 50% overstress condition relative to normal operating current for the pump lasers. End of life for the pump lasers was defined very conservatively as a 20% increase in the operating current required in order to maintain this constant power condition. The actual operating current is monitored periodically during the course of the test.

Accelerated Aging Results

During 1000 hours of operation under these conditions, slight degradation of the performance characteristics is observed, but no failures were observed during this time. Linear regression is therefore used to extrapolate the degradation slope for each device to project the time to achieve 20% increase in operating current. These extrapolated times to failure are tabulated and subjected to statistical analysis.

The statistical analysis demonstrates that the failure rate in time fits very well to  the lognormal distribution, which is generally associated with wear-out failure mechanisms. From the Lognormal distribution analysis, we determine an MTTF at the elevated stress condition of 24,299 hours.

Extrapolation to Normal Operating Temperature

In order to translate this measure into failure rates under expected operating conditions, we must implement a model for the temperature dependence of the reliability.  It is well known, that these lasers commonly exhibit temperature dependence to their reliability that is described well by an Arrhenius model,[1] where failure statistics are exponentially dependent on temperature. In order to extract the “activation energy” which determines the temperature dependence of these devices, it is necessary to evaluate the aging characteristics at multiple temperatures. Since the present study has only used a single temperature, we will rely on literature reported values of the activation energy to make our projection[2]. The literature shows us different degradation processes having associated with them different activation energies.

With our present uncertainty as to the relative importance of the different degradation mechanisms in these devices, we select the lowest of these activation energies to further our analysis. This will result in the most conservative reliability projection. Taking this approach and using the 0.3eV activation energy, we project a room temperature MTTF for these lasers of 112,353 hours, or somewhat in excess of 12 years, under continuous operation. We are thus quite encouraged that fiber lasers based on these pump lasers will meet the reliability requirements for a variety of demanding industrial applications.

Conclusions:

In summary, accelerated aging studies have been used to extract failure rates for fiber laser modules based on the failure rates for the diode laser pump components. We have obtained an MTTF of 112,353 hours for our pump lasers. We emphasize that measures have been taken to maximize the likelihood that this is a conservative estimate relative to the actual MTTF. Firstly, the test is conducted under an electrical current stress condition of 50% greater starting current than normal operating current. Secondly, we have set the failure criterion as a 20% increase in operating current. With appropriate control circuitry, operating current increases of twice this value can be readily accommodated. In our model, this would translate directly into a doubling of the MTTF. Finally, in extrapolating the results to operating temperature, we have assumed the lowest physically plausible value of the activation energy. It is therefore reasonable to expect that actual field performance of these devices will exceed the estimate that we have projected from this study.

Note:   This Application Note is available for comment and discussion.

References:

[1]   J. Spencer, “Calculating laser diode reliability,” Global Telecommunications Conference, 1988, and Exhibition. 'Communications for the Information Age.' Conference Record, GLOBECOM '88., IEEE, 1988, pp. 63-69 vol.1.

[2] T. Kim, D. Kim, K. Kim, K. Jang, G. Moon, J. Park, S. Lee, M. Kim, and J. Koh, “Continuous wave operation of a 250 mW AlGaAs laser diode,” Solid-State Electronics,  vol. 49, Oct. 2005, pp. 1674-1677.

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Moisture Sensing: A key process in Industrial Process Optimization

Moisture sensing by the use of Near and Mid-Infrared Spectroscopy has been established as a powerful technique for process monitoring in a wide variety of industrial applications. The industrial settings range from the paper industry through the food industry into the pharmaceutical industry and beyond. The power of the technique rests in the ability to perform a "standoff" detection, where the sensing head is located typically several inches away from the material being measured. This sensing at a distance capability allows meaningful measurement of the moisture content of a product to be be performed continuously and non-invasively at points distributed throughout a process flow line by measuring the spectrum of the scattered light from the product as it is conveyed through a factory.

The Companies in the Industry

Foundational work on instruments using this approach was performed to address the paper industry by Chiba et.al. at Yokogawa[1]. Today, several companies produce instruments to address this industrial sensing application, with units commonly priced in the range of $10,000 to $30,000 USD. Some of the industry leaders in this market are Grecon (www.grecon.com ), MoistTech (www.moisttech.com) , NDC ( www.ndcinfrared.com ) Process Sensors Corporation ( www.processsensors.com ) Yokogawa ( www.yokogawa.com ) and Kett ( www.kett.com ). Several of these companies currently deploy a platform which is well described by the Grecon IR 5000 Moisture Meter Analyzer below.

The measurement performed is basically a multi-wavelength spectral analysis at two or perhaps more wavelengths. One of the wavelengths corresponds to an absorption peak of water, typically 1400nm or 1940nm, and a nearby reference wavelength is selected at which water absorption is weak.

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Moisture Sensing Industry Standard The output of a halogen lamp is collected to generate the desired wavelengths. Filters mounted on a rotating wheel are used to alternately illuminate the material under test with the probing and reference wavelengths. Light scattered from the sample is detected, and information on the moisture content of the sample is extracted from the ratio of the scattered light intensity at the probing and reference wavelengths. From www.grecon-us.com/html/moisture.htm

The advantages of the halogen lamp/rotating filter wheel based system are that the components are simple, extremely well understood, and can be engineered to have acceptable reliability and precision. However, there is no escaping the realities that halogen lamps decay over time and motors wear out. These and other limitations of the current systems create a driving force for alternate solutions to the moisture sensing process.

Development of Light Emitting Diodes as a Light Source Alternative

Replacement of the Halogen lamp and rotating filter wheel with an electronically modulated solid state light source would potentially result in a moisture sensing system with substantially improved sensitivity, reliability and immunity to operating environment.

Mid infrared Light Emitting Diodes have been recently evaluated for this purpose by Aikio et. al of VTT Technical Research Center of Finland[2]. In their work, they have demonstrated that proper design can produce a moisture sensing system with performance comparable to that of the halogen lamp based system.

However, even this advanced, state of the art LED based approach has it’s shortcomings. Referring to the VTT work, it requires the development of a custom LED module integrated onto a thermal electric cooler (TEC) in order to provide the light sources. The custom LED sources, plus the integration of these sources into the temperature controlled package, will negatively impact the cost of the LED based solution. The limited power available from the LEDs limits the accuracy at which product may be monitored using the LED based approach. Additionally, the broad spectral output of the LED extends beyond the water absorption peak, to the extent that the reference LED and the signal LED have overlapping spectra. This spectral overlap further compromises the sensitivity of the measurement.

Fiber Laser Based Alternative

The UBQ-7810-1925 has been designed as a light source for robust, compact, reliable next generation solutions for the moisture sensing industry. Providing high spectral output centered on the peak of the water absorption band, all of the photons from this device are put to good use in extracting moisture information. Furthermore, the industry leading insensitivity to temperature of the UBQ-7810-1925 can completely eliminate the need for external temperature control of the unit using power consuming thermal electric coolers. Being a laser light source, the beam may be readily directed, efficiently collected, and adjusted with simple optics to any appropriate illumination beam size down to a diffraction limited spot.

Moisture Sensing System Comparison

We can compare the performance of moisture sensing systems based on the use of Halogen lamps, LEDs, and Fiber Lasers. Kett Produces commercial units using both Halogen lamps and LEDs, we incorporate the VTT results from [2], and, largely extrapolating from these LED results, we include the theoretical performance improvement with a high brightness fiber laser source. Key noteworthy items are the extremely fast refresh rates enabled by both LED nd fiber laser sources relative to halogen lamps. Note however, the compromise in accuracy of the VTT prototype driven primarily by the limited brightness of the LED source. By contrast, the high brightness coherent laser source enables industry leading 1.4 ppm level accuracy, while enabling substantial reduction in measurement diameters to 5mm or below.

 Conclusions

We conclude that, while present LED based solutions offer certain substantial advantages in terms of refresh rate relative to Halogen lamps, several opportunities exists to even further enhance system performance with the use of fiber lasers.

See the product page for the UBQ-7810-1925 for details on pricing and availability.

Note:   This Application Note is available for comment and discussion.

References

 [1]          H. Hara, R. Chiba, K. Isozaki, and T. Yamada, “Development of an infrared-ray moisture meter for the paper-making process,” Industrial Electronics, Control, Instrumentation, and Automation, 1992. Power Electronics and Motion Control., Proceedings of the 1992 International Conference on, 2002, pp. 1604–1607.

[2]          R. Aikio, H. Lindström, P. Suopajarvi, J. Malinen, and M. Mantyla, “Low-noise moisture meter with high-speed LED techniques,” Next Generation Spectroscopic Techniques III, Applications of Novel Spectrometers",  Orlando, Florida, USA: SPIE Proceedings Volume 7680 , 2010, pp. 768008-768008-10.