Agilent Measurement Journal - Issue 3 - 2007 - (Page 70)

pd mirror shutter to/from ﬁber sensor wf wf mirror wf ld Figure 1. The integrated optical assembly in the Agilent DTS includes a low-power semiconductor laser (ld) and a single-receiver optical detector (pd). The concept uses a series of wavelength ﬁlters (wf) and mirrors direct the backscattered Stokes and Anti-Stokes lines from the ﬁber sensor to the photo diode. (Figure adapted from Reference 1.) Looking inside the Agilent DTS The core of the Agilent DTS is an integrated optical block that contains a laser source, ﬁlters and a single optical detector in a bulk optical assembly. The entire assembly is hermetically sealed and ﬁlled with an inert gas, isolating the components and preventing condensation from degrading instrument performance over its range of operating temperatures. A schematic of the optical assembly is shown in Figure 1. Two unique aspects of the design are its use of a low-power semiconductor laser and its single-receiver optical detector. The DTS uses a low-power external-cavity diode laser operating at 1064 nm. A semiconductor laser ensures a long operating life, eliminating the need for ﬁeld-replaceable parts. This is an important requirement for reliable, maintenance-free measurements in remote locations. Additionally, the average optical power from the source is ~17 mW, which classiﬁes the laser as a Class 1M “eye safe” device — unlike other commercial instruments that use solid-state YAG lasers. The optical path for the single-receiver design is also shown in Figure 1. Light from the source is coupled into the sensing ﬁber and backscattered light is directed toward the detector through a series of ﬁlters and reﬂectors. The Agilent DTS measures both the Anti-Stokes (1018 nm) and Stokes (1112 nm) lines using a single optical receiver. A single receiver improves the instrument’s measurement accuracy over a wide range of operating temperature by eliminating drift, which can occur in dual-receiver designs. The apparent change in temperature reported by the DTS is less than 1º C as the ambient operating temperature of the DTS changes over its entire 70º C range. The choice of a power-efﬁcient semiconductor laser carries a technical challenge: the resulting backscattered light has limited signal strength at the receiver, and this translates into a lower signal-to-noise ratio. Agilent engineers overcame this issue using a code-correlation technique to boost signal level and improve the signal-to-noise ratio, making it comparable to higher-power, single-pulse laser sources.2 This approach was leveraged from Agilent’s 20 years of experience designing and manufacturing rugged, ﬁeld-reliable optical time-domain reﬂectometers (OTDRs) and external-cavity diode lasers. Making it ﬁeld-ready To address rugged, outdoor ﬁeld applications, the Agilent DTS is designed around an integrated optical block. The instrument also includes an IP66 (NEMA 4) enclosure to prevent moisture from interfering with instrument operation (Figure 2). Additionally, the optical block is temperature stabilized, allowing operation from –10º C to +60º C. Operation at lower temperatures is made possible by adding insulation that traps heat generated by the instrument: With this extra warmth, the DTS will continue to function even if the external temperature drops below –10° C. In a trial, the DTS operated down to –40° C using only external insulation with no internal heating elements. The instrument can be operated with standard telecom ﬁbers for normal temperature ranges or with special ﬁbers that cover a temperature span of –273º C to +700º C, depending on sensor coating. 70 Agilent Measurement Journal

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 3 - 2007

Applying Ingenuity to the Measurement of Emerging Technologies Contents Emerging Innovations Department Delivering Bigger Benefits by Optimizing Customer Workflows Measuring Material Properties at the Nanoscale Utilizing In Situ Atomic Force Microscopy in Life Science, Pharmaceutical and other Bio-Related Applications WiMAX™: Plotting a New Path to Global Mobility Addressing the Triple Complexity of Triple-Play Networks What Next for Mobile Telephony? Exploring the Inner Workings of Tire-Pressure Monitoring Systems Developing, Assessing and Applying a High-Resolution Thin-Film Magnetic Probe Making Accurate Settling-Time Measurements Using a Vector Network Analyzer Applying Metabolomics Methods to the Study of Bacterial Leaf Blight in Rice Plants Measuring Stream Dynamics with Fiber Optics survey

Agilent Measurement Journal - Issue 3 - 2007

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