Page 186 - Instrumentation Reference Book 3E

12 Fiber optics in sensor

instrumentation

B. T. MEGGITT

12.1 Introduction feats that were not possible with the techniques
already available.
Fiber optic instrumentation has progressively Despite the conservative attitude shown to the
evolved since early 1980 following the successful early work on optical fiber sensors, however.
development of optical fiber networks for tele- some commercial instruments did emerge for spe-
communication applications. To a large degree cialist applications, and this has led, or is leading,
optical fiber sensor instrumentation has taken to internationally accepted measurement tech-
advantage of the maturing component market niques. Some of these methods, as will be seen,
for telecommunications use and its progress has are specific for a single measurement parameter,
been dictated by it. Initially a significant level of e.g., vibration, acoustic sensing. etc., while others
research effort was put into developing sensor offer the credibility of being developed into a
systems based on the use of multimoded optical generic measurement technique that can be used
fibers, but as the bias in long-haul telecommuni- to monitor a wide range of parameters, e.g., tem-
cations moved towards the use of single moded perature, pressure, or strain.
optical fibers for improved performance, so op- When considering which design options to
tical fiber sensor systems began to accept the use adopt for a particular sensor system, several per-
of single mode techniques as well as multimode formance-and market-related factors need to be
fiber methods. In addition, the development of taken into account. These can be summarized as:
compact disc (CD) systems has made available (1) resolution requirement
low-cost, high-reliability laser diode devices (both (2) dynamic range capability
single-mode and multimode) that have added (3) response time requirements
further to the advancement of optical fiber sensor (4) operating environment conditions
systems. The initial research work carried out on (5) reliability
optical fiber sensor instrumentation included a (6) long-term stability
wide range of ideas and methods, many taken (7) production capability
over from more traditional open path optical (8) maintenance requirements
sensing methods. (9) target costs
By the end of the first decade of research into
this new sensing method the ideas and techniques (10) market acceptability
followed a degree of rationalization as the advan- In reality, the chosen method will lead to a
tages of certain methods became favored for spe- compromise on the above criteria depending
cific applications (see, for example, Grattan and upon the most dominant requirement factors.
Meggitt 1994). The early optimism over the intro- One should not underestimate the final two items
duction of this new measurement method did not (cost and market acceptance) since these have
initially live up to the earlier expectations. The been two of the most overriding factors that have
subject was then largely technology driven with slowed the general acceptance of optical fiber
factors that make for a successful commercial sensor systems. However impressive the operat-
development being largely overlooked. The con- ing performance and technology design behind a
servative nature of users of the existing electronic sensor system, the end user will have their own
and electrical sensor systems meant that optical unique set of selection criteria, and unavoid-
fiber sensor systems, despite their advantages able cost of ownership (both capital and subse-
over the electronic rivals, had to compete on a quent maintenance costs) will play a leading role
level playing field: they had to compete on price in this process. Many large end-users have
maintainability and reliability as well as perform- remained conservative in their approach to new
ance criteria. The products therefore had to display sensor technology. They often will not be pre-
credible advantages over competing systems or pared to take unnecessary risks on, to them,
had to be able to perform certain nieasurenient unproved techniques, bearing in mind that systems