A low cost and robust temperature controlled optical source for fibre optic measurement systems has been developed The temperature control has been achieved by means of an active heat sink thermally coupled via a copper block housing An embedded 80188 system with the appropriate Input / Output boards has enabled a flexible and low cost controlled source to be realized. The hardware is mounted in a 3 slot STE BUS miniature rack which includes the PSU The software was developed on a 286 host and downloaded to the 80188 card using Sourceview [l] software The use of Borland C++ as a programming language has enabled relatively simple PID Control Algorithms to be developed, though more sophisticated ones may be incorporated in the future
The control system accuracy has been demonstrated as better than 0 05 °C with a high degree of long term stability with tests of many hours duration having been conducted The Instrument is configured to enable the wavelength and/or the Intensity of the source to be modulated This provides the added advantage of being suitable for a wide range of sensor applications
It is considered that this high performance, low cost and robust Instrument is ideal for applications such as optical fibre sensors which could be used in undersea environments Additionally the system is easily adapted for use as stabilization of temperature sensitive detectors e g PIN diodes and APDs A range of sources and detectors can be easily incorporated into this system with fibre coupling being achieved by connectors to standard device housings or direct fibre pigtail
The instrument developed as a result of the work reported in this paper is suitable for a wide range of fibre optic sensor measurement systems including those which can be used for subsea applications The Instrument is capable of housing LEDs (Light Emitting Diodes) Laser Diodes and/or Photo detectors in a robust and transportable unit. This makes it particularly suitable for subsea applications which involve remote measurement e g pressure, temperature, displacement by fibre optics The general use of fibre optics for subsea systems is wide ranging and well established [2,3,4] Advantages offered by fibre optics for subsea monitoring and measurement Include immunity from effects of a harsh operating environment and safe operation In potentially hazardous environments e g explosion risk. Much research has been directed towards the development of subsea fibre optic systems However, one of the major practical limitations of such systems has been the long term performance of the light source and to a lesser extent the detector All LEDs and Laser Diodes exhibit performance limiting characteristics e g drift of output power due to temperature and ageing (time) [5,6] These factors also have an Influence on photo detectors e g PIN photodiodes and APDs through variations of the dark current The Instrument developed In the work described can accommodate light sources and detectors simultaneously. However, the bulk of the experimental results were for a 780 nm (normal centre wavelength) Laser Diode which represents the most severe experimental conditions for stabilisation
Temperature stability was achieved by the use of an active heat sink (Peltier Heat Pump) which is driven from a conventional electronic amplifier power output stage Control was established using PID algorithms resident on a target 8088 based embedded system The development using embedded software rather than a full hardware implementation was intended to promote a system which was highly versatile and easily reconfigurable This approach was necessary so that the full range of sources and detectors were available for testi
