The Bentham Fibre Spectral Loss System is a modular system consisting of chopped light source, monochromator, launch optics, detector and lock-in amplifier. The whole system is controlled by a PC which also calculates loss in dB/km as a function of wavelength.

Spectrol Loss in fibres is usually measured by the cut-back method. Modulated while light is passed through a monochromator before being launched into one end of the fibre under test. The light emitted from the other end of the dibre is collected by a large area indium gallium arsenide or silicon detector and the resulting signla amplfied by a lock-in amplfier. The detector end of the fibre is then cut back by a known length and the cleaved end reinserted in the detector assembly. The spectral loss of the fibre is calculated from the two stored curves as follows:

Spectral Loss (dB/km) = log₁₀ (short/long)/cutback

The prime system requirements are:

• light throughput of the monochromator
• stability of light output
• mechanical stability

Stability

Due to the sequential nature of the measurement, both optical and mechanical stability over the period of the two spectral runs is of critical importance.

For the Bentham range, optical stability is ensured by the IL1 light source, which uses a 100W QH lamp controlled by a highly stabilised constant current power supply. This power supply is at least 10 times more stable than a normal laboratory stabilised supply.

Mechanical instability in such systems results in short or long term wander of the focussed spot over the end of the fibre at the launch end. In the Bentham system the light source and launch optics are bolted firmly to the monochromator, which is constructed from a single rigid casting.