Fibre Spectral Loss/WDM Device Characterisation

Fibre spectral loss system

Spectral loss is a key metric in quality control, material research and development of optical fibre used for example in single transmission and in optical amplifiers.

In the FSL300, this parameter is evaluated using the cut-back technique, the internationally recognised reference test method offering the highest measurement accuracy.

The high degree of configurational flexibility of the FSL300 allows the choice of light source, monochromator configuration, launch optics and detector station to allow measurements over very wide spectral range and a large range of fibre types, including silica, doped glass, plastic and photonic bandgap fibres.

This system may also be used to determine fibre cut-off wavelength, in the measurement of mode field diameter, the determination of macrobending loss and evaluation of the spectral properties of a wide range of passive WDM components such as taps and couplers.

The Cut-Back Technique

Light transmission over the spectral range of interest is measured through a long piece of fibre (typically 1-2km long) which is then cut back by a known amount and the light transmission measurement repeated. spectral loss formula

From these two measurements and knowing the length of fibre cut-back we can determine the spectral loss (dB/km ).


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.

Softwarespectral loss measurement software

Our software controls the entire system via USB. The operator may select the start and finish wavelengths for the scan as well as spectral resolution and signal averaging period.

Automatic zero routines deal with any drift in electronic offset by making a zero level measurement prior to each spectral run. Complete spectral loss files may be stored in ASCII format, allowing easy transfer to other software packages.


If you would like to know more about our fibre spectral loss systems, please contact one of our sales offices.

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