Fibre Sprectral Loss / WDM Device Characterisation

 

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 white 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 fibre is collected by a large area indium gallium arsenide or silicon detector and the resulting signal amplified by a lock-in amplifier. 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

The final signal-to-noise ratio and hence speed and accuracy of measurement is dominated by the level of light reaching the detector. In all real systems this light level is limited by the efficiency and light throughput of the monochromator.

When comparing various commercially available monochromators you find that most have an f number in the region of 4. It is a common misconception to assume that light throughput can be assessed by comparing f number alone. This is not the case. For a given spectral bandwidth, a large f/4 monochromator (long focal length, large mirrors and gratings) will operate with wider slits than a small f/4 unit. This wider slit allows more power through and into the fibre.

The Bentham system uses a 300mm focal length, f/4 monochromator to launch 5nW or -53dBm (cw) into a single mode fibre (1300nm, 10nm bandwidth).

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.

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.