Each application needs a well suited photo detector to monitor the property of the used laser light as power, divergence, pulse shape, ...etc.
Since the lasers are covering a manifold of different spectral lines, ranging from the UV to the IR, a specially selected detector with suited spectral response must be found for each spectral range. To obtain satisfactory results from the performed application, other characteristics of the photo detector, like its sensitivity and time response, should also be taken into consideration. The ideal choice is a wavelength independent detector which is able to cover very low and high intensities with a very short response time. Unfortunately, this detector does not yet exist. Detection of light means converting photons to electrical signals that can be amplified and displayed. In other words, the photon detector can be considered as a photon to electron transducer. Due to the manifold of different interaction with light and matter a large number of different types of photo detectors exist. Within the scope of this experimental set-up different photo detectors based on semi conduction and thermoelectric effects are investigated as these types are the most important ones used in industrial applications. For the characterisation of the spectral sensitivity, a combination of a white light lamp and tuneable prism monochromator with a tuning range from 400 nm to 1500 nm is used. By means of a provided light chopper, the time response of each detector can be measured. Especially for semiconductor photo detectors, the influence of the outer electrical circuit on the response time and sensitivity will be determined. Beside photo detectors, other specimen with spectral characteristics like optical filters or mirrors can also be investigated within this set-up. To compensate for the spectral power distribution of the light source, a wavelength independent detector is provided. For the measurement of absolute values an optional, calibrated power meter can be ordered.

Principle of operation

The light source (LS) generates white light by means of a halogen lamp. The light is collected with a concave mirror and focused into a glass fibre bundle (FB) whose crossection varies from a circular shape at the entrance to a slit at its exit. The cylindrical lens (L1) collimates the emerging light from the slit to a parallel slit image entering the set of prisms. To fully illuminate the sensitive area of the object to be measured, a set of two identically cylindrical lenses (L2 and L3) is used. The selection of the wavelength is achieved by tilting the set of prisms (P1) through (P3). The used three prisms configuration (Försterling arrangement) has the advantage that the prisms are used in the minimum of beam deviation and the exit beam stays constant under 90° with respect to the optical axis of the incoming beam. The prism (P4) is used to bend the beam back to the optical axis of the set-up. Where the prisms (P1 and P3) are responsible for the guidance of the beam, prism (P2) is responsible for the wavelength separation. The shape of (P2) corresponds to the well known Abbe type. Behind the prism monochromator, the light chopper (CH) is arranged in such a way that it can be used to investigate the time response of a particular detector. In addition, the chopper will be used in connection with the subsequent detection system to eliminate the influence of disturbing stray light.
The prism monochromator is designed for a continuously tuning range from 400 nm to 1500 nm with
a resolution of 10 nm. The read out of the micrometer screw along with a calibration table is used to determine the selected wavelength. To compensate
for the wavelength dependent intensity of the white light source, which nearly behaves as a black body radiator, a wavelength independent thermoelectrical detector is provided.