Why choose InGaAs over Germanium?
The choice of detector is affected by several parameters: beam size, power and wavelength. In general a photodiode will give best results when used at wavelengths below the wavelength of peak sensitivity. Although the head may operate at higher wavelengths, the sensitivity of the photodiode will be strongly temperature dependant. Thus, for wavelengths beyond 1550nm, InGaAs will be the material of choice.
Also, at wavelengths below 800nm, our visible enhance InGaAs exhibits markedly higher responsivity than Germanium.
Which size of integrating sphere should I use?
The size of an integrating sphere is specified by its internal diameter.
The size of an integrating sphere mainly determines the attenuation of the detector since the input power is spread over the full internal surface of the sphere. The larger the sphere, the lower the power at the detector and therefore the higher the detectable input power of the sphere. Since the total area of the ports must not be a significant portion of the sphere surface area, smaller spheres must have smaller input ports.
The sphere must be chosen to have an input port large enough to accept the full beam to be measured. Given that, the smallest sphere compatible with the maximum power to be measured should be chosen. This ensures that the signal to noise ratio will be optimized.
Should I use a polymer or a gold integrating sphere?
The indications “polymer” and “gold” refer to the reflecting material inside the sphere.
The choice depends on economics and technical requirements.
Polymer spheres are less expensive than gold spheres.
Polymer spheres can be used from 250-2200nm; gold spheres from 700nm-20µm.
Polymer spheres are quite limited in handling average power (see specifications). The peak power may be as high as for gold spheres if the duty cycle, and hence average power, is low enough. Gold spheres can be used at very high average powers.