Low-Frequency Noise Measurements
Large efforts have been taken to understand the nature of the low frequency fluctuations with a 1/f power spectrum which is omnipresent in almost all physical systems. There have been numerous attempts to discover a common underlying mechanism describing 1/f noise in the various systems. Presently, there is not even agreement on a common mechanism in the various systems. A detailled understanding of the origin of low frequency fluctuations in the various systems is strongly desirable both from an application and basic research point of view.
To characterize the low frequency fluctuations in a wide temperature range (T = 10-300 K) we have developed a specially designed noise measurement setup which uses a dc SQUID system as a highly sensitive voltage amplifier. This setup enables to investigate extremely small voltage signals. A high sensitivity of 60 pV/Hz1/2 down to below 1 Hz can routinely be achieved. The complete setup is situated in a rf shielded room. Special care has been taken to reduce ambient magnetic fields (several cryoperm and mumetal shields). All electrical supplies are heavily filtered. To avoid impairing electrical stray fields from a conventional resistance heater we developed an optical heating system consisting of a laser diode situated outside the sample holder at room temperature. The back side of the sample is irradiated with light from the laser diode which is delivered through a light wave guide.