With the availability of a wide choice of detectors, choosing the best system is a challenging task. The best match of detector sensitivity to signal characteristics is graphically illustrated in Figure 1. The four criteria for selecting the best match of detector sensitivity to signal characteristics are wavelength, bandwidth, time, and field of view.
Figure 1. Examples of poor and good matches of detector responsivity to signal.
Wavelength and Bandwidth
The detector response needs to cover a signal’s wavelength range to avoid noise. Losing some signal information will be the case if there is a mismatch. The signal information is fully captured in the ‘good’ and ‘best’ cases shown in Figure 1, with wavelength mismatch contributing little or nothing to system noise.
Adjusting frequency bandwidth is the most common approach for reducing noise contribution, as this factor is correlated in the form of (BANDWIDTH)1/2 in the noise equations. Bandwidth can be controlled by lowering the 1/f noise by moving up in frequency.
Time and Field of View
Particular events may take place for a specific period at specific time intervals. This periodic behavior can be artificially created and used for signal-to-noise ratio improvement. There must be a significant difference between the characteristic signal frequency and the natural modulation frequencies of the noise sources. For this purpose, choppers are often used. Narrow bandwidth AC detection methods are applied to leverage the signal modulation.
Gated averagers, or integrators, are employed for the signal-to-noise ratio improvement in the measurement of pulsed sources. Field of view is a key aspect for infrared detectors. Narrow field of view is essential to obtain radiation, mostly from the desired source, due to the emission of infrared photons by room temperature objects.
General Noise Reduction Methods
The noise and responsivity characteristics of all detectors and signal conditioning electronics are temperature dependent, to some extent. Stabilizing the thermal environment of an analysis can produce more reproducible results. Cooling the detector to run below room temperature will typically improve its detectivity limits. The degree of gain in performance relies on detector type and post detection electronics. Cooled photon detectors deliver even better performance, as they are responsive to the infrared part of the spectrum. Dark currents decrease a factor of 2 for every 5 to 20°C, based on the characteristic energy of the system.
Ratios and AC Techniques
Since no source is completely stable, the response signal needs to be ratioed to that of the source, whenever possible, to get the most accurate results. The signal can be encoded using a known modulation, and this characteristic modulation can then be used to differentiate against noise contribution that has its power spread over a diverse and wide band of frequencies.
An AC coupled amplifier, featuring a narrow band filter centered on the modulation frequency, can be used for achieving a significant improvement in signal-to-noise ratio. The filter frequency pass band can be narrowed to the limit of the modulator stability. If the noise level is still very high, then it is necessary to revert to lock-in techniques, which rely on the amplifier actively following the modulator frequency. This, in turn, enables using much narrower band filtering. Oriel Instruments often perform this filtering in the digital part of its Merlin™ Radiometer System, to avoid phase and amplitude drifts related to the older analog lock-ins.
Gated Integration/Boxcar Averaging
These techniques are named after the appearance of the pictorial representation of the process (Figure 2). Gates, boxes, or ‘windows’ are employed to describe the signal acquisition times. The signal to noise ratio is already optimized during those gated times, due to the absence of noise contributions that would be built up during the off times.
Figure 2. Repetitive signal and detection “windows”.
The repetition of the process of N pulses will result in signal-to-noise ratio improvement of N1/2 when the noise is of the white variety, Johnson, or shot. This is due to the increase of the integrated signal contribution as N, while the increment of the noise contribution is only as N1/2. Subjecting AC coupled detectors to DC saturation leads to loss of linearity.
Understanding the DC response limits of a detector is essential when it is operated in the AC coupled mode. Frequency roll-offs needs to be carefully monitored. The post detection electronics often limit the detector sensitivity. There may be multiple bandwidths related to a specific piece of instrument that rely on the selected gains.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.