Theoretical background: Spectrometers
The AvaSpec line of spectrometers can be equipped with several types of detector arrays. Presently we offer silicon-based CCDs, back-thinned CCDs, and Photo-Diode Arrays for the 200-1100 nm range. A complete overview of each is given in the next section “ Sensitivity “. For the NIR range (1000-2500 nm) InGaAs arrays are implemented.
All detectors are tested in incoming goods inspection, before they are used in our instruments. Avantes offers full traceability on following detector specifications:
• Dark noise
• Signal to noise
• Photo Response Non-Uniformity
• Hot pixels
StarLine CCD Detectors (AvaSpec-ULS2048/2048L/3648)
The Charged Coupled Device (CCD) detector stores the charge, dissipated as photons strike the photoactive surface. At the end of a controlled time-interval (integration time), the remaining charge is transferred to a buffer and then this signal is being transferred to the AD converter. CCD detectors are naturally integrating and therefore have enormous dynamic range, only limited by the dark (thermal) current and the speed of the AD converter. The 3648-pixel CCD has an integrated electronic shutter function, so an integration time of 10µs can be achieved.
+ Advantages for the CCD detectors are large numbers of pixels (2048 or 3648), high-sensitivity and high-speed.
- Main disadvantage is the lower S/N ratio relative to other detector types.
For applications below 350 nm with the AvaSpec-ULS2048/2048L/3648 a special DUV-detector coating is required. The uncoated CCD-response below 350 nm is very poor; the DUV lumogen coating enhances the detector response in the region 150-350 nm. The DUV coating has a very fast decay time, typ. in ns range and is therefore useful for fast-trigger LIBS applications.
Photo Diode Arrays (AvaSpec-128)
A silicon photodiode array consists of a linear array of multiple photo-diode elements, for the AvaSpec-128 this is 128 pixels. Each pixel consists of a P/N junction with a positively doped P-region and a negatively doped N-region. When light enters the photodiode, electrons will become excited and generate an electrical signal. Most photodiode arrays have an integrated signal processing circuit with readout/integration amplifier on the same chip.
+ Advantages for the Photodiode detector are high NIR sensitivity and high-speed.
- Disadvantages are limited amount of pixels and no UV-response.
SensLine Back-thinned CCD Detectors (AvaSpec-ULS2048x16/x64/XL/HS1024x58/122)
For applications requiring high quantum efficiency in the UV (200-350 nm) and NIR (900-1160 nm) range, combined with good S/N and a wide dynamic-range, back-thinned CCD detectors are the right choice. Both uncooled and cooled backthinned CCD detectors are offered, the uncooled backthinned CCD detector has 2048 pixels with a pixel pitch of 14 µm and a height of 500 µm, to have more sensitivity and a better S/N performance.
For even better sensitivity and S/N the cooled backthinned CCD detector is the best choice, it has 1024 pixels, each of them with 58 or 122 vertically binned pixels, giving an effective detector height of 1.4 mm or nearly 3.0 mm
+ Advantage of the back-thinned CCD detector is the good UV and NIR sensitivity, combined with good S/N and dynamic range.
- Disadvantage is the relatively higher cost.
InGaAs linear image sensors (AvaSpec-NIR256/512)
The InGaAs linear image sensors deliver high-sensitivity in the NIR wavelength range. The detector consists of a charge- amplifier array with CMOS transistors, a shift-register and timing generator. For InGaAs detectors the dynamic range is limited by the dark noise. For ranges up to 1.75 µm no cooling is required and these detectors are available in both 256 and 512 pixels. Detectors for the extended range 2.0-2.5 µm all have 2- stage TE-cooling to reduce dark noise and are available in 256 and 512 pixel versions (1.7 and 2.2 detectors only).
7 versions of detectors are available:
- 256 pixel non-cooled InGaAs detector for the 900-1750 nm range
- 256/512 pixel cooled InGaAs detector for the 900-1750 nm range
- 256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2000 nm range
- 256/512 pixel 2-stage cooled Extended InGaAs detector for the 1000-2200 nm range
- 256 pixel 2-stage cooled Extended InGaAs detector for the 1000-2500 nm range