New position detector shrinks spectrometer

09 Jun 2008

Optics.org speaks to researchers in Sweden who have developed 1D and 2D position sensitive detectors as well as a compact and cost-effective spectrometer.

Position sensitive detector

Combining a position-sensitive detector (PSD) with a Fabry-Perot interferometer creates an extremely compact spectrometer ideal for use in the process industry (Mid Sweden University doctoral thesis 48).

"The overall aim was to develop and improve PSDs and to examine their potential use in compact spectrometers," Henrik Andersson, a researcher at Mid Sweden University, told optics.org. "We combined the two technologies to construct a spectrometer that measures 50x50x5 mm and PSD that works in the UV and NIR range up to 1.1 µm."

A PSD is used to measure the position of a light spot incident on its surface and is typically based on a p-n junction photodiode, which is formed by implanting a p-layer in a silicon wafer. In a novel approach, the team constructed 1D and 2D PSDs based on the metal-oxide-semiconductor (MOS) principle commonly used for MOS transistors.

"These detectors are based on the MOS principle in which a conducting channel is created beneath the isolated gate contact by applying an electric field," explained Andersson. "The advantages of MOS-type PSDs are that we can change the interelectrode resistance and switch the detector on and off by adjusting the gate bias."

A higher interelectrode resistance leads to lower thermal noise and higher position resolution. This will especially affect the resolution for lower light intensities where thermal noise dominates. "The drawback of high interelectrode resistance is that the detector will saturate earlier," commented Andersson. "But the ability to change the resistance means that the PSDs can be adjusted according to the light intensities used at the time."

PSD cross-section

The MOS PSDs have two or four contacts corresponding to a one or two dimensional detector respectively. The current generated by the incident light will diffuse through the bulk resistance separated by the junction field. The current is divided between the two contacts at the edges of the PSD in proportion to the resistance of the active layer from the position of the incident light to the contacts. By comparing the relative magnitude of the currents recorded from each contact, the position can be determined.

The MOS PSDs have two or four contacts corresponding to a one or two dimensional detector respectively. The current generated by the incident light is divided between the contacts in proportion to the resistance between the generation site and the contacts. The position of the light is determined by the ratio of these currents. By comparing the relative magnitude of the currents recorded from each contact, the position can be determined.

The team also wanted to develop a cheaper alternative to the high cost, high performance spectrometers that are used in industrial applications. "In many cases a less expensive instrument with sufficient enough resolution of around 5-10 nm is better suited," commented Andersson. "It is possible to construct a cheaper and more compact spectrometer by using a multi-channel Fabry-Perot interferometer."

The approach involves attaching a wedge-shaped array of Fabry-Perot interferometers on top of a pixel detector array or PSD. The interferometer array produces an interference pattern, which is measured by the detector and processed to extract the spectral information.

"The principle is that collected and collimated light passes through a linear array or matrix of Fabry-Perot interferometers where the thickness changes linearly," explained Andersson. "The change in thickness must be so small that the period of the interference maxima for the shortest wavelength is at least twice the distance between the measurement points. This design requires no intermediate optics between the interferometer and array detector resulting in a reliable and extremely compact spectrometer."

According to Andersson the key problem that needs to be solved is to simplify the manufacture of the spectrometer. "Currently, the devices are built by hand and we need to find a process that is suitable for mass production," concluded Andersson. "This is possible by mounting an interferometer wedge directly onto the detectors instead of a standard cover glass."