A diffractive optical element (DOE) is a passive component that redirects chosen wavelengths of the incoming light to a set of predefined positions. The number of wavelengths needed in charaterisation is limited. This is seen in commercial solutions that comprise filter wheels or grating spectrometers. This DOE can be replicated to a very low cost in large numbers.

To make a low-cost near infrared scanning spectrometer, we use a single detector and a scanning mechanism, and integrate the functions of grating, collimating lenses and wavelength references into a Diffractive Optical Element (DOE). A reflective DOE alters an incoming light wave by shifting the wave’s phase in a prescribed manner at each point of its area in the reflection. This is obtained by the miniature grooves in the DOE surface [fig.1].

By scanning the DOE, light of a complete set of desired wavelengths is sequentially focused on the detector [fig.2].  On the figure only the waves reaching two of the detector position is shown. When we move a detector (or equally tilts the DOE), the detector scans along the shaded rectangle on Fig.2.

The signal of the desired wavelengths will be detected in sequence, and the 5 peaks measured at the detector during a scan cycle, are known to be the 5 target wavelengths [fig. 3]. The wavelengths are optimized for the object materials. Fig. 3 shows in dark blue the background and in lighter blue the signal for PEN (Polyethylen naphalate). The resulting DOE has the function of four beam splitters, five band pass filters and lenses, and self-alignment is obtained through the design.

Applications

• The DOE spectrometer has been developed for Tomra Systems ASA for use in their material recycling machines to charaterise plastics and glass qualities with high precision. This new business area is represented by their new Tomra Automated Recycling Center (ARC).
• A spectrometer to monitor the CO₂-level for ventilation control in buildings has been developed for Optosense AS.
• We foresee potential applications as dedicated spectrometers for detecting other gases, in food control,  in biological and in medical applications.

Related article

Dedicated spectrometers based on diffractive optics. Design, modelling and evaluation
Løvhaugen, O.; Johansen, I.-R.; Bakke, K.A.H.; Fismen, B.G.; Nicolas, S.
Journal of Modern Optics : Special issue on Wave-Optical Engineering, (2004)

For further information contact Odd Løvhaugen. 

This work has been supported by the Norwegian Research Council (NFR). 

Application of DOE technology: Tomra Automated Recycling Center (ARC).  

Application of DOE technology: Optosense Sensors for Demanding Gas Measurements. 

The core of the spectrometric sensors - names HoloChip - contains the swinger and all the optics of the system.