OPTICAL FILTERS

cal density (OD) blocking coupled with nearly vertical transitions from low signal to high transmission.

Filter manufacturers are becoming adept at

FIGURE 1. To evaluate a 785 nm Raman longpass edge filter for integration into a

spectroscopy setup, the optical engineer faces three measurement challenges: confirm transmission of the passband, resolve and verify that steepness of the nearly vertical edge is <1%, and measure the blocking to ensure >OD6 @ 785 nm.

OPTICAL FILTERS continued

measurement instruments.

Let’s say you need to evaluate a 785 nm Raman longpass edge filter for integration into a spectroscopy setup for a fluorescence imaging system using confocal Raman microscopy, similar to Figure 1. The excitation and emission modes of the fluorophore of interest can be similar in wavelength, so the transition width of the filter (distance in nanometers from 50% to 785 nm laser-line) often needs to be <1% or better. In addition, the intensity of the returned Raman signal is inversely proportional to the fourth power of the excitation wavelength—that’s on the order of 1012 smaller than the excitation intensity.

This means the filter requires very deep attenuation at 785 nm to block any residual laser light emitted from the sample, typically requiring an optical density (OD) >6. Any superfluous light needs to be completely attenuated to detect the extremely small emission signal. Likewise, the transmission of the passband needs to be optimized to transmit as much of the signal wavelength as possible. The spectral performance for the Raman longpass edge filter would resemble Figure 2.

For components like this Raman precision filter, the measurement problem becomes three-fold:

1. Confirm transmission of the passband

Transmission (%)

100

785 Wavelength (nm)

FIGURE 2. Spectral performance curve for a typical 785 nm Raman longpass edge filter with OD ≥6.0 coating performance.

2.Resolve and verify the steepness of the nearly vertical edge is <1%

3.Measure the blocking to ensure >OD6

@ 785 nm.

Confirming the transmission of the passband is straightforward and can be performed with any well-calibrated spectrophotometer. The difficulty comes when moving toward the transition edge and beyond to the verification of the OD of the blocking. Determining these values often requires more than one scan, with tradeoffs as to what can be achieved with each measurement. One could bump up the integration time and scan at 0.25– 0.50 nm step intervals to achieve the accuracy needed, but time becomes a major factor and single scans could last up to 30 minutes or longer depending on the desired results.

Effects of spectrophotometer bandwidth

When measuring the very narrow spectral edge of precision filters, you need special care to avoid misleading artifacts. First, the resolution of the instrument can cause a theoretical “square” edge to become a “rounded” edge during the transition from the high transmission region. This is directly related to the spectral bandwidth and area of the diffraction grating—the larger the area of diffraction, the higher the resolution.

For a grating with a given number of lines/mm, a larger grating area provides higher resolution, but adds to the size and cost of the instrument. Similarly, the resolution can be increased by reducing the spectral bandwidth (SBW) parameter, but this has the effect of reducing the amount of light through the instrument, which, in turn, reduces the sensitivity at the detector.

The location of the edge in wavelength space is also important—if the light source is non-collimated and some f/# (cone angle) is present, the edge will exhibit a minor shift. Most spectrophotometers are not perfectly collimated, so it’s best to use very small apertures when measuring to restrict the light to on-axis performance. Again, this decreases the amount of light in and the sensitivity of the system.

Traveling further down the edge of the filter to the 0.1–0.01% region, a “shoulder” or a “sideband measurement artifact” may appear. This feature is unique to very steep transition measurements from high blocking to high transmission, much like this Raman longpass edge filter. It arises from the non-monochro- matic illumination source and SBW effect discussed above.

The instrument scans over a wavelength on the very steep edge and with a non-zero SBW. The finite spectral bandwidth means that instead of being attenuated at the actual OD level of the filter, additional periphery wavelength noise is transmitted by the filter within the band, registering as a signal on the detector. The sum of primary and periphery signals leads to a higher intensity reading than what is actually present. In a commercial instrument, there is little that can be done to reduce this shoulder, except add

ANNOUNCEMENT OF A

COMPETITIVE CALL FOR ADDITIONAL PROJECT PARTNERS

Project acronym: LASHARE

Project grant agreement number: 609046

Project full name: Laser Equipment Assessment for High Impact Innovation in the Manufacturing European industry

The project LASHARE is currently active in the Seventh Framework programme of the European Community for research, technological development and demonstration activities contributing to the creation of the European research area and to innovation (2007-2013). LASHARE requires the participation of new partners to carry out certain tasks within the project. Please note that the Seventh Framework programme offers part-funding not full-funding of research activities. LASHARE has a vacancy in its consortium for partners established in an EU Member State or FP7 Associated State.

LASHARE aims to advance innovative equipment solutions that have been demonstrated at laboratory level, Technology Readiness Level (TRL) 3-4, towards robust prototypes at TRL 7-8. The development is guided by an Assessment Framework that involves an industrial user, an RTD partner and the supplier of the equipment. The laser based equipment assessment (LEA) should focus on the benefit of the supplier which should be an SME. The role of the users is to provide guidance towards requirements and provide the basis for validation.

All assessments shall be related to equipment for laser-based material processing. Suppliers of innovative high-tech equipment are expected to install and assess their prototypes or products in production-like environments and validate them in a manufacturing line or in an industrial environment that is very close to manufacturing conditions. The primary aim is to strengthen the ICT equipment supplier base, predominantly SMEs, through a close cooperation with globally acting manufacturers, by improving the manufacturing processes in relation to quality, speed, environmental and resource efficiency.

The LEAs shall cover innovative laser applications in laser-based material processing and address improvements related to quality, speed, flexibility and resource efficiency of laser-based material processing. The project aims to select between 10 and 12 new LEAs for funding in this competitive call. Each of the LEA teams will consist of a user, a supplier and an RTD partner. The RTD partner must be one of the existing LASHARE RTD partners.

The core task of the laser-based equipment assessment shall focus on sensors and knowledge based ICT systems that enable at least one of the following aspects:

ïautonomous set up of machine and laser parameters for fast and flexible manufacture

ïprocess and quality control for robust processes and

fully documented production

in the area of complex manufacturing tasks.

Opening: 1 February 2014

Deadline: 16 March at 17h00

Expected duration of participation: 24 months

Duration of an assessment: max. 22 months

Funding: Total funding available for new partners amounts to 1,767,000 €; the requested funding for each LEA should not exceed 250,000 €

Call identifier: LASHARE Competitive Call 1

Language of submission: English

Email address: cc@lashare.eu

Web address for further information: www.lashare.eu

LASHARE is also calling for experts to evaluate proposals. If you would like to be considered please visit www.lashare.eu

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