Laser Focus World - January 2015

Wavelength (nm)

Transmission (%)

Blocking Passband

1075 1025

100

975 925 875 825

Transmission
width

785

775 725

80

60

40

20

0

675

Berlin-Brandenburg.

The region for precision.

www.photonics-bb.com

measurement instruments.

Let’s say you need to evaluate a 785 nm Raman longpass edge filter for integration into a spectroscopy setup for a flu- orescence 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 sam- ple, typically requiring an optical density (OD) > 6. Any superfluous light needs to be completely attenuated to detect the ex- tremely small emission signal. Likewise, the transmission of the passband needs to be optimized to transmit as much of the signal wavelength as possible. The spec- tral performance for the Raman long-pass 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

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 per- formed with any well-calibrated spectro- photometer. 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) param- eter, 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.