Biophotonics_Concepts_to_Applications

Example 7.1 As described in Chap. 9 ,inafluorescent spectroscopy setup a

laser emitting in a narrow spectral band is used to excite electrons in some

substance to higher energy levels. The excited electrons lose some of their

energy to molecular vibrations. Because the excited electrons now have a

lower energy, during the transition back to the ground state they emit photons

at a longer wavelength than that of the excitation light. Suppose the substance

being excited is agreenfluorescent protein(GFP) that absorbs light at a peak

wavelength of 469 nm and emits light at a peak wavelength of 525 nm. In the

setup in Fig.7.1a, what characteristics should the dichroicfilter have?

Solution:The following example characteristics can be found from com-

mercial data sheets on dichroic edgefilters, as Fig.7.4illustrates:

• Cutoff wavelength = 497 nm


• Reflection spectral band range: 450–490 nm


• Transmission spectral band range: 505–800 nm
  The reflection and transmission bands could have an average ripple of
  10 %.
  Example 7.2The use of a double-cladfiber (DCF) is another setup possi-
  bility for a single-fiber probe. Show a possible probe configuration using a
  combination of a 2×2 DCF coupler, a single-modefiber (SMF), a multi-
  modefiber (MMF), and a DCF single-fiber probe.
  Solution:An example probe setup using a DCF coupler and three types of
  fibers is shown in Fig.7.5. Here a single-modefiber delivers laser light to the
  single-mode core in the port 1 DCF branch of the DCF coupler. Output port 2
  of the DCF coupler is blocked and thefiber from port 3 is attached to a
  bidirectional DCF probe. The light is delivered to the tissue sample using the
  single-mode core of the DCF and the return path uses the large inner mul-
  timode cladding of thefiber at port 3 for increased light collection efficiency.

450 500 550 600 650 700 750 800

Wavelength (nm)

0

0.8

1.0

0.6

0.4

Transmittance0.2

Reflection band Transmission band

Cutoff wavelength (497 nm)

Fig. 7.4 Example of a dichroic edgefilter with a 497-nm cutoff wavelength

202 7 Optical Probes and Biosensors