Biophotonics_Concepts_to_Applications

parameters for commercial 3-port and 4-port optical circulators. In the biophotonics
field this device is used in conjunction with thin-filmfilters or FBGs to combine or
separate a series of narrow wavelength bands for applications such as
laser-scanning microscopy, optical coherence tomography, imaging techniques (for
example, monitoring tissue changes induced by laser thermal therapy), spectro-
scopic diagnostics, and processing of biosensor signals.

5.8 Summary

Photodetection or light measuring devices for biophotonics applications are needed
in the spectral range from about 190 nm to 10μm, which covers the ultraviolet,
visible, and infrared regions. Depending on the particular discipline in which the
device is used, the performance characteristics can include sensing very low light
levels, responding to light pulses as fast as femtoseconds, and possessing low
internal noise characteristics. The devices used in biophotonics include
semiconductor-basedpinand avalanche photodiodes, photomultiplier tubes, and
detector arrays based on complementary metal-oxide-semiconductor (CMOS) and
charge-coupled device (CCD) technologies.
The photodetectors can be either single-channel elements or multichannel
devices consisting of one-dimensional rows or two-dimensional arrays of detectors.
Multichannel devices can measure multiple spectral or spatial channels simulta-
neously in different time sequences. Two common multichannel devices used
extensively in cameras are based on CMOS and CCD technologies.
For many biophotonics applications it may be necessary to select a narrow
spectral band or to reject light above or below a particular wavelength. Optical
filters are devices that selectively transmit light in one or more specific bands of
wavelengths while absorbing or reflecting the remainder. The absorption or
reflection properties give rise to the two broad categories of absorptivefilters and
dichroicfilters, respectively.

5.9 Problems.

5 :1 An InGaAs pin photodetector has an absorption coefficient of 1.0μm−^1 at

1550 nm. Show that the penetration depth at which 50 % of the photons are

absorbed is 0.69μm.

5 :2 If the absorption coefficient of silicon is 0.05μm−^1 at 860 nm, show that the

penetration depth at which P(x)/Pin=l/e=0.368 is x = 20μm.

5 :3 A particular InGaAs pin photodiode has a bandgap energy of 0.74 eV. Show

that the cutoff wavelength of this device is 1675 nm.

5 :4 If an optical power level Pinis incident on a photodiode, the electron–hole

generation rate G(x) is given by

5.7 Optical Couplers and Optical Circulators 143