Biophotonics_Concepts_to_Applications

Example 1.4Consider an optical link that has four optical components

hooked together sequentially. If the losses of these four components are 0.5,

0.1, 1.5, and 0.8 dB, what is the loss in dB of the link?

Solution: The total power loss is found by adding the losses between the two

link end points, that is,

Link loss¼ 0 :5dBþ 0 :1dBþ 1 :5dBþ 0 :8dB¼ 2 :9dB

Because the decibel is used to refer to ratios or relative units, it gives no indi-
cation of the absolute power level. However, a derived unit can be used for this
purpose. Such a unit that is particularly common is thedBm. This unit expresses the
power level P as a logarithmic ratio of P referred to 1 mW. In this case, the power in
dBm is an absolute value defined by

Power level in dBm¼10 log

P(in mW)

1mW

ð 1 : 5 Þ

Example 1.5Consider three different light sources that have the following

optical output powers: 50μW, 1 mW, and 50 mW. What are the power

levels in dBm units?

Solution: Using Eq. (1.5) the power levels in dBm units are−13, 0, and

+17 dBm.

Example 1.6A product data sheet for a certain photodetector states that an

optical power level of−28 dBm is needed at the photodetector to satisfy a

specific performance requirement. What is the required power level inμW

(microwatt) units?

Solution: Eq. (1.5) shows that−28 dBm corresponds to a power inμWof

P¼10expð 28 = 10 ÞmW¼ 0 :001585 mW¼ 1 : 585 lW

1.6 Structures of Biological Cells and Tissues.

A basic knowledge of biological cell and molecular structures, cellular and
molecular functions, and the vocabulary used to describe these structures and
functions is essential to understanding the biophotonics tools and light-tissue
interaction processes described in this book. This section gives a brief tutorial of

14 1 Overview of Biophotonics