Biophotonics_Concepts_to_Applications

greenfilter passes 505±15 nm, and a redfilter passes 610±15 nm. In the
near-IR region, thin-filmfilters are available in a wide range of passbands varying
from 50 to 800 GHz, or equivalently, spectral widths ranging from 0.3 to 4.5 nm in
the 1300-nm wavelength region or from 0.4 to 6.4 nm in the 1550-nm wavelength
region. Many different standard and custom TFF designs are available commer-
cially for other wavelength regions.

5.7 Optical Couplers and Optical Circulators.

Two optical elements that are widely used in opticalfiber-based equipment setups
are optical couplers and optical circulators. The concept of anoptical coupleror
beam splitterencompasses a variety of functions, including splitting a light signal
into two or more streams, combining two or more light streams, tapping off a small
portion of optical power for monitoring purposes, or transferring a selective range
of optical power betweenfibers. When discussing couplers it is customary to
designate couplers in terms of the number of input ports and output ports on the
device. In general, an N×M coupler has N≥1 input ports and M≥2 output
ports. For example, a coupler with two inputs and two outputs is a 2×2 coupler.
A1×N device (one input and N outputs) is popularly known as astar coupler.
These devices can be made in the form of planar waveguides or they can be made
from opticalfibers that are fused together along a specific coupling length. The
latter device often is called afused-fiber coupler. Biophotonics applications of
fused-fiber couplers include laser surgery, optical coherence tomography, endo-
scopy, optical power level monitoring in light therapy, spectroscopy, and
biosensors.
A2×2 opticalfiber coupler, shown in Fig.5.14, takes a fractionαof the power
from input 1 and places it on output 1 and the remaining fraction 1-αexits from
output 2. Similarly, a fraction 1-αof the power from input 2 is directed to output 1
and the remaining power goes to output 2. The parameterαis called thecoupling
ratio. As an example of nomenclature, if the parameterα= 50 %, the device is
called a 50/50 coupler or a 3 - dB coupler. For light level monitoring purposes, atap
couplermight haveαbe a few percent (e.g., 1–5 %). A coupler can be designed to
be either wavelength-selective or wavelength-independent (sometimes called
wavelength-flat) over a wide usable range. In a wavelength-independent device,αis
independent of the wavelength; in a wavelength-selective device,αdepends on the
wavelength. Table5.4lists some common specifications for optical couplers.

Coupler body

Input fiber 1 Output fiber 1

Input fiber 2 Output fiber 2

Fused fiber region

Fig. 5.14 Operational
concept of an opticalfiber
coupler or splitter

5.6 Optical Filters 141