Page 428 - Tunable Lasers Handbook

388 Paul Zorabedian

the first pass. Therefore, the angular dispersion of the retroreflected light is twice
that of the light diffracted on one pass:

The dispersion of the grazing-incidence configuration is therefore twice that of the
Littrow configuration for the same angle of incidence. In addition, the grazing-
incidence configuration is typically used with a much higher angle of incidence,
for example, 8, - 85".

7.2.1.1.5 Grating Efficieizcy
7.2.1.1.5.1 Blazed Gratings Blazing refers to an enhancement in effi-
ciency that is obtained at a particular wavelength when the grooves on the grat-
ing surface have a triangular shape. A simple explanation for this effect is that
when the specular reflection from the top surface of each groove coincides with
the direction of diffraction, the reflections reinforce the diffraction effect and the
efficiency is maximized. The wavelength h, at which this reinforcement occurs
is called the "blaze wavelength." The angle 8, of the top surface of the groove
with respect to the macroscopic surface of the grating is called the "blaze angle."
The terminology derives from the observation that a grating will light up or
"blaze" when viewed at the correct angle.
The blaze angle of ruled gratings is defined during the process of ruling the
master grating and is transferred to the replica. The simplest type of holographic
grating has a sinusoidal shape. However, after interferometric recording, the
grooves of holographic gratings can be shaped to approximate blazing by an ion-
beam milling process.
In a Littrow mounting the blaze condition is satisfied when the tops of the
grooves are perpendicular to the incident beam. The diffraction efficiency rises
as the angle of incidence is increased up to -8, and falls thereafter. This simple
description is only valid for low blaze angles (up to -10'). Working near 1, for
small blaze angles implies a small diffraction angle as well, so that k<a. This is
the regime of validity for scalar diffraction theory, in which the diffraction effi-
ciency is nearly independent of polarization.

7.2.1.1.5.2 Polarization Effects To obtain greater angular dispersion it is
necessary to use larger blaze and diffraction angles, which implies IL - a. This is
the regime of vector diffraction theory in which polarization effects become sig-
nificant. For blaze angles above -lo", the diffraction efficiency strongly depends
on the orientation of optical polarization with respect to the direction of the
grooves. A particularly useful regime for tuning ECLs is the range of blaze
angles from about 22" to 38". For this regime, there is a broad plateau of high
efficiency for €Il > 8, when the incident polarization is perpendicular to the