Page 431 - Tunable Lasers Handbook

8 Tunable External-Cavity Semiconductor Losers 391
that was coupled to the fiber evanescent field. In this way, a fiber reflective grat-
ing was obtained that had a reflectance of -60 to 80% for 1280 nm < h < 1340
nm. The grating FWHM was between 0.7 to 1.2 nm.

7.2.1.3 Fabrg-Perot Etalon
7.2.1.3.1 Principle oJf Operation
The filtering effect of the Fabry-Perot etalon utilizes the interference fringes
produced in the transmitted light after multiple reflections between two highly
reflective mirrors [65]. The Fabry-Perot etalon has periodic transmission peaks
at wavelengths that satisfy the relation

2nd cos 0 = nzh . (58)

where d is the mirror spacing. 12 is the index of refraction of the space between
the mirrors, 0 is the angle of incidence, and m is an integer. Tuning can be accom-
plished by changing the mirror separation or by varying the angle of incidence.
7.2.1.3.2 Resolution
The ratio of the wavelength of a fringe peak to the FWHM of the peak of a
Fabry-Perot etalon is called the chr-omnnc I-esohing pow'en The chromatic
resolving power is given by

where I' is the amplitude reflectance of the mirrors.
7.2.1.3.3 Free Spectral Range
For a typical air-spaced or solid etalon, d is equal to a few millimeters. The
u avelength spacing between maxima is given by the free spectral range.

For example. for h = 1300 nm. d = 1 mm, and tz = 1.5, the free spectral range is
0.56 nm,
7.2.1.3.4 Finesse
The spacing between orders relative to the width of a single order is given
by the finesse -6 The finesse is defined as