Page 165 - An Introduction to Microelectromechanical Systems Engineering

144 MEM Structures and Systems in Photonic Applications

beam is not necessarily monochromatic, as multiple modes may participate in lasing.
A wavelength filter, typically a grating, selects only one desired wavelength to gener-
ate a monochromatic laser beam (see Figure 5.7).
The tuning of a laser requires two simultaneous operations: the tuning of the fil-
ter to the new desired wavelength and the tuning of the optical length of the cavity
such that one of the resonant longitudinal modes defined by c/2L overlaps the
desired wavelength (see Figure 5.7). Often referred to as the phase tuning, this is a
condition for resonance. Additionally, at any output wavelength of a tunable
laser, the amplification medium must possess a reasonable gain before lasing can
occur—this is strictly a material property that dictates the choice of the material.
The two lasers described here achieve the same objective using two radically differ-
ent approaches. The Iolon approach achieves both tuning steps by using a MEMS-
type microactuator [10]. The Santur approach [11] does it by heating and cooling
the gain medium to change the index of refraction.
The main specifications of a tunable laser are wavelength in nanometers (or the
corresponding optical frequency in Hz), tuning range in nanometers, spectral
linewidth at the lasing frequency in Hz (the narrower the linewidth, the higher the
coherence of the output beam), output optical power expressed in milliwatts or in
dBm (the reference 0 dBm level is at 1 mW), relative intensity noise over a given fre-
quency bandwidth (RIN) expressed in db/Hz, and side-mode suppression ratio
(SMSR) in dB, which measures the power ratio at the lasing fundamental mode or
wavelength to its nearest allowed mode. For applications in telecommunications,
the specifications vary between short-distance (a few kilometers) and long-distance
(>800 km) transmission. The latter requires more stringent specifications; for
instance, the power is typically 13 ± 0.25 dBm (20 ± 1 mW) over the entire C-Band,
the RIN needs to be lower than –120 dB/Hz, and the SMSR is higher than 45 dB.

The External Cavity Tunable Laser from Iolon
The laser design used by Iolon [10] belongs to a family of external-cavity lasers
known after their inventors as Littman-Metcalf (see Figure 5.8) [12]. The three key
building blocks are physically separate and hence can be optimized individually.
External cavity lasers can also deliver superior properties in the form of stable power
as well as high monochromaticity (measured as narrow line width) [13].
In this laser, the amplification medium consists typically of an InGaAsP/InP
semiconductor diode with multiple quantum wells (a laser diode) because its gain
spectrum covers the entire C-Band [14]. A thermoelectric cooler (TEC) maintains
the temperature of the laser diode at approximately 25°C to increase diode lifetime
and minimize chromatic thermal drift—the gain spectrum is a strong function of
temperature. The wavelength filter is a glancing-angle ruled blazed or holographic
grating [15] with a typical periodicity of 1,200 lines per millimeter. A partially
reflective coating on one facet of the laser diode and a reflective mirror bound the
external cavity [see Figure 5.8(a)]. With an effective cavity length of 8 mm, the spac-
ing between the cavity modes is approximately 18 GHz (~ 0.2 nm) (i.e., nearly 190
distinct modes fit within the C-Band). The other facet of the laser diode must be
highly transmissive (coated with an antireflective multilayer coating) in order to
avoid forming a spurious resonant cavity within the diode itself—the reflectance is
−3
often significantly less than 10 . Light emanates from the laser diode through a