Fiber-Optic Sensors - Applied Optics Fundamentals and Device Applications - page 160

width, low power consumption, high coupling efficiency (modest output
beam divergence), and optimum wavelength matching at single longitudi-
nal mode operation available in commercially available devices preclude the
use of HeNe or other types of laser sources in many applications. Although
a wide variety of laser diodes is available, the group consisting of Al x Ga 1- x As
(0 < x < 0.5) in a double-heterostructure configuration is the most suitable for
fiber sensor design. Low-threshold continuous operation at room tempera-
ture has been achieved, and a stable Gaussian beam is produced without
the use of a spatial filter or other apodizer. Feedback diodes are typically
incorporated in the devices to maintain power levels. Proper heat sinking
diode junction thermistor temperature controls have been developed to con-
trol variations of emitted wavelength with temperature. Most laser diode
degradation mechanisms have been identified and eliminated so that 10 6 h
lifetimes are projected. Finally, the output spectrum of these devices shows
no significant shift with current level changes up to 1.5 times threshold.
Wavelengths between 630 and 900 nm, spanning the fiber low-loss region,
can be achieved with appropriate Al concentrations. Injection laser beam
divergence is asymmetrical, with typical half angles of less than 5° × 10°-
10° × 30°. The major portion of the output of these lasers falls within an ellipse
of f /6 × f /12- f /1.8 × f /6 (numerical aperture values of 0.17 × 0.08-0.55 × 0.17).
Although circularizing optics in the form of cylindrical lenses or prism com-
binations can render this input circular, and spherical lenses then used to
couple the laser output to a single-mode fiber, this arrangement requires
extensive lens positioning, fixturing, and unnecessarily large volume. A pre-
ferred approach for coupling the laser output is to pigtail the single-mode
fiber directly to the laser diode package. This arrangement provides excel-
lent coupling efficiency at a reduced volume and is available with several of
the commercial devices. Although the coupling coefficient between the laser
and the fiber depends on the far-field pattern of the laser and the diameter
and acceptance angle of the fiber, coupling loss for a well-designed system is
typically −2 to −3 dB.
Extremely low current thresholds are available (less than 12 mA) and in
fact the electrical-to-optical power conversion efficiency of injection lasers is
among the highest of all candidate lasers (up to 0.45 W/A for diodes in the las-
ing region). The diode emits a linearly polarized TE mode with the electric field
vector parallel to the p-n junction plane. Typical coherence length for a single
longitudinal mode laser is about 6 m for a 50 MHz linewidth (4 km for 75 kHz).
Noise performance of these diodes is excellent. Most of the output is generated
via a quantum statistical process by the spontaneous emission of light and by
small fluctuations in the pumping current and temperature variations.
Temperature affects all of the material parameters of a semiconductor
that are important for lasing. These factors combine to give an overall slight
decrease in output as temperature rises. Many laser diode modules contain a
small monitor photodiode that is used in a feedback loop to control the drive
current in order to maintain the laser output at a constant level. To the extent