Special Report: Optical Patterns
Coherence Coherence is one of the most important attributes of
a light source for optical information processing and pattern recognition.
The spectral linewidth of a microlaser is very narrow [14-16]. This narrow
linewidth is attributed to the short cavity length of the microlaser that
can support only a single longitudinal mode, an effect similar to that of
a distributed-feedback (DFB) laser or a gas laser with a Fabry Perot etalon.
In addition, the high reflectivity of the reflector functions as a spectral
filter. Therefore a microlaser has a longer coherence length than a conventional
Fabry Perot laser diode (See Table 12.1). A short linewidth of 65 MHz has
been measured with heterodyne techniques for a microlaser (850 nm to 865
nm wavelengths) operated near threshold [15]. This corresponds to a coherence
length of 5 m (0.00014nm linewidth) and is comparable with that of an Ar-ion
laser with an etalon. Although microlasers operate inherently at a single
longitudinal mode, the coherence length still decreases at high current
levels. This is due to the multiple transverse modes at high currents. Fig.
12.3 illustrates output light power versus current (L-I) and voltage versus
current (V-I) curves for a typical top-surface emitting gain-guiding microlaser
[18]. The apparent kinks in both curves are associated with changes in the
optical mode and corresponding carrier distributions. The first threshold
corresponds to the TEM00 lasing mode. The first plateau in the L-I characteristic
is associated with the saturation of the TEM00 mode. The subsequent threshold
corresponds to the onset of the TEM01* mode. Lasing begins in the TEM00
mode, but further increases in the current result in the occurrence of additional
higher order transverse modes. These higher-order transverse modes generates
additional spectral modes that are typically separated by approximately
0.7 nm, which greatly reduces the coherence length and the beam profile
of an expanded beam. Such a problem was solved with a spatial filtering
technique, which was achieved by a simple reduction in the contact aperture
size with respect to the width of the confinement (surrounded by the implantation)
[18]. In this way, the kinks were eliminated and a single transverse mode
operation leading to a high coherence length was achieved even at high current
levels without sacrificing the output power (approximately 63%) too much.
Also, each laser of the array operates independently; they are not phase
locked with each other [19].
Fig. 12.3. Light output power versus current and voltage versus current
curves of a typical microlaser. (Reprinted with permission from Ref. [18]
by Morgan et al., © 1993 IEEE.)
12.2.3 Visible microlasers Current microlasers operate mainly in the wavelength
range of 0.75 mm-1.0 mm which can be achieved with GaAs-based alloys. However,
researchers are developing shorter-wavelength microlasers. This is motivated
by the fact that the shorter wavelength would allow a larger capacity of
an optical storage medium as well as easy alignment. The visible operation
is crucial particularly for holographic information processing because most
holographic recording materials are sensitive to only visible wavelengths.
The main difficulty in the fabrication of visible microlasers lies in the
creation of mirrors with high reflectance and low loss in the visible region.
Room-temperature lasing to wavelengths as short as 0.63 mm was demonstrated
with InGaAlP alloys [20-23]. Recently an electrically pumped blue microlaser
array was also demonstrated [24,25]. Laser action was achieved at a wavelength
of 484 nm under a pulsed-current injection at 77 K. The microlaser structure
was composed of a CdZnSe/ZnSe multiple-quantum-well active layer, ZnSe cladding
layers, and SiO2TiO2 distributed Bragg reflectors (DBR's). This result shows
promise of the microlaser for holographic information processing based on
photorefractive crystals, many of which are sensitive at blue-green wavelengths.
Likewise, additional work on blue-green lasers with homoepitaxial deposition
of ZnSe is currently being pursued. The recent demonstration of a blue laser
diode by Nakamura at Nichia Chemical Ind. Ltd. (Tokushima, Japan) [26] shows
great promise in the blue-green operation of the microlaser. The laser is
II-VI based (GaN material with InGaN multiple quantum wells) and is currently
operated at room temperature in a pulsed mode at 390 nm to 440 nm for an
edge emitting structure.
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