Laser Diode Modules
Laser diodes are often used in the form of laser diode modules, i.e. packages which contain one or several laser diodes, in most cases combined with some optics and electronics. Such modules are much easier to use than bare laser diodes, as they serve a number of functions, as explained in the following. The optical output is either into free space – typically as a collimated beam, through a connected fiber or with a fiber connector.
For a laser diode module with free-space output, some optics (e.g. consisting of collimating lenses, micro-optics, and anamorphic prism pairs) can be used to shape the output beam, e.g. in order to obtain an approximately circular (or sometimes elliptical) beam with small divergence. Such a collimated beam can be more easily transmitted over some distance and more efficiently coupled to an optical fiber.
Important parameters e.g. for collimated outputs are the beam radius and beam divergence angle in both directions, the beam quality factor <$M^2$> and some measure for the beam pointing fluctuations (or the possible beam direction change during the warm-up time).
Other modules for specialized applications offer certain beam shapes, e.g. that of a line, a cross, an open circle, or a square.
Some laser diode modules are fiber-coupled, i.e., directly launch the generated light into an optical fiber (“pig-tailed” laser diodes). For limited optical power, this may be a single-mode-fiber, which might even be polarization-maintaining, as the output is often linearly polarized. Otherwise, multimode fibers are common, and are the only option at high power levels.
Power and Wavelength Stabilization
The output power may be stabilized with an internal feedback loop with a monitor photodiode (often built into the actual laser diode). Some modules are very carefully stabilized to achieve a small relative intensity noise.
There are wavelength-stabilized modules, e.g. using passive stabilization with frequency-filtered optical feedback, or just temperature stabilization (see below). The output wavelength may drift somewhat more during the warm-up time after switching on the module.
In some cases, the built-in electronics have an input for power modulation. Others are meant for continuous-wave operation with constant power only.
It is possible to obtain nanosecond or even picosecond light pulses from laser diode modules. For example, there are gain-switched modules with integrated driver electronics – which is important as very short electrical connections should be used for that mode of operation. In other cases, one uses quasi-continuous-wave operation with much longer pulse durations.
A laser diode module may contain not only simple connections to the pins of the laser diode, but also additional electronic circuits, e.g. for protecting the laser diode against electrostatic discharge (ESD protection), wrong poling and too high operating voltages. Power stabilization (see above) and/or power monitoring is another possibility, also electronics for gain switching.
A module can often be operated directly with a battery or with an unstabilized power supply.
The module case may also provide effective shielding against external electromagnetic influences, which might otherwise affect the drive current and output power.
Cooling and Temperature Stabilization
For high powers, a laser diode module can facilitate the cooling, e.g. by offering a metallic surface which can easily be mounted on a cooler.
Temperature stabilization is also important for low-power diode modules, since the junction temperature influences the emission wavelength. A thermoelectric cooler (TEC) may also be included, often with a feedback system to stabilize the diode temperature. This leads to a more stable output wavelength and output power.
Applications of laser diode modules include precise pointing and alignment of optical elements, printing and imaging systems, displays, bar code scanning, optical data storage, optical sensors, pumping of solid-state lasers, free-space optical communications, and medical applications (e.g. photodynamic therapy, ophthalmology).
Many modules are used as OEM laser modules, i.e., integrated into larger devices by a manufacturer who does not want to deal with various details of the laser diodes.
There are also laser pointers, which can be considered as laser diode modules with integrated batteries, normally used as hand-held devices.
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