What is a Diode Bar?

Mal Baxter

A diode bar is a bank of semiconducting laser emitters combined with driver circuitry and optics to form a diode laser module. These units resemble metallic transistors. Lasers are arrayed in a line of broad-range emitters or subarrays of 10 to 20 thin strips. They typically accompany a monitor photodiode chip for feedback control of power output. These modules function in technologies from household disk drives and printers to laser pointers. Higher powered diode bars provide laser light for applications such as laser surgery and industrial surface treatments of materials.

Diode bars function in equipment like laser pointers.
Diode bars function in equipment like laser pointers.

A typical diode bar contains 20 to 50 emitters, each 100 micrometers (μm) wide — the width of two or three human hairs. They may pump tens of watts of output power; their wavelengths fall between 780 and 860 nanometers (nm), and 940 to 980nm, as typical of solid state pumping lasers in fiber optics networks. High-powered lasers of over 100 watts (W) can be formed by stacking diodes vertically. These lasers are highly efficient, energetic, and bright; this makes them suitable for numerous applications across industries.

Common laser beams fall near the infrared portion of the electromagnetic spectrum.
Common laser beams fall near the infrared portion of the electromagnetic spectrum.

Common lasers produced by diode bar modules can be found in computer drives and audio disc players. These beams fall near the infrared portion of the electromagnetic spectrum, at 780 nm. Such lasers output at around five milliwatts (mW), but are collimated through optics that reduce their power to 0.3 to 1mW. The higher-powered infrared (IR) laser diodes of computer optical drives output up to 30 mW. The familiar deep red lasers created by the earliest visible laser diodes operated at about 670 nm, within range of the visible spectrum.

Collimation refers to the focusing of disorganized light rays into a parallel beam. In electronics this might be achieved with a single element plastic lens, which is a convex lens with short focal length. Diode laser modules utilize aspheric lenses; that is, they are not spherical.

In optical fiber networks, the diode bar is produced as a fiber-coupled component. This permits greater utilization of output and the ability to distance the module and cooling system from a laser hot point, such as a diode-pumped laser head. It is typically coupled with single and multimode fibers, or bundled with a single fiber per emitter.

Standard and custom wavelengths are possible with these components. Diode bar units come available in numerous stripe and bar widths and cavity lengths. Safety precautions are warranted, especially with collimated beams, as these can cause harm to vision. Lasers with output higher than 3 to 5 mW, or with wavelengths near the visible spectrum, present the greatest hazards. Beams should be calibrated indirectly using a tester, a laser power meter, IR detector, or even reflected off a card.

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