broad area laser diodes (original) (raw)

Definition: laser diodes with a strongly asymmetric shape of the emitting region

Alternative terms: broad stripe emitters, broad emitter laser diodes, multimode single-emitter laser diodes, high-brightness diodes

Category: optoelectronics

• semiconductor lasers
  • laser diodes
    * broad area laser diodes
    * high-brightness laser diodes
    * diode bars
    * diode stacks
    * tapered laser diodes
    * Fabry–Pérot laser diodes
    * photonic crystal surface-emitting lasers

Related: laser diodesdiode barssemiconductor lasersbeam qualitytapered amplifiershigh-brightness laser diodes

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📦 For purchasing broad area laser diodes, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.

Contents

What are Broad Area Laser Diodes?

Broad area laser diodes (also called broad stripe, multimode single emitters or broad emitter laser diodes, single-emitter laser diodes, and high brightness diode lasers) are edge-emitting laser diodes where the emitting region at the front facet has the shape of a broad stripe (see Figure 2), with dimensions of e.g. 1 μm × 100 μm. Due to the asymmetry of the emitter, the beam properties are also completely different for the two directions:

• In the vertical (short) direction, the height (e.g. 1 μm) is small enough to obtain single-mode guidance and thus an essentially diffraction-limited beam quality with an _M_2 factor only slightly above 1. Because of the small aperture size, the beam divergence in this direction is relatively high, with a beam divergence half-angle of e.g. 370 mrad, corresponding to an FWHM angular range of 25°. Due to that fast divergence, this is called the fast axis direction.
• In the long direction (slow axis direction), the stripe width may be e.g. 50, 100, 200 μm, or even larger, so that the light is distributed over many spatial modes in this direction. As a result, the beam divergence is much larger than for a diffraction-limited beam with that size, although still significantly smaller than for the fast axis direction. (Typical values are around 5–10° FWHM.) The beam quality in terms of focusability is reduced; the _M_2 factor might be of the order of e.g. 20 for a 100-μm stripe. Furthermore, the beam profile may be multi-peaked in the horizontal direction, and the shape of the intensity pattern may depend on the injection current.

Figure 1: Evolution of the beam radii of the output of a broad area laser diode in “fast” and “slow” direction. The beam radius starts from a much smaller value in the fast axis direction, but increases rapidly.

The wavefronts at the output facet are approximately plane in horizontal and vertical direction, but there can be some astigmatism, i.e., a slightly different focus position for the two directions.

Figure 2: Broad area laser diode.

The broader the stripe, the higher is the achievable power, but the worse is the beam quality in the “slow” direction. The technological trend is to obtain increasingly high powers even from narrow stripes, but this is limited by the high optical intensity at the front facet (which can lead to catastrophic optical damage) and by thermal issues. Special techniques of facet passivation can be used to allow for higher powers. For a 100 μm wide aperture, the output power of a commercial device (e.g. in the 0.8-μm wavelength region) is typically a couple of watts or up to the order of 10 W. Similar devices are also available in other wavelength regions, e.g. around 480 nm (blue) or 1550 nm, but typically with lower performance.

The laser resonator is in most cases monolithic, with reflections from dielectric coatings on the facets of the semiconductor chip. In less common cases, a gain chip with an anti-reflection coating on one side is used in an external laser resonator (→ external-cavity diode lasers).

Asymmetric Beam Properties

The strongly asymmetric beam profile and the large divergence in the “fast” direction requires special care, e.g. for properly collimating the output of a broad area laser. A common method is the use of a cylindrical “fast axis collimator” lens with high numerical aperture in close proximity to the diode facet. Such a lens collimates the beam in the fast axis direction, before the beam radius becomes too large. A second cylindrical lens at a larger distance may then be used for collimation in the slow axis direction. By choosing lenses with suitable focal lengths, a circular beam can be obtained, which however will have different divergence angles in the two directions due to the different beam quality values.

Figure 3: Stacked beamlets from multiple broad area emitters after a suitable beam shaper — with one column (left) or two columns (right).

Much higher powers (hundreds of watts or even several kilowatts) can be launched into a multimode fiber with given parameters when using multiple (e.g. 10 to 20) broad area emitters, the outputs of which are properly combined with suitable optics (Figure 3). Here, one stacks the elongated emission patterns of such diodes such that overall a roughly quadratic area is filled with radiation, the shape of which fits better to the circular cross-section of a multimode fiber core. In the case of a large number of emitters, one may arrange the emitters in two columns.

Special Features

Some diode bars are offered with special features:

• A fast axis collimation lens may be integrated into the laser package. This is helpful, since the alignment tolerance for external mounting would be tight. There are also laser diode modules with built-in beam collimation in both directions.
• A built-in Bragg grating can stabilize the emission wavelength and make the emission spectrum significantly narrower. The same could also be achieved with an external volume Bragg grating.
• There are also tapered laser diodes, having a region where the width and thus the area of the active region is significantly increased along the propagation direction. Due to a straight region with smaller width, the beam quality and radiance (brightness) achieved are better than in a laser diode with the maximum width along the whole active region. Therefore, such diodes are often called high-brightness laser diodes.

Tapered Amplifier Devices

As an alternative to a broad area diode laser, one can use a tapered amplifier either as part of a MOPA device or as an external-cavity diode laser. With that technology, one can obtain similar output powers in conjunction with much better beam quality.

Comparison with Diode Bars

The combination of several broad area emitters in a single device leads to a diode bar, which can emit tens of watts or even more than 100 W of optical power. However, a diode bar has a lower brightness than a single-emitter laser, despite the higher output power because the beam quality in the horizontal direction is much lower. For that reason, the design of a diode-pumped laser is generally simpler when using broad-area diodes for pumping. Even for high-power lasers, including high-power fiber lasers and amplifiers, pumping with a significant number of broad-area lasers instead of fewer diode bars has some advantages. One of those is that broad-area diode lasers, other than diode bars, can usually be turned on and off fairly often without shortening the lifetime.

Applications of Broad Area Laser Diodes

Broad area laser diodes are often used for pumping solid-state lasers. A device with a 100-μm or 200-μm broad emitter may easily emit several watts. The laser diode is often mounted on a thermoelectric cooler, which makes it possible to tune the emission wavelength within a few nanometers, so that the emission peak can be matched to the absorption maximum of the laser crystal.

One can also use such diodes for pumping fiber lasers. In other cases, diode bars are used, containing multiple broad area emitters.

Another important application area is laser material processing, see the article on direct diode lasers.

Frequently Asked Questions

What is a broad area laser diode?

A broad area laser diode is an edge-emitting laser diode where the light-emitting region at the facet has a wide, stripe-like shape, such as 1 µm high and 100 µm wide. This geometry allows for high optical output power from a single semiconductor chip.

Why does a broad area laser diode have an asymmetric beam?

The beam is asymmetric because of the emitter's rectangular shape. The narrow dimension (fast axis) is single-mode, causing a highly divergent but high-quality beam. The wide dimension (slow axis) is highly multimode, resulting in lower beam quality and slower divergence.

What are the 'fast axis' and 'slow axis' of a broad area diode laser?

The 'fast axis' corresponds to the narrow dimension of the emitter, where the beam diverges very rapidly. The 'slow axis' corresponds to the wide dimension, where the beam diverges more slowly but has a much lower beam quality (a high M2 value).

What is the typical output power of a broad area laser diode?

A commercial broad area laser diode with a 100-µm-wide emitter can typically produce an output power of several watts, up to roughly 10 W. The maximum power is limited by potential optical damage to the facet and by thermal effects.

How do broad area diodes differ from diode bars?

A broad area diode is a single light emitter, whereas a diode bar is a linear array of multiple emitters on a single chip. While a diode bar offers much higher total power, a single broad area emitter provides higher brightness (radiance) due to its superior beam quality.

What are the main applications of broad area laser diodes?

They are frequently used for pumping solid-state lasers and fiber lasers due to their high power and brightness. Another major application is direct material processing, such as in welding or soldering, where they are used in direct diode laser systems.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 19 suppliers for broad area laser diodes. Among them:

⚙ hardware

Innolume’s broad area (BA) laser diodes are high-power, multimode light sources designed for demanding photonic applications, offering optical output powers of up to 15 W from a single emitter. These devices are available in C-mount, submount configurations, and as bars.

All devices are optimized for effective thermal management through p-side down soldering, ensuring minimal thermal resistance and reliable operation at high current densities. Innolume’s BA laser diodes are customizable in both aperture width (up to 250 µm) and cavity length (up to 5 mm).

We deliver flexible, scalable solutions for applications requiring high brightness and beam shaping, including materials processing, pumping, and free-space optics.

⚙ hardware

Serving North America, RPMC Lasers offers a selection of broad area laser diodes among our semiconductor laser diodes from UV (375 nm) to LWIR (17 µm) with the widest wavelength and package range, from mW TO-cans to kW fiber-coupled turnkey systems, covering components to subsystems.

Our diodes feature free-space or fiber-coupled outputs with connector/fiber options, multimode, single mode, polarized, low noise, stabilized, SLM, narrow linewidth, and single-frequency configs.

Tailored solutions from wafers and chips to OEM and turnkey systems, including single unit, ultra-compact, flexible multi-wavelength combiners with up to 4 diode laser wavelengths, USB-powered, for lightweight, handheld and portable applications or even for lab use.

Let RPMC help you find the right laser today!

⚙ hardware

The Lumics multi-mode fiber-pigtailed flat-pin diode laser module features an optimized GaAs-based quantum well high-power laser diode, meeting stringent reliability requirements. Available in wavelengths of 785, 808, 915, 940, 975, and 1064 nm, and compatible with 50, 105, and 200 µm core NA 0.12—0.22 fibers, these modules are sealed with a single emitter. Each module is serialized for traceability and shipped with specific test data, ensuring high reliability and easy integration. Ideal for applications in pumping, material processing, illumination, and medical laser therapy, these modules offer long lifetime and cost-effective solutions.

⚙ hardware

QPC Lasers manufactures diode lasers with the highest powers and brightness in the industry at wavelengths ranging from 780 to 2000 nm.

Full vertical integration from epitaxy through packaging allows us to offer standard and custom diode solutions in packages ranging from C-mounts to complete OEM light engines that provide performance without compromise. Features include unique Brightlock monolithic "on-chip" wavelength control for unmatched linewidth and spectral control.

⚙ hardware

Broad area lasers operate spatially and longitudinally multimode. They are used for solid-state laser pumping, sensor technology, material processing, medical applications (e.g. photodynamic therapy), as well as in scientific research. eagleyard offers broad area lasers at wavelengths between 670 and 1120 nm and output powers between 1 and 18 W in cw mode. Stripe widths from 60 μm to 400 μm are available to optimize beam structure and power for your application.

Bibliography

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