broad area laser diodes (original) (raw)
Author: the photonics expert
Definition: laser diodes with a strongly asymmetric shape of the emitting region
Alternative terms: broad stripe emitters, multimode single emitters, high-brightness diodes
More general term: laser diodes
optoelectronicsDOI: 10.61835/bx0 [Cite the article](encyclopedia%5Fcite.html?article=broad area laser diodes&doi=10.61835/bx0): BibTex plain textHTML Link to this page LinkedIn
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 higher and higher powers even from narrow stripes, but this is limited by the high optical intensity at the front facet (which can lead to catastrophic failure) 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 end faces 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 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.
More to Learn
Encyclopedia articles:
Bibliography
| [1] | R. J. Lang et al., “Spontaneous filamentation in broad-area diode laser amplifiers”, IEEE J. Quantum Electron. 30 (3), 685 (1994); https://doi.org/10.1109/3.286155 |
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| [2] | H. Adachihara et al., “Spatiotemporal chaos in broad-area semiconductor lasers”, J. Opt. Soc. Am. B 10 (4), 658 (1993); https://doi.org/10.1364/JOSAB.10.000658 |
| [3] | A. Jechow, V. Raab and R. Menzel, “Tunable 6.8 W narrow bandwidth emission from a single-stripe continuous-wave broad-area laser diode in a simple external cavity”, Appl. Opt. 47 (10), 1447 (2008); https://doi.org/10.1364/AO.47.001447 |
| [4] | J. Piprek, “Inverse thermal lens effects on the far-field blooming of broad area laser diodes”, IEEE Photon. Technol. Lett. 25 (10), 958 (2013); https://doi.org/10.1109/LPT.2013.2255590 |
| [5] | V. Svetikov et al., “Selection of high-order lateral modes in broad area laser diode by digital planar hologram”, J. Opt. Soc. Am. B 30 (3), 610 (2013); https://doi.org/10.1364/JOSAB.30.000610 |
| [6] | M. Niebuhr et al., “Mode stabilization of a laterally structured broad area diode laser using an external volume Bragg grating”, Opt. Express 23 (9), 12394 (2015); https://doi.org/10.1364/OE.23.012394 |
| [7] | L. Wang et al., “Loss tailoring of high-power broad-area diode lasers”, Opt. Lett. 44 (14), 3562 (2019); https://doi.org/10.1364/OL.44.003562 |
| [8] | L. Wang et al., “High-power laser diode at 9xx nm with 81.10% efficiency”, Opt. Lett. 47 (13), 3231 (2022); https://doi.org/10.1364/OL.452048 |
(Suggest additional literature!)
Suppliers
The RP Photonics Buyer's Guide contains 18 suppliers for broad area laser diodes. Among them:
RPMC Lasers
Serving North America, RPMC Lasers offers one of the widest wavelength selections of broad area laser diodes available, ranging from the UV to the LWIR regimes with up to 3000 W output power. Our multimode laser diode offerings include single emitters, multi-emitters, stacks, arrays (bars), volume Bragg gratings, laser diode modules, and multi-wavelength laser diodes, available with free-space and fiber-coupled output options and many packaging options including, but not limited to chip on carrier, TO can, HHL, butterfly, MCC, and turn-key systems. Standard and custom options available. Let RPMC help you find the right laser today!
QPC Lasers
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.
eagleyard Photonics
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.
Lumics
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.
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