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CONTRACT NUMBER Wavelength Beam-Combined Laser Diode Arrays 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Massachusetts Institute of Technology,Lincoln Laboratory,244 Wood REPORT NUMBER Street,Lexington,MA,02420-9108 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Same as 2 unclassified unclassified unclassified Report (SAR) Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 2012 | www.ll.mit.edu Tech Notes Wavelength The wavelength beam-combining a spot d = D θ/Φ where Φ is the focus- fiber-coupled diode laser is the first ing angle. The local intensity, I = P/d2, direct-diode laser that is bright enough can then be expressed as I = P (Φ/(Dθ))2 Beam-Combined to cut and weld metal. Although diode = BΦ2 where B = P/(Dθ)2 is defined as lasers are the highest efficiency lasers, the laser beam brightness. An optical Laser Diode until recently their brightness has been system with a given focusing angle Φ relatively low compared to that of will then allow for local intensity pro- Arrays other kW-class lasers used in industrial portional to the laser brightness. Bright- applications. Lincoln Laboratory’s fun- ness (not just power) determines the damental breakthrough in wavelength achievable intensity. beam-combining (WBC) has enabled Wavelength beam combin- the development of a system, shown in The Wavelength ing of diode lasers produces Figure 1, that has the laser brightness Beam-Combining Solution necessary for industrial use. WBC spatially merges multiple wave- the intensity and brightness length sources into a single high-inten- necessary for metal cutting Intensity versus Brightness sity beam with an order-of-magnitude Laser cutting and welding require that improvement in brightness compared and welding that previously the intensity of the focused laser beam to a single source. The fundamental be sufficiently high so as to raise the building block of fiber-coupled diode required more powerful and temperature and locally melt the metal. lasers is the diode-laser bar. A diode- more expensive laser systems. A collimated laser beam is character- laser bar typically consists of an array ized by its power P, its beam diam- (e.g., 19 elements over a typical 1 cm eter D (matching the size of the optical length) of electrically driven, edge- aperture), and its angular divergence, θ, emitting, semiconductor lasers. in the far field. It is a well-known fact Conceptually, WBC can be thought that as a laser beam is transformed by of as the spatial superposition of many an optical system, the product Dθ is independent diode-laser external cavi- conserved; in going through the focus- ties. For simplicity, only three indepen- ing optics, the beam is focused down to dent external cavities are shown in Technical Point of Contact Antonio Sanchez-Rubio Laser Technology and Applications Group [email protected] 781-981-7821 For more information, contact: Communications and Community Outreach Office MIT Lincoln Laboratory 244 Wood Street Figure 1. The wavelength beam-combining fiber-coupled diode laser head developed by Tera- Lexington, MA 02420-9108 Diode is shown cutting through 0.26 inch steel. TeraDiode was formed in 2009 as a spin-off from 781-981-4204 Lincoln Laboratory. f f Folding Diode Current lead mirrors array Microlens Lens with Output array focal length f port Water in/out Diffraction Concave mirror Grating grating (in lieu of lens) Output Figure 3. Lincoln Laboratory-designed WBC “laser in a box.” To coupler reduce the overall size of the WBC device, multiple folding mirrors Figure 2. Schematic diagram of wavelength beam combining. were implemented between the diode array and the concave mirror. Figure 2. Each source is illustrated with entire system is mounted in a standard brightness beams (high power with a different color (red, green, and blue), 19-inch rack. The laser is housed in a near diffraction-limited beams). When which indicates a distinct wavelength stainless steel enclosure. The system compared to these lasers, the WBC characteristic to that external-cavity- also includes the chiller and plumbing direct-diode technology offers a num- stabilized diode laser element. Each manifold, control computer, power sup- ber of improvements: cavity consists of an antireflection- plies, power conditioning/distribution • Higher efficiency when compared to coated and lensed laser gain element, unit, and emergency stop switch. The diode-pumped lasers (fiber, solid state) a transform lens, a diffraction grating, output of the diode laser is coupled to • Graceful degradation when some and an output coupler. Figure 3 shows a 200 µm core processing fiber, which individual diodes age—no single- a tabletop version of the optical system has end connectors that are compatible point of failure The diffraction grating has an angle- with LLK-B industrial fiber connectors. • Ability to withstand unplanned (but to-wavelength conversion property that The output end of the LLK-B fiber is typical) optical feedback—will not allows feedback to each diode-laser coupled to a processing head, which fail if optical feedback occurs from element in the array via the transform can be configured for cutting or weld- the metal sample being cut lens at a different wavelength. Exter- ing processes. • Relative lower cost nal cavity feedback is provided by the • Compact size output coupler mirror. WBC allows for Industrial Applications • Wavelength-versatility—may be brightness scaling of a diode-laser array Multi-kilowatt-class lasers are used in important depending on the specific because all of the diode-laser elements cutting, welding, and other industrial welding/cutting details (both the are spatially overlapped at the output applications where the high-intensity materials and the gas environment coupler, maintaining the output beam laser beam is used to locally raise the surrounding the beam) quality of a single element while scal- temperature of the material. These The WBC technology combines high ing the output power by the number of kW-class lasers include CO, fiber, bulk brightness, efficiency, and reliability 2 elements in the array. solid-state lasers, and disk lasers. with the power to perform the func- As the diode-laser array size In the past, individual diode lasers, tions of current industrial lasers. Direct- increases, both the power and bright- while having a number of attractive diode lasers using WBC technology ness of the output beam increase. Ulti- features (they are compact, low-cost, may also, in time, replace fiber, disk, mately, the number of elements that can reliable, and wavelength-versatile), and other lasers for the most demand- be combined is limited by the overall were limited in power and brightness. ing material-processing applications. ■ bandwidth of the semiconductor gain Individual diode lasers are limited media and the bandwidth allocation in power to the ~1 W class, and con- per element (grating dispersion and ventional diode laser arrays, used for resolution). For a typical semiconduc- power scaling, do not provide beams tor gain bandwidth of ~40 nm, an array with the required brightness. with hundreds of elements could be Compared to other direct-diode wavelength beam combined. technologies in the kW-class, the WBC approach provides the very significant Commercialization advantage of an order of magnitude The technology described above has increase in brightness over other direct- been utilized by TeraDiode to develop diode products. This high brightness the first direct-diode lasers that are enables the WBC technology to cut and This work is sponsored by the High Energy Laser— Joint Technology Office, under Air Force contract bright enough to cut and weld metal. weld metals. FA8721-05-C-0002. Opinions, interpretations, con- The 1 kW TeraDiode laser system cut- Currently, CO, fiber, and solid-state 2 clusions, and recommendations are those of the ting head is shown cutting through lasers are used for industrial applica- author and are not necessarily endorsed by the United 0.26 inch steel plate in Figure 1. The tions because they can generate high- States Government.