Visible diode lasers are the subject of intense research. Red and blue diode lasers are used as an alternative to arc lamps in cinema or home projectors and facilitate the development of mobile projectors for use in smartphones. The development of high power diode lasers in the blue and red spectral region opens the door for new applications, for instance medical, pumping and LIDAR applications.
To meet the requirements of these new applications, DILAS has created a host of fiber coupled and free space lasers in the blue and red spectral range. The proven T-Bar concept devised at DILAS is used in the red spectral range. In the blue spectral range, high power output is enabled by coupling single emitters side by side.
A modular combining concept for packaged single emitters developed at DILAS allows for output powers as high as 100W from a 400µm fiber. Beam sources of 10W and 25W based on this concept were recently launched into the marketplace.
Laser Module Concepts
Fiber Coupled 100W Red Laser Diode Module
In this 638nm laser diode module, the beam source relies on two electro optical base plates, which integrate up to seven laser diode bars and collimation and stacking optics. These plates also include a cooling structure to remove heat efficiently.
The completely automatic assembly of base plates and alignment of all optics enables cost-efficient mass production. Two of these base plates are assembled together in a turnkey package designed for IR direct diode applications (Figure 1).
Figure 1. Package for coupling of 2 T-bar base plates into a single fiber. The laser module has a size of 200x200x50mm3.
Figure 2 shows the beam path of the polarization and fiber coupling section. The two plates’ beam paths have equal length and are redirected towards a thin film polarizer (TFP) using folding mirrors.
An imaging and magnifying relay optic is used to address the problem due to introduction of substantial astigmatism caused by the combination of the high slow axis divergence and long beam path and to adapt the different divergence angles in the fast and slow axis.
Figure 2. Simulated beam path from exit of the base plates to the fiber. On the top right the exit of the two base plates is shown in different beam color.
The imaging and magnifying relay optic is interleaved with the TFP and contains three plano-convex cylindrical lenses, of which two before TFP to form an intermediate image and one after the TFP for the combined beam.
This combined beam recollimates the beam and creates a magnified image of the collimated laser diode bars before the fiber coupling lens. The fiber coupling optics finally comprises a single aspheric lens.
Fiber Coupled Blue Laser Diode Module
The beam quality of an existing advanced single emitter at 450nm is roughly 1-2mm mrad in the slow axis, thereby enabling to integrate over 100 emitters into a 400µm NA0.22 fiber in principle.
However, it is practically difficult to cost effectively align several pre-packaged single emitters for highly efficient fiber coupling. A new submodule with 6 single emitters that are aligned along their fast axis was developed for improving the beam quality (Figure 3).
Figure 3. Submodule for 6 pre-packaged single emitters on a water cooled heatsink for up to 4 submodules.
This module contains six diodes and an aspheric collimation lens for each diode, which are attached to a common heat sink. A turning prism for each single emitter facilitates accurate pointing alignment for each single laser beam and allows increasing fill factor.
It is possible to fit up to four of these modules onto a water cooled heat sink by coupling side by side along the fast axis. This yields a total raw output power of up to 38W at 1.6W per single emitter.
The raw beam obtained has an asymmetrical far field divergence and enables coupling to a 200µm fiber with two cylindrical lenses. Modules with 10W and 25W Output power were constructed based on this concept (Figure 4).
Figure 4. Single emitter module for up to 24 single emitters. The module has a size of 120x150x65mm3.
Performance Results of Fiber Coupled Red Laser Diode Modules
The output power and power conversion efficiency (PCE) results measured for the 100W module are presented in Figure 5. The targeted output power of 100W is attained at a current of 14A.
A total coupling efficiency of 75% is estimated from the raw output power of a single base plate (Figure 6). The PCE achieved at 100W output power and 20°C cooling plate temperature is 24%, which is only 3% less than the previously published data for a fiber coupled single base plate at 50W output power and exhibits the good combining efficiency of polarization coupling.
Figure 5. Output power from a 400µm NA0.22 fiber and power conversion efficiency of the 100W 638nm laser source.
Figure 6. Raw output power from a typical single base plate.
The remaining losses are predominantly caused by Fresnel losses at the uncoated fiber ends and a larger BPP of the single base plate in the slow axis when compared to the fiber coupled single base plate at 50W output power.
The latter problem could be addressed if 50µm fiber is acceptable in a specific application. The wavelength spectrum of the 100W module is depicted in Figure 7. A line width of 1.7nm is attained.
Figure 7. Wavelength spectrum of the 100W 638nm laser source at 80W output power.
A prototype module for combining one base plate to a 200µm NA0.22 fiber was constructed and characterized. The output power obtained and PCE are delineated in Figure 8. Although the polarization coupling is good for low power losses, there is a considerable degradation in coupling efficiency when compared to the 400µm 50W and 100W modules.
This is because the beam parameter product (BPP) in the slow axis is only halved, but remains the same in the fast axis, causing increased coupling losses in the diagonal axis.
Figure 8. Output power from a 200µm NA0.22 fiber and power conversion efficiency of the one base plate polarization coupled module.
It is possible to optimize the position of the polarization coupling optics and the second telescope by utilizing a different housing, which gives additional space for the optics.
In spite of this optimization, potential over 40W in the 200µm NA0.22 fiber with a power conversion efficiency of 20% is attained.
Performance Results of Fiber Coupled Blue Laser Diode Modules
The output power and efficiency of the blue laser module with 24 single emitters connected to a 400µm NA0.22 fiber is illustrated in Figure 9, achieving an output power of 29W at a nominal current of 1.2A and optical coupling efficiency of 84%.
The remaining coupling losses are predominantly due to Fresnel losses at the uncoated fiber ends and absorption, scattering and reflection losses at the optical elements within the module.
Figure 9. Output power and efficiency of The blue laser module with 24 single emitters coupled into a 400µm NA0.22 fiber
The PI curve and power conversion efficiency results of a 10W module designed for 200µm fiber coupling are presented in Figure 10. The WPE is decreased to 18%, a decrease of 3% when compared to 400µm fiber coupling.
Nevertheless, a 10% higher coupling efficiency is predicted when considering the design of the module and single diode measurements. The unclear additional losses in coupling efficiency may be due to stray light owing to aberrations or the single emitters themselves.
Figure 10. Output power and efficiency from a module with 12 single emitters from a 200µm NA0.22 fiber.
Polarization coupling helped achieving a module with over 100W output power at 639nm from a 400µm NA0.22 fiber and more than 40W from a 200µm NA.022 fiber. Advances in packaging enable fiber coupled modules in the blue spectral range of up to 25W, which can be integrated to a 200µm fiber. Wavelength selected modules can be employed to get customized wavelength distributions for specific requirements.
The developed scalable and modular diode laser concepts can be employed to further increase power and brightness of fiber coupled visible diode lasers if required. All blue laser modules are still polarized, meaning that the brightness can be doubled instantaneously by polarization combining.
This information has been sourced, reviewed and adapted from materials provided by DILAS Diode Laser.
For more information on this source, please visit DILAS.