Optics Design and Diode Lasers

Diode laser modules are used as pump sources for solid-state and fiber lasers, to amplify signals in telecommunications as well as in direct materials processing (e.g. plastics welding). The main advantages of diode laser modules are their high electro-optical efficiency, compact size and cost-efficiency.

At the Fraunhofer ILT, diode laser modules are designed and built according to our customer’s specific requirements. Among these count fiber coupling concepts for multimode or single mode radiation as well as frequency stabilization and incoherent spectral scaling of output power (Dense Wavelength Multiplexing). In addition to optics design, including tolerance analysis, we emphasize thermal and mechanical design as well as assembly engineering all the way to fully automatic adjustment of optical components.

By means of various measuring and testing procedures, we investigate assembled high-performance diode laser bars on their electro-optical properties and identify how the structural design engineering influences their efficiency:

• Electro-optical characterization: The relevant characteristic data of the diode laser are recorded in dependence upon the operating current. From these measurements further data are derived, for example, the thermal resistance of the diode laser.
  
• Space-resolved characterization: Individual emitters of the diode laser bar can be characterized regarding power density, wavelength and polarization. These results enable conclusions to be made on the thermal and thermomechanical influence of the structural design engineering.
  
• Measurement of the spatial power density distribution: Beam profiles can be determined by using a fiber sensor (NFSM). The measurement can also be conducted for extended distributions and maximum output power.
  
• Measurement of the angular power density distribution: The power density distribution of diode laser bars is determined in dependency upon the radiation angle.
  
• Determination of the diode laser smile: The deformation of the diode laser is determined with a resolution in the range of sub micrometers. The results enable an evaluation to be made of the structural design engineering as well as of the quality of the microscopic components used.

In addition to the classical requirements upon the design of optical systems, such as diffraction-limited imaging and focussing, the generation of process-adapted power density distributions is gaining significance. Intelligent optical systems can be generated by connecting conventional optics with innovative beam forming elements and open up a wide range of applications. The implementation of these designs in robust and industry-ready prototypes rounds off the development of optical systems.

On the basis of long-term experience, we develop for our customers tailor-made optical systems that can be applied in materials processing, illumination and as pump sources for solid-state lasers. To realize these systems, our highly qualified employees have a comprehensive technical infrastructure at their disposal.

Freeform optics pass beyond the limits of classical optics and set a new trend in the sector of optics design. They cannot be described analytically, but rather can only be defined by the local interpolation of control points. In particular, in the sector of LED illumination applications, this technology has found great interest. Numerically complex optimization algorithms are at the center of this development at the Fraunhofer ILT. Moreover, we also utilize freeform surfaces in laser systems.

We offer a comprehensive package to develop customer-specific prototypes ready for series production:

• Dimensioning of classical optical systems using sequential ray tracing
  
• Non-sequential ray tracing and tolerance analysis of high-performance diode lasers, based on measuring farfield distributions
  
• New concepts to symmetrize the beam parameter products by using beam transformation
  
• One and two-dimensional homogenization of high-performance laser emission
  
• Micro optics for beam forming
  
• Frequency stabilization and spectral superposition
  
• Error and risk analysis including CE certification of developed prototypes