The performance of diode lasers in the visible spectral range has been continuously improved within the last few years, which was mainly driven by the goal to replace arc lamps in cinema or home projectors. In addition, the availability of such high power visible diode lasers also enables new applications in the medical field, but also the usage as pump sources for other solid state lasers. This paper summarizes the latest developments of fiber coupled sources with output power from 1.4 W to 120 W coupled into 100 μm to 400 μm fibers in the spectral range around 405 nm and 640 nm. New developments also include the use of fiber coupled multi single emitter arrays at 450 nm, as well as very compact modules with multi-W output power.
We present a direct diode laser with an optical output power of more than 800 W ex 100 μm with an NA of 0.17. The system is based on 6 commercial pump modules that are wavelength stabilized by use of VBGs. Dielectric filters are used for coarse and dense wavelength multiplexing. Metal sheet cutting tests were performed in order to prove system performance and reliability. Based on a detailed analysis of loss mechanisms, we show that the design can be easily scaled to output powers in the range of 2 kW and to an optical efficiency of 80%.
We report on wavelength stabilized high-power diode laser systems with enhanced spectral brightness by means of Volume Holographic Gratings. High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser systems is an important method to increase the efficiency of diode pumped solid-state lasers. It also enables power scaling by dense wavelength multiplexing. To ensure a wide locking range and efficient wavelength stabilization the parameters of the Volume Holographic Grating and the parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the Volume Holographic Grating, the reflectivity of the diode laser bar as well as its angular and spectral emission characteristics. In this paper we present detailed data on wavelength stabilized diode laser systems with and without fiber coupling in the spectral range from 634 nm up to 1533 nm. The maximum output power of 2.7 kW was measured for a fiber coupled system (1000 μm, NA 0.22), which was stabilized at a wavelength of 969 nm with a spectral width of only 0.6 nm (90% value). Another example is a narrow line-width diode laser stack, which was stabilized at a wavelength of 1533 nm with a spectral bandwidth below 1 nm and an output power of 835 W.
The Beam Parameter Product (BPP) of a passive, lossless system is a constant and cannot be improved upon but the
beams may be reshaped for enhanced coupling performance. The function of the optical designer of fiber coupled diode
lasers is to preserve the brightness of the diode sources while maximizing the coupling efficiency. In coupling diode
laser power into fiber output, the symmetrical geometry of the fiber core makes it highly desirable to have symmetrical
BPPs at the fiber input surface, but this is not always practical. It is therefore desirable to be able to know the 'diagonal'
(fiber) BPP, using the BPPs of the fast and slow axes, before detailed design and simulation processes. A commonly
used expression for this purpose, i.e. the square root of the sum of the squares of the BPPs in the fast and slow axes, has
been found to consistently under-predict the fiber BPP (i.e. better beam quality is predicted than is actually achievable in
practice). In this paper, using a simplified model, we provide the proof of the proper calculation of the diagonal (i.e. the
fiber) BPP using BPPs of the fast and slow axes as input. Using the same simplified model, we also offer the proof that
the fiber BPP can be shown to have a minimum (optimal) value for given diode BPPs and this optimized condition can
be obtained before any detailed design and simulation are carried out. Measured and simulated data confirms satisfactory
correlation between the BPPs of the diode and the predicted fiber BPP.