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Through the systematic exploitation of Quantum mechanical effects, we will experience technological revolutions in 21st century. With the foundation of Q.ANT, we have made it our mission to make a significant contribution to this revolution.

Magnetic field sensors that exploit quantum effects have shown that they can outperform classical sensors in terms of sensitivity enabling a range of novel applications in future, such as a brain machine interface. Negatively charged nitrogen‐vacancy (NV) centers in diamond have emerged as a promising high sensitivity platform for measuring magnetic fields at room temperature. Transferring this technology from laboratory setups into products and applications, the total size of the sensor, the overall power consumption, and the costs need to be reduced and optimized. Here, a fiber‐based NV magnetometer featuring a complete integration of all functional components is demonstrated without using any bulky laboratory equipment. This integrated prototype allows portable measurement of magnetic fields with a sensitivity of 344 pT Hz−1/2.

We present a compact, modular and cross talk free approach for dense wavelength division multiplexing of high power diode lasers based on ultra-steep dielectric filters. The mini bars consist of 5 narrow stripe broad area emitters with a beam parameter product in the range of 2 mm mrad and a wavelength spacing of 2.5 nm between 2 adjacent emitters. Experimental results for fiber coupling (35 μm core diameter, NA < 0.2) of internally and externally stabilized diode lasers are presented. Optical losses are analyzed and alternative optical designs to overcome the current limitations of the setup are discussed.

Multiplexing technologies enable the development of high-brightness diode lasers for direct industrial applications. We present a High-Power Dense Wavelength Division Multiplexer (HP-DWDM) with an average channel spacing of 1.7 (1.5) nm and a subsequent external cavity mirror to provide feedback for frequency stabilization and multiplexing in one step. The "self-optimizing" multiplexing unit consists of four reflective Volume Bragg Gratings (VBGs) with 99% diffraction efficiency and seven dielectric mirrors to overlay the radiation of five input channels with an adjustable channel spacing of 1-2 nm. In detail, we focus on the analysis of the overall optical efficiency, the change of the beam parameter product and the spectral width. The performance is demonstrated using five 90 μm multimode 9xx single emitters with M2≤17. Because of the feedback the lateral (multimodal) spatial and angular intensity distribution changes strongly and the beam parameter product decreases by a factor of 1.2 to 1.9. Thereby the angular intensity distribution is more affected than the width of the beam waist. The spectral width per emitter decreases to 3-200 pm (FWHM) depending on the injection current and the reflectance of the feedback mirror (0.75%, 1.5%, 4%, 6% or 8%). The overall optical multiplexing efficiency ranges between 77% and 86%. With some modifications (e.g. enhanced AR-coatings) we expect 90-95%.

The combination of a dense wavelength division multiplexer based on volume Bragg gratings (VBGs) and a subsequent feedback mirror forms an efficient single-stage frequency stabilization and multiplexing system. The laser emission of the connected diode laser sources is automatically forced to amplify the wavelengths with the maximum feedback and the minimum losses, respectively. Therefore, different transversal modes of a single emitter may be linked to different wavelength regions that fulfill the Bragg conditions of the VBGs with high efficiency. We demonstrate a multiplexing system with a channel spacing of 1.5 nm and an optical efficiency of 86%, up to 90% being feasible with modifications. The emission bandwidth of the diode laser single emitters is reduced to

We present a measurement platform to characterise combining elements used for dense wavelength division multiplexing, such as dielectric edge filters or Volume Bragg Gratings (notch filters). This platform enables the characterisation of the spectral and angular selectivity, the homogeneity of reflectance or transmittance and temperature-induced spectral shift of the band edge. The determination of the diffraction efficiency, the reflectance and the transmittance is carried out for both polarisations with high accuracy.

We present the improvement of the lateral beam quality due to frequency stabilisation in a Dense Wavelength Division Multiplexing setup using Volume Bragg Gratings. The spectral width of each frequency stabilised channel amounts to less than 300 pm and an overall multiplexing efficiency of 86 % has been measured. The near and far field intensity distributions consist of discrete modes and were observed with a post positioned beam analysis module. When operated with frequency stabilisation, the lateral beam quality factor M2 of the single emitters decreased by a factor of 1.4 on average compared to the free running source.

High powers can be produced within narrow spectral widths by stabilizing diode lasers with external volume holographic gratings, but this typically introduces additional optical losses. We compared the influence of diode laser design on stabilization performance by comparing devices with super large optical waveguides (4.8 $\mu{\rm m}$) and narrow vertical far fields with reference designs with thinner waveguides (1.6 $\mu{\rm m}$). We found that the use of diode lasers with super large optical waveguides substantially improves the stabilization performance, with lower losses and wider operation ranges sustained even in the presence of significant $({}{\rm 1}~\mu{\rm m})$ bar smile.

Spatial and spectral emission characteristics and efficiency of high-power diode laser (HPDL) based pump sources enable and define the performance of the fundamental solid state laser concepts like disk, fiber and slab lasers. HPDL are also established as a versatile tool for direct materials processing substituting other laser types like CO2 lasers and lamp pumped solid state lasers and are starting to substitute even some of the diode pumped solid state lasers. Both, pumping and direct applications will benefit from the further improvement of the brightness and control of the output spectrum of HPDL. While edge emitting diodes are already established, a new generation of vertical emitting diode lasers (VCSELs) made significant progress and provides easy scalable output power in the kW range. Beneficial properties are simplified beam shaping, flexible control of the temporal and spatial emission, compact design and low current operation. Other characteristics like efficiency and brightness of VCSELs are still lagging behind the edge emitter performance. Examples of direct applications like surface treatment, soldering, welding, additive manufacturing, cutting and their requirements on the HPDL performance are presented. Furthermore, an overview on process requirements and available as well as perspective performance of laser sources is derived.

We present a flexible, modular and cross-talk free concept for dense wavelength beam combining of HPDL by use of ultra-steep dielectric filters. Five emitters of one mini bar are coupled into a 35 µm fiber. Experimental results of internally and externally stabilized HPDL are presented.

Four different external resonator concepts including VBGs for spectral stabilization of HPDLs are modelled and numerically evaluated to be compared to each other with respect to stabilization efficiency and sensitivity to the “smile-error". The coupled resonators including the external system and the diode laser are solved with a Fox-Li approach. The paper gives a brief summary about the applied simulation model and proceeds with the results for the different feedback concepts. The effective reflectivity, losses in the optical system, losses due to the back-coupling into the waveguide and the averaged optical confinement factor are calculated.

The active alignment of fast axis collimator lenses (FAC) is the most challenging part in the manufacturing process of optical systems based on high power diode laser bars. This is due to the high positioning accuracy in up to 5 degrees of freedom and the complex relations between FAC misalignment and properties of the resulting power density distribution. In this paper an experimental approach for FAC alignment automation is presented. The alignment algorithm is derived from a beam propagation model based on wave optics. The model delivers explicit relations between FAC misalignment and properties of the distorted power density distribution in the near and far field. The model allows to calculate the FAC misalignments and to correct them in one or multiple steps. The alignment algorithm is tested with a demonstrator system. The demonstrator contains an optical system which allows a real time analysis of the near field and far field power distribution of individual emitters. For the tests two different types of FAC lenses and high power diode laser bars are used. The FAC lenses are prealigned within a range of ±50 μm and 0.5 degree around the suitable position. During the automated alignment process the translational and rotational remaining misalignment and the properties of the far field power density distribution are recorded. The experimental results are evaluated regarding reliability and flexibility of the presented FAC alignment algorithm.

We present a compact High-Power DenseWavelength Division Multiplexer (HP-DWDM) based on Volume Bragg Gratings (VBGs) for spectrally stabilized diode lasers with a low average beam quality M 2 ≤50. The center wavelengths of the five input channels with a spectral spacing of 1.5 nm are 973 nm, 974.5 nm, 976 nm, 977.5 nm and 979 nm. Multiplexing efficiencies of 97%±2% have been demonstrated with single mode, frequency stabilized laser radiation. Since the diffraction efficiency strongly depends on the beam quality, the multiplexing efficiency decreases to 94% ( M 2 = 25) and 85%±3% ( M 2 = 45) if multimode radiation is overlaid. Besides, the calculated multiplexing efficiency of the radiation with M 2 = 45 amounts to 87:5 %. Thus, calculations and measurements are in good agreement. In addition, we developed a dynamic temperature control for the multiplexing VBGs which adapts the Bragg wavelengths to the diode laser center wavelengths. In short, the prototype with a radiance of 70GWm -2 sr -1 consists of five spectrally stabilized and passively cooled diode laser bars with 40Woutput after beam transformation. To achieve a good stabilization performance ELOD (Extreme LOw Divergence) diode laser bars have been chosen in combination with an external resonator based on VBGs. As a result, the spectral width defined by 95% power inclusion is < 120pm for each beam source across the entire operating range from 30 A to 120 A. Due to the spectral stabilization, the output power of each bar decreases in the range of < 5 %.

We present design and first performance data of a broadly tunable Alexandrite laser longitudinally pumped by a newly developed high brightness single emitter diode laser module with output in the red spectral range. Replacing the flashlamps, which are usually used for pumping Alexandrite, will increase the efficiency and maintenance interval of the laser. The pump module is designed as an optical stack of seven single-emitter laser diodes. We selected an optomechanical concept for the tight overlay of the radiation using a minimal number of optical components for collimation, e.g. a FAC and a SAC lens, and focusing. The module provides optical output power of more than 14 W (peak pulse output in the focus) with a beam quality of M 2 = 41 in the fast axis and M 2 = 39 in the slow axis. The Alexandrite crystal is pumped from one end at a repetition rate of 35 Hz and 200μs long pump pulses. The temperature of the laser crystal can be tuned to between 30 °C and 190 °C using a thermostat. The diode-pumped Alexandrite laser reaches a maximum optical-optical efficiency of 20 % and a slope efficiency of more than 30 % in fundamental-mode operation (M 2 < 1.10). When a Findlay-Clay analysis with four different output couplers is conducted, the round-trip loss of the cavity is determined to be around 1 %. The wavelength is tunable to between 755 and 788 nm via crystal temperature or between 745 and 805 nm via an additional Brewster prism.

Laser systems based on edge-emitting diodes are established in materials processing applications like soldering and welding and show the potential to substitute solid state and fiber lasers in cutting and deep penetration applications. Another type of diode lasers based on vertical emitters has made significant progress in the past few years. Power scaling to multi-kW level and a significant improvement in efficiency have been achieved. In this publication, the fundamental properties of both laser types are compared and the process regimes that can be addressed with state of the art commercial lasers based on edge-emitters and vertical emitters are presented.

Time and cost for manufacturing prototypes can be significantly decreased by using stock lenses. An f-θ lens design based on our approach and a comparison to the Zemax stock lens matching tool are presented.

A high brightness diode laser module based on single emitters is presented, offering a maximum output power of 15.5 W at 35 Hz repetition rate and 100 μs pulse length. While the presented module (6xx nm) is designed and used for longitudinally pumping an alexandrite laser for atmospheric measurements, the conceptual design is suited for the whole range of diode laser wavelengths from UV to NIR as well as for fiber coupling. The beam propagation factor has been measured to be M2 = 34. To the authors' knowledge this is the highest brilliance ever reported at the specific wavelength.

We present the concept of a modular fiber coupled building block based on chirped DFB or DBR diode laser bars and a Dense Wavelength Division Multiplexer including Volume Bragg Gratings. The minimum channel spacing is 1.5 nm. Multiplexing efficiencies of 97% have been demonstrated in a pre-testing of the multiplexing unit with single mode, frequency stabilized diode laser radiation. For the presented building block with a beam parameter product of 2.2 mm mrad per emitter, the expected multiplexing efficiency is between 90% and 95% for a channel spacing of 1.5 nm, assuming a spectral width between 100 pm and 400 pm per channel.

A diode laser prototype based on a compact dense wavelength division multiplexer (DWDM) for diode laser bars with low average beam quality (M2 = 45) is presented. The prototype with a radiance of 70 GW/(m2sr) (without polarization coupling, at an injection current of I = 120 A) consists of five spectrally stabilized and passively cooled diode laser bars with the center wavelengths 973 nm, 974.5 nm, 976 nm, 977.5 nm and 979 nm. To achieve a good stabilization performance ELOD (Extreme LOw Divergence) diode laser bars have been chosen. The spectral width defined by 95 % power inclusion is

Broad area lasers with narrow spectra are required for many pumping applications and for wavelength beam combination. Although monolithically stabilized lasers show high performance, some applications can only be addressed with external frequency stabilization, for example when very narrow spectra are required. When conventional diode lasers with vertical far field angle, ΘV 95% ~ 45° (95% power) are stabilized using volume holographic gratings (VHGs), optical losses are introduced, limiting both efficiency and reliable output power, with the presence of any bar smile compounding the challenge. Diode lasers with designs optimized for extremely low vertical divergence (ELOD lasers) directly address these challenges. The vertical far field angle in conventional laser designs is limited by the waveguiding of the active region itself. In ELOD designs, quantum barriers are used that have low refractive index, enabling the influence of the active region to be suppressed, leading to narrow far field operation from thin vertical structures, for minimal electrical resistance and maximum power conversion efficiency. We review the design process, and show that 975 nm diode lasers with 90 μm stripes that use ELOD designs operate with ΘV 95% = 26° and reach 58% power conversion efficiency at a CW output power of 10 W. We demonstrate directly that VHG stabilized ELOD lasers have significantly lower loss and larger operation windows than conventional lasers in the collimated feedback regimes, even in the presence of significant (≥ 1 μm) bar smile. We also discuss the potential influence of ELOD designs on reliable output power and options for further performance improvement.

The simulation of spectral stabilization of broad-area edge-emitting semiconductor diode lasers is presented in this paper. In the reported model light-, temperature- and charge carrier-distributions are solved iteratively in frequency domain for transverse slices along the semiconductor heterostructure using wide-angle finite-difference beam propagation. Depending on the operating current the laser characteristics are evaluated numerically, including near- and far-field patterns of the astigmatic laser beam, optical output power and the emission spectra, with central wavelength and spectral width. The focus of the model lies on the prediction of influences on the spectrum and power characteristics by frequency selective feedback from external optical resonators. Results for the free running and the spectrally stabilized diode are presented.

We present design and first performance data of a broadly tunable Alexandrite laser longitudinally pumped by a newly developed high brightness single emitter diode laser module in the red spectral range.

High power diode lasers are established pump sources for solid state lasers due to high efficiency and reliability. Based on spectral scaling techniques, diode laser systems will become suited for high intensity material processing applications.

Integrating volume holographic gratings into micro-optical components such as cylindrical fast-axis collimation lenses (VHG-FAC) for diode lasers constitutes a promising concept for wavelength stabilization by forming an external cavity laser. Compared to standard wavelength stabilization configurations the integrated element reduces the alignment complexity and is furthermore insensitive to the smile-error of diode laser bars. In order to configure and optimize these components the diffraction of the divergent field distribution of a broad area semiconductor laser must be calculated. The present paper presents the extension of the coupled-mode theory in order to calculate the spectral distribution of the diffracted field and the coupling efficiency within the external cavity. The model was extended to three-dimensional space and supplemented to include surface effects, polarization dependency and wave-optical propagation. The asymmetric spectral distribution emitted by an external cavity laser with VBG-FAC is tracked back to the feedback of highly divergent radiation diffracted at the holographic grating. Power losses due to the coupling efficiency within the cavity are also calculated for various field distributions and compared with experimental data. In summary the mathematical model allows to estimate the minimum spectral width and the losses using a VHG-FAC in an external cavity. Thus the injection locking concept using the VHG-FAC can be compared to the spectral characteristics and estimated power losses of standard wavelength stabilization configurations, e.g. the alignment of the grating in the collimated beam.

With the technological progress of tapered diode lasers brightness and output power of fiber coupled modules can be improved. Tapered diodes bear the potential to achieve high coupling efficiencies in multimode as well as single mode fibers. Within the BRIGHTER Project of the European Union several modules are designed to exploit this potential. The optical systems, the mechanical design and the experimental results of these modules will be presented. A design for a telecom pump module with a coupled power of 50 W in a 50 μm fiber with an NA of 0.22 at 975 nm will be presented. 16 collimated tapered single emitters aligned in four groups of four emitters are combined by mirrors and a polarizing beam splitter and coupled into the fiber. As a variant of this module four emitters are fiber coupled to achieve a power of 12 W of a 50 μm fiber with a NA of 0.13. A single mode fiber coupled module with a maximum output power of 1 W will be presented. Based on a tapered DFB Laser with a wavelength of 1060 nm it serves as a free space communication module. In another application this module is utilized as pump source for second harmonic generation. Equipped with a 975 nm tapered laser diode this module serves as a powerful pump source for Raman amplification.

We developed a high brightness fiber coupled diode laser module based on single diode lasers providing more than 60 Watts output power from a 100 micron fiber at the optimum fiber laser pump wavelength of 976 nm. The advantage of using multiple single emitters on a submount compared to using bars or mini bars is the direct fiber coupling by use of optical stacking and the fact that no beam transformation is needed. We achieved best brightness with a high fill factor, optical efficiency of more then 80% and wall-plug efficiency of more then 40%. The use of single emitters on a submount also extends the life span due to reduced failure (x n vs. x) per device (n individual emitters vs. n emitters on a bar (mini array)). Low drive current enables modulation.

We present a dense wavelength division multiplexer based on volume Bragg gratings (VBGs) with a channel spacing of Delta lambda = 1.5 nm. Multiplexing efficiencies of eta(SM) = 97% have been demonstrated with single-mode, frequency-stabilized diode laser radiation. By use of VBGs in an external-cavity laser we constrict the spectral bandwidth of passively cooled multimode diode laser bars with 19 broad-area emitters to delta lambda(95%) = 120 pm. When the multimode high-power diode laser radiation with a beam propagation factor of M-2 approximate to 45 is overlaid, the multiplexing efficiency decreases to eta(MM) = 85%. Temperature control of the VBGs expands the high-efficiency operation range.

The brightness of high-power diode-laser systems can be significantly increased by using dense wavelength-multiplexing technologies. Among these technologies, state-of-the-art volume holographic gratings (VHGs) are suitable wavelength-selective filters for scaling the power of frequency-stabilized high-power lasers. The frequency spacing is limited due to the sidelobes of the spectral filter characteristic. In this Letter, we present the simulation results of novel apodized VHGs produced by Gaussian-beam interference. Apodized VHGs offer increased sidelobe suppression of up to 22 dB with conventional grating dimensions and, moreover, will improve the spectral brightness of dense wavelength-multiplexing systems by a factor of approximately six.