Photonics integration is a key technology for realizing large-scale photonic quantum information processing. Here, we demonstrate the largest reconfigurable photonic processor based on low-loss silicon nitride waveguide networks for InGaAs quantum dots to date. The 20-mode processor is used for quantum information processing with demultiplexed single photons from a quantum dots source which are detected at the output using single photon detectors. In this talk, we will present the newest results of the system using our programmable quantum photonic processors. Furthermore, we show the challenges of scaling up quantum photonic computers and the range of potential applications and use cases.
Spaceborne lidar (light detection and ranging) systems have a large potential to become powerful instruments in the field of atmospheric research. Obviously, they have to be in operation for about three years without any maintenance like readjusting. Furthermore, they have to withstand strong temperature cycles typically in the range of -30 to +50 °C as well as mechanical shocks and vibrations, especially during launch. Additionally, the avoidance of any organic material inside the laser box is required, particularly in UV lasers. For atmospheric research pulses of about several 10 mJ at repetition rates of several 10 Hz are required in many cases. Those parameters are typically addressed by DPSSL that comprise components like: laser crystals, nonlinear crystals in pockels cells, faraday isolators and frequency converters, passive fibers, diode lasers and of course a lot of mirrors and lenses. In particular, some components have strong requirements regarding their tilt stability that is often in the 10 μrad range. In most of the cases components and packages that are used for industrial lasers do not fulfil all those requirements. Thus, the packaging of all these key components has been developed to meet those specifications only making use of metal and ceramics beside the optical component itself. All joints between the optical component and the laser baseplate are soldered or screwed. No clamps or adhesives are used. Most of the critical properties like tilting after temperature cycling have been proven in several tests. Currently, these components are used to build up first prototypes for spaceborne systems.
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%.
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.
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.
Time and costs for manufacturing prototypes can be signficantly decreased by using stock lenses. An f –Ѳ lens design based on our approach and a comparison to the Zemax stock lens matching tool are presented. The designed lens is used for a high power laser cleaning application, and the requirements of this application are discussed in detail.
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 M2 ≤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% (M2 = 25) and 85%±3% (M2 = 45) if multimode radiation is overlaid. Besides, the calculated multiplexing efficiency of the radiation with M2 = 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 M2 = 41 in the fast axis and M2 = 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 (M2 < 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.
KEYWORDS: Semiconductor lasers, Laser stabilization, Rutherfordium, Collimation, Waveguides, Near field optics, Near field, Reflectivity, Laser development, Refractive index
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, ΘV95% ~ 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.
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 (xn vs. x) per device (n individual emitters vs. n emitters on a
bar (mini array)). Low drive current enables modulation.
Pumping fiber lasers is the driving force for the development of high brightness, mid power, passively cooled, fiber
coupled diode lasers. We compare concepts for providing 50 W in a 100 micron fiber at the optimum fiber laser pump
wavelength of 976 nm. The set up is experimentally demonstrated and compared to the optical analysis.
Three basic diode laser concepts are included into this comparison: single emitters, high density emitter arrays and low
density emitter arrays on bars.
Low density stacking in the horizontal direction with increasing the filling factor by a microlens array is the first concept.
For this concept two diode bars with low filling factor are fast and slow axis collimated. Beam transformation, shaping
and focusing are similar to the second concept.
In the second concept a diode laser array with high filling factor is regarded. An 800 μm diode laser bar consists of an
array of four or five emitters. Two bars are polarization coupled and collimated with single lenses. Beam symmetrization
is performed by the well known step mirror. A simple anamorphotic optic enables beam shaping and fiber coupling.
The third one, single emitters, represents optical beam combining of laser diodes that are high density stacked in the
vertical direction. Five emitters are placed in an optical stack, each one collimated with its own lens. Two optical stacks
are polarization coupled and focused on the fiber end. The three concepts are compared in terms of power efficiency and
complexity, and the results of prototype systems are presented.
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