Topic: Diode lasers for sensor & analytics applications

FBH diode lasers for sensor applications – highly capable, flexible and easy-to-use

The FBH has long-term experience in developing diode laser based light sources tailored for sensor and analytic applications. These lasers can be utilized very flexibly in industry, health and well-being as well as in environmental and scientific applications. As different as these applications may be, what they have in common is their need for high-power, diffraction-limited and tunable narrow-linewidth laser sources – with adjusted properties, of course.

In order to develop the optimum device for the respective application, FBH scientists coordinate closely with each customer before starting the design process. Developments are accompanied by comprehensive testing. Potential optimizations can be easily integrated in the next development steps, since the institute’s competencies cover the full value chain, from design and processing of the laser diodes up to operational devices. Moreover, tailor-made demonstrators and prototypes are conceptualized and realized in house. As a result, industrial and scientific partners can easily test FBH’s R&D results in their specific applications.

Diode lasers with advanced functionalities – meeting customer’s demands

FBH developments include wavelength-stabilized, narrowband-emitting diode lasers and diode laser modules, which are suitable for a great variety of demanding applications such as Raman, fluorescence, and absorption spectroscopy. Using Y-branch DBR diode lasers with electrically controlled micro-heaters, which are implemented above the gratings, provides dual-wavelength laser emission and wavelength tuning over several nanometers. These sources are ideally suited for shifted excitation Raman difference spectroscopy (SERDS), enabling to extract Raman signals efficiently and rapidly from disturbing backgrounds such as fluorescence and ambient light, thus improving Raman spectroscopy in real-world applications.

With sampled grating lasers, recently developed at FBH using GaAs based devices for the first time, wider wavelength tuning of several tens of nanometers is obtained. With these diode lasers, the FBH provides capable laser sources whose spectral distance can be adjusted flexibly to the desired bandwidth, matching the sample under investigation.

FBH diode lasers are also developed as pulsed light sources in LiDAR systems for autonomous driving and for 3D object detection. One quite specific pump laser source for a LiDAR system has only recently passed its practical test on board the Aeolus satellite, which now helps to measure Earth’s winds. Aeolus is expected to play a key role in better understanding the workings of the atmosphere and thus will improve weather forecasting.

Diode lasers from the FBH also aim at nonlinear frequency conversion, which enables access to specific wavelengths that currently cannot be addressed by direct-emitting diode lasers. They target wavelengths, ranging from the ultraviolet to the visible spectral range.

In summary, FBH diode lasers enable very robust, small-sized laser sensors that offer stable measurements with highest accuracy – even under challenging conditions when the “wanted” signals are superimposed by disturbing background signals. They therefore allow for particularly compact systems that can be used very flexibly.

FBH developments enable rapid shifted excitation Raman measurements

Raman spectroscopy is an established method to analyze organic and inorganic substances. The FBH bridges the gap between various life science applications and high-end diode laser technology. This enables the development of suitable light sources for high-precision Raman measurements that were already successfully performed on food, soil, plants, and human skin. For several years now, mobile devices like handheld sensors are commercially available and open up application fields including point-of-care diagnostics, on-site food inspection and detection of hazardous substances. Still, Raman signals are weak, and additional challenges have to be considered. Laser-induced fluorescence originating, for example, from biological samples or ambient light such as daylight could mask the Raman signals and hence complicate identification, especially for unknown substances.

However, the shifted excitation Raman difference spectroscopy (SERDS) approach is a powerful and easy-to-use spectroscopic technique to overcome these drawbacks. SERDS requires an excitation light source with two individually controllable emission lines. As a result, Raman signals can be clearly separated from disturbing background interferences. The FBH has demonstrated suitable monolithic dual-wavelength diode lasers emitting at 785 nm and 671 nm, respectively. The spectral distance of the two excitation wavelengths was chosen in a way that they are close to the full width at half maximum (FWHM) of the Raman bands under study. However, for various substances the spectral width of the Raman signals can differ between a few to several tens of wavenumbers. To improve SERDS, the excitation source should provide two emission lines with a flexible spectral distance adjustable with respect to the bandwidth of the Raman signals under investigation.

Flexible and rapid measurements using monolithic dual-wavelength diode lasers

Recently, the FBH has realized dual-wavelength diode lasers at 785 nm with electrically adjustable spectral distance. These monolithic devices have a footprint of only 3 mm x 0.5 mm and provide two excitation lines with an optical power up to 170 mW. Two separate heater elements are implemented close to each distributed Bragg reflector (DBR) grating at the rear side of the semiconductor chip. These on-chip resistor heater elements allow to flexibly adjust the spectral distances between 0 and 2.3 nm (0 and 36 cm-1) simply by changing the heater current. Moreover, these dual-wavelength diode lasers enable a fast alternating operation between both laser lines for rapid SERDS and thus investigations with exposure times down to the millisecond range.

At FBH, Raman and SERDS experiments with Irish cream as test sample were successfully carried out using SERDS distances between 1 cm-1 and 30 cm-1. The liquor was excited using an alternating operation between both laser lines with exposure times down to 50 ms. Separation and identification of closely neighbored Raman bands of different components of the Irish cream could be improved with a 15-fold increased signal-to-background noise using the selected SERDS distance of 15 cm-1.

These FBH innovations make Raman spectroscopy accessible for rapid measurements which show huge disturbing background signals, and whose intensities are often close to the saturation level of the used detector.

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Advanced Raman analysis and imaging

FBH has recently expanded its measurement technology with a confocal Raman microscope. Thus, the institute possesses another powerful analytical tool to conduct spatially resolved investigations and chemical imaging of different samples such as solids, biological material, and polymers. Raman spectra are generated point by point over an area, rendering 2D and 3D spatial distributed chemical information of the sample under study. The Raman microscope alpha300 R offers two excitation wavelengths at 532 nm and 785 nm and can be flexibly used for both scientific projects and industrial services. The tool is additionally equipped with a TrueSurface option to measure the topography of the sample surface with an optical profilometer. This information helps to guide the focus of the excitation and collection spot for Raman imaging experiments of real-world samples.

More information on related project I4S (Intelligence for Soil, Bonares)

High-performance light sources for LiDAR applications – from autonomous driving to weather monitoring

Light Detection and Ranging (LiDAR) is a well-established method for remote sensing. Various parameters can be measured by detecting backscattered light. These include distances and velocity of objects, aerosols, and dust as well as concentrations of gases, particles, and droplets over larger distances. The FBH has developed suitable diode laser based light sources that allow replacing commonly used pulsed high-power solid-state lasers with ps or ns diode lasers.

Ultra-precise distance measurements

Key components for distance measurements, which are required for autonomous driving and 3D object detection, are diode lasers generating short optical pulses in the range from 200 ps to 20 ns. FBH has developed the appropriate laser sources for this purpose, which use a tailored diode laser design for pulse generation and optimized RF components as electronic drivers. Both fields are core competencies of the institute. The design of the output circuit is optimized for high peak current up to 250 A, short optical pulse widths between 3 ns and 20 ns, high repetition rates, and high power efficiency. For automotive driving, 905 nm wavelength-stabilized diode lasers with a pulse peak power per emitter of up to 40 W are being developed. 3-emitter laser chips, whose emission can easily be optically combined into one single spot, yield up to 100 W. Further wavelengths between 650 nm and 1200 nm are possible.

Improving weather forecasts – on earth and in space

For weather monitoring applications, micro-pulse LiDAR (MPL) is using a short laser pulse with moderate peak power transmitted from earth to the atmosphere. As this pulse travels along, part of it is scattered by molecules, water droplets and other objects in the atmosphere. This effect can be used to detect aerosols as well as to determine water vapor concentrations, e.g., for weather forecasts – which makes a high output power along with a narrow spectral linewidth essential. In the case of determining gas concentrations, a dual wavelength operation is required to measure within (lon) and outside (loff) the absorption. The institute has been developing suitable monolithic and hybrid master oscillator power amplifier systems, which are refined with FBH’s RF electronics components. When using a dual wavelength Y-branch laser at 960 nm, similar to the devices used for SERDS at 785 nm, the systems reach peak powers up to 16 W with a narrow spectral linewidth below 10 pm, which enables gas measurements under atmospheric conditions.

With the launch of ESA’s Aeolus satellite, spaceborne measurements of wind speed have attracted a great deal of public attention. This can be attributed to the satellite’s importance for accurate weather forecasts and climate modelling – especially in view of extreme weather phenomena due to climate change. Aeolus utilizes a 3D imaging system based on the ultraviolet (UV) LiDAR instrument ALADIN. FBH delivered the space-qualified diode lasers as pump sources for solid-state resonators. Their third harmonic is 355 nm UV light, which is well suited for the scattering of droplets in the atmosphere. FBH pump lasers exhibit outstanding reliability with mean-time-to-failure of over 10 million hours and proven resilience against environmental stress and space irradiation.

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High-power laser drivers for ultra-short pulse generation

The maximum range of a LiDAR system depends on the available optical pulse power, the range resolution is determined by the pulse width. Typical scenarios demand for power levels exceeding 100 watt at pulse widths from 3 to 10 ns. To generate such short and powerful laser pulses, pulse drivers with a peak current value of more than 100 A are needed. The unique challenge is providing the high-speed high-current switching circuit as well as handling the parasitic inductances due to the assembly of the laser diode and the driver board. FBH drivers use GaN devices in the final stage, in order to allow for optimum high-current switching as well as for high efficiency and high repetition rates. They achieve record performance: 100 W optical pulse peak power are obtained in a pulse-width range between 3 ns and 10 ns, with pulse currents between 180 A and 250 A.

Further information on FBH's high-power pulse laser drivers.

Diode lasers for nonlinear frequency conversion

Wavelengths on demand by second harmonic generation

Recent developments at FBH have enabled access to new wavelengths, which currently cannot be addressed by direct-emitting diode lasers. Compact single-pass second harmonic generation of diode lasers and modules has opened the door to the ultraviolet and gaps in the visible spectral range. For the first time, external wavelength-stabilized GaN diode lasers have been frequency converted to 225 nm with up to 160 µW optical output power. This UV-C wavelength range is highly attractive for applications such as absorption and Raman spectroscopy. By using the developed FBH devices Raman signals can be spectrally separated from disturbing fluorescence contributions. To address the visible spectral range, 920 - 1180 nm ridge waveguide and tapered diode lasers with superior beam quality and internal Bragg gratings have been developed. Compact assemblies on CS mount or as miniaturized modules provide frequency conversion to 460 - 590 nm with watt-class output powers. These lasers are applied in Raman spectroscopy applications like food quality control and medical point-of-care detection as well as for direct pumping of laser systems such as titanium sapphire lasers. Operating these devices in pulsed mode opens up further demanding applications such as STED microscopy and fluorescence spectroscopy.

Further information on laser modules using frequency doubling and for the yellow spectral range

Widely tunable diode lasers for up-conversion and imaging

Based on achievements regarding wavelength-stabilized diode lasers, FBH has recently developed widely tunable ridge-waveguide diode lasers. Multi-branch diode lasers with micro-heaters above the internal Bragg gratings enable multi-wavelength laser emission with close to 10 nm thermal tuning. In a second approach, wavelength tuning of 23 nm was obtained using sampled-grating lasers. Necessary watt-class power scaling of these devices for nonlinear frequency conversion was demonstrated in master oscillator power amplifier configurations on compact 25 x 25 mm2 CS mounts. Sum-frequency generation using devices emitting at 976 nm enables to up-convert infrared (IR) light at 2 - 25 µm to wavelengths < 1 µm, thus a spectral range that can be detected with standard Si-based detectors. The applicable wavelength tuning allows for multiple phase matching conditions and enables hyperspectral imaging in the IR using CCD cameras. These features open up a multitude of industrial and biomedical sensor applications such as combustion analysis, absorption or differential absorption, LIDAR spectroscopy, and cancer diagnostics.

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