Precision agriculture relies on detailed knowledge about the current soil composition to make informed treatment decisions for site-specific farming. Fast and non-destructive optical methods are a promising approach to complement existing complex and time-consuming standard laboratory analyses. Raman spectroscopy is well-suited for this task as it permits to obtain molecule-specific soil information but it suffers from the weak Raman effect and dominant background interferences, e.g., caused by fluorescence or daylight. Here, shifted excitation Raman difference spectroscopy (SERDS) is applied to overcome this challenge using a physical approach with two slightly shifted laser wavelengths. An in-house developed dual-wavelength diode laser with two emission lines around 785 nm is used as excitation light source for SERDS. In preparation for on-site soil analysis, initial SERDS laboratory investigations are performed on 150 samples collected from an agricultural field in Germany. Results show the ability of our approach for qualitative (detection of soil constituents) and quantitative (prediction of organic matter and carbonate contents) soil investigations. The transfer of SERDS from the laboratory setting to the field environment is realized by an in-house developed portable SERDS system specifically designed for soil analysis. On-site investigations on the above-mentioned field are exemplarily carried out at selected points along a distance of 480 m revealing the spatial distribution of carbonaceous matter and selected soil minerals. Moreover, a successful prediction of the contents of organic carbon (R-2 = 0.89, root mean square error of cross-validation RMSECV = 0.3 %) and carbonate (R-2 = 0.86, RMSECV = 2.5 %) as important soil parameters could be realized. Results demonstrate the capability of portable SERDS for qualitative and quantitative on- site soil analysis directly on agricultural fields thus paving the way for applications in precision agriculture.

Shifted excitation Raman difference spectroscopy (SERDS) enables the separation of weak Raman signals from intense background disturbances such as sample fluorescence and ambient light. It requires dual-wavelength excitation for subsequent measurements. Using grating-stabilized diode lasers, the required functionalities can be specifically designed and implemented at chip level. In this work, tailor-made dual-wavelength distributed Bragg reflector (DBR) ridge waveguide diode lasers emitting around 785 nm will be discussed. The devices provide optical output powers in the 100-mW range and alternating wavelength operation at switching frequencies up to 100 Hz. Heater elements next to the DBR gratings enable adjusting the spectral distance within a 2 nm range in 50 ms. Master oscillator power amplifier configurations help to enter Watt-class level optical output powers. Selected Raman and SERDS experiments will demonstrate the capabilities of these devices for spectroscopic applications.

In this study, a dual-wavelength monolithic Y-branch distributed Bragg reflector (DBR) diode laser at 633 nm is demonstrated that is well-suited for Raman spectroscopy and especially shifted excitation Raman difference spectroscopy (SERDS). The device provides 30 mW optical output power with an electrical power consumption of less than 1 W. Its spectrally stabilized dual-wavelength laser emission is narrowband with spectral widths of 12 pm (0.3 cm(-1)). The spectral distance of the two emission wavelengths is 0.4 nm (10 cm(-1) at 633 nm), as targeted for SERDS, and their side mode suppression ratios are in excess of 40 dB. SERDS measurements on soil, exemplarily chosen as highly fluorescent sample, showed a 17-fold improvement in the signal-to-background noise ratio in comparison to measured Raman spectra. Moreover, it revealed weak Raman signals, which were not detectable in Raman spectra and allowed for identification of quartz and calcite as selected soil constituents.

Shifted excitation Raman difference spectroscopy (SERDS) has emerged as a powerful and easy-to-use tool for efficiently separating Raman signals from intense background interferences applying a physical approach. In the last decade, we have developed and applied dual-wavelength diode lasers and diode laser systems for Raman spectroscopy and SERDS with emission wavelengths from the visible, e.g., at 488 nm up to the near-infrared spectral range, e.g., at 785 nm. In this contribution, an overview of our developed and fabricated diode lasers and diode laser systems for Raman spectroscopy and SERDS will be given. The design and key properties will be discussed with respect to the requirements for the experiments. Selected Raman experiments in application fields such as agri-photonics and food quality control will be presented. The results demonstrate the suitability of these devices for SERDS improving Raman spectroscopy for various applications, e.g., for on-site investigations under real-world conditions.

An experimental study of straight and bent distributed Bragg reflector (DBR) ridge waveguide (RW) lasers and Fabry-Perot (FP) RW lasers emitting at 785 nm is presented. To determine the losses introduced by the bent waveguides within DBR-RW lasers, different laser designs were manufactured and characterized. The bent waveguides investigated here within DBR-RW laser diodes are sine-shaped S-bends. S-bends with three different lateral offsets are manufactured. The experimental characterization of FP lasers and the straight DBR-RW lasers with different coatings at the rear facet enables a rough estimation of the losses caused by the DBR grating and the determination of the DBR reflectivity. Furthermore, additional losses in the bent DBR-RW lasers caused by the S-bend (i.e. radiation and scattering losses) are quantified by comparing them to the straight DBR-RW lasers. Within the active resonator, the S-bend losses amount to alpha Bend = 0.6 cm-1 (alpha Bend = 0.5 dB) for the smallest manufactured lateral S-bend offset H = 40 mu m. For both straight and bent DBR-RW lasers spectrally narrow single-mode emission is obtained. A lateral beam width of 3.8 mu m (using second moments) and a lateral far-field angle of about 18 degrees and 19.5 degrees (using second moments) for the straight and S-bend DBR-RW are measured, respectively. This gives a lateral beam propagation ratio of 1.2 and 1.3 (using second moments) for straight and S-bend DBR-RW, respectively. The radiation loss in dependency of the lateral S-bend offset is simulated and compared to experimentally estimated S-bend losses for bent DBR-RW lasers (H = 40 mu m, H = 60 mu m and H = 70 mu m).

Portable Shifted Excitation Raman Difference Spectroscopy (SERDS) using two excitation wavelengths around 785 nm is applied for selected applications in the field of agri-photonics. In the presence of daylight and laser-induced fluorescence, SERDS effectively separates Raman signals of green apple leaves and soil substances with more than 10-fold improved signal-to-background-noise ratios. Major ingredients of bovine milk are clearly detected and identified. A quantitative determination of the fat content in milk is performed and shows a limit-of-detection of 0.1 g / 100 mL. These results show a great potential of portable SERDS for real-world applications, e.g., for precision agriculture and food monitoring.

A monolithically integrated dual-wavelength multimode interference coupler-based master oscillator power amplifier is presented. It consists of two shallowly etched, laterally separated ridge waveguide laser cavities as master oscillators with individual distributed Bragg reflector gratings as cavity mirrors. A deeply etched coupling section containing S-bend shaped waveguides and a multimode interference coupler is used to couple the laser emission of the master oscillators into a shallowly etched single waveguide serving as power amplifier. Changing the etch depth for the coupling section enables a compact device layout. In addition, increased radiation angles of modes not coupled into the power amplifier help to suppress beam steering, otherwise indicated by laterally separated far-field intensity distributions. The device provides 0.5 W of dual-wavelength emission around 830 nm in individual and common operation. As designed, both emission wavelengths are separated by 0.5 nm with spectral widths below 20 pm, limited by the spectral resolution of the spectrometer. Both peak wavelengths remain within spectral windows of 50 pm within the available power range. This enables full flexibility selecting operating points for applications such as shifted excitation Raman difference spectroscopy and the generation of THz emission by photomixing. The emission wavelengths can additionally be non-continuously tuned by applying a heater current to resistors implemented next to the distributed Bragg reflector gratings. As an example, selected spectral distances of 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm are demonstrated. Near field widths of 5 μ m and far field angles of 17° result in beam propagation ratios of 1.4 (1/e ^2 ) in all operation modes and enable easy beam shaping or optical single-mode fiber coupling.

Diode laser-based light sources allow the implementation of features to meet specific application requirements. Besides output power and specific wavelength, this can also include spectral tunability, an alternating operation between two different wavelengths, or a well-defined parallel operation at two wavelengths. Potential applications involve spectroscopic systems and applications, e.g., in Raman spectroscopy, especially shifted excitation Raman difference spectroscopy (SERDS) or sequentially shifted Raman spectroscopy, absorption spectroscopy, or the generation of THz radiation. To meet these demands, diode laser based light sources often use a multi-contact layout. Their operation may require multiple individually adjustable current sources, adjustable galvanically isolated current sources, regulated temperature stabilization and heat removal. Moreover, an interface, e.g., a fiber coupling unit is required to transfer the laser light to the experiment. In this paper, a compact turnkey system is reported meeting these requirements with up to 10 current sources, 2 galvanically isolated current sources, integrated temperature control and an interface for laser light transfer is presented. The system has dimensions of 177 mm x 124 mm x 48 mm and is controlled via a standard USB interface. It additionally provides a synchronization signal for implementation into multi-instrument setups. The current sources are based on implemented dual continuous wave p-type laser diode drivers. Ten current sources with 750 mA enabling switching frequencies up to 1000 Hz and four galvanically isolated current sources with 2 W are available. For temperature control, a TEC controller is used. An effective cooling system allows 10 W of thermal load to be removed. The integration of this turnkey-system into a portable shifted excitation Raman difference spectroscopy sensor system is briefly presented as an example application.

An experimental comparison between individual and common wavelength-operation of a Y-branch distributed Bragg reflector (DBR) ridge waveguide (RW) laser at 785 nm with an electrically adjustable spectral distance is presented. The dual-wavelength Y-branch laser combines two laser cavities via a Y-section to a common output section. DBR gratings with different grating periods are associated with the two cavities, which set the emission wavelengths of the two branches. Implemented resistive heater elements allow separate wavelength tuning of the two branches, which can be operated individually for alternating emission wavelengths in applications such as differential absorption spectroscopy or shifted excitation Raman difference spectroscopy. Common wavelength operation simultaneously generates two emission lines suitable for the generation of THz radiation using difference frequency mixing. Hereby, the devices could potentially be used as single-chip light sources for a combination of Raman and THz applications. For the wavelength-operation comparison presented, the devices were operated up to optical output powers of about 105 and 185 mW in individual and common wavelength-operation mode, respectively. In individual operation mode, the devices show spectral single-mode emission over the whole operation range. In common operation mode, the spectral emission is predominantly single mode up to an optical output power of 65 mW. In both operation modes, mode hops typical for DBR lasers occur. At an optical output power of 50 mW, tuning of the spectral distance between the two wavelengths using the implemented resistor heaters is demonstrated. In both modes of wavelength operation, a flexible frequency difference between 0 and 0.8 THz (0 and 1.6 nm) with predominantly single-mode spectral emission is obtained.

A wavelength adjustable distributed Bragg reflector diode laser suitable for background-free Raman spectroscopy at 785 nm is presented. It is based on a GaAsP single quantum well embedded in a 1 μm thick AlGaAs waveguide. The 3 mm long device consists of a 2.2 μm wide ridge waveguide and a 10th order DBR surface grating as wavelength selective rear side mirror. On-chip resistors implemented as heater elements next to the grating allow applying a current that enables a flexible wavelength adjustment by Joule heating. At a heatsink temperature of 25°C, the laser provides 100 mW of optical output power and narrowband laser emission with spectral widths of 20 pm (0.3 cm-1) along the whole power range. A current up to 0.6 A applied to the on-chip resistors shifts the excitation wavelength by 2.18 nm (35 cm-1) with narrowband emission at all settings and an optical output power remaining within a span of 14 mW. Along that available wavelength range, alternating dual-wavelength operations for five wavelengths separated from another by about 8 cm-1 are presented (f = 1 Hz, 50% duty cycle). At 50 ms after switching, the spectral distance between selected target wavelengths and measured peak wavelengths is ≤ 1.1 cm-1 (≤ 0.07 nm). This enables flexible selections of excitation wavelengths with low latencies for background-free Raman spectroscopy.

In this contribution, the properties of 4 mm long DBR-tapered lasers with different DBR grating lengths and different ridge waveguide lengths will be investigated. For the different device designs, the influence of the ridge waveguide current on the spectral and spatial parameters will be presented. The vertical layer structure of the devices is based on a GaAsP single quantum well in a large optical cavity. Three different grating lengths, i.e., 500 μm, 750 μm, and 1 mm, were manufactured using e-beam lithography. For the first two gratings, the tapered section has a length of 2.5 mm, for the latter 2.0 mm. All devices have a full tapered angle of 6°. The devices having the longer tapered sections reach output powers up to 7 W with a narrow spectral width. Increasing the ridge waveguide current increases the optical output power up to a saturation level. At saturation level, although the output remains approximately stable, differences in the spectral behavior and the beam quality occur. The dependence of spectral properties and beam parameters on the ridge waveguide current will be discussed. A correlation between spectral properties and lateral beam profile measured at beam waist position can be supposed. The measured emission width amounts to 19 pm, which is limited by the resolution of the used spectrometer. In best cases, at an output power of 5 W the lateral beam propagation ratio is below 3.5 (1/e2) and 8.4 (2nd moments). These lasers are well-suited as pump lasers for Tm:YAG lasers and as excitation light sources in Raman spectroscopic experiments with large excitation areas.

Exciting the Raman effect at a wavelength in resonance with the absorption spectrum of the sample, typically in the visible spectral range, can increase the strength of Raman lines by orders of magnitude. Particularly in this case, the lines can be obscured by fluorescence but also by background light. Shifted excitation Raman difference spectroscopy (SERDS) can recover Raman signatures. This method uses an excitation light source with alternating operation of two neighboring, spectrally stabilized, narrow emission wavelengths. Only the Raman lines follow that change in the excitation wavelength and can be separated from the background. Up to now, internally wavelength stabilized dual wavelength diode lasers for the blue and green spectral range are unavailable. Other concepts, as presented in this work, had to be evaluated. First, the combination of two external cavity stabilized GaN diode lasers will be presented. Low reflection coated laser diodes are externally wavelength stabilized using VBGs and their beams are superimposed. Output powers in the 10- mW range for emission wavelengths of 454 nm and 456 nm will be presented. Second, devices based on frequency doubled GaAs diode lasers will be tested. The wavelength shift is realized by thermal tuning of the heat sink or by applying a current to internal heater elements in the GaAs-DBR-RW-lasers. In this case output powers, up to 50 mW at 488 nm or 515 nm were achieved. Third, dual wavelength Y-branch diode lasers at 1064 nm were frequency converted towards 532 nm with output powers in the 10-mW range using customized SHG waveguide crystals.

We present AlGaAs-based monolithically integrated dual-wavelength lasers and master-oscillator power-amplifiers emitting around 785 nm which combine a near diffraction limited beam quality with single longitudinal mode emission, both achieved over a wide range of output powers. This holds for alternating dual wavelength operation which is of interest for applications in spectroscopy as well as for simultaneous dual wavelength operation which can be used to generate THz-radiation. Compared to previous designs the lateral optical confinement is provided by two differently deep etched ridge waveguides. Shallow etching is used in lasing and amplifying sections to ensure single-lateral mode operation, whereas deep etching is employed to form more compact S-bends and to enable the use of multi-mode interference (MMI) couplers instead of Y-couplers.

Solar radiation is a challenge for laser-based daylight imaging since it decreases the signal-to-noise ratio (SNR) of the imaging. Here we demonstrate a micro-integrated external-cavity tapered diode laser system for daylight imaging. It emits light at a wavelength of 762 nm, chosen because of its overlap with an oxygen absorption band in the solar radiation spectrum. The integrated laser system consists of a tapered diode amplifier as gain medium and an external volume Bragg grating for spectrum stabilization and narrowing, thus a narrow bandpass optical filter can be used to improve the SNR further. The laser system can be operated in both continuous wave (CW) mode and pulsed mode by modulating the injected current to the amplifier. In CW mode operation, 1.3 W of output power is obtained with an emission spectral linewidth of 4 pm, and a beam propagation factor in the slow axis, M-2, of 1.7 (4 sigma). In pulsed mode operation with a trigger signal of a 50 mu s pulse width and a 10 kHz repetition rate, 2.0 W of peak output power is achieved with an emission spectral bandwidth of 0.2 nm, and an M2 in the slow axis of 1.9 (4 ). The modulation depth is almost 100%.

785 nm micro-integrated, dual-wavelength master oscillator power amplifiers with a footprint of 5 mm × 25 mm are presented. They are based on Y-branch distributed Bragg reflector ridge waveguide diode lasers and anti-reflection coated tapered amplifiers. In order to reduce the impact of potential optical feedback, devices with master oscillator front facet reflectivities of 5% and 30% as well as with an integrated miniaturized optical isolator have been realized. A comparison up to 1 W shows narrowband dual wavelength laser emission with a spectral distance of 0.6 nm (10 cm−1) and individual spectral widths <20 pm. As expected, a higher front facet reflectivity leads to a significant reduction of feedback related mode hops. Longitudinal modes corresponding to the master oscillator resonator length remain within spectral windows <0.15 nm (3 cm−1), suitable for applications such as Raman spectroscopy and especially shifted excitation Raman difference spectroscopy. Integrating a compact 30 dB optical isolator completely eliminates the observed optical feedback effects. Lateral beam propagation ratios of 1.2 (1/e2) enable easy beam shaping and fiber coupling. Outside of the experimental comparison, the developed MOPAs provide up to 2.7 W of optical output power available for applications.

Alternating dual-wavelength second harmonic generation at 532 nm using a 1064 nm Y-branch distributed Bragg reflector diode laser will be presented. The light source is based on single-pass frequency conversion in a periodically poled lithium niobate waveguide crystal with superimposed poling periods. All components, including the diode laser, optics for beam shaping and the nonlinear crystal, are mounted on a 5 x 25 mm2 aluminum nitride micro-optical bench soldered on a 25 x 25 mm2 conduction cooled package mount. Separated electrical contacts for the diode laser allow for alternating, wavelength stabilized dual-wavelength laser emission with spectral widths of 0.02 nm (0.2 cm-1). Heater elements implemented above the DBR gratings enable wavelength tuning, which is utilized for phase-matching at both wavelengths. At a heat sink temperature of 25°C, optical output powers of 5.6 mW at 532.45 nm and 6.7 mW at 531.85 nm with a spectral width of 0.01 nm (0.35 cm-1) are obtained. The developed light source is suitable for demanding applications such as Raman spectroscopy and shifted excitation Raman difference spectroscopy.

Wavelength stabilized distributed Bragg reflector (DBR) tapered diode lasers at 783 nm will be presented. The devices are based on GaAsP single quantum wells embedded in a large optical cavity leading to a vertical far field angle of about 29° (full width at half maximum). The 3-inch (7.62 cm) wafers are grown using metalorganic vapor phase epitaxy. In a full wafer process, 4 mm long DBR tapered lasers are manufactured. The devices consist of a 500 µm long 10th order surface DBR grating that acts as rear side mirror. After that, a 1 mm long ridge waveguide section is realized for lateral confinement, which is connected to a 2.5 mm long flared section having a full taper angle of 6°. At an injection current of 8 A, a maximum output power of about 7 W is measured. At output powers up to 6 W, the measured emission width limited by the resolution of the spectrometer is smaller than 19 pm. Measured at 1 / e 2 level at this output power, the lateral beam waist width is 11.5 µm, the lateral far field angle 12.5°, and the lateral beam parameter M 2 2.5. The respective parameters measured using the second moments are 31 µm, 15.2°, and 8.3. 70% of the emitted power is originated from the central lobe.

Wavelength stabilized, high-power diode lasers in the spectral range around 783 nm are requested as e.g. pump lasers for Tm:YAG lasers and as excitation light sources for Raman spectroscopy. In the latter case, the emission width should be narrowband enough to resolve Raman lines of e.g., solid and liquids, i.e., smaller than 0.6 nm (10 cm -1 ). Regardless of this, fibre-coupled light sources are preferred for the above-mentioned applications. Here, multimode fibres are often used with core diameters in the range of 50 – 500 µm for guiding the laser light. This requires a sufficiently good beam quality for an efficient coupling.

Wide field Raman imaging using the integral field spectroscopy approach was used as a fast, one shot imaging method for the simultaneous collection of all spectra composing a Raman image. For the suppression of autofluorescence and background signals such as room light, shifted excitation Raman difference spectroscopy (SERDS) was applied to remove background artifacts in Raman spectra. To reduce acquisition times in wide field SERDS imaging, we adapted the nod and shuffle technique from astrophysics and implemented it into a wide field SERDS imaging setup. In our adapted version, the nod corresponds to the change in excitation wavelength, whereas the shuffle corresponds to the shifting of charges up and down on a Charge-Coupled Device (CCD) chip synchronous to the change in excitation wavelength. We coupled this improved wide field SERDS imaging setup to diode lasers with 784.4/785.5 and 457.7/458.9 nm excitation and applied it to samples such as paracetamol and aspirin tablets, polystyrene and polymethyl methacrylate beads, as well as pork meat using multiple accumulations with acquisition times in the range of 50 to 200 ms. The results tackle two main challenges of SERDS imaging: gradual photobleaching changes the autofluorescence background, and multiple readouts of CCD detector prolong the acquisition time.

A micro-integrated diode laser based dual-wavelength master oscillator power amplifier (MOPA) at 785 nm is presented. The device is realized on a 5 x 25 mm2 micro-optical bench and consists of a Y-branch distributed Bragg reflector ridge waveguide (RW) diode laser as MO with a front facet reflectivity of 30%, micro-cylindrical lenses for beam shaping and a tilted RW amplifier as PA. This approach allows power scaling of 785 nm dual-wavelength diode lasers that have already been applied for Raman spectroscopy and terahertz frequency generation. The optical concept is designed to reduce unwanted optical feedback to the MO and avoids integrating an optical isolator, which was used in a previous tabletop configuration. At T = 25°C and 20 mW pump power, diffraction limited laser emission with 0.5 W optical output power and beam propagation parameters of 1.3 (M24σ) are obtained. At both emission wavelengths of 784.6 nm and 785.2 nm, spectral bandwidths below 0.02 nm at full width at half maximum and side mode suppression ratios of 30 dB are measured. A negligible wavelength shift of < 0.02 nm/A between threshold and maximum power corresponds to a temperature rise during operation of only 0.3 K. This indicates a low thermal influence from the PA to the MO and allows a free choice of excitation power for applications. Compared to previously reported free-running Y-branch diode lasers, the MOPA does not show lateral spatial tilts between the two far field intensity distributions at both wavelengths. This compact MOPA allows addressing applications such as shifted excitation Raman difference spectroscopy under in-situ conditions and confocal Raman microscopy without the need of a spectral recalibration during the measurements. In addition, simultaneous dual-wavelength operation also enables terahertz frequency generation.