Raman spectroscopy is a well-established analytical tool that provides a molecular fingerprint of the sample [1]. Utilization of diode lasers as a light source had a big impact in this field, especially for the realization of portable Raman sensors. However, weak Raman signals can be masked by the background interferences. Among others, an effective technique to address this issue is shifted excitation Raman difference spectroscopy (SERDS). It requires a narrowband light source with two slightly shifted emission wavelengths. Corresponding dual-wavelength Y-branch distributed Bragg reflector (DBR) diode lasers suitable for SERDS have been demonstrated and applied in the near-infrared spectral range [2]. At lower excitation wavelengths for higher Raman intensities, the realization of such light sources in the red spectral range around 633 nm has yet been difficult due to the material system and technological challenges. In this paper, for the first time, a monolithic dual-wavelength Y-branch DBR diode laser emitting at 633 nm suitable for Raman spectroscopy and SERDS is presented.

A dual-wavelength monolithic Y-branch distributed Bragg reflector diode laser at 633 nm is presented, which is suitable for shifted excitation Raman difference spectroscopy to effectively address fluorescence and background interference. The device provides 30 mW optical output power at an electrical power consumption of less than 1 W. At a spectral distance of 0.4 nm (10 cm−1), both laser emission wavelengths show narrowband operation with spectral widths of 12 pm (0.3 cm−1) and side mode suppression ratios of more than 40 dB. Performing shifted excitation Raman difference spectroscopy measurements on a highly fluorescent soil sample exemplarily showed the efficient separation of characteristic Raman signals of the soil constituents quartz and calcite from intense fluorescence interference with a 17-fold improvement in the signal-to-background noise ratio in comparison to the individual Raman measurements.

Diode laser based light sources (implemented monolithically or in a hybrid configuration) offer various functionalities to meet the requirements of specific applications. This includes tuning or switching between different wavelengths, modulating the optical output power, or implemented frequency conversion. Such light sources often contain multisection diode lasers or several active elements. Their operation requires multiple individually adjustable current sources, galvanically isolated current sources, and temperature control. A suitable optical interface should be available for a subsequent integration of the turnkey into the addressed application. In this contribution, a versatile turnkey system meeting the above-mentioned requirements will be presented. Ten p-type current sources, each with currents up to 750 mA, and four galvanically decoupled current sources are implemented. The ten individual sources enable switching frequencies up to 1 kHz and can be combined to provide currents up to 7.5 A. A temperature control unit capable to remove 10 W thermal load using a Peltier element completes the system, which contains an internal microcontroller, trigger in- and outputs, and an USB interface for the integration into various environments. Moreover, fiber coupling and free space optics to transfer the laser emission are offered. Turnkey systems containing in-house developed light sources at 488 nm or 785 nm were implemented into portable Raman spectroscopic measurement systems. To separate Raman signals from background disturbances, shifted excitation Raman difference spectroscopy (SERDS) was applied using dual-wavelength light sources. Systems addressing the measurement of carotenoids under clinical conditions and soil properties in the field will be presented.

Y-branch distributed Bragg reflector (DBR) diode lasers with a stable narrowband emission in simultaneous dual-wavelength operation with spectral distances below 3.2 nm are presented. The Y-branch laser consists of two laser branches with different DBR gratings serving as wavelength-selective rear-side mirrors. Therefore, two emission wavelengths with a spectral distance defined by the DBR grating periods can be generated simultaneously. A Y-coupler combines the two ridge waveguide (RW) branches into a single straight output RW. Devices with a spectral distance of 0.6 nm and 2.0 nm emitting around 785 nm are manufactured. Selecting the operation parameters carefully, stable narrowband emission for both wavelengths is obtained. Resistors serving as heaters implemented next to the DBR gratings allow for wavelength adjustment and a tuning of the spectral distance. At an optical output power of 100 mW, the spectral distance can be shifted from 0 to 1.55 nm (0–0.76 THz) for the former device or from 1.00 to 3.15 nm (0.49–1.54 THz) for the latter device, respectively. This makes the Y-branch DBR diode laser particularly interesting for the generation of THz beat-note signals, needed to generate THz radiation via photo-mixing.

The use of aspherical and freeform optics is increasingly prevalent to overcome aberrations and facilitate compact optical systems. However, achieving simultaneously fast, flexible, and accurate measurements of such surfaces remains a challenge. Multiple Aperture Shear-Interferometry (MArS) utilizes multi-spot illumination to measure a wide spectrum of such surfaces without adjusting the measurement system. Using the wave vectors of wave fields reflected by the surface as well as the positions of the individual light sources, the surface form is reconstructed using an inverse ray-tracing approach. Hence, the accuracy of the measurement directly depends on knowledge of these entities. In this publication we present a calibration approach for wave vectors incident across the measurement plane. Secondly, we introduce a geometric and spectral calibration method for the multi-spot illumination, ensuring accurate and consistent measurements across diverse surfaces.

We explore the principles, implementation details, and performance characteristics of a lensless multi-spectral digital holographic sensor and demonstrate its potential for quality assurance in semiconductor manufacturing. The method is based on capturing multi-spectral digital holograms, which are subsequently utilized to evaluate the shape of a reflective test object. It allows for a compact setup satisfying high demands regarding robustness against mechanical vibrations and thus overcomes limitations associated with conventional optical inspection setups associated with lens-based white light interferometry. Additionally, the tunable laser source enhances the versatility of the system and enables adaptation to various sample characteristics. Experimental results based on a wafer test specimen demonstrate the effectiveness of the method. The axial resolution of the sensor is ±2.5 nm, corresponding to 1σ.

Digital holography allows for the recording and reconstruction of three-dimensional images using interference and diffraction principles. The propagation of light from the hologram plane to the reconstruction plane is a crucial step, often achieved through Fresnel propagation, a method that inherently transforms the reconstructed pixel pitch to provide diffraction-limited imaging. However, the accuracy of this method is limited by the Fresnel approximation, especially in applications such as digital holographic microscopy. We present a simple method that significantly improves the accuracy of the Fresnel approximation by incorporating higher orders of the binomial approximation. We validate the effectiveness of our approach through high numerical aperture simulations and experimental results, demonstrating superior sub-micron resolution and reduced distortions compared with standard Fresnel propagation.

Diode laser-based dual-wavelength light sources are experimentally compared in individual and common operation. First, a Y-branch distributed Bragg reflector diode laser is presented. It consists of two laser cavities with a single output waveguide. The device provides 180 mW and dual-wavelength laser emission around 785 nm. The measured spectral widths and spectral distance are 20 pm and 0.6 nm, respectively. Resistors implemented next to the gratings allow changing the wavelength spacing within a range of 0.0 nm - 1.7 nm. Lateral far field profiles show a strong modulation and a lateral shift of 1° between both far fields indicates beam steering. Second, a multimode interference coupler-based master oscillator power amplifier is presented. It provides 500 mW dual-wavelength laser emission. Within the available power range, spectral widths of 20 pm and nearly constant peak emission wavelengths are measured. In comparison to quasicontinuous wavelength tuning obtained for the Y-branch laser, the MMI MOPA enables non-continuous wavelength tuning. As an example, selected spectral distances of 0.0 nm, 0.5 nm, 1.0 nm, 1.5 nm, and 2.0 nm are demonstrated in individual operation. Beam steering is successfully eliminated. Near field widths of 5 μm and far field angles of 15° result in beam propagation ratios of M2 = 1.2 at the 1/e2 level in all operation modes. This enables easy beam shaping or efficient single-mode fiber coupling. Both devices are suitable for spectroscopic applications such as Raman spectroscopy and shifted excitation Raman difference spectroscopy as well as for the generation of THz radiation by photomixing.

Flash-profilometry is a novel measurement approach based on the fullfield lensless acquisition of spectral holograms. It is based on spectral sampling of the mutual coherence function and the subsequent calculation of its propagation along the optical axis several times the depth-of-field. Numerical propagation of the entire coherence function, rather than solely the complex amplitude, allows to digitally reproduce a complete scanning white-light interferometric (WLI) measurement. Hence, the corresponding 3D surface profiling system presented here achieves precision in the low nanometer range along an axial measurement range of several hundred micrometers. Due to the lensless setup, it is compact, immune against dispersion effects and lightweight. Additionally, because of the spectral sampling approach, it is faster than conventional coherence scanning WLI and robust against mechanical distortions, such as vibrations and rigid body movements. Flash-profilometry is therefore suitable for a wide range of applications, such as surface metrology, optical inspection, and material science and appears to be particularly suitable for a direct integration into production environments.

Monolithic multi-wavelength diode lasers are of great interest in application fields such as THz-generation or shifted excitation Raman difference spectroscopy. Combining the laser light of multiple laser resonators placed next to each other on the same chip is necessary to realize multi-wavelength laser emission from a common output aperture. This can be obtained by implementing bent waveguides, which launch the laser light into a Y- [1] or MMI-coupler[2]. However, bent waveguides can introduce additional waveguide losses, which potentially reduce the overall laser efficiency. Therefore, it is of great interest to determine the losses introduced by different bent waveguide designs.

Shifted excitation Raman difference spectroscopy (SERDS) with alternating dual-wavelength excitation enables extracting Raman signals that provide molecular target information from background disturbances such as ambient light and fluorescence [1]. Simultaneous dual-wavelength emission is needed for the generation of THz emission to obtain structural target information [2]. A diode laser suitable for such applications should provide dual-wavelength laser emission with a few hundred milliwatts in the near-infrared spectral range. At 785 nm, Y -branch distributed Bragg reflector (DBR) diode lasers have been presented and applied but showed a power-dependent beam steering [3]. Dual-wavelength multi-mode inference (MMI) based devices without beam steering have been presented as a suitable alternative [4].

Complex optical surfaces such as aspheres and freeforms are used in optical systems to reduce aberrations or to achieve high optical performance with a compact design and less optical surfaces. Due to limited acceptance angles of conventional interferometric techniques, there is still no satisfactory solution for their form measurement that is at the same time precise, flexible, and fast. Often these surfaces are surveyed by pointwise measurement, or the aperture problem is overcome by elaborately compensating wave front deviations either through compensator lenses or computer-generated holograms. Alternatively, several subapertures are used to capture the whole surface. These, however, have to be recorded in sequence since the superposition of multiple independent wave fields cannot be assigned a time-independent wave front. Instead, we present a compressive sensing approach for Multiple Aperture Shear-Interferometry (MArS) which captures multiple overlapping subapertures simultaneously and allows a flexible measurement of aspheres with multi-spot illumination. MArS uses the mutual intensity as the primary measurand which is still well defined for superposed mutually incoherent wave fields. The mutual intensity is sparse in phase space for there are only a finite number of distinct wave fields at every surface point. Utilizing this sparsity, the presented compressive sensing approach avoids superflously large space-bandwidth products and significantly reduces the number of necessary measurements.

Today's 3D dynamic holographic display techniques suffer from severe limitations due to an available number of pixels that is several orders of magnitude lower than required by conventional approaches. We introduce a solution to this problem by introducing the concept of functional pixels. This concept is based on pixels that individually spatially modulate the amplitude and phase of incident light with a polynomial function, rather than just a constant phase or amplitude. We show that even in the simple case of a linear modulation of the phase, the pixel count can be drastically reduced up to 3 orders of magnitude while preserving most of the image details. This scheme can be easily implemented with already existing technology, such as micro mirror arrays that provide tip, tilt and piston movement. Even though the individual pixels need to be technologically more advanced, the comparably small number of such pixels required to form a display may pave the way towards true holographic dynamic 3D displays.

In this paper, we present a multicolor display via referenceless phase holography (RELPH). RELPH permits the display of full optical wave fields (amplitude and phase) using two liquid crystal phase-only spatial light modulators in a Michelson-interferometer-based arrangement. Complex wave fields corresponding to arbitrary real or artificial 3D scenes are decomposed into two mutually coherent wave fields of constant amplitude whose phase distributions are modulated onto the wave fields reflected by the respective light modulators. Here, we present the realization of that concept in two different ways: firstly, via temporal multiplexing using a single setup, switching between wavelengths for temporal integration of the respective wavefields; secondly, using spatial multiplexing of different wavelengths with multiple Michelson-based setups; and finally, we present an approach to magnify the 3D scenes displayed by light modulators with limited space–bandwidth product for a comfortable viewing experience.

Lasers in the spectral range around 785 nm are requested as pump lasers for 2 μm eye-safe Tm:YAG lasers and as excitation light sources in Raman spectroscopic experiments with large excitation areas. The output power should be in the range of several watts together with wavelength stabilization and good beam quality. Distributed Bragg Reflector (DBR) tapered diode lasers offer a potential solution. In this contribution 785 nm DBR tapered lasers with a narrow spectral emission width below 1 pm will be presented. The devices are based on GaAsP single quantum wells embedded in a 1 μm thick large optical cavity leading to a vertical far field angle of 29° (FWHM). The 3-inch wafers were grown using metalorganic vapor phase epitaxy. In a full wafer process 4 mm long DBR tapered lasers were manufactured. Two different layouts were processed. One device type (A) consists of a 1.0 mm long 10th order surface DBR grating acting as rear side mirror, a 1.0 mm long ridge waveguide section, and a 2.0 mm long flared section having a full taper angle of 6°, whereas the other one (B) has a 500 μm long DBR grating, a 1.0 mm long RW section and a 2.5 mm long 6° flared section. Both types of devices reach output powers larger than 5 W at 25°C. The device with the shorter tapered section (A) is limited to this output power, whereas the device with longer taper (B) reach up to 7 W within the studied current range up to 8 A. At 3 W output power the latter device has an emission linewidth below 1 pm. Measured at 1/e2 level at this output power the beam waist width of 8.5 μm and the far field angle of 14.4° lead to a beam propagation ratio M2 of 2.1. More than 71% of the emitted power is within the central lobe of the beam waist.

We show that the shape of a surface can be unambiguously determined from investigating the coherence function of a wave-field reflected by the surface and without the requirement of a reference wave. Spatio-temporal sampling facilitates the identification of temporal shifts of the coherence function that correspond to finite height differences of the surface. Evaluating these finite differences allows for the reconstruction of the surface using a numerical integration procedure. Spatial sampling of the coherence function is provided by a shear interferometer whereas temporal sampling is achieved by means of a Soleil-Babinet compensator. This low coherence profiling method allows to determine the shape of an object with sub-micrometer resolution and over a large unambiguity range, although it does not require any isolation against mechanical vibration. The approach therefore opens up a new avenue for precise, rugged optical metrology suitable for industrial in-line applications.

Multiple Aperture Shear-Interferometry (MArS) is a shape measurement technique that uses multi-spot illumination to overcome the problem of a limited observation aperture of conventional interferometric techniques and thus considerably simplifies the measurement of optical aspheres and freeform surfaces. Using a shear interferometry setup, MArS measures the coherence function in order to obtain wave vector distributions created from multi-spot LED illumination reflected by the specimen. Based on the wave vectors we reconstruct the surface topography of aspheric lenses using an inverse ray tracing approach and prior knowledge about the individual source locations. We present the topographic measurement of two aspheric lenses with different global curvature radii measured with the same identical reflection setup. In addition, we examine the achievable accuracy of the wave vector measurement using a single light source to find physical limits of MArS.