KEYWORDS: Laser stabilization, Design, Field effect transistors, Control systems, Analog to digital converters, Semiconductor lasers, Field programmable gate arrays, Signal processing, Sampling rates
Narrow line width lasers with high stability are a key enabling technology in high-impact applications such as quantum technologies and laser surgery. On the one hand, stabilizing such lasers implies the design of efficient Proportional-Integral-Derivative (PID) controllers. On the other hand, high end applications require PID controllers with very high regulation bandwidth, relocking feature and easy to adjust parameters. In order to tackle these challenges, we designed and enhanced a PID controller using the pipeline technique. This allowed us to improve the sample rate at which the controller operates. The designed PID has been successfully used in a VBG based external cavity for laser stabilization with ultra-narrow line width.
To meet the requirements for multi-species gas analysis, Quartz-Enhanced Photo-Acoustic Spectroscopy (QEPAS) is used in combination with an IC-based External Cavity Laser system (IC-ECL). The laser system allows the coverage of a wavelength range of 285 nm with an output power of several mW. By integrating piezoelectric actuators as well as resonantly driven MEMS actuators, extremely high sampling rates can be achieved. In this work, results on the detection of multiple trace gases by sequential quasi-simultaneous measurements are presented. The requirements of multi-species detection, output power, tuning range and detection rate are met by our work.
We use a slotted Y-branch Laser for Terahertz thickness measurements of high resistive float zone silicon wafers of different thicknesses. The laser provides two-color emission in the 1550 nm region with an optical beat frequency of 1 THz. It is used as a photonic source for thickness measurements of high resistive silicon wafers with continuous wave Terahertz radiation. Frequency tuning is obtained through segment current tuning of the individual branches. We determine the sample´s refractive index and thickness by MSE fitting of the theoretical etalon transmission to the experimental results without additional knowledge.
External resonator diode lasers are the appropriate choice for species detection in application areas such as medicine, climate and industry due to their excellent properties, but have limitations in terms of high detection rates and commercial availability in the MIR region. Especially in the MIR region, many molecules have particularly strong absorption bands, which can result in very low detection limits and is therefore of particular interest. In this paper, we present our new ICbased laser chips with straight and curved waveguides with a center wavelength at 3.4 μm. These are integrated into an external resonator setup and characterized. The IC-based system enables continuous wave operation at room temperature over a wavelength range of 285 nm with several mW output power. With respect to the problem of high sampling rates, one promising technique is MEMS technology integrated as a tuning element in the external resonator structure. This enables planar drive control for high-frequency resonance-driven MEMS scanners, where the sampling frequency corresponds to the resonance frequency. These will be tested for their suitability and integrated into an ECDL setup and evaluated. Our work will address new requirements in terms of tuning range, output power, and acquisition rate.
This work investigates a monolithic slotted Y-branch diode laser as a beating source to drive a continuous wave Terahertz spectrometer. Both arms of the Y-branch laser exhibit spectral selective feedback, which causes simultaneous emission at two frequencies. At first, a thorough optical characterisation with 5400 individual setpoints is performed to find the best point of operation. Two operational regimes with difference frequencies of 1 THz ± 10.5 GHz and 0.85 THz ± 6.5 GHz were identified. While validating the laser as a beating source to drive a cw-THz spectrometer, it was demonstrated that the device supports current-induced tuning of the emitted difference frequency. This technique allows frequency sweeps in the terahertz regime that can be used to measure the transmitted field without a mechanical delay stage. Finally, this technique is demonstrated to independently determine the thickness and refractive index of high resistive float zone silicon wafers of 2, 3.5, 4 and 8 mm thickness without a priori knowledge.
The detection and identification of molecular gases are of high relevance in many applications within healthcare, production monitoring and safety as well as environmental monitoring. One of the major difficulties of trace gas analysis is due to the bulky and expensive systems, what excludes both mobile and handheld use. For this purpose we present our new system based on the Quartz Enhanced PhotoAcoustic Spectroscopy (QEPAS), which can provide the required properties for gas analysis. We have developed a compact detection unit where DFB laser, collimation optics and QTF are integrated in a 14-pin butterfly housing. Therefore an optimization of the DFB laser chips will be presented too. The results show, that the laser diodes not only provide excellent performance, but also allow a detection limit for the greenhouse gas methane and carbon dioxide in the ppm range.
Although external cavity diode lasers have become firmly established for their excellent properties for species detection, they have severe limitations in terms of high acquisition rates. In this paper, we present our new ECDL design based on a resonantly driven MEMS scanner. By using the MEMS technology, a defined frequency range can be tuned extremely fast and without mode-hops. This allows scanning frequencies in the high kHz range to be achieved. The results of the characterization of the spectral properties of the MEMS-based system and its use for rapid detection of trace gases are presented.
External cavity diode lasers (ECDLs) are a well-established laboratory tool due to their excellent emission properties. However, if the ECDLs are used outside the laboratory, they have limitations in terms of tuning speed and robustness. For overcoming these limitations, we developed a new micro-electro-mechanical system (MEMS) based ECDL cavity concept. The 1D MEMS actuator defines the angle of incidence at the diffraction grating as well as the cavity length of the ECDL. Due to the high resonance frequency of the MEMS actuator in the kHz range, the switching speed of the ECDL emission wavelength is drastically reduced. Furthermore, the MEMS actuator minimizes the sensitivity to external disturbance which opens a path to handheld wide mode-hop free tunable ECDLs in the near future. Therefore we have also optimized our curved waveguide concept based on GaSb for the ECDL design, whereby a wavelength range from NIR to the MIR range can be better covered. These features qualify the new developed MEMS tunable ECDL for the high demands of the high resolution multi-species molecular spectroscopy. Application examples of the MEMS based ECDL and the curved gain chips will be provided.
GaSb based types of diode lasers may cover the spectral regime from below 1.8 μm up to 5 μm. For the wavelength regime of 1.8 μm to 2.5 μm InGaAsSb/GaSb MQW material is used. For 2.5 μm to 3.4 μm InAlGaAsSb/GaSb MQW material is used. For above 3μm, an ICL type of design is required. We realized a growth campaign of 10 GaSb based wavers for covering the wavelength regime from 1.9μm to 3μm. We report on the test, performance and applications results in molecular gas sensing of both, gain chips within an external cavity laser as well as on digital DFB lasers.
Tunable laser sources are used in a wide range of novel applications such as spectroscopy, biomedicine or gas sensing techniques. New requirements in terms of size, tuning speed and output power are addressed with our work.
We present a miniaturized external cavity diode laser concept which will be compared with well-known laser systems such as distributed feedback (DFB) lasers. DFB lasers suffer high internal losses due to the overlap of the DFB grating with the optical waveguide. Our concept of Micro Electro Mechanical Systems (MEMS) based lasers are stabilized with a transmission grating, resulting in significantly less losses. Furthermore, the tuning of the diffraction efficiency of the gratings allows the optimization of the output power and the overall tuning range, which is measured to be one order of magnitude larger than what can be achieved with DFBs. It is also important to point out the tuning speed of the MEMS lasers due to the fast nature of the tilting capabilities of the MEMS actuators. Excellent relative intensity noise and narrow linewidth features are present in these laser systems due to the low noise driving electronics for both the diode lasers and the MEMS actuators.
The high output power and the low linewidth will enable a higher sensitivity and resolution for a wide range of applications. The performance of the MEMS laser systems will be presented, being suitable for applications such as Raman spectroscopy or tunable diode laser absorption spectroscopy (TDLAS) in the wavelength ranges of 780 nm and 920 nm.
The NIR/MIR region between 1.8μm and 3.5μm contains important absorption lines for gas detection. State of the art are InP laser based setups, which show poor gain above 1.8μm and cannot be applied beyond 2.1μm. GaSb laser show a significantly higher output power (100mW for Fabry-Perot, 30mW for DFB). The laser design is presented with simulation and actual performance data. The superior performance of the GaSb lasers is verified in gas sensing applications. TDLAS and QEPAS measurements at trace gases like CH4, CO2 and N2O are shown to prove the spectroscopy performance.
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