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.
With emerging technologies, which result in more intensive industry and manufacturing, fully portable and multi-gas detectors are increasingly being required on to handle situations where a wider range of hazardous substances may be present and proper detection equipment is essential.
Most gas monitoring devices focus on a single species. However, there are many sensing applications (e.g.: industrial chemical processes) where several gases need monitoring at the same time and with a single detector. In this regard, a novel portable QEPAS (Quartz-Enhanced Photo-Acoustic Spectroscopy) multi-gas detector with non overlapping QTFs (Quartz Tuning Forks) resonance frequencies can help meet these challenges.
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.
Sacher Lasertechnik demonstrates a saturated Rubidium Absorption experiment based on our next generation Micro X Cavity 780nm laser system. This laser system is integrated with a compact saturated Rubidium Absorption Setup. Contact: Sacher Lasertechnik GmbH, Tel.: +49 6421 305290, Email: Contact@Sacher-Laser.com, Web: https://micron.sacher-laser.com
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.
KEYWORDS: Field programmable gate arrays, Signal to noise ratio, Absorption, Amplifiers, Modulation, Linear filtering, Spectroscopy, Signal detection, Methane, Photoacoustic spectroscopy
Lock-in amplifiers are a powerful tool for signal detection within a noise environment. Commercial Lock-in amplifiers are bulky and disqualified for handheld operation. We report recent progress on FPGA based lock-in real-time detection scheme with the application in Quartz-Enhanced-Photo-Acoustic Spectroscopy (QEPAS). The new QEPAS configuration is tested and verified on a fast Methane detection scheme in the 1650 nm spectral regime. The novel FPGA detection scheme can be easily transferred into other spectral regimes and offers the opportunity of multi-species real-time measurements.
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 diode lasers are an important tool for spectroscopy and as laser sources for a wide range of applications. In this paper, an improvement of External Cavity Diode Lasers (ECDLs) is presented. The present generation of ECDLs is designed as a laboratory instrument which is sensitive against ambient disturbance like shock, noise, and temperature fluctuations. In addition, state of the art ECDLs in Littrow and Littman/Metcalf configuration have limitations in terms of tuning range, tuning speed, and size. These technologically disadvantages make it difficult to use ECDLs for various applications. Therefore, we developed a new miniaturized mode-hop free tunable next-generation ECDL design based on a Micro Electro Mechanical System (MEMS) device. It includes the benefits of the current ECDL technology and allows an outstanding improvement in terms of efficiency, stability, repeatability and tuning range. Moreover, the tuning speed is increased into the kHz regime due to the fast nature of the tilting capabilities of the MEMS actuators. The focus will be set on the initial use of this new design in connection with semiconductor laser chips based on GaAs, InP, GaSb and IC. This makes it possible to cover a large area from the near-infrared up to the mid-infrared. Especially the midinfrared contains stronger absorption lines of significant gases, which are of great interest in the field of biomedicine, process control and environmental monitoring. The excellent performance of this innovative ECDL cavity design as well as the low noise promises better possibilities of gas detection for the previously mentioned applications.
The MIR wavelength regime promises lower gas detection limits than the NIR or the VIS region due to higher absorption levels as one can read for simulation listed in HITRAN. Methane shows moderate absorbance below 3 μm which results into detection limits in the range of low ppm. IC and QC based lasers emit higher wavelengths, where the absorbances of methane are higher. TDLAS and QEPAS measurements to the trace gas CH4 are shown to display the spectroscopy performance of the different lasers with and without influences from the detector material. In this manuscript only QEPAS measurements will be presented. Scope of this paper is a quantitative comparison of the absorption and QEPAS behaviour of Methane in four important spectral regimes.
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.
We report the development of a platform, based-on a Field-Programmable Gate Arrays (FPGAs) and suitable for Time-Division-Multiplexed DFB lasers. The designed platform is subsequently combined with a spectroscopy setup, for detection and quantification of species in a gas mixture. The experimental results show a detection limit of 460 ppm, an uncertainty of 0.1% and a computation time of less than 1000 clock cycles. The proposed system offers a high level of flexibility and is applicable to arbitrary types of gas-mixtures.
Narrow linewidth tunable diode lasers are an important tool for spectroscopic instrumentation. Conventional external cavity diode lasers are designed as laboratory instrument and do not allow hand-held operation for portable instruments. A new miniaturized type of tunable external cavity tunable diode laser will be presented. The presentation will focus on requirements on the assembly technology of micro-optic components as well as on the physical properties of such devices. Examples for the realization of this new technology will be given in the NIR for Alkaline Spectroscopy as well as in the MIR at 1908nm.
This paper, originally published on 27 April 2016, was replaced with a corrected/revised version on 8 June 2016. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.
The purpose of wavelength-beam combining (WBC) is to improve the output power of a multi-wavelength laser system while maintaining the quality of the combined beam. This technique has been primarily proposed for industrial applications, such as metal cutting and soldering, which require optical peak power between kilowatts and megawatts. In order to replace the bulkier solid-state lasers, we propose to use the WBC technique for photoacoustic (PA) applications, where a multi-wavelength focused beam with optical peak power between hundreds of watts up to several kilowatts is necessary to penetrate deeply into biological tissues. In this work we present an analytical study about the coupling of light beams emitted by diode laser bars at 808 nm, 880 nm, 910 nm, 940 nm, and 980 nm into a < 600-μm core-diameter optical fiber for PA endoscopy. In order to achieve an efficient coupling it is necessary to collimate the beams in both fast and slow axes by means of cylindrical lenses and to use partial reflection mirrors at 45° tilt. We show an example of beam collimation using cylindrical lenses in both fast and slow axes. In a real PA scenario, the resulting beam should have a sufficient peak power to generate significant PA signals from a turbid tissue>.
A new setup for efficient blue light generation that consists of two passively coupled optical resonators is presented. The
first resonator is based on a broad area laser diode (BAL) in a Littrow external cavity with a special off-axis design. This
external cavity diode laser provides more than 450 mW diffraction limited and narrow bandwidth emission at 976 nm. A
compact cavity design with 40 mm length could be realized. The second resonator is a monolithic high finesse ring
cavity containing a 10 mm bulk periodically poled lithium niobate (PPLN) crystal for resonant second harmonic
generation. This ring resonator consists of four small mirrors with appropriate reflectivities and two GRIN lenses for
stability reasons. All parts of this ring cavity are mounted monolithically on a glass substrate with a size of 19.5 mm x
8.5 mm. First experiments showed good passive matching of both cavities without any active closed-loop control. With
this setup efficient SHG was achieved. A maximum optical output power of 70 mW blue light at 488 nm was obtained.
The conversion efficiency was better than 15%.
Coherent cw-THz-radiation allows access to new applications in the field of medicine, industrial process control, data
communication and security applications. Major advantages of radiation in this spectral range are that it penetrates
through e.g. plastics but is strongly reflected by metals and that molecules show distinct and distinguishable spectra so
that a selective sensing of single species is possible. However, existing THz-sources are either very bulky or expensive.
THz sources can require cryogenic temperatures or emit only low power radiation. Furthermore the setup is often very
complicated and sensitive so that field measurements are not possible.
Generation of THz radiation based on the technology of frequency mixing requires laser radiation with a difference
frequency in the order of 0.1-2 THz. Due to the low efficiency of frequency mixers, high optical power is required for
pumping frequency mixers. Furthermore, the small efficiency requires short optical pulses for avoiding a high heat
dissipation of the frequency mixers.
We investigated an ultra stable 1W two colour THz pump source for the generation of a THz beat signal with rapid
single mode tuning over several THz. The system consist of a fixed wavelength and a motorized tuneable laser pump
sources which are optical amplified within a pulse operation module. One laser is stabilized to an atomic reference while
the other is locked to an optical cavity which can be tuned continuously.
This signal is pump source for a state of the art frequency mixer, which is typically realized as LT-GaAs crystal with an
antenna design.
The combination of high power, small linewidth and fast tunability is essential for many fields in high resolution spectroscopy. External cavity laser diode systems are limited in tuning speed to several kHz by the resonance frequency of the mechanical assembly together with the actuator. We report on the application of a directly modulated DFB laser as master laser within a master laser power amplifier (MOPA) configuration. This DFB MOPA system combines fast frequency tuning up to more then 100kHz tuning speed, a tuning amplitude of more than 10GHz, a narrow linewidth below 5MHz with high output power of 1500mW and an almost Gaussian shaped beam quality (M2<1.2). The coupling efficiency to optical waveguides as well as single mode fibers exceeds 60%. This concept can be realized within the wavelength regime between 730 and 1060nm. We approved this light source for high resolution spectroscopy by frequency locking to the saturated Rubidium absorption at 780nm. Applying two DFB lasers as master lasers of the MOPA configuration opens the choice to high frequency modulated THz radiation.
High resolution spectroscopy of environmental and medical gases requires reliable, fast tunable laser light sources in the mid-infrared (MIR) wavelength regime between 3 and 5 μm. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources. We report a new, very compact, alignment insensitive, robust, external cavity diode laser system in Littman/Metcalf configuration with an output power of 1000 mW and an almost Gaussian shaped beam quality (M2<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The center wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally single mode with a mode-hop free tuning range of up to 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept can be realized within the wavelength regime between 750 and 1060 nm. We investigated this light source for high resolution spectroscopy in the field of Cavity Ring-Down Spectroscopy (CRDS). Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes difference-frequency generation in Periodically Poled Lithium Niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four resolved isotopic H2O components. This application clearly demonstrates the suitability of this laser for high-precision measurements.
KEYWORDS: Semiconductor lasers, High power lasers, Laser systems engineering, Absorption, Spectroscopy, Chemical species, Diodes, Light sources, High power diode lasers, Resonators
The combination of high power, small linewidth and rapid tuneability is essential for many fields in high resolution spectroscopy. Furthermore these optical features are essential for laser-cooling techniques. Enhancement of high power lasers with excellent spectral and spatial quality is currently an important research subject. The requirements for a laser system applied in both fields of application are demanding: a mode-hop free tuning range of a few GHz, with a linewidth in the order of 1MHz and an output power of a few 100mW. We report a very compact external cavity diode laser system (ECDL) with an output power of up to 800mW with an almost Gaussian shaped beam quality (M2<1.2). The coupling efficiency for a single mode fibre exceeds 60%. The centre wavelength can be preadjusted within the tuning range of 20 nm. This laser operates single mode with a mode-hop free tuning range of up to 15GHz without current compensation and a side-mode-suppression better than 50dB at different wavelength between 730 and 1060nm.
To demonstrate the suitability for neutral atom cooling we used this laser as light source in the production of a BEC of over a million 87Rb atoms. In addition we approved this light source for high resolution spectroscopy, more precisely for the Cavity-Ring-Down-Spectroscopy (CRDS). Our ECDL was part of a MIR-light source which utilizes difference-frequency-generation in PPLN. At the wavelength of 3.3μm we were able to perform a high resolution absorption measurement of 50ppb Ethane. Both applications clearly demonstrate the suitability of this laser for high-precision measurements.
KEYWORDS: High power lasers, Chemical species, Semiconductor lasers, Laser systems engineering, Modulation, Rubidium, Tunable lasers, Single mode fibers, Laser stabilization, Diodes
Since the introduction of laser-cooling techniques for neutral atoms, the enhancement of high-power lasers with excellent spectral and spatial quality has been an important research subject. We report a new principle of using high-power laserdiodes directly in an external cavity. The very compact design offers an output power of up to 1 W and an excellent beam quality (M2 < 1.2). The coupling efficiency for a single mode fiber exceeds 60%. The center wavelength can be tuned between 775 nm and 785 nm. This laser operates single mode with a mode-hop free tuning range of up to 15 GHz without current modulation and a side-mode suppression better than 55 dB. Demonstrating the suitability for neutral atom cooling we used this laser as light source in the production of a BEC of over a million 87Rb atoms.
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