Lars Hildebrandt

DFB Lasers Between 760 nm and 16 µm for Sensing Applications

Recent years have shown the importance of tunable semiconductor lasers in optical sensing. We describe the status quo concerning DFB laser diodes between 760 nm and 3,000 nm as well as new developments aiming for up to 80 nm tuning range in this spectral region. Furthermore we report on QCL between 3 μm and 16 μm and present new developments. An overview of the most interesting applications using such devices is given at the end of this paper.

Antireflection-coated blue GaN laser diodes in an external cavity and Doppler-free indium absorption spectroscopy

Commercially available GaN-based laser diodes were antireflection coated in our laboratory and operated in an external cavity in a Littrow configuration. A total tuning range of typically 4 nm and an optical output power of up to 30 mW were observed after optimization of the external cavity. The linewidth was measured with a beterodyne technique, and 0.8 MHz at a sweep time of 50 ms was obtained. The mode-hop-free tuning range was more than 50 GHz. We demonstrated the performance of the laser by detecting the saturated absorption spectrum of atomic indium at 410 nm, allowing observation of well-resolved Lamb dips.

Compact tunable diode laser with diffraction-limited 1 Watt for atom cooling and trapping

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.

DFB interband cascade lasers for tunable laser absorption spectroscopy from 3 to 6 μm

Tunable Laser Absorption Spectroscopy in the mid-infrared wavelength region, especially between 3 and 6 µm, is of great interest for high performance gas sensing applications. Interband Cascade Lasers can provide monomode continuous wave (cw) emission above room temperature in this wavelength range. We present the simulation, design and manufacturing of application-grade distributed feedback laser devices based on this concept. The fabricated devices successfully target specific, technologically relevant, wavelengths in cw operation above room temperature with low energy consumption. Output powers above 5 mW, high Side Mode Suppression Ratio around 30 dB and tuning ranges of up to13 nm were achieved.

Kompakte External Cavity Dioden Laser für die industrielle Messtechnik (Compact External Cavity Diode Laser for Industrial Analysis)

Fabry-Perot (FP)-Laserdioden sind kompakte, zuverlässige und kostengünstige Laserlichtquellen für eine Reihe von Anwendungsgebieten in Wissenschaft und Technik [1]. Jedoch sind die Emissionseigenschaften bei FP-Laserdioden nicht ideal, insbesondere ist die Durchstimmbarkeit der Wellenlänge sowie die Seitenmodenunterdrückung meist zu gering. Weit bessere Emissionseigenschaften zeigen distributed feedback (DFB)-Diodenlaser, die jedoch wegen der höheren Kosten und der - im Vergleich zu FP-Laserdioden in externen Resonatoren - geringen Durchstimmbarkeit und Leistung für technische Anwendungen häufig nicht in Frage kommen. Entspiegelte FP-Laserdioden in externen Resonatoren können mit ihrem großen Durchstimmbereich, schmaler Linienbreite und hoher Leistung eine Alternative darstellen.

Distributed feedback interband cascade lasers for applications in research and industry

In recent years, the use of laser sources in gas sensing applications has been increasing continuously. Tunable Laser Absorption Spectroscopy (TLAS) has proven to be a versatile tool in a variety of sectors including industry, health and security and modern environmental analysis. Especially the mid-infrared wavelength range is of great interest for high accuracy gas sensing applications, since many technologically and industrially relevant gas species have their strongest absorption features in the spectral region between 3 and 6 μm. These include, e. g., important hydrocarbons like methane or propane, as well as nitric oxide and formaldehyde. Interband cascade lasers (ICL) provide mono mode continuous wave (CW) operation above room temperature in this wavelength range. Application-grade complex coupled distributed feedback (DFB) laser devices based on the ICL concept are presented, using lateral metal gratings as wavelength selective elements. The fabricated devices operate at specific, technologically relevant, emission wavelengths in the spectral region from 3 to 6 μm. CW operation up to 80 °C and mono mode wavelength tuning ranges above 20 nm were achieved with low energy consumption. Application examples in industry and research are presented that demonstrate the high potential of DFB ICLs for the use in TLAS. E. g., formaldehyde gas sensor systems based on DFB ICL devices operating around 3.6 μm can provide realtime in-situ measurements with resolution limits in the low ppb range, even in dense background atmospheres. The low power consumption of ICL based devices makes them especially favorable for battery-powered or portable sensor applications.

External-Cavity-QCL-Systeme (External Cavity QCL Systems)

Abstract Für zahlreiche Anwendungen in der industriellen Messtechnik werden durchstimmbare Laser im mittleren infraroten Spektralbereich benötigt. Aufgrund charakteristischer Spektren vieler umwelt- und medizinisch relevanter Moleküle ist dieser Bereich von besonderem Interesse. Um einen Laser mit einem großen monomodig spektral durchstimmbaren Wellenlängenbereich zu realisieren, sind Quantenkaskadenlaser entspiegelt und in einem externen Resonator betrieben worden.

Novel mono mode interband cascade laser sources for challenging TLAS applications in the MIR

Tunable Laser Absorption Spectroscopy (TLAS) in the mid-infrared (MIR) wavelength region from 3 to 6 μm has proven to be a versatile tool for a variety of challenging gas sensing applications, especially for many technologically or environmentally relevant gases like e.g., methane or sulfur dioxide. In general, semiconductor lasers used for Tunable Laser Absorption Spectroscopy comprise also quantum cascade lasers (QCLs) and diode lasers, but both approaches experience problems for TLAS in the mid-infrared wavelength region. While conventional diode lasers lack the ability to cover the entire target wavelength range from 3 to 6 microns, quantum cascade lasers can reach this spectral range of interest, but they still suffer from extensive threshold power densities as well as a high overall power consumption compared to interband cascade lasers (ICLs). Interband cascade lasers are able to offer suitable application-grade device performance figures with low power consumption in the wavelength range of interest for Tunable Laser Absorption Spectroscopy applications. TLAS requires single mode emitting devices, like distributed feedback (DFB) lasers. To fabricate DFB ICLs, we use metal grating structures laterally to the laser waveguide ridge defined by e-beam lithography. Based on different epitaxial material, the current short-wavelength as well as long-wavelength records for GaSb based DFB ICLs were observed. Distributed feedback lasers with tuning ranges up to 25 nm, side mode suppression ratio> 30 dB and low power consumption below 250 mW at room temperature in continuous wave operation have been achieved. These devices and the related performance enable a wide variety of new applications, from process control to environmental monitoring or applications in medical diagnosis like breath analysis.

Cost-effective tunable laser gas-sensor module for high-volume applications, using DFB laser diodes in the NIR, and ICL in the MIR

This talk gives an insight into the development and the device performance of various tunable Laser Diodes (LD) in the near infrared (NIR) region between 760 nm and 3000 nm, and Interband Cascade Laser (ICL) based sources in the mid infrared (MIR) wavelength region between 3000 nm and 6000 nm for application in high accuracy gas sensing. Distributed Feedback (DFB) type ICL and LD devices based on a lateral metal grating structure were fabricated using an overgrowth free processing route. These devices have been realized in the entire spectral range from 760 nm to 5200 nm up to now. Total tuning ranges of up to 20 nm can be covered by these DFB devices providing typically a few mW of optical output power with low power consumption based on a low operating current and voltage. All fabricated NIR and MIR sources are targeting high performance sensor applications in fields like industrial process control or environmental monitoring. Various application examples utilizing this Tunable Laser Absorption Spectroscopy (TLAS) will be highlighted in the talk.

Distributed feedback interband cascade lasers for spectroscopy from 3-6 μm

In recent years the importance of lasers in optical gas sensing has been continuously increasing. Tunable Laser Absorption Spectroscopy (TLAS) has proven to be a versatile tool in modern environmental analysis. In the mid-infrared wavelength region between 3 and 6 µm, which is of high interest for sensing applications, Interband Cascade Lasers (ICL) can provide monomode continuous wave (CW) emission at room temperature. We present the simulation, design and manufacturing of distributed feedback (DFB) laser devices based on this concept, with focus on devices that target specific, technologically and industrially relevant, wavelengths with low energy consumption. Finally application-grade devices from 3 to 6 µm are presented. CW operation above room temperature and tuning ranges of 11 nm with Side Mode Suppression Ratios (SMSR) greater 30 dB were achieved.