Smart laser sources for materials analysis, medicine, metrology, and displays
Progress achieved in diode laser design and technology during recent years is the prerequisite for new kinds of laser sources that are assembled as compact, hybrid diode laser modules. These advancements include improved efficiency, output power, and implementation of gratings as wavelength filters. The novel diode laser modules can be very flexibly designed according to specific requirements, thus making entirely new applications possible. For material analysis in medicine, food control, and environmental analytics, for example, bulky equipment has been used so far. With the sophisticated diode laser modules, they can now be replaced by significantly smaller, thus mobile devices. This also applies to display technology. In addition to the small size, superior brilliance is the key argument for the use of diode lasers in displays. Another example is space applications requiring very compact, "nearly weightless", and robust laser sources. They are used as transmitters in communication channels between satellites, in ultra-precise optical clocks, and in fundamental physics experiments. Such niche products, by the way, are important technology drivers accelerating developments in various further application fields. The smart and compact laser modules are, of course, also highly attractive for stationary devices making the overall system structure much simpler. This way, weight, energy consumption, and service efforts can be reduced.
The FBH has extensive experience and long-term know-how in diode laser chip technology. Core competencies in this field include design, epitaxy, and wafer processing. FBH successfully developed diode lasers with high wall-plug efficiency, very high output powers, and cutting edge brilliance by implementing gratings and other optical mode filters. Thus, diode laser technology provides the basis for tailored chips. To fully exploit the potential of these achievements and to go a step ahead into application, FBH developed its particularly flexible technology for hybrid integration of diode laser chips with further optical and electronic components. As a next step, the development of fiber coupling is now in progress. Chip technology and sophisticated assembly to smart modules under one umbrella turned out to be very successful, leading to effective and short development processes.
Within all these activities, FBH keeps the application in mind right from the start. Thus, working closely with industrial and research partners already during the R&D process enables the FBH to develop customized modules including the necessary adjustments in the course of all stages of module development.
Flexible light sources - customized solutions for specific applications
High-end diode laser chips are the base for small and tailored light sources to be used in a great variety of applications. However, what makes FBH’s compact diode laser sources effectively smart and custom-made, are further sophisticated steps implemented additionally to chip technology. To achieve the desired properties, diode laser chips are flexibly assembled together with appropriate micro-optics and electronic components onto a micro-bench. The overall hybrid-integrated diode laser module is only as small as a matchbox, enabling, for example, beam shaping, modulation, frequency transformation, building external microresonators, and fiber coupling. Thus, it can be flexibly adjusted to the requirements of the respective application.
In its current research activities, FBH is dealing with the specific challenges of each module and application. For a Raman sensor used in material analytics, for example, an optode with optical elements will be integrated to enable direct probing of material. High-power RGB laser sources developed at the FBH require cutting-edge diode laser chips, but also high-end nonlinear crystals and thermal management. In space, extremely robust, vibration-proof, reliable, and stable light sources are needed for harsh environment operation. The small dimensions make these laser modules also ideally suited for portable systems.
Quantum sensors based on cold atoms are currently moving into the focus of various application fields like precision time keeping, geodesy, geo-physics and exploration, navigation, and fundamental physics. Up to now, the operation of these sensors requires an infrastructure that is, in principle, only provided by an optics laboratory. There is also the fact that no equipment existed so far which could survive and operate properly under the harsh conditions of a field deployment and much less in space. Consequently, the German Space Agency DLR, the European Space Agency ESA, and the European Commission through the EU-FP 7 program fund projects that aim at maturing all aspects of the technology relevant for quantum sensor applications.
FBH is contributing to these activities by developing a hybrid micro-integrated diode laser technology. The corresponding compact laser modules meet the electro-optic (EO) requirements and can, at the same time, cope with the operating conditions determined by the harsh environment. In terms of electro-optical performance, the main challenge common to all cold atom quantum sensor applications is the spectral stability and spectral purity of the laser system. While atom interferometry requires lasers with sub-100 kHz linewidth or a fractional frequency instability less than 1 part in 109, the laser linewidth eventually has to be reduced to the sub-1 Hz level (1 part in 1015) for optical atomic clocks. These EO requirements are complemented by the request for minimum mass, minimum volume, maximum energy efficiency, and maximum resilience against mechanical and thermal stress.
Within a public-funded project, FBH has developed suitable compact laser modules for Bose-Einstein condensation and atom interferometry experiments on rubidium atoms. Tests shall be carried out on board a sounding rocket, with a launch in 2013. For that purpose, diode laser modules with external cavity (ECDL) have been micro-integrated onto 50 x 25 mm2 AlN ceramic micro-optical benches. The modules each comprise a semiconductor laser chip, micro lenses for beam collimation, and a volume holographic grating for frequency selection and spectral stabilization. Volume and mass of these laser systems are reduced by a factor of 100 or more as compared to commercially available systems. The laser system design omits any moveable parts in order to provide the mechanical stability and resilience against random vibration and mechanical shock – a relevant precondition for space applications. FBH laser systems have successfully passed random vibration tests up to 20 g (root-mean-square) and are also in operation at the ZARM drop tower in Bremen. Here, the laser system even undergoes accelerations up to 50 g.
H. Müntinga, H. Ahlers, M. Krutzik, A. Wenzlawski, S. Arnold, D. Becker, K. Bongs, H. Dittus, H. Duncker, N. Gaaloul, C. Gherasim, E. Giese, C. Grzeschik, T.W. Hänsch, O. Hellmig, W. Herr, S. Herrmann, E. Kajari, S. Kleinert, C. Lämmerzahl, W. Lewoczko-Adamczyk, J. Malcolm, N. Meyer, R. Nolte, A. Peters, M. Popp, J. Reichel, A. Roura, J. Rudolph, M. Schiemangk, M. Schneider, S.T. Seidel, K. Sengstock, V. Tamma, T. Valenzuela, A. Vogel, R. Walser, T. Wendrich, P. Windpassinger, W. Zeller, T. van Zoest, W. Ertmer, W.P. Schleich, E.M. Rasel, "Interferometry with Bose-Einstein Condensates in Microgravity", Phys. Rev. Lett., vol. 110, no. 093602 (2013).
E. Luvsandamdin, S. Spießberger, M. Schiemangk, A. Sahm, G. Mura, A. Wicht, A. Peters, G. Erbert, G. Tränkle, "Development of narrow linewidth, micro-integrated extended cavity diode lasers for quantum optics experiments in space", Appl. Phys. B, online view (2013).
A. Wicht, E. Luvsandamdin, A. Kohfeldt, M. Schiemangk, S. Spießberger, A. Sahm, F. Bugge, J. Fricke, H. Wenzel, A. Peters, G. Erbert, G. Tränkle, "Micro-integrated, high power, narrow linewidth diode lasers for precision quantum optics experiments in space", Photonics West (2012).
T.-P. Nguyen, M. Schiemangk, S. Spießberger, H. Wenzel, A. Wicht, A. Peters, G. Erbert, G. Tränkle, V. Kueller, U. Zeimer, M. Weyers, C. Reich, and M. Kneissl, "Optimization of 780 nm DFB diode lasers for high-power narrow linewidth emission", Appl. Phys. B, vol. 108, no. 4, pp. 767-771 (2012).
C. Kürbis, A. Kohfeldt, E. Luvsandamdin, M. Schiemangk, S. Spießberger, A. Wicht, A. Peters, G. Erbert, G. Tränkle, "Mikrointegrierte Lasersysteme für die höchstauflösende Atomspektroskopie und die kohärente Nachrichtenübertragung im Weltraum", DGLR (2011).
S. Spießberger, M. Schiemangk, A. Sahm, A. Wicht, H. Wenzel, A. Peters, G. Erbert, G. Tränkle, "Micro-integrated 1 Watt semiconductor laser system with a linewidth of 3.6 kHz", Opt. Express, vol. 19, no. 8, pp. 7077-7083 (2011).
T. van Zoest, N. Gaaloul, Y. Singh, H. Ahlers, W. Herr, S. T. Seidel, W. Ertmer, E. Rasel, M. Eckart, E. Kajari, S. Arnold, G. Nandi, W. P. Schleich, R. Walser, A. Vogel, K. Sengstock, K. Bongs, W. Lewoczko-Adamczyk, M. Schiemangk, T. Schuldt, A. Peters, T. Könemann, H. Müntinga, C. Lämmerzahl, H. Dittus, T. Steinmetz, T. W. Hänsch, J. Reichel, "Bose-Einstein Condensation in Microgravity", DOI: 10.1126/science.1189164, Science 328, 1540 (2010). http://www.sciencemag.org/content/328/5985/1540.full
Narrow linewidth, hybrid integrated extended cavity diode lasers for quantum optics precision experiments in space, International Conference on Atomic Physics (ICAP), Palaiseau, France, July 23-27, 2012.
Towards packaged micro-integrated semiconductor laser modules for the deployment of cold atom based quantum sensors in space, International Conference on Atomic Physics (ICAP), Palaiseau, France, July 23-27, 2012.
Micro-integrated, high power, narrow linewidth laser system for Rb precision quantum optics experiments in space, European Time and Frequency Forum, Goetborg, Sweden, April 23-27, 201.
Diode lasers are attractive excitation light sources for Raman spectroscopy to investigate and detect substances even at low concentrations. Current activities aim at portable measuring systems for different applications, such as food control and medical applications. FBH recently developed a compact diode laser module suitable for Shifted Excitation Raman Difference Spectroscopy (SERDS) allowing to detect weak Raman signals covered by fluorescence or background light. To excite the SERDS signals, FBH’s 671 nm monolithic distributed Bragg reflector (DBR) diode laser uses two neighboring excitation wavelengths. The monolithic laser source together with hybrid-integrated micro-optical elements enables highly compact sensor systems.
The dual-wavelength diode laser (Fig. 1) is the key component with a footprint of 0.5 x 3 mm2, delivering an output power up to 110 mW. The spectral width is smaller than 0.5 cm-1, the spectral distance between both emission lines 10 cm-1. SERDS capabilities were successfully demonstrated by measurements of an analyte (polystyrene) using a broad-band light source simulating a disturbing background (Fig. 2). The Raman lines were covered by the disturbing light. SERDS allows recovering these weak lines from the sample spectrum and shows a 25-fold improvement of signal-to-background noise.
J. Fricke, A. Klehr, O. Brox, W. John, A. Ginolas, P. Ressel, L. Weixelbaum and G. Erbert, "Y-branch coupled DFB-lasers based on high-order Bragg gratings for wavelength stabilization", Semicond. Sci. Technol., vol. 28. no. 035009 (2013).
M. Maiwald, J. Fricke, A. Ginolas, J. Pohl, B. Sumpf, G. Erbert, G. Tränkle, "Dual wavelengths monolithic Y-branch DBR diode laser at 671 nm suitable for shifted excitation Raman difference spectroscopy", accepted for Journal Laser & Photonics Reviews (2013).
Publications - predecessor module:
B. Sumpf, M. Maiwald, A. Müller, A. Ginolas, K. Häusler, G. Erbert, G. Tränkle, "Reliable Operation for 14 500 h of a Wavelength-Stabilized Diode Laser System on a Microoptical Bench at 671 nm", IEEE Trans. Compon. Packag. Technol., vol. 2, no. 1, pp. 116-121 (2012).
M. Maiwald, H. Schmidt, B. Sumpf, G. Erbert, H.-D. Kronfeldt, G. Tränkle, "Microsystem 671 nm light source for shifted excitation Raman difference spectroscopy", vol. 48, no. 15 / Applied Optics (2009).
M. Maiwald, A. Ginolas, A. Müller, A. Sahm, B. Sumpf, G. Erbert, G. Tränkle, "Wavelength-Stabilized Compact Diode Laser System on a Microoptical Bench With 1.5-W Optical Output Power at 671 nm", IEEE Photonics Technology Letters, vol. 20, no. 19, (2008).
Pico-projectors based on diode lasers are already commercially available. However, projectors requiring a higher light output of 500 lumens and above are still based on lamps. FBH is currently developing hybrid-integrated diode laser modules in the basic colors with the following desired properties: > 1 W output power and radiance > 100 MW/cm²/sr. These compact laser modules integrate customized diode laser chips and beam forming optics. While red light can be generated directly from the chip, for green and blue light, nonlinear frequency conversion by single-pass second harmonic generation (SHG) from near-infrared to visible light is used. Key element for the infrared light source is an internal wavelength-stabilized tapered diode laser providing high output power of up to 8 W – measured after beam forming optics – and good beam quality to deliver 1 W in the visible range. Separate contacts on chip additionally enable modulation. Careful design and precise positioning of the micro lenses, which form the beam by focusing it into the optical nonlinear crystal, is needed. For chip mounting, submounts with highest possible thermal conductivity are used. FBH successfully integrated the laser and the SHG crystal into a small module package, featuring high precision and minimal thermal crosstalk between laser chip and crystal. Prototype modules with more than 1 W output power and a nearly diffraction-limited beam were demonstrated. Integration of multiple emitters is planned and should enable even higher light fluxes and reduced speckles.
D. Feise, W. John, F. Bugge, C. Fiebig, G. Blume, K. Paschke, "High-spectral-radiance, red-emitting tapered diode lasers with monolithically integrated distributed Bragg reflector surface gratings", Opt. Express, vol. 20, no 21, pp. 23374-23382 (2012).
G. Blume, D. Feise, C. Kaspari, A. Sahm, K. Paschke, "High luminance tapered diode lasers for flying-spot display applications", Proc. SPIE, vol. 8280, no. 82800E (2012).
G. Blume, C. Kaspari, D. Feise, A. Sahm, B. Sumpf, B. Eppich, K. Paschke, "Tapered Diode Lasers and Laser Modules near 635nm with Efficient Fiber Coupling for Flying-Spot Display Applications", Opt. Rev., vol. 19, no. 6, pp. 395-399 (2012).
C. Fiebig, J. Fricke, M. Uebernickel, D. Jedrzejczyk, A. Sahm, K. Paschke, "Watt-Class Green-Emitting Laser Modules Using Direct Second Harmonic Generation of Diode Laser Radiation", Opt. Rev., vol. 19, no. 6, pp. 405-408 (2012).
A. Sahm, C. Fiebig, S. Spießberger, M. Schiemangk, E. Luvsandamdin, K. Paschke, G. Erbert, G. Tränkle, "Modular Assembly of Diode Lasers in a Compact and Reliable Setup for a Wide Range of Applications", IEEE 62nd Electronic Components and Technology Conference (ECTC) Proc., San Diego, CA, USA, May 29 - Jun 1, pp. 1852-1857 (2012).
C. Fiebig, S. Pekarek, K. Paschke, M. Uebernickel, T. Südmeyer, U. Keller, G. Erbert, "High-brightness distributed-bragg-reflector tapered diode lasers: pushing your application to the next level", Proc. SPIE, vol. 7918, no. 79180R (2011).
P.Q. Liu, C. Fiebig, M. Uebernickel, G. Blume, D. Feise, A. Sahm, D. Jedrzejczyk, K. Paschke, G. Erbert, "High-power (1.1W) green (532nm) laser source based on single-pass second harmonic generation on a compact micro-optical bench", Proc. SPIE, vol. 7917, no. 791704 (2011).
Hybrid-integrated, matchbox-size laser modules are assembled on micro-benches consisting of AlN ceramics. This material offers a high thermal conductivity, a low thermal expansion coefficient, and an economic price at the same time. At FBH, various bonding technologies are available for mounting of laser components including beam forming optics. Different solder materials are used for active light-emitting components, which makes this technology very flexible. A flip-chip bonder allows careful sequencing with highest precision of ±1 µm and ±0.1 mrad to position the diode laser and amplifier chips. Passive components like micro lenses and components with low thermal load can be glued with FBH’s high-precision mounting equipment. In conjunction with a precisely designed geometry, this enables accurateness in the range of ±0.3 µm and ±0.03 mrad using UV glue. For HF modulation and low-noise applications, electronic elements are additionally implemented on the micro-bench near the active semiconductor chips.
Diode lasers are the most efficient source of optical energy. However, the emission lies within a wavelength range of several nanometers, which is too broad for many applications, for example in spectroscopy. When a periodic grating is built into the semiconductor, the wavelength range can be narrowed a million times to femtometers. In the past decade, FBH scientists have developed the advanced process techniques and laser designs required to deliver such grating-stabilized diode lasers with very high performance. Two technologies have been developed:
- surface-etched gratings
- gratings that are buried in the semiconductor using two-step epitaxial growth techniques.
These allow the FBH team to produce wavelength-stabilized lasers that are tailored for each application.
A. Maaßdorf , C.M. Schultz, O. Brox, H. Wenzel, P. Crump, F. Bugge, A. Mogilatenko, G. Erbert, M. Weyers, G. Tränkle, "In-situ etching of patterned GaAs/InGaP surfaces for highly efficient 975 nm DFB-BA diode lasers", J. Cryst. Growth, vol. 370, pp. 226-229 (2013).
P. Crump, O. Brox, F. Bugge, J. Fricke, C. Schultz, M. Spreemann, B. Sumpf, H. Wenzel, G. Erbert, "High Power, High Efficiency Monolithic Edge-Emitting GaAs-Based Lasers with Narrow Spectral Widths", Semiconductors and Semimetals, vol. 86, SEMSEM, Advances in Semiconductor Lasers, Burlington, UK: Academic Press, ISBN-13: 978-0-12-391066-0, pp. 49-91 (2012).
J. Fricke, W. John, A. Klehr, P. Ressel, L. Weixelbaum, H. Wenzel, G. Erbert, "Properties and fabrication of high-order Bragg gratings for wavelength stabilization of diode lasers", Semicond. Sci. Technol., vol. 27, no. 055009 (2012).