Research in Focus

Digital power amplifier with improved efficiency

Chips (detail) of class-D power amplifier module
Chips (detail) of class-D power amplifier module for mobile communication base stations

Digital power amplifier (PA) concepts are highly attractive for the future development of the wireless infrastructure. Today, the base stations' transmitter architecture is already almost fully digitized, except for the radio-frequency (RF) power amplifier. This is the reason why FBH has been working on digital PA modules for a number of years. Recently, in a joint experiment with the Japanese company NEC, the FBH successfully transferred the Doherty concept to the "digital world". So far, it has only been applied for analog input signals. To achieve this, two PAs were operated in parallel - one of them is effectively only switched on at full-scale, which significantly enhances efficiency at power back-off. The FBH H-bridge PA module was driven by an envelope delta-sigma modulator from NEC. Compared to conventional balanced digital PAs the amplifier shows up to 20% higher efficiency when driven between 6 dB and 12 dB below full-scale. For broadband communication signals with modern modulation schemes like WCDMA the improvement in efficiency is around 10%. Current efforts are devoted to further enhancing PA efficiency for back-off operation beyond 10 dB.


K. Motoi, A. Wentzel, M. Tanio, S. Hori, M. Hayakawa, W. Heinrich, K. Kunihiro, "Digital Doherty Transmitter with Envelope ΔΣ Modulated Class-D GaN Power Amplifier for 800 MHz band", IEEE MTT-S Int. Microw. Symp. Dig., Tampa Bay, USA, Jun 1-6, TU4F-1 (2014).

Dual-wavelength diode lasers - enabling miniaturized Raman sensor systems for real-world applications

Dual-wavelength diode laser
Dual-wavelength diode laser - basis for portable real-world Raman sensor systems

The FBH develops dual-wavelength diode lasers meeting the respective demands of different spectroscopic applications including shifted excitation Raman difference spectroscopy (SERDS). SERDS allows to distinguish clearly between Raman and disturbing background signals like fluorescence and ambient light, effectively paving Raman spectroscopy its way out of the lab into real-world applications. Their small size and low power consumption enables to integrate these devices into miniaturized systems for in-situ measurements in security, food control, and point-of-care diagnostic areas.

These diode lasers are developed within the BMBF-funded project DiLaRa reaching output powers up to 100 mW and are available at 785 nm and 671 nm wavelengths. Moreover, they deliver grating-stabilized laser light, enabling to switch between the two necessary excitation wavelengths for SERDS by current. The applicability of these light sources for in-situ Raman measurements is currently investigated in precision agriculture within the EU-funded project USER-PA. Here, Raman spectroscopy is utilized to yield information on growth, irrigation, and fertilization status of fruits.

Further information on laser sensor developments at FBH.


M. Maiwald, J. Fricke, A. Ginolas, J. Pohl, B. Sumpf, G. Erbert, and G. Tränkle, "Dual-wavelength monolithic Y-branch distributed Bragg reflection diode laser at 671 nm suitable for shifted excitation Raman difference spectroscopy", Laser Photonics Rev., vol. 7, no. 4, pp. L30-L33 (2013).

B. Sumpf, J. Fricke, M. Maiwald, A. Müller, P. Ressel, F. Bugge, G. Erbert, and G. Tränkle, "Wavelength stabilized 785 nm DBR-ridge waveguide lasers with an output power of up to 215 mW", Semicond. Sci. Technol., vol. 29, no. 045025 (2014).

M. Maiwald, B. Eppich, J. Fricke, A. Ginolas, F. Bugge, B. Sumpf, G. Erbert, G. Tränkle, "Dual-Wavelength Y-Branch Distributed Bragg Reflector Diode Laser at 785 Nanometers for Shifted Excitation Raman Difference Spectroscopy", Appl. Spectrosc., vol. 68, no. 8, pp. 838-843 (2014).

M. Maiwald, A. Klehr, B. Sumpf, G. Erbert, and G. Tränkle, "Dual-Wavelength Master Oscillator Power Amplifier Diode-Laser System at 785 nm", IEEE Photonics Technol. Lett., vol. 26, no. 11, pp. 1120-1123 (2014).

UV irradiation - to boost health-promoting compounds in plants

LED module irradiating broccoli sprouts with UV-B light (© IGZ)
FBH's LED module irradiating broccoli sprouts with UV-B light at 310 nm wavelength (© IGZ)

Epidemiological studies revealed that a high consumption of fruits, vegetables, and herbs comes along with a lower risk for both cancer and cardiovascular diseases. This protective effect is mostly due to secondary metabolites to be found in plant tissues. Recently, it has been shown that low dosage exposure to UV-B radiation may positively influence the biosynthesis of these organic compounds during plant growth. The issue is examined jointly by FBH and Leibniz Institute of Vegetable and Ornamental Crops (IGZ). Up to now, conventional low-pressure mercury gas-discharge fluorescent lamps have been used as UV-B radiation sources. These lamps deliver only a broadband UV-B radiation making it impossible to determine the wavelength-dependent action spectra of secondary plant metabolites. However, UV-B LEDs recently developed at FBH feature a narrow emission spectrum (half width < 10 nm) and a peak emission wavelength that can be tailored to ideally trigger specifically health-promoting secondary plant metabolites.

First experiments have been accomplished with an UV-B LED-based module with an emission at 310 nm. An adjustable uniform irradiance of up to 0.1 W m–2 was obtained at a working distance of 30 cm. That way, the formation of secondary metabolites in Arabidopsis leaves and broccoli sprouts could be successfully enhanced. The research activities will now be intensified within the Advanced UV for Life consortium, funded in the context of the BMBF competition Zwanzig20.

Further information on FBH's UV LEDs

Further details on UV-B Induced Secondary Plant Metabolites (article in Optik & Photonik, 2/2014)