Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Humboldt Center for Modern Optics

1 W MOPA laser modules at 780 nm

1 W MOPA laser modules at 780 nm for cold atom experiments featuring narrow linewidth emission

Fig. 1. Micro-integrated MOPA module consisting of a DFB laser as a master oscillator and a tapered amplifier. Beam forming is realized by micro lenses.

Narrow-linewidth high-power lasers find application in a variety of fields like coherent optical communication, precision measurements, spectroscopy and laser cooling.
Due to their compactness, low weight, high efficiency and good mechanical stability, semiconductor lasers are usually favored over optically pumped solid-state and fiber lasers in respective applications. However, solitary semiconductor lasers do not feature stable narrow-linewidth operation at the 1 Watt level as a result of mode instability and reliability problems. A monolithic master oscillator (MO) power amplifier (PA) configuration can provide high-power single-mode operation at high output power. However, such devices suffer from optical feedback from the amplifier to the oscillator which broadens the linewidth. A standard hybrid bench-top MOPA configuration including an isolator might be used to suppress optical feedback though the advantage of compactness and mechanical stability gets lost. The Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik succeeded to realize a compact and rugged micro-integrated MOPA that features both, narrow-linewidth and high-power operation. After having realized a hybrid integrated MOPA emitting near 1060 nm FBH has now successfully transferred the hybrid MOPA concept to 780 nm.

Fig. 2. Optical spectrum vs. injection current into the master oscillator section of a micro-integrated MOPA module. The injection current into the ridge waveguide entry section and the tapered amplifier is 250 mA and 1600 mA, respectively.

For the MOPA modules emitting at 780 nm a dedicated application exists in cold atom experiments to cool, coherently manipulate, and detect atoms. These cold atoms can be used as quantum sensors to evaluate fundamental laws of physics. The corresponding project aims at testing the weak equivalence principle on a space mission. The modules presented here will be used in preliminary experiments at the ZARM drop tower in Bremen. The next step towards space mission is an already scheduled sounding rocket mission that will be equipped with next generation MOPA modules which are just being prepared at the Ferdinand-Braun-Institut.

The output power of these MOPA modules at 780 nm exceeds 1 Watt but with a conversion efficiency of 25%. To ensure mode hop free tuneability, as required for the desired application, we use a 1.5 mm long DFB diode as master oscillator with a FWHM linewidth (10 µs) of 1.3 MHz and an intrinsic linewidth of 140 kHz. Since the linewidth is preserved during amplification, we reach an output power of more than 1 W with the same spectral purity. The amplifier consists of a ridge waveguide section followed by a tapered amplifier section. Optical feedback from the amplifier or the experimental setup to the oscillator is suppressed by a micro-isolator. All semiconductor components and optics are integrated on a footprint of only 50 x 10 mm2.

The module features single-mode operation over the entire current range (0 - 2000 mA). To our knowledge this is the first time that a micro-integrated narrow-linewidth MOPA concept has been realized.

website: Ferdinand-Braun-Institut, Laser Metrology group


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