Institute of Applied Physics

The IAP was founded on June 2nd, 1961 as part of the faculty of natural sciences. Today about 56 employees research and teach in four divisions focusing on Biomedical Photonics, Laser Physics, Nonlinear Optics and Microwave Physics.

Events

Room A97, ExWi, Sidlerstrasse 5, 3012 Bern BMP Sminar, Dr. Martin Schneiter, Institut of Applied Physics, University of Bern

07 December

Room A97, ExWi, Sidlerstrasse 5, 3012 Bern MW Seminar MW Guochun Shi, Institute of Applied Physics & Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland

09 December

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News

Meteor radar vertical wind observation biases and mathematical debiasing strategies including the 3DVAR+DIV algorithm

Precise and accurate vertical wind observations are very challenging at the mesosphere and lower thermosphere.
However, these vertical motions are essential to understand the residual circulation and associated transport processes
In the middle atmosphere. In this study, we discuss mathematical de-biasing strategies to retrieve more reliable vertical
winds from standard monostatic meteor radars. Furthermore, we developed a new 3DVAR+DIV retrieval for multistatic
meteor radar networks to derive precise and accurate vertical wind velocities. This was achieved by implementing the
continuity equation into the forward model of the retrieval. The algorithm computes two vertical wind solutions assuming
an incompressible flow and also estimates a compressible flow solution to account for small-scale gravity waves.

Pulse-echo speed-of-sound imaging using convex probes

Computed ultrasound tomography in echo mode (CUTE) is a new ultrasound (US)-based medical imaging modality with promise for diagnosing various types of disease based on the tissue’s speed of sound (SoS). It is developed for conventional pulse-echo US using handheld probes and can thus be implemented in state-of-the-art medical US systems. One promising application is the quantification of the liver fat fraction in fatty liver disease. So far, CUTE was demonstrated using linear array probes where the imaging depth is comparable to the aperture size. For liver imaging, however, convex probes are preferred since they provide a larger penetration depth and a wider view angle allowing to capture a large area of the liver. With the goal of liver imaging in mind, we adapt CUTE to convex probes, with a special focus on discussing strategies that make use of the convex geometry in order to make our implementation computationally efficient. We then demonstrate in an abdominal imaging phantom that accurate quantitative SoS using convex probes is feasible, in spite of the smaller aperture size in relation to the image area compared to linear arrays. A preliminary in vivo result of liver imaging confirms this outcome, but also indicates that deep quantitative imaging in the real liver can be more challenging, probably due to the increased complexity of the tissue compared to phantoms.

All-fibre heterogeneously-integrated frequency comb generation using silicon core fibre

Originally developed for metrology, optical frequency combs are becoming increasingly
pervasive in a wider range of research topics including optical communications, spectroscopy,
and radio or microwave signal processing. However, application demands in these fields can
be more challenging as they require compact sources with a high tolerance to temperature
variations that are capable of delivering flat comb spectra, high power per tone, narrow
linewidth and high optical signal-to-noise ratio. This work reports the generation of a flat, high
power frequency comb in the telecom band using a 17 mm fully-integrated silicon core fibre
as a parametric mixer. Our all-fibre, cavity-free source combines the material benefits of
planar waveguide structures with the advantageous properties of fibre platforms to achieve a
30 nm bandwidth comb source containing 143 tones with <3 kHz linewidth, 12 dB flatness,
and >30 dB OSNR over the entire spectral region

Perseid meteor shower

During the weekend the Perseid meteor shower was visible above Switzerland. The Microwave Physics department Conducted a joined meteor campaign using two ALLSKY7 camera systems to observe the stream. The cameras are now permanently Installed at the Zimmerwald observatory and the Jungfraujoch station – Sphinx Observatory. The campaign was supported by Prof. Peter Slansky from HFF (Hochschule für Film und Fernsehen) in Munich. For three nights from 11th August-14th August a joined team operated an additional set of cameras with the goal to measure a potential substructure of the Perseid meteor stream source radiant and the velocity distribution.
The cameras were SONY alpha 7 looking at selected parts of the sky to avoid direct moonlight and record meteors with UHD quality
at a frame rate of 25 fps.

We collected a data set of about 1000 Perseid meteors during August at the Sphinx observatory and about stream 750 meteors at the Zimmerwald observatory. The meteor detection of the other cameras is done in collaboration with other colleagues from HFF
Munich.

Reducing the noise of fiber supercontinuum sources to its limits by exploiting cascaded soliton and wave breaking nonlinear dynamics

The low-noise and phase-coherent nonlinear transformation of a narrowband laser into a broadband supercontinuum (SC) in an optical fiber forms the basis of extremely precise applications ranging from optical frequency comb technology to ultrafast photonics and biomedical imaging. A major challenge of this process is the avoidance of incoherent nonlinear effects that amplify random quantum noise, requiring careful birefringence and dispersion engineering of the fiber. However, fundamental trade-offs exist between working in normal or anomalous dispersion regimes. Here, we combine the benefits of nonlinear dynamics in both regimes by cascading soliton compression and optical wave breaking in a hybrid fiber, formed by joining two widely available, commercial, polarization-maintaining step-index fibers exhibiting anomalous and all-normal dispersion, respectively. We experimentally demonstrate that this hybrid approach results in an ultra-low-noise fiber SC source covering the 930–2130 nm range with phase coherence near unity, spectrally resolved relative intensity noise (RIN) as low as 0.05%, and averaging 0.1% over a bandwidth of 750 nm, approaching the theoretical limits close to the pump laser noise. This corresponds to a doubling of the generated spectral bandwidth
and a decrease of RIN by up to 1 order of magnitude compared to direct pumping of the individual fibers, where modula-tional polarization instabilities play a limiting role. Owing to its simplicity and its scalability to high repetition rates, our hybrid scheme is readily applicable to various laser platforms and could enhance the performance of applications such as hyperspectral nonlinear microscopy, coherent optical communications, and photonic signal processing.

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