PhD defence by Muhammad Tahir Jamal

On Monday 25 January 2021, Muhammad Tahir Jamal will defend his PhD thesis "Stability, noise and beam shaping of direct and frequency doubled
diode lasers for pumping Ti:S lasers".

Time: 25 January, at 10:00

Place: Zoom, registration is required:

Supervisor: Senior Researcher Ole Bjarlin Jensen, DTU Fotonik
Co-Supervisor: Senior Researcher Peter E. Andersen, DTU Health Tech
Co-Supervisor: Associate Professor Anders Kragh Hansen, DTU Fotonik

Assessment comittee:
Associate Professor Peter Tidemand-Lichtenberg, DTU Fotonik
Group leader Bernd Jungbluth, Fraunhofer Institute for Laser Technology
Associate Professor Erwin A.J.M. Bente, Technische Universiteit Eindhoven

Senior Researcher Michael Linde Jakobsen, DTU Fotonik

Ultrafast titanium sapphire (Ti:S) laser is widely considered as a powerful tool in the biomedical field. Its wide wavelength tunability and the ability to generate ultrashort pulses makes it very suitable for multi-modal biophotonic imaging. The Ti:S laser require a  multi-watt level pumping source in the blue-green spectral region. Conventionally, a frequency doubled diode-pumped solid-state (DPSS) laser is used for this purpose. However, its bulkiness, complexity and high cost is the critical limiting factor that restricts its widespread adoption for such applications.

A high power diode-based efficient and reliable blue-green pump source can significantly bring down the cost and complexity of ultrafast Ti:sapphire lasers, enabling its widespread adoption for diagnostics via multimodal imaging. Tapered lasers (TPL) and tapered-amplifier-based MOPA devices offer an efficient way of obtaining watt-level blue-green lasers through frequency doubling. Currently, the maximum achieved power per emitter in these devices seems to be limited by the device design. For further power scaling, beam combining of several emitters is necessary. The coherent beam combining (CBC), which offers power scaling while preserving or even improving the spatial and spectral beam quality, is used for frequency doubling in this work. A significant enhancement in the nonlinear frequency conversion efficiency in a single-pass second harmonic generation process is demonstrated by the CBC of multiple tapered amplifiers in MOPA configuration yielding more than 2~W cyan (488~nm) laser light. The high optical feedback sensitivity (OFS) of DBR/DFB tapered lasers and monolithic MOPA devices typically demands 30-60 dB optical isolation for frequency conversion applications. Such high OFS is currently the main factor that limits their adoption as a long-term reliable pump source for Ti:S lasers. Unexpectedly, different OFS has been observed in the TPL devices with a similar design, which shows that further research about the origin of this phenomenon at the device design level is required.

We demonstrated a simple speckle-based wavemeter with high spectral resolution (<30 MHz) for the purpose of using it to stabilize a DBR-TPL. The speckle-wavemeter offers much reduced cost and complexity compared to other stabilization techniques. Furthermore, we showed its capability to determine the multimode behavior of the laser. Due to limited time and resources, the stabilization part was not achieved in this work.

We used recently developed cyan and green BA-DL for pumping KLM Ti:S laser as an alternative to frequency-converted (FC) TPLs. Despite their far-from diffraction-limited beam quality and high RIN compared to FC-TPL or a commercial DPSSL, with careful beam shaping of BA-DL pump, a stable KLM operation of Ti:S is demonstrated yielding 190~mW with slope efficiency comparable to commercial DPSSL. However, the noise performance of BA-DL pumped KLM Ti:S laser was much lower than the DPSSL pumped KLM Ti:S laser but not far from FC-TPL pumped KLM Ti:S laser. It appears the current main limiting factors in BA-DL based pumping are their high inherent noise and limited power per emitter. The noise performance of BA-DL pumped KLM Ti:S laser suggests its suitability for a wide range of life sciences applications. However, for applications that are extremely sensitive to RIN of the laser source, redesign of the BA-DL devices for low inherent noise is inevitable.


Mon 25 Jan 21
10:00 - 13:00


DTU Sundhedsteknologi