Stability, noise and beam shaping of direct and frequency doubled diode lasers for pumping Ti:S lasers

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. 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.
Diode lasers, due to their high efficiency, compactness, reliability and low manufacturing cost, have replaced many solid state, dye and gas lasers and are increasingly attractive for many applications. Recent developments in the broad-area diode lasers (BA-DLs) and tapered diode lasers (TPLs), yielding unprecedented multi-watt output power per emitter, presented very attractive opportunity to develop efficient diode based pumping source for Ti:S laser. Such diode based pump source can be made either by using high power BA-DL emitting in the blue-green (488-520 nm) spectral region or by frequency doubling the high power TPL emitting in the NIR region (976-1064 nm). For further power scaling beam combining of multiple emitters is used.
Coherent beam combining of high power tapered amplifiers in master oscillator power amplifier (MOPA) configuration was investigated for frequency doubling and significant enhancement in frequency conversion efficiency is reported in a journal publication. However, optical feedback induced instabilities were noticed in this laser system.
In order to develop a long-term stable pump source based on TPLs, better understanding and characterization of optical feedback sensitivity of TPLs is vital. A joint study between Technical University of Denmark (DTU) and Ferdinand-Braun-Institute (FBH) was performed and the results showing high optical feedback sensitivity of TPLs were reported in a joint publication. FBH recently developed monolithic MOPA devices with tilted tapered PA (TPA) section to reduce optical feedback sensitivity. Tilted TPA causes a special type of beam distortion. A novel phase retrieval technique was developed and used to characterize the wave front and the results are reported in a journal publication that is currently under-review.
Although TPL devices usually have an intrinsic wavelength stabilization implemented such as frequency doubling in long nonlinear crystals. A high resolution (~30 MHz) speckle-based wavemeter was developed and investigated for the purpose of TPL wavelength stabilization. A manuscript comprised of its characterization emphasizing its high potential for wavelength stabilization, is under preparation.
Finally, BA-DL pumped Ti:S performance was analyzed and compared with two types of pump lasers: frequency doubled TPL and DPSS laser. The results are recently published where its suitability for applications moderately sensitive to noise was argued.