Increasing brightness in multiphoton microscopy with low-repetition-rate, wavelength-tunable femtosecond fiber laser

Many experiments in biological and medical sciences currently use multiphoton microscopy as a core imaging technique. To date, solid-state lasers are most commonly used as excitation beam sources. However, the most demanding applications require precisely adjusted excitation laser parameters to enhance image quality. Still, the lag in developing easy-to-use laser sources with tunable output parameters makes it challenging. Here, we show that manipulating the temporal and spectral properties of the excitation beam can significantly improve the quality of images. We have developed a wavelength-tunable femtosecond fiber laser that operates within the 760 - 800 nm spectral range and produces ultrashort pulses (below 70 fs) with a clean temporal profile and high pulse energy (1 nJ). The repetition rate could be easily adjusted using an integrated pulse picker unit within the 1 - 25 MHz range and without strongly influencing other parameters of the generated pulses. We integrated the laser with a two-photon excited fluorescence (TPEF) scanning laser microscope and investigated the effect of tunable wavelength and reducing the pulse repetition rate on the quality of obtained images. Using our laser, we substantially improved the images brightness and penetration depth of native fluorescence and stained samples compared with a standard fiber laser. Our results will contribute to developing imaging techniques using lower average laser power and broader use of tailored fiber-based sources

Diode laser modules based on laser-machined, multi-layer ceramic substrates with integrated water cooling and micro-optics

This thesis presents a study on the use of low temperature co-fired ceramic (LTCC) material as a new platform for the packaging of multiple broad area single emitter diode lasers. This will address the recent trend in the laser industry of combining multiple laser diodes in a common package to reach the beam brightness and power required for pumping fibre lasers and for direct-diode industrial applications, such as welding, cutting, and etching. Packages based on multiple single emitters offer advantages over those derived from monolithic diode bars such as higher brightness, negligible thermal crosstalk between neighbouring emitters and protection against cascading failed emitters. In addition, insulated sub-mounted laser diodes based on telecommunication standards are preferred to diode bars and stacks because of the degree of assembly automation, and improved lifetime. At present, lasers are packaged on Cu or CuW platforms, whose high thermal conductivities allow an efficient passive cooling. However, as the number of emitters per package increases and improvements in the laser technology enable higher output power, the passive cooling will become insufficient. To overcome this problem, a LTCC platform capable of actively removing the heat generated by the lasers through impingement jet cooling was developed. It was provided with an internal water manifold capable to impinge water at 0.15 lmin-1 flow rate on the back surface of each laser with a variation of less than 2 °C in the temperature between the diodes. The thermal impedance of 2.7°C/W obtained allows the LTCC structure to cool the latest commercial broad area single emitter diode lasers which deliver up to 13 W of optical power. Commonly, the emitters are placed in a “staircase” formation to stack the emitters in the fast-axis, maintaining the brightness of the diode lasers. However, due to technical difficulties of machining the LTCC structure with a staircase-shaped face, a novel out-plane beam shaping method was proposed to obtain an elegant and compact free space combination of the laser beam on board using inexpensive optics. A compact arrangement was obtained using aligned folding mirrors, which stacked the beams on top of each other in the fast direction with the minimum dead space

Single-Frequency EYDFA with polarization-maintaining fibers for gravitational wave detection

In 2015, the space-time distortion caused by GW150914 was found - a pivotal event that inaugurated the era of interferometric gravitational wave astronomy. As of today, gravitational wave observations are routinely made with proper sky localization by the world-wide operating detector network of the second generation. The implementation of cryogenic cooling can reduce the coating thermal noise in the next detector generation. In this case, optics made of fused silica are not suitable because of fused silica's large mechanical loss at low temperatures. Crystalline silicon is an alternative material but not transparent at 1064nm; therefore, other laser wavelengths, e.g. 1.5um, must be used. Single-frequency EYDFAs based on LMA fibers can deliver the required output power at 1.5um. A PM setup, however, has not been demonstrated on the desired ~100W power level so far; also, there has been no demonstration of any successful longterm operation (> hours) even of a non-PM setup. In this work, a prototype amplifier with PM fibers is presented on a laboratory- and advanced engineering-level. A numerical FEM analysis of the pump wavelength dependence of the Yb3+ ASE and non-linear SBS has been performed; off-peak pumping was found to suppress the unwanted Yb3+ ASE considerably. The achievable output power at 1.5um was limited by the Yb3+ ASE if the simulated amplifier was pumped from 880nm to 990nm; the onset of the Yb3+ ASE was linked to a deterioration of the Yb3+-to-Er3+ energy transfer. The simulated amplifier was limited by SBS if pumped at wavelengths shorter than 880nm or longer than 990nm. The power threshold was approximatable by adapting a well-known threshold approximation for passive fibers. Uncontrolled gain, e.g. resulting from a seed laser failure, must be prevented by interlocking the pumping process. In this work, the required reaction time has been studied with single-mode fibers by a combined experimental and numerical approach. It was found that a potential emergency-off system must switch-off the pumping process well below ~100us and/or ~300us to prevent catastrophic gain for the Yb3+ ASE and/or Er3+ ASE, respectively. An electronic circuit was designed; the board in PCB format was found capable to meet this requirement. The PCB prototype was installed as part of the engineering-level amplifier. A high-power single-frequency EYDFA made from 25/300 PM fibers is presented; the amplifier was implemented with low seed input power to match available GWD-compatible seed laser sources. A pump wavelength of 940nm was used. The pre-amplifier delivered 1.07W output power with low ASE power levels and operated free of SBS. The maximum output power of the high-power amplifier was 110W with 44.4+-0.3% optical-to-optical efficiency. The Er3+ ASE extinction ratio was 48.34dB at maximum output power; the Yb3+ ASE was negligible. SBS-free operation was confirmed by monitoring the amplifier noise at MHz frequencies. The PER ranged from 9.8dB to 12.6dB, probably owed to the used gain fiber. Further power scaling was limited by thermal fiber damage assumed to originate from photodarkening. Moreover, an advanced prototype with a revised cooling approach is presented. The performance of two suitable 25/xxx gain fibers was compared at the ~50W level over a 2-week period. The Nufern fiber showed a growing attenuation, i.e. 14.7+-2.2% per 13 days, that was tentatively attributed to the formation of P1 type color centers from POHCs; further research needs to be undertaken to confirm. The iXblue fiber seemed more heat resilient under operation. Furthermore, the PER from the iXblue fiber was in the range of 15.2dB to 20.7dB; the fundamental mode power was 95.7%. It was concluded that the iXblue fiber is suited to be used in GWD-compatible laser sources

Power and efficiency scaling of GaAs-based edge-emitting high-power diode lasers

Current progress in the scaling of continuous wave optical output power and conversion efficiency of broad-area GaAs-based edge emitters, broad-area lasers (BALs), operating in the 900…1000 nm wavelength range is presented. Device research and engineering efforts have ensured that BALs remain the most efficient of all light sources, so that in the past 10 years, power conversion efficiency at 20 W continuous wave (CW) output power from BA lasers with a 90…100 μm wide stripe has increased 1.5-fold to 57% (via epitaxial layer design developments), whilst peak CW power per single emitter has increased around 3-fold to 70 W (via scaling of device size), with further scaling underway, for example via use of multi-junction designs. However, the peak achievable CW power conversion efficiency and CW specific output power (defined here as peak output power from a 100 μm stripe diode lasers with a single p-n junction) has changed remarkably little, remaining around 70% and 25 W, respectively, for the past decade. Fortunately, research to understand the limits to peak efficiency and specific output power has also shown progress. Specifically, recent studies indicate that spatial non-uniformity in optical field and temperature play a major role in limiting both power and conversion efficiency. Technological efforts motivated by these discoveries to flatten lateral and longitudinal temperature profiles have successfully increased both power and efficiency. In addition, epitaxial layer designs with very high modal gain successfully reduce threshold current and increase slope at 25 °C to values comparable to those observed at 200 K, offering a path toward the 80% conversion efficiency range currently seen only at these cryogenic temperatures. Overall, whilst operating efficiency and power continue to scale rapidly, a technological path for increased specific power and peak efficiency is also emerging

Near diffraction limited high-power narrow-linewidth Er3+-doped fiber amplifiers : developments towards laser sources at 1.5 [my]m wavelength for gravitational wave astronomy

[no abstract

Development of High-Repetiton-Rate Picosecond Thin-Disk Lasers

Předkládaná dizertační práce je věnována výzkumu a vývoji pevnolátkových laserů̊ s vysokým středním výstupním výkonem, velmi krátkou délkou pulzu, vysokou opakovací frekvencí a s kvalitou svazku blízkou difrakční mezi.The doctoral thesis pursues the research and development of solid-state lasers with high average output power. Yb:YGAG ceramics was proposed as an attractive laser material for low-temperature ultrashort pulse generation and a laser based on ceramic erbium-doped yttria was demonstrated as an efficient mid-IR source. The main outcome of the work is a detailed description of the developed 0.5-kW picosecond thin-disk Yb:YAG laser system PERLA C