High-power laser systems are extremely important for many applications in defence, industry and medical sectors. In the last 40 years, important scientific progress has been achieved in the development of the continuous-wave (CW) laser by using several geometries of the gain structures and mediums. Most of the achieved results are based on the solid-state laser technology where the active medium is in rod shaped. However, the efficiency and the beam quality of solid-state laser system in high-power operations are very low due to the heat dissipation problems. These expensive and complicated solid-state laser systems operate efficiently only in a particular laboratory environment. Furthermore, optical cavity structures of the solid-state laser systems frequently require alignment in operation which makes the system extremely difficult to use.
New generation fiber optic cables and components are used to develop powerful fiber laser systems. Since fiber laser systems are high-quality laser sources, user friendly, and low cost, they are preferable for the high-average power and energy laser systems. In fiber laser systems the pumped light in active medium is being guided through the entire cable instead of a particular area. For this reason, thermal problems are reduced, since the absorption coefficient of pumped light increases, efficient and high quality laser light can be produced and fiber laser systems requires minimum optical cavity optimization in field applications.
The scope of these research activities is to develop user friendly powerful CW laser sources and produce high quality light with low cost for different applications. Firstly, we perform theoretical investigations on our laser systems for better understanding and control of system parameters. Various cavity design including new pumping configurations such as tandem pumping are under consideration. Our first experimental approach is to develop fiber laser systems with commercial available fiber optical components. On the other hand, our favourite ways to motivate our group is to develop powerful fiber optical components in our labs and demonstrate high performance on fiber laser systems.
kW-Class CW All-Fiber Laser Oscillators
We have developed an alignment-free monolithic Yb-doped all-fiber laser oscillator generating 1 kW of CW signal power. The oscillator operates at 1080 nm with 79 % slope efficiency. The schematic representation of compact high power fiber laser system is shown in Figure 1. The laser is pumped with 6 fiber pigtailed multimode diodes through all-fiber pump-signal combiner. The total available pump power of the pump diodes at the combiner is about 1325 W at 976 nm. The oscillator was then formed with high reflector fiber Bragg grating, double-clad active fiber and output coupler fiber Bragg grating. In order to develop compact and efficient fiber oscillator concept, we have provided high quality fiber integration process and developed new thermal management approach. We have also done additional efforts for high quality light generation from fiber laser by developing advanced laser engine.
Fig.1: The schematic representation of 1 kW high power fiber laser system.
The laser optical spectrum is shown in Fig. 2. The optical spectrum reveals a typical emission of a high-power fibre laser operating at 1080 nm. The laser also has a remarkably high signal-to-noise ratio of 60 dB because of the small reflection bandwidth of the Bragg grating, allowing a wavelength-selective feedback into the cavity. Furthermore, this measurement shows that no stimulated Raman scattering, which could appear at about 1120-1150 nm, occurs even at this high power level, because of the reduced intensity in the large-mode-area fibre. In addition, we can assume as a first observation that there is no significant ASE background in the system. The ratio between the signal and the unabsorbed pump is approximately 20 dB and it corresponds to 100 times in linear scale.
Fig.2: Optical spectrum of 1 kW high power fiber laser system.
The output power characterization of the fiber laser system is illustrated in Fig. 3. The graph shows the laser output power vs. launched pump power. At total launched pump power of 1.325 kW we obtained up to 1.04 kW of continuous-wave output power. The slope efficiency of our high power fiber laser is about 79 %. No stimulated Raman scattering is observed at highest power level. The signal power of such a single all-fiber laser oscillator showed no evidence of rollover, and the highest output was limited only by available pump power. In future, we are planning to develop all-fiber laser generating 2 kW of CW signal power.
Fig.3: Output power characterization of 1 kW high power fiber laser system.