Fiber lasers have been established as a reliable power-scalable laser architecture in the last years. Continuous-wave output powers well above the kW-barrier have been demonstrated with diffraction-limited beam quality. However, even a state-of-the-art fiber laser is eventually limited by nonlinear effects, fiber facet damage or thermal issues. Spectral combining of continuous-wave fiber lasers is a well known and very promising power scaling technique to obtain power levels far beyond the limitations of a single fiber emission without significant beam quality degradation. Spectral beam combination setups provide an overlap of the individual beams in the near and far fields without the need for complex phase control. With the technique of spectral beam combination several beams with different wavelengths are superposed. Each beam has a different incident angle on a grating in such a way that the diffraction angles for all beams are identical. Thus, spectral beam combination trades brightness enhancement with loss of spectral purity.
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.
There are many advantages of using lasers and optical fibers in biomedical and clinical applications. First of all, this procedure is non or minimally invasive. The damage risk of the neighbouring tissues is quite low since the laser energy is specifically absorbed by the target tissue. The laser operations are highly controlled thanks to focusing the laser energy with micron precision. The laser parameters such as power, frequency, number of pulses and energy are adjustable. The wavelength of the laser can be selected according to target chromophore absorption such as water, hemoglobin. Laser energy can be transmited with different beam shapes by manipulating the fiber tip geometry. In our group studies, we are focused on IR-range wavelength laser systems operating both continuous wave (cw) and modulated modes for medical applications. Our motivation is to develop medical lasers which are used in mainly endovenous laser ablation (EVLA). Veneous insufficiency in the lower-extremity is a widespread medical situation, in fact 25% of women and 15% of men suffer from this disease in the USA . The varicose veins are a effected or worsened by some conditions, such as pregnancy, aging, standing and sitting for a long time. The primary symptoms cannot be noticed easily by patients, thus, the disease may progress and cause worse symptoms such as night cramps, pain and fatigue . If the varicose vein is not treated before progression, approximately 50% of patients may ultimately suffer from chronic venous insufficiency . The most probable cause of distinctive varicose vein is great saphenous vein (GSV) reflux and even stripping and surgical ligation has been the most certain solution, it causes perioperative morbidity . Since it is identied that the main reason of the varicose vein is the GSV reflux, eliminating of GSV should be the solution. As an alternative to surgical operation, the laser energy was firstly used in an endoluminal operationvby Bone in 1999 . This study opened a door for varicose vein treatment, and a procedure had been developed for treatment of GSV by using 810nm laser diode The delivery of laser energy into blood vessel lumen has been approved by US Food and Drug Administration in 2002 . There are also other options for treatment of GSV reflux, such as, surgery and RF-ablation. Endovenous laser ablation seems to provide important benefits that are avoidance of general anesthesia and lower rates of complication. Our group developed two medical laser systems. The first one is operating at 980nm with cw and modulated modes. The second one is a dual wavelength laser system operating at both 980nm and 1470nm with cw and modulated modes.