## 1 Introduction  
Medical laser applications in the photothermal region rely heavily on heat generation to achieve desired thermo-physiological effects [1, 2]. Achieved physiological effects are tissue temperature dependent. Tissue temperature, in turn, is a function of generated heat, optical, and thermal properties of the tissue including perfusion characteristics [3]. Depending on the medical procedure, it is necessary to ensure that some of these thermo-physiological effects are generated while preventing some others. Laser tissue welding (LTW) [4, 5, 6, 7, 8], ablation of soft tissues [9, 10], photocoagulation and photothermal denaturation [11], angioplasty [12, 13], treatment of granulation tissues [14], cartilage regeneration [15], and reshaping [16] are among possible medical photothermal laser treatments (PTT). Thermo-physiological interactions occur within a narrow temperature range. Any deviation from this range may result in treatment failure or excessive thermal damage [6]. For instance, photothermal treatment of granulation tissue aims to trigger apoptosis by hyperthermia (45-50 \({}^{\circ}\)C) induced heat shock protein production without causing necrosis (>50 \({}^{\circ}\)C) [14]. Another example for a highly temperature sensitive application is LTW. Optimally, LTW requires temperatures between 65 \({}^{\circ}\)C and 70 \({}^{\circ}\)C (skin tissue) to ensure collagen denaturation (>63 \({}^{\circ}\)C for treatment success) [17]. Keeping the tissues at this temperature range for a moderate duration of time (ranging from 0.06 s to minutes) also prevents excessive thermal damage (coagulative necrosis, heat fixation), which results in scarring and prolonged recovery time [18]. Currently, both treatment of granulation tissue and LTW are experimental procedures, not regularly used in clinical environment due to reproducibility and safety issues. Thermal and optical characteristics of tissues vary among individuals and can change even within the same individual over time [3; 19]. Dosimetry studies are widely used for determining laser parameters and exposure durations for PTT. While providing generally satisfactory laser parameters for treatments, dosimetry studies conducted in controlled environments of laboratories (using rather uniform research animals) do not ensure optimal results for all subjects. A predetermined dose may result in successful treatments in the majority of subjects, but may also lead to treatment failure and/or excessive thermal damage in some subjects. Therefore, conducting PTT with real-time tissue temperature feedback control is necessary. A tissue temperature controlled medical laser system ensures treatment success and standardization, as well as patient safety. It also compensates for inexperienced operators increasing reproducibility of results for current PTT modalities used in the clinic. Consequently, a temperature controlled system may endorse the translation of experimental laser techniques (e.g., LTW, granulation tissue treatment, cartilage reshaping), which currently are limited to the controlled environment of the laboratory, into regular clinical practice. Such a system may also be used in the research of thermal characteristics of tissues and development of new laser treatment techniques. Additionally, it can be used for preventing undesirable temperature increase during photochemical procedures (e.g., photodynamic therapy and low-level laser therapy), which are non-thermal processes by nature [20; 21]. Various research groups utilized different techniques to develop temperature measurement and control systems for medical laser applications [4; 5; 6; 7; 9; 11; 12; 16; 22; 23; 24; 25]. These systems excel at different aspects of temperature control (e.g., fast response time, high accuracy, high spatial resolution) but mostly share a common drawback: the requirement of multiple and costly apparatus that are complicated to use in the clinical environment.