The frequency-domain multiple-distance (FDMD) method is capable of measuring the absolute absorption and reduced scattering coefficients of optically turbid media. Absolute measurement of absorption at two near-infrared (NIR) wavelengths makes possible the quantitation of tissue haemoglobin concentration and tissue haemoglobin oxygen-saturation (StO2). However, errors are introduced by the uncertainties of background absorption and the dissimilarities between real tissues and the simplified mathematical model on which these measurements are based. An FDMD-based tissue instrument has been used for the monitoring of tissue haemoglobin concentration and oxygenation in the brain of newborn piglets during periods of hypoxia and hyperoxia. These tissue haemoglobin saturation values were compared with arterial saturation (SaO2) and venous saturation (SvO2) measured by blood gas analyses. A linear correlation was observed between StO2 and the average of SaO2 and SvO2. However, StO2 is not equal to any fixed weighted average of SaO2 and SvO2 unless we introduce an effective background tissue absorption. The magnitude of the background absorption was about 0.08 cm(-1) at 758 nm and 0.06 cm(-1) at 830 nm, and it was nearly consistent between piglets. The origin of this 'effective' background absorption may be real, an artefact caused by the application of a simplified model to a complex sample, or a combination of factors.

Lasing characteristics of a CW Tm:LuAG with a set of double cavity is presented in this letter. As the air gap etalon keeping at 342 µm, the highest output power reaches 107.8 mW when the totally input pump power is 1.95 W. The output center wavelength is 2027.0 nm. A single longitude mode output power of 17.1 mW is obtained when incident pump power is 855.1 mW. With the changing of DC voltage on the Piezoelectric Transducer (PZT) holding the output coupler, the reflection of the effective mirror changes correspondingly. The oscillating wavelength shifts from 2022 nm to 2031 nm. To our knowledge, this work is the first time to investigate the double cavity design in Tm:LuAG to achieve single longitude mode and tuning oscillation wavelength output laser. Output power, a.u.