An optical temperature sensor was created using a femtosecond micromachined diffraction grating inside transparent bulk 6H-SiC, and to the best of our knowledge, this is a novel technique of measuring temperature. Other methods of measuring temperature using fiber Bragg gratings have been devised by other groups such as Zhang and Kahrizi [in MEMS, NANO, and Smart Systems (IEEE, 2005)]. This temperature sensor was, to the best of our knowledge, also used for a novel method of measuring the linear and nonlinear coefficients of the thermal expansion of transparent and nontransparent materials by means of the grating first-order diffracted beam. Furthermore the coefficient of thermal expansion of 6H-SiC was measured using this new technique. A He–Ne laser beam was used with the SiC grating to produce a first-order diffracted beam where the change in deflection height was measured as a function of temperature. The grating was micromachined with a $20\mathrm{\mu m}$ spacing and has dimensions of approximately $500\mathrm{\mu m}\times 500\mathrm{\mu m}$ ( $l\times w$ ) and is roughly $0.5\mathrm{\mu m}$ deep into the 6H-SiC bulk. A minimum temperature of $26.7°\mathrm{C}$ and a maximum temperature of $399°\mathrm{C}$ were measured, which gives a $\mathrm{\Delta }T$ of $372.3°\mathrm{C}$ . The sensitivity of the technique is $\mathrm{\Delta }T=5°\mathrm{C}$ . A maximum deflection angle of $1.81°$ was measured in the first-order diffracted beam. The trend of the deflection with increasing temperature is a nonlinear polynomial of the second-order. This optical SiC thermal sensor has many high-temperature electronic applications such as aircraft turbine and gas tank monitoring for commercial and military applications.