DVD writer laser temperature???

[the guy]

Does anyone know the temperature of the beam that a DVD writer laser diode (200mw, 650nm) produces?

[swansont]

Does anyone know the temperature of the beam that a DVD writer laser diode (200mw, 650nm) produces?

The light is not from a thermal source, so temperature is not defined.

[the guy]

Ok but if I were to focus the beam onto a thermometer what would it read?

[swansont]

If you assume all of the energy is absorbed and not reflected, it would depend on the material used for the thermometer and what type of thermometer it is.

 

If you shine it on a blackbody with some surface area and it was perfectly conductive, it would absorb the energy and reach a temperature such that the emitted radiation power was equal to what was absorbed, i.e. 200 mW. The Stefan-Boltzmann law tells us that the emitted power varies as T^4 and scales with the area. If you shined it on a surface of 1 cm^2 and we ignore all other effects, it would eventually reach a temperature of 433K, or 160 ºC. In reality, there will be reflection and other cooling effects, and you already have radiation heat transfer occurring. e.g. if you cut the efficiency in half, owing to reflection, then the temperature will be limited to ~90 ºC. If the area is larger, the temperature goes down — you'd get the same effect from doubling the area.

[the guy]

ah right ok cheers, that makes sense.

[Enthalpy]

"The" temperature is defined at equilibrium, of which we're very far here. We could define various temperatures but these differ an awful lot then.

 

For instance, red colour is associated with a temperature, say 700°C.

 

The fine line width of the laser light defines a very low temperature. Nothing abstract: this is what cools ions to some mK in an ion trap.

 

The power concentration defines a very high temperature. Take 20mW (not 200mW) from your laser diode, concentrated to 0.5µm² as on DVD: if absorptivity and emissivity were equal and through a single face, it would attain 30,000 K. Better keep moving.