# Temperature impact on laser emission

• I
• Q.B.
In summary, the conversation discusses the relationship between temperature and the input current needed for laser diodes to start lasing. The rate equations governing the evolution of quantum level populations are mentioned, along with the role of thermal transitions. It is noted that non-radiative processes may also play a role in population inversion at higher temperatures. Finally, the concept of equilibrium and its application in laser diodes is explained.
Q.B.
Hi everyone,

I'm trying to understand why laser diodes need a stronger input current to start lasing when their temperature increases.

If I tried to add thermal transitions to the rate equations governing the evolutions of an atom quantum level populations (let's say ##n_{up}## and ##n_{down}## for a two-level problem) in a laser amplification medium, I would write:
$$\frac{dn_{up}}{dt}= partwithstimulatedemission, absorption, andpumping + (somefactor)(n_{down}e^{(E_{down}-E_{up})/kT} - n_{up}e^{(E_{up}-E_{down})/kT})$$
$$\frac{dn_{down}}{dt}= partwithstimulatedemission, absorption, andpumping - (somefactor)(n_{down}e^{(E_{down}-E_{up})/kT} - n_{up}e^{(E_{up}-E_{down})/kT})$$
Which seems to make population inversion easier when temperature increases.

However, non-radiative transitions (with phonons for instance) might also be supported by the temperature increase. Are these non-radiative processes in the end taking over the thermal fluctuations which help population inversion, and explain the phenomenon observed in diodes?

Q.B.
The Boltzmann factor ## exp( - \frac {E}{k_bT} ) ## applies only for a system at equilibrium. At equilibrium, the probability of population of the higher energy level will always be lower than that for a lower energy level. To get laser operating you need to create a population inversion, that is more electrons in higher energy levels than lower energy levels and the Boltzmann factor is simply not applicable in a non-equilibrium case.

In laser diodes, the population inversion is achieved by injection using a p-n junction. The number of electrons in the upper state depends on the injection current minus recombination. At higher temperature, the recombination rate increases and that's why you need higher current to achieve the population inversion.

## What is the relationship between temperature and laser emission?

The emission of a laser is affected by temperature in several ways. As temperature increases, the atoms or molecules in the laser medium have more thermal energy, leading to higher collision rates and broadening of the emission spectrum. This can result in a decrease in laser output power and a shift in the wavelength of the emitted light.

## How does temperature affect the efficiency of a laser?

Higher temperatures can lead to a decrease in the efficiency of a laser. This is because the increased thermal energy can cause the laser medium to become less transparent, resulting in more energy being lost as heat instead of being converted into laser light. Additionally, higher temperatures can affect the alignment of the laser components, leading to decreased efficiency.

## What is the optimal temperature for laser emission?

The optimal temperature for laser emission varies depending on the type of laser and its specific design. In general, a lower temperature is preferable for achieving a higher efficiency and a more stable output. However, some lasers may require higher temperatures for optimal operation, so it is important to consult the manufacturer's specifications.

## How can temperature fluctuations affect laser emission?

Temperature fluctuations can have a significant impact on laser emission, especially in precision applications. Fluctuations can cause changes in the laser's output power, wavelength, and beam quality. They can also affect the stability and reliability of the laser, making it difficult to maintain consistent results.

## What measures can be taken to minimize the impact of temperature on laser emission?

To minimize the impact of temperature on laser emission, it is important to control the temperature of the laser medium and its components. This can be achieved through proper insulation, cooling systems, and temperature regulation. Additionally, using materials with low thermal expansion coefficients and designing the laser for stability can also help mitigate the effects of temperature fluctuations.

Replies
3
Views
2K
Replies
19
Views
4K
Replies
3
Views
2K
Replies
3
Views
6K
Replies
2
Views
2K
Replies
6
Views
2K
Replies
2
Views
1K
Replies
3
Views
971
Replies
4
Views
3K
Replies
3
Views
16K