# Why would mode-hopping occur as the current is changed?

If some diode laser is given an input current, why should we see mode-hopping if the current is changed?

I'm not 100% sure, but I think that temperature is usually the primary culprit. More current means more resistive heating.

You can't just use a temperature controller to hold the temperature constant: The laser doesn't have just one temperature -- it has hotter parts and colder parts. (The resistive heating only occurs in certain areas within the laser.) No matter how much money you spend on a temperature controller, you cannot hold the laser's entire temperature profile constant when you change the laser current.

When the temperature profile changes, the refractive index profile changes too, therefore the modes' shape and frequency change, which changes the gains and losses of each mode in a different way. Therefore, mode hopping can occur.

• That's an interesting thought. I'll research this myself and get back you. If you're right, I'll mark your answer as correct (and add any bits I find) :) – User 17670 Oct 13 '13 at 14:20
• @SteveB You can use an IR camera/sensor to monitor the part and then adjust the temperature controller more dramatically to help stabilize the junction temperature. – user6972 Oct 13 '13 at 22:05
• @user6972 -- No. The material's temperature affects the mode everywhere that there is light in the mode. So temperature profile matters -- temperature as a function of x, y, z. In principle, you can stabilize the temperature of one particular point $(x_0, y_0, z_0)$ as current is changed, by adjusting the temperature controller as a function of current. But even if you do that perfectly, you'll find that the temperature at other points $(x_1, y_1, z_1)$ in the laser is not stable as the current is changed. – Steve Byrnes Oct 14 '13 at 19:38
• @SteveB I'm not sure what you're saying no to when you appear to agree with me. With an IR camera you can characterized how/when modes occur and either avoid them or induce them depending on what you want. You don't necessarily need to monitor input current. – user6972 Oct 31 '13 at 23:45

EM waves can have many different modes of propagation and when a single mode laser is driven at a different current level sometimes these conditions are met allowing other modes of oscillation.

In multi-mode operation of a laser, there may be transitions between different sets of modes. However, simultaneous oscillation on many modes is then most common, and instead of complete mode hops there are often more continuous transitions, with the optical power being gradually redistributed. The dynamics can be further influenced by nonlinear effects such as spatial hole burning. Mode hops can also involve higher-order modes, or modes with different polarization in lasers with polarization-independent gain.

Typically when a diode is driven at a different power level the previously lasing mode may no longer be the mode with highest gain, so that the power of a competing mode with higher gain can quickly rise causing modes to form or switch. Also a drift of the temperature of the gain medium will shift the wavelength of maximum gain while not shifting the frequencies of the resonator modes.

• You haven't answered the question :( I am asking 'why does a change in current lead to these effects?' – User 17670 Oct 12 '13 at 18:18
• See physics.stackexchange.com/questions/80392/…, User 17670 knows about modes and the effect of temperature. This question is specifically why current also causes mode hopping when the temperature is held constant. – John Rennie Oct 12 '13 at 18:43
• @User17670 It wasn't clear what you wanted or knew from your question. But I thought 2nd paragraph covered that. @ John Rennie You can't keep temperature constant for the internal parts. – user6972 Oct 12 '13 at 20:53
• "the optical power [is] being gradually redistributed" - why? "Mode hops can also involve higher-order modes" - why? – User 17670 Oct 13 '13 at 13:55
• @User17670 per last sentence...by temperature changes which occur in the substrate due to current drive level changes in the junction. If you have an IR temperature measurement meter you can monitor the diode temperature more accurately and you'll find the part changes in temperature much quicker than most temperature controlled environments can even detect or compensate. It's the gain medium that causes this mode jump. – user6972 Oct 13 '13 at 22:02