Laser diode divergence as a function of driving current.

[samchan]

Dear readers,

What happens to the divergence values (along and perpendicular to the emitting surface) when the driving current is increased or the output power is increased.
Does the divergence:

1. Increase along one axis
2. Increase along both axes
3. Decrease along both axes
4. Decrease along one axis.
5. Or is there no trend at all

If you could cite any results to support your answers, it would be very helpful.

 

[samchan]

Samchan, start here:

Laser Diode Technology

www.newport.com

For the laser diodes we use in pointers, they do not function as variable divergence devices. You can however overdrive them, such as some single mode diodes which can be pushed into multimode propagation out of the dye and then have greater divergence due to that. That said, I would not be surprised there is some amount of variability, just that it would not be significant.

Thank you. I should have mentioned that I am interested in the diodes extracted from Blue-ray drives. Do you think they have variable divergence values?

 

[samchan]

Most of the spec sheets for laser diodes quote the divergence values at specific output power (say 100mW) measured at FWHM (50% intensity). But none of the spec sheets which I have seen mention anything about how the divergences are affected when the output power changes.

 

[logsquared]

The slow axis of MM diodes is very dependent on current (gets bigger with higher currents). The fast axis seems pretty stable. Im not sure why. If I had to guess the fast axis is basically diffraction limited due to its very small dimension. This probably keeps divergence stable in that axis. The slow axis may widen due to modal effect or thermal, or both... not sure. Would love to know the mechanism behind it.

 

[samchan]

The slow axis of MM diodes is very dependent on current (gets bigger with higher currents). The fast axis seems pretty stable. I'm not sure why. If I had to guess the fast axis is basically diffraction-limited due to its very small dimension. This probably keeps divergence stable in that axis. The slow axis may widen due to modal effect or thermal, or both... not sure. Would love to know the mechanism behind it.

Very grateful for your reply. Have you seen any journal publications to support this? Considering how ubiquitous laser diodes are, I find it strange that there are hardly any search results for this topic.

 

[Encap]

The slow axis of MM diodes is very dependent on current (gets bigger with higher currents). The fast axis seems pretty stable. Im not sure why. If I had to guess the fast axis is basically diffraction limited due to its very small dimension. This probably keeps divergence stable in that axis. The slow axis may widen due to modal effect or thermal, or both... not sure. Would love to know the mechanism behind it.

"Broad area laser diodes (also called broad stripe or broad emitter laser diodes, single-emitter laser diodes, and high brightness diode lasers) are edge-emitting laser diodes where the emitting region at the front facet has the shape of a broad stripe (see Figure 1), with dimensions of e.g. 1 μm × 100 μm. Due to the asymmetry of the emitter, the beam properties are also completely different for the two directions:

• In the vertical (short) direction, the height (e.g. 1 μm) is small enough to obtain single-mode guidance and thus an essentially diffraction-limited beam quality with an M2 factor only slightly above 1. Because of the small aperture size, the beam divergence in this direction is relatively high, with a beam divergence half-angle of e.g. 370 mrad, corresponding to a full width at half maximum (FWHM) angular range of 25°. Due to that fast divergence, this is called the fast axis direction.
• In the long direction (slow axis direction), the stripe width may be e.g. 50, 100, 200 μm, or even larger, so that the light is distributed over many spatial modes in this direction. As a result, the beam divergence is much larger than for a diffraction-limited beam with that size, although still significantly smaller than for the fast axis direction. (Typical values are around 5–10° FWHM.) The beam quality in terms of focusability is reduced; M2 might be of the order of e.g. 20 for a 100-μm stripe. Furthermore, the beam profile may be multi-peaked in the horizontal direction, and the shape of the intensity pattern may depend on the injection current.

The wavefronts at the output facet are approximately plane in horizontal and vertical direction, but there can be some astigmatism, i.e., a slightly different focus position for the two directions.

Figure 1: Broad area laser diode.
The broader the stripe, the higher is the achievable power, but the worse is the beam quality in the “slow” direction. The technological trend is to obtain higher and higher powers even from narrow stripes, but this is limited by the high optical intensity at the front facet (which can lead to catastrophic failure) and possibly by thermal issues. Special techniques of facet passivation can be used to allow for higher powers. For a 100 μm wide aperture, the output power of a commercial device is typically a few watts or up to the order of 10 W."
From: https://www.rp-photonics.com/broad_area_laser_diodes.html