Blue frenzy hits the holographer too

[Joe Farina]

Perhaps splitting up the elongated beam profile into several, ie ref and object, beams may be a way to mitigate the highly asymmetric pattern.

An interesting note on this subject was published in the American Journal of Physics, Vol. 44, No. 7, July 1976, which is attached below.

[a_k]

Hello Wolfgang

Thanks a lot for clarifying the mystery and correcting the belief about the multiple emitters. According to one text i found, the filamentation in broad area LDs can be reduced by slightly misaligning the feedback from the grating and by using a quarter wave plate in the external resonator.The amount of needed feedback seems to be current dependent. (810nm, 500mW LD, ECDL efficiency 50%, linewidth 5MHz) To accomplish this, the grating angle was left fixed and the feedback ratio varied with the quarter wave plate. One proposed explanation why the misaligned grating helps avoiding the filamentation is that the different transverse modes are coupled to each other this way and lock onto the same frequency: [url http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf][/url]

The quarter wave plate is used to control the amount of feedback because at a fixed feedback ratio it was not possible to get single mode output throughout the whole operation range.

Ahmet

[MfA]

This is cazy - engineers spent efforts in designing these projectors, a lot of high tech parts go into manufacturing them, and then they are bought like crazy so that some stores are sold out already, however just for ripping out the diodes. No wonder if single pieces of those cost thousands of $....

Well they charge thousands of $, they obviously cost next to nothing to produce.

[Sergio]

Hello Wolfgang

Thanks a lot for clarifying the mystery and correcting the belief about the multiple emitters. According to one text i found, the filamentation in broad area LDs can be reduced by slightly misaligning the feedback from the grating and by using a quarter wave plate in the external resonator.The amount of needed feedback seems to be current dependent. (810nm, 500mW LD, ECDL efficiency 50%, linewidth 5MHz) To accomplish this, the grating angle was left fixed and the feedback ratio varied with the quarter wave plate. One proposed explanation why the misaligned grating helps avoiding the filamentation is that the different transverse modes are coupled to each other this way and lock onto the same frequency: [url http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf][/url]

The quarter wave plate is used to control the amount of feedback because at a fixed feedback ratio it was not possible to get single mode output throughout the whole operation range.

Ahmet

So if I understand the "filamentation" correctly you may get single mode operation even with various transversal modes in a ECDL cavity, you could used a fixed grating and wave plate and use the current control only to achieve a single mode? By mounting a unit with fixed grating and WP you could mount it as the same way of an ordinary DPSS laser pointer and control it electronically.

[wler]

So if I understand the "filamentation" correctly you may get single mode operation even with various transversal modes in a ECDL cavity, you could used a fixed grating and wave plate and use the current control only to achieve a single mode? By mounting a unit with fixed grating and WP you could mount it as the same way of an ordinary DPSS laser pointer and control it electronically.

Yes these diodes can run single longitudinal mode (at low powers) despite non-transverse single mode, I always thought that would not be possible. So far an ECDL setup fuer 445nm didn't work too well, probably because of too strong feedback. I also noticed for red diodes that more feedback can make things worse, up to some degree. What is clear from the article is that several factors need to conspire together. They add a wave plate as extra degree of freedom, and apparently that helps. This is something to try asap! Thanks for telling, Ahmet.