Why does my cheap green laser emit two different wavelengths

[John Cuthber]

Well, I guess the question says it all.
It's a laser pointer- nothing special. It's nominally 532nm so it's a frequency doubled Nd type of some sort and yet it gives two different wavelengths of light.

It certainly gives two wavelengths, I first spotted this using a diffraction grating but my direct vision spectroscope verifies the fact (as do two other gratings and they each give just 1 spot with a second 532 nm laser and a HeNe)

As far as I can tell they are 532 and about 536 nm. but it's difficult to judge with my rather low resolution spectrometer

The divergence between the two beams after they hit a grating is about 1.5 cm in 2 metres which pretty much tallies with that estimate.

Neither beam is significantly polarised

 

As far as I understand it the 532 is a direct consequence of the 1064 nm emission from Nd+++ so a second emission frequency should be impossible.

 

Has anyone noticed this sort of thing before and does anyone have a viable explanation for it?

 

[imatfaal]

Neodymium-doped yttrium lithium fluoride apparently has two very close transitions at 1047nm and 1053nm (weaker) - the gap is about right for what you are measuring although the two wavelengths are a bit lower than your measurement.

 

Doesn't seem a common lasing medium for laser pointers - but maybe... and I must admit that I thought you could only get one of the transitions at a time.

[Griffon]

Green laser pointers are more commonly frequency doubled Nd vanadate lasers I believe.

 

I agree Nd:YLF is a possibility. The two lasing transitions have orthogonal polarisations. Just a guess, but may be the laser is flipping between the two wavelengths if there's poor polarisation selection.

[John Cuthber]

Thanks for that. I may have to try to tie down the wavelengths more precisely to see if that gives me a clue.

[Griffon]

Maybe you could check the manufacturer's website to find out exactly what sort of laser you've got there.

 

By the way, I've just discovered that "In addition to the common 1,064 nm wavelength, Nd:YAG has over a dozen other weaker lasing transitions between 1,052 nm and 1,444 nm". http://www.repairfaq.org/sam/laserssl.htm#sslclm

 

I had a vague recollection there are other lines near 1064 nm in YAG lasers.

[John Cuthber]

I could check the distributor's website and it says 532nm.

http://www.maplin.co.uk/slim-green-laser-pointer-340984

 

I would struggle to track down the maker.

My current best estimates are 532.3 and 537.9 nm (though the last digit is probably spurious accuracy: the spectrometer scarcely resolves the difference in angle between the two yellow mercury lines I was using as part of the calibration)

[Griffon]

Is there an original manufacturer's mark or model number on the laser by any chance?

 

Just to confuse matters I see there's a vanadate laser using neodymium lutetium vanadate (Nd:LuVO4) crystals which has two lasing wavelengths at 1066nm and 1076nm which double to 533nm and 538nm - not far from your measured wavelengths. That might be a long shot though.

Edited March 25, 2013 by Griffon

[Enthalpy]

Admitting that the two wavelengths come from the laser and both are doubled, this implies that they are emitted at different times. If simultaneous, the doubler would create a third frequency which is the sum of both - in addition to the double of each.

 

That is, [cos(w₁t)+cos(w₂t)]² contains

cos²(w₁t), cos²(w₂t) and cos(w₁t)*cos(w₂t) which expand into

cos(2w₁t), cos(2w₂t) AND cos[(w₁+w₂)t]

 

Since lasers tend to produce many frequencies erratically, even if they have only one transition, this explanation is natural.

 

By the way, the difference between both frequencies is only 2.6THz, or 115µm, far less than the ambient temperature, meaning that the transition is naturally wide enough to lase at both wavelengths. Lasing makes emission lines much narrower than spontaneous emission, but occasionnally the oscillation can hop from one cavity mode to an other within one lasing transition that is wide enough. This is prevented in more expensive lasers.

 

Did you open the pointer to observe if both wavelengths exist before the doubler?

[John Cuthber]

Interesting point.

One complicating factor is that the effect isn't consistent, the weaker beam is variable (sometimes absent).

I'm not certain and I'd need to check the maths but since frequency doubling is a 2nd order effect I think that the "cross term" would be very weak.

[Enthalpy]

The intensity of the doubled frequency relative to the fundamental is variable (experimenters would say, erratic) but the intensities of the products of nonlinearity are fully linked. Take a second-order nonlinearity, since even powers in Taylor's expansions produce the even harmonics:

 

[a₁cos(w₁t)+a₂cos(w₂t)]²

=a₁²cos²(w₁t) + 2a₁a₂cos(w₁t)cos(w₂t) + a₂²cos²(w₂t)

=0.5a₁²cos(2w₁t) + a₁a₂cos[(w₁+w₂)t] + 0.5a₂²cos(2w₂t)

plus some terms at w=0 and a w₁-w₂

 

so the field amplitude at w₁+w₂ is double (power is quadruple) the geometric mean of the amplitudes at 2w₁t and 2w₂t.

 

This has practical implications in radiocomms. Nonlinearity is sought for mixers of heterodyne receivers

http://en.wikipedia.org/wiki/Heterodyne_receiver

but nonlinearity isn't desired among the many signals arriving at the mixer from different transmitters. To improve mixers, designers prefer parts with even nonlinearity

http://en.wikipedia.org/wiki/Intermodulation

and combine them in symmetric circuits so the mixer is as linear as possible against the input signals.

Edited March 28, 2013 by Enthalpy