by Dinesh » Wed Feb 05, 2014 1:42 pm
When you change the polarisation of the input beam via the broadband halfwave plate, the condition for altering the polarisation angle is dependent on the thickness of the halfwave plate. However, for broadband applications, the thickness is larger than need be. This is known as a "multi-order" halfwave plate (as opposed to a zero order halfwave plate). The polarisation changes as a result of making the thickness create a phase change of an exact multiple of pi and taking advantage of the birefringence. The exact relationship is:
delta phi = (2pi/lambda)*delta(n)*d
and so if you arrange the thickness of the crystal (d) so that delta phi = pi/2 (hence "halfwave plate"), then you have a twisting of the plane of polarisation. Notice however, that the delta phi is lambda dependent. Thus, if you calculate d for a specific lambda, it wouldn't alter any other wavelength. This is known as a zero order halfwave plate. However, in practice, this value of d is extremely small - on the order of 20 microns or so - making it impractical. So, what's done is to stack up lots of d's and create a multiple order halfwaveplate. However, this has the effect of de-sensitising the wavelength selectivity thus making it "broadband". The polarisation is still lambda dependent, but the lambda dependence is 'mushy'. Thus, the halfwave plate will alter the amount of twist of the polarisation of different wavelengths by different amounts (albeit, very close) for different wavelengths. This will have the effect of altering the ratio at the output of the cube. So, once it's fixed on the wavelength/polarisation relationship, it'll probably remain pretty stable. However, stacking the waveplates to create a multiorder waveplate makes it pretty sensitive to temperature fluctuations, and you're hitting it with a laser that might warm it up. I'd suggest that you make (or buy) a low frequency grating, pass the altered polarised beams , ie the beams after passing through the waveplate, into the grating, which will split them, and then place a detector at each beam. Monitoring the detector will give you an idea of the stability.
If you're interested, I wrote something about this at the behest of the head of R&D at Applied Holographics (Hamish Shearer, some forum members might recognise the name) lo these many years ago:
http://www.triple-take.com/publications ... 201984.pdf
When you change the polarisation of the input beam via the broadband halfwave plate, the condition for altering the polarisation angle is dependent on the thickness of the halfwave plate. However, for broadband applications, the thickness is larger than need be. This is known as a "multi-order" halfwave plate (as opposed to a zero order halfwave plate). The polarisation changes as a result of making the thickness create a phase change of an exact multiple of pi and taking advantage of the birefringence. The exact relationship is:
delta phi = (2pi/lambda)*delta(n)*d
and so if you arrange the thickness of the crystal (d) so that delta phi = pi/2 (hence "halfwave plate"), then you have a twisting of the plane of polarisation. Notice however, that the delta phi is lambda dependent. Thus, if you calculate d for a specific lambda, it wouldn't alter any other wavelength. This is known as a zero order halfwave plate. However, in practice, this value of d is extremely small - on the order of 20 microns or so - making it impractical. So, what's done is to stack up lots of d's and create a multiple order halfwaveplate. However, this has the effect of de-sensitising the wavelength selectivity thus making it "broadband". The polarisation is still lambda dependent, but the lambda dependence is 'mushy'. Thus, the halfwave plate will alter the amount of twist of the polarisation of different wavelengths by different amounts (albeit, very close) for different wavelengths. This will have the effect of altering the ratio at the output of the cube. So, once it's fixed on the wavelength/polarisation relationship, it'll probably remain pretty stable. However, stacking the waveplates to create a multiorder waveplate makes it pretty sensitive to temperature fluctuations, and you're hitting it with a laser that might warm it up. I'd suggest that you make (or buy) a low frequency grating, pass the altered polarised beams , ie the beams after passing through the waveplate, into the grating, which will split them, and then place a detector at each beam. Monitoring the detector will give you an idea of the stability.
If you're interested, I wrote something about this at the behest of the head of R&D at Applied Holographics (Hamish Shearer, some forum members might recognise the name) lo these many years ago: http://www.triple-take.com/publications/Polarization%201984.pdf