Measuring Extinction Ratio of a fiber laser

[alex]

I am getting poor polarization measurements from a fiber laser made from PM maintaining fiber. Firstly I would like to ask if my measurement method is ok.

I collimate the beam with an aspherical lens of 8mm focal distance. Then there is a Glan Tayler Polarizer.
https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=815

I tune the angle for maximum and minimum transmission, noting the power. I am only getting a 20:1 ratio between minimum and maximum transmission.

I am concerned that my minimum measurement is my big source of error..Any change in this makes a large change in ratio. I am using a low and high power powermeter for each of the measurements for best accuracy.

Is there another way of doing this measurement?
Thanks
Alex

[JTS]

Post by alex
I am getting poor polarization measurements from a fiber laser made from PM maintaining fiber. Firstly I would like to ask if my measurement method is ok.
I collimate the beam with an aspherical lens of 8mm focal distance. Then there is a Glan Tayler Polarizer.
https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=815
I tune the angle for maximum and minimum transmission, noting the power. I am only getting a 20:1 ratio between minimum and maximum transmission.
I am concerned that my minimum measurement is my big source of error..Any change in this makes a large change in ratio. I am using a low and high power powermeter for each of the measurements for best accuracy.
Is there another way of doing this measurement?
Thanks
Alex

I do not know what is the best way of doing this measurement, but I do
know how to test your method: measure the polarization of light directly
out of the laser (attenuated if necessary). If it works for that, then
the method is ok.

BTW a detector with a measurement range of 10^6 is standard (I do not
know the reason why one can get such a wide range, I took the datum for
one of the Thorlabs detectors), so it is likely that the method will be
ok. Perhaps stray light???

For alternative methods, hopefully someone else will give you some
suggestions.

[JTS]

Post by JTS
I do not know what is the best way of doing this measurement, but I do
know how to test your method: measure the polarization of light directly
out of the laser (attenuated if necessary).

Here I mean: of *a* laser whose polarization you are sure about.

[alex]

Post by JTS

Post by JTS
I do not know what is the best way of doing this measurement, but I do
know how to test your method: measure the polarization of light directly
out of the laser (attenuated if necessary).

Here I mean: of *a* laser whose polarization you are sure about.

Thanks for the idea... I will give that a go tmrw, but I think it might be harder tha it should be to find a laser whose polarisation we know... I'll let you know...
Thanks
Alex

[JTS]

Post by alex

Post by JTS

Post by JTS
I do not know what is the best way of doing this measurement, but I do
know how to test your method: measure the polarization of light directly
out of the laser (attenuated if necessary).

Here I mean: of *a* laser whose polarization you are sure about.

Thanks for the idea... I will give that a go tmrw, but I think it might be harder tha it should be to find a laser whose polarisation we know... I'll let you know...
Thanks
Alex

I think Phil Hobbs' suggestions are more precise than mine, but another
approach is - polarize the light with a polarizer (a polarizer with
1:100 ratio is good enough for this) and repeat your measurements. If
you can measure 1:100, it means the 1:20 you measured is ok.

[Phil Hobbs]

Post by JTS

Post by alex

Post by JTS

Post by JTS
I do not know what is the best way of doing this measurement, but I do
know how to test your method: measure the polarization of light directly
out of the laser (attenuated if necessary).

Here I mean: of *a* laser whose polarization you are sure about.

Thanks for the idea... I will give that a go tmrw, but I think it
might be harder tha it should be to find a laser whose polarisation we
know... I'll let you know...
Thanks
Alex

I think Phil Hobbs' suggestions are more precise than mine, but another
approach is - polarize the light with a polarizer (a polarizer with
1:100 ratio is good enough for this) and repeat your measurements. If
you can measure 1:100, it means the 1:20 you measured is ok.

Yup. A Glan-Taylor is good for 1:10,000 if you use it right. Almost as
good as a Wollaston, at least in the transmitted beam. A "beam
splitting Thompson" improves the refracted beam a fair amount, but
they're not that common. Stick with the transmitted one.

Cheers

Phil Hobbs

[g***@gmail.com]

Post by Phil Hobbs
Yup. A Glan-Taylor is good for 1:10,000 if you use it right. Almost as
good as a Wollaston, at least in the transmitted beam. A "beam
splitting Thompson" improves the refracted beam a fair amount, but
they're not that common. Stick with the transmitted one.

I became curious and I read a bit on Wikipedia. The reflected (I think you mean reflected, right?) beam is only partially polarized in these kind of polarizers because they work by total internal reflection of one polarization (which is then completely absent in the transmitted beam) but the other polarization is also reflected a bit (so it is present in the reflected beam).

I have also read about the difference between Glan-Thompson (is this the one you called "beam splitting Thompson"?) and Glan-Taylor, but to figure it out I would have to spend some time with calculations, so I stop here for the moment.

[Phil Hobbs]

Post by g***@gmail.com

Post by Phil Hobbs
Yup. A Glan-Taylor is good for 1:10,000 if you use it right. Almost as
good as a Wollaston, at least in the transmitted beam. A "beam
splitting Thompson" improves the refracted beam a fair amount, but
they're not that common. Stick with the transmitted one.

I became curious and I read a bit on Wikipedia. The reflected (I think you mean reflected, right?) beam is only partially polarized in these kind of polarizers because they work by total internal reflection of one polarization (which is then completely absent in the transmitted beam) but the other polarization is also reflected a bit (so it is present in the reflected beam).

The transmitted beam is the good one. In a Wollaston, both beams are
equally good, and for a bonus there are no etalon fringes. Glan-Taylors
make fringes because the transmitted beam enters and exits perpendicular
to the facets, so there are multiple coaxial reflections.

Post by g***@gmail.com
I have also read about the difference between Glan-Thompson (is this the one you called "beam splitting Thompson"?) and Glan-Taylor, but to figure it out I would have to spend some time with calculations, so I stop here for the moment.

Normal Glan-Thompsons have black wax to absorb the reflected beam. It
makes many bounces between the hypotenuse and the wax, so it gets
absorbed very efficiently.

Cheers

Phil Hobbs

[Phil Hobbs]

Post by alex
I am getting poor polarization measurements from a fiber laser made from PM maintaining fiber. Firstly I would like to ask if my measurement method is ok.
I collimate the beam with an aspherical lens of 8mm focal distance. Then there is a Glan Tayler Polarizer.
https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=815
I tune the angle for maximum and minimum transmission, noting the power. I am only getting a 20:1 ratio between minimum and maximum transmission.

PM fibre is only good for ~20 dB extinction ratio, so you're not doing
that badly.

It's worse if the injected light isn't aligned with one of the fibre axes.

Also note that the high birefringence of PM fibre means that it has a
high temperature coefficient of birefringence, so that the polarization
is very unstable, though it will stay generally aligned along one axis
if it's launched correctly.

Post by alex
I am concerned that my minimum measurement is my big source of error..Any change in this makes a large change in ratio. I am using a low and high power powermeter for each of the measurements for best accuracy.
Is there another way of doing this measurement?

What power level are you measuring, what's the beam diameter, and what
meters are you using?

Cheers

Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

[r***@gmail.com]

Post by alex
I am getting poor polarization measurements from a fiber laser made from PM maintaining fiber. Firstly I would like to ask if my measurement method is ok.
I collimate the beam with an aspherical lens of 8mm focal distance. Then there is a Glan Tayler Polarizer.
https://www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=815
I tune the angle for maximum and minimum transmission, noting the power. I am only getting a 20:1 ratio between minimum and maximum transmission.
I am concerned that my minimum measurement is my big source of error..Any change in this makes a large change in ratio. I am using a low and high power powermeter for each of the measurements for best accuracy.
Is there another way of doing this measurement?
Thanks
Alex

Hello,

What minimum power values are you seeing for the "min" measurement? The smaller the numbers the more you have to worry about ambient light and scatter etc. Also what is the structure of your fiber laser? The portion that you're measuring is it LMA or standard fiber and what is the NA?

Robert