Fantanstic as always! The only thing i'm a bit confused about is what the role of the first polarizor is when aligning the axis of a polarizer to be perpendicular to the optical table. could you please clarify this? Thank you!
@ParTaban There are a couple of reasons we placed an input linear polarizer at the output of the light source. One is to ensure the light incident on polarizer #2 (the polarizer to be aligned) is linearly polarized with a high extinction ratio. The higher the light's extinction ratio, the larger the change in transmitted power as polarizer #2 is adjusted, and the more precisely we can align polarizer #2. Note that linear polarizers almost always have extinction ratios significantly higher than most lasers. The second reason for including the input polarizer is because this polarizer-alignment approach will not work if the incident light is vertically or horizontally polarized - rotating the input polarizer allows us to easily control the orientation of the input linearly polarized light, at the price of transmitting less power.
I have viewed (and learned a lot!) a good number of videos, including these ThorLabs series of videos, on laser alignments, optics applications, etc., produced by both large and small players in the industry and it brings up a question that comes up quite often in the lab and in the field - when are gloves necessary? When are gloves not necessary?
It is often up to the user's discretion whether to wear gloves meant to protect the optics' surfaces. While weighing risks and consequences, it helps to consider factors including the characteristics of the optic(s) and the likelihood of touching the optic's surface. Gloves are recommended if the optic can be permanently damaged by fingerprints or is difficult or inconvenient to clean. Keep in mind that dirty gloves can also leave smudges on or damage the optic. We're glad you're finding these videos to be helpful! If you think of a topic you'd like to see discussed in one of our videos, let us know!
That's a lovely explanation... But I'm confused if the 2nd polarizer is horizontal, then also we would get the same power after flipping isn't... In that case, how can we distinguish between vertical and horizontal polarization of the polarizer?
In this video we used the engraving to orient the first polarizer's transmission axis since we assumed the engraving approximately locates the polarizer's transmission axis. We rotated the first polarizer to orient the engraving (and transmission axis) nearly, but not exactly, horizontal. We aligned the second polarizer's transmission axis to be vertical since, in the following alignment procedure, we minimized the optical power transmitted through the second polarizer. (Here I'm assuming the first polarizer is the fixed polarizer in front of the source, and the second polarizer is the one whose transmission axis is being aligned.) If using the engraving is not possible, there are other approaches we use to approximately locate the transmission axis. One approach only requires the room lights and a reflection from the floor. The light incident on the floor is randomly polarized, but the reflected light is mostly S-polarized when the angle of reflection is around 60° with respect to the normal. (This is because the Brewster's angle is around 60°.) Since light reflected at this angle is mostly linearly polarized along a direction parallel to the floor, it is possible to use the linear polarizer to block it. Look through the linear polarizer at the reflection, then rotate the polarizer until the reflection appears dimmest. When this happens, the polarizer's transmission axis is perpendicular to the floor (perpendicular to the S-polarized light). An alternative approach is to use a polarizing beamsplitter. The beamsplitter outputs S-polarized light from one face and P-polarized light from another. These directions are well-defined and can be used to identify the transmission axis of a polarizer.
@Kashishkarera The technique will not work when the light incident on the polarizer is circularly polarized or when the input linear polarization state is already parallel or perpendicular to the table. The technique will work for other linear polarization states and elliptically polarized light, but the technique becomes increasingly difficult to use as the input polarization approaches circular polarization. To use this technique, it is necessary to be able to see a difference in the transmitted power when the mis-aligned polarizer is flipped around the vertical axis. When circularly polarized light is incident on the polarizer, the power transmitted by the polarizer will be the same for every orientation of the polarizer axis, and therefore the measured power will not change as the polarizer is flipped around the vertical axis. The same will occur if the input polarization state is already aligned with the table. For more information about the polarization ellipses describing different polarization states, see: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14199 .
I am confused why you set minimum power of second polarizer to 60 value .Though it can decrease further. For orthogonal, we have no light at the end . is that true?
@sehersaleem8581 If every component in the setup was ideal, the optical power measured after the orthogonal polarizers would have been zero. However, since real components are not perfect, it is expected that a small amount of light would be measured. For example, some light with the unwanted polarization state is actually transmitted through the polarizer. This amount can be estimated using the Extinction Ratio (ER), which is a specification provided by the manufacturer. This specification is the amount of transmitted light that is polarized parallel to the polarizer’s axis divided by the amount of transmitted light that is polarized perpendicular. A challenge (and some of the fun) of designing an experiment, and analyzing the results, is the imperfect behavior of the components. Another thing to consider is that our demonstration was performed while room lights and spot lights were turned on for filming, so it is likely the lens tube in front of the power sensor did not block all of the ambient light from reaching the power sensor. You are right that it is necessary to block all of the stray light to in order to orient the orthogonal polarizers with the greatest accuracy. Normally, we would do this procedure in a dark room, and possibly cover the setup to minimize the amount of ambient light.
A member of our ultrafast laser group has the answer to your question: If you generate white light in a bulk medium with polarized input light (i.e. from a femtosecond laser), the white light supercontinuum will generally have the same polarization state as the input light. For example, to generate a P-polarized supercontinuum, input P-polarized light in your sapphire crystal. However, it is important to make sure the light is properly aligned with the sapphire crystal, since sapphire is birefringent (the refractive index depends on the light’s direction of propagation and polarization orientation). A good approach to avoiding birefringence effects that could affect the light’s polarization state is to use a sapphire crystal that is C-cut, so that the optic axis (C-axis) is perpendicular to the input and output surfaces of the crystal. Then, make sure the input light is normally incident on the front surface of the crystal, so that the light propagates parallel to the sapphire’s optic axis. The light will be polarized perpendicular to the optic axis and experience a single refractive index.
In our application with your He- Ne laser with output power ~ 5mW, laser beam diameter ~ 1mm, beam divergence should not exceed 1mrad, linear polarization ratio ~ 500:1, what suitable arrangement should be there to get a s-polarized beam ?
There are a few options. Often, a half-wave plate (retarder) is placed in the linearly polarized laser beam, since rotating the wave plate's fast axis rotates the transmitted light's polarization direction. A way to align the fast axis to provide an s-polarized beam is to first place a linear polarizer, aligned orthogonal to the s-polarization direction, in the beam output by the wave plate. Then, rotate the wave plate until the power transmitted by the polarizer is minimized. It is also possible to obtain an s-polarized beam by rotating the body of the laser around its long axis, to rotate the laser light's polarization direction. The wave plate is not needed in this case, but it may not be convenient to rotate the laser, particularly since adjusting the laser typically changes the beam path and makes it necessary to realign the optical system. Let us know if your application needs another approach, or if you have other questions.
Thank you for reaching out! The readout of your power meter depends on a number of factors. For example, if a fast update rate is selected, the values displayed will change more frequently than with a slower update rate. It’s also true that thermal power sensors are more sensitive to local temperature drifts and air currents than photodiode power sensors. It can be beneficial to place a lens tube in front of the sensor element; doing so reduces air currents as well as the effects of ambient light. Feel free to reach out to tech support if you would like help with your specific situation!
![[Live] : ONE FIGHT NIGHT 25 วันนี้!! "อเล็กซิส vs รีเกียน"](http://i.ytimg.com/vi/suVr9a6BM2g/mqdefault.jpg)
Fantanstic as always!
The only thing i'm a bit confused about is what the role of the first polarizor is when aligning the axis of a polarizer to be perpendicular to the optical table. could you please clarify this?
Thank you!
@ParTaban There are a couple of reasons we placed an input linear polarizer at the output of the light source. One is to ensure the light incident on polarizer #2 (the polarizer to be aligned) is linearly polarized with a high extinction ratio. The higher the light's extinction ratio, the larger the change in transmitted power as polarizer #2 is adjusted, and the more precisely we can align polarizer #2. Note that linear polarizers almost always have extinction ratios significantly higher than most lasers.
The second reason for including the input polarizer is because this polarizer-alignment approach will not work if the incident light is vertically or horizontally polarized - rotating the input polarizer allows us to easily control the orientation of the input linearly polarized light, at the price of transmitting less power.
I have viewed (and learned a lot!) a good number of videos, including these ThorLabs series of videos, on laser alignments, optics applications, etc., produced by both large and small players in the industry and it brings up a question that comes up quite often in the lab and in the field - when are gloves necessary? When are gloves not necessary?
It is often up to the user's discretion whether to wear gloves meant to protect the optics' surfaces. While weighing risks and consequences, it helps to consider factors including the characteristics of the optic(s) and the likelihood of touching the optic's surface. Gloves are recommended if the optic can be permanently damaged by fingerprints or is difficult or inconvenient to clean. Keep in mind that dirty gloves can also leave smudges on or damage the optic.
We're glad you're finding these videos to be helpful! If you think of a topic you'd like to see discussed in one of our videos, let us know!
I would wear gloves when using any kind of grids and coated surfaces (AR, HR, ...) and always when you put these components in ultra-hig-vacuum!
That's a lovely explanation... But I'm confused if the 2nd polarizer is horizontal, then also we would get the same power after flipping isn't... In that case, how can we distinguish between vertical and horizontal polarization of the polarizer?
In this video we used the engraving to orient the first polarizer's transmission axis since we assumed the engraving approximately locates the polarizer's transmission axis. We rotated the first polarizer to orient the engraving (and transmission axis) nearly, but not exactly, horizontal. We aligned the second polarizer's transmission axis to be vertical since, in the following alignment procedure, we minimized the optical power transmitted through the second polarizer. (Here I'm assuming the first polarizer is the fixed polarizer in front of the source, and the second polarizer is the one whose transmission axis is being aligned.)
If using the engraving is not possible, there are other approaches we use to approximately locate the transmission axis. One approach only requires the room lights and a reflection from the floor. The light incident on the floor is randomly polarized, but the reflected light is mostly S-polarized when the angle of reflection is around 60° with respect to the normal. (This is because the Brewster's angle is around 60°.) Since light reflected at this angle is mostly linearly polarized along a direction parallel to the floor, it is possible to use the linear polarizer to block it. Look through the linear polarizer at the reflection, then rotate the polarizer until the reflection appears dimmest. When this happens, the polarizer's transmission axis is perpendicular to the floor (perpendicular to the S-polarized light). An alternative approach is to use a polarizing beamsplitter. The beamsplitter outputs S-polarized light from one face and P-polarized light from another. These directions are well-defined and can be used to identify the transmission axis of a polarizer.
Does this apply for any input(circular, elliptical, or linear)? or just linear input
@Kashishkarera The technique will not work when the light incident on the polarizer is circularly polarized or when the input linear polarization state is already parallel or perpendicular to the table. The technique will work for other linear polarization states and elliptically polarized light, but the technique becomes increasingly difficult to use as the input polarization approaches circular polarization.
To use this technique, it is necessary to be able to see a difference in the transmitted power when the mis-aligned polarizer is flipped around the vertical axis. When circularly polarized light is incident on the polarizer, the power transmitted by the polarizer will be the same for every orientation of the polarizer axis, and therefore the measured power will not change as the polarizer is flipped around the vertical axis. The same will occur if the input polarization state is already aligned with the table. For more information about the polarization ellipses describing different polarization states, see: www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14199 .
I am confused why you set minimum power of second polarizer to 60 value .Though it can decrease further. For orthogonal, we have no light at the end . is that true?
@sehersaleem8581 If every component in the setup was ideal, the optical power measured after the orthogonal polarizers would have been zero. However, since real components are not perfect, it is expected that a small amount of light would be measured. For example, some light with the unwanted polarization state is actually transmitted through the polarizer. This amount can be estimated using the Extinction Ratio (ER), which is a specification provided by the manufacturer. This specification is the amount of transmitted light that is polarized parallel to the polarizer’s axis divided by the amount of transmitted light that is polarized perpendicular. A challenge (and some of the fun) of designing an experiment, and analyzing the results, is the imperfect behavior of the components.
Another thing to consider is that our demonstration was performed while room lights and spot lights were turned on for filming, so it is likely the lens tube in front of the power sensor did not block all of the ambient light from reaching the power sensor. You are right that it is necessary to block all of the stray light to in order to orient the orthogonal polarizers with the greatest accuracy. Normally, we would do this procedure in a dark room, and possibly cover the setup to minimize the amount of ambient light.
What are the procedure if I want to get s or p polarised light from a white light supercontinum generated from 2mm sapphire crystal?
A member of our ultrafast laser group has the answer to your question: If you generate white light in a bulk medium with polarized input light (i.e. from a femtosecond laser), the white light supercontinuum will generally have the same polarization state as the input light. For example, to generate a P-polarized supercontinuum, input P-polarized light in your sapphire crystal. However, it is important to make sure the light is properly aligned with the sapphire crystal, since sapphire is birefringent (the refractive index depends on the light’s direction of propagation and polarization orientation).
A good approach to avoiding birefringence effects that could affect the light’s polarization state is to use a sapphire crystal that is C-cut, so that the optic axis (C-axis) is perpendicular to the input and output surfaces of the crystal. Then, make sure the input light is normally incident on the front surface of the crystal, so that the light propagates parallel to the sapphire’s optic axis. The light will be polarized perpendicular to the optic axis and experience a single refractive index.
In our application with your He- Ne laser with output power ~ 5mW, laser beam diameter ~ 1mm, beam divergence should not exceed 1mrad, linear polarization ratio ~ 500:1, what suitable arrangement should be there to get a s-polarized beam ?
There are a few options. Often, a half-wave plate (retarder) is placed in the linearly polarized laser beam, since rotating the wave plate's fast axis rotates the transmitted light's polarization direction. A way to align the fast axis to provide an s-polarized beam is to first place a linear polarizer, aligned orthogonal to the s-polarization direction, in the beam output by the wave plate. Then, rotate the wave plate until the power transmitted by the polarizer is minimized.
It is also possible to obtain an s-polarized beam by rotating the body of the laser around its long axis, to rotate the laser light's polarization direction. The wave plate is not needed in this case, but it may not be convenient to rotate the laser, particularly since adjusting the laser typically changes the beam path and makes it necessary to realign the optical system.
Let us know if your application needs another approach, or if you have other questions.
why does your power meter show so steadily? While mein varies constantly.
Thank you for reaching out! The readout of your power meter depends on a number of factors. For example, if a fast update rate is selected, the values displayed will change more frequently than with a slower update rate. It’s also true that thermal power sensors are more sensitive to local temperature drifts and air currents than photodiode power sensors. It can be beneficial to place a lens tube in front of the sensor element; doing so reduces air currents as well as the effects of ambient light.
Feel free to reach out to tech support if you would like help with your specific situation!