21st Day of Christmas: Optical Isolator

This Advent during the 24 days leading up to Christmas I thought it would be fun to have a quick introduction of scientific apparatus, in particular tools used in atomic physics research. This is a way to say thanks to their hard work for our research, and a look at how many things are really needed to make a laboratory work!

Optical Isolator

Diode lasers are very sensitive to external feedback, meaning any part of their light that goes back into the diode. This sensitivity can be exploited to improve the laser’s properties, but when the feedback is uncontrolled, it can cause trouble, most of the time in the form of chaotic behaviour (the laser wavelength is jumping around). Since we want to have nice stable laser beam, these unwanted feedback needs to be removed, and that is done by an optical isolator. The optical isolator lets light through one way, but blocks it from returning the same way.

To create an optical isolator, we need some kind of device that treats going one way and going the other way differently, so we can distinguish between the two situation. One such thing is the Faraday rotator, which rotates the polarization angle of the light going through it through the magneto-optical Faraday effect in a crystal. A magnet surrounds the Faraday crystal, and when the light goes through it, the polarization angle is rotated the same direction compared to the magnetic field – but that’s the opposite direction from the point of view of the light going one way or another!

To filter the two directions we send the light through a polarizer first, and filter just – let’s say – the vertical polarization (the case titled “Forward” below, the light’s coming from the top). Our Faraday crystal has such a length, that a certain wavelength of light is rotated 45° after travelling through it. Afterwards, there’s a 45° polarizer, which in this case lets through all the light since it’s at the right polarization angle. Travelling backward (light coming from the bottom of the picture of “Backward” below), a 45° angle is selected, then the crystal rotates it by 45° – and because of how the Faraday effect works, it’s in the same direction of the previous rotation, adding up to 90°. But our polarizer at that end, where our input came originally, is vertical, while now our backward propagating light is horizontal at 90° – and get’s filtered, that is blocked…

The principle of operation of the Faraday-rotator based optical isolator
The principle of operation of the Faraday-rotator based optical isolator (source)

A lot depends on the quality of the polarization filters, as they are the ones that do the actual light preparation and blocking. Good Faraday isolators seem to go up to 30dB isolation, which is 1:1000 extinction ratio, only letting through 0.1% of the light trying to go the wrong way. This figure also depends on the fine-tuning of the polarizers for the particular wavelength, and the isolator adjusted by sending in the laser beam from the “wrong end” first, and adjusting the polarizers’ angles minimizing the amount of light coming out, using a power meter (at a sensitive setting).

1:1000 extinction sounds like a lot, but actually it can be still too much light possibly going back to some laser and causing trouble. In those cases multiple isolators might be used as their extinction adds up (i.e. 1:1,000,000 ideally), though some care needs to be taken as their losses add up as well (might lose 10-25% per isolator, and thus 19-44% after two).

There are a number of different designs of these Faraday rotators, some free space, some optical fibrer coupled for integrated systems. The key specs to watch is what wavelength they are designed for, their extinction ratio, and their transmission for the specific wavelength range.

A variety of optical isolators
A variety of optical isolators (source)

There are also other, somewhat more complicated designs that don’t depend on polarization.

When do we need to use optical isolators? Pretty much every time when any kind of reflective surface is used with a diode laser (or most other lasers). This means when coupling into an optical fiber (the coupling lens can reflect, even with anti-reflection coating), using reference cavities, vapor cells, vacuum system windows, and so on… This also one reason why so many optics in the lab has anti-reflection coating (other reasons are laser safety and more economical use of optical power).

And as a side-note, even though diode lasers don’t like uncontrolled light to enter them, controlled light can be of great use, as in the case of external feedback for the External Cavity Diode Lasers, or providing a reference beam for an optical amplifier.

Further info:

If you liked this, come back tomorrow for another apparatus! And send this link to someone who you think would be interested! :)