Tools of the Laser Optics Lab Trivia Quiz

Answer: The wavelength of the laser in use

Lasers above a certain power (energy output per second) are very dangerous to eyesight. Vision works because the lens of your eye focuses light onto your retina, which contains light-sensitive cells -- but if you focus too much energy there, your retina will burn. Unlike damage from the sun, you can't blink fast enough to avoid damage from a high-powered laser -- and if the beam is invisible, your blink reflex won't even trigger at all.

Laser safety goggles are designed to solve this problem, absorbing enough photons at the laser wavelength that the light reaching your eyes is safe. These only work at limited wavelength ranges -- otherwise you wouldn't be able to see anything! -- so it's important to make sure you're wearing the right goggles for your laser.

Answer: To keep the laser system from shaking or vibrating

Laser optics systems require precise alignment -- often to accuracies of a few nanometers (those are millionths of a millimeter!) When such levels of precision are needed, scientists must be very careful to remove sources of error -- and vibration, even the sound of a cough or the impact of a foot on the floor, is a big source of error. Optical tables, with their complex innards, are designed to damp vibrations and keep laser systems in alignment.

Optical tables have a second advantage for alignment: they're generally topped with a steel or aluminum breadboard, a grid of standard screw holes on a flat surface. Mirrors, lenses and other optical goodies can thus be easily and securely mounted along straight lines or right angles, simplifying the setup of a complex system.

Answer: Pulsed lasers can produce higher peak powers than continuous-wave lasers.

A laser -- the name stands for "Light Amplification by Stimulated Emission of Radiation" -- has at its core a particular gain medium, which could be (among other things) a semiconductor, a gas, or a crystal. By applying some energy, you can "invert the population" -- moving the atoms or molecules to a state with a higher energy. Then, when an atom or molecule is perturbed by a photon with the right energy, it decays back to the lower-energy state, emitting the excess energy in the form of a photon. In this process, called stimulated emission, the original photon becomes two -- with the same energy, the same direction of motion, and the same phase (that is, the peaks and valleys of their waveforms line up). The gain medium is enclosed by two mirrors, so that light bounces back and forth, amplified every time; with constant energy input to keep the population inverted in the gain medium, one ends up with a powerful beam.

A laser can be designed to run continuously (that is, continuous-wave or CW operation) or in a pulsed mode. High-powered CW lasers suffer from heating up too quickly; pulsed lasers achieve greater peak power partly because there's time for the gain medium to recover between pulses. By the same token, however, CW lasers are always on; although some types of pulsed laser have high repetition rates, others require waiting seconds, minutes or even hours between pulses! Choosing a laser requires carefully weighing the pros and cons for your particular application.

Answer: The diameter of the beam's cross section at its narrowest point

A Gaussian laser beam, seen from the side, looks a bit like a bow-tie, or like two cones placed tip to tip. The beam begins with a relatively large diameter (or "spot size"), and gradually tapers to its narrowest extent, the waist - after which it gradually spreads out again.

The quickness of this spreading - called the beam divergence - depends on the laser wavelength and on the beam waist: the narrower the waist, the faster the beam diverges from it on either side. With the right optics, one can get a laser beam very close to a straight line; the process is called collimation, and it relies on maintaining a large beam waist and small divergence.

Answer: A "viewing card" that responds to infrared light by fluorescing at a visible wavelength

An ideal laser produces a beam of coherent light, all at a single wavelength; real-world lasers can never be quite as good, but the wavelengths of laser light should fall in a tight range around the central value. You'll never see a single laser producing a beam that includes both red and green light, let alone light that spans both visible and infrared! Remember, too, that the wavelength of a photon depends on its energy -- and the energy of a particle should not change if all it's doing is moving across the room. A nonlinear optical crystal can change the wavelength of a laser beam passing through it, but the process is both inefficient and finicky. It requires extremely precise beam steering and focusing, so it isn't terribly useful as a diagnostic tool.

A viewing card, however, is a crucial tool for any laser lab. If you're taking proper safety precautions (namely laser safety goggles), you won't be able to see your laser beam whether it falls in the visible range or not -- but you CAN see its image on the viewing card. And it's as portable as it sounds: by moving it around, you can see how the beam's spot size changes with distance, or search for stray reflections. Don't leave home without it!

Answer: A lens

Manipulating laser light is much like manipulating regular light -- except that, since lasers pack much more energy into a smaller area, the necessary optics must be sturdier, more precise and (naturally) more expensive. A laser optics lens is a piece of glass, curved in such a way that it converges or diverges a passing beam of light at a specified angle.

It's the same principle that allows a magnifying glass to make an image appear larger, and that allows the lens of your eye to focus the results on your retina! All of these lenses alter the shape or size of the beam, so they're crucial tools for optimizing the laser beam -- whether it's too large, too small, or too elliptical.

Answer: With a pair of plane mirrors

If you want to send the beam off at some angle but leave it otherwise unchanged, a plane mirror is your best bet. By turning the face of the mirror relative to the incident laser light, you can get an outgoing beam at any angle, although it's most common to try for a right angle. Changing the height of a beam with a pair of plane mirrors is simple. First, you tilt a mirror so that it takes a horizontal beam and directs it straight up. Next, where the upward-traveling beam would reach the correct height, place a second mirror to make it horizontal again.

Laser optics scientists usually try to place the beam at some standard height to take advantage of mass-produced optics mounts, like pedestal posts of a standard height or mirror mounts of a standard diameter.

Answer: The orientation of its electric and magnetic fields

We can think of a photon as a traveling electromagnetic wave, carrying an electric field E and a magnetic field B along with it. E and B are at right angles both to each other and to the direction in which the photon is moving, but those requirements leave a lot of freedom. Take a photon traveling horizontally across an optics table. Its electric field could point straight up, or horizontally at a right angle to its path, or at a 45-degree angle between the table and the floor. It could even rotate about the axis of its momentum! Each of these choices describes a possible photon polarization state (the first three "linear" and the last one "circular"); if most of its photons share a polarization, a laser beam is said to be polarized too.

A photon interacts with matter via its electromagnetic fields, so its polarization affects how some physical processes unfold. As a laser scientist, you probably want to control these effects, so you might start with a polarized laser beam and use quarter-wave plates and half-wave plates to get the precise polarization state you want. (Broadly, a half-wave plate rotates a linear polarization state, and a quarter-wave plate switches between linear and circular polarizations.)

Answer: A beamsplitter

There are many reasons you might want to "split" a laser beam. For example, suppose you want to use a camera to measure the beam's shape -- but your lowest laser power setting is more than enough to fry the camera. A pellicle beamsplitter could save the day: it's a thin film that transmits about 92% of the light while reflecting the other 8%. Or you might have been playing with nonlinear optical crystals, resulting in a beam made up of both infrared and green light -- in which case a dichroic mirror, which reflects one wavelength and transmits the other, can separate those frequencies to give you two monochromatic laser beams.

But beamsplitters' most common use may be in interferometers -- like the ones used to look for gravitational waves. Here the basic idea is to begin with laser light from a single source, split it in two, and send the two beams along different paths that are supposed to be the same length. When you bring them together again, any difference in the path lengths -- such as might be caused by gravitational waves! -- will result in interference between the beams.

Answer: Total internal reflection

A typical fiber-optic cable is a long, thin cylinder of glass, coated in "cladding" (often another type of glass). You might expect that a lot of light would escape from such a fiber -- but optical engineers make use of a special property called total internal reflection.

Think of how light passes from one material to another. Some of it is reflected back from the boundary; some of it is refracted, passing across the boundary but bent by some angle that's dictated by the properties of the two materials. When the light is approaching from the denser side of the boundary, there's a "critical angle" above which all light is reflected and none is refracted -- so that it's all contained in that first material. So all you need to do is make sure that your laser beam enters the fiber core at some angle greater than the critical angle, and it will travel the length of the cable with very little loss. (Sadly, in real-world systems, there's always SOME loss.) Scientists have sent laser beams traveling for miles in this way!