- What a laser is and how it works
- How lasers are used to underpin astronomical research
- Some of the technological laser developments done at ESO
The first question any self-respecting laser physicist is asked at a party is ‘Wait, you work on lasers? Like in Star Wars?’ Despite this, lasers remain fairly enigmatic, with the majority of people not knowing what they are, how they work, or what they’re used for (maybe with the exception of sawing secret agents in half). But don't worry, we've got you covered with this blog post!
Lasers were first demonstrated in 1960 by Theodore Maiman, an American engineer and physicist. Since then, they have gone on to revolutionise the world: guiding navigation systems, performing precision surgery, and delivering the internet globally via the medium of vast, underwater fibre-optic cables. They also underpin much of the high-quality astronomy undertaken by astronomers. But how does a laser work? And how can astronomers make use of them to better see the stars?
What’s in a laser?
The word laser is actually an acronym for Light Amplification by the Stimulated Emission of Radiation — a description of the process which generates the laser beam. Though the components and specifics may vary, every laser follows the same basic principles, and relies on the same three things:
- The laser medium: a material which produces the laser light. It can be made from a number of materials such as a dye, crystal, or gas.
- The energy provider or ‘pump source’: gives energy to the laser material, enabling the laser to operate. This is often a strong electrical current, bright flash of light from a lamp, or a secondary laser.
- Reflectors: they trap the laser light in the laser medium, causing an amplification effect. The reflector depends on the type of laser, but can be as simple as a pair of mirrors.
So, how does a laser work?
Electrons in the laser medium absorb energy from the pump source, becoming excited. These electrons are ready and willing to release this energy, and they do so by emitting particles of light, or photons.
There are two ways in which an excited electron can release a photon. The first is known as spontaneous emission, where the electron randomly emits its excess energy in any direction, similar to a light bulb in the centre of a room.
An excited electron can also be coaxed into releasing its extra energy using a second photon, provided by the pump source. This is known as stimulated emission, because a photon is required to stimulate the process. In stimulated emission, the photon which coaxes the electron and the emitted photon travel onwards in the same direction, in sync with one another.
By surrounding the laser medium with reflectors such as mirrors, both the coaxing photons and the released photons become trapped, causing a cascading effect in which each photon goes on to cause more stimulated emission. In this way, a large number of photons are generated, all travelling in the same direction. Traditionally, one of the mirrors surrounding the laser medium is also designed to be partially transparent. This allows some of the generated photons to escape, forming the laser beam.
The different types of lasers that exist are almost limitless. They can be the size of a room, or fit onto a microchip. Some lasers emit continuously, while others emit in short bursts or ‘pulses’ of light. Their applications are almost as varied, including being integral to modern ground-based astronomy.
Adaptive optics: seeing the stars more clearly
Stars twinkle due to distortions caused by the Earth’s atmosphere. While this twinkling may be beautiful, it blurs the finer details of celestial objects, posing a problem for astronomers. One of the most important uses for lasers in astronomy is to reduce this distortion on images taken with ground-based telescopes. To do this, astronomers employ a technique known as adaptive optics.
In adaptive optics, a deformable mirror reshapes itself in real time to counteract atmospheric turbulence. In order to measure this turbulence one needs to use the light of either the scientific target itself — if it’s bright and compact enough — or of a bright nearby star to provide feedback to the mirror. If such a reference star isn’t available, lasers are used to create an artificial star in the Earth’s upper atmosphere, in the direction where the telescope is pointing. The most commonly used lasers are orange-yellow; they operate at a very specific wavelength of 589.1 nm and excite sodium atoms located about 90 km above the ground. These atoms absorb and re-emit the light, forming an artificial star or ‘laser guide star’ (LGS). By looking at LGSs, astronomers can measure and compensate for the blurring effects of the Earth’s atmosphere in real time. Adaptive optics corrections happen up to 1000 times per second, allowing astronomers to take crystal clear images of the cosmos.
One such LGS system is used on ESO’s Very Large Telescope (VLT), specifically on its cutting-edge Unit Telescope 4 (UT4) or Yepun. The 4 Laser Guide Star Facility (4LGSF) was inaugurated in 2016, and relies on four lasers, each with a power of 22 W. For comparison, this is about 4000 times the maximum allowed power of a standard laser pointer. The beams are 30 cm in diameter, about the size of a dinner plate. The system is equipped with cameras that monitor the sky close to the lasers, and the lasers are automatically switched off when planes are detected incoming in the telescope field of view.
The 4LGSF replaced the previous system which used a single, less powerful laser. Using several lasers allows to better characterise the turbulence at different heights, and to correct it over a larger field of view.
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  The planetary nebula NGC 6563 observed with the MUSE instrument at the VLT, with adaptive optics provided by the 4LGSF. Use the interactive slider to compare the images with and without adaptive optics.
Credit: ESO/P. Weilbacher
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Lasers developed as a result of ESO’s internal research and development have gone on to be patented and produced by industry partners such as MPB Communications and TOPTICA. As part of that partnership, and in collaboration with the European Space Agency, the power of the guide star lasers has recently been increased to 63 W , triple the previous power.
Engineers have also implemented a ‘frequency chirping’ system in their guide star lasers to improve the efficiency of the process. Frequency chirping involves rapidly changing the colour of the laser, which allows the laser to adapt to the tiny changes in motion of the sodium atoms in the atmosphere: a truly staggering work of precision. Both raising the laser power and frequency chirping improve the efficiency of the process, allowing the sodium guide stars to shine more brightly.
Frequency Combs: hunting for planets and examining the building blocks of nature
To answer some of the most mysterious questions in astronomy, be it searching for Earth-sized exoplanets or probing the expansion of the Universe, astronomers need to measure the wavelength of their targets with unprecedented precision. To achieve this precision, astronomers use laser technology to generate some of the most precise rulers in the world, known as frequency combs.
A frequency comb is a laser source with a peculiar spectrum, composed of narrow, evenly spaced lines, each of which corresponds to a specific wavelength. These spectral lines can be used as references against which light from astronomical sources can be compared, allowing astronomers to precisely determine what colours they are seeing. The accuracy that scientists can obtain using frequency combs is unparalleled –– no other instrument can achieve the same level.
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The HARPS laser frequency comb.
Credit: Dinko Milaković
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The laser frequency comb’s developer was awarded the 2005 Nobel Prize in physics , for its contributions to both atomic physics and the development of ultra-accurate clocks. Since then, ESO scientists and engineers, in collaboration with the Max Planck Institute for Quantum Optics, used this breakthrough to develop a new type of calibration device for astronomical measurements .
To generate a frequency comb, scientists use a laser with ultrashort pulses, only a few femtoseconds long. To put that into perspective, a femtosecond is to a second what 7 minutes is to the lifetime of the Universe. The length of time between the pulses of light governs the spacing of the lines in the frequency comb. To ensure that there is no drift in timing of the pulses, frequency combs are synchronised to ultraprecise atomic clocks, the same timing mechanism which underpins national timing standards and high-frequency trading.
ESO uses frequency comb technology on its most precise planet hunters: the HARPS instrument at the 3.6-metre telescope at La Silla Observatory, and the ESPRESSO instrument on the VLT at the Paranal Observatory. Together, HARPS and ESPRESSO have enabled discoveries such as the detection of a new exoplanet around our nearest stellar neighbour, Proxima Centauri.
Continuing to develop its frequency comb technology, ESO will implement one of these fine-toothed optical combs on the Extremely Large Telescope (ELT)’s ANDES instrument. As well as searching for exoplanets, ANDES will also probe the fundamental constants of nature, numerical values which underpin all the phenomena we experience on a day to day basis. In particular, ANDES’s extremely high precision will allow astronomers to look for variations in the fine structure constant, which determines the strength of interaction between light and matter. ANDES will also be able to directly measure the acceleration of the expansion of the Universe over the years, allowing astronomers to test cosmological models.
Laser machining: developing the hardware for studying the Universe
Have you ever wondered how the glass in your phone screen is cut to shape? You may be surprised to find that the answer is using lasers. Laser pulses can be energetic enough to cut through matter. As such, they can be used as part of the manufacturing process. In astronomy, lasers are particularly useful for cutting slits in spectroscopic masks.
When light from astronomical objects passes through a slit and is then dispersed through a special optical element, like a prism (see the Pink Floyd Dark Side of the Moon album cover), it produces a spectrum. This spectrum may contain dark and bright lines at specific wavelengths. These lines tell astronomers not only which chemical elements are present in astronomical objects, but also their temperature and density, and how fast the material is moving. In a sense, a spectrum is rather like the barcode of a particular cosmic object.
Tens or even hundreds of slits can be cut into an individual mask and each slit can be as narrow as a human hair. These incisions must be made in precise locations, so that light from the selected stars or galaxies can go through them. Lasers achieve this through a process called ablation, in which a short pulse of laser light is used to literally evaporate material away from a surface. In doing this, material can be removed without melting or boiling. This means that less residue is left behind, and sharper cuts can be made.
At ESO, a laser machining unit cuts the slits in the masks used for the FORS2 spectrograph on the VLT, as well as the now retired VIMOS spectrograph. To do this, the laser head is mounted statically, and a sheet of black invar is moved according to a computer programme. Invar is an alloy of nickel and iron which doesn’t expand or contract if the temperature changes; this way the position of the slits will remain the same. Instruments can be loaded with different masks, and astronomers at the VLT select the appropriate masks for the measurements they want to make on each observing shift.
In summary…
It’s hard to summarise in a single article all the ways in which lasers contribute to astronomy. For instance, lasers are also key for aligning the optical components of telescopes and instruments. Lasers are some of astronomy’s unsung heroes, underpinning much of the research that happens at ESO’s telescopes, from manufacturing all the way through to taking astronomical images. As laser technology develops, astronomers are finding new ways of implementing lasers into telescope technology, ever bettering our ability to illuminate the cosmos.
Biography Anita Chandran
Anita Chandran is a science journalism intern at ESO. She recently completed her PhD in laser physics at Imperial College London, where she also worked with the science communication department on the ethics of artificial intelligence. She is a writer and editor, having co-founding Tamarind, a literary magazine focusing on the intersections between the arts and sciences.