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Eye-safe Lasers

Definition: lasers emitting in a wavelength region with relatively low hazards for the human eye

More general term: lasers

German: augensichere Laser

Category: laser devices and laser physics

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Cite the article using its DOI: https://doi.org/10.61835/pie

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Lasers with emission wavelengths longer than ≈ 1.4 μm are often called "eye-safe because light in that wavelength range is strongly absorbed in the eye's cornea and lens and therefore cannot reach the significantly more sensitive retina. This makes e.g. erbium lasers and erbium-doped fiber amplifiers used in 1.5-μm telecom systems or 2-μm thulium lasers far less dangerous than e.g. 1-μm lasers with similar output powers. Another class of eye-safe laser sources are optical parametric oscillators.

On the other hand, the absorption length of the cornea reaches tiny values (well below 0.1 mm) at longer wavelengths, particularly around 3 μm and around 10 μm (near the wavelength of CO2 lasers). This means that light pulses at such wavelengths are absorbed in a very thin layer so that the cornea can easily be damaged. Therefore, lasers emitting around 3 μm or 10 μm are less eye-safe than e.g. lasers emitting around 1.5 μm, even though they are “retina-safe”. While the outer surface of the cornea (the epithelium) can at least heal within a couple of days after damage, this is not the case for the inner part (the endothelium). Also, corneal injuries can be very painful.

Obviously, the quality “eye-safe” depends not only on the emission wavelength, but also on the optical power (or energy) level and the optical intensity which can reach the eye. With sufficiently high power, such as is reached with a fiber amplifier or with a Q-switched laser, the eye can still be damaged. However, it can already be very helpful if at least weak parasitic reflections of some main beam are not dangerous for the eyes. Thus, such lasers may at least be relatively eye-safe – comparing with other lasers having the same output power but in some more critical wavelength region.

Note that the laser power alone (or the intensity at the laser output) is not sufficient to assess the possible intensity in the eye; that also depends on other factors, such as the beam divergence and beam quality. Therefore, one cannot simply state a power or intensity limit for eye safety at a given wavelength.

Types of Eye-safe Lasers

Here are some examples for typical types of eye-safe lasers:

  • Erbium lasers emitting in the 1.5-μm spectral region can be realized either as bulk lasers or as fiber lasers. With bulk lasers, e.g. based on an Er:glass piece, one generate pulses with higher pulse energy for long-distance range-finding and other LIDAR applications, for example.
  • Thulium lasers often emit at wavelengths somewhat below 2 μm and can also be either bulk lasers or fiber lasers.
  • Many semiconductor lasers also emit in eye-safe spectral regions.

Applications of Eye-safe Lasers

Eye-safe lasers are particularly important in application cases where light needs to be transmitted over substantial distances in open air. Typical examples are laser rangefinders, e.g. in the context of autonomously driving cars (see also the article on LIDAR), wind sensors for wind turbines and free-space optical communications.

Even in optical fiber communications, eye-safe wavelengths are beneficial, as a fiber cable may be broken such that light can exit the fiber.

There are also eye-safe lasers in military applications, for medical procedures and in various other fields.

Suppliers

The RP Photonics Buyer's Guide contains 33 suppliers for eye-safe lasers. Among them:

See also: laser safety, laser rangefinders, free-space optical communications

Questions and Comments from Users

2021-09-21

How about the safety of quantum cascade lasers used in microscopes?

The author's answer:

Quantum cascade lasers are operating in the long-wavelength region where the radiation is absorbed already in the cornea. It could be harmful if it were intense enough to substantially heat the cornea. That should usually not be the case in the mentioned application.

2022-08-05

How can infrared lasers as used by optometrists for optical coherence tomography be classified as perfectly eye-safe?

The author's answer:

They can be perfectly safe provided that the resulting optical intensity on the retina is small enough and/or the exposure time is sufficiently limited. Laser light is not necessarily dangerous if it is in the infrared.

2023-07-25

Manufacturers often advertise their lasers at 1550 nm as “eye-safe” independently of the maximum power. It is hard to find numbers to rely on to know what is the limit for a laser to remain eye-safe at this wavelength. Do you have an order of magnitude in term of maximum power that would still be considered eye-safe, or a number that relates to what is considered dangerous for the human eye?

The author's answer:

The risk for the eye depends on various factors, including e.g. spatial properties of the beam. See the article on laser safety for more details.

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