Interlocks

Definition: safety devices for automatically switching off a laser power or interrupting a laser beam

German: Sicherheitsverriegelung

Category: laser devices and laser physics

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

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The interlock of a laser is a mechanism which can contribute to laser safety by automatically turning off the laser or by blocking a laser beam via a beam shutter, e.g. when a protective box or a door is opened. Commercial lasers often have interlock connections on the back side of the laser driver (i.e. the electronic controls). This is suitable for connecting some interlock device which recognizes potentially dangerous situations:

An electrical contact may be installed at a door, such that people entering that door are protected. Similarly, contacts may be attached to laser enclosures, covers and blinds.
A light curtain or an infrared detector may be used to monitor the presence of persons in a designated laser area.
A people counter at some entry can fulfill the same purpose. It counts the persons entering or leaving the dangerous area.

Once and interlock has been triggered, it will usually require a manual reset for resuming operation. For example, it would be inappropriate to block a laser beam only while a door is open, automatically releasing it again when it is closed.

Interlock functions may be built into a laser controller, but there are also separate interlock devices which can be connected to laser controllers.

In any case, an interlock system should be designed to be a fail-safe as possible. For example:

In the case of failure of a beam shutter (which may be monitored with an additional optical sensor, for example), it should take appropriate measures.
If it loses optical power, it should bring systems into a safe state. (Note that dangerous lasers may be powered separately, i.e., power loss of the interlock system does not necessarily guarantee that the laser is out of operation.)

Being highly relevant for laser safety, interlock devices are subject to various laser safety standards. The planning of safety installation, including any interlocks, should be done by personnel with detailed experience in laser safety. Some suppliers of interlock devices also offer laser safety consulting.

As interlocks can be annoying in real life, they are often manipulated, particularly in research labs. Therefore, their usefulness for laser safety should not be overestimated.

Features of Interlocks

Interlocks can have a number of additional features, for example:

Many of them have a key switch, so that they can be operated only by an authorized user, having the key. In other cases, authorization is done by entering a code on a numerical pad or with a swipe card.
Some devices allow authorized users to override the interlock under certain conditions (“defeatable access controls”).
Laser interlocks may be combined with access control systems, which can prevent unauthorized persons to enter certain sensitive areas. It may be desirable to physically prevent unauthorized access instead of blocking laser beams, if the latter could seriously affect operations. However, there should still be a fail-save method of entering the room in emergency situations, without requiring keys or key codes which may not be immediately available in an emergency.
An interlock can have one or several built-in electronic drivers for beam shutters.
Laser warning lights may be connected, which are automatically turned on when the laser beam is active.

Suppliers

The RP Photonics Buyer's Guide contains three suppliers for interlocks. Among them:

Kentek

The ENTRY-GUARD™ laser area access control systems provide fail-safe access controls to ensure safe and authorized access to areas with laser radiation. Our systems provide design flexibility for customization to your laser radiation areas. When the power to the controller is interrupted, critical outputs, such as door magnetic locks and laser interlocks, revert to a safe state: magnetic locks off, laser interlocks open.

ENTRY-GUARD™ products can be integrated with many components of access control systems, such as lasers, laser enclosures, laser shutters, curtain systems, light curtains, entryway doors, warning signs, status lights, audible signals, keypads, card readers, biometric scanners, safety interlocks, proximity sensors, safety mats, crash bar sensors, laser scanners, PLC circuits, switches (emergency stop, remote start, exit request, bypass key), magnetic locks and electronic latches.

Questions and Comments from Users

2023-06-27

Do laser fibers also need interlocking?

The author's answer:

As long as the light stays in a fiber, it should be harmless. However, a fiber may be damaged, even when it is protected within a fiber cable. Therefore, some industrial fiber cables have built-in sensors which allow one to switch off or block the laser source when damage is detected.

2023-09-08

Is there any requirement or recommendations on how fast the laser must be shut down (no light) after an interlock event? We are not using any circuitry controlled by software in the loop. We are using solid state lasers and have relays that remove the power and even short across the diodes. Our response time is less than 10ms.

The author's answer:

There are certainly such rules, that depends on which regulations are applied in your case, and probably on some circumstances – a very special question for which there is not general answer.

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