Laser Security By Emile SCHWEICHER OMRA RMA Brussels

Laser Security By Emile SCHWEICHER, OMRA, RMA, Brussels emile. schweicher@gmail. com

Laser Security We are not concerned with the dangers for the skin ! We’ll only investigate the dangers for the eye which arise from picture aside : a 1 m. W laser at 6 m is 10 times more dangerous than the Sun ! This danger is described in • Belgian Army Reglement G 901 (recently updated) • STANAG 3606 LAS (STANAG means Standardization NATO Agreement). There exists also Agreement FINABEL H-FIN/SEC/23. 502 -H issued 05 Aug 1983. The danger for the eye is a function of • The emitted power (obvious!) • The wavelength ; this is less obvious but will be demonstrated by next slide describing this danger in various wavebands

The danger is given by the class : the higher the class number the more dangerous the laser. The class shall be indicated by a sticker on the laser. Class 3 and 4 lasers may only be turned ON by means of a removable key. Class 2 exists only for visible lasers. Class 1: single not dangerous class!

Typical laser safety glasses; compulsory in each lab & workshop Very recent (Oct 07) new classification in Europe & in US ANSI Z 136 standard

An Overview of the LED and Laser Classification System in EN 60825 -1 and IEC 60825 -1 In 2001 the standard governing the safety of laser products in Europe (EN) and Internationally (IEC), was substantially revised and the Classification system was overhauled. This resulted in the introduction of three new laser classes (1 M, 2 M and 3 R) and the abolition of Class 3 A. Below is a brief description of each of the current laser classes. The 60825 -1 standards apply equally to lasers and LEDs. In most places we have used the word "laser", but it can be replaced by "LED". Generally speaking LEDs would be in the lower Classes (1, 1 M, 2, 2 M, 3 R), but very exceptionally may be Class 3 B. At the time of writing we are not aware of any Class 4 LEDs*. In 1 M & 2 M , M comes from Magnification. The phrase "eye-safe" is used below. Please note that "eye-safe" is applicable to the whole optical spectrum from 180 nm to 1 mm wavelength, not just in the retinal hazard range of 400 nm to 1400 nm. Outside the retinal hazard range there is potentially a hazard to the cornea. A wavelength outside the retinal hazard range is therefore not automatically eye-safe! Class 1 This class is eye-safe under all operating conditions. Class 1 M This class is safe for viewing directly with the naked eye, but may be hazardous to view with the aid of optical instruments. In general, the use of magnifying glasses increases the hazard from a widelydiverging beam (e. g. , LEDs and bare laser diodes), and binoculars or telescopes increase the hazard from a wide, collimated beam (such as those used in open-beam telecommunications systems). Radiation in classes 1 and 1 M can be visible, invisible or both. Class 2 These are visible lasers. This class is safe for accidental viewing under all operating conditions. However, it may not be safe for a person who deliberately stares into the laser beam for longer than 0. 25 s, by overcoming their natural aversion response to the very bright light.

Class 2 M These are visible lasers. This class is safe for accidental viewing with the naked eye, as long as the natural aversion response is not overcome as with Class 2, but may be hazardous (even for accidental viewing) when viewed with the aid of optical instruments, as with class 1 M. Radiation in classes 2 and 2 M is visible, but can also contain an invisible element, subject to certain conditions. Classes 1 M and 2 M broadly replace the old class 3 A under IEC and EN classification. Prior to the 2001 amendment there were also lasers which were Class 3 B but were eye-safe when viewed without optical instruments. These lasers are Class 1 M or 2 M under the current Classification system. Class 3 R (R from low Risk) Radiation in this class is considered low risk, but potentially hazardous. The class limit for 3 R is 5 x the applicable class limit for Class 1 (for invisible radiation) or class 2 (for visible radiation). Hence CW visible lasers emitting between 1 and 5 m. W are normally Class 3 R. Visible class 3 R is similar to class IIIA in the US regulations Class 3 B Radiation in this class is very likely to be dangerous. For a continuous wave laser the maximum output into the eye must not exceed 500 m. W. The radiation can be a hazard to the eye or skin. However, viewing of the diffuse reflection is safe. Class 4 This is the highest class of laser radiation. Radiation in this class is very dangerous, and viewing of the diffuse reflection may be dangerous. Class 4 laser beams are capable of setting fire to materials onto which they are projected. Any laser product of a given Class may contain 'embedded' lasers which are greater than the Class assigned to the product, but in these cases engineering controls (protective housings and interlocks) ensure that human access to radiation in excess of product Class is not possible. Notable examples of this are CD and DVD players which are Class 1 laser products while containing Class 3 R or Class 3 B lasers and laser printers which are Class 1 laser products but contain Class 4 embedded lasers.

Note: - for a product to be classified correctly, it must be tested at the maximum output accessible under reasonably foreseeable single-fault conditions (e. g. , in the drive circuitry). A non-M class product must pass both Condition 1 and Condition 2 of Table 10 in IEC/EN 60825 -1, and an M-class product (which by definition has failed either Condition 1 or 2) must pass the irradiance condition in the same table. • Generally speaking lasers are point sources while LEDs are extended sources. Extended sources have higher power limits than point sources for a given laser Class. Therefore a visible LED emitting 10 m. W may be Class 2, while a visible laser pointer of the same power would be Class 3 B. NB Laser pointers above Class 2 are banned for sale to the public by trading standards. CW : Continuous Wave - i. e. not pulsed Diffuse reflection : the reflection of radiation from a matt surface such as a wall Extended source : having an apparent source size with angular subtense of greater than 1. 5 mradian Optical instruments : binoculars, telescopes, microscopes, magnifying glasses (but not prescription glasses) Point source: having an apparent source size with angular subtense of less than 1. 5 mradian

More about various laser safety standards

Typical laser protection glasses

The laser subdivision in 4 classes is evidently a function of power and wavelength Analysis of STANAG 3606 1. EMP = MPE = maximum blootstelling = maximum bestralingsdosis = Maximum Permissible Exposure = Exposition Maximum Permise ; is a power density (W/m²) for CW (Continuous Wave) lasers & is an energy density (J/m²) for pulsed lasers. We consider the case of pulsed lasers ; for CW lasers it suffices to replace energy by power. Laser classification scheme

2. DOCN = NOHD = NKO = Nominale Kritische Oogafstand = Distance Oculaire Critique Nominale = Nominal Ocular Hazard Distance. This is the laser safety distance! A laser safety officer shall only be interested by the NOHD. Unfortunately , the NOHD is not given by the STANAG 3606 nor by G 901 ! STANAG & G 901 only give the MPE (or EMP). So we must be able to compute the NOHD out of the MPE ! a and b are the extreme rays of the laser beam ; like all other laser rays they intersect in the center of the waist (waist=cercle de gorge= keelcirkel) a Clearly , the NOHD is the radius of the sphere where the energy density=EMP Consequently the center of the waist behaves as the origin of a spherical wave involving an energy density proportional to 1/R². NKO b

Fig. V. 65 b Following formula gives the NKO=DOCN=NOHD P is the energy in one laser pulse and d is the beam diameter at laser output; Φ = 2θ is the laser divergence. A large value of the NOHD means a dangerous laser. Formula shows that a large NOHD corresponds to a small EMP. Consequently a dangerous laser is characterized by a small value of the EMP ! EMP varies as PRF-1/2 , where PRF is the Pulse Repetition Frequency.

Computation of the NOHD Fig. V. 66

Using a sight (binoculars) multiplies the NOHD by the magnification M

Fig. V. 67 Table V. 4 Excerpt of STANAG 3606 True eye safe is in the deep pit 1. 4µ-1. 57µ ! In table V. 4, the ratio between 1 st and 4 th row of the EMP is 2000 ! The ratio between 4 th & 5 th row is 100 ; thus the ratio between 1 st and 5 th is 200, 000=2. 105! NLO=Non Linear Optics Raman laser is obtained by a Raman effect (NLO) shifting Nd: YAG 1. 064µ wavelength through a cell filled with CH 4 providing so a coherent output radiation at 1. 54µ; same λ directly from Er: glass or Er: YAG lasers. Better: OPO!

LANTIRN TGP (Targeting Pod)

OPO enables to obtain any eye safe wavelength out of the Nd: YAG 1. 064µ wavelength c. OPO-based laser designator of the Belgian F-16 LANTIRN TGP (Targeting Pod) OPO=Optical Parametric Oscillator; OPO is essentially a NLO (Non Linear Optics) crystal; crystal rotation ensures tunability of output wavelength Fig. 42 BIS. Working principle of the laser designator of the Belgian LANTIRN Targeting Pod, radiating "eye safely" 1. 54µ in peace time, and 1. 064µ in war time. The pumping laser diodes radiate at 808 nm, i. e. , in the middle of the YAG absorbing band • Flash lamp pumped Nd: YAG laser features = 0. 7%, PRF 20 Hz • (laser) diode pumped Nd: YAG laser features = 10%, PRF 5 k. Hz, because the laser diode, being much more monochromatic, extends less out of the YAG absorbing band centered at 808 nm • The LANTIRN (low Altitude Navigation & Targeting by IR at Night) TGP comprises also a LWIR Targeting FLIR (with small FOV) and a TV camera.

F-16 with • the LANTIRN TGP under its air intake • the typical seeker of a laser guided bomb • one pylon tank (not stealthy at all) and one tip carriage • blending (fuselage merges into the wings) and Au-coated canopy: two stealthy features.

How the NOHD can be reduced to 0 : use a beam expander ! m²) ) D is the beam diameter at the laser output , D’ is the beam diameter at the distance=NOHD. Use a beam expander with magnification M=D’/D; is the half divergence without beam expander ; is the half divergence with beam expander

The two kinds of beam expanders; les 2 afocaux; de 2 bundelverbreders Galilean Beam Expander (for HEL) LASER Eyepiece LASER Keplerian beam expander Magnification M = fobj/ Ifoc. I. The laser beam diameter is multiplied by M while the laser divergence is divided by M. Eyepiece Keplerian beam expander with pinhole PH as lowpass spatial filter