STANDARD OPERATING PROCEDURE: Safe Use of Femtosecond Laser System

Oregon State University, Department of Chemistry
Standard Operating Procedure (SOP): Safe Use of Femtosecond Laser System (ver. 1.0)
Author: Weimin Liu and Chong Fang (chong.fang@oregonstate.edu)
Safety Web Inception Date: 5/02/2011
Revised: N/A

Chemistry Department Safety Office: Gilbert Hall Room 153
Emergency Medical Services: 911
Campus Student Health Center: 541-737-9355
Poison Control: 9-1-800-222-1222
OSU Environmental Health and Safety: 541-737-2273
Campus Security: 541-737-7000

A. Introduction

Ultrafast pulsed-lasers are one of the most important experimental tools for investigating fast evolving atomic and molecular dynamics in physics, chemistry, and biology. Our understanding of the relaxation of elementary excitations in condensed matter, carrier dynamics in ultrafast semiconductor devices, or the temporal evolution of chemical reactions has been dominantly coming from experiments performed with picosecond (1 ps = 10^–12 s) and femtosecond (1 fs = 10^–15 s) laser pulses. Moreover, the development and availability of high-power ultrashort duration pulses from solid-state laser systems using the chirped-pulse amplification (CPA) concept have opened up entirely new research avenues on light-matter interactions.

The high peak intensities and the broad bandwidth of the laser pulses provided by short-pulse systems raise special, and sometimes counteracting, requirements for optical materials applied for radiation protection. For instance, the nominal optical density of a filter has to be maintained for a wide range of illumination and pulse durations, while out of the absorption bandwidth the filter should be as transparent as possible. Here, an overview of the possible hazards that may be encountered when operating ultrafast pulsed-lasers is presented. A procedure detailing how to switch our specific laser system (Coherent Legend Elite Amplifier, 1 KHz repetition rate, 35 fs pulse duration and 4 W average power) on and off is available upon request in Gilbert 218.

This SOP is intended to inform on safe working practices to follow whenever lasers are used in the research laboratory; however, it is not a substitute for hands-on training by an experienced co-worker. The SOP should be read and understood prior to the commencement of relevant work and used to complement supervised practical familiarization with the various techniques described.

 

 

B. Hazard Identification

Hazards resulting from the use of laser equipment can be grouped into the two following categories. Appropriate PPE that should be worn to manage these various risk factors is described below in Section C.

 

 

B1. Beam hazards presenting optical radiation hazards to the eye and the skin: The laser produces an intense, highly directional beam of light. If directed, reflected, or focused on an object, laser light will be partially absorbed, raising the temperature of the surface and/or the interior of the object, potentially causing an alteration or deformation of the material. These properties, which have been applied to laser surgery and materials processing, can also cause tissue damage. In addition to these obvious thermal effects upon tissue, there can also be photochemical effects when the wavelength of the laser radiation is sufficiently short, i.e., in the ultraviolet (UV) or blue region of the visible spectrum. Today, most high-power lasers are designed to minimize access to laser radiation during normal operation (thick black covers and sealed compartments). Lower-power lasers may emit levels of laser light that are not a hazard. The human body is vulnerable to the output of certain lasers, and under certain circumstances, exposure can result in severe damage to the eye and skin. Research relating to injury thresholds of the eye and skin has been performed in order to understand the biological hazards of laser radiation. It is now widely accepted that the human eye is more vulnerable to injury than human skin. Depending upon the wavelength different parts of the eye can be injured due to a different absorption behavior. Cornea and lens of the eye absorb ultraviolet light with wavelengths <400 nm and infrared light (>1,400 nm). Above critical laser intensity values, burn injuries and cataract can be induced. In the wavelength range of 400 to 1,400 nm, light passes through the cornea and the lens of the eye and is focused onto the retina. A comparatively low radiation power (per area) can lead to a severe damage of the retina due to the fact that the light intensity at the entrance of the pupil can be amplified by a factor of 10^6 resulting in tremendous intensities at the surface of the retina. The damage effect on the retina is more or less thermal. An illumination of the skin with sufficiently high exposure levels results in a burning of the skin.

 

 

B2. Non-beam hazards as device-related or application-related hazards: Device-related hazards are electrical hazards (e.g. by using high-voltage laser power supplies), chemical hazards (e.g. caused by laser dyes and gases), and danger by pump radiation. Application-related hazards can be e.g. fire hazards, emission of harmful substances or particles, and generation of secondary radiation with wavelengths different from the primary optical radiation.

(1) Chemical hazards

• Compressed gases – great care should be taken while handling tanks of compressed gas

• Fumes from the lasing of target material – industrial hygiene considerations should be addressed to determine adequate ventilation of the system

• Laser dyes or solvents – may be toxic or carcinogenic and should be handled appropriately

(2) Electrical hazards

• Power supplies – high voltage precautions should be designed to prevent electrocution

• Voltages greater than 15 kV – may generate X-rays that penetrate many things

(3) Fire hazards

• Electrical components, gases, fumes and dyes – can constitute a fire hazard; use of flammable materials should be avoided, and flame resistant enclosures should be used when applicable

 

 

C. Safety Equipment and Safe Working Practices: Eye Protection and Beyond

C1. General points and safe practices: Laser protective eyewear is specific to the types of laser radiation in the lab. Each laser laboratory must provide laser-specific appropriate eye protection for personnel working with the laser. Windows or acrylic sheets where Class 2, 3, or 4 beams could be transmitted causing hazards in uncontrolled areas shall be covered or otherwise protected during laser operation. The following guidelines are suggested for maximum eye protection.

(a) Whenever possible, identify the beam path, trace all the stray light, confine the beam, and provide non-reflective, nonflammable beam stops (Figure 1), to minimize the risk of accidental exposure or fire. Use fluorescent screens or secondary viewers (Figure 2) to align the laser beam (a white name card can be used for visible light); always avoid direct beam exposure to the eyes. So, never lower your head to make your eyes similar in height with the laser or any of the optics on the table!

 

Figure 1. A non-flammable beam stop.

 

Figure 2. A secondary viewing device (IR).

 

(b) Use the lowest power possible for beam alignment procedures. Use lower class lasers for preliminary alignment procedures, whenever possible. Keep optical benches free of unnecessary reflective items. Papers can be put and attached to the optics mount to cover weak stray lights.

(c) Confine the beam to the optical bench unless necessary for an experiment, e.g., use barriers at side of benches or other enclosures. Do not use room walls to align Class 3b or 4 laser beams.

(d) Use non-reflective tools. Remember that some tools seem to be non–reflective for visible light may be very reflective for non–visible spectrum.

(e) Do not wear reflective jewelry (including watches, rings, etc.) when working with lasers. Metallic jewelry also increases electrocution hazards. Wear protective glasses whenever working with Class 4 lasers with open beams or when reflections can occur.

 

C2. Eye protection: In general, laser safety glasses may be selected on the basis of protection against various reflections—especially diffuse reflections, and providing protection to a level where the natural aversion reflex will prevent eye injuries, unless intra-beam viewing is required. Generally, protective eyewear may be selected to be adequate to protect against stray reflections. Wearing such glasses allows some visibility of the beam, preventing skin burns, making it more likely that persons will wear the eye protection and work. Also, the increased visibility afforded by this level of protection decreases potential for other accidents in the lab, i.e., tripping, etc. Factors to consider in selection of laser protective eyewear include the following:

 

 

• wavelength(s) or spectral region(s) of laser radiation

• optical density at the particular wavelength(s)

• maximum irradiance (W/cm^2) or beam power (W)

• type of the laser system

• power mode, single pulse, multiple pulse, or continuous wave (cw)

• possibilities of reflections, specular and diffuse

• field of view provided by the design

• availability of prescription lenses, or sufficient size of goggle frames to permit wearing of prescription glasses inside of goggles

• comfort

• ventilation ports to prevent fogging

• effect upon color vision

• impact resistance

• ability to perform required tasks while wearing eyewear (some visibility is required)

Finally, since laser protective eyewear is subject to damage and deterioration over time, the lab safety program should include periodic inspection of these protective items.

 

 

C3. Other PPE: A Laboratory coat is optional but strongly recommended. Long sleeve clothes and pants are suggested to avoid skin exposure to laser radiation. Always be attentive when operating the laser system, when feeling tired or unable to concentrate, turn off the laser system and go home.

 

 

D. Laser Safety Signage

Be familiar with the following standard warning signs and be sure to post appropriate signage in the vicinity of laboratories that pose a risk from laser radiation. Outside the room, a red light will be lit to indicate the laser operation. Be careful when entering or exiting the laser room, and try to maintain it as clean and dust-free as possible.

 

 
 

 

 
This chemical safety advisory document was prepared solely for the use of researchers affiliated to Oregon State University. As stated above (Section A), the content is designed to inform on good working practices and it is not intended to replace hands-on practical training in the techniques described. It is the responsibility of the Principal Investigator to see to it that his/her co-workers are properly trained and informed on hazard management, including the possibility of customization of the information herein as appropriate to meet specific needs. Neither Oregon State University, nor any of its employees (including the author), makes any warranty, express of implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, or represents that its use would not infringe privately owned rights. Reference herein to any specific commerical product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by Oregon State University.