Michael B. Woods, SLAC National Accelerator Laboratory
In September 2009, a graduate student working at SLAC suffered a laser eye injury while adjusting a polarizing beamsplitter being used with a femtosecond Ti:sapphire laser.1 The laser parameters were 800-nm wavelength, 100-fs pulse width, 1-kHz repetition rate and 100-mW beam power.
The polarizing beamsplitter, P1, was part of an optics configuration used for intensity control (Figure 1). During adjustment of the P1 optic, an unblocked reflected beam – the dashed beam in Figure 1 – hit the laser operator’s eye. Although the standard operating procedure document and the area warning signs specified that laser eyewear protection (LEP) was required for this work, the operator was not wearing any at the time of the accident, resulting in a small blind spot sustained in the peripheral vision region. Fortunately, the damage was relatively minor, but the incident could have resulted in significant vision impairment.
Figure 1. Optics configuration associated with the accident. DBS = dichroic beamsplitter; λ/2 = half- wave plate; o-ray = ordinary ray, and e-ray = extraordinary ray; and P1 and P2 = polarizers. Images courtesy of Michael Woods, SLAC National Accelerator Laboratory.
The P1 polarizer optic has three components: a polarizer cube, which has an escape window for the reflection of one polarization component (the polarizer transmits one of the two linear polarization states and reflects the other); a beam tube, which also has an escape window; and a rotation mount. The reflected beam from P1 was not used in the experiment, and, initially, the beam tube and polarizer were correctly secured in the rotation mount with the tube blocking the reflected beam.
The laser operator wanted good extinction capability for laser intensity control and needed the polarizer optics to be normal to the beam path. The operator observed the backreflection from P1 on a fluorescent card while wearing LEP but thought the position information was not precise enough because of a blooming effect resulting from saturation on the card. The operator removed the LEP to allow viewing of the backreflected beam on a white card. The operator also wanted to align the polarizer axis with a 0° marking on its rotation mount, and loosened the polarizer and associated beam tube in the rotation mount to allow this adjustment. While doing so, the escape windows for the polarizer and beam tube accidentally became aligned, allowing the unblocked reflected beam to come up out of the horizontal plane directly into one eye.
The direct causes of the accident were deficiencies in administrative alignment procedures, failure to wear LEP and deficiencies in engineering controls.
Two engineering controls, or configuration changes, that would have prevented the accident are: 1) using beam tubes with no escape window, and 2) using nonrotating mounts for the polarizing beamsplitters. The beamsplitters would then be mounted so that the reflected beam would be in the horizontal plane only (using λ/2 as needed to rotate the polarization vector of the incident laser beam).
Significant mistakes were made in administrative alignment procedures. Good alignment practices would include the following:
• For aligning an optic normal to the beam, choose a method where LEP is worn, such as centering an iris on the incident beam and using a CCD camera or IR viewer to see the backreflected beam from the optic being aligned.
• Block or disable laser beams when not needed. The accident occurred when the worker was rotating the polarizer in its mount, which did not require the beam. Proper protocol would require blocking the beam before adjusting the optic.
• Do not perform unnecessary optics adjustments near accessible laser beams. Installing the polarizer and adjusting its angle in the rotation mount should have been done in an optics preparation area away from accessible laser beams.
Root causes, corrective actions
The postaccident analysis determined seven root causes:
• Inadequate training: in particular, on-the-job training (OJT).
• Inadequate supervision.
• Inadequate work planning.
• Inadequate adherence to laboratory rules for laser alignment.
• Deceptive hazard of a dimly visible 800-nm beam.
• Out-of-plane beams from a polarizer.
• Inadequate intervention following (prior) laser eyewear safety violations by other operators in the facility. (The injured laser operator had observed more senior laser operators remove required LEP to perform some tasks.)
Actions were taken to address the direct and root causes listed above, which included significant improvements to training and supervision, and which directly addressed hazards associated with dimly visible beams and optics that generate out-of-plane beams.
SLAC had a laser supervisor training course at the time of the accident, although it was relatively new at that time. This course has been subsequently strengthened. It is a two-hour classroom course given by the laser safety officer and emphasizes responsibilities for supervisors.
Supervisor responsibilities include:
• Providing good site-specific OJT. This training requires a good syllabus and must be documented. A template example for an OJT syllabus has been created for this.
• Communicating expectations for safe operations and accountability for actions by laser operators.
• Conducting prejob briefings when appropriate and encouraging laser operators to request or conduct these; e.g., for new, unfamiliar or infrequently performed tasks.
• Ensuring that the laser facility is well managed, performing frequent facility visits and interacting with laser operators in their work.
• Effectively addressing problems as they arise. This requires good understanding of a problem’s cause.
• Being proactive about improving procedures and acquiring new equipment to improve operations and safety.
• Ensuring the availability of good equipment, procedures and laser eyewear; i.e., making it easy to comply with laser safety requirements.
• Modeling a good safety culture.
Figure 2. Schematic for SLAC’s laser alignment practical training class.
Two new training requirements have been added:
• Laser Accidents & Lessons Learned. This 90-minute course, taught by the laser safety officer, reviews the SLAC accident as well as other accidents and near-miss situations and how they can be avoided. SLAC’s eyewear policy and requirements for proper work planning and control also are discussed.
• Laser Alignment Practical. This course2 takes one to three hours to complete. It is given by laser supervisors to a maximum of three students at a time. Standardized practical training is given in core laser safety practices that are not site-specific. The course educates prospective laser operators on safe alignment techniques and common mistakes that are made. It also assists supervisors in determining the student’s skill level and how much supervision will be needed. A schematic and photo of the training course setup are shown in Figures 2 and 3.
Figure 3. SLAC’s laser alignment practical training course setup.
Two specific issues for the SLAC accident are hazards associated with polarizing beamsplitters and dimly visible 800-nm beams.
• Deceptive hazard of a dimly visible 800-nm beam: An 800-nm beam is outside the normal visible range but still dimly visible when viewed on a white card if it has high enough power. Some laser personnel unwisely take advantage of this to perform certain tasks while not wearing LEP. They can become complacent about the associated hazard because the perceived hazard is less than what one observes with a low-power visible alignment laser or laser pointer. The diffuse reflection hazard at 0.5-m viewing distance from a white card is typically 104 to 105 times less than the direct beam hazard. And even if the diffuse reflection hazard is below physiological damage thresholds, unintended stray beams at 0.1–1 percent of the direct beam are very hazardous. If LEP is removed for a task, one often is giving up OD5 (attenuation of 105) or more in protection – a huge amount of safety to give up. If these beams were operating instead at 532 nm in the green, they likely would be intimidating enough that laser operators would want to wear the required LEP.
• Hazards from polarizing beamsplitters: Laser beams should always – to the extent practical – be kept in the horizontal plane below eye level, with any associated stray beams (e.g., partial transmission in a dielectric mirror) also blocked and kept in the horizontal plane. It is sometimes necessary, however, to use optics that generate out-of-plane beams, including periscopes, polarizing beamsplitters and diffraction gratings. Extra caution and special training are needed to use these optics safely, with good awareness of common mistakes made and how to avoid them.
Meet the author
Dr. Michael B. Woods is the laser safety officer at SLAC National Accelerator Laboratory, Menlo Park, Calif.; email: email@example.com. This work is supported by the US Department of Energy under contract number DE-AC02-76SF00515.
1. M. Woods (2011). Lessons learned from a recent laser accident. Intl Laser Safety Conf, SLAC-PUB-14346, http://slac.stanford.edu/pubs/slacpubs/14250/slac-pub-14346.pdf
2. M. Woods and S. Edstrom (2011). Laser safety: a laser alignment practical training course. Intl Laser Safety Conf, SLAC-PUB-14345, http://slac.stanford.edu/pubs/slacpubs/14250/slac-pub-14345.pdf