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OFS Fiber and the World's Largest Laser

The football stadium-sized National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in California is one of the most complex experimental facilities in scientific history, and the largest laser ever constructed. Scientists interested in the behavior of materials at high temperatures and pressures will be able to explore new states of matter and generate accurate data at temperatures and pressures that are comparable to what's inside the Sun.

Optical fiber from OFS Specialty Photonics is integral to the precise diagnosis and control of 192 laser beams that deliver 1.8 million joules of ultraviolet laser energy and 500 terawatts of power to compress and heat small capsules of fusion fuel to create thermonuclear ignition. This awesome power begins, however, with an initial 1 billionth of a joule pulse which is amplified 10,000 times, then split into 48 separate laser pulses. Each of these pulses is further amplified 20 billion times, then split 4 ways to create 192 pulses which are each amplified another 15,000 times.

OFS Specialty Photonics worked closely with engineers at the NIF to develop specialty optical fiber for sensor packages in the Injection Laser System, which generates, amplifies, and shapes the laser beams before they enter the main amplifiers. "Fiber is key to the measurement because it preserves the analog features of the waveform while transmitting a sample of the light to the diagnostic’s waveform recorder," says Philip Datte, an NIF Program Manager. The NIF engineers were also able to take advantage of the qualities of the fiber to reduce some hardware expenditures.

Fiber's Role in Diagnostics
Diagnostics are essential to ensuring that the beam is extremely uniform. "The temporal shape of the laser beam is recorded and compared to a model of what you would expect in that region. In addition, we use an energy diagnostic sampling the same light to determine the laser power," says Datte. "Based on these diagnostics and others, we can make any necessary adjustments," he adds.

OFS 400 µm optical fibers not only transmit light to the sensors, they make the process more efficient and significantly less costly thanks to their use as delay coils to multiplex the signals. "When the laser fires and all the diagnostics are sampling the light, if the delay coils were not there the light would show up on all the diagnostics at about the same time," said Datte.

Instead, varying lengths of optical fiber are used to impose optical delays in the range of 50 to 100 nanoseconds spacing so that pulses arrive at the sensor waveform recorder one after the other. With this time spacing the maximum fiber length is approximately 100 meters. "Think of it as a train going by with each boxcar being recorded separately," Datte explains.

Eight to 12 beams can be multiplexed onto a single diode, which reduces the cost considerably compared to one diode and recording channel for each beam. According to Datte, it is the quality of the fiber that makes such multiplexing possible. "A Pre-pulse that is generated by cladding modes because of the unique way light is coupled into the fiber was a problem. When a fiber is long, light can propagate down the cladding and arrive ahead of the core pulse. This pre-pulse can be a problem for sensitive measuring devices," he says.

OFS Exceeds Fiber Requirements
The fiber developed by OFS engineers had to meet a number of criteria, including a large core diameter to match the light collection capability of photodiodes; a graded index multimode fiber to minimize modal and speckle noise; and the right attenuation and dispersion qualities.

According to Adam Hokansson, an OFS engineer, "Our final design utilized a lower NA core than the first version, which was a larger clad, higher NA, polyimide-coated fiber which worked, but was difficult to handle in the delay coils that they needed."

The lower NA of the final iteration helped to reduce the pulse dispersion further. "We also reduced cladding diameter to improve flexibility and bending, but enough to maintain reasonable optical bend loss performance," he adds. The fiber has a mode stripping hard coating to minimize the pre-pulse and make the fiber and fiber terminations more rugged. The fiber has a reduced-diameter ETFE buffer to maintain flexibility and package size, and to provide additional environmental protection.

OFS worked with the NIF engineers on a fiber that had the right attenuation to squelch the pre-pulse.. Initially NIF engineers looked at the fiber only for sampling but were pleasantly surprised that the measured dispersion (<0.4 ps/meter @ 1053nm) was much lower than they expected, making the delay coils possible. Because it was better than what they had specified, they needed less diagnostic equipment and realized a cost saving.

As work continues on the NIF, scientists and engineers are exploring other diagnostic applications for OFS fiber. "We know the fiber and can do a lot with it. Having a good fiber is part of the real essence of the system," says Datte.

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