Noiseless optical amplifier could boost biomedical imaging

Promising for biomedical imaging, researchers at NIST have demonstrated that they can amplify weak light signals without adding noise, while also carrying more pixels than other low-noise amplifiers.

Most optical amplifiers make the image brighter, but add distortion. Recognizing this, researchers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have demonstrated that they can amplify weak light signals without adding noise, while also carrying more pixels than other low-noise amplifiers. The new development could improve biomedical imaging as well as other optical applications.

Researchers have developed other light amplifiers using nonlinear crystals and optical fibers that don't add noise, but they're limited when it comes to amplifying images. Crystals need high laser intensities, which can distort the image. Amplifying light with fibers works well, but the fibers have to be long and the beam is confined to a small area, which constrains the complexity of the image to single pixels.

Overcoming these limitations, the NIST researchers developed a four-wave mixing technique that amplifies images by intersecting the light from two pump lasers and a probe laser carrying the image at precise angles inside a gas of hot rubidium atoms. After passing through a stencil in the shape of the image they want to amplify, the probe laser, whose frequency is halfway between those of the pump lasers, bisects the angle made by the pump lasers. The combination of the lasers' color, their angle of intersection, and their interaction with the rubidium gas creates the conditions for noiseless amplification of complex images with potentially thousands of pixels.

But this kind of amplifier is phase-sensitive; for the amplification to be noiseless, the pump and signal beams going into the amplifier have to remain stable with respect to each other to within a small fraction of a wavelength so that the beams interfere and add up properly. Such a condition on the beams makes it harder to keep them aligned and stable than for the more common "phase-insensitive" amplifiers.

According to NIST physicist Paul Lett, the technique can amplify images by a factor of up to 4.6 times the original signal strength. "The light we use is infrared, which is good for biological and astronomical imaging," says Lett. "Now we just need to show that our technique amplifies the image faithfully, pixel by pixel, so that we can be assured that it is fully practicable."

The work has been published in Physical Review Letters; for more information, please visit http://prl.aps.org/abstract/PRL/v109/i4/e043602.

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