Mapping noise in optical devices

07 Jul 2008

Device designers of the future could benefit from a near-field optical scanning microscopy method that is mapping optical noise in cascade lasers.

Researchers in France claim to have made the first maps of optical noise in a photonic device. The work could lead to a better understanding of optical noise and help optimize device design (Optics Express 16 9513).

"This work is fundamental," Jean-Marie Moison told optics.org. "We would like to have the lowest noise possible in a device, and ultimately eliminate noise modes altogether through adequate design. But to do this we must identify the noise modes first."

Noise scanner

Optical noise directly affects the efficiency of a variety of optical sources, from high-power pump lasers to lasers for CD readout, and ultra-low-noise sub-quantum emitters. Noise occurs when the optical cavity in a device switches from operating in desired modes (or particular harmonics) to other unwanted noise modes. However, identifying and eliminating these noise modes remains a challenge.

Now, Moison and colleagues at the CNRS Laboratory of Photonics and Nanostructures in Marcoussis have used near-field scanning optical microscopy (NSOM) to make the first maps of noise in cascade lasers.

The technique works by using the tip of an optical fibre to probe the optical field pixel by pixel with sub-wavelength resolution. The tip is mechanically scanned across the output of the laser and scatters light from the source. The average noise components of the scattered light intensity are used to generate maps of the optical signal and noise distribution. The researchers then analyse these maps by comparing them to "classic" noise models.

The method can measure the optical noise in any photonic device as long as the device has an easily accessible output face. It will prove useful for diagnosing noise origins in optical devices that require low noise to function efficiently, and particularly anything that works in the analogue regime.

Moison says that the technique clearly identifies basic instabilities between modes and provides a direct insight into the origins of noise in the device. In future, the noise could eventually be eliminated – something that will help in designing subsequent devices.

The team would now like to extend its technique to analyse optical noise in the high-frequency bands used in all-optical signal processing and optical communications. "For this to be successful, we will have to adapt our method of noise measurement in the NSOM scattered light for each different frequency band," explained Moison.

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