Fiber Gratings Affect Noise of Semiconductor Lasers

Michael D. Wheeler

PASADENA, Calif. -- Researchers at the California Institute of Technology's department of applied physics have found a way to manipulate the noise intensity of a semiconductor laser using fiber Bragg gratings -- an advance that could lead to breakthroughs in fiber optic technology. Minimizing intensity noise is desirable for clear transmission over fiber optic telecommunications lines.
The first task before the research team, led by applied physics Professor Amnon Yariv, was to demonstrate how fiber Bragg grating affected the relationship between a laser's frequency noise and its intensity noise. Frequency noise refers to the random fluctuations that take place in a laser beam's frequency; intensity noise refers to fluctuations in a laser's power. The researchers hypothesized that a grating served to convert intensity noise to frequency noise -- important for achieving a high signal-to-noise ratio. The grating also could have an adverse effect: converting frequency noise to intensity noise.

Reducing noise
To confirm its hypothesis, the team set up a 1.54-µm distributed- feedback laser from Ortel Corp. with a low-noise constant-current controller, which eliminated extraneous noise from the laser. The output then passed through a fiber pigtail isolator and through a fiber Bragg grating. After traveling through the grating, the beam passed through a high-speed photodiode detector.
Next the output was amplified and fed to an electronic spectrum analyzer that calculated the noise and frequency fluctuations. The group performed the experiment numerous times, changing the spectral features of the fiber Bragg grating and tuning the laser to different frequencies. Each time the researchers conducted a test, the results varied, sometimes dramatically.
At frequencies below 5 GHz the intensity noise increased more than 20 dB. In other instances, the researchers met with more favorable results. When the laser was tuned above 5 GHz and up to 15 GHz, the team witnessed a reduction of 2 dB.
"It's not going to revolutionize anything, but for any given laser it can make it much less noisy," said Matt McAdams, a member of the research team and recent graduate of Caltech. The group predicts that with the proper tuning and an appropriate grating, users will be able to reduce the relative intensity noise by more than 5 dB.