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.