Fiber-optic liquid-level sensors

NASA Tech Briefs, Jul 2002

Low-jitter pulses can be generated with controllable shape, duration, and repetition frequency. NASA's Jet Propulsion Laboratory, Pasadena, California

A technique for Fourier synthesis of optical pulses involves radio-frequency (RF) phase modulation of laser beams, Brillouin selective amplification of modulation sidebands, and, finally, generation of pulses through coherent superposition of (and thus interference among) the sidebands. (Brillouin amplification is a consequence of a nonlinear interaction of the pump and signal beams with an optical fiber via the electrostrictive effect, and has been described in several prior articles in NASA Tech Briefs.) Coherent superposition is possible because the Brillouin selective sideband amplification (BSSA) automatically locks the various sidebands together in phase. The shape and duration of the pulses can be controlled by controlling the gain for each sideband, while the pulse-repetition frequency can be controlled by controlling the frequency of the modulation. Other attractive features of this technique include built-in optical amplification, simple electronic control, insensitivity to polarization, and conversion of a low-phase noise RF signal into low fitter optical pulses.

One apparatus that has been used to demonstrate the technique (see figure) includes two diode-pumped yttrium aluminum garnet (YAG) lasers, denoted the "signal" and "pump" lasers, that operate at wavelengths =1,319 nm. The output beams from both lasers are phase-modulated by the same continuous-wave signal at a suitable RF (e.g., 7.7 GHz) that equals the desired frequency of repetition of optical pulses. The modulated signal beam is coupled, via a polarizing beam splitter (PBS), into a 4-kmlong single-mode optical fiber on a spool. The polarization axis of the signal beam is made to coincide with the transmission polarization axis of the PBS.

At the far end of the long optical fiber, the signal beam is reflected by a 900 Faraday mirror, so that the polarization axis of the reflected signal beam is orthogonal to that of the forward-going signal beam everywhere along the fiber. Consequently, the reflected signal beam is further reflected, by the PBS, toward an optical circulator, from whence it is coupled into a photodetector.

The modulated pump beam is directed via the optical circulator to the PBS. The polarization axis of the pump beam is made parallel to the reflection polarizes tion axis of the PBS, so that the pump beam is also made to travel along the long optical fiber. Like the signal beam, the pump beam is reflected at the far end by the 900 Faraday mirror so that the reflected pump beam is orthogonal to the forward-going pump beam everywhere along the fiber. Finally, the pump beam passes through the PBS toward the signal laser and is suppressed by an optical isolator before it reaches the signal laser. It is important that the forward-going pump beam always has the same polarization as that of the backward-going signal beam; this condition is optimum for Brillouin amplification everywhere along the fiber and it eliminates polarization sensitivity of the Brillouin-amplification process.

The carrier frequency of the pump laser is adjusted so that the frequency of its peak Brillouin gain coincides with the 2 modulation sideband of the signal beam. Because both the signal and pump beams are modulated by the same RF signal, other Brillouin gain peaks generated by the corresponding modulation sidebands of the pump beam are automatically aligned with the corresponding modulation sidebands of the signal beam.

The apparatus includes a simple circuit that prevents relative frequency drift between the signal and the pump lasers. The circuit is based on the fact that when the signal sidebands are optimally amplified, the DC output of the photodetector attains a maximum value. The DC output of the photodetector can be extracted via a bias T and used to control the frequency of the pump laser.

This work was done by X. Steve Yao of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP)free on-line at www.nasatech.com/tsp under the Physical Sciences category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to Intellectual Assets Office JPL Mail Stop 202-233 4800 Oak Grove Drive Pasadena CA 91109 (818) 354-2240 Email: ipgroup@jpl.nasagov

Refer to NPO-20870, volume and number of this NASA Tech Briefs issue, and the page number