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-jitter 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-km-long 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 90° 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 polarization 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 90° 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 Optoelectronic Apparatus synthesizes optical pulses through superposition of phase-locked, Brillouin-amplified modulation sidebands of a laser beam.
This work was done by X. Steve Yao of Caltech for NASA's Jet Propulsion Laboratory.
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
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Refer to NPO-20870.
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Synthesis of Optical Pulses Using Brillouin Amplification
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Overview
The document presents a technical overview of a method for synthesizing optical pulses using Brillouin amplification, developed by X. Steve Yao at NASA's Jet Propulsion Laboratory (JPL). This technique employs radio-frequency (RF) phase modulation of laser beams, specifically utilizing two diode-pumped yttrium aluminum garnet (YAG) lasers operating at approximately 1,319 nm. The method involves the modulation of both the signal and pump beams by the same RF signal, which facilitates the generation of low-jitter optical pulses with controllable shape, duration, and repetition frequency.
The apparatus described includes a 4-km-long single-mode optical fiber, where the modulated signal beam is coupled through a polarizing beam splitter (PBS) and reflected by a 90° Faraday mirror. This setup ensures that the polarization of the reflected signal beam is orthogonal to that of the forward-going signal beam, optimizing the conditions for Brillouin amplification. The pump beam follows a similar path, maintaining the same polarization as the backward-going signal beam, which is crucial for effective amplification.
Brillouin amplification arises from the nonlinear interaction between the pump and signal beams within the optical fiber, leveraging the electrostrictive effect. The technique allows for coherent superposition of the modulation sidebands, which are automatically locked in phase due to Brillouin selective sideband amplification (BSSA). This coherence enables precise control over the gain of each sideband, thus influencing the shape and duration of the resulting optical pulses.
The document highlights several advantages of this method, including built-in optical amplification, simple electronic control, and insensitivity to polarization. Additionally, it converts low-phase noise RF signals into low-jitter optical pulses, making it a promising approach for various applications in optical communications and signal processing.
The work is part of NASA's ongoing research and development efforts, and inquiries regarding commercial use of the technology are directed to JPL's Intellectual Assets Office. The document emphasizes that the information provided does not imply any endorsement by the U.S. Government or JPL and is intended for informational purposes only. Overall, this synthesis of optical pulses using Brillouin amplification represents a significant advancement in the field of photonics.
