An improved type of optoelectronic oscillator is based on Brillouin amplification. An oscillator of this type converts light energy into an electro-optical oscillation, typically at a frequency of several gigahertz. It offers several advantages over other electro-optical and over all-electronic oscillators:
- There is no need for a relatively bulky, expensive, power-consuming electronic microwave amplifier.
- Without the flicker noise associated with an electronic microwave amplifier, it should be possible to obtain lower oscillator phase noise.
- The narrow gain bandwidth of Brillouin amplification provides band-pass filtering, making it unnecessary to use a relatively bulky electrical band-pass filter; this feature contributes potential for miniaturization and integration.
- The frequency of oscillation can be tuned over a broad range by changing the frequency of one or two pump laser(s).
- A phase modulator can be used in place of a more-expensive and higher-loss amplitude modulator. Because no bias is necessary for a phase modulator, the bias drift associated with an amplitude modulator is eliminated.
The device is based on Brillouin selective sideband amplification (BSSA), the basic principles of which were described in the two preceding articles, "Exploiting Brillouin Scattering in Analog Signal Processing" (NPO-20091) and "More Uses for Brillouin Amplification" (NPO-20092). To recapitulate: Brillouin scattering is the scattering of photons by phonons and is the most sensitive nonlinear optical effect in optical fibers. At a laser-signal power level above a threshold specific to a given optical fiber, SBS generates an acoustic grating via the electrostrictive effect, and the grating gives rise to back-scattering of the forward-propagating optical signal.
Suppose that one laser beam, regarded as a pump signal, is coupled into one end of a long optical fiber. Suppose that another, weaker laser beam, regarded as a carrier signal, is modulated with a radio-frequency signal and coupled into the other end of the long optical fiber. Suppose, further, that the pump or the carrier laser is adjusted so that the carrier frequency is lower than the pump frequency by such an amount as to place the lower sideband of the modulated carrier signal at the SBS peak of the pump signal. As a result, the lower sideband joins the back-scattered pump signal and becomes amplified by the nonlinear SBS effect. The amount of amplification diminishes gradually with departure of the lower sideband frequency from the SBS peak. The carrier and upper sideband are sufficiently distant in frequency from the SBS peak that they are not amplified.
The figure illustrates a basic oscillator of the present type, which exploits Brillouin amplification in an optoelectronic feedback loop. Continuous-wave laser 1 is the carrier laser. The carrier laser beam, at frequency ν1, passes through an electro-optical modulator and an optical isolator into one end of a long optical fiber that serves as a delay line and as the amplifying medium. Continuous-wave laser 2 is the pump laser. The pump laser beam, at frequency ν2, is introduced through an optical circulator or optical coupler at the other end of the optical fiber. At the end where the pump beam is introduced, a photodetector detects the modulated carrier beam that has propagated along the fiber to this end. The electrical output of the photodetector is applied to the modulator.
Initially, the signal applied to the modulator consists of noise, and thus the carrier signal is modulated initially by noise. However, the only part of the optical signal spectrum that becomes amplified is the component at the Brillouin-scattering frequency; this component gives rise to a beat signal in the photodetector output. The beat signal then drives the modulator to produce a stronger sideband at the beat frequency below the carrier frequency. Thus, through positive feedback, the beat signal (which constitutes the desired oscillation) becomes stronger until the gain saturates. The frequency of oscillation is given by
fosc = n1- ν2+ νBS,
where νBSis the Stokes frequency shift of Brillouin scattering.
A prototype oscillator was constructed, using an optical fiber 12.8 km long with a Stokes frequency shift of 12.8 GHz at a laser wavelength of 1,320 nm. The beam from a single yttrium aluminum garnet (YAG) laser with an output power of 70 mW was split to generate both the pump and carrier signals at this wavelength (ν1 = ν2in this case). This apparatus was found to generate an electro-optical oscillation with a power of about -11 dBm at the output of the photodetector. Because the Brillouin gain bandwidth was 10 MHz but the mode spacing associated with the 12.8-km fiber length was <10 kHz, mode hopping was observed. Further development effort would be necessary to eliminate mode hopping.
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.techbriefs.com under the Electronic Components& Circuits category, or circle no. 172 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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-20097 , volume and number of this NASA Tech Briefs issue, and the page number.