Hybrid brillouin/erbium doped fiber amplifier apparatus and method

A method and apparatus to enable the use of low cost high RIN optical sources for microwave photonic links by combining the wide-gain bandwidth and low cost of an EDFA, with the narrow gain bandwidth of a Brillouin amplifier. The hybrid Brillouin/EDFA (&#8220;hybrid amplifier&#8221;) apparatus of the present invention includes at least two couplers, a phase modulator, an Erbium-doped fiber, a laser source to pump the Erbium fiber, an optical circulator, and a length of fiber used for Brillouin amplification. Optical signals from the laser source are split into two optical paths by a polarization maintaining coupler. Optical signals passing through a first path are amplified by the Erbium doped fiber, and the amplified signals are passed through the optical circulator before sending the optical signals into one end of a spool of fiber in order to pump the Brillouin acoustic wave. Optical signals that follow a second path are phase modulated in a phase modulator at the Brillouin frequency and injected into an opposite end of the fiber spool in order to seed the Brillouin amplifier and improve its low frequency stability. The output of the Brillouin section of the hybrid amplifier is directed through the optical circulator, and the output may be used as an optical source for a photonic link.

FIELD OF THE INVENTION

The present invention relates to fiber optic transmission systems. More particularly, it relates to an apparatus and method for improving light amplification and noise figure in fiber optic links by using a high-output-power, low relative intensity noise (RIN) optical sources.

BACKGROUND OF THE INVENTION

Analog photonic links are attractive for remote distribution of optical signals (referred to as optical remoting ) and control of phased array antenna systems due to their low-loss, large intrinsic bandwidth, and immunity to electromagnetic interference. These characteristics of fiber optic links make them desirable for communicating information at RF frequencies including making them as valuable tools for remoting antennas. Photonic links between remote antennas and control stations may be used to remote local oscillator signals or to use RF signals transmitted from an antenna to a remote centralized location. The complexity of the signals received from the remote antennas requires efficiently relaying the information while processing it.

Although analog photonic links may be used as signal processing tools, the complexity of the received RF signals when combined with the dynamic range requirements of antennas imposes strain on the noise figure requirements of photonic links. For example, a typical photonic link composed of a 30 mW DFB laser, a 5 Volt V MZM, and a photodetector, as illustrated in FIG. 1 , may have a RF noise figure of greater than 30 dB with no microwave preamplifier circuitry.

The RF noise figure may deteriorate when laser sources with a high relative intensity noise(RIN) are used in photonic links. Although using high power solid state lasers enable shot-noise-limited photocurrents, and may reduce the noise figure, this approach may be economically prohibitive, thus making high-power shot-noise-limited microwave links uneconomical.

In one approach, Erbium Doped Fiber Amplifiers (EDFAs) are used as in-line amplifiers in fiber transmission systems because of their wide bandwidth and low noise figure. These amplifiers are typically distributed throughout a transmission system in order to maintain high optical power while minimizing signal-to-noise ratio degradation. EDFAs, however, generate amplified spontaneous (ASE) noise, which limits a system's electrical signal-to-noise ratio through spontaneous-to-spontaneous and signal-spontaneous beat noise at a receiver, which may typically be a photodetector. Because of the wide bandwidth of EDFAs, the ASE may extend well past 1 THz. The beat noise resulting from ASE leads to a large RF noise figure, thus limiting their use in microwave photonic links.

Brillouin amplifiers use stimulated Brillouin scattering to produce large gains over a narrow bandwidth. Brillouin scattering is the inelastic interaction of light with acoustic phonons that result in a shift in frequency of the scattered radiation. This effect is similar to Raman scattering, but information is obtained about acoustic branches of the phonon spectrum. It results from statistical density fluctuations due to acoustical vibrations in the scattering medium. These fluctuations travel at the local speed of sound and the frequency of the scattered light is Doppler-Shifted. The relative velocity of the density fluctuation waves depends on the angle of observation, ranging from the speed of light to zero. Therefore the amount of the frequency shift also varies with the angle of observation.

Stimulated Brillouin Scattering (SBS) is a highly efficient mechanism for producing large gains (>30 dB) over a very narrow bandwidth (<30 MHz). Considering an electrorestrictive material where acoustic noise is due to the Bownian motion of its molecules (thermal noise), SBS occurs via interaction of an incident light wave (pump beam), an acoustic wave, and the backscattered (Stokes) lightwave. The backscattered light (Stokes light) propagates in a direction opposite to the pump light/beam to interfere with the pump beam. When two lightwaves of different optical frequencies counter-propagate in the same electrorestrictive material, their superposition yields interference fringes, creating zones of high electrical field intensity and zones of weaker field intensity. Due to electrorestriction, periodical compression zones moving at the speed given by the optical frequency difference are induced in the material. If this speed corresponds to the speed of sound in the material, an acoustic wave is created. Thus, due to the electrorestriction, an acoustic wave is generated that stimulates the Brillouin scattering even more, thereby reinforcing the acoustic wave.

The backscattered light wave (Stokes light) is shifted by the acoustic photon frequency of the optical fiber, or the Brillouin frequency. In optical fibers, a significant portion of the optical power traveling through the fiber is converted into the Stokes wave traveling in the opposite direction to the pump beam. Although fiber based Brillouin amplifiers have a narrow gain bandwidth, the baseband noise in the Stokes light may include strong frequency components to the order of 1 GHz, thus impairing their performance in microwave photonic applications where an amplifier is used for amplifying signal information.

Thus, there is a need to overcome the problems encountered by the prior art by enabling the use of low cost high RIN optical sources for microwave photonic links.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an exemplary method and apparatus in order to enable the use of low-cost high RIN optical sources for microwave photonic links by combining the widegain-bandwidth of an EDFA with the narrow gain-bandwidth of a Brillouin amplifier to produce an inexpensive light source for photonic links. The Brillouin gain is preferably used as a carrier amplifier for the present invention.

The exemplary hybrid Brillouin/EDFA ( hybrid amplifier ) apparatus of the present invention includes at least two couplers, a phase modulator, an Erbium-doped fiber, a laser source to pump the Erbium fiber, an optical circulator, and a length of fiber used for Brillouin amplification. Optical signals from the laser source are split into two optical paths by a polarization maintaining coupler, such as, for example, a 3 dB coupler. Optical signals passing through a first path are amplified by the Erbium doped fiber, and the amplified signals are passed through the optical circulator before passing the optical signals into one end of a spool of optical fiber to pump the Brillouin acoustic wave created in the optical fiber. Optical signals that follow a second path are phase modulated in a phase modulator, such as, for example, a LiN b O 3 phase modulator, at the Brillouin frequency and injected into an opposite end of the optical fiber spool to provide input ( seed ) to the Brillouin amplifier and improve its low frequency stability. The output of the Brillouin section of the hybrid amplifier is directed through the optical circulator, and the output from the circulator may be used as an optical source for a photonic link.

The present invention also provides a method for improving the noise figure, dynamic range, and loss in fiber optic links. In addition, the present invention enables inexpensive, high RIN sources to be used in analog photonic links.

In one embodiment, the present invention comprises a hybrid amplifier with a DFB laser as a pump with a photodetector attached to the output of the hybrid amplifier.

In the drawings, like or similar elements are designated with identical reference numerals throughout the drawings, and the various elements depicted are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a typical photonic link wherein a continuous wave optical signal from a source 120 is received by a Mach Zender Modulator (MZM) 130 . The modulated light is received in an optical fiber 140 , and the output from the optical fiber is detected in a photodetector 150 .

FIG. 2 shows a schematic disclosing the details of an exemplary hybrid Brillouin/Erbium fiber amplifier ( hybrid amplifier ) 200 in accordance with the present invention. Light from an optical source 202 are received in first and second optical paths 206 , 208 , respectively. Light passing through the first path 206 is amplified in an Erbium doped fiber 216 . An Erbium pump 210 provides seed to pump optical signals in the first path, the optical signals being received in the Erbium doped fiber 216 . Light amplified in Erbium doped fiber 216 is then passed through an optical circulator 218 prior to forwarding the light into one end 213 of a spool of fiber 214 to pump the Brillouin acoustic wave created in the fiber spool 214 , the fiber spool being used for Brillouin amplification.

Light following the second path 208 is phase modulated at the Brillouin frequency and injected into the opposite end 215 of the fiber spool 214 . Phase modulated optical signals from second path 206 are used to seed the Brillouin amplifier created in the fiber spool 214 in order to improve the Brillouin amplifier's low frequency stability. The output of the Brillouin section of amplifier 200 is directed through an optical circulator 218 for use as an optical source for a photonic link. The output from circulator 218 is detected by a photodetector 220 .

FIG. 3 shows an exemplary schematic illustrating an experimental configuration of the present invention as shown in FIG. 2 . Here, the Brillouin amplifier/EDFA ( hybrid amplifier ) is generally shown at 300 . Optical signals from a laser source 202 are split by a coupler 302 into first and second optical paths 206 , 208 , respectively. Optical signals in the first path 206 are amplified by an Erbium doped fiber amplifier (EDFA) 304 , and the amplified signals are passed through circulator 218 , and subsequently, into one end of an optical fiber spool 214 to pump the Brillouin acoustic wave created therein. The optical fiber 214 may be, for example, a single mode fiber, such as SFM-28. Light from the second path 208 is passed through a phase modulator 212 where it is modulated at the Brillouin frequency, for example, at 10.8 GHz, and the modulated light is supplied to the other end of fiber 214 as a signal input to the Brillouin amplifier. The combined effect of the two paths ( 206 , 208 ) is that light from EDFA 304 generates an acoustic wave in fiber spool 214 . The acoustic wave creates a scattered light wave (Stokes wave). The scattered light wave amplifies the signals entering from the opposite end 215 of the fiber spool 214 . The amplified light is passed through circulator 218 before the output of the Brillouin section of the amplifier is detected by the photodetector 220 . The output noise of the hybrid amplifier 300 is characterized with the photodetector 220 .

Referring to FIG. 4 , there is shown an exemplary graphical chart comparing the performance of the hybrid amplifier 300 of the present invention to typical EDFA amplified photonic link. Specifically, FIG. 4 shows an exemplary graphical chart comparing the output noise power of the hybrid amplifier and the EDFA amplified links relative to the shot-noise limited Nd:YAG laser link. At photocurrent of 10 mA, the EDFA amplified link was as high as 15 DB above the shot-noise limit. Implementing the hybrid amplifier ( FIG. 3 ) caused the link-noise to decrease to the shot-noise level. The noise reduction may be due to the narrow gain bandwidth, for example, less than 30 MHz, of Brillouin amplification in the optical fiber spool 214 . As a result, a portion of the spontaneous emission from the EDFA 304 was filtered out of the optical beam. The reduced ASE noise for frequencies greater than 500 MHz from the carrier decreases signal-spontaneous beat noise at the photodetector 220 (FIG. 3 ). As graphically illustrated, this is a significant improvement over the noise performance of the EDFA amplified link.

FIG. 5 shows an exemplary graphical chart illustrating the effect of added phase modulator 212 ( FIG. 3 ) versus no modulation signal in the hybrid amplifier 300 in accordance with an example embodiment of the present invention as shows in FIG. 3 . As shown in FIG. 5 , injecting a phase modulated signal beam into the Brillouin amplifier section of fiber spool 214 ( FIG. 3 ) of the hybrid amplifier 300 caused an overall reduction in the noise of the hybrid amplifier 300 for frequencies below 500 MHz. In addition to the noise reduction below 500 MHz, hybrid amplifier 300 also showed a reduction in the burst noise caused by the Brillouin amplifier, while trace to trace variations in the analyzed spectrum were eliminated.

FIG. 6 shows a graphical representation of the effect of RF modulation on the output of an optical link. The output of the hybrid amplifier 300 ( FIG. 3 ) was monitored, with and without phase modulation, using a d.c. coupled photodetector 220 . As illustrated in the graph, the d.c. output appears to be more stable with phase modulation of the Brillouin signal beam. The increased stability in the output indicates that inserting a phase-modulated seed signal into the Brillouin amplifier, created in the optical fiber spool 214 , improves the overall performance of an optical link. The present invention thus provides a new method of achieving shot-noise limited performance from a 0.5 to 0.8 GHz analog photonic link, without having to resort to expensive solid state lasers.

Referring now to FIG. 7 , there is shown another embodiment of the present invention wherein hybrid amplifier 700 is provided with an optical feedback loop 702 . In this approach, the feedback loop 702 is created so that the phase modulated light passing from the optical fiber spool 214 and then through the phase modulator 212 is fed back via feedback loop 702 into the EDFA 304 . This approach increases the output power of the amplifier 700 while maintaining the low-RIN characteristics of the amplifier 700 . FIG. 7 a shows an exemplary embodiment of the present invention wherein hybrid amplifier 700 is provided with an optical feedback loop 702 for feeding back light from optical fiber spool 214 and phase modulator 212 back into the EDFA 304 .

FIG. 8 shows another exemplary embodiment of the present invention. In this embodiment, EDFA 304 ( FIG. 7 ) is removed from amplifier 800 , thus enabling the use of the amplifier 800 as a high-power laser noise filter. The amplifier 800 functions over a broad optical-bandwidth of the optical fiber, such as, for example, a silica fiber provided the laser 202 emits light at a power above the Brillouin threshold. A phase modulator may be included in link 208 located between the 3 dB coupler 302 and the fiber spool 214 .

FIG. 9 shows another exemplary embodiment of the present invention. In this embodiment, a small portion of the Brillouin amplified light (seed) is injected into the opposite end of the fiber spool. The seed light has been wavelength shifted by the Brillouin amplification process and acts to stabilize the amplification process of amplifier 900 . A phase modulator may be included in the feedback loop.

It is believed that the operation and construction of the present invention will be apparent from the foregoing Detailed Description. While the apparatus and method shown and described have been characterized as being preferred, it should be readily understood that various changes, modifications and enhancements could be made therein without departing from the scope of the present invention as set forth in the following claims.

While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent methods and apparatus.