Stimulated brillouin scattering optical amplifier

The stimulated Brillouin scattering optical amplifier includes a first control optics assembly, a driver element, a second control optics assembly, a Brillouin active medium, and egressing optics. The first control optics assembly receives an incoming laser beam and adjusts that incoming laser beam in accordance with first desired wavelength, polarization and beam propagation parameters. A driver element produces a driver laser beam. A second control optics assembly receives the driver laser beam and adjusts that driver laser beam in accordance with second desired wavelength, polarization and beam propagation parameters. A Brillouin active medium receives an output from the first control optics assembly and an output from the second control optics assembly. The Brillouin active medium provides a non-linear optical interaction between the outputs such that the incoming laser beam is amplified producing an amplified Brillouin active medium output laser beam and a depleted driver laser beam. Egressing optics receives the amplified Brillouin active medium output laser beam and the depleted driver laser beam. The egressing optics controllably transmits the amplified Brillouin active medium output laser beam in accordance with third desired wavelength, polarization, and beam propagation parameters and prevents transmission of the depleted driver laser beam. The output of the egressing optics includes an amplified egressing optics output laser beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical amplifiers and more particularly to an optical amplifier that uses stimulated Brillouin scattering for providing optical amplification.

2. Description of the Related Art

To this point, optical signal amplifiers have been dominated by Raman amplifiers. For example, U.S. Pat. No. 3,414,354, issued to E. H. Siegler Jr., entitled Raman Spectrometers, is a seminal publication disclosing the use of stimulated Raman scattering, to provide optical amplification. In a later example, U.S. Pat. No. 3,515,897, issued to W. H. Culver, discloses a design for implementing stimulated Raman scattering as an amplifier.

Use of Raman scattering; however, has limitations in its operation and implementation. Examination of the equations that govern stimulated Raman scattering break down into two terms. The first term is associated with the wave that is being amplified, also known as the Stokes wave. The second term is associated with a material excitation that is a product of the Raman scattering. That material excitation causes inherent inefficiencies and engineering difficulties that cannot be removed. Consequently, stimulated Raman scattering can be considered as a parametric or coupled generation process in which the optical pump wave generates a Stokes wave (i.e. the amplified input) and a material excitation wave. This material excitation wave is part of the coupled wave physical process which allows the input beam to be amplified, but does not contribute anything to the desired amplification. The energy that is distributed to the material excitation is lost to the optical output. Furthermore, this material wave eventually couples its energy into thermal excitations within the media, so that it contributes to waste heat in the process. This heat can lead to immediate distortions in the efficiency of the optical amplifier and long-term deterioration of the amplifier medium itself. Considerable engineering must take place in the optical design to handle this problem, causing the system to be bulkier and heavier than it might be otherwise.

The inherent difficulty with Raman scattering is that the material excitation itself is a high energy excitation. In order to use a Raman active medium for amplification, the optical implementation is constrained to excite the material parameter inherent to said medium. These excitations are associated with vibrational resonances in the infrared segment of the electromagnetic spectrum. The associated wavelengths of these excitations will be in the 3 to 10 micron regime. A typical amplifier beam will be in the mid-visible, at a wavelength of approximately 0.5 micron. Consequently, 10% of the pump beam will be lost to the material excitation, even if the optical system is lossless otherwise. For high power long term operation, this is a considerable loss.

SUMMARY

In a broad aspect, the stimulated Brillouin scattering optical amplifier of the present invention includes a first control optics assembly, a driver element, a second control optics assembly, a Brillouin active medium, and egressing optics. The first control optics assembly receives an incoming laser beam and adjusts that incoming laser beam in accordance with first desired wavelength, polarization and beam propagation parameters. A driver element produces a driver laser beam. A second control optics assembly receives the driver laser beam and adjusts that driver laser beam in accordance with second desired wavelength, polarization and beam propagation parameters. A Brillouin active medium receives an output from the first control optics assembly and an output from the second control optics assembly. The Brillouin active medium provides a non-linear optical interaction between the outputs such that the incoming laser beam is amplified producing an amplified Brillouin active medium output laser beam and a depleted driver laser beam. Egressing optics receives the amplified Brillouin active medium output laser beam and the depleted driver laser beam. The egressing optics controllably transmits the amplified Brillouin active medium output laser beam in accordance with third desired wavelength, polarization, and beam propagation parameters and prevents transmission of the depleted driver laser beam. The output of the egressing optics includes an amplified egressing optics output laser beam.

The use of Brillouin scattering allows parametric amplification of a weak signal with considerably less energy loss to the excitation-coupling medium than Raman scattering. This makes the overall system operation more energy efficient. It reduces the amount of engineering and design necessary to remove the large amount of waste heat associated with the Raman process. As a result, the hardware associated with the use of this amplifier in an optical system, such as an optical communication system, minimizes volume and weight. Furthermore, it provides enhanced energy efficiency.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings and the characters of reference marked thereon,FIG. 1illustrates a preferred embodiment of the present invention, designated generally as10. An incoming laser beam12is received by a first control optics assembly, designated generally as14. The laser beam, λ1, may be generally described as an electromagnetic or light beam with a single narrow wavelength in the optical regime (0.1-10 microns), which is propagating in a uniform well-defined direction, made possible by its coherence properties. The laser beam could represent an image or could be a digitally encoded optical beam for data transmission.

The first control optics assembly14adjusts the incoming laser beam in accordance with first desired wavelength, polarization and beam propagation parameters. These parameters can include, for example, precise wavelength filtering to the expected signal wavelength, the optical bandwidth of the incoming signal, or the polarization of the light. The wavelength may be controlled to fit within the transparency range of the ensuing steerer. It may be more precisely filtered to fit a known input signal, either from an image or from a digitally encoded communication beam.

The assembly14preferably includes a wavelength control element16such as a color filter, an etalon, a Fabry-Perot interferometer, a Fizeau interferometer, a diffraction grating, or a notch filter, etc. A polarization control element18polarizes the wavefront. This may comprise, for example, a polarization plate, a Brewster's angle polarizer, or a thin film polarizer. The precise polarizer to be selected depends on the particular application's engineering requirements such as polarization rejection ratio, size and weight of the polarizer, and the wavelength range over which the steerer must operate, etc. The wavefront is then received by a propagation control element20such as a single lens, double lens, refractive elements or other system up to a fully engineered telescope.

A driver element22for encoding produces a driver laser beam24. The driver element22may comprise, for example, a single frequency laser, with sufficiently high intensity to affect a nonlinear optical interaction with the incoming beam described previously. This could be a solid state laser, a high power diode laser or any number of high intensity lasers.

A second control optics assembly26adjusts the driver laser beam24in accordance with desired wavelength, polarization and beam propagation parameters. The assembly26preferably includes wavelength control element30such as a color filter, an etalon, a Fabry-Perot interferometer, a Fizeau interferometer, a diffraction grating or a notch filter. A polarization control element32and a propagation control element34are utilized, as described above.

A Brillouin active medium36receives an output38from the first control optics assembly14and an output40from the second control optics assembly26. The Brillouin active medium36provides a non-linear optical interaction between the outputs38,40such that an amplified Brillouin active medium output laser beam42and a depleted driver laser beam43are provided. Using stimulated Brillouin scattering as a means of amplification could provide a lower cost solution than other stimulated optical scattering techniques which do not have so much energy coupled into the material excitation. In this process, as in all stimulated processes, there is a material excitation present as the physical entity that couples the pump and signal waves. However, in this case, the excitation is an acoustic wave. A typical frequency shift associated with a Brillouin event is on the order of a few GHz, 106times smaller than that associated with a Raman excitation. Consequently, the energy loss to the material excitation and subsequently to the medium, are negligible as compared to the Raman case.

The gain coefficient for the stimulated Brillouin process is given in Chapter 11 of Principles of Nonlinear Optics by Y. R. Shen. Quoted here, the gain coefficient, GB, is given byGB=4⁢πω22k2⁢c2⁢Im⁢⁢χB(3)⁢E12,
where ω2is the pump laser frequency, k2is the pump wave vector, c is the speed of light, χ8(3)is the third order Brillouin susceptibility, and |Ε1| is the absolute magnitude of the pump laser.

The driver output40enters the Brillouin active medium36along with the weak beam38whose intensity is to be amplified. Via a coupled wave χ(3)process, energy is transferred from the pump or driver beam43to the weak beam38. The material excitation present as the physical entity that couples the pump and signal waves is an acoustic wave. A typical frequency shift associated with a Brillouin event is on the order of a few GHz. The physical process that leads to the growth of the acoustic wave also leads to the growth of the weak beam38, as the wave processes are coupled.

Examples of Brillouin active media include carbon disulphide (CS2), toluene, doped optical fiber, acetone, n-hexane, toluene, CCl4, methanol, benzene, H2O, and cyclohexane. Such materials allow good efficiency in the nonlinear optical interaction.

The Brillouin active medium may be a stimulated thermal Brillouin material comprising a multi-component material. An acoustic wave, which can be described as a time dependent density variation in the material in question, is more general than that described in the above paragraphs. In the previous case, the light scattering is assumed to contain low-frequency thermodynamic fluctuations in a single-component medium. The density fluctuation is actually a function of pressure, p, and entropy, S. Then, the material excitation will include pressure, wave and entropy wave components. Rather than using the thermodynamic variables p and S, one can use the independent quantities, ρ and T. Including absorption of the light energy (which is neglected in normal stimulated Brillouin scattering), heating of the sample can occur. Light scattering under a condition of heating caused by material absorption is called thermal Brillouin scattering. The gain coefficient for thermal Brillouin scattering is given byGTB=ω2⁢γαβT8⁢π⁢⁢CP⁢ρ0⁢ΓB,
where ω2is the frequency of the amplified signal, γ is the electrostrictive coefficient for the thermal Brillouin case, α is the linear absorption coefficient of the Brillouin medium, βTis the isothermal compressibility, Cpis the heat capacity at constant volume, ρ0is the medium density, and ΓBis the Brillouin damping coefficient. Materials which exhibit thermal Brillouin scattering are typically multi component systems, composed primarily of a Brillouin active medium, combined with a small amount of material which is absorptive at the material excitation resonance. For example, CS2or CCl4doped with I2will produce a medium which exhibits thermal Brillouin scattering. This multi-component material includes a Brillouin active material that exhibits thermal Brillouin scattering, combined with a small amount of material that is absorptive at the material excitation resonance. Such a material may be, for example, CS2or CCl4doped with I2.

Egressing optics44receives the output42of the Brillouin active medium36and adjusts that laser beam in accordance with desired wavelength, polarization, and beam propagation parameters. The output of the egressing optics has the laser beam propagation direction shifted relative to the incoming laser beam direction. Egressing optics44includes an egressing wavelength control element46, an egressing propagation control element48and an egressing polarization control element50. These components may be discussed above with respect to assemblies14and26.

Referring now toFIG. 2, integration of the stimulated Brillouin scattering optical amplifier of the present invention is illustrated into an optical communication system, designated generally as52. The communication system52includes an optical receiver54that receives a relatively weak signal56entering via, for example, a fiber or free space. The receiver54may be, for example, a telescope or commercially available fiber terminator for collecting a free space propagated signal or fiber optically propagated signal, respectively. The optics associated with the receiver will be a combination of refractive or reflective elements which couple the weak input into the amplifier stage. The optical amplifier10receives the output from the receiver54and provides an output to an optical transmitter58. The optical transmitter58may typically be a telescope, if free space, or fiber launcher for fiber optic based propagation. The optics associated with the transmitter is a suitable a combination of refractive or reflective elements which couple the amplified signal from the amplifier stage.

The optical communication system may be used for a number of applications. For example, it may be an optical repeater for a telecommunication system, a long distance internet communication system or short haul distribution system for connecting to individual users.

Thus, while the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.