1. An arrangement for reducing the effects of relative intensity noise (RIN) associated with an optical pump source in a Raman fiber amplifier, the arrangement comprising:
a pump source for supplying a first optical pump signal capable of generating gain in an optical information signal propagating along the Raman fiber amplifier;
a compensating, low power laser source for providing a second optical pump signal input to said Raman fiber amplifier; and
a power monitoring arrangement responsive to a portion of the output signal from the pump source for generating therefrom an intensity-modulated bias current input for said compensating, low power laser, said low power laser thus exhibiting an output power controlled by said modulating bias current to stabilize the net gain provided to the optical information signal propagating along said Raman fiber amplifier and minimize the effects of RIN associated with said pump source.
2. An arrangement as defined in claim 1 wherein the compensating, low power laser source comprises a semiconductor laser diode device.
3. An arrangement as defined in claim 1 wherein the arrangement further comprises
a signal tap for removing a portion of the first optical pump signal output from the pump source and applying the tapped-out portion as an input to the power monitoring arrangement;
a first coupler for multiplexing the second optical pump signal output signal from the low power laser source with the first optical pump signal output from said pump source; and
a second coupler for coupling the multiplexed first and second pump signals with the optical information signal propagating along a second fiber, and introducing the coupled signals into a fiber span of the Raman fiber amplifier.
4. An arrangement as defined in claim 3 wherein the first and second couplers comprise wavelength division multiplexers.
5. An arrangement as defined in claim 1 wherein the power monitoring arrangement comprises
a photodetector for recovering a portion of the first optical pump signal output from the pump source and converting said first optical pump signal output into an electrical representation;
an inverting amplifier coupled to the output of the photodetector for generating a phase-matched \u201cdifference\u201d signal output indicative of power fluctuations in the pump source output signal;
a constant current source of a bias current I; and
a summing amplifier, receiving as a first input the difference signal output from the inverting amplifier and as a second input the bias current from the constant current source, providing as an output an intensity-modulated bias current that varies as a function of power fluctuations monitored in the pump source, the bias current applied as an input to the compensating low power laser source.
6. The arrangement as defined in claim 1 wherein the pump source comprises a Raman fiber laser.
7. The arrangement as defined in claim 1 wherein the Raman fiber amplifier comprises a second-order amplifier, with the low power compensating laser source comprising a seed laser source, operating at a wavelength different from the pump source, said seed laser source thus generating as an output a seed compensating signal for reducing RIN transfer between the pump and the optical information signal.
8. The arrangement as defined in claim 7 wherein the pump source operates at a wavelength of approximately 1365 nm and power of 1 W, the seed laser source operates at a wavelength of approximately 1455 nm and the optical input signal operates at a wavelength of approximately 1550 nm.
9. The arrangement as defined in claim 1 wherein the Raman fiber amplifier comprises a higher-order amplifier, with a plurality of low power compensating laser sources used to generate the required Stokes shifts.
10. The arrangement as defined in claim 1 wherein the low power compensating laser source operates at essentially the same wavelength as the pump source, said compensating laser source thus generating as an output a compensation signal for essentially removing the RIN present in the pump signal prior to supplying said pump signal as an input to the fiber in the Raman fiber amplifier.
11. The arrangement as defined in claim 1 wherein the pump source and the low power compensating laser source propagate in the same direction as the optical input signal through the Raman fiber amplifier.
12. The arrangement as defined in claim 1 wherein the pump source and the low power compensating laser source propagate in the opposite direction as the optical input signal.
13. An arrangement for reducing relative intensity noise in an optical pump source for a Raman fiber amplifier, the arrangement comprising
an optical tap for removing a portion of the output signal from the optical pump source to form a tapped-off monitoring signal;
an optical signal splitter responsive to the tapped-off monitoring signal to form a first optical monitoring signal and a second optical monitoring signal;
an optical-to-electrical converter responsive to the first optical monitoring signal to generate an electrical bias current representative of the power fluctuations present in the output signal from the optical pump source;
a variable optical attenuator responsive to both the second optical monitoring signal and the electrical bias current to generate as an output a compensation signal representative of power fluctuations in the optical pump output signal; and
a signal coupler for combining the pump source output signal and the compensation signal to form a stabilized optical pump signal with reduced relative intensity noise.
14. The arrangement as defined in claim 13 wherein the variable optical attenuator comprises a Mach-Zehnder interferometer.
The claims below are in addition to those above.
All refrences to claim(s) which appear below refer to the numbering after this setence.
1. A sound absorber comprising:
at least one elongated gas channel, through which a flowing gas passes in operation of the sound absorber; and
at least one helical fixture mounted in at least one longitudinal section of the at least one gas channel,
wherein the at least one helical fixture defines a helical gas passage through the at least one longitudinal section of the gas channel; and
wherein the at least one helical fixture defines at least one longitudinal Helmholtz resonator within the gas channel which is excited by sound waves propagating in the flowing gas passing through the at least one gas channel.
2. The sound absorber of claim 1, wherein the at least one longitudinal Helmholtz resonator comprises a longitudinal cavity within the at least one gas channel, which is delimited by opposite impedance steps for the sound waves longitudinally propagating in the gas passing through the at least one gas channel.
3. The sound absorber of claim 2, wherein the opposite impedance steps are provided by opposite changes of at least one of the pitch and of the diameter of the at least one helical fixture.
4. The sound absorber of claim 3, wherein the at least one helical fixture and an outer wall enclosing the at least one gas channel in the at least one longitudinal section comprise at least one local radial contraction.
5. The sound absorber of claim 3, wherein the at least one helical fixture and an outer wall enclosing the at least one gas channel in the at least one longitudinal section comprise at least one local radial expansion.
6. The sound absorber of claim 2, wherein the longitudinal cavity is essentially free of any helical fixture.
7. The sound absorber of claim 1, wherein the at least one gas channel has a circular cross-sectional area which is spanned by the at least one helical fixture by means of a double helix.
8. The sound absorber of claim 1, wherein the at least one gas channel comprises a ring-shaped cross-sectional area which is spanned by the at least one helical fixture by means of at least one helix.
9. The sound absorber of claim 1, wherein gas passing through the sound absorber is distributed over the at least one gas channel and at least one further gas channel which is connected in parallel to the at least one gas channel.
10. The sound absorber of claim 9, wherein at least one further longitudinal Helmholtz resonator is defined in the at least one further gas channel by at least one further helical fixture mounted in the at least one further gas channel.
11. The sound absorber of claim 10, wherein that the at least one longitudinal Helmholtz resonator and the at least one further longitudinal Helmholtz resonator are tuned to a same resonant frequency.
12. The sound absorber of claim 10, wherein that the at least one longitudinal Helmholtz resonator is constructed as an acoustic parallel circuit and the at least one further longitudinal Helmholtz resonator is constructed as an acoustic series circuit.
13. The sound absorber of claim 1, wherein the at least one longitudinal Helmholtz resonator at least one further longitudinal Helmholtz are longitudinally connected in series within the at least one gas channel.
14. The sound absorber of claim 13, wherein that the at least one longitudinal Helmholtz resonator and the at least one further longitudinal Helmholtz resonator are tuned to a same resonant frequency.
15. The sound absorber of claim 13, wherein that the at least one longitudinal Helmholtz resonator and the at least one further longitudinal Helmholtz resonator are tuned to different resonant frequencies which are equal to discrete sound frequencies of the sound waves propagating in the gas passing through the at least one gas channel.
16. The sound absorber of claim 1, wherein the at least one helical fixture is actively deformable.
17. The sound absorber of claim 16, wherein the at least one helical fixture comprises at least one functional material.
18. The sound absorber of claim 1, wherein the at least one helical fixture comprises at least one longitudinal end region in which an outer diameter of the at least one helical fixture tapers and gets away from a wall circumferentially enclosing the at least one gas channel.
19. A sound absorber comprising:
at least one elongated gas channel, through which a flowing gas passes in operation of the sound absorber; and
at least two helical fixture mounted at a longitudinal distance in at least one longitudinal section of the at least one gas channel,
wherein the at least two helical fixture define helical gas passages through the at least one longitudinal section of the gas channel;
wherein the at least two helical fixture define between them at least one longitudinal Helmholtz resonator cavity within the gas channel which is excited by sound waves propagating in the gas passing through the at least one gas channel; and
wherein the at least two helical fixtures comprise longitudinal end regions at their far ends in which outer diameters of the at least two helical fixture taper and get away from a wall circumferentially enclosing the at least one gas channel.
20. A system comprising:
at least one of an engine and of a pump; and
a tube including a sound absorber, the sound absorber comprising:
at least one elongated gas channel, through which a gas passes in operation of the sound absorber; and
at least one helical fixture mounted in at least one longitudinal section of the at least one gas channel,
wherein the at least one helical fixture defines a helical gas passage through the at least one longitudinal section of the gas channel; and
wherein the at least one helical fixture defines at least one longitudinal Helmholtz resonator within the gas channel which is excited by sound waves propagating in the