L band amplifier with distributed filtering

• US 6,459,526 B1
• Filed: 08/09/1999
• Issued: 10/01/2002
• Est. Priority Date: 08/09/1999
• Status: Expired due to Term

• Current Assignee: II-VI Delaware, Inc. (Coherent Corp.)
• Original Assignee: Corning Incorporated
• Inventors: Yadlowsky, Michael J., Minelly, John D.
• Primary Examiner(s): Tarcza, Thomas H.
• Assistant Examiner(s): Hughes, Deandra M.
• Application Number:
  US09/370,735
• Publication Number:
  US 20020105720A1
• Time in Patent Office: 1,149 Days
• Field of Search: 359/134, 359/194, 359/290, 359/337, 359/339, 359/341, 359/342, 359/885, 385/126, 372/4
• US Class Current: 359/337.1
• CPC Class Codes: H01S 2301/02 ASE (amplified spontaneous emission), noise; Reduction thereof
• CPC Class Codes: H01S 3/06754 Fibre amplifiers H01S3/06708 takes precedence
• CPC Class Codes: H01S 3/10023 by functional association of additional optical elements, e.g. filters, gratings, reflectors
• CPC Class Codes: H01S 3/1608 erbium
• CPC Class Codes: H04B 10/291 in which processing or amplification is carried out without conversion of the main signal from optical form

Abstract

An L-band optical amplifier has a rare earth doped gain medium including a filter distributed over a finite physical portion of the gain medium. The filter is distributed over between about 25% to substantially the entire length of the gain medium. The distributed filter substantially eliminates out-of-band light emission (C-band ASE, 1520 nm-1565 nm) and thus improves the performance of L-band amplification (1565 nm-1620 nm). Examples of distributed filters include discrete type filters such as long period gratings, or continuous type filters such as rare earth doped, twin core fibers, non-adiabatically tapered fibers and coaxial resonant ring fibers.

Claims

1. An optical amplifier for optical fiber telecommunications lines operating with an in-band transmission signal in a longer wavelength, tail region of a gain spectrum associated with the amplifier, said longer wavelength, tail region being between 1570 nm and 1620 nm, said amplifier comprising:

a rare earth doped gain medium providing a first gain stage for the amplifier;

a source of pump power connected to the gain stage; and

a filter distributed over the gain medium wherein the filter attenuates light associated with amplified spontaneous emission (ASE) in the amplifier, such that the transmission signal in the longer wavelength region of the gain spectrum is amplified wherein the filter includes a continuous filter and the continuous filter comprises a doped waveguiding core that is unpumped so as to absorb the ASE light, and a doped waveguiding core that is pumped so as to amplify the transmission signal, further wherein the unpumped waveguiding core and the pumped waveguiding core exhibit waveguide dispersions such that coupling from the pumped core to the unpumped core occurs substantially only for light in the ASE gain spectrum.

2. The amplifier of claim 1 wherein the rare earth dopant includes erbium, further wherein the gain spectrum associated with the amplifier extends from about 1520-1620 nm, the ASE extends from about 1520-1565 nm, and the longer wavelength, tail region of gain spectrum is between 1570 nm and 1620 nm.

3. The amplifier of claim 1 wherein the filter is distributed over a finite physical portion of the gain medium ranging from between about 25% thereof to substantially the entire gain medium.

4. The amplifier of claim 3 wherein the filter includes a plurality of discrete filters.

5. The amplifier of claim 4 wherein the discrete filters have an inter-filter spacing corresponding to less than or equal to about a 20 dB gain length at a peak gain wavelength of the gain spectrum of the amplifier.

6. The amplifier of claim 4 wherein the discrete filters are long period gratings.

7. The amplifier of claim 4 wherein the long period gratings are written into the gain medium.

8. The amplifier of claim 1 wherein the continuous filter comprises an axial doped waveguiding core and one of a doped and an undoped annular waveguiding core that is coaxial with the axial core.

9. The amplifier of claim 1 wherein the filter has a bandwidth that is substantially coincident with the spectral bandwidth of the ASE, and a depth that is approximately equal to, or greater than, a peak gain coefficient in the gain spectrum of the amplifier.

10. The amplifier of claim 1, further comprising another gain medium providing another gain stage coupled to the first gain stage.

11. The amplifier of claim 10, wherein the other gain stage is coupled closer to an output of the first gain stage than to an input thereof.

12. The amplifier of claim 10 further wherein an ASE filter is distributed over the other gain medium.

13. The amplifier of claim 1, wherein the first and second gain stages each comprises a rare earth doped fiber gain medium.

14. The amplifier of claim 1 wherein the rare earth doped fibers have different host glass compositions.

15. The amplifier of claim 8 wherein at least one of the rare earth doped fibers comprises a phosphor-silicate host glass composition.

16. An erbium doped fiber amplifier having a known gain bandwidth for providing a signal amplification in a long wavelength region of the known gain bandwidth said long wavelength region being between 1570 and 1620 nm, said amplifier comprising:

first gain stage comprising an erbium fiber exhibiting a first gain spectrum within the known gain bandwidth extending from about 1520 nm to 1565 nm;

a source of pump energy coupled to the first gain stage;

a second gain stage comprising an erbium fiber serially connected to the first gain stage exhibiting a useful gain spectrum within the known gain bandwidth between 1570 nm and 1620 nm,

wherein the first gain stage further comprises a filter distributed along the first gain stage, said filter having a depth and a bandwidth sufficient to filter an out-of-band light emission having an emission spectrum substantially coincident with the first gain spectrum to the extent that any unfiltered out-of-band light emission in the first gain stage is insufficient to self-saturate the first gain stage, further wherein the useful gain spectrum exhibits a peak gain that is the peak gain of the known gain bandwidth wherein the filter comprises two erbium doped waveguiding cores in the erbium doped fiber of the first gain stage, further wherein no pumping energy is delivered to one of the doped cores such that said unpumped core attenuates the out-of-band light emission coupled into said unpumped core.

17. The amplifier of claim 16 wherein the first gain spectrum is substantially constant and has a neutral gain magnitude, and the useful gain spectrum includes a peak gain of the known gain bandwidth.

18. A method of operating an optical amplifier for amplification of a signal in a longer wavelength tail region of a known gain bandwidth of the amplifier, said longer wavelength tail region being between 1570 nm and 1620 nm, said method comprising the steps of:

distributing a filter over a finite physical length portion of a phosphor-silicate based gain medium of a first gain stage of the amplifier wherein the filter has a depth and bandwidth sufficient to attenuate a wavelength spectrum associated with amplified spontaneous emission from the amplifier, wherein the step of distributing a filter over the medium of the amplifier comprises distributing continuous filter over the gain medium.

19. A method of operating an optical amplifier for amplification of a signal in a longer wavelength tail region of a known gain bandwidth of the amplifier, amplifier, said longer wavelength tail region being between 1570 nm and 1620 nm, said method comprising the steps of:

distributing a filter over a gain stage of the amplifier said gain stage containing phosphor-silicate based fiber to attenuate an out-of-band light emission associated with amplified spontaneous emission from the amplifier such that an average inversion value associated with the amplifier is higher than the average inversion value associated with the amplifier without a distributed filter.

20. The method of claim 19 wherein the filter is distributed over an input gain stage of the amplifier.

21. The method of claim 20 wherein the filter is distributed over a second gain stage of the amplifier coupled to the first gain stage.

22. The method of claim 19 wherein he amplifier is an erbium doped optical amplifier, the known gain bandwidth is from about 1520 nm-1620 nm, and the out-of-band light emission is from about 1620-1565 nm.