11:15am - 11:45am (Invited) OFPW1.3

1.3 pm Raman Fiber Amplifiers

Stephen G. Grubb AT&T Bell Laboratories

600 Mountain Avenue Murray Hill, NJ 07974

The majority of installed terrestrial fiber optic communications systems currently operated in the second communications window at wavelengths near 1.3 pm. A low-noise fiber optical amplifier is a strongly desired component for many of these systems. Until recently, PI3‘ - doped fiber amplifiers were the only candidates in this spectral region. While many satisfactory systems results have been reported for PI3‘ -doped fiber amplifiers, they suffer a number of disadvantages. The necessity of a fluoride host is perhaps the most serious of these limitations, eliminating the option of fusion splicing to the silica fiber network. The lower strength and reliability of fluoride fibers are also of concern.

Recently the demonstration of fiber Raman amplifiers at 1.3 microns have presented a silica fiber based alternative for the 1.3 pm window. 1,2 Prior to the advent of the Er3’ fiber amplifier, there was considerable work of Raman fiber amplifier for use in the 1.5 pm transmission win do^.^ The main disadvantage at the time was the lack of availability of high-power diode-pumped sources for these amplifiers. Given the recent emergence of diode-pumped high-power cladding-pumped fiber lasers (up to 5 Watts CW .from a single- mode core)4, Raman amplifiers merit a reinvestigation. The ability to fabricate low-loss Bragg fiber gratings directly in the fiber core also is a key advance. These low-loss nested resonators allow the cladding pumped lasers near 1.06 pm to be efficiently shifted to the required pump wavelength of 1.24 pm. Though high pump powers are required for a Raman amplifier (compared to Er3’ amplifiers) are comparable to many of the P? amplifiers. Many of the recent P? pump amplification schemes have in fact been based on diode-pumped solid state lasers near 1.06 pm.’ Recently a 30 dB net gain in a Raman amplifier with 350 mW of pump power has been reported.6

We have investigated two configurations for Raman amplification in silica fibers at 1.3 pm. The first is to generate the third Stokes order at 1240 nm in a cascaded Raman laser pumping with a Nd3’ fiber laser at 1060 nm, and then pumping a separate length of silica fiber to achieve amplification at 1.3 pm. This has the advantages separated optimization of the Raman laser and amplifier sections, choosing between CO- and counter-propagating amplification, and the ability to achieve distributed Raman amplification in a standard silica transmission fiber. The second configuration is to write highly reflective Bragg fiber gratings in a section of silica fiber through the first three orders, thereby increasing the circulating power at the third Stokes frequency, and then inject a 1.3 pm signal (4th Stokes ) through this resonant structure. This “intracavity” configuration is made possible through the use of Bragg fiber gratings, which can be fabricated to be highly reflecting to

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any given Stokes order, but transparent to light at other Stokes orders, especially the signal wavelength of 1.3 pm, where there is negligible backreflection.

While we have generated gains in excess of 40 dB (and output powers of +24 dBm) using both of these configurations, the systems performance is the key indicator of the usefulness of these amplifiers. An initial successfid digital systems test was performed on a counter-pumped Raman amplifier. However, the maximum gain that could be obtained while keeping a reasonable systems performance was limited to 17 dB.’ We have since performed a careful investigation into the noise sources of a Raman fiber amplifier. We have constructed a new amplifier with the goal of reducing the various amplifier noise components. With this configuration we were able to demonstrate 2.5 Gbh transmission with no power penalty with a gain of 30 dB and an output power of +16 dBm. The noise figure measured by the optical technique (signal-spontaneous beating component) was measured to be less than 5 dB. The systems performance of various configurations of Raman amplifiers will be discussed.

References:

1. S.G. Gnibb et. al., Proc. Optical Amplifiers and Their Applications, paper PD-3, August 3-5, Breckenridge, CO (1 994).

2. E.M. Dianov et. al., Optical Fiber Technology 1,236, (1995).

3. Y. Aoki, J. Lightwave Tech., 6(7), 1225, (1988).

4. H. Po et. al. , Electr. Lett., 29(17), 1500, (1993).

5. M. Yamata et. al., IEEE Photon. Tech. Lett., 7(8), 869, (1995).

6. E.M. Dianov et. al., Proc. Optical Amplifiers and Their ApplicationS, paper SaA2, June 15- 17 , Davos, Switzerland (1 995).

7. D.R. Dykaar et. al., Proc. OFC ‘95, paper PD1, San Diego, CA, February 26-March 3, (1995).

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