Fiber Amplifiers

The goal of this Optics Rotation project was to familiarize myself with fiber optics in general and with fiber amplifiers in partucular. Prof. Mecalf's Research Group purchased recently two fiber amplifiers from Keopsys (Fig. 1), which needed to be tested.

Fig. 1: Keopsys FPS-BT2-YFA-NLS Fiber Amplifier

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Fiber Amplifiers: basic concepts

Optical amplifiers amplify incident light through stimulated emission, the same mechanism used by laser. Indeed an optical amplifier is just a laser without feedback.
The basic concept is shown in Fig. 2. A fiber is made from a solid state laser material, typically glass doped with an ion which emits at the desired wavelength lambda1. It is illuminated from one end by a weak signal at lambda1 and a stronger steady beam at a second wavelength lambda2 which excites the ions in the fiber to its upper laser level and achieves population inversion. As the weak signal passes through the fiber amplifier it stimulates emission from exited ions at lambda1.

Fig. 2: Basic consept

A FA can be designed to operate in such a way that the pump and signal beams propagate in opposite directions, a configuration referred to as backward pumping to distinguish it from the forward pumping configuration in which both beams propagate along the same direction.

FA is a travelling wave device which amplifies the strength of signals making a single pass along its length. Such devices have received the most attention because they are attractive for signal amplification in fiber optics systems.

FA are similar in structure to other optical fibers with a high refractive index core and a lower index cladding. (Fig. 3) Total internal reflection confines light to the fiber core. The rare earths dopands are also confined to the core.
FA were studied as early as 1954. But their use become practical only in 1988 after the techniques for fabrication and characterization of low loss rare earth doped fibers were perfected.
Amplifier characteristics such as the operating wavelength and the gain bandwidth are determined by dopants rather than by the fiber which plays the role of a host medium. The rare earth dopands are triply ionized by removal of two outer 6s electrons and an inner 4f electron. Keopsys amplifier uses ytterbium as the dopant, but also erbium and neodymium are very common in telecommunication devices.


Fig. 3: Configuration of a fiber ( From Keopsys Ytterbium Products Presentation )

The level structure of the Yb ion is very simple with only the ground state manifold 2F7/2 and one excited state manifold 2F5/2 involved.


Fig. 4: Yb Absorption and Emission ( From Keopsys Ytterbium Products Presentation)
Nearly complete inversion of the population is possible for a broad range of pump wavelengths around 910 nm. But under these conditions the amplifier would produce a very large gain on the 975 nm peak which would then lead to strong amplified spontaneous emission, thus limiting the gain obtainable at the operating wavelength. This problem is avoided by pumping the amplifier at 975nm. Absorption and emission cross section are essentially equal at this wavelength so that inversions approaching 50% are achieved for powers well above the pump saturation power. The maximum gain then appears around the 1027 nm peak while at 1083nm one has about one third of this peak gain.

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Fig. 5: The optical spectrum (From Keopsys Manual)
The optical spectrum of the FA is sown in Fig. 5. The wavelength range is 1050-1090nm. We used 1083nm laser as input.

The Keopsys FA consists of 2 stages. (Fig. 6) The preamplifier, pumped with 920nm laser diode and booster, pumped with 975nm. The booster provides high output saturated power. The optical isolators prevent back reflections in optical source. Photodiodes convert the optical signal into power and current characteristics. These characteristics are compared to the set points defined by the user. From actual values, set points and other characteristics such as temperature, the electronic unit controls the laser diode current and therefore the amplification of the optical signal.


Fig. 6: Keopsys FA configuration (From Keopsys Ytterbium Products Presentation)
The booster in Keopsys FA is pumped by two laser pump diodes, LD1 and LD2. LD1 is the COUNTER pump laser diode and LD2 is the CO pump laser diode. The counter pumping is favoured due to increased threshold of non linear effects.
The maximum current through LD1 is 5 A and 2 A for LD2. The current for the booster can be set by the user.

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Measurements and Results

The experimental setup is shown in Fig. 7. The first step was to couple light into the FA. The orange cable with green end is the input fiber for the amplifier. The output fiber is in the middle of the photo, across from it is a powermeter. By inserting a mirror, the outcoming beam can be redirected to a Fabry-Perot cavity. The FA itself is on the left edge of the photo.


Fig. 7: Experimental setup
Fig. 8 is taken from the manual and shows output power versus pump current. Fig. 9 is the measured power output. Both figures are consistent, with about 35dBm (3W) output power at 5A through LD1. For a better comparison we requested from Keopsys the original data for the diagram in Fig. 8, but we didn't receive it yet. It is important to compare both data sets, since a drop in the output power is a sign of stimulated Brillouin scattering. This process generates a backward propagating Stokes wave that can damage the laser diode. For this reason we also didn't turn on the LD2.


Fig. 8: Output power versus pump current ( From Keopsys Manual)

Fig. 9: Measured output power versus pump current

Acknowledgements

I thank Prof. Metcalf, Matt Partlow, Xiyue Miao and Jorg Bochmann for their help and advice.

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References

Govind P. Agrawal, Nonlinear Fiber Optics
J. Hecht, The Laser Guidebook
R. Paschotta et al., Opt Commun. 136, 243 (1997)
Keopsys Ytterbium Products Presentation
Keopsys FA Manual