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Schematic of the experimental Raman-amplified dispersive Fourier-transform reflectometer. WDM: wavelength-division multiplexer; DCF: dispersion compensation fiber; FC: fiber collimator. Each interferometer arm has a DCF module with equal round-trip dispersion of and loss of . Dispersive FT is performed in the DCF of each arm, mapping the spectrum into a temporal waveform. During the dispersive Fourier transformation in the sample arm, distributed Raman amplification is implemented by pumping it with two diode lasers. The Raman pumps are injected into and removed from the DCF by the WDMs. Inverse FT is performed on the digitizer output to map the temporal waveform into the depth profile.
Basic performance of the Raman-amplified reflectometer showing the spectrum of the optical source and single-shot point spread functions at various imaging depths. After filtering and amplification, a spectrum centered at with a full width at half maximum bandwidth of is obtained. The axial resolution is limited by the modest bandwidth of the source centered at in this proof-of-principle demonstration.
Single-shot interference spectrum measured on a digital oscilloscope, with and without the Raman amplification. The sample is a partial reflector with two layers (both with about reflectivity) at imaging depths of 6.6 and . The spectrum is mapped into the time-domain waveform by the dispersive FT. Raman amplification improves the fringe visibility, which is otherwise invisible. The spikes at are due to the reflection of the input beam from the fiber coupler without going into the interferometer and are removed from the spectra before inverse FT. The calibrated wavelength axis is shown above the figure.
Depth profile of the sample obtained by performing inverse FT on the pulses in Fig. 3. The depth profile only becomes visible with the Raman amplification.
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