Spatial-mode-selective frequency conversion is potentially useful for both classical and quantum communication applications. By a judicious choice of the quasi-phase-matching period in a Kai(2) multimode waveguide, such conversion can be achieved with high efficiency (close to 100%) and with low crosstalk (< -20 dB). For space-division multiplexing application with classical signals, where each spatial mode represents a separate signal channel, the selective conversion of a spatial mode without disrupting other signal modes can be used for reconfigurable spatial-mode de-multiplexing. This classical de-multiplexing capability can be also extended to the quantum regime, where the quantum state of the signal is preserved during frequency conversion, owing to the unitary nature of the sum-frequency generation (SFG) process.
Building upon our previous experimental demonstration of the classical spatial-mode-selective frequency up-conversion in a two-mode PPLN waveguide, here we report the extension of this work into the single-photon-level regime. The signal (1540 nm) in either a single mode (TM00 or TM01) or a superimposition mode (TM00+TM01, TM00+iTM01) of the waveguide is selectively up-converted into TM01 SFG mode, by interacting with an appropriate pump mode (1560 nm). An accurate measurement of the single-photon-level SFG signals requires thorough filtering of the unwanted photons contributed by the second harmonic of the pump, residual pump noise extending to the signal band, and the Raman noise generated in the waveguide. We have investigated these unwanted photon sources and suppressed them by a combination of thin-film-interference and volume-Bragg-grating filters. Resulting single-photon-counting measurements show >70% internal conversion efficiency, better than -12dB crosstalk, and >100 ratio of the signal to background photon counts for all selected modes and mode superpositions.
We propose an all-optical regeneration scheme for 16-QAM signal. An incoming signal is 50/50 split into I and Q arms. Each arm contains three phase sensitive amplifiers (PSAs); between the PSAs the signal propagates through highly nonlinear fiber (HNLF) and acquires nonlinear phase shift due to self-phase modulation (SPM). At the end, another 50/50 coupler combines the regenerated I and Q signals.
In each arm, the first PSA is used to amplify one quadrature of incoming signal and deamplify the other quadrature in order to squeeze the phase noise. Since data encoded on the deamplified quadrature is also erased by the first PSA, signal in each arm retains only the data on amplified quadrature. After the first PSA, only amplitude noises remain on the two power levels of each signal.
The amplitude noise is regenerated by the SPM in the HNLF, followed by the PSA. The SPM converts the amplitude noise to the phase noise and thus causes rotation of the uncertainty (noise) ellipse around the signal phasor in the complex plane. For a proper nonlinear phase shift, the long axis of the noise ellipse can be significantly compressed by the PSA. To suppress the noise on both signal levels with 1:9 power ratio, there are two HNLF+PSA combinations in each arm.
Our modeling shows better than 4 dB noise reduction for all constellation points. Moreover, the higher signal levels experience even greater noise suppression, which is beneficial for their nonlinear propagation through the transmission fiber.