Efficient net-gain integrated optical parametric amplifier in the quantum regime

Abstract

Optical parametric amplifiers (OPAs) are promising to overcome the wavelength coverage and noise limitations in conventional optical amplifiers based on rare-earth doping and semiconductor gain. However, the high power requirement remains a major obstacle to the widespread use of OPAs. Integrated OPAs can in principle improve the pump efficiency with tight mode confinement; however, challenges associated with propagation loss, limited nonlinearity, and susceptibility to nanoscale fabrication imperfections prevent them from competing with conventional bulk and fiber-based OPAs. Here, we demonstrate a highly efficient integrated OPAs with continuous-wave net gain. The pump efficiency is improved by over one order of magnitude. Phase-sensitive gain of 23.5 dB is demonstrated, significantly exceeding previous integrated OPAs, using only 110 mW pump power and no cavity enhancement. This is achieved with parametric down-conversion in thin-film lithium niobate waveguides using the adapted poling technique to maintain the coherence of nonlinear interactions. Moreover, the high parametric gain exceeds fibre-chip-fibre losses, leading to appreciable net gain up to 10 dB. The 3 dB bandwidth is approximately 120 nm, covering telecommunication S-, C-, and Lbands. Quantum-limited noise performance is confirmed through the measurement of output field fluctuation below the classical limit. We further demonstrate that signalto-noise ratio in noisy optical communications can be increased by leveraging this efficient integrated OPA. Our work marks a significant step towards ideal optical amplifiers with strong amplification, high efficiency, quantum-limited noise, large bandwidth, and continuous-wave operation, unlocking new possibilities for next-generation photonic information processing systems.

Figures (4)

• Figure 1: Integrated OPA with TFLN waveguides using adapted poling.a, Schematic of OPAs based on parametric down-conversion in single-pass waveguides. In the degenerate phase-sensitive case, the pump wavelength is half of the signal wavelength. The signal is exponentially amplified along the waveguide. b, Phase dependence of OPA gain. The gain is maximized when the phase difference between signal and pump is zero (in-phase), and minimized when the phase difference is $\pi/2$ (out-of-phase). c, Simulated gain spectrum of a 14-mm long integrated OPA. The simulated 3-dB bandwidth is around 140 nm, covering the S-, C- and L-band for telecommunication. d, Scanning electron microscopy (SEM) image of fabricated TFLN waveguides with 2.2 $\mu$m width. Green arrows indicate the crystal direction of lithium niobate. e, Measured film thickness (grey) and designed poling period (orange) along TFLN waveguides. The poling period is adjusted based on the measure film thickness to ensure perfect phase-matching along the waveguide. f, Piezoelectric force microscopy (PFM) image of the lithium niobate poling regime. g, Measured second-harmonic generation spectrum of TFLN waveguides with adapted poling. The spectrum shows a nearly theoretical sinc-squared shape with peak nonlinear efficiency 4700 $\pm$ 500 %/W
• Figure 2: Performance characterization of the integrated OPA.a, Experimental setup to characterize the integrated OPA. The upper and lower panels show the direct power measurement and homodyne detection respectively. SHG: commercial second-harmonig generation module; FS: fiber strecher, PD: photodetector. b, Measured OPA gain with phase scanning with 110 mW pump power. The output signal power without pump light used as the reference (0 dB). The red dashed line (13.5 dB) indicates the fiber-chip-fiber transmission loss. c, Measured integrated OPA gain as a function of pump power. d, Measured spontaneous emission spectrum of the integrated OPA with pump power 110 mW, showing 3-dB bandwidth over 120 nm. e, Time-domain homodyne measurement of output field quadrature under phase scan. The red (green) lines mark the squeezing (anti-squeezing) conditions. The sampling rate is 2 GS/s. f, Statistical distributions of time-domain homodyne measurement over 10 $\mu$s (20,000 samples) for shot noise with the pump blocked, squeezing (red line in e), and anti-squeezing (green line in e). The distributions are fitted with Gaussian functions to extract the variances (See methods). g, Measured squeezing and anti-squeezing level as a function of pump power. Uncertainties in pump power in c and g is determined by the variance of measured coupling efficiencies in different devices on the chip. Uncertainties in gain value in c and noise level in g are evaluated by the standard deviation of the fitted parameters.
• Figure 3: Comparison between EDFA and integrated OPA for signal-to-noise ratio improvementa, Experimental schematic. A single-frequency laser at 1549.6 nm is mixed with broadband noise generated from unseeded EDFA. After amplification with EDFA or OPA, the output light is detected by an optical spectrum analyzer (OSA). b, Phase diagram for EDFA and OPA amplification. The signals before and after amplification are labeled by orange circles respectively, and grey areas show the noise. EDFA amplify the signal and noise uniformly for all phases. OPA selectively amplifies in-phase signal and noise and de-amplifies out-of-phase noise. c, d, Optical spectrum before and after 13 dB EDFA amplification. e, SNR change after EDFA amplification with respect to gain. The SNR change has near-zero dependence on the gain, and shows an average of -0.15 dB. f, g, Optical spectrum before and after 13 dB OPA amplification. h, SNR change after OPA amplification with respect to gain. The SNR change increases with the OPA gain, and approaches 6 dB. Different optical SNR values before amplification are tested in e and h including 0 dB (blue), 5 dB (orange), 8 dB (green), and 12 dB (red). Uncertainties in e and h are obtained by calculating the variance of repeated measurements.
• Figure 4: Improvement of optical communications with the integrated OPAa, Experimental schematic. An on-off keying signal is generated using an intensity electro-optic modulator (EOM). After mixing with broadband noise generated from unseeded EDFA, optical signals are amplified by either EDFA or OPA. The optical signal is detected with a fast photodiode and an oscilloscope. b, Time-domain measurement of the optical signal before mixing noise (blue), after mixing noise and before amplification (red), after mixing noise and EDFA amplification (green), and after mixing noise and OPA amplification (purple). c, Normalized spectrum of photodiode signals without amplification (red), with EDFA amplification (green), and OPA amplification (purple). The gains of EDFA and OPA are both 14 dB. The spectrum is calculated from time-domain signal through fast Fourier transformation. The spectrum is normalized to the power at zero frequency. The shaded area indicates the regime the power is dominated by noise, as the modulation spectrum vanishes. d Statistical distributions of photodetector output for bit 0 and bit 1 with EDFA and OPA amplifications respectively. Distributions are fitted with Gaussian distribution (see methods). The overlap between bit-0 and bit-1 distributions is proportional to the bit error rate. e Bit error rate with different EDFA and OPA gain values. f Eye diagrams of received optical signals after EDFA and OPA amplification with 14 dB gain.