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Optics Letters

Noiseless optical amplification in quasi-phase-matched bulk lithium niobate

D. J. Lovering, Jonas Webjörn, Philip St.J. Russell, Juan Ariel Levenson, P. Vidaković

Published Sep 15, 1996• 34 citations
Publisher

Abstract

An optical parametric amplifier (OPA) has been demonstrated in bulk, periodically poled lithium niobate and is shown to operate with a noise figure well below the classical limit. In contrast to conventional OPA's, this device uses quasi-phase matching to provide the coupling between the pump and the signal. Comparison of the measured performance with that of a theoretical model reveals that the main intrinsic contribution to the output noise is due to spatial and temporal mode mixing, which arises as a consequence of tight focusing of the incident beams. Factors that affect the performance of this amplifier are identified theoretically and their relative importance investigated for both amplification and squeezing.

DOI
10.1364/ol.21.001439

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Noiseless, quasi-phase-matched parametric amplifier in bulk lithium niobate

D. J. Lovering, Juan Ariel Levenson, P. Vidaković +2 more

Although noiseless optical amplification has been demonstrated in bulk KTP crystals, and is well understood, there have been no observations of noiseless optical amplification in bulk quasi-phase-matched (QPM) systems. That such an amplifier is possible was predicted recently and indeed squeezing has already been observed in QPM waveguides (using KTP and LiNbO3). QPM materials are attractive for use in quantum-optical devices because they give access to certain (large) components of the nonlinear susceptibility tensor that cannot be used in conventional birefringent phase-matching schemes. They also extend the range of materials accessible for use in parametric amplifiers to include some low-birefringence crystals exhibiting large nonlinearities. Even with the technological advantages offered by certain QPM crystals, efforts to find practical applications for optical parametric amplifiers are hampered by the need for enormous pump-field strengths. The obvious solution to the problem of the pump-field requirement is to confine the incident energy to a narrow interaction region thereby increasing the energy density. However, this approach can only be realised at the expense of sacrificing the homogeneity of the parametric interaction between pump and signal waves. This in turn causes vacuum field fluctuations from parasitic modes to be mixed into the signal mode thereby degrading the signal to noise ratio and negating the advantage of using an optical parametric amplifier. In the experiments presented here, optical signals have been amplified with up to 9dB of gain, yet with a noise figure (NF) of only 1.09, as compared to the minimum possible NF of a classical amplifier with the same gain which would equal 1.86. Furthermore, the interaction is achieved with tightly focused incident fields in a sample of bulk QPM lithium niobate. The entire interaction region is only 200µm thick and 2.8mm long. This performance was achieved with the aid of a theoretical model m which the effect of wavefront curvature and mode distortion are treated within a quantised mode framework using a spatial mode basis. Such a simulation permits the fundamental origin of the excess quantum noise to be easily traced to zero-point fluctuations of high-order spatial modes which couple to the signal mode via the parametric interaction. Furthermore the simulation enabled an experimental geometry to be selected in which an optimal compromise is made between wavefront matching bump beam smaller than signal) and pump-homogeneity (pump larger than signal), so that the overall noise figure remains well into the quantum-performance region.

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Noiseless optical amplification–a non-classical process in which a weak optical signal may be amplified without any degradation in the signal to noise ratio (SNR)–is generating interest in the telecommunications community. Although successfully demonstrated in bulk KTP crystals1, practical applications are severely hampered by the enormous pump-field strength required to obtain a sizeable gain. Efforts to reduce this requirement are centred on the search for better non-linear materials and stronger confinement of the interacting fields. In this article a noiseless optical parametric amplifier (OPA) is demonstrated with tightly-focused incident fields in quasi-phase-matched (QPM) lithium niobate (LiNbO3), for the first time. The quantum noise properties are analysed within a quantised field framework which reveals a strong similarity between guided and free wave interactions.

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We report on an initial squeezing of 3.3 dB below the shot noise level, generated in bulk, quasi-phase-matched lithium niobate. This squeezing was generated by degenerate parametric amplification, at 1047 nm, of a mode-locked pulse train. We present a detailed comparison of the process performed in both the quasi-phase-matched material and in ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3},$ which was phase matched using conventional birefringent methods. The single-pass gain measured was 2.5 from ${\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}$ and 3.9 from periodically poled lithium niobate (PPLN), although the former was affected by photorefractive damage. We directly recorded $11%$ of noise reduction in the vacuum using the PPLN parametric amplifier, purely because the detection efficiency was so low in our experiment. The experimental data was fitted to a model which includes the initial squeezing level and the homodyne detector efficiency as free parameters. The observed squeezing agrees well with the a total detection efficiency of just $16%$ and an initial squeezing level of 3.3 dB (53%) in PPLN.

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Optical parametric amplification (OPA) of arbitrarily polarized light is proposed in a multi-section periodically poled Lithium Niobate (PPLN). External electric field is applied on selected sections to induce the polarization rotation of involved lights, thus the quasi-phase matched optical parametric processes exhibit polarization insensitivity under suitable voltage. In addition to the amplified signal wave, an idler wave with the same polarization is generated simultaneously. As an example, a ~10 times OPA showing polarization independency is simulated. Applications of this technology are also discussed.

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Optical parametric amplification is expected as a key technique for high-speed optical fiber communication and optical quantum computing.We developed a high-gain and broadband optical parametric amplifier (OPA) module by using a periodically poled lithium niobate (PPLN) waveguide, the single-mode core of which is directly bonded to a lithium tantalate substrate.The OPA is assembled as a pump-combiner-integrated module with four optical-fiber input/output ports.By improving the fabrication processes of the PPLN waveguide and the assembly techniques of OPA modules, we achieved over-30-dB phase sensitive amplification with a continuous-wave pump.By using the PPLN waveguide, we successfully obtained over-6-dB squeezed light, the spectral bandwidth of which was 2.5 THz.This high-gain broadband OPA module will help build an advanced information society.

NTT technical review•Nov 1, 2021•0 citations
An optical parametric amplifier (OPA) has been demonstrated in bulk, periodically poled lithium niobate and is shown to operate with a noise figure well below the classical limit. In contrast to conventional OPA's, this device uses quasi-phase matching to provide the coupling between the pump and the signal. Comparison of the measured performance with that of a theoretical model reveals that the main intrinsic contribution to the output noise is due to spatial and temporal mode mixing, which arises as a consequence of tight focusing of the incident beams. Factors that affect the performance of this amplifier are identified theoretically and their relative importance investigated for both amplification and squeezing.
Although noiseless optical amplification has been demonstrated in bulk KTP crystals, and is well understood, there have been no observations of noiseless optical amplification in bulk quasi-phase-matched (QPM) systems. That such an amplifier is possible was predicted recently and indeed squeezing has already been observed in QPM waveguides (using KTP and LiNbO3). QPM materials are attractive for use in quantum-optical devices because they give access to certain (large) components of the nonlinear susceptibility tensor that cannot be used in conventional birefringent phase-matching schemes. They also extend the range of materials accessible for use in parametric amplifiers to include some low-birefringence crystals exhibiting large nonlinearities. Even with the technological advantages offered by certain QPM crystals, efforts to find practical applications for optical parametric amplifiers are hampered by the need for enormous pump-field strengths. The obvious solution to the problem of the pump-field requirement is to confine the incident energy to a narrow interaction region thereby increasing the energy density. However, this approach can only be realised at the expense of sacrificing the homogeneity of the parametric interaction between pump and signal waves. This in turn causes vacuum field fluctuations from parasitic modes to be mixed into the signal mode thereby degrading the signal to noise ratio and negating the advantage of using an optical parametric amplifier. In the experiments presented here, optical signals have been amplified with up to 9dB of gain, yet with a noise figure (NF) of only 1.09, as compared to the minimum possible NF of a classical amplifier with the same gain which would equal 1.86. Furthermore, the interaction is achieved with tightly focused incident fields in a sample of bulk QPM lithium niobate. The entire interaction region is only 200µm thick and 2.8mm long. This performance was achieved with the aid of a theoretical model m which the effect of wavefront curvature and mode distortion are treated within a quantised mode framework using a spatial mode basis. Such a simulation permits the fundamental origin of the excess quantum noise to be easily traced to zero-point fluctuations of high-order spatial modes which couple to the signal mode via the parametric interaction. Furthermore the simulation enabled an experimental geometry to be selected in which an optimal compromise is made between wavefront matching bump beam smaller than signal) and pump-homogeneity (pump larger than signal), so that the overall noise figure remains well into the quantum-performance region.
Noiseless optical amplification–a non-classical process in which a weak optical signal may be amplified without any degradation in the signal to noise ratio (SNR)–is generating interest in the telecommunications community. Although successfully demonstrated in bulk KTP crystals1, practical applications are severely hampered by the enormous pump-field strength required to obtain a sizeable gain. Efforts to reduce this requirement are centred on the search for better non-linear materials and stronger confinement of the interacting fields. In this article a noiseless optical parametric amplifier (OPA) is demonstrated with tightly-focused incident fields in quasi-phase-matched (QPM) lithium niobate (LiNbO3), for the first time. The quantum noise properties are analysed within a quantised field framework which reveals a strong similarity between guided and free wave interactions.
We report on an initial squeezing of 3.3 dB below the shot noise level, generated in bulk, quasi-phase-matched lithium niobate. This squeezing was generated by degenerate parametric amplification, at 1047 nm, of a mode-locked pulse train. We present a detailed comparison of the process performed in both the quasi-phase-matched material and in MgO:LiNbO3,{\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3},MgO:LiNbO3​, which was phase matched using conventional birefringent methods. The single-pass gain measured was 2.5 from MgO:LiNbO3{\mathrm{M}\mathrm{g}\mathrm{O}:\mathrm{L}\mathrm{i}\mathrm{N}\mathrm{b}\mathrm{O}}_{3}MgO:LiNbO3​ and 3.9 from periodically poled lithium niobate (PPLN), although the former was affected by photorefractive damage. We directly recorded 1111%11 of noise reduction in the vacuum using the PPLN parametric amplifier, purely because the detection efficiency was so low in our experiment. The experimental data was fitted to a model which includes the initial squeezing level and the homodyne detector efficiency as free parameters. The observed squeezing agrees well with the a total detection efficiency of just 1616%16 and an initial squeezing level of 3.3 dB (53%) in PPLN.
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.
Optical parametric amplification (OPA) of arbitrarily polarized light is proposed in a multi-section periodically poled Lithium Niobate (PPLN). External electric field is applied on selected sections to induce the polarization rotation of involved lights, thus the quasi-phase matched optical parametric processes exhibit polarization insensitivity under suitable voltage. In addition to the amplified signal wave, an idler wave with the same polarization is generated simultaneously. As an example, a ~10 times OPA showing polarization independency is simulated. Applications of this technology are also discussed.
Optical parametric amplification is expected as a key technique for high-speed optical fiber communication and optical quantum computing.We developed a high-gain and broadband optical parametric amplifier (OPA) module by using a periodically poled lithium niobate (PPLN) waveguide, the single-mode core of which is directly bonded to a lithium tantalate substrate.The OPA is assembled as a pump-combiner-integrated module with four optical-fiber input/output ports.By improving the fabrication processes of the PPLN waveguide and the assembly techniques of OPA modules, we achieved over-30-dB phase sensitive amplification with a continuous-wave pump.By using the PPLN waveguide, we successfully obtained over-6-dB squeezed light, the spectral bandwidth of which was 2.5 THz.This high-gain broadband OPA module will help build an advanced information society.