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Nonlinear Optics

All-optical, polarization-independent switching using the Sagnac interferometer

H. Avramopoulos, N. A. Whitaker, M. C. Gabriel, P. M. W. French

Published Jan 1, 1992• 0 citations
Publisher

Abstract

Polarization-dependent operation is a serious obstacle in the development of practical photonic devices. In photon-photon devices polarization dependence can arise from different reasons. For example, orthogonal linear polarizations are often used to isolate the two input channels of a three terminal device. In such a case polarization dependence is essential for proper operation of the circuit. Also, in devices where interferometry is used, the polarization of the signal and reference beams must be the same or the interference is incomplete. Schemes such as polarization diversity do not work with nonlinear optical devices, as the interaction is intensity dependent. One successful approach is active polarization control, which requires detection and control circuitry combined with a LiNbO3 electrooptic polarization controller [1]. Active control however, entails extra complexity and expense, and the difficulty involved leads one to the search for simpler schemes.

DOI
10.1364/nlo.1992.we22

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Polarization-dependent operation is a serious obstacle in the development of practical photonic devices. In photon-photon devices polarization dependence can arise from different reasons. For example, orthogonal linear polarizations are often used to isolate the two input channels of a three terminal device. In such a case polarization dependence is essential for proper operation of the circuit. Also, in devices where interferometry is used, the polarization of the signal and reference beams must be the same or the interference is incomplete. Schemes such as polarization diversity do not work with nonlinear optical devices, as the interaction is intensity dependent. One successful approach is active polarization control, which requires detection and control circuitry combined with a LiNbO3 electrooptic polarization controller [1]. Active control however, entails extra complexity and expense, and the difficulty involved leads one to the search for simpler schemes.
A bulk-optics, polarization-independent phase shifter for photonic quantum information applications is demonstrated. The device is based on a transverse electro-optic modulator in a Sagnac-type loop containing Faraday rotators. This direction-dependent polarization rotation in the loop is combined with a Mach-Zehnder interferometer to form an optical switch with an on/off contrast of 96% and a switching time of 1.6 ns. We demonstrate that the device's operation maintains any arbitrary polarization state.
An essential component of future quantum networks is an optical switch capable of dynamically routing single photons. Here we implement such a switch, based on a fiber-optical Sagnac interferometer design. The routing is implemented with a pair of fast electro-optical telecom phase modulators placed inside the Sagnac loop, such that each modulator acts on an orthogonal polarization component of the single photons, in order to yield polarization-independent capability that is crucial for several applications. We obtain an average extinction ratio of more than 19 dB between both outputs of the switch. Our experiment is built exclusively with commercial off-the-shelf components, thus allowing direct compatibility with current optical communication systems.
In a search for all-optical signal processing elements operating at a picosecond time scale nonlinear phenomena in quadratic media, extensively studied since many years in a different context were revisited lately. However, all devices proposed and implemented to date (see [1,2] and Refs, therein) have certain drawbacks complicating their use as all-optical amplifying/switching devices, viz., low amplification because of a strong control signal in intensity-controlled devices, smooth switching characteristics of Mach-Zehnder interferometers, and a requirement of fast phase control in phase-sensitive devices [3]. A key requirement of all devices proposed until now is the coherence of pump and signal beams because the χ(2)-process strongly depends on the relative phase. Here we demonstrate that this phase-insensitive amplification/switching can be achieved by a three wave interaction even with reduced pump power requirements. In what follows we focus on the so-called vectorial or type II SHG where two orthogonally polarized fundamental frequency (FF) components A1 and A2 interact with the only second harmonic (SH) wave B polarized parallel to one of the fundamentals [2].
Dynamically routing single photons without disturbing their quantum state is an essential capability for photonic quantum information technology. Here, we experimentally demonstrate a heralded single-photon switch that combines low loss, strict polarization independence, and high process fidelity in a simple architecture based on a free-space Sagnac interferometer incorporating a single Pockels cell. The routing of the photon between two spatial output modes is governed solely by the phase modulation applied to the Pockels cell, and is therefore independent of the input polarization state, while the Sagnac geometry provides passive interferometric stability without active phase locking. Operating at the telecom wavelength of 1550 nm, the switch exhibits an insertion loss of approximately 0.4 dB and an average extinction ratio of 19.1 dB. Quantum process tomography yields process fidelities of 0.985 and 0.979 with respect to the ideal identity operation for the two output modes, demonstrating that the polarization information encoded in the single photons is preserved through the switch. These features make the device a practical building block for fundamental quantum optics experiments, photonic quantum information processing, and quantum networks at telecom wavelengths.
Grover multiports are higher-dimensional generalizations of beam splitters, in which input to any one of the four ports has equal probability of exiting at any of the same four ports, including the input port. In this paper, we demonstrate that interferometers built from such multiports have novel features. For example, when combined with two-photon input and coincidence measurements, it is shown that such interferometers have capabilities beyond those of standard beam-splitter-based interferometers, such as easily controlled interpolation between Hong-Ou-Mandel (HOM) and anti-HOM behavior. Further, it is shown that the Grover-based analog of the Mach-Zehnder interferometer can make three separate phase measurements simultaneously. By arranging the transmission lines between the two multiports to lie in different planes, the same interferometer acts as a higher-dimensional Sagnac interferometer, allowing rotation rates about three different axes to be measured with a single device.