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Optics Express

Wide band optical diode based on passive white light cavities

Jianshan Wang, Xiuwen Xia, Yu-Bo Liang, Zijian Lin, Jingping Xu, and Yaping Yang

Published Sep 4, 2024• 0 citations
Publisher

Abstract

Usually, most optical diodes with high nonreciprocal transmission ratio were accompanied with high insertion loss. The diode made of the asymmetry cavity filled with two-level atoms can simultaneously possess high nonreciprocal transmission ratio and low insertion loss, because the cavity can lower insertion loss and the bistability of the atom can enhance nonreciprocal transmission ratio. However, such diodes can only operate in a very narrow frequency band. In this article we explore two methods to achieve efficient diodes with a wide frequency range. One is using mixed atomic gas instead of a single atom and the other is replacing single-mode cavity with passive white light cavities. By using the two methods, a wide band optical diode with high nonreciprocal transmission ratio and low insertion loss is provided. It will be beneficial for improving the performance of broadband photonic chips and photonic communication.

DOI
10.1364/oe.531387

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Usually, most optical diodes with high nonreciprocal transmission ratio were accompanied with high insertion loss. The diode made of the asymmetry cavity filled with two-level atoms can simultaneously possess high nonreciprocal transmission ratio and low insertion loss, because the cavity can lower insertion loss and the bistability of the atom can enhance nonreciprocal transmission ratio. However, such diodes can only operate in a very narrow frequency band. In this article we explore two methods to achieve efficient diodes with a wide frequency range. One is using mixed atomic gas instead of a single atom and the other is replacing single-mode cavity with passive white light cavities. By using the two methods, a wide band optical diode with high nonreciprocal transmission ratio and low insertion loss is provided. It will be beneficial for improving the performance of broadband photonic chips and photonic communication.
Optical nonreciprocity is important in photonic information processing to route the optical signal or prevent the reverse flow of noise. By adopting the strong nonlinearity associated with a few atoms in a strongly coupled cavity QED system and an asymmetric cavity configuration, we experimentally demonstrate the nonreciprocal transmission between two counterpropagating light fields with extremely low power. The transmission of 18% is achieved for the forward light field, and the maximum blocking ratio for the reverse light is 30 dB. Though the transmission of the forward light can be maximized by optimizing the impedance matching of the cavity, it is ultimately limited by the inherent loss of the scheme. This nonreciprocity can even occur on a few-photon level due to the high optical nonlinearity of the system. The working power can be flexibly tuned by changing the effective number of atoms strongly coupled to the cavity. The idea and result can be applied to optical chips as optical diodes by using fiber-based cavity QED systems. Our work opens up new perspectives for realizing optical nonreciprocity on a few-photon level based on the nonlinearities of atoms strongly coupled to an optical cavity.
Optical nonreciprocity is an essential property for a wide range of applications, such as building nonreciprocal optical devices that include isolators and circulators. The realization of optical nonreciprocity relies on breaking the symmetry associated with Lorentz reciprocity, which typically requires stringent conditions such as introducing a strong magnetic field or a high-finesse cavity with nonreciprocal coupling geometry. Here we discover that the collision effect of thermal atoms, which is undesirable for most studies, can induce broadband optical nonreciprocity. By exploiting the thermal atomic collision, we experimentally observe magnet-free and cavity-free optical nonreciprocity, which possesses a high isolation ratio, ultrabroad bandwidth, and low insertion loss simultaneously. The maximum isolation ratio is close to 40 dB, while the insertion loss is less than 1 dB. The bandwidth for an isolation ratio exceeding 20 dB is over 1.2 GHz, which is 2 orders of magnitude broader than typical resonance-enhanced optical isolators. Our work paves the way for the realization of high-performance optical nonreciprocal devices and provides opportunities for applications in integrated optics and quantum networks.
Optical bistability of a single-mode cavity containing a two-level atom in the Purcell regime has been analyzed. Compared with the case of a symmetric cavity, we find that the bistable regime of input power in an asymmetric cavity can be controlled by adjusting the cavity-loss rates of the asymmetric walls. Such results provide an optimal window of input power to realize giant optical nonreciprocity, which has promising application as the smallest all-optical diode unit with low operation power and high transmitted contrast.
A scheme for magnetic-free optical nonreciprocity in an ensemble of four-level cold atoms is proposed by exploiting the directional four-wave mixing effect. Using experimentally achievable parameters, the nonreciprocal optical responses of the system can be observed and the conversion on nonreciprocal transmission and nonreciprocal phase shift can be implemented. These nonreciprocal phenomena originate from the directional phase matching, which breaks the time-reversal symmetry and dynamic reciprocity of the cold atomic system. Moreover, by embedding the cold atoms into a Mach-Zehnder interferometer and choosing proper parameters, a two-port optical isolator with an isolation ratio of 79.70 dB and an insertion loss of 0.35 dB and a four-port optical circulator with a fidelity of 0.9985 and a photon survival probability of 0.9278 can be realized, which shows the high performance of isolation and circulation. The proposal may enable a new class of optically controllable cavity-free nonreciprocal devices in optical signal processing at the low light level.
Optical nonreciprocity and nonreciprocal devices such as optical diodes have broad and promising applications in various fields, ranging from optical communication to signal process. Here, we propose a magnet-free nonreciprocal scheme based on the four-wave mixing (FWM) effect in semiconductor quantum dots (SQDs). Via controlling the directions of the coupling fields, the probe field can achieve high transmission in the forward direction within a certain frequency range due to the FWM effect. And the transmission of the probe field in the backward direction undergoes significant reduction, as the FWM effect is absent. The calculation results show a wide nonreciprocal transmission window with isolation greater than 12 dB and insertion loss lower than 0.08 dB. The influences of the Rabi frequencies of the coupling fields, the medium length, and the decay rates on the nonreciprocal propagation of the probe field are also studied, showing the requirements of these parameters for good nonreciprocal performances. Our work may offer an insight for developing optical nonreciprocal devices based on the FWM process and the SQD system.
White light cavities with broadband resonance are usually filled with negative dispersion medium, which inevitably leads to gain. In this article, pure passive white light cavities are designed, in which negative dispersion medium is no longer necessary. Theoretically, if the reflection phase of the cavity wall can exhibit a negative dispersion slope, then it can also satisfy white light cavities conditions without medium. In practice, the negative dispersion property of the cavity wall can be realized by two metal coatings with different reflection coefficients. Therefore, our white light cavities are composite cavities, in which the main cavity provides resonance while the auxiliary cavity forms the cavity wall, providing negative dispersion reflection phase. Further, atomic gas can be employed to improve the performance of the white light cavities. Atomic gas exploits effects such as Electromagnetic Induced Transparency (EIT), enabling the white light cavities to be controlled by coherent driving field. With the passive characters, our design can be realized and implemented much more easily.