Optical Isolation in a Fiber

Illustration of ultralow-loss complete optical isolation in a fiber. Light in one direction is absorbed by the spherical resonator (yellow arrows) while light in the opposite direction (red arrows) passes through unaffected. (Source: G. Bahl)

Researchers from the Uni­versity of Illinois at Urbana-Champaign have demon­strated a new level of optical iso­lation necessary to advance on-chip optical signal processing. The technique invol­ving light-sound inter­action can be implemented in nearly any photonic foundry process and can signi­ficantly impact optical computing and communi­cation systems.

“Low-loss optical iso­lators are critical components for signal routing and pro­tection, but their chip-scale inte­gration into photonic circuits is not yet practical. Isolators act as optical diodes by allowing light to pass through one way while blocking it in the opposite direc­tion,” explained Gaurav Bahl, an assistant pro­fessor of mechanical science and engi­neering at Illinois. “We demon­strated that complete optical iso­lation can be obtained within any dielec­tric wave­guide using a very simple approach, and without the use of magnets or mag­netic materials.”

The key charac­teristics of ideal optical iso­lators are that they should permit light with zero loss one way, while absorbing light perfectly in the opposite direc­tion, i.e. the condition of ‘complete’ isolation. Ideal isolators should also have a wide bandwidth and must be linear. The optical signal wave­length does not change through the device and the pro­perties are inde­pendent of signal strength. The best method, to date, for achieving iso­lation with these characteristics has been through the mag­neto-optic Faraday rotation effect occurring in special gyro­tropic materials, e.g. garnet crystals. Unfor­tunately, this technique has proven chal­lenging to implement in chip-scale pho­tonics due to fabri­cation complexity, difficulty in locally confining magnetic fields, and signi­ficant material losses. In light of this challenge, several non-magnetic alter­natives for breaking reci­procity have been explored both theore­tically and experi­mentally.

In a previous study, Bahl’s research team experi­mentally demonstrated, for the first time, the pheno­menon of Brillouin Scattering Induced Trans­parency (BSIT), in which light-sound coupling can be used to slow down, speed up, and block light in an optical wave­guide. “The most signi­ficant aspect of that dis­covery is the obser­vation that BSIT is a non-reci­procal pheno­menon – the trans­parency is only generated one way. In the other direction, the system still absorbs light,” Bahl said. “This non-reci­procal behavior can be exploited to build isolators and circulators that are indis­pensable tools in an optical designer’s toolkit.”

“In this work, we experi­mentally demonstrate complete linear optical isolation in a wave­guide-resonator system composed entirely of silica glass, by pushing the BSIT interaction into the strong coupling regime, and probing optical trans­mission through the waveguide in the forward and backward directions simulta­neously,” stated JunHwan Kim, a graduate student. “Experi­mentally, we have demonstrated a linear isolator capable of generating a record-breaking 78.6 dB of contrast for only 1 dB of forward insertion loss within the isolation band,” J. Kim added. “This means that light propa­gating backwards is nearly 100-million times more strongly suppressed than light in the forward direction. We also demonstrate the dynamic optical recon­figurability of the isolation direction.”

“Currently the effect has been demonstrated in a narrow bandwidth. In the future, wider bandwidth isolation may also be approached if the wave­guide and reso­nator are integrated on-chip, since remaining mecha­nical issues can be eliminated and the inter­acting modes can be designed precisely, “ Bahl said. “Achieving complete linear optical isolation through opto-mecha­nical inter­actions like BSIT that occur in all media, irre­spective of crystal­linity or amorphi­city, material band structure, magnetic bias, or presence of gain, ensures that the technique could be imple­mented with nearly any optical material in nearly any commercial photonics foundry.” Since it avoids magnetic fields or radio­frequency driving fields, this approach is parti­cularly attractive for chip-scale cold atom micro­systems techno­logies, for both isolation and shuttering of optical signals, and on-chip laser pro­tection without loss. (Source: U. Illinois)

Reference: J. Kim et al.: Complete linear optical isolation at the microscale with ultralow loss, Sci. Rep. 7, 1647 (2017). DOI: 10.1038/s41598-017-01494-w

Link: Bahl Research Group, Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, USA

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