How does it ensure that the emitted light and the returned light do not interfere with each other?
An optical circulator achieves the “uninterrupted transmission” of “emitted light” and “reflected light” and their directional separation among different ports by utilizing the Magneto-Optic Effect to break the Lorentz Reciprocity of the optical system.
The specific physical principle for achieving unidirectional light path conduction and spatial isolation internally is as follows:
1. Core Physical Basis: Reciprocal vs. Non-Reciprocal Devices
- Reciprocal Devices (e.g., ordinary fiber splitters, beam splitters): Their optical paths are strictly bidirectional and reversible. If light can travel from point A to point B, then light incident in the reverse direction from point B will return to point A with exactly the same transmission characteristics.
- Non-Reciprocal Devices (Optical Circulators, Optical Isolators): These utilize an external magnetic field to alter the anisotropy of the medium, causing the forward propagation (e.g., \text{Port 1} \to \text{Port 2}) and backward propagation (e.g., \text{Port 2} \to \text{Port 1}) of light to undergo completely different polarization evolutions and refractive paths, thereby preventing the backward light from returning to the incident end and instead directing it to a third port (\text{Port 2} \to \text{Port 3}).
2. Internal Structure and Key Components
Commercial three-port optical circulators typically adopt a polarization-insensitive micro-optic architecture, primarily composed of three types of core components:
- Birefringent Crystals (e.g., Calcite or \text{YVO}_4 ): These split input light into two spatially separated beams (ordinary ray ‘o’ and extraordinary ray ‘e’) based on their polarization states and recombine them at the output.
- Faraday Rotator (Non-Reciprocal Component): Made of magneto-optic crystals (like Bismuth-substituted Iron Garnet BIG/YIG) placed within the magnetic field of a permanent magnet. Regardless of whether the light wave is propagating forward or backward, its polarization direction is fixed to rotate unidirectionally along the direction of the applied magnetic field (standard design is a 45^\circ rotation).
- Half-Wave Plate (Reciprocal Component): Relies on the fast and slow axes of the crystal to produce a fixed phase difference. When light propagates forward, its polarization rotates by +45^\circ, and when propagating backward, it undergoes a -45^\circ relative rotation.
3. Full Process of Optical Path Transmission and Isolation
① Forward Propagation: Port 1 \to Port 2 (Emitted Light)
- Polarization Decomposition: Arbitrarily polarized light emitted from \text{Port 1} enters the front birefringent crystal, where it is split into orthogonal horizontal and vertical polarization components.
- Cooperative Polarization Rotation: The two beams sequentially pass through the half-wave plate (rotating +45^\circ) and the Faraday rotator (rotating +45^\circ). Since the rotation directions are the same, the total polarization state rotates by 90^\circ (+45^\circ + 45^\circ = +90^\circ).
- Beam Combination and Output: The beams with altered polarization states enter the rear crystal. Due to the spatial refractive displacement aligning perfectly with the design, the two beams recombine into one and are focused into \text{Port 2} (sent to the sensing fiber or Fiber Bragg Grating FBG).
② Backward Propagation: Port 2 \to Port 3 (Reflected Light)
- Polarization Decomposition: Light reflected from the FBG or the system’s end enters \text{Port 2} in reverse and is similarly decomposed into two orthogonal components by the rear crystal.
- Non-Reciprocal Polarization Cancellation:
- When the backward light passes through the Faraday rotator, its polarization direction still rotates by +45^\circ along the magnetic field direction (non-reciprocity).
- Subsequently, upon passing through the reciprocal half-wave plate, backward propagation causes its polarization to rotate in the opposite direction by -45^\circ.
- Cancellation: +45^\circ + (-45^\circ) = 0^\circ, meaning the polarization state of the light beam remains unchanged.
- Spatial Deflection and Separation: When the light beam with its original polarization state enters the front crystal, it undergoes a completely different refractive path (spatial walk-off effect) compared to the forward light. The light spot is physically guided to \text{Port 3} (the detector port) and cannot be focused or coupled back to \text{Port 1}.
4. Isolation and Technical Specifications
Through the aforementioned non-reciprocal polarization interference and spatial separation:
- Ultra-high Isolation: The reverse isolation between circulator ports typically reaches 40\text{ dB} \sim 50\text{ dB} (meaning the light intensity leaking back to the light source end, \text{Port 1}, is attenuated to less than one in ten million), completely preventing reflected light from disrupting intra-cavity oscillation of the laser or introducing phase noise.
- Low Insertion Loss: Single-pass transmission loss is typically < 0.8\text{ dB}, allowing the reflected sensing signal to be sent to the demodulation system with virtually no loss.