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Fiber Optic Circulators

The function of an optical circulator is similar to that of a microwave circulator. It is a three or more ports multiport device. Lightwave is transmitted from one port to the next sequential port with minimum loss, but is blocked from from one port to the previous port.

 

Optical circulators were first used in telecom systems. However, with the ready availability of low-cost and high-performance circulators, they are also used in sensing and imaging fields.

Optical Circulator Applications

  • Add-Drop Multiplexing
  • Fiber Sensors
  • Bidirectional Pumping
  • Bidirectional Signal Transmission Systems
  • Coupling In-Line Chromatic Dispersion Compensation Devices

Optical circulators were first used in telecom systems to increase transmission capacity of existing networks. By using optical circulators in a bidirectional transmission system, the capacity is easily doubled as shown below.

Circulators can be used to send optical signals through a single fiber in two directions.

Optical circulators are powerful devices to extract optical signals from a reflective device. It can be used with a mirror for double passing an optical element to increase efficiency as shown below with a reflective erbium-doped fiber amplifier(EDFA).

  1. Signal light (1550nm) is launched into port 1 and passed through port 2 with minimum loss.
  2. The signal is combined with the pump light (980nm) by a WDM coupler and both lights are launched into an erbium-doped fiber
  3. The signal is amplified along the erbium-doped fiber
  4. Both the signal and residual pump light are reflected by the mirror
  5. The signal and residual pump light pass through the erbium-doped fiber again and the signal is amplified again by the residual pump light
  6. At the WDM coupler, the signal light (1550nm) passes through while the pump light (980nm) is guided away into the pump laser (but absorbed by a built-in isolator)
  7. The signal light is guided into port 3 by the circulator

The erbium-doped fiber is used twice, reducing the required length of erbium-doped fiber. And the residual pump power is also re-used to increase the pump efficiency.

Optical Circulator Types

Polarization Dependent or Not

  1. Polarization-dependent circulator
    Functional only for a light with a particular polarization state. Since the polarization state of a light is not maintained in a regular optical fibers due to the birefringence caused by the imperfection of the fiber, polarization-dependent circulators have limited applications such as free-space communications between satellites, and optical sensing.
  2. Polarization-independent circulator
    Functional independent of the polarization state of a light. Widely used in fiber optic telecom networks.

Functionality

  1. Full circulator:
    Light passes through all ports in a complete circle (i.e., light from the last port is transmitted back to the first port)
  2. Quasi-circulator:
    Light passes through all ports sequentially but light from the last port is lost and cannot be transmitted back to the first port.

In most applications only a quasi-circulator is required.

Working Principles of Optical Circulators

Two main design ideas are used to construct an optical circulator.

Type I (currently the most popular):

  1. Polarization beam splitting and recombining
  2. Non-reciprocal polarization rotation by Faraday effect

Type II (not popular due to manufacturing challenges and worse performance):

  1. Asymmetric field conversion
  2. Non-reciprocal phase shift

Design 1 (Type I)

This design uses polarization beam splitter cubes that has dielectric thin-film coatings to either pass or reflect the light based on two perpendicular polarizations, thus split the incoming beam into two beams with orthogonal polarizations.

This design was used in the early days of circulator development. However, the isolation of this type was relatively low due to limited extinction ratio (20 dB) of the polarization beam splitter, It has since been replaced by several improved designs using birefringent crystals as shown in the next section.

From port 1 → Port 2
  1. A light beam launched into port 1 is split into two beams by the polarization beam splitter since it transmits the light with horizontal polarization and reflects the light with vertical polarization
  2. The optic axis of the half-waveplate is arranged at 22.5° to the x-axis, so that the vertically polarized light is rotated by +45°
  3. The thickness of the Faraday rotator is selected for providing 45° polarization rotation and the rotation direction is selected to be counter-clockwise when light propagates along the z-axis
  4. Thus the polarization rotation made by the half-waveplate (+45°) is cancelled by that of the Faraday rotator (-45°).
  5. Therefore, the polarization of the two beams is unchanged after passing through the half-waveplate and the Faraday rotator
  6. The two beams are recombined by the second polarization beam splitter and coupled into port 2
From port 2 → port 3
  1. A light beam is launched into port 2. It is split into two beams with orthogonal polarization by the second polarization beam splitter.
  2. Due to the non-reciprocal rotation of the Faraday rotator, in this direction the polarization rotations made by both the half-waveplate and Faraday rotator are in the same direction (not cancelled out), resulting in a total rotation of 90°
  3. Therefore, the two beams are combined by the first polarization beam splitter in a direction orthogonal to port 1 and coupled into port 3
From port 3 → port 4
  1. This works the same as that from port 1 → port 2

Design 2 (Type I)

This design takes advantage of the high extinction ratio property of birefringent crystals and is a much more improved design than the first. It uses birefringent beam displacers for splitting and combining of the orthogonally polarized light beams.

Each circle indicates the beam position and the arrow inside the circle indicates the polarization direction of the beam.

From port 1 → port 2
  1. A light beam launched into port 1 is split into two beams with orthogonal polarization states along the y-axis
  2. Two half-waveplates, one (upper) with its optic axis oriented 22.5° and the other (lower) at -22.5°, are used to rotate the two beams so that their polarization direction becomes the same
  3. The Faraday rotator rotates the polarization of both beams 45° counter-clockwise and the two beams are vertically polarized (along the y-axis)
  4. These two beams are passed through the second birefringent crystal without any spatial position change because the polarization directions of the the two beams match the ordinary ray direction of the crystal
  5. After passing through another Faraday rotator and half-waveplate set, the two beams are recombined by the third birefringent crystal, which is identical to the first one
From port 2 → port 3
  1. A light beam launched into port 2 is split into two beams and passed through the half-waveplate set and the Faraday rotator
  2. Due to the non-reciprocal rotation of the Faraday rotator, the two beams become horizontally polarized (along the x-axis)
  3. These two beams are spatially shifted along the x-axis by the second birefringent crystal because they match the extraordinary ray direction of the crystal
  4. The two beams are recombined by the first crystal at a location different from port 1 after passing through the Faraday rotator and half-waveplate set.
  5. The distance between port 1 and port 3 is determined by the length of the second birefringent crystal

Design 3 (Type I)

Design 2 uses collimated beam and each port is collimated using a lens; therefore, relative large size elements have to be used due to the beam size. Thus a compact circulator design as shown below is used.

 

This design places optical elements in a diverging beam instead of in a collimating beam to reduce the overall use of expensive materials (birefringent crystals).

Two identical groups of elements are placed near the focal point of the lens. Each group of elements consists of two birefringent crystals, one Faraday rotator with 45° rotation angle, and two half-waveplates with their optic axes oriented in opposite directions (22.5° and -22.5°).

From port 1 → port 2
  1. A light beam from port 1 is split into two orthogonally polarized beams in the y-axis by the first birefringent crystal
  2. The two half-waveplates and the Faraday rotator are arranged such that after passing through the rotators the polarization directions of the two beams are the same and match the ordinary ray direction of the second birefringent crystal
  3. Therefore, the two beams pass through the second birefringent crystal without any displacement
  4. Two lenses are used for for providing a one-to-one imaging system
  5. Because the second group of elements is the same as the first one, the two beams are recombined and launched into port 2
From port 2 → port 3
  1. Similarly, a light beam launched into port 2 is split and passed through to the rotators
  2. However, due to the non-reciprocal rotation of the Faraday rotator, the polarization directions of the two beams are rotated matching the extraordinary ray direction of the second birefringent crystal
  3. Therefore, the two beams are shifted a certain amount along the x-axis and shifted again the same amount by the second birefringent crystal in the other group
  4. The sum of beam shifting by the two birefringent crystals is designed such that it is the same as the distance between the first and third ports, the two beams are recombined and coupled into port 3

In this design, because port 1 and port 3 share a single lens and the beam shifting is done at the diverging beam, the required beam shifting is very small and typically equal to the fiber diameter of 125µm.

To further reduce the required thickness of the birefringent crystals, mode-field diameter of the input and output fibers is expanded to reduce the divergence angle of the beam.

In contrast, in design 2 the required beam shifting is determined by the diameter of a lens due to the use of collimated beams, it is typically in the order of millimeters.

Design 4 (Type I)

This design uses collimated beam deflection as shown above. In this design, polarization-dependent angle deflection is used instead of the polarization-dependent position shift.

A single lens is used to collimate the light for both port 1 and 3 and all elements are positioned in the collimated beam. The main difference is that a Wollaston prism is used as a beam displacer instead of birefringent crystals.

From port 1 → port 2
  1. A light beam launched into port 1 is collimated and split into two beams with orthogonal polarization by the first birefringent crystal
  2. The polarization directions of the two beams are rotated by the half-waveplates and Faraday rotator so that they become the same
  3. Because port 1 is off-axis of the lens, the resulting collimated beam from the lens forms an angle θ to the propagation axis
  4. This angle is corrected by the Wollaston prism and the two beams are propagated straight to the second Faraday rotator (solid lines shown)
  5. After passing through the half-waveplates and being recombined by the third birefringent crystal, the combined beam is focused by the second lens into port 2
From port 2 → port 3
  1. Similarly, light launched into port 2 is collimated and split into two beams with their polarization direction rotated
  2. Due to the non-reciprocal rotation of the Faraday rotator, the two beams from port 2 are deflected to a direction opposite to the angle θ by the Wollaston prism (dotted lines)
  3. Therefore, after passing through the polarization rotators and the first birefringent crystal, the combined beam is focused by the first lens to a position different from that of port 1
  4. The required deflection angle of the Wollaston prism can be determined by the position distance between port 1 and port 3 and the focal length of the lens

This design reduces the size of materials considerably. However, because the beam splitting and recombining is still performed in the collimated beam, it still requires relatively large crystals compared to design 3.

Design 5 (Type I)

This design uses imaging folding to redirect the light beam and reuse the common elements. It can significantly reduce the overall device size and cost.

A single lens and a mirror are used to couple lights between all ports that are at the same side of the circulator. Thus, all elements are passed through twice to reduce the element amount to half of a conventional circulator.

From port 1 → port 2
  1. A light beam launched into port 1 is split into two beams by the first birefringent crystal
  2. These two beams pass through the second crystal without any lateral position change, because the rotation angles of the polarization rotators (+45° or -45° rotation) are designed such that the polarization directions of the two beams match the ordinary ray direction of the second birefringent crystal.
  3. After being collimated by the lens and reflected by the mirror, the two beams are passed through the same elements again except for half-waveplates and recombined into port 2
From port 2 → port 3
  1. Similarly, a light beam launched into port 2 is split into two beams with orthogonal directions
  2. After passing through the polarization rotators, the polarization directions of both beams are aligned with the extraordinary ray direction of the second birefringent crystal due to the non-reciprocal rotation of the Faraday rotator, and the physical locations of the two beams are shifted after passing through the crystal.
  3. The two beams have a location shift again after being reflected by the mirror and passed through the crystal
  4. After the proper polarization rotation the two beams are recombined into port 3

There are many other variations of circulator design, but all these designs share a common idea: polarization splitting and recombining, non-reciprocal polarization rotation (Faraday rotator), and polarization-dependent beam steering (angular or positional).

Design 6 (Type II)

In this design, there is no polarization beam splitting, instead non-reciprocal phase shifting is used to control two-beam interference(phase shift of 0 or π) and thus steer them into different ports.

From port 1 → port 2
  1. A light beam launched into port 1 is split into two beams with equal intensity (power) by the first power splitter
  2. The two beams passes through two sets of phase-shifting elements (half-waveplate and Faraday rotator)
  3. These two sets of half-waveplate and Faraday rotator are designed such that they provide no phase shift (0 shift) between the two beams in one direction, but in the reverse direction, a phase shift of π is added between the two beams (caused by the non-reciprocal nature of Faraday rotator)
  4. Therefore, these two beams are in phase and will be constructively recombined by the second power splitter into port 2
From port 2 → port 3
  1. A light beam launched into port 2 is split into two beams with equal intensity by the second splitter
  2. They pass through the phase shifter set (half-waveplate and Faraday rotator). However, this time, a phase shift of π is added between these two beams (non-reciprocal nature of the Faraday rotators)
  3. These two beams are now out of phase and no long will be coupled into port 1 (destructive interference), but instead will be coupled into port 3 (constructive interference)

This design is very simple, however, the control of the phase in each element and the path length difference between two beams are VERY critical. So this design is only practiced in waveguide devices.

Reference:

  1. Faraday effect
  2. Waveplates
  3. How light propagates in birefringent crystals
  4. Beam displacer
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