Optical isolators

Abstract

An optical isolator on a silicon photonic integrated circuit. The optical isolator includes a polarization beam splitter, a polarization rotator, and a Faraday rotator. The Faraday rotator includes one or more magnets that provide a magnetic field, and a silicon spiral delay line. The silicon spiral delay line is formed from a silicon waveguide shaped into a spiral region without a built-in phase shifter and a central region within the spiral region. The central region has a phase shifter with a total length of no more than 180 degrees.

Description

Technical Field

[0001] The present invention relates to an optical isolator and a Faraday rotator. Background Art

[0002] Optical isolators are often required in systems where there is strong back reflection to the laser and anti-reflection lasers cannot be used.

[0003] Isolators are typically made using a combination of birefringent materials, polarizing films, and magneto-optical materials.

[0004] Integrating these materials with silicon photonic devices remains an outstanding challenge.

[0005] Silicon itself has a small Faraday rotation similar to magneto-optical materials.

[0006] This requires extremely small TE / TM modal birefringence and can only be achieved in large (3 μm) waveguide platforms.

[0007] Silicon isolators are generally considered to include the following components:

[0008] Polarization beam splitter

[0009] Polarization rotator - for example, this can take the form of a reciprocal (i.e., non-Radial) 45-degree rotator

[0010] Faraday rotator - for example this can take the form of a long (5-15 cm) waveguide in a magnetic field of, for example, 0.1-1 Tesla which provides a non-reciprocal 45 degree rotation

[0011] Currently known designs place the Faraday rotator in a single helical delay line in a uniform magnetic field.

[0012] This requires two 90-degree phase rotators for each turn of the helix, so that the Faraday rotation increases constructively. Due to manufacturing tolerances, this many 90-degree phase rotators are considered impractical and may result in poor performance. Summary of the Invention

[0013] Thus, in a first aspect, embodiments of the present invention provide an optical isolator on a silicon photonic integrated circuit, the optical isolator comprising:

[0014] Polarization beam splitter;

[0015] a polarization rotator; and

[0016] Faraday rotator;

[0017] The Faraday rotator comprises:

[0018] one or more magnets for providing a magnetic field; and

[0019] A silicon spiral delay line is formed of a silicon waveguide shaped into a spiral region without built-in phase shifters and a central region within the spiral region, the central region having no more than 180° phase shifters in total.

[0020] Such optical isolators may be easier to fabricate and further produce higher degrees of optical isolation than previously seen. Some examples of the present invention can achieve optical isolation on the order of 7 dB, 14 dB, and 29 dB.

[0021] The optical isolator may have any one or any combination of the following optional features, as long as they are compatible.

[0022] The one or more magnets may include two U-shaped magnets positioned in opposing relationship to generate a magnetic field between respective ends thereof.

[0023] The central region may include two bends, each bend including a 90° phase shifter.

[0024] Each turn of the spiral region of the silicon spiral delay line may have a rectangular shape formed by four straight portions and four curved corners.

[0025] The one or more magnets are positioned so that field lines of the magnetic field follow the path of the waveguide within the spiral region. Each turn of the spiral region of the silicon spiral delay line may have a rectangular shape formed by four straight sections and four curved corners, and the one or more magnets may include four magnets, each magnet being located at a corner of the rectangular shape.

[0026] The central region may not include phase shifters.

[0027] The silicon spiral delay line may be formed of a 3 μm silicon waveguide.

[0028] The polarization rotator may be a 45° polarization rotator.

[0029] The optical isolator may further include an additional polarization beam splitter. The polarization beam splitter may be located at an input side of the Faraday rotator, and the additional polarization beam splitter may be located at an output side of the Faraday rotator.

[0030] In a second aspect, embodiments of the present invention provide a Faraday rotator for use in a silicon waveguide, the Faraday rotator comprising:

[0031] one or more magnets for providing a magnetic field; and

[0032] A silicon spiral delay line is formed of a silicon waveguide shaped into a spiral region without built-in phase shifters and a central region within the spiral region, the central region having a total of no more than 180 degrees of phase shifters.

[0033] The Faraday rotator may have any one or any combination of the following optional features, as long as they are compatible.

[0034] The one or more magnets may include two U-shaped magnets positioned in opposing relationship to generate a magnetic field between respective ends thereof.

[0035] The central region may include two bends, each bend including a 90° phase shifter.

[0036] Each turn of the spiral region of the silicon spiral delay line may have a rectangular shape formed by four straight portions and four curved corners.

[0037] The one or more magnets are positioned such that field lines of the magnetic field follow the path of the waveguide within the helical region.

[0038] Each turn of the spiral region of the silicon spiral delay line may have a rectangular shape formed by four straight portions and four curved corners, and the one or more magnets may include four magnets, each magnet being located at a corner of the rectangular shape.

[0039] The central region may not include phase shifters.

[0040] The silicon spiral delay line may be formed of a 3 μm silicon waveguide. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0042] Figure 1 A schematic diagram showing an optical isolator with a Faraday rotator; and

[0043] Figure 2 Schematic diagram showing an alternative optical isolator with a Faraday rotator. DETAILED DESCRIPTION

[0044] The present invention encompasses the configuration of a silicon Faraday rotator and surrounding photonic integrated circuits.

[0045] See below Figure 1 and Figure 2 to describe two configurations of Faraday rotators.

[0046] Including in Figure 1In the first Faraday rotator 110 of the illustrated optical isolator 100, a silicon helical delay line 114 is placed between two U-shaped magnets 112a and 112b (or other magnet arrangements that produce a similar field 116). This configuration requires only a 180-degree phase rotation (or two 90-degree rotations) in the middle of the helix, so that the Faraday rotation is constructively additive. This greatly simplifies the design, and the manufacturing tolerances of the single rotating element at the center are not limited. The Faraday rotator 110 applies a 45-degree rotation to the polarization of the signal in a non-reciprocal manner (i.e., the rotation does not reverse if the light passes through the rotator in the opposite direction). The magnetic field strength of the magnet can be at least 0.1 Tesla and no more than 1 Tesla.

[0047] Either end of the Faraday rotator 110 is connected to the other components of the optical isolator 100. To the left and 'upstream' of the Faraday rotator are the polarization beam splitter 102 and the polarization rotator 104. In use, light enters the polarization beam splitter and is split into a transverse electric component and a transverse magnetic component. A portion of the signal is then provided to the polarization rotator 104, which applies a 45° rotation to the signal's polarization. This rotation is reciprocal, meaning that if light passes through the rotator 104 in the opposite direction, the rotation is reversed.

[0048] The combination of the Faraday rotator 110 and the polarization rotator 104 is that the light is passed to the Figure 1 The arrangement 110 is configured to rotate the polarization of light by 90° in the second polarization beam splitter 118 on the right side of the Faraday rotator. When the rotation applied by the Faraday rotator is non-reciprocal, the arrangement 110 acts as an optical isolator or optical diode when light can only pass from left to right. This is because light passing from right to left will be in an improper polarization state when received by the first polarization beam splitter 102 and will therefore be provided to the blanked output port.

[0049] Including in Figure 2 In the second Faraday rotator 220 of the illustrated optical isolator 200, the silicon helical delay line 214 is a regular spiral with no central rotation. A plurality of magnets 212a-212d are positioned around the spiral so that the magnetic field 116 follows the path of the waveguide. As previously described, the Faraday rotator 210 applies a 45° rotation to the polarization of the signal in a nonreciprocal manner. The magnetic field strength of the magnets can be at least 0.1 Tesla and no more than 1 Tesla.

[0050] Either end of the Faraday rotator 210 is connected to the other components of the optical isolator 200. To the left and 'upstream' of the Faraday rotator are the polarization beam splitter 102 and the polarization rotator 104. In use, light enters the polarization beam splitter and is split into a transverse electric component and a transverse magnetic component. A portion of the signal is then provided to the polarization rotator 104, which applies a 45° rotation to the signal's polarization. This rotation is reciprocal, meaning that if light passes through the rotator 104 in the opposite direction, the rotation is reversed.

[0051] The combination of the Faraday rotator 210 and the polarization rotator 104 is that the light is passed to Figure 2 The arrangement 210 is configured to rotate the polarization of light by 90° in the second polarization beam splitter 118 on the right side of the Faraday rotator. When the rotation applied by the Faraday rotator is non-reciprocal, the arrangement 210 acts as an optical isolator or optical diode when light can only pass from left to right. This is because light passing from right to left will be in an improper polarization state when received by the first polarization beam splitter 102 and will therefore be provided to the blanked output port.

[0052] As already discussed, both configurations are equipped with two polarizing beam splitters and a 45-degree polarization rotator.

[0053] The present invention encompasses such configurations for producing silicon Faraday rotators with few or no 90-180 degree phase shifters and surrounding silicon photonic integrated circuits to form optical isolators.

[0054] Although the present invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art upon giving this disclosure. Therefore, the exemplary embodiments of the present invention set forth above are intended to be illustrative rather than restrictive. Various changes may be made to the described embodiments without departing from the spirit and scope of the present invention.

Claims

1. An optical isolator on a silicon photonic integrated circuit, the optical isolator comprising: Polarizer beam splitter; Polarization rotator; as well as Faraday rotator; The Faraday rotator comprises: one or more magnets for providing a magnetic field; as well as A silicon spiral delay line is formed of a silicon waveguide shaped into a spiral region without built-in phase shifters and a central region within the spiral region, the central region having phase shifters totaling no more than 180 degrees.

2. The optical isolator of claim 1 , wherein the one or more magnets comprise two U-shaped magnets positioned in opposing relationship to generate the magnetic field between respective ends thereof.

3. An optical isolator as claimed in claim 1 or claim 2, wherein the central region comprises two bends, each bend comprising a 90 degree phase shifter.

4. The optical isolator of claim 1, wherein each turn of the spiral region of the silicon spiral delay line has a rectangular shape formed by four straight portions and four curved corners.

5. The optical isolator of claim 1, wherein the one or more magnets are positioned so that field lines of the magnetic field follow the path of the waveguide within the helical region.

6. The optical isolator of claim 5 , wherein each turn of the spiral region of the silicon spiral delay line has a rectangular shape formed by four straight portions and four curved corners, and wherein the one or more magnets include four magnets, each magnet being located at a corner of the rectangular shape.

7. The optical isolator of claim 1, wherein the silicon spiral delay line is formed from a 3 μm silicon waveguide.

8. The optical isolator of claim 1 , wherein the polarization rotator is a 45 degree polarization rotator.

9. The optical isolator as claimed in claim 1, comprising an additional polarization beam splitter.

10. The optical isolator of claim 9, wherein the polarization beam splitter is located at an input side of the Faraday rotator, and the additional polarization beam splitter is located at an output side of the Faraday rotator.

11. A Faraday rotator for use in a silicon waveguide platform, the Faraday rotator comprising: one or more magnets for providing a magnetic field; as well as A silicon spiral delay line is formed of a silicon waveguide shaped into a spiral region without built-in phase shifters and a central region within the spiral region, the central region having phase shifters totaling no more than 180 degrees.

12. The Faraday rotator of claim 11, wherein the one or more magnets include two U-shaped magnets positioned in opposing relationship to generate the magnetic field between respective ends thereof.

13. A Faraday rotator as claimed in claim 11 or claim 12, wherein the central region comprises two bends, each bend comprising a 90 degree phase shifter.

14. The Faraday rotator of claim 11, wherein each turn of the spiral region of the silicon spiral delay line has a rectangular shape formed by four straight portions and four curved corners.

15. The Faraday rotator of claim 11, wherein the one or more magnets are positioned such that field lines of the magnetic field follow the path of the waveguide within the helical region.

16. The Faraday rotator of claim 11 , wherein each turn of the spiral region of the silicon spiral delay line has a rectangular shape formed by four straight portions and four curved corners, and wherein the one or more magnets include four magnets, each magnet being located at a corner of the rectangular shape.

17. The Faraday rotator of claim 11, wherein the silicon spiral delay line is formed of a 3 μm silicon waveguide.

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