WO2015195674A1

WO2015195674A1 - Manipulating guided light

Info

Publication number: WO2015195674A1
Application number: PCT/US2015/036046
Authority: WO
Inventors: Ned Tabat; Eric GRANSTROM; Peter Mach
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-06-16
Filing date: 2015-06-16
Publication date: 2015-12-23
Anticipated expiration: 2016-12-16

Abstract

A system for manipulating guided light includes an optical fiber including a core surrounded by a cladding. The cladding has a first portion defining a first diameter and a second portion defining a second diameter less than the first diameter so as to allow access to a non-negligible fraction of propagating mode amplitude. A birefringent material is situated about the second portion of the cladding, and an electrode is positioned proximate the birefringent material so as to influence optical properties of the birefringent material.

Description

MANIPULATING GUIDED LIGHT

This application is being filed on 16 June 2015, as a PCT International patent application, and claims priority to U.S. Provisional Patent Application No.

62/012,584, filed June 16, 2014, the disclosure of which is hereby incorporated by reference herein in its entirety.

Background

[0001] Optical fibers were originally developed, and are commonly used as conduits for telecommunication signals. They provide a superb medium for guided wave optics since the optical energy is conveniently confined in two dimensions in a cylindrical fashion and is guided along the third dimension. Silica glass is typically the material of choice for fiber construction primarily due to its extremely low transmission loss at near-infrared wavelengths. Furthermore, the amorphous nature of glass allows for significant mechanical flexibility of the fibers Summary

[0002] In accordance with aspects of the present disclosure, a system for manipulating guided light includes an optical fiber including a core surrounded by a cladding. The cladding has a first portion defining a first diameter and a second thinned portion defining a smaller diameter than the first diameter. The thinned portion allows access to a non-negligible fraction of propagating mode amplitude, and a birefringent material is situated about the second portion of the cladding. One or more electrode(s) is positioned proximate the birefringent material so as to influence optical properties of the birefringent material. A current applied to the electrode, which is in the form of a helical coil in some examples, generates a magnetic and/or electric field such that the birefringent material influences at least one of phase, amplitude and polarization of light transmitted by the optical fiber

Brief Description of the Drawings

[0003] Figure 1 is a schematic view illustrating an example of an optical phase controller in accordance with aspects of the present disclosure. [0004] Figure 2 illustrates magnetic fields generated in an example optical phase controller such as that shown in Figure 1.

[0005] Figure 3 conceptually illustrates an example of a ferronematic material.

Detailed Description

[0006] In the following Detailed Description, reference is made to the

accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as top, bottom, front, back, etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense.

[0007] Wave guiding in an optical fiber is generally achieved by utilizing a core plus cladding construction where a central core region has a larger optical index of refraction than a surrounding cladding region. The index change between the core and cladding in combination with the core diameter determine the number and types of transverse optical modes that are guided within the fiber. A special class of fibers designed to support only one guided mode, known as single-mode fibers, are of particular interest for applications involving coherent light as might be generated by a laser. Coherent light propagates in a very predictable way in single-mode fibers and therefore the combination is often used in applications where wavefront manipulation and control are important. The principal wavefront parameters are phase and polarization. Devices that manipulate phase and polarization without disrupting the guided path are difficult to construct but are of particular importance since they greatly simplify system construction, reduce power loss, and improve reliability. [0008] Figure 1 schematically illustrates aspects an example of an optical phase controller in accordance with aspects of the present disclosure. The optical phase controller 100 includes an optical fiber 1 10. As noted above, optical fibers are commonly used as conduits for transmission of various signals. An optical fiber is a cylindrical waveguide that transmits light along its axis. The optical fiber 1 10 includes a core 1 12 surrounded by a cladding layer 1 14, both of which are made of dielectric (non-conducting) materials, such as silica glass. To confine the optical signal in the core 1 12, the refractive index of the core 1 12 must be greater than that of the cladding 1 14.

[0009] Coupling of light between the fiber and many other optical devices often involves significant loss of energy since the latter generally does not have the cylindrical symmetry of the fiber and, hence, significant mismatch exists between their guided modes and the fibers modes. Consequently, there is a strong incentive to fabricate optical components, both active and passive, within the basic fiber configuration. A notable such device is the rare earth doped fiber laser. When constructed in a single mode fiber, a coherent source of light with excellent wave properties is achieved. The lasing wavelength is defined by the atomic transitions of the rare earth dopants and is, therefore, invariant from device to device.

Consequently, light from several fiber lasers can be combined to achieve high power monochromatic radiation.

[0010] The coherent nature of the laser light also provides for an intriguing possibility. Consider a fiber bundle where each element is fed by an identically designed fiber laser. If the light emanating from all fiber end terminations has the exact same polarization and phase, the resulting combined beam behaves as if it originated from a larger effective cross section than that of single fibers. Larger beam cross section at the source reduces beam divergence and results in enhanced brightness at distance from the bundle. If the fiber terminations are arranged in a regular geometric array, their combined output beams form a well-defined array of sharply peaked beams (lobes), a highly desirable outcome.

[0011] An even more intriguing outcome is possible if the phases of individual emitters are varied controllably relative to their neighboring elements. A uniform (linear) variation of phase with element location within the array, for example, results in the deflection of the lobes array. If the relative phases are varied while maintaining their spatial order, the lobes move in the "far field," that is they are scanned. The ability to move beam directions in this fashion is known as phased array scanning. Such systems are highly sought after due to their significant brightness and speed advantage over mechanically scanned beams using mirrors.

[0012] Practical implementation of optical phased array scanners have not hitherto been realized due to substantial difficulties in phase control of large element arrays. This disclosure describes example embodiments that provide a novel approach to precise and agile in-fiber phase controllers with important practical applications including optical phased arrays. The disclosed approach relies on the generation of a uniform axial stimulus to modify the index of refraction of the fiber cladding layer. In that way, phase retardation can be achieved without affecting the polarization. Since the impact of index change is small, the system must be configured to allow for a long interaction length compared to the light wavelength. In some

embodiments, magnetic field is the choice for the stimulus as uniform axial fields can be readily generated inside a solenoid shaped electrode. Liquid crystals (LC) exhibit large index changes as their molecules are rotated relative to the direction of light propagation. To achieve fast rotation of LC with a magnetic field, dopant particles must be added to create a ferronematic (FN) composite. Ferronematics are discussed further herein below.

[0013] Phase, polarization, and amplitude of guided light may be manipulated in optical fibers by manipulating the effective index of refraction that the evanescent portion of the light experiences. Referring again to Figure 1 , the cladding layer 1 14 of the fiber 1 10 includes a first portion having a first diameter Di, and a second, thinned portion 120 that has a reduced diameter D¾. The provision of the thinned portion 120 allows access to a non-negligible fraction of propagating mode amplitude. A birefringent electro-optic material 130 is situated about around the reduced cladding diameter fiber 120 and anchored to it using, for example, surfactants.

[0014] An electrode 140 is positioned proximate the birefringent material 130 so as to influence optical properties of the birefringent material 130. Various electrode arrangements and the associated influence on optical properties such as phase, amplitude and polarization are possible. For example, in the example shown in Figure 1 the electrode 140 is configured as a helical coil 142 that surrounds the thinned portion 120 of the fiber cladding 1 14 and at least a portion of the birefringent material 130.

[0015] In some examples, the birefringent material 130 is a liquid crystal (LC). LCs are desirable electro-optic material due to their large birefringence. LCs have been extensively developed for and are in common use in flat panel displays, for example. Their molecular structure is optimized for switching response to external electric fields. While there is also a small magnetic response from the conventional LCs, the effect is some ten million times smaller as compared to the electric response.

[0016] There is a significant problem with electrical switching of LC for known in- fiber phase controllers. The ideal orientation of the switching field is along the guided dimension of the fiber. Since the electrodes need to be placed on the outside of the optically active region, it is not possible to create a radially uniform field along the fiber. Non-uniform electric field leads to non-uniform switching of the LC which in turn leads to non-trivial changes in the wave front polarization when only phase modulation is intended.

[0017] A uniform magnetic field can readily be generated within the core region of a solenoid parallel to its axis of symmetry, as shown in Figure 2. However, as was mentioned earlier, torque response of conventional LCs to external magnetic fields is very weak. In order to achieve the desired performance for an in-line phase control element, Figure 3 illustrates a type of LC 150 with increased sensitivity to magnetic excitation. Magnetic response of LCs can be enhanced when doped with certain types of magnetic particlesl 50, as shown schematically in Figure 3.

[0018] Composite LCs such as the LC 150 shown in Figure 3 are referred to as ferronematics (FN). Magnetic particle dopants with small switching fields, large saturation magnetization, and large shape anisotropy are preferred. Electrodeposited nickel-iron (NiFe) alloy particles, for example, have been found to function well as FN dopant. For example, a 50%-50% Ni-Fe composition alloy provides large magnetic moment and soft switching properties for constructing FN composites. Using porous anodized aluminum forms, nearly cylindrical shaped Ni-Fe particles with diameters in the 15-30 nm and lengths in the 75-150 nm can be produced.

Conventional liquid crystals such as E7 can doped with NiFe particles to form a very sensitive FN material, retaining the large birefringence of the native LC while achieving strong linear torque with the applied magnetic field.

[0019] As noted above, some disclosed embodiments of the in-line fiber phase controller 100 have a single mode optical fiber where a section of the fiber has a thinned cladding layer such that a non-negligible portion of the optical mode reaches beyond the cladding diameter. The fiber cladding thickness can be reduced, for example, by chemical etching of the outer portions of the glass cladding 1 14 while monitoring the insertion loss of that section of the fiber. In some implementations, the glass cladding 1 14 is etched using hydrochloric acid. Thinning is then stopped when a certain insertion loss is achieved. The previous effect can be amplified if the fiber is bent to exaggerate the loss. The thinned cladding section 120 is surrounded by FN material used as variable index medium. The outer layer of the thinned cladding section is treated to establish hometropic anchoring of the LC molecules so that the director of LC in its relaxed state is generally pointed in the radial direction as defined by the fiber. The helical coil 142 is placed on the outside of the FN+fiber combination as shown in Figure 1.

[0020] Application of the electric current to the coil 142 generates the uniform magnetic field that exerts torque on the FN. The resulting rotation of the LC molecules increases the effective index of the cladding layer 1 14 experienced by the light propagating within the fiber. The light phase front, as observed at any point beyond the section of the fiber described above will be retarded by an amount proportional the strength of the current flowing in the electrode. Optical phase control is, therefore, achieved

[0021] Various modifications and alterations of this disclosure may become apparent to those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that the scope of this disclosure is not to be unduly limited to the illustrative examples set forth herein.

Claims

What is claimed is:

1. A system for manipulating guided light, comprising:

an optical fiber including a core surrounded by a cladding;

the cladding having a first portion defining a first diameter and a second portion defining a second diameter less than the first diameter, such that the second portion allows access to a non-negligible fraction of propagating mode amplitude;

a birefringent material situated about the second portion of the cladding; an electrode positioned proximate the birefringent material so as to influence optical properties of the birefringent material.

2. The system of claim 1 , wherein the electrode generates an electric field.

3. The system of claim 1, wherein the electrode generates a magnetic field.

4. The system of claim 1 , wherein the birefringent material influences least one of phase, amplitude and polarization of light transmitted by the optical fiber.

5. The system of claim 1 , wherein the birefrigent material is a liquid crystal doped with magnetic particles so as to enhance the material response to an external magnetic field.

6. The system of claim 5, wherein the second portion of the cladding includes a surface that anchors the liquid crystal in a predefined orientation.

7. The system of claim 6, wherein the liquid crystal is anchored to an outer surface of the cladding using surfactants.

8. The system of claim 6, wherein the liquid crystal is anchored perpendicular to an outer surface of the cladding.

9. The system of claim 1 , wherein the electrode is a helical coil that surrounds the second portion of the fiber cladding and at least a portion of the birefringent material.

10. The system of claim 8, wherein the electrode is configured to receive an electric current that generates a magnetic field within the helical coil for manipulation of the birefringent material orientation relative to the core.