WO2006053444A1

WO2006053444A1 - Device and methods utilizing optical waveguide embedded blazed refractive gratings

Info

Publication number: WO2006053444A1
Application number: PCT/CA2005/001769
Authority: WO
Inventors: Chun Yang; Chang-Qing Xu
Original assignee: Mcmaster University
Priority date: 2004-11-22
Filing date: 2005-11-22
Publication date: 2006-05-26
Also published as: CN100345013C; CN1869747A

Abstract

In a broad aspect, the invention is an optical device that comprises an optical waveguide having a blazed Bragg grating such that at least a portion of the light coupled from a guided mode into radiation light beam or beams. The light outcoupled by the blazed fiber grating is focused onto a intermediate image on an intermediate image plane at a first predetermined position peculiarly depending on the wavelength of the light and the effective refractive index of the guided mode from which the light is coupled. The invention may comprise a spatial light modulator placed at the intermediate image plane which serves to control phase and amplitude components of the light beam or beams. More significantly, the invention further comprises a final focusing component which may be a lens used to focus the light beam exiting the spatial light modulator onto a final image on small area such that light intensity is increased and thereby the detection sensitivity is improved. The invention furthermore comprises a light receiver which could be a photodetector or a photodetector array used to detect the final image.

Description

Device and Methods Utilizing Optical Waveguide Embedded Blazed

Refractive Gratings

Field of the Invention

The invention relates to blazed refractive gratings embedded in optical waveguides.

Background of the Invention

In optical fiber communication and sensor systems, a blazed Bragg grating embedded in an optical waveguide is an efficient means for coupling guided light out of the optical waveguide. A refractive index grating is "blazed" if the plane of the index perturbations in the waveguide is not perpendicular to the propagation direction of the guided mode or modes. The blazed Bragg grating has many potential applications, e.g., as a wavelength monitor, as an optical spectrum analyser (OSA) , or for measuring mode power distribution (MPD) for a multimode waveguide.

The OSA is a universal and critical experimental instrument used in the fields of chemistry, physics, and bioscience. Improvements in sensitivity and resolution of the OSA can improve the ability to analyze structures and concentration of materials, enlarge the capacity and reliability of optical communication systems, and provide the ability to sense weak signals.

Up to now, the MPD of a multimode optical fiber has been very difficult to measure. Although the MPD of multimode fiber can be derived from near field or far field measurements, dependencies on refractive index profile, fiber end quality, and misalignments make near field or far field measurement methods very difficult to be applied. Besides its application in optical fiber sensors, MPD may become important in "last mile" optical communication systems in the near future, for example, to control and ensure the alignment of Semiconductor lasers to rnultimode optical fibers.

Summary of the Invention

In a first broad aspect of the invention, there is provided an optical device comprising: an optical waveguide for guiding light; an optical tap and focusing component comprising a refractive index grating for coupling at least a portion of said light into one or more radiation light beams at a location of the refractive index grating in the optical waveguide and focusing each of the one or more radiation light beams in at least one dimension at a respective location on an intermediate image plane; a final focusing component for focusing the one or more radiation light beams of the intermediate image plane in a single image on a final image plane, wherein optical signal characteristics of the guided light in the optical waveguide are determined based on the single image on the final image plane ia uee.

In an embodiment of the first aspect of the invention., the optical tap and focusing component comprises: the refractive index grating which is a blazed in-waveguide refractive index grating for coupling at least a portion of light of a first wavelength λ₁ from a first guided mode m;_t into a radiation light beam of the one or more radiation light beams; and a first focusing component for focusing said light of the first wavelength λ₁ coupled from the first guided mode m_& in at least one dimension into a corresponding intermediate image at a first location on the intermediate image plane.

In another embodiment of the first aspect of the invention, the optical tap and focusing component comprises: the refractive index grating which is a chirped and blazed in- waveguide refractive index grating for coupling at least a portion of light of a first wavelength λ₁ from a first guided mode m_k into a radiation light beam of the one or more radiation light beams, wherein said refractive index, grating has a chirp selected such that the light of the first wavelength λ₁ coupled our from said first guided mode m^ is substantially focused in at least one dimension into a corresponding intermediate image at a first location on the intermediate image plane.

In another embodiment of the first aspect of the invention, the optical device further comprises a spatial light modulator for filtering amplitude and/or phase of light beams focused onto said intermediate image plane at respective locations.

In another embodiment of the first aspect of the invention, the optical device further comprises a light receiver for receiving light focused onto the final image plane, the received light used in determining optical signal characteristics.

In another embodiment of the first aspect of the invention, the blazed in-waveguide refractive index grating is a superstructure blazed in-waveguide refractive index grating with more than one different Fourier grating component for coupling at least a portion of said light of the first wavelength λ₁ from said first guided mode m_k into one or more radiation light beams with more than one different corresponding oblique angle to the longitudinal axis of the optical fiber.

In another embodiment of the first aspect of the invention, the one or more radiation light beams is of a first wavelength λ₁ from a first guided mode m_k and is focused in at least one dimension at a first location on the intermediate image plane.

In another embodiment of the first aspect of the invention, the optical waveguide is further adapted for guiding light of a second wavelength λj, which is a different wavelength from the first wavelength λ_i, in the first guided mode m_k, and said optical tap and focusing component couples at least a portion of said light of the second wavelength λj from the first guided mode m_k into a radiation light beam of the one or more radiation light beams which is focused in at least one dimension into a corresponding intermediate image at a location on the intermediate image plane different from that of the first location.

In another embodiment of the first aspect of the invention, the optical waveguide is further adapted for guiding light of said first wavelength λ₁ in a second guided mode ma different from said first guided mode m_k, and said optical tap and focusing component couples at least a portion of said light of the first wavelength λ₁ from the second guided mode mi into a radiation light beam of the one or more radiation light beams which is then focused in at least one dimension into a corresponding intermediate image at a location on the intermediate image plane different from that of the first location.

In another embodiment of the first aspect of the invention, the optical waveguide is an optical fiber.

In another embodiment of the first aspect of the invention, the optical fiber is a multimode optical fiber optical fiber. In another embodiment of the first aspect of the invention, the optical fiber is a single-mode optical fiber.

In another embodiment of the first aspect of the invention, optical waveguide is any one of a group consisting of : a planar waveguide, a channel waveguide or a ridge waveguide

In another embodiment of the first aspect of the invention, the optical tap and focusing component furuher comprises coupling means that operate in co-operation with the optical waveguide to substantially eliminate a cladding mode in the optical waveguide in a portion of the optical waveguide that includes a blazed in-waveguide refractive index grating.

In another embodiment of the first aspect of the invention, the coupling means comprises a member that is substantially transparent for light at wavelengths of interest.

In another embodiment of the first aspect of the invention, the spatial light modulator is an adjustable amplitude and/or phase filter,

In another embodiment of the first aspect of the invention, the spatial light modulator is a Blit having an adjustable aperture.

In another embodiment of the first aspect of the invention, the slit is moveable within the intermediate image plane.

In another embodiment of the first aspect of the invention, said first focusing component is a lens.

In another embodiment of the first aspect o£ the invention, said lens comprises discrate appropriately shaped and positioned lens portions of the same or different focal lengths.

In another embodiment of the first aspect of the invention, said final focusing component is a lene.

In another embodiment of the firsc aspect of the invention, said lens comprises discrete appropriately shaped and positioned lens portions of the same or different focal lengths.

In another embodiment of the firat aspect of the invention, said light receiver is one of a group consisting of an optical fiber, an optical fiber array, a photodetector and a photodeteαtor array.

In another embodiment of the first aspect of the invention, the optical signal characteristics are any one of a group consisting of wavelength, optical power for a particular wavelength, optical power across a range of wavelengths, optical power in a particular guided mode and optical power in more than one guided mode.

In a second aspect of the invention, there is provided a method of detecting optical Bignal characteristics, the method comprising: coupling at least a portion of light being guided in an optical waveguide out of the optical waveguide as one or more radiation light beams using a refractive index grating, the one or more light beams being coupled out of the optical waveguide at a location of the refractive index grating; focusing each of said one or more light beams in at least one dimension at respective locations on an intermediate image plane,- focusing the one or more light beams of the intermediate image plane in a single image on a final image plane; detecting optical power of the one or more focused light beams at the final image plane. In an embodiment of the second aspect of the invention, the coupling at least a portion of the light being guided in the optical waveguide comprises coupling a single wavelength having multiple guided πiodes.

In an embodiment of the second aspect of the invention, the coupling at least a portion of the light being guided in the optical waveguide comprises coupling multiple wavelengths, each having a single guided mode.

In an embodiment of the second aspect: of the invention, the method further comprises filtering the one or more light beams at the intermediate image plane with a spatial light modulator.

In an embodiment of the second aspect of the invention, filtering comprises modulating the amplitude and/or phase of the light beam,

In another embodiment of the second aspect of the invention, the optical signal characteristics are any one of a group consisting of wavelength, optical power for a particular wavelength, optical power across a range of wavelengths, optical power in a particular guided mode and optical power in more than one guided mode.

In an embodiment of the second aspect of the invention, the method further comprises: the one or more focused light beams are at least two guided modes coupled out of the optical waveguide each having a first wavelength, determining the first wavelength from the detected optical power of an interference pattern formed by interference of the at least two modes focused at the final image plane.

In an embodiment of the second aspect of the invention, the method further comprises: wherein the one or more focused light beams are light beams of one or more guided modes coupled our of the optical waveguide having a first wavelength, determining the modal power of a first focused light beam of the one or more focused light beams by filtering the remaining one or more focused light beams at the intermediate image plane in a manner that only the first focused light beam is focused and detected at the final image plane.

In an embodiment of the second aspect of the invention, the method further comprises: wherein the one or more focused light beamø are light beams of one or more wavelengths of a. first guided mode coupled out of the optical waveguide, determining the modal power of a of a first focused light beam having a first wavelength by filtering the remaining one or more focused light beams at the intermediate image plane in a manner that only the first focused light beam is focused and detected at the final image plane.

In an embodiment of the second aspect of the invention, filtering the remaining one or more focused light beams at the intermediate image plane comprises: using a moveable adjustable-aperture slit located at the intermediate image plane to allow the first focused light beam to continue beyond the intermediate image plane and effectively block the remaining one or more focused light beams..

In an embodiment of the second aspect of the invention, the method further comprises: determining an optical power or wavelength in at least one mode of the light being guided in an optical waveguide using the detected optical power of the one or more focused light beams at the final image plane and temporal and/or positional information of a movable slit of the spatial light modulator. In a third aspect of the invention there is provided a system for determining optical signal characteristics, the system comprising: the optical device of the first aspect of the invention for detecting optical signal characteristics in a waveguide; processing means for determining the optical signal characteristics based on the single image on the final image plane.

Other aspects and features of the present invention will become apparent to chose ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures .

Brief Description of the Drawings

Preferred embodiments of the invention will now be described with reference to the attached drawings in which:

Fig, 1 is a schematic diagram of an optical device according to an embodiment of the invention, including a blazed grating and a first focusing component;

Fig, 2 is a schematic diagram of an opbical device according to another embodiment of the invention, including a chirped and biased grating;

Fig. 3 is a schematic diagram of an optical device according to yet another embodiment of the invention, including a blazed grating and multimode optical fiber;

Fig. 4 is a schematic diagram of an optical device according to a further embodiment of the invention, including a blazed grating and single-mode optical fiber;

Fig. 5 is a schematic diagram of an optical device according to yet a further embodiment αf the invention, including a single-mode optical, fiber and a superstructure blazed grating; and

Fig. e is a flow chart for a method of detecting optical signal characteristics according to an embodiment of rhe invention.

Detailed Description of the Preferred Embodiments

Fig. 1 schematically depicts an exemplary optical device provided by an embodiment of the invention. An optical waveguide 11 is shown comprising a core 13 and a cladding 15. A blazed refractive index grating 17 is formed in the optical waveguide 11. An optical coupler 19 is located adjacent to the optical waveguide 11 in the region of the grating 17. A first focusing component 21 ie located near the opbical coupler 19, but farther way from the optical waveguide 11. The grating 17 within the optical waveguide 11, the optical coupler 19 and the first focusing component 21 collectively form an optical tap and focusing component 20. A spatial light modulator 23 is located at an intermediate image plane 25. The intermediate image plane 25 is located farther away from the optical waveguide 11 in the same direction as the first focusing component: 21. A final focusing component 27 is located on the opposite side of the intermediate image plane 25 from that of the firsr focusing component 21. A light receiver 29 is located on the opposite side of the final focusing component 27 from that of the intermediate image plane 25 at a final image plane

28.

In the illustrated example, the optical waveguide 11 is a conventional silica-based optical fiber, and the grating 17 is written into the fiber using a phase mask or a holographic method. The grating 17 is selected to couple light of a predetermined wavelength range from a guided mode or modes into a radiation mode or modes. Guided modes are the propagating modes in the waveguide. The guided mode in a single mode conventional optical fiber for example is the LP₀₁ mods . Non-guided modes are modes other than a guided mode, for example a cladding mode or a radiation mode. Radiation modes or a radiation light beam are optical modes/light beam that are not completely localised to the waveguide structure. Radiation modes spread away from the waveguide structure, such that at some point along the length, of the waveguide there is an arbitrary small amount of optical power located in the waveguide structure.

The optical coupler 19 aids in coupling light out of the optical waveguide 11. It is preferable to use the optical coupler 19, in optical co-operation with the optical waveguide 11, to reduce the effect of cladding; modes in the cladding 15 of the optical waveguide 11 as a guiding structure. Cladding modes are the optical modes of the waveguide structure that have an effective refractive index less than the refractive index of the cladding material of the waveguide. These modes are bound, in the sense that the optical power in these modes is always localized around the waveguide, and is not spreading out in a direction orthogonal to the propagation direction . After the light is coupled out from the optical waveguide 11, it is referred to as a radiation light beam 31. In some embodiments, an index matching medium (not shown) may be used between the optical coupler 19 and the optical waveguide 11 to improve coupling efficiency out of the optical waveguide 11.

The first focusing component 21 focuses the radiation light beam 31 output from the optical coupler 19 into an intermediate image 33 in at least one dimension at a predetermined location in the intermediate image plane 25, where the spatial light modulator 23 is located. The spatial light modulator is used to modulate or filter amplitude and/or phase components of lighr beam 31. The location of the intermediate image 33 in the intermediate image plane 25 depends on the wavelength of the light coupled out of the optical waveguide 11 and an effective refractive index, of the guided mode from which the light is coupled.

The radiation light beam 31 that is modulated and/or filtered by the spatial light modulator 23 is output to the final focusing component 27.

The final focusing component 27 focuses the radiation light beam 31 output from the spatial light modulator 23 into a single final image 35 within a localized area on the final image plane 28. Focusing the radiation light beam 31 into a single final image 35 increases the intensity of the final image for detection, thereby improving detection sensitivity. The light receiver 29 serves to receive the radiation light beam 31 focused by the final focusing component 27. The focused light can then be utilised in some manner. For example, in some embodiments the light is detected by an optical detector to measure optical power intensity.

In Fig. 1, the optical waveguide 11 is any structure that is capable of guiding light. For instance, a silica optical fiber, a polymer optical fiber, an integrated optical waveguide, a planar optical waveguide, a channel waveguide or a ridge waveguide. The core of the waveguide may be circular, rectangular, elliptical, single layer, multi-layer, single section, multi-section, ring or multi-ring. These are but examples of types of cores that might be found in wavelengths and are not meant to limit the invention.

in some embodiments, the optical waveguide 11 is a multimode waveguide. If light of wavelength λ is coupled out from one guided mode m_k and light of the same wavelength is coupled out from at least another guided mode m_l with a different effective refractive index from that of guided mode m_k, the different modes are focused into intermediate images at different locations on the intermediate image plane 25 and interfere at the final image 35. In such an example, the wavelength λ can be determined from an interference pattern at the final image 35.

In some embodiments, the optical waveguide 11 is a single-mode waveguide. The blazed grating 17 may comprise at least two discrete Fourier grating components. Light of wavelength λ_i coupled out from optical waveguide 11, is focused into intermediate images at different locations on the intermediate image plane 25 corresponding to respective Fourier grating components, which interfere at the final image 35.

In some embodiments, the optical coupler 19 is omitted if the blazed grating is selected to couple a guided mode to a radiation mode without need of the optical coupler 19. For example, if the blazed angle of grating 17 in the waveguide 11 is large enough to couple the guided mode to the radiation mode without the optical coupler 19.

In embodiments where the optical coupler 19 is used, the optical coupler 19 is substantially transparent to light at a wavelength of interest so ap to avoid any undue attenuation of light being coupled out of the optical waveguide 11.

In aome embodiments, the intermediate image plane 25 is located at the focal plane of the first focusing component 21.

In some embodiments, the first focusing component 21 is a lens with appropriate shape and focal length. In some embodiments, the first focusing component 21 consists of discrete appropriately shaped and positioned lenses of different focal lengths. In some embodiments, the first focusing component 21 consists of a jingle cylindrical lens.

In some embodiments, the final focusing component 27 is a lens with, appropriate shape and focal length. In some embodiments, the final focusing component 27 consists of discrete appropriately shaped and positioned lenses of different focal lengths.

Implementation of the spatial light: modulator 23 used for performing filtering and/or modulating can take many different forms. In some embodiments, the spatial light modulator 23 is a static modulator. In some embodiments, the spatial light modulator 23 is a real-time modulator. In some embodiment;s, the spatial light modulator 23 is a slit. In some embodiments, the spatial light modulator 23 is an aperture that is adjustable or configurable to have an appropriate shape. Further components that can be used to implement the spatial light modulator 23 include, but are not limited to, an optical wedge, a light dispersive means, or an electro-optic phase modulator such as a liquid crystal display (LCD) or a digital mirror device (DMD) .

In some embodiments, the spatial light modulator 23 is omitted from the optical device. In the case of a single wavelength guided in two modes of an optical waveguide, light beams coupled from the two modes and having an equal optical power result in two separate images at the intermediate image plane. If no filtering of the two intermediate image plane images is performed at the intermediate image plane by a spatial light modulator then the single final image that is focused on the final image plane by the final focusing component is an interference pattern of the two intermediate image plane images. The interference pattern will allow the wavelength to be determined based on the fact that fringes of the interference pattern are separated by an amount proportional to one wavelength. Other factors that may be involved in the proportionality of distance between fringes of the interference pattern and the wavelength are effective magnification of the first focusing component 21 and the final focusing component 27 in the optical device and the respective effective indices of refraction for each of the two guided modes. Therefore, if the optical power of the two beams is the same resulting in a highest visibility interference pattern is be obtained on the final image, there is no need to use a spatial light modulator. Equal power in two modes can be realized by a fixed excitation condition when light was launched into the two-mode waveguide.

Examples of different implementations of the light receiver 29 are photodetectors, photodetector arrays, optical fibers, an optical fiber guiding light to a detector, and an optical fiber used to multiplex/demultiplex the received light. In some embodiments, the light receiver may be a camera, such as a CCD (charged coupled device) camera.

Turning now once more to Fig. 1, the final focusing component 27 focuses light exiting the spatial light modulator 23 into the final image 35 within a small localized area on a final image plane 28, such that light intensity is high for detection and thereby high sensitivity is obtained. This is advantageous because the amplitude of optical power coupled out from the waveguide may be only a small percentage of the amplitude of optical power originally propagating in the waveguide, which is typically in the range of nanowatts to milliwatts, depending on an application for which the waveguide is being used.

Light coupled out from optical waveguide 11 is focused by the first focusing component 21 into a narrow strip or strips at the intermediate image plane 25. In conjunction with the final focusing component 27 and light receiver 29, the spatial light modulator 23 located at the intermediate image plane 25 can accurately determine the position of intermediate images such that the wavelength of the light beam can be derived with high resolution as will be describe below. If the intermediate images were detected at the intermediate image plane 25 by an optical photodetector or photodetector array, the positional resolution of the detected intermediate image would be limited by the size of the detector active area, for example the positional resolution would be limited to approximately 1 micron. However, by using a moveable slit having positional control on the order of nanometers to filter the intermediate image, for example mounting the slit on a piezo-electric translation stage, it is possible to have improved positional resolution for wavelength, determination at the intermediate image plane, which is detected at final image plane 28 by the light receiver 29,

For example, in the case of a single mode optical waveguide propagating multiple wavelengths, where the spatial light modulator 23 is a movable slit with a slit shape matching the narrow strip of the focused radiation light beam, the movable slit of the spatial light modulator; 23 can be scanned across the intermediate image plane 25. Different wavelengths are focused at different locations on the intermediate image plane. When positional information of the moveable slit from the scanning process is coordinated with measurement information, such as amplitude or intensity information from the light receiver 29, a location of the narrow strip on the intermediate image plane 25 can be accurately measured so that high wavelength resolution is obtained.

In another example as described in more detail above, light of a single wavelength λ coupled out from two guided modes with different effective refractive indices interfere at the final image 35. The interference pattern which is received by the light receiver 25 can be used to further increase the wavelength resolution.

The spatial light modulator 23 can be used to modulate or filter the amplitude and/or phase components of the radiation light beam that is produced by the first focusing component 21 at the intermediate image plane 25. As described above, the interference pattern with a best visibility ia obtained when the amplitude of the two beams is almost equal. However, the power of the two beams may be different. A spatial light modulator capable of amplitude modulation can modulate the amplitude component of one of the beams to improve the interference visibility. In another example, if the power of the two beams is equal, but it is desirable to shift the position of the interference pattern, a spatial light modulator capable of phase modulation can modulate the phase component of one of the light beams and allow the fringes of the interference pattern to move with the phase change. This is advantageous for shifting a linear and sensitive segment of the fringes (rising and falling edge, not peak and valley, because the differential is very small at peaks and valleys) to an appropriate position. The phase of the light beams can be adjusted by a spatial light modulator made of liquid-crystal array or matrix.

As can be seen from the above description, the arrangement in Fig. 1 has both high sensitivity and high wavelength resolution.

In some embodiments, an optical chopper can be place at any position along the light beam outaide the optical waveguide 11. Such an optical chopper can be used to modulate the amplitude of the light beam so as to further increase sensitivity of detection.

Additional embodiments are described with respect to Figs . 2 to 5 .

Referring to Fig. 2, an arrangement is shown that is eimilar to that of Pig, l, except for two main differences. One main difference is that there is no discrete first focusing component 21 in the arrangement of Pig, 2. Another difference is that the blazed refractive index grating 17 in Fig. 1 is now a chirped and blazed grating 43 in Fig. 2. A refractive index grating is "chirped" if the repeat distance Λ of the index perturbations is not constant as a function of the longitudinal axial co-ordinate z of the optical waveguide, i.e., if Λ=Λ(Y) , Light in one guided mode is coupled out by the chirped and biased grating 43 and brought to a focus in at least one dimension outside of the optical coupler 19 into an intermediate image at a first location on the intermediate image plane 25. Applications and functions realized by the arrangement in Fig. 1 can also be realised by the arrangement in Fig. 2. Working collectively, the chirped and blazed, grating 43 and the optical coupler 19 form the optical tap and focusing component 20.

AB seen from Pigs. 1 and 2 the purpose of the optical tap and focusing component 20 ia to couple light out of the optical waveguide 11 and focus it at a particular location. It ia to be understood that Figs. 1 and 2 provide specific examples of two different combinations of elements that can be combined to produce this effect, but it is to be understood that the scope of the invention is not to be limited to these particular examples. Fig. 3 allows another arrangement similar to that of the arrangement in Fig. 1. The optical waveguide is a conventional multimode optical fiber 8 comprising a core io and a cladding 9. A biased refractive index grating 17 is formed in the optical waveguide 8. An optical coupler 49 is located adjacent to the optical waveguide 8 in the region of the grating 17. An index matching medium 47 is located between the optical coupler 49 and the optical waveguide 8 to improve coupling efficiency out of the optical waveguide 8.

Convex lens 51 is a first focusing component as described in Fig. 1 and is located near the optical coupler 49, but farther way from the optical waveguide 8. The grating 17 within the optical waveguide 8, the optical coupler 49 and convex lens 51 collectively form the optical tap and focusing component. The spatial light modulator 23 is located at intermediate image plane 26. The intermediate image plane 26 is located farther away from the optical waveguide 8 in the same direction as convex lens 51. Convex lens 53 is located on the opposite side of the intermediate image plane 26 from that of convex lens 51. The light receiver 29 is located on the opposite side of convex lens 53 from that of the intermediate image plane 26 at a final image plane 28.

In operation, the components of Fig. 3 that are found in Fig. 1 perform essentially the same as those of Fig. 1 and as such will not be described again in detail.

The combination of the grating 17, optical coupler 49 and the index matching medium 47 couple light out of the optical waveguide 8. The optical coupler 49 and the index matching medium 47 aid in eliminating the effects of the waveguide cladding 9 as a guiding structure, and allow light that otherwise would have been coupled into a cladding mode or modes by the blazed grating 17 to propagate away from the optical fiber 8. Index matching medium 47, has a refractive index chosen to be at or slightly above the refractive index of the cladding 9.

In some embodiments, convex lens 51 is a lens having one focal length and the intermediate image plane 26 is coincidentally located at the focal plane of the lens. Convex lens 53 focuses light beams exiting the spatial light modulator 23 into the final image 35 at the final image plane 28. In some embodiments, convex lens 53 is a lens having one focal length and the intermediate image plane 26 and final image plane 28 are coincidentally located at the respective focal planes on each side of the lens.

Numeral 55 indicates a light beam of wavelength λ_i, coupled out from guided mode m_k. Numeral 57 indicates a light beam of wavelength λ_i. coupled out from guided mode m_l. The effective refractive index of the two guided modes m_k and m_l are different. Light beam 55 is focused into an intermediate image 56 on the intermediate image plane 26. Light beam 57 is focused into an intermediate image 58 at a different location on the intermediate image plane 26 from that of intermediate image 56.

While Fig. 3 is described with regard to a single wavelength and multiple guided modes, it is to be understood that used of the device is not limited to this particular example. Multiple wavelengths each having multiple guided modes αould be analyzed with such a device. However, the process does become somewhat more complicated. An example of an optical waveguide propagating a narrow wavelength range having two guided modes will be briefly described. If the refractive indices of the two guided modes are large enough, the intermediate images corresponding to the two modes are located at separate locations on the intermediate image plane. Using two separately controlled slits located at the intermediate image plane, such that each slit is localized to a respective intermediate image, each slit will allow one wavelength exit the slit. The light beams exiting the two slice are focused onto to a single final image. Only, if the two sure select beams of the same wavelength can the interference pattern appear on the final image plane. The position of the two slits and the resulting interference pattern can be used to determine the exact wavelength of the light beams. The position control of the two slits should be very accurate to enable the interference pattern at the final image plane to determine the wavelength measurement with high resolution.

In some embodiments, index matching medium 4-7 is Cargill oil. However, Cargill oil is not the only suitable index matching medium 47. In some embodiments, other index matching mediums are used. For instance, index-matching epoxy can be used to additionally bond optical coupler 49 to the optical fiber 8.

In some embodiments, optical coupler 49 is a block of material with a refractive index at or slightly different from index matching medium 47 and transparent to wavelengths of interest. For instance, in some embodiments optical coupler 49 is a glass prism, such that the light beam exits from face 50 of the block.

The optical device of Fig. 3 has many different applications wherein specific spatial light modulators 23 and light receivers 29 are used. For instance, in an optical device for measuring the mode power distribution (MPD) of multimode optical fibers, optical fiber S is a multimode optical fiber that has n guided modes with different effective refractive indices from each other. Light beams coupled out from the n guided modes are focused at corresponding intermediate images 56,58 on the intermediate image plane 25 at n different locations.

In some embodiments, the spatial light modulator 23 is a movable slit with a shape matching the shape of an intermediate image. By scanning the movable slit of spatial modulator 23 across the intermediate image plane 26, the light beam from each guided mode can be selectively filtered before exiting the spatial light modulator 23. Therefore, optical power in the light beam guided from each mode can be separately received by light receiver 29. The spatial light modulabor 23 is not limited to the implementation of a scanning slit. Other shapes in addition to a slit that have known filtering characteristics can also be used by the spatial light modulator 23. In other embodiments, the spatial light modulator 23 is an opaque plate with one edge matching the shape of the intermediate image 56,58.

In another application, as an optical spectrometer, light beams coupled out from at least two guided modes are focused at corresponding intermediate images 56,58. Light beams exiting the intermediate image plane 26 are focused and interfere at the final image 35. The interference pattern and optical power can be received by light receiver 29. In some embodiments, the spatial light modulator 23 is an optical wedge to control a phase component of the light beams. In some embodiments, the spatial light modulator 23 is omitted.

AB identified above with regard to Fig. 1, in some embodiments the light receiver 29 is a photodetector. In some embodiments the light receiver 29 is a photodetector array, which is used to further increase the wavelength resolution.

Fig. 4 is another embodiment provided by the invention based on the optical device of Fig. 1, wherein the optical waveguide is a single-mode optical fiber 34. The single-mode optical fiber 34 is shown comprising a core 37 and a cladding 32 , A blazed grating 17 is formed in the fiber 34. Other than the optical waveguide being a single-mode fiber instead of a multimαde fiber, the structure of the example of

Fig. 4 is essentially the same as the example of Fig. 3.

Whereas Fig. 3 is used to describe the invention in relation to light of the same wavelength (λ_i) being coupled out of different modes (m_k and m_l) of the optical waveguide, Fig. 4 will be used to describe the invention in relation to light of the different wavelengths (λ_i, and λ_j) being coupled out of the optical waveguide.

Numeral ai indicates a light beam of wavelength λ_i coupled out from the optical fiber 34. Numeral 83 indicates a light beam of wavelength λ_j, which is a different wavelength than λ_i, coupled out from the optical fiber 34. The location of the intermediate image on the intermediate image plane 26 depends on the wavelength. Light beam 81 is focused onto an intermediate image 82 on intermediate image plane 26. Light beam 33 is focused onto an intermediate image 84 at a different location on intermediate image plane 26 from that of intermediate image 82.

The optical device of Fig. 4 has many different applications wherein specific spatial light modulators 23 and light receivers 29 are used. For instance, as an optical spectrometer, light in a predetermined wavelength range is guided in the optical fiber 34. Light beams of different wavelength coupled out from the optical fiber 34 are focused onto corresponding intermediate images on intermediate image plane 26 at different locations. In some embodiments, the spatial light modulator 23 is a movable slit with a shape matching the shape of the intermediate image. By scanning the movable slit of spatial light modulator 23 across intermediate image plane 26, light beams of different wavelength can. be selectively filtered before exiting the spatial light modulator 23. Therefore, optical power in the light beam of specific wavelengths can be separately received by light receiver 29. The spatial light modulator 23 is not limited to the scanning slit. For example, in some embodiments the spatial light modulator 23 is an opaque plate with one edge matching the shape of the intermediate image.

In another application, as an optical spectrometer, light of a single wavelength λ is guided in the optical fiber 34. The spatial light modulator 23 is an appropriately shaped aperture that results in optical power in the light beam exiting the spatial light modulator 23 dependent upon the wavelength λ. The light beam exiting the spatial light modulator 23 is focused by the final focusing component 53 and received by a light receiver 29. The output of the light receiver 29 can be used to determine the wavelength λ.

Fig. 5 is another embodiment provided by the invention baaed on the optical device of Fig. 1, which includes a single-mode optical fiber 34 and a superstructure blazed grating 70. The single-mode optical fiber 34 is shown comprising a core 37 and a cladding 32. The waveguide may be other than a single-mode fiber, for example the waveguide may be a single-mode planar waveguide. A superstructure blazed grating 70 that comprises at least two discrete Fourier grating components is formed in the optical fiber 34. The remaining physical components in Fig. 5 are the same as those in Fig. 4 and operate in a similar manner. The location of an intermediate image on the intermediate image plane 26 depends on the wavelength of the light and the Fourier component: of the grating 70 through which the light is coupled out from the optical fiber 34, In the exemplary case of Fig. 5 the different intermediate images are located at oblique angles to the longitudinal axis of the optical fiber.

Numeral 65 indicates a light beam of wavelength λ coupled out through a first Fourier grating component of the superstructure grating 70. Numeral 67 indicates a light beam of wavelength λ coupled out through a second Fourier grating component of the superstructure grating 70. Light beams exiting the spatial light modulator 23 are focused and interfere at the final image 35. The interference pattern and optical power can be received by light receiver 29. In soma embodiments, the spatial light modulator 23 may be an optical wedge to control phase of the light beams. In some embodiments, the spatial light modulator can also be omitted.

A method for detecting optical signal characteristics of a signal propagating in a waveguide will now be described with regard to Fig. 6. Optical signal characteristics that may be detected are one or more of: wavelength of the signal, optical power of a particular wavelength, optical power across a range of wavelengths, optical power in a particular guided mode and optical power in more than one guided mode.

A first step 610 of the method includes coupling at least a portion of light being guided in an optical waveguide out of the optical waveguide ae a radiation light beam using a refractive index grating. The light beam being coupled out of the optical waveguide is coupled out of the optical waveguide at the location of the refractive index grating, A second step 620 includes focusing the light beam in at least one dimension at a location on an intermediate image plane. A third step 630 includes focusing the light beam of the intermediate image plane in a single image on a final image plane. A fourth step 640 includes detecting optical power of the focused light beam. In. some embodiments, the coupling of light out of the optical waveguide includes coupling out a single wavelength having multiple guided modes. In some embodiments, the coupling of light out of the optical waveguide includes coupling out multiple wavelengths, each having a single guided mode.

In some embodimente, the method of includes filtering the light beams at the intermediate image plane, for example between steps 620 and 630. A spatial light modulator such as that described above can be used to perform the filtering.

Implementations of a filtering step may include scanning a movable slit across the intermediate image plane or adjusting the amplitude and/or phase of the radiation light beam at the intermediate image plane as described in detail above. However, in some embodiments filtering is not performed and optical signal characteristics are be determined froτn the resulting unfiltered single image detected at the final image plane.

In some embodiments, detecting the optical power of the focused light beam of step 640 involves measuring the optical power.

The method may by used with any of the optical devices described in Pigs. 1-5. More generally, the method can be used with optical devices having functionality capable of implementing the steps.

In some embodiments, following detecting the optical power of the focused light beam at step 640, additional processing of the detected optical power in conjunction with other information such as temporal and/or positional information of the movable slit of the spatial light modulator is performed to determine particular optical signal characteristics of the light coupled from the optical waveguide.

The following descriptions are examples of methods for determining particular optical signal characteristics.

In a case where the one or more focused light beams are at least two guided modes coupled, out of the optical waveguide each having a first wavelength, the first wavelength is determined from the detected optical power of an interference partern formed by interference of the at least two modes focused at the final image plane,

In a case where the one or more focused light beams are light beams of one or more guided modes coupled out of the optical waveguide having a first wavelength, modal power of a first focused light beam of the one or more focused light beams is determined by filtering the remaining one or more focused light beams at the intermediate image plane in a manner that only the first focused light beam is focused and detected at the final image plane. The remaining focused light beams are blocked by a spatial light modulator at the intermediate image plane.

In a case where the one or more focused light beams are light beams of one or more wavelengths of a first guided mode coupled out of the optical waveguide, the modal power of a of a first focused light beam having a first wavelength is determined by filtering the remaining one or more focused light beams at the intermediate image plane in a manner that only the first focused light beam is focused and detected at the final image plane. The remaining focused light beams are blocked by a spatial light modulator at the intermediate image plane.

Examples of methods for measuring modal power distribution using an optical device of the type described herein can be found in "A Novel Method to Measure Modal Power Distribution in. Multimode Fibers Using Tilted Fiber Bragg Gratings", IEEE Photonics Technology Letters, Vol. 17, No. io , October 2005, "A novel merhod to measure modal power distribution in few-mode and multimode fibers using tilted fiber Bragg gratings" , Photonic Applications in Devices and Communication Systems", Proc. of SPIE Vol. 5970, and "Measurement of Modal Power Distribution in Multimode Fibers Using Tilted Fiber Bragg Gratings" Pacific Rim Conference on Lasers and Electro-Opcics 2005 (IQEC/CLEO-PR) , Tokyo, Japan, July 11-15, 2005, pp,1074-1075, all of which are incorporated herein by reference.

In some embodiments, the optical devices of Figs. 1-5 may operate in combination with a device that provides processing power to utilize and display results detected and measured by the optical devices . The processing power required can be provided by hardware means and/or software means. Implementation of the processing power can be a processing element. Examples of the processing element are an application- specific integrated circuit (ASIC) , a microprocessor with hardwired digital logic or a digital signal processing chip that can perform mathematical calculations based on algorithmic code stored in a computer readable memory. These examples are not meant to limit the invention, but to provide examples of elements used in some embodiments for determining desired results. The optical devices of any one of Figs. 1-5 and the processing power described above together form a system for detecting optical signal characteristics in an optical waveguide by processing the detected image on the final image plane.

Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised, otherwise than as specifically described herein.

Claims

CL AIMS :

1, An optical device comprising:

an optical waveguide for guiding light;

an optical tap and focusing component comprising a refractive index grating for coupling at least a portion of said light into one or more radiation light beams at a location of the refractive index grating in the optical waveguide and focusing each of the one or more radiation light beams in at least one dimension at a respective location on an infcenrmediate image plane;

a final focusing component for focusing the one or more radiation light beams of the intermediate image plane in a single image on a final image plane,

wherein, optical signal characteristics of the guided light in the optical waveguide are determined based on the single image on the final image plane is use.

2. The optical device of claim 1, wherein the optical tap and focusing component comprises:

the refractive index grating which is a biased in- waveguide refractive index grating for coupling at least a portion of light of a first wavelength λι from a first guided mode mjc into a radiation light beam of the one or more radiation light beams; and

a first focusing component for focusing said light of the first wavelength λ_i coupled from the first guided, mode m_k in at least one dimension into a corresponding intermediate image at a first location on the intermediate image plans.

3. The optical device of claim 1, wherein the optical tap and focusing component comprises:

the refractive index grating which is a chirped and blazed in-waveguide refractive index grating for coupling at least a portion of light of a first wavelength λ.₁ from a first guided mode m_k into a radiation light beam of the one or more radiation light beams, wherein said refractive index grating has a chirp selected such that the light of the first wavelength λ₁ coupled out from said first guided mode m^ is substantially focused in at least one dimension into a corresponding intermediate image at a first location on the intermediate image plane.

4. The optical device of any one of claims 1-3, further comprising a spatial light: modulator for filtering amplitude and/or phase of light beams focused onto said intermediate image plane at respective locations.

5. The optical device of any one of claims 1-4, further comprising a light receiver for receiving light focused onto the final image plane, the received light, used in determining optical signal characteristics.

6. The optical device of claim 2, wherein said biased in-waveguide refractive index grating is a superstructure blamed in-waveguide refractive index grating with more than one different Fourier grating component for coupling at least a portion of said light of the first wavelength λi from said first guided mode m_k into one or more radiation light beams with more than one different corresponding oblique angle no the longitudinal axis of the optical fiber.

7. The optical device of claim 1, wherein the one or more radiation light beams is of a first wavelength λi from a first guided mode m_k and is focused in at least one dimension at a firat location, on the intermediate image plane.

8. The optical device of claim 7, wherein the optical waveguide is further adapted for guiding light of a second wavelength λ_j, which is a different wavelength from the first wavelength λ_i, in the first guided mode 1%, and said optical tap and focusing component couples at least a portion of said light of the second wavelength λ_j from the first guided mode m_k into a radiation light beam of the one or more radiation light beams which is focused in at least one dimension into a corresponding intermediate image at a location on the intermediate image plane different from that of the first location.

9. The optical device of claim 7, wherein the optical waveguide is further adapted for guiding light of said first wavelength λ_i in a second guided mode m_l different from said first guided mode m_k, and said optical tap and focusing component couples at least a portion of said light of the first wavelength λ_i from the second guided mode m_l into a radiation light beam of the one or more radiation light beams which is then focused in at least one dimension into a corresponding intermediate image at a location on the intermediate image plane different from that of the first location.

10. The optical device of any one of claims 1-9, wherein said optical waveguide is an optical fiber.

ll. The optical device of claim 10, wherein said optical fiber is a multimode optical fiber.

12. The optical device of any one of claims 1-8, wherein said optical waveguide is a single-mode optical fiber.

13. The optical device of any one of claims 1-9, wherein said optical waveguide is any one of a group consisting of: a planar waveguide, a channel waveguide or a ridge waveguide

14. The optical device of claim 1, wherein, said optical tap and focusing component comprises coupling means that operabe in co-operation with the optical waveguide to substantially eliminate a cladding mode in the optical waveguide in a portion of the optical waveguide that includes a blazed in-waveguide refractive index grating.

15. The optical device of claim 14, wherein said coupling means comprises a member that is substantially transparent for light at wavelengths of interest.

16. The optical device of claim 4, wherein said spatial light modulator is an adjustable amplitude and/or phase filter.

17. The optical device of claim 4, wherein Baid spatial light modulator is a slit having an adjustable aperture.

18. The optical device of claim 17, wherein said slit is moveable within the intermediate image plane.

19. The optical device of claim 4, wherein said spatial light modulator is one of a group consisting of an optical wedge, a light dispersive means, and an electro-optic phase modulator.

20. The optical device of claim 2, wherein said first focusing component is a lens .

21. The optical device of claim 20, wherein said lens comprises discrete appropriately shaped and positioned lens portions of the same or different focal lengths.

22, The optical device of claim 1, wherein said final focusing component is a lens.

23. The optical device of claim 22, wherein said lens comprises discrete appropriately shaped and positioned lens portions of the same or different focal lengths.

24. The optical device of claim 5, wherein said light receiver is one of a group consisting of an optical fiber, an optical fiber array, a photodetector and a phαtodetector array.

25. The optical device of claim 1, wherein the optical signal characteristics are any one of a group consisting of wavelength, optical power for a particular wavelength, optical power across a range of wavelengths, optical power in a particular guided mode and optical power in more than one guided mode.

26. A method of detecting optical signal characteristics, the method comprising:

coupling at least a portion of light being guided in an optical waveguide out of the optical waveguide as one or more radiation light beams using a refractive index grating, the one or more light beams being coupled out of the optical waveguide at a location of the refractive index grating;

focusing each of said one or more light beams in at least one dimension at respective locations on an intermediate image plane;

focusing the one or more light beams of the intermediate image plane in a single image on a final image plane;

detecting optical power of the one or more focused light beams at the final image plane.

27. The method of claim 26, wherein the coupling at least a portion of the light being guided in the optical waveguide comprises coupling a single wavelength having multiple guided modes.

28. The method of claim 26, wherein the coupling at least a. portion of the light being guided in the optical waveguide comprises coupling multiple wavelengths, each having a single guided mode.

29. The method of claim 26 further comprising filtering the one or more light beams at the intermediate image plane with a spatial light modulator.

30. The method of claim 29, wherein filtering comprises modulating the amplitude and/or phase of the light beam.

31. The method of claim 26, wherein the optical signal characteristics are any one of a group consisting of wavelength, optical power for a particular wavelength, optical power across a range of wavelengths, optical power in a particular guided mode and optical power in more than one guided mode.

32. The method of claim 26 further comprising:

wherein the one or more focused light beams are at least two guided modes coupled out of the optical waveguide each having a first wavelength, determining the first wavelength from the detected optical power of an interference pattern formed by interference of the at least two modes focused at the final image plane.

33. The method of claim 26 further comprising:

wherein the one or more focused light beams are light beams of one or more guided modes coupled out of the optical waveguide having a first wavelength, determining the modal power of a first focused light beam of the one or more focused .ight beams by filtering the remaining one or more focused Iiglit beams at the intermediate image plane in a manner that only the first focused light beam is focused and detected at the final image plane.

34. The method of claim 26 further comprising:

wherein the one or more focused light beams are light beams of one or more wavelengths of a first guided mode coupled out of the optical waveguide, determining the modal power of a of a first focused light beam having a first wavelength by filtering the remaining one or more focused light beams at the intermediate image plane in a manner that only the first focused light beam is focused and detected at the final image plane.

35. The method of claim 33 or 34, wherein filtering the remaining one or more focused light beams at the intermediate image plane comprises .

using a moveable adjustable-aperture slit located at the intermediate image plane to allow the first focused light beam to continue beyond the intermediate image plane and effectively block the remaining one or more focused light beams.

36. The method of claim 26, further comprising:

determining an optical power or wavelength in at least one mode of the light being guided in an optical waveguide using the detected optical power of the one or more focused light beams at the final image plane and temporal and/or positional information of a movable slit of the spatial light modulator.

37. A system for determining optical signal characteristics, the system comprising: the optical device of any one of claims 1-24 for detecting optical signal characteristics in an optical; waveguide;

processing means for determining the optical signal characteristics based on the single image on the final image plane.