KR20010089449A - Optical waveguide structure - Google Patents

Abstract

The present invention relates to an optical device, the optical device comprising a waveguide structure (1) and at least one grating structure (4) formed in the waveguide structure, the grating structure is incident on the waveguide structure at a predetermined angle of incidence with respect to the grating structure Position the light of a predetermined wavelength to a selected path in the waveguide structure.

Description

Optical waveguide structure and manufacturing method {OPTICAL WAVEGUIDE STRUCTURE}

In optical waveguides it is often necessary to guide light around a bend, for example, which reduces the size of the device incorporating the optical waveguide. However, due to the refractive index characteristics of the waveguide and the material surrounding the waveguide, the light will be diffracted out of the bend, especially the abrupt bend, and thus there is an inherent problem commonly referred to as bending loss. This loss limits the performance of the device.

It is also necessary to direct light signals in different directions, for example in devices that need to confine light in a predetermined path in a waveguide in an optical filter or optical resonator structure.

FIELD OF THE INVENTION The present invention relates generally to optical devices comprising waveguides and methods of manufacturing the same.

1 is a schematic representation of an apparatus embodying the invention.

2 is a schematic representation of an apparatus embodying the invention.

3 is a schematic representation of an apparatus embodying the invention.

4 is a schematic representation of an apparatus embodying the invention.

5 is a schematic representation of an apparatus embodying the invention.

6 is an isometric view of a method of manufacturing a lattice defining waveguide embodying the present invention.

7 is an isometric view of another method of making a grating confining waveguide embodying the present invention.

8 is a cross-sectional schematic diagram illustrating an apparatus embodying the present invention.

9 is a plot of grating period versus resonance angle for a grating confined waveguide.

10 is a schematic isometric view of an apparatus embodying the present invention.

11 is a schematic isometric view of an apparatus embodying the present invention.

12 is a schematic cross-sectional view of an apparatus embodying the present invention.

Fig. 13 is a schematic isometric view of a resonator structure embodying the present invention.

14 is a schematic isometric view of an apparatus embodying the present invention.

The present invention includes a waveguide structure and at least one grating structure formed within the waveguide structure, the grating structure being arranged to guide along a selected path in the waveguide structure light of a predetermined wavelength incident on the waveguide structure at a predetermined angle of incidence with respect to the grating structure. Provide an optical device.

Since light guidance is achieved using a grating structure, a substantial reduction in bending loss can be achieved.

The waveguide structure is composed of photosensitive material, and the grating structure is constructed by UV-induced refractive index variation in the waveguide.

The present invention allows for angular dispersion to be added to the propagated light which can be controlled by the properties of the grating structure. This can be used, for example, for variance correction, pulse chirping or pulse compression. This is because different wavelengths have different angular paths for the grating structure.

Such devices can be used in complex light conditioning circuits in the spectroscopic and time domains.

The grating structure includes a chirped grating.

The grating structure is arranged to guide light in a reflective or transmissive mode.

The present invention includes at least one grating structure formed by a waveguide of a photosensitive material, a UV-induced refractive index change in the waveguide, the path selected in the waveguide for the light of a predetermined wavelength to enter the waveguide at a predetermined incident angle with respect to the grating structure It can optionally be defined by providing an optical device arranged to limit the.

Because of the angle dependence of the received wavelength in the grating confining waveguide, such a device relies on an angular sweep to isolate the wavelength or signal, for example.

The grating structure comprises a continuous grating. Optionally, the grating structure comprises two gratings that are symmetric to each other.

In one embodiment, the grating structure includes regions with a constant refractive index extending in the direction of propagation of the wavelength.

This region may extend parallel to the direction of propagation.

This region can extend cylindrically parallel to the direction of propagation.

This area may extend elliptical parallel to the direction of propagation.

Such an apparatus may further comprise at least one optical reflector positioned transverse to the direction of propagation to assist in confining light to the path.

Such a device may include two or more grating structures positioned angularly with respect to each other to channel light around a selected path.

Thus, various confining conditions can be realized at various boundaries of the waveguide.

The grating structure can be formed by UV-holography.

The grating may be a chirped grating.

The grating may be a sampled grating.

Such a device may be a filter, a resonator or a sensor.

In one embodiment, the apparatus further comprises means for measuring the intensity of the light at a given point along the selected path to determine a change in intensity due to an inductive change in the confining conditions of the sensor.

This change is induced by gas molecules entering the waveguide.

The present invention is optionally limited to providing a method of manufacturing an optical device comprising a waveguide made of photosensitive material, the method comprising forming at least one grating structure by a UV-induced refractive index change in the waveguide; The grating structure is arranged to limit light of a predetermined wavelength incident on the waveguide at a predetermined angle of incidence with respect to the grating structure to a selected path in the waveguide.

Preferred embodiments of the present invention will be described below by way of example with reference to the accompanying drawings.

Referring to FIG. 1, a first embodiment is shown schematically in which the waveguide 1 has a sharp bend in the desired path, and the light 2 is projected under the waveguide. Around the sharp bend, the grating structure 4 is located. The grating structure 4 has a photonic band gap which prevents leakage of effervescent light and thus increases the efficiency of the light coupled to the output 5. This results in a substantial reduction in bend loss by using a diffraction grating 4 which allows the sharp bends to be defined within the waveguide structure. The wavelength of the grating 4 can be tuned to match the frequency required for operation.

Optionally, as shown in FIG. 2, the grating 6 may be in reflection mode to provide reflection of the desired frequency along the path 7 with a loss 8 for frequencies that do not have the desired characteristics.

By using the arrangement of FIG. 2 it can be extended to provide wavelength division multiplexing capability on the waveguide structure. This is the waveguide bottom with multiple frequencies (λ1, λ2, λ3 in FIG. 3) coupled out from the waveguide using corresponding matched Bragg gratings 12, 13 and 14 operating to filter out the required frequencies. 3, which can be projected to.

4 shows that the light coupled along the waveguide 15 is coupled to the outputs 16, 17 by a Bragg grating 18 having the desired periodic characteristics and properly matched to the desired frequency required for coupling. The refractive index region 19 which audits the waveguide may be sharpened at the end for stronger bonding. Preferably, the splitter device of FIG. 4 has a Bragg grating coupled such that 50% of the light traverses along each path 17, 18. This can be achieved with twice the wavelength of the Brac period. Of course, the braking period can be adjusted to match the output angle and coupling efficiency.

Similarly, in FIG. 5 a black grating 20 is provided to couple around the bend for light traveling along paths 21 and 22.

In FIG. 6, a waveguide 10 in the form of a layer of photosensitive material is placed on a silicon wafer having a pure oxide layer for optical isolation of the substrate 112, for example waveguide material 110.

The UV beam 116 from the UV source 114 is focused (via the optical element 118) in the plane of the waveguide 110. Substrate 112 moves horizontally, as indicated by arrows 120 and 122, to line 124 of first grating 126 of grating structure 127 via a UV-induced refractive index change of waveguide 110. It is located in the plane shown.

After the first grating 126, the second grating 128 of the grating structure 127 is positioned by proper movement of the substrate 112.

The light incident on the waveguide 110 at a predetermined angle of incidence on the gratings 126 and 128 is limited to a path in which light of a predetermined wavelength extends in the in-plane propagation direction 130 of the waveguide 110. Therefore, the propagation characteristics of the waveguide 110 depend on the optical signal 131 and the angle θ incident on the waveguide 110.

It is noted here that the grating confinement within the planar structure described above is confined in one dimension within the plane of the waveguide 110. However, the waveguide can be made of photosensitive waveguide material in which the grating is limited to two or three dimensions.

For example, as shown in FIG. 7, the holographic UV grating arrangement using the phase mask 140 is performed by gratings 146 and 148 of the first grating structure 147 and gratings of the second grating structure 151, respectively. It is used to fabricate waveguide 142 (in the direction of propagation as indicated by arrow 141) within block 144 of photosensitive waveguide material, which is a two-dimensionally defined grating through 150, 152.

One or more grating structures of the device may optionally include a continuous grating defining light of a predetermined wavelength incident at a predetermined angle of incidence with respect to the grating structure.

For example, the resonator 250 shown in FIG. 14 may be configured to channel light 256 of a predetermined wavelength that enters the resonator 250 at a predetermined angle of incidence on the grating structure 252 around the ring path 258. Two continuous grating structures 252 and 254.

Grating confinement may be achieved, for example, by using cylindrical grating structure 320 around the guiding core 322 (which propagates in a direction perpendicular to the drawing plane) of the optical fiber 324 as shown in FIG. 12. . The grating structure 320 is limited to a path extending in the propagation direction of light of a predetermined wavelength incident on the grating structure 320 at a predetermined incident angle.

Those skilled in the art will appreciate that for non-cylindrical grating structures the limiting conditions may vary in various radial directions.

The basic principle of grating confined waveguide propagation is the Brack condition. For light rays traveling in a medium with a refractive index n, the highest reflection is when the wavelength λ satisfies the following condition:

(One)

Where m is the diffraction order of the grating and [theta] is the angle of light with respect to a single groove of the grating. This mononomial formula encompasses the overall properties of the lattice definition, such as, for example, a so-called photonic crystal optical fiber.

FIG. 8 shows a plot of grating period versus resonance angle for the wavelength region 1200-1600 nm for first, second and third order diffraction. Although effective for first order diffraction, at long periods, the change in resonance angle converges within a few degrees. Physical interpolation is the identification of diffraction characteristics with similar angles of incidence for many multiple wavelengths. Hence, grating confinement occurs with a larger bandwidth for less input coupling angles at longer periods under the same projection conditions. Radiation losses occur outside these areas.

Other important characteristics are indicated. There are other regions of incidence angle where total internal reflection occurs to allow propagation along the grating confined waveguide. Light coupled with higher diffraction orders at much larger angles of incidence also satisfies Bragg reflections, resulting in higher order bandgaps. The effective bond strength is reduced for higher order mode propagation in this area, and is thus characterized as a large mode area. Since the effective refractive indices are different, it is possible to have the basic mode behavior simultaneously with other propagation characteristics. Thus, for example, a photon optical fiber has a different projection area than a conventional effective refractive index optical fiber. This region exists because there is an angular photon bandgap in which light cannot propagate through the surrounding grating cladding. Moreover, this bandgap is robust and does not change much in angular properties with increasing periods, and thus is relatively less sensitive to bends in long periods.

Angular photon bandgap is described by the angular reflectance of the grating. This reflectance bandwidth is very small and depends on the grating size, its coupling coefficient and the angle of incidence. For normal (incident angle, θ = 90 °) or angular incidence, the reflectivity is given by the combined mode theory,

(2)

here,

(3)

K is the angle-dependent coupling coefficient for the grating, L is the length of the grating, and Δβ is the detunig of the wavevector, thus

(4)

The highest reflectance occurs for Δθ = 0 and decreases as Δθ exceeds the magnitude of K. In the grating confined waveguide, it can be seen that the acceptance angle of the reflectance becomes quite narrow with a change from the near normal incidence angle (as indicated by the decreasing slope of FIG. 8). As a result, higher order photon bandgaps are wider and less spatially selective, which implies a single mode of robustness for large input angles. The change δ (Δβ) of the detuning of the angle δθ is easily calculated:

(5)

From this sensitivity to the capture angle, it is possible to significantly change the angular dispersion by appropriately selecting the period. Because the angles of incidence are similar over long periods of time (see Figure 8), the propagation constants and thus the sensitivity to capture angles tend to converge on increasing lattice periods-thus achieving a flat dispersion profile in the form of a numerical representation. It is possible to do

Even for light guided alone under an effective refractive index image when the core refractive index is higher than the ambient cladding, the light can quickly couple to the radiation mode and leak if it does not have a resonant angle with the cladding. Moreover, this intolerance to the mode angle causes high spatial sensitivity of this angular bandgap such that single mode propagation is particularly robust for long grating periods. Therefore, the proposed mode profile is similar to the radial positioning of the grating around the core region, which must be different from conventional waveguide guidance, where this strict limitation does not exist.

By recognizing the importance of diffraction within the periodic grating, it will be appreciated that grating confinement propagation can be readily achieved in a so-called photonic crystal optical fiber. Moreover, the associated angular photon bandgap is responsible for the developmental range that distinguishes such optical fibers from conventional effective refractive index fibers. Extending the application to such fiber-optic resonators, it is expected that very interesting behavior will occur as a result of the strict vector angle of the propagation mode including the ring resonance when the end reflector is tilted. The polarization characteristics of these structures are also different from conventional resonators, and entirely new passive filters, active filters and resonators are possible.

In FIG. 9, the grating periods (which may be chirped) of the gratings 182 and 184 of the first grating structure 183 and the gratings 186 and 188 of the second grating structure 187 are different for wavelengths with different ring resonances. The resonator 181 can be used for WDM (wavelength division multiplexing) filtering if it is different and thus the output is carefully chosen to be spatial at other points. This is schematically illustrated by paths 190 and 192 and example outputs 194 and 196. Grating structures 182 and / or 187 may be sampled grating structures.

Complex designs using sampled profiles and the like can be used for WDM operation. In particular, intensity dependence means that it can have much closer peaks with higher contrast than normal normal incidence. Note that this is applicable to optical fiber (eg photonic crystal fiber) geometry.

As shown in FIG. 10, in a resonant laser design 300, a photonic crystal fiber 302 is positioned on line within a (optional) ring laser 304 to provide line width, laser stability, and mode selectivity (multi-mode active fiber). Improves both, including traversal when is used to increase output. Note that similar designs can be applied to (arbitrary) linear lasers.

As shown in FIG. 11, in an alternative embodiment, the helical ring fiber laser 310 is an optical fiber 312 having a grating confining core structure 314 and concave reflectors 315, 316 spaced within the core structure 314. ). Thus, the helical ring fiber laser 310 can provide a circular birefringent output (indicated by arrow 311).

Moreover, a high performance fiber laser is provided without using a cladding pump configuration. For such lasers, single mode operation, good reliability and large mode area are possible. In this embodiment, the mode relies on grating diffraction, unlike conventional optical fibers, which rely on aperture diffraction.

Those skilled in the art will appreciate that various modifications and / or changes to the present invention are possible as described in the specific embodiments without departing from the spirit or scope of the invention. Therefore, these embodiments are illustrative only and not intended to be limiting.

Claims (20)

Waveguide structures; And At least one grating structure formed in the waveguide structure, And the grating structure is positioned to guide light of a predetermined wavelength incident along the waveguide structure at a predetermined angle of incidence with respect to the grating structure along a selected path in the waveguide structure. The optical device of claim 1, wherein the different waveguide structure comprises a photosensitive material and the grating structure is formed by a UV-induced refractive index change in the photosensitive material. The optical device of claim 1 or 2, wherein the grating structure comprises a chirped grating. The optical device of claim 1, wherein the grating structure comprises a sampled grating. The optical device of claim 1, wherein the grating structure is positioned to guide light in a reflective mode. 6. An optical device according to any one of the preceding claims wherein the grating structure is positioned to guide light in a transmission mode. A waveguide made of a photosensitive material; And At least one grating structure formed by the UV-induced refractive index change in the waveguide, And the grating structure is positioned to limit light of a predetermined wavelength incident on the waveguide at a predetermined angle of incidence with respect to the grating structure to a selected path in the waveguide. 8. The optical device of claim 7, wherein the grating structure comprises a continuous grating. 9. An optical device according to claim 7 or 8 wherein the grating structure comprises two gratings that are symmetrical to each other. 10. The optical device according to any one of claims 7 to 9, wherein the grating structure comprises a constant refractive index region extending in the direction of propagation of the waveguide. 11. An optical device according to claim 10, wherein said region extends parallel to said propagation direction. 12. The optical device of claim 11, wherein the region extends cylindrically parallel to the propagation direction. 12. The optical device of claim 11, wherein the region extends elliptical parallel to the propagation direction. 14. The optical device according to any one of claims 7 to 13, wherein the optical device further comprises at least one optical reflector positioned transverse to the propagation direction to assist in confining light to the path. Optical devices. The optical device according to claim 7, wherein the optical device comprises at least two grating structures channeling light around the selected path and positioned angularly with respect to each other. The optical device according to any one of claims 7 to 15, wherein the grating structure is formed by UV-holography. 17. The optical device according to any one of claims 8 to 16, wherein the grating is chirped. 18. The optical device according to any one of claims 8 to 17, wherein the grating is sampled. 19. The optical device of any one of claims 7 to 18, wherein the optical device is a sensor and measures the intensity of light at a point along the selected path to measure a change in intensity due to induced changes in the confining conditions of the sensor. Further comprising means for. A method of manufacturing an optical device comprising a waveguide composed of a photosensitive material, Forming at least one grating structure by the UV-induced refractive index change in the waveguide, And the grating structure is positioned to limit light of a predetermined wavelength incident on the waveguide at a predetermined angle of incidence with respect to the grating structure to a selected path in the waveguide.