RU2712985C1 - Mode converter device - Google Patents

Abstract

FIELD: physics; manufacturing technology.

SUBSTANCE: invention relates to a device of a mode converter for connection of optical fiber with a channel waveguide of a planar optical crystal (PLC) in form of an integral element of light input from optical fiber into a channel waveguide and vice versa. Device of the mode converter comprises an optical fiber which is connected to the channel waveguide of the planar optical crystal by means of a series of waveguide segments of equal height and rectangular section with smoothly decreasing width and smoothly increasing pitch of arrangement, as well as rectangular section with smoothly decreasing width and smoothly increasing pitch of arrangement. Waveguide segments and the conical segment are arranged integrally with the planar optical crystal and provide efficient light input into the planar optical crystal channel waveguide at much shorter length of input element, than it is required for usual adiabatically expanding cone element of light input, and also other segmented input elements.

EFFECT: connection of optical fiber with a planar optical crystal with improved characteristics in terms of compactness and ease of manufacture.

1 cl, 4 dwg

Description

The device of the mode converter generally relates to optical waveguides with a channel structure, which are rectangular waveguides with a refractive index n1, surrounded on all sides by a medium with a lower refractive index n2 (n1> n2). In particular, the invention relates to a device for optical waveguides with a channel structure with low loss of coupling between optical waveguides with a channel structure and an optical fiber.

A communication element transmitting optical waves between a fiber waveguide used to transmit signals over long distances and a waveguide that is part of an optical integrated circuit is one of the key elements of integrated optical systems. The main characteristics of any communication element is its efficiency, compactness, ease of adjustment. The coupling efficiency, as a rule, is determined by part of the total energy of the optical beam introduced into or removed from the waveguide. In addition, it can be determined based on the coupling loss (in dB). The coupling efficiency very much depends on the matching between the fields of the optical beam and the wave mode.

If an optical waveguide with a channel structure is coupled to a conventional optical fiber (for example, to a standard single-mode optical fiber type SMF 28 with a weakly limited optical mode), large coupling losses appear between the planar optical waveguide and the ordinary optical fiber. In this case, the coupling loss between an optical waveguide with a planar structure and an optical fiber should be understood as the loss of electromagnetic energy at the connection between an optical waveguide with a planar structure and an optical fiber, where electromagnetic energy is transferred between the optical fiber and an optical waveguide with a planar structure. These coupling losses are due to the fact that the optical mode of an optical waveguide with a planar structure is more limited than the optical mode of a conventional optical fiber, i.e., the width at half the lateral height of the optical mode in an optical waveguide with a planar structure is less than the width at half the lateral height of the optical mode known optical fiber.

The work demonstrates an example of a classical end connection between an optical waveguide with a channel structure (with a core - “core” and a sheath - “cladding”), and a conventional single-mode optical fiber / 1 /.

A conventional optical fiber has a core that transmits electromagnetic energy along the optical fiber and which is surrounded by a sheath. The optical mode of an optical waveguide with a channel structure is more limited than the optical mode of a conventional optical fiber, and this leads to large loss of coupling between the optical waveguide with a channel structure and a conventional optical fiber. These losses can be calculated on the basis of the overlap integral between the two optical modes Ein and Eout, where Ein is the spatial distribution of the electric field in the optical waveguide with the spatial distribution of the electric field in the optical waveguide with the channel structure, and Eout is the spatial distribution of the electric field in the optical fiber.

Thus, if the optical mode of a waveguide with a channel structure has a profile that is much different from the optical mode profile of a conventional optical fiber (which can be estimated by comparing the widths at half the height of the optical modes), the overlap integral is very small, as is the electromagnetic energy transmitted between optical fiber and an optical waveguide with a channel structure.

The traditional approach to this problem is to provide the planar waveguide with an enlarged end element for receiving light from optical fiber and gradually (adiabatically) narrow the waveguide core in the lateral direction to the optimal size of a single-mode channel waveguide / 2 /.

The lateral cone (teiper) is in the same plane as the optical scheme. This approach reduces conjugation loss, but, unfortunately, such taping is inefficient for compact planar optical crystals with a large difference in the refractive indices of the core and shell (high-contrast), and, in addition, this approach assumes a significant length of the lateral cone.

The design implements a two-dimensional narrowing of the modes by introducing gaps between the waveguide segments. As a result, the effective refractive index of a segmented waveguide gradually decreases and light smoothly flows from the fiber into the planar optical crystal. This approach is very effective in reducing pairing losses. However, for high-contrast waveguides, the core thickness is small compared to the fiber core; therefore, mode matching through two-dimensional segmented sections is difficult to achieve.

In the US patent, an optical fiber is connected to a planar waveguide through a series of planar waveguide segments with an enlarged cross section and a segment with a conical cross section / 3 /. The combination of spaced segments and the conical segment provides efficient (without loss) waveguide binding to the fiber over a shorter length of the joint portion than is required for conventional adiabatic binding. However, such an approach requires the use of a shadow mask, a significant complication of the technology for manufacturing optical elements. The US patent is taken as a prototype.

The objective of the invention is the creation of a mod converter for connecting optical fiber with a planar optical crystal with improved characteristics in terms of compactness and ease of manufacture.

The invention relates to a modular converter device for connecting an optical fiber to a channel waveguide (mode converter) in the form of an integral element for introducing light from an optical fiber into a channel waveguide and vice versa, characterized in that in accordance with the invention, the optical fiber is connected to a channel waveguide of a planar optical crystal through a series of waveguide segments by the waveguide of the plenary optical crystal through a series of waveguide segments of the same height and rectangular cross section, gradually decreasing scheysya width and gradually increasing lead placement and tapered section segment disposed directly beneath waveguide segment with rectangular cross section gradually decreasing in width and smoothly increasing placing step, wherein, waveguide segments, and the tapered segment, are placed on a single chip with a planar optical crystal.

The length and width of the segments decreases towards the plenary optical crystal with a fixed gap, therefore, the effective index of refraction of the segmented layer decreases towards the planar optical crystal. Therefore, the light moving from the fiber into the segmented waveguide smoothly flows into the underlying cone-shaped layer of “core” with a high refractive index, which is in the same plane with the channel waveguide of the optical crystal (PLC). The final width of the cone section (near the planar optical crystal) can narrow to a single-mode waveguide size. Thus, light through a cone-shaped segment with minimal losses enters the planar optical crystal.

In FIG. 1a (side view) and FIG. 1b (top view) presents the structure of the mode converter, where:

1A, 1B, 1C - sections of a segmented waveguide;

2 - conical segment

3 - sheath ("cladding") of the channel waveguide

4 - fiber core

PLC is a planar optical crystal.

In FIG. Figure 2 shows the structure of a mode converter - the result of modeling.

In FIG. 3 shows the structure of the connection of the optical fiber with the channel waveguide without using a mode converter,

Where:

5 - the core of the channel waveguide - "core"

6 - optical fiber

7 - fiber optic sheath.

In FIG. 4 shows the structure of a two-dimensional mode converter,

Where:

8 - extended end element of the channel waveguide.

a planar optical crystal on a silicon substrate (a quartz substrate is possible), a sequence of operations for manufacturing a segmented section follows.

In general, the manufacturing cycle of this mod converter can look like this:

- the lower buffer layer is formed on the silicon wafer, as a rule, by oxidation (on both sides of the wafer, which gives the wafer minimal curvature), followed by annealing (SiO2).

- a core layer (oxynitride - SiON) is deposited with a higher refractive index using plasma vapor deposition techniques (PECVD) or chemical vapor deposition at low pressure (LPCVD), followed by annealing.

- a channel optical waveguide is formed using photolithography through a chromium mask and subsequent reactive ion etching.

- a layer of chromium is sprayed, photolithography is carried out, and chromium is chemically etched according to the pattern opposite to the future segmented waveguide.

- a layer of “segmented waveguide” is deposited (PECVD or LPCVD technique) (oxynitride - SiON1)

- using photolithography using a chromium mask and subsequent reactive ion etching of the oxy nitride layer, the segments are etched, while the stop layer is to prevent the overlay of the underlying core from being chromed. The size of the elements of the segments can vary from 1 to 30 microns, which can easily be achieved using contact printing. A variant with a fixed length of segments and an increasing gap between them is possible, but technologically, using reactive ion etching, a variant with a fixed gap is preferable.

- chemically removes a layer of chromium, selectively to the underlying layer of the core.

- deposition of the upper buffer layer (SiO2, BFSS) and final annealing of the finished structure are carried out.

It is important that when etching segments using a stop-layer of chromium and subsequent chemical removal of chromium, the main layer of oxynitride - “bark” is subjected to minimal impact. In conventional plasma-chemical etching (reactive ion etching) of the overlying layer, overgrazing or under-etching is inevitable, as well as the rough structure of the etched surface of the underlying layer (in this case, the core), which inevitably causes additional input losses. The choice of chromium as the material of the stop layer is due to the possibility of selective etching of the waveguide dielectric layers with respect to chromium, as well as the possibility of selective chemical removal of chromium with respect to the underlying dielectric layer of oxynitride (SiON).

To solve the problems of alignment of a channel waveguide with a single-mode fiber when using a butt connection with the end of the waveguide, methods of V-shaped or U-shaped grooves in silicon, etched by a liquid or plasma method at the ends of the optical crystal, with subsequent laying of fibers in them, can be used.

The layout of the mode converter involves the use of a three-layer approach containing cores (“cores”) 1 (1A, 1B, 1C) and 2, with refractive indices n1 and n2, respectively, and a buffer layer (“cladding”) 3, with refractive index n3. Moreover, n2> n1> n3. The light from the fiber core 4 passes into a channel waveguide of rectangular cross section. The input element uses three segments of variable length with a fixed gap and a gradually decreasing width to connect the optical fiber through the core 1 to the core 2. The closest segment 1C to the fiber preferably has a refractive index and cross section comparable to the fiber core 4 and provides an initial connection to the fiber core .

The presented structure of this device is a simulation result in which the height of the segmented sections was 8 μm, the width of the first segment closest to the optical fiber was 8 μm, and the width of the last (third) segment was 3 μm. The dimensions of the channel waveguide of the optical crystal: height 3 μm, width 3 μm. The total length of the mod converter was 100 μm.

(n1-n2) / n1 × 100% = 3%,

where n1, n2 are the refractive index of the core ("core") and the sheath (buffer layer - "cladding") of the channel waveguide of the optical crystal, respectively

Calculations show that the coupling loss with the SMF28 single-mode fiber should not exceed 0.2 dB, that is, with comparable communication losses, the length of the mode converter of this design is about three times shorter than that of its analogs.

In this example, the sizes, the number of segments, and in particular the segment thicknesses, can be optimized using a three-dimensional finite-difference beam propagation (FD-BPM) computer program that uses standard methods for modeling waveguides and optimizing devices.

Thus, using this design of the mod converter, it is possible to create an improved version of the input of light from an optical fiber into a channel waveguide and vice versa from the point of view of compactness, simplicity, and development of manufacturing technology.

Sources of information.

1. Hansperger R. "Integral Optics: Theory and Technology", Moscow: Mir, 1985

2. "Analysis of Periodically Segmented", Z. Weissman and I. Hendel, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 13, NO. 10, OCTOBER 1995.

3. US patent: US 2005/0152648 - prototype

Claims (1)

A modal converter device comprising an integral element for introducing light from a fiber into a channel waveguide of a planar optical crystal and vice versa, characterized in that the optical fiber is connected to a channel waveguide of a planar optical crystal by a series of waveguide segments of the same height and rectangular cross section with a gradually decreasing width and a continuously increasing step placement, as well as by means of a segment of a cone-shaped section located directly below the waveguide segments a rectangular section, with a gradually decreasing width and a gradually increasing placement step, the waveguide segments and the cone-shaped segment being integrated with the planar optical crystal.

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