GB2388958A

GB2388958A - Optical device

Info

Publication number: GB2388958A
Application number: GB0212091A
Authority: GB
Inventors: William James Stewart
Original assignee: Marconi Corp PLC; Marconi UK Intellectual Property Ltd; Telent Ltd
Current assignee: Marconi UK Intellectual Property Ltd
Priority date: 2002-05-25
Filing date: 2002-05-25
Publication date: 2003-11-26
Anticipated expiration: 2022-05-25
Also published as: GB0212091D0; GB2388958B

Abstract

An optical device for operation at a predetermined wavelength comprises an array of equally spaced parallel linear optical waveguides 7 to 11 located in slabs 12 to 16. Known laser arrays have been fabricated in this configuration, but the optical device of the invention is much smaller, since the spacing of the slabs lies below one half of the predetermined wavelength in the medium surrounding the optical waveguides and is preferably above one half of the predetermined wavelength in the optical path in the waveguide itself The optical device may act as a laser array, in which the field at the ends of the waveguides is evanescent only and power is withdrawn using a transverse waveguide 17 running along the ends of the waveguides 7 to 11. In other embodiments, the waveguides 7 to 11 are not lasers, and the optical device can be used simply to reproduce the field at the input of the waveguides exactly in amplitude and relative phase at the other end of the waveguides, or to couple with a transverse guide at one or both ends to provide a frequency selective tap or delay.

Description

1 1 2388958

OPI ICAL DEVICE,

This invention relates to optical devices.

The behaviour of closely-coupled arrays of linear waveguides has been studied for a number of reasons, including their use as laser arrays (Somckh, Garrnire et al (Applied s Phys Lett 22 (1973) 46-47), MAJOR, Jun. , J. S., MEHUYS, D. and WELCH, D. F.: 11.5 W Pulsed operation of antiguided laser diode array', Electron. Lett., 1992, 28, (12), pp. 1 1011 102).

Referring to Figures 1, 3, a known array of similar parallel linear optical waveguides 1 lo to 3 is defined by regions of higher refractive index grown in one crystal together with adjacent regions of lower refractive index, in slabs 4 to 6. The slabs 4 to G are made from a solid block of material. The waveguides are defined vertically by the index differences and horizontally by the air spaces between the slabs. The regions of higher index could be Gallium Arsenide, and the regions of lower index Gallium Aluminium 5 Arsenide. Terminals on the top of each slab, and a common terminal on the bottom of the block (not shown) permit current to be injected into the slab to generate photons in the waveguides, thus providing the optical gain or laser action, if required.

A typical wavelength for such a laser would be around 1.5 x 10 metros (light in the 20 near infra-red region). A typical slab period "a" would be betwocn 10 and 100 x 10-6 metros.

( Referring to Figure 2, the electric field E of the electromagnetic radiation generated in

each individual waveguide is not totally confined to that waveguide, so that coupling occurs between the electromagnetic radiation generated in each individual waveguide.

This coupling can cause a phase change such that light in adjacent guides runs 180 out 5 of phase with the respect to each other. This causes the output beam to consist of two symmetrically angled beams A, B (Figure 3). A high-reflectivity mirror applied to the other end confines the emergent radiation to the one end.

The invention provides an optical device for operation at a predetermined wavelength, lo which comprises an array of parallel linear optical waveguides, the spacing of adjacent waveguides being less than one half of the predetermined wavelength in the medium surrounding the optical waveguides.

The spacing of the optical waveguides permits a non-propagating evanescent field to be

5 produced outside the end of the waveguides when power propagates in adjacent guides in antiphase. Such a field does not have the capability to carry power away from the

guide ends into the surrounding medium, so that power can be reflected back into the guides, or coupled to an appropriate transverse guide. Equally, an evanescent field in

the vicinity of the ends of the waveguides for example in a transverse guide can couple Jo to the optical device, driving adjacent waveguides in antiphase. The optical device could instead be used to transfer field, including evanescent field, with any phase

pattern, from one end of the waveguides to the other.

Advantageously, the spacing of adjacent waveguides is greater than one half of the predetermined wavelength in the optical path in the waveguides.

Optical devices constructed in accordance with the invention will now be described in s detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a known laser array; Figure 2 is a schematic view of the distribution of the electric field of electromagnetic

0 radiation in the laser array of Figure 1; Figure 3 is u plan view of the laser array shown in Figure 1; Figure 4 is a perspective view of part of a laser array forming a first embodiment of the 5 invention, the thickness of the optical waveguides along the length of the array being exaggerated for clarity; Figure 5 is a schematic plan view of the laser array forming the first embodiment of the invention; Figure 6 shows the theoretically calculated electric field at the end of the laser array

forming the first embodiment of the invention when adjacent guides are running in anti phase;

Figure 7 shows the theoretically calculated electric field at the end of the laser array

forming the first embodiment of the invention when adjacent guides are running in phase with each other; and s Figure 8 shows a schematic plan view of a further embodiment of the invention. i Like reference numerals apply to like parts throughout all the drawings.

Referring to Figures 4 and 5, the first embodiment of the invention is a laser array and i to consists of an array of similar linear optical waveguides 7 to 11 in slabs 12 to 16. The thickness of the waveguides along the length of the array is exaggerated for clarity in Figure 4, and only some of the waveguidcs shown in Figure 5 are shown in Figure 4.

The laser array of Figures 4 and 5 differs from that of the prior art of Figures 1 and 3 in

that the slab spacing i.e. the pitch "a", is much smaller and lies between one half of the 5 laser wavelength in air and approximately one half of the laser wavelength in the waveguide optical path, i.e. in the waveguide cores. Further, an output guide 17 is provided (not shown in Figure 4) to collect the laser output. In addition, the etching is much deeper than for Figures 1 and 3, to reduce coupling through the base, from which the blocks extend.

It will be recalled with reference to Figure 3 that the known laser array produced two diverging beams A, B. The angle between these beams increases as the guide spacing a' decreases. When the guide spacing falls below a half wavelength in the surrounding medium the two beams vanish, because the 180 angle has been exceeded. If the guide

spacing is decreased sufficiently, the coupling between the guides progressively increases and, at a spacing about equal to a half wavelength in the guide material, the guides become so strongly coupled that they effectively form a continuous medium.

The invention is concerned with slab spacings lying between one half a wavelength in s air and about one half wavelength in the material of the guide.

In this intermediate region, the field in the air beyond the ends of the guides, if adjacent

guides are running in anti-phase, is evanescent in form and carries no power away from the structure. Figure 6 represents a calculation showing the variation of local energy lo density against position along the array shown in Figures 4 and 5, where the guide ends are at the bottom of the figure and there is a residual weak wave running across them which vanishes for a large enough array, this model being finite one. The higher energy density regions are the lighter ones. The variation in height of the peaks is due to the numerical calculation being performed over a finite aperture.

Figure 7 is a calculation showing the variation of local energy density position along the array shown in Figure 4, for the case where the light in the guides is in-phase between the various guides, where the residual structure across the figure (which is exaggerated by the plot) again reflects the finite width of the waveguide array. The plot looks just 20 like a plain wave running vertically and, indeed, the field beyond the guides is not

evanescent in form with in-phase operation, and would therefore not couple to the guide 17.

Referring to Figures 4 and 5, each waveguide 7 to 11 has coherent radiation generated in it which is reflected between the ends. Because the radiation is coherent, it has a particular phase at each end. The guides are in antiphase when the phase at one end of the guides, in this case, the end adjacent the transverse guide 17, differs by 180 from s each individual guide to the next. This is the condition required for generation of a non-

propagating evanescent wave at that end of the waveguides and hence reflection of the power back into the guides. However, some of the effects of evanescence such as coupling to the transverse guide, will occur when the phase difference is not exactly 180 , and over a range of differences from exact antiphase.

Referring to Figures 4 and 5, with the guides working in anti-phase, light cannot escape from the waveguide array and must be reflected. The laser array will therefore be favoured as having a much higher Q factor. In effect, the closely spaced guide ends form a perfect mirror. This is remarkable and allows much higher finesse laser cavities 5 than would normally be possible.

The laser array of Figures 4 and 5 also includes an output guide 17. The propagation constant of this guide is given by 20 p=2 Ma where a is the period defined above. The propagation constant depends on the width of the guide and the material of which it is made. These are chosen so that the phase in the

guide goes through one complete cycle over a length corresponding to twice the array period 'a', since adjacent waveguides are operating in antiphase.

In these circumstances, the evanescent field at the end of the laser waveguide couples

s with the external field (F) from the output guide 17, and the output laser radiation

propagates in both directions along the guide 17.

A range of spacings of the output guide 17 from the waveguides is possible. Although unlikely, the guide 17 could be in contact with the waveguide ends. At the other lo extreme, a spacing significantly greater than a half-wavelength of so will probably give insufficient coupling.

This process provides in-built coupling of the laser power to a single output guide.

I', The laser array may be fabricated from Gallium Arsenide and Gallium Aluminium Arsenide as for the known laser array, and could be micromachined or lithographically etched. The terminals for generating current and therefore photons in the waveguide are connected to the tops of the waveguides and the bottom of the whole structure.

20 The wavelength of light in Gallium Arsenide is approximately one quarter of that in air.

Assuming a typical wavelength of around 1.S x 10-6 metros for the laser wavelength, the slab spacing would lie in the range of 0.75 x 10-6 metros to 0.16 x 10-6 metros. Such devices can be readily fabricated using lithography. It is envisaged that the laser array could operate over any optical frequency including ultra-violet and infra-red.

( 8 The waveguide spacing 'a' is uniform throughout the length of the array. The refractive indices of the core regions (the dark regions 7 to 11) and the cladding regions (the regions of the slabs 12 to 16 above and below the core regions) of the waveguides are chosen to provide the most compact structure. A typical ratio of slab thickness to 5 separation between the slabs, all in the direction along which the array extends, is approximately 1:2.

The waveguides 7 to 11 have the same optical length to a high degree of accuracy, their input and output faces being respectively co-planar.

While nine slabs are shown in Figure S. as few as three could be utilised in some circumstances, but in practice a number between at least ten and many thousands could be employed. The array described is surrounded by air, but it could be immersed in some other medium if desired e.g. oil.

The waveguides can be driven in anti-phase by coupling from another transverse guide like the guide 17, but at the other end of the array. AItcrnatively, the further transverse guide could drive the waveguides in phase, for example, with an incident plane wave, and the required phase shifts could be induced in alternate guides. This could be done so electro-optically, or by arranging for changes in the lengths or widths of alternate guides. Variations may of course be made to the first embodiment without departing from the scope of the invention. Thus, it is desired, the transverse thickness ot the optical

waveguides as seen in Figure 5 need not all be the same. For example, alternating wider and thinner waveguides are possible, as are variations down the length, and in the waveguide lengths, at one end at least.

s While the invention has been described in terms of a dielectric, the array could, if desired, have dielectric or air slabs, defining waveguides, in place of the air spaces between the slabs shown in Figure 4. The spacers could be for example of Gold, which would then be somewhat thinner relative to the waveguides than in the dielectric case.

Metallic-walied waveguides will be lossier than all dielectric ones, but may be attractive lo for infra-red wavelengths where this effect will be reduced.

The second embodiment of the invention is a coherent fibre face plate imaging device, and consists of the array of waveguides shown in Figures 4 and 5 but without the output guide 17. Also, the terminals necessary to provide current to generate photons for the is laser action are also not present, as the device is simply an imaging device.

Because the guides are assumed to have the same optical length to a high accuracy, that is, within a small fraction for example, a tenth of the wavelength, any field above the

structure will be reproduced below it. As compared with a conventional image 20 transferring array of glass fibres, which transmits light intensity at the input of each fibre to its output but randomises the relative phase, the guides of the second embodiment of the invention reproduce at one end of the array, the amplitude and relative phase of light entering at the other end. Whilst a standard fibre face plate can only 'see' objects which are either in contact with or imaged onto its end face, this

10 ' '

structure is like a 'thick window' that allows one to see through it to an object in any position, just putting it further off by the thickness of the structure. The face plate transfers the field, including the evanescent field at one end of the waveguides to the

other end. While the waveguides are of the same optical length, this is not essential, s and could be different if it was desired to induce any pattern to an image viewed at one end. For this embodiment, in-phase phase patterns such as shown in Figure 7 are possible, since coherent image transfer, not reflection, takes place.

Referring to Figure 8, the third embodiment of the invention is the same as the first 10 embodiment, except that an input guide 18 is provided in addition to the output guide 17, and in that the terminals needed for the laser action are not provided.

The input guide 18 has the same propagation co-efficient as the output guide 17, so that it's evanescent field (G) also has the same wavelength as the period of the guides

5 running in anti-phase. Hence, radiation travelling along guide 18 at this particular wavelength (in practice a small band of wavelengths) couples to the waveguide array and out of the guide 17. Because the periodic guide array structure will match the transverse guide's evanescent field only over a limited frequency band, power will

couple from one transverse guide through the array into the other only over this band, 20 forming a wavelength-selective tap. This is similar in operation to grating assisted couplers and could be tuned in a similar way. Likely advantages arise in extinction out-

of-band because of the length of the array. Hence, if a wider band of optical frequencies is travelling along the guide 18, the narrow band referred to will be tapped off. The device thus forms a tap suitable for wave division multiplexing.

Equally, a narrow band of wavelengths input on the guide 17 could combine with other wavelengths travelling along the guide 18.

The fourth embodiment is the same as the first embodiment, except that the terminals 5 necessary to provide the laser operation are not provided. Light travailing down guide 17 couples to the array of waveguides 7 to 11 over a particular wavelength (a narrow band of wavelengths) for which the evanescent field F corresponds to antiphase

excitation of adjacent waveguide ends, so that this wavelength is extracted from a broader band travelling along the guide 17.

The wavelength extracted propagates to the end of the waveguide array and is reflected and returns to the end at which it entered, and again couples with the guide 17 to rejoin the broader band. Now, however, the particular wavelength which couples to the waveguide array has been delayed relative to the remainder of the band of wavelengths 5 travelling along the guide 17.

This provides a way of compensating for dispersion in a waveguide i.e. the fact that different wavelengths travel at different speeds along an optical fibre resulting in an undesirable spreading in time of a pulse of light formed of different wavelengths. With 0 the arrangement described, only one wavelength can be delayed. However, a complex pattern may be achieved by grading the lengths of the waveguides across the array similar to a SAW filter - slowly enough to allow the reflection - fast enough to give the variable delays.

The structure in effect provides a closely spaced series of high Q resonators coupled to the transverse guide. These will load it to produce large changes in group velocity and dispersion in a very compact structure. This can also be done, for example, with coupled ring resonators, but these are difficult to space closely on the guide because s they cannot be easily overlaid.

Claims

! 13 CLAIMS

1. An optical device for operation at a predetermined wavelength, which comprises an array of parallel linear optical waveguides, the spacing of adjacent waveguides being less than one half of the predetermined wavelength in the medium surrounding the optical waveguides.

2. An optical device as claimed in Claim 1, in which the spacing of adjacent

waveguides is greater than one half of the predetermined wavelength in the optical path in the waveguide.

3. An optical device as claimed in Claim 1 or Claim 2, in which the optical waveguides have a uniform spacing.

4. An optical device as claimed in Claim 3, in which the ratio of waveguide thickness to separation between waveguides is approximately 1.2.

5. An optical device as claimed in any one of Claims I to 4, in which the optical waveguides have the same optical length as each other.

6. An optical device as claimed in any one of Claims l to 5, in which there are at least ten waveguides in the array.

7. An optical device as claimed in Claim 6, in which there are at least one hundred waveguides in the array.

( 14 8. An optical device as claimed in any one of Claims 1 to 7, including a transverse guide coupled with one end of the optical waveguides, the evanescent field of which has

a wavelength equal to twice the waveguidc spacing.

9. An optical device as claimed in Claim 8, in which the optical waveguides are lasers. to. An optical device as claimed in Claim 8 or Claim 9, in which the lasers are arranged to be driven so that the phase of the coherent optical radiation at the ends of the each waveguide which couple to the transverse guide, differs by 180 from that of the adjacent one along the array.

11. An optical device as claimed in Claim 8, including a second transverse guide coupled with the other end of the optical waveguides, the evanescent field of which has

a wavelength equal to twice the wavcguide spacing.

12. An optical device as claimed in Claim 8, in which reflection is arranged to take place at the other end of the waveguides, so that light which couples from the transverse guide is reflected at the end of the waveguides to rejoin the transverse waveguide after a delay.