WO2006001000A1

WO2006001000A1 - A method and a device for delaying an optical signal

Info

Publication number: WO2006001000A1
Application number: PCT/IE2005/000071
Authority: WO
Inventors: John Donegan; Yury Rakovich; John Boland; Terence Martin Connolly
Original assignee: The Provost, Fellows And Scholars Of The College Of The Holy And Undivided Trinity Of Queen Elizabeth Near Dublin
Priority date: 2004-06-25
Filing date: 2005-06-27
Publication date: 2006-01-05

Abstract

An optical delay device (1) comprises a photonic molecule (3) comprising a pair of identical optically coupled microspheres (8a, 8b) of light conducting material, the centres of which are located on a common axis of symmetry (10), and the outer surfaces (11) of which touch at (12) on the common axis of symmetry (10). An input fibre optic cable (14) directs an optical signal at the microsphere (8a) along an input optical path (18) which is disposed at an angle θ greater than zero to the common axis of symmetry (10), while the delayed optical signal is detected by an output fibre optic cable (16) along an output optical path (19) from the microsphere (8b) which is disposed at an angle α greater than zero to the common axis of symmetry (10). The input fibre optic cable (14) and the output fibre optic cable (16) are moveable relative to the photonic molecule (3) for facilitating varying of the angle θ an the angle α for in turn varying the delay to which the optical signal is subjected in the photonic molecule (3). Additionally, the microsphere (8a) is moveable along the common axis of symmetry (10) relative to the microsphere (8b) for further facilitating varying of the delay to which the optical signal is subjected in the photonic molecule (3).

Description

"A method and a device for delaying an optical signal"

Field of the Invention The present invention relates to a method and to an optical delay device for delaying an optical signal, and in particular, though not limited to a method and to an optical delay device for delaying an optical signal of laser light comprising optical pulses of duration of the order of picoseconds, and at repetition rates of the order of gigahertz (GHz).

Background to the Invention Optical communications, in particular, in the field of telecommunications involve the use of short optical pulses, typically, of picosecond duration at gigahertz repetition rates for the transmission of data on an optical network. In general, signals from two sources must be synchronised, and synchronisation of such optical signals is typically carried out by passing the signals from one of the sources through a delay line for delaying the signals from that source until they are synchronised with those of the other source. Delay lines may provide a fixed or a variable delay, however, preferably, the delay is variable, and ideally, the delay line should delay the signal with relatively low distortion in both space and time. Such delay lines operate by extending the light path through which the optical signal must pass, thereby delaying the optical signal. One type of delay line comprises optical fibres of different lengths, and by selecting the optical fibre through which the optical signal is passed, the delay to which the optical signal is subjected may be selected, the lengths of the optical fibres determining the delays to which the optical signals are subjected. An alternative type of delay line comprises a moveable mirror, which may be moveable by a motorised drive or a piezo-electric stage, and by varying the position of the mirror, the light path through which the optical signal must pass is varied. Other optical delay lines comprise acousto-optical modulators which move the optical signal in space, thereby delaying the optical signal.

It is also known to delay an optical signal by passing the optical signal into the cavity of a microsphere, within which the optical signal is subjected to multiple reflections due to the "whispering gallery mode" effect, thereby delaying the optical signal. It is further known to optically couple a pair of microspheres to form a photonic molecule whereby a pair of microspheres are arranged on a common axis of symmetry, typically touching each other or if not touching, sufficiently close together for facilitating exchange of light therebetween. Light is directed into one of the microspheres along the common axis of symmetry and is detected also along the common axis of symmetry.

However, such arrangement of photonic molecules formed by optically coupled microspheres, when used in accordance with known methods, while they do subject the optical signal to a delay which is greater than that which can be achieved from a single uncoupled microsphere, the increase in the delay over a single microsphere is limited, and furthermore, the quality factor is relatively poor. The quality factor is defined as being the quotient of the resonance frequency of the detected light divided by half the bandwidth of the resonance frequency of the detected light, or stated another way, the quotient of the wavelength divided by half the difference of the maximum and minimum wavelengths of the detected light. It is well accepted that there is a direct correlation between quality factor and delay, the higher the quality factor, the greater the delay.

There is therefore a need for a method and an optical delay device for delaying an optical signal, which provides improved delay times over methods and devices known heretofore.

The present invention is directed towards providing such a method, and the invention is also directed towards providing an optical delay device.

Summary of the Invention According to the invention there is provided a method for delaying an optical signal, the method comprising the steps of: providing at least two optically coupled microelements of a light conducting medium, for producing multiple whispering gallery mode reflections of the light internally therein and for exchanging light therebetween for delaying the optical signal, the microelements defining a common axis of symmetry with their respective centres lying on the common axis of symmetry, directing the optical signal at one of the microelements along an input optical path, and detecting the delayed optical signal from any one of the microelements along an output optical path, characterised in that at least one of the input optical path and the output optical path is disposed at an angle greater than zero to the common axis of symmetry.

In one embodiment of the invention the centres of the microelements are spaced apart along the common axis of symmetry.

In another embodiment of the invention the microelements touch each other. Preferably, the microelements touch each other adjacent the common axis of symmetry.

Alternatively, the microelements are spaced apart from each other.

Preferably, at least one of the microelements is moveable relative to the other for varying the delay to which the optical signal is subjected. Advantageously, the moveable one of the microelements is moveable along the common axis of symmetry.

In one embodiment of the invention the delayed optical signal is detected from the microelement at which the optical signal is directed.

In another embodiment of the invention the input optical path and the output optical path coincide.

In a further embodiment of the invention the delayed optical signal is detected from one of the microelements other than the microelement at which the optical signal is directed.

In one embodiment of the invention the input optical path is disposed to the common axis of symmetry at the angle greater than zero. Preferably, the angle at which the input optical path is disposed to the common axis of symmetry lies in the range of 1 ° to 180°. Advantageously, the angle at which the input optical path is disposed to the common axis of symmetry lies in the range of 10° to 50°. Ideally, the angle at which the input optical path is disposed to the common axis of symmetry lies in the range of 35° to 40°.

Preferably, the angle at which the input optical path is disposed to the common axis of symmetry is variable for varying the delay to which the optical signal is subjected.

In one embodiment of the invention the output optical path is disposed to the common axis of symmetry at the angle greater than zero. Preferably, the output optical path is disposed to the common axis of symmetry at an angle in the range of 1° to 180°. Advantageously, the output optical path is disposed to the common axis of symmetry at an angle in the range of 10° to 50°, and ideally, the output optical path is disposed to the common axis of symmetry at an angle in the range of 35° to 40°.

Advantageously, the angle at which the output optical path is disposed to the common axis of symmetry is variable for varying the delay to which the optical signal is subjected.

In one embodiment of the invention the microelements are of substantially similar size. Alternatively, the microelements are of different sizes.

In another embodiment of the invention the microelements are of size up to 50 microns. Preferably, the microelements are of size in the range of 3 microns to 20 microns. Advantageously, the spacing between adjacent surfaces of the microelements does not exceed 2 microns. In one embodiment of the invention the light conducting media of the respective microelements are of relatively high refractive indices. Preferably, the refractive indices of the light conducting media of the respective microelements lies in the range of 1.1 to 3.0. Advantageously, the refractive indices of the light conducting media of the respective microelements lies in the range of 1.5 to 2.0. Ideally, the refractive indices of the light conducting media of the respective microelements is approximately 1.8.

In one embodiment of the invention the light conducting media of the respective microelements are of similar refractive indices. Alternatively, the light conducting media of the respective microelements are of different refractive indices.

In one embodiment of the invention the light conducting medium of at least one of the microelements is selected from any one or more of the following materials: melamine formaldehyde latex, polystyrene, glass, glass doped with ions of one or more rare earths, glass doped with ions of one or more transition metals, glass doped with dye molecules, and a semiconductor material.

In another embodiment of the invention when the light conducting medium of the at least one of the microelements is of glass doped with ions of one or more rare earths, the rare earths are selected from any one or more of the following rare earths: Erbium (Er), Thulium (Tm), and other first row rare-earth ions.

In a further embodiment of the invention when the light conducting medium of the at least one of the microelements is of glass doped with ions of one or more transition metais, the transition metais are selected from any one or more of the following transition metals'. Chromium (Cr), Cobalt (Co), Nickel (Ni), Manganese (Mn)₁ and Iron (Fe).

In another embodiment of the invention when the light conducting medium of at least one of the microelements is a semiconductor material, the semiconductor material is selected from any one or more of the following semiconductor materials: Gallium Arsenide (GaAs), Aluminium Arsenide (AIAs), Indium Phosphide (InP), Indium Nitride (InN), Gallium Nitride (GaN), Gallium Phosphide (GaP), and any and all alloys of any one or more of the above listed semiconductor materials.

In one embodiment of the invention the light conducting media of the respective microelements are similar to each other. Alternatively, the light conducting media of the respective microelements are different to each other.

Preferably, at least one of the microelements is coated with nanocrystals of a light emitting medium. Advantageously, the coating of nanocrystals with which the at least one microelement is coated is selected from any one or more of the following media: cadmium telluride (CdTe), cadmium selenide (CdSe), zinc selenide (ZnSe), zinc sulphide (ZnS), zinc telluride (ZnTe), zinc oxide (ZnO), and any and all alloys of any one or more of the above listed media.

In one embodiment of the invention each of the microelements is coated with nanocrystals of a light emitting medium.

In another embodiment of the invention the microelements are microspheres.

In a further embodiment of the invention the respective microspheres are of substantially similar diameter. Alternatively, the respective microspheres are of different diameters.

In one embodiment of the invention the microspheres are of diameter up to 50 microns.

In another embodiment of the invention the microspheres are of diameter of at least 1.5 microns.

In another embodiment of the invention the microelements form a photonic molecule.

The invention also provides an optical delay device for delaying an optical signal, the optical delay device comprising: at least two optically coupled microelements of a light conducting medium for producing multiple whispering gallery mode reflections of light internally therein and for exchanging light therebetween for delaying the optical signal, the microelements defining a common axis of symmetry with their respective centres lying on the common axis of symmetry, a means for directing the optical signal at one of the microelements along an input optical path, and a means for detecting the delayed optical signal from any one of the microelements along an output optical path, characterised in that at least one of the input optical path and the output optical path is disposed at an angle greater than zero to the common axis of symmetry.

In one embodiment of the invention a means is provided for moving the moveable one of the microelements.

In another embodiment of the invention the means for detecting the delayed optical signal detects the delayed optical signal from the microelement at which the optical signal is directed by the means for directing the optical signal.

In a further embodiment of the invention the means for detecting the delayed optical signal detects the delayed optical signal from one of the microelements other than the microelement at which the optical signal is directed by the means for directing the optical signal.

In one embodiment of the invention the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle disposed to the common axis of symmetry greater than zero. Preferably, the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 1° to 180°. Advantageously, the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 10° to 50°. Ideally, the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 35° to 40°.

In another embodiment of the invention one of the means for directing the optical signal and the optically coupled microelements is moveable relative to the other for varying the angle at which the input optical path is disposed to the common axis of symmetry for varying the delay to which the optical signal is subjected. Preferably, a first drive means is provided for moving one of the means for directing the optical signal and the optically coupled microelements relative to the other for varying the angle at which the input optical path is disposed to the common axis of symmetry.

In one embodiment of the invention in that the means for detecting the optical signal from one of the microelements detects the optical signal along the output optical path disposed at an angle to the common axis of symmetry greater than zero. Preferably, the means for detecting the optical signal from one of the microelements detects the optical signal along the output optical path disposed at an angle disposed to the common axis of symmetry in the range of 1 ° to 180°. Advantageously, the means for detecting the optical signal from one of the microelements detects the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 10° to 50°. Ideally, the means for detecting the optical signal from one of the microelements detects the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 35° to 40°.

Preferably, one of the means for detecting the optical signal and the optically coupled microelements is moveable relative to the other for varying the angle at which the output optical path is disposed to the common axis of symmetry for varying the delay to which the optical signal is subjected.

Advantageously, a second drive means is provided for moving one of the means for detecting the optical signal and the optically coupled microelements relative to the other for varying the angle at which the output optical path is disposed to the common axis of symmetry.

In one embodiment of the invention a means for moving one of the microelements relative to the other is provided for varying the delay to which the optical signal is subjected. Advantages of the Invention The advantages of the invention are many. In particular, the optical device according to the invention provides significantly greater delay to an optical signal than optical delay devices known heretofore, which comprise a pair of optically coupled microelements. This is achieved by virtue of the fact that either the optical signal is directed at one of the optically coupled microelements along an input optical path disposed to the common axis of symmetry of the microelements at an angle greater than zero, and/or the delayed optical signal is detected from one of the optically coupled microelements along an output optical path disposed to the common axis of symmetry at an angle greater than zero. By varying the angle at which the input optical path is disposed to the common axis of symmetry, and/or by varying the angle at which the output optical path is disposed to the common axis of symmetry, the delay to which the optical signal is subjected in the optically coupled microelements can be varied. The delay to which the optical signal is subjected is increased by increasing the angle at which one or both of the input and output optical paths are disposed to the common axis of symmetry. Additionally, a further advantage of the invention is provided when at least one of the microelements is moveable relative to the other, since by altering the spacing between the optically coupled microelements along the common axis of symmetry, the delay to which the optical signal is subjected in the optically coupled microelements can be varied, and the relative moveability of the microelements may be used for fine tuning the delay to which the optical signal is subjected.

A further advantage of the invention is that the spacing between whispering gallery or resonance modes of the optical signal is increased by increasing either or both of the angles at which the optical input path along which the optical signal is directed at the optically coupled microelements is disposed to the common axis of symmetry, or the angle at which the output optical path along which the delayed optical signal is detected from the optically coupled microelements is disposed to the common axis of symmetry. The invention and its many advantages will be more clearly understood from the following description of some preferred embodiments thereof, which are given by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a diagrammatic representation of an optical delay device according to the invention for delaying an optical signal,

Fig. 2 is a diagrammatic representation of a portion of the optical delay device according to another embodiment of the invention for delaying an optical signal,

Fig. 3 is a diagrammatic representation of a portion of a prior art optical delay device for delaying an optical signal,

Fig. 4 is a plot of emission intensity against wavelength of the emission spectra of an optical signal detected from the optical delay device of Fig. 2,

Fig. 5(a) is a plot of emission intensity against wavelength of the emission spectra of an optical signal detected from the prior art optical delay device of Fig. 3,

Fig. 5(b) is a plot of emission intensity against wavelength of the emission spectra of an optical signal detected from a pair of optically uncoupled microspheres similar to the microspheres of the photonic molecule of the optical delay device of Fig. 2,

Fig. 6 is a representation of the photon lifetime plotted against wavelength in the optical delay device of Fig. 2,

Fig. 7 illustrates a fit with Lorentzian functions to measured lineshapes of the emission intensity against wavelength, the Lorentzian function is shown in broken lines, while experimental data from the optical device of Fig. 2 is shown in full lines,

Fig. 8 illustrates an analogy of a photonic molecule of the type used in the optical delay devices according to the invention with a hydrogen molecule,

Fig. 9(a) illustrates a plot of the mode spacing against wavelength of the resonance modes of the optical signal obtained from optical delay devices similar to Fig. 2 but with different sizes of microspheres,

Fig. 9(b) is a plot of photon lifetime against wavelength obtained from the optical delay device from which the waveforms of Fig. 9(a) were obtained,

Fig. 10(a) is a plot of emission intensity against wavelength of one of the optical delay devices from which the waveforms of Fig. 9(a) were obtained,

Fig. 10(b) is a plot of the relative increase in photon storage time against wavelength for the resonance modes of the optical delay device from which the waveform of Fig. 10(a) was obtained,

Fig. 11 is a plot of resonance mode spacing plotted against wavelength of one of the optical delay devices from which the waveform of Fig. 9(a) has been derived, illustrating the effect on resonance mode spacing of altering the spacing between the microelements,

Fig. 12(a) is a diagrammatic representation of a portion of an optical delay device according to another embodiment of the invention,

Fig. 12(b) is a diagrammatic representation of a portion of an optical delay device according to another embodiment of the invention,

Fig. 13 is a diagrammatic representation of a portion of an optical delay device according to a further embodiment of the invention,

Fig. 14 is a diagrammatic representation of a portion of an optical device according to a still further embodiment of the invention,

Fig. 15 is a diagrammatic representation of a portion of an optical device according to a still further embodiment of the invention, and

Fig. 16 is a diagrammatic representation of a portion of an optical device according to a still further embodiment of the invention.

Detailed Description of Preferred Embodiments Referring to the drawings and initially to Fig. 1 , there is illustrated an optical delay device according to the invention, indicated generally by the reference numeral 1 , for delaying an optical signal of laser light, for example, an optical telecommunications signal comprising pulses of picosecond duration at gigahertz repetition rates. The optical delay device 1 comprises a support base 2 supporting a substrate 5 of semiconductor material, in this case silicon. A photonic molecule 3 is located in a cavity 4 of the substrate 5. Light accommodating openings 6 and 7 in the substrate 5 accommodate optical signals respectively to and from the cavity 4. The photonic molecule 3 comprises a pair of optically coupled microelements, which in this embodiment of the invention are provided by a pair of optically coupled microspheres 8a and 8b each of a relatively high refractive index light conducting medium, which in this embodiment of the invention is melamine formaldehyde latex.

The microspheres 8 are of substantially similar diameter, and preferably, are of identical diameter, and in this case are each of diameter of the order of 3 microns. Each microsphere 8 defines a spherical micro-cavity within which multiple whispering gallery mode reflections of the optical signal occur from the inner surface of each of the microspheres 8, which cause distinct optical resonance modes of the optical signal to be propagated, which are commonly referred to as whispering gallery modes. The separation between the whispering gallery modes of the optical signal depends on the diameter of the microspheres, larger separation of the modes being obtained from smaller diameter microspheres.

The microspheres 8 are arranged in the cavity 4 so that centres 9 of the respective microspheres 8 lie on a common axis of symmetry 10 defined by the microspheres 8. The microsphere 8a, as will be described below, is moveable relative to the microsphere 8b along the common axis of symmetry 10 from one extreme position illustrated in Fig. 1 with outer surfaces 11 of the microspheres 8 touching at 12 adjacent the common axis of symmetry 10, to another extreme position (not shown) with the microspheres 8 spaced apart with the adjacent outer surfaces 11 not touching, for varying the optical coupling of the microspheres 8, for in turn, varying the delay to which the optical signal is subjected in the photonic molecule 3. Light, and in turn the optical signal, is exchanged between the microspheres 8 adjacent the common axis of symmetry 10 for so long as the microspheres 8 are sufficiently close to each other to be optically coupled.

An input means for directing the optical signal into the photonic molecule 3 comprises an input fibre optic cable 14 rigidly mounted on a first carrier member 15 which is moveably mounted on the support base 2 as will be described below. An output means for detecting the optical signal from the photonic molecule 3 comprises an output fibre optic cable 16 rigidly mounted on a second carrier member 17, which is also moveably mounted on the support base 2 as will also be described below. The input fibre optic cable 14 is arranged relative to the photonic molecule 3 for directing the optical signal along an input optical path represented by the line 18, through the light accommodating opening 6, at one of the microspheres, namely, the microsphere 8a. The input optical path 18 along which the optical signal is directed at the microsphere 8a extends at an angle θ to the common axis of symmetry 10, which is variable as will be described below.

The output fibre optic cable 16 is arranged relative to the photonic molecule 3 so that the delayed optical signal is detected from the other of the microspheres, namely, the microsphere 8b along an output optical path represented by the line 19, through the light accommodating opening 7. The output optical path 19 extends at an angle α to the common axis of symmetry 10, which is variable as will be described below.

The first and second carrier members 15 and 17 are moveably mounted on the support base 2 for facilitating varying the angles θ and α at which the optical signal is directed at the microsphere 8a along the input optical path 18, and at which the optical signal is detected along the output optical path 19 relative to the common axis of symmetry 10. A first guide means comprising a first arcuate guide track 20 is located on the support base 2, and the first carrier member 15 is engageable with the first guide track 20 for constraining the first carrier member 15 to move along an arcuate path in the direction of the arrows A and B for altering the angle θ at which the input optical path 18 is disposed to the common axis of symmetry 10, for in turn selectively varying the delay to which the optical signal is subjected in the photonic molecule 3. A first drive means for moving the first carrier member 15 along the first guide track 20 comprises a first servomotor 21 mounted on the first carrier member 15. A second guide means comprising a second arcuate guide track 23 is located on the support base 2 for engaging the second carrier member 17 for constraining the second carrier member 17 to move along an arcuate path in the direction of the arrows C and D for altering the angle α at which the output optical path 19 is disposed to the common axis of symmetry 10, for in turn selectively varying the delay to which the optical signal is subjected in the photonic molecule 3. A second drive means for moving the second carrier member 17 along the second guide track 23 comprises a second servomotor 24 mounted on the second carrier member 17.

In this embodiment of the invention the first carrier member 15 and the first guide track 20 co-operate to permit approximately 40° of movement of the first carrier member 15 along the first guide track 20 in the direction of the arrows A and B, and the second carrier member 17 co-operates with the second guide track 23 for permitting approximately 40° of movement of the second carrier member 17 along the second guide track 23 in the direction of the arrows C and D. The first and second carrier members 15 and 17 and the first and second guide tracks 20 and 23 are located relative to the photonic molecule 3 for permitting the angles θ and α, at which the input optical path 18 and the output optical path 19, respectively, are disposed to the common axis of symmetry 10 each to be varied between 10° and 50°.

It has been found that by varying the angle θ of the input optical path 18 at which the optical signal is directed towards the photonic molecule 3 relative to the common axis of symmetry 10, the delay to which the optical signal is subjected by the optical delay device 1 may be varied. Additionally, it has been found that by varying the angle α of the output optical path 19 at which the optical signal is detected from the photonic molecule 3 relative to the common axis of symmetry 10, the delay to which the optical signal is subjected by the optical delay device 1 may also be varied. Increasing either or both of the angles θ and α increases the delay to which the optical signal is subjected. Additionally, the spacing between the whispering gallery or resonance modes of the optical signal increases with increases in either or both of the angles θ or α, but splitting occurs at angles of θ or α greater than zero.

In this embodiment of the invention the microspheres 8 are partially metallic coated, and a means for moving the microsphere 8a relative to the microsphere 8b along the common axis of symmetry 10 comprises an electromagnetic field generator 26 provided by an electromagnetic field generating coil, which extends around the substrate 5, and which is illustrated in block representation, for co-operating with the metallic coating on the microspheres 8 for moving the microsphere 8a relative to the microsphere 8b. The electromagnetic field generator 26 generates an electromagnetic field of varying strength under the control of a control circuit (not shown) for selectively varying the spacing between the microspheres 8 along the common axis of symmetry 10 for varying the optical coupling of the microspheres 8, and in turn the delay to which the optical signal is subjected in the photonic molecule 3. The microspheres 8 are constrained in the cavity 4 so that the as the microsphere 8a moves relative to the microsphere 8b under the effect of the electromagnetic field, the centre of the microsphere 8a describes a locus which coincides with the common axis of symmetry 10, thus, as the microsphere 8a is being moved, the respective centres of the microspheres 8 are disposed on the common axis of symmetry 10. Additionally, movement of the microsphere 8a relative to the microsphere 8b is constrained in the cavity 4 so that the microsphere 8a is moveable between one of the extreme positions with the outer surfaces 11 of the microspheres 8 touching at 12 adjacent the common axis of symmetry 10 as illustrated in Fig. 1 , and the other extreme position (not shown) with the spacing between the outer surfaces 11 of the microspheres 8 along the common axis of symmetry 10 not exceeding 2 microns. It is believed that the benefits of optical coupling two microspheres each of 3 microns diameter falls off rapidly at spacings of greater than 2 microns between the outer surfaces of the microspheres.

In use, an optical signal to be delayed is passed through the input fibre optic cable 14 and is directed at the photonic molecule 3 along the input optical path 18 at the angle θ relative to the common axis of symmetry 10. The optical signal is subjected to multiple reflections within the respective microspheres 8 before it is detected by the output fibre optic cable 16 along the output optical path 19 at the angle α relative to the common axis of symmetry 10, thereby producing a delay in the optical signal. By adjusting the angle θ at which the optical signal is directed to the photonic molecule 3 relative to the common axis of symmetry 10 and/or the angle α at which the optical signal is detected from the photonic molecule 3 relative to the common axis of symmetry 10, the desired delay to which the optical signal is to be subjected in the optical delay device 1 can be selected. Additionally, further varying of the delay to which the optical signal is subjected can be achieved by varying the spacing between the microspheres 8 by appropriately selecting the electromagnetic field generated by the electromagnetic field generator 26. However, for given selected values of the angles θ and α, the delay to which the optical signal is subjected in the optical delay device 1 is further increased by urging the microsphere 8a towards the microsphere 8b, and is further reduced by urging the microsphere 8a away from the microsphere 8b. It is believed that varying the spacing between the microspheres 8 could be used for fine tuning the delay to which the optical signal is subjected in the delay device 1 , after coarse tuning has been carried out by varying one or both the angles θ and α. It has been found that by directing an optical signal of laser light at the photonic ^" molecule 3 just described along an input optical path 18 at an angle θ greater than zero to the common axis of symmetry 10 of the microspheres 8, even when the output optical path 19 coincides with the common axis of symmetry, the delay to which the optical signal is subjected in the photonic molecule 3 is surprisingly greater than when the input optical path 18 and the output optical path 19 coincide with the common axis of symmetry 10, in other words, the delay is greater than that which is achieved when the optical signal is directed to give end fire excitation and when the delayed optical signal is detected along the common axis of symmetry 10. While it has been known that when an optical signal is directed at a photonic molecule along the common axis of symmetry defined by a pair of microspheres, and is detected along the common axis of symmetry, the delay to which the optical signal is subjected is greater than that which can be achieved by the individual uncoupled microspheres separately, the increase in the delay by directing the optical signal at the photonic molecule 3 at an angle θ to the common axis of symmetry 10 greater than zero is significantly and surprisingly greater than that which can be achieved when the optical signal is directed at the photonic molecule along the common axis of symmetry 10. The delay to which the optical signal is subjected increases as the angle θ is increased. Additionally, the delay to which the optical signal is subjected increases as the angle α is increased. However, it is believed that optimum results may be achieved at an angle of θ or α lying between 10° and 50° to the common axis of symmetry 10, although further experiments are required to confirm this.

Referring now to Figs. 2 to 6, comparative tests have been carried out on an optical delay device 30 according to another embodiment of the invention which is illustrated in Fig. 2, and a prior art optical delay device 31 which is illustrated in Fig. 3, in order to determine the improvement in the delay to which an optical signal is subjected in a photonic molecule similar to the photonic molecule 3 when the optical signal is directed at the photonic molecule along an input optical path disposed at an angle θ to the common axis of symmetry 10, which is greater than zero, and when the delayed optical signal is detected along an output optical path disposed at an angle α to the common axis of symmetry 10, which is greater than zero. Both optical delay devices 30 and 31 comprise identical photonic molecules 34, which are substantially similar to the photonic molecule 3 described with reference to the optical delay device 1 , with the exception that the microspheres 8 of the optical delay device 30 and the prior art optical delay device 31 are not moveable relative to each other, and accordingly, are not metal-coated. However, the microspheres 8 of both the optical delay device 30 and the prior art optical delay device 31 are coated with nanocrystals of cadmium telluride (CdTe), which emit light in the visible spectral region for facilitating detecting of the optical signal from the photonic molecules 34. The microspheres 8 of the respective photonic molecules 34 are similar to each other, and of similar diameter to the microspheres 8 of the photonic molecule 3 of the optical delay device 1. The photonic molecules 34 of the optical delay devices 30 and 31 are located in a cavity 35 of a substrate 36, which in each case is a polystyrene substrate containing three-dimensionally ordered arrays of pores of 5 microns in size prepared by a thermocapillary convection process. The pores form the respective cavities 35. The microspheres 8 of the photonic molecules 34 of the optical delay devices 30 and 31 are located in the respective cavities 35 touching each other adjacent the common axis of symmetry. In the optical delay device 30 according to the invention the input means for directing the optical signal at the photonic molecule 34 comprises an objective lens 37 which directs the optical signal at one of the microspheres, namely, the microsphere 8a of the photonic molecule 34 along an input optical path 18, which is disposed at an angle θ of approximately 30° to the common axis of symmetry 10 of the photonic molecule 34. The objective lens 37 also acts as an output means for detecting the delayed optical signal from the same microsphere 8a. Accordingly, in this case the output optical path 19 coincides with the input optical path 18, and both are thus disposed to the common axis of symmetry at the angle of approximately 30°.

In the prior art optical delay device 31 the optical signal is directed at the microsphere 8a of the photonic molecule 34 by an objective lens 37, which is similar to the objective lens 37 of the optical delay device 30. Also in the prior art optical delay device 31 , the optical signal is detected from the same microsphere 8a of the photonic molecule 34 by the objective lens 37, and the input and output optical paths 18 and 19 coincide. However, the photonic molecule 34 of the prior art optical delay device 31 is end fire excited by arranging the objective lens 37 for directing the optical signal at and detecting the optical signal from the photonic molecule 34 along the input and output optical paths 18 and 19, both of which coincide with the common axis of symmetry 10.

The quality factor of a. microsphere, or a photonic molecule is accepted as a measure of the length of the optical path through which an optical signal must traverse within a microsphere or a photonic molecule, and in turn is a measure of the delay produced by a microsphere or a photonic molecule. From the comparative tests carried out on the optical delay device 30 according to the invention, and the prior art optical delay device 31, it has been demonstrated that a significant increase in the quality factor has been achieved by the optical delay device 30 according to the invention over and above the quality factor achieved by the prior art optical delay device 31, thus indicating a significant increase in the delay to which the optical signal is subjected in the optical delay device 30 according to the invention over the delay to which the optical signal is subjected in the prior art optical delay device 31.

Referring now to Figs. 4 and 5. Figs. 4 and 5(a) illustrate a plot of the emission intensity in arbitrary units plotted against wavelength of the emission spectra of the optical signal detected from the photonic molecule 34 of the optical delay device 30 and the prior art optical delay device 31 , respectively. Fig. 5(b) illustrates a plot of the emission intensity plotted against wavelengths of the emission spectra of an optical signal detected separately from two non-optically coupled microspheres, which are substantially identical to the two microspheres 8 of the photonic molecule 34 of the optical delay device 30. The results illustrated in Fig. 5(b) were obtained by separately exciting the respective microspheres 8, and superimposing the spectrum of the detected optical signal from one of the microspheres on the spectrum of the detected optical signal from the other microsphere. The non- optically coupled resonance modes of the respective microspheres are illustrated by the peaks G and H. The spectral region A of the spectrum of Fig. 4 represents the emission spectra of the optical signal detected from the photonic molecule 34 of the optical delay device 30 according to the invention, while the spectral region B of the spectrum of Fig. 5(a) represents the corresponding emission spectra of the optical signal detected from the photonic molecule 34 of the prior art optical delay device 31. As can be seen from the spectral region A of Fig. 4, the coupled resonance modes of the detected optical signal are sharply defined, thus indicating a high quality factor, and thus indicating that the optical delay device 30 according to the invention provides a substantial and significant delay of the optical signal. By contrast, in the spectral region B of the spectrum of Fig. 5(a), the optically coupled resonance modes of the detected optical signal, which are indicated by the reference letters E and F, are not nearly as sharply defined as in the spectral region A of the spectrum of Fig. 4, thus indicating a relatively low quality factor for the prior art optical delay device 31 , and thus indicating that the prior art optical delay device 31 provides a significantly shorter delay to the optical signal than that provided by the optical delay device 30 according to the invention. Indeed, as can be seen from Fig. 5(a), the optically uncoupled resonance modes of the optical signal which are indicated by the reference letters C and D are still present in the detected optical signal, thus indicating that a significant amount of energy of the optical signal is lost in the photonic molecule 34 of the prior art optical delay device 31 in driving the uncoupled modes, which could otherwise be used in providing the coupled modes of the optical signal. The optically uncoupled resonance modes C and D of Fig. 5(a) correspond to the resonance modes G and H of the spectra of the detected optical signals from the optically uncoupled microspheres 8 illustrated in Fig. 5(b). Furthermore, it can be seen from the spectral region A of the spectrum of Fig. 4 that the uncoupled modes are virtually entirely suppressed in the photonic molecule 34 of the optical delay device 30, thus indicating that the optical delay device 30 according to the invention is significantly more efficient resulting in significantly less energy being lost from the optical signal than is lost in the prior art optical delay device 31.

Referring now to Fig. 6, the photon lifetime calculated for /n-resonances forming bonding and anti-bonding branches of the photonic molecule 34 of the optical delay device 30 according to the invention are illustrated, where m is an integer representing the azimuthal number of the whispering gallery mode. This is discussed in more detail below. The wavelength of the detected optical signal is plotted on the X-axis in nanometers, while the photon lifetime is plotted on the Y-axis in picoseconds. The photon lifetime for the m-resonances forming the bonding branches is illustrated by the triangles, while the squares illustrate the photon lifetime for the m-resonances forming anti-bonding branches of the photonic molecule. The dark circular dots show the photon lifetime for an individual optically uncoupled microsphere similar to one of the microspheres 8 which form the photonic molecule 34 of the optical delay device 30. As can be seen, there is a significant increase in the photon storage lifetime for the photonic molecule 34 over the individual optically uncoupled microsphere. The resonance modes are numbered 18 to 25 along the top of Fig. 6. For the resonance mode 22 a lifetime of 3.3 picoseconds has been achieved for one of the bonding resonances. With this increase in photon lifetime, the photonic molecule 34 clearly increases the delay to which the optical signal is subjected by a significant amount.

The following is an explanation of the theory behind the invention. An optical signal from the nanocrystal coating of the microspheres 8 of the photonic molecule 3 of the optical delay device 30 is optically coupled into the whispering gallery modes which are observed as strong sharp resonances in the optical spectra. Following an analogy with quantum mechanics, three integers, n, I and m, describing whispering gallery modes, correspond to the angular, radial and the azimuthal quantum numbers, respectively. This approach has enabled small dielectric microspheres to be considered as "photonic atoms". It is well known that the whispering gallery modes extend into the surroundings up to a few micrometers. This effect allows the optical coupling of two microspheres. Such a system of coherently optically coupled "photonic atoms" may be called a "photonic molecule". In analogy to the formation of molecular electronic orbits, two combinations for the electromagnetic field in a system of interacting microspheres: bonding and anti-bonding states are formed. An analogy is illustrated in Fig. 8 of the H-atoms forming a H-molecule with the photonic atoms forming the photonic molecule. Experimentally, the coupling of the photon modes of individual microspheres of the photonic molecule can cause a narrow resonance of a "photonic atom" to split into two modes that are broader than the original resonance i.e. they have a lower quality factor. This phenomenon has been clearly demonstrated in the prior art in a system of two square, photonic dots coupled by a narrow channel, in a dye-stained bisphere system, and in chains of polymer-blend microparticles. The quality factor Q is defined as follows: „ ω λ ^Q = τ Aω^{~ or} AX where ω is the resonance frequency and Δω is the frequency width of the resonance, and λ is the resonance wavelength and Δλ is the wavelength width of the resonance.

The photon lifetime r_c, is directly proportional to the quality factor Q, and therefore a large value of the quality factor Q will result in a large photon lifetime τ_c which is necessary for an optical delay line. To increase the value of the quality factor Q, the width of the resonance Δω or Δλ must be reduced.

Referring now to Fig. 7, Fig. 7 shows the deconvolution of the lineshape of the m-resonances belonging to the transverse-electric TE_n ^l anti-bonding branch of the photonic molecule using Lorentian functions. It has been found that m-resonances of the bonding branch are always sharper than that of the anti-bonding branch, thereby providing a higher quality factor Q value, and in turn longer photon storage lifetime for these modes. However the most remarkable fact is that the quality factor of m-resonances in spectra of the photonic molecule (and therefore lifetime of photon in the resonant modes τ) exceeds the quality factor Q value (and τ) of individual microspheres before contact across the whole spectral region, see Figs. 4, 5(a) and 5(b). This fact along with estimated value of bonding and anti-bonding splitting (-5-7 nm) allows the development of an optical delay line with controllable spectral and temporal tuneability.

Theoretical analysis and computer simulations have been carried out in order to investigate the strong coupling phenomenon in the spectra of photonic molecules when subjected to optical signals in the visible and near-infrared spectral regions. In the theoretical analysis, an m-dependent structure has been analysed for modes accommodated within the communications band (C-band) in optical communications ranging from 1525 nm to 1565 nm. In order to simplify the computer simulations, only photonic molecules comprising identical microspheres with whispering gallery modes having a radial quantum number /equal to one have been considered, taking into account only the interaction between whispering gallery modes of the same n value.

For a given whispering gallery mode polarisation, the value of the splitting between the bonding and anti-bonding modes of a photonic molecule is obtained from the following equation:

te_m = 2T_n X ,M. (1) where X_n = 2πR/λ_π is the size parameter of a resonance with mode number n, λ_n is the corresponding resonant wavelength, T_n is the width of mode n in a single microsphere, which can be calculated within Mie-theory.

For a given radius a of a single microsphere, the coefficient A_n,m(x_n) can be calculated by using the maximum term approximation (MTA) of the single-mode tight-binding method as:

A_n>m(x_n) = -2n(- ϊ)"^+mh% (k₀Z)x J-^-% v x _? ^ r—, (2) V π[n + m\n - m) (n + m) (n - m) where ko = x_n/a is the wavenumber, Z is the characteristic length, and the spherical Hankel function of the first order can be estimated from: _J1O) _(J. ₇Ϊ~ ■«φ[(2» + l/2X« - tanhflf)] _j

(n + 1 / 2 Jy sec « α tannα

Here, α is defined by k₀Z = (2n+1/2)/cosh α. The positions of the m-resonances were estimated as x_m = x ± Ax_m/2. Equations (1) to (3) above give a qualitative guide to analyse a variety of phenomena observed in the spectra of photonic molecules.

Referring now to Figs. 9(a), Fig. 9(a) illustrates the values of the spacing between adjacent m-resonances calculated in the spectral region of the C-band for three photonic molecules, each of which is formed by two identical glass SiOa microspheres, one of the photonic molecules being formed of microspheres of 10 microns diameter, another of the photonic molecules being formed of microspheres of 12 microns diameter, and another of the photonic molecules being formed by microspheres of 16 microns diameter. The mode spacing in GHz which is plotted on the Y-axis is plotted against the difference between the corresponding wavelength and the resonance wavelength of a single one of the microspheres, which is plotted on the X-axis. The inverted triangles on the waveform 1 represent the mode spacing of the m-resonance for the photonic molecule formed by the microspheres of 16 microns diameter. The squares on the waveform 2 represent the mode spacing of the m-resonances of the photonic molecule comprising the microspheres of 12 microns diameter, while the upright triangles on the waveform 3 represent the mode spacing of the m-resonances for the photonic molecules comprising the microspheres of 10 microns diameter. The portions of the waveforms 1 , 2 and 3 to the left-hand side of the zero value on the X-axis represent the mode spacings for the anti-bonding resonance modes, while the portions of the waveforms 1 , 2 and 3 to the right-hand side of the zero value on the X-axis represent the mode spacings for the bonding resonance modes.

The standard for channel spacing in today's optical communication systems is 40GHz-10OGHz, and this was taken into account in deciding on the microsphere sizes. In the model each m-resonance in the optical signal from the photonic molecules can be considered as a channel with spacing between channels being dictated by the size of the microspheres of the photonic molecules and the m number.

For the photonic molecule comprising two microspheres of 10 microns diameter, only one transverse-electric (TE) mode of the individual spheres (λ₂₄ = 1542.4 nm) is found in the C-band. The maximum splitting between bonding and anti-bonding modes when the two spheres are coupled to form a photonic molecule (that is, for m = 1) is 15.7 nm in this case.

In calculating the position of the resonances obtained from the photonic molecule comprising the two microspheres of 16μm diameter, two whispering gallery modes of the individual microspheres have been found in the region of the C-band, namely, TE₄₀(λ=1544.6 nm) and TM₃₉ (λ=1559.6 nm). The maximum splitting in the case of the photonic molecule comprising the two microspheres of 16 microns diameter is much smaller at 8.8 nm for TE₄₀ whispering gallery mode. However, the photonic molecule comprising the two microspheres of 16 microns diameter accommodates a larger number of m-resonances. These results clearly demonstrate that two coupled whispering gallery microspheres generate a modal structure with a controllable number of peaks distributed across the major C-band in contrast to that which can be achieved from single uncoupled microspheres. The passband of the optical delay device according to the invention can be as wide as the width of all communication bands, and can be controlled by the size of the microspheres of the photonic molecules.

Fig. 9(b) illustrates a plot of the photon lifetime for the photonic molecules comprising the respective identical microspheres of sizes of 10 microns, 12 microns and 16 microns diameter for which the mode spacing is illustrated in Fig. 9(a). The photon lifetime in picoseconds of the respective resonance modes plotted in Fig. 9(a) is plotted in Fig. 9(b) on the Y-axis against the difference between the corresponding wavelength and the resonance wavelength of a single one of the microspheres, which is plotted on the X-axis, in the same way as on the X-axis of Fig. 9(a). The squares on the waveform 1 of Fig. 9(b) represent the photon lifetime for the photonic molecule comprising the microspheres of 16 microns diameter for the corresponding resonance modes of Fig. 9(a), while the squares on the waveforms 2 and 3 of Fig. 9(b) similarly represent the photon lifetime for the photonic molecules comprising the microspheres of 12 microns and 10 microns diameters, respectively. As can be seen, the photon lifetime is greatest in the photonic molecule comprising the microspheres of 16 microns diameter, while the photon lifetime in shortest in the photonic molecule comprising the microspheres of 10 microns diameter. Additionally, the photon lifetime is greater in the bonding modes than in the anti-bonding modes.

It is evident from Fig. 9(a) that the interaction between spherical microcavities results in periodic group delay spectra with peaks occurring at each of the m-resonant frequencies with bigger delay time for higher m-values, which implies that the spectral components near these m-resonance spend more time travelling within the photon molecule.

The calculation of the photon lifetime in a photonic molecule is highly complex and has not been carried out. However, in order to estimate the photon lifetimes in the three photonic molecules discussed with reference to Fig. 9(a) in the C-band spectral region, the fact that the relative increase in photon storage time

shows almost linear dependence on azimuthal number m for given mode number n in wide spectral region (n = 17-25) has been relied on. However, it has been found that the slope of this dependence (ATlAm) in its turn varies linearly with n. This finding provides the possibility of extrapolating the ATfAm dependence into region of mode numbers n, which are characteristic of the C band, as is discussed below with reference to Fig. 10.

The observed non-uniformity of spacing between m-resonances (Fig. 9(a)) together with controllable number of modes provide a unique possibility to engineer the distribution of photon storage times in a desired spectral region. Fig. 9(b) shows the estimated distribution of photon lifetime between m-modes of the photonic molecules taking values of photon storage time in single microspheres to be τss=55 picoseconds for TE₄ ^l _Q , 3 picoseconds for

and 0.9 picoseconds for

whispering gallery mode as follows from the Mie-theory calculations. For the smallest photon molecule size, 18 m-modes are available within a window of 12-200 GHz (Fig. 9(b)). The smaller quality factor Q of whispering gallery mode in the individual microspheres forming the photon molecule results in a relatively moderate increase in delay time - from 1.5 picoseconds (obtained for anti-bonding resonance with m - 1 ) up to 7 picoseconds calculated for the corresponding m-resonance of bonding mode of photon molecule. For microspheres of larger size, the number of available modes increases. Indeed, for the photonic molecule comprising the optically coupled microspheres of 12 microns diameter 20 m-resonances fit into the above indicated spacing window, with the biggest value of intermode spacing being 130 GHz. Delay times distributed between these m-modes increase from 5.2 picoseconds up to 32 picoseconds. The upper limit of intermode spacing drops even more for the photonic molecule formed from 16 microns diameter microspheres. In that case, 22 /77-resonances with spacing between 12 GHz and 76 GHz can be seen in Fig. 9(a), providing discrete time delays, which were found to be distributed between 94.5 picoseconds and 960 picoseconds.

Remarkably, this maximum storage lifetime gives an indication on the possibility of achieving tuneable delays in the range of nanoseconds that are of great practical importance and nonetheless cannot be easily obtained with other microcavity structures.

Referring now to Figs. 10(a) and 10(b), Fig. 10(a) is a plot of the emission intensity in arbitrary units plotted on the Y-axis against wavelengths in nanometers plotted on the X-axis for the photonic molecule comprising the identical microspheres of 10 microns diameter described with reference to Figs. 9(a) and 9(b). The waveform A in Fig. 9(a) represents the emission intensity of the anti-bonding and bonding modes of the photonic molecule, while the waveforms C and D illustrate the emission , intensity of the optically uncoupled individual microspheres from which the photonic molecule is constructed. Fig. 10(b) illustrates a plot of the relative increase in the photon storage time T plotted on the Y-axis against wavelengths in nanometers plotted on the X-axis similar to the X-axis of Fig. 10(a). As can be seen, the relative increase in photon storage time T for the m-resonances increase with wavelength for the anti-bonding modes and for the bonding modes, but the slope is significantly greater for the bonding modes than the anti-bonding modes.

In order to demonstrate the effect of altering the spacing between the microspheres of the photonic molecules discussed with reference to Figs. 9(a) and 9(b), computer simulations were carried out on the photonic molecule comprising the identical microspheres of 10 microns diameter, and the effect on the modal spacing of altering the spacing between the microspheres is illustrated in Fig. 11. In Fig. 11 mode spacing in GHz is plotted on the Y-axis in similar fashion as in Fig. 9(a), and the difference between the corresponding wavelength and the resonance wavelength of a single one of the microspheres is plotted on the X-axis in similar fashion as in Figs. 9(a) and 9(b). The squares on the waveform A of Fig. 11 show the spacing of the resonant modes for the photonic molecule with the microspheres of 10 microns diameter touching each other, in other words, the distance d between the microspheres along the common axis of symmetry is zero. The circles on the waveform B of Fig. 11 show the spacing of the resonance modes of the photonic molecule with the microspheres of 10 microns diameter spaced apart along the common axis of symmetry a distance d of 0.1 microns. In other words, the distance d between the adjacent surfaces of the microspheres along the common axis of symmetry is 0.1 microns. The triangles of the waveform C show the spacing of the resonance modes for the photonic molecule with the microspheres of 10 microns diameter, and with a spacing between the adjacent surfaces of the microspheres along the common axis of symmetry of 0.2 microns, in other words, the distance d is 0.2 microns. The anti-bonding resonant modes are shown by the waveforms A, B and C to the left-hand side of the zero value of the X-axis while the bonding resonance modes are shown to the right-hand side of the zero value of the X-axis.

The efficiency of coupling between spherical microcavities forming photonic molecule strongly depends on spacing d between microspheres. It is known that the splitting of the bonding and anti-bonding modes decreases exponentially with increasing separation. For the present values of microsphere sizes, mode numbers and small spacing d, the condition In + 1 > X_n (2 + d 1 a) is satisfied and an approximate equation for coefficient A_n,_m{Xn, d) is found:

Replacing A_n,_m(x_n) in Equation (1 ) by A_n,m(Xn.d) we can estimate the m-mode spacing as a function of the spacing d between the microspheres.

Taking for example a photonic molecule formed from 10 microns diameter microspheres the maximum splitting between bonding and anti-bonding modes shows a decrease by a factor 1.3 as the spacing between spheres increases from zero to 0.1 microns. The corresponding modification of the A_nι!r){x_n,d) coefficient results in a significant decrease of the values of mode spacing. While the delay time and number of m-modes available between 12GHz to 200 GHz are unaffected, the upper limit of mode spacings drops down to 139GHz as can be seen in Fig. 11. For the photonic molecule comprising the two microspheres of 10 microns diameter separated by a distance d = 0.2 microns the spacings of all 18 m-modes are distributed between 12GHz and 100GHz as shown in Fig. 11.

Referring now to Fig. 12(a), there is illustrated a photonic molecule indicated generally by the reference numeral 40 for use with an optical delay device according to the invention. In this embodiment of the invention the photonic molecule comprises two optically coupled microspheres 41 and 42. The microsphere 41 is located within the microsphere 42, and both microspheres 41 and 42 are concentric with each other. The outer surface of the outer microsphere 42 is coated with a reflective metallic coating for reflecting optical signals internally, and the outer microsphere 42 is supported on a substrate 43.

Referring now to Fig. 12(b), there is illustrated a photonic molecule indicated generally by the reference numeral 45 for use with an optical delay device according to the invention. In this embodiment of the invention the photonic molecule 45 comprises two optically coupled microspheres 46 and 47. The microsphere 46 is located within the microsphere 47, and both microspheres 46 and 47 are concentric with each other. The outer surface of the outer microsphere 47 is coated with a reflective metallic coating for reflecting an optical signal internally, and the outer microsphere 47 is supported on a light conducting substrate 48. The outer microsphere 47 has its top and bottom removed to accommodate an input optical signal to the photonic molecule 45 at the top, and to accommodate an output optical signal from the photonic molecule 45 at the bottom through the substrate 48.

Referring now to Fig. 13, there is illustrated a photonic molecule 50 for use with an optical delay device also according to the invention. The photonic molecule 50 comprises a pair of optically coupled microspheres, namely, a microsphere 51 and a microsphere 52. The microsphere 51 is located within the microsphere 52, and lies on a common axis of symmetry with the microsphere 51 , however, the centre of the microsphere 51 is offset from the centre of the microsphere 52 on the common axis of symmetry. In this embodiment of the invention the microsphere 52 is formed by a mirror metallic coating 53 formed on the inner surface of a substrate 54 of dielectric material. An opening 55 is formed in the substrate 54 for accommodating an optical signal into and out of the microspheres 51 and 52 of the photonic molecule 50. The respective microspheres 51 and 52 touch at 57 adjacent the common axis of symmetry for facilitating the transfer of the optical signal therebetween.

Referring now to Fig. 14, there is illustrated a photonic molecule, indicated generally by the reference numeral 60, for use in an optical delay device according to another embodiment of the invention. In this embodiment of the invention the photonic molecule comprises a microsphere 61 and a microsphere 62 which is optically coupled with the microsphere 61 , the microsphere 61 being located within the microsphere 62 and defining with the microsphere 62 a common axis of symmetry. However, in this embodiment of the invention the centre of the microsphere 61 is offset from that of the microsphere 62. Additionally, in this embodiment of the invention the microsphere 62 is formed by the reflective inner spherical surface of a metal substrate 64. An opening 65 is formed in the substrate 64 for accommodating an optical signal into and out of the microspheres 61 and 62 of the photonic molecule 60. The respective microspheres 61 and 62 touch at 66 adjacent the common axis of symmetry for facilitating the transfer of the optical signal therebetween.

Referring now to Fig. 15, there is illustrated an optical delay device, indicated generally by the reference numeral 70 according to another embodiment of the invention. In this embodiment of the invention the optical delay device comprises a photonic molecule 71 having a pair of optically coupled microspheres 72a and 72b. An input means comprising an input optical fibre 73 directs the optical signal to the photonic molecule 71. An output means comprising a plurality of output optical fibres 74 detect the delayed optical signal from the photonic molecule 71. Since the delay to which the optical signal is subjected is dependent on the angle α which the output optical path 19 makes with the common axis of symmetry 10 along which the optical signal is detected from the photonic molecule 71 , the delay to which the optical signal is subjected varies from output optical fibre 74 to output optical fibre 74. Accordingly, in this embodiment of the invention five different delays to which the optical signal is subjected can be selected by selecting the appropriate output optical fibre 74 for detecting the optical signal.

Referring now to Fig. 16, there is illustrated an optical delay device, indicated generally by the reference numeral 80, according to a further embodiment of the invention. The optical delay device comprises a photonic molecule 81 having a pair of optically coupled microspheres 82a and 82b. In this embodiment of the invention an input/output means is provided for directing the optical signal to and detecting the optical signal from the photonic molecule 81. The input/output means comprises a bundle 83 of input/output optical fibres 84, which are provided for both directing one or more optical signals at the photonic molecule 81 along respective input optical paths 18, and also for detecting the delayed optical signal from the photonic molecule 81 along respective output optical paths 19 which coincide with the input optical path 18. An output means in this embodiment of the invention is provided by a bundle 86 of output optical fibres 87, which also detect the delayed optical signals from the photonic molecule 80 along respective output optical paths 19. Since the delay to which the optical signal is subjected is dependent on the angle θ which the input optical path 18 makes with the common axis of symmetry 10 and the angle α which the output optical path 19 makes_/ with the common axis of symmetry 10, the delay to which the optical signal is subjected varies from one of the input/output optical fibres 84 to the other, and from one of the output optical fibres 87 to the other.

While the optical delay devices according to the invention which have been described with reference to Figs. 1 to 16 have all been described as comprising a photonic molecule comprising a pair of optically coupled microspheres, it is envisaged that the photonic molecules may be formed by microelements other than microspheres. Indeed, it is envisaged that instead of providing the optical delay devices according to the invention with a photonic molecule, the optical delay devices may be provided with a pair of optically coupled microelements, which would not necessarily form a photonic molecule. For example, such microelemerits may comprise micro-discs, or other microelements which are suitable for subjecting light or an optical signal to multiple reflections, which in general would be multiple whispering gallery mode reflections.

It will also be appreciated that while the photonic molecule of the optical delay device of Fig. 1 has been described with one of the microspheres of the photonic molecule being moveable along the common axis of symmetry relative to the other microsphere for facilitating further varying the delay to which the optical signal is subjected in the photonic molecule, in certain cases both microspheres may be moveable relative to each other. Alternatively, optical delay devices with photonic molecules in which the spacing between the microspheres is fixed may be provided. In this case, in order to maximise the optical coupling, the microspheres would be located on the common axis of symmetry as close as possible, and preferably, along the common axis of symmetry in order to maximise the optical coupling between the microspheres. However, in certain cases, it may be desirable to provide the photonic molecules with spacing between the microspheres.

It will be appreciated that either or both the input means and the output means may be arranged for directing and detecting the optical signal to and from the optically coupled microelements along the input and output optical paths with both the input and output optical paths disposed at an angle to the common axis of symmetry greater than zero, or with only one of either the input optical path or the output optical path disposed at an angle to the common axis of symmetry greater than zero, and in such a case, the other of the input or output optical path would coincide with the common axis of symmetry.

While the optical devices according to the invention have been described as comprising photonic molecules formed by two microspheres, it is envisaged that in certain cases, optical delay devices according to the invention may be provided with photonic molecules comprising more than two microspheres or other such microelements, which would be optically coupled. The sizes of the microelements may be similar or different, and the spacing between the respective microelements may be similar or different. Additionally, some or all of the microelements may touch each other or may touch in pairs along their respective common axes of symmetry. Needless to say, where a photonic molecule comprises more than two microelements, pairs of microelements may define respective common axes of symmetry. For example, where a photonic molecule comprises three microelements, one of the microelements may be optically coupled to another one of the microelements along one common axis of symmetry while being coupled to another one of the microelements along a different common axis of symmetry.

While specific means for moving the carrier members which carry the input and output fibre optic cables for varying the angles at which the respective input optical path and output optical path are disposed to the common axis of symmetry, any other suitable means for moving the carrier members may be provided. In certain cases, the carrier members may be manually moveable. Needless to say, in certain cases, instead of providing the carrier members as being moveable relative to the photonic molecule, in certain cases, it is envisaged that the photonic molecule may be moveable relative to the carrier member for altering one or both of the angles at which the input optical path and/or the output optical path are disposed to the common axis of symmetry.

Needless to say, while the means for moving one of the microelements relative to the other microelement of the photonic molecule of the optical delay device of Fig. 1 has been described as comprising an electromagnetic field generator, any other suitable means for moving one or both of the microelements relative to each other may be used. For example, it is envisaged that one or both of the microelements may be mounted on a moveable stage, for example, a piezo-electric stage, which would allow one or both of the microelements to be moved relative to the other with relatively fine control. Alternatively, an optical tweezers may be used for moving one of the microelements relative to the other. Such an optical tweezers would typically comprise intense highly focused light, which would move one or both of the microspheres over relatively short distances, in the order of microns. Needless to say, any other suitable means for moving one or both of the microelements relative to the other may be provided.

Claims

Claims 1. A method for delaying an optical signal, the method comprising the steps of: providing at least two optically coupled microelements of a light conducting medium, for producing multiple whispering gallery mode reflections of light internally therein and for exchanging light therebetween for delaying the optical signal, the microelements defining a common axis of symmetry with their respective centres lying on the common axis of symmetry, directing the optical signal at one of the microelements along an input optical path, and detecting the delayed optical signal from any one of the microelements along an output optical path, characterised in that at least one of the input optical path and the output optical path is disposed at an angle greater than zero to the common axis of symmetry.

2. A method as claimed in Claim 1 characterised in that the centres of the microelements are spaced apart along the common axis of symmetry.

3. A method as claimed in Claim 1 or 2 characterised in that the microelements touch each other.

4. A method as claimed in Claim 3 characterised in that the microelements touch each other adjacent the common axis of symmetry.

5. A method as claimed in Claim 1 or 2 characterised in that the microelements are spaced apart from each other.

6. A method as claimed in any preceding claim characterised in that at least one of the microelements is moveable relative to the other for varying the delay to which the optical signal is subjected.

7. A method as claimed in Claim 6 characterised in that the moveable one of the microelements is moveable along the common axis of symmetry.

8. A method as claimed in any preceding claim characterised in that the delayed optical signal is detected from the microelement at which the optical signal is directed.

9. A method as claimed in Claim 8 characterised in that the input optical path and the output optical path coincide.

10. A method as claimed in any of Claims 1 to 7 characterised in that the delayed optical signal is detected from one of the microelements other than the microelement at which the optical signal is directed.

11. A method as claimed in any preceding claim characterised in that the input optical path is disposed to the common axis of symmetry at the angle greater than zero.

12. A method as claimed in Claim 11 characterised in that the angle at which the input optical path is disposed to the common axis of symmetry lies in the range of 1° to 180°.

13. A method as claimed in Claim 12 characterised in that the angle at which the input optical path is disposed to the common axis of symmetry lies in the range of 10° to 50°.

14. A method as claimed in Claim 13 characterised in that the angle at which the input optical path is disposed to the common axis of symmetry lies in the range of 35° to 40°.

15. A method as claimed in any preceding claim characterised in that the angle at which the input optical path is disposed to the common axis of symmetry is variable for varying the delay to which the optical signal is subjected.

16. A method as claimed in any preceding claim characterised in that the output optical path is disposed to the common axis of symmetry at the angle greater than zero.

17. A method as claimed in Claim 16 characterised in that the output optical path is disposed to the common axis of symmetry at an angle in the range of 1° to 180°.

18. A method as claimed in Claim 17 characterised in that the output optical path is disposed to the common axis of symmetry at an angle in the range of 10° to 50°.

19. A method as claimed in Claim 18 characterised in that the output optical path is disposed to the common axis of symmetry at an angle in the range of 35° to 40°.

20. A method as claimed in any preceding claim characterised in that the angle at which the output optical path is disposed to the common axis of symmetry is variable for varying the delay to which the optical signal is subjected.

21. A method as claimed in any preceding claim characterised in that the microelements are of substantially similar size.

22. A method as claimed in any of Claims 1 to 20 characterised in that the microelements are of different sizes.

23. A method as claimed in any preceding claim characterised in that the microelements are of size up to 50 microns.

24. A method as claimed in Claim 23 characterised in that the microelements are of size in the range of 3 microns to 20 microns.

25. A method as claimed in any preceding claim characterised in that the spacing between adjacent surfaces of the microelements does not exceed 2 microns.

26. A method as claimed in any preceding claim characterised in that the light conducting media of the respective microelements are of relatively high refractive indices.

27. A method as claimed in any preceding claim characterised in that the refractive indices of the light conducting media of the respective microelements lies in the range of 1.1 to 3.0.

28. A method as claimed in Claim 27 characterised in that the refractive indices of the light conducting media of the respective microelements lies in the range of 1.5 to 2.0.

29. A method as claimed in Claim 28 characterised in that the refractive indices of the light conducting media of the respective microelements is approximately 1.8.

30. A method as claimed in any preceding claim characterised in that the light conducting media of the respective microelements are of similar refractive indices.

31. A method as claimed in any of Claims 1 to 29 characterised in that the light conducting media of the respective microelements are of different refractive indices.

32. A method as claimed in any preceding claim characterised in that the light conducting medium of at least one of the microelements is selected from any one or more of the following materials: melamine formaldehyde latex, polystyrene, glass, glass doped with ions of one or more rare earths, glass doped with ions of one or more transition metals, glass doped with dye molecules, and a semiconductor material.

33. A method as claimed in Claim 32 characterised in that when the light conducting medium of the at least one of the microelements is of glass doped with ions of one or more rare earths, the rare earths are selected from any one or more of the following rare earths: Erbium (Er), Thulium (Tm), and other first row rare-earth ions.

34. A method as claimed in Claim 32 or 33 characterised in that when the light conducting medium of the at least one of the microelements is of glass doped with ions of one or more transition metals, the transition metals are selected from any one or more of the following transition metals: Chromium (Cr), Cobalt (Co), Nickel (Ni), Manganese (Mn), and Iron (Fe).

35. A method as claimed in any of Claims 32 to 34 characterised in that when the light conducting medium of at least one of the microelements is a semiconductor material, the semiconductor material is selected from any one or more of the following semiconductor materials: Gallium Arsenide (GaAs), Aluminium Arsenide (AIAs), Indium Phosphide (InP), Indium Nitride (InN), Gallium Nitride (GaN), Gallium Phosphide (GaP), and any and all alloys of any one or more of the above listed semiconductor materials.

36. A method as claimed in any preceding claim characterised in that the light conducting media of the respective microelements are similar to each other.

37. A method as claimed in any of Claims 1 to 35 characterised in that the light conducting media of the respective microelements are different to each other.

38. A method as claimed in any preceding claim characterised in that at least one of the microelements is coated with nanocrystals of a light emitting medium.

39. A method as claimed in Claim 38 characterised in that the coating of nanocrystals with which the at least one microelement is coated is selected from any one or more of the following media: cadmium telluride (CdTe), cadmium selenide (CdSe), zinc selenide (ZnSe), zinc sulphide (ZnS), zinc telluride (ZnTe), zinc oxide (ZnO), and any and all alloys of any one or more of the above listed media.

40. A method as claimed in Claim 38 or 39 characterised in that each of the microelements is coated with nanocrystals of a light emitting medium.

41. A method as claimed in any preceding claim characterised in that the microelements are microspheres.

42. A method as claimed in Claim 41 characterised in that the respective microspheres are of substantially similar diameter.

43. A method as claimed in Claim 41 characterised in that the respective microspheres are of different diameters.

44. A method as claimed in any of Claims 41 to 43 characterised in that the microspheres are of diameter up to 50 microns.

45. A method as claimed in any of Claims 41 to 44 characterised in that the microspheres are of diameter of at least 1.5 microns.

46. A method as claimed in any preceding claim characterised in that the microelements form a photonic molecule.

47. An optical delay device for delaying an optical signal, the optical delay device comprising: at least two optically coupled microelements of a light conducting medium for producing multiple whispering gallery mode reflections of light internally therein and for exchanging light therebetween for delaying the optical signal, the microelements defining a common axis of symmetry with their respective centres lying on the common axis of symmetry, a means for directing the optical signal at one of the microelements along an input optical path, and a means for detecting the delayed optical signal from any one of the microelements along an output optical path, characterised in that at least one of the input optical path and the output optical path is disposed at an angle greater than zero to the common axis of symmetry.

48. An optical delay device as claimed in Claim 47 characterised in that the centres of the microelements are spaced apart along the common axis of symmetry.

49. An optical delay device as claimed in Claim 47 or 48 characterised in that the microelements touch each other.

50. An optical delay device as claimed in Claim 49 characterised in that the microelements touch each other adjacent the common axis of symmetry.

51. An optical delay device as claimed in Claim 47 or 48 characterised in that the microelements are spaced apart from each other.

52. An optical delay device as claimed in any of Claims 47 to 51 characterised in that at least one of the microelements is moveable relative to the other microelement for varying the delay to which the optical signal is subjected.

53. An optical delay device as claimed in Claim 52 characterised in that a means is provided for moving the moveable one of the microelements.

54. An optical delay device as claimed in any of Claims 47 to 53 characterised in that the means for detecting the delayed optical signal detects the delayed optical signal from the microelement at which the optical signal is directed by the means for directing the optical signal.

55. An optical delay device as claimed in any of Claims 47 to 53 characterised in that the means for detecting the delayed optical signal detects the delayed optical signal from one of the microelements other than the microelement at which the optical signal is directed by the means for directing the optical signal.

56. An optical delay device as claimed in any of Claims 47 to 55 characterised in that the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle disposed to the common axis of symmetry greater than zero.

57. An optical delay device as claimed in any of Claims 47 to 56 characterised in that the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 1° to 180°.

58. An optical delay device as claimed in Claim 57 characterised in that the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 10° to 50°

59. An optical delay device as claimed in Claim 58 characterised in that the means for directing the optical signal at one of the microelements directs the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 35° to 40°.

60. An optical delay device as claimed in any of Claims 47 to 59 characterised in that one of the means for directing the optical signal and the optically coupled microelements is moveable relative to the other for varying the angle at which the input optical path is disposed to the common axis of symmetry for varying the delay to which the optical signal is subjected.

61. An optical delay device as claimed in Claim 60 characterised in that a first drive means is provided for moving one of the means for directing the optical signal and the optically coupled microelements relative to the other for varying the angle at which the input optical path is disposed to the common axis of symmetry.

62. An optical delay device as claimed in any of Claims 47 to 61 characterised in that the means for detecting the optical signal from one of the microelements detects the optical signal along the output optical path disposed at an angle to the common axis of symmetry greater than zero.

63. An optical delay device as claimed in any of Claims 47 to 62 characterised in that the means for detecting the optical signal from one of the microelements detects the optical signal along the output optical path disposed at an angle disposed to the common axis of symmetry in the range of 1 ° to 180°.

64. An optical delay device as claimed in Claim 63 characterised in that the means for detecting the optical signal from one of the microelements detects the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 10° to 50°.

65. An optical delay device as claimed in Claim 64 characterised in that the means for detecting the optical signal from one of the microelements detects the optical signal along the input optical path disposed at an angle to the common axis of symmetry in the range of 35° to 40°.

66. An optical delay device as claimed in any of Claims 47 to 65 characterised in that one of the means for detecting the optical signal and the optically coupled microelements is moveable relative to the other for varying the angle at which the output optical path is disposed to the common axis of symmetry for varying the delay to which the optical signal is subjected.

67. An optical delay device as claimed in Claim 66 characterised in that a second drive means is provided for moving one of the means for detecting the optical signal and the optically coupled microelements relative to the other for varying the angle at which the output optical path is disposed to the common axis of symmetry.

68. An optical delay device as claimed in any of Claims 47 to 67 characterised in that the microelements are of substantially similar size.

69. An optical delay device as claimed in any of Claims 47 to 67 characterised in that the microelements are different sizes.

70. An optical delay device as claimed in any of Claims 47 to 69 characterised in that the microelements are of size up to 50 microns.

71. An optical delay device as claimed in Claim 70 characterised in that the microelements are of size in the range of 3 microns to 20 microns.

72. An optical delay device as claimed in any of Claims 47 to 71 characterised in that the microelements are spaced apart such that the spacing between adjacent surfaces of the microelements does not exceed 2 microns.

73. An optical delay device as claimed in any of Claims 47 to 72 characterised in that a means for moving one of the microelements relative to the other is provided for varying the delay to which the optical signal is subjected.

74. An optical delay device as claimed in any of Claims 47 to 73 characterised in that the light conducting media of the respective microelements is relatively high refractive index.

75. An optical delay device as claimed in any of Claims 47 to 74 characterised in that the refractive indices of the light conducting media of the respective microelements lies in the range of 1.1 to 3.0.

76. An optical delay device as claimed in Claim 75 characterised in that the refractive indices of the light conducting media of the respective microelements lies in the range of 1.5 to 2.0.

77. An optical delay device as claimed in Claim 76 characterised in that the refractive indices of the light conducting media of the respective microelements is approximately 1.8.

78. An optical delay device as claimed in any of Claims 74 to 77 characterised in that the light conducting media of the respective microelements are of substantially similar refractive indices.

79. An optical delay device as claimed in any of Claims 47 to 77 characterised in that the light conducting media of the respective microelements are of different refractive indices.

80. An optical delay device as claimed in any of Claims 47 to 79 characterised in that the light conducting medium of at least one of the microelements is selected from any one or more of the following materials: melamine formaldehyde latex, polystyrene, glass, glass doped with ions of one or more rare earths, glass doped with ions of one or more transition metals, glass doped with die molecules, and a semiconductor material.

81. An optical delay device as claimed in Claim 80 characterised in that when the light conducting medium of the at least one of the microelements is of glass doped with ions of one or more rare earths, the rare earths are selected from any one or more of the following rare earths: Erbium (Er), Thulium (Tm), and other first row rare-earth ions.

82. An optical delay device as claimed in Claim 80 or 81 characterised in that when the light conducting medium of the at least one of the microelements is of glass doped with ions of one or more transition metals, the transition metals are selected from any one or more of the following transition metals: Chromium (Cr), Cobalt (Co), Nickel (Ni), Manganese (Mn), and Iron (Fe).

83. An optical delay device as claimed in any of Claims 80 to 82 characterised in that when the light conducting media of the at least one of the microelements is a semiconductor material, the semiconductor material is selected from any one or more of the following semiconductor materials: Gallium Arsenide (GaAs), Aluminium Arsenide (AIAs), Indium Phosphide (InP), Indium Nitride (InN), Gallium Nitride (GaN), Gallium Phosphide (GaP)₁ and any and all alloys of any one or more of the above listed semiconductor materials.

84. An optical delay device as claimed in any of Claims 47 to 83 characterised in that the light conducting media of the respective microelements are similar.

85. An optical delay device as claimed in any of Claims 47 to 83 characterised in that the light conducting media of the respective microelements are different.

86. An optica! delay device as claimed in any of Claims 47 to 85 characterised in that at least one of the microelements is coated with nanocrystals of a light emitting medium.

87. An optical delay device as claimed in Claim 86 characterised in that the coating of nanocrystals with which the at least one of the microelements is coated is selected from any one or more of the following media: cadmium telluride (CdTe), cadmium selenide (CdSe), zinc selenide (ZnSe), zinc sulphide (ZnS), zinc telluride (ZnTe), zinc oxide (ZnO), and any and all alloys of any one or more of the above listed media.

88. An optical delay device as claimed in Claims 86 or 87 characterised in that each microelement is coated with nanocrystals of a light emitting medium.

89. An optical delay device as claimed in any of Claims 47 to 88 characterised in that the microelements are microspheres.

90. An optical delay device as claimed in Claim 89 characterised in that the respective microspheres are of substantially similar diameter.

91. An optical delay device as claimed in Claim 89 characterised in that the respective microspheres are of different diameters.

92. An optical delay device as claimed in any of Claims 89 to 91 characterised in that the microspheres are of diameter up to 50 microns.

93. An optical delay device as claimed in any of Claims 89 to 92 characterised in that the microspheres are of diameter of at least 1.5 microns.

94. An optical delay device as claimed in any of Claims 47 to 93 characterised in that the microelements are provided in the form of a photonic molecule.