GB2175682A

GB2175682A - Optical devices using optical fibres

Info

Publication number: GB2175682A
Application number: GB08512385A
Authority: GB
Inventors: Terry Bricheno
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-05-16
Filing date: 1985-05-16
Publication date: 1986-12-03
Also published as: GB2175682B; GB8512385D0

Abstract

A low loss light diffuser is comprised by an optical fibre input (8) whose output end (9) is disposed within an assembly of small particles such as microspheres (12). An optical signal applied to the fibre is uniformally diffused by multiple fresnel reflections at the surfaces of the small particles. <IMAGE>

Description

SPECIFICATION Optical devices This invention relates to optical devices and in particular but not exclusively to a device for uniformally spreading light input thereto.

According to the present invention there is provided an optical device comprising an assembly of small particles and an input for an optical signal to the interior of the assembly, which assembly of small particles serves to diffuse an optical signal applied to the input by multiple fresnel reflection at the small particles' surfaces.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates use of a probe with a low loss diffuser at the tip thereof; Figure 2 illustrates multiple fresnel reflection at the surface of a number of microspheres; Figure 3 illustrates on an enlarged scale compared to Fig. 1 the probe tip and diffuser; Figure 4 illustrates a conventional integrating sphere, and Figure 5 illustrates a low loss diffuser and detector array for use in stable power measurement schemes.

As part of a treatment to remove tumours 1 (Fig. 1) formed on the inside of the bladder 2, it is required to inject Argon ion laser light at a moderately high throughput ( 10 watts) and provide uniform illumination over the whole of the internal, essentially spherical, surface. The size of the probe 3 to do this should be t3mm diameter.

A large core multimode optical fibre 4 is ideally suited to the purpose of delivering the light to the probe tip 5, but no simple lens, mirror or taper arrangement can perform the uniform spreading function.

The solution proposed here involves the use of a small diffuser which relies for its action upon multiple Fresnel reflections at the surface of small particles, for example microspheres, of optical glass or silica, although irregular shapes will work just as well, thus avoiding any self-heating of the probe tip due to absorption at the power levels envisaged.

If the light output from the fibre end passes through a region of radius R at the probe tip, which region is for example packed with microspheres 6 of radius r and refractive index n1, the spaces 7 between the microspheres being filled with air of refractive index n2, then at each interface the light will suffer a reflection of approximately (nt-n2)2/(n,+nJ2. The number of interfaces to be negotiated is approximately R/r, and therefore the fraction which arrives at the outside world is approximately (1-((nl-n2)2/(n,+n2)2))R' and this must be kept small, say < 10-6. Putting R--lmm, n11.45(silica) and n2~1(air) gives r= ;,um.

Fig. 3 illustrates a proposed embodiment of probe tip which somewhat resembles the bulb end of a thermometer. An input fibre 8 is provided with a tapered end 9, that is it is drawn down to a taper, and extends into the bulb element 10 which is manufactured from optical transparent material for example silica.

The chamber 11 of the bulb element 10 is packed with silica microspheres 12. Typically the input fibre 8 is approximately 300,us in diameter (large core, large numerical aperture (NA)), the bulb tipe diameter D is 2.5mm and the microspheres are 10Am in diameter. The tapered end 9 of the input fibre glass an initial spreading of the beam into the diffuser and suppresses reflections back to the driving laser (not shown). Thus an optical signal applied to the input fibre is diffused and uniformally spread by the bulb tip.

Whereas the optical diffuser device of the invention has so far been described in terms of medical applications, its also has other applications, for example in stable optical power measurement schemes as a sort of integrating sphere turned inside out.

An integrating sphere is essentially a device for scrambling the mode distribution of input light in order to render the detected power level independent of the input mode distribution. A conventional integrating sphere system thus comprises a sphere 13 (Fig. 4), an input hole 14 in the sphere, an input fibre 15 and a detector 16 disposed to detect light on a respective internal surface portion of the sphere.

The efficiency of such integrating spheres, small ones in particular, is limited by losses in the diffusing coating on the internal surface of the sphere and is oftenil0%.

An optical diffuser device of the present invention may be employed to measure optical power input to a fibre 17, if a diffuser tip 18 thereof filled with microspheres is disposed within an array of detectors. A possible detector array shown in Fig. 5 comprises four large area detectors 19 each disposed on a respective surface of an internally polished tetrahedron 20 in which the diffuser tip 18 is disposed, there being an input hole 21 for the input fibre 17. Such an arrangement has an efficiency greater than 60% and is capable of handling high power inputs.

1. An optical device comprising an assembly of small particles and an input for an optical signal to the interior of the assembly, which assembly of small particles serves to diffuse an optical signal applied to the input by multiple fresnel reflection at the small particles' surfaces.

2. An optical device as claimed in claim 1, wherein the input comprises an optical fibre one end of which is disposed within the assembly.

3. An optical device as claimed in claim 2, wherein the one end of the optical fibre is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical devices This invention relates to optical devices and in particular but not exclusively to a device for uniformally spreading light input thereto. According to the present invention there is provided an optical device comprising an assembly of small particles and an input for an optical signal to the interior of the assembly, which assembly of small particles serves to diffuse an optical signal applied to the input by multiple fresnel reflection at the small particles' surfaces. Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates use of a probe with a low loss diffuser at the tip thereof; Figure 2 illustrates multiple fresnel reflection at the surface of a number of microspheres; Figure 3 illustrates on an enlarged scale compared to Fig. 1 the probe tip and diffuser; Figure 4 illustrates a conventional integrating sphere, and Figure 5 illustrates a low loss diffuser and detector array for use in stable power measurement schemes. As part of a treatment to remove tumours 1 (Fig. 1) formed on the inside of the bladder 2, it is required to inject Argon ion laser light at a moderately high throughput ( 10 watts) and provide uniform illumination over the whole of the internal, essentially spherical, surface. The size of the probe 3 to do this should be t3mm diameter. A large core multimode optical fibre 4 is ideally suited to the purpose of delivering the light to the probe tip 5, but no simple lens, mirror or taper arrangement can perform the uniform spreading function. The solution proposed here involves the use of a small diffuser which relies for its action upon multiple Fresnel reflections at the surface of small particles, for example microspheres, of optical glass or silica, although irregular shapes will work just as well, thus avoiding any self-heating of the probe tip due to absorption at the power levels envisaged. If the light output from the fibre end passes through a region of radius R at the probe tip, which region is for example packed with microspheres 6 of radius r and refractive index n1, the spaces 7 between the microspheres being filled with air of refractive index n2, then at each interface the light will suffer a reflection of approximately (nt-n2)2/(n,+nJ2. The number of interfaces to be negotiated is approximately R/r, and therefore the fraction which arrives at the outside world is approximately (1-((nl-n2)2/(n,+n2)2))R' and this must be kept small, say < 10-6. Putting R--lmm, n11.45(silica) and n2~1(air) gives r= ;,um. Fig. 3 illustrates a proposed embodiment of probe tip which somewhat resembles the bulb end of a thermometer. An input fibre 8 is provided with a tapered end 9, that is it is drawn down to a taper, and extends into the bulb element 10 which is manufactured from optical transparent material for example silica. The chamber 11 of the bulb element 10 is packed with silica microspheres 12. Typically the input fibre 8 is approximately 300,us in diameter (large core, large numerical aperture (NA)), the bulb tipe diameter D is 2.5mm and the microspheres are 10Am in diameter. The tapered end 9 of the input fibre glass an initial spreading of the beam into the diffuser and suppresses reflections back to the driving laser (not shown). Thus an optical signal applied to the input fibre is diffused and uniformally spread by the bulb tip. Whereas the optical diffuser device of the invention has so far been described in terms of medical applications, its also has other applications, for example in stable optical power measurement schemes as a sort of integrating sphere turned inside out. An integrating sphere is essentially a device for scrambling the mode distribution of input light in order to render the detected power level independent of the input mode distribution. A conventional integrating sphere system thus comprises a sphere 13 (Fig. 4), an input hole 14 in the sphere, an input fibre 15 and a detector 16 disposed to detect light on a respective internal surface portion of the sphere. The efficiency of such integrating spheres, small ones in particular, is limited by losses in the diffusing coating on the internal surface of the sphere and is oftenil0%. An optical diffuser device of the present invention may be employed to measure optical power input to a fibre 17, if a diffuser tip 18 thereof filled with microspheres is disposed within an array of detectors. A possible detector array shown in Fig. 5 comprises four large area detectors 19 each disposed on a respective surface of an internally polished tetrahedron 20 in which the diffuser tip 18 is disposed, there being an input hole 21 for the input fibre 17. Such an arrangement has an efficiency greater than 60% and is capable of handling high power inputs. CLAIMS

3. An optical device as claimed in claim 2, wherein the one end of the optical fibre is tapered.

4. An optical device as claimed in any one of the preceding claims wherein the assembly is disposed within a bulb element of optically transparent material.

5. An optical device as claimed in any one of the preceding claims wherein the small particles are microspheres of optical glass or silica.

6. An optical device as claimed in any one of the preceding claims and comprising optical detector means disposed externally of the assembly.

7. An optical device substantially as herein described with reference to and as illustrated in Fig. 3 or Fig. 5 of the accompanying drawings.