WO2007045842A2

WO2007045842A2 - Silica waveguides for upconversion lasers

Info

Publication number: WO2007045842A2
Application number: PCT/GB2006/003836
Authority: WO
Inventors: Greg Parker
Original assignee: University Of Southampton
Priority date: 2005-10-17
Filing date: 2006-10-16
Publication date: 2007-04-26
Also published as: GB0521072D0; WO2007045842A3; GB2427748A

Abstract

An optical up-conversion device comprises an optical waveguide having an optical core region at least partially surrounded by an air cladding, wherein the core region comprises silica doped with a rare-earth element. The ultraviolet-hard silica core provides an excellent photodarkening-resistant host for an upconversion dopant such as thulium or erbium, whilst the air cladding ensures optical confinement. A particularly suitable arrangement for planar geometries comprises a supported silica membrane with a ridge waveguide.

Description

SILICA WAVEGUIDES FOR UPCONVERSION LASERS

Field of the Invention

The present invention relates to doped silica waveguides and in particular their use in optical upconversion devices.

Background to the Invention

Upconversion lasers utilise low energy photons, typically infra-red photons generated by relatively cheap mass-produced semiconductor lasers, and convert these into "high value" photons in the visible part of the spectrum, usually by using the energy-levels of a rare-earth element (or combination of rare-earth elements) as "stepping stones" for the upconversion.

Fibre upconversion lasers utilising praseodymium (Pr) as the rare earth are commercially available, the fibre typically being ZBLAN (a fluoride based fibre with low phonon energy). It is essential for praseodymium to work effectively in an upconversion laser configuration that a low phonon energy host (such as ZBLAN) is used; otherwise the upconversion mechanism is compromised. Although Pr-doped ZBLAN fibres can produce red (635nm, 605nm), green (521 nm) and blue (492nm, 488nm) photons, only the red is a strong transition capable of giving up to one Watt of laser radiation at 635nm (red). The blue radiation wavelength is too long (and too weak) to be useful in display applications, and the green transition is also rather weak.

It has been known for a long time that thulium [Tm] possesses the right energy levels to produce strong blue emission at wavelengths suitable for displays and photofinishing applications at 455nm, and also at the slightly longer wavelength of 482nm. In fact, the production of 203mW of 482nm laser radiation using a ZBLAN fibre has been reported in Paschotta, R., Moore, N., Clarkson, W., Tropper, A.C., Hanna, D.C. and Maze, G. "230 mW blue light emission from a thulium-doped upconversion fibre laser", IEEE J. Selected Topics Quant. Electron., 3, 1100-1102 (1997). However, as reported by Risk, P.W., Gosnell, T. R., and Nurmiko, A.V. in "Compact Blue-Green Lasers", Cambridge pp441-444 (2003), this fibre suffered from "photodarkening", a phenomenon that defeats attempts at trying to produce blue upconversion lasers based on Tm as the rare-earth dopant. It appears that trying to use Erbium (Er) for green emission at 543-546nm is adversely affected by photodarkening in a similar manner. Photodarkening is the progressive increase in transmission loss of the host material (ZBLAN in the case of fibres) due to the production of ultraviolet (U.V.) light in the upconversion process. The damaging U. V. radiation generates defects in most host materials leading to a steady increase in loss over time, and eventually the extinction of the laser when the loss becomes greater than the overall gain. It is this well-known photodarkening problem that has prevented commercialisation of Tm for high-power blue upconversion lasers, and Er for high-power green upconversion lasers.

There is therefore a need for a suitable host material and geometry for rare- earth based upconversion devices that addresses the photodarkening problem.

Summary of the Invention According to one aspect of the present invention, an optical up-conversion device comprises an optical waveguide having an optical core region at least partially surrounded by an air cladding, wherein the core region comprises silica doped with a rare-earth element.

The provision of a waveguide having an ultraviolet-hard silica core doped with a rare-earth element with an air cladding provides the ideal combination of optical confinement, photodarkening-resistent host material and dopant for applications in up-conversion.

Any suitable rare-earth element may be used as dopant. Preferably, the rare-earth element is thulium (Th). Alternatively, it may be erbium (Er). A combination of different rare-earth materials may also be used.

In one embodiment, the optical waveguide comprises a planar structure including a silica layer having an optical core region doped with the rare-earth element. A planar structure can easily be integrated with other devices and fabricated by a range of techniques. In another embodiment the optical waveguide comprises a silica fibre having a central core region doped with the rare-earth element and substantially surrounded by a plurality of air holes formed in the silica fibre. A fibre geometry has the advantage that it can be quite long and therefore a lower concentration of rare- earth dopant may be used as compared to the planar geometry whilst still achieving a similar level of up-conversion gain.

For the planar geometry, it is preferred that the optical core region of the waveguide further comprises a ridge structure formed from silica doped with the rare-earth element. The ridge waveguide provides lateral optical confinement whilst the air cladding ensures vertical confinement. Preferably, the optical device further comprises a supporting member disposed adjacent a region of the silica layer distal the optical core region. More than one supporting members may be present. In this way, the silica layer may be supported at a point or points removed from the core region, thereby ensuring an air cladding in the region directly adjacent the core region.

In the case of a ridge waveguide it is preferred that the supporting member and the ridge structure are disposed on opposing surfaces of the silica layer. Typically, the ridge will be located on an upper surface and the supporting member will be located below a lower surface.

The supporting member may be formed from any suitable material and will typically comprise a material selected from a group which includes metals, dielectrics and silicon. It is preferred that the device further comprises a substrate, wherein the supporting member is disposed between the substrate and the silica layer. A substrate provides strength and structural integrity to the device and may also be the starting point for fabricating the device. The substrate may comprise silicon, but alternatively may be formed from silica. According to another aspect of the present invention, an up-conversion laser comprises an optical up-conversion device according to the first aspect and means for providing optical feedback at an up-conversion wavelength.

The feedback means may be dielectric mirrors, which are separate from the optical device or may formed on opposite ends of the waveguide. Alternatively, the feedback means could be one-dimensional or even two-dimensional (Bragg) gratings, which may be formed within the waveguide.

An optical up-conversion laser according to the present invention provides high up-conversion efficiency and minimal degradation due to UV-induced photodarkening.

Brief Description of the Drawings

Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a cross-sectional view of a standard ridge-waveguide geometry;

Figure 2 shows a cross-sectional view of a supported silica ridge waveguide membrane according to the present invention;

Figure 3 shows a processing sequence for fabricating a Silicon-based device; Figure 4 shows a processing sequence for fabricating a glass-based device;

Figure 5 shows an upconversion laser with a planar waveguide geometry;

Figure 6A shows a doped holey-fibre waveguide; and, Figure 6B shows an upconversion laser using a holey-fibre waveguide geometry.

Detailed Description It had been expected that Alumina (Aluminium Oxide) would be a robust material against the damaging effects of U.V. radiation, and would not show photodarkening. However, measurements have shown that this is not the case, and that Alumina does indeed show some photodarkening. It is therefore not a suitable host material for a reliable Tm-based upconversion laser. The only other known dielectric oxide material that is U.V.-hard is silica

(silicon dioxide), which is used extensively in the silicon microfabrication Industry. Measurements made on silica have demonstrated that silica does not exhibit photodarkening when exposed to intense short wavelength U.V. radiation. Silica is thus a promising host material for doping with rare-earth elements, including Tm and Er, for use in upconversion laser applications.

However, in order to fabricate a laser, it is necessary to first make a waveguide, whether a planar (flat two-dimensional) waveguide, or a fibre waveguide. If rare-earth doped silica is to be used as the active core material, then in order to form a waveguide it is necessary to clad the active core with a U.V.-hard material of lower refractive index than silica. This cladding would also have to have a similar melting point and expansion coefficient if it were to be drawn into a fibre, or made into a planar waveguide. The only "cladding" that appears to satisfy all the necessary criteria is "air", and so a waveguide design needs to be formulated, either planar or fibre, that has air as the cladding layer. Due to the problems previously identified with upconversion lasers, this type of waveguide geometry has not been required in planar devices, and there has therefore been no motivation to fabricate it.

In contrast, similar geometries have already been implemented for the fibre case, and these are commonly known today as "holey" fibres, whereby a silica waveguiding core is surrounded by a "honeycomb" of air holes that force waveguiding to occur in the core. However, a Tm-doped (or Er-doped) silica cored holey fibre has not been fabricated with a view to making a photodarkening-resistant upconversion fibre laser. Moreover, the planar waveguide equivalent has also not been fabricated, as this requires an unusual (MEMs-type) approach for its fabrication, and up until now there has been no practical need for such a structure.

In order to fabricate a planar silica waveguide, it is necessary to produce an isolated silica membrane, and a means to keep the propagating (waveguided) light away from any support for the silica membrane. Figure 1 shows a cross-sectional view of a standard ridge-waveguide geometry 10 based on a silica layer 11 having a ridge 12 and using air as the "cladding" material 13. As illustrated, the light field is constrained to the region 14 of the silica waveguide. By fabricating a ridge waveguide 10 of this type it becomes possible to confine the light in the central portion of the air-clad waveguide, well away from any potential support structures. Such support structures are likely to have higher index than the silica waveguide and would introduce severe optical loss if the confined (waveguided) light could reach them. Support structures 24 may be incorporated by adapting the ridge waveguide based structure 20 as shown in Figure 2. In this way, a supported "silica membrane" is achieved, whereby the light is confined in the lateral direction to a region 24 directly beneath the ridge 22, thereby preventing it from reaching the support structures 25 which would otherwise lead to a loss of the light. The supports 25 may be made of any suitable material including metals, dielectrics, or even silicon. The whole structure may be fabricated on or attached to a substrate 26, which provides structural integrity to the device and facilitates handling of the otherwise delicate structure. As the confined light field 24 does not reach the support structure 25, the optical properties of the supports 25 are not important. Thus, the basis of the present invention in a planar geometry is the provision of a silica membrane waveguide 21 incorporating a ridge 22 in order to keep the light field 24 from reaching the membrane support structures 25. Furthermore, the choice of silica as the waveguiding material is key in view of its proven resistance to the damaging effects of U.V. radiation. A possible practical fabrication sequence for making such a waveguide using standard Silicon processing techniques is shown in Figure 3. Such processing sequences are commonly used in fabricating MEMs devices. The key process steps are as follows:

Step 301 : Start with a plain silicon wafer 324, which could be a thinned wafer.

Step 302: Form the silica waveguide layer 320 by either oxidation of the silicon substrate 324, which will require implantation of the rare-earth dopant as a final step, or deposit the rare-earth doped oxide by any suitable deposition technique, including sputtering, CVD and MBE. Step 303: Lithographically mask the back of the silicon wafer 324 and then use any suitable etch technique (such as KOH etching) to etch the silicon away, stopping at the oxide layer. In this way, the support structures 325 are produced leaving an intervening air gap 323.

Step 304: Bond a silicon handle wafer 326 to the back of the structure using any suitable technique, for example anodic bonding. Ion implantation of the rare- earth dopant could be carried out at the end of this step, or at the next step.

Step 305: Using lithography and etching, define the ridge waveguide 321,

322 in the oxide. If the oxide were thermally grown, then the next step would be implantation of the rare-earth dopant, unless done at the previous step as indicated.

Note the waveguide is also defined laterally to isolate from any other waveguides on the wafer.

However, it is not necessary to use silicon as the substrate material, and it is quite possible to fabricate the whole structure using glass (silica/quartz) but again using semiconductor fabrication techniques. An example of such a structure, and a possible processing sequence for fabricating it is shown in Figure 4. The process steps are as follows:

Step 401: Start with a quartz (i.e. silica) wafer 424.

Step 402: Using lithography and etching, remove the silica from what will become the air region 423 below the membrane ridge waveguide, leaving behind what will become the supports 425 on a silica substrate 426. Step 403: Bond a second quartz wafer 420 to the first wafer. This top wafer can be thinned.

Step 404: Use lithography and etching to define the ridge 422 in the silica waveguide (top bonded wafer) and to laterally define the extent of the waveguide 421 , if necessary. Ion implantation of the rare-earth dopant (or combination of rare earths) can be carried out at this step, or at the previous step.

It should be noted that, although the processes described above are quite suitable, there are many other ways of producing a supported silica ridge waveguide according to the invention.

Once a suitable waveguide structure 51 has been fabricated, it can be incorporated in a planar-waveguide upconversion laser geometry 50 as shown in

Figure 5. Optical feedback is provided by means of feedback structures 52 on the ends of the silica waveguide. These could be simple dielectric mirrors but could also be one-dimensional or even two-dimensional (Bragg) gratings. Grating structures may be patterned in the waveguide. When used in a planar upconversion laser, the rare-earth doped silica ridge waveguiding membrane structure 51 is likely to be up to a few centimetres in length. The up-conversion medium will typically be pumped by an Infrared semiconductor pump laser 53, which can be mounted on the initial substrate, silicon or quartz, in predefined slots for easy manufacture and alignment. As illustrated, an optional focusing lens 54 may be employed to achieve a suitable spot size for end pumping of the up-conversion medium.

Turning now to the fibre-based geometry, although the general concept of the holey fibre itself is known, as is the use of rare-earth materials as dopants, it has not been appreciated that this geometry allows the use of certain rare earths that would normally photodarken materials in up-conversion applications. However, in a similar manner to the planar geometry of Figure 5, a holey-fibre based up- conversion device can be constructed. Figure 6A shows a schematic representation of a silica-cored "holey fibre" structure 61 in cross-section, with the air holes 66 clearly visible. The geometry and size of these air holes 66 defines how light will be confined to the waveguiding core. For up-conversion laser applications the central silica core (shaded) 67 is doped with a rare earth, or combination of rare earths. An up-conversion fibre laser 60 employing a suitable "holey fibre" structure 61 is shown in Figure 6B. As with the planar embodiment, the feedback structures 62 may be deposited dielectric mirrors or grating structures, either written or etched into the waveguide. The length of the fibre 61 may exceed tens of centimetres allowing lower concentrations of rare-earth dopant to be used if required. The up-conversion medium will typically be pumped by a semiconductor infrared pump laser 63, and a an optional focussing optic 64 may be employed to produce a tighter pump beam spot size for end pumping of the material.

The low intrinsic loss of holey fibres means they can be made quite long (several metres) if required. This might be beneficial in up-conversion laser design as it means that a lower concentration of rare earth dopant per unit volume can be used. Lower concentrations of rare earth dopant means less likelihood of one rare- earth ion interacting with another in a way that could enhance unwanted electronic transitions and lead to lower efficiency of the up-conversion. On the other hand, the limitations of wafer size, and lithographic definition, mean that the planar up- conversion waveguides are limited in length to a few centimetres. This length restriction will define the concentration of rare-earth dopant required to ensure gain (optical amplification for lasing) and so this geometry offers less flexibility in design than the holey fibre.

Claims

1. An optical up-conversion device, the device comprising an optical waveguide having an optical core region at least partially surrounded by an air cladding, wherein the core region comprises silica doped with a rare-earth element.

2. A device according to claim 1 , wherein the rare-earth element is thulium (Th).

3. A device according to claim 1 , wherein the rare-earth element is erbium (Er).

4. A device according to any of claims 1 to 3, wherein the optical waveguide comprises a planar structure including a silica layer having an optical core region doped with the rare-earth element.

5. A device according to claim 4, wherein the optical core region of the waveguide further comprises a ridge structure formed from silica doped with the rare-earth element.

6. A device according to claim 4 or claim 5, further comprising a supporting member disposed adjacent a region of the silica layer distal the optical core region.

7. A device according to claim 6 when dependent on claim 5, wherein the supporting member and the ridge structure are disposed on opposing surfaces of the silica layer.

8. A device according to claim 6 or claim 7, further comprising a substrate, wherein the supporting member is disposed between the substrate and the silica layer.

9. A device according to any of claims 6 to 8, wherein the supporting member comprises a material selected from a group which includes metals, dielectrics and silicon.

10. A device according to any of claims 1 to 3, wherein the optical waveguide comprises a silica fibre having a central core region doped with the rare-earth element and substantially surrounded by a plurality of air holes formed in the silica fibre.

11. An upconversion laser comprising an optical up-conversion device according to any preceding claim and means for providing optical feedback at an upconversion wavelength.