GB2242756A

GB2242756A - Antireflection coating

Info

Publication number: GB2242756A
Application number: GB9007455A
Authority: GB
Inventors: Mark Cunningham Farries; Mark Quinton Kearley; Jens Buus
Original assignee: GEC Marconi Ltd; Plessey Co Ltd; Marconi Co Ltd
Current assignee: Plessey Co Ltd; BAE Systems Electronics Ltd
Priority date: 1990-04-03
Filing date: 1990-04-03
Publication date: 1991-10-09
Anticipated expiration: 2010-04-03
Also published as: GB9007455D0; GB2242756B

Abstract

An antireflection coating comprises a first transparent layer which is not stable and a second layer, covering and protecting the first layer, of a transparent chemically and physically stable material. The thicknesses of the layers, relative to their respective refractive indices being chosen so that the coating comprises a quarter wavelength filter at a predetermined wavelength(s). The layer thicknesses may be measured and more deposited or some etched away to give the desired thickness. The first layer may comprise HfO2 and the second layer may be SiO2. The antireflection coating may be on the windowed 16 end facet 14 of a semiconductor optical amplifier 10 e.g. a buried ridge laser. <IMAGE>

Description

ANTIREFLECTIVE COATINGS This invention relates to antireflective coatings, particularly, but not exclusively, for the facets of semiconductor optical amplifiers.

To avoid wavelength dependant ripples when such amplifiers operate in a Fabry Perot mode, it is essential that the reflectivity of each facet be less than 10-4. Quarter wavelength coatings or angled facets may be used to reduce the facet reflectivity.

Radiation of wavelength 1.3 or 1.5pm is used extensively in fibre optic transmissions and a semiconductor amplifier should show less than 10-4 reflectivity at its facets at these wavelengths. For example a layer of thickness 1900Ao + 1 8AO and a refractive index of 1.819 + 0.048 would give a reflectivity less that 10-4 at such wavelengths. Thickness tolerance may be achieved, for example, by electron beam evaporation techniques. Similarly, the appropriate refractive index may be achieved by coating with a material such as SiO and then partially oxidising the material to SiOx, where SiOx has the required refractive index.However, the refractive index of the requisite SiOx cannot be maintained as SiOx oxidises in air at normal operating temperatures to a more stable form e.g. SiO2 having a refractive index of 1.42 ; too low to achieve the required antireflection effect.

It is an object of the present invention to provide an antireflection coating, and a method of forming the same. wherein the aforesaid disadvantages and difficulties are minimised.

According to the present invention, an antireflection coating comprises a substrate. such as a facet of a semiconductor optical amplifier, an outer layer of a chemically stable material of a first refractive index superimposed upon and protecting a lower layer on the substrate of a material having a second refractive index, and in which the compound coating presents to radiation impinging thereon, a quarter wavelength antireflection coating.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a fragmentary diagrammatic perspective view of a laser having a facet coated in accordance with the present invention; and Fig. 2 is a graph illustrating a manner of selecting materials for forming the component layers of the coating.

Referring to Fig. 1, there is diagrammatically shown a semiconductor optical amplifier 10, for example, a buried ridge laser.

The amplifier 10 comprises a body 12 having an end facet 14 windowed at 16.

If light is reflected from the window 16 region of the end facet 14 back into the body 12, this causes an undesired ripple on the output of the optical amplifier means.

The amplifier 10 is provided with an anti-reflection coating on the end facet 14. Additionally, the facet 14 may be angled to reduce reflection.

In accordance with the present invention, the antireflection coating on the end facet 14 at least in the region of the window 16, comprises a compound coating including an exterior coating 18 of a transparent substantially chemically insert, physically stable material chosen for such properties. A preferred material is silicon dioxide SiO2. Such a material has a refractive index of approximately 1.46 to radiation of wavelength 1 .3m to 1 .5pom, much lower than is preferred for an optimum quarter wavelength antireflection layer.

To compensate for such a low refractive index, the end facet 14 is first coated with a lower layer 20. The layer 20 is formed of a transparent physically stable material. It need not be chemically stable as it is protected by the exterior coating 18. A preferred material for the lower layer 20 is hafnium dioxide Hf02 which has an approximate refractive index, at the same wavelengths, of 1.9. The optical thickness of the two layers 20 and 18 is such as to provide a k/4 antireflective coating on the end facet 14.

The invention also provides a method of forming such an antireflection coating. The method comprises ascertaining a minimum thickness of a layer of SiO2 which will prevent atmospheric deterioration of a subjacent layer of, for example, Hf02, calculating a maximum thickness of the subjacent layer which, together with the aforesaid minimum thickness, provides a quarter wavelength antireflection coating at the stated wavelengths.

Thereafter, a first layer, the lower layer 20, is deposited upon the end facet 14 of a semiconductor optical amplifier 10. The refractive index and the thickness of the deposited material are ascertained. If the thickness exceeds the maximum thickness, the lower layer 20 can be etched back to a value below the maximum thickness. The necessary thickness of the second, outer layer 18, is then calculated and the SiO2 deposited as a protective layer on the lower layer 20 to the calculated thickness and the overall optical thickness of the compound layers 18, 20 determined. Further SiO2 may be deposited, or deposited SiO2 may be etched away to fine tune the coating to an optimum antireflectivity level.

Fig. 2 of the accompanying drawings graphs normalised thickness of an upper layer of SiO2 against normalised lower layer thickness for a range of refractive indices of the lower layer between 1.8 and 2.1.

A choice is thus available as to the constitution of the lower layer, the stability of which can be assured by the inert upper layer.

If the latter is chosen to be SiG2 of which the refractive index is 1.46, then any transparent material, stable when so protected, having a refractive index between the values 1.8 to 2.1 may be used.

The outer layer 18 may be other than SiO2 provided it is transparent and has the necessary chemical and physical stability.

For wavelengths other than the exemplified 1.3Rm and l.5Rm other thicknesses of the layers can be calculated.

As stated above, the end facet 14 may itself be angled to reduce reflectivity in a known manner.

Other variations are possible within the scope of the present invention.

Claims

1. An antireflection coating comprising a first transparent layer of a first refractive index covered by a second transparent chemically and physically stable layer of a second refractive index and coated upon a substrate in respective thickness as together to provide a quarter wavelength antireflection coating at a predetermined wavelength or wavelengths.

2. A coating as claimed in claim 1 wherein the second layer comprises silicon dioxide.

3. A coating as claimed in claim 1 or 2 wherein the first layer comprises hafnium dioxide.

4. A semiconductor optical amplifier comprising a body having an end facet windowed for the egress of radiation, the end facet having an antireflection coating as claimed in claim 1, 2 or 3.

5. An amplifier as claimed in claim 4 wherein the end facet is angled to reduce reflection of emitted radiation.

6. An amplifier as claimed in claim 4 or 5 in the form of a buried ridge laser.

7. A method of forming an antireflection coating on a substrate comprising the steps of depositing a first transparent layer of a first refractive index upon the substrate, measuring the thickness and refractive index of so-deposited first layer material, etching the first layer material if the thickness thereof exceeds a predetermined maximum, depositing a second transparent layer of a chemically and physically stable material as a covering on the first layer, measuring the thickness and refractive index of the so-deposited first and second layers, and depositing further or etching away second layer material to provide a quarter wavelength antireflection coating on the substrate.

8. A method as claimed in claim 6 or 7 including depositing silicon dioxide as the second layer material.

8. A method as claimed in claim 7 including depositing hafnium dioxide as the first layer material.

9. A method as claimed in claim 8 or 9 including depositing silicon dioxide as the second layer material.

Amendments to the claims have been filed as follows CLAIMS

1. A semiconductor optical amplifier comprising a body having an end facet windowed for the egress of radiation, the end facet having an antireflection coating comprising a first transparent layer of a first refractive index covered by a second transparent chemically and physically stable layer of a second refractive index and coated upon a substrate in respective thickness as together to provide a quarter wavelength antireflection coating at a predetermined wavelength or wavelengths.

2. An amplifier as claimed in claim 1 wherein the second layer comprises a silicon dioxide.

3. An amplifier as claimed in claim 1 or 2 wherein the first layer comprises hafnium dioxide.

4. An amplifier as claimed in any of claims 1 to 3 wherein the end facet is angled to reduce reflection of emitted radiation.

5. An amplifier as claimed in any preceding claim in the form of a buried ridge laser.

6. A method of forming an antireflection coating on a windowed end facet of the body of a semiconductor optical amplifier comprising the steps of depositing a first transparent layer of a first refractive index upon the end facet, measuring the thickness and refractive index of so-deposited first layer material, etching the first layer material if the thickness thereof exceeds a predetermined maximum, depositing a second transparent layer of a chemically and physically stable material as a covering on the first layer, measuring the thickness and refractive index of the so-deposited first and second layers, and depositing further or etching away second layer material to provide a quarter wavelength antireflection coating on the end facet.

7. A method as claimed in claim 6 including depositing hafnium dioxide as the first layer material.