GB2246905A - Multi-layered antireflective coatings for optoelectronic components - Google Patents

Abstract

To prevent oscillations at high gain, a semiconductor laser amplifier is provided with antireflection coatings comprising a zone 12 of alternating first and second layers (14, 15) of different refractive indexes, sufficiently thin so as to give the zone an overall refractive index between that of the first and second layers, the zone 12 being 1/4 wavelengths thick. The layers comprise respectively Si and SiO2. The device is made by physical deposition and layer thickness monitored using a quartz resonator or by reflectometry. The layers are 2.4 nm and 1.9 nm thick and there may be more than 25 in number. <IMAGE>

Description

OPTICAL COMPONENTS BT CASE: A24215 (PRTY) WP NO: 1193P This invention relates to optical components and in particular it relates to semiconductor optoelectronic components (e.g. semiconductor optical amplifiers) incorporating anti-reflection coatings.

Optical telecommunications use lasers for two major functions: as oscillators and as amplifiers. These applications are based upon similar semiconductor laser structures. In the case of an oscillator, the lasing structure is associated with a resonating, reflective system whereby radiation is continuously generated. where it is desired to amplify an optical signal it is necessary to suppress, and ideally to eliminate, any tendency of the amplifier to act as an oscillator because oscillation would introduce unwanted noise into the amplified signal.

As was mentioned above, oscillation is driven by reflection of signals back into the amplifying medium and, therefore, it is important to reduce back-reflection in order to achieve the satisfactory performance.

One important use of optical amplifiers is in repeaters in long-distance optical telecommunications systems, eg for submarine telecommunications. In such systems the distance between repeaters is controlled by the gain available, and higher gains make it possible to space the repeaters at greater intervals which is a substantial operational advantage. It is worth mentioning two aspects of amplifier design which, independently, limit the available gain. The intrinsic performance of the semiconductors is clearly one aspect. The gain is limited by a minimum threshold necessary for satisfactory operation, eg by the need to retain at least a minimum signal to noise ratio and the need to provide at least a minimum number of quanta to initiate the lasing process.

Similarly, the gain is also limited by the maximum power which the semiconductor material can handle. These, and similar, considerations set a maximum gain defined by the semiconductor performance. However, within this maximum, if the gain is set too high, the amplifier is liable to oscillate and this tendency to oscillate also limits the maximum available gain. As mentioned above, oscillation is primarily caused by the reflection of signals back into the amplifying medium and, at the present time it is the tendency to oscillate which sets the gain which can be achieved in practice and the capabilities of the semiconductor technologies are thus not fully utilised.

The identification of good anti-reflection coatings would, therefore, enable higher gains to be achieved by more fully exploiting the potential of the semiconductors. In other words, it appears that the identification of good anti-reflection coatings could represent a key to the design of optical amplifiers with higher gain.

The physical theory of anti-reflection coatings is well developed and this theory specifies both the thickness and refractive index of the region or regions which together constitute a good coating. In order to put these theories into practice it is necessary to provide materials which have the specified refractive indices. In order to get good performance it is necessary that the refractive index conforms very closely to the theoretically calculated value. This is a major problem for materials science since it often turns out that there is no composition having the required refractive index.

The problem is made even worse by the fact that an acceptable material has to meet many chemical requirements. For example, the material must be solid and chemically inert at the temperatures of operation.

Furthermore, each material must be compatible with its neighbours and each material must make a strong bond with its neighbours. Finally, the material must be chemically stable and usable in a deposition process. It generally turns out that there is no material which meets all of these criteria which is why the provision of anti-reflection coatings constitutes a very difficult problem.

According to this invention the problem of reducing reflection in an optoelectronic component is solved by providing an antireflection coating comprising a zone of specified refractive index comprising a plurality of super-fine layers each super-fine layer having a different composition to its neighbour. Some of the super-fine layers have refractive indices below the target value whereas the remainder of the super-fine layers have a refractive index about the target value. It has been observed that, provided all the layers are sufficiently thin, the composite zone acts as if it were homogenous with a refractive index equal to the weighted average of the various layers. This observation greatly facilitates the provision of a refractive index over a wide range of specified values.It is emphasised that the condition for apparent homogeneity is that every layer is thin enough, eg less than l/lOth and preferably less than 1/20th of a wavelength (said wavelength being measured in vacuo).

where the component is for coupling to another component, only one such zone is necessary, but where the component couples into an airgap, it is necessary to provide at least one further zone having a different refractive index to the first.

It is preferred to use non-crystalline (ie glassy) materials for the layers, because of their greater stability over time. The layers according to the invention can either be prepared having a constant refractive index, by maintaining the thickness ratios constant over the whole zone, or the layer can be prepared with a graded refractive index by varying the ratio over the zone. For the applications described in this specification, a uniform refractive index is preferred and it is, therefore, preferable to maintain a ratio uniform over the zone.

A particularly important application of this invention is to semiconductor optical amplifiers, primarily amplifiers in which the semiconductor substrate is formed of a compound containing at least one of indium and germanium and at least one of phosphorous and arsenic.

In one embodiment, the anti-reflection coating has two zones at least one of which is formed of super-fine layers as described above. For amplifiers based on indium and/or gallium with phosphorous and/or arsenic we have found that silica and silicon are suitable for forming the antireflection coating wherein the zone adjacent the semiconductor is formed of interleaved super-fine layers of the two materials and the other zone is a homogenous layer formed of silica.

This invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates a semiconductor amplifier having an anti-reflection coating in accordance with the invention, and Figure 2 illustrates an apparatus for depositing the layers.

The device shown in Figure 1 comprises a semiconductor laser amplifier 10 to which anti-reflection coatings generally indicated by the numeral 11, have been applied; in fact two anti-reflection coatings 11 are applied, ie one to the input end and one to the output end, but only one description is given since the two coatings are identical. Each anti-reflection coating 11 comprises an inner zone 12 adjacent to the laser 10 and an outer zone 13 in contact with the inner zone 12. It is known that, for good performance, inner zones 12 and 13 should be 1/4 of a wavelength thick, ie 1/4 of a wavelength as measured in the zone. (It will be recognised that this wavelength is the wavelength in vacuo times the zone refractive index).

Inner zone 12 is formed of a plurality of first and second super-fine layers indicated as 14 and 15 in Figure 1. It should be noted that (with the exception of the layer adjacent to the laser 10 and the layer adjacent to the outer zone 13) each first super-fine layer 14 is in contact with two second super-fine layers 15 and each second super-fine layer 15 is in contact with two first super-fine layers 14. For convenience of drawing, only five super-fine layers, ie three first layers 14 and two second layers 15 have been shown in Figure 1. In reality many layers are used; typically over 25 first layers 14 and over 25 second layers 15. Since large numbers of layer are used it is not of great importance which of the two materials is used for the outer layers.

The laser 10 is a complicated component and, since it is conventional, it will not be shown in detail. However, it can have a path region 16 located between two confining layers 17. The light transmission, and therefore the amplification, takes place in the path region 16 which means that the anti-reflection coatings are designed to cooperate with the path region. The other layers of the laser 10 have little relevance to the design of the anti-reflection coating 11.

A specific amplifier device as shown in Figure 1 was prepared in which the path region 16 was formed of a semiconductor material represented by the formula: GaaInl~aAsbPl-b where a = 0.395 b = 0.906 The path region 16 has a refractive index of 3.56 at X = 1.5# and the anti-reflection coating was designed to match this path region.

The inner zone 12 consisted of thirty-three first layers 14 each of which was formed of silicon (refractive index 3.48 at X = 1.5of) and each of which was 2.4nm thick. As shown in Figure 1, first layers 14 alternated with second layers 15 and the inner zone 12 also consisted of thirty-three second layers 15 each of which was formed of SiO2 (refractive index 1.46 at X = l.5u) and was l.9nm thick. Although the inner zone 12 was formed of sixty-six different layers, it behaved as an homogenous zone with a uniform refractive index of 2.77.The total thickness of the inner zone 12 was 142nm which is 1/4 of a wavelength as measured in the medium (ie optical thickness = The outer zone 13 was an homogenous region of Si02 (refractive index 1.46 at = 1.5u). The optical thickness of the outer zone 13 is also W/4.

The confining layers 17 Nay for example be In P, refractive index 3.17 at 1.5;.

A high gain was obtained using the amplifier just described and this high gain indicated that a very efficient anti-reflection coating was achieved.

The optical amplifier just described was prepared by methods which are substantially conventional but a brief description will now be given, with reference to Figure 2.

The 'superfine optical coatings' are produced within a conventional optical coating unit (manufactured by Balzers; type BAK 640). The process steps are detailed below: Preparation of the coating chamber Prior to deposition the jigs containing the devices to be coated are loaded onto the substrate holder and the chamber evacuated.

Evacuation -5 The process chamber is reduced to 2 x 10 5 pa. The partial pressure of water vapour is closely monitored by means of a Nass Spectrum analyser. The substrate holder is heated to 200 degrees.

Deposition The coating comprises 33 pairs of interleaved first and second layers comprising the inner zone and is followed by a final thick outer zone.

The pairs of layers are deposited in the following way: 1/ Both E-beam sources are ramped up to the evaporation power for the individual material. Initially shutters over both sources prevent deposition onto the devices. A shutter directly above the source of the first material is opened and deposition onto the device takes place. After the correct first layer thickness is deposited, the shutter is replaced over the source and the shutter above the source of the second material opened.

The correct second layer thickness is deposited. The process continues by repeating the deposition of another pair of layers. (33 pairs of layers are deposited in all).

If the materials are Si and Si02, the rates of evaporation are 0.3 nN/sec Si 0.1 nN/sec Si02 Small adjustments to the thickness (from the calculated or "ideal" values) can be made to "fine tune" the equivalent index to the practical optimum value required.

The outer layer comprises of a relatively thick layer of a low index material which is usually chosen to be one of the materials used in the previous layer sequence; in this case Si02.

Monitoring of coating thickness Two methods of thickness monitoring may be used Optical: A single beam reflectometer is used to monitor the growth of the layer structures. The reflectance of the monitor exhibits a minimum which is used to calculate the in-situ refractive index of the equivalent structure. The final layer is deposited to a minimum position in the reflectance curve (at the wavelength of interest).

Quartz: A vibrating quartz crystal is used to monitor the rate of deposition of the layers, by measuring changes in the oscillation frequency to provide a feedback signal. The power to the sources is adjusted accordingly to correct deviations in rate of evaporation from the set (ideal) parameters. This method can be used after the desired layer parameters have been calibrated using the above optical method.

Venting The vacuum chamber is allowed to cool and subsequently vented to dry nitrogen.

Claims (10)

1. A semiconductor optoelectonic component having an antireflection coating which includes a zone exhibiting a refractive index and having a thickness such that the optical thickness of the zone is X/4, where X is the wavelength at which the component operates, the zone comprising a multiplicity of interleaved first and second layers, having respectively first and second thicknesses and comprising respectively first and second materials; the layers being sufficiently thin that the zone exhibits substantially the behaviour of a homogenous material having a refractive index intermediate those of said first and said second materials.

2. A component according to claim 1 further comprising a second zone having an optical thickness of #/4 and exhibiting a different refractive index to the first.

3. A component according to claim 2 in which the second zone is formed of the same material as one of said layers.

4. A component according to any preceding claim in which the first and second materials are non-crystalline.

5. A component according to claim 4 in which the first material is Si and the second is Si02.

6. A component according to any preceding claim in which the refractive index exhibited by the first zone lies between 2 and 3.

7. A component according to any preceding claim comprising a laser configured to operate as an optical amplifier.

8. A method of manufacturing a coating for a component according to any preceding claim comprising the use of physical vapour deposition.

9. A method acccording to claim 6 in which the thickness of the layers is monitored using optical reflectometry.

10. A method according to claim 6 in which the thickness of the layers is monitored using a quartz resonator sensitive thereto.