GB2253480A

GB2253480A - Optical waveguide photodetector

Info

Publication number: GB2253480A
Application number: GB9104835A
Authority: GB
Inventors: Stephen Cecil Gratze; Charles Gordon Crookes
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-03-07
Filing date: 1991-03-07
Publication date: 1992-09-09
Also published as: GB9104835D0

Abstract

A leaky optical waveguide detector incorporates a control layer (40) to regulate the rate of leakage of light from an optical waveguide (30) to a detector (38) and the detector terminals form stripline conductors (42, 43) of a travelling wave microwave transmission line. <IMAGE>

Description

OPTICAL WAVEGUIDE PHOTODETECTOR This invention concerns photodetectors arranged to detect light passing along an optical waveguide. Such detectors are known, the most common being the leaky waveguide detector.

Referring to Figs. la to lc and Figs. 2a and 2b, there is shown an evanescent leaky waveguide structure incorporating features known in the prior art (See, for example, "Impedance matching for enhanced waveguide/photodetector integration", by R. J. Deri and O. Wada; App. Phys. Lett. 55, 25 December 1989, published by The American Institute of Physics).

In the detector diagrammatically shown in Figs. la to ic, an optical waveguide 10 is formed of InP(n) as a channel in an InP(n+) substrate 12. A photodiode 14 comprising a first layer 16 of InP(p+) on an absorber layer 18 of InGaAs (n-) is mounted so as to bridge the waveguide 10 in the substrate 12. The p/n junction so formed acts as a photodetector of optical energy leaking into the absorber layer 18 from the waveguide 10 due to the higher refractive index of the layer 18. As described by Deri et al in the above publication, an impedance matching layer 20 is interposed between the absorber layer 18 and the waveguide 10. By so doing, the coupling length C necessary to ensure transfer of, for example, 90% of the light in the waveguide 10 may be reduced from about lmm to 100 um; a factor of 10.

Light travelling along a rib waveguide 10, diagrammatically illustrated in Fig. 2a. In the region of the detector, light is perturbed and couples through the matching layer 20 into the absorber layer 18 as shown in the channel waveguide structure of Fig. 2b.

As shown in Figs. la to lc (and in Fig. 2b), contacts 22 and 24 are provided for the detector and it is conventional to apply a bias across the p/n junction between the layers 16 and 18 to deplete the layers of free electrons.

In the detector described above, light energy absorbed by the layer 18 from the waveguide 10 generates electron/hole pairs. The so generated electrons and holes drift, in accordance with the applied bias, to provide a detectable current. The speed of drift is one limiting factor in the frequency response of the device. However, other factors have a more deletrious effect on the operation of the detector. The detector effectively forms a capacitor. Its capacitance is dependant upon its physical parameters and the properties of the layer materials. As the capacitance increases, so the upper frequency detectable decreases. Further, if the optical input power is too high, saturation occurs due to the generation of large numbers of photocarriers and the electrical response is reduced.

Harmonies may be generated. The use of a matching layer as described above, whilst reducing the coupling length (and hence favourably reducing the capacitance of the device) tends to increase the problem of saturation. The favourable reduction in capacitance leads, of course, to an increase in bandwidth of the detector but, as noted above, at the expense of the range of possible optical input powers.

Coupled optical waveguides are known in which a pair of like optical waveguides extend parallely and closely adjacent in or on a substrate. Energy is transferred sinusoidally from one such waveguide to the other. At a predetermined repetitive length, 100% of the energy in one waveguide is transferred to the other.

A matching layer, as described above, could be used in conjunction with optical coupled waveguides to reduce the transfer length of such known devices. Coupled waveguide detectors in which evergy is partially or completely transferred to a waveguide including a detector, are known but the disadvantages of the above described leaky waveguide detector are applicable, at least in part, to coupled waveguide detectors.

It is an object of the present invention to provide a photodetector for detecting light travelling in a waveguide, wherein the aforesaid disadvantages are overcome.

According to the present invention, an optical waveguide photodetector comprises an optical waveguide and detection means for detecting light absorbed from the waveguide, the detector means forming a stripline of a travelling wave microwave transmission line.

The term "stripline" as used herein describes a first conductor extending parallel to at least a second conductor forming a ground plane, with a dielectric medium therebetween.

The detector means includes a photoresponsive detector for generating photocarriers and a p/n junction whereat the photocarriers are detected, output terminals of the detector means being in the form of a planar stripline, parallel to the waveguide, of the travelling wave transmission line.

The travelling wave transmission line preferably includes at least one further electrode or stripline conductor parallel to the waveguide and constituting a further ground terminal of the transmission line.

The waveguide may be of ridge or channel structure.

Dependant upon the waveguide of the light to be detected, the waveguide may be formed of InP(n) on or in an InP(n+) substrate (for wavelengths in the range 1.3 to 1.5 um) or may be formed using GaAs/AlGaAs technology for wavelengths of the order of 0.8um).

In order to prevent saturation of the photodetector, a control layer may be interposed between the waveguide and the detector to ensure that the respone of the photodetector, to the detected light, is maintained at a level below that of saturation of the detector.

The control layer is advantageously a layer of InGaAsP and may be epitaxially grown on the waveguide structure.

The or each further stripline conductor may be supported on the control means layer.

The travelling wave transmission line is advantageously constructed so that the phase velocity of the modulated travelling microwave matches closely the phase velocity of the optical wave in the waveguide.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which:- Figs. la to lc and Figs. 2a and b are illustrative of prior art leaky waveguide detectors, as above described; Fig. 3 is illustrative of the structure of a microwave travelling wave detector in accordance with a first embodiment of the present invention; and Figs. 4a, b and c are diagrams similar to Figs. la b and c of a microwave travelling wave detector in accordance with a second embodiment of the present invention.

In the drawings, layer thicknesses are not shown to scale.

The structure shown in Fig. 3 provides an optical waveguide constituted by the layer 30 which will normally be provided as a rib on or a channel in a substratel layer 32a of an intrinsic material itself superimposed on a substrate 32. A detector of the structure is formed of a control layer 40 bridging the waveguide 30 whereon is an absorbent layer 38 and a highly doped p+ layer 36 together providing a p/n junction. A first conductor 42 forms, together with a ground plane conductor 43a, a microstrip forming part of a travelling wave transmission line.

The intrinsic nature of the layers 30, 32a, 38 and 40 provides, the dielectric of the microstrip. A bias applied between the conductors 42 and 43a ensures that any free carriers in such layers are fully depleted.

The refractive index n3 of the layer 30, the optical waveguide, is greater than that n4 of the substrate layer 32a. The refractive index n2 of the layer 40 is also preferably less than that of the optical waveguide 30.

However, the refractive index n2 of the layer 40 is chosen so that the energy transferring per unit length from the waveguide 30 to the absorbent layer 38 is less than that which would saturate the detector's response. Energy absorbed by the layer 38, the energy gap EG1 of which is low (relative to that of the other layers) generates electron/hole pairs of which the electrons and hole drift, in accordance with the applied bias, enabling detection of the optical energy in the waveguide 30.

Suitable materials for the detector and the waveguide are dependant upon the wavelength of light travelling along the waveguide. For wavelengths of the order of 1.3um to 1.5um, the substrate 32 may be of heavily (n+) doped InP, the waveguide cladding substrate layer 32a, of substantially undoped InGaAsP, the waveguide 30 per se, of substantially undoped InP, the control layer 40, of substantially undoped InGaAsP, the absorbent layer 38 is of substantially undoped, or lightly n doped InGaAs and the superjacent layer 36 is highly doped InP (p+).

It will be appreciated, that the various layers have substantially matching latices and each can be gorwn epitaxially upon the preceding layer with appropriate masking and etching to define their physical shapes.

For example, a substrate layer 32a of InGaAsP may be epitaxially grown on a substrate 32 of InP (n+). A channel may be etched into but not through the layer 32a. A waveguide 30 may be epitaxially grown on the layer 32a to fill such a channel whereafter surplus waveguide material may be removed by etching. The control layer 40 may be epitaxially grown on the surface of the waveguide 30 and the surface of the lateral cladding of the waveguide 30 formed by the layer 32a. The absorbent layer 38 may be epitaxially grown on the surface of the control layer 40 and the highly doped p+ layer 36 may be epitaxially grown on the surface of the layer 38. The metallic strip conductor 42 and the wider ground plane conductor 43a may then be plated on the surfaces of the layer 36 and the substrate 32 respectively.

Other known semiconductor manufacturing techniques of macking, deposition, doping, implanting and etching may be utilised as appropriate.

It will be understood that the layer order may be inverted or arranged side-by-side. This latter structure may reduce the resistance of the detector.

The p+ layer 36 may be replaced by an n- intrinsic layer and a Schottky barrier contact formed, such a device would have a lower microwave loss than one using a heavily doped p+ layer.

As shown in Figs. 4a, b and c, a travelling wave detector constructed in accordance with a second embodiment of the present invention is comprised of an optical waveguide 30 formed as a ridge on a substrate 32. The waveguide 30 may be alternatively be formed in a channel in the substrate 32.

Epitaxially grown upon and straddling the waveguide 30 so as to clad the sides thereof is a control layer 40 of, for example, InGaAsP. The control layer has epitaxially grown thereon, an absorber layer 38 of a detector 34. The absorber layer 38 may be formed of InGaAs(n). A p+ layer 36 of, for example InP(p+) is grown upon the absorber layer 38 so as to form therewith a p/n junction at the interface therebetween. A stripline metalic conductor 42 is plated upon the layer 36. The control layer 40, as can be seen, is wider than the absorber layer 38 and provides shoulders at each side of the absorber layer wherealong planar stripline conductors 43 extend parallel to the conductor 42.The placement of the conductors 43 at such a location defines a slot width of the transmission line and assists in velocity matching of the phase velocity of an optical wave in the waveguide 30 to that of a microwave travelling along the conductors 42 and 43.

The conductors 42 and 43 then form part of a substantially coplanar strip-type transmission line.

As described in the above referenced publication, a layer 20 is employed to enhance the capture of light from the waveguide 10 into the absorber layer 18 to reduce the detector length by a factor of 10. In accordance with the present invention, the layer 40 may be used to control the leakage of light into the absorber layer 38. The control is provided as follows. The absorber layer 38 and the detector 34 have a maximum permissible light absorption per unit length governed by the saturation level of the detector 34.

Knowing a maximum optical power in the waveguide 30, the control layer 40 is arranged to control, by its presence, its refractive index and/or by its thickness, the amount of light received per unit length by the absorber layer 38 to less than that at which saturation of the detector occurs.

Thus, the layer 40 may enhance or attenuate the transfer of energy from the waveguide 30 to the absorber layer 38.

As maximum transfer of energy per unit length is not sought or achieved, a consequent penalty is incurred in that the capacitance of the detector 34 is increased as capacitance is length dependent. In accordance with the present invention, by forming the detector 34 as part of a microwave transmission line, the capacitance becomes part of the transmission line parameters and does not restrict the bandwidth of the response of the detector 34.

In order to maximise the advantages so obtained (no saturation and bandwidth (of for example, 20 GHz) maintenance), a further disadvantage of known microwave optical modulators must be overcome.

It is known, in travelling microwave optical modulators, that the phase velocity of the optical wave and of the microwave should be matched for optimum modulation and response.

The provision of stripline conductors 43 adjacent, substantially coplanar and parallel to the waveguide 30 in the region of the detector 34 together with the use of mainly intrinsic material layers enables velocity matching to be optimised.

With the arrangement illustrated in Figs. 4a to 4c, saturation of the detector 34 can be avoided, capacitative bandwidth limitation can be minimised, and phase velocity matching of the input optical wave and of an input microwave or a generated microwave, can be optimised.

With the arrangement illustrated, a constant frequency microwave fed to the transmission line formed by the stripline conductors 42 and 43 will be modulated in accordance with the modulation of the detected optical input wave. Alternatively, the detected optical input wave may generate a correspondingly modulated microwave in the stripline conductors 43 and 43 due to field changes, for subsequent amplification and use.

The invention is not confined to the precise details of the foregoing example and variations may be made thereto.

For instance, the waveguide 30 may be formed of InGaAsP(n).

The lengths a b and c shown in Fig. 1 are applicable also to Fig. 4. The lengths a and b are of the order of several micrometers whereas the length c may be of the order of 10 millimetres.

The length c may be determined knowing the maximum input optical power in the waveguide 30 and the maximum absorption rate of the layer 38 (to avoid saturation) then: c max = input optical power/max. absorption rate/mm (in mm) Biasing of the photodetector 34 can be effected by the application of DC between the strip line conductors 42 and 43 or, alternatively, a single or further ground plane contact 43a (see Fig. 3) may be provided on the substrate 32.

It is to be understood that only a portion of the optical power in the waveguide 30 may be tapped off, by absorption into the layers 40 and 38, for detection. Thus, the waveguide 30 may provide a continuing optical path beyond the detector. If c is calculated as above, then C/2 would provide a 50% tap of the optical energy in the waveguide.

If a lower wavelength of the light is to be detected, for example, 0.8um, then AlGaAs/GoAs may be used instead of InP for the waveguide and substrate.

Claims

1. An optical waveguide photodetector comprising an optical waveguide and detector means for detecting light coupled thereinto from the waveguide, the detector means simultaneously forming a stripline of a travelling wave microwave transmission line.

2. A photodetector as claimed in claim 1 wherein the detector means includes a photoresponsive detector for generating photocarriers and a p/n junction whereat the photocarriers are detected, output terminals of the detector means being in the form of a planar stripline conductors, parallel to the waveguide, of the travelling wave transmission line.

3. A photodetector as claimed in claim 1 or 2 wherein at least one further electrode or stripline conductor is provided, parallel to the optical waveguide and constituting a further ground terminal of the travelling wave transmission line.

4. A photodiode as claimed in claim 1, 2 or 3 wherein the optical waveguide is located in a channel in a substrate.

5. A photodetector as claimed in any preceding claim wherein the optical waveguide is formed of InP(n) in an InP(n+) substrate.

6. A photodetector as claimed in any preceding claim further including a control layer between the waveguide and the detector for controlling the coupling of light energy into the detector.

7. A photodetector as claimed in claim 6 wherein the control layer comprises a layer of InGaAsP epitaxially grown on the optical waveguide structure.

8. A photodetector as claimed in claim 3 and 6 or any claim appendant thereto wherein the at least one further stripline conductor is supported on the control layer.

9. A photodetector as claimed in claim 6 or any claim appendant thereto, wherein the control layer also serves as cladding for the optical waveguide.

10. A photodetector as claimed in any preceding claim wherein the stripline conductors of the travelling wave transmission line are arranged, relative to the optical waveguide, for matching of the phase velocity of an optical wave in the waveguide to the phase velocity of a microwave in the transmission line.

11. An optical waveguide photodetector substantially as hereinbefore described with reference to and as illustrated in Figs. 3 or Fig. 4a, 4b or 4c of the accompanying drawings.