GB2386013A

GB2386013A - Full duplex optical transceiver with remote light source for transmitter

Info

Publication number: GB2386013A
Application number: GB0204815A
Authority: GB
Inventors: Ian Edward Day
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2002-03-01
Filing date: 2002-03-01
Publication date: 2003-09-03
Also published as: GB0204815D0

Abstract

An optical transceiver for transmitting and receiving optical signals comprises an optical signal receiver and a light modulator, the receiver and modulator being arranged such that, in use, light modulated by the modulator is received by the modulator from a source remote from the transceiver, optical signals received by the receiver are not modulated by the modulator, and light modulated by the modulator is transmitted as optical signals from the transceiver simultaneously with the reception of signals by the receiver. The light destined for the modulator may pass through the receiver first. A reflection based configuration is also disclosed.

Description

OPTICAL TRANSCEIVER This invention relates to an optical transceiver, and in particular a full duplex optical transceiver, i. e. one which transmits and receives optical signals simultaneously.

International Patent Application published as WOOO/55994 (Bookham Technology pic) discloses a single wavelength optical transceiver for transmitting and receiving optical signals of the same wavelength comprising a light source, such as a laser diode or a reflector, a light receiver such as a photodiode, and an input-output for receiving light from and transmitting light to a bi-directional optical fibre. An optical switch, e. g. a Mach-Zehnder interferometer comprising one or more p-i-n diode phase modulators, is provided to selectively provide optical communication between the light source and the optical fibre, and between the optical fibre and the light receiver to provide low-loss paths therebetween. With a reflector acting as the light source, the optical switch can also be used to modulate the output of the transceiver. The transceiver of WOOO/55994 is a half-duplex transceiver, i. e. it cannot transmit and receive optical signals simultaneously.

European patent application published as EP 0822676A2 discloses another half-duplex optical transceiver in which a photodiode may selectively either receive optical signals when in a reverse biased receive mode, or may reflect light when in a bias-free or forward biased transmit mode, by being

transparent to the light and allowing it to impinge on a reflective coating.

When the photodiode is in its transmit mode, the light which it reflects is polarisation plane modulated by a piezoelectric element, thereby creating optical signals which are transmitted by the transceiver. The transceiver of EP0822676A2 also reflects light of a different wavelength by means of a wavelength selective filter, the different wavelength light being diverted from the transceiver, for example to an external video receiver.

The present invention seeks to provide an optical transceiver which is able to transmit and receive optical signals simultaneously (i. e. a full duplex transceiver) but which does not require its own light generator (e. g. a laser), thereby enabling the transceiver to be simple and low cost.

Accordingly, the invention provides an optical transceiver for transmitting and receiving optical signals, comprising an optical signal receiver and a light modulator, the receiver and modulator being arranged such that, in use, light modulated by the modulator is received by the modulator from a source remote from the transceiver, optical signals received by the receiver are not modulated by the modulator, and light modulated by the modulator is transmitted as optical signals from the transceiver simultaneously with the reception of signals by the receiver.

Preferably the light modulator is a light intensity modulator, and consequently light modulated by the modulator preferably is intensity modulated.

Optical signals received by the optical signal receiver are absorbed by the signal receiver, and preferably are converted into electrical signals.

A second aspect of the invention comprises an array of a plurality of optical transceivers according to the first aspect of the invention. Preferably the array is arranged to be connectable to a plurality of optical fibres (e. g. ribbon fibre) such that each transceiver of the array is connectable to a respective fibre or pair of fibres.

Preferred and optional features of the invention are described in the dependent claims, and in the accompanying drawings and the description thereof.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 shows, schematically, an embodiment of an optical transceiver according to the invention;

Figure 2 (views (a) and (b), ) shows, schematically, two additional embodiments of an optical transceiver according to the invention; Figure 3 (views (a) and (b) ) shows, schematically, two further embodiments of an optical transceiver according to the invention; and Figure 4 shows, schematically and in cross-section, an embodiment of a silicon rib pin diode modulator of an embodiment of a transceiver according to the invention.

Figure 1 shows, schematically, an embodiment of an optical transceiver 1 according to the invention. The transceiver 1 comprises an optical signal receiver 3 comprising a photodiode, and a light modulator 5 comprising a pin diode light intensity modulator. The transceiver 1 is optically connected to an optical fibre, the cable 7 of which is indicated schematically. The receiver 3 and the modulator 5 are arranged in series on one and the same optical path 9, with the receiver situated between an optical input 11 of the transceiver and the modulator. The receiver 3, the modulator 5 and the optical path 9, are all integrated on one and the same optical chip, with which the optical fibre is optically connected by known methods. The optical chip preferably comprises a silicon chip, and the optical path 9 preferably comprises a silicon rib waveguide (see for example Figure 4, described below).

The modulator 5 may be a semiconductor optical amplifier (SOA) instead of a pin diode modulator. This has the advantage of enabling the use of higher modulation rates, and would enable optical amplification (thus minimising optical loss).

It is to be understood that the optical path 9 may be integrated on an optical chip such that it loops back on itself, as shown. However, it will generally be more desirable for the integrated optical path 9 not to be looped back on itself (for example instead comprising a generally straight path extending between opposite edges of the chip) since a looped integrated path may require a chip of excessive size (due to minimum bend radius requirements). In such cases, it is preferably the optical fibre which loops back on itself rather than the integrated part of the optical path 9. Figure 1 is merely a schematic diagram illustrating generally the path taken by the light.

With the receiver 3 and the modulator 5 arranged in series on the optical path 9 as shown, the receiver and the modulator are preferably arranged to receive different respective optical wavelengths. For example, the receiver 3 may be arranged to receive optical signals carried at a wavelength of 1310nm (for example CATV data), whereas the modulator 5 may be arranged to receive light at a wavelength of 1550nm, both wavelengths being delivered by the same optical fibre. The receiver 3 therefore preferably receives modulated light of a first optical wavelength (e. g.

1310nm) and the modulator 5 preferably receives unmodulated light of a

second, different wavelength (e. g. 1550nm). The receiver 3 is preferably substantially transparent to the second wavelength, and preferably removes substantially all of the first wavelength from the optical path, such that the modulator receives substantially only the second wavelength. That is, the receiver 3 (which, as already mentioned, is preferably a photodiode) acts as a wavelength splitter by absorbing light of a first wavelength (e. g. 1310 nm) and transmitting light of a second wavelength (e. g. 1550 nm). Because the receiver is situated upstream of the modulator, optical signals received by the receiver are not modulated by the modulator. Light modulated by the modulator is transmitted as optical signals from the transceiver simultaneously with the reception of signals by the receiver. The direction of travel of the optical signals received by the receiver, the unmodulated light received by the modulator, and the modulated light transmitted by the modulator, is as indicated by the arrows (and all in the same direction).

The transceiver as shown in Figure 1 (and, in fact all embodiments of transceiver according to the invention) has the advantage that optical data may be received by the transceiver at the same time as data is transmitted optically from the transceiver (i. e. full duplex operation, as mentioned above) even though the light which is modulated by the transceiver and transmitted therefrom is not generated within the transceiver but is instead received by the transceiver from a remote location simultaneously with the optical signal data received by the transceiver. This is a vast improvement over the transceivers disclosed in WOOO/55994 and EP0822676A2 (referred to earlier) which,

although they use light received from a remote location for onward transmission of data by modulating the received light, they are unable to send and receive data at the same time (i. e. they are capable only of half-duplex operation).

Figure 1 also shows an alternative arrangement of the optical signal receiver 3 and the light modulator 5. Shown in dashed outline is the receiver 3 arranged on a separate optical path to that of the modulator 5. In this arrangement, the transceiver 1 includes an optical splitter 13, which divides the light received via the optical fibre into two separate portions. The receiver 3 is located on a branch optical path 15, whereas the modulator 5 is located in the main optical path 9. Consequently, optical signals to be received by the receiver and light to be modulated by the modulator are separated from each other so that they are received, respectively, by the receiver and the modulator via separate optical paths subsequent to their reception by the transceiver via one and the same optical input 11.

This general arrangement may be used, for example, in either of two embodiments of the invention. In a first embodiment, the optical splitter 13 is a wavelength dependent splitter, and two differing wavelengths (as described above) are received by the transceiver 1. In this embodiment, the splitter 13 directs the optical signals to the receiver 3 (which signals, for example may be carried at a wavelength of 1310nm) and directs the unmodulated light to the modulator 5 (which light, for example, may be at a wavelength of 1550nm).

Alternatively, in a second embodiment of the invention, only a single wavelength of light is used, and the splitter 13 directs a portion of the light (e. g. 50%) to the receiver 3 and another portion the light (e. g. 50%) to the modulator 5. Although the light received by the modulator 5 is itself already pre-modulated, (i. e. the optical signals received by the receiver) if the modulation rate of the modulator 5 and the modulation rate of the premodulated light are sufficiently different, this will not substantially affect the modulation provided by the modulator. For example, if the modulation rate provided by the modulator 5 is very much lower than the modulation rate of the pre-existing modulation, then the modulation provided by the modulator 5 will be substantially unaffected by the pre-existing modulation. Preferably the modulation rate of the modulation provided by the modulator 5 is no more than a fifth, more preferably no more than a tenth, of the modulation rate of the optical signals received by the receiver (i. e. the pre-existing modulation). For example, the pre-existing modulation rate may be 1 Mbit/s, while the modulation rate provided by the modulator 5 may be 56Kbit/s.

Figures 2 (a) and (b) show embodiments of transceivers 1 according to the invention which are generally equivalent to the two arrangements shown in Figure 1 (i. e. in which the receiver 3 is either in-line with the modulator 5 or is located on a branch of the optical path 9). However, in these embodiments, instead of the optical path 9 being one-way such that the light modulated by the modulator 5 continues in the same direction as that in which

it entered the transceiver (and for example the optical path 9 looping back on itself as shown in Figure 1) the transceiver according to these embodiments includes a reflector 15 at an end of the optical path 9 opposite to the input 11 end thereof. Consequently, the optical path 9 of the transceivers shown in Figure 2 is two-way, with the light modulated twice by the modulator 5, initially when the light propagates through the modulator in a first direction A (as indicated by the respective arrow) and subsequently when the light propagates through the modulator in the opposite direction B (also as indicated by the respective arrow) after having been reflected by the reflector 15. This has the advantage of doubling the modulation depth, and is made possible by the round trip time between the modulator, the reflector and back to the modulator, (determined by the distance between the modulator and the reflector) being arranged to be significantly less than the modulation rate provided by the modulator. Preferably the modulation rate provided by the modulator 5 is at least 50Mbit/s, more preferably at least 80Mbit/s, e. g.

100Mbit/s. Preferably the round trip time between the modulator, the reflector, and back to the modulator is no more than 1 ns, more preferably no more than 100ps. Such round trip times easily enable modulation rates of 1 OOMbit/s to be provided by the modulator, for example.

In all other respects, the embodiments of the invention illustrated in Figures 2 (a) and 2 (b) are the same as the embodiments shown in Figure 1, i. e. the Figure 2 (a) embodiment functions using two differing wavelengths, and the Figure 2 (b) embodiment may function using two differing wavelengths,

with the optical splitter 13 being a wavelength dependent splitter, or alternatively only a single wavelength maybe used, with the splitter 13 directing a portion of the light to the receiver 3 and directing the remainder of the light to the modulator 5.

The modulator 5 and reflector 15 may comprise a single component, for example a reflective semiconductor optical amplifier (RSOA).

Figures 3 (a) and 3 (b) show two embodiments of the invention which are generally equivalent to the embodiments shown in Figure 1, but instead of the optical path 9 being continuous (and, for example, looping back on itself) it includes a pair of reflectors 15a and 15b by which it loops back on itself. This has the advantage of enabling substantially the entire optical path 9 to be integrated onto an optical chip, for example. The reflectors 15a and 15b (and the reflectors 15 of the embodiments shown in Figures 2 (a) and 2 (b) ) may comprise edges etched in or on an optical chip (which chip also includes the receiver 3 and the modulator 5), for example, and the reflection of the light being by means of total internal reflection.

As indicated above, the optical receiver 3 is preferably a photodiode.

The photodiode may be a waveguide photodiode or a vertical entry photodiode, for example. The skilled person would be able to select an appropriate photodiode for each particular embodiment.

The modulator 5 may be a semiconductor optical amplifier (SOA).

Preferably, however, the modulator 5 comprises a pin diode, and more preferably a silicon rib laterally doped pin diode. Preferably the pin diode modulator is as disclosed in UK Patent Application No. 0123245.3 (Bookham Technology), the entire disclosure of which is incorporated herein by reference. Advantageously, the modulator is a semiconductor optical waveguide device, which comprises a semiconductor rib waveguide and one or more doped regions situated adjacent to the waveguide, the waveguide comprising an elongate rib portion and slab regions on immediately adjacent opposite lateral sides of the rib portion, the rib portion extending above the slab regions, at least one of the slab regions including a recess spaced apart from the rib portion, an un-doped lateral wall of the recess providing a lateral boundary (referred to herein as"the recess lateral wall boundary") of the rib waveguide such that in use it laterally confines an optical wave propagated by the waveguide. Preferably there is at least one recess on each side of the rib portion. More preferably, a doped region extends from the base of each recess.

By"lateral"is meant lateral with respect to the direction of propagation of an optical wave along the waveguide, i. e. lateral with respect to the direction of elongation of the rib portion of the waveguide.

Such a modulator has the advantage that because the un-doped lateral wall of the recess laterally confines the optical wave propagated by the

waveguide, the waveguide may be smaller in width than known semiconductor waveguides, without inducing significant optical losses. It has the additional advantage that because the un-doped lateral wall of the recess laterally confines the optical wave propagated by the waveguide, the (or each) doped region may be positioned closer to the optical wave than in known devices, without inducing significant optical losses, and this in turn has the advantage of increasing the efficiency of the device. Additionally, the modulator provides the possibility of reducing the lateral dimensions (e. g. the diameter) of the optical wave (i. e."squashing"the optical wave), thereby enabling adjacent waveguides to be positioned closer together and/or enabling two or more doped regions on opposite sides of the optical wave to be positioned closer together. This latter advantage has the further advantage of enabling the switching speed of the modulator to be increased, for example, thereby making such modulators particularly suitable for use in the present invention.

Particularly preferred dimensions of a pin diode modulator as used in the invention are as follows. Preferably the rib portion has a lateral width of 3um or less, more preferably 2 urn or less. The distance by which each recess lateral wall boundary of the waveguide is spaced apart from the rib portion of the waveguide is preferably 2um or less, more preferably 1 um or less. Preferably the, or each, recess has a depth of at least 0. 2um.

The rib portion of a waveguide used in the present invention preferably comprises an upper surface and two lateral surfaces. By"upper"is meant upper with respect to the slab regions of the waveguide"above"which the rib portion extends; it is not meant to denote any particular orientation of the waveguide with respect to gravity. An example of a preferred embodiment of a silicon rib pin diode is shown schematically and in cross-section in Figure 4.

Figure 4 shows an SOI (silicon-on-insulator) rib waveguide with a lateral injection p-i-n diode made by ion implantation or diffusion into a layer of silicon. In this arrangement, the waveguide comprises a rib portion 20 extending above upper surfaces 21 of slab regions 23 on opposite lateral sides of the rib portion, the slab regions also forming part of the waveguide.

The rib portion 20 comprises an upper surface 25 and lateral surfaces 27.

Above the rib portion 20 is a thin layer of silica 36. Each slab region 23 includes a recess. On one lateral side of the rib portion 20 is recess 46 and on the opposite lateral side of the rib portion is recess 48. An n±doped region 28 extends from the base 29 (only) of recess 46, and a p±doped region 30 extends from the base 31 (only) of recess 48. Each recess 46,48, comprises a pair of lateral walls 49 and 50. Lateral wall 49 of each recess, namely the lateral wall closer to the rib portion 10, provides a lateral boundary ("the recess lateral wall boundary") of the rib waveguide such that in use it laterally confines an optical wave propagated by the waveguide. The silica layer 36 situated above the slab regions 23 and the rib portion 20, extends into each recess 46 and 48 and covers each lateral wall 39,40 of each

recess. This prevents the lateral walls of the recesses being doped when the base of the recesses is doped, for example by ion implantation. This is particularly important, for example, for lateral walls 49 which provide a lateral boundary ("the recess lateral wall boundary") of the rib waveguide, so that there is substantially no overlap between an optical wave propagated through the waveguide, and the doped regions 28 and 30, which would tend to cause unwanted optical losses.

Electrically conductive contacts 32 and 34 (formed from metal) are arranged in electrical contact with the p±doped and n'-doped regions, respectively, and these enable the p-i-n diode to be electrically biased. The silica layer 36 extends above the slab regions and away from the diode adjacent to the doped regions, with the contacts 32 and 34 extending above it. Below the rib portion 20, the doped regions 28 and 30, and the slab regions 23 is a silica layer 24 which functions as a lower optical confinement layer.

Below the silica layer 24 is a substrate layer of silicon 26.

Claims

CLAIMS : 1. An optical transceiver for transmitting and receiving optical signals, comprising an optical signal receiver and a light modulator, the receiver and modulator being arranged such that, in use, light modulated by the modulator is received by the modulator from a source remote from the transceiver, optical signals received by the receiver are not modulated by the modulator, and light modulated by the modulator is transmitted as optical signals from the transceiver simultaneously with the reception of signals by the receiver.
2. A transceiver according to Claim 1 in which the light modulator is a light intensity modulator, and light modulated by the modulator is intensity modulated.
3. A transceiver according to Claim 1, or Claim 2, further comprising an optical input via which optical signals received by the receiver and light to be modulated by the modulator are received by the transceiver in use.

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4. A transceiver according to Claim 3, which is arranged such that optical signals to be received by the receiver and light to be modulated by the modulator are separated from each other so that they are received, respectively, by the receiver and the modulator via separate optical paths subsequent to their reception by the transceiver via one and the same said optical input.
5. A transceiver according to Claim 4, further comprising an optical splitter which separates the optical signals received by the receiver and the light to be modulated by the modulator.
6. A transceiver according to any preceding claim, in which the receiver and the modulator are arranged to receive one and the same optical wavelength.
7. A transceiver according to any one of Claims 1 to 5, in which the receiver and the modulator are arranged to receive different respective optical wavelengths.
8. A transceiver according to Claim 7, when dependent upon Claim 3, in which the receiver and the modulator are arranged in series on one and the same optical path, with the receiver situated between the optical input and the modulator.

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9. A transceiver according to Claim 8, in which the receiver is arranged to receive a first optical wavelength and to propagate a second optical wavelength to the modulator, and the modulator is arranged to modulate said second optical wavelength.
10. A transceiver according to Claim 7 when dependent upon Claim 5, in which the optical splitter is a wavelength dependent optical splitter which is arranged to propagate substantially only a first respective wavelength to the receiver and substantially only a second respective wavelength to the modulator.
11. A transceiver according to any preceding claim, further comprising a reflector arranged to reflect the light which has been, or which is to be, modulated by the modulator.
12. A transceiver according to Claim 11, in which the reflector is arranged such that it reflects light which has already passed through the modulator, back through the modulator.
13. A transceiver according to Claim 12, in which the modulator modulates the light twice, as it passes therethrough in opposite directions.
14. A transceiver according to any preceding claim, in which the receiver and modulator are integrated on an optical chip.

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15. A transceiver according to Claim 14 when dependent upon Claim 5, in which the optical input, the separate optical paths, and the optical splitter are integrated on said optical chip.
16. A transceiver according to Claim 14 when dependent upon Claim 8, in which said one and the same optical path is integrated on said optical chip.
17. A transceiver according to Claim 14 when dependent upon Claim 11, in which said reflector is integrated on said optical chip.
18. A transceiver according to any one of Claims 14 to 17, in which said integrated optical chip, is a silicon chip.
19. A transceiver according to Claim 15 or Claim 16, in which the, or each, said optical path is a silicon rib waveguide.
20. A transceiver according to any preceding claim in which the receiver comprises a photodiode.
21. A transceiver according to any preceding claim, in which the modulator comprises a pin diode.

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22. A transceiver according to Claim 21, in which the pin diode comprises a silicon rib lateral pin diode.
23. A transceiver according to any one of claims 1 to 20, in which the modulator comprises a semiconductor optical amplifier (SOA).
24. A transceiver according to Claim 23, in which the modulator comprises a reflective semiconductor optical amplifier (RSOA).
25. A transceiver according to Claim 18 when dependent upon Claim 11, in which the reflector comprises an etched edge in or on the silicon chip.
26. A transceiver according to Claim 7 or any claim dependent thereon, in which the different respective optical wavelengths are 1310nm and 1550nm.
27. An array of a plurality of optical transceivers according to any preceding claim.
28. An array according to Claim 27, which is arranged to be connectable to a plurality of optical fibres such that each transceiver of the array is connectable to a respective fibre or pair of fibres.

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29. An array according to Claim 28, in which the plurality of optical fibres is in the form of ribbon fibre.
30. An array according to Claim 28 or Claim 29, which is so connected to said plurality of optical fibres.