GB2377104A

GB2377104A - Integrated optical emitters and multiplexer on the same substrate

Info

Publication number: GB2377104A
Application number: GB0116137A
Authority: GB
Inventors: Christopher Richard Hard White
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-06-29
Filing date: 2001-06-29
Publication date: 2002-12-31
Also published as: GB0116137D0

Abstract

An optical transceiver comprises a plurality of light emitting devices seated on a substrate, on which is also formed a multiplexer and a plurality of waveguides arranged to convey the output of the light emitting devices to the multiplexer. In this way, the entire device can be kept at a uniform temperature in a straightforward fashion. Temperature control of single substrates is an easier matter than temperature control of multiple devices. The light emitting devices are typically InP lasers and the substrate is SOI. As noted, this is a close match to InP-based lasers in terms of its temperature characteristics. Thus, the use of SOI and InP-based lasers means that (potentially) no temperature control is required since all parts will be at a uniform temperature and hence any drift will affect all equally. The multiplexer can be a simple combining region for the waveguides or a suitable arrayed waveguide grating. The present invention also allows a pin detector to be provided on the substrate and coupled to a second waveguide arranged to convey part of the output of the light emitting device, the second waveguide merely approaching the first waveguide, thereby to respond to the evanescent part of the mode in the first waveguide and conveys a true representation of the signal in the first waveguide. To provide a two way device, ie one which can receive as well as transmit, pin detectors can be positioned in place of at least some light emitting devices. A pair of such two way devices can be used to form a multi channel link with improved properties. One channel (or more) can be used to transmit temperature information, enabling the temperature of the two transceivers to be matched for example by heating the cooler transceiver to match the warmer. A method of fabricating an optical transceiver, comprises the steps of providing a substrate, defining thereon a multiplexer, waveguides, and a plurality of pits, placing solder in the pits, placing light emitting devices in the pits on the solder, and heating the combination to melt the solder.

Description

Optical Transceiver The present invention relates to an optical transceiver.

Optical signals are commonly transmitted over extended distances via optical fibre. To use the fibre to its fullest extent, signals can be multiplexed and this is typically achieved by selecting separate wavelengths for separate signals (WDM). This requires a transceiver which includes a number of lasers coupled to a multiplexer such as an AWG (arrayed waveguide grating) or other signal combining apparatus. The laser devices are coupled to the multiplexer via short optical fibres. The entire assembly, lasers, fibres, and multiplexer, are supported in a thermally controlled environment in order to ensure that the entirety operates at a controlled temperature. If one or more parts of the assembly were to drift from the intended temperature, the overall efficiency of the device would suffer as the peak wavelength of the laser (s) might no longer match the pass band of the multiplexer.

The present invention seeks to alleviate those difficulties. It therefore provides an optical transceiver comprising a plurality of light emitting devices seated on a substrate on which is also formed a multiplexer and a plurality of waveguides arranged to convey the output of the light emitting devices to the multiplexer.

In this way, the entire device can be kept at a uniform temperature in a straightforward fashion. Temperature control of single substrates is an easier matter than temperature control of multiple devices.

The light emitting devices are typically lasers. In-based lasers have temperature characteristics which match SOI devices well, and are therefore preferred.

The substrate is preferably silicon, ideally SOI. As noted, this is a close match to InP-based lasers in terms of its temperature characteristics. Thus, the use of SOI and In-based lasers means that (potentially) no temperature control is required since all parts will be at a uniform temperature and hence any drift will affect all equally. In effect, one part may well drift but the other will drift with it.

The multiplexer can be a simple combining region for the waveguides or a suitable arrayed waveguide grating.

It is usually necessary to monitor the output of the light emitting devices.

This is usually done by providing a slightly transmissive facet on the rear face of a laser and positioning a p-i-n detector to respond to light escaping therefrom. However, this affects the laser characteristics and measures only the intensity of the light in the laser cavity. It does not measure the optical signal which is actually coupled into the fibre. Degradation of the front facet will not be detected, nor will coupling errors between the front facet and the fibre.

The present invention allows a pin detector to be provided on the substrate and coupled to a second waveguide arranged to convey part of the output of the light emitting device. It is preferred that the second waveguide merely approaches the first waveguide, ie the waveguide conveying the output of the light emitting device to the multiplexer, thereby to respond to the evanescent part of the mode in the first waveguide. In this way, the second waveguide conveys a true

representation of the signal in the first waveguide, as opposed to the internal mode in the laser, but does not substantially interfere with the signal.

To provide a two way device, ie one which can receive as well as transmit, pin detectors can be positioned in place of at least some light emitting devices. Thus, some (first) waveguides lead from light emitting devices to the multiplexer and some lead from the multiplexer to pin detectors.

A pair of such two way devices can be used to form a multi channel link with improved properties. Each device is provided at either end of an optical fibre, the fibre being coupled at each end to the respective multiplexer. One channel (or more) can be used to transmit temperature information, enabling the temperature of the two transceivers to be matched. Ideally this will be by heating the cooler transceiver to match the warmer. The temperatures need not then be held to a specific target, since although the actual temperature will vary from time to time, the optical devices will all be at a matched temperature. This is a much simpler temperature control system, and hence more reliable.

The present invention also provides a method of fabricating an optical transceiver, comprising the steps of providing a substrate, defining thereon a multiplexer, waveguides, and a plurality of pits, placing solder in the pits, placing light emitting devices in the pits on the solder, and heating the combination to melt the solder.

In this way, the solder melts and the light emitting devices"float"on the solder, occupying a predictable height when the solder solidifies. Thus, the light emitting devices of the present invention experience (on average) fewer temperature cycles, and each experiences the same number of cycles. As integrated devices can degrade at elevated temperatures, this should result in more reliable devices both in terms of the intrinsic reliability of each laser and also in terms of the matching of associated lasers.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying figures, in which; Figure 1 is a perspective view of the main module embodying the present invention; Figure 2 shows the module of figure 1 mounted and with laser drivers; Figure 3 shows a two way device; Figure 4 shows the effect of temperature on the device of figures 1 and 2; Figure 5 shows the effect of temperature on known devices; and Figure 6 shows a multi channel link.

Referring to figure 1, a silicon on insulator (SOI) substrate 10 is provided, comprising an epitaxial layer of silicon separated from a support substrate (which may be of silicon) by an insulating layer, eg of Si02. An array of lasers 12, in this case 4 although greater or fewer could be provided, are held in etched pits 14 on the substrate by a layer of solder (not visible) which attaches them to the base of the pit. The lasers are, in this example, In-based lasers acting by distributed feedback (DFB) in which a grating in the active region of the laser establishes a periodicity which dictates the resonant wavelength of the laser. Each laser can be tuned to a specific wavelength, thus permitting WDM when their respective signals are combined. DFB lasers are an established item of manufacture.

The etch depth of the pits 14 is selected to place the output facet of the lasers 12 at a suitable level to couple into a rib waveguide 16 formed on the substrate 10 in front of each laser 12. These waveguides 16, one per laser 12, lead to a multiplexer 18 shown as a generic device. Multiplexers for optical signals

guided by rib waveguides are known, and existing combinatory structures or arrayed waveguide gratings can be employed.

An output waveguide 20 leaves the multiplexer with the combined signals of the array of lasers 12. It is then coupled into an optical fibre in a conventional fashion.

Figure 2 shows the basic device of figure 1 mounted on a ceramic support 22. Also mounted on the support 22 is a set of parallel laser drivers 24. Link conductors 26 transfer power from the drivers 24 to the lasers 12 via conductors 28 on the substrate 10.

Figures 1 and 2 show p-i-n detectors 30 located adjacent each waveguide 26 conveying an optical signal from a laser 12 to the multiplexer 18. The p-i-n detectors 30 are fed by sensor waveguides 32 which approach the waveguides 26 closely but do not make contact. Thus, the sensor waveguides are influenced by the evanescent part of the mode propagating in the relevant waveguide 26 and convey a small but detectable signal to the p-i-n detector 30. This signal has the advantage of being detectable without substantially affecting or degrading the main output signal, and is representative of the actual output of the device, not of an intermediate stage of the device.

Figure 3 shows a two-way device. Some lasers 12 are replaced with p-i-n signal detectors 34 fed by waveguides 16 from the multiplexer 18. The multiplexer 18 will, in this case, be configured so as to combine signals from the lasers 12 into the input/output waveguide 20 and pass incoming signals therefrom to the appropriate waveguide 16 leading to a p-i-n signal detector 12. An arrayed waveguide grating will usually be the preferred means for doing so. In the example shown, lasers 12 and p-i-n signal detectors 34 alternate but this is not essential. The outputs of the p-i-n signal detectors 34 are fed in parallel to drive circuitry integrated together with the laser drivers 24.

Figures 4 and 5 show the need for temperature control and the benefit of using matched systems such as an Si substrate 10 and In-based lasers. Figure 3 shows the temperature response of InP-based DFB lasers (solid lines) in terms of their output wavelength against temperature. A range of wavelengths are emitted, centred on a peak. On the same scale, in dotted lines, the transmission intensity of an incoming signal of unit intensity through an arrayed waveguide grating (AWG) formed of Si rib waveguides on an Si substrate is shown. It will be apparent that there is a peak transmission wavelength with a relatively broad tail on either side. This allows the AWG to be used in WDM where the adjacent wavelengths used for other channels will fall at a wavelength where the transmissive qualities of the AWG are not at peak but are adequate.

For each device, three plots are provided showing the responses at 0 C, 35 C and 70De, labelled accordingly. It will be seen that as temperature increases, there is a distinct shift to longer wavelengths in both the laser and AWG. However, the shifts of both devices, while unequal, are closely correlated. This means that over a range of 70 C, variations in temperature will not substantially affect the efficiency of the device in that the wavelength emitted by the laser will be a wavelength that is transmitted by the AWG.

Thus, for the device to operate alone, control of temperature is in fact unnecessary. Variations in temperature will not reduce its efficiency since drift in one device is substantially matched by drift in another. A temperature differential between the devices is unlikely since they are integrated onto the same substrate.

It may be necessary to control the temperatures of the devices in order to match with other remote devices, but this is a less onerous task especially as they are integrated.

Figure 4 shows a similar graph for the same laser with an AWG formed of 8i02 rib waveguides on an 8i02 substrate. 8i02 AWGs exhibit less drift with temperature, which is normally a beneficial attribute. However, in this case the

distinct drift of the lasers means that they will only match the Si02 AWG at a specific temperature. Accordingly, temperature control will be required. However, this will (as before) be less onerous as the devices are integrated.

Figure 6 shows a multi channel link employing the present invention. It shows, schematically, a total of 10 transmit and receive channels at location A which are combined via WDM into a single fibre link and connected to corresponding receive and transmit channels at remote location B. The channels are encoded as optical signals and multiplexed using the module of figure 2, although this is not shown in figure 6.

The 10 channels are designated as 5 transmit channels T1-T4 and TT and 5 receive channels R1 to R4 and RT at location A. These correspond to 5 receive channels R1-R4 and RT and 5 transmit channels T 1- T 4 and TT at location B, respectively. R1-R4 and T1-T4 at each location enable data to be passed between the locations. At each location, channel TT sends temperature information for that location, and thus channel RT receives temperature information for the other location. A greater or lesser number of channels could be provided, so long as at least one is set aside for temperature data. It is possible to combine the two (transmit & receive) channels shown into a single channel using suitable timing or handshake techniques.

Each location has a temperature control device to which the incoming temperature data and the current temperature information is supplied. These are programmed to compare the two temperatures, and if the other location is warmer, (within certain limits) raise the temperature locally to match. Thus, the absolute temperature is indeterminate but, as explained above, this need not matter. What is achieved is a matching temperature which ensures that both devices perform a corresponding WDM encoding and operate at the same peak wavelengths.

It will of course be appreciated that many variations may be made to the

above described embodiments without departing from the scope of the present invention. Some such variations are described above; others will be apparent to the skilled person.

Claims

CLAIMS 1. An optical transceiver comprising a plurality of light emitting devices seated on a substrate on which is also formed a multiplexer and a plurality of waveguides arranged to convey the output of the light emitting devices to the multiplexer.
2. An optical transceiver according to claim 1 in which the light emitting devices are lasers.
3. An optical transceiver according to claim 2 in which the lasers are InP- based.
4. An optical transceiver according to any one of claims 1 to 3 in which the substrate is silicon.
5. An optical transceiver according to claim 4 in which the substrate is SOI.
6. An optical transceiver according to any preceding claim in which the multiplexer is an arrayed waveguide grating.
7. An optical transceiver according to any preceding claim in which a p-i-n detector is provided on the substrate, coupled to a second waveguide arranged to convey part of the output of the light emitting device.
8. An optical transceiver according to claim 7 in which the second waveguide merely approaches the first waveguide so as to respond to the evanescent part of the mode in the first waveguide.
9. An optical transceiver according to any preceding claim in which p-i-n detectors are positioned in place of at least some light emitting devices.

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10. A multi channel link comprising two transceivers according to claim 9, each provided at either end of an optical fibre, the fibre being coupled at each end to the respective multiplexer.
11. A multi channel link according to claim 10 in which at least one channel is used to transmit information relating to the temperature of one or more of the transceivers.
12. A method of fabricating an optical transceiver, comprising the steps of providing a substrate, defining thereon a multiplexer, waveguides, and a plurality of pits, placing solder in the pits, placing light emitting devices in the pits on the solder, and heating the combination to melt the solder.
13. An optical transceiver substantially as herein described with reference to and/or as illustrated in the accompanying figures.