GB2381656A

GB2381656A - Hybridised laser transmitters

Info

Publication number: GB2381656A
Application number: GB0126247A
Authority: GB
Inventors: Joseph Alan Barnard
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-11-01
Filing date: 2001-11-01
Publication date: 2003-05-07
Also published as: GB0126247D0

Abstract

A hybridised laser transmitter comprises a laser diode 31 for supplying an optical output signal, and a monitor photodiode 34 receiving an optical signal from a back face of the laser diode 31 for monitoring the output power of the laser diode 31. A laser diode driver 36 is arranged closely adjacent the laser diode 31 between the laser diode 31 and the photodiode 34 and is electrically connected to the laser diode 31, and the driver 36 is transparent to the optical signal passing from the back face of the laser diode 31 towards the photodiode 34. Such an arrangement enables the electrical connection between the driver 36 and the laser diode 31 to be minimised, thus reducing the parasitic inductance and capacitance and improving the high frequency optical performance. As well as improving the optical performance, such an arrangement can enable the transmitter to be operated from a lower voltage supply, such as a 3.3 V d.c. supply.

Description

"Hybridised Laser Transmitters" The present invention relates to hybridised laser transmitters.

In the field of optical communications, it is known to make use of a hybridised laser transmitter, incorporating a laser source, a laser driver and a monitor photodiode, for supplying an output signal to an optical fibre. Typically the arrangement of the components of such a hybridised laser transmitter on a substrate is as shown in Figure 1 of the accompanying drawings. In this arrangement the laser diode 1 (LD) is mounted on the substrate in conventional manner and is arranged to supply an optical output signal to an optical fibre 2 by way of a lens 3 which is preferably a ball lens. A back reflected signal emitted from a rear face of the laser diode 1 is detected by a photodiode 4 attached to a ceramic block 5, the photodiode 4 providing an electrical output signal which may be used to monitor or control the power output of the laser diode 1. A laser diode driver 6 is separately mounted on the substrate and is electrically connected to conductive tracks 7 and 8 on the substrate receiving differential input signals.

Furthermore the driver 6 is electrically connected to the laser diode 1, and optionally also to the photodiode 4 by relatively long bond wires 9 and 10.

However the provision of a relatively long bond wire connected to the laser diode in such an arrangement is disadvantageous when operating at very high data rates, of the order of 10 Gbit/sec, such as are required in systems utilising wavelength division multiplexing (WDM) in which different data channels within an optical signal transmitted along a single optical fibre are differentiated according to the wavelength band of the transmitted light corresponding to that channel. This is because the parasitic inductive reactance of the bond wire can swamp the low dynamic resistance of the laser diode, thus reducing the rise and fall times of the driver signal which can in turn cause eye closure and poor optical signal quality It is an object of the invention to provide a hybridised laser transmitter which is particularly suitable for operation at very high data rates, for example at 10 Gbit/sec.

According to the present invention there is provided a hybridised laser transmitter comprising a laser source for supplying an optical output signal, a monitor photodetector for monitoring the output power of the laser source, the photodetector being arranged to receive an optical signal from a back face of the laser source, and a laser source driver which is arranged closely adjacent the laser source between the laser source and the photodetector and which is electrically connected to the laser source, whilst permitting passage of the optical signal from the back face of the laser source towards the photodetector.

Such an arrangement enables the electrical connection between the driver and the laser source to be minimised, thus reducing the parasitic inductance and capacitance and improving the high frequency optical performance. As well as improving the optical performance, such an arrangement can enable the transmitter to be operated from a lower voltage supply, such as a 3. 3 V d. c. supply.

In a preferred embodiment of the invention at least a part of the driver is transparent to the optical signal passing from the back face of the laser source towards the photodetector.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which : Figure 1 is a schematic diagram illustrating a conventional hybridised laser transmitter arrangement ; Figures 2 and 3 are schematic diagrams of further possible hybridised laser transmitter arrangements ; Figures 4 and 5 are schematic diagrams of a hybridised laser transmitter in accordance with the invention ; and

Figure 6 is a schematic diagram illustrating a possible fabrication scheme for a laser driver which may be used in the transmitter of Figures 4 and 5.

As described above it is a particular disadvantage of the arrangement of Figure 1 that the relatively long bond wire 9 between the driver and the laser diode provides an inductive reactance which swamps the low dynamic resistance of the laser diode. Considering, for example, the case in which the bond wire length is 1 mm and the wire has a diameter of 25 urn and an inductance of 0.7 nH/mm of bond wire length, the total inductance of the bond wire will be 0.7 nH and the inductive reactance at 8 GHz will be

Since the dynamic resistance of a laser diode is in the region of 3 to 50, it will be appreciated that this inductive reactance of the bond wire will swamp the low dynamic resistance of the laser diode. Furthermore, if the laser diode is to be modulated with a 60mA delta current step (Heoviside step function drive) this would require a voltage of V = 0.060 x 50 = 3.0 volts. Since the typical drop across the laser diode is about 1.1 volt, and the voltage drop when the step function is applied across the, say, 50 resistance would be 0.3 volt, a total voltage supply of 3.0 + 1.1 + 0.3 + 0.2 = 4.6 volts would be required (allowing about 0.2 volt for the minimum voltage drop across the collector-emitter junction of a bipolar modulating transistor). Clearly it would not therefore be possible to operate such a circuit with a 3.3 volt supply.

In attempting to overcome the disadvantages of the arrangement of Figure 1, various alternative arrangements have been considered, such as those shown diagrammatically in Figures 2 and 3 of the drawings.

In the case of the arrangement of Figure 2, the laser diode 11 is arranged adjacent both the monitor photodiode 14 and the driver 16 which is itself supplied with differential input signals by way of conductive tracks 17 and 18. In such an arrangement the bond wires 19 and 20 between the driver 16 and the laser diode 11 and between the driver 16 and the photodiode 14 are relatively short. However such an

arrangement may not be appropriate where the laser diode is of a type requiring the electrical drive signals to be supplied to the end of the laser diode package (rather than to one side of the package as shown in Figure 2).

In the case of the arrangement of Figure 3, the laser diode 21 is mounted on a silicon optical circuit 22 having a waveguide structure 23 for conducting the optical output signal to an optical fibre 25. A driver 26 is mounted on the substrate adjacent the laser diode 21 and is supplied with differential input signals by way of conductive tracks 27 and 28. In addition a photodiode 24 is mounted on the circuit 22 to receive a proportion of the output signal from the laser diode 21 by way of a tap-off coupler 23A.

The electrical output signal from the photodiode 24 is supplied to the driver 26 by way of a conductive track 30 on the substrate and associated bond wires, and the driver 26 is electrically connected to the laser diode 21 by a relatively short bond wire 29. However this arrangement is both more expensive to produce, since it requires use of the silicon optical circuit 22, and supplies less output power to the optical fibre 25 (approximately 3 dB or less) as a result of the supply of the output signal to the optical fibre by way of the waveguide structure 23 of the circuit 22, and also as a result of the proportion of the output power supplied to the photodetector 24.

Figure 4 shows an arrangement in accordance with the invention which overcomes these problems. In this case the driver 36 is positioned between the laser diode 31 and the pholodiode 34, as well as being positioned intermediate the symmetrically arranged differential input conductive tracks 37 and 38. This arrangement ensures that the bond wire 39 between the driver 36 and the laser diode 31 is as short as possible, as well as enabling electrical connection to the rear of the laser diode 31 opposite to the front face from which the output signal is supplied to the optical fibre 32. The photodiode 34, which may be a fast edged detecting photodiode is mounted on a ceramic chip 35 which is itself mounted on the substrate.

In such an arrangement the back reflected signal from the rear face of the laser diode 31 is transmitted to the photodiode 34 by passing through the free space above the driver 36 and/or by passing through a transparent part of the driver 36. For example a

part of the driver 36 may be rendered transparent to the wavelength of the signal emitted by the laser diode 31, which is typically 1310 nm or 1550 nm, by being fabricated from Si, SiGe, GaAs or InP.

In a preferred arrangement the driver 36 comprises an integrated circuit (IC) disposed intermediate the laser diode 31 and the photodiode 34, with the IC being fabricated from Si or SiGe so as to be transparent to 1310 nm and 1550 nm light from the laser diode 31. A ball lens 33 is provided so as to focus the output signal from the laser diode 31 on the end of the optical fibre 32, which is itself coupled to the substrate by an output connector which may be in the form of a V-groove. Further conductive tracks 41,42, 43 and 44 are provided for supplying d. c. voltage to the driver 36.

Additionally the chip 35 is mounted on a conductive plate 45 on the substrate, and the photodiode 34 is electrically connected to a further conductive plate 46 on the substrate by a bond wire 47 so as to enable output of the monitoring current.

Such an arrangement is particularly advantageous as it enables the parasitic inductance of the bond wire between the driver 36 and the laser diode 31 to be minimised whilst ensuring high optical output power from the laser diode 31 to the optical fibre 32. The arrangement can also be produced at low cost since it uses a minimum number of components. Furthermore, if the output signal from the photodiode is to be used to control the driver 36, it is advantageous to provide a relatively short bond wire 40 for supplying this signal from the photodetector 34 to the driver 36, as shown in Figure 5.

Figure 6 diagrammatically shows an example of the structure of a possible laser driver which may be used in such an arrangement in order to render the driver transparent to 1310 nm and 1550 nm light. In this example doped layers of InP are grown on a silicon substrate, but these layers may be formed from SiGe or GaAs in other examples.

Claims

CLAIMS:

1. A hybridised laser transmitter comprising a laser source for supplying an optical output signal, a monitor photodetector for monitoring the output power of the laser source, the photodetector being arranged to receive an optical signal from a back face of the laser source, and a laser source driver which is arranged closely adjacent the laser source between the laser source and the photodetector and which is electrically connected to the laser source, whilst permitting passage of the optical signal from the back face of the laser source towards the photodetector.

2. A transmitter according to claim 1, wherein at least a part of the driver is transparent to the optical signal passing from the back face of the laser source towards the photodetector.

3. A transmitter according to claim 1 or 2, wherein the optical source and the driver are mounted on a substrate and electrically connected to conductive tracks on the substrate.

4. A transmitter according to claim 3, wherein the driver is electrically connected to two conductive tracks on the substrate for receiving differential input signals.

5. A transmitter according to claim 4, wherein the two differential input tracks are arranged symmetrically on the substrate on opposite sides of the driver.

6. A transmitter according to claim 3,4 or 5, wherein the photodetector is attached to a chip mounted on the substrate and is electrically connected to at least one further conductive track on the substrate.

7. A transmitter according to any preceding claim, wherein the electrical connections are made by bond wires.

8. A transmitter according to any preceding claim, wherein the photodetector is arranged to supply an electrical control signal to the driver for controlling the power output of the laser source.

9. A transmitter according to any preceding claim, wherein an output connector is provided for supplying the optical output signal from the laser source to an optical fibre.

10. A transmitter according to claim 9, wherein an optical fibre is optically coupled to the output connector.

11. A transmitter according to any preceding claim, wherein the photodetector comprises at least one photodiode.

12. A transmitter according to any preceding claim, which is formed by silicon-oninsulator (SOI) technology.

13. A hybridised laser transmitter substantially as hereinbefore described with reference to Figures 4 and 5 of the accompanying drawings.