GB2353898A

GB2353898A - A semiconductor laser amplifier using waveguides

Info

Publication number: GB2353898A
Application number: GB9920173A
Authority: GB
Inventors: Andrew Michael Tomlinson; Daniel John Kitcher
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 1999-08-25
Filing date: 1999-08-25
Publication date: 2001-03-07
Also published as: CA2382723A1; WO2001015287A2; AU6854400A; WO2001015287A3; GB9920173D0

Abstract

A semiconductor laser has a semiconductor optical amplifier 11 with first and second semiconductor waveguides 18, 19. The first waveguide forms at least part of a first optical path in the amplifier between reflectors, and the second waveguide is optically coupled to the first waveguide in the optical amplifier. A laser output is connected to a second optical path formed at least in part by the second waveguide 19. Alternatively, one of the reflectors may be a reflective grating 25 which provides substantially total reflection. One of the reflectors in the amplifier may be provided by coating one end facets 20 of the optical amplifier in gold. The waveguides may be formed as part of an integrated semiconductor amplifier. The waveguides may be rib waveguides forming a four port evanescent coupler. The semiconductor amplifier 11 may a rectangular chip mounted in a recesss 12 in an integrated circuit optoelectronic device 13. The laser may be in the form of an external cavity arrangement or may be a distributed feedback laser.

Description

2353898 SEMICONDUCTOR LASER The invention relates to semiconductor lasers

including external cavity lasers.

The invention is particularly applicable to narrow line width lasers for use in telecommunication systems. Such systems may comprise wavelength division multiplexing systems.

External cavity lasers are known in which the laser cavity is formed between two reflectors one of which is a partial reflector so as to allow transmission of the laser output. The output power is related to the intracavity power and the ratio of the reflection and transmission coefficients of the reflectors used in the cavity. The threshold current to commence laser operation as well as the output power and intracavity power are all related to the reflectivity of the reflectors used. Normally a low threshold current is incompatible with a high output power where the output is derived by transmission through one of the reflectors. Furthermore the intracavity power may need to be accurately controlled to ensure correct behaviour of the laser in distributed wavelength division multiplex systems. These may require the laser to operate on a single longitudinal mode to ensure narrow line width.

The object of the present invention is to provide an improved semiconductor laser.

The invention provides a semiconductor laser comprising a semiconductor optical amplifier having first and second semiconductor waveguides, a first waveguide forming at least part of a first optical path in the amplifier between reflectors and a second waveguide optically coupled to said first waveguide in the optical amplifier, and a laser output connected to a second optical path formed at least in part by said second waveguide.

The invention also provides a semiconductor laser comprising a 2 semiconductor optical amplifier having first and second semiconductor waveguides, a first waveguide forming at least part of a first optical path between a reflector in the amplifier and a reflective grating, and a second waveguide optically coupled to said first waveguide in the optical amplifier, and a laser output connected to a second optical path formed at least in part by said second waveguide.

Preferably the reflective grating is arranged to provide substantially total reflection.

Preferably said reflector in the amplifier is provided by one end facet of said optical amplifier.

Preferably said first and second waveguides extend from the optical amplifier through an end face inclined to the normal to said end face.

Preferably both said first and second waveguides are parallel at said end of the optical amplifier.

Preferably both said waveguides are formed as part of an integrated semiconductor amplifier.

Said first and second waveguides in the optical amplifier may form a four port evanescent coupler.

Alternatively said first and second waveguides in the optical amplifier form an evanescent coupler in which a reflector in the amplifier is located where the two waveguides are closest.

Preferably said semiconductor amplifier is mounted in a recess in a supporting member on which said reflective grating is provided, said first waveguide forming part of an optical waveguide path extending to said reflective grating.

Preferably the supporting member is an integrated circuit 3 optoelectronic device.

Preferably said supporting member includes further waveguides, one further waveguide connecting said first waveguide to said reflective grating and another further waveguide connecting said second waveguide to said laser output.

Preferably each of said waveguides comprises a semiconductor rib waveguide.

Preferably each of said optical paths includes a silicon rib waveguide.

Preferably the output of the laser includes a junction with an optical fibre.

Preferably electrical pumping circuit is connected to active semiconductor material in said optical amplifier.

Preferably means is provided to vary the optical gain in said second waveguide.

Preferably said means may vary the gain in said second waveguide independently of the gain of the first waveguide.

Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 shows a schematic view of one embodiment of the invention, Figure 2 shows a reflective grating for use in a rib waveguide in the device of Figure 1, Figure 3 shows a vertical section through one structure of use in the device of Figure 1, and 4 Figure 4 shows an alternative arrangement for part of the device of Figure 1.

In this example a reflective semiconductor optical amplifier 11 is formed as an integrated semiconductor chip and located in a recess 12 in an integrated silicon support 13. The optical amplifier 11 is formed as a rectangular chip having two faces 15 and 16 which abut edges of the recess 12 in order to provide correct alignment of the amplifier 11 within the recess 12. The amplifier 11 has two rib waveguides 18 and 19 which extend across the amplifier and form an evanescent coupler within the amplifier. The end facet 20 of the chip 11 has a high reflective coating such as a gold coating. The opposite end face 21 has an antireflective coating so as to allow high level of transmission through the end face 21. The waveguides 18 and 19 are inclined to the normal at the face 21 and are in accurate alignment with corresponding aligned silicon waveguides 22 and 23 in the support 13. Waveguides 22 and 23 are each rib waveguides which extend parallel to each other in the region adjacent the face 21. Waveguide 22 is connected to a reflective grating 25 of the type shown in Figure 2. This comprises a silicon rib 26 with a plurality of transverse grooves 27 formed in the rib so as to provide reflection of light. The grating 25 is arranged on the waveguide 22 so that light transmitted from the amplifier 11 passes along the waveguide 22 to the reflective grating 25 where there is substantially total reflection. The waveguide 22 terminates at the grating 25 so that there is no transmitted output through the grating 25. Light is reflected back into the optical amplifier 11 so that the laser cavity is formed between the highly reflective end 20 of the optical amplifier and the reflective grating 25. The coupler 29 formed in the optical amplifier 11 acts as a tap-off coupler in order to tap off output from the amplifier for use as the laser output. Light which is tapped of f in this way passes along the rib waveguide 23 to a laser output 30 in which an optical fibre 31 is located in a V groove support aligned with the end of the rib waveguide 23. A further waveguide coupler 33 is located adjacent the rib waveguide 23 so as to provide a signal to a photodiode 34 in order to monitor the light output of the laser. Power to drive the laser is derived from an electrical pumping circuit 35 which is connected to electrodes adjacent active semiconductor light transmitting material in the amplifier 11 as will be described with reference to Figure 3.

In the example of Figure 1, the coupler 29 is a four port evanescent coupler such that the distance between the midpoint of the coupler 29 and the reflective end face 20 for each of the waveguides 18 and 19 is either exactly equal for both waveguides or if different they need to be an exact integer of the number of wavelengths in order to get constructive interference at the output.

An alternative to this is shown in Figure 4 which shows a modified optical amplifier 11 in which the waveguides 18 and 19 form an evanescent coupler in which the highly reflective end facet 20 is now located at the midpoint of the coupler where the two waveguides are closest. In other words, only half the coupler 29 of Figure 1 need be used if the midpoint of the coupler coincides with the high reflective end face 20.

one example of a semiconductor structure which may be used for the optical amplifier 11 is shown in Figure 3. This illustrates one of the rib waveguides 19 although both rib waveguides 18 and 19 may be similarly constructed on a single chip. In this case the rib 19 projects upwardly from an upper layer of semiconductor material 40 which may be a mixture of indium, galium, arsenic and phosphorous. These semiconductor materials may be active or passive depending on the concentrations used in any mixture of these semiconductor materials. The layer 40 may be passive. An active layer 41 formed from a mixture of the same semiconductor materials is located below layer 40. A passive substrate 42 is formed of the same semiconductor materials. Electrical terminals 43 and 44 are placed above the rib 19 and below the substrate 42. These metal contacts are connected to the electrical pumping 6 circuit 35. In the example shown in Figure 3 the optical mode for light transmission is shown by the broken line 45.

It will be appreciated that by putting the metal contacts separately over the waveguides 18 and 19 in the optical amplifier, the tapped off output signal through waveguide 19 can be separately controlled so as to provide a high gain in the optical output without changing the intracavity power. Similarly the intracavity power and the laser threshold can be determined by operation of the active material in the waveguide path formed by the rib 18 and has a degree of independence f rom the required output power.

The supporting structure 13 is formed by known integrated silicon techniques for optical waveguides in which the waveguides 22 and 23 are formed as known silicon rib waveguides.

The invention is not limited to the details of the foregoing example.

The laser may be a distributed feed back (DFB) laser.

Although the grating 25 of Figure 1 is highly reflective, it is not practical to achieve 100% reflectivity with such a grating. The photodetector 34 may therefore, in an alternative embodiment, be located adjacent the end of the grating 25 remote from the recess 12 so as to receive light which is transmitted through the grating and thereby avoid unnecessary waste of light energy.

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Claims

CLAIMS:

1. A semiconductor laser comprising a semiconductor optical amplifier having first and second semiconductor waveguides, a first waveguide forming at least part of a first optical path in the amplifier between reflectors and a second waveguide optically coupled to said first waveguide in the optical amplifier, and a laser output connected to a second optical path formed at least in part by said second waveguide.

2. An semiconductor laser comprising a semiconductor optical amplifier having first and second semiconductor waveguides, a first waveguide forming at least part of a first optical path between a reflector in the amplifier and a reflective grating, and a second waveguide optically coupled to said first waveguide in the optical amplifier, and a laser output connected to a second optical path formed at least in part by said second waveguide.

3. A laser according to claim 2 in which the reflective grating is arranged to provide substantially total reflection.

4. A laser according to claim 2 or claim 3 in which said reflector in the amplifier is provided by one end facet of said optical amplifier.

5. A laser according to any one of the preceding claims in which said first and second waveguides extend from the optical amplifier through an end face inclined to the normal to said end face.

6. A laser according to claim 5 in which both said first and second waveguides are parallel at said end of the optical amplifier.

7. A laser according to any one of the preceding claims in which both said waveguides are formed as part of an integrated 8 semiconductor amplifier.

8. A laser according to any one of the preceding claims in which said first and second waveguides in the optical amplifier form a four port evanescent coupler.

9. A laser according to any one of claims 2 to 7 in which said first and second waveguides in the optical amplifier form an evanescent coupler in which a reflector in the amplifier is located where the two waveguides are closest.

10. A laser according to any one of the preceding claims in which said semiconductor amplifier is mounted in a recess in a supporting member on which said reflective grating is provided, said first waveguide forming part of an optical waveguide path extending to said reflective grating.

11. A laser according to claim 10 in which the supporting member is an integrated circuit optoelectronic device.

12. A laser according to claim 10 or claim 11 in which said supporting member includes further waveguides, one further waveguide connecting said first waveguide to said reflective grating and another further waveguide connecting said second waveguide to said laser output.

13. A laser according to any one of the preceding claims in which each of said waveguides comprises a semiconductor rib waveguide.

14. A laser according to claim 13 in which each of said optical paths includes rib waveguides comprises a silicon rib waveguide.

15. A laser according to any one of the preceding claims in which the output of the laser includes a junction with an optical fibre.

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16. A laser according to any one of the preceding claims in which an electrical pumping circuit is connected to active semiconductor material in said optical amplifier.

17. A laser according to any one of the preceding claims in which means is provided to vary the optical gain in said second waveguide.

18. A laser according to claim 17 in which said means may vary the gain in said second waveguide independently of the gain of the first waveguide.

19. A laser according to any one of the preceding claims in which the laser is an external cavity laser.

20. A laser according to claim 1 in which the laser is -a distributed feed back laser.