WO2006134395A2

WO2006134395A2 - Integrated gain block and modulator for hybrid cavity laser device

Info

Publication number: WO2006134395A2
Application number: PCT/GB2006/002271
Authority: WO
Inventors: Ian Francis Lealman
Original assignee: The Centre For Integrated Photonics Limited
Priority date: 2005-06-17
Filing date: 2006-06-19
Publication date: 2006-12-21
Also published as: GB0512384D0; WO2006134395A3

Abstract

A semiconductor optoelectronic device comprises a gain section (21) and a modulator section (25) formed on a common substrate. The device further comprises a reflector (20) arranged to reflect light emerging from the gain section (21) back into the gain section (21), and a coupler (23) associated with the reflector (20) and arranged to couple a proportion of the light from the gain section (21) into the modulator section (25). The device provides a compact arrangement in which the modulator and the gain section are already aligned.

Description

INTEGRATED GAIN BLOCK AND MODULATOR FOR HYBRID CAVITY

LASER DEVICE

Field of the Invention The present invention relates to a semiconductor optoelectronic device, and particularly to a device designed to act as the gain block in a hybrid tuneable laser capable of modulation at rates of 10 GHz and above.

Background to the Invention There is a requirement for low cost wavelength tuneable or settable sources for low cost applications such as FTTH ("fibre to the home") where the wavelength of operation can be set over a wide wavelength range e.g. 40nm. Semiconductor lasers suitable for wavelength tuning over such a range are well known. There are two main approaches, the first of which is based on monolithic devices where the tuning elements are incorporated into the laser cavity. The second is to use a hybrid device where the tuning element is external to the laser gain block and can be realised using a number of approaches including micro-electro-mechanical systems (MEMS) tuning elements.

This second approach is often seen as a better approach to the realisation of a low cost device since the tuning is simpler to implement. However, while monolithic devices can be designed to include features such as electroabsorption modulators to enable the output of the device to be modulated at speeds of 10GHz and above, this is not possible in hybrid devices since the modulator must be kept outside the laser cavity, for example as shown in the arrangement in Figure 1.

Figure 1 shows a prior art hybrid cavity laser using an external modulator. In Figure 1 the gain block 1 has an anti-reflection (AR) coated facet 7 and a high-reflection (HR) coated facet 8. The gain block 1 is located in a cavity with a moveable grating 2 for wavelength selection and a rear cavity mirror 3 and lens 4 for focusing of the light. An output lens 5 is used to focus the light onto an external modulator 6 and into an output fibre 7.

Including the modulator inside the cavity will result in the modulation of the gain of the laser cavity which would increase chirp, and in the case of long external cavity lengths of above 3 cm would limit the modulation speed to below 2 GHz due to the cavity round trip time.

An active element that includes both the gain and modulation elements, for example as shown in Figure 2, enables the assembly cost of the packaged device to be greatly reduced since it reduces the number of discrete elements that must be aligned.

Figure 2 shows a prior art reflective cavity hybrid device with monolithic modulator inside the cavity. The identical components are given identical reference numerals to those used in Figure 1. The additional elements are a monolithic gain block and modulator device 9 where the front facet 10 is AR coated, the back facet 11 is HR coated and the device 9 is composed of a front gain section 13 and a rear modulator section 14 joined by a common waveguide 12. Light is coupled out of the cavity using a beam splitter 15 and coupled into an output fibre 17 using a lens 16.

The design of monolithic chips that integrate semiconductor lasers or semiconductor lasers with electroabsorption modulators is known. The integration of a laser with an electroabsorption modulator was described by Takemi in US 5,383,216. While the integration of a semiconductor optical amplifier (SOA) and electroabsorption modulator (EAM) was described by Behringer et al in the Optical Fibre Communications 1997 (OFC '97) technical digest paper ref WGl . In both of these device configurations the gain and modulation sections are aligned along a single axis. The use of such devices in hybrid laser cavities is also known and is described by Koren in US 6,862,136. In addition the use of reflective device configurations, as disclosed by Mahony in US 5,015,964, to make devices for hybrid cavities or reflective system architectures and the extension of the technology to include angled and curved waveguides are also reported by Campbell in US 5,978,400 and Lealman et al, Electronics Letters, 1998, VoI 34, No 23, pp2247-2249.

None of these devices however disclose how to make such a monolithic element suitable for use in a hybrid cavity tuneable laser, or fixed wavelength device such as a fibre grating laser.

This invention, at least in the preferred embodiments, seeks to provide a method of realising such an element without the design compromises of the arrangement shown in Figure 2.

Summary of the Invention

In embodiments of the current invention the problems of the prior art are overcome using a new folded geometry where one arm contains the gain medium for the laser cavity and the other arm contains an optical modulator.

Thus, viewed from one aspect, the invention provides a semiconductor optoelectronic device comprising a gain section and a modulator section formed on a common substrate, the device further comprising a reflector arranged to reflect light emerging from the gain section back into the gain section, and a coupler associated with the reflector and arranged to couple a proportion of the light from the gain section into the modulator section.

According to the invention a proportion (greater than 0% and less than 100%) of the light from the gain section is coupled into the modulator section by the coupler. In this way, it is not necessary to locate the modulator within the laser cavity, but the gain section and modulator section can still be located on the same chip in order to improved the ease of alignment.

In the preferred embodiment, the reflector is formed by a rear facet of the device. In general, the light from the gain section coupled into the modulator section has been - A - reflected by the reflector. For example, the coupler may be arranged to couple a proportion of the reflected light back into the gain section and a proportion of the reflected light into the modulator section. Alternatively, the coupler may be arranged to couple proportions of the light from the gain section into separate paths for reflection into the gain section and the modulator section respectively.

The gain section and the modulator section may each comprise a respective waveguide and the waveguides from the two sections may be arranged side-by-side for at least a portion of their lengths. In this way, a "folded" geometry of the device may be achieved. At least one of the waveguides may be curved in order to be substantially perpendicular to the reflector in the region of the reflector. Thus, the input and/or output waveguides may include a curved waveguide such that the guide is normal to the rear facet to ensure good reflectivity but angled with respect to the input and output facet to ensure low residual facet in the laser cavity or feedback from the output facet of the device.

Viewed from a broad aspect, the invention may be considered as a single device for incorporating gain and modulation functions by the use of two separate waveguides, wherein the device consists of an input waveguide to provide gain for a hybrid cavity laser joined to a waveguide containing a modulating element coupler region such that the modulation element is optically discrete from the laser cavity and where the coupler region includes a mirror formed by the rear facet of the active element.

The coupler element may be a Y-coupler, a multi-mode interference coupler or a twin waveguide coupler, for example.

The output waveguide may include a booster amplifier after the modulating element.

The invention extends to a hybrid tuneable laser utilising the element described above as the core element. In particular, a hybrid tuneable laser comprising a device as described above and at least a further reflector. Brief Description of the Drawings

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of a prior art hybrid cavity laser using an external modulator; Figure 2 is a schematic view of a prior art reflective cavity hybrid device with monolithic modulator inside the cavity;

Figure 3 is a schematic plan view of a device with integrated gain block and modulator according to a first embodiment of the invention;

Figure 4 is a schematic plan view of a device according to a further embodiment of the invention including a booster semiconductor optical amplifier on the output of the device;

Figure 5 is a cross section from A to B through the waveguide core of Figure 4; and

Figure 6 is a schematic diagram showing how the device of Figure 4 could be configured inside a hybrid cavity.

In the Figures, corresponding components are given the same reference numerals.

Detailed Description of an Embodiment

This application discloses the design of a monolithic element comprising a gain section and a modulator section where the device is configured such that light supplied to the modulator section has little or no interaction with the light remaining inside the laser cavity. It further describes how such a device may be utilised within a hybrid tuneable laser.

The device according to the invention incorporates a folded cavity geometry, as shown for example in Figure 3, in which a proportion of the light is fed back into the laser cavity to ensure a stable narrow linewidth mode is maintained. The rest of the signal is coupled out of the cavity using a wavelength tolerant tap. This signal is then fed into a modulating element such as an electroabsorption modulator that can impart a high bandwidth modulation signal to the device. Since the modulator is outside the laser cavity it does not impart chirp to the laser. In addition other active elements such as semiconductor optical amplifiers may be added to the output arm of the device, as shown for example in Figure 4, to increase the output power of the device. Turning to Figure 3, a first embodiment of the invention with integrated gain block and modulator is shown schematically and in plan view. The chip (device) 18, has an anti- reflection (AR) coated front facet 19 and a high-reflection (HR) coated rear facet 20. At the input to the device 18 there is a gain section 21, with a waveguide core 22. At the end of the gain section 21 there is a coupler 23 which may be either an multimode interference (MMI) or Y junction. This feeds a portion of the light from the gain section 21 into a second passive guide 24 which leads to a modulator section 25. The modulated output light is coupled out from the device through the front facet 19.

In the first embodiment the monolithic element comprises a semiconductor chip 18, as shown in Figure 3. This chip is fabricated from a compound semiconductor material such as, but not limited to, indium phosphide (InP) or gallium arsenide (GaAs). The device substrate is then overgrown by conventional epitaxial growth techniques such as metal organic vapour phase epitaxy (MOVPE) to form a wafer with a high index waveguide core 22 surrounded by lower index cladding material. The material is then patterned to form waveguides to confine the light and supply current or voltage to the various sections of the device. The waveguide geometry used could include, but is not limited to, ridge waveguide or buried heterostructure.

The chip 18 is comprised of a front facet 19 which is generally AR coated and a rear facet 20 which may be left uncoated or be HR coated. There is an input waveguide 22 for the gain section 21 which is angled at the AR coated facet 19 to remove unwanted facet reflections. Outside the gain section 21 the waveguide composition is altered to reduce the waveguide loss by shifting the band edge to a shorter wavelength. The band-gap shift may be realised by a number of established techniques including, but not limited to, selective area growth, butt coupling or quantum well intermixing. At or close to the rear facet 20 of the chip is a coupler 23 to allow a proportion of the light from the gain section 21 to be coupled into the output waveguide 24. The coupler 23 could be either a simple Y splitter or multimode interference coupler. In addition, the strength of the coupling into guide 24 can be tailored to best meet the output power requirements of the combined device. The output waveguide 24 is then curved to provide an angle at the output facet 19. The final section of the device is the modulator section 25 which imparts the high speed signal modulation to the device. The preferred device to perform the modulation would be an electroabsorption modulator (EAM) although other modulators such as a Mach Zehnder Interferometer could also be employed. In addition, to maintain the low capacitance required for modulation speeds of 10Gbit/s or above, the p-contacts between the gain section 21 and the modulator section 25 would also require isolation. This isolation may be achieved by a number of methods including, but not limited to, etching of the p- contact, regrowth of un-doped or semi-insulating (SI) InP or proton isolation.

A further enhancement to the preferred embodiment is shown in Figure 4, which is a schematic plan view of a device 18 including a booster semiconductor optical amplifier (SOA) 26, on the output of the device. In this refinement, the modulator section 25 is moved back from the output facet 19 to permit the inclusion of an SOA 26 at the output to further boost the output power of the device 18.

Figure 5 is a cross section through the waveguide core along the dashed line A-B of Figure 4 to show the waveguide geometry. As shown in Figure 5, the booster SOA 26 and modulator 25 share a common substrate 27, but are isolated on the p-side by semi- insulating doped InP 28.

Figure 5 clearly shows how the output amplifier 26 and modulator section are located on a common substrate 27. The composition of the waveguide core in the amplifier section 22 is of lower band-gap than the modulator section, the band gap shift being obtained by the methods described earlier. The electrical isolation regions 28 between the devices being defined by the techniques described above.

While this monolithic gain and modulation chip 18 could be employed in a number of hybrid laser configurations. Figure 6 shows for clarity one potential configuration. Figure 6 shows how the chip could be configured inside a hybrid cavity, including tuning elements, (a moveable grating 2 for wavelength selection and a rear cavity mirror 3 and lens 4 for focusing of the light), an output fibre 7 and an optional rear facet lens 29 and wavelength locker 30. The active element 18 shown in Figure 3 and Figure 4 is located inside a hybrid cavity tuneable laser in Figure 6. The output from the gain section 22 is coupled via a collimating lens 4 to some form of grating or MEMS based tuning element 2 that reflects from a back mirror 3. The combination of these elements and the rear facet 20 of the gain chip forms the wavelength selective element of the device. At the rear facet 20 a small proportion of the light from the laser cavity could also be tapped off via a second collimating lens 29 to provide the signal for an optional wavelength locker 30. The modulated output of the device would be coupled out of the device by an optical fibre 7. The output coupling could also be achieved using a further collimating lens.

In summary, a semiconductor optoelectronic device comprises a gain section 21 and a modulator section 25 formed on a common substrate. The device further comprises a reflector 20 arranged to reflect light emerging from the gain section 21 back into the gain section 21, and a coupler 23 associated with the reflector 20 and arranged to couple a proportion of the light from the gain section 21 into the modulator section 25. The device provides a compact arrangement in which the modulator and the gain section are already aligned.

Claims

1. A semiconductor optoelectronic device comprising a gain section and a modulator section formed on a common substrate, the device further comprising a reflector arranged to reflect light emerging from the gain section back into the gain section, and a coupler associated with the reflector and arranged to couple a proportion of the light from the gain section into the modulator section.

2. A device as claimed in claim 1, wherein the reflector is formed by a rear facet of the device.

3. A device as claimed in claim 1 or 2, wherein the light from the gain section coupled into the modulator section has been reflected by the reflector.

4. A device as claimed in any preceding claim, wherein the gain section and the modulator section each comprise a respective waveguide and the waveguides from the two sections are arranged side-by-side for at least a portion of their lengths.

5. A device as claimed in claim 4, wherein at least one of the waveguides is curved in order to be substantially perpendicular to the reflector in the region of the reflector.

6. A hybrid tuneable laser comprising a device as claimed in any preceding claim and at least a further reflector.