WO2012085596A1

WO2012085596A1 - A laser-locker assembly

Info

Publication number: WO2012085596A1
Application number: PCT/GB2011/052578
Authority: WO
Inventors: Giacinto Busico; Andrew Ward; Neil WHITBREAD; Ruifen ZHANG
Original assignee: Oclaro Technology Ltd
Priority date: 2010-12-23
Filing date: 2011-12-23
Publication date: 2012-06-28
Also published as: GB201021889D0; GB2486715A

Abstract

A laser-locker assembly comprising an optoelectronic chip, an off-chip wavelength discriminator (104), an off -chip photodetector (106), and a laser controller (108). The optoelectronic chip comprises a laser (110), an optical waveguide (114), and an on-chip photodetector (116). The laser has a transmission end for optically coupling to an optical modulator and an opposite end. The optical waveguide and the on-chip photodetector are optically coupled to receive light emitted from the opposite end. The optical waveguide optically connects to a chip outlet provided at a facet of the optoelectronic chip. The wavelength discriminator and off -chip photodetector are optically coupled to receive light from the chip outlet in use. The laser controller is adapted to receive respective first and second electrical signals from the on-chip to control an operating wavelength of the laser in correspondence with the first and second electrical signals in use.

Description

A LASER-LOCKER ASSEMBLY

The present invention relates to a laser-locker assembly for use in controlling the operating wavelength of a semiconductor laser, and more particularly to a wavelength locker assembly for use with a semiconductor laser tunable across a broad range of wavelengths.

BACKGROUND

The present invention is suitable for use in controlling the operating wavelength of a wavelength tunable semiconductor laser, including a distributed feedback laser and distributed Bragg reflectors lasers (DBR lasers). An example of a high performance, widely tunable DBR laser is disclosed in US7145923. Such lasers are commonly used in the optical telecommunications industry. Commonly, the laser is housed within a hermetically sealed laser package with off-chip optical components, and the laser package is then build into a laser transmission module comprising electronics. Commonly the laser transmission module also comprises an optical receiver package.

Wavelength lockers are well known and are used, for example, to ensure that an optical signal generated by a laser for transmission over an optical communications network has the correct wavelength. Wavelength drift of the optical signal may otherwise occur due to ageing effects, temperature variations, or fluctuations in power supplied to the module circuitry. This is particularly important, for example, in wavelength division multiplex (WDM) optical communications systems, and is even more important in dense wavelength division multiplex (DWDM) systems, in each of which a plurality of wavelength channels is used to transmit optical signals via a single optical fibre. If the wavelength of one or more of the optical signals does not fall within its correct pre-assigned wavelength channel, corruption of the signals and/or problems with detection of the signals may occur, for example.

Within two principal telecommunications bands, namely the C Band (191.6 - 196.2 THz) and the L Band (186.4 - 191 .6 THz), there are standard wavelength channels defined by the International Telecommunications Union (ITU) at spacings of 100 GHz (0.8nm), 50 GHz (0.4nm), or 25 GHz (0.2nm). (In the future, additional bands, and narrower spacings of wavelength channels within the bands may be used.) There is therefore a need to "lock" optical signal wavelengths at these standardised wavelengths for example, and wavelength lockers are used for this purpose. Thus, a wavelength locker is typically used to monitor the light output of a laser and provides electrical feedback to a laser controller to control the operating wavelength of the laser. The wavelength locker typically comprises an etalon and/or other filter with a pair of photodetectors, with these optical components being located off the semiconductor chip of the semiconductor laser. The etalon or other filter transmits light that is a function of wavelength, and the level of light that is detected by the photodetector can therefore be related to the operating wavelength of the laser.

US7161725 discloses a known wavelength locker, in which light transmitted from an output facet of a semiconductor laser chip is sampled by a cube-type optical splitter, with light transmitted by an etalon being received by a first photodetector, and light reflected from the etalon being received by a second photodetector. The laser controller receives electrical signals from the photodetectors. The optical output power of the laser is determined from the sum of the electrical signals. The operating wavelength of the laser is tracked by the laser controller by monitoring the difference between the electrical signals produced by the two photodetectors, when normalised by the optical output power.

The laser controller controls the operating wavelength and the optical output part of the laser by controlling the electrical operating signals provided to the laser, to compensate for any divergence between the operating wavelength and optical output power of the laser and their target values. An example of a control circuit for a DBR laser is disclosed in US7394838, and the interaction of a laser controller with a wavelength locker assembly is disclosed in WO2005091451.

Disadvantageously, space is very restricted within optoelectronic modules, and the beam splitter, etalon and two photodetectors consume a significant footprint, thereby restricting the amount of space available for other components.

Further disadvantageously, the manufacturing cost of a laser-locker assembly is a function of the number of optical components in the assembly, due to the cost of the components themselves, and due to the associated complexity of placing, aligning and bonding the components in the assembly, with some of the components requiring alignment to a very high degree of precision.

A yet further disadvantage of conventional, known laser-locker assemblies, is that larger numbers of components produce larger numbers of unwanted optical reflections within an optical module, producing unwanted optical noise, in particular where incident upon the off- chip photodetectors.

Accordingly, a need remains in the art for an improved laser-locker locker assembly.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the invention, there is provided a laser-locker assembly comprising

an optoelectronic chip,

an off-chip wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein

the optoelectronic chip comprises a laser, an optical waveguide, and an on-chip photodetector,

the laser has a transmission end for optically coupling to an optical modulator and an opposite end,

the optical waveguide and the on-chip photodetector are optically coupled to receive light emitted from the opposite end,

the optical waveguide optically connects to a chip outlet provided at a facet of the optoelectronic chip,

the wavelength discriminator and off-chip photodetector are optically coupled to receive light from the chip outlet in use,

the laser controller is adapted to receive respective first and second electrical signals from the on-chip photodetector and the off-chip photodetector in use, and

the laser controller is further adapted to control an operating wavelength of the laser in correspondence with the first and second electrical signals in use.

According to a second aspect of the invention, there is provided a laser package comprising a housing that houses a laser-locker assembly comprising

an optoelectronic chip,

an off-chip wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein the optoelectronic chip comprises a laser, an optical waveguide, and an on-chip photodetector,

According to a third aspect of the invention, there is provided a laser transmission module comprising a laser-locker assembly comprising

an optoelectronic chip,

an off-chip wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein

the laser controller is further adapted to control an operating wavelength of the laser in correspondence with the first and second electrical signals in use. According to a fourth aspect of the invention, there is provided a method of controlling the wavelength of a laser in a laser-locker assembly comprising

an optoelectronic chip,

an off-chip wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein

the method comprising

receiving light from the chip outlet with the wavelength discriminator and off-chip photodetector,

receiving respective first and second electrical signals from the on-chip photodetector and the off-chip photodetector with the laser controller, and

controlling an operating wavelength of the laser in correspondence with the first and second electrical signals.

Advantageously, the assembly of the laser-locker assembly of the present invention requires fewer optical components than in commonly used, known assemblies. In particular, the laser-locker assembly of the present invention does not require a second off-chip photodetector. Further, some laser-locker assemblies of the present invention also do not require an off-chip optical splitter. Accordingly, the laser-locker assembly of the present invention can advantageously be manufactured at reduced cost and more rapidly, due to the reduced number of components required and the correspondingly reduced level of assembly complexity.

Additionally, such simplification can advantageously enhance the manufacturing yield of laser-locker assembly and associated laser transmission modules. A further advantage is that through use of fewer components, the laser-locker assembly of the present invention has a smaller package-footprint, which advantage within a size limited laser transmission module.

A yet further advantage of the laser-locker assembly of the present invention is that by having fewer components, the amount of unwanted stray light within an associated optical package can be reduced, due to there being fewer optical interfaces from which unwanted reflections can occur. Advantageously, this can reduce the level of optical noise. Further, advantageously, the on-chip photodetector may be less susceptible to optical noise than the second off-chip photodetector of known assemblies.

The on-chip photodetector may be optically connected between the laser and the chip outlet.

The optoelectronic chip may further comprise an on-chip optical splitter optically connected between the laser and the chip outlet, and the on-chip photodetector may be optically connected to the on-chip optical splitter by a branch optical waveguide.

The on-chip photodetector may be adapted to be selectable between operation as a photodetector and operation as an optical amplifier.

The laser controller may be adapted to select between controlling operation of the on-chip photodetector as a photodetector and as an optical amplifier.

Advantageously, the provision of an on-chip optical splitter may make the on-chip photodetector particularly suited to continuous operation as a photodetector (as opposed to being adapted to be switched between operation as a photodetector and a semiconductor optical amplifier). Further advantageously, when the on-chip photodetector is adapted for continuous operation as a photodetector, it is not necessary for the laser controller to be provided with a bias controller adapted to switch operation of the on-chip photodetector between operation as a photodetector and as an optical amplifier.

The laser-locker assembly may comprise an optical amplifier optically connected between the laser and the on-chip photodetector.

The laser-locker assembly may comprise an optical amplifier optically connected between the on-chip photodetector and the chip outlet. The optoelectronic chip may comprise an optical amplifier optically connected between the laser and the chip outlet.

Advantageously, the provision of such a semiconductor optical amplifier between the laser and the on-chip photodetector amplifies the optical signal incident on the on-chip and off- chip photodetectors, thereby enhancing the amplitude and accuracy of the electrical signals that they produce.

The laser-locker assembly may further comprise an off-chip optical splitter arranged to receive light from the outlet in use, and the wavelength discriminator and off-chip photodetector may be arranged to receive an optical output from the off-chip optical splitter in use.

The on-chip photodetector may comprise material adapted to be operable to produce optical gain. The on-chip photodetector may comprise material adapted to be tunable with respect to refractive index.

The laser may be a distributed Bragg reflector laser having a phase control section and/or wavelength tunable distributed Bragg reflector section, and the on-chip photodetector may have an epitaxial structure that is the same as that of the phase control section and/or wavelength tunable distributed Bragg reflector section.

The wavelength discriminator may be a passive wavelength discriminator. The wavelength discriminator may be an etalon.

A region of electrical isolation may be provided in the optoelectronic chip, being located intermediate the laser and the on-chip photodetector. The region of electrical isolation may be a region of ion implantation.

Advantageously, such a region of electrical isolation may reduce electrical interference between the laser and the on-chip photodetector, particularly in the case that the on-chip photodetector is switched between operation as a photodetector and operation as an optical amplifier.

The laser-locker assembly may further comprise an optical modulator optically coupled to the transmission end of the laser. The optical waveguide optically connected to the laser may comprise a surface waveguide, such as an optical ridge waveguide. The optical waveguide optically connected to the laser may comprise a buried waveguide. The optical waveguide optically connected to the on- chip photodetector may comprise a surface waveguide, such as an optical ridge waveguide. Alternatively, the optical waveguide optically connected to the on-chip photodetector may comprise a buried waveguide.

The method may comprise controlling an operating wavelength of the laser in correspondence with the ratio of the first and second electrical signals.

The method may further comprise switching operation of the on-chip photodetector between operation as a photodetector and operation as an optical amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

• Figure 1 shows a laser-locker according to a first embodiment of the present invention;

• Figure 2A, 2B and 2C show laser-lockers according to a second embodiment of the present invention;

• Figure 3 shows a laser-locker according to a third embodiment of the present invention;

• Figure 4 shows a laser-locker according to a fourth embodiment of the present invention;

• Figure 5 shows a laser-locker according to a fifth embodiment of the present invention;

• Figure 6 shows a laser-locker according to a sixth embodiment of the present invention;

• Figures 7A and 7B show laser transmission modules comprising a laser-lockers according to a seventh embodiment of the present invention; and

• Figure 8 shows a laser transmission module comprising a laser-locker according to an eighth embodiment of the present invention. DETAILED DESCRIPTION

In the described embodiments, like features have been identified with like numerals, albeit being incremented by integer multiples of 100, and/or with the addition of suffix letters and/or typographical marks. For example, in different figures 102, 202, 202', 202", 302, 402, 502, 602, 702A, 702B, 702A', 702B' and 802 have been used indicate an optoelectronic chip.

The term "optically connected" has been used with reference to an on-chip optical waveguide to describe that the optical waveguide passes between optical elements provided on the same optoelectronic chip, e.g. a laser and an on-chip photodetector, which are provided on a common optoelectronic chip, may be optically connected by an optical waveguide that passes between them such that light emitted from the laser can pass to the photodetector, with or without further intervening optical elements.

The term "optically coupled" has been used to describe that optical elements are arranged such that light can pass between them, through an optical waveguide and/or through free space between discrete optical elements, e.g. a laser and an off-chip photodetector are optically coupled such that light emitted from the laser passes along an optically connected waveguide, to a chip outlet in a facet of the optoelectronic chip, and passes through free space and a wavelength discriminator to the off-chip photodetector.

The term "on-chip" has been used to describe that an optoelectronic chip has been provided with the corresponding "on-chip" feature. However, the term "on-chip" does not limit the location of the corresponding feature to being provided at or on a surface of the chip. For example, an on-chip optical waveguide may be a surface optical waveguide, such as an optical ridge waveguide, or an on-chip optical waveguide may be a buried optical waveguide, within the optical chip and typically guided by a high refractive index layer that is spaced apart from a surface of the chip.

Figure 1 illustrates a laser and wavelength locker assembly 1 00 com prising an optoelectronic chip 102, a passive wavelength discriminator 104, an off-chip photodetector 106, and a laser controller (laser monitoring and control circuit) 108. The passive wavelength discriminator 104 is an optical etalon. For example, in the laser-locker assembly 100, the etalon 104 is orientated to have a free spectral range (FSR) of 100GHz with respect to light emitted from the chip 102, for operation at channels with 50GHz channel spacings (similarly a 50GHz FSR etalon may be used for 25GHz spaced channels). The off-chip photodetector 106 is of a known type. The optoelectronic chip 102 is a lll-V semiconductor chip, comprising a laser 1 10, a front (transmission) optical waveguide 1 12, a back (monitoring) optical waveguide 1 14, and an on-chip photodetector (e.g. photodiode) 1 16. The transmission optical waveguide 1 12 extends between a front end 1 18 of the laser 1 10 and a transmission output of the chip 102 located at the front facet of the chip 102. The back optical waveguide 1 14 extends between a back end 122 of the laser 1 10 and a back, monitoring chip outlet 124 located at the back facet of the chip 102.

The laser 1 10 has a resonant optical cavity, and the ends of the laser 1 18 and 122 are provided by ends of the resonant optical cavity. The front end 1 18 provides the main (majority) optical output from the laser 1 10, with the light emitted in use being optically connected to the front optical waveguide 1 12 and optically coupled to an optical telecommunications fibre (not shown), albeit typically with intervening passive and/or active optical components that are provided on and/or off the chip 102. The back end 122 emits a lower power optical output than the front (transmission) end 1 18, for use in monitoring the performance of the laser 1 10.

It will be appreciated that the terms "front" and "back" are terms in the art that are used in reference to ends of a semiconductor laser, with front referring to the end of the laser from which the optical signal that is transmitted through an optical fibre is emitted, and back refers to the other end of the laser, from which an optical monitoring signal may be emitted. Accordingly, the use of the terms front and back should not be regarded as limiting with respect to the orientation of the laser and chip.

The on-chip photodetector 1 16 is optically connected within the back optical waveguide 1 14. The on-chip photodetector 1 16 is additionally operable as a semiconductor optical amplifier, to amplify the optical signal emitted through the monitoring chip outlet 124 towards the etalon 104 and the off-chip photodetector 106.

The laser 1 10 may be a distributed Bragg reflector (DBR) laser that comprises an optical gain section and refractive index tunable sections (e.g. a phase control section and sections having wavelength tunable distributed Bragg reflectors). The on-chip photodetector may be formed from material used to produce optical gain in the optical gain section. Alternatively, the on-chip photodetector 1 16 may be formed from material used to produce a refractive index tunable section, for example having an epitaxial structure that is substantially the same as that of the phase control and/or wavelength tunable distributed Bragg reflector sections. Alternatively, the laser may be a distributed feedback (DFB) laser, and the on-chip photodetector may have an epitaxial structure that is similar to that of the DFB laser (e.g. without the provision of a grating between two epitaxial layers of differing refractive index).

An electrical isolation region 126 may be provided between the on-chip photodetector 1 16 and the laser 1 10, to reduce electrical interference between the on-chip photodetector and the laser. In Figure 1 , such an electrical isolation region is provided by the ion implanted region 126 of the chip 102, which crosses the optical waveguide 1 14 optically connecting the laser 1 10 and the on-chip photodetector 1 16. Advantageously, this region of electrical isolation reduces electrical interference between the laser 1 10 and the on-chip photodetector 1 16 in the case that the on-chip photodetector is switched between operation as a photodetector and operation as an optical amplifier.

The laser controller 108 is electrically connected to the on-chip photodetector 1 16, the off- chip photodetector 106, and control electrodes (not shown) of the laser 1 10. The laser controller 108 has the features of a current-to-voltage converter 108A, a bias controller 108B and a laser wavelength controller 108C. The current-to-voltage converter 108A produces voltage signals in correspondence with electrical current signals from the photodetectors 1 16 and 106. The bias controller 108B controls operation of the on-chip photodetector 1 16, being adapted to switch the on-chip photodetector between operation as a photodetector and an optical amplifier. The laser wavelength controller 108C is adapted to provide electrical control of the laser 1 10, for controlling the operating wavelength of the laser, and may also be adapted to control the optical power output of the laser. The laser wavelength controller 108C may control the laser through the application of electrical signals to: one or more tunable distributed Bragg reflectors; a phase tuning section of the laser; a gain section; or through varying the temperature of the laser by controlling a thermo-electric cooling element (e.g. a Peltier cooler) associated with the laser. In the case of an external cavity laser, in which the resonant cavity of the laser extends beyond the optoelectronic chip on which the laser is provided, the controller may apply electrical drive signals to actuate mechanical elements of the external laser cavity.

In use, the main optical output from the resonant optical cavity of the laser 1 10 is emitted from the front end 1 18, into the front optical waveguide 1 12, out of the transmission chip outlet 120, and is coupled to an optical fibre (not shown). It will be appreciated by those skilled in the art that there may be passive and/or active optical components provided on- chip and/or off-chip through which light passes from the front end 1 18 of the laser 1 10 to the optical fibre. Such components may include lenses, an optoelectronic modulator (e.g. an electro-absorption or a Mach-Zehnder interferometer modulator), and an optical isolator.

The resonant optical cavity of the laser 1 10 is adapted to produce a secondary optical output from the back end 122 of the laser (the opposite end to that from which the main optical output is emitted, for example towards an optical fibre). The secondary optical output is emitted into the back optical waveguide 1 14, and transmitted to the back, monitoring chip outlet 124. When the on-chip photodetector 1 16 is operable as a photodetector, the light incident into the on-chip photodetector is at least sampled, or may be fully absorbed, within the on-chip photodetector, producing an electrical signal in correspondence with the intensity of the light received by the on-chip photodetector from the laser controller 108. When the on-chip photodetector 1 16 is operable as a semiconductor optical amplifier, the light received by the on-chip photodetector is amplified and transmitted onwards towards the monitoring chip outlet 124. Light exiting the monitoring chip outlet 124 is transmitted through the etalon 104 to the off-chip photodetector 106, with the proportion that is transmitted through the etalon 104 being a function of the operating wavelength of the laser 1 10 and the optical transmission function of the etalon. The off-chip photodetector 106 produces an electrical signal in correspondence with the intensity of the light incident upon it, and this electrical signal is received by the laser controller 108.

In use, the laser controller 108 periodically alternates the bias of the on-chip photodetector 1 16 between a first mode of operation as a photodetector (the reverse bias condition) and a second mode of operation as a semiconductor optical amplifier (the forward bias condition). In the first mode of operation the laser controller 108 applies an electrical control signal to the photodetector 1 16 (e.g. applying a reverse bias signal to a top electrode, with respect to a further electrode that is connected to the underside of an active layer in the photodetector), and receives a first (optical power reference) electrical signal from the on-chip photodetector 1 16 in correspondence with the optical output power of the laser 1 10. In this mode of operation, the photodetector 1 16 absorbs a significant proportion of the incident light, preferably absorbing substantially all of the incident light to maximise the electrical signal generated and provided to the laser controller 108. In the second mode of operation the laser controller 108 applies an alternative electrical control signal (e.g. forward bias), such that the photodetector 1 16 operates as an optical amplifier, such that the optical signal is amplified as it passes onwards to the etalon 104. The light that is transmitted through the etalon 104 is absorbed by the off-chip photodetector 106, from which the laser controller 108 receives a second electrical signal in correspondence with the operating wavelength of the laser (i.e. the optical power received by the off-chip photodetector is subject to the transmission of the light emitted from the monitoring chip outlet 124 through the etalon 104, which has an optical transmission characteristic that varies as a function of wavelength). In this second mode of operation, the alternative function of the photodetector 1 16 as an optical amplifier increases the electrical signal generated by the off-chip photodetector 106. Thus, advantageously, the process of switching the bias and mode of operation of the photodetector 1 16 enables the provision of enhanced electrical signals from the photodetectors 1 16 and 106 to the laser controller 108, thereby enhancing the performance of the laser controller and the accuracy of the wavelength control of the laser 1 10.

The first electrical signal, from the on-chip photodetector 1 16, is used to monitor the optical output power of the laser 1 10, since the intensity of light emitted from the rear end 122 of the laser corresponds with the intensity of light emitted from the front end 1 18. The second electrical signal, from the off-chip photodetector 106, is normalised with respect to any variation in the optical output power of the laser 1 10, to provide a value in correspondence with the operating wavelength of the laser. Accordingly, the laser controller 108 controls the operating wavelength of the laser 1 10 in correspondence with the electrical signals received from the photodetectors 1 16 and 106. Conveniently, the laser controller 108 may also control the optical output power of the laser 1 10 in correspondence with one or both of the electrical signals received by the photodetectors 1 16 and 106.

Advantageously, the assembly of the laser-locker assembly 100 of Figure 1 requires fewer optical components than in known assemblies such as that disclosed in US7161725. In particular, the embodiment of Figure 1 does not require an off-chip optical splitter and a second off-chip photodetector. Accordingly, this embodiment can be manufactured at reduced cost and more rapidly, due to the reduced number of components required and the reduced amount of assembly complexity. Such simplification may further enhance the manufacturing yield of the associated laser modules.

The laser-locker 100 of Figure 1 has been described having a bias controller 108B adapted to switch operation of the on-chip photodetector 1 16 between operation as a photodetector and as an optical amplifier. Alternatively, the on-chip photodetector 1 16 may be operated continually as a photodetector, in which case the bias controller 108B does not require to be adapted to provide a switchable electrical drive signal to the on-chip photodetector, but may alternatively apply a constant level of bias. Advantageously, such a non-switchable bias controller 108B would be electrically less complex. In the case that the on-chip photodetector 1 16 is used non-switchably, it is adapted to absorb a portion of the incident signal received from the laser 1 10, and to transmit a complementary portion, which is emitted through the chip outlet 124, transmits through the etalon 104, and is absorbed by the off-chip photodetector 106.

The laser-locker assembly 100 may be provided as a sub-assembly housed within a hermetically sealed laser package, typically with additional off-chip optical components, such as lenses, an optical isolator, an optoelectronic modulator (such as a Mach-Zehnder interferometer or an electro-absorption modulator), and an optical fibre "pig-tail" that passes through a wall of the package. The laser locker assembly may be built into a laser transmission module. The laser transmission module may comprise a laser package and further control electronics. The laser transmission module may also comprise an optical receiver package.

Figure 2A illustrates a further laser-locker assembly 200 that differs from that of Figure 1 in that the back optical waveguide 214 is provided with a semiconductor optical amplifier 230 between the on-chip photodetector 216 and the back (monitoring) end 222 of the laser 210.

Advantageously, the provision of such a semiconductor optical amplifier 230 between the laser 210 and the on-chip photodetector amplifies the optical signal incident on the on-chip and off-chip photodetectors 216 and 206, thereby enhancing the amplitude and accuracy of the electrical signals that they produce.

As with the laser-locker assembly 100 of Figure 1 , in use, the laser controller (not shown) of Figure 2A also switches the on-chip photodetector 216 between operation as a photodetector and an optical amplifier. However, again it will be appreciated that, alternatively, the on-chip photodetector 216 may be operated continually as a photodetector. Electrical isolation is provided by a region 226 of ion implantation between the on-chip photodetector 216 and the laser 210.

Figure 2B illustrates a further laser-locker assembly 200' that differs from that of Figure 1 in that the back optical waveguide 214' is provided with a semiconductor optical amplifier 230' that is optically connected between the on-chip photodetector 216' and the chip outlet 224'.

Advantageously, the provision of such an optical amplifier 230' optically connected between the on-chip photodetector 216' and the chip outlet 224' amplifies the optical signal that is optically coupled to the off-chip photodetector 206', thereby enhancing the amplitude and accuracy of the electrical signal that it produces. To reduce electrical interference between the on-chip photodetector 21 6' and other components on the chip 202', first and second electrical isolation regions 226A' and 226B' are provided, crossing the optical waveguide 214'.

Figure 2C illustrates a yet further laser-locker assembly 200" that differs from that of Figure 1 in that the back optical waveguide 214" is provided with a first semiconductor optical amplifier 230A" optically connected between the laser 210" and the on-chip photodetector 216", and a second semiconductor optical amplifier 230B" optically connected between the on-chip photodetector 216" and the chip outlet 224".

Advantageously, the laser-locker assembly 200" of Figure 2C may provide the advantages of the assemblies 200 and 200' of both Figures 2A and 2B.

Figure 3 illustrates a laser-locker assembly 300 that differs from that of Figure 1 with respect to the arrangement by which the on-chip photodetector 316 is optically connected to the laser 310. An on-chip optical splitter 332 (for example, a known multi-mode interference coupler) is optically connected within the back optical waveguide 314 between the back end 322 of the laser 300 and the monitoring outlet 324 of the chip 302, and the on-chip photodetector 316 is optically connected to the on-chip optical splitter 332 by a branch optical waveguide 334 of the back optical waveguide 314. In this embodiment, in use, light emitted from the back end 322 of the laser 310 is split by the on-chip optical splitter 332, with a portion of the light being transmitted to each of the photodetectors 316 and 306.

Advantageously, the provision of the on-chip optical splitter 332 makes the on-chip photodetector 316 particularly suited to continuous operation as a photodetector, and it is not necessary for the laser controller 308 to be provided with a bias controller adapted to switch operation of the on-chip photodetector 316 between operation as a photodetector and as an optical amplifier.

Electrical isolation 326 may be provided by a region of ion implantation between the on-chip photodetector 316 and the laser 310. However, advantageously, in the case that the on-chip photodetector 316 is not being switched between two modes of operation, the benefit of such electrical isolation may be reduced.

In alternative embodiments, the on-chip photodetector may be provided in the front optical waveguide 312, and/or the off-chip photodetector 306 may be optically coupled to the light emitted from the front end 318 of the resonant optical cavity of the laser 310. Figure 4 illustrates a laser-locker assembly 400 (shown without the associated laser controller), in which an on-chip photodetector 416 is optically connected by a branch optical waveguide 434 to an on-chip optical splitter 432, that is optically connected within the front optical waveguide 412 between the front end 418 of the laser 410 and a transmission outlet 420 in the front facet of the optoelectronic chip 402. Light emitted from the transmission outlet 420 of the chip 402 is coupled through an off-chip optical splitter 436 to an optical fibre (not shown). The off-chip optical splitter 436 samples the transmitted light, and reflects a sampled portion of the light to the etalon 404 and to the photodetector 406. In the laser 410 of Figure 4, the emission of light from the back end of the laser 410 is not required, in contrast to the embodiments of Figures 1 , 2 and 3, and the laser may be adapted accordingly.

Conveniently, a semiconductor optical amplifier (not shown) may additionally be provided within the branch optical waveguide 434 to enhance the optical signal provided to the on- chip photodetector 416. Alternatively, conveniently, a semiconductor optical amplifier (not shown) may additionally be provided within optical waveguide between the laser 410 and the outlet 420, for example being located between the laser and the on-chip optical splitter 432 or between the on-chip optical splitter and the outlet.

Advantageously, the assembly of the laser-locker assembly 400 of Figure 4 requires fewer optical components than in known assemblies such as that disclosed in US7161725. In particular, the embodiment of Figure 4 does not require a second off-chip photodetector. Accordingly, this embodiment can be manufactured at reduced cost and more rapidly, due to the reduced number of components required and the reduced amou nt of assembly complexity. Such simplification may further enhance the manufacturing yield of the associated laser modules.

Although Figure 4 has illustrated an embodiment in which the on-chip photodetector 416 receives light from the front end of the laser 410 along a branch waveguide 434 of the front optical waveguide 412, it will be appreciated that alternatively the on-chip photodetector 416 may be provided within the communication optical waveguide 412 between the front (transmission) end 418 of the laser and the transmission outlet 420.

In the embodiments illustrated in Figures 1 to 4, the on-chip and off-chip photodetectors have been optically coupled to the same end of the laser in each case. However, it will be appreciated that the on-chip and off-chip photodetectors may be coupled to opposite ends of the laser. Examples of embodiments in which the on-chip and off-chip photodetectors are coupled to opposite ends of the laser are illustrated in the further embodiments of laser- locker assemblies 500 and 600 illustrated in Figures 5 and 6 (shown without the associated laser controllers).

Figure 7A illustrates a further laser-locker assembly 700 that differs from that of Figure 1 in that the on-chip photodetector 716 is provided on a separate optoelectronic chip 702B from the optoelectronic chip 702A on which the laser 710 is provided. The photodetector 716 is provided within an optical waveguide 732, connected between the chip inlet 746 and an electro-optic modulator 734 (e.g. a Mach-Zehnder interferometer modulator).

The laser chip 702A and the modulator chip 702B are housed within a laser package 740, together with other optical elements. Light emitted from a front chip outlet 720 of the front optical waveguide 712 of the laser chip 702A is coupled through a lens 742A to the chip inlet 746 and an optical waveguide 732 of the modulator chip 702B . Light emitted from the chip outlet 748 of the modulator chip 702B is optically coupled to an optical fibre 744 through a second lens 742B. Other elements may be provide within the laser package 740, but are omitted from Figure 7A for the benefit of clarity.

The laser package 740 and the laser controller 708 are assembled into a laser transmission module 750, which may also comprise further optical and/or electrical components, which are also omitted from Figure 7A for the benefit of clarity.

In Figure 7A the on-chip photodetector 716 is optically connected between the chip inlet 746 of the modulator chip 702B and the modulator 734. Alternatively, the on-chip photodetector 716 could be optically connected between the modulator 734 and the chip outlet 748 of the modulator chip 702B.

Figure 7B illustrates a further alternative laser-locker assembly 700' that differs from that of Figure 7A in that a complementary pair of on-chip photodetectors 716A' and 716B' are optically connected within the arms of an electro-optic modulator 734' (e.g. one is provided in each optical waveguide arm of a Mach-Zehnder interferometer modulator). The bias controller 708C receives electrical signals from the pair of on-chip photodetectors 716A' and 716B'. In a further alternative, the on-chip photodetector may be provided as a complementary tap photodetector, for example being optically connected to a complementary output waveguide from a 2x2 multimode interference coupler.

In a yet further alternative embodiment the laser 710 and modulator 734 may be provided on a common optoelectronic chip, with the laser being optically connected to the modulator.

Figures 7A and 7B illustrate laser locker assemblies 700 and 700' in which the electro-optic modulator 734 and 734' is a Mach-Zehnder interferometer modulator. Alternatively, the modulator 734 may be an electro-absorption modulator.

Figure 8 illustrates a laser transmission module 850, comprising a laser package 840, which comprises a laser-locker assembly 800. The laser-locker assembly 800 of Figure 8 differs from that of Figure 7A in that the on-chip photodetector 816 and the electro-optic modulator 834 (e.g. electro-absorption modulator) are monolithically integrated onto a common optoelectronic chip 802 with the laser 810. The on-chip photodetector 816 is optically connected in the common optical waveguide 812, in-line between the laser 810 and the modulator 834. In an alternative embodiment to, the on-chip photodetector 816 may be optically connected in-line between the modulator 834 and the transmission chip outlet 820 at a front facet of the chip 802.

The figures provided herein are schematic and not to scale.

Throughout the description and claims of this specification , the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

The present invention will now be described with reference to the following clauses:

1 . A laser-locker assembly comprising

an optoelectronic chip comprising a laser and an optical waveguide that optically connects the laser to a chip outlet provided at a facet of the optoelectronic chip,

an optoelectronic chip comprising an on-chip photodetector optically connected to an optical waveguide,

a wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein

2. A laser-locker assembly according to clause 1 , wherein the on-chip photodetector is adapted to be selectable between operation as a photodetector and operation as an optical amplifier.

3. A laser-locker assembly according to clause 2, wherein the laser controller is adapted to select between controlling operation of the on-chip photodetector as a photodetector and as an optical amplifier. 4. A laser-locker assembly according to any preceding clause, wherein a common optoelectronic chip comprises the laser and the on-chip photodetector.

5. A laser-locker assembly according to clause 4, wherein the on-chip photodetector is optically connected between the laser and the chip outlet.

6. A laser-locker assembly according to clause 4, wherein the optoelectronic chip further comprises an on-chip optical splitter optically connected between the laser and the chip outlet, and the on-chip photodetector is optically connected to the on-chip optical splitter by a branch optical waveguide.

7. A laser-locker assembly according to any preceding clause, comprising an optical amplifier optically connected between the laser and the on-chip photodetector.

8. A laser-locker assembly according to any preceding clause, comprising an optical amplifier optically connected between the on-chip photodetector and the chip outlet.

9. A laser-locker assembly according to any one of clauses 1 , 2 or 3, wherein the optoelectronic chips comprising the laser and the on-chip photodetector are different optoelectronic chips.

10. A laser-locker assembly according to any preceding clause, wherein the optoelectronic chip further comprising an optical amplifier optically connected between the laser and the outlet.

1 1 . A laser-locker assembly according to any preceding clause, wherein the laser has a transmission end and an opposite end, and the on-chip photodetector is optically coupled to receive light emitted from the transmission end.

12. A laser-locker assembly according to any one of clauses 1 to 10, wherein the laser has a transmission end and an opposite end, and the on-chip photodetector is optically coupled to receive light emitted from the opposite end.

13. A laser-locker assembly according to any preceding clause, wherein the laser has a transmission end and an opposite end, and the chip outlet is optically connected to receive light emitted from the transmission end. 14. A laser-locker assembly according to any one of clauses 1 to 12, wherein the laser has a transmission end and an opposite end, and the chip outlet is optically connected to receive light emitted from the opposite end.

15. A laser-locker assembly according to any preceding clause, further comprising an off- chip optical splitter arranged to receive light from the outlet in use, and the wavelength discriminator and off-chip photodetector being arranged to receive an optical output from the off-chip optical splitter in use.

16. A laser-locker assembly according to any preceding clause, wherein the on-chip photodetector comprises material adapted to be operable to produce optical gain.

17. A laser-locker assembly according to any of clauses 1 to 12, wherein the on-chip photodetector comprises material adapted to be tunable with respect to refractive index.

18. A laser-locker assembly according to any preceding clause, wherein the wavelength discriminator is a passive wavelength discriminator.

19. A laser-locker assembly according to cla use 1 8 , wh erei n the wavelength discriminator is an etalon.

20. A laser-locker assembly according to any preceding clause, wherein a region of electrical isolation is provided in the optoelectronic chip, being located intermediate the laser and the on-chip photodetector.

21 . A laser-locker assembly according to clause 20, wherein the region of electrical isolation is a region of ion implantation.

22. A laser-locker assembly according to any preceding clause, wherein the laser is a distributed Bragg reflector laser.

23. A laser-locker assembly according to any preceding clause, further comprising an optical modulator optically coupled to the laser.

24. A laser-locker assembly according to any preceding clause, wherein the optical waveguide optically connected to the laser is an optical ridge waveguide. 25. A laser-locker assembly according to any preceding clause, wherein the optical waveguide optically connected to the on-chip photodetector is an optical ridge waveguide.

26. A laser package comprising a housing that houses a laser-locker assembly according to any preceding claim.

27. A laser transmission module comprising a laser-locker assembly according to any preceding claim.

28. A method of controlling the wavelength of a laser in a laser-locker assembly comprising

a wavelength discriminator,

an off-chip photodetector, and

a laser controller,

the method comprising

29. A method according to clause 28, comprising controlling an operating wavelength of the laser in correspondence with the ratio of the first and second electrical signals.

30. A method according to clause 28 or 29, further comprising, switching operation of the on-chip photodetector between operation as a photodetector and operation as an optical amplifier.

Claims

1 . A laser-locker assembly comprising

an optoelectronic chip,

an off-chip wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein

2. A laser-locker assembly according to claim 1 , wherein the on-chip photodetector is optically connected between the laser and the chip outlet.

3. A laser-locker assembly according to claim 1 , wherein the optoelectronic chip further comprises an on-chip optical splitter optically connected between the laser and the chip outlet, and the on-chip photodetector is optically connected to the on-chip optical splitter by a branch optical waveguide.

4. A laser-locker assembly according to any one of claims 1 , 2 or 3, wherein the on-chip photodetector is adapted to be selectable between operation as a photodetector and operation as an optical amplifier.

5. A laser-locker assembly according to claim 4, wherein the laser controller is adapted to select between controlling operation of the on-chip photodetector as a photodetector and as an optical amplifier.

6. A laser-locker assembly according to any preceding claim, comprising an optical amplifier optically connected between the laser and the on-chip photodetector.

7. A laser-locker assembly according to any preceding claim, comprising an optical amplifier optically connected between the on-chip photodetector and the chip outlet.

8. A laser-locker assembly according to any preceding claim, wherein the optoelectronic chip further comprising an optical amplifier optically connected between the laser and the outlet.

9. A laser-locker assembly according to any preceding claim, further comprising an off- chip optical splitter arranged to receive light from the outlet in use, and the wavelength discriminator and off-chip photodetector being arranged to receive an optical output from the off-chip optical splitter in use.

10. A laser-locker assembly according to any preceding claim, wherein the on-chip photodetector comprises material adapted to be operable to produce optical gain.

1 1 . A laser-locker assembly according to any of claims 1 to 9, wherein the on-chip photodetector comprises material adapted to be tunable with respect to refractive index.

12. A laser-locker assembly according to claim 1 1 , wherein the laser is a distributed Bragg reflector laser having a phase control section and/or a wavelength tunable distributed Bragg reflector section, and the on-chip photodetector has an epitaxial structure that is the same as that of the phase control section and/or wavelength tunable distributed Bragg reflector section.

13. A laser-locker assembly according to any preceding claim, wherein the wavelength discriminator is a passive wavelength discriminator.

14. A laser-locker assembly according to claim 13, wherein the wavelength discriminator is an etalon.

15. A laser-locker assembly according to any preceding claim, wherein a region of electrical isolation is provided in the optoelectronic chip, being located intermediate the laser and the on-chip photodetector.

16. A laser-locker assembly according to claim 15, wherein the region of electrical isolation is a region of ion implantation.

17. A laser-locker assembly according to any preceding claim, further comprising an optical modulator optically coupled to the transmission end of the laser.

18. A laser-locker assembly according to any preceding claim, wherein the optical waveguide optically connected to the laser is an optical ridge waveguide.

19. A laser-locker assembly according to any preceding claim, wherein the optical waveguide optically connected to the on-chip photodetector is an optical ridge waveguide.

20. A laser package comprising a housing that houses a laser-locker assembly according to any preceding claim.

21 . A laser transmission module comprising a laser-locker assembly according to any preceding claim.

22. A method of controlling the wavelength of a laser in a laser-locker assembly comprising

an optoelectronic chip,

an off-chip wavelength discriminator,

an off-chip photodetector, and

a laser controller,

wherein

the optical waveguide optically connects to a chip outlet provided at a facet of the optoelectronic chip, the method comprising

23. A method according to claim 22, comprising controlling an operating wavelength of the laser in correspondence with the ratio of the first and second electrical signals.

24. A method according to claim 22 or 23, further comprising, switching operation of the on-chip photodetector between operation as a photodetector and operation as an optical amplifier.

25. A laser-locker assembly substantially as hereinbefore described with reference to the accompanying description and any one of the Figures.

26. A laser package substantially as hereinbefore described with reference to the accompanying description and any one of the Figures.

27. A laser transmission module substantially as hereinbefore described with reference to the accompanying description and any one of the Figures.

28. A method of operating a wavelength locker assembly substantially as hereinbefore described with reference to the accompanying description and any one of the Figures.