WO2005012971A1

WO2005012971A1 - Quantum well intermixing for improved isolation in photonic devices

Info

Publication number: WO2005012971A1
Application number: PCT/GB2004/003309
Authority: WO
Inventors: Yuen Chuen Chan; Hiroshi Nakamura
Original assignee: Denselight Semiconductors Pte Ltd.; Finnie, Peter, John
Priority date: 2003-07-30
Filing date: 2004-07-30
Publication date: 2005-02-10
Also published as: GB0317856D0

Abstract

A photonic device (40) with improved optical isolation and a method to fabricate it. The device is grown on a substrate with a quantum well layer so as to be intentionally absorbing at a particular wavelength. Regions (41, 42, 44) of the device that are to be partially or wholly transparent to light at the particular wavelength, or capable of emission at the particular wavelength, are then bandgap engineered by post-growth quantum well intermixing to render them so. The non-QW intermixed regions (43) remain absorbing and so provide the required isolation at the particular wavelength. The QW intermixing may be tailored to provide a particular behaviour at other predetermined wavelengths.

Description

QUANTUM WELL INTERMIXING FOR IMPROVED ISOLATION IN PHOTONIC DEVICES Field of the Invention The present invention relates to a method for improving isolation in photonic devices, particularly optical transceivers, using a combination of growth techniques and quantum well intermixing. Background to the Invention The development of photonic integrated circuits (PICs) has attracted much attention because of the potential of additional functionality, compact size and other attendant advantages associated with monolithic integration. Monolithic PICs involve the integration of both active devices, such as laser diodes and photodetectors, and passive devices, such as waveguides, couplers and spot-size converters. To achieve optimal performance from each device, the energy bandgap of the material must be varied locally to meet the requirement of the device. For example, the energy bandgap of a passive device should be made larger than the laser source so as to realize low optical absorption loss. On the other hand, the energy bandgap of a monitoring photodetector will have to be matching or smaller than that of the laser source to obtain high responsivity. Quantum well intermixing is one of the most promising techniques for changing the energy bandgap locally on a chip or wafer. Here, vacancy defects are introduced into the semiconductor crystal lattice to promote interdiffusion of atoms between the quantum well and barrier layers. A consequence of the intermixing process is that the rectangular shaped quantum well profile becomes graded and the effective bandgap of the intermixed quantum well is increased. The intermixing process is usually followed by an annealing step to restore good crystallinity to the semiconductor material. The intermixing activation process can involve impurities (as in impurity-induced disordering), dielectric films (as in impurity-free vacancy disordering), laser light (as in laser induced disordering), or plasma. This post-growth bandgap tuning technique is both simple and relatively easy to control. Generally, quantum well intermixing has been used for bandgap tuning of active sections, such as in a multi-wavelength laser diode array, or as a means to shift the bandgap of passive waveguides to transparency to reduce propagation loss. Figure 1A illustrates the typical integration on a single chip 10 of a passive waveguide 11 with a light emitting device 12 and Figure 1B illustrates the integration on a single chip 15 of a passive waveguide 16 with a photodetector 17. In both these cases, there is a need to render the passive waveguide transparent. Optimization of bandgap energy becomes critical for larger scale integration, such as the case of the monolithic optical transceiver 20 shown in Figure 2. In this example, three components are integrated on the same chip 21, namely a Fabry-Perot (FP) laser diode 22, a photodiode (PD) 23 and a wavelength division multiplexing (WDM) filter 24 for wavelength sensitive light splitting. In this case, we consider as an example the FP laser diode 23 emitting at a wavelength of 1310nm. This output light is to exit the chip 21 via the WDM filter 24. At the same time, light at a wavelength of 1490nm is incident on the chip 21 and is to be channelled by the WDM filter 24 to the receiver photodiode 23. Here, in addition to the issue of ensuring transparency for both the 1310nm and 1490nm light in the WDM filter 24, there is the problem of 1310nm transmission light leakage to the receiver PD 23, which degrades the detector sensitivity and so must be suppressed to a high degree. For a typical application operating at a data rate of 1.25Gb/s transmission, a receiver sensitivity of around -30dBm is required. This implies a stringent requirement of at least 40dB~50dB of optical and electrical isolation in order to ensure very low crosstalk. As the optical transceiver 20 is not cooled typically, the transmitter FP 22 wavelength can vary across a wide wavelength band from 1260nm to 1360nm. In this situation, it is very difficult to realize a satisfactory filtering operation over such a wideband simply with a WDM filter 24, which may consist of a directional coupler or a Mach-Zehnder interferometer. Thus, as a result of the requirement for a high degree of isolation in such integrated devices, there is a strong motivation for providing a technique that can greatly enhance the degree of optical isolation currently available.

Summary of the Invention According to one aspect of the present invention, a photonic device comprises an optical section formed on a substrate, the optical section including a quantum well, a first region of the optical section absorbing light at a first optical wavelength and a second quantum well intermixed region of the optical section partially or wholly transparent to light at the first wavelength, thereby, in use, the first section providing optical isolation at the first wavelength. Thus, a device is initially grown on a substrate with a quantum well layer so as to be intentionally absorbing at a particular wavelength. Regions of the device that are to be partially or wholly transparent to light at the particular wavelength are then bandgap engineered by post-growth quantum well intermixing to render them so. The non-QW intermixed regions remain absorbing and so provide the required isolation. Preferably, both the first region and the second quantum well intermixed region is transparent to light at a second wavelength. Alternatively, the first region of the optical section is absorbing to light at a second wavelength and the second quantum well intermixed region is transparent to light at a second wavelength, or both the first region and the second quantum well intermixed region of the optical section is absorbing to light at a second wavelength. Furthermore, the second region may be quantum well intermixed such that the band gap matches a desired emission wavelength, in particular the first wavelength. In this way, the second region, or part thereof, may act as a light emitter, including a laser diode. At the same time, the second non-QW intermixed region remains absorbing

(isolating) at the first wavelength. Preferably, the photonic device is an optical transceiver having a transmitter transmitting at the first wavelength and a receiver receiving at a second wavelength, the receiver proximate the first region of the optical section. In this way, the first region isolates the receiver from light at the first optical wavelength. Alternatively, the device may comprise a superluminescent light emitting diode (SLED) emitting light at the first wavelength, wherein the second region of the optical section includes an active light emitting part and an output facet, light exiting the second region remote from the output facet being absorbed by the first region. In this way, light propagating in an unwanted direction is prevented from reaching another facet and being reflected back towards the active part. According to another aspect of the present invention, a method for fabricating a photonic device comprises the steps of: forming on a substrate an optical section which absorbs light at a first optical wavelength, the optical section including a quantum well, and quantum well intermixing a region of the optical section so as to render the region partially or wholly transparent to light at the first wavelength. Preferably, the optical section as formed on the substrate is transparent to light at a second wavelength and remains so after the step of quantum well intermixing. It is further preferred that the step of quantum well intermixing comprises a technique selected from a group which includes: impurity-induced disordering, dielectric films for impurity-free vacancy disordering, laser induced disordering and plasma induced disordering. According to yet another aspect of the present invention, a method for providing optical isolation at a first wavelength in a first region of a photonic device, the first region to be transparent at a second wavelength and a second region of the device to be transparent at both the first wavelength and the second wavelength, comprises the steps of: providing an optical section which is absorbing to light at the first wavelength and partially or wholly transparent to light at the second wavelength; and, quantum well intermixing the optical section in the second region of the device so as to render the optical section in that region partially or wholly transparent to light at the first wavelength, wherein the optical section in the first region of the device is not quantum well intermixed, thereby remaining absorbing to light at the first wavelength and providing the desired optical isolation.

Brief Description of the Drawings Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figures 1A and 1B show known quantum well intermixed passive waveguide devices; Figure 2 shows a known optical transceiver with transmitting laser diode and receiving photodiode; Figure 3 shows a known photonic integrated circuit (PIC) with WDM filter and photodetector; Figure 4A shows the PIC of Fig 3 QW-intermixed according to the present invention, for wavelength specific photodetection; Figure 4B indicates the respective key wavelengths for the device of Fig 4A; Figure 5 shows a PIC comprising an FP laser diode and passive waveguide; Figure 6A shows the PIC of Fig 5 QW-intermixed according to the present invention, for wavelength-specific photodetection; Figure 6B indicates the respective key wavelengths for the device of Fig 6A; Figure 7 shows a first differentially compensated optical transceiver, QW- intermixed for improved optical isolation; Figure 8 shows a second differentially compensated optical transceiver, QW- intermixed for improved optical isolation; Figure 9 shows a third differentially compensated optical transceiver, QW- intermixed for improved optical isolation Figure 10 shows a fourth differentially compensated optical transceiver, QW- intermixed for improved optical isolation Figure 11 shows a fifth differentially compensated optical transceiver, QW- intermixed for improved optical isolation Figure 12 shows a plan view of a grating for secondary filtering in an optical transceiver, QW-intermixed for improved optical isolation; Figure 13 shows a side view of vertically-coupled waveguides for secondary filtering in an optical transceiver, QW-intermixed for improved optical isolation; Figure 14 shows a monolithic PIC optical transceiver, QW-intermixed for improved optical isolation; Figure 15 shows a monolithic PIC optical transceiver, QW-intermixed for improved optical isolation; Figure 16 shows a known superluminescent diode (SLED) with absorbing section; Figure 17A shows the SLED of Fig 16, QW-intermixed for enhanced absorption of back-reflected light; and, Figure 17B indicates the respective key wavelengths for the device of Fig 17A;

Detailed Description of the Invention In the present invention, there is provided a new implementation of quantum well intermixing for improving optical isolation and reducing optical cross-talk in photonic devices and photonic integrated circuits (PICs). The photonic device is initially fabricated with an optical waveguide that is absorbing for certain wavelengths of light, and particularly in certain parts of the device. Following this, regions of the device are quantum well (QW) intermixed to achieve a desired band gap. This (QWI) band-gap engineering may result in a region becoming partially absorbing, transparent or capable of stimulated emission (when excited) at particular wavelengths. At the same time, the non- QW intermixed parts of the device remain absorbing at the (unwanted) wavelengths of light, thereby providing isolation. The technique is particularly applicable to optical transceiver devices, such as that shown in Figure 2, but can also be applied to multi- section devices, where absorption is to be enhanced at one end to suppress back reflection from the facet. Figure 3 shows a known device to which the present invention can be applied. Here, two wavelengths of light, λi and λ₂ (<λι), are propagating in a PIC 30, which includes several passive waveguides. The PIC consists of an input waveguide 31 to a WDM filter 32, which routes the two wavelengths to two separate output ports via two output waveguides 33 and 34. A photodetector (PD) 35, for example, is placed at the output of the upper waveguide 33. The requirement here is for the photodetector 35 to be responsive to λ, light only and not to be affected by λ₂ light at all. The input waveguide 31 leading to the WDM filter 32 should be transparent to light at both λi and λ₂. However, it is noted that, if the upper waveguide 33 is also transparent to both wavelengths, any λ₂ light leaking into this upper waveguide 33 will reach the receiver PD 35, thereby degrading the optical isolation. Figures 4A and 4B illustrates how QWI is used to improve optical isolation in the device of Figure 3. Firstly, the epitaxial structure for the waveguide PIC 40 with WDM filter is grown such that it is just transparent to λi light, that is λ₂ < λ_as^_rown < λi. Next, the input waveguide 41, the WDM filter region 42, and one of the waveguides 44 leading from it, are intermixed to have a bandgap of Q _I-W_G. such that QW_I-WG < λ₂, thereby rendering these parts of the PIC 40 substantially transparent to light at both λi and λ₂. However, the portion of the device including the waveguide 43 leading to the photodetector 45 remains non-QW mixed. Consequently, light propagating in the non-intermixed passive waveguide 43 at wavelength λ₂ is absorbed and does not reach the detector 45. In this way, the isolation of the active PD device 45 from signals at wavelength λ₂ is greatly improved. The technique is particularly applicable to a simple optical transceiver, of the type 20 shown in Figure 2, for isolating the receiver PD 23 from leakage signals at the wavelength at which the transmitter 22 operates. Figure 5 shows another type of known optical transceiver 50, in which input light at λ^ enters an FP laser diode transmitter 51 via a facet 52 and then propagates through the laser 51 and a passive waveguide 53 before being detected by a photodetector (PD) receiver 54 located behind. The FP laser 51 emits light via the facet 52 at a wavelength of λ₂ (<λι). It is noted that the input light at λi passes straight through the waveguide of the FP laser diode 51 as the laser diode has a larger bandgap (λ₂ < λi). This arrangement makes for a very simple transceiver structure, but there is again the problem that the photodetector 54 be responsive only to light at λi not to be affected by leakage light from the FP laser diode 51 at λ . As shown in Figures 6A and 6B, the solution is the application of QWI to achieve the above requirement. The epitaxial structure for the transceiver 60, including a quantum well layer, is first grown such that it is transparent to λi light, but of shorter bandgap than the desired FP emission wavelength λ (that is λ₂ < λa_S-g_ro _n < i). Then, the FP laser active region 61 is quantum well intermixed such that the bandgap matches the desired wavelength of emission, i.e. λowi = λ₂. The portion of the device 63 leading to the PD 64 remains non-intermixed so that light at λ₂ propagating from the FP laser 61 towards the PD 64 is absorbed in the non-intermixed waveguide 63 and only light at λi reaches the PD 64. Of course, the same technique can be applied to a more sophisticated optical transceiver. Figure 7 shows the configuration for a transceiver 70 that uses a correction technique described in a co-pending International patent application, Agent's Reference PJF01596WO. As usual, the transceiver comprises a WDM filter 71 with input port P1 and output ports P2 and P3, an FP laser diode transmitter 72 operating at 1310nm and a PD receiver 73 that detects incoming 1490nm signals. Moreover, in this example, a "mirror" arrangement of WDM filter 74, FP laser 75 and PD 76 is also included to provide a signal that can be used to compensate for leakage of 1310nm light (1260-1360nm band) to the PD 73. By driving the two FP lasers 72 and 75 in anti-phase, any leakage signal at 1310nm is cancelled when the signals from the two PDs 73 and 76 are combined. The WDM filters 71,74 and FP lasers 72,75 are located on one chip C1 and the PDs 73,76 on a separate chip C2 (a monitor PD 77 is also included on chip C3). However, despite the use of this compensation technique, the correction may not be perfect. Therefore, to reduce the stray 1310nm light to lower levels prior to applying the compensation, QWI is implemented according to the present invention. As shown in Figure 7, the chip C1 is fabricated with a quantum well layer such that the as-grown waveguides are absorbing at 1310nm but transparent at 1490nm. Quantum well intermixing is then performed over much of the chip to render the waveguides (and WDM filters) in this region transparent to the 1310nm light. However the waveguides 78 leading to the PDs remain unmixed and hence absorbing at 1310nm, thereby providing additional isolation against stray 1310nm signals. Figures 8 to 11 illustrate other optical transceiver designs described in the co- pending International patent application, Agent's Reference PJF01596WO, that use the differential correction technique described above. These designs are based on the "through laser" approach adopted in the transceiver 50 of Figure 5, rather the than the more conventional WDM based design shown in Figure 2. In the transceivers 80,100 of Figures 8 and 10, the FP lasers (81,83 and 101,103) are driven in anti-phase, whereas in the transceiver 90 of Figure 9 the signals from the two PDs 92,94 are added in anti-phase. The transceiver 110 of Figure 11 does not use a "mirror" arrangement, but rather employs an additional high speed PD 112 to provide a correction signal, the amplitude of which can be adjusted prior to being added in anti-phase to the signal from PD 113. In each of these cases, the as-grown fabrication technique for absorption followed by QWI according to the present invention is employed to further improve PD isolation from stray 1310nm transmitter signals. The as-grown, non-intermixed waveguides 86,96,105,114 behind the FP lasers absorb any back propagating 1310nm light, whereas the regions of the chips where the FP laser diodes themselves are located are QWI for emission at 1310nm. A further technique for improving isolation in an optical transceiver is described in another co-pending International patent application, Agent's Reference PJF01600WO. The basic idea here is to cascade at least two optical filtering stages between the transmitter and receiver so as to isolate the receiver from stray transmitter signals. Figures 12 to 15 illustrate four different designs that implement this technique. Figures 12 and 13 illustrate a secondary filtering stage that would be employed after the WDM filter 24 in a transceiver 20 of the type shown in Figure 2. Figures 14 and 15 show the overall design for an optical transceiver that employs one type of filter component integrated within another. However, despite the improvement in isolation that is obtained by these designs, employing the technique of as-grown fabrication followed by QWI according to the present invention can make still further gains in isolation. Indeed, the QWI technique is particularly appropriate to monolithic integrated PIC transceivers of the types shown in Figures 12 to 15. The secondary filters 120,130 shown in Figures 12 and 13 are designed to remove residual 1310nm transmitter light resulting from the non-ideal filtering by the primary WDM filter 124,134 and thereby raise the optical isolation level beyond that obtainable from the WDM filter alone. In Figure 12 the secondary filter 120 comprises a passive waveguide 121 with integral diffraction grating 122 that allows the incoming 1490nm signal to propagate through it unaffected towards the receiver PD 123, whereas the unwanted stray 1310nm transmitter light is reflected by the grating 122. In Figure 13 the secondary filter 130 comprises a dual vertically-coupled waveguide structure 131,132. The evanescent coupling between the two waveguides 131,132 is wavelength sensitive, with only the incoming 1490nm signal being efficiently coupled from waveguide 131 to waveguide 132 for onward propagation to the receiver PD 133. Now, on top of these measures, the QWI process may be implemented to reduce the optical crosstalk to even lower levels. Firstly, the PIC chip is grown with an epitaxial layer structure having a bandgap that allows for absorption of the 1490nm light (and 1310nm light), i.e. λ₂ < λ-_l < λas-gro π. This allows optimization of the region of the chip that acts as the receiver photodiode 123,133 detecting the incoming 1490nm light. Next, QW intermixing is carried out on regions to the left of the PD. The bandgap of the WDM filter region 124,134 is shifted beyond 1310nm to achieve transparency at both wavelengths λow.2 λ₂ < λi, while that of the passive waveguide 120,130 between the WDM region 124,134 and the receiver PD 123,133 is shifted by a smaller amount. By tailoring the bandgap shift of the passive waveguide 120,130 such that it lies between 1310nm and

1490nm, i.e. λ₂ < λ-Qwi∑ λi, the waveguide is absorbing to the stray 1310nm light, but transparent to the incoming 1490nm signal light. With such an implementation, the realizable optical crosstalk for both transceivers is easily reduced to approximately -50dB. Figure 14 shows a monolithic PIC transceiver 140 with a 1310nm FP laser diode 141 as the light source and a directional coupler 142 as a type of WDM filter. The directional coupler 142 is optimized for maximum throughput of incoming 1490nm signal light, which enters at port P1 and propagates through the directional coupler 142 before exiting via port P3 to reach the receiver PD 143. The 1310nm signal generated by the FP laser diode transmitter 141 enters via port P2 and propagates through the directional coupler 142, with some of the light remaining in the lower branch 144 and some of the light coupling to the upper branch 145. Diffractive gratings 146 are patterned onto the upper arm 145 of the directional coupler 142 so as to reflect the 1310nm light that couples into it towards port P1, for onward transmission. The diffractive gratings 146 are also designed to have minimal effect on the 1490nm light, so that the 1490nm light sees a very low loss during propagation from port P1 to port P3. The performance of this optical transceiver can be improved in accordance with the present invention by performing QW-intermixing on three different regions of the monolithic chip, the degree of QWI being different in each region. Initially, the PIC chip 140 is grown with an epitaxial layer structure having a bandgap that allows for absorption of the 1490nm light (and 1310nm light), i.e. λ₂ < λi < λas-grown. This allows optimization of the region of the chip that acts as the receiver photodiode 143 detecting the incoming 1490nm light. Next, QW intermixing is carried out on regions to the right of the PD. Firstly, the region in which FP laser diode 141 is located is intermixed such that the bandgap matches the desired wavelength of emission, i.e. λαwo = λ₂ (< λi). This optimizes the performance of the FP laser diode 141. The main portion of the directional coupler 142 is intermixed beyond 1310nm to provide transparency for both 1310nm and 1490nm light, i.e. λαw_.2 < λ₂ < λi. Finally, and most importantly, the part 147 of the directional coupler that leads towards the receiver PD 143 is intermixed such that its bandgap lies between 1310nm and 1490nm, i.e. < λ₂ < λαwii < , so that it will absorb any stray unwanted 1310nm light whilst being transparent to the 1490nm signal light. Consequently, an effective optical cross-talk of lower than -40dB can be achieved. Figure 15 shows another monolithic PIC transceiver 150 with a 1310nm FP laser diode 151 as the light source, but with a Mach-Zehnder interferometer (MZI) 152 as a type of WDM filter. The device operates in a similar manner to that of Figure 14, with the exception that light is coupled to and from the input, output and upper/lower arm 154,155 waveguides via two 3dB beam couplers/splitters 158,159, which will typically be of the multimode interferometer (MM I) type. Here, gratings 156 are introduced to part of both the upper 155 and lower 154 arms of the MZI to provide the means to reflect the 1310nm FP laser light to port P1, for launching into an optical fibre. The Mach-Zehnder interferometer 152 is designed to allow the 1490nm light to be guided efficiently to the receiver PD 153 at port P3 and the diffractive gratings 156 are also designed to have minimal effect on the 1490nm light, so that it sees a very low loss during propagation from port P1 to port P3. As with the design shown in Figure 14, the optical isolation achievable in the transceiver 150 of Figure 15 can also be improved by implementing the appropriate amount of intermixing at specific portions of the monolithic PIC. The structure of the monolithic PIC 150 is grown for 1490nm absorption at the receiver PD 153, the region of the Mach-Zehnder interferometer 152 is bandgap tuned for transparency at both wavelengths and the FP laser diode region 151 is shifted for 1310nm emission. As before, the region 157 of the interferometer located proximate the receiver PD 153 is intermixed such that it is absorbing to the 1310nm light but not to the 1490nm light. Thus, an effective optical crosstalk of lower than -40dB is again possible. Another example of a photonic device whose performance can be improved by the present invention is the superluminescent diode (SLED). A SLED is a high-power, low coherence light source, suitable for many fibre-sensing applications. A typical design for a SLED 160 is shown in Figure 16, where an optical output is taken from the device via a front facet 162 adjacent the active section 161, and an absorbing section 163 follows the active section to absorb any back-propagating light that would otherwise be reflected from the rear facet 164. Normally, the absorbing section 163 is not injected with current to keep it absorptive. However, at high optical output, some light does reach the rear facet 164 and is reflected back into the active section, leading to degradation of the device performance. However, improved isolation and performance can be obtained from a SLED device according to the present invention. As shown in Figures 17A and 17B, the epitaxial layer structure of the SLED device 170 is grown such that its effective bandgap is absoφtive at the emission wavelength, that is λ_βmission < λ_as^_τamt. The region that is to be the active section 171 of the SLED is then intermixed so that the effective bandgap matches with the desired emission wavelength (λ_emfe_sj_on = λ_actjv_e λ_aS-_grown). In this manner, the active region 171 is optimized while the non-intermixed rear section 173 remains highly absorbing to light at the emission wavelength that is propagating toward the rear facet 174. Consequently, absorption of light in the absorbing section 173 is greatly enhanced, leading to better suppression of unwanted reflection from the back facet 174. As has been illustrated, the technique according to the present invention is widely applicable to photonic devices, and particularly those comprising a monolithic photonic integrated circuit. Good isolation as a particular wavelength and in a particular region of the device is assured by growing the structure for the whole device so as to be absorbing at the particular wavelength. Other regions of the device are then bandgap engineered by quantum well intermixing so as to achieve the desired bandgap, which may correspond to optical transparency or a particular emission wavelength, for example.

Claims

1. A photonic device comprising an optical section formed on a substrate, the optical section including a quantum well, a first region of the optical section absorbing light at a first optical wavelength and a second quantum well intermixed region of the optical section partially or wholly transparent to light at the first wavelength, thereby, in use, the first section providing optical isolation at the first wavelength.

2. A device according to claim 1, wherein both the first region and the second quantum well intermixed region of the optical section is transparent to light at a second wavelength.

3. A device according to claim 1, wherein the first region of the optical section is absorbing to light at a second wavelength and the second quantum well intermixed region is transparent to light at the second wavelength.

4. A device according to claim 1, wherein both the first region and the second quantum well intermixed region of the optical section is absorbing to light at a second wavelength.

5. A device according to any preceding claim, wherein the second quantum well intermixed region is capable of emitting light at the first wavelength.

6. A device according to claim 5 when dependent on claim 2, comprising an optical transceiver having a transmitter transmitting at the first wavelength and a receiver receiving at the second wavelength, the receiver proximate the first region of the optical section, thereby isolated from light at the first optical wavelength.

7. A device according to claim 6, wherein the first wavelength lies within the range 1260nm to 1360nm and second wavelength lies within the range 1440nm to 1540nm

8. A device according to claim 5, comprising a superlumiπescent light emitting diode (SLED) emitting light at the first wavelength, the second region of the optical section including an active light emitting part and an output facet, light exiting the second region remote from the facet being absorbed by the first region.

9. A device according to any preceding claim, wherein the device comprises a photonic integrated circuit.

10. A device according to any preceding claim, wherein the optical section includes a passive waveguide.

11. A method for fabricating a photonic device, comprising the steps of: forming on a substrate an optical section which absorbs light at a first optical wavelength, the optical section including a quantum well; and, quantum well intermixing a region of the optical section so as to render the region partially or wholly transparent to light at the first wavelength.

12. A method according to claim 11, wherein the optical section as formed on the substrate is transparent to light at a second wavelength and remains so after the step of quantum well intermixing.

13. A method according to claim 11 or claim 12, wherein the step of quantum well intermixing comprises a technique selected from a group which includes: impurity-induced disordering, dielectric films for impurity-free vacancy disordering, laser induced disordering and plasma induced disordering.

14. A method for providing optical isolation at a first wavelength in a first region of a photonic device, the first region to be transparent at a second wavelength and a second region of the device to be transparent at both the first wavelength and the second wavelength, comprising the steps of: providing an optical section which is absorbing to light at the first wavelength and partially or wholly transparent to light at the second wavelength; and, quantum well intermixing the optical section in the second region of the device so as to render the optical section in that region partially or wholly transparent to light at the first wavelength, wherein the optical section in the first region of the device is not quantum well intermixed, thereby remaining absorbing to light at the first wavelength and providing the desired optical isolation.