WO2005012972A1

WO2005012972A1 - Photonic integrated circuit based optical transceiver

Info

Publication number: WO2005012972A1
Application number: PCT/GB2004/003320
Authority: WO
Inventors: Yee Loy Lam; Yuen Chuen Chan; Keisuke Kojima
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: GB0317859D0

Abstract

A monolithic integrated optical transceiver having a single port for input and output optical signals, the transceiver comprising an optical transmitter and an optical receiver with an intervening photonic integrated circuit that comprises a plurality of optical filtering components which discriminate between the wavelengths of the incoming and outgoing optical signals and thereby isolate the receiver to a very high degree from signals generated by the transmitter.

Description

PHOTONIC INTEGRATED CIRCUIT BASED OPTICAL TRANSCEIVER

Field of the Invention The present invention relates to a monolithic optical transceiver with improved isolation, and in particular to a transceiver with multiple filter components in a photonic integrated circuit.

Background to the Invention The sharp increase in Internet users has generated a strong and robust demand for high-speed access networks to customers' premises, including fibre-to- the-home and fibre-to-the-business. Besides realizing stable high speed and large capacity communication, optical access networks have to break into an environment that is very cost sensitive and requires a big reduction in the cost of optical fibre and components. The Passive Optical Network (PON) architecture has been viewed favorably as one of the promising candidates for the access network. One of the key elements for the deployment of PON is the optical transceiver, which has simultaneously the capability to emit, receive and also discriminate the two optical wavelengths used. In the most basic configuration, two optical fibres are necessary for optical communication, with one fibre for each direction. However, this two optical fibre requirement is not economical, and also wastes huge space just for optical fibre connection. In order to reduce the number of fibres, bi-directional communication can be used. In this configuration, two widely separated wavelengths (typically 1480- 1600nm in one direction, and 1260-1360nm for another direction) are carried on a single fibre. This type of system 10 is illustrated in Figure 1. The downstream data 11 from the central office, at the Optical Line Terminal (OLT) 12, is transmitted in the 1490nm wavelength band, while the upstream transmission 13 from the customer premise, the so-called Optical Network Unit (ONU) 14, is based on the 1310nm wavelength band. In order to economically realize this type of PON access system and to reduce the size of the system, a laser light source, photodiodes, and the wavelength division multiplexing (WDM) filters need to be integrated into one compact package, while meeting the performance specifications defined in the international standards. For a typical optical transceiver, the mean transmitter output power from the laser source is around +2dBm, while the receiver sensitivity should be at least -30dBm. Considering a signal-to-noise ratio of at least 15dB suggests the need to suppress both the optical and electrical cross-talk down to a level of around -47dB (=-2dB- 30dB-15dB). This stringent requirement is not easy to meet, and either higher electrical or/and optical cross-talk will compromise the operation of the transceiver. In a prior art system shown in Figure 2, the transceiver 20 comprises a laser diode packaged in a transistor outline (TO) can 21 with, a TO-packaged photodiode 22, a WDM filter (beam splitter 23), and a glass lens 24, all assembled together in a single package coupled to an optical fibre 25. The WDM filter 23 separates the outgoing light from the incoming light, based on the difference of wavelengths. The drawbacks of this approach are: firstly, because of the use of bulk optics, the package size is large, secondly it requires separate optical alignment for both the laser and a photodiode, and thirdly it requires separate TO cans for a laser 21 and a photodiode 22 and also precision bulk optics, making the package cost higher. The stringent optical isolation requirement also places a constraint as to how close the transmitter and receiver chips can be mounted to realize a more compact module. In another configuration used in a prior art, a compact hybrid monolithic integration on a silicon optical bench 31 is used. Figure 3 shows the schematic of the module's optical subassembly 30. In this case, the 1.3μm transmission light is generated by a laser diode 32, coupled into an optical waveguide 33 and then passes through a WDM filter 34 before being coupled into a single mode optical fibre 35, which is overlaid with glass 36 and has a ferrule 37 for assembly. On the other hand, the 1.55μm light to be received exits the optical fibre 35, is coupled into the optical waveguide 33 and then reflected by the WDM filter 34 before being detected by the 1.55μm photodiode 38, and the resulting signal amplified by a pre- amplifier 39. However, optical coupling between the optical waveguide 33 and the optical fibre 35 or between the laser diode 32 and the optical waveguide 33 is not very high. Therefore, there is a large power loss for the outgoing 1.3μm laser light as well as incoming 1.55μm light. More importantly, the power loss for the incoming light results in poor receiver sensitivity, which is the crucial parameter for this type of optical system. As shown in Figure 4(a), a further prior art system 40 utilizes even more compact monolithic integration, whereby a laser diode 41 having a grating rear reflector, a wavelength-selective absorbing section 42, and a photodiode 43, are monolithically integrated on the same Indium Phosphide (InP) substrate 44, and the whole assembly, including a preamplifier 45, is packaged into a small module 46, as shown in Figure 4(b). The 1.3μm laser light is emitted directly from the laser facet, while the light exiting the laser section 41 in the other direction, via the grating reflector, is partially reflected by the grating and partially absorbed by the absorbing section 42, which comprises the same material as the laser diode 41. The incoming 1.55μm light is coupled into the laser first 41, passes through the absorber 42, and finally fed into the photodiode 43. The absorbing section does not absorb very much of the 1.55μm light. The drawbacks of this known approach are: firstly, the efficiency of coupling the 1.55μm incoming light into the laser section is not large (typically 20-30%); secondly, the incoming 1.55μm light is partially absorbed in the absorbing section; thirdly, optical isolation between the laser and the photodiode is not large enough, resulting in high cross-talk; and fourthly, the electrical isolation and cross-talk is not low enough, as the laser and the photodiode are fabricated in relatively close proximity on the same InP substrate. From the above, it is clear that simple monolithic/hybrid integrated photonic integrated circuit (PΙC)-based solutions are not able to meet the electrical and optical cross-talk requirements. The working alternative is that based on precision bulk optics, which results in a large module and requires strict alignment of both the transmitter and receiver in a 90° relative orientation. Packaging and mounting of such a module is not easy. As a result, the expensive and stringent manufacturing processes involved means that it is very difficult to realize a cost-competitive solution.

Summary of the Invention According to one aspect of the present invention, a monolithic integrated optical transceiver comprises: an optical transmitter for generating an optical output signal at a first waveband in dependence on an electrical driving signal; an optical receiver for detecting an optical input signal at a second waveband, the optical receiver generating an electrical received signal in dependence on the optical input signal; and, a photonic integrated circuit optically coupled to the optical transmitter and the optical receiver, the integrated circuit comprising: an optical port for coupling optical output signals at the first waveband out of the optical transceiver and coupling optical input signals at the second waveband into the optical transceiver; and, a plurality of optically coupled filtering components disposed between the optical transmitter and the optical receiver, said components discriminating between the first waveband and the second waveband, wherein, in use, the optical receiver is substantially isolated from optical signals at the first waveband by the plurality of filtering components. In this way, a monolithic integrated transceiver may be fabricated using photonic integrated circuit technology, in which the necessary high levels of optical isolation (and, indirectly, electrical isolation) between transmitter and receiver is achieved by a cascade of suitably designed filter components that discriminate between the wavelengths of the incoming and outgoing optical signals. The filter chain may comprise a first stage WDM filter followed by a second stage filtering to isolate the receiver from light at the transmitter wavelength. Preferably, the second stage filter comprises a pair of wavelength-selective vertically coupled waveguides, or a waveguide grating. Alternatively, the filter chain may comprise waveguide-based wavelength selective device with an embedded grating for additional wavelength discrimination and isolation. Preferably, the device comprises a directional coupler, or a Mach- Zehnder interferometer. It is preferred that any grating is reflective at the first waveband and transmissive at the second waveband. When designed for optical communication purposes, the first waveband will typically lie within the wavelength range 1260- 1360nm and the second waveband within the wavelength range 1480-1600nm. Preferably, the whole transceiver will be formed on a common Indium Phosphide substrate, as this facilitates monolithic integration of a laser diode transmitter, photodiode receiver and intervening filter components. According to a second aspect of the present invention, a method for improving isolation in an optical transceiver between an optical transmitter generating an optical output signal at a first waveband and an optical receiver for detecting an optical input signal at a second waveband, comprises the step of: disposing a photonic integrated circuit comprising a plurality of optically coupled filter components between the transmitter and the receiver, said filter components discriminating between the first waveband and the second waveband so as to substantially prevent optical signals at the first waveband from reaching the receiver. Brief Description of the Drawings Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 illustrates bi-directional communication between the central office (OLT) and a customer premises (ONU); Figure 2 shows a known transceiver system using bulk optics; Figure 3 shows a known transceiver system utilizing hybrid integration on a Si optical bench; Figure 4A shows a known transceiver system utilizing monolithic integration of a laser diode, an absorbing section and a photodiode; Figure 4B shows a module incorporating the monolithically integrated chip shown in Fig 4A; Figure 5 shows a schematic block diagram of an optical transceiver according to the present invention; Figure 6 shows a side view of vertical coupled waveguides for enhanced optical isolation in a transceiver according to a first embodiment of the present invention; Figure 7 shows a top view of a grating-patterned waveguide for enhanced optical isolation in a transceiver according to a second embodiment of the present invention; Figure 8 shows a top view of a grating-integrated directional coupler for enhanced optical isolation in a transceiver according to a third embodiment of the present invention; and, Figure 9 shows a top view of a grating-integrated Mach-Zehnder interferometer for enhanced optical isolation in a transceiver according to a fourth embodiment of the present invention.

Detailed Description of the Invention The present invention provides a new design for an optical transceiver, which achieves good electrical and optical isolation performance in a compact module, such that it could meet the requirements of PON with speeds of 1Gbps and above. In discussing the various embodiments, the description is focused on the optical transceiver at the customer premise, the ONU. However, the same concept could equally be applied to that at the central office, at the OLT. As has been described, electrical and optical isolation of the two wavelengths of light propagating in an optical transceiver, typically the 1310nm and 1490nm bands, is critical, especially at the receiver photodiode (PD). Preferably, the receiver PD should only be sensitive to the downstream 1490nm light, but not the 1310nm upstream light. However, as a result of the close proximity of the 1310nm transmitter light source (e.g. a Fabry Perot (FP) laser) to the receiver PD in the ONU transceiver, as well as the relatively strong transmitter signal versus the very weak receiver signal, it is difficult to completely eliminate any effect of the 1310nm light on the receiver PD. In the present invention, two or more wavelength sensitive devices, or components, are monolithically integrated on an InP platform, so that their net effective isolation meets the required cross-talk level in a compact optical transceiver module. Figure 5 shows a simple block schematic diagram of an optical transceiver 50, which employs this approach. Located on a single substrate 51 are an optical transmitter 52 (laser source with modulation means) and an optical receiver 53 (photodiode), separated by two filtering components, 54 and 55. Also provided is a single optical port 56, by means of which transmitter signals at wavelength λ_out leave the transceiver and receiver signals at wavelength λ_in enter the transceiver. The optical port is typically coupled to an optical communications fibre, which provides the long distance propagation medium for the transmitter and receiver signals. The whole transceiver may be fabricated so as to form a single photonic integrated circuit, using planar waveguide technology. Individual components are coupled together by passive waveguides, and the components themselves may comprise active or passive waveguides, with appropriate structures and/or electrical driving signals to perform the required function. As shown in Figure 5, filter component 55 provides the primary wavelength discrimination that routes light at wavelength λout from the transmitter to the optical port, whilst exhibiting transparency to light at wavelength λ*_π propagating from the optical port towards the receiver. In this arrangement filter component 54 provides additional isolation, allowing light at wavelength λ_in to reach the receiver whilst preventing unwanted residual light at wavelength λ_out from doing so. Of course, various configurations are possible for the filtering operation, including multiple cascaded optical filters. Moreover, one filter may be embedded within another filter, to provide the necessary level of discrimination and isolation. By adopting the monolithic integration approach, it is possible to eliminate the optical alignment and coupling issues associated with hybrid integration solutions. Since the various devices and components are built on the same substrate, their temperature-dependent characteristics are expected to be similar. More importantly, the provision of devices in a single compact chip means efficient temperature tracking between them, ensuring optimal performance across the entire operating temperature range. In addition, ageing characteristics of the devices are also expected to be similar, meaning that electrical and optical isolation in the transceiver can be maintained over time. A monolithic integrated PIC solution in InP also means a more compact module with lower manufacturing cost. As in known transceiver devices, a WDM filter provides a good means for separating the incoming and outgoing optical signals (cf. component 55 in Figure 5). Ideally, the passive output port waveguide from the WDM filter should ideally guide only the incoming 1490nm light into the receiver photodiode (PD). However, in reality, there is stray 1310nm light propagating towards the receiver PD and this will degrade the optical isolation. Therefore, in one configuration of the present invention, a second stage isolation filter (cf. component 54 in Figure 5), located prior to the receiver PD, supplements the first stage WDM filter. It is assumed that the first stage WDM filter has largely performed the role of separating out the 1490nm and 1310nm light, and therefore we focus on enhancing the optical isolation by introducing a second stage isolation filtering before the receiver PD. A first embodiment of this second stage filtering is shown in Figure 6. Here, the second stage wavelength selectivity is introduced through vertical coupling of two proximate waveguides 60, to improve upon the imperfect filtering operation of the first stage WDM filter. The coupling length between the waveguide 61 and waveguide 62 is designed so that, of the light propagating from the WDM filter and into waveguide 62, only the 1490nm signal light is coupled into waveguide 61, which subsequently guides the 1490nm light into the receiver PD 63 for detection. The unwanted residual 1310nm light is guided away in waveguide 62. The optical isolation provided for by this vertical coupling scheme is around 20dB, and together with that realized at the first stage WDM filter, a net effective optical isolation of at least 40dB could be achieved. A second embodiment of this second stage filtering is shown in Figure 7. Again, the same assumption is made of a first stage WDM filter in this arrangement of the optical transceiver. Here, second stage filtering 70 is achieved by introducing wavelength selectivity in the waveguide 71 leading to the receiver PD 73 by fabricating gratings 72 in the waveguide. The diffractive gratings 72 are designed to reflect only the 1310nm and to allow the 1490nm light to be transmitted through to the receiver PD 73. The optical isolation provided for by this grating scheme is also around 20dB, and together with the contribution by the first stage WDM filter, an overall effective optical isolation of at least 40dB could be achieved. In this configuration, the gratings could be chirped to ensure that it reflects the whole 1310nm band of light, from 1260nm to 1360nm. Figure 8 shows a third embodiment of an optical transceiver 80 according the present invention. Again, multiple filtering is used to achieve enhanced optical isolation, but a first stage WDM filter is not employed. In this arrangement, "first- stage" filtering is provided by a directional coupler 81, comprising two waveguides 82 and 83 located between the transmitter 84 and receiver 85, whilst a grating 86 embedded inside the directional coupler provides the "second-stage" filtering. The directional coupler is designed such that its coupling length allows the 1490nm light to pass straight through as the bar state. As illustrated in Figure 8, the incoming 1490nm signal light enters into waveguide 82 via port P1 and leaves waveguide 82 of the directional coupler via port P3 and is guided towards the receiver PD 85. Here, chirped diffractive gratings 86 are introduced to waveguide 82 of the directional coupler to provide broad reflection to the 1310nm band of light, while offering good transmission to the 1490nm light. Light output from the 1310nm FP laser diode 84 is launched into waveguide 83 via port P2. Some of this light is coupled across to the waveguide 82, but the wavelength selective gratings 86 reflect it towards the output port P1, where it is coupled into an optical fibre. Furthermore, any stray 1310nm light propagating back from the optical fibre towards the receiver PD 85 will either be channeled towards waveguide 83 and on to port P4 or else be reflected back to port P1. With the gratings 86 and directional coupler 81 each contributing around 20dB of optical isolation, the effective optical isolation from the grating-integrated directional coupler filter should achieve at least 40dB. Figure 9 shows a fourth embodiment of an optical transceiver 90 according the present invention. In this arrangement, first-stage filtering is provided by a Mach-Zehnder interferometer (MZI) 91, comprising two waveguides 92 and 93, and two beam splitters/combiners 94 and 95. The MZI 91 is located between the transmitter 96 and receiver 97 and a grating 98 embedded inside the MZI provides the second-stage filtering. The MZI 91 can be implemented in planar waveguide form as part of the PIC, with broad area multimode interferometers (MMIs) providing the necessary beam splitters/combiners 94 and 95. For this implementation, the MZI is designed such that it couples the 1490nm light from port P1 and waveguide W1 to waveguide W3 and port P3, and finally into the receiver PD 97. Chirped gratings 98 are patterned into waveguides 92 and 93 immediately after the MMI coupler 94 to provide for broad reflection of the 1310nm band light but good transmission for the 1490nm band light. Here, the light output from the 1310nm FP laser diode is coupled into port P2 of the MZI and then into waveguides 92 and 93 after traversing the MMI coupler 94. However, the existence of the diffractive gratings 98 means that the 1310nm light will be reflected back towards the MMI coupler 94, and then channeled towards the upper waveguide W1 and into port P1 for launching into the optical fibre. Similarly, stray 1310nm light coming back from the optical fibre will also be reflected back and not propagate to the receiver PD 97. Consequently, the net optical isolation realizable from this grating-integrated Mach-Zehnder interferometer configuration will be at least 40dB, ensuring very low optical cross talk for the transceiver 90. All of the embodiments described above realize a compact monolithic integrated optical transceiver, which exhibits the high level of optical (and electrical) isolation demanded by today's rigorous international standards. Accurate alignment of all constituent components is assured during fabrication of the photonic integrated circuit, negating the alignment problems associated with discrete components. The cascaded or embedded filter components may be optimized individually and also in terms of the composite peformance, according to the specific application of the transceiver.

Claims

1. A monolithic integrated optical transceiver comprising: an optical transmitter for generating an optical output signal at a first waveband in dependence on an electrical driving signal; an optical receiver for detecting an optical input signal at a second waveband, the optical receiver generating an electrical received signal in dependence on the optical input signal; and, a photonic integrated circuit optically coupled to the optical transmitter and the optical receiver, the integrated circuit comprising: an optical port for coupling optical output signals at the first waveband out of the optical transceiver and coupling optical input signals at the second waveband into the optical transceiver; and, a plurality of optically coupled filtering components disposed between the optical transmitter and the optical receiver, said components discriminating between the first waveband and the second waveband, wherein, in use, the optical receiver is substantially isolated from optical signals at the first waveband by the plurality of filtering components.

2. An optical transceiver according to claim 1 , wherein the plurality of optically coupled filtering components comprises a wavelength division multiplexing (WDM) filter and a pair of vertically coupled waveguides, the coupling being wavelength selective.

3. An optical transceiver according to claim 1 , wherein the plurality of optically coupled filtering components comprises a wavelength division multiplexing filter and a waveguide grating.

4. An optical transceiver according to claim 1 , wherein the plurality of optically coupled filtering components comprises a wavelength-selective directional coupler and a waveguide grating, the grating formed in a waveguide of the coupler.

5. An optical transceiver according to claim 1 , wherein the plurality of optically coupled filtering components comprises a wavelength-selective Mach-Zehnder interferometer and a waveguide grating, the grating formed in both waveguide arms of the interferometer.

6. An optical transceiver according to any of claims 3 to 5, wherein the grating is reflective at the first waveband.

7. An optical transceiver according to any of claims 3 to 6, wherein the grating is transmissive at the second waveband.

8. An optical transceiver according to any preceding claim, wherein the first waveband lies within the wavelength range 1260-1360nm and the second waveband lies within the wavelength range 1480-1600nm.

9. An optical transceiver according to any preceding claim, wherein the optical transmitter, the optical transceiver and the photonic integrated circuit is formed on a common Indium Phosphide (InP) substrate.

10. An optical transceiver according to any preceding claim, wherein the optical transmitter comprises a laser diode.

11. An optical transceiver according to any preceding claim, wherein the optical receiver comprises a photodiode.

12. A method for improving isolation in an optical transceiver between an optical transmitter generating an optical output signal at a first waveband and an optical receiver for detecting an optical input signal at a second waveband, comprising the step of: disposing a photonic integrated circuit comprising a plurality of optically coupled filter components between the transmitter and the receiver, said filter components discriminating between the first waveband and the second waveband so as to substantially prevent optical signals at the first waveband from reaching the receiver.