DETECTOR REMODULATOR

Abstract

A detector remodulator comprising a silicon on insulator (SOI) waveguide platform including: a detector coupled to a first input waveguide; a modulator coupled to a second input waveguide and an output waveguide; and an electrical circuit connecting the detector to the modulator; wherein the detector, modulator, second input waveguide and output waveguide are arranged within the same horizontal plane as one another; and wherein the modulator includes a modulation waveguide region at which a semiconductor junction is set horizontally across the waveguide.

Claims

1. A detector remodulator comprising: a silicon on insulator (SOI) chip; a detector coupled to a first input waveguide; an electroabsorption modulator, on the SOI chip, the electroabsorption modulator being coupled to a second input waveguide and to an output waveguide and comprising: an SOI waveguide; an active region, the active region comprising a multiple quantum well (MQW) region; and a coupler for coupling the SOI waveguide to the active region; the coupler comprising: a transit waveguide coupling region; a buffer waveguide coupling region; and a taper region; and an electrical circuit connecting the detector to the modulator; wherein the transit waveguide coupling region is configured to couple light between the SOI waveguide and the buffer waveguide coupling region; and the buffer waveguide coupling region is configured to couple light between the transit waveguide region and the active region via the taper region.

2. The detector remodulator of claim 1, wherein the taper region comprises a multi-segment mode expander.

3. The detector remodulator of claim 2, wherein the multiple quantum well region is a Ge/SiGe multiple quantum well region.

4. The detector remodulator of claim 3, wherein: the transit waveguide coupling region comprises a first portion of a transit waveguide; and the buffer waveguide coupling region comprises a buffer waveguide located on top of a second portion of the transit waveguide.

5. The detector remodulator of claim 4, wherein: the transit waveguide has a refractive index bigger than that of the SOI waveguide but smaller than that of the buffer waveguide.

6. The detector remodulator of claim 5, wherein each of the buffer waveguide and transit waveguide are SiGe waveguides.

7. The detector remodulator of claim 6, wherein the active region comprises: a P-doped region between the buffer layer and the lower surface of a spacer layer underneath a multiple quantum well; and an N-doped region located at the upper surface of the spacer layer on top of the multiple quantum well.

8. The detector remodulator of claim 7, further comprising multiple N-type doped layers with different germanium compositions and doping concentrations.

9. The detector remodulator of claim 8, wherein the electrodes are arranged in a ground-signal (GS) configuration, where a ground electrode is located at an opposite side of the active region from the signal electrode.

10. A detector remodulator comprising: a silicon on insulator (SOI) chip; a detector coupled to a first input waveguide; a modulator, on the SOI chip, the modulator being coupled to a second input waveguide and to an output waveguide; at least one of: the detector, and the modulator comprising: an optically active region (OAR), including a waveguide ridge, the OAR having an upper surface and a lower surface; a lower doped region, wherein the lower doped region is located at and/or adjacent to at least a portion of the lower surface of the OAR, and extends laterally outwards from the waveguide ridge in a first direction; an upper doped region, wherein the upper doped region is located at and/or adjacent to at least a portion of the upper surface of the waveguide ridge of the OAR, and extends laterally outwards from the waveguide ridge in a second direction; and an intrinsic region located between the lower doped region and the upper doped region.

11. The detector remodulator of claim 10, further comprising a first electrode contacting the lower doped region at a first contact surface, and a second electrode contacting the upper doped region at a second contact surface; wherein the first contact surface is laterally offset from the waveguide ridge in the first direction; and wherein the second contact surface is laterally offset from the waveguide ridge in the second direction.

12. The detector remodulator of claim 11, wherein the upper doped region comprises a first doped zone and a second doped zone; wherein the dopant concentration in the second doped zone of the upper doped region is higher than the dopant concentration in the first doped zone of the upper doped region; and wherein the second doped zone of the upper doped region comprises the second contact surface.

13. The detector remodulator of claim 12, wherein first doped zone of the upper doped region is at and/or adjacent to the upper surface of the waveguide ridge of the OAR, and the second doped zone is located at a position which is laterally displaced from the waveguide ridge in the second direction.

14. The detector remodulator of claim 13, wherein the lower doped region comprises a first doped zone and a second doped zone; wherein the dopant concentration in the second doped zone of the lower doped region is higher than the dopant concentration in the first doped zone of the lower doped region; and wherein the second doped zone of the lower doped region comprises the first contact surface.

15. The detector remodulator of claim 14, wherein the first doped zone of the lower doped region is located directly underneath the OAR; and the second doped zone of the lower doped region is located within the OAR, laterally displaced from the waveguide ridge, the second doped zone of the lower doped region having an upper surface which comprises the first contact surface, and a lower surface which is in direct contact with the first doped zone of the lower doped region.

16. The detector remodulator of claim 15, wherein the second doped zone of the lower doped region is located within a portion of the OAR having a reduced height.

17. The detector remodulator of claim 16, wherein the upper doped region is fully located within the OAR.

18. The detector remodulator of claim 17, wherein the OAR is formed from an electro-absorption material in which the Franz-Keldysh effect occurs in response to the application of an applied electric field.

19. The detector remodulator of claim 18, wherein the OAR is formed from a light absorbing material suitable for generating a current upon detection of light when a voltage bias is applied across the upper and lower doped regions.

20. The detector remodulator of claim 19, wherein the optically active region (OAR) includes a waveguide ridge, a first slab on a first side of the waveguide ridge and a second slab on a second side of the of the waveguide ridge, the OAR having an upper surface and a lower surface; wherein the lower doped region is located adjacent to a portion of a lower surface of the OAR; the lower doped portion also extending laterally along and adjacent to the first slab of the OAR, away from the ridge in a first direction; and wherein the upper doped region is located within at least a portion of an upper surface of the ridge of the OAR, and extends laterally outwards along the second slab of the OAR in a second direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0082] FIG. 1 shows a schematic circuit diagram of a wavelength conversion chip including a detector remodulator according to the present invention;

[0083] FIG. 2 shows a schematic top view of a silicon on insulator detector remodulator comprising an EAM modulator;

[0084] FIGS. 3A, 3B, 3C, and 3D show cross sectional views of the detector remodulator taken along the line A-B of FIG. 2 where: (a) the electrical circuit includes a metal strip; (b) and (c) the electrical circuit includes a monolithic doped conductor; and (d) the electrical circuit includes a surface mounted chip;

[0085] FIG. 4 shows a schematic top view of an alternative modulator in the form of a Mach-Zehnder modulator;

[0086] FIG. 5 shows a side view of the modulator of FIG. 4 taken along the line X-Y of FIG. 4;

[0087] FIG. 6 shows a schematic top view of an alternative modulator in the form of a Fabry-Perot resonator modulator;

[0088] FIG. 7 shows an example drawing of a transmittance spectrum for the Fabry-Perot resonator modulator;

[0089] FIG. 8A shows a peak in the transmittance spectrum of the Fabry-Perot resonator tuned to the laser emission wavelength (on state) and FIG. 8B shows a peak in the transmittance spectrum of the Fabry-Perot resonator de-tuned from the laser emission wavelength (off state);

[0090] FIG. 9 shows a schematic top view of an alternative modulator in the form of a ring resonator modulator;

[0091] FIG. 10 shows a side view of the ring resonator modulator of FIG. 9 taken along the line M-N of FIG. 9;

[0092] FIG. 11 shows an example of a transmittance spectrum for the ring resonator modulator;

[0093] FIG. 12 shows a schematic top view of a further alternative modulator in the form of an alternative ring resonator modulator;

[0094] FIG. 13A shows an example of a transmittance spectrum for the ring resonator modulator of FIG. 12 tuned to the laser emission wavelength (on state) and FIG. 13B shows an example of a transmittance spectrum for the ring resonator modulator of FIG. 12 de-tuned from the laser emission wavelength (off state); and

[0095] FIGS. 14A and 14B show the positioning of an ASIC chip on a DRM or multiple DRMs of the present invention.

DETAILED DESCRIPTION

[0096] FIG. 1 shows a conversion chip 10 including a detector remodulator (DRM) 1 according to the present invention. The detector remodulator 1 comprises a silicon on insulator (SOI) waveguide platform which includes: a detector 2, a modulator 3 and an electrical circuit 4 which electrically connects the detector to the modulator. The detector 2 is coupled to an input waveguide 5 and the modulator 3 is coupled to an output waveguide 6.

[0097] The detector 2, modulator 3, input waveguide 5 and output waveguide 6 are arranged within the same horizontal plane as one another within the SOI waveguide platform. In the embodiment shown, a portion of the electrical circuit is located directly between the detector and the modulator.

[0098] The conversion chip includes a waveguide for a (modulated) first optical signal 7 of a first wavelength .sub.1. In the embodiment shown in FIG. 1, the waveguide is coupled to the input waveguide 5 of the detector 2 via a first and second optical amplifier 71, 72, although in an alternative embodiment (not shown) the first optical signal may be directly coupled to the input waveguide 5 of the detector. The detector converts the modulated input signal into an electrical signal which is then applied to the modulator via the electrical circuit 4.

[0099] The conversion chip also includes a waveguide for an unmodulated optical input 8 corresponding to a second wavelength .sub.2. This waveguide is coupled to an input waveguide 9 of the modulator 3 via an optical amplifier 81 (although may alternatively be directly coupled to input waveguide 9). The input waveguide 9 of the modulator also forms a part of the DRM and is oriented along the horizontal plane which includes the detector and modulator as well as the detector input waveguide and modulator output waveguide.

[0100] The electrical signal from the electrical circuit 4 will modulate the (unmodulated) optical input 8 thereby generating a modulated optical signal of the second wavelength .sub.2 which is outputted by the modulator via the modulator output waveguide 6. This modulated output of the second wavelength may then me amplified via an optical amplifier 61 coupled to the modulator output waveguide 6.

[0101] A power monitor may be present (not shown).

[0102] Examples of detectors, electrical circuit components and modulators that can form part of embodiments of the DRM 1 shown in FIG. 1 are described below in relation to FIGS. 2 to 12 where like reference numbers are used to refer to features described above in relation to FIG. 1.

[0103] FIG. 2 shows a top view first embodiment of a DRM 21 in which the modulator 23 is an electro-absorption modulator (EAM). The DRM 21 of FIG. 2 includes a detector 22, modulator 23 and electrical circuit, a portion of which 24 is located between the detector and the modulator.

[0104] The detector 22 is made up of a bulk semiconductor material, in this case germanium, and includes waveguide portion 25 across which the semiconductor junction of the detector is set horizontally. The horizontal semiconductor junction of the detector 22 is made up of three regions: a first doped region 26a, a second doped region 26b and a third region 26c between the first and the second doped regions. This third region may be an intrinsic region or may also be doped.

[0105] In the variation of this embodiment shown in FIG. 2 (and labelled as option a)), the first region is an n-type region; the second region is a p-type region; and the third region is an intrinsic region, such that the semiconductor junction of the detector 22 is a p-i-n junction.

[0106] In other variations, the first, second and third regions may instead form a p-i-p; n-i-n or n-p-n junction (as shown as options b)-d) in FIG. 2). In each of these three variations, the detector functions as a phototransistor.

[0107] In the embodiment shown in FIG. 2, the first doped region (in this case a p-type region) 26a is located at one side of the waveguide 25 of the detector and extends into the waveguide past the waveguide walls. The second doped region (in this case an n-type region) 26b is located at the opposite side of the waveguide to the first region and also extends into the waveguide 25 of the detector. The third region 26c corresponding to the intrinsic part of the p-i-n junction therefore has a width along the horizontal plane which is less than the width w of the waveguide of the detector.

[0108] A first electrode for applying a bias to the first doped region is located above the first doped region, a second electrode for applying a bias to the second doped region is located above the second doped region, and a third electrode for applying a bias to the third region is located above the third region. In all three cases, the electrodes are located directly on top of the relevant doped region.

[0109] The electro-absorption modulator 23 of the DRM also has a modulation waveguide region in the form of an amplitude modulation region at which a semiconductor junction is set horizontally across the waveguide. The modulator 23 is made up of a bulk semiconductor material, in this case doped silicon germanium (SiGe), and includes waveguide portion 28 across which the semiconductor junction of the detector is set in horizontally. The horizontal semiconductor junction of the modulator 23 is made up of three regions: a first doped region 27a, a second doped region 27b and a third region 27c between the first and the second doped regions.

[0110] In the embodiment shown, the first doped region (in this case a p-type region) 27a is located at one side of the waveguide 28 of the modulator and extends into the waveguide past the waveguide walls. The second doped region (in this case an n-type region) 27b is located at the opposite side of the waveguide to the first region and also extends into the waveguide 28 of the detector. The third region 27c corresponding to the intrinsic part of the p-i-n junction therefore has a width along the horizontal plane which is less than the width of the waveguide of the modulator.

[0111] In an alternative embodiment (not shown) the doped region may include a plurality of doped regions (e.g. a total of 5 regions including p+, p, intrinsic, n and n+, or even a total of 7 regions including p++, p+, p, intrinsic, n, n+ and n++).

[0112] A semiconductor optical amplifier (SOA) is located within the waveguide platform before the input waveguide which couples light into the detector.

[0113] The modulator 23 includes a first waveguide transition region 244 between the modulator input waveguide 9 and the modulation waveguide region at which the semiconductor junction is set horizontally across the waveguide. The modulator also includes a second transition region 245 between the modulation waveguide region and the modulator output waveguide 6.

[0114] At the first transition region 244, the waveguide height and/or width are reduced from larger dimensions to smaller dimensions, and at the second transition region 245, the waveguide height and/or width are increased from smaller dimensions to larger dimensions. In this way, the waveguide dimensions within the modulator are smaller than those of the input and output waveguides. This helps to improve the operation speed of the modulator (although it does so at the expense of higher losses).

[0115] The detector 22 includes a transition region 243 between the input waveguide 5 of the detector and the actual waveguide of the detector at which the height and/or width of the waveguide are reduced from larger dimensions to smaller dimensions. In this way, the waveguide dimensions within the detector are smaller than the input waveguide which helps to improve the operation speed of the detector.

[0116] A portion of the electrical circuit 24 is located between the second doped region of the detector and the first doped region of the modulator forming an electrical connection between the detector and the modulator. Cross sectional views of different configurations for this connecting portion taken through line A-B of FIG. 2 are shown in FIGS. 3A, 3B, 3C, and 3D. In the configuration shown in FIG. 3A the connecting portion of the electrical circuit is stripline circuit 221 in the form of a metal strip, the metal strip extending from the electrode on top of the second doped region of the detector to the electrode on top of the first doped region of the modulator. The second doped region of the detector and the first doped region of the modulator are separated by a given distance d, and the in-plane space between the detector and modulator doped regions can be kept as silicon or Ge or SiGe or can be filled with insulating dielectric material 225 such as SiO.sub.2. The metal strip forms a connection above this insulating filler.

[0117] In the variations shown in FIGS. 3B and 3C, the electrical circuit is a monolithic doped conductor 222, 223. This conductive layer may extend the entire depth of the platform thickness down to the box level (i.e. t-h) as shown in FIG. 3B or may extend for only part of the platform thickness as shown in FIG. 3C, in which case an insulating layer 226 is located underneath the monolithic layer. In another variation shown in FIG. 3D, the connecting portion of the electrical circuit 224 is a surface mounted chip such as an Application-Specific Integrated Circuit (ASIC) in which case, conductive pads are located on the platform such that they can be connected to the pads or pins of the chip. FIGS. 14A and 14B give an alternate view of this embodiment. In FIG. 14A the ASIC chip 224 is shown mounted above the optical waveguides. Electrical pads 30 and 39 on the ASIC chip are connected, respectively, to electrical pads 20 and 29 on the optical chip. FIG. 14A is a cross-section view in a region where the waveguides are of silicon. Such waveguides are contiguous with the active photodetector and modulator waveguides illustrated, for example, in FIG. 3D. In FIG. 14B is another view of the same embodiment with the ASIC chip mounted above the active regions of the waveguides of two or more DRMs as well as some parts of the passive (silicon) regions. It will be apparent that one ASIC chip can be mounted above a plurality of DRMs and be configured to make electrical connection between the modulator and photodetector in a plurality of DRMS.

[0118] As can be seen from the cross sections in FIGS. 3A, 3B, 3C, and 3D, the doped regions extend into the detector waveguide and modulator waveguide and do so throughout the entire ridge height h of the waveguides.

[0119] An alternative modulator is described below in relation to FIGS. 4 and 5. This modulator can replace the EAM in the embodiment shown in FIG. 2 to form an alternative DRM according to the present invention, where the remaining features and options of the DRM (other than the EAM) described in relation to FIG. 2 still apply. In this alternative DRM embodiment, the modulator is a Mach-Zehnder modulator 33.

[0120] The Mach-Zehnder modulator is made up of two waveguide branches forming a first interferometric arm 31 and a second interferometric arm 32; each arm including one or more phase shift modulation regions. In fact, in the embodiment shown, each arm contains a plurality of phase shift modulation regions 311, 312, 321, 322 (two of which are shown in each arm) as well as an additional phase shift region 313, 323.

[0121] Each modulation region is a phase modulation region made up of a bulk semiconductor material which has been doped to form a horizontal semiconductor junction in the form of a p-n junction (although an alternative semiconductor junction in the form of a horizontal p-i-n junction would be viable). The p-n junction is made up of a p-type region 331, 341 and an n-type region 332, 342. The p-type regions are each graded into three layers of varying different doping strengths: p, p+ and p++ and the n-doped regions are also graded into three layers of varying doping strengths n, n+ and n++arranged so that the p and n layers overlap the arm waveguide and so that the p++ and n++ layers are furthest away from the waveguide. Electrodes are located directly above the outward-most doped regions. In particular, the electrodes are located directly above the p++ and n++ layers of the doped regions. Suitable bulk semiconductor material for the modulation region includes SiGe or homogeneous silicon.

[0122] The graded p-n junction structure extends the size of the horizontal junction and enables electrodes which apply a bias to the doped regions to be placed advantageously away from the ridge. Each extra pair of layers results in further spaced electrodes as the electrodes are preferably located directly over the most heavily doped regions. This increase in separation of the electrodes gives rise to an increased flexibility of the device design without compromising speed.

[0123] Doping of a bulk semiconductor material to form an electro-optical region is known in the art, both in the case of modulators and also detectors. In all of the devices described herein, the doping concentrations used would correspond to typical values found in the state of the art. For example, the doped regions of the detector may include regions with concentrations of up to 1010.sup.19 cm.sup.3. Doped regions of the modulator may take typical values of 1010.sup.15 cm.sup.3 to 1010.sup.17 cm.sup.3 for p doped regions and 1010.sup.15 cm.sup.3 to 1010.sup.18 cm.sup.3 for n doped regions. However, doped regions (p and/or n) may have higher values of as much as 1010.sup.20 cm.sup.3 or 1010.sup.21 cm.sup.3.

[0124] The additional phase shift region has a lower speed than the modulation regions so may be made of an alternative material such as homogeneous silicon. In the embodiment shown, the additional phase shift region comprises a horizontal semiconductor junction in the form of a p-i-n junction, the p and n doped regions of which do not extend into the waveguide of the first or second waveguide arm. In fact, the intrinsic regions 335, 345 extend beyond the boundary. Electrodes 339a, 349a which apply a bias to the p-doped regions are located directly above the respective p-doped regions 333, 343 and electrodes 339b, 349b which provide a bias to the n-doped regions are located directly above the n-doped regions 334, 344.

[0125] The electrodes above both the modulation regions and phase shift regions are strips which lie along the length of the doped region (along a direction parallel to the longitudinal axis of the waveguide). It is desirable for the electrodes to have as much contact with the respective doped regions as possible whilst also retaining the small sizes that are advantageous to speed of modulation.

[0126] An input 12 coupler couples unmodulated light from the input waveguide 9 into the two arms of the modulator and an output 21 coupler couples the light from the two arms into the output waveguide 6 to form a modulated output signal having the same wavelength as the unmodulated input signal. High-speed Mach-Zehnder modulators are known to the person skilled in the art and may take the form of the Mach-Zehnder modulators described by Dong et al., Optics Express p. 6163-6169 (2012) or D. J. Thompson et al, Optics Express pp. 11507-11516 (2011). The phase difference between modulated light exiting the first arm and modulated light exiting the second arm will affect the interference pattern generated (in time) when light from the two arms combine, therefore altering the amplitude of the light in the output.

[0127] Each arm includes a waveguide transition region 314, 324 between the input 12 coupler and the phase shift region and another waveguide transition region 315, 325 between the modulation regions and the output 21 coupler. In this way, the waveguide dimensions within the resonator modulator can be smaller than those of the input and output waveguides. This helps to improve the operation speed of the modulator (although it does so at the expense of higher losses).

[0128] A central electrical circuit 35 (which is an extension of the DRM electrical circuit) is located between the modulation regions of one arm and the modulation regions of the second arm. This circuit is required where the respective modulation regions of the two arms of the MZM are driven in series in a single drive condition or in a dual drive condition. The nature of this central electrical circuit 35 will control both whether the MZM is single drive or dual drive, but also whether the two arms are driven in series or in parallel.

[0129] The electrical circuit connection 34 between the M-Z modulator and the detector (detector not shown) and the central circuit connection 35 between modulation regions in the two arms can each take the form of any one of the electrical circuit connections described above in relation to FIGS. 3A to 3D but is depicted in FIG. 5 as a stripline circuit in the form of a single metal strip with insulating filler material located underneath the strip. In addition to this electrical connection, the Mach-Zehnder modulator includes a further electrical connection 35 located between a phase modulating region of the first arm 310 and a corresponding phase modulating region in the second arm 320 to connect an electrode 319e over an n++ doped region of the phase modulating region 312 of the first arm 310 with an electrode 329d over a p++ doped region of the corresponding phase modulating region 322 of the second arm. A further alternative modulator is described below with reference to in FIGS. 6, 7 and 8. This modulator can replace the EAM in the embodiment shown in FIG. 2 to form a further alternative DRM according to the present invention, where the remaining features and options of the DRM (other than the EAM) described in relation to FIG. 2 would still apply. In this alternative DRM embodiment, the modulator is a Fabry-Perot (F-P) resonator modulator 43.

[0130] The F-P resonator modulator 43 is formed in a single waveguide section by two reflectors in series with one or more modulation regions 411, 412, 413 located between the two reflectors. In the embodiment shown in FIG. 6, the reflectors take the form of Distributed Bragg Reflectors (DBRs) DBR1, DBR2.

[0131] The Fabry-Perot resonator cavity shown in FIG. 6 actually includes a plurality of modulation regions 411, 412, 413 (3 of which are shown). These are formed in a bulk semiconductor medium and comprise a p-n junction the same as those of the modulation regions described above in relation to FIG. 4.

[0132] Each modulation region 411, 412, 413 is made up of a bulk semiconductor material which has been doped to form a horizontal semiconductor junction in the form of a p-n junction (although an alternative semiconductor junction in the form of a horizontal p-i-n junction would also be viable). Each p-n junction is made up of a p-type region 431 and an n-type region 432. The p-doped regions are each graded into three layers of varying different doping strengths: p, p+ and p++; and the n-doped regions are also graded into three layers of different doping strengths n, n+ and n++. These layers are arranged so that the p and n layers overlap the waveguide, followed by the p+ and n+layers and the p++ and n++ layers so that the p++ and n++ layers are furthest away from the waveguide. Electrodes are located directly above the outward-most doped regions. In particular, the electrodes are located directly above the p++ and n++ layers of the doped regions. Suitable material for the modulation region includes SiGe or homogeneous silicon.

[0133] The Fabry-Perot resonator cavity also includes an additional phase shift region 414 with a lower speed of operation than the modulation regions. As with the phase shift regions described above in relation to the Mach-Zehnder modulator, the function of this phase shift region 414 is to provide low speed cavity FSR fine tuning and therefore operating wavelength fine-tuning and thermal drift compensation. The phase shift region is shown in FIG. 6 as a p-i-n semiconductor junction operating in a carrier injection mode (but could alternatively comprise a p-n phase shift region operating in a carrier depletion mode). As with the p-i-n phase shift regions described above, the p and n doped regions do not extend into the waveguide of the first or second waveguide arm. In fact, the intrinsic regions extend beyond the boundary. Electrodes 439a which apply a bias to the p-doped regions are located directly above the respective p-doped regions 433 and electrodes 439b which provide a bias to the n-doped regions are located directly above the n-doped regions 434.

[0134] The electrodes above both the modulation regions and phase shift regions are strips located over the doped regions and lie along the length of the doped region (along a direction parallel to the longitudinal axis of the waveguide). The electrodes lie along the entire length of the doped regions (length parallel to the longitudinal axis of the waveguide) because is desirable for the electrodes to have as much contact with the respective doped regions as possible whilst also retaining the small sizes (small thicknesses) that are advantageous to speed of modulation.

[0135] An electrical circuit connection 44 between the F-P modulator and the detector (detector not shown) can take the form of any one of the electrical circuit connections described above in relation to FIGS. 3A to 3D.

[0136] The F-P resonator is a resonant F-P filter (also infinite-impulse-response, or IIR filters) which increases the modulation tuning efficiency at the expense of tuning speed, increased temperature sensitivity, and increase manufacturing complexity due to the need for inclusion of the DBR gratings. In an IIR filter, the effect of the index change induced by the phase shifter is enhanced by the number of round-trips in the resonator cavity, thus a smaller injected current density (in the carrier injection case) or bias voltage (in the carrier depletion case) is needed to perform modulation with the same extinction ratio. Thus less optical or electrical amplification would be needed to perform the modulation as compared to the EAM and M-Z embodiments previously described. However manufacturing complexity and tolerances are increased because to reach high modulation speeds of 25 or 40 Gb/s, the photon lifetime of the cavity must be kept small (in addition to the requirement to make a high-speed phase modulator) meaning the cavity length must be short and the Finesse sufficiently low. Therefore the fabrication and design complexity is high due to the need to incorporate DBR gratings with potentially short lengths and deep etch depths.

[0137] The F-P modulator includes a waveguide transition region 444 between the input waveguide 9 and the first DBR and another waveguide transition region 445, between the second DBR and the output waveguide. At the first transition region 444, the waveguide height and width are reduced, and at the second transition region, the waveguide height and width are increased. In this way, the waveguide dimensions within the cavity are smaller than those of the input and output waveguides. This can be used to help to improve the operation speed of the modulator (although it does so at the expense of higher losses).

[0138] Modulation of the resonator is described below in relation to FIGS. 7 and 8. Referring to the reflectance spectra of FIG. 7, it is clear that DBR gratings DBR1 and DBR2 are broad-band reflectors which have equal reflectance over the operating bandwidth of the tunable laser. The reflectance values R1 and R2 are chosen to give a Finesse value that is large enough to create enough cavity round trips to enhance the effect of n (a sufficient X factor of the resonator) to sufficiently reduce the amount of drive current or voltage needed to perform the modulation with the desired extinction ration, but small enough to give a cavity lifetime that is still <1/(bit period). The transmittance of the resonator preferably has a maximum value of between 0.8 and 1 and may be 0.8 as shown in FIG. 7.

[0139] Referring to the transmittance spectra 92, 93 shown in FIG. 8, a resonant peak of the F-P cavity must be tuned to the wavelength of the (non-modulated) laser (P.sub.laser()) in the on-state (FIG. 8A). However, in the off-state (FIG. 8B), the phase of the cavity is altered to detune the resonance peak away from the wavelength of the laser thereby producing a sufficient modulation extinction ratio. When a bias is applied to the electrodes of the p-n junctions of the modulation regions, and the bias is modulated between the on and off states, the transmittance spectrum is therefore switched between on and off positions resulting in the output being modulated from on to off or vice versa. By actively adjusting the bias to the phase shift regions, the alignment of the resonant peak of the F-P cavity to the wavelength of the laser can be maintained in the presence of a thermal drift.

[0140] Further alternative modulators are described below with reference to in FIGS. 9 to 13B. Each of these modulators can replace the EAM in the embodiment shown in FIG. 2 to form a further alternative DRM according to the present invention where the remaining features and options of the DRM (other than the EAM) described in relation to FIG. 2 would still apply. In each of these alternative embodiments, the modulator is a ring resonator modulator 53, 153.

[0141] Taking the first of two ring resonator DRM embodiments and referring in particular to FIGS. 9 to 11, the ring resonator modulator 53 is formed from a ring waveguide section, a first straight waveguide 59 coupled at one side of the ring waveguide and a second straight waveguide 60 coupled to the other side of the ring waveguide. The ring waveguide is defined between an inner waveguide ridge edge 56 and an outer waveguide ridge edge 57. The cross section across dashed line M-N in FIG. 9 is shown in FIG. 10. The ring resonator modulator also comprises a of modulation region 512 formed in a bulk semiconductor medium doped to give a circular p-n junction which is set horizontally across the waveguide (An alternative semiconductor junction in the form of a horizontal p-i-n junction would also work).

[0142] Throughout this document, ring waveguides may take the form of any ring shape including: a circle (as shown in FIGS. 9 and 12), a race track; or an elliptical shape. Furthermore, the circular doped regions may take the form of a circle with constant radius; a race-track shape; or an elliptical shape.

[0143] In the embodiment shown in FIG. 9, the circular p-n junction becomes discontinuous along a portion of its circumference where a continuous circular doped region would otherwise overlap with the input and output straight waveguides. Suitable bulk semiconductor materials for the modulation region include SiGe and homogeneous silicon.

[0144] The p-n junction is made up of a p-type region 551 and an n-type region 552. The p-doped regions are each graded into three concentric layers of varying different doping strengths: p, p+ and p++ and the n-doped regions are also graded into three concentric layers of varying doping strengths n, n+ and n++ arranged so that the p and n layers overlap the ring waveguide and extend radially outwards and inwards respectively within the horizontal plane of the junction beyond the outer and inner waveguide ridge edges. The p++ and n++ doped layers lie furthest away from the ring waveguide. Because of the discontinuous nature of the outer doped portions, the p+, p++, n+ and n++ layers are each made up of two opposing crescent shaped regions rather than complete circular shape as they do not extend the full way around the ring waveguide. This gives clearance for the straight waveguides 59, 60 which couple light in and out of the ring waveguide thereby ensuring that the p-n junction does not modify the refractive index in the light-coupling regions, and therefore does not modify the coupling ratio between the ring and the straight waveguides.

[0145] A ring gap separation 55 exists on either side of the ring waveguide between the ring waveguide and each of the straight waveguides 59, 60. The magnitude of this gap determines the value of the coupling coefficient of the resonator.

[0146] Electrodes are located directly above the outer-most and inner-most respective doped regions. In particular, the electrodes are located directly above the p++ and n++ layers of the doped regions. A central circular electrode 439b is located above the n++ doped region to apply a bias to the n-doped region. A bias is applied to the p-doped region via a looped electrode 439a which extends above and along the crescent shaped p++ regions forming two crescent shaped electrode portions which are then joined together by further electrode portions crossing over one of the straight waveguides to form a closed single electrode.

[0147] An electrical circuit connection 54 between the ring resonator modulator and the detector (detector not shown) can take the form of any one of the electrical circuit connections described above in relation to FIGS. 3A to 3D.

[0148] The ring resonator modulator 53 includes a first waveguide transition region 544 between the modulator input waveguide 9 and the first straight waveguide 59 which couples light into the ring resonator and a second transition region 545 between the second straight waveguide which couples light out of the waveguide and the modulator output waveguide 6.

[0149] At the first transition region 544, the waveguide height and/or width are reduced, and at the second transition region, the waveguide height and/or width are increased. In this way, the waveguide dimensions within the ring resonator modulator are smaller than those of the input and output waveguides. This helps to improve the operation speed of the modulator (although it does so at the expense of higher losses).

[0150] The transmittance spectrum of the ring resonator is shown in FIG. 11 as a periodic set of peaks, each peak separated from the adjacent two peaks via a wavelength difference equal to the free spectral range (FSR) of the ring resonator. The free spectral range of the transmittance signal being set by the size of the ring waveguide. The transmittance of the resonator preferably has a maximum value of between 0.8 and 1 and may be 0.8.

[0151] Modulation of the light occurs via the same process as the F-P modulator, the ring resonance must be tuned to the wavelength of the (non-modulated) laser (P.sub.laser()) in the on-state (FIG. 8A). However, in the off-state (FIG. 8B), the phase of the cavity is altered to detune the resonance peak away from the wavelength of the laser thereby producing a sufficient modulation extinction ratio. When a bias is applied to the electrodes of the p-n junctions of the ring, and the bias is modulated between the on and off states, the transmittance spectrum is therefore switched between on and off positions resulting in the output being modulated from on to off or vice versa.

[0152] The ring resonator modulator 53 also includes a fine tuning region in the form of a heater (not shown) for thermal tuning.

[0153] By actively adjusting the voltage across the phase tuning heater pads 58a and 58b, the alignment of the resonant peak of the F-P cavity to the wavelength of the laser can be maintained in the presence of ambient thermal drift.

[0154] Referring to FIGS. 10, 12, 13A and 13B, the ring resonator modulator 153 according to the second of the two ring resonator DRM embodiments is described. The difference between the ring resonator modulator of FIG. 12 and that of FIG. 9 is the fact that the ring waveguide of the resonator modulator of FIG. 12 is coupled to no more than one straight waveguide. A single straight waveguide 159 only is coupled to the ring waveguide at one side. In this embodiment, the single straight waveguide is therefore configured to couple light both into and out of the ring waveguide.

[0155] As with the previous ring resonator embodiment, the ring waveguide is defined between an inner waveguide ridge 56 and an outer waveguide ridge 57. The cross section across dashed line M-N for this embodiment is also shown by FIG. 10 and the parts of the description above relating to FIG. 10 therefore apply here. In particular, the ring resonator embodiment of FIG. 12 also includes a modulation region 512 formed in a bulk semiconductor medium doped to give a circular p-n junction which is set along a horizontally across the waveguide.

[0156] The p-n junction is made up of a p-type region 551 and an n-type region 552. The p-doped regions are each graded into three concentric layers of varying different doping strengths: p, p+ and p++ and the n-doped regions are also graded into three concentric layers of varying doping strengths n, n+ and n++ arranged so that the p and n layers overlap the ring waveguide and extend radially outwards and inwards respectively beyond the waveguide ridge edges 56, 57 within the horizontal plane of the semiconductor junction.

[0157] The p, n, n+ and n++ regions are ring shaped. However the p+ and p++ regions on the outside of the p-type region are C-shaped; defining a discontinuity where the ring waveguide comes into close contact with the straight waveguide (i.e. where the outer-most doped regions would otherwise overlap the straight waveguide). The clearance between the doped regions and the straight waveguide ensures that the p-n junction does not modify the refractive index in the light-coupling regions, and therefore does not modify the coupling ratio between the ring and the straight waveguide.

[0158] A ring gap separation 155 exists between the ring waveguide and the single straight waveguide 159, the magnitude of which determines the value of the coupling coefficient of the resonator.

[0159] Electrodes are located directly above the respective outer-most and inner-most doped regions that they apply a bias to. In particular, the electrodes are located directly above the p++ and n++ layers of the doped regions. A central circular electrode 439b is located above the n++ doped region to apply a bias to the n-doped region. A bias is applied to the p-doped region via a looped electrode 439a which extends along the C-shaped (i.e. the full length of the discontinuous circumference of the p++ region).

[0160] An electrical circuit connection 54 between the ring resonator modulator and the detector (detector not shown) can take the form of any one of the electrical circuit connections described above in relation to FIGS. 3A to 3D.

[0161] The ring resonator modulator 153 includes a first waveguide transition region 544 between the modulator input waveguide 9 and the single straight waveguide 59 which couples light into the ring resonator and a second transition region 546 between the single straight waveguide 59 and the modulator output waveguide 6.

[0162] At the first transition region 544, the waveguide height and width are reduced, and at the second transition region, 546 the waveguide height and width are increased. In this way, the waveguide dimensions within the ring resonator modulator are smaller than those of the input and output waveguides.

[0163] The transmittance spectrum of the ring resonator is shown in FIGS. 13A and 13B and takes the form of a periodic set of sharp troughs, each trough separated from the two directly adjacent troughs via a wavelength difference equal to the free spectral range (FSR) of the ring resonator. As this transmittance spectrum is the inverse of that for the dual straight waveguide embodiment, the ring resonator modulator of FIGS. 12, 13A and 13B will require an opposite drive signal (bias applied across the p-n junction) as compared to the single coupled waveguide version in order to give rise to the same modulation effect.

[0164] The transmittance of the resonator in the troughs preferably has a maximum value of between 0.8 and 1, and may be 0.8. As with the previous ring resonator embodiment, modulation is achieved when a bias is applied across the p-n junctions from the electrical circuit connector via the electrodes. This tunes the transmittance spectrum on and off resonance with the wavelength of the (unmodulated) laser which in turn results in the transmitted output signal being turned on 94 and off 95. However, because the transmittance is a trough on resonance, the magnitude of bias change is larger to get the same extinction ratio for the dual straight waveguide embodiment.

[0165] An advantage of this embodiment is that there is only one straight waveguide and one discontinuous portion in the p-n junctions around the circumference, meaning the electrode for the p-doped region does not have to cross over a straight waveguide. When metal electrodes cross a waveguide additional optical loss is introduced.

[0166] The ring resonator modulator 153 also includes a fine tuning region in the form of a heater (not shown) for thermal tuning. By actively adjusting the voltage across the phase tuning heater pads 58a and 58b, the alignment of the resonant peak of the F-P cavity to the wavelength of the laser can be maintained in the presence of ambient thermal drift.

[0167] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. All references referred to above are hereby incorporated by reference.