OPTOELECTRONIC DEVICE

Abstract

An optoelectronic device. The device comprising: an input waveguide, which receives an optical signal; a multistage photodiode detector, comprising a plurality of photodiode elements connected in series, one of the photodiode elements of the plurality of photodiode elements being connected to the input waveguide and configured to receive the optical signal therefrom; and a first electrode and a second electrode, wherein the first electrode is connected to a first contact of each of the plurality of photodiode elements, and the second electrode is connected to a second contact of each of the plurality of photodiode elements.

Claims

1. An optoelectronic device comprising: an input waveguide, which is configured to receive an optical signal; a multistage photodiode detector, comprising a plurality of photodiode elements connected in series, one of the photodiode elements of the plurality of photodiode elements being connected to the input waveguide and configured to receive the optical signal therefrom; and a first electrode and a second electrode, wherein the first electrode is connected to a first contact of each of the plurality of photodiode elements, and the second electrode is connected to a second contact of each of the plurality of photodiode elements.

2. The optoelectronic device of claim 1, wherein each photodiode element comprises a p-i-n junction extending across a waveguide.

3. The optoelectronic device of claim 1, wherein each photodiode element is located within a cavity of a silicon-on-insulator wafer.

4. The optoelectronic device of claim 1, further comprising a plurality of passive waveguides, each of which connects a pair of the plurality of photodiode elements.

5. The optoelectronic device of claim 1, wherein a final photodiode element of the plurality of photodiode elements is connected at a terminating end to a high reflectivity mirror.

6. The optoelectronic device of claim 1, wherein a passive waveguide, located between a pair of the plurality of photodiode elements, has a U-shape.

7. The optoelectronic device of claim 6, wherein the first electrode and second electrode each have a U-shape.

8. The optoelectronic device of claim 6, wherein pairs of the plurality of photodiode elements which are spatially adjacent are located within a same cavity of a silicon-on-insulator wafer.

9. The optoelectronic device of claim 1, wherein the input waveguide is connected to a splitter, a first output of the splitter being connected to a first photodiode element and a second output of the splitter being connected to a second photodiode element.

10. The optoelectronic device of claim 9, wherein the first photodiode element is part of a first sub-group of the plurality of photodiode elements and the second photodiode element is part of a second sub-group of the plurality of photodiode elements, and wherein a final photodiode element in the first sub-group is connected to a final photodiode element in the second sub-group via a U-shaped passive waveguide.

11. The optoelectronic device of claim 9, wherein the first photodiode element is part of a first sub-group of the plurality of photodiode elements and the second photodiode element is part of a second sub-group of the plurality of photodiode elements, and wherein a final photodiode element in the first sub-group connected at a terminating end to a first high reflectivity mirror and a final photodiode element in the second sub-group is connected at a terminating end to a second high reflectivity mirror.

12. The optoelectronic device of claim 1, wherein the input waveguide is formed from silicon, and the photodiode elements are formed from silicon germanium.

13. The optoelectronic device of claim 1, wherein the optoelectronic device is a power monitor.

14. An array comprising a plurality of the power monitors according to claim 13, each power monitor being connected to a respective laser or electro-absorption modulator.

15. A method of monitoring a power output of an electro-absorption modulator, using the power monitor of claim 13, comprising the steps of: providing a signal from a laser or an electro-absorption modulator to the input waveguide; and detecting, using the multistage photodiode detector, a power level of the signal.

16. A photonic integrated circuit, for use as a transmission device, comprising the array of claim 14.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0033] FIG. 1 is a top view of an optoelectronic device;

[0034] FIG. 2 is a top view of a variant optoelectronic device;

[0035] FIG. 3 is a top view of an end photodiode element;

[0036] FIG. 4 is a top view of a variant optoelectronic device; and

[0037] FIG. 5 is a top view of a variant optoelectronic device.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES

[0038] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference

[0039] FIG. 1 is a top view of an optoelectronic device 100. The device comprises an input waveguide 102, which receives an optical signal 104. This optical signal can be, for example, from a tap on a laser output or a modulator, so as to monitor the power output of the laser or modulator.

[0040] The device 100 also includes a plurality of photodiode elements 101a, 101b, etc. to 101n, which combined form a multistage photodiode detector. The first photodiode element 101a is connected to the input waveguide at a first end, and to a passive waveguide 106a at a second end. The passive waveguide connects photodiode element 101a to the adjacent photodiode element 101b. Similarly, passive waveguide 106b connects photodiode element 101b to the next photodiode element, and so on.

[0041] A first electrode connects to a first contact of each of the photodiode elements. For example, a first contact 120a of the electrode connects to a portion of the first photodiode element 101a. Similarly, a second contact 120b of the electrode connects to a same portion of the second photodiode element 101b. A second electrode connects to a first contact of each of the photodiode elements. For example, a first contact 112a of the electrode connects to a portion of the first photodiode element 101a. Similarly, a second contact 112b of the electrode connects to a same portion of the second photodiode element 101b.

[0042] Each photodiode element comprises a cavity 122a, etched into a silicon-on-insulator wafer. The input waveguide 102 and passive waveguides 106a-106n are silicon waveguides formed in the silicon-on-insulator (or silicon device layer) of the silicon-on-insulator wafer, and are themselves formed from silicon. The depth of each cavity 122a is such that an active waveguide 108a-108n of each photodiode element is aligned with the adjacent passive waveguides.

[0043] The active waveguides 108a-108n are formed from silicon germanium (SiGe), and comprise a ridge or rib region in the centre and a slab region on either lateral side of the ridge or rib (lateral being a direction typically perpendicular to the guiding direction of the waveguide). Each ridge or rib includes an n doped sidewall 116a-n and a p doped sidewall 114a-n. A centre portion of each ridge or rib is undoped or only unintentionally doped, and so the ridge or rib forms a P-I-N junction and thereby operates as a photodiode when connected to electrodes. In an alternative embodiment, not shown, the ridge or rib is entirely doped and so forms a P-N junction.

[0044] The p doped sidewall 114a-n is connected to a p+ doped region 110a-n of the respective slab. By p+, it is meant that the region of the slab is doped to a dopant concentration level higher than the respective p doped sidewall 114a-n. Similarly, then doped sidewall 116a-n is connected to an n+ doped region 118a-n of the respective slab. By n+, it is meant that the region of the slab is doped to a dopant concentration level higher than the n doped sidewall 116a-n. The electrode contacts 120a-n and 112a-n connect to the p+ and n+ doped regions of the respective slab. The increased dopant concentration in the p+ and n+ doped regions reduces series resistance in the connections to the electrodes.

[0045] In use, the optical signal 104 is passed from the input waveguide 102 to the first photodiode element 101a. The P-I-N or P-N junction in the photodiode element 101a then partially converts the optical signal to an electrical current 124, which is read from the photodiode element. The partially converted optical signal then passes along the passive waveguide 106a to the second photodiode element 101b, which also partially converts the optical signal, and so on.

[0046] A last photodiode element 101n terminates with the end of the active waveguide 106n, and so the number of photodiode elements should be chosen so as to ensure optimum power monitoring. n may take a value of between 2 and 5, typically fewer elements will be used if a mirror is provided at the end of the optoelectronic device (as discussed below).

[0047] As a result, despite the photodiode elements having not been individually optimised, an accurate power reading can be taken from the optoelectronic device to which the photodiode elements are connected. Moreover, this can be achieved given the restraints discussed above relating to the choice of material system, and also the maximum length ‘x’ of the cavities 122a.

[0048] FIG. 2 is a top view of a variant optoelectronic device 200. Where the device 200 shares features with the device 100, like features are indicated by like reference numerals. As before, the device comprises a plurality of photodiode elements 101a-101n. However, in contrast to the device 100, one of the passive waveguides 202 between a pair (101x and 101x+1) of photodiode elements has a U-shape. Therefore light passing through the multistage photodiode detector initially travels in a first direction (from the input waveguide 102 in the direction indicated by arrow 104) before passing through an approximately 180° turn, and so traveling in a second direction antiparallel to the first.

[0049] Moreover, each cavity 122 in device 200 is wider than the cavities in device 100. This is because a pair of photodiode elements (e.g. 101a and 101n or 101x and 101x+1) reside in each. In an alternative example, the cavities 122 in device 200 are the same width as the cavities in device 100 e.g. because the cavities in device 100 are oversized.

[0050] These differences allows the overall footprint of the multistage photodiode detector to be reduced. Moreover, each of the first 112 and second 120 electrodes can have a U-shape (one rotated 180° relative to the other). The U-shaped electrodes overlap in that a ‘leg’ (i.e. the elongate members extending parallel to the active waveguides 108a-108n) from each of the U-shaped electrodes are adjacent to one another, so as to further reduce the footprint of the multistage photodiode detector. Said another way, the footprint of the U-shaped electrodes overlaps in that the ‘leg’ of one U-shaped electrode sits within the bend of U of another.

[0051] FIG. 3 is a top view of a variant end photodiode element 101n. Where it shares features with the photodiode elements discussed previously, like features are indicated by like reference numerals. In contrast to the end photodiode elements shown in FIGS. 1 and 2, the photodiode element 101n in FIG. 3 terminates in high reflectivity mirror 302. This could be, for example, a facet (e.g. etched facet) with HR (high reflectivity) coating or a Sagnac reflector (also known as a Sagnac loop reflector, Sagnac mirror, or Sagnac loop mirror). This high reflectivity mirror causes the signal passing through the multistage photodiode detector to perform a double pass through the detector and so the overall number of elements can be reduced (as each is used twice).

[0052] FIG. 4 is a top view of a variant optoelectronic device 400. Where the device 400 shares features with the devices 100 and 200 shown previously, like features are indicated by like reference numerals. The device 400 differs from those shown previously in that input waveguide 102 does not connect directly to any photodiode element of the multistage photodiode detector. Instead, the input waveguide 102 connects to a splitter 402, for example a multi-mode interference splitter (MMI) which divides the signal and provides outputs at a first output 404 and a second output 406.

[0053] The first output 404 is connected to a first photodiode element 101a of a first sub-group of the plurality of photodiodes. The second output 406 is connected to a first photodiode element 101a′ of a second sup-group of the plurality of photodiodes. In this way, light is provided to both the first and second sub-groups of the plurality of photodiodes. Thereafter each sub-group can be considered its own multistage photodiode detector.

[0054] In the example shown in FIG. 4, the end photodiode elements 101n and 101n′ of each sub-group are connected via a U-shaped waveguide 202. Therefore any residual optical signal, which has not yet been converted to photocurrent in the preceding sub-group of photodiode elements, will then travel back towards the input waveguide and so pass through the other sub-group of photodiode elements and so be converted to photocurrent.

[0055] FIG. 5 is a top view of a variant optoelectronic device 500. Where the device 500 shares features with the devices 100, 200, and 400 shown previously, like features are indicated by like reference numerals. The device 500 in FIG. 5 differs from the device 400 in FIG. 4 in that the end photodiode elements 101n and 101n′ of each sub-group do not connect to each other via a U-shaped waveguide. Instead, each terminates in a high reflectivity mirror 302 and 302′ in the same manner as the end photodiode element shown in FIG. 3. Therefore after passing through all of the photodiode elements in a given sub-group, any residual optical signal is reflected back through the same sub-group towards the input waveguide.

[0056] The devices 100, 200, 400, and 500, can be disposed in an array, each connected to a respective tap on a laser output or electro-absorption modulator and so the power of different signals (e.g. different wavelengths, or on different channels) can be monitored.

[0057] 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.

LIST OF FEATURES

[0058] 100 Optoelectronic device [0059] 101n Photodiode element [0060] 102 Input waveguide [0061] 104 Optical signal [0062] 106n Passive waveguide [0063] 108n Active waveguide [0064] 110n p+ doped region [0065] 112n Contact [0066] 114n p doped region of active waveguide [0067] 116n n doped region of active waveguide [0068] 118n n+ doped region [0069] 120n Contact [0070] 122n Cavity [0071] 124 Photocurrent [0072] 202 U-shaped passive waveguide [0073] 302 High reflectivity mirror [0074] 400 Splitter [0075] 401 First splitter output [0076] 402 Second splitter output