GB2557694A

GB2557694A - Optical modulators

Info

Publication number: GB2557694A
Application number: GB1703716.9A
Authority: GB
Inventors: Singh Nagra Amit; Abediasl Hooman; Kai See Poon Joyce; Yong Zheng
Original assignee: Rockley Photonics Ltd
Current assignee: Rockley Photonics Ltd
Priority date: 2016-12-15
Filing date: 2017-03-08
Publication date: 2018-06-27
Anticipated expiration: 2037-03-08
Also published as: GB2557694B; GB201703716D0

Abstract

An optoelectronic device, which is operable to provide a PAM-N modulated output, the device comprises M optical modulators (104, 109), M being an integer greater than 1, the M optical modulators being arranged in a cascade, the device is configured to operate in N distinct transmittance states 115, as a PAM-N modulator, wherein, in each transmittance state (each level) of the N distinct transmittance states, each of the M optical modulators has applied to it a respective control voltage equal to one of a first voltage or a second voltage. In an embodiment, each of the M optical modulators has applied to it a respective control voltage equal to one of k different control voltages, wherein k is an integer greater than 1 and less than N. The device may include intermediate waveguides 108 connecting the modulators. The intermediate waveguides may be angled with respect to the guiding direction (Fig. 20).

Description

(54) Title of the Invention: Optical modulators

Abstract Title: An optoelectronic device to provide a PAM-N modulated output (57) An optoelectronic device, which is operable to provide a PAM-N modulated output, the device comprises M optical modulators (104, 109), M being an integer greater than 1, the M optical modulators being arranged in a cascade, the device is configured to operate in N distinct transmittance states 115, as a PAM-N modulator, wherein, in each transmittance state (each level) of the N distinct transmittance states, each of the M optical modulators has applied to it a respective control voltage equal to one of a first voltage or a second voltage. In an embodiment, each of the M optical modulators has applied to it a respective control voltage equal to one of k different control voltages, wherein k is an integer greater than 1 and less than N. The device may include intermediate waveguides 108 connecting the modulators. The intermediate waveguides may be angled with respect to the guiding direction (Fig. 20).

101

At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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Application No. GB1703716.9

RTM

Date :19 April 2017

Intellectual

Property

Office

The following terms are registered trade marks and should be read as such wherever they occur in this document:

IEEE (Page 5)

Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

OPTICAL MODULATORS

Cross-Reference to Related Application [0001] The present application claims priority to and the benefit of U.S.

Provisional Application No. 62/435,004, filed December 15, 2016, entitled ELECTRO-OPTIC MODULATORS, the entire content of which is incorporated herein by reference.

Field [0002] Embodiments of the present invention relate to an optoelectronic device and method of operating such a device, it particularly relates to devices within the field of silicon photonics and to an optoelectronic device comprising a first optical modulator and a second optical modulator.

Background [0003] Pulse-amplitude modulation (PAM) is an example modulation format where information comprising a message is encoded in the amplitude of a series of pulses comprising the signal. In PAM-4 modulation, 2² (4) discrete pulse amplitudes are available, which are generally equally spaced (i.e., equally spaced in optical power, for optical data transmission). For example, as shown in Figure 1, a PAM-4 modulation scheme enables pulses to be modulated into one of four discrete amplitudes: [0,0], [0,1], [1,0], or [1,1], Generally adjacent amplitudes are separated by 1/3 of the largest amplitude (the so called largest bit value).

Summary [0004] At their broadest, embodiments of the invention provide PAM modulation using optoelectronic devices including optical modulators, through provision of two or more optical modulators each operable in a respective transmittance state.

[0005] Therefore, in a first aspect, embodiments of the invention provide an optoelectronic device operable to provide a PAM-N modulated output, the device comprising:

M optical modulators, M being an integer greater than 1, the M optical modulators being arranged in a cascade,

- 1 _ the device being configured to operate in N distinct transmittance states, as a PAM-N modulator, wherein, in each transmittance state of the N distinct transmittance states, each of the M optical modulators has applied to it a respective control voltage equal to one of:

a first voltage or a second voltage.

[0006] In a second aspect, embodiments of the invention provide an optoelectronic device operable to provide a PAM-N modulated output, the device comprising:

M optical modulators, M being an integer greater than 1, the M optical modulators being arranged in a cascade, the device being configured to operate in N distinct transmittance states, as a PAM-N modulator, wherein, in each transmittance state of the N distinct transmittance states, each of the M optical modulators has applied to it a respective control voltage equal to one of k different control voltages, wherein k is an integer greater than 1 and less than 1.

[6067] A first optical modulator and a second optical modulator are operable to provide distinct transmittance states (also referred to as modulated outputs). Therefore, the device may allow at least three distinct modulated outputs corresponding to: modulation by the first modulator only, modulation by the second modulator only, modulation by the first and second modulators. This can reduce the requirement for complex drivers to implement PAM-N modulation. N may typically take any value greater than or equal to 4; generally N obeys the equation A^? = 2* where χ {Z} (i.e. is within the set of real integers). For example, N may take values of 2, 4, 8, 16, 32, 64, 128, etc.

[6068] There is disclosed a method of operating an optoelectronic device to provide a PAM-N modulated signal having the steps of:

receiving an electromagnetic wave at a first optical modulator, the first optical modulator being operable to produce a first modulated output;

receiving an electromagnetic wave at a second optical modulator, the second optical modulator being operable to produce a second modulated output which is different to the first modulated output; and

-2 independently operating the first optical modulator and the second optical modulators such that the optoelectronic device is operable to produce three distinct modulated outputs as well as an unmodulated output.

[0009] Optional features of embodiments of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention. [0010] In some embodiments, Μ ~ N -1. For example, if PAM-4 modulation was desired, M = 4 -1 ~ 3, and so three optical modulators may be used.

[0011] In some embodiments, N ~ 4; M = 3; the first voltage is between 0 V and 0.2 V; and the second voltage is between 1.8 V and 2.0 V.

[0012] The optical modulators may modulate various properties of an electromagnetic wave for example: amplitude, phase, or polarisation. Typically, the optical modulators may be electro-absorption modulators, and so modulate the amplitude, [0013] The optical modulators are in a cascade arrangement. As such, each of the optical modulators in the cascade may output electromagnetic waves into an adjacent modulator in the cascade (with the exception of the last modulator, which will output electromagnetic waves info the output waveguide). In some examples, all of the optical modulators may be in a cascade arrangement.

[0014] The device may further comprise an intermediate waveguide, disposed between a first optical modulator of the M optical modulators and a second optical modulator of the M optical modulators, and operable to convey electromagnetic waves from the first optical modulator to the second optical modulator. The intermediate waveguide may have a length of between 0.5 pm and 50 pm. A first interface between the first optical modulator and the intermediate waveguide may be at a first angle relative to a guiding direction of the intermediate waveguide, and a second interface between the second optical modulator and the intermediate waveguide may be at an opposite angle to the first angle, the first angle and opposite angle may not be right angles. By opposite, it may be meant that the opposite angle has an equal magnitude to the first angle but in an opposite sense. For example, if the first angle was 5° clockwise, the opposite angle may be 5° anti-clockwise (or, said another way, 355° clockwise). This may be represented via a minus sign, such that the first angle = 5° and the opposite angle = -5°.

[001S] The optical modulators may be modulating sections of a single device i.e. regions of the device which may independently modulate an electromagnetic wave

-3being transmitted through the device. For example, the optical modulators may be ridge waveguides in an optically active region.

[0016] A first optical modulator of the M optical modulators may have a first doped region having a first doping polarity; a second optical modulator of the M optical modulators may have a second doped region having a second doping polarity; the first doped region may be immediately adjacent to the second doped region; and the first doping polarity may be opposite to the second doping polarity. [0617] One of the M optical modulators may have a first length along which received electromagnetic waves will travel of between 5 pm and 50 pm, and another optical modulator of the M optical modulators may have a second length along which received electromagnetic waves will travel of between 5 pm and 80 pm. Optionally the first length may vary between 15 pm and 20 pm and / or the second length may vary between 35 pm and 50 pm.

[6018] One of the M optical modulators may have a respective waveguide width substantially perpendicular to the direction in which electromagnetic waves are conveyed of between 450 nm and 1100 nm. Optionally the waveguide widths may be between 650 nm and 760 nm or 700 nm and 800 nm.

[6619] The optoelectronic device may have: a first transmittance in a first transmittance state of the N distinct transmittance states; a second transmittance in a second transmittance state of the N distinct transmittance states; and the first transmittance may be one half of the second transmittance. For example. If the first transmittance was 1/3 A, where A is 100% transmittance, then the second transmittance may be equal to 2/3A.

[6626] The device may further comprise a heater, such that the M optical modulators are tuneable with respect to wavelength.

[6621] In a first transmittance state of the N distinct transmittance states, all of the M optical modulators may have applied to them a control voltage equal to the first voltage; and in a second transmittance state of the N distinct transmittance states, all of the M optical modulators have applied to them a control voltage equal to the second voltage.

[6622] M may be equal to 2, N may equal 4, M may equal 2, and k may equal 3.

In a first transmittance state of the 4 distinct transmittance states: a first optical modulator of the 2 optical modulators has maximum transmittance; and a second optical modulator of the 2 optical modulators has maximum transmittance; in a

-4 second transmittance state of the 4 distinct transmittance states: the first optical modulator has a first transmittance, the first transmittance being less than maximum transmittance; and the second optical modulator has maximum transmittance; in a third transmittance state of the 4 distinct transmittance states: the first optical modulator has maximum transmittance; and the second optical modulator has a second transmittance, the second transmittance being less than the first transmittance; and in a fourth transmittance state of the 4 distinct transmittance states: the first optical modulator has a third transmittance, the third transmittance being less than the first transmittance; and the second optical modulator has the second transmittance. In the method, when neither modulator is used to modulate incoming light, the maximum transmittance output of the optoelectronic device may correspond, according to the IEEE P802.3bs/D1.4 Ethernet standard draft, to the “3” level of a PAM-N modulated signal representing the Gray code of the two bits {0,0}; when only the first optical modulator is used (i.e., has a transmittance less than its respective maximum transmittance), the modulated output of the optoelectronic device may correspond to the “2” level of a PAM-N modulated signal representing the Gray code of {0,1}; when only the second optical modulator is used the modulated output of the optoelectronic device may correspond to the “1” level a PAM-N modulated signal representing the Gray code of {11}; and when both the first optical modulator and second optical modulator are used the modulated output of the optoelectronic device may correspond to the “0” level of a PAM-N modulated signal representing the Gray code of {10}.

Brief Description of the Drawings [0023] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0024] Figure 1 is an eye diagram showing the modulation states of a PAM-4 modulated signal;

[0025] Figure 2 is a schematic view of an optoelectronic device capable of PAM-4 modulation;

[0026] Figure 3 Is a detailed schematic of the device of Figure 2;

[0027] Figures 4A and 4B schematically show variant waveguide ridge structures; [0028] Figures 5 schematically shows a further variant waveguide ridge structures;

-5[0029] Figure 6 shows a variant waveguide ridge structure;

[0030] Figure 7 shows an example of an input or output waveguide;

[0031] Figure 8 shows a variant waveguide ridge structure;

[0032] Figure 9 shows a further variant waveguide ridge structure;

[0033] Figure 10 shows a further variant waveguide ridge structure;

[0034] Figure 11 shows a variant optoelectronic device capable of PAM-4 modulation;

[0035] Figures 12A and 12B show further variant optoelectronic devices;

[0036] Figures 13A and 13B show further variant optoelectronic devices;

[0037] Figures 14A and 14B show further variant optoelectronic devices;

[0038] Figure 15 shows a further variant optoelectronic device;

[0039] Figure 16 shows an optoelectronic device according to Figure 2 including a heater;

[0040] Figures 17A and 17B show simulation results for a first optoelectronic device;

[0041] Figures 18A and 18B show simulation results for a second optoelectronic device;

[0042] Figure 19 shows a variant optoelectronic device;

[0043] Figure 20 shows a further variant optoelectronic device; and [0044] Figure 21 shows a further variant optoelectronic device [0045] Figure 2 shows, schematically, an optoelectronic device capable of PAM-4 modulation. Electromagnetic waves 101 may be presented to an input waveguide 102 at a first end of the device. The waves are guided from the input waveguide, through a tapered region 103 and into the first optical modulator 104. as the optical modulator may be an electro-optical modulator (EOM) in that it may utilize an electro-optical effect (for example, the Franz-Keldysh effect if the electro-optical modulator was an electro-absorption modulator). The first optical modulator has an associated length L-i 105 along which the waves are guided, as well as an associated waveguide width Wi 106 which is substantially perpendicular to the length. The first optical modulator is operable, via electrodes 107, to operate in various transmittance states, by applying to this and each optical modulator a respective control voltage selected from three different control voltages (e.g., 0 V,

- 6 1.4 V, and 2 V, as shown in the list below). For example, the optical modulator may be operable in either a maximum transmittance state or in a state with less than maximum transmittance, depending on the desired output of the device. In the examples where the optical modulator is an electro-absorption modulator, the optical modulator may be operated in either (I) a transmittance state of maximum transmittance or (ii) a transmittance state of less than maximum transmittance. By switching between two or more such transmittance states, the optical modulator may modulate the light propagating through the device, where “modulating” the light propagating through the device means imposing a time-varying attenuation on light transmitted through the device.The electrodes may be operable in three states for the first optical modulator, or in two states (for the second optical modulator), each state corresponding to a control voltage. For example, the optical modulators may have applied io them the control voltages listed below, to produce the four PAM-4 levels:

Level 3: ER4-0dB: V1=0V, V2-0V Level 2: ER3=1.05dB: V1=1.4V, V2=0V Level 1: ER2=2.44dB: V1=0V, V2-2V

Level 0: ER1=4.5dB: V1=2V, V2=2V.

In the list above, ER (e.g., ER1, ER2, etc.) is the respective extinction ratio of the optoelectronic device (and the transmittance is the opposite, in dB, e.g., an extinction ratio of 2.44 dB corresponds to a transmittance of -2.44 dB). In an alternate embodiment, Level 2 is produced using V1 = 1V and V2 = 1V. Each optical modulator may have maximum transmittance when the applied control voltage is 0V, and reduced transmittance for higher control voltages. As used herein, an “optical modulator” is an optical element with electrically controlled optical transmittance, such as an electro-absorption modulator (EAM) as discussed above.

[0046] The waves are then guided into a first end of an intermediate waveguide 108, which has a taper so as to match the waveguide width of a second optical modulator 109 disposed at the opposing end. The waveguide width may affect the degree to which light is attenuated by any given optical modulator. The intermediate waveguide 108 can be formed of either undoped Si or undoped SiGe and may have a length of between 0.5 pm and 10 pm or 1 pm and 2 pm. The second optical modulator has an associated length L₂110 along which the waves are guided, as well as an associated waveguide width W₂111 which is substantially perpendicular _ j _ to the length. The length and waveguide width of the second optical modulator is different to the length and waveguide width of the first optical modulator. In this example, this allows the first and second optical modulators to operate in different transmittance states as the length and width of the optical modulators is a factor in determining the degree to which electromagnetic waves are attenuated by the optical modulator. As with the first optical modulator, the second optical modulator is controllable by an electrode 112, Table 1 below show indicative values for the length and waveguide widths of the optical modulator as well as the length of the intermediate waveguide:

Parameter	Base design value	Range
Li	15-20 [pm]	5-50 [pm]
L2	35 - 50 [pm]	5-80 [pm]
di	1 - 2 [pm]	0.5-10 [pm]
W₁	650 - 750 [nm]	450-1100 [nm]
w₂	700 - 800 [nm]	450-1100 [nm]

Table 1 [0047] The waves are then guided into an output waveguide 113, and exit the device. In the lower-right comer of Figure 2 an example 115 of transmittance states is shown indicating the optical power output corresponding to various transmittance states of the device. Level 4 corresponds to maximum transmittance when both optical modulators have zero control voltages (i.e. 0 V and 0 V) applied to them.

Level 3 and 2 correspond to the cases where a non-zero control voltage is applied to only one optical modulator while the other has 0 V applied to it, and finally level 1 corresponds to both the first and second optical modulators having non-zero control voltage applied to them. By design all of the 4 optical output power levels are equally spaced, [0048] Figure 3 shows in more detail the device 200 shown in Figure 2. In this example, the input waveguide 201 is formed of Si and connects via tapered region 202 with a waveguide ridge comprising SiGe having width w-t 209. The waveguide ridge could instead comprise any one or more of Ge, SiGeSn, or GeSn. The

-8waveguide ridge has a lightly doped region of a first species 204, and adjacent to this, in a slab portion, is a heavily doped region 205 of the same species. Opposing this in the width direction, and again present in the ridge waveguide, is a lightly doped region of a second species 206 which is adjacent to, in the slab portion, a heavily doped region 207 of the same second species. The heavily doped regions 205, 207 are connected to respective electrodes 210a and 210b. The lightly doped regions have a length Li 208 which corresponds to the length of the first optical modulator.

[0049] Adjacent to a distal end of both of the first optical modulator’s lightly doped regions is an intermediate waveguide 211 which can be formed of either undoped SiGe or undoped Si. As the second optical modulator 213 has a different waveguide width 219 to the first optical modulator 203, the intermediate waveguide acts to taper from the first width to the second width. Therefore, the waves are guided from the first optical modulator, through the intermediate waveguide and into the second optical modulator.

[0050] The second optical modulator 203, like the first optical modulator has lightly and heavily doped regions. Innermost, and within a waveguide ridge of the second optical modulator, is a region 214 lightly doped with a first species and a region 216 lightly doped with a second species. These regions oppose each other in the width direction. Adjacent to each lightly doped region, and within a slab, are respective heavily doped regions 215 and 217. The region 215 is heavily doped with the same species of dopant as lightly doped region 214, and region 217 is heavily doped with the same species of dopant as lightly doped region 216.

[0051] At a distal end of the second optical modulator 203 to the intermediate waveguide 211 is an output waveguide 222. The output waveguide has a tapered region 221, which tapers from the waveguide width w₂ 219 of the second optical modulator to a second width.

[0052] Figures 4A and 4B show variant waveguide ridge structures that could be used in either optical modulator. Each is formed of a Si slab which provides a slab layer. In the example shown in Figure 4A the Si slab has heavily doped regions 302 and 303 which are doped with a different polarity dopant e.g. N-h- and P++ or P-hand N++ respectively. The slab has been etched to provide a ridge portion of Si, and the heavily doped regions partially extend along this ridge into regions 307 and 308. On top of the ridge portion of Si is a SiGe (or any other optically active materia!)

-9ridge 304, which has been lightly doped in regions 305 and 306, The dopant in region 305 has the same polarity as the dopant in region 302, but its concentration is reduced in comparison. Similarly, the dopant in region 306 has the same polarity as the dopant in region 303 but its concentration is also reduced in comparison.

[0053] Figure 4A is annotated with various dimensions. These include h_wg - the total height of the ridge waveguide; h₂ - the height of the slab; h₃ - the height along which the heavily doped regions extend; d_n - the depth into the sidewall of the ridge that the first dopant penetrates; d_p -- the depth into the sidewall ridge that the second dopant penetrates; d_np - the depth into the sidewall that the heavily doped first dopant penetrates; and d_pp the depth into the sidewall that the heavily doped second doped penetrates. h_wg may have a value of around 3pm (+/-5 %).

[0054] The tolerances of the values are shown in table 2 below, and table 3 indicates example dopant ranges for example dopants:

Dimension	Tolerance [nm]
hwg	100-800
h₂	100-400
h₃	0-400
d_np	50 - 300
d_pp	50 - 300
dp	50 - 300
dn	50 - 300

[0055] A variant ridge waveguide structure is shown in Figure 4B. In this example, the Si slab layer 301 has a SiGe layer 304 immediately on top. The SiGe

-10layer is doped to produce the heavily doped regions 302 303 as well as the lightly doped regions 305 306. The lightly doped regions are, again, present in a ridge 304 of the ridge waveguide structure. In this example, the heavily doped regions do not extend up the ridge of the waveguide. Of course, the features described in relation to the first variant (Figure 4A) are applicable where appropriate to the second variant. For example, a SiGe layer 304 could be provided atop the Si slab layer 301 in the first variant, where the heavily doped regions partially extend up the side of the ridge 304 of the waveguide.

[0056] Figures 5 shows a further variant ridge waveguide structure. Figure 5 is an example where the heavily doped regions (N++ and P++) are provided in the Si slab layer, and do not extend into the SiGe ridge.

[0057] Figure 6 shows a variant ridge waveguide structure. This structure shares a number of features with previously discussed ridge waveguide structures, and so like reference numerals are used for like parts. The structure in Figure 6 differs from that in Figure 4A in that a Si, SiGe, or SiGeSn layer 601 is provided between the ridge 304 and each of the lightly doped regions 305 and 306. This layer 601 should have a lower refractive index with respect to the material making up the ridge 304. The distances d_np2 and d_pp2 indicate, respectively, the depth into the slab 301 that the heavily doped regions are doped. The tolerance on this measurement should be between 50 and 300 nm.

[0658] Figure 7 shows an example of an input or output Si passive waveguide 201 or 222, The input and output waveguides respectively comprise a slab portion 801 having height hi and a ridge portion 802, the ridge and slab having a combined height of h_Wg. hi should have a tolerance of between 100 - 800 nm, and h_Wg should have a value of around 3 pm (+ or -- 5%).

[6659] Figure 8 shows a variant ridge waveguide structure. Here, instead of the doped regions extending vertically up sidewalls of the ridge waveguide, they are disposed in upper and lower doped regions of the waveguide ridge. As can be seen in Figure 8, an N doped region extends across the top of the central portion of the waveguide, and Is connected to an N+ doped region which is in turn connected to an electrode contact. Similarly, a P doped region extends below the central portion of the waveguide, and is connected to a P+ doped region which is in turn connected to a separate electrode contact. The lower doped region exhibits a multilayer structure. The multilayer structure comprises a first doped region (i.e. a first layer) formed from

-11 an implanted doped portion of the silicon-on-insulator (SOI) located directly below the optically active region (OAR), A second doped zone (i.e. a second layer) is formed by implanting dopants into the OAR itself at a region of the OAR located directly above the first doped zone. The second doped zone has a dopant concentration greater than that of the first doped zone. The lower surface of the second doped zone forms the interface between the first doped zone and the second doped zone. The upper surface of the second doped zone forms the contact surface for a corresponding electrode.

[0060] As discussed above, the first doped zone of the lower doped region is P doped, and the second doped zone of the lower doped region is P+ doped (where P+ denotes a P doped region with a greater concentration of P dopants). The upper doped region contains an upper doped region in the form of an N doped region which comprises: an upper N doped waveguide region extending across the upper surface of the OAR waveguide; a lateral N doped region which extends outwards away from the waveguide; and a connecting N doped region which extends vertically along a side of the waveguide to connect the upper N doped waveguide region with the upper lateral N doped region. The connecting N doped region, the upper later N doped region, and the upper N doped waveguide region form a single contiguous N doped region. The OAR comprises the waveguide ridge and slab regions at either side of the waveguide so that the OAR has an inverted T-shape cross section (the cross section taken transverse to the longitudinal axis of the waveguide). The P+, N, and N+ doped regions are all located within the OAR material, whilst the N region extends along the fop and the side of the waveguide ridge as well as the slab, the N+ and P+ regions are only found within the slab sections of the OAR, either side of the waveguide ridge.

[6061] In other embodiments (not shown) the P and N doped regions are reversed so that the lower doped region contains an N doped zone and an N+ doped zone so that the upper doped region is P doped and P+ doped.

[0062] In this embodiment, an extra step of etching a region of the OAR (e.g. SiGe) has occurred before that region is implanted to form a P+ doped region. This etching process creates a P+ region of the OAR which has a reduced height as compared to the slab within which it is located.

[6663] By etching the slab region of the OAR before the P+ doping takes place, it is easier to ensure that the P and P+ doped regions are connected; that is to say that

-12 the P+ dopant region (the second zone of the multilayer lower doped portion) reaches through the thickness of the slab from the contact surface at the top surface to the P doped region at the bottom surface. The thickness of the second zone of the multilayer lower doped portion is 0 pm - 0.2 pm. Where the thickness has a value of 0 pm, this should be understood to mean that the P+ dopant region is completely inside of the P region.

[0064] In Figure 9, an annealing process has taken place which causes the P doped region to diffuse into the central portion of the waveguide from below by a distance of between 50 - 200 nm. This “migrated” area caused by the dopant diffusion can be beneficial in reducing the series resistance and increasing the bandwidth of the optical modulator. The diffusion can occur in any of the embodiments described herein.

[0065] Figure 10, in contrast to Figure 9, shows a device where the P doped region has diffused from a side of the central portion of the waveguide horizontally into the central portion of the OAR waveguide. Furthermore, this device is based on an 800 nm cross-section, whereas some previous examples are based on a 3 pm cross-section. Both forms of devices are suitable for implementing the PAM-N modulation scheme as discussed.

[0666] Figure 11 shows a variant optoelectronic device comprising three optical modulators, a first 701, second 702, and third 703. Each of the optical modulators has a different length as can be seen. In this example > l₂ > (where corresponds to the rf^h optical modulator). As will be understood, the length of an optical modulator correlates with the magnitude of the modulation it will impart on electromagnetic waves being guided through It. Therefore, in this example, the first optical modulator will impart a greater magnitude of modulation to the electromagnetic waves than both the second or third.

[6067] Thus, in some embodiments, pairs of optical modulators can be operated substantially simultaneously in order to provide further tuning between the modulation outputs of the device, to give an additional degree of freedom for the device driver to drive PAM-4 modulation. In other embodiments, the three levels can be controlled independently in such a manner as to avoid relying upon the product of two optical modulator extinction settings (e.g., to produce PAM-4, three optical modulators may be used, and in each of the four transmittance states corresponding to the four levels of PAM-4, at least two of the three optical modulators may have 0 V

-13applied to them as the control voltage). The following table, Table 4, gives an example of where the three levels can be controlled independently:

PAM-4 Level	si Optical Modulator	2^nd Optical Modulator	3^rd Optical Modulator
3	0	0	0
2	1	0	0
1	0	1	0
0	0	0	1

Table 4 [0068] The following table, Table 5, gives an example where three optical modulators can be operated to produce PAM-4 modulated outputs (where 1 indicates that the optical modulator is being used to attenuate the electromagnetic waves, and 0 indicates that it is not). In one embodiment, each value of 0 listed below an optical modulator in Table 4 corresponds to applying a respective control voltage equal to a first voltage (e.g., 0 V) to the optical modulator, and each value of 1 corresponds to applying a respective control voltage equal to a second voltage (e.g., 2 V) to the optical modulator. A cascade of three modulators configured to operate in this manner may be driven by a particularly simple drive circuit having three outputs and including, for example, (i) a first set of three transistors, each of which, when turned on, connects a respective output, of the three outputs, to ground (i.e., 0 V), and (ii) a second set of three transistors, each of which, when turned on, connects a respective output, of the three outputs, to a power supply voltage (e.g., 2 V).

PAM-4	j si	2^nd	3^rd
Level	Optical Modulator	Optical Modulator	Optical Modulator
3	0	0	0
2	1	0	0
1	1	1	0
0	1	1	1

-14Table 5 [0069] A further example is shown in Table 6, where the third optical modulator is operated simultaneously with the second optical modulator (in one embodiment, for example, V-ι = 0 V, V₂ = 1,4 V, and V₃ = 2 ):

PAM-4	1^st Optical	2^nd Optical	3^ra Optical
Level	Modulator	Modulator	Modulator
3	Vt	Vi	Vi
2	v₂	Vi	Vt
1	Vt	V₃	V₃
0	v₃	v₃	v₃

Table 6 [0070] Alternatively, the third optical modulator could be operated independently of the first and second optical modulator i.e. if could be used to generate a PAM leve of modulation without necessarily being used in conjunction with another optical modulator. Therefore, a PAM-8 level modulation scheme would be possible. In some embodiments another modulation scheme, referred to herein as “N-level modulation”, may be implemented using Log₂(N) modulators. Unlike PAM-N modulation (in which the N levels are equally spaced), N-level modulation may have optical power levels that are not equally spaced (they may be logarithmically spaced, for example). 8-level modulation, for example, may be generated using three cascaded modulators, each operated in one of two respective transmittance states, as shown in Table 7:

PAM- 8 Level	1^si Optical Modulator	2 fid Optical Modulator	3^!d Optical Modulator
7	0	0	0
6	1	0	0
5	0	1	0
4	0	0	1
3	1	1	0
2	1	0	1
1	0	1	1
0	1	1	1

Table 7 [0071] As will be recognised, an N-level modulation scheme is possible by providing at least M optical modulators where M - Log₂(N). Said another way, by providing M optical modulators, an N-level modulation scheme is possible with 2^Mlevels.

[0072] In the example shown in Figure 11, the three optical modulators 701, 702, and 703 are provided within a single SiGe ridge waveguide. This can reduce the number of interfaces where the electromagnetic waves must move from a material of one refractive index to another (this having associated reflection losses). However, this is not necessary. As shown in Figures 12A and 12B, it is possible to provide Si waveguides between each of the optical modulators. This may aid manufacturability and have improved performance since intermediate Si waveguides will have less loss.

[0073] For example, as is shown in Figures 13A and 13B, the lengths and / or widths of the optical modulators within a device according to embodiments of the invention may differ markedly. In the example in Figure 13A, the length Li and width Wi of the first optical modulator is much reduced in comparison to the length L₂ and width W₂ of the second optical modulator. However, the length and width can be varied in order to tune the optical modulators in accordance with a particular device requirement. In the example in Figure 13B, the length Li of the first optical modulator is markedly greater than the length L₂ of the second optical modulator, however the

-16first optical modulator’s width Wi is much reduced in comparison to the width W₂ of the second optical modulator.

[0074] The optical modulators may be arranged in order of active length, the longest being first - i.e. exposed first to the input light. This applies to ail of the PAM-N modulators of embodiments of the invention.

[0075] Another feature to note in Figures 13A and 13B is that the intermediate waveguides are (i) formed of bulk Si, and (ii) taper from a first width corresponding to the first optical modulator to an output width (the width of the input and output waveguides) and then from the output width to a second width corresponding to the second optical modulator. Figures 14A and 14B illustrate similar principles to Figures 14A and 14B, however the intermediate waveguides (between the first and second optical modulators) are formed of undoped SiGe and taper directly from the first width io the second width.

[0076] The above figures illustrate that the particular parameters of any given optical modulator may vary (or indeed be tuned) whilst still allowing the optoelectronic device comprising said optical modulators to perform PAM-4 modulation.

[0077] Another variant configuration is illustrated in Figure 15. Previously hereto, each optical modulator has been separated by an electrical insulator for example undoped SiGe or bulk Si. However, it is possible to dope the regions of the ridge waveguide such that dopants of opposing polarity are immediately adjacent. For example, as shown in Figure 15, a heavily doped region 205 of a first species of dopant is immediately adjacent to a heavily doped region 217 of a second species of dopant. The first and second species of dopant are of opposing polarities for example N++ and P++ or P++ and N++ respectively. This is also true for the lightly doped regions 204 and 216.

[0078] By doing so, it is possible to reduce the overall length of the optoelectronic device (and so may also reduce the transmission losses associated with the device) whilst retaining the first and second optical modulators which facilitate PAM-4 modulation.

[0079] Figure 16 illustrates a variant optoelectronic device 100 which further includes a heater 900 disposed near to the first and second optical modulators. The heater can be used to tune the optical modulators (and therefore the optoelectronic device) with respect to wavelength. The heater can be formed form either doped

-17 SiGe, Si, or a metal. The heater is connected to electrodes which allow it to be controlled by the device, [0080] Figures 17A and 17B show, respectively, plots of extinction ratio (measured in dB) and Δα / a against wavelength for a simulated optoelectronic device, a is the absorption coefficient of the absorbing material (e.g. Ge, SiGe, SiGeSn, or GeSn), Δα represents the change in this absorption coefficient as a function of wavelength from when non-zero voltage is applied to the optical modulator to when zero voltage is applied, and Δα/α is the normalised change in absorption coefficient as a function of wavelength, and the optoelectronic device has the following parameters: Li = 15 pm, L₂ = 40 pm, Wi 800 nm, and W₂ --- 700 nm.

In Figure 17A, the dotted lines 402, 404, and 406 correspond to minimum target extinction ratios for the PAM-4 levels 1, 2, and 3 respectively. The solid lines 401, 403, and 405 correspond with the extinction ratio of the optoelectronic device when operated in the respective PAM-4 levels: 1, 2, and 3. The solid line 407 indicates the insertion loss in dB associated with the device.

[0081] In Figure 17B, the solid lines 401,403 and 405 indicate the normalised change in a.

[0082] Figures 18A and 18B show plots similar to Figures 17A and 17B but for a variant optoelectronic device. Here the device had the following parameters: Li ~ 15 pm, L₂ ~ 45 pm, and Wi ~ W₂ = 700 nm. Also, indicated in Figure 18A is the optimum working wavelength bandwidth 408 which is between 1526 nm and 1559 nm.

[0083] These plots show that an optoelectronic device formed in accordance with embodiments of the present invention is operable to provide PAM-4 modulated outputs.

[0084] Figure 20 shows a variant device which differs from previous devices by having angled interfaces. The devices share a number of features with previous devices, and so like features are indicated by like reference numerals. The input waveguide 104 can be described as guiding light along a central axis 1601. The axis is at an angle φ_Ί from a line 1602 which is parallel with a longitudinal direction of the device. Therefore, the input waveguide includes an angled interface 2101. The interface 2101 itself may be at an angle a1 relative to the line 1602 which is not equal to 90°. Similarly, an interface 2104 between the second optical modulator 213 and the output waveguide 222 may be at an angle β2 which is not equal to 90°. As

-18with the input waveguide 104, the output waveguide 222 may also guide light along an axis 1603, and this axis 1603 may be at an angle y2 to the line 1602.

[0085] Figure 21 and 22 show variant devices which, like Figures 19 and 20, differs from the previous devices by having angled interfaces. The devices share a number of features with the previous devices, and so like features are indicated by like reference numerals. Taking the device shown in Figure 21 first, the input waveguide 201 can be described as guiding light along a central axis 1601. The axis is at an angle φ_Ί from a line 1602 which is parallel with a guiding direction of the first and second optical modulators. Therefore, the input waveguide includes an angled interface 2101.

[0086] In the example shown in Figure 21, there is an intermediate waveguide 211 which is formed of a different material to the first 203 and second 213 optical modulator. Therefore, there is a change in refractive index. In order io reduce back reflections caused by this change in refractive index: the intermediate waveguide 211 may extend at an angle yi relative to the longitudinal direction 1602. Finally, the output waveguide 222 extends at an angle γ₂ relative to the longitudinal direction 1602 (which is parallel to the guiding directions in the first and second optical modulators). The angle y₂ can be chosen to help mitigate back reflections which may occur as light moves from the second optical modulator 213 info the output waveguide 222.

[0687] Generally, the angles obey the following:

-14° < φι, φ₂ < 14°

-14° <yi, y₂< 14° [6688] In some cases, α-ι = βι, a₂ = β₂,, Ϊ Yi I =| Y21 and | φι | =| Φ2 i[6089] The interface 2101 between the input waveguide 201 and the first optical modulator 203 may be at an angle not equal to 90° relative to the guiding direction of the first optical modulator. As shown in Figure 21, the interface 2101 may be at an angle Oi which may be less than 90° relative to the guiding direction of the first optical modulator. Similarly, the interface 2102 between the first optical modulator 203 and the intermediate waveguide 211 may be at an angle not equal to 90° relative to the guiding direction of the first optical modulator. As shown in Figure 21, the interface 2102 may be at an angle β_Ί which may be less than 90° relative to the guiding direction of the first optical modulator.

-19[0096] The intermediate waveguide 211 may project from the first optica! modulator 203 at an angle yi relative to the guiding direction of the first optical modulator. This means the intermediate waveguide 211 may guide light at an angle Yi relative to the guiding direction of the first optical modulator. The guiding direction of the intermediate waveguide 211 may also lie at an angle φ₂ to the guiding direction of the second optical modulator 213.

[6091] The interface 2103 between the intermediate waveguide 211 and the second optical modulator 213 may be at an angle which is not equal to 90° relative to the guiding direction of the first optical modulator. As shown in Figure 21, the interface 2103 may be at an angle a₂ relative to the guiding direction of the first optical modulator which may be greater than 90°. Similarly, the interface 2104 between the second optical modulator and the output waveguide 222 may be at an angle not equal to 90° relative to the guiding direction of the first optical modulator.

As shown in Figure 21, the interface 2104 may be at an angle β₂ relative to the guiding direction of the first optical modulator which may be greater than 90°.

[6692] The output waveguide 222 may project from the second optical modulator 213 at an angle y₂ relative to the guiding direction of the first optical modulator.

[6693] In a typical example of the device shown in Figure 21: ai = 80°; βι = 80°; φ-ι = 3°; γ-ι = 3°; a₂ = 100° = 90 + (90-a-f); β₂ ~ 100° ~ a₂; φ₂ = -3° = -φυ and γ₂ = -3° =

-Yi.

[6694] In general however:

«1

Pi

Yi [0

89° to 50° 89° to 50° 0.3° to 14° 0.3° to 14° a_?-91° to 130° β₂ = 91° to 130° φ₂ = -0.3° to-14 γ₂ = -0.3° to -14 and, in some embodiments:

Oh = 89° to 80° β₁ = 89° to 80° 1 = 0.3° to 3° a₂ = 91° to 100° β₂ = 91°1ο100° φ₂ = -0.3° to -3° γ₁ = 0.3° to 3° γ₂ = -0.3° to -3° [6696] The configuration of the device shown in Figure 22 is the same as that shown in Figure 21 insofar as the input waveguide and first optical modulator are concerned. However, the intermediate waveguide 2201 of the device shown in Figure 22 is not linear as shown previously. Instead, the intermediate waveguide

-20bends as the second optical modulator 213 has been rotated by 180° relative to the second optical modulator shown in Figure 21. Therefore light guided by the intermediate waveguide 2201 of Figure 22 follows substantially curved path relative to the intermediate waveguide 211 of Figure 21.

[0097] In a typical example of the device shown in Figure 22: α-ι - 80°: β_Ί = 80°:

φι = 3°; γ! = 3°; a₂ = 80° = a₂; β₂ = 80° = a₂; φ₂ = 3° = and γ₂ = 3° = η general however:

αι = 89° to 50° β₁ - 89° to 50° φι =0.3° to 14' Υί = 0.3° to 14^c and, in some embodiments:

α₂ = 89° to 50° β₂ = 89° to 50° φ₂ = 0.3° to 14° γ₂ = 0.3° to 14° a·, = 89° to 80° β₁ = 89° to 80° φι = 0.3° to 3° Y! = 0.3° to 3° a₂ = 89° to 80° β₂ = 89° to 80° φ₂ = 0.3° to 3° [0098] The angled interfaces in the example shown in Figure 22 may be used instead of those shown in Figure 21, as the angled interfaces 2102 and 2103 are non-parallel to each other and so may supress resonator or Fabry-Perot cavity effects within the intermediate waveguide.

[0099] 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.

[90100] All references referred to above are hereby incorporated by reference.

-21 List of Features

100	Optoelectronic device
101	Input light
102, 201	Input waveguide
103, 202	Tapered region of input waveguide
104, 203	First optical modulator
105, 208	Length - Li - of first optical modulator
106, 209	Waveguide width - Wi - of first optical modulator
107, 112	Electrodes
210a, 210b 220a, 220b 108, 211 109, 213	Electrodes Electrodes Intermediate waveguide Second optical modulator
110, 218	Length - L₂ - of the second optical modulator
111,219	Waveguide width - W₂ - of the second optical modulator
113, 222	Output waveguide
114	Modulated light
115	Example levels of optical modulator
221	Tapered region of output waveguide
204, 214	Lightly doped region (first species)
205, 215	Heavily doped region (first species)
206, 216	Lightly doped region (second species)
207, 217	Heavily doped region (second species)
301	Si slab
302	Heavily doped region (first species)
303	Heavily doped region (second species)
304	SiGe ridge waveguide
305	Lightly doped region (first species)
306	Lightly doped region (second species
307	Heavily doped region in ridge (first species)
308	Heavily doped region in ridge (second species)

401.403.405 First, second, and third optical modulator extinction ratios

402.404.406 Minimum target optical modulator extinction ratio

407 Insertion loss

-22 10

701 First optical modulator

702 Second optical modulator

703 Third optical modulator

900 Heater

1601 Guiding direction of input waveguide

1602 Longitudinal axis of the device / guiding direction of the electro absorption modulator

2101 Input waveguide-optical modulator interface

2102 Optical modulator-intermediate waveguide interface

2103 Intermediate waveguide-optical modulator interface

2104 Optical modulator-output waveguide interface

2201 Curved intermediate waveguide

Claims

WHAT IS CLAIMED IS:

1. An optoelectronic device operable to provide a PAM-N modulated output, the device comprising:

M optical modulators, M being an integer greater than 1, the M optical modulators being arranged in a cascade, the device being configured to operate in N distinct transmittance states, as a PAM-N modulator, wherein, in each transmittance state of the N distinct transmittance states, each of the M optical modulators has applied to it a respective control voltage equal to one of:

a first voltage or a second voltage.

2. The device of claim 1, wherein M = N -1.

3. The device of claim 1, wherein:

N = 4;

M = 3;

the first voltage is between 0 V and 0.2 V; and the second voltage is between 1.8 V and 2.0 V.

4. The device of claim 1, wherein an optical modulator of the M optical modulators is an electro-absorption modulator.

5. The optoelectronic device of claim 1, further comprising an intermediate waveguide, disposed between a first optical modulator of the M optical modulators and a second optical modulator of the M optical modulators, and operable to convey electromagnetic waves from the first optical modulator to the second optical modulator.

6. The optoelectronic device of claim 5, wherein a first interface between the first optical modulator and the intermediate waveguide is at a first angle relative to a guiding direction of the intermediate waveguide, and a second interface between the

-24second optical modulator and the intermediate waveguide is at an opposite angle to the first angle, wherein the first angle is not a right angle and the opposite angle is not a right angle.

7. The optoelectronic device of claim 1, wherein the M optical modulators are optically active regions of a ridge waveguide.

8. The optoelectronic device of claim 1, wherein:

the optoelectronic device has a first transmittance in a first transmittance state of the N distinct transmittance states;

the optoelectronic device has a second transmittance in a second transmittance state of the N distinct transmittance states; and the first transmittance is one half of the second transmittance.

9. The optoelectronic device of claim 1, further comprising a heater, such that the M optical modulators are tuneable with respect to wavelength.

10. The optoelectronic device of claim 1, wherein:

in a first transmittance state of the N distinct transmittance states, ail of the M optical modulators have applied to them a control voltage equal to the first voltage; and in a second transmittance state of the N distinct transmittance states, all of the M optical modulators have applied to them a control voltage equal to the second voltage.

11. The optoelectronic device of claim 1, wherein M = 2.

12. An optoelectronic device operable to provide a PAM-N modulated output, the device comprising:

M optical modulators, M being an integer greater than 1, the M optical modulators being arranged in a cascade, the device being configured to operate in N distinct transmittance states, as a PAM-N modulator,

-25wherein, in each transmittance state of the N distinct transmittance states, each of the M optical modulators has applied io it a respective control voltage equal to one of k different control voltages, wherein k is an integer greater than 1 and less than N.

13. The optoelectronic device of claim 12, wherein the M optical modulators are electro-absorption modulators.

14. The optoelectronic device of claim 12, wherein:

N ---4:

M ~ 2: and k = 3.

15. The optoelectronic device of claim 14, wherein:

in a first transmittance state of the 4 distinct transmittance states:

a first optical modulator of the 2 optical modulators has maximum transmittance; and a second optica! modulator of the 2 optical modulators has maximum transmittance;

in a second transmittance state of the 4 distinct transmittance states: the first optical modulator has a first transmittance, the first transmittance being less than maximum transmittance; and the second optica! modulator has maximum transmittance;

in a third transmittance state of the 4 distinct transmittance states: the first optical modulator has maximum transmittance; and the second optical modulator has a second transmittance, the second transmittance being less than the first transmittance; and in a fourth transmittance state of the 4 distinct transmittance states:

the first optical modulator has a third transmittance, the third transmittance being less than the first transmittance; and the second optical modulator has the second transmittance.

16. The optoelectronic device of claim 12, further comprising an intermediate waveguide, disposed between a first optical modulator of the M optical modulators

-26and a second optical modulator of the M optical modulators and operable to convey electromagnetic waves from the first optical modulator to the second optical modulator.

17. The optoelectronic device of claim 16, wherein a first interface between the first optical modulator and the intermediate waveguide is at a first angle relative to a guiding direction of the intermediate waveguide, and a second interface between the second optical modulator and the intermediate waveguide is at an opposite angle to the first angle, wherein the first angle is not a right angle and the opposite angle is not a right angle.

18. The optoelectronic device of claim 12, wherein a first optical modulator of the M optical modulators and a second optical modulator of the M optical modulators are optically active regions of a ridge waveguide.

19. The optoelectronic device of claim 12, wherein:

20. The optoelectronic device of claim 12, further comprising a heater, such that an optical modulator of the M optical modulators is tuneable with respect to wavelength.

21. The optoelectronic device of claim 12, wherein:

in a first transmittance state of the N distinct transmittance states, all of the M optical modulators have applied io them a control voltage equal to the first voltage; and in a second transmittance state of the N distinct transmittance states, all of the M optical modulators have applied to them a control voltage equal to the second voltage.

-27

22. The optoelectronic device of claim 12, wherein M is greater than or equal to Log2(N).

-2829

Intellectual

Property

Office

Application No: GB 1703716.9 Examiner: Mrs Rachel Morgans