Patent Description:
Intensity modulation of optical signals in telecommunications applications is often achieved with electro-absorption modulators (EAMs). EAMs are semiconductor devices whose optical absorption characteristics can be changed by the application of an electrical field, allowing an incoming high-speed voltage signal to be converted into a high-speed optical signal. Integrated with a laser diode to provide the optical input, an EAM can form an optical data transmitter within a photonic integrated circuit (PIC). During operation, EAMs are subject to self-heating, which can entail performance degradation.

<CIT> discloses a system for electro-absorption modulation, in which there is a waveguide (<NUM>) and a photodetector (<NUM>). An electro-absorption modulator (<NUM>) is configured with respect to the waveguide (<NUM>) for electro- absorption modulation of the optical signal. An integrated heating element (<NUM>) is located alongside and spaced apart from both the photodetector (<NUM>) and the electro-absorption modulator (<NUM>).

The invention is defined in the claims. The present disclosure relates to integrated EAMs with improved performance characteristics achieved by better photocurrent uniformity.

Integrated EAMs are usually structured as vertical diode mesas including a light-guiding intrinsic semiconductor layer sandwiched between p-type and n-type doped layers, disposed above a waveguide that couples light into and out of the intrinsic layer, and with electrical contacts on the top and at the side(s) of the mesa for applying a voltage across the intrinsic layer perpendicularly to the direction of light propagation; the region across which the voltage is applied forms the active region of the EAM, where modulation takes place. Silicon-photonics EAMs are commonly implemented with III-V materials bonded to a silicon-on-insulator (SOI) substrate, but EAMs are also widely implemented in other material platforms that do not involve attachment to an SOI substrate. In many implementations, the intrinsic layer includes a quantum well structure to exploit the quantum-confined Stark effect for high extinction ratios.

At the start of electro-absorption modulation of an optical signal, prior to self-heating, the absorption rate in the EAM is typically constant along the length of the active region, meaning that an approximately constant fraction of the optical power is absorbed every micrometer between the front (where light enters) and rear (where light exits) of the active region. However, as light propagates and is absorbed along the device (by generating photocurrent), the optical power decreases towards the rear, as does the current density of the generated photocurrent, which is a constant fraction of the optical power at a given location. As the EAM self-heats primarily at the front due to the higher photocurrent density at that location, the absorption rate at the front increases (due to the higher temperature), exacerbating the nonuniformity of the photocurrent density. In many cases, an electrical current density that is very high at the front end of the active region of the EAM and very low at the back end of the active region results. This nonuniform current density negatively affects the performance of the EAM, as it increases the voltage drop associated with the series resistance of the EAM in the regions with the highest photocurrent, such that a lower voltage swing remains across the intrinsic layer in the active region. The lower voltage swing, in turn, reduces the degree of electro-optic change and, thus, optical modulation amplitude (i.e., the difference between maximum and minimum power levels in the modulated optical signal) and extinction ratio (i.e., the ratio of maximum to minimum power levels of the modulated optical signal) of the EAM; in some instances, the photocurrent density at the front of the active region becomes so high as to result in a negligible extinction ratio, effectively reducing the length of the active region. At high optical powers (e.g., greater than 10mW for an effective mode area of about <NUM><NUM> or less) and associated high photocurrent density, EAM performance can be further impacted by carrier saturation at the front of the active region, which can reduce the electrical bandwidth of the device. High current density can also cause excessive self-heating (e.g., a temperature rise of more than <NUM>), resulting in component damage or reduced component operating lifetime. It is desirable to mitigate these various effects to achieve improved optical modulation amplitude, extinction ratio, bandwidth, and optical power handling.

Disclosed herein, in various embodiments, are EAM structures and associated calibration and operation methods that achieve a more uniform current density along the length of the active region of the EAM (i.e., in the direction of light propagation within the EAM), resulting in increased optical modulation amplitude and extinction ratio, greater bandwidth (enabling higher signal speeds), and better power handling (reducing the risk of thermal runaway and device failure). Some embodiments involve increasing the optical absorption coefficient of the EAM from the front of the active region towards the back to even out the photocurrent density. In one embodiment, this variation of the absorption coefficient along the active region is achieved by applying heat to the rear of the active region with an integrated heater, which raises the absorption coefficient in that region. In another embodiment, the EAM is equipped with metal contacts that provide two or more separately controlled direct-current (DC) bias voltages at different locations along the length of the active region, and the applied DC bias voltage is increased towards the rear of the active region to increase absorption. In a third embodiment, the material properties (e.g., elemental composition or quantum well dimensions ) of the intrinsic layer are varied along the length of the active region, with the highest-absorbing material being located at the rear end. In another approach, instead of the absorption coefficient, the coupling of light into the EAM diode is varied along the length of the device, e.g., by having the front of the active region overlap with a down-tapered region of the (silicon) waveguide coupling light into the EAM diode. In the following, the various embodiments will be described in detail with reference to the accompanying drawings.

<FIG> is schematic top view of an example EAM <NUM>, which includes an elongated diode structure <NUM> (illustrated in this figure by the outline of the bottom layer of the diode structure) disposed above a ridge waveguide <NUM> that couples light into and out of the diode structure <NUM>. The EAM <NUM> further includes top and lateral electrical contacts <NUM>, <NUM>, whose extent along the length of the diode structure <NUM> defines the active region <NUM> of the EAM, where light modulation via an applied electrical signal takes place. The direction in which light propagates through the waveguide <NUM> and diode structure <NUM>, indicated by the arrows <NUM>, <NUM> for the optical input and the modulated optical output of the EAM <NUM>, respectively, defines the front and rear end <NUM>, <NUM> of the active region <NUM>.

<FIG> provide two schematic cross-sectional views of the example EAM <NUM>, one taken through the active region <NUM> (<FIG>), and the other taken through a region adjacent (e.g., preceding or following) the active region <NUM>. As shown, in this embodiment, the waveguide <NUM> is a rib waveguide formed in the device layer <NUM> of a semiconductor-on-insulator substrate <NUM>, such as, e.g., a silicon-on-insulator (SOI) substrate including a silicon handle, buried oxide layer, and silicon device layer <NUM>. Instead of silicon, the device layer <NUM> may, for instance, be a diamond or germanium layer. The diode structure <NUM>, which may, optionally, be separated from the waveguide by a thin dielectric (e.g., oxide) layer (not shown), includes a doped bottom layer or "strip" <NUM> and, formed on top of the doped bottom strip <NUM>, a layered mesa <NUM> (flat-top table-like structure), much narrower than the bottom strip <NUM> and elongated in the direction of light propagation to form a ridge above the waveguide <NUM>. The mesa <NUM> includes an intrinsic layer <NUM> and a doped top layer <NUM>. Although shown as equal in width, the intrinsic and doped top layers <NUM>, <NUM> may alternatively differ in width, with a slightly wider or narrower intrinsic layer <NUM>. The diode structure <NUM> may be made of compound semiconductor (i.e., semiconductor materials made from two or more elements), thus forming, together with the waveguide <NUM>, a heterogeneous waveguide structure. Suitable compound semiconductors include, e.g., III-V materials (such as, e.g., indium phosphide (InP) or gallium arsenide (GaAs)) or II-VI materials (such as, e.g., cadmium selenide (CdSe) or zinc oxide (ZnO)). The intrinsic layer <NUM> may be made of a different semiconductor compound than the bottom and top layers <NUM>, <NUM>; for example, the diode structure <NUM> may include an intrinsic InAlGaAs layer in between doped InP layers. Further, the intrinsic layer <NUM> may be a bulk semiconductor layer, or may, alternatively, be composed of quantum wells, quantum dots, or quantum dashes. The EAM <NUM> can be made with standard semiconductor-fabrication techniques, such as by lithographic patterning and etching of the substrate <NUM> to create the waveguide <NUM>, followed by bonding of a stack of compound-semiconductor material, and lithographic patterning and etching of the compound semiconductor to create the diode structure <NUM>.

Functionally, the diode structure <NUM> is a p-i-n diode (with p-doped, intrinsic, and n-doped layers). The bottom layer <NUM> of the diode structure <NUM> is often n-doped and the top layer <NUM> p-doped (as hereinafter assumed for specificity), but the roles of the bottom and top layers <NUM>, <NUM> as n-type and p-type layers may also be reversed. The EAM diode is reverse biased in operation, such that the terminal connected to the n-type layer (cathode) has a larger voltage than the terminal connected to the p-type layer (anode). In the active region <NUM> as shown in <FIG>, a p-type (ground or negative voltage) electrical terminal <NUM> provides the electrical top contact <NUM> with the p-type top layer <NUM> of the diode structure <NUM>, either directly or via an optional p-type contact layer <NUM> disposed on top layer <NUM>, and an n-type (positive or ground voltage) electrical terminal <NUM> establishes one or more lateral electrical contacts <NUM> with the bottom strip <NUM> on either or both sides of the mesa <NUM>, likewise either directly (as shown) or optionally via a thin contact layer (not shown). The electrical terminals <NUM>, <NUM> serve to apply a direct-current (DC) bias voltage and/or an alternating-current (AC), radiofrequency (RF) signal voltage for modulation across the diode. Structurally, the electrical terminals <NUM>, <NUM> may be conductive vias implemented as vertical channels formed in a top cladding enclosing the diode structure <NUM>, filled with a suitable metal (e.g., gold (Au), platinum (Pt), titanium (Ti), aluminum (Al), tungsten (W), titanium nitride (TiN), or zinc (Zn)) or other electrically conductive material.

In operation of the EAM <NUM>, light is coupled from the (silicon) waveguide <NUM> into the intrinsic layer <NUM> of the diode structure <NUM> (which functions as a compound semiconductor waveguide) in a region at or immediately preceding the front <NUM> of the active region <NUM>, and back from the diode structure <NUM> into the waveguide <NUM> at or immediately following the rear <NUM> of the active region <NUM>. Such coupling is achieved by tapering the waveguide <NUM> between a greater width outside the active region <NUM>, shown in <FIG>, and a smaller width inside the active region <NUM>, shown in <FIG>. As conceptually illustrated in <FIG>, in the active region <NUM>, the optical mode <NUM> is carried primarily in the intrinsic layer <NUM>, whereas, outside the active region (where the diode structure is not contacted by electrical terminal <NUM>, <NUM>), the optical mode <NUM> is carried primarily in the waveguide <NUM>.

<FIG> is a simplified conceptual circuit diagram for the EAM of <FIG>. Herein, the diode structure <NUM> is represented by an ideal diode <NUM> with junction capacitance CJ and an ohmic resistor <NUM> with series resistance RS, connected between the positive terminal <NUM> and ground (providing the negative terminal) <NUM>. A driver circuit <NUM> applies the DC bias voltage VDC and the AC modulation signal voltage VAC. <FIG> is an another conceptual circuit diagram, which takes the physical extent of the diode structure <NUM> along the length of the active region <NUM> into account by representing the diode structure <NUM> with three parallel segments <NUM>, <NUM>, <NUM>, each including an ideal diode and resistor in series. A third of junction capacitance CJ and three times the series resistance RS are associated with each of the segments <NUM>, <NUM>, <NUM>, for a total junction capacitance CJ and series resistance RS of the three segments <NUM>, <NUM>, <NUM> together.

<FIG> is a graph of a nonuniform photocurrent density <NUM> as is often associated with EAMs such as the EAM <NUM> shown in <FIG>. The photocurrent density <NUM> is plotted as a function of position along a <NUM> long active region, with <NUM> being the front and <NUM> being the back of the region. Also plotted, for comparison, is a uniform photocurrent density <NUM> corresponding to the average of the nonuniform current density <NUM>. As can be seen, the nonuniform photocurrent density <NUM> in a front portion of the active region (e.g., corresponding to segment <NUM> in <FIG>) is high. As a consequence, the front portion experiences a high voltage drop through the series resistance and, accordingly, a reduced voltage swing over the EAM diode junction. This reduced voltage swing, in turn, results in a reduced optical transmission change in this region, and thus a reduced extinction ratio and optical modulation amplitude. In a back portion of the active region, the photocurrent density is low, causing a low voltage drop through the series resistance, and no impairment to the voltage swing over the EAM diode junction. Consequently, the extinction ratio in the back portion is good and the modulation amplitude high. However, the fraction of the optical power in the back is smaller than in the front, and, therefore, the high optical modulation amplitude in the back does not fully compensate for the degradation in optical modulation amplitude in the front. For a fixed input optical power, the EAM can provide a higher extinction ratio and optical modulation amplitude when the generated photocurrent (corresponding to the absorbed optical power) is uniform along the length of the active region.

In addition to decreasing EAM performance, the nonuniform photocurrent density can also result in excessive heating of the front portion of the active region, e.g., in a single hot spot that can go through thermal runaway, which is a well-known failure mode of EAMs. The risk of overheating limits the maximum optical power that can be coupled into the device. Another detriment of nonuniform photocurrent density is its effect on bandwidth. In general, the EAM bandwidth improves (i.e., increases, facilitating faster signal modulation) as the photocurrent increases, due to a lower microwave impedance. This increase with photocurrent continues until carrier build-up occurs in the quantum wells; once photogenerated carriers cannot be swept out quickly enough, the bandwidth degrades (resulting in slower signal modulation). Thus, the portion of the active region with the highest current density has the highest bandwidth until it saturates from charge build-up, but also the smallest voltage swing due to the series resistance voltage drop discussed above. The portion with the lower current density has a lower bandwidth and a larger voltage swing. A uniform photocurrent density creates a constant bandwidth along the active region, which may provide overall better device bandwidth than sections of high and low bandwidth.

Accordingly, to improve EAM performance in terms of optical modulation amplitude and bandwidth, and to counter the risk of overheating, EAMs are configured, in various embodiments, to achieve a more uniform photocurrent density.

<FIG> is schematic top view of an example EAM <NUM> including, in accordance with a first embodiment, a heater <NUM> at the rear end of the diode structure <NUM>, e.g., immediately following the rear <NUM> of the active region <NUM> (the EAM <NUM> otherwise being similar to the EAM <NUM> of <FIG>). The heater <NUM> improves photocurrent uniformity by exploiting the increase in the optical absorption coefficient of the intrinsic layer <NUM> with increasing temperature. Placing the heater <NUM> at the back of the diode structure <NUM> causes the temperature to be higher in a rear portion of the active region <NUM> than in a front portion, resulting in a higher absorption rate in the rear portion, which counteracts the decrease in optical power from the front <NUM> to the rear <NUM> of the active region <NUM>, and thus keeps the rate with which carriers are generated (corresponding to the photocurrent density) more uniform. In this manner, a heater <NUM> at the back can improve optical modulation amplitude and power handling (see also <FIG> described below).

While it is not uncommon to include a heater with an EAM, conventionally, such a heater serves to stabilize the EAM temperature (i.e., compensate for ambient temperature changes to maintain a constant EAM response despite such changes), and is typically placed laterally next to the center section of the active region. For temperature stabilization, heater placement at the back of the device would be inefficient. In its instant application for evening out the photocurrent density, on the other hand, placement of the heater <NUM> at the back is important to achieve the desired thermal gradient towards the rear <NUM> of the active region, with a concomitant improved optical absorption profile.

<FIG> and <FIG> provide schematic cross-sectional views and a more detailed top view, respectively, of the EAM <NUM> in the region of the heater <NUM> (e.g., a region at the back of the diode structure <NUM>, following the active region <NUM>); collectively, <FIG> illustrate an example heater configuration and associated electrical connections. As shown in <FIG>, the heater <NUM> may include winding heater filaments <NUM> to both sides of the diode mesa <NUM>, each running across a rectangular region disposed, parallel to the substrate, above the bottom strip <NUM> of the diode structure <NUM> and the surrounding device layer <NUM>. The heater filaments <NUM> may be made of a metal or metal alloy such as, e.g., tungsten, platinum, titanium-nitride, nickel-chrome, etc. A heater terminal <NUM> of one (e.g., positive) polarity, shown in <FIG>, contacts the heater filaments <NUM> on one end, farther away from the active region <NUM>; and a heater terminal <NUM> of the other (e.g., negative) polarity, shown in <FIG>, contacts the heater filaments <NUM> at the other end, closer to the active region <NUM>. For reference, the locations of the electrical terminals <NUM>, <NUM> contacting the top and bottom layers <NUM>, <NUM> of the diode structure <NUM> are also indicated. Although the use of two heater filaments <NUM> is beneficial for heating the active region effectively and symmetrically (about the waveguide <NUM>), the heater <NUM> may, in principle, also be implemented with only one heater filament <NUM>.

<FIG> is a conceptual circuit diagram for the EAM of <FIG>. As shown, the EAM <NUM> with heater <NUM> can be represented by two separate electrical circuits: One circuit corresponds to the EAM diode structure <NUM> (here represented, as in <FIG>, by three parallel segments each including an ideal diode and resistor in series) and associated driver circuit <NUM>. The other circuit, which is not electrically connected to the diode circuit, corresponds to the heater <NUM>, represented by an ohmic resistor <NUM>, which is placed in physical proximity to the rear end <NUM> of the active region <NUM> of the diode, and its power supply <NUM>.

<FIG> is a schematic top view of an example EAM <NUM> with multiple bias voltages, in accordance with a second embodiment. In this EAM <NUM>, the metal contacts on the top and bottom layers of the diode structure <NUM> are broken up, along the length of the active region <NUM>, into multiple segments (e.g., as illustrated, three segments) forming (three) separate electrical contact regions <NUM>. The electrical contact regions <NUM> are separated by electrical isolation regions <NUM>. Within each of the contact regions <NUM>, as shown in <FIG> in a schematic cross-sectional view, negative and positive electrical terminals <NUM>, <NUM> contact the top and bottom layers <NUM>, <NUM>, respectively, of the diode structure <NUM> (directly or indirectly, e.g., via top contact layer <NUM>) to apply a bias voltage across the diode structure <NUM>. For an active region <NUM> of a given length, the terminals <NUM>, <NUM> are shorter (in the direction along the length of the active region <NUM>) than the terminals <NUM>, <NUM> of the conventional EAM <NUM>, but, otherwise, they may be similar. In each of the electrical isolation regions <NUM>, as shown in <FIG> in a schematic cross-sectional view, there simply are no electrical terminals establishing contact with the top and bottom layers <NUM>, <NUM> of the diode structure <NUM>. As shown in <FIG>, the EAM <NUM> may, in addition to using multiple DC bias voltages, include a heater <NUM> at the rear.

<FIG> is a conceptual circuit diagram for the EAM <NUM> of <FIG>. The EAM diode structure <NUM> is again represented by three segments, each including an ideal diode and series resistor. However, the segments, corresponding to the three electrical contact regions <NUM>, are now connected to their own respective terminals <NUM>, <NUM>, <NUM>, such that three separate DC bias voltages VDC1, VDC2, VDC3 can be applied to the diode structure in the respective contact regions. The higher the bias voltage, the greater is the absorption coefficient in the diode structure. Therefore, to even out the photocurrent density along the active region, the driver circuit <NUM> applies the highest bias voltage (i.e., largest reverse bias voltage over the EAM diode) to terminal <NUM> contacting the back of the active region, and the lowest bias voltage (i.e., smallest reverse bias voltage over the EAM diode) to terminal <NUM> contacting the front of the active region. The high-speed AC signal voltage can be the same for all contact regions, as shown in <FIG>. Alternatively, different signal levels may be applied to different contact regions, using a distributed, multi-terminal driver.

<FIG> illustrate an example EAM <NUM> with an intrinsic layer <NUM> whose material composition changes along the length of the active region, in accordance with a third embodiment. <FIG> are schematic cross-sectional views taken through a region at the front of the active region, a region in the middle of the active region, and a region at the rear of the active region, respectively. As can be seen, the structure of the EAM <NUM>, including the layered diode structure <NUM>, may be constant along the length of the active region, except for the material that makes up the intrinsic layer <NUM>, which varies between the cross sections (as depicted by different shadings). <FIG> is a conceptual circuit diagram for the EAM <NUM>, again representing the diode structure as three parallel segments <NUM>, <NUM>, <NUM>, and indicating that each segment corresponds to a different layer stack in the diode structure <NUM> (with <FIG> showing the stack for front segment <NUM>, <FIG> showing the stack for middle segment <NUM>, and <FIG> showing the stack for rear segment <NUM>). The materials in the different regions of stacks along the active region are selected such that optical absorption is highest at the back of the active region.

Variability in the intrinsic-layer material can be achieved by changing the elemental composition of the layer <NUM>. For example, a quantum well structure in the intrinsic layer <NUM> may have an elemental composition including gallium (Ga), indium (In), arsenic (As), and phosphorous (P), in proportions specified by the formula GaxIn<NUM>-xAsyP<NUM>-y, where the quantum well absorption, and thus the optical absorption coefficient, changes with x and y. Thus, a gradient in the absorption coefficient can be created by changing x and y along the length of the active region, and photocurrent nonuniformity can be compensated for by locating the highest-absorbing material at the rear. Apart from varying elemental composition, the absorption characteristics of an intrinsic quantum well layer, which is generally structured as a stack of quantum wells alternating with barrier sub-layers, can also be modified by varying the quantum well number in the stack or the quantum well thickness. For example, the EAM III-V material could contain seven <NUM> thick quantum wells at the start of the EAM and <NUM> thick quantum wells at the end of the EAM; with the barrier and separate-confinement-heterostructure layer thicknesses or composition adjusted to minimize modal refractive index changes between these regions to prevent optical reflections. The EAM quantum well thickness may vary, e.g., within the range from <NUM>-<NUM>, depending on material design. Similarly, for bulk quaternary, quantum dash, and quantum dot layers, thickness and/or elemental composition may be changed along the length of the active region to effect a variation in optical absorption. For bulk material, this is done by shifting the GaxIn<NUM>-xAsyP<NUM>-y composition such that the EAM material bandgap moves closer to the operating wavelength towards the back of the EAM (e.g., starting from a bandgap wavelength <NUM> below the operating wavelength at the start of the EAM and moving to <NUM> below the operating wavelength at the end). For quantum dashes and dots, the variation in optical absorption can be achieved by increasing the thickness or density of the dash and dot layer, and modifying the composition of surrounding layers or reducing the surrounding layer thicknesses to keep the modal refractive index roughly constant.

A diode structure <NUM> with an intrinsic layer whose material properties vary along one dimension can be made by bonding a single epi piece (i.e., piece of epitaxially grown layers, e.g., singulated or cleaved from a compound semiconductor wafer with varying compound-semiconductor (e.g., III-V) material composition to the substrate <NUM> (followed by patterning the epi piece to create the mesa). The material variation in the epi piece is achieved by varying material properties across a wafer during material growth prior to singulation into epi pieces.

<FIG> is a sequence of wafer top views illustrating steps of manufacturing EAM diodes <NUM> with varying material composition in accordance with the third embodiment. The first step in manufacturing is to (epitaxially) grow a III-V (or other semiconductor compound) wafer <NUM> with an active region of type <NUM>. The wafer <NUM> is patterned using photoresist and lithography (a dielectric deposition before the photoresist and etch after lithography may optionally be added) and etched to create channels <NUM> in the surface of the wafer, removing the type <NUM> material The photoresist is then removed, dielectric is deposited over the wafer, photoresist is reapplied and lithography is repeated to redefine equal or slightly larger channels <NUM> over the type <NUM> areas. Note that this step may be combined with the preceding step if a dielectric mask is used during that step; the decision is typically based on wafer cleaning requirements prior to regrowth. Active regions of type <NUM> are (epitaxially) regrown in the exposed regions of the channels and the dielectric mask is removed, resulting in a wafer <NUM> that includes two types of active regions. The patterning and etch of the type <NUM> material adjacent to the type <NUM> material, and application of dielectric and the regrowing of an active region are then repeated for type <NUM>, which achieves a periodic variation across the wafer <NUM> between three materials. The finished wafer may optionally go through chemical mechanical polishing (CMP) to remove any height variation, especially at the interfaces between material types, which can interfere with bonding. The completed wafer may be planar with height variation on the surface of less than <NUM> root-mean-square over each region to be bonded. The wafer can now be singulated, as shown at <NUM>, into III-V epi pieces <NUM> all characterized by a stepwise variation in its materials along one direction. These epi pieces <NUM> are then bonded to a patterned SOI wafer <NUM>, which can be further processed to create an EAM diode structure in each of the bonded epi pieces. The resulting SOI wafer <NUM> can be diced into multiple EAM devices. While <FIG> illustrates how EAMs with three different material regions are created, it will be evident to those skilled in the art that the process can be extended to any number of different material regions. In practice, EAMs with only a few (e.g., two or three or four) different intrinsic-layer regions along the EAM generally suffice to achieve photocurrent uniformity in accordance with this disclosure, and finer gradations may, accordingly result in unnecessary cost.

The foregoing approaches all achieve, by various means, an increase of the optical absorption towards the rear of the EAM, counteracting the decrease in optical power in that direction. In the following, an alternative concept, in which light is coupled continuously from the silicon waveguide into the intrinsic layer of the diode over a portion of the EAM, is described.

<FIG> is a schematic top view of an example EAM <NUM> having its active region <NUM> overlapping with a tapered section of the (silicon) waveguide <NUM> coupling light into the EAM diode, in accordance with a fourth embodiment. Ordinarily, the (silicon) waveguide of an EAM (such as EAMs <NUM>, <NUM>, <NUM>, <NUM>) tapers down (that is, decreases in width) in a section overlapping with the diode mesa (compound semiconductor waveguide), but preceding the active region <NUM>, where modulation occurs. Thus, at the front end <NUM> of the active region <NUM>, the optical mode is usually carried predominantly in the intrinsic layer of the compound semiconductor waveguide, with only minimal if any light remaining in the (silicon) waveguide. In the EAM <NUM> depicted in <FIG>, by contrast, the down-tapered section <NUM> of the waveguide significantly overlaps with the active region <NUM> (e.g., by at least <NUM>% the length of the active region). As a result, as shown in the cross-section of <FIG>, the waveguide <NUM> still carries an optical mode <NUM> and at least <NUM>% of the total optical power as it reaches the start <NUM> of the active region <NUM>. As the waveguide further decreases in width in the direction of light propagation, this optical mode <NUM> is gradually coupled into the intrinsic layer <NUM> of the diode structure <NUM>, replenishing the optical power of the optical mode <NUM> that is guided in the intrinsic layer <NUM> and absorbed along the way to create photocurrent. With this configuration, the photocurrent density at the front of the active region <NUM> is reduced (compared with a configuration in which all light has been coupled into the intrinsic region <NUM> at the front of the active region <NUM>), for the benefit of increased photocurrent density at locations closer towards the rear, which tends to even out the photocurrent density along the length of the device. The length of the overlap between the down-tapered section <NUM> of the waveguide <NUM> and the active region <NUM>, and the rate at which the width of the waveguide <NUM> decreases along that length, may be optimized for the best achievable photocurrent uniformity. For example, in one embodiment, as illustrated, the waveguide <NUM> is designed such that, at or near the middle <NUM> of the active region <NUM>, all light has been coupled into the intrinsic layer <NUM>, and the down taper of the (silicon) waveguide <NUM> may end there. In the rear portion of the active region <NUM>, therefore, the optical mode <NUM> is entirely in the intrinsic layer <NUM>, as shown in the cross-sectional view of <FIG>.

Having described various structural embodiments of EAM devices designed for a more uniform photocurrent density (e.g., devices <NUM>, <NUM>, <NUM>, <NUM>), methods <NUM> of calibrating and operating such devices will now be described with reference to the flow chart shown in <FIG>. The calibration generally begins with setting the DC bias of the EAM to 0V, and turning on the laser to operating power (<NUM>). The insertion loss of the EAM (which is <NUM> minus the fraction of the optical input power that reaches the output) can then be measured, e.g., using optical taps before and after the EAM, and the DC bias voltage can be adjusted until the target insertion loss is reached (<NUM>). The subsequent procedure then serves to set the device to the maximum optical modulation amplitude at the set fixed optical loss (where the fixed optical loss can be easily measured through pre- and post-EAM taps, without the RF voltage swing turned on). Alternatively, with the RF voltage swing turned on, the following procedure can serve to set the device to the maximum optical modulation amplitude at a set eye crossing value. The DC bias voltage may be adjusted until <NUM>% or a target eye crossing is reached (<NUM>), where the eye crossing is defined as, <MAT>, and Poptical at crossing is the optical power on an eye diagram where the rising <NUM>-to-<NUM> crosses the falling <NUM>-to-<NUM> level optical pattern on an oscilloscope. The eye crossing may be measured on a benchtop oscilloscope tool such as a digital communication analyzer (DCA), or through an eye monitor circuit built into a post-EAM tap. For digital communication, an eye crossing between <NUM>-<NUM>% typically provides the highest bit-error-rate for a fixed RF swing voltage and optical power. The fixed insertion loss method requires less measurement time and is roughly correlated to eye crossing; thus, it can provide much faster modulator calibration, but it is less accurate for a specific eye crossing value.

Depending on the approach taken to achieve greater uniformity in the photocurrent density, different calibration sequences may then be performed to determine the operating setpoint of the EAM. For an EAM <NUM> with a heater at the rear (branch <NUM>), the heater power is increased by small increments, and the bias voltage is concomitantly decreased to maintain the fixed target insertion loss (<NUM>). An RF voltage swing (i.e., AC signal voltage) is then applied to the EAM <NUM>, and the optical modulation amplitude of the output optical signal is measured (<NUM>). This process (<NUM>, <NUM>) is repeated across the full heater power range (<NUM>). In the end, the heater power and corresponding DC bias voltage are set to their values at which the optical modulation amplitude is maximized, and the EAM <NUM> is operated at that setpoint (<NUM>). Similarly, for an EAM <NUM> with segmented electrical terminals to apply multiple bias voltages along the length of the device (branch <NUM>), the DC bias voltage at the rear end of the active region is increased in small steps as the DC bias voltage at the front of the active region is decreased to maintain the set optical insertion loss (<NUM>). An RF voltage swing is then applied across the EAM <NUM>, and the optical modulation amplitude of the output optical signal is measured (<NUM>). This process (<NUM>, <NUM>) is repeated across the full DC voltage sweep range (which may be, e.g., 1V) (<NUM>). The DC bias voltages are then set to the combination of values achieving the maximum optical modulation amplitude, which serves as the setpoint for operating the EAM <NUM> (<NUM>). For EAMs <NUM>, <NUM> that achieve uniform photocurrent density based on their fixed design features (material variation along the active region, or silicon waveguide overlap with the active region) (branch <NUM>), no further calibration is to be performed, and the EAM <NUM>, <NUM> is simply operated (<NUM>).

The various embodiments described above with respect to <FIG> all tend to increase the photocurrent uniformity along the EAM, as compared with a conventional EAM as illustrated in <FIG>, and thereby improve the performance and operating life of the EAM. The photocurrent uniformity in accordance with various embodiments may be characterized, e.g., by a minimum photocurrent linear density along the active region that is no less than <NUM>%, preferably no less than <NUM>%, or even no less than <NUM>% of the maximum photocurrent density. <FIG> illustrate the improvement for an EAM <NUM> with integrated heater at the rear, operated with <NUM> mW optical input power and a drive swing voltage of 2V. The thermal impedance of the EAM <NUM> tested was about <NUM>/W.

<FIG> provides, for comparison, a graph of the temperature <NUM> and normalized photocurrent density <NUM> along an example conventional EAM <NUM> with a <NUM> long active region. As can be seen, the temperature <NUM> is highest at the front of the EAM, with about <NUM>, and decreases towards the rear to <NUM>. The photocurrent density <NUM> likewise decreases along the device, dropping at the rear end to about <NUM>% of its value at the front of the active region.

<FIG> is a graph of the temperature <NUM> and normalized photocurrent density <NUM> along an example EAM <NUM> with a heater at the rear end. Here, the temperature profile is reversed, i.e., the temperature <NUM> increases in the direction of light propagation, from about <NUM> at the front of the active region to about <NUM> at the back. The photocurrent density <NUM> is, in this case, relatively uniform across the device, having a value at the rear that is about <NUM>% of the photocurrent density at the front. Similar improvements may be expected for EAMs with multiple DC bias voltages, properly varied material properties, and/or waveguide taper overlap with the active region. Multiple different approaches to achieving photocurrent uniformity may also be used in conjunction to augment each other in their effect on the photocurrent density along the device.

Thus, from one perspective, there have now been disclosed integrated electro-absorption modulators (EAM) that are structured and/or operated to improve uniformity of the photocurrent density along the active region. In various embodiments, this improvement results from increased optical absorption at the rear of the EAM, e.g., as achieved by heating a region at the rear, increasing a bias voltage applied across the EAM towards the rear, or changing a material composition of an intrinsic layer towards the rear. In another embodiment, the improvement is achieved by coupling light from a waveguide into the EAM active region continuously along a length of the EAM, using overlap between a tapered section of the waveguide and the EAM.

Claim 1:
An integrated electro-absorption modulator (<NUM>) comprising:
a layered diode structure (<NUM>) formed above a device layer of a substrate, the diode structure comprising a bottom diode layer, an intrinsic diode layer, and a top diode layer;
a waveguide (<NUM>) formed in the device layer underneath the layered diode structure to couple light in and out of the intrinsic diode layer;
electrical terminals (<NUM>, <NUM>) contacting the top and bottom diode layers in an active region (<NUM>);
a driver circuit (<NUM>) connected between the electrical terminals and configured to apply a direct-current bias voltage and an alternating-current signal voltage across the layered diode structure; and
a heater (<NUM>) disposed at a rear end of the electro-absorption modulator to heat a rear portion of the active region, causing a temperature in the rear portion of the active region to be higher than a temperature in a front portion of the active region.