Externally modulated laser device

Abstract

In some examples, an externally modulated laser device comprises a laser emitter for generating a laser signal, comprising a first doped layer, a first intrinsic layer disposed on the first doped layer and a second doped layer disposed on the first intrinsic layer, wherein the laser signal propagates within the laser emitter through the first intrinsic layer, an optical modulator connected with the laser emitter, the optical modulator comprising a third doped layer, a fourth doped layer, and a second intrinsic layer disposed between the third doped layer and the fourth doped layer, wherein the third doped layer, the second intrinsic layer and the fourth doped layer are arranged laterally along a plane parallel to the first doped layer, such that the laser signal propagates within the optical modulator through the second intrinsic layer.

Description

[0001] EXTERNALLY MODULATED LASER DEVICE

[0002] TECHNICAL FIELD

[0003] The present disclosure relates, in general, to an externally modulated laser device. Aspects of the disclosure relate to a hybrid junction externally modulated laser.

[0004] BACKGROUND

[0005] The optical transmission capacity and the number of components integrated into a single optical transmitter chip have been rapidly increasing, driven by the growing demands of data centres and artificial intelligence (Al) technologies. Indium phosphide (InP)-based large-scale photonic integrated circuits (PICs) offer substantial advantages, including significant reductions in device size, packaging complexity, fibre coupling loss, and power consumption. Consequently, InP-based PICs have become promising candidates for use in optical transmitters and receivers. For short-reach interconnects within data centres and Al systems, intensity -modulation direct-detection (IM / DD) systems are particularly attractive due to their simpler architecture and reduced power consumption. Operating in the O-band is generally preferred for these applications, as it offers better fibre dispersion characteristics.

[0006] Electro-absorption modulated lasers (EMLs), which consist of a distributed feedback laser (DFB) and an electro-absorption modulator (EAM), are well suited for short-reach optical interconnect applications. These devices offer a compact footprint, high bandwidth (BW), high extinction ratio (ER), and low modulation voltage swing (Vpp), potentially allowing direct drive from the optical digital signal processor. However, the increasing requirements for higher Baud rate optical transmitters, driven by the recent rapid expansion of data centre and Al applications, present several challenges.

[0007] In conventional vertical PIN junction EMLs, the EAM operates based on the quantum confinement Stark effect. Here, the term “PIN junction” refers to a type of semiconductor structure consisting of three layers: a p-type (positively charged doped) layer, an intrinsic (undoped) layer, and an n-type (negatively charged doped) layer, arranged in sequence. P-type semiconductors are formed by introducing an electron acceptor element into an intrinsic semiconductor during the manufacturing process. The designation "p-type" reflects the predominance of positive charge carriers, known as holes. In contrast to n-type semiconductors, p-type materials have a higher concentration of holes than electrons. N-type semiconductors are created by doping an intrinsic semiconductor with an electron donor element. The term "n-type" arises from the negative charge of the electrons. In n-type semiconductors, electrons are the majority carriers, while holes are the minority carriers.

[0008] As the demand for higher ER, lower modulation voltage swing (Vpp), and greater BW intensifies, it becomes increasingly difficult to achieve an optimal balance among these parameters. For instance, achieving a higher ER often necessitates a longer device, higher Vpp, or increased optical confinement, each of which has trade-offs. Reducing the driving voltage typically requires a thinner intrinsic layer or an extended modulator length, but these modifications can lead to increased capacitance, ultimately resulting in a reduction in bandwidth, resulting in lower baud rate.

[0009] Lateral junction EMLs have been developed to address some of these challenges, where both the laser and modulator utilise lateral PIN junction structures. In these designs, p- and n-doping are achieved through ion implantation, diffusion, overgrowth, or a combination of these techniques. However, precise control of doping profiles in lateral PIN junctions is inherently more complex than in vertical PIN junctions. This complexity often results in less satisfactory laser performance and reliability compared to vertical PIN junction lasers. Consequently, the parametric performance of lateral PIN junction lasers, such as output power, linewidth, and efficiency, may not match that of their vertical PIN junction counterparts.

[0010] The difficulties associated with both vertical and lateral junction EMLs highlight the critical issues in optimising these devices for high-performance optical transmitters. Vertical PIN junctions, while offering superior control over doping profiles and better laser performance, struggle to balance high ER, low Vpp, and wide bandwidth. Conversely, lateral PIN junction designs, although potentially offering simpler integration and fabrication processes, suffer from less precise control over doping, leading to compromised device performance and reliability.

[0011] SUMMARY

[0012] An objective of the present disclosure is to provide an externally modulated laser addressing the issues present in the currently available solutions.

[0013] The foregoing and other objectives are achieved by the features of the independent claims.

[0014] Further implementation forms are apparent from the dependent claims, the description and the Figures.

[0015] A first aspect of the present disclosure provides an externally modulated laser device, comprising a laser emitter for generating a laser signal, comprising a first doped layer, a first intrinsic layer disposed on the first doped layer and a second doped layer disposed on the first intrinsic layer, wherein the laser signal propagates within the laser emitter through the first intrinsic layer, an optical modulator connected with the laser emitter, the optical modulator comprising a third doped layer, a fourth doped layer, and a second intrinsic layer disposed between the third doped layer and the fourth doped layer, wherein the third doped layer, the second intrinsic layer and the fourth doped layer are arranged laterally along a plane parallel to the first doped layer, such that the laser signal propagates within the optical modulator through the second intrinsic layer.

[0016] Accordingly, a mechanism to combine the advantages of a vertical PIN junction DFB laser with a lateral PIN junction electroabsorption modulator (EAM) to form a hybrid junction electro-absorption modulated laser can be provided. Advantageously, the invention leverages the established benefits of the vertical PIN junction DFB laser, such as high output power and wide temperature performance, due to the mature epitaxy growth, doping control, and reliability. By integrating a lateral PIN junction EAM with the vertical PIN junction DFB laser, the proposed hybrid PIN junction EML can achieve a low driving voltage and higher bandwidth, thereby targeting applications with high data rates without compromising the extinction ratio. In particular, the lower Vpp enables the optical digital signal processor (oDSP) to directly drive the modulator at high Baud rates, reducing the demands on the oDSP and eliminating the need for an additional driver.

[0017] The externally modulated laser device may further comprise a substrate layer on which the laser emitter and the optical modulator are arranged. The first doped layer may be embedded in the substrate layer.

[0018] The third doped layer, the second intrinsic layer and the fourth doped layer may be disposed over the substrate layer.

[0019] The optical modulator may further comprise a first intermediate layer, wherein the first intermediate layer is arranged below the third doped layer and the fourth doped layer, and wherein the first intermediate layer comprises a first semiconductor.

[0020] The substrate layer may comprise a p-doped substrate, an n-doped substrate, or a semi-insulated substrate.

[0021] The externally modulated laser device may further comprise a transition section arranged between the laser emitter and the optical modulator on the substrate layer, wherein the transition section comprises a waveguide structure arranged to facilitate an optical mode transition from the laser emitter to the optical modulator. The transition section may a fifth doped layer, a third intrinsic layer, a second intermediate layer, and a sixth doped layer, wherein the third intrinsic layer and the sixth doped layer are stacked vertically on top of the fifth doped layer, the fifth doped layer may be embedded in the substrate layer.

[0022] The laser emitter may further comprise a first metal contact arranged below the first doped layer and a second metal contact arranged above the second doped layer.

[0023] A doping of the first doped layer may comprise a doping in an opposite sense to a doping of the second doped layer, wherein a doping of the third doped layer comprises a doping in an opposite sense to a doping of the fourth doped layer.

[0024] The optical modulator may further comprise a top layer disposed over the second intrinsic layer, wherein the top layer comprises at least one of a second semiconductor and a dielectric.

[0025] The first intrinsic layer and the second intrinsic layer may each comprise multiple quantum wells.

[0026] The externally modulated laser device may further comprise a semiconductor optical amplifier (SOA) integrated with the externally modulated laser device.

[0027] The laser emitter may comprise distributed feedback (DFB) laser, a Fabry-Perot (FP) laser, or a distributed Bragg reflector (DBR) laser, wherein the optical modulator comprises a Mach-Zehnder modulator.

[0028] A second aspect of the present disclosure provides a method of fabricating an externally modulated laser device, the method comprising providing a laser emitter for generating a laser signal, comprising a first doped layer, a first intrinsic layer disposed on the first doped layer, and a second doped layer disposed on the first intrinsic layer, wherein the laser signal propagates within the laser emitter through the first intrinsic layer; and providing an optical modulator connected with the laser, the optical modulator comprising a third doped layer, a fourth doped layer, and a second intrinsic layer disposed between the third doped layer and the fourth doped layer, wherein the third doped layer, the second intrinsic layer, and the fourth doped layer are arranged laterally along a plane parallel to the first doped layer, such that the laser signal propagates within the optical modulator through the second intrinsic layer.

[0029] Providing the laser emitter and the optical modulator may comprise providing a substrate layer on which the laser and the optical modulator are to be formed, the method further comprising: forming, on the substrate layer, a transition section comprising a waveguide structure.

[0030] These and other aspects of the invention will be apparent from the embodiment(s) described below.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0033] Figure 1 is a schematic representation of an externally modulated laser device according to an example;

[0034] Figure 2a is a cross-section representation of a laser emitter according to an example;

[0035] Figure 2b is a cross-section representation of a laser emitter according to another example;

[0036] Figure 3 is a cross-section representation of a transition section according to an example;

[0037] Figure 4 is a cross-section representation of an optical modulator according to an example; and Figure 5 is a flow chart of a method of fabricating an externally modulated laser device according to an example.

[0038] DETAILED DESCRIPTION

[0039] Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein.

[0040] Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.

[0041] The terminology used herein to describe embodiments is not intended to limit the scope. The articles “a,” “an,” and ‘The” are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and / or “including,” when used herein, specify the presence of stated features, items, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and / or groups thereof.

[0042] Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein.

[0043] Electro-absorption modulated lasers (EML) have traditionally employed vertically grown PIN junctions, where the layers are deposited sequentially using epitaxial techniques such as metal-organic chemical vapour or molecular beam epitaxy. A PIN junction is a type of semiconductor structure consisting of three layers: a p-type (positively doped) layer, an intrinsic (undoped) layer, and an n-type (negatively doped) layer, arranged in sequence. The intrinsic layer, sandwiched between the p-type and n- type layers, plays a critical role by increasing the depletion region, which enhances the device's ability to control the flow of electric current and its response to external electric fields.

[0044] The vertical PIN configuration offers excellent compatibility with epitaxial growth processes, allowing for precise control over p- and n-type doping. Commonly, the electro-absorption modulator (EAM) in these devices utilises a lumped electrode structure, with bandwidth limitations primarily governed by capacitance. Achieving higher bandwidth requires reducing capacitance; however, this introduces trade-offs in device design. Furthermore, the voltage swing from optical digital signal processors to the EAM is typically constrained to less than 1.5 V, while an extinction ratio of over 4.0 dB is generally required. For a vertical PIN EAM, a 3 dB bandwidth exceeding 50 GHz is standard, with a typical EAM waveguide length of approximately 100 pm and an intrinsic layer thickness of around 0.2 pm.

[0045] For semiconductor lasers, vertically grown PIN structures have been the standard for over fifty years, with stringent requirements for p- and n-doping control to ensure optimal laser performance and reliability. Accurate positioning of the doping layers is critical, particularly for DFB lasers operating under forward bias, where the tolerance for precise doping positions is typically within ±10 nm for optimal device performance. In contrast, EAMs, which operate under reverse bias, generally have more relaxed requirements regarding doping precision compared to DFB lasers. Lateral PIN junctions have been explored since the mid-1980s and have gained renewed interest due to their potential for lower parasitic capacitance and the integration of electronic and photonic components on aflat surface. Lateral junction EAMs, which rely on excitonic peak broadening effects under reverse bias, offer advantages such as reduced voltage swing, higher optical confinement factors, and the possibility of achieving negative chirp. The separation between the p- and n-doped regions in lateral structures can range from approximately 0.5 to 2 pm, resulting in significantly lower capacitance compared to vertical PIN junctions. This combination of reduced capacitance, lower voltage swing, and higher confinement can lead to modulators with substantially increased bandwidth while maintaining the required extinction ratio.

[0046] However, vertically grown PIN junctions in EMLs, which rely on the quantum Confined stark effect within the EAM, face increasing challenges in balancing key performance parameters such as high ER, low voltage swing, and wide bandwidth. For example, achieving a higher ER often necessitates either a longer modulator length, a higher driving voltage, or increased optical confinement, each of which has associated drawbacks. Similarly, reducing the driving voltage usually requires a thinner intrinsic layer or a longer modulator length, both of which can increase capacitance and thereby decrease the bandwidth.

[0047] In contrast, lateral junction EMLs utilise lateral structures for both the laser and modulator, with p- and n-doping achieved through techniques such as ion implantation, diffusion, overgrowth, or combinations thereof. However, achieving precise control over doping profiles in lateral junctions is inherently more complex compared to vertical PIN junctions, often leading to suboptimal laser performance and reliability. Consequently, the performance metrics of lateral junction lasers, including output power, linewidth, and efficiency, generally do not match those of their vertical junction counterparts.

[0048] According to an example, there is provided a hybrid junction electro-absorption modulated laser, integrating the advantages of both vertical and lateral junction technologies. More specifically, aspects relate to combining the benefits of a vertical laser emitter, such as high output power, reliable performance across a wide temperature range, and well-established epitaxy growth and doping control, with the advantages of a lateral junction optical modulator. For example, the integration allows for achieving a high bandwidth (potentially exceeding 100 GHz 3 dB bandwidth) suitable for high-speed data transmission applications (e.g., >400 Gbps per channel) while maintaining a low driving voltage. The lower driving voltage also facilitates direct drive from the optical digital signal processor at high Baud rates, thereby reducing the need for additional driver circuitry and simplifying the overall system design.

[0049] Examples in the present disclosure can be provided as methods, systems or machine-readable instructions, such as any combination of software, hardware, firmware or the like. Such machine-readable instructions may be included on a computer readable storage medium (including but not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0050] The present disclosure is described with reference to flow charts and / or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. In some examples, some blocks of the flow diagrams may not be necessary and / or additional blocks may be added. It shall be understood that each flow and / or block in the flow charts and / or block diagrams, as well as combinations of the flows and / or diagrams in the flow charts and / or block diagrams can be realized by machine readable instructions.

[0051] Figure 1 is a schematic representation of an externally modulated laser device according to an example. The externally modulated laser device 100 comprises a laser emitter 101 for generating a laser signal. The laser emitter 101 may comprise, for example, a DFB laser, a FP laser or a DBR laser, depending on the application. The externally modulated laser device 100 also comprises an optical emitter 103 connected with the laser emitter 101. The optical emitter 103 may comprise, for example, a Mach-Zehnder modulator (MZI).

[0052] Figure 2a is a cross-section representation of a laser emitter according to an example. The laser emitter 101 may be fabricated (i.e., grown) using metal-organic chemical vapour deposition (MOCVD) or molecular beam epitaxy (MBE) on a substrate 104. MOCVD may the chemical reaction of metal-organic precursors in a gas phase to deposit thin layers, while MBE may use the deposition of evaporated materials in a high- vacuum environment to form the layers. While these two methods are mentioned, the skilled person would readily appreciate that any suitable method for providing a laser emitter may be used, and that the invention is not limited thereto. In the example depicted in Figure 2a, the substrate 104 may comprise an n-doped indium phosphide (InP) substrate 104. However, the substrate 104 may comprise any p-doped substrate, an n-doped substrate or a semi-insulated substrate.

[0053] The laser emitter 101 may comprise a vertical PIN junction. The PIN junction is a type of semiconductor structure consisting of three layers: a p-type (positively doped) layer, an intrinsic (undoped) layer, and an n-type (negatively doped) layer, arranged in sequence. As such, the laser emitter 101 comprises a first doped layer 111, a first intrinsic layer 112 disposed on (over) the first doped layer 111, and a second doped layer 113 disposed on the first intrinsic layer 112. The first doped layer 111 may serve as the p-type layer, while the second doped layer 113 may serve as the n-type layer.

[0054] The laser signal propagates within the laser emitter 101 mainly through the first intrinsic layer 112. In other words, the laser signal transmits primarily through the first intrinsic layer 112 of the laser emitter 101, with some leakage into the first doped layer 111 and the second doped layer 113. Considering the drawing, the laser signal transmits ‘into’ the first intrinsic layer 112 of Figure 2a, or through (from the side of) the laser emitter 101 of Figure 1. Specifically, when examining the schematic representation, the laser signal transmits "into" the intrinsic layer 112 of Figure 2a, and this is effectively illustrated as moving through the laser emitter 101 from the side in Figure 1.

[0055] In other words, the laser signal predominantly propagates within the laser emitter 101 from the first doped layer 111, through the first intrinsic layer 112, and to the second doped layer 113. In this vertical configuration, the laser signal primarily propagates through the first intrinsic layer 112, which is situated between the two doped layers. This intrinsic layer is crucial for the generation of light, as it is here that the electrons and holes recombine to emit photons. However, the transmission of the laser signal is not strictly confined to the intrinsic layer, and also extends into the adjacent doped layers. This means that, while the primary pathway for current is through the intrinsic layer, there may be some leakage of current into both the first doped layer 111 and the second doped layer 113.

[0056] The doping of the first doped layer 111 may comprise a doping in an opposite sense to a doping of the second doped layer 113. For example, the first doped layer 111 may comprise an n-doped material, while the second doped layer 113 may comprise a p-doped material, or vice versa. The first doped layer 111 may comprise the substrate 104 - that is, the substrate 104 may be used to form the first doped layer 111. As mentioned above, in the example of Figure 2a, the substrate comprises an n-doped InP substrate. As such, the second doped layer 113 may comprise a p-doped material.

[0057] In the vertical PIN junction comprising the three layers 111-113, the first doped layer 111 may serve as the top layer, which is positively doped to create an excess of holes (positive charge carriers). The undoped intrinsic layer 112, located beneath the first doped layer 111, may provide a region where charge carriers can recombine to produce light. The second doped layer 113, positioned at the bottom, may be negatively doped to create an excess of electrons (negative charge carriers). When a voltage is applied across the junction, current flows vertically through the device from the first doped layer 111 to the second doped layer 113. This flow of current may create an electric field within the intrinsic layer 112, causing the electrons and holes to recombine and emit light. The light then propagates through the laser emitter 101, through the intrinsic layer 112.

[0058] The first intrinsic layer 112 may comprise multiple quantum wells, which are thin semiconductor layers separated by barriers. These quantum wells are designed to confine charge carriers in the direction perpendicular to the layers, enhancing the material's optical and electronic properties. Through the presence of the quantum wells in the first intrinsic layer 112, the efficiency of the emission process can be improved.

[0059] The laser emitter 101 may further comprise a top metal contact 115 and a bottom metal contact 114, enabling electrical connections to the laser structure. The top metal contact 115 may allow for the application of electrical current to the active region of the laser emitter 101, while the bottom metal contact 114 may provide a ground connection. The top metal contact 115 and the bottom metal contact 114 may comprise a conductive material such as gold or aluminium.

[0060] Figure 2b is a cross-section representation of the laser emitter 101 according to another example. In this example, the substrate 104 may comprise a semi- insulated substrate. In this case, the substrate 104 may not be suitable to serve as the first doped layer 111 due to its semi-insulating properties, which prevent it from being electrically conductive. Instead, an additional n-doped layer 160 may be included. This n-doped layer 160 may consist of n-type indium phosphide (n-InP) or other n-doped materials.

[0061] As such, the vertical PIN junction of the laser emitter 101 according to this second example may be formed using the n-doped layer 160, the first intrinsic layer 112, and a second doped layer 113, where the second doped layer 113 may comprise a p- doped material. In this configuration, instead of using a single top metal contact 115, a top p-metal contact 115 and a top n- metal contact 170 may be used. This is because separate metal contacts are needed to connect to the distinct p-doped and n- doped layers.

[0062] The externally modulated laser device 100 may comprise a transition section 102 arranged between the laser emitter 101 and the optical modulator 103. The transition section 102 is depicted in Figure 3. The transition section 102 may also be arranged on the substrate 104, same as the laser emitter 101. Advantageously, all the elements of the externally modulated laser device 100 may be arranged / grown on the same substrate. As such, the transition section 102 may comprise a p-doped substrate, an n-doped substrate or a semi-insulated substrate. The exact choice of substrate is dependent on design and manufacturing choices / constraints.

[0063] The transition section 102 may comprise a waveguide to manage the optical signal as it moves from the laser emitter to the optical modulator. For example, overgrown p-InP material may be processed into a taper waveguide structure, which gradually adjusts the waveguide’s dimensions to match those of the optical modulator 103. In other words, the transition section 102 may help in aligning the spatial distribution of the optical field emitted by the laser 101 with that of the modulator 103, reducing mode mismatch losses and ensuring efficient coupling of light between the two components.

[0064] The transition section 102 may further comprise a series of layers: a fifth doped layer 121, a third intrinsic layer 122, a second intermediate layer 124 and a sixth doped layer 123. The third intrinsic layer 122 and the sixth doped layer 123 may be stacked vertically on top of one another. The intrinsic layer 122 may comprise multiple quantum wells, aiding optical confinement and support effective light transfer. Similarly to the laser emitter 101, the fifth doped layer 121 may comprise the substrate 104: that is, the substrate 104 may be considered to form the fifth doped layer 121. Alternatively, the fifth doped layer 121 may be embedded within the substrate. Figure 4 is a cross- section representation of an optical modulator according to an example. The optical modulator 103 comprises a third doped layer 131 , a fourth doped layer 133 and a second intrinsic layer 132 disposed between the third doped layer 131 and the fourth doped layer 133. In the optical modulator 103, the third doped layer 131, the second intrinsic layer 132, and the fourth doped layer 133 are arranged laterally along a horizontal plane that is parallel to the first doped layer 111 of the laser emitter 101. This lateral arrangement may facilitate horizontal propagation of the laser signal through the optical modulator 103. Specifically, the laser signal may propagate primarily through the second intrinsic layer 132, which is the undoped layer situated between the third doped layer 131 and the fourth doped layer 133.

[0065] The layers of the optical modulator 103 may be disposed on the substrate 104. That is, the third doped layer 131, the second intrinsic layer 132 and the fourth doped layer 133 may be disposed over the substrate layer 104. Advantageously, it is the same substrate as the substrate on which the transition section 102 and the laser emitter 101 are arranged / grown.

[0066] In contrast to the laser emitter 101, the optical modulator 103 comprises a lateral PIN junction. Unlike the vertical PIN junction of the laser emitter 101, the lateral PIN junction in the optical modulator 103 arranges its layers in a horizontal plane. Specifically, the optical modulator 103 comprises a third doped layer 131, a second intrinsic layer 132, and a fourth doped layer 133. As mentioned above, these layers are disposed laterally. The laser signal propagates mainly through the second intrinsic layer 132. While the primary path of the laser signal is through the second intrinsic layer 132, there may be some leakage of current into the adjacent third doped layer 131 and the fourth doped layer 133. However, the main modulation effect takes place within the second intrinsic layer 132.

[0067] In this configuration, the third doped layer 131 may act as the p-type layer, providing a region with an excess of holes, while the fourth doped layer 133 may function as the n-type layer, providing an excess of electrons. The second intrinsic layer 132, positioned between the third doped layer 131 and the fourth doped layer 133, is undoped and serves as the region where the optical signal is being modulated. Alternatively, the third doped layer 131 may act as the n-type layer, while the fourth doped layer 133 may act as the p-type layer. Importantly, the doping of the third doped layer 131 may comprise a doping in an opposite sense to a doping of the fourth doped layer 133. That is, if the third doped layer 131 comprises a p-type layer, then the fourth doped layer 133 will comprise a n-type layer. Conversely, if the third doped layer 131 comprises an n-type layer, the fourth doped layer 133 will comprise a p-type layer.

[0068] The lateral arrangement of these layers means that photon current flows horizontally through the device. When a voltage is applied across the junction, an electric field is created within the second intrinsic layer 132, along with the built-in field of the PIN structure. The built-in field is an inherent electric field in the intrinsic region (i.e., the second intrinsic layer 132) of the PIN junction, formed due to the difference in charge between the p-type and n-type regions. The voltage swing modulates the optical signal passing through the device. The lateral PIN junction design can facilitate efficient modulation with potentially lower capacitance and a higher bandwidth compared to vertical configurations.

[0069] The optical modulator 103 may further include a first intermediate layer 134 positioned beneath the third doped layer 131 and the fourth doped layer 133. This intermediate layer 134 may be composed of a semiconductor material. The inclusion of semiinsulated InP layer (Sl-InP) in the first intermediate layer 134 can serve multiple functions. For instance, it may enhance the electrical and optical performance of the modulator 103 by providing a stable and well-defined interface between the doped layers and the underlying substrate. Additionally, it can contribute to the uniform distribution of the electric field across the device, which in turn improves the efficiency of the modulation process and minimises signal distortion.

[0070] In addition, the optical modulator 103 may comprise a top layer 135 situated above the second intrinsic layer 132. The top layer 135 may consist of a semiconductor material, such as indium phosphide (InP), or alternatively, it may be composed of a dielectric material and the semiconductor material. This top layer 135 may serve to protect the intrinsic layer 132 and the underlying structures from environmental factors and mechanical stresses. Furthermore, the top layer 135 can contribute to optical confinement, enhancing the interaction between the modulated signal and the modulator 103. The optical modulator 103 may also comprise a top-p metal contact 136-1 (arranged above the p-type layer) and a top-n metal contact 136-2 (arranged above the n-type layer), providing the electrical connections to the respective layers.

[0071] Figure 5 is a flow chart of a method of fabricating an externally modulated laser device according to an example. The method comprises, in block 501, providing a laser emitter comprising a first doped layer, a first intrinsic layer, and a second doped layer, wherein the first intrinsic layer and the second doped layer are stacked vertically on top of the first doped layer. Within the laser emitter, the laser signal may propagate mainly through the first intrinsic layer. The laser emitter may comprise the laser emitter 101 described herein.

[0072] In block 502, the method comprises providing an optical modulator integrated with the laser, the optical modulator comprising a third doped layer, a second intrinsic layer, and a fourth doped layer, wherein the third doped layer, the second intrinsic layer, and the fourth doped layer are arranged laterally along a plane parallel to the first doped layer, such that a current within the optical modulator will flow through the second intrinsic layer. In other words, the laser signal within the optical modulator may propagate mainly through the second intrinsic layer. The optical modulator may comprise the optical modulator 103.

[0073] For example, the laser emitter may be grown using MOCVD or MBE on an n-doped InP substrate. As discussed above in relation to the apparatus, the laser emitter comprises a vertical PIN junction structure with multiple quantum wells integrated within the intrinsic layer. To facilitate the integration of an electro-absorption modulator, any unnecessary laser emitter layers can be selectively etched away, enabling the growth of EAM material with a second set of quantum wells directly on the wafer. Subsequent etching of the EAM section materials may be performed, leaving only the waveguide structure intact. Overgrowth of InP material on either side of the layer comprising the second set of quantum wells may then establish the necessary doping profiles to form a lateral PIN junction. Techniques such as ion implantation, diffusion, or a combination thereof may be employed to achieve the required n-type and p-type doping on either side of the vertical PIN junction structure. Finally, top p- doped InP layers and indium gallium arsenide (InGaAs) layer can be grown to prepare for the deposition of the top metal contacts on the laser section.

[0074] For the section between the laser and modulator - i.e., the transition section described above - the overgrown p-doped InP material can be processed into a taper waveguide structure. In the EAM section, the overgrown p-doped InP material may be retained with a taper structure to enhance waveguiding properties or, alternatively, it may be removed if not required. Top metal contacts can be deposited on both sides of the p- and n-doped InP materials to establish the necessary electrical connections.

[0075] Alternatively, if the laser emitter is to be grown on a semi-insulated substrate, the intrinsic layers in the laser emitter, transition section and the optical modulator may be identical.

[0076] The preceding description has been provided to enable others skilled in the art to best utilize various aspects of the exemplary embodiments disclosed herein. This exemplary description is not intended to be exhaustive or to be limited to any precise form disclosed. Many modifications and variations are possible without departing from the spirit and scope of the instant disclosure. The embodiments disclosed herein should be considered in all respects illustrative and not restrictive. Reference should be made to the appended claims and their equivalents in determining the scope of the instant disclosure.

Claims

CLAIMS1. An externally modulated laser device (100), comprising: a laser emitter ( 101 ) for generating a laser signal, comprising a first doped layer ( 111 ), a first intrinsic layer (112) disposed on the first doped layer (111) and a second doped layer (113) disposed on the first intrinsic layer (112), wherein the laser signal propagates within the laser emitter (101) through the first intrinsic layer (112); an optical modulator (103) connected with the laser emitter (101), the optical modulator (103) comprising a third doped layer (131), a fourth doped layer (133), and a second intrinsic layer (132) disposed between the third doped layer (131) and the fourth doped layer (133), wherein the third doped layer (131), the second intrinsic layer (132) and the fourth doped layer (133) are arranged laterally along a plane parallel to the first doped layer (111), such that the laser signal propagates within the optical modulator (103) through the second intrinsic layer (132).

2. The externally modulated laser device (100) of claim 1, further comprising a substrate layer (104) on which the laser emitter (101) and the optical modulator (103) are arranged, wherein the first doped layer (111) is embedded in the substrate layer (104).

3. The externally modulated laser device (100) of claim 2, wherein the third doped layer (131), the second intrinsic layer (132) and the fourth doped layer (133) are disposed over the substrate layer (104).

4. The externally modulated laser device (100) of claim 3, wherein the optical modulator (103) further comprises a first intermediate layer (134) disposed over the substrate layer (104), wherein the first intermediate layer (134) is arranged below the third doped layer (131) and the fourth doped layer (133), , and wherein the first intermediate layer (134) comprises a first semiconductor.

5. The externally modulated laser device (100) of claim 2, 3 or 4, wherein the substrate layer (104) comprises a p-doped substrate, an n-doped substrate, or a semi-insulated substrate.

6. The externally modulated laser device (100) of any one of claims 2 to 5, further comprising a transition section (102) arranged between the laser emitter (101) and the optical modulator (103) on the substrate layer (104), wherein the transition section (102) comprises a waveguide structure arranged to facilitate an optical mode transition from the laser emitter (101) to the optical modulator (103).

7. The externally modulated laser device (100) of claim 6, wherein the transition section (102) comprises a fifth doped layer (121), a third intrinsic layer (122), a second intermediate layer (124) and a sixth doped layer (123), wherein the third intrinsic layer (122) and the sixth doped layer (123) are stacked vertically on top of the fifth doped layer (121), wherein the fifth doped layer (121) is embedded in the substrate layer (104).

8. The externally modulated laser device (100) of any preceding claim, wherein the laser emitter ( 101) further comprises a first metal contact (114) arranged below the first doped layer (111) and a second metal contact (115) arranged above the second doped layer (113).9 The externally modulated laser device (100) of any preceding claim, wherein a doping of the first doped layer (111) comprises a doping in an opposite sense to a doping of the second doped layer (113), wherein a doping of the third doped layer (131) comprises a doping in an opposite sense to a doping of the fourth doped layer (133).

10. The externally modulated laser device (100) of any preceding claim, wherein the optical modulator (103) further comprises a top layer (135) disposed over the second intrinsic layer (132), wherein the top layer (135) comprises at least one of a second semiconductor and a dielectric.

11. The externally modulated laser device (100) of any preceding claim, wherein the first intrinsic layer (112) and the second intrinsic layer (132) each comprises multiple quantum wells.

12. The externally modulated laser device (100) of any preceding claim, further comprising a semiconductor optical amplifier, SOA, integrated with the externally modulated laser device (100).

13. The externally modulated laser device (100) of any preceding claim, wherein the laser emitter (101) comprises a distributed feedback, DFB, laser, a Fabry-Perot, FP, laser, or a distributed Bragg reflector DBR laser, wherein the optical modulator comprises a Mach-Zehnder modulator.

14. A method of fabricating an externally modulated laser device, the method comprising: providing a laser emitter for generating a laser signal, comprising a first doped layer, a first intrinsic layer disposed on the first doped layer, and a second doped layer disposed on the first intrinsic layer, wherein the laser signal propagates within the laser emitter through the first intrinsic layer (501); and providing an optical modulator connected with the laser, the optical modulator comprising a third doped layer, a fourth doped layer, and a second intrinsic layer disposed between the third doped layer and the fourth doped layer, wherein the third doped layer, the second intrinsic layer, and the fourth doped layer are arranged laterally along a plane parallel to the first doped layer, and the laser signal propagates within the optical modulator through the second intrinsic layer.

15. The method of claim 14, wherein providing the laser emitter (501) and the optical modulator (502) comprises providing a substrate layer on which the laser emitter and the optical modulator are to be formed,the method further comprising: forming, on the substrate layer, a transition section comprising a waveguide structure.

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