Photonic device with variable optical power tap

Abstract

A photonic device (200) is fabricated on an epitaxial layer structure (144, 146) comprising a direct-bandgap vertical confinement region. A laser strip (240) and a tap strip (222) are patterned on the

Description

[0001] PHOTONIC DEVICE WITH VARIABLE OPTICAL POWER TAP

[0002] The present invention relates to photonic devices, photonic chips and assemblies, and associated methods of fabrication. In particular, it relates to photonic devices with optical power taps, used for optical power monitoring and control.

[0003] Background Art

[0004] Power monitoring of lasers allows the lasers’ optical output power to be measured and controlled, in order to ensure consistent optical output.

[0005] A compound semiconductor laser typically has an AR (anti-reflective) coating on its cavity’s front facet and an HR (highly-reflective) coating on its cavity’s back facet. Dielectric HR coatings allow some radiation transmission, whereas metallic HR coatings allow no transmission as the signal is either reflected or lost by absorption.

[0006] Although most of the optical output power is emitted through the front AR coating, a dielectric HR coating on the back facet allows some transmission from the back facet. This optical radiation coming out from the back facet may be detected by a photodetector, which converts the optical power in a voltage. This voltage can be used to monitor the laser optical output power.

[0007] Solutions with discrete components and sub-assemblies can be created by coupling laser sources, directional couplers and detectors. On-wafer, integrated and miniaturised solutions require optimised features on devices such as etched facets, on-wafer coating and couplers.

[0008] Dielectric HR coatings are multilayer stacks and the uniformity and the thickness of each individual layer are engineered to ensure an optimal performance of the device. Sputtering or evaporation deposition techniques can be used for high-throughput manufacturing. More accurate deposition techniques such as atomic layer deposition (ALD) may also be used, but these require specialist deposition tools that notoriously have a very low deposition rate and therefore are not suitable for high-throughput manufacturing.

[0009] For cleaved-facet devices, bars are coated at bar level after cleaving. The deposition of an HR coating can be tailored to achieve the required reflectivity required for a subsystem for power monitoring.

[0010] Alternatively, for integrated solutions, facets can be etched in a full-wafer process. This allows a wafer-level coating across the full wafer. A full-wafer process is also desirable as it allows an accurate control of the dimension of the cavity, and fullwafer testing of the coated lasers before dicing into chips of singulated lasers or individual laser arrays. However, in such a full-wafer process, when the facets have been etched, and before cleaving, it is difficult to perform a wafer level HR coating with uniform thickness across the entire area of the wafer with common evaporation or sputtering techniques The angle of incidence of the deposition and the relative distance between the facet of each device and the deposition or sputtering source are enough to cause relatively large variation in the dielectric HR coating transmission, dramatically affecting uniformity and yield Furthermore, shadowing effects of the topography on the wafer near the facets can interfere with coating uniformity across individual facets.

[0011] In contrast, metallic HR coatings are easier to deposit across a whole wafer, as their reflectivity is less dependent on the thickness with negligible or no influence on the resulting optical power of the device. But, as mentioned above, metallic HR coatings do not allow any transmittivity as the light is either reflected or absorbed on the metal surface and transformed in Joule heating. Therefore, metallic HR coatings are not compatible with on-wafer back facet optical power monitoring.

[0012] As an alternative to back facet monitoring, power monitoring can be performed by tapping a portion of the optical power from the laser cavity.

[0013] U. Koren et al. “A 1 .3 pm Wavelength Laser with an Integrated Output Power Monitor Using a Directional Coupler Optical Power Tap”, IEEE Photonics Technology Letters, Vol. 8, No. 3, March 1996, pp. 364-366, discloses a 1.3-pm Fabry-Perot laser with a directional coupler power tap inside the laser cavity, used for diverting optical power to an integrated waveguide monitoring photodetector.

[0014] However, a problem with this approach is that power is lost from the laser optical output for the operating life of the laser, despite using an HR coating on the back facet, because the monitoring power tap is always on as for the devices with AR- coated back facets.

[0015] Transient Tap Couplers for Wafer-Level Photonic Testing Based on Optical Phase Change Materials Zhang et al, ACS Photonics 2021 , 8, 1903-1908, discloses a transient tap coupler design based on optical phase change materials (O-PCMs). In its “on” state, the coupler acts as a tap to extract a fraction of optical power from the photonic integrated circuit PIC. Once the testing is complete, all transient couplers on a wafer can be switched “off” using a low temperature (280 °C) annealing process, leaving negligible (0.01 dB) residual optical loss.

[0016] A problem with this approach is that once the couplers are turned “off”, they cannot be turned on again, so power monitoring cannot be performed during the operating life of the PIC.

[0017] Summary of invention

[0018] It is desirable to provide active photonic devices, chips and photonic chip assemblies that overcome at least some of the above-identified problems.

[0019] According to a first aspect of the present invention, there is provided a photonic device comprising:

[0020] - a substrate having an epitaxial layer structure comprising a direct-bandgap vertical confinement region;

[0021] - a channel strip patterned on the substrate;

[0022] - a tap strip patterned on the substrate;

[0023] - a channel waveguide formed by the confinement region and the channel strip; - facets at each end of the channel waveguide; and

[0024] - an electrically variable optical power tap comprising:

[0025] - a tap waveguide formed by the confinement region and the tap strip monolithically with the channel waveguide;

[0026] - facets at each end of the tap waveguide; and

[0027] - a coupler electrode in contact with the tap strip, for varying the optical power tap, wherein the channel and tap strips are arranged on the substrate such that the tap waveguide is variably optically coupleable to the channel waveguide in dependence on an electrical bias applied to the coupler electrode.

[0028] Preferably, the tap waveguide comprises a first longitudinal portion separated by a first distance from the channel waveguide and at least one second longitudinal portion separated by a second distance, larger than the first distance, from the channel waveguide.

[0029] Preferably, the coupler electrode is in contact with the tap strip on the first longitudinal portion of the tap waveguide.

[0030] Preferably, the photonic device further comprises a channel electrode in contact with the channel strip, for powering the channel waveguide.

[0031] Preferably, the facets are etched facets.

[0032] Preferably, the photonic device further comprises a high-reflection coating on a facet at a first end of the tap waveguide.

[0033] Preferably, the photonic device further comprises a high-reflection coating on a facet at a second end of the tap waveguide opposite the first end, and further comprises a photodiode electrode in contact with the tap strip for using a portion of the tap waveguide as a photodiode.

[0034] Preferably, the high-reflection coatings comprise a metallic layer. Preferably, the high-reflection coatings comprise a multilayer film comprising at least one dielectric layer.

[0035] Preferably, the photonic device further comprises an anti-reflection coating on a facet at a second end of the tap waveguide opposite the first end.

[0036] Preferably, the photonic device further comprises a photodiode formed monolithically with the channel waveguide and tap waveguide, and arranged to receive optical radiation from the tap waveguide.

[0037] Preferably, the channel strip is a laser strip and the channel waveguide is a laser waveguide.

[0038] Preferably, the vertical confinement region comprises a plurality of epitaxial layers.

[0039] According to a second aspect of the present invention, there is provided a photonic chip comprising the photonic device of the first aspect.

[0040] According to a third aspect of the present invention, there is provided a photonic chip assembly comprising the photonic chip of the second and a photonic integrated circuit.

[0041] According to a fourth aspect of the present invention, there is provided a photonic chip assembly comprising the photonic chip of the second aspect and a photodiode arranged to receive optical radiation from the tap waveguide.

[0042] According to a fourth aspect of the present invention, there is provided a method of fabricating a photonic device, the method comprising the steps:

[0043] - providing a substrate having an epitaxial layer structure comprising a direct- bandgap vertical confinement region;

[0044] - fabricating a channel strip patterned on the substrate and arranged with the confinement region to form a channel waveguide and fabricating, monolithically with the channel waveguide, a tap strip patterned on the substrate and arranged with the confinement region to form a tap waveguide; - fabricating facets at each end of the channel waveguide and facets at each end of the tap waveguide; and

[0045] - fabricating a coupler electrode in contact with the tap strip, wherein the channel and tap strips are arranged on the substrate such that the tap waveguide is variably optically coupleable to the channel waveguide in dependence on an electrical bias applied to the coupler electrode.

[0046] Preferably, the tap waveguide is fabricated comprising a first longitudinal portion separated by a first distance from the channel waveguide and at least one second longitudinal portion separated by a second distance, larger than the first distance from the channel waveguide.

[0047] Preferably, the coupler electrode is fabricated in contact with the tap strip on the first longitudinal portion of the tap waveguide.

[0048] Preferably, the method further comprises fabricating a channel electrode in contact with the channel strip, for powering the channel waveguide.

[0049] Preferably, the method comprises fabricating the facets by etching.

[0050] Preferably, the method further comprises fabricating a high-reflection coating on a facet at an end of the channel waveguide and fabricating a high-reflection coating on a facet at a first end of the tap waveguide.

[0051] Preferably, the method further comprises fabricating a high-reflection coating on a facet at a second end of the tap waveguide opposite the first end, and further comprising fabricating a photodiode electrode in contact with the tap strip for using a portion of the tap waveguide as a photodiode.

[0052] Preferably, the high-reflection coatings comprise a metallic layer.

[0053] Preferably, the high-reflection coatings comprise a multilayer film comprising at least one dielectric layer. Preferably, the method further comprises fabricating an anti-reflection coating on a facet at a second end of the tap waveguide opposite the first end.

[0054] Preferably, the method further comprises fabricating a photodiode formed monolithically with the channel waveguide and tap waveguide, and arranged to receive optical radiation from the tap waveguide.

[0055] Brief description of drawings

[0056] Embodiments of the present invention will now be described, by way of example only, with reference to the drawings, in which:

[0057] Figures 1 a to 1 c illustrate, in schematic form, a known etched-facet distributed feedback (DFB) laser chip, in orthographic and cross-section views. Note that in absence of grating the device will work as a FP and the same consideration apply

[0058] Figure 2 illustrates, in schematic form, a photonic device in plan view, in accordance with an embodiment of the present invention.

[0059] Figure 3 illustrates, in schematic form, the photonic chip including the device of Figure 2, in cross section view, while cutting along the ‘d’ plane

[0060] Figure 4 illustrates a graph of coupling coefficient versus coupler voltage.

[0061] Figure 5 illustrates a graph of photodiode voltage versus coupling coefficient.

[0062] Figure 6 illustrates, in schematic form, a photonic device and controller arranged for a closed-loop power control mode, in accordance with another embodiment of the present invention.

[0063] Figure 7 illustrates, in schematic form, a photonic device with a separate monolithic photodiode, in accordance with another embodiment of the present invention. Figure 8 illustrates, in schematic form, a photonic chip assembly with an off-chip photodiode, in accordance with another embodiment of the present invention.

[0064] Figure 9 illustrates, in schematic form, a photonic device with a U-shaped tap in plan view, in accordance with another embodiment of the present invention.

[0065] Figure 10 illustrates, in schematic form, a photonic device with two separate monolithic photodiodes, in accordance with another embodiment of the present invention.

[0066] Figure 11 illustrates, in schematic form, a photonic chip assembly with two off-chip photodiodes, in accordance with another embodiment of the present invention.

[0067] Figure 12 is a flowchart of a method of fabricating a photonic device, in accordance with an embodiment of the present invention.

[0068] Detailed description

[0069] In this description and claims, optical radiation relates to electromagnetic radiation over a range of wavelengths not limited to visible radiation, such as wavelengths spanning ultraviolet, visible and infrared radiation. An InP (indium phosphide) distributed feedback (DFB) laser is described as an example of a photonic device. Other compound semiconductor based devices may be used with embodiments. For example photonic devices based on GaAs, GaSb, InSb or GaN, or photonic devices based on other direct bandgap material systems may be used. Rather than the DFB laser example described herein, other active photonic devices may be used, such as Fabry Perot lasers, modulators, semiconductor optical amplifiers (SOAs), and reflective semiconductor optical amplifiers (RSOAs) used stand-alone or in external cavity lasers.

[0070] The examples described herein relate to photonic devices fabricated with a ridge or buried waveguide with an etched facet, but the skilled person will appreciate that embodiments may include photonic devices fabricated with one or more cleaved facet. Figure 1 illustrates a known photonic device, in this example an etched-facet DFB laser chip 100, with a compound semiconductor laser on an InP substrate. Figure 1 a is an orthographic view of the laser chip 100. Figure 1 b is a cross-section (not to scale) along a-a shown in Figure 1 a. Figure 1 b is thus a lengthwise cross-section through the waveguide 104 along its propagation direction (length). Figure 1c is a cross-section (not to scale) along b-b shown in Figure 1 a. Figure 1 c is thus a crosssection across the waveguide 104 perpendicular to its propagation direction.

[0071] The structure of the laser chip 100 is now described in the context of its wafer-scale fabrication.

[0072] A ridge waveguide 104 is defined by a waveguide etch. A pattern of openings in a hard mask in a lithographic step defines trenches 102, 106 that are etched to define the ridge waveguide 104 in between them. An insulating dielectric material 1 18 covers most of the top surface, and a contact window is opened up in the dielectric along the top of the ridge 104. Subsequently, metal 116 is deposited covering the ridge waveguide and making contact through the contact window to the top of the ridge waveguide 104.

[0073] A pad of the metal 116 at one side of the ridge waveguide is used as an area for soldering or bonding to the metal. In subsequent fabrication steps, a patterned hard mask and facet etch defines front and rear etched facets 110, 108 at either end of the ridge waveguide 104. The facet etch creates an etched surface which extends either side of the etched facet. The facet etch is deeper than the ridge etch.

[0074] A small horizontal spacing is provided between the ridge trenches 102, 106 and the facet etch features, so that the front and rear etched facets 110, 108 are etched as flat planes rather than having corners with the ridge waveguide, which would etch unevenly and would be detrimental to the smoothness of the facet at the end of the waveguide. This results in a structure shaped like a T, with the waveguide being the trunk of the T, and walls 112 being the crossbar of the T in the form of a flange. The effect of the spacing and resulting flanged T-shaped structure is to ensure that the facet is smooth to provide efficient and reproducible transmission through optical coupling regions, or internal reflection at, the facets.

[0075] After the facet etch, an anti-reflective (AR) coating 138 is applied to one etched facet 110 and a high-reflectance (HR) coating 140 (or an AR coating, not shown) is applied to the other etched facet 108 at the other end of the waveguide 104. Anti- reflective coatings may be applied to just one facet for a laser, or one or both facets for Semiconductor Optical Amplifiers (SOAs) or Electro-Absorption Modulators (EAMs).

[0076] Finally, a metallisation step coats the underside of the wafer with metal 143.

[0077] With reference to Figures 1 a and 1 b, in operation the laser cavity, comprising the waveguide 104 bounded by facets 108 and 110 at either end, outputs optical radiation 142 through an optical coupling region 114.

[0078] With reference to Figures 1 b and 1c, the layer structure will now be described in detail. From the top in Figure 1 b, a p-metal layer 116 extends down through a window in the dielectric layer 1 18. The p-metal layer 116 makes contact to a p-type InGaAs contact layer 120, which is the top epitaxially-grown layer. Below that, a p- type InP cladding layer 122 is followed by a p-type etch stop / grating layer 124. The etch that stops on that layer 124 is the waveguide ridge etch, as illustrated in Figure 1c. Next, a p-type InP spacer layer 126 is followed by a p-type separate confinement heterostructure (SCH) layer 128, an undoped multi-quantum well (MQW) layer 130, and an n-type SCH layer 132. The SCH and MQW layers are the optically active layers in the laser.

[0079] The n-type InP buffer layer 134 is the first of the epitaxial layers that is grown on the n-type InP substrate 136.

[0080] In a distributed feedback (DFB) laser, a grating is superimposed on the waveguide to provide optical feedback in the laser cavity. In this example, the grating is made by performing the epilayer growth in two stages and in between the stages patterning the grating. First, a lower epitaxial layer structure 144 is grown on the substrate, starting with the n-type InP buffer layer 134 then the SCH and MQW active layers 125 then the p-type InP spacer layer 126 and the etch stop / grating layer structure 124. Electron beam lithography is used to define a grating pattern 150 (shown in Figure 1 b), which is transferred by etching into the etch stop / grating layer structure 124.

[0081] After the grating patterning, the upper epitaxial layer structure 146 is overgrown on the lower epitaxial layer structure 144, using for example metalorganic vapour-phase epitaxy (MOVPE / MOCVD). The upper epitaxial layer structure 146 includes the p- type InP cladding layer 122 under the p-type InGaAs contact layer 120.

[0082] This specific layer structure is suitable for a laser as well as an SOA. However, the layer structure may be optimised for different active photonic devices.

[0083] In this laser example, the front and rear etched facets 110, 108 are coated with a PECVD-deposited silicon nitride AR coating 138 and an HR coating 140 respectively. The AR coating is selectively removed after deposition to allow bonding to metallic layers. Finally, the n-metal layer 143 is shown.

[0084] In operation, as shown at the left of Figure 1 b, a beam of optical radiation 142, illustrated bounded with dashed lines, is output from the etched facet 1 10 at the optical coupling region 114. In this example, the optical radiation is output from the optically active layers of the ridge waveguide 128, 130, 132 (collectively labelled 125 in Figure 1c) into the air to the left of the front etched facet 110. For an SOA example (not shown), instead of an HR coating 140 another AR coating is applied to the rear etched facet 108 and radiation is input to the waveguide at the rear etched facet 108.

[0085] With reference to Figure 1 c, the p-metal layer 116 can be seen on top of the dielectric layer 118 as it covers trenches 102, 106 either side of the ridge waveguide 104. The p-metal layer 1 16 contacts the top of the ridge 104 through a window in the dielectric 118. The trenches 102, 106 are etched by the waveguide ridge etch, which selectively stops on the p-type etch stop / grating layer 124.

[0086] The location of the optical coupling region 114 is shown projected along the waveguide from the etched facet 110 onto this cross-section plane b-b. It is centred horizontally with respect to the ridge waveguide 104 and centred vertically with respect to the undoped MQW layer 130. The optical mode roughly corresponds to the projected coupling region 1 14.

[0087] A contact is made by the metal layer 116 at the top of ridge 104 to the p-type InGaAs contact layer 120.

[0088] In the drawings, features labelled with numerals correspond to the same features with the same numerals in subsequent drawings. Therefore, a description of a feature in any drawing should also apply to a feature labelled with the same numeral elsewhere in this description.

[0089] As discussed above, etched (as opposed to cleaved) facets advantageously allow full-wafer processing. However full-wafer processing of dielectric HR coatings is difficult at least in part because coating on to the back facet is obstructed by the rest of the uncleaved wafer. Therefore metal (as opposed to dielectric) HR coatings are used with etched facet lasers. However, metal HR coatings block transmission of light from the cavity.

[0090] Embodiments may be used for power monitoring of etched facet lasers, which use such a metal HR coating.

[0091] Also as discussed above, conventional passive intracavity taps for power monitoring suffer from loss of power while the laser operates.

[0092] In embodiments, a variable coupler is used as an intracavity tap to couple optical radiation into a tap strip or “read tail” defined by a tap strip for monitoring by a monolithically integrated photodiode (PD) in the read tail, or an external off-chip photodiode.

[0093] An embodiment is a semiconductor laser with etched facets and metal HR back facet coating. Instead of monitoring light escaping from the back facet, it is tapped from the side of the laser optical cavity. Instead of a directional coupler tap (like Koren et al), a variable and switchable coupler is used as a tap, which allows the monitoring and testing of the laser to be switched on and off and adjusted.

[0094] The variable coupler couples the read-tail to the active region of the laser, for example comprising Multi-Quantum Wells (MQWs). The coupling mechanism involves the Franz-Keldysh effect or the quantum confined Stark effect (QCSE).

[0095] Figure 2 illustrates a photonic device in plan view, in accordance with an embodiment of the present invention.

[0096] The photonic device 200 is fabricated on the InP-based epitaxial layer structure 144, 146 (described with reference to Figure 1 ) comprising a direct-bandgap vertical confinement region, in this case the optically active epitaxial layers 125, grown on the n-type InP substrate 136.

[0097] A channel strip 240 and a tap strip 222 are patterned on the substrate. With reference to Figure 3, a channel waveguide 310 is formed by the confinement region 125 and the channel strip 240, and a tap waveguide 304 is formed by the confinement region 125 and the tap strip 222. The tap waveguide 304 is formed monolithically with the channel waveguide 310.

[0098] In this embodiment, the channel strip 240 is a laser strip (in this example a ridge, but the strip could alternatively be for example a buried heterostructure strip) and the channel waveguide 310 is a laser waveguide. In other embodiments, the channel strip may be a gain portion of a semiconductor optical amplifier (SOA) or reflective semiconductor optical amplifier, for example for use in an external cavity laser.

[0099] With reference again to Figure 2, facets 236, 244 are fabricated at each end of the laser waveguide 310 with facets 220, 228 at each end of the tap waveguide 304. In this example, the facets are etched facets An electrically variable optical power tap is formed by the tap waveguide and its facets and a coupler electrode 216 in contact with 226 the tap strip 222, for varying the optical power tap.

[0100] The laser 240 and tap 222 strips are arranged on the substrate such that the tap waveguide (304 in Figure 3) can be variably optically coupled to the laser waveguide (310 in Figure 3) in dependence on an electrical bias applied to the coupler electrode 216.

[0101] There is a high-reflection (HR) coating 246 on a facet 244 at an end of the laser waveguide formed under the laser strip 240. There is a high-reflection (HR) coating 230 on a facet 228 at a first end (right-hand side of Figure 2) of the tap waveguide formed under the tap strip 222.

[0102] In this example, the high-reflection (HR) coatings 246, 230 comprise a metallic layer with Ti / Au 20nm / 200nm metallisation deposited onto the full wafer by sputtering and patterned with a lift-off photolithographic process. In other embodiments, the high- reflection (HR) coatings may comprise a multilayer film comprising at least one dielectric layer. The multilayer film with a dielectric layer may also comprise a metallic layer.

[0103] A channel electrode (in this example a laser electrode) 238 in contact with 242 the laser strip 240 is provided for powering the laser waveguide.

[0104] The tap waveguide comprises a first longitudinal portion (corresponding to a first longitudinal portion 223 of the tap strip 222) adjacent to the laser waveguide and at least one second longitudinal portion (corresponding to a second longitudinal portion 225 of the tap strip 222) further away from the laser waveguide than the first longitudinal portion. A longitudinal portion means a section along the length of the strip. Thus, the first longitudinal portion 223 is separated by a first distance from the laser waveguide and the second longitudinal portion 225 is separated by a second distance, larger than the first distance, from the laser waveguide. The coupler electrode 216 is in contact 226 with the tap strip on the first (coupling) longitudinal portion of the tap waveguide.

[0105] There is also a high-reflection (HR) coating 218 on a facet 220 at a second end (lefthand side of Figure 2) of the tap waveguide formed under the tap strip 222 opposite the first end.

[0106] A photodiode electrode 214 in contact 224 with the tap strip 222 is provided for using a portion of the tap waveguide as a photodiode.

[0107] Alternatively, as shown in Figures 7 and 8, the photodiode is separate from the tap waveguide, and an anti-reflection (AR) coating 704, 804 is provided on the second end (left-hand side of Figures 7 and 8) of the tap waveguide, opposite the first end. The separate photodiode 706, 802 is arranged to receive optical radiation passing through the AR coating from the tap waveguide (as described with reference to Figures 7 and 8).

[0108] The separate photodiode may be formed with the confinement region monolithically with the laser waveguide and tap waveguide (as described with reference to Figure 7) or may be a separate component 802 (as described with reference to Figure 8) assembled with the photonic device.

[0109] Figure 3 illustrates the photonic device of Figure 2, in cross section view.

[0110] The substrate and layer structure are the same as described with reference to Figure 1 .

[0111] As described above, a channel waveguide 310 is formed by the confinement region 125 and the channel strip 240, and a tap waveguide 304 is formed by the confinement region 125 and the tap strip 222. The refractive index profile of the laser strip 240, and the air and coatings surrounding it, patterned in the epilayers 302, together with the confinement region 125, form the laser waveguide 310 to produce respectively lateral and vertical optical confinement for an optical mode 308. In the same way, the refractive index profile of the tap strip 222, and the air and coatings surrounding it, patterned in the epilayers 302, together with the confinement region 125, form the laser waveguide 304 to produce optical confinement for an optical mode 306.

[0112] Again as described above, the laser 240 and tap 222 strips are arranged on the substrate 136 such that the tap waveguide 304 is variably optically coupleable to the laser waveguide 310 in dependence on an electrical bias applied to the coupler electrode (216 in Figure 2).

[0113] Figure 4 illustrates a graph of coupling coefficient k on the vertical axis versus coupler voltage Vc on the horizontal axis. The plotted line 402 is shown as being sinusoidal, however it may be another shape having a minimum and a maximum.

[0114] As described with reference to Figures 2 and 3, the laser 240 and tap 222 strips are arranged on the substrate136 such that the tap waveguide 304 is variably optically coupleable to the laser waveguide 310 in dependence on an electrical bias Vc applied to the coupler electrode 216. The varying of the optical coupling can be represented by variation of coupling coefficient k.

[0115] Figure 5 illustrates a graph of photodiode voltage on the vertical axis versus coupling coefficient k on the horizontal axis. The plotted line 502 is shown as being a section of a sinusoid, however it may be another shape having a minimum and a maximum.

[0116] With reference to Figures 2, 4 and 5, in a coupler calibration mode, the variable coupler is calibrated:

[0117] • for a fixed laser voltage VL (i.e. fixed optical emission power) and for a fixed photodiode current,

[0118] • the coupler voltage Vc is swept and photodiode voltage VPD is measured to determine a calibration of photodiode voltage VPD as a function of coupler voltage Vc, • Vcmin is determined at a minimum 506 of photodiode voltage VpDmin, for minimum 406 coupling coefficient kmin corresponding to minimum coupling between the laser waveguide and the tap waveguide and

[0119] • Vcmax is determined at a maximum 504 of photodiode voltage VpDmax, for maximum 404 coupling coefficient kmax corresponding to maximum coupling between the laser waveguide and the tap waveguide.

[0120] The coupling coefficient and photodiode voltage may also be calibrated as a function of temperature, by performing the variable coupler calibration with the device in a temperature-controlled chuck. Once the temperature dependence of the coupling is determined, the device can be used as a temperature monitor. For example, when Vcmin and / or Vcmax varies as a function of temperature, the calibration of those values can indicate that the laser voltage VL can be adjusted accordingly (using a temperature versus VL calibration) to correct the coupler voltage for optimum coupling and / or to correct the laser power.

[0121] In a non-coupled (transparent) mode (with optical power tap switched off)

[0122] • for a fixed laser voltage VL,

[0123] • the coupler voltage Vc is set to Vcmin to achieve minimum or zero coupling k = kmin = 0

[0124] • the photodiode voltage VPD is measured and may be used for monitoring if the laser parameters drift enough to affect the coupling coefficient.

[0125] In the non-coupled (transparent) mode, the parasitic power losses to the tap waveguide are minimised, allowing full output optical power from the laser.

[0126] The coupler calibration mode and non-coupled (transparent) mode described above with reference to Figure 5 may be applied to the devices described with reference to Figures 2, 3, 7, 8, 9, 10 and 11 .

[0127] Figure 6 illustrates a photonic device and controller 606 arranged for a closed-loop power control mode 600, in accordance with another embodiment of the present invention. With reference to Figures 4, 5 and 6, in a coupled power monitoring mode or coupled closed-loop power control mode (with the tap switched on):

[0128] • for the coupled power monitoring mode, a fixed laser voltage VL is used with a fixed photodiode current IPD,

[0129] • alternatively, for the coupled closed-loop power control mode, an initial laser voltage VL is used with a fixed photodiode current IPD,

[0130] • the coupler voltage Vc is set to Vctap 408 to achieve non-zero coupling 508 k = ktap,

[0131] • the photodiode voltage Vpotap is measured and used for power monitoring and / or closed-loop power control of the laser diode

[0132] Closed-loop power control of the laser diode is performed, for example, by comparing 604 measured photodiode voltage VPD to a setpoint photodiode voltage 602 to generate an error value for a PID (proportional, integral, differential) controller 606. The controller 606 outputs a control output 608 which changes the applied laser voltage VL (the manipulated variable in the control loop) through the laser electrode 238. Thus the laser operation is the controlled process of the control loop, and the measured photodiode voltage VPD is the measured process variable of the control loop.

[0133] If the output power of the laser varies, this variation will be detected via the tap waveguide and photodiode, and the resulting error signal causes the controller 606 to correct the variation in the laser output power by manipulating the laser voltage VL.

[0134] The coupled power monitoring mode described above with reference to Figure 6 may be applied to the devices described with reference to Figures 2, 3, 7, 8, 9, 10 and 1 1 , with the addition of a setpoint 602, comparison 604 controller 606 and control output 608.

[0135] Figure 7 illustrates a photonic device 700 with a separate monolithic photodiode 706, in accordance with another embodiment of the present invention. In this example, the photodiode 706 is separate from the tap waveguide formed under tap strip 712, and an anti-reflection (AR) coating 704 is provided on the second end facet 710 of the tap waveguide opposite the first end facet 228. The separate photodiode 706 is arranged to receive optical radiation passing through the AR coating 704 from the tap waveguide formed under the tap strip 712. In another example (not shown) there is no coating on the second end facet 710, but the separate photodiode 706 is still arranged to receive optical radiation from the tap waveguide.

[0136] The separate photodiode 706 is formed (in this example with the confinement region) monolithically with the laser waveguide and tap waveguide. A photodiode electrode 702 in contact 708 with the separate photodiode, is used to measure 210 the photodiode voltage VPD.

[0137] Figure 8 illustrates a photonic chip assembly 800 with an off-chip external photodiode 802, in accordance with another embodiment of the present invention.

[0138] In this example, the photodiode 802 is not only separate from the tap waveguide formed under tap strip 812 but also external to the chip comprising the monolithic laser waveguide and tap waveguide. The separate photodiode 802 is an external component assembled with the photonic device in the photonic chip assembly 800, or may be within external test apparatus for example in the calibration mode.

[0139] An anti-reflection (AR) coating 804 is provided on the second end facet 810 of the power tap waveguide 812 opposite the first end facet 228. The separate photodiode 802 is arranged to receive optical radiation passing through the AR coating 804 from the tap waveguide 812 optionally via one or more of a lens and optical waveguide (not shown). In another example (not shown) there is no coating on the second end facet 810, but the separate photodiode 802 is still arranged to receive optical radiation from the tap waveguide 812. An output from the external photodiode 802 is used to measure 210 the photodiode voltage VPD.

[0140] Figure 9 illustrates a photonic device with a U-shaped tap in plan view, in accordance with another embodiment of the present invention.

[0141] This device is the same as described with reference to Figure 2, except the tap strip 222 of Figure 2 is replaced by a U-shaped tap strip 922. This means that in Figure 9, the high-reflection (HR) coating 218 on the facet 220 at the second end of the tap waveguide (formed opposite the first end from the perspective of radiation guided along the curved length of the waveguide) is next to the high-reflection (HR) coating 230 on the facet 228 at the first end (right-hand side of Figure 9) of the tap waveguide formed under the tap strip 922. This has the advantage that all the HR coatings 218, 230, and 246 are at the same side of the device. This is an advantage if on-wafer HR coating is directionally deposited or if cleaving is done before coating, because only one deposition needs to be applied to the right-hand side facets to provide all the required HR coatings for the device. In the example of Figure 9, only an AR coating is applied to the left-hand side, which is advantageous for the same reasons as the HR coating.

[0142] Figure 10 illustrates a photonic device with two separate monolithic photodiodes, in accordance with another embodiment of the present invention.

[0143] This device 1000 is the same as described with reference to Figure 7, except there is an AR coating 1030 instead of a high-reflection (HR) coating 230 on the facet 228 at a first end (right-hand side of Figure 10) of the tap waveguide formed under the tap strip 712. Furthermore, there is a second photodiode 1006 separate from the tap waveguide. The separate photodiode 1006 is arranged to receive optical radiation passing through the AR coating 1030 from the tap waveguide 712. In another example (not shown) there are no coatings on the first end facet 228 and second end facet 710, but the separate photodiodes 1006, 706 are still arranged to receive optical radiation from the tap waveguide 712. Like the separate photodiode 706 described with reference to Figure 7, the separate second photodiode 1006 of Figure 10 is formed with the confinement region monolithically with the laser waveguide and tap waveguide. A photodiode electrode 1002 in contact 1008 with the second separate photodiode 1006, is used to measure 1210 the photodiode voltage VPD.

[0144] Figure 11 illustrates a photonic chip assembly 1 100 with two off-chip photodiodes, in accordance with another embodiment of the present invention.

[0145] This device is the same as described with reference to Figure 8, except there is an AR coating 1130 instead of a high-reflection (HR) coating 230 on the facet 228 at a first end (right-hand side of Figure 1 1 ) of the tap waveguide formed under the tap strip 812. Furthermore, there is a second photodiode 1102 separate from the tap waveguide. Like the separate photodiode 802 described with reference to Figure 8, The photodiode 1102 is both separate from the tap waveguide formed under tap strip 812 and external to the monolithic laser waveguide and tap waveguide. The separate second photodiode 1102 is an external component assembled with the photonic device in the photonic chip assembly 1 100, or may be within external test apparatus for example in the calibration mode The separate photodiode 1 102 is arranged to receive optical radiation passing through the AR coating 1030 from the tap waveguide formed under tap strip 812. In another example (not shown) there are no coatings on the first end facet 228 and second end facet 810, but the separate photodiodes 1102, 802 are still arranged to receive optical radiation from the tap waveguide 812.

[0146] An output from the external second photodiode 1102 is used to measure 1210 the photodiode voltage VPD.

[0147] Figure 12 is a flowchart 1200 of a method of fabricating a photonic device, in accordance with an embodiment of the present invention. With reference to Figure 12 (and also to Figure 2), the method has the steps:

[0148] 1202: Providing a substrate, such as an InP substrate, having an epitaxial layer structure comprising a direct-bandgap vertical confinement region 125. 1204: Fabricating a channel strip 240 patterned on the substrate and arranged with the confinement region 125 to form a channel waveguide 310 and fabricating, monolithically with the channel waveguide, a tap strip 222 patterned on the substrate and arranged with the confinement region 125 to form a tap waveguide 304. The tap waveguide is fabricated to have a first longitudinal portion separated by a first distance from the channel waveguide and at least one second longitudinal portion separated by a second distance, larger than the first distance from the channel waveguide. As described with reference to Figures 7 and 10, the method may also include, at step, fabricating a photodiode formed with the confinement region monolithically with the channel waveguide and tap waveguide, and arranged to receive optical radiation from the tap waveguide.

[0149] 1206: Fabricating facets 236, 244 at each end of the channel waveguide 310 and facets 220, 228 at each end of the tap waveguide 304. Facets are fabricated at each end of the tap waveguide, by etching, or cleaving.

[0150] 1208: Fabricating electrodes. A channel electrode 238 is fabricated in contact 242with the channel strip 240, for powering the channel waveguide. A coupler electrode 216 is fabricated in contact with 226 the tap strip 222.

[0151] The channel 240 and tap 222 strips are arranged on the substrate such that the tap waveguide 304 is variably optically coupleable to the channel waveguide 310 in dependence on an electrical bias applied to the coupler electrode 216. The coupler electrode in this example is fabricated in contact with the tap strip on the first longitudinal portion of the tap waveguide.

[0152] 1210: Depositing coatings. A high-reflection (HR) coating is deposited on a facet at an end of the channel waveguide and a high-reflection (HR) coating is deposited on a facet at a first end of the tap waveguide.

[0153] The high-reflection (HR) coatings may comprise a metallic layer. The high-reflection (HR) coatings may comprise a multilayer film comprising at least one dielectric layer. As described with reference to Figures 2, 3 and 9, in step 1208 a photodiode electrode 214 may be fabricated in contact with the tap strip 222 for using a portion of the tap waveguide as a photodiode. Then in step 1210 a metallic high-reflection (HR) coating may be deposited on a facet at a second end of the power tap waveguide opposite the first end.

[0154] Alternatively, as described with reference to Figures 7, 8, 10 and 11 , in step 1210 an anti-reflection (AR) coating may be deposited on a facet at a second end of the tap waveguide opposite the first end.

Claims

Claims1 . A photonic device comprising:- a substrate having an epitaxial layer structure comprising a direct-bandgap vertical confinement region;- a channel strip patterned on the substrate;- a tap strip patterned on the substrate;- a channel waveguide formed by the confinement region and the channel strip;- facets at each end of the channel waveguide; and- an electrically variable optical power tap comprising:- a tap waveguide formed by the confinement region and the tap strip monolithically with the channel waveguide;- facets at each end of the tap waveguide; and- a coupler electrode in contact with the tap strip, for varying the optical power tap, wherein the channel and tap strips are arranged on the substrate such that the tap waveguide is variably optically coupleable to the channel waveguide in dependence on an electrical bias applied to the coupler electrode.

2. The photonic device of claim 1 wherein the tap waveguide comprises a first longitudinal portion separated by a first distance from the channel waveguide and at least one second longitudinal portion separated by a second distance, larger than the first distance, from the channel waveguide.

3. The photonic device of claim 2 wherein the coupler electrode is in contact with the tap strip on the first longitudinal portion of the tap waveguide.

4. The photonic device of any preceding claim further comprising a channel electrode in contact with the channel strip, for powering the channel waveguide.

5. The photonic device of any preceding claim wherein the facets are etched facets.

6. The photonic device of any preceding claim further comprising a high-reflection coating on a facet at a first end of the tap waveguide.

7. The photonic device of claim 6 further comprising a high-reflection coating on a facet at a second end of the tap waveguide opposite the first end, and further comprising a photodiode electrode in contact with the tap strip for using a portion of the tap waveguide as a photodiode.

8. The photonic device of claim 6 or claim 7 wherein the high-reflection coatings comprise a metallic layer.

9. The photonic device of any of claims 6 to 8 wherein the high-reflection coatings comprise a multilayer film comprising at least one dielectric layer.

10. The photonic device of claim 6 further comprising an anti-reflection coating on a facet at a second end of the tap waveguide opposite the first end.1 1 . The photonic device of any preceding claim further comprising a photodiode formed monolithically with the channel waveguide and tap waveguide, and arranged to receive optical radiation from the tap waveguide.

12. The photonic device of any preceding claim wherein the channel strip is a laser strip and the channel waveguide is a laser waveguide.

13. The photonic device of any preceding claim wherein the vertical confinement region comprises a plurality of epitaxial layers.

14. A photonic chip comprising the photonic device of any preceding claim.

15. A photonic chip assembly comprising the photonic chip of claim 14 and a photonic integrated circuit.

16. A photonic chip assembly comprising the photonic chip of claim 14 and a photodiode arranged to receive optical radiation from the tap waveguide.

17. A method of fabricating a photonic device, the method comprising the steps:- providing a substrate having an epitaxial layer structure comprising a direct- bandgap vertical confinement region;- fabricating a channel strip patterned on the substrate and arranged with the confinement region to form a channel waveguide and fabricating, monolithically with the channel waveguide, a tap strip patterned on the substrate and arranged with the confinement region to form a tap waveguide;- fabricating facets at each end of the channel waveguide and facets at each end of the tap waveguide; and- fabricating a coupler electrode in contact with the tap strip, wherein the channel and tap strips are arranged on the substrate such that the tap waveguide is variably optically coupleable to the channel waveguide in dependence on an electrical bias applied to the coupler electrode.

18. The method of claim 17 wherein the tap waveguide is fabricated comprising a first longitudinal portion separated by a first distance from the channel waveguide and at least one second longitudinal portion separated by a second distance, larger than the first distance from the channel waveguide.

19. The method of claim 18 wherein the coupler electrode is fabricated in contact with the tap strip on the first longitudinal portion of the tap waveguide.

20. The method of any of claims 17 to 19 further comprising fabricating a channel electrode in contact with the channel strip, for powering the channel waveguide.21 . The method of any of claims 17 to 20 comprising fabricating the facets by etching.

22. The method of any of claims 17 to 21 further comprising fabricating a high- reflection coating on a facet at an end of the channel waveguide and fabricating a high-reflection coating on a facet at a first end of the tap waveguide.

23. The method of claim 22 further comprising fabricating a high-reflection coating on a facet at a second end of the tap waveguide opposite the first end, and further comprising fabricating a photodiode electrode in contact with the tap strip for using a portion of the tap waveguide as a photodiode.

24. The photonic device of claim 22 or claim 23 wherein the high-reflection coatings comprise a metallic layer.

25. The photonic device of any of claims 22 to 24 wherein the high-reflection coatings comprise a multilayer film comprising at least one dielectric layer.

26. The method of claim 22 further comprising fabricating an anti-reflection coating on a facet at a second end of the tap waveguide opposite the first end.

27. The method of any of claims 17 to 26 further comprising fabricating a photodiode formed monolithically with the channel waveguide and tap waveguide, and arranged to receive optical radiation from the tap waveguide.

Images