Distributed feedback lasers with frequency trimming

Abstract

A distributed feedback (DFB) laser apparatus has a distributed feedback (DFB) laser (650) comprising a DFB structure (511), a heater (508) arranged to heat the DFB structure to change a lasing frequency of the DFB laser and a heater circuitry hard-wiring configuration (556) hard-wired to route power to a selected portion (616) of the heater such that heating the selected portion (616) changes refractive index of the DFB structure (511) at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency.

Description

[0001] DISTRIBUTED FEEDBACK LASERS WITH FREQUENCY TRIMMING

[0002] The present invention relates to distributed feedback (DFB) laser apparatuses and methods of trimming wavelength of a DFB laser apparatus.

[0003] Background Art

[0004] In the field of optical wavelength-division multiplexing (WDM), the wavelength of each laser in an array needs to be positioned accurately in a grid having a precise defined optical frequency. Distributed-feedback (DFB) semiconductor lasers, can be fabricated to have specific lasing frequencies. However normal variation in the laser manufacturing process results in unwanted variation of the lasing frequency, with respect to the target.

[0005] This variation can be compensated by frequency trimming (equivalent to wavelength trimming) or active control. Trimming shifts the frequency distribution, increasing device yield with respect to meeting the specified frequency target. Active control tunes the frequency during operation.

[0006] An example of trimming is given in Sudoh et al., Wavelength trimming of distributed- feedback lasers by photo-absorption-induced disordering, Conference Proceedings LEOS'96 9th Annual Meeting IEEE Lasers and Electro-Optics Society 18-21 November 1996, which discloses wavelength trimming by focusing a CW Nd:YAG laser on the laser chip for 30 min. Another example is given in PhD thesis, TK Sudoh, 1996, Wavelength Trimming Technology for Multiple-Wavelength Distributed-Feedback Semiconductor Laser Arrays, which discloses wavelength trimming was achieved by a He:Ne laser irradiation. The shift reached saturation after 570 sec of irradiation where the total amount of wavelength shift was 0.14 nm. Problems with such trimming approaches are that irradiation is time consuming and difficult to implement in high-volume manufacturing as it requires long irradiation and complex testing, and also complex fabrication in the case of a chalcogenide glass refractive index change approach.

[0007] US2014010249A1 discloses a method to tune an emission wavelength of a wavelength tunable laser diode (LD) to a target wavelength. The wavelength tunable LD providing two regions each having at least one micro heater to modify refractive index of respective regions. One region is a chirped sampled grating distributed Bragg reflector (CSG-DBR), while the other region is a sampled grating distributed feedback (SG-DFB) region. A controller sets biases, stored in a look-up table (LUT) according to the target wavelength, in respective heaters and electrodes, to make the emission wavelength of the wavelength tunable LD close to or around the target wavelength. Biases are measured in advance of the operation of the wavelength tunable LD 10 and stored in the memory by the configuration of the LUT. Power to compensate the wavelength difference is evaluated independently for the respective micro heaters.

[0008] A problem with active control approaches is that complex circuitry and processing is needed to provide the active control. Thus, frequency tuning using active heater control is costly in terms of design complexity and semiconductor real-estate.

[0009] Summary of invention

[0010] It is desirable to provide a DFB laser apparatus, and a method of trimming wavelength of a DFB laser apparatus, that overcome at least some of the aboveidentified problems. It is desirable to provide a DFB laser apparatus with improved frequency conformance to a frequency grid, without complex frequency tuning control circuitry or time-consuming frequency trimming.

[0011] According to a first aspect of the present invention, there is provided a distributed feedback (DFB) laser apparatus comprising:

[0012] - a distributed feedback (DFB) laser comprising a DFB structure; - a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and

[0013] - a heater circuitry hard-wiring configuration comprising heater circuitry hard-wired to route power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency.

[0014] Preferably, the heater comprises a plurality of separate heater segments.

[0015] Preferably, the heater circuitry hard-wiring configuration selects a heater portion or size to be powered.

[0016] Preferably, the heater circuitry is hard-wired to route the power via one or more circuitry element added to the heater circuitry.

[0017] Preferably, the one or more circuitry element comprises one or more bond.

[0018] Preferably, the one or more circuitry element comprises one or more conducting track.

[0019] Preferably, the heater circuitry is hard-wired to route the power with one or more open circuit where one or more circuitry element has been removed from the heater circuitry.

[0020] Preferably, the one or more circuitry element comprises one or more fuse and the heater circuitry is hard-wired to route the power by blowing the one or more fuse.

[0021] Preferably, the one or more circuitry element comprises one or more conducting track and the heater circuitry is hard-wired to route the power by electrically opening the one or more conducting track to form the one or more open circuit.

[0022] Preferably, the DFB structure and heater are fabricated on a first substrate and the heater circuitry comprises components fabricated on a second substrate, the first and second substrates assembled together. Preferably, the heater circuitry hard-wiring configuration comprises sizing of the heater.

[0023] Preferably, the heater comprises a resistive layer arranged at least partially on top of the DFB structure.

[0024] According to a second aspect of the present invention, there is provided a substrate comprising a plurality of the DFB laser apparatus of any preceding claim, having different respective heater circuitry hard-wiring configurations to adjust each laser’s frequency towards a target value, by adjusting the heating of the respective DFB laser structure.

[0025] According to a third aspect of the present invention, there is provided a distributed feedback (DFB) laser apparatus comprising:

[0026] - a distributed feedback (DFB) laser comprising a DFB structure;

[0027] - a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and

[0028] - a heater circuitry hard-wiring configuration comprising:

[0029] - heater circuitry hard-wireable to route power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency; and

[0030] - a computer-readable medium comprising information describing a hard-wired configuration of heater circuitry to route power to a selected portion of the heater.

[0031] According to a fourth aspect of the present invention, there is provided a distributed feedback (DFB) laser apparatus comprising:

[0032] - a distributed feedback (DFB) laser comprising a DFB structure;

[0033] - a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and

[0034] - a heater circuitry hard-wiring configuration comprising:

[0035] - heater circuitry hard-wireable to route power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency; and

[0036] - a computer-readable medium comprising a result of a lasing frequency measurement of the DFB laser and information describing how to use the result to hard wire the heater circuitry to route power to a selected portion of the heater, the portion selected based on the measurement.

[0037] According to a fifth aspect of the present invention, there is provided a method of trimming wavelength of a DFB laser apparatus, the method comprising the steps:

[0038] (a) fabricating a distributed feedback (DFB) laser comprising a DFB structure;

[0039] (b) fabricating a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and

[0040] (c) producing, based on a lasing frequency measurement of the fabricated DFB laser, a heater circuitry hard-wiring configuration to route power to a selected portion or size of the heater such that heating the selected portion or size changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion or size, thereby adjusting the lasing frequency.

[0041] Preferably, fabricating the heater comprises fabricating a plurality of heater segments.

[0042] Preferably, the step of producing a heater circuitry hard-wiring configuration comprises fabricating heater circuitry hard-wired to route power to a portion of the heater, the portion selected based on the measurement.

[0043] Preferably, the step of fabricating heater circuitry comprises adding one or more circuitry element to the heater circuitry in a location based on the measurement, so as to route the power via the added one or more circuitry element.

[0044] Preferably, adding one or more circuitry element comprises adding one or more bond.

[0045] Preferably, adding one or more circuitry element comprises adding one or more conducting track. Preferably, fabricating heater circuitry comprises removing one or more circuitry element from the heater circuitry in a location based on the measurement, so as to route the power with one or more open circuit.

[0046] Preferably, the one or more circuitry element comprises one or more fuse and the heater circuitry is hard-wired to route the power by blowing the one or more fuse.

[0047] Preferably, the one or more circuitry element comprises one or more conducting track and the heater circuitry is hard-wired to route the power by cutting the one or more conducting track to form the one or more open circuit.

[0048] Preferably, the DFB structure and heater are fabricated on a first substrate and at least some heater circuitry components are fabricated on a second substrate, and the method further comprises assembling the first and second substrates together.

[0049] Preferably, the heater circuitry hard-wiring configuration comprises sizing of the fabricated heater based on the measurement.

[0050] Preferably, the method comprises fabricating the heater as one or more resistive layer arranged on top of the DFB structure.

[0051] Preferably, the method comprises fabricating a plurality of the DFB laser apparatuses on a substrate, the DFB laser apparatuses having different respective heater circuitry hard-wiring configurations based on different respective measurements of the respective fabricated DFB structure.

[0052] Preferably, the step of producing a heater circuitry hard-wiring configuration comprises recording a computer-readable medium comprising information describing a hard-wired configuration of heater circuitry to route power to a portion of the heater, the portion selected based on the measurement.

[0053] Preferably, the step of producing a heater circuitry hard-wiring configuration comprises recording one or more computer-readable medium comprising a result of the measurement and information describing how to use the result to hard wire the heater circuitry to route power to a portion of the heater, the portion selected based on the measurement.

[0054] Preferably, the method further comprises the step of using the heater circuitry hardwiring configuration to route power to the selected portion or size of the heater such that heating the selected portion or size changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion or size, thereby adjusting the lasing frequency.

[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.

[0058] Figures 2a and 2b illustrate, in schematic form, a known DFB laser and a DFB laser with heater.

[0059] Figure 3 is a graph of the distribution of lasing frequency for a population of DFB lasers, before and after trimming heating, based on a measurement of the DFB structure without heating during the measurement.

[0060] Figure 4 is a graph of the distribution of lasing frequency for a population of DFB lasers, before and after trimming heating, based on a measurement of the DFB structure with heating during the measurement.

[0061] Figures 5a and 5b illustrate, in schematic form, a DFB laser apparatus with measurements of a fabricated DFB structure, without and with heating, in accordance with an embodiment of the present invention. Figures 6a and 6b illustrate, in schematic form, two DFB lasers on a laser chip, each with different heater circuitry hard-wiring configurations with a bond added to the heater circuitry to route power to a portion of the heater, in accordance with an embodiment of the present invention.

[0062] Figure 7 illustrates, in schematic form, a DFB laser apparatus configured with a bond added to the heater circuitry to bridge a gap to route power to a portion of the heater, in accordance with an embodiment of the present invention.

[0063] Figure 8 illustrates, in schematic form, a DFB laser apparatus configured by blowing fuses in the heater circuitry to route power with open circuits to a portion of the heater, in accordance with an embodiment of the present invention.

[0064] Figure 9 illustrates, in schematic form, a DFB laser apparatus configured by laser cutting tracks to form open circuits in the heater circuitry to route power to a portion of the heater, in accordance with an embodiment of the present invention.

[0065] Figures 10a and 10b illustrate, in schematic form, a DFB laser apparatus with measurements of a fabricated DFB structure, and subsequent configuration with a conducting track added to the heater circuitry to route power to a portion of the heater, in accordance with an embodiment of the present invention.

[0066] Figure 11 illustrates, in schematic form, a DFB laser apparatus configured with a conducting track and pad added to the heater circuitry to route power to a portion of the heater, in accordance with an embodiment of the present invention.

[0067] Figures 12a and 12b illustrate, in schematic form, two DFB lasers on a substrate, each having different respective heater circuitry hard-wiring configurations, in accordance with an embodiment of the present invention.

[0068] Figure 13 illustrates, in schematic form, a DFB laser apparatus where the heater circuitry hard-wiring configuration comprises sizing of the heater length based on the measurements, in accordance with an embodiment of the present invention. Figure 14 illustrates, in schematic form, a DFB laser apparatus where the heater circuitry hard-wiring configuration comprises sizing of the heater length based on the measurements, and using a conducting track to route power to the heater, the conducting track in a location commensurate with the sizing of the heater, in accordance with an embodiment of the present invention.

[0069] Figures 15 illustrates, in schematic form, a DFB laser apparatus configured with bonds added to the heater circuitry to route power to a portion of the heater corresponding to a selected separate heater segment, in accordance with an embodiment of the present invention.

[0070] Figure 16 and 17a to 17b illustrate, in schematic form, a DFB laser apparatus assembly where the DFB structure and heater are fabricated on a first substrate and routing of power is configured by blowing fuses in heater circuitry fabricated on a sub-mount, in accordance with an embodiment of the present invention.

[0071] Figure 18 is a flowchart of a method of trimming wavelength of a DFB laser apparatus, in accordance with an embodiment of the present invention.

[0072] Figure 19 shows a simulation of optical power spectra, showing a single mode wavelength shift before and after trimming of the heating of the DFB by the heater.

[0073] Figure 20 illustrates, in schematic form, optical power spectra, showing multimode wavelength shift before and after trimming of the heating of the DFB by the heater.

[0074] Detailed description

[0075] In this description and claims, optical and optical radiation relate to electromagnetic radiation over a range of wavelengths not limited to visible radiation, such as wavelengths spanning ultraviolet, visible and infrared radiation. An InP distributed feedback (DFB) laser is described as an example of a DFB laser apparatus. Other compound semiconductor based devices may be used with embodiments. For example photonic devices based on GaAs, GaSb, or GaN, or photonic devices based on other material systems, may be used.

[0076] The examples described herein relate to DFB lasers fabricated with an etched facet, but the skilled person will appreciate that embodiments may include DFB lasers fabricated with a cleaved facet.

[0077] Figure 1 illustrates a known DFB laser, in this example an etched-facet laser chip 100, with a compound semiconductor laser on an InP (indium phosphide) 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 cross-section through the waveguide 104 along its propagation direction (length). Figure 1c is a crosssection (not to scale) along b-b shown in Figure 1 a. Figure 1c is thus a cross-section across the waveguide 104 perpendicular to its propagation direction.

[0078] The structure of the laser chip 100 is now described in the context of its wafer-scale fabrication.

[0079] 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.

[0080] 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.

[0081] 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. The effect of the spacing and resulting 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.

[0082] 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.

[0083] Finally, a metallisation step coats the underside of the wafer with metal 142.

[0084] 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.

[0085] 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 1 c. 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.

[0086] The n-type InP buffer layer 134 is the first of the epitaxial layers that is grown on the n-type InP substrate 136.

[0087] 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.

[0088] 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 (MOCVD). The upper epitaxial layer structure 146 includes the p-type InP cladding layer 122 under the p-type InGaAs contact layer 120.

[0089] In this 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.

[0090] Finally, the n-metal layer 142 is shown.

[0091] In operation, as shown at the left of Figure 1 b, a beam of optical radiation 140, 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.

[0092] 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 etch, which selectively stops on the p-type etch stop / grating layer 124. 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.

[0093] With reference to Figure 1 c, a contact is made by the metal layer 116 at the top of ridge 104 to the p-type InGaAs contact layer 120. Current spreads outwards from under the ridge 104 towards the active layer 125 because it is not laterally confined when it leaves the ridge downwards towards the active layers. Therefore the whole of the optical mode is supplied with current. The optical mode roughly corresponds to the projected coupling region 1 14.

[0094] In the drawings of Figure 1 and in subsequent drawings, features labelled with the same numerals correspond to the same features 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.

[0095] DFB laser arrays for WDM need accurate frequency control. Rather than wavelength trimming, frequency trimming can be referred to because specifications applied to dense WDM (DWDM) applications are defined with respect to the nominal central frequencies (ITU-T G.694.1 , Spectral grids for WDM applications: DWDM frequency grid).

[0096] Compared to irradiation trimming approaches, embodiments of the present invention provide faster wavelength trimming. For example bonding each laser in specific configuration is quicker than irradiating each laser, not least because the bonding often has to be performed anyway, even without trimming.

[0097] Compared to active heater control, an advantage of the present invention is that the setting of the heating is hard-wired once at the beginning of the DFB laser life cycle. Passive offset correction does not require control circuitry of active wavelength tuning approaches. This reduces the cost of the DFB laser system. In the description herein, an example of a DFB laser apparatus is an InP DFB laser chip, referred to as a DFB laser. Another example is a DFB laser chip flip-chip assembled with a sub-mount and / or silicon photonics (SiPh) chip. The embodiments can be used with other material systems, including for example GaAs and GaN DFB lasers with edge emission (e.g. ridge or buried heterostructure) or surface emission lasers (e.g. VCSELs) in for example InP, GaAs and GaN material systems.

[0098] In embodiments, post-fabrication (of the DFB structure) and post-test DFB wavelength / frequency trimming is achieved by hard-wiring a portion of a heater, a subset of heater circuits, or the size of the heater itself, to trim heating of the DFB structure in each laser in an array. The heater circuit wiring is hard-wired to trim the heater configuration to adjust each laser’s frequency towards a target value, by adjusting the heating of the respective DFB laser structure. The lasing frequency of a single mode can be adjusted (See Figure 19), and / or the lasing frequency can be adjusted to a different mode (see Figure 20).

[0099] In examples, the heater circuitry hard-wiring configuration for a laser is based on a measurement of the as-fabricated lasing frequency of that laser. The term “based on a measurement” in this description and claims means determined or calculated in dependence on a result or results of the measurement.

[0100] In the embodiments in this description and the claims, a lasing frequency measurement includes direct or indirect measurements of the lasing frequency of the DFB laser. The lasing frequency measurement may comprise a measurement of the lasing frequency or a measurement of the lasing wavelength measurement. The measurement may use for example an optical spectrum analyser, or a Fabry-Perot interferometer. The lasing frequency measurement may not be an absolute measurement, but may be a measurement of a value that depends on the DFB’s lasing frequency, for example, which is related by scaling or transformation of the lasing frequency.

[0101] A corresponding laser frequency trimming method involves:

[0102] - Designing a DFB laser with a target frequency ftgt. - Fabricating the lasers (thereby introducing a process-induced variation from the nominal frequency).

[0103] - For each laser, measuring the laser frequency offset relative to the target frequency ftgt (measured with no heating and / or with heating).

[0104] - For each laser, selecting a heater portion or size to be powered, based on the offset for the respective laser.

[0105] - For each laser, hard-wiring a heater circuitry configuration.

[0106] - For each laser, heating the laser in operation with its hard-wired heater circuitry configuration, thereby obtaining a lasing frequency trimmed to correct the offset.

[0107] The heater may be a plurality of discrete heater elements, or selected lengths of a heater element may be selectable (for example with pads along its length) or fabricated. One heater element can meander on and off the laser DFB structure. Heaters can be hard-wired for example by bonding, bonding across an insulating gap, track cutting or fuse blowing.

[0108] An embodiment may be a laser having unwired heaters, and a configuration map for subsequent hard-wiring, based on the frequency offset measurement.

[0109] Figure 2a shows a known DFB laser. A metal laser electrode 204, corresponding to metal layer 116 in Figures 1 a to 1c, provides power to the laser. The DFB structure 211 lies under the laser electrode 204 and corresponds to DFB grating 150 at the base of the ridge 104 in Figures 1 b and 1 c. Power is provided to the laser active region by a bond 228. For the bond 228, the bond wire is illustrated with a ball bond at the end, in the same way as for the other bonds illustrated in the Figures. The bond wire applies a voltage of 1 .5V to the laser electrode. 1 .5V is used throughout this description as an example, to represent the same or other supply voltages being used in practice. GND represents ground.

[0110] Figure 2b shows a known DFB laser with a heater 208. A dielectric layer (not shown) on top of and overlapping the metal laser electrode 204 isolates the heater from the laser electrode. The DFB structure 211 again corresponds to DFB grating 150 at the base of the ridge 104 in Figures 1 b and 1 c. Power is routed to the whole length of the heater 208 by bonds 214 and 226, via respective bond pads 210, 212. Again, for the bonds 214 and 216, bond wires are illustrated with a ball bond at their top ends.

[0111] Figures 3 and 4 illustrate the process of trimming frequency, which reduces variation in lasing frequency in a population of fabricated DFB laser apparatuses.

[0112] Figure 3 is a graph of the distribution of lasing frequency for a population of DFB lasers, before and after trimming heating, based on a lasing frequency measurement of the DFB laser, without heating during the measurement. The vertical axis is the quantity (QTY) of DFB lasers in the population. The horizontal axis is the frequency (f) of operation of the lasers. After lasers comprising a DFB structure are fabricated, they have a distribution 310 of measured lasing frequencies, centred on a nominal frequency fnom. The distribution 310 is the measured distribution before trimming. An example of trimming of one DFB laser apparatus is shown. A measurement 312 of one device results in a measured frequency fmea. Based on the frequency measurement, the target frequency ftgt, and a previous set of calibration measurements, and / or simulations, a heater circuitry hard-wiring configuration is produced and the heater circuitry is configured using the heater circuitry hard-wiring configuration to trim heating by the heater. This results in a shift 308 of the lasing frequency closer to a target frequency ftgt to result in a new lasing frequency 306. When this is done for a whole population of DFB lasers, it results in a distribution 304 of lasing frequencies, corrected after using a heater circuitry hard-wiring configuration to trim heating by the heater for each respective DFB laser in the population. Ideally, the distribution after trimming 304 is centred on the target frequency ftgt.

[0113] Figure 4 is a graph of the distribution of lasing frequency for a population of DFB lasers, before and after trimming heating, based on a measurement of the DFB structure with heating during the measurement. As for Figure 3, the vertical axis is the quantity of DFB lasers in the population and the horizontal axis is the frequency of operation of the lasers. After lasers comprising a DFB structure are fabricated, with DFB heating, they have a distribution 410 of measured lasing frequencies, centred on a nominal frequency fnom. The distribution 410 is the measured distribution before trimming. An example of trimming of one DFB laser apparatus is shown. A measurement 412 of one device with heating results in a measured frequency fmea. Based on the frequency measurement, the target frequency, and a previous set of calibration measurements, and / or simulations, a heater circuitry hardwiring configuration is produced and the heater circuitry is configured using the heater circuitry hard-wiring configuration to trim heating by the heater. This results in a shift 408 of the lasing frequency closer to a target frequency ftgt to result in a new lasing frequency 406. When this is done for a whole population of DFB lasers, it results in a distribution 404 of lasing frequencies, corrected after using the heater circuitry hard-wiring configuration to trim heating by the heater for each respective DFB laser in the population. Ideally, the distribution after trimming 404 is centred on the target frequency ftgt.

[0114] Trimming may also be achieved based on frequency measurements at two or more levels of heating, such as no heating and full heating, or levels in between. With two or more frequency measurements, the frequency as a function of heating can be more accurately characterised. This improves the accuracy of the configuration of the hard wiring of the heaters, to achieve a frequency distribution more tightly centred on the targe frequency.

[0115] Figures 5a and 5b illustrate, in schematic form, a DFB laser apparatus with measurements of a fabricated DFB structure, without and with heating.

[0116] Figure 5a shows a DFB laser with a metal laser electrode 504, corresponding to metal layer 116 in Figures 1 a to 1c, that provides power to the laser. The DFB structure 511 lies under the laser electrode 504 and corresponds to DFB grating 150 at the base of the ridge 104 in Figures 1 b and 1 c. The heater 508 is arranged to heat the DFB structure to change its refractive index, when power is supplied to the heater. The heater 508 comprises one or more resistive layer arranged on top of the DFB structure. Being on top of the DFB structure provides efficient heating of the DFB structure, because it is close to the DFB structure. In other examples (not shown) the heater may be partially on top of the DFB structure, or adjacent but close to the DFB structure. A dielectric layer (not shown) on top of and overlapping the metal laser electrode 504 isolates the pads 506 and heater 508 from the laser electrode 504. In this example, the heater 508 is defined lithographically after the bond pads 506, so the heater lies on top of the bond pads where they overlap. On other examples, the heater 508 may be defined lithographically before the bond pads 506, so the bond pads lie on top of the heater where they overlap. In both cases, the heater is advantageously on the ridge, so that it is arranged as close as possible to the DFB structure being heated. In other examples (not shown), the heater may be adjacent but close to the DFB structure. The heater may have power routed to it via bond pads and / or contact electrode metal, or other metal layers lithographically formed separately from the contact electrode and bond pads.

[0117] In Figures 5a and 5b, for the lasing frequency measurement of the fabricated DFB laser, power is routed to the laser active region by a probe tip 516.

[0118] In Figure 5a, for lasing frequency measurement without heating, the heater is not powered so is inactive. Pads 506 are provided as heater circuitry that can be later hard-wired to route power to a portion of the heater 508. Pads 510 and 512 for powering the heater are not used in Figure 5a, as no heating is required.

[0119] After measuring the lasing frequency of the DFB structure with no heating 550 and optionally measuring the heater material resistance, a heater circuitry hard-wiring configuration 552 is produced, based on the measurement of frequency of the fabricated DFB structure, for trimming heating by the heater.

[0120] In Figure 5b, for frequency measurement with heating, power is routed to the whole length of the heater 508 by probes 514 and 518, via respective bond pads 510, 512. The heater 508 thus powered is depicted in a heating state by being filled in black. After testing the DFB structure with heating 554, a heater circuitry hard-wiring configuration 556 is produced, based on the measurement of frequency of the fabricated DFB structure, for trimming heating by the heater. The heater circuitry hard-wiring configuration 552, 556 may have heater circuitry that is either hard-wired or hard wireable to route power to one or more selected portion or a size of the heater such that heating the selected one or more portion of the heater or sized heater changes refractive index of the DFB structure 511 at a position along the DFB structure corresponding to the selected one or more portion of the heater or sized heater, thereby adjusting the lasing frequency. The heater circuitry hard-wiring configuration 552, 556 may comprise a physical configuration as described with reference to Figures 6 to 15. The heater circuitry hard-wiring configuration 552, 556 is in this case hard-wired to route power to a selected portion or size of the heater.

[0121] The heater circuitry hard-wiring configuration 552, 556 may have an unconfigured physical configuration and computer file. The unconfigured heater circuitry is in this case hard-wirable to route power to a selected portion or size of the heater. The heater circuitry hard-wiring configuration 552, 556 further comprises a computer- readable medium (such a computer file) comprising information describing a hardwired configuration of heater circuitry to route power to a portion of the heater, the portion selected based on the lasing frequency measurement of the fabricated DFB laser.

[0122] Alternatively, the computer-readable medium may comprise a result of the lasing frequency measurement and information describing how to use the result to configure hard wire the heater circuitry to route power to the selected portion of the heater, the portion selected based on the measurement.

[0123] The heater circuitry hard-wiring configuration 552, 556 may also be based on measurements or predictions of resistance of the heater material. Wafer-to-wafer and cross-wafer variation in the heater material thickness and / or linewidth can be measured using process control structures. With knowledge of which wafer and where on the wafer the specific DFB laser was fabricated, the heater circuitry hardwiring configuration 552 can be adjusted to compensate for the resistance variation.

[0124] The heater circuitry hard-wiring configuration 554 may also be based on measurements of resistance of the specific DFB laser’s heater 506 itself (if fabricated before testing). The four pads with two at each end of the heater allow a 4-point probe measurement of heater resistance for all or a sample of the heaters. This can provide an accurate value of the specific laser heater resistance, or wafer-to-wafer and cross-wafer heater resistance variation. In both cases, the knowledge of the actual heater resistance can be taken into account in the determination of the heater circuitry hard-wiring configuration.

[0125] Figures 6a and 6b illustrate, in schematic form, two DFB lasers 650, 652 on a laser chip substrate 657 comprising a plurality of DFB lasers in a laser array 658 suitable for a Continuous-Wave Wavelength Division Multiplexing (CW-WDM) application. In Figure 6a, the dashed-line rectangle 657’ represents a part of the laser chip substrate 657. The lasers have different respective heater circuitry hard-wiring configurations based on different respective on-chip frequency measurements of the respective fabricated DFB structures, with a bond added to the respective heater circuitry to route power to a portion of the respective heater. This laser-chip embodiment may have the plurality of DFB lasers with different respective heater circuitry hard-wiring configurations of the other types described with reference to the other figures, such as using straps (see Figures 12a and 12b), fuse or laser cut or sizing configurations.

[0126] Power is routed to the laser active region by a bond 630 and electrode 504. The bond wire 630 applies a voltage of 1 .5V to the laser electrode.

[0127] In Figure 6a, the heater circuitry hard-wiring configuration 556 such as produced from the measurements in Figure 5a and 5b, is implemented 656, in each DFB laser 650,652 respectively, as heater circuitry hard-wired to route power to a portion 616, 618 of the heater, the portion selected based on the respective measurement. In one laser 650 shown in Figure 6a, power is routed to the selected portion 616 of the heater 508 by bonds 624 and 628, via respective bond pads 622, 626. In another laser 652 shown in Figure 6a, power is routed to the selected portion 618 of the heater 508 by bonds 634 and 638, via respective bond pads 632, 636. The bonds are circuitry elements.

[0128] The skilled person will appreciate that one or more lengths of the heater can be powered, depending on the hard-wiring configuration of the bond pads. Thus the heating may be trimmed in magnitude and / or position along the DFB structure, thus power is routed to a selected portion 616 of the heater such that heating the selected portion 616 changes refractive index of the DFB structure 511 at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency. The heater circuitry is thus hard-wired to route the power via one or more bond added to the heater circuitry in a location based on the measurement. For example, power can be routed through four of the bond pads to two portions of the heater along the DFB structure. In an example (not shown) power is routed from a supply through a first bond, via a fist selected portion of the heater, then via a second bond to a short (between second and third bonds) then via the third bond to a second selected portion of the heater, then via a fourth bond to ground. Instead of being shorted, the second and third bonds may be separately connected to a power supply.

[0129] Figure 7 illustrates, in schematic form, a DFB laser apparatus configured with a bond added to the heater circuitry to bridge a gap to route power to a portion of the heater. Most of the features are numbered and described as for Figure 6a. But instead of a simple strip heater 508 as illustrated in Figure 6a, the heater 708 in Figure 7 and subsequent figures has protrusions or stubs 709 extending to the bond pads. These stubs may be made wider (not shown) than the heater strip to reduce the stub resistance and any resulting heating effect outside the area of the DFB structure.

[0130] In this example, power is routed to the laser active region by bond 724 and electrode 704. The bond 724 bridges the gap between electrode 704 and bond pad 725 to power both the laser and the heater simultaneously. The bond 724 applies a voltage of 1 .5V to the laser electrode and heater. Power is thus routed to the selected portion 716 of the heater 708 by bonds 724 and 728, via respective bond pads 725, 512.

[0131] Figure 8 illustrates, in schematic form, a DFB laser apparatus configured by blowing fuses in the heater circuitry to route power with open circuits to a portion of the heater.

[0132] In this example, the heater circuitry is hard-wired to route the power with open circuits 807 where fuses have been removed from the heater circuitry in a location based on the frequency measurement, by blowing the fuses. The power is routed through the one fuse 809 that has not been blown. A fuse may be blown by passing a large current through the fuse, via probe needles (not shown) contacting pads 822, 806 on either side of the fuse. Power is routed to the selected portion 816 of the heater 708 by the unblown fuse 809 and bonds 824 and 828, via respective bond pads 822, 512. For arranging the laser in an array, the contacting pads 822, 806 on either side of the fuses can be placed differently to the example shown, to allow the parallel laser DFB structures to be closer together in the array.

[0133] Figure 9 illustrates, in schematic form, a DFB laser apparatus configured by laser cutting a track to form open circuits in the heater circuitry to route power to a portion of the heater.

[0134] In this example, the heater circuitry is hard-wired to route the power with open circuits 907 where fuses have been removed from the heater circuitry in a location based on the frequency measurement, by electrically opening conducting tracks. The power is routed through the one conducting track 909 that has not been cut. A conducting track may be electrically opened to form an open circuit for example by laser cutting or lithographically patterned etching. Power is routed to the selected portion 916 of the heater 708 by the remaining conducting track 909 and bonds 924 and 928, via respective bond pads 922, 512.

[0135] Figures 10a and 10b illustrate, in schematic form, a DFB laser apparatus with frequency measurements of a fabricated DFB structure, and subsequent configuration with a conducting track added to the heater circuitry to route power to a portion of the heater.

[0136] Figure 10a shows a heater 1008 is arranged to heat the DFB to change its refractive index, when power is supplied to the heater. In this example, the heater 1008 is defined lithographically after the bond pads 510, 512, so the heater lies on top of the ridge and on the bond pads where they overlap. For lasing frequency measurement of the fabricated DFB laser (in this example a frequency or wavelength measurement, as described in relation to Figures 5a and 5b), power is routed to the laser active region by a probe tip 516. For frequency measurement without heating, the heater is not powered so is inactive. Pads 510 and 512 for powering the heater need not then be used, as no heating is required. For frequency measurement with heating, power is routed to the whole length of the heater 1008 by probes 514 and 518, via respective bond pads 510, 512. After frequency testing of the DFB structure without and / or with heating, a heater circuitry hard-wiring configuration is produced, based on the measurement of frequency of the fabricated DFB structure, for trimming heating by the heater.

[0137] With reference to to Figure 10b, power is routed to the laser active region by a bond 630 and electrode 504. The bond 630 applies a voltage of 1 .5V to the laser electrode.

[0138] The heater circuitry hard-wiring configuration, produced from the measurements in Figure 10a, is implemented in heater circuitry hard-wired to route power to a portion 1016 of the heater, the portion selected based on the frequency measurement. Power is routed to the selected portion 1016 of the heater 1008 by adding a strap circuitry element 1007, which is a conducting track, along with contact windows (not shown) under each end of the strap through a dielectric layer (not shown) that lies on the heater. Bond 630, connected to end of the selected portion by the strap 1007, and another bond 638 provide power via respective electrode 504 and bond pad 512. The heater portion 1016 thus powered is depicted in a heating state by being filled in black.

[0139] Figure 11 illustrates, in schematic form, a DFB laser apparatus configured with a conducting track and pad added to the heater circuitry to bridge a gap to route power to a portion of the heater. The difference between Figure 10b and 11 is that the strap 1007 is extended by a large rectangle 1122 which can be bonded to by a separate bond 1124.

[0140] With reference to Figures 10b and 11 , the skilled person will appreciate that one or more lengths of the heater can be powered, depending on the hard-wiring configuration of one (shown) or more (not shown) straps and bond pads. Thus the heating may be trimmed in magnitude and / or position along the DFB structure, thus power is routed to a selected portion 1016 of the heater such that heating the selected portion 1016 changes refractive index of the DFB structure 511 at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency. The heater circuitry may thus be hard-wired to route the power via one or more conducting track added to the heater circuitry in a respective location based on the measurement.

[0141] Figures 12a and 12b illustrate, in schematic form, two DFB lasers 1250, 1252 on a wafer substrate 1257 comprising a plurality 1258 of DFB lasers. In Figure 12a, the dashed-line rectangle 1257’ represents a part of the substrate 1257. The lasers have different respective heater circuitry hard-wiring configurations based on different respective on-substrate frequency measurements of the respective fabricated DFB structures.

[0142] In this example, two DFB lasers 1250, 1252 are shown having different selected portion lengths configured as described with reference to Figure 10b. This embodiment may have the plurality of DFB lasers with different respective heater circuitry hard-wiring configurations of the other types described with reference to the other figures, such as using bond pad, fuse or laser cut or sizing configurations.

[0143] Figure 13 illustrates, in schematic form, a DFB laser apparatus where the heater circuitry hard-wiring configuration comprises sizing of the heater length based on the frequency measurements. The DFB laser of Figure 13 is similar to that of Figure 10b. However, in Figure 13, the heater 1308 is formed after the frequency measurement. With no DFB heaters on the wafer at the measurement stage, an on-wafer frequency measurement of the DFB lasers is therefore performed without DFB heating.

[0144] Different respective heater length sizing is based on the on-substrate measurements of the respective fabricated DFB structures. Then the heater is fabricated to the required size, to trim the heating, such that heating the selected size of heater 1308 changes refractive index of the DFB structure 511 at a position along the DFB structure corresponding to the selected size of heater 1308, thereby adjusting the lasing frequency. In this example, a stub 1323 protrudes from the heater 1308, to make contact with a pad 1322. The long pad 1322 can accommodate such connection by a range of heater lengths, as it is as long as the DFB structure 511 on which the heater might extend. Bond 1324, connected to end of the heater 1308 by the stub 1323, and another bond 638 can provide power to the heater via respective bond pads 1322, 512. For clarity, the heater 1308 is depicted in an unpowered heating state.

[0145] In other embodiments (not shown) sizing of the heater width may be used in addition or alternatively to the sizing of the heater length.

[0146] Figure 14 illustrates, in schematic form, a DFB laser apparatus where the heater circuitry hard-wiring configuration comprises sizing of the heater length based on the measurements, and using a conducting track to route power to the heater, the conducting track in a location commensurate with the length of the heater.

[0147] Compared to the DFB laser of Figure 13, the sized heater 1408 is a straight bar. Subsequent to the heater fabrication a conducting track 1423 and pad 1422 are fabricated to route power to the heater. The conducting track 1423 is in a location based on the frequency measurement and commensurate with the sizing of the heater.

[0148] Bond 1424, connected to end of the sized heater 1408 by the conducting track 1423, and another bond 638 can provide power to the heater via respective bond pads 1422, 1412. Again for clarity, the heater 1408 is depicted in an unpowered heating state.

[0149] Figures 15 illustrates, in schematic form, a DFB laser apparatus configured with bonds added to the heater circuitry to route power to a portion of the heater corresponding to a selected separate heater segment.

[0150] With reference to figure 15, in this example, the DFB laser comprises four separate heater segments 1514, 1516, 1520, 1522. These segments are castellated (or serpentine) and interleaved. This has the advantage that on a section of the DFB structure, one or two heaters may be activated to trim the heating by routing power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency. The heater segments 1514, 1516, 1520, 1522 thus comprise resistive layers arranged partially on top of the DFB structure.

[0151] Power is routed to the laser active region by bonds 1528, 1534, via the contact electrode 504. The heater circuitry hard-wiring configuration, such as produced from measurements similar to those described with reference to Figure 5a and 5b, is implemented in heater circuitry hard-wired to route power to a portion 1522 of the heater, the portion selected based on the frequency measurement. Power is routed to the selected portion 1522 of the whole heater 1514, 1516, 1520, 1522 by bonds 1530 and 1538, via respective bond pads 1532, 1540. Other segments 1514, 1516, 1520 can be selected similarly by placement of bond wires to these and the other pads 1506, 1508, 1510, 1526 and supplying power through bond wires, based on the frequency measurement.

[0152] Figure 16 and 17a to 17b illustrate, in schematic form, a DFB laser apparatus assembly where the DFB structure and heater are fabricated on a first substrate and routing of power is configured by blowing fuses in heater circuitry fabricated on a sub-mount.

[0153] With reference to Figure 16 and Figure 17a, the DFB structure 511 and heater 708 are fabricated on a in InP chip 1600. With reference to Figure 16 and Figure 17b, the heater circuitry comprises components including fuses 1670 fabricated on a submount substrate 1640. The InP chip 1600 and sub-mount 1640 are flip-chip assembled together, as illustrated in Figure 16.

[0154] Power is routed to the laser active region from the sub-mount pad 1632 by solder 1622 and electrode 504. The sub-mount pad 1632 applies a voltage of 1 .5V to the connected laser electrode.

[0155] The heater circuitry hard-wiring configuration 552, 556 such as produced from the measurements in Figure 5a and 5b, is implemented in heater circuitry hard-wired on the sub-mount to route power to a portion 916 of the heater on the InP chip, the portion selected based on the frequency measurement of the DFB laser on the InP chip. In this example, the heater circuitry is hard-wired to route the power with open circuits 1670 where fuses have been removed from the heater circuitry on the submount in a location based on the measurement, by blowing the fuses. The power is routed through the one fuse 1669 that has not been blown. A fuse 1667 may be blown by passing a large current through the fuse, before flip-chip assembly, via probe needles (not shown) contacting pads 1638, 1634 on either side of the fuse 1667. Power is routed to the selected portion 916 of the heater 708 along the following path: The sub-mount pad 1638 applies a voltage of 1 .5V. Sub-mount pad 1638 is connected by track 1664 and unblown fuse 1669 via sub-mount pad 1636 and solder 1626 to InP chip pad 1606. This pad 1606 is connected to the selected portion of the heater 916. The other end of the portion 916 is connected via InP chip pad 1608 and solder 1628 to sub-mount pad 1638, which is connected to ground.

[0156] Instead of fuse blowing, other ways of hard-wiring may be used to route the power with one or more open circuit by removing one or more circuitry element from the heater circuitry in a location on the sub-mount based on the measurement. For example laser cutting or etching could be used. Alternatively additive methods such as bonding to the assembled flip-chip package could be configured to route power in a heater circuitry hard-wiring configuration.

[0157] Figure 18 is a flowchart of a method of trimming wavelength of a DFB laser apparatus.

[0158] The method has the steps:

[0159] 1810: Designing a DFB laser with a target frequency ftgt.

[0160] 1812: Fabricating a distributed feedback (DFB) structure 511 , 150. This step may involve fabricating a heater 508 arranged to heat the DFB structure to change its refractive index. Fabricating the heater 508 may involve fabricating a plurality of heater segments, as described with reference to Figure 15. Fabricating the heater may be done before or after performing the frequency measurement: with this step 1812 of fabricating the DFB structure, or with the later step 1818 of producing a heater circuitry hard-wiring configuration, when that step involves heater fabrication.

[0161] 1816: Obtaining a lasing frequency measurement of the DFB laser, such as laser frequency or wavelength, of the fabricated DFB laser. An optical spectrum analyser (OSA) or wavelength meter may be used to obtain the lasing wavelength from which the lasing frequency is obtained.

[0162] 1818: Producing a heater circuitry hard-wiring configuration 1820, based on the measurement, for trimming heating by the heater, by routing power to a selected portion or size of the heater such that heating the selected portion or size changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion or size, thereby adjusting the lasing frequency. Preferably, the target frequency will be taken into account, for example to determine an offset of the measured frequency from the target. The magnitude of the offset will be related to the amount of heating.

[0163] As well as the actual measured frequency and the target frequency, the characteristics of the heater circuitry 1808 are taken into account in producing a heater circuitry hard-wiring configuration 1820. For example, in a characterisation process ahead of volume laser production, DFB lasers can be fabricated 1802 and a range of heater circuitry configurations are defined 1804. Next the lasing frequency is tested 1806 over some or all of the range of heater circuitry configurations. This yields the characteristics of the heater circuitry 1808, for example a graph or table of frequency shift as a function of heater length (and / or heater material resistivity). The characteristics of the heater circuitry 1808 can be used to calculate a frequency shift for a specific DFB laser, to correct the measured frequency offset from the target frequency.

[0164] This step of producing a heater circuitry hard-wiring configuration may involve fabricating heater circuitry hard-wired to route power to a portion 517 of the heater, the portion selected based on the frequency measurement, for example as described with reference to Figures 5a to 12b. Fabricating heater circuitry may involve adding one or more circuitry element to the heater circuitry in a respective location based on the measurement, so as to route the power via the added one or more circuitry element. Adding one or more circuitry element may involve adding one or more bond as described with reference to Figures 5a to 7 and 15. Alternatively, adding one or more circuitry element may involve adding one or more conducting track as described with reference to Figures 10a to 12b. Conducting tracks may be added by for example photolithography and metal deposition or by 3D printing of a conductor.

[0165] Fabricating heater circuitry may involve removing one or more circuitry element from the heater circuitry in a respective location based on the measurement, so as to route the power with one or more open circuit. The removed circuitry element(s) may be one or more fuse. In this case the heater circuitry is hard-wired to route the power by blowing the one or more fuse as described with reference to Figures 8 and 16 to 17b. The removed circuitry element(s) may be one or more conducting track. In this case, the heater circuitry may be hard-wired to route the power by cutting the one or more conducting track to form the one or more open circuit as described with reference to Figure 9.

[0166] This step 1818 of producing a heater circuitry hard-wiring configuration may involve sizing the fabricated heater based on the measurements, the sizing different from corresponding dimensions of heaters on other DFB laser apparatuses fabricated on a substrate with the photonic device, such as described with reference to Figures 13 and 14. In this case the heater is fabricated after the measurements. When sizing is used, a conducting track may be fabricated to route the power to the sized heater, the conducting track in a location based on the measurement and commensurate with the sizing of the heater, as described with reference to Figure 14.

[0167] This step 1818 of producing a heater circuitry hard-wiring configuration may involve fabricating a plurality of the DFB laser apparatuses on a substrate, the DFB laser apparatuses having different respective heater circuitry hard-wiring configurations based on different respective measurements of the respective fabricated DFB structure, as described with respect to Figures 12a and 12b. This step 1818 of producing a heater circuitry hard-wiring configuration may involve recording a computer-readable medium 1820 comprising information describing a hard-wired configuration of heater circuitry to route power to a portion 517 of the heater, the portion selected based on the measurement.

[0168] This step 1818 of producing a heater circuitry hard-wiring configuration may involve recording one or more computer-readable medium 1820 comprising a result of the lasing frequency measurement and information describing how to use the result to hard wire the heater circuitry to route power to a portion of the heater, the portion selected based on the measurement.

[0169] 1822: Using the heater circuitry hard-wiring configuration to trim heating by the heater. This may be done by routing power to the selected portion or size of the heater such that heating the selected portion or size changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion or size, thereby adjusting the lasing frequency

[0170] If the step 1818 of producing a heater circuitry hard-wiring configuration involves fabricating heater circuitry, then this step 1822 of using the heater circuitry hardwiring configuration involves powering up the fabricated circuitry to trim heating by the heater.

[0171] If the step 1818 of producing a heater circuitry hard-wiring configuration involves recording a computer-readable medium 1820, then this step 1822 of using the heater circuitry hard-wiring configuration involves fabricating heater circuitry as described in relation to step 1818 then powering up the fabricated circuitry to trim heating by the heater.

[0172] 1824: In some applications, the DFB laser can be assembled with for example a submount or silicon photonics chip. Thus the DFB structure and heater may be fabricated on a first substrate (e.g. a laser chip) and at least some heater circuitry components fabricated on a second substrate (e.g. a sub-mount), such as described with reference to Figures 16 to 17b. Figure 19 shows a simulation of optical power spectra, showing a single mode wavelength shift by trimming the heating in accordance with embodiments of the present invention. The horizontal axis is wavelength (nm) and the vertical axis is optical power P (dBm / nm). The spectra before 1902 (dashed line) and after 1094 trimming of the heating of the DFB by the heater are shown. It can be seen that the lasing wavelength (i.e. highest emission peak) shifts to a lower wavelength, after trimming of the heating.

[0173] Figure 20 shows optical power spectra, showing multimode wavelength shift by trimming the heating in accordance with embodiments of the present invention. The horizontal axis is wavelength (nm) and the vertical axis is optical power P (dBm / nm). The spectrum before 2002 (dashed line) and after 2004 trimming of the heating of the DFB by the heater are shown. It can be seen that the lasing mode (i.e. highest emission peak) switches to another mode on the other side of the stopband 2006 at a lower wavelength, after trimming of the heating.

Claims

Claims1 . A distributed feedback (DFB) laser apparatus comprising:- a distributed feedback (DFB) laser comprising a DFB structure;- a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and- a heater circuitry hard-wiring configuration comprising heater circuitry hardwired to route power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency.

2. The DFB laser apparatus of claim 1 , wherein the heater comprises a plurality of separate heater segments.

3. The DFB laser apparatus of claim 1 or claim 2, wherein the heater circuitry hardwiring configuration selects a heater portion or size to be powered.

4. The DFB laser apparatus of any preceding claim, wherein the heater circuitry is hard-wired to route the power via one or more circuitry element added to the heater circuitry.

5. The DFB laser apparatus of claim 4, wherein the one or more circuitry element comprises one or more bond.

6. The DFB laser apparatus of claim 4, wherein the one or more circuitry element comprises one or more conducting track.

7. The DFB laser apparatus of any of claims 3 to 6, wherein the heater circuitry is hard-wired to route the power with one or more open circuit where one or more circuitry element has been removed from the heater circuitry.

8. The DFB laser apparatus of claim 7, wherein the one or more circuitry element comprises one or more fuse and the heater circuitry is hard-wired to route the power by blowing the one or more fuse.

9. The DFB laser apparatus of claim 7, wherein the one or more circuitry element comprises one or more conducting track and the heater circuitry is hard-wired to route the power by electrically opening the one or more conducting track to form the one or more open circuit.

10. The DFB laser apparatus of any preceding claim, wherein the DFB structure and heater are fabricated on a first substrate and the heater circuitry comprises components fabricated on a second substrate, the first and second substrates assembled together.11 . The DFB laser apparatus of any preceding claim wherein the heater circuitry hard-wiring configuration comprises sizing of the heater.

12. The DFB laser apparatus of any of preceding claim wherein the heater comprises a resistive layer arranged at least partially on top of the DFB structure.

13. A substrate comprising a plurality of the DFB laser apparatus of any preceding claim, having different respective heater circuitry hard-wiring configurations to adjust each laser’s frequency towards a target value, by adjusting the heating of the respective DFB laser structure.

14. A distributed feedback (DFB) laser apparatus comprising:- a distributed feedback (DFB) laser comprising a DFB structure;- a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and- a heater circuitry hard-wiring configuration comprising:- heater circuitry hard-wireable to route power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency; and- a computer-readable medium comprising information describing a hard-wired configuration of heater circuitry to route power to a selected portion of the heater.

15. A distributed feedback (DFB) laser apparatus comprising:- a distributed feedback (DFB) laser comprising a DFB structure;- a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and- a heater circuitry hard-wiring configuration comprising:- heater circuitry hard-wireable to route power to a selected portion of the heater such that heating the selected portion changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion, thereby adjusting the lasing frequency; and- a computer-readable medium comprising a result of a lasing frequency measurement of the DFB laser and information describing how to use the result to hard wire the heater circuitry to route power to a selected portion of the heater, the portion selected based on the measurement.

16. A method of trimming wavelength of a DFB laser apparatus, the method comprising the steps:(a) fabricating a distributed feedback (DFB) laser comprising a DFB structure;(b) fabricating a heater arranged to heat the DFB structure to change a lasing frequency of the DFB laser; and(c) producing, based on a lasing frequency measurement of the fabricated DFB laser, a heater circuitry hard-wiring configuration to route power to a selected portion or size of the heater such that heating the selected portion or size changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion or size, thereby adjusting the lasing frequency.

17. The method of claim 16, wherein fabricating the heater comprises fabricating a plurality of heater segments.

18. The method of 16 or claim 17, wherein the step of producing a heater circuitry hard-wiring configuration comprises fabricating heater circuitry hard-wired to route power to a portion of the heater, the portion selected based on the measurement.

19. The method of claim 18, wherein the step of fabricating heater circuitry comprises adding one or more circuitry element to the heater circuitry in a location based on the measurement, so as to route the power via the added one or more circuitry element.

20. The method of claim 19, wherein adding one or more circuitry element comprises adding one or more bond.21 . The method of claim 19, wherein adding one or more circuitry element comprises adding one or more conducting track.

22. The method of any of claims 18 to 21 , wherein the step of fabricating heater circuitry comprises removing one or more circuitry element from the heater circuitry in a location based on the measurement, so as to route the power with one or more open circuit.

23. The method of claim 22, wherein the one or more circuitry element comprises one or more fuse and the heater circuitry is hard-wired to route the power by blowing the one or more fuse.

24. The method of claim 22, wherein the one or more circuitry element comprises one or more conducting track and the heater circuitry is hard-wired to route the power by cutting the one or more conducting track to form the one or more open circuit.

25. The method of any of claims 18 to 24, wherein the DFB structure and heater are fabricated on a first substrate and at least some heater circuitry components are fabricated on a second substrate, and the method further comprises assembling the first and second substrates together.

26. The method of claim 18 wherein the heater circuitry hard-wiring configuration comprises sizing of the fabricated heater based on the measurement.

27. The method of any of claims 16 to 26 comprising fabricating the heater as one or more resistive layer arranged on top of the DFB structure.

28. The method of any of claims 16 to 27 comprising fabricating a plurality of the DFB laser apparatuses on a substrate, the DFB laser apparatuses having different respective heater circuitry hard-wiring configurations based on different respective measurements of the respective fabricated DFB structure.

29. The method of claim 16 or claim 17, wherein the step of producing a heater circuitry hard-wiring configuration comprises recording a computer-readable medium comprising information describing a hard-wired configuration of heater circuitry to route power to a portion of the heater, the portion selected based on the measurement.

30. The method of claim 16 or claim 17, wherein the step of producing a heater circuitry hard-wiring configuration comprises recording one or more computer- readable medium comprising a result of the measurement and information describing how to use the result to hard wire the heater circuitry to route power to a portion of the heater, the portion selected based on the measurement.31 . The method of any of claims 16 to 30 further comprising the step of using the heater circuitry hard-wiring configuration to route power to the selected portion or size of the heater such that heating the selected portion or size changes refractive index of the DFB structure at a position along the DFB structure corresponding to the selected portion or size, thereby adjusting the lasing frequency.

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