METHOD AND SYSTEM FOR HYBRID INTEGRATED IXN DWDM TRANSMITTER

Field of the Invention

The present invention is directed to fiber optical transport systems More particularly, the invention provides a method and system for integrating semiconductor (for example. hiP) laser/modulator chips on an aluminum nitride submount with a silica/silicon AWG ^" Merely b> way of example, the invention has been applied to a wav elength multiplexed tiansmitter array But it would be recognized that the invention has a much broader range of applicability

Background of the Invention

Since its deployment in the middle of 1990s, dense wavelength div ision multiplexing (DWDM) has become a dominant technology for all long haul and regional backbone transport networks, and is gradually making its way to metro area networks As the technology is maturing, the cost for making such systems and related components has been decieasmg Now the price bottleneck is moving toward the packaging cost of each individual optical component, such as lasers, modulators, and MUX BEMUX filters

Currently optical components in a DWDVI transport system are indiv idually packaged The packaging cost of these components (e g . a 1 OG transmitter) becomes a bottleneck, pϊ eventing further reduction in price For example, the cost of a bare distributed feedback (DFB) laser chip is only a feu dollars, while a packaged DFB laser sells for several hundred dollars Thus, for a DWDM system vendor to be competitiv e, it is desirable to remo\ e this price bottleneck, making the packaging cost a less concern in manufacturing optical components

In a conventional DWDM transport system, each optical component, either a laser or a ^" MUX filter, is individually packaged For example, a line card is built around a transmitter receiv er module which includes a laser, a modulator (or an integrated laser/modulator) and a receiver The laser comes typically as an tndiuni phosphide (InP) chip sitting inside a Butterfly package The optical output of the line card is combined with other line cards of different wav elengths v ia a wavelength multiplexer which often includes an arras ed wa\ eguide giating (λWG) made of

silica-on-silicon. These iine cards are connected to the multiplexer card using fiber jumpers and the combined optical output is then amplified before launching into a fiber network.

Even though these conventional DWDM systems are useful in some areas., they have many limitations that restrict their effectiveness in broader applications. Some of these limitations are discussed below, and improved techniques based on embodiments of the present invention are presented.

The present invention is directed to fiber optical transport systems. More particularly, the invention provides a method and system for integrating semiconductor (InP) laser/modulator chips on an aluminum nitride subraount with a silica/si! icon AWG. Merely by way of example, the invention has been applied to a wavelength multiplexed transmitter array. But it would be recognized that the invention has a much broader range of applicability.

According to a specific embodiment of the invention, an integrated DWDM transmitter apparatus includes a first support component and a silica-on-silicon substrate overlying the first support component. The sϋica-on-siiicon substrate includes a silica layer overlying a silicon substrate. A coefficient of thermal expansion of the first support component is substantially matched to a coefficient of thermal expansion of the silicon substrate. The apparatus also includes an optical multiplexer within the silica layer, which includes a plurality of input waveguides and at least an output waveguide. Additionally, the apparatus includes a second support component attached to a side surface of the first support component. One or more semiconductor laser array chips are mounted to overlie the second support component. A coefficient of thermal expansion of the one or more semiconductor chips is substantially matched to a coefficient of thermal expansion of the second support component. Moreover, each of the one or more laser array chips includes one or more lasers, each of which is optically coupled to a corresponding one of the plurality of input waveguides.

According to an alternative embodiment of the invention, an integrated DWDM transmitter apparatus includes a first support component and a silica-on~silicon

substrate overlying the first support component The sϋica-on-silicon substrate includes a silica layer overlying a silicon substrate A coefficient of thermal expansion of the first support component is substantially matched to a coefficient of thermal expansion of the silicon substrate. The apparatus also includes an optical multiplexer within the silica Saver., which includes a plurality of input waveguides and at least an output waveguide. Additionally, the apparatus includes a second support component attached to a side surface of the first support component. One or more semiconductor laser array chips are mounted to overlie the second support component. A coefficient of thermal expansion of the one or more semiconductor chips is substantially matched to a coefficient of thermal expansion of the second support component. Moreover, each of the one or more laser array chips includes one or more lasers, each of which is optical Iy coupled to a corresponding one of the plurality of input waveguides. The transmitter apparatus also includes a plurality of micro heaters., each of the plurality of micro heaters being iocated adjacent to a correspond one of the one or more lasers. In a specific embodiment of the transmitter apparatus, each of the one or more semiconductor laser array chips includes one or more InP lasers, and the second support component comprises aluminum nitride (AlN).

According to another embodiment, the invention provides a method for making an integrated DWDM transmitter apparatus. The method includes forming an optical multiplexer in a siiica-on-silicon substrate. The silica-on-silicon substrate includes a silica layer overlying a silicon substrate, and the optical multiplexer has a plurality of input waveguides and at least an output waveguide. The method also includes providing a first support component A coefficient of thermal expansion of the first support component is substantially matched to a coefficient of thermal expansion of the silicon substrate For example, the first support component can be a silicon substrate characterized by a thickness of approximately lmm The method further includes attaching the sϋica-on-silicon substrate to the first support component. According to the method, a second support component is provided Additionally; one or more semiconductor laser array chips are mounted to a top surface of the second support component A coefficient of thermal expansion of the one or more semiconductor laser array chips is substantially matched to a coefficient of thermal expansion of the second support component. Each of the one or more semiconductor laser array chips includes one or more lasers. The method also includes aligning

each of the one or more lasers to a corresponding one of the plurality of input waveguides tbf prov iding an optical coupling Additionally, the method furthei includes attaching the second support component to the first support component

According to yet another embodiment, the invention provides an integrated dense wavelength division multiplexing (DWDM) transmitter apparatus that converts multiple electrical signals to a multiple-channel DWDM signal The apparatus includes a plurality of input terminals for receiving the multiple electrical signals The apparatus also includes a first support component and a silica-oπ-silicoπ substrate overlying the first support component The siliea-on-silicon substrate includes a silica iayei overlying a silicon substrate A coefficient of thermal expansion of the first support component is substantially matched to a coefficient of thermal expansion of the silicon substrate The apparatus further includes a second support component attached to the silica-on-silicon substrate and the first support component One or more semiconductor laser at ray chips are mounted overlying the second suppott component The one or more semiconductor laser array chips are coupled to the plurality of input terminals for receiving the multiple electrical signals and converting the multiple electrical signals to corresponding multiple optica! signals bach of the one or more laser array chips includes one or more lasers Additionally, the appatatus includes an optical multiplexer coupled to the one or mote semiconductor laser array chips for conv erting the multiple optical signals to the multiple-channel DWDM signal The optical multiplexer is located in the silica-on-silicon substrate which is mounted on the first support component, e g , a second silicon substrate The optical multiplexer includes a plurality of input waveguides each of which is coupled to a corresponding laser foi teceiving a corresponding optical signal and at least an output w aveguide for outputting the multiple-channel DWDM signal

Many benefits are achieved by way of the present invention over conventional techniques" For example, in certain embodiments, the invention prov ides" a method of integrating various optical components using excellent CTt match between InI* and AIK as well as good thermal conduct! \ ϊfy of AlN According to embodiments of the present i mention, the method is readily scaSeable to large laser arrav chips for further reduction of cost and s\ stem footprint as well as increasing product reliability and manufacture v ϊeid According to certain embodiments of the invention, iarge

array chips can enable further improvements of optical coupling between AWG and transmittei s For example, vertical variation in heigh! jtelative to the \\a\ egυides can be minimized from transmitter to transmitter if these transmitters are formed on a single array chip In contrast, large arrav chips can not be used in conventional flip-chip bonding due to CTt mismatch and, therefore, the vertical height \ arses from chip to chip, giv ing rise to non-uniform coupling efficiencies

Some embodiments of the present invention provides techniques that reduce the complexities in product manufacturing in that quality control can he easii) carried out at different levels of the integration For example, the laser arrav on AlN ma> be manufactured by a specialist, and the final integration can be reduced to a simple alignment of the array with λWG and can be manufactured by using conventional packaging houses in the industry Thciefoie, performance dev iation of the final product is minima! and its packaging yield may be substantial!) higher than integrating InP chips directly on PI C In specific embodiments, techniques are foi using a butt joint method for optical coupling between laser ami) and AWG As a result, the coupling efficiency is expected to be high In an embodiment, both lasei waveguide surfaces can be an ti reflection (^R) coated to reduce reflection loss as well as possible back reflection into the laser cavit\ for high laser performance and stability In some embodiments, the invention provides a method and svstera for using micro heaters to adjust a center f requeue j of each individual laser in the transmitter

Various additional objects, features, and advantages of the present invention can be more full) appiecialed with reference to the detailed description and accompany ing drawings that follows

Brief Description of the Drawings

Figure IA is a simplified top view diagram of a hybrid integrated DWDM transmitter according to an embodiment of the present invention

Figure IB is a simplified cross-sectional view diagram of the hybrid integrated DWDM transmitter of Kiguie IA according to an embodiment of the invention

Figures 2\ is a simplified expanded top s tew diagram of a h) brid integrated

DWDM transmitter according to an embodiment of the present invention

Figure 2B is a simplified expanded cross-sectional view diagram of a hybrid integrated DWDM transmitter of Figure 2 A according to an embodiment of the invention

Figure 3 a simplified view diagram of an integrated DWDM transmitter s>siem according to another embodiment of the present invention

Figure 4 A is a simplified flowchart of a method for maintaining a target wavelength in an integrated DWDM transmitter according to an embodiment of the invention

Figures 4B-4D are simplified wavelength diagrams illustrating the method for maintaining a target wavelength in an integrated DWDM transmitter according to the embodiment of the invention

Figure 5 is a simplified flowchart of a method for making in an integrated DWDM transmitter according to another embodiment of the present invention

Figure 6A is a simplified top \iew diagram of a packaged hybrid integrated

DWDM transmitter according to yet another embodiment of the piesent in\ ention

Figure 6B is a simplified cioss-seciiona! view diagram of the packaged hybrid integrated DWDM transmitter of Figure 4A according to an embodiment of the invention

The present invention is directed to fiber optical transport svstems More particularly, the invention provides a method and sv stem for integrating semiconductor (InP) laser/modulator chips on an aluminum nitride subrøount with a silica/silicon AWG Merely by of example, the invention has been applied to a wavelength multiplexed transmitter array But it would be recognized that the invention has a much broader range of applicability

As discussed above, the optical components in a conventional DWDM system aie usually indh idualls packaged Disciete optical components often hav e large

footprint and hence bulky in size For example, a I OG transmitter in a Butterfly package has typical dimensions of approximately 14x1 1 5mm ϊf the line side of a DWDM system requires i2 such transmitters, it occupies a sizable space too large to fit onto a conventional line card Multiple line cards and hence shelves arc needed to support high DW DM channel counts, giving rise to a bottleneck in system dimensions Furthermore, these multiple line cards need to be wavelength multiplexed by a multiplexer before launching into a network Connection to the multiplexer card often requires a large number of fiber patch cores, increasing the complexity as well as cost of the fiber management

There have been great efforts in the past several years to monolithically integrate multiple lasers, modulators, receivers, and AWCJ onto a single InP chip in an attempt to reduce the footprint of a DWDM terminal as well as simplifying liber management and field deployment f conventional monolithic integration method relies heavily on InP technologies, which has yet to reach its maturity The cuiient yield of the technologies is low compared to silicon processing, even for single DWDM laser chips With multiple elements integrated on the same chip, the yield may be expected to decrease exponentially In addition, the passh e AWG usually occupies much larger area of the integrated chip than the active elements, such as lasers and detectors, resulting in inefficient use of the expensive InP materials The diameters of InP wafers are typically 2" or T\ as compared to 8" or even 12" for silicon wafer which is an order of magnitude larger The processing cost per unit area for InP wafers can be two orders of magnitude higher than that for silicon wafers fhe low chip yield, coupled with high processing cost, nialces it uneconomical to monolithicaily integrate a DWDM transmitter

λn altemath e technique involves brid integration With the successful development of mode corners! on (beam expanders) in the InP chips, hybrid integration of InP chips with PLC becomes feasible Coupling loss from InP laser to PLC waveguide as low as 2dB ha\e been demonstrated and multiple hybrid integrated devices were fabricated However, these hybrid integration approaches, although promising in increasing yield and reducing size, have other integration/packaging issues For example, the base materials of the active/passhe components in hybrid integration are often different due to different technologies As a result.

conventional hybrid integration methods suffer from many limitations

In conventional techniques, semiconductor lasers or modulators arc mostly InP-based, while AWG and other passive devices are silica'silicon based The two materials have different thermal and mechanical properties For example, the coefficient of theimal expansion (CTE) of InP is about 4 6 pprci _' K, but CTE of silicon is - 3 0 ppm K The temperature for bonding the DMLs and the λVVG is -300" C, while the operating temperature of the transmitter is -3O ⁰ C Thus a 2mm chip about the si/e of a four DML lasers, will shrink by -1 I μro relative to the silicon substrate (AWG) after bonding Such mismatch would not only affect the waveguide alignment, but also introduces strains on the laser chip, which could degrade laser performance, such as stain-induced line broadening Another problem associated with CTE mismatch is the build-up of stress at the lasei-'PL€ interface T his build-up will lead to the complexities in meeting the emironmental requirements, for example, temperature cycling requirement between -6^ and 75 "C

From the above, it is seen that an improved technique for integrating seimeofiducloi (InP) laser modulator chips with an AWCT is desired

Figure IA is a simplified top view diagram of a hybπcϊ integrated DWDM transmitter according to an embodiment of the present invention This diagram is merely an example, which should not undul> limit the scope of the claims herein One of ordinaπ skill in the art would recognize other \ ariations, modifications, and alternatives As shown, hybrid integrated DWDM transmitter 100 includes a silicon bench 101 In a specific embodiment the silicon bench 101 includes a silica-oil-silicon substrate, i e a silica laver overling a silicon substrate Hybrid transmitter 100 also includes an optical rnultipSexei in the silicon bench In a specific embodiment, the optical multiplexer includes an arrayed waveguide grating (AWG) 1 10 made in a silica-on-silicon planar lightwave circuit (PLC) in the silicon bench Hybrid transmitter 100 further includes one or more multiple laser array chips, e g , 114 and 115 mounted on a support component, for example, submount 126 In a preferred embodiment the laser array chips include DML lasers made in InP in a specific embodiment, each hiP lasei arraj chip includes two or more lasers Of course, there can be other variations, modifications, and alternatives

In a specific embodiment, the AWG 1 iO includes one optical output port U 2, multiple input ports I S 3, and grating waveguides 116 ϊn an embodiment, the output port 112 is optically coupled to an optical fiber 1 19, which may be coupled to an optical transmission system. The output and input ports, for example, can all be implemented in the form of waveguides. In a specific embodiment, the grating waveguides 1 16 include a number of waveguides for coupling to the input and output polls. ^' These waveguides have varying lengths for performing wavelength division multiplexing and demultiplexing functions In some embodiments, each input port of the AWG has a center wavelength and pass band associated with Sight transmission.

Figure I B is a simplified cross-sectional view diagram of the hybrid integrated

DWDM transmitter 100 according to an embodiment of the invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary' skill in the art would recognize other variations, modifications, and alternatives As shown, a waveguide includes doped silica region 12! enclosed in an undoped silica layer 122 on a silicon substrate 124 In a specific embodiment, the doped silica region 121 has a higher index of refraction than the undoped silica region, In a specific example, the doped silica region S 21 has an index of refraction of about 1.47, and the undoped silica region has an index of refraction of about 1 ,45.

According to certain embodiments of the present invention, integrated transmitter 100 includes one or more laser array chips, and each laser array chip may include two or more lasers, in the specific embodiment shown in Figure IA, the integrated transmitter 100 includes two direct-modulated laser (DML) array chips S 14 and 115.

As shown in Figure SB, the DML array chips are mounted on a submount 126 In an example, the DML array chips are soldered on an aluminum nitride (AlN) submount S 26. Further, the silicon substrate 124 is mounted on a silicon submount 132, as shown in Figure IB. The AIN submount 126 is attached to the silicon submount 132. In a specific embodiment, the submount 132 is mounted on a temperature adjustment component 134.. such as a thermal electric cooler (TEC).

As shown in Figure IA and IB, the hybrid integrated DWDM transmitter KXJ includes an arrayed waveguide grating (AWG) made of silica-on-silicon planar

lightwave circuit (PLC) and one or more multiple direct-modulated laser (DML) array chips made of InP Depending on the embodiments, the laser chips are mounted on an aluminum nitride (AL-N) submount in either an epi-dowπ (epitaxial-growth-surface-down) or epi-up geometry to form a transmitter array and then butt join with AWG for wavelength multiplexing. In certain embodiments, the AWG has one optical output port and multiple (N) input ports, all in the form of waveguides. Each input port transmits light at pre-selected wavelength defined by ITU-T standards (for example, 193.1 THz) with a known width of the pass band.

In a specific embodiment, each DML array can include more than one DML For example, the DMLs can be of the types of distributed feedback (DFB) lasers and hence are operated in single frequency mode. The spacing between two adjacent

DMLs matches with that of the waveguides along with their operating wavelengths for efficient transmission. More generally, in alternative embodiments, the DML arrays can also be single DML chips The DMLs can also be substituted by integrated CW lasers and modulators, for example, an integrated DFB laser with an electro-absorption (EA) modulator The lasers can also be distributed Bragg grating

(DBR) lasers. Of course, there can be other variations, modifications, and alternatives.

According to embodiments of the invention, to minimize the OTE mismatch, the chips are mounted on to an aluminum nitride submount Thermal mismatch problems can occur with some submount materials. For example, when solder (e.g ,

AuSn) solidifies after chip attach, it freezes the chip to the submount. As the assembly is cooled from the solder ^' s solidus point (e.g , 280' C) to room temperature, different CTE of the chip and the submount can introduce undesirable stresses into the laser structure This effect is often more severe the harder the solder and the higher the temperature difference between the solder's solidus point and operating temperature

According to embodiments of the present invention, aluminum nitride (AlN) has certain desirable properties as a submount material. For example, its CTE (4.5 ppni/ K) is closely matched to that of hiP. This property can minimize undesirable stresses on the laser structure, both those frozen-in after chip mounting and those arising from thermal cycling during device operation. In addition, high-grade AlN has been

known to exhibit high thermal conductivity of 230 WMrK at room temperature Although no! as high as other materials such as Cu, this value is 3 times higher than the thermal conductivity of InP (70 W ^' ra-K at room temperature), which naturally enhances heat transfer from the laser's active zone into the heat sink by reducing thermal resistance in the epi-down or epi-up geometry.

Moreover, AIN is an electrical insulator, hence it permits easy formation of electrical interconnects on the submount for incorporating various passive monitoring elements into the integrated chip on carrier (CoC) assembly. Therefore, according to embodiments of the invention, the AlN AuSn system simultaneously ensures high thermal performance of the mounted semiconductor lasers and offers the benefits of highly scalable, manufacturing-level fabrication, In some embodiments of the present invention, ALN holds promise to eventually enable economical mass production of high-performance, high-reliability, low-cost integrated DWDM transmitters. In alternative embodiments of the present invention, other materials with similar properties to AlN can also be used as the laser submount

Figures 2A is a simplified expanded top view diagram of a hybrid integrated DWDM transmitter according to an embodiment of the present invention Figure 2B is a simplified expanded cross-sectional view diagram of a hybrid integrated DWDM transmitter of Figure 2A according to an embodiment of the invention. These diagrams are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives.

As shown in Figure 2 A, hybrid integrated DWDM transmitter 200 includes waveguides 212 and 213 that are coupled to an optical multiplexer, such as an arrayed waveguide grating (AWG) (not shown). As an example, the waveguides and the AWG are made in silica-on-silicon planar lightwave circuit (PLC), as described in Figure I A. integrated transmitter 200 also includes lasers 214 and 215 mounted on submount 226 Examples of lasers are discussed above in connection with Figures I A and IB.

Figure 2B shows silica waveguide 213 enclosed in an υndoped silica layer 222 on a silicon substrate 224. In a specific embodiment, the silicon substrate is

mounted on a support component 232. for example, a silicon submount There are various considerations regarding alignment of the components For example, the laser 215 is aligned to the silica waveguide 213 both vertically and horizontally with accuracies about ±2 μm. m another example, there is no direct contact between facets (output ports) of laser 215 and the silica waveguide 21}. In specific example, the distance 2 i 8 between the facets is kept to about - 10-20 μm Of course, there can be other variations, modifications, and aJtematives.

In a specific embodiment, the physical separation between the DMLs on the array, and hence the separation between the AWG input waveguides, is kept large enough to minimize the thermal crosstalk, and the electrical crosstalk due to the high speed data modulations. For example, a suitable distance between lasers 214 and 215 in Figure 2A can be about 0 3-0.5mm in an embodiment. Of course, there can be other variations, modifications, and alternatives.

Although the above has been shown using a selected group of components for the integrated DWDM transmitter system, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced.

According to an embodiment of the present invention, after being mounted on the AlN submount with desirable spacing, the DML array bar is butt joined with the silica-on-silicon AWG as shown in Figures 2A and 2B. In an embodiment, the DML chips or chip arrays ha\e integrated mode converters, and the alignment of the two array devices is relatively simple. For example, at l,55Oπm mode diameters of standard silica PLC are typically -8- i0μm. with output beam divergence -7-1O ⁰ , similar to those of the InP chip after mode conversion. Thus, in a specific embodiment, an alignment tolerance of as large as í/-2μrn can be expected, simplifying considerably the alignment process

According to embodiments of the invention, active alignment is required to obtain good optical coupling for every channel between the transmitter array bonded on the AlN bar and silica; silicon AWG. Once the alignment reaches the desired

precision, the AIN bar is then giued on to the AWG by epoxies with good thermal conductance Special care is taken to ensme thai the joint of diffetem raateiials will not crack or deform under temperature cycling due to different C I Es In a specific embodiment to prevent such a mechanical failure at the joint, a silicon submoimt with a thickness of - 1 -mm is used Silica/si Neon λW G is first glued b\ epoxies with good thermal conductance on to the silicon submount 232 as shown in Figure 2B to increase its thickness for a strongei joint The two pieces are flush aligned at the side where the waveguide input is located After the epoxy sets, this side is polished to have a good surface normal to the waveguide core In an embodiment, the input of the AWG is coated with an anti -reflection coating at 1550 nni for good optical coupling as well as preventing back reflection of light into laser cavity In a specific embodiment the entire thickness of the AWG chip including the submount is around 2-3 mm In alternative embodiments, more generally, the AWG submount can be of any materia! with similar properties as silicon Still more generally, the AWG can be replaced by a PLC broadband coupler, or a PLC waveguide fan-in concentrator The latter is to match the wide waveguide spacing required by the DMLs and the narrow spacing typically used in standard AWGs

Merely as an example, an AlN bar holding an array of 12 DML transmitters spaced by 0 5mm has preferably dimensions of approximately 1 The 1 5x6mm surface with conducting film electrodes is used to mount the laser chip The output surface of the chip is chosen not to be Hush with the 4.1N submount with a small recess of - 10-20 μm (see Figures 2A and 2B) to prevent direct contact with AWG as well as to ensure excellent optical coupling The side of the AiN submount is then glued to the input side of the AWG via active alignment The entire unit is then placed on a thermal electrical cooler (THC) for the control of operating tempeia.ure normally set at - 25 "'C Preferably, the dimensions of the TIlC ts chosen such that it fits to dimensions of the silicon submount holding the λWG, the at ray bar untouched by the TL ⁷ C

According to a specific embodiment of the invention, the AWG submount uses same material as the λWG and this joint geometry will not cause a vertical tilt of the array bar due to temperature changes λ temperature ramp from -40 to -85 °C (the standard range for stoiage teinperatuies) will have a negligible change of the iecess

n

less than -w-0 045 μm ^or example, the AWG is typically operated at -25 ^C C λt this tempeiatuie. the ceπtej wavelengths of the DMLt. ate matched roughly to those of the AW G input ports, for example, 193 1 I Hz. 193 2 I Hz. 193 3 FHz, etc The center wavelengths of the AWG shift with temperature by -0 01 nm ^C C, and the center wavelengths of the InP lasers shift with temperature by -0 InIn "! ^" Due to the manufacturing tolerance, the center wavelengths of the lasers do not fall exactly on the I ^' ll J-T grid at the TIiC operating temperature The \ arialion is l\ picall\ on the order of lnm In a specific embodiment, a micro heater is placed adjacent to each DML waveguide, either on the lasei chip or on the AlN lasei submount By raising the local temperature to -0-10 ⁰ C relative to the substrate, one can fine time the center wavelengths of the DMLs to the ITU grids

Various embodiments of the invention also include hybrid integration with Vernier AWG to further improve the λYYG yield and hence reduce the manufacture cost In an embodiment, Vernier 4.WG compensates for the possible center wavelength shift due to the slight fabrication error in waveguide parameters such as the effective index of the waveguides by adding additional ports at both input and output Of course, one of ordinary skill in the art will lecogni/c other variations, modifications, and aitemathes

According to an embodiment of the present invention, a method is prov ided for tine adjustment of the center wavelengths of the DMI s Due to the manufacturing tolerance, the center wavelensths of the lasers raav not fal! exact! v on the l i ^' U- f arid at the temperature adjustment component operating temperature The variation, for example is typically on the order of 1 nm In certain embodiments of the invention, a micro heater is used to raise a temperature of a DML waveguide For example, in a specific embodiment, a micro heater is placed adjacent to each DML wav eguide, either on the laser chip or on the PLC According to a specific embodiment of the invention. b\ raising the local temperature to about 0-lOT relative to the substrate, one can fine tune the center wavelengths of the DMLs to the HlJ grids Further details of the method are discussed below with reference to Figure 3

Figure 3 is a simplified view diagram of an integrated DWDM transmitter system according to another embodiment of the present invention This diagram is merely an example, which should not υnduK limit the scope of the claims herein u

One of ordinary skill in the art would recognize other variations, modifications, and alternatives λs shown, integiated uansmitter system 300 includes a hsbrid integrated transmitter 350 similar to transmitter 100 discussed hi connection with Figures 1 A and IB For easy reference, corresponding parts of the devices are marked by identical numerals λs shown, hybrid integrated transmitter 350 includes a laser 1 15 overlying a support component such as an aluminum nitride (AlN) submount 12(>, a silica waveguide 121 formed in a silicon bench 101 which includes u n doped silica layer 122 overlying a silicon substrate 124 The silicon substrate 124 overlies a support component 130, which includes submount 132, e g , a silicon substrate, and a temperature adjustment component 134, such as a thermal electric cooler (TEC) In a specific embodiment, integrated transmitter system 300 also includes a micro heater 335 in a vicinity of the laser 115. an optical anal\zer 362. and a controller 364 The optical analyzer 362 is optically coupled to an output waveguide in the integrated DWDM transmitter which may be optically coupled to an optical communication ss stem through optical fiber i l ^c > The controller 364 is electrically coupled to the optical analyzer 362 and the micro heater 335 In Figure 3. the micro heater 335 is shown to be on top of the laser 115 Depending on the embodiment, the micro heatei can be placed in a location so as to adjust the temperature of the laser For example, the micro heater can be placed on the silicon substrate and adjacent to the laser In an embodiment, a micro heater is placed adjacent to each laser, either on the laser chip or on the PLC Jn a specific embodiment, the micro heater h a tost stive element, such as a metal strip, deposited in a of laser 115 as shoun in Figure 3

Although the above has been shown using a selected group of components foi the integrated DWDM transmitter system, there can be man\ alternatives, modifications, and variations For example, some of the components may be expanded and/or combined Other components may be inserted to those noted abo\e

Depending upon the embodiment, the arrangement of components may be interchanged with others replaced f ^j or example, integrated transmitter 350 may include features in transmitter 200 discussed above in connection with Figures 2 λ and

2B

Figure 4A is a simplified flowchart of a method foi maintaining a target

i ?

wavelength in an integrated DWDM transmitter according to an embodiment of the im ention Figuies 4B-4D aie simplified wavelength diagjaras aecoiding to the method These diagrams are merely examples, which should not unduly limit the scope of the claims herein One of ordinary skill in the art would recognize other variations, modifications, and alternatives The method can be briefly outlined below, with reference to the integrated DWDM system in Figure 3, the flowchart in Figures 4A, and the wavelength diagrams in Figure 4B-4P

( Process 410) Determine laser wavelengths distribution at a predetermined global TEC temperature An example of wavelength distribution at TEC temperature of 25°C is shown in Figure 4B

(Process 420) Adjust the TF.C to a second global temperature to shirt the all laser wavelengths to below the target wa\elengths for the corresponding H L ^' - P grids λn example is shown in Figure 4C

(Process 430) For each laser, determine a center clength at an output wasegutde, using the optical analyzer 362,

( Process 440) ϋeteimine a dev iation between the measured center eiength and the target wavelength, using the controller 364,

(Process 450) Adjust a temperature of the micro heater 335, using the controller ^64, to increase the center ^■ wavelength of the laser to approach the corresponding target wavelength according to the I HJ-I grids Figure 4 D is an example of wavelengths shifted to the corresponding target wavelengths according to the HO-T grids

The above sequence of processes prowdes a method for maintaining a target waxeiength associated with an integrated DWDM transmitter according to an embodiment of the (m ention As shown, the method uses a combination of processes including a way of using the TEC to shift all laser wavelengths to the shorter wavelength side of the grids and using local micro heaters to increase the local temperature at each laser as needed to shift all laser wavelengths to the I R ⁾ -T grids Other alternatives can also be provided in which steps are added, one or more steps are or one or more steps are provided in a different sequence without

departing from the scope of the claims herein. Further details of the present method can be found throughout the present specification.

Figure 5 is a simplified flowchart of a method for making an integrated DWDM transmitter according to an embodiment of the invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives. The method can be briefly outlined below, with reference to the flowchart in Figure 5.

(Process 510) Form an optical multiplexer in a silica-on-sϋicon substrate;

(Process 520) Provide a first support, component;

(Process 530) Attach the silica-on-silicon substrate to the first support component;

(Process 540) Provide a second support component;

(Process 550) Mount one or more semiconductor laser array chips to the second support component;

(Process 560) Align the lasers to a corresponding input waveguides; and

(Process 570) Attach the second support component to the first support component,

As shown, Figure 5 provides a method for making an integrated DWDiVI transmitter apparatus. The method includes (Process 510) forming an optical multiplexer in a silica-on-silicon substrate. The silJca-on-silicon substrate includes a silica layer overlying a silicon substrate, and the optical multiplexer has a plurality of input waveguides and at least an output waveguide. In an embodiment, the optical multiplexer includes an arrayed waveguide grating In a specific embodiment, forming the optica! multiplexer includes the following processes.

forming a lirst un-doped silica sub-layer on the silicon substrate;

forming a doped silica sub-layer on the first un-doped silica sub-layer;

etching at least a second portion of the doped silica sub-layer, and

depositing a second un -doped silica sub-layer on the etched doped silica sub-layer and the first un-doped silica sub-layer

In Process S2G, the method includes providing a first support component A coefficient of thermal expansion of the first support component is substantially matched to a coefficient of thermal expansion of the silicon substrate The first support component has a thickness that allows alignment between each of the one or more lasers and a corresponding one of the plurality of input waveguides As an example, the first component can be a silicon substiate characterized b; a thickness of approximate^ lmm In Process 530, the method includes attaching the silica-on-silicon substiate to the first support component

According to Process 540, a second support component is provided In Process 550, one or more semiconductor laser array chips are mounted to a top surface of the second support component A coefficient of thermal expansion of the one or more semiconductor lasei aπay chips is substantial I \ matched to a coefficient of thermal expansion of the second support component Each of the one or more semiconductor lasei array chips includes one or more lasers In an embodiment, the laser an ay chips include one or more InP lasers For matching of the coefficients of thermal expansion, the second support component may be an aluminum nitride (AlN) subrøount in a specific embodiment the laser array chips are mounted on the second support component using a soldering process in a specific embodiment, a micro heater is mounted in a vicinity of each laser for fine tuning a center frequency of the laser

In Process 560, the method also includes aligning each of the lasers to a corresponding one of the plurality of input wa\ eguides for providing an optical coupling Additional Iv, the method includes attaching the second support component to the first support component (Process 570} In an embodiment, the second support component is butt joined to the side surface of the first support component Foi example, the second support component can be glued to the fust support component epoxies with good thermal conductance In another embodiment the method also includes adding a temperature adjustment component

underlying the first support component. As an example, the method is implemented according to Figures I A, I B, 2A, and 2B and the associated text.

The above sequence of processes provides a method for making an integrated

DWDM transmitter apparatus according to an embodiment of the invention. As shown, the method uses a combination of processes including a way of making an optical multiplexer in a siϊ ica-on -silicon substrate and mounting laser array chips on a portion of the substrate. Other alternatives can also be provided in which steps are added, one or more steps are removed, or one or more steps are provided in a different sequence without departing from the scope of the claims herein. Further details of the present method can be found throughout the present specification

In a specific embodiment, the DML chips, the AWG. the submount, and the TEC, after proper electrical wire bonding, are put inside a single package to form the DWDM transmitter. In an embodiment, the transmitter has multiple electrical inputs that control and monitor the temperatures of the AWG and I)MLs. the DC currents and RF modulations of the DMLs, etc. hi a particular embodiment, the transmitter has a single optical output, typically through an optical fiber pigtail, sending out the multiple-channel DWDM signals Of course there can be other variations, modifications, and alternatives

Figure 6A is a simplified top view diagram of a packaged hybrid integrated DWDM transmitter according to yet another embodiment of the present invention.

Figure 6B is a simplified cross-sectional view diagram of the packaged hybrid integrated DWDM transmitter of Figure 6A. These diagrams are merely examples, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize other variations, modifications, and alternatives As shown in Figure όA, packaged integrated dense wavelength division multiplexing

( DWDM) transmitter apparatus 600 is capable of converting multiple electrical signals to a multiple-channel DWDM signal. In a specific embodiment transmitter apparatus 600 includes a plurality of input terminals 610 for receiving the multiple electrical signals and a functional block 100 inside a package 601 . In an embodiment, functional block 100 may be a hybrid integrated transmitter similar to block 100 discussed above in connection with Figures IA and I B.

Block iOO in transmitter apparatus 600 is discussed below with reference to

Figures I λ and I B In a specific embodiment, hsbύd integrated transmitter 100 includes a support component 132 and a silicon bench 101 which includes a silica-on-silicon substrate overlying the support component The silica-on-silicon substrate includes a silica la\er 122 overlying a silicon substrate 124

T he hybrid integrated transmitter 100 also includes one or more semiconductor laser array chips (e g 114 and 115). w hich are coupled to the input termi rials 610 for receiving the multiple electrical signals as shown in Figure 6 A The lasei array chips convert the multiple electrical signals to corresponding multiple optical signals The one or more semiconductor laser array chips (e g 1 14 and 1 15) o\ eriie a second support component 120 in a specific embodiment each of the one or more laser arrav chips (e g 114 and 115) includes two or more lasers

The hybrid integrated transmitter 100 also includes an optica! multiplexer I SO which may include an arrayed waveguide grating (λWG> 116 made in a silica-on-silicon planar lightwaxe circuit (PLC) shown in Hgures 1λ and IB The optical multiplexer 1 10 is coupled to the one or more semiconductor lasei array chips (e g 1 14 and 1 1 5) for converting the multiple optical signals to a multiple-channel DWDM stgnal As shown, the optical multiplexer I K* is located within the silica layer 122 o\erlying at least a portion of the silicon substrate 124 In an embodiment, the optical multiplexer U O includes a plurality of input waveguides 1 13, each of which is coupled to a corresponding laser in the one or more laser array chips for receiving a corresponding optical signal The optical multiplexer HO converts the multiple optical signals to a multiple-channel DWDM signal w hich may be transmitted outside the transmitter through an output waveguide 1 12 Of course, there may be othei variations, modifications, and alternatives For example, depending on the embodiments, there ma\ be more than one output waveguide in the transmitter 100

In an embodiment, input terminals 610 arc coupled to the laser array chips in transmitter 100 via electrical connections such as 605 ϊn some embodiment, packaged DWDM transmitter 600 may include electrical amplifiers 603 for boosting the electrical signals and coupling them io the laser an ay chips In art embodiment. the output waveguide 1 12 of the transmitter may be coupled to an optical fiber 61 ^c > for

transmitting the multiple-channel DWDM signal to, for example, an external optical communication ss stem In a specific embodiment, the transmittei also has multiple electrical input and output connections for control and monitor of the temperature of the AWG and other components of flic packaged transmitter apparatus 600

Depending upon the embodiment, the psesent invention includes various features, which may be used These features include the following

1 \ hybrid integrated DWDM traπsmittei arra> including one or more multiple direct-modulated laser (DML) array chips made of InP and an arravcd waveguide grating (AW G) made of silica-on-MUcon planar lightwave circuit (PLC),

2 An Integrating laser chips or array chips on an aluminum nitride submoimt to prevent O I L mismatch and the complexities associated with the mismatch [ ^' his array bar on AlN is then butt joined with PLC for high coupling efficiency,

3 \ method and s> stem for using micro heaters to adjust a center frequency of each indhiduaϊ laser in the transmitter,

4 \ method of using antireflcction (AR.) coating to prevent back reflection of light in to laser cavity, and

5 A method of using a silicon subrøount to increase the thickness of the AWG chip for a strong butt joint

As shown, the above features may be in one or more of the embodiments These features are merely examples, which should not unduly limit the scope of the application One of ordinary skill in the art would recognize many variations, modifications, and alternatives

Many benefits are achieved by way of the present invention o\ er conventional techniques For example, in certain embodiments, the invention provides a method of integiating \ arious optical components using excellent CTE match between InP and ALN as λVCII as good thermal conducth ity of Alls According to embodiments of the present invention, the method is readily seal cable to large laser array chips for further reduction of cost and system footprint as well as increasing product reliability and manufacture \ ield According to certain embodiments of the invention, large

array chips can enable further improvements of optical coupling between AWG and transmitters. For example, vertical variation in height relative to the waveguides can be minimized from transmitter to transmitter if these transmitters are formed on a single array chip. In contrast, large array chips can not be used in conventional flip-chip bonding due to CTE mismatch and, therefore, the vertical height varies from chip to chip, giving rise to non-uniform coupling efficiencies

Some embodiments of the present invention provides techniques that reduce the complexities in product manufacturing in that quality control can he easily carried out at different levels of the integration. For example, the laser array on AlN may be manufactured by a specialist, and the final integration can be reduced to a simple alignment of the array with AWG and can be manufactured by using conventional packaging houses in the industry. Therefore, performance deviation of the final product k minima! and its packaging yield may be substantially higher than integrating InP chips directly on PLC In specific embodiments, techniques are provided for using a butt joint method for optical coupling between laser array and AWG. As a result, the coupling efficiency is expected to be high. In an embodiment, both laserwaveguide surfaces can be AR coated to reduce reflection loss as well as possible back reflection into the laser cavity for high laser performance and stability.

It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.