Full Transcript
https://www.youtube.com/watch?v=XH3Mzelu0j0
[00:00] hello everybody and and welcome to
[00:04] hello everybody and and welcome to Europe practices webinar series on
[00:06] Europe practices webinar series on advanced packaging
[00:07] advanced packaging I'm Ramsey Salim and you will practice
[00:10] I'm Ramsey Salim and you will practice lead Institute where we focus on system
[00:14] lead Institute where we focus on system ingredient and advanced photonics
[00:17] ingredient and advanced photonics packaging this webinar continues our
[00:20] packaging this webinar continues our series where we step into the world of
[00:22] series where we step into the world of advanced photonics packaging in the last
[00:25] advanced photonics packaging in the last episode we looked at fiber teacher edge
[00:28] episode we looked at fiber teacher edge coupling in today's episode we'll be
[00:31] coupling in today's episode we'll be looking at grating couplers in fact the
[00:34] looking at grating couplers in fact the next for this episode included with the
[00:38] next for this episode included with the next three as well we will be looking in
[00:40] next three as well we will be looking in more detail at grating couplers led
[00:44] more detail at grating couplers led before which I've been to that let's get
[00:45] before which I've been to that let's get familiar with the zoom platform I think
[00:47] familiar with the zoom platform I think most of you probably already familiar
[00:49] most of you probably already familiar with zoom and essentially if you want to
[00:52] with zoom and essentially if you want to ask questions the best way to do that is
[00:54] ask questions the best way to do that is to click on the Q&A button and the Q&A
[00:58] to click on the Q&A button and the Q&A button there's a whole section where you
[01:00] button there's a whole section where you can paint your questions and they get
[01:01] can paint your questions and they get collated for a question and answer
[01:04] collated for a question and answer session at the end of the talk or after
[01:07] session at the end of the talk or after the talk section but without further
[01:10] the talk section but without further delay let me introduce you to my co-host
[01:12] delay let me introduce you to my co-host dr. francesco flores the head of the
[01:17] dr. francesco flores the head of the training programs and a luca ciganlija
[01:21] training programs and a luca ciganlija hello who you're helping develop the
[01:24] hello who you're helping develop the grating couplers technology here at
[01:26] grating couplers technology here at Tyndall our new Luca yep this am
[01:29] Tyndall our new Luca yep this am Francesco over to you for the grating
[01:33] Francesco over to you for the grating couplers about you and tell us about the
[01:34] couplers about you and tell us about the content today yeah thanks a lot
[01:36] content today yeah thanks a lot hi everybody so today we start talking
[01:41] hi everybody so today we start talking about grating couplers has Ramsey told
[01:44] about grating couplers has Ramsey told you just a few seconds ago we have four
[01:48] you just a few seconds ago we have four webinars planned to talk quite deeply
[01:52] webinars planned to talk quite deeply about plating couplers this is just part
[01:55] about plating couplers this is just part one so we will introduce you about again
[01:59] one so we will introduce you about again the mode mismatch this is a concept that
[02:01] the mode mismatch this is a concept that we already told you during the previous webinar.
[02:04] then the main features and they key parameters geometrical and electromagnetic.
[02:10] so we will show you how to manage the electromagnetic behavior of the scattering processes behind the grating coupling opportunity tuning the geometrical features of these devices.
[02:26] how then to read these electromagnetic parameters in terms of the spectrum and some additional hints about the scattering process.
[02:37] then we will jump deeply into the coupling and optimization and packaging techniques.
[02:46] and I will show you the two geometrical approaches that you can use vertical and horizontal fiber coupling and two examples of final devices and obviously at the end the Q&A session so let's
[03:02] at the end the Q&A session so let's start again main problem mode knees.
[03:05] start again main problem mode knees mention we have a fiber with a core that is of the order of ten point five micron.
[03:12] is of the order of ten point five micron and the mode inside the core with the mode field diameter around ten point four microns.
[03:18] mode field diameter around ten point four microns we want to shine the light that is running inside the core on a grating coupler and the dimension of the grating cover are averaged 12 by 20 microns.
[03:29] grating cover are averaged 12 by 20 microns then we have the SOI so in this case our web guides has a geometrical dimensions around to order of magnitudes smaller with respect to the core of the fiber.
[03:43] smaller with respect to the core of the fiber we are talking four hundred and fifty times two hundred and twenty nanometers.
[03:48] fifty times two hundred and twenty nanometers so again we have to face this mode mismatch how can we do that.
[03:52] nanometers so again we have to face this mode mismatch how can we do that we have as I told you before two geometrical approaches.
[03:58] as I told you before two geometrical approaches the first one here on the left is this so-called vertical fiber.
[04:03] left is this so-called vertical fiber coupling so we have a fiber that is
[04:06] coupling so we have a fiber that is vertically this place on top of the
[04:10] vertically this place on top of the beaker here you can see the grating
[04:12] beaker here you can see the grating coupler the main disadvantage of this
[04:14] coupler the main disadvantage of this type of coupling is the fact that we can
[04:17] type of coupling is the fact that we can apply a torque on the fiber and since
[04:21] apply a torque on the fiber and since the fiber is made of glass we can break
[04:23] the fiber is made of glass we can break it quite easily so we need to help the
[04:27] it quite easily so we need to help the far
[04:27] far with the external sustain that in this
[04:31] with the external sustain that in this case it means that we are increasing the
[04:33] case it means that we are increasing the difficulties of the packaging a better
[04:36] difficulties of the packaging a better way to couple the light on top of the
[04:40] way to couple the light on top of the rating coupler is to use the so called
[04:42] rating coupler is to use the so called or example geometrical approach in this
[04:45] or example geometrical approach in this case we have a fiber that is a
[04:48] case we have a fiber that is a horizontally placed on top of the
[04:50] horizontally placed on top of the grating coupler and we polish the end
[04:54] grating coupler and we polish the end facet of the fiber with a specific angle
[04:58] facet of the fiber with a specific angle in this case it's 40 degrees in order to
[05:01] in this case it's 40 degrees in order to achieve total internal reflection so
[05:03] achieve total internal reflection so what happen is that the light traveling
[05:05] what happen is that the light traveling inside the core heater at the end of the inside the core heater at the end of the fiber the hem facet and in Stoughton.
[05:12] fiber the hem facet and in Stoughton lindana reflected downward towards the lindana reflected downward towards the grating coupler the grating coupler can.
[05:17] grating coupler the grating coupler can collect this light and inject properly collect this light and inject properly the light inside the silicon webknight.
[05:22] the light inside the silicon webknight main advantages let's say we have quite main advantages let's say we have quite moderate insertion loss we are talking.
[05:28] moderate insertion loss we are talking about 2.5 degrees which means roughly 56 about 2.5 degrees which means roughly 56 70 % of transmission then we have a 1 DB.
[05:36] 70 % of transmission then we have a 1 DB bandwidth of the order of 30 nanometers.
[05:39] bandwidth of the order of 30 nanometers so the alignment tolerances are quite so the alignment tolerances are quite relaxed but what is really important is.
[05:46] relaxed but what is really important is the fact that the grating culture is the fact that the grating culture is polarization sensitive so remember that.
[05:50] polarization sensitive so remember that you have to tune design specifically the you have to tune design specifically the grating coupler for TE or TM mode so.
[06:01] grating coupler for TE or TM mode so this is a side view of the entire this is a side view of the entire grating coupler structure and starting.
[06:06] grating coupler structure and starting from the bottom there is the first layer.
[06:08] from the bottom there is the first layer that is the silicon substrate usually it.
[06:11] that is the silicon substrate usually it has a thickness of hundreds of microns.
[06:13] has a thickness of hundreds of microns then there is the bottom oxide layer.
[06:15] then there is the bottom oxide layer with a typical thickness of 2 microns.
[06:18] with a typical thickness of 2 microns and the last layer is this another.
[06:20] and the last layer is this another silicon layer with a thickness of 220.
[06:23] silicon layer with a thickness of 220 nanometers and in this layer the grating.
[06:26] nanometers and in this layer the grating copper is created and as you can see the.
[06:29] copper is created and as you can see the periodic structure usually has a length.
[06:31] periodic structure usually has a length of 20 microns and then the other key.
[06:33] of 20 microns and then the other key parameters are the etch depth and the.
[06:36] parameters are the etch depth and the period so the distance between 2 or.
[06:38] period so the distance between 2 or Nauticus points.
[06:41] Nauticus points the other key parameters are related to.
[06:44] the other key parameters are related to the electromagnetic field that strikes.
[06:46] the electromagnetic field that strikes the grating coupler so first of all the.
[06:48] the grating coupler so first of all the polarization in this case the electric.
[06:50] polarization in this case the electric field oscillates on a directions and.
[06:52] field oscillates on a directions and points out of your screen then the mode.
[06:56] points out of your screen then the mode has a dimension that fits the.
[06:59] has a dimension that fits the geometrical dimension of the grating and.
[07:01] geometrical dimension of the grating and it has a certain angle of incidence in.
[07:03] it has a certain angle of incidence in this way we were able to couple the.
[07:05] this way we were able to couple the light inside the waveguide in the.
[07:07] light inside the waveguide in the channel that we call in this light seee
[07:10] channel that we call in this light seee so coupling efficiency and we are able
[07:13] so coupling efficiency and we are able to reduce the losses due to the back
[07:16] to reduce the losses due to the back reflection or the our channel and the
[07:18] reflection or the our channel and the light that passes through the grating
[07:19] light that passes through the grating without interacting with it that is the
[07:21] without interacting with it that is the T channel transmission channel exactly
[07:24] T channel transmission channel exactly so again super important is remember
[07:28] so again super important is remember that two key parameters in this case you
[07:32] that two key parameters in this case you can see here the grating coupler and
[07:35] can see here the grating coupler and it's fundamental that your beam profiles
[07:38] it's fundamental that your beam profiles heat and shine completely inside the
[07:42] heat and shine completely inside the geometrical cross-section of the grating
[07:44] geometrical cross-section of the grating coupler in this case you can be sure
[07:47] coupler in this case you can be sure that 100% of the light is scattered
[07:50] that 100% of the light is scattered properly from your grating coupler
[07:52] properly from your grating coupler inside the waveguide and then again the
[07:56] inside the waveguide and then again the polarization state here we have a te
[07:59] polarization state here we have a te polarized light so it means that the
[08:03] polarized light so it means that the electric field is oscillating parallel
[08:06] electric field is oscillating parallel to the silicon tits this is fundamental
[08:10] to the silicon tits this is fundamental yeah and the electromagnetic spectrum is
[08:13] and the electromagnetic spectrum is quite important to see the behavior of
[08:16] quite important to see the behavior of the interaction of the light with this
[08:18] the interaction of the light with this type of structure and as you can see the
[08:22] type of structure and as you can see the grating coupler is able to couple one
[08:26] grating coupler is able to couple one waylynn efficiently in this case it's
[08:29] waylynn efficiently in this case it's called the working Wayland and it is
[08:30] called the working Wayland and it is 1550 nanometers as you can see from the
[08:33] 1550 nanometers as you can see from the dash product line this is a video from
[08:39] one of our simulation as you can see
[08:41] one of our simulation as you can see there is the light that comes from the
[08:42] there is the light that comes from the top
[08:43] top and interact with the grating some of
[08:45] and interact with the grating some of the light is back reflected some passes
[08:46] the light is back reflected some passes for the gratings and the majority of it
[08:48] for the gratings and the majority of it in this case 56% is injected inside the
[08:53] in this case 56% is injected inside the way
[08:53] way at the bottom of this image you can see
[08:57] at the bottom of this image you can see that there is the Bragg law that
[08:59] that there is the Bragg law that basically links the working way length
[09:01] basically links the working way length with the period of the grating copper
[09:03] with the period of the grating copper and theta that is the angle of incidence
[09:07] and theta that is the angle of incidence of the light yeah exactly so now that we
[09:10] of the light yeah exactly so now that we have in mind the geometrical and
[09:13] have in mind the geometrical and electromagnetic key parameters of the
[09:15] electromagnetic key parameters of the grating coupler you can maybe better see
[09:18] grating coupler you can maybe better see why is then so important to align
[09:21] why is then so important to align properly the fibers no matter if single
[09:24] properly the fibers no matter if single fibers has here on the left or fiber
[09:28] fibers has here on the left or fiber erased on top of the grating Cutler if
[09:30] erased on top of the grating Cutler if you consider the basic procedure what
[09:32] you consider the basic procedure what you have is imagining two fibers in this
[09:35] you have is imagining two fibers in this case a single fibers but the concept is
[09:38] case a single fibers but the concept is the same also for fiber arrays you need
[09:40] the same also for fiber arrays you need to go geometers and two branches both
[09:44] to go geometers and two branches both able to move resorting to six degrees of
[09:47] able to move resorting to six degrees of freedom in this case you can scan the
[09:50] freedom in this case you can scan the surface of the silicon peak you can find
[09:53] surface of the silicon peak you can find integrating cutler this is called
[09:55] integrating cutler this is called passive alignment then resorting to an
[09:57] passive alignment then resorting to an external source of light usually a laser
[10:01] external source of light usually a laser source you can then finally align the
[10:05] source you can then finally align the core of the fiber on top of the center
[10:07] core of the fiber on top of the center of your grating Cutler in order to
[10:09] of your grating Cutler in order to maximize the injection of the light or
[10:12] maximize the injection of the light or the collection of the light in or out.
[10:15] the collection of the light in or out from your wave vector in this case in.
[10:18] from your wave vector in this case in the central image you can see for.
[10:20] the central image you can see for example array of fibers that is cut or.
[10:24] example array of fibers that is cut or on top of a silicon beaker in this case.
[10:27] on top of a silicon beaker in this case you can see that we are using the.
[10:28] you can see that we are using the vertical geometry and it's clear that.
[10:30] vertical geometry and it's clear that from the mechanical point of view as I.
[10:32] from the mechanical point of view as I told you before this is not the best.
[10:35] told you before this is not the best option you can break for example the.
[10:37] option you can break for example the fiber here while here at the contact.
[10:41] fiber here while here at the contact surface between the fiber array and the.
[10:45] surface between the fiber array and the silicon picker you can see the epoxy now.
[10:48] silicon picker you can see the epoxy now on the right an example of a fully.
[10:52] on the right an example of a fully packaged device again this is not the.
[10:55] packaged device again this is not the best solution is also not easy to take.
[10:58] best solution is also not easy to take with your hands packaged like this so.
[11:01] with your hands packaged like this so how can we improve from the mechanical.
[11:04] how can we improve from the mechanical point of view the fiber cut me.
[11:07] point of view the fiber cut me using grating cutler we can resort to.
[11:10] using grating cutler we can resort to the so called or example geometry in.
[11:13] the so called or example geometry in this case the core of the fiber is for.
[11:15] this case the core of the fiber is for example which means that the fiber it's.
[11:18] example which means that the fiber it's parallel with respect to the surface of.
[11:20] parallel with respect to the surface of the beaker what is important in this.
[11:23] the beaker what is important in this case is that you can handle easily your.
[11:27] case is that you can handle easily your vehicle yeah here the only critical.
[11:31] vehicle yeah here the only critical point is that you have to be sure that.
[11:33] point is that you have to be sure that the polishing angle of your fiber must.
[11:36] the polishing angle of your fiber must respect the total internal reflection.
[11:37] respect the total internal reflection condition so the lightest travels inside.
[11:40] condition so the lightest travels inside the core of the waveguide and strikes.
[11:42] the core of the waveguide and strikes the tilt facet of the fiber must have an.
[11:45] the tilt facet of the fiber must have an incident angle respect to the normal of.
[11:48] incident angle respect to the normal of the facet that is higher than the.
[11:51] the facet that is higher than the critical angle you can see that is.
[11:53] critical angle you can see that is roughly 43 degrees in this case plus the.
[11:56] roughly 43 degrees in this case plus the the polishing angle in this case 40.
[11:59] the polishing angle in this case 40 degrees allows you to get the 10 degrees.
[12:02] degrees allows you to get the 10 degrees angle that you want to couple.
[12:04] angle that you want to couple efficiently the light inside the grating.
[12:05] efficiently the light inside the grating powder now of course we have tolerances.
[12:11] powder now of course we have tolerances when we try to align this fiber respect.
[12:13] when we try to align this fiber respect to the grating copper for example in to the grating copper for example in this case we can see the alignment tolerances respect to the grating copper.
[12:21] tolerances respect to the grating copper position and so if we put an offset position and so if we put an offset between the fiber and the grating copper between the fiber and the grating copper as we can see the relaxed the alingment tolerances are quite relaxed in this case 2.5 microns and the other tolerances that we have to consider are the one related to the road angle so if you imagine to have an axis passes through the core of the fiber then if you rotate the fiber about this axis then you get that you have a tolerance of 2.5 degrees right so with all this concept in mind we can see two examples of our example geometry in particular here top right you can see two single fibers that have been properly aligned on top of a grating coupler inner and outer and here in the SEM image you can
[13:15] outer and here in the SEM image you can also see again the epoxy that we use to fix and secure the fiber on top of the vehicle.
[13:21] if you want to increase the number of channels you can use fiber erase and here again you can see an example so we have a 5 min array for the input and another fiber array for the output from the mechanical point of view this is super advantageous because your system is almost flat and again you can see here the epoxy that we use to glue the beaker and the fiber arrays together.
[13:46] how can we do that I already introduced you to our machines in the lab that is called how to aligner mainly the process is divided into two steps the first step is the passive alignment so we just resorting to two cameras put the geometrically the optical fiber on top of the beaker then next step resorting to an external source we use a laser source we just refine the alignment now active.
[14:15] refine the alignment now active alignment so we move super carefully the alignment.
[14:19] so we move super carefully the optical fiber on top of the peak till we maximize the signal that is injected inside the grating Cutler's final devices.
[14:30] inside the grating Cutler's final devices again on the left an example of fiber or a cap on resorting to grating Cutler on a silicon beaker vertical at geometry.
[14:42] geometry let's see again that this is not from the mechanical point of view the best solution you can break the fiber on the right we have to turn speaks up in San Diego for the Z nature.
[14:55] this is a example of the integrated photonics national heat you can see instead here an example geometric Calicut whole fiber array on top of a silicon pika.
[15:07] it's clear that your example geometry is the best one you can just imagine how easy is to endl this solution with
[15:16] easy is to endl this solution with respect to this solution so that's the respect to this solution so that's the story with the grating couplers thank you very much Francisco thank you very much Luca it was very interesting.
[15:27] few questions myself actually but Before we jump into the Q&A session let me just tell you a bit about what's coming up and again you know this is a webinar series and so we've been looking at previous episodes to sign Romans five Mirta educating today we're looking at creating couplers other episodes about thermal management laser welding process of fiber attaches but yeah so for the next few episodes we've got a dedicated few sessions to grating couplers in fact next episode is about a special type of grating coupler the epidermis grating couplers are right Francesca yeah let's
[16:17] Couplers are right Francesca.
[16:19] Yeah, let's say next episode we will focus on how to improve the coupling efficiency.
[16:22] And we will show you essentially two ways.
[16:25] The first one is to increase the thickness of the grating coupler, adding an additional, it's called overlay of silicone.
[16:33] The second approach is to switch from the standard grating coupler, so pitch constant, to an analyzed grating coupler.
[16:43] Which means essentially that the pitch is changing along the grating coupler.
[16:48] That's alright, if you switch the next slide.
[16:58] So the next episode, like Francesca saying, is a particular type of grating coupler.
[17:02] And that would be on Tuesday the 19th of May.
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