Patent Publication Number: US-2015080982-A1

Title: Window in a Case of an Implantable Medical Device to Facilitate Optical Communications With External Devices

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a non-provisional of U.S. Provisional Patent Application Ser. No. 61/877,871, filed Sep. 13, 2013, which is incorporated herein by reference in its entirety, and to which priority is claimed. 
     This application is related to U.S. Provisional Patent Application Ser. No. 61/877,877, filed Sep. 13, 2013, entitled “Optical Communications Between an Implantable Medical Device and External Charger,” which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to wireless communications with and wireless charging of an implantable medical device such as an implantable pulse generator. 
     BACKGROUND 
     Implantable stimulation devices are devices that generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system. 
     As shown in  FIGS. 1A and 1B , a SCS system typically includes an Implantable Pulse Generator (IPG)  10 , which includes a biocompatible device case  12  formed of metallic material such as titanium for example. The case  12  typically holds the circuitry and battery  14  ( FIG. 2B ) necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG  10  is coupled to electrodes  16  via one or more electrode leads (two such leads  18  are shown), such that the electrodes  16  form an electrode array  20 . The electrodes  16  are carried on a flexible body  22 , which also houses the individual signal wires  24  coupled to each electrode. In the illustrated embodiment, there are eight electrodes on each lead, although the number of leads and electrodes is application specific and therefore can vary. The leads  18  couple to the IPG  10  using lead connectors  26 , which are fixed in a header  28  comprising epoxy for example, which header is affixed to the case  12 . In a SCS application, distal ends of electrode leads  18  are typically implanted on the right and left side of the dura within the patient&#39;s spinal cord. The proximal ends of leads  18  are then tunneled through the patient&#39;s tissue  100  to a distant location such as the buttocks where the IPG  10  is implanted, where the proximal leads ends are then connected to the lead connectors  26 . 
     As shown in cross section in  FIG. 2B , the IPG  10  typically includes an electronic substrate assembly including a printed circuit board (PCB)  30  containing various electronic components  32  necessary for operation of the IPG  10 , some of which are described subsequently. Two coils are generally present in the IPG  10 : a telemetry coil  34  used to transmit/receive data to/from an external controller  50  ( FIG. 2A ); and a charging coil  36  for charging or recharging the IPG&#39;s battery  14  using an external charger  70  ( FIG. 4A ). These coils  34  and  36  are also shown in the perspective view of the IPG  10  in  FIG. 1B , which omits the case  12  for easier viewing. Although shown as inside in the case  12  in the Figures, the telemetry coil  34  can alternatively be fixed in header  28 . Coils  34  and  36  may alternative be combined into a single telemetry/charging coil. 
       FIG. 2A  shows plan views of the external controller  50 , and  FIG. 2B  shows it in cross section and in relation to the IPG  10  during a communication session. The external controller  50 , such as a hand-held portable programmer or a clinician&#39;s programmer, is used to set or adjust the therapy settings the IPG  10  will provide to the patient (such as which electrodes  16  are active, whether such electrodes sink and source current, and the duration, frequency, and amplitude of pulses formed at the electrodes, etc.). The external controller  50  can also act as a receiver of data from the IPG  10 , such as various data reporting on the IPG&#39;s status, the level of the IPG  10 &#39;s battery  14 , and other parameters measured or logged at the IPG  10 . Such communications can occur bi-directionally via link  75 . 
     As shown in  FIG. 2B , the external controller  50  contains a PCB  51  on which electronic components  52  are placed to control operation of the external controller  50 . The external controller  50  is powered by a battery  53 , but could also be powered by plugging it into a wall outlet for example. A telemetry coil  54  is also present in the external controller  50 , which will be discussed further below. A case  59 , typically made of plastic, houses the internal components of the external controller  50 . The external controller  50  typically comprises a user interface  55  similar to that used for a portable computer, cell phone, or other hand held electronic device, including touchable buttons  56  and a display  57 . A port  58  allows the external controller to be electrically coupled to a power source, to other computer devices, etc. 
     Wireless data transfer between the external controller  50  and the IPG  10  via link  75  takes place via magnetic inductive coupling between coils  54  and  34 , either of which can act as the transmitter or the receiver to enable two-way communication between the two devices. Referring to  FIG. 3 , which depicts circuitry in these devices, when a series of digital data bits (FSK data  47 ) is to be sent from the external controller  50  to the IPG  10 , control circuitry  60  (e.g., a microcontroller) provides these bits in sequence to a modulator  61 . Modulator  61  energizes coil  54  with an alternating current (AC) whose frequency is modulated in accordance with the state of the data bit currently being transferred in accordance with a Frequency Shift Keying (FSK) protocol. For example, the coil  54  may nominally be tuned to resonate at 125 kHz in accordance with the inductance of the coil  54  and a tuning capacitor (not shown), which data states ‘0’ and ‘1’ altering this center frequency to f 0 =121 kHz and f 1 =129 kHz respectively. The frequency-modulated current through the coil  54  in turn generates a frequency-modulated magnetic field comprising link  75 , which in turn induces a frequency-modulated current in the IPG&#39;s telemetry coil  34 . This received signal is demodulated 43 back into the series of digital data bits, and sent to control circuitry  38  (e.g., a microcontroller) in the IPG  10  for interpretation. Data telemetry in the opposite direction from IPG  10  to external controller  50  occurs similarly via modulator  41  and demodulator  62 . Inductive coupling via link  75  occurs transcutaneously, i.e., through the patient&#39;s tissue  100 . 
     Other means for electro-magnetically communicating between the external controller  50  and IPG  10  via link  75  are known as well, including RF communications such as Bluetooth, Zigbee, etc., that are enabled patch, wire, or slot antennas. In this instance, link  75  would comprise a longer-range electromagnetic field, rather than the near-field magnetic field enabled by coils  54  and  34 . 
       FIG. 4A  shows a plan view of the external charger  70 , and  FIG. 4B  shows it in cross section and in relation to the IPG  10  during a charging session. The external charger  70  is used to wirelessly charge (or recharge) the IPG&#39;s battery  14 , and includes at least one PCB  72  (two are shown; see U.S. Patent Application Publication 2008/0027500); electronic components  74 , some of which are subsequently discussed; a charging coil  76 ; and a battery  78  for providing operational power for the external charger  70  and for the production of a magnetic charging field  80  from the coil  76 . These components are typically housed within a case  77 , which may be made of plastic for example. 
     The external charger  70  has a user interface  82 , which typically comprises an on/off switch  84  to activate the production of the magnetic charging field  80 ; an LED  86  to indicate the status of the on/off switch  84 ; and a speaker  88 . The speaker  88  emits a “beep” for example if the external charger  70  detects that its charging coil  76  is not in good alignment with the charging coil  36  in the IPG  10  during a charging session, as discussed further below. The external charger  70  is sized to be hand held and portable, and may be placed in a pouch around a patient&#39;s waist to position the external charger  70  in alignment with the IPG  10  during a charging session. Typically, the external charger  70  is touching the patient&#39;s tissue  100  during a charging session as shown, although the patient&#39;s clothing or the material of the pouch may intervene. 
     Wireless power transfer from the external charger  70  to the IPG  10  occurs by magnetic inductive coupling between coils  76  and  36 . Referring to  FIG. 5 , when the external charger  70  is activated (e.g., on/off switch  84  is pressed), a charging circuit  94  under control of control circuitry  92  (e.g., a microcontroller) energizes coil  76  with a non-data-modulated AC current (Icharge) to create the magnetic charging field  80 . The frequency of the magnetic charging field may be on the order of 80 kHz for example, and may be set by the inductance of the coil  76  and the capacitance of a tuning capacitor (not shown). The magnetic charging field  80  induces a current in the IPG  10 &#39;s charging coil  36 , which current is rectified  44  to DC levels and used to provide a charging current (Ibat) to recharge the IPG&#39;s battery  14 , perhaps under the control of charging and battery protection circuitry  46  as shown. This again occurs transcutaneously. 
     The IPG  10  can also communicate data back to the external charger  70  using Load Shift Keying (LSK) telemetry. Relevant data, such as the capacity of the battery, is sent from control circuitry  38  in the IPG  10  to a LSK modulator  40 , which creates a series of digital data bits (LSK data  48 ). This data is input to the gate of a load transistor  42  to modulate the impedance of the charging coil  36  in the IPG  10 . Such modulation of the charging coil  36  is detectable at the external charger  70  due to the mutual inductance between the coils  76  and  36 , and will change the magnitude of the AC voltage needed at coil  76  (Vcoil) to drive the charging current, Icharge. If coil  36  is shorted (LSK data=1), Vcoil increases (Vcoil 1 ) to maintain Icharge; if not shorted (LSK data=0), Vcoil decreases (Vcoil 0 ), as shown in the waveform in  FIG. 5 . LSK demodulator  96  in the external charger  70  can detect these changes in Vcoil (ΔV) to recover the series of digital data bits, which data is then received at control circuitry  92  so that appropriate action can be taken, such as ceasing production of the magnetic charging field  80  (i.e., setting Icharge to zero) because the battery  14  in the IPG  10  is full. Note that the nature of LSK telemetry as described here only allows for telemetry from the IPG  10  to the external charger  70  when a magnetic charging field  80  is being produced. See, e.g., U.S. Patent Application Publication 2013/0123881 for further details regarding the use of LSK telemetry in an external charger system. 
     The inventor is concerned about certain problems with traditional means of wireless communications between an external controller  50  and the IPG  10 , and with traditional means of charging an IPG  10  using an external charger  70 . The inventor&#39;s concerns regarding communications are discussed first. 
     As is known, wireless communications to and from the IPG  10  can be attenuated by the conductive material of the case  12  as well as other conductive structures present in the IPG  10  and the external controller  50 . Especially when magnetic induction is used as the means for establishing communication link  75  for example, the generated AC magnetic fields will create eddy currents in such conductive structures, which essentially act as an unwanted sink for the energy in the field, thus reducing the distance at which communications and charging can reliably occur. See, e.g., U.S. Pat. No. 8,457,756. 
     Previous IPGs  10  have used non-conductive ceramic materials for the case  12 , see, e.g., U.S. Pat. No. 7,351,921, which would reduce attenuation of wireless communications in IPGs using internal coils. However, ceramic materials are also brittle and difficult to work with. Ceramic case components further require brazing to mechanically couple them together or to other metallic components, which can be difficult to perform. 
     Previous approaches have used optical radiation instead of electromagnetic fields as the means to communicate with an implantable medical device. For example, U.S. Pat. No. 5,556,421 discloses a pacemaker which has photoemitter such as a Light Emitting Diode (LED), and a photodetector such as a phototransistor, for respectively transmitting data to and receiving data from a device external to the patient. See FIG. 15 of the &#39;421 patent. However, in the &#39;421 patent, the photoemitter and photodetector are contained within the header of the pacemaker, similar to the header  28  for the IPG  10  described earlier ( FIG. 1A ). The header is described in the &#39;421 patent as suitably translucent to the wavelengths of optical radiation at which the LED and photodiode operate (within the range of 640 to 940 nm). 
     The inventor however finds the optical communication approach of the &#39;421 patent to be problematic, in particular because the optical elements are contained within the header of the implantable medical device. The three-dimensional shape of the header makes optical transmission and reception difficult, as optical radiation will reflect at the outer surfaces of the header and other reflective components in the header, such as the lead connectors  26  (see, e.g.,  FIG. 1A ). Optical radiation will also refract, attenuate, and disperse in the header material. Additionally, there may be little room in the header to accommodate optical elements. This is particularly problematic in a SCS IPG, which comprises many electrodes (e.g., 16 or 32), and hence requires long lead connectors  26 , or more lead connectors, in the header  28 . Providing optical elements in the header provides further concerns that additional feedthrough pins between the header and the interior of the case would be necessary, complicating IPG design and potentially impacting reliability. 
     U.S. Pat. No. 6,243,608 also discloses a pacemaker that can communicate optically with an external device, although once again in this reference, the optical element is contained in the header, thus suffering from the same problems discussed above with reference to the &#39;421 patent. (Specifically, this pacemaker has only a photoemitter and thus can only communicate optically with the external device in one direction; communication from the external device to the pacemaker occurs via magnetic induction between two coils). In the text associated with one embodiment, see FIG. 6 of the &#39;608 patent, it is mentioned that the photoemitter can be located in an electronics module inside the pacemaker case. But in this instance, the photoemitter transmits light from inside the case to the translucent header. This too is not practical. Although not discussed in detail in the &#39;608 patent, this approach requires porting the optical radiation through the feedthrough between the case and header in some fashion, which would attenuate the radiation, and complicate feedthrough design. It is noted that a mirror may need to be provided in the header to direct the optical radiation to the external device, or that a portion of the outer surface of the header be shaped as a lens, both of which are complicated, expensive, and could be expected to attenuate the radiation. 
     U.S. Pat. No. 7,447,533 discloses a pacemaker in which a photoemitter and photodetector are used to detect a physiological parameter, such as blood flow (photoplethysmography). In one example, see FIGS. 7 and 8 of the &#39;533 patent, an aperture is formed in one of the flat sides of the case that accommodates an assembly containing the optical elements. Once positioned in place, the assembly is welded to the case. Nonetheless, the &#39;533 patent is not relevant to the inventor&#39;s concern regarding communications between an implant and an external device. The optical elements in the &#39;533 are not used to send and receive a series of data bits, and are not used to communicate optical radiation externally to the implant. Instead, the photoemitter provides radiation that reflects off the patient&#39;s tissue, which reflection is detected at the implant&#39;s photodetector to determine the physiological parameter. (If a dual-wavelength photoemitter is used, the wavelengths are enabled in an alternating fashion). For communications between the implant and the external device, the &#39;533 patent instead uses an electromagnetic antenna operable with radio waves (e.g., 10-15 MHz). U.S. Pat. No. 5,902,326 is similar, although in this patent the optical elements are used to detect a different physiological parameter, namely blood oxygen content (oximetry). 
     U.S. Patent Application Publication 2009/0076353 also comprises a pacemaker having an aperture on one of the flat sides of the case that accommodates an optical sensor assembly, which again can be welded to the case. However, the unique particulars of the &#39;353 Publication render it unsuitable for data communication external to the implant. The optical sensor assembly is designed to detect yet another physiological parameter, in this case analytes such as Potassium ions. As described in the &#39;353 Publication, such analytes are designed to diffuse through the optical sensor assembly where they meet with a chemical sensing element. Photoemitters in the assembly are made to reflect off of this chemical sensing element. The chemical sensing element&#39;s optical properties change in the presence of the analyte, and so reflections are received at a photodetector in the optical sensor assembly to measure the analyte. Indeed, the unique particulars of this publication render it unsuitable for external data communications, as an overlying cover layer is included to block ambient light from entering the optical sensor assembly, and also to prevent the light from the photoemitters from escaping the optical sensor assembly. 
     The inventor is also concerned about shortcomings concerning charging an implantable medical device battery. In particular, the inventor is concerned that charging is hampered by difficulty in determining the alignment between the external charger  70  and the IPG  10 . 
     It is generally desirable to charge the IPG&#39;s battery  14  as quickly as possible to minimize inconvenience to the patient. One way to decrease charging time is to increase the strength of the magnetic charging field  80  by increasing Icharge in the charging coil  76  of the external charger  70 . Increasing the magnetic charging field  80  will increase the current/voltage induced in the coil  36  of the IPG  10 , which increases the battery charging current, Ibat, hence charging the battery  14  faster. 
     However, the strength of the magnetic charging field  80  can only be increased so far before heating becomes a concern. Heating is an inevitable side effect of inductive charging using magnetic fields, and can result because of activation of relevant charging circuitry in the external charger  70  or IPG  10 , or as a result of eddy currents formed by the magnetic charging field  80  in conductive structures in either device. Heating is a safety concern. The external charger  70  is usually in contact with the patient&#39;s tissue  100  during a charging session, and of course the IPG  10  is inside the patient. If the temperature of either exceeds a given safe temperature, the patient&#39;s tissue may be aggravated or damaged. 
     The alignment between the external charger  70  and the IPG  10  can affect heating, as shown in  FIGS. 6A and 6B . In  FIG. 6A , the charging coils  76  and  36  in the external charger  70  and the IPG  10  are well aligned, because the axes  76 ′ and  36 ′ around which the coils  76  and  36  are wound are collinear. As such, these coils  76  and  36  are well coupled electrically, meaning that a higher percentage of the power expended at coil  76  in creating the magnetic charging field  80  is actually received at coil  36 , which leads to higher values for Ibat. In  FIG. 6B , the charging coils  76  and  36  are laterally misaligned (d), which reduces the electrical coupling between the coils. Increasing the vertical distance x between the coils  76  and  36  ( FIG. 6C ), or increasing the angle (θ) between the preferably parallel planes in which they reside ( FIG. 6D ), will also reduce coupling. 
     If it is desired that the alignment scenarios of  FIGS. 6A and 6B  charge the battery  14  at the same rate (Ibat=Y), then a higher value for Icharge (Icharge&gt;X) will be needed in the misaligned scenario of  FIG. 6B  compared to the well-aligned scenario of  FIG. 6A  (Icharge=X). A higher value for Icharge in  FIG. 6B  will create a more intense magnetic charging field  80  that tend to increase the temperature of the environment (T&gt;Z) when compared to the temperature of the environment in  FIG. 6A  (T=Z). If it is desired that the temperature be the same for both scenarios, then Icharge can be lowered in  FIG. 6B , but this will also lower Ibat, and hence the battery  14  in that scenario would take longer to charge. In short, misalignment between the external charger  70  and the IPG  10  is not desired. 
     Accordingly, the art has disclosed several manners for determining misalignment between an external charger and an IPG, which techniques usually result in some form of user-discernible output letting the patient know when alignment is poor (such as via speaker  88  discussed earlier). Such techniques may also inform a patient how to fix the alignment, such as by indicating a direction the external charger should be moved relative to the IPG  10 . See, e.g., U.S. Pat. Nos. 8,473,066 and 8,311,638. 
     Previous external charger alignment techniques however are difficult to implement, and may not precisely determine alignment as they rely on inferences gleaned from electrical measurements taken during the charging session. For example, one prior art alignment techniques relies on determining the loading of the charging coil in the external charger during production of the magnetic charging field. Specifically, the voltage across the charging coil (Vcoil) is reviewed at the external charger and compared to a Vcoil threshold to determine alignment. This technique though suffers in its inability to distinguish between the scenarios of  FIGS. 6B and 6C  for example. In either of these scenarios, Vcoil would be higher due to poor coupling, but in  FIG. 6B  the poor coupling arises from misalignment, whereas in  FIG. 6C  the alignment is as good as it can be given the IPG  10 &#39;s depth (x). A modification to this technique helpful in distinguishing these scenarios requires transmitting the magnetic charging field at different frequencies and measuring the input current to the charging coil in the external charger to estimate an implant depth (x), and thus to set an appropriate Vcoil threshold. See, e.g., U.S. Patent Application Publication 2010/0137948. However, the additional overhead of having to produce magnetic charging fields at different frequencies makes this technique complicated. 
     Other alignment techniques require the external charger to have positioning coils in addition to the main charging coil (e.g.,  76 ). In these techniques, measurements taken from the positioning coils during the charging session are used to determine misalignment, and to indicate a direction the external charger can be moved to improve alignment (coupling). See, e.g., U.S. Pat. Nos. 8,473,066 and 8,311,638. The requirement of additional coils beyond the main charging coil though complicates the design of the external charger. 
     Still other alignment techniques employ electromagnetic (EM) telemetry from the IPG, see, e.g., U.S. Patent Application Publications 2013/0096651 and 2011/0087307, which adds complexity to both the IPG and the external charger. Moreover, EM telemetry may be difficult to employ while the external charger is generating a magnetic charging field (e.g.,  80 ), because such field is relatively strong, and may add significant noise to the EM telemetry signal. Thus it may be necessary to periodically cease the production of the magnetic charging field during a charging session to allow such telemetry from the IPG to the external charger to occur, which inconveniently lengthens the duration of the charging session. 
     The inventor is further concerned that LSK telemetry is limited in its ability to communicate information from the IPG the external charger. First, as noted earlier, LSK telemetry is only useful when the external charger is producing a magnetic charging field, thus hampering the ability of the IPG to communicate with the external charger, prior to starting a charging session for example. Moreover, LSK telemetry may be difficult to demodulate (e.g.,  FIG. 5 ,  96 ). Vcoil, the parameter assessed by LSK demodulator  100 , can vary in magnitude as the alignment between the external charger and IPG varies during a charging session, which is typical. Likewise, ΔV, the difference in Vcoil for each of the logic states being transmitted by the IPG, can vary and may also be relatively small and hard to detect depending on the coupling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show an Implantable Pulse Generator (IPG) and the manner in which electrodes are affixed in accordance with the prior art. 
         FIGS. 2A and 2B  show an external controller for an IPG and the manner in which they communicate in accordance with the prior art. 
         FIG. 3  shows the communication circuitry in the external controller and the IPG in accordance with the prior art. 
         FIGS. 4A and 4B  show an external charger for an IPG and the manner in which they communicate in accordance with the prior art. 
         FIG. 5  shows the communication circuitry in the external charger and the IPG in accordance with the prior art. 
         FIGS. 6A-6D  show different alignment scenarios between the external charger and the IPG in accordance with the prior art. 
         FIGS. 7A and 7B  show an improved IPG having a window assembly for optical communications. 
         FIGS. 8A and 8B  show the improved IPG in cross section. 
         FIG. 9  shows an improved external controller for optically communicating with the improved IPG. 
         FIG. 10  shows the optical communication circuitry in the improved external controller and the improved IPG. 
         FIG. 11  shows a modified improved external controller having an optical communication head. 
         FIG. 12  shows an improved external charger for communicating with the improved IPG. 
         FIG. 13  shows the optical communication circuitry in the improved external charger and the improved IPG. 
         FIGS. 14A-14C  show use of the optical communication circuitry to determine external charger/IPG alignment. 
         FIGS. 15A-15D  show use of the optical communication circuitry to determine a direction of external charger/IPG misalignment. 
         FIG. 16  shows a modified improved external charger having a charging head with optical communication capabilities. 
         FIG. 17  shows an improved combined external controller/charger for optically communicating with the improved IPG. 
         FIG. 18  shows a modified improved combined external controller/charger having a combined communication/charging head. 
     
    
    
     DETAILED DESCRIPTION 
     The inventor discloses an improved medical device system in which system devices communicate optically. An Implantable Medical Device (IMD) such as an IPG is disclosed having a hermetic window assembly on one side of its case, through which a photoemitter and photodetector can directly transmit and receive optical signals to and from the outside of the patient. The optical radiation in the optical signals is preferably visible, which permits communications from the IMD to be seen prior to implantation and even after implantation through a patient&#39;s tissue. External controllers for adjusting therapeutic operation of the IMD, external chargers for providing a magnetic charging field to charge a battery in the IMD, and combined external controllers/chargers are also disclosed that optically bi-directionally communicate with the IMD through the patient&#39;s tissue, all of which may include distal communication heads which perform optical communications with the IMD, and which may also include a charging coil. The optical communication capabilities of the external charger are particularly useful in determining and indicating misalignment with the IMD, and optical communications can occur between the external charger and the IMD regardless whether the external charger is producing a magnetic charging field or not, and without the need to cease production of the magnetic charging field. Such optical communications can also displace legacy Load Shift Keying means of communicating with the external charger, which can be difficult to demodulate. 
     An improved IPG having optical communication capabilities is discussed first ( FIGS. 7A-8B ). External devices that can optically communicate with the improved IPG  110  are then discussed, such as an improved external controller ( FIGS. 9-11 ), and an improved external charger ( FIGS. 12-16 ). An integrated external device combining both communication and charging capabilities that can optically communicate with the improved IPG is discussed last ( FIGS. 17-18 ). 
       FIG. 7A  shows an improved IPG  110  having an optical window assembly  112  on a top flat side of its case  12  that will face outwardly of the patient when implanted. Underlying and generally centered with the window assembly  112  are optical devices, namely a photoemitter  114  and a photodetector  116 , as seen in  FIG. 7B  (with the case  12  removed). As shown in the cross section of  FIG. 8A , the photoemitter  114  and a photodetector  116  are electrically coupled to the PCB  30  of the IPG  110 . Photodetector  116  may be a photo-sensitive transistor in one example, and photoemitter  114  may comprise a Light Emitting Diode (LED) or a laser diode, although other types of optical devices may also be used. As will be explained further below, having both a photoemitter  114  and photodetector  116  enables the IPG  110  to optically communicate in two directions ( 101   a  and  101   b ). However, this is not strictly necessary, and in uni-directional applications, only one of these devices  114  and  116  may be needed. 
       FIG. 8B  shows further details of the construction of the window assembly  112 , and the manner in which it is affixed to the case  12 . The window assembly  112  comprises a biocompatible material and may be comprised of a glassy material, such as Schott BK10, Corning 7056, sapphire, fused silica, or quartz for example. The window  118  is made to fit in a ring-shaped collar  122  comprising titanium for example, or any other biocompatible material affixable to the material of the case  12 . A filler material  120 , such as gold or glass, fills the seal between the window  118  and the collar  122 , and brazing is performed to melt the filler  120  and create a hermetic seal between the window  118  and the collar  122 . Other brazing filler materials  120  could be used as well. As shown, the collar  122  may include a step to keep the window  118  and filler material  120  in place during the brazing process. 
     The side of IPG  110 &#39;s case  12  is formed with a hole  13  to accommodate the window assembly  112  once its manufacture is completed. In this example, the case  12  and collar  122  include steps to allow the window assembly  112  to be placed within the hole  13  without falling through. Once positioned in place, the window assembly  112  can be welded  124  to the case  12  to create a hermetic seal. As depicted, the window assembly  112  is substantially flush with the outside surface of the case (e.g., less than 10 mils difference), which is preferred to prevent aggravation of tissue that the window assembly  112  will contact. 
     This design for window assembly  112 , and the manner in which it is affixed to the case  12 , are merely examples. Other designs and methods for providing a window  118  in an implantable medical device case  12  with good hermeticity can be used. 
     The photoemitter  114  and a photodetector  116  are positioned to receive and transmit optical radiation through the window  118  in the window assembly  112 . In preferred embodiments, the photoemitter  114  and a photodetector  116  operate at visible wavelengths (e.g., from approximately 380 to 740 nm). This is particularly preferred when a patient, clinician, or manufacturer desires to see optical radiation emitted from the IPG  110 , which is useful in several circumstances explained below. However, in other examples, the photoemitter  114  and a photodetector  116  can operate at non-visible wavelengths, such as near-Ultraviolet (e.g., 10 nm-400 nm) and near-Infrared wavelengths (e.g., 700 to 2500 nm). Essentially, any wavelength of optical radiation can be used in the context of IPG  110 , so long as it (1) is not significantly attenuated by the window  118  and the patient&#39;s tissue  100 , and (2) does not risk damaging the patient&#39;s tissue  100 . 
     Because the photoemitter  114  and a photodetector  116  are hermetically sealed inside the case  12  with other IPG electronics, special care does not need to be taken to ensure that such devices are biocompatible, and thus typical, inexpensive, off-the-shelf optical components can be used for each. Photoemitter  114  and photodetector  116  may comprise a number of emitters or detectors, which may be integrated into a single optical device, although they are illustrated here separately here for clarity. Photoemitter  114  may additionally emit optical radiation at different wavelengths (e.g., different colors), while photodetector  116  may likewise be sensitive to such wavelengths. 
     Note that the inclusion of photoemitter  114  in the IPG  110  provides several benefits. During manufacturing or even during implantation when the IPG  110  is not yet covered by a patient&#39;s tissue  100 , the photoemitter  114  (particularly if it operates at visible wavelengths) provides an easy means of verifying IPG  110  operation. For example, a manufacturer of IPG  110  can test the device and receive optical feedback concerning IPG operation by viewing the illumination of photoemitter  114  through the window assembly  112 . Visual feedback can come from the photoemitter  114  in any number of forms. For example, a green light may indicate proper IPG  110  operation, and a red light may indicate faulty operation, etc. Various operational conditions can also be visually indicated. For example, a solid light may indicate one condition, a slow blinking pulse a second condition, a fast blinking pulse a third condition, etc. Combinations of blinking pulses can visually indicate various operational codes, including failure codes. For example, repeating a single pulse might indicate a first code; repeating two pulses might indicate a second code, etc. Combinations of these types of visual feedback can also be used to indicate operation, conditions, or codes. Such visual feedback can be issued by the IPG  110  of its own accord, or in response to a communication sent from the external controller  150  ( FIG. 9 ), the external charger  170  ( FIG. 12 ), or other external device. 
     A clinician can also benefit from such visual feedback provided by IPG  110 . For example, when the clinician attaches the leads  18  ( FIG. 1A ) to the lead connectors  26  during surgery, it is important to verify that good electrical contact is established, and that there are no open or short circuits at any of the electrode contacts  16 . This can be visually indicated to the clinician via photoemitter  114  before surgery is complete, in any of the foregoing manners. 
     A patient may also benefit from visual feedback even after the IPG  110  has been implanted in her tissue  100 . In this regard, and referring to  FIG. 8A , photoemitter  114  will illuminate a portion  101  of the patient&#39;s tissue  100  after implantation. The optical radiation will scatter and attenuate in the illuminated tissue  101 , particularly if the IPG  110  is implanted deeper in the patient (x). Still, many implantable medical devices, including SCS IPGs  110 , are purposely implanted reasonably close to the patient&#39;s skin to make implanting and explanting easier, which would reduce optical attenuation. If optical attenuation is significant enough that optical radiation from the photoemitter  114  cannot been seen at the surface of the patient&#39;s skin, the power of the photoemitter  114  can be increased. Alternatively, a patient may be instructed to view the illuminated tissue  101  in a darker environment to review relevant IPG operations, conditions, or codes. 
     If necessary, the window  118  in the window assembly  112  can be formed as a lens, instead of flat, to better focus the optical radiation, and to reduce the volume of the illuminated tissue  101  in which the optical radiation disperses. For example, use of a convex lens  118  would tend to focus optical radiation in the illuminated  101  tissue if radiation emitted from the photoemitter  114  is not well collimated. A convex lens  118  would also focus optical radiation dispersed in the illuminated tissue  101  toward the photodetector  116  when receiving radiation in the other direction, as subsequently explained. Traditional bulk lenses or Fresnel lenses could be used for window  118 . 
     It should also be noted that window  118  need not be significantly optically translucent, or “see through,” as glass would generally be to visible light for example. Instead, window  118  could be made of otherwise generally opaque materials that are still able to pass significant amounts of optical radiation in and out of the IPG  110 . For example, window  118  could comprise a ceramic material, which could pass suitable amounts of optical radiation if made thin enough, and/or if the power and sensitivity of the photoemitter  114  and photodetector  116  are suitably high. In short, window  118  may be comprised of any material able to pass suitable amounts of optical radiation to enable the various means of optical communications disclosed, and “window” should not be construed to cover simply translucent materials. 
     Through-the-skin visual indications can occur in any of the ways discussed above, and can provide different sorts of information to the patient. For example, a patient could be provided with a card describing various indications important to IPG  110  operation. For example, a first indication might denote a first type of fault in the IPG  110 ; a second indication a second type of fault, etc. A flashing red light might indicate potentially unsafe stimulation setting, while a solid red light indicates a severe failure that caused the IPG  110  to shut down. Red indications may denote that the patient needs to contact the clinician, while green indications indicate normal IPG operation while still providing particular information. 
     Through-the-skin visual feedback can also be used to indicate information relevant to the battery  14  in the IPG  110 , such as when it needs to be recharged. In one simple example illustrated in  FIG. 6A , the photoemitter  114  may pulse  102  when charging is required (when Vbat is low. More complicated visible feedback scenarios are also possible, such as changing the pulse frequency (f) as a function of Vbat. Such pulses  102  can be of constant intensity (I) over their pulse width (pw), or could also include modulated data interpretable by the external controller  150  ( FIG. 9 ) or the external charger  170  ( FIG. 12 ). For example, pulses  102  could include data such as an instruction to the external charger  170  start charging and/or the level of the battery  14  (Vbat). Pulses  102  could also include error codes interpretable by the external controller  150 , which the patient could review on that device&#39;s graphical user interface. Such data modulation within pulses  102  may occur on a time scale not visually resolvable by the patient, who instead may simply see single pulses  102  despite the data included within them. Data-modulated portions of pulses (if any), may be small compared to the pulse width of the pulses, and can occur at the end or beginning of such pulses, or may be interleaved with pulses of constant intensity. Although the pulses  102  are shown as periodic, they don&#39;t have to be. Moreover, using photoemitter  114  in the IPG  110  to produce visual feedback, or pulses  102  specifically, is not strictly required. 
       FIG. 9  shows an improved external controller  150  operable to optically communicate with the improved IPG  110 , which in this example comprises modifications to the external controller  50  disclosed earlier ( FIG. 2A ). Like the external controller  50  described earlier, external controller  150  is used to set or adjust the therapy settings the IPG  110  will provide to the patient, and to receive relevant data from the IPG  110 . 
     External controller  150  includes an optical window assembly  163  formed in the bottom of its case  59  that will face inwardly of a patient during a communication session between the external controller  150  and the IPG  110 . External controller  150  further includes a photoemitter  164  and a photodetector  166  affixed to its PCB  51  which are generally centered with the window assembly  172 . As with the IPG  110 , external controller  150  may have only one of photoemitter  164  or photodetector  166  in a uni-directional application. Because the external controller  150  is not governed by the same hermeticity requirements as the IPG  110 , the manner in which the window assembly  172  is affixed to a hole in the case  59  is less critical, and can occur in any manner suitable for an external device. 
     Moreover, the photoemitter  164 ′ and photodetector  166 ′ can also be modified to pass through one or more holes in the case  77 , or photoemitter  164 ″ and a photodetector  166 ″ may be located outside of the case  77 , with their lead wires passing through one or more smaller holes in the case, as shown in the bottom of  FIG. 9 . Coatings or epoxies can be useful to hold the photoemitters  164 ′ or  164 ″ the photodetectors  166 ′ or  166 ′ to the case  77  and for mechanical protection in these modification. Despite such modified placement of these optical devices of the external controller  150 , the subsequent discussion focuses for simplicity on placement using a windows assembly  153 , even though these modified placements could be used subsequently as well. 
     The photoemitter  164  and photodetector  166  in the external controller  150  may operate at the same wavelengths described earlier for the photoemitter  114  and a photodetector  116  in the IPG  110 . Photoemitter  164 , like photoemitter  164 , will illuminate tissue  101  at a sufficient depth (x) to reach the optical window assembly  112  of the IPG  110 . As such, at least some amount of the optical radiation from photoemitter  164  in the external controller  150  will reach the photodetector  116  in the IPG  110 , and at least some amount of the optical radiation from photoemitter  114  in the IPG  110  will reach the photodetector  166  in the external controller  150 . This allows the two devices  110  and  150  to bi-directionally optically communicate through the patient&#39;s tissue  100 . 
     The window assembly  153  can appear anywhere on the external controller  150 , but the window assembly  112  of the IPG  110  is preferably generally centered with its charging coil  36 . This placement of window assembly  112  is useful if the IPG  110  is additionally charged by an external charger  170  ( FIG. 12 ) that employs optical communications, as described below. Another place where the window assembly  153 ′ and its corresponding photoemitter  164  and photodetector  166 ′ may be placed on the external controller  150  is on its edge next to the port  58  ( FIG. 2A ) as shown in dotted lines, which may make accessing the user interface of the external controller  150  easier during a communication session with the IPG  110 . As with the window assembly  112  of the IPG  110 , the window assembly  153  could comprise a lens to better focus optical radiation emitted from photoemitter  164 , or received at photodetector  166 , and may be comprised of any material able to pass suitable amounts of optical radiation. 
     The window assembly  153  of external controller  150  preferably touches the patient&#39;s tissue  100  during a communication session in the center of the IPG  110 , or at the illuminated tissue  101  if photoemitter  114  in the IPG  110  is providing visual feedback as discussed earlier. It is helpful to reduce ambient light in any space  149  between the external controller  150  and the tissue  100  that could otherwise interfere with optical communications, although if the optical devices in both are tuned to specific wavelengths, this is not as critical. Optical communications can still be had through a patient&#39;s clothing if it permits sufficient optical radiation to pass through. 
       FIG. 10  shows circuitry that can be used for bi-directional optical communications between the external controller  150  and the IPG  110  via optical communication links  101   a  and  101   b  (comprising the illuminated tissue  101 ), which are used to optically transmit and receive a series of digital data bits  147 . Such circuitry includes optical transmitters ( 144 ,  134 ) coupled to the photoemitters ( 164 ,  114 ) in the external controller  150 , and optical receivers ( 146 ,  136 ) coupled to the photodetectors ( 166 ,  116 ) in the IPG  110 . The optical transmitters  144  and  134  may include necessary modulation circuitry to convert the digital data bits  147  into appropriate modulated analog signals to drive the photoemitters  164  and  114  per the modulation scheme chosen. The optical receivers  146  and  136  may likewise include necessary demodulation circuitry to convert the modulated analog signals received at the photodetectors  166  and  116  into the series digital data bits  147  per the same modulation scheme. 
     The optical data modulation scheme used in the optical transmitters  144  and  134  can include Phase Shift Keying (PSK), which can occur at 9600 bits-per-second in one example. See, e.g., K. Inoue et al., “Transcutaneous Optical Telemetry System with Infrared Laser Diode,” ASAIO J. at 841 (1998), which is submitted in the Information Disclosure Statement filed herewith. If PSK is used as the modulation scheme, the optical receivers  146  and  136  could comprise Phase Locked Loops (PLLs) for example. However, use of PSK modulation is merely one example. For example, Pulse Width Modulation (PWM), On-Off Keying (OOK), Differential Phase Shift Keying (DPSK), Pulse Amplitude Modulation (PAM), Quadrature Amplitude Modulation (QAM), M-ary varieties of the foregoing, etc., could also be used. The nature of optical communications would also allow for significant higher bit rates, although 9600 bits-per-second would generally be sufficient for communications between the external controller  150  and the IPG  110 . 
     The optical receivers  146  and  136  in the external controller  150  and IPG  110  may not be strictly necessary, or could be modified. For example, if control circuitries  60  and  38  are additionally programmed to provide necessary demodulation functionality, the optical receivers  146  and  136  may comprise Analog-to-Digital (A/D) converters that can digitize the analog intensity signal provided by the photodetectors  176  and  116 , and provide these digitized intensity values to their respective control circuitries  60  and  38  to determine the individual data bits. If the control circuitries  60  and  38  provide A/D inputs capable of digitizing the data, separate optical receivers  146  and  136  may not be necessary at all, and the control circuitries can be programmed to perform the demodulation. Likewise discrete optical modulators  144  and  134  may also be modified or may not be required if control circuitries  60  and  38  are additionally programmed to provide necessary modulation functionality, or if they include D/A outputs that can drive the photoemitters  114  and  164  directly. 
     The optical receivers  146  and  136  in the external charger  170  and IPG  110  can vary in design depending on whether what is important in received optical signals is modulated data (D), intensity (Ia), or both, which may also depend of the modulation scheme being used. For example, received intensity Ia would be important in modulation schemes employing amplitude modulation, and can be important if a received optical signal is of constant intensity and without data, as discussed further below. In this regard, the optical receivers  146  and  136  can also report such received intensities Ia to the control circuitries  60  and  38  along with any digital data. Received intensity may involve filtering or integrating the analog signal from the photodetectors  166  or  166  either before or after they are digitized, or may be gleaned by processing in the control circuitries  92  and  38 . 
       FIG. 11  shows another embodiment of an external controller  150 ′ system that can optically communicate with IPG  110 . In this example, optical components have been moved out of the external controller  150  (compare  FIG. 9 ), and into an optical communication head  210 , including the photoemitter  164 , the photodetector  166 , and if necessary, optical transmitter and receiver circuitry  144  and  146 . These components may be integrated on a PCB  216 , and contained within a housing  218 , which may be significantly smaller and less complicated that the housing  59  used by the external controller  150 . As shown, the optical communication head  210  includes a window assembly  153 , similar to that described earlier for external controller  150 , but could include the other optical device placement options discussed in  FIG. 9 . 
     The optical communication head  210  communicates with a mobile controller with a graphical user interface, such as the external controller  50  described earlier ( FIG. 2A ), or a mobile device  200  such as a cell phone, a tablet computer, or another hand-holdable portable control device. As depicted, the optical communication head includes a cable  212  and a connector  214  that can couple to appropriate ports  58  ( FIG. 2A ) or  208  on the mobile controller. However, such wired connection of the optical communication head  210  is not necessary, and instead it may communicate with the external controller  50  or mobile device  20  wirelessly using a suitable short-range protocol, such as Near Field Communication (NFC), Bluetooth, Bluetooth Low Energy (BLE), Wifi, Zigbee, etc, that is supported by the mobile controller. If such wireless communications were used, the optical communication head  210  may also additionally include a battery (as it could not receive power from the mobile controller by the cable  212 ), and telemetry circuitry compliant with the short-range protocol used. As disclosed in U.S. Provisional Patent Application Ser. Nos. 61/873,314, filed Sep. 3, 2013, and 61/874,863, filed Sep. 6, 2013, which are both incorporated herein by reference in their entireties, the mobile device  200  can include an executable application to provide a graphical user interface, which like external controller  50  can allow a patient to set or adjust the therapy settings the IPG  110  will provide to the patient, and to receive relevant data from the IPG  110 . 
     Under control of the graphical user interface provided by the mobile controller, digital data bit to be transmitted to the IPG  110  can be serialized and sent to ports  58  or  208  and down the cable  212  to the head  210 , or the data can be wirelessly transmitted from the mobile controller via a short-range protocol and recovered at the head  210 . Once received at the head  210 , the data is modulated at optical transmitter  144 , with the modulated data driving photoemitter  144  in the head to provided optical data to the IPG  110  via link  101   a . Optical data received from the IPG  110  via link  101   b  is received at photodetector  166  in the head  210 , and is demodulated at optical receiver  146  to recover the series of digital data bits. Thereafter, the head  210  can provide the bits to the mobile controller via the cable  212 , or wirelessly using the short range protocol. Note that providing optical transmitter and receiver circuitry  144  and  146  in the optical communication head  210  is not strictly necessary if the mobile controller can provide the proper signals at the ports  58  and  208  to drive and receive data from the photoemitter  164  and photodetector  166  directly. 
     The external controller  150 ′ of  FIG. 11 , while having different pieces, may be more convenient for a patient, because it allows the optical communication head  210  to be placed proximate to the IPG  110  (such as in a belt with a pocket, or adhered to the patient&#39;s tissue  100  using double sided tape), while the mobile controller can remain relatively distant from the IPG  110  by virtue of the length of cable  212  or the length of the short-range protocol. This makes IPG optical communications easier, particularly if the IPG  110  is located in an area behind the patient, as occurs in an SCS application, as it permits the graphical user interface of the mobile controller to be held and seen in front of the patient. If the optical communication head  210  is held in place with a pouch, provisions can be made in the pouch to render it transparent to the optical radiation used to communicate between the head  210  and the IPG  110 , such as by forming the patient-facing wall of the pouch with transparent plastic or a mesh material, by providing a hole in the patient-facing side for the optical window  153 , etc. 
     The design of IPG  110  and external controllers  150  and  150 ′ provide reliable means for optically communicating through the patient&#39;s tissue  100 . The path optical communications take in the disclosed devices is direct compared to more-complicated prior art optical communication approaches discussed earlier. For example, when transmitting from the IPG  110  to the external controller  150 , optical radiation generated at photoemitter  114  passes straight to a flat and relatively thin window  118  in window assembly  112  and thus will experience little attenuation in the IPG  110 , as the optical radiation need not pass through curved or bulky translucent materials (like header  28 ) that can reflect or refract such radiation, or which contain additional components that could interfere with such radiation (such as feedthroughs, lead connectors, mirrors, etc.). The window assembly  112  is beneficially provided on a side of the IPG case that is already naturally facing outwardly of the patient when implanted, thus sending optical radiation trough the tissue  100  in a direct route out of the patient without re-direction and with little attenuation. After passing through the patient&#39;s tissue in this manner, the radiation again experiences little attenuation coming into the external controller  150 , as it travels straight through another thin, flat window assembly  153  overlying photodetector  166 , or directly to the photodetector  166  if alternatively mounted to the external controller  150  (see  FIG. 9 ). Moreover, this optical communication path can be symmetric, as the window assemblies  153  and  112 , the optical components, and their positions relative to the window assemblies can be essentially the same. These factors result in optical communications that can be transmitted with the same low levels of energy at both devices. 
     Discussion now turns to external charging, and to an improved external charger  170  that can optically communicate with the IPG  110 .  FIG. 12  shows the improved external charger  170 , which includes an optical window assembly  172  formed in the bottom of its case  77  that will face inwardly of a patient during a charging session. External charger  170  further includes a photoemitter  174  and a photodetector  176  affixed to its PCB  72  which are generally centered with the window assembly  172 , although again, only one of photoemitter  174  or photodetector  176  may be required in a uni-directional application. Because the external charger  170  is not governed by the same hermeticity requirements as the IPG  110 , the manner in which the window assembly  172  is affixed to a hole in the case  77  is less critical, and can occur in any manner suitable for an external device. Moreover, the photoemitter  174  and photodetector  176  can be positioned through or on the bottom of case  77 , as discussed with respect to the external controller  150  of  FIG. 9 , although these modifications are not again depicted for simplicity. Again, the window assembly  172  could comprise a lens to better focus emitted and received radiation, and may be comprised of any material able to pass suitable amounts of optical radiation. 
     The photoemitter  174  and photodetector  176  in the external charger  170  may operate at the same wavelengths described earlier for the photoemitter  114  and a photodetector  116  in the IPG  110 . Photoemitter  174 , like photoemitter  114 , will illuminate tissue  101  at a sufficient depth (x) to reach the optical window assembly  112  of the IPG  110 . As such, at least some amount of the optical radiation from photoemitter  174  in the external charger  170  will reach the photodetector  116  in the IPG  110 , and at least some amount of the optical radiation from photoemitter  114  in the IPG  110  will reach the photodetector  176  in the external charger  170 . This allows the two devices  110  and  170  to bi-directionally optically communicate through the patient&#39;s tissue  100 . 
     The window assembly  172  can appear anywhere on the external charger  170 , but is preferably generally centered with its charging coil  76  as shown. As will be discussed further below, this is useful in optically determining external charger/IPG alignment. It should however generally be easy for the patient to align the external charger  170  with the IPG  110  in preparation for a charging session. For example, if the patient sees pulses  102  ( FIG. 8A ) illuminating her tissue  101 , this may indicate that the battery  14  in the IPG  110  is low, and that charging is required. The patient can thus visually place the window assembly  172  on the bottom of the external charger  170  over the illuminated tissue  101 , as shown in  FIG. 12 . The external charger  170  generally touches the patient&#39;s tissue  100  during a charging session to reduce ambient light that could otherwise interfere with optical communications, as described earlier. As such, the external charger  170  will tend to block such ambient light, and as a result, a less-powerful photoemitter  174  and less-sensitive photodetector  176  may be used in external charger  170  when compared to such optical devices in the external controller  150  described earlier. Optical communications can still be had with the external charger  170  through a patient&#39;s clothing or using a modified pouch as described earlier. 
       FIG. 13  shows circuitry that can be used for bi-directional optical communications between the external charger  170  and the IPG  110  via optical communication links  101   a  and  101   b  (comprising the illuminated tissue  101 ), which includes optical transmitters ( 184 ,  134 ) coupled to the photoemitters ( 174 ,  114 ), and optical receivers ( 186 ,  136 ) coupled to the photodetectors ( 176 ,  116 ). The optical data modulation scheme used in the optical transmitters  184  and  134  can again include Phase Shift Keying (PSK) or any of the other modulation schemes mentioned earlier with respect to the external controller  150 . In short, the optical components in the external charger  170  can largely mimic those appearing in the external controller  150  described earlier. 
     The external charger  170  and IPG  110  can communicate optically whether or not the external charger  170  is producing a magnetic changing field  80  to charge the IPG  110 &#39;s battery  14  during a charging session. This is in distinction to LSK communications, which as noted earlier can only occur while the magnetic charging field  80  is produced. Moreover, optical communications will not interfere with the magnetic charging field  80  and so can occur during a charging session, unlike systems that use EM telemetry schemes to determine alignment that were discussed earlier. 
     The external charger  170  and IPG  110  can optically communicate for many useful reasons. For example, the IPG  110  can send an optical signal with data via link  101   b  to instruct the external charger  170  to start generating a magnetic charging field  80  or to send the battery voltage Vbat, which as already noted may be included in pulses  102  ( FIG. 8A ). Other examples of optical communications along link  101   b  are discussed in detail below. 
     The external charger  170  can also optically communicate data to the IPG  110  along link  101   a . For example, the external charger  170  when positioned over the IPG  110  can optically send an instruction to the IPG  110  to report its battery capacity, Vbat, along link  101   b , which may be beneficial if pulses  102  are not used to indicate Vbat, or if pulses  102  are merely of constant intensity and do not contain data such as Vbat. Once Vbat is then optically reported from the IPG  110  to the external charger  170 , the external charger  170  can initiate a charging session by generating a magnetic charging field  80  if necessary (i.e., if Vbat is low). Alternatively, the external charger  170  may optically send further instructions to the IPG  110  via link  101   a  informing the IPG  110  of the desired to begin charging, with such charging commencing only after the external charger  170  optically receives an acknowledgement from the IPG  110  via link  101   b . This might be desired to allow the IPG  110  time to prepare itself for charging. Link  101   a  can also be used during a charging session. For example, the external charger  170  might periodically provide information to the IPG  110  during the charging session, such as an estimation of how much longer the external charger  170  believes charging may last. Alternatively, the external charger  170  may not have a photoemitter  174 , and thus may only allow for one-way optical communications from the IPG  110  (via link  101   b ). 
     Once the IPG  110  starts receiving the magnetic charging field  80 , it preferably periodically optically sends data to the external charger  170  via link  101   b , which is shown in  FIG. 13  as pulses  103 . Pulses  103  may be the same as pulses  102  ( FIG. 8A ), which may simply continue after receipt of the magnetic charging field  80 , and which may now contain additional data relevant to the charging session. Pulses  103  may include for example the IPG&#39;s battery voltage (Vbat), the temperature of the IPG (T), information indicative of the electrical coupling between the external charger  170  and the IPG  110  such as the battery charging current (Ibat), or all of these or still other parameters relevant to the current charging session. Again, although pulses  103  are shown as periodic, they don&#39;t have to be, so long as they are sent with a suitable frequency to allow the external charger  170  to optically receive data on a reasonable time scale during the charging session (e.g., every second). 
     The external charger  170  may use such optically-received data to control the magnetic charging field  80  it is producing. For example, if the external charger  170  understands that the optically-reported temperature data (T) from the IPG  110  is above a threshold for example, it can reduce the energy of the magnetic charging field  80 , for example by lowering Icharge, or by reducing the duty cycle of the field. See, e.g., U.S. Patent Application Publication 2011/0087307. The external charger  170  can also monitor optically-reported coupling data (e.g., Ibat), and adjust the magnetic charging field  80  accordingly, for example by increasing Icharge or the duty cycle if Ibat is lower than a threshold, or by reducing these magnetic charging field parameters if Ibat is higher than a threshold. Monitoring optically-reported data for Vbat may also be used by the external charger  170  to control the magnetic charging field  80  or to monitor the progress of charging generally, as well as to understand when Vbat is fully charged (e.g., to a threshold), so that generation of the magnetic charging field  80  can cease. 
     Optical communications can also be used in determining external charger  170 /IPG  110  alignment, which as noted earlier is important to ensure fast IPG battery charging without overheating. Additional circuitry is shown in  FIG. 14A  to assist with the determination of alignment, including an alignment detector, which in the example shown comprises an intensity module  190  programmed in the external charger  170 &#39;s control circuitry  92 . In this example, alignment is determined by assessing the received intensity (Ia) of optical signals transmitted by the photoemitter  114  in the IPG  110 , and detected at the photodetector  176  in the external charger  170  via optical link  101   b . Optical receiver  186  in the external charger  170  can be configured to provide the received intensity, Ia, as describer earlier in conjunction with the external controller  150 . 
     These optical signals are shown as alignment pulses  104  in  FIG. 14A , and can comprise the same pulses  102  or  103  discussed earlier, and thus may contain modulated data (D). If the pulse width (pw) of pulses  104  is relatively large compared to portions that might contain optically-modulated data, the presence of such data in pulses  104  should not affect their received intensities, Ia. Alternatively, pulses  104  may comprise pulses of constant intensity over its pulse width. Pulses  104  can also be discrete from pulses  103  ( FIG. 13 ), and may be interleaved with these pulses during a charging session. As with earlier pulses, pulses  104  do not have to be periodic, but are preferably sent with a suitable frequency to allow the external charger  170  to receive data on a reasonable time scale to assess alignment in real time from the patient&#39;s perspective (e.g., every second). 
     In this example, the photoemitter  114  in the IPG  110  issues optical pulses  104  during a charging session of a known intensity, I. As the pulses  102  pass through the illuminated tissue  101  ( 101   b ), they will attenuate, and thus will be received at the external charger  170  with a lower intensity, Ia. This intensity Ia can be determined at the optical receiver  186  in the external charger  170  as explained earlier. 
     Once Ia is received, it is presented to intensity module  190 , which compares Ia to an intensity threshold, Ith, to determine if Ia is lower than it should be, which may warrant a conclusion that the external charger  170  is misaligned with respect to the IPG  110 .  FIGS. 14B and 14C  show the effect of external charger  170 /IPG  110  alignment on received optical intensity, Ia. In  FIG. 14B , the external charger  170  and IPG  110  are well aligned, and the axes through their window assembly  172  and  112  are collinear. Because the window assemblies  172  and  112  are also generally centered with respect to charging coils  76  and  36  respectively, these coils  76  and  36  are thus also collinear, which as noted earlier ( FIG. 6A ) represents good alignment and electrical coupling between the external charger  170  and IPG  110 . When well aligned, the window assembly  172  of the external charger will cover a broader area of the tissue  101  illuminated by the photoemitter  114  in the IPG  110 , and thus the photodetector  176  in the external charger  170  will receive a larger amount of optical radiation (i.e., Ia&gt;Ith), and thus Ia will be relatively high. 
     By contrast, in  FIG. 14C , the window assembly axes and the charging coils are not collinear; the window assembly  172  covers a smaller area of the illuminated tissue  101 ; photodetector  176  receives a smaller amount of optical radiation; and Ia is relatively low (e.g., Ia&lt;Ith), indicating poor alignment and coupling. Should the intensity module  190  detect this condition, it can notify the patient of the misalignment condition using speaker  88  as described earlier so that the patient can readjust the position of the external charger  170 . 
     An IPG can be implanted at different depths (x;  FIG. 8A ) in different patients, and so the received intensity Ia may vary from patient to patient. Thus, the intensity threshold, It, is preferably established for a given patient during a training or learning phase. For example, when first using the external charger  170 , a patient may be instructed to very carefully align the two window assemblies  172  and  112 , to press the external charger  170  firmly against her tissue  100 , and to turn on the external charger  170  to start generating a magnetic charging field  80 . The external charger  170  can then assess the strength of Ia for this known good alignment condition, and then set Ith in the intensity module  190  at an appropriate lower value. 
       FIGS. 15A-15D  illustrate a different embodiment of an external charger  170 ′ that as well as determining alignment optically can also determine a direction in which the external charger  170 ′ is misaligned with respect to the IPG  110 , and indicate this to the patient to assist in adjustment. In  FIG. 15A , the bottom side of the external charger  170 ′ includes a larger window assembly  172 , and mounted to the PCB  72  are a number of photodetectors  176   x  arranged radially around a central photoemitter  168 , and also arranged radially with respect to the charging coil  76  ( FIG. 7 ).  FIG. 15B  shows the photodetectors  176   x  at the same locations, but with each having its own window assembly  172   x  in the external charger&#39;s case  77 . A central photodetector could also be present at the central location of the photoemitter  174  to assist in communications or alignment as described earlier, but this is not shown. 
     In either example, the photodetectors  176   x  will receive different intensities Ia x  from the central photoemitter  114  in the IPG  110  depending on external charger  170 ′/IPG  110  alignment. An underlying IPG  110  is shown in dotted lines in  FIG. 15A  to illustrate this. Notice that photodetector  176   1  is relatively aligned with the window assembly  112  and photoemitter  114  in the IPG  110 , and thus will receive a relatively strong intensity Ia 1 . Photodetectors  176   2  and  176   3  are more distant, and will receive smaller intensities Ia 2  and Ia 3 . Photodetector  176   4  is far away from the IPG window assembly  112 , and thus will receive a small intensity Ia 4 , which may be zero. 
     As shown in  FIG. 15C , optical receivers  186   x  corresponding to the photodetectors  176   x  in the external charger  170 ′ report these intensity values Ia x  to the control circuitry  92 . Although separate optical receivers  186   x  are shown, a single optical receiver  186  could also be used to sample the received intensities Ia x  from the various photodetectors  176   x  at different points in time. 
     The control circuitry  92  is programmed with a position determination module  194  that assesses the reported intensities Ia x  to triangulate the position of the external charger  170 ′ with respect to the IPG  110 , and to indicate misalignment to the patient. Such misalignment indication can include use of the speaker  88  as discussed earlier, but in addition direction indicators  196   x  are used to inform the patient in which direction the external charger  170 ′ should be moved to improve alignment and electrical coupling with the IPG  110  during the charging session. These direction indicators  196   x  in one example can comprise LEDs on the top face of the external charger  170 ′ that may (but need not) generally coincide with the location of the photodetectors  176   x  as shown in  FIG. 15D . 
     Continuing the example misalignment condition of  FIG. 15A  in  FIG. 15D , upon determining that the photoemitter  114  in IPG  110  is near photodetector  176   1 , somewhat near photodetectors  176   2  and  176   3 , but far from photodetector  176   4 , the position determination module  194  may light LED  196   1  with a strong brightness, LED  196   2  and  196   3  with a moderate brightness, and  196   4  with a low brightness, to indicate the direction that the patient should move the external charger  170 ′, i.e., to the north-east as shown by the arrow in  FIG. 15D . In other words, the position determination module  194  can control the direction indicators  196   x  in accordance with the receive intensities values Ia x , and may do so in manners other than by controlling their brightness, such as by pulse rate or color. If alignment pulses  104  are provided frequently enough to the external charger  170 ′, control of the LEDs  196  by the position determination module  194  can occur in essentially real time to allow a patient to visually assess their progress toward achieving better alignment as she moves the external charger  170 ′ by viewing the LEDs. 
     Once suitably aligned, none of the LEDs  196   x  may be lit, at which time indications from the speaker  88  might also cease. Enablement or disablement of the speaker  88  though need not coincide with enablement or disablement of the LEDs  196   x , and instead use of the speaker may be limited to gross misalignment conditions, for example, when no photodetector  176  in the external charger  170 ′ is receiving a suitable level of intensity Ia x . Speaker  88  may also be dispensed with. 
     Because the disclosed optical techniques for determining and indicating external charger/IPG alignment do not depend on electrical measurements taken during production of a magnetic field  80 , the techniques may be used prior to a charging sessions, i.e., prior to use of the external charger  170  to produce a magnetic charging field  80 . This is beneficial, as it allows a patient to set the positioning of the external charger  170  before the charging session begins, which hopefully the patient would not need to revisit later during the charging session. However, should alignment change during the charging session, the disclosed techniques can still notify the user and to suggest corrections, as explained above. 
     It should also be noted that the optical alignment techniques described above ( FIG. 14A-15D ) with respect to the external charger  170  ( FIG. 14A-15D ) can also be used with the external controllers  150  and  150 ′. This may be beneficial to ensure good optical coupling, and hence reduced optical attenuation, between the two before or during the communicating data. The disclosed optical alignment techniques may also be used in conjunction with the prior art alignment techniques described earlier. For example, the prior art techniques can be used to enable coarse alignment between the external device and the IPG  110 , with the disclosed optical techniques then used for fine alignment assessment and adjustment. 
       FIG. 16  shows another embodiment of an external charger  170 ″ system that can optically communicate with IPG  110 . In this example, optical components have been moved out of the external charger  170  ( FIG. 12 ), and into a charging head  250 , including the photoemitter  174 , the photodetector  176 , and if necessary, optical transmitter and receiver circuitry  184  and  186 . The charging head also includes the charging coil  76 . These components may be integrated on a PCB  256 , and contained within a housing  258 , which may be significantly smaller and less complicated that the housing  77  used by the external charger  170 . As shown, the charging head  250  includes a window assembly  172 , similar to that described earlier for external charger  170  but could include the other optical device placement options discussed in  FIG. 9 . The window assembly  172  and optical devices  176  and  174  are preferably centered within the charging coil  76  in the head, which as noted earlier assists with determining alignment between the charging head for the same reasons discussed earlier. Charging head  250  could also include different photodetectors  176   x  to allow for misalignment position determination, and could include direction indicators  196   x  (see  FIGS. 15A-15D ), but this is not shown for convenience. 
     The charging head  250  communicates with a mobile controller such as the external controller  50  ( FIG. 2A ) or mobile device  200  as discussed earlier (see  FIG. 11 ), or an external charger  70  (not shown). As depicted, the charging head  250  includes a cable  252  and a connector  254  that can couple to appropriate ports  58  or  208  on the relevant external device, which can provide a graphical user interface providing functionality similar to the user interface  82  of the external charger  70  ( FIG. 4A ), as described previously. Power for the charging head  250 , as well as to generate the magnetic charging field  80  from the charging coil  76 , can come from the external device via cable  252 . 
     The external charger  170 ″ can otherwise operate similarly with the external chargers  170  and  170 ′ described earlier to optically communicate relevant charging information, and to allow the external device to determine charging head  250 /IPG  110  alignment using such optical communications. The graphical user interfaces of the external device are further useful in that they can provide indications of misalignment (either using their speakers, or by displaying information on their graphical user interfaces). If the charging head  250  has multiple photodetectors  176   x  allowing for misalignment direction assessment (see  FIGS. 15A-15D ), the direction indications concerning how to move the charging head  250  for better alignment can be displayed on the graphical user interfaces of these devices, essentially taking the place of LEDs  196   x  in  FIG. 15D . 
     The external charger  170 ″ of  FIG. 11 , while having different pieces, may be more convenient for a patient, because, like optical communication head  210  ( FIG. 11 ), it allows the charging head  250  to be placed proximate to the IPG  110  to allow optical communications and wireless charging to take place, while the external device can remain relatively distant from the IPG  110  by virtue of the length of cable  252 , making it easier to access. 
     The design of IPG  110  and external chargers  170 ,  170 ′ and  170 ″ provided reliable means for optically communicating through the patient&#39;s tissue  100 , and the same benefits discussed earlier for external controller/IPG communications apply here as well. 
     Optical communications between the IPG  110  and the external controller may render certain aspects of the prior art IPG  10  and external controller  50  unnecessary. For example, as FSK communications are not used, FSK modulation and demodulation circuitry ( FIGS. 3 ,  41 ,  43 ,  61  and  62 ) as well as telemetry coils  34  and  54  are not necessary, and these coils are thus shown in dotted lines in IPG  110  and the various external controllers in recognition of this fact. This is significant, as coils  34  and  54  take up room in their respective, which may now be made smaller when optical communications are employed. Eliminating the telemetry coil  34  from the IPG  110  is particularly beneficial, as space is at a premium in an implantable devices, and because it is always desirable to make such implantable devices smaller to minimize inconvenience to the patient. That being said, FSK communications still could be used between IPG  110  and an external controller in addition to the optical communications disclosed herein. As an additional benefit, optical communications will not be subject to or provide electromagnetic interference. This can simplify circuit design in the disclosed IPG and external controllers, as coupling between such circuitry and the coils  34  or  54  is a non-issue. 
     Optical communications between the IPG  110  and the external charger may also render certain aspects of the prior art IPG  10  and external charger  70  unnecessary. For example, because the external charger and IPG  110  can communicate optically via link  101   b , LSK communications from the IPG  110  to the external charger may not be necessary, and hence relevant LSK circuitry in both devices are shown in dotted lines ( FIG. 14A ). That being said, LSK communications still could be used between IPG  110  and an external charger in addition to the optical communications disclosed herein. 
     Optical communications also allows for the integration of the external controller and the external charger in a single device. For example,  FIG. 17  shows an integrated external controller/charger  300  in the form of a modified external controller  150  (see  FIG. 9 ). In external device  300 , coil  54  (previously used for FSK telemetry) has been replaced by a charging coil  302  used to provide the magnetic charging field  80  to charge the IPG&#39;s battery. To assist with alignment determinations, the window assembly  304  and underlying photoemitter  164  and photodetector  166  have been moved to be generally centered with respect to the coil  302 . In this example, the external controller/charger  300  is integrated in a unitary housing, i.e., case  59 . 
     The external device  300  can operate in an IPG communications mode having functionality similar to the external charger to set or adjust the therapy settings the IPG  110  provides to the patient, and to receive relevant data from the IPG  110 . The external device  300  can also operate in a charging mode to produce the magnetic charging field. These modes can operate and use optical communications to beneficial ends as discussed previously. Note that redundancies in the external controller and the external charger can be eliminated in the integrated external device  300 . For example, the external device  300  may have only one optical transmitter (e.g.,  144 ;  FIG. 10 ) coupled to photoemitter  164 , and one optical receiver (e.g.,  146 ,  FIG. 10 ) coupled to one photodetector  166 , which would handle optical communications for all modes of operation. External device  300  may also have additional photodetectors to allow for alignment direction determinations ( FIGS. 15A-15D ), and may position the optical devices differently with respect to the case  59  ( FIG. 9 ). 
       FIG. 18  shows a modified external device  300 ′ having integrated communication and charging capability, which uses a combined communication/charging head  350 . Like the heads  210  and  250  described earlier ( FIGS. 11 and 16 ), the head  350  is coupled by a cable  352  to a mobile controller. The head  350  is essentially similar to the charging head  250  described earlier and can be modified as explained earlier (additional photodetectors, different placement of the optical devices, etc.), although optical communications related to external controller functionality are also passed between the head  350  and the IPG  110  in addition to optical communications used for charging. Otherwise, the external device  300 ′ can operate as external device  300 , providing both communication and charging functionality, with the additional convenience of separating the graphical user interface from aspects of the system that need to be proximate to the IPG  110 . 
     Other modifications to the disclosed devices and techniques are possible. For example, the optical radiation used in optical communications need not have a fixed wavelength, but can comprise radiation with a wider frequency spectrum. Optical communications used in different directions (e.g., link  101   a ,  101   b ), can occur at different wavelengths, which may facilitate full duplex communications on these links. More than one set of photoemitters and corresponding photodetectors may be used to respectively transmit and receive optical radiation in a given direction along a communication link, which may operate at different wavelengths. The circuitry disclosed herein can also be modified in any number of ways. For example, instead of programming modules in the various control circuitries of the devices, such modules can exists as discrete circuits outside of their control circuitries. 
     While it is preferred to use a single window assembly  112  in the IPG  110 , number of window assemblies  112  can be used as well. For example, although not depicted, the IPG  110  could contain two window assemblies  112  on its top side, one of which contains a photoemitter  114  and the other which contains a photodetector  116 . The external controller or charger could likewise contain two window assemblies, one of which contains a photoemitter and the other which contains a photodetector. This would allow different optical links to be supported in different directions ( 101   a  and  101   b ) through different sets of windows. Providing different numbers of window assemblies  112  on the IPG  110 , or a larger window assembly containing a number of photoemitters or photodetectors, may also be useful in determining and indicating alignment. For example, the alignment procedure described above can essentially be reversed, with the external charger providing pulses from its photoemitter, which intensity is detected at a number of photodetectors in the IPG, with such intensities optically reported to the external charger for alignment interpretation. Different window assemblies  112  operating at different wavelengths that correspond with different window assemblies in the external charger can also be used for alignment. 
     Another window assembly placed elsewhere on the IPG, for example on its bottom side facing inwardly of the patient, may also be useful to allow the IPG  110  to optically communicate with other implanted devices, such as another IPG  110 . 
     Use of the window assembly  112  on the side of the IPG  110 &#39;s case  12  is preferred, but not necessary in all manners in which optical communications are useful. For example, if optical communications are used to provide visual feedback regarding IPG operations, conditions, or codes to a manufacturer, clinician, or patient as described earlier, positioning of the photoemitter  114  that provides such indications is less critical. If used for such purposes, the photoemitter  114  could be placed elsewhere, such as in the header  28  ( FIG. 1A ), and electrically coupled to the circuitry inside the case via additional feedthrough wires between the case  12  and the header  28 . Or the photoemitter  114  could be placed inside the case, with radiation optically ported through the feedthrough, using a lens, a fiber optical cable, a window, or other structure. A special optical feedthrough different from that used for the electrodes wires could also be provided. Suitable visual indications can be provided even if the optical radiation in these scenarios will travel through more complicated or optically bulky structures. 
     Visual indications are especially useful to display operations, conditions, or codes before implantation, such as during manufacturing, or surgery. Particularly useful is optically providing a visual indication of a “go/no-go” signal to the implanting clinician before the IPG is implanted in the patient. In this regard, the IPG  110  can be configured to detect the impedance at each electrode  16  after the electrode lead(s)  18  are connected to the IPG  110 . If any electrode is not within a proper impedance range (e.g., if an electrode is measured to have an open or short circuit), a “no-go” signal could be visually indicated by a photoemitter placed anywhere on the IPG. The implanting clinician can then attempt to re-secure the leads until a “go” signal is visually indicated, at which point the clinician could then safely implant the IPG. Alternatively, the “no-go” signal might indicate a particular electrode that is not being measured with a suitable impedance. 
     Providing optical communication functionality to the external devices (e.g., the external controller and external charger) may also be beneficial as an easier way to communicate with or test such devices, or to allow them to optical communicate with each other or with other external devices. 
     The following claims at times recite “a” structure, but this should not be construed as limiting scope to devices that only contain a singular one of such structures. 
     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.