Abstract:
An embodiment of a light launching portion of a photoplethysmographic device having a laser ( 20 ) light source and a light guide ( 40 ). The coupled end of the light guide ( 40 ) includes an anti-reflection coating ( 30   a ) to prevent or minimize the back reflection of light emitted by the laser ( 20 ). This minimizes the extent to which back reflected light can re-enter the laser and adversely alter the optical output properties of the laser ( 20 ) and additionally minimizes the associated light loss thus helping to maximize the optical coupling efficiency. Other embodiments are described and shown.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with government support under R44HL073518 awarded by the National Institutes of Health. The government has certain rights in the invention. 
     
    
     BACKGROUND-PRIOR ART 
       [0002]      
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 U.S. Patents 
                   
                   
                   
               
               
                   
                 Pat. No. 
                 Kind Code 
                 Issue Date 
                 Patentee 
               
               
                   
                   
               
             
             
               
                   
                 7,313,427 
                 B2 
                 Dec. 25, 2007 
                 Benni 
               
               
                   
                 7,047,054 
                 B2 
                 May 16, 2006 
                 Benni 
               
               
                   
                 6,184,521 
                 B1 
                 Feb. 6, 2001 
                 Coffin 
               
               
                   
                   
               
             
          
         
       
     
       BACKGROUND OF THE INVENTION 
       [0003]    In the science of photoplethysmography, light is used to illuminate or trans-illuminate living tissue for the purpose of providing noninvasive measurements of blood analytes or other hemodynamic parameters or tissue properties. In this monitoring modality light is directed into living tissue and a portion of the light which is not absorbed by the tissues, or scattered in some other direction, is detected a short distance from the point at which the light entered the tissue. The detected light is converted into electronic signals that are indicative of the received light intensity exiting the tissue. These signals one for each emitter, or spectral band of light incident on the living tissue (referred to in this specification as the tissue-under-test), vary with the pulsation of the blood through the tissue-under-test. These time varying signals are referred to as photoplethysmographic signals. The photoplethysmographic signals are used to calculate blood analytes such as arterial blood oxygen saturation and/or hemodynamic variables such as heart rate, cardiac output, or tissue perfusion. Among the blood analytes that may be measured by photoplethysmography are various types of hemoglobin, including the percentages of oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin in the arterial blood. A device which detects and processes photoplethysmographic signals to measure the levels of various blood analytes and/or various hemodynamic parameters is referred to as a photoplethysmographic measurement apparatus, photoplethysmographic device, or photoplethysmographic instrument. 
         [0004]    The first widespread commercially-used photoplethysmographic device in medicine was the pulse oximeter, a photoplethysmographic device designed to measure arterial blood oxygen saturation. To measure oxygen saturation two different bands of light must be used, with each light band possessing a unique spectral content. Each spectral band, or light band, is typically referred to by a center wavelength, the centroid (or first moment of area of the wavelength distribution of the spectral band), or sometimes by a peak wavelength (the wavelength of maximum optical power). In conventional pulse oximetry two different emitters such as light emitting diodes (LEDs) are commonly used to generate the desired spectral bands. Usually one LED has a center, or peak, wavelength near 660 nanometers (nm) and a second LED has a center, or peak, wavelength near 900 nm. More recently photoplethysmographic instruments have been developed in which more than two light bands are utilized to allow the measurement of a larger number of blood analytes, including such blood analytes as oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin. 
         [0005]    Use of a photoplethysmographic instrument requires that fight from each emitter (each light band) is incident on the tissue-under-test. On a person the tissue-under-test usually consists of a finger, earlobe, toe, foot, cheek, forehead, or other site on or, for invasive use, inside the body. The emitter light is delivered via a sensor positioned on the tissue-under-test. The tissue-under-test is preferably well perfused with blood which helps provide a strong photoplethysmographic (or pulsatile) optical signal to be received at the detector that is typically also integral to the sensor. The detector is located a short distance from where the light enters the tissue-under-test, which allows for attenuation of the light signal by the pulsating blood flow within the tissue-under-test. 
         [0006]    As the science of photoplethysmographic monitoring has progressed, an increasing number of light bands have been required to measure an increasing number of blood analytes. Furthermore, to improve the accuracy of measurement and the ability to discriminate between an ever-increasing number of blood analytes, it is most desirable to use spectrally-stable narrowband light sources. One type of spectrally-stable narrowband light source is a laser. 
         [0007]    The use of one or more lasers in photoplethysmography creates some unique challenges. These include the requirement that the laser be located at a distance from the sensor and that certain instabilities are caused by light emitted by the laser being reflected back toward the laser. 
         [0008]    When using one or more lasers as light sources, or emitters, in a photoplethysmographic device, the lasers often cannot be placed in the sensor that is positioned in close proximity to, or directly on, the tissue-under-test, as has been typical with LED based photoplethysmographic sensors. This might be due to the physical size of the laser device being too large for placement in a small sensor designed for application to commonly-used sensing sites such as a finger. It also might be due to the need to position the laser in close proximity to its driver electronics, to one or more heat sinks, or to other electro-mechanical devices that, as a whole, create a package that is too large or cumbersome to place in the sensor or to conveniently position in immediate proximity to the tissue-under-test. 
         [0009]    A typical photoplethysmographic instrument consists of a monitor, which provides the user interface for the instrument; a cable, which connects the monitor to a sensor; and the sensor, which is placed on the tissue-under-test. Many different but substantially equivalent configurations of the instrument are also possible. The lasers, given that they are not housed in the sensor, might be housed in the monitor or at some intermittent point along the cable connecting the monitor to the sensor. Regardless of exactly where the lasers are housed, as long as they are not at the sensor, the light emitted by the laser (or lasers) must be transmitted from the laser housing to the sensor, or at least to the sensing location on the tissue-under-test. 
         [0010]    In such cases; this light transmission from the laser to the sensor is typically accomplished by employing a light guide. The light guide may be any one of a number of elements, or a chain of elements, including optical elements such as glass or plastic optical fibers, liquid filled light guides, fiber optic bundles, or other light pipes. 
         [0011]    Light guides have been used in photoplethysmographic devices since the late 1970s when the first commercially available pulse oximeter went on the market. One early photoplethysmographic instrument used a pair of light guides in the form of two fiber bundles to both deliver the light, from a tungsten lamp source, to the tissue-under-test and to receive the light from the tissue-under-test and return the photoplethysmographic signals to the monitor for analysis. More recently, light guides have been used in pulse oximeters specifically designed for use on patients undergoing MRI (Magnetic Resonance Imaging) examination. None of these light guide-based devices, however, addressed the problems associated with the instabilities caused by a portion of the light incident on the light guide reflecting back toward the emitter, and in specific, a laser-based emitter. 
         [0012]    Coupling, or launching, light emitted by a laser into a light guide for use in a photoplethysmographic measurement device can cause a portion of the light emitted by the laser to reflect back into the laser cavity. This back reflection occurs due to the discontinuity in index of refraction that the light encounters after exiting the laser into the air and then entering the light guide. These effects are well-known in the field of optics and described by the equations for Fresnel reflections. As an example, if light is passing from air into a light guide, such as a glass optical fiber, then the light is passing from an index of refraction of approximately 1.0 to an index of refraction that might be near 1.5. If the lights is entering the light guide at an angle that is perpendicular to the surface of this particular light guide, the back reflection of the light from the surface of the light guide would be approximately 4% of the incident light intensity. 
         [0013]    Back reflections of this magnitude can cause several adverse effects, any one of which can be detrimental to the accuracy of a photoplethysmographic measurement technology that is using laser light sources. These detrimental effects occur because the light reflected off the surface of the light guide can re-enter the laser cavity and interfere with the performance of the laser. 
         [0014]    Depending on the exact type of laser used, light emitted by the laser and reflected off the front surface of the light guide back toward the laser cavity can cause problems such as reducing the mode hop spacing as a function of temperature, inducing additional mode hops because of secondary and tertiary resonant cavities formed between the laser facets and the light guide end face, and increasing the magnitude of the wavelength shift associated with any individual mode hop. (In this specification the term mode refers to resonant modes, also called longitudinal modes, of the laser cavity; and mode hopping refers to sudden jumps in the optical frequency, or spectral content, of the light output by the laser. Changes in output intensity occur concurrently with, and because of, the mode hops. See the following article:  Romanian Reports in Physics , Vol. 59, No. 1, P. 87-92, 2007, included herein by reference, for additional understanding of the modal behavior in diode laser use.) 
         [0015]    For photoplethysmographic measurement of blood analytes to be accurate, the light incident on the tissue-under-test must be stable in amplitude and in spectral content (or at least very controlled in amplitude and spectral content and devoid of unintended fluctuations in intensity or wavelength to the greatest extent possible). In laser-based photoplethysmographic instrument systems, the back reflection of light towards a source laser when launching its light into a light guide causes fluctuations in intensity and spectral content (or wavelength) that are large enough to dramatically reduce the accuracy of the photoplethysmographic measurements. 
         [0016]    It should be noted that these fluctuations in intensity and spectral content can be small enough that they do not adversely affect other non-photoplethysmographic uses of lasers coupled to light guides. But in the case of photoplethysmography, changes in output intensity as small as 0.5% can obscure the signals that are required for accurate blood analyte measurement. Similarly, wavelength shifts of only one nanometer can induce errors in the measurement of certain blood analytes which are large enough to make these blood analyte measurements clinically useless. 
         [0017]    U.S. Pat. Nos. 7,313,427 and 7,047,054 both mention the use of anti-reflective coatings on ball lenses to improve coupling efficiency when combining the output of several fibers into a single fiber in a near infrared spectrophotometric monitor. This use of anti-reflective coatings does nothing to prevent light that is being emitted by a laser and launched into a light guide from reflecting backward toward the laser and inducing the exact problems described earlier. Additionally, U.S. Pat. Nos. 7,313,427 and 7,047,054 were designed for use in near-infrared spectroscopic monitoring, which is not a photoplethysmographic technique. The only mention that U.S. Pat. Nos. 7,313,427 and 7,047,054 make to a photoplethysmographic technique, is a brief reference regarding pulse oximetry stating that “Since venous blood is not pulsatile, pulse oximetry cannot provide any information about venous blood.” and how “Conversely, NIRS [Near-Infrared Spectroscopy] does not require pulsatile blood volume to calculate parameters of clinical value.” 
         [0018]    U.S. Pat. No. 6,184,521 presents the use of an anti-reflective coating on a photodiode in a pulse oximeter sensor. This is an application of anti-reflective coatings on the receiving side of a photoplethysmographic sensor where the light exits the tissue, not the light launching side. This application of anti-reflective coatings once again does nothing to prevent light emitted by a laser, and being launched into a light guide, from reflecting back toward the laser and causing the problems described earlier. 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    In accordance with one embodiment a light launching apparatus for a photoplethysmographic device comprises a laser-based emitter coupled to a light guide wherein the coupled end of the light guide is coated to minimize back reflection. Accordingly, several advantages of one or more aspects are as follows: that the light exiting the laser does not sufficiently reflect off the light guide and become incident on the laser and thus minimizes the likelihood of adversely affecting the wavelength or intensity stability of the light emitted by the laser; that a percentage of the light emitted by the laser and incident on the light guide is lost to back reflection, thus maximizing the light available for sensing of the desired blood analytes, hemodynamic parameters, or tissue properties. 
     
    
     
       DRAWINGS 
         [0020]      FIG. 1 . Light launching apparatus. 
           [0021]      FIG. 2 . Light launching apparatus with discrete launch-optics. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    One embodiment of a light launching portion of a photoplethysmographic device is shown in  FIG. 1 . An emitter housing  10  contains a light source, also called an emitter. The emitter is a laser  20  but it can be any one of a number different types of lasers. The emitter shown in this figure, by way of example only, is a diode laser. A few of the possible laser types include gas lasers, diode lasers, dye lasers, or vertical cavity surface emitting lasers (VCSEL), to name a few. 
         [0023]    The light emitted by the laser  20  is directly coupled, or launched, into a light guide  40 . That is, the light emitted by the laser  20  is directly incident on and injected-into the light guide  40 . The light guide  40  can accept light that is incident on its entrance face, however there may be limits to the incidence angle on the entrance face at which light can propagate into the light guide. The limits to this angle of incidence may be quantified in part by the numerical aperture of the light guide. The light guide  40  has an end with an applied coating  30   a  that is positioned between the laser  20  and the light guide  40 . The coating  30   a  is specifically selected and/or designed to minimize back reflection of light emitted by the laser  20 . 
         [0024]    In conventional photoplethysmographic devices the light sources, also called emitters, generate the light that is used for sensing the blood analytes or the physiological parameters to be measured. The analytes or physiological parameters to be measured may include arterial blood oxygen saturation or level (also referred to as O 2 Hb, [O 2 Hb], SaO 2 , or S p O 2 ), carboxyhemoglobin level (also referred to as COHb, [COHb], or S p CO), methemoglobin level (also referred to as metHb, [metHb], or S p met), pulse rate (also called heart-rate; HR, or PR), and perfusion index (also called PI), along with others. In pulse oximetry, a common photoplethysmographic device, the emitters used for these measurements typically consist of light emitting diodes (LEDs), although several other light sources have been used including, in the earliest pulse oximeters, tungsten lamps. 
         [0025]    In a conventional pulse oximeter the LEDs are housed in the sensor. Light emitted by the LEDs may pass through a diffuser, or other intervening optics, and then the light passes through an output window, or aperture, and is incident directly on the tissue-under-test. A small portion of the light then passes through the tissue-under-test and is received by a photodetector that is typically positioned a short distance from where the light originally entered the tissue-under-test. The photodetector signal is measured by the photoplethysmographic instrument and processed into the desired blood analyte measurements. The conventional pulse oximeter is only capable of measuring oxygen saturation (SpO 2 ) and perhaps heart rate (HR) and perfusion index (PI). 
         [0026]    With the increasing desire to measure more blood analytes and physiological parameters, and with ever-increasing accuracy, the emitter types now being used include lasers. Lasers are a type of emitter that can generate light with a much narrower spectral bandwidth than conventional LEDs. The use of lasers in photoplethysmographic devices provides the opportunity for increased measurement accuracy and precision as well as the opportunity to measure additional parameters and/or blood analytes that were not attainable with more broadband light sources. 
         [0027]    The difficulty in using a laser light source is that many laser types are sensitive to back reflection of the laser light into the laser cavity. As discussed earlier, back reflection, or reflection of some portion of the light emitted by the laser back into the laser cavity, can increase the fluctuations in intensity and fluctuations in spectral content of the output light. This diminishes the inherent value of using a laser light source for photoplethysmographic measurements. To prevent this problem this embodiment includes a light guide  40  to which a specially-designed optical coating  30   a  has been applied. The coating  30   a  is referred to as an anti-reflection or an anti-reflective coating. Anti-reflection coatings can reduce the reflection of incident light off of an optical surface to less than 0.1% of the incident intensity, compared to reflections of approximately 4% that may result from the same optical surface that is uncoated. The actual magnitude of the reduction of the back reflection of the light incident on the coated surface depends on the design of the coating  30   a  and on how well its anti-reflection properties are optimized for the spectral band of the specific emitter, in this case some type of laser  20 . 
         [0028]    With the proper anti-reflection coating  30   a  the back reflection of light into laser  20  is reduced to a low enough level that the laser operation is not adversely affected. For use in photoplethysmography this is more specifically a low enough back reflection so that the laser&#39;s light output is sufficiently stable in spectral content, center wavelength, and output intensity to allow accurate measurement of the desired blood analytes and/or physiological parameters. Unlike a laser coupled into an uncoated light guide, a laser coupled to an appropriately anti-reflection coated light guide behaves similarly to a laser emitting into free space, with regards to power intensity and spectral content. The anti-reflection coating  30   a  eliminates adverse back-reflection effects from being induced in the laser  20  output that could otherwise make it nearly impossible to use laser  20  for practical photoplethysmographic measurements. 
         [0029]    Note that to couple light efficiently and directly into a light guide  40  it is typical to position the light guide  40  very close to the laser  20 . The separation distance may be considerably less than 0.5 millimeters. The need for the small separation distance may be due, in part, to the diverging emission pattern of the light as it exits laser  20 . If the light guide  40  is placed at too great a distance, only a small portion of the light exiting the laser  20  may be incident on an entrance face of the light guide  40 , and the light level that reaches the light guide and ultimately the tissue-under-test may be insufficient for good instrument performance. Thus to maximize coupling efficiency (the percentage of the light emitted by laser  20  that actually enters the light guide  40 ), the distance between the laser  20  and the light guide  40  is typically minimized so that the maximum amount of the light emitted by the laser is incident on the entrance face of the light guide  40 . The negative implication of the close placement of the light guide  40  to the laser  20  is that the closer these two elements are placed to each other, the less the divergence of the reflected light and therefore the greater the intensity of light that will be back reflected toward the laser cavity, which further increases the need for the anti-reflection coating  30   a.    
         [0030]    It is not uncommon to use launch-optics when coupling laser to a light guide. One possible embodiment is shown in  FIG. 2 . In this embodiment a laser  20 , housed in a laser housing  10 , emits light to be coupled, or launched, into a light guide  40 . In this configuration, however, there are a set of one or more elements that make up the launch-optics  50  and  60 . These launch-optics typically consist of lenses or other light shaping optics which typically perform the function of conditioning the light to increase coupling efficiency into the light guide  40 . In the particular example, diagrammed in  FIG. 2 , the laser  20  is followed by a cylindrical lens  50  which is then followed by a ball lens  60  and finally by the light guide  40 . 
         [0031]    The exact number, type, and configuration, of the optical elements that make up the launch-optics  50  and  60  is dependant on the spatial output of the laser  20  and the geometry and numerical aperture of the light guide  40 . If the laser  20  is a device with an asymmetrical light output pattern, which is typical with a diode laser then a cylindrical lens such as  50  might be used as an element in the launch-optics  50  and  60  to first reshape the light into a more circular emission pattern. This emission pattern is then incident on the next element  60 , which in this example is a ball lens. This element  60  then focuses or conditions the light to best match the numerical aperture of the light guide  40 . If, for example, the light guide is a step index fiber with a fiber core diameter of 50 micrometers (um) and a numerical aperture of 0.2, then the light incident on the entrance face of light guide  40  would ideally have an entrance angle of not greater than 11.5 degrees, as measured from a line perpendicular to the entrance face. The light would focus down to a spot size of 50 um or smaller on the entrance face and be centered on the fiber core of light guide  40 . 
         [0032]      FIG. 2  shows only one example of many possible configurations of launch-optics that might be used in launching light from the laser  20  into the light guide  40 . Regardless of the exact configuration of the launch-optics or the number of elements used, these elements would be coated with an anti-reflection coating to prevent, or at least minimize back reflection. In  FIG. 2  launch-optics  50  and  60  are shown to have anti-reflection coatings  30   b  and  30   c , respectively, each specifically designed or selected to minimize back reflection of the incident light emitted by laser  20 . In  FIG. 2  the light guide  40  also has an anti-reflection end coating  30   a  because any optical surface in the optical path from the laser  20  up to and including the light guide  40  could cause significant and destructive back reflection. 
         [0033]    Conventional mounting hardware and/or adhesives (not shown) are used to hold the various optical components in place and in proper alignment within the photoplethysmographic device. The light guide  40  then carries the light, emitted by the laser  20  and launched into the light guide  40 , to the sensor (not shown) located at the tissue-under-test. 
         [0034]    Photoplethysmographic devices may require several emitters to allow accurate measurement of numerous blood analytes and/or physiological parameters. The same light launching portion, of a photoplethysmographic device described above may be utilized multiple times within the device. In addition to preventing or at least minimizing back reflection of light into the laser, the anti-reflection coating incorporated as described herein has the additional advantage of reducing die light losses in the optical system because light that is reflected off the entrance face of the light guide  40  is light that is not launched into the light guide  40 . 
         [0035]    The previous discussion of the embodiments has been presented for the purposes of illustration and description. The description is not intended to limit the invention to the form disclosed herein. Variations and modifications commensurate with the above are considered to be within the scope of the present invention. The embodiments described herein are further intended to explain the best modes presently known of practicing the invention and to enable others skilled in the art to utilize the invention as such, or in other embodiments, and with the particular modifications required by their particular application or uses of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.