Patent Publication Number: US-2020297222-A1

Title: Optical sensing apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an optical sensing apparatus. 
     2. Description of the Related Art 
     In recent years, an optical sensing apparatus has been used to obtain, in a non-contact manner, useful information on the interior of a physical object such as a living body or food. The optical sensing apparatus includes a laser device and a photodetector. The laser device irradiates the physical object with laser light. The photodetector detects reflected scattered light coming out from a surface of the physical object after being multiply scattered in the interior of the physical object. 
     In a case where the physical object is a living body, light emitted from the laser device penetrates inside of the living body through the skin. After that, the reflected scattered light coming out from the skin contains biometric information such as a condition of the blood by having passed through a blood vessel or the like. By detecting the reflected scattered light, information such as the pulse, blood pressure, blood flow, and oxygen saturation can be obtained. These pieces of information can be used in a physical examination. 
     For example, laser light with a near-infrared wavelength of 700 to 950 nm has the property of passing through body tissue such as muscles, fat, and bones with a comparatively high transmissivity. Meanwhile, laser light in a near-infrared wavelength region also have the property of being easily absorbed into oxyhemoglobin (HbO 2 ) and deoxyhemoglobin (Hb) in the blood. Accordingly, biometric information measurements commonly involve the use of laser devices that irradiate physical objects with laser light in a near-infrared wavelength region. 
     Through the use of such a laser device, information about intracerebral blood flow can be acquired by irradiating the forehead with laser light and detecting reflected scattered light. For example, the amount of change in intracerebral blood flow and the respective amounts of change in concentration of oxyhemoglobin and deoxyhemoglobin in the blood can be measured. A state of brain activity can be estimated on the basis of the amount of change in blood flow, an oxygen state of hemoglobin, or the like. 
     Further, information such as the freshness and sugar content of food can be obtained in a non-destructive manner by irradiating the food with laser light and detecting reflected scattered light from inside of the food. 
     Japanese Unexamined Patent Application Publication No. 2003-337102 discloses an optical cerebral function measuring apparatus that measures a cerebral function in a non-contact manner. Japanese Unexamined Patent Application Publication No. 2007-260123 discloses an imaging system that performs a time-resolved measurement with improvement in S/N ratio of signal light returning from a deep place in body tissue. 
     Further, WO 2003-077389 and Japanese Unexamined Patent Application Publication No. 2012-169375 disclose a laser device that enlarges an apparent light source size by irradiating a scatterer with laser light and using scattered light thus spread. 
     SUMMARY 
     In one general aspect, the techniques disclosed here feature an optical sensing apparatus including a laser device, a photodetector, and a control circuit. The laser device includes a light source that emits laser light, a diffusing member having a diffusing surface that crosses an optical path of the laser light, the diffusing member refracting or diffracting the laser light to make the laser light lower in intensity in a first portion including a center of a cross-section of the laser light that crosses the optical path, to make the laser light higher in intensity in a second portion of the laser light that surrounds the first portion in the cross-section, and to enlarge a beam diameter of the laser light, and a screen that crosses an optical path of the laser light having passed through the diffusing member. The laser device irradiates a physical object with the laser light having passed through the screen or the laser light reflected by the screen. The photodetector detects at least a portion of the laser light returning from the physical object and outputs an electric signal. The control circuit controls the laser device and the photodetector. The control circuit causes the laser device to irradiate the physical object with at least one optical pulse of the laser light and causes the photodetector to perform a time-resolved measurement of at least one reflected optical pulse of the laser light returning from the physical object. 
     Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view for explaining a configuration of a laser device and how a physical object is irradiated with light emitted from the laser device; 
         FIG. 2A  is a plan view schematically showing a structure of a diffusing member of the laser device; 
         FIG. 2B  is a cross-sectional view schematically showing the structure of the diffusing member of the laser device; 
         FIG. 3A  is a diagram schematically showing a shape of light on a diffusing surface of the laser device; 
         FIG. 3B  is a diagram schematically showing a light intensity distribution of the light on the diffusing surface of the laser device; 
         FIG. 3C  is a diagram schematically showing a shape of light on a screen of the laser device; 
         FIG. 3D  is a diagram schematically showing a light intensity distribution of the light on the screen of the laser device; 
         FIG. 3E  is a diagram schematically showing a shape of light on a surface of the physical object; 
         FIG. 3F  is a diagram schematically showing a light intensity distribution of the light on the surface of the physical object; 
         FIG. 3G  is an example of a diagram showing a light intensity distribution on the diffusing surface; 
         FIG. 3H  is an example of a diagram showing a light intensity distribution on the screen; 
         FIG. 4  is a schematic view for explaining a configuration of an optical sensing apparatus and how a biometric measurement is carried out; 
         FIG. 5  is a diagram schematically showing an internal configuration of a control circuit and a photodetector of the optical sensing apparatus and the flow of signals; 
         FIG. 6A  is a diagram showing an example of a time distribution of a single optical pulse serving as emitted light; 
         FIG. 6B  is a diagram showing total optical power (solid line) detected in a stationary state, an amount of change (dotted line) in optical power corresponding to an amount of change in cerebral blood flow, and a time distribution of degrees of modulation (dot-and-dash line); 
         FIG. 7  is a diagram schematically showing a time distribution of optical pulses emitted from the laser device, a time distribution of optical power detected by the photodetector of the optical sensing apparatus, and timings and charge storage of an electronic shutter; 
         FIG. 8A  is a front view showing changes in blood flow that are present inside the physical object; 
         FIG. 8B  is a side cross-sectional view showing changes in blood flow that are present in the physical object; 
         FIG. 9A  is a diagram schematically showing changes in blood flow in the interior of the physical object; 
         FIG. 9B  is a diagram schematically showing changes in blood flow in the interior of the physical object as image-corrected by image computations; 
         FIG. 10  is a schematic view for explaining a configuration of a laser device and how a physical object is irradiated with light emitted from the laser device; 
         FIG. 11  is a schematic view for explaining a configuration of a laser device and how a physical object is irradiated with light emitted from the laser device; 
         FIG. 12  is a schematic view for explaining a configuration of a laser device and how a physical object is irradiated with light emitted from the laser device; 
         FIG. 13  is a schematic view for explaining a configuration of a laser device and how a physical object is irradiated with light emitted from the laser device; 
         FIG. 14  is a diagram schematically showing an internal configuration of a control circuit and a photodetector of an optical sensing apparatus and the flow of signals; 
         FIG. 15  is a diagram schematically showing a time distribution of optical pulses emitted from the laser device, a time distribution of optical power detected by the photodetector of the optical sensing apparatus, and timings and charge storage of an electronic shutter; 
         FIG. 16  is a schematic view for explaining a configuration of a conventional laser device and how a physical object is irradiated with light emitted from the laser device; 
         FIG. 17A  is a diagram schematically showing a time distribution of optical power of an optical pulse emitted from a light source of the conventional laser device; and 
         FIG. 17B  is a diagram schematically showing a time distribution of optical power of light with which a physical object was irradiated by the conventional laser device. 
     
    
    
     DETAILED DESCRIPTION 
     First, prior to a description of embodiments of the present disclosure, underlying knowledge forming the basis of an optical sensing apparatus according to the present disclosure is described. 
     Japanese Unexamined Patent Application Publication No. 2003-337102 discloses an optical cerebral function measuring apparatus that measures a cerebral function in a non-contact manner by irradiating the forehead of a subject with laser light. In Japanese Unexamined Patent Application Publication No. 2003-337102, in irradiating the forehead of a subject with the laser light, the subject wears a light-shielding member such as a sleeping mask for protection of the eyes. 
     Japanese Unexamined Patent Application Publication No. 2007-260123 discloses an imaging system that performs a time-resolved measurement with improvement in S/N ratio of signal light returning from a deep place in body tissue. In Japanese Unexamined Patent Application Publication No. 2007-260123, optical pulses are used as illuminating light to delay an imaging timing, whereby intense noise light that returns temporally early is not imaged. This brings about improvement in S/N ratio of the signal light. In Japanese Unexamined Patent Application Publication No. 2007-260123, an endoscopic apparatus is used to observe blood flow information on a blood vessel buried in body tissue covered with visceral fat. 
     WO 2003-077389 discloses a laser device that enlarges an apparent light source size by using scattered light coming out spread by multiple reflection that occurs inside a scatterer covering an exit end of a semiconductor laser. 
     Japanese Unexamined Patent Application Publication No. 2012-169375 discloses an automotive headlight unit. In Japanese Unexamined Patent Application Publication No. 2012-169375, a scattering part containing TiO 2  microparticles and a phosphor is irradiated with laser light. Wavelength-converted fluorescence is produced by the phosphor being excited by scattered light spread by multiple reflection within the scattering part. The fluorescence and the scattered light coming out spread turns into parallel light through a reflecting mirror. Light obtained by the parallel light passing through a diffusing member is emitted from the automotive headlight unit. 
     In a case where the eye of a subject are fitted with a light-shielding member as in the case of Japanese Unexamined Patent Application Publication No. 2003-337102, a task that is executed in performing a brain measurement is limited to the audio. This results in a low degree of freedom in measurement. In a case where a measurement is carried out without a light-shielding member fitted on the eyes, a laser product that meets the safety standards of the eyes is used. The laser device is limited in maximum permissible exposure (MPE) and accessible emission limit (AEL) for Class 1 provided for by the safety standards of laser products. Therefore, the power of the laser light is set low to meet MPE and AEL for Class 1. Parallel light such as that of Japanese Unexamined Patent Application Publication No. 2003-337102 easily affects the eyes in particular. In a case where the laser light is a continuum of emissions with a wavelength of, for example, 850 nm, the maximum value of AEL is 0.78 mW, which is very small. In a case where such laser light is used, the S/N ratio of a brain measurement becomes very poor. 
     In WO 2003-077389 and Japanese Unexamined Patent Application Publication No. 2012-169375, an apparent light source size is enlarged by irradiating a scatterer with laser light and using scattered light coming out spread and fluorescence. The term “apparent light source size” here means the size of a currently light-emitting region as viewed by a subject. The scatterer, which is a radiation source of diverging light coming out spread, can be considered as an extended source. In general, diverging light provides more enhanced safety than parallel light. AEL and MPE for Class 1 are greater in the case of diverging light than in the case of parallel light. In the case of diverging light from an extended source, AEL and MPE for Class 1 are fixed at a minimum distance, included in an eye-focusing range, at which the eyes are brought to a focus under the most dangerous conditions. 
     For example, in the case of an apparent light source size of larger than 10 mm, the eyes are brought to a focus at a distance of 100 mm or longer from a light source. Accordingly, with the eyes having an aperture diameter of 7 mm in the case of a distance of 100 mm, AEL and MPE are determined by optical power within the range of an aperture diameter of 7 mm. In the case of an apparent light source size of smaller than 10 mm, the smaller a light source is in size, the shorter the distance from light source at which the eyes are brought to a focus becomes. For example, in the case of a light source size of 1.5 mm, the minimum distance at which the eyes are brought to a focus is 39. 3 mm. In the case of a light source size of 3 mm, the minimum distance at which the eyes are brought to a focus is 55.2 mm. Accordingly, in the case of a light source size of smaller than 10 mm, the smaller a light source is in size, the shorter the minimum distance at which the eyes are brought to a focus is. Accordingly, determining the values of AEL and MPE by optical power within the range of an aperture diameter of 7 mm at that minimum distance results in decreases in the values of AEL and MPE. 
     When an apparent light source size is smaller than 10 mm, AEL and MPE for Class 1 can be increased in proportion to the square of the apparent light source size. This makes it possible to irradiate a physical object with increased power while maintaining safety. 
     Assume a case where the S/N ratio of an optical sensing apparatus is improved by irradiating a physical object with laser light from the laser device of WO 2003-077389 or Japanese Unexamined Patent Application Publication No. 2012-169375 (hereinafter referred to as “conventional laser device”) and performing a time-resolved measurement with the optical sensing apparatus of Japanese Unexamined Patent Application Publication No. 2007-260123 (hereinafter referred to as “conventional optical sensing apparatus”). The inventors found that in the conventional optical sensing apparatus, multiple scattering inside a scatterer that has an effect of spreading laser light reduces the S/N ratio of a time-resolved measurement. The following gives a detailed description. 
       FIG. 16  is a schematic view for explaining a configuration of a conventional laser device  106  and how a physical object is irradiated with light emitted from the laser device  106 . 
     The conventional laser device  106  includes a light source  101  and a scatterer  136  having a thickness d s . The conventional optical sensing apparatus includes the laser device  106  and a photodetector (not illustrated). 
     In the example shown in  FIG. 16 , emitted light  131  emitted from the light source  101  falls on the scatterer  136 . Light  138  on a surface of incidence of the scatterer  136  has an optical beam diameter f s1 . The incident light  138  is repeatedly multiply scattered in the interior of the scatterer  136 . Scattered light  137  is emitted spread from the scatterer  136 . Light  139  on a surface of emergence of the scatterer  136  has an optical beam diameter f s2 . The optical beam diameter f s2  is larger than the optical beam diameter f s1 . 
     The apparent light source size is enlarged from f s1  to f s2  by the scatterer  136 . As a result, AEL and MPE for Class 1 can be increased by the size of the square of a scale of enlargement (f s2 /f s1 ). Using a thicker scatterer  136  can make the scale of enlargement larger and therefore can make AEL and MPE for Class 1 greater. However, a thicker scatterer  136  reduces the intensity of scattered light that is emitted from the scatterer  136 . 
     In the example shown in  FIG. 16 , irradiating light  108 , which is scattered light, gets further spread. Irradiating a physical object  105  with spread light  126  turns the light  126  into internally-scattered light  109 . The internally-scattered light  109  is emitted outward as reflected scattered light (not illustrated) containing internal information on the physical object  105  and detected by a photodetector (not illustrated). 
       FIG. 17A  is a diagram schematically showing a time distribution of optical power of an optical pulse emitted from a light source of the conventional laser device. In a case where a short optical pulse of, for example, approximately 1 to 20 ns has been emitted from the light source  101 , a time distribution of optical power of light emitted from the light source  101  is for example shaped as shown in  FIG. 17A . During a period from time t s  to t be , a trapezoidal optical pulse having a maximum optical power P S  is emitted from the light source  101 . A period of time from time t bs  to t be  during which the optical power falls is a falling period of this optical pulse, and t f  (=t be −t bs )) is a falling time. 
       FIG. 17B  is a diagram schematically showing a time distribution of optical power of light with which a physical object was irradiated by the conventional laser device in a case where an optical pulse has passed through a thin scatterer or a thick scatterer. 
     In the example shown in  FIG. 16 , the point of intersection between an optical path indicated by a dot-and-dash line and the surface of emergence of the scatterer  136  is the central part of the surface of emergence of the scatterer  136 . A time distribution of optical power of the light  139  in the central part of the surface of emergence of the scatterer  136  is for example shaped as shown in  FIG. 17B . The light  139  emitted from the scatterer  136  turns into the irradiating light  108 . The thickness d s  assumes two different types of scatterer. A time distribution of optical power in the case of a thick scatterer is represented by a dotted line, and a distribution of optical power in the case of a thin scatterer is represented by a solid line. 
     In the case of a thick scatterer, the irradiating light  108  is present during a period from time t s1  to t be2  and has a maximum optical power P S2 . The irradiating light  108  is emitted as a trapezoidal optical pulse C 2  whose lower slope leaves a trail in a back-end region in the second half of a falling period. On the other hand, in the case of a thin scatterer, the irradiating light  108  is present during a period from time t s1  to t be1  and has a maximum optical power Psi. The irradiating light  108  is emitted as a trapezoidal optical pulse C 1  whose lower slope leaves a trail in a back-end region in the second half of a falling period. A thicker scatterer is longer in falling period length and smaller in optical power than a thinner scatterer. The falling time in the case of a thick scatterer is t f2 , and the falling time in the case of a thin scatterer is t f1 . The falling time t f2  in the case of a thick scatterer is longer than the falling time t f1  in the case of a thin scatterer. 
     The term “scattered light” refers to light that becomes spread by a change in direction of propagation of light by microparticles contained in the scatterer  136 . As shown in  FIG. 16 , the scattered light  137  gets spread while being repeatedly multiply scattered while taking a zigzag optical path or changing optical paths, for example, from back to front or from front to back. 
     In the example shown in  FIG. 16 , all of the light  139  on the surface of emergence of the scatterer  136  is scattered light. Accordingly, in a case where a short optical pulse has been emitted from the light source  101 , times of arrival of scattered light at the surface of emergence of the scatterer  136  are not identical but vary. A thicker scatterer  136  leads to a larger degree of multiple scattering. This results in a greater temporal variation in the light  139  and results in a longer falling time. Therefore, as shown in  FIG. 17B , a thicker scatterer leads to a longer falling time of irradiating light. 
     The inventors studied use of the conventional optical sensing apparatus to illuminate a physical object with light with a long falling time that has passed through a scatterer and to perform a time-resolved measurement by detecting reflected scattered light from the physical object. As a result of their study, the inventors found that a thicker scatterer leads to not only a fall in irradiating power due to a great loss of light but also a longer falling time and that a longer falling time leads to a reduction in S/N ratio of a detected signal. 
     On the basis of the foregoing findings, the inventors conceived of a novel laser device and an optical sensing apparatus including the same. 
     The present disclosure encompasses optical sensing apparatuses according to the following items. 
     First Item 
     An optical sensing apparatus according to a first item of the present disclosure includes: a laser device including
         a light source that emits laser light,   a diffusing member having a diffusing surface that crosses an optical path of the laser light, the diffusing member refracting or diffracting the laser light to make the laser light lower in intensity in a first portion including a center of a cross-section of the laser light that crosses the optical path, to make the laser light higher in intensity in a second portion of the laser light that surrounds the first portion in the cross-section, and to enlarge a beam diameter of the laser light, and   a screen that crosses an optical path of the laser light having passed through the diffusing member,       

     the laser device irradiating a physical object with the laser light having passed through the screen or the laser light reflected by the screen; 
     a photodetector that detects at least a portion of the laser light returning from the physical object and outputs an electric signal; and 
     a control circuit that controls the laser device and the photodetector. 
     The control circuit causes the laser device to irradiate the physical object with at least one optical pulse of the laser light and causes the photodetector to perform a time-resolved measurement of at least one reflected optical pulse of the laser light returning from the physical object. 
     In the laser device of this optical sensing apparatus, the beam diameter of the light on the screen is larger than the beam diameter of the light on the diffusing member. Further, the light intensity on the screen is lower than the light intensity on the diffusing member. This makes it possible to increase irradiating power while maintaining safety. Furthermore, for example, a falling time of an optical pulse emitted from the light source is substantially the same as a falling time of an optical pulse on the screen. This makes it possible to improve the S/N ratio of a time-resolved measurement. 
     Second Item 
     In the optical sensing apparatus according to the first item, the beam diameter of the laser light on the diffusing surface may be smaller than 10 mm. 
     In this optical sensing apparatus, AEL and MPE for Class 1 can be increased in proportion to the square of an apparent light source size between the diffusing member and the screen. 
     Third Item 
     In the optical sensing apparatus according to the first or second item, the beam diameter of the laser light on the screen may be not smaller than 10 mm. 
     In this optical sensing apparatus, the maximum light intensity of an apparent light source on the screen becomes lower. This ensures the safety of the eyes of a subject. 
     Fourth Item 
     In the optical sensing apparatus according to any of the first to third items, the beam diameter of the laser light on a surface of the physical object may be larger than the beam diameter of the laser light on the screen. 
     In this optical sensing apparatus, the light intensity on the surface of the physical object becomes lower than the light intensity on the screen. This brings about further improvement in safety on the surface of the physical object. 
     Fifth Item 
     In the optical sensing apparatus according to any of the first to third items, the diffusing member may include a lens array on the diffusing surface. 
     In this optical sensing apparatus, emitted light from the laser light source having a Gaussian distribution of light intensity is converted by a diffusing member including a lens array into light having a wholly flat distribution of light intensity. 
     Sixth Item 
     In the optical sensing apparatus according to any of the first to fifth items, the diffusing member may be configured such that the laser light spreads at a first spread angle when the laser light passes through the diffusing member, the screen may be configured such that the laser light spreads at a second spread angle when the laser light passes through the screen or is reflected by the screen, and the first spread angle may be larger than the second spread angle. 
     In this optical sensing apparatus, the distance between the diffusing member and the screen can be shortened. 
     Seventh Item 
     In the optical sensing apparatus according to any of the first to sixth items, the screen may include depressions and projections alternately arranged two-dimensionally on a surface of the screen, and a depth of each of the depressions and a height of each of the projections may each range from 2 μm to 30 μm. 
     Eighth Item 
     In the optical sensing apparatus according to any of the first to sixth items, the screen may include a first layer and a second layer, and a refractive index of the first layer may be different from a refractive index of the second layer. 
     Ninth Item 
     In the optical sensing apparatus according to any of the first to sixth items, the screen may include first parts and second parts alternately arranged two-dimensionally on a surface of the screen, and a refractive index of each of the first parts may be different from a refractive index of each of the second parts. 
     In this optical sensing apparatus, the screen refracts or diffracts the light. This makes it possible to decrease the intensity of the central part of the light, increase the intensity of the peripheral part of the light, and enlarge the beam diameter of the light. 
     Tenth Item 
     In the optical sensing apparatus according to any of the first to ninth items, 1/μs≤d≤1/μs′, where d is a thickness of the screen, μs′ is an equivalent scattering coefficient of the screen, and μs is a scattering coefficient of the screen. 
     In this optical sensing apparatus, multiple scattering hardly occurs in the screen. As a result, a falling time of an optical pulse emitted from the laser light source can be considered to be substantially the same as a falling time of an optical pulse on the screen. This makes it possible to perform a time-resolved measurement with a high S/N ratio. 
     Eleventh Item 
     In the optical sensing apparatus according to any of the first to tenth items, a distance from the diffusing member to the screen may be longer than a distance from the light source to the diffusing member. 
     This optical sensing apparatus makes it possible to increase the scale of enlargement of the apparent light source size while holding the sum of the two distances constant. 
     Twelfth Item 
     The optical sensing apparatus according to any of the first to eleventh items may further include an optical member, disposed between the diffusing member and the screen, that has a first surface and a second surface opposite to the first surface. The diffusing member may be disposed on the first surface, and the screen may be disposed on the second surface. 
     In this optical sensing apparatus, the diffusing member and the screen are integrally formed. Forming the diffusing member and the screen so that they face each other brings about a structure stabilization effect. 
     Thirteenth Item 
     The optical sensing apparatus according to any of the first to eleventh items may further include a first optical member having a first surface and a second surface opposite to the first surface; and a second optical member having a third surface and a fourth surface opposite to the third surface. The first optical member and the second optical member may both be disposed between the diffusing member and the screen, the diffusing member may be disposed on the first surface, the screen may be disposed on the fourth surface, and the second surface may face the third surface. 
     A diffusing structure formed by a combination of the diffusing member and the first optical member can be fabricated by forming, on a large optical member, a diffusing member of the same size as the optical member and cutting out a desired size. The diffusing stricture is smaller in size than a screen structured formed by a combination of the screen and the second optical member. Accordingly, the cost of the diffusing structure can be reduced. 
     Fourteenth Item 
     The optical sensing apparatus according to any of the first to eleventh items may further include: a first optical member having a first surface and a second surface opposite to the first surface; and a second optical member having a third surface and a fourth surface opposite to the third surface. The first optical member may be disposed between the diffusing member and the screen, the diffusing member may be disposed on the first surface, the screen may be disposed on the third surface, the second surface may face a surface of the screen, the second surface may be tilted with respect to the surface of the screen, and the laser device may irradiate the physical object with the laser light reflected by the surface of the screen. 
     In this optical sensing apparatus, the optical path from the light source to the physical object is not linear but folded. This makes it possible to make the size of the laser device smaller in the direction of propagation of the light reflected by the screen. 
     Fifteenth Item 
     The optical sensing apparatus according to any of the first to twelfth items may further include a collimator lens located between the laser light source and the diffusing member. 
     In this optical sensing apparatus, the light emitted from the light source is converted by the collimator lens into parallel light. The diffusing member is designed in accordance with parallel light. Accordingly, it is not necessary to design the diffusing member in accordance with diverging light serving as the emitted light from the light source, so that the design of the diffusing member is simplified. 
     Sixteenth Item 
     In the optical sensing apparatus according to any of the first to fifteenth items, the control circuit may cause the photodetector to detect a component of light included in a falling period of the at least one reflected optical pulse and output an electric signal representing an amount of the component, and the falling period may be a period from beginning to end of a decrease in intensity of the at least one reflected optical pulse. 
     Seventeenth Item 
     In the optical sensing apparatus according to the sixteenth item, the electric signal may contain information on an internal state of the physical object. 
     Eighteenth Item 
     In the optical sensing apparatus according to the seventeenth item, the physical object may be a living body, and the electric signal may contain information on blood flow in the living body. 
     Nineteenth Item 
     In the optical sensing apparatus according to the seventeenth item, the physical object may be a human forehead, and the electric signal may contain information on cerebral blood flow. 
     Twentieth Item 
     In the optical sensing apparatus according to any of the first to nineteenth items, the electric signal may contain information on a distance from the physical object to the photodetector. 
     Twenty-First Item 
     In the optical sensing apparatus according to the twentieth item, the photodetector may be a time-of-flight (TOF) camera. 
     In the present disclosure, all or a part of any of circuit, unit, device, part or portion, or any of functional blocks in the block diagrams may be implemented as one or more of electronic circuits including, but not limited to, a semiconductor device, a semiconductor integrated circuit (IC) or an LSI. The LSI or IC can be integrated into one chip, or also can be a combination of plural chips. For example, functional blocks other than a memory may be integrated into one chip. The name used here is LSI or IC, but it may also be called system LSI, VLSI (very large scale integration), or ULSI (ultra large scale integration) depending on the degree of integration. A Field Programmable Gate Array (FPGA) that can be programmed after manufacturing an LSI or a reconfigurable logic device that allows reconfiguration of the connection or setup of circuit cells inside the LSI can be used for the same purpose. 
     Further, it is also possible that all or a part of the functions or operations of the circuit, unit, device, part or portion are implemented by executing software. In such a case, the software is recorded on one or more non-transitory recording media such as a ROM, an optical disk or a hard disk drive, and when the software is executed by a processor, the software causes the processor together with peripheral devices to execute the functions specified in the software. A system or apparatus may include such one or more non-transitory recording media on which the software is recorded and a processor together with necessary hardware devices such as an interface. 
     In the following, embodiments of the present disclosure are more specifically described. Note, however, that an unnecessarily detailed description may be omitted. For example, a detailed description of a matter that has already been well known and a repeated description of substantially identical configurations may be omitted. This is intended to prevent the following description from becoming unnecessarily redundant and facilitate understanding of persons skilled in the art. It should be noted that the inventors provide the accompanying drawings and the following description so that persons skilled in the art can sufficiently understand the present disclosure, and do not intend to thereby limit the subject matters recited in the claims. In the following description, identical or similar constituent elements are given the same reference signs. 
     In the following, embodiments are described with reference to the drawings. 
     Embodiment 1 
     First, a laser device according to Embodiment 1 of the present disclosure is described in detail with reference to  FIGS. 1 to 3C .  FIGS. 1 to 3C  show XYZ coordinates whose X, Y, and Z directions are orthogonal to one another. 
       FIG. 1  is a schematic view for explaining a configuration of a laser device  6  according to Embodiment 1 of the present disclosure and how a physical object  5  is irradiated with light  8  emitted from the laser device  6 . 
     The laser device  6  of Embodiment 1 includes a laser light source  1 , a diffusing member  20 , and a screen  21 . 
     The laser light source  1  is for example a semiconductor laser that repeatedly emits optical pulses. The laser light source  1  emits light of, for example, not shorter than 650 nm to not longer than 950 nm. This wavelength range is included in a wavelength range of red to near infrared radiation. The aforementioned wavelength range is called “biological window” and known to be low in absorptance in the body. The laser light source  1  according to Embodiment 1 is described as one that emits light falling within the aforementioned wavelength range, but light falling within another wavelength range may be used. The term “light” as used herein means not only visible light but also infrared radiation. 
     In a visible light region of shorter than 650 nm, absorption by hemoglobin in the blood is high, and in a wavelength range of longer than 950 nm, absorbance by water is high. Meanwhile, in a wavelength range of not shorter than 650 nm to not longer than 950 nm, the absorption coefficients of hemoglobin and water are comparatively low and the scattering coefficients of hemoglobin and water are comparatively high. Accordingly, light falling within this wavelength range is subjected to strong scattering after entering the body and returns to the body surface. This makes it possible to efficiently acquire information on the interior of the body. Therefore, the present embodiment mainly uses light falling within this wavelength range. 
     The diffusing member  20  has a diffusing surface  20   s  that crosses the optical path of light  31  emitted from the laser light source  1 . The diffusing member  20  refracts or diffracts light to make the light  31  lower in intensity in a central part serving as a first portion including the center of a cross-section of laser light that crosses the optical path and make the light  31  higher in intensity in a peripheral part serving as a second portion of the laser light that surrounds the first portion in the cross-section. For example, in a case where light having a Gaussian distribution of intensity has fallen on the diffusing member  20 , the light is converted into light having an approximately-flat distribution of intensity and emitted from the diffusing member  20 . A specific example of a configuration of the diffusing member  20  will be described later. 
     The screen  21  crosses an optical path  27  of light  23  having passed through the diffusing member  20 . On a surface of the screen  21 , there appears a spot of light having passed through the screen  21  or light reflected by the screen  21 . In the present embodiment, the spot of light can be construed as being an apparent light source. The term “screen” as used herein refers to a member that does not have a function of greatly converting an intensity distribution of light and reflects the apparent light source. The screen  21  enlarges the apparent light source size. In the absence of the screen  21 , the apparent light source size is the beam diameter f 1  of light  24  on the diffusing surface  20   s . In the presence of the screen  21 , the apparent light source size is the beam diameter f 2  of light  25  on the screen  21 . The beam diameter f 2  of the light  25  on the screen  21  is larger than the beam diameter f 1  of the light  24  on the diffusing surface  20   s . The apparent light source size on the screen  21  may be herein sometimes referred to as “light source size of the laser device  6 ”. A specific example of a configuration of the screen  21  will be describer later. 
     In the example shown in  FIG. 1 , it is assumed that d 1  is the distance from the laser light source  1  to the diffusing member  20 , that d 2  is the distance from the diffusing member  20  to the screen  21 , and that WD is the distance from the screen  21  to the physical object  5 . The light  31 , which is emitted light, the light  23 , which is deflected light or diffracted light emitted from the diffusing member  20 , and the light  8 , which is irradiating light, are each diverging light oriented in the Z direction on the optical path. The diffusing member  20  and the screen  21  are each placed orthogonal to the optical path  27  and parallel to the Y direction. An example is illustrated in which the optical path of the light  23  emitted from the diffusing member  20  is parallel to the Z direction. Without being bound by such a configuration, the optical path of the light  23  may be set at a slant from the Z direction. In that case, the screen  21  may be placed at a tilt from the Y direction so that the screen  21  is perpendicular to the optical path. 
     The physical object  5  is irradiated with the light  8  having passed through the screen  21  or the light  8  reflected by the screen  21 . The beam diameter f 3  of light  26  on a surface  5   s  of the physical object  5  is larger than both the beam diameter f 1  of light on the diffusing surface  20   s  and the beam diameter f 2  of light on the screen  21 . The distance WD does not need to be that great in order for the physical object  5  to be illuminated at a wide angle. The light  8  enters the physical object  5 . A portion of the light  8  turns into directly-reflected light (not illustrated) that is reflected by the surface  5   s , and another portion of the light  8  turns into internally-scattered light  9 . 
     The laser device  6  further includes an optical member  28  having a first surface  1  and a second surface that face each other. In the example shown in  FIG. 1 , the diffusing member  20  and the screen  21  are formed integrally with the optical member  28 . The optical member  28  has translucency. The optical member  28  is for example a glass is a planar substrate of glass or resin. The diffusing member  20  is disposed on the first surface, and the screen  21  is disposed on the second surface. The optical member  28 , on which the diffusing member  20  and the screen  21  are disposed, is placed so that the diffusing member  20  faces the laser light source  1 . Placing the diffusing member  20  and the screen  21  on the optical member  28  so that the diffusing member  20  and the screen  21  face each other brings about a structure stabilization effect. Further, the optical member  28 , the diffusing member  20 , and the screen  21  may be used as a single optical component by being integrally molded by injection molding of resin. The integral molding is advantageous in terms of cost and positioning. 
     The inventors found the following. The light  31 , which is diverging light, emitted from the laser light source  31  enters the diffusing member  20  and turns into the light  23 , which is refracted light or diffracted light emitted spread from the diffusing member  20 . The light  23  travels in a straight line in a particular direction without being multiply scattered. Therefore, there are no temporal variations in time distribution of optical power no matter where on the screen  21  a time distribution is observed. Further, a time distribution of optical power is the same in shape as a time distribution of the optical power of the light  31 . That is, in a case where a short optical pulse of, for example, approximately 1 to 20 ns has been emitted from the laser light source  1 , the falling time of an optical pulse of the laser light source  1  can be considered to be the same as the falling time of an optical pulse  25  on the screen  21 . 
     The screen  21  may include, for example, a plurality of depressions and a plurality of projections alternately arranged two-dimensionally on the surface of the screen  21 , and the depth of each of the plurality of depressions and the height of each of the plurality of projections may each range from 2 μm to 30 μm. Alternatively, the screen  21  may include a first layer and a second layer, and the refractive index of the first layer may be different from the refractive index of the second layer. For example, the second layer may be formed by applying paint to a surface of the first layer. Alternatively, the screen  21  may include a plurality of first parts and a plurality of second parts alternately arranged two-dimensionally on the surface of the screen  21 , and the refractive index of each of the plurality of first parts may be different from the refractive index of each of the plurality of second parts. It is desirable that multiple scattering not occur on the screen  21 . The illustrated structure is a structure in which multiple scattering hardly occurs. The inventors found that multiple scattering hardly occurs when such a screen  21  having a surface to which paint has been applied or a screen  21  varying in refractive index from position to position satisfies 1/μ s ≤d≤1/μ s ′, where d is the thickness of the member, μ s ′ is the equivalent scattering coefficient of the member, and μ s  is the scattering coefficient of the member. In a case where the physical object  5  is irradiated with the light  8  having passed through such a screen  21 , the falling time of an optical pulse on the surface  5   s  of the physical object  5  can be considered to be substantially the same as the falling time of an optical pulse of the laser light source  1 , so that the influence of multiple scattering is small. Accordingly, the laser device  6  is applicable to a high S/N ratio time-resolved measurement. 
     Further, the apparent light source size is enlarged from f 1  to f 2 . As a result, AEL and MPE for Class 1 can be increased by the size of the square of a scale of enlargement (f 2 /f 1 ). 
       FIGS. 2A and 2B  are a plan view and a cross-sectional view, respectively, schematically showing a structure of the diffusing member  20  of the laser device  6  according to Embodiment 1 of the present disclosure. The cross-section in the example shown in  FIG. 2B  is equivalent to the cross-section taken along IIB-IIB in the example shown in  FIG. 2A . 
     The diffusing member  20  is placed on one of the two surfaces of the optical member  28 . The diffusing member  20  has a function of further spreading the light  31 , which is emitted light from the laser light source  1 . As shown in  FIG. 1 , the diffusing member  20  converts the light  31  into the light  23 , which is refracted light or diffracted light. The light  31  has a maximum angle of emergence θ. The light  23  emitted from the diffusing member  20  has a spread angle θ 1  that is larger than an angle sin −1  (sin θ/n) of incidence on the optical member  28  with a refractive index n in the absence of the diffusing member  20 . Therefore, θ 1 &gt;sin −1 (sin θ/n) holds. In order to perform a time-resolved measurement with a reduced loss of light, it is desirable that the scattering member  20  not be a microparticle-containing scatterer. 
     In the laser device  6  according to the present embodiment, the diffusing member  20  includes a lens array  32  on the diffusing surface  20   s . The lens array  32  diffuses light by refracting or diffracting it. The lens array  32  is formed, for example, by a transparent resin containing no microparticles. Although, in the examples shown in  FIGS. 2A and 2B , the lens array  32  is a 4×4 refractive lens array having four lenses arranged in both the X direction and the Y direction, the lens array  32  may alternatively be a diffractive lens array. The number of lenses may be set according to the specifications. The number of lenses may be increased by reducing the size of each lens. Further, although the lenses shown are convex in shape, they may alternatively be concave in shape. Concave lenses and convex lenses may be randomly arranged to form a lens array. 
     Random variations in the centers  34  of the lenses, in the film thickness distributions of the lenses, and in lens boundaries  35  in an XY plane in the lens array  32  bring about an effect of reducing diffraction noise in the light  8 . In the example shown in  FIG. 2A , the film thickness distributions are represented by contour lines  33  of certain heights. 
     The light  31  emitted from the laser light source  1  has a Gaussian distribution of light intensity. By passing through each convex lens in the lens array  32 , the light  31  having a Gaussian distribution of light intensity turns into a plurality of rays from each separate convex lens that overlap one another to form a wholly flat distribution of light intensity. 
       FIG. 3A  is a diagram schematically showing a shape of light on the diffusing surface  20   s  of the laser device  6  according to Embodiment 1 of the present disclosure, and  FIG. 3B  is a diagram schematically showing a light intensity distribution of the light on the diffusing surface  20   s  of the laser device  6 .  FIG. 3C  is a diagram schematically showing a shape of light on the screen  21  of the laser device  6  according to Embodiment 1 of the present disclosure, and  FIG. 3D  is a diagram schematically showing a light intensity distribution of the light on the screen  21  of the laser device  6 .  FIG. 3E  is a diagram schematically showing a shape of light on the surface  5   s  of the physical object  5  according to Embodiment 1 of the present disclosure, and  FIG. 3F  is a diagram schematically showing a light intensity distribution of the light on the surface  5   s  of the physical object  5 . 
     In a case where a common semiconductor laser light source is used as the laser light source  1 , the light  31 , which is emitted from the laser light source  1 , has a Gaussian distribution with different angles of emergence in the X direction and the Y direction. Accordingly, as shown in  FIG. 3A , the shape of the light  24  on the diffusing member  20  is for example an elliptical shape having a long axis in the X direction. C 1  represents the center of the elliptical shape. As shown in  FIG. 3B , the maximum light intensity of the light  24  is P 1 . The beam diameters of 1/e 2  of the light  24  in the X direction and the Y direction are f 1x  and f 1y , respectively, and for example, when d 1 =4 mm, f 1x =2 mm and f 1y =1 mm. The apparent light source size in the diffusing member  20  is the average of the beam diameters of 1/e 2  in the X direction and the Y direction. The apparent light source size is 1.5 mm. The average of the beam diameters is herein sometimes referred to simply as “beam diameter”. 
     Using the lens array  32  as the diffusing member  20  makes it possible to convert a Gaussian distribution of light intensity into a substantially flat distribution of light intensity on the screen  21 . The beam diameters of 1/e 2  in the X direction and the Y direction are f 2x  and f 2y , which are substantially the same; for example, f 2x =f 2y =10 mm. As shown in  FIG. 3C , the shape of the light  25  on the screen  21  can be made equal to the shape of a lens boundary  35  forming one lens. C 2  represents the center of the rectangular shape. 
     As shown in  FIG. 3D , the maximum light intensity becomes P 2  and can be made much smaller than P 1 . In terms of safety of the eyes, it is desirable that the maximum light intensity of the apparent light source be low. This is for the following reason. In a case where laser light has fallen on an eye, a speckle appears on the retina, as the laser light has coherence. This causes the light intensity to reach its maximum in certain places on the retina. If the maximum light intensity is too high, the retina might be damaged. 
     Accordingly, the diffusing member  20  refracts or diffract the light  31  emitted from the laser light source  1 , thereby reducing the intensity of the central part of the light  31  and increases the intensity of the peripheral part of the light  31 . The laser device  6  is required to have such characteristics as to be able to maximize total optical power while minimizing light intensities in positions on the screen  21 , which is the apparent light source. A light intensity distribution of the light  25  on the screen  21  may be a flat distribution such as that shown in  FIG. 3D . 
     For a reduction in size of the laser device  6  in the Z direction, it is preferable that d 1 +d 2  be small. Meanwhile, the beam diameter of the light  25  on the screen  21 , which is the apparent light source size, may be not smaller than 10 mm. Further, d 1  may be reduced so that the beam diameter of the light  24  on the diffusing surface  20   s  is smaller than 10 mm. This is because, as mentioned above, when the apparent light source size is smaller than 10 mm, AEL and MPE for Class 1 can be increased in proportion to the square of the apparent light source size. For minimization of d 2 , the beam diameter f of light on the screen  21  may be 10 mm. 
     A substantial spread angle θ 1 −sin −1 (sin θ/n) that is obtained by the diffusing member  20  may be made larger than a substantial spread angle θ 2 −sin −1 (n sin θ 1 ) that is obtained by the screen  21 . This brings about an effect of making d 2  smaller. The substantial spread angle that is obtained by the diffusing member  20  means a measure of an angle by which oblique incident light is spread by the diffusing member  20 . This substantial spread angle is equivalent to the difference between an angle of a ray of light in the absence of the diffusing member  20  and an angle of a ray of light in the presence of the diffusing member  20 . The substantial spread angle that is obtained by the screen  21  means a measure of an angle by which oblique incident light is spread by the screen  21 . This substantial spread angle is equivalent to the difference between an angle of a ray of light in the absence of the screen  21  and an angle of a ray of light in the presence of the screen  21 . 
     In the laser device  6  of the present embodiment, the apparent light source size is enlarged, for example, from f 1 =1.5 mm to f 2 =10 mm. The scale of enlargement is f 2 /f 1 =6.7 times. Then, AEL and MPE for Class 1 can be increased by 44 times, which is the size of the square of the scale of enlargement. 
     The light  26  on the surface  5   s  of the physical object  5  is light obtained by spreading the light  25  on the screen  21 . Therefore, as shown in  FIG. 3E , the beam diameters of 1/e 2  in the X direction and the Y direction are fax and f 3y , which are substantially the same; for example, f 3x =f 3y =60 mm. As shown in  FIG. 3F , the maximum light intensity P 3  of the light  26  is smaller than P 2 , so that the light  26  has a substantially flat light intensity distribution. 
     The distance d 2  from the diffusing member  20  to the screen  21  may be made longer than the distance d 1  from the surface of emergence of the laser light source  1  to the diffusing member  20 . This makes it possible to increase the scale of enlargement of the apparent light source size while holding d 1 +d 2  constant. 
       FIG. 3G  is an example of a diagram showing a light intensity distribution on the diffusing surface  20   s  in a case where the diffusing member  20  having depressions and projections has been actually irradiated with red laser light, and  FIG. 3H  is an example of a diagram showing a light intensity distribution on the screen  21  in a case where the diffusing member  20  having depressions and projections has been actually irradiated with red laser light. In the example shown in  FIG. 3G , a Gaussian distribution with a small beam diameter and a high light intensity is shown in the diffusing surface  20   s . The color of white indicates a high light intensity. Meanwhile, in the example shown in  FIG. 3H , a Gaussian distribution with a large beam diameter and a low light intensity is shown on the screen  21 . The color of black indicates a low light intensity. As shown in  FIGS. 3G and 3H , it is found that light having passed through the diffusing member  20  exhibits a substantially flat distribution with a much-enlarged beam diameter and a low intensity distribution. 
     In the absence of the screen  21 , light having a high light intensity distribution as shown in  FIG. 3G  is directly viewed by a subject as the apparent light source. This causes the subject to feel dazzled and have a feeling of discomfort. Further, in terms of safety of the eyes, it is not desirable to directly view light having a high light intensity distribution. Meanwhile, in the presence of the screen  21 , a light having a low light intensity distribution as shown in  FIG. 3H  can be safely directly viewed by the subject as the apparent light source. 
     Next, a laser sensing apparatus according to Embodiment 1 of the present disclosure is described. 
       FIG. 4  is a schematic view for explaining a configuration of an optical sensing apparatus  17  according to Embodiment 1 of the present disclosure and how a biometric measurement is carried out.  FIG. 5  is a diagram schematically showing an internal configuration of a control circuit  7  and a photodetector  2  of the optical sensing apparatus  17  according to Embodiment 1 of the present disclosure and the flow of signals. 
     The light sensing apparatus  17  according to Embodiment 1 includes the aforementioned laser device  6 , the photodetector  2 , and the control circuit  7 . 
     The control circuit  7  controls the laser device  6  and the photodetector  2 . The laser light source  1  can generate almost any optical pulse by starting and stopping the emission of light and changing light emission powers in accordance with instructions from the control circuit  7 . Further, the control circuit  7  includes a signal processing circuit  36  that processes an electric signal  15  (hereinafter referred to simply as “signal”) that is outputted from the photodetector  2 . The electric signal  15  contains information on an internal state. The signal processing circuit  36  generates internal information on the physical object  5  by performing a computation involving the use of a plurality of signals outputted from the photodetector  2 . 
     The control circuit  7  may be an integrated circuit having a processor such as a central processing unit (CPU) and a memory. For example, by executing a program stored in the memory, the control circuit  7  causes the laser device  6  to emit light and causes the photodetector  2  to detect the light in synchronization with the emission of the light by the laser device  6 . Although, in the present embodiment, the optical sensing apparatus  17  includes the control circuit  7 , the control circuit  7  may be an element that is external to the optical sensing apparatus  17 . 
     The photodetector  2  detects reflected scattered light  11  reflected and/or scattered by a physical object  5  located away from the laser device  6  and outputs an electric signal  15 . The photodetector  2  includes a photoelectric converter  3  that generates signal charge corresponding to the amount of light received, a storage  4  in which signal charge is stored, and a drain  12  through which signal charge is discharged. The photoelectric converter  3  may include, for example, a photodiode. Signal charge produced by the photoelectric converter  3  is stored in the storage  4  or discharged through the drain  12 . The timings of signal storage and discharge are controlled by the control circuit  7  and an internal circuit of the photodetector  2 . The internal circuit of the photodetector  2  involved in this control is herein sometimes referred to as “electronic shutter”. 
     In the present embodiment, the physical object  5  is the forehead of a person. Information on cerebral blood flow can be acquired by irradiating the forehead with light and detecting the resulting scattered light. The “scattered light” contains reflected scattered light and transmitted scattered light. In the following description, the reflected scattered light is sometimes simply referred to as “reflected light”. 
     Present in the interior of the forehead, which is the physical object  5 , are the scalp (approximately 3 to 6 mm thick), the skull (approximately 5 to 10 mm thick), the cerebrospinal fluid layer (approximately 2 mm thick), and the brain tissue, starting from the surface. The ranges of thicknesses in parentheses mean that there are differences between individuals. Blood vessels are present in the scalp and in the brain tissue. Accordingly, blood flow in the scalp is called “scalp blood flow”, and blood flow in the brain tissue is called “cerebral blood flow”. In a cerebral function measurement, a measurement object is a part where there is cerebral blood flow. 
     A living body is a scatterer. A portion of the light  8  emitted toward the physical object  5  returns as directly-reflected light  10  to the optical sensing apparatus  17 . Another portion of the light enters the interior of the physical object  5  and gets diffused, and a portion of it is absorbed. The light having entered the interior of the physical object  5  turns into internally-scattered light  9  containing information on blood flow that is present in a range of depth of approximately 10 to 18 mm from the surface, i.e. cerebral blood flow. The internally-scattered light  9  returns to the optical sensing apparatus  17  as reflected scattered light  11  from the interior. The directly-reflected light  10  and the reflected scattered light  11  are detected by the photodetector  2 . 
     The time from the emission of the directly-reflected light  10  from the laser device  6  to arrival of the directly-reflected light  10  at the photodetector  2  is relatively short, and the time from the emission of the reflected scattered light  11  from the interior from the laser device  6  to arrival of the reflected scattered light  11  from the interior at the photodetector  2  is relatively long. Among them, the component required to be detected at a high S/N ratio is the reflected scattered light  11  from the interior, which has the information on cerebral blood flow. 
     It should be noted that the transmitted scattered light, as well as the reflected scattered light, may be used in carrying out a biological measurement other than a cerebral blood flow measurement. In a case where information on blood other than cerebral blood flow is acquired, the part being tested may be a part other than the forehead (e.g. an arm, a leg, or the like). In the following description, unless otherwise noted, the physical object  5  is the forehead. The subject is a human, but may alternatively be a non-human animal having skin and having a hairless part. The term “subject” as used herein means specimens in general including such animals. 
     In order to quantify the light amounts of the directly-reflected light  10  and the reflected scattered light  11  that are detected by the photodetector  2 , the inventors ran a simulation of an optical pulse response assuming, as the physical object  5 , a phantom mimicking the head of a typical Japanese. Specifically, the inventors calculated through a Monte Carlo analysis a time distribution of optical power, i.e. an optical pulse response, that is detected by the photodetector  2  in a case where an optical pulse  8  is emitted toward a physical object  5  located at a distance of, for example, 15 cm from the laser device  6 . 
       FIG. 6A  is a diagram showing an example of a time distribution of a single optical pulse that is emitted light. In this example, the optical pulse has a wavelength λ of 850 nm and a full width at half maximum of 11 ns. This single optical pulse has a typical trapezoidal shape whose rising and falling times are each 1 ns. The term “rising time” as used herein means the time it takes for an optical power to increase from a peak value of 0% to 100%, and the period of time is referred to as “rising period”. The term “falling time” means the time it takes for an optical power to decrease from a peak value (100%) to zero (0%), and the period of time is referred to as “falling period”. The example shown in  FIG. 6A  assumes that the emission of the single optical pulse starts at a time t=0 and completely stops at t=12 ns. 
     Since the velocity of light c is 300000 km/s and the distance from the laser device  6  to the physical object  5  is 15 cm, the time t from the emission of the light  8 , which is irradiating light, to the arrival of the light  8  at the surface of the physical object  5  is 0.5 ns. The time it takes for the light  8  to arrive at a surface of the photodetector  2  after being directly reflected by the surface of the physical object  5  and turning into the directly-reflected light  10  is 1 ns. Accordingly, the time it takes for the light to be detected on the photodetector  2  is 1 ns or longer. 
     The optical sensing apparatus  17  measures the amount of change in light amount of the reflected scattered light  11  from the interior of the physical object  5  on the basis of changes in oxyhemoglobin concentration and deoxyhemoglobin concentration in the cerebral blood flow. The brain tissue has an absorber whose absorption coefficient and scattering coefficient vary according to changes in cerebral blood flow. In a stationary state, it is possible to model the interior of the brain as uniform brain tissue and conduct a Monte Carlo analysis. The term “changes in blood flow” as used herein means temporal changes in blood flow. 
       FIG. 6B  is a diagram showing total optical power (solid line) detected in a stationary state, an amount of change (dotted line) in optical power corresponding to an amount of change in cerebral blood flow, and a time distribution of degrees of modulation (dot-and-dash line). The term “degree of modulation” means a value obtained by dividing, by the total optical power detected in the stationary state, the amount of change in optical power corresponding to the amount of change in cerebral blood flow. In  FIG. 6A , the vertical axis is expressed by a linear display, and in  FIG. 6B , the vertical axis is expressed by a logarithmic display. 
     The amount of change in optical power corresponding to the amount of change in cerebral blood flow, which is included in the total optical power detected in the stationary state, is only approximately 2×10 −5 . 
     Let it be assumed that t bs  is the time at which the light power starts to decrease on the photodetector  2  and t be  is the time at which the light power completely decreases to a noise level. As shown in  FIG. 6B , it is found that the proportion of signals that indicate changes in cerebral blood flow becomes higher in a falling period  13  of light during which the time t is not shorter than t bs  and not longer than t be . As the second half of the falling period  13  of light passes, the light amount decreases and noise increases accordingly. However, the degree of modulation becomes higher. Of the light falling period  13  of light t bs ≤t≤t be , the power of light at and after t=13.5 ns, for example, is approximately 1/100 of the total optical power at which the light  8 , which is an optical pulse, was detected. In a case where light arriving during the falling period  13  is subjected to a time-resolved measurement with use of the function of an electronic shutter of the photodetector  2 , the proportion of optical power corresponding to the amount of change in cerebral blood flow increases to 7% of the total optical power that is detected at and after t=13.5 ns. This makes it possible to sufficiently acquire signals that indicate changes in cerebral blood flow. Without use of the electronic shutter, the proportion of changes in cerebral blood flow is approximately 2×10 −5 . 
     Accordingly, signals that indicate changes in cerebral blood flow can be detected by using the photodetector  2  to receive a component of the reflected scattered light  11  included in the falling period  13  of an optical pulse from the physical object  5  and detect changes in optical power thereof. 
     The emission of an optical pulse and the detection of light in the light sensing apparatus  17  of to the present embodiment are described on the basis of the aforementioned principle of measurement of changes in cerebral blood flow. 
     In a case where the subject  5  is a human as in the case of the present embodiment, AEL and MPE for Class 1 need to be met for safety of the eyes. As described above, the laser device  6  of the present embodiment can increase AEL and MPE. However, the S/N ratio of signal light is not enough in many cases of cerebral function measurements. Accordingly, improvement in S/N ratio is usually brought about by repeatedly performing optical pulse emission and signal detection. 
       FIG. 7  is a diagram schematically showing a time distribution (upper row) of optical pulses  38  emitted from the laser device  6 , a time distribution (middle row) of optical power detected by the photodetector  2  of the optical sensing apparatus  17 , and timings and charge storage (lower row) of an electronic shutter according to Embodiment 1 of the present disclosure. 
     In the optical sensing apparatus  17  of Embodiment 1, the control circuit  7  causes the laser device  6  to irradiate the physical object  5  with at least one optical pulse  38 . The control circuit  7  causes the photodetector  2  to detect a component of light included in a falling period of at least one reflected optical pulse  19  returning from the physical object  5  and output an electric signal  15  representing the amount of light detected. 
     As shown in the upper row of  FIG. 7 , the laser light source  1  emits optical pulses  38  in sequence, for example, in a cycle Λ 1 . An optical pulse  38  has a pulse width T 1  and a maximum optical power P 1 . T n =(=Λ 1 −T 1 ) represents a duration of time during which no optical pulse  38  is present. 
     As shown in the middle row of  FIG. 7 , a distribution of a reflected optical pulse  19  detected by the photodetector  2  in correspondence with an optical pulse  38  has a pulse shape whose lower slope is slightly spread. This is attributed to the occurrence of a time lag under the influence of internal scattering in the physical object  5 . The pulse width T d1  is slightly wider than T 1 . 
     The photodetector  2  photoelectrically converts, through the photoelectric converter  3  of the photodetector  2 , a component of light in a reflected optical pulse  19  included in a falling period  13  and stores signal charge  18  in the storage  4 . 
     In the present embodiment, the pulse width T 1  of an optical pulse  38  ranges from 11 to 22 ns. These optical pulses  38  may be repeatedly emitted, for example, approximately 1000 times to 1000000 times in a time cycle Λ 1  of approximately 55 ns to 110 ns. In this way, one frame is composed. Laying frames side by side can compose a moving image. 
     In the optical sensing apparatus  17  of the present embodiment, the photodetector  2  includes an electronic shutter that switches between storing signal charge and not storing signal change and the storage  4 . The electronic shutter is a circuit that controls storage and discharge of signal charge generated by the photoelectric converter  3 . 
     A component of light included in a falling period  13  of a reflected optical pulse  19  is photoelectrically converted by the photoelectric converter  3 . After that, the storage  4  is selected (that is, the electronic shutter is kept open) in accordance with a control signal  16   a  from the control circuit  7 , and signal charge  18  is stored for a period of time T S  of, for example, 11 to 22 ns. After passage of the period of time T S , the drain  12  is selected (that is, the electronic shutter is kept close) in accordance with a control signal  16   c  from the control circuit, and charge from the photoelectric converter  3  is released. 
     Accordingly, a repetitive string of components of light included in falling periods  13  of reflected optical pulses  19  is stored in the storage  4  by photoelectric conversion as one frame of signal charge  18  in correspondence with a repetitive pulse string of optical pulses  38 . After the end of one frame, the signal charge  18  is outputted to the control circuit  7  as an electric signal  15 . The electric signal  15  contains information on cerebral blood flow. 
     After the emission of optical pulses  38 , ambient noise may be measured by keeping the electronic shutter open and closed for the same length of time and the same number of times in the absence of light emission. The S/N ratios of the signals can be improved by subtracting the value of the ambient noise from each of the signal values. 
     The configuration of the photodetector  2  shown in  FIG. 5  is equivalent to one pixel. This makes it possible to acquire biological information about averaged blood flow within the physical object  5 . 
     Alternatively, as the photodetector  2 , an image sensor including, for each pixel, a photoelectric converter  3 , a storage  4 , and an electronic shutter that switches between storing signal charge and not storing signal charge in the storage  4  may be used. In this case, the photodetector  2  is an image sensor having a plurality of photodetection cells arrayed two-dimensionally. Each of the photodetection cells stores, as signal charge  18 , a component of light included in a falling period of a reflected optical pulse  19 . Furthermore, each of the photodetection cells outputs an electric signal  15  representing the total amount of signal charge stored. This makes it possible to acquire biological information about the blood flow of the physical object  5  as a moving image including a plurality of frames. 
     Next, the superimposition of the information on cerebral blood flow onto the electric signal  15  is described with reference to  FIGS. 8A and 8B . 
       FIG. 8A  is a front view showing changes in blood flow that are present inside the physical object  5 .  FIG. 8B  is a side cross-sectional view showing changes in blood flow that are present in the physical object  5 . In the examples shown in  FIGS. 8A and 8B , regions  14   a  and  14   b  are shown that indicate internal blood flow at approximately 10 to 18 mm from the surface. The internal blood flow here is cerebral blood flow. Attention is paid to the optical path through which the light  8 , which is irradiating light, enters the physical object  5  and is detected as the internally-scattered light  9  from the interior by the photodetector  2 . The internally-scattered light  9 , albeit depending on a blood flow distribution, passes through the regions  14   a  and  14   b . Furthermore, the internally-scattered light  9  is repeatedly scattered or absorbed and comes out of the physical object  5  as the reflected scattered light  11  from the interior. 
     Next, a method for acquiring a distribution that indicates changes in blood flow in the physical object  5  is described. 
     First, the control circuit  7  causes the photodetector  2 , which is an image sensor, to output the following first and second image signals. The first image signal represents a two-dimensional distribution of the total amount of signal charge  18  stored in the plurality of photodetection cells during a first period. The second image signal represents a two-dimensional distribution of the total amount of signal charge  18  stored in the plurality of photodetection cells during a second period preceding the first period. 
     Next, the signal processing circuit  36  receives the first and second image signals from the photodetector  2 . After that, the signal processing circuit  36  generates a difference image representing the difference between an image represented by the first image signal and an image represented by the second image signal. 
     The difference image is equivalent to a distribution that indicates changes in cerebral blood flow in a part being tested  60 . It is assumed herein that the difference image is an image that uses the second image signal as a reference value and displays an increase or decrease in the first image signal from the reference value. When the signal processing circuit  36  receives the second image signal only once and repeatedly receives the first image signal every one-frame cycle, a moving image representing a distribution that indicates changes in blood flow in the physical object  5  is obtained. 
     Next, a method for improving the size of a cerebral blood flow region detected to an actual size is described. 
       FIG. 9A  is a diagram schematically showing changes in blood flow in the interior of the physical object  5  as detected by irradiation with an optical pulse. FIG.  9 B is a diagram schematically showing changes in blood flow in the interior of the physical object  5  as image-corrected by image computations. 
     The signal processing circuit  36  generates blood flow information on the interior of the physical object  5  through the use of an electric signal  15  representing an amount of signal charge  18 . The electric signal  15  contains the blood flow information on the interior of the physical object  5 . 
     In the example shown in  FIG. 9A , the two-dimensional image represents a distribution of a region  14   c  of change in cerebral blood flow. The region  14   c  of change in cerebral blood flow is in a spread state due to scattering of cerebral blood flow in the interior. To address this problem, the signal processing circuit  36  makes an image correction by guessing the scattering state through a diffusion equation or a Monte Carlo analysis. By so doing, the signal processing circuit  36  generates an actual-size two-dimensional image representing a distribution of a region  14   d  of change in cerebral blood flow such as that shown in  FIG. 9B . This two-dimensional image is a desired image that indicates changes in cerebral blood flow. 
     Next, laser devices  6  according to modifications of Embodiment 1 of the present disclosure are described. 
       FIG. 10  is a schematic view for explaining a configuration of a laser device  6  according to a first modification of Embodiment 1 of the present disclosure and how a physical object  5  is irradiated with light  8  emitted from the laser device  6 . 
     Instead of the optical member  28 , the laser device  6  according to the first modification of Embodiment 1 further includes an optical member  28   a  having two surfaces opposite to each other and an optical member  28   b  having two surfaces opposite to each other. In the laser device  6  according to the first modification of Embodiment 1, the diffusing member  20  and the screen  21  are not disposed on an identical optical member but disposed on the separate optical members  28   a  and  28   b , respectively. The diffusing member  20  is on one of the two surfaces of the optical member  28   a , and the screen  21  is on one of the two surfaces of the optical member  28   b . The other of the two surfaces of the optical member  28   a  faces the other of the two surfaces of the optical member  28   b . The phrase “two surfaces opposite to each other” as used herein encompasses a case where the two surfaces are not parallel to each other. In this example, the optical members  28   a  and  28   b  have translucency. The optical members  28   a  and  28   b  are for example planar substrates of glass or resin. 
     The diffusing member  20  and the optical member  28   a  are combined to serve as a diffusing structure  29 , and the screen  21  and the optical member  28   b  are combined to serve as a screen structure  30 . The diffusing structure  29  can be fabricated by forming, on a larger optical member  28   a , a diffusing member  20  of the same size as the optical member  28   a  and cutting out a desired size. The diffusing structure  29  is much smaller in size than the screen structure  30 . Accordingly, the cost of the diffusing structure  29  can be reduced. 
     Further, since the diffusing member  20  is on that one of the two surfaces of the optical member  28   a  which faces the laser light source  1 , it is possible to make the scale of enlargement of the apparent light source larger than it is when the diffusing member  20  is on the opposite surface. Further, since the screen  21  is on that one of the two surfaces of the optical member  28   b  which is opposite to the surface facing the laser light source  1 , it is possible to make the scale of enlargement of the apparent light source larger than it is when the screen  21  is on the surface facing the laser light source  1 . 
       FIG. 11  is a schematic view for explaining a configuration of a laser device  6  according to a second modification of Embodiment 1 of the present disclosure and how a physical object  5  is irradiated with light  8  emitted from the laser device  6 . 
     The laser device  6  according to the second modification of Embodiment 1 further includes a collimator lens  22  located between the laser light source  1  and the diffusing member  20 . In the laser device  6 , the emitted light  31  from the laser light source  1  enters the diffusing member  20  after being converted by the collimator lens  22  into parallel light. The diffusing member  20  is designed in accordance with parallel light. Accordingly, it is not necessary to design the diffusing member  20  in accordance with diverging light serving as the light  31  emitted from the laser light source  1 , so that the design of the diffusing member  20  is simplified. 
       FIG. 12  is a schematic view for explaining a configuration of a laser device  6  according to a third modification of Embodiment 1 of the present disclosure and how a physical object  5  is irradiated with light  8  emitted from the laser device  6 . 
     In the laser device  6  according to the third modification of Embodiment 1, the screen structure  30  including the screen  21  is placed at a slant of, for example, 45 degrees with respect to the optical path of the light  23 , which is refracted light or diffracted light emitted from the diffusing member  20 . The diffusing member  20  is on one of the two surfaces of the optical member  28   a , and the screen  21  is on one of the two surfaces of the optical member  28   b . The other of the two surfaces of the optical member  28   a  faces the surface of the screen  21 . The other of the two surfaces of the optical member  28   a  is tilted with respect to the surface of the screen  21 . In this example, the optical member  28   a  has translucency, and the optical member  28   b  has reflectivity. The optical member  28   a  is for example a planar substrate of glass or resin. The optical member  28   b  is for example a planar substrate of metal. A usable example of the metal is aluminum. 
     The light  31  emitted in the Y direction is reflected by the screen  21  to be bent in the Z direction. The physical object  5  is irradiated with light bent in the Z direction. Such a folding optical system makes it possible to reduce the Z-direction size of the laser device  6 . 
     Embodiment 2 
     Next, an optical sensing apparatus according to Embodiment 2 of the present disclosure is described with reference to  FIGS. 13 to 15  with a focus on differences from the optical sensing apparatus  17  according to Embodiment 1. 
       FIG. 13  is a schematic view for explaining a configuration of a laser device  6  according to Embodiment 2 of the present disclosure and how a physical object  5  is irradiated with light  8  emitted from the laser device  6 .  FIG. 14  is a diagram schematically showing an internal configuration of a control circuit  7  and a photodetector  2  of an optical sensing apparatus  17  according to Embodiment 2 of the present disclosure and the flow of signals.  FIG. 15  is a diagram schematically showing a time distribution (upper row) of optical pulses emitted from the laser device  6 , a time distribution (middle row) of optical power detected by the photodetector  2  of the optical sensing apparatus  17 , and timings and charge storage (lower row) of an electronic shutter according to Embodiment 2 of the present disclosure. 
     The laser device  6  according to Embodiment 2 differs from the laser device  6  according to Embodiment 1 in that the laser light source  1  is a multiwavelength light source that emits at least two wavelengths of light and emits optical pulses in sequence for each separate wavelength. The optical sensing apparatus  17  of Embodiment 2 differs from the optical sensing apparatus  17  of Embodiment 1 in that the optical sensing apparatus  17  of Embodiment 2 includes the laser device  6  of Embodiment 2, which has a multiwavelength light source. The other components are the same as those of the optical sensing apparatus  17  of Embodiment 1. 
     The laser light source  1  includes, for example, a plurality of light-emitting elements  1   a  and  1   b  arranged side by side in the Y direction. The light-emitting element  1   a  emits light of a first wavelength range, and the light-emitting element  1   b  emits light of a second wavelength range that is different from the first wavelength range. The light-emitting elements  1   a  and  1   b  are for example laser chips. 
     The absorptance of oxyhemoglobin and deoxyhemoglobin varies, for example, at wavelengths of λ 1 =750 nm and λ 2 =850 nm. Therefore, computing two electric signals respectively obtained by using these two wavelengths makes it possible to measure the proportions of oxyhemoglobin and deoxyhemoglobin in the physical object  5 . 
     When the physical object  5  is a forehead area of the head of a living body, the amount of change in cerebral blood flow in the frontal lobe, the amounts of change in oxyhemoglobin concentration and deoxyhemoglobin concentration, or the like can be measured. This makes sensing of information such as emotions possible. For example, in a centered state, there occur an increase in cerebral blood flow volume, an increase in amount of oxyhemoglobin, and the like. 
     Various combinations of wavelengths are possible. At a wavelength of 805 nm, the rates of absorption of oxyhemoglobin and deoxyhemoglobin become equal. Accordingly, in view of the biological window, for example, a wavelength of not shorter than 650 nm and shorter than 805 nm and a wavelength of longer than 805 nm and not longer than 950 nm may be combined. Furthermore, a third wavelength of 805 nm may be used in addition to the two wavelengths. In a case where three wavelengths of light are used, three laser chips are needed; however, since information on the third wavelength is obtained, utilizing the information may make computations easy. 
     The photodetector  2  of the optical sensing apparatus  17  of the present embodiment includes an electronic shutter that switches between storing signal charge and not storing signal charge and two storages  4   a  and  4   b . An optical pulse  38   a  of a wavelength λ 1  is emitted from the light-emitting element  1   a . The photoelectric converter  3  photoelectrically converts a component of reflected scattered light  11  included in a falling period  13  of a reflected optical pulse  19   a  returning to the photodetector  2  in correspondence with the optical pulse  38   a . After that, the storage  4   a  is selected in accordance with control signals  16   a ,  16   b , and  16   c  from the control circuit  7 , and a first signal charge  18   a  is stored for a period of time T S1  of, for example, 11 to 22 ns. After passage of the period of time T S1 , the drain  12  is selected in accordance with the control signals  16   a ,  16   b , and  16   c  from the control circuit  7 , and, and charge from the photoelectric converter  3  is released. 
     After this, the light-emitting element  1   a  is replaced by the light-emitting element  1   b , which similarly emits an optical pulse  38   b  of a wavelength λ 2 . The photoelectric converter  3  photoelectrically converts a component of reflected scattered light  11  included in a falling period  13  of a reflected optical pulse  19   b  returning to the photodetector  2  in correspondence with the optical pulse  38   b . After that, the storage  4   b  is selected in accordance with the control signals  16   a ,  16   b , and  16   c  from the control circuit  7 , and a second signal charge  18   b  is stored for a period of time T S2  of, for example, 11 to 22 ns. After passage of the period of time T S2 , the drain  12  is selected in accordance with the control signals  16   a ,  16   b , and  16   c  from the control circuit  7 , and, and charge from the photoelectric converter  3  is released. These optical pulses  38   a  and  38   b  are repeatedly emitted in sequence and stored as one frame of first signal charge  38   a  and one frame of second signal charge  18   b , respectively. After the end of one frame, the first signal charge  18   a  and the second signal charge  18   b  are outputted to the control circuit  7  as a first electric signal  15   a  and a second electric signal  15   b , respectively. 
     From two pieces of image information acquired, two images representing two-dimensional concentration distributions of oxyhemoglobin and deoxyhemoglobin, respectively, can be generated as images that indicate changes in cerebral blood flow. 
     The present embodiment, too, makes it possible to provide a laser device  6  that emits light that increases MPE and AEL for Class 1 and can be applied to a time-resolved measurement and an optical sensing apparatus  17  including the same that can improve an S/N ratio. 
     The optical sensing apparatuses  17  according to Embodiments 1 and 2 have been described with reference to a case where information on the physical object  5  is cerebral blood flow. The information on the physical object  5  may be information on the distance from the physical object  5  to the photodetector  2 . In this case, the electric signal  15  contains a signal representing the distance. The optical sensing apparatuses  17  according to Embodiments 1 and 2 may be configured as TOF (time-of-flight) cameras that can increase irradiating power while ensuring safety. 
     In the foregoing, the laser devices  6  according to Embodiments 1 and 2 and the optical sensing apparatuses  17  including the same have been described. However, the present disclosure is not limited to these embodiments. 
     The laser devices according to Embodiments 1 and 2, the optical sensing apparatuses including the laser devices, and a laser device based on a combination of the configurations of the optical sensing apparatuses are also encompassed in the present disclosure and can bring about similar effects.