U.S. patent application number 11/079472 was filed with the patent office on 2005-09-29 for light source device.
Invention is credited to Benitez-Gimenez, Pablo, Hirohashi, Kazutoshi, Lopez-Hernandez, Francisco-Jose, Minano-Dominguez, Juan-Carlos, Sakai, Masahisa.
Application Number | 20050213180 11/079472 |
Document ID | / |
Family ID | 18995232 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050213180 |
Kind Code |
A1 |
Lopez-Hernandez, Francisco-Jose ;
et al. |
September 29, 2005 |
Light source device
Abstract
The present invention provides a light source device which is
safe for human eyes and whose switching is performed at high speed.
The light source device comprising one or more laser light source 1
for monochromatically or polychromatically emitting, a diffuser 3
(transmissive, reflective or mixture) for diffusing the light
bundle injected directly from the laser light source 1 or via the
optical focusing system 2, and the optical system 4 which is
referred to as a collimator for collimating the diffused light
bundle emitted from the diffuser 3.
Inventors: |
Lopez-Hernandez,
Francisco-Jose; (Madrid, ES) ; Minano-Dominguez,
Juan-Carlos; (Madrid, ES) ; Benitez-Gimenez,
Pablo; (Madrid, ES) ; Sakai, Masahisa;
(Yokohama, JP) ; Hirohashi, Kazutoshi; (Yokohama,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
18995232 |
Appl. No.: |
11/079472 |
Filed: |
March 15, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11079472 |
Mar 15, 2005 |
|
|
|
10143923 |
May 14, 2002 |
|
|
|
6867929 |
|
|
|
|
Current U.S.
Class: |
359/237 |
Current CPC
Class: |
H01S 5/06825 20130101;
H01S 5/005 20130101; H01S 3/005 20130101; G02B 5/0278 20130101;
G02B 5/0284 20130101 |
Class at
Publication: |
359/237 |
International
Class: |
G02F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 18, 2001 |
JP |
JP2001-150178 |
Claims
What is claimed is:
1. A light source device comprising: a laser light source; diffuser
for diffusing a light emitted from said laser light source; and a
collimator for collimating a light diffused by said diffuser and
emitting the light.
2. The light source device according to claim 1, including an
optical focusing system for focusing a light emitted from said
laser light and making said diffuser inject the light.
3. The light source device according to claim 1, wherein said
diffuser is any one of a transmissive diffuser for transmitting and
diffusing an incident light, a reflective diffuser for reflecting
and diffusing an incident light, or a mixture diffuser which
combines said transmissive diffuser and said reflective
diffuser.
4. The light source device according to claim 1, wherein said
diffuser is situated on a plane having normal having an incident
light direction and an optional angle from said laser light
source.
5. The light source device according to claim 2, wherein said
diffuser, said collimator and said optical focusing system are
constituted in an integrated manner.
6. The light source device according to claim 5, wherein one
portion of one surface is coated by a mirror surface, and said
diffuser is mounted on one portion of other surface.
7. The light source device according to claim 1, wherein said
optical focusing system includes at least one optical system using
a polyhedral body reflecting or refracting.
8. The light source device according to claim 1, including multiple
laser light sources, wherein a light emitted from said multiple
laser light source is injected into said diffuser.
9. The light source device according to claim 1, including a
diaphragm for determining an acceptable emission limit of said
collimator.
10. The light source device according to claim 9, wherein an
aperture of said diaphragm is variable.
11. The light source device according to claim 1, wherein an
incident surface of said diffuser is a concave surface.
12. The light source device according to claim 1, wherein a set of
microlenses or micromirrors is used instead of said diffuser.
13. The light source device according to claim 12, wherein said
microlens or micromirror attenuates a fraction of power which does
not exceed 10% of a light emitted from said laser light source.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light source device in
which a laser diode is employed as a light source and used for a
wireless optical communication.
[0003] 2. Description of the Related Art
[0004] In recent years, there has been growing interest in wireless
optical communications. There are several reasons for this as
follows: the electromagnetic spectrum they use is not covered by
current legislation, they are reliable systems, and they are not
expensive. Moreover the possibility of an information leakage to
the external due to rectilinear propagation property of the light
is low and secrecy is high. The most widespread example of this
type of communications can be found in the great majority of remote
controls for electronic consumer goods. In this case the
communication is usually unidirectional and very low-speed. Among
the disadvantages of this type of device, the most important are
the need for a visual link without obstacles and the limitation of
the level of exposure to which the human eye can be subjected,
which restricts the power and collimation characteristics of the
bundle. This limitation has obliged the majority of these devices
to use as radiation source an LED (Light Emitting Diode) or IRED
(Infra Red Light Emitting Diode) instead of an LD (laser diode).
The use of LEDs or IREDs has restricted in practice not only the
irradiance (power per unit of area) of the bundle transmitted, but
also the transmission speed, since the switching time of these
devices (in the order of a few nanoseconds) is comparatively long
if they generate radiant power of around tens of milliwatts or
more, which is generally necessary for wireless optical
communications. To these LEDs and IREDs diode, a laser diode is
obviously advantageous with respect to switching time and
generating radiant power, and is also advantageous in cost because
it is popular as a device for optical discs such as MD, CD. Its
only disadvantage is that it is necessary to modify the
characteristics of the bundle (for example, in general, it is
necessary to reduce the irradiance of the bundle) if there exists
the possibility of its reaching the eyes, which is the case in most
applications of wireless communications and illumination. The
maximum permissible exposure level where this possibility exists is
laid down in the European CENELEC EN 60825-1 (CEI 825-1:1993),
CENELEC EN 60825-2 (CEI 825-2:1993), JISC6802:1997 and
IEC60825-2:1993 standards relating to "Safety of laser products".
Maximum permissible exposure levels depend on several factors,
important among which are radiation wavelength, duration and
frequency of light pulses and type of source, extended or
collimated.
[0005] In order to achieve that the irradiance is within the
limits, it is always possible to use the solution of attenuating
the bundle by means of an appropriate filter. However, this
produces very high optical losses. There is also the possibility of
reducing the irradiance and, moreover, taking advantage of this
reduction to increase the cross-sectional size of the bundle or
expand its angular divergence, or both, without causing high
optical losses. When the bundle is expanded, its irradiance
necessarily decreases, thus achieving the objective; furthermore,
the power transmitted in the far field axial direction of the
bundle can be increased.
[0006] Nevertheless, if this expansion is carried out by means of a
conventional optical system, that is, through a combination of
lenses and mirrors, the laser system is not eye-safe. This is due
to the fact that viewing the bundle through optical instruments
such as binoculars or telescopes is unsafe, since such instruments
accomplish the inverse process to that of the expansion of the
bundle, that is, they concentrate the laser bundle on the eyes, so
that permitted exposure levels may be exceeded. Furthermore, this
procedure for expanding the bundle reduces angular divergence,
which is a disadvantage in those applications in which a wide
pointing error tolerance is required.
[0007] Irradiance can also be reduced by means of diffusers.
Diffusers achieve an increase in the angular divergence of the
bundle without modifying the cross-section the bundle has when it
reaches the diffuser. Obviously, mean radiance (power per unit of
surface and unit of solid angle) of the bundle decreases, since the
conservation of energy must be fulfilled. Where there is diffusion,
the mean irradiance of the bundle decreases more rapidly with
distance from the point of diffusion, since angular divergence is
bigger than the one of the non-diffusion case. In this way it is
possible to fulfil the requirements of CENELEC EN 60825-1
(CEI825-1:1993), JISC6802:1997 and IEC60825-1:1993 with effect from
a short distance away from the point of emission. Moreover, this
solution reduces the problem of the possible viewing of the bundle
with binoculars or telescopes, since, on reducing the mean radiance
of the bundle, the maximum irradiance that can be achieved with
such optical instruments is limited. This fact is related to loss
of (spatial) coherence of the laser bundle after diffusion. These
diffusers can be made in various ways. For example, a transmissive
diffuser can be made with sheets of transparent material, one of
whose surfaces is matt, that is, one of whose surfaces is such that
the direction of the normal to the surface on which the refraction
occurs can be considered as a random variable. From the probability
distribution function of this normal and the distribution of
radiant intensity of the laser bundle, it is possible to calculate
the radiation intensity distribution after the diffusion. A more
precise calculation would require the consideration of Fresnel
reflections, which contribute to greater diffusion. If the
irregularities are of the order of the wavelength, than it is
necessary to use the wave optics theory to obtain the intensity
distribution on the diffuser exit. This is the case of another type
of diffuser that is made using holograms.
[0008] Reflection diffusers constitute a second type. A reflection
diffuser can be achieved simply by using matte paint. The laser
radiation impinges on the reflector with a small angular divergence
and is reflected with a large angular divergence. A good diffusive
reflector, such as those used as the inner coating of integrating
spheres, produces a reflected intensity with a Lambertian pattern,
that is, the reflector emits isotropically in the hemisphere it is
facing.
[0009] The main problem of diffusers is that the only way to reduce
irradiance is through expansion of the angular divergence, and this
means that the divergence of the bundle is determined by the safety
criteria. These are such that the irradiance achieved at a distance
of 10 cm from the point of diffusion must be below the maximum
permissible (10 cm is the minimum distance established by the
regulations CENELEC EN 60825-1 (CEI825-1:1993), JISC6802:1997 and
IEC60825-1:1993). If, in order to achieve this objective, it has
been necessary to reduce the irradiance by a factor 1/C (C>1),
then, and given that angular divergence does not vary, the
reduction of irradiance at a distance D will be C=D.sup.2/(10
cm).sup.2. This result is approximately correct when the divergence
of the bundle is much greater than the solid angle subtended by the
illuminated area of the diffuser at a distance of 10 cm, which is
the case if the irradiance of the laser bundle has to be reduced.
In this way, the transportation of light through the air involves
high propagation losses, and this solution becomes useful only for
small distances.
[0010] Diffusers have a second problem: Given the random nature of
the principle of diffusion, intensity distributions, as a function
of direction of emission, present gentle variations, and their form
is not totally controllable (in the case of diffusers based on
rough surfaces, these distributions tend to be Gaussian). In
general, the desired type of intensity distribution is one that
fulfils the eye-safety criteria within a given angular field of
emission, and that emits nothing outside of it (so as not to lose
laser power). This type of sharp distribution cannot be achieved
with diffusers, which necessarily emit some power outside of the
design angular field and, moreover, cannot maintain a constant
intensity within that field. The overall result is a loss of
transmission efficiency of the laser radiation.
SUMMARY OF THE INVENTION
[0011] The present invention is proposed in consideration of the
above-described circumstances, and an object of the present
invention is to provide a light source device such that the
divergent angle and width of the light bundle are controlled, and
the monochromatic or polychromatic incoherent light bundle having a
short switching time is emitted.
[0012] To solve the above-described problem, a light source device
of the present invention Comprising a laser light source, a
diffuser for diffusing a light emitted from the laser light source,
and a collimator for collimating a light diffused by the diffuser
and emitting it.
[0013] Specifically, a light source device of the present invention
comprising one or more laser diodes which are laser light sources,
one or more diffusers, a collimation optical system for collimating
a light emitted from one or more diffusers. The collimator optical
system is referred to as a collimator. The diffuser may be any one
of a transmissive diffuser through which laser bundle transmits and
are diffused, a reflective diffuser on which laser bundle is
reflected and diffused, or a mixture diffuser which combines these
diffusers. A reflective diffuser is simply a reflector whose
diffuse reflection coefficient is higher than its mirror surface
reflection coefficient. A laser bundle is emitted toward the
diffuser. A light source device may have an optical focusing system
for making the diffuser focusing the laser bundle. Using an optical
focusing system, control of distribution of irradiance on the
diffuser is easily performed. After the laser bundle is diffused by
the diffuser, the laser bundle is collimated by the collimator.
Usually, the diffuser is disposed on the focal plane. In this case,
the intensity distribution of the entire device is determined by
the distribution of irradiance in the diffuser. Therefore, the
shape and value of intensity distribution of the light source
device can be controlled by appropriately designing the optical
focusing system of the laser bundle. Moreover, outgoing angle of
view can be controlled by employing a diaphragm which limits
illuminated area in the diffuser, or by limiting a diffuser
area.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 is a schematic diagram of the light source
device.
[0015] FIG. 2 is a diagram of an example of the light source
device.
[0016] FIG. 3 is a diagram of another example of the light source
device without optical focusing system.
[0017] FIG. 4 is a diagram of another example of the light source
device using one of the collimators described in U.S. provisional
patent application No. 60/190,130.
[0018] FIG. 5 is a diagram of another example of the light source
device using the so-called RXI collimator described in U.S.
provisional patent application No. 60/190,130.
[0019] FIG. 6 is a diagram of another example of the light source
device in which the collimator and the diffuser form a single-solid
piece.
[0020] FIG. 7 is a scheme of Cartesian oval of reflection when
there are no optical elements between the oval and point 7 and
point 8.
[0021] FIG. 8 is a diagram of system of coordinates x, y, z given
by Cartesian axis 10, 11 and 12.
[0022] FIG. 9 is a diagram of polyhedral lens on completing the
iteration i=2.
[0023] FIG. 10 is a diagram of polyhedral lens in which the x
coordinate (axis 10) of point 7 is 8 mm and the value of L in
equation 8 is L=11.16 mm.
[0024] FIG. 11 is a schematic graph displaying the result of
calculation of the irradiance obtained in the plane of the diffuser
(in arbitrary units), obtained through ray tracing.
[0025] FIG. 12 is a diagram of another example of the light source
device containing various laser devices with different
wavelengths.
[0026] FIG. 13 is a diagram showing a function of the integrating
sphere acting as a diffusing element.
[0027] FIG. 14 is a diagram of another example of the light source
device employing a concave reflective diffuser surface.
[0028] FIG. 15 is a diagram of another example of the light source
device employing a set of microlenses instead of a transmissive
diffuser.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] Hereinafter, a light source device of embodiments of the
present invention will be described with reference to the
drawings.
[0030] The present invention is a light source device comprising a
laser light source for emitting monochromatic light or
polychromatic light, a diffuser for diffusing the light bundle
emitted from the laser light source and a collimated optical system
referred to as a collimator which collimates the light bundle
emitted from the diffuser, which is safe for human eyes and whose
switching is performed at high speed. As a diffuser, a transmissive
diffuser, a reflective diffuser, or a mixture diffuser which
combines the transmissive diffuser and the reflective diffuser can
be used, a laser light source directly emits the light bundle
toward the diffuser, or injects the light bundle into the diffuser
via the optical focusing system.
[0031] FIG. 1 is a schematic view showing a light source device
applying the present invention.
[0032] The light source device comprising a laser light source 1
for emitting a laser bundle, an optical focusing system 2 for
focusing the light bundle injected from the laser light source 1, a
transmissive diffuser 3 for diffusing the light bundle focused by
the optical focusing system 2, and a collimated optical system 4,
that is, a collimator for collimating the light bundle diffused by
the diffuser 3.
[0033] FIG. 2 is a diagram showing a concrete example of a light
source device applying the present invention.
[0034] This light source device comprising the laser light source
1, the optical focusing system 2 consisted of a single lens, the
transmissive diffuser 3 and the collimation optical system 4
consisted of a signal lens.
[0035] It is not always necessary to employ an optical focusing
system for collimating the laser bundle. This is because it is
possible to adjust the distance from the laser light source to the
diffuser in such a way that the illuminated area of the diffuser is
precisely required by the collimator.
[0036] FIG. 3 is a diagram showing a concrete example of a light
source device which does not have an optical focusing system.
[0037] This light source device comprising the laser light source
1, the reflective diffuser 3, and the collimated optical system 4
consisted of a parabolic mirror. In this light source device, the
laser bundle emitted the laser light source 1 directly injects into
the reflective diffuser 3 without going through the optical
focusing system.
[0038] The diffuser does not necessarily have to be a plane
perpendicular to the central direction of the laser bundle. In
fact, when the diffuser is on a flat surface whose normal forms any
angle with the central direction of the incident laser bundle it is
possible to correct the astigmatism common to many semiconductor
lasers.
[0039] The diffusers that are easiest to make, and consequently the
least expensive, are Lambertian or quasi-Lambertian diffusers. The
radiation impinging at a point of a diffuser of this type is
diffused isotropically on one of the two hemispheres defined by the
plane tangent to the diffuser at that point. If the diffuser works
by reflection, then the radiation is diffused on the hemisphere
that contains the direction of incidence on the diffuser. If the
diffuser works by transmission, then the radiation is diffused on
the other hemisphere. In general, it is difficult to achieve a
highly efficient transmissive diffuser, since part of the radiation
diffused is reflected. An efficient reflection diffuser is easier
to achieve. For these cases it is very useful to employ as
collimators the optical systems described in "Method of Design and
Apparatus Derived From Method for Ultra Compact, High Efficiency,
Optical Non-imaging Concentrators, Collimators and
Couplers"--Provisional Patent Application (PPA) No. 60/190,130
filed with the US Patent and Trademark Office (USPTO) on Mar. 16,
2000. These optical systems are highly efficient for collimating
isotropic radiation, which is the case in question when the
diffuser is Lambertian. Moreover, they are very simple and
compact.
[0040] FIG. 4 is a diagram showing a concrete example of a light
source device employing one collimator according to the
above-described U.S. Provisional Patent Application No.
60/190,130.
[0041] This collimator is called the RX. The device of FIG. 4
consists of the laser bundle emitter 1, the optical focusing system
2, formed in this case of a single lens, a reflection diffuser 3,
and finally, the RX 4, which constitutes the optical system of
collimation. The RX 4 is formed of a single solid piece with a
reflective surface 5 and a refractive surface 6. The radiation
diffused by 3 is reflected by 5 and refracted by 6, so that it
exits collimated. In the example of FIG. 4, the optical focusing
system 2 is incorporated in the collimator 4, so that the diffuser,
the optical focusing systems and the optical systems of collimation
form a single solid piece. In this example it can also be seen that
the solid piece has some of its surfaces coated with a specular
reflector and the diffuser is attached to another of the
surfaces.
[0042] FIG. 5 is a diagram showing a concrete example of a light
source device employing the RXI collimator according to the
above-described U.S. Provisional Patent Application No.
60/190,130.
[0043] This light source device comprising the laser light source
1, the optical focusing system 2 consisted of a single lens, the
transimissive diffuser 3 and the RXI 4 constituting the collimation
optical system. The RXI is formed of a single solid piece with a
reflective surface 5 in the rear part, as well as a small specular
reflective area at the front, and a refractive surface 6 that also
acts as a reflector through internal reflection. The radiation
diffused by 3 is reflected by the frontal part of 5 and by the
surface 6, which now acts as a reflector by internal reflection.
After this reflection the radiation is reflected by the rear part
of 5 and refracted by 6, so that it exits collimated. In the
example of FIG. 5, the diffuser and the optical system of
collimation form a single solid piece. Also in this example it can
be seen that the solid piece has some of its surfaces coated with a
specular reflector and the diffuser is attracted to another of the
surfaces.
[0044] FIG. 6 is a diagram showing a concrete example of a light
source device constituting a single constructed body that is
configured by the diffuser and the collimation optical system.
[0045] This light source device comprising the laser light source
1, the optical focusing system 2 consisted of a single lens, the
transmissive diffuser 3, and the RXI 4 constituting the collimation
optical system.
[0046] By way of an example, suppose that it is desired to effect a
communication link at wavelength .lambda.=780 mm with a Class 1
security level. This is the maximum-security level, which
guarantees that the laser product is safe in all reasonably
foreseeable conditions of use. For this link it is estimated that
time of exposure to a human eye may reach the maximum allowed for
in the regulations CENELEC EN 60825-1 (CEI 825-1:1993),
JISC6802:1997 and IEC60825-1:1993(t=30000s), and the emitter is
required to have an angular divergence of .theta.=60 mrad.
[0047] It is aimed to compare the maximum permissible power on exit
from the emitter in the following two cases: (1) if the emitter
consists simply of a laser and a conventional lens for adjusting
its divergence to the specified value .theta., (2) if the emitter
of FIG. 4 is used, with an exit aperture diameter of 20 mm and a
Lambertian diffuser. In order to make the comparison, it is
supposed that both produce a far field diagram of Gaussian
intensity with rotation symmetry, with angular divergence
.theta.=60 mrad as defined in the regulations CENELEC EN
60825-1(CEI 825-1:1993), JISC6802:1997 and IEC60825-1:1993: a cone
centered on the emitter that subtends a complete angle of e=60 mrad
encloses 63% of the far field power radiated by the emitter.
[0048] Regulations CENELEC EN 60825-1 (CEI 825-1:1993),
JISC6802:1997 and IEC60825-1:1993 establish not only the maximum
safe exposure level, but also the measurement conditions of this
exposure level. For Class 1 safety level, t=30000 s and
.lambda.=780 nm, the power measurement should be taken at a
distance r=10 cm from the emitter. Both the size of the sensor that
measures this power and the maximum value of this power (called
Accessible Emission Limit, or AEL) depend on the angular size of
the source, .alpha., at a distance r=10 cm from the emitter. This
angular size of the source coincides with its angular divergence e
in the emitter of case (2), whilst for the emitter in case (1) ,
assuming the Gaussian laser bundle, it is given by the angular size
that produces maximum narrowing of the bundle at that distance.
Using the basic relationship between the divergence and maximum
narrowing of the Gaussian bundle, we obtain: 1 ( emitter 1 ) = 2
tan - 1 ( r ) = 41.3 rad ( emitter 2 ) = = 60 m rad ( 1 )
[0049] According to regulations CENELEC EN 60825-11 (CEI
825-1:1993), JISC6802:1997 and IEC60825-1:1993, sources with values
of .alpha.<1.5 rnrad (which includes the case of emitter 1) are
equivalent from the point of view of safety for all exposure
conditions. This is due to the combined effect of the resolution of
the human eye and its unconscious natural movements (which prevent
the point of focus on the retina from remaining stationary).
[0050] The maximum power values to be measured in the conditions
indicated by the regulations are given by:
AEL=1.2*10.sup.-4*C.sub.4*C.sub.6(W) (2)
[0051] where
C.sub.4=10.sup.0.002.times.(.lambda.(nm)-780)=1.445 (3)
[0052] and 2 C 6 ( emitter 1 ) = 1 C 6 ( emitter 2 ) = ( m rad ) 11
m rad = 60 11 = 5.455 ( 4 )
[0053] Therefore:
AEL(emitter1)=173.4 .mu.W
AEL(emitter2)=945.9 .mu.W (5)
[0054] In accordance with modifications to the regulations CENELEC
EN 60825-1/A11:1996, JISC6802:1997 and IEC60825-1 Amendament1:
1997, the measurement should be carried out in each case with a
sensor of the following diameter d: 3 d ( emitter 1 ) = 50 mm d (
emitter 2 ) = ( 7 mm ) 100 ( m rad ) + 0.46 = 9 mm ( 6 )
[0055] All of the power emitted by emitter 1 is collected by the
sensor (since at r=10 cm, width of the divergent bundle with
.theta.=60 mrad is around 3 mm). Emitter 2 verifies that the
irradiance it produces at any point of its exit aperture is
constant and equal to that received by the sensor situated at r=10
cm. Taking this into account, together with equations (5) and (6),
it is obtained that the power emitted by emitters 1 and 2 is
limited by the values:
P.sub.OUT<173.4 .mu.W (emitter1)
P.sub.OUT<4.67 .mu.mW (emitter2) (7)
[0056] As it can be seen in equation (7), the emitter proposed in
this patent (emitter 2) permits, in this example, working with
Class 1 safety level with powers 27 times higher than a
conventional emitter (emitter 1) of equal angular divergence and
safety level.
[0057] In general, it is required that the intensity (power
radiated per unit of solid angle) emitted by the collimator is the
maximum permissible within a given angular region. The reason for
this is that if for any direction within the angular region of
interest that intensity is lower than the permitted maximum, then
in that direction there is less emission, but with no increase in
safety for the human eye. When the diffuser is situated in the
focal plane of the collimator, the requirement that the intensity
emitted by the collimator is constant within an angular region is
equivalent to requiring that the irradiance produced by the laser
beam on the diffuser (through the focusing optical system) is also
constant for a given area of the focal plane.
[0058] In order to achieve an approximately constant irradiance on
the diffuser surface, conventional optics components can be used.
In general, semiconductor lasers have a radiation diagram that
varies greatly from one unit to another. This hinders enormously
the design of an optical focusing system valid for all units, if
what is required is that this focusing system produces a constant
irradiance and that it is efficient from the energy point of view.
That is, if in addition to the constant irradiance on the diffuser
it is required that a large proportion of the power emitted by the
laser is that which illuminates the diffuser with constant
irradiance. If uniformity of the irradiance is more important than
energy efficiency, then it is advantageous to use polyhedral
lenses. These are lenses which have one ore more refractive
surfaces with a polyhedral shape.
[0059] Polyhedral lenses have the advantage of being able to
provide an almost uniform irradiance on the diffuser surface,
regardless of the radiation diagram. It is sufficient for any point
of the lens to be at a large distance from any point of the area of
the diffuser in which it is required to achieve uniform irradiance
and that the irradiance produced by the laser bundle does not vary
greatly within the points of a single face of the polyhedral
surface. In order to know when the distance is sufficiently large
or when the irradiance varies little within a face of the
polyhedron, it is necessary to know how uniform the irradiance on
the diffuser must be. The polyhedral lens can be satisfactorily
used with laser diodes whose radiation diagram is widely dispersed
in the manufacturing process. There follows a description of how to
design one of these polyhedral lenses. The same basic procedure can
be followed to design polyhedral mirrors.
[0060] First of all, it is necessary to identify the area of the
diffuser in which it is required to obtain uniform irradiance. This
area will be referred to as the active area of the diffuser. Choose
a central point of this area as the central point of the
diffuser.
[0061] Next, select the distance D between the waist of the laser
bundle (point 7) and the central point of the diffuser (point 8).
High values of D produce greater uniformity of irradiance, though,
in general, they imply greater size.
[0062] Subsequently, design a Cartesian oval that focuses point 7
on point 8. The design of the Cartesian oval is simple: it is
sufficient to establish that the optical path between point 7 and
point 8 is constant for the rays that are refracted (reflected for
mirrors) on the surface of the Cartesian oval. If x, y, z are the
coordinates of a point of the oval, the equation of the surface of
the oval is
L.sub.1(x, y, z)+L.sub.2(x, y, z)=L (8)
[0063] Where L1(x, y, z) is the optical path between point 7 and
the point of the oval given by the coordinates (x, y, z) and L2 (x,
y, z) is the optical path between the point of the oval given by
the coordinates (x, y, z) and point 8. L is a constant that
establishes the total optical path between A and B. The value of L
is selected at the beginning of the procedure. Different values of
L will permit greater or les ser uniformity of illumination in the
plane of the diffuser, since they will give rise to lenses (or
mirrors) more or less separated from the diffuser or from the
laser. In a Cartesian oval of refraction not the entire surface
defined by equation (8) is Cartesian oval (for a fuller explanation
of this concept, see, for example: O. N. Stravoudis. "The Optics of
Rays, Wavefronts and Caustics". Academic Press, London 1972). In
this case the surface given by equation (8) use to be a closed
surface, so that the straight lines on which the rays rest cut this
surface at least twice. They are Cartesian oval points the points
of the surface given equation (8) for which the ray coming from
point 7 forms with the normal to the surface (at that point) an
angle whose cosine has the same sign as the cosine of the angle
formed by that normal with the ray that goes to point 8.
[0064] FIG. 7 is a diagram showing a Cartesian oval line of
refraction in the case where there are no optical elements between
the oval line and the points 7 and 8.
[0065] The points of the surface in the Cartesian oval line 9 are a
subset of the points given by the equation (8).
[0066] FIG. 8 is a diagram showing the coordinate system x, y and z
given by the orthogonal axes 10, 11 and 12.
[0067] Any point 15 of the surface of the oval line that focuses
the rays of point 0.7 on the point 8 can be defined by the angles
13 .theta. and 14 .phi.. The plane tangent to the oval line that
passes through a predetermined point is also univocally defined by
angles 13 and 14. Now assume that this plane is a refractive
(reflective) surface. By using ray tracing we can identify the area
of this plane through which rays emitted from 7 and eventually
reaching the active area of the diffuser. This area is referred to
as an active area corresponding to planes 13 and 14. The procedure
for calculating the faces of the polyhedral surface that form the
lens will be described below:
[0068] 1. Calculate the active area that corresponds to a plane
with a given direction, for example, that corresponding to the
angle 13=0. The polyhedron is now formed by a single plane.
Establish the value of the variable i=0.
[0069] 2. Increase i by one unit.
[0070] 3. Next, increase angle 13 by a small value (for instance
.pi./20) and choose the number n of faces that will be the number
of faces of the row i. These faces will be tangent to the oval at
points of the oval that have an angle 13 equal to the value just
increased, which will be called .theta..sub.i. Choose an arbitrary
value .phi..sub.0i that lies between 0 and 2.pi./n.
[0071] 4. Calculate the active areas corresponding to the values of
the angle 13=.theta..sub.i and angle 14=.phi..sub.0i+j2.pi./n where
j=0,1, . . . , n-1.
[0072] 5. Calculate the polyhedron Pi formed by the faces contained
in the planes corresponding to the angle 13=.phi..sub.i and angle
14=.phi..sub.0i+j2.pi./n where j=0, 1, . . . , n-1.
[0073] 6. If the active areas calculated in step 4 are not
contained in the surface of the polyhedron, return to step 3 and
take a smaller integer value for n until achieving that these
active areas are contained in the polyhedron P.sub.i. In order to
achieve greater efficiency of the lens (or mirror) it is desirable
that the active areas are tangent to the sides of the polyhedron,
or that they approximate to this situation.
[0074] 7. Form the polyhedron T.sub.i with all the faces calculated
up to now. If the active areas calculated in step 4 are not
contained in the surface of the polyhedron T.sub.i, return to step
3 and take a lower value for the increase of angle 13 or modify the
value of .phi..sub.0i until achieving that these active areas are
contained in the polyhedron T.sub.i. In order to achieve greater
efficiency of the lens (or mirror) it is desirable that the active
areas are tangent to the sides of the polyhedron, or that they
approximate to this situation.
[0075] 8. Go to step 2 until either the surface of the polyhedron
T.sub.i intercepts the majority (80% or more) of the power emitted
by the laser bundle, or until no further advance can be made
because on increasing angle 13 in step 3, points are obtained that
do not belong to the Cartesian oval.
[0076] 9. Calculate by ray tracing the irradiance produced by the
laser and the lens on the active surface of the diffuser and the
efficiency of the polyhedral surface (power collected on the active
surface of the diffuser divided by power emitted by the laser),
without taking into account interference effects.
[0077] 10. If the uniformity of the irradiance in the active area
is not sufficient, make a new design situating the diffuser or the
laser further away from the Cartesian oval. If the efficiency is
insufficient, choose values greater than n or lower values for the
increase of angle 13 in step 3. In general, it is not possible to
achieve efficiencies of 100% or absolutely uniform irradiances, and
a compromise solution must be reached. Finally, it is possible to
discard some of the designed faces either because they are
inconvenient from a mechanical point of view or because their
contribution to uniformity and efficiency is not particularly
significant. After this discarding it is advisable to make a new
analysis.
[0078] FIG. 9 is a diagram showing a constitution of a polyhedral
lens on completing the iteration i=2.
[0079] The refractive surface of the lens 17 delimits the region of
high refractive index, which corresponds to low values of the axis
x 10 of the region of low refractive index, which corresponds to
high values. This surface is formed of flat faces. FIG. 9 also
shows the active area 18 of each face, which looks oval. This is
the area of the flat face through which pass rays coming from point
7 that will fall into the active area of the diffuser 16.
[0080] FIG. 10 is a diagram showing an actual constitution of
polyhedral lens in the case where the x axis (axis 10) of the point
7 is 8 mm, and the value of L in the equation (8) is L=11.16
mm.
[0081] The refractive index of the lens is 1.48.
[0082] FIG. 11 is a diagram showing the results of calculating the
irradiance obtained in the plane of the diffuser (in arbitrary
units), obtained through ray tracing without taking into account
interference effects.
[0083] It is not necessary to begin with the plane corresponding to
angle 13=0 (step 1). The design can also be carried out limiting
step 1 to establishing i=0.
[0084] The above procedure is valid for both polyhedral lenses. and
mirrors. When the focusing system of the laser bundle is made up of
an optical system that contains at least one polyhedral surface
that works by reflection or by refraction, then it is possible to
achieve uniformity of irradiance on the surface of the
diffuser.
[0085] FIG. 12 is a diagram showing a concrete example of a light
source device including several laser light source having different
wavelengths focused on the same area of the diffuser, in such a way
that the radiation diffused and the radiation emitted by the
collimation optical system are those of the color resulting from
the mixing of the radiations of the different lasers.
[0086] The light source device comprises three laser devices having
different wavelengths 1, 19 and 20, focusing the laser bundle
emitted by the devices on reflective diffuser 3 through 23 focusing
system 2.
[0087] The interest of this application is in the field of
illumination. By means of the different lasers it is possible to
achieve, for example, the color white, and through appropriate
design it is possible to achieve that its radiation is safe for the
human eye. Laser devices, as against LED devices, present the
advantage of greater efficiency in the conversion of electrical
energy into light energy, although at present their cost is far
higher.
[0088] Sometimes it is interesting to utilize a device that makes
use of a diaphragm for delimiting the area of the diffuser that
emits radiation towards the collimator. This diaphragm may define a
circular outline of an outline of any other type, and mayor may not
have a variable aperture. If the optical system of collimation is
such that the diffuser is situated in its focal plane, then the
angular field of emission of the laser radiation diffused will also
be delimited through the use of a diaphragm. If, moreover, the
aperture of this diaphragm is variable, it is possible to vary the
angular field of emission accordingly.
[0089] In addition to polyhedral lenses, there exists at least one
other simple possibility for improving the uniformity of the
irradiance emitted by the diffuser. This possibility consists in
the use of concave surfaces such that the incident laser radiation
undergoes several diffuse reflections before leaving the diffuser
and moving towards the collimator. This is the principle of the
integrating sphere or Helmholtz sphere (for more information on
this concept, see, for example, J. C. Miano "Optical confinement in
Photovoltaics" in Physical Limitations to Photovoltaic Energy
Conversion". A. Luque, G. L. Araujo (Editors). Adam Hilger, Bristol
1990).
[0090] FIG. 13 is a diagram showing a function of the integrating
sphere acting as a diffusing element.
[0091] The sphere is coated in its interior with diffusive
reflective material (for example, white paint), and has an aperture
through which radiation enters and exits. The incident laser
radiation 21 is reflected in the interior of the sphere. Part of
this reflected diffuse radiation 22 is reflected again in the
interior of the sphere, so that the radiation that leaves through
the aperture 23 improves the uniformity of irradiance with respect
to the case in which the sphere is not used. In order to achieve
that the irradiance on exit from the diffuser is uniform, it is
enough for the surface to be concave. This idea is also applicable
to transmissive diffusers. In the transmissive diffuser there is,
in general a part of the radiation that it is reflected. This part
is lost in a flat diffuser but it can be used to increase
uniformity in a concave transmissive diffuser. When the surface of
the diffuser is a concave surface, as seen from the incident laser
beam, some increase in uniformity is achieved with respect to the
case in which the surface is flat or convex. The increase is higher
the greater the ratio between the area of the diffuser and the area
of its aperture.
[0092] FIG. 14 is a diagram showing a concrete example of a light
source device employing a concave reflective diffuser surface.
[0093] This light source device employs an integrating sphere as
the concave reflective diffuser 3 in the light source device shown
in FIG. 4.
[0094] In all of the previous cases the diffuser can be substituted
by a set of microlenses or micromirrors. If the focal distance of
each one of these microlenses or micromirrors is small compared to
the maximum diameter of the active area of the diffuser, then an
effect similar to that of the diffuser can be achieved. However, it
should be noted that the treatment given by the regulations CENELEC
EN 60825-1, CENELEC EN 60825-2 (CEI825-2:1993), JISC6802:1997 and
IEC60825-2:1993 to this case is different from that given to the
case in which a diffuser is used, and that, in general, it is not
as beneficial from the point of view of improving safety for the
human eye, unless the number of microlenses or micromirrors is very
large, that is, when each one of these microlenses or micromirrors
intercepts a small fraction of the power transported by the
incident laser bundle. When the diffuser is substituted by a set of
microlenses or micromirrors in such a way that each one of these
microlenses or micromirrors intercepts a fraction not exceeding 10%
of the power emitted by the laser bundle it is achieved that the
power of the laser bundle that must be considered in the
regulations cited above is reduced.
[0095] FIG. 15 is a diagram showing a concrete example of a light
source device employing a set of microlenses instead of a
transmissive diffuser.
[0096] The light source device comprises a set of microlenses
24.
[0097] The laser radiation 21 impinges on the set of microlenses 24
with short focal distance. On leaving the microlenses the radiation
25 has greater angular dispersion. This radiation is directed
towards the collimator.
[0098] When the focal plane of the microlenses or micromirrors
coincides with the focal plane of the collimator, the object of the
present invention can be used as an emitting source for multi-spot
configurations in wireless optical communications (these
configurations are described, for example, in S. Jivkova and M.
Kavehrad, "Multi-spot diffusing configuration for wireless infrared
access; joint optimization of multi-beam transmitter and angle
diversity receiver", in Optical Wireless Communications II, Eric
Korevaar, Editor, Proceedings of SP1E Vol. 3850, pp 72-79), since
the effect it has is to separate the incident laser in to several
laser bundles of lower intensity (as many as there are illuminated
microlenses or micromirrors).
[0099] Another way of achieving a spectral composition on exit
different from the spectral composition of the entry laser bundle
is to use phosphorescent or fluorescent materials that absorb the
incident radiation and reemit it at other wavelengths. When the
diffuser is formed of a florescent or phosphorescent material that
reemits in diffuse form incident laser radiation, and which also
modifies its spectral composition, then the radiation that
illuminates the collimator, and therefore that which leaves the
device that is the object of this invention, has a different
spectral composition from that of the incident laser. In this way
it is possible, for example, to achieve that the object of this
invention emits white light when the laser emits monochromatic
(blue or ultra-violet) radiation.
[0100] The optical system that is the object of the invention can
be manufactured by turning on a diamond-tip lathe with numerical
control (CNC) using a plastic material such as PMMA. The reflective
diffuser can be manufactured by coating a white color coating on
the substrate, the transmissive diffuser can be manufactured by
treating the surface chemically or mechanically (for example, with
an abrasive and the like), or by adding spherical transparent
material in a size of several--several tens of .mu.m, that is, by
employing a transparent substrate having a matt finish surface (for
example, glass or PMMA).
[0101] The invention presented here has direct application in a
variety of fields. In general, it can be used as a laser radiation
emitting system in all applications in which the laser device
produces irradiance greater than desired, so that it is necessary
to reduce it without having to renounce the collimation of the
bundle. In particular, it can be used as a commuted laser radiation
source in a wireless optical communication system, or as a source
of continuous radiation for illumination (both within the visible
spectrum and outside of it), as well as in medical
applications.
[0102] As described above, according to the present invention, a
light source device which employs a laser diode as a light source
is safe for human eyes, and its switching is performed at high
speed can be provided. This light source device is preferably used
for a wireless optical communication.
[0103] 1. Laser device
[0104] 2. Optical device for focussing the laser beam on the
diffuser
[0105] 3. Diffuser
[0106] 4. Optical device for collimating the diffused beam
[0107] 5. Reflector
[0108] 6. Refractive surface
[0109] 7. Laser beam waist
[0110] 8. Diffuser central point
[0111] 9. Cartesian oval surface
[0112] 10. x axis
[0113] 11. y axis
[0114] 12. z axis
[0115] 13. angle .theta.
[0116] 14. angle .phi.
[0117] 15. A point of the oval
[0118] 16. Diffuser active region
[0119] 17. Polyhedral lens
[0120] 18. Active region of a face of the polyhedron
[0121] 19. Laser device emitting radiation of wavelength
.lambda..sub.2.
[0122] 20. Laser device emitting radiation of wavelength
.lambda..sub.2.
[0123] 21. Laser beam
[0124] 22. Reflected diffuse radiation
[0125] 23. Diffuser aperture
[0126] 24. Microlens array
[0127] 25. Radiation exiting the microlens array
* * * * *