WO2007072334A1

WO2007072334A1 - Rod integrator that reduces speckle in a laser-based projector

Info

Publication number: WO2007072334A1
Application number: PCT/IB2006/054821
Authority: WO
Inventors: Oscar Hendrikus Willemsen; Marcellinus Petrus Carolus Michael Krijn
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2005-12-19
Filing date: 2006-12-13
Publication date: 2007-06-28

Abstract

The present invention provides an optical component (100 700 800 900), projection engine (200-300), and method that reduces laser speckle and at the same time ensures homogeneous illumination of a projected image. By combining a laser with a rod integrator (100 700 800 900), part of the laser light can be forced to pass through the rod integrator (100) several times before exiting and hitting a spatial light modulator, such as a transmissive LCD panel. When the length of the rod integrator (100 700 800 900) is in the order of the coherence length of the laser or larger, significant speckle reduction is obtained, while at the same time the illumination profile is homogenized.

Description

ROD INTEGRATOR THAT REDUCES SPECKLE IN A LASER-BASED

PROJECTOR

The present invention relates to the reduction of speckle in laser-based projection. More particularly, the present invention is related to a rod integrator that reduces laser speckle and at the same time ensures homogeneous illumination of projected images.

At present the Ultra High Pressure (UHP) lamp is the most established light source for rear and front projection applications, since it combines a high lumen efficacy with a high source brightness at affordable cost. In the last few years solid-state light source technology has improved so much that it is expected to compete with UHP technology. This is because solid- state light sources offer some unique advantages such as high color purity, fast optical response and mercury free operation.

The most mature solid-state light source technology applicable to displays is the high brightness LED. This is because high brightness LEDs are available in all display primaries at low cost, with high lumen efficacy and with a small form factor. However, since the light output of an LED is rather low and since the etendue is comparable to that of an UHP lamp, a projector based on LEDs has low lumen output and moderate size. Hence, such projectors cannot (yet) compete with UHP lamps on applications that require large screen size, but they can be used very well in new application areas such as handheld and mobile projection. However, it remains unclear whether LED based projectors can keep up with the ever-increasing demands of smaller size and higher lumen output.

Another type of solid-state light source, a laser, has extremely high source brightness combined with a very small etendue. In fact, it can be considered as a point source and this enables the construction of a smallest possible light engine for a projection display. In addition, lasers are available in output powers that can range several Watts, thus enabling high lumen output.

Keeping the above in mind, it is expected that lasers will become the ultimate light source for all types of projection applications. However, there are still some issues that impede the application of lasers in projection displays. The most important of these issues are cost, availability in green and blue, lumen efficacy and laser speckle. Although the former three issues can be expected to be solved in the foreseeable future, the speckle issue presents a more difficult problem to solve. This is because laser speckle is related directly to the coherence of the light and, hence, solving the speckle problem of laser-based projection systems requires a major change of laser architecture. This is very challenging, especially when small size and low cost are required. There is yet another problem associated with laser illumination. Almost all lasers emit light in the TEMoo mode, which has a Gaussian intensity distribution. The image projected, on the other hand, should be as homogeneous as possible. Although this problem can be solved with a careful design of the illumination optics of a projection engine, this type of solution requires additional components and/or size of the projection engine.

The present invention provides an optical system, component and method that reduce laser speckle and at the same time ensure homogeneous illumination of a projected image. In a preferred embodiment of a laser-based projection system, by combining a laser with a rod integrator, part of the laser light can be forced to pass the integrator several times before hitting a spatial light modulator, such as a transmissive LCD panel. When the length of the integrator is one fifth or greater than the coherence length of the laser, significant speckle reduction is obtained while at the same time the illumination profile is homogenized.

FIG. 1 illustrates a first preferred embodiment of a rod integrator, according to the present invention;

FIG. 2 illustrates the position of the rod integrator component, according to the present invention, with respect to the display panel in projection engine systems with proximity illumination;

FIG. 3 illustrates the position of the rod integrator component, according to the present invention, with respect to the display panel in projection engine systems using relay optics when the display panel is of the transmissive type (architectures with reflective or refractive types of light engines are possible also);

FIG. 4 illustrates a rod integrator component, according to the present invention;

FIG. 5 illustrates a rod integrator component with the relevant light fluxes in the device, according to the present invention; FIG. 6 illustrates a graph of the transmission of the rod integrator component, according to the present invention, as a function of the reflectivity of the exit face;

FIG. 7 illustrates a second preferred embodiment of a rod integrator component, according to the present invention;

FIG. 8 illustrates a third preferred embodiment of a rod integrator component, according to the present invention; and

FIG. 9 illustrates a fourth preferred embodiment of a rod integrator component, according to the present invention.

It is to be understood by persons of ordinary skill in the art that the following descriptions are provided for purposes of illustration and not for limitation. An artisan understands that there are many variations that lie within the spirit of the invention and the scope of the appended claims. Unnecessary detail of known functions and structure may be omitted from the current descriptions so as not to obscure the present invention.

The present invention provides a system, apparatus and method to reduce speckle and homogenize the illumination pattern in a laser-based light engine for a projection display. By using a rod integrator to homogenize a Gaussian distributed laser beam, a more or less uniformly distributed profile is obtained at the exit of the integrator. By ensuring that the laser beam, on the average, reflects inside the integrator several times before it leaves the exit face of the integrator, significant speckle reduction can be obtained. This is only possible if the length of the integrator is in the order of the coherence length of the laser light used or higher. The homogenized light distribution at the exit face is used to illuminate a display panel by means of relay optics or proximity illumination. The details of the invention are illustrated in FIGs. 1-3.

In FIG. 1 a rod integrator is illustrated that can be made of glass or a transparent plastic. The entrance plane of the integrator has been covered with a layer for high reflectivity, for instance silver or a dielectric stack, in which a hole has been made. The hole is of such a diameter that laser light, which has an extremely low etendue and thus can be focused into a very small spot, can pass through the hole without significant light loss or diffraction effects. As an example, a laser beam has been drawn that is focused such that it enters the integrator with a half angle θ . A beam with such a half an le can be focused to a diameter d that is given by:

It should be noted that the diameter of the hole 102 must exceed this size by at least a factor of two to minimize diffraction effects. The light that hits the sidewalls of the integrator will have an angle that is larger than the critical angle and will be reflected by total internal reflection. After reflection it will hit the exit face of the integrator. This exit face has been covered by a dielectric stack, which is dimensioned such that it has a transmission coefficient T_c that is significantly lower than unity and a reflection coefficient R₀ = 1 - T_c. Hence, at the interface, part of the light is transmitted and part is reflected back into the integrator. The reflected light will travel back to the entrance face, at which it is almost completely reflected for a second pass in the integrator. The net effect is that light exiting the integrator at the exit face, will be composed of light that has passed the integrator 1 time, 2 times and so on. If the coherence length of the integrator is smaller than or on the order of the integrator length, the light contributions will be incoherent and significant speckle reduction is obtained.

FIG. 2 illustrates the position of a rod integrator component (100), according to the present invention, with respect to a display panel (201) of a projection engine employing proximity illumination.

FIG. 3 illustrates the position of a rod integrator component (100), according to the present invention, with respect to the display panel (201) in projection engine systems using relay optics (301) when the display panel is of the transmissive type (architectures with reflective or refractive types of light engines are possible also).

A first preferred embodiment is illustrated in FIG. 4. The rod integrator is the same as the one depicted in FIG. 1, but the half angle of the beam entering the integrator is smaller. This is equivalent to a beam having a higher F/#. A person skilled in the art may consider this first embodiment as being quite counterintuitive, since a conventional rod integrator uses reflection at the sidewalls to homogenize the light. The reason for choosing this particular combination of beam and integrator is discussed in the following sections. Arguably, one disadvantage of combining a laser source with a rod integrator is a severe increase of the etendue of the system. As is well known in the art, the etendue of a laser is extremely low, so this will also be the case for the beam that enters the integrator. Although the beam will experience multiple reflections in the integrator, the angular distribution of the beam will not change. However, the area, under which it emits light, will be much larger, resulting in a larger etendue. In fact, it has grown by a factor that is equal to the ratio of the area of the exit surface of the integrator to the area of the entrance hole. By choosing a beam entering the integrator with a large F/# (in the drawing F/5 is chosen, but also F/ 10 should be possible) still an efficient illumination system is obtained. Since the etendue of the beam at the exit of the integrator is still much smaller than the etendue of conventional projection systems based on UHP lamps or LEDs, the following advantages apply for the complete projection engine:

• The acceptance aperture of the spatial light modulator can be made smaller, resulting in smaller panels that are cheaper;

• The projection lens can be designed for a larger F/# and can thus be made smaller and cheaper;

• The depth of focus is much larger which is especially important for mobile projection on tilted and curved surfaces.

The use of multiple passes in a rod integrator inevitably will result in a decrease of the optical throughput. An estimate of the light loss to be expected is provided below.

The transmission through the rod integrator is now described.

The light flux at different positions in the rod integrator (100) is indicated in FIG. 5. In FIG. 5 the following symbols are defined: φ_o(r) is the light flux that enters the integrator through the entrance hole 501 with relative area r (i.e. the fraction of the total area). Of course, the light flux is a function of the radius of the hole. Inside the integrator 500 the light flux going from left to right is defined as φ_λ and the light flux going from right to left as ψ_λ . Finally, the light flux leaving the integrator is defined as φ_τ .

The reflection coefficient of the side walls is defined as unity because of total internal reflection, the reflection of the entrance face as R₁, the reflection of the exit face as Rg and the transmission of the exit face as T_e. The ratio of the area of the hole in the entrance plane and the area of the entrance plane itself is defined as r.

The flux at different positions inside and outside the integrator can now be described in the following set of equations:

The transmitted intensity is calculated by eliminating φi and ψi:

With this formula we can calculate the transmission efficiency, which is defined by A ■ The

/ ro result of a calculation for a laser beam that illuminates a 0.55" WVGA HTPS transmissive LCD panel by proximity illumination is a follows:

• The area of the exit face is approximately 7*14 mm².

• A beam with F/# 5 is focused onto the hole 501 of the integrator 500. The spot diameter of the (diffraction limited) beam is 6 μm in that case. To completely rule out diffraction effects we have chosen to use a hole diameter of 120 Dm.

• The value of r is 1 ^• 10 ⁴.

FIG. 6 illustrates a graph of the transmission of the integrator as a function of the reflectivity of the exit face.

If the reflectivity of the entrance face is assumed to be 98%, which can be accomplished relatively easily with a dielectric stack, the transmission of the integrator is still greater than 85 % for reflections of the end face up to 90 %. With 90 % reflection a beam, on average, passes through the integrator 10 times, resulting in significant speckle reduction.

It should be noted that the reflectivity of the end face can be made much higher, since the integrator is meant for a single wavelength. If this is the case the transmission will also be higher, or, alternatively, the number of passes can be increased to enhance speckle reduction. In conclusion, a preferred embodiment has the following characteristics: • A high quality laser beam that can be focused to a spot that is close to the diffraction limit;

• The laser beam at the same time should have a coherence length of a few cm or smaller;

• The laser beam enters a rod integrator through a small hole in the entrance face and with an F/# that is substantially higher than in conventional projection systems (F/# higher than 2.8);

• The hole in the rod integrator should be large enough such that almost all of the laser light can pass it without severe diffraction effects;

• The remaining part of the entrance face should have a high reflection coefficient for the wavelength used;

• The exit face of the integrator should be covered with a dielectric stack such that the reflection coefficient is reasonably high (at least 50 %);

• The laser speckle of the laser light exiting through the exit face of the integrator is reduced by at least 10% and transmission thereof is decreased by at most 10%;

• The preferred value of half angle of the laser beam entering the rod integrator is between 0.5 and 6 degrees and the maximum value of the half angle is 30 degrees;

• In an alternative preferred embodiment, the length of the rod integrator is greater than or in the order of the coherence length of the laser light, preferable the length is larger than one fifth of the coherence length; and

• The rod integrator can be a cube.

It should be noted that this embodiment is especially suited for lasers that emit perpendicular to the surface, such as VCSELs, VECSELs and NECSELs. These types of lasers can simply be grouped in an array on the same wafer and the integrator can simply be mounted on top of this array.

In FIG. 7 a second preferred embodiment 700, according to the present invention, is illustrated. Instead of homogenizing the intensity distribution of a single laser, the distribution of a number of lasers of the same wavelength is homogenized in an integrator 700 with multiple entrance holes 701. Although the aperture factor r in equation (3) is increased, the transmission intensity is still quite high. The advantage of using multiple lasers is that the speckle is reduced by an additional factor of N^1/2, in which N is the number of lasers used.

A third preferred embodiment 800 is illustrated in FIG. 8. The exit face 804 of the integrator 800 is partly covered by a high reflective coating 804, for instance the same as the first reflective coating 801 used for the entrance face 803. The central part 806 of the exit face 805 is not covered at all or coated with a layer that has a much lower reflection coefficient. In this embodiment the F/# must be much lower than in the second embodiment and the integrator must be so long that the intensity distribution at the exit face 805 is homogeneous. Part of the light at the exit face 805 will be reflected and part will be transmitted. The ratio of these two is determined by the ratio of the areas of transmissive to the reflective parts of the exit face 805. Hence, a substantial part of the light will have a second pass in the integrator and speckle reduction is obtained.

In a fourth preferred embodiment, a different geometry is chosen for the wave-guide to introduce optical delay between the individual beams. An example of such a geometry is depicted in FIG. 9. It is clear that the wave-guide depicted in FIG. 9 introduces an optical path difference between the beam traces indicated between the solid 902 line and the dashed 901 line. If this optical path length difference is larger than the coherence length of the light then the speckle is reduced. A disadvantage of the wave-guide depicted in FIG. 9 is that the homogeneity of the intensity distribution is less than in the first through the third embodiments described previously.

The optical integrator and speckle reducer together with its light source is intended to be used as an illumination unit in a projection system. It can be used for proximity illumination or for illumination by means of relay lenses. It is also possible to enlarge or decrease the size of the exit face by in an optical imaging system.

The panel that is illuminated can be a two-dimensional or one-dimensional spatial light modulator. The former case was described in detail in the embodiments. In the latter case a planar wave-guide with the shape of the embodiments described has to be used. The thickness of the wave-guide will be much smaller than the other dimensions of the wave-guide. The reflection coefficient and transmission coefficient of the exit face need not necessarily be constant across the exit face.

While the preferred embodiments of the rod integrator present invention have been illustrated and described, it will be understood by those skilled in the art that the embodiments of the present invention as described herein are illustrative and various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from its central scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the claims appended hereto as well as all implementation techniques.

Claims

CLAIMS:

1. A rod integrator (100) for a beam of laser light having a value given F/#, comprising: a rod- shaped body (106) having an interior that passes laser light therethrough and an interior surface (107) that reflects light incident thereon back into the interior of the rod-shaped body (106); an entrance face (103) positioned on a first end of the rod-shaped body covered with a first reflective coating (101) having a hole (102) therein to allow light to pass therethrough into the interior of the rod-shaped body, said first reflective coating to reflect light incident thereon back into the interior of the rod-shaped body (103); and an exit face (105), positioned on a second end of the rod-shaped body (106) opposite to and parallel to the entrance face (103), and covered with a second reflective coating (104) that allows a part of light incident thereon to pass therethrough and reflects a part of light incident thereon back into the rod-shaped body (106), wherein, part of the laser light entering said rod integrator via said hole (102) is forced to pass through the interior thereof several times before passing through said exit face (105) and then exiting through said exit face (105) is homogenized.

2. The rod integrator (100) of claim 1, wherein a length of the rod-shaped body (105) > one fifth of the coherence length of the laser light.

3. The rod integrator (100) of claim 1, wherein the rod-shaped body (105) is a cube.

4. The rod integrator (100) of claim 1, wherein laser speckle of the laser light exiting through said exit face (105) is reduced by at least 10% and transmission thereof is decreased by at most 10%.

5. The rod integrator ( 100) of claim 1 , wherein: the rod-shaped body comprises a material selected from the group consisting of glass and a transparent plastic; the first reflective coating (101) is a material selected from the group consisting of silver and a dielectric stack or an other reflective metal such a aluminum, or a combination thereof; the hole (102) is of a diameter such that the laser light can be focused into said hole and can pass therethrough without significant light loss and diffraction effects.

6. The rod integrator (100) of claim 5, wherein: said laser light is focused to enter the hole (102) with a half angle θ , wherein the

maximum value of the half angle is 30 degrees; said hole (102) has a diameter > 2d where d is determined by the equation

to minimize diffraction effects; and said second reflective coating (104) is dimensioned to have a transmission coefficient T_c that is less than .5 and a reflection coefficient R_c= 1 - T_c such that part of the light is transmitted and part is reflected back into the rod integrator (100) thereby, wherein, the reflected light travels back to the entrance face (103), at which it is almost completely reflected for a second pass through the rod integrator (100) with a net effect that light exiting the rod integrator at the exit face (105) is composed of light that has passed through the rod integrator (100) at least 1 time.

7. The rod integrator (100) of claim 6, wherein the half angle of the laser beam entering the rod integrator is between 0.5 and 6 degrees.

8. The rod integrator (700) of claim 1 , wherein: said beam of laser light is a plurality N of beams of laser light having an identical wavelength; and said hole (102) is a plurality of holes (702), wherein, the laser speckle is reduced by an additional factor of N^1/2.

9. The rod integrator (800) of claim 8, further comprising a second reflective coating (104) that is a highly reflective coating (804) having a central hole therein such that the highly reflective coating (804) does not cover a central part (806) of the exit face (805), wherein, a ratio of the part of the laser light reflected by the highly reflective coating (804) to the part of the light transmitted through the central hole (806) is determined by a ratio of an area of the central hole (806) to an area of the highly reflective coating (804).

10. The rod integrator (900) of claim 9, wherein said rod-shaped body is further configured to have a geometry that introduces an optical delay between individual beams of laser light introduced into the rod integrator via the hole (102).

11. The rod integrator (800) of claim 1, further comprising a second reflective coating (104) that is a highly reflective coating (804) having a central hole therein such that the highly reflective coating (804) does not cover a central part (806) of the exit face (805), wherein a ratio of the part of the laser light reflected by the highly reflective coating (804) to the part of the light transmitted through the central hole (806) of the exit face (805) is determined by a ratio of an area of the central hole (805) to an area of the highly reflective coating (804).

12. The rod integrator (900) of claim 11, wherein said rod-shaped body is further configured to have a geometry that introduces an optical delay between individual beams of laser light introduced into the rod integrator via the hole (102).

13. The rod integrator (900) of claim 8, wherein said rod-shaped body is further configured to have a geometry that introduces an optical delay between individual beams of laser light introduced into the rod integrator via the hole (102).

14. A method for reducing laser speckle that ensures homogeneous illumination of a projected image, comprising: providing a beam of a laser light having a given value F/#; providing a rod-shaped body (106) that includes an entrance face (103) positioned on a first end and an exit face (105) positioned on a second end of the rod-shaped body (106) opposite to and parallel to the entrance face (103) and having an interior that passes laser light therethrough and an interior surface (107) that reflects light incident thereon back into the interior of the rod-shaped body (106); covering the entrance face (103) of the provided rod-shaped body with a first reflective coating (101) having a hole (102) therein to allow light to pass therethrough into the interior of the rod-shaped body, said first reflective coating to reflect light incident thereon back into the interior of the rod-shaped body (103); and covering the exit face (105) with a second reflective coating (104) that allows a part of light incident thereon to pass therethrough and reflects a part of light incident thereon back into the rod-shaped body (106), wherein, part of the laser light entering said rod integrator via said hole (102) is forced to pass through the interior thereof several times before passing through said exit face (105) and then exiting through said exit face (105) is homogenized.

15. The method of claim 12, wherein a length of the rod-shaped body > one fifth of the coherence length of the laser light.

16. The method of claim 12, wherein laser speckle of the laser light exiting through said exit face (105) is reduced by at least 10% and transmission thereof is decreased by at most 10%.

17. The method of claim 16, further comprising the steps of: focusing said laser light to enter the hole (102) with a half angle θ , wherein the

maximum value of the half angle is 30 degrees ; sizing said hole (102) to have a diameter > 2d where d is determined by the equation

to minimize diffraction effects; and dimensioning said second reflective coating (104) to have a transmission coefficient T_c that is less than .5 and a reflection coefficient R_c= 1 - T_c such that part of the light is transmitted and part is reflected back into the rod integrator (100) thereby, wherein, the reflected light travels back to the entrance face (103), at which it is almost completely reflected for a second pass through the rod integrator (100) with a net effect that light exiting the rod integrator at the exit face (105) is composed of light that has passed through the rod integrator (100) at least 1 time.

18. The method of claim 17, wherein the half angle of the laser beam entering the rod integrator is between 0.5 and 6 degrees.

19. The method of claim 18, wherein: said beam of laser light is a plurality N of beams of laser light having an identical wavelength; and said hole (102) is a plurality of holes (702), wherein, the laser speckle is reduced by an additional factor of N^1/2.

20. The method of claim 19, further comprising the step of providing a second reflective coating (104) that is a highly reflective coating (804) having a central hole therein such that the highly reflective coating (804) does not cover a central part (806) of the exit face (805), wherein a ratio of the part of the laser light reflected by the highly reflective coating (804) to the part of the light transmitted through the central hole is determined by a ratio of an area of the central hole to an area of the highly reflective coating (804).

21. The method of claim 20, further comprising the step of configuring the rod-shaped body (106) to have a geometry that introduces an optical delay between individual beams of laser light introduced into the rod- shaped body (106) via the hole (102).

22. A projection engine system (200), comprising: a rod integrator including: a rod- shaped body (106) having an interior that passes the laser light therethrough and an interior surface (107) that reflects light incident thereon back into the interior of the rod-shaped body (106), an entrance face (103) positioned on a first end of the rod-shaped body covered with a first reflective coating (101) having a hole (102) therein to allow light to pass therethrough into the interior of the rod-shaped body, said first reflective coating to reflect light incident thereon back into the interior of the rod-shaped body (103), and an exit face (105), positioned on a second end of the rod-shaped body (106) opposite to and parallel to the entrance face (103), and covered with a second reflective coating (104) that allows a part of light incident thereon to pass therethrough and reflects a part of light incident thereon back into the rod-shaped body (106), wherein, part of the laser light entering said rod integrator via said hole (102) at a given half angle θ < 30 degrees is forced to pass through the interior thereof several times

before passing through said exit face (105) and then exiting through said exit face (105) is homogenized, has speckle therein reduced; and a display panel (201) configured in proximity to said exit face (105) of said rod integrator (100) such that said display panel (201) is illuminated by the homogenized light exiting through said exit face (105) having reduced speckle.

23. The engine (200) of claim 22, wherein a length of the rod-shaped body (105) > one fifth of the coherence length of the laser light.

24. The engine (200) of claim 22, wherein the rod-shaped body (105) is a cube.

25. The engine (200) of claim 22, wherein laser speckle of the laser light exiting through said exit face (105) is reduced by at least 10% and transmission thereof is decreased by at most 10%.

26. The engine (200) of claim 22, wherein the half angle of the beam of laser light entering the rod integrator (100) is between 0.5 and 6 degrees.

27. The engine (200) of any of claim 25, wherein: said beam of laser light is a plurality N of beams of laser light having an identical wavelength; and said hole (102) is a plurality of holes (702), wherein, the laser speckle is reduced by an additional factor of N^1/2.

28. The engine (200) of claim 22, further comprising a second reflective coating (104) that is a highly reflective coating (804) having a central hole therein such that the highly reflective coating (804) does not cover a central part (806) of the exit face (805), wherein, a ratio of the part of the laser light reflected by the highly reflective coating (804) to the part of the light transmitted through the central hole is determined by a ratio of an area of the central hole to an area of the highly reflective coating (804).

29. The engine (200) of claim 27, wherein said rod-shaped body (106) is further configured to have a geometry that introduces an optical delay between individual beams of laser light introduced into the rod integrator via the hole (102).