WO2009040725A2

WO2009040725A2 - Laser-pumped lamp using a light guide with a pinhole in its entrance mirror

Info

Publication number: WO2009040725A2
Application number: PCT/IB2008/053849
Authority: WO
Inventors: Marcellinus P. C. M. Krijn; Willem L. Ijzerman; Youbin Zheng
Original assignee: Koninklijke Philips Electronics N. V.
Priority date: 2007-09-26
Filing date: 2008-09-23
Publication date: 2009-04-02
Also published as: WO2009040725A3

Abstract

A laser-based lamp comprising a laser (10) coupled to a light guide core (14) via apinhole (26). The light guide (12) is clad with a light-emissive and/or scattering layer (16). In an alternative embodiment, light from the laser (10) is coupled into the light guide (12) via a semi-transparent mirror (50). High light efficiency is achieved during beam reshaping and speckle is minimised.

Description

LASER-BASED LAMP

FIELD OF THE INVENTION

This invention relates generally to laser-based lamps and, more specifically, to the use of lasers to create substantially speckle-free light sources suitable for use in applications such as automotive lighting, projection, general lighting and backlighting.

BACKGROUND OF THE INVENTION

Japanese patent application no. JP2000275444 describes a light emitting device in which light emitted from a semiconductor laser is guided to the core of an optical fibre. The fibre is clad with a phosphor-containing material which emits part of the laser light guided into the fibre, thus re-shaping the beam and providing an incoherent light source. Light leakage can be reduced by minimising the diameter of the core, but then there would be a high degree of speckle because there would be little reflection of the light within the core before the light is coupled back out. This can be reduced by increasing the diameter of the core (and hence the diameter of the coupling means), but then there is a high degree of light leakage.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved laser-based lamp in which the laser beam is reshaped into an incoherent light source with a high degree of light efficiency and minimal speckle.

In accordance with the present invention, there is provided a lamp comprising a laser coupled to a light guide, said light guide having first and second opposing ends with a core therebetween and coupling means at said first end for coupling light from said laser to said core of said light guide, the outer surface of said light guide being at least partially clad with a light emissive and/or scattering material, wherein said coupling means is arranged to transmit light output by said laser into said core and substantially prevent said light from exiting said core via said coupling means. In a first exemplary embodiment, the coupling means may comprise an aperture in the first end wall, the aperture having a diameter smaller than that of the core of the light guide. In this case, the first and second end walls may have a reflective surface to minimise light leakage. Preferably, converging means (e.g. a lens) is provided for converging light from the laser into the core via the aperture.

In an alternative embodiment, the coupling means may comprise a semi- transparent mirror adjacent to the first end of the light guide.

The outer surface of the light guide between the first and second end walls may be clad with a diffusely scattering material. Alternatively, the light guide may be at least partially clad with a material containing particles that convert laser light into a different wavelength or wavelength spectrum, or the cladding layer may have luminescent material dispersed therein.

The laser is preferably a semiconductor laser.

In an exemplary embodiment, two or more lasers of different colors may be provided, wherein light from the two or more lasers is combined and then coupled into the core of the light guide via the coupling means. This enables a color-tunable lamp to be made.

An inner reflective layer may be provided on the bottom of the light guide and a cladding layer may be provided on the opposing upper surface thereof.

The laser and the light guide are integrated with each other, with a semi- transparent mirror being provided between the laser cavity and the light guide core, through which mirror laser light leaks into the light guide core.

The light guide may be made from a light emissive and/or scattering material.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic side view of a lamp according to a first exemplary embodiment of the present invention; Figure 2 is a schematic view of a lamp according to a second exemplary embodiment of the present invention;

Figure 3 illustrates schematically some cross-sectional shapes suitable for the light guide of a lamp according to the respective exemplary embodiments of the present invention; Figure 4 illustrates schematically two lamps according to further respective exemplary embodiments of the present invention; Figure 5 illustrates schematically a lamp according to an exemplary embodiment of the present invention, suitable for automotive applications.

Figure 6 illustrates schematically a lamp according to an exemplary embodiment of the present invention, suitable for backlighting or ultra-thin lighting applications; and

Figure 7 is illustrative of an analytical one-dimensional model of light emitted by a 'leaky' light guide.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to Figure 1 of the drawings, there is illustrated a first exemplary embodiment of the present invention that aims to solve the aforementioned problems of reshaping a laser beam and reducing speckle and thus achieve the above-mentioned object. The light from a laser 10 (preferably a semiconductor laser) is coupled into a light guide 12. This light guide 12 can, for example, be a fibre. The fibre has a transparent core 14 in combination with a diffusely scattering cladding layer 16. The light travels through the fibre, primarily by means of total- internal-reflection (TIR). Occasionally, a light ray 18 is coupled out of the fibre by the diffusely scattering cladding layer 16. The light travels back and forth within the light guide core until all the light is coupled out. To minimize the amount of light leaking from the fibre, mirrors 20, 22 are located at the beginning and the end. By means of a lens 24 the light is coupled into the fibre through a pinhole 26 in the entrance mirror 20. The smaller the pinhole 26, the less light leakage will occur.

Compared to the prior art, the arrangement described above has several advantages:

• The shape of the light guide (more or less) equals the shape of the resulting light source. Thus, there is a high degree of freedom in choosing this shape (e.g. thin and short versus flat and thin, depending on the desired application).

• The light distribution can be made homogeneous. • Speckle is much reduced: on average, the light travels back and forth in the light guide several times before being coupled out. Light that is coupled out after a number of round trips will no longer be coherent with respect to light coupled out in previous or subsequent round trips. Effectively, many independent sources of light are created, thereby reducing the speckle amplitude.

• The implementation is simple and inexpensive.

In addition, a high light efficiency can be achieved. With reference to Figure 7 of the drawings, the illustrated model describes a light guide into which light is coupled through a pinhole in a mirror at the entrance. The light travels trough the light guide, mainly by total- internal-reflection (TIR) and gradually leaks out of the light guide. A mirror reflects the light that is left at the end of the light guide. The net flux of the light, Φ, travelling back and forth inside the light guide is governed by the relation

Φ(x) = _e-^{a x}Φ^R -e^{a x}Φ^L .

This net flux is the sum of a flux directed to the right, Φ^R, and a flux directed to the left, Φ^L (representing the light reflected from the mirror at the end). In this expression, α describes the light that is lost. This loss factor is the sum of two contributions α = α_afa +α_em! ,

The first contribution refers to the absorption of light inside the light guide (preferably this absorption is negligible). The second contribution describes the emission of light from the light guide. One can derive that

and φ^L = R . _e-^{2a L} - Φ^R .

In these relations, R refers to the reflection coefficient of the mirrors, A is the surface area of the entrance of the light guide and a is the surface area of the hole in the mirror covering this entrance. L is the length of the light guide.

The efficiency (i.e. the fraction of light emitted by the light guide, relative to the amount of light coupled into the light guide) is obtained from the relation

η = [φ(0) -Φ(Z,)]- - α_em! α The light emitted from the light guide is proportional to -dΦ/dx. In experiments, taking R=O.95, A=LO mm², a=0.01 mm², L=2.0 mm,a=0.2 mm^'1 , r\=87%, which is relatively high.

Alternatively, instead of a diffusely reflecting cladding, one can also use a cladding with embedded phosphor particles that convert light from the laser into a different wavelength or wavelength spectrum. For example, light from a blue laser is coupled into the light guide and is converted into yellow light by the phosphor. Both blue and yellow light are coupled out of the fibre (blue light is coupled out by diffuse scattering from the phosphor particles and yellow light is coupled out directly by emission of the phosphor particles). Together they can form white light. In another method, a luminescent material is dispersed inside the cladding layer, converting near-UV light into visible light.

A second exemplary embodiment according to the invention is illustrated schematically in Figure 2.

First, light emitted by each of a red, green, and blue emitting laser 30, 32, 34 is combined into a single optical path. For this purpose, so-called dichroic filters 36, 38, 40 (also called dichroic mirrors) can be used. These filters reflect a certain part of the wavelength spectrum and transmit the complementary part. Next, the combined light from the three lasers is coupled into the fibre 12 by means of a lens 24. The advantage of this arrangement is that a color-tunable light source with a large color gamut can be created. The color of the lamp can be tuned by tuning the input current of the lasers.

In other embodiments, different shapes can be used for the light guide. It can be cylindrical as shown in Figure 3(a), as would be the case with a fibre. The advantage is that it is easy to fabricate.

It can be rectangular, as shown in Figure 3(b), the advantage being its convenience during assemby.

It can also have a diffuse cladding layer 16 at the top and a mirror 17 at the bottom, as shown in Figure 3(c). The advantage of this is that it is more suitable for planar illuminators.

In yet another exemplary embodiment, the light guide can be integrated with the laser itself. The laser has a cavity in which the lasing action takes place. In general, light leaves the cavity by leaking through a semi-transparent mirror at the end of the laser cavity. In this exemplary embodiment, the end mirror 50 of the laser cavity also functions as the entrance mirror of the light guide as illustrated in Figure 4(a). The mirror can have a relatively high reflection coefficient (e.g. reflection = 95%, transmission =5%). This means that the mirror has the same effect as the pinhole of the embodiment described with reference to Figure 1. Although, in this particular case, the laser light only has a 5% chance of entering the fibre, this arrangement can be very advantageous for some applications. This is because a diode laser (the most common and inexpensive laser) essentially consists of a semiconductor sandwiched between two mirrors (one of which typically has a transmission coefficient of around 5%). These mirrors form a cavity in which light is reflected back and forth. Thus, in the embodiment of figure 4(a), the mirror that is already part of the laser can be used as the coupling means to couple laser light into the fibre. Such an integrated light source can be very compact and the efficiency can be even higher compared to that of previous embodiments. The laser cavity can be an external cavity.

Moreover, the light guide 12 itself can be made of scattering material or a phosphorous material, as illustrated in Figure 4(b).

The present invention has many potential applications. By combining a laser with a thin and short light guide and a curved mirror 52 (or a mirror with facets), it can be used as an automotive headlight or as a replacement of UHP lamps in projection displays as illustrated in Figure 5. The advantage of this is that it combines a high efficiency with a very long lifetime.

Alternatively, if a very thin fibre is used as the light guide, the resulting linear light source could be used to realise a very thin edge-lit backlight, as illustrated schematically in Figure 6. The backlight can be made much thinner than would be possible when using LEDs.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. For example, in case the light guide is flexible and very long relative to its width and height, it can be made to replace, for example, signage based on neon tubes or it can be folded into a meandering lamp.

A laser can be located at each end of the light guide. Instead of a single laser, an array of lasers can be focused through the same pinhole by means of a lens or more complex optics. Alternatively, each laser could have its own pinhole.

In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A lamp comprising a laser (10) coupled to a light guide (12), said light guide having first and second opposing ends with a core (14) therebetween and coupling means (26, 50) at said first end for coupling light from said laser (10) into said core (14) of said light guide (12), the outer surface of said light guide being at least partially clad with a light emissive and/or scattering material (16), wherein said coupling means (26, 50) is arranged to transmit light output by said laser (10) into said core (14) and substantially prevent said light from exiting said core (14) via said coupling means (26, 50).

2. A lamp according to claim 1, wherein said coupling means comprises an aperture (26) in said first end wall, said aperture (26) having a diameter smaller than that of said core (14) of said light guide (12).

3. A lamp according to claim 2, wherein said first and second end walls have a reflective surface (20, 22).

4. A lamp according to claim 2, further comprising converging means (24) for converging light from said laser (10) into said core (14) via said aperture (26).

5. A lamp according to claim 1, wherein said coupling means comprises a semi- transparent mirror (50) adjacent said first end of said light guide (12).

6. A lamp according to claim 1, wherein the outer surface of the light guide between said first and second end walls is clad with a diffusely scattering material (16).

7. A lamp according to claim 1, wherein said light guide (12) is at least partially clad with a material (16) containing particles that convert laser light into a different wavelength or wavelength spectrum.

8. A lamp according to claim 1, wherein said light guide (12) is at least partially clad with a cladding layer (16) having luminescent material dispersed therein.

9. A lamp according to claim 1, comprising two or more lasers (30, 32, 34) of different colors, wherein light from said two or more lasers (30, 32, 34) is combined and then coupled into said core (14) of said light guide (12) via said coupling means (26, 50).

10. A lamp according to claim 1, wherein an inner reflective layer (17) is provided on the bottom of said light guide (12) and a cladding layer (16) is provided on the opposing upper surface thereof.

11. A lamp according to claim 5, wherein said laser (10) and said light guide (12) are integrated with each other, a semi-transparent mirror (50) being provided between the laser cavity and said light guide core (14), through which mirror (50) laser light leaks into said light guide core (14).

12. A lamp according to claim 14, wherein said light guide (12) is made from a light emissive and/or scattering material.