Design and development of speckle-free high-power laser-driven phosphor converted compact automotive headlamp module

The applicability of diode-lasers in automobile headlights is an advanced innovation for the automobile illumination industry due to the extraordinary properties of laser light over conventional light sources, such as high brightness, wide colour gamut, high directionality, low energy consumptions and long lifetime. Lasers are highly coherent in nature, so they encounter the problem of unwanted speckles and spurious fringes and always require a high level of opto-thermal engineering along with speckle reduction mechanisms for high lumen laser applications. Targeting such challenges, in this paper, we report an innovative design and development scheme for a high lumen laser-based automotive headlamp module. The headlamp prototype comprises a set of four cylindrical diffusers which distribute the high energy laser radiation via scattering along the length of the diffusers within a metallic mirro-based pyramidal cavity reflector. The scattered laser light from cylindrical diffusers interacts with a remote phosphor layer that prevents phosphor–resin burning. The pyramidal cavity reflector plays an important role in making the laser light uniform and speckle-free, via spatial and angular diversity, as light exits from the cavity after multiple internal reflections. This reflector redirects the highly concentrated white light over a long range without using any projection lens. The design and performance of the headlight system was studied using TracePro simulation software and tested experimentally in a photometric laboratory. The International Commission on Illumination (CIE) coordinates of the light generated by the headlamp was (0.3947, 0.4908) and the correlated colour temperature was 4240 K, which represents warm white light illumination.


Introduction
In recent years, the illumination industry has grown rapidly due to modern computer-generated headlamp and luminaire design.Conventional headlamps are round because of the native shape of a parabolic reflector [1].In the present scenario, LED-based headlamps replace the filament-based automobile headlamps due to their improved energy efficiency and brighter illumination capabilities [2,3].Hung et al [4] reported a design of an LED-based automotive headlight using a digital micromirror device.Laser-based headlamps are the future of the automobile illumination industry because of their extraordinary properties such as high brightness, wide colour gamut, high directionality, low energy consumption and long lifetime.Lasers are sources that can provide light intensity thousands of times greater than conventional LEDs [5].Laser diodes have a typical lifetime in the range from 10 000 to 30 000 h [6].It is attractive to users because a one-time investment can provide a long period of service.Bavarian Motor Works (BMW) has designed a laser-based headlight for the first time for its i8 electric sport cars along with an LED headlight supplement [7].In BMW's design three lasers were arranged in triangular pattern and redirected towards a yellow phosphor based lens with the help of three mirrors and the white light generated from the phosphor was used in the reflection mode to illuminate the road [8].In BMW's hybrid illumination system, a laser beam was used in high beam mode as a booster along with a combination of many powerful LEDs and there was no role of laser light in low beam mode illumination [7].Chang et al [9] reported a glass phosphor-based laser headlight module using a 5 × 1 array of blue lasers with 1.2 W optical power for each laser, i.e. a total 6 W of optical power with 1860 lm flux.
Laser-based illumination causes a problem known as speckled noise which is due to mutual interference of highly coherent light [10].This problem degrades the illumination conditions as the human eye is very sensitive to the intensity distribution of visible light.There are a few methods by which it is possible to generate white light using solid state laser diodes [11].The most common method for white light generation is the combination of red, green and blue (RGB) lasers [12,13].This method is known to achieve a wide colour gamut, but is associated with the problems of laser speckles and spurious fringes [14].However, it is possible to achieve speckle-free white light illumination using the RGB laser system but at the cost of a loss in light intensity.Angular diversity, wavelength diversity, temporal diversity, spatial diversity and polarisation diversity are conventional methods for the reduction of the speckle noise in laser-based systems [10,11].Another method of white light generation is the combination of blue laser and yellow phosphor by down-conversion approach [15].In this method, a uniform layer of yellow phosphor material is excited by blue laser light to achieve white light illumination.This generates white light as well as making the illumination almost speckle-free.Hence, this is an effective method to achieve speckle-free white light illumination.Micro-vibrated phosphor paper along with colour-mixing techniques have been utilised to generate speckle-free white light illumination from blue laser diodes [16].A reduction in speckle contrast from 50% to 7.4% was achieved using micro-vibrated phosphor paper.It was reported in research that 5000 K is the boundary colour-temperature to establish a speckle-free white light system.In a study of laser pumped phosphor converted light sources, Aquino et al [17] reported that speckles are not exhibited in down-converted light but exhibited in the residual pumped light.In most laser-driven phosphor converted white light-source systems the laser beam is generally focused directly onto the phosphor layer, leading to heating and finally burning of the phosphor layer.This causes laser converted sources to fail.Therefore, an innovative mechanism is required to avoid the phosphor heating and burning of resin so that the lifetime of the source can be increased.
In the present paper, the architecture of a laser-based headlamp module was designed in Autodesk's 123D design software and developed with 3D printing technology.One of the main innovations behind this design is the optical engineering within a pyramidal cavity based reflector system that supports speckle reduction as well as uniformly redirecting the down converted white light over a long distance.In addition, the laser beams are redirected along the length of the cylindrical diffusers so that heating and burning of the phosphor layer is avoided.This design is very efficient compared to other light guide shapes or light reflectors, as demonstrated [18].Pyramids exhibit unique properties such as light concentration and multiple resonance for the electromagnetic radiation [18,19].To simulate the illumination conditions of the headlamp module, a system with a road and headlamp was modelled.In this model, the length and width of the road were considered as 30 m and 3 m, respectively, for a single headlamp module.The headlamp was placed 24 inch above the road.This model was simulated with a popular ray tracing tool 'TracePro,' which is well known for optical architecture designs and simulations.In addition to the simulation, the optical performance of the headlamp prototype was tested experimentally in the laboratory and also on the road.A charge-coupled device camera with a blue filter was used to record the images of speckle patterns in space geometry and the speckle contrast was computed by image processing on MATLAB.The illumination with this headlamp module was found to be almost speckle free and uniform.

Design and development
A blue laser and yellow phosphor based automobile headlamp system was designed in Autodesk's 123D design software.Replacing conventional parabolic reflectors, this innovative design consists of a pyramidal cavity-based reflector associated with a set of four cylindrical diffusers and a remote phosphor plate.
Pyramidal cavity reflectors are special lightguides that provide an efficient and uniform illumination [18,19].Figure 1 shows the 3D design of a laser diode module based headlamp system.An illustration of the assembled prototype of the headlamp is shown in the inset of figure 1.This design is composed of four high power blue diode lasers.The light beams from all these lasers were combined into cylindrical diffusers using a laser-to-diffuser beam coupler.
Figure 2 shows the structural design of the headlamp module in steps (a)-(f).In figure 2(a) the laser-to-diffuser beam coupler mounted on a coupler base is shown, (b) four cylindrical diffusers are mounted on the beam coupler, (c) a phosphor plate is arranged after the cylindrical diffusers, (d) the pyramidal cavity reflector is introduced around the phosphor plate and four diode lasers are aligned with the  coupler, (e) the protective glass holder and a cooling fan are fixed, and in (f) the compact laser headlamp with 3D printed housing is shown.

Development of the laser-to-diffuser beam coupler
The laser-to-diffuser beam coupler shown in figure 2(a) was made of aluminium metal using a computer numerical control machine.Its head contains a diffuser holder of area of 20 mm × 20 mm and a thickness of 5 mm.The diffuser holder was mounted on a 100 mm long aluminium rod with a cross sectional area of 6 mm × 6 mm.A system of four aluminium-based conductive mirrors was joined to the aluminium rod, and the tail of this rod was connected to the coupler base.The coupler base is made up of polylactic acid plastic and developed using a 3D printer.

Development of cylindrical diffusers
As shown by design in figure 2(b), four cylindrical diffusers were mounted on the head of the laser-to-diffuser coupler.Cylindrical diffusers were developed by sandblasting the glass rods shown in the upper row of figure 3(a).The length and diameter of each rod was 25 mm and 6 mm, respectively.One end  of each cylindrical rod was diffused uniformly by a length of 20 mm using 600 grit sandpaper as shown in the bottom row of figure 3(a).
For a clearer realisation of the cylindrical diffuser system, the photographs of the glass rods before and after sandblasting can be seen in figures 3(b) and (c), respectively.The role of cylindrical waveguide diffusers is to diffuse the highly directional laser beams and to scatter the light uniformly within the pyramidal cavity before interacting with the phosphor plate.Direct interaction of a high-power laser beam can harm the phosphor layer.This problem is known as phosphor-resin burning.Applying a remote phosphor plate followed by a set of cylindrical diffusers solves this problem.

Alignment of diode lasers
Four diode lasers from four directions were allowed to throw high power laser beams on the metallic mirrors of the laser-to-diffuser coupler as shown in figure 2(d).Aluminium-based mirrors were positioned at an angle of 45 • from the incident laser beam.The laser beams were redirected at 90 • after reflection from the mirrors.The reflected beams were then coupled with four cylindrical diffusers.This is how the highly directional laser beams from four powerful lasers are coupled and diffused before interacting with the phosphor plate.This arrangement helps to protect the phosphor-resin burning due to high power laser radiation.Figure 4 shows the coupling of laser beams with four cylindrical diffusers mounted on a laser-to-diffuser coupler, (a) redirection of laser beams after reflection from aluminium mirrors, (b) arrangement of four laser diode modules with the laser-to-diffuser coupler.

Development of remote phosphor plate
All four sides of a glass plate of 4 mm thickness and 35 mm × 35 mm area were ground to make tapered edges so that it can be fixed properly in the pyramidal shaped cavity after the cylindrical diffusers, as shown in figure 2(c).The glass plate was then sandblasted from its tapered face using 200 grit sized sandpaper followed by a 600 grit sized sandpaper.Another smooth face of the glass plate diffuser was coated with a YAG:Ce +3 phosphor (Zhuhai Hanbo Trading Co., Ltd., China) using a screen printing method.For the coating, YAG:Ce +3 phosphor was mixed with UV adhesive (Norland Optical Adhesive 61 LOT 352, Thor Labs) with a weight ratio of 6:5.The thickness of the phosphor layer on the 35 mm × 35 mm glass plate was 185 µm.The phosphor coated plate was then placed in a UV chamber to expose the UV light for 10 min.Figure 5(a) shows the diffused glass plates before and after phosphor coating.
All four laser beams were diffused into a set of four cylindrical diffusers to uniformly scatter the highly directional beam.Then the scattered blue light interacts with the remote phosphor plate placed within the pyramidal cavity reflector (figure 2 The figure shows many images of a phosphor plate generated due to the multiple reflections inside the pyramidal cavity, which is a unique property of a pyramid-shaped reflector.Speckle reduction and light uniformity were achieved by the phenomenon of multiple reflections.

Development of pyramidal cavity reflector
The pyramidal cavity shown in figure 2(d) was designed in Autodesk's 123D design software and developed using 3D printer (Ultimaker).The length of the pyramidal cavity was 135 mm, the front cross-sectional area was 75 mm × 75 mm and back cross-sectional area was 22.5 mm × 22.5 mm with a wall thickness of 1.2 mm.Four aluminium-based highly reflecting mirrors were installed inside the pyramidal cavity to make it a pyramidal reflector.The role of the pyramidal reflector was to collect and redirect the down-converted white light over a long distance following multiple internal reflections.A 12 V DC cooling fan was installed at the end of the pyramidal reflector as shown in figure 2(e).The fan dissipates the heat generated due to the interaction of the high-power laser beam with the cylindrical diffusers and phosphor plate.

System modelling and simulations
The complete system was modelled in Autodesk's 123D design software with a road of length 30 m and width 3 m for a headlamp module placed at a height of 24 inch from the road.Figure 6 shows the model of a headlamp on a road, viewed from different angles.The illumination conditions of this model were simulated in TracePro software in terms of an irradiance map and a radiance map.ABg BSDF model was used to generate diffuser and mirror properties in this work.Table 1 shows ABg BSDF model parameters used in the TracePro for the cylindrical diffuser and reflectors (mirrors) of the pyramidal cavity.The practical cylindrical  diffusers were made up of 6 mm thick glass rod with 4% reflectivity of the inlet surface and other surfaces were diffused with 600 grit size sandpaper.
As in most LED-based headlamps, a costly projection lens with a parabolic reflector is used to project the light beam over a long distance, we simulate a pyramidal reflector based laser-headlamp with and without a projection lens.We applied a projection lens with the pyramidal reflector at the place of protective glass as shown in figure 1.An irradiance map shown in figure 7(a) associated with the model shown in figure 6 was obtained.
It can be observed from the irradiance map that the illumination conditions are not uniform across the width of the road when we apply a projection lens with a pyramidal cavity.Figure 7(b) shows a simulated irradiance map without any projection lens.It can be observed from the figure that the illumination across the width of the road is uniform and there is no significant difference in the range of illumination.Similarly, figures 7(c) and (d) show the illumination pattern as a radiance map on the road with and without applying a projection lens, respectively.We conclude that long-range and uniform illumination can be obtained without using a projection lens with pyramidal reflector, unlike the costly projection lenses that are required in LED and parabolic reflector-based headlamps.
It is demonstrated that the proposed system is efficient and cost effective as it can uniformly illuminate the road over long distances without using a costly and bulky projection lens.The main benefit of replacing conventional parabolic reflectors and projection lenses from automobile headlamps by the innovative pyramidal reflector is to make the headlamp a compact and cost-effective system.After proper modelling and simulations, it was decided that we do not need any projection lens with our pyramidal reflector based laser-headlamp module.In the simulations, the pyramidal reflector was found capable of projecting the light beam over long distances uniformly without using projection optics.

Results and discussion
Four high-power laser diode modules having wavelength 450 nm were used in the present setup.Table 2 gives the specifications of all four laser diode modules in terms of maximum voltage (V max ), maximum current (I max ), electrical power (P e ) and optical power (P o ), while figure 8 gives the electro-optic response of all four laser diode modules (LD-A, LD-B, LD-C and LD-D).
After switching-on all four laser diode modules, high power laser beams (shown in green in figure 1) from the lasers were incident onto a set of four cylindrical diffusers after reflection from four metallic mirrors.The metallic mirrors are part of the laser-to-diffuser beam coupler and aligned at 45 • from the incident laser beams.The metallic mirrors redirect the laser beams for coupling the beams with the cylindrical diffusers.Four highly directional laser beams are incident onto the cylindrical diffusers, which are scattered along the length of the diffusers to spread the high energy light field within the pyramidal cavity reflector and then interact with the phosphor plate.In this way scattered light uniformly illuminates the surface of the phosphor film.Scattered blue laser light having a total 6.261 W power and wavelength 450 nm excites the yellow phosphor to generate down-converted concentrated white light.Highly concentrated white light is redirected in the forward direction by the pyramidal cavity reflector.A spectroradiometer (LISUN-6000) and the headlamp module were mounted on a goniometer.The spectroradiometer was allowed to rotate from −90 • to +90 • on the goniometer around the headlamp.The distance of the spectroradiometer was varied from 20 cm to 1 m to record the angular luminance uniformity (ALU) with increasing distance.The ALU response curve of the headlamp module is shown in figure 9(a).Figure 9(a) shows that the headlamp module spreads the concentrated light within an angular region of −40 • to +40 • on both side of the optical axis of the module.
A very high illuminance (>8000 lx at 20 cm) was recorded near the headlamp.The distribution of this highly intense light has increased uniformity across the angular range from −40 • to +40 • as the headlamp to spectroradiometer distance is increased.The curve shows that the angular distribution of the light from the headlamp becomes almost uniform at 1 m. Figure 9     coordinates of the light generated by the headlamp was (0.3947, 0.4908) which represent warm white light illumination.
Figure 11 shows the thermal stability curve of the headlamp module.To record this curve, a spectroradiometer (LISUN-6000) was mounted in front of the module at 1 m distance and the luminance response of the module with respect to the time was recorded by continuously operating the module for more than 4 h.No significant degradation in the system's normalised intensity was observed and the CCT of the light was uniform for the whole period of the testing.Constant CCT over time represents that the phosphor layer does not degrade due to the heat generated by continuous exposure of high-power laser radiation (6.261 W).In the case of an unstable system, the phosphor layer is degraded due to resin burning by continuous exposure of intense laser light and the direct blue photons reach the detector which shifts the CCT values to the blue region, but the present system is effectively engineered for high thermal stability.The illumination of the laser headlamp module was also tested for speckle noise, and the results were compared with the speckle noise generated by a stationary diffuser and a rotating diffuser.
Figure 12 shows the speckle patterns generated by blue laser light that are incident onto (a) a stationary diffuser, (b) a rotating diffuser, (c) cylindrical diffusers inside a pyramidal cavity and (d) cylindrical diffusers and phosphor plate inside the pyramidal cavity.Speckle patterns shown in figure 12 were recorded in free space geometry using a colour CCD camera (Lumenera, model number infinity 2-1RC).The values of computed speckle contrast for configurations are written in the top right corner of each speckle pattern.
Figures 12(e)-(h) show the intensity line profiles associated with the speckle patterns shown in figures 12(a)-(d), respectively.The computed speckle contrast is 0.32 when the direct laser beam passes through a static diffuser glass plate and 0.14 when the beam passes through the rotating diffuser, which is the most used method of speckle reduction.The speckle contrast was 0.09 when the laser beam passed through a cylindrical diffuser and pyramidal cavity system and 0.05 when a phosphor plate was introduced into this arrangement.Figure 13 shows a photograph of the prototype of the developed laser light-based headlamp module.

Conclusion
We have designed and developed a speckle-free laser light-based headlamp module.The innovative design and development method for a high power laser-based automotive headlamp module is presented.The headlamp prototype comprises a set of four cylindrical diffusers and a pyramidal shaped reflector.Simulated results show that without using projection optics, the headlamp can illuminate the road uniformly over a long distance.Four high-power blue laser diode modules were efficiently engineered to generate eye-safe and user-friendly illumination for automobile headlights.The CIE coordinates of the light generated by the headlamp were (0.3947, 0.4908) and CCT was 4240 K, which represents warm white light illumination.The electrical power of the four laser-diode based system was 40 W. The experimentally observed range of illumination was 60 ft when all four laser diode modules produced a total optical power of about 6 W. The measured luminous intensity of the system was 317 cd at 1 m from the source.The white light received from the headlamp module was almost speckle-free with a computed speckle contrast 5%.The thermal stability of the system was tested by continuously operating the device for more than 4 h.The system was found to be highly stable and efficient.

Figure 1 .
Figure 1.3D design of the laser-based automobile headlamp system and visualisation of its various parts.

Figure 2 .
Figure 2. Various development steps of the laser-based automobile headlamp.(a) Laser-to-diffuser coupler, (b) cylindrical diffusers mounted on coupler, (c) remote phosphor plate, (d) alignment of pyramidal reflector and lasers, (e) protective glass holder and a cooling fan, (f) the compact laser headlamp with 3D printed housing is shown.

Figure 3 .
Figure 3. Images of the cylindrical diffuser system (a) cylindrical glass roads (upper row) and cylindrical diffusers (bottom row), (b) set of transparent cylindrical rods before sandblasting, (c) set of diffused rods after sandblasting.

Figure 4 .
Figure 4. Coupling of laser beams with cylindrical diffusers mounted on the laser-to-diffuser coupler, (a) redirection of laser beams after reflection from the aluminium mirrors, (b) arrangement of four laser diode modules with the laser-to-diffuser coupler.

Figure 5 .
Figure 5. (a) Diffused glass plate before and after phosphor coating, (b) images of a phosphor coated plate generated by multiple reflections inside the pyramidal cavity.
(d)) to generate the down-converted white light.The pyramidal cavity reflector shown in figure 2(d) redirects the bright white light in the forward direction after multiple reflections.Multiple reflections within the pyramidal cavity contribute to speckle reduction and uniform illumination.Figure 5(b) is a photograph of the phosphor plate placed inside the pyramidal cavity reflector.

Figure 6 .
Figure 6.Model of the road and headlamp viewed from different angles.The length of the road in this model was 30 m and the width was 3 m.The headlamp was placed 24 inch above the road surface.

Figure 7 .
Figure 7. (a) and (b) show the irradiance maps of the headlight on the modelled road with and without projection lens respectively, (c) and (d) radiance maps of the system with and without projection lens, respectively.The simulation was performed in TracePro software.
(b)  shows the angular colour uniformity of the laser-based headlamp module in terms of correlated colour temperature (CCT).
Figure 9(b) shows that the CCT of the headlamp remains almost constant across the whole angular range of illumination (−40 • to +40 • ) and at various distances from the headlamp.Figure 10(a) shows the spectrum and (b) shows the CIE chromaticity diagram of the laser-based headlamp module.The chromaticity diagram shows that the CIE

Figure 8 .
Figure 8. Electro-optic response curve of four laser diode modules (LD-A, LD-B, LD-C and LD-D).

Figure 10 .
Figure 10.(a) and (b) Shows the spectrum and CIE chromaticity diagram, respectively, of the of phosphor converted laser light based headlamp module.

Figure 11 .
Figure 11.Thermal stability curve of laser light based headlamp module.CCT: correlated colour temperature.

Figure 12 .
Figure 12.Shows the speckle patterns generated by a blue laser beam incident onto (a) a stationary diffuser, (b) a rotating diffuser, (c) cylindrical diffusers inside a pyramidal cavity and (d) cylindrical diffusers and phosphor plate inside the pyramidal cavity.

Figure 13 .
Figure 13.Photograph of the prototype of the developed laser-based headlamp module.

Table 1 .
ABg BSDF model parameters used in the TracePro simulations.

Table 2 .
Specifications of all four laser diode modules in terms of maximum voltage (Vmax), maximum current (Imax), electrical power (Pe) and optical power (Po).