FR2777124A1 - Microlaser mosaic image projection - Google Patents

Abstract

The image projector consists of a number of microlasers arranged in a matrix. Each microlaser unit comprises an optical cavity bounded by an entrance mirror (8) and an exit mirror (6). An active lasing medium (2) and modulation media (10,12) are also placed in the microlaser resonator

Description

MICROLASER MOSAIC IMAGE PROJECTOR
Technical field and prior art
The present invention relates to the production of an image projector using a mosaic of solid microlasers. The device is capable of high brightness and good resolution with a low manufacturing cost.

 Projecting images finds applications wherever a large number of people must be affected by the information. We identify applications in an industrial environment, with screens, or in public places with signboards, vectors of evolving information (display in stations) or animated (advertising). Such applications require simple and efficient projection means.

 Other applications relate to the field of optical correlators for which high gloss image planes are possibly sought.

 The device described is indeed an image projection system, and not a product competing with imagers of the CRT or LCD screen type (although these fields of application can be envisaged). One of the main applications therefore relates to the field of projecting large images (for example greater than 25 inches).

 Three projection techniques are known.

A first technique uses optical "valves" acting as a spatial filter on the beam of a white light source. Different types of valves have been used - liquid crystals, with active matrix or not, - liquid crystals in diffractive mode, - mobile micromirrors produced in microtechnology
(described for example in the article "The mirror
Matrix Tube: A novel Light Valve for projection
displays ", R. Noel Thomas, Jens Guldberg, IEEE Trans
on Elect. Devices, 22, no. 9, September 75.

 Another technique uses image tubes (CRT) of very high gloss (eg Barco ...).

 A third technique involves scanning three laser beams of different colors.

 More information on these techniques can be found in the article "Optical efficiency of lightwave projectors", W.E. Glenn, Proc. of IEEE, proj. Display II, San Jose, vol 2650, 1996.

 The disadvantage of the first two techniques mentioned is the limitation in power of the headlight, due to the limited transmission of the filters for the first and, for the other, the limited performance of the existing phosphors.

 The drawback of the third method is the complexity of the device used, linked to the need for a high modulation rate in order to obtain high resolution images.

Recently, has been proposed ("Laser pixels may form images in projection TV", Nabil Lawandy, Joel
Firehammer, Laser Focus World, May 1997) an image projection device based on the fragmentation of a laser beam (dye) by a liquid crystal modulator.

 No device is known which combines the advantages of the previously mentioned techniques, namely a high resolution and a high gloss.

Statement of the invention
The invention firstly relates to an elementary device for projecting images, comprising a plurality of microlaser sources arranged in a matrix, each source comprising a cavity delimited by an input mirror and an output mirror, as well as an active laser medium and means for modulating, in intensity, each microlaser source.

 The image projection system, object of the invention is based on laser sources arranged in a mosaic of transmitters.

 The laser sources envisaged are of the solid laser type pumped by diodes which, although emitting wavelengths generally located in the near IR, are capable, by optical frequency doubling, of providing wavelengths in the visible: the YAG: Nd3 + in particular, by doubling the three fundamental lines (1318, 1064, 946 nm) provides the monochromatic wavelengths R, V, B: 659, 532, 473 nm.

 The device which is the subject of the invention can therefore be of the three-color projection type and thus makes it possible to obtain images with bright colors, possibly even in strong ambient light (good contrast) with a specific screen.

 The projector system can therefore consist of the parallel connection of three analog image projection devices, dedicated to each of the wavelengths (R, G, B). The paralleling can be carried out by optical covering of the three beams, for example by means of a wavelength mixing prism.

 In the invention, according to a first embodiment, an image is considered as a set of light points (Pixels) with specific intensity levels and this in each of the basic color components (R, G, B mode) . When the projected image is at high resolution (for example at least 1024 × 768) and when the mosaic must remain of a size compatible with the manufacturing processes, each pixel must be small.

Typically a mosaic then has a size of the order of 50 mm, and the pattern of an elementary transmitter does not exceed 50pu.

 The manufacturing method of "solid" microlasers based on a collective process, lends itself to the production of emitter mosaics for each elementary projection device.

 A precise description of this manufacturing method is given in document EP-653,824 (US-5,395,494). The manufacturing process consists of making all the transmitters together (etching the shape of the cavities, triggering technology, frequency doubling, depositing the mirrors, etc.).

Finally, the microlasers are individualized by cutting, just at the end of the process. This last cutting step is no longer necessary in the case of the invention. It is indeed in mosaic of transmitters that microlasers are used.

 The small size of the transmitter pattern can be the cause of the crosstalk between transmitters. The small thickness of solid microlasers makes it possible to avoid it.

According to the article by N. Lawandy already cited above, the number of Fresnel NF defined by ab
NF =, 2 # nL with:
a: pixel size,
b: distance between pixels
L: thickness of the laser,
n: index of the laser material (1.5), must be at least equal to 2.

 With 50um mosaic pattern steps, the length of the laser cavity (L) must, according to this condition, be less than 0.8mm.

 According to this geometric constraint, two possible approaches for configuring optical frequency doubling (intra or extra cavity) can be chosen.

 Intracavity dubbing can be envisaged with the advantage of a doubling yield a priori greater. However, the constraint of laser cavity thickness could in certain cases induce a limited pumping efficiency of the microlaser (thickness of the absorbent medium low). The solution of the external doubling is then preferred.

 The extracavity lining configuration is perfectly compatible with (current) microlaser technology, and allows the limited thickness of the cavity to be respected. The triggering process described in document EP-653,824, with the mastery of a saturable absorbent technology in thin thickness allows, in fact, a triggered operation (passively) of the microlasers, with the generation of giant light pulses, of high peak power. This high peak power can then be used to achieve wavelengths in the visible range, by frequency doubling (optical non-linear effect).

Each of the emitters of the mosaic is intensity-modulated in order to provide, on each of the color components, information in “gray” level. The command used is preferably of the all or nothing type, with variable duty cycle (PWM,
Pulse Width Modulation), to define these gray levels or the average intensities.

The operation, or stopping, of the laser emission can be controlled by various means such as those described in the document EP-657 976, or (preferably) by devices of the MOEMS type ("Micro-Opto
Electro-Mecanical Systems "). Several solutions can be envisaged - by intracavity shutter, produced by a crew
mobile, - by modification of the geometry of the laser cavity,
with transition from stable to unstable cavity states.

 This second solution is available in variants defined by the method of controlling the stability of the cavity.

 For example, the mirror can be mobile and tiltable, or deformable. The change in shape of the laser mirror (plane / concave) can for example be carried out by deformation of a membrane produced in microtechnology (MOEMS).

 All of the membranes are for example made of silicon. The thickness of the membranes does not exceed, for example, 0.5 µm.

 The laser cavity mirrors (metallic or multi-electric) with a high reflection coefficient are then obtained by deposition, on all of the membranes, at the end of the process, during a collective manufacturing process.

 The invention also relates to an image projection device comprising several elementary devices as described above.

Each elementary device emits at its own wavelength.

 The covering of beams from the mosaics of transmitters of the elementary devices can be obtained with an optical device, for example of the cube corner type.

Each mosaic is preferably optically pumped from the side through which the beams of the microlasers are emitted. Thus, several "front" pumping modes are possible - front pumping, normal to the face by which the
microlaser beams are emitted, - pumping like power lasers of the type
disk lasers, - pumping of the mosaic, in wave configuration
guided.

 The image projection system, described above, is based on a point-to-point correspondence of the elements of the emitter mosaic with the pixels of the image.

 Another embodiment implements a mosaic with a lower number of points, but integrating a scanning of the beams (the extreme case of this configuration corresponds to image projection systems with three lasers (R, G, B) scanned).

 A typical configuration of this device can for example consist of a mosaic of lasers with only a thousand emitters (= 30 × 30 emitters in square distribution) associated with a device for scanning all the beams.

Compared to the first configuration described, this second configuration optimizes a certain number of parameters
the smaller number of emitters of the mosaic
(1000 instead of a million) reduces the
cut. With a step between transmitters of 100wum (which is
also more tolerant with regard to relationship (1) already
given above), the mosaic size is only
3 mm. The control circuit (MOEMS) is also
smaller and ultimately the price of the component
consisting of modular transmitters can be
reduced (collective manufacturing).

Reducing the size of the mosaic is also
favorable to an efficient pumping configuration. The
"laser disc" pumping configuration is thus
relatively easy to implement.

 The beams of several mosaics can be superimposed for example according to the methods described above (separator cube) or more simply by proximity of the mosaics: these being small, the parallax error remains low.

Brief description of the figures
In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to the appended drawings in which
- Figure 1 is an embodiment of a device according to the invention.

 - Figure 2 gives the power emitted by a microlaser, as a function of the incident pump power, for two types of stable and planar microlaser cavities.

 - Figure 3 shows a device comprising three elementary image projection devices according to the invention.

 - Figure 4 shows, for three elementary image projection devices according to the invention, a front pumping configuration.

 - Figure 5 is a detail of the optical system allowing the projection of divergent beams and the focusing of the pumping beam at multiple points.

 - Figure 6 shows the pumping of an elementary image projection device according to 1 invention, in "laser-disc" configuration.

 - Figure 7 shows the pumping of an elementary image projection device according to 1 invention, in "guided wave" configuration.

 - Figure 8 is a projection device comprising an elementary image projection device according to the invention and a scanning device, according to another embodiment of the invention.

Detailed description of embodiments of the invention
FIG. 1 represents a first embodiment of an elementary device for projecting images according to the invention. This device comprises a layer 2 of an active laser material. This material is generally a dielectric material. Such a material consists of a base material (for example: YAG) provided with a dopant giving it properties of active laser medium (for example: Nd3 +, in YAG for an emission at 1.06 μm). Various possible materials and dopants, depending on the desired emission wavelengths, are given in document EP-653,824.

In FIG. 1, the laser material is associated with a layer 4 of a saturable absorbent material. Various examples of this material, as well as the properties of the saturable absorbent element, are described in the aforementioned document EP-653,824. This document also describes the production of a saturable absorbent in the form of a thin layer, directly on the active laser medium 2.

 The cavity of each microlaser is defined by an inlet mirror 6 and an outlet mirror 8 from the cavity.

FIG. 2 gives the power Pp emitted by each microlaser as a function of the power P of an incident pumping beam, and this for a plane-plane cavity (curve I) and for a stable cavity (curve
II).

 Consequently, according to the invention, it is sought to modulate the emission power of each microlaser by transforming the initially stable cavity of each microlaser into an unstable cavity. With an equal pumping power, the transition from the stable state to the unstable state therefore makes it possible to reach a higher emission level of the microlaser.

 In practice, it is possible to seek to effect a change in shape of the mirror 8 of the microlaser, by deformation of a membrane 10, produced in microtechnology. This type of membrane can for example be made of silicon, according to the method described in the document by P. Reey et al., "A high density capacitive pressure sensor array for fingerprint sensor application" IEEE transducers, 1997, (Chicago, pp.

1453-1456). The thickness of the membranes is, for example, less than or equal to 0.54 μm. The membrane 10 is associated with an addressing circuit 12. The addressing circuit makes it possible to control several microlasers individually (in FIG. 1, two microlasers are shown, one in the off state "0", the other in transmission "1"). A projection device according to the invention may even comprise, for example, around 106 microlasers produced from the same wafer 2 of active laser material.

 The assembly of all of the membranes 10, with the mosaic of the sources 2, is ensured by “slice” hybridization methods, which are conventional methods in silicon technology (bonding, or adhesion, or fusion of beads) . Such methods are described for example in Journal of Micromechanics and Microengineering, vol. 1, no 3, up. 139-166 (1991).

 The mirror 8, metallic or multi-electric, with a high reflection coefficient, can be obtained by depositing the material of the mirror on all of the membranes 10, at the end of the process, or during a collective manufacturing process.

 The control circuit 12 is either integrated in the silicon wafer which supports the membranes 10, or is hybridized by transfer methods, for example using balls 14, 12 (see the reference above for the techniques of 'assembly).

 In FIG. 1, the device is also associated with means 20 for doubling the frequency of the beam emitted by the microlasers. It is, for example, a crystal of titanium and potassium phosphate (KTP).

This crystal can be plated on the mirror 6, the assembly then constituting a monolithic assembly. This crystal can also be incorporated inside the microlaser cavity, as explained in the document
FR-95 05650.

 The diagram in FIG. 1 shows two microlasers of the elementary image projection device, one of them being off (state "0ss') while the other is on (state" 1 ") and emitting at a wavelength defined by the nature of the laser material 2.

 An elementary image projection device can comprise a number N of elementary microlaser emitters. This number N can be arbitrary and important, for example: N = 106.

It is possible to assemble or bring together several elementary image projection devices so as to produce a complex image projection device such as that illustrated in FIG. 3, where three elementary devices, 22, 24, 26 are combined . Each of these elementary devices, by choosing the corresponding active laser medium 2, emits at a determined wavelength. For example, one emits in blue (B), another in green (V) and the third in red (R). The beams of the three devices are superimposed using an optical device 28, for example a cube corner device. Such a device is for example marketed by the company Balzers ("Thin Films Color
Cube").

 The pumping of the elementary devices is carried out using optical pumping means 30 (FIG. 4). When this optical pumping takes place at wavelengths in the near infrared (typically: 808 nm), the silicon of the control circuit 12 is not transparent for these wavelengths. We therefore seek to ensure a "front" pumping of the mosaics.

 FIG. 4 represents a completely frontal pumping, normal to the emission surface of each mosaic. A dichroic plate 32 makes it possible to reflect, in the direction of the mosaics, the pumping beam emitted by the pumping device 30. This dichroic plate 32 allows the beam 36 which results from the projection to pass in the direction of an optical projection system 34 beams emitted by the various elementary emitting devices 22, 24, 26.

 A set of microlenses 42, 44, 46 is placed in the vicinity of each of the emitter mosaics. This set allows on the one hand to obtain the divergence of all the beams from the different sources of the mosaic (to form the enlarged image) and, on the other hand, reciprocally, to focus the pumping beam 48 on each microlaser in the mosaic.

 FIG. 5 shows more precisely the arrangement of a set of microlenses 44 relative to the elementary device 24. The emission axis of each of the elementary microlaser emitters is designated in this FIG. 5 by the reference 24-1, 24 -2, 24-3, 24-4. It can be noted in this figure that the pitch of the microlenses is different from the pitch of the microlasers. This configuration allows the different beams to "bloom" in order to enlarge the projected image of the mosaic: in fact, the optical centers of the different lenses not being on the optical axis of the microlasers, the emitted beams are spread out. Such a system allows the projection of divergent beams 25-1, 25-2, 25-3, 25-4, and the focusing of the pumping beam at multiple points, that is to say for the different microlasers.

 The emitter mosaics have planar geometry. It is therefore possible to carry out pumping, as for power lasers, of the laser-disc type. Such a pumping structure is described in the document by A. Giesen et al., "Scalable concept for diode-pumped high-power solid-state lasers", Applied Physics, B58, p. 365-372 (1994).

 A planar pumping structure of a mosaic and microlasers according to the invention is shown diagrammatically in FIG. 6. The pumping beam of one, or more, pump diodes 50, 52, or a set of laser diodes power, is shaped and homogenized by a microoptic system which ensures a "mixing" of the different areas of the initial wavefront by flipping images, for example of the type marketed with excimer lasers (for example: by SOPRA or LAMBDA PHYSIK).

The emission beam of the microlaser matrix is then shaped by an optical projection system 54 adapted to the desired image.

 Pumping of the mosaic in guided wave configuration is also possible. This type of pumping achieves a high density of pump power thanks to the confined environment. This high pump power may be necessary to efficiently pump a laser whose operating temperature is likely to be high, due to the powers involved (population inversion of atomic levels is disadvantaged by the thermal population of almost fundamental level). A guiding structure, for example a high index layer between two areas of lower indices, can be obtained by co-doping of the amplifying medium 2. For example, for a YAG amplifying medium: Nid3, one can produce a high doping layer in neodymium on the active laser medium, by epitaxial growth in the liquid phase. The pumping structure is then that illustrated in FIG. 7. The pumping diodes 50, 52 emit a pumping beam in the direction of the guide layer 54, formed on the surface of the active laser medium 2. The coupling of the beams of the laser diodes with the guiding layer 54 can be produced by prisms or by networks 56, 58 on the surface of the layer 54.

Classic image projections (style
Barco) produce luminous fluxes of the order of 1000 lm, which represent - in energy units - a power close to 1.5W. Such power is readily available with mosaics of microlasers. Thus, even for a higher projector power, of the order of 20W (for use in daylight), the individual power of each of the microlasers does not exceed 20uW (value which can be compared to the power levels presented in figure 2).

To have good visibility in ambient light, an image emittance of 1000 lm / m2 is required, which implies a projector illumination of 3000 lux on the screen. We deduce the power of the projector on the basis of 10 W / m2, which makes it possible to envisage displays of several m2, with mosaics of transmitters of several tens of
Watts.

An important point for the use of such powerful projectors with regard to the operating conditions in eye safety. These are carried out as long as an observer cannot intercept the beam too close to the projector. Eye safety - class 1 - imposes a limit of 1 mW in a pupil of 7 mm in diameter, or 26 W / m2 (the surface power at the level of the mosaic is still around 104 W / cm2 for a projector of 20
W). It is therefore necessary not to risk intercepting the beam, coming from the projector, as long as its size is not at least 1m2 (for the envisaged case of a 20 W projector). Under normal conditions, eye safety is perfectly achieved on the projection plane (possibly in an area where spectators pass) because the illumination does not exceed 10 W / m2.

 This constraint is incommensurate with that of a laser beam projector. Indeed, in this case, it is the full power of the beam (20 W) which can destroy the eye of an observer, even if the scanning speed means that the interaction time is short.

 The projection system which has been described above can be used in point-to-point correspondence with elements of a projected surface: there are then as many elements of the mosaic of emitters as there are pixels of the image. An elementary image projection device 60 can also be used in combination with a scanning device 62. In this case, the emitter mosaic is smaller than the projected surface 64. A typical configuration of this device could then be consisting of a mosaic of lasers from only 1 thousand emitters (about 30x30 emitters squared) associated with a scanning device 62 of all the beams.

 Compared to the point-to-point configuration already described, this second configuration optimizes a certain number of parameters.

 In particular, the smaller number of emitters of the mosaic makes it possible to reduce its size.

 Furthermore, the reduction in the size of the mosaic is also favorable to an efficient pumping configuration. The "laserdisc" pumping configuration is thus relatively easy to implement.

 When we reduce the number of transmitters in the mosaic, and in order to keep the overall power, we seek to increase the power of the individual transmitters. This can easily be achieved since the individual transmitters can reach a power of 10 mW (see FIG. 2), which allows an overall power of the mosaic of at least 10W.

 The surface swept by each of the beams of the mosaic is only a fraction of the projected surface (1 / number of transmitters). The beam scanning angle remains small (of the order of a degree). Scanning can be performed (in two dimensions) with devices such as fluid prisms (used in some camcorders to "counter" vibrations), or vibrating mirrors. These deflection modes can be used simultaneously on all the beams as long as the "emission surface" is small.

 In this configuration, as a transmitter of the mosaic "scans" several points of the image, this one is modulated (PWM) at a frequency higher than that of the refreshment of the image: typically this frequency is 100 Hz x number of points, ie of the order of a few hundred kHz. This is still accessible to MOEMS techniques, and in any case achievable with modulation techniques such as that described in document US Pat. No. 5,502,737.

 The divergence of the beams (between them) will advantageously be obtained by a configuration of projection optics of the micro-optical type, such as that presented in FIG. 5.

 In this configuration with scanning, it is also possible to superimpose beams of individual transmitters, for example according to the methods already described above (for example separator cube). It is also possible to superimpose the beams by simple proximity of the mosaics, these being smaller and the parallax error remaining low.

Claims (25)

 1. Elementary image projection device, comprising a plurality of microlaser sources arranged in a matrix, each source comprising a cavity delimited by an input mirror (8) and an output mirror (6), as well as an active medium laser (2) and means (10, 12) for modulating, in intensity, each microlaser source.  2. Image projection device according to claim 1, each microlaser source being provided with means (4) for passive triggering.  3. Device according to claim 1 or 2, further comprising means (20) for frequency doubling of the radiation emitted by the microlaser sources.  4. Device according to claim 3, the frequency doubling means (20) being arranged inside each microlaser cavity.  5. Device according to claim 3, the frequency doubling means (20) being arranged outside each microlaser cavity.  6. Device according to one of claims 1 to 5, the microlasers of the matrix being arranged in a pitch of 50ut.  7. Device according to one of the preceding claims, the length of each microlaser cavity being less than or equal to 0.8mm.  8. Device according to one of the preceding claims, the means (10, 12) for modulating in intensity each microlaser source being means making it possible to change the cavity from stable to unstable, and vice versa.  9. Device according to one of claims 1 to 8, one of the mirrors (8) of each microlaser unit being mobile and tiltable, or deformable.  10. Device according to claim 9, the movable or deformable mirror being produced on a deformable membrane (10), controlled by an addressing circuit (12).  11. Device according to claim 10, the membrane (10) being made of silicon.  12. Device according to claim 11, the thickness of the membrane (10) being less than, 511m.  13. Image projection device comprising several elementary devices (22, 24, 26) according to one of claims 1 to 12, each elementary device emitting at a determined wavelength, different from that at which each other device emits. elementary.  14. An image projection device according to claim 13, comprising means (28) for overlapping the beams emitted by the various elementary devices.  15. An image projection device according to claim 14, the means (28) for overlapping the beams emitted by the elementary devices comprising a cube corner.  16. Device according to one of claims 13 to 15, comprising a set of three elementary devices (22, 24, 26).  17. Device according to one of claims 1 to 16, further comprising means (30, 32) for forming an optical pumping beam of the active laser medium of the elementary image projection device or of each elementary image projection device. images, each elementary device having an emission surface, through which the beams of the microlasers of this elementary device are emitted, the optical pumping beam penetrating into each elementary device normally at its emission surface.  18. Device according to claim 17, each elementary device being further provided with a set of microlenses (42, 44, 46), placed opposite its emission surface.  19. Device according to claim 18, the microlenses being arranged in a pitch different from that of the microlaser sources.  20. Device according to one of claims 1 to 16, further comprising means (50, 52) for forming an optical pumping beam of the active laser medium of the elementary image projection device or of each elementary device, and means for shaping the pumping beam, for pumping in laser-disc configuration.  21. Device according to one of claims 1 to 16, further comprising means for forming an optical pumping beam of the active laser medium of the elementary image projection device, or of each elementary device, and means (54, 56, 58) for guiding the pumping beam towards, the microlaser (s).  22. Device according to claim 21, the means (54, 56, 58) for guiding comprising a layer of high refractive index between two zones of lowest refractive index.  23. Device according to one of claims 1 to 22, further comprising a device (62) for scanning the beams emitted by the microlasers of the elementary device (60).  24. Device according to claim 23, except claim 6, the pitch between each microlaser emitter of the elementary device (60) being 100 µm.  25. Device according to claim 23 or 24, the scanning device (62) comprising fluid prisms or vibrating mirrors.