Patent ID: 12218478

DETAILED DESCRIPTION OF EMBODIMENTS

Semiconductor lasers, such as vertical-cavity semiconductor lasers (VCSELs), can be used for illumination tasks especially in portable devices, in which compact light sources are required. Some applications require that the waist of the Gaussian beam emitted by a VCSEL be located at a specific distance from the VCSEL in order to match the locations of the other components of the device. For example, the application may require that the beam be focused so as to pass through a small aperture, such as an aperture between the pixels of a display. In back-emitting VCSELs, which are formed on a first face of a semiconductor substrate, the focal distance of the Gaussian beam (i.e., the distance to the waist of the beam) exiting through the second face of the substrate may be controlled for this purpose by a microlens formed on the second face. Due to size and process constraints, however, the focal distances of such microlenses may be too short to match the locations of the other components.

The embodiments of the present invention that are described herein address these problems by providing a compact optoelectronic device in which the beam emitted by a solid-state emitter through a semiconductor substrate, such as a back-emitting VCSEL, is reflected multiple times through the substrate and focused, so that the Gaussian waist of the beam is formed at a large distance from the substrate, relative to solutions that are known in the art.

In the disclosed embodiments, a microlens is formed on the semiconductor substrate opposite the emitter. Two reflectors are formed on the two faces of the substrate: a first reflector formed on the first face near the emitter, and a second reflector disposed on the second face on or near the curved optical surface of the microlens. The emitter emits a beam of optical radiation through the substrate toward the second reflector, which reflects the beam back through the substrate onto the first reflector. The first reflector further reflects the beam toward the curved optical surface, which refracts the beam and transmits it out of the substrate.

In the disclosed embodiments, the first reflector is planar, whereas the second reflector may be either concave, planar, or convex. A planar second reflector increases the effective optical thickness of the substrate (by causing the beam to pass through the substrate multiple times before exiting through the curved optical surface), and in this way increases the distance of the beam waist from the substrate. When employing a curved second reflector, the radii of curvature of the second reflector and the curved optical surface are chosen so that reflectors and the curved optical surface together form a telescope, which projects the beam waist of the emitted beam to the required distance.

In some embodiments, the second reflector is tilted so that the reflected beam is shifted laterally. This shift prevents the second reflector from obscuring the beam as it exits through the curved optical surface. An offset of the curved optical surface may be used to compensate for the tilt introduced by the second reflector. The direction of the lateral offset in the plane of the substrate may be controlled by the direction of the tilt and the respective offsets of the second reflector and the curved optical surface. By appropriate selection of the sizes and directions of these lateral offsets, a group of two or more (possibly up to six) emitters may be formed with the lateral distances between the emitted rays smaller than the distance between the respective VCSELs.

FIG.1is a schematic sectional view of an optoelectronic device20, in accordance with an embodiment of the present invention.

Optoelectronic device20is formed on a semiconductor substrate22. Typically, substrate22comprises a wafer of a III-V semiconductor compound, such as gallium-arsenide (GaAs), which is transparent to near-infrared radiation. A VCSEL24is formed on a first face25of substrate22using semiconductor fabrication methods and processes that are known in the art. A laser cavity for VCSEL24is formed by epitaxial deposition of thin-film layers to produce a lower distributed Bragg-grating (DBR)28and an upper DBR30, wherein the DBRs comprise highly reflective multilayer mirrors. A multiple-quantum-well (MQW) stack32, comprising a series of quantum wells disposed between a series of barriers, is deposited over lower DBR28, and upper DBR30is deposited over the MQW stack. For example, MQW stack32may comprise alternating InAlGaAs quantum wells and InAlGaAs barriers. VCSEL24is etched to form a mesa-structure with sidewalls26, and an oxide aperture is formed to define the optical and electrical current aperture of the VCSEL.

For the sake of simplicity, additional layers of VCSEL24, such as electrical conductors, have been omitted from the figure.

A curved optical surface36has been formed on a second face38of substrate22, forming a microlens40with a focal length of f. Such curved optical surfaces can be formed on substrate22by methods known in the art, such as gray-scale photolithography or the Confined Etchant Layer Technique (CELT). In the present embodiment, curved optical surface36is a spherical surface with a radius of curvature of R1. Alternatively, curved optical surface36may be an aspheric surface, such as, for example, a parabola, or may have any other suitable form.

Optical device20also comprises a second reflector deposited on second face38in proximity to curved optical surface36. In the pictured embodiment, second reflector42is formed over an apex44of microlens40. The area for second reflector42is formed in the same processing steps as curved optical surface36. Alternatively, additional patterning and etch steps, such as those, for example, described with reference toFIG.2, may be used to flatten apex44. Second reflector42may be either flat or have a concave or convex shape toward VCSEL24.

After etching of curved optical surface36and apex44, second face38is passivated, and an anti-reflective coating46, for example, comprising a quarter-wave layer of silicon-nitride (SiN) is deposited on curved optical surface36. Second reflector42is formed on apex44by coating the apex with gold, which provides a reflectance of 97% for near-infrared radiation emitted by VCSEL24(for example at 940 nm), or with another suitable metal or multi-layer thin film coating.

Lower DBR28and a metallic redistribution layer (RDL) (a metal layer between electrodes of VCSEL24and substrate22) together form a first reflector50on first face25. At the near-IR wavelength of VCSEL24, the reflectance of lower DBR28is typically 99%, and the reflectance of RDL48exceeds 94%. In an alternative embodiment (not shown in the figures), some of the epitaxial layers of lower DBR28are not etched away in forming the mesa structure of VCSEL24, and thus extend across face25of substrate22beyond sidewalls26and define the entire area of reflector50. In this case, reflector50may have higher overall reflectance, without gaps at the boundary between lower DBR28and RDL48.

VCSEL24emits a diverging beam52of optical radiation, which is reflected by second reflector42toward first reflector50as a beam54. First reflector50reflects beam54into a beam56, which propagates toward second face38. When second reflector42is planar (infinite radius of curvature), beam54continues diverging with the same divergence as beam52. Due to the planar form of first reflector50, this divergence is preserved in beam56, too, thus causing the outer part of beam56to impinge on curved optical surface36outside second reflector42. This part of beam56is refracted by curved optical surface36into a beam58, having a waist60located a distance zoutabove apex44. Due to the back-and-forth reflections between first and second reflectors50and42, respectively, the optical path for the beam emitted by VCSEL24and exiting through curved optical surface36is effectively tripled, as compared to a beam exiting without these reflections.

Assuming beam52to be Gaussian, the distance zoutis given by:

1zout=1f-1teff+zr2/(teff-f)(1)
wherein the parameters of the equation are:f=the focal length of microlens40.teff=the optical thickness of substrate22, tsub, multiplied by the number of passes of the beam through the substrate in the multi-pass geometry.zr=the Rayleigh range of VCSEL24.

This equation illustrates that for a given focal length f, the multi-pass geometry of device20can be used to position waist60at a substantially greater distance from second face38than would be possible in a conventional, single-pass geometry. Typical values for these parameters are tsub=100-200 μm, f=50-100 μm, and zr=50 μm, although other parameter values may alternatively be used.

In alternative embodiments, second reflector42is formed with a curved optical surface. Forming second reflector42with a convex shape towards VCSEL24provides for additional optical power for the optics of optoelectronic device20and requires a larger diameter for microlens40, as compared to a planar shape of the second reflector. Conversely, forming second reflector42with a concave shape towards VCSEL24reduces the optical power and the required diameter for microlens40.

Increasing the distance zoutto beam waist60in device20is particularly useful when device20is to be mounted behind a panel62, such as a circuit substrate, and is required to emit beam58through a transparent window64in the panel. By proper choice of the thickness of substrate22and the parameters of optical surface36, it is possible to achieve values of zoutin the range of 300-500 μm, which is sufficient to enable mounting substrate22behind panel62so that waist60is aligned within window64with reasonable design tolerances. When aligned in this manner, beam58will pass through panel62with only minimal scatter and loss.

For example, electronic display layouts can be designed with a transparent window in a gap between the pixel circuit elements within each pixel of the display. Such a display typically comprises a substrate, such as glass, which is transparent to optical radiation at wavelengths in the visible and near infrared ranges. An array of display cells is formed on the substrate by methods of display fabrication that are known in the art. Each display cell comprises pixel circuit elements66, such as an OLED (organic light-emitting diode) and a TFT (thin-film transistor) for switching the OLED, as well as conductors connecting the pixel circuits to electronics external to the display.

The display cells are spaced on the substrate (for example, on panel62in the example shown inFIG.1) at a certain pixel pitch, with gaps of a predefined size, defining one or more transparent windows64, between the pixel circuit elements. One or more VCSELs24are placed behind respective windows64and are aligned so that each beam58passes through the corresponding window. Specifically, device20is designed, based on the principles explained above, and aligned with the display so that the beam waist60of each device falls within window64.

A single VCSEL24or an array of VCSELs behind an array of windows64in a display panel can thus provide illumination for applications of a mobile computing device, such as a smartphone or tablet computer. In this manner, the area of the display can be maximized, relative to the size of the computing device, without requiring that panel space be allocated for the illumination source. For example, the VCSELs can illuminate the area in front of the display for applications such as3D mapping or face recognition.

FIG.2is a schematic sectional view of an optoelectronic device100, in accordance with another embodiment of the present invention.

Optoelectronic device100comprises, similarly to optoelectronic device20, VCSEL24on first face25of substrate22and microlens40on second face38. A planar second reflector102, having a normal103to its surface, is tilted with respect to a normal105of substrate22(i.e., normal103is not parallel to normal105of substrate22). As in the embodiment ofFIG.1, the tilted area is coated with gold or other suitable materials for high reflectance. Alternatively, second reflector102may have a curved shape.

Beam104emitted by VCSEL24impinges on second reflector102, which reflects it as a beam106toward first reflector50. As second reflector102is tilted, beam106impinges on first reflector50at a non-normal angle and with a lateral offset from VCSEL24. Consequently, with a suitable tilt angle of second reflector102, a beam108reflected from first reflector50impinges on curved optical surface36of microlens40avoiding second reflector102, thus reducing multiple reflections and increasing the output coupling efficiency. Beam108is refracted by microlens40and exits as a beam110. In the embodiment shown inFIG.2, beam110exits from microlens40at a non-zero exit angle α with respect to normal105of substrate22. In an alternative embodiment, beam110is directed along a normal to substrate22(i.e., exit angle α is brought to zero) by a suitable lateral offset of microlens40relative to VCSEL24, as shown inFIG.3, below.

FIG.3is a schematic sectional view of an optoelectronic assembly150comprising a pair of optoelectronic devices200and202, in accordance with an additional embodiment of the present invention. Optoelectronic devices200and202, formed on a substrate203, are similar to optoelectronic device100. Optoelectronic device200comprises a VCSEL204, a microlens206, and a tilted second reflector208, corresponding respectively to VCSEL24, microlens40, and second reflector102of optoelectronic device100. Microlens206is positioned with an offset210between its optical axis212and an optical axis214of VCSEL204. Offset210has been selected so that a beam216, exiting from microlens206, is perpendicular to substrate203, with an offset of218from optical axis214of VCSEL204. Additional labels, such as inFIG.2, have been omitted for the sake of clarity.

Optoelectronic device202comprises a VCSEL220, a microlens222, and a tilted second reflector224. Optoelectronic device202is identical to optoelectronic device200, except that it is oriented with a 180° rotation around a normal to substrate203by comparison with device200. Thus, a beam226emitted by optoelectronic device202is perpendicular to substrate203with an offset228to an optical axis230of VCSEL220, equal and opposite to offset218of device200.

Due to the relative orientations of optoelectronic devices200and202, a spacing (or pitch)232between beams216and226is substantially smaller than a spacing234between optical axes214and228of VCSELs204and220. Thus, beams216and226may pass together through a relatively narrow aperture (not shown) above device150and create a dense pattern of beams or a combined beam of high intensity in the far field.

In alternative embodiments, a larger number of optoelectronic devices, similar to devices200and202, may be arranged in close proximity to each other and with suitable rotations around normals to the substrate, thus achieving beam-to-beam pitches that are smaller than the corresponding VCSEL-to-VCSEL pitches. A reduction of beam-to-beam pitches may be achieved with up to six devices.

It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.