FIELD

The present application relates to an optoelectronic module, a method for operating an optoelectronic module and a Head-Mounted Display.

It is an object to provide an optoelectronic module comprising improved optical characteristics.

SUMMARY

According to at least one embodiment of the optoelectronic module the optoelectronic module comprises at least one semiconductor laser and a photonic chip. A semiconductor laser is intended for the emission of a coherent electromagnetic radiation. A photonic chip preferably comprises a radiation permeable material and optical structures which are suitable for the manipulation of an electromagnetic radiation, for example a waveguide.

According to at least one embodiment of the optoelectronic module the semiconductor laser emits a primary electromagnetic radiation. The primary electromagnetic radiation is coupled into the photonic chip. The primary electromagnetic radiation comprises a dominant wavelength. The dominant wavelength of an electromagnetic radiation is the wavelength at which the spectrum of the electromagnetic radiation has the highest intensity.

According to at least one embodiment of the optoelectronic module the photonic chip comprises at least one first waveguide and at least one optical Bragg reflector having a reflectivity which is modulated by an electrical modulation signal. The first waveguide transfers the primary electromagnetic radiation from the semiconductor laser to the optical Bragg reflector. The first waveguide may comprise a material which is permeable for the primary electromagnetic radiation. For example, the first waveguide is at least partially surrounded by a material having a lower refractive index for the primary electromagnetic radiation than the material of the first waveguide itself. Thus, the primary electromagnetic radiation is confined within the waveguide and propagates along the first waveguide.

According to at least one embodiment of the optoelectronic module a secondary electromagnetic radiation is coupled out of the photonic chip by means of at least one second waveguide. The second waveguide is arranged downstream of the Bragg reflector. The secondary electromagnetic radiation has a dominant wavelength which is modulated in dependence of the electrical modulation signal. The modulation of the electrical modulation signal can be any time-dependent variation and does not necessarily have to be periodic.

The optoelectronic module preferably emits a secondary electromagnetic radiation which lies at least partially within a spectral range which is visible to the human eye.

According to at least one embodiment the optoelectronic module comprises at least one semiconductor laser and a photonic chip, whereinthe semiconductor laser emits a primary electromagnetic radiation,the primary electromagnetic radiation is coupled into the photonic chip,the photonic chip comprises at least one first waveguide and at least one optical Bragg reflector having a reflectivity which is modulated by an electrical modulation signal, anda secondary electromagnetic radiation is coupled out of the photonic chip by means of at least one second waveguide, wherein the secondary electromagnetic radiation has a dominant wavelength which is modulated in dependence of the electrical modulation signal.

An optoelectronic module described herein is, inter alia, based on the following considerations: Semiconductor lasers emit a primary electromagnetic radiation which has a good beam quality, making them particularly suitable for use in Head-Mounted display units or projection devices. In addition to their good beam quality, semiconductor lasers also have a particularly small extension of an emission area that corresponds approximately to a point light source. This results in an advantageously high luminance, which can contribute to the miniaturization of optical systems. When using a semiconductor laser in a visible wavelength range, however, undesirable interference effects, for example in the form of speckles, can sometimes be perceived by an observer. These interference effects lead to uneven illumination and disturbing illumination patterns.

The optoelectronic module described herein is, among other things, based on the idea of generating a secondary electromagnetic radiation which has a varying dominant wavelength. The dominant wavelength of the secondary electromagnetic radiation can be modulated by means of a Bragg reflector whose reflectivity can be varied by means of an electrical modulation signal. Furthermore, by superimposing a plurality of primary electromagnetic radiations of a plurality of semiconductor lasers, a secondary electromagnetic radiation with an increased secondary spectral bandwidth can be generated. By varying the dominant wavelength of the secondary electromagnetic radiation, an increase in spectral bandwidth and a decrease in coherence length can be achieved. A less coherent radiation advantageously reduces the intensity of any unwanted interference effects. This has the advantage of reducing or eliminating the appearance of speckles.

According to at least one embodiment of the optoelectronic module the semiconductor laser has a front facet which is covered with an anti-reflection coating. The anti-reflection coating decreases the amount of radiation which is reflected back at the front facet and thus cannot be coupled out of the semiconductor laser. Preferably the front facet has a reflectivity for the primary electromagnetic radiation of at most 10%, preferably of at most 1% and particularly preferably of at most 0.001%. A low reflectivity of the front facet improves an efficiency of the optoelectronic module. Furthermore, if a reflectivity of the front facet is lowered, the optical feedback for the semiconductor laser is advantageously dominated by the modulated reflectivity of the Bragg reflector.

According to at least one embodiment of the optoelectronic module the primary electromagnetic radiation has a primary spectral bandwidth, and the secondary electromagnetic magnetic radiation has a secondary spectral bandwidth which is larger than the primary spectral bandwidth and which is controlled by the electrical modulation signal. A higher spectral bandwidth results in a decreased coherence of the electromagnetic radiation and thus decreases or eliminates disturbing optical interference effects, like speckles.

According to at least one embodiment of the optoelectronic module the dominant wavelength of the primary electromagnetic radiation and of the secondary electromagnetic radiation is in the visible spectral range. Here and in the following the visible spectral range shall be defined as the spectral range of wavelength being equal to or greater than 380 nm and being equal to or smaller than 780 nm.

According to at least one embodiment of the optoelectronic module the optoelectronic module comprises a plurality of semiconductor lasers, first waveguides and Bragg reflectors, wherein a first waveguide and a Bragg reflector are assigned to each semiconductor laser. The use of a plurality of semiconductor lasers can increase the power output. The power output of the optoelectronic module can thus easily be made scalable. For instance, a broad spectral range can be covered by an optoelectronic module which uses a plurality of different semiconductor lasers, wherein each semiconductor laser emits a primary electromagnetic radiation having a different dominant wavelength.

According to at least one embodiment of the optoelectronic module the semiconductor lasers are monolithically integrated. Preferably, all semiconductor lasers of the optoelectronic module are monolithically integrated. A monolithically integrated component can for example be a multi-ridge element which comprises a multiplicity of laser ridges integrated into one single semiconductor body. This facilitates an arrangement of the semiconductor lasers with a small lateral distance from each other. A lateral distance is a distance parallel to a main extension plane of the optoelectronic module.

According to at least one embodiment of the optoelectronic module the second waveguides extend to a side face of the photonic chip and are arranged within a lateral distance of less than 10 μm to each other at the side face of the photonic chip. An arrangement of the second waveguides in a small lateral distance at the side face of the photonic chip ensures a particularly small emission region. Such an arrangement is advantageous for focusing and a projection of the secondary electromagnetic radiation.

According to at least one embodiment of the optoelectronic module the second waveguide is a beam combiner which couples the secondary electromagnetic radiation of the semiconductor lasers in a common waveguide with a common output facet. Preferably, the second waveguide is a beam combiner which couples the secondary electromagnetic radiation of each semiconductor lasers in a common waveguide with a common output facet. Such a design allows for example the projection of a single RGB pixel by superposition of the primary electromagnetic radiation of several semiconductor lasers and simplifies a projection of the secondary electromagnetic radiation via optics arranged downstream of the common output facet.

According to at least one embodiment of the optoelectronic module the optoelectronic module comprises at least three different semiconductor lasers wherein each semiconductor laser emits primary electromagnetic radiation having a different dominant wavelength. For example, one semiconductor laser emits a primary electromagnetic radiation having a dominant wavelength in the spectral region of red light, one semiconductor laser emits a primary electromagnetic radiation having a dominant wavelength in the spectral region of green light and one semiconductor laser emits a primary electromagnetic radiation having a dominant wavelength in the spectral region of blue light. Such an arrangement allows manufacturing an RGB optoelectronic module which is capable of emitting a primary electromagnetic radiation having any desired color that lies within a color triangle that is spanned by the three different dominant wavelengths.

According to at least one embodiment of the optoelectronic module the semiconductor laser is arranged on the photonic chip. The arrangement of a semiconductor laser on the photonic chip facilitates an alignment of the semiconductor laser with respect to the first waveguide. Furthermore, temperature dependent misalignments can be decreased or avoided and a mechanical stability can be improved.

According to at least one embodiment of the optoelectronic module a region extending from the front facet to the first waveguide is filled with a filler material which is permeable for the primary electromagnetic radiation. The filler material can protect the front facet from deteriorating environmental influences. Thus, advantageously no hermetical encapsulation of the front facet is necessary. For example, the filler material comprises a silicone or epoxy material.

According to at least one embodiment of the optoelectronic module the first waveguide, the second waveguide and/or the Bragg reflector are made of one of the following materials: LiNb, ITO, SiN, SiO, liquid crystal-materials. Preferably, the first waveguide, the second waveguide and the Bragg reflector are made of one of these materials. The refractive index of ITO depends on an electric current flowing through it. The refractive index of LiNb, SiN, SiO and liquid crystal-materials depends on an electric voltage attached to it. Consequently the refractive indices of these materials can advantageously be modulated by using an electric modulation signal.

According to at least one embodiment of the optoelectronic module the first waveguide is a single mode waveguide. A single mode waveguide only allows the propagation of the transverse mode. Advantageously this results in a particularly homogenous beam intensity profile.

According to at least one embodiment of the optoelectronic module the first waveguide is tapered starting from the front facet towards the Bragg reflector. The tapering can increase an incoupling efficiency for the primary electromagnetic radiation which is generated in the semiconductor laser. For example, the first waveguide has a larger diameter at the end facing towards the semiconductor laser, and the diameter decreases with an increasing distance from the semiconductor laser.

According to at least one embodiment of the optoelectronic module the photonic chip comprises a substrate which is made from silicon, glass or sapphire and preferably provides mechanical stability. These substrate materials can be made particularly pure and thus used particularly well for the growth of additional layers.

According to at least one embodiment of the optoelectronic module the photonic chip comprises at least one optical modulator downstream of the Bragg reflector. The optical modulator can be used to modulate an intensity of the secondary electromagnetic radiation. Consequently, a pulse length of the secondary electromagnetic radiation can be adjusted. This is especially advantageous if the semiconductor laser should be operated in a continuous mode. Preferably the optical modulator is a Mach-Zehnder-modulator.

According to at least one embodiment of the optoelectronic module the photonic chip comprises an optical deflector which deflects the secondary electromagnetic radiation out of the main extension plane of the photonic chip. Preferably the optical deflector deflects the secondary electromagnetic radiation in a vertical direction, particularly preferably in in a direction perpendicular to the main extension plane of the photonic chip. For example the optical deflector is a prism, a grating or a diffractive META-optic component. The optical deflector may be arranged directly on the photonic chip. The direct arrangement of the deflector on the photonic chip provides for a particularly compact optoelectronic module.

According to at least one embodiment of the optoelectronic module each Bragg reflector is electrically connected to a common electrical potential or each Bragg reflector is electrically connected to an individual electrical potential. For example, each Bragg reflector comprises first electrical terminals which are preferably connected to different electrical potentials. Bragg reflectors which are individually electrically connected can be configured to be controlled simultaneously or to be controlled individually.

According to at least one embodiment of the optoelectronic module the front facet is slanted at an angle between 2° and 32° inclusive with respect to a main extension direction of the semiconductor chip or an incoupling facet of the first waveguide is slanted at an angle between 2° and 32° inclusive with respect to a main extension direction of the first waveguide. The main extension direction is parallel to a main emission direction of the semiconductor laser. The incoupling facet is an end surface of the first waveguide facing towards the semiconductor laser. Preferably, the primary electromagnetic radiation is mainly coupled into the first waveguide through the incoupling facet.

By slanting the front facets of the semiconductor laser and/or the incoupling facet of the first waveguide a reflection of the impinging primary electromagnetic radiation back into the semiconductor laser can advantageously be decreased or prevented. For example, part of the electromagnetic radiation is reflected back in such a way that it gains little or no further optical amplification. This can be helpful to suppress amplification of any unwanted side modes.

Preferably, the front facet and/or the incoupling facet is slanted according to a Brewster Angle for the primary electromagnetic radiation in order to minimize a reflectivity of the front facet and/or the incoupling facet.

According to at least one embodiment of the optoelectronic module the photonic chip comprises a photodiode, wherein part of the secondary electromagnetic radiation impinges on said photodiode. Preferably 10% or less of the secondary electromagnetic radiation are impinging on the photodiode.

The photodiode can be used to monitor the optical power of the semiconductor lasers.

According to at least one embodiment of the optoelectronic module the photodiode is realized by introducing Ge or Si in the second waveguide.

According to at least one embodiment of the optoelectronic module the module comprises a wave plate downstream of the photonic chip to alter the polarization state of the secondary electromagnetic radiation.

According to at least one embodiment of the optoelectronic module the module comprises a collimation optic downstream of the photonic chip to collimate the secondary electromagnetic radiation.

According to at least one embodiment of the optoelectronic module the photonic chip comprises at least one semiconductor optical amplifier (SOA).

The aforementioned exemplary embodiments, features and properties of a semiconductor laser relate both to optoelectronic modules with a single semiconductor laser and to modules with a plurality of semiconductor lasers. Furthermore, the disclosed features and properties can be implemented in all or only in some of the semiconductor lasers.

The aforementioned exemplary embodiments, features and properties of a first waveguide relate both to optoelectronic modules with a single first waveguide and to modules with a plurality of first waveguides. Furthermore, the disclosed features and properties can be implemented in all or only in some of the first waveguides.

The aforementioned exemplary embodiments, features and properties of a second waveguide relate both to optoelectronic modules with a single second waveguide and to modules with a plurality of second waveguides. Furthermore, the disclosed features and properties can be implemented in all or only in some of the second waveguides.

The aforementioned exemplary embodiments, features and properties of a Bragg reflector relate both to optoelectronic modules with a single Bragg reflector and to modules with a plurality of Bragg reflectors. Furthermore, the disclosed features and properties can be implemented in all or only in some of the Bragg reflectors.

A method for operating an optoelectronic module is also disclosed. The method for operating an optoelectronic module is particularly suitable for operating an optoelectronic module described here. This means that all features disclosed in connection with the optoelectronic module are also disclosed for the method for operating the optoelectronic module and vice versa.

According to at least one embodiment of the method for operating an optoelectronic module the Bragg reflector modulates the dominant wavelength of the secondary electromagnetic radiation with a modulation rate of at least 2 nm per ns. A high modulation rate allows for a significant increase of a spectral bandwidth in the secondary electromagnetic radiation without disturbing a perception of the emitted light for a human viewer by flickering for example.

According to at least one embodiment of the method for operating an optoelectronic module the Bragg reflector modulates the dominant wavelength of the secondary electromagnetic radiation within a spectral range of at least 10 nm. By varying the dominant wavelength of the secondary electromagnetic radiation within a spectral range of at least 10 nm a coherence length of the secondary electromagnetic radiation is advantageously reduced.

A Head-Mounted Display comprising an optoelectronic module is also disclosed. The Head-Mounted Display is particularly suitable for using an optoelectronic module described herein. This means that all features disclosed in connection with the optoelectronic module are also disclosed for the Head-Mounted Display and vice versa. A Head-Mounted Display is for example a near eye display, a Head-Up Display or a Virtual-Reality Headset.

An optoelectronic module described herein is particularly suitable for use in so-called “smart eyewear products” with which augmented reality (AR), or virtual reality (VR) units are realized. The optoelectronic module described herein can also be used in various projection systems for displaying image content, for example in glasses, close to the eye or for direct projection of an image into a human eye.

Further advantages and advantageous designs and further developments of the optoelectronic module result from the following exemplary embodiments, which are described below in association with the figures.

