Optical wireless power transmission using laser diodes

Abstract

A system for transmitting laser power to a remote receiver, comprising a laser diode, an optical system for collimating and focusing the laser beam to the receiver, and a scanning system for directing the beam towards the receiver. The beam has a wavelength of 1150 to 1550 nm, a power exceeding 300 mW, an emitter width of 15 to 250 μm, is a spatial multimode beam, and more than 50% of its power is transmitted via TEM. 00 In the mode, less than 15% of its power, and a higher-order TEM with more than 20 (n+m) nm The mode has a laser oscillation cavity with a Fresnel number between 0.01 and 20, and a low-speed axis M of less than 15. 2 , high-speed axis M less than 1.5 2 It has a high-speed axial divergence angle exceeding 60°. The collimated laser beam has a single lobe when imaged up to a range of 10 meters.

Description

[Technical Field]

[0001] The present invention relates to the field of laser-based wireless power transmission for supplying power to a remote receiver, and more particularly to the need for a power source capable of providing a beam to ensure efficient and secure transmission of wireless power to the remote receiver over a required distance. [Background technology]

[0002] Systems exist that transmit optical power to remote locations without requiring physical wire connections. This need has become increasingly important in recent decades with the proliferation of portable electronic devices that operate on batteries requiring periodic recharging. Currently, the battery capacity of state-of-the-art technology and the typical battery usage of heavily used smartphones mean that batteries may require multiple charges per day, making the need for remote wireless battery recharging crucial. Several conventional systems have been proposed to securely transmit power to remote locations characterized by distances significantly greater than the dimensions of the transmitting or receiving device. A typical configuration involves transmitting power to a smartphone-sized receiver over typical distances in a home room setting. Reception of the transmitted power is generally done by using a photovoltaic cell, allowing for the secure transmission of optical power from the transmitted laser beam to the mobile device. Therefore, the transmitted laser beam must have sufficiently high power to perform its intended function, but it must provide that power level while maintaining a tightly collimated beam that transmits most of its power to the photovoltaic cell. As a result, efficient transmission of optical energy is achieved without endangering the environment or people along the transmission area due to excessive beam divergence and leakage.

[0003] A favorable power source for generating such laser power is the laser diode, used in many industrial, analytical, and medical applications that utilize laser power. Many of these applications using diode lasers require very different beam characteristics, and the laser diode industry has provided devices to meet these diverse needs. For example, laser diodes for analytical spectroscopy or microscopy should generally have low power levels but narrow linewidth wavelength emission and a beam with very low divergence, typically less than 2 mrad when collimated. On the other hand, laser diodes, or laser diode bars, or arrays of laser diodes, for use in industrial processes such as cutting or welding should have the highest possible power, but beam divergence is generally less important. Laser beams for medical applications such as ablation or laser scalpels require a well-focused beam but can be multimode with relatively high divergence from a complex mode structure.

[0004] In attempts to increase the efficiency of laser diodes, and consequently the power output as a function of input power, laser diodes with asymmetric structures have recently been developed as an alternative to lasers with symmetric structures, i.e., structures referred to in this industry as "nearly symmetric." Several such asymmetric diode structures are described in U.S. Patent No. 8,798,109, "High-Efficiency Diode Laser," by G. Erbert et al., in the paper "High-Power 1.5 μm Pulsed Laser Diode with Asymmetric Waveguide and Active Layer Near p-Cladding," by L. W. Allman et al. (IEEE Photonics Technology Letters, Vol. 31, No. 20, October 2019, pp. 1635-1638), and in the paper "High-Efficiency High-Power Laser Diode," by P. Crump et al. (IEEE J. of Selected Topics in Quantum Electronics, Vol. 19, No. 4, July-August 2013).

[0005] The disclosure of each publication referred to in this section of the specification and other sections is hereby incorporated by reference in its entirety into this specification.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

[0008] The application of laser beams for wireless transmission of optical power to remote receivers involves a combination of two inherently conflicting characteristics. First, a sufficient power level is required to meet the needs of the power receiver, which typically means a power level only available from multimode diode lasers that operate to emit multimode beams, given the current state of laser diode technology. Second, the beam needs to be collimated to such an extent that the majority of its power is incident on the receiving element, which means generating a beam from a laser diode with low-order modes. Thus, such a combination is generally considered a contradiction in the characteristics of laser diodes, making it difficult to achieve the construction of an efficient laser power transmission system to a remote receiver. Because of this difficulty, some conventional wireless optical power transmission systems are based on lasers other than diode lasers, or alternatively, even when diode lasers are used, employ receivers with large-aperture photovoltaic detectors that eliminate the need to focus the beam into a small spot.

[0009] The requirements for laser projection, which means forming a small, high-power laser spot at a certain distance from the projector, are complex. On the other hand, the laser beam needs to have good optical quality to be focused into a small spot. Therefore, some prior art systems require the use of single-space-mode diodes, which are suitable for low-power applications but unsuitable for high-power applications because single-mode diodes generally have limited power output. Currently, no single-mode diode lasers with optical power outputs exceeding 600mW are commercially available. Other prior art systems use multi-mode diodes, but this limits the projection range over which the laser can form a sufficiently small spot, because multi-mode beams cannot be sufficiently collimated without significant power loss.

[0010] The present invention discloses an optical power transmission system having a laser diode that is suitable for projection applications requiring high power, but does not impair the laser diode's ability to project onto a small spot at a desired long distance, such as in optical wireless power transmission applications.

[0011] To implement and operate such a projection system, the laser beam should have at least the following three characteristics. Alongside each beam characteristic, specific features or structures required of the laser diode to achieve such laser beam characteristics are described.

[0012] wavelength The laser beam should have a wavelength between 1150 nm and 1550 nm to ensure eye safety, and the power emitted outside this range should be less than 2 mW. Since the wavelength emitted from the laser diode depends on the band gap of the active gain medium used in the laser diode, the band gap of the gain layer should be selected to be in the range of approximately 0.8 eV to approximately 1.2 eV with respect to the aforementioned beam wavelength range. (A high band gap of 1.2 eV corresponds to a higher energy wavelength, i.e., a wavelength shorter than 1150 nm, because the band gap tends to decrease somewhat as the gain medium heats up as the diode oscillates.) Such a band gap can be achieved by constructing the laser diode on a III-V or II-VI semiconductor substrate and having a gain layer consisting of one of the following: a. Quantum dot structure, b.GaInAs composition, c.GaAsSb composition, d.InPAs composition, e. InAlAs composition, or f. Quaternary material.

[0013] Beam Power The beam output power should be at least 300mW to provide, for example, enough power to charge a typical mobile phone battery within an acceptable time after conversion in a receiver. Considering the efficiency of the laser diode itself, the input drive current to the diode should be at least 800mA with an applied voltage of at least 0.8V, and its value is determined by the gain medium used and its IV characteristic curve.

[0014] Beam Mode • As a preliminary comment, it should be clarified that in the context of this application, the terms “single-mode” and “multi-mode” are used to describe a single-space (or transverse) mode and a multi-space (or transverse) mode, respectively, and not a single-longitudinal mode or a multi-longitudinal mode. A laser diode can be a multi-mode device to ensure a sufficiently high power level, but the design parameters of a laser diode are such that the emitted multi-mode beam has a specific mode structure, and at least 50% of the power of its output beam is Hermitian Gaussian TEM 00 Higher-order TEMs are those that focus on modes, with less than 15% of their power being when the mode order is greater than 20 (n+m). nm The design parameters should be such that the beam is in a specific mode. These design parameters are described in more detail below. As mentioned above, these mode characteristics are carefully selected to provide a beam that is compatible with the generally conflicting characteristics of a multimode beam: high power output associated with a multimode beam and good collimation characteristics associated with a single-mode beam.

[0015] These characteristics are obtained by carefully selecting the geometry of the laser diode, particularly the width of the gain layer. As shown below, the cladding layer of the diode structure adjacent to the gain layer is a layer that defines the outer range of the active cavity or resonator region in the fast axis direction of the laser diode, and can be regarded as the emission height in the fast axis. The positive and negatively doped layers adjacent to the thin laser amplification layer called the gain layer have a refractive index n1, while the cladding layer has a refractive index n2 lower than that of the gain layer. Typically, there are two such layers with the same refractive index n2. In the case of different refractive indices n2 and n3, when calculating the laser diode characteristics shown below, the lower of the two should be used.

[0016] To achieve the above-mentioned mode distribution from the diode where most of the power is located in the low-order mode, the Fresnel number FN of the cavity in the slow axis, which is the larger dimension of the gain region cross-sectional area, should be within the range given by the formula 0.01 < FN < 20. FN is given by the formula w 2 / λL, where L is the total length of the laser oscillation cavity. To provide these values, the emitter width w should be within the range of 15 to 250 microns.

[0017] With such a mode distribution, the M 2 value of the beam in the slow axis direction is allowed to be less than 15.

[0018] Since the height of the gain area (defined as the positively doped layer, the laser amplification layer, and the negatively doped layer) is small, the M 2 value of the fast axis is less than 1.5, and the divergence angle of the fast axis is less than 60°.

[0019] Such a selection of the M 2 value and the mode structure allows the beam mode to have a single lobe when imaged at a distance of 10 m, and allows the focused spot size r at such a distance to reach an optimal size.

[0020] The spot size r generated by the focused beam will be within the following range:

number

[0021] It should be understood that in many such systems, the spot is not circular, so throughout this disclosure, the term “effective radius” refers to a measurement that is half the average lateral dimensions of the irradiated spot, including 95% of the spot power.

[0022] A brief overview of the notable features of a typical implementation of the system described in the claims of this disclosure, along with a brief explanation of the motivation for each feature where necessary, can be expressed as follows:

[0023] A system for transmitting laser power from a transmitter to a remote receiver, A laser diode light source containing a gain medium and supplied with current from a laser driver, An optical system that collimates the laser beam emitted from the laser diode light source to generate an illumination spot on the remote receiver, The system comprises a scanning system that directs the collimated laser beam towards the aperture of a photovoltaic cell in the remote receiver, (i) The laser diode emits a beam with a wavelength λ between 1150 nm and 1550 nm that is invisible to the human eye, and provides enhanced safety due to water absorption at longer wavelengths in that range. (ii) The laser beam has a power of at least 300 mW so as to be strong enough to supply the desired power to the client device. (iii) The laser diode light source has an emitter width w between 15 and 250 μm that enables the laser beam to be achieved as a spatial multimode beam, and at least 50% of the power of the spatial multimode beam is concentrated in the Hermite-Gauss TEM 00 mode, and less than 15% of its power becomes a higher-order TEM nm mode having an order where (n + m) exceeds 20, thereby facilitating the generation of a beam having an appropriate M 2 value, (iv) The laser diode light source has a laser oscillation cavity having dimensions such that the laser beam emitted therefrom has a fast axis and a slow axis, (v) The laser oscillation cavity of the laser diode has a Fresnel number w 2 / (λL) between 0.01 and 20, where L is the length of the laser resonator, The above characteristics result in the resulting beam having (vi) An M value of less than 15 in the slow axis direction, 2 and, (vii) An M value of less than 1.5 in the fast axis direction, and a divergence angle of less than 60° of the laser beam in the fast axis direction, 2 and, (viii) The mode of the collimated laser beam having a single lobe when imaged at a distance of 10 m by the optical system, (ix) And a spot size r of the generated laser beam given by the following formula

Equation

[0024] Therefore, according to a typical implementation example of the device described in this disclosure, a system is provided for transmitting laser power from a transmitter to a remote receiver, and the system is The laser diode includes a laser diode that emits a laser beam for optical wireless power transmission, the laser diode is (i) comprising a first cladding layer having a first refractive index and a second cladding layer having a second refractive index, wherein an emission region exists between them, and the emission region is (ii) A positive doped layer having a third refractive index and a negative doped layer having a fourth refractive index, wherein the third and fourth refractive indices are each greater than either of the first and second refractive indices, (iii) A gain medium layer deposited between the positive doped layer and the negative doped layer, the gain medium layer having an energy band gap between 0.8 eV and 1.2 eV, The width w of the emission region in meters is,

number

[0025] In such a laser diode, the first and second refractive indices may have substantially the same value, and the third and fourth refractive indices may have substantially the same value.

[0026] Furthermore, according to further implementation examples of such laser diodes, the first predetermined percentage should be 50%, the second predetermined percentage should be 15%, the number of presets should be 20, and the desired level of laser beam output power should be 300mW. In the last case, the laser beam power level for wavelengths outside the range of 1150nm to 1550nm should be less than 2mW.

[0027] In any of the laser diodes described above, the cavity has a Fresnel number w in the low-speed axis with respect to the range of w in claim 1. 2 The length L may be such that / λL is between 0.01 and 20, where L is measured in meters.

[0028] Furthermore, in any of the above laser diodes, the laser beam is (i) M less than 15 in the low-speed axis direction 2 The value and, (ii) The laser beam may have an M2 value of less than 1.5 in the high-speed axis direction and a divergence angle of less than 60° in the high-speed axis direction.

[0029] In addition, according to yet another implementation example of such a laser diode, (i) The mode of the collimated laser beam is such that when the beam is imaged at a distance of 10 m by a suitable optical system, the beam has a single lobe. (ii) The effective radius r of the focused laser beam generated at the distance of 10 m is given by the formula

number

[0030] In all of the above implementation examples, the effective radius r of the focused laser beam should include 95% of the laser beam power.

[0031] The laser gain medium layer of the laser diode described above is (i) Quantum dot real layer (ii) GaInAs composition (iii) GaAsSb composition (iv) InPAs composition (v) AlAs composition or (vi) Quaternary materials It may include any of the following:

[0032] Finally, in any of the laser diodes described above, the cladding layer and the doped diode layer may have a structure that is symmetrical or nearly symmetrical with respect to the gain medium layer.

[0033] According to yet another typical implementation example of the device described in this disclosure, a system for transmitting laser power from a transmitter to a remote receiver is further provided. The system is (i) A laser diode light source comprising a gain medium sandwiched between p-type and n-type doped layers, wherein a laser diode light source is supplied with current from a laser driver, (ii) An optical system that collimates the laser beam emitted from the laser diode light source to generate an illumination spot on the remote receiver, (iii) A scanning system that directs the collimated laser beam towards the aperture of the photovoltaic cell in the remote receiver, (a) The gain medium of the laser diode is selected such that the laser diode emits a beam at a wavelength λ between 1150 nm and 1550 nm. (b) The current supplied to the gain medium by the laser driver is selected such that the laser beam has a power of at least 300 mW. (c) The laser diode light source has an emitter width w between 15 and 250 μm, The aforementioned laser beam is a spatial multimode beam, and at least 50% of its power is Hermitian Gaussian TEM 00 Higher-order TEM focused on modes where less than 15% of its power is greater than 20 (n+m). nm In mode, The length of the gain medium of the laser diode is such that the Fresnel number w of the laser diode is in the low-speed axis. 2 / λL is selected to be between 0.01 and 20, where L is the length of the laser cavity. Less than 15 M in the low-speed axis direction 2 The value and, M less than 1.5 in the high-speed axial direction 2 The value and, Divergence angle of less than 60° of the laser beam in the high-speed axis direction, The single lobe mode of the collimated laser beam when imaged at a distance of 10 m by the optical system, The resulting focused spot of the laser beam is,

number

[0034] In such a system, the bandgap of the gain medium of the laser diode should be in the range of 0.8 eV to 1.2 eV. In addition, such a laser diode may be constructed on a substrate of a Group III-V or Group II-VI semiconductor. Furthermore, the gain medium layer may include a quantum dot structure, a GaInAs composition, a GaAsSb composition, an InPAs composition, an InAlAs composition, or a quaternary material.

[0035] Furthermore, in such a system, the focused spot size may have an effective radius r that includes 95% of the beam power. In addition, the power of the laser diode emitted at wavelengths outside the range of 1150 nm to 1550 nm must not exceed 2 mW.

[0036] Finally, in such a system, the gain medium may be placed between an n-type doped layer and a p-type doped layer having an average refractive index n1, and these layers themselves are placed between cladding layers having a refractive index n2. In such a system, the width w of the gain medium is given by:

number

[0037] In a further typical implementation example of the above system for transmitting laser power from a transmitter to a remote receiver, the electrical connection for supplying current from the laser driver to the laser diode should be an isolated connection adapted to prevent the possibility of accidental electrical contact with the laser diode, thereby increasing the safety of the system.

[0038] In such a system, the electrical connections supplying current from the laser driver to the laser diode should have at least one gate switch to control the flow of current through each electrical connection. These gate switches should be activated by a gate driver having an operating voltage higher than the operating voltage of other electronic circuits in the system that provide control functions to the system. The gate driver may be configured to hold each gate switch in a conducting state when the gate driver is commanded to activate the gates. In such a case, when the operating voltage of the gate driver drops to a level below the operating voltage of other electronic circuits in the system that provide control functions to the system, the gate switches return to a non-conducting state. Thus, this non-conducting state isolates the laser diode from any current sources, even if it results from an accidental electrical connection to the laser diode. [Brief explanation of the drawing]

[0039] The present invention will be better understood and appreciated by referring to the following detailed description in conjunction with the accompanying drawings.

[0040] [Figure 1] Figure 1 schematically shows a typical laser power transmission system described in this disclosure for supplying optical power to a remote receiver. [Figure 2] Figure 2 shows the first set of multiple Hermitian-Gaussian beam modes TEMmn and their relative spatial extent. [Figure 3] Figure 3 schematically shows a perspective view of a laser diode structure according to a typical implementation example of the present invention. [Figure 4] Figure 4 is a top view of the diode laser shown in Figure 3. [Figure 5] Figure 5 schematically shows a block diagram of the key functions of the laser diode power supply scheme, which provides a high level of operational safety even in the event of an accidental electrical connection to the laser diode. [Modes for carrying out the invention]

[0041] First, refer to Figure 1. Figure 1 schematically illustrates a type of laser power transmission system used in this disclosure to power a remote receiver. The transmitter 10 includes a laser source, which is advantageously a laser diode 16, and elements necessary to control and direct the emitted laser beam 12. The transmitter also incorporates a laser driver 15 that powers the laser diode 16, a controller 13 that operates to keep the laser running in the desired and required manner, and a safety system 14 that ensures the laser beam transmission is carried out without posing a danger to the environment in which the transmission system operates or to any person in that environment. The laser beam 12 emitted from the diode laser 16 diverges substantially up to 60° along the high-speed axis and up to 30° along the low-speed axis, although the typical low-speed divergence is significantly smaller, as low as 10°. Therefore, the beam needs to be collimated by a focusing system 17, which forms a focal or virtual focal point of the beam at a certain distance from the transmitter, typically at a distance of at least 200 times the diameter of the focusing lens. Subsequently, the beam is directed by the beam deflection unit 18 along the correct path to the target receiver 11 as a collimated beam 12, or a beam 12 that is nearly collimated due to the impossibility of complete collimation, where it collides with the photovoltaic cell and is converted into power. The method described below shows how to construct a system using a laser diode 16 having a combination of characteristics such that the laser beam 12 reaches the target focused enough to transfer most of its power to the photovoltaic cell of the receiver 11, while at the same time maintaining the power level necessary for the receiver to perform its intended function.

[0042] Laser diodes are generally classified into single-space-mode laser diodes and multi-mode laser diodes. Single-mode diodes generally provide good beam quality, while multi-space-mode laser diodes provide inferior beam quality. However, single-mode diodes have power limitations and therefore cannot be used for projection applications such as the wireless power supply applications described in this disclosure.

[0043] Throughout this document, the abbreviated terms “single-mode” and “multi-mode” refer to single-space (or transverse) mode and multi-space (or transverse) mode outputs, respectively, and do not refer to single / multi-longitudinal modes of a laser resonator. Single / multi-longitudinal modes are different terms related to changes in beam cross-section along the laser resonator and are largely irrelevant to this application. Furthermore, throughout this disclosure, the term TEM mode generally refers to Hermitian-Gaussian modes, but may in some cases refer to Laguerre modes.

[0044] Generally, the beam quality of single-mode lasers is nearly perfect, and TEM 00 It emits a nearly pure Gaussian beam, also known as a mode, which can be easily collimated or focused into a diffraction-limited spot. Single-mode lasers are M 2 The value is close to 1. Here, M 2 Lasers with a value less than 1.2 are generally considered single-mode lasers. In such single-mode lasers, almost all of the emitted power is focused to the central Gauss lobe of the collimated beam at optical infinity and can be focused to a diffraction-limited spot regardless of the dimensions of the laser emitter aperture.

[0045] On the other hand, multimode lasers are TEM mn Supporting many spatial modes designated as modes results in inferior beam quality. Therefore, the output of a multimode laser cannot be collimated without significant power loss. Multimode lasers are used in many TEMmn Supports mode. Here, M 2 The factor values ​​are given by (2n+1) in the x direction and (2m+1) in the y direction. As a result, low-order TEM 01 Even in mode, 3 M in the Y direction 2 It has a value, TEM 05 11 M in the Y direction 2 It has a value. The M of the beam 2 Since the value is directly proportional to the ability to collimate and / or focus the beam, a multimode beam from a standard multimode laser cannot be collimated or focused to a diffraction-limited spot without significant power loss caused by the loss of the portion of modes that form the outer lobes of the beam.

[0046] Due to inherent power losses from the beam, projecting multimode beams over long distances is inefficient and typically unsafe. Therefore, when laser diodes are used as light sources in optical power transmission applications, generally only single-mode diode lasers are used, and since single-mode diode lasers have limited power, the available power is also limited.

[0047] It should be noted that, due to the ratio of the cavity dimensions of the multiple diode lasers, their beams will have different M values ​​in different directions. 2 It can have a value close to 1. A single-mode diode laser beam typically has a value close to 1 in the "high-speed axis," which is the narrow axis of the diode laser cavity. 2 It has a value close to 1 in the "low-speed axis," which is the wide axis of the diode laser cavity. 2 While having a value, multimode diode lasers have a value significantly greater than 1.2 in the "low-speed axis," typically nearly 10 or more. 2 It holds.

[0048] Refer to Figure 2 here. Figure 2 shows the first Hermitian-Gaussian beam-mode TEM, as is well known in the industry. mnA set of images and their relative extent are shown. TEM 00 The extended spatial extent of modes other than the fundamental mode is evident from the mode shapes shown in Figure 2, and therefore it is clear that a compact focused spot cannot be obtained using higher-order mode beams.

[0049] Laser beams emitted from single-mode diodes are generally TEM 00 Includes only Hermitian Gaussian modes. In some cases, the modes may be slightly distorted by diffraction from the edges of various apertures in the laser, such as in an emitter, but the M of the beam in both lateral directions. 2 As long as the value is less than 1.3, preferably less than 1.2, the laser is still generally considered a single-mode laser. Currently, single-mode diode lasers with the necessary wavelengths suitable for laser power transmission are limited to power levels of less than 300 mW, which limits their usability in projection applications.

[0050] On the other hand, multimode diode lasers are mixtures of many modes, typically TEM 00 , 01 , 02 , 03 , 04 , 05 , 06 ... 020 It emits a beam containing a variable mixture of many TEM modes, such as [example of TEM modes].

[0051] A single-mode beam can be focused to or near its diffraction limit. Therefore, as can be easily derived from optical design principles, a single-mode beam with a wavelength of 1 μm emitted from an emitter of any size, when focused by an optical system with a numerical aperture of 0.01, forms a spot of approximately 122 microns at its optimal focusing distance. The projected spot size does not change based on the focal length of the lens used; only the numerical aperture of the focusing beam affects the spot size. These conclusions, of course, only apply to perfect lenses without aberrations, and similarly perfect lasers and intermediate media.

[0052] On the other hand, higher-order modes are (2n+1) M 2 The beam has a value and cannot be focused to the diffraction limit. A multimode beam typically only forms an image of an approximate size obtained by multiplying the size of the emitter by the magnification of the focusing optical system. Therefore, a typical multimode beam emitted from a 500 μm × 1 μm emitter and focused by a focal length lens of f = 10 mm placed 10.1 mm from the emitter will form a minimum spot at a distance of 10 m. Since the magnification of that system is approximately 1:1000, the length of the image formed at a distance of 10 m will be approximately 0.5 m. The beam is diffraction-limited in other dimensions, and the focused "spot" becomes an elongated line. Since such a long line cannot be absorbed by conventionally shaped and sized photovoltaic cells, such diode lasers are not suitable for use in long-distance projection applications.

[0053] To form a spot smaller than 1 cm from that multimode diode, the system magnification should be 20 or less, and the lens must be positioned 50 cm from the emitter. Such a system, even with a complex optical system instead of a single lens, would make the transmitter large and expensive, exceeding the acceptable limits for most applications.

[0054] Returning to the system shown in Figure 1, the laser diode 16 emits an expanding light beam, and the divergence angle of the emitted light depends mainly on the thickness of the p-type and n-type doped layers (including a thin gain medium layer). The thinner this combination of layers, the greater the divergence in the high-speed axial direction. Therefore, in order to transmit light through an aperture small enough to accommodate a reasonably sized device while effectively focusing the light, the optical surface of the focusing system 17 facing the laser diode 16 must be positioned at a distance d from the diode beam emission surface that lies between the following two limits, as indicated by the basic optical design principles.

number

[0055] This creates a spot in the receiver, where 95% of the power is contained within a circle of radius r measured in meters, and its range is given by:

number

[0056] Laser diodes with a large emission width w form spots that are too large for simple projection applications, and as h decreases, the resulting devices become large and expensive because the high-velocity axis of the beam diverges rapidly, requiring a very large lens placed very close to the diode for effective collimation. This would make achieving tolerances for placement and structure difficult, as we have seen previously, because the distance between the diode and the lens is determined by the magnification required to form a sufficiently small spot on the photovoltaic cell. The closer the lens is to the diode, the larger the spot size. In addition, the diameter of the lens is determined by the divergence of the high-velocity axis on its way to the lens. Therefore, selecting an h value outside the desired range above results in a large device, because by making the lens diameter very large, it is necessary to include a significant portion of the light emitted by the laser diode and collimate or focus it to the desired range.

[0057] Refer to Figure 3 here. Figure 3 schematically shows a schematic perspective view of the semiconductor layer structure of a laser diode according to a typical implementation example of the present invention. This figure is taken from a point perpendicular to the wafer surface and perpendicular to the beam emission direction. The laser diode for this beam projection application is advantageously either a III-V or II-VI semiconductor diode. The general structure of the diode includes layers grown on a semiconductor wafer, typically many layers, but only some of which provide the resonator and gain for the laser.

[0058] The resonator and gain-providing layer for the laser typically includes a pair of outer cladding layers, p-type and n-type doped layers within the cladding layers, and a gain-generating quantum well layer 34 located between the doped layers. The structure in which the gain layer is surrounded by p-type and n-type doped layers may be repeated several times.

[0059] At the top and bottom of the diode are anode and cathode electrodes 31, 38 for supplying power to the diode from the laser driver 15. The wafer is typically a GaAs, Ge, Si, InP, or other common semiconductor wafer. Describing the structure from the bottom electrode 38 and wafer 37, the terms “top” and “bottom” refer only to what is graphically shown in the drawing in Figure 3, and below the diode structure itself, a number of lattice matching layers 37 may grow just above the wafer, followed by a low refractive index cladding layer 36. Above the first cladding layer 36, a doped layer 35, which is either p-doped or n-doped, grows, and this layer has a refractive index greater than that of the first cladding layer 36, thereby defining the waveguide height.

[0060] Furthermore, a thin gain layer 34, generally having a higher refractive index, is grown on the first doped layer 35. Typically, the thickness of the gain layer 34 is less than a single wavelength of the laser light. The gain layer 34 has a band gap of approximately 0.75 to 1.2 eV and, when powered by electrodes 31, 38, provides gain at the laser oscillation wavelength. The gain medium, or quantum well composition, 1. Quantum dots, 2.GaInAs composition, 3.GaAsSb composition, 4.InPAs composition, 5. InAlAs composition, or 6. Quaternary materials It may be any of the following:

[0061] A second doping layer 33 is grown on top of the gain layer 34, having the opposite doping to the first doping layer 35. Both p-type and n-type doping layers typically have similar refractive indices. The combined height of the two doping layers and the thin gain layer 34 between them (which together defines what is known as the laser resonator height h) determines, among other things, the divergence of the high-speed axis of the laser diode. A second upper cladding layer 32, having a refractive index lower than that of the gain layer and doping layers, is deposited on top of the second doping layer 33.

[0062] The diode structure shown in Figure 3 is known as a symmetric or nearly symmetric diode structure. In this structure, the center of the fundamental resonant mode is substantially symmetrically and centrally located in the coupled doped layer and gain medium layer waveguide structure contained between the outer cladding layers. Such diode structures generally provide a beam with the cleanest output mode.

[0063] As mentioned earlier, in attempts to increase the efficiency and thus the output power of laser diodes as a function of input power, laser diodes with asymmetric waveguide structures have been developed. In these structures, the active layer is intentionally placed very close to the cladding layer in the p-type doped layer to reduce current-induced non-uniform carrier accumulation and associated carrier losses on the p-side of the waveguide structure. The cladding layer on this p-type doped side of the diode waveguide itself is highly p-doped to reduce its series resistance. All of these features contribute to reducing the losses of the laser diode, thereby enabling higher output power and efficiency. However, such asymmetric diode structures generally result in a decrease in the purity of the output modes of the laser diode resonator. In addition, multiple quantum well diode structures have been used to increase output power, and these structural features also generally result in a decrease in the purity of the output modes of the laser diode resonator. Both of these methods for increasing the output power of the diode can be adapted to use with the laser diodes described above, but the results may still be considered less desirable in providing the desired beam collimation.

[0064] Refer to Figure 4 here. Figure 4 is a top view of the diode laser of Figure 3, that is, a view from above the outermost electrode 31 (or 38). Figure 4 shows the upper electrode 42 on the outer surface of the device. In Figure 4, the upper electrode 42 is shown to cover the entire width of the diode structure, but it may be slightly narrower than the entire width of the wafer. The current flowing through the diode between the electrodes generates a population inversion in the gain layer portion through which the current flows, as well as a small-signal gain, and a saturation gain during laser oscillation. The laser beam is formed between the back mirrors 44, passing through the gain medium toward the output coupler 48. Part of the beam is reflected into the cavity resonator, and the other part is transmitted to the outside of the laser as the output beam 46.

[0065] The resonator length L is typically in the range of 0.5 to 10 mm and is typically the distance between the rearview mirror 44 and the output coupler 48, while the width w is determined by the limiting aperture of the resonator. This is either the width of the gain region or the width of the output coupler.

[0066] For a typical laser diode of the present invention, the emitter width w should preferably be in the range of 15 to 250 μm.

[0067] The low-speed axis is a low-speed axis with cavity width w, and the high-speed axis is a high-speed axis with thickness (height) h of the gain medium having its two associated doped layers.

[0068] Cavity Fresnel number FN(w) in low-speed shaft 2 / λL) should be within the range given by the following: 0.01 <FN<20 (4) This ensures that limited higher-order modes, which allow for high power generation by the diode, develop, while extremely high-order modes that limit focusing and safety are suppressed. These low Fresnel numbers prioritize lower-order spatial modes, but do not limit the diode to single-mode operation.

[0069] The width w should be adjusted to be within the following range:

number

[0070] For diodes of reasonable length, typically less than a few millimeters, which are necessary to achieve high yield and low cost, the above Fresnel numbers can be achieved with a width w of the order of 20 to 100 microns.

[0071] If the emitter width w is wider than the range given by equation (5), too many higher-order modes are generated, preventing the diode from focusing at the desired distance, and the spot becomes too large for projection applications. If the emitter width w is too narrow, not enough modes are generated to provide the power required in many power projection applications, and the diode does not have sufficient optical power.

[0072] A laser diode cavity / resonator structure having the above-defined width and Fresnel number ensures that the emitted beam has satisfactory optical quality. Specifically, such a beam should have the following favorable qualities:

[0073] When focused using the above optical system at a desired distance from the emitter, it forms a single lobe, allowing for focusing onto a small receiver.

[0074] The laser oscillation wavelength is between 1150 and 1550 nm, which advantageously provides eye safety and invisibility. Preferably, the laser oscillation wavelength should be between 1200 and 1450 nm.

[0075] This beam is a multimode laser beam that is particularly restricted in the "low-speed axis" (direction w), but single-mode emission should be obtained when the current flowing through the diode is low.

[0076] The beam obtained by selecting the above parameters is at least 50% TEM 00 All Hermitian Gaussian modes TEM consisting of Hermitian Gaussian modes where m > 0 and n > 20 mn The resulting beam will have less than 15% of its original size. These limitations are necessary to achieve the desired ability to focus the beam into a small lobe with the required spread, while simultaneously providing sufficient power to perform the required tasks.

[0077] Due to the selected values ​​of width w and Fresnel number of the resonator, the beam is TEM 00 It has a low-speed axial (w-direction) spatial mode that is not 6 M 2 It needs to have a value. Such beams are combined and mixed with many TEM mn It consists of modes, but the resulting M 2 The value is guaranteed to be less than 6. Each pure TEM mode is 2m+1 or 2n+1 M 2 The above TEM has a value, mn The mode combination is determined by the percentage of higher-order mode components in the output beam and the M of each component mode in the beam. 2 The overall M depends on the value. 2 It brings about a value. M 2 The value can be easily measured experimentally using a standard monitor for that purpose.

[0078] Due to the selected value of cavity height h, the divergence angle of the high-speed axis will be less than 60 degrees. Furthermore, the high-speed axis (in the h direction) is M 2 It has a spatial mode of less than 1.4. Unlike a true single-mode diode laser, the distance between the high-speed axis waist (above the emitter) and the low-speed axis waist (inside the diode) is less than 1 mm, but is always greater than zero.

[0079] Such laser diodes can operate at voltages exceeding 0.8 volts and emit light exceeding 300 mW when supplied with a current of at least 800 mA, typically up to several amperes.

[0080] The band gap changes as a function of junction temperature and current. Consequently, the Fresnel number, wavelength, and diode characteristics also change, and the control unit needs to monitor the diode temperature and be programmed to maintain the laser oscillation stability of the beam.

[0081] At the threshold current, the diode generates a single-mode beam focused to a very small diffraction-limited spot (but with low power). As the current increases, the power increases, the bandgap, and sometimes the Fresnel number change, and the beam becomes similar to the beam described above. It is important to characterize the beam at its operating current, but it is also important to characterize it at 25% above and below that operating current, and at twice the threshold current.

[0082] When focused by a lens placed at a distance d (measured in meters and within the following range) from the diode emitter, d is measured in meters and within the following range.

number

number

[0083] A wide diode cavity forms a spot that is too large for such projection applications, and as h decreases, the resulting device becomes large and expensive. The beam uses TEM to obtain at least 50% of its power. 00 It includes less than 15% of that power, and higher-order TEM where (n+m) is greater than 20. nm This must be included in the mode. This value should function at diode currents between 2 and 4 times the threshold current, as well as at typical operating current levels.

[0084] The laser diode described above provides efficient and safe laser beam transmission to the receiver power detector. However, the system also needs to include safety features. The safety features not only allow the beam to be focused on the receiver photovoltaic cell but also warn of situations in which the beam may collide with another object. This situation may indicate a laser hazard and mandate the cessation of diode emission. The system described now incorporates a number of features that provide the system with protection to prevent unintended laser diode emission in situations in which a physical short circuit or an electronically virtual short circuit allows the operating current to pass through the laser diode. Such protective features are described in PCT application PCT / IL2022 / 051040, “System for Localization and Charging of Wireless Receivers,” which is commonly owned by the applicant and incorporated as a whole by reference. The main features of these additional safety systems are (i) improved physical electrical insulation of the laser diode power supply leads, (ii) independently controlled switches in the anode and cathode leads of the laser diode, and (iii) system startup by a two-level power supply voltage arrangement.

[0085] Refer to Figure 5 here. Figure 5 schematically shows a block diagram of the main functions of this power supply scheme for the laser diode. The laser diode 50 is powered by a laser diode power supply 51 which receives drive commands from the system main controller 52. The main controller 52 is programmed to use the laser diode power supply 51 to turn the laser diode on and off and adjust its power level during conventional operation of the system, providing a certain level of safety from laser hazards. The diode power supply 51 delivers the appropriate drive current to the laser diode 50 via the input and output current connections of the laser diode, i.e., to the anode 54 of the laser diode and from the cathode 55 of the laser diode, advantageously by a cable having a fully covered insulator 54 including the legs and casing or the legs themselves of the laser diode mount. These current leads include two auxiliary gate switches S1 and S2 which are controlled by a gate controller which can be incorporated into the main controller 52. Therefore, the activation of the current from the laser diode power supply 51 to the anode 55 of the laser diode, and the activation of the current from the cathode 56 of the laser diode to the circuit ground or the negative terminal of the laser diode power supply, are controlled by two switches S1 and S2. This ON / OFF control is in addition to the basic level control of the laser current from the laser diode power supply 51 itself, whose output level is controlled by the main controller 52. These two switches S1 and S2 are held in a conducted state (hereinafter "closed") by a control voltage on their gates and are used for additional safety, enabling two additional independent and redundant methods of terminating the current to the laser, in addition to the conventional control of the diode current by the controller 52, and these can be implemented separately or together. However, the conventional control of the laser diode current does not necessarily achieve its desired function if a short circuit occurs that supplies current to the laser diode without going through the laser diode power supply 51.For example, in such a situation, the two switches S1 and S2 provide an additional safety method to shut down the laser emission when the situation requires such a shutdown.

[0086] The additional safety feature of this switching process, which interrupts the laser diode current, arises from the manner in which the switch is powered, compared to other electronic modules and features of the system. The operation of these two gate switches takes advantage of the fact that most infrared laser diodes typically operate at low voltages in the region of less than 1.5V. This is significantly lower than the voltages used by most other electronic components associated with the electronic circuits of systems generally based on Si semiconductor technology. Such Si technology devices cannot operate at such low voltages and typically use higher operating voltages such as 1.7V, 3.3V, 5V, or 12V.

[0087] The function of the gate controller, either within the main controller 52 or separately, is to stop laser oscillation by opening at least one switch S1, S2, under conditions where the main laser driver controller 52 fails to do so even if instructed to do so. The gate controller function can be incorporated as an additional unit of the main controller 52, or it may be implemented as an additional separate circuit module (not shown in Figure 5).

[0088] At least one of the two switch gates is positioned to be normally non-conductive unless it is actively held in a conductive state by applying the required voltage to the switch gate. The laser current is enabled during normal operation by holding the gate in a conductive state with a voltage supplied by the gate controller. When its latch voltage drops, the gate returns to an open non-conductive state. The switch gate, specifically the gate controller circuit, is driven by another operating voltage V2 that is higher than the voltage V1 supplied from the system's main power supply (not shown in Figure 5) to the main controller 52 or the laser diode power supply 51 or other electronic functions in the system. If a physical short circuit occurs and a voltage exceeding 1.5V, the typical voltage at which the infrared laser diode operates, is applied to the anode lead wire 54 of the laser diode, the laser diode 50 will turn on and emit a laser beam, even though the controller 52 is instructed to turn off the laser driver and the anode switch S1 is instructed to be non-conductive. The same situation applies if such a circuit malfunction occurs in the laser diode power supply 51 and current is supplied to the laser even though it is not instructed by the controller 52. The laser diode operates at less than 1.5V, and any accidental application of another voltage present in the circuit will be higher than 1.5V. Therefore, an increase in current drawn from the mains power supply can cause a drop in the mains power supply voltage to all control functions of the system, or a drop to a level not high enough to reliably operate the controller 52. Since the gate of the switch is operated by the controller 52 at a voltage higher than the main controller 52 itself, this voltage drop will switch the gate switch to a non-conducting state, independently of the controller's commands to the laser power supply 51. When either of these switches S1, S2 is put into a non-conducting state, the laser diode current stops, regardless of the operation of the main controller 50 or any other circuit controllers such as the system's electronic protection mechanisms, and the system becomes safe.

[0089] In conclusion, the use of a high power supply voltage V2 for the gate controller ensures that, in the event of a failure that causes a drop in the overall voltage supplied by the system power supply, the gate controller will be the first circuit to drop out because it operates at a higher voltage than other circuit elements, and therefore, prior to or independently of what is happening in other controller functions, it will cut off the gate holding voltage, and consequently the power to the laser diode.

[0090] In a second alternative scenario, if the voltage applied to power the main controller 50 drops sufficiently low, causing the main controller to malfunction and fail to respond by reducing an unexpected and uncontrollable laser diode current, the function of performing a switch operation via the gate controller 53, which relies on an operating voltage higher than the operating voltage of the system controller 50 or laser driver 48, means that the switch will become non-conductive, and consequently the laser diode current will terminate, regardless of what the system controller or laser driver is trying to do.

[0091] All points in the circuit that could short-circuit to ground or other energized metal contacts within the laser generator enclosure should be adequately electrically insulated. This protection is especially important when C-mount laser diodes are used, as such C-mounts have a large area of ​​exposed metal surface that is part of the diode conductor and could short-circuit to ground or other energized metal contacts within the laser generator enclosure if mechanical failure occurs, such as mechanical intrusion or loose wire connections becoming free. This is not a simple task to achieve entirely without affecting the cooling requirements of the laser diode.

[0092] The exemplary embodiments are provided to elaborate on the disclosure and to fully convey its scope to those skilled in the art. Numerous specific details, such as examples of particular parts, devices, and methods, are described to provide a complete understanding of the embodiments of the disclosure. As will be obvious to those skilled in the art, it is not necessary to adopt specific details, and the exemplary embodiments may be embodied in many different forms, none of which should be construed as limiting the scope of the disclosure. Furthermore, it will be understood by those skilled in the art that the invention is not limited to those specifically shown and described above. Rather, the scope of the invention includes both combinations and partial combinations of the various features described above, as well as variations and modifications thereof that will be recalled by those skilled in the art when reading the above description and that are not in the prior art.

Claims

1. A laser diode that emits a laser beam for optical wireless power transmission, It comprises a first cladding layer having a first refractive index and a second cladding layer having a second refractive index, with an emission region between them, and the emission region is A positive doped layer having a third refractive index and a negative doped layer having a fourth refractive index, wherein the third and fourth refractive indices are each greater than either of the first and second refractive indices, A gain medium layer deposited between the positive doped layer and the negative doped layer, the gain medium layer having an energy band gap between 0.8 eV and 1.2 eV Includes, The width w of the emission region in meters is, [Math 1] It is within the range, Here, E is the band gap of the gain layer measured in joules, and n １ is the average refractive index of the doped layer, n ２ is the average refractive index of the cladding layer, The upper limit of w is when the beam in the low-speed axis has at least a first predetermined percentage of its power in Hermitian Gauss TEM. ００ A higher-order TEM that focuses on a mode and has a second predetermined percentage of its power less than or equal to the number of presets (n + m). ｎｍ By selecting this mode, it is ensured that the required quality of spatial multimode format is achieved. The lower limit of w is selected to ensure that the beam in the low-speed axis has a combination of higher-order modes sufficient to generate at least the required level of laser beam output power, in the laser diode.

2. The laser diode according to claim 1, wherein the first and second refractive indices may have substantially the same value, and the third and fourth refractive indices may have substantially the same value.

3. The laser diode according to any one of claims 1 and 2, wherein the first predetermined percentage is 50%.

4. The laser diode according to any one of claims 1 and 2, wherein the second predetermined percentage is 15%.

5. The laser diode according to any one of claims 1 to 4, wherein the number of presets is 20.

6. The laser diode according to any one of claims 1 to 5, wherein the desired level of laser output beam power is 300 mW.

7. The laser diode according to claim 6, wherein the laser beam power level having wavelengths outside the range of 1150 nm to 1550 nm is less than 2 mW.

8. The laser diode has a Fresnel number w in the low-speed axis relative to the range of w in claim 1. ２／ A laser diode according to any one of claims 1 to 7, having a cavity of length L in meters such that λL is between 0.01 and 20.

9. The aforementioned laser beam, M less than 15 in the direction of the low-speed axis ２ The value and, M less than 1.5 in the direction of the high-speed axis ２ The value and the divergence angle of the laser beam less than 60° in the direction of the high-speed axis and A laser diode according to any one of claims 1 to 18, having the following characteristics.

10. The mode of the collimated laser beam is such that when the beam is imaged at a distance of 10 m by a suitable optical system, it has a single lobe. As a result, the effective radius r of the focused laser beam generated at the distance of 10 m is given by the formula [Math 2] Given by, Here, h is the height of the laser emitter measured in meters. w is the width of the emission area measured in meters, r is the effective spot radius measured in meters. The laser diode according to any one of claims 1 to 9, wherein the tan function is in radians.

11. The laser diode according to any one of claims 1 to 10, wherein the effective radius r of the focused laser beam spot includes 95% of the laser beam power.

12. The laser gain medium layer Quantum dot gain layer, GaInAs composition, GaAsSb composition, InPAs composition, AlAs composition, or quaternary material A laser diode according to any one of claims 1 to 11, comprising any one of the following:

13. The laser diode according to any one of claims 1 to 12, wherein the cladding layer and the doped diode layer have a structure that is symmetrical or nearly symmetrical with respect to the gain medium layer.

14. A system for transmitting laser power from a transmitter to a remote receiver, A laser diode light source comprising a gain medium sandwiched between p-type and n-type doped layers, wherein the laser diode light source is supplied with current from a laser driver, An optical system that collimates the laser beam emitted from the laser diode light source to generate an illumination spot on the remote receiver, A scanning system that directs the collimated laser beam towards the aperture of the photovoltaic cell in the remote receiver. Equipped with, The gain medium of the laser diode is selected so that the laser diode emits a beam with a wavelength λ between 1150 nm and 1550 nm. The current supplied to the gain medium by the laser driver is selected so that the laser beam has a power of at least 300 mW. The laser diode light source has an emitter width w between 15 and 250 μm. The laser beam is a spatial multimode beam, and at least 50% of its power is concentrated in the Hermite-Gaussian TEM ００ mode, and less than 15% of its power is in a higher-order TEM with (n + m) greater than 20 ｎｍ mode, and The length of the gain medium of the laser diode is such that the Fresnel number w of the laser diode is in the low-speed axis. ２ / λL is selected to be between 0.01 and 20, where L is the length of the laser cavity. M less than 15 in the direction of the low-speed axis ２ The value and, M less than 1.5 in the direction of the high-speed axis ２ The value and, A divergence angle of less than 60° of the laser beam in the direction of the high-speed axis, The single lobe mode of the collimated laser beam when imaged at a distance of 10 m by the optical system, The resulting focused spot of the laser beam is, [Math 3] The things that are given A beam having the following characteristics is generated, Here, h is the combined height of the p-type and n-type doped layers and the gain medium, measured in meters. w is the width of the gain medium measured in meters. r is the effective radius of the focusing spot, The tan function is a system of units in radians.

15. The system according to claim 14, wherein the band gap of the gain medium of the laser diode is in the range of 0.8 eV to 1.2 eV.

16. The system according to any one of claims 14 and 15, wherein the laser diode is configured on a substrate of a Group III-V or Group II-VI semiconductor.

17. The system according to any one of claims 14 to 16, wherein the laser diode has a gain medium layer comprising a quantum dot structure, a GaInAs composition, a GaAsSb composition, an InPAs composition, an InAlAs composition, or a quaternary material.

18. The system according to any one of claims 14 to 17, wherein the focused spot size has an effective radius r that includes 95% of the beam's power.

19. The system according to any one of claims 14 to 18, wherein the power of the laser diode emitted at wavelengths outside the range of 1150 nm to 1550 nm does not exceed 2 mW.

20. The gain medium has an average refractive index n １ An n-type doped layer having a refractive index n is placed between an n-type doped layer and a p-type doped layer, and these layers themselves have a refractive index n ２ The system according to any one of claims 14 to 19, which is disposed between cladding layers having

21. The width w of the gain medium is given by the formula [Math 4] It is within the range given by, Here, E is the band gap of the gain medium layer measured in joules, n １ is the average refractive index of the doped layer, n ２ The system according to claim 20, wherein is the average refractive index of the cladding layer.

22. The system according to any one of claims 14 to 21, wherein the electrical connection for supplying current from the laser driver to the laser diode is an insulated connection adapted to prevent the possibility of accidental electrical contact with the laser diode, thereby increasing the safety of the system.

23. The system according to any one of claims 14 to 22, wherein the electrical connection for supplying current from the laser driver to the laser diode has at least one gate switch that controls the flow of current through each electrical connection.

24. The system according to claim 23, wherein the gate switch that controls the flow of current through each electrical connection is activated by a gate driver having a higher operating voltage than the operating voltage of other electronic circuits in the system that provide control functions to the system.

25. The system according to claim 14, wherein the gate driver is configured to hold each gate switch in its conductive state when the gate driver is commanded to activate the gate.

26. The system according to claim 25, wherein when the operating voltage of the gate driver drops to a level below the operating voltage of other electronic circuits in the system that provide control functions to the system, the gate switch returns to a non-conductive state.

27. The system according to claim 26, wherein the non-conductive state is isolated from all current sources, even if the non-conductive state is caused by an accidental electrical connection to the laser diode.

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