Optical wireless power transfer using laser diodes

Abstract

A system for transmitting laser power to a remote receiver includes a laser diode, an optical system for collimating and focusing the laser beam onto the receiver, and a scanning system for directing the beam towards the receiver. The light beam has a wavelength of 1150 to 1550 nm, and has gt; a power of 300 mW, having an emitter width of between 15 and 250 [mu] m, and being a spatial multimode beam, having gt in TEM00 mode; 50% of its power, and at (n + m) gt; 20 in a higher order TEMnm mode having lt; and the power is 15%. The laser cavity has a Fresnel number of between 0.01 and 20, and the slow axis has a lt; 15, and the fast axis has lt; m2 of 1.5, and has lt; and 60-degree fast axis divergence is realized. When imaged up to a range of 10 meters, the collimated laser beam has a single lobe.

Description

Technical Field

[0001] The present invention relates to the field of laser-based wireless power transfer for providing power to a remote receiver, and more particularly to the need for a power source for a beam that can provide for the efficient and safe transmission of wireless power over the desired distance to the remote receiver. Background Art

[0002] There are systems for transmitting optical power to a remote location without the need for a physical wired connection. This need has become important over the past few decades with the proliferation of battery-operated portable electronic devices that require periodic recharging. Currently, the capacity of batteries in the art and the typical battery usage of smart phones intended for use may make it such that the battery may need to be recharged more than once per day, making the need for remote wireless battery recharging important. A number of prior art systems have been proposed for safely transmitting power to a remote location, which can be characterized as being at distances significantly greater than the size of the transmitting or receiving device. A typical configuration would be to transmit power to a smart phone-sized receiver at a typical distance in a home room setting. The reception of the transmitted power is typically performed by using one or more photovoltaic cells, thus allowing the optical power to be safely transferred from the emitted laser beam to the mobile device. Therefore, the emitted laser beam needs to have a high enough power to achieve its intended function, but this power level must be provided while maintaining a tightly collimated beam that transfers most of its power onto the photovoltaic cells, such that the effective transmission of light energy is performed without endangering the environment or people along the transmission area due to excessive beam divergence and leakage.

[0003] A favorable power source for such laser power generation is a laser diode, as in many industrial, analytical, and medical aspects where laser power is used. The many applications using diode lasers require very different characteristics of the beam, and the laser diode industry has provided devices for these various needs. Thus, for example, laser diodes for analytical spectroscopy or microscopy typically have a low power level, but should have a narrow linewidth wavelength emission and a very low divergence beam, which is typically less than 2 mrad when collimated. On the other hand, laser diodes or laser diode bars or laser diode arrays for industrial processes such as cutting or welding should have as high a power as possible, while beam divergence is typically less critical. Laser beams for medical uses such as ablation or as a laser scalpel require a well-focused beam, but can be multimode, having a relatively high divergence with a complex mode structure.

[0004] In an attempt to improve the efficiency of a laser diode and thus increase the power output as a function of the input power, instead of a laser having a symmetric or what is known in the art as a "near-symmetric" structure, laser diodes having an asymmetric structure have recently been developed. Some such asymmetric diode structures have been described in U.S. Patent No. 8,798,109 "High Efficiency Diode Laser" by G. Erbert et al., and in "High Power 1.5μm Pulsed Laser Diode with Asymmetric Waveguide and Active Layer near p-Cladding" by L. W. Hallman et al. in IEEE Photonics Technology Letters, Vol. 31, No. 20, Oct. 2019, pages 1635 - 1638, and in the article titled "Efficient High Power Laser Diodes" by P. Crump et al. in IEEE J. of Selected Topics in Quantum Electronics, Vol 19, Issue 4, July - Aug. 2013.

[0005] The disclosure of each publication mentioned in this section and in other parts of the specification is hereby incorporated by reference in its entirety. SUMMARY OF THE INVENTION

[0006] A laser beam for wirelessly transmitting optical power to a remote receiver is an application that involves a combination of two inherently conflicting characteristics. First, a power level sufficient to provide the needs of the power receiver is required, which typically means, in the current state of laser diode technology, a power level that can only be obtained from a multimode diode laser that operates in a manner that emits a multimode beam. Second, the beam needs to be collimated so that most of its power impinges on the receiving element, which means generating a beam with a low-order mode from the laser diode. Such a combination is generally considered a contradiction in the nature of laser diodes, making it difficult to achieve the construction of an efficient laser power transmission system to a remote receiver. Due to this difficulty, some prior art wireless optical power transmission systems are based on lasers other than diode lasers, or alternatively, if a diode laser is used, a receiver with a large-aperture photovoltaic detector is employed, which eliminates the need to focus the beam into a small spot.

[0007] The requirements for laser projection mean that at a certain distance from the projector, the formation of a small laser spot with high power is complex. On the one hand, the laser beam needs to have good optical quality in order to be focused onto a small spot. Therefore, some prior art systems require the use of single spatial mode diodes, which are indeed suitable for low power applications but cannot be used for higher power applications because single mode diodes are typically power limited. Currently, single mode diode lasers with an output optical power of more than 600 mW are not available on the market. Other prior art systems use multimode diodes, but this limits the projection range within which the laser can form a sufficiently small spot because multimode beams cannot be well collimated without significant power loss.

[0008] The present invention discloses an optical power transmission system having a laser diode that is suitable for projection applications requiring high power without compromising the ability of the laser diode to project a small spot at a desired remote distance such as in optical wireless power transmission applications.

[0009] To implement and operate such a projection system, the laser beam should have at least the following three characteristics. For each beam property, specific characteristics or structures required of the laser diode are specified in order to achieve such a laser beam property.

[0010] Wavelength - The laser beam should have a wavelength between 1150 nm and 1550 nm to ensure eye safety, with no more than 2 mW of power emitted outside this range. The wavelength emitted by the laser diode depends on the bandgap of the active gain medium used in the laser diode. Therefore, for the above beam wavelength range, the bandgap of the gain layer should be selected to be in the range from approximately 0.8 eV to approximately 1.2 eV. (A higher bandgap of 1.2 eV corresponds to a higher energy wavelength (i.e., below 1150 nm) because as the gain medium heats up as the diode emits laser light, the bandgap tends to decrease slightly). Such a bandgap can be achieved by constructing the laser diode on a substrate of III-V or II-VI semiconductors having a gain layer made of any of the following:

[0011] a. Quantum dot structures,

[0012] b. GaInAs compositions,

[0013] c. GaAsSb compositions,

[0014] d. InPA compositions,

[0015] e. InAlAs compositions, or

[0016] f. Quaternary materials.

[0017] Beam power - The optical beam output power should be at least 300 mW in order to provide an electrical power level after conversion in the receiver to charge, for example, the battery of a typical mobile phone within a user-acceptable time. Considering the efficiency of the laser diode itself, the input drive current to the diode should be at least 800 mA at an applied voltage of at least 0.8 V, which value is determined by the gain medium used and its I-V characteristic curve.

[0018] Beam mode - As a preliminary clarification, it should be noted that in the context of the present application, the terms "single-mode" and "multi-mode" are used to describe single spatial (or transverse) modes and multi-spatial (or transverse) modes, respectively, rather than single or multiple longitudinal modes. The laser diode can be a multi-mode device to ensure a sufficiently high power level, but the design parameters of the laser diode should be such that the emitted multi-mode optical beam will have a specific mode structure such that its output optical beam should have at least 50% of its power concentrated in the Hermite-Gaussian TEM 00 modes and less than 15% of its power in the higher-order TEM nm modes, where the mode order is such that (n + m) is greater than 20. Such 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 substantially contradictory properties of the high output power associated with a multi-mode optical beam and the good collimation properties associated with a single-mode optical beam.

[0019] These properties result from the careful selection of the geometry, in particular the width of the gain layer of the laser diode. As will be shown below, the cladding layer adjacent to the gain layer of the diode structure is the layer that defines the outer extent of the active cavity or resonator region in the fast axis direction of the laser diode and can be regarded as the radiation emission height in the fast axis. The p-doped and n-doped layers (referred to herein as the gain layer) adjacent to the thin laser amplification layer have a refractive index n1, while the cladding layer has a refractive index n2 that is lower than the refractive index of the gain layer. There are typically 2 such layers, and these layers typically have the same refractive index n2. If the refractive indices are different (n2 and n3), the lower of the two is applied to perform the calculations of the laser diode properties shown below.

[0020] To achieve the above mode distribution from the diode, where most of the power is in the low-order modes, the Fresnel number FN of the cavity in the slow axis (which is the wide dimension of the cross-section of the gain region) should be in the range given by the expression 0.01 < FN < 20. FN is given by the expression w 2 / λL, where L is the total length of the laser cavity. To provide these values, the emitter width w should be in the range of 15 microns.

[0021] This mode distribution will allow the optical beam to have an M in the slow axis direction 2The value is less than 15.

[0022] Due to the small height of the gain regions defined as the positive-doped layer, the laser amplification layer, and the negative-doped layer, the M 2 value should be less than 1.5, and the fast-axis divergence should be less than 60°.

[0023] For the M 2 value and this selection of the mode structure will allow the beam mode to have a single lobe when imaged at 10 m, and will allow the focused spot size r such as the distance to reach the optimal size.

[0024] The spot size r generated by the beam after focusing should be in the following range:

[0025]

[0026] where:

[0027] h is the height of the gain layer measured in meters,

[0028] w is the width of the gain layer measured in meters,

[0029] r is the effective radius of the spot containing 95% of the power of the projected beam, also measured in meters, and

[0030] gives the angle of the tangent measured in radians.

[0031] It should be understood that since in many such systems the spot will not be circular, the term "effective radius" throughout this disclosure refers to the measurement that is half of the average lateral outer dimension of the illuminated spot that contains 95% of the spot power.

[0032] A brief overview of the significant features of an exemplary embodiment of the claimed system of this disclosure, where a necessary brief explanation of the motivation for each feature, can be expressed as:

[0033] A system for transmitting laser power from a transmitter to a remote receiver, comprising:

[0034] A laser diode source that includes a gain medium and is supplied with current from a laser driver,

[0035] An optical system for collimating the laser beam emitted from the laser diode source to produce an illuminated spot on the remote receiver; and

[0036] A scanning system for directing the collimated laser beam towards a hole in a photovoltaic cell on the remote receiver, where:

[0037] (i) The laser diode emits a light beam with a wavelength λ between 1150 nm and 1550 nm, making it invisible to the human eye and providing greater safety due to the water absorbency at the longer wavelengths in this range.

[0038] (ii) The laser beam has a power of at least 300 mW, making it powerful enough to provide the desired power to the client device.

[0039] (iii) The emitter width w of the laser diode source is between 15 and 250 μm, which enables the laser beam to be realized as a spatially multimode beam, with at least 50% of its power concentrated in the Hermite-Gaussian TEM 00 mode, and less than 15% of its power concentrated in higher-order TEM nm modes, where the order of the TEM nm mode is such that (n + m) is greater than 20, thus contributing to the generation of a beam with a suitable M 2 value.

[0040] (iv) The laser diode source has a laser cavity that has dimensions of the laser beam emitted therefrom in the fast axis and slow axis directions, and

[0041] (v) The laser cavity of the laser diode has a Fresnel number w 2 / λL between 0.01 and 20 on the slow axis, where L is the length of the laser cavity.

[0042] The above characteristics enable the resulting beam to have:

[0043] (vi) An M 2 value of less than 15 in the slow axis direction.

[0044] (vii) An M 2 value of less than 1.5 in the fast axis direction, and the laser beam has a divergence angle of less than 60° in the fast axis direction.

[0045] (viii) The mode of the collimated laser beam has a single lobe when imaged by an optical system at a distance of 10 m, and

[0046] (ix) The spot size r of the laser beam is thus generated and is given by the following expression:

[0047]

[0048] where h is the height of the p-doped and n-doped layers of the gain medium of the laser source measured in meters.

[0049] w is the width of the gain medium measured in meters.

[0050] r is the effective radius, and

[0051] The tan function is in radians.

[0052] Thus, according to an exemplary embodiment of the device described in the present disclosure, there is provided a system for transmitting laser power from a transmitter to a remote receiver, the system comprising:

[0053] A laser diode that emits a laser beam for optical wireless power transfer, the laser diode comprising:

[0054] (i) A first cladding layer having a first refractive index and a second cladding layer having a second refractive index, an emission region existing between the first cladding layer and the second cladding layer, the emission region comprising:

[0055] (ii) A p-doped layer having a third refractive index and an n-doped layer having a fourth refractive index, each of the third refractive index and the fourth refractive index being greater than either the first refractive index or the second refractive index, and

[0056] (iii) A gain medium layer deposited between the p-doped layer and the n-doped layer and having a bandgap between 0.8 eV and 1.2 eV,

[0057] Wherein the width w of the emission region in meters is in the following range:

[0058]

[0059] Where E is the bandgap of the gain layer measured in joules, n1 is the average refractive index of the doped layer, and n2 is the average refractive index of the cladding layer,

[0060] And wherein:

[0061] (a) The upper limit of w is selected to ensure that on the slow axis, the beam has a spatial multimode form of the required quality (by concentrating at least a first predetermined percentage of its power in the Hermite-Gaussian TEM 00 mode and not exceeding a second predetermined percentage of its power in higher-order TEM nm modes, (n + m) being greater than a preset number), and

[0062] (b) The lower limit of w is selected to ensure that on the slow axis the beam has a sufficient combination of higher-order modes such that it produces at least the required level of laser beam output power.

[0063] In such a laser diode, the first refractive index and the second refractive index may have substantially the same value, and the third refractive index and the fourth refractive index may have substantially the same value.

[0064] In addition, according to another embodiment of such a laser diode, the first predetermined percentage should be 50%, the second predetermined percentage should be 15%, the preset number should be 20, and the required level of the laser beam output power should be 300 mW. In the last case, the level of the laser beam power outside the range of 1150 nm to 1550 nm should be less than 2 mW.

[0065] In any of the foregoing laser diodes, the cavity can have such a length L that for the range of w of claim 1, the Fresnel number w in the slow axis 2 / λL should be between 0.01 and 20, where L is measured in meters.

[0066] In addition, in any of the foregoing laser diodes, the laser beam can have:

[0067] (i) an M value less than 15 in the slow axis direction, and 2 and

[0068] (ii) an M value less than 1.5 in the fast axis direction, and the divergence of the laser beam in the fast axis direction is less than 60°. 2

[0069] Furthermore, according to yet another embodiment of such a laser diode,

[0070] (i) the mode of the collimated laser beam is such that when imaged through a suitable optical system to a distance of 10 m, the beam has a single lobe, and

[0071] (ii) the effective radius r of the focused laser beam generated thereby at a distance of 10 m is given by:

[0072]

[0073] where h is the height of the laser emitter measured in meters,

[0074] w is the width of the emission region measured in meters,

[0075] r is the effective spot radius measured in meters, and

[0076] the tan function is in radians.

[0077] In any of the above embodiments, the effective radius r of the focused laser beam should include 95% of the laser beam power.

[0078] The laser gain medium layer of the above laser diode can include any of the following:

[0079] (i) a quantum dot gain layer.

[0080] (ii) a GaInAs composition​

[0081] (iii) GaAsSb composition

[0082] (iv) InP composition

[0083] (v) AlAs composition, or

[0084] (vi) Quaternary material.

[0085] Finally, in any of the foregoing laser diodes, the cladding layer and the doped diode layer may have a symmetric or approximately symmetric structure with respect to the gain medium layer.

[0086] According to an exemplary embodiment of the device described in the present disclosure, a system for transmitting laser power from a transmitter to a remote receiver is further provided, the system comprising:

[0087] (i) A laser diode source including a gain medium sandwiched between a p-doped layer and an n-doped layer and supplied with current from a laser driver,

[0088] (ii) An optical system for collimating the laser beam emitted from the laser diode source to produce an illumination spot on the remote receiver; and

[0089] (iii) A scanning system for directing the collimated laser beam towards an aperture of a photovoltaic cell on the remote receiver, wherein:

[0090] (a) The gain medium of the laser diode is selected such that the laser diode emits a beam having a wavelength λ between 1150 nm and 1550 nm,

[0091] (b) The current supplied by the laser driver to the gain medium is selected such that the laser beam has a power of at least 300 mW, and

[0092] (c) The laser diode source has an emitter width w between 15 and 250 μm such that:

[0093] The laser beam is a spatially multimode beam, at least 50% of whose power is concentrated in the Hermite-Gaussian TEM 00 mode, and less than 15% of whose power is in the higher-order TEM nm mode, (n + m) being greater than 20, and

[0094] The gain medium length of the laser diode is selected such that the laser diode has a Fresnel number w 2 / λL in the slow axis between 0.01 and 20, where L is the length of the laser cavity, thereby producing a beam having the following:

[0095] An M 2 value less than 15 in the slow axis direction,

[0096] An M value less than 1.5 in the fast axis direction 2 value

[0097] The divergence of the laser beam in the fast axis direction is less than 60°

[0098] When imaged by an optical system at a distance of 10 m, it has a single-lobe mode with a collimated laser beam; and

[0099] The sequential focused spots of the laser beam given by the following expression:

[0100]

[0101] where h is the combined height of the p-doped layer, n-doped layer and gain medium measured in meters

[0102] w is the width of the gain medium measured in meters

[0103] r is the effective radius of the focused spot, and

[0104] The tan function is in radians

[0105] 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. Additionally, such a laser diode can be fabricated on a substrate of III-V or II-VI semiconductors. Furthermore, the gain medium layer can include any of a quantum dot structure, GaInAs composition, GaAsSb composition, InPAs composition, InAlAs composition or a quaternary material

[0106] Moreover, in such a system, the focused spot size can have an effective radius r that includes 95% of the beam power. Additionally, the power emitted by the laser diode at wavelengths outside the range between 1150 nm and 1550 nm should not exceed 2 mW

[0107] Finally, in such a system, the gain medium can be disposed between the n-doped layer and the p-doped layer, having an average refractive index of n1, and they are themselves disposed between cladding layers with a refractive index of n2. In such a system, the width w of the gain medium should be in the range given by the following expression:

[0108]

[0109] where E is the bandgap of the gain medium layer measured in joules

[0110] n1 is the average refractive index of the doped layer, and

[0111] n2 is the average refractive index of the cladding layer

[0112] According to yet another exemplary embodiment of the system for transmitting laser power from a transmitter to a remote receiver as described above, the electrical connection for providing current from a laser driver to a laser diode should be an insulated connection, and the insulated connection is adapted to prevent the possibility of an inadvertent electrical connection to the laser diode, thereby enhancing the safety of the system.

[0113] In such a system, the electrical connection for providing current from a laser driver to a laser diode should have at least one gated switch for controlling the flow of current through each electrical connection. These gated switches should be activated by a gate driver having an operating voltage that is higher than the operating voltage of other electronic circuits of the system that provide control functions for the system. When the gate driver is instructed to activate the gate, the gate driver can be configured to hold each gated switch in its conductive state. In this case, a drop in the operating voltage of the gate driver to a level below the operating voltage of other electronic circuits of the system that provide control functions for the system causes the gated switch to revert to a non-conductive state. This non-conductive state thus isolates the laser diode from any current source, even one resulting from an inadvertent electrical connection to the laser diode. BRIEF DESCRIPTION OF THE DRAWINGS

[0114] The present invention will be more fully understood and appreciated in conjunction with the accompanying drawings, in which:

[0115] Figure 1 Schematically illustrates an exemplary laser power transmission system described in the present disclosure for providing optical power to a remote receiver;

[0116] Figure 2 Shows the first Hermite-Gaussian beam mode TEM mn and its relative spatial extent;

[0117] Figure 3 Schematically shows a schematic isometric view of a laser diode structure according to an exemplary embodiment of the present disclosure;

[0118] Figure 4 is Figure 3 a top view of the diode laser; and

[0119] Figure 5 Schematically illustrates a block diagram of the main features of a power supply scheme for a laser diode, which provides a high level of operational safety even in the event of an accidental electrical connection to the laser diode. DETAILED DESCRIPTION

[0120] First refer to Figure 1 , Figure 1Schematically shown is a laser power transmission system of the type used in the present disclosure for providing power to a remote receiver. The transmitter 10 includes a laser source, advantageously a laser diode 16, and the necessary elements for controlling and directing the emitted laser beam 12. The transmitter also incorporates a laser driver 15, a controller 13, and a safety system 14. The laser driver 15 provides electrical power for the laser diode 16. The controller 13 is operable to maintain the laser operating in a desired and required manner. The safety system 14 ensures that the laser beam transmission is being performed without posing a danger to the environment in which the transmission system is operating or to the people therein. The laser beam 12 emitted from the diode laser 16 diverges up to 60° substantially along the fast axis and up to 30° along the slow axis, although typical slow-axis divergence will be significantly less than 30°, even only 10°. The beam must thus be collimated by a focusing system 17 that forms a focal or virtual focus of the beam at a distance from the transmitter that is typically at least 200 times the diameter of the focusing lens. The beam is then directed by a beam deflection unit 18 along the correct path towards its target receiver 11 as a collimated or, since perfect collimation is not possible, as an almost collimated beam 12, where the beam 12 impinges on a photovoltaic cell to be converted into electrical power. The methods described below show how the system can be constructed with a laser diode 16 having a combination of characteristics such that the laser beam 12 reaches its target in a sufficiently focused state where it delivers most of its power to the photovoltaic cell on the receiver 11 while maintaining the power level required by the receiver to meet its intended function.

[0121] Laser diodes are generally classified into single spatial mode laser diodes and multimode laser diodes. Single-mode diodes typically provide good beam quality, while multimode spatial laser diodes provide poorer beam quality. However, single-mode diodes are more power-limited and thus cannot be used in the projection applications of the wireless power supply applications described in the present disclosure.

[0122] Throughout this document, the shortened terms "single-mode" and "multimode" refer to single spatial (or transverse) mode and multiple spatial (or transverse) mode outputs, rather than to the single / multiple longitudinal modes of the laser resonator, which are different terms related to the variation of the beam cross-section along the laser resonator and are largely irrelevant to the current application. Additionally, throughout the present invention, the term TEM mode generally refers to the Hermite-Gaussian mode, but in some cases may refer to the Laguerre mode.

[0123] Typically, the beam quality of single-mode lasers is nearly perfect because they emit an almost pure Gaussian beam, also known as the TEM 00 mode, which can be easily collimated or focused into a diffraction-limited spot. Single-mode lasers have an M 2 value close to 1, where an M less than 1.22 Lasers with values are generally considered single-mode lasers. For such single-mode lasers, almost all of the emitted power can be collected in the central Gaussian lobe of a collimated beam at optical infinity and can be focused into a diffraction-limited spot, regardless of the size of the emitter aperture of the laser.

[0124] On the other hand, multimode lasers support many spatial modes, which are labeled TEM mn modes and have poor beam quality. The output of a multimode laser cannot therefore be collimated without significant power loss. Multimode lasers support many TEM mn modes, where the value of the M 2 factor is given by (2n + 1) in the x direction and by (2m + 1) in the y direction. Thus, even for a low-order TEM 01 mode with an M 2 value of 3 in the y direction, while TEM 05 has an M 2 value of 11 in the y direction. Since the M 2 value of a beam is proportional to the ability to collimate and / or focus the beam, a multimode beam from a conventional multimode laser cannot be collimated or focused into a diffraction-limited spot without significant power loss due to the loss of the mode components that form the outer lobes of the beam.

[0125] Since projecting a multimode beam over long distances is inefficient and generally unsafe due to the inherent power loss in the beam, when laser diodes are used as sources in optical power transmission applications, usually only single-mode diode lasers have been used, and thus the available power is limited because single-mode diode lasers are power-limited.

[0126] It should be noted that due to the ratio of the cavity dimensions of diode lasers, their beams can have different M 2 values in different directions. A single-mode diode laser beam typically has an M 2 value close to 1 in the "fast axis", which is the narrow axis of the diode laser cavity, and an M 2 value close to 1 in the "slow axis", which is the wider axis of the diode laser cavity, while multimode diode lasers have an M 2 significantly greater than 1.2 and typically around 10 or more in the "slow axis".

[0127] Now referring to Figure 2 which shows a set of images of the first Hermite-Gaussian beam mode TEM mn and their relative extents, as is known in the art. The extended spatial extents of the different modes, different from the TEM 00 fundamental mode, range from Figure 2The mode shape shown in is clear, and thus it is obvious that a compact focused light spot cannot be obtained using a higher mode beam.

[0128] A laser beam emitted from a single-mode diode typically includes only TEM 00 Hermite-Gaussian modes. In some cases, the mode can be slightly distorted by diffraction from the edges of various apertures in the laser (such as the emitter), but as long as the M 2 value in the two transverse directions of the beam is less than 1.3, and more preferably less than 1.2, the laser is still generally considered a single-mode laser. Currently, single-mode diode lasers are limited to power levels below 300 mW at the desired wavelengths suitable for laser power transmission, which limits their availability in projection applications.

[0129] On the other hand, the beam emitted by a multimode diode laser contains a mixture of many modes, typically a variable mixture of many TEM modes (such as TEM 00,01,02,03,04,05,06…020 ).

[0130] A single-mode beam can be focused to its diffraction limit or close to its diffraction limit. Thus, for example, and as can be easily derived from the principles of optical design, a single-mode beam emitted from a laser transmitter of any size and having a wavelength of 1 μm, focused by an optical system with a numerical aperture of 0.01, will form a light spot of approximately 122 microns at its optimal focusing distance. The size of the projected light spot will not change based on the focal length of the lens used; only the numerical aperture of the converging beam affects the light spot size. For a perfect lens without any aberrations, as well as a similar perfect laser and intermediate medium, these conclusions will be clearly true.

[0131] On the other hand, higher-order modes have an M 2 value of (2n + 1) and cannot be focused to the diffraction limit. Generally, a multimode beam can only form an image approximately equal to the emitter size multiplied by the magnification of the focusing optical system. Thus, a typical multimode beam emitted from a 500 μm x 1 μm emitter and focused by a f = 10 mm focal length lens placed 10.1 mm from the transmitter will form a minimum light spot at a distance of 10 m. The magnification of this system is approximately 1:1000, and thus the length of the image formed at a 10 m distance is approximately half a meter. The beam will be diffraction-limited in the other dimension, such that the focused "point" will be an elongated line. Since such a long line cannot be absorbed by a photovoltaic cell of conventional shape and size, such a diode laser will not be suitable for long-distance projection applications.

[0132] To form a light spot smaller than 1 cm from the multimode diode, the system magnification should be 20 or less, and the lens must be located 50 cm from the emitter. Even with complex optics instead of a single lens, such a system would make the transmitter large and expensive, beyond the acceptable limit for most applications.

[0133] Now return to reference Figure 1 In the system shown, the laser diode 16 emits an expanded beam, and the divergence angle of the emitted light mainly depends on the thicknesses of the p-doped layer and the n-doped layer, including the thin gain medium layer. The thinner this combination of layers, the greater the divergence in the fast axis direction. Therefore, in order to be able to successfully focus the light while emitting the light with a small enough aperture to allow for a reasonably sized device, the optical surface of the focusing system 17 facing the laser diode 16 should be placed at a distance d from the diode beam emission surface, which can be shown from basic optical design principles between the following two limits:

[0134]

[0135] where d is the distance of the lens from the emitter aperture of the diode, measured in meters;

[0136] tan is the tangent function, and the angle is measured in radians;

[0137] and h is the thickness (height) of the combined p-doped layer and n-doped layer and the gain layer between them, measured in meters.

[0138] This forms a light spot at the receiver, where 95% of the power is contained in a circle with a radius r measured in meters, given by the following range:

[0139]

[0140] where h is the height (thickness) of the combined p and n doped layers, measured in meters,

[0141] w is the emitter width of the gain layer, measured in meters, and the tan function is given in radians.

[0142] A laser diode with a large emission width w will form a spot that is too large for simple projection applications, and if h is reduced, the resulting device will need to be made larger and more expensive because the fast axis of the beam will diverge rapidly, requiring a very large lens for effective collimation, which is positioned at a very close distance from the diode. This will require placement and structural tolerances that are difficult to achieve. This is because, as previously seen, the distance between the diode and the lens is set by the required magnification needed to allow for a small enough spot to be formed on the photovoltaic cell, thus moving the lens closer to the diode makes the spot size larger. Additionally, the diameter of the lens will be determined by the divergence of the fast axis along its path towards the lens. Therefore, choosing an h value outside of the above desired range will result in a larger device because the lens diameter will need to become very large in order to encompass and collimate or focus most of the light emitted by the laser diode within the desired range.

[0143] Reference is now made to Figure 3 , Figure 3 , which schematically shows a perspective isometric view of a semiconductor layer structure of a laser diode according to an exemplary embodiment of the present disclosure. The view is 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 a III-V or II-VI semiconductor diode. The general structure of the diode includes layers grown on a semiconductor wafer, where typically there are many layers, but only some of the layers provide the resonator and the gain for the laser.

[0144] The layers that provide the resonator and the gain for the laser typically include a pair of outer cladding layers, with a p-doped layer and an n-doped layer inside the cladding layers, and a gain-generating quantum well layer 34 located between the doped layers. The structure made of the gain layer surrounded by the p-doped layer and the n-doped layer can be repeated multiple times.

[0145] On the top and bottom of the diode are an anode electrode 31 and a cathode electrode 38 for powering the diode from a laser driver 15. The wafer is typically a GaAs, Ge, Si, InP, or another common semiconductor wafer. Describing the structure from the lower electrode 38 and the wafer 37, the terms "upper" and "lower", and "on" and "under" are related to what is graphically shown in the Figure 3 accompanying drawings. Several lattice-matched layers 37 can be directly grown on top of the wafer, below the diode structure itself, followed by a low-index cladding layer 36. On top of the first cladding layer 36, a p-doped or n-doped doped layer 35 is grown, which has a refractive index greater than that of the first cladding layer 36, thus defining the waveguide height.

[0146] A thin gain layer 34, which typically has a higher refractive index, is grown on the first doped layer 35. Generally, the thickness of the gain layer 34 is less than a single wavelength of the laser. The gain layer 34 has a bandgap between approximately 0.75 and 1.2 eV and provides gain at the laser wavelength when powered by electrodes 31, 38. The gain medium or quantum well composition can be any of the following:

[0147] 1. Quantum dots;

[0148] 2. GaInAs composition;

[0149] 3. GaAsSb composition;

[0150] 4. InPA composition;

[0151] 5. InAlAs composition; or

[0152] 6. Quaternary materials.

[0153] On top of the gain layer 34, a second doped layer 33 with a doping opposite to that of the first doped layer 35 is grown. The p-doped layer and the n-doped layer typically have similar refractive indices. The total height of these two doped layers and the thin gain layer 34 between them together define the height h of the laser resonator, which particularly determines the divergence of the fast axis of the laser diode. A second upper cladding layer 32 with a refractive index lower than that of the gain layer and the doped layers is deposited on top of the second doped layer 33.

[0154] Figure 3 The diode structure shown is a so-called symmetric or nearly symmetric diode structure, in which the fundamental resonant mode is centered substantially symmetrically and centered on the combined doped layer and the gain medium layer waveguide structure contained between the cladding layers. This diode structure typically provides a beam with the cleanest output mode.

[0155] As described above, in order to increase efficiency and thus the power output of a laser diode as a function of the input power, laser diodes with an asymmetric waveguide structure have been developed, where the active layer is intentionally positioned very close to the cladding layer at the p-doped layer in order to reduce the non-uniform carrier accumulation caused by the current on the p-side of the waveguide structure and the associated carrier losses. The cladding layer on this p-doped side of the diode waveguide itself is highly p-doped in order to reduce its series resistance. All of these features contribute to reducing the losses in the laser diode, thus enabling higher output power and efficiency. However, such an asymmetric diode structure is typically accompanied by a reduction in the purity of the output mode of the laser diode resonator. Additionally, multiple quantum well diode structures have been used to increase the power output, and these structural features are also typically accompanied by a reduction in the purity of the output mode of the laser diode resonator. These two methods of increasing the power output of the diode can also be customized for use with the currently described laser diodes, but the results can still be considered less than ideal for providing the desired beam collimation.

[0156] Now referring to Figure 4 , Figure 4 which Figure 3 is a view of the diode laser looking from the top, i.e., from above the outermost electrode 31 (or 38). Figure 4 The top electrode 42 at the outer surface of the device is shown. Although the top electrode 42 is shown in Figure 4 as covering the entire width of the diode structure, it can be slightly narrower than the entire wafer width. The current flowing through the diode between the electrodes creates population inversion and small-signal gain in the portion of the gain layer where the current flows, and also creates saturated gain during laser operation. The laser beam passes through the gain medium and is formed between the rear mirror 44 and the output coupler 48. Some of the light in this beam is reflected back into the cavity resonator, while another portion is transmitted as the output beam 46 outside the laser.

[0157] The resonator length L, which is typically in the range of 0.5 to 10 mm, is usually the distance between the rear mirror 44 and the output coupler 48, while the width w is determined by the limiting aperture of the resonator. This would be the width of the gain region or the width of the output coupler.

[0158] For the exemplary laser diode of the present disclosure, the transmitter width w should preferably be in the range of 15 to 250 μm.

[0159] The slow axis is the axis of the cavity width w, and the fast axis is the axis of the thickness (height) h of the gain medium and its two associated doped layers.

[0160] The cavity Fresnel number FN(w 2 / λL) in the slow axis should be in the range given by the following formula:

[0161] 0.01 < FN < 20(4)

[0162] This will ensure that a limited number of higher-order modes are developed to allow the diode to generate higher power while still suppressing the very high-order modes that limit focusability and safety. These low values of the Fresnel number prioritize low-order spatial modes but do not confine the diode to single-mode operation.

[0163] The width w should be adjusted to be within the following range:

[0164]

[0165] where E is the bandgap of the gain layer measured in joules, and n1 and n2 are the refractive indices of the doped layer and the confinement or cladding layer, respectively.

[0166] For diodes with a reasonable length (typically less than a few millimeters), such as required for high-volume and low-cost production, the above Fresnel numbers can be achieved with a width w in the range of 20 to 100 micrometers.

[0167] If the emitter width w is wider than the range given in expression (5), the diode will not be able to focus at the desired distance because too many higher-order modes will be created and the spot size will be too large for projection applications. If the emitter width w is too narrow, the diode will not have sufficient optical power because an insufficient number of modes will be generated to provide the power required in many power projection applications.

[0168] A laser diode cavity / resonator structure with the above-defined width and Fresnel number ensures that the emitted beam has a satisfactory optical quality. Specifically, such a beam should have the following advantageous qualities:

[0169] When focused using the above optical system, it will form a single lobe that, at the desired distance from the transmitter, allows it to be focused onto a small receiver.

[0170] The laser wavelength will be between 1150 and 1550 nm, advantageously providing eye safety and invisibility. More preferably, the laser wavelength should be between 1200 and 1450 nm.

[0171] The beam will be a confined multimode laser beam, especially in the "slow axis" (the w direction), but single-mode emission should be obtained when the current through the diode is low.

[0172] The beam resulting from the selection of the above parameters will be a beam that will include at least 50% TEM 00 Hermite-Gaussian modes and have less than 15% of all Hermite-Gaussian modes TEM mn, where m > 0 and n > 20. These limitations are necessary to achieve the desired ability to focus the light beam into the small bumps in the desired range and, at the same time, provide sufficient power to perform the desired task.

[0173] Due to the selected values of the width w and the resonator Fresnel number, the light beam will have a spatial mode in the slow axis (w direction) that is not TEM 00 , but should still have an M 2 value less than 6. Such a light beam will consist of many TEM mn modes, combined and mixed, but ensuring that the subsequent M 2 value is less than 6. Each pure TEM mode has an M 2 value of 2m + 1 or 2n + 1, and the above combination of TEM mn modes results in an overall M 2 value that depends on the percentage of the higher modes in the output light beam and the M 2 value of each component mode of the light beam. The M 2 value can be easily measured experimentally using a standard monitor for this purpose.

[0174] Due to the selected value of the cavity height h, the fast axis divergence is less than 60 degrees. In addition, the fast axis (h direction) will have a spatial mode with an M 2 less than 1.4. Different from a true single-mode diode laser, the distance between the waist in the fast axis (on the emitter) and the waist in the slow axis (inside the diode) is less than 1 mm but always greater than zero.

[0175] Such a laser diode will operate at a voltage V > 0.8 volts and is capable of emitting >300 mW of light when supplied with a current of at least 800 mA and typically up to several amperes.

[0176] The bandgap varies as a function of the junction temperature and current. As a result, the Fresnel number, wavelength, and diode characteristics will also change, such that the control unit should monitor the diode temperature and be programmed to maintain the laser stability of the beam.

[0177] At the threshold current, the diode produces a single-mode light beam that will be focused into a very small diffraction-limited spot (but with low power). When the current increases, the power increases, the bandgap, and sometimes the Fresnel number change, and the light beam becomes similar to the above light beam. It is important to characterize the light beam at its operating current, but also at 25% above and below its operating current and twice the threshold current.

[0178] When focused by a lens positioned at a distance d from the emitter of the diode, where d is measured in meters and within the range:

[0179]

[0180] where the angle of the tan function is measured in radians, and

[0181] h is the height of the p-doped and n-doped layers measured in meters,

[0182] When focused at a distance with a numerical aperture of 0.01, the diode is expected to produce a single lobe with a radius containing 95% of the power, limited to a radius r measured in meters, given by the following range:

[0183]

[0184] where h is the height of the p-doped and n-doped layers measured in meters,

[0185] w is the width of the gain layer measured in meters, and

[0186] The tan function is in radians.

[0187] A wider diode cavity will form a light spot that is too large for such projection applications, and if h is to be reduced, the resulting device will be larger and more expensive. The beam should contain at least 50% of its power in TEM 00 and less than 15% of its power in higher-order TEM nm modes, where the order is such that (n + m) is greater than 20. This value should apply to diode currents that are 2 to 4 times the threshold current and to conventional levels of operating current.

[0188] The laser diode described above provides efficient and safe laser beam transmission for a receiver power detector. However, the system must also include safety features that not only enable the beam to be focused on the receiver photovoltaic cell but also will warn of any situation where the beam might strike another body, which could indicate a laser hazard and should force the diode emission to stop. The currently described system incorporates several features that ensure that in such a possibility, the system is provided with protection that will prevent unintended laser diode emission in such situations where a physical short circuit or an electronic virtual short circuit enables the operating current to pass through the laser diode. Such protection features are described in PCT application PCT / IL2022 / 051040, "A System for Location and Charging of Wireless Receiver", which is jointly owned by the present applicant and is incorporated herein by reference in its entirety. The main features of these additional safety systems are (i) improving the physical electrical insulation of the laser diode power leads, (ii) independently controlling the switches in the anode and cathode leads of the laser diode, and (iii) activating the system through a two-stage power voltage arrangement.

[0189] Referring now to Figure 5 , Figure 5 which schematically shows a block diagram of the main features of the power supply scheme for a laser diode. The laser diode 50 is powered by a laser diode power supply 51 which receives its drive instructions from a system main controller 52. This main controller 52 is programmed to turn the laser diode on and off and to regulate its power level during normal operation of the system using the laser diode power supply 51 so as to provide a safety level from laser hazards. The diode power supply 51 sends an appropriate drive current to the laser diode 50 via a cable having a fully covered insulator 54, i.e. 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), the cable advantageously including the legs and housing of the laser diode mount or legs themselves. These current leads include two auxiliary gated switches S1 and S2 controlled by a gate controller which may be incorporated within the main controller 52. Thus, the enabling of the current from the laser diode power supply 51 to the anode 55 of the laser diode, and from the cathode 56 of the laser diode to ground of the circuit or to the negative terminal of the laser diode power supply is controlled by the two switches S1, 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, the output level of which is controlled by the main controller 52. These two switches S1, S2 which are held in the conducting state (hereinafter referred to as "closed") by a control voltage on their gates are used for additional safety, thus providing two additional and independent redundant means of terminating the current to the laser in addition to the normal control of the diode current by the controller 52, which may be implemented individually or together. However, in the case of a short circuit supplying current to the laser diode, the conventional control of the laser diode current may not always be able to perform its desired function other than via the laser diode power supply 51. For example, it is in such a case that the two switches S1, S2 provide an additional safety means of shutting off the laser emission when the conditions require it to be turned off.

[0190] This additional safety feature for interrupting the switching process of the laser diode current comes from the way the switches are powered relative to the other electronic modules and functions of the system. The operation of these two gated switches makes use of the fact that most infrared laser diodes typically operate at a low voltage (in the region below 1.5V). This is a significantly lower voltage than that used by most other electronic components associated with the electronic circuitry of a system typically based on Si semiconductor technology. Such Si technology devices cannot operate at such a low voltage and use a higher operating voltage, typically 1.7V, 3.3V, 5V or 12V etc.

[0191] The function of the gate controller, either within or separate from the main controller 52, is to stop laser emission by turning on at least one of the switches S1, S2 when the main laser driver controller 52 does not stop laser emission when so instructed. The gate controller function can be incorporated as an additional unit of the main controller 52, but it can be implemented as an additional and separate circuit module ( Figure 5 not shown in

[0192] When not actively held in the conductive state by applying the required voltage to the switch gate, at least one of the two switch gates is arranged to be in a normally non-conductive state. The voltage provided by the gate controller holds the gate in the conductive state, enabling the laser current during normal operation. When the latch voltage drops, the gate will revert to the open non-conductive state. The switch gate, or more specifically, the gate controller circuit, is driven by the system main power supply ( Figure 5 not shown) through a separate operating voltage V2, which is higher than the voltage V1 supplied to the main controller 52 or the laser diode power supply 51 or any other electronic function in the system. If a physical short occurs, causing a voltage of more than 1.5V (which is a typical voltage for infrared laser diode operation) to be applied to the anode lead 54 of the laser diode, the laser diode 50 will turn on and emit a laser beam even when the controller 52 instructs the laser driver to be in the off state and the anode switch S1 is instructed to be non-conductive. The same applies if such a circuit fault occurs in the laser diode power supply 51 and current is passed to the laser even when the controller 52 does not instruct current to be delivered to the laser. Since the laser diode operates at 1.5V or lower and another voltage inadvertently present in the circuit will be higher than 1.5V, the increased current drawn from the main power supply may cause the main power supply voltage of all control functions of the system to drop, or alternatively, drop to a level insufficient to reliably operate the controller 52. Since the gates of the switches are actuated by the controller 52 at a higher voltage than the main controller 52 itself, this voltage drop will cause the gated switches to switch to their non-conductive state independently of the controller's instruction to the laser power supply 51. Therefore, putting any one of these switches S1, S2 in the non-conductive state will stop the laser diode current and put the system in a safe state, regardless of the functional actions of any other circuit controllers (such as the main controller 50) or the electronic safety mechanisms of the system.

[0193] In summary, using a higher supply voltage V2 for the gate controller ensures that in the event of a fault that causes a reduction in the voltage provided by the overall system power supply, the gate controller should be the first circuit to lose power because it operates at a higher voltage than other circuit components and will cut off the gate holding voltage and thus the power to the laser diode before and independently of other controller functions.

[0194] In a second alternative scenario, if the voltage supplied to power the main controller 50 drops low enough to cause the main controller to malfunction and thus be unable to respond by reducing the accidental and uncontrolled laser diode current, the fact that the switching operation of the gate controller 53 relies on a higher operating voltage than the system controller 50 or the laser driver 48 means that the switch will become non-conductive and thus terminate the laser diode current regardless of what the system controller or laser driver attempts to do.

[0195] All points in the circuit (which could short to ground or another energized metal contact within the laser generator housing) should be well electrically insulated. This protection is particularly important when using a C-mount laser diode because such a C-mount has a large area of exposed metal surface that is part of the diode conductor and in the event of mechanical intrusion or mechanical failure (such as a loose wire connection becoming free), the diode conductor could short to ground or another energized metal contact within the laser generator housing. Achieving this fully without compromising the cooling requirements of the laser diode is not a simple task.

[0196] Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those skilled in the art. Numerous specific details are set forth, such as examples of specific components, devices, and methods, to provide a thorough understanding of the embodiments of this disclosure. It will be apparent to those skilled in the art that specific details need not be employed and that the example embodiments may be embodied in many different forms and should not be construed as limiting the scope of this disclosure. Additionally, those skilled in the art should understand that the present invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention includes combinations and sub-combinations of the various features described above and variations and modifications thereof that would occur to those skilled in the art upon reading the above description and not being in the prior art.

Claims

1. A laser diode that emits a laser beam for optical wireless power transfer, the laser diode comprising: A first cladding layer having a first refractive index and a second cladding layer having a second refractive index, with an emission region therebetween, the emission region comprising: A positively doped layer having a third refractive index and a negatively doped layer having a fourth refractive index, each of the third refractive index and the fourth refractive index being greater than either of the first refractive index and the second refractive index; and A gain medium layer deposited between the positively doped layer and the negatively doped layer and having a bandgap between 0.8 eV and 1.2 eV, wherein the width w of the emission region in meters is in the range: where E is the bandgap of the gain layer measured in joules, n1 is the average refractive index of the doped layer, and n2 is the average refractive index of the cladding layer, and wherein: The upper limit of w is selected such that at least a first predetermined percentage of the power of the beam is concentrated in the Hermite-Gaussian TEM 00 mode, and does not exceed a second predetermined percentage of the power of the beam in the higher-order TEM nm mode, to ensure that the beam has a spatially multimode form with the desired quality on the slow axis, where (n + m) is greater than a preset number, and The lower limit of w is chosen to ensure that on the slow axis, the beam has a sufficient combination of higher-order modes such that the beam produces at least the required level of laser beam output power.

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

3. The laser diode according to any one of the preceding claims, 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 the preceding claims, wherein, The preset number is 20.

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

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

8. The laser diode according to any one of the preceding claims, wherein, The laser diode has a cavity with a length L in meters such that for the range of w according to claim 1, the Fresnel number w 2 / λL is between 0.01 and 20.

9. The laser diode according to any one of the preceding claims, wherein, The laser beam has: an M value of less than 15 in the direction of the slow axis 2 value; and M less than 1.5 in the direction of the fast axis 2 and the divergence of the laser beam in the direction of the fast axis is less than 60°.

10. The laser diode according to any one of the preceding claims, wherein, The mode of the collimated laser beam is such that when imaged through a suitable optical system to a distance of 10 m, the beam has a single lobe; and The effective radius r of the focused laser beam produced at the 10 m distance is given by the following expression: where h is the height of the laser emitter measured in meters, w is the width of the emission region measured in meters, r is the effective spot radius measured in meters, and The tan function is in radians.

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

12. The laser diode according to any one of the preceding claims, wherein, The laser gain medium layer includes any of the following: Quantum dot gain layer; GaInAs composition; GaAsSb composition; InPA composition; AlAs composition; or Quaternary material.

13. The laser diode according to any one of the preceding claims, wherein, The cladding layer and the doped diode layer have a symmetric or approximately symmetric structure with respect to the gain medium layer.

14. A system for transmitting laser power from a transmitter to a remote receiver, the system comprising: A laser diode source that includes a gain medium sandwiched between a p-doped layer and an n-doped layer and is supplied with current from a laser driver; An optical system for collimating the laser beam emitted from the laser diode source to produce an illumination spot on the remote receiver; And A scanning system for directing the collimated laser beam towards a hole in a photovoltaic cell on the remote receiver, wherein: Select the gain medium of the laser diode such that the laser diode emits a beam having a wavelength λ between 1150 nm and 1550 nm; Select the current supplied by the laser driver to the gain medium such that the laser beam has a power of at least 300 mW, and The laser diode source has an emitter width w between 15 and 250 μm such that: The laser beam is a spatially multimode beam, at least 50% of the power of the laser beam being concentrated in the Hermite-Gaussian TEM 00 mode, and less than 15% of the power of the laser beam being in the higher-order TEM nm mode, where (n + m) is greater than 20; and Select the laser diode gain medium length such that the laser diode has a Fresnel number w in the slow axis between 0.01 and 20 2 / λL, where L is the length of the laser cavity, thereby generating a light beam having the following M less than 15 in the direction of the slow axis 2 value M less than 1.5 in the direction of the fast axis 2 value; The divergence of the laser beam in the direction of the fast axis is less than 60°; When imaged by an optical system to a distance of 10 m, it has a single-lobe mode with a collimated laser beam; and The sequential focusing spots of the laser beam are given by the following expression: where h is the combined height of the p-doped layer, the n-doped layer, 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, and The tan function is in radians.

15. The system according to claim 14, wherein The bandgap 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 fabricated on a substrate of III-V or II-VI semiconductor.

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

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

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

20. The system according to any one of claims 14 - 19, wherein, The gain medium is disposed between an n-doped layer and a p-doped layer, the n-doped layer and the p-doped layer have an average refractive index of n1, and the n-doped layer and the p-doped layer are themselves disposed between cladding layers having a refractive index of n2.

21. The system according to claim 20, wherein, The width w of the gain medium is in the range given by the following expression: where E is the bandgap of the gain medium layer measured in joules, n1 is the average refractive index of the doped layer, and n2 is the average refractive index of the cladding layer.

22. The system according to any one of claims 14-21, wherein The electrical connection for supplying current from the laser driver to the laser diode is an insulated connection that is adapted to prevent the possibility of inadvertently electrically connecting to the laser diode, thereby enhancing the safety of the system.

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

24. The system according to claim 23, wherein The gated switch for controlling the flow of the current through each of the electrical connections is activated by a gate driver having an operating voltage that is higher than the operating voltage of other electronic circuits of the system for providing control functions to the system.

25. The system according to claim 24, wherein, The gate driver is configured to hold each of the gated switches in its conductive state when the gate driver is instructed to activate the gate.

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

27. The system according to claim 26, wherein, This non-conductive state thus isolates the laser diode from any current source, even one resulting from an unintentional electrical connection to the laser diode.

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