Laser based on composite pumping of Nd:YAG crystal by LED and LD

Abstract

The present invention discloses a laser based on an LED and LD composite-pumped Nd:YAG crystal. This is a dual-wavelength near-infrared all-solid-state laser. By designing a novel cavity coupling device, effective composite pumping of LD end-face pumping and LED side-face pumping, as well as dual temperature control functions, is achieved. A large-mode-volume flat-flat cavity metastable structure is employed to directly composite-pump an Nd:YAG crystal with a high doping concentration of 1.5 at.%, achieving simultaneous dual-wavelength laser output at 938 nm and 946 nm. The composite pumping structure, dual temperature control function, and increased laser crystal doping concentration significantly improve light-to-light conversion efficiency, effectively enhancing the efficiency of dual-wavelength laser output. The device of the present invention has the advantages of low cost, long life, simple system structure, and stable operation.

Description

Technical Field

[0001] The present invention relates to a photoelectric device, in particular to a laser based on LED and LD composite pumping Nd:YAG crystal, which is a dual-wavelength near-infrared all-solid-state laser. Background Art

[0002] Traditional solid-state lasers are pumped by flash lamps. The emergence of high-power semiconductor lasers has combined the advantages of semiconductor lasers—high efficiency, high repetition rate, miniaturization, and long life—with the narrow pulse width, small divergence angle, spectral purity, and high beam quality of solid-state lasers, driving the development of both solid-state and semiconductor lasers. However, solid-state lasers pumped by semiconductor lasers (abbreviated as all-solid-state lasers (LDPSSLs)) have a significant drawback: the central wavelength range of the LD semiconductor laser output is narrower than that of an LED. Once the LD's central wavelength deviates due to changes in ambient temperature or other factors, its energy relative to the specific output laser wavelength will immediately drop, compromising the stable operation of the laser system.

[0003] Compared to laser diodes, using light-emitting diodes (LEDs) as pump sources offers advantages such as longer lifespan, compact systems, and low cost. Early development of LED-pumped solid-state lasers was limited by the low pump power of LEDs. Today, the optical power of a single LED die can be scaled up to the watt level with smaller chip sizes. Furthermore, by operating the LED in quasi-continuous-wave (QCW) mode, peak output light intensities can reach hundreds of watts per square centimeter. Furthermore, the energy-level transition absorption mechanism of narrowband LD pumping differs theoretically from that of broadband LED pumping. The output spectral width of an LD is typically less than 5 nm; therefore, when an LD is used as a pump source, matching a single absorption peak in the absorption spectrum of the working laser material is often required. In some cases, the spectral width can be neglected, and the pumping method can be considered narrowband. In contrast, since LED sources typically have a spectral bandwidth of 20–60 nm, they can be considered broadband pumps, eliminating the need for spectral matching between the pump light spectrum and the crystal absorption spectrum. Multiple absorption peaks can be covered within the absorption spectrum of the working laser material, making it easier to achieve dual-wavelength or broadband laser output. However, the laser output energy is still limited by the current loading limit of the LED array, the relatively low coupling efficiency of the LED optical output to the gain medium, and the limited fill factor of the available LED array. Existing LED pumping systems can only achieve threshold gain, low light-to-light conversion efficiency, or low slope efficiency laser output.

[0004] Whether using LD or LED pumping alone, both have inherent drawbacks. LD end-pumping offers high energy density but a narrow spectral tunability range, while LED side-pumping offers low energy density but a wide spectral range. Therefore, the present invention combines the advantages and disadvantages of both pump sources to design a cavity coupling device capable of composite pumping, achieving dual-band laser emission at 938nm and 946nm.

[0005] Yang Boda et al. published "High-Temperature LDAs Side-Pulsed Nd:YAG Lasers" in the March 2021 issue of the Journal of Photonics, Volume 50, Issue 3. They reported a compact, high-temperature laser diode array side-pumped Nd:YAG pulsed laser. The pump source's operating temperature was controlled at 60°C by a semiconductor cooler, with an emission center wavelength of 808 nm and a spectral linewidth of 4 nm. The temperature field distribution of the pump source was simulated over a 60-second period at 40°C, 50°C, and 60°C. While this pioneering research has been conducted on this technology, the application of composite pumping has not yet been proposed, nor has the aforementioned technical issues been resolved. Summary of the Invention

[0006] The present invention aims to effectively combine side pumping and end pumping, improving light-to-light conversion efficiency through composite pumping and facilitating dual-wavelength laser emission at 938nm and 946nm. This composite pumping combines the high energy density of an LD light source with the broadband pumping of an LED light source, creating a unique pumping method.

[0007] The specific technical solution of the present invention is as follows: a dual-wavelength near-infrared all-solid-state laser based on LED and LD composite pumping high-doping concentration Nd:YAG crystal, including LD semiconductor laser, LED light bar, input mirror, output coupling mirror, cavity coupling device, laser crystal and cooling system;

[0008] The LD semiconductor laser is emitted by an LD laser diode, and the light spot is focused on the crystal end face through a focusing lens with a focal length of 75mm. The maximum optical power can reach 15W, the wavelength range is 801~809nm, and the half-peak width is 2nm.

[0009] The LD semiconductor laser is precisely temperature-controlled by the LD temperature control system and operates stably at room temperature of 25°C;

[0010] The LED light bar is a light-emitting module consisting of an LED substrate or printed circuit board and SMD LEDs. The LED substrate is an aluminum substrate 80mm long and 8mm wide, on which up to 19 SMD LEDs can be soldered. M2 threaded holes are provided at both ends for easy fixing. The SMD LEDs measure 3.5*3.5*2.5mm, have a divergence angle of 90°, operate at a voltage of 2.8-3.4V, and have a maximum operating current of 1.5A (continuous) / 5A (pulsed). The optimal operating temperature is 0-5°C, and the optical power can reach up to 2W. The central wavelength during operation is 810nm, the peak wavelength is 815nm, and the half-peak width is 30nm.

[0011] The LED strip is cooled by a TEC directly contacting the aluminum substrate and a novel cavity coupling device (copper heat sink). To avoid excessive surface condensation and achieve high-power operation of the LEDs, we chose 0°C as the operating point for the LEDs.

[0012] The input mirror has a lens coating with a transmittance of 99% in the 808nm band and a high reflectivity of 99.5% in the 900-1000nm band;

[0013] The output coupling mirror has a lens coating with a transmittance of 99% in the 808nm band and a transmittance of 3% in the 938-946nm band;

[0014] The novel cavity coupling device is made of brass. A central projection 501 for mounting the crystal is located within the cavity. The upper and lower protrusions, combined with indium foil, secure the laser crystal and keep it level. M2 threaded holes 502 on either side allow for the attachment of an LED light strip, ensuring the SMD LED is aligned with the crystal. M4 threaded holes 503 on both ends of the cavity allow for temperature control of the crystal and LED light strip using a pagoda connector and cooling pump. The front of the cavity serves as a water inlet, while the rear serves as an outlet.

[0015] The novel cavity coupling device has four M6 threaded holes 504 at the bottom, which facilitates fixing to the goniometer.

[0016] The cavity coupling device helps to clamp and cool the long block crystal, and can also fix and cool the LED light bar, simplifying the system while providing stable and effective composite pumping.

[0017] The laser crystal is Nd:YAG with a doping concentration of 1.5 at.%, a crystal size of 5*5*72 mm, and the crystal end faces are coated with a 938~946 nm anti-reflection film;

[0018] The cooling system includes a cooling pump and an LD temperature control system.

[0019] The cooling pump is a low-temperature coolant circulation pump with an adjustable temperature range of -40°C to 25°C, and the medium is anhydrous ethanol;

[0020] The LD temperature control system consists of a control mainboard, a thermistor, and a semiconductor cooler (TEC). The control mainboard achieves precise temperature control of the LD laser through temperature change feedback from the thermistor and cooling of the LD laser diode by the TEC.

[0021] Preferably, the device is provided with at least three identification units, which respectively identify the coating parameters of the input mirror and the output mirror and the specification parameters of the cavity coupling device.

[0022] Preferably, it further comprises an identification unit for identifying various performance parameters of the LD laser diode, wherein the parameters at least include the brightness and wavelength range of the LD laser diode.

[0023] Preferably, it includes an identification unit for identifying various parameters of the cooling system, and the parameters at least include the water cooling range.

[0024] Compared with the prior art, the present invention has the following advantages: 1. The present invention adopts two completely different pump sources, and effectively solves the problem of difficult matching of LD narrowband pump spectrum and the problem of low energy density of LED broadband pump by designing a new cavity coupling device for composite pumping. The advantages of the two pump sources are integrated to achieve more effective pumping. 2. The Nd:YAG crystal used in the present invention is highly doped, and its optical properties change differently from those of general crystals. When using 808nm pumping, 938nm and 946nm dual-band lasers are simultaneously generated instead of the common 1064nm band. 3. The present invention is suitable for long block crystals. The temperature of the crystal is controlled by the boss in the new cavity coupling device and the indium foil covering the upper and lower surfaces of the crystal, which solves the problem of difficult cooling of the side pump of the long block crystal. 4. The device of the present invention is a new pump-coupled dual-wavelength laser operation system with the characteristics of low cost, long life, simple system structure, and stable operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic structural diagram of the all-solid-state laser of the present invention.

[0026] Figure 2 Schematic diagram of the structure of the new cavity coupling device designed.

[0027] Figure 3 Schematic diagram of the structure of the LED substrate.

[0028] Figure 4 This is a structural diagram of a SMD LED.

[0029] Figure 5This is a schematic diagram of the pagoda joint.

[0030] Figure 6 This is the fluorescence spectrum without adding a resonant cavity.

[0031] Markings in the figure: 1. LD semiconductor laser, 2. LED light bar, 3. Input mirror, 4. Output coupling mirror, 5. New cavity coupling device, 6. Laser crystal Nd:YAG, 7. Focusing mirror with focal length of 75mm, 501. Boss, 502. M2 threaded hole, 503. M4 threaded hole, 504. M6 threaded hole. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further explained below with reference to the accompanying drawings.

[0033] Attachment Figure 1 A schematic structural diagram of the dual-wavelength near-infrared all-solid-state laser based on LED and LD composite pumping of a high-doping concentration Nd:YAG crystal according to the present invention is shown.

[0034] The following is combined with Figure 1 The content and working principle of the present invention are further described in detail.

[0035] The present invention is a dual-wavelength near-infrared all-solid-state laser based on LED and LD composite pumping high-doping concentration Nd:YAG crystal, such as Figure 1 As shown, the laser includes an LD semiconductor laser 1 and two LED light strips 2 as pump sources, a Nd:YAG crystal 6 with a doping concentration of 1.5at.% as the working material, a resonant cavity composed of an input mirror 3 (HT@808nm & HR@900~1000nm) and an output coupling mirror 4 (HT@808nm & T=3% @938~946nm), a new cavity coupling device 5, and a 75 mm focusing mirror 7.

[0036] The LD semiconductor laser is a tunable pulsed laser diode with a peak power of 15W and a center wavelength of 808nm. The output center wavelength can be controlled by adjusting the temperature of the PN junction. The LED light bar consists of 19 SMD LEDs with a 90° divergence angle arranged in a rectangular pattern with a spacing of 0.2mm. Each SMD LED outputs a maximum peak optical power of 2W, with a center wavelength of 810nm, a peak wavelength of 815nm, and a half-width of 30nm during operation. The LED light bar is driven by an external programmable pulse power supply, which can be freely switched between constant current and constant voltage modes. The maximum output voltage is 150V, the maximum output current is 30A, the frequency is 10-2000Hz, and the duty cycle is tunable from 1% to 100%. This pulse power supply also supports pulse signal output. The pulse signal of the LD semiconductor laser can be synchronized via a dual BNC interface cable, enabling signal cascading between pump sources.

[0037] The size of the laser crystal is 5*5*72mm, and both ends of the crystal are coated with a 938~946nm band anti-reflection film. When the pump light with a central wavelength of 808nm is incident on the crystal Nd:YAG, it is absorbed by the electrons of the appropriate energy level of the outer layer of the Nd ions doped in the crystal. Under normal circumstances, 1064nm laser is easier to excite because its branch emission ratio is about 60%, and it is a 4-level system with a low laser threshold. However, the laser crystal used in the present invention has a high Nd ion doping concentration, which shortens the fluorescence lifetime, broadens the line width, causes strain in the crystal, and ultimately causes changes in its optical properties, such as Figure 6 The figure shows the excitation of dual-wavelength lasing at 938nm and 946nm, which is more difficult to achieve than 1064nm. The challenge with the 938nm and 946nm laser transitions is that they are quasi-three-level systems, resulting in significant absorption losses and a very small stimulated emission surface at room temperature. Therefore, this system must operate at temperatures below room temperature.

[0038] Attachment Figure 2 A schematic diagram of the structure of a novel cavity coupling device suitable for compound pumping, designed for the present invention. Because the crystal size is 5*5mm, the width of boss 501 is 5mm, and the spacing between the upper and lower bosses is 5.05mm, several layers of indium foil can be placed on the upper and lower surfaces of the crystal to both secure the crystal and prevent scratches on the crystal surface, achieving effective heat dissipation while protecting the crystal. Marked 502 is an M2 threaded hole, which can be used with the threaded holes on the LED light bar to secure the LED light bar and ensure that the light emitted by the LED is horizontally incident on the side of the crystal. Marked 503 is an M4 threaded hole, which can be used with the pagoda interface to connect a cooling pump to dissipate heat for the entire cavity. Marked 504 is an M6 threaded hole, which can be used with a tilting device to secure the crystal, facilitating adjustment of the pitch angle of the crystal within the cavity.

[0039] Attachment Figure 3 This is a schematic diagram of the structure of a substrate suitable for SMD LEDs designed by the present invention. As shown in the figure, up to 19 SMD LEDs can be soldered on it, and two M2 threaded holes are provided on the left and right sides for easy fixing in the cavity.

[0040] Attachment Figure 4 It is a SMD LED with specifications of 3.5*3.5*2.5mm. There is a square mark in the upper left corner, representing the positive terminal, which is convenient for soldering.

[0041] Attachment Figure 5 It is a pagoda-shaped interface with a through hole inside. It can quickly dissipate heat from the cavity when combined with the M4 threaded hole 503 on the cavity, a soft rubber tube with an inner diameter of 3mm, and a cooling pump.

[0042] Attachment Figure 6This is the fluorescence spectrum without a resonant cavity. As can be seen in the figure, when pumped at 808nm, the laser crystal's fluorescence intensity at 938nm and 946nm is much higher than at 1064nm, unlike when pumping Nd:YAG crystals in general. Adding a resonant cavity mirror, combined with LD end-pumping and LED side-pumping, effectively stimulates dual-band laser emission at 938nm and 946nm.

[0043] The embodiments of the present invention are used as a preferred technical solution to illustrate the concept of the present invention, and the scope of protection shall be subject to the description of the claims.

Claims

1. A laser based on LED and LD composite pumped Nd:YAG crystal, the laser is a dual-wavelength near-infrared all-solid-state laser, characterized in that: It includes at least two LED light bars and at least one LD laser, a resonant cavity, a cooling system, and an Nd:YAG crystal; The LED light strips have the same brightness, wavelength, package, size and shape, and the center wavelength is 810nm, the peak wavelength is 815nm, and the half-peak width is 30nm. The resonant cavity includes an input mirror, an output coupling mirror, and a cavity coupling device for mounting a crystal and an LED light bar. The input mirror and the output coupling mirror are made of the same material, have the same size, thickness, and shape, and are both plane mirrors. The input mirror lens coating has a transmittance of 99% in the 808nm band and a high reflectivity of 99.5% in the 900-1000nm band. The output coupling mirror lens coating has a transmittance of 99% in the 808nm band and a transmittance of 3% in the 938-946nm band. The LD laser is emitted by an LD laser diode, and the light spot is focused on the end face of the crystal through a focusing lens. The maximum optical power is 15W, the wavelength range is 801~809nm and can be tuned, and the half-peak width is 2nm. The cooling system includes a cooling pump and an LD temperature control system; The cooling pump can achieve any adjustment of the room temperature from -40°C to 25°C, and can cool the LED light bar and laser crystal by pumping cold water to circulate in the copper cavity coupling device. The LD temperature control system consists of a control motherboard, a thermistor, and a semiconductor cooler (TEC). The control motherboard achieves precise temperature control of the LD laser through temperature change feedback from the thermistor and cooling of the LD laser diode by the TEC. The laser crystal is a Nd: YAG long block crystal with a doping concentration of 1.5 at.%, and both ends of the crystal are coated with an antireflection coating in the 938~946nm band; While the two LED light strips are pulse pumped on the side, the composite pumping method is to use externally driven synchronous LD laser diode end face pulse pumping.

2. The laser based on LED and LD composite pumped Nd:YAG crystal according to claim 1, characterized in that: The SMD LEDs in the LED light bar are connected in series and arranged in a rectangular shape. The distances between the SMD LEDs are the same, and the distances between the two light bars and the crystal are also the same.

3. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: Each SMD LED light emitting diode in the LED light bar is an LED light emitting diode of the same sorting bin and its adjacent sorting bin.

4. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: It also includes at least three identification units, which respectively identify various performance parameters of the patch LED, and the performance parameters at least include the brightness, central wavelength and half-peak width of the LED light-emitting diode.

5. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: It also includes at least three identification units, which respectively identify the coating parameters of the input mirror and the output mirror and the specification parameters of the cavity coupling device.

6. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: It also includes an identification unit for identifying various performance parameters of the LD laser diode, and the parameters at least include the brightness and wavelength range of the LD laser diode.

7. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: It includes an identification unit for identifying various parameters of the cooling system, and the parameters at least include a water cooling range.

8. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: The surface of the crystal is neat and smooth; both end faces of the crystal are coated with an anti-reflection film for the 938-946 nm wavelength band.

9. The laser based on LED and LD composite pumping Nd:YAG crystal according to claim 1, characterized in that: The cavity coupling device is fixed on the oscillator, and there is a boss in the middle of the cavity for mounting the crystal. The upper and lower protruding parts cooperate with the indium foil to fix the laser crystal to make it horizontal. The LED light strips are fixed through threaded holes on both sides so that the SMD LED and the crystal are on the same horizontal plane. The two ends of the cavity are connected to the cooling system to control the temperature of the crystal and the LED light strip. One end is the water inlet and the other end is the water outlet.

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