An optical-pumped semiconductor disk laser based on photonic crystal

Abstract

An optical-pumped semiconductor disk laser based on a photonic crystal relates to the field of semiconductor lasers. While eliminating the waste heat generated by the pump light that causes high-order modes and boosting the power, it achieves a fundamental transverse mode with M2 = 1 and equal M2 factors for the X-axis and Y-axis, presenting a fundamental transverse mode Gaussian spot. The laser includes: an incident pump light source, a gain chip, and an external resonant cavity mirror; the incident pump light source performs surface pumping on the gain chip under the condition of oblique incidence to achieve population inversion, enabling the lasing laser to transmit in the cavity surface formed by the gain chip and the external resonant cavity mirror; the gain chip reflects the excess pump light to control the thermal effect and the transverse mode quality. The photonic crystal structure of the present invention has the characteristic of high reflection for the pump light source, so it can suppress high-order modes and reduce the thermal effect that causes high-order mode pump spots. The range of the pump spot is restricted to pump only within the fundamental transverse mode range on the chip.

Description

Technical Field

[0001] The present invention relates to semiconductor lasers, and particularly to an optically pumped semiconductor disk laser based on a photonic crystal. Background Art

[0002] An optically pumped vertical-external-cavity surface-emitting laser (OP-VCSEL), also known as an optically pumped semiconductor disk laser (OP-SDL), is an external-cavity structure laser that is pumped by an external pump light source 9, has a high absorption efficiency, can cover a large area in terms of wavelength band, and can achieve high-power operation. It is widely used in many fields such as scientific research, biomedicine, information communication, and industrial production.

[0003] OP-VECSEL is similar to a conventional surface-emitting semiconductor laser. Their main component is a semiconductor chip epitaxially grown on a GaAs or GaSb substrate 5. However, different from a surface-emitting semiconductor laser, its cavity is completed by an external resonant cavity mirror 10. All surface-emitting semiconductor lasers are electrically pumped, while OP-VECSEL is pumped by an external pump light source 9.

[0004] OP-VECSEL is surface-pumped by an external pump light source 9 and is severely affected by the pump light. The heat generated by the pump light will cause the laser to heat up. The size and shape of the pump light spot 6 will directly affect the beam quality of the output laser. Generally, the pump light is incident at an angle of 30 degrees - 50 degrees with respect to the laser lasing axis direction, which will cause the lengths of the pump light spots 6 in the X-axis and Y-axis directions to be inconsistent, forming an elliptical pump light spot 6, resulting in higher-order mode lasing, affecting the laser transverse mode quality, and there is a deviation in the M2 factor. The change in the shape of the disk pump light spot 6 results in different powers at different positions of the pump light. The thermal effect causes a change in the refractive index, generating a thermal lens effect, and the laser stability deteriorates. Conventional methods for controlling the beam quality cannot eliminate the heat from the pump light that causes higher-order modes, resulting in a serious thermal effect. The thermal effect will cause thermal rollover, making it impossible to further increase the power.

[0005] Currently, there are four methods for controlling the beam quality of OP-VECSEL: 1. Perform spatial filtering outside the laser. 2. Use an intracavity aperture. 3. Etch grooves on the disk. 4. Design the resonant cavity to make the fundamental transverse mode size smaller than the pump light spot 6.

[0006] Method 1.2.3 can achieve a laser beam quality of M2 = 1. However, experiments show that using this method will lead to a significant power drop. The reason is that a large number of higher-order modes are filtered, but the waste heat generated by the pump light that generates higher-order modes on the gain chip cannot be eliminated, resulting in earlier thermal inversion and making the power lower than the theory. Method 4 can only achieve near-diffraction-limited laser with M2 = 1.2 at best, with a deviation of more than 10% in the M2 factors of the X-axis and Y-axis, and it is extremely unstable. The beam quality factors are different under different pump powers. OP-VECSEL is severely affected by heat. Although the above four methods can control higher-order modes to a certain extent, each of the four methods has its advantages and disadvantages. Either the power is small and a circular spot with M2 = 1 can be achieved, or the power is large, but the beam quality is approximately a near-diffraction-limited elliptical spot, and the beam quality changes significantly with the change of the pump light. Therefore, none of the above four methods can achieve both the control of the transverse mode and the elimination of the waste heat generated by the pump light that generates higher-order modes on the gain chip, and it is impossible to achieve the fundamental transverse mode at a higher power level. Summary of the Invention

[0007] In order to solve the problems existing in the prior art, the present invention provides an optically pumped semiconductor disk laser based on a photonic crystal, which can eliminate the waste heat generated by the pump light that causes higher-order modes, improve the power, and at the same time achieve a fundamental transverse mode with M2 = 1 and equal M2 factors on the X-axis and Y-axis, that is, a fundamental transverse mode Gaussian spot.

[0008] The technical solutions adopted by the present invention to solve the technical problems are as follows:

[0009] An optically pumped semiconductor disk laser based on a photonic crystal, the laser comprising:

[0010] The laser comprises: an incident pump light source, a gain chip, and an external resonant cavity mirror; the incident pump light source performs surface pumping on the gain chip under the condition of oblique incidence to achieve population inversion, so that the lasing laser transmits in the cavity formed by the gain chip and the external resonant cavity mirror; the photonic crystal structure on the surface of the gain chip reflects the redundant pump light, so that the gain within the range of the photonic crystal structure is reduced, the loss is increased, the intracavity mode resonance is suppressed, the thermal effect is reduced, and the transverse mode quality is controlled.

[0011] Preferably, the pump light spot located on the surface of the gain chip is elliptical.

[0012] Preferably, the reflectivity of the gain chip for reflecting the pump light at the photonic crystal structure is higher than 80%.

[0013] Preferably, the gain chip includes, successively fabricated on the substrate: a distributed Bragg reflector, a periodic gain structure, a cover layer, and a photonic crystal structure; the photonic crystal structure is located within the cover layer, and the depth of the photonic crystal structure is less than the depth of the cover layer.

[0014] Preferably, the lattice of the photonic crystal structure is a triangular lattice or a square lattice; the lattice holes of the photonic crystal structure are triangular or circular.

[0015] Preferably, the inner diameter size of the photonic crystal structure should be greater than the surface fundamental transverse mode size of the gain chip and less than the excitation size of the TEM01 or TEM10 mode; the outer diameter size of the annular structure of the photonic crystal structure should be greater than the major axis size of the pump light spot.

[0016] Preferably, the periodic gain structure is a multi-quantum well structure, and the standing wave peaks within the gain chip coincide with the quantum wells.

[0017] Preferably, it further includes a heat sink disposed under the substrate.

[0018] Preferably, the heat sink is a bonded metallized diamond heat sink bonded under the substrate and a gold-plated copper heat sink bonded under the metallized diamond heat sink.

[0019] The beneficial effects of the present invention are as follows: The photonic crystal structure of the present invention has the characteristic of high reflection for the pump light source, so it can suppress high-order modes and reduce the thermal effects that cause high-order mode pump light spots. The range of the pump light spot is restricted so that it is only pumped within the fundamental transverse mode range on the chip. Since the pump light power in the chip area where high-order transverse modes are excited is small, the power of the pump light is difficult to reach the excitation threshold of the high-order transverse mode, and the high-order transverse mode cannot oscillate. Even if the threshold is reached at a very high pump power, due to the reflection effect of the photonic crystal structure on the broad spectrum, it cannot oscillate through the external cavity. Description of the Drawings

[0020] Figure 1 Schematic diagram of the structure of an optically pumped semiconductor disk laser based on a photonic crystal according to the present invention.

[0021] Figure 2 Three-dimensional view of the photonic crystal structure of an optically pumped semiconductor disk laser based on a photonic crystal according to the present invention.

[0022] Figure 3 Cross-sectional view of the photonic crystal structure of an optically pumped semiconductor disk laser based on a photonic crystal according to the present invention.

[0023] Figure 4 Photonic crystal structure diagram provided by an embodiment of the present invention, with the air columns arranged in an equilateral triangle.

[0024] Figure 5 Top view of the gain chip for etching a photonic crystal ring structure provided by an embodiment of the present invention.

[0025] Figure 6 Schematic diagram of the pump light spot provided by an embodiment of the present invention, where the semi-transparent ellipse is the actual shape of the pump light spot.

[0026] Figure 7 Schematic diagram of the gain chip package provided by an embodiment of the present invention

[0027] Figure 8 Schematic diagram of the simulation result of the photonic crystal reflection spectrum provided by an embodiment of the present invention

[0028] In the figure: 1. Photonic crystal structure, 2. Cover layer, 3. Periodic gain structure, 4. Distributed Bragg reflector, 5. Substrate, 6. Pump light spot, 7. Heat sink, 8. Gain chip, 9. Incident pump light, 10. External resonant cavity mirror, 11. Laser emission, 12. Diamond heat sink, 13. Gold-plated copper heat sink. Detailed implementation manners

[0029] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0030] As Figure 1 shown, an optically pumped semiconductor disk laser based on a photonic crystal, the laser comprising: an incident pump light source 1, a gain chip 8, and an external resonant cavity mirror 10; the incident pump light source 1 performs surface pumping on the gain chip 8 in an oblique incidence case to achieve population inversion, so that the laser emission 11 is transmitted in the cavity surface formed by the gain chip 8 and the external resonant cavity mirror 10; the gain chip 8 reflects the excess pump light to control the thermal effect and the transverse mode quality. Wherein the pump light spot 6 located on the surface of the gain chip 8 is elliptical. The reflectivity of the gain chip 8 for reflecting the excess pump light is higher than 80%.

[0031] As Figure 2 shown, the gain chip 8 includes, grown in sequence on the substrate 5: a distributed Bragg reflector 4, a periodic gain structure 3, a cover layer 2, and a photonic crystal structure 1; the photonic crystal structure 1 is located in the cover layer 2, and the depth of the photonic crystal structure 1 is less than the depth of the cover layer 2. The photonic crystal structure 1 is prepared in the cover layer 2 by etching or adding materials with other dielectric constants, and its dielectric constant has the property of periodic change. The photonic crystal structure 1 provides mode control and the function of reflecting pump light. The periodic gain structure 3 is used as the active region to generate laser radiation, and can be a single quantum well or a multi-quantum well structure. Generally speaking, in order to obtain greater gain, a multi-quantum well structure is often adopted. The distributed Bragg reflector 4 is used as the resonant cavity mirror for the laser emission 11 to provide a high reflectivity.

[0032] Light is an electromagnetic wave. After passing through a material with a periodically varying dielectric constant (periodically varying refractive index), the electromagnetic field intensity at certain wavelength bands will sharply attenuate and cannot propagate through, thus having a bandgap structure, which can control the movement of light. The light that cannot propagate in the bandgap structure is called a photonic bandgap. The photonic bandgap is divided into two types: an incomplete bandgap and a complete bandgap. The incomplete bandgap means that the optoelectronic confinement effect exists in a specific direction or polarization mode, while the complete bandgap means that light cannot propagate in all directions and for any polarization.

[0033] According to the above principle, by etching a photonic crystal structure 1 in the cover layer 2 to make the surface structure have a high reflectivity, a OP-VECSEL with a photonic crystal structure that realizes the pump light in the reflection region and suppresses the high-order mode lasing is achieved. Its specific working principle is that through the reflective film layer of the photonic crystal structure 1, that is, the distributed Bragg reflector 4, the pump light that excites the high-order mode is reflected, making it difficult to reach the threshold. Therefore, the pump light is only absorbed in the fundamental transverse mode region and reflected in the high-order mode region, improving the heat on the surface of the gain chip 8, delaying its thermal inversion, and thus obtaining a higher output power.

[0034] The photonic crystal structure 1 is preferably etched in the cover layer 2, and its etching depth should not be deeper than the window layer or the epitaxial cover layer. It can be an epitaxial layer such as epitaxial silicon dioxide, aluminum gallium arsenide, etc. that does not damage the original performance of the chip, or it can be the window layer or substrate of the gain chip 8 and other outermost surfaces that originally exist. The thickness of the epitaxial cover layer should ensure that the periodic gain structure 3 in the gain chip 8 is not damaged. The specific epitaxial thickness can be given by MODE solution simulation. The periodic gain structure 3 is a multi-quantum well structure, and the standing wave peak in the gain chip coincides with the quantum well.

[0035] The photonic crystal structure 1 is generally an annular structure, and it can also be other structures, such as polygons or irregular shapes like squares and hexagons. The inner diameter size of the photonic crystal structure 1 should be larger than the fundamental transverse mode size on the gain chip 8 and smaller than the excitation size of the TEM01 or TEM10 mode. The outer diameter size should be larger than the major axis size of the elliptical pump light spot 6 formed by oblique incidence. Such a size design can effectively limit the high-order mode and reflect the excess pump light. Its photonic crystal lattice can be a triangular lattice and a square lattice, or other lattices; the cross-sectional shape of its photonic crystal holes can be triangular and circular, or other shapes. The photonic crystal structure 1 constructed on the surface of the gain chip 8 has a high reflectivity and can be used to control the shape of the pump light spot 6 and reflect the excess pump light spot. Because the incident pump light source 9 for pumping has a relatively wide wavelength band, and the photonic bandgap corresponding to different materials is different, specific numerical values for the lattice constant and hole radius are not restricted. However, for any situation that can achieve a high reflectivity in the case of oblique incidence, its reflectivity should be between 80% and 100%.

[0036] Due to the special properties of photonic crystals and semiconductor materials, the incident pump light 9 is preferably linearly polarized. Thermal effects such as thermal drift, thermal inversion, and thermal lensing in semiconductor materials can cause problems such as wavelength variation of the laser, low power, and unstable output. Therefore, the gain chip 8 with a photonic crystal structure 1 is indium-bonded to a metallized diamond heat sink 12, as Figure 7 shown, and then this metallized diamond heat sink 12 is encapsulated on a gold-plated copper heat sink 13 through a Au-In solid-liquid bonding process for efficient heat dissipation.

[0037] Typically, OP-VECSEL is grown based on III-V or II-VI group compound semiconductor materials as the substrate 5, such as GaN-based, GaAs-based, and GaSb-based. Materials such as GaAs and AlGaAs are widely used in the research of photonic crystals. In this embodiment, a near-infrared light-pumped vertical external cavity surface-emitting laser with GaAs as the substrate 5 material will be taken as an example.

[0038] For OP-VECSEL, barrier pumping or in-well pumping is generally adopted. Therefore, the selected pump laser has a relatively wide energy band gap and a relatively small wavelength, generally less than the lasing wavelength. For the design of the gain chip, due to the need for a high population inversion density and high carrier confinement, a quantum well structure is adopted. It can be seen from the Anderson energy band model that in order to prevent the increase of electron and hole losses at the barrier, a wide-bandgap material is usually selected as the cap layer 2, that is, the outermost structure. In this embodiment, Al x GaAs 1-x is used as the outermost cap layer 2, and its refractive index is 3.46. Materials such as SiO 2 and GaAs can also be selected as the cap layer 2. In this embodiment, for the light-pumped vertical external cavity surface-emitting laser as Figure 1 shown, a flat-concave straight cavity is selected, a straight cavity composed of the gain chip 8 and the output coupling mirror 10. V-shaped cavities, Z-shaped cavities, etc. can be selected. The external resonant cavity mirror 10 is selected as a flat-concave mirror with a radius of curvature R of 250 mm, and the cavity length is 200 mm. Through the calculation of the transfer matrix, the diameter of the fundamental transverse mode size on the surface of the gain chip 8 can be obtained as 353.2 mm. The incident pump light source 9 is usually selected to be obliquely incident at a 45-degree angle. Under the condition of sufficient focusing, the diameter of the pump light spot 6 is 400 um. After irradiating on the surface of the gain chip 8, it becomes an elliptical light spot, as Figure 6 shown, the horizontal direction pump light spot 6 is 565.68 um, and the vertical direction pump light spot 6 size is 400 um. According to the TEM mode cutoff size and the incident angle of the incident pump light source 9, photonic crystal structures 1 with different inner and outer diameters are selected. In this embodiment, the photonic crystal structure 1 is as Figure 5Shown is an annular structure etched in the cladding 2. The inner diameter of the annular structure is 360 μm, and the outer diameter is 570 μm. In particular, the photonic crystal structure for controlling the transverse mode and the pump light reflected to cause the excitation of higher-order modes can also be of other shapes, such as square, hexagonal, or other shapes with different inner and outer shapes. However, no matter what shape it is, while ensuring the maximum existence of the transverse mode, the higher-order modes and the excess part of the reflected elliptical pump light should be suppressed.

[0039] In this embodiment, a linearly polarized fiber laser with a wavelength of 920 nm is selected as the external pump light source 9, and the in-well pumping method is adopted. In this embodiment, as Figure 4 shown, the photonic crystal structure 1 selects an equilateral triangle arrangement with a lattice period a = 400 nm and a radius r = 100 nm. As Figure 3 shown, for the photonic crystal at the position of the cladding 2, the cladding in the gain chip is generally several hundred nm thick, and the depth of the air holes of the photonic crystal generally needs to etch through the cladding 2 without penetrating the active region to ensure that the active region is not damaged. During implementation, it is necessary to comprehensively consider without destroying the periodic gain chip 8 and according to the FDTD simulation results at different depths. In this embodiment, the thickness of the selected cladding 2 is 240 nm, and the depth of the air holes of the photonic crystal is also 240 nm. After determining that the lattice type of the photonic crystal is an equilateral triangle lattice, the lattice period a = 400 nm, and the radius of the air holes r = 100 nm, the reflection of the incident light with a wavelength range of 700 nm - 1000 nm is simulated by the FDTD solution software. The FDTD contains one lattice, and the number of detector sampling points is set to 5000 at both the upper and lower positions. As many sampling points as possible can be set according to the wavelength range. The incident light is at a 45-degree angle to the plane of the photonic crystal, and the angle phi is 90 degrees. It is found that the reflection spectrum has a very high reflectivity in the 870 nm - 1000 nm band, as Figure 8 shown. This photonic crystal ring has a reflectivity close to 100% for the 920 nm pump laser incident at 45 degrees. If the external pump light source is partially polarized light, a cladding and holes with a high dielectric constant can be selected according to the actual substrate material. By adjusting the lattice size and duty cycle, the bandgaps of the TE mode and the TM mode can be made to coincide, and the change of the FDTD simulation reflection spectrum can be observed. Finally, the lattice period and radius are determined to achieve high reflection.

[0040] The preparation method of the photonic crystal structure 1 is specifically as follows:

[0041] Step 1: Cleave the gain chip 8, and use a dicing machine to cleave the epitaxial wafer into small pieces of 1 cm * 1 cm along the crystal orientation direction.

[0042] Step 2: Clean the gain chip 8. After placing the cleaved chip in acetone, methanol, and isopropyl alcohol and heating it in a water bath at 68 °C, rinse it repeatedly with deionized water.

[0043] Step 3: Epitaxially grow a silica layer, which serves as a mask for etching the photonic crystal structure 1, and the thickness is determined according to the depth to be etched.

[0044] Step 4: Spin-coat the photoresist. Place the gain chip 8 on the wafer stage, cover the air holes, evacuate the air, drop the ZEP photoresist, start spin-coating, and bake at 180 °C for 3 minutes after spin-coating.

[0045] Step 5: Pattern the photoresist on the silica by electron beam lithography, and the patterned structure is the desired photonic crystal structure 1.

[0046] Step 6: Develop and fix the photoresist. Immerse it in xylene for 70 s, transfer it to IPA and immerse for 30 s, dry it with air, and then bake at 120 °C for 3 minutes.

[0047] Step 6: Use ICP to etch the silica and transfer the pattern of the photoresist to the silica mask.

[0048] Step 7: Alternately boil the sample in dimethylpyrrolidone, acetone, and IPA by ultrasonic waves, and finally perform plasma cleaning using RIE.

[0049] Step 8: Use ICP dry etching to transfer the photonic crystal structure 1 of the silica layer to the AlGaAs capping layer 2 of the chip.

[0050] Step 9: Use BOE to remove the silica layer.

[0051] Step 10: Place the diamond in a 10% sodium hydroxide solution and boil it with ultrasonic waves for 20 minutes, then place it in a 10% dilute nitric acid solution and boil it with ultrasonic waves for 20 minutes. After that, place the diamond in a high-vacuum electron beam evaporation system for metallization treatment, and metallization is required on both sides.

[0052] Step 11: Boil and ultrasonically clean the chip, the metallized diamond heat sink 12, and the gold-plated copper heat sink 13 in acetone, isopropyl alcohol, and methanol for 10 minutes respectively.

[0053] Step 12: Deposit a metal indium film on one side of the metallized diamond heat sink 12 and the central area of the gold-plated copper heat sink 13, and the deposition thickness is 2 - 3 μm.

[0054] Step 13: Align the gain chip 8, the metallized diamond heat sink 12, and the gold-plated copper heat sink 13 at the center in sequence, and use an elastic fixture to fix them and place them in a reflow furnace for eutectic soldering.

Claims

1. A light-pumped semiconductor disk laser based on a photonic crystal, characterized in that, the laser includes: an incident pump light source, a gain chip, and an external resonant cavity mirror; the incident pump light source performs surface pumping on the gain chip under the condition of oblique incidence to achieve population inversion, so that the lasing laser transmits in the cavity formed by the gain chip and the external resonant cavity mirror; the gain chip includes, sequentially fabricated on a substrate: a distributed Bragg reflector, a periodic gain structure, a cover layer, and a photonic crystal structure; the photonic crystal structure is located within the cover layer, and the depth of the photonic crystal structure is less than the depth of the cover layer; the distributed Bragg reflector reflects the pump light that excites the high-order mode, making it difficult to reach the threshold, and the pump light is only absorbed in the fundamental transverse mode region, and the photonic crystal structure reflects the pump light in the high-order mode region; the photonic crystal structure on the surface of the gain chip reflects the excess pump light, so that the gain within the range of the photonic crystal structure decreases, the loss increases, the intracavity mode resonance is suppressed, the thermal effect is reduced, and the transverse mode quality is controlled.

2. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 1, characterized in that, the pump light spot located on the surface of the gain chip is elliptical.

3. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 1, characterized in that, the reflectivity of the gain chip for reflecting the pump light at the photonic crystal structure is higher than 80%.

4. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 1, characterized in that, the lattice of the photonic crystal structure is a triangular lattice or a square lattice; the lattice holes of the photonic crystal structure are triangular or circular.

5. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 1 or 2, characterized in that, the inner diameter size of the photonic crystal structure should be greater than the fundamental transverse mode size on the surface of the gain chip and less than the excitation size of the TEM01 or TEM10 mode; the outer diameter size of the annular structure of the photonic crystal structure should be greater than the major axis size of the pump light spot.

6. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 1, characterized in that, the periodic gain structure is a multi-quantum well structure, and the standing wave peaks in the gain chip coincide with the quantum wells.

7. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 1, characterized in that, it further includes a heat sink disposed under the substrate.

8. The light-pumped semiconductor disk laser based on a photonic crystal according to claim 7, characterized in that, the heat sink is a bonded metallized diamond heat sink bonded under the substrate and a gold-plated copper heat sink bonded under the metallized diamond heat sink.

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