Multi-cavity cascade coupling vertical cavity surface emitting laser and preparation method thereof

Abstract

The invention discloses a multi-cavity cascade coupling vertical cavity surface emitting laser and a preparation method thereof, and relates to the technical field of preparation of vertical cavity surface emitting lasers. The laser comprises a top mirror, an oxide layer, an active area, a preset number of middle mirror and passive cavity assemblies, a bottom mirror and a substrate from top to bottom, a first electrode is arranged on the side, away from the oxide layer, of the top mirror, and second electrodes are arranged at the two ends of the sides, close to the top mirror, of the middle mirror and passive cavity assemblies. Each middle mirror and passive cavity assembly comprises a middle mirror and a passive cavity arranged on one side, far away from the active area, of the middle mirror. A light field is effectively coupled from an active cavity to enter a plurality of cascaded passive cavities, photons are emitted after being circulated for multiple times in the passive cavities, the service life of the photons is prolonged, the line width is reduced, and the problems that an existing vertical cavity surface emitting laser is short in effective cavity length and large in spectral line width are solved. The invention can be applied to atomic clock light sources. The preparation method is convenient and easy to widely popularize and apply.

Description

Technical Field

[0001] The present invention relates to the technical field of vertical cavity surface emitting laser fabrication, and particularly to a multi-cavity cascaded coupled vertical cavity surface emitting laser and a fabrication method thereof. Background Art

[0002] Vertical cavity surface emitting lasers (VCSELs) have rapidly occupied the semiconductor laser market in recent years and play an important role in the fields of optical communication, sensing, storage, etc. An atomic clock is an advanced timekeeping device with an accuracy that can reach an error of 1 second every 20 million years and is widely used in global navigation systems. Atomic clock sensors rely on the spectral analysis of alkali atoms (rubidium or cesium), which are present in small vapor chambers. The corresponding spectral wavelengths of cesium are 894.6 nm (D1) and 852.3 nm (D2), and those of rubidium are 795.0 nm (D1) and 780.2 nm (D2). Due to the characteristics of low power consumption and circular output beam of VCSELs, they have become the preferred light sources for atomic clocks.

[0003] Chip-scale atomic clocks (CSACs) have become an ideal choice in the fields of satellite communication and global navigation satellite systems (GNSS) due to their compact structure and high efficiency. The core mechanism of CSACs is based on the coherent population trapping (CPT) effect, and their performance depends to a large extent on the linewidth of the light source. Vertical cavity surface emitting lasers (VCSELs), due to their small size, light weight, and low power consumption characteristics, can meet the size, weight, and power consumption (SWaP) requirements of CSACs. However, the typical linewidth of traditional VCSELs is about 100 MHz, which is much higher than the 1 - 10 MHz range required by advanced atomic systems. This range matches the natural transition linewidth of cesium atoms (5 MHz) and can be achieved through atomic beam, trapped atom, or trapped ion technologies.

[0004] However, several methods for achieving a narrow linewidth of semiconductor lasers in the prior art all have relatively obvious defects. For example, the method of reducing the linewidth enhancement factor requires considering complex factors such as the charge carrier concentration and differential gain in the active region; traditional optical feedback techniques rely on complex secondary epitaxial growth and precise manufacturing control, increasing the process difficulty and cost. Extending the effective cavity length can relatively simply narrow the linewidth, but the high electric field strength in the active region of existing extended cavity VCSELs will result in a large oxidation confinement factor, and it is very difficult to achieve single-mode operation. Summary of the Invention

[0005] The invention aims to solve the problems of the existing vertical cavity surface emitting laser with a short effective cavity length and a large spectral linewidth. The invention proposes a novel multi-cavity cascaded coupled vertical cavity surface emitting laser based on inter-cavity electric field modulation. By effectively coupling the optical field from the active cavity into multiple cascaded passive cavities, in the passive cavities, photons are emitted after experiencing multiple cycles, prolonging the lifetime of photons to reduce the linewidth, which has obvious advantages.

[0006] To achieve the above object, the present invention specifically adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a multi-cavity cascaded coupled vertical cavity surface emitting laser, which includes, from top to bottom, a top mirror, an oxide layer, an active region, a preset number of intermediate mirrors and a passive cavity assembly, a bottom mirror, and a substrate. A first electrode is provided on a side of the top mirror away from the oxide layer, and second electrodes are provided at both ends of a side of the intermediate mirror and the passive cavity assembly close to the top mirror.

[0008] Further, each of the intermediate mirror and the passive cavity assembly includes an intermediate mirror and a passive cavity provided on a side of the intermediate mirror away from the active region.

[0009] Further, the preset number of the intermediate mirror and the passive cavity assembly is 2 to 6.

[0010] Further, the top mirror includes 21 to 25 pairs of p-type doped Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 2×10 18 ~3×10 18 cm -3 .

[0011] Further, the active region includes three repeating units, and each repeating unit includes a 6-nm In 0.135 Ga 0.865 As quantum well separated by an 8-nm Al 0.25 Ga 0.75 As barrier.

[0012] Further, each intermediate mirror includes, from bottom to top, an n-type doped M1 layer, an M2 layer, and an M3 layer;

[0013] M1 includes 11 to 13.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 1×10 18 ~1.5×10 18 cm-3 ; M2 includes Al with a preset optical thickness 0.16 Ga 0.84 As layer with a doping concentration of 2×10 18 cm -3 ; M3 includes 7 - 9.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As with a doping concentration of 1.5×10 18 ~2×10 18 cm -3 。

[0014] Furthermore, the preset optical thickness is 1 - 4λ.

[0015] Furthermore, there is a dip in the reflection spectrum of the intermediate mirror.

[0016] Furthermore, the reflectivity of the top mirror is 99.3% - 99.7%, the reflectivity of the intermediate mirror is 15% - 45%, and the reflectivity of the bottom mirror is 99.9%.

[0017] In a second aspect, the present invention also provides a method for manufacturing a multi - cavity cascaded coupled vertical - cavity surface - emitting laser based on the above, including the following steps:

[0018] S1. Provide a substrate, and epitaxially grow a bottom mirror, a preset number of intermediate mirrors and a passive - cavity combination body, an active region, an oxide layer, and a top mirror on the substrate in sequence to obtain a first pre - fabricated component;

[0019] S2. Set a first electrode on the top mirror of the first pre - fabricated component, and set a second electrode on the first intermediate mirror and the passive - cavity combination body on the side close to the top mirror in the first pre - fabricated component to complete the fabrication of the laser.

[0020] Compared with the prior art, the advantages of the present invention are as follows:

[0021] 1. For the multi - cavity cascaded coupled vertical - cavity surface - emitting laser involved in the present invention, it breaks through the problems of limited effective cavity length and large lasing spectral linewidth, which are difficult to meet the requirements such as the light source of precision time - keeping atomic clocks. Without relying on the multi - cavity cascaded coupling linewidth compression of an external cavity, a laser spectral linewidth less than 1 MHz is achieved. In the multi - cavity cascaded coupled vertical - cavity surface - emitting laser of the present invention, by effectively coupling the optical field from the active cavity into multiple cascaded passive cavities, through the first intermediate mirror after the active region, part of the light is extracted and stored in the "optical reservoir" composed of passive cavities.

[0022] In a passive cavity, photons are emitted after experiencing multiple cycles, thus significantly extending the lifespan of photons. This method can achieve a substantial increase in the photon lifespan compared to conventional extended cavities and three-mirror VCSELs, thereby greatly compressing the linewidth of the laser. It can effectively extend the photon lifespan to reduce the linewidth and has obvious advantages.

[0023] 2. The preparation method of a multi-cavity cascaded coupled vertical cavity surface emitting laser involved in the present invention is simple and easy to be widely promoted and applied. The prepared vertical cavity surface emitting laser has excellent performance and a relatively long cavity length. Brief Description of the Drawings

[0024] Figure 1 It is a schematic structural diagram of the coupled vertical cavity surface emitting laser provided by the present invention. Figure 1 。

[0025] Figure 2 It is a schematic structural diagram of the coupled vertical cavity surface emitting laser provided by the present invention. Figure 2 。

[0026] Figure 3 It is a schematic structural diagram of the three-cavity cascaded coupled vertical cavity surface emitting laser 1 provided in Embodiment 1 of the specification.

[0027] Figure 4 It is a schematic diagram of the variation results of the electric field intensity and refractive index distribution of the three-cavity cascaded coupled vertical cavity surface emitting laser 1 provided in Embodiment 1 of the specification with distance.

[0028] Figure 5 It is a schematic diagram of the variation law of the cold cavity linewidth of the three-cavity cascaded coupled vertical cavity surface emitting laser 1 provided in Embodiment 1 of the specification with the electric field intensity ratio.

[0029] Among them, AC represents the electric field intensity in the active cavity, and PC1 represents the electric field intensity in the first passive cavity.

[0030] Figure 6 It is a schematic diagram of the variation law of the cold cavity linewidth of the three-cavity cascaded coupled vertical cavity surface emitting laser 1 provided in Embodiment 1 of the specification with the electric field intensity ratio.

[0031] Among them, PC1 represents the electric field intensity in the first passive cavity, and PC2 represents the electric field intensity in the second passive cavity. Figure 7 It is a schematic diagram of the variation law of the spectral linewidth of the two-cavity cascaded, three-cavity cascaded, four-cavity cascaded, and five-cavity cascaded coupled vertical cavity surface emitting lasers with the reciprocal of the output power in Embodiment 6 of the specification.

[0032] Among them, 2C represents a two-cavity cascaded VCSEL, 3C represents a three-cavity cascaded VCSEL, 4C represents a four-cavity cascaded VCSEL, and 5C represents a five-cavity cascaded VCSEL.

[0033] Figure 8 It is the phase noise diagram of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascaded structure provided in Embodiment 1 of the specification.

[0034] Figure 9 It is the L-I diagram of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascaded structure provided in Embodiment 1 of the specification.

[0035] Figure 10 It is the diagram of the variation law of the light intensity of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascaded structure provided in Embodiment 1 of the specification with respect to wavelength and temperature.

[0036] Figure 11 It is the structure and electric field distribution diagram of the coupled vertical cavity surface emitting laser 2 with a four-cavity cascaded structure provided in Embodiment 4 of the specification.

[0037] Figure 12 It is the structure and electric field distribution diagram of the coupled vertical cavity surface emitting laser 3 with a five-cavity cascaded structure provided in Embodiment 5 of the specification.

[0038] Reference numerals in the drawings: 1 - first electrode, 2 - top mirror, 3 - oxide layer, 4 - active region, 5 - combination of intermediate mirror and passive cavity, 51 - intermediate mirror, 52 - passive cavity, 6 - second electrode, 7 - bottom mirror, 8 - substrate; 151 - first intermediate mirror, 152 - first passive cavity, 251 - second intermediate mirror, 252 - second passive cavity, 351 - third intermediate mirror, 352 - third passive cavity, 451 - fourth intermediate mirror, 452 - fourth passive cavity.

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention.

[0040] Therefore, the following detailed description of the provided embodiments of the present invention is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention. Detailed implementation manners

[0041] In a first aspect, the present invention provides a multi-cavity cascaded coupled vertical cavity surface emitting laser. Please refer to Figure 1, the laser includes, from top to bottom, a top mirror 2, an oxide layer 3, an active region 4, a preset number of intermediate mirrors and a passive cavity assembly 5, a bottom mirror 7, and a substrate 8. A first electrode 1 is provided on a side of the top mirror 2 away from the oxide layer 3, and second electrodes 6 are provided at both ends of a side of the intermediate mirror and passive cavity assembly 5 close to the top mirror 2.

[0042] In some embodiments of the present invention, please continue to refer to Figure 1 and Figure 2 , each of the intermediate mirror and passive cavity assembly 5 includes an intermediate mirror 52 and a passive cavity 52 provided on a side of the intermediate mirror 52 away from the active region. That is, each assembly includes an intermediate mirror 51 and a passive cavity 52.

[0043] It can be understood that the multi-cavity cascaded coupled vertical cavity surface emitting laser of the present invention breaks through the problems of limited effective cavity length, relatively large lasing spectral linewidth, and thus difficulty in meeting the requirements such as the light source of precision timing atomic clocks. Without relying on an external cavity, multi-cavity cascaded coupling linewidth compression realizes a laser spectral linewidth of less than 1 MHz.

[0044] By effectively coupling the optical field from the active cavity into multiple cascaded passive cavities, through the first intermediate mirror after the active region, part of the light is extracted and stored in the "optical library" composed of passive cavities. In the passive cavity, photons are emitted after experiencing multiple cycles, thus significantly prolonging the lifetime of photons. This method can achieve a significant increase in the photon lifetime compared to conventional extended cavities and three-mirror VCSELs, and thus greatly compress the linewidth of the laser, and can effectively prolong the photon lifetime to reduce the linewidth.

[0045] To further elaborate, the method for reducing the cold cavity linewidth of the multi-cavity cascaded VCSEL based on field regulation in the present invention is proposed according to the formula Δν c = 1 / (2πτ c ). Δν c represents the cold cavity linewidth, and τ c represents the photon lifetime in the cavity. This formula shows that the cold cavity linewidth is inversely proportional to the photon lifetime. Increasing the photon lifetime in the cavity can reduce the cold cavity linewidth, thereby reducing the laser spectral linewidth. Therefore, by setting multiple intermediate mirrors and passive cavity assemblies, the present invention can effectively improve the photon lifetime, thus significantly reducing the spectral linewidth and making the performance of the laser excellent.

[0046] Through theoretical calculation and analysis and comparison with simulation results, it can be obtained that for a common VCSEL laser, the cold cavity linewidth of the resonant cavity is generally greater than 0.2 nm. For a VCSEL laser with an output optical power of less than 1 mW, its output spectral linewidth is above 1 GHz. For a multi-cavity cascaded VCSEL, due to the repeated oscillation and frequency selection of photons in multiple cavities, the cold cavity linewidth of its resonant cavity can reach below 0.01 nm, such asFigure 5 As shown, when the output power is 1 mW, the spectral linewidths of the two-cavity cascaded, three-cavity cascaded, four-cavity cascaded, and five-cavity cascaded VCSELs are 0.9 MHz, 0.1 MHz, 0.02 MHz, and 0.01 MHz respectively, which are more than two orders of magnitude smaller than that of the traditional VCSEL.

[0047] In some embodiments of the present invention, the preset number of the intermediate mirror and passive cavity combination is 2 to 6.

[0048] That is, a laser with two intermediate mirrors and passive cavity combinations can be called a coupled vertical cavity surface emitting laser with a three-cavity cascaded structure (see the present invention Figure 1 、 Figure 2 ); a laser with three intermediate mirrors and passive cavity combinations can be called a coupled vertical cavity surface emitting laser with a four-cavity cascaded structure (see the present invention Figure 11 ); a laser with four intermediate mirrors and passive cavity combinations can be called a coupled vertical cavity surface emitting laser with a four-five cavity cascaded structure (see the present invention Figure 12 ).

[0049] In some embodiments of the present invention, the top mirror includes 21 to 25 pairs of p-type doped Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, and the doping concentration is 2×10 18 ~3×10 18 cm -3 .

[0050] In some embodiments of the present invention, the active region includes three repeating units, and each repeating unit includes a 6-nm In 0.135 Ga 0.865 As quantum well separated by an 8-nm Al 0.25 Ga 0.75 As barrier.

[0051] In some embodiments of the present invention, a layer of oxide is placed between the spacer layer and the first pair of p-DBR. The diameter of the oxidation hole is 3 μm, and it is a p-type Al 0.98 Ga 0.02 As with a thickness of 30 nm and a doping concentration range of 2×10 18 cm -3 . Lateral oxidation generates Al2O3 to form an oxidation confinement layer with good insulation. In addition to restricting the aperture of the light-emitting region, the oxidation confinement layer can also effectively inhibit electron leakage, enabling the multi-cavity cascaded coupled vertical cavity surface emitting laser to achieve low-threshold single-mode laser output, thereby further optimizing the performance of the device.

[0052] In some embodiments of the present invention, under high-temperature water vapor conditions, aluminum oxide is formed on the surface of the Al 0.98 Ga 0.02 As material. After processing it into a columnar platform structure using etching technology, its side is oxidized into insulating aluminum oxide, while the unoxidized part in the center still retains conductivity. This design enables the current to flow only along the central conductive region, thus forming a current confinement layer, also known as the oxide layer. The open part of this structure is called the oxide aperture, which is also the light-emitting region of the laser. The refractive index of AlGaAs in the middle of the oxide aperture is different from that of the aluminum oxide on the outside, enabling the VCSEL to form a cylindrical waveguide with a refractive index difference Δn eff between the core layer and the cladding layer in the radial direction, achieving transverse optical confinement in the oxide-confined VCSEL. In the present invention, the passive cavity optical field is higher than the active cavity optical field, reducing the effective refractive index difference inside and outside the VCSEL light-emitting aperture, thereby suppressing the generation of higher-order modes. The light beam of the higher-order mode has a larger divergence angle. Therefore, after suppressing the light beam of the higher-order mode, the remaining light beam of the lower-order mode can achieve a smaller divergence angle. The effective refractive index N core of the core layer and the effective refractive index N clad of the cladding layer are calculated as follows:

[0053]

[0054] The effective refractive index difference inside and outside the VCSEL light-emitting aperture is determined based on the following formula:

[0055] Δn eff =N core -N clad

[0056] where n core and n clad are the refractive indices of the core layer and the cladding layer materials respectively, and E Z is the standing wave distribution of the electric field along the Z direction. A smaller effective refractive index difference can reduce the number of modes and make the spectrum pure.

[0057] In some embodiments of the present invention, each intermediate mirror is, from bottom to top, an n-type doped M1 layer, an M2 layer, and an M3 layer in sequence;

[0058] M1 includes 11 to 13.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 1×10 18 to 1.5×10 18 cm -3 ; M2 includes Al 0.16 Ga 0.84The As layer has a doping concentration of 2×10 18 cm -3 ; M3 includes 7 to 9.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 1.5×10 18 ~2×10 18 cm -3 。

[0059] In some embodiments of the present invention, the preset optical thickness is 1 to 4λ. It should be noted that the optical thickness must be an integer multiple of λ / 2. The preset optical thickness is preferably 4λ.

[0060] It should be noted that although the intermediate mirror is n-type doped in the above description, the intermediate mirror can also be p-type doped and placed between the oxide layer and the top mirror. This inverted design has an obvious disadvantage compared to the present invention. This disadvantage is that due to the lower mobility of p-type carriers, they must move a longer lateral distance in the intermediate DBR. This lower mobility leads to an increase in the overall resistance of the coupled-cavity VCSEL, resulting in a decrease in efficiency.

[0061] In some embodiments of the present invention, there is a dip in the reflection spectrum of the intermediate mirror. It can be understood that the present invention forms the intermediate mirror by introducing a defect in a DBR. The defect is specifically manifested as follows: when the two DBRs adjacent to the first intermediate mirror and the second intermediate mirror have the same refractive index, a dip appears in the reflection spectrum of the intermediate mirror; when the two DBRs adjacent to the first intermediate mirror and the second intermediate mirror have different refractive indices, no dip appears in the reflection spectrum of the intermediate mirror. This design is significantly different from that of a VCSEL with multiple extended cavities. For a VCSEL based on multiple extended cavities, the intermediate mirror does not require a dip in the reflection spectrum.

[0062] In some embodiments of the present invention, the reflectivity of the top mirror is 99.3% to 99.7%, the reflectivity of the intermediate mirror is 15% to 45%, and the reflectivity of the bottom mirror is 99.9%. It can be understood that, different from the traditional design, the optical field in the passive cavity of the multi-mirror coupled-cavity VCSEL is higher than that in the active region. This is because the reflectivities of each mirror group are different, and photons circulate in the passive cavity between the bottom mirror and the intermediate mirror, increasing the lifetime of photons in the cavity. It can be seen that the present invention can effectively adjust the circulation of photons in the passive cavity by adjusting the reflectivities of each mirror group, thereby increasing the lifetime of photons in the passive cavity from another dimension, further reducing the spectral linewidth, and further improving the performance of the laser.

[0063] In some embodiments of the present invention, the bottom mirror comprises 36 pairs of n-doped Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 1.5×10 18 cm -3 ~2×10 18 cm -3 。

[0064] It should be noted that the bottom mirror comprises a plurality of alternating high-index and low-index refractive layers and can have any suitable combination of composition and thickness. The composition of the high-index and low-index layers is preferably selected to minimize the total number of alternating high-index and low-index layers. Generally, the reflectivity of the bottom DBR mirror is a function of the number of periods, and the more periods, the greater the reflectivity. Since a reflectivity of about 99.9% or higher is preferred, the number of periods is usually greater than 30, depending on the composition of the layers employed.

[0065] In some embodiments of the present invention, the passive cavity can have any suitable composition and thickness, but the composition and thickness can be selected to minimize optical loss and light scattering within the passive cavity.

[0066] Similar to the bottom mirror, the composition of the passive cavity is preferably selected to minimize the difference in its lattice constant from that of the remaining materials of the multi-cavity cascaded VCSEL.

[0067] The thickness of the passive cavity is equal to an integer multiple of half of the operating wavelength within the passive cavity. And most preferably, it is substantially equal to a thickness of at least 8 times the operating wavelength within the passive cavity.

[0068] In some embodiments of the present invention, the substrate can be any suitable substrate, including semiconductor wafers such as GaAs or other III-V materials, wafers. The substrate is preferably a semiconductor substrate that is substantially lattice-matched to the materials used to form the remainder of the multi-cavity cascaded VCSEL.

[0069] In some embodiments of the present invention, the material of the first electrode and / or the second electrode is any one of gold, copper, graphite, silver, or tin.

[0070] Second, the present invention also provides a preparation method of a multi-cavity cascaded coupled vertical cavity surface emitting laser based on the above, please continue to refer to Figure 1 ,including the following steps:

[0071] S1. Provide a substrate 8, and epitaxially grow a bottom mirror 7, a preset number of intermediate mirrors, and a passive cavity assembly 5, an active region 4, an oxide layer 3, and a top mirror 2 on the substrate 8 in sequence to obtain a first preform;

[0072] S2. A first electrode 1 is disposed on the top mirror of the first preform, and a second electrode 6 is disposed on the first intermediate mirror and the passive cavity combination 5 on one side of the first preform close to the top mirror 2, thus completing the fabrication of the laser.

[0073] It should be noted that during the specific epitaxial process, there are buffer layers between the bottom mirror, intermediate mirror, passive cavity, active region, oxide layer, and top mirror that have been involved. The parameters are adjusted according to the actual preparation requirements for epitaxy of each buffer layer to meet the actual needs. By using the transition layer, the resistance of the intermediate DBR mirror and the top DBR mirror is reduced. When the driving current passes through the intermediate DBR mirror and the top DBR mirror, this improves the overall efficiency of the multi-cavity cascaded VCSEL.

[0074] In some embodiments of the present invention, the preparation method of the present invention can select metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) to perform epitaxy on each layer.

[0075] Preferably, the preparation method of the present invention uses metal organic chemical vapor deposition (MOCVD) to perform epitaxy on each layer.

[0076] The present invention also provides an application of the multi-cavity cascaded coupled vertical cavity surface emitting laser prepared by the above method or the above.

[0077] It can be understood that the preparation method provided by the present invention does not rely on the multi-cavity cascaded coupled linewidth compression technology of an external cavity. The linewidth of the output spectrum of the prepared laser can reach a level less than 1 MHz, and the performance of the laser is extremely excellent, so it has a better application space. Therefore, the applicable space is larger. In the design, all cavity structures resonate at the 894.6 nm wavelength of cesium atom transition. This design method can also be used for other wavelengths required for precision measurement or coherent optical communication such as 852 nm and 795 nm. The multi-cavity cascaded coupled structure can also be used for linewidth compression of edge-emitting lasers. Therefore, the preparation method of the present invention and the multi-cavity cascaded coupled vertical cavity surface emitting laser prepared by the preparation method can be applied to special fields such as atomic clocks, and have obvious progressiveness and superiority.

[0078] Embodiment 1

[0079] In this embodiment, a three-cavity cascaded structure coupled vertical cavity surface emitting laser 1 is provided. Please refer to Figure 3Schematic diagram of a three-cavity cascaded VCSEL structure. The structure of the laser from top to bottom includes a top mirror 2, an oxide layer 3, an active region 4, a first intermediate mirror 151, a first passive cavity 152, a second intermediate mirror 251, a second passive cavity 252, a bottom mirror 7, and a GaAs substrate 8. A first electrode 1 is provided on the side of the top mirror 2 away from the oxide layer 3, and second electrodes 6 are provided at both ends of the side of the first passive cavity 152 close to the top mirror 2.

[0080] Please continue to refer to Figure 3 , the multi-cavity cascaded coupled vertical cavity surface emitting laser 1 has a total of four mirrors (top mirror 2, first intermediate mirror 151, second intermediate mirror 251, bottom mirror 7) and three cavities. The optical thickness of the entire three-cavity cascaded coupled VCSEL must be an integer multiple of λ / 2. Compared with the traditional VCSEL, there are two more intermediate mirrors and two passive cavities.

[0081] The top mirror includes 23 pairs of p-type doped Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 2×10 18 cm -3 . The active region includes three repeating units, and each repeating unit includes a 6-nm In 0.135 Ga 0.865 As quantum well separated by an 8-nm Al 0.25 Ga 0.75 As barrier. Between the spacer layer and the first pair of p-DBRs, an oxide layer is placed. The oxidation hole has a diameter of 3 μm and a thickness of 30 nm of p-type Al 0.98 Ga 0.02 As, with a doping concentration range of 2×10 18 cm -3 , and lateral oxidation produces Al2O3 to form an oxidation confinement layer with good insulation. The two passive cavities, the first passive cavity and the second passive cavity, are composed of Al 0.16 Ga 0.84 As with an optical thickness of about 8λ (the thickness must be an integer multiple of λ / 2), n-type doped, with a doping concentration of 1.5×10 18 cm -3 . The bottom mirror is 36 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, n-type doped, with a doping concentration of 2×10 18 cm -3。The design of the middle mirror is the difficulty of the entire multi-cavity cascaded coupled VCSEL design and is also the key to differentiating multi-cavity cascaded VCSELs from multiple extended cavity VCSELs.

[0082] First, the composition design of the middle mirror of the multi-cavity cascaded VCSEL in this embodiment is specifically as follows:

[0083] The middle mirror is composed of three parts, all of which are n-type doped. The first middle mirror from bottom to top is MM 11 , MM 12 , MM 13 . MM 11 is close to the active region and is composed of 11 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 1×10 18 cm -3 ; MM 12 is an Al 0.16 Ga 0.84 As layer with an optical thickness of approximately 4λ (the thickness must be an integer multiple of λ / 2), and a doping concentration of 2×10 18 cm -3 ; MM 13 is close to the first passive region and is composed of 7 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 2×10 18 cm -3 .

[0084] The second middle mirror from bottom to top is MM 21 , MM 22 , MM 23 . MM 21 is composed of 13.5 pairs of MM 23 (MM 23 ), with a doping concentration of 1.5×10 18 cm -3 ; MM 22 is an Al 0.16 Ga 0.84 As layer with an optical thickness of approximately 4λ (the thickness must be an integer multiple of λ / 2), and a doping concentration of 1.5×10 18 cm -3 ; MM 23 is composed of 9.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, with a doping concentration of 1.5×10 18cm -3 。

[0085] Although the middle mirror is n-type doped in the above description, the middle mirror can also be p-type doped and placed between the oxide layer and the top mirror. This inverted design has an obvious disadvantage compared to the present invention. This disadvantage is that due to the lower mobility of p-type carriers, they must move a longer lateral distance in the middle DBR. This lower mobility results in an increase in the overall resistance of the coupled-cavity VCSEL, thereby reducing the efficiency.

[0086] Secondly, the structural design of the middle mirror of the multi-cavity cascaded VCSEL in this embodiment is as follows: The structure of the multi-mirror requires that the middle mirror has a certain transmittance, which is manifested as a dip in the reflection spectrum.

[0087] For this reason, the present invention forms the middle mirror by introducing a defect in a DBR. The defect is specifically manifested as follows: When the two DBRs adjacent to MM 12 and MM 22 have the same refractive index, a dip appears in the reflection spectrum of the middle mirror; when the two DBRs adjacent to MM 12 and MM 22 have different refractive indices, no dip appears in the reflection spectrum of the middle mirror. This design is significantly different from that of a VCSEL with multiple extended cavities. For a VCSEL based on multiple extended cavities, a dip in the reflection spectrum is not required for the middle mirror.

[0088] Finally, the reflectivity design of the middle mirror of the multi-cavity cascaded VCSEL in this embodiment is as follows: Different from the traditional design, the optical field in the passive cavity of the multi-mirror coupled-cavity VCSEL is higher than that in the active region. This is because the reflectivities of the four mirrors are different. Among them, the reflectivity of the bottom mirror is about 99.9%; the reflectivity of the middle mirror is about between 15% and 45%; the reflectivity of the top DBR is about 99.3% to 99.7%. Photons circulate in the passive cavity between the bottom and middle DBR mirrors, increasing the lifetime of photons in the cavity.

[0089] Please continue to refer to Figure 3 for its manufacturing method:

[0090] S1. Epitaxially grow the bottom mirror 7, the second passive cavity 252, the second middle mirror 251, the first passive cavity 152, the first middle mirror 151, the active region 4, the oxide layer 3, and the top mirror 2 on the GaAs substrate 8 in sequence to obtain a first preform;

[0091] S2. Set the first electrode 1 on the upper part of the top mirror 2 of the first preform, and set the second electrode 6 on the first intermediate mirror and the passive cavity combination on one side of the first preform close to the top mirror 2 to complete the fabrication of the three-cavity cascaded structure coupled vertical cavity surface emitting laser 1.

[0092] Example 2

[0093] 2.1 Experimental design

[0094] In this example, the three-cavity cascaded structure coupled vertical cavity surface emitting laser 1 prepared in Example 1 is selected for dynamic analysis of the changes of the electric field strength and refractive index distribution with distance. The analysis results are shown in Figure 4 .

[0095] 2.2 Analysis of experimental results

[0096] See Figure 4 , the quantum well is located at the antinode of the standing light field to improve the coupling efficiency of carriers and photons. The oxide layer is designed at the first node of the standing light field close to the quantum well to reduce losses. The electric field strength rapidly decays in the relatively thick and high-reflectivity bottom DBR mirror; similarly, the electric field strength rapidly decays in the relatively thick and high-reflectivity top DBR mirror. In the passive cavity, the electric field strength basically remains constant at its maximum value.

[0097] In the intermediate DBR mirror, MM 11 is adjacent to the active region part, and MM 13 is adjacent to the first passive cavity part. The electric field strength in the doped contact region MM 12The internal attenuation is a constant value. It can be seen that the optical and electric fields of the multi-cavity cascaded VCSEL are different from those of the conventional VCSEL. The optical and electric field distribution of the conventional extended cavity VCSEL is that the optical field in the active region is higher than that in the passive region, while the optical and electric field distribution of the multi-cavity cascaded VCSEL is that the optical field in the passive region is higher than that in the active region. Such an optical and electric field design can not only effectively narrow the linewidth, but also make the mode purer. The improvement of the single-mode performance is due to the additional cavity spacing increasing the diffraction loss of the high-order modes with larger diffraction angles. The mechanisms of reducing the linewidth of the two structures are different. The extended cavity VCSEL reduces the linewidth by increasing the effective cavity length, that is, the distance that photons circulate in the cavity; the multi-cavity cascaded VCSEL reduces the linewidth by increasing the photon lifetime, that is, the number of times photons circulate in the cavity. When the photons generated in the active region propagate towards the first intermediate DBR mirror, some photons are reflected back, while the rest are transmitted into the first passive cavity. In the passive cavity, the electric field strength is higher than that in the active region, resulting in a higher energy concentration. The total electric field energy stored in the optical reservoir is several times larger than the electric field energy in a traditional extended cavity of the same spatial volume. These transmitted photons then encounter additional reflections and transmissions at the next intermediate DBR mirror. As a result, the photons experience multiple reflection and transmission cycles, circulating between the active cavity and the passive cavity. This process effectively extends the photon lifetime by increasing the number of oscillation cycles in the cavity, while the extended cavity extends the photon lifetime by extending the photon propagation distance.

[0098] Example 3

[0099] 3.1 Experimental design

[0100] In this example, the coupled vertical cavity surface emitting laser 1 with a three-cavity cascaded structure prepared in Example 1 is selected to analyze the relationship between the cold cavity linewidth and the electric field strength. The analysis results are shown in Figure 5 and Figure 6 .

[0101] 3.1 Analysis of experimental results

[0102] See Figure 5 and Figure 6 , the first intermediate mirror extracts light from the active region and stores it in the first passive cavity. The second intermediate mirror transmits the light from the first passive cavity to the second passive cavity, forming two interconnected optical reservoirs. These reservoirs act as optical dams, storing photons and enhancing their intensity. The intermediate DBR mirror acts as an adjustable valve. By adjusting the DBR layer numbers in MM 11 and MM 13 , the electric field distribution can be precisely adjusted. Reducing the DBR layer number in MM 11The DBR layer in [it] can increase its transmittance, enabling most photons from the active region to enter the first passive cavity. A portion of these photons passes through the middle mirror on the second side and enters the second passive cavity, undergoing repeated oscillations, effectively increasing the photon lifetime in the entire laser cavity. The present invention emphasizes that the total number of DBR layers and the total cavity length in the multi-cavity cascaded VCSEL remain unchanged throughout the process.

[0103] To simplify the following research, the present invention defines the electric field strength ratio between the active cavity and the first passive cavity as r 01 , and the electric field strength ratio between the first and second passive cavities as r 12 . When r 12 is fixed, as r 01 decreases, the cold cavity linewidth narrows. For example, in a VCSEL, when the r 01 ratio is ~0.3, a cold cavity linewidth of ~7.3 pm can be achieved. However, when r 01 is increased to ~5, the cold cavity linewidth is ~74.7 pm, which is 10 times wider than that of a multi-cavity cascaded VCSEL with a low r 01 ratio design. Similarly, when r 01 remains unchanged, as r 12 decreases, the cold cavity linewidth of the VCSEL also slowly narrows. It can be observed that changing the number of DBR layers in MM 21 and MM 23 can control the photon transmission between the first and second passive cavities, thereby affecting the photon lifetime. However, this effect is not as obvious as adjusting the first intermediate DBR mirror.

[0104] Example 4

[0105] In this embodiment, a coupled vertical cavity surface emitting laser 2 with a four-cavity cascaded structure is fabricated. For its specific structure, please refer to Figure 11 , and the laser 2 is similar to the vertical cavity surface emitting laser 1 in Example 1. The difference lies in that there are three intermediate mirrors.

[0106] The structure of this four-cavity cascaded coupled vertical cavity surface emitting laser 2 from top to bottom includes, the laser from top to bottom includes, top mirror 2, oxide layer 3, active region 4, first intermediate mirror 151, first passive cavity 152, second intermediate mirror 251, second passive cavity 252, third intermediate mirror 351, third passive cavity 352, bottom mirror 7, substrate 8. On the side of the top mirror 2 away from the oxide layer 3, there is a first electrode 1, and on both ends of the side of the first passive cavity 152 close to the top mirror 2, there are second electrodes 6.

[0107] Its fabrication method is the same as that in Example 1, except that there are additional settings of the third intermediate mirror 351 and the third passive cavity 352 in the intermediate film layer.

[0108] Example 5

[0109] In this example, a coupled vertical cavity surface emitting laser 3 with a five-cavity cascade structure is prepared. For its specific structure, please refer to Figure 12 , and the laser 3 is similar to the vertical cavity surface emitting laser 1 in Example 1. The difference is that it has four intermediate mirrors.

[0110] The structure of this five-cavity cascade coupled vertical cavity surface emitting laser 3 from top to bottom includes, and the laser from top to bottom includes a top mirror 2, an oxide layer 3, an active region 4, a first intermediate mirror 151, a first passive cavity 152, a second intermediate mirror 251, a second passive cavity 252, a third intermediate mirror 351, a third passive cavity 352, a fourth intermediate mirror 451, a fourth passive cavity 452, a bottom mirror 7, and a substrate 8. A first electrode 1 is provided on the side of the top mirror 2 away from the oxide layer 3, and second electrodes 6 are provided at both ends of the side of the first passive cavity 152 close to the top mirror 2.

[0111] Its preparation method is the same as that of Example 1, except that in the intermediate film layer, the third intermediate mirror 351, the third passive cavity 352, the fourth intermediate mirror 451, and the fourth passive cavity 452 are additionally provided.

[0112] Example 6

[0113] 6.1 Experimental design

[0114] In this example, a commercially available ordinary VCSEL laser, a VCSEL laser with an output optical power less than 1 mW, and the coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1 are selected to calculate and analyze the output spectral linewidth respectively.

[0115] 6.2 Analysis of experimental results

[0116] Through theoretical calculation and analysis and comparison with the simulation results, it can be obtained that for an ordinary VCSEL laser, the cold cavity linewidth of the resonant cavity is generally greater than 0.2 nm. For a VCSEL laser with an output optical power less than 1 mW, its output spectral linewidth is above 1 GHz. For a multi-cavity cascade VCSEL, due to the repeated oscillation and frequency selection of photons in multiple cavities, the cold cavity linewidth of its resonant cavity can reach below 0.01 nm.

[0117] Example 7

[0118] 7.1 Experimental design

[0119] In this embodiment, a commercially available coupled vertical cavity surface emitting laser with a two-cavity cascade structure, the coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1, the coupled vertical cavity surface emitting laser 2 with a four-cavity cascade structure prepared in Example 4, and the coupled vertical cavity surface emitting laser 3 with a five-cavity cascade structure prepared in Example 5 are selected to measure the variation law of the spectral linewidth with the reciprocal of the output power; the results are shown in Figure 7 。

[0120] 7.2 Analysis of Test Results

[0121] As Figure 7 shown, when the output power is the same at 1 mW, the spectral linewidths of the two-cavity cascade, three-cavity cascade, four-cavity cascade, and five-cavity cascade VCSELs are 0.9 MHz, 0.1 MHz, 0.02 MHz, and 0.01 MHz respectively. It is more than two orders of magnitude smaller than the linewidth of traditional VCSELs.

[0122] Example 8

[0123] 8.1 Test Design

[0124] In this embodiment, the coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1 is selected to measure the phase noise diagram, as shown in Figure 8 。The coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1 is measured for the L-I diagram, as shown in Figure 9 。The variation law diagram of the light intensity with the wavelength of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1 is measured, as shown in Figure 10 。

[0125] 8.2 Analysis of Test Results

[0126] See Figure 8 , the Lorentz linewidth of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1 is less than 1 MHz.

[0127] See Figure 9 , for the L-I diagram of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascade structure prepared in Example 1, for 3 μm VCSELs, the maximum optical output power at 85 °C is ~1.3 mW, and the threshold current (Ith) is ~0.45 mA, meeting the low power consumption requirements.

[0128] See Figure 10, Variation diagram of the light intensity of the coupled vertical cavity surface emitting laser 1 with a three-cavity cascaded structure prepared in Example 1. The emission wavelength of the three-cavity cascaded VCSEL at 85 °C is 894.6 nm, which exactly corresponds to the cesium D1 line wavelength, and the side mode suppression ratio (SMSR) is above 30 dB. From the spectrum, the three-cavity cascaded VCSEL shows a very pure single mode at 85 °C.

[0129] In summary, according to the preparation of Examples 1-8 and the experimental determination of each parameter, it can be found that the coupled vertical cavity surface emitting lasers 1-3 prepared by the present invention have excellent device performance, showing a laser spectral linewidth of less than 1 MHz, meeting the requirements of low power consumption and showing a very pure single mode. It can be seen that the multi-cavity cascaded coupled vertical cavity surface emitting laser of the present invention solves the problems of limited effective cavity length and large lasing spectral linewidth of the existing vertical cavity surface emitting lasers. Due to its excellent performance, it can be applied to scenarios such as atomic clock light sources. And the preparation method is relatively convenient, easy to be widely promoted and applied, and has high superiority.

[0130] The above embodiments are only one implementation manner of the present invention, and its description is relatively specific and detailed, but it should not be understood as a limitation on the scope of the present invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A multi-cavity cascade coupled vertical cavity surface emitting laser, characterized in that: The laser comprises, from top to bottom, a top mirror, an oxide layer, an active region, a preset number of intermediate mirrors and a passive cavity assembly, a bottom mirror, and a substrate. A first electrode is provided on a side of the top mirror away from the oxide layer, and second electrodes are provided at both ends of a side of the intermediate mirror and the passive cavity assembly close to the top mirror.

2. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 1, characterized in that: Each of the intermediate mirror and passive cavity combinations includes an intermediate mirror and a passive cavity arranged on a side of the intermediate mirror away from the active area.

3. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 1, characterized in that: The preset number of the intermediate mirror and passive cavity assembly is 2 to 6.

4. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 1, characterized in that: The top mirror includes 21 to 25 pairs of p-type doped Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, doping concentration is 2×10 18 ~3×10 18 cm -3 .

5. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 1, characterized in that: The active region includes three repeating units, each of which includes 6 nm of In 0.135 Ga 0.865 As quantum wells are surrounded by 8nm Al 0.25 Ga 0.75 As separated by a potential barrier.

6. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 2, characterized in that: Each intermediate mirror consists of n-type doped M1 layer, M2 layer, and M3 layer from bottom to top; M1 includes 11 to 13.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, doping concentration is 1×10 18 ~1.5×10 18 cm -3 ; M2 includes Al 0.16 Ga 0.84 As layer, doping concentration is 2×10 18 cm -3 ; M3 includes 7 to 9.5 pairs of Al 0.92 Ga 0.08 As / Al 0.16 Ga 0.84 As, doping concentration is 1.5×10 18 ~2×10 18 cm -3 .

7. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 6, characterized in that: The preset optical thickness is 1-4λ.

8. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 2, characterized in that: There is a dip in the reflection spectrum of the intermediate mirror.

9. The multi-cavity cascade coupled vertical cavity surface emitting laser according to claim 2, characterized in that: The reflectivity of the top mirror is 99.3% to 99.7%, the reflectivity of the middle mirror is 15% to 45%, and the reflectivity of the bottom mirror is 99.9%.

10. A method for preparing a multi-cavity cascade coupled vertical cavity surface emitting laser according to any one of claims 1 to 9, characterized in that: The following steps are involved: S1, providing a substrate, and sequentially epitaxially growing a bottom mirror, a preset number of middle mirrors and a passive cavity assembly, an active region, an oxide layer, and a top mirror on the substrate to obtain a first preform; S2. A first electrode is arranged on the top mirror of the first preform, and a second electrode is arranged on the first intermediate mirror and the passive cavity assembly on the side of the first preform close to the top mirror, to complete the manufacture of the laser.

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