Multijunction laser diode with improved wavelength stability.

Abstract

To provide a multi-junction laser diode with improved wavelength stability.SOLUTION: A multi-junction laser diode is found in an edge emitting laser (EEL). The disclosed system and method are suited for improving wavelength stability of multi-junction EEL without coupling laser modes of individual junctions and without using any external elements such as Fiber Bragg Gratings (FBR), Volume Bragg Gratings (VBG), or Thermoelectric Cooling (TEC).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 480,353, filed January 18, 2023, entitled "MULTI-JUNCTION LASER DIODE WITH IMPROVED WAVELENGTH STABILITY," the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002]

[0002] The limitations and disadvantages of conventional multi-junction laser diodes will become apparent to those skilled in the art through a comparison of such approaches with certain aspects of the present method and system discussed in the remainder of this disclosure with reference to the drawings. Summary of the Invention

[0003]

[0003] As more fully discussed in the claims, systems and methods for producing multi-junction laser diodes with improved wavelength stability are provided, substantially as illustrated by and / or described with respect to at least one of the figures. [Brief explanation of the drawings]

[0004] [Figure 1]

[0004] FIG. 1 illustrates an example diagram of an edge-emitting laser (EEL) for LiDAR applications, with typical chip dimensions of 400×600 μm 2 , in accordance with various implementations of the present disclosure. [Figure 2]

[0005] 1A-1C illustrate example cross-sectional schematics of multi-junction lasers with improved wavelength stability, in accordance with various implementations of the present disclosure. [Figure 3]

[0006] 1A-1C illustrate examples of the reflectance of a standard front surface mirror as a reference with weak wavelength dependence and a wavelength-stabilized mirror with strong wavelength dependence, according to various implementations of the present disclosure. [Figure 4]

[0007] 1A-1C illustrate example emission spectra for different heat sink temperatures of a GaAs-based multi-junction laser with a standard mirror (left) and a wavelength-stabilized mirror (right), according to various implementations of the present disclosure. [Figure 5a]

[0008] FIG. 5a illustrates example spectral shifts vs. temperature for a standard chip and a chip with λ stabilization according to various implementations of the present disclosure, where the wavelength shift, averaging less than 0.04 nm / K, in the λ stabilization chip is reduced from over 30 nm to approximately 5 nm over the plotted temperature range, with only approximately 2 nm of wavelength shift recorded between 5° C. and 105° C. [Figure 5b]

[0009] FIG. 5b illustrates that the power over temperature performance under 100 ns and 0.1% dc pulsed operation according to various implementations of the present disclosure is similar for both chips, but slightly higher at low T and lower at high T for the λ-stabilized chip. DETAILED DESCRIPTION OF THE INVENTION

[0005]

[0010] High-power lasers generating nanosecond pulses are a key component for LiDAR systems, which provide fast, reliable, and far-range 3D depth information. A variety of applications exist based on this technology, ranging from rangefinders for industrial sensing to autonomous guidance systems for self-driving vehicles and robots. Each of these applications has its own specific set of requirements, and therefore LiDAR comes in many different forms. The most mature technology is direct time-of-flight (dToF). Such systems essentially measure the time delay of reflected short pulses to extract spatially resolved distance information. These systems come in two variants: flash systems illuminate the entire scene at once, while scanning systems use complex optics to scan the laser across the field of view. These systems often include edge-emitting lasers (EELs). EELs exhibit excellent brightness, making them an ideal technology for use in long-range scanning dTOF LiDAR systems.

[0006]

[0011] EELs can be fabricated based on GaAs at near-infrared (NIR) wavelengths ranging from 800 to 1100 nm. Different applications require different choices for the wavelength used. 905 nm is superior when it comes to detecting obstacles through rain and fog, allowing for higher quantum efficiency at the detector side. This disclosure presents EELs with a 905 nm wavelength. However, EELs with any wavelength are also possible.

[0007]

[0012] It is important to use a narrow spectral bandwidth filter on the detector side to filter out the remaining solar background. The main limitation on the type of filter that can be used for EEL is the large wavelength shift that occurs according to the gain shift of the material system—approximately 0.3 nm / K in the case of GaAs. The smaller the bandpass wavelength window, the better the laser signal versus solar background noise. Wavelength stabilization is therefore key to further improving the detection range of LiDAR systems. This disclosure presents a wavelength-stabilized EEL chip that reduces the spectral shift by a factor of 6 between -35°C and 105°C, to an average of <0.04 nm / K.

[0008]

[0013] This disclosure presents lasers with any number of tunnel junctions, for example, three junctions connected by two tunnel junctions, or four junctions connected by three tunnel junctions. An example EEL operates at 905 nm and has a 400 x 600 μm 2 FIG. 1 illustrates a 400×600 μm footprint according to various implementations of the present disclosure. 2 1 illustrates an example diagram of an edge-emitting laser (EEL) for LiDAR applications, with chip dimensions of 200×10 μm, depending on the number of junctions. 2 or 200 × 13 μm 2 However, the present disclosure is not limited by a particular number of junctions or by a particular size of the light-emitting area.

[0009]

[0014] To achieve stable wavelength behavior without changing the gain material, the laser needs to be designed in such a way that the mode is governed by the cavity itself, similar to a VCSEL structure, rather than solely following the gain shift. Traditionally, distributed Bragg reflector (DBR) and distributed feedback (DFB) lasers have been used to internally stabilize the wavelength shift in EELs. In multi-junction devices, the situation is more complicated. Each junction may operate as an individual laser without any mode coupling. Controlling the laser cavity with a surface grating can therefore only affect the dominant laser and is not a realistic option for stabilizing the wavelength of the entire device. Different laser designs can address this limitation, but typically at the expense of using higher-order vertical modes, which lead to non-standard far-field characteristics that can challenge integration of the laser into systems using conventional optics.

[0010]

[0015] This disclosure describes techniques suitable for improving the wavelength stability of multi-junction EELs without coupling the laser modes of the individual junctions and without using any external elements such as fiber Bragg gratings (FBRs), volume Bragg gratings (VBGs), or thermoelectric coolers (TECs). It will be apparent to those skilled in the art that this disclosure also works for single-junction EELs, where traditional stabilization methods are also applicable, as well as for designs using higher-order or coupled waveguides and modes.

[0011]

[0016] Figure 2 illustrates an example cross-sectional schematic of a multi-junction laser with improved wavelength stability according to various implementations of the present disclosure. The EEL consists of a laser cavity including multiple lasing junctions 201, a highly reflective back mirror 203 deposited on a first facet, and a front mirror with variable reflectivity with respect to wavelength deposited on a second facet. The n lasing junctions 201 are electrically connected in series by means of tunnel junctions. Laser emission 205 occurs at the second facet, which includes a front mirror 207 with variable reflectivity as further described with respect to Figure 3.

[0012]

[0017] FIG. 3 illustrates example reflectivities of a standard front surface mirror as a reference with weak wavelength dependence and a wavelength-stabilized front surface mirror (e.g., front surface mirror 207 in FIG. 2 ) with strong wavelength dependence, according to various implementations of the present disclosure. The front surface mirror (e.g., front surface mirror 207 in FIG. 2 ) is designed so that the reflectivity of the front surface mirror decreases rapidly with increasing wavelength, as shown in FIG. 3 , and within the wavelength range in which the laser medium provides gain. The reflectivity slope of the wavelength-stabilized front surface mirror (e.g., front surface mirror 207 in FIG. 2 ) differentiates it from a standard mirror. For example, for a section extending from 2% below the emission wavelength to 3% above the emission wavelength, the wavelength-stabilized mirror may have a maximum reflectivity value that is more than twice the minimum reflectivity value. FIG. 3 illustrates an example wavelength-stabilized mirror with an emission wavelength of 900 nm. For a section extending from 882 nm to 927 nm, this example wavelength-stabilized mirror has a maximum reflectivity of 20% and a minimum reflectivity of 5%.

[0013]

[0018] Wavelength-stabilized mirrors can be achieved by either single-layer or multi-layer dielectric coatings. In the example presented below, an AlOx coating deposited by ion beam sputtering with a thickness of approximately 1000 nm is used to stabilize the wavelength at approximately 900 nm. However, other coating materials and thicknesses are also suitable as long as they exhibit a sufficiently steep, declining reflectivity at longer wavelengths (e.g., 750 nm or 1300 nm AlOx for a wavelength of 900 nm).

[0014]

[0019] Lasing is typically achieved by feedback generated in a cavity, which may be formed by dielectric mirrors on the front and back facets. The greater the feedback (i.e., reflectivity), the easier it is to satisfy the lasing condition. If the reflectivity is not constant across competing lasing modes with different wavelengths, it is therefore possible to reduce the lasing conditions for some of the modes (i.e., make some of them lase more easily) and increase them for others. In the case of the mirrors presented in Figure 3, modes with wavelengths shorter than 885 nm will begin to lase more easily with a wavelength-stabilized mirror than with a standard mirror, and the opposite is true for modes with longer wavelengths. Because the gain of the active material shifts to longer wavelengths at higher temperatures, this effect can therefore be offset by introducing such a mirror that favors modes with lower wavelengths. Applying a mirror on the front facet rather than on the back facet helps reduce lasing from the back facet and the resulting reduced efficiency of the laser diode. The threshold current is increased at larger wavelengths when such stabilization is applied. Therefore, this technique is particularly suitable for lasers with operating points well above (eg, 30x to 50x) the threshold current to reduce the relative power penalty.

[0015]

[0020] FIG. 4 illustrates example emission spectra for different heat sink temperatures of a GaAs-based multijunction laser with standard mirrors (left) and wavelength-stabilized mirrors (right) according to various implementations of the present disclosure. FIG. 4 illustrates experimental results obtained using the disclosed techniques. Two multijunction laser diodes of otherwise identical design are coated with a conventional standard front surface mirror coating and a wavelength-stabilized front surface mirror coating, respectively. Their emission spectra are recorded for several heat sink temperatures from −35° C. to 105° C. The emission of the laser with standard mirrors (left) shifts (and broadens) by approximately 0.27 nm / K due to the gain shift in the GaAs system. Instead, the laser with wavelength-stabilized mirrors shifts wavelength only to approximately 900 nm, after which its wavelength remains relatively stable.

[0016]

[0021] The reason is that at larger wavelengths, low reflectivity prevents the laser from lasing efficiently, and modes at lower wavelengths are favored, even considering that these modes are not located at the gain maximum. A mean wavelength shift of less than 0.04 nm / K is recorded between 35°C and 105°C. Furthermore, because the lasing mode is constrained, the typical broadening at higher temperatures is not observed, and the spectral width remains nearly constant.

[0017]

[0022] FIG. 5a illustrates example spectral shifts versus temperature for a standard chip and a chip with λ stabilization according to various implementations of the present disclosure. For the λ stabilization chip, the wavelength shift, averaging less than 0.04 nm / K, reduces from over 30 nm to approximately 5 nm over the plotted temperature range, with only a wavelength shift of approximately 2 nm recorded between 5°C and 105°C. While EELs can deliver high output power at very high brightness, their drawback is a relatively large wavelength shift of approximately 0.3 nm / K, traditionally consistent with the gain shift in GaAs systems. This behavior can be seen in FIG. 5a, and is quite similar for all EELs published to date. Such wavelength shifts are detrimental to building LiDAR systems because they limit the spectral width available on the detector side and, therefore, the achievable signal-to-noise ratio.

[0018]

[0023] This disclosure develops a new EEL chip design that stabilizes wavelength over a large temperature range. Figure 5a shows the wavelength shift of a conventional laser as a reference and an equivalent laser with λ stabilization. While the spectrum of a standard laser shifts at a constant rate over the entire measured temperature window, the new design enables significantly more stable operation. From -35°C to 105°C, a spectral window of only 5 nm is utilized, corresponding to an average wavelength shift that is six times smaller than the reference and less than 0.04 nm / K.

[0019]

[0024] At the same time, the power remains nearly as stable as for the standard chip, as indicated in Figure 5b, which illustrates that the power over temperature performance under pulsed operation at 100 ns and 0.1% duty cycle according to various implementations of the present disclosure is similar for both chips, but is slightly higher at low T and lower at high T for the λ-stabilized chip. The λ-stabilized chip as presented in the present disclosure allows for the use of much narrower bandpass filters on the detector side, and is therefore ideal for improving the signal-to-noise ratio, and thus the detection range, of dTOF LiDAR systems.

[0020]

[0025] While the present methods and / or systems have been described with reference to certain implementations, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present methods and / or systems. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the scope of the present disclosure. Therefore, it is not intended that the present methods and / or systems be limited to the particular implementations disclosed, but rather that the present methods and / or systems will include all implementations falling within the scope of the appended claims. [Explanation of symbols]

[0021] 201 Laser joining 203 Highly reflective rear mirror 205 Laser Emission 207 Front mirror

Claims

1. 1. A system including a multi-junction edge-emitting laser diode, the multi-junction edge-emitting laser diode comprising: a laser cavity including a plurality of lasing junctions electrically connected in series by one or more tunnel junctions; a first mirror, which is a highly reflective back mirror, deposited on a first facet of the laser cavity; a second mirror formed on a second facet of the laser cavity, the second mirror being a front surface mirror with variable reflectivity; the reflectivity of the second mirror decreases with increasing wavelength over a wavelength range that includes the emission wavelength, and the reflectivity of the second mirror has a maximum value that is greater than twice the minimum value for an interval extending from 2% below the emission wavelength to 3% above the emission wavelength, thereby enabling the emission wavelength to change by less than 0.04 nm / K between −35° C. and 105° C.; feedback occurs between the first mirror and the second mirror in the laser cavity; system.

2. 10. The system of claim 1, wherein the system comprises an edge-emitting laser (EEL).

3. 3. The system of claim 2, wherein the first facet is a back or highly reflective facet of the EEL and the second facet is a front or light-emitting facet of the EEL.

4. 3. The system of claim 2, wherein the first facet is a front or light-emitting facet of the EEL and the second facet is a back or highly reflective facet of the EEL.

5. 10. The system of claim 1, wherein the system comprises a plurality of the multi-junction edge-emitting laser diodes individually disposed on a chip.

6. The system of claim 1 , wherein one or both of the first mirror and the second mirror include a multiple dielectric layer coating.

7. 10. The system of claim 1, wherein the second mirror comprises an AlOx coating.

8. 10. The system of claim 1, wherein the second mirror includes a dielectric coating having a thickness of 1 micron.

9. 10. The system of claim 1, the maximum value of the reflectance in the section is greater than twice the minimum value of the reflectance in the section, The interval extends from 2% below the emission wavelength to 3% above the emission wavelength. system.

10. 1. A method comprising: forming a laser cavity with a plurality of lasing junctions electrically connected in series by one or more tunnel junctions; depositing a first mirror, the first mirror being a highly reflective back mirror, deposited on a first facet of the laser cavity; depositing a second mirror formed on the second facet, the second mirror being a front surface mirror with variable reflectivity, the reflectivity of the second mirror decreasing with increasing wavelength over a wavelength range that includes the emission wavelength, the reflectivity of the second mirror having a maximum value that is greater than twice the minimum value for an interval extending from 2% below the emission wavelength to 3% above the emission wavelength, thereby enabling the emission wavelength to change less than 0.04 nm / K between -35°C and 105°C; generating feedback in the laser cavity between the first mirror and the second mirror; 20. A method for fabricating a multi-junction edge-emitting laser diode, comprising:

11. 11. The method of claim 10, wherein the method includes fabricating an edge-emitting laser (EEL).

12. 12. The method of claim 11, wherein the first facet is a back or highly reflective facet of the EEL and the second facet is a front or light-emitting facet of the EEL.

13. 12. The method of claim 11, wherein the first facet is a front or light-emitting facet of the EEL and the second facet is a back or highly reflective facet of the EEL.

14. The method of claim 10 , wherein one or both of the first mirror and the second mirror include a multiple dielectric layer coating.

15. 11. The method of claim 10, wherein depositing the second mirror comprises ion beam sputtering an AlOx coating.

16. 11. The method of claim 10, wherein the second mirror includes a dielectric coating having a thickness of 1 micron.

17. 11. The method of claim 10, the maximum value of the reflectance in the section is greater than twice the minimum value of the reflectance in the section, The interval extends from 2% below the emission wavelength to 3% above the emission wavelength. method.

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