A method of obtaining a laser diode with distributed waveguide and a laser diode with distributed waveguide obtained by the method

Abstract

The present invention relates to a method (100) of obtaining a laser diode with distributed waveguide which improves laser diode performance and lifetime by reducing laser self-heating, that is the main cause of catastrophic optical damage, and a laser diode with distributed waveguide obtained by the method (100).

Description

[0001] DESCRIPTION

[0002] A METHOD OF OBTAINING A LASER DIODE WITH DISTRIBUTED WAVEGUIDE AND A LASER DIODE WITH DISTRIBUTED WAVEGUIDE OBTAINED BY THE METHOD

[0003] Technical Field

[0004] The present invention relates to a method of obtaining a laser diode with distributed waveguide which improves laser diode performance and lifetime by reducing laser self-heating, that is the main cause of catastrophic optical damage, and a laser diode with distributed waveguide obtained by the method.

[0005] Background of the Invention

[0006] Utilization of laser diode (LD) has increased rapidly over the past two decades due to demands in industrial, consumer and medical applications. GaAs-based high-power LD has become an established technology for most of these applications. Despite having the highest electro-optical conversion efficiency and record levels of output power among all light sources, the main issue the limiting power and reliability of LD output is catastrophic optical damage (COD). Therefore, it has been comprehensively studied and many methods have been developed as a solution. COD not only limits the performance of the LD, but also directly affects fiber and solid-state lasers which rely on high-power LDs as pump sources. For this reason, there is a need for a method which will directly improve the performance, reliability and cost of high-power fiber, direct-diode and solid- state lasers to be developed in order to solve COD in LDs.

[0007] The key factors determining the value and impact of LDs are performance and lifetime. Epitaxial design is a determining factor in performance, whereas reliability / lifetime is a more complex and difficult problem to be studied / solved / improved. Therefore, what is industrially valuable is not "power" but to be a "reliable power". One of the main life-limiting factors in LDs is catastrophic optical bulk damage (COBD). In order to make LDs resistant to COBD, studies carried out so far have aimed to reduce the temperature which triggers COBD or material defects leading to COBD. These methods have mainly been incremental approaches based on material-oriented and optimization.

[0008] It is well understood that heat affects the performance and lifetime of semiconductor devices adversely. And this also applies to LDs. Despite their high efficiency, these devices generate a large amount of heat. Studies which have been carried out since the early years agree that the COD mechanism resulting in the death of the LD starts with thermal runaway. The discovery of the fact that the laser must reach the critical temperature for thermal runaway to start has been a turning point in this respect. To date, experimental studies conducted on different laser structures have revealed that the critical temperature required for the formation of COD is (Tc=120-160°C). For a laser reaching the critical temperature, the process is unstoppable and irreversible. Thermal runaway may occur at the facet (catastrophic optical mirror damage, COMD) or anywhere on the device (catastrophic optical bulk damage, COBD) as well. The starting point is that this high temperature is reached on the mirror surface or anywhere on the device. Laser deterioration is an interconnecting reaction process driven by heat, electric current and light. In all these cases, heat provides more heat generation by amplifying reactions which increase with temperature and it leads to a positive feedback loop. Therefore, when determining the values of temperatures and currents where lasers should / recommended be operated, a power level that will not reach the critical temperature and an operating region that will take a long time to reach it (e.g. 10,000 hours) are selected. Although record high- performance LDs have been demonstrated, COD is still the primary cause of death for commercial high-power LDs. For this reason, there is a need to develop new and novel methods to achieve COD-free chips. Studies which have been carried out to date so as to make LDs resistant to COBD can be categorized into three main areas: (1) using a substrate or packaging method with high thermal conductivity (LD packaging); (2) improving the efficiency of the laser (LD internal structure design); (3) improving epitaxial growth methods. In these studies, it is aimed to reduce the temperature which triggers COBD or the material defect causing COBD. It can be said that these methods are essentially incremental approaches which are material-oriented and based on optimization. Although many significant progresses have been made in the external material used for packaging and the GaAs-based materials used for growth, the lifetime of COBD LDs still remains to be limiting.

[0009] The United States patent document no. US2020028324, an application included in the state of the art, discloses a manufacturing method of a distributed Bragg reflector laser diode wherein a recess region can be easily provided. The manufacturing method of a distributed Bragg reflector laser diode includes forming gratings on a subcoating, forming a waveguide comprising a passive waveguide and an active waveguide coupled to the passive waveguide on the gratings; forming a top coating on the waveguide; forming a plurality of top electrodes on the top coating; forming a lower electrode layer below the lower clad; and etching a part of the lower electrode layer below the gratings and a part of the lower coating to form a lower electrode and a recess region. The laser diode further includes a cooling device, a distributed Bragg reflector laser diode having a subcoating comprising a recess region on one side of the cooling device and connected to the other side of the cooling device, and an air gap between the cooling device and the distributed Bragg reflector laser diode.

[0010] Summary of the Invention

[0011] An objective of the present invention is to realize a method of obtaining a laser diode with distributed waveguide which improves laser diode performance and lifetime by reducing laser self-heating, the main cause of catastrophic optical damage, and a laser diode with distributed waveguide obtained by the method.

[0012] Another objective of the present invention is to realize a method for reducing the temperature of the laser diode by distributing the heat, which is generated on the laser diode by the power of the laser that is not converted into light, effectively by means of the waveguide design of the heat and a laser diode with distributed waveguide obtained by the method.

[0013] Detailed Description of the Invention

[0014] A “Method of Obtaining a Laser Diode with Distributed Waveguide and a Laser Diode with Distributed Waveguide Obtained by the Method” realized to fulfil the objectives of the present invention is shown in the figures attached, in which:

[0015] Figure l is a flow chart of the inventive method.

[0016] Figure 2 illustrates the inventive b) laser diode structure with distributed waveguide (D-LD) and a) a standard laser diode (S-LD) comparatively. Figure 3 is a graph of a comparison of DL-D and S-LD heat profile.

[0017] Figure 4 is a graph showing the variation of laser lifetime with temperature.

[0018] Figure 5 is a schematic view of a laser diode with distributed waveguide.

[0019] Figure 6 is a schematic illustration of the working principle of a laser diode with distributed waveguide under a single current.

[0020] Figure 7 is a schematic representation of a laser diode with distributed waveguide a) laterally and b) from top.

[0021] Figure 8 is a schematic representation of a laser diode with distributed waveguide, which can operate under a single current, a) laterally and b) from top.

[0022] Figure 9 illustrates template examples which can be used for region B. The components illustrated in the figures are individually numbered, where the numbers refer to the following:

[0023] 100. Method

[0024] The inventive method (100) of obtaining a laser diode with distributed waveguide which improves laser diode performance and lifetime by reducing laser selfheating, that is the main cause of catastrophic optical damage, comprises steps of

[0025] - manufacturing of a laser diode by passing it through fabrication steps (101); and

[0026] - forming a waveguide structure distributed along a laser diode (102).

[0027] In the step of manufacturing of a laser diode by passing it through fabrication steps (101) of the inventive method (100); an etching process of waveguide and insulation is carried out by applying lithography and etching methods on the wafer used in laser diode production. The top surface of the wafer is coated with a dielectric material for the blocking function of top surface current. A p-metal coating process is carried out by opening the p-metal window required to open the areas where the current will be provided. Then, the fabrication process is finalized upon the n-metal coating.

[0028] In the step of forming a waveguide structure distributed along a laser diode (102) of the inventive method (100); the laser diode wafer coated with n-metal is broken into bars such that regions A and B are successively arranged along the wafer and made ready for mirror coating. Multilayer mirror coating is performed to achieve high reflectivity (>90%) on the rear surface of the wafer and one or two-layers mirror coating is performed to achieve low reflectivity (-0.1-10%) on the front surface where the light will exit. The region A is a region where high electric current is supplied to the laser diode and laser light is generated. The length of the A region is in the range of 5-500 pm. The laser diode heats up in the region A. The region B is a region where a lower current is supplied to the laser diode in comparison to the region A. The length of the region B is in the range of 5-500 pm. The laser diode cools down in the region B. The region B is designed to allow the passage of low electric current. The B region can also be designed to consist of open and closed areas. In this case, the width of the openings is set to be in the range of 0.1-10 pm and the ratio of the open area to the closed area is set to be greater than 1%. The shape of the apertures to be used in the B region can be any of circular, rectangular or rod-shaped geometries or have any other geometry. On the laser diode, there is a C area which separates the A and B regions and which is current-tight. The area C enables to control the A and B regions electrically independently of the currents supplied to the laser diode. The C region is electrically isolated by etching, ion implantation or dielectric coating method between the A and B regions on the laser diode. The cooling of the laser diode is ensured by controlling the laser region fill factor, i.e. the ratio of the region A to the region B on the laser diode.

[0029] The said invention relates to a laser diode with distributed waveguide which is obtained by following the steps of the above-mentioned method (100). When the laser diode with distributed waveguide is exposed to high current, the region A heats up whereas the region B enables to distribute the heat effectively by allowing the laser to cool down, it prolongs the performance and lifetime of the laser diode.

[0030] The structure of the laser diode with distributed waveguide (D-LD), which is set forth in the invention, is shown in the Figure 2 in comparison with the standard laser diode (S-LD).

[0031] S-LD: A single waveguide (Figure 2a) is included in standard high-power LDs. In high-power lasers, laser current (lias) is delivered through a single electrical contact with a width (width, W) of 5-300 pm and a length (Ltot) of 1-7 mm. Despite the high-power conversion efficiency in these lasers with industrially high output power (1-50 W), there is an intense heat load generated by the electrical power which cannot be converted into light. This heat, which can raise the LD to high temperatures, limits its output power and shortens its lifetime by causing sudden death (COMD or COBD).

[0032] D-LD: In this structure disclosed in the invention, the high heat generated in the LD is removed by being distributed effectively. As shown in the Figure 2b), regions where the current injected in the D-LD structure consist of regions A (laser) and B (passive guide) distributed along the laser cavity. The region A, where high current is injected and laser light is generated, will heat up; while the region B, where current is injected at the transparency level (at or slightly below the laser threshold current density), will allow the laser to cool down. In this structure, the device is allowed to cool down by controlling the laser area fill factor (laser fill factor, FFias.: the ratio of the region A to the region B) and the period length (Lper). In D-LDs, there is a current-tight area which separates the regions A and B. This area separates the two regions electrically and enables to control their currents independently. For this purpose, interval of the regions A and B is etched or electrically isolated by means of an ion-implantation method. The optical mode to be transmitted between the regions A and B must not be affected by the etched region and it must be transmitted without experiencing any loss. At the optimum depth, there is no optical loss and the regions A & B can be controlled with different currents. For example, in a D-LD with FFias=50%, the region A which generates the laser light will operate at high currents (e.g: Iias~l 0- 50 A), the current values at which the region B must operate will be very small (e.g: Itran=0.5-2A). In this way, the heat generated in region A is effectively dissipated and thereby the heating of the device is substantially reduced.

[0033] One of the simulation studies illustrating the operation of the invention is shown in the Figure 3. In the simulations, a 100 pm wide waveguide and a 5 W thermal load (1 W / mm) were used in accordance with the LD structure with an output power of 10 W. In the D-LD structure, structures with a 200 pm period and a 50% laser fill factor were formed. In the S-LD, since the device consists of a single part, the temperature increase relative to the environment was ~70 °C; whereas the highest temperature increase was obtained as ~43 °C in the D-LD, which is suitable to cooling. This result shows that D-LD devices have the potential to delay or prevent COBD since they will operate at much lower temperature than S- LD.

[0034] Arrhenius model is one of the most common equations used to analyze the lifetimes of laser diodes. This model utilizes temperature and activation energy in order to predict the laser's lifetime (time to failure). The following equation (1) shows how the Arrhenius model can be expressed to show the lifetime when the temperature and activation energy are known. > (1)

[0035] Herein, tf is the laser time to failure in hours, A is the scaling coefficient, Eais the activation energy [eV], k is the Boltzman constant [8.617 x 10-5 eV / K] and T is the laser temperature [Kelvin], As can be clearly seen herein, the lifetime of the laser strongly depends on the temperature. The result of the relative calculation of the laser lifetime versus the internal temperature of the laser we obtained by using this model is presented in the Figure 4. In the calculations, 0.45 eV -which is a typical value for these devices- is used as the activation energy. Standard S-LD devices are operated at a level where the internal temperature does not exceed approximately 100°C (TOda+75°C) so as to be away from the COD critical temperature (Tc~120-160°C). To facilitate understanding the idea, we addresses a case where the S-LD lifetime is 1 year at this temperature (Figure 4), but these values can be scaled linearly. In D-LD devices, the internal temperature is expected to be 50°C (TOda+25°C) in a design where the transparent region has twice the area of the gain area. In this case, the laser lifetime is expected to increase by about 8.5 times. A schematic view of a laser diode with distributed waveguide is included in the Figure 5 and the Figure 6. The said laser diode with distributed waveguide can be described as follows: Waveguide width (W): 5-1000 gm (typical width 50-300 pm), Laser length (L): 1-20 mm (typical S-LD length 1-6 mm, typical D-LD length 2-20 mm), Laser area length (LA): Less than 500 pm (preferably length 10- 100 pm), Laser area current (IA): 1-100 A (typical values 10-50 A), Passive guide area length (LB): Less than 500 pm (preferably length 10-100 pm) and Passive guide area current (IB): 0.1-10 A (typical values 0.5-2 A).

[0036] For example, when a typical S-LD laser with dimensions of (5mm)x(100pm) is operated at 25 A, a temperature increase in the order of 100°C occurs in the laser region. For the equivalent D-LD laser with dimensions of (lOmm)x(lOOpm) and LA=LB, a much more efficient and long-lasting lasers are obtained by reducing this temperature raise in the order of 50°C. Alternatively, the region B can be designed so as to allow partial current flow. Examples of this are shown below schematically. In this case, the width of the apertures in the region B can be in the range of 0.1-10 pm and the ratio of aperture to the enclosed area can be greater than 1%. The shape of the apertures to be used in region B can be of any geometry (e.g: circle, rectangle, bar).

[0037] In the experimental studies carried out, output surface temperature measurements of standard diode laser and laser diode with distributed waveguide were performed. In standard diode lasers (S-LD), the output surface temperature is much higher than the lasers with distributed waveguide (D-LD). This suggests that D-LD type lasers will have a much longer lifetime.

[0038] A schematic representation of the side and top view of the laser diode with distributed waveguide is included in the Figure 7. Whereas a schematic representation of the side and top view of a laser diode with distributed waveguide which can operate under single current is included in the Figure 8. Here, the region B is shaped at the fabrication stage and then limits the amount of current that will pass through this area. Thereby, a current is applied to a standard laser diode in a similar way, however by limiting the current that will pass through the region B; the laser diode effectively operates as if it is under two different currents. An example of a template that can be used for the region B is presented in the Figure 8. Here, since it is required to keep the amount of amount of current that will through this area small, the ratio of the aperture to the enclosed area can be greater than 1%.

[0039] Within these basic concepts; it is possible to develop various embodiments of the inventive “A Method (100) of Obtaining a Laser Diode with Distributed Waveguide and a Laser Diode with Distributed Waveguide Obtained by the Method (100)”; the invention cannot be limited to examples disclosed herein and it is essentially according to claims.

Claims

CLAIMS1. A method (100) of obtaining a laser diode with distributed waveguide which improves laser diode performance and lifetime by reducing laser self-heating, that is the main cause of catastrophic optical damage, characterized by steps of- manufacturing of a laser diode by passing it through fabrication steps (101); and- forming a waveguide structure distributed along a laser diode (102).

2. A method (100) according to Claim 1; characterized in that in the step of manufacturing of a laser diode by passing it through fabrication steps (101); an etching process of waveguide and insulation is carried out by applying lithography and etching methods on the wafer used in laser diode production.

3. A method (100) according to Claim 2; characterized in that in the step of manufacturing of a laser diode by passing it through fabrication steps (101); the top surface of the wafer is coated with a dielectric material for the blocking function of top surface current.

4. A method (100) according to any of the preceding claims; characterized in that in the step of manufacturing of a laser diode by passing it through fabrication steps (101); a p-metal coating process is carried out by opening the p-metal window required to open the areas where the current will be provided.

5. A method (100) according to any of the preceding claims; characterized in that in the step of manufacturing of a laser diode by passing it throughfabrication steps (101); the fabrication process is finalized upon the n-metal coating.

6. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the laser diode wafer coated with n-metal is broken into bars such that regions A and B are successively arranged along the wafer and made ready for mirror coating.

7. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); multilayer mirror coating is performed to achieve high reflectivity (>90%) on the rear surface of the wafer and one or two-layers mirror coating is performed to achieve low reflectivity (-0.1-10%) on the front surface where the light will exit.

8. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the region A is a region where high electric current is supplied to the laser diode and laser light is generated, and its length is in the range of 5-500 pm.

9. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the region B is a region where a lower current is supplied to the laser diode in comparison to the region A and which enables to cool down the laser diode and its length is in the range of 5-500 pm.

10. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laserdiode (102); the region B is designed to allow the passage of low electric current.

11. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the B region is designed to consist of open and closed areas and the width of its openings is set to be in the range of 0.1-10 pm and the ratio of the open area to the closed area is set to be greater than 1%.

12. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the shape of the apertures to be used in the B region can be any of circular, rectangular or rod-shaped geometries or have any other geometry.

13. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); on the laser diode, there is a C area which separates the A and B regions and which is current-tight.

14. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the area C enables to control the A and B regions electrically independently of the currents supplied to the laser diode.

15. A method (100) according to any of the preceding claims; characterized in that in the step of forming a waveguide structure distributed along a laser diode (102); the C region is electrically isolated by etching, ion implantation or dielectric coating method between the A and B regions on the laser diode.

16. A laser diode with distributed waveguide which is obtained by following the above-mentioned method (100) steps.

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