Optical element

Abstract

The present invention provides an optical device including a first cladding layer formed on the upper part of a substrate, a MQW (Multiple Quantum Well) active layer formed on the upper part of the first cladding layer over the entire light source region and modulator region, a second cladding layer formed on the upper part of the MQW active layer, and a heater that applies heat to the MQW active layer corresponding to the light source region to adjust the energy bandgap of the light source.

Description

Technical Field

[0001] The present invention relates to an optical device that integrally realizes a light source and a modulator having the same active layer in a DFB-LD (Distributed Feedback laser diode) or DBR-LD (Distributed Bragg Reflector) structure.

Background Art

[0002] Currently, for optical communication, a laser light source with a direct modulation structure such as a DFB-LD (Distributed Feedback laser diode) or a DBR-LD (Distributed Bragg Reflector) is used, or an external modulation method that integrates a DFB-LD or a DBR-LD with an EA (Electro-absorption) modulator or an MZ (Mach Zhender) modulator is used.

[0003] An EML (Electro Modulated Laser) in which a light source such as a DFB-LD and an EA (Electro-absorption) modulator or an MZ (Mach Zhender) modulator are integrated minimizes the chirp phenomenon that occurs when directly modulating the current of a light source such as a DFB-LD, enabling high-speed and long-distance communication.

[0004] In particular, in the case of an EML light source with an integrated EA modulator, it is preferable that the band gap wavelength of the semiconductor material in the DFB-LD region and the band gap wavelength of the semiconductor material in the EA modulator region have different characteristics. Typically, it is preferable that the band gap wavelength of the optical gain region, such as the DFB-LD, is about 20 nm to 40 nm longer than the band gap wavelength of the EA modulator region. Methods for making the band gap wavelengths of the DFB-LD region and the EA modulator region different in epitaxial crystal growth for laser light source fabrication have been applied in a very limited manner, such as the SAG (Selective Area Growth) technique. Typically, after crystal growth in the DFB-LD region, the epitaxial crystal for the DFB-LD region is removed in the area where the modulator will be placed, and then an epitaxial crystal suitable for the modulator is regrow. This has resulted in the fabrication method being very difficult and having disadvantages such as high unit costs for optical elements.

[0005] Figure 1 is a diagram illustrating a conventional modulator-integrated light source (EML).

[0006] As shown in Figure 1, the modulator integrated light source (EML) 100 may include a substrate 110, a first cladding layer 120, a first MQW active layer 131, a second MQW active layer 132, a second cladding layer 141, and a third cladding layer 142. Here, MQW is an abbreviation for Multiple Quantum Well.

[0007] The first cladding layer 120 is formed on top of the substrate 110.

[0008] The first MQW active layer 131 is formed on one side of the upper part of the first cladding layer 120 ("left side in reference to Figure 1").

[0009] The second MQW active layer 132 is formed on the other side of the upper part of the first cladding layer 120 ("right side with respect to Figure 1"). Here, the first MQW active layer 131 and the second MQW active layer 132 may be provided on the same plane and can be formed on the upper part of the first cladding layer 120 in various ways.

[0010] The first MQW active layer 131 region corresponds to the light source, and the second MQW active layer 132 region corresponds to the modulator.

[0011] The energy band gap of the second MQW active layer 132 is larger than the energy band gap of the first MQW active layer 131.

[0012] The second cladding layer 141 is formed on top of the first MQW active layer 131.

[0013] The third cladding layer 142 is formed on top of the second MQW active layer 132.

[0014] Thus, in conventional modulator-integrated light sources (EMLs), a first MQW active layer 131 and a second MQW active layer 132 with different energy band gaps are formed so that the energy band gap of the light source region is smaller than the energy band gap of the modulator region.

[0015] To form the first MQW active layer 131 and the second MQW active layer 132, which have different energy band gaps, the first MQW active layer 131 is grown first, the region of the first MQW active layer 131 where the modulator will grow is etched, and the second MQW active layer 132 of the modulator is grown in the etched region. In this process, controlling the etching depth, stabilizing the etched surface, and regrowing the epitaxial crystal are extremely difficult steps, and various problems arise during this process.

[0016] Figure 2 is a diagram illustrating the heat transfer path from a conventional heater.

[0017] As shown in Figure 2, the first cladding layer 210 is formed on top of the substrate 200.

[0018] The second cladding layer 211 is formed on both sides of the upper part of the first cladding layer 210.

[0019] . The MWQ active layer 220 is formed in the center of the second cladding layer 211. The width of the MWQ active layer 220 is very narrow (for example, about 1.5 μm to 2 μm), as shown in Figure 2.

[0020] In such conventional technologies, the heat generated from the heater (not shown) is not concentrated and transferred to the MWQ active layer 220, but is transferred to the entire surrounding region (240). As a result, not all of the heat generated from the heater contributes to the temperature rise of the MWQ active layer 220, leading to inefficient heat transfer and a significant decrease in overall energy efficiency.

[0021] Furthermore, since the spatial resolution of semiconductor lithography, which is commonly used for fabricating optical devices, is only about 1 µm, it is extremely difficult to fabricate heaters on a narrow MWQ active layer 220 with a width of about 1.5 µm to 2 µm.

[0022] Currently, in internationally standardized communication systems such as NG-PON2 (IEEE G.989.2), the light source uses a Time Domain Multiplexing (TDM) method, where multiple subscribers share a single optical wavelength channel over time. That is, the laser light source of subscribers not using communication must be completely off, and therefore, in the time-division multiplexing (TDM) method, the light source is switched on and off alternately. Semiconductor lasers are highly sensitive to temperature fluctuations in their oscillation wavelength, and therefore, the light source of an EML element, which is a combination of a light source such as a DFB-LD element and a modulator, must also be switched on and off ("burst mode"). In the time-division multiplexing method, when the light source is turned on, the temperature of the light source changes due to the heat generated by the light source, which causes a problem in that the wavelength of the light source changes.

[0023] The information contained in this background section is provided to facilitate understanding of the background of the invention and may include matters that are not prior art known to a person with ordinary skill in the art to which this technology belongs.

[0024] [National R&D projects that supported this invention] [Problem Identification Number] 1425179521 [Project Number] RS-2023-00281122 [Ministry Name] Small and Medium-Sized Enterprises Division [Project Management (Specialist) Organization Name] Small and Medium Enterprise Technology Information Promotion Agency [Research Project Name] Small and Medium-Sized Enterprise Technology Innovation Development Project (Export-Oriented) [Research Topic Name] Development of the NG-PON2 PON-stick transceiver [Contribution Rate] 1 / 1 [Project Implementing Organization Name] PHOVEL Co., Ltd. [Research Period] 2023-07-17~2027-07-16

Summary of the Invention

Problems to be Solved by the Invention

[0025] An object of the present invention is, in a situation where the energy band gaps of the light source region and the modulator region are the same, to reduce the energy band gap of the light source region by using a heater so that the energy band gap of the light source region is smaller than the energy band gap of the modulator region, enabling the modulator to operate normally.

[0026] Another object of the present invention is to form the second cladding layer in a structure where the width increases upward, so that electrodes and the like can be conveniently formed in the wider upper part, making the manufacturing very easy.

[0027] Another object of the present invention is to form the second cladding layer in a structure where the width increases upward, so that the heat generated from the heater can be concentrated in the active region without being dispersed, enabling the desired heat to be transferred to the active region in a short time and providing an optical element with excellent thermal energy efficiency because the heat is not dispersed.

[0028] Another objective of the present invention is to maintain a constant total thermal power throughout the entire time, thereby ensuring that not only does the wavelength of the light source remain constant even as time changes, but the energy band gap also remains constant, by controlling the heater's thermal power to cancel out the thermal power generated when the light source is turned on during the burst mode. [Means for solving the problem]

[0029] The optical element according to this embodiment includes a first cladding layer formed on the upper part of a substrate, an MQW (Multiple Quantum Well) active layer formed on top of the first cladding layer over the entire light source region and modulator region, and a heater that applies heat to a second cladding layer formed on top of the MQW active layer and to the MQW active layer corresponding to the light source region to adjust the energy band gap of the light source.

[0030] The heater is characterized by applying heat to the MQW active layer corresponding to the light source region to increase its temperature, thereby reducing the light source energy band gap, so that the light source energy band gap becomes smaller than the modulator energy band gap in the modulator region.

[0031] The second cladding layer in the light source region is characterized by having a shape in which the width of the lower side is smaller than the width of the upper side.

[0032] The optical element further includes a first electrode formed on the upper part of the second cladding layer and an insulating film formed on the upper part of the first electrode, and the heater is formed on the upper part of the insulating film.

[0033] The second cladding layer is formed such that the height of the cladding layer formed in the light source region and the modulator region is greater than the height of the cladding layer formed between the light source region and the modulator region.

[0034] The optical element further includes a second electrode formed on top of the second cladding layer in the modulator region.

[0035] The heater maintains the thermal power (P) necessary to form the desired light source energy band gap in the light source region. However, when the optical element is driven in burst mode, the heater's thermal power (P) is changed to "thermal power (P) - thermal power (R)" during the time when the light source is turned on in burst mode and generates thermal power (R), so that it is maintained at a constant thermal power (P) even when driven in burst mode. [Effects of the Invention]

[0036] The present invention has the effect of making the energy band gap of the light source region smaller than the energy band gap of the modulator region by using a heater to reduce the energy band gap of the light source region when the energy band gaps of the light source region and the modulator region are the same, thereby enabling the modulator to operate normally.

[0037] Furthermore, by forming the second cladding layer with a structure that widens towards the top, the present invention makes it possible to conveniently form electrodes and the like on the wider upper part, thus greatly simplifying the manufacturing process.

[0038] Furthermore, by forming the second cladding layer with a structure that widens towards the top, the present invention not only allows the heat generated from the heater to be concentrated in the active region without being dispersed, thereby enabling the desired heat to be transferred to the active region in a short time, but also results in excellent thermal energy efficiency because the heat is not dispersed.

[0039] Furthermore, the present invention aims to maintain a constant total thermal power throughout the entire time, thereby ensuring that not only is the wavelength of the light source constant even as time changes, but the energy band gap also remains constant, by controlling the heater's thermal power to cancel out the thermal power generated when the light source is turned on during the burst mode. [Brief explanation of the drawing]

[0040] [Figure 1]This is a diagram illustrating a conventional modulator-integrated light source (EML). [Figure 2] This diagram illustrates the direction in which heat is radiated from a conventional heater. [Figure 3-4] This is a diagram to explain the principles of light emission and light absorption. [Figure 5-6] This diagram illustrates the optical modulation principle in an optical element that integrates a light source and a modulator. [Figure 7-10] This is a diagram illustrating an optical element according to one embodiment of the present invention. [Figure 11] This figure illustrates a method for adjusting the energy band gap in an optical element according to one embodiment of the present invention. [Figure 12] This diagram illustrates a method for controlling a heater when an optical element according to one embodiment of the present invention is operating in burst mode. [Modes for carrying out the invention]

[0041] The specific details for carrying out the invention will be described below with reference to the attached drawings.

[0042] Figures 3 and 4 are diagrams illustrating the principles of light emission and light absorption.

[0043] As shown in Figure 3, when an electric current is injected into a pn junction structure, electron transfer occurs that exceeds the energy band gap 330 between the conduction band 310 and the valence band 320, which are formed by the semiconductor epicrystalline structure, and photons with wavelengths corresponding to this energy are emitted.

[0044] As shown in Figure 4, when light is absorbed by a semiconductor, photon energy exceeding the energy band gap of 330 is absorbed, causing electrons to move from the valence band 320 to the conduction band 310, resulting in light absorption.

[0045] Therefore, if the light source and the modulator have the same energy gap, the light emitted from the light source is easily absorbed by the modulator, reducing the transmittance in the modulator.

[0046] Figures 5 and 6 illustrate the optical modulation principle in an optical element that integrates a light source and a modulator.

[0047] In Figures 5 and 6, optical modulation is explained based on the case where the energy bandgap 430 in the modulator region is larger than the energy bandgap 330 in the light source region.

[0048] As shown in Figure 5, the light source and modulator are divided into a light source area 300 and a modulator area 400.

[0049] In the light source region 300, electron transfer occurs that exceeds the energy band gap 330 of the light source between the conduction band 310 and the valence band 320, and photons with wavelengths corresponding to this energy are emitted.

[0050] In the modulator region 400, no reverse voltage is applied to the modulator. Therefore, if the size of the modulator's energy band gap 430 between the conduction band 410 and the valence band 420 does not change, the energy band gap 430 in the modulator region 400 is larger than the energy band gap in the light source region. As a result, the light emitted from the light source passes through without being absorbed by the modulator.

[0051] As shown in Figure 6, when a reverse voltage is applied to the modulator, the energy band gap of the modulator 430 is reduced (to 440) due to the QCSE (Quantum Confined Stark Effect) (for example, it can become the same as the energy band gap of the light source 330). In this case, the modulator region absorbs the light emitted from the light source well and exhibits the basic characteristics (function) of the modulator. Thus, it can be confirmed that the modulator operates well when the energy band gap of the modulator region is made larger than the energy band gap of the light source region.

[0052] Figures 7, 8, 9, and 10 are diagrams illustrating an optical element according to one embodiment of the present invention.

[0053] As shown in Figure 7, the optical element 700 may include a substrate 710, a first cladding layer 720, an MQW active layer 730, a second cladding layer 740, a first electrode 750, an insulating film 760, a heater 770, a second electrode 780, and a control unit.

[0054] The light source included in the optical element 700 may have a DFB-LD or DBR-LD structure.

[0055] The first cladding layer 720 may be formed on top of the substrate 710.

[0056] The MQW active layer 730 may be formed on top of the first cladding layer 720. The MQW active layer 131 may be formed in a single epicrystalline growth process across both the light source region 800, which is the left region with reference to Figure 7, and the modulator region 900, which is the right region with reference to Figure 7, and instead of different MQW active layers, one identical MQW active layer may be formed on top of the first cladding layer 720.

[0057] The second cladding layer 740 may be formed on top of the MQW active layer 730. The height of the cladding layer 740 formed in the light source region 800 and the modulator region 900 may be approximately the same, and the cladding layer formed between the light source region 800 and the modulator region 900 may be formed lower than the cladding layer formed in the light source region 800 and the modulator region 900.

[0058] The second cladding layer 740 may further include a grating section 740 on its underside.

[0059] In another embodiment, the grating portion 740 may be formed on top of the first cladding layer 720 rather than on the second cladding layer 740.

[0060] The first electrode 750 may be formed on top of the second cladding layer 740 of the light source region 800.

[0061] The first electrode 750 is a P electrode, and electrons in the electrode move to the MQW active layer 730 via the second cladding layer 740.

[0062] The insulating film 760 may be formed between the first electrode 750 and the heater 770. The insulating film 760 can provide insulation between the first electrode 750 and the heater 770.

[0063] The heater 770 generates heat, which can be transferred to the MQW active layer 730 via the first electrode 750 and the second cladding layer 740.

[0064] The second electrode 780 may be formed on top of the second cladding layer 740 of the modulator region 900.

[0065] The second electrode 780 is used to apply a reverse voltage to the modulator. When a reverse voltage is applied to the modulator using the second electrode 780, the modulator's energy bandgap 930 is reduced due to the QCSE (Quantum Confined Stark Effect).

[0066] Since the MQW active layer 730 is formed identically across the light source region 800 and the modulator region 900, the energy band gap 830 between the conduction band 810 and the valence band 820 of the light source region 800 and the modulator energy band gap 930 between the conduction band 910 and the valence band 920 of the modulator region 900 are the same.

[0067] A control unit (not shown) can control the driving of components formed on the optical element. For example, the control unit (not shown) can control the first electrode, second electrode, heater, etc., by the input control signal.

[0068] The connection point between the light source and the modulator conversion section can be an electrically isolated region.

[0069] Figure 8 shows the second cladding layer 740, the first electrode 750, the insulating film 760, and the heater 770 as viewed from direction A shown in Figure 7.

[0070] As shown in Figure 8, the second cladding layer 740 has a shape in which the lower width 742 is smaller than the upper width 743. For example, the second cladding layer 740 may have a reverse mesa structure.

[0071] Because the second cladding layer 740 has the structure shown in Figure 8, the first electrode 750, insulating film 760, and heater 770 can be installed in a wider area such as the upper width 743 of the second cladding layer 740, rather than in a narrow area such as the lower width 742 of the second cladding layer 740, making the manufacturing process very easy.

[0072] Figure 9 is a diagram illustrating the path through which heat generated from the heater in Figure 8 is transferred.

[0073] As shown in Figure 9, the heat generated from the heater 770 can be concentrated (744) along the structure of the second cladding layer 740 and not dispersed into the active region 731 of the MQW active layer 730.

[0074] According to this embodiment, the structure allows the heat generated from the heater 770 to be concentrated in the active region 731 without being dispersed. This not only enables the transfer of the desired heat to the active region 731 in a short time, but also results in excellent thermal energy efficiency because the heat is not dispersed.

[0075] Figure 10 shows the second cladding layer 740 and the second electrode 780 as viewed from the "B direction" shown in Figure 7.

[0076] As shown in Figure 10, the second cladding layer 740 in the modulator region may be formed perpendicular to the MQW active layer 730. For example, the second cladding layer 740 may have a vertical mesa structure, but is not limited to this structure and may be formed in a variety of structures.

[0077] The second electrode 780 may be formed on the upper side of the second cladding layer 740.

[0078] Figure 11 is a diagram illustrating a method for adjusting the energy band gap in an optical element according to one embodiment of the present invention.

[0079] Referring to Figure 11, as explained in Figure 7, the light source energy bandgap 1130 of the light source region 1000 and the modulation energy bandgap 1230 of the modulator region 1020 have the same value.

[0080] However, in order to exhibit normal modulator characteristics (performance), the light source energy bandgap 1130 must be smaller than the modulation energy bandgap 1230 of the modulator region 1020.

[0081] Therefore, in the present invention, a heater can be used to apply heat to the light source region 1000 to increase the temperature and reduce the light source energy band gap 1130.

[0082] For example, the reduction in the energy band gap due to temperature typically translates to a reduction of 0.4-0.5 nm / °C in wavelength. In other words, a temperature increase of around 40°C results in either a shift in the band gap wavelength to a longer wavelength of around 20 nm or a reduction in the energy band gap.

[0083] As described above, by applying heat to the light source region 1000, the light source energy band gap 1130 can be reduced to the same size as the light source energy band gap 1130 of the light source region 1010. This makes the light source energy band gap 1130 smaller than the modulator energy band gap 1230 of the modulation region 1020.

[0084] Thus, after the light source energy bandgap 1130 becomes smaller than the modulator energy bandgap 1230 in the modulation region 1020, if no reverse voltage is applied to the modulator in the modulator region 1020 and the size of the modulator energy bandgap 1230 between the conduction band 1210 and the valence band 1220 does not change, the energy bandgap 1230 in the modulator region 1020 is larger than the reduced energy bandgap 1230 in the light source region 1010. Therefore, the light emitted from the light source passes through without being absorbed by the modulator.

[0085] If a reverse voltage is applied to the modulator, the energy band gap 1230 of the modulator region 1030 will shrink due to the QCSE (Quantum Confined Stark Effect) (for example, it may become the same as the reduced energy band gap 1130 of the light source region 1010). In this case, the modulator region will absorb the light emitted from the light source well and exhibit the basic characteristics (function) of the modulator.

[0086] In this embodiment, the modulator can operate normally by using a heater to reduce the energy bandgap of the light source region, thereby making the energy bandgap of the light source region smaller than the energy bandgap of the modulator region.

[0087] Figure 12 is a diagram illustrating a heater control method when an optical element according to one embodiment of the present invention is operating in burst mode.

[0088] Referring to Figures 7, 8, 9, 10, 11, and 12, in the time-division multiplexing (TDM) method, the light source is alternately switched on and off completely. Therefore, the light source of the optical element in this embodiment must also be switched on and off ("burst mode").

[0089] The heater 770 can generate thermal power (P) to obtain the desired energy bandgap 1130 of the light source (1200).

[0090] In burst mode, when the light source is turned on, thermal energy (R) is generated, and when the light source is turned off, the thermal energy becomes 0 (1210).

[0091] If the thermal power (P) generated from heater 770 and the thermal power (R) generated when the light source is turned on in burst mode are simply added together, the thermal power will not be constant but will change into "P+R" and "P" power (1220). In this case, not only will the wavelength of the light source change due to the thermal power, but the energy band gap will also change over time.

[0092] Therefore, in burst mode, the heater 770 changes the thermal power of the heater 770 from "P to PR" during the time when the light source is turned on and thermal power (R) is generated (1230). A control unit (not shown) can control the heater 770 to operate as described above.

[0093] By changing the thermal power of heater 770 from "P" to "PR" and controlling it, even when the light source is turned on in burst mode and generates thermal power (R), which is then combined with the thermal power of heater 770, the thermal power (P) can be kept constant.

[0094] This allows the total thermal power to remain constant even when the light source is turned on in burst mode, so that not only does the wavelength of the light source remain constant over time, but the energy band gap also remains constant.

[0095] The aforementioned embodiments can also be configured by selectively combining all or part of each embodiment to allow for a variety of modifications.

[0096] Furthermore, it should be noted that the examples are for illustrative purposes only and not limiting. Also, a typical expert in the technical field of the present invention will understand that a variety of embodiments are possible within the scope of the technical concept of the present invention.

Claims

[Claim 1] An optical element comprising a light source and a modulator, It has a light source region, a modulator region, and a region between the light source and the modulator. A first cladding layer formed on the upper part of the substrate, An MQW (Multiple Quantum Well) active layer is formed on top of the first cladding layer over the entire light source region, the modulator region, and the region between the light source and the modulator, A second cladding layer formed on top of the MQW active layer, A first electrode formed on the upper part of the second cladding layer in the light source region, A second electrode formed on the upper part of the second cladding layer in the modulator region, An insulating film formed on the upper part of the first electrode, The insulator includes a heater formed on the upper part thereof, A grating portion is formed on the upper part of the first cladding layer or on the lower part of the interior of the second cladding layer. The MQW active layer is formed across the light source region, the modulator region, and the region between the light source and the modulator, and instead of different MQW active layers, a single identical MQW active layer is formed on top of the first cladding layer. The second cladding layer is formed such that the height of the second cladding layer formed in the light source region and the modulator region is greater than the height of the second cladding layer formed in the region between the light source and the modulator, the second cladding layer in the light source region has a shape in which the lower width is smaller than the upper width, and the second cladding layer in the modulator region is formed in a direction perpendicular to the MQW active layer. The second electrode is used to apply a reverse voltage to the modulator, and when a reverse voltage is applied to the modulator using the second electrode, the modulator energy bandgap in the modulator region is reduced by QCSE (Quantum Confined Stark Effect). The heater applies heat to the MQW active layer corresponding to the light source region to increase its temperature, thereby reducing the light source energy band gap, so that the light source energy band gap becomes smaller than the modulator energy band gap. Furthermore, the heater maintains a thermal power (P) necessary to form the desired light source energy band gap in the light source region, but when the optical element is driven in burst mode, the thermal power (P) of the heater is changed to "thermal power (P) - thermal power (R)" during the time when the light source is turned on in burst mode and thermal power (R) is generated, so that it is maintained at a constant thermal power (P) even when driven in burst mode.

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