Laser and optical phase-locked loop system

Abstract

The embodiment of the invention provides a laser device and an optical phase-locked loop system, and relates to the technical field of laser devices, the laser device is specifically a wavelength continuous tunable laser device, and the laser device can simultaneously realize large sweep frequency speed and narrow line width. The laser comprises a gain chip and an external cavity filter. The gain chip comprises a first wrapping layer, an active layer and a second wrapping layer which are sequentially arranged in a stacked mode. The gain chip comprises a gain region and a phase modulation region which are arranged along a first direction, and the first direction is perpendicular to the stacking direction. Wherein the active layer comprises a first part and a second part which are arranged in the first direction, the first part is located in the gain region, and the second part is located in the phase modulation region. The central wavelength of the photoluminescence spectrum of the first part is different from the central wavelength of the photoluminescence spectrum of the second part. And the outer cavity filter is coupled with one end, far away from the gain region, of the phase modulation region in the gain chip.

Description

Technical Field

[0001] The present application relates to the field of laser technology, and in particular to a laser and an optical phase-locked loop system. Background Art

[0002] A wavelength continuously tunable laser can output laser with continuously changing wavelength. It has very important applications in wavelength division multiplexing (WDM) systems, frequency modulated continuous wave (FMCW) radar, quantum key distribution (QKD) systems and coherent optical communication systems.

[0003] Among them, existing wavelength continuously tunable lasers include wavelength continuously tunable lasers based on thermal tuning and wavelength continuously tunable lasers based on electrical tuning. The wavelength continuously tunable laser based on thermal tuning has good line width and output power, but due to the long response time of thermal tuning, the frequency sweep speed of the wavelength continuously tunable laser based on thermal tuning is usually low. The wavelength continuously tunable laser based on electrical tuning has a higher frequency sweep speed, but because the waveguide (optical transmission medium) in the wavelength continuously tunable laser based on electrical tuning needs to be doped, the doping will enhance the light absorption of the waveguide, thereby causing the quality factor (Q value) in the resonant cavity to decrease, thereby increasing the line width of the wavelength continuously tunable laser based on electrical tuning. Summary of the Invention

[0004] The embodiments of the present application provide a laser and an optical phase-locked loop system. The laser may be a wavelength continuously tunable laser that can simultaneously achieve a higher frequency sweep speed and a narrower linewidth.

[0005] In a first aspect, a laser is provided, comprising a gain chip and an external cavity filter. The gain chip comprises a first cladding layer, an active layer, and a second cladding layer stacked in sequence. The gain chip comprises a gain region and a phase modulation region arranged along a first direction, the first direction being perpendicular to the stacking direction. The active layer comprises a first portion and a second portion arranged along the first direction, the first portion being located in the gain region, and the second portion being located in the phase modulation region. The center wavelength of the photoluminescence spectrum of the first portion is different from the center wavelength of the photoluminescence spectrum of the second portion. The external cavity filter is coupled to an end of the phase modulation region in the gain chip that is distal from the gain region. In this laser, the active layer comprises a first portion and a second portion arranged along the first direction, the first portion being located in the gain region. When a voltage is applied to the gain region and a current is injected, the first portion radiates photons. The external cavity filter and the side of the gain chip distal from the external cavity filter form a resonant cavity of the laser. The second portion is located in the phase modulation region. When a voltage is applied to the phase modulation region, the refractive index of the phase modulation region can be adjusted, thereby enabling the laser to output laser light of different wavelengths. A voltage is applied to the phase modulation region to adjust its refractive index. This laser is electrically tuned, resulting in a high sweep speed. Furthermore, the center wavelength of the photoluminescence spectrum of the first portion differs from the center wavelength of the photoluminescence spectrum of the second portion. This reduces the absorption of photons generated by the gain region by the phase modulation region, thereby reducing losses in the laser's resonant cavity, increasing the quality factor (Q value) of the laser's resonant cavity, and narrowing the linewidth of the output laser light. This means that the laser can simultaneously achieve a high sweep speed and a narrow linewidth.

[0006] Optionally, the center wavelength of the photoluminescence spectrum of the first portion is greater than the center wavelength of the photoluminescence spectrum of the second portion. In this optional embodiment, the center wavelength of the photoluminescence spectrum of the first portion needs to be greater than the center wavelength of the photoluminescence spectrum of the second portion to reduce the absorption of photons generated by the gain region by the phase modulation region. When the center wavelength of the photoluminescence spectrum of the first portion is less than the center wavelength of the photoluminescence spectrum of the second portion, the phase modulation region absorbs photons generated by the gain region more significantly.

[0007] Optionally, the difference between the center wavelength of the photoluminescence spectrum of the first portion and the center wavelength of the photoluminescence spectrum of the second portion is greater than or equal to 60 nanometers. In this optional embodiment, the greater the difference between the center wavelengths of the photoluminescence spectrum of the first portion and the center wavelengths of the photoluminescence spectrum of the second portion, the less the phase modulation region absorbs photons generated in the gain region, the lower the loss in the laser resonant cavity, and the higher the Q value of the laser resonant cavity. Specifically, the center wavelength of the photoluminescence spectrum of the first portion can be greater than the center wavelength of the photoluminescence spectrum of the second portion, and the difference between the center wavelengths of the photoluminescence spectrum of the first portion and the center wavelengths of the photoluminescence spectrum of the second portion can be greater than or equal to 60 nanometers, thereby significantly reducing the phase modulation region's absorption of photons generated in the gain region.

[0008] Optionally, the second portion includes quantum wells or quantum dots; and a reverse bias voltage is applied to the phase modulation region. In this optional method, the second portion includes quantum wells, and the first portion may also include quantum wells or quantum dots. For example, when the second portion includes quantum wells, the center wavelength of the photoluminescence spectrum of the first portion can be made different from the center wavelength of the photoluminescence spectrum of the second portion by adjusting the width, composition, stress, etc. of the quantum wells in the second portion, or by changing the energy band center of the quantum well structure in the second portion through quantum well intercalation (QWI) technology. Specifically, when the second portion includes quantum wells or quantum dots, a reverse bias voltage needs to be applied to the phase modulation region to reverse bias the phase modulation region, thereby generating a quantum confined Stark effect (QCSE) in the quantum wells or quantum dots in the second portion, ultimately causing the refractive index of the phase modulation region to change.

[0009] Optionally, the second portion includes a first bulk waveguide having dopants disposed therein; and a reverse bias voltage is applied to the phase modulation region. In this optional embodiment, the second portion includes a first bulk waveguide having dopants disposed therein. When a reverse bias voltage is applied to the phase modulation region, the phase modulation region can be reverse biased, and the dopants in the first bulk waveguide diffuse outward, thereby reducing the concentration of the dopants in the first bulk waveguide and thereby changing the refractive index of the phase modulation region.

[0010] Optionally, the dopant is an N-type dopant, and the doping concentration of the dopant is less than or equal to 1e18 atoms / cm3. In this optional manner, the first bulk waveguide is lightly doped.

[0011] Optionally, the second portion includes a first bulk waveguide; a forward bias voltage is applied to the phase modulation region. In this optional embodiment, the second portion includes the first bulk waveguide, and no additional dopants are provided in the first bulk waveguide. When a forward bias voltage is applied to the phase modulation region, the phase modulation region is forward biased, and the carrier concentration in the first bulk waveguide continuously increases, thereby causing the refractive index of the phase modulation region to change.

[0012] Optionally, the active layer further includes a second bulk waveguide disposed between the first portion and the second portion along the first direction, with one end of the second bulk waveguide connected to the first portion and the other end of the second bulk waveguide connected to the second portion. The second bulk waveguide is configured to match the mode field between the first portion and the second portion. In this optional embodiment, the second bulk waveguide is configured to match the mode field between the first portion and the second portion, thereby reducing transmission loss incurred when photons propagate between the first portion and the second portion, thereby reducing loss in the laser's resonant cavity and improving the Q factor of the laser's resonant cavity.

[0013] Optionally, a central wavelength of the photoluminescence spectrum of the first portion is greater than a central wavelength of the photoluminescence spectrum of the second bulk waveguide.

[0014] Optionally, a difference between a center wavelength of the photoluminescence spectrum of the first portion and a center wavelength of the photoluminescence spectrum of the second bulk waveguide is greater than or equal to 100 nanometers. In this embodiment, the greater the difference between the center wavelength of the photoluminescence spectrum of the first portion and the center wavelength of the photoluminescence spectrum of the second bulk waveguide, the less absorption of photons generated in the gain region by the second bulk waveguide, and the higher the Q value of the laser resonant cavity.

[0015] Optionally, a microstructure formed by etching is provided in the second bulk waveguide. In this optional manner, the microstructure is formed by etching the second bulk waveguide so that the second bulk waveguide can match the mode field between the first portion and the second portion.

[0016] Optionally, the second portion includes a first bulk waveguide, and the first bulk waveguide and the second bulk waveguide are integrally provided. In this optional embodiment, the second portion includes the first bulk waveguide, and thus the first bulk waveguide and the second bulk waveguide can be integrally provided, for example, by depositing a bulk waveguide material once, thereby forming the first bulk waveguide and the second bulk waveguide in the bulk waveguide material.

[0017] Optionally, the portion of the second cladding layer near the external cavity filter is curved in a plane perpendicular to the stacking direction. In this optional embodiment, the curved portion of the second cladding layer can reduce the Fabry-Perot (FP) effect generated at both ends of the gain chip, thereby reducing the loss of the laser resonator and improving the Q factor of the laser resonator.

[0018] Optionally, the gain chip further includes an antireflection film disposed at one end of the gain chip near the external cavity filter. In this optional embodiment, the antireflection film increases the transmission ratio of photons between the phase modulation region and the external cavity filter, thereby reducing the FP effect generated at both ends of the gain chip.

[0019] Optionally, the gain chip further comprises a high-reflection film disposed on an end of the gain chip away from the external cavity filter. In this optional embodiment, the high-reflection film can enable the output end of the laser to be located on a side of the external cavity filter away from the gain chip.

[0020] Optionally, the gain chip further includes a first electrode and a second electrode arranged on a side of the second cladding away from the active layer, and a third electrode arranged on a side of the first cladding away from the active layer, the first electrode and the second electrode are not in contact, the first electrode and the third electrode are used to inject current into the gain region; the second electrode and the third electrode are used to apply voltage to the phase modulation region.

[0021] Optionally, the gain chip further includes a first cap layer disposed between the second cladding layer and the first electrode, and a second cap layer disposed between the second cladding layer and the second electrode, wherein the first cap layer is not in contact with the second cap layer.

[0022] Optionally, the external cavity filter includes an antireflection film disposed at one end of the external cavity filter near the gain chip. In this optional embodiment, the antireflection film increases the transmission ratio of photons between the gain chip and the external cavity filter, thereby reducing the loss of the laser resonant cavity and improving the Q value of the laser resonant cavity.

[0023] Optionally, the external cavity filter includes a filtering structure; the filtering structure includes any one of the following: a grating, a microring resonator, and a multi-channel interference filter.

[0024] Optionally, the external cavity filter also includes a metal heating strip arranged on the filtering structure. In this optional manner, the metal heating strip is connected to the control electrode, and applying a voltage to the metal heating strip through the control electrode can change the performance of the filtering structure, thereby achieving a change in parameters such as the frequency sweep range of the laser output by the laser. For example, when a voltage is not applied to the metal heating strip through the control electrode, the initial value of the wavelength of the laser output by the laser is a first value, which has a certain error with the target value (the reason may be that there is a process error when the filtering structure is manufactured). Then, a voltage can be applied to the metal heating strip through the control electrode to change the parameters of the filtering structure, thereby making the initial value of the wavelength of the laser output by the laser the target value. Subsequently, a voltage is applied to the phase modulation zone to adjust the refractive index of the phase modulation zone to change the wavelength of the laser output by the laser and achieve frequency sweep.

[0025] Optionally, the external cavity filter further includes a first mode field matching structure disposed on a side of the filter structure proximal to the gain chip; the first mode field matching structure is configured to match the mode field between the gain chip and the filter structure. In this optional embodiment, the first mode field matching structure is configured to match the mode field between the gain chip and the filter structure to reduce photon transmission loss between the gain chip and the filter structure, thereby reducing loss in the laser's resonant cavity and improving the Q factor of the laser's resonant cavity.

[0026] Optionally, the external cavity filter further includes a second mode field matching structure disposed on a side of the filter structure away from the gain chip; the second mode field matching structure is used to adjust the mode field of the laser light output by the filter structure. In this optional embodiment, when the laser light output by the filter structure needs to be transmitted through an optical fiber or optical waveguide, the second mode field matching structure is used to adjust the mode field of the laser light output by the filter structure, thereby enabling better coupling of the laser light into the optical fiber or optical waveguide. When the laser light output by the filter structure needs to be transmitted through space to another device, the second mode field matching structure is used to adjust the mode field of the laser light output by the filter structure, thereby enabling better transmission of the laser light to the other device, thereby improving the flexibility of the laser and its compatibility with other devices.

[0027] Optionally, the external cavity filter comprises any of the following materials: silicon, silicon nitride, silicon-based silicon dioxide, and lithium niobate. In this optional embodiment, the external cavity filter includes a waveguide (also called a waveguide layer), which is used to guide light to propagate within the external cavity filter, thereby passing through the filter structure and other structures within the external cavity filter. When the external cavity filter comprises any of the following materials: silicon, silicon nitride, silicon-based silicon dioxide, and lithium niobate, the loss coefficient of the waveguide within the external cavity filter is relatively small, further reducing the loss of the laser resonant cavity and improving the Q value of the laser resonant cavity.

[0028] In a second aspect, an optical phase-locked loop system is provided, comprising an optical phase-locked loop and a laser as described in any one of the first aspects above; the optical phase-locked loop is used to receive a first laser output by the laser and adjust a control signal transmitted to the laser according to the phase of the first laser; and the laser is used to output a second laser according to the control signal.

[0029] Among them, the technical effects brought about by any possible implementation method of the second aspect can refer to the technical effects brought about by the different implementation methods of the above-mentioned first aspect, and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 A schematic diagram of the structure of a distributed feedback laser provided in an embodiment of the present application;

[0031] Figure 2 A diagram showing changes in phase and amplitude of laser light output by a distributed feedback laser provided in an embodiment of the present application at different modulation frequencies;

[0032] Figure 3 A three-dimensional structural diagram of a laser provided in an embodiment of the present application;

[0033] Figure 4 for Figure 3 Top view of the laser shown Figure 1 ;

[0034] Figure 5 for Figure 3 Cross-section of the gain chip in the laser shown along AA' Figure 1 ;

[0035] Figure 6 A graph showing how the refractive index of the phase modulation region varies with the applied reverse bias voltage according to an embodiment of the present application;

[0036] Figure 7 A schematic diagram of the photoluminescence spectrum of the quantum well in the second part provided in the embodiments of the present application;

[0037] Figure 8 A graph showing a change in the effective refractive index of a phase modulation region according to an embodiment of the present application as a function of a reverse bias voltage applied to the phase modulation region;

[0038] Figure 9 for Figure 3 Cross-section of the gain chip in the laser shown along AA' Figure 2 ;

[0039] Figure 10 A graph showing a change in carrier concentration in a phase modulation region in a stacking direction according to an embodiment of the present application;

[0040] Figure 11 A graph showing a change in the effective refractive index of a phase modulation region according to an embodiment of the present application as a function of a reverse bias voltage applied to the phase modulation region;

[0041] Figure 12 for Figure 3 Top view of the laser shown Figure 2 ;

[0042] Figure 13 A graph showing how the line width of the laser output by the laser varies with the current injected into the gain region when the loss coefficient of the waveguide in the external cavity filter is different according to an embodiment of the present application;

[0043] Figure 14 The frequency noise power spectrum density diagram of the laser output when the waveguide of the external cavity filter provided in the embodiment of the present application is 5 mm long and the loss coefficient of the waveguide is 0.02 dB / cm;

[0044] Figure 15 for Figure 3 Cross-section of the gain chip in the laser shown along AA' Figure 3 ;

[0045] Figure 16 for Figure 3 Top view of the laser shown Figure 3 ;

[0046] Figure 17 for Figure 3 Top view of the laser shown Figure 4 ;

[0047] Figure 18 for Figure 3 Top view of the laser shown Figure 5 . DETAILED DESCRIPTION

[0048] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0049] Unless otherwise defined, all scientific and technological terms used herein have the same meaning as those known to those of ordinary skill in the art. In the embodiments of the present application, "at least one" refers to one or more, and "a plurality" refers to two or more. "And / or" describes the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: the existence of A alone, the existence of A and B at the same time, and the existence of B alone, wherein A and B can be singular or plural. The character " / " generally indicates that the associated objects before and after are in an "or" relationship. "At least one of the following items" or its similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, at least one of a, b or c can represent: a, b, c, a and b, a and c, b and c or a, b and c, wherein a, b and c can be single or multiple. In addition, in the embodiments of the present application, words such as "first" and "second" do not limit quantity and order.

[0050] In addition, in the embodiments of the present application, directional terms such as "upper" and "lower" are defined relative to the orientation of the components in the drawings. It should be understood that these directional terms are relative concepts. They are used for relative description and clarification, and they can change accordingly according to changes in the orientation of the components in the drawings.

[0051] In the embodiments of this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in the embodiments of this application should not be interpreted as being more preferred or advantageous than other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0052] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0053] The following are the technical terms used in the embodiments of the present application:

[0054] Semiconductor: A semiconductor is a material whose electrical conductivity at room temperature is between that of a conductor and an insulator; semiconductors include intrinsic semiconductors and impurity semiconductors. A pure semiconductor without impurities and defects, with equal concentrations of electrons and holes, is called an intrinsic semiconductor. A semiconductor doped with a certain amount of impurities is called an impurity semiconductor or an intrinsic semiconductor. Among them, the impurities doped into the impurity semiconductor can provide a certain concentration of carriers (such as holes or electrons). Impurity semiconductors that dope electron impurities (such as pentavalent phosphorus) are also called electron-type semiconductors or N (negative)-type semiconductors, and impurity semiconductors that dope hole impurities (such as trivalent boron) are also called hole-type semiconductors or P (positive)-type semiconductors. Doping can improve the conductivity of intrinsic semiconductors. Generally, the greater the carrier concentration, the lower the resistivity of the semiconductor and the better the conductivity. In the embodiments of the present application, the layer structure in a device made of semiconductors (or semiconductor materials) is called a semiconductor material layer.

[0055] Photoluminescence spectroscopy (PL spectrum) refers to the spectrum of light gain, intensity or energy distribution of light of different wavelengths formed by the recombination of electrons and holes in a substance under the stimulation of light.

[0056] Central wavelength of photoluminescence spectrum: In the photoluminescence spectrum, the wavelength corresponding to the center of the light gain, intensity or energy distribution spectrum is the central wavelength of the photoluminescence spectrum.

[0057] The following describes in detail the implementation of the embodiments of the present application in conjunction with the accompanying drawings.

[0058] Lasers include continuously tunable wavelength lasers, which can output lasers with continuously changing wavelengths. Continuously tunable wavelength lasers have very important applications in wavelength division multiplexing (WDM) systems, frequency modulated continuous wave (FMCW) radars, quantum key distribution (QKD) systems, and coherent optical communication systems.

[0059] The wavelength continuously tunable laser includes a distributed feedback (DFB) laser. Figure 1 As shown, an embodiment of the present application provides a schematic structural diagram of a DFB laser 10. The DFB laser 10 includes a stacked substrate 101, a lower cladding layer 102, an active layer 103, a grating layer 104, and an upper cladding layer 105. In the DFB laser 10, the size of the upper cladding layer 105 along the x-axis is smaller than the size of the active layer 103 along the x-axis, forming a ridge waveguide structure. When current is injected into the DFB laser 10, carrier recombination occurs in the active layer 103, radiating photons. These photons are reflected by the grating layer 104, and the directions of the incident light and reflected light between the grating strips of the grating layer 104 are exactly opposite. Only light of a specific wavelength that meets the Bragg condition is strongly reflected, thereby outputting a monochromatic laser beam. The laser light is transmitted along the z-axis in the ridge waveguide structure. The wavelength of the laser light output by the DFB laser 10 is related to the refractive index of each semiconductor material layer in the DFB laser 10. In order to make the DFB laser 10 wavelength tunable, the magnitude of the current injected into the DFB laser 10 is usually changed, thereby changing the refractive index of each semiconductor material layer in the DFB laser 10 and ultimately changing the wavelength of the laser output by the DFB laser 10.

[0060] When current is injected into the DFB laser 10, carriers in each semiconductor material layer in the DFB laser 10 undergo a scattering effect, which can change the refractive index of each semiconductor material layer in the DFB laser 10. In addition, when current is injected into the DFB laser 10, the DFB laser 10 generates Joule heat, causing each semiconductor material layer in the DFB laser 10 to produce a thermo-optical effect, thereby changing the refractive index of each semiconductor material layer in the DFB laser 10. Specifically, the carrier scattering effect can cause the refractive index of each semiconductor material layer in the DFB laser 10 to change negatively, while the thermo-optical effect can cause the refractive index of each semiconductor material layer in the DFB laser 10 to change positively.

[0061] For example, refer to Figure 2 As shown, Figure 2 The figure shows the changes in the phase and amplitude of the laser light output by the DFB laser 10 at different modulation frequencies. The modulation frequency is specifically the frequency of the current injected into the DFB laser 10. Curve 1 shows the change in the phase of the laser light output by the DFB laser 10 as the modulation frequency changes; Curve 2 shows the change in the amplitude of the laser light output by the DFB laser 10 as the modulation frequency changes. From Curves 1 and 2 in the figure, it can be seen that the modulation frequency corresponding to the phase reversal point of the DFB laser 10 is A, and A is between 100 kHz and 1 MHz. Specifically, when the modulation frequency is greater than or equal to 10 kHz and less than or equal to A, the amplitude of the refractive index change of each semiconductor material layer in the DFB laser 10 due to the thermo-optical effect is greater than the amplitude of the refractive index change caused by carrier scattering in each semiconductor material layer in the DFB laser 10. At this time, the refractive index change of each semiconductor material layer in the DFB laser 10 is mainly the forward refractive index change brought about by the thermo-optical effect in each semiconductor material layer in the DFB laser 10. Therefore, the slope of the phase change of the laser shown in curve 1 continues to increase, and the absolute value of the difference between the amplitude of the laser shown in curve 2 and the initial amplitude continues to increase. When the modulation frequency is greater than A and less than or equal to 100 MHz, the thermal relaxation response time of the thermo-optical effect is very long, wherein the thermal relaxation response time is in the order of hundreds of microseconds to milliseconds. Therefore, when the modulation frequency is high, the thermo-optical effect generated by each semiconductor material layer in the DFB laser 10 can be ignored. At this time, the refractive index change of each semiconductor material layer in the DFB laser 10 is mainly the negative refractive index change caused by the carrier scattering of each semiconductor material layer in the DFB laser 10. Therefore, the slope of the phase change of the laser shown in curve 1 continuously decreases, and the absolute value of the difference between the amplitude of the laser shown in curve 2 and the initial amplitude continuously decreases. Specifically, when the modulation frequency is greater than 100 MHz, the refractive index change caused by the carrier scattering of each semiconductor material layer in the DFB laser 10 has reached its maximum. Therefore, the slope of the phase change of the laser shown in curve 1 does not change, and the absolute value of the difference between the amplitude of the laser shown in curve 2 and the initial amplitude does not change.

[0062] For example, in an optical phase-locked loop (OPLL) system, the system includes Figure 1The DFB laser 10 and the optical phase-locked loop are shown. The optical phase-locked loop receives the laser light output by the DFB laser 10 and adjusts the control signal transmitted to the DFB laser 10 according to the phase of the laser light. Specifically, the control signal of the DFB laser 10 may be a current injected into the DFB laser, thereby causing the DFB laser 10 to output laser light according to the control signal, so as to reduce the phase noise of the laser light output by the DFB laser 10. However, since the refractive index change mechanism of the DFB laser 10 is different when the modulation frequency is low and when the modulation frequency is high, the optical phase-locked loop cannot simultaneously suppress the phase noise generated by the low modulation frequency and the phase noise generated by the high modulation frequency, resulting in a limited loop bandwidth of the OPLL system. The loop bandwidth of the OPLL system is the range of modulation frequencies corresponding to which the optical phase-locked loop can suppress the phase noise. Figure 2 In the example, the loop bandwidth of the OPLL system is approximately 1MHz-100KHz=900KHz.

[0063] Exemplarily, wavelength continuously tunable lasers also include some structures without phase inversion points, such as wavelength continuously tunable lasers based on thermal tuning and wavelength continuously tunable lasers based on electrical tuning. The wavelength continuously tunable laser based on thermal tuning has good line width and output power, but due to the long thermal relaxation response time of thermal tuning, the frequency sweep speed of the wavelength continuously tunable laser based on thermal tuning is usually small. The wavelength continuously tunable laser based on electrical tuning has a higher frequency sweep speed, but because the waveguide (optical transmission medium) in the wavelength continuously tunable laser based on electrical tuning needs to be doped, the doping will enhance the light absorption of the waveguide, thereby causing the quality factor (Q value) in the resonant cavity to decrease, thereby increasing the line width of the wavelength continuously tunable laser based on electrical tuning.

[0064] To this end, an embodiment of the present application provides a laser, which is specifically a wavelength continuously tunable laser (also called a swept laser, swept light source, etc.). The laser has no phase reversal point and can achieve a faster sweep speed and a narrower linewidth.

[0065] Reference Figure 3 、 Figure 4 and Figure 5 As shown, Figure 3 A three-dimensional structural diagram of a laser 20 provided in an embodiment of the present application, Figure 4 for Figure 3 A top view of the laser 20 is shown, Figure 5 for Figure 3 The cross-sectional view of the gain chip 21 in the laser 20 is shown along AA'. Figure 3 As shown, the laser 20 includes a gain chip 21 and an external cavity filter 22. Figure 5As shown, the gain chip 21 includes a cladding layer 212 (also called a first cladding layer), an active layer 213, and a cladding layer 214 (also called a second cladding layer) stacked in sequence along the z-axis direction, wherein the stacking direction is specifically the y-axis direction. Figure 3 As shown, the gain chip 21 includes a gain region D1 and a phase modulation region D2 arranged along the z-axis direction (also referred to as the first direction), and the z-axis direction is perpendicular to the y-axis direction. Figure 3 In the embodiment, a portion of the cladding 212 along the x-axis is located in the gain region D1, and another portion is located in the phase modulation region D2; a portion of the cladding 214 along the x-axis is located in the gain region D1, and another portion is located in the phase modulation region D2.

[0066] Reference Figure 3 As shown, the active layer 213 includes a first portion 2131 and a second portion 2132 arranged along the x-axis (also referred to as the first direction). The first portion 2131 is located in the gain region D1, and the second portion 2132 is located in the phase modulation region D2. Injecting current into the gain region D1 radiates photons, while applying voltage to the phase modulation region D2 adjusts the refractive index of the phase modulation region D2. The center wavelength of the photoluminescence spectrum of the first portion 2131 is different from that of the second portion 2132.

[0067] For example, the central wavelength of the photoluminescence spectrum of the first portion 2131 differs from the central wavelength of the photoluminescence spectrum of the second portion 2132, thereby reducing the absorption of photons generated in the gain region D1 by the phase modulation region D2. Furthermore, the greater the difference between the central wavelengths of the photoluminescence spectrum of the first portion 2131 and the central wavelengths of the photoluminescence spectrum of the second portion 2132, the less the absorption of photons generated in the gain region D1 by the phase modulation region D2. In some examples, the central wavelength of the photoluminescence spectrum of the first portion 2131 may be greater than the central wavelength of the photoluminescence spectrum of the second portion 2132, and the difference between the central wavelengths of the photoluminescence spectrum of the first portion 2131 and the central wavelengths of the photoluminescence spectrum of the second portion 2132 may be greater than or equal to 60 nanometers (nm). When the central wavelength of the photoluminescence spectrum of the first portion 2131 is less than the central wavelength of the photoluminescence spectrum of the second portion 2132, the absorption of photons generated in the gain region D1 by the phase modulation region D2 is greater.

[0068] Reference Figure 3 and Figure 4 As shown, the external cavity filter 22 is coupled to an end of the phase modulation region D2 in the gain chip 21 away from the gain region D1 .

[0069] exist Figure 3In the illustrated laser 20, the active layer 213 includes a first portion 2131 and a second portion 2132 arranged along the z-axis. The first portion 2131 is located in the gain region D1. When current is injected into the gain region D1, photons are radiated from the first portion 2131 of the gain region D1. The external cavity filter 22 and the side of the gain chip 21 facing away from the external cavity filter 22 form the resonant cavity of the laser 20. The second portion 2132 is located in the phase modulation region D2. Applying a voltage to the phase modulation region D2 adjusts the refractive index of the phase modulation region D2, thereby enabling the laser 20 to output laser light of varying wavelengths. Applying a voltage to the phase modulation region D2 adjusts the refractive index of the phase modulation region D2. The laser is electrically tunable, resulting in a high frequency sweep speed for the laser 20. Furthermore, the central wavelength of the photoluminescence spectrum of the first portion 2131 is different from the central wavelength of the photoluminescence spectrum of the second portion 2132. This can reduce the absorption of photons generated by the gain region D1 by the phase modulation region D2, thereby reducing the loss of the resonant cavity of the laser 20, thereby increasing the quality factor (Q value) of the resonant cavity of the laser 20 and narrowing the linewidth of the output laser light. In other words, the laser can simultaneously achieve a high sweep speed and a narrow linewidth.

[0070] For example, refer to Figure 3 and Figure 5 As shown, in some embodiments, the first portion 2131 includes a quantum well 2131a, wherein when current is injected into the gain region D1, it is necessary to control the gain region D1 to be forward-biased, thereby causing carrier recombination in the quantum well 2131a to radiate photons. The quantum well 2131a in the first portion 2131 is specifically a multi-quantum well (multi-quantum well) structure. In other embodiments, the first portion 2131 may also include quantum dots, wherein when current is injected into the gain region D1, it is necessary to control the gain region D1 to be forward-biased, thereby causing carrier recombination in the quantum dots to radiate photons. The embodiments of the present application are described using the example of the first portion 2131 including the quantum well 2131a.

[0071] For example, the central wavelength of the photoluminescence spectrum of the first portion 2131 is specifically the central wavelength of the photoluminescence spectrum of the quantum well 2131a or quantum dot in the first portion 2131. The photoluminescence spectrum of the quantum well 2131a or quantum dot can reflect the band gap energy of the quantum well 2131a or quantum dot. Based on the photoluminescence spectrum of the quantum well 2131a or quantum dot, the position of the gain peak of photons radiated when current is injected into the gain region D1 can be predicted.

[0072] For example, refer to Figure 5As shown, the first portion 2131 further includes a separation confinement heterojunction (SCH) layer 2131b disposed below the quantum well 2131a along the z-axis, a separation confinement heterojunction layer 2131c disposed above the quantum well 2131a along the z-axis, a barrier layer 2131d disposed on the side of the separation confinement heterojunction layer 2131b away from the quantum well 2131a, and a barrier layer 2131e disposed on the side of the separation confinement heterojunction layer 2131c away from the quantum well 2131a. The barrier layer 2131e is located near the cladding 214, and the barrier layer 2131d is located near the cladding 212. Specifically, when current is injected into the gain region D1, carriers (electrons and holes) are injected into the quantum well 2131a, causing carrier recombination (coincidence effect of electrons and holes) in the quantum well 2131a and radiating photons. The barrier layer 2131d is used to block carriers from being transmitted to the cladding layer 212, and the barrier layer 2131e is used to block carriers from being transmitted to the cladding layer 214. The refractive index distribution of the separated confinement heterojunction layer 2131b, the quantum well 2131a, and the separated confinement heterojunction layer 2131c is low-high-low. The separated confinement heterojunction layer 2131b and the separated confinement heterojunction layer 2131c are used to limit the transmission of photons in the quantum well 2131a along the z-axis in the first portion 2131.

[0073] In this example, the layer structure of the gain chip 21 stacked along the y-axis direction is specifically a semiconductor material layer, and the two sides of the quantum well 2131a of the gain chip 21 along the y-axis direction are respectively an N-type semiconductor material layer side (referred to as the N-type side) and a P-type semiconductor material layer side (referred to as the P-type side). Figure 5 In the example shown in FIG, cladding layer 212 is specifically an N-type cladding layer, barrier layer 2131d is specifically an N-type barrier layer, and separation and confinement heterojunction layer 2131b is an N-type separation and confinement heterojunction layer. Cladding layer 214 is specifically a P-type cladding layer, barrier layer 2131e is specifically a P-type barrier layer, and separation and confinement heterojunction layer 2131c is a P-type separation and confinement heterojunction layer.

[0074] In some embodiments, reference Figure 3 and Figure 5As shown, in order to inject current into the gain region D1 and apply voltage to the phase modulation region D2, the gain chip 21 further includes an electrode 215 and an electrode 216 disposed on a side of the cladding 214 away from the active layer 213, and an electrode 211 disposed on a side of the cladding 212 away from the active layer 213. The electrode 215 and the electrode 216 are not in contact with each other. The electrode 215 and the electrode 211 are used to inject current into the gain region D1 to radiate photons in the quantum well 2131a in the first portion 2131 of the gain region D1; the electrode 216 and the electrode 211 are used to apply voltage to the phase modulation region D2 to adjust the refractive index of the phase modulation region D2. Among them, the present application only illustrates one possible arrangement of electrodes. In some embodiments, electrode 211, electrode 215 and electrode 216 can also be arranged as coplanar electrodes, or electrode 211 can include two sub-electrodes, etc. Electrode 215 and one sub-electrode of electrode 211 are used to inject current into the gain region D1; electrode 216 and the other sub-electrode of electrode 211 are used to apply voltage to the phase modulation region D2. The embodiments of the present application do not limit the arrangement of electrodes.

[0075] In some embodiments, reference Figure 5 As shown, the gain chip 21 further includes a substrate 201 disposed between the electrode layer 211 and the cladding layer 212, and a buffer layer 202 disposed between the cladding layer 212 and the active layer 213. The buffer layer 202 is used to adjust the lattice mismatch between the substrate 201 and the active layer 213 and relieve stress. For example, in the example of the present application, the cladding layer 212 is specifically an N-type side cladding layer, and the substrate 201 is specifically an N-type substrate. In other embodiments, the substrate 201 may be specifically a P-type substrate, the cladding layer 212 may be a P-type side cladding layer, the barrier layer 2131d may be a P-type side barrier layer, the separation and confinement heterojunction layer 2131b may be a P-type side separation and confinement heterojunction layer, the cladding layer 214 may be specifically an N-type side cladding layer, the barrier layer 2131e may be an N-type side barrier layer, and the separation and confinement heterojunction layer 2131c may be an N-type side separation and confinement heterojunction layer.

[0076] In some embodiments, reference Figure 5 As shown, the gain chip 21 further includes a spacer layer 203 and an etch stop layer 204 disposed between the active layer 213 and the cladding layer 214. In the z-axis direction, the etch stop layer 204 is close to the cladding layer 214, and the spacer layer 203 is close to the active layer 213. The spacer layer 203 is also called a filling layer. Figure 3 As shown, the cladding layer 214 needs to be etched so that the dimension of the cladding layer 214 along the x-axis direction is smaller than the dimension of the active layer 213 along the x-axis direction, thereby forming a ridge waveguide structure. When etching the cladding layer 214, the etching is specifically carried out until the etch stop layer 204 is reached. In other examples, a buried waveguide structure may be formed instead of a ridge waveguide structure. The embodiments of the present application take a ridge waveguide structure as an example.

[0077] In some embodiments, reference Figure 5 As shown, gain chip 21 further includes a cap layer (cap) 205 (also referred to as a first cap layer) disposed between electrode 215 and cladding 214, and a cap layer (cap) 206 (also referred to as a second cap layer) disposed between electrode 216 and cladding 214. Cap layer 205 and cap layer 206 do not contact each other, and cap layer 205 is located in gain region D1, while cap layer 206 is located in phase modulation region D2. Cap layer 205 is specifically used to achieve ohmic contact between electrode 215 and cladding 214, while cap layer 206 is specifically used to achieve ohmic contact between electrode 216 and cladding 214.

[0078] The frequency sweep range of the laser 20 is related to the material composition of the gain region D1 and the material composition of the phase modulation region D2. The frequency sweep range of the laser 20 is specifically the wavelength variation range (or frequency variation range) of the laser 20.

[0079] For example, in Figure 3 、 Figure 4 、 Figure 5 In the laser 20 shown, a voltage is applied to the phase modulation region D2 to adjust the refractive index of the phase modulation region D2, thereby changing the wavelength of the laser output by the laser 20. The second portion 2132 specifically includes the following three different structures:

[0080] The first type: the second portion 2132 includes quantum wells or quantum dots.

[0081] When the second portion 2132 includes a quantum well 2132a, the quantum well 2132a is a multi-quantum well (multi-quantum well) structure, and the second portion 2132 further includes a separation and confinement heterojunction layer 2132b disposed below the quantum well 2132a along the z-axis, a separation and confinement heterojunction layer 2132c disposed above the quantum well 2132a along the z-axis, a barrier layer 2132d disposed on the side of the separation and confinement heterojunction layer 2132b away from the quantum well 2131a, and a barrier layer 2132e disposed on the side of the separation and confinement heterojunction layer 2132c away from the quantum well 2132a. The barrier layer 2132e is close to the cladding 214, and the barrier layer 2132d is close to the cladding 212. Separate confinement heterojunction layer 2132b, quantum well 2132a, and separate confinement heterojunction layer 2132c have a low-high-low refractive index profile. Separate confinement heterojunction layer 2132b and separate confinement heterojunction layer 2132c are used to confine photons propagating along the z-axis in second portion 2132. When first portion 2131 envelops quantum well 2131a, the structure of second portion 2132 is similar to that of first portion 2131.

[0082] Exemplarily, the second portion 2132 includes quantum wells 2132 a or quantum dots, and the central wavelength of the photoluminescence spectrum of the second portion 2132 is specifically the central wavelength of the photoluminescence spectrum of the quantum wells 2132 a or quantum dots in the second portion 2132 .

[0083] In this type of structure, the first portion 2131 includes a quantum well 2131a, and the second portion 2132 includes a quantum well 2132a. The center wavelength of the photoluminescence spectrum of the first portion 2131 is specifically the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131, and the center wavelength of the photoluminescence spectrum of the second portion 2132 is specifically the center wavelength of the photoluminescence spectrum of the quantum well 2132a in the second portion 2132. For example, by adjusting the width, composition, or stress of the quantum well 2132a in the second portion 2132, or by changing the energy band center of the quantum well 2132a in the second portion 2132 through quantum well intercalation (QWI) technology, the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131 can be made different from the center wavelength of the photoluminescence spectrum of the quantum well 2132a in the second portion 2132.

[0084] The second portion 2132 includes quantum wells or quantum dots. A reverse bias voltage needs to be applied to the phase modulation region D2 to reverse bias the phase modulation region D2. For example, when the cladding 212 is an N-type side cladding and the cladding 214 is a P-type side cladding, the voltage received by the electrode 211 can be greater than the voltage received by the electrode 216, thereby reverse biasing the phase modulation region D2. When the phase modulation region D2 is reverse biased, a quantum confined Stark effect (QCSE) is generated in the quantum well 2132a or quantum dot, ultimately causing the refractive index of the phase modulation region D2 to change.

[0085] Reference Figure 6 As shown, the embodiment of the present application provides a graph showing how the refractive index of the phase modulation region D2 changes with the applied reverse bias voltage when the second portion 2132 includes a quantum well 2132a. Figure 6 Curve 5 in FIG is a curve showing the change of the reverse bias voltage applied to the phase modulation region D2. Figure 6 Curve 6 in FIG represents a curve showing the percentage change of the refractive index of the phase modulation region D2. Figure 6 The horizontal axis represents time in nanoseconds (ns). Figure 6 The vertical axis in represents the reverse bias voltage applied to the phase modulation region D2, and the unit is volt (V). Figure 6Point A of the curve 6 in FIG. 1 represents the starting point at which the reverse bias voltage is applied to the phase modulation region D2 and the refractive index of the phase modulation region D2 changes. Figure 6 Point B of the curve 6 in FIG. 1 indicates that the refractive index change of the phase modulation region D2 reaches 90% of the target change. Figure 6 Point C of the curve 6 in FIG. 1 indicates that the refractive index change amount of the phase modulation area D2 reaches 100% of the target change amount. Figure 6 Point D of the curve 6 in FIG represents the starting point of the refractive index change of the phase modulation region D2 when the reverse bias voltage is applied to the phase modulation region D2 in the next cycle. According to the time difference between point A and point B, the response time of the phase modulation region D2 is less than or equal to 1 nanosecond (ns), and Figure 6 In the example shown in FIG, the response time of the phase modulation region D2 is approximately equal to 0.3 ns. Figure 6 It can be seen that when the second portion 2132 includes the quantum well 2132 a , the response time of the phase modulation region D2 is shorter, thereby increasing the frequency sweep speed of the laser 20 .

[0086] Reference Figure 7 As shown, the embodiment of the present application provides a schematic diagram of the photoluminescence spectrum of the quantum well 2132a in the second part 2132, Figure 7 The horizontal axis is the wavelength, and the unit is micrometer (μm). Figure 7 The vertical axis represents the material light gain, and the unit is 1 / cm. Figure 7 It can be seen that the central wavelength of the photoluminescence spectrum of the quantum well 2132a in the second portion 2132 is 1420 nm. Furthermore, during simulation, the central wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131 is 1550 nm. This indicates that the central wavelength of the photoluminescence spectrum of the quantum well 2132a in the second portion 2132 is 80 nm shorter than that of the quantum well 2131a in the first portion 2131. This indicates that the phase modulation region D2 absorbs less photons generated in the gain region D1.

[0087] Then the reverse bias voltage applied to the phase modulation area D2 is controlled to change from 0V to 10V, and the simulation results are Figure 8 The effective refractive index change of the phase modulation region D2 is shown as follows, wherein Figure 8 The horizontal axis represents the reverse bias voltage applied to the phase modulation area D2, in volts (V). Figure 8 The vertical axis in represents the effective refractive index change of the phase modulation area D2. Figure 8It can be seen that when the second portion 2132 includes a quantum well 2132a, the change in the effective refractive index of the phase modulation region D2 is positively correlated with the reverse bias voltage applied to the phase modulation region D2. Furthermore, the reverse bias voltage applied to the phase modulation region D2 also generates a microampere-level current, which in turn generates Joule heating, causing the phase modulation region D2 to produce a very weak thermo-optical effect that can be essentially ignored. Because the thermo-optical effect can cause a positive change in the refractive index of the phase modulation region D2, and the quantum confined Stark effect can also cause a positive change in the refractive index of the phase modulation region D2, the laser 20 does not have a phase reversal point during frequency sweeping.

[0088] The second type: the second portion 2132 includes a first bulk waveguide, and dopants are disposed in the first bulk waveguide.

[0089] In this case, refer to Figure 9 As shown, Figure 9 for Figure 3 The cross-section of the laser along AA' is shown, and Figure 9 , the second portion 2132 includes a first bulk waveguide, Figure 9 The structure of the first part 2131 is the same as Figure 5 The structure of the first part 2131 is the same as that of Figure 9 In the embodiment, the structure of the second part 2132 is different from that of the first part 2131, and the dopant in the first body waveguide can specifically be an N-type dopant, which can provide electrons, and the doping concentration of the N-type dopant is less than or equal to 1e18 atoms / cubic centimeter, which is lightly doped.

[0090] In this type of structure, the center wavelength of the photoluminescence spectrum of the first portion 2131 is specifically the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131, and the center wavelength of the photoluminescence spectrum of the second portion 2132 is specifically the center wavelength of the photoluminescence spectrum of the first bulk waveguide. Furthermore, the center wavelength of the photoluminescence spectrum of the second portion 2132 is at least 100 nm shorter than the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131.

[0091] In particular, the second portion 2132 includes a first bulk waveguide. When the dopant in the first bulk waveguide is N-type and lightly doped, a reverse bias voltage needs to be applied to the phase modulation region D2 to reverse bias the phase modulation region D2. For example, when the cladding 212 is an N-type side cladding and the cladding 214 is a P-type side cladding, the voltage received by the electrode 211 can be greater than the voltage received by the electrode 216, thereby reverse biasing the phase modulation region D2. When the phase modulation region D2 is reverse biased, the dopant in the second portion 2132 diffuses outward, reducing the concentration of the dopant in the second portion 2132, thereby changing the refractive index of the phase modulation region D2.

[0092] For example, refer to Figure 10 As shown, the embodiment of the present application provides a concentration change curve of carriers (N-type carriers) in the phase modulation region D2 in the y-axis direction (stacked direction), wherein, Figure 10 The horizontal axis in the figure represents the position in the y-axis direction (stack direction) and the unit is micrometer (μm). Figure 10 The vertical axis represents the concentration of carriers (N-type carriers) in units of 1e18 atoms / cubic centimeter (cm 3 ).in, Figure 10 The position of the rectangular frame shown in the y-axis direction corresponds to the position of the second part 2132 in the y-axis direction. It can be seen from the carrier concentration change curve that when the reverse bias voltage applied to the phase modulation area D2 continues to increase, the carrier concentration in the second part 2132 continues to decrease, so this area is also called the waveguide depletion region.

[0093] Reference Figure 11 As shown, an embodiment of the present application provides a graph showing the effective refractive index change of the phase modulation region D2 as a function of the reverse bias voltage applied to the phase modulation region D2, wherein: Figure 11 The horizontal axis represents the reverse bias voltage applied to the phase modulation area D2, in volts (V). Figure 11 The vertical axis in represents the effective refractive index change of the phase modulation area D2. Figure 11 It can be seen that the refractive index of the phase modulation zone D2 is positively correlated with the reverse bias voltage applied to the phase modulation zone D2, and when the reverse bias voltage applied to the phase modulation zone D2 is greater than or equal to 4V, the carriers in the phase modulation zone D2 are approximately cleared, and the refractive index of the phase modulation zone D2 does not change much.

[0094] Applying a reverse bias voltage to the phase modulation region D2 also generates a microampere-level current, which in turn generates Joule heating. This produces a very weak, essentially negligible thermo-optic effect in the phase modulation region D2. Because the thermo-optic effect can cause a positive change in the refractive index of the phase modulation region D2, and because changes in carrier concentration in this example can also cause a positive change in the refractive index of the phase modulation region D2, the laser 20 does not experience a phase reversal point during frequency sweeping.

[0095] The third type: the second portion 2132 includes a first bulk waveguide.

[0096] In this case, Figure 3 The cross-sectional view of the laser along AA' is shown in FIG. Figure 9 As shown, the difference from the second type of structure is that in the third type of structure, no additional dopant is provided in the first body waveguide included in the second part 2132.

[0097] In this type of structure, the center wavelength of the photoluminescence spectrum of the first portion 2131 is specifically the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131, and the center wavelength of the photoluminescence spectrum of the second portion 2132 is specifically the center wavelength of the photoluminescence spectrum of the first bulk waveguide. Furthermore, the center wavelength of the photoluminescence spectrum of the second portion 2132 is at least 100 nm shorter than the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first portion 2131.

[0098] In particular, when the second portion 2132 includes a first bulk waveguide and no additional dopants are provided in the first bulk waveguide, a forward bias voltage needs to be applied to the phase modulation region D2 to forward-bias the phase modulation region D2. For example, when the cladding 212 is an N-type side cladding and the cladding 214 is a P-type side cladding, the voltage received by the electrode 211 can be less than the voltage received by the electrode 216, thereby forward-biasing the phase modulation region D2. When the phase modulation region D2 is forward-biased, the carrier concentration in the phase modulation region D2 continuously increases, thereby causing the refractive index of the phase modulation region D2 to change.

[0099] Exemplarily, the second portion 2132 includes a first bulk waveguide, in which no additional dopants are disposed. When a forward bias voltage is applied to the phase modulation region D2, the refractive index of the phase modulation region D2 is negatively correlated with the forward bias voltage applied to the phase modulation region D2. The forward bias voltage applied to the phase modulation region D2 generates a current in the milliampere range or even tens of milliamperes, thereby generating Joule heat, causing the phase modulation region D2 to produce a thermo-optical effect. The thermo-optical effect can cause a positive change in the refractive index of the phase modulation region D2. In this example, a change in carrier concentration can cause a negative change in the refractive index of the phase modulation region D2. In this type of structure, the change in refractive index caused by the change in carrier concentration is greater than the change in refractive index caused by the thermo-optical effect. Therefore, the laser 20 does not experience a phase reversal point during frequency sweeping.

[0100] For example, refer to Figure 3 and Figure 4 As shown, the laser 20 includes an external cavity filter 22, and the external cavity filter 22 includes a filter structure 221; the filter structure 221 includes any of the following: a grating, a micro ring resonator (MRR), a multi-channel interference filter (MCI), etc. For example, refer to Figure 4 As shown, Figure 4 The filter structure 221 shown is specifically a grating. The grating can be an on-chip integrated grating or a fiber grating, and the structure of the grating can be a distance Bragg grating (DBR) structure. Figure 12 As shown, compared with Figure 4 The laser 20 shown, Figure 12 The filter structure 221 in the laser 20 shown is specifically an MRR.

[0101] The external cavity filter 22 includes a waveguide, which is used to guide light to propagate within the external cavity filter 22, and then pass through the filtering structure 221 and other structures within the external cavity filter 22. The material of the external cavity filter 22 includes any of the following: silicon, silicon nitride, silicon-based silicon dioxide, and lithium niobate. This also means that the material of the waveguide within the external cavity filter 22 includes any of the following: silicon, silicon nitride, silicon-based silicon dioxide, and lithium niobate. When the external cavity filter 22 includes silicon, the external cavity filter 22 is also referred to as a silicon optical integrated structure; when the external cavity filter 22 includes silicon-based silicon dioxide, the external cavity filter 22 is also referred to as a planar light waveguide (PLC) integrated structure.

[0102] Reference Figure 13 As shown, the embodiment of the present application provides a graph showing how the line width of the laser output by the laser 20 changes with the current injected into the gain region D1 when the loss coefficient of the waveguide in the external cavity filter 22 is different. Figure 13 The horizontal axis represents the magnitude of the current injected into the gain region D1, in milliamperes (mA). Figure 13 The vertical axis in represents the line width of the laser output by the laser 20. Figure 13 Curve 7 in FIG. 2 corresponds to a curve showing that the loss coefficient of the waveguide in the external cavity filter 22 is 5 dB / cm, and the line width of the laser output by the laser 20 varies with the current injected into the gain region D1; Figure 13 Curve 8 in FIG. 2 corresponds to a curve showing that the loss coefficient of the waveguide in the external cavity filter 22 is 4.5 dB / cm, and the line width of the laser output by the laser 20 varies with the current injected into the gain region D1; Figure 13 Curve 9 in FIG. 2 corresponds to a curve showing that the loss coefficient of the waveguide in the external cavity filter 22 is 4 dB / cm, and the line width of the laser output by the laser 20 varies with the current injected into the gain region D1; Figure 13 Curve 10 in FIG. 1 is a curve showing the change in line width of the laser output by the laser 20 with the current injected into the gain region D1, when the loss coefficient of the waveguide in the external cavity filter 22 is 3.5 dB / cm. Figure 13It can be seen that when the current injected into gain region D1 is fixed, the smaller the loss coefficient of the waveguide in the external cavity filter 22 of laser 20, the narrower the linewidth of the laser output by laser 20. The material of the typical external cavity filter 22 is indium phosphide (InP), and the loss coefficient of the InP waveguide is approximately 5dB / cm. When the material of the external cavity filter 22 is silicon nitride (SiN), the loss coefficient of the SiN waveguide is 0.01dB / cm to 0.02dB / cm; when the material of the external cavity filter 22 is silicon-based silica, the loss coefficient of the SiN waveguide is 0.01dB / cm to 0.02dB / cm, which is more than two orders of magnitude lower than the loss coefficient of the InP waveguide. When the material of the external cavity filter 22 includes any of the following: silicon, silicon nitride, silicon-based silicon dioxide, and lithium niobate, the loss of the resonant cavity of the laser 20 can be reduced, the Q value of the resonant cavity of the laser 20 can be improved, and the line width of the laser output by the laser 20 can be narrower.

[0103] Reference Figure 14 As shown, an embodiment of the present application provides a frequency noise power spectrum density diagram of the laser output by the laser 20 when the waveguide length of the external cavity filter 22 is 5 mm and the loss coefficient of the waveguide is 0.02 dB / cm, wherein, Figure 14 The horizontal axis in represents the frequency of the laser output by the laser 20, in Hertz (Hz). Figure 14 The vertical axis in the figure represents the noise power spectrum density of the laser output by the laser 20, in units of (Hz 2 / Hz). Figure 14 Point E in the figure is the base position of the high-frequency noise, and its intensity is related to the line width (Lorentz line width) of the laser output by the laser 20. Figure 14 The noise power spectrum density at point E is 342.8723Hz 2 / Hz. Calculation shows that the linewidth of the laser output by the laser 20 corresponding to the noise power spectral density is 1.077kHz. When the material of the external cavity filter 22 is indium phosphide (InP), the linewidth of the laser output by the laser is generally tens of kHz to 100 kHz, which is two orders of magnitude higher than that of the embodiment of the present application.

[0104] The resonant cavity of the laser 20 is specifically along the z-axis direction, from the side of the filter structure 221 away from the gain chip 21 to the side of the gain chip 21 away from the filter structure 221 .

[0105] In some embodiments, the external cavity filter 22 further includes a metal heating strip disposed on the filter structure 221. The metal heating strip is connected to the control electrode, and applying a voltage to the metal heating strip through the control electrode can cause the metal heating strip to heat the filter structure 221, thereby changing the performance of the filter structure 221, thereby changing the parameters such as the frequency sweep range of the laser output by the laser 20. For example, when no voltage is applied to the metal heating strip through the control electrode, the initial value of the wavelength of the laser output by the laser 20 is a first value, which has a certain error with the target value (the reason for this may be that there is a process error when the filter structure 221 is manufactured). Then, a voltage can be applied to the metal heating strip through the control electrode to change the parameters of the filter structure, thereby making the initial value of the wavelength of the laser output by the laser 20 the target value. Subsequently, a voltage is applied to the phase modulation area D2 to adjust the refractive index of the phase modulation area D2 to change the wavelength of the laser output by the laser 20 and achieve frequency sweep.

[0106] In order to reduce the loss of the resonant cavity of the laser 20, increase the Q value of the resonant cavity of the laser 20, and further optimize the line width of the laser output by the laser 20, the present application proposes the following embodiments:

[0107] Example 1, reference Figure 4 or Figure 12 As shown, the gain chip 21 further includes an anti-reflection film m1 disposed at one end of the gain chip 21 close to the external cavity filter 22, wherein the anti-reflection film m1 increases the transmission ratio of photons between the phase modulation region D2 and the external cavity filter 22, so as to reduce the Fabry-Perot (FP) effect generated at both ends of the gain chip 21 along the x-axis direction.

[0108] Example 2: Reference Figure 15 As shown, Figure 15 for Figure 3 The cross-section along AA' is Figure 15In the embodiment, the active layer 213 further includes a second bulk waveguide 2133, which is disposed between the first portion 2131 and the second portion 2132 along the z-axis. One end of the second bulk waveguide 2133 is connected to the first portion 2131, and the other end of the second bulk waveguide 2133 is connected to the second portion 2132. The second bulk waveguide 2133 is configured to match the mode field between the first portion 2131 and the second portion 2132. Specifically, the second bulk waveguide 2133 is provided with a microstructure formed by etching, thereby enabling the second bulk waveguide 2133 to match the mode field between the first portion 2131 and the second portion 2132. The embodiments of the present application do not limit the size of the first portion 2131 along the y-axis, nor do they limit the size of the second portion 2132 along the y-axis. When the size of the first portion 2131 along the y-axis differs from the size of the second portion 2132 along the y-axis, the second bulk waveguide 2133 can be used to match the mode fields between the first portion 2131 and the second portion 2132, thereby reducing the transmission loss of photons between the gain region D1 and the phase modulation region D2. Exemplary materials for the second bulk waveguide 2133 include indium gallium arsenide phosphide or indium phosphide gallium arsenide quaternary material.

[0109] Among them, the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first part 2131 is different from the center wavelength of the photoluminescence spectrum of the second bulk waveguide 2133; the difference between the center wavelength of the photoluminescence spectrum of the quantum well 2131a in the first part 2131 and the center wavelength of the photoluminescence spectrum of the second bulk waveguide 2133 is greater than or equal to 100 nm, and the second bulk waveguide 2133 has little absorption of photons generated by the gain region D1.

[0110] Exemplarily, when the active layer 213 further includes a second bulk waveguide 2133 and the second portion 2132 includes a first bulk waveguide, the second bulk waveguide 2133 and the second portion 2132 are integrally provided, for example, by depositing a bulk waveguide material once, thereby forming the first bulk waveguide and the second bulk waveguide 2133 in the bulk waveguide material. For example, a microstructure may be formed on a portion of the bulk waveguide material through an etching process to form the second bulk waveguide 2133. When additional dopants are required in the first bulk waveguide included in the second portion 2132, ion implantation may be used to implant a dopant with a concentration of less than or equal to 1e18 atoms / cm3 into another portion of the bulk waveguide material. 3 ) to form a first body waveguide (ie, the second portion 2132).

[0111] Example 3: Reference Figure 16 , Figure 16 for Figure 3In the top view of the laser 20 shown, the portion of the cladding 214 near the external cavity filter 22 is bent in a plane perpendicular to the stacking direction. Specifically, the portion of the cladding 214 near the external cavity filter 22 is bent in a plane formed by the x-axis direction and the y-axis direction. The bending of the portion of the cladding 214 near the external cavity filter 22 can further reduce the FP effect generated at both ends of the gain chip 21 along the z-axis direction.

[0112] Example 4: Reference Figure 4 or Figure 12 or Figure 16 As shown, the external cavity filter 22 further includes a mode field matching structure 222 (also referred to as a first mode field matching structure) disposed on a side of the filter structure 221 close to the gain chip 21; the mode field matching structure 222 is used to match the mode field between the gain chip 21 and the filter structure 221 to reduce the transmission loss of photons between the gain chip 21 and the filter structure 221. Exemplarily, the mode field matching structure 222 includes a spot size converter (SSC), a conical lens, a lens group, a bonding structure formed by optical wire bonding technology, a reflector, etc. Figure 17 As shown, Figure 17 The illustrated mode field matching structure 222 specifically includes a conical lens.

[0113] Example 5: Reference Figure 4 or Figure 12 or Figure 16 or Figure 17 As shown, the external cavity filter 22 further includes an antireflection film m2 disposed on a side of the external cavity filter 22 close to the gain chip 21 . The antireflection film m2 increases the transmission ratio of photons between the gain chip 21 and the external cavity filter 22 .

[0114] Among them, the same laser 20 can include one or more of the above-mentioned embodiments 1 to 5, thereby reducing the loss of the resonant cavity of the laser 20 and increasing the Q value of the resonant cavity of the laser 20. The embodiments of the present application are not limited to this.

[0115] In some embodiments, the gain chip 21 and the external cavity filter 22 are independent components with a gap therebetween. When photons propagate between the gain chip 21 and the external cavity filter 22, they pass through the gap between the gain chip 21 and the external cavity filter 22. In other embodiments, the gain chip 21 and the external cavity filter 22 may be an integrated structure fabricated by heteroepitaxial growth or heterogeneous integration. In this case, no gap exists between the gain chip 21 and the external cavity filter 22. When providing an antireflection film, only the antireflection film m1 or the antireflection film m2 may be provided, or the antireflection films m1 and m2 may be mounted together.

[0116] For example, refer to Figure 4 or Figure 12 or Figure 16 or Figure 17 As shown, the gain chip 21 further includes a high-reflection film m3 disposed on the end of the gain chip 21 away from the external cavity filter 22, so that the output end of the laser 20 can be located on the side of the external cavity filter 22 away from the gain chip 21. Exemplarily, in this case, the external cavity filter 22 may further include a mode field matching structure 224 (also referred to as a second mode field matching structure) disposed on the side of the filter structure 221 away from the gain chip 21; the mode field matching structure 224 is used to adjust the mode field of the laser output by the filter structure 221. The mode field matching structure 224 includes a spot size converter (SSC), a conical lens, a lens group, a bonding structure formed by optical wire bonding technology, a reflector, and the like. For example, when the laser output by the filter structure 221 needs to be transmitted through an optical fiber or an optical waveguide, the mode field matching structure 224 is used to adjust the mode field of the laser output by the filter structure 221, so that the laser can be better coupled into the optical fiber or the optical waveguide; when the laser output by the filter structure 221 needs to be transmitted through space to another device, the mode field matching structure 224 is used to adjust the mode field of the laser output by the filter structure 221, so that the laser can be better transmitted to another device, thereby improving the matching degree between the laser 20 and other devices.

[0117] In other examples, refer to Figure 18 As shown, the end of the gain chip 21 away from the external cavity filter 22 may not be provided with a high-reflection film, and this end is a natural cleavage surface, so that the output end of the laser 20 can be located on the side of the gain chip 21 away from the external cavity filter 22.

[0118] Exemplarily, an embodiment of the present application further provides an optical phase-locked loop (OPLL) system, the OPLL system including the laser 20 and the optical phase-locked loop shown in any of the above figures. Wherein, the optical phase-locked loop receives the first laser output by the laser 20, and adjusts the control signal transmitted to the laser 20 according to the phase of the first laser. Specifically, the control signal of the laser 20 includes a voltage applied to the gain region D1 or a voltage applied to the phase modulation region D2, thereby causing the laser 20 to output a second laser according to the control signal, with the phase noise of the second laser. Wherein, since the laser 20 has no phase reversal point, the optical phase-locked loop can suppress the phase noise generated by the low and high frequencies of the modulation frequency, thereby increasing the loop bandwidth of the OPLL system.

[0119] Although the present application has been described with reference to specific features and embodiments thereof, it is apparent that various modifications and combinations may be made thereto without departing from the spirit and scope of the present application. Accordingly, this specification and the drawings are merely illustrative of the present application as defined by the appended claims and are deemed to cover any and all modifications, variations, combinations or equivalents within the scope of the present application. Obviously, those skilled in the art may make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, the present application is intended to include such modifications and variations as fall within the scope of the claims of the present application and their equivalents.

Claims

1. A laser, characterized in that: Including gain chip and external cavity filter; The gain chip comprises a first cladding layer, an active layer, and a second cladding layer stacked in sequence; The gain chip includes a gain region and a phase modulation region arranged along a first direction, wherein the first direction is perpendicular to the stacking direction; The active layer includes a first portion and a second portion arranged along the first direction; the first portion is located in the gain region, and the second portion is located in the phase modulation region; a central wavelength of the photoluminescence spectrum of the first portion is different from a central wavelength of the photoluminescence spectrum of the second portion; The external cavity filter is coupled to an end of the phase modulation region in the gain chip away from the gain region.

2. The laser according to claim 1, characterized in that The central wavelength of the photoluminescence spectrum of the first portion is greater than the central wavelength of the photoluminescence spectrum of the second portion.

3. The laser according to claim 2, characterized in that A difference between a central wavelength of the photoluminescence spectrum of the first portion and a central wavelength of the photoluminescence spectrum of the second portion is greater than or equal to 60 nanometers.

4. The laser according to any one of claims 1 to 3, characterized in that The second portion includes a quantum well or a quantum dot; and a reverse bias voltage is applied to the phase modulation region.

5. The laser according to any one of claims 1 to 3, characterized in that: The second portion includes a first bulk waveguide in which dopants are arranged; and a reverse bias voltage is applied to the phase modulation region.

6. The laser according to claim 5, characterized in that The dopant is an N-type dopant, and the doping concentration of the dopant is less than or equal to 1e18 atoms / cubic centimeter.

7. The laser according to any one of claims 1 to 3, characterized in that: The second portion includes a first bulk waveguide; and a forward bias voltage is applied to the phase modulation region.

8. The laser according to any one of claims 1 to 7, characterized in that: The active layer further includes a second bulk waveguide, which is arranged between the first portion and the second portion along the first direction, one end of the second bulk waveguide is connected to the first portion, and the other end of the second bulk waveguide is connected to the second portion; The second bulk waveguide is used for matching the mode field between the first portion and the second portion.

9. The laser according to claim 8, characterized in that A central wavelength of the photoluminescence spectrum of the first portion is greater than a central wavelength of the photoluminescence spectrum of the second bulk waveguide.

10. The laser according to claim 9, characterized in that A difference between a center wavelength of the photoluminescence spectrum of the first portion and a center wavelength of the photoluminescence spectrum of the second bulk waveguide is greater than or equal to 100 nanometers.

11. The laser according to any one of claims 8 to 10, characterized in that: The second bulk waveguide is provided with a microstructure formed by etching.

12. The laser according to any one of claims 8 to 11, characterized in that: The second portion includes a first bulk waveguide, and the first bulk waveguide and the second bulk waveguide are integrally arranged.

13. The laser according to any one of claims 1 to 12, characterized in that: A portion of the second cladding layer close to the external cavity filter is bent in a plane perpendicular to a stacking direction.

14. The laser according to any one of claims 1 to 13, characterized in that: The gain chip further includes an antireflection film arranged at one end of the gain chip close to the external cavity filter.

15. The laser according to claim 14, characterized in that The gain chip further comprises a high-reflection film arranged at an end of the gain chip away from the external cavity filter.

16. The laser according to any one of claims 1 to 15, characterized in that The gain chip further includes a first electrode and a second electrode disposed on a side of the second cladding away from the active layer, and a third electrode disposed on a side of the first cladding away from the active layer. The first electrode and the second electrode are not in contact with each other. The first electrode and the third electrode are used to inject current into the gain region; the second electrode and the third electrode are used to apply voltage to the phase modulation region.

17. The laser according to claim 16, characterized in that The gain chip further includes a first cap layer disposed between the second cladding layer and the first electrode, and a second cap layer disposed between the second cladding layer and the second electrode, wherein the first cap layer is not in contact with the second cap layer.

18. The laser according to any one of claims 1 to 17, characterized in that: The external cavity filter includes an antireflection film arranged at one end of the external cavity filter close to the gain chip.

19. The laser according to any one of claims 1 to 18, characterized in that: The external cavity filter includes a filtering structure; the filtering structure includes any one of the following: a grating, a microring resonator, and a multi-channel interference filter.

20. The laser according to claim 19, characterized in that The external cavity filter further includes a metal heating strip arranged on the filtering structure.

21. The laser according to claim 19 or 20, characterized in that The external cavity filter further includes a first mode field matching structure provided on a side of the filter structure close to the gain chip; The first mode field matching structure is used to match the mode field between the gain chip and the filtering structure.

22. The laser according to any one of claims 19 to 21, characterized in that The external cavity filter further comprises a second mode field matching structure arranged on a side of the filter structure away from the gain chip; The second mode field matching structure is used to adjust the mode field of the laser output by the filtering structure.

23. The laser according to any one of claims 1 to 22, characterized in that The material of the external cavity filter includes any one of the following: silicon, silicon nitride, silicon-based silicon dioxide, and lithium niobate.

24. An optical phase-locked loop system, characterized in that: comprising an optical phase-locked loop and a laser according to any one of claims 1 to 23; The optical phase-locked loop is configured to receive the first laser light output by the laser and adjust the control signal transmitted to the laser according to the phase of the first laser light; The laser is used to output a second laser according to the control signal.

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