Littrow external cavity tunable laser

Abstract

The application provides a Littman external cavity tunable laser, and relates to the technical field of lasers.The Littman external cavity tunable laser comprises a semiconductor photoelectric device for outputting a wide-spectrum light signal; a light condensing module for collimating and focusing the wide-spectrum light signal and projecting focused light to a mode spot conversion polarization rotation beam splitter; the mode spot conversion polarization rotation beam splitter is used for adapting the mode spot size and regulating the polarization state of the focused light, filtering out non-target polarization components, and outputting a target polarization light signal with a single polarization state, and is also used for adapting the mode spot size of reflected external cavity feedback light; and a tuning light emitting module is used for spatially dispersing and separating different wavelength components, generating zero-order diffraction light as laser output light, and generating first-order diffraction light as external cavity feedback light, and realizing laser wavelength tuning by changing the wavelength of the external cavity feedback light. The application can solve the defect of unstable polarization state in the tuning process of a traditional Littman laser.

Description

Technical Field

[0001] This application relates to the field of laser technology, and more particularly to a Littman external cavity tunable laser. Background Technology

[0002] The field of integrated circuit manufacturing has placed more stringent requirements on the output characteristics of lasers. Not only do they need to achieve single-frequency output with narrow linewidth and high side-mode rejection ratio, but they also need to have tunability with a wide range of mode-free switching capabilities. At the same time, the output beam direction must be stable, the structure compact, and the loss controllable to adapt to the integration and application needs of different scenarios.

[0003] The core structure of the Littman laser is based on the traditional semiconductor laser, incorporating a diffraction grating and an independently tuned mirror to form an external cavity resonant system. The Littman external cavity tunable laser, with its advantages of wide tuning range, narrow linewidth, and compact structure, has been widely used in various fields.

[0004] However, traditional Littman lasers suffer from severe polarization mode competition and unstable polarization states during tuning, which can easily lead to fluctuations in the output power and wavelength of the external cavity laser. Summary of the Invention

[0005] In view of this, this application proposes a Littman external cavity tunable laser.

[0006] The technical solution of this application is implemented as follows: A Littman external cavity tunable laser includes: a semiconductor optoelectronic device, a focusing module, a mode-spot conversion polarization rotating beam splitter and a tuned output module arranged sequentially along the laser propagation direction; The semiconductor optoelectronic device is used to output broadband optical signals; The focusing module is used to receive the broadband optical signal, collimate and focus the broadband optical signal, and project the focused light onto the mode-spot conversion polarization rotating beam splitter. The mode-spot conversion polarization rotating beam splitter is used to adapt the mode size and control the polarization state of the focused light, filter out non-target polarization components, and output a target polarized light signal with a single polarization state. The tuned output module is used to perform spatial dispersion separation of different wavelength components, generate zero-order diffraction light as laser output light, and generate first-order diffraction light as external cavity feedback light, and achieve laser wavelength tuning by changing the wavelength of the external cavity feedback light. The mode-spot conversion polarization rotating beam splitter is also used to adapt the mode spot size of the reflected external cavity feedback light.

[0007] In one embodiment, the semiconductor optoelectronic device is a superluminescent diode.

[0008] In one embodiment, the focusing module includes: a first collimating optical unit and a first focusing optical unit; The first collimating optical unit is an aspherical collimating lens, used to receive the broadband optical signal, collimate the broadband optical signal into parallel light, and then project it to the first focusing optical unit; The first focusing optical unit is an aspherical focusing lens, used to focus the parallel light output from the first collimating optical unit onto the incident end face of the polarization rotating beam splitter with mode spot conversion.

[0009] In one embodiment, the mode-spot conversion polarization rotating beam splitter includes: a substrate, a lower cladding layer, a waveguide core layer, and an upper cladding layer; The lower cladding layer is located on the upper surface of the substrate; The waveguide core layer is located on the surface of the lower cladding layer away from the substrate. The waveguide core layer includes a first mode conversion region, a polarization rotation region, a polarization beam splitting region, and a second mode conversion region. The first mode conversion region is used to adapt the mode size of the focused light. The polarization rotation region is used to rotate the polarization state and fine-tune the angle of the target polarization component. The polarization beam splitting region is used to separate and filter out non-target polarization components, retaining the target polarized light signal with a single polarization state. The second mode conversion region is used to output the target polarized light signal and adapt the mode size of the reflected external cavity feedback light. The upper cladding is located on the side of the waveguide core layer away from the substrate, and the upper cladding covers the waveguide core layer.

[0010] In one embodiment, the first mode conversion region and the second mode conversion region respectively adopt an adiabatic conical waveguide structure; The waveguide width of the first mode conversion region gradually changes from wide to narrow along the light propagation direction, while the waveguide width of the second mode conversion region gradually changes from narrow to wide along the light propagation direction.

[0011] In one embodiment, the polarization rotation region includes a tilted ridge waveguide structure and a microheater; The inclined ridge waveguide structure is connected to the first mode conversion region and the polarization beam splitting region, respectively. The microheater is disposed on the upper cladding and is used to adjust the effective refractive index difference of the tilted ridge waveguide structure by thermal tuning, so as to adjust the polarization rotation angle.

[0012] In one embodiment, the polarization beam splitting region employs an asymmetric directional coupler structure, which includes a main waveguide and a sub-waveguide. The first ends of the main waveguide and the sub-waveguide are respectively connected to the polarization rotation region, the second end of the sub-waveguide is connected to the on-chip optical absorption region, and the second end of the main waveguide is connected to the second mode conversion region.

[0013] In one embodiment, the tuned output module includes: a second collimating optical unit, a blazed grating, a MEMS mirror, an optical isolation unit, a second focusing optical unit, and an output fiber unit; The second collimating optical unit is used to collimate the optical signal output from the mode-spot-converting polarization rotating beam splitter into parallel light and project it onto the blazed grating; The blazed grating is used to disperse the incident parallel light, generating first-order diffraction feedback light and zero-order diffraction light. The MEMS mirror is used to reflect the first-order diffraction feedback light generated by the blazed grating, so that the first-order diffraction feedback light returns to the semiconductor optoelectronic device along the original optical path through the blazed grating, the mode-spot conversion polarization rotating beam splitter and the focusing module, forming a stable external cavity resonance; The optical isolation unit is used to receive the zeroth-order diffracted light and project it onto the second focusing optical unit; the optical isolation unit is also used to isolate the reflected light from the laser output end. The second focusing optical unit is used to focus the parallel light output from the optical isolation unit onto the core of the output optical fiber unit, so as to output single-mode laser through the output optical fiber unit.

[0014] In one embodiment, the extended plane lines of the semiconductor optoelectronic device, the blazed grating, and the MEMS mirror intersect at a point, which is used as a virtual rotation center. The MEMS mirror rotates around the virtual rotation center during the tuning process.

[0015] In one embodiment, the Littman external cavity tunable laser further includes a temperature control unit for stabilizing the temperature of the semiconductor optoelectronic device, the mode-spot conversion polarization rotating beam splitter, and the blazed grating.

[0016] The Littman external cavity tunable laser of this application has the following advantages over related technologies: 1. The Littman external cavity tunable laser of this application comprises a semiconductor optoelectronic device, a focusing module, a mode-spot conversion polarization rotating beam splitter, and a tuned output module arranged sequentially along the laser propagation direction. The semiconductor optoelectronic device outputs a broadband optical signal, which is collimated and focused by the focusing module and then projected onto the mode-spot conversion polarization rotating beam splitter. The mode-spot conversion polarization rotating beam splitter completes the mode-spot size adaptation and polarization state control of the focused light, filters out non-target polarization components, and outputs a single polarization state target polarized light signal. Then, the tuned output module performs spatial dispersion separation on the target polarized light signal, using the zero-order diffracted light as the output light and the first-order diffracted light as the external cavity feedback light. By changing the wavelength of the external cavity feedback light, the laser achieves mode-hopping-free wavelength tuning, and finally stably outputs a single-mode laser with narrow linewidth, high polarization purity, and low power jitter.

[0017] 2. By using a built-in polarization rotation beam splitter with mode conversion, polarization mode competition is suppressed at the source, completely solving the defect of unstable polarization state during the tuning process of traditional Littman lasers, avoiding jitter in output power and wavelength, and ensuring laser output stability; the mode size adaptation function can improve the optical field coupling efficiency, and the mode adaptation of external cavity feedback light can optimize the resonant feedback effect. Combined with broadband light output and precise wavelength tuning, the output quality and operational reliability of the laser are comprehensively improved. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments or related technologies of this application, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a Littman external cavity tunable laser in one embodiment of this application; Figure 2 This is a schematic cross-sectional view of a mode-spot conversion polarization rotating beam splitter in one embodiment of this application; Figure 3 This is a schematic diagram of the regional structure distribution of the waveguide core layer in one embodiment of this application; Figure 4 This is a schematic diagram of the tuning process of a Littman external cavity tunable laser in one embodiment of this application; Figure 5 This is a schematic diagram of the wavelength drift curve of a Littman external cavity tunable laser at a working wavelength of 635nm in one embodiment of this application.

[0020] Explanation of reference numerals in the attached figures: 1-Semiconductor optoelectronic device, 2-Focusing module, 21-First collimating optical unit, 22-First focusing optical unit, 3-Mode-spot conversion polarization rotating beam splitter, 31-Substrate, 32-Lower cladding, 33-Waveguide core layer, 4-Tune output module, 41-Second collimating optical unit, 42-Blazed grating, 43-MEMS mirror, 44-Optical isolation unit, 45-Second focusing optical unit, 46-Output fiber unit. Detailed Implementation

[0021] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the embodiments of this application. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0022] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0023] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0024] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0025] In some embodiments, such as Figure 1As shown, this application provides a Littman external cavity tunable laser, comprising: a semiconductor optoelectronic device 1, a focusing module 2, a mode-spot conversion polarization rotating beam splitter 3, and a tuned output module 4 arranged sequentially along the laser propagation direction.

[0026] Semiconductor optoelectronic device 1 is used to output a broadband optical signal. Focusing module 2 receives the broadband optical signal, collimates and focuses it, and projects the focused light onto a mode-spot conversion polarization rotating beam splitter 3. Mode-spot conversion polarization rotating beam splitter 3 is used to adapt the mode size and control the polarization state of the focused light, filtering out non-target polarization components and outputting a single-polarization-state target-polarized optical signal. Tuning output module 4 is used to perform spatial dispersion separation of different wavelength components, generating zero-order diffracted light as the laser output light, and generating first-order diffracted light as the external cavity feedback light, and achieving laser wavelength tuning by changing the wavelength of the external cavity feedback light. The mode-spot conversion polarization rotating beam splitter 3 is also used to adapt the mode size of the reflected external cavity feedback light.

[0027] Among them, the semiconductor optoelectronic device 1 can be a superluminescent diode (SLD). The superluminescent diode operates based on the principle of spontaneous emission amplification (ASE) of the active region of semiconductors. It combines the high output power and high brightness of laser diodes with the wide spectrum and low coherence of light-emitting diodes. It can cover the entire wavelength range from visible light to near infrared, with an output spectral width of 15~100nm. It can provide sufficient and stable broadband optical signals for laser wavelength tuning, and fundamentally solve the defects of traditional gain chips with single wavelength and narrow gain bandwidth.

[0028] It is understandable that the focusing module 2 is responsible for the transmission and shaping of the optical signal. It can efficiently receive the broadband optical signal emitted by the semiconductor optoelectronic device 1, first convert the divergent broadband light into parallel light through collimation to reduce transmission loss, and then focus the parallel light into a matched and coupled focused light through focusing. This light is then accurately projected onto the mode-converting polarization rotating beam splitter 3, ensuring that the optical signal is transmitted to the core functional device with low loss and high accuracy. The mode-converting polarization rotating beam splitter 3 has a dual function of mode matching and polarization control. It performs mode size matching processing on the incident focused light, so that the optical field mode is efficiently matched with the subsequent optical path structure. At the same time, it performs polarization state control on the optical signal, accurately filtering out non-target polarization components and outputting a pure single polarization state target polarized optical signal. In addition, this device can also perform mode size matching on the external cavity feedback light reflected back along the optical path, ensuring that the feedback light can be smoothly transmitted back to maintain resonance stability. The tuned output module 4 is responsible for wavelength separation, laser output and wavelength tuning. It can perform spatial dispersion separation of different wavelength components in a single polarized light signal, use the separated zero-order diffracted light as the laser output, generate first-order diffracted light as the feedback light to maintain the external cavity resonance, and achieve flexible tuning of the laser output wavelength by changing the wavelength of the external cavity feedback light to adapt to different application requirements.

[0029] The aforementioned Littman external cavity tunable laser comprises, along the laser propagation direction, a semiconductor optoelectronic device 1, a focusing module 2, a mode-spot conversion polarization rotating beam splitter 3, and a tuned output module 4. The semiconductor optoelectronic device 1 outputs a broadband optical signal, which is collimated and focused by the focusing module 2 and then projected onto the mode-spot conversion polarization rotating beam splitter 3. The mode-spot conversion polarization rotating beam splitter 3 completes the mode-spot size adaptation and polarization state control of the focused light, filters out non-target polarization components, and outputs a single-polarization target polarized light signal. Then, the tuned output module 4 performs spatial dispersion separation on the target polarized light signal, using the zero-order diffracted light as the output light and the first-order diffracted light as the external cavity feedback light. By changing the wavelength of the external cavity feedback light, the laser achieves mode-hopping-free wavelength tuning, ultimately outputting a stable single-mode laser with narrow linewidth, high polarization purity, and low power jitter. By incorporating a built-in polarization rotation beam splitter with mode conversion, polarization mode competition is suppressed at the source, completely solving the defect of unstable polarization state during the tuning process of traditional Littman lasers, avoiding jitter in output power and wavelength, and ensuring laser output stability. The mode size adaptation function can improve the optical field coupling efficiency, and the mode adaptation of the external cavity feedback light can optimize the resonant feedback effect. Combined with broadband light output and precise wavelength tuning, the output quality and operational reliability of the laser are comprehensively improved.

[0030] In some embodiments, the focusing module 2 includes a first collimating optical unit 21 and a first focusing optical unit 22.

[0031] The first collimating optical unit 21 is an aspherical collimating lens used to receive broadband optical signals and collimate the broadband optical signals into parallel light before projecting them to the first focusing optical unit 22.

[0032] The first focusing optical unit 22 is an aspherical focusing lens used to focus the parallel light output from the first collimating optical unit 21 onto the incident end face of the polarization rotating beam splitter 3 with mode spot conversion.

[0033] In applications, the first collimating optical unit 21 has a collimation accuracy of no less than 0.3 mrad, its surface is coated with an anti-reflection film for the corresponding wavelength band, and its single-sided reflectivity is no higher than 0.2%. For example, the first focusing optical unit 22 has a focal length of 3 mm, a numerical aperture of 0.52, a collimation accuracy of 0.25 mrad, a surface coated with an anti-reflection film for the 630-640 nm wavelength band, and a single-sided reflectivity of 0.15%.

[0034] The focusing accuracy of the first focusing optical unit 22 is not less than 0.5 μm. For example, the focal length of the first focusing optical unit 22 is 10 mm, the numerical aperture is 0.35, and the focusing accuracy is 0.4 μm.

[0035] The first collimating optical unit 21 is an aspherical collimating lens adapted to the transmission characteristics of broadband optical signals. With its aspherical optical surface, the lens can effectively eliminate aberrations such as spherical aberration and astigmatism that exist in traditional spherical lenses. It can accurately receive the broadband optical signal output by the semiconductor optoelectronic device 1, and convert the originally divergent beam of the signal into parallel light with the same propagation direction through optical refraction and surface matching. It then stably projects this parallel light onto the first focusing optical unit 22, providing a low-loss and highly uniform parallel light field for subsequent focusing operations.

[0036] The first focusing optical unit 22 also adopts an aspherical focusing lens adapted to the requirements of broadband optical band and mode spot matching. It can efficiently receive the parallel light output by the first collimating optical unit 21. Based on the high focusing accuracy characteristics of the aspherical lens, it accurately focuses the parallel light onto the incident end face of the mode spot conversion polarization rotating beam splitter 3, realizing the initial adaptation of the focused light spot and the mode spot size of the incident end face of the beam splitter. At the same time, it minimizes the energy loss and scattering loss of the optical signal in the focusing coupling process, ensuring that the broadband optical signal is transmitted to the mode spot conversion polarization rotating beam splitter 3 with high alignment accuracy and high coupling efficiency, laying a stable optical transmission foundation for subsequent mode spot size adaptation and polarization state control.

[0037] In some embodiments, such as Figure 2 and Figure 3 As shown, the mode-spot conversion polarization rotating beam splitter 3 includes: a substrate 31, a lower cladding layer 32, a waveguide core layer 33, and an upper cladding layer (not shown in the figure).

[0038] The lower cladding layer 32 is located on the upper surface of the substrate 31.

[0039] The waveguide core layer 33 is located on the surface of the lower cladding layer 32 away from the substrate 31. The waveguide core layer 33 includes a first mode conversion region, a polarization rotation region, a polarization beam splitting region, and a second mode conversion region. The first mode conversion region is used to adapt the mode size of the focused light. The polarization rotation region is used to rotate the polarization state and fine-tune the angle of the target polarization component. The polarization beam splitting region is used to separate and filter out non-target polarization components and retain the target polarized light signal with a single polarization state. The second mode conversion region is used to output the target polarized light signal and adapt the mode size of the reflected external cavity feedback light.

[0040] The upper cladding is located on the side of the waveguide core layer 33 away from the substrate 31, and the upper cladding covers the waveguide core layer 33.

[0041] The substrate 31 can be a thick single-crystal silicon substrate 31. A silicon dioxide (SiO2) lower cladding layer 32 is grown on the surface of the substrate 31 to achieve total internal reflection confinement of the optical field. The waveguide core layer 33 can be a low-loss silicon nitride (Si3N4) thin film, which is prepared by plasma-enhanced chemical vapor deposition (PECVD). The upper cladding layer uses the same SiO2 material as the lower cladding layer 32. The upper cladding layer covers the entire waveguide structure, protecting the waveguide and suppressing surface scattering loss.

[0042] For example, the substrate 31 can be a single-crystal silicon substrate 31 with a thickness of 400μm to 600μm, and a silicon dioxide under-cladding layer 32 with a thickness of 1.5μm to 2.5μm is prepared on the surface of the single-crystal silicon substrate 31; the waveguide core layer 33 is a silicon nitride thin film with a thickness of 200nm to 400nm, which is prepared by plasma-enhanced chemical vapor deposition process and has a refractive index of 2.00 to 2.04 at a wavelength of 635nm.

[0043] For example, the substrate 31 is a single-crystal silicon substrate 31 with a thickness of 500 μm, and a silicon dioxide (SiO2) under-cladding layer 32 with a thickness of 2 μm is prepared on the surface of the substrate 31; the waveguide core layer 33 is a silicon nitride (Si3N4) thin film with a thickness of 300 nm, which is prepared by plasma-enhanced chemical vapor deposition (PECVD) process and has a refractive index of 2.02 at a wavelength of 635 nm.

[0044] It can be understood that the mode-spot conversion polarization rotating beam splitter 3 includes a substrate 31, a lower cladding layer 32, a waveguide core layer 33, and an upper cladding layer. The lower cladding layer 32 is located on the upper surface of the substrate 31, providing stable support and a foundation for limiting the lower boundary of the optical field for the waveguide core layer 33. The waveguide core layer 33 is disposed on the surface of the lower cladding layer 32 away from the substrate 31. The waveguide core layer 33 integrates four functional regions: a first mode conversion region, a polarization rotation region, a polarization beam splitting region, and a second mode conversion region. The first mode conversion region performs mode size adaptation on the incoming focused light, so that the optical field mode is efficiently matched with the subsequent functional waveguide to reduce transmission loss. The polarization rotation region performs polarization state orientation rotation and precise angle fine adjustment on the target polarization component in the optical signal to ensure that the target polarization component is matched with the main axis of the subsequent optical path. The polarization beam splitting region efficiently separates different polarization components, directionally filters out non-target polarization components, and retains the pure target polarized light signal with a single polarization state. The second mode conversion region, on the one hand, stably outputs the target polarized light signal to the subsequent optical path, and on the other hand, adapts the mode size of the reflected external cavity feedback light to ensure that the feedback light can be smoothly transmitted back to maintain the stability of the external cavity resonance. The upper cladding layer is disposed on the side of the waveguide core layer 33 away from the substrate 31 and completely covers the waveguide core layer 33. It is used to protect the structure of the waveguide core layer 33, suppress surface scattering loss, and cooperate with the lower cladding layer 32 to achieve total internal reflection confinement of the optical field.

[0045] In this embodiment, the mode-spot conversion polarization rotation beam splitter 3 is fabricated based on silicon-based silicon nitride (Si3N4) monolithic integrated waveguide technology. It has no discrete component splicing and air interface, and integrates three major functions: mode conversion, polarization rotation and polarization beam splitting. It realizes mode matching of input light field, controllable rotation of polarization state and on-chip filtering of non-target polarization modes, suppresses polarization mode competition from the source of resonant cavity, and covers the working band from visible light to near infrared, with strong wavelength adaptability.

[0046] In some embodiments, such as Figure 3 As shown, the first mode conversion region and the second mode conversion region adopt thermally adiabatic conical waveguide structures respectively.

[0047] The waveguide width of the first mode conversion region gradually changes from wide to narrow along the direction of light propagation, while the waveguide width of the second mode conversion region gradually changes from narrow to wide along the direction of light propagation.

[0048] For example, the input width of the modulus conversion region is 5μm~7μm, the output width is 1μm~3μm, and the length is 1.5mm~2.5mm, with a modulus conversion efficiency of not less than 98%. Alternatively, the input width of the modulus conversion region is 6μm, the output width is 2μm, and the length is 2mm, with a modulus conversion efficiency of 98.5%.

[0049] The performance indicators of the first and second mode conversion regions are: mode conversion efficiency ≥98%, insertion loss ≤0.2dB, and operating wavelength coverage 400~1700nm.

[0050] It is understandable that the waveguide width of the first mode conversion region along the forward light propagation direction exhibits a linearly gradual change from wide to narrow. Relying on the mode evolution characteristics of the adiabatic gradually changing waveguide, the large-mode Gaussian beam coupled by the first focusing optical unit 22 can be adiabatically compressed into a small-mode fundamental mode beam that matches the single-mode waveguides in the polarization rotation region and polarization beam splitting region. This avoids the excitation of higher-order modes and energy loss during mode conversion, achieving efficient and low-loss mode size adaptation. The waveguide width of the second mode conversion region along the forward light propagation direction exhibits a linearly gradual change from narrow to wide. It can restore the small-mode target polarized light signal after polarization modulation to a mode size suitable for subsequent optical path transmission. At the same time, it can perform reverse mode size adaptation for the external cavity feedback light reflected and folded back along the optical path, ensuring that the feedback light can be stably and with low loss transmitted back to the front-end optical path. Both adiabatic conical waveguide structures are designed with linearly gradual waveguide widths to achieve precise mode matching while minimizing optical field transmission loss and mode interference.

[0051] In some embodiments, such as Figure 3 As shown, the polarization rotation region includes an inclined ridge waveguide structure and a microheater.

[0052] The inclined ridge waveguide structure is connected to the first mode conversion region and the polarization beam splitting region, respectively.

[0053] A microheater is disposed on the upper cladding. The microheater is used to adjust the effective refractive index difference of the tilted ridge waveguide structure by thermal tuning, so as to adjust the polarization rotation angle.

[0054] For example, the polarization rotation region adopts a tilted ridge waveguide structure with a ridge height of 100nm~200nm, a tilt angle of 12°~18°, and a length of 2.5mm~3.5mm. The polarization rotation region integrates a Ti / Au microheater with a heating current adjustable range of 0mA~10mA, enabling continuous adjustment of the polarization rotation angle from 0° to 90°, and achieving a TE (Transverse Electric Mode) rotation efficiency of no less than 95%. As another example, the polarization rotation region has a ridge height of 150nm, a tilt angle of 15°, and a length of 3mm. This region integrates an Au microheater with a heating current adjustable range of 0~8mA, enabling continuous adjustment of the polarization rotation angle from 0° to 90°, and achieving a TE mode rotation efficiency of 96%.

[0055] It is understandable that the tilted ridge waveguide structure, as the core transmission carrier for the polarization rotation of optical signals, is directly connected at both ends to the first mode conversion region and the polarization beam splitting region, forming a continuous waveguide transmission channel. This ensures that the optical signal, after mode matching, can smoothly enter the polarization rotation stage and be successfully transmitted to the polarization beam splitting region. The geometric birefringence effect of the tilted ridge waveguide breaks the XY plane symmetry of the waveguide, creating a difference in the effective refractive index between the TE mode and the TM (Transverse Magnetic Mode), causing continuous rotation of the polarization state of the optical field during transmission. The microheater is integrated on the surface of the upper cladding. It can change the effective refractive index difference of the tilted ridge waveguide structure through thermal tuning. By applying different heating currents (0~10mA) to the microheater, the waveguide temperature is changed and the magnitude of geometric birefringence is finely adjusted. This allows for continuous adjustment of the polarization rotation angle within the range of 0°~90°, ensuring that the target polarization component (TE mode) of the input light is completely rotated to match the principal axis of the subsequent polarization beam splitting region. This achieves precise control of the polarization rotation angle, ensuring that the target polarization component is rotated to match the subsequent polarization beam splitting region and guaranteeing the effectiveness of polarization control and separation.

[0056] In some embodiments, such as Figure 3 As shown, the polarization beam splitting region adopts an asymmetric directional coupler structure, which includes a main waveguide and a sub-waveguide.

[0057] The first ends of the main waveguide and the sub-waveguide are connected to the polarization rotation region, the second end of the sub-waveguide is connected to the on-chip optical absorption region, and the second end of the main waveguide is connected to the second mode conversion region.

[0058] For example, the polarization beam splitter adopts an asymmetric directional coupler structure, with a main waveguide width of 1.5μm~2.5μm, a secondary waveguide width of 1.5μm~2.0μm, a main-secondary waveguide spacing of 0.4μm~0.6μm, a coupling length of 1.0mm~2.0mm, and a doped silicon light absorption region connected to the end of the secondary waveguide. The TM mode suppression ratio is not less than 25dB. In conjunction with the aforementioned embodiment, the overall size of this mode-spot conversion polarization rotating beam splitter 3 is 7mm~9mm×1.5mm~2.0mm×0.5mm~1.0mm, the operating wavelength range is 620nm~650nm, the input mode spot size is 2μm~4μm, the output mode spot size is 0.5μm~1.5μm, the TE mode transmittance is not less than 90%, and the total insertion loss is not more than 1.0dB. For example, the main waveguide width of the polarization beam splitter is 2μm, the secondary waveguide width is 1.8μm, the spacing between the main and secondary waveguides is 0.5μm, the coupling length is 1.5mm, and a doped silicon (Si) absorption region is connected to the end of the secondary waveguide. The TM mode suppression ratio is 25dB. The overall size of this polarization rotating beam splitter 3 with mode conversion is 8mm×1.8mm×0.7mm, the operating band covers 620~650nm, the input mode spot size is 3μm, the output mode spot size is 1μm, the TE mode transmittance is 93%, and the total insertion loss is 0.8dB.

[0059] The performance indicators of the polarization beam splitter region are: TE mode transmittance ≥90%, TM mode suppression ratio ≥30dB, insertion loss ≤0.3dB, and polarization-dependent loss ≤0.1dB.

[0060] It is understandable that by utilizing the different coupling length characteristics of the TE mode and TM mode in the directional coupler, the coupling length of the polarization beam splitter can be designed so that the TM mode is completely coupled to the sub-waveguide. Since the end of the sub-waveguide is connected to the on-chip light absorption region (doped with Si material), the non-target polarization mode (TM mode) light coupled to the sub-waveguide will be converted into heat energy dissipation, while the TE mode hardly undergoes coupling and continues to propagate along the main waveguide to the output end, realizing on-chip screening and suppression of polarization modes. This embodiment relies on the asymmetric structural design of the main waveguide and the sub-waveguide to achieve efficient separation of target polarization components and non-target polarization components. On this basis, this application integrates the mode-spot conversion polarization rotation beam splitter 3 into the external cavity resonant system, completing polarization mode screening before the light field enters the blazed grating 42, allowing only a single TE polarization state to participate in the subsequent diffraction and resonance process, fundamentally eliminating the mode competition between the TM mode and the TE mode; at the same time, the mode-spot conversion function realizes efficient coupling between the SLD and the waveguide, and between the waveguide and the subsequent optical system, solving the problems of low coupling efficiency and difficult assembly and adjustment of traditional discrete polarization elements.

[0061] In some embodiments, such as Figure 1As shown, the tuned optical module 4 includes: a second collimating optical unit 41, a blazed grating 42, a MEMS (Micro-Electro-Mechanical System) mirror 43, an optical isolation unit 44, a second focusing optical unit 45, and an output optical fiber unit 46.

[0062] The second collimating optical unit 41 collimates the optical signal output from the mode-spot-converting polarization rotating beam splitter 3 into parallel light and projects it onto the blazed grating 42. The blazed grating 42 disperses the incident parallel light, generating first-order diffraction feedback light and zero-order diffraction light. The MEMS mirror 43 reflects the first-order diffraction feedback light generated by the blazed grating 42, so that the first-order diffraction feedback light returns to the semiconductor optoelectronic device 1 along the original optical path through the blazed grating 42, the mode-spot-converting polarization rotating beam splitter 3, and the focusing module 2, forming a stable external cavity resonance. The optical isolation unit 44 receives the zero-order diffraction light and projects it onto the second focusing optical unit 45; the optical isolation unit 44 also isolates the reflected light from the laser output end. The second focusing optical unit 45 focuses the parallel light output from the optical isolation unit 44 onto the core of the output fiber unit 46, so that a single-mode laser is output through the output fiber unit 46.

[0063] The second collimating optical unit 41 adopts an aspherical collimating lens with the same structure as the first collimating optical unit 21. The focal length can be 4mm and the numerical aperture can be 0.52. The second collimating optical unit 41 can accurately collimate the light signal output by the mode-spot conversion polarization rotating beam splitter 3 into parallel light and project it onto the blazed grating 42, effectively improving the grating diffraction efficiency. The blazed grating 42 is a planar holographic blazed grating 42. The grating line density can be flexibly selected according to the target working wavelength. In the 635nm wavelength band, it can be 2400 lines / mm. The grating constant is 416.7nm, the incident angle is 83°, and the blaze angle is 24.8°. A gold reflective film can be deposited on the surface to disperse the incident parallel light, generating -1st order diffraction feedback light and zeroth order diffraction light as output to meet the resonance requirements. The blazed grating 42 achieves a -1st order diffraction efficiency of 50% and a zeroth order diffraction efficiency of 40% for TE polarization mode. Under general operating conditions, the -1st order diffraction efficiency is not less than 30% and the zeroth order diffraction efficiency is not more than 60%, which can adapt to multi-band application requirements.

[0064] MEMS reflector 43 can be electrostatically driven, with a mirror diameter of 3mm, a mechanical deflection angle of ±4°, a response time of 30us, a repeatability of 0.8urad, a driving voltage of 24V, and a power consumption of 3mW. The mirror is coated with an anti-reflection film for the corresponding wavelength band, with a reflectivity of ≥99.5%. It adopts chip-level miniaturized packaging and can reflect the -1st order diffraction feedback light emitted from blazed grating 42 along the original optical path, so that it passes sequentially through blazed grating 42, mode-spot conversion polarization rotating beam splitter 3 and focusing module 2 back to semiconductor optoelectronic device 1, thereby forming a stable external cavity resonance.

[0065] The optical isolator 44 uses a 630~640nm polarization-dependent optical isolator in the 635nm band, with an isolation of 38dB and an insertion loss of 0.7dB. Under normal operating conditions, the isolation is not less than 35dB and the insertion loss is not more than 0.8dB. The optical isolator 44 receives the zero-order diffracted light emitted from the blazed grating 42 and projects it onto the second focusing optical unit 45. At the same time, it effectively isolates the reflected light from the laser output end, preventing it from returning to the external cavity resonant system and interfering with the resonant stability and reducing mode switching.

[0066] The second focusing optical unit 45 adopts an aspherical focusing lens with a focal length of 6mm, a numerical aperture of 0.42, and a focusing efficiency of 88%. Under general operating conditions, it can efficiently focus the parallel light output from the optical isolation unit 44 onto the fiber core of the output fiber unit 46, with a fiber coupling efficiency of not less than 80%.

[0067] The output fiber unit 46 can use PM630 polarization-maintaining fiber in the 635nm band. The fiber core diameter is 6μm, the numerical aperture is 0.13, the coupling efficiency is 82%, and the polarization extinction ratio at the output end is 25dB. Under general operating conditions, single-mode or polarization-maintaining fiber can be selected. The fiber core diameter is matched with the output mode spot size of the polarization rotating beam splitter 3 with mode spot conversion. The polarization extinction ratio at the output end is not less than 30dB. The power fluctuation during long-term operation (≥1000 h) is not higher than 2.5%. Finally, the single-mode laser is stably output through the output fiber unit 46.

[0068] In some embodiments, such as Figure 4 As shown, the extended lines of the semiconductor optoelectronic device 1, the blazed grating 42, and the MEMS mirror 43 intersect at a point. The intersection point is taken as the virtual rotation center, and the MEMS mirror 43 rotates around the virtual rotation center during the tuning process.

[0069] It can be understood that the semiconductor optoelectronic device 1, the blazed grating 42, and the MEMS mirror 43 are located on the same optical plane, and their corresponding planar optical path extensions precisely intersect at the same point. This intersection point is the preset virtual rotation center. During the wavelength tuning process of the laser, the deflection action of the MEMS mirror 43 always rotates smoothly around this virtual rotation center. This ensures that when the mirror changes its deflection angle, the diffraction conditions and equivalent optical path of the external cavity resonant optical path can maintain a stable match. From the structural design, optical path offset and optical path abrupt change are avoided, ensuring that the laser can achieve mode-free, high-precision continuous wavelength tuning in the entire tuning range, while maintaining the stability of the grating diffraction efficiency and the external cavity optical feedback coupling efficiency.

[0070] In some embodiments, the Littman external cavity tunable laser further includes a temperature control unit (not shown), which is used to perform temperature stabilization control on the semiconductor optoelectronic device 1, the mode-spot conversion polarization rotating beam splitter 3, and the blazed grating 42.

[0071] The temperature control unit includes a thermoelectric cooler (TEC) and a temperature sensor (thermostat temperature sensor). The SLD, the mode-spot conversion polarization rotating beam splitter 3, and the blazed grating 42 are all thermally coupled to the thermoelectric cooler. Temperature is detected by the thermistor temperature sensor, and the operation of the thermoelectric cooler (TEC) is controlled based on the temperature detection structure, achieving synchronous and stable temperature control of the core components. In application, the temperature control unit achieves a temperature control accuracy of no less than ±0.03℃ and a temperature stability of no less than 0.005℃ / h, effectively suppressing wavelength drift and polarization state fluctuations caused by temperature changes, ensuring long-term stability of the laser output performance.

[0072] In conjunction with the foregoing embodiments, in one example, a 635nm tunable narrow linewidth laser for wavelength-tunable laser interferometers is used as a typical application scenario to describe the working process of this external cavity tunable narrow linewidth laser source.

[0073] like Figure 1 As shown, after the system is powered on and the temperature control unit stabilizes the overall operating temperature at 25℃±0.1℃, the SLD begins to output broadband spontaneous emission light in the range of 625~645nm. The broadband light is coupled into the mode-spot conversion polarization rotating beam splitter 3 via the first collimating optical unit 21 and the first focusing optical unit 22. The mode-spot conversion polarization rotating beam splitter 3 performs mode-spot shaping and polarization purification on the input light field, filtering out the TM mode component and retaining only the high-purity TE mode for forward transmission.

[0074] The TE-mode beam is collimated into parallel light by the second collimating optical unit 41 and incident on the blazed grating 42 at a specific incident angle. The grating performs spatial dispersion separation on different wavelength components, among which the target working wavelength 635nm component satisfies the Littman diffraction condition and forms the -1st order diffraction feedback light returning along the original incident direction. The remaining wavelength components are mainly lost in the form of zero-order transmission, thus achieving preliminary wavelength selection.

[0075] The -1 level 635nm feedback light is reflected by MEMS mirror 43, folds back along the original optical path, passes through blazed grating 42, mode-spot-converting polarization rotating beam splitter 3, and the first focusing and collimating unit, and is coupled back to the SLD gain region to form wavelength-locked external cavity resonant feedback.

[0076] By applying a driving voltage to electrostatically tune the deflection angle of the MEMS mirror 43, the equivalent incident angle of the grating can be continuously changed, allowing the resonant wavelength to be continuously scanned within the range of 635nm ± 0.1nm. Taking the target wavelength of 635nm as an example, when the MEMS deflection angle is adjusted to the corresponding resonant position, the system quickly establishes a stable single-mode oscillation.

[0077] The narrow-linewidth laser output from the resonant circuit is blocked from reverse light interference by the optical isolation unit 44, and then efficiently coupled into the output fiber unit 46 by the second focusing optical unit 45. Finally, at the 635nm operating point, the output laser linewidth is measured to be ≤200kHz, the polarization extinction ratio is ≥25dB, and it exhibits high wavelength stability and excellent polarization purity. It can be directly used in precision imaging scenarios such as laser interferometry and scattering detection, effectively improving the imaging signal-to-noise ratio and measurement resolution.

[0078] The collimated light emitted from the SLD from the left input contains optical signals with two polarization modes, TE0 and TM0, such as... Figure 3 As shown, the input light is first adapted to the mode size by a mode-spot conversion polarization rotating beam splitter 3, allowing light of different modes to smoothly enter the splitting region. Then, a polarization-sensitive waveguide structure is used to separate the TE0 and TM0 modes. The TE0 mode is output in its original mode, while the TM0 mode is converted to the TE0 mode for output, ultimately achieving polarization beam splitting and mode unification. Finally, the output is processed through the right-end mode converter. Simultaneously, the feedback light from the right side is fed back through a grating and adapted to the mode size by the right-side template converter, ensuring that the TE0 mode light maintains its original mode and is fed back into the SLD from the left end, forming an external cavity feedback lasing.

[0079] like Figure 4As shown, the blazed grating 42 is used to diffract the laser collimated at the feedback end, selecting light of a specific wavelength with a diffraction order of -1 to be incident on the MEMS mirror 43; the MEMS mirror is used for total internal reflection grating to diffract the perpendicularly incident laser, which returns along the original path after being diffracted by the blazed grating 42 and then returns to the SLD1 to form an external cavity resonance; the MEMS mirror 43 is deflected at high speed and with high precision through electrostatic control, thereby changing the equivalent optical path of the external cavity and achieving the tuning of the output wavelength; the SLD, the blazed grating 42 and the MEMS mirror 43 intersect at a point on the plane extension line, which is marked as the virtual rotation center. The mirror tuning process rotates around the virtual rotation center to achieve the purpose of mode-skipping tuning.

[0080] As shown in Table 1 below, compared with the 17dB polarization extinction ratio measured by a conventional SLD, this embodiment improves the polarization extinction ratio to 25dB. This embodiment can achieve stable single-polarization output, with polarization state fluctuation ≤1.5dB during tuning, high coupling efficiency, low insertion loss, good long-term working stability, and can flexibly adapt to 635nm and other visible and near-infrared bands, solving the problems of polarization instability, low coupling efficiency, and poor band adaptability of traditional lasers. Figure 5 As shown, this embodiment has a wavelength stability of ±0.5 pm, which meets the requirements of semiconductor measurement lasers.

[0081] Table 1 compares the performance of the mode-conversion polarization rotating beam splitter in this embodiment with that of traditional discrete polarization elements.

[0082] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application. Therefore, the scope of this application should not be limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A Littman external cavity tunable laser, characterized in that, include: Semiconductor optoelectronic devices, a focusing module, a mode-spot conversion polarization rotating beam splitter, and a tuned light output module are arranged sequentially along the laser propagation direction; The semiconductor optoelectronic device is used to output broadband optical signals; The focusing module is used to receive the broadband optical signal, collimate and focus the broadband optical signal, and project the focused light onto the mode-spot conversion polarization rotating beam splitter. The mode-spot conversion polarization rotating beam splitter includes: a substrate, a lower cladding layer, a waveguide core layer, and an upper cladding layer; the lower cladding layer is located on the upper surface of the substrate; The waveguide core layer is located on the surface of the lower cladding layer away from the substrate. The waveguide core layer includes a first mode conversion region, a polarization rotation region, a polarization beam splitting region, and a second mode conversion region. The first mode conversion region is used to adapt the mode size of the focused light. The polarization rotation region is used to rotate the polarization state and fine-tune the angle of the target polarization component. The polarization beam splitting region is used to separate and filter non-target polarization components, retaining the target polarized light signal with a single polarization state. The second mode conversion region is used to output the target polarized light signal and adapt the mode size of the reflected external cavity feedback light. The upper cladding layer is located on the side of the waveguide core layer away from the substrate. The upper cladding layer covers the waveguide core layer; the first mode conversion region and the second mode conversion region each adopt an adiabatic conical waveguide structure; the waveguide width of the first mode conversion region gradually changes from wide to narrow along the light propagation direction, and the waveguide width of the second mode conversion region gradually changes from narrow to wide along the light propagation direction; the polarization rotation region includes an inclined ridge waveguide structure and a microheater; the inclined ridge waveguide structure is connected to the first mode conversion region and the polarization beam splitting region respectively; the microheater is disposed on the upper cladding layer, and the microheater is used to adjust the effective refractive index difference of the inclined ridge waveguide structure by thermal tuning to achieve the adjustment of the polarization rotation angle; The tuned output module is used to perform spatial dispersion separation of different wavelength components, generate zero-order diffracted light as laser output light, and generate first-order diffracted light as external cavity feedback light, and achieve laser wavelength tuning by changing the wavelength of the external cavity feedback light.

2. The Littman external cavity tunable laser as described in claim 1, characterized in that, The semiconductor optoelectronic device is a superluminescent diode.

3. The Littman external cavity tunable laser as described in claim 1, characterized in that, The focusing module includes: a first collimating optical unit and a first focusing optical unit; The first collimating optical unit is an aspherical collimating lens, used to receive the broadband optical signal, collimate the broadband optical signal into parallel light, and then project it to the first focusing optical unit; The first focusing optical unit is an aspherical focusing lens, used to focus the parallel light output from the first collimating optical unit onto the incident end face of the polarization rotating beam splitter with mode spot conversion.

4. The Littman external cavity tunable laser as described in claim 1, characterized in that, The polarization beam splitting region adopts an asymmetric directional coupler structure, which includes a main waveguide and a sub-waveguide. The first ends of the main waveguide and the sub-waveguide are respectively connected to the polarization rotation region, the second end of the sub-waveguide is connected to the on-chip optical absorption region, and the second end of the main waveguide is connected to the second mode conversion region.

5. The Littman external cavity tunable laser as described in claim 1, characterized in that, The tuned light output module includes: a second collimating optical unit, a blazed grating, a MEMS mirror, an optical isolation unit, a second focusing optical unit, and an output fiber unit; The second collimating optical unit is used to collimate the optical signal output from the mode-spot-converting polarization rotating beam splitter into parallel light and project it onto the blazed grating; The blazed grating is used to disperse the incident parallel light, generating first-order diffraction feedback light and zero-order diffraction light. The MEMS mirror is used to reflect the first-order diffraction feedback light generated by the blazed grating, so that the first-order diffraction feedback light returns to the semiconductor optoelectronic device along the original optical path through the blazed grating, the mode-spot conversion polarization rotating beam splitter and the focusing module, forming a stable external cavity resonance; The optical isolation unit is used to receive the zeroth-order diffracted light and project it onto the second focusing optical unit; the optical isolation unit is also used to isolate the reflected light from the laser output end. The second focusing optical unit is used to focus the parallel light output from the optical isolation unit onto the core of the output optical fiber unit, so as to output single-mode laser through the output optical fiber unit.

6. The Littman external cavity tunable laser as described in claim 5, characterized in that, The extended planes of the semiconductor optoelectronic device, the blazed grating, and the MEMS mirror intersect at a point. This intersection point is used as a virtual rotation center, and the MEMS mirror rotates around this virtual rotation center during the tuning process.

7. The Littman external cavity tunable laser as described in claim 5, characterized in that, The Littman external cavity tunable laser also includes a temperature control unit, which is used to perform stable temperature control on the semiconductor optoelectronic device, the mode-spot conversion polarization rotating beam splitter, and the blazed grating.

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