High repetition frequency mode-locked external cavity semiconductor laser

Abstract

The invention provides a high repetition frequency mode-locking external cavity semiconductor laser. The high repetition frequency mode-locking external cavity semiconductor laser comprises a saturable absorber region, a gain region, a phase adjustment region, a light splitting region and a micro-ring resonance region. The phase adjusting area, the light splitting area and the micro-ring resonance area form an outer cavity, the gain area provides optical gain, one side of the saturable absorption area is plated with a high-reflection film, and the saturable absorption area and the outer cavity structure jointly form an optical resonant cavity. When free spectrums of the Fabry-Perot resonant cavity and the micro-ring resonant cavity are accurately matched, the repetition frequency of the laser can be multiplied, passive mode locking is realized through the nonlinear absorption characteristic of the saturable absorber, and high repetition frequency pulse output is formed. The scheme provided by the invention has the advantages of high integration level, high stability, high repetition frequency and the like, and has important application value in the fields of data centers, optical communication, high-speed optical sampling, double-optical comb measurement and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of semiconductor lasers, and particularly relates to a mode-locked external cavity semiconductor laser with high repetition frequency, which is suitable for dense wavelength division multiplexing systems, coherent optical communication, high-speed optical sampling, laser radar, optical coherence tomography, and optical computing and optical interconnection, and is applied to application scenarios with high stability and high repetition frequency requirements for ultra-short pulse sequences. BACKGROUND

[0002] High-repetition-frequency mode-locked lasers can generate ultra-short pulse sequences, and have wide applications in data centers and optical communication, high-speed optical sampling, dual-optical comb measurement, and the like. With the development of silicon optical technology, low-power and stable on-chip high-repetition-frequency light sources have important application values in dense wavelength division multiplexing and coherent communication, clock or signal sources in optical computing and optical interconnection, laser radar, and optical coherence tomography, and the like. At present, the methods for increasing the repetition frequency of a passively mode-locked laser mainly include: (1) harmonic mode-locking. For example, Chen Jie et al. of Taiyuan University of Technology proposed a mixed mode-locked fiber laser for generating high-order harmonic solitons with tunable repetition frequency, and generated high-order harmonic solitons with tunable repetition frequency in a mixed mode-locked erbium-doped fiber laser by adjusting the cavity loss related gain and spectral filtering effect. (2) Shortening the cavity length. For example, Sun Long et al. proposed a laser resonator with multiple selectable repetition frequencies and a control method thereof, and set different resonant cavities by selectively changing a polarization reflection unit. Fei et al. of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposed a high-repetition-frequency fiber laser based on an active multi-core optical fiber, which comprises an ultra-short resonant cavity based on an active multi-core optical fiber, and realizes GHz repetition frequency. The Institute of Semiconductors, Chinese Academy of Sciences, proposed a mode-locked laser and a driving method thereof, and realized different mode-locked pulse repetition frequencies by designing sub-resonant cavities with different lengths. In the fiber system, the repetition frequency of the order of GHz requires the cavity length to be shortened to the order of 0.1 m, which is difficult to achieve in the fiber device system. In addition, the existing high-repetition-frequency mode-locked lasers mostly use discrete elements or hybrid integrated structures, and it is difficult to realize true on-chip system integration, which restricts their large-scale application in the silicon optical platform. Although micro-ring resonators have been widely used in the fields of filtering, modulation, and sensing, their key role in external cavity feedback and repetition frequency multiplication in high-repetition-frequency mode-locked lasers has not been fully explored and applied. How to combine the filtering and reflection characteristics of the micro-ring resonator to design a mode-locked laser capable of realizing high repetition frequency, good stability, and high integration degree at the same time has become a technical problem to be solved in the field. SUMMARY

[0003] The application aims to provide a high-repetition-rate mode-locked external cavity semiconductor laser to solve the problems of cavity length limitation, insufficient stability and low integration in the existing mode-locked laser when realizing high repetition rate. By introducing a micro-ring resonant structure as the core element of the external cavity, combining a saturable absorber and a semiconductor gain region, high repetition rate and high stability of ultrashort pulse output are realized, and the structure is compact and can be integrated, thereby meeting the increasing performance requirements of high-repetition-rate mode-locked lasers in the field of photonics.

[0004] The technical solution of the application is as follows: A high-repetition-rate mode-locked external cavity semiconductor laser, characterized in that: a saturable absorber region, a gain region, a phase adjustment region, a light splitting region and a micro-ring resonant region are arranged along an optical path. The saturable absorber region and the gain region constitute an active gain part of the laser, and the phase adjustment region, the light splitting region and the micro-ring resonant region constitute an external cavity structure; the saturable absorber region is coated with a high reflection film away from one side of the external cavity, and the high reflection film and the external cavity structure jointly constitute an optical resonant cavity. The micro-ring resonant region is designed to make the free spectral range thereof an integer multiple of the free spectral range of an equivalent Fabry-Perot cavity constituted by the high reflection film and the reflection surface of the micro-ring resonant region, so as to realize multiplication of the repetition rate of the laser, and to realize passive mode-locking by using the nonlinear absorption characteristics of the saturable absorber region, thereby forming high-repetition-rate pulse output. The saturable absorber is a Ⅲ-Ⅴ group material and works in a reverse bias state. The saturable absorber is coated with a high reflection film away from one side of the external cavity. The gain region is made of a quantum well structure of a Ⅲ-V group material and realizes particle inversion under electrical excitation, thereby providing optical gain for the semiconductor laser. The phase adjustment region is a certain length of a thermal tuning electrode made on an optical waveguide, and the effective refractive index of the waveguide is adjusted by a bias current, so as to adjust the effective length of the resonant cavity of the external cavity semiconductor laser and change the longitudinal mode position and interval. The light splitting region can split a certain proportion of light and input the light into different optical paths for transmission. The micro-ring resonant region has a specific optical resonant wavelength and can filter incident light signals. The coupling coefficient of the micro-ring resonator determines the optical reflectivity, and the radius of the micro-ring resonator determines the repetition rate. The micro-ring is both a resonant filter and a reflector. The working process of the application: when the laser works, the gain area emits photons under electrical excitation, enters the light splitting area after phase adjustment, and the optical path is divided into two paths, which are coupled into the micro-ring resonator respectively, and part of the light is coupled back to the gain area after micro-ring resonance, forming optical resonance feedback. When the Fabry-Perot cavity composed of high-reflection film and micro-ring reflection structure and the free spectral range of the micro-ring resonator are accurately matched, the repetition frequency of the laser is doubled, and passive mode locking is realized through the nonlinear absorption characteristics of the saturable absorber, and finally a high-repetition-frequency ultrashort optical pulse sequence is output.

[0005] Compared with the prior art, the application has the following advantages: By using the micro-ring resonator as an external cavity reflection structure, the repetition frequency is doubled when the main cavity mode is matched, breaking through the physical limitations of the traditional short cavity scheme By dynamically adjusting the cavity length and longitudinal mode spacing through the thermal tuning electrode, the long-term matching of the double-cavity mode is ensured, and the stability and reliability of the mode locking process are improved. Combined with the nonlinear absorption effect of the saturable absorber and the filtering reflection characteristics of the micro-ring, a stable passive mode locking state is achieved, which is suitable for high-repetition-frequency and narrow-pulse-width application scenarios. The laser can be realized through monolithic integration, heterogeneous integration or hybrid integration on silicon, silicon nitride, lithium niobate and other material platforms, and has high structural compactness and process compatibility. BRIEF DESCRIPTION OF DRAWINGS

[0006] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed in the embodiment description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0007] Figure 1 The optical path diagram of the high-repetition-frequency mode-locked external cavity semiconductor laser. In the figure: 1-saturable absorption area, 2-gain area, 3-phase adjustment area, 4-light splitting area, 5-micro-ring resonance area. DETAILED DESCRIPTION

[0008] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor belong to the scope of protection of the present application.

[0009] The embodiments of the present application are further described in detail below with reference to the accompanying drawings.

[0010] Referring to Figure 1 as shown, Figure 1 A structure schematic diagram of a high-repetition-rate mode-locked external-cavity semiconductor laser is provided in the embodiments of the present application. The laser in the embodiments adopts a heterogeneous integration scheme, adopts a silicon-based optoelectronic chip as a passive integration platform, and prepares a waveguide structure of a phase adjustment region 3, a light splitting region 4 and a micro-ring resonant region 5 through a CMOS compatible process. A saturable absorber region 1 and a gain region 2 are separately prepared by using III-V group materials (such as an InGaAsP / InP multi-quantum well structure), and are flip-chip integrated into a predetermined region of the silicon optical chip through wafer bonding technology, so as to realize effective coupling of optical modes. The saturable absorber 1 is made of III-V group materials and works in a reverse bias state. At a low light intensity, the absorption coefficient is large, and the light is strongly absorbed. When the light intensity increases to a certain extent, the absorption coefficient decreases with the increase of the light intensity, and gradually becomes transparent. The high-intensity pulse is less absorbed and more amplified, thereby realizing mode locking. The side of the saturable absorber away from the external cavity is coated with a high-reflection film. The gain region 2 is made of a quantum well or quantum dot structure of III-V group materials, and realizes particle inversion under electrical excitation, thereby providing optical gain for the semiconductor laser. The phase adjustment region 3 is a certain length of a thermal tuning electrode prepared on an optical waveguide, which adjusts the effective refractive index of the waveguide through a bias current, and further adjusts the effective length of the resonant cavity of the external-cavity semiconductor laser, changes the longitudinal mode position and interval. The light splitting region 4 can split a certain proportion of light and input into different light paths for transmission. In the embodiments, a multimode interference coupler (MMI) is adopted. The design is of a 1x2 type, and the splitting ratio is preferably 50:50, so as to ensure that the optical powers of the two output lights are equal, thereby symmetrically exciting two input ports of the micro-ring resonator, and obtaining the best signal-to-noise ratio and filtering characteristics. The micro-ring resonant region 5 has a specific optical resonant wavelength, can filter the incident light signal, and the coupling coefficient of the micro-ring resonator determines the optical reflectivity, and the radius of the micro-ring resonator determines the repetition frequency. The micro-ring is both a resonant filter and a reflector. The structure proposed in the present application can be prepared on a silicon, lithium niobate, silicon nitride or other material platform, or prepared on a heterogeneous platform in a heterogeneous integration manner, including monolithic integration, heterogeneous integration, hybrid integration, and integration containing a micro-lens. Only the following design conditions need to be met: Resonant frequency of the Fabry-Perot cavity: Resonant frequency of the micro-ring structure: The resonance frequency interval of the micro-ring is an integer times of the F-P cavity. When the laser works, firstly, the pump current higher than the threshold is injected into the gain region 2, and the appropriate reverse bias is applied to the saturable absorber region 1. By adjusting the current of the phase adjustment region 3, the effective cavity length is changed, and the output is monitored by the oscilloscope and the spectrometer. When the longitudinal mode of the equivalent F-P cavity and the resonance peak of the micro-ring are accurately aligned, the passive mode-locked state is automatically started and maintained by slightly increasing the gain region current and fine-tuning the phase. Finally, the sequence of ultra-short optical pulses with high repetition frequency is obtained at the output end of the laser. By fine-tuning the current of the phase region, the repetition frequency can be fine-tuned in a small range. The embodiment successfully multiplies the repetition frequency by 8 times through the micro-ring resonator, avoids the gain deficiency and process difficulty caused by directly making an extremely short cavity. The hetero-integration scheme combines the high gain advantage of III-V materials and the high integration and low cost advantage of the silicon-based platform. The introduction of the phase adjustment region effectively compensates for the process deviation and environmental temperature fluctuation, ensures the long-term stability of the mode matching of the double cavities, and thus obtains stable and reliable high repetition frequency mode-locked output.

[0011] The above description is merely that of a specific embodiment of the present application, which enables a person skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A high-repetition-rate mode-locked external-cavity semiconductor laser, characterized in that: The laser device comprises a saturable absorber region (1), a gain region (2), a phase adjustment region (3), a light splitting region (4) and a micro-ring resonator region (5) arranged along an optical path. The saturable absorber region (1) and the gain region (2) constitute an active gain part of the laser device, and the phase adjustment region (3), the light splitting region (4) and the micro-ring resonator region (5) constitute an external cavity structure. The micro-ring resonator region (5) is designed to have a free spectral range which is an integer multiple of the free spectral range of an equivalent Fabry-Perot cavity formed by the high reflection film and the reflecting surface of the micro-ring resonator region (5), so as to realize multiplication of the repetition frequency of the laser device and passive mode-locking by using the nonlinear absorption characteristics of the saturable absorber region (1) to form high repetition rate pulse output.

2. The high-repetition-rate mode-locked external-cavity semiconductor laser of claim 1, wherein: The saturable absorber region (1) is made of a III-V semiconductor material and works in a reverse bias state to realize controllable nonlinear absorption and mode-locking start.

3. The high-repetition-rate mode-locked external-cavity semiconductor laser according to claim 1 or 2, characterized in that The gain region (2) is made of a quantum well or quantum dot active structure of a III-V semiconductor material to generate particle inversion and provide optical gain under electrical excitation.

4. The high-repetition-rate mode-locked external-cavity semiconductor laser of claim 1, wherein: The phase adjustment region (3) comprises a thermal tuning electrode made on an optical waveguide to adjust the effective refractive index of the optical waveguide by applying a bias current, and further adjust the longitudinal mode position and interval of the external cavity.

5. The high-repetition-rate mode-locked external-cavity semiconductor laser of claim 1, wherein: The light splitting region (4) is made of a directional coupler or a multimode interference coupler to divide the incident light into two paths according to a predetermined splitting ratio and couple the two paths to the double input ports of the micro-ring resonator region (5) respectively.

6. The high-repetition-rate mode-locked external-cavity semiconductor laser of claim 1, wherein: The micro-ring resonator region (5) has a specific optical resonance wavelength and can filter the incident light signal, and the coupling coefficient of the micro-ring resonator determines the optical reflectivity, and the radius of the micro-ring resonator determines the repetition frequency.

7. The high-repetition-rate mode-locked external-cavity semiconductor laser according to any one of claims 1 to 6, characterized in that The structures of the laser device are made on the same substrate by monolithic integration, heterogeneous integration or hybrid integration, and the substrate material includes silicon, silicon nitride, indium phosphide or lithium niobate.

8. The high-repetition-rate mode-locked external-cavity semiconductor laser of claim 7, wherein, The saturable absorber region (1) and the gain region (2) are made of a III-V semiconductor material and are bonded on a silicon or silicon nitride optical chip by a heterogeneous integration technology.

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