Method for improving mode-locked performance of single-section quantum dot mode-locked laser based on double-loop feedback

Abstract

This invention provides a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback. The method includes the following steps: Step 1, connecting the output of the single-region quantum dot mode-locked laser to port a of an optical coupler via a prism fiber; Step 2, connecting an external feedback loop consisting of an optical delay line, a first polarization controller, a fiber-based polarization beam splitter, and a second polarization controller to the optical coupler via ports c and d; Step 3, disconnecting the single-region quantum dot mode-locked laser from the optical coupler, observing the RF linewidth using a spectrometer, and analyzing and calculating it using a photodetector and an electro-spectroscopy system; Step 4, connecting the single-region quantum dot mode-locked laser to the optical coupler, connecting port b to a photodetector and an electro-spectroscopy system to observe the RF linewidth, and reducing the RF linewidth by adjusting the length of the external feedback loop and by adjusting the first and second polarization controllers.

Description

Technical Field

[0001] This invention relates to a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser, specifically a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback. Background Art

[0002] Mode-locked lasers are lasers whose time-domain output waveform is pulsed, and whose longitudinal modes have a fixed phase relationship in the frequency domain. Mode-locked lasers have wide applications in communication fields such as high-speed data transmission, optical time-division multiplexing, optical clock recovery, ultrafast signal processing, and optical combs. Semiconductor-based mode-locked lasers have many advantages, including small size, easy integration, and high repetition rate. Since the density of states of electrons in quantum dots is a delta function of energy, quantum dot-based semiconductor lasers possess ultra-low threshold current density, good temperature stability, near-zero linewidth gain factor, and high modulation bandwidth, showing great development potential and broad application prospects.

[0003] Single-region quantum dot mode-locked lasers (SMD lasers) have only a gain region in their cavity, and mode-locking is caused by strong nonlinearity within the quantum dot cavity. Since the repetition rate of a mode-locked laser is inversely proportional to its cavity length, SMD lasers have a shorter cavity length, allowing them to generate higher-frequency and narrower pulses. However, because SMD lasers lack a saturation absorption region, mode-locking cannot be optimized by adjusting the bias voltage and the length of the saturation region. Therefore, SMD lasers can be optimized through external control to reduce RF linewidth and timing jitter. Currently used optoelectronic optimization methods include injection locking, coupled opto-oscillation, and external cavity feedback. Injection locking requires an externally stabilized laser, while coupled opto-oscillation requires additional components such as opto-modulators, electrical filters, and photodetectors. In contrast, external cavity feedback only requires feeding a portion of the laser light back into the laser cavity through an external optical fiber. This method is simpler, more direct, and more economical, as it does not require an external light source or optoelectronic devices.

[0004] External cavity feedback includes single-cavity feedback and dual-loop feedback. Compared to single-cavity feedback, dual-loop feedback can better reduce RF linewidth, decrease timing jitter, and achieve a higher side-mode rejection ratio. In dual-loop feedback, the length of the two loops and the feedback strength both affect the RF linewidth. By adjusting the loop lengths and feedback power parameters, it was found that when the two loops are symmetrical, the narrowest RF linewidth is achieved under controlled feedback power. Symmetry requires identical loop lengths and polarization, and also necessitates adjustment of the attenuation of each loop. These factors require precise control of the parameters of both loops, increasing the complexity of the control process. Summary of the Invention

[0005] This invention is made to solve the above-mentioned problems, and aims to provide a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback.

[0006] This invention provides a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback. The method comprises the following steps: Step 1, connecting the output of the single-region quantum dot mode-locked laser to an optical coupler via a prism fiber. The optical coupler has ports a, b, c, and d, and the output of the single-region quantum dot mode-locked laser is connected to port a; Step 2, sequentially connecting an optical delay line, a first polarization controller, an optical fiber-based polarization beam splitter, and a second polarization controller to form an external feedback loop. The optical delay line is connected to port c, and the second polarization controller is connected to port d, thus completing the connection of the single-region quantum dot mode-locked laser, the optical coupler, and the external feedback loop; Step 3, disconnecting the single-region quantum dot mode-locked laser from port a of the optical coupler, observing the frequency spectrum and mode spacing using a spectrometer, and analyzing and calculating using a photodetector and an electro-spectroscopy spectrometer to obtain the result when the external feedback loop is not connected. RF linewidth; Step 4: Connect the single-region quantum dot mode-locked laser to port a of the optical coupler, and connect port b to a photodetector and an electro-spectroscopy instrument to observe the RF linewidth. Adjust the length of the external feedback loop and the first and second polarization controllers to adjust the feedback optical power to reduce the RF linewidth and improve the mode-locking performance. The external feedback loop has two feedback loops. The first feedback loop is where the output of the single-region quantum dot mode-locked laser passes through port a, enters port c, passes through the first polarization controller, the fiber-based polarization beam splitter and the second polarization controller, and then returns to port a and the single-region quantum dot mode-locked laser through port d. The second feedback loop is where the output of the single-region quantum dot mode-locked laser passes through port a, passes through port d, the second polarization controller, the fiber-based polarization beam splitter and the first polarization controller, and then returns to port a and the single-region quantum dot mode-locked laser through port c. The first and second feedback loops follow the same optical path and have the same length.

[0007] The method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback provided by the present invention may also have the following feature: the length of the external feedback loop is coarsely adjusted by adding fiber jumpers or finely adjusted by using optical delay lines.

[0008] The method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback provided by the present invention may also have the following features: the optical power of the first feedback loop is changed by adjusting the first polarization controller to change the polarization direction of the light entering the fiber-based polarization beam splitter, thereby changing the optical power emitted from the fiber-based polarization beam splitter and the optical power fed back to the single-region quantum dot mode-locked laser; the optical power of the second feedback loop is changed by adjusting the second polarization controller to change the polarization direction of the light entering the fiber-based polarization beam splitter, thereby changing the optical power emitted from the fiber-based polarization beam splitter and the optical power fed back to the single-region quantum dot mode-locked laser.

[0009] The method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback provided by the present invention may also have the following features: wherein, port a, port b, port c, and port d are all reversible ports, the splitting ratio of port a and port b is 50%, and the splitting ratio of port c and port d is 50%.

[0010] The role and effect of invention

[0011] According to the method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback, since a single-region quantum dot mode-locked laser only has a gain region and no saturated absorption region, its mode-locking performance cannot be improved by adjusting the parameters of the gain and absorption regions. Therefore, this invention optimizes the performance through an external cavity feedback method. Furthermore, the external cavity feedback loop contains only a few passive components, resulting in a simple and practical structure. This invention reduces the RF linewidth of a single-region quantum dot mode-locked laser through a relatively simple and direct technical solution, thereby improving the mode-locking performance of the single-region quantum dot mode-locked laser. Because the external feedback loop uses a polarization controller and... The feedback power is controlled by a fiber-optic polarization beam splitter, eliminating the need for an optical attenuator and reducing costs. Furthermore, the combination of the polarization controller and the fiber-optic polarization beam splitter allows for continuous and flexible adjustment of the feedback power, unaffected by the precision of the optical attenuator. Because the single-zone quantum dot mode-locked laser is connected to the external feedback loop via an optical coupler, the bidirectional operation of the optical coupler allows for the direct connection of the two output ports to form two optical feedback loops of equal length and identical paths. Therefore, no additional length matching of the two feedback loops is required, reducing the complexity of the external cavity feedback. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback, as described in an embodiment of the present invention.

[0013] Figure 2This is a structural diagram illustrating the specific application of the method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback, as described in an embodiment of the present invention. Detailed Implementation

[0014] To make the technical means and effects of the present invention easy to understand, the present invention will be specifically described below in conjunction with embodiments and accompanying drawings.

[0015] <Example>

[0016] This embodiment presents a method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback. The method includes the following steps:

[0017] Figure 1 This is a schematic diagram illustrating the method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback, as described in an embodiment of the present invention. Figure 2 This is a structural diagram illustrating the specific application of the method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback, as described in an embodiment of the present invention.

[0018] like Figure 1 and Figure 2 As shown, in step 1, the output end of the single-region quantum dot mode-locked laser 10 is connected to the optical coupler 30 through the prism fiber 20. The optical coupler 30 has port a, port b, port c and port d. The output end of the single-region quantum dot mode-locked laser 10 is connected to port a.

[0019] In this embodiment, the single-region quantum dot mode-locked laser used has a center wavelength of 1550 nm, a threshold current of 40 mA, a cavity length of 850 μm, and a mode-locking frequency of 50.25 GHz (mode spacing). One end face of the laser is coated with a high-reflectivity film with approximately 95% reflectivity, while the other end face is a cleaved surface with approximately 33% reflectivity.

[0020] Step 2: Connect an optical delay line 40, a first polarization controller 51, an optical fiber-based polarization beam splitter 60, and a second polarization controller 52 in sequence to form an external feedback loop. Connect the optical delay line 40 to port c and the second polarization controller 52 to port d, so that the single-region quantum dot mode-locked laser 10, the optical coupler 30, and the external feedback loop are connected.

[0021] Ports a, b, c, and d are all reversible ports.

[0022] The splitting ratio between port A and port B is 50%, so that 50% of the light enters the external feedback loop through port A, and 50% of the light is used for testing through port B.

[0023] The splitting ratio of port C and port D is 50%, ensuring that the light intensity of the feedback light in the two feedback loops is equal.

[0024] The external feedback loop has two feedback loops. The first feedback loop is where the output of the single-region quantum dot mode-locked laser 10 enters port c after passing through port a, then passes through the first polarization controller 51, the fiber-based polarization beam splitter 60, and the second polarization controller 52, before returning to port a and the single-region quantum dot mode-locked laser 10 through port d.

[0025] The second feedback loop is that the output of the single-region quantum dot mode-locked laser 10 passes through port a, then through port d, the second polarization controller 52, the fiber-based polarization beam splitter 60, and the first polarization controller 51, before returning to port a and the single-region quantum dot mode-locked laser 10 through port c.

[0026] The first feedback loop and the second feedback loop follow the exact same optical path and have the same length.

[0027] Step 3: Disconnect the connection between the single-region quantum dot mode-locked laser 10 and port a of the optical coupler 30. Observe the spectrum and mode spacing in the frequency domain using a spectrometer. Analyze and calculate the RF linewidth when the external feedback loop is not connected using a photodetector and an electro-spectrometer.

[0028] In this embodiment, the gain spectrum of a single-region quantum dot mode-locked laser with a center frequency of 1550 nm and a mode spacing of 50.25 GHz was observed using a spectrometer. Then, using a high-frequency photodetector (bandwidth greater than 50 GHz), the RF signal was observed on an electro-spectroscopy spectrometer. The center frequency of the RF signal was 50.25 GHz, equal to the mode spacing of the single-region quantum dot mode-locked laser, and the linewidth of the RF signal was approximately on the order of 100 kHz.

[0029] Step 4: Connect the single-region quantum dot mode-locked laser 10 to port a of the optical coupler 30, and connect port b to a photodetector and an electro-spectroscopy instrument to observe the RF linewidth. Adjust the length of the external feedback loop and the first polarization controller 51 and the second polarization controller 52 to adjust the feedback light power to reduce the RF linewidth and improve the mode-locking performance.

[0030] The length of the external feedback loop can be coarsely adjusted by adding fiber optic patch cords or finely adjusted by using optical delay line 40.

[0031] In this embodiment, 17 modes are observed within a 100MHz range on the optical spectrometer, indicating that the length of the external feedback fiber is approximately 16m (the external feedback loop is 32m long). By adjusting the optical delay line 40, the length of the external feedback loop is fine-tuned to an integer multiple of the laser cavity length, which corresponds to the resonant point. At this point, the most stable output can be observed on the RF spectrum, with increased RF peak power and a corresponding decrease in RF linewidth. During the adjustment of the optical delay line, a periodic trend in RF peak power and RF linewidth can be observed. The RF peak power is highest at the resonant point, and the RF linewidth can decrease to the 10kHz level at the resonant point.

[0032] The optical power of the first feedback loop is adjusted by the first polarization controller 51, which changes the polarization direction of the light entering the fiber-based polarization beam splitter 60, thereby changing the optical power emitted from the fiber-based polarization beam splitter 60 and the optical power fed back to the single-region quantum dot mode-locked laser 10. The optical power of the second feedback loop is adjusted by the second polarization controller 52, which changes the polarization direction of the light entering the fiber-based polarization beam splitter 60, thereby changing the optical power emitted from the fiber-based polarization beam splitter 60 and the optical power fed back to the single-region quantum dot mode-locked laser 10.

[0033] In this embodiment, the feedback power of the two feedback loops is controlled by adjusting the first polarization controller 51 and the second polarization controller 52. Because the output light power of the fiber-based polarization beam splitter 60 varies with the polarization of the input light, different polarizations of incident light will cause different power feedback to the laser, affecting the mode-locking performance and thus changing the RF linewidth. An optimal feedback power ratio can narrow the linewidth to the 1kHz level.

[0034] Because the optimization of RF linewidth by feedback power only works within a certain range, if an RF linewidth on the order of 10kHz cannot be obtained when adjusting the length of the external feedback loop, the first polarization controller 51 and the second polarization controller 52 can be roughly adjusted. After obtaining an RF linewidth on the order of 10kHz, the first polarization controller 51 and the second polarization controller 52 can be used for fine-tuning. That is, first obtain an RF linewidth on the order of 10kHz within a certain feedback power range, and then fine-tune the feedback power to further reduce the RF linewidth to the order of 1kHz.

[0035] The role and effect of the embodiments

[0036] According to the method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback involved in this embodiment, since the single-region quantum dot mode-locked laser only has a gain region and no saturated absorption region, its mode-locking performance cannot be improved by adjusting the parameters of the gain and absorption regions. Therefore, this invention optimizes the performance through an external cavity feedback method. Furthermore, the external cavity feedback loop contains only a few passive components, resulting in a simple and practical structure. This invention reduces the RF linewidth of the single-region quantum dot mode-locked laser through a relatively simple and direct technical solution, thereby improving the mode-locking performance of the single-region quantum dot mode-locked laser. Because the external feedback loop uses a polarization controller and... The feedback power is controlled by a fiber-optic polarization beam splitter, eliminating the need for an optical attenuator and reducing costs. Furthermore, the combination of the polarization controller and the fiber-optic polarization beam splitter allows for continuous and flexible adjustment of the feedback power, unaffected by the precision of the optical attenuator. Because the single-zone quantum dot mode-locked laser is connected to the external feedback loop via an optical coupler, the bidirectional operation of the optical coupler allows for the direct connection of the two output ports to form two optical feedback loops of equal length and identical paths. Therefore, no additional length matching of the two feedback loops is required, reducing the complexity of the external cavity feedback.

[0037] The above embodiments are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention.

Claims

1. A method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback, characterized in that, The following steps are involved: Step 1: Connect the output of the single-zone quantum dot mode-locked laser to an optical coupler via a prism fiber. The optical coupler has port a, port b, port c, and port d. The output of the single-zone quantum dot mode-locked laser is connected to port a. Step 2: Connect an optical delay line, a first polarization controller, an optical fiber-based polarization beam splitter, and a second polarization controller in sequence to form an external feedback loop. Connect the optical delay line to the c port and the second polarization controller to the d port, so that the single-region quantum dot mode-locked laser, the optical coupler, and the external feedback loop are connected. Step 3: Disconnect the single-region quantum dot mode-locked laser from the a-port of the optical coupler, observe the frequency domain spectrum and mode spacing using a spectrometer, and analyze and calculate using a photodetector and an electro-spectrometer to obtain the RF linewidth when the external feedback loop is not connected. Step 4: Connect the single-region quantum dot mode-locked laser to port a of the optical coupler, and connect port b to a photodetector and an electro-spectroscopy instrument to observe the RF linewidth. Adjust the feedback optical power by adjusting the length of the external feedback loop and by adjusting the first polarization controller and the second polarization controller to reduce the RF linewidth and improve the mode-locking performance. The external feedback loop has two feedback loops. The first feedback loop is where the output of the single-region quantum dot mode-locked laser enters the c port after passing through the a port, then passes through the first polarization controller, the fiber-based polarization beam splitter, and the second polarization controller, and finally returns to the a port and the single-region quantum dot mode-locked laser through the d port. The second feedback loop consists of the output of the single-region quantum dot mode-locked laser passing through port a, then through port d, the second polarization controller, the fiber-based polarization beam splitter, and the first polarization controller, before returning to port a and the single-region quantum dot mode-locked laser via port c. The first feedback loop and the second feedback loop follow exactly the same optical path and have the same length. The optical power of the first feedback loop is adjusted by regulating the first polarization controller, which changes the polarization direction of the light entering the fiber-based polarization beam splitter, thereby altering the optical power emitted from the fiber-based polarization beam splitter and the optical power fed back to the single-region quantum dot mode-locked laser. The optical power of the second feedback loop is adjusted by regulating the second polarization controller, which changes the polarization direction of the light entering the fiber-based polarization beam splitter, thereby changing the optical power emitted from the fiber-based polarization beam splitter and the optical power fed back to the single-region quantum dot mode-locked laser.

2. The method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback according to claim 1, characterized in that: in, The length of the external feedback loop can be coarsely adjusted by adding fiber optic jumpers or finely adjusted by the optical delay line.

3. The method for improving the mode-locking performance of a single-region quantum dot mode-locked laser based on dual-loop feedback according to claim 1, characterized in that: in, Port a, port b, port c, and port d are all reversible ports. The splitting ratio of port a and port b is 50%, and the splitting ratio of port c and port d is 50%.

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