A semiconductor laser linewidth compression system based on hybrid optical feedback

Abstract

This invention discloses a semiconductor laser linewidth compression system based on hybrid optical feedback, comprising a semiconductor laser, an optical fiber beam splitter, a micro-ring resonator, and an optical fiber ring resonator. The optical fiber ring resonator achieves self-injection locking linewidth compression, while the micro-ring resonator achieves frequency stabilization through backscattering Rayleigh scattering. A temperature sensor and a thermoelectric cooler control the micro-ring temperature. This system eliminates the need for a complex external feedback system, resulting in a compact structure and low cost. This invention effectively suppresses mode hopping, achieves stable narrow-linewidth laser output, significantly reduces laser linewidth, and improves frequency stability. This system is suitable for applications such as optical communication and optical detection, offering advantages of high stability and low cost.

Description

Technical Field

[0001] This invention relates to a system for linewidth compression of semiconductor lasers, and more particularly to a laser linewidth compression system based on hybrid optical feedback. Background Technology

[0002] Semiconductor lasers, with their significant advantages such as narrow linewidth, high coherence, low noise, and small size, have shown broad application potential in fields such as communication, sensing, detection, and imaging. To meet the requirements of high precision and stability, narrow-linewidth semiconductor lasers are particularly important. Currently, most mainstream narrow-linewidth technologies employ distributed feedback (DFB) configurations, which can stabilize laser linewidths to the megahertz level. However, with technological advancements and evolving application demands, even more stringent challenges are posed to the linewidth and stability of lasers.

[0003] Lasers based on self-injection locking technology are frequently used as narrow-linewidth semiconductor laser sources in experimental and practical applications due to their excellent linewidth compression effect. Early self-injection locking primarily employed methods such as Fabry-Perot cavities with high quality factors. By tuning the semiconductor laser frequency to the resonant frequency of the Fabry-Perot cavity, the laser's output linewidth was significantly reduced. However, these linewidth compression systems require precise adjustment of the relative positions of the laser and the locking cavity, resulting in poor overall system stability and a substantial increase in laser cost.

[0004] With continuous technological advancements, narrow-linewidth laser systems based on fiber ring self-injection locking can compress linewidth to some extent, addressing the aforementioned issues and reducing linewidth to the kHz level. However, fiber ring-based self-injection locking systems can cause mode hopping in semiconductor lasers, making it difficult to stabilize the laser in a specific longitudinal mode. Furthermore, the better the self-injection locking effect, the more severe the mode hopping phenomenon, significantly limiting the application range of self-injection locked semiconductor lasers.

[0005] In self-injection locked lasers utilizing fiber rings, researchers primarily employ active feedback frequency stabilization to suppress mode hopping. This method involves identifying the laser frequency, converting the optical signal into an electrical signal, and then stabilizing the frequency by altering the laser current, thereby suppressing mode hopping. However, the introduction of microcontrollers, photodetectors, and other components hinders improvements in the stability and reliability of semiconductor lasers and drastically increases their cost, making mass production difficult.

[0006] Therefore, there is an urgent need for a stable and easily integrated linewidth compression system that can be combined with a semiconductor laser to enable the semiconductor laser to achieve stable sub-kHz linewidth laser output without mode hopping. Summary of the Invention

[0007] To address the mode hopping phenomenon in fiber ring self-injection locking and the current requirements for narrow linewidth semiconductor laser output linewidth in communication and sensing applications, this invention proposes a laser linewidth compression system based on hybrid optical feedback. The semiconductor output laser compressed by this hybrid optical feedback system has an extremely narrow linewidth output and high frequency stability. Furthermore, the compression system can effectively suppress the generation of mode hopping phenomenon and has advantages such as simple structure, no need for an additional active feedback system, and low cost.

[0008] The technical solution of this invention is as follows:

[0009] A laser linewidth compression system based on hybrid optical feedback includes a semiconductor laser 1, a 1×3 polarization-maintaining fiber beam splitter 2, a micro-ring resonator 4, a temperature sensor 7, a semiconductor cooler 5, a fiber optic FPC connector 3, a butterfly-shaped package 6, a fiber optic circulator 8, and a fiber optic ring resonator composed of a pair of 2×2 fiber optic couplers. The semiconductor cooler, temperature sensor, and micro-ring resonator are fixedly mounted on the butterfly-shaped package 6. The semiconductor cooler is connected to the butterfly-shaped package 6. One end of the micro-ring resonator is connected to the input end of the 1×3 fiber beam splitter 2, and the other end is equipped with the fiber optic FPC connector 3. The output end of the 1×3 polarization-maintaining fiber beam splitter 2 is connected to the semiconductor laser 1 and the fiber optic circulator 8, respectively. The last remaining port is used as the laser output port. The fiber optic circulator is connected to the fiber optic ring resonator.

[0010] The pair of 2×2 couplers are connected to each other via flanges to form an optical fiber ring resonator. The splitting ratio of each 2×2 optical fiber coupler is 50:50, and the pigtail length is 0.9m.

[0011] The micro-ring resonant cavity, temperature sensor, and semiconductor cooler are all fixed to the inner surface of the butterfly laser housing by thermally conductive adhesive. The semiconductor cooler is placed at the bottom of the butterfly-shaped package housing, and the micro-ring resonant cavity and other components are placed on the surface of the cooler.

[0012] The semiconductor cooler and temperature sensor are connected to the corresponding pins of the butterfly laser housing via copper wires. The two ports of the micro-ring resonator are led out to the outside of the butterfly laser housing via output optical fibers, and the through end is connected to the optical fiber FPC connector.

[0013] The output light of the semiconductor laser is split into three paths by a 1×3 polarization-maintaining fiber beam splitter. One path serves as the output light, another path is self-injected and locked through a fiber ring to compress the linewidth of the laser, and the last path is backscattered into the laser through a micro-ring resonator for frequency stabilization.

[0014] Temperature sensors and semiconductor coolers are attached to the surface of the microring resonator to monitor and control the temperature of the microring resonator.

[0015] Furthermore, the aforementioned semiconductor laser is a distributed feedback semiconductor laser with a built-in temperature controller, and its output wavelength is adjusted by the built-in temperature controller and current.

[0016] Furthermore, the aforementioned microring resonator is a through-type microcavity made of high refractive index difference glass, with a free spectral range of 49 GHz.

[0017] Furthermore, the optical fibers used to connect the various optical devices are all single-mode polarization-maintaining fibers.

[0018] The advantages of this invention are as follows:

[0019] 1. This invention uses a semiconductor laser as the light source. Based on the principle of self-injection locking, it utilizes an optical fiber ring cavity as a feedback device and a micro-ring resonator as a frequency stabilization device to construct a simple semiconductor laser linewidth compression system that requires no external feedback system and has high stability and low cost.

[0020] 2. The micro-ring resonator used in this invention is made of high refractive index difference glass material, fabricated using CMOS compatible technology, and coupled with a straight waveguide using a through-type coupling method, which facilitates mass production and packaging.

[0021] 3. Compared with servo feedback frequency stabilization systems, the core frequency stabilization device used in the invention has a smaller size and higher stability, and can be packaged and integrated within a certain size, which has the potential to be applied in fields such as optical communication and optical detection. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of an optical fiber ring cavity structure.

[0023] Figure 2 This is a schematic diagram of the structure of the present invention.

[0024] Figure 3 This is a schematic diagram illustrating the working principle of the present invention.

[0025] Figure 4 This is a linewidth diagram of the laser when it is in free motion.

[0026] Figure 5 This is a linewidth diagram of the laser after linewidth compression and frequency stabilization.

[0027] The attached figures are labeled as follows:

[0028] 1—Semiconductor laser; 2—1×3 polarization-maintaining fiber beam splitter; 3—Fiber optic FPC connector; 4—Micro-ring resonator; 5—Semiconductor cooler; 6—Butterfly-shaped package shell; 7—Temperature sensor; 8—Fiber optic circulator; 9—2×2 coupler; 10—2×2 coupler; 11—Flange. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0030] A schematic diagram of the fiber optic ring cavity structure is shown below. Figure 1 As shown, two 2×2 couplers 9 and 10 are connected by flange 11 to form an upper and lower channel annular cavity structure, wherein the splitting ratio of each individual 2×2 coupler is 50:50.

[0031] like Figure 2 As shown, the laser linewidth compression system based on hybrid optical feedback provided by the present invention includes:

[0032] Semiconductor laser 1;

[0033] The input end of the 1×3 polarization-maintaining fiber beam splitter 2 is connected to the semiconductor laser 1, and the output end is connected to the micro-ring resonator 4, the fiber optic circulator 8, and the third port.

[0034] The fiber optic circulator 8 has its input end connected to one output end of a 1×3 polarization-maintaining fiber optic beam splitter, and its output end connected to a fiber optic ring resonator.

[0035] The fiber optic ring resonator consists of a pair of 2×2 couplers connected to each other via flanges. The splitting ratio of a single 2×2 coupler is 50:50, and the pigtail length is 0.9m.

[0036] The micro-ring resonant cavity 4 is connected at one end to the other output end of the 1×3 fiber optic beam splitter, and the other end is fitted with a fiber optic FPC connector.

[0037] Temperature sensor 7 and semiconductor cooler 5 are both fixed to the inner surface of butterfly-shaped package 6 and are in close contact with micro-ring resonant cavity 4 for monitoring and controlling the temperature of micro-ring resonant cavity 4.

[0038] The butterfly-shaped package 6 has a temperature sensor 7, a semiconductor cooler 5 and a micro-ring resonant cavity 4 fixed on its inner surface, and the semiconductor cooler 5 is connected to the butterfly-shaped package 6.

[0039] The thermoelectric cooler 5 is placed on the inner surface of the butterfly-shaped package 6 using thermally conductive adhesive, and its wires are connected to the corresponding pins of the butterfly-shaped package by soldering. The temperature sensor 7 and the micro-ring resonant cavity 4 are placed on the surface of the thermoelectric cooler and fixed with thermally conductive adhesive. Metal gaskets are installed on the surface of the package for heat conduction. The temperature sensor 7 is placed near the micro-ring resonant cavity 4 to detect the operating temperature of the micro-ring resonant cavity 4. The thermoelectric cooler 5 changes the resonant wavelength of the micro-ring resonant cavity 4 by changing the operating temperature of the micro-ring resonant cavity 4.

[0040] The output light of the semiconductor laser is split into three paths by a 1×3 polarization-maintaining fiber beam splitter. One path serves as the output light, another path undergoes self-injection locking via a fiber ring resonator to achieve linewidth compression, and the final path generates backscattered Rayleigh light through a micro-ring resonator, which is then fed back into the laser for frequency stabilization. A through-type micro-ring resonator 4 is connected to the polarization-maintaining beam splitter 2 at one end for laser input; the other end is fitted with a fiber optic FPC connector 3, which allows for detection of the coupling state via optical power. The backscattered Rayleigh light generated by the micro-ring resonator 4 is reflected back into the laser 1 to stabilize the frequency.

[0041] The semiconductor laser is a distributed feedback semiconductor laser with a built-in temperature controller, and its output wavelength is adjusted by the built-in temperature controller and current.

[0042] The microring resonator is a through-type microcavity made of high refractive index difference glass, with a high quality factor and a free spectral range of 49 GHz.

[0043] All optical fibers connecting optical devices are single-mode polarization-maintaining fibers.

[0044] The micro-ring resonator 4 in this invention is made of a glass material with a high refractive index difference and is fabricated using a CMOS-compatible process. It adopts a through-type coupling structure and has a free spectral range of 49 GHz. The light reflected back to the laser through the micro-ring resonator 4 changes the refractive index of the semiconductor medium due to the plasma effect in the laser, thereby stabilizing the output frequency of the laser. At the same time, in the semiconductor laser linewidth compression system provided by this invention, the ring cavity composed of 2×2 fiber couplers 9 and 10 serves as a self-injection locked feedback system. After the laser passes through the ring resonator, the light with a wavelength of the fiber ring resonant wavelength is injected into the distributed feedback laser 1 through the transmission end, which significantly compresses the output linewidth of the laser.

[0045] The following is combined with Figure 3 The working principle of the semiconductor laser linewidth compression system of the present invention will be described in detail below:

[0046] Turn on the distributed semiconductor laser and adjust the operating temperature and current. Due to the small free spectral range of the fiber ring resonator, self-injection locking of the fiber ring can occur without complex adjustments. Adjust the operating temperature of the micro-ring resonator 4 to position it at the rising point of the resonance peak.

[0047] When the fiber optic ring resonator generates mode hopping due to other reasons, the operating wavelength of the semiconductor laser 1 will change. Due to the filtering effect of the micro-ring resonator 4, the frequency change of the semiconductor laser 1 will cause a change in the intensity of the feedback light passing through the micro-ring resonator 4, and the micro-ring resonator 4 will feed the changed intensity signal and frequency signal back to the semiconductor laser 1.

[0048] The specific mechanism is:

[0049] When mode hopping occurs in the self-injection locking system based on a fiber ring resonator, the distributed feedback laser 1 will lock to the adjacent fiber ring mode, causing a change in the output frequency of the distributed semiconductor laser 1. Taking the semiconductor laser frequency 1 shifting to a lower fiber ring cavity mode as an example, due to the filtering effect of the micro-ring resonator 4, the Rayleigh scattering light field intensity generated by the semiconductor laser 1 through the micro-ring resonator 4 will decrease. This reduces the number of photons in the laser cavity of the semiconductor laser 1, and stimulated emission of photons will consume fewer charge carriers. Based on the waveguide plasma effect, the refractive index of the waveguide in the laser cavity will decrease, which pulls the emission frequency of the semiconductor laser to a higher frequency direction, causing the laser output frequency to return to the set operating frequency, thereby stabilizing the output frequency of the semiconductor laser 1. When mode hopping occurs, based on the above theory, the output frequency of the semiconductor laser 1 can return to the locked frequency, thus achieving stable narrow-linewidth laser output without mode hopping for a longer period of time.

[0050] When a wireless wide compression system is involved, the output linewidth of a freely operating laser is as follows: Figure 4 As shown, the linewidth measurement system is built based on the time-delay self-heterodyne method. The measured and calculated linewidth is approximately 65 kHz. After linewidth compression via a self-injection locking and frequency stabilization system, the laser output linewidth is as follows: Figure 5 As shown, its linewidth was measured and calculated to be 20kHz. Tests show that by adopting the solution of the present invention, the output linewidth of the laser can be significantly reduced on the one hand, and the mode hopping phenomenon generated in the self-injection locking of the fiber ring cavity can be effectively suppressed on the other hand, so that the linewidth and frequency of the laser output are kept at a relatively stable level.

[0051] In summary, this hybrid optical feedback semiconductor laser linewidth compression system possesses advantages such as simple structure, low cost, and excellent robustness, and eliminates the need for an external, complex servo system to stabilize the laser frequency. It provides a highly reliable, low-cost, and highly stable semiconductor laser linewidth compression system for various technical fields, including resonant optical gyroscope systems, long-distance coherent detection, high-precision laser imaging, optical computing networks, aerospace exploration, and coherent optical communication.

Claims

1. A laser linewidth compression system based on hybrid optical feedback, characterized in that: The device includes a semiconductor laser (1), a 1×3 polarization-maintaining fiber beam splitter (2), a micro-ring resonator (4), a temperature sensor (7), a semiconductor cooler (5), a fiber optic FPC connector (3), a butterfly-shaped package shell (6), a fiber optic circulator (8), and a fiber optic ring resonator composed of a pair of 2×2 fiber optic couplers connected to each other by a flange. The butterfly-shaped package shell (6) is fixedly equipped with the semiconductor cooler (5), the temperature sensor (7), and the micro-ring resonator (4). The semiconductor cooler (5) is connected to the butterfly-shaped package shell (6). One end of the micro-ring resonator (4) is connected to the output end of the 1×3 polarization-maintaining fiber beam splitter (2), and the other end is equipped with the fiber optic FPC connector (3). The input end of the 1×3 polarization-maintaining fiber beam splitter (2) is connected to the semiconductor laser (1), and the output end is connected to the micro-ring resonator (4) and the fiber optic circulator (8). The last remaining port is used as the laser output port. The fiber optic circulator (8) is connected to the fiber optic ring resonator. The output light of the semiconductor laser (1) is split into three paths by a 1×3 polarization-maintaining fiber beam splitter (2). One path is used as the output light, another path is self-injected and locked through a fiber ring to compress the linewidth of the laser, and the last path is entered into the laser through a micro-ring resonator (4) for frequency stabilization via backscattering Rayleigh.

2. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: Each 2×2 fiber coupler has a splitting ratio of 50:50 and a pigtail length of 0.9m.

3. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The micro-ring resonant cavity (4), temperature sensor (7), and semiconductor cooler (5) are all fixed to the inner surface of the butterfly laser housing by thermally conductive adhesive. The semiconductor cooler (5) is placed at the bottom of the butterfly package housing (6), and the micro-ring resonant cavity (4) is placed on the surface of the cooler.

4. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The semiconductor cooler (5) and temperature sensor (7) are connected to the corresponding pins of the butterfly laser housing via copper wires. The micro-ring resonant cavity (4) is a through-type microcavity made of high refractive index difference glass. Its two ports are led out to the outside of the butterfly laser housing through output optical fibers, and the through end is connected to the optical fiber FPC connector (3).

5. A laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: Temperature sensor (7) and semiconductor cooler (5) are attached to the surface of microring resonator (4) to monitor and control the temperature of microring resonator (4).

6. A laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The semiconductor laser (1) is a distributed feedback semiconductor laser with a built-in temperature controller, and its output wavelength is adjusted by the built-in temperature controller and current.

7. A laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The free spectral range of the micro-ring resonator (4) is 49 GHz.

8. A laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The optical fibers used to connect the various optical devices are single-mode polarization-maintaining fibers.

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