Semiconductor laser linewidth compression system based on mixed optical feedback

Abstract

The invention discloses a semiconductor laser linewidth compression system based on hybrid optical feedback. The semiconductor laser linewidth compression system comprises a semiconductor laser, an optical fiber beam splitter, a micro-ring resonant cavity, an optical fiber ring resonator and the like. The self-injection locking compression line width is realized through the optical fiber ring resonator, the frequency of the micro-ring resonant cavity is stabilized through backward Rayleigh scattering, and the temperature of the micro-ring is controlled by the temperature sensor and the semiconductor cooler. The system does not need an external complex feedback system, and is compact in structure and low in cost. The mode hopping phenomenon is effectively inhibited, stable narrow linewidth laser output is realized, the linewidth of the laser is remarkably reduced, and the frequency stability is improved. The system is suitable for the fields of optical communication, optical detection and the like, and has the advantages of high stability and low cost.

Description

Technical Field

[0001] The invention relates to a system for compressing the line width of a semiconductor laser, and in particular to a laser line width compression system based on mixed light feedback. Background Art

[0002] Semiconductor lasers have shown extensive application potential in the fields of communications, sensing, detection, and imaging due to their significant advantages such as narrow linewidth, strong coherence, low noise, and small size. In order to meet the requirements of high precision and stability, narrow linewidth semiconductor lasers are particularly important. At present, most mainstream narrow linewidth technologies use a distributed feedback (DFB) configuration, which can stabilize the laser linewidth to the megahertz level. However, with the advancement of technology and the upgrading of application needs, more stringent challenges have been posed to the linewidth and stability of lasers.

[0003] Lasers based on self-injection locking technology are often used as narrow-linewidth semiconductor laser light sources in experiments and practical applications due to their excellent linewidth compression effect. Early self-injection locking was mainly performed through high-quality factor Fabry-Perot cavities, etc., and the output linewidth of the laser was greatly reduced by tuning the frequency of the semiconductor laser to the resonant frequency of the Fabry-Perot cavity. However, the above-mentioned linewidth compression system requires precise adjustment of the relative position of the laser and the locking cavity, which makes the stability of the entire laser system poor and greatly increases the cost of the laser.

[0004] With the continuous advancement of technology, the narrow linewidth laser system based on fiber ring self-injection locking can compress the linewidth to a certain extent, solving the above problems to a certain extent and compressing the linewidth to the kHz level. However, the self-injection locking system based on fiber ring will cause the semiconductor laser to experience mode hopping, making it difficult to stabilize the laser in a specific longitudinal mode. And the better the self-injection locking effect, the more serious the mode hopping phenomenon, which greatly limits the application scope of self-injection locked semiconductor lasers.

[0005] In the self-injection locked laser using an optical fiber ring, the researchers mainly suppress the mode hopping phenomenon by the active feedback frequency stabilization method, which mainly identifies the laser frequency, converts the optical signal into an electrical signal, and stabilizes the frequency by changing the laser current, thereby suppressing the mode hopping. The introduction of components such as microcontrollers and photodetectors is not conducive to improving the stability and reliability of semiconductor lasers, and the cost of lasers has risen sharply, making it difficult to achieve mass production.

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

[0007] In view of the mode hopping phenomenon occurring in the self-injection locking of an optical fiber ring and the current requirements for the output linewidth of narrow-linewidth semiconductor lasers in communications, sensing, etc., the present invention proposes a laser linewidth compression system based on hybrid optical feedback. The semiconductor output laser of the hybrid optical feedback compression system has an extremely narrow linewidth output and high frequency stability. The compression system can effectively suppress the occurrence of the mode hopping phenomenon, and has the advantages of simple structure, no need for an additional active feedback system, and low cost.

[0008] The technical solution of the present invention is: A laser linewidth compression system based on hybrid optical feedback comprises a semiconductor laser 1, a 1×3 polarization-maintaining optical fiber beam splitter 2, a micro-ring resonant cavity 4, a temperature sensor 7, a semiconductor refrigerator 5, an optical fiber FPC connector 3, a butterfly-shaped package shell 6, an optical fiber circulator 8, and an optical fiber ring resonator composed of a pair of 2×2 optical fiber couplers, wherein the semiconductor refrigerator, the temperature sensor, and the micro-ring resonant cavity are fixedly arranged on the butterfly-shaped package shell, wherein the semiconductor refrigerator is connected to the butterfly-shaped package shell, one end of the micro-ring resonant cavity is connected to the input end of the 1×3 optical fiber beam splitter 2, and the other end is provided with the optical fiber FPC connector 3; the output end of the 1×3 polarization-maintaining optical fiber beam splitter 2 is respectively connected to the semiconductor laser 1 and the optical fiber circulator 8, and the remaining last port is used as a laser output port, and the optical fiber circulator is connected to the optical fiber ring resonator.

[0009] The pair of 2×2 couplers are connected to each other through a flange to form a fiber ring resonator. The splitting ratio of each 2×2 fiber coupler is 50:50, and the length of the pigtail is 0.9 m.

[0010] The micro-ring resonant cavity, temperature sensor and semiconductor refrigerator are all fixed on the inner surface of the butterfly laser housing by heat-conducting adhesive, wherein the semiconductor refrigerator is placed on the bottom of the butterfly packaging shell, and the micro-ring resonant cavity is placed on the surface of the refrigerator.

[0011] The semiconductor refrigerator and the temperature sensor are connected to the corresponding pins of the butterfly laser housing through copper wires. The two ports of the micro-ring resonant cavity are led out of the butterfly laser housing through output optical fibers, and the through ends are connected to the optical fiber FPC connectors.

[0012] The output light of the semiconductor laser is divided into three paths through a 1×3 polarization-maintaining fiber beam splitter, one path is used as output light, one path is self-injection locked through an optical fiber ring to compress the laser line width, and the last path enters the laser through a microring resonant cavity through backward Rayleigh scattering for frequency stabilization.

[0013] The temperature sensor and the semiconductor cooler are closely attached to the surface of the micro-ring resonant cavity and are used for monitoring and controlling the temperature of the micro-ring resonant cavity.

[0014] Furthermore, the semiconductor laser is a distributed feedback semiconductor laser with a built-in temperature controller, and the output light wavelength is adjusted by the built-in temperature controller and current.

[0015] Furthermore, the above-mentioned microring resonant cavity is a straight-through microcavity made of high refractive index difference glass, and its quality factor is and its free spectrum range is 49 GHz.

[0016] Furthermore, the optical fibers used to connect the optical devices are all single-mode polarization-maintaining fibers.

[0017] The advantages of the present invention are as follows: 1. The present invention adopts a semiconductor laser as a light source, based on the principle of self-injection locking, uses a fiber ring cavity as a feedback device, and a microring resonant cavity as a frequency stabilization device to construct a semiconductor laser linewidth compression system with a simple structure, no need for external feedback system intervention, high stability and low cost.

[0018] 2. The micro-ring resonant cavity used in the present invention is made of high refractive index difference glass material, manufactured by CMOS compatible process, and adopts straight-through coupling mode and straight waveguide coupling, which is convenient for mass production and packaging.

[0019] 3. Compared with the servo feedback frequency stabilization system, the core frequency stabilization device used in the invention has a smaller size and higher stability, can achieve packaging integration under a certain volume, and has the potential to be applied in optical communications, optical detection and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the fiber ring cavity structure.

[0021] Figure 2 It is a schematic diagram of the structure of the present invention.

[0022] Figure 3 It is a working principle diagram of the present invention.

[0023] Figure 4 This is the laser line width diagram during free operation.

[0024] Figure 5 This is the laser linewidth diagram after linewidth compression and frequency stabilization.

[0025] The reference numerals are as follows: 1—semiconductor laser; 2—1×3 polarization-maintaining fiber beam splitter; 3—fiber FPC connector; 4—microring resonator; 5—semiconductor refrigerator; 6—butterfly-shaped package shell; 7—temperature sensor; 8—fiber circulator, 9—2×2 coupler; 10—2×2 coupler; 11—flange. DETAILED DESCRIPTION

[0026] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Schematic diagram of the fiber ring cavity structure Figure 1 As shown, two 2×2 couplers 9 and 10 are connected by flange 11 to form an upper and lower voice path type ring cavity structure, wherein the splitting ratio of a single 2×2 coupler is 50:50.

[0027] like Figure 2 As shown, the laser linewidth compression system based on hybrid light feedback provided by the present invention includes: Semiconductor laser 1; A 1×3 polarization-maintaining fiber beam splitter 2, whose output ends are respectively connected to a semiconductor laser, a fiber circulator and a third port; A fiber circulator 8, whose input end is connected to an output end of a 1×3 polarization-maintaining fiber beam splitter, and whose output end is connected to a fiber ring resonator; The fiber ring resonator is composed of a pair of 2×2 couplers connected to each other by a flange, wherein the splitting ratio of a single 2×2 coupler is 50:50 and the pigtail length is 0.9m; A micro-ring resonator 4, one end of which is connected to the other output end of the 1×3 optical fiber coupler, and the other end of which is provided with an optical fiber FPC connector; The temperature sensor 7 and the semiconductor refrigerator 5 are both fixed to the inner surface of the butterfly-shaped package shell 6 and are closely attached to the micro-ring resonant cavity 4, and are used to monitor and control the temperature of the micro-ring resonant cavity 4; A butterfly-shaped package shell 6, on the inner surface of which a temperature sensor 7, a semiconductor refrigerator 5 and a micro-ring resonant cavity 4 are fixed, and the semiconductor refrigerator 5 is connected to the butterfly-shaped package shell 6; The semiconductor refrigerator 5 is placed on the inner surface of the butterfly-shaped packaging shell 6 through thermal conductive glue, and its wires are connected to the pins of the corresponding butterfly shell by soldering; the temperature sensor 7 and the micro-ring resonant cavity 4 are placed on the surface of the semiconductor refrigerator and fixed by thermal conductive glue, and a metal gasket is installed on the surface of the shell for heat conduction; the temperature sensor 7 is placed near the micro-ring resonant cavity 4 to detect the working temperature of the micro-ring resonant cavity 4, and the semiconductor refrigerator 5 changes the resonant wavelength of the micro-ring resonant cavity 4 by changing the working temperature of the micro-ring resonant cavity 4.

[0028] The output light of the semiconductor laser is divided into three paths through a 1×3 polarization-maintaining fiber beam splitter, one path is used as output light, one path is self-injection locked through a fiber ring resonator to achieve line width compression, and the last path is fed back to the laser for frequency stabilization by generating backward Rayleigh scattered light through a micro-ring resonator. One end of the straight-through micro-ring resonator 4 is connected to a polarization-maintaining beam splitter 2 for laser input; the other end is equipped with a fiber FPC connector 3, which can detect the coupling state through optical power. The backward Rayleigh scattered light generated by the micro-ring resonator 4 is reflected back to the laser 1 to stabilize the frequency.

[0029] The semiconductor laser is a distributed feedback semiconductor laser with a built-in temperature controller, and the output light wavelength is adjusted by the built-in temperature controller and current.

[0030] The micro-ring resonant cavity is a straight-through micro-cavity made of high-refractive index difference glass, has a high quality factor, and a free spectrum range of 49 GHz.

[0031] All optical fibers connecting optical devices are single-mode polarization-maintaining fibers.

[0032] The micro-ring resonant cavity 4 in the present invention is made of glass material with a high refractive index difference, is made by a CMOS-compatible process, and adopts a straight-through coupling structure. The free spectrum range of the micro-ring resonant cavity is 49 GHz, and the quality factor is; the light reflected back to the laser by the micro-ring resonant cavity 4 changes the refractive index in 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 line width compression system provided by the present invention, the ring cavity composed of 2×2 fiber couplers 9 and 10 serves as a feedback system for self-injection locking; after the laser passes through the ring resonant cavity, light with a wavelength of the fiber ring resonance wavelength will be injected into the distributed feedback laser 1 through the transmission end, so that the laser output line width is significantly compressed; Combine the following Figure 3 The working principle of the semiconductor laser line width compression system of the present invention is described in detail: The distributed semiconductor laser is turned on, and the operating temperature and current are adjusted. Since the free spectrum range of the fiber ring resonator is small, the self-injection locking of the fiber ring can occur without complex adjustment. The operating temperature of the micro-ring resonator 4 is adjusted to make it at the rising point of the resonance peak.

[0033] When the fiber ring resonator produces mode hopping due to other reasons, the operating wavelength of the semiconductor laser 1 will change. Due to the filtering effect of the microring resonator 4, the frequency change of the semiconductor laser 1 will cause the change of the feedback light intensity passing through the microring resonator 4, and the changed light intensity signal and frequency signal will be fed back to the semiconductor laser 1 through the microring resonator 4.

[0034] The specific mechanism is: When the self-injection locking system based on the fiber ring resonator has a mode hopping phenomenon, the distributed feedback laser 1 will be locked to the adjacent fiber ring mode, thereby causing the output frequency of the distributed semiconductor laser 1 to change. Taking the semiconductor laser frequency 1 moving to the fiber ring cavity mode at a lower frequency as an example, due to the filtering effect of the microring resonator 4, the intensity of the Rayleigh scattered light field generated by the semiconductor laser 1 through the microring resonator 4 will decrease; this reduces the number of photons in the laser cavity of the semiconductor laser 1, and the stimulated radiation of photons will consume fewer carriers; based on the waveguide plasma effect, the waveguide refractive index in the laser cavity will decrease, which pulls the emission frequency of the semiconductor laser to a higher frequency direction, so that the laser output frequency returns to the set working point frequency, thereby achieving the stability of the output frequency of the semiconductor laser 1. When the mode hopping phenomenon occurs, based on the above theory, the output frequency of the semiconductor laser 1 can return to the locking frequency, thereby achieving a stable narrow linewidth laser output without mode hopping for a long time.

[0035] When the wireless bandwidth compression system is involved, the output line width of the free-running laser is as Figure 4 As shown in the figure, the line width measurement system is built based on the principle of delayed self-heterodyne method. After measurement and calculation, its line width is about 65khz. After the line width is compressed by self-injection locking and frequency stabilization system, the laser output line width is as follows Figure 5 As shown, the line width is measured and calculated to be 20kHz; the test shows that the scheme of the present invention can significantly reduce the output line width of the laser on the one hand, and can effectively suppress the mode hopping phenomenon caused by the self-injection locking of the optical fiber ring cavity on the other hand, so that the line width and frequency of the laser output are maintained at a relatively stable level.

[0036] In summary, this type of hybrid optical feedback semiconductor laser linewidth compression system has the advantages of simple structure, low cost, excellent robustness, and no external complex servo system is required to stabilize the frequency of the laser. It provides a semiconductor laser linewidth compression system with high reliability, low cost and high stability for various technical fields such as resonant optical gyroscope systems, long-distance coherent detection, high-precision laser imaging, optical computing networks, aerospace detection and coherent optical communications.

Claims

1. A laser linewidth compression system based on hybrid optical feedback, characterized in that: The invention comprises a semiconductor laser (1), a 1×3 polarization-maintaining optical fiber beam splitter (2), a micro-ring resonant cavity (4), a temperature sensor (7), a semiconductor refrigerator (5), an optical fiber FPC connector (3), a butterfly-shaped package shell (6), an optical fiber circulator (8), and an optical fiber ring resonator composed of a pair of 2×2 optical fiber couplers, wherein the semiconductor refrigerator, the temperature sensor, and the micro-ring resonant cavity are fixedly arranged on the butterfly-shaped package shell, wherein the semiconductor refrigerator is connected to the butterfly-shaped package shell, one end of the micro-ring resonant cavity is connected to the input end of the 1×3 optical fiber beam splitter (2), and the other end is provided with an optical fiber FPC connector (3); the output end of the 1×3 polarization-maintaining optical fiber beam splitter (2) is respectively connected to the semiconductor laser (1) and the optical fiber circulator (8), and the remaining last port is used as a laser output port, and the optical fiber circulator is connected to the optical fiber ring resonator.

2. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The pair of 2×2 couplers are connected to each other through a flange to form a fiber ring resonator. The splitting ratio of each 2×2 fiber coupler is 50:50, and the length of the pigtail is 0.9 m.

3. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The micro-ring resonant cavity, temperature sensor and semiconductor refrigerator are all fixed on the inner surface of the butterfly laser housing by heat-conducting adhesive, wherein the semiconductor refrigerator is placed on the bottom of the butterfly packaging shell, and the micro-ring resonant cavity is placed on the surface of the refrigerator.

4. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The semiconductor refrigerator and the temperature sensor are connected to the corresponding pins of the butterfly laser housing through copper wires. The two ports of the micro-ring resonant cavity are led out of the butterfly laser housing through output optical fibers, and the through ends are connected to the optical fiber FPC connectors.

5. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The output light of the semiconductor laser is divided into three paths through a 1×3 polarization-maintaining fiber beam splitter, one path is used as output light, one path is self-injection locked through an optical fiber ring to compress the laser line width, and the last path enters the laser through a microring resonant cavity through backward Rayleigh scattering for frequency stabilization.

6. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The temperature sensor and the semiconductor cooler are closely attached to the surface of the micro-ring resonant cavity and are used for monitoring and controlling the temperature of the micro-ring resonant cavity.

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

8. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The micro-ring resonant cavity is a straight-through micro-cavity made of high refractive index difference glass, with a quality factor of , and a free spectrum range of 49 GHz.

9. The laser linewidth compression system based on hybrid optical feedback according to claim 1, characterized in that: The optical fiber used to connect the optical devices is a single-mode polarization-maintaining fiber.

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