A self-starting 9-shaped cavity laser based on external signal injection

Abstract

The invention discloses a self-starting 9-shaped cavity laser based on external signal injection, and relates to the field of laser technology. The laser includes a nonlinear amplifying ring mirror, a linear arm, a polarization-maintaining fiber coupler, an external optical signal source and a polarization-maintaining fiber circulator. The polarization-maintaining fiber coupler is respectively connected to the linear arm, the polarization-maintaining fiber circulator and the nonlinear amplifying ring mirror; the polarization-maintaining fiber circulator is connected to the external optical signal source. The external optical signal is injected into the cavity through the polarization-maintaining fiber circulator to form a high-intensity initial light field, guide the nonlinear enhancement of the intracavity signal, enter the positive feedback zone in the cavity, and make the cavity enter the single-pulse mode-locked state faster. There is no need to use a high-power pump source, which avoids power waste and reduces costs.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber lasers, and in particular to a self-starting 9-shaped cavity laser based on external signal injection. Background Art

[0002] In fiber lasers, mode locking technology is a key method to achieve ultrashort pulse laser output. Normally, there are many longitudinal mode oscillations in the free-running laser cavity. The phase relationship of these longitudinal modes is random and independent of each other, resulting in continuous wave characteristics of the laser output. However, under certain conditions, by adjusting the state in the laser cavity, a fixed and stable phase difference is established between the longitudinal modes to meet the interference conditions, so that the laser output is transformed from a continuous wave to a periodic light pulse form.

[0003] As a new type of fiber laser mode-locking technology, the figure-9 cavity mode-locked fiber laser has many advantages, such as high repetition rate, high stability and simplified structural design, but it also has some disadvantages and challenges: Generally speaking, the figure-9 cavity (phase-biased nonlinear amplifying ring mirror) mode-locking generally requires a high pump current to achieve rapid self-starting, and after starting, it is in a multi-pulse state. It is necessary to gradually reduce the pump current and then gradually consume the excess pulses to obtain a single pulse state at the fundamental frequency. In addition, in traditional figure-9 cavity lasers, the mode-locking start time shows a long-tail distribution (part of the mode-locking process may be extremely long). This results in the need to use a high-power pump source for system startup, which has problems of power waste and high cost. Summary of the invention

[0004] The purpose of the present invention is to solve the problems of power waste and high cost mentioned in the above background technology, and to propose a self-starting 9-shaped cavity laser based on external signal injection.

[0005] The present invention provides a self-starting 9-shaped cavity laser based on external signal injection, wherein the laser comprises a nonlinear amplifying ring mirror, a linear arm, a 2x2 polarization-maintaining fiber coupler, an external optical signal source, a 3-port polarization-maintaining fiber circulator, a photodetector and a control module; wherein:

[0006] The first port and the second port of the 2x2 polarization-maintaining fiber coupler are connected to the linear arm and the second port of the 3-port polarization-maintaining fiber circulator respectively;

[0007] The third port and the fourth port of the 2x2 polarization-maintaining fiber coupler are connected to the nonlinear amplifying loop mirror;

[0008] The first port and the third port of the three-port polarization-maintaining fiber circulator are connected to the external optical signal source and the photodetector respectively;

[0009] The control module is connected to the nonlinear amplifying ring mirror, the external optical signal source and the photodetector respectively.

[0010] Specifically, the mode locking process of the laser includes:

[0011] Step 1, the control module loads a preset current to the nonlinear amplifying ring mirror to form a continuous optical signal in the ring;

[0012] Step 2, the control module drives the external optical signal source to inject the external optical signal into the nonlinear amplifying loop mirror through the 3-port polarization-maintaining fiber circulator and the 2x2 polarization-maintaining fiber coupler, and turns off the external optical signal source after a period of time;

[0013] Step 3, the photodetector receives the intracavity optical signal through the 3-port polarization-maintaining fiber circulator and converts it into an electrical signal;

[0014] Step 4, the control module determines whether the mold locking is successful according to the electrical signal;

[0015] Step 5: If the locking fails, return to step 2.

[0016] Specifically, the nonlinear amplifying loop mirror includes a polarization-maintaining active optical fiber, a polarization-maintaining wavelength division multiplexer, a phase shifter, a polarization-maintaining passive optical fiber and a pump source; wherein:

[0017] Under the excitation of a preset current, the pump source emits pump light, and the pump light is incident on the polarization-maintaining active optical fiber through the polarization-maintaining wavelength division multiplexer, generating a continuous optical signal in the ring, and the continuous optical signal is bidirectionally transmitted in the ring in a clockwise direction and a counterclockwise direction;

[0018] The two noise signals transmitted clockwise and counterclockwise in the ring generate nonlinear phase shift under the effect of the asymmetry of the positions of the phase shifter and the polarization-maintaining active optical fiber; the noise signal includes the continuous optical signal and the external optical signal.

[0019] Specifically, the polarization-maintaining active optical fiber is a polarization-maintaining ytterbium-doped high-gain optical fiber PM-YSF-HI-HP, and the pump source is a semiconductor diode with a wavelength of 980 nm.

[0020] Specifically, the phase shifter has a phase delay of -π / 2.

[0021] Specifically, the linear arm includes an output jumper head, a polarization-maintaining fiber isolator and a polarization-maintaining fiber grating; wherein:

[0022] The polarization-maintaining fiber grating is used to split the light transmitted by the 2x2 polarization-maintaining fiber coupler into two paths, one path is reflected back to the 2x2 polarization-maintaining fiber coupler, and the other path is transmitted to the polarization-maintaining fiber isolator;

[0023] The polarization-maintaining optical fiber isolator is used to transmit the optical signal unidirectionally to the output jumper head;

[0024] The output jumper is used to lead the optical signal after the intracavity mode locking is stabilized out of the system to achieve external applications.

[0025] Specifically, the reflectivity of the polarization-maintaining fiber grating is 20%, the central wavelength is 1064 nm, and the bandwidth is 2 nm.

[0026] Specifically, the splitting ratio of the 2x2 polarization-maintaining fiber coupler is 60:40, wherein 60% is output from the third port and 40% is output from the fourth port.

[0027] Specifically, the external optical signal source is any one of a pulsed semiconductor laser, a Q-switched fiber laser, a modulated CW laser and a mode-locked laser.

[0028] Beneficial effects of the present invention:

[0029] The invention provides a self-starting 9-shaped cavity laser based on external signal injection. The laser comprises a nonlinear amplifying ring mirror, a linear arm, a 2x2 polarization-maintaining fiber coupler, an external optical signal source, a 3-port polarization-maintaining fiber circulator, a photodetector and a control module. The first port and the second port of the 2x2 polarization-maintaining fiber coupler are respectively connected to the linear arm and the second port of the 3-port polarization-maintaining fiber circulator; the third port and the fourth port of the 2x2 polarization-maintaining fiber coupler are connected to the nonlinear amplifying ring mirror; the first port and the third port of the 3-port polarization-maintaining fiber circulator are respectively connected to the external optical signal source and the photodetector; and the control module is respectively connected to the nonlinear amplifying ring mirror, the external optical signal source and the photodetector.

[0030] The external optical signal is injected into the cavity through a 3-port polarization-maintaining fiber circulator to form a high-intensity initial light field, which guides the nonlinear enhancement of the intracavity signal. The strong signal is more likely to enter the positive feedback region of the cavity, allowing the cavity to enter the single-pulse mode-locked state more quickly. There is no need to use a high-power pump source, which avoids power waste and reduces costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below in conjunction with the accompanying drawings.

[0032] Figure 1 A schematic diagram of the structure of a 9-shaped cavity laser is provided for an embodiment of the present invention;

[0033] Figure 2 A component port schematic diagram is provided for an embodiment of the present invention;

[0034] Figure 3 A schematic diagram of a startup effect is provided in an embodiment of the present invention.

[0035] The meanings of the numbers in the figure are: 1-nonlinear amplifying ring mirror, 101-polarization-maintaining active fiber, 102-polarization-maintaining wavelength division multiplexer, 103-phase shifter, 104-polarization-maintaining passive fiber, 105-pump source; 2-linear arm, 201-output jumper head, 202-polarization-maintaining fiber isolator, 203-polarization-maintaining fiber grating; 3-2x2 polarization-maintaining fiber coupler; 4-external optical signal source; 5-3-port polarization-maintaining fiber circulator; 6-photodetector; 7-control module. DETAILED DESCRIPTION

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] The embodiment of the present invention provides a self-starting 9-shaped cavity laser based on external signal injection. Figure 1 , Figure 1 A schematic diagram of the structure of a 9-shaped cavity laser provided in an embodiment of the present invention. Figure 1 For several components in Figure 2 , Figure 2 (a) shows four ports of the 2x2 polarization-maintaining fiber coupler 3; Figure 2 (b) shows three ports of the 3-port polarization-maintaining fiber circulator 5 .

[0038] The laser comprises a nonlinear amplifying ring mirror 1, a linear arm 2, a 2x2 polarization-maintaining fiber coupler 3, an external optical signal source 4, a 3-port polarization-maintaining fiber circulator 5, a photodetector 6 and a control module 7; wherein:

[0039] The first port and the second port of the 2×2 polarization-maintaining fiber coupler 3 are connected to the linear arm 2 and the second port of the 3-port polarization-maintaining fiber circulator 5 , respectively.

[0040] The third port and the fourth port of the 2×2 polarization-maintaining fiber coupler 3 are connected to the nonlinear amplifying loop mirror 1 .

[0041] The first port and the third port of the three-port polarization-maintaining fiber circulator 5 are connected to the external optical signal source 4 and the photodetector 6 respectively.

[0042] The control module 7 is connected to the nonlinear amplifying ring mirror 1 , the external optical signal source 4 and the photodetector 6 respectively.

[0043] The nonlinear amplifying loop mirror 1 comprises a polarization-maintaining active optical fiber 101, a polarization-maintaining wavelength division multiplexer 102, a phase shifter 103, a polarization-maintaining passive optical fiber 104 and a pump source 105; wherein the polarization-maintaining active optical fiber 101, the polarization-maintaining wavelength division multiplexer 102, the phase shifter 103 and the polarization-maintaining passive optical fiber 104 are connected in sequence, and the pump source 105 is connected to the polarization-maintaining wavelength division multiplexer.

[0044] The linear arm 2 comprises an output jumper head 201, a polarization-maintaining fiber isolator 202 and a polarization-maintaining fiber grating 203; wherein the output jumper head 201, the polarization-maintaining fiber isolator 202 and the polarization-maintaining fiber grating 203 are connected in sequence.

[0045] In one implementation, the mode locking process of the laser includes:

[0046] Step 1: The control module 7 loads a preset current to the nonlinear amplifying loop mirror 1 to form a continuous optical signal in the loop.

[0047] Specifically, under the excitation of a preset current, the pump source 105 emits pump light, and the pump light enters the polarization-maintaining active optical fiber 101 through the polarization-maintaining wavelength division multiplexer 102. The polarization-maintaining active optical fiber 101 generates stimulated radiation to form a continuous optical signal in the ring. The continuous optical signal is bidirectionally transmitted in the ring in a clockwise direction and a counterclockwise direction.

[0048] Step 2: The control module 7 drives the external optical signal source 4 to inject the external optical signal into the nonlinear amplifying loop mirror 1 through the 3-port polarization-maintaining fiber circulator 5 and the 2x2 polarization-maintaining fiber coupler 3, and turns off the external optical signal source 4 after a period of time. The external optical signal source can be any one of a pulsed semiconductor laser, a Q-switched fiber laser, a modulated CW laser, and a mode-locked laser. The wavelength of the external signal light is the same as the operating wavelength of the laser, which is 1064nm, the power is 1mW, the pulse width is 1ns, and the repetition frequency is 2000KHz.

[0049] In this process, the two noise signals transmitted clockwise and counterclockwise in the ring generate nonlinear phase shift under the effect of the asymmetry of the positions of the phase shifter 103 and the polarization-maintaining active fiber 101; the noise signal includes a continuous optical signal and an external optical signal. The optical signals in two directions in the ring interfere at the 2x2 polarization-maintaining fiber coupler 3, and enter the polarization-maintaining fiber grating 203 and the 3-port polarization-maintaining fiber circulator 5 through the first port and the second port respectively.

[0050] Step 3: The photodetector 6 receives the intracavity optical signal through the 3-port polarization-maintaining fiber circulator 5 and converts it into an electrical signal.

[0051] It should be noted that the three-port polarization-maintaining fiber circulator 5 has the characteristic of high isolation. Light incident from the first port is emitted from the second port; light incident from the second port is emitted from the third port, and the external optical signal source will not be damaged.

[0052] Step 4: the control module 7 determines whether the mold locking is successful according to the electrical signal;

[0053] Step 5: If the mold locking fails, return to step 2.

[0054] Mode-locked lasers usually rely on spontaneous emission noise in the cavity to trigger the formation of short pulses. Due to internal nonlinear effects, inhomogeneity of pump light and other factors, figure-9 cavity lasers will generate spontaneous noise signals. These noise signals are usually random and have a wide spectrum. They reflect, propagate and interfere in the cavity. As the light signal in the cavity propagates, these noise signals begin to match the mode spectrum in the cavity. Factors such as the interaction between the modes in the cavity, nonlinear effects and phase mismatch in the optical fiber will cause these noise signals to focus on a specific frequency and phase. Eventually, the light signal in the cavity tends to stabilize and reaches a mode-locked state. However, at low pump power, the cavity noise is weak, making it difficult to start mode locking. Therefore, when starting, traditional figure-9 cavity lasers require higher pump power to trigger mode locking, and this self-starting mode locking relies on random fluctuations and takes a long time.

[0055] The self-starting 9-shaped cavity laser based on external signal injection provided in this embodiment provides additional random fluctuations by introducing an external optical signal, thereby enhancing the noise signal energy in the cavity, making it easier for the noise signal in the cavity to reach the intensity required to trigger the mode locking, thereby reducing the requirement for pump power; and by providing a strong initial signal, there is no need to wait for a long time for the spontaneous radiation noise to gradually accumulate, thereby shortening the mode locking time. It should be noted that, unlike the injection mode locking or external modulation mode locking in the active mode locking technology, the external optical signal in this embodiment does not need to strictly match the cavity mode frequency, thereby avoiding the cost of additional precision narrow linewidth laser source and control system.

[0056] See also Figure 3 , Figure 3 This is a schematic diagram of a startup effect provided by an embodiment of the present invention. Figure 3 (a) shows the mode locking process without external optical signal injection; Figure 3 (b) in the figure shows the mode-locking process when an external optical signal is injected. It can be seen that the self-starting 9-shaped cavity laser based on external signal injection proposed in the embodiment of the present invention avoids the characteristic that a high pump current needs to be applied for the rapid self-start of the traditional 9-shaped cavity, realizes mode-locking self-starting under low pump current, and reduces the waste of pump power.

[0057] In one embodiment, the polarization-maintaining active fiber 101 may be a polarization-maintaining ytterbium-doped high-gain fiber PM-YSF-HI-HP, and the corresponding pump source may be a semiconductor diode with a wavelength of 980 nm.

[0058] In one embodiment, the phase shifter 103 is a non-reciprocal phase shifter with a phase delay of -π / 2.

[0059] In one embodiment, the reflectivity of the polarization-maintaining fiber grating 203 is 20%, the central wavelength is 1064 nm, and the bandwidth is 2 nm.

[0060] Specifically, the polarization-maintaining fiber grating 203 divides the light transmitted by the 2x2 polarization-maintaining fiber coupler 3 into two paths, one of which reflects 20% of the optical power back to the 2x2 polarization-maintaining fiber coupler 3, and then enters the nonlinear amplifying loop mirror 1. The splitting ratio of the 2x2 polarization-maintaining fiber coupler 3 can be 60:40, specifically, 60% is output from the third port, and 40% is output from the fourth port.

[0061] The other path is transmitted to the polarization-maintaining fiber isolator 202; the polarization-maintaining fiber isolator 202 transmits the optical signal unidirectionally to the output jumper 201; the output jumper 201 leads the optical signal after the intracavity mode locking and stabilization out of the system to achieve external applications.

[0062] The above is a detailed description of an embodiment of the present invention, but the content is only a preferred embodiment of the present invention and cannot be considered to limit the scope of implementation of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A self-starting 9-shaped cavity laser based on external signal injection, the laser comprising a nonlinear amplifying ring mirror (1), a linear arm (2), and a 2x2 polarization-maintaining fiber coupler (3); characterized in that: The laser further comprises an external optical signal source (4), a three-port polarization-maintaining optical fiber circulator (5), a photodetector (6) and a control module (7); wherein: The first port and the second port of the 2x2 polarization-maintaining optical fiber coupler (3) are respectively connected to the linear arm (2) and the second port of the 3-port polarization-maintaining optical fiber circulator (5); The third port and the fourth port of the 2x2 polarization-maintaining optical fiber coupler (3) are connected to the nonlinear amplifying loop mirror (1); The first port and the third port of the three-port polarization-maintaining optical fiber circulator (5) are connected to the external optical signal source (4) and the photoelectric detector (6) respectively; The control module (7) is respectively connected to the nonlinear amplifying ring mirror (1), the external optical signal source (4) and the photoelectric detector (6).

2. A self-starting 9-shaped cavity laser based on external signal injection according to claim 1, characterized in that: The mode locking process of the laser includes: Step 1: the control module (7) loads a preset current to the nonlinear amplifying ring mirror (1) to form a continuous optical signal in the ring; Step 2: the control module (7) drives the external optical signal source (4) to inject the external optical signal into the nonlinear amplifying loop mirror (1) through the 3-port polarization-maintaining fiber circulator (5) and the 2x2 polarization-maintaining fiber coupler (3), and turns off the external optical signal source (4) after a period of time. Step three, the photodetector (6) receives the intracavity optical signal through the three-port polarization-maintaining fiber circulator (5) and converts it into an electrical signal; Step 4, the control module (7) determines whether the mold locking is successful based on the electrical signal; Step 5: If the locking fails, return to step 2.

3. A self-starting 9-shaped cavity laser based on external signal injection according to claim 2, characterized in that: The nonlinear amplifying loop mirror (1) comprises a polarization-maintaining active optical fiber (101), a polarization-maintaining wavelength division multiplexer (102), a phase shifter (103), a polarization-maintaining passive optical fiber (104) and a pump source (105); wherein: Under the stimulation of a preset current, the pump source (105) emits pump light, the pump light is incident on the polarization-maintaining active optical fiber (101) via the polarization-maintaining wavelength division multiplexer (102), and a continuous optical signal is generated in the ring, and the continuous optical signal is bidirectionally transmitted in the ring in a clockwise direction and a counterclockwise direction; Two noise signals transmitted clockwise and counterclockwise in the ring generate nonlinear phase shift under the effect of the asymmetry of the positions of the phase shifter (103) and the polarization-maintaining active optical fiber (101); the noise signals include the continuous optical signal and the external optical signal.

4. A self-starting 9-shaped cavity laser based on external signal injection according to claim 3, characterized in that: The polarization-maintaining active optical fiber (101) is a polarization-maintaining ytterbium-doped high-gain optical fiber PM-YSF-HI-HP, and the pump source is a semiconductor diode with a wavelength of 980 nm.

5. The self-starting 9-shaped cavity laser based on external signal injection according to claim 3, characterized in that: The phase shifter (103) has a phase delay of -π / 2.

6. A self-starting 9-shaped cavity laser based on external signal injection according to claim 2, characterized in that: The linear arm (2) comprises an output jumper head (201), a polarization-maintaining optical fiber isolator (202) and a polarization-maintaining optical fiber grating (203); wherein: The polarization-maintaining fiber grating (203) is used to split the light transmitted by the 2x2 polarization-maintaining fiber coupler (3) into two paths, one path being reflected back to the 2x2 polarization-maintaining fiber coupler (3), and the other path being transmitted to the polarization-maintaining fiber isolator (202); The polarization-maintaining optical fiber isolator (202) is used to unidirectionally transmit the optical signal to the output jumper head (201); The output jumper head (201) is used to lead the optical signal after the intracavity mode locking is stabilized out of the system to achieve external applications.

7. A self-starting 9-shaped cavity laser based on external signal injection according to claim 6, characterized in that: The polarization-maintaining fiber grating (203) has a reflectivity of 20%, a central wavelength of 1064 nm, and a bandwidth of 2 nm.

8. The self-starting 9-shaped cavity laser based on external signal injection according to claim 2, characterized in that: The splitting ratio of the 2x2 polarization-maintaining optical fiber coupler (3) is 60:40, wherein 60% is output from the third port and 40% is output from the fourth port.

9. The self-starting 9-shaped cavity laser based on external signal injection according to claim 1, characterized in that: The external optical signal source (4) is any one of a pulsed semiconductor laser, a Q-switched fiber laser, a modulated CW laser and a mode-locked laser.

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