WO2019200886A1 - Method for realizing synchronous tuning of wavelength and repetition frequency in passive mode-locking laser - Google Patents

Abstract

A method for realizing synchronous tuning of a wavelength and a repetition frequency in a passive mode-locking laser. One end of a section of active optical fiber or passive optical fiber in a resonant cavity of the laser is polished into a mirror surface and fixed on a multi-dimensional tuning frame (9); a semiconductor saturable absorption mirror (4) with copper radiators is mounted on a fixing support, and by rotating a micrometer of a rotating platform on the multi-dimensional tuning frame, the coupling angle of the section of optical fiber and the semiconductor saturable absorption mirror is mechanically tuned, so that the laser wavelength and the repetition frequency are synchronously tuned. Compared with a conventional method adopting piezoelectric ceramic or a fiber grating, the apparatus is simpler and more compact. By rotating the micrometer of the rotating platform on the multi-dimensional tuning frame, high-precision synchronous tuning of the laser wavelength and the repetition frequency can be achieved, and the repetition frequency tuning range can reach 1 MHz. The tuning method is simple and easy to realize, and can be applied to the fields of precise gas detection, gas spectrum analysis, frequency calibration, nonlinear biophotonics and the like.

Description

Method for realizing synchronous adjustment of wavelength and repetition frequency in passive mode-locked laser Technical field

The invention relates to the field of passive mode-locked lasers, and in particular to a method for realizing synchronous adjustment of wavelength and repetition frequency in a passive mode-locked laser.

Background technique

The wavelength and repetition frequency tunable characteristics of femtosecond lasers extend their applications to precision gas detection, gas spectroscopy, frequency calibration, and nonlinear biophotonics. The wavelength and re-frequency tunability of the mode-locked femtosecond laser can be divided into two concepts according to the two types of active mode-locking and passive mode-locking. For the active mode-locked laser, changing the frequency of the electrical signal modulation to change the repetition frequency of the femtosecond pulse can directly realize that the laser wavelength can be tuned within a certain range. The advantage of this method is that the tuning range is large (for repetition frequency) and the operation is intuitive. However, due to the introduction of complex electrical equipment, it will undoubtedly increase the cost and complexity of the system. For passive mode-locked lasers, since the passive mode-locking itself does not require an external electrical modulation signal, the system structure is simple, so the re-frequency and wavelength tunability can be realized directly by regulating optical components such as laser resonators or gains.

At present, there are two main methods for tuning in passive mode-locked lasers. First, by changing the polarization state of the system to change the polarization loss introduced by the optical device in the cavity, the power in the cavity is adjusted, and finally the adjustment of the output wavelength and the re-frequency is realized. However, this tuning is very limited, the tunable range of the wavelength is very small, and the change of the repetitive frequency is weak to negligible. The second is to adjust the cavity length or reflection wavelength of the laser cavity by changing the temperature of the laser gain fiber or the reflection grating, thereby changing the repetition frequency and the laser wavelength. However, this change in cavity length due to temperature changes tends to be on the order of micrometers. When applied to conventional passive mode-locked lasers with a frequency of several tens of megahertz, the cavity length is generally in the range of ten meters to several tens of meters. Inside, the relative change in cavity length is extremely small, and the range of repetition frequency of regulation is very limited. In addition, it is difficult to achieve synchronous tuning of laser wavelength and repetition frequency by changing the reflection grating, which affects the practical application to some extent.

Under the above background, the present invention proposes a new method for simultaneously tuning the laser wavelength and the repetition frequency to obtain a larger tuning range. The method is very simple and easy to implement, and only requires mechanical adjustment of the coupling angle of a length of fiber and a semiconductor saturable absorption mirror. This method is not limited by the cavity length and is suitable for all passive mode-locked femtosecond lasers, but especially for ultra-short cavity (<10cm) passive mode-locked femtosecond lasers, the tuning range and sensitivity can be greatly improved, so More suitable for the output wavelength and repetition frequency of high-repetition femtosecond lasers. In summary, this method provides a new idea for the tuning of the femtosecond laser wavelength and repetition frequency, and has great application potential.

Summary of the invention

One of the objects of the present invention is to provide a method for synchronizing the wavelength and repetition frequency in a passive mode-locked laser, which has a tuning range for the femtosecond laser wavelength and repetition frequency much larger than that of the conventional passive mode-locked laser. And very simple and easy to implement.

The invention is achieved by the following scheme.

A method for synchronously adjusting the wavelength and the repetition frequency in a passive mode-locked laser, which fixes a section of the optical fiber and the semiconductor saturable absorption mirror in the laser cavity to the multi-dimensional adjustment frame, and rotates the micrometer of the rotating platform in the multi-dimensional adjustment frame. The coupling angle of the fiber and the semiconductor saturable absorption mirror is adjusted to achieve tuning of the laser wavelength and the repetition frequency.

Further, the length of the optical fiber is an active optical fiber or a passive optical fiber.

Further, the repetition frequency is adjusted from 0 to 1 MHz.

Further, the apparatus used in the method of the present invention comprises, in order from left to right, a laser diode, a polarization controller, a wavelength division multiplexer, a dichroic mirror, a ceramic tube wearing a length of optical fiber, and a semiconductor saturable absorption mirror. It also includes an isolator and a multi-dimensional mount; the wavelength division multiplexer is also connected to the isolator, the output of the isolator acts as a laser output; the ceramic tube that passes through a length of fiber is mounted on a multi-dimensional mount.

Further, the inner diameter of the ceramic tube is the same as the outer diameter of the optical fiber, and a length of the optical fiber is inserted into the ceramic tube; the end surface of the ceramic tube is polished and mirror-polished.

Further, another device used in the method of the present invention includes a laser diode, a wavelength division multiplexer, a gain fiber, and a coupler output in a clockwise direction as a laser output terminal, an isolator, a polarization controller, and a ring. The device further comprises a glass tube, a multi-dimensional adjustment frame and a semiconductor saturable absorption mirror which are inserted through a fiber; the circulator is also connected with a wavelength division multiplexer, one end of the glass tube which is passed through the optical fiber; and the glass tube which is inserted through the optical fiber is installed in the multi-dimensional The other end of the glass tube that is inserted through the optical fiber is connected to the semiconductor saturable absorption mirror.

The invention uses a section of an active fiber or a passive fiber in a laser cavity that is passively mode-locked by a saturable absorber to be mirrored and fixed on a multi-dimensional mount. The semiconductor saturable absorption mirror is mounted on the fixed bracket, and the coupling angle of the length of the optical fiber and the semiconductor saturable absorption mirror is mechanically adjusted by rotating the micrometer of the rotating platform in the multi-dimensional adjustment frame, thereby synchronously tuning the wavelength and repetition of the passive mode-locked laser frequency. The repetition frequency can be tuned up to 1MHz.

Compared with the prior art, the present invention has the following advantages:

The method of tuning the laser wavelength and repetition frequency of the passive mode-locked laser of the present invention is very simple and easy to implement. It is only necessary to mechanically rotate the micrometer of the rotating platform in the multi-dimensional adjustment frame to achieve high-precision synchronous tuning of the laser wavelength and the repetition frequency, and the adjustment range of the re-frequency is up to 1 MHz. The device of this method is simpler and more compact than the method using piezoelectric ceramic or fiber grating.

The invention has wide application range and is not limited by the wavelength of the laser, and is suitable for the passive mode-locked laser cavity of the polarization maintaining structure and the passively mode-locked laser cavity of the non-polarization maintaining.

DRAWINGS

1 is a structural diagram of a coupling angle tuning in Embodiment 1 of the present invention.

2 is a structural diagram of a coupling angle tuning in Embodiment 2 of the present invention.

FIG. 3 is a schematic diagram of coupling angle tuning in an embodiment of the present invention.

Figure 4 is a graph showing the evolution of the laser wavelength as a function of the coupling angle in the test case.

Figure 5 is a graph showing the evolution of the repetition frequency with the coupling angle in the test case.

Fig. 6 is a laser wavelength chart in the test example.

Fig. 7 is a partially enlarged view of the laser wavelength in the test example.

Fig. 8 is a pulse width diagram in the test example.

detailed description

The present invention will be further described in detail below with reference to the embodiments and the accompanying drawings. However, embodiments of the invention are not limited thereto.

Example 1

1 is a structural diagram of a femtosecond laser wavelength and a repetition frequency adjustable for a passive mode-locked laser of the present embodiment, from left to right, a laser diode 1, a polarization controller 2, a wavelength division multiplexer 7, and a dichroic mirror. 3. A ceramic tube 8 through which a fiber is inserted, a semiconductor saturable absorption mirror 4, the wavelength division multiplexer 7 is also connected to the isolator 6, the output of the isolator 6 is used as a laser output end 5; the ceramic tube 8 penetrating a length of optical fiber is mounted on Multi-dimensional adjustment frame 9. The multi-dimensional adjustment frame can adopt the manual precision aligning unit of the Japanese Shenjin Seiki (KOHZU), and the ceramic tube 8 penetrating a length of fiber is opposed to the semiconductor saturable absorption mirror 4 by adjusting the θy axis of the multi-dimensional adjustment frame with an accuracy of 0.0059°. The angle of the gradual change gradually realizes the synchronous adjustment of the laser wavelength and the repetition frequency. The inner diameter of the ceramic tube is the same as the outer diameter of the optical fiber, and a length of the optical fiber is inserted into the ceramic tube. After the end surface of the ceramic tube is flattened, mirror polishing is performed, and the polished ceramic tube is fixed to the multi-dimensional adjustment frame 9.

The dichroic mirror has a transmittance of 95% for pump light and 5% for signal light. By rotating the micrometer of the rotating platform in the multi-dimensional adjustment frame 9, the coupling angle of the ceramic tube 8 passing through the optical fiber and the semiconductor saturable absorption mirror 4 is mechanically adjusted, thereby simultaneously tuning the laser wavelength and the repetition frequency.

Example 2

2 is a structural diagram showing the adjustable wavelength and repetition frequency of the femtosecond laser of the passive mode-locked laser of the present embodiment, and the output of the laser diode 1, the wavelength division multiplexer 7, the gain fiber 10, and the coupler 11 in the clockwise direction. As the laser output terminal 5, the isolator 6, the polarization controller 2, and the circulator 12, the circulator 12 is also connected to the wavelength division multiplexer 7, one end of the glass tube 13 penetrating the passive optical fiber, and the glass tube of the passive optical fiber. 13 is mounted on the multi-dimensional mount 9, and the other end of the glass tube 13 penetrating the passive optical fiber is connected to the semiconductor saturable absorption mirror 4. Wherein, the inner diameter of the glass tube is the same as the outer diameter of the optical fiber, and the optical fiber is inserted into the glass tube. After the end surface of the glass tube is flattened, mirror polishing is performed, and the polished glass tube is fixed to the multi-dimensional adjustment frame 9.

The coupler 11 employs a coupling ratio of 95:5, the 5% output of the coupler 11 as the laser output 5, and the output of the system signal light is 5%. By rotating the micrometer of the rotating platform in the multi-dimensional adjustment frame 9, the coupling angle of the glass tube 13 passing through the passive optical fiber and the semiconductor saturable absorption mirror 4 is mechanically adjusted, thereby simultaneously tuning the laser wavelength and the repetition frequency.

Test case:

The following test cases are mainly tested for the examples.

In the test case, the coupling angle of the optical fiber and the semiconductor saturable absorption mirror is mechanically adjusted, and the theoretical mechanism of the laser wavelength and repetition frequency of the synchronously tuned passive mode-locked laser is as follows:

FIG. 3 is a schematic diagram of coupling angle tuning in an embodiment of the present invention. Due to the micrometer of the rotating platform in the rotating multi-dimensional adjustment frame, the ceramic tube end face and the semiconductor saturable absorption mirror which pass through the optical fiber have a coupling angle and an air gap, so that the length of the entire laser cavity is changed, resulting in a change in the repetition frequency. The laser exiting from the fiber end face will have a filtering effect when it returns to the fiber, so that the laser wavelength can be tuned. For synchronous tuning of the laser wavelength and repetition frequency, the following functions can be mathematically described:

Where f is the laser repetition frequency, and c, n, and L are the speed of light, the refractive index, and the length of the cavity. The coupling angle of the fiber and the semiconductor saturable absorption mirror is mechanically adjusted to change the cavity length of the entire laser cavity, resulting in a change in the repetition frequency.

Due to the mechanical adjustment of the coupling angle, the fiber is tilted relative to the semiconductor saturable absorption mirror, resulting in a deviation x ₁ when the exiting laser returns to the fiber plane. The corresponding description function:

Where x ₁ is the deviation when the laser returns to the plane of the fiber, x ₀ is the distance perpendicular to the incident end point of the fiber to the saturable absorption mirror, and θ and w are the incident angle and frequency of the outgoing laser, respectively, and n ₂ and d are respectively The refractive index and thickness of the saturated absorption mirror, n' ₂ is the first derivative of n ₂ versus w. Since n' _{2 is} greater than 0, the lower optical frequency corresponds to a larger deviation x ₁ , so long-wavelength light is easily filtered out during the adjustment of the angle, allowing the spectral center wavelength to be tuned.

The test results are as follows:

Fig. 4 and Fig. 5 are correlation diagrams of the laser wavelength and repetition frequency of the passive mode-locked laser in the test example, which are tuned with the coupling angle. In the test example, the equivalent length of the mode-locked laser cavity is 3 cm. According to the calculation of the relationship between the speed of light c, the cavity length L, the fundamental frequency repetition frequency f and the refractive index n of the fiber, f=c/(2nL), the base of the laser cavity is known. The frequency repetition frequency is approximately 3.2 GHz. It can be seen from the spectrum and spectrogram that the coupling angle of the mechanical adjustment gain fiber and the semiconductor saturable absorption mirror can realize the synchronous tuning of the laser wavelength and the repetition frequency. The repetition frequency can be tuned from 3.19331965 GHz to 3.19338015 GHz, and the tuning range is 60.5. KHz; the spectral peak wavelength can be tuned from 1591.4 nm to 1586.1 nm with a tuning range of 5.3 nm. Figure 6 is a pulse spectrum of the center wavelength of 1590nm; Figure 7 is a partial enlarged view of the pulse spectrum of Figure 6, the longitudinal mode spacing in the spectrum can be seen, corresponding to the pulse base repetition frequency of 3.2GHz; Figure 8 is the autocorrelation diagram of the pulse, pulse The width is 639fs, indicating that the pulse width is narrow.

The above test examples are one of the embodiments of the present invention, but the embodiments of the present invention are not limited by the embodiments and the test examples, and any other changes, modifications, and modifications made without departing from the spirit and principles of the present invention. Alternatives, combinations, and simplifications are all equivalents and are included in the scope of the present invention.

Claims (6)

1. A method for synchronously adjusting wavelength and repetition frequency in a passive mode-locked laser is characterized in that a section of an optical fiber and a semiconductor saturable absorption mirror in a laser cavity are respectively fixed on a multi-dimensional adjustment frame, and a rotating platform of the multi-dimensional adjustment frame is rotated. The micrometer adjusts the coupling angle of the fiber and the semiconductor saturable absorption mirror to achieve tuning of the laser wavelength and repetition frequency.
2. The method of claim 1, wherein the length of the fiber is an active fiber or a passive fiber.
3. A method of realizing wavelength and repetition frequency synchronization adjustment in a passive mode-locked laser according to claim 1, wherein said repetition frequency is adjusted from 0 to 1 MHz.
4. Apparatus for implementing the method of any one of claims 1 to 3, characterized by comprising laser diode (1), polarization controller (2), wavelength division multiplexer (7), and two colors from left to right Mirror (3), ceramic tube (8) wearing a length of fiber, semiconductor saturable absorption mirror (4), isolator (6) and multi-dimensional adjustment frame (9); wavelength division multiplexer (7) is also isolated The device (6) is connected, the output of the isolator (6) is used as a laser output end (5), and the ceramic tube (8) penetrating a length of fiber is mounted on the multi-dimensional adjustment frame (9).
5. The apparatus according to claim 4, wherein the inner diameter of the ceramic tube is the same as the outer diameter of the optical fiber, and a length of the optical fiber is inserted into the ceramic tube; the end surface of the ceramic tube is smoothed and mirror-polished.
6. Apparatus for implementing the method of any one of claims 1 to 3, characterized by comprising a laser diode (1), a wavelength division multiplexer (7), a gain fiber (10), a coupler (11) in a clockwise direction The output is used as a laser output (5), an isolator (6), a polarization controller (2), a circulator (12), and a glass tube (13), a multi-dimensional mount (9) and a semiconductor that pass through a length of fiber. The saturable absorption mirror (4); the circulator (12) is also connected to the wavelength division multiplexer (7), one end of the glass tube (13) penetrating the optical fiber; the glass tube (13) penetrating the optical fiber is installed in the multi-dimensional adjustment The other end of the glass tube (13) through which the optical fiber is inserted is connected to the semiconductor saturable absorption mirror (4).

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