CN108767642B - Method for generating low-repetition-frequency high-energy pulse from mode-locked laser - Google Patents

Method for generating low-repetition-frequency high-energy pulse from mode-locked laser Download PDF

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CN108767642B
CN108767642B CN201810191924.2A CN201810191924A CN108767642B CN 108767642 B CN108767642 B CN 108767642B CN 201810191924 A CN201810191924 A CN 201810191924A CN 108767642 B CN108767642 B CN 108767642B
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laser
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repetition
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CN108767642A (en
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谢国强
陈迎安
乔桢
钱列加
袁鹏
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Shanghai Jiaotong University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/086—One or more reflectors having variable properties or positions for initial adjustment of the resonator

Abstract

A method for generating low-repetition-frequency high-energy pulse from mode-locked laser, in the mode-locked laser, using multi-transverse-mode phase locking to modulate mode-locked pulse sequence periodically, when the frequency interval of adjacent transverse modes is M/N of longitudinal-mode frequency interval (M/N is positive rational number, M and N are positive integer and are mutually prime), the mode-locked laser repetition frequency can be changed into 1/N of fundamental frequency (c/2L, c is light speed, L is cavity length), thereby realizing mode-locked laser output with low repetition frequency and high pulse energy. The invention can realize the great reduction of the repetition frequency (reduced to 1/N of the fundamental frequency) without increasing the cavity length, thereby having the effect of generating high pulse energy from the mode-locked laser, and the N value can be adjusted according to the requirement.

Description

Method for generating low-repetition-frequency high-energy pulse from mode-locked laser
Technical Field
The invention belongs to the technical field of ultrafast lasers, and particularly relates to a method for generating low-repetition-frequency high-energy pulses from a mode-locked laser.
Background
The ultrashort pulse generated by the mode-locked laser has short duration and high peak power, and is widely applied to various fields such as manufacturing industry, medicine and the like.
The pulse repetition frequency of the mode-locked laser is an important parameter, and determines the pulse energy of the mode-locked laser under the condition of certain average power of the mode-locked laser. Generally, the repetition rate of the laser pulses depends on the cavity length, for example, a repetition rate of 100 MHz for a laser having a cavity length of 1.5 m, i.e., 10 laser pulses per second8And (4) respectively. Therefore, to achieve high pulse energies in mode-locked lasers, this can be achieved by reducing the repetition rate. Reducing the repetition rate can usually only be achieved by increasing the cavity length, but this can be problematic in practical lasers: to reduce the repetition frequency from 100 mhz to 1 mhz, the cavity length needs to be increased from 1.5 m to 150 m, so that the volume of the solid laser becomes very large, and multiple optical elements are needed to fold the optical path, but the fiber laser has the limit of nonlinearity after adopting an excessively long fiberAnd (5) preparing.
Disclosure of Invention
The present invention provides a method of generating low repetition frequency high energy pulses from a mode-locked laser. The method can realize the great reduction of the repetition frequency (reduced to 1/N of the fundamental frequency) without increasing the cavity length, thereby having the effect of generating high pulse energy from the mode-locked laser, and the value of N can be adjusted according to the requirement.
The technical solution of the invention is as follows:
a method for generating low-repetition-frequency high-energy pulse from mode-locked laser features that in the mode-locked laser with multiple transverse modes, the phases of said transverse modes are locked, and when the Gouy phase shift in cavity is M pi/N, the repetition frequency of output laser pulse is changed to 1/N of fundamental frequency c/2L, where M/N is positive rational number, M and N are both positive integers and are mutually prime, c is light speed and L is cavity length. By adjusting the position of the laser cavity mirror, the size of N can be changed.
A method for generating low repetition frequency high energy pulse from mode-locked laser includes following steps:
1) exciting multiple transverse modes to oscillate simultaneously: the output end of the mode-locked laser is provided with a CCD, and for the solid laser, after the laser emits light, the position of a concave mirror is finely adjusted so as to increase the mode size ratio of pump light and laser and excite multiple transverse modes to simultaneously oscillate; for the optical fiber laser, a multimode optical fiber is used for exciting multiple transverse modes to oscillate simultaneously, and a light spot pattern is observed through the CCD to judge that the multiple transverse modes are generated;
2) and (3) simultaneous locking of multiple transverse molds: connecting the output end of the CCD with the input end of an oscilloscope, then finely adjusting the position of the concave mirror, observing a pulse sequence on the oscilloscope, and observing a stable pulse sequence, namely a modulated pulse sequence, on the oscilloscope;
3) adjusting the value of N: because the pulse sequence on the oscilloscope can reflect the size of the N value, the position of each cavity mirror is finely adjusted, and the pulse sequence on the oscilloscope is observed at the same time, so as to obtain the expected low-repetition-frequency high-energy pulse, and the repetition frequency of the pulse is reduced to 1/N of the fundamental frequency.
Under the condition that the phase of each transverse mode is locked in the mode-locked laser, the expression of the photoelectric field of the mode-locked pulse in the time domain is as follows:
Figure BDA0001591955460000021
wherein A ism,nIs the amplitude of each transverse mode, the natural number q is the longitudinal mode number, the natural numbers m and n are the transverse mode numbers, Δ νLc/2L is the longitudinal mode frequency spacing, Δ νTIs the transverse mode frequency interval, Δ νTAnd Δ νLIs equal to the ratio of the Gouy phase shift within the cavity, which is determined by the cavity structure and is tunable, to a constant pi. The first term of the optical electric field represents a modulation function generated by multi-transverse mode phase locking, and the second term represents a fundamental frequency pulse sequence generated by longitudinal mode locking. When the phases of different transverse modes are locked and delta vTAnd Δ νLWhen the ratio of (A) is M/N (M/N is a positive rational number, M and N are both positive integers and are relatively prime, and the ratio can represent a positive rational number), the least common multiple of the modulation function period locked by the transverse mode phase and the pulse period generated by the longitudinal mode locking is the period of the output pulse, the reciprocal of the period is solved to obtain the repetition frequency of the output pulse, and the repetition frequency is 1/N of the fundamental frequency (independent of the value of M).
The beneficial effects of the invention can be summarized as follows:
1) the invention realizes the mode locking pulse output with low repetition frequency and high pulse energy by using the phase locking of a plurality of transverse modes in the mode locking laser;
2) the repetition frequency can be greatly reduced without changing the cavity length, and the compactness of the mode-locked laser is kept.
Drawings
FIG. 1 is a schematic of the present invention. (a) Traditional single transverse mode-locked laser: the output pulse period is the cavity period (T); (b) the invention relates to a low repetition frequency mode-locked laser: through multi-transverse-mode phase locking, and the Gouy phase shift in the cavity meets M pi/N (M/N is a positive rational number, M and N are both positive integers and are coprime, and the ratio of M to N can represent a positive rational number), the period of the output pulse is NT, namely the repetition frequency is changed into 1/N of the fundamental frequency.
Fig. 2 is a low repetition frequency pulse generation principle. (a) Pulse sequence of longitudinal mode locking; (b) intensity modulation function due to transverse mode phase locking; (c) the mode-locked laser generates a low repetition frequency (repetition frequency becomes 1/N of the fundamental frequency) pulse train.
Fig. 3 is an example of a calculation to achieve a low repetition frequency. (a) A corresponding base frequency pulse sequence with the cavity length of 1.5 meters; (b) when the intra-cavity Gouy phase shift is pi/5, the multi-transverse mode phase locks the low repetition frequency (1/5 where the repetition frequency becomes the fundamental frequency) mode-locked pulses generated by the mode-locked laser.
Fig. 4 is a low repetition frequency mode-locked laser embodiment of the present invention.
Detailed Description
For a mode-locked laser with a cavity length of 1.5 m, the pulse sequence generated by single transverse mode locking is shown in fig. 3(a), and the repetition frequency of the fundamental frequency is 100 mhz. If the 10 transverse modes (00 mode, 01 mode, … …, 09 mode) are locked at the same phase, when the Gouy phase shift in the cavity is pi/5, then the repetition frequency becomes 20 mhz, 1/5 of the fundamental frequency, as shown in fig. 3 (b).
We will further describe the experimental examples. Referring to fig. 4, fig. 4 is a diagram of an embodiment of a low repetition frequency mode-locked laser according to the present invention.
Fig. 4(a) is an optical path diagram of a mode-locked laser. Wherein LD is the pump source, L1And L2Two focusing lenses, M1、M2And M3Three concave mirrors, the SESAM is a semiconductor saturable absorber, and the OC is an output coupling mirror. To achieve the reduction of the repetition frequency, the specific operation comprises the following steps:
1) exciting multiple transverse modes to oscillate simultaneously: the output of an output coupling mirror OC of the mode-locked laser is provided with a CCD, and for the solid laser, after the laser emits light, the positions of concave mirrors M1, M2 or M3 are finely adjusted so as to increase the mode size ratio of pump light and laser and excite multiple transverse modes to simultaneously oscillate; simultaneously observing the light spot pattern through the CCD to judge that a plurality of transverse modes are generated;
2) and (3) simultaneous locking of multiple transverse molds: connecting the output end of the CCD with the input end of an oscilloscope, then finely adjusting the position of the concave mirror, observing a pulse sequence on the oscilloscope, and observing a stable pulse sequence, namely a modulated pulse sequence, on the oscilloscope;
3) adjusting the value of N: because the pulse sequence on the oscilloscope can reflect the size of the N value, the position of each cavity mirror is finely adjusted, and the pulse sequence on the oscilloscope is observed at the same time, so as to obtain the expected low-repetition-frequency high-energy pulse, and the repetition frequency of the pulse is reduced to 1/N of the fundamental frequency.
(b) Locking the multi-transverse-mode phase in the mode-locked laser to obtain a light spot pattern; (c) the repetition frequency obtained in the experiment was a pulse sequence with a fundamental frequency of 1/5. As shown in fig. 4(a), a mode-locked laser with a cavity length of 1.39M (the fundamental frequency c/2L is 107.6 mhz), and a concave mirror M is adjusted2And M3Thereafter, three phase-locked transverse modes (00 mode, 02 mode, and 04 mode) oscillate in the laser as shown in fig. 4 (b). With the cavity mirrors adjusted, we obtained a pulse sequence as shown in FIG. 4(c) with a pulse period of 46.5 ns and a repetition rate of 21.5 MHz, equal to 1/5 at the fundamental frequency. At this time, N is 5 from the repetition frequency, and the Gouy phase shift in the cavity is 13 pi/5 by calculation.
It should be noted that the above examples are only for better illustrating the present invention and are not meant to be limiting. It should be understood by those skilled in the art that various modifications and equivalent arrangements can be made without departing from the spirit and scope of the present invention and the present invention is covered by the appended claims.

Claims (3)

1. A method for generating low-repetition-frequency high-energy pulse from a mode-locked laser is characterized in that in the laser, a saturable absorption element is placed in a laser cavity and a cavity mirror is finely adjusted after a solid laser is excited to oscillate in multiple transverse modes by increasing the size of a pump light mode or using multimode fibers for a fiber laser; when the modulated pulse sequence is observed on an oscilloscope, the transverse mode is locked, and the expression of the optical electric field at the center of the light spot in the time domain is as follows:
Figure FDA0002573016180000011
where t is time, Am, n is amplitude of each transverse mode, q is a natural number, m and n are longitudinal number, f (ω) isq,0,0) Is the amplitude, Δ ν, of each longitudinal mode corresponding to the fundamental modeLc/2L is the longitudinal mode frequency interval, where c is the speed of light and L is the cavity length; Δ νTIs the transverse mode frequency interval, Δ νTAnd Δ νLThe ratio of the first factor to the second factor is equal to the ratio of Gouy phase shift to constant pi, the Gouy phase shift is determined by the cavity structure and is adjustable, the first factor of the optical-electric field represents the modulation function generated by multi-transverse mode phase locking, the second factor represents the fundamental frequency pulse sequence generated by longitudinal mode locking, when the phases of different transverse modes are locked and delta v is equal toTAnd Δ νLWhen the ratio of (A) to (B) is M/N, M/N is a positive rational number, M and N are both positive integers and are relatively prime, the ratio can represent a positive rational number, and the repetition frequency is changed into 1/N of the fundamental frequency and is irrelevant to the value of M.
2. The method of claim 1 in which the magnitude of N is varied by adjusting the position of the cavity mirrors of the laser.
3. A method of generating low repetition frequency high energy pulses from a mode-locked laser as claimed in claim 1 or 2, characterized in that the method comprises the following steps:
1) exciting multiple transverse modes to oscillate simultaneously: the output end of the mode-locked laser is provided with the CCD, and for the solid laser, after the laser emits light, the position of the cavity mirror is finely adjusted to increase the mode size ratio of pump light and laser light, so that multiple transverse modes are excited to oscillate simultaneously; for the optical fiber laser, a multimode optical fiber is used for exciting multiple transverse modes to oscillate simultaneously, and a light spot pattern is observed through the CCD to judge that the multiple transverse modes are generated;
2) and (3) simultaneous locking of multiple transverse molds: placing a saturable absorption element in a laser cavity, placing a detector at the center of a light spot, connecting the output end of the detector with the input end of an oscilloscope, finely adjusting the position of the cavity mirror, observing a pulse sequence on the oscilloscope, and locking a transverse mode when a stable modulated pulse sequence is observed on the oscilloscope;
3) adjusting the value of N: because the pulse sequence on the oscilloscope can reflect the size of the N value, the position of each cavity mirror is finely adjusted, and the pulse sequence on the oscilloscope is observed at the same time, the expected low-repetition-frequency high-energy pulse can be obtained, and the repetition frequency is reduced to 1/N of the fundamental frequency.
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