CN115016133B - Novel carrier modulation pulse generation device and method - Google Patents

Abstract

The invention provides a novel device and method for generating carrier-modulated pulses, comprising: a first half-wave plate, a polarization beam splitter and a cyclic delay loop; the first half-wave plate is used to adjust the polarization direction of the incident pulse; The beam splitter is used to split the incident pulse after the polarization direction is adjusted to obtain the split pulse; the cyclic delay loop is used to reflect and adjust the polarization direction of the split pulse; the polarization beam splitter is also used to The beam-splitting pulse after the polarization direction is adjusted, obtains a modulated pulse train with equal pulse intervals. The present invention combines the method of polarization modulation and cyclic time delay to obtain a modulated pulse train with the same time interval between adjacent sub-pulses after intensity modulation of a single pulse, which has certain universality.

Description

A Novel Carrier Modulated Pulse Generating Device and Method

technical field

The invention belongs to the field of laser technology, and in particular relates to a novel device and method for generating carrier-modulated pulses.

Background technique

The existing technologies at home and abroad can be divided into the following three types:

The principle of splitting delay splicing method based on beam splitter is shown in Figure 1. The pulse modulation system consists of three 50:50 beam splitters and four mirrors. The 532nm pulse can be divided into Four sub-pulses transmitted along four delay lines of different lengths, the four delay lines are shown in Figure 2, and the lengths of the four delay lines (marked in red) are arranged in an arithmetic sequence, namely 2x, 4x, 2x+ 2y, 4x+2y form an arithmetic progression, then y=2x is required. A pulse train with the same time interval between four adjacent pulses can be obtained. Its time interval Δt=2x/c, and the modulation frequency is equal to the reciprocal f=c/2x of the time interval of adjacent pulses. The modulated pulse obtained in this way is shown in Figure 3. The interval between adjacent pulses is 666ps, and the corresponding modulation frequency is 1.5GHz. This method can directly modulate the 532nm laser pulse, but this method needs to adjust four mirrors and three beam splitters, there are 7 degrees of freedom in the optical path, and it is difficult to align the optical path. In addition, there are two output ends of the last beam splitter, and this method only selects one end as the output, which loses half of the pulse energy.

The principle of the cyclic frequency shift method based on the acousto-optic modulator is shown in Figure 4. The idea of this method is to perform high-frequency modulation on the 1064nm pulse first, and then frequency multiplies to obtain the 532nm modulated pulse. Reflector 1 and reflector 2 form the linear resonant cavity of the passive Q-switched pulsed laser at the left end. The generated pulse is output through reflector 2, and the frequency shift after passing through the acousto-optic modulator is the driving frequency f _AO , and then reflected by the reflector The frequency shift amount through the AOM is 2f _AO , so every time the pulse goes back and forth to the AOM, the frequency shift amount of 2f _AO will be obtained, and after being reflected by the mirror 2, it will go back and forth through the AOM again, and the above cycle will continue. In the process, the frequency-shifted 1064nm laser pulse will beat with the initial pulse generated by the passive Q-switched pulse laser on the left to generate a high-frequency modulated pulse. Its pulse waveform is shown in Figure 5(a), and its frequency spectrum is shown in Figure 5(b). Although this method obtains high-frequency modulated pulses by passing through the acousto-optic frequency shifter multiple times, its spectral energy is dispersed in each harmonic. In addition, it will cause loss every time it passes through the acousto-optic frequency shifter. The spectral energy is greatly attenuated.

The principle of the circular delay method based on dichroic mirrors and frequency-doubling crystals is shown in Figure 6. In this method, frequency-doubling crystals are placed in two dichromatic mirrors to form a linear cavity to modulate a 1064nm pulse laser to obtain a 532nm modulated pulse. . After a single laser pulse of 1064nm is input on the left side, the left dichromatic mirror is a full lens of 1064nm, so the pulse passes through the left dichromatic mirror and then enters the linear cavity to obtain a 532nm pulse. The right dichromatic mirror is partially transparent to the 532nm pulse However, after outputting a 532nm sub-pulse at this time, part of the 532nm pulse is reflected back and forth in the cavity and then part of the pulse is output, thus obtaining a modulated pulse with constant pulse intensity attenuation and the same time interval. Its waveform is shown in Figure 7(a), and its frequency spectrum is shown in Figure 7(b). This method does not directly modulate the 532nm pulse. Because it uses a dichroic mirror and a frequency doubling crystal to form an external cavity modulator, additional parameters such as the length of the frequency doubling crystal need to be designed to achieve the optimal conversion efficiency, which increases the system design. difficulty. In addition, the spot distribution of the incident pulse, the change of power, and even the change of ambient temperature will also affect the conversion efficiency, which reduces the general applicability of the method.

Contents of the invention

In order to solve the above technical problems, the present invention proposes a new device and method for generating carrier-modulated pulses. By combining polarization modulation and cyclic delay method, the intensity modulation of a single pulse is obtained to obtain the modulation with the same time interval between adjacent sub-pulses. Pulse train has certain universality.

On the one hand, in order to achieve the above object, the present invention provides a novel carrier modulation pulse generating device, comprising: a first half-wave plate, a polarization beam splitter and a cyclic delay loop;

The first half-wave plate is used to adjust the polarization direction of the incident pulse;

The polarization beam splitter is used to split the incident pulse after the polarization direction is adjusted to obtain a split pulse;

The cyclic delay loop is used to reflect and adjust the polarization direction of the split pulse;

The polarization beam splitter is also used to obtain a modulated pulse train with equal pulse intervals based on the beam-splitting pulse after reflection and polarization direction adjustment.

Optionally, the cyclic delay loop includes: a first reflector, a second reflector, a third reflector and a second half-wave plate;

The first mirror and the second mirror are respectively used to reflect the split pulse;

The second half-wave plate is used to adjust the polarization direction of the reflected beam splitting pulse;

The third mirror is used to reflect the beam-splitting pulse after the polarization direction has been adjusted.

Optionally, the incident pulse is divided into: s-polarized light, p-polarized light and sp-polarized light after the polarization direction is adjusted.

Optionally, the beam splitting pulse includes: 1s polarized light and 1p polarized light.

Optionally, the modulated pulse train is: the 1s polarized light, 2p polarized light, 3p polarized light....Np polarized light.

Optionally, the light intensity I _total (t) of the modulated pulse train is:

τ为入射高斯脉冲宽度。Among them, T ₁ = sin ² θ ₁ ,

τ is the incident Gaussian pulse width.

On the other hand, to achieve the above object, the present invention provides a novel method for generating carrier-modulated pulses, comprising the following steps:

Adjust the polarization direction of the incident pulse;

beam-splitting the incident pulse after the polarization direction is adjusted to obtain a beam-splitting pulse;

performing reflection and polarization adjustment on the split pulse;

Based on the beam-splitting pulses after reflection and polarization direction adjustment, a modulated pulse train with equal pulse intervals is obtained.

Optionally, the method for reflecting and adjusting the polarization direction of the split pulse is:

reflecting the split pulse;

adjusting the polarization direction of the reflected beam splitting pulse;

and reflecting the split pulse after the polarization direction is adjusted.

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

The invention proposes a novel device and method for generating carrier-modulated pulses. The invention can simultaneously realize direct modulation of 532nm green light pulses, simple optical path alignment and relatively concentrated spectrum energy. Since this external cavity modulation method belongs to direct modulation and does not involve external frequency doubling, the spot distribution and power changes of the 532nm incident pulse will not affect the intensity modulation effect, so this method has certain universality.

Description of drawings

The drawings constituting a part of the application are used to provide further understanding of the application, and the schematic embodiments and descriptions of the application are used to explain the application, and do not constitute an improper limitation to the application. In the attached picture:

Fig. 1 is the schematic diagram of the principle of the branch delay splicing method based on the beam splitter in the background technology;

2 is a schematic diagram of four-way delay lines based on beam splitter-based split delay splicing method in the background technology, wherein (a) is a schematic diagram of the first delay line, and (b) is a schematic diagram of the second delay line, (c) is a schematic diagram of the third delay line, and (d) is a schematic diagram of the fourth delay line;

Fig. 3 is a schematic diagram of modulated pulses based on beam splitter-based splitting delay splicing method in the background technology;

4 is a schematic diagram of the principle of the cyclic frequency shift method based on the acousto-optic modulator in the background technology;

Fig. 5 is a pulse waveform and a schematic diagram of a frequency spectrum based on a circular frequency shift method of an acousto-optic modulator in the background technology, wherein (a) is a schematic diagram of a pulse waveform, and (b) is a schematic diagram of a spectrum;

6 is a schematic diagram of the principle of the cycle delay method based on dichroic mirrors and frequency doubling crystals in the background technology;

Fig. 7 is the waveform and the schematic diagram of spectrum based on the cycle delay method of dichroic mirror and frequency doubling crystal in the background technology, wherein (a) is a schematic diagram of waveform, (b) is a schematic diagram of spectrum;

FIG. 8 is a schematic structural diagram of a novel carrier-modulated pulse generating device according to Embodiment 1 of the present invention;

9 is a schematic diagram of the polarization directions of sub-pulses at the four surfaces of the polarization beam splitter according to Embodiment 1 of the present invention;

10 is a schematic diagram of the angle between the polarization direction of the light beam and the s-axis after being modulated by the first half-wave plate according to Embodiment 1 of the present invention;

11 is a schematic diagram of the angle between the main axis direction and the s-axis of the second half-wave plate according to Embodiment 1 of the present invention;

12 is a schematic diagram of the polarization rotation direction after passing through the second half-wave plate according to Embodiment 1 of the present invention;

Fig. 13 is a schematic diagram of the modulated pulse waveform and frequency spectrum of Embodiment 1 of the present invention, wherein (a) is a schematic diagram of the modulated pulse waveform, and (b) is a schematic diagram of the modulated pulse spectrum.

Detailed ways

It should be noted that, in the case of no conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and embodiments.

It should be noted that the steps shown in the flowcharts of the accompanying drawings may be performed in a computer system, such as a set of computer-executable instructions, and that although a logical order is shown in the flowcharts, in some cases, The steps shown or described may be performed in an order different than here.

Embodiment one

As shown in Figure 8, the present invention provides a novel carrier modulation pulse generating device, which is characterized in that it includes: a first half-wave plate, a polarization beam splitter and a cyclic delay loop;

The first half-wave plate is used to adjust the polarization direction of the incident pulse;

The polarization beam splitter is used to split the incident pulse after the polarization direction is adjusted to obtain the split pulse;

The circular delay loop is used to reflect and adjust the polarization direction of the split pulse;

The polarizing beam splitter is also used to obtain a modulated pulse train with equal pulse intervals based on the reflected and polarized split pulses.

The circular delay loop includes: a first reflector, a second reflector, a third reflector and a second half-wave plate;

The first reflector and the second reflector are respectively used to reflect the beam splitting pulse;

The second half-wave plate is used to adjust the polarization direction of the reflected beam splitting pulse;

The third reflecting mirror is used for reflecting the beam-splitting pulse after the polarization direction is adjusted.

Further, the optical components required in this embodiment include two half-wave plates, three mirrors and a polarization beam splitter. Assuming that the incident pulse is linearly polarized light, the beam passes through the first half-wave plate, adjusts the polarization direction, and after being split by the polarization beam splitter, part of the beam is directly reflected into the detector, and the other part is transmitted into the circular delay loop. The beam passes through the second half-wave plate to adjust the polarization state, and then splits the beam again through the polarization beam splitter. After multiple cycles, a modulated pulse train with equal pulse intervals can be obtained.

From the above description, it can be seen that the polarization state of the pulse before entering the polarization beam splitter determines the energy splitting ratio, so it is necessary to further explore the polarization directions of the sub-pulses at the four surfaces of the polarization beam splitter (PBS) during the cycle, as shown in Figure 9 shown. The number in 0sp represents the pulse number, 0 is the initial incident pulse, the subscript s represents s-polarized light (the polarization direction is perpendicular to the direction of the paper), the subscript p represents p-polarized light (the polarization direction is along the direction of the paper), and the subscript sp represents polarized light with both polarization directions. 0sp light is divided into 1s polarized light and 1p polarized light after passing through the polarization beam splitter, and the polarization direction of 1p polarized light is changed into 1sp polarized light after being modulated by the second half-wave plate; 1sp polarized light passes through the polarization beam splitter to complete the first After that, it is divided into 2p polarized light and 2s polarized light, and the 2s polarized light is continuously circulated through the above modulation process to obtain a modulated pulse train with equal pulse intervals.

Assuming that the incident light pulse is linearly polarized light, after passing through the first half-wave plate to rotate the polarization direction and then pass through the polarization beam splitter to split the light, the s-polarized light is reflected and the p-polarized light is transmitted; if the polarization is rotated when passing through the second half-wave plate The direction is then split by a polarization beam splitter, p-polarized light is reflected, and s-polarized light is transmitted. The polarization change for each cycle is shown in Fig. 9. It is worth noting that the light entering the circulation loop for the first time is p-polarized light, and the light entering the circulation loop for the second time and thereafter is s-polarized light.

After being modulated by the first half-wave plate, the included angle between the polarization direction of the beam and the s-axis (the s-axis is in the same direction as the s-polarization direction, and the p-axis is in the same direction as the p-polarization direction) is θ ₁ , as shown in Figure 10, the black two-way Arrows represent polarization directions, and black unidirectional arrows represent coordinate axes. Therefore, the s-polarized light intensity component can be expressed as: I _1s (t)=I ₀ (t)cos ² θ ₁ , and the p-polarized light is transmitted into the loop, and its light intensity is expressed as: I _1p (t)=I ₀ (t)sin ² θ ₁ .

s偏振光透射进入循环回路，其光强表示为：

I _1p is p-light, assuming that the angle between the main axis of the second half-wave plate and the s-axis is θ ₂ , as shown in Figure 11, the black dotted double-headed arrow is the main axis of the second half-wave plate, and the curved arrow points to the direction of polarization rotation. Therefore, the p-polarized light intensity component after passing through the polarization beam splitter can be expressed as:

The s-polarized light is transmitted into the loop, and its light intensity is expressed as:

s光透射进入循环回路，其光强表示为：

The included angle between the main axis direction of the second half-wave plate and the s-axis is also θ ₂ , and the polarization rotation direction after passing through the second half-wave plate is shown in FIG. 12 . After passing through PBS, the light intensity component of p-polarization can be expressed as:

The s light is transmitted into the loop, and its light intensity is expressed as:

可以得到最终输出端的光强I_total(t)：Let T ₁ = sin ² θ ₁ ,

The light intensity I _total (t) at the final output can be obtained:

τ为入射高斯脉冲宽度；所以转动波片的角度可以起到改变透射比的效果。转动第一半波片(HWP)可以改变进入循环延时回路的光强与直接反射的光强比，转动第二HWP可以改变循环回路的反射、透射比，可以通过选取这两个角度来达到最佳的调制效果。In the formula,

τ is the width of the incident Gaussian pulse; therefore, turning the angle of the wave plate can change the transmittance. Rotating the first half-wave plate (HWP) can change the ratio of the light intensity entering the circular delay circuit to the light intensity directly reflected, and rotating the second HWP can change the reflection and transmittance of the circular circuit, which can be achieved by selecting these two angles The best modulation effect.

Further, the modulated pulse waveforms obtained by optimizing the two parameters θ ₁ and θ ₂ are shown in Figures 13(a) and 13(b), and the parameters are as follows: θ ₁ =68°, θ ₂ =25°, time delay The line length is 0.3m and the initial incident Gaussian pulse width is 500ps.

Judging from the structure of this embodiment, the alignment of the optical path only needs to adjust three mirrors and one polarizing beam splitter, and the four degrees of freedom are significantly less than the seven degrees of freedom in the beam splitter-based split delay splicing method , the difficulty of optical path alignment is greatly reduced. In addition, because this embodiment can directly modulate the 532nm pulse, the modulation system does not involve a frequency doubling crystal, and the spot distribution and power of the incident pulse will not affect its intensity modulation effect. Compared with the cyclic delay method, this embodiment adapts pulsed lasers with various parameters, which is more universal.

From the modulation results of this embodiment, it can be seen that the spectrum of the modulated pulse is more concentrated at the modulation frequency of 1 GHz, which overcomes the shortcoming of spectrum energy dispersion in the cyclic frequency shift method based on the acousto-optic modulator.

Embodiment two

The present invention also provides a novel method for generating a carrier-modulated pulse, comprising the following steps of adjusting the polarization direction of the incident pulse;

beam-splitting the incident pulse after the polarization direction is adjusted to obtain a beam-splitting pulse;

performing reflection and polarization adjustment on the split pulse;

Based on the beam-splitting pulses after reflection and polarization direction adjustment, a modulated pulse train with equal pulse intervals is obtained.

Further, the method for reflecting and adjusting the polarization direction of the split pulse is as follows:

reflecting the split pulse;

adjusting the polarization direction of the reflected beam splitting pulse;

and reflecting the split pulse after the polarization direction is adjusted.

The above are only preferred specific implementation methods of the present application, but the scope of protection of the present application is not limited thereto. Anyone skilled in the art can easily think of changes or substitutions within the technical scope disclosed in the present application. All should be covered within the scope of protection of this application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims (5)

1. The generating device of a kind of novel carrier modulation pulse is characterized in that, comprises: the first half-wave plate, polarization beam splitter and loop delay loop; The first half-wave plate is used to adjust the polarization direction of the incident pulse; The polarization beam splitter is used to split the incident pulse after the polarization direction is adjusted to obtain a split pulse; The cyclic delay loop is used to reflect and adjust the polarization direction of the split pulse; The polarizing beam splitter is also used to obtain a modulated pulse train with equal pulse intervals based on the beam-splitting pulses after reflection and polarization direction adjustment; The beam splitting pulse includes: 1s polarized light and 1p polarized light; The modulated pulse train is: the 1s polarized light, 2p polarized light, 3p polarized light....Np polarized light; The light intensity of the modulated pulse train

for:

, in,

is the width of the incident Gaussian pulse, θ ₁ is the angle between the polarization direction of the beam modulated by the first half-wave plate and the s-axis, and θ ₂ is the angle between the main axis direction of the second half-wave plate and the s-axis. 2. The generating device of the novel carrier-modulated pulse according to claim 1, wherein the cyclic delay loop comprises: a first reflector, a second reflector, a third reflector and a second half-wave plate ; The first mirror and the second mirror are respectively used to reflect the split pulse; The second half-wave plate is used to adjust the polarization direction of the reflected beam splitting pulse; The third mirror is used to reflect the beam-splitting pulse after the polarization direction has been adjusted. 3. The device for generating novel carrier-modulated pulses according to claim 1, wherein the incident pulses are divided into s-polarized light, p-polarized light and sp-polarized light after the polarization direction is adjusted. 4. A generation method of a novel carrier-modulated pulse, used for the control of the generating device of the novel carrier-modulated pulse described in any one of claims 1-3, is characterized in that, comprises the following steps: adjust the polarization direction of the incident pulse; beam-splitting the incident pulse after the polarization direction is adjusted to obtain a beam-splitting pulse; performing reflection and polarization adjustment on the split pulse; Based on the beam-splitting pulses after reflection and polarization direction adjustment, a modulated pulse train with equal pulse intervals is obtained. 5. the generation method of novel carrier modulation pulse according to claim 4, it is characterized in that, the method that described beam splitting pulse is carried out reflection and polarization direction adjustment is: reflecting the split pulse; adjusting the polarization direction of the reflected beam splitting pulse; and reflecting the split pulse after the polarization direction is adjusted.

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