Dispersion Control Device and Method for Ultrashort Pulse Laser System

Abstract

A device and method for dispersion control of an ultrashort pulse laser system. The device includes a light source system, a dispersion detection system, and a control system. The light source system consists of a narrowband tunable light source, a wavelength meter, an amplitude modulator, an electrical amplifier, and a radio frequency signal source, and is used to achieve wavelength adjustment and amplitude modulation of the optical signal. The dispersion detection system consists of a high-speed photodiode and a vector network analyzer, and is used to measure the group delay and dispersion of the ultrashort pulse laser system and provide the results to the control system. The control system is used to control the output wavelength of the light source system to achieve optical frequency sweeping, and according to the measurement results of the dispersion detection system, output control signals to separately adjust the fine compressor and the delay regulator in the ultrashort pulse laser system, and achieve precise control of the system group delay dispersion and group delay through cyclic iteration. In this method, after optical signals with different central wavelengths pass through the system under test and are converted into high-frequency electrical signals by the photodiode, only by measuring the phase change amount between different electrical signals can the dispersion measurement of the laser system be achieved. It is applicable not only to the dispersion control of the ultrashort pulse laser system, but also to the dispersion calibration of a single component.

Description

Technical Field

[0001] The present invention relates to the fields of optical measurement technology and laser technology, etc., and in particular to a device and method for dispersion control of an ultra-short pulse laser system. Background Art

[0002] Dispersion is an important parameter in the field of laser technology, which affects the output characteristics of an ultra-short pulse laser system. The output peak power of an ultra-short laser pulse system depends on the compressed laser pulse width and energy. On the one hand, it can be obtained by increasing the output energy, but this will increase the scale of the experimental device and the cost is also relatively high; on the other hand, it can also be achieved by increasing the spectral width and realizing a compressed pulse width output close to the Fourier transform limit, thereby obtaining a laser pulse with ultra-high peak power. When an ultra-short pulse passes through a homogeneous medium or a dispersion component, due to the relatively wide spectral bandwidth, under the action of group velocity delay, the pulse phase will be distorted and the pulse width will also change. All optical elements will introduce dispersion, and in a laser system, the influence of dispersion should be minimized or reasonably utilized. Taking a picosecond petawatt laser system as an example, when the grating load capacity is limited, the ultimate output energy of the system is directly related to the pulse width. Therefore, it is necessary to achieve precise characterization of the laser time-domain characteristics (pulse width) through dispersion control to support the requirements of precision physics experiments while ensuring the safe operation of the picosecond petawatt laser system. Through the dispersion detection and control of the laser system, not only can an ideal transform-limited pulse width output of the laser system be obtained, but also the precise characterization of the pulse time-domain characteristics when the laser system outputs at a non-transform-limited pulse width can be achieved. Therefore, precise control of dispersion is one of the very important control means for an ultra-short pulse laser system.

[0003] Traditional dispersion measurements include the time-delay method, the interference method, etc. The time-delay method directly measures the relative delay amount between pulses of different wavelengths transmitted in a dispersion component, and obtains the dispersion value through a data fitting model. As mentioned in the literature "High-precision group-delay dispersion measurements of optical fibers via fingerprint-spectral wavelength-to-time mapping," (Photon. Res. 4, 13-16 2016), this method requires extremely accurate timing of the pulse arrival time. At the same time, the measurement accuracy is affected by the pulse shape, the measurement accuracy is low, and it is not suitable for measuring components with a small dispersion amount. The interference method is based on the configuration of a Mach-Zehnder interferometer or a Michelson interferometer. By measuring the interference spectrum, the phase information is extracted therefrom to obtain the dispersion value. For example, as mentioned in the literature "Comparison of single-mode fiber dispersion measurement techniques" (Lightwave Technol, vol. 3, no. 5, pp. 958-966, 1985), although the measurement system structure of this method is relatively simple, due to the reference arm or the arm to be measured being easily interfered by the external environment (such as temperature, vibration, etc.), its measurement accuracy is difficult to guarantee. Summary of the Invention

[0004] The purpose of the present invention is to provide a dispersion control device and method for an ultrashort pulse laser system, so as to solve the problems of complex device structure, low measurement accuracy, complex measurement process, and being easily affected by other factors in current dispersion measurements.

[0005] The technical solution of the present invention is as follows:

[0006] On the one hand, the present invention provides a dispersion control device for an ultrashort pulse laser system. The ultrashort pulse laser system is composed of a mode-locked laser, a stretcher, a delay regulator, a fine-tuning compressor, an amplifier, and a compressor connected in sequence. The feature is that the dispersion control device includes a light source system, a dispersion detection system, and a control system. The dispersion detection system is composed of a high-speed photodiode and a vector network analyzer connected in sequence; the light source system is composed of a narrowband tunable light source, a wavelength meter, an amplitude modulator, an electrical amplifier, and a radio frequency signal source, and is used to realize the amplitude modulation, center wavelength adjustment, and detection of optical signals; the narrowband tunable light source can be a narrowband light source or realized by a combination of a broadband light source and a tunable narrowband filter; the amplitude modulator can be a Mach-Zehnder modulator or other electro-absorption modulators, etc.

[0007] The described dispersion detection system is composed of a high-speed photodiode and a vector network analyzer, and is used to measure the group delay and dispersion of an ultrashort pulse laser system, and provide the results to the control system.

[0008] The described control system is used to control the output wavelength of the light source system, realize optical frequency sweeping, and realize the dispersion regulation of the ultrashort pulse laser system according to the dispersion measurement results of the described dispersion detection system. The typical ultrashort pulse laser system consists of a mode-locked laser, a stretcher, a delay regulator, a fine-tuning compressor, an amplifier and a compressor. The delay regulator is used for synchronous delay adjustment of the ultrashort pulse laser system, and the fine-tuning compressor is used to adjust the dispersion amount of the ultrashort pulse laser system.

[0009] The output beam of the described narrowband tunable light source is divided into two beams of different sizes. The large beam is connected to the first input end of the amplitude modulator, the small beam is connected to the input end of the wavelength meter, the output end of the wavelength meter is connected to the second input end of the control system, and the output end of the amplitude modulator is connected to the input end of the stretcher; the first output end of the radio frequency signal source is connected to the second input end of the amplitude modulator through the power amplifier, the second output end of the radio frequency signal source is connected to the second input end of the vector network analyzer, the output end of this vector network analyzer is connected to the first input end of the control system, the first output end of this control system is connected to the control end of the narrowband tunable light source, the second output end of the control system is connected to the control end of the fine-tuning compressor, the third output end of the control system is connected to the control end of the delay regulator, and the output end of the compressor, which is the output end of the ultrashort pulse laser system, is connected to the input end of the high-speed photodiode.

[0010] On the other hand, the present invention also provides a method for regulating the dispersion of an ultrashort pulse laser system, including the following steps:

[0011] S1. The output wavelength of the described narrowband tunable light source is controlled by the control system, and a part of the sampled light enters the wavelength meter for detection, and the wavelength information λ k (k = 1, 2, 3, ……, n) is fed back to the described control system;

[0012] S2. Most of the light of the narrowband tunable light source in S1 enters the amplitude modulator. The radio frequency signal source outputs a high-frequency sine signal, which is amplified by the power amplifier and then drives the amplitude modulator to modulate the continuous and stable light source into a sine signal in the time domain, and further transmits the modulated optical signal to the ultrashort pulse laser system to be measured;

[0013] S3. The output of the amplitude modulator is connected to the stretcher described in claim 1. The modulated sine signal passes through the stretcher, delay regulator, fine-tuning compressor, amplifier, and compressor in the ultrashort pulse laser system and then enters the dispersion detection system;

[0014] S4. In the dispersion detection system, the optical signal is first converted into an electrical signal by a high-speed photodiode and then enters port 1 of the vector network analyzer. The RF signal source in S2 provides a reference electrical signal to port 2 of the vector network analyzer. In the vector network analyzer, by demodulating the two electrical signals, the relative phase difference between the electrical signal after photoelectric conversion and the reference electrical signal is measured. According to the phase difference of the measured signal, the group delay can be solved by combining the following formula:

[0015] where GD is the solved group delay, is the measured phase difference, f m is the operating frequency of the loaded sine modulation signal, v k is the frequency of light.

[0016] S5. Repeat steps S1 - S4, and each time change the output wavelength through the light source system to achieve optical frequency sweeping until the measurement of the group delay of all wavelengths (λ k , where k = 1, 2, 3, ……, n) is completed.

[0017] S6. Since the group delay dispersion is caused by the change of the group delay with the optical frequency, the group delay dispersion can be solved by polynomial fitting of the group delays of lights with different wavelengths. The specific formula is as follows:

[0018]

[0019] where GDD is the solved group delay dispersion, v is the frequency of light, which is obtained by converting the spectral information detected by the wavelength meter during optical frequency sweeping.

[0020] S7. The control system feeds back and adjusts the signal to the fine-tuning compressor in the ultrashort pulse laser system according to the measurement result of the dispersion. The electric displacement stage of the fine-tuning compressor is adjusted according to the feedback signal to realize the adjustment of the GDD dispersion amount in the ultrashort pulse laser system. After adjustment, it is detected by the dispersion detection system again, and the detection result is input into the control system;

[0021] S8. The control system calculates the optical path adjustment amount ΔGD(v c ) according to the change amount ΔGD(v c) × C, where C is the speed of light, and feedback the adjustment signal to the delay regulator in the ultrashort pulse laser system. The delay regulator synchronously adjusts the ultrashort pulse laser system according to the feedback signal. After adjustment, it is detected by the dispersion detection system, and the detection result is input into the control system. After cyclic iteration, the dispersion detection and regulation of the ultrashort pulse laser system can be realized.

[0022] Furthermore, the dispersion regulation device of the ultrashort pulse laser system can separately measure various types of dispersion components, such as stretchers, compressors, etc., and can also be applied to optical links such as ultrashort pulse laser systems.

[0023] The beneficial effects of the present invention are as follows:

[0024] 1) By detecting the light source system and controlling the control system, optical frequency sweeping can be achieved.

[0025] 2) After optical signals with different central wavelengths pass through the system to be measured and are converted into high-frequency electrical signals by a photodiode, only by measuring the phase change amount between different electrical signals can the dispersion measurement and regulation of the laser system be realized. It is not only applicable to the dispersion regulation of ultrashort pulse laser systems, but also applicable to the dispersion calibration of individual components.

[0026] 3) Within the measurement frequency range, the vector network analyzer can automatically statistically measure the relative phase change amount that changes with the laser frequency, and then analyze the phase change amount of different wavelengths, thereby realizing the rapid dispersion measurement of the system and improving the accuracy of dispersion measurement.

[0027] 4) The present invention can realize the dispersion detection and regulation of the ultrashort pulse laser system by adjusting the delay regulator and the fine-tuning compressor in the ultrashort pulse laser system through the control system. Description of the Drawings

[0028] Figure 1 It is a schematic structural diagram provided by Embodiment 1 of the present invention;

[0029] Figure 2 It is a schematic control flow diagram provided by Embodiment 2 of the present invention;

[0030] Figure 3 It is a schematic diagram of the phase difference between different wavelength modulation signals provided by Embodiment 2 of the present invention. Detailed Embodiments

[0031] The following further elaborates on the present invention in detail with reference to the drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0032] Embodiment 1:

[0033] Figure 1It is a schematic structural diagram of the dispersion synchronization control device of the ultrashort pulse laser system of the present invention. Among them, the components of each system include: a light source system 1, a dispersion detection system 2, and a control system 108.

[0034] The described light source system 1 is composed of a narrowband tunable light source 101, a wavelength meter 102, an amplitude modulator 103, an electrical amplifier 104, and a radio frequency signal source 105, and is used to achieve high-frequency modulation, central wavelength adjustment, and detection of optical signals. The narrowband tunable light source 101 is controlled for output by port 1 of the control system 108. The output end of the light source is respectively connected to the wavelength meter 102 and the amplitude modulator 103. The optical signal emitted by the narrowband tunable light source 101 is sampled and detected by the wavelength meter 102. The output end of the wavelength meter 102 is connected to port 2 of the control system 108 and transmits the detected wavelength information to the control system 108. One port of the amplitude modulator 103 is connected to the output end of the electrical amplifier 104, and the other port is connected to the stretcher 202 in the ultrashort pulse laser system. The input end of the electrical amplifier 104 is connected to the output end of the radio frequency signal source 105, which is used to provide the radio frequency drive signal for the amplitude modulator 103. Another output end of the radio frequency signal source 105 is connected to the vector network analyzer 107. In this example, the narrowband tunable light source 101 is a narrowband light source with a tunable central wavelength, and the amplitude modulator 103 selects a Mach-Zehnder modulator.

[0035] The described dispersion detection system is composed of a high-speed photodiode 106 and a vector network analyzer 107, and is used to measure the group delay and dispersion of the ultrashort pulse laser system and provide the results to the control system 108. The described typical ultrashort pulse laser system is composed of a mode-locked laser 201, a stretcher 202, a delay regulator 203, a fine-tuning compressor 204, an amplifier 205, and a compressor 206. The output end of the ultrashort pulse laser system is connected to the high-speed photodiode 106. The high-speed photodiode 106 is connected to port 1 of the vector network analyzer 107, and port 2 of the vector network analyzer 107 is connected to the radio frequency signal source 105 to receive the radio frequency signal.

[0036] The described control system 108 is respectively connected to the narrowband tunable light source 101, the wavelength meter 102, the vector network analyzer 107, the delay regulator 203, and the fine-tuning compressor 204 in the ultrashort pulse laser system. The control system 108 is used to control the output wavelength of the light source system, achieve optical frequency sweeping, and realize the dispersion control of the ultrashort pulse laser system according to the dispersion measurement results of the vector network analyzer 107.

[0037] Example 2:

[0038] Figure 2Schematic diagram of the process of the dispersion synchronization control device and method for an ultrashort pulse laser system. The output of the narrowband tunable light source 101 is controlled by the control system 108. A part of the sampled light enters the wavelength meter 102 for detection, and the wavelength information λ k (k = 1, 2, 3, ……, n) is fed back to the control system 108.

[0039] Most of the light from the narrowband tunable light source enters the amplitude modulator 103. The radio frequency signal source 105 outputs a high-frequency sine signal, which is amplified by the electrical amplifier 104 and then drives the amplitude modulator 103 to modulate the continuous and stable light source into a sine signal in the time domain. Further, the modulated optical signal is transmitted to the stretcher 202 of the ultrashort pulse laser system.

[0040] The typical composition of an ultrashort pulse laser system is as described in Embodiment 1. The system includes components such as a stretcher, a compressor, a delay regulator, and a fine-tuning compressor. The delay regulator is used for equal optical path adjustment in the ultrashort pulse laser system, and the fine-tuning compressor is used to control the dispersion amount in the ultrashort pulse laser system.

[0041] The output of the amplitude modulator is connected to the stretcher. The modulated sine signal passes through the stretcher, delay regulator, fine-tuning compressor, amplifier, and compressor and then enters the dispersion detection system.

[0042] In the dispersion detection system, the optical signal is first converted into an electrical signal by a high-speed photodiode and then enters Port 1 of the vector network analyzer. The radio frequency signal source provides a reference electrical signal to Port 2 of the vector network analyzer. In the vector network analyzer, by demodulating the two electrical signals, the relative phase difference measurement between the electrical signal after photoelectric conversion and the reference electrical signal is realized. According to the phase difference detection of the vector network analyzer, the result is input into the control system, and the light source system is controlled again according to the feedback to change the output wavelength. The corresponding phases between different wavelengths are different, and the phase difference between the two signals also reflects the synchronization accuracy. Data can be further obtained through the demodulated signal. As Figure 3 shown, it reflects the corresponding phase difference relationship after different wavelengths of light are converted into electrical signals. Among them, the phase difference between the wavelength λ 1 and the reference signal is The wavelength is λ 2 and the phase difference between the reference signal is λ 1 and λ 2 The corresponding phase difference between them is According to the phase difference of the measured signal, the group delay can be solved by combining the following formula:

[0043]

[0044] where GD is the solved group delay, is the measured phase difference, f m is the operating frequency of the loaded sinusoidal modulation signal, v k is the frequency of light. Repeat the above operations to measure the phase difference passing through the ultrafast pulsed laser system. Each time, change the output wavelength through the light source system to achieve optical frequency sweeping until the measurement of the group delay of all wavelengths (λ k , where k = 1, 2, 3, ……, n) is completed.

[0045] Since group delay dispersion is caused by the change of group delay with optical frequency, group delay dispersion can be solved by polynomial fitting and can be solved by the following formula:

[0046]

[0047] where GDD is the solved group delay dispersion, v is the optical frequency, which is obtained by converting the spectral information detected by the wavemeter during optical frequency sweeping. In this example, we use a Mach-Zehnder modulator to perform amplitude modulation on the laser, the modulation frequency is 5 GHz, the vector network analyzer has a phase resolution of ~0.1°, and the measurement accuracy in the time domain can be better than the femtosecond level. Within a certain range, the measurement accuracy of the group delay increases with the increase of the modulation frequency.

[0048] The control system adjusts the feedback signal to the fine-tuning compressor in the ultrafast pulsed laser system according to the measurement result of the dispersion. The electric displacement stage of the fine-tuning compressor is adjusted according to the feedback signal to realize the adjustment of the GDD dispersion amount in the ultrafast pulsed laser system. After adjustment, it is detected by the dispersion detection system, and the detection result is input into the control system.

[0049] The control system measures the change amount ΔGD(v c ) of the group delay corresponding to the central wavelength before and after the dispersion adjustment by the dispersion measurement system, calculates the optical path adjustment amount ΔGD(v c )×C, where C is the speed of light, and feeds back the adjustment signal to the delay regulator in the ultrafast pulsed laser system. The delay regulator synchronously adjusts the ultrafast pulsed laser system according to the feedback signal. After adjustment, it is detected by the dispersion detection system, and the detection result is input into the control system. After cyclic iteration, the dispersion detection and control of the ultrafast pulsed laser system can be realized.

[0050] The present invention provides a device and method for dispersion synchronization control of an ultrashort pulse laser system, which solves the problems of complex device structure, low measurement accuracy, complex measurement process, and susceptibility to other factors in current dispersion measurements. Through the detection of the light source system and the control of the control system, optical frequency sweeping can be achieved. Within the measurement frequency range, the vector network analyzer can automatically statistically measure the relative phase change amount that varies with the laser frequency, and then analyze the phase change amounts of different wavelengths, thereby realizing the rapid dispersion measurement of the system and improving the accuracy of dispersion measurement. In this method, after the optical signals of different central wavelengths pass through the system under test and are converted into high-frequency electrical signals by a photodiode, only the phase change amount between different electrical signals needs to be measured to achieve the dispersion measurement of the laser system. It is not only applicable to the dispersion control of ultrashort pulse laser systems but also to the dispersion calibration of individual components. By adjusting the delay regulator and fine-tuning compressor in the ultrashort pulse laser system through the control system, the dispersion control of the ultrashort pulse laser system is further realized.

[0051] The above description is only the preferred embodiment of the present invention and the technical principles applied. Those skilled in the art should understand that the present invention is not limited to the specific embodiments described herein. Various obvious changes, re-adjustments, and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments. Without departing from the concept of the present invention, more other equivalent embodiments can be included, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A dispersion control device for an ultrashort pulse laser system, wherein the ultrashort pulse laser system is composed of a mode-locked laser (201), a stretcher (202), a delay regulator (203), a fine compressor (204), an amplifier (205) and a compressor (206) connected in sequence. It is characterized in that the dispersion control device includes a light source system, a dispersion detection system and a control system (108), and the dispersion detection system is composed of a high-speed photodiode (106) and a vector network analyzer (107) connected in sequence; the light source system is composed of a narrowband tunable light source (101), a wavelength meter (102), an amplitude modulator (103), an electrical amplifier (104) and a radio frequency signal source (105). The output beam of the narrowband tunable light source (101) is divided into two beams of different sizes. The large beam is connected to the first input end of the amplitude modulator (103), the small beam is connected to the input end of the wavelength meter (102), the output end of the wavelength meter (102) is connected to the second input end of the control system (108), and the output end of the amplitude modulator (103) is connected to the input end of the stretcher (202); the first output end of the radio frequency signal source (105) is connected to the second input end of the amplitude modulator (103) through the electrical amplifier (104), the second output end of the radio frequency signal source (105) is connected to the second input end of the vector network analyzer (107), the output end of this vector network analyzer (107) is connected to the first input end of the control system (108), the first output end of this control system (108) is connected to the control end of the narrowband tunable light source (101), the second output end of the control system (108) is connected to the control end of the fine compressor (204), the third output end of the control system (108) is connected to the control end of the delay regulator (203), and the output end of the compressor (206), which is the output end of the ultrashort pulse laser system, is connected to the input end of the high-speed photodiode (106).

2. The dispersion control device for an ultrashort pulse laser system according to claim 1, It is characterized in that the narrowband tunable light source (101) is a narrowband light source or a combination of a broadband light source and a tunable narrowband filter.

3. The dispersion control device for an ultrashort pulse laser system according to claim 1, It is characterized in that the amplitude modulator (103) is a Mach-Zehnder modulator.

4. A method for controlling the dispersion of an ultrashort pulse laser system by using the dispersion control device for an ultrashort pulse laser system according to claim 1, It is characterized in that it includes the following steps: 1) The output wavelength of the narrowband tunable light source (101) is controlled by the control system (108). Let k = 1, and the wavelength of the light output by the narrowband tunable light source (101) is λ k (k = 1, 2, 3, ……, n), where n is set according to the tunable wavelength range and the wavelength adjustable step size; 2) A part of the sampled light output by the narrowband tunable light source (101) enters the wavelength meter (102) for wavelength detection and feeds the wavelength information λ k back to the control system (108); 3) Most of the light from the narrowband tunable light source (101) enters the amplitude modulator (103). The radio frequency signal source (105) outputs a high-frequency sine signal, which is amplified by the electrical amplifier (104) and then drives the amplitude modulator (103) to modulate the continuous and stable light source into a sine signal in the time domain; 4) The sine signal output by the amplitude modulator (103) enters the stretcher (202), and after passing through the stretcher (202), the delay regulator (203), the fine-tuning compressor (204), the amplifier (205) and the compressor (206), it enters the dispersion detection system; 5) The optical signal output by the compressor (206) is first converted into an electrical signal by the high-speed photodiode (106) and then enters the vector network analyzer (107) through the first input terminal. The reference electrical signal provided by the radio frequency signal source (105) enters the vector network analyzer (107) through the second input terminal. In the vector network analyzer (107), by demodulating the two electrical signals, the relative phase difference between the electrical signal after photoelectric conversion and the reference electrical signal is measured. According to the phase difference of the measured signal, the group delay can be solved by combining the following formula: wherein, GD is the group delay to be solved, is the measured phase difference, f m is the modulation frequency output by the amplitude modulator (103), v k is the optical frequency output by the narrowband tunable laser when n = k currently; 6) If K < n, and let K = K + 1, return to step 2). When K > n, then go to the next step; 7) Since the group delay dispersion is caused by the change of the group delay with the optical frequency, the group delay dispersion can be solved by fitting the group delay polynomials of different wavelengths of light. The specific formula is as follows: where GDD is the solved group delay dispersion, and v is the optical frequency. When performing optical frequency sweeping, the optical frequency v is obtained by converting the spectral information detected by the wavelength meter (102) through the formula v = c / λ; 8) The control system (108) feeds back and adjusts the signal to the fine-tuning compressor (204) according to the measurement result of the dispersion. The electric displacement stage of the fine-tuning compressor (204) adjusts the grating spacing or the grating deflection angle in the fine-tuning compressor (204) according to the feedback signal, changes the dispersion amount in the ultrashort pulse system, realizes the adjustment of the GDD dispersion amount in the ultrashort pulse laser system. After the adjustment, it is detected by the dispersion detection system again, and the detection result is input into the control system (108); 9) The described control system (108) calculates the change in group delay ΔGD(v corresponding to the center wavelength before and after the measured dispersion adjustment c ), converts it to the optical path adjustment amount ΔGD(v c ) × C, where C is the speed of light, and feeds back an adjustment signal to the delay adjuster (203). The delay adjuster (203) synchronously adjusts the ultrashort pulse laser system according to the feedback signal. After adjustment, it is detected by the dispersion detection system, and the detection result is input into the control system (108). After cyclic iteration, the dispersion control of the ultrashort pulse laser system can be achieved, minimizing the residual dispersion amount in the ultrashort pulse laser system and compressing the output pulse width to the limit, that is, the ultrashort pulse laser system outputs a time-domain pulse with Fourier transform limit.

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