1. Introduction
Burst-mode lasers combine a series of closely spaced pulses into short bursts to achieve a high repetition rate and a high peak power. Compared to noise-like pulse generation [
1,
2], an unique advantage of burst-mode lasers is the tunability of their intra-burst pulse parameters, including the intra-burst pulse waveform and duty cycle. This enables burst-mode lasers to be used not only for precision machining [
3,
4], but also for fiber optic communications [
5,
6] and LIDAR [
7,
8]. Recently, a novel application is the use of tunable high-frequency (GHz) burst-mode lasers to illuminate photoconductive semiconductor switches (PCSSs), enabling the generation of high-power, frequency-agile RF waves and microwaves, which is named photoconductive microwave technology (PMT) [
9,
10,
11]. Where the burst-mode laser is used as the optical drive source, the characteristics of the laser can determine the characteristics of the output microwave (including pulse width and main frequency.) and the peak power of the laser affects the power of the output microwave [
12]. Compared to traditional pure electric microwave technology (powered by vacuum tubes and high electron mobility transistors) and microwave photon technology, the main advantage of PMT is that it can generate high-power (~100 kW) and tunable wide bandwidth (~GHz) microwave signals [
13,
14]. PMT not only overcomes the limitations of pure electric microwave technology, which struggles to achieve a large frequency tuning range and a high frequency (~GHz) bandwidth [
15,
16], but also compensates for the challenges faced by microwave photonic (MWP) technology, with which it is difficult to output a high power (~mW) [
17,
18,
19]. The high-power (~100 kW) and wide-bandwidth (~GHz) characteristics of PMT exhibit great potential for high-power MWP radar systems.
Figure 1 briefly depicts a schematic of a high-power MWP radar utilizing a high-power burst-mode laser, where the high-power burst-mode laser plays an important role as the optical drive source. To make the burst-mode laser suitable for MWP radar applications, some output characteristics need to be considered, Firstly, a long pulse width (~μs) and high energy can help increase the radar’s detection distance. Secondly, an adjustable intra-burst repetition rate in the GHz range can be beneficial for improving the radar’s detection accuracy, viability, and working modes [
19]. Additionally, a flat-top envelope pulse can improve the duration of radar detection. Therefore, a high-power microsecond pulse duration burst-mode laser with a flat-top envelope and GHz tunable frequency is required.
In 2015, Yu et al. [
20] investigated a compact burst-mode pulse fiber amplifier utilizing a Q-switched mode-locked laser at a wavelength of 1064 nm. The amplifier operated at a repetition rate of 154 kHz, generating a burst pulse with a full width at half maximum (FWHM) of 4 μs that comprised 280 of 20 ps pulses. Although the total burst energy of 552 μJ per pulse was achieved by a 30/250 μm fiber amplification, the shape of the burst pulse at maximum energy was an approximate Gaussian envelope with a FWHM of 4 μs; in addition, the intra-burst pulse frequency is difficult to adjust due to the complex structure of the mode-locked laser. In 2023, Zhang et al. [
21] reported a frequency-quadrupling microsecond burst-mode laser that employs an EOM to extract micro-pulses from a 1064 nm laser source. The burst pulse had a duration of 16 µs and a repetition rate of 550 Hz, while the micro-pulse width was 18 ps with a repetition rate of 178 MHz. When the 1064 nm laser pulse energy was 787 nJ, the amplifier obtained a flat-top burst pulse envelope. However, the frequency tunability of the intra-burst pulse was not characterized. Recently, our research group has preliminarily developed an adjustable microsecond burst-mode fiber seed [
22]. The pulse duration of the amplifier was 5 μs, and the adjustable frequency range of the intra-burst pulse was 1–2 GHz. Since the main amplification fiber amplifier was not built, the output energy was only 26.6 μJ and the peak power was about 10 W. Further characteristics of the seed source were not given. So far, there are few studies that have reported on microsecond burst-mode lasers designed for high-power MWP radar. Almost all burst-mode laser systems use a master oscillator power amplifier (MOPA) configuration to provide flexibility of the pulse parameters. To achieve high-energy or -power burst-mode pulse lasers, a hybrid amplification structure comprising a microsecond burst-mode pulse fiber laser and a solid-state amplifier (Nd: YAG) is generally adopted [
23], where a high-power microsecond burst-mode pulse fiber seed with flexible pulse parameters is critical to MOPA. The seeds with flexible parameters can provide a tunable intra-burst pulse frequency and waveform compensation, and the higher seed output power can simplify the amplification structure of the solid-state laser. Several methods can be applied to generate a microsecond pulse burst-mode fiber seed laser. One method is to use a mode-locked fiber laser to generate a pulse train, with the Q-switch being periodically chopped into a microsecond pulse train laser [
24,
25,
26]. However, the frequency tuning range of mode-locked lasers is limited for complex laser structures, making it difficult to achieve wide-range tuning in the GHz range. Another way to obtain a high frequency intra-burst pulse is to stack the pulses by using a fiber pulse multiplier [
27]. However, using this method to generate tunable burst parameters requires adjusting the number of fiber multipliers, making it challenging to achieve tuning over a wide range. An option is to use an EOM to generate an adjustable intra-burst pulse repetition rate over GHz. Due to the small capacity of the EOM, the power of the fiber seed is limited. Multi-stage amplification is necessary to increase the power, but ASE and nonlinear effects limit this process (especially at low repetition rates). Additionally, maintaining a flat-envelope microsecond envelope at a high power is challenging due to the gain saturation effect [
28]. As a result, achieving a high-power nearly flat-top envelope microsecond pulse burst-mode laser is challenging.
In this paper, we present a 520 μJ burst-mode pulse laser that maintains a nearly flat-top pulse envelope for 5 μs. The laser features an adjustable repetition rate intra-burst pulse range of 1–5 GHz. First, a pulse laser is used to generate a 5.5 μs/10 Hz narrow linewidth seed laser. Secondly, the pulse laser is modulated as a burst-mode pulse laser by an EOM with an intra-burst pulse tunable range of 1–5 GHz. Finally, a synchronous pumping scheme is employed to suppress ASE from the laser amplification system and achieve a 520 μJ energy output. Additionally, an AOM is used for waveform pre-compensation to achieve a 5 μs nearly flat-top pulse envelope. The fiber amplifiers can be further amplified as light-driven sources to generate high-power microwaves for applications in microwave photonic radar systems.
2. Experimental Setup
The schematic structure of the laser system setup is illustrated in
Figure 2a and comprises four primary sections: a pulse seed source, a high-frequency modulation unit, a waveform compensation module, two pre-amplifier stages, and two main amplification stages. The pulse seed source (homemade) is a polarization-maintaining (PM) 1064 nm laser. The pulse duration is 5.5 µs and the repetition rate is 10 Hz. An isolator (ISO) is used to protect the pulse seed from optical feedback. The seed laser is directed into a high-bandwidth EOM (iXblue, Denver, CO, USA). The EOM has a bandwidth of 10 GHz and an insertion loss of 3.5 dB and is controlled by a signal generator (N5181B, Keysight Inc, Santa Rosa, CA, USA). The seed laser is modulated into a burst-mode laser by an EOM. To compensate for the energy loss caused by the EOM and AOM, two PM fiber pre-amplifier stages are incorporated into the system. A highly doped PM Yb
3+ single-mode fiber with an absorption coefficient of 250 dB/m at 976 nm is used as the gain medium for the pre-amplifier stages. Four hundred-milliwatt single-mode 976 nm laser diodes (LDs) are used as pump sources. During the pre-amplifier stage, a taper/wavelength division multiplexing (TWDM) setup is employed to couple the pump laser to the single-mode gain fiber. The TWDM has a 1:99 coupling ratio, with the 1% port serving to monitor the burst seed characteristics. The waveform compensation section comprises a high-extinction-ratio AOM (extinction ratio > 20 dB, Gooch & Housego PLC and G&H Ilminster, Somerset, UK) and an arbitrary waveform generator (AWG). The AOM enables the pre-compensation waveform program to reform the temporal shape of the pulses. In the second stage of the pre-amplifier, an isolator/bandpass/taper/wavelength division multiplexing (IBPTWDM) device with a 3 dB bandwidth of 2 nm is connected to the AOM. This component serves to suppress ASE in the frequency domain and isolate any reverse return light. The 1% port of the IBPTWDM allows for monitoring of the characteristics of the pre-compensation waveform generated by the AOM.
The microsecond burst-mode seed laser is subsequently amplified through two primary fiber amplifier stages. The first main amplifier is composed of a 1.2 m Yb3+-doped double-cladding polarization-maintaining fiber, which has core and cladding diameters of 10 μm and 125 μm, respectively, as well as a cladding absorption value of 4.8 dB/m at 976 nm. The pump source is a 976 nm laser diode (LD) with a maximum average power of 9 W. Both the burst-mode seed laser and the pump laser are introduced into the first main amplifier via a (2 + 1) × 1 signal-pump combiner. The secondary main amplifier utilizes a PM Yb3+-doped fiber, the core and cladding diameters of which are 20 μm and 130 μm, respectively. It is pumped by a 976 nm LD with an output power level of 9 W. The absorption coefficient of the gain fiber is 5.1 dB/m at 976 nm, and its length is 1 m. To maintain the polarization characteristics of the system, all optical components within the amplifier are polarization-maintaining devices. Meanwhile, to reduce the amount of cladding light and suppress backward reflections, an endcap with a cladding light stripper (CLS, homemade) is spliced to the gain fiber.
Considering that the system operates at a low repetition rate of 10 Hz, a multi-stage synchronous pulse pumping method is used in all amplifier stages to reduce the ASE [
29,
30]. The time sequence for synchronous triggering at each stage is as shown in
Figure 2b. The AOM pre-compensation pulse is strictly aligned with the laser seed pulse, and the trailing edge of the laser seed pulse coincides with the leading edge of the pump pulse, thus allowing the upper stored energy to be consumed by the pump and reducing the extraction of stored energy by the ASE [
31]. The pump pulse widths are 300 μs (first pre-amplifier) and 250 μs (second pre-amplifier), and 200 μs (first main amplifier) and 250 μs (second main amplifier) for the pre-amplifier and main amplifier, respectively. It is worth noting that the pump pulse width is experimentally optimized to achieve a different pump pulse energy, suppress ASE, and maximize the pumping efficiency.
In this study, a high-bandwidth oscilloscope (10Zi-A, Teledyne Lecroy Inc, New York, NY, USA) is utilized for monitoring the temporal laser pulse train and radio frequency (RF) spectrum. Additionally, an optical spectrum analyzer (AQ6370, Yokogawa Test & Measurement Corporation, Tokyo, Japan) is employed to detect the optical spectrum. Moreover, an energy meter (PE9-ES-C, Ophir Inc, Jerusalem, Israel) and a power meter (3A, Ophir Inc, Jerusalem, Israel) are utilized for relative measurements of the output energy (<100 μJ) and the pump pulse power (<3 mW).
4. Conclusions
In this study, we have successfully demonstrated a 520 μJ microsecond burst-mode pulse laser with a GHz-adjustable intra-burst pulse. The laser operates at a FWHM of 6.5 μs and a burst laser repetition rate of 10 Hz. By incorporating a synchronous pumping technique, we effectively suppress ASE and nonlinear effects within the system. The laser provides an adjustable intra-burst pulse repetition rate range of 1–5 GHz with a 50% duty cycle sinusoidal modulation, achieving a 5 μs flat-top pulse envelope through the utilization of a pre-compensation waveform generated by an AOM. The beam quality factors M2x and M2y are measured as 1.04 and 1.1, respectively, at the maximum energy output, indicating excellent beam quality. The jitter time for varying energy levels ranges from 4.31 ps to 0.78 ps across the 1–5 GHz intra-burst frequency range.
In addition, the energy of this high-energy microsecond burst-mode laser is significantly enhanced compared to that of reference [
21]. This high-energy microsecond pulse cluster source serves as an important reference for the design of future high-energy burst-mode lasers. In comparison with that of reference [
22], this laser exhibits a notable increase in both energy and tuning range, positioning it as a promising high-energy tunable seed source. This advancement is beneficial for the further amplification of energy and the effective deployment of large-range tunable optical-guided microwave radar systems.
In the future, the tunable burst-mode laser can be further amplified as an optical drive source for high-power, large-bandwidth microwave photon (MWP) radar applications. At the same time, the high energy, long duration, and wide bandwidth of burst-mode lasers also make them potentially applicable as pulse sources for high-speed optical communications, the high-precision processing of special materials, and LIDAR ranging.
