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Abstract
GHz pulsed thulium-doped fiber laser with stabilized repetition rate can enable a wide range of applications. By employing regenerative mode-locking and cavity stabilization technique, we have for the first time demonstrated a 10 GHz polarization-maintaining thulium-doped fiber laser, which has a long-term repetition-rate stabilization and picosecond timing-jitter. In our experiment, a RF circuitry is designed to extract the 10 GHz longitudinal clock signal so that stable regenerative mode-locking is achieved. A piezo actuator-based phase-lock-loop is used to lock the regeneratively mode-locked pulses to a local reference synthesizer. The regeneratively mode-locked pulses with picosecond pulse width exhibit a high super-mode suppression ratio of 60 dB. In addition, the repetition rate of the laser shows good long-term stability with a variation of 8 Hz in 8 hours, corresponding to a cavity free spectral range fluctuation of less than 16 mHz. Meanwhile, the Allan deviation of the stabilized 10 GHz regeneratively mode-locked pulses is measured to be as low as 2 × 10−12 over 1000 s average time, which is only limited by the stability of the reference synthesizer. Such an ultra-stable 10 GHz pulsed thulium fiber laser may find potential application in 2 µm optical communication, material processing and spectroscopy.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Mode-locked thulium-doped fiber lasers with gigahertz repetition rate, operating in the atmospheric window, are attractive for a variety of applications such as high-speed optical communication [1–3], material processing [4–6], and spectroscopy [7–9]. The advantages of a thulium fiber system mainly arise from their broad amplification bandwidth, high quantum efficiency and compatibility with fiber communication systems. For these applications, the accuracy and signal-to-noise metrics will also depend on the repetition-rate stability of the gigahertz mode-locked lasers. Thulium-doped fiber lasers which are passively mode-locked by saturable absorbers [10,11], nonlinear-polarization-evolution [12,13] and nonlinear-amplifying-loop-mirror [14,15] have been intensively investigated in the past few years, but their repetition rate is typically limited to tens of MHz due to the relatively long cavity length [16]. Active harmonic mode-locking has been considered an effective way of generating gigahertz pulses for thulium-doped fiber laser at 2 µm, usually by periodically modulating the cavity loss or phase at a integer multiple of the cavity fundamental free spectral range (FSR) [17,18]. Precise tuning of the modulation frequency to the desired harmonic of the FSR frequency is required to ensure long-term mode-locking. However, in many cases, the detuning frequencies between the fixed modulation signal and cavity FSR imposed by environmental perturbations could result in unavoidable phase noise or jitter, and very often lead to the failure of mode-locking. By extracting the longitudinal clock signal from the fiber oscillator, rather than driving the modulator with a fixed frequency, the regeneratively mode-locking technique can effectively circumvent the detuning effects. The longitudinal clock signal extraction circuitry in the feedback loop is always composed of a high-speed photodetector, a narrow-band filter and a high-gain electrical amplifier. Since the clock signal is extracted from the oscillator itself, the modulation frequency matches well with the FSR frequency of the laser cavity. Therefore, regeneratively mode-locked (RML) pulses will always experience the maximum transmission as they pass through the modulator. Regenerative mode-locking is applicable for all kinds of lasers and has been implemented to obtain long-term stable ultrashort pulses in solid [19] and dye [20] lasers, and was later extended to fiber lasers with repetition rate up to 40 GHz at 1550 nm and 1064 nm [21–23]. Although the mode-locked state of the RML lasers is stable, the repetition rate still fluctuates with time due to the unlocked optical cavity length. In order to stabilize the repetition rate, several techniques have been reported for lasers at the near infrared region [24]. For example, X. Shan et al. employed a fiber-wounded piezo-electric transducer (PZT) to lock the optical pulse phase to that of the synthesizer [25]. H. Takara et al. extracted the relaxation oscillation component to drive an optical delay line for cavity length stabilization [26]. D. Hudson et al. stabilized the longitudinal mode spacing by modulating the refractive index of the intracavity modulator [27]. Of the above stabilization techniques, gigahertz repetition-rate pulses have been achieved in RML fiber lasers in 1 µm and 1.5 µm wavelength bands [28–30].
Recently, we have pushed the repetition rate of 2 µm actively mode-locked lasers to 20 GHz [31]. However, its long-term stabilization is limited due to the environmental perturbations. In this paper, we have demonstrated an all-polarization-maintaining RML fiber laser at 2 µm to overcome the detuning problem. To this end, a clock extraction circuitry is introduced to extract the longitudinal clock signal from the oscillator and ensure its long-term synchronization with the optical pulses. Furthermore, a phase-lock-loop (PLL) circuitry is designed to lock the repetition rate of this RML pulses to a standard 10 GHz synthesizer. The super-mode suppression ratio for the 10 GHz RML pulses is measured to be ∼ 60 dB. This robust 10 GHz RML laser also shows a good long-term stability (>8 hours) with repetition rate frequency fluctuation of less than 8 Hz, corresponding to a fundamental FSR fluctuation of less than 16 mHz. The Allan variance of the repetition rate over 1000 s averaging time is as low as 2 × 10−12, which is mainly limited by the synthesizer (1.5 × 10−12@ 1000 s). The single sideband phase noise of the 10 GHz RML laser pulses is −78 dBc/Hz at 10 kHz, while the timing jitter of the pulses is calculated to be 1.28 ps when the noise spectrum is integrated from 10 Hz to 30 MHz offset frequency. Such a long-time stable 10 GHz repetition rate thulium-doped fiber laser with low phase noise feature is well suited for emerging 2 µm high-speed photonics.
2. Experimental setup
The experimental setup of the RML laser is shown in Fig. 1, which consists of three main parts: the laser cavity, the clock extraction circuitry and the PLL circuitry. In the schematic setup, the solid blue lines indicate the optical fiber while the solid black lines indicate the RF cable connections. With respect to the laser cavity, all the fiber components are polarization-maintaining and its temperature is well controlled. Therefore, the external perturbation from the environment can be avoided. A continuous-wave (CW) 1550 nm laser is used as the pump for the RML laser which is coupled into the ring cavity through a 1550/2000 nm wavelength division multiplexer (WDM). 2.2 meters long thulium-doped fiber (Nufern Inc. PM-TSF 9/125) with core/cladding diameter of 9/125 µm is used as the gain medium. A 10 GHz lithium niobate phase modulator (EOSPACE Inc. LNPM) is inserted in the cavity to perform harmonic active mode-locking. The modulator is driven by the signal from the longitudinal clock extraction circuitry. Output pulses are coupled from the cavity by a 10:90 optical coupler. The total fiber length of the ring cavity is about 11.2 m, corresponding to a FSR of Δν = c∕nL = 18.162 MHz.
Fig. 1. The schematic setup of the PLL-RML fiber laser system. WDM: wavelength division multiplexer; ISO: isolator; PZT: piezo-electric transducer; LNPM: lithium niobate phase modulator; PD: photodetector; BPF: bandpass filter; AMP: amplifier; OC: optical coupler; RF-C: RF coupler; PS: phase shifter; DBM: double balanced mixer.
The clock extraction circuitry used for regenerative mode-locking is composed of a high-speed photodetector, a 10 GHz band-pass filter (Filter1, 500 MHz bandwidth), a 10 GHz narrow band-pass filter (Filter2, 15 MHz bandwidth), a phase shifter, three RF amplifiers and two RF couplers. At first, part of the laser output is detected by the high-speed photodetector. Then, the longitudinal clock signal is extracted by the following two 10 GHz band-pass filters. The first filter is employed to suppress parasitic passbands. The bandwidth of the second filter is specially tailored to be narrower than the cavity FSR in order to get a clean clock signal. The extracted longitudinal clock signal is very weak. To meet the requirement of the driving-power for the LNPM, two low noise amplifiers (AMP1, AMP2) are used for clock signal amplification while AMP3 is used for power amplification. In addition, a 1:10 RF coupler (RF-C2) is inserted between the AMP3 and the LNPM for real-time monitoring of the modulation signal. By finely adjusting the phase (by setting PS) between the optical pulse and modulation signal, regenerative harmonic mode-locking can be achieved. Stable regenerative mode-locking operation can be sustained for hours since the modulation signal is extracted directly from the laser itself.
To stabilize the repetition rate frequency, a PLL circuitry which consists of a synthesizer, a mixer, a PI controller and a PZT is introduced to stabilize the cavity length. A segment of the intra-cavity fiber is attached onto the PZT, so that the cavity length can be changed by controlling the voltage to the PZT. The full driving voltage to the PZT will introduce a cavity length change of 44.2 µm, leading to a repetition-rate variation of 44.28 kHz around 10 GHz. This wide control range ensures full compensation to the repetition-rate drift of the laser. The repetition-rate change per volt is 295.2 Hz/V.
3. Results and discussion
Figure 2(a) shows the average output power of the RML laser from the 10% port of OC1 under different pump power. The pump threshold of the laser is ∼420 mW and the laser system starts mode-locking at a pump power of 500 mW, while the output power of the RML laser is 1.5 mW. At first, the modulation signal is not connected to the LNPM. In this case, the free-running thulium-doped fiber laser is in CW operation. The CW laser from the 90% port of OC2 is detected by a high-speed photodetector (Discovery Semiconductor Inc.) and then the signal is recorded by a RF signal analyzer (R&S Inc. FSV 30). Figure 2(b) shows the RF spectrum of the detected signal. The initial RF spectrum of the signal consists of longitudinal beat-signal located at frequencies of integer multiple of the cavity FSR. To extract the longitudinal clock signal out of the beat signal, the cavity length is finely tuned to match with the narrow passband of the RF filters. The frequency of the filtered beat signal is 10.008 GHz, which is 551th harmonic order of the cavity FSR. The typical RF spectrum of the monitoring clock signal from RF-C2 after filtering and amplification is shown in Fig. 2(c). The RF spectrum of the monitoring signal in the figure exhibits a super-mode suppression ratio of 90 dB under a resolution bandwidth (RBW) of 10 kHz, which indicates that a clean regenerative longitudinal clock signal has been successfully extracted. The intensity of the monitored regenerative longitudinal clock signal is measured to be 11 dBm. Therefore, the actual driving power of the regenerative longitudinal clock signal attached to the LNPM is 21 dBm. When the extracted longitudinal clock signal is connected to the LNPM and an appropriate pump power is set, RML thulium-doped fiber laser with 10 GHz repetition rate can be achieved. In this situation, the 10% port of OC2 acts as the output for the RML laser.
Fig. 2. (a) Average output power from the 10% port of OC1 under different pump power. (b) The initial RF spectrum of the thulium-doped fiber oscillator under CW operation. (c) The RF spectrum of the regenerative modulation signal.
The output spectrum of the RML laser is monitored by an optical spectrum analyzer (Yokogawa AQ6375) with a resolution of 0.05 nm, which is shown in Fig. 3(a). For comparison, the optical spectrum of the laser under CW operation is also shown in the figure. The output spectrum of the RML laser is centered at 1888 nm with a full-width-at-half-maximum (FWHM) bandwidth of 0.81 nm. The spectral bandwidth of the laser is obviously broadened by phase modulation and a comb of new frequency components are produced on both sides of the center wavelength. The interval between the new frequency components is 0.13 nm, which matches well with the frequency of the longitudinal clock signal. To characterize the RML pulses, the output laser is detected by a InGaAs photodetector (EOT ET-5000F, 11 GHz, rise time<28 ps). The corresponding RML pulse train is monitored by a real-time oscilloscope (KEYSIGHT, 14 GHz, rise time<16 ps). As shown in Fig. 3(b), the pulse train exhibits a good flatness in large time scales, indicating good stability of the regenerative mode-locking. In addition, Fig. 3(c) indicates the pulse width of the RML laser is ∼49 ps, which is limited by the resolution of the photodetector. To further investigate the performance of the RML laser, the RF spectrum of the detected pulses is recorded at the same time. As shown in Fig. 3(d), the RF spectrum of the RML pulses exhibits a super-mode suppression ratio of 60 dB with a span range of 50 MHz (resolution bandwidth of 5 kHz). In the meanwhile, the RF spectrum with broad span range (30 GHz) is also shown in the inset. This clean RF spectrum further proves the stability of the regenerative mode-locking.
Fig. 3. (a) Optical spectra of the laser under RML operation and CW operation. (b) The temporal pulse train. (c) Pulse width and (d) RF spectrum of the RML laser.
The regenerative mode-locking of the thulium-doped fiber lasers is stable, but its repetition rate may fluctuate due to the environmental perturbation. Combining the regenerative mode-locking technology and the piezo actuator based PLL circuitry, repetition-rate stabilized RML thulium-doped fiber laser is demonstrated. To investigate the long-term stabilization, the repetition-rate fluctuation of the RML thulium-doped fiber laser is monitored. Figure 4(a) shows the repetition-rate variation of the free running RML laser and the phase-lock-loop controlled and regeneratively mode-locked (PLL-RML) laser, which is measured by the RF signal analyzer with a resolution of 100 mHz. There is a periodical frequency fluctuation of 25 kHz for the free running RML laser. This means the cavity length experiences a periodical change of 28 µm. For comparison, a stabilized repetition rate is obtained when the PLL circuitry is introduced. The repetition-rate variation of the PLL-RML laser in 8 hours’ time is shown in Fig. 4(b). The mean value of the repetition rate is 10,008,865,000 Hz and its variation is around 8 Hz with a standard deviation about 1.15 Hz. As the thulium-doped fiber laser is mode-locked at the 551th harmonic order, the relative variation of the fundamental cavity FSR is less than 16 mHz. It is noted the frequency stability of the PLL-RML pulses is comparable with the reference synthesizer. In other words, the stability of the PLL-RML laser is mainly limited by the synthesizer used in the experiment. Besides, the Allan deviation of the laser’s repetition rate is an important figure-of-merit for stabilization characterization, which can be measured through the frequency counter option of the RF signal analyzer. The Allan deviation values of the PLL-RML pulses and the synthesizer are shown in Fig. 4(c). It is clear to see that the Allan deviation decreases when the averaging time increases. This indicates that the system is in a well stabilized state. The Allan deviations of the repetition rate are 5 × 10−11 and 2 × 10−12 for integration times of 1s and 1000s, respectively. It can be also seen that Allan deviation of the PLL-RML pulses is similar to that of the reference synthesizer, which indeed proves the stability of the PLL-RML pulses is constrained by the synthesizer used in the experiment. These results indicate that the PLL-RML laser can be tracked completely to the synthesizer without introducing residual phase noise during the stable operation. To further investigate the long-term stabilization, the phase noises of the RML laser with and without PLL operation are measured at the repetition rate frequency of 10 GHz. Figure 4(d) shows the typical phase noise curves of the RML laser with and without PLL operation. The single sideband phase noise is about −78 dBc/Hz at 10 kHz in both situations (shot noise floor is ∼−131 dBc/Hz). We can also see that the high-frequency (∼10 kHz−30 MHz) phase noise is not significantly impacted by the PLL operation but mainly depends on the regenerative mode-locking performance of the thulium-doped fiber laser itself. However, the low-frequency (∼Hz-kHz) phase noise can be effectively suppressed by the PLL operation, which means the PLL operation is beneficial for long-term repetition-rate stabilization. According to the phase noise spectrum in Fig. 4(d), the timing jitter of the PLL-RML pulses is calculated to be 1.28 ps (integrated from 10 Hz to 30 MHz offset frequency). Besides, we have also characterized the relative intensity noise (RIN) of the RML laser, as shown in Fig. 4(e). It can be seen that the low-frequency RIN of the RML is also improved with PLL-operation.
Fig. 4. (a) The repetition rate variation of free running and stabilized RML thulium-doped fiber laser. (b) The repetition stability of the PLL-RML laser and the reference synthesizer. (c) The Allan deviation of the PLL-RML fiber laser and the synthesizer. (d) The single sideband phase noise and integrated jitter of the RML fiber laser with and without PLL operation. (e) The RIN of the RML fiber laser with and without PLL operation.
4. Conclusion
In conclusion, a regeneratively mode-locked (RML) thulium-doped fiber with phase-locked-loop (PLL) operation at 10 GHz repetition rate is experimentally demonstrated for the first time. The PLL-RML scheme ensures the laser works at a stabilized repetition rate in 8 hours. The variation of the stabilized repetition rate is measured to be 8 Hz, corresponding to a fundamental free-spectral-range fluctuation of less than 16 mHz. The Allan deviation of the repetition rate over 1000s averaging time is as low as 2 × 10−12 (limited by the synthesizer used for the PLL operation). In the meanwhile, the super-mode suppression ratio for the PLL-RML pulse train is measured to be 60 dB. The single sideband phase noise is about −78 dBc/Hz at 10 kHz offset frequency and the timing jitter of the laser is calculated to be 1.28 ps. As the thulium-doped fiber laser is polarization maintained and temperature stabilized, it is immune to the environmental perturbations. Such a 10 GHz RML thulium-doped fiber laser with stabilized repetition rate and low timing jitter shows potential application in 2 µm optical communication, material processing and spectroscopy.
Funding
State Key Project of Research and Development of China (2018YFB2200500); National Key Research and Development Program of China (2011CB301900, 2014CB921101); National Natural Science Foundation of China (61378025, 61427812, 61805116); ‘Jiangsu Shuangchuang Team’ Program; Natural Science Foundation of Jiangsu Province (BK20140054, BK20170012, BK20180056, BK20192006).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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