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Abstract
A robust all polarization-maintaining (PM) passively mode-locked Er-doped fiber laser is demonstrated based on the biased nonlinear amplifier loop mirror (NALM). With a π/2 nonreciprocal free-space phase shifter, stable single pulse mode locking can be obtained at the central wavelength of 1565.7 nm with a 3 dB spectral bandwidth of 24.6 nm in the soliton regime. The repetition rate of the pulse train is 98.13 MHz. The direct output pulse duration is 109 fs, which is nearly transform-limited. After the intracavity dispersion management, the robust self-started mode-locking in the stretched-pulse regime is realized at 1564 nm, and the 3 dB spectral bandwidth reaches up to 78 nm. The repetition rate of the pulse train is 199.6 MHz. In particular, the direct output pulse width is only 77 fs with a low integrated relative intensity noise (RIN) of only 0.0044% (integrated from 1 Hz to 1 MHz). To the best of our knowledge, this is the shortest pulse width directly from the all-PM NALM laser oscillator.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrashort pulse fiber lasers with high repetition rate have been the focus of intense interest in a large number of applications, such as optical frequency metrology [1,2], biomedical imaging [3], biophotonics [4], microscopy [5], low-noise microwave generation [6], distance metrology [7–9] and so on. The lasers with the fundamental repetition rate exceeding gigahertz based on liner cavity and saturable absorber (SA) have been experimentally realized over the past few years [10–12]. However, the pulse shaping mechanism of SA limits the full width at half maximum (FWHM) of the optical spectrum to a few nanometers, corresponding to a transform-limited pulse width of hundreds of femtoseconds [13,14]. Although utilization of the pre-chirp managed nonlinear amplification can further broaden the optical spectrum and compress the pulse width down to sub-100 fs [15,16], the device has also become more complex. Fast saturable absorber such as nonlinear polarization evolution (NPE) has been shown to directly generate a broader spectrum. The previous work has demonstrated an Er-doped fiber laser in the stretched-pulse regime with the FWHM of the optical spectrum of 135 nm and the pulse width of 37.4 fs at the repetition rate of 225 MHz [17]. Li et al. achieved the FWHM of 148 nm and the pulse width of 44.6 fs [18].
Although NPE has shown a great development potential to generate sub-100 fs pulses, the environmental stability of the fiber laser is always an issue because of the non-polarization maintaining structure. In 2017, Hänsel et al. reported a robust all-PM mode-locked fiber laser based on the reflective Sagnac loop with a free space phase shifter at the repetition rate of 250 MHz and the 3 dB optical spectral bandwidth of 43 nm [19]. This kind of configuration combines the advantage of the low losses of the transmissive Sagnac loop with the short physical length of the reflective Sagnac loop. In 2018, Gao et al. demonstrated the same but non-PM structure and attained the pulse width of 44.6 fs after compression with the fundamental repetition as high as 257 MHz [20]. However, the study didn’t provide the approximate intracavity net dispersion and the FWHM of the optical spectrum. In 2019, an investigation showed that the dispersion-managed biased NALM has the potential to direct generate sub-100 fs ultrashort pulses without any external dispersion compensation [21]. In addition, more studies have shown that the biased NALM has good noise characteristics as an ultrafast laser source [22,23].
In this letter, we demonstrate an all-PM erbium-doped NALM-based fiber laser. Optimizing the net dispersion in the cavity to near zero enabled self-started mode-locking at the pump power of 650 mW in the stretched-pulse regime. The oscillator works in the single pulse region by reducing the pump power direct to 400 mW. The FWHM of the optical spectrum is 78 nm and the direct output pulse width is 77 fs at the repetition rate of 199.6 MHz. Moreover, the integrated RIN and the timing jitter for mode locking are measured to be 0.0044% (integrated from 1 Hz to 1 MHz) and 321 fs (integrated from 100 Hz to 1 MHz), respectively.
2. Experimental setup
The configuration of the laser is illustrated in Fig. 1. All fibers in the cavity are polarization maintaining (PANDA type). A segment of 425 mm long highly doped Er-doped fiber (Liekki Er80-4/125-HD-PM) is used as the gain fiber, marked as PM-EDF. This kind of gain fiber has normal dispersion with group-velocity dispersion (GVD) around 20 fs2/mm and the peak core absorption at 1530 nm is 80 dB/m. The gain fiber is spliced close to a collimator with a 58 mm pigtail (PM-SMF1), marked as Col1, providing an asymmetrical nonlinear phase shift for the two counter-propagating beams. The other end of the gain fiber is spliced to the common port of a 980/1550 nm PM wavelength division multiplexer (WDM). The gain fiber is pumped by a 976 nm PM single-mode laser diode (LD) with 960 mW maximum average power. The pigtail length of the common port (PM-SMF2) is 160 mm. The WDM is fast axis blocked to ensure the pulses propagate along the slow axis while transferring along the PM fiber. The signal port of the WDM is spliced to another collimator (Col2). For convenience, the fiber pigtail of the WDM single port and the Col2 are jointly marked as PM-SMF3. The GVD of the single-mode fiber (PM-SMF) in the cavity is around -22 fs2/mm.
Fig. 1. The configuration of the all-PM NALM fiber laser. Col: collimator; PM-EDF: polarization-maintaining erbium-doped fiber; PM-SMF: polarization maintaining single mode fiber; WDM: wavelength division multiplexer; PBS: polarization beam splitter; FR: 45 ° faraday rotator; 1/8 WP: 1/8 waveplate; Reflector: silver mirror.
To reduce the losses introduced by the free space component, we rotated the collimators carefully to make the polarization of the two counter-propagating beams output from Col1 and Col2 parallel to the p and s state of the polarization beam splitter (PBS1), respectively. Therefore, the two beams with orthogonal polarization directions are combined at PBS1 and propagate through a non-reciprocal phase shifter. The phase shifter, which introduces π/2 non-reciprocal phase bias in one round-trip to reduce the mode-locked threshold, is consisted of a 45° faraday rotator, a 1/8 waveplate (central wavelength at 1550 nm), and a reflector. Benefiting from this configuration, NALM behaves as a saturable absorber with relatively moderate modulation depth. Thus, it can serve as an appropriate mode locker to realize robust mode locking at relatively low pump power [19]. The two beams with orthogonal polarization which have accumulated nonlinear phase differences in the fiber loop, obtain a non-reciprocal linear phase difference after passing through the phase shifter. They are then projected on the p and s directions of PBS2. The interference between the two beams at PBS2 guarantees that, the high-power optical pulses with large nonlinear phase difference transmit through the PBS2 with lower loss and the low-power pulses with high loss. This process is equivalent to an artificial saturable absorber. The optical length of free space in one round-trip is about 163 mm.
3. Results and discussion
Firstly, soliton mode locking is realized. The parameters of every segment of fibers in the soliton mode locking region are illustrated in Table 1. When the length of PM-SMF3 is initially set at about 1260 mm, the net dispersion in the cavity is -24016 fs2. By rotating the 1/8 waveplate to find the optimal angle, self-started mode locking operation in the multiple-pulse region can be achieved at the pump power of 560 mW without disturbance to the fiber. The relationship between the output power and the pump power is shown in Fig. 2(a). The continuous wave (CW) laser oscillation occurs at the pump power of 35 mW. In the region of [35 mW, 560 mW], the CW output power increases linearly with the increasing pump power, shown as the red dots. Once the pump power is set to 560 mW, self-started mode locking can be achieved in the multiple-pulse region, shown as the blue dots line. When the pump power drops to 255 mW, single pulse mode locking can be realized. The range of pump power for the single pulse region is [140 mW, 255 mW]. When the pump power is 255 mW, the maximum power of single-pulse output from PBS2 is 28 mW. Because of the fast-axis blocked WDM, the intracavity optical field is linearly polarized along the slow axis of the PM fiber with over 49 dB extinction ratio. The optical power from the other port of PBS1 is only sub-1 µW level. Therefore, we mainly measure the characteristics of the mode locking from PBS2.
Fig. 2. (a). Variation of the output power (CW and mode-locked) with the pump power, and the self-starting pump threshold is 560 mW. (b) Optical spectrum of the fundamental mode-locking operation at a pump power of 255 mW. (c) Measured interferometric autocorrelation trace of the direct output pulses from PBS2. (d) Output pulse train of fundamental mode-locked fiber laser with 98.13 MHz repetition rate. (e) Measured RF spectrum of the fundamental mode-locking operation. Inset: the broad-span RF spectrum.
Table 1. The GVD and length of fibers in the all-PM NALM
The optical spectrum of the laser output is measured by an optical spectrum analyzer (YOKOGAWA AQ6375B). Figure 2(b) illustrates the optical spectrum at the pump power of 255 mW. The optical spectrum that is measured at the resolution of 0.5 nm has a FWHM of 24.6 nm at the central wavelength of 1565.7 nm. The pulse width is measured by an autocorrelator (APE Pulsecheck 150). The measured autocorrelations trace of the single pulse at the pump power of 255 mW is illustrated in Fig. 2(b). Without any extra-cavity dispersion compensation, the pulse width is 109 fs, with the Sech2 fit profile assumed, nearly transform limited (104.6 fs). The pulse trains are detected by a 2-GHz photodetector (EOT ET-3000A) and analyzed by a 1-GHz-bandwidth oscilloscope (LeCroy Waverunner 104Xi). Figure 2(c) shows the oscilloscope trace of the single pulse trains with the fundamental repetition rate of 98.13 MHz. The radio frequency (RF) spectrum is obtained by an RF spectrum signal analyzer (Agilent N9340B). The RF spectrum from 93 MHz to 103 MHz at a 30 Hz resolution bandwidth is shown in Fig. 2(e). The signal-to-noise ratio (SNR) is around 82.4 dB. Inset in Fig. 2(e) shows the RF spectrum with a 1-GHz span and a resolution bandwidth of 300 kHz.
Thanks to the all-PM structure, the mode locking is stable even when the fiber is shaken violently or the laser base plate is hardly knocked.
To obtain a broader spectral bandwidth and a higher repetition rate, the cavity dispersion needs to be close to zero to achieve stretched-pulse mode locking. The parameters of every segment of fibers in the stretched-pulse mode-locked laser are also shown in Table 1. After optimizing the intracavity dispersion by cutting the PM-SMF3 to around 225 mm, the net dispersion in the cavity is estimated to be -1246 fs2. Measurements of the relationship between the output power of the laser and the pump power are summarized in Fig. 3(a). The CW laser oscillation occurs at the pump power of 35 mW. The mode locking starts with a CW component at the central wavelength at the pump power of 740 mW, which is higher than that of the soliton mode locking (Fig. 2(a)). The increase of the self-started power threshold might be caused by the increase in repetition rate, which will lead to a decrease in pulse peak power when assuming the same average power, in turn, it will reduce the nonlinear phase difference. Reducing the pump power will eliminate the CW component. The single pulse region is turned to be [230 mW, 370 mW], due to the increase of repetition rate and the close-to-zero cavity dispersion, the pulse can accumulate higher nonlinear phase shift without breaking. The maximum single pulse output power is 37.72 mW at the pump power of 370 mW.
Fig. 3. (a). Variation of the output power (CW and mode-locked) with the pump power, and the self-starting pump threshold is 740 mW. (b) Optical spectrum of the fundamental mode locking operation at a pump power of 370 mW. (c) Measured interferometric autocorrelation trace of the direct output pulses from PBS2. (d) Output pulse train of fundamental mode-locked fiber laser with 199.6 MHz repetition rate. (e) Measured RF spectrum of the fundamental mode locking operation. Inset: the broad-span RF spectrum.
Figure 3(b) shows the optical spectrum directly output from PBS2 with the resolution of 0.5 nm. Compared with Fig. 2(b), the spectrum is significantly broadened to reach a FWHM of up to 78 nm at the central wavelength of 1564 nm, corresponding to the transform-limited pulse width of 33 fs (Sech2 fit). Significantly, Kelly sideband is also suppressed. Fringe-resolved autocorrelation trace of the output pulse from PBS2 without extra-cavity dispersion compensation is shown in Fig. 2(c), the pulse duration is measured as 77 fs (Sech2 fit) at the pump power of 370 mW. Time domain single pulse train with 199.6 MHz repetition rate is shown in Fig. 3(d). The RF spectrum from 194.5 MHz to 204.5 MHz at a 30 Hz resolution bandwidth is illustrated in Fig. 3(e). The fundamental frequency is located at the repetition rate of 199.6 MHz with the SNR of 84.5 dB. Inset in Fig. 3(e) shows the fundamental repetition frequency and its harmonics with a frequency range of 1 GHz. The clear intensity demonstrated that the laser was well operated at CW mode locking.
We also monitor the average output power with and without an enclosure of the laser at room temperature, respectively, to check the power stability of the stretched-pulse mode locking laser. The power’s recording rate is 1 Hz. The power fluctuation is approximately 0.064% root mean square (RMS) when the laser continuously runs for 24 hours, shown as Fig. 4(a). With a simple plastic enclosure to prevent airflow, the measured power fluctuation in 2 days is illustrated in Fig. 4(b). The power stability has been significantly improved.
Fig. 4. Power stability of the laser (a) running in 24 hours without any enclosure and (b) running in 2 days with a simple plastic enclosure.
To study the noise characteristics of this kind of laser with different cavity dispersion, the phase noise power spectral density (PSD) of the repetition rate is measured by a phase noise analyzer (Microsemi 5125A). The phase noise PSD of a microwave signal scales with the square of carrier frequency as follows:
where Sφ(f) is the phase noise, Sx(f) is the timing jitter, ν0 is the carrier frequency and f is the Fourier frequency. Thus, it is reasonable to compare the phase noise PSDs of the two pulse trains with similar carrier frequency ν0. In this case, we measure the phase noise of the second harmonic (196.26 MHz) of the soliton mode locking with the cavity dispersion of -24016 fs2 at the pump power of 255 mW, and the first harmonic (199.6 MHz) of the stretched-pulse mode locking with the cavity dispersion of -1246 fs2 at the pump power of 370 mW, respectively. The photodetector receives the same optical power during detection and the measurement is shown in Fig. 5. The difference between the offset frequency from 1 Hz to 10 Hz might be caused by the influence of external disturbance. The spikes at 10 Hz to 1 kHz are mainly caused by the AC power (50 Hz and harmonics) and acoustic noise that is coupled to the lasers [24]. From 10 Hz to 1MHz, the stretched-pulsed mode locking has a relatively low phase noise compared with the soliton mode locking. The timing jitter of the soliton mode locking and the stretched-pulse mode locking integrated from 1 MHz to 100 Hz was calculated to be 374 fs and 321 fs, respectively.Fig. 5. The measured phase noise of the second harmonic (196.26 MHz) of the soliton mode locking (blue curve) and the first harmonic (199.6 MHz) of the stretched-pulse mode locking
In addition, the RIN of the optical pulses from the stretched-pulse laser cavity is shown as the blue curve in Fig. 6. The optical pulses are detected by using a 17 MHz photodetector (Thorlabs PDA10CS-EC). With a 22 MHz low bandpass filter at the output, the RIN is obtained by a phase noise analyzer (Agilent Technologies E5052B) using the baseband mode. At low frequency, the long-term drifting of the RIN is caused by the environmental changes. In the Fourier frequency ranging from 40 Hz to 2 kHz, the RIN power spectral density (PSD) is flat at around ∼ 130 dBc/Hz which originates from the intensity fluctuation of the pump laser. For the Fourier frequency beyond 2 kHz, the gain medium acts as a low-pass filter, which filters out the intensity fluctuations from the pump laser at high frequency [25]. The integration curve of RIN is plotted in the same figure (red curve). It can be noted that integration from 1M Hz to 1 Hz is only 0.0044%.
Fig. 6. RIN (blue curve) of the stretched-pulse mode-locking with 199.6 MHz repetition rate and integrated RIN (red curve).
4. Conclusion
In conclusion, we have presented a robust, dispersion-managed all-PM erbium-doped NALM mode-locked in the soliton and the stretched-pulse regime. At the repetition rate of 98.13 MHz and the net dispersion of -24016 fs2, self-started soliton mode locking can be achieved at the pump power of 560 mW. The optical spectral bandwidth is measured to be 24.6 nm in the single pulse region. To achieve a broader spectral bandwidth, the net dispersion in the cavity is managed to be -1246 fs2, close to zero. The stretched-pulse mode locking can self-start at the pump power of 740 mW. In the single pulse region, the measured spectral bandwidth is as broad as 78 nm and the direct output pulse duration is 77 fs. Moreover, the stretched-pulse mode locking operation can keep for over one week with reliable repeatability. The integrated RIN and the timing jitter are measured to be 0.0044% and 321 fs, respectively. This robust, high-repetition-rate and ultrashort laser source could have great potential in optical frequency comb based applications in space under microgravity [26].
Funding
The Scientific Research Foundation of Key Laboratory of Metrology and Calibration Technology, China (JLJK2021001B001); National Natural Science Foundation of China (61905205).
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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