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Abstract
We demonstrate a 1.7 µm femtosecond Tm-doped fiber laser system featuring an all-polarization-maintaining architecture. The seed oscillator is mode-locked by carbon nanotubes and delivers stable pulse centered at 1787.6 nm. With two backward pumped amplifiers, the average power of the laser is amplified to ∼458 mW. Employing proper dispersion management in an all-fiber chirped pulse amplification scheme and the soliton compression effect, we obtained a femtosecond pulse of 206 fs with a pulse energy of 8.8 nJ at a repetition rate of ∼52 MHz. To the best of our knowledge, this is the first demonstration of 1.7 µm femtosecond laser based on a thulium-doped oscillator with all-polarization-maintaining architecture.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Intense efforts are dedicated to the development of ultrafast fiber lasers, due to the inherent compactness, excellent heat dissipation and high beam quality of a fiber-based platform. A wide range of operation wavelengths are accessible due to the maturity of ytterbium (Yb), erbium (Er), and thulium (Tm) doped silica gain fibers, as well as the help of fiber nonlinearity. In the area of bio-imaging, particular multi-photon microscopy, due to the advantages of decreased water absorption and lower scattering loss compared with shorter wavelengths (0.8 µm and 1.3 µm) [1,2], ultrafast fiber lasers with a center wavelength of ∼ 1.7 µm is becoming an increasingly popular excitation source [3,4]. Incentive to develop compact, efficient and commercial-grade ultrafast all-fiber lasers at 1.7 µm is ever growing.
Nonlinear wavelength conversion was previously employed to obtain 1.7 µm pulses [5,6]. However, frequency shift generally involves complex coupling with special fibers like large-mode-area fiber [7] and photonic crystal rods [8], which could increase the instabilities, and complexity of the sources. Also, the achievable energy is limited by mode-locked Er-fiber laser pump source. Actually, the pulses at 1.7 µm can be directly generated in rare-earth-ions doped fiber lasers, such as bismuth-doped fiber laser [9] and thulium-doped fiber laser [10,11]. However, bismuth-doped fiber is not commercially mature and has relatively low optical gain at 1.7 µm, leading to the limitation of pulse amplification. In recent years, a number of papers demonstrate Tm-doped fiber (TDF) lasers operating at wavelength around 1.7 µm [10,11]. Commercial TDF has a wide gain bandwidth spanning 1600 nm to 2100 nm in the 3F4 − 3H6 transition [12,13], and both mode-locking and dispersion compensating components are conveniently available, making it effective to directly produce 1.7 µm femtosecond pulses. However, TDF have gain saturation effects and strong re-absorption of light when operating at relatively short wavelength region below 1.8 µm [14]. To work around this, optical filters and wavelength selective components are usually applied in the oscillator cavity to suppress unwanted long wavelength emissions [15,16]. Li et al demonstrated an nonlinear polarization rotation (NPR) mode-locked TDF laser, and bandpass filter was employed to generate noise-like pulse at a center wavelength of 1750nm [15]. Chen et al reported 1.7 µm dissipative solitons with pulse duration of 230 fs from a mode-locked TDF laser by using bandpass filter in the oscillator cavity [16]. Moreover, bending special structure fibers can also suppress amplified spontaneous emission (ASE) at long wavelength [17]. Chen et al proposed a parabolic W-type TDF for the 1.7 µm high-energy femtosecond pulsed laser by utilizing the bending-induced bandpass effect, and delivered 174 fs pulses with energy of 128 nJ from an all-fiber chirped pulse amplification configuration [17]. However, intra-cavity filters and special fibers could increase the instabilities, complexity and the insertion loss of the sources. He et al controlled the length of the gain fiber to achieve wavelength selection, and demonstrated an all-fiber mode-locked laser centered at 1780nm without optical filters in the cavity [18]. Despite a great number of reports on mode-locked TDF lasers operating at 1.7-µm region [19], there are no report on all-polarization maintaining (PM) lasers. In a non-PM cavity, a polarization controller is usually required to control the polarization state of the propagating light and start the mode-locking. Such oscillator might be sensitive to external perturbations, such as changes in arrangement and movement of fibers and passive components [20]. Therefore, there is great interest in developing all-PM 1.7 µm femtosecond lasers based on all fiber configuration without optical filter and spatial fibers.
In this letter, we demonstrate for the first time an all-PM mode-locked TDF fiber laser operating at ∼1.7 µm. The oscillator is mode-locked by carbon nanotubes (CNTs) and amplified with two backward pumped amplifiers. In the compression stage, the amplified pulse is compressed by anomalous dispersion fiber to achieve an all-fiber chirped pulse amplification (CPA) scheme. Finally, we obtain a femtosecond pulse of 206 fs with a pulse energy of 8.8nJ at a repetition rate of ∼ 52 MHz. This all-PM stable femtosecond laser provides considerable potential for biomedical imaging and spectroscopy applications.
2. Experiment setup
The experimental setup of the all-PM TDF mode-locked ultrafast laser is illustrated in Fig. 1. In the oscillator, a segment of PM TDF (PM-TSF-9/125, Nufern) is used as the gain medium, which is pumped by an amplified 1560 nm laser source via a 1560/1750nm wavelength division multiplexer (WDM). The CNTs films, prepared using the same method as our previous work [21,22], is inserted between two fiber connectors as the mode-locker. Meanwhile, an isolator (ISO) is employed to ensure the unidirectional operation of light. Followed by the ISO1, another WDM is placed in the cavity to remove the residual pump at 1560 nm. A 50/50 fiber output coupler (OC) is employed as the output coupler. Another OC (99/1) is used to split a small proportion of the output to an oscillator to monitor the temporal waveforms, and the 99% output is fed to the pre-amplifier. Importantly, with our scheme there is no need to use polarization controllers in the cavity, which simplifies the whole setup. All the passive components in the laser cavity are made from PM1550. In order to control the cavity dispersion, a segment of PM2000D is used as a normal dispersion component. All the optical fibers and devices in the setup are polarization-maintaining and there are no free-space optical filters in the oscillator, which ensures stable mode-locked operation.
Fig. 1. Schematic of the all-PM Tm-doped ultrafast laser with all fiber configuration. CNT-SA: carbon nanotubes saturable absorber; OC: coupler; ISO: isolator; WDM: wavelength division multiplexer.
In order to enhance the pulse energy, two backward pumped amplifiers are constructed. All the gain medium in the amplifier are PM-TSF-9/125, which both are pumped by 1560 nm laser source. Before the first amplifier, ISO2 is spliced after the oscillator to ensure unidirectional signal propagation and isolate the backpropagating signal, and ISO3 is used to avoid unabsorbed pumping light at 1560 nm. In the same way, ISO4 and ISO5 inserted between two amplification stages play the same role as ISO2 and ISO3. Followed by the first amplifier, PM2000D is placed as pulse stretcher to sufficiently stretch the seed pulses and suppress nonlinearity for CPA amplification in the last fiber amplifier. In order to keep all-fiber configuration of the system, a segment PM1550 is used to compress the amplified pulse instead of free space compressor such as diffraction gratings and prism pair.
3. Experiment results
Initially, the oscillator is constructed without PM2000D and all fibers in the cavity show negative dispersion characteristics. In this case, the length of the gain fiber is adjusted to control the center operating wavelength of the oscillator. Figure 2 shows the output spectra of the oscillator with different TDF lengths. At first, the length of 3 m is used in the cavity. When the pump power is increased to an appropriate level, stable self-starting single-pulse soliton mode-locking is always initiated and stable oscilloscope traces can be observed. As shown in Fig. 2(a), obvious Kelly sidebands can be seen from the spectrum with a center wavelength of 1822nm, indicating that the laser operates in the soliton regime with anomalous dispersion. By reducing the length of TDF, the center wavelength is considerably blue shifted due to the balance between absorption and amplification in the TDF [23]. The center wavelength for TDF lengths of 2 m and 0.95 m are 1811nm and 1784nm, respectively, as shown in Fig. 2(b) and (c).
In order to mitigate temporal wave breaking and spectral distortion in the amplification stage, a segment of 0.7 m PM2000D is placed in the oscillator to compensate the cavity dispersion and decrease Kelly sidebands of the output spectrum, corresponding to a total cavity length of 3.75 m and the net dispersion is about -0.0594 ps2. In fact, the pulses are alternately stretched and compressed as they transmit in the oscillator when employ a segment of positive-dispersion fiber, which can reduce phase-matched coupling to resonant sidebands and make the output spectrum more cleaner [24]. But further increase of the length of PM2000D results in unstable mode-locked operation due to the intra-cavity loss. The oscillator can operate in the mode-locked regime by itself at a pump power of 813 mW, which produced an average output power of 3.6 mW. It was noteworthy that the oscillator can be self-started with no polarization controller in the cavity. Due to the all-PM configuration, the mode-locked operation is immune to any external perturbations such as movement of the fibers and components [25]. As displayed in Fig. 3(a), the spectrum, recorded by an optical spectrum analyzer (OSA, Yokogawa AQ6375), is centered at 1787.6 nm and has a full width at half-maximum (FWHM) of 9.2 nm. It is fairly obvious that the Kelly sidebands of this spectrum are much smaller than the spectra from the oscillator without PM2000D. As the signal output power is too low, the pulse duration cannot be measured with an autocorrelator. Figure 3(b) shows the pulse train with a fundamental repetition rate of ∼52 MHz, corresponding to 19.2 ns pulse spacing. In addition, radio frequency (RF) spectrum measured with 0.5 MHz and 1 GHz frequency span are depicted in Fig. 3(c) and (d) respectively. The signal-to-noise ratio (SNR) is about 67 dB, indicating stable mode-locked operation.
Fig. 3. (a) Output spectrum of the mode-locked laser, (b) corresponding pulse train with 19.2 ns pulse spacing, (c) RF spectrum measured with 0.5 MHz frequency span and (d) 1 GHz frequency span.
The seed pulse then is injected into two ISOs and amplified in the first backward pumped amplifier. In order to suppress ASE at wavelength range longer than 1800nm, only 1.68 m gain fiber is used in the pre-amplifier. At a pump power of 814 mW, the seed pulse is amplified to 14 mW. As presented in Fig. 4(a), the output spectrum is centered at 1787.2 nm and the FWHM bandwidth of the central part is about 8.3 nm. Significantly, there are no obvious spectral broadening after amplification and the ASE at long wavelength is sufficiently suppressed. For suppressing nonlinear effects during the main amplification stage, PM2000D with a length of 11.62 m is used to sufficiently stretch the amplified pulses for fiber CPA configuration [26]. A relatively long piece of gain fiber ∼3.5 m is used, which can supply more gain to boost the power. As shown in Fig. 4(b), with the increase of pump power, the output power increases linearly. With backward pumping, the average output power can reach 476 mW at the maximum pump power of 2.6 W, corresponding to a pulse energy of 9.1 nJ. The conversion efficiency in the main amplifier is about 18.3%. Figure 4(c) demonstrates output spectra evolution with different pump power. As the spectra show, the ASE at long wavelength slightly increase compared with the spectrum from pre-amplifier, which is caused by the high emission cross section of the TDF at long-wavelength band [19]. The spectrum with maximum output power of 476 mW is centered at 1786.7 nm and the ASE baseline is about 23 dB below the spectral peak. Figure 4(d) shows autocorrelation traces of the laser after main amplifier and the pulse duration is measured as 6.1 ps. Notably, there are no optical filter and wavelength selective component in the system.
Fig. 4. (a) Output spectrum from the pre-amplifier, (b) the relationship between the pump and output power and (c) output optical spectra with different launched pump and (d) autocorrelation trace of amplified pulses in the main amplifier with maximum output power.
Instead of using diffraction grating-pair compressor with free-space beam section [27], PM1550 is placed for all-fiber temporal compression configuration. In order to ensure sufficient spectral broadening and compressed pulse quality, the length of 11 m is used. After PM1550, the output power is reduced to 458 mW, corresponding to a pulse energy of 8.8 nJ. As shown in Fig. 5(a), spectral broadening is obvious and the 3-dB bandwidth is broadened to 37.9 nm. Figure 5(b) illustrates autocorrelation trace of the compressed pulse and the deconvoluted pulse width is ∼206 fs assuming a Gaussian profile, which is 1.6 times the transform-limited value.
4. Conclusion
In summary, we have presented an all-polarization-maintaining Tm-doped laser at 1.7 µm. The oscillator is mode-locked by CNT-SA and produce pulses with a center wavelength of 1787.6 nm and a repetition rate of ∼ 52 MHz. The seed pulses are amplified to 14 mW via backward pumped pre-amplifier. After the all-fiber CPA system incorporating a fiber-based pulse compressor, pulses are compressed to 206 fs with an average power of 458 mW, corresponding to pulse energy of 8.8 nJ. Notably, all the optical fibers and device are polarization maintaining and there is no free space optical in the system, which ensures stable laser operation. To the best of our knowledge, this is the first report of 1.7 µm femtosecond laser based on a thulium-doped oscillator with all-polarization-maintaining system
Funding
National Key Research and Development Program of China (2018YFB2200500).
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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