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Flexible picosecond thulium-doped fiber laser using the active mode-locking technique Ke Yin, Bin Zhang, Weiqiang Yang, He Chen, Shengping Chen, and Jing Hou* College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China *Corresponding author: [email protected] Received May 6, 2014; revised June 7, 2014; accepted June 9, 2014; posted June 10, 2014 (Doc. ID 211461); published July 15, 2014 An all-fiber actively mode-locked thulium-doped fiber laser (AML-TDFL) based on a 10 GHz bandwidth electrooptic intensity modulator (EOM) providing flexible picosecond pulses at 1980 nm is presented. The EOM is driven by electrical pulses rather than traditional sine-wave signals. The repetition rate of output pulses was 21.4 MHz at fundamental mode-locking, which could be scaled up to 1.498 GHz through the 70th order harmonic mode-locking, and the shortest measured output pulse width was 38 ps. Furthermore, the output pulse width could be tuned by either adjusting the modulation frequency with small detuning or changing the width of these driving electrical pulses without frequency detuning. In our work, the stability of these mode-locked pulses obtained from the AML-TDFL was superior; for instance, the measured supermode suppression ratio of 1.498 GHz pulses train was up to 48 dB. © 2014 Optical Society of America OCIS codes: (140.7090) Ultrafast lasers; (140.0140) Lasers and laser optics; (140.4050) Mode-locked lasers. http://dx.doi.org/10.1364/OL.39.004259

Both continuous-wave and pulsed thulium-doped fiber lasers (TDFLs) in the approximately 2.0 μm wavelength region can be used in many research fields such as optical sensing [1], material processing [2], medical treatment [3], and nonlinear optical frequency conversion to the mid-infrared region [4]. The passively mode-locked TDFLs with output ultrafast pulses [5–8] are being investigated by making use of nonlinear optical effects in the laser cavities as well as various saturable absorbers such as semiconductor saturable absorber mirrors [5], carbon nanotubes [6], graphemes [7], and topological insulators [8]. Unfortunately, the unchangeable parameters of the laser cavity limit the output performance of a passively mode-locked fiber laser once the laser is built [5]. For instance, the output pulse width depends heavily on the cavity dispersion and nonlinearity, and the output fundamental pulse repetition rate is often limited to megahertz (MHz) with respect to the total cavity length. In order to increase the repetition rate, the cavity length should be reduced; for example, 430 MHz output pulses around 2.0 μm can be realized by shortening the cavity to about 23 cm [9]. However, it is not an ideal solution because sufficient gain has to be afforded by adopting long enough rare-earth doped fiber as well as some necessary optical components. Therefore, it will be very hard for a passively fundamentally mode-locked fiber laser to operate at a repetition rate over 1 GHz. Even that harmonic mode-locking technique can offer a much higher repetition rate [10] by coupling more pump power into a passively mode-locked fiber laser cavity. This would decrease the lifetime of the saturable absorbers involved and the stability of the laser and eventually would limit its practical operation conditions. Recently, a gain-switched semiconductor laser diode was proposed by Heidt et al. [11,12]. They offer tunable picosecond seed pulses for a master oscillator power amplifier (MOPA) system in the 2.0 μm wavelength region. Both the pulse width and the pulse repetition rate can be widely tuned. Their seed diodes emit picosecond pulses with peak power of only a few milliwatts, which means 0146-9592/14/144259-04$15.00/0

the amplified spontaneous emission noise during power amplification must have special care and a complex experimental structure must be in place to reach a total gain up to ∼70 dB [12]. In general, versatile pulses with much higher peak/average power with great tunability in repetition rate, pulse width, or wavelength will be more beneficial for seeding a MOPA system or for other application fields of interests [13]. In actively mode-locked (AML) fiber lasers [14,15], ultrafast pulses are generated in ways differing from the passively mode-locked fiber lasers, and their repetition rate can be easily tuned with the of actively harmonic mode-locking technique. Further, the peak power of the output mode-locked pulses is far beyond milliwatts meaning these pulses will be more suitable than gain-switched semiconductor laser diodes for power scaling. Traditionally, active mode-locking in lasers are induced with optic modulators by amplitude modulation, phase modulation, or frequency modulation. Further, traditional AML fiber lasers driven by sine-wave signals can provide ultrafast pulses with a highly flexible pulse repetition rate (through the actively harmonic mode-locking technique) operating at 1.0 and 1.5 μm wavelength regions [16]. The AML-TDFLs around 2.0 μm driven by a traditional sine-wave signal have also been demonstrated in recent years [17,18]. Hübner et al. present a high-power AML-TDFL based on bulk optics utilizing an acousto-optic modulator in 2011 [17], and an all-fiberized AML-TDFL using an intracavity phase modulator is also demonstrated [18]. However, the above AML-TDFLs operate only in the fundamental modelocking regime, which limits the output pulse repetition rate and the pulse width. Different from the above-mentioned works on AMLTDFLs, in fact, modulators in AML fiber lasers, either intensity modulator or phase modulator, can also be driven by electrical pulses [19–21] rather than sinusoidal waves. The modulation provides a time-dependent dark pulse-like loss curve in laser cavities that is critical and favorable for stable mode-locked operation. Schlager et al. [19] present an AML erbium-doped fiber © 2014 Optical Society of America

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laser (EDFL) in which mode-locking is realized by driving an intensity modulator with a pulse generator through fundamental mode-locking technique. In 2012, an electrooptic fiber phase modulator based Sagnac loop driven by short voltage pulses is used as the loss modulator in an all-fiber AML-EDFL that is capable of generating femtosecond pulses [20]. It has been demonstrated that modulators driven by electrical pulses offer high stability operation in AML fiber lasers with either fundamental or harmonic mode-locking [21]. To our knowledge, this concept is still novel for AML-TDFLs at 2.0 μm wavelength region and needs to be further explored. In this Letter, we present what we believe to be the first report on a novel all-fiberized AML-TDFL at 1980 nm. The intracavity electro-optic intensity modulator (EOM) is driven by picosecond electrical pulses. The average output power of the AML-TDFL is milliwatt scale. The laser also has a great flexibility in tuning of the output pulse repetition rate (from 21.4 MHz to 1.498 GHz) and the pulse width (from 38 to 860 ps). The 70th order harmonically mode-locked pulses (1.498 GHz) have a measured RF spectrum with a 48 dB supermode suppression ratio and >60 dB signal to noise ratio (SNR), which shows high stability. So far, it is the highest output pulse repetition rate in existing all-fiberized TDFLs at 2.0 μm wavelength region. The experimental setup of the AML-TDFL is shown in Fig. 1. The gain medium is a 1.5 m single mode thuliumdoped fiber (TDF) that is pumped by our own 1550 nm continuous-wave fiber laser via a 1550∕2000 nm wavelength division multiplexer (WDM). The ring cavity length L is about 9.6 m. The single mode TDF has a core/cladding diameter of 9∕125 μm and its effective core numerical aperture is 0.15. The core absorption coefficient is ∼10 dB∕m at 1550 nm. An optical circulator ensures the unidirectional laser propagation in the ring cavity that is also spliced with a high-reflectivity fiber Bragg grating (HR-FBG) to select the signal wavelength. The HR-FBG has a peak reflectivity of >99% at 1980 nm and a 3 dB bandwidth of 1.3 nm. A fiber pigtailed LiNbO3 high-speed (bandwidth 10 GHz) EOM is inserted to modulate the loss of the ring cavity driven by amplified electrical pulses generated from a pulse generator. A polarization controller (PC) is employed to maximize the coupling of light from the fiber to the EOM. Output pulses are extracted from a 30∶70 optical coupler (OC). The cavity dispersion is roughly estimated as −0.76 ps2 at 1550 nm fiber laser

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Fig. 1. Experimental setup of the AML-TDFL. TDF, thuliumdoped fiber; WDM, wavelength division multiplexer; HR-FBG, high-reflectivity fiber Bragg grating; EOM, electro-optic intensity modulator; PC, polarization controller; OC, optical coupler; RF amplifier, radio frequency amplifier.

1980 nm. In the experiment, the pulse generator could produce rectangular electrical pulses with temporal width from ∼80 ps to several nanoseconds and with a pulse repetition rate up to ∼6 GHz, which is very critical for the realization of our AML-TDFL. The measurement of temporal width of the modelocked pulses (>80 ps) is done by combining an InGaAs photodector (with a bandwidth of 7 GHz) with a sampling oscilloscope (with a bandwidth of 30 GHz). A second harmonic generation based autocorrelator (Femto-chrome Research Inc., FR-103XL) is used to measure the pulse width of output pulses that are shorter than 80 ps. A TDF amplifier is placed before the autocorrelator in order to get high peak power of the output pulses to induce a second harmonic signal. Other measuring equipment includes an optical spectrum analyzer, a thermal power meter, a digital oscilloscope (with a bandwidth of 1.5 GHz) and an RF spectrum analyzer (with a bandwidth of 26.5 GHz). The mode-locking of the AML-TDFL is realized by driving the EOM with a modulation frequency equaling a multiple of the longitudinal mode spacing Δν of the ring cavity. We estimate the longitudinal mode spacing using, Δν
c∕nL, where c is the light speed in the vacuum, and n is the effective refractive index of the fiber core at 1980 nm. The result is ∼21.6 MHz with n
1.442 at 1980 nm. In experiments, both fundamental and harmonic mode-locked operations of the AML-TDFL are investigated. First, the electrical pulse width is set to ∼80 ps and the modulation frequency is tuned to around 21.6 MHz to fit the longitudinal mode spacing. Stable output pulses without detuning are obtained at 21.4 MHz when the AML-TDFL is fundamentally mode-locked. The little difference from the estimated longitudinal mode spacing Δν (∼21.6 MHz) and modulation frequency (21.4 MHz) is caused by the measuring error of cavity length. The threshold pump power of the AML-TDFL is 380 mW under stable fundamental mode-locking operation. The linear fitting of the average output power provides a slope efficiency of only ∼4.9% with respect to the pump power. The relatively low efficiency of the AML-TDFL mainly originates from the high insertion losses of the EOM (∼5.4 dB) and the circulator (∼2.1 dB). Furthermore, the AML-TDFL can operate in stable mode-locking mode for over an hour without feedback control. Figure 2 depicts the measured characteristics of the actively fundamental mode-locked pulses at an average power of 5 mW. In Fig. 2(a), the spectrum of the mode-locked pulses is centered at 1979.6 nm measured with a resolution of 0.05 nm, and it has a 3 dB bandwidth of 0.23 nm. Figure 2(b) plots the measured pulse train with a time interval of ∼46.7 ns between each pulse corresponding to a pulse repetition rate of 21.4 MHz. As shown in Fig. 2(c) the SNR is suppressed better than 55.3 dB in the RF spectrum with a resolution bandwidth (RBW) of 1 kHz. Figure 2(d) shows the measured autocorrelation trace with a full width at half-maximum (FWHM) of 58 ps. If a sech2 pulse profile is assumed, the pulse width is 38 ps. Then the calculated timebandwidth product of the output pulses is 0.67, which is a little larger than the theoretical value for

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transform-limited sech2 pulse indicating a small amount of negative dispersion in laser pulses. It is known that stable AML operation can still be guaranteed as there is a locking region (small detuning between the modulation frequency and the longitudinal mode spacing) in AML fiber lasers [22], which has been experimentally demonstrated [23]. Therefore, we experimentally studied the effects of small detuning on the output pulse width of our AML-TDFL. Figure 3(a) depicts the measured autocorrelation traces with different amounts of detuning (0.51, 1.08, and 1.82 kHz) in the AML-TDFL at fundamental mode-locking regime compared to the one without detuning. The FWHM levels of the three autocorrelation traces are 74, 86, and 92 ps corresponding to the mode-locked pulse widths of 48, 56, and 60 ps, respectively. We fins that the output pulse width in mode-locking regimes increases in proportion to the detuning [15]. In addition, we find the pulse width of the AML-TDFL increases when tuning the electrical pulse width from ∼80 ps to 1 ns continuously. Some typical output pulse shapes measured by the InGaAs photodector on the sampling oscilloscope are shown in Fig. 3(b). The pulse widths are 211, 287, 388, 414, 530, and 860 ps, respectively. Meanwhile, it shows that the measured spectral width gets narrow slightly with the increasing of output pulse width. But the output pulses are still chirped. It must be noted that pulse shapes with widths of 211

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and 287 ps are distorted in view of their steep rising edges which are supposed to be limited by the bandwidth of the used InGaAs detector (rise time