Gain-switched and mode-locked Tm/Ho-codoped 2 μm fiber laser for mid-IR supercontinuum generation in a Tm-doped fiber amplifier

Pulsed fiber lasers operating in an eye-safe wavelength range around 2 μm have attracted significant attention. These 2 μm pulsed fiber lasers have a variety of attractive applications, such as in medical treatment, spectroscopy, telecommunications and gas remote sensing. 2 μm pulsed fiber lasers can also be used as the ideal pump sources to generate a mid-IR supercontinuum (SC) [1, 2]. Q-switching and mode-locking are two general methods for pulsed laser generation, and both Q-switched and mode-locked 2 μm fiber lasers have been investigated and reported by a few groups [3–11]. Compared with Q-switching and mode-locking, gain switching is not necessary for Q-switched or mode-locked devices in the laser cavity, and it can be a simple and effective candidate to obtain 2 μm pulsed lasers. Gain-switched operation of a thulium-doped or holmium-doped fiber laser by pumping with a 1.55 μm pulsed laser has been demonstrated in several systems [12–14]. In this letter, a 1.55 μm pulsed laser is used for fast gain switching and a simultaneously gain-switched and mode-locked 2 μm Tm/Ho-codoped fiber (THDF) laser is obtained.

Recently, Eckerle et al reported a simultaneously actively Q-switched and actively mode-locked 2 μm Tm-doped fiber laser [15]. It has been demonstrated to be a useful pump source for mid-IR SC generation in fluoride fibers. However, the fiber laser needed two antireflection coated acousto-optic modulators (AOMs), for Q-switching and mode-locking respectively. The bulky AOMs are expensive and not suitable for the all-fiber compact setups. Similar to the simultaneously actively Q-switched and actively mode-locked fiber laser, this letter presents a simultaneously gain-switched and mode-locked 2 μm THDF laser. There are many mode-locked sub-pulses within one gain-switched pulse envelope. Compared with the actively Q-switched and mode-locked fiber laser, the simultaneously gain-switched and mode-locked pulses generated from the THDF laser form an attractive approach to the simple, robust, all-fiber-integrated, and more cost-effective pulsed laser, as there is no need for the rather expensive and bulky AOMs.

In the field of SC generation, the main results have been obtained using microstructure fibers, fiber tapers and other highly nonlinear fibers [16–19]. Recently, we demonstrated flat SC generation in an Er/Yb-codoped double-clad fiber amplifier and 70 W high-power SC generation in a Yb-doped fiber amplifier [20, 21]. Kurkov et al reported mid-IR SC generation in a Ho-doped fiber amplifier [22]. Geng et al reported high-spectral-flatness mid-IR SC generation in a Tm-doped fiber amplifier (TDFA) [23]. In those systems, rare-earth doped fiber amplifiers exhibited an excellent performance for SC generation.

In this letter, we present a simultaneously gain-switched and mode-locked 2 μm THDF laser. In the SC generation experiment, this gain-switched and mode-locked laser is used as a seed source and amplified by two stages of fiber amplifiers. A flat mid-IR SC is generated in the second stage Tm-doped fiber amplifier, where the Tm-doped fiber acts both as a nonlinear and a gain medium.

The experimental setup of the gain-switched and mode-locked THDF laser is shown in figure 1. The pump source is a modulated 1550 nm diode laser amplified by a fiber amplifier with an output power up to 514 mW. The repetition rate and pulse width of the pump laser are 20 kHz and 6.4 ns, respectively. The master oscillator comprises a 1.5 m length of THDF, two fiber Bragg gratings (FBG1 and FBG2) and a wavelength division multiplexer (WDM). The total length of the fiber laser cavity is ∼7 m. The THDF has a core/cladding diameter of 8/125 μm, an effective numerical aperture (NA) of 0.18, and an absorption coefficient of ∼13 dB m⁻¹ at 1550 nm. The FBG1 and FBG2 have the same peak reflectivity of 50% at ∼1958 nm and the same bandwidth of ∼2 nm. Simultaneously gain-switched and mode-locked pulses can be observed in the experimental results, which will be discussed in detail in the following sections.

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In a second experiment, the gain-switched and mode-locked laser is amplified by two stages of fiber amplifiers. A flat mid-IR SC is generated in the second stage TDFA. The amplifier system is shown in figure 2. The pump laser of the first stage amplifier is a CW fiber laser at 1570 nm with a pump power up to 1.5 W. The THDF used here is the same as that in the gain-switched and mode-locked THDF laser. A 1.5 m length of THDF with an absorption coefficient of ∼20 dB m⁻¹ at 1570 nm is used. An isolator is used after the THDF to avoid the effect of the backward propagating light. In the second stage amplifier, a 7 m length double-clad Tm-doped fiber (TDF) is used as the gain medium. The TDF has a core/cladding diameter of 10/130 μm, an effective core NA of 0.15, a cladding NA ≥ 0.46 and a peak cladding absorption of ∼3 dB m⁻¹ at 793 nm. The pump diode used in the second stage amplifier is a fiber-coupled 12 W, 793 nm pump laser with a 105/125 μm core/cladding diameter and a 0.22 NA fiber pigtail. The pump laser is injected into the TDF via a pump and signal combiner. The end of TDF is angle cleaved to avoid light being reflected back into the pump system.

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The output signal is monitored by a high-speed photodetector with a wavelength range of 1200–2600 nm (THORLABS, DET10D) and a 20 GHz sampling rate oscilloscope (Tektronix, TDS7154). The laser spectrum is acquired at 1 nm intervals using a monochromator and a liquid-nitrogen-cooled InSb detector.

In the gain-switched and mode-locked THDF laser experiment, simultaneously gain-switched and mode-locked pulses can be observed when the incident pump power is beyond 160 mW. The pulse train of the gain-switched and mode-locked fiber laser for an incident pump power of 210 mW is shown in figure 3. In this case the output power is 15.3 mW. The repetition rate of the gain-switched pulse envelopes is 20 kHz, which is the same as the pump laser. One single gain-switched pulse envelope for an incident pump power of 210 mW is shown in figure 4. The pulse train contains many mode-locked sub-pulses within one gain-switched pulse envelope. The sub-pulses play an important role in increasing the peak power. The high peak power is useful for SC generation [16], as is demonstrated in our following SC generation experiment. The inset of figure 4 shows an amplified section of sub-pulses. The repetition rate of these mode-locked sub-pulses is Δν ≈ 14.8 MHz, which is determined by the fiber laser cavity length L, given by Δν = c/2nL, where c is the speed of light in vacuum and n is the refractive index of the fiber core. The exact pulse width of the sub-pulse is not measured, as it is limited by the resolution of our detection system. The high-speed photodetector used in the experiment has a rise time of ≤25 ns, so the repetition rate of the sub-pulses can be detected but the pulse width is beyond its resolution.

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Besides the gain-switched pulse envelope, there is a small signal pulse, which is labeled in figure 4. Different from the mode-locked sub-pulses, this small signal pulse comes from the incident pump laser. In fact, the spectrum of the incident pump laser already extends to 2 μm when the incident pump power is greater than 131.4 mW. The spectrum of the incident pump laser for an incident pump power of 210 mW is shown in figure 5. In our experiment, the 1550 nm pump laser comes from a fiber laser which contains an EYDF amplifier. The spectrum of the pump laser is extended in the EYDF amplifier, based on a modulation instability-initiated SC generation process. The physical mechanism of spectrum extension is the same as in our previous report [20], in which we demonstrated flat SC generation in an EYDF amplifier. As can be seen in figure 5, the spectrum covers the reflection bands (∼1958 nm) of FBG1 and FBG2. The 1958 nm part of the incident pump laser can be reflected and amplified in the gain-switched fiber laser. This small signal pulse is significant when the pump power is lower. The gain-switched pulse envelope and mode-locked sub-pulses for an incident pump power of 178 mW is shown in figure 6. With the increasing incident pump power, the small signal pulse becomes smaller because of its lower gain than the gain-switched and mode-locked pulses. There is a time delay between the small signal pulse and the gain-switched pulse envelope. This is because the 2 μm output pulse is delayed relative to the pump pulse due to the time needed for the photon density buildup in the upper energy level [12]. With increasing pump pulse energy, the buildup time decreases. So, the delay time is 4 μs in figure 6, but only 1.7 μs in figure 4.

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On further increasing the incident pump power to 325 mW, the output power is 52 mW. In this case, the gain-switched pulse becomes unstable and exhibits multi-pulsing, due to the excitation population becoming too large to be depleted by the first gain-switched pulse. For an incident pump power of 325 mW, the pulse shape with multi-pulsing is shown in figure 7. There are no mode-locked sub-pulses within the gain-switched pulse envelopes. The time delay between the pulses is 2.2 μs, which is determined by the photon density buildup time of the upper energy level.

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In the second experiment, this simultaneously gain-switched and mode-locked fiber laser is used as a seed source with an average output power of 31.2 mW. As figure 2 shows, the seed laser is amplified by two stages of fiber amplifiers. The first stage amplifier produces an output power of 222 mW from an incident pump power of 975 mW. The average output power of the TDFA as a function of the incident pump power is shown in figure 8. The second stage TDFA operates with a slope efficiency of ∼21%. The maximal output power of the TDFA is 2.17 W for an incident pump power of 11.6 W. A flat mid-IR SC is generated in the second stage TDFA, where the Tm-doped fiber acts both as a nonlinear and a gain medium. The output spectra of the TDFA at different average output powers is shown in figure 9. The low repetition rate of the gain-switched pulse envelope and the short pulse duration of the mode-locked sub-pulses make it easy to get high peak power pulses, which are necessary for SC generation. The SC spectrum is shifted towards the longer wavelength, as similarly observed in our previous research [20, 21]. For an average output power of 2.17 W, the observed spectrum covers the spectral range ∼1953–2593 nm with a 10 dB bandwidth and the long wavelength edge of the SC spectrum is extended to 2750 nm.

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In conclusion, a simultaneously gain-switched and mode-locked 2 μm THDF laser has been demonstrated in this letter. The pulse train contains many mode-locked sub-pulses within one gain-switched pulse envelope. The repetition rate of the gain-switched pulse envelopes is 20 kHz, which is the same as the pump laser. The repetition rate of these mode-locked sub-pulses is 14.8 MHz, which is determined by the fiber laser cavity length. In the supercontinuum generation experiment, this gain-switched and mode-locked laser is used as a seed source and amplified by two stages of fiber amplifiers. A flat mid-IR SC is generated in the second stage Tm-doped fiber amplifier where the Tm-doped fiber acts both as a nonlinear and a gain medium. For the maximum output power of 2.17 W, the long wavelength edge of the SC spectrum is extended to 2750 nm, and the SC has a 10 dB bandwidth of 640 nm (a spectral range of ∼1953–2593 nm).

This work is supported by the Projects of National Natural Science Foundation of China (Grant no. 61077076) and International Science and Technology Cooperation Program of China (Grant no. 2012 DFG 11470).