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Abstract
We report a single-frequency, linearly polarized gain-switched, distributed feedback (DFB), 2-µm thulium doped silica fiber laser (TDFL), with an effective cavity length of 2.5 mm. The cavity is based on a heavily thulium doped non-polarization-maintaining silica fiber and composed of a π-phase-shifted fiber Bragg grating (FBG) with a total FBG length of 35 mm. The DFB FBG was written by femtosecond-laser point-by-point (PbP) method. In-band pumping scheme is chosen with a 1550 nm nanosecond pulsed erbium-doped silica fiber laser pump. Single-longitudinal, linearly polarized, gain-switched TDFL at 2002 nm, with a recorded shortest pulse duration of 4.7 ns, a repetition rate of 20 kHz, a maximum peak power of 170 W, and single pulse energy of 0.8 µJ, has been obtained, benefitting from the ultrashort DFB cavity made by the femtosecond laser direct-writing method.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-infrared (mid-IR) 2-5 µm fiber lasers are widely demanded for many applications including military, security, environmental monitoring, medical surgery, and so on [1]. Both high power continuous-wave (CW) and high-energy ultrafast 2-µm TDFLs have been rapidly developed in the last decade [2–4]. A 2-µm pulsed laser, rather than a CW laser, is required for many applications, such as eye-safe Lidars (light detection and ranging), optical countermeasures, mid-IR nonlinear frequency conversion, and so on. For example, 2-µm Tm: YAG pulsed laser with kW-level peak power was successfully employed in the very first airborne Lidar to determine wind profiles and predict wind hazards for aircraft guidance and navigation, utilizing the laser radar atmospheric backscatter data [5]. Since the significant global climate change in the past decades has caused a significant increase of the reported flight incidents due to the strong but invisible air turbulence, such an airborne Lidar system becomes necessary for aircrafts. And with the progress of high-power fiber laser technology in the past 30 years, a compact all-fiber 2-µm pulsed fiber laser shows advantages in the compactness, light-weight, robustness, and power scalability over its crystalline counterpart in Lidar technology.
High-power/high-energy TDFL covering 2.0-2.05 µm and the later emerging holmium doped fiber lasers (HDFLs) covering 2.04-2.17 µm [6–8] play important roles in eye-safe laser applications such as remote sensing and free-space optical communications in the 2.0-2.4 µm atmospheric transmission windows [9]. However, the primary absorption bands of holmium ions are located at 1.15 µm and 1.95 µm [10], where there is no suitable laser diodes available as the high-power pump. TDFL is therefore the most suitable pump to realize high-power HDFL with low quantum defect. Nevertheless, 2-µm thulium fiber laser is one of the most efficient sources to excite many 2-5 µm fiber lasers [11–13] and solid-state lasers [14], and therefore TDFL technology acts as the special doorway for 2-5 µm laser generation.
Highly sensitive remote sensing technique requires a compact all-fiberized pulsed fiber laser with single longitudinal mode, linear polarization with sufficiently high pulse energy or peak power [15]. In general, high-sensitivity coherent Lidars utilize heterodyne interferometry between the laser light scattered from a remote target and a reference local coherent laser oscillator. High sensitivity can be achieved by effectively suppressing phase detection errors, arising from the power fluctuation, phase fluctuation, and polarization fluctuation, in the measurements [16–19], while the ranging distance can be benefitted from using a source with high pulse energy or high peak power [5,15]. In addition, with the massive usage of unmanned aerial vehicles (UAVs) for the low-altitude remote sensing [20,21], compact lightweight mid-infrared laser sensing system started to be equipped on the UAV platforms [22]. A compact 2-µm TDFL with high performances, including high spectral purity, spectral brightness, polarization purity, and high peak power/high pulse energy, is therefore also desired for promoting the development of the mini-airborne mid-IR Lidar systems.
To build such a neat laser, first, short-length distributed Bragg reflector (DBR) [23–28] or distributed feedback (DFB) [29–33] cavity is the typical approach to obtain single frequency, highly linearly polarized lasing in a fiber laser oscillator. Next, polarization selective elements should also be involved, in order to have mode-hopping-free, stable single longitudinal mode operation. For having polarization selectivity in the fiber laser oscillator, both ends of the polarization maintaining (PM) gain fiber should be connected with active or passive PM fiber Bragg gratings. The involvement of such polarization selective FBGs is time-consuming due to the difficulty in having the principal polarization direction precisely aligned along the whole length of the fiber cavity.
For realizing an all-fiberized pulsed laser, gain-switched technology [34,35] is such a suitable approach, by periodically modulating the pump laser. Essentially, the gain-switching process involves selectively switching the laser gain only to the first spike in the sequence of the relaxation oscillations. Different from other pulsed fiber lasers, a gain-switched fiber laser requires no intra-cavity modulator and hence provides a simple all-fiber-format pulse generation method, while still mostly retaining the inherent advantages of the fiber laser as an efficient, compact, and robust device. So far, various types of rare earth (e.g., Nd3+, Yb3+, Er3+, Tm3+, Ho3+, etc) doped gain-switched fiber lasers [11–13,34–42] have been reported, indicating the versatility of such a fiber laser technology to achieve lasing at the wavelength range from 1 µm to 3.55 µm. A gain-switched fiber laser can produce laser pulses with duration in a wide range between tens of nanoseconds and milliseconds. The repetition rate of the gain-switched laser can be widely tuned between kHz and MHz, in which the upper limit is only limited by the relaxation oscillation frequency.
Additionally, it should be noted that the slope efficiency with respect to launched pump power of a gain-switched fiber laser can be high and close to the quantum efficiency [35], while in the case of single-frequency, gain-switched fiber lasers such a slope efficiency is much lower [43–45]. For instance, for 2-µm gain-switched TDFLs, one of the recorded shortest pulse width was 10 ns with a peak power over 1 kW and a slope efficiency of 50% [38], while the slope efficiency of a single frequency gain-switched TDFL with an ultrashort DBR cavity was only 11.7% [45]. This is because when the short-length linear fiber laser cavity is adopted in a single-frequency fiber laser, the utilization of the pump power becomes low, since the absorbed pump power through such a short fiber cavity is low. Such pump utilization will further be nearly halved for linearly polarized lasing using a conventional unpolarized pump.
For many application scenarios like coherent Lidars for 100m-level short-distance ranging usage [15] and mid-IR nonlinear frequency generation, nanosecond pulsed single-frequency TDFL with µJ-level pulse energy will be sufficiently useful. Beside the pulse energy, high peak power will be also a critical parameter for the excitation source. For example, for mid-IR nonlinear frequency generation utilizing Kerr nonlinearity, laser with high peak power of 100W-1 kW is desired. Given that the typical pulse energy of the gain-switched TDFL is 1 µJ when a commercially available watt-level unpolarized nanosecond pulsed pump is employed [38,39], the gain-switched pulse width should be no more than 10 ns.
When the pump power is well above the threshold, the pulse duration of the gain-switched laser, τLP, is approximately proportional to $\sqrt {A \cdot {L_c} \cdot h{\nu _p}/{P_{abs}}}$, where Pabs is the absorbed pump power, Lc the cavity length, A the fiber core area, and hvp the pump photon energy [34–37]. Short gain-switched pulse can be obtained by largely enhancing pump power or shrinking cavity size (either cavity length or core size). An ultrashort 1950 nm single-frequency gain-switched DBR TDFL has been reported recently, with a pulse duration of 19 ns and a maximum peak power of 34.2 W (corresponding to single pulse energy of 0.65 µJ) [45]. A piece of non-PM thulium doped germinate fiber with a length of 1.6 cm was employed as the gain medium, and two FBGs on passive silica fiber were spliced on both sides of the Tm fiber. To obtain linearly polarized gain-switched lasing from the non-PM active fiber, the low-reflectivity FBG was made in a passive PM fiber as the polarization selective element in the cavity, while the rest elements of the fiber cavity were based on non-PM fibers. Note that the same sample was employed for realizing continuous wave (CW) single-frequency DBR TDFL at 1950 nm [46]. But neither of the above works [45,46] mentioned that linearly polarized laser output was achieved, probably because the polarization extinction ratio (PER) of the polarized laser oscillation was not largely enhanced with only one PM FBG employed at the output port of the laser.
Figure 1(a) plots the essential structure of a short linear cavity required for achieving a single-frequency, linearly polarized, gain-switched TDFL. The principle of using PM FBGs as the polarization selecting elements in the conventional fiber lasers is to introduce orthogonal anisotropy into the cross-section of the fiber cavity. It is required to precisely align the incident beam of the exposure laser (either femtosecond laser or UV laser) with the polarization axes of the PM gain fiber before writing the FBGs. Misalignment along the fiber cavity could significantly affect the efficiency and PER of the polarized laser oscillation. A simple and efficient approach is preferable for constructing a compact 2-µm all-fiber-format high-performance pulsed fiber laser.
Fig. 1. (a) Essential fiber laser structure for gain-switched single-frequency, linearly polarized lasing. (b) Schematic top view (upper) and side view (lower) of fiber FP cavity made by fs laser writing method. Note that femtosecond laser is launched along the positive y direction into the paper, while laser-induced index modulation is inscribed in the active fiber core when the focused fs laser spot moves along the z direction along the fiber. (c) Cross-sectional view of birefringent area of laser-induced refractive-index modulation inside non-PM active fiber core using PbP method.
It has been known that the FBGs made by the femtosecond laser direct-writing method have large polarization-dependent birefringence [32,33,47–49]. The high birefringence of the fs-laser directly written FBGs arises from the anisotropy of the induced refractive index change in the focus spot of femtosecond laser pulses in the fiber core. The discrimination of the gains on the two orthogonal polarization directions leads into highly linearly polarized laser oscillation. Therefore, using such an fs-laser writing approach can build the desired high-performance single-frequency, linearly polarized gain-switched 2-µm fiber laser cavity using conventional non-PM thulium-doped silica fiber. Still, precisely aligning the principal polarization direction of the directly-written FBGs and high overlapping between focused oval-shaped laser spot with the central axis of the active core cylinder through the entire cavity is necessary for having highly linear polarization laser oscillation. Mind that the birefringence has also been observed in the FBGs made by continuous-wave UV radiation [32], but the typical birefringence in UV written FBGs is only ∼10−6, much weaker than the birefringence of 10−5-10−4 observed in 800-nm fs laser directly-written FBGs [33,49].
Figure 1(b) shows the schematic of fs-laser PbP written short linear cavity in the core of a non-PM gain fiber. Due to the geometrical difference of the laser-induced index-modified oval-shaped patterns in the orthogonal directions of the cross section of the fiber core (see Fig. 1(c)), the FBGs have large polarization-dependent birefringence. With precise alignment and high overlapping of the focused fs laser spot with the central axis (z axis shown in Fig. 1(b) and (c)) of the active fiber core cylinder along the designed cavity, high polarization selectivity can be created in such a short linear Tm-doped fiber laser cavity. By carefully designing the defect gap between the pair of FBG cavity mirror, a DBR cavity or phase-shifted DFB cavity can be fabricated.
Additionally, in comparison with the traditional phase-mask [50–55] or interferometric techniques [56–59] using either UV-laser or near-infrared femtosecond laser exposure, fs-laser direct-writing technique is much more versatile for fabricating FBGs (i) in any glass fiber host even without UV photosensitivity, (ii) with any FBG structure parameters (including periodicity Λ of the FBGs, length of each FBG, and the defect gap between the pair of uniform FBGs, as shown in Fig. 1(b)) without relying on expensive pre-designed phase masks, (iii) high spatial resolution of the inscribed microstructured array benefitted from the nm-resolution high-precision motorized three-dimensional translation stages in the direct-writing platform, and (iv) much higher resistance to high laser power or high temperature [60–62]. An ultrashort fiber cavity could lead into the gain-switched TDFL with significant enhanced performances, for example large reduction of pulse duration, and high peak power. However, single-frequency, linearly polarized gain-switched 2-µm DFB TDFL oscillator made by the versatile femtosecond laser direct-writing method has not been reported so far.
In this work, for the first time, we report a single-frequency, linearly polarized gain-switched 2-µm DFB TDFL, fabricated by fs-laser direct-writing method. Commercial single-cladding heavily thulium doped non-PM silica fiber is selected as the gain medium. A π-phase-shifted DFB FBG is inscribed inside the Tm-doped core by PbP writing method. In-band pumping scheme is chosen, by using a 1550 nm nanosecond pulsed erbium-doped silica fiber laser as the pump. The laser wavelength of the gain-switched DFB TDFL is located at 2002 nm, with a repetition rate ranging between 20-100 kHz. Benefitted from the high laser damage threshold of silica glass material and the femtosecond laser written FBGs, high photon density can therefore achieved in the small volume of the ultrashort fiber laser cavity. A recorded shortest pulse duration of 4.7 ns and a maximum peak power of 170 W has been obtained in the single-longitudinal, linearly polarized, gain-switched DFB TDFL.
2. DFB grating structure in Tm-doped silica fiber by fs-laser direct-writing
An 800-nm Ti:sapphire laser system (Coherent) with 80 fs pulse width and 1-kHz repetition rate was employed for FBG fabrication. A commercial thulium doped silica fiber (Coherent/Nufern SM-TSF-5/125) was chosen as the gain medium. The single-cladding Tm fiber has a core diameter of 5.5 ± 1.0 µm and an absorption of ∼340 ± 50 dB/m at 1560 nm.
The DFB grating structure was written in the Tm fiber by fs-laser PbP method [33,48]. First, the fs laser beam was focused into the active fiber core through the coating layer and the cladding. Then precise alignment between the fs laser spot and the central axis of the core cylinder along the writing length was carried out using our self-developed imaging-recognition method [63]. After the alignment, the laser spot moved along the central axis of the core with a constant velocity of 0.69307 mm/s. A first-order DFB grating structure with a period of 693.07 nm was written inside the active fiber core along its central axis. The entire length of the DFB structure was 35 mm and the π-phase-shift was offset from the center of the DFB structure by 2.5 mm in order to obtain dominant laser emission from the side of the cavity closer to the phase shift. Out of each end of the DFB cavity, 5 mm un-written Tm fiber was left for splicing with the WDM couplers. Note that during the FBG writing process, the single pulse energy of fs laser was kept at 220 nJ and high tension was applied on the Tm fiber.
A homemade 2-µm ASE (amplified spontaneous emission) source and an optical spectrum analyzer (OSA, Yokogawa, AQ6375E 1200-2400 nm, resolution: 0.05 nm) were used to monitor the growth of FBG transmission spectra in-situ during the FBG writing and characterize the FBG spectral performance after the sample was unloaded from the direct-writing platform with released tension. The transmission spectrum of the DFB FBG with tension released was shown in Fig. 2(a), in which the ultrafine bandpass spectral line of the DFB FBG was not resolved, due to the ultranarrow bandpass spectral features and the limited OSA resolution [33]. The coupling coefficient, κ, of the DFB gratings is estimated ∼0.4 mm−1, based on the uniform test gratings written prior to the phase-shifted DFB grating. Such an FBG coupling coefficient corresponds to an index modulation amplitude of ∼2.5x 10−4. The uniform grating length on each side of the π phase shift was L1 = 15 mm and L2 = 20 mm, respectively (see Fig. 2(b)). The reflectivity R of the uniform gratings on each side of the DFB structure is thus calculated to be both close to unity. Note that R of the uniform grating is calculated by the formula of $R = tanh{(\kappa L)^2}$ [64], in which L is the length of the uniform grating. The effective length, Leff, of the DFB cavity is estimated to be ∼2.5 mm, according to the formula of ${L_{eff}} = R/\kappa$ [24,65]. To the best of our knowledge, this is the recorded shortest laser cavity in the reported gain-switched TDFLs.
Fig. 2. (a) Normalized measured transmission spectra of DFB grating with center-offset π-phase-shift. (b) Schematic experimental set-up of gain-switched DFB TDFL. (BW: backward output; FW: forward output; ISO: isolator).
The scattering loss of the fs-laser written FBG in the DFB cavity is then calculated according to the FBG coupling coefficient. Firstly, the pump threshold of the DFB fiber laser is estimated by the following formula [33,66]:
3. Experimental results and discussion of 2-µm gain-switched DFB TDFL
3.1 Set-up of 2-µm gain-switched DFB TDFL
Figure 2(b) illustrates the schematic experimental set-up of the gain-switched TDFL. A home-made 1550-nm nanosecond pulsed erbium-doped silica fiber laser was selected as the pump. The cavity gain can thus be switched on and off nearly instantaneously following the on-off modulation of the pump. Under such an in-band pumping scheme, stable pulsed lasing can be obtained without chaotic pulsation [38]. The pump pulse duration was fixed at 50 ns, while its repetition rate was adjusted from 100, 50 to 20 kHz.
A fiberized 1550-nm isolator was spliced at the output of the 1550-nm pump source to prevent the back reflection of the pump light. The pump was coupled into the DFB cavity via a 1550/2000 nm WDM coupler (Thorlabs, WD1520AB). The π phase shift was offset from the center of the DFB towards the pump input port to ensure that the backward laser output is dominant. The forward output end of the DFB cavity was also spliced with a second 1550/2000 nm WDM coupler, in order to separate the 2-µm laser signal and the residual 1.55µm pump. Note that the length of the Tm fiber section out of the DFB structure was then cleaved and kept no more than 5 mm, before both ends of the DFB laser were spliced with two WDM couplers.
In addition, to further suppress the reflection backward to the pump, all fiber ends at the forward and backward ports of the WDM couplers were cleaved with an angle of ∼8°. Thermo-electric cooler (TEC1-12706) was attached underneath the DFB laser to keep the oscillator at a constant temperature of 25.0 ± 0.1 °C.
3.2 Spectral characteristics of 2-µm gain-switched DFB TDFL
Figure 3(a) shows the backward output spectra of the 2-µm gain-switched TDFL under different average pump power, when the 1550-nm pump is with a pulse duration of 50 ns and a repetition rate of 20 kHz. It is seen that the peak wavelength of the gain-switched TDFL is located at 2002 nm. With the increase of the launched average pump power from 121 mW to 413 mW, the peak wavelength of the gain-switched TDFL showed slightly red-shifted, mainly due to the heat generated within the ultrashort DFB cavity. Figure 3(b) plots the backward output spectrum of the gain-switched DFB TDFL in the whole 1.50-2.05 µm range, under the maximum average pump power of 413 mW (repetition rate: 20 kHz). It is seen that the backward 2-µm lasing shows excellent signal-to-noise ratio (SNR) of >60 dB without Tm3 + ASE in the background. No 1.55-µm residual pump was seen from the backward output. The total active fiber length used is ∼ 4.5 cm, including 35mm-long FBGs and 10mm-long unwritten Tm fiber out of the cavity. The single-pass pump absorption of the entire thulium fiber is then estimated to be ∼15 dB.
Fig. 3. (a) Evolution of BW spectra under various average pump power of gain-switched DFB TDFL. (b) Global BW spectrum of gain-switched DFB TDFL under maximum average pump power (repetition rate: 20 kHz). (c) Red-shifting of peak wavelength and (d) slope efficiency of BW output of gain-switched DFB TDFL under various average pump power (repetition rate: 20, 50 and 100 kHz).
On the other hand, strong 1.7-2 µm Tm3+ background ASE spectrum and weak residual 1.55 µm pump were observed from the forward output of the TDFL. And the forward 2-µm output power was measured to be <0.3 mW under all the pump power level and conditions. Therefore, only the characteristics of the backward gain-switched DFB TDFL output will be discussed in the following sections.
Figure 3(c) summarizes the peak wavelength shifting of the backward 2-µm laser output, with the increase of the launched average pump power. Within a wide range of pump power and pulse duration, the peak wavelength of the gain-switched TDFL shows linearly red-shifting with a slope of ∼0.55 nm/W.
Figure 3(d) plots the relation between the backward 2-µm average output power of the gain-switched TDFL and the launched average pump power with varying repetition rate of 20, 50, and 100 kHz. The gain-switched DFB TDFL shows a pump threshold of ∼120 mW. The fitted slope efficiency and the maximum average output power is 5.7%, 5.3%, and 4.5%, 16 mW, 26 mW, and 15 mW, corresponding to the pump repetition rate of 20, 50 and 100 kHz, respectively.
3.3 Temporal characteristics
The temporal characteristics of the backward output of the fabricated gain-switched TDFL were recorded by using a high-speed extended InGaAs detector (Newport, 818-BB-51) and an oscilloscope (Siglent, SDS5104X).
Figure 4(a) illustrates the pulsed traces of 2-µm TDFL, when the average power of launched average pump power (rep. rate: 20 kHz) increases from 153 mW to 413 mW. It is seen that the gain-switched pulses exhibit good Gaussian lineshape, in the whole pump power range. For example, the purple trace in Fig. 4(a) under the maximum average pump power of 413 mW, exhibits a Gaussian profile with a weak tail well below the 1/e2 of the peak intensity. The formation of stable Gaussian shaped gain-switched nanosecond pulses is consistent with the observation in gain-switched TDFL under fast gain-switching pump (i.e., pump pulse duration <100 ns) and high pump ratio (i.e., ratio of pump power to pump threshold >2) [40].
Fig. 4. (a) Temporal characteristics of gain-switched laser pulses under various launched average pump power (rep. rate: 20 kHz). (b) Gain-switched laser pulse train under pump with various repetition rate. (c) FWHM pulse width and (d) build-up time of 2-µm gain-switched DFB TDFL under various average pump power (repetition rate: 20, 50 and 100 kHz).
Figure 4(b) plots the gain-switched pulse trains under pump with different repetition rate. Essentially, the gain-switched pulse trains exhibit the same repetition rate as the modulated pump pulses.
Figure 4(c) and Fig. 4(d) summarize the evolution of the pulse FWHM (full width at half maximum) width and build-up time of the backward 2-µm gain-switched TDFL, under various launched average pump power (repetition rate: 20, 50 and 100 kHz). Note that the build-up time of the gain-switched pulse is defined as the interval between the pump pulse and the gain-switched pulse. It is seen that both the pulse width of the 2-µm gain-switched TDFL and the pulse build-up time reduce with the launched average pump power. For instance, when the launched average pump power (repetition rate: 20 kHz) increases from 121 mW to 413 mW, the pulse width of the gain-switched laser decreases from 30 ns to 4.7 ns, while the gain-switching build-up time decreases from 206 ns to 57 ns. The former trend of pulse width decreasing with the pump power is in good agreement with the above approximation of ${\tau _{\textrm{LP}}} \propto 1/\sqrt {{P_{abs}}}$. In the latter case of the gain-switching build-up time, it is known that the build-up time is approximately inversely proportional to the square root of the pump pulse energy [37]. To the best of our knowledge, the pulse width of 4.7 ns is the recorded shortest pulse duration that can be obtained from a single-frequency gain-switched TDFL. The correspondingly peak power and the pulse energy of the gain-switched TDFL is 170 W and 0.8 µJ, respectively.
3.4 Longitudinal mode characteristics
The longitudinal mode characteristics of the gain-switched TDFL were measured by launching the backward laser output into a scanning Fabry-Perot interferometer (SFPI, Thorlabs, SA200-18C). The SFPI beat traces were recorded by the oscilloscope (Siglent, SDS5104X). Note that the SFPI has a free space range (FSR) of 1.5 GHz and a resolution of 7.5 MHz.
In general, only one peak is observed within the 1.5 GHz span, for all the gain-switched output under the pump with a variation of pump power and repetition rate. Figure 5(a)–5(c) show the selected observed SFPI beat signals of the gain-switched laser, under launched average pump power of 153 mW (pulse duration: 50 ns; repetition rate: 20 kHz), 243 mW (pulse duration: 50 ns; repetition rate: 50 kHz), 275 mW (pulse duration: 50 ns; repetition rate: 100 kHz), respectively. Single longitudinal behavior is thus confirmed for the gain-switched TDFL within the whole pump power range. Indeed, such a DFB cavity with the ultrashort effective length of 2.5-mm can ensure truly single longitudinal mode laser oscillation.
Fig. 5. Observed SFPI beat signals of gain-switched DFB TDFL with pump repetition rate of (a) 20 kHz, (b) 50 kHz, and (c) 100 kHz, respectively. (d) Measured PER of gain-switched TDFL under various launched average pump power.
3.5 Polarization extinction ratio (PER)
In the PER measurement, the maximum power Pmax and the minimum power Pmin, corresponding to the principal linear polarization state and the orthogonal polarization state, of the backward output, were measured in free space. A pair of holographic wire grid linear polarizers (WP25H-C CaF2 2-8 µm) were inserted between the backward output of the TDFL and the power meter. Each polarizer has an extinction ratio >150:1 at 2 µm and such a pair of polarizers provides a total extinction ratio >20000:1 in our measurement. A high-sensitivity thermal power meter (Ophir Nova II, 3A) was used to precisely detect low-level Pmin. Note that all the fiber pigtails were taped on the optical bench to minimize the influence of mechanical vibration, since all the fiber components in the TDFL are non-polarized. The polarization extinction ratio (PER) of the backward gain-switched TDFL was then obtained, according to the relation of $PER(dB) = 10{\log _{10}}\frac{{{P_{\max }}}}{{{P_{\min }}}}$. The uncertainty of the measured PER is estimated to be ±1 dB.
Figure 5(d) plots the measured PER of the gain-switched DFB TDFL under various launched average pump power (pulse duration: 50 ns; repetition rate: 20-100 kHz). It is seen that the measured PER is >18 dB, within the whole launched pump power range with various pump repetition rate. It therefore proves that the fabricated 2-µm single-frequency gain-switched DFB TDFL is also highly linearly polarized, since the monolithic Tm-doped DFB fiber laser made by the femtosecond laser PbP method is with a high birefringence of ∼10−4 (see Sec. 2) and causes highly polarization-selectivity in the gain-switching process. This explains why only one peak is observed in the SFPI spectrum (see Fig. 5(a)–5(c)).
3.6 Transform limited behavior of 2-µm gain-switched DFB laser pulses
The linewidth of the 2-µm single-frequency, linearly polarized, gain-switched DFB TDFL was retrieved from the zoomed beat signal shown in the SFPI spectrum (e.g., see Fig. 5(a-c)), under different launched average pump power (pulse duration: 50 ns; repetition rate: 20, 50 and 100 kHz).
Figure 6(a)–6(e) shows the evolution of 2-µm gain-switched pulses, under the launched average pump power of 121, 195, 272, 338, and 413 mW (pulse width: 50 ns; repetition rate: 20 kHz). The peak power (Ppeak) of the corresponding 2-µm gain-switched DFB TDFL is 1.55 W, 11.1 W, 60.1 W, 121 W, and 170 W, respectively. Note that the discrete gain-switched pulses shown in Fig. 6(a)–6(e) are with a periodic time interval of 50 µs, in good agreement with the repetition rate of 20 kHz. When the launched average pump power is 121 mW, the FWHM linewidth of the fitting envelope is 17.1 MHz, corresponding to the measured pulse width of 29.9 ns. When the launched average pump power increases to 413 mW, the FWHM linewidth of the fitting envelope is 159 MHz, corresponding to the measured pulse width of 4.7 ns. The correspondingly time-bandwidth product (TBP) is calculated to be 0.56 and 0.89, respectively (see Fig. 6(f)). On the other hand, when the repetition rate of the launched pump is 50 and 100 kHz, the TBP of the gain-switched TDFL pulses is virtually constant within the range of 0.60-0.70, across the whole range of gain-switched pulse peak power.
Fig. 6. (a)-(e) Measured laser linewidth of DFB TDFL with gain-switched pulse peak power Ppeak of 1.55 W, 11.1 W, 60.1 W, 121 W, and 170 W (pump pulse width: 50 ns; rep. rate: 20 kHz), respectively. (f) Summary of relation between measured TBP of gain-switched DFB TDFL pulsed and gain-switched pulse peak power Ppeak with various rep. rate. Note that the straight lines in the figure are guides to the eye.
First, the above TBP varying trend of the gain-switched DFB TDFL pulses under different pump repetition rate shows intensity-dependent behavior. The pulse energy of the pump and correspondingly the gain-switched laser under the pump with repetition rate of 20 kHz are much higher than those under the pump with repetition rate of 50 and 100 kHz. Second, one can see from Fig. 6(f) that the TBP of the gain-switched TDFL pulses does not exceed 0.7, when the peak power of the 2-µm gain-switched pulses is below 80 W, regardless of the repetition rate of the pump. All in all, when the pump pulse energy is low, the TBP of the gain-switched DFB TDFL pulses is close to the transform limit of 0.441 for Gaussian-shaped pulses; when the pump pulse energy is high, the TBP of the gain-switched pulses shows minor degradation away from the original Gaussian profile since the TBP is below 0.9 across the whole pump pulse energy range.
Additionally, sideband components appear in the linewidth profile of the gain-switched DFB TDFL, when the peak power of the gain-switched pulses exceed 100 W (for example see Fig. 6(d) and 6(e)). As shown in Fig. 6(e), the linewidth of the gain-switched pulses measured from SFPI is 159 MHz, equal to ∼2.1 × 10−3 nm. The peak of the first sideband is apart from the main peak by ∼120 MHz, equivalent to a spectral red-shift of ∼1.6 × 10−3 nm. Both figures are smaller than the resolution of the OSA by more than one order of magnitude.
In the time domain, a Gaussian pulse I(t) can be expressed as $\textrm{I}(\textrm{t} )= {\textrm{I}_0} \cdot {\textrm{2}^{{{( - 2t/{\tau _L}_P)}^2}}}$, where I0 the maximum magnitude, t the time, and τLP the FWHM pulse duration, respectively. We can reasonably assume that the distortion of Gaussian pulses originates from the accumulation of nonlinear effect arising from the Kerr nonlinearity and the dispersion. Essentially, the initial generated gain-switched pulses are with transform-limited Gaussian profile. Effectively, within each round-trip journey, tiny distortion, which is related to the exponent term of t/τLP, is applied onto the pulse profile in the time domain. Such distortion accumulates and deviates significantly from the Gaussian transform limit, after the gain-switched pulses propagate and grow within the laser cavity after multiple round trips. For an ultrashort cavity with a length Lc, the round trip time (2nLc/c) of the gain-switched photons is << τLP. In our case, the round-trip time of the gain-switched pulses is below 30 ps, less than 0.5% of the pulse width. Benefitted from the ultrashort DFB cavity, the accumulation of nonlinearity becomes largely relieved, in comparison with the gain-switched fiber laser with cavity length of tens of centimeters. Hence, gain-switched TDFL pulsed output with TBP close to the transform limit is obtained.
As a comparison, in another reported single-frequency ultrashort-cavity gain-switched DBR TDLF, the pulse width and the TBP is reported to be 19 ns and 0.70, respectively [45]. Due to the low peak power (<35 W) of the gain-switched laser achieved in Ref. [45], the TBP should show constant in the whole power range, similar to the TBP varying trend shown in the low peak power range (see Fig. 6(f)). Therefore, it suggests that the transform-limited behavior of the single-frequency, linearly polarized DFB TDFL pulses is the intrinsic behavior of the laser cavity itself, which is related to the fiber nonlinearity and dispersion.
3.7 Predication of ultimate pulse width and peak power of gain-switched DFB TDFL
The pulse width τLP of a gain-switched fiber laser can be derived from the early established model [34, 36]:
The damage threshold of silica glass under nanosecond laser irradiation was reported to be ∼500 GW/cm2 [68]. Furthermore, the fs-laser written FBGs in silica fiber show excellent resistibility to high laser power density and high temperature and allow the silica fiber laser working under high laser density up to its material damage threshold [63–65]. Therefore, for the employed Tm-doped silica fiber, which has a core diameter of 5.0 ± 0.5 µm, the maximum average pump power can be launched and absorbed is calculated to be between 15 and 25 W, assuming that the pulse duration and the repletion of the pump is 10 ns and 20 kHz, respectively. Given that the absorbed pump power at 1550 nm is 15 W, the ultimate pulse width of a single frequency, gain-switched 2-µm TDFL can access 1 ns in a 1-mm long ultrashort linear cavity (see Fig. 7(a)). Instead, using a 1950 nm in-band pump, the gain-switched 2-µm laser pulse width can be further reduced by another 10%, i.e., down to 0.9 ns, under the same pulse pump condition, because the pump photon energy hvp at 1950 nm is lower than that at 1550 nm. In the case of using a 1950 nm pump, given that the slope efficiency of such a single-frequency, gain-switched DFB TDFL is 5%, the peak power and the pulse energy of the gains-switched laser output will be 4.2 kW and 3.8 µJ, respectively (see Fig. 7(a) and (b)). Mind that here we assume that the Kerr nonlinear effect will not influence the temporal profile of the laser pulse significantly, when the cavity length is reduced down to 1 mm.
4. Conclusion
In conclusion, we report a 2-µm single-frequency, linearly polarized gain-switched silica TDFL, using an ultra-short DFB cavity with an effectively length of 2.5 mm. The DFB cavity with a pair of FBGs with high reflectivity is built in a commercial non-polarized Tm fiber by femtosecond-laser PbP direct-writing method. Gain-switched TDFL at 2002 nm with a pulse width of 4.7 ns, a repetition rate of 20 kHz, a peak power of 170 W, and a single pulse energy of 0.8 µJ, has been demonstrated. To the best of our knowledge, the pulse duration of 4.7 ns is the recorded shortest pulses generated from a single-frequency gain-switched TDFL. Linear polarization with a PER > 18 dB has also been confirmed in such a single frequency gain-switched 2-µm laser. The ultrashort DFB gain-switched TDFL shows a time-bandwidth product close to the transform limit of Gaussian-shaped pulses. Further works on optimizing the single-frequency, linearly polarized gain-switched TDFL, e.g., reducing pulse width, enhancing peak power and slope efficiency, is currently ongoing. We expect that the ultimate peak power of a compact all-fiber-format 2-µm single-frequency, linearly polarized gain-switched TDFL can be enhanced to kilowatt-level, using a femtosecond-laser written 1-mm long ultrashort single-stage oscillator.
Funding
National Natural Science Foundation of China (62175096, 62275111); Jiangsu Innovation and Entrepreneurship Team; Priority Academic Program Development of Jiangsu Higher Education Institutions; Jiangsu Collaborative Innovation Centre of Advanced Laser Technology and Emerging Industry.
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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