Highly adaptable gain-switched fiber laser with improved efficiency

A highly adaptable fiber laser with pulse-on-demand and precision pulse-duration tuning is presented. It is based on a compact optical design combining the gain-switching technique with the all-fiber master oscillator and pump-recovery amplifier architecture. The approach of laser-pulse stability control by compensation pumping and pulse-duration control by changing the pump wavelength are introduced. In order to prove the concept, a laser setup capable of producing laser pulses with an average power of up to 30 W and a peak power of approximately 1 kW at an improved efficiency and an arbitrary repetition rate is presented. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Today's laser-based manufacturing processes and some laser treatments in medicine require highly flexible production tools to make more advanced patterns and structures, potentially at high speed. Such processes could benefit from highly adaptable laser systems that can produce an on-demand pulse and also control other pulse parameters in combination with a high-speed scanning system based on a polygon or resonant scanner. Such laser systems are usually complex because they must include additional optical elements, i.e., modulators and their drivers. For several applications, such as micromachining, laser direct color marking, laser transfer printing and even some special medical treatment (like selective photocoagulation), laser pulses with a duration of several 10s of ns at relatively low peak power (the kW range or even below) are required. Here, an example of an efficient solution is the gain-switched laser [1], which can effectively maintain pulse parameters across a broad range of repetition rates [2]. It was shown that with the right optimization of the laser's physical parameters, short nanosecond pulses can be achieved [3]. Such pulses typically have a peak power that is 10 times larger than the absorbed peak pump power.
An unwanted characteristic of gain-switched lasers is the laser pulse's dependence on the physical parameters of the oscillator (length and pump absorption), which for the given laser system fixes the output pulse duration to a peak power ratio [4]. For example, short duration pulses (generated in the gain-switched mode) require a short resonator length, which leads to a decrease in the overall efficiency (typically less than 30%). This can be addressed by adding an additional active fiber that is pumped by a residual (unabsorbed) pump from the main oscillator [5]. Such a fiber-laser architecture maintains the simplicity and compactness of the gain-switched laser and can be applied to lasers operating in applications where robustness and low maintenance are a requirement, pulsed operation at exotic wavelengths, where there is lack of the appropriate active optical elements for pulse generation, or as a high-brightness pump source for core pumping applications.
The gain-switched technique was mainly developed with ytterbium-or thulium-doped active fibers. For example, to the best of our knowledge, the shortest pulses achieved to date from Yb-doped fiber lasers were 28 ns [3], with even shorter pulses (17 ns) being produced from a core-pumped Tm-doped fiber [6] by a 1550-nm pulsed pump source. Also, the clad pumping of Tm-doped fibers can be realized [7], enabled by efficient pumping at 790 nm, which is not susceptible to photo darkening and can achieve up to 64% slope efficiency [8]. By combining the technique with thulium-and holmium-doped active fibers, a spectral region of up to 2.1 μm was achieved [9], while an erbium-doped fluoride fiber [10] produced pulses at 2.8 μm.
In this paper we show an advanced, all-fiber, gain-switched, ytterbium-doped fiber laser that is capable of operating at arbitrary repetition rates from a single pulse to approximately 1 MHz using a technique based on gain control to maintain the laser always slightly below the operation threshold. This allows the repetition rate of the laser to be changed instantly and real pulse-on-demand can be achieved. Furthermore, the laser is capable of changing its pulse duration while preserving the peak power while operating at 1030 nm, which is otherwise hard to obtain with gain-switched lasers based only on the oscillator. This is achieved by introducing another tuning parameter: an effective pump absorption that allows for pump power distribution control between the oscillator and the amplifier, and thus changing of the pulse duration while maintaining the total efficiency of the laser.
The biggest advantage of the proposed gain-switched laser system compared to the gainswitched oscillator with a separate added amplifier is its low complexity. The proposed laser is much simpler and more cost effective to build, as it eliminates the need for a feed-through combiner and uses only one pump system based on pump laser diodes for providing a pulseon-demand and pulse-tuning functionality. In addition, the existing solutions of gain-switched lasers have the peak power changed when setting the pulse duration. For short laser pulses a short oscillator length is required, which then lowers the efficiency. The efficiency is also lowered when operating in pulse-on-demand mode. In comparison, the pump-recovery amplifier together with the compensation pumping presented in this paper allows for high efficiency at a constant laser-pulse peak power when producing pulses on demand and changing the pulse duration.
The approach presented in this paper for a laser pulse-on-demand operating mode and pulse tuning is not limited to ytterbium-doped fibers, but can benefit the design and applications of gain-switched fiber lasers based on different kinds of active fibers covering a currently very interesting range of wavelengths that are longer than 2 μm.

Theory
The gain-switched operation of a pulsed-laser system is based on the high-speed modulation of the pump power that produces relaxation oscillations of the laser output power. By switching off the pump laser diodes fast enough, only the first relaxation spike can be selected. This spike can be formed in a controllable manner and represents the output laser pulse. The dynamics of gain switching were already described in [4,11,12] and a good approximation connecting the laser pulse duration t 0 to the laser parameters can be derived from rate equations describing the laser photon and inversion populations: Equation (1) is written in such a way that the important parameters influencing the pulse duration are grouped together in the second square-root term: L p is the passive fiber length, L d is the doped fiber length, P p is the pump peak power and α is the absorption coefficient. The other parameters are as follows: n is the refractive index of the fiber, A is the doped area cross-section, h is Planck's constant, Γ is the overlap integral between the signal and the fiber core, σ e is the emission cross-section and λ p is the pump wavelength.

Pulse-duration tuning
As can be seen from Eq. (1), most of the parameters that affect the laser pulse must be predefined when designing the laser system. The only free parameter left is the absorbed pump power, which depends on the available power of the pump diodes and the absorption coefficient of the pump light. In order to keep the laser peak power constant, only the latter is available for t regenerative a makes it poss can be seen in This allow of the model pulse duration take into acco doped oscillat 1(c)).

Pulse on dem
The gain-swi recovery amp further analy developing a the repetition derived.
For the os consequence t [2]: that can be fu Because the y occupancy of the tuning and amplifier setup sible to tune th n Fig. 1(a). ws for maximum shown are the n at the corresp ount the reduc tor length to pr   Fig. 1(b)). Th uration to the m oscillator. We m cillator, so the ween 0.2 and 0. he pump ciency, is e aim of ndency on plifier are s and as a equations (2) (3) e thermal (4) where E 2 is th fiber), λ s is th represents the The system (P abs ) in the o As the activ consequence h amplifier appr in Fig. 2 for pulse power o this time the a 1.6%, which m described exa occurs when corresponds t increasing the generation be to amplify th pumped cond deplete the up to hold the wh emission can

Experime
The gain-swi recovery amp ytterbium-dop μm using a se was formed b reflectivity of spliced direct output end to simulation as he energy of th he signal wave e amplifier thre m is pumped w scillator and th e fiber volum have a differen roaches the thr the setup desc of 100 W the average exited means that the ample is the c operating at to the spontan e repetition rat etween the osci he generated l dition is due to pper laser level hole pumped la be mitigated. The measu 4(a), where th oscillations. T after the laser Fig. 4(c)). Fig  Fig. 4

Results a
The presented available cont pulses, as sho driving curren laser pulse is a stack of 250 Fig. 5(b), with ore, the tuning ficiency, was te re shown in Fig  was achieved, in Fig. 1(b). T the whole sy nly the oscillat ne ranging from h the pump-rec llator, the effi being absorbed cy, but shows umping power velength select on m can operate s and laser dio at a repetition , the pump pu 5(a). To check ulses plotted to ment between th train at 500 kHz s 0 successive pulse of the pulse d ested on the sy g. 6 for a repet which is in g The peak pow ystem did not tor. Another b m 10% to 30% covery amplifi ciency increas d in the laser. a slight drop r being availab tion, this reduct with repetition de driver. Othe n rate of 500 kH lse with a peak k the laser-puls gether and fitte he measuremen shown together w es plotted together duration, togeth ystem at the sam tition rate of 10 ood agreemen er in this exp t change sign benefit of the % was increased ier. When usin ses with the ef Adding the pu with the incre ble in the amp tion in efficien n rates up to erwise, the las kHz. The relati ak power of 10 se duration and ed with the sec nts and the fit.
with the pump puls r is shown in b) a her with the pe me pump-pulse 00 kHz. A lase nt with the mod eriment was p nificantly, in concept is tha d to a range fro ng a gain-switc ffective absorp ump-recovery eased effective lifier. With fu ncy could be av Using nor between the p a factor of 5 i pump-pulse d relaxation osc The reduc laser optical p pulse before t condition is r doped fibers a the gain betw recovery amp different pum The comp high pump pu diodes in CW shown in Fig.   6. Gain-switched, l icient are shown i ency between the m consisting of an rmal pumping pulses from 2 μ in the tested ra duration (Fig. 7 cillation from th ction in the pe path. As show the amplifier re elated to the li and is the main ween the pump plifier must use mped conditions pensation pump ulses. Setting t W mode, the las 7(b) (blue circ laser-output, pulse in a) for pulse FW e two setups: osc oscillator and a pu with nearly s μs to 1 ms, prod ange ( Fig. 7(b)) 7(a)), which is he laser oscilla ak power is co wn in Fig. 2, fo eaches the thre ifetime of the e n mechanism t pulses. This sh e an adapted pu s induced by th ping can be re the compensat ser's peak-pow cles). achieved with ere the measu hievable output aser peak powe n pumping. In of 0.9 W betw n depleting the power pump pu g at different wn in Fig. 9(a ere omitted. B mp pulses are u by the low pu h fluctuation in iminate that a he pulse's FWH shown for differen ower P Lp b) for the es. It is clear that arts to hinder the e h the system urements are p ut laser power a er was kept con n this mode t ween the highexited-ion pop ulse. repetition rate a). In Fig. 9

Conclusio
The concept pulses with a presented. Th the all-fiber, tunability and peak laser pow laser processi One advan while maintai with gain-swi pump-recover absorbed pum absorption of Besides th same concept nm absorption with the chan allow the bui adaptable with 9. Arbitrary laserssive pulses "zoom he standard deviati .7% for the FWHM on of a highly a an average pow he novel conce master-oscilla d improved eff wers that can g ng unit. ntage of the sy ining a constan itched fiber la ry fiber amplif mp power di the system wh he demonstrate t could be appl n peak. This p nge in absorptio lding of comp h respect to the -pulse pattern gen m in" shown in b) ion for the peak po M.
adaptable, gain wer of up to 3 pt combines th ator, pump-re ficiency. It is r generate a puls ystem is its ab nt output peak asers. The inno fier and a tuna stribution thro hile keeping its ed modes of op ied to gain-sw peak is narrow on by a factor pact, pulsed 2e application.