Hong-Yu Luo | Yong-Zhi Wang

Abstract—We demonstrated the～2.8-μm and～3.5-μm linearly polarized continuous wave (CW) laser outputs from a polarization-maintaining (PM) Er3+-doped fluoride fiber laser.By introducing a film polarizer into the cavity to select the laser polarization orientation,the～2.8-μm linearly polarized CW laser with a high polarization extinction ratio(PER) of～23 dB and maximum output power of 2.37 W was achieved under double-end pumping at 976 nm.By adding another 1981-nm pump source simultaneously,the～3.5-μm linearly polarized CW laser was also obtained,giving higher PER of～27 dB and maximum output power of 307 mW which is only limited by the available power of 1981-nm pump.To the best of our knowledge,this is the first report on a mid-infrared linearly polarized CW PM fiber laser in the >2.5-μm mid-infrared region.This work not only opens up opportunities for some new mid-infrared applications,but also provides a promising platform for developing high-stability and versatile mid-infrared laser sources.

Index Terms—Fiber laser,linearly polarized,mid-infrared,polarization-maintaining (PM).

1.lntroduction

Mid-infrared laser sources emitting at 2 μm to 20 μm have already received researchers’ continuous attention due to their huge potential in a number of application fields (e.g.,medicine,spectroscopy,remote sensing,and defense[1]-[4]).Thereinto,the laser sources based on active/passive optical fibers (commonly called fiber lasers) have been developed to a popular platform for mid-infrared laser generation in the recent decade,by virtue of the superior conversion efficiency,heat dissipation,beam quality,and structure[5].Until now,different rare-earth ions including Tm3+,Er3+,Ho3+,and Dy3+have been doped into the low-phononenergy hosts,especially fluoride glasses,to achieve emissions at 2 μm to 5 μm[6]-[12].And the corresponding lasers have also already covered the wavelength of 2 μm to 4 μm[6]-[11].Among them,the Er3+-doped fluoride fiber laser may be the most attractive one because of the availability of the commercial 976-nm pump laser diode (LD) and its ability of dual mid-infrared emissions at～2.8 μm (4I11/2→4I13/2) and 3.5 μm (4F9/2→4I9/2),which have been substantially reported.

For the4I11/2→4I13/2transition,the first report on the laser operation of an Er3+-doped fluoride fiber laser could date back to 1988,in which the pumping at 476.5 nm was used by Brierley and France[13].Due to the natural self-terminating as a result of a shorter lifetime of the laser lower level (i.e.,6.9 ms) than the upper level (i.e.,9 ms),a high dopant concentration of Er3+is usually needed,since the enhanced energy transfer upconversion (ETU) process (i.e.,4I13/2,4I13/2→4I9/2+4I15/2) with the increased dopant can not only effectively depopulate the laser lower level,but also recycle the ions,hence improving the efficiency,although there is a simultaneously increased heat load.Based on this highly Er3+-doped scheme,the output was significantly scaled with the developments of the heat management and diode pumping technology at～976 nm in the following more than 30 years,during which there were some impressive results reported[14]-[18].In 1999,the output from a 970-nm diode pumped M-profile Er3+-doped fluoride fiber laser at 2.8 μm had already reached the 1-W level[14].In 2007,Zhu and Jain realized the first 10-W-level 2.8-μm Er3+-doped fluoride fiber laser with the slope efficiency of 21.3% in a free-space alignment,where the active water cooling was exploited[15],and the output was further scaled to 41.2 W with the slope efficiency of 22.9% in an optimized double-end pumped all-fiber cavity by Aydinet al.[16].Recently,Newburgh and Dubinskii achieved a nearly 70-W quasicontinuous wave (CW) output with the slope efficiency of 20% from a double-end pumped free-space Er3+-doped fluoride fiber laser at 2.8 μm,where the dry nitrogen protection of both fiber ends was adopted[17].Fortinet al.demonstrated a 30-W all-fiber Er3+-doped fluoride fiber cavity closed by a pair of fiber Bragg gratings (FBGs) at the extended wavelength of 2.94 μm with the scarified slope efficiency of 16%[18].By recycling the ions in the laser lower level back to the upper level by the excited state absorption (ESA)process (i.e.,4I13/2→4I9/2),which highly resonates the adjacent～1.6-μm transition,a 10-W-level Er3+-doped fluoride fiber laser was achieved with the significantly improved slope efficiency of 50%,exceeding the Stoke limit by 15%[19].These results almost represent the current highest levels of fiber lasers at～3 μm.

For the4F9/2→4I9/2transition,the first demonstration of the～3.5-μm Er3+-doped fluoride fiber laser could also date back to 30 years ago.In 1991,T?bben utilized a 653-nm 4-dicyanomethylene-2-methyl-6-pdimethyl-amino-styryl-4H-pyran (DCM) dye laser to pump an Er3+-doped fluoride fiber based cavity at a low temperature of 77 K,achieving the CW laser output in the range of 3.43 μm to 3.48 μm,although only 8.5-mW power with the slope efficiency of 3% was obtained[20].After that,the CW laser output was realized from an optimized system at room temperature using the same pumping method.However,the output power was decreased to even worse～2 mW[21].Such a poor performance is mainly caused by the long lifetimes of the lower excited states (i.e.,4I11/2and4I13/2),which prevent the ions decaying from the laser lower level returning to the ground state.Until 2014,Henderson-Sapiret al.proposed the concept of the dual-wavelength pumping to solve the ions bottleneck[22],where LD at 985 nm was used to excite the ions to the long-lived virtual ground state (VGS)4I11/2,while another fiber laser at 1973 nm continued to excite the ions to the laser upper level.As a result,significantly enhanced power of >200 mW was achieved with the optical-to-optical efficiency of 16% at room temperature.In the following several years,many efforts have been made to improve the performance of the dual-wavelength pumped system[23]-[26].Some important transitions,e.g.,energy transfer (ET) (i.e.,4I11/2+4F9/2→4I13/2+4S3/2)[23]and virtual excited state absorption (VESA) (i.e.,4F9/2→4F7/2)[24],which have the direct effects on the～3.5-μm emission,have been discovered.While some experimental results have been also reported.Impressively,Maeset al.demonstrated a compact all-fiber double-clad 1-mol% lightly Er3+-doped fluoride fiber laser whose cavity was formed by a pair of inscribed FBGs at 3.55 μm.It yielded the current recorded output power of 5.6 W with the slope efficiency of 36.9%with respect to the 1976-nm laser[25].Due to the action of excessive 1976-nm pumping induced VESA,however,the laser quenching behavior,previously predicted in [24],was observed.To circumvent this issue,they proposed to use a 7-mol% heavily Er3+-doped fluoride fiber as the gain medium instead.With the help of the enhanced ETU and cross relaxation (CR) processes resulted from the increased dopant concentration,the 3.42-μm CW laser without any quenching behavior was obtained,giving the maximum output power of 3.4 W with the slightly improved slope efficiency of 38.6%[26],although such a high dopant led to a significantly increased heat load as mentioned before.

However,almost all the previous reports on the rare-earth ions doped CW fiber lasers in the >2.5-μm mid-infrared region (not only limited to the above Er3+-doped systems) are based on non-polarizationmaintaining (PM) fiber schemes,only providing randomly polarized outputs which may be not suitable for some specific applications (e.g.,harmonic generation,parametric conversion,coherent detection,and coherent and spectral beam combining)[27].In addition,the non-PM systems suffer from serious environmental instability as a result of their strong sensitivity for mechanical perturbations and temperature variations.

In this paper,we report the linearly polarized CW laser outputs at～2.8 μm and～3.5 μm from a PM Er3+-doped fluoride fiber laser based on different pump schemes.The output characteristics including the power,polarization extinction ratio (PER),and wavelength with the varied pump power are studied in detail.To our knowledge,this is the first report on the direct linearly polarized laser generation from a PM system at>2.5-μm mid-infrared wavelengths.

2.Experimental Setup

The proposed experimental setups are schematically shown inFig.1.

Fig.1.Experimental setups (left) and the related energy levels and transitions (right) of the established linearly polarized PM double-clad Er3+-doped fluoride fiber lasers operating at (a)～2.8 μm and (b)～3.5 μm.L1 to L8:Different lenses with different parameters;DM1 to DM6:Dichroic mirrors with different parameters (DM1 and DM2:High transmission (HT) at 976 nm and high reflection (HR) at～2.8 μm;DM3:HT at 976 nm and HR at 1981 nm;DM4 and DM5:HT at 976 nm and 1981 nm while HR at～3.5 μm;DM6:HT at 976 nm and 1981 nm while 60% reflection at～3.5 μm);FP:Film polarizer;GM:Gold mirror;BF:Bandpass filter;BS:Beam splitter;BT:Beam trap.

Fig.1 (a)shows the schematic of the～2.8-μm linearly polarized PM double-clad Er3+-doped fluoride fiber laser,which is a typical double-end pumping structure.The pump source includes two same 976-nm LDs,each of which has a 105-μm core diameter fiber pigtail and the maximum output power of 30 W.The gain medium is a segment of 4-m commercial PM double-clad 7-mol% Er3+-doped fluoride fiber (Le Verre Fluoré)with a birefringence of 7×10-5,corresponding to a beat length of～4 cm measured based on the conoscopic interference phenomenon[28].It has a core diameter of 15 μm (NA=0.12) and an inner clad diameter of 260 μm (NA=0.4),where its circular symmetry is broken by two parallel flats separated by 240 μm.Our measured fast-and slow-axis background losses of the PM fiber at～2.8 μm are 42.5 dB/km and 43.8 dB/km,respectively,close to the average loss of 40 dB/km provided by the manufacturer.The left fiber end is perpendicularly cleaved as the output coupler with the aid of 4% Fresnel reflection,while the right one is angle-cleaved at 8° to avoid parasitic lasing.For two fiber ends,the same pump coupling alignment is adopted,which can contribute to the clad coupling efficiency of 77%.The laser from the angle-cleaved fiber end is guided by DM2 and projected onto GM as the terminated feedback.Between DM2 and GM,FP is inserted to keep the intra-cavity linearly polarized oscillation.The laser from the other fiber end is guided by DM1 for the spectrum,power,and PER monitoring after purifying with BF (2500 nm to 3000 nm).Fig.1 (b)shows the experimental setup of the～3.5-μm linearly polarized PM double-clad Er3+-doped fluoride fiber laser,in which the same gain fiber is used.The pump source includes two lasers.One is 976-nm LD,the same as before,and the other is a home-made Tm-doped fiber laser at 1981 nm with an SMF28e fiber pigtail which can provide 8-W maximum output power.After combining by using DM3,the two beams are launched into the clad and core of the fiber via L7 with the efficiency of 76% and 86%,respectively.Different from the above～2.8-μm cavity,external DM6 is butted against the perpendicularly cleaved fiber end acting as the output coupler instead due to the lower gain of the～3.5-μm transition.DM4 is placed between DM3 and DM6 to guide the laser output for following measurement after purifying by using another BF (3250 nm to 3750 nm).The laser from the angle-cleaved fiber end is collimated by using L8 and then projected onto GM as the terminated feedback after removing the residual pump by DM5.The same FP is inserted between DM5 and GM to keep the intra-cavity linearly polarized oscillation.In the experiments,the power is recorded using a thermal power sensor (Thorlabs,S405C),and the optical spectrum is measured using a monochromator with～0.4-nm resolution (Princeton Instruments,Acton SP2300).

3.Experimental Results

3.1.Operation in 2.8-μm Band

In the～2.8-μm laser system as shown inFig.1 (a),FP was firstly rotated to make its polarization orientation parallel to the fast or slow axis of the PM fiber,thus keeping the linearly polarized oscillations.The laser output characteristics with the varied pump power in both cases were recorded,as plotted inFigs.2 (a)and(b).It is observed that the output power increases almost linearly with the pump power in either case,while the laser oscillation along the fiber fast axis has the slope efficiency of 17.9% which is higher than that(i.e.,16.1%) along the slow axis,although it is still significantly lower than its non-PM counterpart,mainly caused by the 17% insertion loss from FP.In this experiment,only the maximum pump power of 13.3 W was allowed,since we found that higher output power would lead to the damage of FP as a result of a large amount of heat loads.At this pump power,the maximum output power of 2.37 W is achieved,of which further scaling is expected by using a high-damage-threshold polarizer (e.g.,polarization beam splitter or Glan-Taylor polarizer) instead in the future.Over the whole pump range,the laser PERs in both cases are almost kept unchanged at～23 dB and～21 dB for the fast-and slow-axis oscillations,respectively.These results indicate that the oscillation along the fast axis can provide a better performance,which is mainly determined by the fiber quality itself.

Fig.2.Laser output characteristics along the fast and slow axes with varied pump power:(a) power and (b) PER evolutions.

In the case of the fast-axis oscillation,the output spectra at different levels of pump power were recorded,as shown inFig.3.It is seen that the wavelength red-shifts from 2776.6 nm to 2794.2 nm with the increased pump power from 1.31 W to 1.75 W,as a result of the up-shifted terminated Stark manifolds of the4I13/2level,and then slightly to 2794.6 nm with the further increased pump power to 11.8 W.While the 3-dB bandwidth is almost kept～2 nm.

Fig.3.Laser spectra at different levels of pump power.

3.2.Operation in 3.5-μm Band

In the～3.5-μm laser system as shown inFig.1 (b),FP was directly rotated to the orientation along the PM fiber fast axis as a result of the better performance.The output power evolutions of the～3.5-μm laser with the varied 976-nm and 1981-nm pump power were recorded,as shown inFigs.4 (a)and(b),respectively.FromFig.4 (a),it is seen that the～3.5-μm laser power first increases monotonously and then tends to saturate with the increased 976-nm pump power.This phenomenon is typical for such a dual-wavelength pumped system due to the insufficient 1981-nm pump power,which has been observed in previous demonstrations of either lightly or heavily non-PM Er3+-doped systems in this band[25],[26].The slight red-shifting of the saturation point with the increased 1981-nm pump power is also a confirmation.In addition,it is clearly seen that the 976-nm pump threshold decreases with the increased 1981-nm power,which is exclusive for the heavily Er3+-doped system since there is no VESA induced quenching behavior[26].At the 1981-nm pump power of 6.6 W,the maximum～3.5-μm laser power of 307 mW is achieved with a 976-nm pump threshold of 0.98 W.Further power scaling is only limited by the available 1981-nm pump power.Fig.4 (b)shows the corresponding evolution with the varied 1981-nm pump power.It is seen that the～3.5-μm output power always increases linearly with the 1981-nm pump power without any quenching phenomena.With the increased 976-nm pump power,the 1981-nm pump threshold decreases while the slope efficiency increases,and both tend to saturate,which are similar to the previously reported non-PM heavily Er3+-doped fluoride fiber system at～3.5 μm and are the result of the population recycling dynamics[26].At the 976-nm pump power of 1.44 W,the slope efficiency and threshold with respect to the 1981-nm pump power are 11.7% and 3.7 W,respectively,worse than the previously reported non-PM system[26].This is mainly caused by the high insertion loss of >2 dB from FM and L8 lens at this wavelength.

Fig.4.Laser output power with the varied pump power at (a) 976 nm and (b) 1981 nm.

Fig.5 (a)plots the measured PER at different values of output power.The higher PER of >27 dB over the whole output power range indicates the greater linearly polarized output,and it is even 4 dB higher than that achieved at～2.8 μm,which is partly caused by the higher extinction ratio of FP at～3.5 μm.While the output optical spectra at different pump levels of 1981-nm pump were recorded,as shown inFig.5 (b).With the increased 1981-nm pump power,the center wavelength red-shifts from 3464.0 nm to 3468.8 nm with an almost unchanged 3-dB bandwidth of 1 nm.

Fig.5.Laser output characteristics:(a) PER as a function of output power and (b) optical spectra at different power levels of 1981-nm pump at the fixed 976-nm pump power of 1.44 W.

4.Conclusions

In summary,we reported the mid-infrared linearly polarized CW laser generations directly from a PM Er3+-doped fluoride fiber laser,for the first time to the best of our knowledge,where FP was used to guarantee the linearly polarized operation.Pumping at 976 nm alone,the linearly polarized～2.8-μm CW laser with PER of >23 dB was achieved,yielding the maximum output power of 2.37 W.Pumping at 976 nm and 1981 nm simultaneously,the laser yielded the linearly polarized～3.5-μm CW laser with higher PER of >27 dB and output power of 307 mW.By employing a high-damage-threshold polarizer and a 1981-nm pump laser with higher power,further power scaling at both bands is expected in the future.This work not only opens up the opportunities for some new mid-infrared applications,but also provides a promising platform for developing high-stability and versatile mid-infrared laser sources.

Disclosures

The authors declare no conflicts of interest.