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Abstract
We demonstrate the generation of both continuous-wave (CW) and Q-switched cylindrical vector beams (CVBs) from a mid-infrared Er3+-doped ZBLAN (Er:ZBLAN) fiber laser at ∼ 2.8 µm. A customized S-waveplate is incorporated as the intracavity mode converter to achieve the mid-infrared CVBs. Switchable modes of CVBs between the radially and azimuthally polarized beam can be realized easily by manipulating the cavity conditions. A maximum output power of ∼250 mW is achieved for the CW CVBs. In the short-pulsed CVBs operation regime, both the active and passive Q-switching modes are realized with a pulse duration of hundreds of nanoseconds. The proposed mid-infrared cylindrical vector lasers can have significant potential for applications in biomedicine, optical trapping, material processing and optical communication.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Cylindrical vector beams (CVBs) have garnered extensive interest due to their unique features, e.g. the centrosymmetric polarization distribution as well as the doughnut-like intensity profile, which enables the CVBs to possess potential applications in high-capacity communications [1–3], surface plasmon excitation [4,5], optical tweezers [6,7], and etc. CVBs can be classified as the radially polarized beam (RPB), the azimuthally polarized beam (APB), and the hybrid polarized beam. It is worth noting that, the RPB can break the traditional optical diffraction limit under the high numerical aperture (NA) focusing condition and has significant prospects in high-resolution imaging [8,9], while the APB can exhibit unique advantages in the material processing area [10].
In general, CVBs can be generated via both the active method and the passive one. The active approach needs to introduce an intracavity mode-selection or mode-conversion mechanism (e.g. using an optical element or the thermal effect) in an active laser cavity, which can then directly output the CVBs. For the passive method, CVBs are not generated within a laser resonator but formed during the propagation process based on the polarization transformation or mode superposition. Compared to the passive methods, the active counterparts can offer superior polarization purity, highly improved beam quality, and a compact configuration [11]. To date, the active generation of CVBs within a laser cavity has been demonstrated primarily using the birefringent crystal [12–14], the annular-shaped pumping regime [15,16], the mode selective coupler (MSC) [17,18], q-plate [19–22], and etc. However, the methods mentioned above are not able to generate high-power CVBs together with a high purity and a good stability. The birefringent crystal based mode-selection method generally requires a high pump power, which can then induce an additional thermal effect, thus possibly deteriorating the output stability. The annular-shaped pumping method suffers from a large pump power loss and low efficiency. The MSC, fabricated using the fiber tapering technique, cannot handle high-power operation and also suffers from a relatively low mode purity. Q-plate, although has been widely applied to generate CVBs, vortex beams, and high-order Poincaré sphere beams due to its simplicity and flexibility. The low damage threshold restricts the application of this liquid crystal based geometric phase retarder in the ultrafast vector beams generation [23]. Recently, another type of geometric phase retarder called S-waveplate has been employed to generate CVBs [24,25]. It is fabricated inside high purity fused silica glass using a femtosecond laser and features self-organized nanostructured gratings. The unique structure of this mode converter, with a spatially varying optical axis, allows for convenient manipulation of the polarization state of the incident beam, enabling the production of various polarization distributions. Additionally, the millimeter-scale thickness of the S-waveplate reduces the complexity of the laser system. Compared to the liquid crystal based Q-plate, the novel mode-converter S-waveplate, fabricated inside high purity fused silica glass using a femtosecond laser, can support a high-power operation regime. The implementation of S-waveplate for high-power RPB generation has been demonstrated within the laser cavity of a Q-switched and gain-switched laser, achieving up to 2.5 kW of peak power and 22.5 W of average output power respectively [26,27].
In contrast to the continuous-wave (CW) CVBs, pulsed CVBs, possessing high energy and high peak power, are critically important for practical applications such as laser processing and particle acceleration. In contrast to the continuous-wave (CW) CVBs, pulsed CVBs, possessing high energy and high peak power, are critically important for practical applications such as laser processing and particle acceleration. Based on the above methods, the generations of pulsed CVBs from solid-state lasers or fiber lasers are mainly operating in near-infrared (1um, 1.5um) regions and have been already demonstrated [15,17,21,25,27,28]. The cylindrical vector beams generation in the mid-infrared range has been achieved only in the 2 µm laser using Tm-doped crystal or Tm/Ho-co-doped fiber [13,18,26]. However, mid-infrared lasers beyond 2 µm have attracted more attention due to their advanced advantages. For instance, mid-infrared pulses at 2.8 µm can be strongly absorbed by polymers, making them desirable for polymer surface waveguide processing and optical fiber stripping [29,30]. Additionally, mid-infrared lasers operate within optically transparent windows in the atmosphere, allowing for important applications such as atmospheric communications, molecular spectroscopy, infrared ranging, and sensing [31–35]. Due to the advantages of the CVBs, the application of mid-infrared lasers can be further expanded. However, generating ∼3 µm pulsed CVBs directly from a mid-infrared active laser source presents significant challenges. For solid-state lasers, the gain medium at ∼3 µm exhibits low gain and high intracavity loss, which requires a relatively short cavity length that restricts the free insertion of devices. Achieving an all-fiber structure laser, where mode-selecting or converting devices are typically spliced to generate CVBs, is difficult to operate at 2.8 µm due to challenges in fusing fluoride and silica fibers. To date, there have been no reports on the intracavity direct generation of CVBs in 2.8 µm fiber lasers.
Here, we demonstrate the generation of both CW and Q-switched CVBs from a polarization-maintaining (PM) Er:ZBLAN fiber laser at ∼ 2.8 µm. A customized S-waveplate manufactured with infrared fused silica (Corning IRFS 7979) as the substrate material is incorporated as an intracavity mode converter to realize the generation of mid-infrared CVBs. Operation modes between the radially polarized beam and the azimuthally polarized one can be easily switched by manipulating the cavity conditions. The CW CVBs have a maximum output power of ∼ 250 mW. For the pulsed operation mode, both active and passive Q-switching CVBs are generated. In the active Q-switching regime based on an acousto-optic modulator (AOM), the output CVBs have a pulse duration as short as 281 ns. The corresponding repetition rate and average output power are 41 kHz and 104.7 mW respectively. In the passive Q-switching regime based on a semiconductor saturable absorber mirror (SESAM), the shortest pulse duration of the output CVBs is 968 ns. The corresponding repetition rate and average output power are 75.3 kHz and 86.3 mW, respectively. The proposed lasing scheme provides an effective way to generate pulsed CVBs at ∼ 3 µm and fills the gap in ∼ 3 µm CVB lasers. Besides, the S-waveplate-based approach combines the high damage threshold, simplicity and flexibility, indicating the significant potential of the S-waveplate in the mid-infrared band to produce high-power and hard-to-achieve structured laser emissions, e.g. the ultrashort mid-infrared CVBs and vortex lasers.
2. Experiment and results
2.1 Experimental setup
The experimental setup of the Er:ZBLAN fiber laser with switchable CVBs emissions and the photograph of the mid-infrared S-waveplate are illustrated in Fig. 1(a). The pump source is a 976-nm fiber-pigtailed multimode (105-µm of core diameter and 0.22 of NA) laser diode (LD) with a maximum output power of 27 W. The gain medium is a segment of 5-m commercial PM double-cladding Er:ZBLAN fiber (Le Verre Fluoré) with an Er3+-ions doping concentration of 70000 ppm. It has a core diameter of 15 µm (NA = 0.12) and a D-shape inner cladding diameter of 240*260 µm (NA = 0.4). The pump light is collimated and focused by a pair of plano-convex lenses L1 (f = 25.4 mm) and L2 (f = 50 mm), respectively, into the inner cladding of the gain fiber through a 0° dichroic mirror DM1. The dichroic mirror DM1, with a high transmission at ∼976 nm and a high reflectivity at ∼2.8 µm, is physically butted to one facet of the gain fiber and acts as one cavity mirror. The other gain fiber facet is perpendicularly cleaved to provide the Fresnel feedback and close the lasing cavity. An uncoated aspherical ZnSe lens L3 with a focal length of 6 mm is used to achieve a small collimated laser spot incident onto the AO Q-switch. A 45° dichroic mirror DM2 with a high reflectivity at the laser wavelength and a high transmission at the pump wavelength is inserted into the optical path to filter out the residual pump light. The output coupling mirror OCM1 has a transmission of 50% at the 2800 nm lasing wavelength. A half-waveplate (HWP) and the S-waveplate are inserted between DM2 and OCM1. The S-waveplate is inscribed with nanostructured gratings (clear aperture of 6 mm) with spatially varying optical axis using a femtosecond laser (300fs, 1064 nm, 200 kHz, 1W), which is working as a mode converter and can convert the incident linearly polarized beam to the CVBs. Both surfaces of the S-waveplate are antireflection coated at 2.8 µm to increase the transmission efficiency. The AOM has an effective aperture of 3 mm, a diffraction efficiency of over 60%, and an insertion loss of 0.5% in the ∼3 µm region.
Fig. 1. Experimental setup of the Er:ZBLAN fiber laser delivering switchable CVBs. (a) In the active Q-switching regime; (b) In the passive Q-switching regime. The inset is the photograph of the mid-infrared S-waveplate.
2.2 Evolution of Jones matrix in the laser cavity
To avoid the additional loss induced by the disruption of the self-consistent intracavity polarization state, it is essential to guarantee that the polarization state of the laser beam at a specific location will recover to its original one after one single round-trip circulation inside the laser cavity. We utilize the corresponding evolution of Jones matrix to describe the entire process. The S-waveplate is written with considerable nanogratings, which can be regarded as a uniaxial crystal with homogeneous phase retardation δ at the operating wavelength of 2800 nm. Besides, the orientation of the nanogratings optical axis is depending on the written geometrical parameter in the transverse plane [23], which can be expressed in the polar coordinate system as follows:
where ${\alpha _0}$ represents the initial orientation of the optical axis in the S-waveplate, q (q = 0.5) is the topological charge of the S-waveplate, and $\mathrm{\varphi }$ is the azimuthal angle in the transverse plane. The transfer matrix of the S-waveplate can be expressed as $\textrm{T}(\mathrm{\varphi } )= \textrm{R}(\mathrm{\varphi } )J{R^{ - 1}}(\mathrm{\varphi } )$, where $\textrm{R}(\mathrm{\varphi } )$ is the rotation matrix, and J is the Jones matrix of the uniaxial crystal. The initial orientation of the optical axis of in the S-waveplate is in the horizontal direction, i.e. ${\alpha _0} = 0$.The obtained radially polarized beam will then be reflected back by OCM1 and transmit through the S-waveplate and the HWP again. The ultimate Jones matrix of the transmitted beam is then derived as,
We can see that the horizontally polarized incident beam can survive in the laser cavity by maintaining its original polarization state after bouncing back. Similarly, for a vertically polarized incident beam, when passing through the HWP and the S-waveplate, an azimuthally polarized beam will be achieved. The reflected beam after passing through the S-waveplate and the HWP reverts to its original polarization state and survive in the intracavity circulation. In this sense, the laser output can be switched easily between the radially polarized beam and the azimuthally polarized one by changing the polarization state of the incident laser beam. Besides, the rotation angle of the S-waveplate will not influence the self-consistent intracavity polarization state.
2.3 Results in the CW operation regime
Firstly, we investigate the CW lasing characteristics of the PM Er:ZBLAN fiber laser without incorporating the intracavity Q-switch. It is worth noting that, the PM Er:ZBLAN fiber laser can produce linearly polarized output in the transverse fundamental mode without including the S-waveplate into the cavity, as already indicated in Ref. [36]. Installed in a two-dimensional manual translation platform (Thorlabs, ST1XY) to adjust the position, the S-waveplate is inserted between the HWP and OCM1 and operated coaxially with the collimated beam. By adjusting the rotation angle of the HWP and slightly modifying the tilt angle of OCM1, the CVBs output is realized and can be switched very easily between the APB and the RPB, which is attributed to the selective oscillation of laser modes during the fine adjustment of intracavity optics. A charge-coupled device (CCD) (Spiricon, PY-IV-C-A) is employed to record the output beam profiles. By placing a polarizer before the CCD, CVBs can be further distinguished and categorized into the APB, RPB and hybrid polarized beams.
The CW output powers of the fundamental Gaussian mode as well as the CVB mode (i.e. the RPB and the APB) versus the incident pump power are investigated and shown in Fig. 2. For the laser output of transverse Gaussian mode, a maximum output power of 321 mW is obtained at an incident pump power of 5.72 W, corresponding to a slope efficiency of 6.04%. For the RPB and APB operation regimes, the maximum output powers are 240.3 mW and 245.6 mW at the same incident pump power, corresponding to the slope efficiencies of 4.38% and 4.47% respectively, indicating an estimated insertion loss of 26.7% for the S-waveplate.
Fig. 2. The output powers of the CW Gaussian mode (blue), the RPB (red), and the APB (green) versus the incident pump power.
At an incident pump power of 1.33 W, the laser output in the CW CVBs operation regime exhibits a typical donut pattern, as shown in Figs. 3(a) and (c). From the intensity profiles after transmitting through the polarizer with different orientations, the transverse polarization distribution of the laser output can be determined. As shown in Figs. 3 (a1) to (a4), the white arrow represents the polarization axis of the polarizer and the double-lobed intensity pattern is always parallel to it, indicating the RPB output. It can be seen from Figs. 3 (c1) to (c4), the double-lobed intensity pattern of the laser output is always perpendicular to the polarizer orientation, which indicates the laser output is the APB. We also measure the output spectrum of the APB and RPB in the CW operation regime based on an optical spectrum analyzer (OSA) (Thorlabs, OSA 207C), as shown in Figs. 3(b) and (d). The center wavelengths of the RPB and the APB are almost the same of 2793.91 nm.
Fig. 3. (a) Intensity profile of the CW RPB; (a1)–(a4) Intensity profiles of the CW RPB after passing through the polarizer with different orientations; (b) Spectrum of the CW RPB; (c) Intensity profile of the CW APB; (c1)–(c4) Intensity profiles of the CW APB after passing through the polarizer with different orientations; (d) Spectrum of the CW APB; The white arrows represent the orientation of the polarizer.
2.4 Results in the active Q-switching operation regime
To achieve the active Q-switching operation regime, we incorporate an AOM into the cavity, as shown in Fig. 1(a), to generate short-pulsed CVBs. Figure 4(a) exhibits the relationship between the average output power of the RPB and the incident pump power, from which we can see that the average output power remains almost the same under the same pump power for different pulse repetition rates. The average output power varies from ∼40 mW to ∼105 mW as the incident pump power increases from 1.08 W to 2.62 W, corresponding to a reduced slope efficiency of 4.09% in contrast to the CW regime. This is attributed to the additional insertion loss of the AOM. Figure 4(b) displays the pulse duration of the RPB versus the incident pump power for different pulse repetition rates. Both the single pulse energy and the peak power of the output RPB pulses increase with the incident pump power while decreasing with the pulse repetition rate. The shortest pulse duration is achieved when the incident pump power is 2.62 W at a pulse repetition rate of 41 kHz. When we further decrease the pulse repetition rate and increase the incident pump power to a relatively high level, satellite pulses start to appear and cannot be eliminated due to the imperfect switching characteristics of the AOM. Stable single-pulse operation regime can be maintained only at a relatively high repetition rate beyond 41 kHz. This limits the further pulse shortening and the scaling up of single pulse energy and peak power. Besides, a further increase in the incident pump power may cause a fiber failure owing to the accumulated thermal effect. Splicing an end cap to protect the two fiber end facets can mitigate this issue, allowing for a higher incident pump power [37]. Figures 4(c) and (d) illustrate the pulse train as well as the single pulse profile of the RPB, respectively, indicating the stable active Q-switching regime. The shortest pulse duration achieved is 281 ns, corresponding to the highest pulse energy and peak power of 2.55 µJ and 9.07W, respectively. Due to the spectral resolution limit of the employed OSA (Thorlabs, OSA 207C, resolution is ∼ 0.1 nm), the spectral bandwidth of RPB seems to be larger than that of APB. We measure the pulse durations of these two types of CVBs (i.e., 284 ns for the APB and 281 ns for the RPB). The peak power of APB is calculated to be 9.05 W under the same condition. Therefore, the output lasing characteristics are not significantly influenced when the operation is transitioned from the RPB regime to the APB one.
Fig. 4. (a) Output powers of the RPB versus the incident pump power for different pulse repetition rates; (b) Pulse duration of the RPB versus the incident pump power for different pulse repetition rates; (c) The Q-switching pulse train of the RPB; (d) The single pulse profile of the RPB. RR: repetition rate. FWHM: full width at half maximum.
The intensity profiles of the active Q-switching CVBs are shown in Figs. 5(a) and (c). Figures 5(b) and (d) are the output spectrum of the two different operation regimes. The center wavelengths of the RPB and the APB are almost the same of 2793.91 nm. We notice that the output spectral widths in the active Q-switching regime are larger than that in the CW operation regime as shown in Figs. 3(b) and (d), which is attributed to the higher nonlinearity induced spectral broadening resulting from higher peak powers in the active Q-switching operation regime.
Fig. 5. (a) Intensity profiles of the actively Q-switched RPB; (a1)–(a4) Intensity profiles of the active Q-switching RPB after passing through a polarizer with different orientations; (b) Output spectrum of the actively Q-switched RPB; (c) Intensity profiles of the actively Q-switched APB; (c1)–(c4) Intensity profiles of the active Q-switching APB after passing through a polarizer with different orientations; (d) Output spectrum of the actively Q-switched APB; The white arrows represent the orientation of the polarizer;
2.5 Results in the passive Q-switching operation regime
The experimental setup of the passively Q-switched Er:ZBLAN fiber laser with switchable CVBs emissions is displayed in Fig. 1(b). The plano-convex lens L3 is replaced with another lens possessing a larger focus length of 12.7 mm, which facilitates the CVBs mode conversion with a larger collimated incident beam onto the S-waveplate. OCM1 is replaced with a SESAM (BATOP, SAM-2800-34-10ps), which is acting as the passive Q-switch. Besides, a plano-convex lens L4 (f = 12.7 mm) is inserted to obtain an appropriate focusing beam onto the SESAM. A 45° output coupling mirror with a transmission of 65% at 2800 nm lasing wavelength is employed to extract the laser output.
By utilizing a two-dimensional translation stage to adjust the position of L4 precisely, we can obtain the passively Q-switched CVBs output. The average output power of the Q-switching RPB increases linearly from 27.3 mW to 86.3 mW when the incident pump power is increasing from 1.08 W to 3.12 W, corresponding to a slope efficiency of 2.89%. By rotating the HWP and slightly modifying the tilt angle of SESAM to change the linear polarization to its orthogonal direction, the output between APB and RPB can be switched easily. The lasing characteristics between the two different vector modes are almost identical in terms of the average output power and output spectrum at various pump powers.
Figure 6(a) illustrates the measured repetition rate and pulse duration of the RPB as a function of the incident pump power. When the incident pump power varies from 1.08 W to 3.12 W, the repetition rate increases almost linearly from 41.31 kHz to 75.3 kHz while the pulse duration decreases from 1.978 µs to 968 ns. The highest pulse energy is calculated to be 1.14 µJ. The mode transition from the RPB to the APB does not change the pulse characteristics too much. Figures 6(b) and (c) exhibit the intensity profiles of the passive Q-switching RPB and APB at the incident pump power of 3.12 W, respectively.
Fig. 6. (a) Measured repetition rate and pulse duration of the passively Q-switched RPB as a function of incident pump power; (b) Intensity profiles of the passively Q-switched RPB; (b1)- (b4) Intensity profiles of the passively Q-switched RPB after passing through a polarizer with different orientations; (c) Intensity profiles of the passively Q-switched APB; (c1)- (c4) Intensity profiles of the passively Q-switched APB after passing through a polarizer with different orientations; The white arrows represent the orientation of the polarizer.
3. Discussion
We demonstrate the CVBs output in both the CW and the Q-switching (including the active one and the passive one) operation regimes from a mid-infrared Er:ZBLAN fiber laser at 2.8 µm. To our knowledge, this is the first report of generating radially and azimuthally polarized beam from the 2.8 µm fiber laser. Discussions will be given concerning the further improvement of the CVBs lasing performance.
In the passive Q-switching operation regime, the fiber laser based on the SESAM can only produce CVBs with a shortest pulse duration of 968 ns. The pulse duration can be further shortened by optimizing the gain fiber length [38] and simply increasing the incident pump power. Other high-performance passive Q-switching components with a higher modulation depth can be applied in future experiment to improve the pulse characteristics, such as some emerging two-dimensional materials based saturable absorbers [39–41]. In the active Q-switching operation regime based on an AOM, pulsed CVBs as short as 281 ns are achieved, which still has room to improve in contrast to our previous results [42] and may be attributed to the unoptimized fiber length. Typically, Q-switched fiber lasers, which are able to produce very short pulses, generally employ short gain fibers with high doping concentrations to further shorten the cavity length [43–45].
In addition, further scaling up of the laser output power and efficiency can be realized by splicing two protective end caps to both facets of the gain fiber, which contributes to the mitigation of the accumulated thermal load. With this laser configuration, more pump power can be launched into the cavity. Besides, the S-waveplate has a high damage threshold of 1 GW / cm2, which can withstand the high intracavity peak power and energy in the Q-switching and even mode-locking operation regimes. Ultrashort 2.8 µm CVBs with high peak power of megawatt or even gigawatt level modulated by the S-waveplate can be expected in future.
4. Conclusion
In this paper, we demonstrate the generation of both CW and Q-switching CVBs directly from an Er:ZBLAN fiber laser at 2793.91 nm. A customized S-waveplate is incorporated as the intracavity mode converter to generate mid-infrared CVBs. The laser output between the radially and azimuthally polarized beam can be freely switched by manipulating the cavity conditions. In the CW operation regime, the maximum output power is ∼ 250 mW. In the active Q-switching operation regime, pulsed CVBs as short as 281 ns are achieved. In the passive Q-switching operation regime based on the SESAM, the pulsed CVBs have a shortest pulse duration of 968 ns. The proposed mid-infrared cylindrical vector laser holds significant potential for applications in biomedicine, optical trapping, material processing and optical communication.
Funding
National Natural Science Foundation of China (123004478, 62105209); Guangdong Basic and Applied Basic Research Foundation (2020A1515110471, 2020A1515111143); Natural Science Foundation of Guangdong Province (2019A1515111060, 2021A1515011532, 2022A1515010326); Shenzhen Government's Plan of Science and Technology (JCYJ20200109105606426, JCYJ20190808145016980, JCYJ20220818100019040, RCYX20210609103157071); Major Key Project of PCL.
Disclosures
The authors declare there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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