We demonstrate high average power, high energy 1.55 μm ultra-short pulse (<1 ps) laser delivery using helium-filled and argon-filled large mode area hollow core photonic band-gap fibers and compare relevant performance parameters. The ultra-short pulse laser beam—with pulse energy higher than 7 μJ and pulse train average power larger than 0.7 W—is output from a 2 m long hollow core fiber with diffraction limited beam quality. We introduce a pulse tuning mechanism of argon-filled hollow core photonic band-gap fiber. We assess the damage threshold of the hollow core photonic band-gap fiber and propose methods to further increase pulse energy and average power handling.
©2011 Optical Society of America
Ultra-short pulse (USP), or ultrafast, lasers emit extremely brief pulses of light, generally with duration of about one picosecond (ps) or less. Owing to this temporal brevity (pulse width <1 ps) pulses with even modest energy (>1 μJ) will sustain peak power (>1 MW) high enough to induce nonlinear interactions with various materials. The nature of the interaction will depend, of course, upon the peak irradiance (peak power per unit area) and the beam propagation distance. The benefit and novelty of many of these interactions has created rapidly expanding demand for robust USP laser sources for research and industrial applications. In particular, the ability to photo-ionize and ablate materials with minimal heat deposit to the bulk substrate makes this form of light incredibly useful for micro-machining, surface marking and texturing, surgical procedures, and sensing applications. In contrast to longer pulse or continuous wave lasers where significant heat diffuses from the absorption volume, each ultra-short laser pulse can deposit its energy and initiate ablation in a time frame too short for appreciable thermal diffusion beyond the optical skin depth . When the USP laser beam is appropriately delivered to the target, the ablation event occurs without leaving behind any heat or collateral damage [2,3]. Despite the compelling properties of USP laser driven processes, few applications have made the jump from research lab to commercial development due to the cumbersome, expensive, and unreliable nature of the incumbent laser sources. New USP laser sources, beam delivery options, and applications know-how are needed to fully leverage the valuable capabilities of this form of light.
Fiber-optic USP lasers provide a highly robust and compact architecture in comparison to conventional bulk-optics based USP systems, and the pulse energy and average power from fiber USP lasers have recently reached levels to enable commercially relevant process speeds . Hence, fiber USP lasers are expected to fulfill the present laser source requirements and to scale in performance, form factor, and cost in order to keep pace with industry growth. For many high value applications—especially medical therapeutic treatments—it is critical to transport and deliver the ultra-short laser pulses using a one to five meter fiber-optic path. In the past few years, substantial progress has been achieved in USP laser beam delivery using optical fibers and other forms of waveguides [5–11]. The previously proposed candidates have comprised expanded area, solid core fibers as well as alternative format hollow core fibers. Each of these alternatives has several inherent limitations that negates consideration in the context of photonic signal fidelity and cost-efficiency necessary to enable meaningful application deployment.
Solid core fibers, such as graded index multimode fiber or high order mode fiber, have been experimentally proven capable of merely nanojoule pulse energy capacity in the ultra-short pulse regime. The salient feature is the small area (about 1000 μm2 or less) of the confined mode within the glass fiber core. The resultant high peak irradiance yields unrecoverable pulse distortion due to self-phase modulation (SPM) or catastrophic damage from dielectric breakdown. Attempts to further expand the mode area have yielded severe multiple mode cross-coupling and distortion, and extremely high loss as a result of weak mode confinement in low numerical aperture (NA) waveguides. Thus alternative optical fiber formats, such as hollow core waveguides, appear to be the most viable option for USP beam delivery. When the waveguide core comprises a gas at low to moderate pressure, the pulse train propagates with a much weaker nonlinear interaction. At atmospheric pressure, the nonlinear refractive indices (n2) of air, argon, and helium are respectively ~4 × 10−23m2/W , ~1 × 10−23m2/W  and ~4 × 10−25m2/W . These nonlinear refractive indices are three orders of magnitude weaker than a silica solid core with n2 ~2.4 × 10−20m2/W . Although some of these nonlinear refractive indices were measured at wavelengths other that 1.55 μm, and n2 has weak wavelength dependence from 800 nm to 2000 nm , the values cited here are used for the analysis through the remainder of this paper. Similarly, the ionization potential of candidate gas species, 24.59 eV for helium , and 15.76 eV for argon , is two to three times higher than in fused silica glass, 8.15 eV . We estimate the ionization potential of air is between 12.07 eV (O2)  and 15.58 eV (N2) . Owing to the higher ionization potential and lower nonlinear refractive indices of argon and helium compared with that of air, pulses delivered through helium- and argon-filled HC-PBGF will experience lower nonlinear effects and higher damage threshold.
Hollow core waveguides have included metallic inner-surface fibers (capillaries) as well as hollow core photonic band-gap fibers (HC-PBGF). Metal coated, hollow core fibers and capillaries have been utilized for infrared laser delivery where the guided mode diameter is fundamentally large, e.g. at the CO2 laser wavelength of 10 μm, or when multimode interference is not detrimental to the application. For near-infrared USP lasers neither of these conditions holds true. The requisite single mode propagation would require scaling the hollow core diameter down to a size that inhibits metal deposition on the inner surface. Near-infrared, single mode metallic hollow core fiber simply cannot be manufactured with current methods. Hollow core plastic one-dimensional (1D) Bragg fibers have shown their high potential for spatially and temporally distortion-free transmission of high peak power USP pulses. Nonetheless, due to fabrication complexities, it has been challenging for plastic Bragg fibers to achieve the low transmission loss domain of glass hollow core fibers. Therefore, a glass-based HC-PBGF is the best candidate for USP laser beam delivery.
HC-PBGFs have been studied recently for purpose of USP laser beam delivery and/or pulse compression. In these hollow core fibers, SPM and other deleterious effects can be reduced by more than 1000 times as compared to solid core fibers, and delivery of microjoule level USP laser pulses has been demonstrated . Table 1 summarizes the pulse energy and average power of USP laser beams delivered by HC-PBGFs and 1D Bragg fiber as described in the literature. Although more than 10 μJ of delivered pulse energy has been achieved for small core HC-PBGFs, the average power was in the milliwatt level—limited by the pulse repetition rate (1 kHz) of these laser systems [9–11]—which limits its industrial usage. For the 1D Bragg fiber, >0.25 W has been delivered. Due to its high transmission loss, however, it remains less desirable for USP laser beam delivery for practical usage.
The theoretical limiting factor in USP fiber delivery is the peak power of the pulses; however, average power also plays an important role in limiting the actual application of ultra short pulse fiber delivery. On one hand, the fiber input end may be heated, and eventually damaged, if the average power is too high while the input coupling is not optimized. In order to reach high average power handling capability, optimized beam quality of the input laser beam as well as input coupling mode matching, i.e. coupling efficiency, are critical factors. High input coupling efficiency remains a challenge for hollow core fibers especially with noncircular core [9,17,20,21]. On the other hand, if the delivery fiber has significant propagation loss, the scattered and radiated light along the fiber, especially when the fiber is bent, will heat up the fiber material, such as the protective coating polymer, and eventually cause failure. Study of average power scaling capability of HC-PBGF for USP laser beam delivery is necessary and essential for industrial applications. In this manuscript, we not only demonstrate high energy USP laser fiber delivery at the “eye-safer” 1.55 μm wavelength, but also high average power which meaningful and appropriate for industry-requirements-driven high speed micromachining applications.
In this paper, we demonstrate USP laser beam delivery using commercially available HC-PBGF with the largest effective mode area among its kind. More than 7 μJ pulse energy with average power of 0.7 W and peak irradiance of 5.3 TW/cm2 is delivered with diffraction limited beam quality and controllable pulse width by manipulating the partial pressure of noble gases in the fiber core. Compared with previous published results [17–19] using HC-PBGF for USP laser beam delivery, the large mode area helium-filled HC-PBGF delivers ultra-short pulses with >50 × average power which is significant for a variety of micromachining applications. The experimental set-up and test conditions are illustrated next in section 2, and then measurement data and analyses are provided in section 3. In addition to beam delivery demonstration data, we reveal nonlinear pulse compression effects and dependencies on pulse energy and gas pressure. Furthermore, we discuss the relevant fiber peak power induced damage threshold issues for this USP fiber delivery scenario.
2. Experimental Setup
The HC-PBGF employed here is manufactured by Crystal Fibre A/S (Model # HC19-1550-01). The hollow core is formed by removing the 19 inner-most hexagonal unit cells from the cladding. The advantages of the larger core fiber include lower loss, lower dispersion and higher breakdown threshold. The attenuation at the center operating wavelength of 1570 nm is < 0.02 dB/m with >80 nm transmission band. The hollow core diameter is 20 μm with NA = 0.13, the effective mode field diameter (MFD) is 13 μm, and the effective mode area (Aeff) is 133 μm2. Figure 1 shows the experimental setup we utilized for investigating the 2 m length of HC-PBGF. Through basic characterization of the fiber, we verified that the facets are readily prepared using a standard fiber cleaver, and the fiber is readily coiled to a diameter of 20 cm without noticeable excess bending loss.
The USP fiber laser source operates at 100 kHz repetition rate and can generate up to 50 μJ pulse energy (5 W average power) with pulse width less than 1 ps. A telescope expands the collimated beam; a half-wave plate (HWP) and a polarization beam splitter (PBS) attenuate the beam; and a triplet lens with a focal length of 30 mm focuses the collimated beam onto the entrance facet of the HC-PBGF. The collimated beam size can be tuned by telescope adjustment in order to maximize the coupling efficiency by matching beam waist to transverse mode profile. The output end of the HC-PBGF is connectorized using a standard FC fiber connector and is connected to a beam collimating assembly. The entire collimating assembly is sealed and a tube fitting is attached to the assembly in order to introduce noble gases into the HC-PBGF from the output end. The input end is open to the atmosphere, and the gas pressure is monitored by a pressure gauge connected to the tube fitting on the focusing assembly.
Positive gas pressure inside the HC-PBGF compared to ambient helps reduce the possibility of dust entering the hollow core and cladding holes which would dramatically increase the likelihood of damage to the fiber. In addition, the gas flow helps cool the fiber facet where the beam generates a modest heat load. An infrared thermal camera is used to monitor the temperature at the input fiber facet. Without inert gas injection, the facet temperature is 35°C with 0.5 W output power from the fiber. With inert gas injection, the input fiber facet temperature drops to ~30°C at 30 psi. A few centimeters of the coating material is removed from the input end of the fiber, and it is placed inside a V-groove mount with a small amount of thermal paste to increase the thermal conductivity. A power meter, USP laser intensity autocorrelator (AC), optical spectral analyzer (OSA), and a scanning slit beam profiler (Nanoscan) are consecutively positioned at the fiber output port to measure the beam characteristics. Throughout this investigation, the fiber input coupling efficiency is optimized at low pulse energy (~0.5 μJ), and the launched energy is slowly increased to minimize thermal shock which we observed to cause slight fiber movement that resulted in damage in some cases. This issue would be negated by rigid mounting of the fiber tip and a more aggressive thermal management design.
3. Results and Analysis
In order to demonstrate fiber delivery of an ablative USP laser beam, we introduce helium into the HC19-1550-01 fiber from the output end (gas flow is opposite direction to light propagation). The helium pressure at the collimator is held at 25 psi, and both the pulse quality and optical spectra are measured at the fiber output using the autocorrelator and OSA. Input beam-to-fiber coupling efficiency is 80%, and the fiber USP beam delivery assembly is stable with 0.7 W average power (7 μJ pulse energy) output from the HC19-1550-01 fiber. Both the near-field beam profile and far-field beam profile reveal a TEM00 Gaussian diffraction limited beam with M2 < 1.3. When additional beam power was injected (for fiber output >0.7 W), the fiber input facet was damaged. To our knowledge, this simultaneous output average power (0.7 W), pulse energy (7 μJ), pulse duration (~1 ps), peak irradiance (5.3 TW/cm2), and spatial beam quality (M2 < 1.3) is the most powerful output demonstrated to date from a HC-PBGF. This combination of performance parameters is essential for delivery fiber evaluation, and this performance benchmark will enable high processing speed and high feature quality for many micro-machining processes.
Figure 2 (A) shows the HC19-1550-01 intensity autocorrelation traces of the delivery fiber input and output pulse trains at the 7 μJ output point. The respective deconvolved pulse duration estimates for the input and output are 1.0 ps and 1.2 ps, hence minor pulse broadening during fiber propagation is indicated. Figure 2 (B) shows the corresponding optical spectra for the input and output pulse trains, and the dip in the middle of the output spectrum is consistent with modest SPM . Due to its low dispersion, <20 ps/nm/km measured with both argon and helium-filled core at 20 to 30 psi, the calculated dispersion length, LD, is larger than 40 m, much longer than the length of the HC-PBGF sample (2 m). On the other hand, the calculated nonlinear length, LNL, is about 0.3 m and 7 m, respectively, for argon-filled and helium-filled HC-PBGF. Therefore pulse evolution is governed by SPM that leads to spectral broadening of the pulse  for both the argon-filled and helium-filled HC-PBGF. As discussed in , for SPM governed pulse evolution assuming unchirped Gaussian-like pulses, the peak SPM nonlinear phase is given approximately by ϕmax ≈(M-1/2)π, where M is the number of peaks in the SPM-broadened spectrum. In order to estimate the peak nonlinear phase shift, we compared the depth of the notch that splits the output spectrum with standard model predictions and estimate the phase shift at 1.5π. Assuming the nonlinear refractive index of helium is five orders of magnitude lower than that of silica [18,23], we solve for the total effective nonlinearity using the relative irradiance introduced in  as η = Isilica/Icore, the ratio of the peak irradiance Isilica in the silica glass to that Icore in the helium-filled hollow core. Using n2 = 2.4 × 10−16 cm2/W as the nonlinear refractive index of silica glass, the calculated relative irradiance η is 4.3 × 10−4, about 2.7 × smaller than the seven cell HC-PBGF, HC7-1550-01 reported in . This confirms that with the increase in the hollow core size, the relative intensity inside the silica glass is reduced; therefore the total nonlinearity is reduced as well.
Figure 3A shows the HC19-1550-01 intensity autocorrelation traces of the input pulse and output pulses at 5 μJ (to avoid damage) with the same set-up as above, but with 25 psi argon injected. The respective deconvolved pulse duration estimates for the input and output are 1.0 ps and 0.63 ps, hence minor pulse compression during fiber propagation is indicated. This is consistent with the pulse compression mechanism for HC-PBGF found in . Figure 3 (B) shows the corresponding optical spectra for the input and output pulse trains, and the periodic modulation across the middle of the output spectrum is consistent with appreciable SPM . HC-PBGF provides the necessary anomalous dispersion for soliton compression , and SPM is apparent from the pulse temporal pedestal enhancement. As above, to estimate the peak nonlinear phase shift, we examine the depth of the modulation in the optical spectrum. Compared with a smooth input optical spectrum, the output optical spectrum exhibits clear splitting with 9 peaks, which is very close to the theoretical prediction for a peak nonlinear phase shift of 8.5π . Based on the peak nonlinear phase shift values here and above, we estimate that the effective nonlinear refractive index of helium inside the HC-PBGF is about 8 × smaller than for the argon inside the HC-PBGF at 1552.5 nm.
Figure 4 shows the pulse duration at the fiber output as a function of pulse energy when the HC-PBGF is filled with helium versus argon. For these data sets, the gas pressure is fixed at 25 psi. When the fiber is filled with argon, significant pulse compression is observed as predicted in , and the FWHM pulse width reaches 0.63 ps at 5 μJ output pulse energy. When the fiber is filled with helium, however, the FWHM of the output pulse exhibits pulse compression only initially—when the pulse energy is increased up to 3.5 μJ—and pulse broadening occurs when the pulse energy goes above this point due to the much smaller nonlinear refractive index of helium, hence less SPM versus the linear waveguide dispersion that must be in balance for soliton-like propagation. Ideal pulse compression is a balance of SPM, waveguide dispersion, and fiber length—all in light of spectral broadening and other changes to the laser signal .
In order to check the pulse duration dependence on the pressure of the noble gas, we tuned the argon pressure from 20 psi to 30 psi, and measured the FWHM pulse duration at 5 μJ, as shown in Fig. 5 . The pulse duration has a clear trend, with the FWHM decreasing from 0.67 ps to 0.605 ps when the argon pressure is increased from 20 psi to 30 psi. This data shows that controlling the noble gas pressure inside a hollow core fiber is a simple way of tuning the output pulse duration with a tuning slope of 6.5 fs/psi in this case. It is expected that similar tuning curves would be proven by varying the length of the fiber and/or expanding the range of gas pressures to achieve the sought after balance of linear and nonlinear dispersion as the laser spectrum expands due to SPM.
It is obvious that one way to raise the power and energy handling capability of the delivery fiber is to increase the core size of the HC-PBGF, hence the irradiance on the fiber facet structures is lower for a given laser signal amplitude. It is vital, nevertheless, to understand the fiber damage mechanisms by analyzing the breakdown threshold of the gas inside the fiber and the ablation threshold of the glass spans in the holey cladding structure. The gas breakdown threshold is proportional to the ionization potential energy, and based on , the ionization of helium, argon and air are in the range of 15 to 25 eV. As example, for ~1 ps duration pulses, the air breakdown optical irradiance is ~2 × 1013 W/cm2 , and the damage threshold of the fiber cladding structure should be similar to the damage threshold of bulk silica . The calculation for peak optical irradiance inside the HC-PBGF using the relative irradiance η (Fig. 6 ) shows that if the input coupled beam mode is perfectly matched with the transverse mode of the HC-PBGF, then increasing pulse energy should lead to the peak optical irradiance reaching the air breakdown threshold before reaching the cladding damage threshold. For the HC19-1550-01 fiber with a core MFD of 13 μm, the calculated air breakdown threshold is 26 μJ and the cladding damage threshold is 49 μJ. The experimental damage threshold of the HC19-1550-01 fiber is a little higher than 7 μJ, which is, of course, much lower than the predicted value of 26 μJ.
There are several factors that may contribute to dramatically reduce the damage threshold of the HC-PBGF: (1) laser beam quality deviations such as astigmatism and ellipticity at the focus; (2) the relative pointing instability between the input beam and the fiber input facet (angular, lateral and longitudinal displacement); and (3) the fiber facet cleaving quality, i.e. flatness and smoothness. Figure 7 (A) shows the near-field transmission optical microscopic image of the HC19-1550-01 right after a fresh cleave, and for comparison, Fig. 7 (B) shows the near-field transmission optical microscopic image of the HC19-1550-01 immediately following a damage event. First, the damaged area is not circular, which indicates either an elliptical beam at the fiber input facet or a relative beam shift at the focal plane. Second, the damaged area is not centered at the HC-PBGF facet, which is likely caused by either input beam pointing instability from the laser or movement of the fiber input facet due to heating. In order to further increase the pulse energy and average power handling (without damage) of the HC-PBGF fiber facet, a stable coupling design along with optimized input beam quality, a clean fiber cleaved facet, and heat-sinking method are required.
We have demonstrated high pulse energy and high average power (7 μJ and 0.7 W, respectively) 1.55 μm wavelength ultra-short pulse laser delivery using a large mode area, noble gas-filled HC-PBGF. To our knowledge, this is the highest pulse energy and average power ultra-short pulse laser beam combination ever to be delivered by a HC-PBGF. Helium versus argon gas-filled HC-PBGF ultra-short pulse delivery has been compared and a pulse tuning method using noble gas pressure control has been introduced. The damage threshold of the HC-PBGF has been analyzed, and issues relevant to further increasing the deliverable pulse energy and average power have been discussed.
This work was supported by the Naval Air Warfare Center under contract number N68335-09-C-0013.
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