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Single-mode operation in a large-mode-area fiber laser is highly desired for power scaling. We have, for the first time, demonstrated a 50μm-core-diameter Yb-doped all-solid photonic bandgap fiber laser with a mode area over 4 times that of the previous demonstration. 75W output power has been generated with a diffraction-limited beam and an efficiency of 70% relative to the launched pump power. We have also experimentally confirmed that a robust single-mode regime exists near the high frequency edge of the bandgap. These fibers only guide light within the bandgap over a narrow spectral range, which is essential for lasing far from the gain peak and suppression of stimulated Raman scattering. This work demonstrates the strong potential for mode area scaling of in single-mode all-solid photonic bandgap fibers.
© 2014 Optical Society of America
1. Introduction
Ytterbium-doped fiber lasers have exhibited excellent efficiency and low thermal loading due to the small quantum defect of ytterbium [1,2]. In addition, high-power pumping sources can be easily obtained from commercial diode lasers at wavelength of ~910 and ~975nm. Yb-doped CW lasers and pulsed lasers have been widely used in many applications. The emission wavelength of ~1μm can also be frequency-doubled to provide high powers in the blue to red wavelengths [3].
For high-power fiber lasers, detrimental nonlinear effects such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and four-wave mixing (FWM) can occur due to the high optical intensity in the core. Mode area scaling is the most effective way to prevent optical nonlinear effects from impairing power scaling. By enlarging the core size, it is also beneficial in that it increases pump absorption and consequently shortens fiber length [4]. However, multimode operation comes into play when enlarging the mode area. Several approaches have been explored to suppress the higher-order-modes (HOMs) using specialty fibers including photonic crystal fibers (PCF), leakage channel fibers (LCF) and photonic bandgap fibers (PBF).
A typical PCF consists of a solid silica-based central core and a lattice of periodically arranged air-hole cladding [5]. The lower numerical aperture (NA) significantly reduces the number of modes supported. The first photonic crystal fiber was proposed with a very small core size, which later inspired the development of large-mode-area photonic crystal fiber lasers [6–8]. Endlessly-single-mode operation can be achieved by appropriately tailoring the air holes in the cladding. The high bending sensitivity due to the very low NA at large core sizes, however, makes a PCF only feasible to be used as a short stiff rod beyond 40μm core diameter [9], rendering it unsuitable for high average power lasers where long fiber length is necessary for thermal management. More efficient HOM suppression in rod-type PCF can be realized by incorporating high-index nodes among air-hole cladding so that HOM can be out-coupled to the cladding modes while fundamental mode is still guided based on PCF structure [10]. LCF takes a different avenue to address the single-mode requirement. In this case, the discontinuity of the core-cladding boundary allows the fiber to be engineered to have high differential mode losses so that the fiber can effectively maintain strong fundamental mode (FM) propagation with negligible HOM transmission [11,12]. The resonant effect and boundary shape can also be exploited to further improve the HOM suppression [13]. The inherent nature of leaky modes enables core diameters to be scaled up with ease. Robust single-mode operation with a flat-top mode in a Yb-doped LCF with ~50μm core was reported recently [14]. The effective mode area was increased by 50% in a straight configuration, owing to the flat-top mode arising from by a slightly lower refractive index in the core center.
The all-solid PBFs have been viewed as a promising candidate for realizing high power fiber lasers and amplifiers [15,16]. They usually have periodically arranged high-index rods embedded in the host materials forming the cladding. The core is created by omitting several high-index rods in the center. The PBF guides light based on the bandgap effect, meaning a mode is supported in the core only when it falls within the photonic bandgap of the cladding lattice. The light guidance only exists within a narrow spectral range, which can be used for lasing off the gain peak [16] and SRS suppression [17]. A PBF can be designed to provide significant differential mode losses for FM and HOMs so that single-mode operation is possible for large core diameters. Theoretical and experimental studies have contributed to a large effective mode area of up to 900μm2 [18–20]. Recently, an innovative PBF design with so-called hetero-structured claddings was also investigated for further enhancing HOM suppression by resonantly out-coupling HOMs into the cladding defect intentionally made [21].
In this work, we have demonstrated an Yb-doped all-solid photonic bandgap fiber laser with a core diameter of ~50μm. The calculated effective mode area is ~1450μm2 in a straight fiber, which is over a factor of 4 increase over that previously demonstrated in a ytterbium-doped all-solid PBF [16]. We have also tested the laser performance and beam quality. We measured 84% and 70% slope efficiencies relative to the absorbed and launched pump power respectively. Strong single-mode propagation was observed, with a measured M2 value less than 1.2. We have also experimentally confirmed the predicted robust single-mode regime near the high frequency edge of the bandgap.
2. Fiber parameters and simulations
One passive PBF and two active PBFs with Yb-doped cores were fabricated using the standard stack and draw technique for this work. Both active fibers were drawn from the same preform while the second active PBF (Active2) was ~4% larger than the first active PBF (Active1) in dimensions. This small increment was calculated to allow true single-mode operation at the lasing wavelength. A more detailed explanation will be given in Section 3. For all three fibers, the nodes in the cladding were made of germanium-doped silica with graded index profile which has a peak value of 1.48. Both active PBFs were coated with low refractive index polymer coating, providing a numerical aperture (NA) of 0.46 for the guidance of pump light. The pump absorption of the active PBF was estimated to be ~1dB/m at 976nm. The cross-section of the Active1 PBF and the dimensions of three fibers are shown in Fig. 1 and Table 1, respectively.
Table 1. Dimensions of Fabricated PBF
The core of the active PBFs consists of 7 Yb-doped rods in the center. Ideally, the refractive index of the active rods should be matched to that of silica background. However, this is very hard to satisfy and our active rods have a small index depression relative to silica [22]. In order to determine the index depression Δn which is defined as the difference of refractive index between the background glass and the active rods, the fiber was cladding pumped well below the lasing threshold and the mode pattern was measured using amplified spontaneous emission (ASE). This was then compared to the simulation, as done in [14]. The simulation studied mode profiles at different index depression Δn, ranging from 0.5 × 10−4 to 3 × 10−4 with increment of 0.25 × 10−4. The mode patterns and the intensity distributions across the white axis are presented in Fig. 2. A shallow dip at the center of the core was observed during the experiment due to the index depression. The simulation showed this phenomenon clearly. It is estimated that the measurement best matches simulation result when Δn equals 2.25 × 10−4.
Fig. 2 (a) Top: Experimental results of mode profile and intensity distribution using ASE; (b) Below: Simulation of mode profile at Δn = 2.25 × 10−4 and intensity distributions at various ∆n with 0.25x10−4 increments.
The depressed refractive index of the Yb-doped rods would directly affect the effective mode area of the non-Gaussian-like mode. Figure 3(a) shows the effective mode area with respect to the index depression in a straight fiber. As the difference in refractive index increases, the FM becomes flatter, resulting in a larger effective mode area. At an index depression of 2.25 × 10−4, the effective mode area reaches ~1450μm2. . On the other hand, the effective mode area at various bending radii when Δn is fixed at 2.25 × 10−4 is plotted in Fig. 3(b). At a bending radius of 0.25m, which is the designed coil configuration, the effective mode area is estimated to be ~1020μm2.
Fig. 3 (a) Simulated effective area versus index depression Δn in a straight fiber; (b) Simulated effective area versus bending radius with Δn = 2.25 × 10−4.
3. Slope efficiencies and beam quality
The two active PBFs were subjected to extensive study to determine the power scalability and robustness of single mode operation. In all the characterizations, the fiber had both ends perpendicularly cleaved to form the cavity and was laid in an aluminum groove with a 50cm diameter coil in accordance to the initial design. The metal plate also served to dissipate heat from outside coating. The fiber was cladding pumped by a commercial laser diode emitting at ~976nm (LIMO200-F200-DL980) through a dichroic mirror. The output power at 1030nm and the residual pump light were measured at the other end. The slope efficiency with respect to the absorbed pump power and lasing threshold as a function of bending diameter using 6m Active1 is shown in Fig. 4(a). The slope efficiency remained above 80% with the threshold below ~6W when bending diameter was kept at and above 50cm. It can be seen that from a bending diameter of 60cm to 50cm, the slope efficiency only dropped 3%, but the mode quality is expected to benefit from a tighter coil size. The optimal coil size was consequently determined to be ~50cm. The Active1 was then repeatedly cut back from its original length. The slope efficiencies versus fiber length are shown in Fig. 4(b). The dashed blue line indicates the slope efficiency with respect to the launched pump power while the solid red line indicates the slope efficiency with respect to the absorbed pump power. Both efficiencies increased as the fiber length was shortened until they reached maximal values of 72% and 83% at the optimal fiber length of 5.2m.
Fig. 4 (a) Efficiency relative to the absorbed pump as a function of bending diameter. (b) Measured optimal fiber length to achieve maximal efficiency.
However, a closer look shows that Active1 did not provide single-mode operation very well at 50cm coil diameter. This was attributed to the lasing wavelength being too close to the low frequency edge of the bandgap, where the fiber is multimode [23]. The Active2 PBF with 4% increase in dimension was drawn to aim at moving the lasing wavelength of ~1030nm closer to the high frequency edge of the bandgap, where robust single-mode operation is expected (see Fig. 6). Laser output from a section of 6m Active2 coiled at 50cm in diameter was characterized for beam quality. A CCD camera traced the output beam propagation over 15cm distance. Calculation of the beam size was based on the second-moment method, yielding M2 = 1.13 and M2 = 1.16 along the horizontal and vertical direction respectively. The fact that M2 is larger than 1 is attributed to the non-Gaussian-like beam shape. The promising result shown in Fig. 5(a) implies robust single mode operation. The expected rotation of the hexagonal mode shape from near field to far field can be clearly observed. The slope efficiencies of Active2 were measured subsequently. The slope efficiencies relative to the launched and absorbed pump power were measured to be 70% and 84% respectively, which is close to the maximum efficiencies of Active1. The highest output power achieved in this configuration was ~75W, limited by the pump power available.
Fig. 5 (a) Beam quality measurement of the output signal. Insets along the curve represent mode profiles at near-field, beam waist and other transition phases. (b) Measured slope efficiencies relative to the launched and absorbed pump power. The dotted line represents a linear fit while the solid circles and triangles represent measured values.
As mentioned before, by bringing the high-frequency bandgap edge closer to the laser wavelength, one can expect more robust single-mode operation. This can be illustrated by the inset in Fig. 6, as the frequency increases, the HOMs are cut off gradually at the high-frequency badgap edge and only FM is supported at the highest frequency [23]. To test the robustness of the single-mode operation, the light from a tunable laser was launched into a section of Active2 with only a single 50cm coil. The launch beam was first carefully aligned then intentionally moved by 6.25μm and 12.5μm in the transverse plane off the optimal launch condition so that HOM may be excited. Figure 6 shows the scaled transmission band obtained from the passive PBF and mode profile captured from Active2 at various wavelengths from 1025nm-1095nm. Since the index depression of the active fiber is very small, we expect its bandwidth to be only slightly narrower than that of the passive fiber while the overall bandgap structure remains the same. Near the short wavelength edge of the bandgap, i.e. at the high frequency edge of the bandgap, the fiber exhibited a clear robust single-mode regime in Active2. HOM was observed at non-optimal launch conditions above 1040nm.
Fig. 6 The transmission band measured from the passive PBF and the measured mode profile from Active2 at different wavelengths. Δx is distance of the launch offset. The inset illustrates modes supported within the bandgap of a PBF.
4. Conclusion
We have demonstrated an Yb-doped all-solid PBF laser with a core diameter of ~50μm. The effective area is ~1450μm2 in a straight fiber and ~1020μm2 when coiled at 50cm diameter. The large effective mode area will help to mitigate nonlinear effects at high powers. Active1, reached 83% and 72% slope efficiencies against the absorbed and launched power respectively. Active2 reached 84% and 70% slope efficiencies with respect to the absorbed and launched power respectively. We have also confirmed robust single-mode operation in Active2 with M2 less than 1.2 in both horizontal and vertical axes. We have also experimentally confirmed that the single-mode regime exists at shorter wavelengths close to the edge of transmission window and small fiber diameter adjustments can be used to fine tune the robustness of single mode operation, a feature unique to PBF. This work demonstrates that the significant power scalability and excellent beam quality is possible in all-solid PBF lasers.
Acknowledgments
This material is based upon work supported in part by the U. S. Army Research Laboratory and the U. S. Army Research Office under contract/grant number W911NF-10-1-0423 through a Joint Technology Office MRI program.
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