An ytterbium-doped solid-core photonic bandgap fiber oscillator in an all-fiber format is investigated for high power at an extreme long wavelength. The photonic bandgap fiber is spliced with two fiber Bragg gratings to compose the cavity. The sharp-cut bandpass distributed filtering effect of the photonic bandgap fibers efficiently suppresses amplified spontaneous emission in the conventional high-gain region. Fine adjustment of the short cut-off wavelength by coiling with tighter diameter is performed to suppress parasitic lasing. A record output power of 53.6 W with a slope efficiency of 53% at 1178 nm was demonstrated.
© 2012 OSA
Yellow-orange (570-600 nm) light sources are useful for various medical applications of dermatology and ophthalmology, high-resolution spectroscopy, laser guide star (LGS) systems and so on. Unfortunately, no gain medium has sufficient gain directly at these wavelengths, except for dye molecules requiring highly intensive maintenance and sensitive alignment. The traditional solid state laser can produce yellow-orange light by nonlinear sum-frequency generation (SFG)  or an external Raman oscillator . Recently frequency doubling of fiber lasers in the long wavelength 1150-1200 nm regions is a new candidate for yellow-orange light sources. Fiber Raman lasers (FRLs) and bismuth-doped fiber lasers operating in the regions have been investigated. Recently an output of 60 W at 1178 nm has been demonstrated by cascaded coherent collinear combination of a triplet of split-seed non-PM fiber Raman amplifier [3,4]. Coherent combining can outperform the single FRA output which is limited  by the SBS effect. However, pulsed operation is difficult, since energy storage is not possible in the Raman process. Both CW and pulsed Bi-doped fiber laser have also been demonstrated [5–7]. All these configurations need core-pumping with 1µm high-brightness sources such as Yb fiber lasers.
Moreover ytterbium-doped fibers have also been investigated at long wavelength of 1150-1200 nm as the frequency doubling source of yellow-orange light [8–10]. The small-signal gain of Yb-doped fiber can be 0.8 dB/m at 1178 nm (with an Yb concentration of 5000 ppmwt) and the net small-signal gain of a typical double-clad Yb fiber can easily exceed 10 dB . However, the large gain band around 1030-1100 nm grows rapidly and strong amplified spontaneous emission (ASE) dominates and thus leads to parasitic lasing. This parasitic lasing has limited power scaling of Yb-doped fiber laser and amplifier [9,10]. In order to overcome the ASE and parasitic lasing, Yb-doped solid-core photonic bandgap fibers (PBGFs) are being extensively investigated.
In the solid-core PBGFs, high-index inclusions arranged periodically in the cladding create PBG in the low-index solid-core. PBGFs can have both short- and long-wavelength distributed filtering with very high-contrast, sharp-cut loss profiles, prompting the recent explorations of special wavelength sources. ASE suppression in the high-gain region in Yb-doped solid-core PBGFs has been reported [12,13]. We have been investigated Yb-PBGF amplifier at 1178 nm over these years [14–17]. 167 W output power with 61% slope efficiency by an Yb-PBGF amplifier at 1178 nm was achieved . 24.6 W single-frequency Yb-doped PBGF amplifier at 1178 nm with 320 kHz linewidth has been reported . Other microstructure PBGF lasers have also been researched [18–20] and a 10 W output linearly polarized Yb-PBGF oscillator at 1180 nm has also been reported . As well known, high power fiber amplifiers are relatively complex owing to the power amplifier chains and the placement of isolators between the stages. Therefore fiber oscillators directly generating the required output power with low cost and compact nature are attractive and significant for applications. It should be noted that high-power and high-efficiency operation of fiber oscillator at 1178 nm is really challenging. In order to suppress ASE and parasitic lasing, the oscillating threshold and hence the operating gain of 1178 nm should be lower . Therefore a high feedback resonator with narrowband and high reflection fiber Bragg gratings is more efficient for suppressing parasitic lasing and high power scaling other than a resonator with dichroic mirrors of broadband reflection in free space. However the technology of directly writing Bragg grating in PBGF is not grown-up yet [21,22], thus high splicing loss between large cladding diameter PBGF and traditional signal mode fiber is inevitable for the oscillating cavity. High splicing loss will not only lead to high cavity loss and high oscillating threshold, but also lower the parasitic lasing threshold because of scattering and return loss in the splice point. Parasitic lasing can be in a self-pulsing operation and damage the laser diodes (LD) and limit the output power. Therefore, to establish a resonator with low cavity loss to efficiently suppress the detrimental parasitic lasing is the key point.
This paper presents an all-fiber Yb-doped solid-core PBGF oscillator directly lasing at 1178 nm. As high as 53.6 W of output power and a slope efficiency of 53% are obtained. This is, to our knowledge, the highest output power directly oscillating from an Yb-doped fiber laser at the long wavelength edge of the gain band. Furthermore, ASE and parasitic lasing are suppressed by 40 dB.
2. Yb-doped photonic bandgap fiber
The microscope image of the PBGF fabricated for the research is shown in Fig. 1(a) and Fig. 1(b), similar to those reported in [11,16]. The Yb-doped core is surrounded by a PBG pump cladding structure with eight rings of high-index Ge-doped rods arranged in a triangular lattice with a pitch of 10.1 µm. Each Ge-rod has a graded index profile with a peak index contrast ∆n = 0.035 against the silica background. The two B-doped rods are introduced on either side of the core to create a birefringence on the order of 10−4. The low refractive index of the B-rods results in confinement by total internal reflection in the direction of the boron rods, while maintaining bandgap guidance in the orthogonal direction. The mode field diameter at 1178 nm is ~10.3 µm. The air-clad structure has a cladding diameter of ~220 µm and a numerical aperture of ~0.6. The pump absorption is 1.1 dB/m at 976 nm. A near field image at 1178 nm is shown in Fig. 1(c). The lack of anti-resonant tails of light in B-doped rods on either side of the core is the evidence of index guidance in the direction of the stress rods and bandgap guiding in the other four directions.
In the Yb-PBGF oscillators, the short wavelength cut-off edge is critically important to suppress ASE/parasitic lasing at short wavelength. Therefore fine adjustment of the short cut-off wavelength was performed by coiling the 43 m PBGF with different diameters. The white light transmission spectra of the PBGF coiled to 26 cm and 20 cm diameter are shown in Fig. 2(a) . The PBGFs were butt-coupled with mode field diameter (MFD)-matched single mode PCFs at both ends to probe only core-mode transmission . The short cut-off wavelength of 20 cm coil diameter is ~15 nm longer and sharper than that of 26 cm and thereby we can get the steeper loss edge. The loss was measured by cut-backing method. The bandgap center is 1220 nm and the propagation loss is ~0.03 dB/m. The loss profiles are compared with the calculated small-signal gain spectrum near the bandgap edge in Fig. 2(b). Though the loss of 20 cm coil diameter is 0.06 dB/m higher by smaller coiling diameter, the cut-off wavelength edge is longer and sharper and will suppress parasitic lasing more efficiently.
3. High power photonic bandgap fiber oscillator
The 1178 nm Yb-PBGF oscillator is composed of two fiber Bragg gratings (FBG) and a 43 m PBGF, as illustrated in Fig. 3 . The 43 m PBGF is coiled to a diameter of either 26 cm or 20 cm. The two FBGs are written in double cladding fiber (DCF, CorActive DCF 8/200) with a core diameter of 8 µm and cladding diameter of 200 µm. PBGF and DCF are spliced by a Furukawa S184PM-SLDF fusion splicer. The microscope image of the spliced fiber is shown in Fig. 4(a) and Fig. 4(b). The two fibers were manual aligned to get the core mode shown in Fig. 1(c) as perfect as possible. The splicing loss estimated from the fraction of the core mode is about ~2.22 dB. Both HR-FBG (R = 99.8%, ∆λ = 0.52 nm) and OC-FBG (R = 50.2%, ∆λ = 0.21 nm) are temperature-controlled by TEC in order to stabilize and match the Bragg wavelengths at 1178.0 nm precisely. The splicing point of the PBGF and the HR-FBG is also cooled by heat sink to remove the residual pump power. Both fiber ends are angle cleaved by 8° to minimize the feedback. The input fiber end is put in a V-groove holder with water cooling, as shown in the inset of Fig. 3. The oscillator is backward-pumped by a 200 µm fiber coupled 976 nm laser diode (LD, DILAS). The laser output is separated by a dichroic mirror (HT for 976 nm and HR for 1020-1300 nm).
The output power of the oscillator as a function of absorbed pump power is shown in Fig. 5(a) . The maximum output power of the PBGF with 26 cm coil diameter was limited to 39.2 W by parasitic lasing at 1140 nm, as shown in Fig. 6(a) . Since the total cavity loss of our 1178 nm oscillator is high due to the splicing loss between PBGF and DCF, the operation gain is high. As stated in , in such case parasitic lasing tends to occur at the wavelength of the net gain peak determined by the Yb gain profile and loss profile of a PBGF, as shown in Fig. 5(b). For comparative convenience, we present all the curves at 100 W of pump power. The typical parasitic lasing threshold is about 40-50 dB when both fiber ends are terminated without reflection. The net gain peak at 1140 nm with 100 W of pump power is as high as 59 dB, which is obviously above the parasitic lasing threshold. Thus both the position and steepness of the short wavelength edge of the bandgap are critically important for power scaling. As a note, we also encountered the same problem with an all-fiber pumping configuration with a pump combiner. After oscillating of 1178 nm laser, the LDs were sometimes damaged by the parasitic lasing at higher pump power. If pump protectors are inserted between the LDs and the resonator, the power scaling of this all-fiber system can also be expected. Here we choose end pumping in free space to avoid the LDs damage by the dichroic mirror.
However coiling the 43 m PBGF with 20 cm diameter can suppress parasitic lasing more efficiently by sharper bandgap edge and shorter cut-off wavelength (Fig. 2). The operating gain is much lower than that of 26 cm and the net gain peak is only 23 dB as shown in Fig. 5(b). We obtained output power of 53.6 W at the absorbed pump power of 107.8 W with a slope efficiency of 53%. The output spectrum at 53.6 W shown in Fig. 6(b) indicates ASE and parasitic lasing are efficiently suppressed. The intensity at the oscillating wavelength of 1178 nm is more than 40 dB higher than the ASE.
The maximum output power of 53.6 W was limited by coating damage in the pump end of the OC-FBG fiber. It is well known that the acrylate coating and low index polymer inner clad are not high temperature resistant. The uncoupled pump power of 40 W may be leaked into the coating layer at the input end when 53.6 W of output power and the interstitial air between V-grooves and fiber coatings may also cause not well thermal conductivity. Therefore the coating temperature will exceed the long term reliability temperature of 85 °C and results in the damage. Higher output power can be expected by precisely designing the fiber holder or filling it with thermal interface material such as thermal grease or phase change material .
We demonstrated an LD pumped, high power, high efficiency all-fiber Yb-PBGF laser directly oscillating at 1178 nm. The cavity was composed of two FBGs spliced with PBGF. 53.6 W output power and 53% slope efficiency were successfully achieved – a record output power for directly oscillating from Yb-doped fiber lasers and PBG fiber lasers at these wavelengths. The ASE and parasitic lasing are highly suppressed by more than 40 dB and further power scaling can be expected.
Since the FBG fibers used here are not polarization maintaining, the output is not linearly polarized. Further investigation for linear polarization by cross-splicing FBGs [9,24] or the use of a fiber polarizer  is on-going. It is also the next objective of our project to obtain a suitable frequency-doubling source for yellow-orange light. A compact fiber-based efficient and powerful laser will be preferred for clinical medical application. We believe the presented high power long-wavelength Yb-PBGF oscillator will be a promising source for yellow-orange light.
This research was partly supported by Grant-in-Aid for Scientific Research and the Photon Frontier Network Program of Ministry of Education, Culture, Sports, Science and Technology of Japan.
References and links
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