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We demonstrate suppression of amplified spontaneous emission at the conventional ytterbium gain wavelengths around 1030 nm in a cladding-pumped polarization-maintaining ytterbium-doped all-solid photonic crystal fibre. The fibre works through combined index and bandgap guiding. Furthermore, we show that the peak of the amplified spontaneous emission can be shifted towards longer wavelengths by rescaling the fibre dimensions. Thereby one can obtain lasing or amplification at longer wavelengths (1100 nm–1200 nm) as the amount of amplification in the fibre is shown to scale with the power of the amplified spontaneous emission.
©2008 Optical Society of America
1. Introduction
In photonic bandgap (PBG) fibres light is confined within a low-index core by a microstructured cladding typically formed by silica and air. In this work an all-solid PBG fibre is used in which the PBG cladding is made by a triangular lattice of germanium-doped high-index micro-rods in a silica background, which raise the overall cladding index as in [1], while the core consists of a single ytterbium-doped micro-rod as gain medium. Combining the wavelength filtering effect of PBG confinement with an ytterbium-doped core results in efficient suppression of amplified spontaneous emission (ASE) at the conventional ytterbium gain wavelengths around 1030 nm and thus a reduction in parasitic lasing outside the bandgap, which typicially limits the operation of long wavelength fibre lasers and amplifiers [2]. Frequency doubled fibre lasers and amplifiers around 1180 nm [3,4] are of interest for yellow light generation in medical and astronomical application, while frequency quadrupling of fibre lasers can generate narrow linewidth UV light in the range 255 nm–295 nm for applications in atomic physics [5]. Previous work on the use of the photonic bandgap filtering effect for shorter wavelength laser operation at 907 nm and 980 nm is reported in [6,7].
In this paper, we demonstrate suppression of ASE for two different all-solid PBG fibres with different bandgap positions. We show that the ASE peak can be shifted towards longer wavelengths by rescaling the dimensions of the fibre and thereby moving the bandgap. It has been shown that the ASE peak of an all-solid PBG fibre can be shifted towards longer wavelengths by reducing the coiling diameter [8], however, this method works by and suffers from significant bend losses [9]. With the present approach, we show that tight coiling of the fibre is not necessary in order to shift towards longer wavelengths. Furthermore, we demonstrate that the amplification in the fibres scales with the ASE power for a given wavelength.
Fig. 1. (a) Microscope image of the fibre structure. The lighter regions are the germanium-doped rods constituting the pump-cladding, while the two darker regions are the boron rods. (b) Microscope image of the core region. (c) Microscope image of the airclad surrounding the pump-cladding structure.
2. Fibre properties
Two all-solid photonic PBG fibres were fabricated with a cladding pitch of 9.45 µm and 9.80 µm, respectively. The signal core consists of an ytterbium-doped rod, which is index matched to the silica background. The core is surrounded by the PBG pump-cladding structure made by an eight period triangular lattice structure of high-index germanium-doped rods. For the two fibres we measured the diameter of the cladding to 214 µm and 220 µm, respectively. Both fibres have pump absorption of 1.1 dB/m at 976 nm. Furthermore, the PBG pump cladding is surrounded by an airclad structure providing the pump guide with a large numerical aperture of 0.57, allowing for efficient high-power cladding-pumping [10,11].
The polarization-maintaining properties of the fibre are obtained by incorporating two low index boron-doped rods on either side of the core. The boron-doped rods act as stress applying parts, inducing high birefringence in the fibre while the lower index results in confinement by total internal reflection (TIR) in one direction and bandgap guiding in other directions [12]. The birefringence is on the order of 10-4.
Microscope images of the fibre structure, the core and the airclad structure surrounding the pump cladding are shown in Fig. 1. The lighter regions are the germanium-doped rods, while the two darker regions near the core are the boron rods.
Core properties of the fibre have been determined by performing measurements on a fibre which is identical to the fibre in question in terms of pump cladding and core properties, but without the surrounding airclad. Near-field measurements at 1150 nm yield a 1/e 2 mode field diameter of 10.0 µm in the axis parallel to the stress rods, a mode field diameter of 10.8 µm in the axis perpendicular to the stress rods and an average numerical aperture of 0.1. A near field image taken in the third order bandgap using 1150nm light is shown in Fig. 2. The lack of an anti-resonant tail of light in the boron-doped stress rods on either side of the core is evidence of index guiding in the direction of the stress rods and bandgap guiding in the other direction. A passive version of this type of hybrid TIR/bandgap fibre has been reported in [13].
Fig. 2. Near field CCD image taken in the third order bandgap using 1150 nm light, where the stress rods are in the horizontal direction. An anti-resonant tail of light is visible in the surrounding high-index germanium-doped rods, but not in the boron-doped stress rods.
Recently, ASE suppression in an ytterbium-doped all-solid photonic PBG fibre for long wavelength applications has been demonstrated [8]. The present fibre design is polarization-maintaining and holds potential for higher power due to the airclad surrounding the pump cladding.
3. Suppression of amplified spontaneous emission
The transmission spectrum of the fibres is measured using the setup in Fig. 3(a). In order to only transmit light in the core, light from a white light source is launched into 1 m of single-mode large mode area (LMA) fibre with a core size of 10 um, which is then butt-coupled to the 10 µm core of 30 m of the ytterbium-doped all-solid PBG fibre. The light from the core of the PBG fibre is subsequently collected by a high NA 10 µm core fibre in order to limit light collected by the optical spectrum analyzer (OSA) to the core region.
Fig. 3. (a) Experimental setup for measuring the bandgap transmission spectrum and (b) setup for measuring the ASE spectrum.
The ASE spectrum in the core is measured using the setup shown in Fig. 3(b). The 976 nm pump light is launched into the pump cladding of the PBG fibre using a multi-mode fibre with a core size of 100 µm and the output core light is collected by the same high NA fibre and detected by the optical spectrum analyzer.
Figure 4 shows the ASE spectrum (red) of a PBG fibre with a bandgap centered around 1140 nm (grey). Compared to the ASE spectrum of an index-guiding photonic crystal fibre (black), the PBG fibre shows significant suppression of ASE outside the bandgap.
Fig. 4. ASE spectrum of an ytterbium-doped all-solid PBG fibre (red) showing suppression of ASE outside the bandgap (grey) compared to the ASE spectrum of an index guiding photonic crystal fibre (black).
Figure 5 shows the ASE spectra of the two different ytterbium-doped all-solid PBG fibres, one designed with a bandgap centered at 1140 nm (red) and another designed with a bandgap centered at 1180 nm (black). Results show that the peak of the ASE spectrum can be shifted towards longer wavelengths by moving the bandgap position in the fibre. This is done by rescaling the fibre dimensions in the drawing process.
Fig. 5. ASE spectra of two ytterbium-doped all-solid PBG fibres with bandgap positions centered at 1140nm (red) and 1180nm (black). The peak of the ASE spectrum is shifted towards longer wavelengths by moving the bandgap.
4. Amplification and laser properties
The amplification properties of the fibres are measured using the forward seeded amplification setup depicted in Fig. 6. The PBG fibre is pumped with up to 3 W of 980 nm light and seeded with 2 mW of 1080 nm–1145 nm light from a laser cavity created from 30 m of the same PBG fibre and a laser dispersing prism. The seed laser cavity is tuned using a mirror. Furthermore, the pump light is launched at an angle in order to reduce guidance in the high-index rods and obtain maximum pump absorption.
Fig. 6. Amplification setup using forward seeding with a seed setup tunable in the range 1080 nm–1145 nm.
The amplification results for the two PBG fibres are presented in Fig. 7. For both fibres results show that the amplification for a given wavelength scales with the ASE power. For the specified pump power an amplification of up to 15dB is obtained in both fibres. Peak amplification occurs at the wavelengths that experience the least loss due to the bandgap combined with the highest gain due to their emission cross section. Increasing the pump power will shift the ASE peak, and hence also the peak amplification, towards the left bandgap edge.
Fig. 7. The amplification properties of the fibres (red dots) are seen to follow the ASE profiles (black).
Laser properties of the fibres are presented in Fig. 8. The fibre with the bandgap centered at 1140 nm is pumped with 1.1 W of 980 nm light while the fibre with the bandgap centered at 1180 nm is pumped with 2.5 W. Approximately 17 dB of pump light is absorbed at 980 nm while 2 dB of pump light is trapped in the high-index rods due to imperfect angled incoupling. The fibres are tunable as shown in Fig. 8(a) with optimum wavelengths at approximately 1100 nm and 1120 nm for the two fibres. These wavelengths lie at the top of the left bandgap edge. The bandgap widens as the cladding pitch is increased [14], which causes the left bandgap edge to shift less than the bandgap center and hence reduces the shift of the optimum lasing wavelength to 20 nm compared to the 40 nm shift of the bandgap center. Furthermore, in Fig. 8(b) the ASE suppression is apparent in the output power spectrum outside the bandgap, when the fibre with a bandgap centered at 1180 nm is lasing at exactly the edge of the bandgap. Comparing the level of ASE inside the bandgap to the level of ASE outside the bandgap, we measure an ASE suppression of 15dB at the bandgap edge.
Fig. 8. (a) Laser properties of the two fibres. (b) Power spectrum of the PBG fibre with a bandgap centered at 1180nm (red) lasing at the edge of the bandgap (grey). ASE suppression of 15 dB is apparent in the power spectrum outside the bandgap.
5. Conclusion
Experimental results show ASE suppression in two different ytterbium-doped all-solid PBG fibres and the ability to shift the ASE peak towards longer wavelengths by rescaling of the fibre design. In addition to the control of the ASE suppression, amplification measurements for the two fibres have shown that also the amplification properties can be controlled by design as the amplification is seen to scale with the ASE power. These features can be useful when designing fibres with potential for even longer wavelength lasing and amplification and can open new possibilities for high-power ytterbium-doped fibre lasers and amplifiers lasing at longer wavelengths above the conventional ytterbium gain wavelengths.
References and links
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