Gravitational wave detectors require linearly polarized single-frequency laser sources with a high fractional TEM00 mode content. We investigated the modal decomposition of a polarization maintaining photonic crystal fiber with a mode field diameter of 29 µm, operating in a single-frequency master-oscillator power-amplifier scheme, with respect to the TEMnm modes. Low degradation of the beam quality with increasing pump power could be observed, while a maximum power in the TEM00 mode of 203 W was achieved.
©2012 Optical Society of America
In the past years high-power single-frequency laser sources have found great interest in a variety of applications, such as Doppler-Lidar, coherent beam combining for power scaling and frequency conversion. Our primary area of interest is the use of such systems as laser sources in interferometric gravitational wave detectors (GWD). Currently, the lasers for the 2nd generation of Laser Interferometer Gravitational Wave Observatories (LIGO) are being installed. These laser systems consist of a solid-state high-power ring oscillator, which is injection-locked to an amplified nonplanar ring oscillator (NPRO) to achieve single-frequency operation. It delivers 220 W of output power at the wavelength of 1064 nm . After filtering the output beam with a non-confocal ring cavity, a pure TEM00 mode with an output power of 168 W was obtained. Still one major limitation of the interferometer sensitivity at high frequencies is shot noise, which decreases proportionally to the square root of the laser power. Therefore, the 3rd generation of these interferometric GWDs will most likely require single-frequency TEM00 laser sources at 1064 nm with output powers in the kW range . To achieve this power level while maintaining single-frequency operation, the master oscillator power amplifier (MOPA) scheme is a promising approach. As the interferometer requires a stable linear polarization state, the laser should have a constant high degree of polarization.
Fiber-based MOPA systems have made great progress in terms of output power over the past years. Double-clad fibers allowed the utilization of low-brightness pump sources, which are capable of delivering the necessary pump power to amplify signals up to several 100 W. In the 1 µm wavelength range, Yb-doped fiber amplifier systems deliver CW output power levels of several kW . However, when amplifying kHz-linewidth signals, one of the major obstacles is the onset of stimulated Brillouin scattering (SBS).
To increase the threshold of this nonlinear power-limiting effect, different approaches have been proposed. Especially in the field of fiber development a lot of progress has been made over the past years. The current power records for different polarization maintaining fiber designs employed in single-frequency master oscillator power amplifier systems are 402 W for a step-index large mode area (LMA) PM fiber , 511 W for a chirally coupled core (CCC) fiber , and 494 W for an acoustically segmented photonic crystal fiber (PCF) . All these fiber designs were customized, and are not yet commercially available. They were primarily tested for their SBS threshold, while the beam quality has been investigated in terms of M2 measurements. Even though the output beams were found to be near diffraction limited, with M2 values between 1.1  and 1.3 , a conclusion about the TEM00-mode content cannot be drawn. In fact, M2 values of <1.1 still allow for the fraction of higher order mode beam content to be as high as 30% .
To date, there have been two high power 1 µm master oscillator fiber amplifier systems, for which the fractional TEM00-mode content has been measured [8, 9]. The amplifier fiber in Ref . was a non-polarization maintaining PCF with a MFD of 22 µm. This system consisted of only a single amplification stage, and was pump- as well as seed-power limited. At a maximum output power of 148 W, the fundamental mode content was measured to be 92.6%. The second amplifier system  employed a polarization maintaining standard step index fiber with a MFD of approximately 21 µm by Nufern. The fundamental mode content was found to be power independent and around 95%, but the maximum output power was limited by the onset of SBS to 100 W. Discussing the suitability of Yb-doped single-frequency fiber MOPAs as laser sources for the 3rd generation of GWDs, the modal decomposition with respect to the free-space TEMnm modes of amplifier configurations, capable of further power scaling, needs to be investigated.
2. Experimental setup
We chose a polarization maintaining PCF (DC-400-40-PZ-Yb) by NKT photonics, containing a 40 µm Yb-doped silica core with a NA of 0.03 for our experiments. The resulting mode field diameter (MFD) of 29 µm of this fiber is the largest commercially available one. It was specified as operating transversely single-mode at a signal wavelength of 1060 nm. The pump-cladding diameter and NA were 400 µm and 0.46, respectively, leading to a cladding absorption of 2.4 dB/m at 976 nm. Furthermore, the fiber had an airclad and an outer silica glass cladding with a diameter of 700 µm, which was finally coated with a high temperature acrylate. Two boron stress rods induced high birefringence to ensure linear-polarization operation, if the seed signal polarization was launched in the slow fiber axis.
A fiber length of 6.8 m was used in the experiments. It was partly coiled on a 40 cm diameter metal spool, but not actively cooled. Only the pump end of the fiber was placed in a water-cooled copper V-groove. The fiber ends were angle-polished, after the airholes had been collapsed to seal the end facets.
The setup is depicted in Fig. 1 . A nonplanar ring oscillator (NPRO) delivered the low-noise ~1 kHz linewidth seed signal with an output power of 500 mW at 1064 nm. This seed signal was preamplified in a step-index PM fiber with a 10 µm core (PLMA-YDF-10/125 by Nufern), which was chosen to assure single-mode seeding of the main-amplifier. For improved long term reliability, an all-fiber set-up was chosen for the pre-amplification stage. It was pumped in a co-propagating scheme to protect the pump diodes. The maximum available power in front of the main amplifier was about 8 W. Two 30 dB isolators protected the preamplifier from backreflections of the main amplifier. A partial reflector was used to sample the backscattered light from the PCF amplifier stage, while the transmitted pump light was separated from the signal beam by a dichroic mirror (DCM). The seed signal was mode-matched to the fundamental mode of the main amplifier fiber by a spherical lens with f = 100 mm and an axial gradient glass lens with f = 10 mm.
A counter-propagating pump scheme was used for the PCF amplifier stage. The pump light from a temperature stabilized laser diode module emitting at 976 nm was delivered by a multimode fiber with a fiber diameter of 600 µm and a NA 0.22. It was collimated by a commercial water-cooled high-power collimator, and subsequently launched into the fiber through an f = 100 mm spherical lens and an axial gradient glass lens with f = 15 mm. A pump coupling efficiency of approximately 85% could be achieved. After separation from the pump light by a dichroic mirror, the amplified signal passed through a silica plate, which sampled part of the beam for diagnostic purposes.
3. Experimental results
3.1 General amplifier characterization
The amplifier output power versus the absorbed pump power is plotted in Fig. 2(a) . The differential optical-to-optical efficiency with respect to the absorbed pump power was 80%. At an absorbed pump power of 363 W and a corresponding output power of 294 W parasitic laser processes occurred, which limited the power scaling of the amplifier. The parasitic laser processes became manifested in additional laser spikes in the forward optical spectrum. Possible sources of a parasitic resonator are backreflections from optical components, even though the amplifier gain was only about 16 dB. In addition, the lasing could have occurred on a higher order mode due to transverse spectral hole burning . In this case, the mode would have experienced a significantly higher gain. No further amplifier characterization at this power level was done to prevent the amplifier fiber and other optical components from being damaged. However, the ASE suppression was >50 dB and therefore still excellent at an output power of 246 W, as can be seen in optical output spectrum (Fig. 2(b)).
To verify that the amplifier was operating below the SBS threshold, the forward relative intensity noise and high resolution backward optical spectra were monitored. No indication of stimulated Brillouin scattering could be observed at the highest power level of 294 W. The polarization extinction ratio (PER) of the system was 23 dB at an output power of 11 W. At 246 W, even a PER of 27 dB could be achieved.
Increasing the seed power should allow for further power scaling, which would require a modified preamplifier allowing for a higher output power. However, the aim of this experiment was to identify the limit of this simple two-stage setup with a truly single-mode all-fiber seed for the higher power final amplifier stage and to measure the fractional TEM00-mode content of the beam at the maximum achieved output power.
3.2 Mode content measurements
To evaluate the beam quality in terms of its modal decomposition, we used a three mirror scanning ring cavity in a non-confocal setup . Consequently, its different transverse eigenmodes, which are by design the free-space TEMnm modes, have different eigenfrequencies. Furthermore, the cavity had a free spectral range (FSR) of 715 MHz, and the finesse was about 200 at 1064 nm. If a single-frequency signal as in our experiments is analyzed, only one distinct transverse mode can be transmitted through the cavity, while all other modes are reflected. By injecting a ramp signal to the piezoelectric transducer attached to one of the cavity mirrors, the cavity length, and therefore the transmitted transverse mode can be selected. A detailed description of the measurement technique and the associated analyzing algorithm can be found in Ref .
The transmission signal of a complete sweep through one FSR of the mode scanning cavity at 246 W of output power is presented in Fig. 3 . At this power level a TEM00 mode content of 91.2%, determined by comparing a complete modal fit (dotted line) of the measured signal (solid line) to a perfect TEM00 fit (dashed line), was achieved. Each distinctive transverse mode is represented by a peak in the obtained mode-scan. For the beam quality not only the number of higher order modes, but also their magnitude has to be taken into account. Please note that the mode scan signal is presented on a logarithmic scale, and the mode-mismatched part of the input beam is decomposed into a set of cavity higher order modes. Therefore even in case of a predominant TEM00 mode content, the mode scan will show many small peaks due to the unavoidable residual mode mismatching between the analyzed beam and the TEM00 mode of the cavity.
Additionally to the power in the higher order modes, 9.5% of the power were lost due to cladding modes. This yields a fractional TEM00 mode power of 203 W in a linear polarization state. However, as this power loss is quite high compared to the seed power of the high power MOPA, it can be rather related to high bending losses than coupling the seed into the cladding. Improvement of the amplifier performance and a higher TEM00-mode power might be achieved by the utilization of a larger coiling diameter. On the other hand this might affect the higher order mode suppression.
Further mode scans were carried out at different power levels. The corresponding measured fractional TEM00 mode contents of the core light are also shown in Fig. 2(a) (blue triangles). At an amplifier output power of 33 W the higher order mode content was found to be already 7.2%. Using a 3 m long sample of a PCF with a slightly smaller core diameter (38 µm) and a different geometry of the stress rods, the higher order mode content of the output beam was less than 5% even at an amplifier output power of 121 W. Anyhow, with that fiber, increasing the output power led to the onset of mode-instabilities , so that the mode scanning experiments with that fiber sample were stopped at this power level. For the single-stage amplifier presented in Ref . the output beam contained only 2% higher order modes at 28 W of output power. Nevertheless, the beam quality decreased to a higher order mode content of 7.4% at the maximum output power of 148 W.
In general, the slight increase in the higher order mode content at higher power levels can be attributed to both gain effects and thermally induced changes in the mode-guiding properties [13, 14]. The different results regarding the modal decomposition of the three fiber amplifiers mentioned above are most probably due to a combination of various aspects. Fiber properties such as core size, geometry and stress rods, as well as gain factors and thermal gradients were different in the setups and all affect the modal composition of the output beam.
In conclusion, we have presented an analysis of the modal decomposition with respect to the TEM modes of an Yb-doped PCF with a MFD of 29 µm operating in a single-frequency MOPA configuration. A total TEM00 mode power of 203 W was measured, and a PER of up to 27 dB could be obtained. At an output power of 246 W, the ASE suppression was >50dB, while further power scaling was only limited by parasitic lasing processes. No evidence of SBS was recorded even at the highest power level. Thus, the tested fiber in the presented amplifier setup meets the power and beam quality requirements for the second generation of GWDs. For the application as a laser source in the 3rd generation of GWDs, a modified setup employing an intermediate amplifier stage that is capable of delivering a significantly higher seed power still needs to be investigated. However, the results regarding the degradation of the fractional TEM00 mode content with increasing amplifier output power indicate that a further TEM00 mode power scaling should be possible with this fiber.
This work was conducted in the framework of the Cluster of Excellence “Centre for Quantum-Engineering and Space-Time Research” (QUEST), funded by the German Research Foundation (DFG).
References and links
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