A continuous-wave all-polarization maintaining ytterbium-doped fiber master oscillator power amplifier, with a tuning range of 70nm addressable at tuning rates of up to 20nm/ms, is described. Up to 10W of linearly polarized output was generated with an amplified spontaneous emission content of less than 0.2% throughout the tuning range.
© 2011 OSA
Ytterbium (Yb) doped fibers are attractive media for tunable near-IR lasers and amplifiers, providing gain in the wavelength range ~1000-1150nm  and high-power tunable operation with diffraction-limited performance . These characteristics make them particularly suited to further nonlinear frequency conversion and incorporation in parametric sources with broad mid-IR coverage. For trace-level gas-phase spectroscopy of simple molecules, such sources require short-range, high resolution tuning to address individual species-specific rovibrational resonances. Coarse, wider-range tuning enables multi-species detection via access to many such resonances . By contrast, the spectra of solids, liquids and volatile organic compounds with more complex molecular structures , do not generally show clearly resolved individual resonances. In such cases, species must be identified by the location of broad spectral features within a wider overall envelope, frequently several hundred cm−1. Frequency-swept acquisition of such spectra requires mid-IR parametric sources capable of tuning over hundreds of cm−1 on millisecond timescales with moderate resolution (a few cm−1), placing similar requirements on the near-IR pump source, which must also provide the multi-watt output power required to pump, for example, a singly-resonant parametric oscillator well above threshold. Yb-doped fibers have been combined with acousto-optic tunable filters (AOTF) for rapid frequency-swept operation, both as lasers  and in a master oscillator power amplifier (MOPA) configuration with a semiconductor master oscillator . However, the demonstrated tuning ranges of 43nm and 36nm, respectively, only partially exploited the Yb gain bandwidth. While capable of wider tuning ranges , and offering rapid gain recovery with consequent resistance to dynamic perturbations associated with rapid tuning, semiconductor-based lasers appear unable to generate the power levels necessary to saturate multi-watt level Yb amplifiers across their full bandwidth. While fiber length allows some control of the gain profile , the tuning range in fiber-based gain media is typically limited by amplified spontaneous emission (ASE), which results in increasing spectral contamination at the extremes of the tuning range, ultimately saturating the gain more effectively than the laser wavelength. In grating-tuned systems, filtering approaches have lead to essentially ASE-free output at low to moderate power levels , with subsequent amplification yielding more than 8W over a 70nm range while retaining acceptable ASE levels . However, the manually-adjusted tuning used does not provide the rapid wavelength adjustment required for applications such as broad-band spectroscopy.
We describe here an all-fiber MOPA system optimized for rapid wide-range tuning. The master oscillator could be tuned over 1030-1110nm in 3.8ms with output powers of 0.225-0.435W essentially ASE-free. Further amplification yielded 8.3-10.0W output with ASE content <0.2% over the range 1035-1105nm. We believe this 70nm fast-tuning range, achieved by combining an acousto-optic element with ASE filtration techniques, to be at least twice that previously reported for a fast-tuning fiber-based source at these power levels.
2. Master oscillator
The master oscillator consisted of a 5m double-clad polarization-maintaining (PM) Yb-doped fiber (nLIGHT, Yb1200-10/125DC-PM) having a 10μm diameter active core and 125μm inner cladding with numerical apertures of 0.08 and 0.46, respectively, placed in a ring cavity, as shown in Fig. 1 . The fiber end facets were polished to an angle of 8° to prevent Fresnel backreflections. Pump radiation from a 976nm fiber-coupled diode laser (JENOPTIK, JOLD-30-FC-12) was coupled to the inner cladding counter to the lasing direction with an estimated efficiency of 80%. Use of PM fiber ensured a stable polarization state over wide tuning ranges with minimal sensitivity to thermal or mechanical perturbations . To minimize wavelength-dependent polarization changes in external components, the fiber polarization axes were carefully aligned to the s- and p-planes of the external optical system. In practice, the fiber ends were oriented with corresponding axes orthogonal; a horizontally polarized input to the fiber yielding a vertically polarized output. The half-wave plate labeled WP1 in Fig. 1, allowed control of the horizontally-polarized component, which exits the cavity directly at polarizing beam splitter PB1, relative to the vertically-polarized component which constitutes the laser feedback and is directed into the tuning section of the cavity via the Faraday rotator. Following further polarization control (WP3 removing the 45° rotation of the Faraday rotator), light in the feedback path was double-passed through the AOTF (Brimrose TEAF7-0.8-1.4-H, having a measured double pass bandwidth (FWHM) of 1.45nm at 1070nm) before returning via the Faraday rotator. A small wavelength-dependent deviation introduced by the AOTF was compensated using a BK7 prism . The net 90° polarization rotation due to the double pass of the Faraday rotator allowed the AOTF-filtered light to traverse PB1 and be fed back to the fiber via half-waveplate WP2 and polarizing beam splitter PB2.
This configuration, based on similar grating-tuned approaches , allows two modes of operation. When direct output from PB1 is desired, WP2 is set to maximize the transmitted component at PB2 and thus the level of feedback into the fiber. In this case, WP1 controls the degree of output coupling at PB1 and the output spectrum reflects that obtained directly from the fiber including any ASE background (direct output mode). Alternatively, WP1 can be set to direct all of the fiber output through the tuning section of the cavity with output coupling obtained by reflection at PB2 controlled using WP2. As the output is now extracted after spectral filtering by the AOTF it is essentially free of background ASE (ASE-free mode). Although less efficient, the ASE-free mode of operation is particularly attractive if wide tuning ranges are desired in a fiber MOPA system .
The master oscillator’s (MO) spectral properties were measured at a resolution of 0.5nm using an optical spectrum analyzer (OSA) and comparative output power and spectral characteristics were investigated in both direct and ASE-free output modes for a pump level of 3.4W. In both modes the relevant waveplate (WP1,2) was adjusted to output-couple around 77% of the incident power via the corresponding beamsplitter (PB1,2). This ratio yielded the maximum output power at 1110nm while being lower than optimum for the shorter wavelengths investigated. As shown in Fig. 2 , ASE-free operation allowed the ASE content, determined by integration across the spectrum, to be suppressed by more than 50dB throughout the entire 1030-1110nm tuning range, this level actually being an upper limit defined by the OSA noise floor. By contrast, for direct-output operation at wavelengths below 1040nm and above 1095nm, the ASE component exceeds 0.1% of the total output (Fig. 2(b)). While this ASE free performance comes at the expense of reduced output power, the power levels of between 0.225W and 0.435W obtained remain sufficient for the MO to saturate the subsequent amplifier stage throughout the tuning range.
To investigate rapidly-tuned operation of the master oscillator in ASE-free mode, the center of the AOTF passband was periodically swept between 1030 and 1110nm. The overall spectral coverage was recorded as a rolling average (Fig. 3(a) ), while real-time monitoring of the laser frequency variation was achieved by observing the reflected fringe pattern from an uncoated BK7 etalon with a free spectral range of ~533GHz (Fig. 3(b)). The continuous low-finesse fringe pattern seen in Fig. 3(b) indicates monotonic tuning behavior without significant discontinuities. The 37 fringes recorded indicate a total coverage of 20.88THz corresponding well with the 1030-1110nm (20.95THz) coverage observed in the OSA spectrum. Increasing the tuning rate beyond the 20nm/ms depicted in Fig. 3(b) caused strong spiking in the laser’s output at around 150kHz, corresponding to the relaxation oscillation frequency. The 20nm/ms rate at which this occurred was consistent across various sweep-frequency / tuning range combinations while strong spiking only occurred on the decreasing wavelength sweep. These observations lead us to attribute the spiking onset to residual wavelength-dependent variations in the intracavity losses driving the relaxation frequency of the laser as the wavelength was swept, and the directional dependence to the interaction of the sweep rate with the AOTF-induced round-trip frequency shift as described by Kodach et al. . While scan rates above 20nm/ms were not investigated in this work, AOTF-based active suppression of relaxation oscillations would offer a clear route to higher tuning rates . The output also exhibited varying levels of oscillation at the mode spacing of 38MHz and its harmonics indicating operation on several longitudinal modes. Detector bandwidths below this frequency range were used in the measurements shown here.
Examination of the output spectrum with the maximum OSA resolution at 1070nm of 0.024nm, revealed typical linewidths of around 0.1nm for fixed wavelength operation, as shown in Fig. 3(c). Fine tuning control adjustment revealed preferred operating wavelengths at 0.2nm intervals implying that while monotonic and lacking major discontinuities, as indicated in Fig. 3(b), tuning actually occurred in discrete steps attributable to weak etalon effects arising from residual reflectivity of intracavity components. To estimate the laser linewidth during swept operation, a 0.1nm resolution monochromator was set to a series of fixed wavelengths and its transmission recorded on a fast photodiode as the laser wavelength was swept at 20nm/ms. As seen in Fig. 3(d), calibration of the sweep rate against the set wavelengths allowed the instantaneous linewidth of the laser to be determined as typically 0.2nm while recording the centre wavelengths of measured transmission maxima overlapping at their half-maximum points (Taylor criterion) confirmed a spectral resolution of 0.2nm.
3. Power amplifier
The power amplifier (PA), shown in Fig. 4 , utilized 7m of PM double-clad Yb-doped fiber (n-LIGHT, Yb1200-20/400DC-PM) having core and inner cladding diameters of 20μm and 400μm with NAs of 0.07 and 0.46, respectively. Coiling the fiber with a 7cm radius ensured single transverse mode operation . An identical pump laser and similar coupling optics to those of the master oscillator were used, the larger fiber diameter allowing an estimated coupling efficiency exceeding 95%. The 7m fiber length, somewhat greater than required for optimal pump absorption, resulted in relative gain enhancement at longer wavelengths , and improved gain uniformity across the MO tuning range. As with the MO, use of a PM fiber with axes aligned to avoid polarization mixing of either the launched seed or the amplified output ensured polarization stability across the tuning range. Output from the MO was coupled into the PA via a single-stage optical isolator (Gsänger Optoelektronik GmbH, FR1060/5HP), with the standard thin-film polarizers replaced by broad-band components. A similar device isolated the amplifier from subsequent optical systems and, combined with a half-waveplate, allowed for a controlled output attenuation.
Investigations of grating-tuned fiber MOPA systems have highlighted the ability of an ASE-free MO to significantly improve ASE suppression in subsequent PA stages . This is of particular value if large tuning ranges, implying operation far from the wavelength of maximum amplifier gain, are desired. To investigate this behavior in our AOTF-tuned system, the PA performance for both modes of MO operation were compared for a PA pump power of 23W and MO seed power of 200mW. The resulting spectra for three representative laser wavelengths, along with data for amplified output power and ASE suppression across the entire tuning range are shown in Fig. 5 . It can be seen that the ASE-free seed significantly increases the useful tuning range of the MOPA. In direct-output mode, background ASE in the MO output competes with the laser wavelength for amplifier gain. ASE in the MO output coincides with wavelengths of maximum amplifier gain and extracts power more efficiently than laser wavelengths at the extremes of the tuning range (Fig. 5(a)). Seeding with the ASE-free MO output, the ASE in the amplifier output is limited to that contributed by the amplifier itself (Fig. 5(a)). This allowed ASE content to be held below 0.2% over the entire 1035-1105nm tuning range, along with 8.3 to 10.0W of polarized output power measured after FI2. By contrast, seeding in direct-output mode reduced the tuning range to 1045-1090nm if similar ASE levels were maintained (Fig. 5(b)). Outside this range the ASE content rose rapidly, the increased output power seen in Fig. 5(b) at the limits of the tuning range actually representing a dramatic increase in ASE content, rather than increased output at the laser wavelength.
The seed power required to efficiently saturate the amplifier was determined using the ASE-free output of the MO at the three representative wavelengths used in the measurements described above. The results are shown in Fig. 6 . Although only 10-20mW was sufficient to saturate the amplifier at 1070nm, 100-200mW were required to approach saturation at the limits of the tuning range. While the ASE-free output power of the MO is therefore far in excess of that required over much of its tuning range, this result emphasizes the necessity of an ASE-free seed able to generate elevated output powers, of the order of several hundred mW, if the effective tuning range of the subsequent PA is to be maximized. The results of Fig. 5 highlight the fact that in wide-tuning applications, such an MO should be able to generate sufficient ASE-free power levels directly, without the use of preamplifiers which will introduce their own ASE background .
In summary, we have described a fast-tuning (20 nm/ms) Yb-doped PM-fiber laser source yielding ASE-free output over a wide spectral range of 1030-1110nm. When amplified in a second Yb-doped PM-fiber, output powers between 8.3 and 10.0W with spectral purities exceeding 99.8% over the range of 1035-1105nm were obtained. The rapidly and widely tunable nature of the MOPA source renders it attractive for pumping wavelength-swept non-linear optical processes. We believe the 70nm rapid-tuning ranges of the MOPA to be the widest reported for a multi-watt acousto-optically-tuned Yb-doped fiber based system.
This work was carried out with the support of the Bristol Centre for Nanoscience and Quantum Information and funded by the UK Engineering and Physical Sciences Research Council. I. D. Lindsay acknowledges support from the Framework 7 program of the European Commission via a Marie Curie European Reintegration Grant.
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