We reportthat the Tm:Ho-doped fiber can be utilized to improve the frequency stabilization of the Er-doped fiber comb. This rare-earth doped fiber provides photon absorption at 1.2 μm and 1.7 μm wavelengths together with emission at wavelengths between 1.8 μm to 2.1 μm. This unique combination of the absorption and emission regions constructively redistributes the spectral power of the supercontinuum generated by a highly nonlinear fiber to detect the carrier-envelope-offset frequency (fceo) via a self-referencing f-2f interferometer. As a result, the signal to noise (S/N) ratio of the detected fceo signal increases by 10 dB, thereby increasing the potential of enhancing the long-term frequency stability of the fiber frequency comb.
©2009 Optical Society of America
The advent of mode-locked femtosecond lasers has enabled optical frequency calibration with direct traceability to the microwave time standard [1,2]. This advance was first demonstrated with Ti:Sapphire crystal lasers, but afterward Er-doped fiber lasers began to draw attention because of their advantages of compact size, robustness to vibration, ease of optical pumping, and spectral extension to near infra-red light [3–5]. However, Er-doped fiber lasers are not yet comparable to crystal lasers in the achievable frequency stability and linewidth of the generated comb. This is attributed to several reasons, one of which is the phase noise encountered in the process of extracting the carrier-envelope-offset frequency (fceo). Nonetheless, the signal to noise (S/N) ratio of the detected signal of fceo should at least be 30 dB to realize a practically reliable fiber comb being faithfully locked to a radio-frequency atomic clock.
Detection of fceo relies on the heterodyne technique using f-2f interferometry . This self-referencing technique requires broadening the original spectrum of the Er-doped fiber laser to a supercontinuum capable of providing strong power concentrations at both the ends of its octave-spanning spectrum. This can be achieved by adopting a specially designed photonic crystal fiber or a highly nonlinear fiber [3–5]. However, some additive means may be more effective in obtaining the desired optimum power distribution to enhance the frequency stability characteristic even for a long time as first demonstrated by elaborating a Bragg-grating within a highly nonlinear fiber . In this investigation, we proposed and tested a more simple and general method to redistribute the spectral power of the Er-doped fiber laser by adopting the Tm:Ho-doped fiber. This method is intended to enhance the S/N ratio of the f ceo signal by making the most of inherent photon absorption and emission characteristics of the rare-earth doped fiber.
2. Common-path f-2f interferometer for fceo stabilization
Figure 1(a) shows the optical layout of our frequency comb system. It is comprised of an Er-doped fiber oscillator, an Er-doped fiber amplifier, a highly nonlinear fiber (HNLF), and an f-2f interferometer . The Er-doped fiber oscillator (C-Fiber, Menlosystems GmbH) is set to produce ultrashort pulses of 100 fs duration at a 100 MHz repetition rate with an average power of 20 mW. The Er-doped fiber amplifier raises the average pulse power to 230 mW. The amplified pulses are then coupled to the highly nonlinear fiber (HNLF, Menlosystems GmbH) to produce an octave-spanning supercontinuum over the spectrum from 1 μm to 2 μm wavelength. The f-2f interferometer configured here is of common-path type, producing the fceo signal by utilizing a periodically poled lithium niobate (PPLN, Crystal Technology) crystal for frequency doubling . The repetition rate (frep) is also monitored using a photodetector. Stabilization of the whole fiber comb system is completed by locking the detected signals of frep and fceo simultaneously to a Rb clock of time standard.
Dispersion control is important for effective operation of our fiber comb in two respects. First, the dispersion arising in the Er-doped fiber amplifier causes temporal pulse broadening with consequent reduction in the peak pulse power. This problem is readily overcome by installing a dispersion-compensation fiber to recover the pulse duration to 60 fs at the exit of the fiber amplifier. Second, a significant amount of dispersion occurs within the HNLF fiber during the process of supercontinuum generation, causing a time delay between 1 μm and 2 μm wavelengths due to group velocity difference [8,9]. When the time delay exceeds a few micrometers, no heterodyne interference signal is produced from the f-2f interferometer. To avoid the problem, a single-mode fiber is usually attached to the HNLF fiber to compensate for the dispersion [5,10].
3. Enhancement of fceo signal to noise ratio using Tm:Ho-doped fiber
Here, we replace the single-mode fiber with the rare-earth Tm:Ho-doped fiber (TH540, CorActive). Figure 2(a) shows the energy levels of the Tm3+ and Ho3+ ions embedded in silica . The energy diagram indicates that the Tm:Ho-doped fiber yields two absorption regions within the range of 1 μm to 2 μm wavelengths; one at 1.2 μm and the other at 1.7 μm. There is also an emission region found between 1.8 μm and 2.1 μm wavelengths. Figure 2(b) and 2(c) present the supercontinuum spectra generated with and without the Tm:Ho-doped fiber being added, respectively, which were readily monitored using an optical spectrum analyzer (MS9710B, Anritsu) up to 1.75 μm wavelength. Comparison of the two spectra clearly indicates two absorption valleys observed at 1.2 μm and 1.7 μm wavelengths.
The emission effect in the range of 2.0 μm wavelength is not directly monitored since it lies beyond the operating limit of our optical spectrum analyzer. An extended spectrometer system was therefore devised as illustrated in Fig. 1(b), in which the shorter wavelength region below 1.5 μm of the supercontinuum under measurement is first filtered out by a long pass filter (LPF). Then, the spectral portion around 2 μm wavelength is frequency-doubled using a PPLN (Part #97-02256-01, Crystal Technology) and its second harmonics are detected using a CCD-based near-IR spectrometer (AvaSpec-2048, Avantes). Test results of the emission effect are shown in Fig. 3 , which summarizes the frequency-doubled spectra obtained by Tm:Ho-doped fibers of different lengths in comparison with those of a conventional single-mode fiber (SMF-28, Corning). Substantial increase in the spectral power around 2 μm wavelength (before frequency doubling) is clearly observed particularly when the length of the Tm:Ho-doped fiber is less than 12 cm.
Finally, the fceo signal is extracted by the self-referencing f-2f interferometer as explained in Fig. 1(a). The S/N ratio is then evaluated by reading the peak amplitude of the fceo signal from the background noise level as shown in Fig. 4 . The S/N ratio is found sensitive to the actual length of the used fiber. When the single-mode fiber is used only to compensate for the f-2f time delay, the S/N ratio reaches only 32 dB. With the Tm:Ho-doped fiber being added to redistribute the spectral power, the S/N ratio can be enhanced by at least 3 dB and generally more than 10 dB for longer fiber lengths in comparison to the single-mode fiber. No noticeable degradation in the noise pedestal is observed due to the relatively low peak power causing insignificant nonlinear effects in the Tm:Ho-doped fiber. In consideration of both the requirements for time-delay compensation and spectral power redistribution, the optimum length of the Tm:Ho-doped fiber is found to be ~12 cm.
The Tm:Ho-doped fiber tested in this study is found to be able to enhance the S/N ratio of the detected fceo signal by redistributing the supercontinuum spectral power as well as by compensating for the group velocity difference between 1 μm and 2 μm wavelengths simultaneously. Compared with the single-mode fiber, the rare-earth doped fiber permits a 3 to 10 dB improvement in the achievable S/N ratio of fceo signal. This benefit is independent of the nonlinear fiber used to generate the supercontinuum, allowing the Er-doped fiber comb to be more robust and reliable for its industrial applications to frequency calibration, spectroscopy, optical clocks and length metrology.
This research was supported by the Creative Research Initiative program and the National Space Laboratory program funded by the Korea Science and Engineering Foundation.
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