We present the characterization of the carrier envelope offset frequency of 490 MHz femtosecond Yb-fiber ring laser. After amplification and compression, 1.7 W 90 fs pulses were produced for octave-span-spectrum generation from 600 nm to 1300 nm. More than 30 dB S/N ratio carrier envelope offset frequency signal was measured through a quasi-common-path interferometer.
©2012 Optical Society of America
Yb-doped fiber frequency combs have attracted much attention in recent years due to their high efficiency, high power and high repetition rate capabilities [1–7], of which, the large mode spacing frequency comb is particularly important in the application of the calibration of astronomical spectrographs.
An important step for making a frequency comb is to stabilize the carrier envelope offset frequency (fceo), where the linewidth of the fceo concerns the absolute stability of the frequency comb. The measurement and stabilizing the fceo require an octave-spanning spectrum and an f-to-2f interferometer. To lock the fceo it is prescribed to have strong enough ratio of signal to noise (S/N>30dB). The noise of optical frequency combs can be from two sources: the intra-cavity noise and the extra-cavity noise. The intra-cavity noise mainly comes from the cavity dispersion and pump noise, which can broaden the linewidth of fceo signal. It is demonstrated that the linewidth of the fceo is the narrowest in a net zero cavity dispersion . The extra-cavity noise mainly comes from the ASE (amplified spontaneous emission) of amplification and noise during octave-span-spectrum generation, which can raise the background of fceo signal . This noise can be suppressed by high-energy seed pulses before entering the amplifier. Both linear cavities and ring cavities can achieve high repetition rate [7, 9–13], and it’s even more difficult for ring cavity to reach high repetition rate since the WDM (wavelength-division multiplexer) needs to be placed inside the cavity. However, compared with the saturable absorber used in reference [7, 9], NPE (nonlinear polarization evolution) mode locking has higher output pulse energy and can generate shorter pulses, which corresponds to a less soliton breakup noise in octave-span-spectrum generation . Besides, with grating pair inside cavity, we can easily adjust the net cavity dispersion to minimize the fceo linewidth.
We have developed a 500-MHz repetition rate Yb-fiber ring laser for the frequency comb generation . In this work we demonstrated the S/N ratio of the fceo signal and characterized its influencing factors, which is to our knowledge the largest mode spacing frequency comb based on NPE mode locked fiber laser.
2. Experimental setup
The system configuration is shown in Fig. 1 . The key component to achieve 490 MHz is our specially designed semi-WDM [11–13], which greatly reduces the cavity length so that the laser can work at the high repetition rate. Although the solid-core photonic band gap fiber has been used to achieve 500 MHz repetition rate fiber laser in our group , the separation adjustable grating pair was employed in this work for investigating the linewidth of the fceo signal. The laser was pumped through the semi-WDM at up to 800 mw by polarization combining two single-mode-fiber-coupled diodes at 976 nm. Stable mode locking was enabled at pump powers above 700 mW. At full pump power we measured a pulse train of more than 250 mW average power. The pump efficiency is more than 30%. The output spectrum is shown in Fig. 2 . The output pulses were compressible to 35 fs FWHM (full width at half maximum).
The pulse-train was amplified by cladding pumped 4 m long Yb-doped double-clad fiber. The absorption of the double-clad fiber is 6.9 dB/m at 976 nm. The pulses were amplified without pre-chirping. SPM induced dispersion compensated part of the third order dispersion. The output average power of amplifier was 3.2 W with 12 W pump. The spectrum width was 20 nm as shown in Fig. 2(a). The amplified pulses were compressed by a 1250 lines/mm fused silica transmission grating pair with a separation of 11 mm. After compression 1.7 W de-chirped pulses were obtained. Figure 2(b) shows the measured interferometric autocorrelation and inferred intensity autocorrelation of the compressed pulses. If assume the pulse has Gaussian shape, the corresponding pulse width was 90 ± 3 fs FWHM.
In order to obtain the low noise octave-spanning spectrum at relatively low pulse energy, a piece of photonic crystal fiber (PCF NKT SC-5.0-1040) was tapered and its core diameter was reduced from 5 μm to 2 μm, and the tapered PCF was around 30 cm. The average power of 1000 mW was coupled into the PCF, corresponding to pulse energy 2 nJ and a peak power of 26 kW. The output spectrum from the PCF is shown in Fig. 3 . The spectrum has enough power at both 1264 nm and 632 nm so that they can offer enough signal to measure the fceo signal over the f-to-2f technique.
As shown in Fig. 1, a quasi-common-path f -to- 2f interferometer was adopted to suppress the acoustical noise and air streaks . The spatial separation of fundamental and harmonics was generated by a pair of SF10 glass Brewster prism. An optical delay line was used to compensate the group delay between second harmonic generation of 1264 nm and the short wavelength part of the pulse. A 5 × 7 × 0.5 mm fan-out PPLN was optimized for efficient frequency doubling of 1264 nm in room temperature. The combined beam was focused on a Si avalanche photodiode (APD) with a filter of 10 nm band-pass-width for the fceo detection.
3. Results and discussions
Since the soliton breakup plays an important role in our octave-span-spectrum generation (Fig. 3), it is necessary to calculate the order of soliton for noise characterization, using the standard definition:Fig. 4 , those two pulses cover same area, so the ideal Gaussian pulse in figure (b) can be an equivalent pulse for high bound calculation. Consequently, the low bound of T0 is 90/1.665 fs, and high bound is 270/1.665 fs.
By tapering the PCF, we can adjust the nonlinearity and dispersion, reduce the γ/|β2| ratio, diminish the inherent quantum noise of soliton breakup and increase the coherence of supercontinuum spectrum. As a result, γ was increased from 11 W−1km−1 to near 57 W−1km−1 and β2 was increased from −2.5 fs2/mm to −102 fs2/mm. Pulse energy was 2 nJ. The soliton number therefore descended from 16.4~28.4 to 5.8~10.1. According to reference , the soliton number is within the coherent region. Tapered PCF can also help to increase the coupling efficiency from the un-tapered fiber and make it more resistant to mechanical drift. The coupling efficiency was measured as high as 60% and it varied no more than 10% during test of 48 hours.
Pump noise and cavity dispersion are two major sources of intra-cavity noise. Both of them can significantly influence the fceo linewidth. We employed two different LD drivers: one is a self-made LD driver with switched DC power supply inside; another is a commercial LD driver (ILX Lightwave LDC-3900). The latter one is considered quieter. A pin photodiode detector was used to detect the pump power and an oscilloscope was for monitoring the output voltage of the photo detector, which can indicate the variation of the pump power. The voltage RMS value of the “quiet” pump was 7.381 V and the standard deviation was 0.066 V. For “noisy” pump the RMS value was 7.417 V and the standard deviation was 0.2 V. Since the variation is too small compare with the absolute value, we added a long pass filter to block DC signal to make it comparable to the background of the photo detector, as shown in Fig. 5 . We can see that when power supply is “quieter”, the linewidth of the fceo became narrower.
On the other hand, Fig. 6 shows that the linewidth of fceo decreased from 10 MHz to less than 1 MHz, as the cavity dispersion was adjusted from anomalous to near zero by slight increase of the grating separation. The corresponding pulse spectrum is shown in the Fig. 6(a) and 6(b) respectively, which are the evidence of the pulse shaping from soliton to stretched pulse.
Optimizing the peak power of input pulses can further reduce the soliton order in supercontinuum generation. Besides, the nonlinearity in the fiber amplifier also leads to additional amplitude noise, which increases the background noise and lowers the visibility of fceo beating signal. Therefore the pump power must be adjusted to maximize the S/N ratio of fceo signal. Figure 7 shows the optimized f-to-2f beat signal. The S/N ratio of fceo signal is above 30 dB (with the resolution bandwidth (RBW) = 100 kHz). The FWHM value of the fceo signal is around 300 kHz. This signal will be used to lock the fceo signal. With the isolation of the system from the environment, frep will be further stabilized.
We have demonstrated the fceo signal characterization of a 490 MHz NPE mode-locked Yb-fiber-ring laser. From the preceding results, we can see the pump noise and net cavity dispersion play an important role in the intra-cavity noise of frequency combs, especially for high repetition rate cases, because ASE and nonlinearity become less significant when the gain fiber is shorter and the peak power is lower. Short pulses combined with the nonlinearity and dispersion adjustment by tapering the PCF can minimize the soliton number, reduce the extra-cavity noise, and increase the coherence of the octave-span-spectrum. The noisy environment makes it hard to lock the fceo signal and fceo S/N ratio is not high enough to achieve a robust CEO locking either, which is mainly limited by the highly structured pump pulses. Therefore after isolating from the environment noise and special designing the amplification system, such as using highly doped Yb fiber and adopting the chirped-pulse-amplification system to reduce the nonlinearity and high order dispersion, we can reach more than 35 dB fceo S/N ratio and achieve a robust optical frequency system.
This research was supported in part by the National Natural Science Foundation of China (No. 60927010, 60907040, 10974006 and 110274046) and the Templeton Foundation.
References and links
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