Tunable optical frequency comb from a compact and robust Er:fiber laser Download: 607次
1 Introduction
Optical frequency comb (OFC) systems[1] are usually described as mode-locked fiber lasers with a certain repetition rate (
Here, we have developed a compact, movable and robust OFC system with a tunable repetition rate. We combined a homemade all-PM, mode-locked fiber laser[20] with another all-PM fiber amplifier and two phase-locked loops to realize an electrically controlled OFC system with attractive features. The all-fiber structure ensures the contraction of volume and makes the system compact. The optimized homemade oscillator with low noise guarantees the noise suppression of phase locking and excellent performance for ensuring the accuracy and reliability of OFC measurements. The modular design with portability and the improved mechanical package design with PM components provide robustness and anti-environment-disturbance ability. The design of electronic control makes it possible to provide a variable repetition rate with simultaneous phase locking by programmed operations on the internal fiber delay and external radio frequency (RF) signals, which makes it more convenient to use OFCs in wider fields.
With an electrically controlled fiber delay line in the laser cavity, the OFC can provide a tunable range of 342 kHz at a repetition rate of 101 MHz, while the laser remains mode-locked with simultaneous phase locking. The stabilized in-loop
2 Comb design
The configuration of a tunable compact fiber frequency comb design is shown in Figure
Fig. 1. (a) Experimental setup. APD, avalanche photodiode; CIR, circulator at 1550 nm; EDF, Er-doped fiber; FC/APC, FC/APC connectors; FDL, electrically controlled fiber delay line; HNLF, highly nonlinear fiber; , homemade -to- interferometer; LD, 976 nm laser diode; PD, fiber-coupled photodiode; PLL, phase-locked loop system; PZT, piezoelectric transducer; SESAM, packaged semiconductor saturable absorber mirror; WDM, 980/1550 nm wavelength division multiplexer. (b) The photograph and movie (Visualization 1) of the ‘optics package’ and one package of the PLLs .
The net intra-cavity dispersion of
Fig. 2. (a) The optical spectrum of the oscillator. (b) Measured RF spectrum of the repetition rate (300 kHz resolution). (c) The tunable repetition rates with FDL modulations (10 Hz resolution). (d) Long-term output power of the oscillator above 12 h and the stability test (Visualization 2). (e) The original amplitude noise (blue line) and the integral amplitude noise (green line) from the oscillator. (f) The pulse duration of the compressed pulse after the amplifier.
The low noise of the optimized fiber oscillator also provided a lower noise level for the following phase locking. To further suppress the technical noise from the environment and make our system robust outside the laboratory, the packaging of optics and the mechanical package design were improved. Different parts of optics were installed in aluminum boxes separately. The main purpose of the separated design was to keep the oscillator independent for possible dismantlement and overhaul in the future. For the same reason, the FC/APC connectors were used between the oscillator and the amplifier while fiber splicing was employed for other connecting points, as shown in Figure
The pulses of the oscillator are separated by a coupler, and the 90% power gets into an all-PM fiber amplifier for boosting pulse energy. Two WDMs, equipped with unidirectional isolators at 1550 nm, are spliced at the two ends of the gain fiber in the amplifier. The gain fiber in the amplifier is
The compressed pulse train with a high peak power is then fiber-coupled to a piece of highly nonlinear germanosilicate fiber[27] with anomalous dispersion at 1550 nm. An octave-spanning spectrum is then obtained in the PM highly nonlinear fiber (HNLF) via a soliton fission process and measured with two optical spectrum analyzers (AQ6370 and 771B-MIR) aimed at different wavelength ranges. Figure
Fig. 3. (a) The supercontinuum after HNLF with the compressed pulses. (b) The RF spectrum of the frequency detected by the APD.
For the phase locking between the carrier and the envelope, the detected RF signal of
3 Results and discussion
Adopting the modular design, the robust and movable fiber OFC has been demonstrated with many attractive features. The self-starting process has been shown in Visualization 1, and a series of other results pertaining to the long-term behavior of the OFC are measured here, as shown in Figure
Fig. 4. (a) The recorded time series of the offset. (b) The counts of the offset. (c) The overlapping Allan deviation of the recorded . (d) The recorded time series of the offset. (e) The counts of the offset. (f) The overlapping Allan deviation of the recorded .
The phase noise performance is another important evaluation for the stability of OFCs. The phase noise of the in-loop
Fig. 5. (a) The noise characterization of . The phase noise of stabilized with the pump current (blue line), the integrated phase noise of the stabilized with the pump current (green line) and the introduced -separation line (orange line). (b) The noise characterization of . The phase noise of stabilized with the PZT (blue line) and the integrated phase noise of stabilized with the PZT (green line).
4 Conclusions
In conclusion, we have developed a compact and robust OFC system. The all-PM fiber design makes the system movable and practical for applications outside the laboratory. By means of an electrically controlled fiber delay in a laser cavity, the repetition rate of the OFC has a tunable range of 342 kHz at 101 MHz for specific applications, especially benefiting those measurements with tunable repetition rates. Tight phase locking of
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Zhiwei Zhu, Yang Liu, Daping Luo, Chenglin Gu, Lian Zhou, Gehui Xie, Zejiang Deng, Wenxue Li. Tunable optical frequency comb from a compact and robust Er:fiber laser[J]. High Power Laser Science and Engineering, 2020, 8(2): 02000e17.