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The operation of an erbium figure-of-eight all-fiber laser with a fundamental repetition rate of 5.68 MHz, peak power of 0.7 kW and pulse width of 420 fs is reported. Four times the fundamental repetition rate is achieved with two passive sub-ring loops at a repetition rate of 22.8 MHz. The sub-ring loop consists of only a single-mode fiber coupler with an input port and output port connected so as to make the time delay an odd multiple half-period of the input pulse period. This method is simple and inexpensive when compared with conventional methods employing sub-ring cavity or special fiber Bragg grating to control repetition rate.
©2006 Optical Society of America
1. Introduction
Recently remarkable progress has been made in the development of figure-of-eight sub-picosecond fiber lasers using a nonlinear amplifying loop mirror as the additive-pulse mode-locking mechanism [1]. Doran and Wood [2] proposed a switching device, which operates on the nonlinear phase induced by self-phase modulation (SPM), known as the nonlinear optical loop mirror (NOLM). Duling [3] reported the first passive mode locking of a fiber laser with a modified NOLM, which is called the nonlinear amplifying loop mirror (NALM) proposed by Fermann [4]. The development of so-called figure-of-eight lasers (F8L) as a passive mode locker has allowed the generation of sub-picosecond pulses directly from all fiber lasers [5]. However, for practical system applications, such as optical CDMA and OTDM system, it is desirable to be able to control the repetition rate with less pulse intensity fluctuation with a simple all-fiber configuration. Harmonic mode locking and extracavity feedback have been demonstrated up to 142.7 MHz, but the pulse amplitude fluctuation was susceptible to the feedback delay and intracavity bulk optic elements were required [5,6]. Multi-gigahertz pulse F8L has also been demonstrated by incorporating sampled fiber Bragg grating, but the pulse train operated in burst mode [7].
In this paper, we propose a simple passive sub-ring loop configuration to increase the repetition rate. The sub-ring loop consists of only a single-mode fiber coupler the input port and output port connected to make a time delay of odd multiple half-period of the input pulse period. This method is simple and inexpensive and has less intensity variation (~4%) when compared with conventional methods (intensity variation >5%) to control repetition rate employing sub-ring cavity and special fiber Bragg grating or passive Mach-Zehnder interferometric pulse multiplexing scheme [8, 9]. Four times the fundamental repetition rate is achieved with two passive sub-ring loops at a repetition rate of 22.8 MHz. This approach is capable of increasing the repetition rate up to ~10 GHz with the passive sub-ring loop scheme. For shorter half-period time delay, high-order odd multiple of half-period delay loop is preferable. The simplicity and small intensity variation of this all-fiber mode-locked laser configuration makes it highly practical for a number of applications including optical CDMA and OTDM systems.
2. Experiment
A schematic diagram of the experimental arrangement is shown in Fig. 1. The figure-of-eight fiber laser consisted of a NALM and a linear loop to recirculate the switched pulse through a polarization-insensitive optical isolator. The NALM consisted of a 90:10 fiber coupler, two 980/1550 wavelength division multiplexers, a length of 10.4 m erbium doped fiber, a length of 8 m dispersion-shifted fiber, and an 80:20 output coupler connected with the external sub-ring loops. The NALM periodically reflects and transmits input optical signal depending on the optical power of the input signal due to the SPM. The polarization controllers were installed in the linear and nonlinear loops to compensate for the stress birefringence of the fiber and initiate pulse oscillation. Since the optical isolator is polarization-insensitive, there is no polarization selection in the linear cavity. Once a pulse is initiated in the cavity of the F8L and the peak power satisfies the total transmission condition, the pulsed signal keeps circulating in the F8L cavity. On the other hand, pulses that do not satisfy the total transmission condition are reflected by the NALM and suppressed by the isolator. The controllers then served to select a preferential polarization eigenstate of the light in the cavity. The fundamental repetition rate was determined by the cavity fiber length. The pulse width is related to cavity gain, cavity dispersion and cavity length. For a fixed gain of the cavity, the pulse width can be reduced by shortening the cavity length or selecting lower dispersion fiber.
Fig. 1. Schematic configuration of F8L with passive sub-ring loops. P.I.ISO.: polarization-insensitive isolator; P.C.: polarization controller; WDM: wavelength division multiplexer; DSF: dispersion shifted fiber; VOA: variable optical attenuator; DL: optical delay line.
Sub-ring loops consist of only fiber couplers and optical delay lines. An optical pulse is split into two pulses after transiting the coupler at port P1, one pulse goes through the coupler directly out of port P2, the other pulse goes to port P4 and comes back to port P3 through an optical delay line DL1. This returning pulse at port P3 is split into two pulses and superimposes with the electric fields. If the time delay is much larger than the coherence time of the pulse source, the vector electric field overlap will reduce to a scalar overlap of the pulse power [10]. When the whole loop time delay, DL1, is odd multiple of half the period of the incoming pulse period, a new pulse stream with twice the incoming pulse repetition rate will appear at port P2. With two sub-ring loops of time delay DL1 and DL2, four times the fundamental repetition rate is achieved. Although the pulse continues to circulate, after the two circulations, the pulse amplitude variation is less than 1.3% when 2/3:1/3 coupler is used. The pulse will broaden or maintain its shape through soliton propagation in the loop depending on the time delay, dispersion and nonlinearity of the loop fiber.
3. Results
The F8L was characterized with the experimental setup as shown in Fig. 1. The average output power of 1.65 mW, for 42 mW pump power, was measured at point P0. To reduce the signal power, a ~50 dB variable optical attenuator was used for spectrum and pulse characterization. A stable pulse train was obtained with a repetition rate of 5.68 MHz, corresponding to a 36 m long cavity with a round trip time 176 ns, as shown in Fig. 2. The autocorrelation trace shown in Fig. 3 was measured with a Femtochrome Research Inc. FR-103MN autocorrelator. The pulse width of the autocorrelator signal was 648 fs and assuming the sec h 2 (t) shape, the full-width half-maximum (FWHM) of the pulses was 420 fs and the peak power was 0.7 kW. The laser output had a 6.2 nm bandwidth with a center wavelength 1555.6 nm as measured with an MS9710A optical spectrum analyzer at point P1 as shown in Fig. 4. Thus, ΔνΔτ of the output pulse was approximately ~0.33 (close to the theoretical time-bandwidth product limit for a hyperbolic-secant-shaped pulse of 0.315), indicating that the pulse was slightly chirped.
Fig. 2. The F8L output pulse train in a mode locked operation mode with a repetition rate of 5.68 MHz.
Fig. 3. Autocorrelation trace of the mode-locked F8L with 420 fs of FWHM for sec h 2 (t) pulse shape.
Fig. 4. Spectrum of the F8L at center wavelength 1555.6 nm with 6.2 nm bandwidth and 8.4 nm sidelobes either side of the main peak.
Two 70:30 fused-taper couplers were used to achieve four times the fundamental repetition rate at 22.8 MHz. The sub-ring loops configuration is shown in Fig. 1. The throughput power coupling ratio of 30% (less than the theoretical power splitting ratio of 33%) was chosen with the consideration of insertion loss. The amplitude of the output pulse stream can be conveniently controlled by slightly bending the fiber-loop to increase loss. The time delay loops DL1 and DL2 are ~18 m and ~9 m long, which correspond to 88 ns and 44 ns time delays, respectively. The time delay can be easily fine-tuned with accuracy less than 1 ps through fiber stretching. To maintain soliton propagation in the loops, dispersion-shifted fiber with a dispersion of -0.8 ps2/km at 1555.6 nm was used. A 1 GHz bandwidth photoreceiver (New Focus 1611) in combination with a 1GHz oscilloscope (LeCroy 9384L) were used to record the pulse train. The output pulse trains measured at points P2 and P5 are shown in Fig. 5(a) and 5(b), respectively. The amplitude fluctuation of ~4%, estimated from the oscilloscope persistence observation, was due to the loop phase and amplitude noises. The oscillation feature near the trailing edge of the pulses is currently under investigation.
Fig. 5. Multiplying repetition rate output at P2 and P5: (a) double repetition rate (11.4 MHz); (b) quadruple repetition rate (22.8 MHz).
4. Conclusion
In conclusion, we have experimentally demonstrated a F8L with passive sub-ring loops to increase the repetition rate. Experimental results show that the passive sub-ring loops pulse multiplication scheme is a simple and inexpensive means of F8L pulse multiplication. Compared with other methods for controlling the repetition rate, the pulse intensity fluctuation is less than ~4%. The stability, short pulse duration, and high repetition rate of this system make it attractive for optical system applications. There is potential that the sub-ring loops can also be used to broaden the pulse duration by selecting proper fiber dispersion and nonlinearity for the fiber delay line
Acknowledgments
The authors acknowledge many helpful discussions with L. Fu, at the Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), University of Sydney. This work was partly funded by the Australian Research Council.
References and links
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