ISSN 1054660X, Laser Physics, 2011, Vol. 21, No. 5, pp. 924–927. © Pleiades Publishing, Ltd., 2011.
Original Text © Astro, Ltd., 2011.
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1. INTRODUCTION
Stable and wavelengthtunable singlelongitudi nalmode (SLM) operations were required for erbiumdoped fiber (EDF) ring lasers for the applica tions of wavelengthdivisionmultiplexing (WDM) communication systems and fiberoptic sensor sys tems [1–4]. Generally, the tunable bandpass filter (TBF), fiber Fabry–Perot tunable filters (FFPTF) and fiber Bragg grating (FBG) could be employed inside the fiber cavity to generate wavelengthtuning [5–7]. However, it was insufficient to stabilize the las ing wavelength and output power of a EDF ring laser due to the modehopping and gain competition effects. In order to solve the unstable optical output, several methods were used to obtain a SLM operation, such as integrating two cascaded FFPTF of widely different free spectral ranges (FSRs) inside ring cavity, using a compound ring resonator composed of a dual coupler fiber ring and dualring scheme, and adding an extra ITUgrid periodic filter in the ring loop [8– 10]. Moreover, utilizing a unpumped EDF inside fiber loop to serve as a saturableabsorborbased filter to achieve SLM output, have been proposed and investi gated [11, 12]. However, these past researches could not be flattened outputs due to homogeneous broad ening effect. In such wavelengthtunable fiber laser, the flattened output performance is also important issue for WDM networks and sensor systems. Hence, the past EDF ring laser schemes, which were pro posed, with flatter output spectrum could be achieved 1The article is published in the original.
by varying the pumping power [13, 14]. But the lasing wavelengths of above studies were not SLM. In addi tion, using Sagnac loop mirror, Mach–Zehnder inter ferometer and allpolarizationmaintaining loop methods inside fiber ring cavity also could generate the single and multiwavelength output in singlemode operations [15–19].
In this study, we propose and experimentally inves tigate an erbiumdoped fiber (EDF) ring laser scheme using a dualsaturableabsorberbased (DSAB) filter inside gain cavity for the stable singlefrequency oper FIBER OPTICS
Employing DualSaturableAbsorberBased Filter
for Stable and Tunable ErbiumDoped Fiber Ring Laser
in SingleFrequency
1C.H. Yeha, *, C.W. Chowb, K.H. Chenc, and J.H. Chend
a Information and Communications Research Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu, 31040 Taiwan
b Department of Photonics and Institute of ElectroOptical Engineering, National Chiao Tung University, Hsinchu, 30010 Taiwan
c Department of Electrical Engineering, Feng Chia University, Taichung, 40724 Taiwan d Department of Photonics, Feng Chia University, Taichung, 40724 Taiwan
*email: [email protected]
Received November 28, 2010; in final form, November 30, 2010; published online April 2, 2011
Abstract—In this demonstration, a stable and wavelengthtunable erbiumdoped fiber (EDF) ring laser using
dualsaturableabsorberbased (DSAB) filter inside loop cavity is proposed and experimentally investigated. The proposed DSAB filter not only can filter the sidemode in singlefrequency output, but also can obtain the flattened output power spectrum within 1 dB variation in the effectively range of 1529 to 1563 nm. In addition, the output stabilities of wavelength and power are also measured experimentally and discussed.
DOI: 10.1134/S1054660X11090271
Fig. 1. Experimental setup of proposed stable and wave
lengthtunable EDF ring laser configuration.
EDFA PC ISO TBF PC 980 nm LD 1.5 m unpumped EDF 1.5 m unpumped EDF Output CP CP 10 m EDF
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ation. The DSAB filter only can obtain the flattened output spectrum within dB power variation among 1529 to 1563 nm by the DSAB filter scheme, but also can achieve the sidemode suppression ratio (SMSR) of >38 dB/0.01 nm. In addition, the output character istics of the proposed fiber laser have also been ana lyzed and discussed.
2. EXPERIMENT AND RESULTS
Figure 1 presents the experimental setup of pro posed stable and wavelengthtunable EDF ring laser configuration. The proposed fiber laser was consisted of an erbiumdoped fiber amplifier (EDFA), a tun
ablebandpass filter (TBF), a 2 × 2 and 50:50 optical coupler (OCP), a 1 × 2 and 50:50 OCP, a polarization controllers (PC), and two unpumped EDFs with 1.5 m long. Two unpumped EDFs and two OCPs were used to act as the DSAB filter for filtering the sidemode of lasing lightwave. The PC in the experiment was employed to control and adjust the polarization status and maintain the maximum output power. The TBF, having a 3dB bandwidth of 0.4 nm, which can be operated in the tuning range of 1525 and 1560 nm, inside the EDF ring cavity, was employed to filter ASE spectrum and generate lasingwavelength. Here, the EDFA was constructed by an optical isolator (ISO), a 980/1550 nm WDM coupler, a 980 nm pumping laser diode, and a 10 m long EDF (Produced by FiberCore DC1550F). In this measurement, the output wave lengths and powers could be measured by an optical spectrum analyzer (OSA) with a 0.01 nm resolution and a power meter (PM).
Here, Fig. 2 shows the amplified spontaneous emission (ASE) spectra of the EDFA used without the ring structure since the 980 nm pumping laser diode is operated at different powers between 18 and 66 mW. As shown in Fig. 2, with increase of pumping power gradually, die observed ASE power is also increase. Besides, the measured maximum power level of around 1533 nm could be observed under different pumping powers.
In the experiment, we also measure the output power under different pumping powers of 18 to 66 mW at the lasing wavelength of 1550 nm, as illustrated in Fig. 3. The threshold power of 980 nm LD is 26 mW in the proposed EDF laser scheme. So, with the increase
18 mW 26 mW 34 mW 42 mW 50 mW 58 mW 66 mW −10 −20 −30 −40 −50 −60 1520 1530 1540 1550 1560 1570 Wavelength, nm Power, dBm
Fig. 2. Output amplified spontaneous emission (ASE) spectra of the EDFA used without the ring structure since the 980 nm
pumping laser diode is operated at different power levels between 18 and 66 mW.
0 −20 −40 −60 10 20 30 40 50 60 70 Pumping power, mW Output power, dBm 1550 nm
Fig. 3. Output power under different pumping powers of 18
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YEH et al.
of pumping power, the retrieved lasing power is also increase. When the pumping powers are 26, 41, and 66 mW, respectively, the observed lasing powers are ⎯7.2, –1.8, and 1.4 dBm, as seen in Fig. 3.
Then, when the TBF was utilized inside cavity loop, the different lasing wavelengths could be retrieved by the proposed EDF ring laser scheme. Thus, Fig. 4 shows the output spectra of proposed EDF ring laser with DSAB filter scheme in the wave length range of 1522 to 1563 nm, while the pumping power of 980 nm LD is 66 mW. As shown in Fig. 4, we can observe the larger ASE noise is introduced around 1533 nm, when the lasing wavelength region is shorter than 1533 nm. It would cause the worse sidemode
suppression ratio (SMSR) in the effective lasing range. With the drift of lasing wavelength in longer waveband gradually, the presented ASE of around 1533 nm could be suppressed simultaneously, as seen in Fig. 4. Besides, Fig. 4 also shows the related optical signal to noise ratios (OSNRs) in the lasing wavelength range of the proposed EDF laser scheme. Here, the OSNRs are measured between 47.8 and 54.2 dB in the wavelengths of 1529 to 1563 nm.
Figure 5 shows the output power and related SMSR curves under the different wavelengths for the pro posed EDF ring laser in 1522 to 1563 nm. Here, the measured output powers and SMSRs are between ⎯12.3 and 2.1 dBm and 12.5 and 54.8 dB/0.05 nm, 15 5 −5 −15 −25 −35 −45 −55 1520 1530 1540 1550 1560 Wavelength, nm OSNR 60 50 40 30 20
Optical signal to noise ratio, dB
Power, dBm
Fig. 4. Output spectra of proposed EDF ring laser with dual SABF scheme in the wavelength range of 1522 to 1563 nm, while the
pumping power of 980 nm LD is 66 mW. 20 15 10 5 0 −5 −10 −15 1520 1530 1540 1550 1560 1570 Wavelength, nm 60 50 40 30 20 10 0 Peak power SMSR ΔP = 1 dB SMSR, dB Power, dBm
Fig. 5. Output powers and related SMSRs versus the differ
ent wavelengths for the proposed EDF laser in the operat ing range of 1522 to 1563 nm. 1551.0 1550.8 1550.6 1550.4 1550.2 1550.0 1549.8 1549.6 0 10 2020 30 4 2 0 −2 −4 −6 −8 −10 Wavelength Power
Observing time, min
Output power, dBm
Wavelength, nm
Fig. 6. Shortterm observation of the proposed fiber laser is
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respectively. Due to the smaller gain region of 1522 to 1530 nm, we could get the smaller output powers (⎯12.3 and –1.1 dBm) in the region. Moreover, we can obtain the flatter output power range in the wave lengths of 1529 to 1563 nm (with 34 nm wavelength bandwidth). Hence, the measured powers and SMSRs are between 1.1 and 2.1 dBm, and 38.0 and 54.8 dB/0.01 nm, respectively, in the range of 1529 to 1563 nm. Here, the maximum power variation (ΔPout) of 1 dB can be maintained in the lasing bandwidth of 1529 to 1563 nm. Comparing with the past studies [3, 14], the proposed EDF ring laser not only can achieve SLM operation, but also can obtain flatter output levels.
To investigate and realize the optical stabilities of output power and lasing wavelength, a shortterm observation of the proposed EDF ring laser is mea sured as shown in Fig. 6. The lasing wavelength is at 1550 nm initially with 1.4 dBm output power and the observation time is over 30 min. In Fig. 6, the pro posed fiber ring laser can dramatically reduce the wavelength variation (Δλ) of 0.01 nm and power fluc tuation (ΔP) of 0.1 dB. During the one hour observa tion time, the stabilized output of the proposed ring laser is still maintained in SLM output.
3. CONCLUSIONS
In summary, we have investigated and demon strated a stable and wavelengthtunable EDF ring laser scheme using DSAB filter inside loop cavity to achieve SLM output. The proposed DSAB filter not only can obtain singlefrequency output, but also can obtain the flattened output power spectrum within 1 dB vari ation (from 1.1 to 2.1 dBm) in the effectively range of 1529 to 1563 nm. In this region, the SMSR is obtained between 38.0 and 54.8 dB/0.01 nm. In addition, the stabilities of output power and wavelength have been measured within 0.1 dB and 0.01 nm in the observing time of 30 min in the experiment.
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