ISSN 1054660X, Laser Physics, 2010, Vol. 20, No. 2, pp. 512–515. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © Astro, Ltd., 2010.
512
1 1. INTRODUCTION
Recently, broadband wavelengthtunable fiber ring lasers are very attractive light sources, operating in the wavelength ranges of S (1480–1520 nm), C (1530– 1560 nm), and Lbands (1560–1620 nm). They can be used in many applications, such as the highresolution spectroscopy, passive component measurement, fiber sensor network, and wavelength division multiplexing (WDM) system backup source [1–3]. However, the fiber ring laser may have modehopping leading to multimode output due to the longer cavity length and very narrow longitudinal modespacing [1]. Generally, the filters are used inside the gain cavity of ring laser for lasing singlelongitudinalmode (SLM) output, such as using fiber Bragg grating (FBG), Fabry–Perot etalon filter, or fiber ring scheme filter etc. [4–7]. Moreover, the SLM fiber ring laser using saturable absorber has also been studied to overcome the limited spectral width of the mode selection filters inside the gain cavity [8, 9].
In this study, we propose and investigate a SLM S band erbiumdoped fiber (EDF) laser with a novel ring scheme using a FBG inside the gain cavity to restrict the lasing mode. The proposed laser is constructed by tworing cavities serving as the mode filters. By using a FBG inside the proposed laser scheme, it will retrieve the lasing wavelength at SLM. Besides, the output per formance of proposed fiber laser is also discussed. Compared with our past research [10], this work doesn’t need to use a shorter unpumped EDF to filter the sidemode to achieve a SLM output. In this new
1 The article is published in the original.
proposed fiber scheme, we only use simple tworing architecture obtaining the SLM operation.
2. EXPERIMENT AND DISCUSSION Figure 1 shows the experimental setup of the pro posed EDF ring laser scheme. The proposed laser is consisted of an Sband EDFA, a 1 × 2 and 50 : 50 cou
FIBER OPTICS
SingleLongitudinalMode ErbiumDoped Fiber Laser
with Novel Scheme Utilizing Fiber Bragg Grating
inside Ring Cavity
1C. H. Yeha, * and C. W. Chowb
a Information and Communications Research Laboratories, Industrial Technology Research Institute,
Chutung, Hsinchu, 31040 Taiwan
b Department of Photonics and Institute of ElectroOptical Engineering, National Chiao Tung University,
Hsinchu, 30010 Taiwan
*email: depew@itri.org.tw
Received August 15, 2009; in final form, August 27, 2009; published online December 23, 2009
Abstract—In this paper, we experimentally investigate a singlelongitudinalmode (SLM) Sband erbium
doped fiber (EDF) laser with dualring scheme using a Sband fiber Bragg grating (FBG) inside the gain cav ity to restrict the lasing mode. The proposed laser is constructed by tworing cavities serving as the mode fil ters. The FBG inside the proposed laser scheme can retrieve the SLM operation. Moreover, the output per formance of the proposed fiber laser is also analyzed and discussed.
DOI: 10.1134/S1054660X10030229 PC EDF(2) EDF(1) OIS OIS OIS C1 WC WC C2 C1 980nm LD OC OC FBG L(2) L(1) 1 2 3 1 2 3 Output
Fig. 1. Proposed erbiumdoped fiber ring laser scheme. C1: 1 × 2 optical coupler; C2: 2 × 2 optical coupler; OC: optical circulator; WC: 980/1550 nm WDM coupler; PC: polar ization controller; OIS: optical isolator; EDF: erbium doped fiber; FBG: fiber Bragg grating.
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pler (C1), a 2 × 2 and 50 : 50 coupler (C2), a FBG with
the central wavelength and reflectivity of 1511.08 nm and 91% respectively, two optical circulators (OCs), and a polarization controller (PC). The C1 and C2 are
used to produce to two ring scheme, as shown in Fig. 1, and the FBG is used to connect the two ring cavities via two OCs. The PC is adjusted to obtain the maximum output power and maintain the polarization state. The Sband amplifier in the experiment, with a depressedcladding design and 280 mW pumping power, can generate the EDF gain bandwidth in the Sband window [3]. The EDF lengths of the first and second amplifier stages are 20 and 30 m long. Thus, the gain and noise figure of EDFA can reach to 32.0 and 5.7 dB at 1500 nm for input power of 25 dBm, and the saturated output power at 1500 nm can be up to 14 dBm for input power of 0 dBm. In this experiment, an optical spectrum analyzer (OSA) with a 0.05 nm resolution and a power meter (PM) are used to mea sure the output spectrum and output power of the pro posed ring laser.
The FBG not only determines a lasing wavelength but also serves as a moderestricting component to provide the first restriction on the possible laser modes. Figure 2 shows the reflective spectrum of FBG used at the wavelength of 1511.08 nm. Due to the combination of a FBG and proposed two ring cavities, a SLM operation in the proposed laser can be guaran teed. It is significant that the two cavities (L1 and L2) are served as the mode filters, as seen in Fig. 1. The two fiber cavities have the free spectral ranges (FSRs), FSR = c/nL, where c is the light speed in vacuum, n is the average refractive index of the singlemode fiber (SMF) of 1.468, and L is the fiber cavity length. In the proposed laser scheme, the cavity lengths of L1 and L2 are 54.6 and 56.5 m long. Here, the two corresponding
FSRs of ring cavities will be 3.74 and 3.61 MHz respectively. In order to realize the SLM selection of proposed fiber laser, the lasing output performance could be measured by the selfhomodyne detection. The optical circuit for a measurement is consisted of a photodetector (PD) with 3 dB bandwidth of 10 GHz and a Mach–Zehnder interferometer with about 20 km long SMF.
For the laser measurement, Fig. 3 presents the original output amplified spontaneous emission (ASE) spectrum (blue dash line) of Sband EDFA used under the effective operating bandwidth of 1480 to 1520 nm. In the experiment, as seen in Fig. 1, the lasing mode can oscillate at the wavelength of 1509 –80 –90 –70 –60 –50 Power, dBm FBG@1511.08 nm 1510 1511 1512 1513 Wavelength, nm
Fig. 2. Reflective spectrum of the Sband FBG at
1511.08 nm. 1490 –30 1480 –60 –50 –20 –40 0 –10 1500 1510 1520 1530 Wavelength, nm Output power, dBm ASE Output λ 1511.08 nm
Fig. 3. Output ASE spectrum of original Sband EDFA
and the lasing wavelength of proposed fiber laser at 1511.08 nm. 0 –80 –60 –40 –20 0 100 200 300 400 500 Frequency, MHz Relative intensity noise, dB/Hz
Single ring Proposed
Fig. 4. Detected homodyne frequency spectra of the pro
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1511.08 nm (red solid line) when the Sband FBG is utilized in ring cavity, as also illustrated in Fig. 3. The output power and sidemode suppression ratio (SMSR) of the lasing wavelength are measured at ⎯1.1 dBm and 47.8 dB. In the proposed EDF ring laser scheme, if we want to obtain different lasing wavelengths, we can use a wavelengthtunable FBG to replace the wavelengthfixed FBG for lasing mode tuning.
As shown in Fig. 1, when the fiber cavity L1 and OC1 are removed in the proposed laser scheme, the
laser would result in the modehopping. Therefore, Fig. 4 shows the detected selfhomodyne frequency spectrum of the proposed laser at the wavelength of 1511.08 nm without the first ring cavity (L1), produc ing a noisy and unstable output signal due to the modehopping, as seen in the dash line of Fig. 4. The behavior of modehopping can be affected by the environment disturbances of temperature and vibra tion. Here, when the proposed fiber laser scheme is used in the experiment, the SLM oscillation is much easier to achieve compared with traditional singlering laser scheme, as illustrated in Fig. 5. Clearly, no beat ing noises are observed in RIN spectrum of the pro posed laser which indicates that single frequency oscillation can be retrieved, as illustrated in solid line of Fig. 4. Thus, Fig. 4 presents a stable SLM output spectrum with side mode suppression in the measuring bandwidth of 500 MHz. In addition, after an hour observation, no spike noise and stable frequency out put are observed in the RF spectrum of the proposed fiber laser.
In order to investigate the stabilities of output power and wavelength in the proposed fiber laser structure, a shortterm stability of the proposed ring laser is experimented and measured as shown in Fig. 6. Here, the observation time is over 30 minutes at the lasing wavelength of 1511.08 nm with –1.1 dBm out
put power. The output variations of power and wave length are observed of <0.5 dB and <0.04 nm, respec tively. Moreover, during the observation time of 60 min, the output stability of the proposed fiber laser is still kept and maintained. Furthermore, for the past studies [10–17], either they used the Brillouin pump (nonlinear effect), intracore FBG or saturable absorberbased filter to complete the SLM operating. However, our fiber laser only use a simple dualring fiber cavity design to guarantee a SLM output. Hence, the proposed fiber laser not only has simple design but also has costeffectiveness.
3. CONCLUSION
In summary, we have proposed and demonstrated a singlelongitudinalmode (SLM) fiber gratingbased Sband erbiumdoped fiber laser with passive two ring cavities, which serve as mode filters. While a fiber Bragg grating (FBG) is used inside the proposed laser scheme, the output lasing wavelength can be guaran teed in a SLM oscillation. In the experiment, the fiber laser effectively suppresses sidemode frequencies of 0.5 GHz and provides an output power of –1.1 dBm with a sidemode suppression ratio (SMSR) of 48.7 dB/0.05 nm at the wavelength of 1511.08 nm. Moreover, the power fluctuation of less than 0.5 dB and the central wavelength variation of less than 0.04 nm are observed for lasing SLM wavelength.
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–1.8
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1511.12 –0.8 –1.0 –1.2 –1.4 –1.6 1511.10 1511.08 1511.06 1511.04 1511.02 Power, dBm Wavelength, nm Power Wavelength
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