A triple function linear fiber laser with passivemode-locking and continuous-wave

single-wavelength and multiwavelength lasingJiajun Tian,1 Yong Yao,1,* Jun Jun Xiao,1 Xiaochuan Xu,2 and Deying Chen1,3

1Department of Electronic and Information Engineering, Shenzhen Graduate School, Harbin Institute of Technology,Shenzhen, Guangdong Province, 518055, China

2Microelectronics Research Center, University of Texas at Austin, Austin, Texas, 78758, USA3National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology,

Harbin, Heilongjiang Province, 150001, China*Corresponding author: yaoyong@hit.edu.cn

Received January 6, 2011; revised March 24, 2011; accepted March 24, 2011;posted March 24, 2011 (Doc. ID 140732); published April 15, 2011

We propose a fiber laser that can operate with passive mode-locking pulse output, continuous single-wavelengthemission, and continuous multiwavelength emission. It consists of a nonlinear optical loop mirror (NOLM) in alinear cavity. The NOLM is used to induce different kinds of intensity-dependent feedback, which deter-mine the operation mode of the laser. We have experimentally demonstrated that the fiber laser can easily beswitched between the operation modes. The results may find useful applications in optical communication andfiber sensing. © 2011 Optical Society of AmericaOCIS codes: 140.3500, 060.2320.

Over the past decades, due to vast and importantapplications in fiber optics [1–4], mode-locking and CWsingle-wavelength and multiwavelength fiber lasers havereceived considerable interest. Various technologieshave been proposed to achieve mode-locking [5–7] andCW single-wavelength [8] and multiwavelength [9–14] fi-ber lasers. For diverse aims and technologies used in thefiber sensor systems, different kinds of lasers are needed[1–3]. It would be remarkably useful and convenient forpractical applications if one fiber laser could operate inmultiple working states. However, there are few reportson multifunction fiber lasers [15].In this Letter, a multifunction fiber laser based on a

nonlinear optical loop mirror (NOLM) in a linear cavityis proposed and demonstrated. The NOLM is used to pro-vide both laser output and an intensity-dependent laserfeedback. Depending on the intensity-dependent laserfeedback, the fiber laser can work at three differentmodes: passive mode-locking and CW single-wavelengthand multiwavelength emission. It is flexible to obtainthese three operation modes and switch one workingstate to another by adjusting the working state of theNOLM. To the best of our knowledge, this is the first re-port of a fiber laser with such triple functionalities.Figure 1 shows the experiment setup. A 15m Erbium-

doped fiber (EDF, Nufern: EDFL-980-HP) serving as thegain medium is pumped by a 1480 nm laser (Amonics:ALD1480-400-B-FA). A power-symmetric NOLM servesas both controllable laser feedback and system output.It consists of a 3 dB coupler, a 1km single mode fiber(SMF, Corning: SMF-28e) twisted by 3 turns=m, and aquarter-wave plate (QWP). The other end of the linearcavity is a circulator inside which a Fabry–Perot (FP) fil-ter (MOI: FFP-TF2) is mounted to provide periodic loss inthe frequency domain. The polarization state is adjustedby a polarization controller (PC). The laser output fromthe NOLM is taken via a 3 dB coupler and simultaneouslycharacterized by both an optical spectrum analyzer

(YOKOGAWA: AQ6370) with 0:05 nm resolution and aserial data analyzer (LeCroy: SDA 6000A) with the band-width 6GHz.

It is important to analyze the linear cavity transmissionto understand the lasing process. The transmission of theNOLM can be calculated as in Ref. [16]. It strongly de-pends on the QWP angle α and the polarization stateof the input beam, which is represented by the Stokesparameter As and the polarization direction φ, whereAs ¼ ðjCþj2 − jC−j2Þ=ðjCþj2 þ jC−j2Þ and Cþ and C− arethe complex amplitudes of the circular right and left po-larization components, respectively [16]. The minimalswitching power of the NOLM in our experiment isaround 7:5W [16,17]. Various power-dependent reflec-tions of the NOLM can be found by adjusting the PCand the QWP. Two typical distinct cases that resemblea saturable absorber (SA) and an intensity limiter (IL)are shown in Fig. 2. By definition, the NOLM reflectionacts as an SA if the input beam with higher power experi-ences a higher reflection, as shown in Figs. 2(a) and 2(c).On the other hand, the NOLM reflection acts as an ILwhen the input beam with higher power experiences alower reflection, as shown in Fig. 2(b). Figure 2 furtherdemonstrates the adjustability of the reflection R0 atzero input power, for example, R0 ¼ 0:53 in the IL state[Fig. 2(b)] and R0 ¼ 0:15 in the SA state 2 [Fig. 2(c)]. It isnoted that R0 determines the fundamental output loss of

Fig. 1. Schematic of proposed fiber laser.

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the laser. The fundamental output loss and the NOLMstate (SA or IL) determine the working mode, e.g., pulseor CW.Particularly, when the NOLM reflection is at SA state

and R0 is adjusted to a relatively low level [like Fig. 2(c)],low-intensity CW light (pulse wings) will be attenuateddramatically by the NOLM. However, because of the ran-dom fluctuations in the signal intensity experienced byan un-mode-locked laser, any random, intense spike ofthe light will be reflected preferentially by the NOLM.As the light in the cavity oscillates, this process repeats,leading to the selective amplification of the high-intensityspikes and the attenuation of the low-intensity light. Aftermany round trips, this leads to a train of pulses andmode-locking of the laser [18].The situation is remarkably different when R0 is ad-

justed to a relatively higher level. Opposite to the mode-locking case, a dramatic part of the low power signals arereflected into the linear cavity to be amplified again.Thus, a CW operation can be achieved. Additionally, ifthe NOLM is set at IL state [Fig. 2(b)], the signal withhigher power has lower reflection than the lower powerone. This means that the input beam with higher powerwill experience higher loss from the NOLM. This featurecan be utilized to suppress the mode competition of theEDF and achieve stable multiwavelength oscillation [19].On the other hand, if the NOLM is set at SA state with a

higher level R0, the CW output characteristics are chan-ged. In this working state, a higher power beam gets high-er reflection through the NOLM [see Fig. 2(a)]. Therefore,the higher power beam experiences lower output loss. Insuch a case, due to the homogeneous broadening of EDF,only the lasing line corresponding to the highest gain canbe built up [8]. This feature can be used to obtain CWsingle-wavelengthoperation.Moreover, as shown inFig. 2,the output loss of the laser, which is related to R0, can becontrolled by the QWP and the PC. This influences thegain distribution of the EDF [8,20]. Thus, the spectrum re-gion of the single lasing line can be controlled.Following the discussions above, we have conducted a

series of experiments to verify the analysis. The pumppower is fixed at 410mW. By simply adjusting the

QWP and the PC to set the NOLM at an appropriateSA state [like Fig. 2(c)], mode-locked pulses can be read-ily obtained. The typical pulse train, pulse shape, and thecorresponding optical spectrum are shown in Fig. 3. Thepulse repetition is of the fundamental cavity frequency192:43 kHz, as shown in Fig. 3(b). Via the linear filteringof the FP filter, the spectrum in Fig. 3(a) exhibits a comb-like shape [15]. Figure 3(c) shows that the pulse durationis below 15 ns and the pulse is of a rectangular shape.This is probably due to the group velocity dispersion(GVD) and self-phase modulation (SPM) effects [21]. Arough estimation shows that the GVD of the SMF couldlead to a pulse broadening of 0:4 ns. Additionally, theSPM could be strong, because the intracavity pulse inten-sity may be very high as the output transient pulse poweris around 0:7W [calculated from the average outputpower 2:076mW, measured by a power-meter (ILX light-wave FPM-8210H)]. It is known in theory [21] and experi-ments [22,23] that SPM can dramatically enhance thebroadening rate of the pulse in the normal-dispersion re-gime and make a Gaussian pulse shape to be a rectangu-lar one. This indicates that the SPM could further

Fig. 2. (Color online) Power-dependent NOLM reflectionfor different values of the QWP angle α and different inputpolarization states, (a) α ¼ 0:35π, As ¼ 0:2, φ ¼ −0:38π;(b) α ¼ 0:45π, As ¼ 0:3, φ ¼ −0:5π; (c) α ¼ 0:64π, As ¼ 0:9,φ ¼ −0:89π.

Fig. 3. Passive mode-locking operation of the fiber laser:(a) comblike spectrum, (b) mode-locked pulse train, (c) wave-form of a single pulse.

Fig. 4. (Color online) Output spectrum of a CW multiwave-length operation.

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broaden the pulse dramatically. These two effects lead tothe broadened pulse, which is highly chirped.Next, we realize the CW multiwavelength output.

Figure 4 displays a four-line multiwavelength operationof the fiber laser when the NOLM reflection is set atan appropriate IL state [like Fig. 2(b)]. The wavelengthspacing is 1 nm due to the FP filter. Compared withthe comblike spectrum in Fig. 3(a), the CW multiwave-length has a much higher signal-to-noise ratio. The for-mer is simply a result of periodic filtering to thespectral envelope of a mode-locked pulse, while the lat-ter results from multichannel CW emission. The multi-wavelength spectrum is relatively flat, as the four lasinglines are in the 3dB bandwidth. We have also scanned themultiwavelength spectrum every 0:5 min for half an hour.The measured results show that the fluctuation ofsingle-wavelength power is within 0:9 dB (figure notshown here).It is useful to switch expediently from CW multiwave-

length to single-wavelength [3]. This is realized by settingthe NOLM at an SA state [as in Fig. 2(a)], and CW single-wavelength operation can be obtained. The results areshown in Fig. 5. It is seen that CW single-wavelength out-puts at three different spectral regions are obtained. Thewavelengths are 1596, 1598, and 1600 nm, with total out-put power 67.5, 60.6, and 55:88mW, respectively. As dis-cussed above, R0 can be controlled by the NOLM, whichadjusts the output loss. The output loss has two differenteffects on the single lasing line output. First, a high out-put loss leads to a high output power. Second, if the out-put loss is higher, the spectrum region with the highestgain is shifted to a shorter wavelength, which is the spec-trum region in which the CW single lasing line should bebuilt up [8]. Therefore, the spectral region of the CW sin-gle lasing line can be adjusted by setting the NOLM indifferent SA states.In summary, a NOLM-based fiber laser with three dif-

ferent working states, i.e., passive mode-locking and sin-gle-wavelength and multiwavelength emission, has been

experimentally demonstrated and theoretically analyzed.The intensity-dependent reflections of the NOLM areresponsible for different laser feedback in the linear cav-ity and therefore determine the working state of the laser.When the NOLM reflection is at different SA states, pas-sive mode-locking and CW single-wavelength operationcan be achieved. When the NOLM reflection is at an ILstate, CW multiwavelength operation can be realized.The operation mode can be conveniently controlled byadjusting a PC and a QWP in the system.

This work was financially supported in part byNational Natural Science Foundation of China (NSFC)(60977034, 11004043), and in part by Shenzhen MunicipalScience and Technology Plan Project (SMSTPR)(JC200903120167A, JC201005260185A). The authorsacknowledge the help from Dr. Y. Lai.

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Fig. 5. (Color online) Output spectrum of a CW single-wavelength operation.

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