Stable and wavelength-tunable erbium-doped fiber double-ring laser in S-band window operation

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Stable and wavelength-tunable erbium-doped ﬁber

double-ring laser in S-band window operation

Hung-Chang Chien

a

, Chien-Hung Yeh

b,*

, Kuo-Hsiang Lai

a,c

,

Chien-Chung Lee

a

, Sien Chi

a,d

a

Department of Photonics, Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan b

Transmission System Department, Computer and Communications Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 310, Taiwan

c

Chunghwa Telecom Co., Ltd., Yang-Mei, Taoyuan 326, Taiwan d

Institute of Electro-Optical Engineering, Yuan Ze University, Chung-Li 320, Taiwan Received 28 August 2004; received in revised form 10 January 2005; accepted 14 January 2005

Abstract

A stable and tunable S-band erbium-doped ﬁber (EDF) double-ring laser has been proposed and experimentally demonstrated. Based on the double-ring conﬁguration, the EDF ring laser exhibits more stable output wavelengths and powers than those of the single-ring. Wide tunable range of 1484–1518 nm, the side-mode suppression ratio (SMSR) of larger than 44.7 dB/0.05 nm and the output power of larger than 2.2 dB m over the operation range of 1484–1510 nm have been retrieved.

Keywords: EDF; Fiber laser; S-band; Double-ring

1. Introduction

Broadband tunable laser sources are the major devices in optical transmission systems. The stable output frequency and power of the ﬁber ring lasers

are always required for wavelength division multi-plexing (WDM) networks and sensing systems. Recently, several stabilization techniques, such as integrating two cascaded FFP filters of wide differ-ent free spectral ranges (FSRs) into cavity [1], using passive multiple-ring cavity[2], have been re-ported. In[3], we also demonstrated a stabilization technique employing a Fabry–Perot laser (FP-LD) and an FFP filter for the multi-mode erbium-doped fiber (EDF) ring laser. However, the tuning

*

Corresponding author. Tel.: +886 939 442785; fax: +886 3 5828187.

E-mail addresses:[email protected],

[email protected](C.-H. Yeh).

Optics Communications 249 (2005) 261–264

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steps of the proposed fiber ring laser were sub-jected to the mode-spacing of FP-LD. Because of the bandwidth limitation of erbium-doped fiber amplifiers (EDFAs), the operation region of EDF ring lasers only covers both the C- to L-band (1530–1610 nm)[4,5], and is not sufficient until an EDFA and a fiber ring laser for S-band operation are proposed [6,7]. In this letter, we propose and experimentally investigate a stable and tunable S-band (1484–1518 nm) EDF ring laser with dou-ble-ring configuration. This fiber ring laser has free tuning steps and features stable output powers and wavelengths in S-band. The behavior of the output power and wavelength stabilities, tuning range and side-mode suppression ratio (SMSR) has also been experimentally studied.

2. Experiments

Fig. 1shows the experimental setup for the sta-ble and tunasta-ble S-band EDF dousta-ble-ring laser. This apparatus consists of two 2· 2 and 50:50 optical couplers, a polarization controller (PC), two fiber Fabry–Perot filters (FFP-TP), and an S-band EDFA module with two amplifier stages and a power-sharing 980 nm pump laser. The total pump power of this S-band amplifier can be up to

280 mW while the bias current is operated at 356 mA. The S-band EDF inside EDFA module has a depressed cladding design in order to provide a sharp, high attenuation, long wavelength cutoff filter into active fibers. Furthermore, the EDFA module comes in two stages EDF of different properties. The fiber in the first stage is 20 m long, and both low noise figure and medium gain are contributed by forward pumping. The fiber in the second stage has the fiber length of 30 m, and large output power can be generated through backward pumping. Besides, the optical isolator is arranged between these two stages in order to re-duce backward amplified spontaneous emission (ASE). Both high gain of 32 dB and low noise fig-ure of 5.7 dB at 1500 nm can be obtained while the input power of 25 dB m is provided. The satu-rated output power at 1500 nm can reach 14 dB m for input signal power of 0 dB m. More-over, two FFP filters are all-fiber components with widely tunable range, FSR of 44.5 nm, finesse of 200, low polarization-dependent loss of 0.1 dB and insertion loss of <0.5 dB. Wavelength selec-tion in the double-ring laser cavity can be achieved by applying external voltage (0–12 V) on the piezo-electric transducer (PZT) of two FFP filters. Two FFP filters are nearly tuned to the same wave-length for single frequency output. The stable out-put wavelength and power of the laser can be easily achieved while the state of polarization ad-justed by the PC in the dual-ring cavity is main-tained. In addition, an optical spectrum analyzer (OSA) with 0.05 nm resolution and a power meter (PM) are used to measure the output wavelengths and powers of the proposed ring laser.

3. Results and discussions

Double-ring configuration can serve as a mode filter and only the particular modes, which coin-cide with the central frequencies of two filters, can oscillate. The double-ring configuration shown inFig. 1can be viewed as the combination of two single-ring cavities called main-ring (52 m long) and sub-ring (60 m long) cavities, respectively. Moreover, as schematically depicted in Fig. 2, the main- and sub-ring cavities have FSRs of

C : Optical Coupler W : 980/1480 nm WDM Coupler PM : Power Meter

PC : Polarization Controller OSA : Optical Spectrum Analyzer FFP : Fiber Fabry-Perot FFP Filter C FFP Filter PC 50 50 980 nm Pump Laser W W C EDF EDF Isolator S - Band EDFA Module

C

OSA 50 50

PM

Fig. 1. Experimental setup of the S-band EDF double-ring laser.

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FSRm and FSRs, respectively. Owing to Vernier effect [2], the value of effective FSR becomes the least common multiple number of both FSRm and FSRs. As a result, the mode suppression can be achieved and governed by the length of the main-ring and sub-ring cavities we choose. In addition, two lasing spectra from FFP filter #1 and #2 are nearly overlapped to provide further mode restriction on possible laser modes. Finally, only 3 modes fs1–3 are selected for oscillation and mode fs2 dominates. Comparing with the single-ring case, the proposed fiber single-ring laser has fewer selected mode and less mode competition effect. Thus, the mode stability can be guaranteed. There-fore, two lasing lights from two ring cavities can interfere mutually to retrieve the stabilized fre-quency output as long as a static state of polariza-tion is provided.

Fig. 3 shows the optical spectra of the stable and tunable S-band EDF double-ring laser over the operation region of 1484–1518 nm when the voltages of 0–12 V are applied on two PZTs of two FFP ﬁlters, respectively. The insert of Fig. 3

is the ASE spectrum of the S-band ampliﬁer.Fig. 4 shows the output power and SMSR versus the tuning wavelength for this double-ring laser over the bandwidth from 1484 to 1518 nm. As seen in

Fig. 4, the maximum output power of 0 dB m is re-trieved at around 1498 nm, and the output powers drop to 8.71 dB m at 1518 nm. The output power level can be kept larger than 2.2 dB m over the

tuning range of 1484–1510 nm. Owing to the ASE compression and gain competition eﬀect, the maximum SMSR value can be up to 56.8 dB/ 0.05 nm at near 1504 nm. The SMSR can be kept larger than 44.7 dB/0.05 nm in a wide tuning range over 30 nm (1484–1514 nm).

To investigate the behaviors of output power and wavelength stabilities, the short-term stability of the proposed conﬁguration (in Fig. 1) is mea-sured and compared with the traditional architec-ture[6]. The operating conditions such as a pump

Filter #1 Filter #2 Effective FSR FSRm FSRs 1 s f f_s₂ f_s₃

Fig. 2. Schematic diagram of mode selection in the double-ring laser cavity.

Fig. 3. The output wavelength spectra of the S-band EDF double-ring laser. Wavelength (nm) 1470 1480 1490 1500 1510 1520 1530 Output Power (dBm) -20 -10 0 10 20 SMSR (dB) 0 20 40 60 80 Output Power SMSR

Fig. 4. The output power and SMSR versus the tuning wavelength for this proposed ring laser over the tuning range from 1484 to 1518 nm.

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power of 280 mW and lasing wavelength of 1948 nm are set equally for both single-ring and double-ring configurations. The measured slope efficiencies are about 2.02% and 1.13% for the S-band fiber laser with single-ring and double-ring structures, respectively; the threshold pump power of the proposed fiber laser is 142 mW and that of the traditional one is 122 mW. As indicated in

Fig. 5(a), the average output power of the single-ring and double-ring lasers are 4.28 and 0.02 dB m, respectively. Moreover, the normalized power ﬂuctuation of the single-ring laser is 7.85%, while that of the double-ring laser is only 0.92%. The observing time is over 900 s. In addition,

Fig. 5(b) shows that the wavelength variations of two conﬁgurations are 0.03 and 0.06 nm, respec-tively. Obviously, the intra-cavity loss of the dou-ble-ring conﬁguration resulting from polarization dependant components is higher than that of the

single-ring structure. Accordingly, the results re-veal that increasing the intra-cavity loss will allow more stability but sacrifice the threshold pump power and slope efficiency of the fiber laser. Dur-ing 3-h observation, the stable output of the pro-posed double-ring laser is still maintained. Therefore, compared with traditional fiber ring laser, this laser has better stability.

4. Conclusion

In conclusion, we propose and experimentally demonstrate a stable and tunable S-band EDF double-ring laser. Based on the proposed conﬁgu-ration, this ﬁber ring laser exhibits more stable output power than that of the traditional one. Wide tunable range of 1484–1518 nm, the SMSR of larger than 44.7 dB/0.05 nm and the output power of larger than 2.2 dB m over the operation range of 1484–1510 nm have been retrieved.

Acknowledgement

This work was supported in part by the Na-tional Science Council (NSC) of R.O.C. under grants NSC 2752-E009-009-PAE, NSC 93-2215-E-115-004, and NSC 93-2215-E-115-005. Authors would like to thank C.Y. Chen for help with the experiments.

References

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[3] C.H. Yeh, C.C. Lee, S. Chi, IEEE Photonics Technol. Lett. 16 (3) (2004) 765.

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[5] R.M. Sova, K. Chang-Seok, J.U. Kang, J.B. KhurginIEEE CLEO 2002 Tech. Dig., vol. 1, 2002, pp. 444–445. [6] C.H. Yeh, C.C. Lee, S. Chi, IEEE Photonics Technol. Lett.

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[7] M. Arbore, Y. Zhou, H. Thiele, J. Bromage, L. NelsonOpti-cal Fiber Communications Conference (OFC), vol. 1, 2003, pp. 374–376. Time (sec) 0 200 400 600 800 1000 Output Power (dBm) -2 -1 0 1 2 3 4 Output Power (dBm) 1 2 3 4 5 6 7 Double-ring Single-ring Time (sec) 0 200 400 600 800 1000 Central Wavelength (nm) 1497.6 1497.8 1498.0 1498.2 1498.4 Double-ring Single-ring (a) (b)

Fig. 5. (a) The output power ﬂuctuation and (b) the wavelength variation of the proposed and traditional conﬁgurations as the wavelength is set at 1498 nm, respectively.