C-band continuously tunable lasers using tunable fiber Bragg gratings

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Optics & Laser Technology 39 (2007) 1214–1217

C-band continuously tunable lasers using tunable ﬁber Bragg gratings

Shien-Kuei Liaw

a,

, Kuan-Luen Hung

a

, Yi-Tseng Lin

a

,

Chia-Chin Chiang

b

, Chow-Shing Shin

c

a

Department of Electronic Engineering, National Taiwan University of Science and Technology, Number 43, Sector 4, Keelung Road, Taipei 106, Taiwan, ROC

b_{Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, ROC} c_{Department of Mechanical Engineering, National Taiwan University, Number 1, Sector 4, Roosevelt Road, Taipei 106, Taiwan, ROC}

Received 4 May 2006; received in revised form 5 August 2006; accepted 5 August 2006 Available online 28 September 2006

Abstract

We report the development of a ring tunable fiber laser based on tunable fiber Bragg gratings (TFBG) integrated with an optical circulator. The TFBG is embedded inside a 3-piont bending device for wavelength tuning. The tunable laser operating in the C-band has power variation, tuning resolution, tuning range and laser line width of70.5 dB, 0.5 nm, 25.0 nm and less than 0.05 nm, respectively. As 40 mW of pump power is used, the ring tunable laser has a side mode suppression ratio of 60 dB and a power conversion efficiency of 25%. These specifications ensure the high-quality operation of a tunable laser.

Keywords: Tunable laser; Fiber Bragg gratings; Fiber laser

1. Introduction

In recent years, lasers have found a variety of applica-tions, including sensing, instrument testing, optical signal processing, optical communications, and photon analog-to-digital conversion (ADC) [1,2]. More specifically, tunable lasers have been flexible for usage in wavelength routing, wavelength protection and optical measurement. Several methods have already been proposed to investigate tunable lasers using photonic crystal mirrors [3] and a multiple quantum-well waveguide. Among them, the use of the erbium-doped fiber ring laser (EDFRL) exhibits potential for success, partially since it features a low-temperature sensitivity of wavelength. The selection of their operation wavelengths has been achieved by using different optical filtering devices, such as Mach–Zehnder filters or in-fiber comb filters. However, the tuning range can be narrow[4], or the line width can be wide due to large filter bandwidth. Another approach is the dual wavelength tunable fiber laser. Lasing occurs at two different

wavelengths, alternatively, by using a circuit consisting of two overlapping circuits with a common gain medium[5]. In this paper, we demonstrate the use of the tunable ﬁber-Bragg-gratings (TFBGs)-based tunable laser. A suitably designed optical device could cover the whole C-band region. The laser is constructed by a 1 2 optical switch (OSW) and optical circulator (OC). The results show that the tuning ability could be realized by appropriately switching the OSW and compressing/straining the TFBGs.

2. Operation mechanism of the TFBG

From the theory of solid mechanics [6], the transverse displacement (n) is related to longitudinal strain (e) applied to the FBG for tuning purpose.

E ¼My I (1) and n ¼PL 3 48EI . (2)

In these equations, E is the elastic modulus, I is the moment of inertia, M( ¼ PL/2) is the bending moment,

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Corresponding author. Tel.: +886 2 2737 6384; fax: +886 3481 2175. E-mail address:skliaw@et.ntust.edu.tw (S.-K. Liaw).

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combined TFBGs, with tuning resolution of 0.5 nm for each TFBG. The output power increases as the reﬂectivity is increased. The average output power is 0.0 dBm (i.e., 1 mW) for each channel and power variation is less than 70.5 dB over the whole tuning range. Thus, no variable optical attenuator (VOA) is need for power equalization laser wavelength. The coupler ratio is 10/90, with 90% of lasing power being looped back to the ring circuit and 10% of lasing power detected by an optical spectrum analyzer (OSA) or power meter. Since the pumping power of the 1480 nm laser diode is 40 mW, the ring tunable laser has a pumping efﬁciency of 25%.

The tunable laser proposed here is of a unidirectional ring circuit design. It is well suited for obtaining nearly single longitudinal mode operation due to the cancellation of spatial hole-burning effect in the traveling wave ﬁeld of the ring circuit. Laser line width can be reduced by means

of inserting sub-ring circuit(s) in the main ring circuit as in our prior work[8]. In this work, only a sub-ring circuit is included composed of a polarization controller (PC) and one 50:50 coupler. The main ring integrated sub-ring serves as a mode filter. The laser mode oscillates only at a frequency that simultaneously satisfies the resonant condi-tions of both the main circuit and the sub-ring circuit. The PC in the sub-ring circuit must be tuned to the same states of polarization as that of the main circuit to ensure that no polarization mode competition effect is observed. The free spectra range (FSR) of the ring circuit is defined as FSR ¼ c/nL. The lasing wavelength is set at 1533 nm by using one of the TFBGs and the mean refractive index of the optical fiber is about 1.5. The lengths of the main and the sub-ring circuits are close to 30 and 4.5 m, correspond-ing to the FSRs of 6.6 and 44.0 MHz, respectively. Although an RF spectrum analyzer is not available for precise measurement of the laser line width, which is estimated to be much narrow than that of the resolution limitation of an OSA of 0.05 nm.

A versatile and cost-effective laser source should have the tunable ability to allow one to choose the wavelength needed or to scan a range of bandwidth. Our proposed TFBG lasers are able to fulﬁll such requirements. In theory, the proposed method could be applied to the whole S, C and L bands by parallel connection of several TFBGs with a 1 N OSW pair. It goes without saying that TFBGs with appropriate original central wavelengths are necessary.

5. Conclusion

We report the investigation of the ring tunable laser based on TFBG technology. Each TFBG is embedded inside a 3-point bending device for wavelength tuning. The power variation, tuning resolution, tuning range and laser line width are 0.5 dB, 0.5, 25 nm and less than 0.05 nm, respectively. The ring tunable laser also has 60 dB of SMSR to ensure high-quality operation. With the features mentioned above, the tunable laser is a potential candidate for high-speed modulation in either a digital or analog system. It may also ﬁnd vast applications in optical communication and optical measurement.

Acknowledgments

The work is partially supported by NSC under projects No. 94-2219-E-011-007 and 94-2622-E-011-021-CC3, Taiwan, ROC. We thank Y.C. Lai, S.C Chen and C.R. Wang for their kind help.

References

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[2] Liaw SK, Hung FC, Xu Z, Jiao Y, Qin T, Zhang H, et al. Wavelength tuning and multiple-wavelength array-waveguide-grating lasers. Opt Eng 2003;42:2178–9.

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OSA 10% 1 2 3 90% PC _TFBG2 TFBG1 ISO ISO OC 1480 nm Pump LD WDM EDF EDFA 10/90 Coupler _2×2 Coupler 1×2 OSW

Fig. 3. TFBG-based ring tunable laser; PC: polarization controller; OSW: optical switch; OC: optical circulator; TFBG: tunable ﬁber Bragg grating; ISO: optical isolator; EDF: erbium-doped ﬁber; OSA: optical spectrum analyzer. 1520 1530 1540 1550 1560 1570 -40 -80 -60 -20 20 0 Second grating 1556.44nm First grating 1540.36nm 1535.36nm -0.37dBm 1530.36nm -0.55dBm 1540.36nm -0.29dBm 1546.44nm 0.43dBm 1561.44nm 0.25dBm 1551.44nm 0.44dBm 1556.44nm 0.38dBm Wavelength (nm) Power (dBm)

Fig. 4. Superimposed optical spectra of the ring tunable laser. The whole C band tuning is realized by using two TFBGs with different original wavelengths.

S.-K. Liaw et al. / Optics & Laser Technology 39 (2007) 1214–1217 1216

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