Thermal treatment method for tuning the lasing wavelength of a DFB fiber laser using coil heaters Ha Huy Thanh and Bui Trung Dzung National Center for Technology Progress (NACENTECH) C6-Thanh Xuan Bac-Hanoi-Vietnam Abstract: The popular method for tuning the lasing wavelength of a DFB fiber laser is to apply strains along the fiber grating using a Piezoelectric Transducer (PZT) [1]. Although the tuning range of 3nm can be obtained by this way, the spliced connections between the grating and the conventional single-mode fibers are often broken down, especially when over stretched. To avoid this risk, in this paper, we propose a safer compact tuning method. By controlling the DC current in the coil heaters around the DFB grating, we obtained a tuning range up to 2.3nm without any mode hoping. Keywords: DFB fiber laser, optical communications, tunable fiber laser. Introduction The development of high bit-rate, coherent optical communication systems using Wavelength Division Multiplexing (WDM) technique requires that the laser source have a very narrow linewidth, single frequency, stable polarization and tunable wavelength. The Distributed Feedback (DFB) fiber laser has been emerging as a promising alternative to the DFB semiconductor laser for use in CATV networks and high bit rate WDM communications. The advantages of the DFB fiber laser include fiber compatibility, compact size, and ultra-narrow linewidth [2]. The tunable DFB fiber laser can be also obtained by applying strains along the DFB grating. H.Yoon et. al. proposed a method using a PZT stretcher to tune the lasing wavelength of a DFB fiber laser [1]. The PZT stretcher creates a nearly uniform strain distribution along the grating, which would make the shift of the lasing wavelength. Although, this method can provide a tuning range up to 3nm without any mode hoping, the force-based methods generally show to be unsafe to the grating, especially when the grating is over-stretched. The structure of a DFB fiber laser includes a phase-shifted DFB grating whose two ends are spliced with two conventional single mode fibers. When we over-stretch the grating or stretch it several times, the two spliced connections are possibly disconnected. Furthermore, the stretching and twisting are two main sources that cause the instability of the polarization mode of the laser. To avoid this risk, in this paper, we propose a new compact and safer thermal treatment method that can provide the tuning range up to 2.3 nm and no mode hoping was seen. The heated Ni-Cr coils were employed to create a nearly uniform temperature distribution along the grating. We also tested the operational parameters of the laser across the tuning range. 279
Theory Basis The single frequency of a DFB laser is obtained by creating a phase-shift of π/2 in the DFB structure. The single wavelength of the laser is equal to the Bragg wavelength of the structure [3]. The Bragg wavelength of the grating depends on the effective index of refraction of the core and the periodicity of the grating. The effective index of refraction, as well as the periodic spacing between the grating planes, will be affected by changes in temperature. Consequently, the shift λ B in the Bragg center wavelength λ B is dependent on the changes in the temperature, which is given as: λ = λ B α Λ + α Bo( Λ ) FBG Where T FBG =(T H -T O ) is the heating temperature in degree oc, λ Bo is the Bragg wavelength at the reference temperature T O, 1 Λ αλ = is the thermal expansion coefficient Λ Τ T α n 1 = neff Λ is thermo-optic coefficient Τ The rate of refractive index change is higher than the period changes, and which is the main contributor to the wavelength shift. The wavelength shows high sensitivity at higher values of T H. Though, under 100 o C, the change is considered linear [4]. In addition, the fiber Bragg gratings show excellent temperature stability in the temperature under 300 o C [5]. It is, therefore, permissible to think of a thermal method for tuning the lasing wavelength of a DFB fiber laser. Experiment The DFB fiber laser used in the experiment has the following specifications: grating length: 5cm, single wavelength: 1551nm, single polarization mode, linewidth: 30KHz, pumping power 9mW, maximum power peak: 40mW, SNR >50dB, SMSR > 40dB [6]. The Figure 1 shows the optical spectrum of the laser. 0 Output Power dbm -20-40 -60-80 1550.0 1550.5 1551.0 1551.5 1552.0 Wavelength Figure 1: Optical spectrum of the laser 280
The basic idea is to create a controllable uniform thermal distribution along the grating. We rolled the Ni-Cr wire to form the coils with the internal diameter of 300µm and external diameter of 550µm around the DFB structure. The Ni-Cr coils were placed on a high temperature resistance Silicon substrate. By controlling the DC current intensity inside the coils, we created a nearly uniform temperature distribution along the grating. DFB fiber grating Ni-Cr wire φ=550µm Figure 2: The Ni-Cr coil for heating the DFB fiber grating We put the DFB fiber grating inside the Ni-Cr coil heaters, pumped the grating by GaAlAsP diode laser 980nm, which was controlled by a Laser Diode Driver Model 525. Changing the DC current inside the Ni-Cr wire, we controlled the temperature range from 22 o C to 135 o C. Increasing the temperature by controlling the DC current intensity, and observing the laser spectrum on the Optic Spectrum Analyzer (OSA) AQ 6317B, we realized the linear shifting of the lasing wavelength. The tuning range was 2.3nm and no mode hoping was seen throughout the tuning range (Fig. 3). 1553.5 1553.0 Lasing Wavelength 1552.5 1552.0 1551.5 1551.0 20 40 60 80 100 120 140 Temperature Figure 3: The shift of the lasing wavelength vs. the changes of the temperature Using Delayed Self-Heterodyne technique (Fig. 4) [7], we tested the stability of the linewidth across the tuning range. At wavelength of 1551nm, the output power is 3dBm and the linewidth is 30KHz. At the wavelength of 1553.3nm, the output power of 3.2dBm, the linewidth is 30.4KHz. 281
980nm WDM 980/1550 DFB inside the coil heaters Isolator 12 km delay fiber Detector + RF Spectrum Analyzer AOM Attenuator Figure 4: The Delayed Self-Heterodyne technique for measuring the linewidth of the laser Using the setup shown in the figure 5, we tested the single polarization mode stability of the laser throughout the tuning range. At 1551nm, the laser operated in the single polarization mode. The polarization extinction ratio was better than 20dB. At 1553.2 nm, the laser still operated in the single polarization mode. The polarization extinction ratio was about 16dB. Extinction ratio reduced due to the birefringence induced by the temperature. Though, the laser still operated in the single polarization mode. 980nm LD WDM 980/1550nm DFB fiber inside the coil heaters Polarization Controller Polarizer OSA Figure 5: The schema for testing the polarization mode of the laser across the tuning range. Conclusion We proposed a new thermal treatment method employing the Ni-Cr coil heaters for tuning the lasing wavelength of the DFB fiber laser. The tuning range is 2.3nm corresponding to the change of the temperature from 22 o C to 130 o C. No mode hoping was seen throughout the tuning process. This method shows safe and compact compared to the popular force-based methods. Across the tuning range, the laser keeps almost all of its important characteristics: single wavelength, single polarization mode, and ultra-narrow linewidth. 282
References [1] H.Yoon, K. M. Cho, S. B. Lee, S. H. Kim, and S. S. Choi, "Tunable fiber DFB laser using PZT-stretcher" OECC 2000, 14B4-5, pp.516-517, Chiba, Japan, 2000. [2] Micheal J. F. Digonnet, "Rare Earth doped fiber lasers and amplifiers," Marcel Dekker Inc., 1993. [3] Kringlebotn, J.T., Archambaultm J.L, Reekie, L., and Payne, D. N., Er3+:Yb3+ codoped fibre distributed feedback laser, Opt. Lett., vol 19, no. 24, 1994, pp. 2101-2103. [4] M. Mahmoud, Z. Ghassemlooy, Lu Chao, Modeling and analysis on the thermal tuning of fiber Bragg gratings for optical communication applications, Technical report, Sheffield Hallam University, UK, 2003. [5] Dong, L., and W. F. Liu, Thermal decay of fiber Bragg gratings of positive and negative index changes formed at 193nm in a Boron-co-doped germanosilicate fiber, App. Opt., vol. 36, 1997, pp. 8222-8226. [6] Ha Huy Thanh, Fabrication and characterization of the tunable DFB fiber laser with the single wavelength and single polarization mode for optical communications, International Workshop on Photonics and Applications IWPA-2004, Hanoi, Vietnam, 2004. [7] T. Okoshi, K. Kiluchi, A. Nakayama, Novel method for high resolution measurement of laser output spectrum, Electron. Lett., vol 16, no. 16, 1980, pp. 630-631. 283