IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 5, MARCH 1, 2009

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An Automatic Wavelength Control Method of a Tunable Laser for a WDM-PON Jung-Hyung Moon, Ki-Man Choi, Sil-Gu Mun, and Chang-Hee Lee, Senior Member, IEEE

Abstract—We propose a novel automatic wavelength control method of a tunable laser for a wavelength-division-multiplexed passive optical network (WDM-PON). By sending a low-power amplified spontaneous emission light generated from a broadband light source (BLS) to a tunable laser through a wavelength-division multiplexer, we match the wavelength of the tunable laser to a peak transmission wavelength of the multiplexer. The deviation of the wavelength of the tunable laser was less than 0.05 nm from the center of the target channel. We demonstrate up to 10-Gb/s transmission in a WDM-PON by using the proposed method. We also show that system impairments induced by the BLS are negligible.

Fig. 1. Experimental setup.

Index Terms—Passive optical network (PON), tunable laser, wavelength control, wavelength-division multiplexing (WDM).

I. INTRODUCTION wavelength-division-multiplexing passive optical network (WDM-PON) is a promising solution for access networks since it provides almost unlimited bandwidth, high security, and protocol transparency [1]. For the WDM-PON, several color-free (or colorless) optical sources are proposed [2]–[4]. Among those color-free sources, a tunable laser is an attractive source for a very high-speed and a long-reach transmission. However, we need an automatic wavelength matching method to achieve true color-free operation. In addition, the wavelength should be stabilized to maintain alignment with the peak transmission wavelength of the WDM multiplexer/demultiplexer. The upstream wavelength can be determined by using a wavelength assignment table after the detection of the downstream wavelength [5]. However, we may need an optical spectrum analyzer or a tunable filter to confirm the downstream wavelength. To stabilize the wavelength of the tunable laser, a Fabry–Pérot etalon filter in each transmitter [6] or a planar lightwave circuit-type Mach–Zehnder interferometer arrayed waveguide grating (AWG) can be used [7]. However, these techniques increase the system cost and require prior setting of the wavelength within locking range of the feedback control.

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Manuscript received May 05, 2008; revised December 01, 2008. First published January 09, 2009; current version published February 19, 2009. This work was supported in part by the IT R&D program of MKE/IITA (2007-S014-02, Metro-access integrated optical network technology). J.-H. Moon, S.-G. Mun, and C.-H. Lee are with the Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea (e-mail: [email protected]; [email protected]; [email protected]). K.-M. Choi is with the Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea, and also with the Next Generation Research Department, Korea Telecom Network Technology Laboratory, Daejeon 305-811, South Korea (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.2011373

In this letter, we propose an automatic wavelength control method of a tunable laser for a WDM-PON. A low-power amplified spontaneous emission (ASE) light filtered by a wavelengthdivision multiplexer is used as a reference light for wavelength control. By maximizing the optical beat noise between the reference light and the tunable laser output, the wavelength of the tunable laser is matched to the transmission peak of the multiplexer. Then, it is stabilized with a control loop. We demonstrated 10-Gb/s transmission in a WDM-PON with the proposed method. We also investigated system impairments including effects of optical back-reflection. II. PROPOSED METHOD AND EXPERIMENTAL RESULTS The experimental setup to investigate the feasibility of the proposed wavelength management method is shown in Fig. 1. Here, we considered the upstream transmission based on a tunable laser for simplicity. We used a broadband light source (BLS) based on the erbium-doped fiber amplifier (EDFA) at the central office (CO) to provide ASE light to a tunable laser located at an optical network termination (ONT). The output of the BLS is spectrally sliced by an AWG with Gaussian-type passband at the remote node (RN) and injected into ONTs. At an ONT, the spectrally spliced ASE light is passed through the coupler and a part of the ASE light is reflected by the mirror. Then, the optical beat noise between the spectrum-sliced ASE light and the tunable laser output is generated at the photodiode (PD). The spectral dependence of the optical beat noise will resemble the spectral shape of the AWG (or the spectrum-sliced ASE). Therefore, we can match the wavelength of the tunable laser to the peak transmission wavelength of the AWG at the RN by maximizing the beat noise. The center wavelength difference between athermal AWGs having temperature tuning pm C located at the RN and the coefficient less than CO can be increased up to 0.035 nm with the variation in the environmental temperature of 0 C–70 C around two AWGs [8]. The measured filtering loss passing through two AWGs with the wavelength mismatch of 0.035 nm was 0.14 dB and the power penalty was less than 0.1 dB for this case. Thus

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 5, MARCH 1, 2009

Fig. 2. Noise power variation as a function of the wavelength difference between the tunable laser and the spectrum-sliced ASE light. Fig. 3. Measured wavelength tuning and tracking characteristics. Insets show enlarged tacking curve. (a) Gaussian AWG; (b) flat-top AWG.

wavelength matching between AWGs located at the RN and the CO could be good enough for WDM-PON application. The spectral dependences of the measured noise powers are shown in Fig. 2. The noise powers were measured from 30 to 40 MHz using the electrical spectrum analyzer (ESA). The channel spacing and 3-dB bandwidth of the AWG were 0.8 and 0.4 nm, respectively. The coupling ratio of the 2 2 coupler was 90 : 10 and the loss of the mirror was 1.1 dB. The output power of the tunable laser was 0 dBm. The measured spectral profiles are sufficient to identify the peak of the transmission wavelength of the AWG. For the realization of the cost-effective system, it is important to use a low-power BLS. The minimum BLS power can be estimated by assuming that the noise power at the center of the target channel should be 3 dB higher than the noise power of the background that can be a region with a wavelength difference of 0.8 nm or above. Then the required minimum ASE injection power into an ONT is about 35 dBm, as shown in Fig. 2. When we consider 11-dB link loss including fiber loss of 5 dB for 20 km, AWG loss of 5 dB, and circulator loss of 1 dB, the required BLS power at the CO for a 32-channel WDM-PON can be estimated to be about 5 dBm ( 35 dBm required ASE power, 11-dB link loss, 3-dB AWG filtering loss, and 16 dB for 32 channels). It can be realized with a low-cost source and will be widely available. For automatic wavelength matching, we used a two-steps approach. At first, we swept the whole wavelength range with a coarse wavelength step and measured the noise power. By identifying a wavelength at the peak noise power, we can find a wavelength for a starting point of the fine tuning. Then, the wavelength of the tunable laser is locked to the transmission peak of the AWG with a control loop. In experimental demonstration, we used a port of the AWG with the transmission peak wavelength of 1553 nm. We used 0.4 nm for a coarse tuning. Then, it is possible to find out a target channel, since the maximum deviation of the coarse tuning point from the center of the target channel is 0.2 nm. We used 0.01-nm step for fine tuning. The ASE injection power after the AWG was 35 dBm. A typical wavelength tuning characteristic using the proposed method is shown in Fig. 3. We scanned the wavelength range from 1547 to 1560 nm to find out a reference point for the fine tuning. The wavelength difference with the target wavelength was about 0.2 nm for this coarse tuning process. After finding out of the reference wavelength, we started the fine tuning to match the tunable laser wavelength to a transmission peak of the AWG.

Fig. 4. Measured wavelength tuning and tracking characteristics using the directly modulated laser.

The deviation of the measured wavelength at the stable state was less than 0.05 nm from the target wavelength as shown in Fig. 3, inset (a). The tuning process was done for 20 min. It may be noted that it took about 13 s to measure a point in the wavelength domain including tuning time of the tunable laser which is mechanically tuned, ESA scanning time, and GPIB control time [9]. This time was limited by the tuning time and data acquisition time. However, it can be reduced with a laser with a fast tuning time and control circuits [10]. It may be noted that the power penalty induced by the wavelength mismatch was less than 0.2 dB, when the wavelength mismatch was increased to 0.1 nm. We also investigated the proposed wavelength control method with a flat-top AWG. However, the measured maximum deviation from the center wavelength of the target channel, i.e., 1554.14 nm, was increased to 0.14 nm due to a plateau of the transmission spectrum as shown in the Fig. 3, inset (b). The proposed method can be also applied to the directly modulated laser. We measured the wavelength tracking characteristic with the directly modulated distributed-feedback laser at 2.5 Gb/s. We use temperature tuning for this measurement. The measured wavelength tracking characteristics and the maximum deviation from the center of the target channel were almost same, as shown in Fig. 4. In the proposed scheme, the back-reflected light of the BLS is entered into the receiver at the CO. The back-reflected light is combination of Rayleigh back-scattering and optical reflection from connectors or splice points [11]. Then, the optical beat noise between the upstream signal and the back-reflected light of the BLS may induce penalty. To investigate the effect of the back-reflection on the system performance, we measured the

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MOON et al.: AUTOMATIC WAVELENGTH CONTROL METHOD OF A TUNABLE LASER FOR A WDM-PON

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III. CONCLUSION AND DISCUSSION

Fig. 5. Measured power penalty according to the SBR.

power penalty at bit-error rate of according to the signal to back-reflected light ratio (SBR), as shown in Fig. 5. Here, we used an external modulator and an EDFA in front of the tunable laser for upstream signal. If we allow the power penalty of 1 dB, the minimum SBR is 11 dB for 2.5 Gb/s and 13.5 dB for 10 Gb/s. In the proposed scheme, the received signal power at the the CO ), the received back-reflected light of the BLS at the CO ( ), and the ASE injection power into the ONT ( ) ( are given by

Since the output of the spectrally sliced BLS is incoherent light, it fluctuates in time. However, the time averaged power and the wavelength are stable. The proposed method uses time average inside of the control loop to correct the lasing wavelength change of the tunable laser and/or to track drift of the transmission wavelength of the AWG at the RN. Thus, effects of short-term fluctuation of the filtered ASE on the wavelength accuracy will be small. We demonstrated a new automatic wavelength control method for a WDM-PON with a tunable laser. A spectrum sliced low-power BLS was used as a reference light to find out the wavelength to be tuned. The required minimum ASE injection power into an ONT was about 35 dBm for the wavelength management of the tunable laser. The fluctuation of the wavelength was less than 0.05 nm in steady state. We also demonstrated the feasibility of 10-Gb/s WDM-PON with the proposed method. The acceptable fiber loss was 22.75 and 21.5 dB for 2.5- and 10-Gb/s transmission, respectively. The system impairments induced by BLS were negligible even in upgrade scenario from an ASE injected high speed WDM-PON. The proposed method can be useful for a general WDM network with tunable lasers. REFERENCES

where

,

, , , , , , and are, respectively, the tunable laser output power, the 90 : 10 2 2 coupler insertion loss between two ports having the same ratio, the AWG insertion loss, the fiber loss, the circulator insertion loss, the BLS output power, the back-reflection coefficient of the BLS into the CO, and the AWG filtering loss. Then the SBR is given by

Therefore, the proposed wavelength control method can be applied with the fiber loss of 22.75 dB satisfying the SBR dBm, of 11 dB for 2.5-Gb/s transmission with dBm, dB, dB, dB. The fiber loss for 10-Gb/s transmisand sion is 21.5 dB to have the SBR of 13.5 dB. Therefore, the back-reflection effects of the BLS are negligible in a WDM-PON. The proposed method is also useful to upgrade a WDM-PON with a wavelength-locked ASE injected Fabry–Pérot laser diode (F-P LD) or reflective semiconductor optical amplifier (RSOA) [2], [3]. It is not easy to provide 2.5 Gb/s or above with these sources. However, we can provide 2.5 or 10 Gb/s by changing an F-P LD (or a RSOA) with a tunable laser. In this case, the injected BLS can be used for the wavelength control of the tunable was 8 dBm which is enough injeclaser. Even though tion power for color-free 1.25-Gb/s transmission [3], [11], [12], we did not observe any penalty due to the injection of the BLS.

[1] S.-J. Park, C.-H. Lee, K.-T. Jeong, H.-J. Park, J.-G. Ahn, and K.-H. Song, “Fiber-to-the-home services based on wavelength-division-multiplexing passive optical network,” J. Lightw. Technol., vol. 22, no. 11, pp. 2582–2591, Nov. 2004. [2] H. D. Kim, S.-G. Kang, and C.-H. Lee, “A low-cost WDM source with an ASE injected Fabry-Pérot semiconductor laser,” IEEE Photon. Technol. Lett., vol. 12, no. 8, pp. 1067–1069, Aug. 2000. [3] P. Healey, P. Townsend, C. Ford, L. Johnston, P. Townley, I. Lealman, L. Rivers, S. Perrin, and R. Moor, “Spectral slicing WDM-PON using wavelength-seeded reflective SOAs,” Electron. Lett., vol. 37, no. 19, pp. 1181–1182, Sep. 2001. [4] J. H. Lee, M. Y. Park, C. Y. Kim, S. H. Cho, W. Lee, G. Jeong, and B. W. Kim, “Tunable external cavity laser based on polymer waveguide platform for WDM access network,” IEEE Photon. Technol. Lett., vol. 17, no. 9, pp. 1956–1958, Sep. 2005. [5] H. Suzuki, M. Fujiwarea, T. Suzuki, N. Yoshimoto, K. Iwatsuki, and T. Imai, “A remote wavelength setting procedure based on wavelength sense random access (-RA) for power-splitter-based WDM-PON,” in Eur. Conf. Opt. Commun. 2006, Cannes, France, Sep. 2006, p. 157, Paper We3. [6] H. Nasu, T. Takagi, M. Oike, T. Nomura, and A. Kasukawa, “Ultrahigh wavelength stability through thermal compensation in wavelength monitor integrated laser modules,” IEEE Photon. Technol. Lett., vol. 15, no. 3, pp. 380–382, Mar. 2003. [7] M. Fujiwara, H. Suzuki, T. Tanaka, N. Ooba, N. Yoshimoto, and M. Tsubokawa, “Centralized frequency stabilization by dithering transmission spectra of PLC-Type MZI-AWG for DWDM-PON,” in Eur. Conf. Opt. Commun. 2007, Berlin, Germany, Sep. 2007, Paper 7.6.2. [8] L. Leick, M. Boulanger, J. G. Nielsen, H. Imam, and J. Ingenhoff, “Athermal AWG’s for colourless WDM-PON with 40 C to 70 C and underwater operations,” in Proc. Optical Fiber Commun. Conf. (OFC 2006), Anaheim, CA, Mar. 2006, Paper PDP31. [9] M. G. Littman and H. J. Metcalf, “Spectrally narrow pulsed dye laser without beam expander,” Appl. Opt., vol. 17, no. 14, pp. 2224–2227, Jul. 1978. [10] C.-K. Chan, K. L. Sherman, and M. Zirngibl, “A fast 100-channel wavelength-tunable transmitter for optical packet switching,” IEEE Photon. Technol. Lett., vol. 13, no. 7, pp. 729–731, Jul. 2001. [11] J.-H. Moon, K.-M. Choi, S.-G. Mun, and C.-H. Lee, “Effects of backreflection in WDM-PONs based on seed light injection,” IEEE Photon. Technol. Lett., vol. 19, no. 24, pp. 2045–2047, Dec. 15, 2007. [12] S.-G. Mun, J.-H. Moon, H.-K. Lee, J.-Y. Kim, and C.-H. Lee, “A WDM-PON with a 40 Gb/s (32 1.25 Gb/s) capacity based on wavelength-locked Fabry–Perot laser diodes,” Opt. Express, vol. 16, no. 15, pp. 11361–11368, Jul. 2008.

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