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Multiwavelength generation by utilizing second-order nonlinearity of LiNbO3 waveguides in fiber lasers
Optics Communications 224 (2003) 125–130 www.elsevier.com/locate/optcom
Short Communication
Multiwavelength generation by utilizing second-order nonlinearity of LiNbO3 waveguides in fiber lasers Junqiang Sun *, Wei Liu Department of Optoelectronic Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PeopleÕs Republic of China Received 9 December 2002; received in revised form 21 April 2003; accepted 7 July 2003
Abstract Multiwavelength oscillation is demonstrated with a novel laser scheme, in which cascaded second-order nonlinearity in a periodically poled LiNbO3 (PPLN) waveguide is used to double the lasing lines. Twenty-two wavelengths approaching S- and L-bands are simultaneously achieved with a channel spacing of 1.6 nm, and the band separation is changeable. The number of wavelengths and channel spacing can be controlled by the adjustment of the tunable Fabry– Perot (FP) etalon and light polarization states in the laser cavity. Ó 2003 Published by Elsevier B.V. PACS: 42.55.Wd; 42.65.)k Keywords: Second-order nonlinearity; Multiwavelength lasing; Erbium-doped fiber; Wavelength-division multiplexing
1. Introduction Wavelength-division multiplexing (WDM) technique has manifested its potentials to satisfy exponential growth of various traffics from IP users. As the transmission capacity of WDM systems tends towards several Tbit/s, multiwavelength laser source becomes more important, taking into account the increments of the source cost and complexity with the increase of the number of channels [1]. Furthermore, the operat*
Corresponding author. Tel.: +86-278-754-3355ext.5; fax: +86-278-755-6188. E-mail address: [email protected] (J. Sun). 0030-4018/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0030-4018(03)01727-9
ing wavelength region of the multiwavelength laser source is required to extend from the S-band (1490–1520 nm) to the L-band (1570–1610 nm) to respond to the rapid development of WDM systems. Erbium-doped fiber (EDF) is considered as an alternative gain medium to achieve multiwavelength oscillation due to its high gain with broad gain bandwidth and compatibility with fiber communication systems [2–4]. However, large homogeneous linewidth of EDF at room temperature poses a major barrier to obtain stable simultaneous multiwavelength lasing. Therefore, multiwavelength oscillation with erbium-doped fiber lasers (EDFLs) was previously proposed by cooling EDF down to liquid nitrogen temperature
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in order to reduce the homogeneous linewidth of EDF and thus the strong mode competition is suppressed [3]. But this method is inconvenient and may impact the device durability, leading to complexity in component configuration. Recent advances have shown that installing various comb filters inside the cavity can realize multiwavelength oscillation even at room temperature and anchor the lasing wavelengths on ITU grid. Such filters can be constructed by the use of sampled fiber Bragg gratings [4], high-birefringence fiber loop mirror [5], and various kinds interferometers [6]. Fiber nonlinearities are also exploited to implement multiwavelength oscillation, such as utilizing the nonlinear Brillouin gain and linear EDF gain in Brillouin/erbium fiber lasers [7], and combining stimulated Brillouin scattering (SBS) with fourwave mixing (FWM) effect in Fabry–Perot (FP) EDFL [8]. In the mentioned approaches above, however, few of them can generate multiple wavelengths covering different bands and hold the tunable characteristics including the changeable lasing wavelengths and channel spacing. In this paper, a novel multiwavelength EDFL is developed for the enhancement of multiwavelength performance. A periodically poled LiNbO3 (PPLN) waveguide is employed to double the number of wavelengths by means of the cascade of second-order nonlinearity. With this configuration, we have successfully obtained dual-band 22 wavelengths individually towards S-band and Lband regions. Additionally, the lasing wavelengths and the channel spacing are changeable. The results imply that such a scheme provides the possibility to broaden wavelength range for multiwavelength laser source.
2. Experimental setup The experimental arrangement for multiwavelength generation is schematically shown in Fig. 1, which embodies two fiber ring lasers. One is composed of a variable attenuator, a tunable FP etalon, a bandpass filter, an isolator, a PPLN waveguide, polarization controllers, EDFA2 and EDFA3, and another is formed by a tunable filter, the PPLN waveguide, the isolator, polari-
Fig. 1. Experimental arrangement for multiwavelength generation. PC, Polarization controller; FC, fiber coupler.
zation controllers, EDFA1 and EDFA2. The lasing wavelength of the upper fiber ring laser can be tuned as a pump wavelength to meet the quasi-phasematching (QPM) condition for the second-harmonic generation (SHG) in the PPLN waveguide. EDFA1, which contains a 15-m long EDF, is pumped by a 980 nm laser diode with a maximum power of 80 mW through a 980/1550 WDM fiber coupler. It can provide the smallsignal gain of 25 dB and its saturated output power is 14 dBm. EDFA3 is the same as EDFA1. EDFA2 is a commercial EDFA with a high saturated output power of 27 dBm and its small-signal gain is 32 dB. Aside from EDFA 2, there are no additional isolators in another EDFA configurations. The PPLN waveguide is fabricated by annealed proton exchange technique [9] with the length of 50 mm. The waveguide width is approximate 10 lm, and the QPM period is 15.31 lm. These device parameters allow phasematching at room temperature between the fundamental mode of the pump at 1544 nm and the SHG wave at 772 nm. The fiber to fiber coupling loss is estimated about 4.7 dB caused by the reflection losses at the uncoated end surfaces, mode mismatching between the fibers and the PPLN waveguide, and intrinsic waveguide losses. The polarization-independent optical isolators are positioned to prevent back reflection and thus to ensure unidirectional laser propagation in the fiber rings. The tunable FP etalon serves as a comb filter with a minimum free spectral range of
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1.6 nm and bandwidth of 0.8 nm, and this etalon can be adjusted to change the free spectral range and the number of channels. The bandpass filter is a wideband filter used in coarse wavelengthdivision multiplexers (CWDMs) centering at the wavelength of 1570 nm. It has a passband width of 16.3 nm at )0.5 dB. This filter here is used to inhibit the lasing wavelength region and filter the amplified spontaneous emission (ASE) noise. The tunable filter can tune the wavelength from 1535 to 1585 nm with a bandwidth of 1 nm. The inline polarization controllers are inserted into the cavity to align the polarization states to realize optimal QPM and enhance birefringence effect. The multiwavelength oscillation is generated in the lower fiber laser thanks to the comb filter employed in the fiber loop. The number of lasing lines and channel spacing are determined by the comb filter and the lasing wavelength range is dominated by the bandpass filter. These lasing wavelengths are then doubled through the difference-frequency generation (DFG) effect in PPLN. The variable optical attenuator is used to adjust the cavity loss and to obtain optimal DFG efficiency. The output multiwavelength lines are extracted through a 9:1 fiber coupler. The output spectra of the laser are monitored by an optical spectrum analyzer (OSA) with the highest spectral resolution of 0.06 nm.
3. Experimental results and discussion The operation principle of the multiwavelength laser can be described as follows. The generated pump wavelength (kp ) in the upper fiber ring laser is first frequency-doubled in the PPLN waveguide through the nonlinear SHG effect. Then the frequency-doubled wavelength (kp /2) will interact with the lasing wavelength (ks1 ) from the lower fiber ring laser and the new wavelength (ks2 ) is created via nonlinear DFG effect. The new wavelength (ks2 ) is governed by the following expression according to the frequency mixing equation [10]: 1 2 1 ¼ : ks2 kp ks1
ð1Þ
127
From (1), we find that the wavelength ks2 satisfies the mirror image relationship relative to the wavelength ks1 as though a mirror were placed at kp (between ks1 and ks2 ). If more than one lasing wavelength are launched in the PPLN, the expression (1) is still available and thus the same number of wavelengths is produced as the number of lasing wavelengths. The lasing band span can also be adjusted by tuning the wavelength ks1 . Based on the nonlinear DFG effect, the optical power at ks2 is mainly dominated by the intracavity pump power at kp . Accordingly, we accommodate the laser parameters to obtain the highest optical power at kp . Considering that there exists gain competition between the pump wavelength kp and lasing wavelength ks1 , we adjust the variable attenuator in the fiber ring to sustain the pump power at the highest level. Fig. 2 illustrates the measured output spectra from the experimental configuration when a narrow-band tunable filter substitutes the tunable FP etalon and no bandpass filter is adopted. The pump wavelength is tuned at 1543.4 nm through the tunable filter to realize strong second-order nonlinear effect. The lasing wavelength is varied from 1547.6 to 1580.2 nm with the adjustment of the narrow-band tunable filter inside the lower fiber ring. Consequently, three-wavelength is observed in the output spectra, which include the lasing wavelength, the pump wavelength, and the new generated wavelength. The new generated wavelength tends towards the short-wavelength side while the lasing wavelength is tuned towards the longwavelength side. The new generated wavelength locates at 1508.5 nm and out of the EDF gain region in Fig. 2(c). This implies that signal at Sband wavelength can be produced with this scheme and no gain medium is required at S-band wavelength side. The output power difference is kept about 25 dB for the entire wavelength conversion range, which corresponds to the constant DFG efficiency within the rather wide wavelength region (about 70 nm) [9]. The intracavity pump power is assessed approximate 15.6 dBm and is less than the pump power requirement for DFG effect reported previously [11]. Fig. 3 shows output optical spectra when the tunable FP etalon and bandpass filter are applied. By adjusting the
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Fig. 2. Measured spectra when the tunable FP etalon is substituted by a narrow-band tunable filter and no bandpass filter is adopted. The lasing wavelength is: (a) 1547.6 nm, (b) 1565.2 nm, (c) 1580.2 nm, respectively.
tunable FP etalon and the polarization controllers, dual- and quad-wavelength oscillation are obtained and their corresponding image wavelengths are also observed. The channel spacing is varied from 1.6 to 5.4 nm during the process of adjusting the tunable FP etalon and light polarization states in the cavity. This phenomenon may be attributed to the high-birefringence effect induced by the PPLN waveguide [12]. Thus, in addition to the FP etalon, another comb-like filter is built up due to the polarization hole burning in the EDF gain profile. The above results indicate that not only can the wavelengths locating within different bands be generated but also the channel separation can be expediently changed. The mirror image relationship for the wavelengths be-
longing to different bands is distinctly manifested in the measured spectra. According to DFG mechanism, the optical fields of different bands are spectral inversion with each other. Therefore, employing wavelengths within different bands as optical carriers can compensate system dispersion in high-speed optical communication network. Eleven-wavelength and its image wavelengths are achieved by the further adjustment of FP etalon and light polarization states in the cavity (shown in Fig. 4). The channel spacing is measured about 1.6 nm. The output power of each wavelength in this case is lower compared to the situations in Fig. 3. From the measured spectra we find that the ASE noise caused by EDFA is successfully suppressed. The reasons can be explained as follows.
J. Sun, W. Liu / Optics Communications 224 (2003) 125–130
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Fig. 3. The multiwavelength spectra when the tunable FP etalon and the bandpass filter are adopted: (a) dual-wavelength with 2.8 nm channel spacing; (b) dual-wavelength with 5.4 nm channel spacing; and (c) quad-wavelength with 1.6 nm channel spacing.
On the one hand, the residual inverted population in EDFA is completely depleted owing to the stimulated emission at the pump and the lasing wavelengths. On the other hand, the ASE noise can be effectively filtered due to the fact that the DFG efficiency is decreased with optical power [9]. As we well know, stable multiwavelength operation in EDFLs is usually difficult due to the EDFA homogeneous broadening gain characteristics that result in strong mode competition and unstable lasing. However, in our experimental scheme, the mode competition and instability are significantly suppressed. The reason is attributed to the following facts. Firstly, according to the EDFA gain property, the lasing wavelengths from
1561 to 1579 nm locate within the gain-flattened region [13]. Secondly, different transmission losses for different polarization states are formed owing to introducing the LiNbO3 optical waveguide in the fiber ring cavity. Thus, the polarization and spatial inhomogeneities are enhanced, which is benefit to the weakening of the mode competition. Finally, the loss corresponding to each lasing line can be balanced by designing the transmission profile of the comb filter. Marking one of the lasing lines and running the OSA repeatedly, we find that lasing wavelength and channel spacing keep unchanged and there are slight power fluctuations in the lasing lines. The maximum output power ripples are measured about 0.8 dB.
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positioned at dual-band are achieved using the suggested configuration. In addition to that the number of wavelengths and channel spacing are varied by the adjustment of the tunable FP etalon and light polarization states in the cavity, this laser can simultaneously generate wavelengths covering two different bands with spectral inversion. The laser has potential applications in dispersion management WDM systems.
Acknowledgements Fig. 4. The measured spectra when both the tunable FP etalon and the bandpass filter are employed. The passband is from 1561 to1579 nm. Twenty-two wavelengths with the channel spacing of 1.6 nm locate within two different bands.
During the above experiments, the pump wavelength is fixed at 1543.4 nm to meet the QPM condition of PPLN waveguide at room temperature. In fact, this pump wavelength may be changed by the design of different QPM period and the adjustment of operation temperature for the PPLN waveguide. Moreover, the QPM condition can be further relaxed by introducing various phase-shifting domains into the QPM gratings [14]. As a result, the lasing wavelengths within the entire EDFA gain bandwidth can be used as the pump wavelength [15]. Broadband multiwavelength laser sources from 1400 to 1700 nm are expected to potentially implement by employing our scheme. Multiwavelength signals covering from S- to L-band will be subject of the later study.
4. Conclusion A novel multiwavelength laser is proposed and demonstrated by introducing the PPLN wavelength waveguide in fiber lasers. This scheme exploits the cascaded second-order nonlinearity in PPLN to double the lasing wavelengths. Twentytwo wavelengths with 1.6 nm channel spacing
We would like to thank Dr. J.R. Kurz of Stanford University for providing the PPLN waveguide for the experiments. This work was supported by the Chinese Natural Science Foundation under Grant No. 60177015.
References [1] J.J. Veselka, S.K. Korotky, IEEE Photon. Technol. Lett. 10 (7) (1998) 958. [2] N. Park, J.W. Dawson, K.J. Vahala, IEEE Photon. Technol. Lett. 4 (6) (1992) 540. [3] S. Yamashita, K. Hotate, Electron. Lett. 32 (14) (1996) 1298. [4] J. Chow, G. Town, B. Eggleton, M. Ibsen, K. Sugden, I. Bennion, IEEE Photon. Technol. Lett. 8 (1) (1996) 60. [5] X.P. Dong, S. Li, K.S. Chiang, M.N. Ng, B.C.B. Chu, Electron. Lett. 36 (19) (2000) 1609. [6] H.L. An, X.Z. Lin, E.Y.B. Pun, H.D. Liu, Opt. Commun. 169 (1) (1999) 159. [7] G.J. Cowle, D.Y. Stepanov, Y.T. Chieng, J. Lightwave Technol. 15 (7) (1997) 1198. [8] D.S. Lim, H.K. Lee, K.H. Kim, S.B. Kang, J.T. Ahn, M.-Y. Jeon, Opt. Lett. 23 (21) (1998) 1671. [9] M.H. Chou, I. Brener, M.M. Fejer, E.E. Chaban, S.B. Christman, IEEE Photon. Technol. Lett. 11 (6) (1999) 653. [10] N. Antoniades, S.J.B. Yoo, K. Bala, G. Ellinas, T.E. Stern, J. Lightwave Technol. 17 (7) (1999) 1113. [11] K.R. Parameswaran, R.K. Route, J.R. Kurz, R.V. Roussev, M.M. Fejer, M. Fujimura, Opt. Lett. 27 (3) (2002) 179. [12] J. Sun, J. Qiu, D. Huang, Opt. Comm. 182 (1) (1999) 193. [13] H. Ono, M. Yamada, Y. Ohishi, IEEE Photon. Technol. Lett. 9 (5) (1997) 596. [14] K. Mizuuchi, K. Yamamoto, Opt. Lett. 23 (24) (1998) 1880. [15] K. Gallo, G. Assanto, G. Stegeman, Appl. Phys. Lett. 71 (8) (1997) 1020.
Short Communication
Multiwavelength generation by utilizing second-order nonlinearity of LiNbO3 waveguides in fiber lasers Junqiang Sun *, Wei Liu Department of Optoelectronic Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PeopleÕs Republic of China Received 9 December 2002; received in revised form 21 April 2003; accepted 7 July 2003
Abstract Multiwavelength oscillation is demonstrated with a novel laser scheme, in which cascaded second-order nonlinearity in a periodically poled LiNbO3 (PPLN) waveguide is used to double the lasing lines. Twenty-two wavelengths approaching S- and L-bands are simultaneously achieved with a channel spacing of 1.6 nm, and the band separation is changeable. The number of wavelengths and channel spacing can be controlled by the adjustment of the tunable Fabry– Perot (FP) etalon and light polarization states in the laser cavity. Ó 2003 Published by Elsevier B.V. PACS: 42.55.Wd; 42.65.)k Keywords: Second-order nonlinearity; Multiwavelength lasing; Erbium-doped fiber; Wavelength-division multiplexing
1. Introduction Wavelength-division multiplexing (WDM) technique has manifested its potentials to satisfy exponential growth of various traffics from IP users. As the transmission capacity of WDM systems tends towards several Tbit/s, multiwavelength laser source becomes more important, taking into account the increments of the source cost and complexity with the increase of the number of channels [1]. Furthermore, the operat*
Corresponding author. Tel.: +86-278-754-3355ext.5; fax: +86-278-755-6188. E-mail address: [email protected] (J. Sun). 0030-4018/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0030-4018(03)01727-9
ing wavelength region of the multiwavelength laser source is required to extend from the S-band (1490–1520 nm) to the L-band (1570–1610 nm) to respond to the rapid development of WDM systems. Erbium-doped fiber (EDF) is considered as an alternative gain medium to achieve multiwavelength oscillation due to its high gain with broad gain bandwidth and compatibility with fiber communication systems [2–4]. However, large homogeneous linewidth of EDF at room temperature poses a major barrier to obtain stable simultaneous multiwavelength lasing. Therefore, multiwavelength oscillation with erbium-doped fiber lasers (EDFLs) was previously proposed by cooling EDF down to liquid nitrogen temperature
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in order to reduce the homogeneous linewidth of EDF and thus the strong mode competition is suppressed [3]. But this method is inconvenient and may impact the device durability, leading to complexity in component configuration. Recent advances have shown that installing various comb filters inside the cavity can realize multiwavelength oscillation even at room temperature and anchor the lasing wavelengths on ITU grid. Such filters can be constructed by the use of sampled fiber Bragg gratings [4], high-birefringence fiber loop mirror [5], and various kinds interferometers [6]. Fiber nonlinearities are also exploited to implement multiwavelength oscillation, such as utilizing the nonlinear Brillouin gain and linear EDF gain in Brillouin/erbium fiber lasers [7], and combining stimulated Brillouin scattering (SBS) with fourwave mixing (FWM) effect in Fabry–Perot (FP) EDFL [8]. In the mentioned approaches above, however, few of them can generate multiple wavelengths covering different bands and hold the tunable characteristics including the changeable lasing wavelengths and channel spacing. In this paper, a novel multiwavelength EDFL is developed for the enhancement of multiwavelength performance. A periodically poled LiNbO3 (PPLN) waveguide is employed to double the number of wavelengths by means of the cascade of second-order nonlinearity. With this configuration, we have successfully obtained dual-band 22 wavelengths individually towards S-band and Lband regions. Additionally, the lasing wavelengths and the channel spacing are changeable. The results imply that such a scheme provides the possibility to broaden wavelength range for multiwavelength laser source.
2. Experimental setup The experimental arrangement for multiwavelength generation is schematically shown in Fig. 1, which embodies two fiber ring lasers. One is composed of a variable attenuator, a tunable FP etalon, a bandpass filter, an isolator, a PPLN waveguide, polarization controllers, EDFA2 and EDFA3, and another is formed by a tunable filter, the PPLN waveguide, the isolator, polari-
Fig. 1. Experimental arrangement for multiwavelength generation. PC, Polarization controller; FC, fiber coupler.
zation controllers, EDFA1 and EDFA2. The lasing wavelength of the upper fiber ring laser can be tuned as a pump wavelength to meet the quasi-phasematching (QPM) condition for the second-harmonic generation (SHG) in the PPLN waveguide. EDFA1, which contains a 15-m long EDF, is pumped by a 980 nm laser diode with a maximum power of 80 mW through a 980/1550 WDM fiber coupler. It can provide the smallsignal gain of 25 dB and its saturated output power is 14 dBm. EDFA3 is the same as EDFA1. EDFA2 is a commercial EDFA with a high saturated output power of 27 dBm and its small-signal gain is 32 dB. Aside from EDFA 2, there are no additional isolators in another EDFA configurations. The PPLN waveguide is fabricated by annealed proton exchange technique [9] with the length of 50 mm. The waveguide width is approximate 10 lm, and the QPM period is 15.31 lm. These device parameters allow phasematching at room temperature between the fundamental mode of the pump at 1544 nm and the SHG wave at 772 nm. The fiber to fiber coupling loss is estimated about 4.7 dB caused by the reflection losses at the uncoated end surfaces, mode mismatching between the fibers and the PPLN waveguide, and intrinsic waveguide losses. The polarization-independent optical isolators are positioned to prevent back reflection and thus to ensure unidirectional laser propagation in the fiber rings. The tunable FP etalon serves as a comb filter with a minimum free spectral range of
J. Sun, W. Liu / Optics Communications 224 (2003) 125–130
1.6 nm and bandwidth of 0.8 nm, and this etalon can be adjusted to change the free spectral range and the number of channels. The bandpass filter is a wideband filter used in coarse wavelengthdivision multiplexers (CWDMs) centering at the wavelength of 1570 nm. It has a passband width of 16.3 nm at )0.5 dB. This filter here is used to inhibit the lasing wavelength region and filter the amplified spontaneous emission (ASE) noise. The tunable filter can tune the wavelength from 1535 to 1585 nm with a bandwidth of 1 nm. The inline polarization controllers are inserted into the cavity to align the polarization states to realize optimal QPM and enhance birefringence effect. The multiwavelength oscillation is generated in the lower fiber laser thanks to the comb filter employed in the fiber loop. The number of lasing lines and channel spacing are determined by the comb filter and the lasing wavelength range is dominated by the bandpass filter. These lasing wavelengths are then doubled through the difference-frequency generation (DFG) effect in PPLN. The variable optical attenuator is used to adjust the cavity loss and to obtain optimal DFG efficiency. The output multiwavelength lines are extracted through a 9:1 fiber coupler. The output spectra of the laser are monitored by an optical spectrum analyzer (OSA) with the highest spectral resolution of 0.06 nm.
3. Experimental results and discussion The operation principle of the multiwavelength laser can be described as follows. The generated pump wavelength (kp ) in the upper fiber ring laser is first frequency-doubled in the PPLN waveguide through the nonlinear SHG effect. Then the frequency-doubled wavelength (kp /2) will interact with the lasing wavelength (ks1 ) from the lower fiber ring laser and the new wavelength (ks2 ) is created via nonlinear DFG effect. The new wavelength (ks2 ) is governed by the following expression according to the frequency mixing equation [10]: 1 2 1 ¼ : ks2 kp ks1
ð1Þ
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From (1), we find that the wavelength ks2 satisfies the mirror image relationship relative to the wavelength ks1 as though a mirror were placed at kp (between ks1 and ks2 ). If more than one lasing wavelength are launched in the PPLN, the expression (1) is still available and thus the same number of wavelengths is produced as the number of lasing wavelengths. The lasing band span can also be adjusted by tuning the wavelength ks1 . Based on the nonlinear DFG effect, the optical power at ks2 is mainly dominated by the intracavity pump power at kp . Accordingly, we accommodate the laser parameters to obtain the highest optical power at kp . Considering that there exists gain competition between the pump wavelength kp and lasing wavelength ks1 , we adjust the variable attenuator in the fiber ring to sustain the pump power at the highest level. Fig. 2 illustrates the measured output spectra from the experimental configuration when a narrow-band tunable filter substitutes the tunable FP etalon and no bandpass filter is adopted. The pump wavelength is tuned at 1543.4 nm through the tunable filter to realize strong second-order nonlinear effect. The lasing wavelength is varied from 1547.6 to 1580.2 nm with the adjustment of the narrow-band tunable filter inside the lower fiber ring. Consequently, three-wavelength is observed in the output spectra, which include the lasing wavelength, the pump wavelength, and the new generated wavelength. The new generated wavelength tends towards the short-wavelength side while the lasing wavelength is tuned towards the longwavelength side. The new generated wavelength locates at 1508.5 nm and out of the EDF gain region in Fig. 2(c). This implies that signal at Sband wavelength can be produced with this scheme and no gain medium is required at S-band wavelength side. The output power difference is kept about 25 dB for the entire wavelength conversion range, which corresponds to the constant DFG efficiency within the rather wide wavelength region (about 70 nm) [9]. The intracavity pump power is assessed approximate 15.6 dBm and is less than the pump power requirement for DFG effect reported previously [11]. Fig. 3 shows output optical spectra when the tunable FP etalon and bandpass filter are applied. By adjusting the
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Fig. 2. Measured spectra when the tunable FP etalon is substituted by a narrow-band tunable filter and no bandpass filter is adopted. The lasing wavelength is: (a) 1547.6 nm, (b) 1565.2 nm, (c) 1580.2 nm, respectively.
tunable FP etalon and the polarization controllers, dual- and quad-wavelength oscillation are obtained and their corresponding image wavelengths are also observed. The channel spacing is varied from 1.6 to 5.4 nm during the process of adjusting the tunable FP etalon and light polarization states in the cavity. This phenomenon may be attributed to the high-birefringence effect induced by the PPLN waveguide [12]. Thus, in addition to the FP etalon, another comb-like filter is built up due to the polarization hole burning in the EDF gain profile. The above results indicate that not only can the wavelengths locating within different bands be generated but also the channel separation can be expediently changed. The mirror image relationship for the wavelengths be-
longing to different bands is distinctly manifested in the measured spectra. According to DFG mechanism, the optical fields of different bands are spectral inversion with each other. Therefore, employing wavelengths within different bands as optical carriers can compensate system dispersion in high-speed optical communication network. Eleven-wavelength and its image wavelengths are achieved by the further adjustment of FP etalon and light polarization states in the cavity (shown in Fig. 4). The channel spacing is measured about 1.6 nm. The output power of each wavelength in this case is lower compared to the situations in Fig. 3. From the measured spectra we find that the ASE noise caused by EDFA is successfully suppressed. The reasons can be explained as follows.
J. Sun, W. Liu / Optics Communications 224 (2003) 125–130
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Fig. 3. The multiwavelength spectra when the tunable FP etalon and the bandpass filter are adopted: (a) dual-wavelength with 2.8 nm channel spacing; (b) dual-wavelength with 5.4 nm channel spacing; and (c) quad-wavelength with 1.6 nm channel spacing.
On the one hand, the residual inverted population in EDFA is completely depleted owing to the stimulated emission at the pump and the lasing wavelengths. On the other hand, the ASE noise can be effectively filtered due to the fact that the DFG efficiency is decreased with optical power [9]. As we well know, stable multiwavelength operation in EDFLs is usually difficult due to the EDFA homogeneous broadening gain characteristics that result in strong mode competition and unstable lasing. However, in our experimental scheme, the mode competition and instability are significantly suppressed. The reason is attributed to the following facts. Firstly, according to the EDFA gain property, the lasing wavelengths from
1561 to 1579 nm locate within the gain-flattened region [13]. Secondly, different transmission losses for different polarization states are formed owing to introducing the LiNbO3 optical waveguide in the fiber ring cavity. Thus, the polarization and spatial inhomogeneities are enhanced, which is benefit to the weakening of the mode competition. Finally, the loss corresponding to each lasing line can be balanced by designing the transmission profile of the comb filter. Marking one of the lasing lines and running the OSA repeatedly, we find that lasing wavelength and channel spacing keep unchanged and there are slight power fluctuations in the lasing lines. The maximum output power ripples are measured about 0.8 dB.
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J. Sun, W. Liu / Optics Communications 224 (2003) 125–130
positioned at dual-band are achieved using the suggested configuration. In addition to that the number of wavelengths and channel spacing are varied by the adjustment of the tunable FP etalon and light polarization states in the cavity, this laser can simultaneously generate wavelengths covering two different bands with spectral inversion. The laser has potential applications in dispersion management WDM systems.
Acknowledgements Fig. 4. The measured spectra when both the tunable FP etalon and the bandpass filter are employed. The passband is from 1561 to1579 nm. Twenty-two wavelengths with the channel spacing of 1.6 nm locate within two different bands.
During the above experiments, the pump wavelength is fixed at 1543.4 nm to meet the QPM condition of PPLN waveguide at room temperature. In fact, this pump wavelength may be changed by the design of different QPM period and the adjustment of operation temperature for the PPLN waveguide. Moreover, the QPM condition can be further relaxed by introducing various phase-shifting domains into the QPM gratings [14]. As a result, the lasing wavelengths within the entire EDFA gain bandwidth can be used as the pump wavelength [15]. Broadband multiwavelength laser sources from 1400 to 1700 nm are expected to potentially implement by employing our scheme. Multiwavelength signals covering from S- to L-band will be subject of the later study.
4. Conclusion A novel multiwavelength laser is proposed and demonstrated by introducing the PPLN wavelength waveguide in fiber lasers. This scheme exploits the cascaded second-order nonlinearity in PPLN to double the lasing wavelengths. Twentytwo wavelengths with 1.6 nm channel spacing
We would like to thank Dr. J.R. Kurz of Stanford University for providing the PPLN waveguide for the experiments. This work was supported by the Chinese Natural Science Foundation under Grant No. 60177015.
References [1] J.J. Veselka, S.K. Korotky, IEEE Photon. Technol. Lett. 10 (7) (1998) 958. [2] N. Park, J.W. Dawson, K.J. Vahala, IEEE Photon. Technol. Lett. 4 (6) (1992) 540. [3] S. Yamashita, K. Hotate, Electron. Lett. 32 (14) (1996) 1298. [4] J. Chow, G. Town, B. Eggleton, M. Ibsen, K. Sugden, I. Bennion, IEEE Photon. Technol. Lett. 8 (1) (1996) 60. [5] X.P. Dong, S. Li, K.S. Chiang, M.N. Ng, B.C.B. Chu, Electron. Lett. 36 (19) (2000) 1609. [6] H.L. An, X.Z. Lin, E.Y.B. Pun, H.D. Liu, Opt. Commun. 169 (1) (1999) 159. [7] G.J. Cowle, D.Y. Stepanov, Y.T. Chieng, J. Lightwave Technol. 15 (7) (1997) 1198. [8] D.S. Lim, H.K. Lee, K.H. Kim, S.B. Kang, J.T. Ahn, M.-Y. Jeon, Opt. Lett. 23 (21) (1998) 1671. [9] M.H. Chou, I. Brener, M.M. Fejer, E.E. Chaban, S.B. Christman, IEEE Photon. Technol. Lett. 11 (6) (1999) 653. [10] N. Antoniades, S.J.B. Yoo, K. Bala, G. Ellinas, T.E. Stern, J. Lightwave Technol. 17 (7) (1999) 1113. [11] K.R. Parameswaran, R.K. Route, J.R. Kurz, R.V. Roussev, M.M. Fejer, M. Fujimura, Opt. Lett. 27 (3) (2002) 179. [12] J. Sun, J. Qiu, D. Huang, Opt. Comm. 182 (1) (1999) 193. [13] H. Ono, M. Yamada, Y. Ohishi, IEEE Photon. Technol. Lett. 9 (5) (1997) 596. [14] K. Mizuuchi, K. Yamamoto, Opt. Lett. 23 (24) (1998) 1880. [15] K. Gallo, G. Assanto, G. Stegeman, Appl. Phys. Lett. 71 (8) (1997) 1020.