Multiwavelength erbium-doped fiber laser based on graphene oxide Xia Hao,1,* Zhengrong Tong,1 Junfa Zhao,2 Ye Cao,1 and Lan Li1 1
School of Computer and Communication Engineering, Tianjin Key Laboratory of Film Electronic and Communication Devices, Tianjin University of Technology, Tianjin 300384, China
2
School of Electronics and Information Engineering, Tianjin Polytechnic University, Tianjin 300387, China *Corresponding author: [email protected] Received 1 April 2014; revised 27 May 2014; accepted 8 June 2014; posted 9 June 2014 (Doc. ID 209174); published 9 July 2014
A multiwavelength erbium-doped fiber (EDF) laser based on graphene oxide (GO) has been proposed, to the best of our knowledge, for the first time, to generate an output of stable wavelengths. The structure mainly comprises a few layers of GO between two single-mode fibers incorporated into a capillary device and a Lyot comb filter. GO can show a good nonlinear optical effect, which is beneficial to suppress the mode competition caused by the EDF and stabilize the multiwavelength output. With assistance from the GO device, 11 stable simultaneous lasing signals with a power nonuniformity of about 1.5 dB are obtained. Wavelength spacing is about 0.42 nm and the linewidth of each wavelength is less than 0.07 nm. © 2014 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (190.4360) Nonlinear optics, devices; (160.4670) Optical materials. http://dx.doi.org/10.1364/AO.53.004519
1. Introduction
Since fiber lasers have attracted attention widely, all kinds of rare-earth-doped lasers, especially erbium-doped fiber (EDF) lasers, are always the mainstream of research. Multiwavelength EDF lasers (MWEDFLs), which are used in wavelengthdivision multiplexing (WDM) systems, certainly attract considerable interest as well [1]. There are many different methods proposed for implementing room-temperature multiwavelength fiber lasers. In order to attain multiwavelength operation, a multicenter filter should usually be used, such as a Fabry– Perot filter, a chirped, or a sampled fiber grating. In addition, a simple Lyot filter can be an alternative. However, homogeneous gain broadening of an EDF leads to unstable mode competition. It is really difficult to obtain stable multiwavelength lasing. At present, a variety of techniques have been come up with in order to overcome this difficulty. The 1559-128X/14/204519-05$15.00/0 © 2014 Optical Society of America
major one is adding a special physical mechanism into the optical cavity, such as nonlinear polarization rotation, a nonlinear optical effect, four-wave mixing, etc. [2–5] A feasible alternative toward acquiring stable multiwavelength lasing in an EDF-assisted fiber laser is a nonlinear optical effect, which is considered a splendid method to suppress the mode competition caused by an EDF at room temperature. For example, nonlinear optical loop mirrors have been used as amplitude equalizers to induce intensity-dependent loss and alleviate the mode competition caused by homogeneous gain broadening in EDFs [6]. Typically, integrating a nonlinear medium, such as a single-mode fiber (SMF) that requires a long length (more than several kilometers), dispersion-shifted fibers, photonic crystal fibers, or highly nonlinear fibers, into the fiber laser can induce a nonlinear optical effect [7–9]. However, these special fibers usually lead to increase of cost and complexity of the systems. To overcome these problems, many experiments using graphene have been proposed and reported previously [3,4,10–14]. Graphene oxide (GO) is also 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
4519
the highly potential medium reported by Loh et al. (2010). It can show a good nonlinear optical effect [15]. In addition, GO is very simple and cheap to manufacture, and has strong adsorption [16]. Several layers of GO may show the same effect as high-cost special fibers, possessing enormous potential as a stabilizing medium for MWEDFLs. In this paper, to the best of our knowledge, a compact and stable multiwavelength fiber laser based on GO films and a capillary that serves as a platform to realize fiber joints is achieved for the first time. The system uses a Lyot structure as a comb filter to generate a multiwavelength laser. The proposed MWEDFL will have stable 11-channel simultaneous lasing with a wavelength spacing of 0.42 nm, providing numerous advantages for a multitude of applications. 2. Experimental Setup and Principle
The proposed schematic diagram of the experimental setup is shown in Fig. 1. A 980 nm laser diode is used as the pump source and is connected to a 7 m long EDF through a 980 nm∕1550 nm wavelengthdivision multiplexer. The other end of the EDF is then connected to a Lyot comb filter that is made up of a polarization-dependent isolator (PD-ISO), two polarization controllers (PCs), and a polarization-maintaining fiber (PMF). On account of superior characteristics such as low insertion loss and controllable wavelength spacing, the Lyot filter has been used widely [17]. The transfer function of the Lyot comb filter can be expressed as [2] jTj2 cos2 α cos2 β sin2 α sin2 β
1 sin 2α sin 2β cosΔφ Δφ0 : 2
(1)
Here, α and β are the angles between the polarization directions of the polarizers and the fast axes of the fiber, and Δφ is the phase difference between the wave components in the two orthogonal birefringent axes, which is introduced by the PCs. Δφ0 is the linear phase shift difference. Note that the transmission of the Lyot filter is a periodic function of Δφ; therefore, the spectral spacing and the transmission peak position of the intracavity birefringence-induced comb filter can be tuned by rotating the PCs, as
Fig. 1. Schematic diagram of the experimental setup. 4520
APPLIED OPTICS / Vol. 53, No. 20 / 10 July 2014
Fig. 2. Simulation result of the Lyot filter with PC2 in state (−π∕4, π∕4), whereas PC1 is in state (π∕3).
shown in Fig. 2. The transmission of the Lyot has a period with equal spacing, which can be expressed as Δλ λ2 ∕Δn × L:
(2)
Apparently, the wavelength spacing can be varied by changing the length of the PMF. Here, Δn is 0.0006 and the length of the PMF is selected to be 4, 10, and 15 m. It can be calculated that the corresponding wavelength spacings are 1.04, 0.42, and 0.24 nm, respectively, according to Eq. (2). Then the light encounters the GO films, which are housed between the two fiber ends. The output from the GO is connected to a 90:10 coupler, where a portion of the output containing wavelengths is extracted by a 10% port into an optical spectrum analyzer (OSA) with a resolution of 0.07 nm and the rest is fed back into the laser cavity. The GO used in our experiment is first created by depositing on the face of a fiber end. First, two segments of SMFs whose cladding diameter is 125 μm are chosen; then the fiber coating is peeled off, with the cladding and core left. The fibers, which are cut flatly, are immersed into GO solution for about 30 min. The GO solution is deposited on the fiber end. Second, the fibers with GO films are left to dry at room temperature. Third, the two segments of fibers are inserted into a capillary whose core diameter is 126 μm. The capillary is perfectly suitable for the SMFs. Figure 3 shows the structure of the fiber that is inserted into the capillary. Then the device is fixed on a table to form a stable platform. The total insertion loss of the GO–capillary
Fig. 3. Structure of the fiber with GO that is inserted into the capillary.
device is about 8 dB. The role of the GO films in this device is to stabilize the multiwavelength output by using the very strong nonlinear optical effect [13]. 3. Results and Discussion
In order to prove the importance of GO, an experiment that uses 4, 10, and 15 m PMFs is performed. First, GO is not used purposely in the ring cavity. The threshold is about 30 mW in our fiber laser. There is only one channel showing lasing at 1531 nm. When the 980 nm pump’s power is increased to 100 mW, several wavelengths are induced around 1531 nm and it is reasonable that they are quite unstable because of the high mode competition caused by the EDF. When the pump power is continued to be increased up to 160 mW, the number of the output lines
reaches the maximum and remains constant. However, the output power does not increase until pump power reaches 260 mW. Next, the GO–capillary device is put into the ring cavity. Wavelengths from 1528 to 1534 nm are excited and the output is more improved than that without GO. Figure 4 shows a record of the output spectra by the OSA at a pump power of 260 mW. Figures 4(a), 4(b), and 4(c) represent, respectively, random outputs with 4, 10, and 15 m long PMFs in the experiment. When GO is used, as shown in Figs. 4(d)–4(f), stable 5-line, 11-line, and 15-line lasing are obtained. The number of lasing signals depends mainly on the length of the PMF. Such wonderful power uniformity partially benefits from the power redistribution of nonlinear
Fig. 4. Output lasing (a)–(c) without GO and (d)–(f) with depositing once. 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
4521
optical effect through a few layers of GO. The number of wavelengths could be increased further by increasing the thickness of the GO films. Besides, the thickness of the GO films cannot be calculated in our device. For a better scheme, the deposition experiment should be repeated 5 and 10 times. When the 10 m PMF is chosen, the corresponding wavelength spacing is close to 0.4 nm, which complies with the international telecommunication union wavelength grid, and has wide applications in optical communication. Figure 5 shows the multiwavelength outputs under different deposition times when the 10 m PMF is chosen. There is no doubt that when depositing five times, the output wavelengths are very flat, with an average output power of −18 dBm and power nonuniformity is less than 1.5 dB. The average output power decreases by about 2 dB when depositing over 10 times. In our experiment, depositing five times can be considered the best condition. By adjusting the PCs to balance the wavelengthdependent gain and loss, we can obtain 11 stable wavelengths at a pump power of 260 mW at room temperature, as shown in Fig. 5(a). When depositing the GO solution on the fiber end once, the side-mode
Fig. 6. Tuning characteristic of the MWEDFL.
Fig. 5. Output lasing with depositing (a) 5 times and (b) 10 times that uses the 10 m PMF. 4522
APPLIED OPTICS / Vol. 53, No. 20 / 10 July 2014
Fig. 7. (a) Repeatedly scanned output spectra at 10 min spacings. (b) Fluctuations of output wavelength and peak power over 1 h.
suppression ratio (SMSR) is about 37 dB. When depositing five times, the SMSR is about 34 dB. When depositing 10 times, the SMSR is about 27 dB, and power fluctuation is bigger than that with depositing five times as shown in Fig. 5(b). It is probably because the nonlinear signal is suppressed at the thick layers of GO just like the optical property of graphene [9]. The linewidth of each lasing is less than 0.07 nm (the resolution of OSA is 0.07 nm). The wavelength spacing is 0.42 nm. Moreover, the multiple wavelengths can be tuned by adjusting the PCs. The tuning range is very small (about 0.3 nm) just due to the property of the Lyot comb filter. The tuning characteristic of the MWEDFL is shown in Fig. 6. In order to measure the stability of the MWEDFL, the proposed experiment is left to run for approximately an hour. The wavelength spectra acquired are shown in Fig. 7(a). The wavelength at 1531 nm is chosen. It can be seen from Fig. 7(b) that the peak power fluctuation and wavelength shift are ∼1.1 dB and 0.05 nm, respectively, when the output is scanned at 10 min intervals over 1 h. The self-stabilizing effect is mainly benefited from GO, which is due to its four-wave mixing nonlinear optical effect [18]. 4. Conclusion
Using the high nonlinear effect of GO to suppress mode competition, a stable and simple MWEDFL is demonstrated by using a Lyot comb filter. Eleven lasing signals with an average power of −18 dBm are obtained at a pump power of 260 mW and the SMSRs are about 34 dB. The MWEDFL creates a good flat output spectrum with little fluctuations and provides a narrow linewidth of
School of Computer and Communication Engineering, Tianjin Key Laboratory of Film Electronic and Communication Devices, Tianjin University of Technology, Tianjin 300384, China
2
School of Electronics and Information Engineering, Tianjin Polytechnic University, Tianjin 300387, China *Corresponding author: [email protected] Received 1 April 2014; revised 27 May 2014; accepted 8 June 2014; posted 9 June 2014 (Doc. ID 209174); published 9 July 2014
A multiwavelength erbium-doped fiber (EDF) laser based on graphene oxide (GO) has been proposed, to the best of our knowledge, for the first time, to generate an output of stable wavelengths. The structure mainly comprises a few layers of GO between two single-mode fibers incorporated into a capillary device and a Lyot comb filter. GO can show a good nonlinear optical effect, which is beneficial to suppress the mode competition caused by the EDF and stabilize the multiwavelength output. With assistance from the GO device, 11 stable simultaneous lasing signals with a power nonuniformity of about 1.5 dB are obtained. Wavelength spacing is about 0.42 nm and the linewidth of each wavelength is less than 0.07 nm. © 2014 Optical Society of America OCIS codes: (060.3510) Lasers, fiber; (190.4360) Nonlinear optics, devices; (160.4670) Optical materials. http://dx.doi.org/10.1364/AO.53.004519
1. Introduction
Since fiber lasers have attracted attention widely, all kinds of rare-earth-doped lasers, especially erbium-doped fiber (EDF) lasers, are always the mainstream of research. Multiwavelength EDF lasers (MWEDFLs), which are used in wavelengthdivision multiplexing (WDM) systems, certainly attract considerable interest as well [1]. There are many different methods proposed for implementing room-temperature multiwavelength fiber lasers. In order to attain multiwavelength operation, a multicenter filter should usually be used, such as a Fabry– Perot filter, a chirped, or a sampled fiber grating. In addition, a simple Lyot filter can be an alternative. However, homogeneous gain broadening of an EDF leads to unstable mode competition. It is really difficult to obtain stable multiwavelength lasing. At present, a variety of techniques have been come up with in order to overcome this difficulty. The 1559-128X/14/204519-05$15.00/0 © 2014 Optical Society of America
major one is adding a special physical mechanism into the optical cavity, such as nonlinear polarization rotation, a nonlinear optical effect, four-wave mixing, etc. [2–5] A feasible alternative toward acquiring stable multiwavelength lasing in an EDF-assisted fiber laser is a nonlinear optical effect, which is considered a splendid method to suppress the mode competition caused by an EDF at room temperature. For example, nonlinear optical loop mirrors have been used as amplitude equalizers to induce intensity-dependent loss and alleviate the mode competition caused by homogeneous gain broadening in EDFs [6]. Typically, integrating a nonlinear medium, such as a single-mode fiber (SMF) that requires a long length (more than several kilometers), dispersion-shifted fibers, photonic crystal fibers, or highly nonlinear fibers, into the fiber laser can induce a nonlinear optical effect [7–9]. However, these special fibers usually lead to increase of cost and complexity of the systems. To overcome these problems, many experiments using graphene have been proposed and reported previously [3,4,10–14]. Graphene oxide (GO) is also 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
4519
the highly potential medium reported by Loh et al. (2010). It can show a good nonlinear optical effect [15]. In addition, GO is very simple and cheap to manufacture, and has strong adsorption [16]. Several layers of GO may show the same effect as high-cost special fibers, possessing enormous potential as a stabilizing medium for MWEDFLs. In this paper, to the best of our knowledge, a compact and stable multiwavelength fiber laser based on GO films and a capillary that serves as a platform to realize fiber joints is achieved for the first time. The system uses a Lyot structure as a comb filter to generate a multiwavelength laser. The proposed MWEDFL will have stable 11-channel simultaneous lasing with a wavelength spacing of 0.42 nm, providing numerous advantages for a multitude of applications. 2. Experimental Setup and Principle
The proposed schematic diagram of the experimental setup is shown in Fig. 1. A 980 nm laser diode is used as the pump source and is connected to a 7 m long EDF through a 980 nm∕1550 nm wavelengthdivision multiplexer. The other end of the EDF is then connected to a Lyot comb filter that is made up of a polarization-dependent isolator (PD-ISO), two polarization controllers (PCs), and a polarization-maintaining fiber (PMF). On account of superior characteristics such as low insertion loss and controllable wavelength spacing, the Lyot filter has been used widely [17]. The transfer function of the Lyot comb filter can be expressed as [2] jTj2 cos2 α cos2 β sin2 α sin2 β
1 sin 2α sin 2β cosΔφ Δφ0 : 2
(1)
Here, α and β are the angles between the polarization directions of the polarizers and the fast axes of the fiber, and Δφ is the phase difference between the wave components in the two orthogonal birefringent axes, which is introduced by the PCs. Δφ0 is the linear phase shift difference. Note that the transmission of the Lyot filter is a periodic function of Δφ; therefore, the spectral spacing and the transmission peak position of the intracavity birefringence-induced comb filter can be tuned by rotating the PCs, as
Fig. 1. Schematic diagram of the experimental setup. 4520
APPLIED OPTICS / Vol. 53, No. 20 / 10 July 2014
Fig. 2. Simulation result of the Lyot filter with PC2 in state (−π∕4, π∕4), whereas PC1 is in state (π∕3).
shown in Fig. 2. The transmission of the Lyot has a period with equal spacing, which can be expressed as Δλ λ2 ∕Δn × L:
(2)
Apparently, the wavelength spacing can be varied by changing the length of the PMF. Here, Δn is 0.0006 and the length of the PMF is selected to be 4, 10, and 15 m. It can be calculated that the corresponding wavelength spacings are 1.04, 0.42, and 0.24 nm, respectively, according to Eq. (2). Then the light encounters the GO films, which are housed between the two fiber ends. The output from the GO is connected to a 90:10 coupler, where a portion of the output containing wavelengths is extracted by a 10% port into an optical spectrum analyzer (OSA) with a resolution of 0.07 nm and the rest is fed back into the laser cavity. The GO used in our experiment is first created by depositing on the face of a fiber end. First, two segments of SMFs whose cladding diameter is 125 μm are chosen; then the fiber coating is peeled off, with the cladding and core left. The fibers, which are cut flatly, are immersed into GO solution for about 30 min. The GO solution is deposited on the fiber end. Second, the fibers with GO films are left to dry at room temperature. Third, the two segments of fibers are inserted into a capillary whose core diameter is 126 μm. The capillary is perfectly suitable for the SMFs. Figure 3 shows the structure of the fiber that is inserted into the capillary. Then the device is fixed on a table to form a stable platform. The total insertion loss of the GO–capillary
Fig. 3. Structure of the fiber with GO that is inserted into the capillary.
device is about 8 dB. The role of the GO films in this device is to stabilize the multiwavelength output by using the very strong nonlinear optical effect [13]. 3. Results and Discussion
In order to prove the importance of GO, an experiment that uses 4, 10, and 15 m PMFs is performed. First, GO is not used purposely in the ring cavity. The threshold is about 30 mW in our fiber laser. There is only one channel showing lasing at 1531 nm. When the 980 nm pump’s power is increased to 100 mW, several wavelengths are induced around 1531 nm and it is reasonable that they are quite unstable because of the high mode competition caused by the EDF. When the pump power is continued to be increased up to 160 mW, the number of the output lines
reaches the maximum and remains constant. However, the output power does not increase until pump power reaches 260 mW. Next, the GO–capillary device is put into the ring cavity. Wavelengths from 1528 to 1534 nm are excited and the output is more improved than that without GO. Figure 4 shows a record of the output spectra by the OSA at a pump power of 260 mW. Figures 4(a), 4(b), and 4(c) represent, respectively, random outputs with 4, 10, and 15 m long PMFs in the experiment. When GO is used, as shown in Figs. 4(d)–4(f), stable 5-line, 11-line, and 15-line lasing are obtained. The number of lasing signals depends mainly on the length of the PMF. Such wonderful power uniformity partially benefits from the power redistribution of nonlinear
Fig. 4. Output lasing (a)–(c) without GO and (d)–(f) with depositing once. 10 July 2014 / Vol. 53, No. 20 / APPLIED OPTICS
4521
optical effect through a few layers of GO. The number of wavelengths could be increased further by increasing the thickness of the GO films. Besides, the thickness of the GO films cannot be calculated in our device. For a better scheme, the deposition experiment should be repeated 5 and 10 times. When the 10 m PMF is chosen, the corresponding wavelength spacing is close to 0.4 nm, which complies with the international telecommunication union wavelength grid, and has wide applications in optical communication. Figure 5 shows the multiwavelength outputs under different deposition times when the 10 m PMF is chosen. There is no doubt that when depositing five times, the output wavelengths are very flat, with an average output power of −18 dBm and power nonuniformity is less than 1.5 dB. The average output power decreases by about 2 dB when depositing over 10 times. In our experiment, depositing five times can be considered the best condition. By adjusting the PCs to balance the wavelengthdependent gain and loss, we can obtain 11 stable wavelengths at a pump power of 260 mW at room temperature, as shown in Fig. 5(a). When depositing the GO solution on the fiber end once, the side-mode
Fig. 6. Tuning characteristic of the MWEDFL.
Fig. 5. Output lasing with depositing (a) 5 times and (b) 10 times that uses the 10 m PMF. 4522
APPLIED OPTICS / Vol. 53, No. 20 / 10 July 2014
Fig. 7. (a) Repeatedly scanned output spectra at 10 min spacings. (b) Fluctuations of output wavelength and peak power over 1 h.
suppression ratio (SMSR) is about 37 dB. When depositing five times, the SMSR is about 34 dB. When depositing 10 times, the SMSR is about 27 dB, and power fluctuation is bigger than that with depositing five times as shown in Fig. 5(b). It is probably because the nonlinear signal is suppressed at the thick layers of GO just like the optical property of graphene [9]. The linewidth of each lasing is less than 0.07 nm (the resolution of OSA is 0.07 nm). The wavelength spacing is 0.42 nm. Moreover, the multiple wavelengths can be tuned by adjusting the PCs. The tuning range is very small (about 0.3 nm) just due to the property of the Lyot comb filter. The tuning characteristic of the MWEDFL is shown in Fig. 6. In order to measure the stability of the MWEDFL, the proposed experiment is left to run for approximately an hour. The wavelength spectra acquired are shown in Fig. 7(a). The wavelength at 1531 nm is chosen. It can be seen from Fig. 7(b) that the peak power fluctuation and wavelength shift are ∼1.1 dB and 0.05 nm, respectively, when the output is scanned at 10 min intervals over 1 h. The self-stabilizing effect is mainly benefited from GO, which is due to its four-wave mixing nonlinear optical effect [18]. 4. Conclusion
Using the high nonlinear effect of GO to suppress mode competition, a stable and simple MWEDFL is demonstrated by using a Lyot comb filter. Eleven lasing signals with an average power of −18 dBm are obtained at a pump power of 260 mW and the SMSRs are about 34 dB. The MWEDFL creates a good flat output spectrum with little fluctuations and provides a narrow linewidth of
