Tunable spacing of O-band Multiwavelength Brillouin Fiber Laser
Siti Fatimah Norizan
Kulliyyah of Science, International Islamic University of Malaysia,
Kuantan, Malaysia
Mohd Zamani Zulkifli Photonics Research Center,
University of Malaya Kuala Lumpur Malaysia
Abstract~ A tuneable spacing of O-band multiwavelength Brillouin fiber laser is demonstrated. The channel spacing is varied by employing two types of cavities. A full closed linear cavity produce multiwavelength Brillouin fiber laser with 12.5GHz meanwhile a ring cavity provide channel spacing of 25.0 GHz. By combining these two cavities and employing optical channel switch the channel spacing are varied. The comparison of Brillouin threshold between O-band and C-band demonstrated. An O-band transmission window is used instead of C-band due to its lower Brillouin threshold.
Keywords- Brillouin scattering, tuneable spacing, fiber laser
I. Introduction
Revolution of optical fiber technology leads to other inventions to support the technology including the transmission source. The frequency of the transmitted signal is around 800 nm and quite recently has improved to 2.0 nm due to current development in the fiber technology. As the traffic demand continues growing, the bit rates have rising in numbers of generations starting from 45 Mbps in 1975 to 100 Tbsp in 2010 which multiply the speed almost 10 times for every 4 years. . Therefore, proactive measures should be taken to ensure that the needs of users can be cater. One of the measures is to maximising the use of all transmission windows available. It consists of expanding the optical amplifier bandwidth, maximise the use of transceiver/receiver and also improves the efficiency of optical fiber. There are numerous ways of producing multiwavelength fiber laser including Sagnac Loop Mirror [1], polarization hole burning (PHB) [2] fiber Bragg grating [3,4], arrayed waveguide [5] and utilizing stimulated Brillouin scattering (SBS) [6]. The SBS is an efficient technique since it requires less power and provided dense channels spacing. Unfortunately, the SBS effects produce multiwavelength with fixed
channel spacing. Therefore, it is hard to be applied in application that requires varied spacing.
In this work, a comparison of Brillouin threshold made between O-band and C-band together with different of channel spacing. The multiwavelength Brillouin fiber laser with variability in the channels spacing is also demonstrated.
II. Experimental Setup
According to the equation 1, the threshold of nonlinearity effect SBS depends on is Brillouin pump (BP) wavelength, where low Brillouin threshold obtain in the short BP wavelength. In comparison with C-band, the O-band multiwavelength required lower BP power. The Brillouin threshold power (Pth) is defined as;
(1)
Where, Aeff is the effective cross section of the fiber used, gB(νB) is the peak value of the Brillouin gain coefficient, for is the ratio of frequency linewidth of the pump over the spontaneous Brillouin linewidth [7] and Leff is the effective length
(2) Where L is the length of the nonlinear fiber
and α is the attenuation.
Figure 1 Backscattered power measurement
2015 International Conference on Automation, Cognitive Science, Optics, Micro Electro-Mechanical System, and Information Technology (ICACOMIT), Bandung, Indonesia, October 29–30, 2015
The Brillouin threshold obtained by measure the backscattered power as illustrated in figure 1. The tunable laser source act as Brillouin pump that generating SBS effect. The generated Stokes moves in backward direction. The angle cleave prevent the Fresnel reflection back into the nonlinear medium. The first Stokes generated if the BP power exceeds its threshold and it propagates in BP backward direction. An ideal Brillouin threshold can be manifested when the power of the backward scattering Stokes is equal to the initial power of the Brillouin pump. The closest approximation of the non-ideal condition, the Brillouin threshold is estimated at the backward scattering power of 10% of the BP power [7,8].
Configuration of the cavity, affect the power and spectrum of the multiwavelength fiber laser. Ring cavity and full closed linear cavity are two most common cavity uses in producing multiwavelength fiber laser. Ring cavity only allows unidirectional propagation which could eliminate the feedback laser. Meanwhile the full closed linear cavity allows bidirectional propagation that maximise the use of its feedback laser. By combining both of the configurations we can have different channel spacing and make it channel spacing variable. Figure 2 below show configuration of tuneable multiwavelength Brillouin fiber laser.
Figure 2 Configuration of tuneable multiwavelength Brillouin fiber laser
The variable spacing multiwavelength Brillouin fiber laser setup is built by combining ring cavity and linear cavity together with switch component. The BP generated from TLS connected to 70% port of C1 and amplifies by BOA after pass through port 3 and 2 of optical circulator OC1 and 3dB of coupler C2. Amplified BP then generates the Stokes which then travels to optical channel selector (OCS). The OCS is an optical switch, that change the route of the Brillouin pump and Stokes generated. By changing the route it create different
type of cavity. Linear cavity generated by connecting the channel 1 of the OCS to the OC2 which looped the beam back into the nonlinear medium. The feedback beam will continuously pass the nonlinear medium bidirectional to perform full closed linear cavity. Meanwhile, ring cavity configuration assemble by connecting channel 2 of the OCS linked to coupler C4 that connected with C2 creating the unidirectional propagation. The 10% power is tapped through the coupler C3 and C4 to be analysed by the OSA.
III. Results
The backward power was measured by using 20km of True Wave Reach Fiber (TWRF) assemble in an open cavity. Comparison of backward power between the 1310 nm (O-band) and 1550 nm (C-band) is demonstrated as indicated in figure 3.
Figure 3 Backscattered stokes power for BP wavelength of 1310 and 1550 nm.
The green dotted line from figure 2 is the 10% of the Brillouin pump. The cross point between the green line and C-band or O-band series indicates the Brillouin threshold. The figure show that the BP threshold for operation in the 1310 nm is lower than that for 1550 nm with values of 5.2 dBm and 8.5 dBm, respectively. Therefore the following experiments operate in O-band region.
The suppression make the spectrum looks like it produce multiwavelength of 25GHz.
Figure 4 Multiwavelength Brillouin fiber laser via (a) full closed linear cavity (b) ring cavity
The spectrum of multiwavelength Brillouin fiber laser is depending on the channel selected by the OCS. Following figures 4 show the spectrum generated by both of the channels.
Figure 5 Spectrum of tuneable multiwavelength Brillouin fiber laser
From the figures 5 it shows that by switching the OCS to 1st channel the multiwavelength Brillouin fiber laser have spacing of 0.08nm/12.5GHz and by changing it to channel 2 its tune to 0.16nm/25GHz spacing. The anti- Stokes
of the full closed linear cavity disappears due to low power effect from loss of OCS (~3dB).
IV. Conclusion
The tuneable of O-band multiwavelength Brillouin fiber laser have been demonstrated. The channels varied from 12.5GHz to 25GHz by employing two cavities with OCS. The O-band also prove to be more effective that C-band in producing Brillouin fiber laser due to its low threshold.
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