ISSN 1054660X, Laser Physics, 2011, Vol. 21, No. 3, pp. 522–525. © Pleiades Publishing, Ltd., 2011.
Original Text © Astro, Ltd., 2011.
522 1. INTRODUCTION
Recently, RF phase shifters that exploit photonic approaches have attracted considerable attention because of their potential use in microwave systems [1, 2]. Photonic devices have numerous advantages over conventional microwave systems that are based on purely electrical devices. These include flexible tun ability, lightness of weight, and immunity to electro magnetic interference (EMI) [3–5]. These advantages have caused photonic RF phase shifter gradually to replace electronic shifters in microwave systems.
Of all photonic phase shifters, semiconducting devices are the most desirable because of their inher ent compactness, ease of integration with other devices, and low power consumption, which enable the phase of an RF signal to be accurately and easily controlled for microwave photonic systems [5–7]. Recently, controllable time delay using population oscillation in a semiconductor optical amplifier (SOA) has been investigated [8]. The time delays can be con trolled by varying the power of the external pump laser. However, this scheme depended on a highly stable and expensive external pump laser, and is difficult to implement.
Recently, fiber ring lasers have been the focus of considerable research, because fiber lasers are simply implemented, have a high output power, a high signal to noise ratio, and the potential to be incorporated into a fiber system [9–18]. This investigation proposes a novel photonic RF phase shifter that is based on a fiber ring laser. The fiber ring laser replaces the exter nal laser source for the photonic RF phase shifter. It is constructed from an optical filter and optical circula tors. The power of the fiber ring laser is controlled using a variable optical attenuator (VA). The time
delay can be tuned by adjusting the power of the laser. A 1 GHz modulation signal with a tunable optical delay of 102 ps is presented. The relationship between the modulation frequencies of the probe signal and the time delay is also investigated.
2. EXPERIMENT SETUP AND RESULTS Figure 1a present the experimental setup for mea suring the tunable time delays in an SOA with an exter nal pump laser source. A 1546.11nm optical signal is generated by a tunable laser source and then modu lated via a Mach–Zehender modulator (MZM). The optical circulator C1 is used to connect the probe sig nal to the SOA, and C2 is used to link the SOA to the pump laser source. The phase shifter can be tuned by adjusting the power of the pump laser. Figure 1b shows the experimental setup of the proposed system. The amplified spontaneous emission from the SOA circu lates in the loop. This feedback continues to oscillate in the loop and create a laser beam. The direction of the feedback signal is counterclockwise as deter mined by the optical circulator. The central wave length of the fiber ring laser is determined by the opti cal filter, and is 1546.11 nm. The inset of Fig. 1b shows the optical spectrum. The variable optical attenuator is used to control the power of the fiber ring laser.
Figure 2 plots the waveforms of the probe signal at various powers of the external pump laser and the fiber ring laser, respectively. The laser powers are used to control the time delay in the SOA. The laser power is varied from –20 to 8 dBm, and the driving current of the SOA is 300 mA. Figure 3 plots the measured time delays at different laser powers. The time delay in the SOA increases with the laser power. Finally, the time FIBER OPTICS
SOABased Fiber Ring Laser Use
in a Photonic RadioFrequency Phase Shifter
1F. M. Wua, P. C. Pengb, *, J. Chena, C. T. Linc, and W. C. Kaoa
a Institute of ElectroOptical Engineering, National Chiao Tung University,
Hsinchu, Taiwan, China
b Department of ElectroOptical Engineering, National Taipei University of Technology,
Taipei, Taiwan, China
c Institute of Photonic System, National Chiao Tung University,
Tainan, Taiwan, China
*email: pcpeng@ntut.edu.tw
Received May 11, 2010; in final form, October 14, 2010; published online February 2, 2011
Abstract—This investigation demonstrates the feasibility of a semiconductor optical amplifier (SOA)— based fiber ring laser use in a photonic radiofrequency (RF) phase shifter. Experimental results indicate that a fiber ring laser can successfully replace an external pump laser in a photonic RF phase shifter. The RF phase shifter can be turned by adjusting the power of the fiber ring laser.
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SOABASED FIBER RING LASER USE 523
delay in the SOA with an external pump laser yield similar results as that of the SOA with the fiber ring laser. The fiber ring laser can therefore replace the external pump laser source.
The time delay is also measured at different driving currents of the SOA. Figure 4 plots the measurements of time delay for the 1 GHz probe signal at various powers of the fiber ring laser. The maximum time delays are 99 and 52 ps at bias currents of 200 and 100 mA, respectively. Moreover, Fig. 5 plots the time delay against the power of the fiber ring laser and the driving current of SOA. The time delay increases with the optical power of the fiber ring laser.
(a) (b) Current EOM DC bias Laser source Signal generator SOA Pump laser (1546.11 nm) Sampling oscilloscope C1 C2 EOM DC bias Laser source Signal generator SOA Sampling oscilloscope C1 C2 Filter OC Current source Power, 10 dB/div Wavelength, 0.5 nm/div 1546.11 nm 1 2 3 1 2 3 1 2 3 1 2 3
Fig. 1. Experimental setup for measuring tunable time delays in SOA (EOM: electrooptic modulator, C: optical circulator, SOA:
semiconductor optical amplifier, VA: variable attenuator, OC: optical coupler).
External pump laser
Fiber ring laser (SOA 300 mA) −20 dBm 8 dBm −20 dBm 8 dBm 101 ps 102 ps 400 ps/div
Fig. 2. Waveforms at various powers of probe signal.
150 120 90 60 30 0 −20 −10 0 10 Power, dBm Delay time, ps
External pump laser Fiber ring laser
Fig. 3. Time delay against optical power of external pump
laser and fiber ring laser.
source
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WU et al.
Figure 6 plots the time delay against the power of the fiber ring laser and the modulation frequency of the probe signal. The laser power is varied from –20 to 8 dBm, and the driving current of SOA is set to at 300 mA. The maximum time delays are 102, 51, and 21 ps at modulation frequencies are 1, 5, and 10 GHz, respectively. The time delay increases as the modula tion frequency declines.
3. CONCLUSIONS
This investigation experimentally demonstrates a fiber ring laser, including an SOA, for application in a photonic RF phase shifter. A fiber ring laser can suc cessfully replace the highly stable and expensive exter nal pump laser. It is compact and can therefore be use in a phase shifter. The photonic phase shifter can be tuned by adjusting the power of the fiber ring laser. This phase shifter has the potential to reduce the size and cost of microwave photonic systems. Moreover, the fiber ring laser could apply to wavelengthdivision multiplexing systems by using an allfiber birefringent filter in the ring cavity [19].
ACKNOWLEDGMENTS
This work was supported by the National Science Council of the Republic of China, Taiwan, under Contract NSC 982221E027007MY3 and Con tract NSC 982622E027038CC3.
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