Original Text © Astro, Ltd., 2012.
1 1. INTRODUCTION
Many rareearthdoped fibers have been used as the active media of fiber lasers [1]. Among these active media, erbiumdoped fibers (EDFs) and ytterbium doped fibers (YDFs) have attracted much attention in the construction of either continuouswave (CW) or pulsed fiber lasers [2–5]. Recently, semiconductor optical amplifiers (SOAs) provide interesting alterna tives as the gain media in fiber laser cavities because of their dominant inhomogeneous broadening properties and the ease of integration with other devices [6–17]. As compared to the rareearthdoped fibers, SOAs can generate multiple stable lasing wavelengths at room temperature, which make them attractive for applica tions in telecommunication systems and photonic technologies [18, 19]. Pleros et al. demonstrated multiwavelength and powerequalized operation in a 1.55μm SOAbased ring laser that uses a fiber Fabry– Perot filter [6]. Simultaneous oscillation of 52 lines with 50GHz spacing is achieved by using singlepass optical feedback. Multiwavelength fiber lasers can also be obtained by incorporating fiber Bragg gratings (FBGs) or arrayed waveguide gratings (AWGs) in the SOAbased cavity [11–13]. Latif et al. presented the design of a quadwavelength fiber laser that covers the 1.3 and 1.55μm bands using two SOAs and an AWG of 100GHz interchannel spacing [13]. Their design is capable of providing more than 24 ways of tuning the wavelength spacings. A recently proposed scheme of SOAbased fiber laser adopted a Sagnac loop mirror (SLM) in the cavity to generate multiple lasing lines 1The article is published in the original.
[20, 21]. Using this scheme, Zulkifli et al. [20] have obtained 60 wavelengths oscillating simultaneously between 1464 and 1521 nm with a spacing of 0.92 nm. Pulse generation in a fiber laser may employ either active or passive technology [22–24]. Using a intrac avity MachZehnder intensity modulator, Tang et al. have generated tunable picosecond pulses from a 10GHz harmonically modelocked SOA ring laser with a center wavelength spanning from 1491 to 1588 nm [16]. In order to generate ultrashort pulses in fiber lasers, passive modelocking (PML) have been widely reported based on several types of mechanisms. Due to its long cavity length, significant interactions between dispersive and dissipative optical effects, such as Kerr nonlinearity, chromatic dispersion, linear and saturable losses, as well as bandwidthlimited gain, can be experienced in a fiber laser even under moderate pump powers. Yang et al. reported the generation of subpicosecond optical pulses using an SOA and a lin ear polarizer in a ring laser cavity [10]. For this laser, nonlinear polarization rotation in the SOA serves as the passive modelocking mechanism. By using a fig ure8 fiber laser cavity, nonlinear amplification can also be designed to favor modelocking [1, 15].
Passively modelocked short pulse lasers often con tain two main elements: (a) a gain medium that is responsible for the amplification of the pulse, and (b) a nonlinear optical device that is responsible for the compression of the pulse. Using nonlinear polariza tion evolution (NPE) technology for pulse generation has avoided the use of saturable absorber and thus sim plifies the cavity configuration. NPE modelocking has been utilized in erbiumdoped fiber lasers
OPTICS
Generation of Optical Waveforms in 1.3
µm
SOABased Fiber Lasers
1J.Y. Wanga, K.H. Lina, *, and H.R. Chenb
a Department of Science, Taipei Municipal University of Education, 1, AiKuo West Road, Taipei 100, Taiwan b Department of Photonics and Institute of ElectroOptical Engineering, National Chiao Tung University,
1001, TaHsueh Road, Hsinchu 300, Taiwan
*email: [email protected]
Received June 19, 2011; in final form, June 29, 2011; published online October 24, 2011
Abstract—Passive optical waveform generation is obtained in fiber lasers using a 1.3µm semiconductor opti cal amplifier (SOA) as the gain medium. Various waveforms, including square wave, staircase wave, triangular wave, pulse, and dark pulse are generated in SOAbased fiber lasers by adjusting intracavity polarization con trollers. The passive waveform generation might be attributed to the SOA gain dynamics and the enhanced nonlinear interaction at the 1.3µm zero dispersion wavelength of traditional singlemode fiber (SMF), as well as the interference effect between the two subcavities of fiber laser. With figure8 cavity configuration, 1250thorder harmonic pulses have been successfully demonstrated. We have also obtained a freerunning SOAbased fiber laser with 3dB spectral width of 16 nm, and the center wavelength can be tuned over 45 nm range.
(EDFLs) and ytterbiumdoped fiber lasers (YDFLs) to generated femtosecond light pulses [3, 25, 26]. However, the waveforms of these passively mode locked fiber lasers are governed by the GinzburgLan dau equation and the commonly observed pulse shapes are sech2, Gaussian, or parabolic. For anoma lous dispersion lasers, soliton formation is mainly attributed to the interplay between the anomalous cav ity dispersion and the selfphase modulation effect. For normal dispersion lasers, however, soliton forma tion is resulted from the interaction between normal dispersion and intracavity bandwidth filtering. Wu et al. have reported the generation of squareprofile dis sipative solitons from an allnormaldispersion erbiumdoped fiber laser which is modelocked with the nonlinear polarization rotation technique [27].
It is interesting to raise a question now: what kind of pulses will be generated in the zero or nearly zero dispersion fiber lasers? In fact, generation and shaping of optical pulses in the 1.3μm regime are rarely reported for SOAbased fiber lasers, despite the enhanced wavemixing effect in the SOA gain medium as well as the zerodispersion regime of conventional singlemode fibers. As inspired by the electronic wave form generators, an optical counterpart capable of providing square wave, sinusoidal wave, and triangular wave, etc., is attractive for applications in optical com munication, signal processing, and device character ization. In this work we present a passive technology for optical waveform generation by using 1.3μm SOAbased fiber lasers. Various waveforms such as square wave, staircase wave, triangular wave, pulse, and dark pulse have been successfully generated with proper cavity design and the adjustment of intracavity polarization.
2. EXPERIMENTAL SETUP
Figure 1 shows the experimental setup of the SOA based fiber lasers. In our experiment, we have adopted three cavity configurations: (1) a conventional NPE ring cavity (Fig. 1a), (2) a figure8 cavity (Fig. 1b), and (3) a σ cavity with the insertion of a semiconductor saturable absorber mirror (SESAM) (Fig. 1c). The SOA has a peak output wavelength of 1290 nm and 3dB bandwidth of 54 nm; the maximum small signal gain is 20.9 dB and the saturation output power is 10.8 dBm (measured from the output port). Since SOA is a bidirectional device, an optical isolator is used in the cavity to ensure unidirectional laser oscil lation. A roll of 2km singlemode fiber (SMF) is inserted in the cavity to enhance the interaction length. The output coupling is 10% for the ring and figure8 cavities, while it is 50% for the σ cavity. Almost all the fiber components in the laser cavity are linked via singlemodefiber jumpers with FC/APC connectors, except that the SOA connections are FC/PC.
For the NPE cavity, the combination of polarizer and polarization controllers works effectively as the nonlinear loss. For the figure8 cavity, a 3dB coupler is used to connect the two fiber loops. The subcavity containing the SOA works effectively as nonlinear gain. The laser operation states are monitored by using a highspeed InGaAs photodetector and a digital oscilloscope (HP 54542C), while the output spectra is recorded by an optical spectrum analyzer (Agilent 86146B). A power meter (Thorlabs Inc.) and a radio frequency (RF) spectrum analyzer (HP 8560E) are used to measure the output power and the longitudinal mode beating. SOA SMF Isolator Coupler Output Polarization controller Polarizer (a) Polarization controller SOA SMF Coupler Output Polarization controller (b) Polarization controller Coupler Isolator Polarization controller Polarization controller Coupler Isolator Polarizer Output SMF SOA (c) SESAM
Fig. 1. Experimental setup of SOA fiber lasers: (a) a con ventional NPE ring cavity, (b) a figure8 cavity, and (c) a σcavity SOA fiber laser with semiconductor saturable absorber mirror.
3. RESULTS AND DISCUSSIONS
Initially we have adopted the conventional NPE laser cavity of 10% output coupling (Fig. 1a). The SOA current is set to 240 mA. By adjusting the polarization controllers, the laser can be operated in the freerun ning state with single, dual, or even 3 to 4 spectral bands. We have obtained a singleband SOA fiber laser with center wavelength of 1324 nm and 3dB spectral width of 16 nm (Figs. 2a and 2b), and the laser output power is –0.2 dBm. The center wavelength of single band operation can be tuned over 45 nm range. By fur ther adjustment of the polarization controllers, we have obtained dualband laser operation with wave length spacing of 56 nm (Figs. 2c and 2d), but the out put power is decreased to –2.8 dBm and the spectral width of each band is reduced. We have carefully adjusted the two polarization controllers in Fig. 1a, but the laser still works in the freerunning mode without any pulse generation.
It is interesting that a freerunning laser can pro vide spectral width as large as 16 nm, since the maxi mum intracavity power of our SOAbased fiber laser is only about 10 mW. Although the bandwidth of oscillo scope is 500 MHz and the sampling rate is 2 Gb/s, we have used a 2.9GHz RF spectrum analyzer (HP 8560E) to monitor the laser output, and still we do not observe pulse signals. Therefore, the broad laser spec trum might be a result of inhomogeneous broadening instead of modelocking. When the 2km single mode fiber is removed from the cavity, the laser spectrum shows some minor dips with 1.4 nm spacing, which should be attributed to the residual reflections from the SOA surfaces and the accompanying parasitic Fabry–Perot modes. The spectral dips are eliminated when the 2km SMF is inserted in the cavity, which
should be attributed to the spectral broadening effect inside the long intracavity fiber.
However, with the figure8 cavity configuration (Fig. 1b), various optical waveforms have been suc cessfully generated with careful polarization tuning. Figures 3a, 3c, 3e, 3g show the observed pulse trains of square wave, staircase wave, pulse, and dark pulse, while the corresponding output spectra are shown in Figs. 3b, 3d, 3f, 3h, respectively. The table summarizes the characteristics for these optical waveforms gener ated in the figure8 cavity. The pulse separation for 0.05 −40 −20 0 20 40 Time, ns 0.04 0.03 0.02 0.01 0 (c) 0.05 −40 −20 0 20 40 Time, ns 0.04 0.03 0.02 0.01 0 Intensity, (a) −20 1260 1300 1340 Wavelength, nm −40 −60 −80 Intensity, dBm (b) −20 1260 1300 1340 Wavelength, nm −40 −60 −80 Intensity, dBm (d)
Fig. 2. Oscilloscope traces and optical spectra for the ring cavity SOA laser. (a) and (b): singleband operation with center wavelength of 1324 nm and 3dB spectral width of 16 nm; (c) and (d): dualband operation with wavelength spacing of 56 nm. 0.05 −400 0 400 Time, ns 0.04 0.03 0.02 0.01 0
Intensity, arb. units
(c) 0.05 −40 −20 0 20 40 Time, ns 0.04 0.03 0.02 0.01 0
Intensity, arb. units
(a) −20 1260 1300 1340 Wavelength, nm −40 −60 −80 Intensity, dBm (b) −20 1260 1300 1340 Wavelength, nm −40 −60 −80 Intensity, dBm (d) 0.05 −40 −20 0 20 40 Time, ns 0.04 0.03 0.02 0.01 0 Intensity,arb. units (g) 0.05 −40 −20 0 20 40 Time, ns 0.04 0.03 0.02 0.01 0 Intensity,arb. units (e) −20 1260 1300 1340 Wavelength, nm −40 −60 −80 Intensity, dBm (f) −20 1260 1300 1340 Wavelength, nm −40 −60 −80 Intensity, dBm (h)
Fig. 3. Optical waveforms and spectra generated in the fig ure8 cavity with the insertion of 2km SMF. (a) and (b): square wave; (c) and (d): staircase wave; (e) and (f): pulse; (g) and (h): dark pulse.
Characteristics of optical waveforms in figure 8 SOAbased fiber laser cavity with 2km SMF
Waveform Output Power, dBm Repetition Rate, MHz
Square 2.6 50
Staircase 2.6 4
Pulse 2.1 125
Dark Pulse 3.9 125
arb. units
square wave is 20 ns (Fig. 3a), and the laser output power is 2.6 dBm. For staircase wave, the output power is also 2.6 dBm, but the pulse spacing is increased to 250 ns (Fig. 3c). Some overshoots are observed on the rising edges of staircase wave. Depending on the polar ization adjustment, the staircase wave could either be upstairs or downstairs. By further adjustment of the polarization controllers, the optical waveform can be switched to pulse or dark pulse. Figures 3e and 3g shows the oscilloscope trace for pulses and dark pulses, respectively. The pulse shape resembles that of soli tons, while the dark pulse looks like dark solitons. Both of them have pulse separations of 8 ns, but the output powers are 2.1 and 3.9 dBm for pulse and dark pulse states. The output spectra show single band around 1325 nm for square wave, pulse, and dark pulse states. However, dualband spectrum is observed for the staircase wace.
With a 2km figure8 cavity, the roundtrip time is about 10 μs and the pulse repetition rate is about 100 kHz; however, we have not observed this funda mental pulse trains, although the intracavity power is only moderate (≈10 mW). The lowestorder pulse rep etition rate achieved is 4 MHz for the staircase wave, corresponding to 40thorder harmonics. For pulse and dark pulse trains, the repetition rate is 125 MHz, which corresponds to 1250thorder harmonic pulse generation. As a comparison, it is easier for rareearth doped fiber lasers, such as erbiumdoped and ytter biumdoped fiber lasers, to operate in fundamental modelocking states than harmonic modelocking states under such low intracavity powers.
When the 2km SMF is removed from the figure8 cavity, dark pulses can be obtained with 5ns spacing (Fig. 4a), and the output power is 3.6 dBm. The output spectrum shows a singleband structure, and the peak wavelength is located at about 1328 nm (Fig. 4b). By
tuning the polarization controllers, triangular pulses have be generated (Fig. 4c). The pulse spacing is also 5 ns for triangular wave and the output power is some what reduced to 3.3 dBm. Again the spectrum shows a singleband structure, but the peak wavelength is shift to about 1335 nm (Fig. 4d). It seems that the triangular wave is a transition state from pulse state to darkpulse state.
The passive waveform generation might be attrib uted to the SOA gain dynamics and the enhanced non linear interaction at the 1.3μm zero dispersion wave length of SMF, as well as the interference effect between the two subcavities of figure8 laser. In addi tion, it seems easier to generate optical waveforms in a coupled cavity than a singlering cavity. We have dem onstrated this by using a σcavity SOA fiber laser, where a semiconductor saturable absorber mirror is connected to the main cavity by a 2 × 2 3dB coupler, as shown in Fig. 1c. The SESAM in the σ cavity is originally designed for the 1550nm band (Batop, SAM 1550232psFC/APC). It has an unsaturated absorption of A0 = 23% and modulation depth of ΔR = 14% at 1550 nm, with saturation fluence of 25 μJ/cm2. For the σcavity SOAbased fiber laser, we find that the squarewave pulses can also be generated by adjusting the polarization controllers. Figure 5a shows the laser output signals observed on the oscilloscope, and the corresponding optical spectrum is shown in Fig. 5b. The oscilloscope trace demonstrates a train of square waves with pulse spacing of 10 ns, while the center wavelength of these pulses is 1310 nm. Since the laser wavelength is far from the operating wavelength (1550 nm) of this SESAM, passive waveform genera tion might be attributed to coupling effect provided by the back reflection from the SESAM as well as the enhanced nonlinear interaction at the zero dispersion wavelength of conventional singlemode fiber.
4. CONCLUSIONS
We have developed a passive technology to generate optical waveforms by using a 1.3μm SOAbased fiber laser. Various waveforms such as square wave, staircase wave, triangular wave, pulse, and dark pulse have been successfully generated with proper cavity design and 0.05 −20 −10 0 10 20 Time, ns 0.04 0.03 0.02 0.01 0
Intensity, arb. units
(c) 0.05 −20 −10 0 10 20 Time, ns 0.04 0.03 0.02 0.01 0
Intensity, arb. units
(a) −20 Wavelength, nm −40 −60 −80 Intensity, dBm (d) 1340 1320 1300 −20 Wavelength, nm −40 −60 −80 Intensity, dBm (b) 1340 1320 1300
Fig. 4. Optical waveforms and spectra generated in the fig ure8 cavity without the 2km SMF. (a) and (b): dark pulse; (c) and (d): triangular wave.
0.08 −40 −20 0 20 40 Time, ns 0.04 0.02 0
Intensity, arb. units
(a) −20 Wavelength, nm −40 −60 −80 Intensity, dBm (b) 1340 1300 1260 0.06
Fig. 5. (a) Oscilloscope trace and (b) optical spectrum for the σcavity SOA fiber laser.
the adjustment of intracavity polarization. Harmonic pulses as high as 1250thorder has been generated with a figure8 laser configuration, although the intracavity power is only at moderate level of about 10 mW. The passive optical waveform generation might be attrib uted to the nonlinear interaction in singlemode fiber and the SOA gain dynamics, as well as the wave cou pling effect between subcavities. Passive generation of optical waveforms may potentially be used in optical function generators as well as applications in optical communication, signal processing, and device char acterization in the future.
ACKNOWLEDGMENTS
This work is supported partially by the National Science Council of Taiwan, Republic of China, under grant NSC992221E133001.
REFERENCES
1. G. P. Agrawal, Applications of Nonlinear Fiber Optics (Academic Press, 2008), ch. 5.
2. O. Okhotnikov, A. Grudinin, and M. Pessa, New J. Phys. 6, 177 (2004).
3. M. E. Fermann and I. Hartl, IEEE J. Sel. Top. Quan tum Electron. 15, 191 (2009).
4. K.H. Lin, J.J. Kang, H.H. Wu, C.K. Lee, and G. R. Lin, Opt. Express 17, 4806 (2009).
5. K.H. Lin, J.H. Lin, and C.C. Chen, Laser Phys. 20, 1984 (2010).
6. N. Pleros, C. Bintjas, M. Kalyvas, G. Theophilopou los, K. Yiannopoulos, S. Sygletos, and H. Avramopou los, IEEE Photon. Technol. Lett. 14, 693 (2002). 7. A. A. Moiseev, G. V. Gelikonov, E. A. Mashcovitch, and
V. M. Gelikonov, Laser Phys. Lett. 7, 505 (2010). 8. H. Ahmad, M. Z. Zulkifli, A. A. Latif, and S. W. Harun,
Laser Phys. Lett. 7, 164 (2010).
9. V. Baby, L. R. Chen, S. Doucet, and S. LaRochelle, IEEE J. Sel. Top. Quantum Electron. 13, 764 (2007).
10. X. Yang, Z. Li, E. Tangdiongga, D. Lenstra, G. D. Khoe, and H. J. S. Dorren, Opt. Express 12, 2448 (2004). 11. H. Ahmad, A. H. Sulaiman, S. Shahi, and S. W. Harun,
Laser Phys. 19, 1002 (2009).
12. Y. Wei and B. Sun, Laser Phys. 19, 1252 (2009). 13. A. A. Latif, M. Z. Zulkifli, N. A. Hassan, S. W. Harun,
Z. A. Ghani, and H. Ahmad, Laser Phys. Lett. 7, 597 (2010).
14. Z. Li, X. Yang, E. Tangdiongga, H. Ju, G.D. Khoe, H. J. S. Dorren, and D. Lenstra, IEEE J. Quantum Electron. 41, 808 (2005).
15. Y. H. Zhong, Z. X. Zhang, and X. Y. Tao, Laser Phys. 20, 1756 (2010).
16. W. W. Tang, M. P. Fok, and Ch. Shu, Opt. Express 14, 2158 (2006).
17. F. M. Wu, P. C. Peng, J. Chen, C. T. Lin, and W. C. Kao, Laser Phys. 21, 522 (2011).
18. H. Ahmad, K. Thambiratnam, A. H. Sulaiman, N. Tamchek, and S. W. Harun, Laser Phys. Lett. 5, 726 (2008).
19. H. Ahmad, M. Z. Zulkifli, K. Thambiratnam, A. A. La tiff, and S. W. Harun, Laser Phys. Lett. 6, 539 (2009). 20. M. Z. Zulkifli, N. A. Hassan, N. A. Awang, Z. A. Gha
ni, S. W. Harun, and H. Ahmad, Laser Phys. Lett. 7, 673 (2010).
21. J. E. Im, B. K. Kim, and Y. Chung, Laser Phys. 20, 1918 (2010).
22. M. Hofer, M. E. Fermann, F. Haberl, M. H. Ober, and A. J. Schmidt, Opt. Lett. 16, 502 (1991).
23. J. Fekete, A. Cserteg, and R. Szipocs, Laser Phys. Lett. 6, 49 (2009).
24. Y. J. Song, M. L. Hu, C. L. Gu, L. Chai, C. Y. Wang, and A. M. Zheltikov, Laser Phys. Lett. 7, 230 (2010). 25. F. W. Wise, A. Chong, and W. H. Renninger, Laser Pho
ton. Rev. 2, 58 (2008).
26. J.H. Lin, D. Wang, and K.H. Lin, Laser Phys. Lett. 8, 66 (2011).
27. X. Wu, D. Y. Tang, H. Zhang, and L. M. Zhao, Opt. Express 14, 5580 (2009).
