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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 14 — Jul. 6, 2009
  • pp: 12167–12173
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Frequency-switchable microwave generation based on a dual-wavelength single-longitudinal-mode fiber laser incorporating a high-finesse ring filter

Shilong Pan and Jianping Yao  »View Author Affiliations


Optics Express, Vol. 17, Issue 14, pp. 12167-12173 (2009)
http://dx.doi.org/10.1364/OE.17.012167


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Abstract

A wavelength-switchable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser (EDFL) incorporating a novel high-finesse ring filter is proposed and demonstrated. The ring filter consists of two optical couplers and a section of pumped erbium-doped fiber (EDF). Thanks to the gain generated by the EDF, the ring filter has spectral response with a high finesse. The incorporation of the ring filter leads to the suppression of undesirable modes in the dual-wavelength EDFL. An experiment is carried out. Two SLM wavelengths are generated. The side mode suppression ratio is greater than 50 dB. The wavelength spacing of the two wavelengths is tunable with a tuning step of ~10 GHz. A frequency switchable microwave signal from ~10 to ~40 GHz is thus generated by beating the two wavelengths at a photodetector (PD). The spectral width of the generated microwave signal is measured to be less than 5 kHz.

© 2009 OSA

1. Introduction

In this paper, we propose and demonstrate a novel fiber laser that incorporates a high-finesse fiber ring filter. The ring filter consists of a section of EDF that is weakly pumped which would generate a small optical gain. Thanks to the optical gain, the ring filter would have a frequency response with a high finesse. A dual-wavelength erbium-doped fiber laser (EDFL) is then constructed incorporating the high finesse ring filter. The gain competition in the gain medium is suppressed by the polarization hole burning (PHB) effect [5

5. S. L. Pan and J. P. Yao, “A wavelength-switchable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17(7), 5414–5419 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-7-5414. [CrossRef]

]. Two wavelengths with a side mode suppression ratio (SMSR) of more than 50 dB are generated. The wavelength spacing is tunable with a tuning step of ~10 GHz. By beating the two wavelengths at a photodetector (PD), a microwave signal with a frequency switchable from ~10 to ~40 GHz is generated.

2. Principle

Fig. 1. (a) Configuration of the proposed SLM dual-wavelength EDFL. EDF: erbium-doped fiber; WDM: wavelength-division multiplexer; PC: polarization controller; PBC: polarization beam combiner; ISO: isolator; FFPI: fiber Fabry-Perot interferometer, ATT: attenuator. (b) Schematic of the high finesse ring filter.
Fig. 2. (a) Transmission of the high finesse ring filter, FSR=140 MHz, Δω 7.3 MHz when γ=0.5 and g=1.2. (b) Finesse vs. effective gain in the ring filter.

The schematic diagram of the proposed SLM dual-wavelength EDFL is shown in Fig. 1(a). A section of EDF with a length of 8 meters (EDF1) is used as the gain medium. To ensure unidirectional operation, an optical isolator is placed before EDF1. A fiber Fabry-Perot interferometer (FFPI, Micron Optics) with an FSR of 10.2 GHz and a finesse of 4 is inserted to provide a coarse wavelength selection. To obtain dual-wavelength oscillation, the light wave is split into two channels by a 50-GHz WDM, with each channel having a polarization controller (PC) and a variable attenuator (ATT). The light waves from the two channels are then polarization multiplexed at a polarization beam combiner (PBC) to ensure that the two lasing wavelengths are orthogonally polarized, which would further introduce the PHB effect in the EDFs, and thus suppress the strong homogeneous line broadening and cross-gain saturation in the EDFs. The PBC is polarization-maintaining-fiber (PMF) pigtailed at the two input ports. The length of the PMF is about 0.5 meter, corresponding to a differential group delay (DGD) of 0.7 ps at 1550 nm. The PMFs and the PBC form two Lyot filters for the two channels. The inset in the Fig. 1(a) shows the wavelength selection for one channel. By simply adjusting the PC in that channel, the transmission spectrum of the Lyot filter is changed from the solid line to the dashed line [5

5. S. L. Pan and J. P. Yao, “A wavelength-switchable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17(7), 5414–5419 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-7-5414. [CrossRef]

]. A switching of the oscillating wavelength in that channel is thus obtained. However, densely spaced longitudinal modes around the central oscillating mode will introduce multi-longitudinal-mode oscillating and mode hopping. To overcome the problem, a novel fiber ring loop is incorporated, which consists of a section of 0.6-meter EDF (EDF2) and a 20:80 optical coupler. EDF1 and EDF2 are co-pumped by a 980-nm laser diode (LD) via a 1550 nm/980 nm WDM coupler and a 3-dB optical coupler. The 3-dB optical coupler has a splitting ratio of 50:50 at 1550 nm and a splitting ratio of 10:90 at 980 nm, with 10% of the pump power being injected into a fiber ring loop. Due to the gain introduced by the weakly pumped EDF, the ring filter has a comb spectral response with a high finesse, which is used to effectively suppress the undesirable modes in the two transmission peaks. The laser output is monitored by an optical spectrum analyzer (OSA, Ando AQ-6317B) with a resolution of 0.01 nm, and the generated microwave signal is observed by an electrical spectrum analyzer (ESA, Agilent E4448A, 3 Hz-50 GHz).

The key device in the dual-wavelength fiber laser is the high-finesse ring filter, which is schematically depicted in Fig. 1(b). Mathematically, when two input light waves with the optical fields represented by E 1 and E 2 injected into the 2×2 optical coupler, the optical fields at the two output ports can be expressed as

[E3E4]=[1γiγiγ1γ][E1E2]
(1)

where γ is the coupling factor. The optical field E 4 in the ring filter is then re-written as

E4=iγE1+1γE2
(2)

E 4 is transmitted through the 1×2 optical coupler and EDF2 in the fiber ring, obtaining an effective gain (or loss) of g and a time delay of τ, and then sent to one of the input ports of the 2×2 optical coupler. If a steady state is achieved, we obtain

E2=gejωτE4
(3)

where ω is the angular frequency of the optical fields. Substituting Eq. (2) into Eq. (3) yields

E2=igeiωτγ1geiωτ1γE1
(4)

Then, substituting Eq. (4) back into Eq. (1) we have

E3=1γgeiωτ1geiωτ1γE1
(5)
E4=iγ1geiωτ1γE1
(6)

The corresponding transmission functions are

T1=E32E12=1γ+g22g1γcosωτ1+g21γ2g1γcosωτ
(7)
T2=E42E12=γ1+g21γ2g1γcosωτ
(8)

T 1 and T 2 correspond to two transmission functions of the comb filters with the output ports at O1 and O2, respectively. In the previously reported schemes [12

12. C.-H. Yeh, T. T. Huang, H.-C. Chien, C.-H. Ko, and S. Chi, “Tunable S-band erbium-doped triple-ring laser with single-longitudinal-mode operation,” Opt. Express 15(2), 382–386 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-2-382. [CrossRef]

,13

13. S. L. Pan, X. F. Zhao, and C. Y. Lou, “Switchable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier,” Opt. Lett. 33(8), 764–766 (2008). [CrossRef]

], the output of the ring filter is from O1, thus the transmission function is T 1. Since there is no gain (no EDF) in the ring, i.e., g 1, we obtain from Eq. (7) T 1 1, which means that the fiber ring is almost an all-pass filter. To form a comb filter, one has to introduce a loss to the ring loop, making g<1. In that case, the finesse of the generated comb filter is very low (<2). To increase the finesse of the filter, in the proposed approach the output of the ring filter is from O2 and EDF2 is weakly pumped to make g>1. Correspondingly, T2 is the transmission function. The FSR and the 3-dB bandwidth of the proposed ring filter are given by

FSR=2πτ
(9)
Δω=1τcos1[4g1γ1g21γ2g1γ]
(10)

With parameters given by γ=0.5, g=1.2 and FSR=140 MHz, we obtain the simulated transmission spectrum of the ring filter, as shown in Fig. 2(a). The 3-dB bandwidth is calculated to be 7.3 MHz, giving a finesse of 19. In the experiment, the total cavity lengths without the ring filter for the two wavelengths are about 15.9 and 15.5 m, corresponding to two FSRs of 13 and 13.1 MHz. The 3-dB bandwidth of the ring filter is less than the FSRs of the cavities, indicating that the SLM condition is well satisfied. Figure 2(b) shows the impact of the effective gain on the finesse of the filter. From Fig. 2(b), we can see that the finesse is monotonically increasing for 0≤g≤1.2. In addition, a smaller γ means that a higher resonance power is fed back into the ring, resulting in a higher finesse.

3. Experimental results and discussion

Fig. 3. Electrical spectra measured at the output of the PD. (a) The spectrum measured when EDF2 is not pumped, and (b) the spectrum measured when EDF2 is weakly pumped.

An experiment based on the setup shown in Fig. 1(a) is performed. To verify the operation of the high finesse ring filter, we measure the RF spectra in the frequency range of -0.1~0.9 GHz when EDF2 is and is not pumped. When EDF2 is not pumped, a strong beat signal at 26 MHz is observed, as shown in Fig. 3(a). Since the FSRs of the laser cavities for the two wavelengths are ~13 MHz, a 13 MHz beat note is expected. However, no 13-MHz component is found in the experiment. It is believed that the 13 MHz beat note is suppressed due to the 0.6-m unpumped EDF in the ring filter. On the other hand, when a proper pump power is injected into the ring filter, the finesse of the filter is greatly increased, and an SLM operation is thus established, as shown in Fig. 3(b).

Figure 4(a) shows the optical spectra at the laser output. Two wavelengths at 1551.79 and 1551.95 nm with a wavelength spacing of 0.16 nm corresponding to a difference frequency of 20 GHz are observed. The SMSR for both wavelengths is ~51dB. It should be noted that the output port of the laser is placed just after EDF1 for high output power. The amplified spontaneous emission (ASE) from EDF1 would greatly reduce the SMSR. If the laser output port is placed before EDF1, an improved SMSR as high as 65 dB would be observed. The two wavelengths are heterodyned at the PD. As shown in Fig. 4(b), only one beat note at 20.6 GHz is observed, which demonstrates again that the dual-wavelength laser is operating at SLM. The inset of Fig. 4(b) gives a zoom-in view of the beat signal, displayed on the ESA with a span of 300 kHz and a resolution bandwidth of 4.7 kHz. We can estimate that the 3-dB bandwidth of the obtained electrical signal is less than 5 kHz. The stability of the system is also investigated. To do so, we let the system operate in a room environment for a period of 60 minutes. The optical spectra recorded at a 3-minunte interval are shown in Fig. 4(a). No significant fluctuations are observed. The wavelengths shift is less than 2 pm and the optical power fluctuation is smaller than 0.5 dB. The center frequency shift of the generated microwave signal is less than 10 MHz and the electrical power fluctuations are less than 1 dB. The frequency drift is mainly originated from the FFPI, which is very sensitive to temperature variations. Mode hopping is occasionally found during the 60-minute observation, but the separation of the modes is 140 MHz, corresponding to the FSR of the ring filter, which confirms that other modes close to the center wavelength are well suppressed. If the FSR of the ring filter is increased or a large-finesse FFPI is used, mode hopping should be completely eliminated.

Fig. 4. Generation of a 20-GHz microwave signal using the proposed fiber laser. (a) The optical spectra measured at a 4-minute interval over a 60-minute period; (b) the electrical spectra measured with RBW=300 kHz; inset: the zoom-in view of the beating signal at SPAN=300 kHz, RBW=4.7 kHz.
Fig. 5. The spectra of (a) the optical signal and (b) the generated electrical signal, with the frequency tuned from 10 to 40 GHz, RBW=100 kHz.

The wavelength-switchability of the proposed fiber laser is also investigated. As shown in Fig. 5(a), the wavelength spacing of the two wavelengths is switched to be 0.08, 0.16, 0.25, 0.33 nm, with a microwave beat note at 10.31, 20.63, 30.95 and 41.30 GHz being observed on the ESA, respectively, as shown in Fig. 5(b).

3. Conclusion

A novel fiber laser incorporating a high finesse ring filter for SLM dual-wavelength operation was proposed and studied. The key device in the proposed fiber laser was the ring filter, which was designed to have a length of EDF that was weakly pumped to generate a gain. Based on our analysis, when the effective gain was 0≤g≤1.2, the finesse of the ring filter was monotonically increasing, leading to a better suppression of undesired modes. An experiment was performed. The results showed that a dual-wavelength operation with each wavelength having a single frequency was demonstrated. The SMSR was measured greater than 50 dB for both wavelengths. The wavelength spacing could be tuned with a tuning step of ~10 GHz. By beating the two wavelengths at a PD, a microwave signal with a switchable frequency from ~10 to ~40 GHz was generated.

Acknowledgments

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through its strategic project grants program.

References and links

1.

L. Goldberg, H. F. Taylor, J. F. Weller, and D. M. Bloom, “Microwave signal generation with injection-locked laser-diodes,” Electron. Lett. 19(13), 491–493 (1983). [CrossRef]

2.

R. T. Ramos and A. J. Seeds, “Fast heterodyne optical phase-lock loop using double quantum-well laser-diodes,” Electron. Lett. 28(1), 82–83 (1992). [CrossRef]

3.

J. Liu, J. P. Yao, J. Yao, and T. H. Yeap, “Single-longitudinal-mode multiwavelength fiber ring laser,” IEEE Photon. Technol. Lett. 16(4), 1020–1022 (2004). [CrossRef]

4.

K. Zhang and J. U. Kang, “C-band wavelength-swept single-longitudinalmode erbium-doped fiber ring laser,” Opt. Express 16(18), 14173–14179 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-18-14173. [CrossRef]

5.

S. L. Pan and J. P. Yao, “A wavelength-switchable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17(7), 5414–5419 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-7-5414. [CrossRef]

6.

G. J. Chen, D. X. Huang, X. L. Zhang, and H. Cao, “Photonic generation of a microwave signal by incorporating a delay interferometer and a saturable absorber,” Opt. Lett. 33(6), 554–556 (2008). [CrossRef]

7.

X. F. Chen, Z. C. Deng, and J. P. Yao, “Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode fiber ring laser,” IEEE Trans. Microw. Theory Tech. 54(2), 804–809 (2006). [CrossRef]

8.

J. Sun, Y. T. Dai, X. F. Chen, Y. J. Zhang, and S. Z. Xie, “Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation,” IEEE Photon. Technol. Lett. 18(24), 2587–2589 (2006). [CrossRef]

9.

J. L. Zhou, L. Xia, X. P. Cheng, X. P. Dong, and P. Shum, “Photonic generation of tunable microwave signals by beating a dual-wavelength single longitudinal mode fiber ring laser,” Appl. Phys. B 91(1), 99–103 (2008). [CrossRef]

10.

D. Chen, H. Fu, and W. Liu, “Single-longitudinal-mode erbium-doped fiber laser based on a fiber Bragg grating Fabry-Perot filter,” Laser Phys. 17(10), 1246–1248 (2007). [CrossRef]

11.

W. Guan and J. R. Marciante, “Dual-frequency operation in a short-cavity ytterbium-doped fiber laser,” IEEE Photon. Technol. Lett. 19(5), 261–263 (2007). [CrossRef]

12.

C.-H. Yeh, T. T. Huang, H.-C. Chien, C.-H. Ko, and S. Chi, “Tunable S-band erbium-doped triple-ring laser with single-longitudinal-mode operation,” Opt. Express 15(2), 382–386 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-2-382. [CrossRef]

13.

S. L. Pan, X. F. Zhao, and C. Y. Lou, “Switchable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier,” Opt. Lett. 33(8), 764–766 (2008). [CrossRef]

14.

J. R. Qian, J. Su, and L. Hong, “A widely tunable dual-wavelength erbium-doped fiber ring laser operating in single longitudinal mode,” Opt. Commun. 281(17), 4432–4434 (2008). [CrossRef]

OCIS Codes
(350.4010) Other areas of optics : Microwaves
(060.2840) Fiber optics and optical communications : Heterodyne
(060.3510) Fiber optics and optical communications : Lasers, fiber

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: May 8, 2009
Revised Manuscript: June 16, 2009
Manuscript Accepted: June 24, 2009
Published: July 2, 2009

Citation
Shilong Pan and Jianping Yao, "Frequency-switchable microwave generation based on a dual-wavelength single-longitudinal-mode fiber laser incorporating a high-finesse ring filter," Opt. Express 17, 12167-12173 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-14-12167


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References

  1. L. Goldberg, H. F. Taylor, J. F. Weller, and D. M. Bloom, “Microwave signal generation with injection-locked laser-diodes,” Electron. Lett. 19(13), 491–493 (1983). [CrossRef]
  2. R. T. Ramos and A. J. Seeds, “Fast heterodyne optical phase-lock loop using double quantum-well laser-diodes,” Electron. Lett. 28(1), 82–83 (1992). [CrossRef]
  3. J. Liu, J. P. Yao, J. Yao, and T. H. Yeap, “Single-longitudinal-mode multiwavelength fiber ring laser,” IEEE Photon. Technol. Lett. 16(4), 1020–1022 (2004). [CrossRef]
  4. K. Zhang and J. U. Kang, “C-band wavelength-swept single-longitudinalmode erbium-doped fiber ring laser,” Opt. Express 16(18), 14173–14179 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-18-14173 . [CrossRef]
  5. S. L. Pan and J. P. Yao, “A wavelength-switchable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for switchable microwave generation,” Opt. Express 17(7), 5414–5419 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-7-5414 . [CrossRef]
  6. G. J. Chen, D. X. Huang, X. L. Zhang, and H. Cao, “Photonic generation of a microwave signal by incorporating a delay interferometer and a saturable absorber,” Opt. Lett. 33(6), 554–556 (2008). [CrossRef]
  7. X. F. Chen, Z. C. Deng, and J. P. Yao, “Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode fiber ring laser,” IEEE Trans. Microw. Theory Tech. 54(2), 804–809 (2006). [CrossRef]
  8. J. Sun, Y. T. Dai, X. F. Chen, Y. J. Zhang, and S. Z. Xie, “Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation,” IEEE Photon. Technol. Lett. 18(24), 2587–2589 (2006). [CrossRef]
  9. J. L. Zhou, L. Xia, X. P. Cheng, X. P. Dong, and P. Shum, “Photonic generation of tunable microwave signals by beating a dual-wavelength single longitudinal mode fiber ring laser,” Appl. Phys. B 91(1), 99–103 (2008). [CrossRef]
  10. D. Chen, H. Fu, and W. Liu, “Single-longitudinal-mode erbium-doped fiber laser based on a fiber Bragg grating Fabry-Perot filter,” Laser Phys. 17(10), 1246–1248 (2007). [CrossRef]
  11. W. Guan and J. R. Marciante, “Dual-frequency operation in a short-cavity ytterbium-doped fiber laser,” IEEE Photon. Technol. Lett. 19(5), 261–263 (2007). [CrossRef]
  12. C.-H. Yeh, T. T. Huang, H.-C. Chien, C.-H. Ko, and S. Chi, “Tunable S-band erbium-doped triple-ring laser with single-longitudinal-mode operation,” Opt. Express 15(2), 382–386 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-2-382 . [CrossRef]
  13. S. L. Pan, X. F. Zhao, and C. Y. Lou, “Switchable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser incorporating a semiconductor optical amplifier,” Opt. Lett. 33(8), 764–766 (2008). [CrossRef]
  14. J. R. Qian, J. Su, and L. Hong, “A widely tunable dual-wavelength erbium-doped fiber ring laser operating in single longitudinal mode,” Opt. Commun. 281(17), 4432–4434 (2008). [CrossRef]

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