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Dual-wavelength single-frequency fiber laser based on FP-LD injection locking for millimeter-wave generation
Optics & Laser Technology 64 (2014) 328–332
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Dual-wavelength single-frequency fiber laser based on FP-LD injection locking for millimeter-wave generation Jingjuan Zhou, Xinhuan Feng n, Yuzhuo Wang, Zhaohui Li, Bai-ou Guan Institute of Photonics Technology, Jinan University, 510632 Guangzhou, China
art ic l e i nf o
a b s t r a c t
Article history: Received 19 January 2014 Received in revised form 15 May 2014 Accepted 7 June 2014 Available online 26 June 2014
We propose and successfully demonstrate a dual-wavelength single-frequency fiber laser based on two cascaded Fabry Pérot laser diodes (FP-LDs) and a WaveShaper. Both the cascaded FP-LDs and the WaveShaper perform the dual-wavelength selection. Injection locking of the FP-LDs in laser cavity and narrow bandwidth filtering effect of the WaveShaper guarantee single-frequency operation of the laser. By adjustment of the drive currents of the two FP-LDs and the filtering characteristic of the WaveShaper, dual-wavelength single-frequency output with switchable wavelength spacing can be obtained, which can be used for switchable millimeter-wave generation. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Fiber laser Dual-wavelength single-frequency Injection locking
1. Introduction Single-frequency fiber laser has attracted much interest for its potential applications in optical communications, fiber sensing, and high-resolution spectroscopy due to the advantage of its good coherence. Lasers operating simultaneously at two wavelengths, each at a single frequency, have been extensively studied because of their special applications in microwave generation [1–16] and optical fiber sensing [17,18] due to their advantages of low phase noise, low power consumption, and narrow linewidth. Since different techniques have to be used to ensure both single-frequency operation and dualwavelength output of the laser, it is always a challenge to realize simultaneously dual-wavelength and single-frequency oscillation from a fiber laser. To date, there are mainly three kinds of techniques reported to achieve dual-wavelength single-frequency fiber laser: (1) The utilization of short cavity such as distributed feedback (DFB) and distributed Bragg reflector (DBR) fiber lasers combined with special fiber Bragg gratings (FBGs) [1–3,17,18]. Such dual-wavelength single-frequency lasers have good stability, but the FBG needs to be specially fabricated and the spacing between the two transmission peak wavelengths is restricted by the reflection bandwidth of the FBG; (2) The insertion of high finesse sub-cavity or ultra-narrow filters in the cavity [4–8], single-frequency operation is easier to be realized because spatial hole burning is effectively avoided, but the operation lack of stability; (3) The utilization of combined filtering effect of dualwavelength filter and unpumped erbium-doped fiber as saturable
absorber [9–16], the lasers achieved have good stability, but the output power is lower. Yeh et al. once reported a tunable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser (EDFL) utilizing the self feedback injection of the Fabry Pérot laser diode (FP-LD) [19]. However, the wavelength spacing of the laser can not be tunable, and the long cavity of the EDFL makes it more difficult to produce stable single-frequency output. In this paper, a dual-wavelength single-frequency fiber laser with switchable wavelength spacing based on FP-LD injection locking and a WaveShaper is proposed and demonstrated. Injection locking of the FP-LDs in laser cavity and narrow bandwidth filtering effect of the WaveShaper guarantee single-frequency operation of the laser. By appropriate adjustment of the drive currents of the two FP-LDs and the passband wavelengths of the WaveShaper, dual-wavelength single-frequency oscillation with switchable wavelength spacing has been achieved. Each wavelength has a linewidth less than 16.5 kHz and a side-mode suppression ratio (SMSR) larger than 50 dB. The wavelength spacing can be switchable among about 0.24 nm, 0.40 nm, 0.60 nm and 0.80 nm, corresponding to millimeter wave generation respectively at frequencies of about 29.7 GHz, 50 GHz, 75 GHz and 100 GHz. The proposed laser has advantages such as switchable wavelength spacing and tunable wavelength, and can find potential application in tunable millimeter-wave signal generation.
2. Experimental setup
n
Corresponding author. E-mail address: [email protected] (X. Feng).
http://dx.doi.org/10.1016/j.optlastec.2014.06.006 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
The experimental setup of the proposed fiber laser is shown in Fig. 1. It mainly consists of a semiconductor optical amplifier (SOA) (S-OEC-1550, CIP), a WaveShaper (4000S, Finisar), two FP-LDs, two
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Fig. 1. Experimental setup of the proposed fiber laser. FP-LD: Fabry Pérot laser diode; SOA: semiconductor optical amplifier; PC: polarization controlle; ISO: isolator; CIR: circulator; TOF: tunable optical filter; OSA: optical spectrum analyzer; ESA: electronic spectrum analyzer; PD: photo-detector.
polarization controllers (PCs), and a 90:10 output coupler. The SOA is used as gain medium and has a maximum gain of about 23 dB and a bandwidth of about 60 nm. Two isolators (ISOs) are inserted before and behind the SOA to ensure unidirectional operation. The FP-LD1 and the FP-LD2 are introduced into the ring cavity respectively through two circulators (CIRs). PCs are used before the SOA and the FP-LDs for optimum gain performance. The 10% output is sent to a tunable optical filter (TOF) (OTF-350, Santec) for high side-mode suppression ratio (SMSR) of the output oscillation. The filtered output is then divided into two parts by a 50:50 coupler: one is measured by an optical spectrum analyzer (OSA) (AQ6370B, Yokogawa) with a resolution of 0.02 nm; the other part is measured using a 26 GHz electronic spectrum analyzer (ESA) (MS2692A, Anritsu)after being detected at a photo-detector (PD) (Newport). One of the key components in the proposed scheme is the WaveShaper. It is a programmable optical filter with full control of filter amplitude, wavelength, bandwidth, shape and phase characteristics. The WaveShaper 4000S supports arbitrary user-generated filter shapes. The required filter profile can be generated by the user and then loaded into the WaveManager software which translates the user specification into the required filter shape. In this experiment, the WaveShaper is programmed to act as a dual-wavelength narrow band-pass filter. Both the passband spacing and the passband bandwidth can be controlled with a minimum step of 0.1 nm. An SOA is used as the gain medium to further ensure the single-frequency operation, since it can greatly shorten the laser cavity and consequently increase the longitudinal-mode spacing, compared with erbium-doped fiber amplifier.
3. Operation principle The dual-wavelength single-frequency operation principle of the laser can be explained as following: On the one hand, the output from a FP-LD injection locked by a multi-longitudinalmode light can have fewer longitudinal-modes and narrower linewidth [20]. Single-frequency operation can be achieved when stable laser oscillation established after many roundtrips through the FP-LDs. On the other hand, the WaveShaper is programmed to act as a dual-wavelength narrow band-pass filter, each channel with a filtering bandwidth of 0.1 nm and a tunable wavelength step of 0.1 nm. When the two passband wavelengths of the WaveShaper and the two different longitudinal modes defined by the two FP-LDs are adjusted to coincide with each other, dualwavelength single-frequency operation can be possibly achieved by combination of the narrowing effect of the FP-LDs injection locking and the WaveShaper. Fig. 2 is given to illustrate the principle more clearly, which shows the optical spectra of FP-LD1 (black solid line) and FP-LD2
Fig. 2. Optical spectra of the FP-LD1 (black solid line) and FP-LD2 (red dashed line) without injection locking. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(red dashed line) without injection locking when the drive currents were respectively set at 50 mA and 58 mA. When dualpassband of the WaveShaper is programmed to coincide with two neighboring longitudinal modes defined by the two FP-LDs, singlefrequency operation at these two wavelengths can be obtained, according to the principle mentioned above. Wavelength spacing of the two neighboring longitudinal modes defined by the two FPLDs can be changed by adjustment of the drive currents of the two FP-LDs, and the passband spacing of the WaveShaper can be programmed to vary with a certain step size. Consequently, dual-wavelength single-frequency output with switchable wavelength spacing can be achieved. Furthermore, when the passband spacing of the WaveShaper is fixed, wavelength tunable dualwavelength output can be obtained by injection locking different longitudinal modes of the FP-LDs through adjustment of the center wavelength of the WaveShaper.
4. Results and discussions When the center wavelength of the WaveShaper was fixed at 1550.05 nm while the passband spacing between the defined two wavelengths was varied from about 0.2 nm to 0.8 nm with a step of 0.2 nm, dual-wavelength single-frequency lasing with switchable wavelength spacing has been achieved by adjusting the drive currents of the FP-LD1 and FP-LD2 respectively from 50 mA to 30 mA and from 45 mA to 65 mA. The output optical spectra before and after being filtered by the TOF are respectively shown in Fig. 3(a) and (b). The wavelength spacing was switched among about 0.24 nm (black solid
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Fig. 3. Optical spectra of the laser with switchable wavelength spacing (a) before and (b) after being filtered by the TOF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
line), 0.40 nm (red dash line), 0.60 nm (blue dash-dot line) and 0.80 nm (green short dash line), corresponding to the passband spacing defined by the WaveShaper. All the output oscillations had a SMSR of at least 35 dB before the TOF and the SMSR could be larger than 50 dB after the TOF, as shown in Fig. 3(a) and (b). To verify the single-frequency operation condition of the proposed laser system, we measured the beating radio frequency (RF) spectrum of the laser output using an ESA with a resolution bandwidth (RBW) of 300 kHz. Fig. 4 shows the RF spectrum of the laser output when the wavelength spacing of the oscillation was 0.6 nm. Since the estimated longitudinal mode spacing of the laser is to be about 20 MHz, the RF spectrum confirms that there was only single longitudinal mode exists and the laser was under single-frequency operation. The linewidth of each wavelength of the laser output was measured using the delayed self-heterodyne method [20]. Fig. 5 (a) and (b) show the delayed self-heterodyne RF spectra of lasing wavelengths of 1549.78 nm and 1550.38 nm (the black line) when the wavelength spacing of the output was 0.6 nm. The Lorentz curve fittings of intensity are also shown (the red line). Assuming the laser spectrum to be Lorentzian-shaped, the full width at half maximums (FWHMs) of the two wavelengths are estimated to be about 16.5 kHz and 14.5 kHz, calculated respectively from the 10 dB bandwidths of 99 kHz and 87 kHz. Since the frequency of the beat signal of the two output wavelengths is out of the measured range of the ESA and it can't be observed directly, we down-converted the beat signal using an electronic mixer. When the wavelength spacing of the laser output was about 0.24 nm (corresponding to a beat frequency of 29.7 GHz), a RF signal at about 9.7 GHz was obtained after down-conversion by mixing the beat millimeter wave signal with a 20 GHz microwave signal from a microwave signal generator. Fig. 6 shows the RF spectrum of the down-converted signal. The corresponding optical spectrum of the laser output is shown in the inset. Due to the operation frequency limitation of the PD, the ESA and the microwave signal generator, the RF spectrum of millimeter-wave signals by beating dual-wavelength oscillation with larger wavelength spacings was not obtained. By tuning the center wavelength of the WaveShaper from 1550.05 nm to 1552.75 nm respectively with a step size of 0.1 nm and 1.3 nm, four dual-wavelength single-frequency lasers with respectively center wavelength of 1550.05 nm (black solid line), 1550.15 nm (red dash line), 1551.45 nm (blue dash-dot line) and 1552.75 nm (green short dash line) were achieved. Fig. 7 shows the optical spectra of the laser output after being filtered by the TOF when the drive currents of the FP-LD1, FP-LD2 and SOA were respectively set at 50 mA, 58 mA and 68 mA. In our experiment, the TOF was placed outside the cavity to filter out the side-modes to improve the SMSR. Furthermore, the
Fig. 4. RF spectrum of the laser output.
optical power difference between the two wavelengths can be balanced by adjusting the TOF. But it should be noted that the power difference between the two wavelengths doesn't affect the generation of the millimeter-wave. The gain of the SOA may have some effects on the singlefrequency operation of the laser. When the wavelength spacing is changed, PC2 has to be adjusted slightly to ensure singlefrequency operation. As known, the gain of the SOA would change with the adjustment of the PC. But single-frequency operation of the laser can be obtained for different gains of the SOA by adjustment of the PCs and the FP-LDs. Another point we have to note is that continuous tunability of wavelength spacing is limited by the WaveShaper. As mentioned previously, each channel defined by the WaveShaper has a filtering bandwidth of about 0.1 nm (the minimum filter bandwidth of the WaveShaper). This minimum filtering bandwidth of the WaveShaper makes it impossible to obtain a continuous tenability of wavelength spacing. As for larger wavelength spacing, there is no limitation. We can just program the WaveShaper and tune the two FP-LDs accordingly. Therefore, it is not limited to just four wavelength spacings as shown in Fig. 3. Since the limitation of continuous tuning of wavelength spacing comes from the WaveShaper, we think other kind of dual-wavelength filter with tunable wavelength spacing may replace the WaveShaper to obtain different wavelength spacing to match the millimeter-wave generation that might be needed for a specific application. For example, a high birefringence fiber Bragg grating can be used for this purpose when stain is applied to its either axis. But it is not as stable as the WaveShaper.
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Fig. 5. Typical delayed self-heterodyne RF spectra of wavelengths of (a) 1549.78 nm and (b) 1550.38 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
cascaded FP-LDs and the WaveShaper perform the dualwavelength selection and introduce narrow bandwidth filtering effect to guarantee single-frequency operation of the laser. By appropriately adjusting the drive currents of the two FP-LDs and the passband wavelengths of the WaveShaper, dual-wavelength single-frequency output with switchable wavelength spacings about 0.24 nm, 0.40 nm, 0.60 nm and 0.80 nm has been obtained. The linewidths of the two wavelength signals are narrower than 16.5 kHz, and the SMSRs are larger than 50 dB. A 9.7 GHz beat frequency signal has been achieved through a down-conversion form 29.7 GHz. It has potential application in switchable millimeter-wave signal generation.
Acknowledgements Fig. 6. Frequency spectrum of the down-converted beating signal.
This work was supported in part by the Natural Science Foundation of Guangdong Province of China (no. S2012010008850), the Fundamental Research Funds for the Central Universities in China (21612201), and the Planned Science and Technology Project of Guangzhou (no. 2012J5100028). References
Fig. 7. Optical spectra of the laser for wavelength tunable output. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Finally, note that the proposed fiber laser can be reconfigured by cascading more FP-LDs. The WaveShaper can also be programmed to have multiple passbands. Consequently, multi-wavelength single-frequency lasing oscillation can be possibly achieved.
5. Conclusion In conclusion, we have successfully proposed and experimentally demonstrated a dual-wavelength single-frequency fiber laser based on FP-LD injection locking and a WaveShaper. Both the
[1] Sun J, Dai YT, Chen XF, Zhang YJ, Xie SZ. Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation. IEEE Photon Technol Lett 2006;18:2587–9. [2] Guan BO, Zhang Y, Zhang LW, Tam HY. Electrically tunable microwave generation using compact dual-polarization fiber laser. IEEE Photon Technol Lett 2009;21:727–9. [3] Mo SP, Feng ZM, Xu SH, Zhang WN, Chen DD, Yang T, et al. Photonic generation of tunable microwave signals from a dual-wavelength distributedBragg-reflector highly Er3 þ/Yb3 þ co-doped phosphate fiber laser. Laser Phys Lett 2013;10:125107–10. [4] Zhou JL, Xia L, Cheng XP, Dong XP, Shum P. Photonic generation of tunable microwave signals by beating a dual-wavelength single longitudinal mode fiber ring laser. Appl Phys B 2008;91:99–103. [5] Chen XF, Deng ZC, Yao JP. Photonic generation of microwave signal using a dual-wavelength single-longitudinal-mode fiber ring laser. IEEE Trans Microwave Theory Tech 2006;54:804–9. [6] Yao Y, Chen XF, Dai YT, Xie SZ. Dual-wavelength erbium-doped fiber laser with a simple linear cavity and its application in microwave generation. IEEE Photon Technol Lett 2006;18:187–9. [7] Pan SL, Yao JP. Frequency-switchable microwave generation based on a dualwavelength single-longitudinal-mode fiber laser incorporating a high-finesse ring filter. Opt Express 2009;17:12167–73. [8] Chen D, Fu H, Liu W, Wei Y, He S. Dual-wavelength single-longitudinal-mode erbium-doped fibre laser based on fibre Bragg grating pair and its application in microwave signal generation. Electron Lett 2008;44:459–61. [9] Pan SL, Yao JP. A wavelength-switchable single-longitudinal-mode dualwavelength erbium-doped fiber laser for switchable microwave generation. Opt Express 2009;17:5414–9. [10] Feng SC, Qi CH, Li Q, Peng WJ, Gao S, Jian SS. Photonic generation of microwave signal by beating a dual-wavelength single longitudinal mode erbium-doped fiber ring laser based on the polarization maintaining fiber Bragg grating. Microwave Opt Technol Lett 2013;55:347–51.
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[11] Feng SC, Xu O, Lu SH, Ning TG, Jian SS. Switchable single-longitudinal-mode dual-wavelength erbium-doped fiber ring laser based on one polarization maintaining fiber Bragg grating incorporating saturable absorber and feedback fiber loop. Opt Commun 2009;282:2165–8. [12] He XY, Fang X, Liao CR, Wang DN, Sun JQ. A tunable and switchable singlelongitudinal-mode dual-wavelength fiber laser with a simple linear cavity. Opt Express 2009;17:21773–81. [13] Lin B, Tjin SC, Jiang M, Shum P. Tunable microwave generation based on a dual-wavelength fiber laser with an inverse-Gaussian apodized fiber Bragg grating. Appl Opt 2011;50:4912–6. [14] Sun JQ, Huang YX, Li H, Jiang C. Photonic generation of microwave signals using dual-wavelength single-longitudinal-mode fiber lasers. Optik 2011;122: 764–8. [15] Kim RK, Chu S, Han YG. Stable and widely tunable single-longitudinal-mode dual-wavelength erbium-doped fiber laser for optical beat frequency generation. IEEE Photon Technol Lett 2012;24:521–3.
[16] Wang F, Xu EM, Dong JJ, Zhang XL. A tunable and switchable singlelongitudinal-mode dual-wavelength fiber laser incorporating a reconfigurable dual-pass Mach–Zehnder interferometer and its application in microwave generation. Opt Commun 2011;284:2337–40. [17] Shao LY, Dong XY, Zhang AP, Tam HY, He SL. High-resolution strain and temperature sensor based on distributed Bragg reflector fiber laser. IEEE Photon Technol Lett 2007;19:1598–600. [18] Guan BO, Jin L, Zhang Y, Tam HY. Polarimetric heterodyning fiber grating laser sensors. J Lightwave Technol 2012;30:1097–112. [19] Yeh CH, Shih FY, Wang CH, Chow CW, Chi S. Tunable and stable singlelongitudinal-mode dual-wavelength erbium fiber laser with 1.3 nm mode spacing output. Laser Phys Lett 2008;5:821–4. [20] Zhang AQ, Feng XH, Wan MG, Li ZH, Guan BO. Tunable single frequency fiber laser based on FP-LD injection locking. Opt Express 2013;21:12874–80.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Dual-wavelength single-frequency fiber laser based on FP-LD injection locking for millimeter-wave generation Jingjuan Zhou, Xinhuan Feng n, Yuzhuo Wang, Zhaohui Li, Bai-ou Guan Institute of Photonics Technology, Jinan University, 510632 Guangzhou, China
art ic l e i nf o
a b s t r a c t
Article history: Received 19 January 2014 Received in revised form 15 May 2014 Accepted 7 June 2014 Available online 26 June 2014
We propose and successfully demonstrate a dual-wavelength single-frequency fiber laser based on two cascaded Fabry Pérot laser diodes (FP-LDs) and a WaveShaper. Both the cascaded FP-LDs and the WaveShaper perform the dual-wavelength selection. Injection locking of the FP-LDs in laser cavity and narrow bandwidth filtering effect of the WaveShaper guarantee single-frequency operation of the laser. By adjustment of the drive currents of the two FP-LDs and the filtering characteristic of the WaveShaper, dual-wavelength single-frequency output with switchable wavelength spacing can be obtained, which can be used for switchable millimeter-wave generation. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Fiber laser Dual-wavelength single-frequency Injection locking
1. Introduction Single-frequency fiber laser has attracted much interest for its potential applications in optical communications, fiber sensing, and high-resolution spectroscopy due to the advantage of its good coherence. Lasers operating simultaneously at two wavelengths, each at a single frequency, have been extensively studied because of their special applications in microwave generation [1–16] and optical fiber sensing [17,18] due to their advantages of low phase noise, low power consumption, and narrow linewidth. Since different techniques have to be used to ensure both single-frequency operation and dualwavelength output of the laser, it is always a challenge to realize simultaneously dual-wavelength and single-frequency oscillation from a fiber laser. To date, there are mainly three kinds of techniques reported to achieve dual-wavelength single-frequency fiber laser: (1) The utilization of short cavity such as distributed feedback (DFB) and distributed Bragg reflector (DBR) fiber lasers combined with special fiber Bragg gratings (FBGs) [1–3,17,18]. Such dual-wavelength single-frequency lasers have good stability, but the FBG needs to be specially fabricated and the spacing between the two transmission peak wavelengths is restricted by the reflection bandwidth of the FBG; (2) The insertion of high finesse sub-cavity or ultra-narrow filters in the cavity [4–8], single-frequency operation is easier to be realized because spatial hole burning is effectively avoided, but the operation lack of stability; (3) The utilization of combined filtering effect of dualwavelength filter and unpumped erbium-doped fiber as saturable
absorber [9–16], the lasers achieved have good stability, but the output power is lower. Yeh et al. once reported a tunable single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber laser (EDFL) utilizing the self feedback injection of the Fabry Pérot laser diode (FP-LD) [19]. However, the wavelength spacing of the laser can not be tunable, and the long cavity of the EDFL makes it more difficult to produce stable single-frequency output. In this paper, a dual-wavelength single-frequency fiber laser with switchable wavelength spacing based on FP-LD injection locking and a WaveShaper is proposed and demonstrated. Injection locking of the FP-LDs in laser cavity and narrow bandwidth filtering effect of the WaveShaper guarantee single-frequency operation of the laser. By appropriate adjustment of the drive currents of the two FP-LDs and the passband wavelengths of the WaveShaper, dual-wavelength single-frequency oscillation with switchable wavelength spacing has been achieved. Each wavelength has a linewidth less than 16.5 kHz and a side-mode suppression ratio (SMSR) larger than 50 dB. The wavelength spacing can be switchable among about 0.24 nm, 0.40 nm, 0.60 nm and 0.80 nm, corresponding to millimeter wave generation respectively at frequencies of about 29.7 GHz, 50 GHz, 75 GHz and 100 GHz. The proposed laser has advantages such as switchable wavelength spacing and tunable wavelength, and can find potential application in tunable millimeter-wave signal generation.
2. Experimental setup
n
Corresponding author. E-mail address: [email protected] (X. Feng).
http://dx.doi.org/10.1016/j.optlastec.2014.06.006 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
The experimental setup of the proposed fiber laser is shown in Fig. 1. It mainly consists of a semiconductor optical amplifier (SOA) (S-OEC-1550, CIP), a WaveShaper (4000S, Finisar), two FP-LDs, two
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Fig. 1. Experimental setup of the proposed fiber laser. FP-LD: Fabry Pérot laser diode; SOA: semiconductor optical amplifier; PC: polarization controlle; ISO: isolator; CIR: circulator; TOF: tunable optical filter; OSA: optical spectrum analyzer; ESA: electronic spectrum analyzer; PD: photo-detector.
polarization controllers (PCs), and a 90:10 output coupler. The SOA is used as gain medium and has a maximum gain of about 23 dB and a bandwidth of about 60 nm. Two isolators (ISOs) are inserted before and behind the SOA to ensure unidirectional operation. The FP-LD1 and the FP-LD2 are introduced into the ring cavity respectively through two circulators (CIRs). PCs are used before the SOA and the FP-LDs for optimum gain performance. The 10% output is sent to a tunable optical filter (TOF) (OTF-350, Santec) for high side-mode suppression ratio (SMSR) of the output oscillation. The filtered output is then divided into two parts by a 50:50 coupler: one is measured by an optical spectrum analyzer (OSA) (AQ6370B, Yokogawa) with a resolution of 0.02 nm; the other part is measured using a 26 GHz electronic spectrum analyzer (ESA) (MS2692A, Anritsu)after being detected at a photo-detector (PD) (Newport). One of the key components in the proposed scheme is the WaveShaper. It is a programmable optical filter with full control of filter amplitude, wavelength, bandwidth, shape and phase characteristics. The WaveShaper 4000S supports arbitrary user-generated filter shapes. The required filter profile can be generated by the user and then loaded into the WaveManager software which translates the user specification into the required filter shape. In this experiment, the WaveShaper is programmed to act as a dual-wavelength narrow band-pass filter. Both the passband spacing and the passband bandwidth can be controlled with a minimum step of 0.1 nm. An SOA is used as the gain medium to further ensure the single-frequency operation, since it can greatly shorten the laser cavity and consequently increase the longitudinal-mode spacing, compared with erbium-doped fiber amplifier.
3. Operation principle The dual-wavelength single-frequency operation principle of the laser can be explained as following: On the one hand, the output from a FP-LD injection locked by a multi-longitudinalmode light can have fewer longitudinal-modes and narrower linewidth [20]. Single-frequency operation can be achieved when stable laser oscillation established after many roundtrips through the FP-LDs. On the other hand, the WaveShaper is programmed to act as a dual-wavelength narrow band-pass filter, each channel with a filtering bandwidth of 0.1 nm and a tunable wavelength step of 0.1 nm. When the two passband wavelengths of the WaveShaper and the two different longitudinal modes defined by the two FP-LDs are adjusted to coincide with each other, dualwavelength single-frequency operation can be possibly achieved by combination of the narrowing effect of the FP-LDs injection locking and the WaveShaper. Fig. 2 is given to illustrate the principle more clearly, which shows the optical spectra of FP-LD1 (black solid line) and FP-LD2
Fig. 2. Optical spectra of the FP-LD1 (black solid line) and FP-LD2 (red dashed line) without injection locking. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(red dashed line) without injection locking when the drive currents were respectively set at 50 mA and 58 mA. When dualpassband of the WaveShaper is programmed to coincide with two neighboring longitudinal modes defined by the two FP-LDs, singlefrequency operation at these two wavelengths can be obtained, according to the principle mentioned above. Wavelength spacing of the two neighboring longitudinal modes defined by the two FPLDs can be changed by adjustment of the drive currents of the two FP-LDs, and the passband spacing of the WaveShaper can be programmed to vary with a certain step size. Consequently, dual-wavelength single-frequency output with switchable wavelength spacing can be achieved. Furthermore, when the passband spacing of the WaveShaper is fixed, wavelength tunable dualwavelength output can be obtained by injection locking different longitudinal modes of the FP-LDs through adjustment of the center wavelength of the WaveShaper.
4. Results and discussions When the center wavelength of the WaveShaper was fixed at 1550.05 nm while the passband spacing between the defined two wavelengths was varied from about 0.2 nm to 0.8 nm with a step of 0.2 nm, dual-wavelength single-frequency lasing with switchable wavelength spacing has been achieved by adjusting the drive currents of the FP-LD1 and FP-LD2 respectively from 50 mA to 30 mA and from 45 mA to 65 mA. The output optical spectra before and after being filtered by the TOF are respectively shown in Fig. 3(a) and (b). The wavelength spacing was switched among about 0.24 nm (black solid
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Fig. 3. Optical spectra of the laser with switchable wavelength spacing (a) before and (b) after being filtered by the TOF. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
line), 0.40 nm (red dash line), 0.60 nm (blue dash-dot line) and 0.80 nm (green short dash line), corresponding to the passband spacing defined by the WaveShaper. All the output oscillations had a SMSR of at least 35 dB before the TOF and the SMSR could be larger than 50 dB after the TOF, as shown in Fig. 3(a) and (b). To verify the single-frequency operation condition of the proposed laser system, we measured the beating radio frequency (RF) spectrum of the laser output using an ESA with a resolution bandwidth (RBW) of 300 kHz. Fig. 4 shows the RF spectrum of the laser output when the wavelength spacing of the oscillation was 0.6 nm. Since the estimated longitudinal mode spacing of the laser is to be about 20 MHz, the RF spectrum confirms that there was only single longitudinal mode exists and the laser was under single-frequency operation. The linewidth of each wavelength of the laser output was measured using the delayed self-heterodyne method [20]. Fig. 5 (a) and (b) show the delayed self-heterodyne RF spectra of lasing wavelengths of 1549.78 nm and 1550.38 nm (the black line) when the wavelength spacing of the output was 0.6 nm. The Lorentz curve fittings of intensity are also shown (the red line). Assuming the laser spectrum to be Lorentzian-shaped, the full width at half maximums (FWHMs) of the two wavelengths are estimated to be about 16.5 kHz and 14.5 kHz, calculated respectively from the 10 dB bandwidths of 99 kHz and 87 kHz. Since the frequency of the beat signal of the two output wavelengths is out of the measured range of the ESA and it can't be observed directly, we down-converted the beat signal using an electronic mixer. When the wavelength spacing of the laser output was about 0.24 nm (corresponding to a beat frequency of 29.7 GHz), a RF signal at about 9.7 GHz was obtained after down-conversion by mixing the beat millimeter wave signal with a 20 GHz microwave signal from a microwave signal generator. Fig. 6 shows the RF spectrum of the down-converted signal. The corresponding optical spectrum of the laser output is shown in the inset. Due to the operation frequency limitation of the PD, the ESA and the microwave signal generator, the RF spectrum of millimeter-wave signals by beating dual-wavelength oscillation with larger wavelength spacings was not obtained. By tuning the center wavelength of the WaveShaper from 1550.05 nm to 1552.75 nm respectively with a step size of 0.1 nm and 1.3 nm, four dual-wavelength single-frequency lasers with respectively center wavelength of 1550.05 nm (black solid line), 1550.15 nm (red dash line), 1551.45 nm (blue dash-dot line) and 1552.75 nm (green short dash line) were achieved. Fig. 7 shows the optical spectra of the laser output after being filtered by the TOF when the drive currents of the FP-LD1, FP-LD2 and SOA were respectively set at 50 mA, 58 mA and 68 mA. In our experiment, the TOF was placed outside the cavity to filter out the side-modes to improve the SMSR. Furthermore, the
Fig. 4. RF spectrum of the laser output.
optical power difference between the two wavelengths can be balanced by adjusting the TOF. But it should be noted that the power difference between the two wavelengths doesn't affect the generation of the millimeter-wave. The gain of the SOA may have some effects on the singlefrequency operation of the laser. When the wavelength spacing is changed, PC2 has to be adjusted slightly to ensure singlefrequency operation. As known, the gain of the SOA would change with the adjustment of the PC. But single-frequency operation of the laser can be obtained for different gains of the SOA by adjustment of the PCs and the FP-LDs. Another point we have to note is that continuous tunability of wavelength spacing is limited by the WaveShaper. As mentioned previously, each channel defined by the WaveShaper has a filtering bandwidth of about 0.1 nm (the minimum filter bandwidth of the WaveShaper). This minimum filtering bandwidth of the WaveShaper makes it impossible to obtain a continuous tenability of wavelength spacing. As for larger wavelength spacing, there is no limitation. We can just program the WaveShaper and tune the two FP-LDs accordingly. Therefore, it is not limited to just four wavelength spacings as shown in Fig. 3. Since the limitation of continuous tuning of wavelength spacing comes from the WaveShaper, we think other kind of dual-wavelength filter with tunable wavelength spacing may replace the WaveShaper to obtain different wavelength spacing to match the millimeter-wave generation that might be needed for a specific application. For example, a high birefringence fiber Bragg grating can be used for this purpose when stain is applied to its either axis. But it is not as stable as the WaveShaper.
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Fig. 5. Typical delayed self-heterodyne RF spectra of wavelengths of (a) 1549.78 nm and (b) 1550.38 nm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
cascaded FP-LDs and the WaveShaper perform the dualwavelength selection and introduce narrow bandwidth filtering effect to guarantee single-frequency operation of the laser. By appropriately adjusting the drive currents of the two FP-LDs and the passband wavelengths of the WaveShaper, dual-wavelength single-frequency output with switchable wavelength spacings about 0.24 nm, 0.40 nm, 0.60 nm and 0.80 nm has been obtained. The linewidths of the two wavelength signals are narrower than 16.5 kHz, and the SMSRs are larger than 50 dB. A 9.7 GHz beat frequency signal has been achieved through a down-conversion form 29.7 GHz. It has potential application in switchable millimeter-wave signal generation.
Acknowledgements Fig. 6. Frequency spectrum of the down-converted beating signal.
This work was supported in part by the Natural Science Foundation of Guangdong Province of China (no. S2012010008850), the Fundamental Research Funds for the Central Universities in China (21612201), and the Planned Science and Technology Project of Guangzhou (no. 2012J5100028). References
Fig. 7. Optical spectra of the laser for wavelength tunable output. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Finally, note that the proposed fiber laser can be reconfigured by cascading more FP-LDs. The WaveShaper can also be programmed to have multiple passbands. Consequently, multi-wavelength single-frequency lasing oscillation can be possibly achieved.
5. Conclusion In conclusion, we have successfully proposed and experimentally demonstrated a dual-wavelength single-frequency fiber laser based on FP-LD injection locking and a WaveShaper. Both the
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