Photonic generation of microwave signal using a dual-wavelength erbium-doped fiber ring laser with CMFBG filter and saturable absorber

Optics & Laser Technology 45 (2013) 32–36

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Photonic generation of microwave signal using a dual-wavelength erbium-doped fiber ring laser with CMFBG filter and saturable absorber Suchun Feng a,n, Shaohua Lu b, Wanjing Peng a, Qi Li a, Chunhui Qi a, Ting Feng a, Shuisheng Jian a a b

Institute of Lightwave Technology, Key Lab of All Optical Network and Advanced Telecommunication of EMC, Beijing Jiaotong University, Beijing, China Safety Engineering Department, Beijing Vocational College of Labour and Social Security, Beijing, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 May 2012 Received in revised form 31 July 2012 Accepted 2 August 2012 Available online 24 August 2012

A simple approach for photonic generation of microwave signal using a dual-wavelength singlelongitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser is proposed and demonstrated. For the first time as we know, a chirped moire´ fiber Bragg grating (CMFBG) filter with ultra-narrow transmission band and a chirped fiber Bragg grating (FBG) are used to select the laser longitudinal mode. The stable SLM operation of the fiber laser is guaranteed by the combination of the CMFBG filter and 3 m unpumped EDF acting as a saturable absorber. Stable dual-wavelength SLM fiber laser with a wavelength spacing of approximately 0.140 nm is experimentally realized. By beating the dualwavelength fiber laser at a photodetector, photonic generation of microwave signal at 17.682 GHz is successfully obtained. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Microwave generation Single-longitudinal-mode CMFBG filter

1. Introduction Photonic generation of microwave signals has attracted considerable interests for their applications in various fields such as radio-over-fiber networks, broadband wireless access network, sensor networks and so on. In the last few years, there are numerous techniques for the photonic generation of microwave signals, which can be classified into four categories [1]: (a) optical injection locking, (b) optical phase-lock loop, (c) microwave generation using external modulation, and (d) dual-wavelength single-longitudinal-mode (SLM) laser source. Recently, more and more researchers have paid attention to photonic generation of microwave signals using dual-wavelength SLM fiber laser sources. It is difficult to obtain simultaneous multi-wavelength lasing at room temperature in erbium-doped fiber (EDF) lasers because EDF is the primary homogeneous gain medium. Chen et al. [2] proposed dual-wavelength SLM laser utilizing an ultranarrow dual-transmission-peak bandpass filter in a ring cavity. They also demonstrated the dual-wavelength SLM lasers in linear laser cavity [3–6]. Nevertheless, the required bandpass filters must be specially designed with the complicated equivalent phase shift technique and the spacing between the two transmission peak wavelengths is restricted by the reflection bandwidth of the uniform fiber Bragg grating (FBG). Liu [7] also proposed the similar technique to realize the dual-wavelength SLM laser

n

Corresponding author. Tel./fax: þ 86 10 51683625. E-mail address: [email protected] (S. Feng).

0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.08.002

in a linear laser cavity. Li et al. [8] proposed dual-wavelength emission from cascaded distributed feedback fiber lasers, but the FBG need to be specially fabricated for the lack of photosensitivity of the phosphate glass fiber. Villanueva et al. [9] obtained dualwavelength DFB fiber laser by introducing two local phase shifts generated by piezoelectric transducers in the periodic structure of an erbium-doped FBG. Polarization maintaining FBG or FBG with birefringence is also used to achieve the SLM dual-wavelength fiber lasers utilizing the polarization hole burning (PHB) effect [10–12]. The Fabry–Pe´rot etalon filters with narrow transmission bandwidth or phase-shifted FBG based on the uniform FBG can be used to obtain the SLM dual-wavelength fiber lasers [13–18], but the spacing between the two transmission peak wavelengths is also restricted by the reflection bandwidth of the FBG. Phaseshifted chirped FBG is also used to achieve the tunable SLM dualwavelength fiber lasers [19–21]. Superimposed chirped FBG (SCFBG) or named chirped moire´ FBG (CMFBG), which can also be seen as the phase-shifted chirped FBG [22] has ultra-narrow transmission band, has been used to achieve multi-wavelength SLM lasing in the photosensitive erbium–ytterbium co-doped fiber [23]. Sun et al. [24] presented SLM dual-wavelength fiber ring laser using Sagnac filters and a saturable absorber with complicated configuration and operating principle. Qian et al. [25] demonstrated SLM dual-wavelength fiber ring laser using the Mach–Zehnder comb filter incorporating with the counterpropagation of the two light beams. Chen et al. [26] also proposed SLM dual-wavelength fiber ring laser using the Mach–Zehnder comb filter and a saturable absorber. However, we all know that the Sagnac or Mach–Zehnder comb filter is more sensitive to

S. Feng et al. / Optics & Laser Technology 45 (2013) 32–36

temperature and the environment vibration compared with the FBGs. Sun et al. proposed SLM dual-wavelength fiber ring laser utilizing two FBGs [27]. Pan et al. [28] presented switchable SLM dual-wavelength fiber ring laser by using a passive triple-ring cavity and a hybrid gain medium. But the parameters of the laser need to be properly adjusted to guarantee the operation. Pan et al. also proposed a wavelength-switchable SLM dual-wavelength erbium-doped fiber laser for switchable microwave generation using the same technique which was presented by Qian [29,30]. Dual-wavelength SLM Brillouin fiber laser has also been proposed to achieve photonic generation of microwave millimeter-wave sources [31]. In this paper, we report a simple approach for photonic generation of microwave signal using a dual-wavelength SLM erbium-doped fiber ring laser. For the first time as we know, a CMFBG filter with ultra-narrow transmission band and a chirped FBG are used to select the laser longitudinal mode. The stable SLM operation of the fiber laser is guaranteed by the combination of the CMFBG filter and 3 m unpumped EDF acting as a saturable absorber. Stable dual-wavelength SLM fiber laser operation is obtained with a wavelength spacing of approximately 0.140 nm, which is in good agreement with the detected generated microwave frequency of 17.682 GHz.

2. System configuration and principle Fig. 1 shows the configuration of the proposed dual-wavelength SLM erbium-doped fiber ring laser. The fiber laser consists of a 980/1550 nm wavelength division multiplex (WDM), a section of homemade EDF1 of 4 m with an absorption coefficient 13 dB/m at 1530 nm as the gain medium, a CMFBG filter, a chirped FBG, a segment of unpumped EDF2, an optical circulator (OC), a polarization controller (PC), and a 90:10 fiber coupler which provides 10% of the optical power for the output and 90% for feedback inside the laser cavity. The homemade EDF1 is pumped by a 980 nm laser diode through the 980/1550 nm WDM. The length of the EDF1 is optimized for having the flat amplified spontaneous role="materials-methods"emission spectrum at a saturation level with spectral hole burning (SHB) effect introduced. The CMFBG filter and the chirped FBG serve as a longitudinal mode discriminator to restrict the laser oscillation to a few longitudinal modes. The OC is employed to help the implementation of the ultranarrow bandpass filtering and sustain the unidirectional oscillation. A segment of 3 m unpumped EDF2 is used as a saturable absorber, and acts as a dynamic narrow band filter [29]. The absorption coefficient of the EDF2 at 1530 nm is also 13 dB/m. With the combination of the CMFBG filter and 3 m unpumped EDF acting as a saturable absorber, stable SLM operation of the fiber laser is guaranteed. The fiber laser power is coupled out from the 10% port of the fiber optical coupler. Then, using another 980/1550 WDM

980nm pump

10% output

EDF1

CMFBG unpumped EDF2 OC CFBG PC

OSA

EDFA

PD/ESA

Fig. 1. Schematic of proposed fiber laser.

33

identical coupler, 10% of the output optical power is sent to an optical spectrum analyzer (OSA) (ANDO AQ6317C) with a wavelength resolution of 0.01 nm, and 90% of the fiber laser power is injected to an erbium-doped fiber amplifier (EDFA). A photodetector (PD) (Tektronix CSA803A SD-48 PD Sub-unit, 33 GHz) is employed to receive the amplified optical power. SLM operation of the fiber laser and generated electrical beating microwave signal at the PD are monitored through an electrical signal analyzer (ESA) (Agilent N9010A, 9 kHz–26.5 GHz). Superimposed chirped FBGs (SCFBGs) or named chirped moire´ FBG (CMFBG), which can be seen as the phase-shifted chirped FBG [22] or the distributed Fabry–Pe´rot (DFP) resonator [23] possesses excellent comb-like filtering characteristics including stable wavelength interval and ultra-narrow transmission band. Thus, it can be used to select the longitudinal modes of the laser cavity effectively. The simplest way to fabricate the CMFBG is using the dual-exposure method with a chirped phase mask. The wavelength interval of the adjacent narrow transmission peak can be expressed as Dl E l2/2nD, where l is the central wavelength of the chirped FBG, n is the effective refractive index of the fiber, and D is the displacement of the phase mask between the two exposures. Thus, through changing the displacement of the phase mask D between the two exposures, different wavelength interval of the adjacent transmission peak can be obtained [22]. It is beneficial to easily design the CMFBG with different wavelength interval through tuning the displacement of the phase mask D and choosing the chirped phase mask for the dual-wavelength fiber laser with different lasing wavelength spacing. The CMFBG with a length of about 13 cm is written in a hydrogen-loaded germanium-doped Corning SMF28 fiber with a 13 cm linearly chirped phase mask which has a chirp rate of 0.0485 nm/cm and a period of 1068 nm scanned by 248 nm KrF excimer laser ultraviolet light. Fig. 2(a) and (b) show the transmission spectra of two typical 13 cm long CMFBGs fabricated with the displacement of the phase mask D 8.2 mm and 5.9 mm as the illustrations, corresponding to the wavelength interval 0.1 nm and 0.14 nm, respectively. Here, we just select the CMFBG with the wavelength interval 0.14 nm for the realization of the dual-wavelength SLM fiber laser. The measured transmission spectrum of the CMFBG filter with the wavelength interval 0.14 nm is plotted again with a solid line in Fig. 3. The two adjacent narrow transmission peaks A1 and A2 with wavelength of 1545.879 nm and 1546.020 nm within the chirped FBG stopband are selected to implement the narrow filtering function for the dual-wavelength SLM fiber laser. Theoretically, the real optical power level of the narrow transmittance peak of the CMFBG is invariable and same as the optical power level outside the optical transmission band of the CMFBG (i.e. the power level of the optical source) according to our previous work [22]. The insertion loss of the CMFBG filter is much small, almost negligible. The power difference between the optical power level of the narrow transmittance peak of the CMFBG and the power level outside the optical transmission band of the CMFBG is not induced by the loss, it is because the true optical spectrum of the narrow transmittance peak of the CMFBG cannot be measured limited by the scanning resolution of the OSA (0.01 nm). That is to say, the CMFBG possesses ultranarrow transmission peak band. The exact value bandwidth of the narrow transmittance peak of the CMFBG should be measured by using an OSA with the higher resolution (http://www.apex-t.com/ apex_instruments.htm), but according to our current laboratory conditions, we do not have the similar equipment to measure the exact value bandwidth of the narrow transmittance peak of the CMFBG. We have estimated the bandwidth of the narrow transmittance peak of the CMFBG, and we think that our estimation is valid and reliable. The estimated 3 dB bandwidth of the narrow transmittance peak of the CMFBG is less than 0.3 pm.

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D = 8.2mm

0

0 -5 Power (dBm)

-10 Power (dBm)

D = 5.9mm

-20 -30 -40

-10 -15 -20 -25 -30

-50 1545.0 1545.5 1546.0 1546.5 Wavelength (nm)

-35 1545.0 1545.5 1546.0 1546.5 Wavelength (nm)

Fig. 2. Transmission spectra of two typical CMFBGs with different displacement of the phase mask D.

CFBG CMFBG

5 0 Power (dBm)

-5

A1 A2

-10 -15 -20 -25 -30 -35 1545.0

1545.5 1546.0 1546.5 Wavelength (nm)

Fig. 3. Transmission spectrum of the CMFBG (solid line) and the reflection spectrum of the CFBG (dashed line).

(The estimation could be deduced and testified from the electrical beating signals of the proposed dual-wavelength fiber laser through the self-homodyne method when only the CMFBG and chirped FBG serve as a longitudinal mode discriminator to restrict the laser oscillation to a few longitudinal modes in Section 3. There are only two beating signals which mean that the amount of the laser longitudinal modes in the narrow transmittance peak of the CMFBG filter is about three. The cavity length of the fiber ring laser is approximately 21 m, corresponding to free spectrum range (FSR) of 9.6 MHz. Thus in the fiber laser cavity, the longitudinal mode spacing of the fiber laser is about 9.6 MHz. Then we can estimate the 3 dB bandwidth of the narrow transmittance peak is about 0.16 pm (20 MHz E 9.6 MHz*2, three laser longitudinal modes in the narrow transmittance peak of the CMFBG filter). For the conservative estimate, the estimated 3 dB bandwidth of the narrow transmittance peak is less than 0.3 pm.). The chirped FBG with a length of 4 cm and a reflectivity of 99% is written in the hydrogen-loaded Corning SMF28 fiber with the same chirped phase mask. The reflection central wavelength and 1 dB bandwidth of the chirped FBG are 1545.951 nm and 0.210 nm, respectively, as the dashed line shown in Fig. 3. Although the reflection spectrum of the chirped FBG is not only overlapping with A1 and A2 but also with the first transmission band (the most left-hand side) as shown in Fig. 3, their power differences is huge (at least 10 dB) due to the chirped FBG acting as feedback mirrors introducing different losses (by different reflectivity). Thus, the lasing oscillation corresponding to the first transmission band cannot be established. The working principle of the stable dual-wavelength fiber laser operation is mainly based on the SHB effect and the fine

adjustment of the gain and loss at each wavelength. The SHB effect can be introduced when the signal input into the erbiumdoped fiber amplifier is deeply saturated. Due to the effect, the inhomogeneous broadening plays a dominant role, effectively weakening the mode competition induced by the homogeneous broadening of EDF [32]. Thus, it is possible to achieve the stable multi-wavelength lasing state at room temperature. To obtain the laser emission, it is necessary to balance the cavity losses with the gain of erbium-doped fiber amplifier at certain wavelengths where the oscillation condition should be satisfied. When the condition is met, the lasing oscillation can be formed and exported. The PC is used to tune the birefringence status of the laser cavity to slightly adjust the gain and loss among the peak wavelength of the CMFBG and initiate the multiple lasing operations in the laser cavity. The gain and loss between the two peak wavelengths of the CMFBG could be carefully balanced and then stable dual-wavelength laser oscillation can be achieved.

3. Experimental results and discussion The pumped EDF1 have the flat spectrum at a saturation level with SHB effect introduced under about 200 mW 980 nm pump power. Through carefully tuning the PC to slightly adjust the gain and loss of the laser cavity under a 200 mW 980 nm pump power, we achieved stable dual-wavelength fiber laser operation. Fig. 4(a) shows the measured optical spectrum of the proposed dual-wavelength fiber laser. The dual-wavelength fiber laser is generated at 1545.884 nm and 1546.024 nm, respectively, which is consistent with the wavelength of the two narrow transmission peaks A1 and A2 of the CMFBG. The 3 dB bandwidth of the laser spectrum measured by using an OSA with a wavelength resolution of 0.01 nm is 0.012 nm for both wavelengths. The optical signal to noise ratio (OSNR) of the dual-wavelength fiber laser is 450 dB. To study the stability of the fiber laser, we measured the optical spectrum of the fiber laser with 16 times repeated scans at 2 min intervals in half an hour as shown in Fig. 4(b), which means the dual-wavelength fiber laser operation is stable. The power fluctuation of the fiber laser was less than 1.5 dB. We noted that the output power of the dual-wavelength fiber laser is not high, is around 20 dBm. This may be due to the fact that the saturable absorber EDF2 has a strong absorption effect, and the pumped EDF1 with a length of 4 m does not provide a sufficient gain. The output power could be increased by using an EDF with highly doped erbium. The 90% output power of the dual-wavelength fiber laser is amplified by an EDFA. When the amplified optical power is injected to the PD, a beating microwave signal of 17.682 GHz is detected, as shown in Fig. 5(a). The beating frequency signal can

S. Feng et al. / Optics & Laser Technology 45 (2013) 32–36

35

-10 -20 0.0

Power (dBm)

-30 Power/dBm

-40 -50 -60 -70

00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

-20.0 REF dBm -40.0 -60.0

-80 -90 1545.0

-80.0 1544.90nm

1545.5 1546.0 1546.5 Wavelength (nm)

1545.90nm Wavelength/nm

1546.90nm

Fig. 4. Dual-wavelength operation of the fiber laser.

-20

-20

-40 Power (dBm)

Power (dBm)

-40

-60

-60 -80 -100

-80

-120 -100 0.0

5.0x109

1.0x1010 1.5x1010 RF frequency (Hz)

2.0x1010

1.7681x1010

1.7682x1010

1.7683x1010

1.7684x1010

RF frenquency (Hz)

-60

-60

-70

-70

Power (dBm)

Power (dBm)

Fig. 5. Electrical spectra of the beating signals (a) 20 GHz span with resolution of 1 MHz, and (b) 3 MHz span with resolution of 10 kHz.

-80 -90 -100 0.0

50.0M 100.0M 150.0M 200.0M RF frequency (Hz)

-80 -90 -100 0.0

50.0M 100.0M 150.0M 200.0M RF frequency (Hz)

Fig. 6. Electrical spectra of the beating signals (a) with the unpumped EDF, and (b) without the unpumped EDF.

be expressed as fRF ¼cnDl/l2, where l is the lasing wavelength of the fiber laser. The lasing wavelength spacing of the dualwavelength fiber laser is about 0.140 nm, we can get the theoretical frequency of the generated microwave signal of 17.572 GHz, which is consistent with the detected beating frequency of 17.682 GHz very well in consideration of the resolution of the OSA. The electrical signal to noise ratio of the beating signal is 440 dB, and the 3 dB bandwidth of the generated microwave signal is approximately 25 kHz. The details of the microwave signal with a resolution of 10 kHz is shown in Fig. 5(b). We

monitored the stability of the frequency of the generated microwave signal. The center frequency shift of the beating signal is measured to be less than 50 MHz. The frequency drift is mainly originated from the CMFBG, which is sensitive to the temperature variations and external vibration. If the CMFBG is controlled by a thermo-electric cooler (TEC) and well vibration isolation, the frequency drift would be greatly reduced. Furthermore, the proposed fiber laser is non-polarization-maintaining, thus, the polarization changes of the laser cavity may also have influence on the center frequency shift of the beating signal. This may

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S. Feng et al. / Optics & Laser Technology 45 (2013) 32–36

be improved through using the all polarization-maintaining configuration. The SLM operation of the fiber laser was verified through the self-homodyne method. Fig. 6(a) shows the detected electrical spectrum of the fiber laser in the range of 200 MHz. The cavity length of the dual-wavelength fiber ring laser is approximately 21 m, corresponding to free spectrum range (FSR) of 9.6 MHz. The scanning range is much wider than the FSR. It is clear that the dual-wavelength fiber laser is in SLM operation. When we remove the EDF2, the dual-wavelength fiber laser is no longer stably operating at SLM. Fig. 6(b) shows the electrical spectrum of single-wavelength laser without the EDF2 but with the CMFBG and chirped FBG retained; there are only two laser longitudinal modes beating signals, which mean that the CMFBG filter can be used to select the laser longitudinal modes effectively. Thus, with the combination of the CMFBG filter and 3 m unpumped EDF acting as a saturable absorber, stable SLM operation of the fiber laser is guaranteed. Note that, it is possible to slightly change the wavelength spacing of dual-wavelength SLM fiber laser by applying strain [20] or temperature control to the CMFBG filter for the future improvement. Therefore, it has a potential to be a tunable fiber laser source for photonic generation of the tunable microwave signal.

4. Conclusion In conclusion, we proposed a simple solution to photonic generation of microwave signal using a dual-wavelength SLM EDF ring laser. A CMFBG filter with ultra-narrow transmission band and a chirped FBG are used to select the laser longitudinal mode. The stable SLM operation of the fiber laser is guaranteed by the combination of the CMFBG filter and 3 m unpumped EDF acting as a saturable absorber. Stable dual-wavelength SLM fiber laser operation is obtained with a wavelength spacing of approximately 0.140 nm. By beating the dual-wavelength fiber laser at a PD, photonic generation of microwave signal at 17.682 GHz is successfully obtained.

Acknowledgments The authors would like to thank the Optical Fiber Group of Institute of Lightwave Technology for supplying the EDF with high quality. This work is jointly supported by the Major State Basic Research Development Program of China (No. 2010CB328206), the Key Program of the National Natural Science Foundation of China (No. 60837002), the National Natural Science Foundation of China (No. 61107094), the Fundamental Research Funds for the Central Universities (Beijing Jiaotong University, No. 2011JBM001) and Research Foundation for Talented Scholars (Beijing Jiaotong University, No. 2010RC027). References [1] Yao JP. Microwave photonics. Journal of Lightwave Technology 2009;27: 314–35. [2] Chen X, Yao J, Deng Z. Ultranarrow dual-transmission-band fiber Bragg grating filter and its application in a dual-wavelength single-longitudinalmode fiber ring laser. Optics Letters 2005;30:2068–70. [3] Dai Y, Chen X, Sun J, Yao Y, Xie S. Dual-wavelength DFB fiber laser based on a chirped structure and the equivalent phase shift method. IEEE Photonics Technology Letters 2006;18:1964–7. [4] Yao Y, Chen X, Dai Y, Xie S. Dual-wavelength erbium-doped fiber laser with a simple linear cavity and its application in microwave generation. IEEE Photonics Technology Letters 2006;18:187–9.

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