Single Longitudinal Mode Brillouin Fiber Laser With ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 2, JANUARY 15, 2014

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Single Longitudinal Mode Brillouin Fiber Laser With Cascaded Ring Fabry–Pérot Resonator Yi Liu, Jin-Long Yu, Wen-Rui Wang, Hong-Gang Pan, and En-Ze Yang Abstract— A novel single longitudinal mode (SLM) Brillouin fiber laser (BFL) with cascaded ring (CR) Fabry–Pérot resonator is proposed and demonstrated. By optimizing the CR length of the single mode fiber cavity at 100 m (or 50 m) and 10 m, stable SLM operation is obtained. Therefore, the threshold power is lower and linewidth is narrower than that of previously reported BFL with 10 m (or 20 m) single cavity. Additionally, there is no additional attenuation except intrinsic loss of optical devices. The measured linewidth with 60 dB (or 45 dB) improved value of side mode suppression ratio is 0.41 kHz (or 3.23 kHz), which is three (or two) orders of magnitude than that of the pump. Using a stabilizing feedback loop based on autotracking technique with polarization maintaining fiber-based optical delay line, the SLM BFL exhibits good performance with 6% power fluctuation in 1 h. Index Terms— Cascaded ring Fabry–Pérot resonator, Brillouin fiber laser (BFL), single longitudinal mode (SLM), narrow linewidth.

I. I NTRODUCTION

S

TIMULATED Brillouin scattering (SBS) with narrow band and low noise has broad applications in frequency shifter [1], microwave frequency generation [2], [3], narrow bandwidth low-noise amplification [4], [5] and fiber laser [6]–[12]. Meanwhile, extra attention has been attracted to single longitudinal mode Brillouin fiber laser (BFL) with high quality, low loss and cost. The investigation is mainly concentrated on the following aspects. On the one hand, in order to separate the pump and Brillouin laser and reduce the threshold power, single ring cavity with one 2 × 2 coupler [7]–[9], an circulator [2], [11], [12] or an unbalanced MachZehnder interferometer (UMZI) [9] is configured. Circulator based single ring cavity has the advantage that the Brillouin pump frequency does not need to match the cavity mode. On the other hand, when the cavity mode spacing of about 10 m or 20 m cavity length [7]–[13] is comparable to the Brillouin gain, the BFL easily operates in SLM (single longitudinal mode) status. However, because of length limitation of SLM operation, it is difficult to improve the threshold power and linewidth. Then, BFL incorporating long fiber length (500 m)

Manuscript received October 24, 2013; revised November 5, 2013; accepted November 6, 2013. Date of publication November 13, 2013; date of current version December 31, 2013. This work was supported in part by the framework of National Basic Research Program of China under Grant 2012CB315704 and in part by the Natural Science Foundation for Young Scientists of China under Grant 61205061. The authors are with the Laboratory of Fiber-Optic Communication, School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China. They are also with the Key Laboratory of Optoelectronics Information Technical and Science, Tianjin University, Ministry of Education, Tianjin 300072, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2290191

Fig. 1.

Experiment setup of SLM BFL with CR-FP.

high nonlinear fiber (HNLF) and an unpumped Erbium-doped fiber (EDF) loop (10 m) [14] which is regarded as a selfinduced fiber Bragg grating (FBG) filter with an ultra-narrow bandwidth is proved. However, accurate power control is necessary to make the FBG reach proper pass-band bandwidth and 500 m HNLF is easy to generate high-order Stokes lines [15]. Lastly, besides of the temperature control, a servo loop is realized by autotracking the pump light frequency or adjusting the cavity length with piezoelectric transducer (PZT) [10] or Pound–Drever–Hall (PDH) frequency-locking scheme [11] resulting from environmental perturbations. However, the polarization of resonance cavity with PZT which is unstable will cause Brillouin gain shift. And not only is configuration of PDH complicated, but also optical reference resonator in that need to been taken simultaneity stabilization scheme. In this letter, SLM BFL with cascaded ring Fabry–Pérot (CR-FP) resonator is firstly proposed. Contrasting with selfinduced FBG, CR-FP hasn’t additional cavity attenuation except devices’ intrinsic loss and is hardly to generate highorder Stokes for low nonlinear coefficient in single mode fiber (SMF). CR-FP (shown in Fig. 1) is constructed by a longer length fiber (100 m or 50 m) and a shorter length fiber (10 m), Meanwhile, an autotracking system with high precision polarization maintaining fiber-based optical delay line (PM-ODL) is configured, so that the unnecessary polarization factor is reduced [16]. II. FABRICATION AND P RINCIPLE The experiment setup of the SLM BFL with CR-FP is shown in Fig. 1. The pump light from a tunable laser source (TLS) with 100 kHz linewidth is amplified by the high power erbium-doped fiber amplifier (EDFA). In order to achieve maximum gain in BFL, the polarization of the pump is kept parallel to that of the Stokes wave by a polarization controller

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Fig. 2.


Principle of SLM BFL with CR-FP operation.

(PC1). The amplifier spontaneous emission (ASE) noise from EDFA is filtered by an optical bandpass filter (OBF) with 1nm bandwidth. Then the pump is injected to the single-pass cavity clockwise by optical circulator (Cir) and passes through the cavity with only one roundtrip. PC2 is used to align the state of polarization of the FP Ring-1 (FP1) and FP Ring-2 (FP2) to guarantee a SLM operation. FP2 is constructed with a 50/50 polarization-insensitive optical coupler (OC2) and a 10 m SMF length of previously reported SLM BFL [7–9]. When power of pump is higher than the threshold of SBS, laser is output from a 1/99 OC1 and the rest circulate in the cavity with multiple roundtrips anticlockwise. For the temperature stability of the BFL, it is enclosed in a temperature control system with better than 0.4° resolution and 3°C∼80°C range. Besides, In order to prevent the frequency drift of the laser modes that results in power fluctuation and multimode operation, an auto tracking feedback loop which consist of a high-bandwidth (150MHz) InGaAs amplified detector with 700∼1800 nm wavelength and an PM-ODL with 0.0017 ps resolution is implemented. Fig. 2 shows the principle of SLM BFL with CR-FP. The frequency shift νB with respect to the pump is given νB = (2νA /c)νP [7], [9], where νA is the acoustic velocity in the medium, c is the vacuum-light velocity, and νP is the optical frequency of the pump beam. νB is about 10GHz in 1550nm wavelength region. According to the vernier effect [17], [18], the effective free spectral range (FSR) of with CR-FP is FSR = n 1 FSR1 = n 2 FSR2

(1)

where FSRm = c/n L m (m = 1, 2) is FSR of FP1 and FP2, L m (m = 1, 2) is the ring length of FP1 and FP2 and n m (m = 1, 2) is the integer. Thus the FSR of FP1 and FP2 are 2 MHz and 20 MHz respectively. According to (1), effective FSR is 20 MHz. When effective FSR exceeds the Brillouin gain bandwidth (20 MHz) and gain is great than loss, the laser mode v L oscillates only at a frequency that satisfies the resonant conditions of FP1 and FP2 simultaneously. III. E XPERIMENT AND D ISCUSSION Fig. 3 shows Brillouin laser output total power as a function of pump power for 100 m (blue line) and 50 m (red line) length of FP1. Pump power was measured at the EDFA fiber pigtail and Brillouin laser output firstly was measured at the 1% port of the OC1. In the BFL, suppose that pump and SBS are polarized along the same direction,

Fig. 3. BFL output power as a function of pump power with 100 m (blue line) and 50 m (red line) length of FP1.

the threshold input power Pth is inversely proportional to the effective cavity length L eff = [1 − exp(−2αL t )]/2α . Pth is given by [7], [13] R exp(g Pt h L e f f /A − αL t ) = 1

(2)

where α = 0.2 dB/km is the attenuation coefficient of SMF, L t = L 1 + L 2 = 110 m (or 60 m) is the total cavity length, g = 5 × 10−11 m/W is the Brillouin gain coefficient, A = 52.18 × 10−12 m 2 is the effective core area and R ≈ 0.99 is the ratio of laser power fed back after each roundtrip. It should be noted that total loss of CR-FP is 3dB. So R = 0.5 × R = 0.495 is caused by total cavity loss. Then Pt h of 100 m and 50 m is equal to 11 mW and 22 mW respectively. Considering the loss of PC1, OBF and Cir (from port 1 to port 2), extra power is about 44 mW and 88 mW. Because polarization effect and peak match between Brillouin gain and resonance peak of cascaded ring are not considered, real threshold input power is 56 mW and 99 mW. The inserted chart in Fig. 3 shows 0.08 nm wavelength interval between the Brillouin pump and BFL and 10dB noise suppression measured by an optical spectrum analyzer (OSA) with a 0.05 nm resolution. To illustrate the SLM operation of BFL with CR-FP in detail, three groups (group A with FP1-230 m, FP2-50 m. group B with FP1-50 m, FP 2-10 m. group C with FP 1-100 m, FP 2-10 m) with different fiber lengths are contrastly analyzed. The lengths of 230 m, 100 m, 50 m and 10 m are corresponding to the FSRs of nearly 870 kHz, 2 MHz, 4 MHz and 20 MHz. Detected homodyne frequency spectrum is shown in Fig. 4(a) and (b) for group A, (c) and (d) for group B and (e) and (f) for group C. Fig. 4 (a), (c), and (e) and (b), (d), and (f) are single-ring cavity and CR-FP respectively. The sidemode under Brillouin gain with group A is not suppressed under Brillouin gain (0∼10 MHz), whereas that with group B and C is suppressed greatly. The improved value of sidemode suppression ratio is 45 dB and 60 dB for group B and C respectively. And Signal to noise ratio of group C is better than group B. The stability of the BFL is now discussed. The stability of the Brillouin fiber laser focuses on three main effects: the temperature effect, the nonlinear Kerr effect and frequency

LIU et al.: SLM BFL WITH CASCADED RING FP RESONATOR

870kHz

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(b)

(a) 5MHz

(d)

(c) 4MHz 45dB

(f)

(e) 2MHz

Fig. 5.

Power fluctuation in 1 h.

60dB

Fig. 4. Experimental results. (a) and (b) Detected homodyne frequency spectrum of the proposed laser with group A. (c) and (d) Detected homodyne frequency spectrum of the proposed laser with group B. (e) and (f) Detected homodyne frequency spectrum of the proposed laser with group C. (a), (c), and (e) with single-ring cavity and (b), (d), and (f) with CR-FP.

pulling effect. Because the FSR and the Brillouin gain mainly depend on the temperature, the laser mode is temperature dependent. According to the reference [19], [20], the temperature range Tmode-hopping with two consecutive mode hops for this BFL is expressed by 1 ∂ν B 1 ∂n 1 ∂ L t + + Tmode−hopping ≈F S Rmin/ ν B νB ∂ T n ∂ T Lt ∂ T (3) where F S Rmin = 2MHz (4MHZ) of the 100m (50m) cavity length is the minimum FSR of the CR-FP. For the SMF, 1 ∂ Lt −6 L t ∂T = 10 /°C is length fluctuation coefficient with temB perature and ∂ν ∂T = 1.04 MHz/°C is the Brillouin frequency shift with temperature at 1550 nm [19], [20]. On the one hand, Tmode−hopping ≈ 1.89°C (3.78°C) is obtained and 0.4°C resolution of temperature controller can’t lead to mode hopping. On the other hand, 0.4°C resolution of temperature control causes 0.416 MHz Brillouin shift and 0.8 Hz (or 1.6 Hz) FSR variation for 100 m (or 50 m) of CR-FP. Furthermore, an autotracking loop with high precise PM-ODL of 0.496 μm resolution (corresponding to 0.01 Hz and 0.02 Hz FSR shift for 100m and 50m cavity respectively) is used to implement variation and stabilize ring length for only one longitudinal mode matching the center of the Brillouin gain spectrum. The main principle of the autotracking is classical dithering method. Fig. 5 shows the 6% power fluctuation in 1 h. An analytical relation connecting Brillouin laser linewidth [21] vBFL and pump laser linewidth vP is given by vBFL = ν P /(1 + γ A / C )2 , where γ A = πvB (Brillouin gain bandwidth vB = 20MHz) and C = −c ln R/n L t are the damping rate of the acoustic wave and the cavity loss rate. Our Brillouin fiber laser has a coupling ratio R = 0.495, the

Fig. 6. Linewidth measurement of BFL with 50 m of FP1 using selfheterodyne method at Center 50 MHz, Span 50 kHz.

Fig. 7.

Heterodyne beat technique using two independent pump lasers.

linewidth is about three (or two) orders of magnitude narrower than that of the pump for 100 m and 50 m cavity length of FP1 respectively. Linewidth measurement for 50 m FP1 and 10 m FP2 is realized by self-heterodyne method with a 25 km optical fiber delay line. 3.23 kHz of linewidth (shown in Fig. 6) is in good agreement with our analysis. However, because of the 4 kHz measurement accuracy of 25 km fiber, the linewidth should be below 4 kHz. In order to measure more accurately the linewidth, heterodyne beat technique [11] using two independent pump lasers is established (shown in Fig. 7). In contrast to the Fig. 1, TLS1 whose performance is the same

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Fig. 8. Linewidth measurement of BFL with 100 m of FP1 using heterodyne beat technique with two independent pump lasers at Center 50 MHz, Span 5 kHz.

as TLS is used as another pump laser. The linewidth result of 50 m FP1 and 10 m FP2 is same as that of the self-heterodyne method. 0.41 kHz linewidth of 100 m FP1 and 10 m FP2 is obtained which is in good agreement with theoretical analysis (shown in Fig. 8). IV. C ONCLUSION In conclusion, A SLM BFL with CR-FP is presented. The BFL is structured by using CR-FP with a longer length fiber (100 m or 50 m) and a shorter length fiber (10 m or 20 m). The autotracking loop constructed by PM-ODL and temperature control is used to stabilize Brillouin frequency shift and FSR shift resulting from environment. The sidemode suppression ratio is improved by 45 dB and 60 dB. And 6% power fluctuation is observed in 1h. 0.41 kHz and 3.23 kHz linewidth of BFL which is three and two order of magnitude than that of the pump is measured by selfheterodyne method with 25 km optical fiber and heterodyne beat technique with two independent pump lasers. In addition, the laser is very simple, practical and cost effective. R EFERENCES [1] K. Kalli, D. Culverhouse, and D. Jackson, “Fiber frequency shifter based on generation of stimulated Brillouin scattering in high-finesse ring resonators,” Opt. Lett., vol. 16, no. 19, pp. 1538–1540, 1991. [2] J. Liu, et al., “Optical generation of tunable microwave signal using cascaded Brillouin fiber lasers,” IEEE Photon. Technol. Lett., vol. 24, no. 1, pp. 22–24, Jan. 1, 2012.

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