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A tunable and single longitudinal mode Er-doped fiber ring laser (SLM-EDFRL) is proposed and demonstrated based on Rayleigh backscattering (RBS) in single mode fiber-28e (SMF-28e). Theory and experimental study on formation of SLM from normal multi-mode ring laser is demonstrated. The RBS feedback in 660 m SMF-28e is the key to ensure SLM laser oscillation. This tunable SLM laser can be tuned over 1549.7-1550.18 nm with a linewidth of 2.5-3.0 kHz and a side mode suppression ratio (SMSR) of ~72 dB for electrical signal power. The tuning range is determined by the bandpass filter and gain medium used in the experiment. The laser is able to operate at S+C+L band.
© 2011 Optical Society of America
1. Introduction
Single longitudinal mode (SLM) operation fiber lasers are of considerable interest for many applications, such as optical precision metrology, high-resolution spectroscopy, sensors and communication systems. A SLM laser can be achieved with a ring cavity using a pumped erbium-doped fiber (EDF) as the gain medium. However, an Er-doped fiber ring laser (EDFRL) usually has a long cavity, which would lead to the generation of a large number of densely spaced longitudinal modes. To achieve SLM operation, several mode suppression techniques have been proposed, such as using two cascaded fiber Fabry-Perot (FP) filters [1], phase-shifted fiber Bragg grating (FBG) with ultra-narrow bandwidth [2], a high finesse FBG based FP etalon [3], a compound ring [4], passive multiple ring cavities [5], a storable absorber inducing spatial-hole burning [6–9] and a FBG as self-injection feedback [10, 11]. Combining these controlling technologies with FBG, intracavity FP filter or FBG based FP etalon, both wavelength tunable and SLM operation have been realized, simultaneously [12–20]. However, they are complicated and costly.
In this paper, we demonstrate a tunable SLM-EDFRL with random distributed feedback based on Rayleigh backscattering (RBS) in the single mode fiber-28e (SMF-28e). In our precious work, SLM oscillation was achieved based on stimulated Rayleigh scattering (STRS) in 5.7 km non-uniform fiber [21]. Because of a long fiber length, STRS can be obtained directly with high pump power, which means only stimulated Brillouin scattering (SBS) suppression fiber (i.e. non-uniform fiber) can be used to achieve SLM operation instead of conventional single mode fiber (SMF). This is because the SBS threshold value of non-uniform fiber is ~7 dB higher than that of SMF [22]. In this work, we directly prove using double-loop laser configuration for the pump and RBS in which RBS acts as mode suppression element and seeded signal to form SLM-EDFRL. By using a much shorter fiber length (660 m) with less than 0.2 dB loss, we demonstrate the SLM creation with lower pump power instead of long length non-uniform fiber (5.7 km) with total loss of ~3 dB (0.45 dB/km). This has reduced the cavity loss significantly. The theoretical model of the SLM operation is given, followed by a series of experiments that proved SLM laser formation from standard multi-mode ring laser. Our experimental results show that the 3 dB linewidth of 2.5-3.0 kHz is achieved with side mode suppression ratio (SMSR) of ~72 dB for the electrical signal power. The tuning range is 1549.7-1550.18 nm, which is dependent on the wavelength range of the tuning filter within gain spectrum.
2. Experimental setup and theory
The schematic of the tunable SLM-EDFRL is shown in Fig. 1 . The laser is constructed by means of feeding back the RBS light to a main ring cavity (MRC). In MRC, a 12 m EDF pumped by 980 nm laser provides a broadband light source. A tunable optical bandpass filter (TOBPF) with bandwidth of ~3GHz is used as a wavelength selector. A 660 m SMF-28e is used to generate the RBS seed light. The stimulated Brillouin scattering at ~10 GHz shift of the pump wavelength is suppressed by the narrow bandwidth TOBPF. The two polarization controllers (PCs) are used to ensure that the state of the polarization at input and output of EDF are orthogonal to reduce the background noise. In the part of RBS feedback injection, the RBS is measured at point B using an optical circulator (OC), and then is fed back to MRC using a coupler. The laser output is monitored by an optical spectrum analyzer (OSA) with a resolution of 0.06 nm and an electrical spectrum analyzer (ESA) as shown in Fig. 2 . The laser output at Point A in Fig. 1 is split into two parts by a 50:50 coupler. One part is monitored by the OSA to give the wavelength reading; the other part is used to measure the linewidth using the delayed self-heterodyne method. In the Mach-Zehnder interferometer (MZI), an acoustic optic modulator (AOM) with a frequency shift of ~200 MHz is added into upper arm of MZI. A normal single mode fiber of 100 Km is used to generate ~500 μs delay in the under arm, which corresponds to a frequency resolution of ~1 kHz [23]. The beating signal of the two arms is detected by a photo-detector (PD) with frequency response range of 100 Hz ~350 MHz. Finally, the linewidth of the SLM-EDFRL can be measured by ESA.
Fig. 1 The configuration of the experimental setup. WDM: wavelength division multiplexing device; EDF: Erbium-doped fiber; TOBPF: tunable optical bandpass filter; OC: optical circulator; PC1, PC2: polarization controller; ISO: isolator; TOBPF: tunable optical bandpass filter; C1: 95:5 coupler or 50:50 coupler; C2: 90:10 coupler; VOA: variable optical attenuator; pump light direction (black arrows); RBS light direction (blue arrows).
Fig. 2 The schematic of the linewidth measurement setup. EDFA: Erbium-doped fiber amplifier; AOM: acoustic-optic modulator; PD: photo-detector; OSA: optical spectrum analyzer; ESA: electrical spectrum analyzer.
The RBS occurs at multiple scattering centers along a fiber. The multiple random reflections play the role of a distributed mirror. Due to the distributed character of such reflections in the ring cavity, they diffuse the effective cavity length parameter forming effective ring cavities of variable lengths, leading to variable mode separation with the strongest one in the front end of the fiber position where the RBS starts to build up with coherent Rayleigh scattering of neighboring scattering centers. The principle of the SLM-EDFRL can be explained as follows. The MRC forms a master laser, which works as a pump for the RBS. The RBS feedback injection works as a slave laser, which modifies the master laser’s performance via side mode suppression due to the distributed mirror. The SLM laser works when both master and slave lasers are synchronized.
The pump light and the RBS light are combined together by using C1. Hence, the electric field can be expressed as
where L is the length of the SMF-28e, l is the length of the other optical elements of the MRC; z is the distributed varying length. The factor Ffilter represents the function of the tunable filter with centre frequency f0, and bandwidth Δf (3GHz in our experiment).The first term corresponds to the standard cavity radiation of the MRC; the second term represents the accumulated effect of the RBS radiation. Both of the two terms should be in phase to get constructive interference as following:where β is the propagation constant with the wavelength centered at the peak of the optical filter, l1 is the length of the feedback path, m and n are positive integers. When the phase conditions are satisfied, the effective intensity is given byThis equation can be explained in Fig. 3 . The first term gives rise to multiple longitudinal modes. The mode separation is given by δƒ = c/[n(L+l)]. The second and third terms contribute to the narrow SLM operation. Due to the distributed reflections of RBS in the ring cavity, they lead to variable mode separation, and the overlapping of modes creates modeless radiation [24]. RBS acts as mode suppression element to modulate the multiple longitudinal modes of the MRC. The variable optical attenuator (VOA) is used to adjust the cavity loss and suppress the side modes. It is critical in seeding RBS source to form SLM operation. Due to the high loss at C1 and VOA for the pump signal, after several round trips, RBS gets more gain from the pumped EDF than the master ring modes and becomes the lasing mode which gets synchronized with the highest gain mode from the master laser ring. The EDF exhibits a fluorescence relaxation time in the millisecond-scale. Since the RBS light travels within nanoseconds for a single round trip of the ring laser, the EDF will allow the RBS light to travel several million times to create high gain and long coherence length, hence narrow linewidth operation.
Fig. 3 The schematic of the theoretical model, the spectrum of bandpass filter (black curve), the multi-modes induced by MRC (blue curve), the spectrum of RBS with single mode (red curve), the cavity loss level (dashed line).
3. Experimental result
In order to minimize the loss of the RBS feedback light, the RBS feedback light is coupled back into the MRC by the 95% port of a 95:5 coupler at C1. Meanwhile, the 5% port of the coupler is inserted into the MRC. Besides the ~10 dB attenuation due to the 5% port, VOA gives attenuation ≥ 3 dB. When pump current is 200 mA, without 660 m SMF-28e, the laser output is multi-modes operation as shown in Fig. 4(a) . Note, the VOA has been removed in Fig. 4(a); otherwise there is no laser output due to large cavity loss induced by VOA comparing with the low gain at 200 mA. When 660 m SMF-28e is inserted to the cavity and the attenuation at VOA is 6 dB, Fig. 4(b) shows the laser output with single mode operation, the role of VOA (6dB) is to suppress the transmission mode contribution to the EDF gain, so that Rayleigh backscattering signal is larger than the transmission modes in the cavity when they reach EDF. Without 660 m SMF-28e, the RBS feedback injection is not established actually, and only the typical MRC works, which gives multi-modes operation. The mode separation is ~6.5 MHz, which matches with the cavity length l = 31 m. With 660 m SMF-28e, besides the master laser, the slave laser is established though the RBS injection. As mentioned in the theory, the RBS light works as mode suppression element to modify the multi-modes performance, and the VOA with proper attenuation is used to suppress the side modes efficiently. The detailed SLM mechanism will be discussed in the following analysis. The electrical power contrast and linewidth vs. the pump current are shown in Fig. 4(c). The electrical power contrast is ~68 dB, the electrical power linewidth is ~2.7 kHz and for the optical linewidth of Lorentzian shape, a factor of 1.58 should be considered, which gives 4.3 kHz. The threshold is 140 mA, which corresponds to 36 mW. When the pump current is fixed at 500 mA, we increased the attenuation at VOA from 0 dB (i.e. not using VOA) to 20 dB, the laser threshold current increased from 130 mA to 240 mA, and the electrical power contrast and linewidth vs. the attenuation are shown in Fig. 4(d). There is an optimized attenuation for the highest contrast, however, the linewidth changes slightly over the range.
Fig. 4 The laser spectrum (a) without 660 m SMF-28e, (b) with 660 m SMF-28e, the evolution of the electrical power contrast and the linewidth (b) vs. the pump current when attenuation is 6 dB, (d) vs. the attenuation when pump current is 500 mA.
In order to further investigate SLM mechanism via master and slave laser, we applied a 50:50 coupler to replace the 95:5 coupler at C1. In the MRC, The attenuation due to the coupler decreased from ~10 dB to ~3dB. The RBS light is detected at point D by connecting a 90:10 coupler along the feedback path. Firstly, we disconnected points B and C, so that only the MRC is formed to achieve a standard ring laser with cavity length of ~690 m (L+l). Whatever attenuation is applied, the laser output and RBS light measured at point B are multi-mode with mode separation of ~300 kHz as shown in Fig. 5(a) . After connecting points B and C, the RBS feedback injection is established and the RBS light is coupled back to the MRC. Without VOA, the laser output is also multi-mode, while the RBS has only one peak instead of multi-modes as shown in Fig. 5(b). When the VOA is inserted with attenuation of 3 dB, the side modes are suppressed dramatically, and the RBS light with only one peak is increased as shown in Fig. 5(c). When using 95:5 coupler at C1 with attenuation of 6 dB at VOA, the laser spectrum and the RBS spectrum are shown in Fig. 5(d). Due to the small loss of the 90:10 coupler, the laser output is comparable with the result without 90:10 coupler as shown in Fig. 4(b). In this case, because the loss of the RBS light is minimized and the cavity loss is optimized, the side modes are suppressed efficiently. All above results prove that the RBS light work as side modes suppression element and the attenuation of the VOA is critical in suppressing the side modes and seeding the RBS light to form SLM operation.
Fig. 5 The laser spectrum at A and RBS spectrum at D. (a) disconnecting points B and C with 50:50 coupler at C1, (b) connecting points B and C with 50:50 coupler and attenuation of 0 dB, (c) connecting points B with C with 50:50 coupler and attenuation of 3 dB, (d) connecting point B with point C with 95:5 coupler and attenuation of 6 dB
4. Laser cavity optimization
In the former experiment, the pump light and the RBS light propagate in the different optical path. Hence, the phase condition is most likely to change due to the variations in the environment (i.e. temperature or vibration). In addition, C1 leads to loss for the pump light and RBS light. In order to reduce overall loss (pump and RBS), an improved setup is proposed as shown as Fig. 6 .
In the improved schematic, pump light and RBS light share the same optical path. The pump light from port 2 of the circulator is lunched into 660 m SMF-28e, and then is reflected by the Faraday rotating mirror (FRM) to form a standard ring laser. This laser worked as the master laser to provide the pump for the RBS. The distributed RBS light in SMF-28e is coupled back to the cavity through port 3 of the circulator. The pump light and the RBS light is combined at port 3. The SLM operation is similar as described in the former setup except that the total cavity length is 2L+l instead of L+l.
Here we need to find an appropriate VOA value for SLM in this shared ring configuration. It is worth pointing out that the cavity loss induced by VOA is twice the attenuation at VOA due to the double passing. All below mentioned attenuation is the cavity loss induced by VOA instead of the attenuation at VOA. Firstly, without VOA, Fig. 7(a) gives multi-modes operation with the mode separation of ~148 kHz, which matches the cavity length of 1350 m (2L+l). When cavity loss is increased to 6 dB, the measured and Lorentz fitting results are shown in Fig. 7(b). We increased the pump current from 130 mA - 500 mA, which correspond to 30 mW – 240 mW. The electrical power contrast and linewidth are shown in Fig. 7(c). Comparing with the previous result, the pump threshold power is decreased by 6 mW, the electrical power linewidth is decreased from ~2.7 kHz to 1.6-1.9 kHz, which is equivalent to optical linewidth of 4.3 kHz and 2.5-3.0 kHz. The electrical power contrast is increased to ~72 dB. Figure 7(d) shows the output power. When we continued to increase the cavity loss, the electrical power contrast decreased and the electrical power linewidth increased due to the high cavity loss as shown in Fig. 7(e).
Fig. 7 (a) the laser spectrum without VOA. (b) the laser spectrum with cavity loss of 6 dB. (c) the evolution of the electrical power contrast and linewidth vs. the pump current. (d) the evolution of the laser power. (e) the evolution of the electrical power contrast and linewidth vs. the attenuation. (f) the tunable characteristic.
The tunable characteristic is investigated when the pump current is fixed at 500 mA and the attenuation is 6 dB. The output spectrum of the tunable laser is shown in Fig. 7(f). There are 7 wavelengths in the range of 1549.70-1550.18 nm. In the whole tuning proceeding, the optical linewidth is 2.5-3.0 kHz with a 72 dB extinction ratio measured with electrical signal power. Because the SLM operation is determined by the RBS feedback and the attenuation of VOA, the laser with wavelength range of S+C+L band can be realized by using proper filter and gain medium. Also, the linewidth depends on the bandwidth of the filter and the relaxation time of the gain medium.
4. Conclusion
In conclusion, we realized a novel tunable SLM-EDFRL based on RBS feedback in SMF-28e. The RBS feedback acts as a side modes suppression element and the attenuation of the cavity is optimized to ensure SLM operation. Our experimental results show the tuning range is 1549.70-1550.18 nm, the optical linewidth is 2.5-3.0 kHz and the electrical power contrast is ~72 dB, respectively. In this experiment, the tuning range is determined by the wavelength range of the filter within the gain medium. Hence, there is potential to expand this configuration to S+C+L band by applying a proper filter and gain medium.
Acknowledgments
We would like to acknowledge the financial support of Canadian Institute of Photonics Innovation (CIPI) and Natural Science and Engineering Research Council of Canada (NSERC) through Discovery Grant. Guolu Yin is supported by ‘the Fundamental Research Funds for the Central Universities’ (Nos. 2011YJS002 and 2011YJS288).
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