A novel tunable fiber Fabry-Perot (FP) filter is proposed and demonstrated by using a hollow-core photonic bandgap fiber (HC-PBF) and a micro-fiber. The interference cavity is a hollow core of HC-PBF. One of the reflection mirrors is the splicing point between a section of HC-PBF and a single mode fiber. The other reflection mirror is a gold-coated end of micro-fiber that uses chemical etching process to obtain the similar diameter as the core of HC-PBF. Hence the movable mirror can be adjusted with long distance inside the hollow core of HC-PBF. Tunable FP filter is used as a mode selecting component in the reflection mode to implement stable single longitudinal mode (SLM) operation in a ring laser. With FP cavity length of 0.25 ± 0.14 mm, the wavelength of SLM laser can be tuned over 1554-1562 nm with a tuning step of 0.2-0.3 nm, a side-mode suppression ratio (SMSR) of 32-36 dB and a linewidth of 3.0-5.1 kHz. With FP cavity length of 2.37 ± 0.37 mm, the SLM laser can be tuned over 1557.3-1560.2 nm with a tuning step of 0.06-0.1 nm, a SMSR of 44-51 dB and a linewidth of 1.8-3.0 kHz.
©2011 Optical Society of America
Cost-effective tunable Fabry-Perot interferometers (FPI) have been successfully commercialized and widely used for measurement of various parameters, such as strain, temperature, pressure, vibration, etc., in many fields [1–4]. In the past decade, most of the FPIs were based on the splicing of different kinds of fibers, such as single mode fiber (SMF) and multimode fiber (MMF) , SMF and photonic crystal fiber (PCF) , SMF and hollow-core photonic crystal fiber (HC-PCF) , SMF and hollow-core photonic bandgap fiber (HC-PBF) , SMF and a tapered SMF tip . However, these FPI sensor configurations have a limited tuning range on the order of micrometers.
Tunable lasers are of considerable interest for many applications, such as wavelength division multiplexed (WDM) communications, fiber sensors, high-resolution spectroscopy and optical coherent tomography. Linewidth is an important performance parameter. Standard distributed feedback (DFB) lasers and external cavity lasers (ECL) have typical linewidths of 5 MHz and 100 kHz, respectively. Tunable single-longitudinal-mode (SLM) lasers have more advantage of narrow spectral linewidth. Complex mode selection mechanisms have been used to achieve the SLM operation in erbium doped fiber ring lasers (EDFRL), such as multiple subring-cavities combined with either a fiber Bragg grating (FBG)  or with an intracavity FP filter  or with a high finesse ring filter , a saturable absorber combined with either a widely tunable FBG [13,14] or with a FP filter [15,16], and a tunable FBG FP etalon [17,18]. With a sub-cavity which is complicated and difficult to control, the side-mode suppression ratio (SMSR) of a conventional SLM-EDFRL can reach 50 dB  or higher . With a saturable absorber, the linewidth is reduced to ~2 kHz  or less [13, 15]. Due to low finesse associated with low reflectivity in the transmission mode, the filter usage as a mode selecting element in ring laser has limited the linewidth (>5KHz) and tuning range (hundreds of GHz) of the fiber laser.
In this paper, a novel tunable fiber FP filter is proposed and demonstrated experimentally. The fiber FP filter is fabricated by using a HC-PBF and a micro-fiber. The splicing point between HC-PBF and SMF forms a reflection mirror. The other reflection mirror is formed by a gold-coated micro-fiber. The micro-fiber has a similar diameter as the core of HC-PBF by using chemical etching process. Hence the movable mirror can be tuned inside the hollow core of HC-PBF with a range of millimeter. The fabrication of FP filter is described in section 2. The tunable cavity length of the compact FP filter structure can be enlarged to several millimeters, or even centimeters. The reflection characteristics of the FP filter can be used as a narrowband notch filter. The FP filter is incorporated in an EDFRL to build a simple and efficient mode selection mechanism. The EDFRL setup and its characteristics are shown in section 3. Both the theoretical discussion and experimental results confirm the SLM operation. With a FP cavity length of 0.25 mm and a tuning range of ±0.14 mm, the SLM-EDFRL can be tuned over 1557.3-1560.2 nm with a tuning step of 0.2-0.3 nm. The SMSR is reached to 32-36 dB and a linewidth of 3.0-5.1 kHz. When the cavity length is increased to 2.37 mm with a tuning range of ±0.37 mm, the SLM-EDFRL with the novel FP filter has a SMSR of ~50 dB and a linewidth less than 2 kHz is achieved without other mode selecting etalons and saturable absorber. The wavelength can be tuned over 1557.3-1560.2 nm with a tuning step of 0.06-0.1 nm.
2. FP filter fabrication and principle
The schematic diagram of the tunable FP filter is shown in Fig. 1(a) . The filter is built by a section of HC-PBF (NKT Photonics, HC-1550) spliced with a standard SMF (Corning, SMF-28) and a section of micro-fiber. As shown in Fig. 1(b), HC-PBF, which guides light by the photonic bandgap (PBG) effect, has an air-core surrounded by a micro-structured cladding comprising a triangular array of air holes in a silica background. The core and cladding diameters of the HC-1550 are 10.9 μm and 120 μm, respectively. The mode field diameter (MFD) of the HC-1550 is 7.5 μm, which is different from that of the SMF (10.5 μm). The fabrication of the etalon is straightforward and highly repeatable. Firstly, a cleaved end of the HC-PBF is spliced to a cleaved end of SMF-28 fiber using repeated arc discharges . Micro-hole collapse effect of HC-PBF should be taken into account. An electric arc fusion splicer (Fitel, FSU995A) with special parameters can repeat a low-loss fusion splicing with good mechanical strength and smooth reflective surface. The Fresnel reflection is generated at the splicing point due to modal field mismatch and refractive index difference. Secondly, the spliced HC-PBF is cleaved to a desired length L m on the order of millimeters. Compared with the high transmission loss in a conventional Fizeau cavity with an air gap, the length of the low-loss FP cavity could be extended to several centimeters [7, 20]. Thirdly, the micro-fiber is fabricated by chemical etching. A section of cleaved SMF is dipped into 5% Hydrofluoric (HF) acid vertically for about 20 hours. The fiber diameter, d, is reduced to 8-10 μm. The micro-fiber with a uniform diameter of d and a length of ℓ can be designed to meet the requirement of the FP cavity. The end of micro-fiber acts as the other reflection mirror in the FP cavity. Fourthly, in order to improve the fringe visibility, a gold film with a thickness of 20 nm is coated at the end of the micro-fiber. The reflection coefficient of the film is ~85%. A Fizeau-type etalon is formed by the reflection from the splicing surface of SMF and HC-PBF and the gold-coated end of the micro-fiber. The spacing of the FP filter can be changed over a large distance by adjusting the position of micro-fiber in the hollow core of HC-PBF. Microscope images of two reflection mirrors in the experiment are shown in Fig. 1(c) and (d). In these experiments, d = 9 μm, ℓ = 3 mm, and L m = 2.74 mm.
For the FP filter, free spectral range (FSR) can be tuned by changing the longitudinal displacement of the micro-fiber, which is given by
The normalized reflective optical intensity can be derived as follows ,20]. η <1 is the excess power loss of reflection on the mirror due to the non-ideal beam collimation in the HC-PBF and micro-fiber section. ϕ is the optical phase induced by the FP cavity, which is given by
Using the experimental data, we have , which means that the fringe visibility V will decreases as η decreases. In addition, we know that η decreases as the cavity length L increases for a fixed length of HC-PBF. Therefore, we obtain , which means that the fringe visibility of the FP filter decreases as the cavity length increases.
3. Laser setup and characteristics
The EDFRL configuration is shown in Fig. 2 . The gain medium, a 7m EDF (R37PM01, OFS Inc.), is forward-pumped through a WDM by a 980 nm laser diode (LD). The EDF has the peak absorption of 18 dB/m at 1530 nm. Two polarization controllers (PC) are used to adjust the polarization state of the light, which is used to optimize SMSR for each wavelength. An isolator (ISO) keeps the laser unidirectional. The laser output is monitored by an optical spectrum analyzer (OSA) through 1% output port of a coupler (C1). 99% output of the coupler is injected into the FP filter via port 1 of a circulator. The reflected light of the FP filter is coupled into the main ring cavity through port 3 of the circulator. The main ring cavity generates enormous number of longitudinal modes. Suppression of side longitudinal modes is achieved by the novel FP filter with millimeter cavity length. The millisecond-scale fluorescence relaxation time of the EDF keeps the light going through a nanosecond round trip several million times to create high gain and long coherence length. In this way, a narrow-linewidth SLM operation is established by a simple combination of the novel FP filter and the single-ring cavity.
Figure 3 presents the reflection spectra (1550-1560 nm) of the FP filter with different cavity lengths. These series of sharp fringes in wavelength space are used as mode selection mechanism. When the cavity length L = 0.36 mm, 0.67 mm, 1.23 mm and 2.57 mm, FSR = 3.22 nm, 1.72 nm, 0.94 nm and 0.45 nm, respectively. Without and with gold coating, the insertion losses of the FP filter are 16.5 dB and 12.7 dB, respectively, with the cavity length of 0.36 mm. The main loss is due to ~3% Fresnel reflection at the splicing point between the HC-PBF and SMF. As shown in Fig. 3, the insertion loss increases as the cavity length of the FP filter increases due to non-ideal beam collimation in the HC-PBF and micro-fiber section. With the help of a 20 nm gold film coated at the end of the micro-fiber, the contrast is improved from 2.9 dB to 12 dB with the cavity length of 0.36 mm. The corresponding fringe visibility increases from 0.32 to 0.88. When the cavity length is increased to 2.57 mm, the contrast is improved from 1 dB to 5 dB. The corresponding fringe visibility increases from 0.08 to 0.39. Because η decreases as the cavity length L increases, the fringe visibility of the FP filter decreases as the cavity length increases. Here is verified experimentally. Comparing the slope of reflection spectra within a fixed wavelength range, the longer cavity length is associated with a sharper slope, which provides more efficient mode selection as an FP filter.
Tuning characteristics of the SLM-EDFRLs are shown in Fig. 4(a) and (b) . The characteristics of the SLM-EDFRLs are list in Table 1 . With a short cavity length of 0.25 mm and a tuning range of ±0.14 mm, the FP filter has a larger FSR with a slow changing slope. The laser has a large tuning range over 1554.0-1562.0 nm with a large step 0.2-0.3 nm. The corresponding resolution of the translation stage is ~0.004 mm during laser wavelength tuning. When the cavity length increases to 2.37 mm and the tuning range increases to ±0.37 mm, the FP filter has a shorter FSR with a sharp slope. The tuning range of EDFRL is decreased to 1557.3-1560.2 nm, while the tuning step is decreased to 0.06-0.1 nm. The corresponding resolution of the translation stage is ~0.002 mm during laser wavelength tuning. The minimum resolvable tuning step of the laser is limited by the 0.0813802 μm resolution of the motorized translation stage and the 0.06 nm resolution of OSA. In practice, two accurate piezoelectrics (PZT) can be used for coarse and fine adjustment of the cavity length. The characteristics of the SLM-EDFRLs can thus be further improved. The power stability is also investigated. When the injected pump current is 400mA, the output power of the lasers are −8.0±0.4 dBm and −7.6±0.1 dBm, with the short and long FP cavities, respectively. Therefore, the high power stability of the proposed laser is confirmed.
As shown is Fig. 2, the linewidth of laser output is measured by self-heterodyne method. The laser output is launched into a Mach-Zehnder interferometer (MZI) which is constructed by two 3 dB couplers C2 and C3. An acousto-optic modulator (AOM, Brimrose) is added into the upper arm of MZI to generate a frequency shift of ~200 MHz. The length of delaying fiber is 100 km, which is corresponding to 1 kHz frequency resolution . The interference signal is detected by using a photodetector (PD, Thorlabs PDB130C-AC). The spectrum is measured by using an electrical spectrum analyzer (ESA, Agilent E4446A). The electrical spectrum of the beating signal of the main EDFRL cavity without the FP filter is inserted into Fig. 5(a) . The measured FSR is ~5.9 MHz. Figure 5(a) indicates the SLM operation and Fig. 5(b) indicates the narrow 3 dB linewidths. For L = 0.36 mm, the SLM-EDFRL has a SMSR of 36 dB and a linewidth of 3.0 kHz. For L = 2.51 mm, the SLM-EDFRL has a SMSR of 51 dB and a linewidth of 1.8 kHz. Several residual side modes are highly suppressed and the spacing of the nearest side modes is ~5.9 MHz. The experimental results confirm that the FP filter with a longer cavity has much better side-mode suppression.
The SMSR is controlled by the polarization state of light, pump power and the mode selecting efficiency determined by the FP cavity length. For L = 0.36 mm and 2.51 mm, the threshold pump currents are 118 mA and 111 mA, respectively. As shown in Fig. 6(a) and (b) , when the threshold pump current is reached, the EDFRL with a longer-cavity FP filter can obtain a SMSR of ~31.2 dB while the shorter one only has a SMSR of ~16.5 dB. In low pump current range, all side modes vanish and the lasting longitudinal mode increases quickly, such as the curve of I EDF = 111 mA in Fig. 6(b). Therefore, the SMSR increases as the pump current increases. The SMSR is reached to the maximum at the optimum pump current of I EDF = 250 mA for both lasers. In high pump current range, the side modes are partly suppressed and will increase as the pump current increases. Hence the SMSR decreases as the pump current increases. As shown in Fig. 6(a), linewidths are always limited in the range of 2-3 kHz.
In conclusion, we designed and demonstrated a tunable FP filter, which is formed by a section of HC-PBF spliced with a SMF and a section of gold-coated micro-fiber. Hollow core of HC-PBF is used as the interference cavity. The FP mirrors is comprised of a splicing point between a section of HC-PBF and a SMF and a gold-coated end of an inserted micro-fiber inside the HC-PBF. The fabrication repeatability is proved in the experiments. The tunable FP filter with a millimeter cavity length builds an efficient mode selection mechanism in an EDFRL. With a cavity length of 0.25 mm and a tuning range of ±0.14 mm, the SLM-EDFRL can be tuned over 1554.0-1562.0 nm with a minimum step of 0.2 nm. The SMSR is reached to 32-36 dB and a linewidth of 3.0-5.1 kHz. With a cavity length of 2.37 mm and a tuning range of ±0.37 mm, the SLM-EDFRL can be tuned over 1557.3-1560.2 nm with a minimum step of 0.06 nm. The SMSR is reached to 44-51 dB and a linewidth of 1.8-3.0 kHz.
X. Wang is supported by the China Scholar Council. We thank Prof. Wojtek Bock (University of Quebec) for doing the gold coating. This research is supported by the Canadian Institute for Photonic Innovations (CIPI) — a Networks of Centres of Excellence program — and the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants.
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