Optical Property of Polarization-Maintaining Fiber Taper for Tunable Multi-Wavelength Fiber Laser Generation

We investigate optical property of polarization-maintaining fiber taper (PMFT) for tunable multi-wavelength laser generation in a fiber ring cavity. A panda-type polarization fiber is fused and tapered into a fiber taper with a diameter of about 2.5 μm. Optical and thermal properties of the PMFT are investigated theoretically and experimentally. As combined with a polarization dependent isolator, tunable multi-wavelength fiber laser operation is achieved by controlling of polarization-dependent loss in the fiber ring cavity. The laser wavelength number can be switched from 1 to 4. The 3 dB bandwidths of laser spectra are less than 0.05 nm for all wavelength numbers, with a maximum OSNR of about 54 dB. For single-wavelength laser operation near 1561.66 nm, the measured wavelength and power fluctuations are <; 0.02 nm and 0.4 dB, respectively.

SECTION 1. Introduction

Recently, fiber lasers have attracted great attention in various fields such as fiber optical sensing, communication and signal processing, due to their advantages such as high flexibility, low noise, high-efficiency, narrow linewidth and high stability [1]–​[3]. In particular, tunable multi-wavelength fiber laser has become an important fiber source for optical wavelength division multiplexing (WDM) communication systems.

Previously, tunable laser operation can be achieved by mode-selection using thin film filter, acousto-optic tunable filter in laser resonators [4], [5]. However, as to increase the compactness and reduce intracavity loss, fiber filters are used, such as fiber Bragg gratings (FBGs) [6], [7], high birefringence (HiBi) Sagnac interferometers [8], [9] and Mach-Zehnder interferometers (MZIs) [10], [11]. Wavelength tuning ability of the FBG is determined by its strain characteristics. A flexible fiber laser with wavelength tuning range of >20 nm is achieved by tension and compression strain controlling of a uniform FBG system [6]. Previously, a triple-wavelength fiber laser is realized by stretching Chirped FBGs [7]. Laser stability of using the high birefringence (HiBi) Sagnac interferometers needs to be improved due to the dimension of the interference ring is relatively complex for integration. As for the MZI, the length of the two interference arms should to be precisely controlled [9]. Nonlinear effects in optical fibers could also be used for tunable laser mode-selection in fiber laser cavities, such as stimulated Brillouin scattering (SBS) [2], nonlinear polarization rotation (NPR) and Four-Wave mixing (FWM), etc [1], [10]. However, their laser cavity lengths are relatively long.

In this paper, we propose and demonstrate a novel tunable, multi-wavelength Erbium-doped fiber laser based on polarization maintaining fiber taper (PMFT). Optical and thermal properties of the PMFT are investigated theoretically and experimentally. Mode-selection of the PMTF in a ring shaped fiber cavity is investigated, which achieves flexible fiber laser operation with wavelength number switched from 1 to 4. The 3 dB bandwidths of laser spectra are less than 0.05 nm for all wavelength numbers, with a maximum OSNR of about 54 dB. The maximum wavelength tuning range is 41.7 nm. Laser wavelength shift and peak power fluctuation are < 0.02 nm and 0.4 dB respectively in one hour for single-wavelength laser operation near 1561.66 nm.

SECTION 2. Principles

Figure 1(a) illustrates the geometrical structure of polarization maintaining fiber (PMF, 1550-XP). During the melting and tapering process, the refractive index distribution and polarization characteristic are preserved, thus the PMFT has high birefringence [11]. The original fiber core diameter of the PMF is about 8 μm, which is much smaller than the fiber cladding diameter. After the melting and tapering process, the PMFT is transformed into a new waveguide with higher refractive index difference between the core and the air-cladding. As shown in Fig. 1(b), mode intensity distributions of the HE11, TE01 and TM01 mode are simulated by finite element method (FEM). The effective refractive indices (RIs) of the HE11, TE01 and TM01 modes are 1.3956, 1.3137 and 1.296, respectively. The radius of the PMFT is set as R = 1.25 μm. The radius of the stress region is r = 0.2 μm and the distance between the two stress regions is set as L = 1 μm. The RIs of the cladding and stress regions are 1.458 and 1.451, respectively. Fig. 1(c) shows the microscope image of the PMFT. The fiber waist diameter D is about 2.5 μm. Because of non-adiabatic, the fundamental-mode of the PMFT core does not satisfy the total reflection condition and will be transformed into the cladding mode during propagation, which could lead to the leakage of fundamental-mode power into the high-order mode. In addition, the asymmetrical index distribution of the PMFT will further strengthen the process of fundamental-mode leakage [12], [13]. During the tapering process, the decreased fiber diameter results in fewer low-order mode energy passing through the waist region and excitation of stronger evanescent field intensity of high-order mode evanescent field, thus forms a stable interference phenomenon. The HE11 mode exchanges power with the TE01 and TM01 mode and generate interference spectra.

Fig. 1.

Illustration of: (a) Geometrical structure of the PMFT. (b) Simulated intensity distributions of HE11, TE01 and TM01 mode in the PMFT. (c) Microscope image of the fabricated PMFT.

Figure 2 shows the experimental setup for measuring the transmission spectra of the PMFT. A broad-band source (BBS, with wavelength tuning from 1250 nm∼1650 nm) is connected to the PMFT by a single mode fiber (SMF). The transmission spectra of the PMFT are recorded by an optical spectrum analyzer (OSA, YOKOGAWA, AQ6370C, resolution bandwidth = 0.02 nm).

Fig. 2.

Experimental setup for measuring the transmission spectra of the PMFT.

The normalized output light intensity ratio T of the TE01 mode and the TM01 mode in the PMFT can be expressed as [12]:

$$\begin{align*} T = {\sin ^2} \left(\frac{{\Delta \varphi }}{2}\right) = \sin^{2} \left(\frac{{\pi \Delta L}}{\lambda }\right) = \sin^2\left (\frac{\pi l({n_1} - {n_2})}{\lambda }\right)\tag{1} \end{align*}$$

Where $\Delta \varphi = (2\pi \Delta L)/\lambda$ is the phase difference between the TE01 mode and the TM01 mode, ΔL is the optical path difference between the TE01 mode and the TM01 mode which is determined by their effective RIs and the length of the PMF waist $l \approx 1\text{ cm}$. λ is the wavelength of the incident light.

According to equation (1), T is determined by $\lambda$, effective RIs and $l$. The effective RIs of the modes and the interference procedure are influenced by $l$. Thus, the PMFT's tapering parameters should be controlled during the manufacturing process. Longer tapering length of the PMFT results in smaller D. Fig. 3. shows the simulated transmission spectra of the PMFT using beam propagation method (BPM). It can be seen that the free spectral range (FSR) is reduced as D decreases. Since the extinction ratio of the spectrum (35 dB @1550 nm) and the FSR are better when D = 2.5 μm, we conclude that D = 2.5 μm is the better for mode selection.

Fig. 3.

Simulated interference spectrum of the PMFT with different waist diameter D.

SECTION 3. Experiment

3.1 Fabrication and Spectral Testing

PMFT with D = 2.5 μm is fabricated by heating and stretching the PMF along the fiber longitudinal direction by moving two fiber holders in the opposite direction. Non-adiabatic cone is fabricated to excite high-order mode and modal interference effect. According to our experimental experiences, the optimized tapering speed is 160 μm/s, for which the polarization-maintaining fiber non-adiabatic taper can be realized, and interference spectra with high extinction ratio (ER) can be achieved. In addition, hydrogen flow is a key factor affecting the size of the flame and the rate at which the fiber melts, which also affects the non-adiabatic shape. The hydrogen flow rate we used is about 160 SCCM (standard cc/min).

Figure 4(a) shows the measured interference spectra of the fabricated PMFTs with different D (D1 = 2.50 μm and D2 = 4.66 μm) at room temperature. The FSR decreases as D is decreased, which agrees well with the simulation results. The 3 dB bandwidth of the PMFT with D2 = 4.66 μm is broader. Fig. 4(b) shows the comparison of the measured and simulated interference spectrum of the PMFT when D is 2.50 μm. The theoretically calculated FSR based on the BPM is 8.4 nm, agrees well with the experimentally measured value.

Fig. 4.

Comparison: (a) Measured interference spectra of the PMFT with different diameters; (b) The simulated and measured interference spectra of the PMFT with D = 2.5 μm.

According to previous simulation results, the interference envelope is determined by the orthogonal fundamental modes with different coupling coefficients. The fast Fourier transform (FFT) of the measured interference spectrum of the PMFT with D = 2.5 μm is shown in Fig. 5. The 1st peak represents the modal intensity of the HE11 mode, which does not participate in the interference process. The 2nd peak represents the modal intensity of interference between the TE01 and TM01 modes. The 3rd peak indicates the interference between other higher-order modes [14].

Fig. 5.

Spatial frequency domain of the interference spectrum of the PMFT with D = 2.5 μm.

3.2 Thermal Property

The PMFT with D = 2.5 μm was placed in a thermal controlled oven with the temperature varied from 50 °C to 70 °C, with a step size of 5 °C. The measured transmission spectra and wavelength shift are shown in Fig. 6. It can be seen that as the temperature is increased for 20°C, the dip-wavelength shift near 1557.19 nm is < 0.02 nm, indicating that the measured thermal wavelength stability of the PFMT is high.

Fig. 6.

Measured wavelength shift of the transmission spectra versus temperature variation from 50 °C–70 °C. Inset: Transmission spectra of the PMFT with different temperatures.

3.3 Experimental Setup of the Laser System

Figure 7 shows the configuration of the proposed fiber laser employing a PMFT filter in n fiber ring cavity. The gain medium was 8 m Erbium doped fiber (EDF-980-HP, Nufern) pumped by a 980-nm pump laser through a 980/1550 nm (WDM). A polarization controller (PC) and a PDI (polarization dependent isolator) are connected sequentially between the EDF and the PMFT. Continuous adjustment of the laser birefringence state in the fiber ring cavity is achieved by rotating the PC. About 90% intracavity laser power is feedback into the fiber ring cavity and 10% laser power is measured by the OSA, via a 10/90 fiber coupler.

Fig. 7.

Experimental setup of the proposed tunable fiber ring laser.

3.4 Results and Discussion

For comparison, we firstly investigate laser performance using PMFT combined with a polarization insensitive isolator (PII) as the intracavity filter. Laser output with different wavelength numbers can be obtained by adjusting the PC, as shown in Fig. 8. The 3 dB bandwidths of the laser spectra are less than 0.04 nm. In Fig. 8(a), a dual-wavelength laser output can be achieved with a pump power of 50 mW, near wavelength of 1554.061 nm and 1562.736 nm with an optical signal-to-noise ratio (OSNR) of > 43 dB. In Fig. 8(b), triple-wavelength laser output is achieved near wavelength of 1554.212 nm, 1562.584 nm and 1562.884 nm with an OSNR of > 30 dB. Fig. 8(c) shows another triple-wavelength laser output near wavelength of 1553.948 nm, 1554.528 nm and 1562.48 nm respectively, the OSNR is > 38 dB. Fig. 8(d) shows the four-wavelength laser output near wavelength of 1553.952 nm, 1554.58 nm, 1562.312 nm and 1563.076 nm with a pump power of 100 mW and an OSNR of > 36 dB. The mode competition is stronger for using the PII and the PMFT since the OSNR is relatively low and mode hopping happens during the measurement. Thus, the mode selection effect using PMFT combined with the PII is still need to be improved for higher OSNR and stability.

Fig. 8.

Measured laser spectra by using PMFT combined with a PII as the intracavity filter. (a) Dual-wavelength; (b) Triple-wavelengths; (c) Another triple-wavelength; (d) Four-wavelength.

PDI is then used for controlling the polarization dependent loss of the fiber ring cavity, combined with the PMFT of high birefringence to achieve tunable multi-wavelengths output. The PC to controls the intracavity polarization state and wavelength number. Due to the polarization hole burning (PHB) effect in the fiber ring cavity, the gain and loss difference for various laser modes is enhanced and as well as the side-mode suppression effect to suppress mode competition in the fiber ring cavity. The PDI filters the light in the cavity, which causes the inconsistent transmission light intensity for different polarization states, and achieve laser output with different wavelength numbers by adjusting the PC. Moreover, because of the high birefringence effect of the PMFT, enhanced mode selection effect is achieved. Thus, laser output performance is determined by three devices: the PMFT, the PDI, and the PC.

Figure 9 shows the measured single-wavelength laser spectra operating near: 1528.49 nm, 1536.42 nm, 1545.44 nm, 1553.81 nm, and 1562.08 nm. These five wavelengths are corresponded to the laser oscillation modes, which have the smallest polarization-dependent loss. The 3 dB bandwidths of the all the single-wavelength laser oscillations are < 0.05 nm with all the OSNRs > 40 dB. The wavelength tuning range is about 33 nm. The single-wavelength output has a minimum OSNR near 1534.9 nm, which is decides by the relative smaller extinction ratio of the PMFT interferometer around 1535 nm as compared with other wavelengths.

Fig. 9.

Measured single-wavelength laser spectra by employing the PDI and the PMFT.

Figure 10(a) shows the measured dual-wavelength laser spectra with wavelength spans of about 8∼9 nm. The wavelengths are: 1528.596 nm and 1536.56 nm, 1536.64 nm and 1545.396 nm, 1545.124 nm and 1553.84 nm, 1553.752 nm and 1562.368 nm, 1561.856 nm and 1570.228 nm. The 3 dB bandwidths of the dual-wavelength laser spectra are < 0.02 nm, and the OSNRs are > 48 dB. The wavelength tuning range is about 41.7 nm. Fig. 10(b) shows the dual-wavelength laser spectra with wavelength spans > 20 nm. The wavelengths are: 1536.062 nm and 1562.336 nm, 1528.74 nm and 1561.972 nm, 1528.35 nm and 1570.325 nm. The 3 dB bandwidths of the dual-wavelength laser spectra are < 0.04 nm, and the OSNRs are > 42 dB. The peak wavelength power nonuniformity is < 2 dB.

Fig. 10.

Measured dual-wavelength laser spectra with (a) wavelength span of 8∼9 nm and (b) wavelength span of ∼20 nm by using the PDI and the PMFT.

Figure 11 shows the triple- and four- wavelength laser spectra. For the triple-wavelength laser oscillation in Fig. 11(a), the 3 dB bandwidths are < 0.04 nm, the OSNRs are > 44 dB, and the peak power nonuniformity is < 4 dB. By increasing the pump power to 50 mW and adjusting the PC, a four-wavelength laser operation is obtained, which is showed in Fig. 11(b). The wavelengths are: 1536.96 nm, 1544.968 nm, 15537.32 nm, and 1562.386 nm. The 3 dB bandwidths of the four-wavelength laser spectrum are < 0.03 nm, the OSNR are about 46 dB, and the peak wavelength power nonuniformity is < 5 dB. Four-wavelength laser oscillation is unstable as compared to the one with fewer wavelengths due to stronger mode-competition effect.

Fig. 11.

Experimentally Measured laser spectra with: (a) Triple-wavelength and (b) Four-wavelength by using the PDI and the PMFT.

Figure 12(a) shows the single-wavelength laser output spectra peaked at 1561.66 nm, measured for 1 hour. The fluctuations of laser wavelength and power are < 0.02 nm and 0.4 dB, respectively. The threshold pump power for the single-wavelength laser operation is about 8 mW, with a slope efficiency of about 1.5%, as shown in Fig. 12(b).

Fig. 12.

Measured single-wavelength laser output: (a) Stable laser spectra peaked at 1561.66 nm, tested every 15 minutes. (b) Measured laser slope efficiency.

SECTION 4. Conclusions

In summary, we report a wavelength flexible erbium-doped fiber ring laser using thermal insensitive PMFT as the all fiber filter. The flexible laser wavelength can be switched from 1 to 4. The 3 dB bandwidths of laser spectra are < 0.05 nm for all wavelength numbers. The largest tuning wavelength range is about 41.7 nm for the dual-wavelength laser operation with a maximum OSNR of about 54 dB. For single-wavelength laser operation near 1561.66 nm, the measured laser wavelength and intensity fluctuations are < 0.02 nm and 0.4 dB. The proposed fiber laser could be a good fiber source candidate for optical communication and sensing system applications.