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Abstract
We propose and demonstrate an adjustable all-fiber comb filter with precisely controlled channel spacing by employing a tapered fiber in one arm of a Mach-Zehnder interferometer (MZI) for the first time. Using fused taper technology to draw the fiber, we can precisely control the optical path difference between the two arms of the MZI, thus realizing a precisely controllable channel spacing. By rotating the polarization controller state in the other arm of the MZI, the transmission spectrum wavelength can be continuously tuned. Comb filters with controllable channel spacings from 0.2 to 3.0 nm have been numerically studied and achieved in experiment. Applications of a filter based on a multi-wavelength tunable all-fiber laser source are also demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Owing to their simple structure, low cost, low insertion losses, and fiber compatibility, fiber comb filters have proven to be useful in wavelength division multiplexed (WDM) systems [1], multi-channel dispersion compensation [2], fiber sensors [3–5] and multi-wavelength lasers [6–8]. In order to enhance the filter efficiency in these applications, researchers have attempted to improve the filter performance in two ways. One is to tune the transmission spectrum of a comb filter to a desired wavelength, and the other is to ensure that the filter channel spacing can be precisely controlled.
Numerous studies have been conducted to realize filter wavelength tuning by employing a piezoelectric ceramic [9] or a semiconductor optical amplifier [10] in a Sagnac loop, by using a polarization-diversity loop configuration [11–14], by exploiting a polarization beam splitter (PBS) in a Mach-Zehnder interferometer (MZI) [15], or by inserting a rotatable polarizer in a double-loop MZI [16]. On the other hand, it has been demonstrated that the filter channel spacing can be precisely controlled by altering the optical path difference between the two arms of the MZI [17–21], using a sampled chirped fiber Bragg grating (FBG) [22], applying a linear strain gradient on a sampled chirped FBG [23,24], or using a cascaded Lyot configuration [25,26].
Furthermore, by employing a PBS-based two stage cascaded MZI [27], or by using a two-stage Lyot filter and exploiting the intensity-dependent loss effect induced by a nonlinear optical loop mirror [28], it has been possible to realize adjustments of the filters in both the transmission spectrum wavelength and the channel spacing. Although the channel spacing can be selected by changing the length of the birefringent fiber of a Lyot filter or the length of one arm of an MZI, it cannot be controlled precisely and repeatably for each expected wavelength or channel spacing.
Recently, because of its unique structure and optical properties, the tapered fiber has attracted much attention [29–32]. By using the fused taper method, the length of one arm of an MZI can be slightly stretched. Since the tapered fiber length can be precisely controlled by using advanced translation stages with movement accuracy on the order of micrometers, the optical path difference between the two arms of the MZI can also be controlled precisely. As a result, based on the fused taper technology, channel spacing precisely controlled, all-fiber comb filters can be realized.
In this paper, we demonstrate the implementation and application of an all-fiber comb filter, in which both the transmission spectrum wavelength and the channel spacing can be precisely controlled. By using a fused taper in one arm and adjusting the polarization controller (PC) in the other arm of the MZI, comb filters with controllable channel spacings from 0.2 to 3.0 nm have been demonstrated in both theory and experiment. Inserting this filter into an Er-doped fiber (EDF) laser, up to 16 stable lasing lines with 0.8-nm wavelength spacing and 37 lasing lines with 0.4-nm wavelength spacing have been obtained.
2. Operation principle and experimental implementation
Figure 1 shows a schematic diagram of the proposed comb filter, which is based on a single-pass MZI composed of two 3-dB optical couplers (OCs), and the MZI consisting of a PC1 in one arm and a segment of tapered fiber in the other arm. The transmission characteristics of the MZI can be analyzed using the matrix equation:
where [Ein1] and [Ein2] are the input fields of the two input ports, and can be expressed as [Ein1] = [Acosθ; Asinθ] and [Ein2] = [0; 0], where A is the amplitude of the input light and θ is the angle respect to one of the principal axes of the input1 fiber. Since no components were used in the experimental apparatus to change the polarization angle θ, the value of θ was constant, and was set at 0.25π in simulations. The matrices [Cn], [Mn] (n = 1, 2), and [P] represent the matrices of the 3-dB coupler, the two arms and the PC1, respectively, and can be expressed as follows: where α is the rotation angle of the light through the PC1, L is the length of the shorter arm, φx and φy are the phase differences of the two axes between the two arms due to the optical path difference ΔL. The transmission function at output1 of the MZI is then: where nx and ny are the refractive indexes along these two axes, and λ is the operating wavelength. Since the birefringence of single mode fiber (SMF) is very small, the transmission spectrum of this MZI has a series of equally spaced transmission peaks in the frequency domain. Therefore, it can be used as a comb filter. The channel spacing, Δλ, between adjacent peaks in the transmission spectrum satisfies the relationship Δλ = λ2/(nΔL), where n is the refractive index of the fiber.
Fig. 1 Schematic diagram of the proposed comb filter with a PC1 in one arm and a segment of tapered fiber in the other.
By rotating the PC1 in the filter, which is equal to changing the rotation angle α, we numerically analyzed the effects on the transmission properties of this filter. Initially, we took the optical path difference ΔL to be 2071 μm with the angle α set at 0.5π, and obtained the normalized transmission curve with a filter channel spacing of 0.8 nm shown in Fig. 2(a) (blue line). By changing the angle α from 0.5π to 1.5π (0.5π, 0.65π, 1.35π, and 1.5π), while maintaining the other parameters constant, the transmission spectrum location could be tuned [the black, red and green lines of Fig. 2(a)] while the channel spacing remained unchanged.
Fig. 2 Simulated tunable transmission spectrum location of the MZI comb filters with channel spacings of 0.8 nm (a) and 0.4 nm (b).
We then changed ΔL to other numbers, such as 552 μm, 4142 μm, or 8284 μm, and correspondingly, the transmission curves with different channel spacing of 3.0 nm, 0.4 nm or 0.2 nm were all obtained. As an example, Fig. 2(b) shows the normalized transmission curve with a filter channel spacing of 0.4 nm. The tunable wavelength properties are consistent with the characteristics of the 0.8-nm filter. For simplicity, we neglected the nonlinear phase shift in the simulation since its magnitude is only 10−10 radians.
To verify the theoretical predictions, we constructed an experimental setup as shown in Fig. 1. In one arm, an extruded PC1 was employed to achieve the tunable wavelength. In the other arm, a fiber flame-heated taper-drawing device with a precision of 1 μm was used to control the filter channel spacing precisely. The stretching length of the fiber was controlled accurately by a computer-controlled dynamoelectric translation stage. Here, the operating parameters of the hydrogen flow rate, flame head height, flame width, and the stretching speed were 160 mL/min, 19.24 mm, 1 cm, and 120 μm/s, respectively.
The fabrication of the single-pass MZI comb filter with a specific channel spacing can be divided into two steps: first, calculating the optical path difference, ΔL, through analyzing the filter channel spacing of an arbitrary MZI comb filter using the expression Δλ = λ2/(nΔL); and secondly, using fused taper technology to draw a fiber in one arm of the MZI to the objective length, then the required channel spacing could be realized. For example, by drawing one arm to make the difference of the two arms to be 552 μm, a comb filter with a channel spacing of 3.0 nm and a extinction ratio of ~17 dB was achieved [the upper of Fig. 3(a)]. Further stretching the tapered fiber arm to 1657-um optical path difference, another specific channel spacing of 1.0 nm was obtained, and 0.6-nm and 0.2-nm comb filters were also presented in Fig. 3(a). Figure 3(b) shows the measured channel spacing as a function of optical path difference of two arms of the MZI. In the experiment, we have also found that, there was hardly change in transmission for about 5578-μm stretching length as shown in Fig. 3(c). When the stretched length was longer than 5578 μm, the oscillations started, which contributed to the interference between different modes in the waist region of the tapered fiber [33,34]. Therefore, 5578 μm was set as the upper limit for the stretching length. It is worth noting that, based on the volume conservation condition and the fiber-optic effective fusing degree [35], with altering the parameters of a fiber flame-heated taper-drawing device, such as the hydrogen flow rate, flame head height, flame width, or the stretching speed, the upper limit value will be changed, and different values have been presented in [33,34]. So in the context, we used the same operating parameters. If the optical path difference needed to change exceeds the upper limit, it can be realized by stretching multi-segment tapered fibers in different locations on one arm. Once a filter with a fixed channel spacing was obtained, by carefully rotating the PC1, the wavelength of the transmission could be tuned. As an example, Fig. 4 shows the tunable transmission spectra of the MZI comb filter with channel spacings of 0.8 nm [See Fig. 4(a)], and 0.4 nm [See Fig. 4(b)], corresponding to the optical path differences of 2071 and 4142 μm, respectively. Note that the channel spacing is unchanged and there is no significantly change in the extinction ratio.
Fig. 3 The optical transmission spectra of the MZI comb filter with channel spacings of 3.0 nm (with 0.1-nm resolution), 1.0 nm (with 0.05-nm resolution), 0.6 nm (with 0.02-nm resolution), and 0.2 nm (with 0.01-nm resolution) (a), the channel spacing versus optical path difference (b), and the oscillations of transmitted optical power during the taper heating-pulling process (c).
Fig. 4 The tunable transmission spectra (with 0.05-nm resolution) of the MZI comb filter with channel spacings of 0.8 nm (a) and 0.4 nm (b).
3. Application
Using the above mentioned filter, we built a tunable multi-wavelength EDF ring laser as shown in Fig. 5. A 1480-nm Raman fiber laser (KPS-BT2-RFL-1480-50-FA) was used to provide pump power through a 1480/1550 nm WDM. A 2-m-long EDF with a group velocity dispersion (GVD) of 66.3 ps2/km was used to provide the linear gain. A 5-km long SMF with a GVD of −22 ps2/km was inserted into the cavity to suppress mode competition in the EDF and broaden the output spectrum. A polarization insensitive isolator (PI-ISO) with an isolation of 45 dB ensured that the signal light propagated along the cavity in an unidirectional manner. The damage thresholds of the WDM and the PI-ISO were both 2 W. A three-ring PC2 was employed to vary the polarization states of the circulating light in the cavity. A 10:90 OC1 was used to output the optical signal.
Fig. 5 Schematic of the proposed multi-wavelength fiber laser. 1480 nm pump laser: a 1480-nm Raman fiber laser, WDM: wavelength division multiplexer, EDF: erbium-doped fiber, SMF: single mode fiber, MZI: Mach-Zehnder interferometer, PI-ISO: polarization-insensitive isolator, PC1 and PC2: polarization controllers, OC1: 10:90 output coupler; 3-dB OC2 and 3-dB OC3: 50:50 output couplers.
The multi-wavelength lasing is a balance between intensity-dependent loss (IDL) and mode competition, and the insertion of the above filter will change the IDL. Consequently, by changing the difference of the two arms and properly rotating the PC1 of the filter, we have obtained tunable multi-wavelength lasing with specific wavelength spacing. As an example, we presented the output characteristics under multi-wavelength operation with a 0.8-nm spacing comb filter in the cavity. As shown in Fig. 6(a), the number of lasing wavelength was increased with the pump power while the polarization states of the PCs were fixed. At a low pump power of 21 dB, there were only 5 lasing lines in a 3-dB bandwidth at the central wavelength of 1545.03 nm. With the increase of the pump power, the lasing line appeared successively in the long wavelength direction. In order to protect the devices, the maximum pump power was set to 33 dB, and the multi-wavelength lasing with 16 lasing lines in a 3 dB bandwidth was obtained as presented in Fig. 6(a). The wavelength spacing was 0.8 nm as determined by the comb filter, and the 3-dB linewidth was 0.182 nm. The side-mode suppression ratio (SMSR) was up to 35 dB. At this pump power, fixing the position of PC2 and properly rotating PC1, we achieved multi-wavelength tunable lasing output as shown in Fig. 6(b). It is worth mentioning that the wavelength tuning range was 1.6 nm, which is consistent with the characteristics of the filter mentioned above. In this case, by further stretching the tapered fiber arm 2071 um, the channel spacing of the comb filter became to be 0.4 nm. Corresponding, the wavelength spacing of the multi-wavelength operation also changed to 0.4 nm as shown in Fig. 7. Figure 7(a) shows the spectrum at the same pump power of 33 dBm. Within the 3 dB bandwidth of the full spectrum, up to 37 simultaneously lasing wavelengths could be observed. The 3-dB linewidth of one channel was about 0.125 nm and the SMSR was more than 30 dB. Using the same pump power and the polarization state of PC2, and adjusting only PC1, multi-wavelength laser operation was also tunable over a range of 0.8 nm as shown in Fig. 7(b). These observations of uniform multi-wavelength operation with equal wavelength intervals show that our filter can be used as an excellent comb filter.
Fig. 6 The output spectra of the proposed fiber laser with a 0.8-nm comb filter. (a) The spectra with different pump powers. (b) The spectra with tunable multi-wavelength operation.
Fig. 7 The output spectra of the proposed fiber laser with a 0.4-nm comb filter. (a) The spectrum at a pump power of 33 dBm. (b) The spectra with tunable multi-wavelength operation.
4. Conclusion
In conclusion, based on a PC and fused tapered fiber technology, we have demonstrated a tunable, channel spacing precisely controlled, all-fiber comb filter. Employing this filter in a fiber laser, we have demonstrated a tunable multi-wavelength laser source with different wavelength spacing. With the advantages of low cost, easy fabrication, broad bandwidth, narrow linewidth, high extinction ratio as well as tunability, this kind of filter has a wide range of potential applications in WDM systems, in the optical sensing field and many other fields.
Funding
National Natural Science Foundation of China (NSFC) (11374089, 61605040); Natural Science Foundation of Hebei Province (NSFHP) (F2017205162, F2017205060, F2016205124); Program for High-Level Talents of Colleges and Universities in Hebei Province (PHLTCUHP) (BJ2017020); Science Foundation of Hebei Normal University (SFHNU) (L2016B07).
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