Fiber Comb Filter Research Articles

Wavelength-tunability of arbitrary transmittance functions in first-order fiber comb filters based on heterogeneous waveplate combination

We investigated the wavelength tunability of arbitrarily chosen transmittance functions (ACTFs) in first-order fiber comb filters using heterogeneous waveplate combinations: specifically, a combination of dual quarter-wave plates (QWPs) and a set of a half-wave plate and a QWP in a polarization-diversified fiber loop (PDFL). PDFL comb filters studied here create polarization interference using two polarization-maintaining fiber (PMF) segments of equal length and a polarization beam splitter. We considered two comb filters with different waveplate combinations to scrutinize the effect of waveplate combination order. By analyzing the effect of the waveplates and PMF on the output polarization state (OPS) of the filter, we identified the orientation angles of the waveplates that resulted in a redshift of the ACTF by one free spectral range (λS) by making all polarization states on the OPS trace of the second PMF move in the direction of wavelength decrease. For each filter, we theoretically derived 360 sets of waveplate angles that enabled the tuning of its ACTF by λS. Among these sets, we picked eight sets inducing a wavelength shift of λS/8 for each increase in set order and displayed wavelength-shifted comb spectra at these eight sets, which corroborate the wavelength tunability of the ACTFs of the filters.

광섬유 빗살 필터의 사분파장 지연기 연속 조합을 이용한 협대역 다파장 스펙트럼의 연속 주파수 조정에 관한 연구

We propose an optical fiber comb filter based on a polarization-diversity loop structure, which can continuously adjust the narrowband multiwavelength spectrum. The proposed comb filter consists of a polarization beam splitter(PBS), a successive combination of quarter-wave retarders(QWRs), a set of a half-wave retarder and a QWR, and two polarization-maintaining fiber(PMF) segments of equal length. For the convenient adjustment of the narrowband multiwavelength spectrum, the successive combination of QWRs was located between the PBS and the first PMF segment. The transmittance function of the proposed filter was theoretically derived using the Jones calculus. Based on the filter transmittance, we theoretically predicted the orientation angle sets of four wave retarders, which allow the extra phase difference φ of the transmittance function to vary from 0˚ to 315˚ with increments of 45˚. Then, the proposed filter successfully demonstrated the predicted results by obtaining ∼0.0915 ㎚ average wavelength shift and a high linearity of 0.99878.

Design and analysis of polarization-beam-splitter-based fiber optical comb filter

In this paper, we proposed and demonstrated a novel switchable fiber comb filter based on polarization beam splitter (PBS). The proposed filter transmission characteristics is theoretically analyzed in detail by using the transmission matrix theory. The simulated results show that the filter can generate two different output channels selected by adjusting the polarization controllers (PC) in front of the PBS, and the two different channels’ wavelength intervals can be changed with polarization-maintaining fiber’s (PMF’s) characteristics. Different PMFs give a new switchable combination of channel intervals. A multi-wavelength fiber laser (MWFL) based on the proposed filter is experimentally demonstrated. Its wavelength interval can be switched among 0.85 nm, 0.40 nm, which is consistent with the simulated results of the filter. The power fluctuation of the two channel interval outputs in one hour is less than 0.88 dB and 1.37 dB, and the wavelength fluctuation is less than 0.06 nm and 0.18 nm, respectively.

Polarization-independent wavelength-tunable flat-top comb filter based on quarter-wave polarization transformation

By harnessing quarter-wave polarization transformation, we propose a polarization-independent wavelength-tunable flat-top fiber comb filter composed of a polarization beam splitter, two polarization-maintaining fiber segments of equal length, a set of a half-wave plate and a quarter-wave plate (QWP), and dual QWPs, the simplest polarization controller. Using the Jones calculus, we derived the polarization-independent filter transmittance and sought the possible loci of the orientation angles (OAs) of the four waveplates as the functions of the transmittance phase (ϕ). With these OA loci, flat-top transmission spectra were calculated for eight ϕ values from 0° to 315° (increment: 45°), which manifests the frequency tunability of the filter. This theoretical prediction was also experimentally verified by using a broadband light source and optical spectrum analyzer. Eight measured flat-top transmission spectra were spaced 0.1 nm apart in wavelength as predicted, and the wavelength-tuning linearity was high enough to exhibit an adjusted R 2 value of ∼0.99925.

Continuously Wavelength-Tunable First-Order Narrowband Fiber Comb Filter Using Composite Combination of Wave Retarders

Here, by harnessing a composite combination of wave retarders, we propose and experimentally demonstrate a first-order narrowband fiber comb filter capable of continuously tuning its wavelength, of which the filter structure is on the fundamental basis of a polarization–diversity loop structure. The demonstrated comb filter consists of a polarizing beam splitter (PBS), two high birefringence fiber (HBF) segments of the same length, an ordered wave retarder combination (WRC) of a quarter-wave retarder (QWR) and a half-wave retarder (HWR) before the first HBF segment, and an ordered WRC of an HWR and a QWR before the second HBF segment. The second HBF segment is butt-coupled to one port of the PBS so that its principal axis should be 22.5° away from the horizontal axis of the PBS. Taking the filter transmittance obtained by Jones calculus into consideration, we found the azimuth orientation angle (AOA) sets of the four wave retarders, which could allow extra phase shifts (ψ’s) ranging from 0° to 360° to be induced in the narrowband transmittance function. From filter transmission spectra calculated according to the AOA sets found above, it is confirmed that the first-order narrowband comb spectrum can be continuously tuned by properly controlling the AOA’s, clearly indicating the continuous wavelength tunability based on a composite combination of ordered wave retarders. This theoretical prediction was verified by actually constructing the proposed filter. Then, it is concluded that our filter employing the composite combination of wave retarders can be continuously frequency-tuned by properly controlling the AOA’s of the wave retarders.

Polarization-Diversity-Loop-Based Optical Fiber Comb Filter with Polarization-Controlled Dual Transmission Channel Spacings.

In this paper, we propose an optical comb filter based on a polarization-diversity loop structure, whose two transmission channel spacings (TCS's) can be alternately switched between two principal axes of the filter by controlling waveplates contained in the filter. The proposed filter consists of a polarization beam splitter (PBS), three half-wave plates (HWPs), one Faraday rotator (FR), and two polarization-maintaining fiber (PMF) segments with different lengths (L and 2L) and equal birefringence (B). As the PMF segments are utilized as birefringence elements, the TCS of the filter is determined by the product of their effective length Le and birefringence B. For an input signal introduced into each principal axis of the filter, Le can be the sum of or difference between the two lengths (L and 2L) of the two PMF segments, that is, L or 3L, resulting in a TCS of S1 or S₂, respectively, according to angular combination of the orientation angles of the three HWPs. At a specific set of the three HWP angles, the proposed filter can create two sinusoidal transmittance functions with different TCS's (S1 and S₂) at the two principal axes, respectively. Also, the two TCS's (S1 and S₂) are switchable with each other depending on three HWP angles for each principal axis. With the fabricated filter, the flexible switching between S1 and S₂ could be successfully implemented for two orthogonally polarized input signals aligned along the two principal axes of the filter by properly controlling the three HWPs, without any modification of the filter configuration.

Chinese medicine injection combined with conventional therapies for acute lung injury: a systematic review and meta-analysis of randomized controlled trials

By incorporating a simple waveplate combination (WPC) set composed of two waveplates, we propose a wavelength-tunable fiber comb filter based on a polarization-diversified loop (PDL). The simple WPC set includes three kinds of waveplate groups such as two quarter-wave plates (QWPs), a set of a QWP and a half-wave plate (HWP), and a set of an HWP and a QWP. The PDL is implemented by making a Sagnac birefringence loop comprised of a four-port polarization beam splitter (PBS), two waveplates, and polarization-maintaining fiber (PMF). In the PDL, one end of PMF is connected to one port of the PBS with its slow axis π/4 (45°) oriented with respect to the horizontal axis of the PBS, and the other end of PMF is concatenated with the waveplates. First, we investigated light polarization conditions required to continuously tune the absolute wavelength location of the proposed filter in terms of input and output states of polarization (SOPs) of a birefringence element, or PMF. Then, three analytic transmittances of the filter were derived for the three WPC sets with arbitrary orientation angles of waveplates through Jones matrix formulation. And eight specific orientation angle sets of two waveplates, which caused phase shifts increasing linearly from 0° to 315° by a step of 45° in a sinusoidal transmittance function, were found for each WPC set. In particular, it has been theoretically proved that an orientation angle set of two waveplates, which can induce an arbitrary phase shift in the sinusoidal transmittance function, always exists for each WPC set. This implies that the comb spectrum of the proposed filter can be continuously tuned within one channel bandwidth by the proper control of the waveplate orientation angles. Finally, the input SOPs of PMF and the wavelength-dependent evolution of its output SOP were examined on the Poincare sphere at the eight specific waveplate angle sets. The relationship between the wavelength tuning and the SOP evolution was also discussed.

Continuous wavelength tuning of the first-order general comb spectrum.

Here we show the continuous wavelength tunability of a general transmittance function (GTF), besides flat-top and narrow band transmittance functions, which is mostly obtainable in the first-order fiber comb filter composed of a polarization beam splitter to form a polarization-diversified loop, four waveplates, and two birefringent fiber segments. On the basis of polarization conditions, which should be satisfied to continuously tune the GTF, waveplate orientation angles (WOAs), which gave an extra phase shift of 1-360° to a specifically chosen GTF, were found using a newly proposed WOA search scheme. Wavelength-tuned comb spectra were calculated at eight WOA sets (i.e., Sets I-VIII) selected from the above WOA sets. In the fabricated filter whose free spectral range was ~0.8 nm, its comb spectrum redshifted step by step by ~0.1 nm, resulting in a total displacement of ~0.7 nm, when the WOA set changed from Set I to VIII. The calculated and measured spectra clearly show that an arbitrary GTF can be continuously wavelength-tuned by properly choosing WOAs.

Polarization-Independent Wavelength-Tunable First-Order Fiber Comb Filter

Here, we propose a polarization-independent wavelength-tunable first-order fiber comb filter based on a polarization-diversified loop configuration. The proposed filter is composed of a polarization beam splitter, two equal-length high birefringence fiber (HBF) segments, three half-wave retarders (HWRs), and two quarter-wave retarders (QWRs). Among these five wave retarders (WRs), a group of an HWR and a QWR is located in front of each HBF. The third remaining HWR is positioned after the second HBF segment and utilized to adjust the effective orientation angle of this HBF. On the basis of the filter transmittance derived through Jones formulation, orientation angle sets of the five WRs, which can induce an arbitrary phase shift from 0 to 2π in the transmittance function, are found at both flat-top and narrow band transmission spectra. From theoretical transmission spectra obtained at eight selected orientation angle sets of the five WRs, which cause phase shifts increasing linearly from 0 to 7π/4 by a step of π/4, it is confirmed that the flat-top or narrow band comb spectrum can be continuously tuned by properly controlling the WR orientation angles. Then, this theoretical prediction is verified by experimental demonstration. In particular, the spectral evolution of the output states of polarization (SOPs) of the third HWR, the last one of optical components contained within the polarization-diversified loop, is investigated on the Poincare sphere. The relationship of the passband shape and wavelength tuning with the spectral evolution of the SOP is also discussed.

Poincaré sphere-based technique for tuning flat-top fiber comb filter

We propose a universal method for designing flat-top passband and wavelength tuning of transmission spectrum of a fiber comb filter based on a polarization-diversified loop structure using Poincaré sphere representation. The proposed method gives a clear visualization of the evolution of polarization state through birefringent optical elements comprising the filter, and the design results are experimentally verified. Our method can also suggest which optical components are necessary for a specifically desired transmission spectrum of the comb filter.

Passively Mode-Locked Fiber Lasers Based on Nonlinearity at 2-μm Band

2- μ m fiber lasers are of wide potential applications in the fields of radar, sensing, and free-space communications. We introduced a simple approach to generate passively mode-locked pulse in thulium-doped fiber lasers based on nonlinear polarization rotation (NPR) in our previous work. The high repetition rate of 1.78 GHz is obtained by both NPR and semiconductor saturable absorption mirror. By using a polarization-maintaining fiber in the experiment, a birefringence Lyot filter as a fiber comb filter can be constructed, and then the tuning range of 94 nm can be obtained for the mode-locked laser. In this paper, we also demonstrated our recent research work, a square-wave mode-locked fiber laser at 2- μ m band based on nonlinear amplifying loop mirror. The square-wave noiselike pulse (NLP) at longer wavelength of 2- μ m band can be observed. With the increase of the pump power, pulse width can be increased from 0.75 to 1.3 ns. The polarization vector characteristics of the square-wave NLP were analyzed experimentally. In addition, with the increase of the pump power, a kind of square-wave mode-locked pulse with low-intensity multipulse bunches can be observed, and these multipulse bunches can be operated at the multipulse mode-locked state. The side-mode suppression ratio decreased from 50 to 30 dB.

Continuously wavelength-tunable passband-flattened fiber comb filter based on polarization-diversified loop structure

Continuous wavelength tuning of optical comb filters, which is an essential functionality for flexible signal processing in reconfigurable optical systems, has been challenging in high order filter structures with two birefringent elements (BEs) or more due to cumbersomeness in finding a combination of waveplates and BEs and complexity in determining their individual azimuthal orientations. Here, we propose a continuously tunable polarization-independent passband-flattened fiber comb filter with two BEs using a polarization-diversified loop structure for the first time. The proposed filter consists of a polarization beam splitter and two groups of a half-wave plate, quarter-wave plate, and polarization-maintaining fiber (PMF). The azimuthal orientation of PMF in the second group is fixed as 22.5°. Orientation angle sets of the four waveplates, which can induce an arbitrary phase shift from 0 to 2π in the passband-flattened transmittance function, are found from the filter transmittance derived using Jones matrix formulation. From theoretical spectral analysis, it is confirmed that passband-flattened comb spectra can be continuously tuned. Theoretical prediction is verified by experimental demonstration. Moreover, the wavelength-dependent evolution of the output state of polarization (SOP) of each PMF is investigated on the Poincare sphere, and the relationship between wavelength tuning and SOP evolution is also discussed.

Widely tunable 2 μm continuous-wave and mode-locked fiber laser.

We propose and experimentally demonstrate a widely tunable mode-locked thulium-doped fiber laser. The laser can operate at both continuous-wave and mode-locked states at different pump power levels. A classic nonlinear polarization rotation structure is employed to obtain the passive mode-locked laser. A birefringence Lyot filter as a fiber comb filter is used to expand the tuning range. Thanks to the filtering component, the tuning range of the continuous-wave laser can reach 127nm (1823-1950nm). The tuning ranges of the single-wavelength and dual-wavelength mode-locked lasers are 94nm (1835-1929nm) and 87nm (1831-1918nm), respectively, with a 3.085MHz repetition rate and 75ps pulse width.

Continuously tunable polarization-independent zeroth-order fiber comb filter based on polarization-diversity loop structure

By selecting some optimal wave retarder combination (WRC) groups, we propose and experimentally implement a continuously tunable polarization-independent zeroth-order fiber comb filter based on a polarization-diversity loop structure. The selected WRC groups contain a set of two quarter-wave retarders (QWRs), a set of a QWR and a half-wave retarder (HWR), and a set of an HWR and a QWR. The filter was formed using a polarization beam splitter (PBS), one of the three selected WRC groups, and high birefringence fiber (HBF). One end of HBF was butt-coupled to the PBS so that its slow axis should be oriented at 45° for the horizontal axis of the PBS, and the other end was connected to the WRC group. Three kinds of comb filters were fabricated with the three selected WRC groups. Through theoretical analysis on light polarization conditions for continuous spectral tuning and filter transmittances, eight special azimuth angle sets of two wave retarders, which gave the transmittance function eight different phase shifts of 0 to –7π/4 with a –π/4 step, were found for each WRC group. Theoretical prediction was verified by experimental demonstration. It was also confirmed that the filter could be continuously tuned by the appropriate control of wave retarders.

Simplified tunable fiber comb filter based on polarization-diversity loop configuration

A simple tunable fiber comb filter composed of two quarter-wave plates and a section of polarization-maintaining fiber is proposed. The states of polarization are investigated through theoretical analysis, and we present the means for continuous wavelength tunability using analytic expression. The optimal angle combination of each birefringent device could be deduced so that an arbitrary wavelength could be a channel center of the comb filter without transmission loss, which is verified experimentally.

Tunable fiber comb filter based on simple waveplate combination and polarization-diversified loop

By incorporating a simple waveplate combination (WPC) set composed of two waveplates, we propose a wavelength-tunable fiber comb filter based on a polarization-diversified loop (PDL). The simple WPC set includes three kinds of waveplate groups such as two quarter-wave plates (QWPs), a set of a QWP and a half-wave plate (HWP), and a set of an HWP and a QWP. The PDL is implemented by making a Sagnac birefringence loop comprised of a four-port polarization beam splitter (PBS), two waveplates, and polarization-maintaining fiber (PMF). In the PDL, one end of PMF is connected to one port of the PBS with its slow axis π/4 (45°) oriented with respect to the horizontal axis of the PBS, and the other end of PMF is concatenated with the waveplates. First, we investigated light polarization conditions required to continuously tune the absolute wavelength location of the proposed filter in terms of input and output states of polarization (SOPs) of a birefringence element, or PMF. Then, three analytic transmittances of the filter were derived for the three WPC sets with arbitrary orientation angles of waveplates through Jones matrix formulation. And eight specific orientation angle sets of two waveplates, which caused phase shifts increasing linearly from 0° to 315° by a step of 45° in a sinusoidal transmittance function, were found for each WPC set. In particular, it has been theoretically proved that an orientation angle set of two waveplates, which can induce an arbitrary phase shift in the sinusoidal transmittance function, always exists for each WPC set. This implies that the comb spectrum of the proposed filter can be continuously tuned within one channel bandwidth by the proper control of the waveplate orientation angles. Finally, the input SOPs of PMF and the wavelength-dependent evolution of its output SOP were examined on the Poincare sphere at the eight specific waveplate angle sets. The relationship between the wavelength tuning and the SOP evolution was also discussed.

편광상이 고리 구조 기반 Lyot형 고차 광섬유 빗살 필터의 출력 편광 분석에 관한 연구

In this paper, we investigated the output polarization of a Lyot-type optical fiber comb filter based on a polarization-diversity loop structure. It was found that the output state of polarization (SOP) of the filter made a wavelength-dependent evolution, and the spectral periods of the output SOP variation in flat-top and lossy flat-top band modes were the channel separation of the filter and its half, respectively. For a certain input SOP, the filter could pass or reject specific spectral sections by adding and controlling an output analyzer. In particular, it was theoretically anticipated that the filter with the output polarizer could provide the fine continuous tuning of its pass band center in a wavelength range corresponding to the ±9.5% of channel spacing(0.8㎚) when the input SOP was properly adjusted. It is expected that this tuning function can be effectively applied to suppress unwanted spectral portions in modulated optical signals.

Study on Transmission and Output Polarization Characteristics of a First-Order Lyot-Type Fiber Comb Filter Using Polarization-Diversity Loop

A first-order Lyot-type fiber comb filter was fabricated by incorporating a polarization-diversity loop structure composed of a polarization beam splitter, two half-wave plates (HWPs), and two polarization-maintaining fiber (PMF) segments. One of the two PMF segments was two times longer than the other, and they were concatenated with a 60 $^{\circ} $ offset between their principal axes. The transmission spectra and output polarization characteristics of the filter were theoretically predicted and experimentally verified. By proper control of the two HWPs, first-order transmission spectra could be obtained, including flat-top and lossy flat-top band modes, and the interleaving operation of the respective spectrum could be also achieved. This flat-top band mode spectrum showed a higher attenuation rate at the cutoff level, in comparison with that of the Solc-type filter. In particular, it was revealed that the output state of polarization of the filter was a periodical function of wavelength and that the evolution period was different, according to transmission band modes. With the help of a linear output polarizer, the specific portion of a passband could be passed or rejected by controlling input polarization. It was also theoretically predicted that the fine tuning of the passband center was possible within a specific wavelength range.

Switchable dual‐wavelength single‐longitudinal‐mode erbium‐doped fiber laser based on a thin‐core fiber comb filter and saturable absorber

ABSTRACTA tunable dual‐wavelength single‐longitudinal‐mode erbium‐doped fiber laser using thin‐core fiber comb filter (TCFCF) and a saturable absorber is proposed. TCFCF is achieved by splicing a thin‐core fiber between two single‐mode fibers. Two lasers incurred shifts of 1 and <3 pm, and corresponding 3‐dB linewidths are 3.4 and 3.0 kHz, respectively. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:287–292, 2015

Thermal effect on the output transmission of single-core fiber Comb filter

We propose and experimentally demonstrate switchable and tunable transmission characteristics of a single-core comb filter based on thermal operation. Its temperature characteristics are investigated to reveal a shift in the peak wavelength position from 0.15 to 0.41 nm/°C and a tunable range of wavelength spacing of 1.18–1.38 nm for maximum and minimum effective lengths, respectively. This configuration provides the unique advantages of an all-fiber structure, tunable wavelength spacing, switchable spectral peaks, independent tuning of the center wavelength and wavelength spacing of the spectral peaks, and low polarization sensitivity. It is relatively simple to fabricate and is expected to have applications in temperature fiber optic sensors and multiwavelength fiber laser sources.