Tunable Optical Bandpass Filter Research Articles

Widely tunable S-band ring-cavity Tm3+-doped fluorotellurite fiber laser.

Tm3+-doped fluorotellurite fibers (TDFTFs) are fabricated by using a rod-in-tube method. A 2.1 m long TDFTF is used as the gain medium, in which both ends of the TDFTF are connected to a short piece of a silica fiber by direct fusion splicing. By inserting the above TDFTF and a tunable optical bandpass filter into a ring cavity and employing a 1400/1570 nm dual-wavelength pumping technique, tunable lasing from 1460 to 1526 nm is obtained, which almost covers the whole S-band. To the best of our knowledge, this is the first report of tunable Tm3+-doped fiber laser with a tunable range almost covering the whole S-band. Furthermore, by removing the tunable optical bandpass filter from the ring cavity, free-running multi-wavelength lasers at 1500 and 1901 nm are achieved. Our results show that TDFTFs are promising gain media for constructing S-band fiber lasers.

Tunable non-polarizing optical bandpass filtering in prism pair coupled planar optical waveguide.

A tunable non-polarizing optical bandpass filter structure, comprising a prism pair coupled planar optical waveguide (POW), is demonstrated, by changing the incident angle of the filter. Experimental measurements show that pass bands for both TM and TE polarized waves are present in the filter simultaneously, and the two passbands overlap on each other. The overlapping of the two passbands can be sustainable for the peak wavelength from 623 to 852 nm as the incident angle of the light tuned within 2°. This POW based optical bandpass filter can be potentially applicable in various fields of optical and laser spectroscopies.

A single passband microwave photonic filter with enhanced flat top and shape factor based on tunable optical bandpass filter and fiber Bragg gratings

A single passband microwave photonic filter with enhanced flat top and shape factor based on tunable optical bandpass filter and fiber Bragg gratings

Design and performance of a repetition rate controllable and wavelength tunable L + U-band actively mode-locked erbium fiber laser

Mode-locked lasers draw great interest in the laser research field due to their diverse applications and easy achievement of ultra-short and high-intensity optical pulse trains. Actively mode-locked (AML) fiber lasers are more flexible compared to passively mode-locked fiber lasers owing to their ability to control repetition rates and pulse intervals electronically. The design and development of wavelength tunable AML fiber lasers operating beyond the C-band have attracted great attention from the research community. In this paper, a repetition rate controllable and wavelength tunable L+U-band AML erbium fiber laser (AML-EFL) based on a single standard 1480 nm pump laser and Mach–Zehnder modulator (MZM) as an intra-cavity intensity modulator is demonstrated using numerical simulation. The AML-EFL is created using a MZM driven by electrical Gaussian pulses and a tunable optical bandpass filter that is used to tune the laser cavity at any wavelength in the 1565–1645 nm range. The repetition rate of the AML-EFL is controlled from 500 kHz to 1 GHz. Trains of pulses with pulse widths of 103 ns and 50 ps and pulse intervals of 2 µs and 1 ns are successfully generated at repetition rates of 500 kHz and 1 GHz, respectively. A pulse energy of 626 nJ is obtained for a 500 kHz repetition rate. Signal-to-noise ratios of 30 and 32 dB are observed for trains of mode-locked pulses with repetition rates of 500 kHz and 1 GHz, respectively. In addition, slope efficiencies of around 66% for an L-band wavelength of 1582.1 nm and 30% for a U-band wavelength of 1633.6 nm are obtained considering the optimized parameters.

Wideband multiwavelength Brillouin fiber laser with switchable channel spacing.

A multiwavelength Brillouin fiber laser (MBFL) with a switchable channel spacing is demonstrated using a 1.55-µm single-mode AlGaInAs/InP hybrid square-rectangular laser as a seeding source. The scheme employs a highly nonlinear fiber loop with a feedback path to generate a 10-GHz-spacing MBFL. Then, assisted by a tunable optical bandpass filter, MBFLs with spacing from 20GHz to 100GHz at a step of 10GHz are generated in another highly nonlinear fiber loop based on the cavity-enhanced four-wave mixing. More than 60 lasing lines with an optical signal-to-noise ratio over 10dB are obtained successfully in all the switchable spacings. The total output power and the channel spacing of the MBFLs are proved to be stable.

Stable and Tunable PT-Symmetric Single-Longitudinal-Mode Fiber Laser Using a Nonreciprocal Sagnac Loop

A stable and tunable parity-time (PT) symmetric single-longitudinal-mode fiber laser using a nonreciprocal Sagnac loop is proposed, which is experimentally and numerically demonstrated. The nonreciprocal Sagnac loop only including a 3-dB optical coupler (OC) and a polarization controller (PC) is incorporated into a fiber ring cavity, in which nonreciprocal light transmission between the frequency-degenerate clockwise (CW) and counterclockwise (CCW) resonator modes is induced to suppress multiple-longitudinal-mode oscillation of the laser. The two light paths along the CW and CCW directions in the Sagnac loop are respectively defined as the gain and loss loop of the PT-symmetric laser, due to a controllable birefringence induced by the PC. By controlling the polarization states of the two light waves, the PT symmetry is broken when the gain coefficient is larger than the coupling coefficient, single-mode lasing is thus achieved. Experimental results show that the optical signal to noise ratio (OSNR) of the single-mode lasing at 1550 nm is 43.0 dB, proving the great superiority of PT symmetry for lasing mode selection. By tuning an optical tunable band-pass filter incorporated into the fiber cavity, the wavelength of output single-mode lasing varies from 1530 to 1560 nm, and their 3-dB Lorentzian linewidth varies within a range from 529 to 687 Hz. During a 30-min observation period, the variations of wavelength drift and OSNR of the single-mode lasing are less than 6 pm and 2.42 dB, respectively. The advantages of the proposed scheme are obvious, which include simple structure, low cost, simple operation, and good stability.

Tunable optical band pass filter using self-imaged hybrid Sagnac etched MMF interferometer

Tunable optical band pass filter using self-imaged hybrid Sagnac etched MMF interferometer

Analysis of Liquid Crystal Tunable Thin-Film Optical Filters Using Signal Flow Graph Technique

In this paper, a liquid crystal tunable thin-film optical bandpass filter is studied and analyzed using the signal flow graph technique. This paper investigates an exact form for calculating the transmission coefficients, reflection coefficients, and the transmission intensity of the filter. The simulation results show the filter performance and the channel shape profile. In addition, the results show the tuning capability of the filter. The signal flow graph technique provides an attractive method for analyzing the thin-film optical filters since it overcomes the difficulty of the refractive index concept in extending to optical applications. Moreover, it simplifies the filter analysis and design process.

All-Optical Non-Inverted Parity Generator and Checker Based on Semiconductor Optical Amplifiers

An all-optical non-inverted parity generator and checker based on semiconductor optical amplifiers (SOAs) are proposed with four-wave mixing (FWM) and cross-gain modulation (XGM) non-linear effects. A 2-bit parity generator and checker using by exclusive NOR (XNOR) and exclusive OR (XOR) gates are implemented by first SOA and second SOA with 10 Gb/s return-to-zero (RZ) code, respectively. The parity and check bits are provided by adjusting the center wavelength of the tunable optical bandpass filter (TOBPF). A saturable absorber (SA) is used to reduce the negative effect of small signal clock (Clk) probe light to improve extinction ratio (ER) and optical signal-to-noise ratio (OSNR). For Pe and Ce (even parity bit and even check bit) without Clk probe light, ER and OSNR still maintain good performance because of the amplified effect of SOA. For Po (odd parity bit), ER and OSNR are improved to 1 dB difference for the original value. For Co (odd check bit), ER is deteriorated by 4 dB without SA, while OSNR is deteriorated by 12 dB. ER and OSNR are improved by about 2 dB for the original value with the SA. This design has the advantages of simple structure and great integration capability and low cost.

Broadband continuously tunable microwave photonic delay line based on cascaded silicon microrings.

We present a novel broadband continuously tunable microwave photonic delay line consisting of a modulator, a four-stage microring resonator delay line, a tunable optical bandpass filter, and a photodetector. Unlike the traditional microring delay lines working at the on-resonant wavelength, the microring resonators in our chip work at the anti-resonant wavelengths, leading to a large delay bandwidth and a small delay ripple. The experimental results show that relative group delay can be continuously tuned from 0 to 160 ps for microwave frequencies in the range of 0 to 16 GHz. The delay ripple is less than 6.2 ps. These results represent an important step towards the realization of integrated continuously tunable delay lines demanded in broadband microwave phased array antennas.

A novel optical signal-to-noise ratio monitoring technique based on Gaussian process regression

We propose and experimentally demonstrate a novel in-band optical signal-to-noise ratio (OSNR) monitoring technique that uses a commercially available widely tunable optical bandpass filter to sample the measured optical power as input features of Gaussian process regression (GPR) can accurately estimate the large dynamic range OSNR and is not affected by the configuration of the optical link, and has the characteristics of distributed and low cost. Experimental results for 32 Gbaud PDM-16QAM signals demonstrate OSNR monitoring with the root mean squared error (RMSE) of 0.429 dB and the mean absolute error (MAE) of 0.294 dB within a large OSNR range of -1 dB~30 dB. Moreover, our proposed technique is proved to be insensitive to chromatic dispersion, polarization mode dispersion, nonlinear effect, and cascaded filtering effect (CFE). Furthermore, our proposed technique has the potential to be employed for link monitoring at the intermediation nodes without knowing the transmission information and is more convenient to operate because no calibration is required.

Modulation format-independent optical performance monitoring technique insensitive to chromatic dispersion and polarization mode dispersion using a multi-task artificial neural network.

We propose and experimentally demonstrate modulation format-independent optical performance monitoring (OPM) based on a multi-task artificial neural network (MT-ANN). Optical power measurements at a series of center wavelengths adjusted using a widely tunable optical bandpass filter (OBPF) are used as the input features for a MT-ANN to simultaneously realize high-precision optical signal-to-noise ratio (OSNR) and launch power monitoring and baud rate identification (BRI). This technique is insensitive to chromatic dispersion (CD) and polarization mode dispersion (PMD). The experimental verification in a 9-channel WDM system shows that for 10 Gbaud QPSK and 32 Gbaud PDM-16QAM signals with OSNR in the range of 1-30 dB, the OSNR mean absolute error (MAE) and root mean square error (RMSE) are 0.28 dB and 0.48 dB, respectively. For launch power in the range of 0-8 dBm, the MAE and RMSE of the launch power monitoring are 0.034 dB and 0.066 dB, respectively, and the identification accuracy for both baud rates is 100%. Furthermore, this technique utilizes a single MT-ANN instead of three ANNs to realize the simultaneous monitoring of three OPM parameters, which greatly reduces the cost and complexity.

Tunable Optical Bandpass Filter via a Microtip-Touched Tapered Optical Fiber*Supported by the National Basic Research Program of China (Grant No. 2017YFA0304502), the National Natural Science Foundation of China (Grant Nos. 11974225, 11634008, and 11974223), the Fund for Shanxi “1331 Project” Key Subjects, the Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices (Grant No. KF201809), and the

We demonstrate a tunable bandpass optical filter based on a tapered optical fiber (TOF) touched by a hemispherical microfiber tip (MFT). Other than the interference and selective material absorption effects, the filter relies on the controllable and wavelength-dependent mode–mode interactions in TOF. Experimentally, a large range of tunability is realized by controlling the position of the MFT in contact with the TOF for various TOF radii, and two distinct bandpass filter mechanisms are demonstrated. The center wavelength of the bandpass filter can be tuned from 890 nm to 1000 nm, while the FWHM bandwidth can be tuned from 110 nm to 240 nm when the MFT touches the TOF in the radius range from 160 nm to 390 nm. The distinction ratio can reach 28 ± 3 dB experimentally. The combined TOF-MFT is an in-line tunable bandpass optical filter that has great application potential in optical networks and spectroscopy, and the principle could also be generalized to other integrated photonic devices.

Modulation-format-independent in-band OSNR monitoring technique using Gaussian process regression for a Raman amplified multi-span system with a cascaded filtering effect.

We propose and experimentally demonstrate an accurate modulation-format-indepen-dent and cascaded filtering effect (CFE) insensitive in-band optical signal-to-noise ratio (OSNR) monitoring technique enabled by Gaussian process regression (GPR) utilizing a widely tunable optical bandpass filter (OBPF) and optical power measurements. By adjusting the center frequency of a widely tunable OBPF and measuring the corresponding output optical power as the input features of GPR, the proposed OSNR monitoring technique is experimentally proven to be transparent to modulation formats and robust to CFE, chromatic dispersion (CD), polarization mode dispersion (PMD), and nonlinear effect (NLE). Experimental results for 9-channel 32Gbaud PDM-16QAM signals with 50GHz channel spacing demonstrate OSNR monitoring with the root mean squared error (RMSE) of 0.429 dB and the mean absolute error (MAE) of 0.294 dB, in the OSNR range of -1∼30 dB. Even better, our proposed technique has the potential to be employed for link monitoring at the intermediation nodes and can eliminate the necessity to know the transmission information.

Multiple passband microwave photonic filter with adjustable bandwidth

We propose a straightforward technique that allows adjusting the bandwidth of microwave filtered signals by using two tunable optical bandpass filters. The principle of operation of the microwave photonic filter that allows the generation of passband windows is described by a full mathematical analysis. The electrical bandwidth of each generated passband window is adjusted by optically filtering a multimode laser diode. We demonstrate that the proposed filter can generate passband windows located at 4.87 GHz and 9.63 GHz exhibiting a bandwidth of 284.54 MHz and 391.24 MHz, respectively. The electrical bandwidth of each passband window can be increased to a maximum value of 1000 MHz by means of an adequate adjustment of the optical bandwidth from 2 nm to 10 nm approximately. A potential application of this proposal resides in the fact that the microwave filtered signals can be used as electrical carriers in optical communication schemes taking advantage of the electrical bandwidth that can be tailored according to a data transmission.

Tuning the optical isotropic point of mixed crystals of ethylenediammonium sulfate/selenate.

In anisotropic crystals, optical isotropic points are wavelengths where linear birefringence disappears because the refractive indices for both eigenmodes are accidentally equivalent. Here, the optical isotropic point of ethylene-diammonium selenate (EDSe) is tuned by generating a solid-solution series of EDSe doped with sulfate. Mueller matrix polarimetry and single-crystal X-ray diffraction are used to correlate changes in linear birefringence with the crystal composition of EDS x Se1-x . A scheme for using mixed crystals with isotropic points as tunable birefringent optical bandpass filters is proposed and their performance is modeled.

A 448-Gb/s PAM4 FSO Communication With Polarization-Multiplexing Injection-Locked VCSELs Through 600 M Free-Space Link

A 448-Gb/s four-level pulse amplitude modulation (PAM4) free-space optical (FSO) communication through 600 m free-space link was constructed, utilizing polarization-multiplexing injection-locked vertical-cavity surface-emitting lasers (VCSELs) for presentation. When uniting four-wavelength polarization-multiplexing and PAM4 modulation schemes, the transmission capacity of FSO communications is substantially multiplied, with an aggregate transmission rate of 448 Gb/s [56 Gb/s PAM4/wavelength × 4 wavelengths × 2 polarizations (xand y-polarizations)]. The results show that four 1.55 μm VCSEL transmitters with injection locking technique are sufficiently powerful for 448 Gb/s PAM4 signal transmission. Adopting a tunable optical band-pass filter and a polarization beam splitter for wavelength filtering and polarization de-multiplexing, the polarized wavelengths are effectually filtered and de-multiplexed in each polarized state. As the polarization state is well preserved in the scenario over 600 m free-space transmission, a sophisticated polarization tracker is not required in this proposed FSO communication. High bit error rate performance and accepted PAM4 eye diagrams are attained through 600 m free-space transmission. The four-wavelength polarization-multiplexing PAM4 FSO communication established here reveals the prominent benefits of high aggregate transmission capacity and long-distance free-space link.

Widely tunable erbium doped fiber ring laser based on loop and double-pass EDFA design

This study presents two novel tunable erbium doped fiber ring laser (EDFRL) configurations based on loop EDFA design and double-pass EDFA design, respectively to achieve an ultra-wideband tuning range and stable single-longitudinal-mode lasing operation. A micro-electro-mechanical system (MEMS) based optical tunable band pass filter was utilized as the wavelength selection element. Output spectral and power characteristics of these EDFRLs were experimentally analyzed and discussed. The EDFRLs had moderate and stable output power level (~-3 dBm) over 70 nm tuning range that covers C+L band. Moreover, the proposed EDFRLs could also suppress the undesired multi-longitudinal-modes successfully and have a high (~60 dB) OSNR level in whole tuning range.

Tunable THz Signal Generation and Radio-Over-Fiber Link Based on an Optoelectronic Oscillator-Driven Optical Frequency Comb

We propose and demonstrate experimentally a photonic THz signal generation technique based on a tunable optoelectronic oscillator (OEO), and its application in a radio over fiber (RoF) link. The OEO's frequency is tuned by varying the bandwidth of a tunable optical bandpass filter (TOBF) that is cascaded with a phase modulator (PM). The resulting tunable microwave photonic filter is used to generate OEO oscillations from 6.58 GHz up to 18.36 GHz (with a phase noise of ≤−103 dBc/Hz at 10 kHz offset from the carrier frequency). The OEO is subsequently used to drive an optical comb, generating 22 comb lines with a frequency spacing of 17.33 GHz covering a bandwidth of 360 GHz within a 20 dB envelope. By selecting two optical comb lines with a wavelength selective switch (WSS) and beating them in a uni-traveling carrier photodiode (UTC-PD), THz signals are generated at 101.5 GHz and 242.6 GHz with phase noise of −90 dBc/Hz and −78 dBc/Hz, respectively at 10 kHz offset from carrier frequency. Tunable mm-wave and THz signals can be generated either by changing the OEO oscillation frequency or the selected comb lines. Using the OEO driven OFCG, we implemented a RoF link at 242.6 GHz with a data rate of 24 Gbps over a wireless distance of 30 cm and with a bit error rate (BER) below the hard decision forward error correction (FEC) limit of 3.8 × 10−3. This method allows the creation of an all-photonic frequency reconfigurable THz signal generator and RoF system.

Tunable single-longitudinal-mode fiber laser based on a chirped fiber Bragg grating

In this paper, a tunable single-longitudinal-mode (SLM) fiber laser based on a chirped fiber Bragg grating (CFBG) is proposed and experimentally demonstrated. Multi-sub-cavities formed by a single CFBG inserted inside the laser loop greatly increase the longitudinal mode spacing so that SLM operation can be obtained. The SLM laser output can be tuned in a wide wavelength range from 1535 to 1565 nm by simply adjusting the central wavelength of a tunable optical band-pass filter (TOBF). A side-mode suppression ratio of larger than 50 dB and a linewidth of less than 8.5 kHz have been respectively achieved.