Wavelength Drift Research Articles

Multi-wavelength switchable random fibre laser based on double Sagnac-loop filter

A multi-wavelength switchable random fibre laser based on a double Sagnac-loop filter is proposed. The threshold of laser is 27 mW and the slope efficiency of the total power output to pump power is about 0.04%. In the experiment, stable and switchable laser outputs of single-, dual-, tri- and quad-wavelength channels are obtained by adjusting the polarization controller at room temperature. The wavelength positions are tuneable within the range of 1556–1564 nm and intervals are switchable between 2.5, 5 and 7.5 nm. Under 300 mW pump power, average power intensity of −24dBm is achieved. Furthermore, the output of fibre laser is relatively stable with less than 0.2 nm wavelength drift and less than 1.5 dBm power fluctuation within one hour. The proposed random fibre laser has promising application prospects for distributed sensing systems and biomedical imaging.

The Performance Analysis of Thermal Effect in Mesh-Based Optical Networks-on-Chips

Optical networks-on-chips (ONoCs) are the key technology for the sustainable development of multiprocessors system-on-chip (MPSoC) in the future. Micro-ring resonators (MRs) are widely used in ONoCs as key device to select and redirect optical signals. However, MRs have the inherent property of being sensitive to environmental temperature. With the fluctuation of environmental temperature, its resonance wavelength drifts, which can introduce more loss and crosstalk noise to ONoCs and make the network performance decline sharply. Therefore, it is very important to analyze the influence of thermal effect in ONoCs to solve this problem. In this paper, the theoretical models of loss and crosstalk noise changing with temperature are established from device level to network level, respectively. And a series of network performances of mesh-based ONoCs caused by thermal effect are systematically modeled and analyzed employing formal methods. Finally, we conduct case studies for mesh-based ONoCs using optimized crossbar optical router and crux optical router to evaluate the proposed method. The simulation results show that the performance of mesh-based ONoCs declines with the increase of temperature, such as the decrease of optical signal-to-noise ratio (OSNR) and the increase of bit error rate (BER), which severely limits the network scalability. The formal analytical models provide a criterion for the performance analysis of thermal effect in ONoCs. In addition, the formal methods have high portability and scalability, and can provide technical support for future research work.

Multi-wavelength Bismuth-doped fiber laser in 1.3 µm based on a compact two-mode fiber filter

In this work, a multi-wavelength Bismuth-doped fiber laser (MWLBDFL) operating in the 1.3 µm wavelength region is presented and experimentally demonstrated. The MWLBDFL leveraged on a 60 m Bismuth-doped fiber as an active gain medium, a compact two-mode fiber filter (TMFF) as a comb filter and a 2 km single-mode fiber section to induce four-wave mixing in the system. By optimising the polarization controller, the MWLBDFL generated 18 stable lasing output channels within 10 dB from the highest lasing line and a reasonably flat spectrum over a range of 11 nm. The stability of the MWLBDFL tested for a 150-minute duration showed a peak power fluctuation of less than 1 dB with a negligible wavelength drift. The extinction ratio of the TMFF was approximately 8.6 dB while the highest and lowest optical-signal-to-noise ratios (OSNRs) of the MWLBDFL were ~ 55 dB and ~ 50 dB, respectively. To the best of the author’s knowledge, the proposed MWLBDFL demonstrates the highest number of output channels compared to other MWLs operating in the same wavelength region.

High Sensitivity Fiber-Optic Strain Sensor Based on Modified Microfiber-Assisted Open-Cavity Mach-Zehnder Interferometer

In this paper, a modified microfiber-assisted open- cavity (MMA-OC) structure is proposed by introducing two pieces of multimode fiber and a novel sub-millimeter in-line Mach-Zehnder interferometer is theoretically and experimentally completed. The cavity-length dependence of MMA-OC on axial strain sensing is characterized and the comprehensive strain tests are conducted under an optimal waist diameter. The experimental results show that the selective intensity-/wavelength- modulated strain response can be achieved by varying the cavity length. For a shorter cavity length, MMA-OC presents more than 20-dB intensity variation and near-zero wavelength drift. But an apparent and flat red-shift of fringes occurs in the structure with >1000-μm cavity length. Moreover, due to inverse feature of temperature response, the calculated cross-sensitivity is merely 0.106 μϵ/°C. And the detection resolution up to ~0.2 μϵ is gained when the cavity length is equal to 503 μm, with the measurement error of ±0.021 μϵ. Our sensor with the merits of compactness, low-cost and ease of fabrication is very promising for high-precision engineering monitoring and sensing.

Method for boosting dispersive spectrograph stability 1000× using interferometry with crossfaded pairs of delays

We demonstrate a key step along a technical route to achieving cm/s scale accuracy for astronomical spectrographs over long (multi-year) time scales, which is critical for the Doppler characterization of Earth sized exoplanets, and measurement of small cosmic redshift drift over many years. This same technique also enables searching exoplanet atmospheres for biosignificant molecules in direct planet imaging using, otherwise, insufficiently low resolution and drift prone dispersive (grating or prism) spectrographs. Using a method called crossfading for externally dispersed interferometers (EDIs) to get highly robust spectra, we recently demonstrated a factor of 1000 × reduction in the net shift of an EDI measured ThAr line to a deliberate simulated wavelength translation of the detector. This 1000 × gain in disperser stability can be combined with conventional stability gains afforded by fiber scramblers, vacuum tanks, and thermal control, to provide an additional 1 to 3 orders of magnitude reduction in the net point spread function shift drift. Crossfading combines high- and low-delay fringing signals that react oppositely in phase to cancel their net reaction to a detector wavelength drift. This can be implemented by an interferometer addition to a facility spectrograph.

Multiwavelength switchable fiber laser employing a DMD as tuning element and variable optical attenuator

Multiwavelength switchable erbium-doped fiber laser is demonstrated by using a digital micromirror device (DMD) as an optical filter and wavelength-dependent variable optical attenuator. The spectrum can be switched from single to multiwavelength lasing output by simple adjustment of the DMD reflection pattern. The mode competition in a fiber ring is efficiently suppressed by finely modulating the reflection column to balance the diffraction loss and EDF gain on each wavelength. The results show the 3 dB-bandwidth of multiwavelength lasing signal is less than 0.06 nm and the side mode suppression ratio is greater than 35 dB. The peak power fluctuation is less than 0.8 dB, and the wavelength drift is less than 0.06 nm over 50 min. The multiwavelength generation from the DMD-based tunable fiber laser is stable, flexible and fast.

3D printed long period gratings and their applications as high sensitivity shear-strain and torsion sensors.

In this work we demonstrate the capability to measure shear-strain and torsion loads by bonding an optical fiber to a 3D printed periodic grooved plate. The device acts as a long period grating where the resonances show loss tunability ranging from ∼0 up to ∼20 dB, achieving sensitivities values for the dip transmission ratio as function of the load of 0.12 /mε and 0.21/deg, for shear-strain and torsion loads ranging from 0-∼8 mε and 1-∼4 deg, respectively. The low wavelength drift allowed us to operate the sensor through intensity demodulation techniques, showing good tracking performance of external stimuli.

PH Sensor Through a Self-Assembled Multifunctional Layer With Clitoria Ternatea Based on Long-Period Fiber Gratings

In this study, Clitoria ternatea was used to provide a quantifiable method for pH measurement. The researchers proposed a new pH measurement method that involves the use of a KrF excimer laser to perform laser-assisted etching of the crown structure of long-period fiber gratings. Processing of the front surface of the optical fibers through grating resulted in periodically structured optical fibers with a sensor diameter of 55 μm and a period of 660 μm; these structured optical fibers were encapsulated using glass slides. On the front surface of the optical fibers, functional group modification of the fiber surface was conducted and a nanogold coating was applied using the self-assembly method. Additionally, Clitoria ternatea was bonded to the surface of the optical fibers for the measurement of refractive index. In the pH sensing experiment, the average loss sensitivity detected by the sensors with no nanogold self-assembled coating was 0.0635 dB/pH. By contrast, the average resonant wavelength sensitivity of the sensors with a nanogold self-assembled coating under localized surface plasmon resonance was increased to 0.173 nm/pH. Compared with the sensors with no nanogold self-assembled coating, the sensors with this coating had a wavelength drift sensitivity enhanced by a factor of 37.

A tunable single-frequency green laser based on a wedged Nd:YVO4 crystal and a KTP crystal

A 532 nm single-frequency laser generated from a V-type oscillator configuration with a wedged Nd:YVO4 crystal and a KTP crystal is reported. An a-cut, 2.at% Nd:YVO4 crystal with a wedge angle of 10° and a KTP nonlinear crystal composes a novel birefringent filter. By adjusting the incident angle of the fundamental mode in the KTP, the laser can change from single-mode operation to two-mode oscillation. The maximum single-frequency output power of lasers with a 5 mm long KTP and a 7 mm long KTP are 284 and 148 mW, respectively, at pump powers of 2 and 1.1 W. The corresponding optical-to-optical conversion efficiencies are 14.2% and 13.5%. For the laser configuration with a 5 mm long KTP, the single-frequency operation at a pump power of 2 W is maintained within the temperature range of 18 °C to 24 °C, and the measured wavelength drift rate with the temperature is approximately 0.071 nm °C−1.

Fiber tip Michelson interferometer for temperature sensing based on polymer-filled suspended core fiber

A low-cost fiber tip temperature sensor is proposed and experimentally demonstrated. It is based on the Michelson interferometer (MI) structure which is partially filled with polymer-UV glue in the suspended core fiber (SCF). Due to the greater thermal properties of UV glue compared with the silica optical fiber, the length of the inner cavity in the MI structure is susceptible to temperature modulation, resulting in the interference wavelength drift. Experimental results show that the sensor achieves a sensitivity of −164 pm/°C in the temperature range of 25–60 °C, and the sensor offers good wavelength and power stability at room temperature. In addition, the sensor has advantages such as robust, small size, and low cost, especially the reflective tip structure is more suitable for remote sensing in practical applications.

Accurate Measurements of a Wavelength Drift in High-Temperature Silica-Fiber Bragg Gratings

Fiber Bragg gratings (FBG) are extensively used to perform high-temperature measurements in harsh environments, however the drift of the characteristic Bragg wavelength affects their long-term stability resulting in an erroneous temperature measurement. Herein we report the most precise and accurate measurements of wavelength drifts available up to date on high-temperature FBGs. The measurements were performed with a set of packaged π-phase-shifted FBGs for high wavelength resolution, in caesium and sodium pressure-controlled heat pipes for stable temperature environment and with a tunable laser for stable wavelength measurements with a 0.1 pm resolution. Using this dataset we outline the experimental caveats that can lead to inconsistent results and confusion in measuring wavelength drifts, namely: influence of packaging; interchangeability of FBGs produced under identical conditions; birefringence of π-phase-shifted FBGs; initial transient behaviour of FBGs at constant temperature and dependence on the previous thermal history of FBGs. In addition, we observe that the wavelength stability of π-phase-shifted gratings at lower temperature is significantly improved upon by annealing at higher temperature. The lowest value of the wavelength drift we obtain is +0.014 pm·h−1 at 600 °C (corresponding to +0.001 °C·h−1) after annealing for 400 h at 1000 °C, the longest annealing time we have tried. The annealing time required to achieve the small drift rate is FBG-specific.

Design and Implementation of FBG Three-dimensional Stress Sensor for Complicated Stress Test in Rock Strata

A fiber Bragg grating (FBG) sensor was presented and designed which was used to monitor the three-dimensional stress independently in rock strata. The sensing elements with FBGs in x, y, and z directions were combined in one intact material, and the stress/strain would be obtained without interference from other directions because each element could deform independently. By numerical modeling, the stress was applied respectively in different directions on the surface of the sensor, and the result revealed that the stress distributed in the direction applied was far more than other directions, which indicated that every element of the sensor was only sensitive to the stress in one direction. To get the relationship between applied stress and wavelength drifts of the FBG sensor, the calibration experiments were carried out, and the sensitivities in x, y, and z direction were obtained. It is of great significance for obtaining complicated stress state in rock strata and achieving intrinsically safe mining in coalmine.

Experimental Study of Shock Wave Spread Rule Monitoring with FBG Sensing for Lithologic Characteristics

To obtain the influence mechanism of lithologic characteristics when rock bust shock waves transmit through rock, the waves in different rock were monitored with fiber Bragg grating (FBG). The result shows that the wave will change the period Λ and effective refractive index n of FBG, and further affect the initial wavelength value. The amplitude, phase and frequency of shock wave are directly related to the wavelength drifts of FBG. The transmitting velocity of shock wave in rock is affected by lithologic characteristics. The Elastic modulus, density and Poisson’s ratio of rock influence the initial wavelength value of FBG in the form of , and are Cosine related to wavelength drifts. The experiments of shock waves monitoring were carried out in sandstone, limestone and marble respectively. The result shows that the lithologic parameters of rock were Cosine related to the initial wavelength value of FBG. This study provided a theoretical basis and practical application guidance for coal or rock burst monitoring with FBG sensing.

Fiber Bragg Grating Wavelength Drift in Long-Term High Temperature Annealing

High-temperature-resistant fiber Bragg gratings (FBGs) are the main competitors to thermocouples as sensors in applications for high temperature environments defined as being in the 600–1200 °C temperature range. Due to their small size, capacity to be multiplexed into high density distributed sensor arrays and survivability in extreme ambient temperatures, they could provide the essential sensing support that is needed in high temperature processes. While capable of providing reliable sensing information in the short term, their long-term functionality is affected by the drift of the characteristic Bragg wavelength or resonance that is used to derive the temperature. A number of physical processes have been proposed as the cause of the high temperature wavelength drift but there is yet no credible description of this process. In this paper we review the literature related to the long-term wavelength drift of FBGs at high temperature and provide our recent results of more than 4000 h of high temperature testing in the 900–1000 °C range. We identify the major components of the high temperature wavelength drift and we propose mechanisms that could be causing them.

Athermal high-Q tantalum-pentoxide-based microresonators on silicon substrates

Integrated microresonators with low thermal drift of resonant wavelength are imperative photonic devices to provide stable multiplexing/filtering functions on silicon substrates with large thermo-optic coefficient. To realize athermal microresonators with high quality factor and small footprint, we present an undercut Ta2O5-based microdisk resonator on the Si substrate using a facile lift-off process on the sputtered film and chemical etching. Dependences of surface morphology, crystalline property, refractive index and extinction coefficient of Ta2O5 film on the annealing temperature are studied to reveal favorable parameters for the microresonator production. The produced 40 μm-diameter Ta2O5 microdisk resonator exhibits low thermal drift (7.40 pm/°C), high quality factor (1.65 × 104) and wide free spectral range (11.5 nm). The measured thermo-optic coefficient at the wavelength 1550 nm is 3.0 × 10−6/°C. The thermal stability is further enhanced by using the TiO2 film with the negative thermo-optic coefficient. The study of Ta2O5/TiO2 composite microdisk resonators achieves high thermal stability as best as 0.26 pm/°C.

Snap-shot topography measurement via dual-VCSEL and dual wavelength digital holographic interferometry

In this paper, we propose a dual-wavelength digital holographic interferometry method based on a compact dual vertical-cavity surface-emitting laser (VCSEL) source. The source simultaneously emits light from two highly stabilized coherent light sources with slightly different wavelengths. A highly stabilized and adjustable current source enables the application of digital holographic dual-wavelength techniques to measure the shape of an object with height steps of a few millimeters. The wavelength drift over 12 h over the entire measurement range, which was evaluated using a wavemeter, was smaller than 1 pm. In addition to the low measurement uncertainty at large height jumps, the dual-wavelength digital holographic system distinguishes itself by its robustness to environmental disturbances such as air turbulence, heat load, and/or mechanical vibrations. This is enabled via a fiber-based almost common-path single-shot digital holographic acquisition of the information of the two different wavelengths using angular multiplexing. The experimental setup and data evaluation are discussed, and we present measurements of non-cooperative objects with specular reflective and/or diffuse reflective surfaces having different colors.

Temperature-Insensitive Label-Free Sensors for Human IgG Based on S-Tapered Optical Fiber Sensors

We demonstrate a temperature-insensitive and high-sensitive label-free biosensor for low-concentration Human IgG detection based on S-tapered optical fiber (STF). The sensor is sensitive to local refractive index changes and has high robustness against temperature noise without a temperature control system. The temperature sensitivity of the sensor is −0.02nm/°C in the range of 25°C-45°C. Within 15 minutes at room temperature, the average drift of the center wavelength is 0.04 nm. The calibration experiments demonstrate that STFs for liquid achieved a sensing precision of 0.04 nm and a sensitivity of 538.62 nm/RIU. The STFs are applied for the concentration measurement of multiple diluted ex vivo IgG biomolecule solutions. The results showed that STFs were reliable for low-concentration IgG detection and its lower limit of measurement was demonstrated as 28 ng/ml. With the advantages of temperature-insensitive, compact size, low cost, high precision, real-time detection, fast response, good specificity, this STF sensors are promising for the antigen detection applications.

Terfenol-D Based Magnetic Field Sensor With Temperature Independence Incorporating Dual Fiber Bragg Gratings Structure

A magnetic field sensor with temperature insensitivity implemented by two fiber Bragg gratings (FBGs) structure is presented and experimentally demonstrated. In the proposed sensing probe, a Terfenol-D piece is bonded with the same type of two FBGs in different direction relative to magnetic field. Two center wavelengths of the notches of FBG filters are mainly determined by the magnetic field, and the two FBGs experience different magnetic-field-induced strain and the same thermal expansion, leading to the linear relationship between the wavelength drift difference and the magnetic field and temperature insensitivity. By monitoring the shift of the wavelength drift difference, the magnetic field measurement for thermal-insensitive interrogation of the magnetic field sensor can be realized. Owing to its compact and temperature independent advantages, the proposed sensor has potential application especially in the unstable direction of magnetic field. Moreover, the sensor can also serve the purpose of temperature measurement by monitoring one of the center wavelengths. The measured sensitivity of the magnetic field intensity of the proposed sensor is 8.77 pm/mT.

Stable Multi-Wavelength Thulium-Doped Fiber Laser With Two Cascaded Single-Mode-Four-Mode-Single-Mode Fiber Interferometers

We propose and experimentally demonstrate a multi-wavelength thulium-doped fiber laser using two cascaded single-mode-four-mode-single-mode (SFS) fiber interferometers and a 150-m-long highly nonlinear fiber. The transmission properties of SFS fiber interferometers with two different four-mode fiber (FMF) lengths are analyzed theoretically. The interferometer with a longer FMF is used to select specific wavelengths, and the one with a shorter FMF is used to stimulate more laser lines. By adjusting the polarization controller and increasing the pump power, the number of output laser channels and the wavelength spacings can be changed and switched. By selecting the proper length of the FMFs, 31 lasing lines are achieved with covering a wavelength range of $\sim 22$ nm. The power fluctuation and the wavelength drift of the 31-wavelength operation for the proposed fiber laser are within 0.706 dB and 0.02 nm, respectively.

High-sensitivity and low-interference gas analyzer with feature extraction from mid-infrared laser absorption-modulated signal

In this paper, we propose a gas analyzer that uses a quantum cascade laser (QCL) and achieves high sensitivity and gas selectivity with simple configuration and signal processing. Feature quantities are extracted from an absorption-modulated signal, which is obtained by the logarithmic conversion of a detector signal receiving a wavelength-modulated laser light. The extracted feature quantities are used for the determination of target and interfering gas concentration with simple simultaneous linear equations. As a result of the demonstration of CO gas measurement with a gas analyzer consisting of a 4.6 μm pulsed QCL, a small-volume Herriott cell with a path length of 5 m and a thermopile as a photodetector, it is shown that the limit of detection is 2.0 ppb at the integration time of 1 s and that interference by N2O can be eliminated. It is also shown that various disturbances such as spectral shift due to laser wavelength drift and spectral broadening due to partial pressure change of coexisting gases can be corrected by extracting appropriate additional feature quantities.