Narrow Bandwidth Optical Filter Research Articles

Coherent high-spectral-resolution lidar for the measurement of the atmospheric Mie-Rayleigh-Brillouin backscatter spectrum.

In this study, a 1550 nm coherent high-spectral-resolution lidar (CHSRL) is developed to measure the optical properties of aerosols and atmospheric wind profiles in the atmospheric boundary layer. To determine the optical properties, a coherent frequency discriminator based on the fast Fourier transform is designed in the CHSRL to separate the Mie and the Rayleigh-Brillouin backscatter spectra to fulfill the needs of high-spectral measurements. The atmospheric wind velocity is retrieved using the simultaneously measured Doppler shift. This non-optical frequency discriminator is a feasible and low-cost solution compared to a narrow-bandwidth optical filter, such as a Fabry-Perot interferometer or an atomic filter. However, shot, amplifier spontaneous emission, and detector noise interfere with the Rayleigh-Brillouin spectrum. Therefore, a spectrum correction algorithm is proposed to recover the interfered Rayleigh-Brillouin spectrum, and the measurement results of the spectral line agree well with those modeled with Tenti S6 at different central frequencies. Finally, field observations for comparison are conducted with the co-located CHSRL, Raman lidar, and coherent Doppler wind lidar. The comparison results indicate that the correlation coefficient of the aerosol backscatter coefficient is 0.84. The correlation coefficient and standard deviation of wind velocity are 0.98 and 0.13 m · s-1, respectively.

Ultra simple low-power RF signal detection based on an optoelectronic feedback DFB laser.

An ultra simple and low cost method to detect low-power RF signal is proposed and experimentally demonstrated based on optoelectronic feedback DFB semiconductor laser. To our knowledge, according to public reports, this is the simplest photonics-assisted method which avoids using high sensitive optical modulators and narrow bandwidth optical filters. The RF signal, which matches the oscillation mode at the relaxation oscillation peak of the DFB laser, is amplified based on optoelectronic feedback. The RF signal from 1 to 4.5 GHz can be detected by adjusting the frequency of relaxation oscillation which is related to the laser bias current. The system provides a maximum gain of 15 dB for the low-power RF signal. The sensitivity of the system can reach up to as high as -97 dBm. Considering the real application of the detection system, the properties like dynamic range, resistance to large signals and performance for detecting modulated RF signal are also investigated.

Tunable high repetition rate actively mode-locked fiber laser at 2 μm

We demonstrate a tunable actively mode-locked fiber laser at 2 μm band. A segment of 4 m Tm-Ho-co-doped fiber is used as gain medium. Active mode locking pulse is realized by using intensity modulation and the signal source is high frequency sinusoidal signal. A tunable narrow bandwidth optical filter is used to narrow laser linewidth, suppress noise and achieve wavelength tuning. Stable actively mode-locked pulses with up to 2.2 GHz repetition rate is obtained, corresponding to 649 order harmonic mode-locked pulse train. The pulse width is about 200 ps. The signal-to-noise ratio of RF spectrum is 68 dB. The optical tuning range is 1907 nm~1927 nm.

An Optoelectronic Equivalent Narrowband Filter for High Resolution Optical Spectrum Analysis.

To achieve a narrow bandwidth optical filter with a wide swept range for new generation optical spectrum analysis (OSA) of high performance optical sensors, an optoelectronic equivalent narrowband filter (OENF) was investigated and a swept optical filter with bandwidth of several MHz and sweep range of several tens of nanometers was built using electric filters and a sweep laser as local oscillator (LO). The principle of OENF is introduced and analysis of the OENF system is presented. Two electric filters are optimized to be RBW filters for high and medium spectral resolution applications. Both simulations and experiments are conducted to verify the OENF principle and the results show that the power uncertainty is less than 1.2% and the spectral resolution can reach 6 MHz. Then, a real-time wavelength calibration system consisting of a HCN gas cell and Fabry–Pérot etalon is proposed to guarantee a wavelength accuracy of ±0.4 pm in the C-band and to reduce the influence of phase noise and nonlinear velocity of the LO sweep. Finally, OSA experiments on actual spectra of various optical sensors are conducted using the OENF system. These experimental results indicate that OENF system has an excellent capacity for the analysis of fine spectrum structures.

Experimental evidence of extreme value statistics in Raman fiber lasers

We present an experiment in which the intracavity broadened spectrum of a Raman fiber laser is sliced by using a narrow-bandwidth optical filter. High-contrast fast fluctuations of the Stokes power are observed at the output of the optical filter. The statistics of the power fluctuations strongly change with the central wavelength of the filter, and rare extreme events are found to be generated in the far wings of the spectrum.

Sensing micrometer-scale deformations via stretching of a photonic crystal

This paper reports micrometer-scale strain measurements that are enabled through the observed wavelength shift in a stretched surface photonic crystal narrow bandwidth optical reflectance filter. The photonic crystal optical filter is comprised of a 550 nm period, dielectric-coated linear grating structure, which was fabricated on the surface of a thin sheet of silicone rubber. When stretched, the change in grating period induced a change in the wavelength of light coupled into the photonic crystal, which could be measured using a spectrometer. The observed wavelength change was 4.53 nm per 1% strain and was linear up to 3.75% strain. The measurement was affected by temperature fluctuations, which could be compensated. The resolution of the measurement was 78 μɛ.

Q value analysis of a first-order IIR microwave photonic filter based on SOA

The Q value of a first-order infinite impulse response (IIR) microwave photonic filter based on a semiconductor optical amplifier (SOA) in a loop is investigated theoretically and experimentally. The maximum Q value measured is around 200 by setting a narrow bandwidth optical filter in the active loop and changing the input optical power of the loop and the pump current of the SOA. Analysis shows that increasing the signal to noise ratio and the gain of the signal light through the loop contributes to achieving higher Q value. The influences of the input optical power, the pump current of the SOA and the bandwidth of the optical filter in the loop on the Q value are calculated, based on the amplified spontaneous emission (ASE) noise of the SOA being investigated. Experimental results are in good agreement with the calculated results.

Practical sensor for nitrogen in direct current glow discharges

This paper presents a method for precise measurement of atomic and molecular nitrogen in an oxygen-nitrogen dc plasma. This is achieved by monitoring the intensities of the atomic nitrogen spectral line at 821.6 nm and the molecular nitrogen bandhead at 337.1 nm, relative to the atomic oxygen spectral line at 844.7 nm. Oxygen is one of the most frequently used gases for surface chemical treatment, including deposition and etching, therefore the ability to measure and control the process and chemical composition of the process is essential. To validate this oxygen actimometry method for N2-xO2 (where x varies from 0 to 1) dc plasmas, threshold tests have been performed with Ar actinometry. The optical measurements have been performed using two methods. The first approach uses a USB2000 fiber optic spectrometer, calibrated with a Gigahertz–Optik BN-0102-1 reference standard source, to record the desired spectral lines. The second approach uses narrow bandwidth optical filters (∼0.7–0.07 nm) with central wavelengths of 821.6, 337.1, and 844.69 nm and photodiodes to detect the emission intensity, also calibrated with the same standard source. Optical data are collected for a range of experimental conditions in a flowing glow discharge of N2-xO2 mixture. The maximum dc voltage is 2.2 kV and maximum chamber pressure is 266 Pa. Data from both optical methods are compared and used to interpret the relative atomic and molecular nitrogen concentrations under various operating conditions.

Improvement of Optical NRZ- and RZ-Duobinary Transmission Systems With Narrow Bandwidth Optical Filters

We show that a narrow bandwidth optical filter at the receiver side can greatly improve the receiver sensitivity of a nonreturn-to-zero (NRZ)-duobinary system. We propose to use the narrow bandwidth filtering at the receiver side of a return-to-zero (RZ)-duobinary system to convert RZ-duobinary to NRZ-duobinary, so that the system tolerance to residual chromatic dispersion can be increased while its resistance to nonlinearity in transmission is still maintained. The idea is demonstrated with experiments.

Tunable Narrow-Bandwidth Optical Filter Based on Acoustically Modulated Fiber Bragg Grating

We demonstrate a novel tunable narrow-bandwidth optical filter based on an acoustically modulated fiber Bragg grating (FBG). A flexural acoustic wave applied to an off-centered core FBG modulates the period of the grating, producing sidebands tunable both in wavelength and magnitude.

Tunable optical filter for wavelength division multiplexing using dynamic interband photorefractive gratings

We demonstrate a narrow-bandwidth tunable optical filter in the telecommunication wavelength region near 1550 nm using dynamic holography. A thick holographic reflection grating is produced in a pure potassium niobate crystal with interband photorefractive effects. The filter is tuned by changing the grating spacing by means of an acousto-optic deflector. High wavelength selectivity with a 3 dB channel bandwidth of 0.13 nm is achieved. Tuning times are in the order of a few hundred microseconds.

The wavelength add/drop multiplexer for lightwave communication networks

Lightwave systems are progressing toward optical networks capable of manipulating data paths by optical means rather than by traditional electronic switching. This is facilitated by wavelength multiplexed transmission, in which narrow bandwidth optical filters can be used to remove specific channels and reinsert new ones anywhere in the optical link. Wavelength add/drop multiplexers performing this optical channel processing can range in capability from providing dedicated add/drop of a single channel to having fully reconfigurable add/drop of many, if not all, of the wavelength division multiplexed (WDM) channels. Careful placement of wavelength add/drop multiplexers can dramatically improve a network's flexibility and robustness while providing significant cost advantages. This paper summarizes the rationale for incorporating wavelength add/drop multiplexers in modern optical networks, outlines their logical and optical characteristics, and introduces the predominant technology choices.

Optical duobinary transmission system featuring improved receiver sensitivity and reduced optical bandwidth

We propose and demonstrate a simple method to improve both the receiver sensitivity and spectral efficiency of the optical duobinary transmission system. Compared to the conventional duobinary signal generated by using an electrical low-pass filter, more than 1-dB improvement of receiver sensitivity and /spl sim/20% reduction in the spectral width are achieved simply by filtering the 10-Gb/s duobinary signals with a narrow-bandwidth (7 GHz) optical filter.

Demonstration of optical duobinary transmission system using phase modulator and optical filter

We experimentally demonstrate an optical duobinary transmission system employing a phase modulator and a narrow-bandwidth optical filter. Compared to the conventional duobinary signal generated by using an electrical low-pass filter, the demonstrated duobinary signal 1) provides improved receiver sensitivity; 2) overcomes the problem associated with word lengths; and 3) substantially relaxes the performance sensitivity to the driver and modulator response, while maintaining narrow spectral bandwidth and high dispersion tolerance.

Narrow-band resonant grating waveguide filters constructed with azobenzene polymers

Resonant grating waveguide structures were used to fabricate narrow-bandwidth optical filters. Azopolymer films were deposited on top of slab waveguides, and surface relief gratings were optically inscribed on them to be used as couplers. This technique is a simple one-step process and produces efficient gratings with high accuracy. Sharp resonant peaks are observed in the transmission and the reflection spectra of these structures. The thickness and the index of refraction of the waveguide can be accurately determined from these resonances by use of modal theory. These parameters are then used in the design of an optical filter. Bandwidths of less than 1 nm and a decrease in transmitted signal of 60% are reported. Measurement of these values was limited by the divergence of the probe beam.

Polarimetric nonincremental fiber-optic strain-gauge

A polarimetric non incremental strain-gauge which allows to obtain a strain resolution of the order of 1 με on 10 cm of measurement base, is shown. The sensing scheme uses a superluminescent source in a configuration which provides two interference signals at different wavelength obtained from the same interference beam by a couple of narrow-bandwidth optical filters. The absence of any interferometer path compensating devices makes the measurement reliable and precise. Experimental trials confirm that the novel scheme provides an effective polarimetric optical fiber strain-gauge with nonincremental measurement capabilities for the experimental mechanics. Accuracy as well as ranging of the strain measurements as function of the source and fiber characteristics are pointed out.

Optimum frequency deviation for noncoherent CPFSK receivers using narrow-bandwidth optical filters

For CPFSK lightwave systems with noncoherent detection using a Fabry–Perot optical filter, the optimum frequency deviation is determined experimentally when the FM response is nonuniform, as for a single-electrode DFB laser, and the filter bandwidth is narrow. Owing to the passband response of the optical filter, the optimum frequency deviation is considerably larger than that for a heterodyne receiver.

Experimental demonstration of a diode laser-excited optical filter in atomic Rb vapor

A narrow-bandwidth optical filter using diode-laser-pumped atomic Rb vapor has been demonstrated. Excellent rejection of off-resonant laser photons has been achieved. The measured detection bandwidth was 1.18 GHz at the Rb cell temperature of 150 degrees C, which shows a good agreement with the calculated detection bandwidth of 1.02 GHz. The quantum efficiency of the atomic Rb vapor laser-excited optical filter is linearly dependent on the diode laser pumping power. >

Long-haul high-speed optical communication systems using a semiconductor laser amplifier

High-speed long-haul systems using semiconductor laser amplifiers, which eliminate the need for high-speed electronics in repeaters and are transparent to the transmission speed are considered for application in undersea high-speed transmission systems. The potential of laser-amplifier-repeated transmission systems has been explored by transmission experiments, showing that a high-speed system above 2 Gb is possible by filtering out the spontaneous emission power of the laser amplifier. A theoretical estimation of SNR degradation due to noise accumulation of chained laser amplifiers shows that systems are possible, using 30 to 40 laser amplifier repeaters, if narrow-bandwidth optical filters are used.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An improved reference interferometer, incorporating an angular positioning servo, used to tune a narrow bandwidth optical filter

A reference interferometer based on an earlier design has been developed, in which a reference wedge is rotated by a direct-drive position servo-system. The reference interferometer is used to set the spacing of Fabry-Perot interferometer plates in a narrow bandwidth optical filter. The instrument is designed to contain a nitrogen atmosphere and to operate under adverse conditions.