Faraday Filter Research Articles

Polarization insensitive efficient ultra-narrow diode laser strictly locked by a Faraday filter.

A Faraday anomalous dispersion optical filter (FADOF) could lock high-power diode lasers to atomic resonance lines with ultra-narrow bandwidth. However, the polarization sensitivity of the Faraday filter limits its applications since the standard diode module often employs polarization combination to increase pumping brightness. We proposed a polarization-insensitive mutual injection configuration to solve this problem and locked a standard polarization combined diode module to Rb D2-line. The laser bandwidth was narrowed from 4 nm to 0.005 nm (2.6 GHz, FWHM) with 38.3 W output and an external cavity efficiency of 80%. This FADOF-based polarization-insensitive external-cavity scheme would find many applications, such as high energy atomic gas laser pumping (alkali lasers, metastable rare gas lasers) and quantum optics, etc.

Better magneto-optical filters with cascaded vapor cells.

Single-cell magneto-optical Faraday filters find great utility and are realized with either "wing" or "line center" spectral profiles. We show that cascading a second cell with independent axial (Faraday) or transverse (Voigt) magnetic field leads to improved performance in terms of figure of merit (FOM) and spectral profile. The first cell optically rotates the plane of polarization of light creating the high transmission window; the second cell selectively absorbs the light eliminating unwanted transmission. Using naturally abundant Rb vapor cells, we realize a Faraday-Faraday wing filter and the first, to the best of our knowledge, recorded Faraday-Voigt line center filter which show excellent agreement with theory. The two filters have FOM values of 0.86 and 1.63 GHz-1, respectively.

Background Noise Resistant Underwater Wireless Optical Communication Using Faraday Atomic Line Laser and Filter

High sensitive underwater wireless optical communication (UWOC) in shallow water or FSO-UWOC convergent applications suffers severely from solar noise and other background noise interference. This paper presents experimental demonstrations of broadband background noise resistant air-water and underwater wireless optical communication by using a pair of spectrum matched 852 nm Cs atomic Faraday line laser and filter, with a narrow passband of only 1 GHz. Experimental results show that by using the Faraday atomic line laser and filter, the Q factor can be improved by 3.37 dB compared to using the interference filter, and 14.67 dB compared to system without using noise resistant filter. A numerical model for UWOC system performance affected by broadband background noise with various filtering conditions is presented, where both the numerical and experimental results match each other quite well. Based on the experimental results substantiated model, performance of UWOC under solar noise interference is numerically studied. The numerical results show that under strong solar noise interference, the maximum depth of the receiver is extended from 0.77 m to 3.79 m for FSO-UWOC system and from 0.87 m to 3.54 m for UWOC system. It proves that FADOF can provide considerable performance improvement for both UWOC and FSO-UWOC system with broadband background noise interferences.

Metastable helium Faraday filter for helium lidar to measure the density of the thermosphere.

We demonstrate a metastable helium Faraday optical filter operating on the 23S1 - 23P1 and 23S1 - 23P2 transition at 1083 nm by using a 3 cm long helium cell. The influence of the magnetic field and gas pressure of the helium cell on the filter characteristics is experimental studied. When the magnetic field is 230 Gs and the gas pressure of helium cell is about 110 Pa, the peak transmission corresponding to the two energy level transitions is about 32% and 57%, respectively. The equivalent noise bandwidth (ENBW) under this working condition is about 1.9 GHz. The metastable helium Faraday filter can be used to improve the optical inefficiency of a helium resonance fluorescence lidar to achieve the metastable helium density detection at 200-1000 km thermosphere.

Diurnal variation of atmospheric metal Na layer and nighttime top extension detected by a Na lidar with narrowband spectral filters at Beijing, China

Diurnal observations of metal Na layer can be implemented by making use of narrowband Faraday filters. On one hand, the narrowband spectral filters help to effectively suppress the background noise; on the other hand, the narrow bandwidth leads to different transmissions of Na layer resonance fluorescence signal and stratosphere Rayleigh scattering signal. The influence of the narrow spectral transmission on Na density retrieval cannot be canceled out by Rayleigh normalization and would result in a systematic error. In this paper, the Na density retrieval method when narrowband Faraday filters are used in the lidar receiver is discussed based on the transmission spectrum of Faraday filters and echo spectra of Rayleigh scattering and Na fluorescence. The results of Na layer diurnal variations that have been obtained from Na lidar observations at Beijing, China are presented. Considerable diurnal variation in Na density on the layer top and bottom are clearly revealed on a logarithmic scale. Intriguingly, larger height-scale nighttime extensions on the Na layer topside have been frequently observed from early summer to autumn. These observational results can provide valuable evidence for studying solar effect on the metal layer variation, atmospheric dynamical and chemical processes in the mesosphere and lower thermosphere region.

Special Structures of Sodium Layer Observed in the Daytime Over Beijing, China

In this paper the observation of sodium (Na) layer in mesosphere and lower thermosphere (MLT) region over complete diurnal cycles based on broadband Na lidar at Yanqing Station, Beijing, China (40.5°N,116°E ) was reported. Faraday filters with dual-channel design were used in the lidar receiving unit to suppress the strong background light in the daytime, which allow observation of Na layer with an acceptable signal-to-noise ratio (SNR) under sunlit condition. Several special structures of Na layer observed in the daytime was discussed. The simultaneous continuous observation of zonal wind by meteor radar was presented for comparison. These observation results can provide direct and reliable supports for the study of mesopause dynamics and solar effect on Na layer.

Isotope 87Rb Faraday filter with a single transmission peak resonant with atomic transition at 780 nm.

We demonstrate an isotope 87Rb Faraday anomalous dispersion optical filter (FADOF) with a single transmission peak resonant with the 5S1/2, F = 2 → 5P3/2, F' = 1, 2, 3 transitions at 780 nm with an enriched 87Rb isotope at low temperature. The isotope 87Rb FADOF achieves a single peak transmission of 74.8% with a bandwidth of 0.96 GHz. Compared with most of FADOFs operated at frequencies far from absorption, the isotope 87Rb FADOF that we have achieved can provide a transmission band exactly covering atomic transitions for many practical applications.

Faraday Filtering on the Cs-D1-Line for Quantum Hybrid Systems

Narrow-band filtering of light is widely used in optical spectroscopy. Atomic filters, which rely on the Faraday effect, allow for GHz-wide transmission spectra, which are intrinsically matched to an atomic transition. We present an experimental realization and a theoretical study of a Faraday filter based on cesium and its D1-line-transition ( $6^{2}S_{1/2}\rightarrow 6^{2}P_{1/2}$ ) around 894 nm. We also present the prospects and visions for combining this filter with the single photon emission of a single quantum dot, which matches with the atomic transition. The option to lock the spectral position of a quantum dot is discussed at the end of this letter.

How anomalous is my Faraday filter?

The Macaluso-Corbino effect describes the optical rotation of light in the spectral proximity to an atomic resonance. One use of this effect is narrowband optical filtering. So-called Faraday filters utilize the difference of the two components of the refractive indices, which are split by the Zeeman effect in a longitudinal magnetic field. This allows for a net rotation of a linearly polarized input beam within the medium. Placing it between crossed polarizers therefore only allows light near resonance to pass. Since any resonant spectrum implies anomalous dispersion on resonance, these filters are often characterized as being based on this anomalous dispersion. This Letter analyses to what extent the anomalous dispersion and the anomalous rotation are relevant for Faraday filters. Considering the sign of the anomalous rotation introduces a strict criterion if the filter is operated in the line center or in the spectral wing of an atomic resonance.

Frequency feedback for two-photon interference from separate quantum dots

We employ active feedback to stabilize the frequency of single photons emitted by two separate quantum dots to an atomic standard. The transmission of a rubidium-based Faraday filter serves as the error signal for frequency stabilization. We achieve a residual frequency deviation of $<30$ MHz, which is less than 1.5% of the quantum dot linewidth. Long-term stability is demonstrated by Hong-Ou-Mandel interference between photons from the two quantum dots. Their internal dephasing limits the expected visibility to $V=40%$. We observe ${V}_{\mathrm{lock}}=(41\ifmmode\pm\else\textpm\fi{}5)%$ for frequency-stabilized dots as opposed to ${V}_{\mathrm{free}}=(31\ifmmode\pm\else\textpm\fi{}7)%$ for free-running emission. Our technique reaches the maximally expected visibility for the given system and therefore facilitates quantum networks with indistinguishable photons from distributed sources.

Optimized ultra-narrow atomic bandpass filters via magneto-optic rotation in an unconstrained geometry.

Atomic bandpass filters are widely used in a variety of applications, owing to their high peak transmission and narrow bandwidths. Much of the previous literature has used the Faraday effect to realize these filters, where an axial magnetic field is applied across the atomic medium. Here we show that by using a non-axial magnetic field, the performance of these filters can be improved in comparison to the Faraday geometry. We optimize the performance of these filters using a numerical model and verify their performance by direct quantitative comparison with experimental data. We find excellent agreement between experiment and theory. These optimized filters could find use in many of the areas where Faraday filters are currently used, with little modification to the optical setup, allowing for improved performance with relatively little change.

Signal intensity influences on the atomic Faraday filter.

Previous studies of the Faraday anomalous dispersion optical filter (FADOF) mainly focus on the weak signal light filtering, without regard for the influences of the signal light intensity on the filter itself. However, in some applications the signal light is strong enough to change the filter's performance. In this work, the influences of the signal light intensity on the transmittance spectrum is experimentally investigated in a 780nm Rb85 FADOF in both the line-center and wings operation modes. The results show that the transmittance spectrum varies significantly with the signal light intensity. As the signal light increases, some existing transmittance peaks decline, some new transmittance peaks appear, and the maximum transmittance peak frequency may change. The spectrum in strong signal lights can be quite different from those calculated by programs in the condition of weak signal lights. These results are important for applications of the FADOF in the condition of strong signal lights.

Simultaneous Faraday filtering of the Mollow triplet sidebands with the Cs-D1 clock transition.

Hybrid quantum systems integrating semiconductor quantum dots (QDs) and atomic vapours become important building blocks for scalable quantum networks due to the complementary strengths of individual parts. QDs provide on-demand single-photon emission with near-unity indistinguishability comprising unprecedented brightness—while atomic vapour systems provide ultra-precise frequency standards and promise long coherence times for the storage of qubits. Spectral filtering is one of the key components for the successful link between QD photons and atoms. Here we present a tailored Faraday anomalous dispersion optical filter based on the caesium-D1 transition for interfacing it with a resonantly pumped QD. The presented Faraday filter enables a narrow-bandwidth (Δω=2π × 1 GHz) simultaneous filtering of both Mollow triplet sidebands. This result opens the way to use QDs as sources of single as well as cascaded photons in photonic quantum networks aligned to the primary frequency standard of the caesium clock transition.

A single-mode external cavity diode laser using an intra-cavity atomic Faraday filter with short-term linewidth

We report on the development of a diode laser system - the "Faraday laser" - using an atomic Faraday filter as the frequency-selective element. In contrast to typical external-cavity diode laser systems which offer tunable output frequency but require additional control systems in order to achieve a stable output frequency, our system only lases at a single frequency, set by the peak transmission frequency of the internal atomic Faraday filter. Our system has both short-term and long-term stability of less than 1 MHz, which is less than the natural linewidth of alkali-atomic D-lines, making similar systems suitable for use as a "turn-key" solution for laser-cooling experiments.

Tunable rubidium excited state Voigt atomic optical filter.

A tunable rubidium excited state Voigt atomic optical filter working at optical communication wavelength (1.5 μm) is realized. The filter achieves a peak transmittance of 57.6% with a double-peak structure, in which each one has a bandwidth of 600 MHz. Benefiting from the Voigt type structure, the magnetic field of the filter can be tuned from 0 to 1600 gauss, and a peak transmittance tunability of 1.6 GHz can thus be realized. Different from the excited state Faraday type filter, the pump efficiency in the Voigt filter is affected a lot by the pump polarization. Measured absorption results of the pump laser and transmittances of the signal laser both prove that the vertical linear polarization pumping is the most efficient in the Voigt filter.

Empirical Determination of Optimal Parameters for Sodium Double-Edge Magneto-Optic Filters

A method is proposed for determining the optimal temperature and magnetic field strength used to condition a sodium vapor cell for use in a sodium Double-Edge Magneto-Optic Filter (Na-DEMOF). The desirable characteristics of these filters are first defined and then analyzed over a range of temperatures and magnetic field strengths, using an IDL Faraday filter simulation adapted for the Na-DEMOF. This simulation is then compared to real behavior of a Na-DEMOF constructed for use with the Chu Research Group’s STAR Na Doppler resonance-fluorescence lidar for lower atmospheric observations.

Magnetically tuned, robust and efficient filtering system for spatially multimode quantum memory in warm atomic vapors

Warm atomic vapor quantum memories are simple and robust, yet suffer from a number of parasitic processes which produce excess noise. For operating in a single-photon regime precise filtering of the output light is essential. Here, we report a combination of magnetically tuned absorption and Faraday filters, both light–direction insensitive, which stop the driving lasers and attenuate spurious fluorescence and four-wave mixing while transmitting narrowband Stokes and anti-Stokes photons generated in write-in and readout processes. We characterize both filters with respect to adjustable working parameters. We demonstrate a significant increase in the signal-to-noise ratio upon applying the filters seen qualitatively in measurements of correlation between the Raman scattered photons.

Diode laser operating on an atomic transition limited by an isotope ⁸⁷Rb Faraday filter at 780 nm.

We demonstrate an extended cavity Faraday laser system using an antireflection-coated laser diode as the gain medium and the isotope (87)Rb Faraday anomalous dispersion optical filter (FADOF) as the frequency selective device. Using this method, the laser wavelength works stably at the highest transmission peak of the isotope (87)Rb FADOF over the laser diode current from 55 to 140 mA and the temperature from 15°C to 35°C. Neither the current nor the temperature of the laser diode has significant influence on the output frequency. Compared with previous extended cavity laser systems operating at frequencies irrelevant to spectacular atomic transition lines, the laser system realized here provides a stable laser source with the frequency operating on atomic transitions for many practical applications.

Optimization of atomic Faraday filters in the presence of homogeneous line broadening

We show that homogeneous line broadening drastically affects the performance of atomic Faraday filters. We study the effects of cell length and find that the behaviour of ‘line-centre’ filters are quite different from ‘wing-type’ filters, where the effect of self-broadening is found to be particularly important. We use a computer optimization algorithm to find the best magnetic field and temperature for Faraday filters with a range of cell lengths, and experimentally realize one particular example using a micro-fabricated 87Rb vapour cell. We find excellent agreement between our theoretical model and experimental data.

Atomic Faraday filter with equivalent noise bandwidth less than 1 GHz.

We demonstrate an atomic bandpass optical filter with an equivalent noise bandwidth less than 1 GHz using the D1 line in a cesium vapor. We use the ElecSus computer program to find optimal experimental parameters and find that, for important quantities, the cesium D1 line clearly outperforms other alkali metals on either D-lines. The filter simultaneously achieves a peak transmission of 77%, a passband of 310 MHz, and an equivalent noise bandwidth of 0.96 GHz, for a magnetic field of 45.3 G and a temperature of 68.0°C. Experimentally, the prediction from the model is verified. The experiment and theoretical predictions show excellent agreement.