Zongjun Huang

The propagation characters of selective-filled photonic crystal fibres

The spectral characteristics and the number of bandgap of photonic liquid crystal fibre are analysed. By changing the filling ratio of air holes, the number of photonic bandgap increases and the output spectra also move to shorter wavelength more significantly, nearly 20 nm with increasing temperature. The light power in the central core disappears and reappears at high temperature when all or half of air holes are filled with liquid crystal. Such structures are quite useful for optical fibre sensor, filter and optical switch.

The propagation characters of two-core selective filling photonic liquid crystal fibres

A single fibre-type transformation which has two adjacent cores has been investigated experimentally. One of six air holes near the central core of the index-guide photonic crystal fibre was filled with liquid crystal to half the total fibre length, almost 10 cm. Light power was provided from the opposite end of the crystal fibre. When the temperature was increased from 65 to 80°C, the light power was transmitted from the central solid core into the liquid crystal-filled air hole. The spectra transmitted were from 550 nm to 850 nm, a range of almost 300 nm, but shorter wavelengths could also be included. The light power was concentrated in the central core, and as the heating area was changed it became weaker and then reappeared at the same temperature.

Detection of extremely small polarization rotation angles based on magneto-optic modulation

We designed and built a measuring system to detect a very small optical rotation. The total measuring system consists of a laser, a Faraday modulator, a photo-detector, and a lock-in amplifier. To test the sensitivity of the system, we built another system to generate the small polarization rotation angle. The rotation-generating system includes a dense coil around a magneto-optic crystal, a power amplifier working at modulation frequency about 1 kHz. The rotationgenerating system can drive probe beam polarization by 2°due to the Faraday effect under 0-2 kHz modulation frequency. We monitor the frequency and amplitude of the modulating signal by observing the second harmonic signal received by the photo-detector. Some factors which influence the detection system have been analyzed experimentally to found the best working setup of the system. Recently we can detect an angle of magnitude at 10-6rad and the SNR of the output of the lock-in amplifier is above 331.The resolution of the angle detection is better than 10-7rad.

Photonic lantern with multimode fibers embedded

A photonic lantern is studied which is formed by seven multimode fibers inserted into a pure silica capillary tube. The core of the tapered end has a uniform refractive index because the polymer claddings are removed before the fibers are inserted. Consequently, the light distribution is also uniform. Two theories describing a slowly varying waveguide and multimode coupling are used to analyze the photonic lantern. The transmission loss decreases as the length of the tapered part increases. For a device with a taper length of 3.4 cm, the loss is about 1.06 dB on average for light propagating through the taper from an inserted fiber to the tapered end and 0.99 dB in the reverse direction. For a device with a taper length of 0.7 cm, the two loss values are 2.63 dB and 2.53 dB, respectively. The results show that it is possible to achieve a uniform light distribution with the tapered end and a low-loss transmission in the device if parameters related to the lantern are reasonably defined.

Light propagation in a fibre-brush-shape converter

Optical fibres play more and more important roles in astronomy, for example, to transfer light from the focus point of telescopes to spectrometers. In this paper, a novel designed, a fibre-brush-shape converter was designed to transfer circle input of a fibre to a line-shape output. The brush-shape converter consists of several bare fibres at one end, one fibre at the other end and a taper between them. The light propagating from the bare fibres to the single fibre will be coupled. According to the theoretical and calculated results, the power of the light could be confined in the core of the fibre if the parameters of the taper are appropriate.

The influence of refractive index profile on the numeric aperture in a double-layer large-core fiber

A special designed fiber with double layers of cores was investigated theoretically and experimentally. This fiber is widely used in astronomy to transform light from the telescope to the optical spectrum analyzer. The refractive index profile of this fiber was measured firstly. The testing result showed that the fiber has three layers of refractive index. A special layer of the highest refractive index is between the uniform general fiber core and the general low-refractive-index fiber cladding. This highest layer has higher local numeric aperture (NA), which can absorb more light when the incidence angle is little larger. A series of experiments have been done to prove that the NA is larger than normal fibers due to the higher-index layer. Two special incidence angles were measured respect to uniform and circle mode pattern.

Proper temperature for Cs atomic magnetometer

Alkali atomic magnetometer is a good choice for magnetic testing devices with very-high sensitivity. Many parameters could affect the sensitivity of the atomic magnetometer. In this paper we described a Cs atomic magnetometer operating at normal temperature. A circularly polarized pumping light is used to polarize Cs atoms, while a linear polarized probing light is used to test the magneto-caused polarization rotation. During the experiment, we found the temperature is a very important parameter for the signal-to-noise ratio(SNR) enhancement. On the one hand, the higher temperature makes the SNR higher, which is good for sensitivity. On the other hand, the higher temperature causes more frequent collisions between the Cs atoms and the Cs atom and container wall, so that the polarization is easy to be broken, which reduces its sensitivity. So there must be a proper temperature for the high-sensitivity atomic magnetometer. We measured the magnetometer linewidth and sensitivity in the experiment. At optimum temperature the magnetometer achieves magnetic field sensitivity of 3.2pT/Hz1/2.

Loss measurement without cutting fibers

The optical loss of large-core fibers was investigated theoretically and experimentally. An optical system is built to realize the measurement of the fiber loss without cutting the fibers. The connector loss and the fiber loss are given simultaneously. The fiber loss at short wavelength is much larger than that at long wave length.

Investigation of a laser frequency stabilization method based on linear magneto-optics effect

Recent progresses in the investigation of nonlinear magneto-optics effects and the field of exploring fundamental physical problems became possible largely due to the development of various frequency stabilized diode laser systems. The typical frequency stabilization method is based on saturated absorption spectrum, which usually just stabilizes the frequency to the center of the resonance line. In this paper, a laser frequency stabilization method is investigated based on linear magneto-optical effect, which can extend the frequency stabilization tuning range to the wings of the resonance line. The optical system is a sequence of a linear polarizer, a cesium atomic vapor cell, a λ/4 plate and a Wollaston prism to separate two polarizing beams. The outgoing lights are detected by two photodiodes with the same type to get the differential signal of intensities. With fixed angle of the λ/4 plate respect to the Wollaston prism, a small rotation of the polarizer will produces a common offset of the differential signal, which will lead to movements of the zero-crossing point along the frequency detuning axis. This behavior can be used to tune the laser frequency on the wings of the resonance line. To analyze the properties and the theoretical value of differential signal of our experimental frequency stabilization system, Jones matrix approach is used. The experimental results show that this method can be used in laser frequency stabilization by extending the frequency tuning range to the wings of a resonance line.

Design and analysis of non-polarizing beam splitter in a glass cube

The reflectance and transmittance of thin films at oblique incident angles exhibit strong polarization effects, particularly for the films inside a glass cube. However, the polarization effects are undesirable in many applications. To solve this problem, non-polarizing beam splitters with unique optical thin films have been achieved employing a method of combination of interference and frustrated total internal reflection, the non-polarizing condition expressions based on frustrated total internal reflection has been derived, and the design examples of non-polarizing beam splitters with an optimization technique have been also presented. The results of Rp=(50±0.5)%, Rs=(50±0.5)% andΔr=(0±0.3) degree in the wavelength range of 400-700nm have been obtained. The thickness sensitivity of NPBSs is also analyzed.