Wave Retarder Research Articles

Exploiting a metal wire grating in total internal reflection geometry to achieve achromatic polarization conversion

We demonstrate how a metal wire grating can work as a 45° polarization converter, a quarter-wave retarder, and a half-wave retarder over a broadband terahertz range when set up in total internal reflection geometry. Classical electromagnetic theory is applied to understand the mechanism, and equations to calculate the polarization state of reflected light are derived. We use a metal grating with a period of 20 μm and width of 10 μm on a fused silica surface: linearly polarized terahertz light incident from fused silica with a supercritical incident angle of 52° is totally reflected by the metal grating and air. The polarization of the terahertz light is rotated by 45°, 90°, and circularly polarized by simply rotating the wire grating. The performance is achromatic over the measured range of 0.1–0.7 THz and comparable to commercial visible light wave retarders.

Continuously tunable polarization-independent zeroth-order fiber comb filter based on polarization-diversity loop structure

By selecting some optimal wave retarder combination (WRC) groups, we propose and experimentally implement a continuously tunable polarization-independent zeroth-order fiber comb filter based on a polarization-diversity loop structure. The selected WRC groups contain a set of two quarter-wave retarders (QWRs), a set of a QWR and a half-wave retarder (HWR), and a set of an HWR and a QWR. The filter was formed using a polarization beam splitter (PBS), one of the three selected WRC groups, and high birefringence fiber (HBF). One end of HBF was butt-coupled to the PBS so that its slow axis should be oriented at 45° for the horizontal axis of the PBS, and the other end was connected to the WRC group. Three kinds of comb filters were fabricated with the three selected WRC groups. Through theoretical analysis on light polarization conditions for continuous spectral tuning and filter transmittances, eight special azimuth angle sets of two wave retarders, which gave the transmittance function eight different phase shifts of 0 to –7π/4 with a –π/4 step, were found for each WRC group. Theoretical prediction was verified by experimental demonstration. It was also confirmed that the filter could be continuously tuned by the appropriate control of wave retarders.

A temporal insight into the rich dynamics of a figure-eight fibre laser in the noise-like pulsing regime

We report on the dynamics of noise-like pulses at the ns scale in a passively mode-locked fibre laser, which grow in complexity as wave retarder adjustments are performed. We can observe that the laser operating in the fundamental mode can be tuned to get different shapes of the noise-like pulse. Following a regime of a very stable waveform, regimes characterized by a much more variable (but still compact) waveform are observed. Then we can get the fragmentation of the main bunch and expulsion of sub-packets and, finally, a variety of puzzling dynamics with increasing complexity are evidenced. Although the collective behaviour of the multiple waveforms is at first sight random we can observe some well-defined patterns in the kinematics of light bunches at the global cavity scale. These results may be useful to unravel the subtle mechanisms at play in complex dissipative nonlinear systems such as passively mode-locked fibre lasers.

Magnetically induced birefringence of randomly aligned liquid crystals in the terahertz regime under a weak magnetic field

Tunable terahertz (THz) wave cells have been demonstrated by using magnetic-controlled birefringence in a randomly aligned liquid crystal (LC) cell embedded with three kinds of thermotropic LCs (5CB, E7 and BNHR). By using the THz time domain spectroscopy, these three LCs have been investigated under different low magnetic fields. Experimental results show that the randomly aligned LCs in 3mm thickness cells still have high birefringence controlled by a low magnetic field in the THz regime. The phase shift of π for BNHR cell is achieved over the entire testing range at 30mT, and the dynamic response process of BNHR under a weak magnetic field has also been investigated. When the initial magnetic field of 5mT is applied, unlike the continuous tunability of the cells filled with 5CB and E7, the BNHR cell has a great phase shift of 1.5π at 0.35THz before reaching the steady state. During that process the refractive index and absorption of BNHR vary with time due to its high viscosity, and a larger magnetic field can significantly shorten the response time. These indicate that the randomly aligned mm-thick LC layer of THz devices can be used as a tunable THz wave retarder with low a driving magnetic field and high phase modulation depth in a broad THz band. Therefore, a simple manufacturing technique and low magnetic control of these randomly aligned LC layers may explore some novel LC based THz devices for spatial light modulation, filtering and tuning tasks.

Analysis of Crosstalk in 3D Circularly Polarized LCDs Depending on the Vertical Viewing Location.

Crosstalk in circularly polarized (CP) liquid crystal display (LCD) with polarized glasses (passive 3D glasses) is mainly caused by two factors: 1) the polarizing system including wave retarders and 2) the vertical misalignment (VM) of light between the LC module and the patterned retarder. We show that the latter, which is highly dependent on the vertical viewing location, is a much more significant factor of crosstalk in CP LCD than the former. There are three contributions in this paper. Initially, a display model for CP LCD, which accurately characterizes VM, is proposed. A novel display calibration method for the VM characterization that only requires pictures of the screen taken at four viewing locations. In addition, we prove that the VM-based crosstalk cannot be efficiently reduced by either preprocessing the input images or optimizing the polarizing system. Furthermore, we derive the analytic solution for the viewing zone, where the entire screen does not have the VM-based crosstalk.

Quarter wave retarder design with subwavelength gratings based on modal method

A novel method for the design of the phase retarder is proposed and analyzed. The method is based on the modal method. In addition, the design of a quarter wave retarder (phase retardance, 90°) has demonstrated using the modal method. The device is made from a surface-relief one-dimensional subwavelength fused-silica grating at normal incidence. And the rigorous couple-wave analysis (RCWA) is used to verify the design parameters of the phase retarder. The results confirmed the geometrical parameters of the quarter wave retarder calculated by the modal method agree well with the numerical simulations results.

Experimental study of the polarization asymmetrical NOLM with adjustable switch power

This work presents a study of a nonlinear optical loop mirror (NOLM) based on polarization asymmetry. In contrast with the previously reported results where a quarter wave retarder was inserted to the loop to provide the polarization asymmetry, we used a wave retarder with variable retardation (VWR). We show that the use of the VWR allows easy adjustment of the switching power. We used in the experiment a NOLM made of 200-m length of standard SMF-28 fiber twisted at the rate of 6turn/m to mitigate linear birefringence. As source of pulses we used a mode-locked ring fiber laser that generates 0.7-ps pulses. A change of nonlinear transmission up to 10 times at the same input power was found in the experiments. The experimental results were corroborated with numerical simulation. The adjustment of the NOLM transmission makes it attractive for applications in optical switching devices or mode-locked fiber lasers.

Thin film characterization with a simple Stokes ellipsometer

Ellipsometry is an important and non-destructive technique to measure the optical constants and thicknesses of layered materials. This technique is based on the simple idea of changes in the polarization state of light upon reflection from the sample surface. However, commercial ellipsometers are rather complicated and expensive devices that are not the best choice for learning the principles of ellipsometry. They usually require precise measurements of the angular position of polarization-sensitive components, such as polarisers and wave retarders. In this paper we demonstrate a simple ellipsometer which uses the concept of Stokes parameters to fully determine the polarization state of the reflected light by measuring the relative intensities at four positions of the analyser. We also describe the essential steps in analysing the ellipsometry data, and provide three real-world examples. The film thickness and index of refraction of layered samples are measured with reasonable accuracy. This work is the result of a graduate-level physics project and can easily be implemented in an educational optics laboratory to help graduate students better understand and use the concepts of light polarization, Stokes parameters, and ellipsometry.

An acoustic lens built with a low dispersion metamaterial

The realization of a near-frequency-independent acoustic metamaterial and the successful application of this material as a convex acoustic lens are introduced. We design a face-centered-orifice-cubic (FCOC) unit cell that has the characteristics of an acoustic wave retarder. The increment in the acoustic refractive index produced by the FCOC unit cell is observed by reducing the size of the orifice. This also shows that the refractive index remains nearly constant with frequency increments in the frequency regime that satisfies the homogeneous medium condition of the metamaterial. Using these unit cells, we create a two-dimensional acoustic metamaterial convex lens that correspond to the conventional optical convex lens and show that this lens focuses acoustic waves in the same manner that an optical lens focuses light waves.

A broadband silicon quarter-wave retarder for far-infrared spectroscopic circular dichroism

The high brightness, broad spectral coverage and pulsed characteristics of infrared synchrotron radiation enable time-resolved spectroscopy under throughput-limited optical systems, as can occur with the high-field magnet cryostat systems used to study electron dynamics and cyclotron resonance by far-infrared techniques. A natural extension for magnetospectroscopy is to sense circular dichroism, i.e. the difference in a material’s optical response for left and right circularly polarized light. A key component for spectroscopic circular dichroism is an achromatic 14 wave retarder functioning over the spectral range of interest. We report here the development of an in-line retarder using total internal reflection in high-resistivity silicon. We demonstrate its performance by distinguishing electronic excitations of differing handedness for GaAs in a magnetic field. This 14 wave retarder is expected to be useful for far-infrared spectroscopy of circular dichroism in many materials.

Two regimes of widely tuneable noise-like pulses from a figure-eight fibre laser

In this paper we study a dispersion-managed figure-eight fibre laser generating noise-like pulses with adjustable characteristics. Non-self-starting mode locking leads to the formation of a single noise-like pulse circulating in the cavity. Both the duration of the pulse and its spectral width can be adjusted by tuning the angle of wave retarders, in particular a half-wave retarder that controls the switching power of the polarization-imbalanced nonlinear optical loop mirror that is used as mode locker. Wave retarder tuning also allows observing an abrupt transition between two clearly distinct noise-like pulse regimes, one characterized by a long (> 1 ns) rectangular pulse envelope with a narrow spectrum and the other characterized by shorter sub-ns bell-shaped pulses whose Raman-enhanced spectrum extends far beyond the doped fibre gain spectrum. The existence of two distinct noise-like pulsing modes can be understood in terms of the periodic variation of the pulse spectrum along the cavity, which is able to shift the effective dispersion regime of the laser. By joining the tuning ranges of each regime, the noise-like pulse duration can be adjusted between 57 ps and 6.3 ns, and its bandwidth between 3.5 and 59 nm.

Liquid crystal polarimetry for metastability exchange optical pumping of 3He

We detail the design and operation of a compact, discharge light polarimeter for metastability exchange optical pumping of 3He gas near 1torr under a low magnetic field. The nuclear polarization of 3He can be discerned from its electron polarization, measured via the circular polarization of 668nm discharge light from an RF excitation. This apparatus measures the circular polarization of this very dim discharge light using a nematic liquid crystal wave retarder (LCR) and a high-gain, transimpedance amplified Si photodiode. We outline corrections required in such a measurement and discuss contributions to its systematic error.

Single shot interferometry using a two-interferogram phase shifting algorithm

Abstract An interferometer capable of retrieving the phase information in a single shot was implemented, the system was made by taking into account the double interferogram output obtained in a Mach Zehnder interferometer and then retrieving the phase information using a two-step algorithm already encountered in the literature. By controlling the polarization properties of the system, a new polarization phase shifting technique was implemented based upon the rotation of two quarter wave retarders placed in each of the arms of the interferometer. We present the analytical model taking into account the necessary conditions presented by the demodulation method. The configuration presented does not require micro-polarizer arrays or diffraction gratings presenting the advantage of avoiding the use of conventional polarizing filters to control the phase shift. Experimental results are presented.

Strong Birefringence Tuning of Optical Waveguides With Femtosecond Laser Irradiation of Bulk Fused Silica and Single Mode Fibers

Birefringence tunability is demonstrated in waveguides formed in bulk fused silica and in the core of single mode fibers, by femtosecond laser writing of stress inducing tracks that are placed with different geometries around the core of the waveguides. The femtosecond laser generated stress effect was probed by the birefringence induced spectral splitting of either Bragg grating waveguides in bulk fused silica or weakly modulated, femtosecond laser induced Bragg gratings in optical fibers. Birefringence values as low as 4 ×10-6 and up to 2 ×10-3 were obtained by controlling the fabrication conditions such as the laser pulse energy, the writing femtosecond laser polarization, the number of overwriting exposures, and the geometry of the induced stress tracks. Wave retarders are developed and characterized by a cross polarization technique to provide the spectral response of the stress induced birefringence, offering the convenient fabrication of short length and broadband in-line polarization devices. With this approach, millimeter length tracks provided 10 nm bandwidth polarization retarders in a single mode fiber and a 65 nm bandwidth retarders in bulk fused silica.

Birefringence dispersion of polyethylene and cellulose triacetate sheets used as photonic wave retarders

In a wide spectral range of 400 to 1000 nm, the channeled spectra formed from the interference of polarized white light are used to extract the dispersion of phase birefringence Δn(λ) of polyethylene and cellulose triacetate (CTA) polymer sheets. The Δn(λ) of polyethylene and CTA sheets is fitted to Cauchy dispersion function with relative error of order 1.2×10 −4 and 4×10 −4 , respectively. The dispersion of group birefringence Δn g (λ) of polyethylene and CTA is also calculated and fitted to Ghosh dispersion function with relative error of order 7×10 −4 and 5×10 −4 , respectively. Furthermore, the phase and group retardations introduced by polyethylene and CTA sheets are also calculated. The amount of phase retardation confirms that polyethylene and CTA sheets can act as multiple order half and quarter wave plates working at many different wavelengths through the spectral range 400 to 1000 nm. For polyethylene and CTA sheets, a large difference between group and phase retardance is observed at a short wavelength (λ≈400 nm ), while such difference progressively diminished at a long wavelength (λ≈1000 nm ). Finally, a zero order polyethylene quarterwave plate working at 544 nm and dual wavelength CTA quarterwave plate working at 510 and 680 nm are experimentally realized.

Optimal achromatic wave retarders using two birefringent wave plates

Two plates of different birefringence material can be combined to obtain an achromatic wave retarder. In this work, we achieve a correction for the overall retardation of the system that extends the relation to any azimuth. Current techniques for the design of achromatic wave retarders do not present a parameter that characterizes its achromatism on a range of wavelengths. Thus, an achromatic degree has been introduced, in order to determine the optimal achromatic design composed with retarder plates for a spectrum of incident light. In particular, we have optimized a quarter retarder using two wave plates for the visible spectrum. Our technique has been compared to previous results, showing significant improvement.

Multiple continuous-wave and pulsed modes of a figure-of-eight fibre laser

We study experimentally a figure-of-eight fibre laser including a polarization-imbalanced nonlinear optical loop mirror and a Mach–Zehnder optical filter formed by two fibre tapers placed in series. Depending on the adjustments of two wave retarders included in the setup, different modes of operation of the laser are found. In continuous-wave mode, tunable single-wavelength operation as well as multiwavelength lasing are observed. For some adjustments, self-pulsing also takes place, although the pulses are very unstable. Finally, for some adjustments a mechanical stimulation (a kick) leads to the onset of passive mode locking. Measurements reveal that the mode-locked pulses actually are noise-like pulses. Both stable fundamental mode locking and second-harmonic mode locking with particular dynamics were obtained. In this work, we analyse how simple wave plate adjustments can lead to such a variety of operational modes of the fibre laser.

Electrically Tunable Terahertz Notch Filters

In this letter we present a switchable THz notch filter. The filter contains a liquid crystal layer that acts as a half wave retarder in one state and as an isotropic layer in the other state. The device combines three unique properties: it can be switched electrically, it provides a filter depth of 35 dB at 350 GHz and it can be tuned over wide frequency range from 350 GHz to 700 GHz.

Generation of long broadband pulses with a figure-eight fiber laser

In this paper we performed the experimental and numerical study of a passively mode-locked fiber laser that generates packets of sub-picosecond pulses instead of individual pulses. The proposed configuration is a figure-eight fiber laser scheme, which includes a Nonlinear Optical Loop Mirror with polarization asymmetry inserted into a ring cavity. No experimental evidence of self-starting mode locking operation of the laser was observed; however, for proper adjustments of the wave retarders included in the setup, a mechanical stimulation triggers the onset of mode locking. The autocorrelation of the generated pulses shows a narrow sub-picosecond peak riding a large sub-nanosecond pedestal whose intensity is half that of the peak, and the optical spectrum is smooth and wide. We show that contrary to conventional ultrashort pulses, these pulses do not vanish rapidly after propagation through a long dispersive fiber, which makes then attractive for super-continuum generation as well as for applications in metrology. Finally, we study the pulse formation in the laser and present arguments based on experimental data and numerical simulations that the observed pulses are actually sets of a large number of solitons.

Plasmonic metamaterial wave retarders in reflection by orthogonally oriented detuned electrical dipoles

We demonstrate that a pair of perpendicular electrical dipolar scatterers resonating at different frequencies can be used as a metamaterial unit cell to construct a nanometer-thin retarder in reflection, designing nanocross and nanobrick plasmonic configurations to function as reflecting quarter-wave plates at ~1520 and 770 nm, respectively. The design is corroborated experimentally with a monolayer of gold nanobricks, transforming linearly polarized incident radiation into circularly polarized radiation at ~780 nm.