Calculation of Refractive Index Changes from Thermal Lens Fringes using Continuous Wavelet Algorithm

Thermal lens fringes are obtained on the nile-blue/ethanol solution by illuminating it with a He–Ne laser in a Mach–Zehnder interferometer. The refractive index change distribution of these fringes is calculated by using two different continuous wavelet transform (CWT) algorithms. It is concluded that the CWT-phase method works better than the CWT-gradient algorithm for the analysis of thermal lens fringes according to theoretical results.     

Measurement of the frequency modulation transfer function of a laser using a Mach–Zehnder interferometer

A technique is presented for determining the frequency modulation transfer function of a laser. The method is based on a Mach–Zehnder interferometer, with a significant difference in the optical path lengths of the two arms. A frequency-modulated laser beam incident on the interferometer produces a phase-modulated photocurrent signal with an effective modulation index that is related to the amplitude of the optical frequency modulation. Techniques for determining both the amplitude and the phase of the optical frequency modulation from the photocurrent signal are described.     

Nanoplasmonic directional couplers and Mach–Zehnder interferometers

We present a novel design and analysis of two nano-scale plasmonic devices: a directional coupler and a Mach–Zehnder interferometer. The designs of the two devices are based on our recent work on the air-gap coupler that resulted in high coupling efficiency between a dielectric waveguide and a plasmonic waveguide. The two devices are embedded between two dielectric waveguides and operate at optical telecom wavelengths. The overall efficiency was 37% for a 2×2 directional coupler switch and above 50% for the proposed designs for a Mach–Zehnder Interferometer. The efficiency in the proposed devices can be increased using broader plasmonic waveguides.     

Mach–Zehnder interferometers in photonic crystals

Photonic crystal technology allows the creation of optical waveguides with low sharp-bending losses as well as ultra-low group velocity. This last property is particularly interesting to develop highly-compact optical devices based on the controlled modification of the optical phase of the signals traveling through the waveguides. Among these devices, the Mach–Zehnder interferometer acquires fundamental importance because it can be used as a building block of more complex optical devices and functionalities such as optical filters, wavelength demultiplexers, channels interleavers, intensity modulators, switches and optical gates. In this paper, the performance of a Mach–Zehnder interferometer consisting of two coupled-cavity waveguides with different lengths created in a two-dimensional photonic crystal is theoretically analyzed. We also provide simulation results using a finite-difference time-domain code that confirm the theoretical analysis. The main limitations in the performance of the structure are addressed and discussed.     

Wavelength-spacing continuously tunable multi-wavelength SOA-fiber ring laser based on Mach–Zehnder interferometer

A Mach–Zehnder interferometer (MZI) which is used as a wavelength-spacing tunable comb filter in a fiber ring laser is built by employing an optical variable delay line (OVDL). Stable multi-wavelength semiconductor optical amplifier (SOA)-fiber ring laser based on an SOA and the MZI comb filter is achieved. Wavelength spacing can be continuously tuned by adjusting the OVDL and, as an example, multi-wavelength lasing with the wavelength spacing of 0.4, 0.8, or 1.6 nm is demonstrated. The output of the proposed multi-wavelength SOA-fiber ring laser is quite stable at room temperature and the output spectrum can be adjusted by controlling the bias current of the SOA.     

A high-precision optical metrology system for determining the thickness of birefringent wave plates

This study develops a non-destructive measurement system for determining the thickness and refractive indices of birefringent optical wave plates. Compared to previous methods presented in the literature, the proposed metrology system provides the ability to measure the thickness of the birefringent optical plate in high-precision. The results show that for a commercially available birefringent optical wave plate with refractive indices of n_e=1.5518, n₀=1.5427 and a thickness of 452.1428 μm, the experimentally determined value for the error in the wave plate thickness measurement is just 0.046 μm. The measurement resolution of the proposed system exceeds that of the interferometer hardware itself. The proposed method provides a simple yet highly accurate means of measuring the principal optical parameters of birefringent glass wave plates.     

Mach–Zehnder interferometer by utilizing phase modulation of transmitted light through magnetic fluid films possessing tunable refractive index

Due to its diverse applications in photonics, bio-sensors, mechanics, etc., Mach–Zehnder interferometer becomes one of important devices. Hence, lots of efforts have been paid to develop advanced Mach–Zehnder interferometers. In this work, we explore new-model Mach–Zehnder interferometer, in which one of arm is consisted of magnetic fluid films. By utilizing the tunable refractive index of magnetic fluid films under external magnetic fields, the traveling phase of a propagating light through the magnetic fluid film is changed. This could lead to a variation in the interfered intensity of the Mach–Zehnder interferometer when an external magnetic field is applied. The modulation in the interfered intensity by the external magnetic field is demonstrated experimentally, and the relevant physical origin is also discussed.     

An improved design of an integrated optical isolator based on non-reciprocalMach–Zehnder interferometry

Non-reciprocal rib waveguide structures can be used to realize integrated optical isolators. In this paper, we propose a concrete design for a Mach–Zehnder interferometer type isolator for TM modes. Just one of the arms, which are of equal length, is a non-reciprocal magneto-optic waveguide. The rest of the interferometer is reciprocal. Required fabrication tolerances are estimated, and the entire isolator is simulated by applying a finite difference beam propagation method.     

Experimental study of a semiconductor laser diode having a Mach–Zehnder interferometer in the cavity

The performance of a semiconductor laser diode that has an asymmetric Mach–Zehnder interferometer all-optical switch in the cavity has been studied experimentally. This novel device was designed to be free from clock pulse insertion, since mode-locked optical pulses are generated internally and change the balance of the interferometer periodically. The device was fabricated using a InGaAsP/InP buried heterostructure and the primary optical properties of the device were investigated. Lasing characteristics that were peculiar to the twin-cavity structure were observed, i.e., continuous-wave lasing power oscillation in relation to the injection current balance between the two arms, and cyclic changes in the single/multiple emission peaks as a function of bias voltage at the saturable absorber. Electrical spectrum analysis indicated 40 GHz modulation of lasing output from the twin-cavity laser.     

Measurements of phase retardation and principal axis angle using an electro-optic modulated Mach–Zehnder interferometer

This paper presents an electro-optic modulated circular heterodyne modified Mach–Zehnder interferometer and a convenient two-phase signal-processing algorithm for the measurement of variations in the magnitude of phase retardation and the angle of principal axis in optical materials. The developed method solves the problems of normalized intensity jump and limited phase retardation measurement range associated with the circular heterodyne interferometer proposed previously. The present method uses a saw-tooth wave signal to drive an electro-optic (EO) modulator, and employs a lock-in amplifier to demodulate the principal axis angle and the phase retardation. Specifically, this paper considers two main sources of measurement errors, namely the misorientation of the EO modulator and the reflection phase retardation of the beam splitter. Furthermore, the study develops calibration procedures and identifies a means to minimize measurement errors induced by the reflection phase retardation of the beam splitter.     

Data-dependent system approach for strain measurement in interferometric fiber optic sensor

A fiber optic interferometric sensor (Mach–Zehnder type) based on data-dependent system is presented for strain measurement. The interferogram recorded from the interferometer has been digitized and characterized by means of an autoregressive model. The phase change due to the measurand has been obtained from the self coherence function of the interferogram. The modulation of group delay and dispersion are obtained from the phase curve. Results for applied strain in the range of 5–10 μ strain are presented.     

Nonlinear effects in blue-shift waveguide EAM's

To exploit the vast bandwidth of optical communication systems for high bit-rate long-haul transmission, external modulators show a better system performance than directly modulated lasers. One of the main advantages of electroabsorption modulators (EAM's) compared with Mach–Zehnder modulators is the possibility to integrate the modulator with a laser having the same active layer. This reduces processing complexity and system costs. Usually the quantum confined Stark effect results in a red shift of the absorption, which leads to a small gain due to a detuned operation of the integrated laser. In contrast, blue shift structures have been proposed for these integrated devices as they show both good laser and modulator properties. These structures suffer from the drawback that saturation effects may occur for higher optical power as these devices absorb the optical power at low applied bias. The aim of this paper is to investigate the influence of nonlinear saturation effects like carrier accumulation, Burnstein–Moss-effect and carrier screening.     

Wave front reconstruction of Fresnel off-axis holograms with compensation of aberrations by means of phase-shifting digital holography

Off-axis holograms recorded with a CCD camera are numerically reconstructed in amplitude by calculating through the Fresnel–Kirchhoff integral. A phase-shifting Mach–Zehnder interferometer is used for recording four-quadrature phase-shifted off-axis holograms. The basic principle of this technique and its experimental verification are described. We show that the application of this algorithm allows for the suppression of the zero order of diffraction and of the twin image and that the contrast of the reconstructed images can be further enhanced by digital compensation of the aberrations introduced by the holographic recording system     

Zygo interferometer for measuring refractive index of photorefractive bismuth silicon oxide (Bi12SiO20) crystal

An application of the Zygo system for measuring the refractive index of optical materials such as bismuth silicon oxide (BSO) having a higher value of the refractive index than that of glass is presented. The setting accuracy is found to be of the order of 1 μm in the Zygo phase measuring interferometer. The mean value of refractive index of BSO crystal measured by the interferometer for several samples of thickness in the range of 3–9 mm is 2.542 at a wavelength of 632.8 nm. The accuracy of measurement is ±0.002. It is possible to achieve an accuracy of ±0.0003 for a sample of thickness of 30 mm in the measurement of refractive index due to high setting accuracy of the Zygo system.     

All-optical DGD monitor for packet-switched networks based on an integrated active Mach–Zehnder interferometer operating as logic XOR gate

An all-optical differential group delay (DGD) monitor for packet-switched networks is proposed. The monitoring approach consists of an integrated active Mach–Zehnder interferometer acting as a logic XOR gate. According to the estimated DGD value, a latching switch is employed to route the input packets “on-the-fly”. The simulation results show a successful operation which has been confirmed with the experimental validation of the XOR-based monitoring subsystem.     

Development of a time-resolved white-light interference microscope for optical phase measurements during fs-laser material processing

A modified Mach–Zehnder interferometer set-up combined with microscope objectives has been developed for the measurement of phase changes in the processed material sample, like modification and melting of glass. The white light is generated by focusing ultrafast laser radiation (t _p=80 fs) in a sapphire crystal using a micro-lens array to minimize temporal and spatial fluctuations in the white-light continuum. Lateral and coaxial pump-probe measurements of the phase changes during material processing are performed using two coupled ultrafast laser sources at different repetition rates (f _rep=1 Hz–1 MHz). The optical phase shift and therefore the refractive index of the material are calculated from the interference images using two approaches. The knowledge of the refractive index during the laser processing with a temporal resolution in the ps-range and a spatial resolution of several microns leads to a better understanding of the initial processes for the permanent material modifications.     

Theoretical analysis of all-optical 2R regeneration in a SOA-based Mach–Zehnder interferometer

All-optical 2R regeneration in a semiconductor optical amplifier-based Mach–Zehnder interferometer is theoretically analyzed. It is shown that the chirp of the regenerated signal is positive, while the input signal has no chirp. The peak extinction ratio (ER) of the regenerated signal is associated with the input wavelength and there exists an input wavelength corresponding to the largest ER of the output signal. The output peak ER is also found influenced by the input power, with lower input power getting higher peak ER.     

A Method for Measuring the Three-Dimensional Refractive-Index Distribution of Single Cells Using Proximal Two-Beam Optical Tweezers and a Phase-Shifting Mach—Zehnder Interferometer

This paper describes a method for measuring the three-dimensional (3D) refractive-index distribution in a single cell. The method can be used to observe the distribution of cell components without fluorescence staining. The two-dimensional optical path length distributions from multiple directions are obtained by non-contact rotation of the cell. These optical path lengths are converted into the line integrals of the refractive index, and the 3D refractive-index distribution is reconstructed by means of computed tomography. The refractive-index distribution in a breast cancer cell can be measured using a phase-shifting Mach—Zehnder interferometer in conjunction with proximal two-beam optical tweezers.     

Measurements of the refractive indices and refractive index increment of a synthetic PMMA solutions at 488 nm

We describe a Mach–Zehnder interferometer (MZI) method for measuring the refractive index (RI) of polymethyl-methacrylate (PMMA) solution in both acetone and methyl-ethyl-ketone (MEK). The measurements are made as a function of concentration values 4, 8, 12, 16 and 20 g/l at a wavelength of 488 nm with a high degree of accuracy tends to 1.4×10⁻⁵. The refractive index increments (RIIs) dn/dc of PMMA in both investigated solvents are determined too. In addition, the RIIs Δn as a function of concentration and the RIIs at zero concentration (dn/dc)_c=0 are determined for both solvents accurately. The PMMA solutions in acetone and MEK solvents are chosen for laser light scattering investigations.     

Determination of path length difference by low coherence interference spectrum

Path length difference is the key parameter in two-beam interferometer, especially in low coherence interferometer. It determines the visibility of the interference fringes. In this study, we present a method to determine the path length difference between two arms of a fiber optic Mach–Zehnder interferometer by evaluating the peaks of power distribution of the interference spectrum with a wide band light source. The experimental results are in close agreement with the theoretical calculations.