Waveguide Interferometer Research Articles

Novel Mach–Zehnder Interferometer Waveguide Design as a Light Delivery System for Heat-Assisted Magnetic Recording

We propose a light delivery system to the near-field transducer (NFT) of a heat-assisted magnetic recording (HAMR) head using a Mach–Zehnder interferometer (MZI) waveguide arrangement with a transverse electric (TE) mode coupled to the planar waveguide from a single mode fiber. The proposed design offers great flexibility in the coupling of light to the NFT in order to match the desired radiation pattern of the antenna and thus to optimize the energy transfer. Furthermore, it allows excitation of particular surface plasmon (SP) resonances of the transducer (quadrupole or higher) by controlling the coupling angle and the phase of the two beams of light. The optimum phase shift between the TE waveguide modes incident on the NFT can be achieved either statically by making one of the MZI arms longer/shorter compared with the other one, or dynamically by changing the mode effective index of the MZI waveguide arm through electro-optic or thermo-optic modulation. In addition, the proposed MZI waveguide arrangement enables easier coupling from a laser with coupling losses from laser to MZI waveguide being below 3 dB and the design enables an arrangement of the NFT placed either on the top of the waveguide at the termination side of the MZI or between the rib and the ridge at the maximum of the electric field of the waveguide, which maximizes the SP resonance enhancement of the NFT. Another advantage is that the magnetic-write pole, in our design, can be easily integrated on the same chip, preferably above the center of the MZI, between the two arms of the interferometer and away from the propagating mode, thus avoiding blocking of the light path.

A prototype stationary Fourier transform spectrometer for near-infrared absorption spectroscopy.

A prototype stationary Fourier transform spectrometer (FTS) was constructed with a fiber-coupled lithium niobate (LiNbO3) waveguide Mach-Zehnder interferometer (MZI) for the purpose of rapid on-site spectroscopy of biological and chemical measurands. The MZI contains push-pull electrodes for electro-optic modulation, and its interferogram as a plot of intensity against voltage was obtained by scanning the modulating voltage from -60 to +60 V in 50 ms. The power spectrum of input signal was retrieved by Fourier transform processing of the interferogram combined with the wavelength dispersion of half-wave voltage determined for the MZI used. The prototype FTS operates in the single-mode wavelength range from 1200 to 1700 nm and allows for reproducible spectroscopy. A linear concentration dependence of the absorbance at λmax = 1451 nm for water in ethanolic solution was obtained using the prototype FTS. The near-infrared spectroscopy of solid samples was also implemented, and the different spectra obtained with different materials evidenced the chemical recognition capability of the prototype FTS. To make this prototype FTS practically applicable, work on improving its spectral resolution by increasing the maximum optical path length difference is in progress.

High-sensitivity and high-Q-factor glass photonic crystal cavity and its applications as sensors.

We investigate the properties of a planar photonic crystal cavity on glass and its applications as sensors. An airbridged twofold defect cavity on Schott glass background and Gorilla glass substrate has been designed for high Q-factor up to 4459. The average sensitivity of the cavity resonance to background refractive index is 388nm/Refractive Index Unit. The resonant wavelength is sensitive to background temperature by 18.5pm/°C. The designed sensors show much higher sensitivity than those based on waveguide interferometers or photonic bandgap structures without cavity resonance. The results are also useful for experimental studies of glass photonic devices.

ALL-OPTICAL LOGIC GATES BASED ON SPATIAL-SOLITON INTERACTIONS IN OPTICAL COMMUNICATION SPECTRAL REGION

New designs of all-optical logic gates based on spatial-soliton interactions in optical communication spectral regions were proposed. The proposed structures are composed of local nonlinear Mach-Zehnder interferometer (MZI) waveguide structures with multi-input ports and two nonlinear output ports. They can be used to design various all-optical logic gates. The nonlinear MZI waveguide structure with local nonlinear waveguides functions like a phase shifter. It employs angular deflection of spatial solitons controlled by the phase modulation created in the local nonlinear MZI. The light-induced index changes in the local nonlinear MZI waveguide structures break the symmetry of structure and make the output signal beam propagate through different nonlinear output waveguides. By properly choosing the input control power, the spatial solitons will be switched to different output ports. The numerical results show that the proposed local nonlinear MZI waveguide structures could really function as all-optical logic gates in the optical communication spectral region.

End-face reflected LiNbO3 waveguide based stationary miniature Fourier transform spectrometer with two-fold enhanced spectral resolution

Miniature Fourier transform spectrometer (FTS) has attracted considerable interest because of its important application in spaceborne spectroscopy and as a portable analytical tool for rapid on-site chemical/biochemical detection. In a previous paper, a stationary miniature FTS constructed with an electro-optic (EO) modulator of a LiNbO3 (LN) waveguide Mach-Zehnder interferometer (MZI) containing push-pull electrodes was demonstrated. This stationary miniature FTS is operated in the near-infrared region with either nonlinear or linear scanning of the modulating voltage. The simple and mirrorless structure renders the device compact, vibration resistant, and cost-effective. However, the spectral resolution of the proposed prototype FTS was not satisfactory due to the limited optical pathlength difference (OPD), thereby limiting the device application. To improve its spectral resolution, the factors affecting the spectral resolution of the LN waveguide-based FTS is investigated in this paper. Findings show that the spectral resolution is inversely proportional to the maximum OPD, which is proportional to the length of the EO modulating region. A simple method for two-fold enhancement of the spectral resolution of the FTS is proposed based on the end-face reflection in LN waveguide interferometer. With the end-face reflection geometry the guided mode can propagate back and forth in the LN waveguide, making the mode passing through the EO modulating region twice and consequently leading to two times enhancement of the OPD. Therefore, the end-face reflection geometry enables to double the maximum OPD of the interferometer without increasing the device size and thus to offer the device a two-fold enhanced spectral resolution according to the equation for FTS resolution. Two prototypes of FTS with and without the end-face reflection structure are prepared using the same commercial LN waveguide EO modulator. The spectral resolutions in terms of the full-width at half maximum (FWHM) at different wavelengths for the two prototypes of FTS are measured using a series of distributed feedback lasers. The FWHM measured at a specific wavelength with the end-face reflection structure is half as large as that obtained without the end-face reflection structure. Experimental results are in excellent agreement with the theoretical data, demonstrating the applicability of the end-face reflection method to the spectral resolution enhancement.

Optical frequency comb generation by utilizing the three-branch waveguide interferometer-based single-sideband modulator with recirculating frequency shifting loop

The property of a three-branch waveguide interferometer (TBWI) used to achieve the desired single-side-band (SSB) frequency shift has been theoretically analyzed in detail. Optical frequency comb generation (OFCG) based on TBWI with a recirculating loop has been also proposed and analyzed. The simulation has been implemented to show the properties of TSSBM and TSSBM-based OFCG (TOFCG). Simulation results show that the TOFCG has the ability to generate high-quality multiple optical frequency combs by applying optimized parameters.

Spatially continuous distributed fiber optic sensing using optical carrier based microwave interferometry.

This paper reports a spatially continuous distributed fiber optic sensing technique using optical carrier based microwave interferometry (OCMI), in which many optical interferometers with the same or different optical path differences are interrogated in the microwave domain and their locations can be unambiguously determined. The concept is demonstrated using cascaded weak optical reflectors along a single optical fiber, where any two arbitrary reflectors are paired to define a low-finesse Fabry-Perot interferometer. While spatially continuous (i.e., no dark zone), fully distributed strain measurement was used as an example to demonstrate the capability, the proposed concept may also be implemented on other types of waveguide or free-space interferometers and used for distributed measurement of various physical, chemical and biological quantities.

Miniature Fourier transform spectrometer based on wavelength dependence of half-wave voltage of a LiNbO_3 waveguide interferometer

A simple and reliable spectrum-retrieval method was proposed for the development of miniature stationary Fourier transform (FT) spectrometers based on a LiNbO₃ (LN) waveguide Mach-Zehnder interferometer (MZI) modulator. The method takes into account the wavelength dependence of the optical pathlength difference (OPD) and allows us to use a nonlinear voltage ramp to modulate the OPD. The method is based on the dispersion of the half-wave voltage, which was measured to be a monotonous polynomial function of the wavelength for the LN waveguide MZI used. With the measured dispersion of the half-wave voltage, the OPD, as a linear function of the modulating voltage, can be accurately determined at each wavelength in the near-infrared region in which the MZI used is a single-mode device. A prototype FT spectrometer was prepared using a LN waveguide MZI modulator based on the above method. The experimental results demonstrated that the spectrometer can be used for accurate determination of the laser wavelength and for liquid absorptiometry.

Electro-optic fringe locking and photometric tuning using a two-stage Mach–Zehnder lithium niobate waveguide for high-contrast mid-infrared interferometry

b Universite de Franche-Comte, FEMTO-ST, Besancon 25030, France Abstract. We present an optimization process to improve the rejection ratio in integrated beam combiners by locking the dark fringe and then monitoring its intensity. The method proposed here uses the electro-optic effect of lithium niobate in order to lock the dark fringe and to real-time balance the photometric flux by means of a two- stage Mach-Zehnder interferometer waveguide. By applying a control voltage on the output Y-junction, we are able to lock the phase and stay in the dark fringe, while an independent second voltage is applied on the first- stage intensity modulator, to finely balance the photometries. We have obtained a rejection ratio of 4600 (36.6 dB) at 3.39 μm in transverse electric polarization, corresponding to 99.98% fringe contrast, and shown that the system can compensate external phase perturbations (a piston variation of 100 nm) up to around 1 kHz. We also show the preliminary results of this process on wide-band modulation, where a contrast of 38% in 3.25- to 3.65-μm spectral range is obtained. These preliminary results on wide-band need to be opti- mized, in particular, for reducing scattered light of the device at the Y-junction. We expect this active method to be useful in high-contrast interferometry, in particular, for astronomical spatial projects actually under study.

Integrated Optical $E$-Field Sensor for Intense Nanosecond Electromagnetic Pulse Measurement

A lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-based integrated optical E-field sensor utilizing Mach-Zehnder optical waveguide interferometer and bow-tie antenna has been designed and fabricated for the detection of nanosecond intense pulsed electric field. Experimental data demonstrate that the average rise time, fall time, and pulsewidth of the input/output electromagnetic pulse (EMP) are 1.31/1.28, 3.63/3.61, and 34.19/34.25 ns, respectively. Correspondingly, the relative errors are 2.3%, 0.6%, and 0.2%, respectively. In addition, the input/output of the sensor system shows a linear relationship as the correlation coefficient between measured and fitting is 0.9991. The minimum and maximum measured EMP fields are approximately equal to 3 and 50 kV/m, respectively.

Proposal of in-line wavelength-selective modulator based on waveguide interferometer

Proposal of in-line wavelength-selective modulator based on waveguide interferometer

Dynamic range enhancement and phase-ambiguity elimination in wavelength-interrogated interferometric sensor

We present a method to resolve the phase-ambiguity problem in interferometer sensors. The proposed method is based on operating a wavelength-interrogated two-wave interferometric sensor close to the critical sensitivity point. We show that in this vicinity, changes in the transmission spectrum take place at two sensitivity scales. One of them is highly sensitive and periodic while the other scale has low sensitivity but is monotonic and quasi-linear and serves for recording the phase periods. The dynamic range of the sensor can thus be increased by 1–2 orders of magnitude as compared to ordinary unbalanced interferometric sensors. The method thus avoids the need for the continuous tracking of data while sensing. Critical sensitivity can exist in a hetero-modal interferometer where the propagation takes place within paths for which the dispersion properties are different. Calculations and detailed numerical simulations are presented for the design of a porous-silicon (PSi) waveguide interferometer which exhibits critical sensitivity. The waveguides of the interferometer are of rib-type and are composed of a guiding layer of low porosity on a cladding layer with higher porosity.

Research technique of metamaterials with the controlled effective negative refractive index and their characteristics

A design of the two-beam hybrid microwave interferometer for measuring refractive indices of metamaterials is described. It is based on the branching of radiation using a waveguide directional coupler. Radiations of the interferometer waveguide arms are summed and the phase disbalance of the interferometer arms is counted on the waveguide measuring line. The frequency dispersion of the negative refractive indices of the metamaterial samples with a variable distance between its constituent plates is experimentally investigated. A possibility of controlling the refractive index of a metamaterial is demonstrated.

Self-repairing, interferometric waveguide sensor with a large strain range

We demonstrate a polymer waveguide, Fabry-Perot interferometer strain sensor fabricated through a self-writing process in a photopolymerizable resin bath between two silica optical fibers. The measurable strain range is extended through sensor self-repair and strain measurements are demonstrated up to 150% applied tensile strain. The sensor fabrication and repair is performed in the ultraviolet wavelength range, while the sensor interrogation is performed in the near-infrared wavelength range. A hybrid sensor is fabricated by splicing a short segment of multimode optical fiber to the input single-mode optical fiber. The hybrid sensor provides the high quality of waveguide fabrication previously demonstrated through self-writing between multimode optical fibers with the high fringe visibility of single-mode propagation. The peak frequency shift of the reflected spectrum Fabry-Perot sensor is extremely linear with applied strain for the hybrid sensor, with a sensitivity of 2.3×10(-3) per nanometer per percent strain. The calibrated peak frequency shift with applied strain is the same for both the original sensor and the repaired sensor; therefore, the fact that the sensor has self-repaired does not need to be known. Additionally, this calibration is the same between multiple sensor fabrications. In contrast to a conventional air gap Fabry-Perot cavity sensor, no decrease in the fringe visibility is observed over the measurable strain range.

Size-selective detection in integrated optical interferometric biosensors

We present a new size-selective detection method for integrated optical interferometric biosensors that can strongly enhance their performance. We demonstrate that by launching multiple wavelengths into a Young interferometer waveguide sensor it is feasible to derive refractive index changes from different regions above the waveguide surface, enabling one to distinguish between bound particles (e.g. proteins, viruses, bacteria) based on their differences in size and simultaneously eliminating interference from bulk refractive index changes. Therefore it is anticipated that this new method will be ideally suited for the detection of viruses in complex media. Numerical calculations are used to optimize sensor design and the detection method. Furthermore the specific case of virus detection is analyzed theoretically showing a minimum detectable virus mass coverage of 4 × 10(2) fg/mm(2) < (typically corresponding to 5 × 10(1) particles/ml).

Inductively coupled plasma etching to fabricate sensing window for polymer waveguide biosensor application

Integrated polymeric waveguide biosensors are usually used for the detection of chemical/biological samples. To improve the sensitivity of the conventional waveguide sensors with only one sensing surface, we design and fabricate an improved sensing window with three sensing surfaces based on inductively coupled plasma etching method, whose sensitivity can be enhanced by a factor of 2.8 in theory. The effects of etching parameters, especially the antenna power, the bias power and the O2 flow rate were systematically studied and the parameters were optimized to fabricate the sensing window. A single-mode balanced Mach–Zehnder interferometer waveguide biosensor with the improved sensing window was fabricated under this optimum etching condition, and presented a higher sensitivity than that of the sensor with conventional sensing window. This type of sensing window will be very useful in polymer waveguide biosensor devices.

Integrated Optical Electric Field Sensor From 10 kHz to 18 GHz

A lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )-based integrated optical sensor with Mach-Zehnder optical waveguide interferometer and tapered antenna array was designed and fabricated for electric field detection. Experimental results reveal that this sensor has a frequency response deviation less than ±10 dB within the range of 10 kHz to 18 GHz, and its minimum detectable electric field intensity is 0.4 V/m. Moreover, a pulsed electric field with nanoseconds width and electric field intensity of 103 V/m have been detected by this sensor.

Measuring protein concentration with entangled photons

Optical interferometry is amongst the most sensitive techniques for precision measurement. By increasing the light intensity, a more precise measurement can usually be made. However, if the sample is light sensitive entangled states can achieve the same precision with less exposure. This concept has been demonstrated in measurements of known optical components. Here, we use two-photon entangled states to measure the concentration of a blood protein in an aqueous buffer solution. We use an opto-fluidic device that couples a waveguide interferometer with a microfluidic channel. These results point the way to practical applications of quantum metrology to light-sensitive samples.

Direct Wafer Bonding and Its Application to Waveguide Optical Isolators.

This paper reviews the direct bonding technique focusing on the waveguide optical isolator application. A surface activated direct bonding technique is a powerful tool to realize a tight contact between dissimilar materials. This technique has the potential advantage that dissimilar materials are bonded at low temperature, which enables one to avoid the issue associated with the difference in thermal expansion. Using this technique, a magneto-optic garnet is successfully bonded on silicon, III-V compound semiconductors and LiNbO3. As an application of this technique, waveguide optical isolators are investigated including an interferometric waveguide optical isolator and a semileaky waveguide optical isolator. The interferometric waveguide optical isolator that uses nonreciprocal phase shift is applicable to a variety of waveguide platforms. The low refractive index of buried oxide layer in a silicon-on-insulator (SOI) waveguide enhances the magneto-optic phase shift, which contributes to the size reduction of the isolator. A semileaky waveguide optical isolator has the advantage of large fabrication-tolerance as well as a wide operation wavelength range.

Waveguide Optical Isolators for Integrated Optics

Waveguide optical isolators are investigated for an application to integrated optics. A nonreciprocal loss shift isolator has the advantage of high compatibility with optical active devices. The interferometric waveguide optical isolator that uses nonreciprocal phase shift brought about by the first-order magneto-optic effect is applicable to a variety of waveguide platforms. In a silicon-on-insulator (SOI) waveguide, the low refractive index of buried oxide layer enhances the magneto-optic phase shift, which contributes to reduce the device size of isolator. We developed the surface activated direct bonding technique to integrate a magneto-optic garnet crystal onto dissimilar crystals. The performance of SOI waveguide optical isolator was demonstrated with an optical isolation of 21 dB at a wavelength of 1559 nm.