Integrated Optical Chip Research Articles

Single-mode waveguides for GRAVITY

Context. Within the framework of the second-generation instrumentation of the Very Large Telescope Interferometer of the European Southern Observatory we have developed the four-telescope beam combiner in integrated optics. Aims. We optimized the performance of such beam combiners, for the first time in the near-infrared K band, for the GRAVITY instrument dedicated to the study of the close environment of the galactic centre black hole by precision narrow-angle astrometry and interferometric imaging. Methods. We optimized the design of the integrated optics chip and the manufacturing technology as well, to fulfil the very demanding throughput specification. We also designed an integrated optics assembly able to operate at 200 K in the GRAVITY cryostat to reduce thermal emission. Results. We manufactured about 50 beam combiners by silica-on-silicon etching technology. We glued the best combiners to single-mode fluoride fibre arrays that inject the VLTI light into the integrated optics beam combiners. The final integrated optics assemblies have been fully characterized in the laboratory and through on-site calibrations: their global throughput over the K band is higher than 55% and the instrumental contrast reaches more than 95% in polarized light, which is well within the GRAVITY specifications. Conclusions. While integrated optics technology is known to be mature enough to provide efficient and reliable beam combiners for astronomical interferometry in the H band, we managed to successfully extend it to the longest wavelengths of the K band and to manufacture the most complex integrated optics beam combiner in this specific spectral band.

Correction of Y-Branches on Proton-exchanged Waveguides in Lithium Niobate By Femtosecond Writing Technology

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Large and accessible conductivity of charged domain walls in lithium niobate

Ferroelectric domain walls are interfaces between areas of a material that exhibits different directions of spontaneous polarization. The properties of domain walls can be very different from those of the undisturbed material. Metallic-like conductivity of charged domain walls (CDWs) in nominally insulating ferroelectrics was predicted in 1973 and detected recently. This important effect is still in its infancy: The electric currents are still smaller than expected, the access to the conductivity at CDWs is hampered by contact barriers, and stability is low because of sophisticated domain structures or proximity of the Curie point. Here, we report on large, accessible, and stable conductivity at CDWs in lithium niobate (LN) crystals – a vital material for photonics. Our results mark a breakthrough: Increase of conductivity at CDWs by more than 13 orders of magnitude compared to that of the bulk, access to the effect via ohmic and diode-like contacts, and high stability for temperatures T ≤ 70 °C are demonstrated. A promising and now realistic prospect is to combine CDW functionalities with linear and nonlinear optical phenomena. Our findings allow new generations of adaptive-optical elements, of electrically controlled integrated-optical chips for quantum photonics, and of advanced LN-semiconductor hybrid optoelectronic devices.

Ultrafast laser inscription in ZBLAN integrated optics chips for mid-IR beam combination in astronomical interferometry.

Astronomical interferometry is a unique technique that allows observation with angular resolutions on the milliarcsec scale by combining the light of several apertures hundreds of meters apart. The PIONIER and GRAVITY instruments at the Very Large Telescope Interferometer have demonstrated that silica-based integrated optics (IO) provide a small-scale and highly stable solution for the interferometric beam combination process. Yet, important science cases such as exoplanet hunting or the spectroscopic characterization of exoplanetary atmospheres are favorable for observation in the mid-IR, namely the atmospheric windows L and L' band (3-4 µm), a wavelength range that is not covered by conventional silica-based IO. Here, we propose laser-inscribed IO 2×2 couplers in ZBLAN and experimentally assess the critical properties of the component for broadband mid-IR interferometry. We measure the splitting ratio over the 2.5 to 5.0 µm range and find excellent broadband contrast over the L (3.1-3.6 µm) and L' (3.6 - 4.0 µm) bands. Furthermore, we quantify the dispersion properties of the coupler and find a phase variation as low as 0.02 rad across the L and L' band, respectively. By optimizing the NA of our injection beam, we measured a very high total throughput of 58% over the L band including Fresnel reflection and coupling losses. We also compare our findings to recent advances in mid-IR IO in GLS and discuss its advantages and disadvantages for the implementation in future mid-IR interferometers.

Measurement error analysis for polarization extinction ratio of multifunctional integrated optic chips.

Measurement error for the polarization extinction ratio (PER) of a multifunctional integrated optic chip (MFIOC) utilizing white light interferometry was analyzed. Three influence factors derived from the all-fiber device (or optical circuit) under test were demonstrated to be the main error sources, including: 1)the axis-alignment angle (AA) of the connection point between the extended polarization-maintaining fiber (PMF) and the chip PMF pigtail; 2)the oriented angle (OA) of the linear polarizer; and 3)the birefringence dispersion of PMF and the MFIOC chip. Theoretical calculations and experimental results indicated that by controlling the AA range within 0°±5°, the OA range within 45°±2° and combining with dispersion compensation process, the maximal PER measurement error can be limited to under 1.4dB, with the 3σ uncertainty of 0.3dB. The variations of birefringence dispersion effect versus PMF length were also discussed to further confirm the validity of dispersion compensation. A MFIOC with the PER of ∼50 dB was experimentally tested, and the total measurement error was calculated to be ∼0.7 dB, which proved the effectiveness of the proposed error reduction methods. We believe that these methods are able to facilitate high-accuracy PER measurement.

Parameter optimization analysis to minimize the polarization error in a localized thermal tunable fiber ring resonator gyro.

The accuracy of the resonant frequency servo loop is a major concern for the high-performance operation of a resonant fiber optic gyro. For instance, a bias error as large as tens or even hundreds of degrees/hour has been observed at the demodulated output of the resonant frequency servo loop. The traditional frequency servo mechanism is not an efficient tool to address this problem. In our previous work, we proposed a novel method to minimize the laser frequency noise to the level of the shot noise by refractive index modulation by a thermally tunable resonator. In this paper, we performed the parameter optimization for the resonator coil, multifunction integrated-optics chip, and couplers by the transition matrix using the Jones matrix methodology to minimize the polarization error. With the optimized parameter values, we achieved the bias value of the resonator fiber optic gyro to 1.924°/h.

Study on the Drift of Modulated Phase in Interference Fiber Optic Gyroscope

Integrated optical chip (IOC) is an essential component in the closed-loop interference fiber optic gyroscope(IFOG) system. One of the main functions of IOC in IFOG is to modulate the phase, which includes the π/2 phase biased by square waves and Sagnac phase biased by ladder waves. In fact, both square waves and ladder waves have high frequency, including multiple harmonic waves and being disturbed easily, which induce the drift of modulated phase in IOC, and has great effect on IFOG performance, such as zero bias and drift. This paper analyzes theoretically the relationship between two kinds of modulated phases and the performance of IFOG drift, and searches for the error information in two kinds of modulated phases. Focusing on different error factors in IOC, the system is optimized in aspects of power source, clock, electromagnetic compatibility (EMC), signal processing and so on, to restrain phase drift error of 2π, then to improve the bias stability of IFOG. Signals of the power source, clock, EMC, and signal processing and so on are tested, the test results show that all these methods are effective to restrain the drift of modulated phase in IOC, the system is stable and reliable. The static testing data of FOG is obtained in Three-axis Turntable, and the static testing data is analyzed quantitatively by using the method of Allan variance to identify random errors related to the drift of modulated phase from all kinds of error sources in the FOG system. The result is shown that the bias stability of the FOG system is better than 0.03°/hr. We also obtained the dynamic testing data of FOG in Three-axis Turntable, the result is shown that scale factor stability of the FOG system is better than 25ppm. To sum up, the whole performance of the IFOG system can meet the requirements of inertial navigation system (INS) with high precision.

Accurate Evaluation of Polarization Characteristics in the Integrated Optic Chip for Interferometric Fiber Optic Gyroscope Based on Path-matched Interferometry

Accurate evaluation of polarization characteristics in the integrated optic chip (IOC) for interferometric fiber optic gyroscope was performed. Spatial distribution of optical wavetrains caused by the polarization parameters such as local polarization cross-coupling and polarization rejection coefficient of the IOC were measured utilizing the path-matched optical coherence domain polarimetry (PM-OCDP). With the analytic model deduced from Jones matrix representation, we could accurately identify the polarization characteristics of the IOC. Both degree of measurement error due to the imperfect equipment conditions in PM-OCDP and birefringence of IOC chip were also characterized.

Optimisation of distributed feedback laser biosensors

A new integrated optical sensor chip is proposed, based on a modified distributed- feedback (DFB) semiconductor laser. The semiconductor layers of different refractive indices that comprise a laser form the basis of a waveguide sensor, where changes in the refractive index of material at the surface are sensed via changes in the evanescent field of the lasing mode. In DFB lasers, laser oscillation occurs at the Bragg wavelength. Since this is sensitive to the effective refractive index of the optical mode, the emission wavelength is sensitive to the index of a sample on the waveguide surface. Hence, lasers are modelled as planar waveguides and the effective index of the fundamental transverse electric mode is calculated as a function of index and thickness of a thin surface layer using the beam propagation method. We find that an optimised structure has a thin upper cladding layer of ~0.15 mum, which according to this model gives detection limits on test layer index and thickness resolution of 0.1 and 1.57 nm, respectively, a figure which may be further improved using two lasers in an interferometer-type configuration.

A polarizer chip based on CMOS Cu-interconnect for optical telecommunications

The construction by CMOS Cu-interconnect fabrication technology of an optical chip that performs the function of linear polarization is described. The chip consists of three rows of a one dimensional array of rods in a square lattice structure. The rods make use of the Cu-interconnects of a CMOS IC for this optical purpose. The whole structure embedded inside a slab waveguide. The optical properties of the device in the optical communication wavelength range (1.475 microm ~ 1.625 microm) are simulated using the finite-difference-time-domain (FDTD) approach. Under double-cascaded operation, a linear polarization ratio as high as 10(6) can be achieved. Our study shows that the CMOS IC technology can be used a tool for mass fabrication of passive integrated optical chips for optical communications.

Sea of Polymer Pillars Electrical and Optical Chip I/O Interconnections for Gigascale Integration

Optical chip-to-chip communication is a promising technology that can mitigate some of the performance short-comings of electrical interconnections, especially bandwidth. Moreover, future high-performance chips are projected to drain hundreds of amperes of supply current. To this end, it is important to develop a high-density and high-performance integrated electrical and optical chip I/O interconnection technology. We describe sea of polymer pillars (or polymer pins), which enables the simultaneous batch fabrication of electrical and optical I/O interconnections at the wafer-level. The electrical and optical I/O interconnections are designed to be laterally compliant to minimize the stresses on the die's low-k dielectric as well as to maintain optical alignment between the coefficient of thermal expansion (CTE)-mismatched board and die during thermal cycling. We demonstrate the fabrication and mechanical performance of various size and aspect ratio electrical and optical polymer pillars. We also describe methods of fabricating polymer pillars with nonflat tip surface area for optical interconnection.

Integration of microfluidics with a four-channel integrated optical Young interferometer immunosensor

This report describes an optical sensing hybrid system obtained by bonding a microfluidic system to an integrated optical (IO) four-channel Young interferometer (YI) chip. The microfluidic system implemented into a glass plate consists of four microchannels with cross-sectional dimensions of 200 μm×15 μm. The microfluidic system is structured in such a way that after bonding to the IO chip, each microchannel addresses one sensing window in the four-channel YI sensor. Experimental tests show that the implementation of the microfluidics reduces the response time of the sensor from 100 s, as achieved with a bulky cuvette, to 4 s. Monitoring the anti-human serum albumine/human serum albumine (α-HSA/HSA) immunoreaction demonstrates the feasibility to use the microfluidic sensing system for immunosensing applications. In this case, a better discrimination between the bulk refractive index change and the layer formation can be made, resulting into higher accuracy and offering the prospect of being able to use the kinetics of the immunoreaction. The microfluidic sensing system shows an average phase resolution of 7×10 −5×2π for different pairs of channels, which at the given interaction length of 4 mm corresponds to a refractive index resolution of 6×10 −8, being equivalent to a protein mass coverage resolution of 20 fg/mm 2.

Multiaxial beam combiner for optical coherence tomography

We propose a new interferometric approach for optical coherence tomography applications. The device uses the Young's-hole principle to produce the fringes. The low-cost and versatile ion-exchange technology has been chosen to fabricate an integrated optics chip. This new approach has allowed us to eliminate optical delay lines presenting mechanical instabilities. We present fringes using both laser and white light.

Wavelength-interrogated optical sensor for biochemical applications

A novel miniature integrated-optical sensor for versatile multichannel applications is described. Wavelength-modulation techniques using laser diodes (vertical-cavity surface-emitting lasers) are used to interrogate multiple-waveguide sensing regions on a single integrated-optical chip for accurate measurement of effective refractive-index changes at a high data rate. With the experimental miniature sensor system, a resolution of DN(PP) = 10(-7) (short term, peak to peak) of the effective refractive index was demonstrated. In terms of surface-mass coverage, this resolution corresponds to D?(PP) = 130 fg/mm(2).

Proton‐induced degradation in interferometric fiber optic gyroscopes

Recent advances in Interferometric Fiber Optic Gyroscope (IFOG) technology have enabled these devices to equal, and in some respects exceed, the performance of the floated, spinning wheel rate integrating gyroscope. However, their ability to perform in a space radiation environment has been a significant concern. Test results are presented addressing the effects of space radiation on the performance of a high precision pointing grade IFOG. Proton-induced degradation of the optical components of an IFOG is evaluated based on testing performed at the Harvard Cyclotron Laboratory (HCL). Rationale is provided for using the HCL proton accelerator as a reasonable simulation of the space environment. An analysis is presented which prioritizes the component-level dose tests based on expected radiation sensitivities. The evaluation addresses both total dose (to about 12 krad) and dose rate effects. Testing was performed at the component level as well as the system level with an expanded version of a closed-loop operational IFOG. Primary concerns include permanent attenuation and spectral transmission (wavelength) sensitivity to total dose, and angle random walk and bias stability degradation as a function of dose rate. Component level results are presented for a superfluorescent light source, integrated optics chip (IOC), coupler, and polarization maintaining fiber coils. Closed-loop transient noise results are evaluated based on dose rate testing of the IOC, coupler, and fiber coil.

Miniature integrated-optical wavelength analyzer chip

A novel integrated-optical chip suitable for realizing compact miniature wavelength analyzers with high linear dispersion is presented. The chip performs the complete task of converting the spectrum of an input beam into a corresponding spatial irradiance distribution without the need for an imaging function. We demonstrate the feasibility of this approach experimentally by monitoring the changes in the mode spectrum of a laser diode on varying its case temperature. Comparing the results with simultaneous measurements by a commercial spectrometer yielded a rms wavelength deviation of 0.01 nm.

Design procedures for passive planar coupled waveguide devices

Many useful optical systems make use of coupled waveguides configured in nonplanar arrangements. This paper deals with the emulation of such systems in planar waveguide arrangements, so that otherwise nonplanar systems can be implemented as compact, integrated optics chips. As an example, design procedures are presented for a wholly planar 3*3 coupler to emulate the symmetric 3*3 fibre coupler; the first procedure is readily implemented yet holds only for weakly coupled guides, whereas the second is unwieldy but can accommodate arbitrarily strongly coupled guides. This discussion leads to general remarks about passive planar coupled waveguide devices and it is proven that, in theory, any lossless nonplanar coupled waveguide device's behaviour can be emulated by a wholly planar coupled waveguide device. Lastly, using these ideas, implementations of planar emulations of the symmetric 4*4 and 5*5, fibre couplers are presented.

High-performance integrated-optical chip for a broad range of fiber-optic gyro applications

A high-performance integrated-optical device for a broad range of fiber-optic gyro applications is reported. The circuit contains four directional couplers and six phase modulators that form the fiber-loop coupler, the bias and serrodyne phase modulators, and an on-chip interferometric serrodyne phase detector. The chip was fabricated by the annealed-proton-exchange method in X-cut LiNbO(3), which yielded a highly polarizing guided circuit with a polarization extinction ratio of at least 60 dB. Other performance parameters include low half-wave voltages (3.5-5.5 V) at 1.3 microm and splitting and tapping directional coupler parameters well within the design needs required by application.

Integrated-optical circuit for fibre gyroscope applications

An integrated-optic chip containing two acousto-optic frequency shifters and a TE-pass polariser is presented. All the elements needed for a fibre-optic gyroscope using the frequency-shift method are realised on one LiNbO3 chip.

Application of guided-wave optics in signal processing and sensing

Research in the use of guided-wave optics for signal processing and sensing is reviewed, and advantages and limitations of the technology are discussed. The signal processors employ electrooptic, acoustooptic, and fiber-optic components to perform such functions as spectral analysis of radio-frequency signals, correlation and matched filtering, code and waveform synthesis, signal delay and storage, and analog-to-digital and digital-to-analog conversion. In most cases, the guided-wave approach is distinguished by the ability to perform a particular function at very high analog bandwidths or digital data rates. The second part of the review is concerned with sensor applications for guided-wave elements fabricated on planar substrates (integrated optics). Waveguides on an integrated optics chip have been used for sensing temperature, humidity, electric field, wavefront angle, and optical disc reflectivity. Also described are integrated optic phase and frequency shifters developed for use in fiber-optic sensors. In concluding remarks, an effort is made to provide some perspective on competing technologies and to indicate some areas where future research might prove fruitful.