The DARWIN breadboard cryogenic optical delay line

Modern optical communication systems require tunable optical delay lines for synchronization of data packages. Chirped fiber grating are typically used as a dispersive element for this application. Fiber-based schemes, however, suffer from bulkiness, slow response and/or low time resolution. Integrated photonics on silicon is one alternative approach. Several more advanced device architectures have been subsequently demonstrated on silicon, including ringresonator and photonic crystal (PhC) line-defect waveguide types. The coauthors of this paper have recently proposed a novel class of tunable optical delay lines based on apodized grating waveguides. It was shown that high bit-rate operation is achievable by cascading two complementary (inward and outward) apodized gratings. In this paper, we have experimentally demonstrated our proposed approach and studied performance of the fabricated delay lines for optical telecommunication applications. The bit-rate was extracted from broadening of transform-limited input pulses of the measured dispersion of the delay lines. Characterization of the compact delay lines shows that the single grating devices offer true-time delays as long as 164 ps, tuning range of ~ 81 ps, and a minimum bit rate of ~ 15 Gb/s. The cascaded scheme offer true-time delays of 82 ps and tuning range of 32 ps and can potentially operate at bit rates as high as 107 Gb/s.

A key technique for optical aperture synthesis instruments is the cophasing of the beams from the individual apertures. A cophasing system includes a number of delay lines for the adjustment of the optical path lengths. The design of a high-resolution delay line is based on the mission requirements of a space-borne interferometer. A cat's-eye type of retroreflector is the selected option for the optics. Three delay lines, including actuators and control electronics, have been built and have been tested in a dedicated setup, which includes a star simulator and a cophasing interferometer. The test results support the feasibility of the presented delay line concept.

In this paper, we demonstrate the relation between the amount of slowing and the amount of insertion loss in an optical slow wave structure when the material used has finite absorption. The delay properties are calculated as a function of wavelength for a single Fabry-Perot resonator and a delay line consisting of multiple coupled resonators. In all of these delay lines, if the amount of delay increases, the insertion loss of the line likewise increases. For low material loss, the normalized increase in delay is equal to the normalized increase in insertion loss. We also compare a coupled resonator delay line to a simple waveguide made of the same material and with the same amount of delay. The simple waveguide always has less insertion loss than the coupled resonator line because of the unavoidable reflection from the coupled resonator line. Even in the case of an asymmetric mirror delay line with zero reflection at the input, the simple waveguide has less insertion loss.

Highly stable and small size local oscillators is a very important component of any remote sensing, imaging, and telecommunication systems. The key factor influencing the frequency stability is quality factor of the resonance structure. Related close-in to carrier phase noise for offset frequencies above active device corner frequency reduces at least at a rate of 20 dB/decade while offset below corner frequency increases at rate of 30 dB/decade. A new approach in realization of the electrical oscillators is to augment electrical quality factor by optical energy storage components (e.g., fiber optic delay lines and ring resonators) to enhance effective quality factor (Q) to 10^4-10^6, well beyond available electrical Q of 2000 in dielectric resonator oscillators (DRO). Moreover, stability of resonance frequency has been impacted by the environmental induced (e.g., piezo and pyro properties of material) sensitivities. In addition, micro-scale design approaches are reported using multi-mode (both semiconductor and solid state) lasers for low phase noise intermodal oscillations. Finally, forced oscillation techniques of injection locking, phase locking, and mode-locking are attractive to further stabilize electrical and optical oscillation methods. In this thesis, development of broadband stable frequency sources is reported covering both X- and K-band using forced opto-electronic oscillator technique. The broadband frequency tuning is achieved using coarse electrical tuning of YIG bandpass filter combined with fine-tuned optical transversal filter using dispersive delay component and wavelength control of a fiber laser, as optical source. Forced opto-electronic oscillation techniques of self-injection locking (SIL), self-phase locked loop (SPLL), and self-injection locked phase locked loop (SILPLL) are optimized for further frequency stabilization of a frequency synthesizer. In SILPLL application, long delay line is crucial for substantial phase noise reduction; therefore, kilometer long optical fiber delay lines have been used to construct the optical feedback since their loss is extremely low compared to electrical delay lines. Meantime, open loop gain is another key parameter to achieve low phase noise in these feedback systems, as the phase noise reduction is proportional to phase locking loop gain. The phase noise and pull-in time can be greatly reduced in SILPLL as opposed to SIL or SPLL alone. A forced oscillation of fixed frequency DRO at 10 GHz system provide -137 dBc/Hz phase noise at 10kHz offset with under 10 fs aperture jitters, while OEO based X-band and K-band synthesizers provide phase noise of -137 dBc/Hz (<10 fs jitters) and -127 dBc/Hz (<12 fs jitters) at offset 10 kHz respectively in a computer controlled 19" rack-mountable OEO systems. After realization of the desired modular frequency synthesizer, development a compact opto-electronic oscillator system is demonstrated in my PhD study. Monolithic design and testing of multi-section quantum well lasers are reported on 5x2 mm2 InP chip using a commercial foundry service (i.e., SmartPhotonics). The designed lasers consist of 4 major sections of semiconductor optical amplifier (SOA), optical phase modulator (PM) and distributed Bragg reflector (DBR) mirrors. The DBR section provides wavelength selectivity while phase section and gain medium provide output wavelength tuning. An electro-absorption modulator (EAM) is prepared for mode number control using its wavelength sensitive absorption coefficient. The experiments supported predicted performance of this multi-mode laser structure and impact of combined forced oscillation of intermodal oscillations with mode locking method to realize a frequency stabilized and tunable RF signal source. Meantime, forced technique SILPLL and optimum mode locking techniques are applied in order to achieve improved phase noise performance. The free-running inter-modal oscillation frequencies suffers from poor phase noise of -5 dBc/Hz at 1 kHz and -32 dBc/Hz at 10kHz offset around 11.54 GHz, while under forced oscillation a -58 dBc/Hz at 1 kHz offset and -98 dBc/Hz at 10 kHz offset are achieved. A frequency tuning range of 800 MHz is also measured. In mode locked laser operation, the RF beat-notes forced with SILPLL topology provides a better performance. Improved phase noise of -79 dBc/Hz at 1kHz, -110 dBc/Hz at 10 kHz is measured with under 50 fs aperture jitters, while a broadband frequency tuning from 11 GHz to 13 GHz range is measured. The tuning range is potentially to be further improved, if the shared DBR laser pair symmetry could be improved. In summary, this thesis provides the solution for building RF synthesizer using forced opto-electronic oscillator and laser beat-notes oscillation output. The experimental results are corroborated with detailed analytical and numerical modeling to optimize oscillator phase noise performance. The modular design solution shows great stability and low phase noise, while the compact monolithic design solutions are quite attractive for a smaller size. Potential for full integration of multi-mode multi-section MQW laser with optical delay lines are also presented as a logical development path towards a compact frequency synthesizer.

To make compact delay lines it is essential to have low-loss single-mode straight guides, and compatible low-loss, small radius comers (or mirrors). Straight guide losses of a few tenths of a dB per centimetre have been achieved in III-V semiconductor waveguides, but these are not usually compatible with small radius comers [1,2]. Altematively, very low excess comer losses have been achieved with a bend radius as small as 30 µm using deep-etched guides, but the corresponding straight guide losses were very high [3]. In this paper we use deep-etched, vertical-walled GaAs/AlGaAs waveguides (λ= 1.064 µm). We compare sets of straight guides and corners for two wafer designs: (1) a smaller-guide wafer, with a 2.6 µm single-mode width; and (2) a larger-guide wafer, with a 4.2 µm single-mode width. We also compare measured losses of different waveguide comer designs, and demonstrate the advantages of using corners with an optimised near-hyperbolic shape, designed to minimise straight to curved guide mode mismatch. We show, for example, results for an optimised 90° comer, which has an excess loss of 0.5 dB for an equivalent radius of 100 µm.

A continuously tunable integrated optical delay line architecture with large bandwidth and bandwidth-delay product is proposed and demonstrated. The non-resonant delay line is based on cascaded Mach-Zehnder interferometers with tunable couplers. The device is operated with a single control signal and outperforms the common ring-resonator-based delay lines in terms of bandwidth-delay product. A device with a delay tuning range of 124 ps is experimentally demonstrated over a transmission bandwidth of 4.5 GHz.

A cophasing system architecture has been derived for an imaging multi-aperture optical space interferometer. This system is based on cophasing on a guide star. Breadboarding and testing of the main part of the system, with emphasis on the critical items (delay line, cophasing interferometer, Optical Path Difference (OPD retrieval methods), has provided an evaluation of the expected performance. This paper describes the reference interferometry mission used for the study, the overall OPD stabilization concept, the cophasing system and associated OPD restitution algorithms, and the first test results.

In this paper, we present a study of integrated optical delay lines (DLs) for application in optical time-division multiplexers. The investigated DLs are formed by spirally folded waveguides. The components were designed in a generic approach and fabricated in multi-project wafer runs on an InP-based platform. The design process and rules, together with characterization results of test structures, are discussed. Static and dynamic measurements were performed for verification of the DLs' performance in the wavelength and time domain. Additionally, a comparison between the simulation and characterization results is given, which confirms good agreement between measured and designed values.

Rapid improvement in built-in-test (BIT) system capability is probably one of the most important activities in an air-vehicle system development test and evaluation program. An effective BIT adequacy evaluation methodology can accomplish this. The methodology must provide the development tester BIT parameters that serve as indicators of air-vehicle BIT system performance. Deficiencies that are pinpointed can then be corrected to improve the overall BIT system's performance. A BIT adequacy evaluation methodology developed and implemented during a recently conducted Air Force development and test program did just that. It provided real-time technical information at a particular period that indicated the status of the BIT system in relation to specified requirements. It also served as a springboard to evaluate the BIT system, and to document the results, which provided the manufacturer with valuable data on the BIT system's performance. Finally, it gave the manufacturer the opportunity to implement corrective actions within the allocated schedule, and develop the air-vehicle BIT system concurrently.

The operation of harmonic SAW delay lines up to 2 GHz has previously been reported [l], [2], using stepped- finger interdigital transducers (IDT) without sub-micron IDT fingers. These IDT's employed single-electrode finger geometries. Here, we examine the second-order distortion of the (sin X)/X passband response of such structures, as exemplified for a 1 GHz 5th harmonic delay line. From time-gated impulse response observations, the primary source of this passband distortion is attributed to finger reflections from the single-electrode IDT's, rather then to electromagnetic feedthrough. Illustrative experiments subsequently conducted on a 5th harmonic delay line with double-electrode IDT's confirmed this observation.

The present paper reports a new unified approach to the design of tunable magnetostatic wave (MSW) filters, delay lines and one-port resonators. This approach is based on the electrodynamic theory of the excitation of surface and volume magnetostatic waves (MSWs) in ferrite films by transducers of an arbitrary type (coplanar, slotline, microstrip etc., including multi-element gratings). An interaction of MSWs with the transducers' magnetic fields that produce them is taken into account. The theory discribes the generation of the magnetostatic modes due to the finite width of ferrite films and the interaction between these modes. Arbitrary loadings and length of transducers are considered. The analysis is carried both in the magnetostatic and electrodynamic approximation for MSW modes and may be extended on the general case of the spin wave excitation, with the exchange interaction and the surface anisotropy of films being taken into account. Simple analytical expressions derived for response functions of MSW filters, delay lines and resonators provide the base for the modeling and the design of these devices.

The optical dispersion can be obtained from the adjacent relative phase between neighbor peaks in the optical frequency comb. Thus, the dispersion measurement becomes possible by measuring the relative phase spectrum. Our group has experimentally confirmed the operation principle by parallel capturing of the dispersion spectrum using an arrayed waveguide grating. We have proposed a dual-heterodyne mixing that obtained relative phases (ΔΦ) by fitting data of beat intensity versus optical path length difference. The path difference was applied by a delay line. In this study, we removed the delay line to realize a fast measurement by measuring simultaneous three relative phases with path length differences corresponding to π⁄2 or π, with which we have measured the dispersion in millisecond speed (250 sec. in previous ). In general, it is effective to measured chromatic dispersion using high-speed signal transmission in the fundamental scientific research, such as the analysis of material properties and telecommunications. It is, however, that limit of cutoff frequency using measurement is the restriction on increasing of the speed. Our proposed method to observe it on a frequency domain is effective for the high-speed signal processing.

Selected aspects related to the calculations and design of a cryogenic transfer line

We introduce an ultra-thin attosecond optical delay line based on controlled wavefront division of a femtosecond infrared pulse after transmission through a pair of micrometer-thin glass plates with negligible dispersion effects. The time delay between the two pulses is controlled by rotating one of the glass plates from absolute zero to several optical cycles, with 2.5 as to tens of attosecond resolution with 2 as stability, as determined by interferometric self-calibration. The performance of the delay line is validated by observing attosecond-resolved oscillations in the yield of high harmonics induced by time delayed infrared pulses, in agreement with a numerical simulation for a simple model atom. This approach can be extended in the future for performing XUV-IR attosecond pump-probe experiments.

Two mirrors, 15 centimeters in diameter and 25 meters apart, form an optical delay line which can store two gaussian beams for 342 round trips or 60 microseconds. This paper reports experiments which studied the intensity profiles, the phase fronts, and the cross scattering between these beams after their retrieval from the delay line. In certain respects, the delay line simulates a multiple beam guide made of 684 mirror periscopes. The experimental results permit an estimate of the beam capacity, the crosstalk, and the transmission length of such a guide.