Measurement driven quantum evolution

We study the problem of mapping an unknown mixed quantum state onto a known pure state without the use of unitary transformations. This is achieved with the help of sequential measurements of two non-commuting observables only. We show that the overall success probability is maximized in the case of measuring two observables whose eigenstates define mutually unbiased bases. We find that for this optimal case the success probability quickly converges to unity as the number of measurement processes increases and that it is almost independent of the initial state. In particular, we show that to guarantee a success probability close to one the number of consecutive measurements must be larger than the dimension of the Hilbert space. We connect these results to quantum copying, quantum deleting and entanglement generation.Comment: 7 pages, 1 figur

Widely tunable erbium-doped fiber laser based on multimode interference effect

A widely tunable erbium-doped all-fiber laser has been demonstrated. The tunable mechanism is based on a novel tunable filter using multimode interference effects (MMI). The tunable MMI filter was applied to fabricate a tunable erbium-doped fiber laser via a standard ring cavity. A tuning range of 60 nm was obtained, ranging from 1549 nm to 1609 nm, with a signal to noise ratio of 40 dB. The tunable MMI filter mechanism is very simple and inexpensive, but also quite efficient as a wavelength tunable filter

Tuning characteristics of cladding-pumped neodymium-doped fiber laser

We describe a cladding-pumped neodymium-doped fiber laser, tunable from 1057 to 1118 nm. The power varies significantly over the tuning range. Improved characteristics are obtained with tuning elements in both ends of the cavity

Mmi-Based 2×2 Photonic Switch

We propose a robust, multi-mode interferometer-based, 2×2 photonic switch, which demonstrates high tolerance to typical fabrication errors and material non-uniformity. This tolerance margin is dependent upon the properties inherent to the MMI design and benefits from the high symmetry of the switch. The key design parameter of the device is to form a pair of well defined self-images from the injected light in the exact center of the switch. In allowing the index modulated regions to precisely overlap these positions, and by creating identical contact features there, any refractive index change induced in the material due to electrical isolation will be duplicated in both self-images. Since the phase relation will remain unchanged between the images, the off-state output will be unaltered. Similarly, offset and dimension errors are reflected symmetrically onto both self-images and, as a result, do not seriously impact the imaging. We investigate the characteristics of the switch under different scenarios using the finite difference beam propagation method. Crosstalk levels better than -20 dB are achievable over a wavelength range of 100 nm when utilizing this configuration. Polarization independence is maintained during device operation

Ultra-Compact Multimode Interference Ingaasp Multiple Quantum Well Modulator

We propose a new structure for an ultra-compact multimode interference (MMI) InGaAsP multiple quantum well modulator. The operating principle is based on restricting the coupling of the self-image produced by the MMI region into a single mode output waveguide. The key is to excite only the even modes within the MMI region, and this is achieved by operating the MMI waveguide under the condition of restricted symmetric interference. By asymmetrically inducing a phase change of π along a selected area within the MMI region, mode conversion of all the even modes to odd modes is achieved. Since only the fundamental mode can be coupled to the output waveguide, neither an individual mode, nor any combination of the modes will be coupled, and therefore the injected light is fully attenuated. The modulation characteristics are analyzed using the finite-difference beam propagation method. Extinction ratios as low as -37 dB are demonstrated without electro-absorption effects. For the case of low electro-absorption, which corresponds to a more realistic situation, this value is only increased to -35 dB. © 2005 Springer

Variable Optical Attenuator Using Active Multimode Interference Waveguide

We propose a new structure for an integrated variable optical attenuator using InGaAsP multiple quantum wells. The principle of operation relies on the self-imaging properties of multimode interference (MMI) waveguides. The device consists of a MMI region that is 12 μm wide by 350 μm long, with input and output waveguides that are 2 μm wide. The dimensions of the MMI are calculated such that an image of the input field is produced at the output waveguide. The last statement is true as long as the phase relation between the modes in the MMI section is kept constant. Therefore, by selectively perturbing the refractive index within the MMI section, the phase relation of the modes is altered, thereby modifying the interference properties at the output of the device. We present numerical simulations using the Finite-Difference Beam Propagation Method (FD-BPM), and demonstrate that optical attenuation is possible by selectively modulating the refractive index of a narrow region within the MMI section. A dynamic range of -37 dB can be easily obtained at a wavelength of 1.55 μm with a device insertion loss of 0.3 dB. The effects of electro-absorption on the device performance are also investigated

A Reconfigurable Multimode Interference Splitter For Sensing Applications

A reconfigurable multimode interference (MMI) coupler is demonstrated. The device operates by modifying the phase of the multiple images that are formed around the midpoint of the MMI section. This modifies the properties of the following set of images, and light can be directed to a specific output waveguide, which is therefore ideal to develop a reconfigurable MMI coupler. In our device the phase change is achieved by current injection, and therefore minimizing current spreading is crucial for optimal operation. A zinc in-diffusion process has been implemented to selectively define p-n regions and effectively regulate current spreading by controlling the depth of the zinc doping. Using this process, a reconfigurable 3 dB MMI coupler has been fabricated. Our experimental results revealed that the device can be easily set to a perfect 3 dB splitter using only 0.7 mA of current injection. In addition, the device can be adjusted all the way from a 90:10 to a 30:70 splitting ratio. The results are very encouraging since, to our knowledge, this degree of tuning of the optical power has never been experimentally demonstrated in MMI devices. Furthermore, this concept can easily be applied to a wide variety of semiconductor photonic switches that operate on MMI effects. © 2007 IOP Publishing Ltd

Tunable Mmi Splitter For Sensing Applications

A tunable MMI splitter is demonstrated. The device can be easily tuned from a 90:10 to a 30:70 splitting ratio. This is extremely useful for integrated interferometric sensors to obtain the ultimate device performance. © 2006 Optical Society of America

Tunable Multimode-Interference Bandpass Fiber Filter

We report on a wavelength-tunable filter based on multimode interference (MMI) effects. A typical MMI filter consists of a multimode fiber (MMF) spliced between two single-mode fibers (SMF). The peak wavelength response of the filter exhibits a linear dependence when the length of the MMF is modified. Therefore a capillary tube filled with refractive-index-matching liquid is used to effectively increase the length of the MMF, and thus wavelength tuning is achieved. Using this filter a ring-based tunable erbium-doped fiber laser is demonstrated with a tunability of 30 nm, covering the full C-band. © 2010 Optical Society of America

All-Fiber Tunable Mmi Fiber Laser

We report on a novel tuning mechanism to fabricate an all-fiber tunable laser based on multimode interference (MMI) effects. It is well known that the wavelength response of MMI devices exhibits a linear dependence when the length of the multimode fiber (MMF) section. Therefore, tuning in the MMI filter is achieved using a ferrule (capillary tube of 127 μm diameter) filled with a liquid with a higher refractive index than that of the ferrule, which creates a variable liquid MMF. This liquid MMF is used to increase the effective length of the MMI filter and tuning takes place. Using this simple scheme, a tuning range of 30 nm was easily achieved, with very small insertion losses. The filter was tested within a typical Erbium doped fiber (EDF) ring laser cavity, and a tunable EDF laser covering the full C-band was demonstrated. The advantage of our laser is of course the simplicity of the tunable MMI filter, which results in an inexpensive tunable fiber laser. ©2009 SPIE