H. Melchior

General self-imaging properties in N × N multimode interference couplers including phase relations.

Self-imaging properties of generalized N × N multimode interference couplers are derived. Positions, amplitudes, and phases of the self-images are directly related to the lengths and widths of the coupler by solving the eigenmode superposition equation analytically for any arbitrary length. Devices of length (M/N) 3L(c), where M is the multiple occurrence of the N self-images, are analyzed in detail. The general formalism is applied to practical N × N couplers used in integrated optics, and simple phase relations are obtained.

Optical bandwidth and fabrication tolerances of multimode interference couplers

Analytical expressions are derived which relate the optical bandwidth and the fabrication tolerances of multimode interference (MMI) couplers to their most important design parameters. These expressions compare adequately with mode analysis simulations. For strong guided structures, the optical bandwidth is shown to he inversely proportional to the number of input and output ports N and to the length of the device. The fabrication tolerances are independent of N and proportional to the output channel separation D. Measurements on MMI couplers in InP/InGaAsP corroborate the theoretical predictions. >

Passively Q-switched 0.1mJ fiber laser system at 1.53µm

We demonstrate a passively Q-switched fiber laser system generating pulses with as much as 0.1 mJ pulse energy at 1.53µm and > 1kHz repetition rate. This was achieved with a simple MOPA (master oscillator, power amplifier) scheme with a single pump source, realized with large mode area fiber and multiple reflections on a semiconductor saturable absorber mirror (SESAM).

Passively Q-switched 1.34-µm Nd:YVO 4 microchip laser with semiconductor saturable-absorber mirrors

We demonstrate a passively Q -switched diode-pumped 1.34- microm Nd:YVO(4) microchip laser. We achieved single-frequency, 230-ps pulses by using an InGaAsP semiconductor saturable-absorber mirror. We can vary the pulse width and the repetition rate from 230ps to 12ns and from 30 kHz to 4 MHz, respectively, by changing the design parameters of the saturable absorber, the thickness of the crystal, and the pump power.

Overlapping-image multimode interference couplers with a reduced number of self-images for uniform and nonuniform power splitting

Overlapping-image multimode interference (MMI) couplers, a new class of devices, permit uniform and nonuniform power splitting. A theoretical description directly relates coupler geometry to image intensities, positions, and phases. Among many possibilities of nonuniform power splitting, examples of 1 × 2 couplers with ratios of 15:85 and 28:72 are given. An analysis of uniform power splitters includes the well-known 2 × N and 1 × N MMI couplers. Applications of MMI couplers include mode filters, mode splitters-combiners, and mode converters.

Multimode interference couplers for the conversion and combining of zero- and first-order modes

Optical waveguide mode-combiners for fundamental and first-order modes, based on multimode interference (MMI) couplers are presented. These devices convert a fundamental mode into a transversal first-order mode and combine it in lossless fashion with a second fundamental mode. They can separate zero- and first-order modes in a common waveguide and allow the splitting and combining of zero- and first-order modes with nonuniform power splitting ratios. Realizations in InGaAsP-InP are demonstrated. These new components have successfully been integrated into all-optical switches and were found to have advantageous characteristics in all-optical devices.

New 2/spl times/2 and 1/spl times/3 multimode interference couplers with free selection of power splitting ratios

We report on the concept and realization of new 2/spl times/2 and 1/spl times/3 multimode interference (MMI) couplers that offer the possibility of a free choice of the power splitting ratio. These MMI devices, using a new butterfly geometrical design, are compact, polarization-insensitive, and tolerant to fabrication parameters. Realized in InGaAsP/InP with large fabrication tolerances, they permit the control of the output powers within a few percentage points for both polarizations. The measurement results fit well the theoretical predictions. Extensions of the design are presented that assure a fully symmetrical geometry and therefore an optimal homogeneity of the device characteristics. Based on the same idea, a new compact mode converter-combiner is proposed.

Four-quadrant CMOS analogue multiplier

A novel two-signal four-quadrant analogue voltage multiplier has been built in CMOS technology. This multiplier relies on the quadratic current/voltage characteristics of MOS transistors in saturation and combines them with switched-capacitor-type voltage dividers and charge transfer circuits to achieve accurate voltage multiplications with low offsets and large dynamic ranges up to several volts throughout the audio frequency range.

Low-power compact 2/spl times/2 thermooptic silica-on-silicon waveguide switch with fast response

5-mm-long and 50-/spl mu/m-wide thermooptic waveguide 2/spl times/2 Mach-Zehnder interferometer (MZI) switches with multimode interference (MMI) couplers have been realized in silica-on-silicon. The use of diffused square waveguides with a separating trench results in low heating power of 110 mW for /spl pi/ phase shift and short rise and fall times of 150 and 180 /spl mu/s, respectively.

All-optical space switches with gain and principally ideal extinction ratios

Asymmetric Mach-Zehnder interferometer (MZI) configurations are proposed to build all-optical space switches with gain and principally ideal extinction ratios. Actually, three asymmetries in MZI configurations with semiconductor optical amplifiers (SOAs) on their arms are discussed. The asymmetries in the all-optical switches are necessary to overcome the extinction ratio limitations that are due to the disturbing gain changes that arise when control signals are introduced into the SOAs to induce the necessary refractive index change for switching. Starting from a generic MZI configuration with SOAs on the arms, a description in terms of transmission matrices is used and applied to identify 1/spl times/2 and 2/spl times/2 all-optical switch configurations with high on-state transmissions and close to ideally large extinction ratios. The theoretical predictions are verified and found to be in excellent agreement with experiments for a switch with symmetric MZI splitters in a monolithically integrated InP waveguide version that allows operation with equally or unequally biased SOAs.