Beam combining of laser arrays with high efficiency and good beam quality for power and radiance (brightness) scaling is a long-standing problem in laser technology. Recently, significant progress has been made using wavelength (spectral) techniques and coherent (phased array) techniques, which has led to the demonstration of beam combining of a large semiconductor diode laser array (100 array elements) with near-diffraction-limited output (M/sup 2//spl sim/1.3) at significant power (35 W). This paper provides an overview of progress in beam combining and highlights some of the tradeoffs among beam-combining techniques.

Laser beam combining for high-power, high-radiance sources

Wavelength-division-multiplexing (WDM) technology is now recognized as one of the key technologies in optical communications systems. This is because it has great potential to enhance system design and flexibility. This paper reviews state-of-the-art optical multi/demultiplexers (MUX/DEMUX) and WDM system design. Various system applications are also summarized.

Review and status of wavelength-division-multiplexing technology and its application

A communications network has a plurality of nodes interconnected by an optical transmission medium. The transmission medium is capable of a carrying a plurality of wavelengths organized into bands. A filter at each node for drops a band associated therewith and passively forwards other bands through the transmission medium. A device is provided at each node for adding a band to the transmission medium. Communication can be established directly between a pair of nodes in the network sharing a common band without the active intervention of any intervening node. This allows the network to be protocol independent. Also, the low losses incurred by the passive filters permit relatively long path lengths without optical amplification.

WDM optical network with passive pass-through at each node

Graded-refractive-index-rod lenses (GRIN-rod lenses) have a number of features that make them particularly suitable for use in optical devices for manipulating and processing the optical signals in fiber communication systems. Such lenses can be cemented directly to the other elements of a device, thus giving a structure that is compact, solid, stable, and rugged. They also have significantly smaller aberrations than equivalent simple homogeneous lenses, and this results in lower insertion losses. Designs for various GRIN-rod lens devices, including connectors, attenuators, directional couplers, switches, isolators and wavelength-division multiplexers are reviewed. A consistent set of loss estimates is provided for all the device designs considered.

Applications of GRIN-rod lenses in optical fiber communication systems.

Multiple channel optical multiplexing/demultiplexing devices that utilize only a single constant, non-variable wavelength selective optical filter have many advantages. The wavelength-selective optical filter (30) is a conventional fixed wavelength optical interference filter having an angle shift property wherein the wavelength-selectivity changes with changing angles of incidence upon the filter. Because such filters are transparent to a different center wavelength and depending on the angle of incidence of a light beam, multiplexing/demultiplexing is achieved by varying the angle of incidence of a light beam (40) upon a single, constant and non-variable optical interference filter. Two such exemplary systems utilize a multiple reflection chamber adapted to transmit a light beam at successively varied angles of incidence upon a single interference filter.

Multiple channel multiplexer/demultiplexer devices

A multireflection multi/demultiplexer using interference filters is proposed. Light waves of different wavelengths are combined or separated while they propagate between two filter arrays. The insertion loss of the 6-channel demultiplexer was around 1dB.

Multireflection optical multi/demultiplexer using interference filters

PURPOSE:To obtain the light wavelength branching filter of low coupling losses and less deviations for respective wavelength bands by subsequently arraying the band- pass filters using interference films from the wavelength bands of larger optical fiber transmission losses. CONSTITUTION:In the transmission path, the wavelength at which the optical path length in glass bulk 10 for the wavelength giving maximum loss is first taken out, whereby the reduction in losses is accomplished. The losses of the respective wavelengths become smaller in the order of lambda1, lambda8, lambda4, lambda2, lambda3, lambda7, lambda5, lambda6. For this purpose, interference film band-pass filters are arrayed in the order of 21, 28, 24, 22, 23, 27, 25 so that the optical path lengths in the bulk become subsequently longer. As a result of such constitution, the loss fluctuations of transmission systems by the transmission line and multiple branching circuit may be reduced.

Light wavelength multiple branching system

PURPOSE:To obtain the photo branching filter of a small size having high dispersion power by forming grating one surface of a prism and making use of the wavelength dispersibility of both. CONSTITUTION:Light emitted from a fiber 100 is collimated by a lens 1' and enters a prism 3' at an incident angle theta'1 through a prism 6. The refractive angle theta'2 of this prism 3' varies with wavelengths. Hence, the incident angle alpha' to a grating 4' varies with wavelengths and the diffraction anyle beta' thereof depends upon wavelengths as well. Further the exit angle theta' from the prism 3' also depends upon the wavelengths. In this way, the light separated by each wavelength by the dispersion effect of the prism 3' and grating 4' is coupled to fibers 101 thru 105 by a rod-form lens 2' via prism 7. Thereby, the photo branching filter having high resolution may be obtained.

Photo branching filter

PURPOSE:To detect fault points such as breakage with high sensitivity by making the light beam modulated with repetitive signals incident to an optical fiber, and detecting the difference between the modulation frequencies of the modulated incident light beam and the backward scattered and reflected light beam. CONSTITUTION:An oscillator 21 changes an oscillation frequency at an F0[Hz/sec] ratio with respect to a time, and this signal is amplified with an amplifier 22, whereby a laser 23 is modulated. The modulated light 14 from the laser 23 is made incident to an optical fiber 11 past a photodirectional coupler 24. The backward scattered and reflected light beam 15 from the fiber 11 again passes through the coupler 24, is detected with a photo-electric transducer 25 and is amplified with an amplifier 26, after which it is frequency-mixed with the output of the oscillator 21 with a mixer 27, whereby the signal of the differential frequency is created. Only the differential frequency signal is extracted by a band-pass filter 28, and the frequency components are observed with a spectrum analyzer 29. From the difference between the modulating frequency F1[Hz] of the incident light beam and the frequency F2[Hz] extracted from the reflected light, the distance S1[m] between the incident point and the reflection point is determined by S1=VX¦F1-F2¦/2XF0.

Kunio Hashimoto

Papers

Multireflection optical multi/demultiplexer using interference filters

Light wavelength multiple branching system

Photo branching filter

Fault searching method by optical fiber