An improved non-planar ring oscillator (NPRO) of the fiber-pigtailed type has been developed. The improvements over prior lasers of this type lie in details of design and construction that are intended to enhance mechanical stability and thereby increase the stability of optical performance.

A description of the prior art is necessary to put the improvements in perspective: A basic NPRO laser includes a specially shaped laser crystal that serves as the laser cavity. The input and output optical paths in the crystal intersect at a common point on the surface of the crystal. The laser crystal is pumped by a laser diode, the output of which is coupled into the crystal via a cylindrical lens and a grating-index-of-refraction (GRIN) rod lens. Because of the monolithic nature of the laser cavity, the laser frequency is highly stable. Magnets placed near the crystal ensure unidirectional lasing. A quarter-wave plate converts the elliptically polarized output laser beam into a linearly polarized beam, and an optical isolator blocks undesired feedback to the laser. In the case of an NPRO of the fiber-pigtailed type, some components for output coupling to an optical fiber are included in the laser package.

In a fiber-pigtailed NPRO laser package of prior design, all of the aforementioned optical components are mounted, variously, by use of solder, epoxy, or screws on two substrates. In turn, the two substrates are joined to each other by solder at three points. The combination of solder, epoxy, and screws is vulnerable to long-term mechanical instability that can degrade critical alignments of optical components, thereby reducing the net output power.

In the improved NPRO ring laser, the need for critical alignment of the laser-diode pump, lenses, and laser crystal is eliminated because the laser-diode light is coupled to the laser crystal via a bare optical fiber. The NPRO crystal, a heater plate, and a thermistor are placed inside a monolithic, 304L-stainless-steel monolithic block housing by use of a precise tool, then bonded together by use of solder; these are the only components bonded by solder. The other optical components are made with 304L-stainless-steel housings, most of which are laser-welded to the monolithic block housing, except one that is laser-welded to another housing that is, in turn, welded to the monolithic block housing, as described in the next paragraph. Laser welding of these parts is superior to soldering and epoxy bonding and is expected to contribute greater mechanical stability.

In the improved laser as in a typical prior NPRO ring laser, the laser beam is coupled to an output single-mode optical fiber via a GRIN rod lens, which, in this case, is held in a 304L-stainless-steel housing. A novel procedure for alignment of the GRIN rod lens is based on the observation that if the beam is centered on the input face of the lens, then the beam emerges perpendicularly to the output face of the lens. Once the GRIN rod lens has been aligned, its housing is laser-welded to the monolithic block housing. The output optical fiber is held in a ferrule in a housing that, in turn, is laser-welded to the housing of the GRIN rod lens housing. Because the axis of the fiber is held parallel with the output beam emerging from the GRIN rod lens, this arrangement helps to maximize output coupling.

This work was done by Duncan Liu of Caltech for NASA's Jet Propulsion Laboratory.


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Improved Fiber-Pigtailed Non-Planar Ring Oscillator Laser

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This article first appeared in the October, 2001 issue of Photonics Tech Briefs Magazine.

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