Three-fold effective brightness increase of laser diode bar emission by assessment and correction of diode array curvature

Abstract

An optical arrangement is used to characterize diode bar curvature which is known to limit the effective brightness achievable with such devices. By introducing an inclined, cylindrical collimating lens in front of the diode bar, the curvature of the diode's beam can be reduced by more than 60%. The correct inclination angle for this method is derived. Using a beam shaper together with this correction mechanism we achieve a homogeneous beam profile with low M² values in orthogonal directions and an effective brightness increase of more than 200%.

Introduction

Diode laser bars are nowadays the basic elements of high power semiconductor lasers. They continuously emit 5–$\text{60}\text{W}$ at specific visible to near infrared wavelengths and find their main applications in materials processing and optical pumping of solid state lasers. A single diode bar is a linear array of 20–50 individual diode emitters of 50–$\text{200}\text{μm}$ width and 100–$\text{400}\text{μm}$ center to center spacing. The overall emitting area of a diode bar has generally a width of $\text{1}\text{cm}$ and is about $\text{1}\text{μm}$ high. In this work, the term effective brightness is defined as the intensity per steradian radiated by the overall emitting area of the bar, which includes the non-emitting area in between the individual diode emitters. Due to the small height, comparable to the emission wavelength, the emitted beam diverges strongly in this direction (fast divergence axis) whereas in the other direction, given by the array of emitters, the divergence is smaller (slow divergence axis) [1]. As a result, the combined output beam is almost diffraction limited in the fast direction but has divergence that is more than thousand times that of a diffraction limited beam in the other direction. This very poor beam quality hampers the usefulness of diode bars in many applications. Specifically, when trying to focus the beam tightly, very different waist sizes, waist positions and depth of foci are obtained for the fast and slow axis, which makes e.g. end-pumping of solid-state lasers difficult. Several beam- shaping techniques have been proposed which generate approximately equal beam quality factors in orthogonal planes, permitting thereby smaller focus diameters [2], [3], [4]. Most of these techniques are accompanied by a strong brightness decrease and only a few are suitable for end-pumping of solid-state lasers [5]. Of these, two of the most well known techniques use either two parallel, high-reflectivity mirrors [6] or two series of micro mirrors (about 20) arranged in a special manner [7] to transform geometrically the diode radiation into a more circular beam with similar beam quality factors in orthogonal transverse directions. Both techniques are based on the same principle, which is cutting the approximately $\text{1}\text{cm}$ wide beam emitted by the diode bar into a series of smaller beams (about 20–30) and then stacking them on top of another thereby achieving a more square and compact beam profile.

The brightness of the laser beam becomes significantly degraded if the diode array is curved. Array curvature, also called “smile”, is mainly a function of the manufacturing process [8]. Once the diode is manufactured, the bar curvature is fixed (Fig. 1). This defect is introduced during the manufacturing process when the laser bar is bonded onto the sub-mount which is then bonded to the copper heat-sink. Some manufacturers also bond the diode bar directly to the copper heat-sink which is referred to as “direct bonding”.

The beam quality becomes even more degraded upon passage of the radiation through the fast-axis collimating lens and the beam shaper. To collimate the fast-axis radiation of a diode bar, with its divergence angle of up to 50°, very high numerical aperture collimating optics is required. When beam quality is not of critical importance, common plano-convex cylindrical lenses of long focal length can be used. However, when the preservation of the diode brightness is of importance, low optical aberrations are required. Because geometrical aberrations depend on lens shape, but not on focal length, they become less important, compared to the diffraction limit, for very short focal length [9]. Therefore, micro-lenses are indicated as fast-axis collimators whenever high brightness is necessary, for example when end-pumping laser rods [10]. These $\text{1}\text{cm}$ long, cylindrically shaped lenses, have diameters of several hundreds of microns and are attached directly to the diode heat-sink, parallel to the array of diode emitters. Good collection efficiency and little cylindrical aberration are obtained with factory installed micro-lenses [11], although an overall brightness decrease of at least 3 times due to aberrations is the case no matter what lens design is chosen [12]. Additional aberrations are introduced due to misalignment of the micro-lens and will be treated at a later point.

Diode array curvature greatly increases off-axis aberrations introduced by the fast-axis collimator depending on the type of collimating lens used. A $\text{5}\text{μm}$ displacement may reduce the beam quality by a factor of 2. It is therefore very important that the lens shows as little sensitivity to array curvature as possible. In at least one publication it has been tried to compensate smile by adjusting the curvature of the flexible fiber-lens with a piezo-electric actuator [12], but we do not know if this procedure has any long term success.

Due to the intrinsic way that beam shapers work, the beam emitted by curved diode bars gets either clipped by the mirrors [6], causing a power loss, or the total area occupied by the beam after it has been re-shaped by the device increases [13]. Either effect results in an effective brightness decrease of the diode radiation. Because array curvature becomes significant at the focus, relative to the spot size, it is also a problem when side-pumping solid-state lasers [14].

This is to our knowledge the first time that the consequences of array curvature are analyzed for the purpose of pumping solid-state lasers and that a simple method is proposed and analyzed which can improve the effective brightness obtained in common diode pumping schemes.