Multielement laser chips spawn ‘super arrays’

A new laser platform-based on individually addressable single-mode laser-array modules-allows increased levels of optoelectronic integration and technical performance. Very large element arrays of high-performance single-mode lasers can be fabricated with high yield.

Laser diodes are used for many printing applications, including the familiar laser printer that usually uses a single low-power emitter with a rotating mirror to scan the beam. High-performance printing applications, however, require high-power lasers, usually with a single transverse-mode beam to provide good image quality. In these cases, several lasers are used in parallel to speed the image process. Systems for these applications, therefore, typically contain a relatively small number of individually packaged lasers (usually fiber-coupled) inside a large, complex, and expensive print head. Packaging individual lasers inside a larger print head, however, is becoming increasingly impractical as the print industry demands faster print speed, smaller form factor, and more cost-effective solutions.

Photonic integration provides an attractive alternative to achieve the specifications required by advances in digital printing. Using proprietary quantum-well intermixing technology, we are creating a very robust semiconductor laser that allows large-scale ­laser integration onto a single chip.

The multielement laser chip

Catastrophic optical damage (COD) of the laser facet is often a limiting factor that restricts the ability to integrate a large number of semiconductor lasers onto a single chip with good reliability. An impurity-free vacancy disordering process has been developed by Intense to achieve spatially selective quantum-well intermixing (QWI).1 The QWI processes are used to create passive nonabsorbing mirrors (NAMs) at the facet regions of a semiconductor laser that dramatically improve the COD power of the laser.2 These NAM regions allow the possibility of large-scale integration on a single chip and improve single-mode stability at high drive current because the facet region remains relatively cold during operation, allowing high single-mode powers to be achieved.3

In addition to the NAM regions, we have developed a novel epitaxy design that includes a “V-profile” layer to reduce the far-field diffraction angle and suppress higher-order transverse modes.4 The graded V-profile layer is located in the aluminum gallium arsenide (AlGaAs) cladding region close to the quantum-well active region. The graded V-profile design is optimized to give low beam divergence and high optical power, and has proven to be robust for full-wafer epitaxy growth and process manufacture.

Full 3-in. wafer processing using state-of-the-art photolithography and dry-etching techniques, in combination with NAMs and the V-profile laser design, allows very large element arrays of high-performance single-mode lasers to be fabricated with high yield. Large arrays of high-power (up to 100 elements, with each element approximately 300-mW single-mode optical power), with excellent laser parametric uniformity (laser threshold, optical power, and beam shape), can be obtained (see Fig. 1).

Also, the QWI process was developed for a wide range of materials systems, including aluminum indium gallium arsenide/aluminum gallium arsenide (AlInGaAs/AlGaAs), which allows lasers to be fabricated over the wide wavelength range of interest (800 to 1000 nm) for commercial thermal-printing applications.

INSlam technology

To take advantage of the excellent laser parametric performance of the large-array laser chip, Intense has developed a generic laser platform. Called INSlam (individually addressable single-mode laser-array modules), the platform consists of individually addressable single-mode laser arrays, drive electronics, optical monitoring, and optics incorporated on a single platform within a small-form-factor package.

To translate the benefits of laser parametric uniformity at chip level into a fully functional module, significant laser package development and assembly issues had to be overcome to handle large-area, multielement, single-mode laser chips. Optical system issues such as thermal, optical, and electrical crosstalk must be considered before the benefits of the INSlam technology can be fully realized.

For many applications, there is a complicated interdependency and tradeoff between the laser and package. Consider, for example, a 100-element INSlam with each laser operating at maximum continuous-wave (CW) single-mode power; such stringent operating conditions introduce a significant heat load into the module that can adversely affect the uniformity of beam profile, spot position, and optical power across the array. The choice of carrier material (thermal conductivity, thermal-expansion coefficient, and mechanical finish), the solder material, and quality of die bond and the assembly process have to be carefully considered for large-area laser-chip bonding onto a multicomponent INSlam module. If such care is not taken, hot spots, thermal-expansion differences, and other effects can give rise to undesirable variations in uniformity.

A thermally optimized INSlam can still produce a nonuniform temperature profile or temperature “smile” under certain operating conditions. This thermal smile can have a deleterious effect on the parametric uniformity of a conventional laser array, giving rise to undesirable mode hops, power variations and beam-steering issues. Nonabsorbing-mirror laser technology has proved to be very robust against thermal “smile” issues because the passive NAM regions are relatively cold during operation and hence act as mode filters, enabling high-power, single-mode uniformity across the array.

The NAM high-temperature performance, the reduced far-field V-profile epitaxy, together with a good thermally matched module design with micro-optics allows for submicron positioning accuracy of each focused spot in the array during operation. Where INSlams contain optical monitoring, precise control of the dynamic and long-term power output is possible.

Significant assembly challenges, in terms of wire and die bonding and positioning and alignment tolerances of multilayer components, also had to be overcome during development of the INSlam module to achieve volume manufacture.

Applications

The optoelectronic integration of multielement laser diodes allowed by INSlam gives significant benefits to the print industry-first, in terms of a small-form-factor, low-cost optical print head; and second, in increasing the number of high-power, single-mode laser elements to give high print speed and submicron spot placement accuracy for high-resolution, full-color imaging.

INSlam provides a viable technology in this market by providing a large, ­laser-element-array integrated solution that offers an alternative to existing imaging heads consisting of a large, bulky package containing a relatively small number of individually packaged lasers, which limits the print speed.

The platform can also provide a building block to construct “super arrays” of many thousands of individually addressable laser elements by using patented automatic pick-and-place technology to place laser chips side by side to create a stitchless array of laser beams (see Fig. 2). The super array consists of several laser-array chips die-bonded side by side and coupled into a single microlens to create a very large array of laser beams. INSlam super-array modules are of interest for page-wide, single-scan laser marking and thermal paper-printing applications, where a “no-contact” laser mark gives reliability benefits compared to a conventional thermal print head that wears out due to friction. A no-contact laser mark also allows the latest security tagging technology to be applied.

Beyond printing, INSlams are suitable for a vast range of materials processing and life-science applications. Medical imaging and scanning applications can also benefit from these modules. In biosensors, for example, INSlams can be used for simultaneous and multiplexed chemical and biological experimentation.

REFERENCES

1. O. P. Kowalski et al., Appl. Phys. Lett. 72, 581 (1998).

2. C. L. Walker, A. C. Bryce, and J. H. Marsh, IEEE Photonics Tech. Lett. 14(10) 1394 (October 2002).

3. J. H. Marsh, Laser Focus World 40(6) 98 (June 2004).

4. S. P. Najda et al., Proc. SPIE 5365, 1 (2004).

JOHN MARSH is chief technical officer and STEVE NAJDA is technical expert at Intense, 4 Stanley Boulevard, Hamilton International Technology Park, Blantyre, Glasgow G72 0BN, Scotland; e-mail: [email protected].