Diffractive Microlens Cuts Laser Costs

Breck Hitz

Assembly and alignment are major cost drivers in the fabrication of photonic components. Nowhere is this more evident than in the packaging of laser modules for metropolitan optical networks, for which demand is growing rapidly. A team at Oki Electric Industry Co. Ltd. in Tokyo has addressed this problem with a diffractive microlens that is compatible with V-groove substrates which enables manufacturers to use less-costly passive alignment techniques for packaging laser modules.

Packaging a module involves alignment between the laser diode and single-mode fiber with submicron accuracy. Typically, the laser is mounted on the surface of a silicon substrate, the fiber is passively aligned in a V-groove in the same substrate, and a microlens is placed between the laser and fiber. But the inevitable variations of lens diameter and focal length in conventional microlenses make them incompatible with V-groove alignment, so further alignment of the fiber and/or lens is required, driving up the cost of the modules that employ them.

The new silicon diffractive microlens has several advantages over conventional, refractive microlenses (Figure 1). First, the focal-length variations among diffractive lenses are much smaller than among refractive lenses because the former are fabricated using ultraprecise photolithographic and etching techniques. Moreover, diffractive microlenses can be fabricated in large numbers using conventional photolithography technology. The researchers selected silicon, rather than silica, as the optical material because of its high refractive index (approximately 3.5). The required depth of the diffractive grooves is inversely proportional to the refractive index.

Figure 1. A new silicon diffractive microlens can be aligned passively in a V-groove, reducing the costs of packaging laser modules.

The diffractive lens, which has the same 125-µm diameter as the fiber, can be passively aligned in the V-groove using a flip-chip bonder, but they actively aligned the lens in their experiments to quantify the optimum performance. Their Monte Carlo calculations demonstrate that the design is suitable for passively aligned, low-cost, high-performance modules.

In their experimental arrangement, the silicon microlens was antireflection-coated to minimize insertion loss (Figure 2). The focal length of the collimating lens (Lens 1) was 80 µm, and that of the focusing lens (Lens 2), 500 µm. The diode laser produced a symmetric output beam, so no cylindrical focusing was necessary. When the lens was aligned for optimal performance, the coupling efficiency between the laser diode and the single-mode fiber was –3 dB.

Figure 2. The diffractive microlens focuses the laser output into a single-mode fiber. Monte Carlo simulations suggest an 89 percent probability of a better than —5-dB total coupling loss for a passively aligned module.

The Monte Carlo simulation took into account the misalignment tolerance for each of the components in the package and calculated the total coupling loss across the package. The result was an 89 percent probability of a loss of –5 dB or better, which the researchers believe can lead to inexpensive mass production of high-performance laser modules for use in metro networks. They are working to fabricate full laser modules using the new silicon microlenses.