Surface mounted optics aid automated assembly

02 Jun 2003

Michael Hatcher reports on a technique developed by a Swiss collaboration that paves the way towards faster, automated assembly of miniature optical subsystems.

From Opto & Laser Europe June 2003

Although optical subsystems are widespread in applications such as sensing and telecoms today, the way in which the optics components are assembled and packaged remains a tricky and time-consuming business. For the most part, optics are passively aligned and then stuck together with glue.

In high-volume semiconductor manufacture, such techniques would be unthinkable. More sophisticated methods for the assembly of photonics modules are essential if their manufacture is to reach the same level of automation as that of electronics while maintaining high-precision alignment and high reliability.

A collaboration between the Swiss Federal Institute of Technology in Lausanne (EPFL) and Leica Geosystems of Switzerland has recently developed a technique that appears to offer a way forward. The group's three-dimensional miniaturized optical surface-mounted devices (TRIMO-SMD) imitate the assembly techniques that were developed for the electronics industry 20 years ago. The method, which uses six-axis robotic motion, automated optical alignment and laser-reflow soldering to make photonics modules, is currently being made commercially available by Leica Geosystems.

Automated assembly Opto & Laser Europe reported on the first generation of this automated assembly equipment back in September 1998. Originally developed by the EPFL, optical surface-mounted devices (O-SMD) were useful for assembling optics of approximately 8-10 mm in size. A spin-off company called BrightPower was set up to commercialize the technology.

However, the O-SMD technique, which involves simultaneous laser-welding of three metal cups that act as a tripod holding the optical component, was judged to be unsuitable for micro-optical assembly. For the past few years, Leica Geosystems has been working closely with both the Institute of Applied Optics and the Institute of Robotics at EPFL on a technique that, while it works on a similar principle to O-SMD, is suitable for use in manufacturing micro-optic systems - TRIMO-SMD.

"We decided that O-SMD was way too large for future projects," said Laurent Stauffer, who has been managing the technological development of the first commercial product to use TRIMO-SMD at Leica Geosystems. "TRIMO works extremely well with laser diodes. In many applications where you have a laser diode and you need an optical component you can use TRIMO, so there is a very wide potential market."

TRIMO-SMD is designed for use with optical components of around 2 mm in diameter. "We regard the high throughput and reliability possible with TRIMO-SMD to be something of a quantum step in optical assembly," Stauffer told Opto & Laser Europe.

Smaller optics Laser-welding a mount to a substrate was not viable for making subsystems that incorporate smaller optics. Instead, laser-soldering or brazing was found to be the best method of attachment. This is the key difference between O-SMD and TRIMO-SMD: rather than holding the optics in place with a tripod of metal cups, the optical element in TRIMO-SMD is suspended up to 400µm away from the substrate material, and is moved into position by a robot. There is no contact between the substrate and the optical mount, which according to Stauffer is an improvement on O-SMD. "Contact between the substrate and the element was a bit of a problem in the past," he said.

In TRIMO-SMD, a mount called a universal holder is produced first. This holder consists of a 2.5mm-diameter round cup and two vertical arms 2.6mm long. It is covered with a tin preform in preparation for the soldering process. "The universal holder is our standard interface between the optics and the ground plate," explained Stauffer. A sub-mount containing the optical element is then laser-welded to the holder. With the holder gripped by a robot-controlled jig, the optical element is aligned using cameras and sensors. The robot can move in six dimensions and, according to Stauffer, has a placement precision of 0.25µm.

When the optical element is suspended precisely above the desired position, an 808nm continuous-wave high-power diode laser fires 20-40W through the substrate underneath the optics. The substrate is partially transparent and part-metallized to enable wetting of the surface during soldering. When the laser hits the preform, the tin melts and then drops onto the mounting plate to form a stable joint with the holder.

This method fixes the optical element into position in just 2s. The soldering causes slight thermal shrinkage that alters the exact position of the optical element. Stauffer says, however, that this can be easily resolved: "With a 200µm gap, there is typically a shrinkage of 3µm - this can be calibrated accurately, and you can simply offset the optical element before soldering," he commented. After soldering, the gripper is relaxed and the optics left in a fixed position. "The placement of each optic is repeatable to within 1µm [to a 99% confidence limit]," said Stauffer.

When Leica Geosystems and EPFL first developed TRIMO-SMD, they had a particular product in mind: a laser rangefinder used in military applications. Leica needed to place a beam-shaping optic directly in front of a laser diode inside the rangefinder. Thanks to the new micro-optic assembly produced using TRIMO-SMD, the distance over which the equipment is effective has been increased from 5 to 10km.

Award-winning technology The technique had a welcome boost in February this year, when its selection as one of the winners of the Swiss Technology Award meant that it was exhibited at the Hannover Messe technology show. "We found five very interested potential customers for TRIMO-SMD at Hannover," Stauffer said. A spin-off company dedicated to the commercialization of TRIMO-SMD is planned and will be set up in the autumn of this year. According to Stauffer, "Leica will support the company over the first few years and the goal is to offer a manufacturing service, technical support and consulting."

In the meantime, Leica is looking to cash in on its investment (the company owns two patents protecting the technology) by licensing the technique to industrial partners. "We think that there is a very wide market for TRIMO-SMD, particularly companies that are involved in optical sensing and telecommunications," Stauffer said. "Leica Geosystems will use TRIMO for applications using optical sensing and we expect other customers to do the same. Medical applications are also possible."

In addition to the laser rangefinder, Leica has built a lidar transceiver module using TRIMO-SMD. The technique enabled a dramatic reduction in the size and weight of the transceiver and improved its stability and robustness. Coupled with a drop in price, the module could open up a new market for transceivers.

A third module that has been built by Leica using TRIMO-SMD is a series of components that produce second-harmonic generation from an Nd:YAG microchip laser. "All of the devices used are ideally sized for TRIMO-SMD, and components like the self-focusing lens can be positioned to a very high accuracy," said Stauffer.

He recommends that anybody considering using the TRIMO technique should carefully plan exactly what they need to assemble: "You need to 'think TRIMO', and design your optics accordingly." He adds that although micro-optic assembly currently takes around 10-15 min using the technique (the active alignment has been only partially automated), this could be shortened and optimized for high-throughput mass-production by any company willing to invest in the technology.

If a major manufacturer of optical subsystems is forthcoming with that kind of investment, the TRIMO-SMD technique could propel photonics manufacturing along the same path as electronics.