Laser modules gain from fibre alignment and characterisation
Manufacturers of laser diode modules (LDMs) are addressing two areas of the fabrication process: fibre alignment and device characterisation.
In a traditional production line, fibre alignment and device characterisation are separate operations that may share little more than the use of an optical power sensor to measure LDM output. Alignment is a mechanical assembly step that uses analogue feedback to locate the best mounting point for the pigtail, while characterisation requires a series of detailed light, current, and voltage (LIV) measurements.
The physical region of the laser diode junction that emits light is microscopic, requiring alignment of the fibre to within 0.001mm, typically. Nano-positioning equipment, which locates the best mounting point for the fibre, can automate the alignment process. This is usually performed at a fixed LDM temperature, with the laser diode driven at maximum power output. A photodetector attached to the other end of the optical-fibre pigtail supplies an analogue signal that drives the positioning mechanism.
Once the beam is detected, the positioning equipment adjusts the pigtail position as light measurements are made. When the point of maximum brightness is located, the tip of the pigtail is mounted permanently to the laser diode.
During LIV testing, the LDM is characterised at various drive current levels. Typically, these tests require transferring the LDM from the alignment stand to a computer-controlled test stand containing current and voltage sources, an instrument to power and control the thermoelectric cooler inside the LDM, one or more photodetectors or other sensors, and sensitive current measurement capability.
It has proved difficult to find a way to combine fibre alignment and LIV testing, since each process has significantly different priorities. Traditionally, the accuracy of fibre alignment has been a function of the mechanical precision of the nano-positioning mechanism. Alignment speed has been governed by the response of the sensor and circuitry used to drive the positioning mechanism.
In contrast to the fibre alignment process, device characterisation demands high accuracy and precision. These differing demands create a conflict when attempting to combine the two processes. This is why, hitherto, they have not been integrated into one operation in the past. However, current generations of instruments, such as Keithley's 2502 Fiber Alignment Meter (Fig.2), offer the flexibility needed to perform high-speed alignment as well as characterisation of LDMs.
Beam alignment is a time-intensive operation. In fact, a major portion of alignment time is spent searching for the laser beam itself.
Typically, nano-positioning controllers operate from an input signal of the order of 0–2VDC, 0–10V, 4–20mA, or a similar standard loop architecture, and digitise the voltage internally to control the positioning mechanism. By selecting a wider drive voltage range to drive the controller (e.g. 0–10V), the analogue signal can provide ample low-level response to indicate detection of the beam skirt farther from the centre of the beam. Another benefit of wide dynamic range is that it minimises the need for range changes as the system converges on the optimum beam location.
A single-point fibre-optic alignment can be performed with the LDM at full power and at a single temperature. This scenario would be adequate if the LDM were always used under the same set of narrowly defined conditions. However, different laser diode applications can involve a broad range of operating conditions.
A more advanced alignment method involves collecting enough information about the actual behaviour of the laser over a range of operating conditions, then selecting an optimal mounting target based on the specific application and expected operating conditions for the device.
The General Purpose Interface Bus (GPIB) is an instrument control bus used to program instruments and retrieve data. However, it can take a considerable time to transmit commands and data between the controlling computer and instrumentation. The use of an instrument's source memory can be used to store several complete system test routines, which allow the instrument to control the entire test system autonomously after receiving an "execute" command.
In the case of alignment and characterisation, both tests can be stored within the same instrument system — one setup optimised for high-speed alignment, and the other for high-precision device characterisation — to allow completion of both processes without compromising speed or precision.