Laser Diode Module Testing Critical to Development of High Bandwidth Communication Systems

By Paul Meyer
Keithley Instruments, Inc.


I. Background

Driven by ever-increasing Internet traffic, the world's appetite for communication bandwidth seems to grow by the day. Having exceeded the capacity of traditional copper, microwave, and single wavelength laser-based fiber-optic solutions, designers have turned to dense wavelength dimension multiplexed (DWDM) systems. These systems have already become the dominant solution for high bandwidth data communications.

DWDM technology allows the bandwidth of existing fiber-optic networks to be increased manyfold beyond their original design characteristics. This is accomplished by allowing several different lasers operating at slightly different wavelengths to use the existing fiber-optic cable infrastructure. Each wavelength can carry thousands of communication channels.

The laser that produces the coherent light beam is part of a complex laser diode module (LDM), which is the optical equivalent of a local oscillator in an RF communication system. The industry's demand for increasing bandwidth is mirrored by the demand for LDMs. Consequently, many LDM manufacturers are ramping up production at rates of several hundred percent per year.

To maintain this frenetic pace, the makers of LDMs are doing everything possible to increase productivity. However, due to the high value added during LDM production, critical components and sub-systems must undergo quality assurance testing at key stages in the assembly process. This requires test systems with both high accuracy and high throughput. Development of automated LDM test systems that provide this kind of performance can strain manufacturers' test engineering teams. Frequently, this means they must abandon in-house development of custom designed test systems, opting for turnkey solutions developed by outside experts that meet or exceed the performance of internally developed systems.

II. LDM Features

A typical LDM consists of a laser diode, fiber-optic coupling, back facet monitor photodiode, thermo-electric cooler (TEC), thermistor, and modulator. These components are integrated on a ceramic substrate and mounted in a metal enclosure. A short length of optical fiber exits the enclosure and is terminated with a standard fiber-optic connector. Each of these components must be tested at appropriate points in LDM production.

A. Laser Diode

The laser diode emits monochromatic coherent light over a range of currents. The lower end of the range is defined by the lasing threshold current, below which light emissions are non-coherent. At the upper end of the useful current range a point is reached where the light emissions are characterized by partial coherence, a broadening of the optical spectra, and a decrease in emission efficiency. In between is a linear operating region that produces coherent light suitable for modulation. Testing of the laser diode typically includes a series of light output measurements over a range of input currents and voltages.

B. Back Facet Monitor Photodiode (BFMPD)

The laser diode typically is designed to radiate a small portion of the stimulated light emissions from the back or rear face of the resonant cavity. This light is directed to a photosensitive detector called the Back Facet Monitor Photodiode (BFMPD), which is integrated into the LDM to facilitate monitoring and control. Testing of the BFMPD usually is limited to verification of forward and reverse I-V characteristics. However, the BFMPD also is an integral part of the laser diode testing.

C. TEC and Thermistor

To maintain coherent light output at the appropriate wavelength, LDMs use a thermo-electric cooler and thermistor for precise regulation of the laser diode's operating temperature. Resistance measurements are used to determine the quality of both the thermistor and the Peltier device that is the heart of the TEC.

III. Description of DC Testing of LDMs for Quality Assurance
Measuring DC characteristics of the principal LDM devices may sound simple, but these tests are quite complex. For example, laser diode testing is accomplished with a technique known as an L-I-V Sweep, so named because (L)ight output, Current (I), and (V)oltage are measured for a range of drive currents. L-I-V sweeps typically are performed at the following steps in the manufacturing process:
  • Wafer processing
  • Finished laser diode chip
  • Chip-on-carrier
  • Laser mounted on TEC with thermistor
  • Fiber coupling
  • Completed Module
  • Post Burn-in

Each of these manufacturing checkpoints requires slightly different test requirements and instruments. The algorithms used by many LDM manufacturers to analyze raw measurement data are customized and proprietary, which must be integrated into the test system by the supplier. The ease of moving measurement results to the control computer for analysis, plus the scaling and programming of the test system, are crucial elements in the value of the test equipment solution.

The L-I-V test sweeps include forward voltage measurements on the laser diode, reverse voltage leakage and breakdown tests, a lasing threshold current test, and light intensity measurement. The latter can involve either AC or DC testing. In DC testing, a reverse biased photodiode is exposed to the output of the laser diode and the resulting photodiode DC current is measured with a picoammeter or electrometer. This current is calibrated to the output level of the laser diode. Current measurements are also conducted on the BFMPD to determine its I-V characteristics and its response to increases in light output as the laser diode drive current is increased. Depending on the test performed, measured current levels can range from picoamps up to a few amps. Obviously, this requires instruments with a large dynamic range and high resolution.

Keithley is currently developing customized test solutions for LDM manufacturers. With proprietary programming of appropriate instruments, Keithley engineers are able to conduct a 100 point sweep in less than 10 seconds. This inexpensive DC test identifies failed assemblies early in the test process so that expensive non-DC domain test systems can be more cost effective on the remaining higher yield components.


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