Design of a precision optical low-coherence reflector - Technical

Hewlett-Packard Journal, Feb, 1993 by D. Howard Booster, Harry Chou, Michael G. Hart, Steven J. Mifsud, Rollin F. Rawson

The HP 8504A precision reflectometer uses the classic Michelson interferometric measurement technique to allow designers and manufacturers to measure reflections easily in optical components and assemblies. Spatial resolution is on the order of tens of micrometers.

An optical low-coherence reflectometer uses a Michelson interferometer configured with a broadband source to make spatially resolved measurements of reflections within optical components and assemblies. The short coherence length* of the source, usually a light-emitting diode, produces a spatial resolution on the order of tens of micrometers, while the measurement range is limited only by the travel of the reference mirror in the interferometer. The objective of the HP 8504A precision reflectometer project was to provide the lightwave industry with the first commercial implementation of the Michelson interferometric measurement technique in which all the components are provided in one package.

While white-light interferometry** has been around for some time in research labs, the measurement setup is complex, typically requiring an optical table and a very skilled person to make the measurement and interpret the results. To be successful, the commercial version of this measurement technique needs to be rugged, reliable, easy to use, and affordable for manufacturing applications. To meet this challenge within the constraints of a short development cycle and the requirement to minimize development cost, a small group of experienced engineers was assembled to build the HP 8504A. Off-the-shelf parts were used wherever possible and every opportunity to leverage previous designs was taken.

The article on page 52 provides a detailed discussion about the Michelson interferometer and white-light interferometry.

Several instrument concepts were considered for the HP 8504A precision reflectometer, including a single, designedfrom-scratch box and an upgrade or retrofit of an existing network analyzer. The final design consists of two packages of reasonable size and weight which can be transported separately and then reattached Without losing factory calibration.

Measurement Example

Fig. 2 shows the display of a measurement taken with the HP 8504A precision reflectometer and an illustration of the device under test. The vertical axis of the display shows the magnitude of the optical reflections seen looking into the device and the horizontal axis shows the positions of the reflections.

In this case, the device is a small packaged laser. The first reflection is from the end of the fiber pigtail. The return loss is approximately 14 dB, which one would expect from the polished end of the fiber. The next two reflections are from the two faces of the package window that provides the hermetic seal for the laser. The reflections are very low because these surfaces are coated with an antireflection film. Reflections from both sides of the spherical lens can also be seen. Because of the spherical shape of the lens the reflected light diverges, causing lens reflections to be small and limiting the amount of reflected light recaptured by the fiber. Finally, reflections from the front and rear facets of the laser chip are clearly visible.

Measurements of this type are nondestructive and valuable to both the component designer who wants to optimize each element of the component and the process engineer who wants to monitor the component's quality and assembly process consistency.

Block Diagram

Fig. 3 shows the block diagram of the HP 8504A precision reflectometer. In its standard configuration, the instrument contains two light-emitting diode sources, one at 1300 nm and the other at 1550 nm. The outputs of the two sources are multiplexed by a wavelength division multiplexer. (Only one source is on at a time.) The output of the multiplexer is split into the two anus of the Michelson interferometer by a 3-dB directional coupler. The test arm is simply one leg of the directional coupler to which a test device can be connected. The reference arm output is polarized and collimated, and after going through a retroreflector, is reflected back along the same path by a stationary mirror and reenters the fiber. This reflection from the reference ann is then combined with the reflection from the test arm by the directional coupler and sent to the receiver.

The retroreflector doubles the optical path length for a given mechanical travel and also improves the stability of the optical assembly. The reference arm also contains an extension cable that can be changed from the instrument front panel. By varying the length of this extension cable, the reference plane of the measurement can be positioned at any desired point that is, the measurement window can be offset to compensate for the fiber pigtail on the device under test.

The retroreflector scans during the measurement and hence the light reflected from the reference arm of the reflectometer is shifted in frequency (because of the Doppler effect) by an amount proportional to the scanning velocity of the retroreflector (a frequency shift of 27.7 kHz in this case). Thus, two beams of different optical frequencies are incident upon the polarization diversity receiver, and it is their difference frequency (or beat frequency) of 27.7 kHz that is processed by the receiver.