Fiber ring laser speeds swept-wavelength measurement

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Philip J. Santangelo, Joel Mock, Jerry Volcy, Keith Chandler, Kevin Hsu, and Calvin Miller

A new approach to measurement provides increased amplitude vs. wavelength information, higher speeds, and improved dynamic range, insertion loss, and wavelength accuracy.

Optical components for dense wavelength-division multiplexing (DWDM) systems and networks generally require characterization over a dynamic range exceeding 60 dB with wavelength accuracy of ±5 pm or better and loss in device accuracy of ±0.1 dB. Available systems that meet these requirements take many seconds, or minutes, per scan, which are insufficient speeds for device characterization during manufacturing.

Typical swept-wavelength measurements are performed with combinations of external-cavity tunable diode lasers, amplified spontaneous emission sources, optical spectrum analyzers (OSAs), power meters, and wavemeters (see table). However, test schemes using these elements have several limitations.

The speeds of swept-wavelength systems generally available are limited by several components. Most lasers can't sweep the band of interest in less than one second. In addition, the bandwidth and gain switching of the power meters can take seconds, and the ability to digital-signal process and transfer data to a computer can account for additional delays.

Using a compilation of several pieces of equipment (external-cavity laser plus OSA plus power meter) and the logic needed for these devices to communicate introduces more processing delays related to integration software. Typical overall measurement times can range from tens of seconds to minutes per single-port device and to tens of minutes for multiport devices.

HIGHER-SPEED MEASUREMENTS
A need has existed for a measurement system that can provide the necessary amplitude vs. wavelength information in real time, especially for active alignment and in-process monitoring during manufacturing. The need for a fast and accurate integrated system is even greater for functional tests of multiport optical devices such as arrayed waveguide gratings and the like.

This requires a laser that can sweep rapidly, with wide bandwidth, wide dynamic range detection, and high-speed data transfer. It also requires an instrument driven by an operating system that can manage large amounts of data without interruption due to user interaction.

To achieve greater performance, we have demonstrated a new approach. The first step was to build a laser that can sweep faster than 1 Hz with improved noise characteristics. This was accomplished by building an erbium-doped fiber-ring laser that takes advantage of the near-theoretical filtering properties of tunable fiber Fabry-Perot filters. By doing so, the laser can sweep 50 nm in 100 ms. Its SSE is >85 dB and its STSE ratio is >65 dB (see Fig. 1). This makes it ideal for measuring deep notch filters and specialty gratings, as well as examining crosstalk in devices like multiplexers and interleavers.

The next step was to achieve wide bandwidth and wide dynamic-range detection to take advantage of the laser's low noise level. This was accomplished by using a high-bandwidth logarithmic amplifier that has exceptional log conformity and accuracy over temperature, and has greater than 60-dB dynamic range.

These innovative building blocks were then combined with a National Institute of Standards and Technology traceable wavelength reference and a proprietary etalon that is used to calibrate the system after every sweep. This setup enables an absolute wavelength accuracy better than 5 pm. In addition, because the system recalibrates on every sweep, it never needs to be externally wavelength-calibrated. The user can check the wavelength calibration by displaying the internal wavelength reference trace.

The last step involves managing and processing large amounts of data. On every sweep, power, wavelength, and device data are acquired to provide 2.5-pm resolution data. The data is managed and processed using high-speed data acquisition, with simultaneous hold and sample functionalities on every sweep for the device-under-test channel. An operating system that allows for complete hardware control and isolation from the user enables the processing to be independent of user interaction and therefore maximizes system speed. An overall measurement time of 200 ms is achieved, which is one to two orders of magnitude faster than conventional techniques. The new solution also provides improved dynamic range, insertion loss, and wavelength accuracy.

APPLYING ADVANTAGES
One of the first applications of the component test system has been in the process of manufacturing tunable and fixed Fabry-Perot filters. The instrument provides real-time feedback for minimization of features 30 to 50 dB down from the transmission peak. Filters aligned using the system exhibit vastly improved performance with significantly improved yield and reduced assembly time.

Data from additional WDM components have also been determined (see Fig. 2). Other passive components including fiber Bragg gratings, arrayed waveguide gratings, interference filters, and multiplexers will probably benefit similarly from the high-speed characterizations during fabrication, assembly, and final functional test.

Thanks to the extremely low noise and high power characteristics of the swept fiber ring laser, the laser signal can be split and amplified with an EDFA to measure 8, 16, 32, 64, or even more DWDM channels, without any significant reduction in performance. Because of the high sweep rate and data-acquisition rate of the swept-laser component-test system, many configurations can be used to interrogate multiple devices under test or devices with multiple inputs and/or outputs. The test system can be combined with optical switches to interrogate 16- or 32-channel devices or 16 or 32 different devices in about 3 to 6 seconds. (see Fig. 3).

One advantage of having a laser with a very low noise floor is that an erbium-doped fiber amplifier (EDFA) can amplify the signal from the laser and allow for the full 70 dB of measurement dynamic range for additional channels. In external cavity systems, the use of an EDFA would be prohibited because of the laser noise floor and the dynamic range is decreased by the use of splitters or couplers.

In addition to these configurations, which measure each channel sequentially, an eight-channel simultaneous solution exits . In this arrangement, an add-on photodetector array provides eight additional receivers and an additional processor deals with the amount of data streaming into the instrument.

The component test system can also be used to characterize EDFAs. The shape and gain can be accurately determined with varying amounts of laser input. A variable optical attenuator is used to change the input laser conditions to the EDFA and the output is directed to the receiver.

This high-speed platform may open the door to a number of other real-time products, such as optical spectrum analyzers and laser interrogation systems. Other capabilities will include PDL measurement capability, group delay measurements and specialized software and hardware for tailoring the component test system to specific types of WDM components.

Philip Santangelo, Joel Mock, and Jerry Volcy are R&D engineers; Keith Chandler is director of industrial systems development; Kevin Hsu is VP of R&D; and Calvin Miller is CEO at Micron Optics, 1900 Century Place, Atlanta, GA 30345. Santangelo can be contacted at psantangelo@micronoptics.com.

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