External-cavity diode lasers provide absolute references for WDM testing
Michael Lang
The rapid growth in data traffic is driving development of higher-capacity optical-fiber-transmission methods based on wavelength-division multiplexing (WDM). In WDM, several lasers at different wavelengths simultaneously transmit separate streams of data along a single fiber. At the receiving end, the different wavelengths are optically separated and individually detected. The transmission capacity of an existing fiber link can, therefore, be instantly increased by a factor equal to the number of individual wavelengths used. The rapid evolution of WDM technology has created the need for laser sources with output that is reliably locked to a known absolute wavelength (frequency) with high precision.
Recently, the International Telecommunications Union (Arlington, VA) created a standard for 32-channel WDM transmission that consists of 40 wavelength channels, separated by only 100 GHz (0.8 nm), eight of which are for overhead--signal management. The close channel spacing is necessary to ensure that all the channels fit under the high-gain portion of the erbium-doped fiber-amplifier gain curve, centered around 1545 nm. Looking to the future, higher-density WDM formats with 64 channels separated by only 50 GHz (0.4 nm) are predicted. And some researchers are already experimenting with channel spacings of less than 2 GHz.
Successful WDM operation depends on eliminating crosstalk between the different wavelength chan nels. This requires that all the active and passive system components operate within the spectral window they were designed for, without undue drift or chirp. At the manufacturing, installation, and servicing/ repair stages, this requirement creates a need to check the wavelength performance of key components such as lasers, filters, and fiber gratings. Wavelength meters, spectrometers, and spectrum analyzers are important in this area. However, while these tools may suffice for field testing of the latest WDM systems, more-accurate, absolute methods are needed for use in quality control and testing laboratories, as well as for the research and development effort to produce future generations of WDM technology.
Absolute wavelength metrics
One method of providing an absolute yet compact wavelength standard is to lock the output of a diode laser to an atomic or molecular spectral line. Indeed, such absorption and emission lines have long been used as absolute wavelength standards in several fields. Until recently, however, there were almost no spectral standards in the 1550-nm spectral region. This absence was partly because there are few strong spectral absorptions in this region and also because there was little commercial demand for such absolute standards. The explosive growth of WDM technology has changed this situation.
In the past two years, tremendous advances have been made in calibrating the spectra of both acetylene (C2H2) and hydrogen cyanide (HCN), because these molecules have a combination of vibrational overtone absorption lines that span the entire 1550-nm erbium gain region (see Fig. 1). The first atlas of acetylene lines has now been published by the Tokyo Institute of Technology,1 which claims 150-kHz absolute precision in the frequency domain--traceable to the cesium atomic-clock frequency via an absorption line of rubidium. Also, researchers at NIST (Boulder, CO) have characterized the small, yet measurable, pressure shifts (up to 200-torr pressure) of these lines.
Third-derivative locking
Unlike the single atomic lines of neon and krypton, acetylene and hydrogen cyanide each have a comb of spectral lines throughout the spectral region of interest. Equally important, their absorption lines can be detected in simple, compact cells with no need for noisy, delicate discharge cells. For these reasons, Environment Optical Sensors Inc. (ESOI; Boulder, CO) chose acetylene and hydrogen cyanide absorption lines as the basis for a series of rugged reference sources for WDM work (see Fig. 2).
The emitting element is a laser diode with a highly efficient (reflectivity <10-5) anti reflection coating on its front facet. This diode operates in a Littman-Metcalf external cavity, where the position of the tuning mirror determines the output wavelength. The collimated output of this cavity is coupled into a FC/APC (fiber connector/angled physical connector) fiber coupler. A beamsplitter, positioned before this coupler, splits off 5% of the collimated laser light, which allows a normalized (dual-channel) measurement of the ab sorption in a cell containing C2H2 or HCN, by a pair of indium gallium arsenide (InGaAs) photo detectors.
Both C2H2 and HCN have line widths on the order of 1 GHz, due to Doppler broadening. To accurately lock to line center, a technique called third-derivative locking is used. A small (about 1 MHz) frequency dither is applied to the laser diode by the piezoelectric actuator that translates the tuning mirror. The detection electronics then take the third derivative of the normalized absorption signal in the frequency domain. This third derivative has the twin advantages of being extremely steep as it passes through zero and being immune to any background drifts in the absorption measurement. A feedback loop actively adjusts the dc position of the tuning mirror to maintain a zero third-derivative signal. Using this approach, the compact, portable source has a long-term stability of ۫ MHz, or one part in 108.
There are many rotational lines in the C2H2 or HCN spectra to which the WDM reference source can be stabilized. In practice, the end user selects from this list of absolute wavelengths--just as RF engineers have selected the frequency of crystal reference oscillators for many years. The angle of the tuning mirror is then permanently set at the factory. To further enhance the economy of this approach, several active heads (at different wavelengths) can be used interchangeably with the same electronic controller.
Heterodyne source testing
These reference sources can be used to calibrate the accuracy of important WDM diagnostic tools such as wavelength meters and high-resolution spectrometers, which are then used to check the passive components such as fiber gratings and receiver filters. These diagnostic tools also can measure the wavelength of the stabilized distributed-feedback diodes and fiber-grating controlled laser-diode sources used in WDM. The most accurate measurements of these sources, however, are made by direct measurement against the WDM reference source. The laser being measured can then be traced back to the cesium clock with ۫ MHz precision.
In this type of heterodyne measurement, the output of the WDM reference source and the test laser are coupled into a high-speed InGaAs photodetector (see Fig. 3). The resulting beat frequency is measured with a high-accuracy spectrum analyzer or frequency counter.
How fast must the photodetector be? When answering this question, another advantage of C2H2 and HCN becomes apparent. The rotational line spacing in both molecules is less than 30 GHz. It is possible, therefore, to select a WDM reference source within roughly 15 GHz of a given frequency. Detecting beat frequencies of 15 and even 30 GHz is straightforward, because there are off-the-shelf InGaAs photodetectors capable of 50-GHz detection and custom devices up to 100 GHz. Larger intervals can be calibrated by "daisy-chaining" the photodetectors. The WDM reference source is used to calibrate one laser at an interval of less than 50 GHz, which is then used to calibrate another laser at an additional interval of less than or equal to 50 GHz, and so on. Passing along a frequency standard from one source to another in this type of linkage has long been the standard method by which all calibrated spectral lines are ultimately referenced back to the cesium atomic clock through intermediate calibrated spectral features.
The overall backbone capacity of fiberoptic telecommunications systems is expected to double approximately every 3.7 months. This growth rate makes development of stabilized, accurate, absolute frequency references a technological imperative. o
REFERENCE
1. K. Nakagawa et al., "Accurate optical frequency atlas of the 1.5 µm bands of acetylene," JOSA, B, vol. 13, 2708 (1996).
FIGURE 1. Vibrational overtone bands of hydrogen cyanide (HCN) and acetylene (C2H2) together span the erbium gain region.
FIGURE 2. WDM reference source from Environment Optical Sensors Inc. consists of an external-cavity diode laser locked to an integral absorption cell containing either acetylene (C2H2) or hydrogen cyanide (HCN).
FIGURE 3. Wavelength-division-multiplexing source is calibrated by measuring a heterodyne beat frequency using a high-speed InGaAs detector.
MICHAEL LANG is vice president of business development, Environmental Optical Sensors Inc., 6395 Gunpark Drive, Boulder, CO; e-mail: [email protected].