Wavelength References at the NMISA Fibre Optic Laboratory to Improve Uncertainty of Measurement for Telecommunication Wavelengths

Transcription

1 Wavelength References at the NMISA Fibre Optic Laboratory to Improve Uncertainty of Measurement for Telecommunication Wavelengths Speaker / Author: M. Nel National Metrology Institute of South Africa Private BagX34, Lynwood Ridge, Pretoria, 0040, South Africa MNel@nmisa.org, Phone: ,Fax: Abstract Absolute wavelength references, hydrogen cyanide (HCN) and carbon monoxide (CO) in a glass gas cell coupled with fibre optic ports is used to get reliable and more accurate wavelength reference to improve uncertainties for fibre optic wavelength calibrations in the S-, C, and L-bands for telecommunication purposes. Method validation using a gas cell and a calibrated laser are discussed. The accuracy of the Optical Spectrum Analyser (OSA) is determined and wavelength sources can now be calibrated with an uncertainty of 0,05 nm at NMISA. 1. Introduction Dense wavelength division multiplexing (DWDM) is a method used to increase the amount of data that can be transmitted across fibre optic networks. DWDM has become the standard technology in high-capacity telecommunications networks, where the fibre length varies from a few kilometres to a few thousands of kilometres. DWDM improves the data-carrying capacity (bandwidth capability) of networks by sending more than one data stream through a single fibre. Every data stream is sent at a specific optical wavelength (i.e. frequency), called a channel. In this way, each fibre in a network can carry many independent channels simultaneously. Wavelength measurements become increasingly more important as the number of channels is increased. This is due to a reduction in the wavelength gap (channel spacing) maintained between adjacent channels. Dense in DWDM refers to the fact that these channels are very closely spaced, less than 1 nm. Accurate wavelength measurements are required to be able to differentiate between consecutive channels, so that the signals can be received independently. Currently, telecommunications applications utilise wavelengths around 850 nm (multimode fibres), as well as in the whole range from nm to nm (single-mode fibres). Owing to its importance for DWDM, the wavelength region in which the highest accuracies are required, is that between nm and nm, which includes the so-called S-, C, and L- bands. An absolute wavelength standard is a standard which does not require calibration, and will remain at the correct wavelength [1]. These absolute wavelength references are available in glass cells coupled with fibre and area great improvement to previously cumbersome, sometimes dangerous and expensive measurement setups. For example, using vacuum pumps, toxic gasses in cylinders and high power open beam lasers to transmit light through

2 the gas and detect the absorption lines. The gas cells are available as a closed system which is already fibre connectorised, only a few centimetres in size, it is compact and safe to use even if it has toxic gas inside. Investigation into capturing the gas molecules in photonic band gap fibres to utilise as wavelength references [2] was also done. This has been demonstrated with acetylene and methane gas. The gas filled fibres are used for monitoring calibration channels. In this paper the implementation of ahydrogen cyanide (HCN) and carbon monoxide (CO) fibre coupled gas cell is discussed to reduce measurement uncertainties in the telecommunication wavelength range. 2. Wavelength references There are a number of gasses that can be used to cover the wavelength spectrum for telecommunication. These include acetylene (C 2 H 2 ), hydrogen cyanide (HCN), carbon monoxide (CO), carbon dioxide(co 2 ), hydrogen sulphide (H 2 S) andmethane (CH 4 ) [1]. The absorption spectra of the molecules all vary, some lines are stronger than other lines and in different wavelengths ranges. It is best to choose stronger lines, lines where more absorption takes place, which cover the wavelength range you require. When the absorption is weak at a specific wavelength it can easily be lost in noise of the light emitting diode (LED) source being used. At the NMISA fibre optic laboratory, hydrogen cyanide (HCN) and carbon monoxide (CO) fibre coupled gas cell were specifically selected because of their near-infrared absorption spectrum also overlapping with that of acetylene. NMISA already had an acetylene cell which is build-in into the Optical Spectrum Analyser (OSA). The selected gas (HCN, CO) reference lines extend as far as nm, which covers the nm telecommunication band. Figure 1 gives a picture of the fibre coupled reference cell. Figure 1: Fibre optic connectorised hydrogen cyanide (HCN) and carbon monoxide (CO) gas cell. 3. Equipment and measurements An Optical Spectrum Analyser (OSA), a broadband LED source (fibre coupled) and the fibre coupled gas cell were used to measure the absorption spectra.as an example, the laser source, is used to show that a measurement uncertainty of 0,05 nm can be achieved at NMISA.

3 A fibre optic coupled LED source with sufficient power and low noise is required in the wavelength range of the absolute standards, nm nm. An OSA with a resolution of 0,02 nm was used to capture the trace from the LED through the gas cell (absolute standard). Figure 2 shows the measurement setup for capturing the absorption spectra on the OSA. OSA LED source Wavelength reference Figure2: The measurement to capture the absorption lines. Reduced measurement uncertainty over a limited wavelength range (1 511 nm to nm) could be achieved with an acetylene (C 2 H 2 ) cell [3], which is internal to the OSA (Ando AQ6317). Thus the hydrogen cyanide (HCN) and carbon monoxide (CO) cell were acquired to improve measurement uncertainties up to nm. HCN [4] was chosen specifically because its near-infrared absorption spectrum overlaps with that of acetylene and also extends past nm. The CO absorption lines extend to past nm [5]. This can clearly be seen in Figure 3, which shows the hydrogen cyanide and carbon monoxide absorption bands and specific lines of the reference gas cell. The absorption lines can be clearly seen and compared to the certified absorption spectra of the gasses [3][4][5] Power [dbm] HCN CO Wavelength [nm] Figure 3: Gas absorption lines of the new dual-gas reference cell.

4 4. Validation A tuneable laser was calibrated on the OSA using the absorption lines as reference. Certified values from another national metrology institute (NMI) were used to compare our measurement results in a validation exercise. Figure 4 shows the measurement setup. OSA Tuneable laser source Step attenuator Figure 4: Measurement setup for measuring the tuneable laser on the OSA. In table 1 the certified values from another NMI and measured results from NMISA are shown. A normalised error, En = (R m R r )/sqrt(u m 2 + U r 2 ) (1) where R m is the measured result of NMISA and U m the associated measurement uncertainty, R r is the certified result and U r the associated measurement uncertainty of the NMI, was calculated. All the results are well below 1, the smaller the value the better. NMISA s measurement uncertainty is large compared to that of that of the NMI, but the accuracy of these results is very good, they compare well to within 0,004 nm. Further investigation could be performed, by NMISA, to reduce the measurement uncertainty even more in the wavelength range nm to nm. Table 1: Tuneable laser measured at specific wavelengths. NMI certified values Measured on OSA at NMISA Wavelength [nm] Uncertainty [nm] Wavelength [nm] Uncertainty [nm] En 1542,3837 0, ,383 0,050-0, ,3837 0, ,385 0,050 0, ,4760 0, ,474 0,050-0, ,4940 0, ,490 0,050-0, ,5390 0, ,538 0,050-0, ,5430 0, ,542 0,050-0,02 5. Conclusion The wavelength range, at which an accuracy of 0,05 nm could be obtained for measurement of the wavelength of a source, has been extended beyond nm, up to nm. This is

5 due to the new gas reference cell, containing hydrogen cyanide and carbon monoxide, with absorption bands occurring beyond those of acetylene. Moreover, there has been a fourfold reduction in the accuracy of wavelength measurements in this interval, from 0,2 nm to 0,05 nm. The wavelength range, at which a measurement uncertainty of 0,05 nm can be achieved for measurement of the wavelength of a source, is nm to nm. Results of the tuneable laser calibration compare well with certified values of the laser. 6. References [1] J. C. Petersen and J. Henningsen, Molecules as absolute standards for optical telecommunication. Requirements and characterisation, URSI 2002, paper [2] J. Tuominen, T. Ritari, H. Ludvigsen and J.C. Petersen, Gas filled photonic bandgap fibres as wavelength references, Optics communications 255 (2005) pp [3] SL Gilbert and WC Swann, Acetylene 12C2H2 absorption reference for 1510 nm to 1540 nm wavelength calibration SRM 2517a, NIST Special Publication (2001). [4] SL Gilbert, WC Swann and C-M Wang, Hydrogen cyanide H13C14N absorption reference for 1530 nm to 1565 nm wavelength calibration SRM 2519a, NIST Special Publication (2005). [5] SL Gilbert and WC Swann, Carbon Monoxide Absorption References for 1560 nm to 1630 nm Wavelength Calibration SRM 2514 ( 12 C 16 O) and SRM 2515 ( 13 C 16 O),NIST Special Publication (2002).