The Ultra High Resolution OSA (UHR-OSA) proposed by APEX Technologies remains a very important tool not only to measure the optical OFDM (Orthogonal Frequency Division Multiplexing) signal spectrums even with few tens of MHz spacing sub-carriers but also to adjust and adapt it to other modulation techniques.
APEX-UHR-OSA incessant need in OFDM research area
APEX UHR-OSAs have found a wide industrial and academic success marked by the incessant demand of researchers and experts in all aspects of the OFDM technique. Indeed, more than 30 APEX-UHR-OSAs are currently used throughout the world by several universities such as Dublin City university (IRELAND) [1], Melbourne university (AUSTRALIA), IT AVEIRO (PORTUGAL), Bangor university[2],… and industries such as Orange Labs [3], KDDI R&D Lab [4],…What is OFDM?
Optical OFDM (Orthogonal frequency division multiplexing) is a promising format for the next generation of long-haul and access networks because of its high spectral efficiency and the resistance to a variety of dispersions including chromatic dispersion (CD). The basic principle of OFDM technique is to carry information using several hundred sub-carriers which transport a fraction of the data rate each. The main feature of OFDM resides in the orthogonality of its sub-carriers obtained by spacing each of them with a multiple of the inverse of symbol duration (of the low bit-rate streams). Its main advantage is to avoid the inter-carrier interference and to allow spectral overlapping in order to ensure a high spectral efficiency. The orthogonality is maintained by adding a cyclic prefix to each OFDM symbol in order to eliminate the inter-symbol interference (ISI). In terms of transmission, OFDM has received increased attention thanks to its robustness to ISI, namely chromatic dispersion (CD) and polarisation mode dispersion (PMD), provided by the low sub-carrier data rate without any need for complex equalisation at the receiver side. For this reason, increasing the sub-carrier number is very crucial so that each sub-carrier transports the lowest possible bit-rate stream (equal to the nominal bit-rate divided by the number of sub-carriers). In frequency domain, it corresponds to a few ten of MHz (typical 20 MHz to 50 MHz) sub-carrier spacing. Fibre-optic OFDM systems can be realised either with direct detection optical (DDO) or with coherent optical detection (COD). What else makes APEX-UHR-OSA so good?[caption id="attachment_3868" align="alignleft" width="300"]
Figure 1: Double Side Band (DSB) OFDM spectrum measured by APEX-UHR-OSA taken from [5][/caption]The key feature of the APEX-UHR-OSA is its capacity to measure the OFDM signal spectrum and to display all the sub-carriers clearly even with a few tens of MHz spacing, this is not possible with a traditional grating based OSA resolution (down to 20 pm/ 2.5 GHz). Based on an interferometric method, APEX Technologies UHR-OSA combines high resolution (up to 5 MHz, 0.04 pm), wavelength accuracy (+/- 3 pm) and high dynamic range. These equipment specifications (in particular the resolution) are good enough to see the details and the separation between adjacent sub-carriers (figure 1).
Figure 1: Double Side Band (DSB) OFDM spectrum measured by APEX-UHR-OSA taken from [5][/caption]The key feature of the APEX-UHR-OSA is its capacity to measure the OFDM signal spectrum and to display all the sub-carriers clearly even with a few tens of MHz spacing, this is not possible with a traditional grating based OSA resolution (down to 20 pm/ 2.5 GHz). Based on an interferometric method, APEX Technologies UHR-OSA combines high resolution (up to 5 MHz, 0.04 pm), wavelength accuracy (+/- 3 pm) and high dynamic range. These equipment specifications (in particular the resolution) are good enough to see the details and the separation between adjacent sub-carriers (figure 1).
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