Optical Complex Spectrum Analyzer: High efficient tool for any kind of usual modulation format measurements

Download Article

This application note proves the high efficiency of the Optical Complex Spectrum Analyzer AP2441B/AP2443B in measuring and testing any kind of usual modulation formats. Compared to a standard oscilloscope, the Optical Complex Spectrum Analyzer AP2441B/AP2443B has a maximum bandwidth > 6 THz due the complex spectral analysis principle giving access to the optical phase and intensity both in frequency and time domains. Therefore, the Optical Complex Spectrum Analyzer has no baud/bit rate limitations. Examples of RZ short pulses (2 ps pulse-width), pulse time resolved chirp, Differential Phase-Shift Keying (DPSK) modulation and Quadrature Phase-Shift Keying (QPSK) modulation measurements are presented.

Optical Complex Spectrum Analyzer utility

Fast development of broadband communication services makes stringent the need for further capacity which creates new challenges for all the segments of the optical communication network. This large increase in the magnitude of the data exchanges is driving the research towards advanced modulation formats. Furthermore, this challenge is correlated with stringent requirements for high performance optical test equipments as an important tool to measure and adjust all kind of modulation formats. In this context, test equipment able to measure the optical phase as function of time remains a very important tool in the telecommunications industry, and in research laboratories.
Based on our core technology, APEX Technologies provides the world’s first commercially available Optical Complex Spectrum Analyzer (OCSA) providing the capacity of optical phase measurement both in time and frequency domain for any modulation formats without any bit/baud rate limitations.
The patented method used by the Apex Technologies OCSA is based upon a spectral analysis of the optical field; it extracts the amplitude and the phase of each frequency component (whereas classic optical spectrum analyzers measure only the power spectral density, giving only the amplitude). By knowing the amplitude and the phase of each spectral component, the temporal variations of the amplitude and the phase can be calculated by means of the Fourier transform, providing the intensity, the chirp and the phase as function of time and display the eye diagram, the BER estimation and the EVM (Error Vector Magnitude). In the following paragraphs, we will present some application examples of the OCSA.

Short optical pulse for RZ modulation

Most alternative measurement solutions are bandwidth limited because they measure RZ short pulses in the time domain, while the Apex Technologies OCSA series uses the complex spectrum to calculate the time domain results. The Apex Technologies method is only limited by the wavelength range of the optical complex spectrum analyzer. Actually, if the RZ modulated signal is inside the wavelength range of the optical complex spectrum analyzer it can be measured without distortion (figure 7). The maximum wavelength range offered by Apex Technologies is currently 110nm, covering both the C and L bands and giving a maximum temporal resolution of 75 fs. The optical complex spectrum analyzer is the perfect tool for RZ pulse analysis as well as for high bit rates NRZ signal (40 Gb/s, 160 Gb/s and up) measurements.


Figure 7: 10 Gb/s RZ modulation (2ps pulse width) measured by the Optical Complex Spectrum Analyzer

Time resolve chirp

Most solutions used for time resolved chirp measurement are based on the frequency discrimination method. This method is based on the conversion of the frequency modulation (chirp) into intensity modulation using a Mach Zehnder interferometer (MZI). The intensity modulation is therefore analyzed with a standard oscilloscope. However, this method is not scalable experimentally to higher data rates (40 Gb/s and up) because of the bandwidth limitation of the MZI and the oscilloscope. With the optical complex spectrum analyzer, time resolved chirp is directly calculated from the optical complex spectrum, giving accurate results and excellent repeatability (figure 8).


Figure 8: Intensity and chirp measurement of a z-cut LiNbO3 external modulator measured by the Optical Complex Spectrum Analyzer

DPSK measurement

With DPSK modulation, a digital signal is represented by the phase of the optical carrier. DPSK modulation can be made either with a phase modulator or an intensity modulator (with the bias point adjusted at the minimum transmission).
Figure 9 depicts the intensity (blue trace) and the phase modulation (red trace) of a 10 Gb/s DPSK signals using an intensity modulator. In this case, the two phase states are exactly separated by 180°. These two phase states can be seen on the red trace.
Because of the intensity modulator, we can clearly see the drop in intensity during the phase transitions. For the phase modulation formats, the OCSA is able to display the optical constellation diagram (this is the fact that each modulation state is assigned by a point, the entire of these points represented in complex domain is called constellation)
With this constellation display (figure 10), we can see that the 180° phase change is well made but the bias adjustment was not correct (assymetry between the left and the right part of the graph).


Figure 9: 10 Gb/s DPSK modulation realized with an intensity modulator and measured by the Optical Complex Spectrum Analyzer


Figure 10: The constellation of a DPSK modulation displayed by the Optical Complex Spectrum Analyzer

DPSK modulation can be realized with a phase modulator also. In this case the intensity is stable over the time scale and only the phase is varying. When using a phase
modulator, every kind of phase state can be obtained, so it is necessary to measure the time resolved phase to verify if the two phase states are spaced by 180°. In this example (figure 11), RF power applied to the modulator was not strong enough and because of this instead of having two phase states spaced by 180° we now only have 135°.


Figure 11: The phase of a DPSK modulation realized with a phase modulator and displayed by the Optical Complex Spectrum Analyzer

QPSK measurement

Compared to DPSK modulation having only 2 different phase states, QPSK is coded with 4 different phase states. So, compared to a DPSK signal, QPSK is doubling the spectral efficiency (same spectral width but double bit rate). Adjustment of a QPSK modulation is difficult to realize and there was a need of having an instrument to measure the phase variations as a function of time. Now with the OCSA, it is now possible to visualize this phase modulation and adjust all the modulation parameters accurately in order to improve the transmission quality (figure 12).


Figure 12:  40 Gb/s QPSK modulation measured by the Optical Complex Spectrum Analyzer

Optical spectrum analysis with ultra high resolution

The OCSA can be used as a normal optical spectrum analyzer but at a 100 times better resolution. With a 20MHz (0.16pm) resolution, 60dB@+/-2pm close-in dynamic range, +/-3pm wavelength accuracy, the OCSA is the perfect instrument for spectrum analysis (figure 13). In addition to its spectral efficiency, ultra high resolution spectrum analysis provides a lot of other details (figure 14) on a modulated signal such as Crosstalk, laser linewidth, relaxation oscillations, and more.


Figure 13: comparison between a 10 Gb/s OOK modulated signal (blue trace) and a 10Gb/s Duobinary modulated signal (red trace)


Figure 14: 10 Gb/s OOK modulated signal measured by the Optical Complex Spectrum Analyzer


The Apex Technologies Optical complex Spectrum Analyzer (OCSA) is the perfect tool for measuring any kind of optical modulation formats without any bit/baud rate limitation. The OCSA most important key features are:

  •  Time resolved intensity measurement with a > 6THz maximum bandwidth
  • Time resolved chirp measurement with an high accuracy and repeatability
  • Time resolved phase measurement for any kind of phase modulation formats
  • Optical complex spectrum for chromatic dispersion measurement
  • Optical spectrum analysis with ultra high resolution

All high speed modulation formats can now be accurately measured and leaves any kind of guesswork behind without the need for multiple instruments demanding a lot of space.