Wideband chirp measurement technique for high bit rate sources

Abstract

the value corresponding to the selected reference point. This was realized by detecting the averaged intensity of the pulse sequence through the etalon, and by applying this signal in a feedback loop to control the temperature of the silicon chip. The control method relies on the average power staying constant during the measurement. The feedback signal was generated by modulating the pulse width of an external frequency generator by the detected average intensity, and by using a microcontroller to generate a current proportional to the width of the pulse. The microcontroller was programmed to perform all of the operations needed in the measurements. Ž . First, the transmission spectrum of the etalon see Fig. 1 is scanned by applying a high heating current. During the scan, the average power of the laser should stay constant to avoid errors when the reference points are determined. The value for the transmission at the maximum and minimum points T and T are stored, and the desired reference point T max min 0 Ž . is calculated. We chose the reference point T see Fig. 1 The performance of the device was tested by measuring the frequency chirp of a DFB laser that was modulated using a pseudorandom-bit sequence at the rate 2.5 Gbitsrs. The time trace of the detected signal intensity for a 12 bit sequence ''010110011101'' and the corresponding time-resolved frequency chirp are shown in Figure 3. The average power of the modulated laser output was y0.8 dBm. The adiabatic part of the chirp varies linearly with the signal power, but there are also larger frequency transients present at the fast changes of the optical power. The detected time trace of the signal is an average of 64 values at each measurement point. CONCLUSIONWe have developed and demonstrated a simple and inexpensive device for measurements of the time-resolved frequency chirp in narrowband light sources used for telecommunication purposes. The device makes use of a solid silicon wafer as a frequency discriminator to convert fluctuations in the laser frequency into variations in the transmitted signal intensity. The transmission of the etalon is tuned by controlling the refractive index of silicon by changing the temperature of Figure 3 Time traces of the modulated laser output power and the measured frequency chirp for a DFB laser operating at 1.55 m the chip. The FSR of 105 GHz allows frequency chirp up to "25 GHz to be measured with a time resolution of about 20 ps. The attractive features of the chirp analyzer include a broad wavelength range, insensitivity to mechanical vibrations, and to the polarization state of the light.

Discrete Multitone Format for Repeater-Less Direct-Modulation Direct-Detection Over 150 km

“…7b) as a function of time for both PFL and DML. The chirp was measured using a delayed line interferometer with a delay of 25 ps as an optical frequency discriminator [25]. For both lasers, the chirp value changes proportionally to the driving waveform, suggesting that the chirp was predominantly adiabatic [26].…”

Computer simulation of high frequency modulation of laser diode radiation

“…Laser's main mode frequency deviations during direct current modulation are referred to as a frequency chirp [3,4]. Chirp dependence on time is usually referred to as a time-resolved chirp.…”

“…where FSR is the free spectral range of the interferometer, V AM is the amplitude modulation term, V FM is the frequency modulation term [3]. Laser's main mode frequency deviations during direct current modulation are referred to as a frequency chirp [3,4].…”

Investigation of High-Speed Transient Processes and Parameter Extraction of InGaAsP Laser Diodes

“…When the bias current was modulated, the optical beam power and optical wavelength became modulated as well. The optical beam is coupled to the tapered fiber (Shani et al, 1989;Alder et al, 2000), which is connected to a Mach-Zehnder fiber interferometer (Saunders et al, 1994;Jeong & Park, 1997;Laverdiere et al, 2003). An optical amplifier can be used if the interferometer output signal is too weak.…”