Clamped gain travelling wave semiconductor optical amplifier for wavelength division multiplexing applications

Abstract

Introduction : Crosstalk induced by gain saturation and short carrier lifetime, intrinsic to the semiconductor medium, can severely limits the application of Semiconductor Optical Amplifiers (SOA) in Wavelength Division Multiplexing (WDM) systems with intensity modulated scheme [ 11. Suppression of signal induced gain fluctuations using gain clamping by laser oscillation at a wavelength different from signal bandwidth has already been demonstrated in the case of a semiconductor amplifier with an external wavelength selective reflector [2]. In this paper, we report a new Clamped Gain SOA (CG SOA) structure with an integrated Bragg grating as a wavelength selective reflector and demonstrate crosstalk suppression in case of WDM application at 2.5 Gbit/s. Device structure and principle : The proposed amplifier structure ( Figure 1) is similar to those reported in [3] and is based on 0.5 pm wide square active stripe made of InGaAsP bulk material for polarisation independant operation. The active stripe is lateraly tapered over 70 pm at both ends for reduction of far field divergence and improvement of coupling efficiency to the fiber. Window regions and antireflection coating are used for suppression of cleaved facet reflectivity. The total length of the device is 600 pm. A distributed Bragg grating is introduced under the active stripe and forms a wavelength selective reflector. The coupling coefficient of the grating is chosen so that it provides a high single pass gain at the oscillation threshold. Assuming homogeneous broadening of semiconductor gain lineshape, the overall gain bandwidth is expected to be clamped once the threshold oscillation is reached. At a wavelength different from the Bragg wavelength, the gain is independent of the signal intensity as long as the lasing oscillation is not switched off through carrier depletion effect. The optical power is, in fact, stored in the lasing wavelength and converted into amplified signal following intensity modulations,with a time response limited by the relaxation oscillation frequency. Device characteristics and results : Static light / current characteristic of CG SOA is shown on figure 2. Lasing oscillation threshold is clearly observed around 60 mA. The maximum output power is 15 mW per facet at 200mA and is mainly limited by thermal effects. Small signal fiber to fiber gain versus driving current is shown on figure 3 for TE and TM polarisations: thanks to the square active stripe, the TE / TM gain difference is less than 1 dB and gain reaches 15 dB at 60 mA where the oscillation occurs. Figure 3 clearly shows the gain clamping effect due to the distributed grating: at 60 mA the gain saturate strongly and is nearly independant, in the one dB range, on the driving current from 60 up to 200 mA. Taking into account 3 to 4 dB coupling losses per facet, the internal gain is in the 21-23 dB range. The spectrum of the light at the output of the amplifier is shown on figure 4 for -25 dBm signal power launched at the input of the device. We can observe the lasing wave...

Optical Bistability in a Traveling-Wave SOA Connected to a DFB Laser Diode: Theory and Experiment

“…The GCSOA used during the experiments was a commercially available component acquired from Alcatel [1] some years ago. The lasing wavelength was 1508 nm.…”

“…Up till now two types of integrated linear optical amplifiers exist, each of them based on the gain clamping by an internal laser cavity. The first type, the so-called gain-clamped SOA (GCSOA) [1], uses a longitudinal laser cavity and is schematically depicted in Fig. 1(a).…”

“…1(b). Both components proved their potential in a number of high-speed applications both using the linear (e.g., [1], [3]- [5]) and the nonlinear properties (e.g., [6]- [8]). …”

Numerical and theoretical study of the crosstalk in gain clamped semiconductor optical amplifiers

“…This is particularly true for switching applications in WDM based optical networks [1], [2], where SOA's could provide very high on-off switching ratios as well as loss compensation. Recent progress in SOA technology has solved many of the problems (e.g., polarization sensitivity [3], [4] and low saturation output power [5]) which have prevented their widespread deployment in optical networks.…”

A novel optical decision circuit based on a Mach-Zehnder or Michelson interferometer and gain-clamped semiconductor optical amplifiers

“…In fact, to our knowledge an optical decision circuit that is bit rate and modulation format transparant, that is suitable for integration, and that operates on the signal itself instead of on a pump or external pulse sequence, and with a truly digital (i.e., nonperiodic) characteristic has not yet been reported. In this letter, we present an optical decision circuit [4], based on Mach-Zehnder or Michelson interferometers (MZI or MI) and gain-clamped semiconductor optical amplifiers (GCSOA's) [5], [6], which does possess all these properties. In the next section, we start with the description of the decision circuits and we explain their principle of operation.…”