A Simple Description of the Nonlinear Optical Amplifier (NLOA)

model and the formalism of Adams [2.2] the static and time-dependent dependence on optical power and wavelength ai*e described.

2.2 A Simple Description of the Nonlinear Optical Amplifier (NLOA)

The semiconductor nonlinear' optical amplifier (NLOA) behaves as an optical gate which can be triggered by an optical (or electrical) signal. The device is an all-optical switch. As suggested by the name the NLOA is an active semiconductor device and has two regions which are independently supplied with an electrical bias current. The two sections are generally of different lengths with the longer section snongly forward biased to produce a gain region and the shorter section only weakly biased to produce a saturable absorption region. It is the saturation of this absorption that results in the nonlinearity and causes the NLOA to behave as a switch. However, this is not the whole story and the performance of the NLOA switch is governed by many parameters and a careful detailed analysis is generally required. A simple and straightforward understanding can be obtained by just considering the device as a tunable optical filter with gain. The NLOA is a Fabiy-Perot (FP) type device and has many FP modes separated in frequency by c/2NL, where L is the cavity length and N is the effective group refractive index experienced by the optical mode. The location of these modes in wavelength is dependent on the single pass optical phase change experienced by the propagating signal. This phase has a dependence on the carrier density in the active waveguide. An increase in carrier density reduces the effective refractive index of the material. The variation of transmission, or gain, across the FP mode (its ‘shape’) is governed by the loss of the cavity, the mode is strongly peaked when the loss is low but flat and broad when high. In the NLOA's case there are two sources of loss. One source of loss is the cavity loss, which is a combination of mirror losses, scattering, free- carrier absoi*ption etc and is unsaturable. The other source of loss is the material absorption. This loss is saturable because the material absorption can be reduced by increasing the carrier density within the material (this will be discussed in the next section). The reduction of this loss can cause the FP mode to change from broad and flat to being peaked. However, although the material absorption can be removed and the finesse of the cavity improved, the carrier density

Ch 2 A Simple Description of the Nonlinear Optical Amplifier (NLOA)

change, associated with the absorption change, also alters the single pass phase change and therefore the FP mode shifts its wavelength as well as changing its spectral profile. This is refened to as a dispersive effect. Figure 2,1 illustrates this effect and is a schematic diagram showing the change in spectral profile of one FP mode due to saturation of an absorption region within a two contact device.

FP mode with absorber saturated

Gain

FP mode with gain and absorber saturated

/ ' \

‘sat

Unstaurated FP mode

Wavelength A

Figure 2.1: A schematic diagram showing the variation ofF P mode shape and position with the saturation o f an absorber section within a two-contact device.

The dotted curve in Figure 2.1 represents a FP mode profile where the NLOA is biased with a saturable absorber region. The FP modes are small in size and the NLOA is said to be 'off. Consider now an optical signal injected into the NLOA at a wavelength slightly shorter than one of the FP mode wavelengths, point A in Figure 2.1. The signal wavelength experiences a low gain G|. Now suppose that the absorption region within the NLOA becomes satuiated due to this input optical signal. The saturation of absorption alters the carrier density which in turn reduces the cavity loss and changes the single pass phase change experienced by the light in the FP cavity. The FP modes of the NLOA now look like those shown by the solid line in Figure 2.1. The NLOA is said to be 'on' as the signal wavelength at A now sees a higher gain G2. It is

this large change in gain, G2-Gj, due to the increased finesse and shift in position of the FP

modes due to the saturation of the absorption that gives the NLOA its switch-like characteristics. As we shall see both theoretically and experimentally, the saturation of absorption can result in changes in signal gain that exceed 10 dB.

If the signal power injected into the NLOA is too great then the reduction of the carrier density due to stimulated emission may exceed the pumping rate from the bias current to the

Ch 2 A Simple Description of the Nonlinear Optical Amplifier (NLOA)

device. This effectively reduces the carrier density in the gain region and causes gain saturation. The saturation of the gain changes the FP modes in the opposite way to absorber saturation, thus reducing the peak gain and shifting the modes to longer wavelengths, see dot-dash curve in Figure 2.1. If gain saturation occurs then the 'on* signal gain will be reduced, Ggat- This reduces the effective conti'ast in gain between 'off and 'on' states.

The above description is a rather simplified outline. A more accurate description of the device performance can be obtained by considering many other parameters. In the following section other parameters are included in a logical order so that a model can be developed to predict the qualitative performance of NLOA's. First we need to be able to predict the material gain (or loss) so that the NLOA's device gain can be determined. Both the steady-state and dynamic chai’acteristics of the devices need to be considered, requiring the modelling of the rate equations describing the inter-relation of the photon and carrier populations in the two regions. By putting these descriptions together a theoretical prediction of the NLOA's characteristics can be obtained. The basis on which to build such a model is the relationship between the material gain and caiTier density.

Conduction Band

heavy holes

Valence Band _{light holes}

b a

Figure 2.2: a) A schematic diagram o f the conduction and valence bands in InGaAsP and b) a schematic o f the occupied states variation with energy.