The radio-frequency (RF) gain and the bandwidth of the analogue fibre link modulated by an electroabsorption modulator (EAM) are examined at high optical powers. With the inclusion of the photocurrent resistance, which is essential in understanding the EAM link under high optical power, the gain-bandwidth product of the fibre link is shown to be compressed, but less severely than the RF gain owing to the enhancement in bandwidth. It is also pointed out that the link with resonantly enhanced RF gain by negative photocurrent resistance can overcome this gain -bandwidth compression under high optical power owing to more rapid increase in gain than the reduction in bandwidth.Introduction: Analogue fibre links have important application areas ranging from antenna remoting and phased-array radars to cable televisions and radio-over-fibre systems owing to their many advantages including high dynamic range and wide bandwidth [1,2]. The analogue links that utilise electroabsorption modulators (EAMs) have been studied extensively to take advantage of the EAM, such as small size, small driving voltage, versatility both as a modulator and a detector, and simple lumped electrodes for relatively high bandwidth (up to 10 GHz) [3][4][5]. As the optical power driving the analogue fibre link has been increased to achieve higher radio-frequency (RF) gain, the operation of the EAM under high optical power has been studied and designs that enable the high-optical-power operation have been suggested. By combining the diluted waveguide that absorbs much reduced optical power per unit length and the modified quantum-well design [6][7][8], an EAM has been demonstrated to handle a very high optical power up to 25 dBm ( 320 mW) [9].Under this very high optical power, however, the analogue fibre link that utilises an EAM has been shown to have saturation in RF gain owing to the photocurrent resistance inherent in the EAM device [9][10][11][12]. The photocurrent resistance R ph is given by (dI ph /dV ) 21 , where I ph is the photocurrent generated in the EAM and V is the reverse bias voltage supplied to the device. As the photocurrent resistance reduces with optical power, the signal voltage drop at the active junction decreases and the saturation in RF gain occurs. In [9], for example, it is shown experimentally that the compression factor in the RF gain reaches up to 23 dB at an optical power of 25 dBm.In this Letter we examine how the bandwidth is affected by the photocurrent resistance, which is an aspect rarely looked at in the analogue fibre link utilising the EAM. As a figure of merit that includes both the RF gain and the bandwidth, the gain -bandwidth product (GBW) of the analogue fibre link is examined under very high optical power. The concept of negative photocurrent resistance has been proposed recently to break the limit posed by the saturation due to the photocurrent [11]. The negative photocurrent resistance is caused by blue shift in electroabsorption, which can be induced by, for example, a step barrier inside the conventio...
