QM4E.3.pdf CLEO:2013 Technical Digest © OSA 2013

Amplitude-invariant Fast Light in a Semiconductor Optical Amplifier for Microwave Photonics

Matthew P. Chang and Paul R. Prucnal Lightwave Communication Research Laboratory, Department of Electrical Engineering, Princeton University, Princeton NJ 08540

mpchang@princeton.edu

Abstract: We experimentally demonstrate tunable fast light in a semiconductor optical amplifier based on cross-gain modulation. Up to 60 ps amplitude-invariant time delay is achieved on a 500 MHz microwave signal. OCIS codes: (250.5980) Semiconductor optical amplifiers; (270.1670) Coherent optical effects; (060.5625) Radio frequency photonics

1. Introduction

The phenomenon of slow and fast light has been an exciting research topic because of its potential applications in optical storage, optical buffering, and ultrafast tunable optical delay lines [1]. The effect refers to controlling the group velocity of a medium, and can be achieved at room temperature using techniques such as coherent population oscillations in semiconductors [2]. To date, slow/fast light has been demonstrated in ruby [2], semiconductor optical amplifiers (SOA) [3,4], and many other media. The delay of signals in these semiconductor devices can occur at GHz bandwidths, and can be tuned quickly by adjusting electrical and optical pumps [4]. However, to tune the delay, one must also concede an amplitude variation [3,4], which is non-ideal for many applications. In this work, we demonstrate amplitude-invariant tunable fast light in a SOA using coherent population oscillations and cross-gain modulation (XGM). The device can delay a 500 MHz signal by up to 60 ps by varying the SOA bias current and optical pump power, and can be used as an ultrafast optical delay line for microwave photonics.

2. Operation Principles and Experimental Setup

The group velocity, vg, of light in a propagating medium is given by the following expression [1]:

g−1v = dk(ω)

dω= 1

cn(ω)+ω dn(ω)

dω⎛

⎝⎜

⎞

⎠⎟ (1)

where k is the wave vector, c is the speed of light, and ω is the angular frequency of the light wave. Slow and fast light is based on changing vg by producing a rapid change in refractive index, dn(ω)/dω. By the Kramer-Kronig relations, this is done by creating a sudden gain peak/trough, or a spectral hole, in the medium’s gain spectrum [1].

To demonstrate this in a SOA, the setup in Fig. 1 is used. A microwave signal is modulated onto an optical carrier (λ=1551.72 nm), the pump, by an electro-absorption modulator. A variable optical attenuator sets the pump power before it enters a SOA via an optical circulator. A weaker, unmodulated optical carrier (λ=1553.33 nm, P=-6.5 dBm), the probe, enters the SOA from the other side. The pump and probe counterpropagate through the SOA, where the microwave signal is transferred from pump to probe by XGM. In addition, the pump burns a spectral hole in the gain spectrum of the SOA. Because a spectral hole is created in the gain, dn(ω)/dω is negative so the group velocity is increased, resulting in fast light [4]. To avoid confusion, the term “delay” is used even if the signal is advanced, as in this case. After exiting the SOA, the modulated and delayed probe is sent to a photodetector by a circulator and bandpass filter. The output microwave signal is compared to the input by a network analyzer to determine delay and gain as a function of the SOA bias current, ISOA, and pump power. Measurements are performed as pump power is swept from 0 dBm to 14 dBm; at each pump power, ISOA is swept from 40 mA to 400 mA.

Figure 1: Setup used to measure fast light in a semiconductor optical amplifier. SOA = Semiconductor Optical Amplifier, EAM = Electro-Absorption Modulator, EDFA = Erbium-Doped Fiber Amplifier, and VOA = Variable Optical Attenuator.

978-1-55752-973-2/13/$31.00 ©2013 Optical Society of America

QM4E.3.pdf CLEO:2013 Technical Digest © OSA 2013

Figure 2: Microwave signal delay (a) and gain (b) measured at 9 dBm pump power and variable ISOA across 3 GHz. Surfaces of delay (c) and gain (d) are plotted as functions of pump power and ISOA for a 500 MHz signal. Red points correspond to larger relative delay/gain while blue points correspond to smaller relative delay/gain. One constant gain curve is shown as a solid black line in (d) while its associated delay is shown in (c).

3. Experimental Results and Analysis

A typical set of delay and gain curves across a 3 GHz bandwidth is shown in Fig. 2a and 2b, respectively, for a fixed pump power of 9 dBm and different ISOA. Signal delay is plotted relative to the signal delay at 14 dBm pump power and ISOA=400 mA, which corresponds to the fastest light. For signal gain, link loss of the modulator and photodetector is normalized out so that only the attenuation or gain of the fast light device is considered. Fig. 2a shows that a signal delay of over 60 ps can be achieved by varying ISOA; however, this is linked to the amplitude (or gain) variation shown in Fig. 2b. The tradeoff between signal delay and bandwidth due to finite carrier lifetime can also be seen in Fig. 2a. Carrier lifetime appears to be on the order of 1 ns, but can be modulated by the pump power and ISOA. It should be noted that the detuning between the pump and probe does not affect signal delay or bandwidth. This indicates that it is not the beating between pump and probe waves that generates the relevant population oscillations, but rather the beating between the pump wave and its modulated sideband. The probe only serves as the canvas onto which the delayed signal is written during XGM.

The importance of using XGM will now be shown. By plotting the delay and gain as a function of both ISOA and pump power, delay and gain surfaces can be built as shown in Fig. 2c and 2d, respectively, for a 500 MHz signal. The surfaces reveal what range of delay is achievable by the SOA, as well as what relationship between ISOA and pump power, if any, enables a constant amplitude while still permitting a tunable delay. In Fig. 2d, it is clear that ISOA is directly related to gain but pump power is inversely related to gain. On the other hand, in Fig. 2c, both ISOAand pump power are directly related to signal delay. Therefore, it is possible to find curves of constant gain, but non-constant delay, by changing ISOA and pump power in the same direction. This relationship is made possible by XGM. One of many constant gain curves is shown as the solid line in Fig. 2d, with its associated (non-constant) delay shown as the solid line in Fig. 2c. For a constant gain of 0 dB, a maximum delay range of 60 ps can be achieved. This is ideal for an optical delay line, where an amplitude-invariant tunable delay is desired.

4. Conclusion

We propose and demonstrate an optical delay line for microwave photonics by using coherent population oscillations and cross-gain modulation in a SOA. A signal delay of about 60 ps can be achieved across ~500 MHz without amplitude variation as long as ISOA and optical pump power are varied along curves of constant gain.

References [1] J. B. Khurgin, “Slow light in various media: a tutorial,” Advances in Optics and Photonics, 2, pp. 287-318 (2010). [2] M. S. Bigelow, N. N. Lepeshkin, and R. W. Boyd, “Observation of Ultraslow Light Propagation in a Ruby Crystal at Room Temperature,” Phys. Rev. Lett. 90, 113903 (2003). [3] W. Xue and J. Mørk, “Tunable true-time delay of a microwave photonic signal realized by cross gain modulation in a semiconductor waveguide,” Appl. Phys. Lett. 99, 231102 (2011) [4] F. G. Sedgwick et. al., “THz-bandwidth tunable slow light in semiconductor optical amplifiers,” Opt. Express 15, pp. 747-753 (2007)