1. Introduction

Optical Bistability (OB) in semiconductor laser amplifiers has been the subject of considerable research because of its potential use in all-optical signal processing applications (for a review, see [1]). The physical mechanism of this OB is a dispersive nonlinearity where the cavity resonance of the device shifts towards longer wavelengths with external light injection as a consequence of nonlinear refraction associated with gain saturation [1]. The phenomenon has been reported in planar Fabry–Perot semiconductor laser amplifiers (FPSLAs) [2–5], distributed feedback semiconductor optical amplifiers (DFBSOAs) [6] [7] and vertical-cavity semiconductor optical amplifiers (VCSOAs) [8–13]. The VCSOA consists of two Distributed Bragg Reflectors (DBRs) forming a vertical cavity that contains an active medium. The use of VCSOAs adds new competitive advantages compared to their edge-emitting counterparts, including reduced manufacturing costs, higher coupling efficiency to optical fibers and the possibility to fabricate 2-D arrays of these devices. Experimental observation of dispersive OB in an optically pumped VCSOA was reported for the first time by Bohn and McInerney [8]. Subsequently, Wen et al [9] and Sanchez et al [10], respectively, reported experimental observation of anticlockwise OB and nonlinear gain in an electrically pumped 850nm-VCSOA operated in reflection. Recently, the authors have reported the first experimental observation of OB and nonlinear gain in a VCSOA working at 1550 nm [11], the most commonly used wavelength in telecommunications. Two different shapes of OB and nonlinear gain, anticlockwise and clockwise, previously only predicted in theory [12], were observed experimentally in that work [11]. Very recently, butterfly-shaped OB and switching thresholds as low as 2 μW were reported in a 1550 nm VCSOA [13].

However, the effect of the polarization of the externally injected signal on dispersive OB in VCSOAs has not yet been analyzed. Pan et al [14] reported experimental observation of polarization bistability in a vertical-cavity surface-emitting laser (VCSEL) under orthogonal polarized optical injection. Later, Yu [15] reported a complete theoretical study of polarization bistability and Hong et al [16–18] reported experimental observation of frequency polarization bistability in a VCSEL under optical injection with orthogonal polarization. Moreover, recently, a wide variety of nonlinear dynamics and polarization switching have been predicted theoretically [19–25] and observed experimentally [26–29] in a VCSEL under orthogonal polarized light injection. In this context it would be relevant to analyze the effect of the polarization of the injected signal on the nonlinear optical characteristics of VCSOAs.

In this work we report an experimental study of dispersive OB and nonlinear gain in a 1550 nm VCSOA under parallel and orthogonal polarized optical injection into the two orthogonal polarizations of the fundamental mode. Two different shapes of OB and nonlinear gain, anticlockwise and clockwise, have been observed experimentally when the polarization of the externally injected signal is parallel to that of the mode under injection, while linear responses are obtained for orthogonal polarized injection. To the best of our knowledge, this is the first time that this diversity of behavior, depending on the polarization of the externally injected signal has been observed in a 1550 nm VCSOA. This offers promise for the potential use of VCSOAs for all-optical switching/routing, optical parallel signal processing and optical interconnects applications in optical telecommunication networks.

 

Fig. 1. Experimental setup

Download Full Size | PDF

 

Fig. 2. (a). L-I curve and (b) spectrum of the 1550 nm VCSOA used in the experiments (I_Bias = 2mA).

Download Full Size | PDF

2. Experimental setup

The experimental setup used in this work is illustrated in Fig. 1. An all-fibre system has been developed in order to inject the light from a tuneable external cavity laser source into the 1550 nm VCSOA used in the experiments. The VCSOA was mounted on a temperature-controlled stage that was held constant at 298 K during the experiments. An optical isolator was used to avoid reflections into the tunable laser. The state of polarization of the injected signal was controlled with a fiber polarization controller whilst the injected optical power was controlled with an optical attenuator. The optical signal is divided into two branches using a commercial 3dB optical fiber directional coupler. The first output of the directional coupler is connected to a power meter to monitor the optical input power. The second output is injected into the VCSOA via a three-port optical circulator. The reflected power of the 1550 nm VCSOA has been measured using an Optical Spectrum Analyzer (OSA). The OSA has also been used to determine the initial wavelength detuning between the resonant peak of the VCSOA and the wavelength of the injected signal.

The 1550 nm VCSOA used in the experiments was a commercial VCSEL. Figure 2(a) shows the measured L-I curve with a threshold current of 2.015mA at 298K. Figure 2(b) shows the spectrum of the device when biased just below threshold with a current of 2.000 mA. The two modes appearing in Fig. 2(b) correspond to the two orthogonal polarizations of the fundamental mode. The first mode at the wavelength of 1551.11 nm has parallel polarization, while the second mode has orthogonal polarization and is shifted 0.578 nm to the long wavelength side. The lasing mode of the device was for all biases the parallel polarized mode located at 1551.11 nm, while the second mode with orthogonal polarization was suppressed. The polarization of the device was stable for all bias currents. For simplicity, throughout this work we will refer to principal and secondary mode, respectively, for the parallel and orthogonal polarized mode.

3. Results and discussion

Figures 3(a)-3(c) show the measured relationships between the reflected and injected optical power under parallel and orthogonal polarized optical injection into the principal mode of the 1550 nm VCSOA when the device is biased with a current of 2.000 mA. These results are for three different values of initial wavelength detuning (0.05 nm, 0.1 nm and 0.15 nm) between the wavelength of the principal mode of the VCSOA and the wavelength of the externally injected signal. The three different values of initial wavelength detuning have been chosen so as to clearly show the evolution in the shape of the nonlinear switching transition with increasing detuning [11]. A variety of nonlinear behavior is observed when the polarization of the externally injected signal is the same (parallel polarization) as that of the principal mode. Anticlockwise and clockwise nonlinear gain are observed when the detuning is 0.05 nm and 0.1 nm, respectively, and clockwise OB is seen for a detuning of 0.15nm. By contrast, when the polarization of the externally injected signal is orthogonal to that of the principal mode, a linear response was exclusively observed in the reflected versus input power relationships, independent of initial wavelength detuning.

The optical characteristics under parallel and orthogonal polarized optical injection into the secondary mode have also been studied. Figures 3(d)-3(f) show the reflected versus input power relationships for three different initial wavelength detunings (0.05 nm, 0.1 nm and 0.15 nm) to the long-wavelength side of this secondary mode. In contrast to the case of injection into the principal mode, now the diversity of nonlinear behavior (anticlockwise and clockwise nonlinear gain and clockwise OB) appears when the injected signal is orthogonally polarized and a linear response is observed no matter which detuning is used for the parallel polarized optical injection.

When the polarization of the external signal matches that of the particular mode selected for injection, the behavior is the same as that reported previously experimentally [11] and theoretically [12]. The injection of external power causes a reduction of the carrier concentration in the active region of the device due to stimulated recombination processes; this increases the refractive index in the active region, thus causing the wavelength of the mode to shift to longer wavelengths. After a threshold in the optical input power is exceeded the nonlinear switching transition appears in the input-output relationship [9–13]. A complete theoretical explanation for the occurrence of different shapes of nonlinear switching can be found in [12]. In summary, the saturation of gain with increasing optical injection modifies the balance between the input, reflected, transmitted and internal optical intensities of the VCSOA and gives rise to different shapes of nonlinear switching (anticlockwise and clockwise) for different values of initial wavelength detuning. In the opposite case, when the polarization of the externally injected signal is orthogonal to that of the particular mode selected, the same physical mechanism is involved but a different behavior results. Now, injecting light with orthogonal polarization to a particular mode does not affect this mode but the other one, as the polarization of this second mode is the same as that of the externally injected signal. Since the modes are separated in wavelength [by 0.578 nm, as seen in Fig. 2(b)], the total optical input power is not enough to reach a nonlinear switching transition, and hence neither bistability nor nonlinear gain can be achieved, and a linear response is observed in the reflected versus input power relationship. For the case of low initial detuning (anticlockwise nonlinear switching), when the injected polarization matches that of the injected mode, an optical gain of approximately 8–10 dB is obtained in the reflected versus input power relationship [9][13]. Further increase of the initial detuning reduces the optical gain and the VCSOA tends to behave as a passive cavity. For the case of orthogonal polarized injection the reflected versus input power characteristic is linear and the device again behaves as a passive cavity.

 

Fig. 3. Reflected versus input power relationships under parallel and orthogonal polarized optical injection into the principal (a-c) and secondary mode (d-f) for different initial wavelength detunings as indicated. Applied bias current of 2 mA, below threshold.

Download Full Size | PDF

The optical characteristics for parallel and orthogonal polarized optical injection have also been studied experimentally when the VCSOA is biased above its threshold current. For these studies a bias current of 2.100 mA (1.0421 I_th) has been used. Figures 4(a)-4(c) show the measured relationships between the reflected and input optical power for parallel and orthogonal optical injection into the principal mode of the device for three different levels of initial wavelength detuning. Analogously, Figs. 4(d)-4(f) show the results for parallel and orthogonal optical injection to the secondary mode of the VCSOA. Again, for the above threshold case, different forms of nonlinear behavior, including OB and nonlinear gain, have been observed experimentally for parallel polarized optical injection into the principal mode as well as for orthogonal polarized optical injection into the secondary mode. Once again, a linear response is observed, respectively, for the cases of orthogonal/parallel optical injection to the principal/secondary mode.

To the best of our knowledge, this is the first time that different nonlinear characteristics are reported in a VCSOA for different polarized optical injections (parallel and orthogonal), into the two orthogonal polarizations of the fundamental mode. Different results have been found for both modes when the polarization of the externally injected signal matches that of the particular injected mode, while a linear response is measured when the polarization is orthogonal to that of the mode under study. This difference in the behavior of the device with input polarization suggest the potential use of this mechanism for different types of applications including all-optical signal processing, all-optical switching applications for optical communications, cross-bar switches, etc.

 

Fig. 4. Reflected versus input power relationships under parallel and orthogonal polarized optical injection into the principal (a-c) and secondary mode (d-f) for different initial wavelength detunings as indicated. Applied bias current of 2.1 mA, above threshold.

Download Full Size | PDF

4. Conclusion

We have reported for the first time, to the best of our knowledge, experimental observation of OB and nonlinear gain in a commercial 1550 nm VCSOA under optical injection with parallel and orthogonal polarization. A diversity of nonlinear responses, including anticlockwise and clockwise nonlinear gain and bistability, have been observed for both modes when the polarization of the external optical signal matches that of the particular mode selected for injection. When the polarization of the external signal is orthogonal to that of the externally injected mode, the input/output power relationship is linear. These results are encouraging for potential applications of VCSOAs in all-optical signal processing either for optical computing or optical switching/routing applications in telecommunications where the potential of conventional SOAs for wavelength conversion and regeneration is already widely recognized.