We demonstrate a 1.5 μm passively mode-locked fiber laser using an intersubband transition (ISBT) in a quantum well as a saturable absorber. The saturable absorption characteristic of faster than 1 ps in the ISBT was utilized for femtosecond pulse generation. We designed the laser cavity as a soliton laser, which enabled us to generate a stable pulse. As a result, an 88 fs, 42 MHz soliton pulse was successfully generated.
©2009 Optical Society of America
There is a growing demand for femtosecond pulse lasers in various industrial fields including optical communication, all-optical signal processing, optical metrology and bio-medical technology. Passively mode-locked fiber lasers have been widely used to generate such ultra-short optical pulses because they have a simple structure that does not require any external active devices for mode-locking. Passive mode-locking can be easily achieved simply by employing a saturable absorption effect, where the light absorption decreases with increasing light intensity. Semiconductor-based saturable absorber mirrors (SESAM)  and single-wall carbon nanotubes (SWNTs) [2–5] are well-known as ultrafast saturable absorbers. The nonlinear optical effect in fibers can also be used as a mode locker in passively mode-locked lasers , such as nonlinear optical loop mirrors (NOLM) , nonlinear amplifying loop mirrors (NALM) [8,9] and nonlinear polarization rotation (NPR) . To improve the laser performance, saturable absorbers with large optical nonlinearity, an ultrahigh-speed response time, and high stability are strongly required.
Recently, ultrahigh-speed absorption saturation with an intersubband transition (ISBT) in a quantum well (QW) has attracted a lot of attention since it can be utilized for ultrafast all-optical switching and signal processing [11–13]. The carrier relaxation time of an ISBT is less than 1 ps, which is much faster than that of electron-hole recombination . Moreover, ISBTs have been realized at the optical communication wavelength (λ =1.5 μm) by using several materials including InGaAs/AlAsSb , GaN/AlGaN  and (CdS/ZnSe)/BeTe . These properties allow us to realize higher bit rate (above 100 Gbit/s) operation at 1.5 μm. In this paper, we apply the ultrahigh-speed absorption saturation characteristics of an ISBT to a passively mode-locked erbium fiber laser. As a result, we successfully demonstrated 88 fs soliton pulse generation using a II-VI-based ISBT waveguide device. The average output power was approximately 3 mW.
2. Principle of intersubband saturable absorption and the structure of ISBT device
Figure 1 shows the principle of an intersubband transition. In a QW, there are several discrete energy levels called subbands. When a high intensity TM pump pulse is injected into a QW subband, the electrons are excited from the ground level to the upper level and the absorption coefficient is saturated. An important feature of the ISBT switch is that the switching response is in the subpicosecond regime due to the ultrafast carrier relaxation time associated with longitudinal optical (LO) phonon emission.
In this work, we employed a II-VI semiconductor-based ISBT device in which the carrier relaxation is more efficient owing to the ease of ionization, and thus the strong interaction between the electron and LO phonon. Figure 2 shows the band structure of a QW employing the CdS/ZnSe/BeTe material system . A CdS/ZnSe well layer was sandwiched between BeTe barrier layers. To realize the ISBT at λ =1.5 μm, the band offset between the well and barrier layer must be sufficiently deep. CdS, ZnSe and BeTe have a type II band structure and this results in a wide-band offset in the conduction band. The band offsets of ZnSe/BeTe and CdS/BeTe were 2.3 and 3.1 eV, respectively. The wide band-gap material system enables us to eliminate the effect of two-photon absorption.
Figure 3 shows the optical waveguide structure of the ISBT device. We employed a mesa waveguide structure with a mesa width of 3 μm and a device length of 300 μm. As shown in the Fig. 3, a multiple QW (MQW) active layer consisting of a multiple CdS/ZnSe/BeTe QW was placed at the center of the core layer. Mixed crystal material, ZnBeSe, was placed above and below the MQW layer to act as optical confinement layers. ZnMgBeSe cladding layers were placed above and below the core layer. To realize highly efficient absorption saturation, a decoupled optic and electronic confinement structure was employed . A photograph of the ISBT device is shown in Fig. 4 , where the module is equipped with a fiber pig-tailed input and output. The fiber-to-fiber linear loss is approximately 3 dB, which is accounted for by the input and output coupling losses between the fiber and waveguide (1.5 dB each). Here, the TE absorption of the waveguide and the scattering loss were negligible compared with the coupling loss. At λ =1.56 μm, the ISBT linear loss, i.e., the difference between the TE and TM losses, was 9.6 dB.
3. Passively mode-locked femtosecond fiber soliton laser with ISBT module
We installed the ISBT module as a saturable absorber in a passively mode-locked fiber laser and performed a femtosecond pulse generation experiment. Figure 5 shows the configuration of the laser cavity. It is composed of a ring resonator whose cavity length is 4.8 m. An erbium-doped fiber (EDF) with an Er3+ concentration of 7100 ppm was used as the gain medium. The average dispersion of the laser was anomalous ( +2.3 ps/nm/km). We installed a polarization controller to maintain a fixed polarization state (TM mode incident into the ISBT module) in the fiber laser cavity. We maintained the ISBT module at a constant temperature of 25 °C by using a temperature controller to prevent the device from heating up as a result of the thermal effect and to maintain stable laser output characteristics.
To generate a stable optical pulse, we employed the soliton effect in the fiber laser cavity. This was made possible by the anomalous dispersion of the laser cavity. For fundamental soliton propagation, the following two conditions must be satisfied .
Here Pp is the peak power of the pulse circulating in the cavity and Psoliton is the peak power required for a fundamental soliton. Z0 is the soliton period, and L is the cavity length. Psoliton and Z0 are given by
Figure 6 shows the laser output characteristics. Figure 6(a) shows the relationship between the pump power and the laser output power, and Fig. 6(b) shows the pulse width and the time-bandwidth product against the pump power. When the pump power was increased, first continuous-wave oscillation was obtained above a threshold power of 80 mW and then pulsed oscillation was achieved above 103 mW. Figure 7 shows a pulse train waveform, which we measured with a photodetector and an oscilloscope. This figure reveals that the repetition rate was 42 MHz, which corresponds to a fiber cavity length of 4.8 m. Figure 8 shows the autocorrelation waveform and optical spectrum of the fiber laser immediately above the pulsed oscillation threshold (pump power of 110 mW). A pulse width of 129 fs with an average power of 0.8 mW was obtained. The time-bandwidth product was 0.43. As the power was increased, a shorter pulse was obtained and the time-bandwidth product approached 0.32 as shown in Fig. 6(b). The shortest pulse was obtained with a pump power of 180 mW. Figure 9 shows the autocorrelation waveform and optical spectrum. A pulse width as short as 88 fs was obtained at a repetition rate of 42 MHz with an average output power of approximately 3 mW. The time-bandwidth product was 0.29, indicating that a nearly transform-limited sech pulse was generated. To check the soliton condition, here we evaluate the peak power required for a soliton, Psoliton, and a soliton period, Z0. Psoliton was calculated to be 802 W, which is close to the peak power of the pulse circulating in the cavity, Pp = 762 W, thus satisfying the condition given by Eq. (1). The soliton period, Z0, was calculated to be 1.3 m, which was shorter than the cavity length of 4.8 m thus satisfying Eq. (2). These results indicate that a stable femtosecond soliton pulse was successfully generated.
We demonstrated a passively mode-locked fiber laser, in which we utilized ultrahigh-speed absorption saturation with an ISBT in a semiconductor-QW as a mode locker. An ISBT operating in the 1.5 μm wavelength region was realized by using the (CdS/ZnSe)/BeTe material system. The fiber laser cavity was optimized as a soliton laser to generate a stable fundamental soliton pulse. With these configurations, we successfully generated an 88 fs soliton pulse with an average power of approximately 3 mW at a repetition rate of 42 MHz.
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