Abstract

Our group has recently demonstrated that a compact master-oscillator power-amplifier semiconductor laser (MOPA-SL) is a good candidate for a coherent light source (operating at 1550 nm) in a Doppler wind Lidar [1]. The MOPA-SL requires two injection currents: I_DFB for the distributed-feedback (DFB) laser section (master oscillator) and IAMP for the tapered amplifier section. For our laser, the specified maximum current values are 0.7 A and 4.0 A for I_DFB and I_AMP, respectively. Figure 1(a) shows the measurement of CW output power of the MOPA-SL versus I_AMP for three different values of I_DFB = 400, 500, and 600 mA. Our measurements were taken at 19 °C set temperature. The L-I curve shows that the output power linearly increases with I_AMP and reaches an ample 1 W level. Although the MOPA-SL has been proven capable of producing single-frequency CW output beam, stable operation at this spectral condition has also been known to highly depend on the drive currents to the laser [2]. It is therefore important to identify pockets of stability in the I_DFB-I_AMP-T_S (T_S = laser set temperature) space where lasting single-frequency operation is achieved. At a chosen set temperature T_s = 19 °C, we searched for combinations of (I_DFB, I_AMP) where the laser operates stably at single-frequency. This was done by observing the spectral characteristic of the laser using an optical spectrum analyzer (OSA, ANDO AQ6315) at different drive current combinations. The resolution of the OSA used is 0.05 nm. Depending on the drive currents, single-frequency or multi-frequency operations of the laser may occur as shown in Fig 1(b). Furthermore, hysteretic mode-hops between single-frequency regimes can occur at higher values of I_AMP. When using the laser for a Doppler wind Lidar application, a combination of (I_DFB, I_AMP) which is close to the center of an identified stable single-frequency regime is used. For the MOPA-SL unit that we use in our Lidar, we typically use (I_DFB = 0.5 A, I_AMP = 3.0 A) at T_s = 19°C. These current settings for the laser result in a highly stable Lidar as shown by a 5-day long continuous measurement of the Doppler shift produced by a constantly rotating diffusely reflecting target (Fig. 2).

© 2009 IEEE