The present TQL design avoids pumping-induced current instabilities, which prevented a previous design from operating at the required field strength as discussed by Giehler et al. [J. Appl. Phys. 110, 103104 (2011)]. We applied photocurrent (PC) spectroscopy to examine the pumping process. The measured PC spectra are found to be in agreement with simulated spectra in the vicinity of the desired operating field strength. We demonstrate that electric-field domain formation can be suppressed using a continuous-wave CO2 laser as the pump.