Project C07 - Photonic integrated THz Image Sensor
Tunable two-color lasers for THz generation
We have developed very flexible two-colour diode laser systems as sources for our optoelectronic THz systems. External cavity arrangements offer the largest flexibility. Figure 1 shows a two-colour double Littrow arrangement (left) enabling large tuning of the difference frequency (right).
THz system for humidity measurements
The laser output was amplified and coupled into a simple THz transmission system with a Photodiode as THz transmitter and a Schottky barrier diode for detection (see. Figure 2). The use of the system for simple THz transmission measurements in the frequency range around 0.3 THz was successfully proven with humidity measurements.
Fig. 2: THz measure¬ment system with two-color laser (RUB), photodiode (UDE) for THz generation, and Schottky barrier diode (UDE) for detection
THz systems with tunable monolithic Y-branch lasers
For future integration external cavity laser diode arrangements are not suitable because of their complexity. Therefore, we have analyzed monolithic two color diode lasers based on DBR (distributed Bragg reflector) and DFB (distributed feedback) laser architectures. This analysis started with Y-shaped double DBR-laser diodes provided by the Ferdinand Braun Institute (FBH). These lasers showed tunable two color operation and we successfully implemented them into a homodyne THz system for application in THz absorption and refractive index measurements. Though a proof of principle was successfully realized, the lasers provided several difficulties for the desired application in this project. First, they were designed for a center wavelength in the 0.8 µm range which is not compatible with the transmitter and receiver technology we use. Second, and more important, the lasers exhibit mode hops and imperfect tuning behavior which induces major challenges for THz measurement applications. Therefore, in the next step, we investigated Y-shaped double DFB two color laser diodes based on InP technology, i.e. emitting in the 1.55 µm wavelength range.
Such a monolithic slotted Y-branch laser diode was used for generation and detection of continuous wave Terahertz radiation at 1THz. The device features two operation regimes, one at 1 THz with a tuning range of about 50 GHz and a second at 850 GHz with a tuning range of about 20 GHz. Moreover current induced frequency tuning was demonstrated, which allows for THz thickness measurements using a single monolithic laser diode without the need for a mechanical delay line. The figure shows a principle sketch of such a laser diode, a continuous wave THz measurement at 1 THz and the corresponding frequency spectrum.
THz asynchronous optical sampling system
In addition to the continuous wave (CW) THz systems discussed above, we have also studied modelocked diode laser systems for time domain spectroscopy (TDS). First, we have developed a THz TDS system based on asynchronous optical sampling of two external cavity modelocked laser diodes. The system setup as shown in the figure was successfully realized and its operation was verified but the realized bandwidth was below 0.3 THz due to electronic problems and timing jitter of the external cavity diode lasers. Therefore, we have started to analyze different concepts for monolitically modelocked diode lasers that potentially enable integration into our THz systems. The stability of these single-chip devices has been and is still analyzed with respect to applications in THz-TDS systems and as frequency combs for CW THz systems.
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