# Integrated IR Modulator with a Quantum Cascade Laser
## 1 Introduction  
Quantum cascade (QC) laser technology is a commonly used radiation source in many applications that operates in mid-and far-infrared wavelengths [1; 2; 3]. These lasers are mainly used in chemical sensing and spectroscopy, in which low noise and high stability of both the light power and spectra are required [4; 5]. To this application, a special low-noise current driver and temperature stabilizing unit were constructed [6]. However, these lasers are also being used in other applications as a result of their features, e.g., high power, modulation bandwidth, direct current control, compact size, room-temperature operation, and operation in high transparent spectral ranges of the atmosphere. These features are important for telecommunication, security, and defense technologies [7; 8]. Some research works of the modeling techniques used for a simulation of QC lasers have been conducted. Their main focus was to describe the physical processes in electron transport in multiple-quantum-well heterostructures to improve properties for operating temperature, efficiency, and spectral range [9]. What is more, some electrical equivalent circuits were also developed to simulate the laser operation, analyzing the laser electrical features and driving electronics. These works use an emulator of simulating electrical circuits without creating additional mathematical functions or numerical calculations [10]. It is not so computationally intensive and can be implemented in lasing structure optimization or an applied-level device. Although a simulation model of a realistic circuit should consider five rate equations of quantum wells, three-level or two-level models with certain approximations have beenused [10; 11; 12]. Based on these models, the light-current characteristics of QC laser were analyzed. These simulations were limited to intensity modulation characteristics for a different ratio to the threshold current. However, only a few papers describe a QC laser pulse response [13]. The obtained results were used to define the influence of different bias currents on the turn-on delay of the pulse current conversion to the light signal. During described simulations, no model of the current driving unit was applied. The results verification was based on data from other circuit models or numerical calculations. These can be critical for some aspects of applied science in which laser-based systems are analyzed. QC lasers require a new class of pulse drivers because their operational characteristics are different from those of a laser diode [14]. These lasers need driving signals (current and voltage) of an order higher than those of a bipolar diode laser [15]. The high voltage results from the applied cascade construction, in which the energy of the emitted photons is multiplied by the number of stages (e.g., 30-50). It can reach 20 V or more. The applied intersubband transitions are characterized by a lower optical gain than the interband, requiring high supplying currents. Therefore, the threshold current density of QC lasers is more than one order higher than that of diode lasers [16]. These features represent a challenge when constructing a laser driver that can consider both current and voltage. In these devices, various methods are applied to connect the signals to the QC laser. The most popular is to design unique signal poles or sockets on the printed circuit board (PCB) to connect the pins of the laser housing, e.g., butterfly, transistor outline package (TO), high heat load housing (HHL), and others. However, in a short pulse transmission, the main issues are impedance matching and inductance minimization. However, the matching can be impossible, because laser impedance depends on its construction and fabrication, temperature, or level of driving current [17]. The most critical is its dynamic change during current switching. The properties of QC laser modulator radiation are matched to its application. Most often, they concern a required shape and the power of the emitted light.