# Integrated IR Modulator with a Quantum Cascade Laser
## 1 Introduction
In the case of gas sensing devices, the applied laser absorption spectroscopy (LAS) technique determines the optical signal shape. This shape has a form ranging from a simple pulse to complex waveforms. For example, a QC laser pulse with a time duration of 500 ns was used in an intra-pulsed tuned LAS to detect the spectra of nitrogen oxide (NO) and nitrous oxide (N\({}_{2}\)O) at around 5.25486 \(\upmu\)m and 4.52284 \(\upmu\)m, respectively [18]. Shorter pulses are also used, for example, in cavity-enhanced absorption spectroscopy to detect ammonia (NH\({}_{3}\)), with a QC laser emitting 50 ns pulses at 200 kHz frequency around 6.8 \(\upmu\)m [19]. More complex optical signals are preferable in a tunable laser absorption spectroscopy (TLAS). To obtain scanning of an absorption line range, the QC laser spectra are tuned applying both current ramp signal and high-frequency wave. Many LAS setups use tunable QC lasers, e.g., direct absorption spectroscopy or wavelength modulation [20]. In these setups, IR modulators generated the current signal consisting of, for example, a slow ramp (10 Hz) and a sinusoid (8 kHz) [21]. A high rate of radiation modulation is the primary goal of IR modulator construction for free-space optics (FSO). Some experimental results with GHz- range modulation of QC lasers have been described. In those tests, some specific architectures of laser structure were used [22]. For example, a laser waveguide with a microstrip line ensured a modulation rate of up to 14 GHz [23]. However, a 'high speed' technology of QC lasers is underdeveloped. The modulation speeds up to 26.5 GHz were obtained using a special design QC laser placed on a cold finger with continuous-flow liquid nitrogen and driven with bias-tee and a microwave signal [24]. There are many other applications for which it is important to design a compact and low-power consumption IR modulator. For example, it could be used as an alarm beacon with optical signals detected at extended ranges, a jamming device to disorientate an incoming missile threat, or a light source for beam riding and ranging systems. There are only a few such devices described in the literature [25]. However, they can work in pulsedmode with closed construction, allowing light pulse generation in strictly defined pulse configurations at the maximum repetition rate of 500 kHz. This study describes an integrated IR pulsed modulator consisting of a QC laser structure and a current switching module. It ensured the generation of short light pulses (tens of ns) with a maximum frequency of 1 MHz, using direct wire bonding of the lasing structure and the current terminals. Various simulations of this configuration were performed considering different QC laser electrical parameters. Finally, the preliminary experiments were performed with the designed modulator. The obtained results provided experimental verification of, e.g., the influence of both QC laser parameters and the driving signal interface on the light pulses, determination of the electrical scheme of the QC laser, and the integrated modulator. In the future, this modulator can be installed in some standard housings, e.g., HHL, butterfly.