## 3 System Configuration
### Mid-Infrared Tunable Lasers
#### 3.1.4 Mid-Infrared Optical Frequency Comb  
OFCs consist of a series of discrete, narrow, stable, equally spaced spectral lines that have a fixed phase relationship between them. These combs can span a broadband of frequency range that have found important applications in areas such as metrology, spectroscopy, and optical communications [79]. To date, OFC sources have extended from the ultraviolet, visible, infrared, and terahertz spectral regions [80]. We only focus on the MIR-OFC, including mode-locked, difference frequency generators (DFG), optical parametric oscillation (OPO), and direct modulation OFC.  
**Mode-Locked Laser**. The most popular way of generating a frequency comb is with a mode-locked laser. Mode-locked lasers produce a series of optical pulses separated in time by the round-trip time of the laser cavity. The spectrum of such a pulse train approximates a series of Dirac delta functions separated by the repetition rate of the laser. In the past decade, there have been hundreds of demonstrations of generating OFC with various lasers including Ti: sapphire solid-state lasers [81], Er: fiber lasers [82], Kerr-lens mode-locked lasers [83], QCLs [84], and ICLs [85] with repetition rates typically between to MHz to 10 GHz. The issues with mode-locked OFC are lower output power, still experimental, and not commercially available.  
**Difference Frequency Generators**. DFG is the most versatile method to generate mode-hop-free tunable broad laser by a nonlinear optical process. The two necessary conditions to achieve MIR OFC output are high-coherence pump lasers and a suitable order nonlinear crystal. There are more than a dozen demonstrated crystals, among which LiNbO3 and ZnGeP2 are the most popular. DFG-based OFC have more power and stability and do not require an oscillating cavity or a high threshold of the optical parametric process [86], whereas the drawbacks are high cost and a relatively low conversion efficiency [37; 86; 87].  
**Optical parametric oscillator**. An OPO is another parametric nonlinear optical process used to provide a high output power and versatile sources of coherent radiation for spectral regions inaccessible to lasers. OPO has been shown providing high power OFCs based on Yb: fiber laser covering several micrometers [88; 89]. Researchers have not only shown that degenerate synchronously OPOs are efficient tools to transfer near-infrared (NIR) frequency combs to the mid-infrared; also, they can utilize pump lasers in NIR and expand the spectrum to the mid-infrared [38; 90]. OPO offer high output power with broad spectral coverage, but require a free-space resonator, external pumping sources, and many optical components [91]. This makes OPO sources bulky, vulnerable to any external disturbances, and very impractical in field applications.  
**Direct Frequency-Modulation Combs**. Direct FM combs have been experimentally demonstrated in semiconductor lasers, such as QCLs [91; 92; 93; 94; 95; 96; 97], ICLs [85], quantum dot (QD) [98], and quantum dash lasers [99]. These lasers are passively mode-locked with cw or quasi-cw output. Direct FM combs enjoy broadband, wide repetition frequency from kHz to THz [100] with line width of kHz to MHz level. Direct FM comb offers the possibility of a portable, chip-scale device with low power consumption, which are desired in spectroscopic trace gas sensing.