Progress in QCL Performance:

Even though the first QCL operation was demonstrated in 1994, 2002 marks the important point in the QCL development because of the report of the first continuous wave room temperature operation of QCLs at ~9.1 μm [4], using a two-phonon resonance de-excitation scheme for emptying the lower laser level. CW operation of QCLs operating at ~4.8 μm was subsequently reported by Evans, et al. [5]. The progress beyond 2004 was significantly aided by two DARPA programs: (a) L-PAS (laser photoacoustic spectroscopy) for the development of a portable chemical warfare agent (CWA) and explosives sensor and (b) EMIL (efficient mid wave infrared laser) program for the development of high power, high wall plug efficiency QCLs for infrared countermeasures. The former resulted in transforming the QCLs from being a laboratory curiosity to a useful device, through innovations that included the first epi-down mounting of QCLs on coefficient of thermal expansion (CTE) matched AlN submounts using Au:Sn hard solder [6]. The second program was instrumental in raising the CW output power levels of QCLs from a few tens of mW to more than one watt. A crucial advance that resulted from the second DARPA program was a revolutionary new QCL structure design that marked a departure from all the earlier designs that relied on longitudinal phonon resonances for depopulating the lower laser level, e.g., one phonon resonance in case of the first demonstration of a QCL operation [1] and two phonon resonance for the first CW operation of the QCL [4]. The new design, called the non-resonance extraction (NRE), removes many, if not all of the severe design constraints imposed by the LO-phonon resonance designs [7]. The NRE design has made it possible to extend the high power, high wall plug efficiency operation of QCLs both to shorter [8] and longer wavelengths [9].

FABRY-PEROT CONFIGURATION QCLS:

Current capabilities of QCLs in terms of power output and wall plug efficiency depend crucially on the wavelength of operation. At the shorter end of the spectrum, shorter than about 3.6 μm, QCL performance is limited by the depth of the quantum wells possible in the presently successful structures that use GaInAs/InAlAs combination of materials. At the longer wavelength end of the spectrum, longer than about 10 μm, the performance of the QCLs is limited by electron free carrier absorption. TableI shows a sample of current best performance of continuous wave operation Fabry-Perot configuration (broad band output around the central design wavelength of the QCL). Generally high power QCLs, when operated CW, require thermoelectric cooling to maintain the QCL active region temperature at a point where good performance can be obtained. Furthermore, since the emitting region is typically 2-3 μm high and 8-10μm wide. Thus the laser output from the end facet emerges in a fan that is ~90° X 60° wide. To be able to utilize all of the power generated by the QCL, it is necessary to collimate the beam by placing appropriate optical elements close to the exit face. The performance data have been obtained from QCLs that fully packaged in hermetically sealed butterfly packages and have beam collimating optics as well as the TEC within. The back facet has a high reflectivity coating a while the front facet has a partially reflective coating whose reflectivity is optimized for extracting the maximum amount of power from the device. The output emerges as a collimated beam rather than a widely diverging beam that would be obtained from the end facet of the QCLs. By convention, the wall plug efficiency given in TableI, does not take into account the power consumption by the TEC. Because of the high nascent wall plug efficiency of the QCL, even when we take into account the power consumption by the TEC, overall system efficiency of near 10% is obtained at ~4.6μm.

Table 1.

Best continuous wave operation performance of QCLs at room temperature (fully packaged lasers in hermetically sealed butterfly packages containing the QCL, thermoelectric cooler and beam collimating optics).

Thermal management at the QCL chip level is important because the superlattice forming the QCL has relatively low thermal conductivity normal to the layers, which makes it advantageous to operate the QCLs in a high duty cycle pulsed mode. This entails operating QCLs with 100ns-500ns pulses with a duty cycle as high as 50%. The “off-time” between pulses is sufficiently long to remove the heat from the active layers before the next pulse comes along. High duty cycle operation assures high average powers for the QCLs. As a rule, QCW operation QCLs provides average powers that are comparable to the CW powers available from the same QCL material. However, QCW operation does not require utilization of a thermoelectric cooler, resulting in a much simpler and much smaller QCLs system for the same power output. For many practical applications, QCW operation with high duty cycle is sufficient and true CW operation is not needed because the relatively slow response of the detection schemes used for many applications. Figure 1 shows a miniaturized high average power QCW QCL package, which includes the QCL in the hermetically sealed butterfly package with beam collimating optics, but no TEC and the necessary pulse drive electronics. The entire package weight 50 g (<2 oz), making it ideal for many applications where size and weight are critically important.

Figure 1.

QCW operation QCL in a miniature package along with pulse driver electronics (package dimensions: 1” W x 4.25” L x 0.5” H; weight: 50 g)

Table 2 provides average power output and wall plug efficiency data for QCW operation of QCLs at room temperature. The stated wall plug efficiency is the true efficiency of the QCL system, including the losses in the pulse generation circuit, i.e., it is the efficiency that a system integrator can utilize.

Table 2.

Best quasi-continuous wave operation performance of QCLs at room temperature (fully packaged lasers in hermetically sealed butterfly packages containing the QCL and beam collimating optics).

There are other laser sources that cover the spectral region form about 3.5 μm to 12 μm. However, QCL stands out as a unique source of laser radiation in this spectral region as seen from the comparison with some of the other contenders (Table 3).

Table 3.

A comparison between available laser sources in the 3.5 μm to 12 μm spectral region.

Compared to all other sources, for powers less than ~10 W, QCLs are ideal because they are solid state devices, convert electrical energy directly into optical energy, are small and efficient. CO₂ lasers have comparable wall plug efficiency and produce very high powers, but they have a limited wavelength reach, are discretely tuned and are gas lasers, which makes them unsuitable for applications where ruggedness and resilience to vibration and shock are required. Thus, where SWaP issues are paramount, QCLs appear to be ideal choices.

TUNABLE LASER CONFIGURATION QCLs:

Laser sources that have broad gain spectra and provide a broad bandwidth radiation when operated in Fabry-Perot configuration but can be made to operate on a single tunable wavelength by inclusion of a wavelength dispersive element within the optical cavity. Even though Fabry-Perot configuration with wide band width output radiation has many important applications as seen above, there is a need for tunable narrow bandwidth radiation in the MWIR/LWIR spectral region, which heretofore could only be covered by optical parametric oscillators or cryogenically cooled lead salt diode lasers. With the advent of broad gain bandwidth QCLs that operate at CW at room temperature, tunable QCLs have become important sources of radiation wherever the application depends on the spectral characteristics of the radiation. These applications include spectroscopic studies of materials, such as laboratory investigation of materials, in-situ detection of pollution and presence of toxic substances including chemical warfare agents (CWAs) and explosives and standoff detection of CWAs and explosives (IEDs).

The most commonly used geometry of a tunable QCL consists of laser gain medium enclosed in an optical cavity that includes a dispersive moving grating. By changing the angle of the grating, the resonant wavelength of the laser cavity can be controlled (Figure 2). This scheme has been widely deployed and has been quite successful. It is, however, susceptible to mechanical shock and vibration, which limits its employment in field applications. Another significant limitation for tunable QCLs with a moving grating is a slow tuning speed on the order of 10s of milliseconds. In real environment, where large urban or rural areas need to be scanned, tuning rate on the order of microseconds is required.

Figure 2.

Traditional EC QCL with angle tuned grating

Rapid Tuning of EC QCLs (Switching Time <700 ns)

A novel external cavity QCL configuration that does not suffer from these two limitations has recently been developed [10]. In these devices the ultra-rapid wavelength tuning is achieved using an electrically controlled acousto-optic modulator. A radio-frequency acoustic wave in AO modulators is generated in a transparent material (germanium in case of LWIR region) and this acoustic wave forms an index grating. The incident light waves from the laser are deflected by this grating (Figure 3). The angle of the deflection is controlled by the choices of optical and acoustic wavelengths. The direction of the lasing is determined by the laser resonator cavity so the output laser wavelength can be electronically selected by changing the acoustic frequency of the AO modulator, thus changing the Bragg angle condition for the selected wavelength. The response time of the modulators and, as a consequence, of the entire laser system is determined by the transit time of the acoustic wave across the material and typically lies in the MHz range.

Figure 3.

Schematic of acousto-optic modulator tuned quantum cascade laser

Figure 4 shows a benchtop version of a AOM tuned EC QCL. The measured switching time for changing the AOM controlled EC QCL lasing wavelength was less than 700 ns for switching between any wavelengths within the lasing region of the QCL. To demonstrate the ultra rapid spectral measurement capability for the AOM-controlled EC QCLs, we measured the absorption spectrum of Freon (R134a, cell length 5 cm, room temperature, concentration of 2.63%) that has a broad infrared spectral signature in the 9 µm region. The measurement was carried out in a single wavelength sweep when acousto-optic frequency was linearly changed from 42 MHz to 48 MHz in 17µs. Figure 5 shows the transmission signal collected using a room temperature MCT detector with and without the Freon cell present in the optical path (red and blue curves, respectively). The change in the AOM frequency resulted in lasing wavelength sweep across the QCL tunable range.

Figure 4.

Benchtop realization of AOM tuned EC QCL

Figure 5.

Experimental transmission data with and without the Freon (R134a) cell present in the optical path. AOM frequency was linearly changed from 42MHz to 49MHz, which resulted in wavelength tuning. QCL was driven in QCW mode with 5MHz repetition rate and 50% duty cycle

Figure 6 shows corresponding absorption spectrum (processed data from Figure 5) and Freon absorption spectrum simulated using PNNL Spectral Database convolved using the measured linewidth of laser emission. A good correspondence was observed between the experimental and simulation data. The fact that the experimental data were collected in 17µs without statistical averaging clearly demonstrates the advantage of rapid tuning for AOM controlled EC QCLs.

Figure 6.

Processed experimental Figure 4 data (blue points) vs PNNL data for Freon (R134a). PNNL data were convolved with the measured linewidth of the laser emission (red line). The Freon spectrum was calculated using 2.36% concentration, temperature of 23^°C, and pressure of 760 Torr.

Figure 7 shows a compact packaged version of the benchtop AOM tuned EC QCL set up for measuring transmission spectra of gases.

Figure 7.

Gas transmission measurement setup using fully packaged AOM tuned EC QCL (VeloXscan).

The above experimental data clearly demonstrate that AOM tuned EC QCL offers unique advantages of very fast tuning capability with spectral scan measurement time of under 20µs and wavelength switching time of < 700 ns for switching wavelengths between any two points within the spectral scan width of the QCL. The AOM tuned EC QCL has no moving parts and therefore it is inherently amenable to be ruggedized for demanding field applications, including mounting on moving or flying platforms. The flexibility offered by the AOM approach will significantly improve the capabilities of external cavity QCLs. The AOM tuned QCLs (VeloXscan) will find immediate use in applications where the ultra-rapid scanning rate is of utmost importance, including standoff detection of IEDs and CWAs, combustion and explosion dynamics, optical frequency domain reflectometry (OFDR) as well as measurements of dynamic spectral changes during chemical and biological reactions.

CONCLUSION

In above brief review we have shown the vitality of the field of quantum cascade lasers that are operated in Fabry-Perot configuration (broad band output) as well as when they are configured in an external cavity geometry, where one of the mirrors forming the laser cavity is a wavelength dispersive element. The new developments are being driven by the recognition that for MWIR and LWIR regions, QCLs offer unique advantages over other sources of coherent radiation in the given spectral region.