Mid-infrared sensing with a Quantum Cascade Laser (QCL) as a light source is expected to offer a high sensitivity, a short measurement time, and a good portability compared to conventional methods. However, commercially available QCLs have high power-consumption, leading to the necessity for a large cooling system. Therefore, a portable sensor using a QCL have not been realized. To address this issue, recently we had developed a low power-consumption DFB-QCL which enables continuous wave (CW) operation up to 80 ◦C. In this study, we performed gas sensing using our QCL mounted on a Φ 5.6 mm TO-CAN package under uncooled condition. For example, even when the package temperature rose to room temperature +18 ◦C by injecting 180mA current (1.9 W power-consumption) into the uncooled QCL, it could CW operation, and emit output power of 9 mW. Lasing wavelength were stable when the power consumption of the QCL was below 2.2 W, and in this stable wavelength range, about 20 nm wavelength tuning range was obtained by sweeping injection current. We performed mid- infrared gas sensing of methane around 7.4 μm wavelength using a measurement system consisting of the QCL, a gas cell and a thermopile. For this measurement, the QCL was kept uncooled and was driven by CW current, which made a lasing wavelength sweep sufficient for sensing possible. Measured absorption wavelength, intensity, and width under uncooled operation were agreed well with the HITRAN simulation. Sensitivity was obtained about 2.3 ppb under uncooled operation, which was comparable to under cooled operation.
Sensing method with Quantum Cascade Laser (QCL) as a light source is expected to offer a high sensitivity, a short measurement time, and a good portability compared to conventional methods. However, commercially available QCLs have high power-consumption of several W. Therefore, a large power supply is required to drive QCL, and most of the input power is released as heat, leading to the necessity for a large cooling system. For these reasons, portable gas sensors using QCL have not been realized. To address this issue, we had recently developed a low power-consumption DFB-QCL in the 7μm wavelength region. In this study, we developed a compact and low power consumption QCL module with Φ 15.4 mm To-CAN package. The QCL device, a thermoelectric cooler (TEC), a thermistor and a window were assembled in this package. The threshold power-consumption and the maximum output power were 0.97 W and 37 mW at 20°C, respectively under continuous wave driving. In addition, it maintained a single mode operation between 20°C and 80°C without a mode hopping. The performance of this QCL module as a light source for gas sensing was evaluated by measuring the mid-infrared absorption spectrum of the methane gas with a multi pass type gas cell. High sensitive methane gas detection was achieved, which was comparable to that of the conventional high heat load (HHL) packaged QCL module reported by other group. It is expected that a compact and low-cost MIR gas sensor with high-sensitivity can be realized with our QCL module.
The authors estimate signal-to-noise ratios (SNRs) and contrasts for both InGaAs SWIR camera (cut-off wavelength λco~1.7 μm) and type II superlattice (T2SL) SWIR camera (λco~2.3 μm), under such situations as human skin as an object and vegetation as surroundings which are illuminated only by OH night airglow. In estimating the number of signal electrons, the measured spectral properties of quantum efficiencies for both InGaAs and T2SL detectors are used along with reflectance spectra of human skin and materials, while atmospheric transmission spectra are calculated with MODTRAN. As to noise electrons, shot noise resulting from dark current of InGaAs or T2SL detector is added to photon noise and ROIC (Read-Out Integrated Circuit) noise. The SNR values for the T2SL camera are found larger than those for the InGaAs camera. The contrasts of human skin vs surroundings are positive for the T2SL camera, while those for the InGaAs camera are negative.
Stray light in focal plane array (FPA) deteriorates the accuracy of hyper spectral imaging. Multiple reflections between FPA window and peripheral region of a sensor chip are considered to be the major sources of stray light. One idea for suppressing the stray light is to shield the incident light on the peripheral region of the sensor chip by narrowing the FPA window. However, it is limited by the tolerance of assembly. In this study we have examined an epoxy coating on the peripheral region such as ROIC contact pad area, AlN substrate and bonding wire. Sensor chip with InGaAs/GaAsSb type-II quantum well structures, which has the cut-off wavelength of 2.35 μm, 320×256 pixels were bonded to ROIC through indium bumps, assembled to AlN substrate and to a four stage TEC. To avoid the degradation by the stress to the chip and bonding wire, low elastic modulus epoxy was selected. Stray light suppression was confirmed by the sensor signal output of epoxy coated samples, 3% contrast improvement was achieved. Further, reliability test of 10,000 heat cycles between -75°C and 25°C was carried out. No degradations were found in sensor characteristics of the epoxy coated sample. These results suggest that the epoxy coating in SWIR FPA is effective in suppressing the stray light and suitable for hyper spectral imaging.
Quantum cascade lasers (QCLs) are promising as compact light sources in the mid-infrared region. In order to put them into a practical use, their relatively high threshold currents should be reduced. Facet reflectivity increase by distributed Bragg reflector (DBR) is effective for this purpose, but there have been few reports on DBR-integrated QCLs (DBRQCLs). In this paper, we report a successful operation of a DBR-QCL in 7 μm wavelength region. With the fabrication, an n-InP buffer layer, a core region consisting of AlInAs/GaInAs superlattices, an n-InP cladding layer, and an n-GaInAs contact layer were successively grown on an n-InP substrate using OMVPE in the first growth. Then, the wafer was processed into a mesa-stripe, and it was buried by an Fe-doped InP current-blocking layer to form a buriedheterostructure (BH) waveguide. After that, a DBR in which semiconductor-walls and air-gaps were alternately arranged was formed at the front or end of the cavity by dry-etching the epitaxial layers of the air-gap regions, and thus a DBRQCL was fabricated. A DBR-QCL chip (Mesa-width:10 μm, Cavity-legth:2 mm) which had a DBR-structure consisting of 1 pair of a 3λ/4-thick semiconductor-wall/3λ/4-thick air-gap at the front end and a high reflective facet at the rear end oscillated successfully under continuous-wave condition at 15°C. This is the first report on the InP-based DBR-QCL to our knowledge. The facet reflectivity at the DBR was 66%, which was about two times larger than that of the cleaved facet. This result clearly shows that the DBR-structure is effective for threshold current reduction of QCL.
Quantum cascade lasers (QCLs) are promising light sources for real time high-sensitivity gas sensing in the mid-infrared
region. For the practical use of QCLs as a compact and portable gas sensor, their power-consumption needs to be
reduced. We report a successful operation of a low power-consumption distributed feedback (DFB) QCL. For the
reduction of power consumption, we introduced a vertical-transition structure in a core region to improve carrier
transition efficiency and reduced the core volume. DFB-QCL epitaxial structure was grown by low-pressure OMVPE.
The core region consists of AlInAs/GaInAs superlattices lattice-matched to InP. A first-order Bragg-grating was formed
near the core region to obtain a large coupling coefficiency. A mesa-strip was formed by reactive ion etching and a
buried-heterostructure was fabricated by the regrowth of semi-insulating InP. High-reflective facet coatings were also
performed to decrease the mirror loss for the reduction of the threshold current. A device (5x500μm) operated with a
single mode in the wavelength region from 7.23μm to 7.27μm. The threshold current and threshold voltage under CW
operation at 20 °C were 52mA and 8.4V respectively. A very low threshold power-consumption as low as 0.44 W was
achieved, which is among the lowest values at room temperature to our knowledge.
Progress of information technology in recent years has led to a rapid expansion in data communication capacity and there has been a strong demand for constructing cost-effective and high-performance optical communication systems. Photonic integrated circuit (photonic IC) technology has offered solutions for these requirements by eliminating the individual packaging and optical connections between devices. This approach is expected not only to reduce the cost, size, and power consumption but also to realize new functions that can never be possible with conventional discrete devices. For the practical use of photonic ICs, it is desirable that they can be used under uncooled conditions and are highly productive. However, it seems difficult for conventional InP-based devices to satisfy these requirements because their temperature characteristics are insufficient due to a weak electron confinement in the active region. In addition, at present, InP substrates used for production are mainly 2 or 3 inches in diameter and it is difficult to enlarge the wafer size with maintaining the quality and mechanical strength. GaInNAs, which has been developed recently as an alternative semiconductor material in the long-wavelength region, seems to be the best candidate to satisfy these requirements. It covers bandgaps corresponding to the wavelength from 1.3 μm to 1.6 μm with lattic-matched to GaAs, which leads to the following advantages. First low-cost and large-scale integrations can be realized with high productivity due to the usage of large GaAs substrates of up to 6 inches in diameter and well-established Ga-As-based process technology. Second as well known in GaInNAs lasers, much stronger electron confinement in the active layer can be realized. Therefore GaInNAs-based devices are expected to have larger gain and better temperature characteristics comparing with conventional InP-based devices. In addition, the low Auger recombination rate and large effective mass of electron would also improve gain and temperature characteristics. The crystal quality of GaInNAs has been improved rapidly in several years. Threshold current densities of GaInNAs lasers have been reduced more than one figure and both molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE) can provide high quality GaInNAs as shown in figure 1. The threshold current density reached 200A/cm2 level at 1.3 μm region which is low enough to be used for practical applications. Therefore GaInNAs is thought to be a key material in photonic ICs in the long wavelength region. Semiconductor lasers and semiconductor optical amplifiers (SOAs) are key components of photonic ICs functioning as light sources, switches, amplifiers, wavelength converters and so on. In this paper, as the first study for photonic integration using GaInNAs, we present very low threshold current GaInNAs lasers and GaInNAs SOAs operating 1.3 μm region with good temperature characteristics.