Single-mode Quantum Cascade Lasers Research Articles

Above 20 GHz high frequency operation of mid-infrared doughnut-shaped microcavity quantum cascade lasers

Microresonator-based high-speed single-mode quantum cascade lasers are ideal candidates for on-chip optical data interconnection and high sensitivity gas sensing in the mid-infrared spectral range. In this paper, we propose a high frequency operation of single-mode doughnut-shaped microcavity quantum cascade laser at ∼4.6 µm. By leveraging compact micro-ring resonators and integrating with grounded coplanar waveguide transmission lines, we have greatly reduced the parasitics originating from both the device and wire bonding. In addition, a selective heat dissipation scheme was introduced to improve the thermal characteristics of the device by semi-insulating InP infill regrowth. The highest continuous wave operating temperature of the device reaches 288 K. A maximum −3 dB bandwidth of 11 GHz and a cut-off frequency exceeding 20 GHz in a microwave rectification technique are obtained. Benefiting from the notch at the short axis of the microcavity resonator, a highly customized far-field profile with an in-plane beam divergence angle of 2.4° is achieved.

Post-Process Frequency Tuning of Single-Mode Quantum Cascade Laser at 4.7 THz

Post-Process Frequency Tuning of Single-Mode Quantum Cascade Laser at 4.7 THz

Relative phase locking of a terahertz laser system configured with a frequency comb and a single-mode laser

Stable operation is one of the most important requirements for a laser source for high-precision applications. Many efforts have been made to improve the stability of lasers by employing various techniques, e.g., electrical and/or optical injection and phase locking. However, these techniques normally involve complex experimental facilities. Therefore, an easy implementation of the stability evaluation of a laser is still challenging, especially for lasers emitting in the terahertz (THz) frequency range because the broadband photodetectors and mature locking techniques are limited. In this work, we propose a simple method, i.e., relative phase locking, to quickly evaluate the stability of THz lasers without a need of a THz local oscillator. The THz laser system consists of a THz quantum cascade laser (QCL) frequency comb and a single-mode QCL. Using the single-mode laser as a fast detector, heterodyne signals resulting from the beating between the single-mode laser and the comb laser are obtained. One of the heterodyne beating signals is selected and sent to a phase-locked loop (PLL) for implementing the relative phase locking. Two kinds of locks are performed by feeding the output error signal of the PLL, either to the comb laser or to the single-mode laser. By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition, we, in principle, are able to experimentally compare the stability of the emission frequency of the single-mode QCL (fs) and the carrier envelope offset frequency (fCEO) of the QCL comb. The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser. The work provides a simple heterodyne scheme for understanding the stability of THz lasers, which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

Continuous-wave single-mode quantum cascade laser at 5.1 THz based on graded sampled grating design

We report on the terahertz (THz) quantum cascade lasers in continuous-wave (CW) operation with an emitting frequency above 5 THz. Excellent performance with a smaller leakage current and higher population inversion efficiency is obtained by one-well bridged bound-to-continuum hybrid quantum design at 5 THz. By designing and fabricating a graded metallic sampled distributed feedback grating in the waveguide, the first single-mode THz quantum cascade laser at 5.13 THz in CW operation mode is achieved. The maximum single-mode optical power of ∼ 48 mW is achieved at 15 K with a side-mode suppression ratio above 24 dB. This will draw great interest in the spectroscopy applications above the 5 THz range for THz quantum cascade lasers.

Continuous-wave distributed Bragg reflector quantum cascade lasers with fine single-mode tuning up to 102 °C at [formula omitted

We have demonstrated an 8.4μm stable single-mode quantum cascade laser using a two-section distributed Bragg reflector design. Output power of 252 mW, a threshold current density of 1.04 kA/cm2, and a characteristic temperature of 206 K were achieved during continuous-wave operation at room temperature. Robust single-mode emissions with all side-mode suppression ratios exceeding 25 dB were obtained at injection currents ranging from 420 mA to 900 mA and heat-sink temperatures ranging from 10 °C to 102 °C. Benefiting from the physical isolation of the two sections and from being mounted epilayer-side down on patterned submounts, the tiny tuning coefficients of the device are −0.003014 cm−1mA−1 and −0.0762 cm−1K−1. This type of single-mode quantum cascade laser with fine tuning characteristics is required to perform accurate detection of trace gases.

Widely tunable single-mode slot waveguide quantum cascade laser array.

We report designs and experimental demonstrations of a widely tunable single-mode quantum cascade laser array based on slot waveguide structures in the mid-infrared region. The laser array device realized a continuous tuning range of 71 cm-1 from 9.66 μm to 10.37 μm at 300K only using the current tuning without any external heatsink temperature adjustments, in good agreement with the design. Stable single-mode operations free of undesired mode-hops have been obtained over the whole tuning range. Another slot waveguide QCL array with a 41 cm-1 continuous tuning range around 7.3 μm has also been realized with the same design principle, demonstrating the universal applicability of the array design. The broadly continuous tuning with simple processing makes the array device a suitable candidate for mid-infrared sensing and spectroscopy application.

Wavelength beam-combining of terahertz quantum-cascade laser arrays.

Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of 50-170mW are overlapped in the far field at frequencies ranging from 3.31-3.54THz. Collinear propagation with a maximum angular deviation of 0.3∘ is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as 25%. This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.

High-speed operation of single-mode tunable quantum cascade laser based on ultra-short resonant cavity

We have demonstrated a room temperature, low power consumption, high-speed, and single-mode tunable quantum cascade laser at λ ∼ 4.6 µm. Combining a 300 µm buried hetero-structure ultra-short cavity with optimized facet coatings, a single-mode tuning range covering ∼40 nm is obtained at room temperature under low electrical power consumption below 1.1 W. The single-mode emission is observed with more than 25 dB side mode suppression ratio in the whole tuning range. The high frequency modulation is achieved with a microwave package, resulting in the −3 dB cutoff frequency of 5.6 GHz.

Terahertz master-oscillator power-amplifier quantum Cascade laser with controllable polarization

We report the realization of controllable linear-to-circular polarization states in single-mode terahertz master-oscillator power-amplifier quantum cascade lasers (THz-MOPA-QCLs). The MOPA device contains a first-order distributed feedback (DFB) laser as the master-oscillator, a preamplifier, and a 2D periodical antenna array as the power extractor. The polarization state is determined by the orientation and the phase relationship between the antennas. The antenna array is carefully designed to efficiently extract the THz radiation and not to induce field oscillation in the array or influence the mode oscillation in the DFB section. Each demonstrated device exhibits single-mode emission with a side mode suppression ratio of ∼26 dB and a single-lobed beam with a low divergence of ∼23°×30°. Realized in different devices, the degree of linear or circular polarization reaches as high as 97.5% or 99.3%. Both the operation frequency and the polarization state of the radiation are lithographically tunable.

High power continuous wave operation of single mode quantum cascade lasers up to 5 W spanning λ∼3.8-8.3 µm.

In this work, we report high power continuous wave room-temperature operation single mode quantum cascade lasers in the mid-infrared spectral range from 3.8 to 8.3 µm. Single mode robustness and dynamic range are enhanced by optimizing the distributed feedback grating coupling design and the facet coatings. High power single mode operation is secured by circumventing the over-coupling issue and spatial hole burning effect. Maximum single-facet continuous-wave output power of 5.1 W and wall plug efficiency of 16.6% is achieved at room temperature. Single mode operation with a side mode suppression ratio of 30 dB and single-lobed far field with negligible beam steering is observed. The significantly increased power for single mode emission will boost the QCL applications in long-range free-space communication and remote sensing of hazardous chemicals.

Frequency noise correlation between the offset frequency and the mode spacing in a mid-infrared quantum cascade laser frequency comb.

The generation of frequency combs in the mid-infrared (MIR) spectral range by quantum cascade lasers (QCLs) has the potential for revolutionizing dual-comb multi-heterodyne spectroscopy in the molecular fingerprint region. However, in contrast to frequency combs based on passively mode-locked ultrafast lasers, their operation relies on a completely different mechanism resulting from a four-wave mixing process occurring in the semiconductor gain medium that locks the modes together. As a result, these lasers do not emit pulses and no direct self-referencing of a QCL comb spectrum has been achieved so far. Here, we present a detailed frequency noise characterization of a MIR QCL frequency comb operating at a wavelength of 8 µm with a mode spacing of ∼7.4 GHz. Using a beat measurement with a narrow-linewidth single-mode QCL in combination with a dedicated electrical scheme, we measured the frequency noise properties of an optical mode of the QCL comb, and indirectly of its offset frequency for the first time, without detecting it by the standard approach of nonlinear interferometry applied to ultrafast mode-locked lasers. In addition, we also separately measured the noise of the comb mode spacing extracted electrically from the QCL. We observed a strong anti-correlation between the frequency fluctuations of the offset frequency and mode spacing, leading to optical modes with a linewidth slightly below 1 MHz in the free-running QCL comb (at 1-s integration time), which is narrower than the individual contributions of the offset frequency and mode spacing that are at least 2 MHz each.

Electrical tuning of a terahertz quantum cascade laser based on detuned intersubband absorption

A mechanism to electrically tune the frequency of terahertz quantum cascade lasers (QCLs) is developed that allows for tuning, while the QCL is operated close to its peak bias and temperature. Two optically coupled but electrically isolated cavities are used in which the bias of a control cavity tunes the resonant-mode of the coupled QCL cavity independent of the QCL's operating bias. Approximately 4 GHz electrical tuning is realized for a 3.6 THz distributed-feedback QCL operating in pulsed mode at 58 K in a Stirling cooler. The single-mode QCL emits near-constant peak-power in the range of 5−5.3 mW through the tuning range and radiates in a narrow single-lobed beam with a far-field divergence of ∼4°×11°. The superlattice structure of the QCL is designed to implement a low-voltage intersubband absorption transition that is detuned from that of its gain transition, the strength of which could be controlled sensitively with applied voltage utilizing resonant-tunneling injection of electrons in the absorption subband. The tuning is realized by the application of small bias voltages (∼6−7 V) and requires a narrow bias range (∼1 V, ∼40 A/cm2) to traverse across the entire tuning range, and the method should be generally applicable to all intersubband lasers including mid-infrared QCLs.

Design and fabrication of a quantum cascade laser with a chirped sampling grating

We demonstrate a quantum cascade laser (QCL) with a chirped sampling grating by altering the sampling duty cycle along the cavity. The chirped sampling grating structure, with an asymmetric coupling coefficient, can be realized by holographic exposure combined with optical photolithography. We simulated and compared three different types of sampling structures; the QCLs with a 1.5 mm chirped region and 0.5 mm uniform region can achieve an average output-power ratio of 1.71 between the front and rear facets. The proposed chirped sampling grating structure is helpful in realizing a single-mode QCL with improved power efficiency and low fabrication cost.

Single-Mode Quantum Cascade Laser at $5.1~\mu \text{m}$ With Slotted Refractive Index Modulation

We presented a technique to realize single-mode quantum cascade lasers by simply etching several large-period slots in the ridge, which leads to refractive index perturbations and mirror-loss spectrum variation at different wavelengths stemming from high-order slot feedback. Stable single-mode operation is realized as a result of the reduced mirror-loss at the designed wavelength. A side-mode-suppression-ratio of about 20 dB for all measured temperatures and peak output power of about 500 mW in pulsed operation at room temperature have been achieved.

Continuous tuning of two-section, single-mode terahertz quantum-cascade lasers by fiber-coupled, near-infrared illumination

The dynamical tuning due to rear facet illumination of single-mode, terahertz (THz) quantum-cascade lasers (QCLs) which employ distributed feedback gratings are compared to the tuning of single-mode QCLs based on two-section cavities. The THz QCLs under investigation emit in the range of 3 to 4.7 THz. The tuning is achieved by illuminating the rear facet of the QCL with a fiber-coupled light source emitting at 777 nm. Tuning ranges of 5.0 and 11.9 GHz under continuous-wave and pulsed operation, respectively, are demonstrated for a single-mode, two-section cavity QCL emitting at about 3.1 THz, which exhibits a side-mode suppression ratio better than −25 dB.

High-resolution and broadband mid-infrared Zeeman spectrum of gas-phase nitric oxide using a quantum cascade laser around 5.2 μm

A broadly tunable single-mode quantum cascade laser operating around 5.2 μm has been used to study the high-resolution and broadband mid-IR Zeeman spectrum of Gas-phase 14N16O. The Zeeman spectrum signals in the R (from R0.5 to R22.5), Q (from Q0.5 to Q6.5) and P (from P1.5 to P14.5) branches of the fundamental vibrational transition, have been recorded with both large spectral region (over the range of 1824 –1947 cm−1) and high spectral resolution (<0.001 cm−1) for the first time. We simulated the patterns of magnetic Zeeman split lines in the R, Q, and P branches, and calculated the Landé g-factor of the and components for pure Hund’s coupling case (a) and (b), and intermediate coupling (a–b) case. The experimental Zeeman spectrum measured at various values of magnetic field strength (from 0.04 to 0.32 T) at conditions of both and was calibrated by a 101.6 mm long solid germanium etalon with a free spectral range (FSR) of 0.012 cm−1. Good agreement between the calculated Landé g-factors using Radford’s theory and their best fit values inferred from the experimental data was obtained, which suggests that the Landé g-factor in Hund’s coupling case (a–b) is a better approximation, either at higher rotational quantum numbers, or at lower rotational levels, for both and subsystems. Our results provide a more general evidence to determine the coupling case of NO than previously available, because we can measure and simulate the high-resolution and broadband mid-IR Zeeman spectrum for both low and high rotational levels of the two subsystems.

Room temperature operation of a deep etched buried heterostructure photonic crystal quantum cascade laser

AbstractHigh power single mode quantum cascade lasers with a narrow far field are important for several applications including surgery or military countermeasure. Existing technologies suffer from drawbacks such as operation temperature and scalability. In this paper we introduce a fabrication approach that potentially solves simultaneously these remaining limitations. We demonstrate and characterize deep etched, buried photonic crystal quantum cascade lasers emitting around a wavelength of 8.5 μm. The active region was dry etched before being regrown with semi‐insulating Fe:InP. This fabrication strategy results in a refractive index contrast of 10% allowing good photonic mode control, and simultaneously provides good thermal extraction during operation. Single mode emission with narrow far field pattern and peak powers up to 0.88 W at 263 K were recorded from the facet of the photonic crystal laser, and lasing operation was maintained up to room temperature. The lasing modes emitted from square photonic crystal mesas with a side length of 550μm, were identified as slow Bloch photonic crystal modes by means of three‐dimensional photonic simulations and measurements. image

Low-dissipation 7.4-µm single-mode quantum cascade lasers without epitaxial regrowth.

We report continuous-wave operation of single-mode quantum cascade (QC) lasers emitting near 7.4 µm with threshold power consumption below 1 W at temperatures up to 40 °C. The lasers were fabricated with narrow, plasma-etched waveguides and distributed-feedback sidewall gratings clad with sputtered aluminum nitride. In contrast to conventional buried-heterostructure (BH) devices with epitaxial sidewall cladding and in-plane gratings, the devices described here were fabricated without any epitaxial regrowth processes, yet they exhibit power consumption comparable to the lowest-dissipation BH QC lasers reported to date. These low-dissipation devices are designed primarily as light sources for infrared spectroscopy instruments with limited volume, mass, and power budgets.

Single-Mode Quantum Cascade Laser Array Emitting From a Single Facet

We present an array of single-mode quantum cascade lasers operating in the range 8– $9.5~\mu \text{m}$ . The resonator features a pair of distributed Bragg reflectors for the mode selection, together with a second-order extraction grating. The radiation of these substrate emitting devices was combined in a single collector ridge. A proof of principle featured seven laser devices with a coverage of 170 cm $^{-1}$ . The functionality of our approach is corroborated with a set of polarization and near-field measurements confirming the collection of multiple frequencies in a unique ridge.

High power and single mode quantum cascade lasers.

We present a single mode quantum cascade laser with nearly 1 W optical power. A buried distributed feedback reflector is used on the back section for wavelength selection. The laser is 6 mm long, 3.5 μm wide, mounted episide-up and the laser facets are left uncoated. Laser emission is centered at 4.68 μm. Single-mode operation with a side mode suppression ratio of more than 30 dB is obtained in whole range of operation. Farfield measurements prove a symmetric, single transverse-mode emission in TM00-mode with typical divergences of 41° and 33° in the vertical and horizontal direction respectively. This work shows the potential for simple fabrication of high power lasers compatible with standard DFB processing.