Quantum Cascade Laser Optical Frequency Comb

Abstract

The present invention relates to the design of quantum cascade laser (QCL)-based optical frequency combs, i.e. lasers whose spectrum consists of a series of discrete equally spaced frequency lines, in particular a waveguide design that controls dispersion. To achieve this, the active region of the laser is sandwiched between two highly doped plasmonic layers. This novel structure is particularly advantageous for mass-produced optical frequency comb QCLs.

Description

[Technical Field]

[0001] This invention relates to an optical frequency comb based on a quantum cascade laser (QCL), and more particularly to a waveguide design method for achieving dispersion compensation in such a device. A frequency comb is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines. Such frequency combs are used, for example, in laser-based spectroscopy, measurement, and microwave signal generation. [Background technology]

[0002] Since the invention of optical frequency combs, they have become an integral part of systems such as dual-comb spectroscopy, measurement, and ultra-low noise microwave signal generation. The first demonstration of a quantum cascade laser (QCL) frequency comb by ETH Zurich and Alpes Lasers SA in 2012 made it possible to apply well-developed frequency comb techniques to the mid-infrared (mid-IR) region of the electromagnetic spectrum, as described in A. Hugi et al., “Mid-infrared frequency comb based on a quantum cascade laser”, Nature 492,229-233 (2012) and G. Villares et al., “Dual-comb spectroscopy based on quantum-cascade-laser frequency combs”, Nat.Commun. 5,5192 (2014).

[0003] Because many molecules have strong absorption lines in the mid-infrared region (4–12 μm), this spectral region is of particular interest to spectroscopic chemical sensing applications, including industrial process monitoring, environmental monitoring, and remote sensing applications. QCLs are ideal laser sources for this spectral region. The central emission wavelength can be tuned to any wavelength between 4 and 12 μm by adjusting the design of the active region. One of the main challenges regarding the generation of frequency combs in QCLs remains the dispersion control of the system, which must be tuned to near zero for efficient phase locking of the modes to generate the comb teeth of the frequency comb by four-wave mixing. This problem becomes even more difficult in short-wavelength QCLs, as the resonant absorption frequencies of the materials make the dispersion of the materials very high and difficult to reduce. This was described in Y. Bidaux et al., “Coupled-Waveguides for Dispersion Compensation in Semiconductor Lasers”, Laser&Photonics Rev.12,1700323(2018).

[0004] A considerable amount of research has been conducted to address this problem. Various solutions have been proposed and demonstrated, including the external Gires-Tournois interferometer (GTI) mirror described in J. Hillbrand et al., “Tunable dispersion compensation of quantum cascade laser frequency combs”, Opt. Lett. 43, 1746-1749 (2018).

[0005] Other solutions include dispersion-compensated end-face coating, as described in G. Villares et al., “Dispersion engineering of quantum cascade laser frequency combs”, Optica 3, 252-258 (2016) or QYLu et al., “Shortwave quantum cascade laser frequency comb for multi-heterodyne spectroscopy”, Appl. Phys. Lett. 112, 1-5 (2018).

[0006] A further solution is the geometric arrangement of coupled double waveguides, as described in Y. Bidaux et al., “Coupled-Waveguides for Dispersion Compensation in Semiconductor Lasers”, Laser & Photonics Rev. 12, 1700323 (2018), and disclosed in U.S. Patent No. 11070030 (B2) (J. Faist et al., “Waveguide Heterostructure for Dispersion Compensation in Semiconductor Lasers”).

[0007] However, all of the above technologies have difficulties in manufacturing (possibly mass production) or in distributed control:

[0008] The use of external GTI mirrors, as described in J. Hillbrand et al., “Tunable dispersion compensation of quantum cascade laser frequency combs”, Opt. Lett. 43, 1746-1749 (2018), requires extremely precise positioning and mechanical stability to achieve and maintain the desired effect.

[0009] On the other hand, GTI mirror endface coatings, such as those described in G. Villares et al., “Dispersion engineering of quantum cascade laser frequency combs”, Optica 3, 252-258 (2016), have the advantage of being monolithically integrated with the QCL chip, but their fabrication requires the deposition of a thick multilayer coating on the QCL endface, which is difficult in the mid-infrared spectral region where standard coating materials are not transparent. Furthermore, different coating structures are required for each wavelength and cavity length.

[0010] The dual waveguide geometric arrangement described in Y. Bidaux et al., “Coupled-Waveguides for Dispersion Compensation in Semiconductor Lasers”, Laser&Photonics Rev.12, 1700323 (2018), requires epitaxial growth of a passive InGaAs waveguide in addition to the waveguide formed by the QCL active region, and processing of the passive waveguide in the embedded heterostructure geometry. As a result, wafer manufacturing requires significantly more steps than the standard QCL process. Another drawback of the dual waveguide method is that its heat dissipation capacity is lower than that of a standard QCL because InGaAs has a lower thermal conductivity compared to InP. This leads to higher temperatures in the active region during operation and thus reduced performance.

[0011] Plasmon-enhanced dispersion-controlled QCL combs have proven to be a robust and reproducible design for efficient QCL frequency comb fabrication, as described in Y. Bidaux et al., “Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs”, Opt. Lett. 42, 1604-1607 (2017) and S. Hakobyan and R. Maulini, “High performance quantum cascade laser frequency combs at λ~6μm based on plasmon-enhanced dispersion compensation”, Opt. Express 28, 20714-20727 (2020).

[0012] To date, various groups have worked on plasmon-enhanced dispersion compensation, as described in Y. Bidaux, R. Maulini, A. Muller et al., “Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs”, Opt. Lett. 42, 1604-1607 (2017). S. Hakobyan, R. Maulini, A. Muller et al. also addressed this issue in “High performance quantum cascade laser frequency combs at λ~6μm based on plasmon-enhanced dispersion compensation”, Opt. Express 28, 20714-20727 (2020). However, both of these publications only show one plasmon layer above the active region.

[0013] However, in the case of short-wavelength QCL combs (4 - 6 μm), this technique is thought not to function due to the limitations of doping concentration and, furthermore, the induced losses to the system, which are well described in Y. Bidaux et al., “Coupled-Waveguides for Dispersion Compensation in Semiconductor Lasers”, Laser&Photonics Rev. 12, 1700323 (2018).

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Non-Patent Documents

[0015]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

[0016] Therefore, the main object of the present invention is to devise a waveguide design for dispersion compensation of short-wavelength QCL combs suitable for mass production. [Means for solving the problem]

[0017] A novel waveguide design for dispersion-compensated QCL is schematically shown in Figure 1. To control dispersion, the waveguide has two highly doped n-segments located above and below the active region, respectively. + -Includes an InP plasmon layer. These two plasmon layers are separated from the active region by two low-doped n-InP spacer layers.

[0018] These two highly doped n + - Carrier concentration and thickness of the InP layer, and the two highly doped n + -By adjusting the thickness of the n-InP spacer layer that separates the InP layer from the active region, the group velocity dispersion in the waveguide can be reduced to 500 fs, which is required for the generation of an optical frequency comb. 2 It can be reduced to a value of less than / mm.

[0019] The fabrication of the dispersion-compensated double plasmon waveguide according to the present invention does not require additional fabrication steps compared to standard QCLs. The lower plasmon layer and spacer layer are grown during the same epitaxial growth process as the active region.

[0020] The QCL comb according to the present invention does not require special end-face coatings or external mirrors because it relies on dispersion compensation within the cavity. Comb production is obtained from QCL chips of various lengths, having as-cleaved end faces and a regular (i.e., non-dispersion-compensated) high-reflectivity coating.

[0021] Unlike the dual waveguide design referenced above, the design of the present invention does not require the use of low thermal conductivity alloys such as InGaAs, and therefore heat dissipation is not affected.

[0022] These two highly doped n molecules are located above and below the active region, or rather, they sandwich the active region. + - The InP layer is a novel structure of the present invention and is particularly advantageous for mass-produced frequency comb QCLs.

[0023] The attached drawing shows the following: [Brief explanation of the drawing]

[0024] [Figure 1] This is a schematic diagram of a novel QCL according to the present invention. [Figure 2] (a) to (c) are three figures showing the electric field distribution, and (d) to (f) are three figures showing the effect of the new QCL. [Figure 3] These are LIV curves for various temperatures. [Figure 4] This figure shows the optical spectrum and relative power of the novel QCL. [Figure 5] This figure shows the dispersion and loss of different novel QCLs at 4 μm. [Figure 6] This is the operation map of the frequency comb for the new QCL. [Modes for carrying out the invention]

[0025] As schematically shown in Figure 1, the waveguide is of the embedded heterostructure (BH) type. The following layers are grown on the n-InP substrate in the first epitaxial growth process: n-InP substrate 1, n-InP lower cladding 2, n + -InP lower plasmon layer 3, n-InP lower spacer layer 4, n-InGaAs lower independent confinement layer (SCL) 5, InGaAs / AlInAs superlattice active region 7, n-InGaAs upper SCL8.

[0026] The purpose of SCLs 5 and 8 is to increase the overlap between the optical mode and the active region. These improve the function but are not essential. The present invention can be implemented using the two SCLs shown above, namely one below the active region and one above the active region. This can also be achieved with only one SCL below the active region, or only one above the active region, or even without any SCLs.

[0027] Also, the n-InP lower cladding layer 2 is not essential. This can be replaced by appropriately doping the substrate 1 so that the growth process starts from the lower plasmon layer 3.

[0028] After the first growth process step, the active region 7 (and optionally the SCL) is etched within the mesa, and semi-insulating InP:Fe6 is selectively regrown on the side surfaces to provide current confinement and waveguiding.

[0029] Finally, the remaining layers are grown in the third growth process step: n-InP upper spacer layer 9, n + -InP upper plasmon layer 10, n-InP upper cladding 11, and n + -InP or n + -InGaAs contact layer 12.

[0030] In FIG. 1, the lower cladding 2 and the upper cladding 11 are shown as single layers, but these are usually composed of several InP layers having different doping levels.

[0031] Variations in the presence, number, and / or position of many layers are possible, but the two plasmon layers 3 and 10 are an essential part of the present invention.

[0032] In the first embodiment of the present invention, an optical power of up to 350 mW and 55 cm ー1A highly efficient optical frequency comb (QCL) exhibiting an optical bandwidth and an emission spectrum centered at 5.3 μm was demonstrated. The QCL shows stable frequency comb operation at measurement temperatures from -10°C to 50°C and at almost all currents. In particular, this novel design showed good dispersion compensation at wavelengths down to 4 μm.

[0033] The embodiments described herein are merely examples, and those skilled in the art should understand that other embodiments can be utilized without departing from the scope of the present invention.

[0034] The design of novel waveguide structures was motivated by prior research by different groups on plasmon-enhanced dispersion compensation, as described in Y. Bidaux, R. Maulini, A. Muller et al., “Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs”, Opt. Lett. 42, 1604-1607 (2017). Furthermore, S. Hakobyan, R. Maulini, A. Muller et al. addressed this issue in “High performance quantum cascade laser frequency combs at λ~6μm based on plasmon-enhanced dispersion compensation”, Opt. Express 28, 20714-20727 (2020).

[0035] However, these publications focus solely on the upper plasmon layer. With upper cladding designs alone, plasmon-enhanced dispersion compensation requires impractically high doping concentrations, resulting in a significant increase in waveguide losses. In addition to adding a highly doped InP plasmon thin layer to the upper cladding, it has been found that adding a further highly doped InP plasmon layer to the lower cladding is advantageous because it achieves much lower losses. The distance between the active region and the plasmon layer is controlled by a low-doped InP spacer.

[0036] Figure 2 shows various diagrams of the second embodiment, i.e., a waveguide structure that includes an upper SCL but does not include a lower SCL. Low doping (n=1~4×10 17 cm -3 The InP:S substrate served the function of the undercladding, and therefore there was no need to grow the undercladding.

[0037] Figure 2 contains various figures, among which, Figure (a) is a horizontal cross-sectional view of the 2D electric field norm along the center of the structure. Figure (b) shows the 2D norm of the electric field distribution. Figure (c) is a vertical cross-sectional view of the electric field norm along the center of the structure. Figure (d) compares the (simulated) mode effective refractive indices of QCLs without two highly doped plasmon layers (upper curve) and with two highly doped plasmon layers, one in the upper cladding and one in the lower cladding (lower curve). Figure (e) compares the (simulated / calculated) group velocity dispersion (GVD) of QCLs with and without a highly doped plasmon layer in the upper and lower cladding (upper curve) respectively. Figure (f) compares the (simulated / calculated) losses of QCLs with and without a highly doped plasmon layer (lower curve) and with a highly doped plasmon layer (upper curve) in the upper and lower cladding.

[0038] The gray areas in Figures (d), (e), and (f) show the emission spectra of the QCL.

[0039] Leakage of fundamental modes into the plasmon layer can be seen in the vertical cross-sectional view of the modes shown in Figure 2c), while the horizontal cross-sectional view remains unchanged, as shown in Figure 2(a). This significantly alters the dispersion of the structure. Figures 2(d) to 2(f) show a comparison of the mode effective refractive index, group velocity dispersion (GVD), and waveguide loss of fundamental modes for semiconductor structures without two plasmon layers and semiconductor structures with two plasmon layers. Adding two plasmon layers according to the present invention brings the waveguide loss to a value of 1 cm, which is appropriate for efficient comb operation. -1 While keeping it below this, the variance is approximately 800 fs 2 / mm to approximately 200fs 2 It decreases to / mm. The composition, thickness, and doping levels of the various layers that make up the structure, including the plasmon layer, Listed in Table 1.

[0040] [Table 1]

[0041] A 6 mm long laser structure with an active region of 3.65 μm to 6.15 μm in width was bonded epitaxially side-up to an AIN submount using AuSn solder. The rear end face was coated with a high-reflectance (HR) coating, but the front end face was not. Subsequently, the chip-on submount was soldered onto a copper mount using In solder.

[0042] The QCL chip was operated within an experimental laser housing equipped with an integrated Peltier cooler for temperature stabilization. The housing allows for temperature control from -20°C to +50°C. The continuous-wave drive current is supplied via a bias tee. The RF component of the system current is drawn from the AC port of the bias tee and analyzed with an RF spectrum analyzer. For optical power measurements, a calibrated thermopile detector was placed in front of the output window of the housing. The optical spectrum was measured with a Fourier transform infrared (FTIR) spectrometer. Details regarding the characterization settings can be found in the above-cited paper by S. Hakobyan, R. Maulini, A. Muller et al., which addresses this issue in “High performance quantum cascade laser frequency combs at λ~6μm based on plasmon-enhanced dispersion compensation”, Opt. Express 28, 20714-20727 (2020).

[0043] Figure 3 shows the continuous-wave photocurrent-voltage (LIV) curves of a 6.15 μm wide laser emitting light at 5.3 μm from -20°C (upper curve) to +50°C (lower curve). This is the QCL identified in Table 1 and related explanations above.

[0044] The laser exhibits maximum output power of 350 mW at -20°C and 70 mW at +50°C, respectively. The temperature coefficient and slope efficiency of the threshold current density are T0 = 110 K and T1 = 100 K, respectively. The laser has a spectral width of approximately 50 cm², which is in very good agreement with the design target. -1 Approximately 1870cm -1 It has an optical spectrum centered on and exhibits comb operation slightly above the threshold current. This is shown in the upper part of Figure 4, which shows the optical spectrum of a laser operating at -20°C with a current of approximately 1.2A.

[0045] The lower part of Figure 4 shows the offset from a typical intermode beat frequency of 7.5 GHz with a span of approximately 80 kHz, measured at -10°C and a current of approximately 1.15 A. The sharp peak at the carrier or intermode beat frequency indicates coherent frequency comb operation. This peak at approximately 7.5 GHz corresponds to the round-trip frequency within a cavity of length 6 mm. Coherent comb operation is demonstrated with a full width at half maximum of less than 5 kHz.

[0046] The full range of coherent frequency comb operation is shown in Figure 6. This figure is a frequency comb operation map of the QCL, showing that comb operation in the hundreds of mA operating range can be observed at all measurement temperatures (-10°C to +50°C).

[0047] Figure 5 shows the calculated group velocity dispersion (GVD) in Figure 5a) and waveguide loss in Figure 5b) for the basic TM mode of waveguide design at 4 μm. The upper curve in Figure 5a) and the lower curve in Figure 5b) show the values ​​for the initial design or standard design, i.e., the design without a plasmon layer, respectively. The lower curve in Figure 5a) and the upper curve in Figure 5b) show the values ​​for two plasmon structures, i.e., structures with lower and upper plasmon layers, respectively. The middle curve shows the dispersion and loss when only a single plasmon layer is applied. A single plasmon layer only on the lower cladding appears to have significantly lower dispersion (branching downwards, see the middle curve in Figure 5a). For loss, it does not appear to matter whether the single plasmon layer is placed on the top or bottom (see the middle curve in Figure 5b).

[0048] By adding two plasmon layers, the variance is approximately 850 fs. 2 The frequency decreases by 500fs / mm, which is considered the baseline for frequency comb operation according to Faist et al., “Quantum-Cascade Laser Frequency Combs”, Nanophotonics 5, 272-291 (2016). 2 The value will be less than / mm.

[0049] Simultaneously, the plasmon layer losses and doping concentrations are kept at acceptable levels for efficient laser oscillation and fabrication. This comparison demonstrates the efficiency of the two-plasmon structure compared to a single-plasmon design.

[0050] The experimental results reported above demonstrate that the present invention enables the realization of high-performance QCL combs at wavelengths as short as 5.3 μm. To demonstrate the possibility of realizing high-performance QCL combs at even shorter wavelengths, a QCL active region and waveguide were designed to emit light at a central wavelength of 4.0 μm, and numerical simulations of the optical modes were performed to evaluate group velocity dispersion (GVD) and waveguide losses.

[0051] In conclusion, a novel waveguide design for dispersion compensation in short-wavelength QCLs for frequency comb operation is presented. By stacking an active region between two highly doped InP plasmon layers, it becomes possible to precisely control the system's dispersion while keeping loss and doping levels within acceptable limits. The usefulness of the present invention was experimentally demonstrated by achieving high-performance frequency comb operation at a wavelength of 5.3 μm. Numerical simulations demonstrated that the present invention can also be applied to the design of high-performance QCL combs at short wavelengths equivalent to 4.0 μm. This application further has the following features. [Aspect 1] A semiconductor quantum cascade laser having a waveguide heterostructure having an active core including an active region (7), wherein the active core is sandwiched between two pairs of passive layers of a semiconductor layer in a frequency comb configuration, the pairs of passive layers are located between the core and a contact layer (12) and between the substrate (1) and the core, and each pair of passive layers includes at least one highly doped semiconductor layer (3, 10) having a high carrier concentration, a so-called plasmon layer, and at least one low-doped spacer layer (4, 9) having a carrier concentration lower than the high carrier concentration of the plasmon layer (3, 10). [Aspect 2] The plasmon layer (3, 10) is ≥ 10 19 cm -3 The carrier concentration is such that the spacer layer (4, 9) is <10 18 cm -3 A semiconductor quantum cascade laser according to embodiment 1 above, having a carrier concentration of the above. [Aspect 3] A semiconductor quantum cascade laser having a waveguide heterostructure having an active core including an active region (7), wherein the active core is sandwiched between two pairs of passive layers of a semiconductor layer in a frequency comb configuration, the pairs of passive layers are located between the core and a contact layer (12) and between the substrate (1) and the core, and each of the pairs of passive layers has a low refractive index at the emission wavelength of the laser, preferably n プラズモン At least one so-called plasmon layer having <2 and a refractive index higher than the low refractive index, preferably n スペーサー A semiconductor quantum cascade laser characterized by comprising at least one so-called spacer layer having >2. [Aspect 4] The plasmon layer is n at the emission wavelength of the laser プラズモン Having a refractive index of <2, each adjacent spacer layer is n スペーサー A semiconductor quantum cascade laser according to embodiment 2, having a refractive index of >2. [Aspect 5] The semiconductor quantum cascade laser according to aspect 1 or 3, wherein the core has a higher refractive index than the set of adjacent layers. [Aspect 6] A semiconductor quantum cascade laser according to aspect 1 or 3, wherein the refractive index of the plasmon layer (3, 10) at the emission wavelength of the laser is lower than the refractive index of the adjacent spacer layer (4, 9), and the refractive index of the adjacent spacer layer (4, 9) is lower than the refractive index of the core. [Aspect 7] A semiconductor quantum cascade laser according to aspect 1 or 3, wherein the difference between the refractive index of the plasmon layer and the refractive index of the spacer layer is greater than the difference between the refractive index of the core and the refractive index of the spacer layer. [Aspect 8] The semiconductor quantum cascade laser according to aspect 1 or 3, wherein the composition of the heterostructure is as follows. [Table 2] [Aspect 9] A semiconductor quantum cascade laser according to embodiment 8, wherein only a single upper or lower independent confinement layer (5 or 8) is provided. [Aspect 10] The semiconductor quantum cascade laser according to embodiment 1 or 3, wherein the composition of the heterostructure is as follows. [Table 3]

Claims

1. A semiconductor quantum cascade laser having a waveguide heterostructure having an active core including an active region (7), wherein the active core is sandwiched between two pairs of passive layers of a semiconductor layer in a frequency comb configuration, and the pairs of passive layers are located between the active core and a contact layer (12), and between the substrate (1) and the active core. A semiconductor quantum cascade laser characterized in that each set of passive layers includes at least one highly doped semiconductor layer (3, 10) having a high carrier concentration, a so-called plasmon layer, and at least one less doped spacer layer (4, 9) having a carrier concentration lower than the high carrier concentration of the plasmon layer (3, 10).

2. The plasmon layer (3, 10) is ≥ 10 １９ cm －３ The carrier concentration is such that the low-doped spacer layer (4, 9) has a carrier concentration of <10 １８ cm －３ A semiconductor quantum cascade laser according to claim 1, having a carrier concentration of the specified value.

3. A semiconductor quantum cascade laser having a waveguide heterostructure having an active core including an active region (7), wherein the active core is sandwiched between two pairs of passive layers of a semiconductor layer in a frequency comb configuration, and the pairs of passive layers are located between the active core and a contact layer (12), and between the substrate (1) and the active core. A semiconductor quantum cascade laser characterized in that each set of passive layers includes at least one so-called plasmon layer having a low refractive index at the emission wavelength of the semiconductor quantum cascade laser and at least one so-called spacer layer having a refractive index higher than the low refractive index.

4. The plasmon layer is n at the emission wavelength of the semiconductor quantum cascade laser. プラズモン It has a refractive index of <2, and each adjacent spacer layer is n スペーサー A semiconductor quantum cascade laser according to claim 2 or 3, having a refractive index of 2.

5. The semiconductor quantum cascade laser according to claim 1 or 3, wherein the active core has a higher refractive index than the set of adjacent layers.

6. The semiconductor quantum cascade laser according to claim 1 or 3, wherein, at the emission wavelength of the semiconductor quantum cascade laser, the refractive index of the plasmon layer (3, 10) is lower than the refractive index of the respective adjacent spacer layer (4, 9), and the refractive index of the adjacent spacer layer (4, 9) is lower than the refractive index of the active core.

7. A semiconductor quantum cascade laser according to claim 1 or 3, wherein the difference between the refractive index of the plasmon layer and the refractive index of the spacer layer is greater than the difference between the refractive index of the active core and the refractive index of the spacer layer.

8. The semiconductor quantum cascade laser according to claim 1 or 3, wherein the composition of the waveguide heterostructure is as follows. Table 1

9. A semiconductor quantum cascade laser according to claim 1 or 3, comprising a single upper or lower independent confinement layer (5, 8).

10. The semiconductor quantum cascade laser according to claim 1 or 3, wherein the composition of the waveguide heterostructure is as follows. Table 2

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