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] The present invention relates to quantum cascade laser (QCL) based optical frequency combs, and in particular to waveguide designs for achieving dispersion compensation in such devices. 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, metrology and microwave signal generation. [Background technology]

[0002] Since the invention of the optical frequency comb, it has become an integral part of systems such as dual-comb spectroscopy, metrology, and ultra-low noise microwave signal generation. The demonstration of the first quantum cascade laser (QCL) frequency comb in 2012 by ETH Zurich and Alpes Lasers SA made it possible to apply the well-developed technology of frequency combs to the mid-infrared (mid-IR) range 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] Since many molecules have strong absorption lines within the mid-infrared range (4-12 μm), this spectral region is of particular interest for spectroscopy-based chemical sensing applications, including industrial process monitoring, environmental monitoring, and remote sensing applications. QCLs are ideal laser sources for this spectral range. 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 for the generation of frequency combs in QCLs remains the dispersion control of the system, which must be tuned to be close to 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 challenging for short-wavelength QCLs, since the resonant absorption frequency of the material makes the dispersion of the material 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 significant amount of research has been done to address this issue. Various solutions have been proposed and demonstrated, including external Gires-Tournois interferometer (GTI) mirrors, as 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-compensating end coatings 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] Further solutions include the coupled double-waveguide geometry 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. Pat. No. 11,070,030 (B2) (J. Faist et al., “Waveguide Heterostructure for Dispersion Compensation in Semiconductor Lasers”)).

[0007] However, all of the above techniques have manufacturing difficulties (to make mass production possible) or are difficult to 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 very precise positioning and mechanical stability to achieve and maintain the desired effect.

[0009] On the other hand, GTI mirror endface coatings, as 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 thick multilayer coatings on the QCL endfaces, which is difficult in the mid-infrared spectral range, where standard coating materials are not transparent. Moreover, different coating structures are required for each wavelength and each cavity length.

[0010] The dual-waveguide geometry 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 said passive waveguide in a buried heterostructure geometry. As a result, wafer fabrication requires significantly more steps than that of standard QCLs. Another drawback of the dual-waveguide approach is that its heat dissipation capability is lower than that of standard QCLs due to the lower thermal conductivity of InGaAs compared to InP. This leads to higher temperatures in the active region during operation and therefore reduced performance.

[0011] Plasmon-enhanced dispersion-controlled QCL combs have been 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] So far, various groups have addressed 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). Also, 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). However, both of these publications show only one plasmon layer above the active region.

[0013] However, for short wavelength QCL combs (4-6 μm), this technique does not seem to work due to limitations in doping concentration and also induced losses in the system, which are well explained 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] US Patent No. 11070030(B2) [Non-patent literature]

[0015] [Non-Patent Document 1] A. Hugi et al., “Mid-infrared frequency comb based on a quantum cascade laser”, Nature 492, 229-233 (2012) [Non-Patent Document 2] G. Villares et al., “Dual-comb spectroscopy based on quantum-cascade-laser frequency combs”, Nat.Commun.5, 5192 (2014) [Non-Patent Document 3] Y. Bidaux et al., “Coupled-Waveguides for Dispersion Compensation in Semiconductor Lasers”, Laser&Photonics Rev.12, 1700323(2018) [Non-Patent Document 4] J. Hillbrand et al., “Tunable dispersion compensation of quantum cascade laser frequency combs”, Opt. Lett. 43, 1746-1749 (2018) [Non-Patent Document 5] G. Villares et al., “Dispersion engineering of quantum cascade laser frequency combs”, Optica 3, 252-258 (2016) [Non-Patent Document 6] QYLu et al., “Shortwave quantum cascade laser frequency comb for multi-heterodyne spectroscopy”, Appl.Phys.Lett.112,1-5(2018) [Non-Patent Document 7] Y. Bidaux et al., “Plasmon-enhanced waveguide for dispersion compensation in mid-infrared quantum cascade laser frequency combs”, Opt. Lett. 42, 1604-1607 (2017) [Non-Patent Document 8] 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) Summary of the Invention [Problem to be solved by the invention]

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

[0017] The novel waveguide design for dispersion-compensating QCL is shown diagrammatically in Fig. 1. To control dispersion, the waveguide is constructed from two highly doped n-doped quartz crystals located above and below the active region, respectively. + These two plasmonic layers are separated from the active region by two lightly doped n-InP spacer layers.

[0018] These two highly doped + The carrier concentration and thickness of the InP layer and the two highly doped n + By tuning the thickness of the n-InP spacer layer that separates the n-InP layer from the active region, the group velocity dispersion in the waveguide was reduced to the 500 fs required for generating an optical frequency comb. 2 / mm.

[0019] The fabrication of the dispersion-compensating dual plasmonic waveguide according to the present invention does not require any additional manufacturing steps compared to a standard QCL: the bottom plasmonic layer and the spacer layer are grown during the same epitaxial growth step as the active region.

[0020] The QCL comb according to the present invention does not require special end facet coatings or external mirrors since it relies on intracavity dispersion compensation. Comb generation is obtained from QCL chips of various lengths with as-cleaved end faces and regular (i.e., non-dispersion-compensated) high-reflectivity coatings.

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

[0022] These two highly doped n-type MOSFETs, located above and below the active region, sandwich the active region, so to speak. + The -InP layer is a novel inventive structure that is particularly advantageous for mass-produced frequency comb QCLs.

[0023] In the accompanying drawings, the following is shown: [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram of a novel QCL according to the present invention. [Diagram 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. [Diagram 3] 1 is a LIV curve for various temperatures. [Figure 4] FIG. 1 shows the optical spectra and relative powers of the novel QCLs. [Diagram 5] FIG. 1 shows the dispersion and loss of different novel QCLs at 4 μm. [Figure 6] This is a map of the operation of the frequency comb of the new QCL. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The waveguide is of the buried heterostructure (BH) type, as shown diagrammatically in Figure 1. The following layers are grown on an n-InP substrate in a first epitaxial growth process: n-InP substrate 1, n-InP lower cladding 2, n + - InP lower plasmonic layer 3, n-InP lower spacer layer 4; n-InGaAs lower separate confinement layer (SCL) 5, an InGaAs / AlInAs superlattice active region 7; and n-InGaAs top SCL8.

[0026] The purpose of SCLs 5 and 8 is to increase the overlap between the optical mode and the active region. They improve performance, but are not required. The invention can be realized with two SCLs, one below and one above the active region, as shown above. It can also be realized with only one SCL below the active region, or one above the active region, or even no SCLs at all.

[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 plasmonic layer 3.

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

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

[0030] Although in FIG. 1 the lower cladding 2 and upper cladding 11 are each shown as a single layer, they will typically be composed of several InP layers having various doping levels.

[0031] Although variations in the presence, number and / or location of the various layers are possible, the two plasmonic layers 3 and 10 are an essential part of the present invention.

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

[0033] It should be understood that the embodiments described herein are merely examples and that those skilled in the art may utilize other embodiments without departing from the scope of the present invention.

[0034] The design of the novel waveguide structure was motivated by previous work done by different groups on plasmon-enhanced dispersion compensation 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).

[0035] However, these publications focus only on the top plasmonic layer. With only the top cladding design, plasmonic enhanced dispersion compensation requires impractically high doping concentrations. This results in a significant increase in waveguide loss. In addition to the addition of a thin highly doped InP plasmonic layer to the top cladding, it turns out to be advantageous to add an additional plasmonic layer of highly doped InP to the bottom cladding, as much lower losses are achieved. The distance between the active region and the plasmonic layer is controlled by a lightly doped InP spacer.

[0036] FIG. 2 shows various views of the second embodiment, i.e., a waveguide structure including an upper SCL but not a lower SCL. 17 cm -3 ) The InP:S substrate served the function of the lower cladding, so there was no need to grow a lower cladding.

[0037] Figure 2 contains various figures, among which: Figure (a) shows a horizontal cross section 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 section of the electric field norm along the center of the structure, Figure (d) compares the (simulated) modal effective refractive indices of a QCL without (top curve) and with two highly doped plasmonic layers, one in the upper cladding and one in the lower cladding (bottom curve); Figure (e) shows a comparison of the (simulated / calculated) group velocity dispersion (GVD) of a QCL without (top curve) and with (bottom curve) highly doped plasmonic layers in the upper and lower claddings; Panel (f) compares the (simulated / calculated) losses of a QCL without (lower curve) and with (upper curve) highly doped plasmonic layers in the upper and lower claddings.

[0038] The grey areas in panels (d), (e) and (f) show the emission spectra of the QCLs.

[0039] The leakage of the fundamental mode into the plasmonic layers can be seen in the vertical cross section of the mode shown in Figure 2c), while the horizontal cross section remains unchanged as shown in Figure 2(a). This significantly modifies the dispersion of the structure. Figures 2(d)-2(f) show a comparison of the modal effective index, group velocity dispersion (GVD), and fundamental mode waveguide loss of a semiconductor structure without and with two plasmonic layers. The addition of two plasmonic layers according to the invention reduces the waveguide loss to 1 cm, a value adequate for efficient comb operation. -1 Dispersion is kept below 800 fs 2 / mm to about 200fs 2 The composition, thickness and doping levels of the various layers that make up the structure, including the plasmonic layer, are Listed in Table 1.

[0040] [Table 1]

[0041] A 6 mm long laser structure with an active area ranging from 3.65 μm to 6.15 μm wide was bonded epitaxial side up on an AlN submount with AuSn solder. The rear facet was highly reflective (HR) coated, whereas the front facet was uncoated. The chip-on-submount was then soldered onto a Cu mount using In solder.

[0042] The QCL chip was operated in an experimental laser housing with an integrated Peltier cooler for temperature stabilization. The housing allows the temperature to be controlled from -20 °C to +50 °C. The continuous wave drive current is sent through a bias tee. The RF component of the system's current is tapped 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 setup can be found in the above cited paper by S. Hakobyan, R. Maulini, A. Muller et al., who 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).

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

[0044] The laser exhibits maximum output powers of 350 mW at -20°C and 70 mW at +50°C, respectively. The temperature coefficient of threshold current density and slope efficiency are T0 = 110 K and T1 = 100 K, respectively. The laser has a spectral width of approximately 50 cm, in close agreement with the design goal. -1 , about 1870cm -1 The laser exhibits comb operation just above the threshold current, with an optical spectrum centered around . This is illustrated in the top panel of Figure 4, which shows the optical spectrum of the laser operating at -20°C with a current of about 1.2 A.

[0045] The bottom diagram in Figure 4 shows the offset from a typical inter-modal beat frequency of 7.5 GHz with a span of about 80 kHz, measured at -10 °C and a current of about 1.15 A. A sharp peak in the carrier or inter-modal beat frequency indicates coherent frequency comb operation. This peak at about 7.5 GHz corresponds to the round trip frequency in a 6 mm long cavity. With a full width at half maximum of less than 5 kHz, coherent comb operation is indicated.

[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 operating range of several hundred mA can be observed at all measurement temperatures (from -10°C to +50°C).

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

[0048] By adding two plasmonic layers, the dispersion is reduced to about 850 fs 2 / mm, and 500 fs, which is considered the standard for frequency comb operation by Faist et al., “Quantum-Cascade Laser Frequency Combs”, Nanophotonics 5, 272-291 (2016). 2 / mm.

[0049] At the same time, the losses and doping concentrations in the plasmonic layers are kept at acceptable levels for efficient lasing and fabrication. This comparison shows the efficiency of the two plasmonic structures compared to a single plasmonic design.

[0050] The experimental results reported above demonstrate that the present invention makes it possible to realize a high-performance QCL comb at wavelengths as short as 5.3 μm. To demonstrate that a high-performance QCL comb can be realized at even shorter wavelengths, we designed a QCL active region and waveguide to emit light at a central wavelength of 4.0 μm, and performed numerical simulations of the optical modes to evaluate the 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. Stacking the active region between two highly doped InP plasmonic layers allows precise control of the dispersion of the system while keeping the loss and doping levels within acceptable ranges. We experimentally demonstrate the utility of the invention by achieving high-performance frequency comb operation at a wavelength of 5.3 μm. Numerical simulations demonstrate that the invention can also be applied to the design of high-performance QCL combs at wavelengths as short as 4.0 μm.

Claims

1. A semiconductor quantum cascade laser having a waveguide heterostructure with an active core including an active region (7), said active core being sandwiched between two passive layer sets of semiconductor layers in a frequency comb configuration, said passive layer sets being located between said core and a contact layer (12) and between a substrate (1) and said core, 1. A semiconductor quantum cascade laser, characterized in that each of the sets of passive layers comprises at least one highly doped semiconductor layer (3, 10) having a high carrier concentration, a so-called plasmonic layer, and at least one lightly doped spacer layer (4, 9) having a carrier concentration lower than the high carrier concentration of the plasmonic layer (3, 10).

2. The plasmonic layer (3, 10) is ≧10 １９ cm －３ and the spacer layer (4, 9) has a carrier concentration of <10 １８ cm －３ 2. The semiconductor quantum cascade laser of claim 1 having a carrier concentration of

3. A semiconductor quantum cascade laser having a waveguide heterostructure with an active core including an active region (7), said active core being sandwiched between two passive layer sets of semiconductor layers in a frequency comb configuration, said passive layer sets being located between said core and a contact layer (12) and between a substrate (1) and said core, Each of the pair of passive layers has a low refractive index at the emission wavelength of the laser, preferably n プラズモン <2 and at least one so-called plasmonic layer having a refractive index higher than said low refractive index, preferably n スペーサー and at least one so-called spacer layer having a n-th order of 1.

4. The plasmonic layer has a wavelength of n プラズモン <2, and each adjacent spacer layer has a refractive index of n スペーサー 3. The semiconductor quantum cascade laser of claim 2 having a refractive index >2.

5. 4. The semiconductor quantum cascade laser of claim 1, wherein the core has a higher refractive index than the adjacent layer set.

6. 4. The semiconductor quantum cascade laser of claim 1, wherein the refractive index of the plasmonic layer (3, 10) at the emission wavelength of the laser is lower than the refractive index of the adjacent spacer layer (4, 9), which in turn is lower than the refractive index of the core.

7. 4. The semiconductor quantum cascade laser of claim 1, 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.

8. 4. The semiconductor quantum cascade laser of claim 1, wherein the composition of the heterostructure is: 【Table 1】

9. 9. The semiconductor quantum cascade laser of claim 8, wherein only a single upper or lower independent confinement layer (5 or 8) is provided.

10. 4. The semiconductor quantum cascade laser of claim 1, wherein the composition of the heterostructure is: 【Table 2】

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