A terahertz quantum cascade laser device is provided comprising a substrate having a top substrate surface, a bottom substrate surface, and an exit facet extending between the top substrate surface and the bottom substrate surface at an angle θ.sub.tap. The device comprises a waveguide structure having a top surface, a bottom surface, a front facet extending between the top surface and the bottom surface and positioned proximate to the exit facet, and a back facet extending between the top surface and the bottom surface and oppositely facing the front facet. The waveguide structure comprises a quantum cascade laser structure configured to generate light comprising light of a first frequency ω.sub.1, light of a second frequency ω.sub.2, and light of a third frequency ω.sub.THz, wherein ω.sub.THz=ω.sub.1−ω.sub.2; an upper cladding layer; and a lower cladding layer. The device comprises a distributed feedback grating layer configured to provide optical feedback for one or both of the light of the first frequency ω.sub.1 and the light of the second frequency ω.sub.2 and to produce lasing at one or both of the first frequency ω.sub.1 and the second frequency ω.sub.2, thereby resulting in laser emission at the third frequency ω.sub.THz at a Cherenkov angle θ.sub.THz through the bottom surface of the waveguide structure into the substrate and exiting the substrate through the exit facet. The device comprises a high-reflectivity coating on the front facet of the waveguide structure.

A vertical external cavity surface emitting laser (VECSEL) based system in a linear single cavity configuration is configured to deliver light in higher-order Hermite-Gaussian transverse modes with Watt-level output power. Simultaneous and independent lasing of spatially-restructurable multiple high-order transverse modes that are collinearly-propagating at the output of such laser cavity is facilitated with the use of an optical pumping scheme devised to control positions of location at which the gain medium of the system is pumped (e.g., locations of focal spots of multiple pump beams on the gain-medium chip). An external astigmatic mode converter is utilized to convert such high-order Hermite-Gaussian modes into corresponding Laguerre-Gaussian modes.

A quantum cascade laser includes a semiconductor substrate, an optical waveguide formed on a first surface of the semiconductor substrate, and a temperature adjusting member. The optical waveguide includes a first region and a second region located on one side with respect to the first region in the optical waveguide direction of the optical waveguide. The first region generates a first light having a first wavelength, and the second region generates a second light having a second wavelength. The optical waveguide generates an output light having a frequency corresponding to a difference between the first wavelength and the second wavelength by difference-frequency generation. A recess for suppressing heat transfer between the first region and the second region is formed at a second surface of the semiconductor substrate. The temperature adjusting member includes a first temperature adjusting member for adjusting the temperature of the second region.

A vertical external cavity surface emitting laser (VECSEL) based system in a linear single cavity configuration is configured to deliver light in higher-order Hermite-Gaussian transverse modes with Watt-level output power. Simultaneous and independent lasing of spatially-restructurable multiple high-order transverse modes that are collinearly-propagating at the output of such laser cavity is facilitated with the use of an optical pumping scheme devised to control positions of location at which the gain medium of the system is pumped (e.g., locations of focal spots of multiple pump beams on the gain-medium chip). An external astigmatic mode converter is utilized to convert such high-order Hermite-Gaussian modes into corresponding Laguerre-Gaussian modes.

A semiconductor light source including a planar optical component that focuses long-wavelength (e.g., infrared) light emitted in a resonant cavity into a nonlinear crystal, which then converts the long-wavelength light into light having a shorter wavelength (e.g., visible light) by frequency doubling. A wavelength-selective reflection layer on the nonlinear crystal reflects the long-wavelength light back into the resonant cavity to form an external cavity and transmits the light having the shorter wavelength out of the external cavity. The resonant cavity includes an active region that emits the long-wavelength light at a high efficiency. The planar optical component includes a micro-lens formed in semiconductor layers or a gradient refractive index lens formed in the nonlinear crystal.

The invention relates to a method for modulating the resonant frequency of an optical resonator (1) in accordance with a periodic, not necessarily harmonic, modulation signal (U.sub.mod(t)). Fast modulation of an optical resonator is intended to be made possible in which the current resonant frequency follows the modulation signal (U.sub.mod(t)) as precisely as possible, and specifically at a fundamental frequency of the modulation signal in the kHz range. To do this, the invention proposes the following method steps: deriving an error signal (E(t)) from a light field circulating in the resonator (1), wherein the error signal (E(t)) indicates the deviation of the optical frequency of the light field from a target value, deriving a first actuating signal (S.sub.1(t)) from the error signal (E(t)) by means of a controller (6), generating a second actuating signal (S.sub.2(t)), which has actuating-signal components at one or more harmonics (f.sub.mod, 2f.sub.mod, . . . ) of the fundamental frequency (f.sub.mod) of the modulation signal (U.sub.mod(t)), and applying a superposition signal made up of the first and the second actuating signal (S.sub.1(t), S.sub.2(t)) to an actuator (3) that changes the optical path length of the resonator (1). In other words, the invention makes use of a combination of control and narrow-band feed-forward control tuned to the spectrum of the modulation signal (U.sub.mod(t)) and of the error signal (E(t)) to modulate the resonant frequency. Preferably, the feed-forward control used for generating the second actuating signal (S.sub.2(t)) is automatically adapted in accordance with the error signal (E(t)). In addition, the invention relates to an accordingly configured optical system.

A laser module including a quantum cascade laser that includes a substrate having a main surface, a first clad layer provided on the main surface, an active layer provided on the first clad layer, and a second clad layer provided on the active layer, and a lens that has a lens plane disposed at a position facing the end surface of the active layer. An end surface of the active layer constitutes a resonator that causes light of a first frequency and light of a second frequency to oscillate, and the active layer is configured to generate a terahertz wave of a differential frequency between the first frequency and the second frequency. The substrate is in direct contact or indirect contact with the lens plane, and the end surface of the active layer is inclined with respect to a portion facing the end surface in the lens plane.

A photonics frequency comb generator includes two integrated dies: an indium phosphide die laser of a first wavelength is grown on from, and a silicon photonics die having a microring resonator connected to the laser and frequency modulators. The microring resonator converts the first wavelength into a number of second wavelengths. One type of the microring resonator is a hybrid non-linear optical wavelength generator, comprising non-silicon materials, such as SiC or SiGe built on silicon to yield a non-linear wavelength generation. The second wavelengths are generated by adjusting the ring's geometric size and a distance between the ring and the traverse waveguide. Another type of microring resonator splits the first wavelength into a plurality of second wavelengths and transmits the multiple second wavelengths to filters and modulators, and each selects and modulates one of the second wavelengths in a one-to-one relationship. This frequency comb generator has applications in WDM/CWDM and multi-chip modules in high speed transceivers.

The laser module includes a QCL element, a diffraction grating unit including a movable diffraction grating, a first lens that transmits light emitted from an end surface of the QCL element and light returning from the movable diffraction grating to the QCL element, a second lens that transmits terahertz waves emitted from the QCL element, a first holder, and a package. The first holder has a support surface on which the QCL element is mounted and a side surface connected to the support surface and facing the first lens in a resonance direction. A distance from an intersection point between the side surface and the support surface to the end surface along the resonance direction is smaller than a distance between the intersection point and the first lens along the resonance direction.

A quantum cascade laser device includes a semiconductor substrate, an active layer provided on the semiconductor substrate, and an upper clad layer provided on a side of the active layer opposite to the semiconductor substrate side and having a doping concentration of impurities of less than 1×10.sup.17 cm.sup.−3. Unit laminates included in the active layer each include a first emission upper level, a second emission upper level, and at least one emission lower level in their subband level structure. The active layer is configured to generate light having a center wavelength of 10 μm or more due to electron transition between at least two levels of the first emission upper level, the second emission upper level, and the at least one emission lower level in the light emission layer in each of the unit laminates.