Terahertz cross-correlation device

Abstract

A terahertz (THz) cross-correlation apparatus for material characterization of a sample, comprising a light source for outputting a continuous wave (CW) optical signal, a terahertz antenna providing a terahertz transmitter and a terahertz receiver optically coupled to the light source, and an optical delay mechanism configured to adjust synchronization of the terahertz receiver to the terahertz transmitter with the CW optical signal. The light source is configured to provide a CW optical signal having an at least substantially continuous broadband spectrum, and an optical path for the CW signal is provided by an optical waveguide, such as an optical fiber, or an integrated waveguide. The optical delay mechanism comprises a double-pass polarization-preserving fiber stretcher comprising a circulator, a fiber stretcher, and a Faraday mirror, and / or a variable solid-state optical delay (SSOD).

Description

Technical Field

[0001] The invention relates to an apparatus for the determination and characterization of materials using terahertz radiation.

Background Art

[0002] Although terahertz (THz) time-domain spectroscopy (TDS) is established, it is still a newly emerging method, especially for the characterization of materials related to spectroscopy and thickness measurement. Terahertz cross-correlation spectroscopy is an alternative approach to time-domain spectroscopy that enables phase-sensitive measurements without the need for an ultrafast pulsed laser source (see, for example, Appl. Phys. Rev. 8, 021311 (2021); doi: 10.1063 / 5.0037395).

[0003] Terahertz is a part of the electromagnetic spectrum and has the property of being able to penetrate materials that are opaque to other electromagnetic frequencies. Therefore, terahertz is attracting increasing attention for commercial material inspection due to its wide range of applications. The determination and characterization of materials using terahertz technology are still waiting for a commercial breakthrough because the available systems are expensive, large in scale, and vulnerable to environmental noise such as temperature and humidity fluctuations, vibrations, and shocks.

Summary of the Invention

[0004] In its broadest aspect, the invention relates to a terahertz cross-correlation apparatus for the characterization of the material properties of a sample by electromagnetic radiation in the spectral range [0.1; 10 THz], the apparatus comprising - a light source for outputting a continuous-wave (CW) optical signal, - a terahertz transmitter optically coupled to the light source and configured to emit terahertz radiation towards the sample when modulated by the CW optical signal, - A terahertz receiver optically coupled to a light source, configured to detect terahertz radiation by generating an electrical detection signal which is the result of interference between terahertz radiation and a CW optical signal, -It features an optical delay mechanism configured to adjust the synchronization of the terahertz receiver with respect to the terahertz transmitter using a CW optical signal. The light source is preferably configured to provide a CW optical signal having at least a substantially continuous broadband spectrum. The optical path for the CW signal from the light source to the optical delay component, terahertz transmitter, and terahertz receiver is preferably provided by an optical waveguide. The optical delay mechanism is preferably, • A double-pass polarization-preserving fiber stretcher comprising a circulator, a fiber stretcher, and a Faraday mirror positioned to receive CW optical signals propagating in a first direction that changes in direction for reflection in the Faraday mirror and in a second direction opposite to that, and / or It features a variable solid-state optical delay (SSOD) comprising two or more optical ports, one or more optical waveguide sections for connecting the two optical ports, and an actuation means for changing the optical path length between the two connected optical ports.

[0005] In another embodiment, the present invention relates to a terahertz (THz) cross-correlation apparatus for material characterization of a sample by electromagnetic radiation at frequencies between 0.1 THz and 10 THz, wherein the apparatus - A light source for outputting a continuous wave (CW) optical signal, - A terahertz transmitter optically coupled to a light source, configured to emit terahertz radiation toward a sample when modulated by a CW optical signal, - A terahertz receiver optically coupled to a light source, configured to detect terahertz radiation by generating an electrical detection signal which is the result of interference between terahertz radiation and a CW optical signal, - A beam splitter for defining a first arm that receives a CW signal from a light source and provides optical coupling between the beam splitter and a terahertz transmitter, and a second arm that provides optical coupling between the beam splitter and a terahertz receiver, - Equipped with an optical delay mechanism configured to adjust the synchronization of a terahertz receiver with respect to a terahertz transmitter using a CW optical signal, - The light source is configured to provide a CW optical signal having a continuous broadband spectrum. -The optical path for the CW signal from the light source to the optical delay component, terahertz transmitter, and terahertz receiver is provided by an optical waveguide. - The optical delay mechanism comprises a first double-pass polarization-preserving fiber stretcher in a first arm and a second double-pass polarization-preserving fiber stretcher in a second arm, each double-pass polarization-preserving fiber stretcher comprising a circulator, a fiber stretcher, and a Faraday mirror positioned to receive CW optical signals propagating in a first direction that changes in direction for reflection at the Faraday mirror and a second direction opposite to that.

[0006] The invention also relates to a method for performing terahertz cross-correlation measurements of a sample using the terahertz cross-correlation apparatus described above.

[0007] The following figures and examples are provided below to illustrate the present invention. These figures and examples are intended to be illustrative and should not be construed as restrictive in any way. [Brief explanation of the drawing]

[0008] [Figure 1] This figure shows an example of a terahertz cross-correlation device related to this disclosure. [Figure 2] An example of a double-pass polarization-preserving fiber stretcher related to this disclosure is shown. [Figure 3]An example of a variable solid-state optical delay related to this disclosure is shown. [Figure 4] This disclosure also shows a terahertz cross-correlation device as a different example. [Figure 5] This disclosure also shows a terahertz cross-correlation device as a different example. [Figure 6] This disclosure also shows a terahertz cross-correlation device as a different example. [Figure 7] An example of a continuous broadband spectrum relating to this disclosure is shown. [Modes for carrying out the invention]

[0009] The invention provides a terahertz cross-correlation apparatus for material characterization of a sample by electromagnetic radiation in the spectral range [0.1;10 THz].

[0010] Terahertz cross-correlation devices include, but are not limited to, a pump coupled to an optical amplifier and / or a light source such as a seed light source. In exemplary embodiments, the frequency spectra from the light source (with / without amplifier) ​​are continuous in the sense that they are at least substantially continuous, without sharp peaks such as mode peaks. This will be discussed in detail below when considering both the application of spectroscopy and the application of time domain. In the frequency domain, a CW light source with a multimode spectrum (e.g., a multimode laser diode) also produces a multimode, i.e., non-continuous terahertz spectrum. This generally contains information about only one frequency (mode) and not information about the rest of the spectrum, thus reducing its usefulness for the application of spectroscopy. When using a cross-correlation device for material characterization, such as determining layer thickness, the focus is primarily on the time domain. In the time domain, a multimode laser creates a "train" of terahertz pulses. When examining material layers of a sample, each layer generates reflections that appear as pulses that are time-shifted with respect to the others. In multimode spectra, the reflection of each pulse in a train of pulses overlaps with the next pulse, making it difficult to distinguish between the differences in reflections and thus difficult to obtain thickness.

[0011] Therefore, it is preferable to use a continuous spectrum source. There are prior art references that use multimode laser diodes as light sources in terahertz-based TDS systems because multimode laser diodes can easily provide the required amount of optical power, they are easy to operate and characterize, and they are inexpensive. Such references are generally proof-of-concept publications that do not achieve the accuracy required for industrial application systems. Therefore, in the exemplary embodiment, the light source is not a multimode laser light source such as a multimode laser diode.

[0012] In an exemplary embodiment, the spectrum from the light source is a broadband spectrum having an optical bandwidth of at least 2 nm, such as at least 5 nm. In current terahertz antennas, the bandwidth of the terahertz signal is proportional to the bandwidth of the CW optical signal. Therefore, in another exemplary embodiment, the spectrum from the light source, when received by a terahertz transmitter, is a broadband spectrum resulting in the generation of a terahertz signal having a bandwidth of at least 0.1 THz, such as at least 0.5 THz or at least 1 THz. For clarification, at 1,550 nm, an optical bandwidth of 8 nm corresponds to a terahertz bandwidth of approximately 1 THz. This conversion holds only for 1,550 nm; for shorter wavelengths, a narrower optical bandwidth is required to obtain the same terahertz bandwidth. In this specification, the broadband spectrum of a light source is defined as cΔλ / λ 2 >0.2 THz, cΔλ / λ 2 >0.5THz, etc., cΔλ / λ 2 This refers to a spectrum with a frequency >0.1 THz, a bandwidth Δλ of -3 dB, and a center wavelength λ. The broader the light source's spectrum, the more spectral information can be recovered from the sample, thus improving the functionality of the instrument for the user.

[0013] The center frequency of the broadband spectrum can be selected depending on the type of the light source or the antenna substrate in the terahertz transmitter and receiver. In one embodiment, the center frequency is about 1,550 nm. This is an advantage since commercial light sources and optical fibers from telecommunication can be used. In other embodiments, the center wavelength is shorter such as 1,064 nm or 960 nm, which is an advantage since a narrower bandwidth of the light source is required to meet the 0.1 THz requirement. A further advantage is that for these shorter wavelengths, cheaper and / or better semiconductor materials for the antenna are available. As will be described in more detail later, the optical CW source can comprise an amplified spontaneous emission (ASE) source, a superluminescent diode (SLED, SLD), a light emitting diode (LED), an erbium doped fiber amplifier (EDFA), and any combination thereof.

[0014] Time domain spectroscopy (TDS) using short laser pulses typically requires the use of free space optics since short and ultrashort laser pulses have to propagate in free space to avoid non-linear effects. Such free space optics includes lenses, prisms, mirrors for controlling the direction and spread of light, and mechanical translation stages for controlling the optical delay between arms. For this, see, for example, "Interferometer-Assisted Terahertz Time Domain Spectroscopy" Opt. Express, 25(7), 7547-7558 (2017); doi:10.1364 / OE.25.007547. These free space optical components are disadvantageous since mirrors, lenses, and mechanical translation stages are bulky and their alignment is susceptible to environmental noise and any movement of the device.

[0015] The terahertz cross-correlation device using a CW optical signal according to the invention means that the free space requirement can be relaxed, which enables a new approach in the design of the device.

[0016] In this specification, an optical waveguide is a system or material designed to confine and direct electromagnetic waves in a direction determined by the physical boundary of the waveguide. Typical types of waveguides include optical fibers, channel waveguides, and planar waveguides. In the terahertz cross-correlation apparatus of the invention, the optical path of the CW signal from the light source to the optical delay component, terahertz transmitter, and terahertz receiver is preferably provided by an optical waveguide, thus ensuring an all-optical waveguide path for the apparatus. This eliminates the need for the apparatus to rely on lenses and mirrors to control the direction and spread of the optical signal along the optical path. In free-space mechanisms, lenses and mirrors are bulky components and require precise and stable alignment. Therefore, an all-optical waveguide path is advantageous because it is less susceptible to fluctuations in humidity and temperature, physical vibrations, and shocks. In addition, the use of optical waveguides allows for a reduction in the overall size of the apparatus. In the alternative configuration, it is preferable that the CW optical signal does not propagate through free space (i.e., through the ambient atmosphere) at any point along its path to the terahertz antenna.

[0017] The optical path between the light source and the optical delay mechanism is provided with a beam splitter for splitting the optical path into a first arm that provides optical coupling between the beam splitter and the terahertz transmitter, and a second arm that provides optical coupling between the beam splitter and the terahertz receiver. Thus, such a beam splitter also splits the optical signal from the light source into two distinct optical signals. In one example, the beam splitter is a 50 / 50 splitter that provides two optical signals with similar power. In other examples, the beam splitter provides two signals, one with considerably greater power than the other, with a split ratio of 60 / 40, 70 / 30, or 80 / 20, where the signal with greater power is typically for the terahertz transmitter. The beam splitter is located between the light source and the optical delay mechanism.

[0018] The disclosed optical delay mechanism provides advantages over conventional free-space translation stage delays, in that the optical delay mechanism does not include mechanically moving parts or separate mirrors and lenses that must maintain precise alignment with other optical components. Thus, the optical delay mechanism is an advantage in that it is more robust and less affected by mechanical vibrations and shocks. The optical delay mechanism is a further advantage in that the optical signal propagates in a solid rather than free space and is thus significantly less affected by environmental parameters such as pressure, temperature, humidity, gas, aerosol, etc. The optical delay mechanism preferably comprises · a double-pass polarization-maintaining fiber stretcher comprising a circulator, a fiber stretcher, and a Faraday mirror arranged to receive CW optical signals propagating in a first direction that changes in direction for reflection at the Faraday mirror and a second direction opposite thereto, and / or · a variable solid state optical delay (SSOD) comprising two or more optical ports, one or more optical waveguide sections for connecting the two optical ports, and actuating means for changing the optical path length between the two connected optical ports.

[0019] To achieve high-precision terahertz cross-correlation measurements, the optical signals driving both terahertz antennas are preferably in-phase. On the other hand, using a broadband spectral light source typically results in a coherence length on the order of centimeters. Thus, in an exemplary embodiment, the optical path length L1 of the first arm and the optical path length L2 of the second arm are equal within a range corresponding to the coherence length of the optical signal. In another exemplary embodiment, the optical path length L1 of the first arm and the optical path length L2 of the second arm are preferably equal within half of the stroke of the optical delay mechanism, where the stroke is the maximum difference in optical path length between the two arms achievable by the optical delay mechanism. In yet another exemplary embodiment, the optical path length L1 of the first arm and the optical path length L2 of the second arm are preferably equal within a length of 5 cm.

[0020] Depending on the specific application of the terahertz cross-correlation device, the scanning range of the optical delay mechanism may be on the order of one-tenth to one-thousandth of a picosecond, and the time step may typically be on the order of 10 to 100 femtoseconds. While continuous-time scanning is not a requirement, it is desirable to satisfy the Nyquist sampling theorem. For example, for a system with a 3 THz bandwidth, the Nyquist theorem states that the time step should be 166 femtoseconds.

[0021] In an exemplary embodiment, the optical delay mechanism includes double-pass polarization-preserving fiber stretchers in each of the first and second arms. These double-pass polarization-preserving fiber stretchers are preferably identical, or identical except for a small difference in the optical path length of the fiber stretchers, which is on the order of the desired scanning range. This mechanism is advantageous because it provides identical or substantially identical optical paths in terms of both optical path length and any distortion effects for the CW optical signal in the first and second arms. This again ensures that the CW optical signal applied to the terahertz antenna is in phase over the stroke of the optical delay mechanism.

[0022] A double-pass polarization-preserving fiber stretcher is an optical delay component comprising an optical circulator, a fiber stretcher, and a Faraday mirror positioned to receive CW optical signals propagating in a first direction that changes direction due to reflection in the Faraday mirror, and a second direction opposite to that. An optical circulator is an optical device having three or more ports, designed so that light incident on any port exits from the next port. This means that when light is incident on port 1, it exits from port 2, but if any of the emitted light is reflected back by the circulator, it does not exit from port 1 but exits from port 3. Optical circulators are typically used, for example, to separate optical signals traveling in opposite directions to achieve bidirectional transmission on a single fiber.

[0023] The double-pass polarization-preserving fiber stretcher also features a Faraday mirror, which is a combination of a 45-degree Faraday rotor and a mirror. The Faraday rotor rotates the polarization of light in the same direction with respect to the propagation direction on both paths, so the optical signal reflected by the Faraday mirror returns with a 90-degree rotated polarization.

[0024] In a double-pass polarization-preserving fiber stretcher, the delay is caused by physically stretching the optical fiber of the fiber stretcher to extend the optical path. In an exemplary embodiment, the fiber stretcher is, for example, a section of optical fiber 50 to 100 meters long, tightly wound around a piezoelectric crystal or other electrostrictive material that can be deformed by applying a voltage. The longer the fiber, the longer it is stretched, and the greater the resulting optical delay. Numerous fiber stretchers that can be used in this mechanism are available on the market.

[0025] One end of the fiber stretcher is optically coupled to a light source via a waveguide and an optical circulator, while the other end of the fiber stretcher is equipped with a Faraday mirror. The optical signal is received by the circulator, acquires a delay in the fiber stretcher, is reflected by the Faraday mirror, and passes through the fiber stretcher again to acquire an additional delay. Upon arriving at the optical circulator from the fiber stretcher, the optical signal is coupled to a different optical waveguide that propagates toward a terahertz transmitter or receiver. The Faraday mirror rotates the polarization of the optical signal by 90 degrees, thereby causing any change in polarization caused by the stretching of the optical fiber in the fiber stretcher during the first pass to reciprocate during the second pass. The 90-degree rotation is advantageous because terahertz transmitters and receivers are susceptible to the effects of the polarization of the optical signal. A double-pass polarization-preserving fiber stretcher is also advantageous because the optical signal passes through the fiber stretcher twice in opposite directions (hence the name double-pass). As explained above, this has the effect of causing the birefringence present in the fiber to reciprocate, which leads to polarization rotation. Furthermore, the double-pass polarization-preserving fiber stretcher has the effect of doubling the optical delay due to the additional optical path length resulting from stretching the fiber, which means that the components can be made smaller. In addition, the fiber stretcher is advantageous because it provides continuous adjustment of the optical delay and can therefore be used to provide time steps of any desired length.

[0026] A variable solid-state optical delay (SSOD) is an optical delay component comprising two or more optical ports, such as ports for coupling input and output signals into an optical waveguide, one or more optical waveguide sections for connecting the two optical ports, and an actuation means for changing the optical path length between the two connected optical ports. Such a solid-state optical delay is also referred to as a non-mechanically variable optical time delay line or solid-state delay line (SSDL).

[0027] In an exemplary embodiment, a variable solid-state optical delay comprises two or more optical waveguide sections of different lengths for connecting two optical ports, and the actuation means is configured to select one or a combination of the two or more optical waveguide sections for connecting the two or more optical ports. Thus, in this embodiment, the optical path length between the two ports can be varied by selecting different routes of different lengths. In this embodiment, the two or more different optical waveguide sections in the variable solid-state optical delay may be fiber loops or channel / planar waveguide sections of varying length formed on a substrate. The actuation means can select one of the optical waveguide sections or connect two or more optical waveguide sections in series to provide a selectable set of isolation optical delays. In addition, some variable solid-state optical delays can provide some continuous adjustment of the optical path length around or between one or more isolation optical delays. The actuation means may be, for example, an optic-mechanical fiber switch or a MEMS (micro-electromechanical system) switch, which have the advantage of not including macroscopic moving parts. In an example embodiment, the variable solid-state optical delay is a non-mechanical optical delay, and the actuation means may comprise one or more of a thermo-optic switch, an electro-optic switch, an acoustic-optic switch, and a magneto-optic switch. In addition to increased robustness and the absence of free-space propagation, a variable solid-state optical delay has the advantage that any moving parts, mirrors, and lenses can be eliminated and reduced to fit onto a single chip. In an example embodiment, the variable solid-state optical delay comprises at least 10 9 It features a durable and highly reliable optical switch capable of 100 switch cycles.

[0028] In another embodiment of a variable solid-state optical delay, the optical path length between two ports is changed by adjusting the material properties (preferably excluding elongation) of the optical waveguide section connecting the ports. In one embodiment, this may be adjustment of the refractive index or birefringence of the waveguide material. One example of such a variable solid-state optical delay may be an acousto-optic delay module, in which an actuation means transmits an acoustic signal through a birefringent quartz crystal, which leads to a change in the lattice position, thereby resulting in a change in the diffraction of the optical signal and ultimately a different optical path length.

[0029] In an example embodiment of a variable solid-state optical delay, the variable solid-state optical delay is equipped with a temperature stabilization process to stabilize the temperature of the variable solid-state optical delay. This is advantageous because changes in temperature alter the refractive index of the waveguide medium, resulting in different path lengths and affecting the interference pattern. In another example embodiment, the variable solid-state optical delay is equipped with a low-loss waveguide medium such that propagation through different paths in the variable solid-state optical delay does not substantially alter the optical power at the output.

[0030] In an exemplary embodiment, the optical delay mechanism of the terahertz cross-correlation device comprises at least a first optical delay component in the first arm and a second optical delay component in the second arm, that is, each arm comprises at least one optical delay component. Preferably, the first and second delay components are, • Double-pass polarized fiber stretcher or • Features a variable solid-state optical delay.

[0031] In one example, the optical delay mechanism of a terahertz cross-correlation device comprises both a double-pass polarization-preserving fiber stretcher and a variable solid-state optical delay. These can be provided in parallel (on different arms) or in series (on the same arm). The variable solid-state optical delay preferably provides a set of separated optical delays selectable by an actuari, where the maximum difference between two subsequent optical delays is D. In one example, the double-pass polarization-preserving fiber stretcher is adapted to provide continuous optical delay adjustment of D or greater. This combination is advantageous because it allows for continuous scanning of a wide range of optical delays.

[0032] The invention also relates to a process for performing terahertz cross-correlation measurements of a sample using the terahertz cross-correlation apparatus described above. Such terahertz cross-correlation measurements may be terahertz cross-correlation spectroscopy, such as that described in “Terahertz Cross-Correlation Spectroscopy Driven by Incoherent Light from a Superluminescent Diode,” Opt. Express 27, 12659-12665 ​​(2019); doi:10.1364 / OE.27.012659. Measurements may be material characterization measurements, such as thickness measurements, which are often referred to as spectroscopy but do not necessarily result in the reflection, absorption, or transmission spectra of the sample provided.

[0033] Hereafter, various examples and details will be described with reference to the figures where applicable. It should be noted that the figures may or may not be drawn with a certain scaling ratio, and that elements of similar structure or function are represented by similar reference numbers throughout the figures. It should also be noted that the figures are intended solely to facilitate the description of the examples. The figures are not intended to describe the disclosure exhaustively, nor are they intended to limit the scope of the disclosure. Furthermore, the examples shown do not necessarily have all the aspects or advantages shown. Aspects or advantages described in relation to a particular example are not necessarily limited to that example and may be practiced in any other example, even if not shown or explicitly described as such.

[0034] Figure 1 shows an example terahertz cross-correlation apparatus 1 relating to the disclosure. The apparatus comprises a light source 2 for outputting a continuous wave (CW) optical signal, a terahertz transmitter 4 optically coupled to the light source 2, configured to emit terahertz radiation 5 toward a sample 8 when modulated by the CW optical signal, a terahertz receiver 6 optically coupled to the light source 2, configured to detect the terahertz radiation 5 by generating an electrical detection signal which is the result of interference between the terahertz radiation 5 and the CW optical signal, and an optical delay mechanism 16 configured to adjust the synchronization of the terahertz receiver 6 with respect to the terahertz transmitter 4 using the CW optical signal. Apparatus 1 may include a beam splitter 15 to facilitate the optical coupling of both the terahertz transmitter 4 and the terahertz receiver 6 to the light source 2.

[0035] The terahertz cross-correlation device 1 can be used in a variety of applications where different signals are measured from a sample. In Figure 1, the terahertz transmitter 4 and terahertz receiver 6 are set up to measure radiation 5 transmitted from sample 8, while in Figures 4 and 5, they are set up to measure radiation 5b reflected from sample 8.

[0036] The optical path of the CW signal from the light source to the optical delay component, the terahertz transmitter, and the terahertz receiver is preferably provided by an optical waveguide 11, a channel waveguide, and a planar waveguide, such as an optical fiber. In an exemplary embodiment, the optical path is an all-optical waveguide path, meaning that no portion of the optical path of the CW optical signal is located outside the optical waveguide 11. This has the effect that the CW optical signal does not propagate in free space at any point, i.e., it does not propagate in the atmosphere of the environment in which the device 1 is located.

[0037] The optical delay mechanism 16 may have one or more optical delay components in both the arm to the terahertz transmitter and the arm to the terahertz receiver, as shown in Figure 1, or it may have one or more optical delay components in only one of the arms (see, for example, Figures 4 and 5). An example embodiment of one or more optical delay components of the optical delay mechanism 16 is described below with reference to Figures 2 and 3.

[0038] Figure 2 shows an embodiment as an example of a double-pass polarization-preserving fiber stretcher 18, which includes a circulator 20, a fiber stretcher 22, and a Faraday mirror 24 positioned to receive a CW optical signal 3 propagating in a first direction whose direction changes due to reflection at the Faraday mirror 24 and in a second direction opposite to that. The input and output ports of the circulator 20 are coupled to an optical waveguide 11. The fiber stretcher 22 may be a commercially available fiber stretcher.

[0039] Figure 3 shows an embodiment as an example of a variable solid-state optical delay 26, comprising two optical ports 28 and optical waveguide sections 30 of different lengths, here comprising optical fiber sections A-D for connecting the optical ports 28, and an operating means 26 for selecting one or a combination of the optical waveguide sections 30 for connecting the optical ports 28. The optical ports 28 are coupled to optical waveguides 11 for receiving and transmitting CW optical signals 3.

[0040] Figures 4 to 6 show embodiments of the terahertz cross-correlation apparatus 1 as further examples relating to the disclosure. Any of these embodiments, or any individual feature described in relation to the embodiment illustrated in Figure 1, can be combined with features of other embodiments.

[0041] In the apparatus 1 illustrated in Figures 4 and 5, the terahertz antennas 4 and 6 are configured to irradiate the sample 8 with terahertz radiation 5a and measure the terahertz radiation 5b reflected from the sample.

[0042] In the apparatus 1 illustrated in Figures 4 and 5, the optical waveguide 11 is supplied by an optical fiber. The type of optical fiber used can be selected from single-mode or multimode optical fiber types available on the market. The optical fiber can be selected depending on parameters such as the wavelength spectrum and field strength of the CW optical signal, the length of the optical waveguide 11, and the desired price / quality of the apparatus.

[0043] In the apparatus 1 illustrated in Figure 6, the optical waveguide 11 is provided by a channel waveguide or planar waveguide formed on or integrated into the substrate 36 of the semiconductor device. In this exemplary embodiment, all components can be integrated on-chip (on the photonic circuit), thereby making the system more compact and smaller, and manufacturing can be scaled up by using existing methods in semiconductor processing.

[0044] In the apparatus 1 illustrated in Figure 1, the optical delay mechanism 16 comprises at least one optical delay component in each arm of the apparatus. In the apparatus 1 illustrated in Figure 4, the optical delay mechanism 16 comprises at least two optical delay components in one arm of the apparatus (here, in the arm to the terahertz receiver, but alternatively, in the arm to the terahertz transmitter). The resulting at least two optical delay components are -Two double-pass polarization preservation fiber stretchers 18, - Two variable solid-state optical delays 26, -Double-pass polarization-preserving fiber stretcher 18 and variable solid-state optical delay 26, -Double-pass polarization-preserving fiber stretcher 18 and different optical delay components, - A variable solid-state optical delay 26 and different optical delay components may be used.

[0045] In the apparatus 1 illustrated in Figure 5, the optical delay mechanism 16 comprises only a single optical delay component in one arm of the apparatus (here, in the arm to the terahertz receiver, but it may instead be in the arm to the terahertz transmitter). The optical delay component may be either a double-pass polarization-preserving fiber stretcher 18 or a variable solid-state optical delay 26.

[0046] While all of the above combinations are applicable, it is preferable to keep the arms as symmetrical as possible, so that environmental parameters, especially any temperature changes, affect both arms as similarly as possible, resulting in higher accuracy. For example, when one arm contains a double-pass polarization-preserving fiber stretcher, it is also advantageous to have a double-pass polarization-preserving fiber stretcher in the other arm as well, so that both arms have approximately the same total path length, or to have a non-stretching fiber e-coil with a circulator and a Faraday mirror to match both the total path length and the reciprocating motion of the fiber's birefringence. Including only a fiber coil twice the length of the fiber stretcher 22 results in an asymmetric birefringence effect.

[0047] In a preferred example embodiment, the optical delay mechanism 16 includes a double-pass polarization-preserving fiber stretcher 18 in each arm and a variable solid-state optical delay 26 in one of the arms. This is a combination of the systems shown in Figures 1 and 4. Here, the fiber stretcher provides a small, variable time step, while the solid-state delay provides a coarse time step.

[0048] Terahertz transmitters and receivers are generally terahertz antennas. One example type is a "CW photomixer" having a semiconductor structure (which itself can consist of many different layers of semiconductor) and a metallic antenna (typically a bowtie or dipole antenna). Here, an optical signal excites the semiconductor, while a voltage bias is applied to the antenna poles (towards the emitter), or the current generated in the antenna is measured (towards the receiver). Other applicable terahertz antennas also exist, and new antennas can be developed, which are equally applicable in terms of ingenuity.

[0049] The terahertz cross-correlation apparatus 1 may be equipped with a terahertz optical instrument 7 in relation to a terahertz transmitter 4 and a terahertz receiver 6, see, for example, Figures 4 and 5. Such a terahertz optical instrument 7 may include lenses, mirrors, polarizers, beam splitters, etc., adapted to EM radiation in the terahertz band and used to control the direction and spread of the terahertz signal 5 to or from the sample 8. The terahertz receiver 6 is configured to detect the terahertz radiation 5 by generating an electrical detection signal, which is the result of interference between the terahertz radiation 5 and the CW optical signal 3. For this purpose, the apparatus 1 may be connected to an electronic processor 34 for receiving and processing the electrical detection signal from the terahertz receiver 6, as illustrated in Figure 1.

[0050] The light source and optical components can operate at any wavelength, typically visible or infrared, and the terahertz transmitter and receiver must be tuned to these wavelengths. In a preferred embodiment, the light source operates at 1,550 nm to take advantage of the availability of optical components for telecommunications.

[0051] In one exemplary embodiment, the light source 2 comprises an optical seed signal connected to an optical amplifier that amplifies the seed signal. In a preferred embodiment, the seed signal is a continuous broadband signal provided by a light-emitting diode (LED) or superluminescent diode (SLED), preferably at 1,550 nm, and by an optical amplifier and an erbium-doped fiber (EDFA). This type of light source is typically referred to as EDFA (erbium-doped fiber) and is illustrated in Figure 5. In other exemplary embodiments (not shown), the optical seed signal can be electrically amplified, such as in a mechanism in which the LED seed is amplified by a semiconductor optical amplifier.

[0052] In another exemplary embodiment, the light source 2 includes a pump signal connected to a medium that absorbs the pump signal and re-emits it via an amplified spontaneous emission (ASE) process at a longer wavelength and larger continuous bandwidth. Preferably, the pump signal is provided by a pump laser, and the ASE medium is an erbium-doped fiber. In an exemplary embodiment, the pump laser may be a 980 nm laser, and the ASE medium is an erbium-doped fiber that absorbs 980 nm and re-emits it at 1,550 nm with a wider bandwidth. This type of light source is typically referred to as an ASE and is illustrated in Figure 5.

[0053] Figure 7 shows an example of a continuous broadband spectrum of the light source of the invention. The spectrum is centered at 1,550 nm, has a bandwidth of 40 nm, and is free of any mode peaks or other discontinuities.

[0054] As previously mentioned, the terahertz cross-correlation apparatus can be used in different applications, and the disclosure provides a method for performing terahertz cross-correlation measurements of a sample using the terahertz cross-correlation apparatus. In an exemplary embodiment, the terahertz cross-correlation measurement is a layer thickness measurement, and the thickness of individual layers of a multilayer structure can be measured non-contact and non-destructively. Here, the sample is placed in a reflection mechanism as illustrated in Figures 4 and 5. While the apparatus is operating, terahertz radiation 5a from the terahertz transmitter 4 strikes the surface of the sample 8, and the reflected terahertz signal 5b is detected by the terahertz receiver 6. In the terahertz receiver 6, the CW optical signal 3 and the incident terahertz radiation 5b overlap, resulting in a photocurrent, which is the cross-correlation between the optical field and the terahertz wave. The photocurrent signal is amplified and recorded and fed to an electronic processor (see 34 in Figure 1) which can determine the layer thickness of the sample and, possibly, optical parameters. In another embodiment, terahertz cross-correlation measurement is used for non-destructive testing or quality assurance, such as non-destructive testing or quality assurance of layer thickness and surface. The inventions disclosed herein include the following: [Aspect 1] A terahertz (THz) cross-correlation apparatus for evaluating the material properties of a sample by electromagnetic radiation at frequencies between 0.1 THz and 10 THz, - A light source for outputting a continuous wave (CW) optical signal, - A terahertz transmitter optically coupled to the light source, configured to emit terahertz radiation toward a sample when modulated by the CW optical signal, - A terahertz receiver optically coupled to the light source, configured to detect terahertz radiation by generating an electrical detection signal which is the result of interference between the terahertz radiation and the CW optical signal, - A beam splitter comprising a beam splitter for defining a first arm that receives the CW signal from the light source and provides optical coupling between the beam splitter and the terahertz transmitter, and a second arm that provides optical coupling between the beam splitter and the terahertz receiver, - An optical delay mechanism configured to adjust the synchronization of the terahertz receiver with respect to the terahertz transmitter using the CW optical signal, - The light source is configured to provide a CW optical signal having a continuous broadband spectrum, -The optical path for the CW signal from the light source to the optical delay component, the terahertz transmitter, and the terahertz receiver is provided by an optical waveguide. - The optical delay mechanism comprises a first double-pass polarization-preserving fiber stretcher in the first arm and a second double-pass polarization-preserving fiber stretcher in the second arm, wherein each double-pass polarization-preserving fiber stretcher comprises a circulator, a fiber stretcher, and a Faraday mirror arranged to receive the CW optical signal propagating in a first direction that changes in direction for reflection in the Faraday mirror and in a second direction opposite to that. Terahertz cross-correlation device. [Aspect 2] The terahertz cross-correlation apparatus according to embodiment 1, characterized in that the first and second double-pass polarization-preserving fiber stretchers are identical. [Aspect 3] The terahertz cross-correlation apparatus according to embodiment 1 or 2, characterized in that the light source comprises a laser diode. [Aspect 4] The terahertz cross-correlation apparatus according to any one of embodiments 1 to 3, characterized in that the light source comprises a superluminescent diode (SLED). [Aspect 5] The terahertz cross-correlation apparatus according to any one of embodiments 1 to 4, characterized in that the light source comprises a light source based on amplified spontaneous emission (ASE). [Aspect 6] The terahertz cross-correlation apparatus according to any one of embodiments 1 to 5, characterized in that the light source comprises a fiber amplifier. [Aspect 7] The terahertz cross-correlation apparatus according to any one of embodiments 1 to 6, characterized in that the optical delay mechanism further comprises a variable solid-state optical delay (SSOD) having two or more optical ports, one or more optical waveguide sections for connecting the two optical ports, and an actuation means for changing the optical path length between the two connected optical ports. [Aspect 8] The variable solid-state optical delay provides a set of isolated optical delays that can be selected by the actuator, wherein the maximum difference between two subsequent optical delays is D. The terahertz cross-correlation apparatus according to embodiment 7, characterized in that at least one of the first and second double-pass polarization-preserving fiber stretchers is adapted to provide continuous optical delay adjustment of D or greater. [Aspect 9] The variable solid-state optical delay (SSOD) comprises two or more optical waveguide sections of different lengths for connecting two optical ports, The terahertz cross-correlation apparatus according to any one of embodiments 1 to 8, characterized in that the operating means is configured to select one or a combination of the two or more optical waveguide sections for connecting the two or more optical ports. [Aspect 10] The terahertz cross-correlation apparatus according to any one of embodiments 1 to 9, characterized in that the light source is not a multimode laser system. [Aspect 11] A method for performing terahertz cross-correlation measurement of a sample using the terahertz cross-correlation apparatus described in any one of embodiments 1 to 10.

[0055] Reference number list 1. Terahertz cross-correlation device 2.Light source 3.CW optical signal 4. Terahertz Transmitter 5. Terahertz radiation / signals 6. Terahertz receiver 7. Terahertz Optical Instruments 8. Sample 10. Optical path of a terahertz cross-correlation device 11.Optical waveguide 12.Light source 13. Optical Amplifier 14. Spectrum of a CW optical signal 15. Beam Splitter 16. Optical delay mechanism 18. Double-pass polarized fiber stretcher 20. Optical Circulator 22. Fiber Stretcher 24. Faraday Miller 26. Variable Solid-State Optical Delay 28. Optical Input / Output Ports 30. Optical waveguide section 32. Operating means 34. Electronic Processor 36. Circuit board

[0056] The use of terms such as "first," "second," "third," "fourth," "primary," "secondary," and "tertiary" does not imply any particular order, but is included to identify individual elements. Furthermore, the use of terms such as "first," "second," "third," "fourth," "primary," "secondary," and "tertiary" does not indicate any order of importance, and these terms are used to distinguish one element from another. Note that while terms such as "first," "second," "third," "fourth," "primary," "secondary," and "tertiary" are used here and elsewhere, they are solely for labeling purposes and are not intended to indicate any particular spatial or temporal order. Moreover, the labeling of the first element does not imply the existence of the second element, and vice versa.

[0057] A feature considered above as a separate implementation can also be implemented by combining it into a single implementation. Conversely, a feature described as a single implementation can also be implemented in multiple separate implementations, or in any suitable smaller combination. Furthermore, a feature can be described above as acting in a certain combination, but in some cases one or more features from a claimed combination can be implemented from the combination, and the combination can be described as any smaller combination, or as a variation of any smaller combination.

[0058] It should be noted that the term "equipped" does not necessarily exclude the existence of other elements or steps other than those listed.

[0059] It should be noted that the term "one" preceding an element does not exclude the existence of multiple such elements.

[0060] No reference numeral limits the scope of the claims, and it should be further noted that examples can be realized at least partially by both hardware and software, and some “means,” “units,” or “apparatus” can be represented by the same items of hardware.

[0061] The degree-indicating terms used herein, such as “almost,” “about,” “generally,” and “substantially,” represent values, quantities, or characteristics that are close to the described value, quantity, or characteristic, and still perform the desired function and achieve the desired result. For example, the terms “almost,” “about,” “generally,” and “substantially” may refer to quantities, qualities, or characteristics such as the continuity of an optical spectrum, within 10%, 5%, 1%, 0.1%, and 0.01% of the described quantity, quality, or characteristic.

Claims

1. A terahertz (THz) cross-correlation apparatus (1) for measuring a sample (8) by electromagnetic radiation at frequencies between 0.1 THz and 10 THz, - A light source (2) for outputting a continuous wave (CW) optical signal (3), - A terahertz transmitter (4) optically coupled to the light source, configured to emit terahertz radiation (5) toward the sample when modulated by the CW optical signal, - A terahertz receiver (6) optically coupled to the light source, configured to detect terahertz radiation interacting with the sample by generating an electrical detection signal which is the result of interference between the terahertz radiation and the CW optical signal, - A beam splitter (15) for defining a first arm that receives the CW optical signal from the light source and provides optical coupling between the beam splitter and the terahertz transmitter, and a second arm that provides optical coupling between the beam splitter and the terahertz receiver, - An optical delay mechanism (16) configured to adjust the synchronization of the terahertz receiver with respect to the terahertz transmitter using the CW optical signal, - The light source is configured to provide a CW optical signal having a continuous broadband spectrum. - The optical path (10) for the CW optical signal from the light source to the optical delay mechanism, the terahertz transmitter, and the terahertz receiver is provided by an optical waveguide (11). - The optical delay mechanism comprises a first double-pass polarization-preserving fiber stretcher (18) in the first arm and a second double-pass polarization-preserving fiber stretcher (18) in the second arm, wherein each double-pass polarization-preserving fiber stretcher comprises a circulator (20), a fiber stretcher (22), and a Faraday mirror (24) positioned to receive the CW optical signal propagating in a first direction that changes in direction for reflection in the Faraday mirror and in a second direction opposite to that direction. Terahertz cross-correlation device.

2. The terahertz cross-correlation apparatus according to claim 1, characterized in that the first and second double-pass polarization-preserving fiber stretchers are identical.

3. The terahertz cross-correlation apparatus according to claim 1 or 2, characterized in that the light source comprises a laser diode.

4. The terahertz cross-correlation apparatus according to claim 1, characterized in that the light source comprises a superluminescent diode.

5. The terahertz cross-correlation apparatus according to claim 1, characterized in that the light source comprises a light source based on amplified spontaneous emission.

6. The terahertz cross-correlation apparatus according to claim 1, characterized in that the light source is provided with a fiber amplifier.

7. The terahertz cross-correlation apparatus according to claim 1, wherein the optical delay mechanism further comprises a variable solid-state optical delay (26) having two or more optical ports (28), one or more optical waveguide sections (30) for connecting the two optical ports, and an actuation means (32) for changing the optical path length between the two connected optical ports.

8. The variable solid-state optical delay provides a set of isolated optical delays that can be selected by the actuator, wherein the maximum difference between two consecutive optical delays in the set of isolated optical delays is D. The terahertz cross-correlation apparatus according to claim 7, characterized in that at least one of the first and second double-pass polarization-preserving fiber stretchers is adapted to provide continuous optical delay adjustment of D or more.

9. The variable solid-state optical delay comprises two or more optical waveguide sections of different lengths for connecting two optical ports. The terahertz cross-correlation apparatus according to claim 7, characterized in that the operating means is configured to select one or a combination of the two or more optical waveguide sections for connecting the two or more optical ports.

10. A method for performing terahertz cross-correlation measurement of a sample using the terahertz cross-correlation apparatus described in claim 1.

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