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 technology]

[0002] Terahertz (THz) time-domain spectroscopy (TDS) is an established but still emerging method for materials characterization, particularly for spectroscopy and thickness measurements. Terahertz cross-correlation spectroscopy is an alternative approach to time-domain spectroscopy that enables phase-sensitive measurements without the need for ultrafast pulsed laser sources (see, e.g., Appl. Phys. Rev. 8, 021311 (2021); doi:10.1063 / 5.0037395).

[0003] Terahertz is a part of the electromagnetic spectrum that has properties that allow it to penetrate materials that other electromagnetic frequencies cannot. Therefore, terahertz is attracting increasing attention for commercial material inspection due to its wide range of uses. Material determination and characterization using terahertz technology is still waiting for a commercial breakthrough because available systems are expensive, large in size, and susceptible to environmental noise such as temperature and humidity fluctuations, vibration, and shock. Summary of the Invention

[0004] In its broadest aspect, the present invention relates to a terahertz cross-correlation device for material characterization of a sample by means of electromagnetic radiation in the spectral range [0.1; 10 THz], the device comprising: a light source for outputting a continuous wave (CW) optical signal; a terahertz transmitter optically coupled to the light source, the terahertz transmitter configured to emit terahertz radiation towards the sample when modulated by a CW optical signal; - a terahertz receiver optically coupled to the light source, the terahertz receiver configured to detect the terahertz radiation by generating an electrical detection signal that is a result of interference between the terahertz radiation and the CW optical signal; - an optical delay mechanism configured to adjust synchronization of the terahertz receiver to the terahertz transmitter by 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, the terahertz transmitter, and the terahertz receiver is preferably provided by an optical waveguide. The optical delay mechanism preferably comprises: a double-pass polarization-preserving fiber stretcher comprising a circulator, a fiber stretcher, and a Faraday mirror arranged to receive a CW optical signal propagating in a first direction that changes in direction due to reflection at the Faraday mirror and in a second direction in the opposite direction, and / or 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 varying the optical path length between the two connected optical ports.

[0005] In another aspect, the present invention relates to a Terahertz (THz) cross-correlation apparatus for material characterization of a sample by electromagnetic radiation of frequencies between 0.1 THz and 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, the terahertz transmitter configured to emit terahertz radiation towards the sample when modulated by a CW optical signal; - a terahertz receiver optically coupled to the light source, the terahertz receiver configured to detect the terahertz radiation by generating an electrical detection signal that is a result of interference between the terahertz radiation and the CW optical signal; a beam splitter for receiving a CW signal from a light source and defining a first arm providing an optical coupling between the beam splitter and a terahertz transmitter and a second arm providing an optical coupling between the beam splitter and a terahertz receiver; an optical delay mechanism configured to adjust the synchronization of the terahertz receiver to the terahertz transmitter by a CW optical signal; the light source is configured to provide a CW optical signal having a continuous broadband spectrum; an optical path for the CW signal from the light source to the optical delay element, the terahertz transmitter, and the terahertz receiver is provided by an optical waveguide; The optical delay mechanism includes 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 including a circulator, a fiber stretcher, and a Faraday mirror arranged to receive a CW optical signal propagating in a first direction, the first direction changing in direction due to reflection at the Faraday mirror, and a second direction in the opposite direction, the circulator and the fiber stretcher.

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

[0007] The following figures and examples are provided below to illustrate the present invention. They are intended to be illustrative and should not be construed as limiting in any sense. [Brief description of the drawings]

[0008] [Figure 1] 1 illustrates an example terahertz cross-correlation apparatus according to the present disclosure. [Diagram 2] 1 illustrates an example double-pass polarization-preserving fiber stretcher according to the present disclosure. [Diagram 3]1 illustrates an example variable solid-state optical delay according to the present disclosure. [Figure 4] 1 illustrates different example terahertz cross-correlation devices according to the present disclosure. [Diagram 5] 1 illustrates different example terahertz cross-correlation devices according to the present disclosure. [Figure 6] 1 illustrates different example terahertz cross-correlation devices according to the present disclosure. [Figure 7] 1 illustrates an example continuous broadband spectrum according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0010] The terahertz cross-correlation device includes a light source, such as, but not limited to, a pump and / or seed light source coupled to an optical amplifier. In an example embodiment, the frequency spectrum from the light source (with or without amplifier) ​​is continuous, so as to be at least substantially continuous, in the sense that there are no sharp peaks, such as mode peaks. This is discussed in more detail below when considering both spectroscopy and time domain applications. In the frequency domain, a CW light source (e.g., a multimode laser diode) with a multimode spectrum creates a terahertz spectrum that is also multimode, i.e., not continuous. This generally reduces its usefulness for spectroscopy applications, since it only contains information about certain frequencies (modes) and not about the rest of the spectrum. When using the cross-correlation device for material characterization, such as determining layer thickness, the time domain is of primary interest. In the time domain, a multimode laser creates a "train" of terahertz pulses. When interrogating material layers of a sample, each layer produces a reflection that is shown as a pulse that is shifted in time with respect to the others. In a multimode spectrum, the reflection of each pulse in the train of pulses overlaps with the next, making it difficult to distinguish the difference between the reflections and therefore difficult to obtain the thickness.

[0011] Therefore, it is preferable to use a continuous spectrum source. There are prior art references that use multimode laser diodes as the light source in near-Terahertz TDS systems because they 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 cannot achieve the accuracy required for industrial application systems. Thus, in the example embodiment, the light source is not a multimode laser source such as a multimode laser diode.

[0012] In an example 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. Thus, in another example embodiment, the spectrum from the light source is a broadband spectrum that, when received by the terahertz transmitter, results 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. To summarize, at 1,550 nm, an optical bandwidth of 8 nm corresponds to a terahertz bandwidth of about 1 THz. This conversion only holds for 1,550 nm, and at shorter wavelengths, a narrower optical bandwidth is required to obtain the same terahertz bandwidth. As used herein, a broadband spectrum of a light source is defined as a spectrum with a bandwidth of at least cΔλ / λ 2 >0.2THz, cΔλ / λ 2 cΔλ / λ, such as >0.5 THz 2 By broadband spectrum we mean a spectrum having a -3 dB bandwidth Δλ and a central wavelength λ that is >0.1 THz. The wider the broadband spectrum of the light source, the more spectral information can be recovered from the sample, thus improving the functionality of the device for the user.

[0013] The center frequency of the broadband spectrum can be selected depending on the type of light source or 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 telecommunications 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 needed 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 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 because short and ultrashort laser pulses must propagate in free space to avoid nonlinear effects. Such free-space optics include lenses, prisms, mirrors to control the direction and spread of the light, and mechanical translation stages to control the optical delay between the arms. 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 because the mirrors, lenses, and mechanical translation stages are bulky and their alignment is susceptible to environmental noise and any movement of the device.

[0015] A terahertz cross-correlation device using a CW optical signal according to the invention means that the free space requirements can be relaxed, which allows new approaches in the design of the device.

[0016] As used herein, an optical waveguide is a system or material designed to confine and direct electromagnetic waves in a direction determined by the physical boundaries of the waveguide. Exemplary types of waveguides include optical fibers, channel waveguides, and planar waveguides. In the inventive terahertz cross-correlation device, 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, thus ensuring an all-optical waveguide path for the device. Thereby, the device does not 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 that 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 vibration, and shock. In addition, the use of optical waveguides allows the overall size of the device to be reduced. In an alternative configuration, it is preferred that the CW optical signal does not propagate through free space (ie, through the ambient atmosphere) at any point during its path to the terahertz antenna.

[0017] The optical path between the light source and the optical delay mechanism includes 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. Such a beam splitter therefore also splits the optical signal from the light source into two separate 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 significantly more power than the other, with a split ratio of 60 / 40, 70 / 30, or 80 / 20, with the signal with the greater power typically being for the terahertz transmitter. The beam splitter is located between the light source and the optical delay mechanism.

[0018] The disclosed optical delay mechanism offers advantages over conventional free-space translation stage delays because it does not contain mechanically moving parts or separate mirrors and lenses that must be kept in precise alignment with other optical components. Thus, the optical delay mechanism is advantageous because it is more robust and less affected by mechanical vibration and shock. An additional advantage of the optical delay mechanism is that it is significantly less affected by environmental parameters such as pressure, temperature, humidity, gases, aerosols, etc., because the optical signal propagates in a solid rather than in free space. The optical delay mechanism preferably comprises: a double-pass polarization-preserving fiber stretcher comprising a circulator, a fiber stretcher, and a Faraday mirror arranged to receive a CW optical signal propagating in a first direction that changes in direction due to reflection at the Faraday mirror and in a second direction in the opposite direction, and / or 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 varying the optical path length between the two connected optical ports.

[0019] To achieve high accuracy terahertz cross-correlation measurements, the optical signals driving both terahertz antennas are preferably in phase. On the other hand, using a broadband spectrum light source typically results in a coherence length on the order of centimeters. Thus, in an example 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 example 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 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 example 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 particular application of the terahertz cross-correlation device, the scanning range of the optical delay mechanism may be on the order of tenths to thousandths of a picosecond, and the time step may typically be on the order of 10-100 femtoseconds. It is not a requirement to have a continuous time scan, but it is preferable 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 example embodiment, the optical delay mechanism comprises a double-pass polarization-preserving fiber stretcher 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 stretcher, which difference is on the order of the desired scanning range. This mechanism is advantageous because it provides identical or nearly identical optical paths in terms of both optical path length and any distorting 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 that includes an optical circulator, a fiber stretcher, and a Faraday mirror arranged to receive a CW optical signal propagating in a first direction that changes direction due to reflection at the Faraday mirror, and a second, opposite direction, where the optical circulator and fiber stretcher change direction. An optical circulator is an optical device with three or more ports designed such that light entering any port exits from the next port. This means that if light enters port 1, it will exit from port 2, but if any of the exited light is reflected back by the circulator, it will not exit from port 1, but will exit from port 3. Optical circulators are typically used to separate optical signals traveling in opposite directions, for example to achieve bidirectional transmission on a single fiber.

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

[0024] In a double-pass polarization-preserving fiber stretcher, the delay is induced by physically stretching the optical fiber of the fiber stretcher to lengthen the optical path. In an example embodiment, the fiber stretcher is a section of optical fiber, e.g., 50-100 meters, tightly wound around a piezo crystal or other electrostrictive material that can be distorted by applying a voltage. The longer the fiber is, the more it is stretched, and the greater the resulting optical delay. There are numerous fiber stretchers available on the market that can be used in this setup.

[0025] One end of the fiber stretcher is optically coupled to the light source via a waveguide and an optical circulator, and 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 into a different optical waveguide that propagates towards a terahertz transmitter or receiver. The Faraday mirror rotates the polarization of the optical signal by 90 degrees, so that any change in polarization caused by the stretching of the optical fiber in the fiber stretcher during the first pass is reciprocated during the second pass. The 90 degree rotation is an advantage since terahertz transmitters and receivers are sensitive to the polarization of the optical signal. A double-pass polarization-preserving fiber stretcher is also an advantage since 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 reciprocating any birefringence present in the fiber, an effect that leads to polarization rotation. Furthermore, a double-pass polarization-preserving fiber stretcher has the effect that the additional optical path length due to stretching the fiber, and therefore the optical delay, is also doubled, which means that the components can be made smaller. Additionally, fiber stretchers are advantageous because they provide 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 that includes two or more optical ports, such as ports for coupling input and output signals to optical waveguides, one or more optical waveguide sections for connecting the two optical ports, and actuation means for varying the optical path length between the two connected optical ports. Such solid-state optical delays are also referred to as non-mechanical variable optical time delay lines or solid-state delay lines (SSDLs).

[0027] In an exemplary embodiment, the variable solid-state optical delay comprises two or more optical waveguide sections of different lengths for connecting two or more 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 is changed by the selection of 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 lengths formed on a substrate. The actuation means may select one of the optical waveguide sections or may connect two or more of the optical waveguide sections in series to provide a set of selectable separate optical delays. In addition, some variable solid-state optical delays may provide some continuous adjustment of the optical path length around or between one or more separate optical delays. The actuation means may be, for example, an opto-mechanical fiber switch or a MEMS (microelectromechanical system) switch, which is advantageous since they do not include any macroscopic moving parts. In an example embodiment, the variable solid-state optical delay is a non-mechanical optical delay and the actuation means can comprise one or more of a thermo-optical switch, an electro-optical switch, an acousto-optical switch, and a magneto-optical switch. In addition to increased robustness and the absence of free space propagation, the variable solid-state optical delay is advantageous because any moving parts, mirrors, lenses are eliminated and can be miniaturized to fit on a single chip. In an example embodiment, the variable solid-state optical delay is at least 10 9 The optical switch is durable and highly reliable, capable of multiple switching cycles.

[0028] In another example 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 adjusting the refractive index or birefringence of the waveguide material. One example of such a variable solid-state optical delay may be an acousto-optical delay module, where the actuation means sends an acoustic signal through a birefringent crystal, which leads to a change in the grating position, which results in a change in the diffraction of the optical signal and ultimately a different optical path length.

[0029] In an example embodiment of the variable solid-state optical delay, the variable solid-state optical delay includes a temperature stabilization process to stabilize the temperature of the variable solid-state optical delay. This is advantageous because changes in temperature change 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 includes a low-loss waveguide medium such that propagation through different paths in the variable solid-state optical delay does not substantially change the optical power at the output.

[0030] In an example embodiment, the optical delay mechanism of the terahertz cross-correlation device comprises at least a first optical delay component in a first arm and a second optical delay component in a second arm, i.e., each arm comprises at least one optical delay component. Preferably, each of the first and second delay components comprises: Double-pass polarization-preserving fiber stretcher or Variable solid-state optical delay.

[0031] In one example, the optical delay mechanism of the terahertz cross-correlation device comprises both a double-pass polarization-preserving fiber stretcher and a variable solid-state optical delay, which can be provided in parallel (in different arms) or in series (in the same arm). The variable solid-state optical delay preferably provides a set of discrete optical delays selectable by an actuation means, with a maximum difference between two subsequent optical delays being D. In one example, the double-pass polarization-preserving fiber stretcher is adapted to provide continuous optical delay adjustment of D or more. This combination is advantageous as it allows a wide range of optical delays to be scanned continuously.

[0032] The invention also relates to a process for performing a terahertz cross-correlation measurement of a sample using the above-described terahertz cross-correlation device. Such a terahertz cross-correlation measurement may be, for example, terahertz cross-correlation spectroscopy as 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. The measurement may be a material characterization measurement such as a thickness measurement, which is often referred to as spectroscopy but does not necessarily result in a reflection, absorption, or transmission spectrum of the sample being provided.

[0033] Various examples and details are described hereinafter, with reference to the figures, where applicable. It should be noted that the figures may or may not be drawn to scale, and elements of similar structure or function are represented by similar reference numerals throughout the figures. It should also be noted that the figures are intended only to facilitate the description of the examples. The figures are not intended as an exhaustive description of the disclosure, nor as limitations on the scope of the disclosure. In addition, the examples shown need not have all aspects or advantages shown. An aspect or advantage described in connection with a particular example is not necessarily limited to that example, and may be practiced in any other example, even if not so shown or explicitly described.

[0034] 1 is a diagram illustrating an example terahertz cross-correlation apparatus 1 according to the disclosure. The apparatus includes a light source 2 for outputting a continuous wave (CW) optical signal, a terahertz transmitter 4 optically coupled to the light source 2 and 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 and configured to detect the terahertz radiation 5 by generating an electrical detection signal that is a result of interference between the terahertz radiation 5 and the CW optical signal, and an optical delay mechanism 16 configured to adjust synchronization of the terahertz receiver 6 to the terahertz transmitter 4 with the CW optical signal. The apparatus 1 may include a beam splitter 15 to facilitate 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 having different signals measured from samples. In Fig. 1 the terahertz transmitter 4 and terahertz receiver 6 are set up to measure transmitted radiation 5 from a sample 8, whereas in Figs. 4 and 5 they are set up to measure reflected radiation 5b from the sample 8.

[0036] The optical path of the CW signal from the optical source to the optical delay components, the terahertz transmitter and the terahertz receiver is preferably provided by optical waveguides 11, such as optical fibers, channel waveguides and planar waveguides. In the example embodiment, the optical path is an all-optical waveguide route, meaning that no part of the optical path of the CW optical signal is located outside of the optical waveguide 11. This has the effect that the CW optical signal does not propagate in free space at any point, i.e. does not propagate in the atmosphere of the environment in which the apparatus 1 is located.

[0037] Optical delay mechanism 16 can have one or more optical delay components in both the arm to the terahertz transmitter and the arm to the terahertz receiver, as in FIG. 1, or can have one or more optical delay components in only one of the arms (see, e.g., FIGS. 4 and 5). Example embodiments of one or more optical delay components of optical delay mechanism 16 are described below with reference to FIGS. 2 and 3.

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

[0039] 3 shows an exemplary embodiment of a variable solid-state optical delay 26 with two optical ports 28 and optical waveguide sections 30 of different lengths, here optical fiber sections A-D for connecting the optical ports 28, and an actuation 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 an optical waveguide 11 for receiving and transmitting a CW optical signal 3.

[0040] Figures 4 to 6 show further example embodiments of the disclosed terahertz cross-correlation device 1. Individual features described with respect to any of these embodiments, or the embodiment illustrated in Figure 1, may 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 set up to illuminate a sample 8 with terahertz radiation 5a and to measure terahertz radiation 5b reflected from the sample.

[0042] In the device 1 illustrated in Figures 4 and 5, the optical waveguide 11 is provided by an optical fiber. The type of optical fiber used can be selected from commercially available single mode or multimode optical fiber types. 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, the desired price / quality of the device, etc.

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

[0044] In the apparatus 1 illustrated in Fig. 1, the optical delay mechanism 16 comprises at least one optical delay element in each arm of the apparatus. In the apparatus 1 illustrated in Fig. 4, the optical delay mechanism 16 comprises at least two optical delay elements in one arm of the apparatus (here in the arm to the terahertz receiver, but could alternatively be in the arm to the terahertz transmitter). The resulting at least two optical delay elements are: - Two double-pass polarization-preserving fiber stretchers18, - 2 variable solid-state optical delays 26, -Double-pass polarization-preserving fiber stretcher 18 and variable solid-state optical delay 26; - a double-pass polarization-preserving fiber stretcher 18 and different optical delay components; - Variable solid-state optical delay 26 and may be different optical delay components.

[0045] 5, the optical delay mechanism 16 includes only a single optical delay element in only one arm of the device (here, in the arm to the terahertz receiver, but could alternatively be in the arm to the terahertz transmitter). The optical delay element could be either a double-pass polarization-preserving fiber stretcher 18 or a variable solid-state optical delay 26.

[0046] Although all combinations above are applicable, it is preferable to keep the arms as symmetrical as possible, so that any change in environmental parameters, especially temperature, affects 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 in order to have approximately the same total path length on both arms, or to have a non-stretching fiber e-coil with circulator and Faraday mirror to match both the total path length and the reciprocating motion of the fiber birefringence. Including only a fiber coil twice as long as the fiber stretcher 22 will result in asymmetric birefringence effects.

[0047] In a preferred example embodiment, the optical delay mechanism 16 comprises 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 of Figures 1 and 4, where 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 typically terahertz antennas. One example type is a "CW photomixer" with a semiconductor structure (which itself can be composed of many different layers of semiconductors) 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 (for the emitter) or the current generated in the antenna is measured (for the receiver). Other applicable terahertz antennas exist and new antennas can be developed that are equally applicable in the invention.

[0049] The terahertz cross-correlation device 1 may comprise terahertz optics 7 for the terahertz transmitter 4 and the terahertz receiver 6, see for example Figs. 4 and 5. Such terahertz optics 7 may comprise lenses, mirrors, polarizers, beam splitters, etc. adapted for 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 that is the result of interference between the terahertz radiation 5 and the CW optical signal 3. For this purpose, the device 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 Fig. 1.

[0050] The light source and optical components can operate at any wavelength, typically visible or infrared wavelengths, to which the terahertz transmitters and receivers must be tuned. 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 example embodiment, the light source 2 comprises an optical seed signal coupled into 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, an optical amplifier, and an erbium doped fiber (EDFA). This type of light source is typically referred to as an EDFA (erbium doped fiber) and is illustrated in FIG. 5. In other example embodiments (not shown), the optical seed signal can be electrically amplified, such as in an arrangement where an LED seed is amplified by a semiconductor optical amplifier.

[0052] In another example embodiment, the light source 2 comprises a pump signal coupled into a medium that absorbs the pump signal and re-emits it at a longer wavelength and a large continuous bandwidth via an amplified spontaneous emission (ASE) process. Preferably, the pump signal is provided by a pump laser and the ASE medium is a doped fiber. In an example 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 in a wider bandwidth. This type of light source is typically referred to as an ASE and is illustrated in FIG. 5.

[0053] 7 shows an example continuous broadband spectrum of an inventive light source. 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 mentioned above, the terahertz cross-correlation device can be used in different applications, and the disclosure provides a method for performing a terahertz cross-correlation measurement of a sample using the terahertz cross-correlation device. In an example embodiment, the terahertz cross-correlation measurement is a layer thickness measurement, and the thickness of individual layers of a multi-layer structure can be measured non-contact and non-destructively. Here, the sample is placed in a reflecting setup as illustrated in Figures 4 and 5. When the device is in operation, terahertz radiation 5a from the terahertz transmitter 4 impinges on 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 that is the cross-correlation of 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) to determine the layer thickness and possibly the optical parameters of the sample. In other example embodiments, the terahertz cross-correlation measurements are non-destructive testing or quality assurance, such as non-destructive testing or quality assurance of layer thicknesses and surfaces.

[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 the 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 polarization-preserving fiber stretcher 20.Optical Circulator 22. Fiber stretcher 24. Faraday Mirror 26. Variable Solid State Optical Delay 28. Optical input / output port 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", "tertiary", etc., 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", "tertiary", etc., does not imply any order of importance, and terms such as "first", "second", "third", "fourth", "primary", "secondary", "tertiary", etc., are used to distinguish one element from another element. It should be noted that while terms such as "first", "second", "third", "fourth", "primary", "secondary", "tertiary", etc. are used herein and elsewhere, they are for labeling purposes only and are not intended to indicate any particular spatial or temporal order. Furthermore, the labeling of a first element does not imply the presence of a second element, and vice versa.

[0057] Certain features discussed above as separate implementations can also be implemented in combination in a single implementation. Conversely, features described as a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Furthermore, although features may be described above as acting in a combination, one or more features from a claimed combination may in some cases be practiced in combination, and the combination may be claimed as any subcombination or as a variation of any subcombination.

[0058] It should be noted that the term 'comprising' does not necessarily exclude the presence of other elements or steps than those listed.

[0059] It should be noted that the word "a" preceding an element does not exclude the presence of a plurality of such elements.

[0060] It should further be noted that any reference signs do not limit the scope of the claims, and that the examples can be implemented at least in part by means of both hardware and software, and that several "means", "units" or "apparatus" may be represented by the same item of hardware.

[0061] As used herein, terms of degree, such as "approximately," "about," "generally," and "substantially," describe a value, amount, or characteristic that is close to a stated value, amount, or characteristic and still performs a desired function or achieves a desired result. For example, the terms "approximately," "about," "generally," and "substantially" can refer to an amount or quality or characteristic, such as optical spectral continuity, within 10%, within 5%, within 1%, within 0.1%, and within 0.01% of the stated amount, quality, or characteristic.

Claims

1. A terahertz (THz) cross-correlation apparatus (1) for measuring a sample (8) by electromagnetic radiation having a frequency between 0.1 THz and 10 THz, comprising: - 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) towards 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 that is a result of interference between the terahertz radiation and the CW optical signal; - A beam splitter (15) that receives the CW signal from the light source and defines a first arm that provides an optical coupling between the beam splitter and the terahertz transmitter, and a second arm that provides an 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 by the CW optical signal; - The light source is configured to provide a CW optical signal having a continuous broadband spectrum; - The optical path (10) of 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 (11); - The optical delay mechanism includes a first double-pass polarization-maintaining fiber stretcher (18) in the first arm and a second double-pass polarization-maintaining fiber stretcher (18) in the second arm. Each double-pass polarization-maintaining fiber stretcher includes a circulator (20), a fiber stretcher (22), and the Faraday mirror (24) arranged to receive the CW optical signal propagating in a first direction that changes in direction due to reflection at the Faraday mirror and a second direction opposite thereto. A terahertz cross-correlation device characterized by that.

2. The terahertz cross-correlation device according to claim 1, wherein the first and second double-pass polarization-maintaining fiber stretchers are identical.

3. The terahertz cross-correlation device according to claim 1 or 2, wherein the light source includes a laser diode.

4. The terahertz cross-correlation device according to claim 1, wherein the light source includes a superluminescent diode.

5. The terahertz cross-correlation device according to claim 1, wherein the light source includes a light source based on amplified spontaneous emission.

6. The terahertz cross-correlation device according to claim 1, wherein the light source includes a fiber amplifier.

7. The terahertz cross-correlation device according to claim 1, wherein the optical delay mechanism further includes a variable solid-state optical delay (26) including two or more optical ports (28), one or more optical waveguide sections (30) for connecting the two optical ports, and actuating 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 discrete optical delays selectable by the actuating means, and the maximum difference between two subsequent optical delays is D. The terahertz cross-correlation device according to claim 7, characterized in that at least one of the first and second double-pass polarization-maintaining fiber stretchers is adapted to provide a continuous optical delay adjustment of D or more.

9. The variable solid-state optical delay includes two or more optical waveguide sections of different lengths for connecting two optical ports. The terahertz cross-correlation device according to claim 7, characterized in that the actuating 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 device according to claim 1.

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