Method and apparatus for dual comb interferometry for optical device characterization and sensing

Abstract

An apparatus determines one or more characteristics of an optical device under test (DUT) includes a laser source, a beam splitter, a first ring resonator, and a second ring resonator generating first and second optical frequency combs. The second optical frequency comb is a reference signal. A beam combiner combines a DUT output light signal and the reference signal to generate a combined light signal. An optical detector detects an interference signal with spectral components in the radio frequency (RF) domain. Acquisition circuitry detects the interference signal and generates a digital interference signal. Processing circuitry, coupled to the acquisition circuitry, processes the digital interference signal to generate a corresponding complex signal and analyzes the complex signal to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters including wavelength, amplitude, and phase.

Description

METHOD AND APPARATUS FOR DUAL COMB INTERFEROMETRY FOR OPTICAL DEVICE CHARACTERIZATION AND SENSING CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority from U.S. provisional application 63 / 614,251, filed on December 22, 2023, the contents of which are incorporated here by reference. INTRODUCTION

[0002] The technology in this application relates to testing and sensing of fiber optic and photonic devices and networks.

[0003] Fiber optic and photonic devices are used to route optical signals for telecommunications and sensing networks. The design and characterization of these devices is critical to performance, and there is an increasing demand for optical characterization systems that can be deployed economically in applications such as fiber optic and photonic test and physical parameters sensing. In particular, fiber optic and photonic devices must be characterized across the full optical wavelength or frequency range of intended use. Characterization systems may be divided into two categories: 1) stepped or swept laser and detector systems and 2) swept wavelength interferometric systems.

[0004] Category 1 characterization systems are relatively low-cost and measure amplitude, wavelength, and polarization information. Therefore, these systems may measure for example insertion loss, return loss, polarization dependent loss as a function of wavelength, but cannot measure optical phase related parameters such as group delay, chromatic dispersion, and polarization mode dispersion. However, these systems take an extensive amount of time to characterize a device under test (DUT) because the laser must be step tuned to each individual spectral point of interest and the full suite of tests must be performed. Because the laser is step tuned, there is no simple method to reconstruct the optical phase as a function of wavelength or optical frequency. This drawback is exacerbated for multichannel devices such as wavelength selectable switches.

[0005] Category 2 characterization systems use sophisticated swept laser and detection schemes to provide a more complete characterization of amplitude, wavelength, polarization, andphase. However, these sophisticated swept laser and detection schemes require substantial cost and complexity and have known performance limitations in dynamic environments such that motion and vibration of the Device Under Test (DUT) imposes artifacts in the measured interferograms and device response. For example, it is common for such systems to cover approximately 70nm wavelength range with a sweeping laser at 50nm / s, which requires a static DUT and environment for over a second. These requirements are often unrealistic outside of laboratory environments, e.g., in a manufacturing environment. Approaches that tune or sweep the laser faster come at the price of degraded laser tuning and detection characteristics such as increased ripple and noise on the measured response, as well as proportionally decreased interferometric signal bandwidth.

[0006] Known dual comb spectroscopy approaches typically require multiple lasers and phase locking mechanisms. Even a system that can generate the combs from a single laser source does not address fiber optic and photonic test and measurement requirements including reference interferogram generation for amplitude and phase corrections, polarization signal synthesis, and / or wavelength registration and monitoring.

[0007] Thus, it would be desirable to have a method and apparatus for performing testing and sensing of fiber optic and photonic devices and networks at high speed.

[0008] It would also be desirable to perform such testing using a relatively low-cost apparatus.

[0009] It would also be desirable to perform measurements with high spectral resolution over a broad range of potential input wavelengths.

[0010] An example objective is to provide a dual optical comb analyzer for rapid, broadband optical characterization of fiber optic and photonic devices and optical sensors including polarization, wavelength, amplitude, and / or phase.

[0011] A further example objective is to provide a dual optical comb analyzer that is solid-state and low-cost.

[0012] A further example objective is to provide an optical characterization apparatus that does not require a broadband swept laser, stepped laser, and / or swept wavelength interference.

[0013] Accordingly, it will be appreciated that new and improved techniques, systems, and processes are continually sought after.SUMMARY

[0014] Example embodiments include a dual comb analyzer that fully characterizes fiber optic and photonic devices and networks as a function of wavelength, polarization, amplitude, and / or phase. The example embodiments may be used for fiber optic and photonic test and measurements and physical sensing.

[0015] In certain example embodiments, an apparatus determines one or more characteristics of an optical device under test (DUT). A laser source generates laser light at a laser frequency, and a beam splitter splits the laser light into first light that propagates over a first optical path and second light that propagates over a second optical path. A first ring resonator receives the first light in the first optical path and generates a first optical frequency comb, and the DUT receives the first optical frequency comb and provides a DUT output light signal. A second ring resonator receives the second light in the second optical path and generates a second optical frequency comb different from the first optical frequency comb. The second optical frequency comb is a reference signal. A beam combiner combines the DUT output light signal and the reference signal to generate a light interference signal. An optical detector detects the interference signal that includes spectral components in the radio frequency (RF) domain. Acquisition circuitry detects the interference signal and generates a digital interference signal. Processing circuitry, coupled to the acquisition circuitry, processes the digital interference signal to generate a corresponding complex signal and analyzes the complex signal to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters including wavelength, amplitude, and phase.

[0016] In certain example embodiments, a method is provided for optical characterization of an optical device under test (DUT) using dual comb interferometry. First and second optical frequency combs are generated using a laser source and ring resonators with different geometric properties. Relative coherence between the first and second optical frequency combs is maintained by thermal and / or electrical tuning which affects ring resonator geometry and index of refraction. The first optical frequency comb is directed through the DUT to generate a DUT optical frequency comb while the second optical frequency comb is provided as a reference optical frequency comb. The DUT optical frequency comb and the reference optical frequency comb are combined to generate an interferogram. The interferogram isanalyzed to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters including wavelength, amplitude, and phase.

[0017] This Summary is provided to introduce a selection of concepts that are further described below in the Detailed Description. This Summary is intended neither to identify key features or essential features of the claimed subject matter, nor to be used to limit the scope of the claimed subject matter; rather, this Summary is intended to provide an overview of the subject matter described in this document. Accordingly, it will be appreciated that the above- described features are merely examples, and that other features, aspects, and advantages of the subject matter described herein will become apparent from the following Detailed Description, Figures, and Claims. BRIEF DESCRIPTION OF THE FIGURES

[0018] These and other features and advantages will be better and more completely understood by referring to the following detailed description of example non-limiting illustrative embodiments in conjunction with the drawings. Unless specific dimensions are given in a figure, the figures are not necessarily to scale and are schematic representations.

[0019] Figure 1 shows an example embodiment of a method for optical characterization of an optical device under test (DUT) using dual comb interferometry.

[0020] Figure 2 shows an example embodiment of a dual comb interferometric apparatus for determining DUT characterization parameters.

[0021] Figure 3 shows an example embodiment of a method for DUT interferogram signal processing that may be used by the signal processing unit in Figure 2.

[0022] Figure 4 shows another example embodiment of a dual comb interferometric apparatus for determining characteristics of a DUT that includes polarization control, an absolute wavelength reference signal, a phase correction signal, a controller for adjusting ring resonator properties, and a modulator for adjusting and generating auxiliary comb lines.

[0023] Figure 5 shows an example embodiment of a method for DUT interferogram signal processing that may be used by the signal processing unit in Figure 4.

[0024] Figure 6 shows an example embodiment of a method for interferogram signal processing that includes wavelength correction.

[0025] Figure 7 shows an example embodiment of a method for interferogram signal processing that includes phase error correction.

[0026] Figure 8 shows another example embodiment of a dual comb interferometric apparatus for simultaneous determination of transmission and reflection characteristics of a DUT.

[0027] Figure 9 shows another example embodiment of a dual comb interferometric apparatus for determining characteristics of a DUT using an optical switch to select between transmission and reflection measurements.

[0028] Figure 10 shows another example embodiment of a dual comb interferometric apparatus for determining characteristics of a DUT also using a secondary interferometer and a tunable laser to provide both optical frequency domain reflectometry (OFDR) and dual comb spectral characterization. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0029] Specific embodiments are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the claims to the particular embodiments disclosed, even where only a single embodiment is described with respect to a particular feature. On the contrary, the intention is to cover all modifications, equivalents and alternatives that would be apparent to a person skilled in the art having the benefit of this disclosure. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise.

[0030] Terms, such as first, second, and the like, may be used herein to describe various components. Each of these terminologies is not used to define an essence, order or sequence of a corresponding component but used merely to distinguish the corresponding component from other component(s). For example, a "first" component may be referred to as a "second" component, and similarly, the "second" component may be referred to as the "first" component. “Based on” as used herein covers based at least on. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). The singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises / comprising" and / or "includes / including" when used herein, specify the presence of stated features, steps, operations, elements, and / or components, but donot preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0031] The same name may be used to describe an element included in the embodiments described above and an element having a common function. Once a component or function is described for one embodiment, that description is not repeated for other embodiments where that component or function operates or performs similarly. Unless disclosed to the contrary, a configuration of components disclosed in any embodiment may be applied to other embodiments, and the specific description of the repeated configuration will be omitted.

[0032] Unless otherwise defined, all terms used herein including technical or scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which an example belongs. Terms, such as those defined in commonly used dictionaries, are to be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0033] The methods and apparatus described below may include electronic and optical circuits and networks, software functions and programs, and data recorded via any number of computer readable media. For example, the signal processing unit 28 referred to below may include one or more of the following: one or more data processors including processing circuitry such as a single- or multi-core processor, a microprocessor (e.g., which may be referred to as a central processing unit or CPU), a digital signal processor (DSP), a microprocessor in association with a DSP core, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) circuit, or a system-on-a-chip (SOC) (e.g., an integrated circuit that includes a CPU and other hardware components such as memory, networking interfaces, and the like); one or more memory devices; one or more network interface devices; one or more display interfaces; and one or more user input adapters. Additionally, in some embodiments, the computing device may be connected to or includes a display device.

[0034] Various embodiments of optical apparatus described in this application generate broadband dual frequency combs of light (possibly of one or more polarizations) and inject one or more of these combs into a fiber optic, photonic device, network, sensor, or other structure under test, collectively included in the term device under test (DUT). The DUT imparts changes to one of the input frequency combs, and those changes are detected as a function of wavelength,amplitude, phase, and / or polarization by combining the light which traverses the DUT with light which traverses a reference path. The light from the two paths maintains relative coherence and impinges on a detector resulting in interference which is recorded on one or more photodetectors, digitized, and processed in a signal processing unit. These imparted optical changes are detected and analyzed by the signal processing unit to fully characterize the DUT. In the example of optical sensing DUTs, the analyzed optical changes may be used to determine or deduce parameters including strain, temperature, pressure, acoustic signals and frequencies, time of flight, rotational motion, distance, shape, material composition, and more.

[0035] Figure 1 shows an example embodiment of a method for optical characterization of an optical device under test (DUT) using dual comb interferometry. A laser source generates a single frequency of light (step S1). The laser light is passed through two ring resonators with different geometric properties to generate first and second optical frequency combs (step S2). Relative coherence between the first and second optical frequency combs is maintained by thermal and / or electrical tuning which affects ring resonator geometry and index of refraction. (step S3). The first optical frequency comb is directed through a polarization controller and DUT to generate a DUT optical frequency comb (step S4). The second optical frequency comb is provided as a reference optical frequency comb (step S5). The DUT optical frequency comb and the reference optical frequency comb are combined to generate an interferogram as the heterodyne detection of the two comb signals (step S7). The interferogram is analyzed to determine amplitude, phase, polarization, and / or wavelength characteristics of the DUT as a function of wavelength or frequency (step S8). In an example embodiment, interferogram may be analyzed to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters including wavelength, amplitude, and phase.

[0036] Figure 2 shows an example embodiment of a dual comb interferometric apparatus 20 for determining characteristics of a DUT. As is common in comb generation a laser 21 generates light having a wavelength or frequency, referred to as a “seed” laser wavelength. Example suitable lasers include but are not limited to continuous wave (cw) lasers and pulsed lasers.

[0037] The laser light is split using an optical splitter into a first optical light path and a second optical light path which are injected into a photonic integrated circuit (PIC) 22. The PIC 22 is designed to guide the same seed laser wavelength to a first ring resonator 23 and a secondring resonator 24 included in the PIC. Alternatively, a PIC need not be used and the first ring resonator 23 and the second ring resonator 24 may be stand-alone components. However, integration of the two combs onto a single PIC provides advantages in cost, size, and achieving and maintaining comb coherence since the same laser may be used to drive both resonators, and closer proximity reduces variation in environmental fluctuations such as temperature and vibration as compared with standalone components.

[0038] The first ring resonator 23 and the second ring resonator 24 may for example be micro-ring resonators but other types of ring resonators may be used. Micro-ring resonators may be used because they are compact which leads to advantages in smaller size, high Q-Factor (a measure of spectral performance and coherence), and greater efficiency in non-linear effects required for comb generation. Thus, micro-ring resonators reduce the requirements of the seed laser and / or increase the power output by the combs which is important as the optical output bandwidth of each comb and desired spectral resolution both increase (resulting in more comb lines, and inherently less power per line per conservation of energy).

[0039] The first ring resonator 23 imparts optical non-linear effects via parametric four- wave mixing to the laser light received over the first optical light path and generates a first optical frequency comb which is directed through a polarization controller 31 used to probe the DUT 25 for specific states of polarization. Optical device characterization as a function of input and output polarization is advantageous for environmental robustness (e.g., optical devices typically must perform well in spite of an uncontrolled input polarization state) as well as the encoding of telecommunication signals onto individual phase and polarization states as is done for example in Dual Polarization Quadrature Phase Shift Keying (DP-QPSK). The DUT 25 may impart changes to the first optical frequency comb in the form of changes in amplitude, phase, and / or polarization. The second ring resonator 24 has slightly different geometry from the first ring resonator 23 that alters the non-linear effects, resulting in a second optical frequency comb with slightly different, but fixed, frequency spacing. The second optical frequency comb is referred to as a reference optical frequency comb.

[0040] The first and second optical frequency combs are combined by an optical combiner and detected in an optical detector such as a photodiode (PD) 26. The coherent interference of the first and second combs impinging on PD 26 results in a radio frequency (RF) comb comprised of heterodyne beats between individual comb frequencies. Any modification toamplitude or phase due to the DUT 25 is imparted and extractable through this heterodyne detection signal. The heterodyne detection of the two comb signals is referred to as an interferogram. The interferogram is digitized using analog to digital conversion circuitry 27. The digitized interferogram is then processed by signal processing unit 28 to analyze the interferogram to determine amplitude, phase, polarization, and wavelength characteristics of the DUT as a function of wavelength or frequency. As mentioned above, in one example, the interferogram is analyzed to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters including wavelength, amplitude, and phase.

[0041] Figure 3 shows an example embodiment of a method for DUT signal processing performed by the signal processing unit 28 on the DUT interferogram. Initially, the signal processing unit 28 performs a fast Fourier transform (FFT) of the time-domain DUT interferogram to the radio frequency (RF) domain (step S10). The signal processing unit 28 applies a window function to the RF domain interferogram to extract each individual RF comb line amplitude and phase (step S11). The signal processing unit 28 corrects a DUT transfer function by applying amplitude and phase corrections from a calibration reference scan (step S12). The signal processing unit 28 may calculate parameters of interest such as amplitude, phase, Insertion Loss (IL), Group Delay (GD), Chromatic Dispersion (CD), Polarization Dependent Loss (PDL), Polarization Mode Dispersion (PMD), etc. (step S13).

[0042] EXAMPLE SIGNAL PROCESSING

[0043] Example signal processing which allows the measurement of the DUT complex response (transfer function) as a function of wavelength, and further how the above-described interferograms may be used to extract parameters of interest of the DUT including amplitude, phase, IL, GD, CD, PDL, PMD, etc. is now described.

[0044] The electric field describing an optical frequency comb (comb 1) may be written in the time domain as follows: ^ such that:

[0045] ^^^represents the spectral envelope, ^^represents the optical frequency of the nth line of comb 1 and is equal to the carrier frequency, ^^^, plus n times the combs repetitionfrequency, ^^^^^, and ^^^represents the phase of the nth line. Equation 1 can be seen as a Fourier decomposition of the optical modes comprising the comb, where in other words ^^^represent the amplitude coefficients for each of the optical modes.

[0046] The expression for a second comb is simply described by substituting subscripts 1->2, n->k, N->K starting from equation 1. ^ ^^^^^ = ^ ^^^ exp ^^2^^^^ + ^^^^^2^ ^^^ ^^^represents the spectral envelope, ^^represents the optical frequency of the kth line of comb 2 and is equal to the carrier frequency, ^^^, plus k times the combs repetition frequency, ^^^^^, and ^^^represents the phase of the kth line.

[0047] Assume that comb 1 traverses a DUT which imparts an effect on the amplitude and phase of comb 1. Equation (1) is modified to include the complex response of the DUT, ^^^ = | ^^^| exp ^^ ^"(^^^, noting that this represents multiplication in the frequency domainand is equivalent to convolution of the field ^^^^^with the time-domain response of the DUT, h(t): # ^^"^^^ = ^ ^^^ ^^2^^^^ + ^^^ + ^"^^^^ ^When comb 1 andphotodiode, their electric fields coherently interfere and the signal is described as the intensity of the sum of the electric fields: &^^^ ∝ |^ ^^"^^^ + ^^^^^| ^4^Equation (4) may be readily&^^^ ∝ |^^"^^^|^ + |^^^^^|^ + 2)*+^^"^^^ ∙ ^∗^^^^. ^5^The first two termsanalysis. The last term is of interest as it encodes the DUT response. The last term may be expanded and simplified by substituting equations 2 and 3: &^^^ ∝ 2)*+^^"^^^ ∙ ^∗^^^^. ^6^Such that 12^and ∆^2^represent the interferogram amplitude and phase respectively at RF line n: 12^ = ^^^^^^| ^^^^| ^8^∆^2^ = 2^^^^ − ^^^^ + ^^^ − ^^^ + ^"^^^^, ^9^Inspecting the phase in equation 9 reveals the frequency components in the RF domain: ^9:^ = ^^ − ^^ = ^^^ + ;^^^^^ − ^^^^ + <^^^^^^ ^10^A simplification is todue to the finite one only one (nearest) line from comb 2 and therefore equation 10 simplifies to ^9:^ = ^^^ − ^^^ + ;^^^^^ − ;^^^^^ = ^^^ − ^^^ + ;∆^^^^ ^11^Therefore, it is clear that the DUT phase response as a function of optical frequency, ^"^^^^, may be estimated by extracting the phase of each of the comb lines in the RF domain: ∆^2^ = 2^+;∆^^^^.^ + ^^^ − ^^^ + ^"^^^^, ^12^In practice, this is inequation (7), resulting in a complex number, and windowing around each of the discrete RF comb frequencies.

[0048] The amplitudes in equation 8 represent the comb signal envelope and errors of the measurement network. These effects must be removed in order to correct the DUT transfer function amplitude. This may be accomplished by measuring the transfer function of a suitable calibration device – one with low loss, and dispersion, such as smf-28 fiber, and dividing by the amplitude.

[0049] Measuring the transfer function of a standard telecom fiber, from equation 8 and 9 we may write: 12^^>^ = ^^^^^^? ^^>^^^^? ^13^Next, we normalize1^ ^ ^=2^ ^^ ^^|"@A^^^|≈^^^ ^The corrected"@A^^^^ = 1"@ABCDDEFGEH exp ^∆^"@ABCDDEFGEH ^17^A similar correction procedure may be performed using the output of the PR channel, described below in conjunction with Figure 4, and following the steps above in equations 13-17. This correction removes the effects of instantaneous laser and comb amplitude and phase perturbations and is performed on every DUT scan, whereas the telecom reference scan is performed less frequently and removes the effects of the measurement network path that are common between the reference scan and DUT scan – for example, optical coupling to the DUT, and electronic amplitude and phase response of the photodiode, filtering, and ADC.

[0050] Once the interferogram is measured and corrected for amplitude and phase response, several parameters of interest may be readily calculated from the device response. Forsimplicity we will convert variables to the following: ^^^ = | ^^^| exp ^^ ^"(^^^. Forcalculation clarity, we will also pivot to a discrete representation of the device’s frequency domain transfer function H(i), such that i is an index of the measurement results over the optical frequency domain. That is, the wavelength vector is calculated as follows: ^L^M ^ ∙ O^ + ^KL^M = = PQR^Q ^18^Here, c is the speed of light incalculated from the wavelength reference (Gas channel), as described above.

[0051] Amplitude may be calculated as the amplitude of the complex transfer function: STUL^M = | L^M| ^19^

[0052] Insertion Loss may be calculated from the amplitude of the complex transfer function by converting it to a log scale: VWL^M = 20log ^STUL^M^ ^20^

[0053] Polarization Dependent Loss (PDL) may be calculated using the all states method to probe over all input polarization states to determine the minimum and maximum Insertion Loss (IL) states (International Electrotechnical Commission (IEC) standard 61300-3-2). Explicitly, a polarization controller, e.g., PC 31, is used to rotate the input polarization states presented to the DUT, and at each polarization state a new measurement for Insertion Loss is calculated. The maximum and minimum loss at each wavelength (optical frequency) is recorded and the difference represents the PDL as a function of wavelength. &[WL^M = VW$R\L^M − VW$]^L^M ^21^

[0054] Group delay, ^_, is by definition the derivative of the phase as a function of angular frequency: ^ `^^a _=^ `a ^22^From the discrete transfer function, web[L^M =2^ ∙ 0.001 ∙ O^ ^23^Such that O^ is the frequency increment per index i in GHz, and the factor 0.001 converts GD to units of picoseconds, as is convention. The value of O^ may be calculated from the wavelength reference (gas channel, e.g., gas 32) or may be output as part of the internal laser control.

[0055] Optical phase may be calculated as the integration of the group delay signal: ^ ^L^M = ^^b[L^M ∙ 2^ ∙ 0.001 ∙ O^^^24^ ]^^

[0056] Chromaticbe calculated as the derivative of the group delay signal with respect to wavelength: GDLi + 1M − b[L^Me[L^M =KL^ + 1M − KL^M , ^25^

[0057] Polarization Mode Dispersion (PMD) may be calculated using the all states method to probe over all input polarization states to determine the minimum and maximum Group Delay states. Explicitly, a polarization controller is used to rotate the input polarization states presented to the DUT, and at each polarization state a new measurement for Group Delay is calculated. The maximum and minimum Group Delay at each wavelength (optical frequency) is recorded and the difference represents the PMD as a function of wavelength. &i[L^M = b[$R\L^M − b[$]^L^M ^26^

[0058] Further parameters may be calculated from the DUT transfer function including min / max loss states, second order PMD, phase error, and more.

[0059] There are potential shortcomings of the example embodiment shown in Figure 2 related to wavelength accuracy, and phase errors. Lasers often drift in wavelength as a function of temperature and cavity dynamics (e.g., index of refraction changes which affect the optical path length and therefore lasing wavelength). For this reason, dual comb generators often require multiple mode-locked laser sources and operate in free-running mode, such that optical wavelength is not actively controlled or adjusted. Although wavelength drift may be acceptablefor some applications, in optical test and measurement, wavelength accuracy and verification are required. A third potential problem is phase errors. Lasers, combs, optical waveguides and other devices may generate phase errors and dispersion which result in non-uniform comb spacing, linewidths, and measurement noise. This causes non-ideal sampling of the optical device’s complex response function or transfer function. These shortcomings are addressed using the apparatus in Figure 4. (Note: like reference numbers refer to like elements throughout the figures).

[0060] Figure 4 shows another example embodiment of a dual comb interferometric apparatus 30 for determining characteristics of a DUT that includes polarization control 31 and an absolute wavelength reference signal 32 for spectral alignment and calibration and a controller 21A for adjusting ring resonator coherence and wavelength coupling. An optional modulator 32A for generating secondary comb lines that enhance comb control and may be used to change the effective repetition rate thereby improving optical frequency sampling resolution. In particular, the example embodiment in Figure 4 illustrates interrogator modifications that may be used to mitigate issues of wavelength drift, polarization control, and phase error.

[0061] As in Figure 2, a polarization controller 31 is included in the first light path to adjust the polarization state of the first optical frequency comb before being passed through the DUT 25. This allows for probing of individual polarization states, balancing detected signals, and the calculation of polarization dependent amplitude and phase values such as Polarization Dependent Loss (PDL) and Polarization mode dispersion (PMD).

[0062] Another feature relating to calibration is shown in Figure 4. Light from the first and second optical frequency combs are combined and traverse a gas cell (not shown), with absorption features spanning the wavelength range of interest, and are interfered on a separate photodiode labeled Gas 32. The detected gas signal provides an absolute wavelength reference by which all detection channels may be spectrally aligned and calibrated by the signal processing unit.

[0063] Figure 4 also illustrates features for resolving polarization fading and provides additional advantages in polarization characterization via detection of orthogonal polarization states. After traversing the DUT, the light is combined into a single path before traversing a polarizing beam splitter (PBS) 33 which divides the light into orthogonal polarization states. This guarantees the signal exiting the dual comb PIC 22 and the DUT 25 will interfere on at leastone P detector 34 and / or S detector 35, alleviating signal loss due to non-aligned polarization states in heterodyne detection. Furthermore, the PBS 33, P detector 34, and S detector 35 allow complex characterization of polarization dependent parameters including but not limited to: PDL, PMD, second order PMD, and more.

[0064] Figure 4 provides an additional signal for amplitude and phase correction via a secondary fiber interferometer. Light from the first optical comb is divided by a beam splitter prior to traversing the DUT 25, and light from second optical comb is divided by a beam splitter in order to traverse a longer optical path represented in the figure as loops 36. These two light signals are interfered and detected upon a photodiode PR 37 and the resulting interferogram is processed in order to calculate amplitude, phase, and / or dispersion errors, for example, due to laser and comb tuning drift and optical waveguides.

[0065] Figure 4 provides an additional controller 21A DCC for adjusting ring resonator coherence and wavelength coupling via temperature and current which may influence geometry and index of refraction in particular which enhances control of the comb outputs including coherence, repetition rates, and wavelength resonance, for example.

[0066] Figure 4 provides an additional modulator 32A EM for generating secondary comb lines that may be used in either or both comb paths to multiply the effective repetition rate by a factor of 3 or more which may be used to improve optical frequency sampling resolution also by a factor of 3 or more.

[0067] Figure 5 shows an example embodiment of a method for DUT signal processing that includes using gas channel spectral alignment and calibration in conjunction with the example dual comb interferometric apparatus in Figure 4. Initially, the signal processing unit 28 performs a fast Fourier transform (FFT) of the time-domain DUT interferogram to the radio frequency (RF) domain (step S20). The signal processing unit 28 applies a window function to the RF domain interferogram to extract each individual RF comb line amplitude and phase (step S21). The signal processing unit 28 spectrally aligns the DUT comb lines, using the absolute optical frequency offset and spectral increment δν obtained from the gas channel processing (step 22). The signal processing unit 28 corrects a DUT transfer function by applying amplitude and phase corrections from calibration reference scan (step S23). The signal processing unit 28 calculates parameters of interest such as amplitude, phase, IL, GD, CD, PDL, PMD, etc. (step S24).

[0068] The signal processing unit 28 analyzes the interferogram to extract the DUT characteristics as a function of wavelength or frequency including one or more polarization characteristics of the DUT and one or more further DUT characterization parameters including wavelength, amplitude, and phase (step S21). The signal processing unit 28 performs an FFT of the time-domain Gas channel interferogram into the RF frequency domain (step S22). The signal processing unit 28 applies a window function to extract each individual RF comb line amplitude and phase (step S23) and calculates a gas channel optical spectrum as sum of power in each RF comb line versus array index (step S24). Array index refers to the storage of the gas channel optical spectrum in a numerical array in memory accessible by the signal processing unit 28. The signal processing unit 28 compares the gas channel optical spectrum with an absorption peak look up table stored in memory accessible to the signal processing unit 28 to determine an absolute optical frequency offset and a spectral increment δν (step S25). The signal processing unit 28 generates a corrected optical frequency array axis as array index*increment + absolute optical frequency offset and calculates a wavelength array using the speed of light (step S26). The absolute optical frequency offset and spectral increments may then be used to spectrally align all acquired and processed spectra including the DUT spectra (e.g., acquired via photodiode PD 26, P 24, S 35), and the phase error correction spectrum acquired via photodiode PR. The corrected optical frequency and wavelength arrays are required to plot and further analyze DUT or interferogram characteristics as a function of frequency or wavelength (e.g., IL vs wavelength or frequency, PDL vs wavelength or frequency, GD vs wavelength or frequency, and so on for CD, PMD, optical phase, or any other parameter derived from the DUT response).

[0069] Note that the various features described above for the example embodiment in Figure 4 need not all be used together (or at all) and also may be used selectively to augment the example embodiment shown in Figure 2.

[0070] Figure 6 shows an example embodiment of a method for interferogram signal processing that includes wavelength correction. The signal processing unit 28 performs an FFT of a time-domain gas channel interferogram to the RF frequency domain (step S30). The signal processing unit 28 applies a window function to extract each individual RF comb line amplitude and phase (step S31). The signal processing unit 28 calculates the gas channel optical spectrum as the sum of power in each RF comb line versus array index (step S32). The signal processing unit 28 compares the gas channel optical spectrum with an absorption peak look up table storedin memory accessible to the signal processing unit 28 to determine an absolute optical frequency offset and a spectral increment δν (step S33). The signal processing unit 28 generates a corrected optical frequency array axis as array index*increment + absolute optical frequency offset and calculates a wavelength array using the speed of light (step S33). This correction may be applied to correct the DUT interferogram for example as shown in Figure 5 (step 22).

[0071] Figure 7 shows an example embodiment of a method for phase error correction. The signal processing unit 28 performs an FFT of a time-domain (phase error) PR channel interferogram to the RF frequency domain (step S40). The signal processing unit 28 applies a window function to extract each individual RF comb line amplitude and phase (step S41) and calculates phase error PR channel spectral amplitude and phase (step S42). This correction may be applied to correct the DUT interferogram for example as shown in Figure 5 (step 23).

[0072] Figure 8 shows another example embodiment of a dual comb interferometric apparatus 40 for determining transmission and reflection characteristics of a DUT. Figure 7 describes an embodiment that provides amplitude, phase, and wavelength characterization in both reflection and transmission. Optical devices are often designed for use in transmission (e.g., couplers, wavelength selectable switches), reflection (e.g., mirrors), or both simultaneously (e.g., switches, isolators, circulators). Light from the bottom comb is split and travels directly to the transmission photodiode PD 26, and light from the top comb reflects from the DUT 25. A reflective response signal is measured on a reflection photodiode RD 41 as the heterodyne interference of these two signals. This example embodiment allows simultaneous capture of both reflective and transmissive responses. As noted above, previous embodiment and features such as orthogonal polarization detection (PBS – S / P), polarization controller (PC), phase error correction interferometer (PR), and wavelength reference (Gas) architectures, laser control, resonator and coherence control (DCC), phase modulation (EM), may be selectively included in this example embodiment.

[0073] Figure 9 shows another example embodiment of a dual comb interferometric apparatus 50 for determining characteristics of a DUT. Figure 9 introduces a switch 51 that allows a single photodiode 26 to capture both reflection and transmission responses utilizing fewer components, but at the price of non-simultaneous measurement. As noted above, previous embodiment and features such as orthogonal polarization detection (PBS – S / P), polarization controller (PC), phase error correction interferometer (PR), and wavelength reference (Gas)architectures, laser control, resonator and coherence control (DCC), phase modulation (EM), may be selectively included in this example embodiment.

[0074] Figure 10 shows another example embodiment of a dual comb interferometric apparatus 60 for determining characteristics of a DUT also using a secondary interferometer and a tunable laser to correct for phase and dispersion effects caused by the laser, combs, and optical paths. Figure 10 combines optical frequency domain reflectometry (OFDR) and dual comb spectral characterization. This provides benefits for device characterization because OFDR provides a map of reflected amplitude and phase as a function of optical path length (e.g., delay down the device, or distance if the index of refraction is known). This can be a powerful tool for device and network characterization and troubleshooting. This embodiment may be applied in free space, utilizing frequency modulated continuous wave (FMCW) light detection and ranging (lidar), or in fiber or other waveguide devices for reflectometry and characterization as a function of optical wavelength or frequency. As noted above, that embodiment and features such as orthogonal polarization detection (PBS – S / P), polarization controller (PC), phase error correction interferometer (PR), and wavelength reference (Gas) architectures, laser control, resonator and coherence control (DCC), phase modulation (EM), may be selectively included in this example embodiment.

[0075] In this example embodiment, the DUT 25 may be connected to 1 of 2 output ports on the interrogator 60. The DUT 1 connection (labeled as 25A) provides spectral characterization and may contain any number of dual comb embodiment features as described previously but not limited to: polarization controllers, orthogonal polarization detection, switches, reflection and transmission measurement, etc. The DUT 2 connection (labeled as 25B) provides OFDR measurement. A single DUT connection may be implemented to provide simultaneous OFDR and spectral measurement by (1) employing an optical or electrical filter to reduce the signal detected at (D), the OFDR received wavelengths, to that of the laser sweep and block the broadband reflected dual comb wavelengths, or by (2) performing signal processing to stitch the swept comb light into a single swept source data set.

[0076] Various example embodiments are now described that may be used to improve wavelength resolution in any of the above example embodiments. One embodiment provides a modulator such as an acousto-optic or electro-optic modulator that may be placed in the path to generate additional sidebands for each comb tooth. This increases the spectral resolution of theheterodyne interference signals since it creates two sidebands for every comb tooth. With careful selection of the modulator frequency (f = repetition rate / 3) it is possible to effectively improve the spectral resolution by a factor of 3, since the comb repetition frequency sets the sampling resolution in the optical frequency domain, and the modulator creates a side band on both sides. The downside of this approach is the addition of potentially expensive components, increase power optical and electrical power consumption, and the complexity of driving / controlling an external phase modulator. Building additional phase modulation into a dual ring resonator chip may provide significant advantages here since it may be integrated via RF thermal, optical amplitude, or electrical modulation.

[0077] A second wavelength resolution improvement embodiment uses a narrow band swept wavelength laser to rapidly sweep out a small range of frequencies (e.g., 10GHz, or a portion or the full spacing between comb lines). This changes the wavelengths at which characterization is performed and “fills in” default sampling points. By changing the lasing wavelength, all individual comb teeth are shifted, and wavelength resolution is then limited only by the ability to subtly and accurately control the laser wavelength. Desirable characteristics include highly controlled in optical output power, absolute wavelength (pm resolution) and linear tuning profile (deviation from linearity < 1MHz), low-cost, and provides 10s-100s GHz or more tunable frequency sweep range.

[0078] A third wavelength resolution improvement embodiment maintains the laser frequency fixed, and thermally or electrically controls the geometry of the resonators 23 and 24, such that the comb spacing of one or both combs is / are swept. This provides heterodyne sampling of separate wavelengths in a time division multiplexed fashion and does not require a sweeping / tunable laser. The third wavelength resolution improvement embodiment is referred to as breathing combs.

[0079] EXAMPLE APPLICATIONS AND ADVANTAGES

[0080] The technology described in this application provides a platform for a new optical interrogator, architectures, and methods for fully characterizing optical devices as a function of wavelength, polarization, amplitude, and / or phase. It provides a significant advancement in the testing and characterization of optical devices and networks, addressing challenges with wavelength corrections, amplitude and phase error corrections, and achieving high spectral resolution and broad wavelength coverage. The technology can achieve sub 100 MHz spectralresolution, and spectral coverage of more than 200nm. The described technology is versatile, serving both fiber optic and photonic test and measurement applications as well as physical, material, and chemical sensing.

[0081] In telecommunications, the technology enables detailed characterization of fiber and photonic components such as couplers, switches, wavelength-selective switches, dispersion compensators, photonic integrated circuits (PICs), optical amplifiers, and more. Its applicability extends to sensing networks, facilitating broad-spectrum analysis of physical parameters like strain, temperature, pressure, acoustics, shape for single point and distributed sensing applications. The technology provided advantages in full optical characterization with high- speed and accuracy represent a powerful tool for manufacturing environments, ensuring reliable quality control of optical devices even under common manufacturing environment conditions such as vibration and thermal changes. Beyond industrial applications, this technology may be used in medical optics, advanced diagnostics, environmental sensing, and general research applications because it accurately characterizes polarization and phase-dependent optical properties as a function of wavelength / optical frequency.

[0082] The disclosed dual comb system embodiments overcome critical limitations in existing optical characterization methods. Existing methods often rely on large, complex, and expensive setups, such as systems with multiple phase-locked pulsed lasers and nonlinear fibers, which are difficult to operate and maintain coherence, particularly in busy, dynamic, or space- constrained environments. In contrast, the technology in this application employs a single laser passed through two ring resonators on a single photonic integrated circuit (PIC). This innovative approach resolves coherence loss and operational complexity while offering a compact, solid- state solution with no moving parts, making it highly suitable for deployment in space or other demanding environments. The parallel structure of the rings allows closer comb line spacing (smaller repetition rates) while avoiding the comb lines of one resonator seeding the second resonator, which is a limitation in sequential ring resonator designs. Additionally, pulsed laser systems and swept laser systems can cost several hundred thousand dollars, whereas the disclosed system achieves comparable or superior performance with a single non-swept low-cost laser driving substantial cost savings.

[0083] The technology in this application provides correction signals for both the input comb light and the light traversing the device under test. While the combs need not be generatedon a PIC, doing so offers distinct advantages, including mutual coherence, simplified tuning for comb resonance, reduced size, and lower cost and complexity. For telecommunications test and measurement applications, the technology in this application employs specific sample coupling configurations, where one comb passes through the device under test while the other serves as a local oscillator. Polarization control, absolute wavelength references, and disclosed techniques to enhance spectral resolution ensure the system’s superior performance compared to standard spectroscopy techniques.

[0084] The technology in this application incorporates advanced features to meet the demanding requirements of high-resolution spectral analysis and accuracy. Techniques described such as modulator-induced sideband generation and dynamic comb “breathing” enhance spectral resolution without the need for bulky or costly external equipment. Unlike traditional spectroscopy or grating-based systems, the technology in this application is compact and is not limited by path length constraints and can utilize single-pixel detectors instead of linear arrays, further reducing cost and complexity. Integration of resonators on a photonic integrated circuit (PIC) platform ensures scalability for future applications, including wavelength-division multiplexing (WDM) networks and ultra-broadband sensing systems. The system also features phase and amplitude correction mechanisms achieved through secondary interferometers and absolute wavelength calibration using gas cell references.

[0085] Although various embodiments have been shown and described in detail, the claims are not limited to any particular embodiment or example. None of the above description should be read as implying that any particular element, step, range, or function is essential. All structural and functional equivalents to the elements of the above-described embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed. Features of the embodiments described above may be combined unless clearly technically impossible. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved, for it to be encompassed by the invention. No embodiment, feature, element, component, or step in this document is intended to be dedicated to the public.

[0086] All methods described herein can be performed in any suitable order unless otherwise indicated herein. The use of any and all examples, or example language (e.g., “suchas”) provided herein, is intended merely to better illuminate the example embodiments and does not pose a limitation on the scope of the claims appended hereto unless otherwise claimed.

[0087] The term “about” or “approximately” means an acceptable error for a particular recited value, which depends in part on how the value is measured or determined. In certain embodiments, “about” can mean 1 or more standard deviations. When the antecedent term "about" is applied to a recited range or value it denotes an approximation within the deviation in the range or value known or expected in the art from the measurement’s method. For removal of doubt, it is understood that any range stated herein that does not specifically recite the term “about” before the range or before any value within the stated range inherently includes such term to encompass the approximation within the deviation noted above.

Claims

What is claimed is:

1. An apparatus for determining one or more characteristics of an optical device under test (DUT), comprising: a laser source configured to generate laser light at a laser frequency; a beam splitter configured to split the laser light into first light that propagates over a first optical path and second light that propagates over a second optical path; a first ring resonator configured to receive the first light in the first optical path and generate a first optical frequency comb, where DUT is configured to receive the first optical frequency comb and provide a DUT output light signal; a second ring resonator configured to receive the second light in the second optical path and generate a second optical frequency comb different from the first optical frequency comb, wherein the second optical frequency comb is a reference signal; a beam combiner configured to combine the DUT output light signal and the reference signal to generate a light interference signal; an optical detector configured to detect the interference signal; acquisition circuitry configured to detect the interference signal and generate a digital interference signal; and processing circuitry, coupled to the acquisition circuitry, configured to process the digital interference signal to generate a corresponding complex signal and analyze the complex signal to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters: wavelength, amplitude, and phase.

2. The apparatus of claim 1, further comprising a photonic integrated circuit (PIC) that comprises the first ring resonator and the second ring resonator.

3. The apparatus of claim 2, wherein the laser is a continuous wave laser and the PIC is configured to guide a same seed laser wavelength to the first ring resonator and the second ring resonator.

4. The apparatus of claim 1, wherein each of the first ring resonator and the second ring resonator is a micro ring resonator designed to impart optical non-linear effects via parametric four-wave mixing.

5. The apparatus of claim 1, further comprising: a polarization controller configured to adjust a polarization state of the first optical frequency comb before the DUT receives the first optical frequency comb, wherein the processing circuitry is configured to analyze the complex signal to determine polarization- dependent loss and dispersion in the DUT based on the adjustment of the polarization state of the first optical frequency comb.

6. The apparatus of claim 5, wherein the apparatus includes: a polarization beam splitter (PBS); a first photodiode; a second photodiode; and a wavelength reference in a third optical path configured to provide one or more absolute wavelength references for spectral alignment and calibration of all detected interferograms 7. The apparatus in claim 1, further comprising: a secondary fiber interferometer coupled to the first optical frequency comb and the second optical frequency comb, wherein the processing circuitry is configured to process an output of the secondary fiber interferometer to correct for phase and dispersion effects caused by one or more of: the laser, the optical frequency comb, the second optical frequency comb, the first optical path, and the second optical path.

8. The apparatus in claim 1, wherein the DUT is characterized in both reflective and transmissive modes, and the optical detector comprises a reflection response photodetector to detect DUT reflection and a transmission response photodetector to detect DUT transmission.

9. The apparatus in claim 1, further comprising comb adjustment circuitry configured to adjust ring resonator geometry and optical index of refraction of one or both of the first optical frequency comb and the second optical frequency comb to dynamically adjust comb spacing and enhance wavelength resolution.

10. The apparatus in claim 1, wherein the laser is a narrow-band tunable laser configured to dynamically sweep at least a portion of optical spectrum to increase spectral resolution of the DUT characterization parameters.

11. The apparatus in claim 1, further comprising: a modulator configured to generate one or more sidebands for each tooth of the first optical frequency comb and / or the second optical frequency comb to increase spectral resolution the DUT characterization parameters.

12. The apparatus in claim 1, further comprising: a switch and optical network configured to provide both transmissive and reflective DUT characterization with a single photodetector.

13. A method for optical characterization an optical device under test (DUT) using dual comb interferometry, comprising: generating first and second optical frequency combs using a single laser source and ring resonators with different geometric properties; maintaining relative coherence between the first and second optical frequency combs; directing the first optical frequency comb through the DUT to generate a DUT optical frequency comb; providing the second optical frequency comb as a reference optical frequency comb; combining the DUT optical frequency comb and the reference optical frequency comb to generate an interferogram; and analyzing the interferogram to determine one or more polarization characteristics of the DUT and one or more further DUT characterization parameters: wavelength, amplitude, and phase.

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