CN112197941B - In-situ loss measuring device and method on-chip waveguide - Google Patents
In-situ loss measuring device and method on-chip waveguide Download PDFInfo
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- CN112197941B CN112197941B CN202010978883.9A CN202010978883A CN112197941B CN 112197941 B CN112197941 B CN 112197941B CN 202010978883 A CN202010978883 A CN 202010978883A CN 112197941 B CN112197941 B CN 112197941B
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Abstract
The invention discloses an in-situ loss measuring device and method on an on-chip waveguide, wherein the method comprises the following steps: an in-situ loss measuring device built on the on-chip waveguide adjusts a light path to enable the end face of the laser to be coupled into the waveguide, and a jittered probe is placed at the center of a certain position of the waveguide to be measured; after the jittered probe and a guided mode evanescent field in the on-chip waveguide are mutually overlapped, modulated signal light is generated, and the modulated signal light is the superposition of a first signal light and a second signal light; sweeping frequency of the frequency sweeping laser, and extracting a demodulation signal of the phase-locked amplifier; carrying out Fourier analysis on the demodulation signal at the position to obtain the position of the probe and the relative amplitude of the position, wherein the relative amplitude is the ratio of the amplitudes of the first signal light and the second signal light; and selecting the position where the probe needs to be placed according to the specific loss type to be measured, and calculating the corresponding loss according to the probe position obtained by analyzing each position and the corresponding relative light intensity information. The invention ensures the possibility of in-situ measurement of loss characteristics of waveguides of any size and photonic circuits on chip of any complexity.
Description
Technical Field
The invention relates to the technical field of on-chip photon loop performance testing, in particular to an in-situ loss measuring device and method on an on-chip waveguide.
Background
With the progress of micro-nano Photonic technology, a Photonic Integrated Circuit (PIC) on chip can integrate thousands of functional devices into one chip, thereby realizing optical communication, optical sensing, optical measurement, and optical computing functions with low power consumption. As the most basic component of the photonic integrated circuit, the on-chip micro-nano waveguide is used for communicating the basic functions of each optical device, and the loss characteristic of the on-chip micro-nano waveguide directly determines the performance of the whole photonic circuit. Therefore, accurate and reliable waveguide loss measurement plays a crucial role in promoting the development of on-chip photonic integration technology. The loss includes both the transmission loss of the waveguide and the insertion loss and return loss of the devices in the waveguide. With the increase of the scale of the on-chip photonic integrated circuit and the extensive application research of various multiplexing technologies represented by the "mode division multiplexing" technology, the "in-situ measurement" of the loss characteristics of one segment of waveguide or a single device becomes a new critical requirement, and is also a significant issue that is not solved by the current testing technology.
The existing on-chip waveguide loss measurement methods mainly include two types. One is a 'black box' method, which mainly comprises a truncation method, a Fabry-Perot cavity spectrum analysis method and an annular cavity spectrum analysis method. Although the method can accurately measure the loss of a single waveguide, there are some human factors affecting the measurement result (for example, the input/output coupling efficiency of the waveguide is unknown, the reflectivity at two ends of the waveguide is susceptible, etc.). Moreover, such measurements do not distinguish between the loss of a waveguide in the photonic loop and do not allow for "in situ measurements". Another class of loss measurement methods is achieved by collecting and resolving the weak scattered light along the waveguide that is generated by its own defects, including far-field direct imaging methods and Optical Frequency-Domain Reflectometry (OFDR). Such methods break the limitations of the "black box" approach to some extent for resolving the composition of transmission losses. However, they still have the problems of insufficient spatial resolution and limited measurement dimension, and still have a large gap from the requirement of "in-situ measurement" in terms of flexibility, especially not applicable to waveguide circuits with more branched structures and using polarization/mode multiplexing.
To make a strictly "in situ measurement" of the on-chip photonic circuits, a flexibly deployable optical probe is a critical tool. The non-invasive probes currently available, with dimensions of ten to one hundred microns, can help one perform optical path tests on localized areas of the photonic circuit. However, because the probes made of the optical fiber head, the optical grating and the capacitor have large space size and low accuracy, the probes are only suitable for qualitative optical path connectivity tests and cannot be used for quantitative loss measurement. In contrast, Near-field Optical microscopy (NSOM) probes have extremely high spatial resolution (less than 100nm), which is a potential solution for making accurate in situ loss measurements. However, in practical applications, the existing NSOM technology has some critical problems. First, both holey and scattering NSOM technologies are expensive and complex in their optical path. Second, the probe scan speed of NSOM is too slow (one scan typically takes several minutes), and the instability of the optical path coupling during this process will seriously affect the measurement accuracy. Thirdly, the NSOM technology is developed for nanoscale samples, the scanning measurement range of which is limited to hundreds of microns, mechanical adjustment is required beyond the range, and the position precision is greatly reduced, so that the NSOM technology is difficult to be applied to the measurement of millimeter-scale and longer waveguides.
Disclosure of Invention
In order to overcome the defects and shortcomings in the prior art, the invention provides an in-situ loss measurement device and method on an on-chip waveguide.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention provides an in-situ loss measuring device on an on-chip waveguide, comprising: the device comprises a sweep frequency single longitudinal mode laser, a polarization controller, a circulator, a lens optical fiber, an on-chip waveguide, a probe, piezoelectric ceramics, a photoelectric detector, a phase-locked amplifier, a control box and a computer;
the swept frequency single longitudinal mode laser is connected with a polarization controller, the polarization controller is connected with a circulator, the circulator is connected with a lens optical fiber, the circulator is connected with a photoelectric detector, and the lens optical fiber is aligned with an on-chip waveguide in space;
the photoelectric detector is connected with a phase-locked amplifier, the phase-locked amplifier is connected with a control box, the control box is connected with piezoelectric ceramics, and the control box is connected with a computer;
the swept-frequency single longitudinal mode laser is used for generating narrow-linewidth swept-frequency laser, and the narrow-linewidth laser sequentially passes through the polarization controller and the circulator and enters the on-chip waveguide through the lens fiber in an end face coupling mode;
the on-chip waveguide is provided with a metal nano structure, is placed right above the on-chip waveguide to be detected and is contacted with the waveguide, or the on-chip waveguide is provided with a rear end face of the waveguide;
the probe shakes along the vertical direction at a set frequency, and after the probe and a guided mode evanescent field in the on-chip waveguide are mutually overlapped, reverse modulation signal light is generated;
a reflected light formed by the contact of the jitter probe and the evanescent field of the waveguide forward guided mode is used as a first signal light;
the power reduction of a forward guided mode caused by the jitter probe, the modulated light which returns to the optical fiber along the original path of the waveguide and is formed by the reflection of the metal nano structure or the rear end surface of the waveguide, and the modulated light which returns to the optical fiber along the original path of the waveguide and is formed by the power reduction caused by the jitter probe after the reflection of the metal nano structure or the rear end surface of the waveguide are completely the same and are superposed to be used as second signal light;
the first signal light and the second signal light are superposed to be used as total modulation signal light;
the narrow linewidth laser generates reflected continuous light at the joint of the lens optical fiber and the on-chip waveguide to serve as reference light;
the modulated signal light and the reference light generate interference, and the formed interference light reversely passes through the lens optical fiber and the circulator and enters the photoelectric detector;
the photoelectric detector is used for converting interference light into a photocurrent signal, the phase-locked amplifier is used for demodulating the photocurrent signal to obtain a demodulated output signal, and a demodulation reference signal of the phase-locked amplifier is provided by the control box;
the piezoelectric ceramic is used for controlling the shaking of the probe, the control box is used for driving the piezoelectric ceramic, and the computer is used for controlling the control box and calculating the position of the probe and the light intensity of the position of the probe through frequency spectrum analysis.
Preferably, the probe is placed at the central axis of the on-chip waveguide.
As a preferable technical scheme, the metal nano structure adopts silver nano wires with the diameter of 300 nm.
The invention provides an in-situ loss measurement method on an on-chip waveguide, which comprises the following steps:
generating narrow-linewidth frequency-sweeping laser by adopting a frequency-sweeping single longitudinal mode laser, enabling the narrow-linewidth laser to sequentially pass through a polarization controller and a circulator, then entering an on-chip waveguide by adopting a lens fiber in an end-face coupling mode, enabling a probe to shake along the vertical direction at a set frequency, generating reverse modulation signal light after the probe and a guided mode evanescent field in the on-chip waveguide are mutually overlapped, arranging a metal nano structure or the rear end face of the waveguide on the on-chip waveguide, placing the rear end face right above the waveguide on the to-be-detected chip, and contacting the waveguide;
the shaking of the probe is controlled by adopting piezoelectric ceramics;
a reflected light formed by the contact of the jitter probe and the evanescent field of the waveguide forward guided mode is used as a first signal light;
the power reduction of a forward guided mode caused by the jitter probe, the modulated light which returns to the optical fiber along the original path of the waveguide and is formed by the reflection of the metal nano structure or the rear end surface of the waveguide, and the modulated light which returns to the optical fiber along the original path of the waveguide and is formed by the power reduction caused by the jitter probe after the reflection of the metal nano structure or the rear end surface of the waveguide are completely the same and are superposed to be used as second signal light;
superposing the first signal light and the second signal light to be used as total modulation signal light;
the narrow linewidth laser generates reflected continuous light at the joint of the lens optical fiber and the on-chip waveguide, and the reflected continuous light is used as reference light;
the modulated signal light and the reference light generate interference, and the formed interference light reversely passes through the lens optical fiber and the circulator and enters the photoelectric detector;
converting interference light into a photocurrent signal by using a photoelectric detector, and demodulating the photocurrent signal by using a phase-locked amplifier to obtain a demodulated output signal;
resolving the position of the probe and the relative amplitude of the position of the probe by adopting frequency spectrum Fourier analysis according to the demodulated output signal;
and moving the probe to the center of the next position of the waveguide, collecting the positions of the plurality of probes and corresponding relative light intensity, and calculating to obtain corresponding loss.
As a preferred technical solution, the demodulation output signal is a function of the laser frequency.
As a preferred technical solution, the probe is moved to the center of a position of the waveguide, and the relative amplitude of the position of the probe is measured, and the specific calculation formula is as follows:
|E1|＝|E0|·α0κrefl·exp(-αL1)
|E2|＝2|E0|·r·α0κext·exp(-αL2)
|Er|≡|E1|/|E2|＝κrefl|(2rκext)·exp[α(L2-L1)]
wherein, | E0I denotes the amplitude of the laser output, α0Representing the total amplitude loss in the optical path of the fiber, alpha being the waveguide transmission loss coefficient, L1And L2Representing the position of the metallic nanostructure on the probe and on-chip waveguide, κreflDenotes the amplitude reflectivity, κ, of the probeextRepresenting the amplitude extinction ratio of the probe, r is the amplitude value of the reflectivity of the rear end face of the metal nano structure or the waveguide, | ErI denotes the relative amplitude, | E1I represents the amplitude of reflected light caused by the contact of the dither probe with the evanescent field of the waveguide forward guided mode, | E2And | represents the amplitude of the modulated light returning to the fiber along the original path of the waveguide.
As a preferred technical solution, the calculation obtains the corresponding loss, and the specific calculation formula is:
wherein α represents a waveguide transmission loss coefficient, L1And L2Indicating the position of metallic nanostructures on probes and on-chip waveguides, | ErThe relative amplitudes of the two end positions of the waveguide are represented by |, the superscript a represents the first point of measurement, and the superscript b represents the second point of measurement;
as a preferred technical solution, the calculation obtains the corresponding loss, or the transmission loss is calculated by linear fitting by measuring the relative amplitudes of a plurality of positions in the whole waveguide.
As a preferred technical scheme, the method further comprises a step of measuring the insertion loss of the on-chip device, and the specific steps comprise:
for a certain on-chip device in the waveguide, respectively placing probes on two sides of the front end and the rear end of the on-chip device, and measuring the respective transmission loss of the front waveguide and the rear waveguide of the device;
and according to the transmission loss, taking the extension line to the position of the on-chip device, and calculating the difference of the light intensity of the waveguides at two ends on the on-chip device to obtain the insertion loss of the on-chip device.
As a preferred technical scheme, the method further comprises a step of measuring the return loss of the on-chip device, and the specific steps comprise:
selecting a reflecting end face to be detected, placing probes at a plurality of positions of a waveguide in front of the reflecting end face to be detected, obtaining a Fourier transform curve, and obtaining the positions of the probes and the reflecting end face from the curve;
and moving the position of the probe to obtain a curve of relative amplitude along the transmission distance, linearly fitting and extending the curve to the position of the reflecting end face, and calculating the reflectivity r:
wherein L is1And L2Representing the position of the metallic nanostructure on the probe and on-chip waveguide, κreflIs amplitude reflectivity, κextRepresents the amplitude extinction ratio, | Er(L1＝L2) And | represents the relative amplitude of the linear fit extension line at the position of the reflecting end face.
Compared with the prior art, the invention has the following advantages and beneficial effects:
(1) the invention integrates the near-field optical probe technology and the optical frequency domain reflection technology (OFDR), the probe with high spatial resolution and the flexibly arranged metal nano structure ensure the possibility of carrying out loss characteristic in-situ measurement on waveguides with any size and on-chip photon loops with any complexity, including polarization multiplexing/mode division multiplexing waveguides and devices in the leading edge research field.
(2) The invention ensures that the space span of the measurement operation can cover the whole on-chip photon loop by means of the optical frequency domain reflection technology.
(3) The invention selects the relative amplitude | ErThe | is taken as a measured value to ensure that the measurement precision is not influenced by an input/output light pathThe effect of instability.
(4) The in-situ loss measuring device on the on-chip waveguide has the advantages of compact structure, strong robustness, low cost, convenience in operation, high signal-to-noise ratio and the like, and is suitable for being applied to precise optical characterization of a complex micro-nano photonic loop.
Drawings
FIG. 1 is a schematic diagram of an in-situ loss measurement device on an on-chip waveguide according to the present invention;
FIG. 2 is a flow chart of a method of in situ loss measurement on an on-chip waveguide in accordance with the present invention;
FIG. 3 is a schematic diagram of the probe dithering of the present invention to generate modulated first signal light and second signal light;
FIG. 4 is a schematic diagram of photocurrent signal demodulation and Fourier transform data processing according to the present invention;
FIG. 5(a) is a schematic diagram of a method for measuring insertion loss according to the present invention;
FIG. 5(b) is a graph showing the measurement results of insertion loss according to the present invention;
FIG. 6(a) is a schematic diagram of a return loss measurement method according to the present invention;
FIG. 6(b) is a diagram showing Fourier transform curves in the step of measuring the return loss according to the present invention;
fig. 6(c) is a graph illustrating the relative amplitude along the transmission distance in the step of measuring the return loss according to the present invention.
The device comprises a 1-sweep frequency single longitudinal mode laser, a 2-polarization controller, a 3-circulator, a 4-lens optical fiber, a 5-on-chip waveguide, a 6-probe, 7-piezoelectric ceramic, an 8-photoelectric detector, a 9-phase-locked amplifier, a 10-control box, an 11-computer, a 12-metal nanostructure, a 13-device and a 14-reflection end face.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the invention.
Examples
As shown in fig. 1, the present embodiment provides an in-situ loss measurement apparatus on an on-chip waveguide, including: the device comprises a sweep frequency single longitudinal mode laser 1, a polarization controller 2, a circulator 3, a lens optical fiber 4, an on-chip waveguide 5, a probe 6, a piezoelectric ceramic 7, a photoelectric detector 8, a phase-locked amplifier 9, a control box 10, a computer 11 and a metal nano structure 12;
the frequency sweeping single longitudinal mode laser 1 is used for generating narrow line width frequency sweeping laser (the bandwidth is hundreds of kHz magnitude), the narrow line width laser enters the on-chip waveguide through the lens fiber 4 in an end face coupling mode after passing through the polarization controller 2 and the circulator 3 in sequence, the upper part of the on-chip waveguide 5 is exposed in the air, and when a probe 6 of an Atomic Force Microscope (AFM) and a guided mode evanescent field in the on-chip waveguide 5 are overlapped with each other, reverse modulation signal light is generated; the narrow linewidth laser generates reflected continuous light at the joint of the lens optical fiber 4 and the on-chip waveguide 5 as reference light; the interference light of the modulated signal light and the reference light sequentially reversely passes through the lens optical fiber 4 and the circulator 3 and enters the photoelectric detector 8;
the computer 11 is used for operating the control box, the control box 10 is used for driving the piezoelectric ceramic, the piezoelectric ceramic 7 controls the probe 6 to vertically shake at a set frequency Ω (Ω of the embodiment is 140-250kHz, and the shaking amplitude is 100nm), so that the modulated signal light is subjected to intensity modulation at the same frequency;
the photoelectric detector 8 is used for converting continuous light and modulated signal light into photocurrent signals, the phase-locked amplifier 9 is used for acquiring a curve of a demodulated signal changing along with laser frequency, demodulating the photocurrent signals to obtain demodulated output signals, and the demodulated reference signals of the phase-locked amplifier are provided by the control box;
the demodulation signals are a function of laser frequency, and Fourier analysis is carried out on the demodulation signals of the probe at a plurality of positions along the waveguide to obtain waveguide loss;
the probe of the embodiment adopts an atomic force microscope probe;
the losses of the on-chip waveguide loop include transmission loss, insertion loss and return loss. The transmission loss refers to gradual loss of light in the process of transmitting along a waveguide, the insertion loss is loss caused by devices or defects in a loop, and the return loss is reflection representing the devices.
In this embodiment, the probe is placed at different positions for measurement, and the transmission loss, the insertion loss and the return loss can be further obtained by measuring the coordinates and the light intensity of the corresponding positions;
as shown in fig. 2, this embodiment further provides an in-situ loss measurement method on an on-chip waveguide, which includes the following specific steps:
s1: the in-situ loss measuring device on the on-chip waveguide is built, and the light path is adjusted to enable laser to be coupled into the waveguide;
s2: placing the dithered probe at the center of a location of the waveguide to be tested;
s3: the frequency sweep laser scans frequency and extracts a demodulation signal (changing along with the frequency) of the phase-locked amplifier;
s4: fourier analysis is carried out on the demodulation signal of the position to obtain the position L of the probe1And relative amplitude | Er|， |ErThe magnitude of L depends on the longitudinal position L of the probe on the waveguide1But is changed;
s5: selecting the position where the probe needs to be placed according to the specific loss type to be measured, and repeating the steps S3 and S4 at the center of each position waveguide;
s6: and calculating corresponding loss according to the probe position obtained by analyzing each position and corresponding relative light intensity information.
In the measurement step S3, the modulated signal light generated by the probe dither returns to the lens fiber along the input optical path, and enters the photodetector 8 via the circulator 3. The modulated signal light is a superposition of a first signal light and a second signal light, as shown in fig. 3, the first signal light is a reflected light (κ) caused by the contact between the dither probe and the evanescent field of the waveguide forward guided modereflAmplitude reflectivity). The second signal light includes two parts: power reduction of forward guided mode (κ) by dither probeextThe amplitude extinction ratio representing the process), the modulated light returning to the optical fiber along the original path of the waveguide formed by the reflection (amplitude reflectivity r) of the metal nanostructure or the rear end face of the waveguide; first through the metal nanostructure or waveguide back end face reflection (r) and then throughPower reduction (κ) by over-dithering the probeext). As shown in fig. 3, the two signals are identical and indistinguishable, and are superimposed to form the second signal light. Considering kappareflAnd kappaextThe amounts are small, the embodiment does not discuss the generation process of the higher-order signal light, and the higher-order signal light will appear at a position with a larger time delay in the OFDR curve, and the analysis of the first signal light and the second signal light will not be affected. The total transmission distance of the first signal light in the waveguide is L1As probe position changes; the total transmission distance of the second signal light in the waveguide is L2And does not change with the position of the probe. The first signal light and the second signal light are both modulated light and cannot be distinguished under a single wavelength.
Amplitude (| E) of the two portions of signal light1I and I E2|) can be expressed by the following formula,
|E1|＝|E0|·α0κrefl·exp(-αL1) (1a)
|E2|＝2|E0|·r·α0κext·exp(-αL2) (1b)
wherein, | E0I represents the amplitude, alpha, of the input laser (swept single longitudinal mode laser 1)0Is the total amplitude loss in the optical fiber light path (polarization controller 2, circulator 3, lensed fiber 4 and two end-face couplings with the waveguide), alpha is the waveguide transmission loss coefficient, r is the amplitude value of the metal nanostructure or the waveguide rear end-face reflectivity, L1And L2Representing the position of the probe and the metal nanostructure. Kappa typereflAmplitude reflectance of the probe, κextThe cross section centers of the same probe at different positions of the waveguide are kept unchanged, wherein the cross section centers are the amplitude extinction ratios of the probes. The coefficient 2 in front of the equation (1b) represents that the second signal light has two identical components.
By measuring a plurality of probe positions L1Corresponding amplitude | E of1The transmission loss α of the waveguide can be theoretically measured by using the formula (1). However, due to the coefficient α in the formula (1)0Including coupling loss of the lensed fiber to the front facet of the on-chip waveguide,this loss is closely related to the degree of optical path alignment. Therefore, directly measuring two physical quantities in equation (1) will cause significant error in the final result α.
To avoid the above problem, in the present embodiment, the ratio of the formula (1a) to the formula (1b) is adopted:
|Er|≡|E1|/|E2|＝κrefl/(2rκext)·exp[α(L2-L1)] (2)
in the formula (2), the ratio of the amplitudes of the two signal lights is completely independent of the properties of the coupling optical path at the front end of the waveguide and is only dependent on the parameter kappa caused by the proberefl/κextAnd a fixed physical quantity such as a reflectance caused by the metal nanostructure 12, thereby greatly improving the measured value | ErReliability and accuracy of l. In each measurement, the probe is placed at the central axis of the cross section of the waveguide on the chip, and no transverse scanning is performed, so that the dwell time at a single position of the waveguide is saved. Of course, in this embodiment, the probe may be placed in a particular off-center position as desired, such as when measuring higher modes of a multimode waveguide.
The present embodiment uses 300nm diameter silver nanowires as a specific form of the metal nanostructures 12. The metal nanostructure 12 is placed directly above the waveguide on the wafer to be tested, and contacts with the waveguide, and provides an amplitude reflectivity r of > 3% for SOI single mode waveguide (cross section 500nm x 220nm, operating wavelength 1550 nm). In the present embodiment, the material and structure of the metal nanostructure 12 are not limited, and only a sufficient amount of reflection is required, for example, the back end surface of the waveguide to be tested can also cause reflection and can replace the metal nanostructure.
At the same time, a part of the continuous light f introduced from the lensed fiber 4 into the waveguide end face0Will be reflected back into the optical path of the fiber, and this part of the continuous light is the reference light. For a typical silicon-on-insulator (SOI) waveguide, the end facet reflectivity is typically on the order of 0.1% to 1%. The end surface reflected light is not modulated by the dither probe 6, and the characteristics of the continuous light are maintained.
The optical signal received by the photodetector 8 is an interference optical signal of modulated signal light (first signal light and second signal light) and reference light, and then the photocurrent signal is demodulated at the lock-in amplifier 9, where the demodulation frequency is the jitter frequency Ω of the probe 6, so as to obtain an output signal η, where the demodulation output signal η is a function of the output optical frequency f of the laser.
In the measurement step S4, the demodulated output signal η (f) is fourier transformed in a computer, as shown in fig. 4, to obtain a graph of the relationship between different signal peaks and propagation delay in the waveguide. For this reason, this embodiment performs a frequency sweep of the 0.6THz (about 5nm) range/1.2 GHz interval around 1550nm (193.4 THz). One-time sweep frequency measurement takes 10 seconds, and the processes of modulation, receiving and demodulation of the optical signals are finished in real time.
In this embodiment, two signal peaks in fig. 4 are extracted, the abscissa represents their transmission delay in the optical waveguide, and the transmission distance L can be calculated by combining the group velocity of the waveguide1And L2. On the other hand, the ordinate of the signal peak in fig. 4 represents the amplitude (| E) of the signal light generated by the probe modulation1I and I E2See below) and measuring their ratio allows to obtain precisely the amplitude of the guided mode in the waveguide at the probe position.
In the present embodiment, for the measurement of the waveguide transmission loss, it is necessary to measure the respective | E' S at both ends (points a and b) of the waveguide in steps S5 and S6, respectivelyrThen the transmission loss of this section of waveguide can be directly calculated:
or | E of multiple positions can be measured in the whole waveguiderAnd then the transmission loss is calculated by linear fitting.
The probe can be deeply inserted into any position of the waveguide, so that the measuring method can be applied to any section of waveguide of an on-chip photonic integrated circuit and can also be applied to waveguides with a plurality of coexisting conduction modes, such as polarization multiplexing/mode division multiplexing. Due to the high spatial resolution and signal modulation/demodulation capability of the NSOM probe, other waveguides connected in series or in parallel with the waveguide to be measured do not affect the measurement result. The present embodiment thus achieves a true sense of "in situ measurement".
In this embodiment, the method further includes a step of measuring the insertion loss of the on-chip device, and for the insertion loss of the on-chip device or the structure sandwiched between the two sections of waveguides, the similar method is adopted to distinguish the insertion loss from the waveguide transmission loss and measure the insertion loss at the same time;
specifically, for a certain device in the waveguide, as shown in fig. 5 (a). In steps S5 and S6, probes need to be respectively placed on both sides of the front and rear waveguides of the device 13, and the respective transmission losses of the front and rear waveguides of the device 13 are measured; according to the transmission loss, the extension line is taken to the position of the device, the difference of the light intensity of the waveguides at the two ends at the device is calculated to be the insertion loss, as shown in fig. 5(b), the two lines at the two sides represent the gradual change of the light intensity caused by the loss of the front and the rear waveguides, and the sudden change of the light intensity in the middle corresponds to the insertion loss.
In this embodiment, the device 13 refers to a device required for realizing a specific function, and includes a coupler, a microcavity, a grating, and the like, and may also be a defect in a waveguide; the devices are connected by waveguides, and the devices can generate certain insertion loss and are targets to be measured; a series of air holes in the waveguide is shown in fig. 5(a) as the device under test.
In this embodiment, the method further includes a step of measuring the return loss of the on-chip device, where the return loss is the reflectivity of the device and is an important indicator related to the performance of the integrated photonic loop. As shown in fig. 6(a), for a certain reflecting end face 14, the present embodiment places the probe at several positions of the waveguide in front thereof in steps S5 and S6. As shown in fig. 6(b), a fourier transform curve is obtained for one of the positions from which the positions of the probe and the reflecting end face can be obtained. Moving the probe position, as shown in FIG. 6(c), yields | Er| is along the curve of the transmission distance. The fitting line is extended to the position of the reflecting end face to obtain | E in formula (2)rL is in L1＝L2The size of (c). At this position, the contribution of the waveguide transmission loss is zero, so the | E, which can be derived from an experimental fitrI and k calculated by simulationrefl/κext(since the materials and structures of the probe and waveguide are known,. kappa.reflAnd kappaextCan be accurately obtained by simulation calculation) to calculate the reflectivity
I.e. the return loss of the device.
The near-field optical probe and the optical frequency domain reflection measurement technology are combined, the near-field optical probe with nanoscale and high spatial resolution and the high-quality metal nanowire/nanosheet structure which can be flexibly deployed and removed ensure the possibility of carrying out in-situ measurement on transmission loss/insertion loss/reflection loss parameters of optical waveguides with any tiny size and on-chip integrated photonic loop samples with any structural complexity.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such modifications are intended to be included in the scope of the present invention.
Claims (10)
1. An in-situ loss measurement device on an on-chip waveguide, comprising: the device comprises a sweep frequency single longitudinal mode laser, a polarization controller, a circulator, a lens optical fiber, an on-chip waveguide, a probe, piezoelectric ceramics, a photoelectric detector, a phase-locked amplifier, a control box and a computer;
the swept frequency single longitudinal mode laser is connected with a polarization controller, the polarization controller is connected with a circulator, the circulator is connected with a lens optical fiber, the circulator is connected with a photoelectric detector, and the lens optical fiber is aligned with an on-chip waveguide in space;
the photoelectric detector is connected with a phase-locked amplifier, the phase-locked amplifier is connected with a control box, the control box is connected with piezoelectric ceramics, and the control box is connected with a computer;
the sweep frequency single longitudinal mode laser is used for generating narrow linewidth sweep frequency laser, the narrow linewidth sweep frequency laser sequentially passes through the polarization controller and the circulator, and enters the on-chip waveguide through the lens optical fiber in an end face coupling mode;
the on-chip waveguide is provided with a metal nano structure, is placed right above the on-chip waveguide to be detected and is contacted with the on-chip waveguide, or the on-chip waveguide is provided with a rear end face of the on-chip waveguide;
the probe shakes along the vertical direction at a set frequency, and after the probe and a guided mode evanescent field in the on-chip waveguide are mutually overlapped, reverse modulation signal light is generated;
the jitter probe is contacted with an evanescent field of the on-chip waveguide forward guided mode to form reflected light serving as first signal light;
the power reduction of a forward guided mode caused by the jitter probe, the modulated light which is formed by the reflection of the rear end face of the on-chip waveguide and returned to the lens optical fiber along the on-chip waveguide original path, and the modulated light which is formed by the power reduction caused by the jitter probe and returned to the lens optical fiber along the on-chip waveguide original path after the direct reflection of the rear end face of the on-chip waveguide and the metal nanostructure are completely the same, and the two modulated lights are superposed to be used as second signal light;
the first signal light and the second signal light are superposed to be used as total modulation signal light;
the narrow linewidth sweep-frequency laser generates reflected continuous light at the joint of the lens optical fiber and the on-chip waveguide, and the reflected continuous light is used as reference light;
the modulated signal light and the reference light generate interference, and the formed interference light reversely passes through the lens optical fiber and the circulator and enters the photoelectric detector;
the photoelectric detector is used for converting interference light into a photocurrent signal, the phase-locked amplifier is used for demodulating the photocurrent signal to obtain a demodulated output signal, and a demodulation reference signal of the phase-locked amplifier is provided by the control box;
the piezoelectric ceramic is used for controlling the shaking of the probe, the control box is used for driving the piezoelectric ceramic, and the computer is used for controlling the control box and calculating the position of the probe and the light intensity of the position of the probe through frequency spectrum analysis.
2. The in-situ loss measurement device on an on-chip waveguide of claim 1, wherein the probe is placed at a central axis position of the on-chip waveguide.
3. The in-situ loss measurement device on an on-chip waveguide of claim 1, wherein the metallic nanostructure employs 300nm diameter silver nanowires.
4. A method of in-situ loss measurement on an on-chip waveguide, comprising the steps of:
generating narrow-linewidth frequency-sweeping laser by adopting a frequency-sweeping single longitudinal mode laser, enabling the narrow-linewidth frequency-sweeping laser to sequentially pass through a polarization controller and a circulator, then entering an on-chip waveguide by adopting a lens fiber in an end face coupling mode, enabling a probe to shake along the vertical direction at a set frequency, generating reverse modulation signal light after the probe and a guided mode evanescent field in the on-chip waveguide are mutually overlapped, arranging a metal nano structure or the rear end face of the on-chip waveguide on the on-chip waveguide, placing the rear end face right above the on-chip waveguide to be detected, and contacting the on-chip waveguide;
the shaking of the probe is controlled by adopting piezoelectric ceramics;
the jitter probe is contacted with an evanescent field of the on-chip waveguide forward guided mode to form reflected light serving as first signal light;
the power reduction of a forward guided mode caused by the jitter probe, the modulated light which is formed by the reflection of the rear end face of the on-chip waveguide and returned to the lens optical fiber along the on-chip waveguide original path, and the modulated light which is formed by the power reduction caused by the jitter probe and returned to the lens optical fiber along the on-chip waveguide original path after the direct reflection of the rear end face of the on-chip waveguide and the metal nanostructure are completely the same, and the two modulated lights are superposed to be used as second signal light;
superposing the first signal light and the second signal light to be used as total modulation signal light;
the narrow linewidth sweep frequency laser generates reflected continuous light at the joint of the lens optical fiber and the on-chip waveguide as reference light;
the total modulation signal light and the reference light generate interference, and the formed interference light reversely passes through the lens optical fiber and the circulator and enters the photoelectric detector;
the photoelectric detector is used for converting interference light into a photocurrent signal, and the phase-locked amplifier is used for demodulating the photocurrent signal to obtain a demodulated output signal;
resolving the position of the probe and the relative amplitude of the position of the probe by adopting frequency spectrum Fourier analysis according to the demodulated output signal;
and moving the probe to the center of the next position of the on-chip waveguide, collecting a plurality of probe positions and corresponding relative light intensity, and calculating to obtain corresponding loss.
5. The method of claim 4, wherein the demodulated output signal is a function of laser frequency.
6. The method of claim 4, wherein the probe is moved to the center of a position of the on-chip waveguide, and the relative amplitude of the position of the probe is measured, and the specific calculation formula is:
|E1|＝|E0|·α0κrefl·exp(-αL1)
|E2|＝2|E0|·r·α0κext·exp(-αL2)
|Er|≡|E1|/|E2|＝κrefl/(2rκext)·exp[α(L2-L1)]
wherein, | E0I represents the amplitude, alpha, of the output of the swept-frequency single-longitudinal-mode laser0Representing the total amplitude loss in the optical path of the fiber, alpha being the on-chip waveguide transmission loss coefficient, L1And L2Representing the position of the metallic nanostructure on the probe and on-chip waveguide, κreflDenotes the amplitude reflectivity, κ, of the probeextRepresenting the amplitude extinction ratio of the probe, r being the reflectivity of the rear end face of the waveguide on the metal nanostructure or chipAmplitude value, | ErI denotes the relative amplitude, | E1I represents the amplitude of reflected light caused by the contact of a jitter probe and an evanescent field of a waveguide forward guided mode on a chip, and E2And | represents the modulated light amplitude returning to the lensed fiber along the on-chip waveguide primary path.
7. The method of claim 4, wherein the relative amplitudes of the two ends of the on-chip waveguide are collected and the corresponding loss is calculated by the following specific calculation formula:
wherein α represents an on-chip waveguide transmission loss coefficient, L1And L2Indicating the position of the metallic nanostructure on the probe and on-chip waveguide, | ErThe relative amplitudes of the two end positions of the waveguide on the chip are indicated by |, the superscript a indicating the first point of measurement and the superscript b indicating the second point of measurement.
8. The method of claim 4, wherein the calculating results in a corresponding loss, or the transmission loss is calculated by linear fitting using relative amplitudes at multiple positions measured over the entire length of the on-chip waveguide.
9. An in-situ loss measurement method on an on-chip waveguide as claimed in any of claims 4-8, further comprising a step of measuring the insertion loss of the on-chip device, the specific steps comprising:
for a certain on-chip device in the on-chip waveguides, respectively placing probes on two sides of the waveguides on the front end and the rear end of the on-chip device, and measuring the respective transmission loss of the waveguides on the front and the rear of the device;
and according to the transmission loss, taking the extension line to the position of the on-chip device, and calculating the difference of the light intensity of the waveguides on the two end chips at the on-chip device to obtain the insertion loss of the on-chip device.
10. An in-situ loss measurement method on an on-chip waveguide as claimed in any one of claims 4 to 8, further comprising a step of measuring the return loss of the on-chip device, the specific steps comprising:
selecting a reflecting end face to be detected, placing probes at a plurality of positions of a waveguide on a front panel of the reflecting end face to be detected, obtaining a Fourier transform curve, and obtaining the positions of the probes and the reflecting end face from the curve;
and moving the position of the probe to obtain a curve of relative amplitude along the transmission distance, linearly fitting and extending the curve to the position of the reflecting end face, and calculating the reflectivity r:
wherein L is1And L2Representing the position of the metallic nanostructure on the probe and on-chip waveguide, κreflAs amplitude reflectivity, κextRepresents the amplitude extinction ratio, | Er(L1＝L2) And | represents the relative amplitude of the linear fit extension line at the position of the reflecting end face.
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