Technical Field

The present disclosure relates to a system and method for inspecting an optical surface of an optical element or an optical system.

Background

Optical components may be inspected for quality purpose either after or during manufacturing process. Performing metrology measurements during manufacturing process may be used as a feedback mechanism to manage the polishing of optical components. For instance, when a certain optical quality has been achieved a signal may be sent to stop polishing. Performing a quality check during manufacturing is advantageous for instance when manufacturing many optical components or when manufacturing large optics, spanning several meters in diameters as found in telescopes for astronomical observatories. For example, a primary telescope mirror of a large telescope may have a diameter of 30m and include over 500 individual segments which all need to be inspected. Conventional high accuracy surface inspection techniques are based on phase-stepping interferometry or carrier fringe interferometry. In phase-stepping interferometry, a series of interferograms are acquired sequentially with a known phase shift between them. The phase shift is typically accomplished by moving a reference mirror axially, by a fraction of a wavelength using a piezo-electric acuator. The sequential nature of the acquisition requires the setup to be extremely stable for the duration of the measurement which limits its use in a noisy environment. The carrier fringe technique requires adding a large optical tilt to the wavefront being measured so that the tilt dominates the aberrations in the wavefront. The tilt is subsequently removed from the measurement by Fourier analysis. The addition of the tilt limits the maximum slope that can be measured on the surface under test.

Performing an optical surface quality check in the presence of vibration and air turbulences as found in a manufacturing environment requires isolating the metrology system. Existing technologies capable of measurement in a noisy environment overcome some of the limitations of phase-stepping interferometry by measuring three or four phase shifted images simultaneously, encoded as different polarisation states. This is achieved by aligning three cameras to a complex assembly of bulk polarization optics as described in US2006146340A1, or by providing a pixelated phase mask in front of a single camera as described in W02005052502A2.

The isolation of metrology systems from environmental noise becomes increasingly difficult and expensive as the size of the object under test increases. Current metrology systems are also relatively slow. It is an object of the disclosure to address one or more of the above-mentioned limitations.

Summary

According to a first aspect of the disclosure, there is provided a device for inspecting an optical surface, the device comprising a source of electromagnetic radiation adapted to provide a first beam having a first frequency and a second beam having a second frequency; a sensor comprising a plurality of phase detectors; and an interferometric optical arrangement comprising an input optically coupled to the source of electromagnetic radiation and an output optically coupled to the sensor; the interferometric optical arrangement being adapted to form an interference beam.

Optionally, the plurality of phase detectors may form a two dimensional array of phase detectors wherein each phase detector in the two dimensional array is adapted to perform heterodyne phase demodulation of a different region of the interference beam in parallel with the other phase detectors.

For example, the optical surface may be an optical surface of an optical element or an optical system. The optical surface may be an optically smooth surface or optically rough surface.

The sensor may be a time of flight sensor such as a time of flight camera.

Optionally, the second frequency is shifted with respect to the first frequency by a difference frequency; the interference beam having an amplitude modulated at the difference frequency; and each phase detector among the plurality of phase detectors is adapted to detect a phase of the interference beam at the difference frequency.

Optionally, each phase detector in the plurality of phase detectors is configured to demodulate the phase of the interference beam in parallel with the other phase detectors.

Optionally, the device comprises a processor coupled to the sensor, the processor being adapted to collect phase data from each detector and to generate a phase map.

Optionally, the difference frequency is greater than about lKHz. For example, the difference frequency may range from about 1MHz to about 100MHz.

Optionally, the first beam has a first polarization state and the second beam has a second polarization state, the first polarization state being substantially perpendicular to the second polarization state.

Optionally, the source of electromagnetic radiation comprises a laser source. Optionally, the laser source is adapted to generate the first beam having the first frequency and the second beam having the second frequency. For example, the laser source may be a Zeeman laser.

Optionally, the laser source is coupled to a frequency shifter. For example, the frequency shifter may be a modulator such as an acousto-optic modulator or an electro-optic modulator. Optionally, the source of electromagnetic radiation is adapted to provide a visible beam, or an infrared beam. For example, the source of electromagnetic radiation may be a laser source such as a He-Ne laser providing a visible beam at about 632 nm. Optionally, the source of electromagnetic radiation is adapted to provide broad spectrum of electromagnetic radiation. For example, the source of electromagnetic radiation may be adapted to generate white light.

Optionally, the interferometric optical arrangement is configured to form a first path for receiving the first beam and a second path for receiving the second beam.

Optionally, the second path is adapted to probe the optical surface. Optionally, the second path comprises an optical system to expand the second beam.

Optionally, the interferometric arrangement is configured to form a common-path interferometer in which the first beam and the second beam travel along a same optical axis. For example, the common-path interferometer may be a Fizeau interferometer, a Sagnac interferometer or a shearing interferometer. Optionally, the interferometric arrangement is configured to form a double-path interferometer in which the first beam and the second beam travel along two separate optical axes. For example, the double-path interferometer may be a Michelson interferometer, a Twyman-Green interferometer or a Mach-Zehnder interferometer.

According to a second aspect of the disclosure, there is provided a manufacturing system for manufacturing an optical element comprising a device for inspecting an optical surface according to the first aspect.

Optionally, the manufacturing system comprises a polishing device to polish the optical surface; the polishing device being adapted to receive a control signal from the device for inspecting the optical surface.

For example, the control signal may be configured to activate the polishing device when an optical defect has been detected and to stop the polishing device when a certain optical quality has been reached. The manufacturing system according to the second aspect of the disclosure may comprise any of the features described above in relation to the device according to the first aspect of the disclosure.

According to a third aspect of the disclosure, there is provided a method of inspecting an optical surface, the method comprising providing a first beam having a first frequency and a second beam having a second frequency; providing a sensor comprising a plurality of phase detectors; probing the optical surface with the second beam; forming an interference beam using the first beam and the second beam and detecting a phase of the interference beam. Optionally the plurality of phase detectors forms a two dimensional array and the method comprises performing parallel heterodyne phase demodulation of the interference beam using the two dimensional array; wherein each phase detector in the two dimensional array is adapted to perform heterodyne phase demodulation of a different region of the interference beam.

Optionally, the second frequency is shifted with respect to the first frequency by a difference frequency and the interference beam has an amplitude modulated at the difference frequency; wherein each phase detector is adapted to detect the phase of the interference beam at the difference frequency.

Optionally, the method comprises forming a first phase map at a first time.

Optionally, the method comprises comparing the first phase map with a reference phase map to identify an optical defect.

Optionally, the method comprises forming a second phase map obtained at a second time and subtracting the first phase map from the second phase map to obtain a deformation map to identify a phase distribution associated with a deformation of the optical surface.

The method of inspecting an optical surface according to the third aspect of the disclosure may share features of the first aspect as noted above and herein.

Description

The disclosure is described in further detail below by way of example and with reference to the accompanying drawings, in which:

figure 1 is a flow chart of a method for inspecting an optical surface; figure 2 is a diagram of a device for implementing the method of figure

1;

figure 3 is an exemplary embodiment of the device of figure 2;

figure 4 is a time chart illustrating the working of the device of figure

2;

figure 5 is another exemplary embodiment of the device of figure 2; figure 6 is a diagram of a manufacturing system including the device of figure 2.

Figure 1 is a flow chart of a method for inspecting an optical surface of an optical component or an optical system. At step 110 a first beam and a second beam of electromagnetic radiation EM are provided. The first beam has a first frequency and the second beam has a second frequency. The second frequency is shifted with respect to the first frequency by a difference frequency. At step 120, a sensor comprising a plurality of phase detectors is provided. At step 130 the optical surface is probed with the second beam. At step 140 an interference beam is formed using the first beam and the second beam. The interference beam has an amplitude modulated at the difference frequency. At step 150 each phase detector of the sensor detects a phase of the interference beam at the difference frequency.

Figure 2 illustrates a metrology device 200 for inspecting an optical surface 232 of an optical element 230 or an optical system comprising multiple elements. Such optical elements may include one or more of a mirror, a lens, a prism, a filter, a beam splitter, a polariser or other element providing an optical function.

The metrology device 200 includes a source of electromagnetic radiation, 210 such as a laser, an interferometric optical arrangement 220 and a sensor 240 provided with a plurality of phase detectors. The interferometric optical arrangement 220 has an input optically coupled to the source of electromagnetic radiation 210 and an output optically coupled to the sensor 240. The sensor 240 is coupled to a processor 250 for processing phase data from the phase detectors. The processor 250 may be integrated as part of the sensor, for instance in a system on chip topology. Alternatively, the processor 250 may be part of a computer. In yet another embodiment, the processor 250 may be implemented in a remote server.

The source of electromagnetic radiation 210 is adapted to provide a first beam having a first frequency vi= v and a second beam having a second frequency V2 = v+f in which the difference frequency f=V2 - vi. One beam may be referred to as the reference beam and the other beam may be referred to as the sample beam. For instance, if the first beam is the reference beam, then the second beam is the sample beam and vice versa.

The sensor 240 may form a 2D array of phase detectors. For example, the sensor 240 may be a time-of-flight sensor. The source of electromagnetic radiation 210 may be a laser system providing visible light or infrared 1R light. Alternatively, a system providing a broad spectrum such as white light may also be used. The sensor 240 is adapted to detect the wavelength of the chosen source of electromagnetic radiation.

The interferometric optical arrangement 220 may be implemented in different fashions. For instance, the optical arrangement may be configured to form a common-path interferometer in which the reference beam and the sample beam travel along a same path or substantially similar path; or a double-path interferometer in which the reference beam and the sample beam travel along divergent paths. Example of double-path interferometers include Michelson, Twyman-Green and Mach-Zehnder interferometers. Example of substantially common-path interferometers include Fizeau interferometers, Sagnac interferometers and various shearing interferometers. In operation, the interferometric optical arrangement 220 forms an interference beam having an amplitude modulated at the difference frequency. The phase of the interference beam modulated at the difference frequency is then detected by each phase detector present in the sensor 240.

The processor 250 is configured to receive the phase information from each phase detector and to generate a phase map. The processor 250 may also be configured to compare the phase map with a reference map to assess the optical quality of the optical surface 232. For example, the reference map may be a map of an optical component having an ideal optical surface.

The noise encountered in a typical manufacturing environment has a frequency in the order of lKHz or less. The difference frequency also referred to as modulation frequency may be chosen greater than the noise frequency. For instance, the modulation frequency may be chosen about several orders of magnitude greater than the noise frequency. For example, the modulation frequency may range from about 1MHz to about 100MHz or more. The metrology device 200 may be implemented as part of an optics manufacturing system. Since the metrology device 200 does not need to be stabilized, a quality inspection may be obtained relatively quickly. This reduces processing time and energy costs. The amount of consumable required to manufacture the optical component, such as polishing slurries, may also be reduced.

Figure 3 illustrates an exemplary embodiment of the device of figure 2. The device 300 includes a laser system 310 coupled to an interferometric optical arrangement 320 and to a sensor 340 to form a heterodyne imaging interferometer. The laser system 310 includes a laser source 312 for providing a laser beam at a first frequency v, coupled to an acousto-optic modulator 314 for generating a second beam at the frequency v+f. The laser system is also equipped with a timer 316 to provide a clock signal at the modulation frequency f. The timer 316 is coupled to the acousto-optic modulator 314 via a driver 318. In this example, the interferometric optical arrangement 320 forms a double-path interferometer that includes a polarization beam splitter 321, a diverging lens 322, a reference mirror 323, first and second quarter wave plates labelled 324 and 325 respectively and a polarizer 326. An imaging lens 329 is provided to image the optical surface 232 onto the sensor 340. The polarization beam splitter 321 is optically aligned with the diverging lens 322 along a first axis 327. The polarization beam splitter 321 is also optically aligned with the reference mirror 323 along a second axis 328 substantially perpendicular to the first axis 327. The second quarter waveplate 325 is provided between the polarization beam splitter 321 and the reference mirror 323, while the polarizer 326 is provided between the polarization beam splitter 321 and the sensor 340 along the second axis 328. The imaging lens 329 is optically aligned between the polarizer 326 and the sensor 340.

The optical element under test 230 is positioned with respect to the device 300 such that the centre of curvature of the surface 232 is at the focus of the diverging lens 322. As a result, the spherical wavefront of the incident beam matches the shape of the surface 232. It will be appreciated that depending on the optical element under test, various other optical components may be added to diverging lens 322 in order to achieve the desired wavefront shape. For instance, converging lens arrangement may be used for the inspection of a convex optical surface. The inspection of aspheric surfaces may require a system adapted to create an aspheric wavefront such as a null lens or a computer-generated hologram. In this example, the second beam is obtained by acousto-optic modulation of a primary beam. However, it will be appreciated that the second beam may be obtained using other approaches. For instance, the laser may be a Zeeman laser having a lasing medium provided in a strong axial magnetic field to produce two beam having different frequencies.

In operation, the timer 316 provides the clock or reference signal to both the sensor 340 and the driver 318. The acousto-optic modulator 314 is then driven at the modulation frequency f. The laser source 312 generates a laser beam at a first frequency v. The acousto-optic modulator 314 receives the input laser beam and produces a second beam at frequency v+f that propagates along the first beam. As a result, the output of the laser system is a combined beam having an enveloped modulated at the difference frequency f.

The laser system 310 may be provided by a Zygo model 7702 two frequency HeNe laser with a nominal wavelength at 632.8nm. The laser system produces a first beam having a first wavelength with a vertical polarization and a second beam having a wavelength l ₂ with a horizontal polarization. The timer 316 produces a clock signal at 20MHz that set the driving frequency of the acousto-optic modulator 314. As a result, the combined beam has an enveloped modulated in frequency with a frequency modulation of 20 MHz.

Upon incidence of the combined beam onto the beam splitter 321, the reference beam having the frequency v and vertical polarization propagates along the second axis 328 towards the reference mirror 323 while the sample beam having the frequency v+f and horizontal polarization propagates along the first axis 327 towards the diverging lens 322. The sample beam initially collimated, diverges at the output of the diverging lens 322, allowing the sample beam to illuminate a portion of the optical surface 232. The reflected reference beam and the reflected sample beam are then combined by the polarizer beam splitter 321 hence forming a reflected combined beam that propagates along the second axis 328 towards the sensor 340 via the imaging lens 329. The quarter waveplate 324 is used to transform the polarisation of the incident beam at frequency v+f from horizontal to circular and to transform the polarisation of the reflected sample beam from circular to vertical. Similarly, the quarter waveplate 325 is used to transform the polarisation of the incident beam at frequency v from vertical to circular and to transform the polarisation of the reflected reference beam from circular to horizontal. As a result, the polarisation of the reflected sample beam and the reflected reference beam after the first and second quarter waveplates respectively are orthogonal, hence preventing interference. The polarizer 326 is used to control the polarisation of the two reflected beams allowing them to form an interference beam. The surface 232 is imaged onto the sensor 340 via the component 329. The sensor 340 determines the phase relationship between the modulation signal and the interference beam received at each pixel. The sensor 340 outputs a signal which compares the phase of the modulated intensity of the light signal falling on each pixel with an electrical reference signal at the modulating frequency, derived from a common clock source. The sensor 340 may output quadrature signals (IQ) from which the phase is calculated. This calculation may be performed on the sensor chip itself or on an associated processor chip provided on a host computer.

Figure 4 is a time chart showing a reference beam 410 having a first wavelength lΐ and a sample beam 420 having a second wavelength A2. Also illustrated are the combined beams 430 and the interference beam 440. The combined beam 430 results from the combination of the reference beam 410 c and the sample beam 420. The reference beam 410 has a frequency v =— li ;

c

and the sample beam 420 has a frequency v ₂ =— . The combined beam has l ₂

an envelope that is modulated in frequency. The modulated frequency may also be referred to as the beat frequency. The interference beam 440 results from the combination of the reflected reference beam and the reflected sample beam. The interference beam 440 has an envelope modulated at a beat frequency equal to the frequency shift Vi— v ₂ , between the reference beam and the sample beam. However, the phase f between the combined beam 430 and the interference beam has now changed. The phase varies according to the optical path difference between the reference beam and the sample beam. A variation of one wavelength results in 2p phase change. This phase difference f is measured by the sensor 340. Each detector or pixel in the sensor 340 performs phase demodulation of the signal received. Heterodyne imaging is performed by simultaneously demodulating the phase at each of the pixels that makes up the image. The phase data provided by each phase detector in the sensor 340 is then transmitted to the processor 350 via the interface 342. The electronic interface 342 may be implemented in different fashions. For instance, the interface may be a parallel bus, a fast serial bus-USB, a Firewire or Ethernet cable.

The sensor 340 may be a camera comprising an array of detectors or pixels, in which each detector is adapted to detect the phase of the received signal simultaneously. For instance, the sensor 340 may be a time-of-flight (ToF) camera for performing parallel heterodyne demodulation. The sensor 340 may have a relatively high resolution for instance 1280x1024 pixel resolution. A ToF camera employs a pixel structure in which the photo-signal is mixed with an electronic reference signal in a manner analogous to a lock-in amplifier. The pixel structure and signal demodulation methodology may vary between different ToF sensors. In the present example the beat or modulation frequency is 20 MHz. As a result, the system is not affected by the low frequency noise, typically less than lKHz, encountered in a typical manufacturing environment.

Figure 5 illustrates another exemplary embodiment of the device of figure 2. In this example, the interferometric optical arrangement is implemented as a modified Fizeau interferometer. The metrology device 500 includes a laser system 510 coupled to an interferometric optical arrangement formed by a polarising beam splitter 522, a diverging lens (not shown), a beam splitter 524, a collimating lens 526 and a reference element 528. The polarising beam splitter 522 may be a Rochon prism or a Wollaston prism. The polarising beam splitter 522 is optically aligned with the diverging lens, the collimating lens 526, the reference element 528 and the optical component under test 530. The beam splitter 524 is provided between the diverging lens and the collimating lens.

The laser system 510 may be implemented according to the laser system 310 of figure 3. Alternatively, the laser system 510 may include a modulator adapted to provide laterally displaced beams. In this case there is no need for the polarising beam splitter 522.

In operation, the laser system 510 provides an input beam having two frequency components v and v+f. The first frequency component has a polarisation that is orthogonal with respect to the polarisation of the second frequency component. The input beam is incident onto the polarising beam splitter 522, which splits the input beam into two beams laterally displaced from each other. The diverging lens is provided at the output of the polarising beam splitter 522. The two diverging beams coming out of the diverging lens pass through the beam splitter 524 and are collimated by the collimating lens 526, hence producing two collimated beams with a slight angular displacement referred to as reference beam and sample beam respectively. The reference beam is reflected by the reference surface 527 of the reference element 528 and the sample beam is reflected at the optical surface 532 of the component under test 530. The reflected sample beam and the reflected reference beam pass through the collimating lens 526 and are deflected by the beam splitter 524 towards an aperture 529 provided in the focal plane of the collimating lens 526. The reflected sample beam and the reflected reference are focused in the plan of the aperture 529 to form four spots: a first spot arising from the reference surface 527 with a frequency v; a second spot arising from the reference surface 527 with a frequency v+f ; a third spot arising from the surface under test 532 with a frequency v; and a fourth spot arising from the surface under test 532 with a frequency v+f .

The reference element 528 and the optical component under test 530 are positioned such that the first and fourth spots overlap and pass through the aperture 529; while the second and third spots are blocked. Alternatively, the components may be aligned such that the second and third spots coincide and pass through the aperture 529; while the first and fourth spots are blocked. The sensor 540 receives an interference beam having an amplitude modulated at the difference frequency f. The phase of the interference beam modulated at the difference frequency is then detected by each phase detector present in the sensor 540.

The optical systems described above with reference to figures 2 to 5 may be used for measuring movement or deformations of an optically rough surface, also referred to as speckle interferometry. An optical surface is considered rough as opposed to smooth, if the surface irregularities over a short distance are large compared with the wavelength of the incident beam. A beam reflecting from an optically rough surface has a grainy wavefront resulting from random statistics of the beam phase. For instance, when an optically rough surface is illuminated with coherent light it appears to consist of a multitude of point sources of light with randomly distributed phases also referred to as speckle pattern. When a beam reflected from an optically rough surface beam is mixed with a reference beam, an interference beam having a random interference pattern is formed. If the optical surface is distorted or moved, then the phase of the speckles observed at the detector changes along with the interference pattern. In classical speckle interferometry, an image of a speckle interference pattern is recorded with the surface in an initial state and subtracted from subsequent images recorded at subsequent states of the surface. The result is a fringe pattern having fringes showing contours of equal deformation or displacement. The fringe patterns result from a random process and are very noisy. By applying the imaging heterodyne technique of the disclosure, an initial phase distribution also referred to as initial phase map of the speckle image may be recorded. The initial phase map can then be subtracted from one or more subsequent phase distributions recorded at a later time to obtain a phase distribution of the deformation or displacement also referred to as displacement map. Since the phase map depends only on the phase and not on the intensity distribution of the image, fringe patterns may be obtained with significantly less noise compared to the conventional approach.

Figure 6 is a diagram of a manufacturing system for manufacturing an optical component. The manufacturing system 600 includes a polishing device 610 for polishing a surface of the optical component coupled to a metrology device 620. The metrology device 620 may be implemented as described above with reference to figures 2 to 5. The metrology device 620 can be implemented as part of a feedback loop to control an operation of the polishing device 610.

In operation the metrology device 620 provides an optical quality parameter based on the phase map of the optical surface. Then the metrology device may generate a control signal to either activate or deactivate the polishing device 610. For instance, the control signal may be configured to activate polishing of a particular surface area or stop polishing of a particular surface area depending on the optical quality parameter obtained. Using such a system, an optical component being manufactured can be inspected in situ, hence without having to move the optical component to another location during the manufacturing process. This is particularly useful when manufacturing large components.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the disclosure. Accordingly, the above description of the specific embodiments is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.