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Sensing & Measurement

Anti-stokes imaging device used in full-field laser metrology

A C-band external cavity diode laser, coupled to an inexpensive phosphor-coated camera, can be used to implement full-field interferometric phase measurement.
27 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0571

Longer IR wavelengths can be beneficial in some optical measurement applications,1–3 owing to the relationship between the refractive index of a material and wavelength. Some promising telecom devices already available at C-band wavelengths (1525–1565nm) may also prove useful in metrological instrumentation. However, imaging devices designed for these wavelengths tend to be expensive.

In this work, we investigated a phosphor-coated CCD camera sensitive to C-band radiation as an inexpensive alternative. When longer wavelength IR radiation is incident on a phosphor-coated sensor, it is converted by anti-Stokes emission to shorter wavelength light, which a CCD sensor can detect.

Our measurement was based on wavelength-tuning interferometry. In an interferometer with arms of unequal optical path, tuning the wavelength of a laser is equivalent to scanning the path length (see Figure 1). This is because interferometric phase is related to the optical path difference and to the wavelength of the light. Since we used a camera, the measurement was full-field, meaning that a 2D region was imaged and measured. Full-field wavelength-tuning interferometry has the following advantages: there are no moving parts, both rough and smooth surfaces can be profiled with the same setup, and the metrology is integral, ultraprecise, and always known (depending on laser quality). The technique has been successfully used for noncontact millimeter surface topography4 and nanometer surface relief.5,6

Figure 1. Experimental setup: tunable laser (TL), optical fiber (OF), microlens collimator (COL), 50/50 beamsplitter (BS), mirrors (M1, M2), phosphor-coated IR CCD camera (CAM). A computer (PC) controls wavelength-tuning and camera acquisition.

Our investigations of camera dynamics revealed a nonlinear (quadratic) response to visible and C-band IR wavelengths (see Figure 2). For this characterization, we monitored the camera output value on multiple pixels as light intensity was decreased from high to low, and separately for laser light at 633nm and 1550nm. At C-band wavelengths, the camera sensitivity range was only 3dB, due to the inefficiency of the photon–electron conversion process in anti-Stokes imaging. The quadratic light response meant that the camera was more sensitive to light at higher intensities, a result of the anti-Stokes phosphor-coating which led to both nonuniform interference fringes and pixel blooming and blotching (see Figure 2).

Figure 2. Camera nonlinear response to light intensity at C-band and visible wavelengths (left), and interference fringes as they appeared on camera at C-band wavelengths (right).

We used an external cavity diode laser as the light source. This device allowed coarse tuning from 1521 to 1599nm, and fine tuning over approximately 0.5nm. By linearly tuning the laser wavelength, we expected a sinusoidal interferometric signal. However, fine wavelength-tuning was not linear: we observed fluctuations of laser linewidth, center wavelength, and output intensity over a 0.2nm tuning range. This meant that the interferometric signal generated by wavelength-tuning was distorted, which made retrieving the interferometric phase quite challenging. One way to overcome these distortions is to use an analytic signal processing technique to retrieve the interferometric phase.7,8 In analytic signal processing, the negative spectral component is suppressed or removed from a real-valued function (for example, a sinusoid), leaving only the positive spectral component to be operated on and manipulated.

We used an analytic signal phase retrieval algorithm9 that entailed building a window in the time-domain equal to 4 periods of the interferometric continuous-wave signal. We selected to use a Hanning window convolved with itself in time: the Hann2 window offered good extinction of negative frequency component (at least 2.815 × 10-5 at f=-fc) while tolerating some carrier frequency variation. The window was multiplied by a factor exp(j2pfct) to center its spectral passband at the positive frequency of the carrier fc. Convolving this window with the time history of each camera pixel yielded the analytic signal. The static phase difference between any two pixels was obtained by dividing the two analytic signals and taking the mean argument.

Despite wavelength-tuning and camera response nonlinearities, a reasonable continuous-wave signal and spectrum was obtained, as shown in Figure 3. The spectrum of the Hann2 window allowed some fluctuation of the carrier frequency around fc=1.75Hz. The distortions of the interferometric signal were attributed to the physical characteristics of the external cavity diode laser, which depend on ambient temperature, operating current, and central wavelength. As a proof-of-principle measurement, we tilted one mirror in the interferometer and recorded 100 images over four seconds during laser wavelength-tuning. This process was repeated for two other tilts of the mirror. The difference in static phase between the three mirror positions then provided an estimate of the mirror tilt (see Figure 4).

Figure 3. Normalized experimental interferometric continuous-wave signal and Hann2 window with spectra (dc component removed). The Hann2 window was centered on the positive component of the interferometric signal to obtain the analytic signal. Convolution in the time-domain is equivalent to multiplication in the frequency-domain.

Figure 4. Phase measurement of mirror tilting at three different inclinations. Tilt was measured as a static phase difference between two mirror positions and converted from phase (rad) to wavelength (nm).

To conclude, we investigated proof-of-principle full-field phase measurement at C-band wavelengths using wavelength-tuning interferometry. The technique used a 1555nm external cavity diode laser and an inexpensive phosphor-coated CCD camera sensitive to C-band radiation. The Michelson interferometer had no moving parts. In spite of wavelength-tuning nonlinearities, an analytic signal-phase-retrieval algorithm achieved accuracy of the retrieved phase within 7nm. However, camera nonlinearity and short response ranges represent drawbacks which may prove difficult to overcome in future applications.

Patrick Egan, 
Photonics Group (IHCP),
European Commission Joint Research Centre
Ispra, Varese, Italy
Optical Communications research Group,
University of Limerick
Castletroy, Limerick, Ireland
Fereydoun Lakestani, Maurice P Whelan
Photonics Group (IHCP),
European Commission Joint Research Centre
Ispra, Varese, Italy
Michael Connelly 
Optical Communications research Group,
University of Limerick
Castletroy, Limerick, Ireland

1. Jaydeep K. Sinha, Hareesh V. Tippur, Infrared interferometry for rough surface measurements: application to failure characterization and flaw detection, Opt. Eng. 36, no. 8, pp. 2223-2239, 1997. doi:10.1117/1.601447.
3. Tony L. Schmitz, Angela Davies, Chris J. Evans, Robert E. Parks, Silicon wafer thickness variation measurements using the National Institute of Standards and Technology infrared interferometer, Opt. Eng. 42, no. 8, pp. 2281-2290, 2003. doi:10.1117/1.1589757.
4. Ichirou Yamaguchi, Akihiro Yamamoto, Masaru Yano, Surface topography by wavelength scanning interferometry,
Opt. Eng.
 39, no. 1, pp. 40-46, 2000. doi:10.1117/1.602333.