A.

Principles

The Stationary-Wave Integrated Fourier Transform Spectrometer (SWIFTS) encapsulated a concept for on-chip spectro-detection described and demonstrated by several papers [1][2]. The detection is achieved along a standing wave that lies in a waveguide by probing the interferogram. Based on this concept a motionless device is able to probe the spectral information of the incoming signal. Several implementations have been investigated in several electromagnetic domains (Visible, Infrared and millimetric).

For all of these cases the basic principle is the same: a set of scattering dot/detectors extracts only a small fraction of the guided energy. The pitch of the detector set will match a convenient sampling of the standing wave, preferably more than 2 samples per fringe. The detectors must be smaller than a sixth of one fringe accounting the refraction index of the propagating material : d_s < λ_min.n_eff / 6

SWIFTS Lippmann

In a first mode named SWIFTS-Lippmann (see Fig. 1a,b), the electromagnetic radiation is coupled into a waveguide ended by a mirror. The standing waves form the interference pattern along the wave-guide, with its central black fringe locked onto the mirror. It provides a Lippmann interferogram starting by a dark fringe. This mode gives access to the spectra of the incoming light, like for a Fourier Transform Spectrometer.

Fig. 1.

SWIFTS principles Nanodetectors are placed in the evanescent field of the waveguide. Each detector samples a small part of the flux over a distance smaller than the fringe size. For polychromatic input light, the resulting superimposition of stationary waves lead on to a Fourier interferogram sampled by the set of nanodetectors. The lower 3D artist views present the corresponding SWIFTS devices for colored entrance light and the resulting interferograms.

Left: a) SWIFTS Lippmann principle. The forward propagating optical field coupled in the waveguide is reflected on the mirror at the waveguide end. An interference is made with returning field forming a stationary wave. b) The stationary waves are locked on the mirror leading to the so-called Lippmann interferogram starting by a black fringe.

Right: c) SWIFTS Gabor principle. The two coupled waves propagate in the waveguide in counter-propagative mode. d) The stationary waves are centered on the zero optical path difference location giving an access to the measurement of the interferogram phase.

B.

Performances

In first approximation, SWIFTS can be considered as a FTS, main characteristics are comparable to those of a Michelson interferometer. The mirror replaces the FTS beam splitter with a very good spectral coverage. The number of used detectors is minimal because only one side of the interferogram is sampled, the mirror defining itself the ideal null value for the dark fringe. The measurement stability only depends of temperature that can be achieved independently.

Spectral resolution

Because in SWIFTS the interferogram takes place in a material medium (waveguide) we have to take in account the effect of the index of refraction.

For instance in the case of a 1.7 cm long device, an apodized resolution of 89600 is achievable at 635 nm with a refractive index of n=1.5. This corresponds to 0.17cm^-1 spectral resolution

Spectral coverage (Shannon, Detection efficiency and transmittance of wave guide …)

As for the spectral resolution we have to take in account the index of refraction:

Where ΔL is the distance between two nano-detectors or scattering centers. Because we still using detectors with a pixel pitch larger than wavelength, the attainable spectral range is only 100cm^-1. To overpass this strong limitation and take benefits from matrix detectors for which we can set 256 parallel waveguides enlighten with the same source. The common mirror placed on the last surface of the substrate is tilted with a small angle in order to gentle shift sampling dot’s place in interferogram. This permits to have a complete Shannon sampling giving an access to the whole domain spectra accessible by detectors.

C.

Some recent developments

To simplify full demonstration of SWIFTS, we have choose to make a system using well known silver exchanged glass waveguides extended in visible domain. Some gold rods are placed at the glass’s surface by an e-beam written lift-off process. As it has been done in previous publication [1,2], we record images with a classical microscope. An delay line permits us to scan fringes between to successive 3μm dots position. The recovered spectra is shown in Fig 2.

Fig. 2.

Visible SWIFTS tests at 852nm : in the SWIFTS 400-1000 framework, we have developed special single mode waveguides on the 400nm to 1000nm domain. Upper : a enlargement of a laser illuminated waveguides covered showing 256 over 512 dots placed every 3μm in the evanescent field. Fringes appears like a Moiré figure. The measured 220 pm spectral resolution is compatible with predictions. A logarithmic representation used in spectra analyzers shows the quality of the recovered peak.

The SWIFTS 400-1000 project propose to glue the glass plate containing the 256 waveguides directly on visible CCD detector to build the spectrometer covering in one shot all the visible domain from 400 to 1000nm with a 0.2cm^-1 apodized spectral resolution. The light is coupled to system by a fiber bundle system permitting one to use a feeding 25μm diameter core fiber. The fig 3. shows a non-running mockup of the developed system.

Fig. 3.

SWIFTS 400-1000 mockup: A 1K frame transfer CCD is surrounded by a glass plate containing parallel waveguides and diffusing dots is fed by optical fiber bundle. A 0.2cm^-1 apodized spectral resolution is foreseen.

Weight and volume

The intrinsic volume and weight of SWIFTS based optical instrument can be 10⁶ times less than classical instrument. For instance an hybrid 2K×2K MCT Hawaii Teledyne detector cover by silicon array of waveguides is equivalent for spectral resolution to the 0.035cm^-1 MIPAS FTS aboard ENVISAT, Nevertheless, to properly estimate weight and volume we must include all feeding optics, cryogenic, electronic and computer associated to instrument which remain constant.

Field of view

As it is developed for SWIFTS 400-1000 project (see Fig 3.), we have considered SWIFTS built with a set parallel waveguides joined to a matrix fed by an image slicer. This permits one to adjust the right number of pixels to properly sample the dedicated spectral domain according to the generalized Shannon’s theorem. Each single mode waveguides intrinsically defines an elementary optical étendue of λ², total acceptance of the system is equal to the number of waveguides × λ², then the diameter of the entrance optics define the field of view. A 256 waveguide SWIFTS in visible spectral domain on a LEO orbit can observe a field of view of 20 meters at background with a 40 cm diameter telescope.

Radiometry

As a consequence of the wavelength depending small acceptance of SWIFTS, with today noisy infrared detectors, the sensitivity expressed in NESR remain greater than 2000 nW/cm²/sr/cm^-1 at 10 μm, and 100 times higher at 1μm. SWIFTS spectrometer cannot compete actually for ultimate detection on large acceptance angle necessary for detection of emission line in earth atmosphere. Inversely, the new generation of high dynamic APD detectors [6] should improve SWIFTS to reach this very demanding goal.

TRL

The SWIFTS concept has already proved in laboratory and published. The aim of the SWIFTS 400-1000 framework is build a running system with well known technologies for background use. The assembly tests should occur next months.

The following table summarizes this study.

Tab. 1.

Overview SWIFTS’s applications for space