Photonics Spectra - September 2009

Two Tools in One It’s Two, Two,

Technique enables simultaneous imaging and laser surgery without mechanical scanning.

BY LAURA S. MARSHALL MANAGING EDITOR

If a surgeon is about to use a probe in the brain, the patient is sure to want hat probe to be as small as possible. And three University of California, Los Angeles (UCLA), researchers have developed technology that could lead to tinier probes that can perform both endoscopic imaging and laser microsurgery at the same time.

The technique, called “spectrally encoded confocal microscopy and microsurgery” – SECOMM for short – uses light for both functions. And it’s the first of its kind that does not require mechanical scanning for either.

Mechanical scanning is useful because it enables acquisition of multidimensional images. But it has its downsides in endoscopy.

“Fluctuations in mechanical scanning introduce image noise and artifacts,” said recent PhD graduate Kevin Kin-Man Tsia, now an assistant professor in the department of electrical and electronic engineering at the University of Hong Kong. He added that endoscopic probes using micro-

electromechanical systems (MEMS) scanners are similarly limited. And MEMS scanners in miniaturized endoscopic probes require a large probe size, up to ;1 cm, which limits their usefulness in clinical settings.

But SECOMM doesn’t have that problem.

UCLA researchers Dr. Kevin Kin-Man Tsia, left, professor Bahram Jalali, center, and Dr. Keisuke Goda have developed an endoscope-compatible single-fiber-based device capable of simultaneous imaging and high-precision laser microsurgery.

With the SECOMM method, a fiber combiner takes a broadband light source (for imaging) and combines it with a wavelength-tunable continuous-wave laser followed by a fiber amplifier (for laser ablation). The 2-D spatial disperser generates a “spectral shower,” and the spatial information about the sample is encoded into the spectrum of the back-reflected spectral shower. The optical circulator routes the back-reflected spectral shower to the spectrometer. Images courtesy of Kevin Kin-Man Tsia.

How it works

“The heart of SECOMM,” Tsia said, “is an optical diffractive component: a two-dimensional spatial disperser which diffracts the different wavelengths of incident light into 2-D space, creating a 1:1 map between 2-D spatial coordinates and the optical wavelengths.”

He said the 2-D spatial disperser delivers broadband light for imaging and wavelength-tunable light for laser surgery. Two optical diffractive elements make this happen: a virtually imaged phase array (VIPA) and a diffraction grating. The figure below, left illustrates the SECOMM design.

“Both the broadband light and high-power tunable laser are coupled into the same single fiber and the same 2-D spatial disperser to perform simultaneous imaging and high-precision laser microsurgery,” Tsia said.

The disperser transforms an incident broadband light beam from a supercontin-uum pulse laser or an incoherent broadband light source into a 2-D spatial spectral pattern resembling a spectral shower, which is used to illuminate the sample.

The sample’s 2-D spatial information is encoded into the back-reflected “spectral shower,” which is transmitted back to the “nondispersed” – but image-encoded – beam by the same 2-D spatial disperser.

The single-mode fiber re-collects the beam, allowing transmission of 2-D images of the sample, and a spectrometer detects the image-encoded spectrum.

“Such imaging is essentially a confocal microscope,” Tsia noted, “as the aperture of the fiber that captures the reflection from the sample rejects the scattered light from out-of-focus axial planes.”