Coherence switching of a degenerate VECSEL for
multimodality imaging
SEBASTIAN KNITTER,
1
CHANGGENG LIU,
2
BRANDON REDDING,
1
MUSTAFA K. KHOKHA,
3,5
MICHAEL A. CHOMA,
1,2,4,5
AND HUI CAO
1,
*
1
Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA
2
Department of Radiology and Biomedical imaging, Yale University, New Haven, Connecticut 06520, USA
3
Department of Genetics, Yale University, New Haven, Connecticut 06520, USA
4
Department of Biomedical Engineering, Yale University, New Haven, Connecticut 06520, USA
5
Department of Pediatrics, Yale University, New Haven, Connecticut 06520, USA
*Corresponding author: hui.cao@yale.edu
Received 4 January 2016; revised 13 February 2016; accepted 19 February 2016 (Doc. ID 256409); published 13 April 2016
Speckle formation is a limiting factor when using coherent
sources for imaging and sensing, but can provide useful infor-
mation about the motion of an object. Illumination sources
with tunable spatial coherence are therefore desirable as they
can offer both speckled and speckle-free images. Efficient
methods of coherence switching have been achieved with a
solid-state degenerate laser, and here we demonstrate a de-
generate laser system that can be switched between a large
number of mutually incoherent spatial modes and few-mode
operation. This technology enables multimodality imaging,
where low spatial coherence illumination is used for tradi-
tional high-speed videomicroscopy and high spatial coherence
illumination is used to extract dynamic information of flow
processes. Our implementation is based on a vertical external-
cavity surface-emitting laser (VECSEL) architecture. This
architecture uses a semiconductor gain module that is electri-
cally pumped, mechanically compact, and supports continuous-
wave emission. As an initial example, we perform dynamic
multimodality biomedical imaging in Xenopus embryo
(tadpole) hearts, an important animal model of human heart
disease.
© 2016 Optical Society of America
OCIS codes: (140.3580) Lasers, solid-state; (110.2945) Illumination de-
sign; (170.3880) Medical and biological imaging.
http://dx.doi.org/10.1364/OPTICA.3.000403
Traditional single-mode lasers are characterized by their bright-
ness, efficiency, and output directionality. These properties have
enabled tremendous advances in imaging and sensing. However,
to date, lasers have not been widely used as illumination sources
for full-field imaging and display applications. This limitation
exists because the high spatial coherence of existing lasers results
in coherent artifacts such as speckle. To avoid coherent artifacts,
sources with low spatial coherence typically are used. Unfortunately,
traditional low spatial coherence sources, such as incandescent
lamps and light emitting diodes (LEDs), are not sufficiently bright
for certain high-speed imaging or wide-area projection applications.
One approach to overcoming the brightness limitations of tradi-
tional low spatial coherence sources is to use bright single-mode
lasers in conjunction with various extracavity compounding
methods to synthesize low spatial coherence light [1]. Although
conceptually straightforward, these methods have not reached
large deployment in imaging, sensing, and display. Moreover, sev-
eral of these methods require multiple sequential image acquisi-
tions or integratio n over a longer exposure time, thereby limiting
their use in high-speed imaging. A new and promising approach
for laser-based illumination focuses on intracavity design as op-
posed to extracavity compounding. Specifically, this design-driven
approach develops laser cavities with large numbers of mutually
incoherent lasing modes (≈10
3
). Consequently, the emission out
of the cavity has low spatial coherence and thus mitigates or elim-
inates speckle artifacts. To this end, different types of laser have
been developed for speckle-free imaging, including dye-based ran-
dom lasers [2], chaotic microcavity semiconductor lasers [3], broad-
area vertical cavity surface emitting lasers (VCSEL) [4–6], VCSEL
arrays [7], and pulsed, solid-state degenerate (self-imaging cavity)
lasers [8].
In this Letter we report our development of a degenerate
laser with spatial coherence switching capability that is designed
around a semiconductor gain element. This laser overcomes bar-
riers presented by previous low spatial coherence lasers and dem-
onstrates the unique potential of spatial coherence tuning for
multimodal biomedical imaging. In terms of overcoming barriers,
the low spatial coherence, speckle-free laser is high power, con-
tinuous wave (CW), and electrically pumped. The semiconductor
laser has a straightforward self-imaging design that can be built in
a laboratory without any custom fabrication. The laser is thermo-
electrically cooled, so that the system can be kept compact with
low maintenance cost. The gain medium is an electrically pumped
vertical external-cavity surface-emitting laser (VECSEL) with a
large active area.
While VECSELs are known for high output power [9,10],
prior VECSELs used a cavity design that enabled single- or
few-mode operation. Our use of a degen erate laser cavity enables
Letter Vol. 3, No. 4 / April 2016 / Optica 403
2334-2536/16/040403-04 Journal © 2016 Optical Society of America