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EP-4493887-B1 - CAT'S-EYE SWEPT SOURCE LASER FOR OCT AND SPECTROSCOPY

EP4493887B1EP 4493887 B1EP4493887 B1EP 4493887B1EP-4493887-B1

Inventors

  • ATIA, WALID A.
  • FLANDERS, DALE C.

Dates

Publication Date
20260506
Application Date
20230315

Claims (15)

  1. A tunable laser, comprising: a gain chip for amplifying light in a laser cavity; a collimating lens for collimating light from the gain chip; an end reflector of the laser cavity; a focusing lens for focusing the collimated light on the end reflector; a thin film bandpass filter between the collimating lens and the focusing lens; and at least one angle control actuator for changing the angle of the thin film filter to the collimated light, characterized in that the gain chip is a single angled facet edge-emitting chip with an anti-reflective coated front facet and a curved ridge waveguide that is perpendicular to a rear facet but is angled at an interface with the front facet.
  2. The tunable laser of claim 1, wherein the gain chip is a GaAlAs chip.
  3. The tunable laser of either of claims 1 or 2, wherein the gain chip is mounted in a TO-can hermetic package.
  4. The tunable laser of any of claims 1-3, wherein a pass band of the thin film bandpass filter is between 0.05 nanometers (nm) and 5 nm wide, full width at half maximum (FWHM), and preferably between 0.1 nm and 2 nm wide, FWHM.
  5. The tunable laser of any of claims 1-4, wherein the at least one angle control actuator is a galvanometer.
  6. The tunable laser of any of claims 1-5, wherein the at least one angle control actuator is a servomechanism.
  7. The tunable laser of any of claims 1-4, wherein the at least one angle control actuator is a motor that continuously spins the thin film bandpass filter.
  8. The tunable laser of any of claims 1-7, wherein the at least one angle control actuator tunes the thin film bandpass filter with a tuning speed between 3,000nm/sec and 11,000nm/sec.
  9. The tunable laser of any of claims 1-8, wherein the at least one angle control actuator tunes the thin film bandpass filter to have a tuning range of 70nm or more.
  10. The tunable laser of any of claims 1-9, wherein a diameter of the collimated light is greater than 1 millimeter (mm) and preferably greater than 2 mm.
  11. The tunable laser of any of claims 1-10, wherein a cone half angle of the collimated light is less than 0.04x0.02 degrees.
  12. The tunable laser of any of claims 1-11, wherein the thin film bandpass filter is oriented to receive an S polarization from the gain chip.
  13. The tunable laser of any of claims 1-12, wherein the angle control actuator tilts the thin film bandpass filter between the collimating lens and the focusing lens to achieve a tuning speed of between 3,000nm/sec and 11,000nm/sec.
  14. A control method for a tunable laser, comprising: amplifying light in a laser cavity with a gain chip; generating collimated light from the gain chip; focusing the collimated light on an end reflector; characterized in that the method further comprises the steps of: controlling an angle of a thin film bandpass filter in the collimated light with a servo mechanism; reducing reflections at the front facet and improving laser performance using a gain chip which is a single angled facet edge-emitting chip with an anti-reflective coated front facet and a curved ridge waveguide that is perpendicular to a rear facet but is angled at an interface with the front facet.
  15. The control method of claim 14 employing a laser as described in any of claims 1-13.

Description

RELATED APPLICATIONS This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 63/319,973, filed on March 15, 2022. BACKGROUND OF THE INVENTION Optical coherence tomography (OCT) is a cross-sectional, non-invasive imaging modality that is used in many areas of medical imaging. For example, in ophthalmology, OCT has been widely used for imaging the retina, choroid and anterior segment. Functional imaging of the blood velocity and vessel microvasculature is also possible. Fourier-domain OCT (FD-OCT) has recently attracted more attention because of its high sensitivity and imaging speed compared to time-domain OCT (TD-OCT), which uses an optical delay line for mechanical depth scanning with a relatively slow imaging speed. The spectral information discrimination in FD-OCT is accomplished either by using a dispersive spectrometer in the detection arm (spectral domain or SD-OCT) or rapidly scanning a swept laser source (swept-source OCT or SS-OCT). Compared to spectrometer-based FD-OCT, swept-source OCT (SS-OCT) has several advantages, including its robustness to motion artifacts and fringe washout, lower sensitivity roll-off and higher detection efficiency. Many different approaches have been implemented to develop high-speed swept sources for SS-OCT. One approach employs a semiconductor optical amplifier (SOA) based ring laser design (see for example Yun et al "High-speed optical frequency-domain imaging" Opt. Express 11:2953 2003 and Huber et al "Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s," Opt. Express 13, 3513 2005). Short cavity lasers (see for example Kuznetsov et al "Compact Ultrafast Reflective Fabry-Perot Tunable Lasers For OCT Imaging Applications," Proc. SPIE 7554:75541F 2010) are another example. SOA based ring laser designs have been practically limited to positive wavelength sweeps (increasing wavelength) because of the significant power loss that occurs in negative tuning. This has been attributed to four-wave mixing (FWM) in SOAs causing a negative frequency shift in intracavity light as it propagates through the SOA (Bilenca et al "Numerical study of wavelength-swept semiconductor ring lasers: the role of refractive-index nonlinearities in semiconductor optical amplifiers and implications for biomedical imaging applications," Opt. Lett. 31: 760-762 2006). A commercially available short cavity laser (Axsun Technologies Billerica, MA) in excess of 100 kHz has been reported (see for example Kuztietsov et al "Compact Ultrafast Reflective Fabry-Perot Tunable Lasers for OCT Imaging Applications," Proc. SPIE 7554: 75541F 2010). Short cavity lasers enable a significant increase in sweep speeds over conventional swept laser technology because the time needed to build up lasing from spontaneous emission noise to saturate the gain medium is greatly shortened (R. Huber et al "Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s," Opt. Express 13: 3513 2005). However, the effective duty cycle of the bidirectional sweeping short cavity laser was limited to less than 50% because of the FWM effects mentioned above. The effective repetition rate of the laser is thus limited. More recently, tunable vertical cavity surface emitting lasers (VCSELs) have been offered by Thorlabs and Axsun Technologies. The short cavities implicit in this technology enables even higher speed sweeping. Other methods have also been proposed to increase the effective repetition rates of SS-OCT systems including sweep buffering with a delay line, and multiplexing of multiple sources, thereby increasing the duty cycle of the laser. The method used to multiplex these sweeps together may include components that introduce orthogonal polarizations to the sweeps originating from different optical paths. Combining diverse polarizations at a polarization beamsplitter is a very light efficient way of transmitting the light to a single beam path. Goldberg et al. demonstrated a ping-pong laser configuration for high-speed SS-OCT system that achieves a doubling of the effective A-line rate by interleaving sweeps of orthogonal polarization in the same cavity (see Goldberg et al "200 kHz A-line rate swept-source optical coherence tomography with a novel laser configuration" Proceedings of SPIE v.7889 paper 55 2011). Potsaid et al. demonstrated another method to double the effective repetition rate of a swept source laser by buffering and multiplexing the sweep of a single laser source (see Potsaid et al "Ultrahigh speed 1050 nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second" Opt. Express 18: 20029-20048 2010). However, the long fiber spool will cause a significant birefringence to the laser output. Zhengbo et al.: "657nm narrow bandwidth interference filter-stabilized diode laser