Method and apparatus for performing optical measurements using a rapidly frequency-tuned laser

• US 5,956,355 A
• Filed: 06/17/1997
• Issued: 09/21/1999
• Est. Priority Date: 04/29/1991
• Status: Expired due to Term

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Chinn, Stephen R., Swanson, Eric A.
• Primary Examiner(s): Bovernick, Rodney
• Assistant Examiner(s): ARROYO, TERESA M
• Application Number:
  US08/877,340
• Time in Patent Office: 826 Days
• Field of Search: 372/20, 372/32, 372/92, 372/97, 356/355, 356/357
• US Class Current: 372/20
• CPC Class Codes: A61B 1/00183 for variable viewing angles
• CPC Class Codes: A61B 2562/0242 for varying or adjusting the optical path length in the tissue
• CPC Class Codes: A61B 3/1005 for measuring distances inside the eye, e.g. thickness of the cornea A61B3/11 takes precedence
• CPC Class Codes: A61B 3/102 for optical coherence tomography [OCT]
• CPC Class Codes: A61B 5/0062 Arrangements for scanning
• CPC Class Codes: A61B 5/0064 Body surface scanning
• CPC Class Codes: A61B 5/0066 Optical coherence imaging
• CPC Class Codes: A61B 5/441 Skin evaluation, e.g. for skin disorder diagnosis
• CPC Class Codes: A61B 5/7225 Details of analog processing, e.g. isolation amplifier, gain or sensitivity adjustment, filtering, baseline or drift compensation A61B5/30, A61B5/316 take precedence; transformers for specific applications H01F38/00; amplifiers H03F
• CPC Class Codes: B82Y 35/00 Methods or apparatus for measurement or analysis of nanostructures
• CPC Class Codes: G01B 11/2441 using interferometry
• CPC Class Codes: G01B 9/02004 using frequency scans
• CPC Class Codes: G01B 9/02045 using the Doppler effect
• CPC Class Codes: G01B 9/02069 Synchronization of light source or manipulator and detector
• CPC Class Codes: G01B 9/02072 by calibration or testing of interferometer
• CPC Class Codes: G01B 9/02091 Tomographic interferometers, e.g. based on optical coherence
• CPC Class Codes: G01J 1/00 Photometry, e.g. photographic exposure meter spectrophotometry G01J3/00; specially adapted for radiation pyrometry G01J5/00 ; exposure meters built in cameras G03B17/06
• CPC Class Codes: G01N 21/4795 spatially resolved investigating of object in scattering medium in vivo A61B
• CPC Class Codes: G02B 27/4244 in wavelength selecting devices spectrometry G01J
• CPC Class Codes: G02B 5/1828 having means for producing variable diffraction controlling the direction of light by means of one or more diffracting elements G02B26/0808; acousto-optical elements G02F1/11, G02F1/33; electro- or magneto-optical diffraction G02F1/292, G02F1/2955
• CPC Class Codes: G11B 2007/0013 : for carriers having multiple discrete layers
• CPC Class Codes: H01S 3/08009 : using a diffraction grating
• CPC Class Codes: H01S 3/0812 : using a diffraction grating
• CPC Class Codes: H01S 3/083 : Ring lasers fibre ring lasers H01S3/06791
• CPC Class Codes: H01S 5/06255 : Controlling the frequency of the radiation
• CPC Class Codes: H01S 5/1215 : Multiplicity of periods
• CPC Class Codes: H01S 5/141 : using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence
• CPC Class Codes: H01S 5/143 : Littman-Metcalf configuration, e.g. laser - grating - mirror
• CPC Class Codes: H01S 5/146 : using a fiber as external cavity

Abstract

An optical system, in one embodiment including an external-cavity frequency-tuned laser having a tunable longitudinal cavity mode and a center tunable wavelength. The external-cavity frequency-tuned laser includes an optical cavity, an optical gain medium positioned within the optical cavity, and a rapid tuning wavelength selecting device positioned to receive light from the optical gain medium and adapted to return selected wavelengths of light to the optical gain medium. The external-cavity frequency-tuned laser is substantially continuously tunable and the tunable wavelength selecting device tunes the center wavelength of the external-cavity frequency-tuned laser at the same rate as the longitudinal cavity mode of the external-cavity frequency-tuned laser is tuned. In another embodiment the optical system includes an interferometer in optical communication with the external-cavity frequency-tuned laser. The interferometer is adapted to project a portion of light from the external-cavity frequency-tuned laser onto a sample, to receive light altered by the sample, and to combine light reflected from the sample and the light from the external-cavity frequency-tuned laser. A detector positioned to receive the combined light from the interferometer and to generate a signal in response thereto. A processing unit in communication with the detector performs digital signal processing to extract spatial information related to the sample in response to the signal from the detector.

Claims

1. An optical system comprising:

an external-cavity frequency-tuned laser having a tunable longitudinal cavity mode and a center timable wavelength, said external-cavity frequency-tuned laser comprising;

an optical cavity;

an optical gain medium positioned within said optical cavity; and

a rapid tuning wavelength selecting device positioned to receive light from said optical gain medium and adapted to return selected wavelengths of light to said optical gain medium;

an interferometer in optical communication with said external-cavity frequency-tuned laser, said interferometer adapted to project a portion of light from said external-cavity frequency-tuned laser onto a sample and to receive light altered by said sample, said interferometer adapted to combine light reflected from said sample and said light from said external-cavity frequency-tuned laser;

a detector positioned to receive said combined light from said interferometer and to generate a signal in response thereto; and

a processing unit in communication with said detector,

wherein said external-cavity frequency-tuned laser is substantially continuously tunable,

wherein said tunable wavelength selecting device tunes the center wavelength of the external-cavity frequency-tuned laser at the same rate as the longitudinal cavity mode of the external-cavity frequency-tuned laser is tuned, and

wherein said processing unit performs digital signal processing to extract spatial information related to the sample in response to said signal from said detector.

2. The system of claim 1 wherein said wavelength selecting device comprises a grating and an angular scanning device.

3. The system of claim 2 wherein said angular scanning device comprises an electro-optical beam deflector.

4. The system of claim 2 wherein said angular scanning device comprises a rotatable prism.

5. The system of claim 2 wherein said angular scanning device comprises a galvanometer scanned mirror.

6. The system of claim 2 wherein said angular scanning device comprises a resonant scanning mechanism.

7. The system of claim 2 wherein said angular scanning device scans so as to generate a temporally non-linear frequency sweep.

8. The system of claim 7 further comprising:

an auxiliary interferometer in optical communication with said external-cavity frequency-tuned laser, said auxiliary interferometer having an auxiliary detector in electrical communication with said processing unit,

wherein data collected in response to said temporally non-linear frequency sweep are corrected by said processing unit in response to signals from said detector of said auxiliary interferometer.

9. The system of claim 7 wherein said temporally non-linear frequency sweep is sinusoidal.

10. The system of claim 7 wherein data collected in response to said temporally non-linear frequency sweep are corrected by said processing unit in response to signals from said detector.

11. The system of claim 2, wherein said scanner includes a drive mechanism whose drive function is pre-distorted in time to precorrect for any nonlinerarity in the laser transfer function so as to achieve a nearly continuous frequency sweep in time.

12. The system of claim 1 wherein said wavelength selecting device comprises tunable optical filter.

13. The external-cavity laser of claim 12, wherein said wavelength selecting device comprises an AOTF.