Linearized Swept Laser Source for Optical Coherence Analysis System
A frequency swept laser source that generates an optical signal that is tuned over a spectral scan band at single discrete wavelengths associated with longitudinal modes of the swept laser source. Laser hopping over discrete single cavity modes allows long laser coherence length even under dynamic very high speed tuning conditions. A ramp drive to the laser is used to linearize laser frequency tuning. A beam splitter is used to divide the optical signal between a reference arm leading to a reference reflector and a sample arm leading to a sample. A detector system detects the optical signal from the reference arm and the sample arm for generating depth profiles and images of the sample.
1 . A swept laser source for an optical coherence analysis system comprising:
a laser cavity;
a semiconductor gain medium in the laser cavity; and
a tuning element in the laser cavity that is tuned over a spectral scan band;
a ramp generator for driving the tuning element with a ramp function substantially to linearize frequency tuning with time of an optical signal over the spectral scan band.
2 . An optical coherence analysis system as claimed in claim 1 , wherein a duty cycle of the linearized output is greater than 50%.
3 . An optical coherence analysis system as claimed in claim 1 , wherein a duty cycle of the linearized output is greater than 80%.
4 . An optical coherence analysis system as claimed in claim 1 , wherein the ramp generator drives the tuning element at greater than 10 kHz.
5 . An optical coherence analysis system as claimed in claim 1 , wherein the ramp generator drives the tuning element at greater than 30 kHz.
6 . An optical coherence analysis system as claimed in claim 1 , wherein the laser cavity is characterized by longitudinal modes and the swept source only generates an optical signal at single discrete wavelengths associated with the longitudinal modes.
7 . An optical coherence analysis system as claimed in claim 1 , wherein the tuning element comprises a tunable pass band that restricts the swept laser source to lasing at the single discrete wavelengths of the longitudinal modes.
8 . An optical coherence analysis system as claimed in claim 1 , wherein the tuning element comprises a Fabry-Perot tunable filter.
9 . An optical coherence analysis system as claimed in claim 1 , wherein the tuning element comprises a MEMS Fabry-Perot tunable filter.
10 . An optical coherence analysis system as claimed in claim 1 , wherein the laser cavity is defined by at least two reflectors.
11 . An optical coherence analysis system as claimed in claim 10 , wherein one of the reflectors is integral with the semiconductor gain medium.
12 . An optical coherence analysis system as claimed in claim 10 , wherein one of the reflectors is integral with tuning element.
13 . An optical coherence analysis system as claimed in claim 1 , wherein a bandwidth of the tuning element and spacing between longitudinal modes enables only individual ones of the longitudinal modes to lase as the optical signal is tuned over the spectral band.
14 . An optical coherence analysis system as claimed in claim 1 , wherein the optical signal is taken from the laser cavity through the tuning element.
15 . An optical coherence analysis system as claimed in claim 1 , wherein the optical signal is taken from the laser cavity through the semiconductor gain medium.
16 . An optical coherence analysis method, comprising:
tuning an optical signal over a scan band by a tuning element in a laser cavity with a ramp function to linearize frequency tuning of the optical signal over the spectral scan band;
dividing the optical signal between a reference arm leading to a reference reflector and a sample arm leading to a sample; and
detecting the optical signal from the reference arm and the sample arm.
17 . An optical coherence analysis method as claimed in claim 16 , wherein a duty cycle of the linearized output is greater than 50%.
18 . An optical coherence analysis method as claimed in claim 16 , wherein a duty cycle of a linearized output is greater than 80%.
19 . An optical coherence analysis method as claimed in claim 16 , further comprising driving the tuning element at greater than 10 kHz.
20 . An optical coherence analysis method as claimed in claim 16 , further comprising driving the tuning element at greater than 30 kHz.
21 . An optical coherence analysis method as claimed in claim 16 , wherein the laser cavity is characterized by longitudinal modes, and further comprising only generating the optical signal at single discrete wavelengths associated with the longitudinal modes.
22 . An optical coherence analysis method as claimed in claim 16 , further comprising selecting a bandwidth of the tuning elements and spacing between longitudinal modes to enable only individual ones of the longitudinal modes to lase as the optical signal is tuned over the spectral scan band.