Optical coherence tomography system

Abstract

An optical coherence tomography system, includes a swept-source laser, a Mach-Zehnder interferometer and a balanced detector. The interferometer includes a first fiber coupler, a second fiber coupler, a sample arm and a reference arm. The reference arm includes a reference arm front section, a reference arm rear section and a delay line. A tail end of the reference arm front section is connected to the reference arm rear section through the delay line. The first fiber coupler is configured to split the output light of the swept-source into a sample light and a reference light and distribute the returned sample light to the second fiber coupler. A difference between the optical path length of a parasitic reflected signal of the delay line reaching the second fiber coupler and the optical path length of the sample light is greater than 8 times the cavity length of the swept-source laser.

Claims

1. An optical coherence tomography (OCT) system, comprising: a swept-source laser; a Mach-Zehnder interferometer; and a balanced detector, wherein the Mach-Zehnder interferometer comprises a first fiber coupler, a second fiber coupler, a sample arm and a reference arm; the reference arm comprises a reference arm front section, a reference arm rear section and a delay line; each of the first fiber coupler and the second fiber coupler comprises a first port, a second port, a third port and a fourth port; an output of the swept-source laser is connected to the first port of the first fiber coupler, the second port of the first fiber coupler is connected to the sample arm, the third port of the first fiber coupler is connected to the reference arm front section, and the fourth port of the first fiber coupler is connected to the first port of the second fiber coupler; a tail end of the reference arm front section is connected to the reference arm rear section through the delay line; the first fiber coupler is configured to split an output light of the swept-source laser into a sample light and a reference light and distribute the returned sample light to the second fiber coupler; a tail end of the reference arm rear section is connected to the second port of the second fiber coupler; the third port and the fourth port of the second fiber coupler are connected to the balanced detector; a resonant cavity length of the swept-source laser is greater than 35 mm; and a difference between an optical path length of a parasitic or a stray reflected signal of the delay line reaching the second fiber coupler and an optical path length of the sample light is greater than 8 times the resonant cavity length of the swept-source laser.

2. The OCT system of claim 1, wherein the delay line comprises fiber tips, collimators and a reflective optical component; the fiber tips comprise a fiber tip of the reference arm front section and a fiber tip of the reference arm rear section; the collimators comprise a transmitting collimator and a receiving collimator; the reference light enters the transmitting collimator through the reference arm front section and is reflected by the reflective optical component; the returned reference light passes to the reference arm rear section through the receiving collimator and reaches the second fiber coupler; and an optical path length of the reference arm front section is greater than an optical path length of the sample arm, with a difference between the two greater than 8 times the cavity length of the swept-source laser.

3. The OCT system of claim 1, wherein the delay line comprises fiber tips, collimators and a reflective optical component; the fiber tips comprise a fiber tip of the reference arm front section and a fiber tip of the reference arm rear section; the collimators comprise a transmitting collimator and a receiving collimator; the reference light enters the transmitting collimator through the reference arm front section and is reflected by the reflective optical component; the returned reference light passes to the reference arm rear section through the receiving collimator and reaches the second fiber coupler; a sum of an optical path length of the reference arm front section and a round-trip optical path length of the delay line is L; and L is less than an optical path length of the sample arm, with a difference between the two greater than 8 times the cavity length of the swept-source laser.

4. The OCT system of claim 1, wherein the delay line comprises a single-collimator a third fiber coupler, which are shared by the reference arm front section and the reference arm rear section; the third fiber coupler comprises a first port, a second port and a third port; the reference arm front section is connected to the first port of the third fiber coupler; the second port of the third fiber coupler is connected to the single-collimator; the reference light enters the single-collimator, reaches a reflective optical component, is reflected back along the same path, and passes to the third port of the third fiber coupler; the third port of the third fiber coupler is connected to the second port of the second fiber coupler; and an optical path length of the reference arm front section is greater than an optical path length of the sample arm, with a difference between the two greater than 8 times the cavity length of the swept-source laser.

5. The OCT system of claim 1, wherein the delay line comprises a single-collimator and a third fiber coupler, which are shared by the reference arm front section and the reference arm rear section; the third fiber coupler comprises a first port, a second port and a third port; the reference arm front section is connected to the first port of the third fiber coupler; the second port of the third fiber coupler is connected to the single-collimator; the reference light enters the single-collimator, reaches a reflective optical component, is reflected along the same path, and passes to the third port of the third fiber coupler; the third port of the third fiber coupler is connected to the second port of the second fiber coupler; a sum of an optical path length of the reference arm front section and a round-trip optical path length of the delay line is L, and L is less than an optical path length of the sample arm, with a difference between the two greater than 8 times the cavity length of the swept-source laser.

6. The OCT system of claim 2, wherein the reflective optical component is a corner reflector.

7. The OCT system of claim 3, wherein the reflective optical component is a corner reflector.

8. The OCT system of claim 4, wherein the reflective optical component is a corner reflector.

9. The OCT system of claim 5, wherein the reflective optical component is a corner reflector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram of an SD-OCT system.

(2) FIG. 2 is a schematic diagram of an SS-OCT system.

(3) FIG. 3 is a schematic diagram of the SS-OCT system based on a Mach-Zehnder interferometer.

(4) FIG. 4 is a schematic diagram of parasitic reflections of a delay line component.

(5) FIG. 5 is an example OCT image with coherence revival artifacts caused by the parasitic reflections of delay line components.

(6) FIG. 6 is a schematic diagram of an SS-OCT system of Embodiment I.

(7) FIG. 7 shows images of coherence revival artifacts of different orders.

(8) FIG. 8 shows the relative artifact signal intensity as a function of the order of coherence revival.

(9) FIG. 9 is a schematic diagram of an SS-OCT system of Embodiment II.

(10) FIG. 10 is a schematic diagram of an SS-OCT system of Embodiment III.

(11) FIG. 11 is a schematic diagram showing typical coupler splitting ratios and optical power values of Embodiment III.

DETAILED DESCRIPTION OF THE INVENTION

(12) The invention will be further described below with reference to the accompanying drawings and specific embodiments.

Embodiment I

(13) An optical coherence tomography (OCT) system includes a swept-source laser 1, a Mach-Zehnder interferometer, and a balanced detector 2. The interferometer includes a first fiber coupler 3, a second fiber coupler 4, a sample arm 5 and a reference arm. The reference arm includes a reference arm front section 61, a reference arm rear section 62 and a delay line 63. Each of the first fiber coupler 3 and the second fiber coupler 4 includes a first port, a second port, a third port and a fourth port; the output of the swept-source laser 1 is connected to the first port of the first fiber coupler 3. The second port of the first fiber coupler 3 is connected to the sample arm 5. The third port of the first fiber coupler 3 is connected to the reference arm front section 61. The fourth port of the first fiber coupler 3 is connected to the first port of the second fiber coupler 4. The tail end of the reference arm front section 61 is connected to the reference arm rear section 62 through the delay line 63. The first fiber coupler 3 is configured to split output light of the swept-source laser 1 into sample light and reference light and distribute the returned sample light to the second fiber coupler 4. The tail end of the reference arm rear section 62 is connected to the second port of the second fiber coupler 4. The third port and the fourth port of the second fiber coupler 4 are connected to the balanced detector 2. The laser cavity length of the swept-source laser is greater than 35 mm.

(14) The delay line 63 includes fiber tips, collimators and a reflective optical component 632 (a corner reflector in the present embodiment). The fiber tips include a fiber tip 611 of the reference arm front section and a fiber tip 621 of the reference arm rear section. The collimators include a transmitting collimator 631 and a receiving collimator 633. The reference light enters the incident collimator 631 through the reference arm front section 61 and is reflected by the reflective optical component 632, and the returned reference light passes to the reference arm rear section 62 through the reflective collimator 633 and reaches the second fiber coupler 4.

(15) In the present embodiment, by decreasing a fiber length of the reference arm rear section 62 and increasing the fiber length of the reference arm front section 61, a total optical path length of B.fwdarw.E.fwdarw.F.fwdarw.G.fwdarw.H remains unchanged. In this way, an optical path length OPL(B.fwdarw.E) of the reference arm front section will be much greater than the optical path length OPL(B.fwdarw.C.fwdarw.D) of the sample arm, with an optical path length difference L′ between them. L′ should be greater than N times the cavity length of the swept-source laser, and N is a positive integer. Experiments show that when N is greater than 8, the effect of coherence revival artifacts on the OCT image is negligible. Experimental results of a specific swept source are shown in FIG. 7 and FIG. 8. A “+” sign for the order in FIG. 7 indicates that the optical path length of the sample arm is greater than the optical path length of the reference arm, and a “−” sign for the order indicates that the optical path length of the sample arm is less than the optical path length of the reference arm. In the present embodiment, if the cavity length of the swept-source laser is 50 mm, the optical path length difference between the reference arm front section and the sample arm should be greater than 400 mm, and if the cavity length of the swept-source laser is 40 mm, the optical path length difference should be greater than 320 mm. That is, the following condition needs to be met: OPL(B.fwdarw.E)>OPL(B.fwdarw.C.fwdarw.D)+8×cavity length of swept-source laser.

Embodiment II

(16) Embodiment II is different from Embodiment I in that Embodiment II achieves the same effect of suppressing the coherence revival artifacts by increasing the fiber length of the reference arm rear section 62 and decreasing the fiber length of the reference arm front section 61. Other parts of the system are the same as those in Embodiment I.

(17) Let us assume that the sum of the optical path length of the reference arm front section 61 and the round-trip optical path length of the delay line 63 be L, then L should be less than the optical path length of the sample arm, with a difference greater than 8 times the cavity length of the swept-source laser. That is, the following condition needs to be met: OPL(B.fwdarw.E.fwdarw.F.fwdarw.G)<OPL(B.fwdarw.C.fwdarw.D)−8×cavity length of swept-source laser.

Embodiment III

(18) The delay line 63 in Embodiment III is different from that in Embodiment I and Embodiment II. In the delay line 63, a single-collimator 631 (which serves as both the transmitting collimator and the receiving collimator) and a third fiber coupler 7 are shared by the reference arm front section and the reference arm rear section. The third fiber coupler 7 includes a first port, a second port and a third port. The reference arm front section 61 is connected to the first port of the third fiber coupler 7. The second port of the third fiber coupler 7 is connected to the single-collimator. The reference light enters the single-collimator, reaches the reflective optical component 632 (a corner reflector in the present embodiment), is reflected back along the same path, and passes to the third port of the third fiber coupler 7. The third port of the third fiber coupler 7 is connected to the second port of the second fiber coupler 4. Other parts are the same as those in Embodiment I and Embodiment II.

(19) As shown in FIG. 10, the optical path length of the sample arm matches the optical path length of the reference arm, which meets the following condition: OPL(B.fwdarw.C.fwdarw.D.fwdarw.C.fwdarw.B.fwdarw.H)≈OPL(B.fwdarw.J.fwdarw.E.fwdarw.F.fwdarw.E.fwdarw.J.fwdarw.H).

(20) Parasitic reflected light from components in the delay line is coupled back to the interference system through the fiber tip 611 of the reference arm front section, and passes to the second fiber coupler 4 through the third fiber coupler 7 and the first fiber coupler 3 to interfere with the returned light of the reference arm. It should be particularly noted that, due to the existence of the third fiber coupler 7, coupling efficiency of the parasitic reflected light increases, and the coherence revival artifacts are more severe. The optical path length of the reference arm front section 61 should be greater than the optical path length of the sample arm 5, and the difference between them should be greater than 8 times the cavity length of the swept-source laser. That is, the following condition needs to be met to achieve the purpose of suppressing the coherence revival artifacts: OPL(B.fwdarw.J.fwdarw.E)>OPL(B.fwdarw.C.fwdarw.D)+8×cavity length of swept-source laser.

(21) Alternatively, let us assume that the sum of the optical path length of the reference arm front section 61 and the round-trip optical path length of the delay line 63 be L, then L should be less than the optical path length of the sample arm 5, with a difference greater than 8 times the cavity length of the swept-source laser, that is: OPL(B.fwdarw.J.fwdarw.E.fwdarw.F.fwdarw.E)<OPL(B.fwdarw.C.fwdarw.D)−8×cavity length of swept-source laser.

(22) Compared with Embodiment I and Embodiment II, in Embodiment III of the invention as shown in FIG. 10, the delay line 63 is simplified, since the single-collimator is more stable and easier to assemble. Moreover, the third fiber coupler 7 has an extra fourth port G, which can be used for other purposes, such as laser power monitoring.

(23) FIG. 11 shows typical coupler splitting ratios and optical power values of Embodiment III. For the purpose of illustration, the power calculation is simplified here, coupling losses in the system are ignored, and the reflection efficiency of the delay line is assumed to be 50%.

(24) In addition to the optical power values shown at different locations in FIG. 11, the power of the light reflected by the delay line back to the interferometer system (path J.fwdarw.B.fwdarw.H in FIG. 10) is 2.4×0.5×0.2×0.2=0.048 mW, which is approximately 5% of the reference arm optical power (0.96 mW). Although the light will only increase the background noise level by 0.2 dB, it has a much stronger intensity than the light signal reflected back from the sample arm. If the optical system design does not meet the optical path difference condition described above (coherence revival order N>8), strong coherence revival artifacts would arise.

(25) By selecting the following fiber lengths (FIG. 11), the order of the coherence revival artifacts can be easily increased beyond 20, which is much greater than the requirement of being higher than the 8th order mentioned above (the higher the order, the better the effect of suppressing the coherence revival artifacts). It is assumed here that the cavity length of the swept-source laser is 50 mm, and the optical path lengths of the optical couplers are ignored. In the calculation of OPL, the group refractive index of the optical fiber is about 1.47.

(26) TABLE-US-00001 Fiber or Air BJ JE EF BC CD BH HJ Length (mm) 2000 1000 150 1830 400 200 200

(27) Forward sample arm optical path length OPL(B.fwdarw.C.fwdarw.D)=1830×1.47+400≈3090 mm.

(28) Backward sample arm optical path length OPL(D.fwdarw.C.fwdarw.B.fwdarw.H)=400+(1830+200)×1.47≈3384 mm.

(29) Total sample arm optical path length OPL(B.fwdarw.C.fwdarw.D.fwdarw.C.fwdarw.B.fwdarw.H)≈3090+3384=6474 mm.

(30) Total reference arm optical path length OPL(B.fwdarw.J.fwdarw.E.fwdarw.F.fwdarw.E.fwdarw.J.fwdarw.H)=(2000+1000×2+200)×1.47+150×2=6474 mm.

(31) Shortest forward optical path length of stray light OPL(B.fwdarw.J.fwdarw.E)≈(2000+1000)×1.47≈4410 mm.

(32) OPL(B.fwdarw.J.fwdarw.E)−OPL(B.fwdarw.C.fwdarw.D)≈4410−3090=1320 mm≈26×cavity length of swept-source laser.

(33) The optical power reflected back to the swept source is 2.4×0.5×0.2×0.8=0.192 mW (not shown in FIG. 11), which is about 1.3% of the output power of the light source. The swept-source laser must tolerate at least 1.3% of optical back reflection.

(34) Although the above three embodiments have different designs, the purpose is the same: to create a difference between the optical path length of the parasitic or stray reflected light from the delay line and the optical path length of the sample light, the difference being greater than 8 times the cavity length of the swept-source laser, by increasing or decreasing the optical path length of the parasitic or stray reflected light, thereby achieving the purpose of suppressing the coherence revival artifacts.