阅读说明：本技术 双非对称光纤耦合器 (Double asymmetric optical fiber coupler ) 是由 尼古拉斯·戈德布特 卢卡斯·马约 于 2019-12-20 设计创作，主要内容包括：描述了一种光纤耦合器,其通常具有：第一光纤,该第一光纤具有纵向延伸的多模引导区域和在第一光纤的第一位置与第二位置之间纵向延伸的第一锥形部分,第一锥形部分具有沿着第一锥形方向从第一位置到第二位置逐渐减小的尺寸；第二光纤,该第二光纤具有纵向延伸的多模引导区域和在第二光纤的第三位置与第四位置之间纵向延伸的第二锥形部分,第二锥形部分具有沿着第二锥形方向从第三位置到第四位置逐渐减小的尺寸；以及耦合区域,在该耦合区域中,第一锥形部分的至少一部分光学耦合至第二锥形部分的至少一部分,且第一锥形方向与第二锥形方向彼此相反。(A fiber optic coupler is described, generally having: a first optical fiber having a longitudinally extending multimode guiding region and a first tapered portion extending longitudinally between a first location and a second location of the first optical fiber, the first tapered portion having a dimension that gradually decreases from the first location to the second location along a first taper direction; a second optical fiber having a longitudinally extending multimode guiding region and a second tapered portion extending longitudinally between a third location and a fourth location of the second optical fiber, the second tapered portion having a size that gradually decreases from the third location to the fourth location along a second tapered direction; and a coupling region in which at least a portion of the first tapered portion is optically coupled to at least a portion of the second tapered portion, and the first and second taper directions are opposite to each other.)

1. An optical fiber coupler comprising:

a first optical fiber having a longitudinally extending multimode guiding region and a first tapered portion extending longitudinally between a first location and a second location of the first optical fiber, the first tapered portion having a dimension that gradually decreases along a first taper direction from the first location to the second location;

a second optical fiber having a multimode guiding region extending longitudinally and a second tapered portion extending longitudinally between a third location and a fourth location of the second optical fiber, the second tapered portion having a size that gradually decreases along a second tapered direction from the third location to the fourth location; and

a coupling region in which at least a portion of the first tapered portion is optically coupled to at least a portion of the second tapered portion, and the first taper direction is opposite the second taper direction.

2. The fiber coupler of claim 1, wherein the first optical fiber is a multi-clad fiber having a core surrounded by at least one inner cladding, the at least one inner cladding of the first optical fiber serving as the longitudinally extending multimode guiding region of the first optical fiber.

3. The fiber coupler of claim 2, wherein the second optical fiber is a multi-clad fiber having a core surrounded by at least one inner cladding, the at least one inner cladding of the second optical fiber serving as the longitudinally extending multimode guiding region of the second optical fiber.

4. The fiber coupler of claim 2, wherein the second optical fiber is a multimode fiber having a multimode core, the multimode core of the second optical fiber serving as the longitudinally extending multimode guiding region of the second optical fiber.

5. The fiber coupler of claim 1, wherein the first optical fiber is a multimode fiber having a multimode core, the multimode core of the first optical fiber serving as the longitudinally extending multimode guiding region of the first optical fiber.

6. The fiber coupler of claim 5, wherein the second optical fiber is a multi-clad fiber having a core surrounded by at least one inner cladding, the at least one inner cladding of the second optical fiber serving as the longitudinally extending multimode guiding region of the second optical fiber.

7. The fiber coupler of claim 5, wherein the second optical fiber is a multimode fiber having a multimode core, the multimode core of the second optical fiber serving as the longitudinally extending multimode guiding region of the second optical fiber.

8. The fiber coupler of claim 1, further comprising a coupling direction extending from either of the first and second optical fibers to the other of the first and second optical fibers through the coupling region, wherein a cross-section of the coupling region has an etendue that is maintained at least along the coupling direction, the etendue defined by an equation equivalent to:

G_i＝πS_i(WA_i)²，

wherein G is_iRepresents the etendue, S, of the coupling region at a longitudinal position i along the coupling direction_iA surface representing a cross-section of the coupling region at the longitudinal position i, and NA_iRepresenting the numerical aperture of the coupling region at the longitudinal position i.

9. The fiber optic coupler of claim 8, wherein the etendue increases along the coupling direction.

10. The fiber optic coupler of claim 1, wherein at least a portion of the first tapered portion is thermally fused to at least a portion of the second tapered portion.

11. The fiber optic coupler of claim 1, wherein:

the coupling region defines a multimode injection direction extending from the third location of the second optical fiber to the first location of the first optical fiber, and a multimode extraction direction extending from the first location of the first optical fiber to the third location of the second optical fiber; and is

The coupling region is configured to allow propagation of a multimode signal having substantially fully satisfied two transmit conditions:

along the multimode injection direction at a given multimode injection rate; and

along the multi-mode extraction direction at a given multi-mode extraction rate.

12. The fiber coupler of claim 11, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 100%.

13. The fiber coupler of claim 11, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 110%.

14. The fiber coupler of claim 11, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 120%.

15. The fiber coupler of claim 11, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 150%.

16. The fiber coupler of claim 11, wherein the given multimode injection rate and the given multimode extraction rate are both higher than 50%.

17. The fiber coupler of claim 11, wherein the given multimode injection rate and the given multimode extraction rate are both higher than 60%.

18. The fiber coupler of claim 11, wherein the given multimode injection rate and the given multimode extraction rate are both higher than 70%.

19. The fiber coupler of claim 11, wherein the coupling region is configured to block the given multimode injection rate to facilitate the given multimode extraction rate.

20. An optical fiber coupler comprising:

a first optical fiber having a longitudinally extending multimode guiding region and a first fiber portion extending longitudinally between a first location and a second location of the first optical fiber;

a second optical fiber having a longitudinally extending multimode guiding region and a second fiber portion extending longitudinally between a third location and a fourth location of the second optical fiber; and

a coupling region in which at least a portion of the first fiber portion is optically coupled to at least a portion of the second fiber portion, thereby defining a multimode injection direction extending from the third location of the second fiber to the first location of the first fiber, and a multimode extraction direction extending from the first location of the first fiber to the third location of the second fiber;

wherein the coupling region is configured to allow propagation of a multimode signal having substantially fully satisfied two transmit conditions:

along the multimode injection direction at a given multimode injection rate; and

along the multi-mode extraction direction at a given multi-mode extraction rate.

21. The fiber coupler of claim 20, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 100%.

22. The fiber coupler of claim 20, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 110%.

23. The fiber coupler of claim 20, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 120%.

24. The fiber coupler of claim 20, wherein the sum of the given multimode injection rate and the given multimode extraction rate is higher than 150%.

25. The fiber coupler of claim 20, wherein the given multimode injection rate and the given multimode extraction rate are both higher than 50%.

26. The fiber coupler of claim 20, wherein the given multimode injection rate and the given multimode extraction rate are both higher than 60%.

27. The fiber coupler of claim 20, wherein the given multimode injection rate and the given multimode extraction rate are both higher than 70%.

28. The fiber coupler of claim 20, wherein the coupling region is configured to block the given multimode injection rate to facilitate the given multimode extraction rate.

Technical Field

The improvements relate generally to fiber optic couplers and, more particularly, to fiber optic couplers for the extraction and/or injection of multimode optical signals.

Background

Fiber optic couplers are used in many types of imaging and/or sensing systems. Such couplers are typically manufactured by thermally fusing and/or tapering a portion of a first optical fiber to a portion of a second optical fiber such that the guided light interacts in their respective guiding regions, leaving four different ports.

One example of a symmetric fiber coupler having two double-clad fibers fused together and tapered is described in U.S. patent No. 8792757B2 to Boudoux et al. The fiber optic coupler has a first port at which a coherent illumination signal can be injected and a second port from which the coherent illumination signal is output, the second port also collecting an output optical signal that includes a coherent single-mode component and a diffuse multi-mode component. The first and second fibers have the same dimensions and/or configuration, and thus the fiber coupler is said to be symmetrical. Such symmetric fiber couplers are known to have theoretical averaging limits that limit the amount of multimode components that can be extracted from the second port to the third port or injected from the third port to the second port to 50% of the incoming multimode signal. In fact, even reaching near or above 43% has proven challenging.

One example of an asymmetric fiber coupler intended to enhance such multimode signal extraction and/or injection rates is described in U.S. patent No. 9753222B2 to Godbout et al. More specifically, the fiber coupler has a first double-clad fiber fused to a second fiber of a different size and/or configuration in a manner that renders the theoretical averaging limits discussed with reference to Boudoux et al no longer applicable. In this way, fiber couplers with multimode extraction and/or injection rates much higher than 50% can be obtained.

While existing fiber optic couplers have proven satisfactory to some extent, there is room for improvement.

Disclosure of Invention

Etendue is defined as the product of the surface of the cross-section of an optical fiber and the square (or equivalently, the solid angle) of its numerical aperture. Etendue conservation is a physical principle applicable to a linear optical system, and it indicates that, when an optical signal propagates through a linear optical system, etendue cannot be reduced without causing optical loss of the optical signal. Since the fiber couplers discussed above are linear optical systems, they are subject to etendue conservation.

With respect to the fiber coupler described by Boudoux et al, a multimode optical signal propagating from the second port to the third port will experience a constant amount of optical expansion along the region where the first and second fibers fuse to each other. More specifically, the etendue at the entrance position of the fusion zone on the first optical fiber will be similar to the etendue at the exit position of the fusion zone on the second optical fiber, since both the first and second optical fibers have the same size and/or structure. Thus, while constrained by the theoretical averaging limits discussed above, it has been found that multimode extraction and/or injection can occur with negligible optical losses using such a fiber coupler.

Although greater multimode extraction and/or transmission rates may be achieved using the fiber coupler described by Godbout et al, the conservation of etendue provided by such a fiber coupler may not be satisfactory. For example, the etendue at the entrance position of a fusion zone on a first optical fiber may be greater than the etendue at the exit position of a fusion zone on a second optical fiber because the first optical fiber and the second optical fiber actually have different sizes and/or structures. In this case, the optical loss may be large.

The inventors have discovered a way to provide a fiber coupler that can help achieve multimode extraction and/or injection rates above the theoretical averaging limit while still achieving satisfactory optical losses.

In one aspect, a fiber optic coupler is described having a first optical fiber with a first tapered portion that tapers in size along a first taper direction; a second optical fiber having a second tapered portion of which the size is gradually reduced along a second tapered direction; and a coupling region in which at least a portion of the first tapered portion of the first optical fiber is optically coupled to the second tapered portion of the second optical fiber. When the first tapering direction and the second tapering direction are opposite to each other, it is expected that the overall shape factor remains slightly constant along the coupling region under given parameters of the first and second optical fibers and the first and second tapering portions, which in turn contributes to the maintenance of etendue. However, as will be appreciated from the embodiments described in this disclosure, the form factor need not be maintained along a given coupling direction but still contributes to the maintenance of etendue.

According to a first aspect of the present disclosure, there is provided an optical fiber coupler comprising: a first optical fiber having a longitudinally extending multimode guiding region and a first tapered portion extending longitudinally between a first location and a second location of the first optical fiber, the first tapered portion having a dimension that gradually decreases from the first location to the second location along a first taper direction; a second optical fiber having a longitudinally extending multimode guiding region and a second tapered portion extending longitudinally between a third location and a fourth location of the second optical fiber, the second tapered portion having a size that gradually decreases from the third location to the fourth location along a second tapered direction; and a coupling region in which at least a portion of the first tapered portion is optically coupled to at least a portion of the second tapered portion, and the first taper direction is opposite to the second taper direction.

Further in accordance with the first aspect of the present disclosure, the first optical fiber may for example be a multi-clad optical fiber having a core surrounded by at least one inner cladding, in which case the at least one inner cladding of the first optical fiber may for example serve as a longitudinally extending multimode guiding region of the first optical fiber.

Still further in accordance with the first aspect of the present disclosure, the second optical fiber may for example be a multi-clad optical fiber having a core surrounded by at least one inner cladding, in which case the at least one inner cladding of the second optical fiber may for example serve as a longitudinally extending multimode guiding region of the second optical fiber.

Still further in accordance with the first aspect of the present disclosure, the second optical fiber may for example be a multimode optical fiber having a multimode core, in which case the multimode core of the second optical fiber may for example serve as a longitudinally extending multimode guiding region of the second optical fiber.

Still further in accordance with the first aspect of the present disclosure, the first optical fiber may for example be a multimode optical fiber having a multimode core, in which case the multimode core of the first optical fiber may for example serve as a longitudinally extending multimode guiding region of the first optical fiber.

Still further, in accordance with the first aspect of the present disclosure, the fiber coupler may, for example, have a coupling direction extending from either one of the first and second optical fibers to the other one of the first and second optical fibers through the coupling region, wherein a cross-section of the coupling region has an etendue maintained at least along the coupling direction, the etendue being defined by an equation equivalent to the following equation: g_i＝πS_i(NA_i)²Wherein G is_iDenotes the etendue, S, of the coupling region at a longitudinal position i along the coupling direction_iSurface representing the cross-section of the coupling region at longitudinal position i, and NA_iRepresenting the numerical aperture of the coupling region at longitudinal position i.

Still further, according to the first aspect of the present disclosure, the etendue may be increased, for example, along the coupling direction.

Still further in accordance with the first aspect of the present disclosure, at least a portion of the first tapered portion may be, for example, heat fused to at least a portion of the second tapered portion.

Still further in accordance with the first aspect of the present disclosure, the coupling region may, for example, define a multimode injection direction extending from the third location of the second optical fiber to the first location of the first optical fiber, and a multimode extraction direction extending from the first location of the first optical fiber to the third location of the second optical fiber; and, the coupling region is configured to allow propagation of a multimode signal having substantially fully satisfied two transmit conditions: along a multimode injection direction at a given multimode injection rate; and along the multi-mode extraction direction at a given multi-mode extraction rate.

Still further, according to the first aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 100%.

Still further, according to the first aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be higher than 110%, for example.

Still further, according to the first aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be higher than 120%, for example.

Still further, according to the first aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 150%.

Still further, according to the first aspect of the present disclosure, both the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 50%.

Still further, according to the first aspect of the present disclosure, both the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 60%.

Still further, according to the first aspect of the present disclosure, both the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 70%.

Still further in accordance with the first aspect of the present disclosure, the coupling region may, for example, be configured to block a given multimode injection rate to facilitate a given multimode extraction rate.

According to a second aspect of the present disclosure, there is provided an optical fiber coupler comprising: a first optical fiber having a longitudinally extending multimode guiding region and a first fiber portion extending longitudinally between a first location and a second location of the first optical fiber; a second optical fiber having a longitudinally extending multimode guiding region and a second fiber portion extending longitudinally between a third location and a fourth location of the second optical fiber; and a coupling region in which at least a portion of the first optical fiber portion is optically coupled to at least a portion of the second optical fiber portion, thereby defining a multimode injection direction extending from the third location of the second optical fiber to the first location of the first optical fiber, and a multimode extraction direction extending from the first location of the first optical fiber to the third location of the second optical fiber; wherein the coupling region is configured to allow propagation of a multimode signal having substantially fully satisfied two transmit conditions: along a multimode injection direction at a given multimode injection rate; and along the multi-mode extraction direction at a given multi-mode extraction rate.

Further, according to the second aspect of the present disclosure, the sum of the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 100%.

Still further, according to the second aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be higher than 110%, for example.

Still further, according to the second aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be higher than 120%, for example.

Still further, according to the second aspect of the present disclosure, a sum of the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 150%.

Still further, according to the second aspect of the present disclosure, both the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 50%.

Still further, according to the second aspect of the present disclosure, both the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 60%.

Still further, according to the second aspect of the present disclosure, both the given multimode injection rate and the given multimode extraction rate may be, for example, higher than 70%.

Still further, in accordance with the second aspect of the present disclosure, the coupling region may be configured to block a given multimode injection rate, for example, to facilitate a given multimode extraction rate.

The term "tapered portion" is intended to be broadly construed to include any portion of an optical fiber that gradually decreases in size along a given direction of the fiber. Examples of reduced dimensions may include, but are not limited to, the cross-section of the optical fiber, the width of the optical fiber, the thickness of the optical fiber, and the like. Accordingly, in the present disclosure, each tapered portion has a direction defined as a direction along which the size gradually decreases.

It has been found convenient in this disclosure to introduce the expressions "descending taper portion" and "ascending taper portion" to refer to some particular taper portion. However, it is understood that a portion that is referred to as a descending taper from one perspective may also be referred to as an ascending taper from a different perspective. However, in the present disclosure, a tapered portion will be referred to as a descending tapered portion or an ascending tapered portion depending on whether the size of the tapered portion decreases or increases in a direction extending from left to right. For example, a tapered portion that decreases in size from left to right will be referred to as a descending tapered portion, while a tapered portion that increases in size from left to right will be referred to as an ascending tapered portion. Such terms should, of course, be construed broadly depending upon the context in which they are used.

Many other features and combinations of features relating to the present improvements will be apparent to those skilled in the art upon reading the present disclosure.

Drawings

In the drawings:

FIG. 1 is an example of a fiber optic coupler in accordance with one or more embodiments;

FIG. 2A is a schematic illustration of a first multimode optical fiber and a second multimode optical fiber parallel to each other, wherein an ascending tapered portion of the first multimode optical fiber is longitudinally aligned with a descending tapered portion of the second multimode optical fiber, in accordance with one or more embodiments;

FIG. 2B is a schematic diagram of an example of a fiber coupler having the first multimode fiber and the second multimode fiber of FIG. 2A, where the rising taper portion of the first multimode fiber is optically coupled to the falling taper of the second multimode fiber, in accordance with one or more embodiments;

FIG. 3A is a schematic illustration of a double-clad optical fiber and a multimode optical fiber parallel to each other with an ascending tapered portion of the double-clad optical fiber longitudinally aligned with a descending tapered portion of the multimode optical fiber in accordance with one or more embodiments;

FIG. 3B is a schematic diagram of an example of a fiber coupler having the double-clad fiber of FIG. 3A and a multimode fiber, where an ascending tapered portion of the double-clad fiber is optically coupled to a descending tapered portion of the multimode fiber, in accordance with one or more embodiments;

FIG. 4A is a schematic illustration of a double-clad optical fiber and a multimode optical fiber parallel to each other, wherein an ascending tapered portion of the double-clad optical fiber is longitudinally aligned with a descending tapered portion of the multimode optical fiber, the multimode optical fiber having a taper ratio greater than that of the double-clad optical fiber, in accordance with one or more embodiments;

FIG. 4B is a schematic diagram of an example of a fiber coupler having the double-clad fiber of FIG. 4A and a multimode fiber, where an ascending tapered portion of the double-clad fiber is optically coupled to a descending tapered portion of the multimode fiber, in accordance with one or more embodiments;

FIG. 5A is a schematic illustration of a double-clad optical fiber and a multimode optical fiber parallel to each other, wherein the rising taper of the double-clad optical fiber is longitudinally aligned with the falling taper portion of the multimode optical fiber, the multimode optical fiber having a multimode core with a size larger than the size of the inner cladding of the double-clad optical fiber, in accordance with one or more embodiments;

FIG. 5B is a schematic diagram of an example of a fiber coupler having the double-clad fiber of FIG. 5A and a multimode fiber, where an ascending tapered portion of the double-clad fiber is optically coupled to a descending tapered portion of the multimode fiber, in accordance with one or more embodiments;

FIG. 6A is a schematic illustration of a double-clad optical fiber and a multimode optical fiber parallel to each other, wherein the rising taper portion of the double-clad optical fiber is longitudinally aligned with the falling taper portion of the multimode optical fiber and the rising taper portion of the double-clad optical fiber is longer than the falling taper portion of the multimode optical fiber, in accordance with one or more embodiments;

FIG. 6B is a schematic diagram of an example of a fiber coupler having the double-clad fiber of FIG. 6A and a multimode fiber, where an ascending tapered portion of the double-clad fiber is optically coupled to a descending tapered portion of the multimode fiber, in accordance with one or more embodiments;

FIG. 7A is a schematic illustration of a first multimode optical fiber and a second multimode optical fiber parallel to each other illustrating a stripped region along which outer cladding layers of the first multimode optical fiber and the second multimode optical fiber are removed in accordance with one or more embodiments;

FIG. 7B is a schematic diagram of an example of a fiber coupler having the first multimode fiber and the second multimode fiber of FIG. 7A in which the rising taper portion of the first multimode fiber is optically coupled to the falling taper of the second multimode fiber in accordance with one or more embodiments;

FIG. 8 is a schematic diagram of an example of a clinical system incorporating a laser marking system and an optical coherence tomography monitoring system in which the fiber coupler of FIG. 4A is incorporated in accordance with one or more embodiments;

FIG. 9 is a schematic diagram of an example of a combined spectroscopy and optical coherence tomography system in which the fiber coupler of FIG. 3B is incorporated in accordance with one or more embodiments; and

FIG. 10 is a schematic diagram of an example of a spectral optical coherence tomography system in accordance with one or more embodiments incorporating the fiber coupler of FIG. 3B, showing a broadband multimode circulator.

Detailed Description

Fig. 1 shows an example of a fiber coupler 100, the fiber coupler 100 having a first optical fiber 102 extending between a first port 102a and a second port 102b, a second optical fiber 104 extending between a third port 104a and a fourth port 104b, and a coupling region 106, wherein at least a portion of the first optical fiber 102 is optically coupled to at least a portion of the second optical fiber 104.

More specifically, the first optical fiber 102 has a first tapered portion 108 that extends longitudinally between a first location 108a and a second location 108b of the first optical fiber 102. The first tapered portion 108 has a dimension 110 that decreases gradually from a first location 108a to a second location 108b along a first tapered direction 112.

The second optical fiber 104 has a second tapered portion 114 extending longitudinally between a third location 114a and a fourth location 114b of the second optical fiber 104. The second conical portion 114 has a dimension 116 that gradually decreases from the third location 114a to the fourth location 114b along a second conical direction 118.

As will be described below with reference to the figures, the coupling region 106 optically couples at least a portion of the first tapered portion 108 to at least a portion of the second tapered portion 114, where the first taper direction 112 is opposite the second taper direction 118.

In this example, the first tapered portion 108 of the first optical fiber 102 is an ascending taper and the second tapered portion 114 of the second optical fiber 104 is a descending taper, and will be referred to as the ascending taper 108 and the descending taper 114 in subsequent paragraphs.

However, as will be appreciated, in some other embodiments, the first tapered portion 108 may be a descending tapered portion and the second tapered portion 114 may be an ascending tapered portion.

As such, the coupling region 106 of the fiber coupler 100 has at least a portion of a descending taper optically coupled to at least a portion of an ascending taper portion, the at least a portion of the descending taper belonging to either of the first and second optical fibers 102, 104 and the at least a portion of the ascending taper portion belonging to the other of the first and second optical fibers 102, 104.

It is understood that depending on the embodiment, the coupling region 106 may have a length 120 that is longer, equal, or shorter than the length of the ascending 108 and descending 114 tapered portions.

For example, in some embodiments, only a portion of the rising taper portion 108 and the falling taper portion 114 are optically coupled to each other such that the length of the coupling region 106 is less than the length of the rising taper portion 108 and the falling taper portion 114.

In alternative embodiments, coupling region 106 extends beyond one or both ends of ascending tapered portion 108 and descending tapered portion 114, which may cause the length of coupling region 106 to be longer than the length of ascending tapered portion 108 and descending tapered portion 114.

It is understood that depending on the embodiment, the ascending conical portion 108 and the descending conical portion 114 may be longer, equal, or shorter than each other.

Further, depending on the embodiment, the initial diameters of the ascending and descending tapered portions 108, 114 and the final diameters of the ascending and descending tapered portions 108, 114 may be larger, equal, or smaller than each other.

Those skilled in the art will also appreciate that the locations of the ascending cone portion 108 and the descending cone portion 114 may be offset, e.g., only a portion of the ascending cone may contact a portion or all of the descending cone, or vice versa.

In some embodiments, the ascending taper portion and/or the descending taper portion may each have a series of tapers, resulting in a taper portion having one or more flat tops and/or regions having hills and valleys.

In this example, the optical coupling 106 includes intimate contact between the rising tapered portion 108 of the first optical fiber 102 and the falling tapered portion 114 of the second optical fiber 104. However, in some other embodiments, the optical coupling 106 is obtained by thermally fusing and/or tapering the rising tapered section 108 of the first optical fiber 102 to the falling tapered section 114 of the second optical fiber 104.

It should be noted that the first optical fiber 102 and the second optical fiber 104 are each configured to propagate at least a multimode optical signal to allow multimode transmission therebetween. Accordingly, the first optical fiber 102 and the second optical fiber 104 each have a longitudinally extending multimode guiding region 132 surrounded by an outer cladding 142. In some embodiments, the outer cladding 142 may be partially or completely removed from one or both of the first and second optical fibers 102, 104. Depending on the embodiment, such overcladding removal may be performed before or after tapering.

For example, in some embodiments, the first optical fiber 102 is a multi-clad optical fiber having a core surrounded by at least one inner cladding. In these embodiments, the inner cladding of the first optical fiber 102 serves as the longitudinally extending multimode guiding region 132 of the first optical fiber 102. In some other embodiments, the first optical fiber 102 is a multimode optical fiber having a multimode core. In these latter embodiments, the multimode core of the first optical fiber 102 serves as a longitudinally extending multimode guiding region 132 of the first optical fiber 102.

As can the second optical fiber 104. Similarly, in some embodiments, the second optical fiber 104 is a multi-clad optical fiber having a core surrounded by at least one inner cladding. In these embodiments, the inner cladding of the second optical fiber 104 serves as the longitudinally extending multimode guiding region 132 of the first optical fiber 104. In some other embodiments, the second optical fiber 104 is a multimode optical fiber having a multimode core. In these latter embodiments, the multimode core of the second optical fiber 104 serves as a longitudinally extending multimode guiding region 132 of the second optical fiber 104.

Note that the multi-mode transmission ratio may be defined as the ratio of the strength of the multi-mode signal transmitted from the first port 102a to the fourth port 104b (or vice versa) or transmitted from the second port 102b to the third port 104a (or vice versa) to the initial strength of the multi-mode signal. The multimode transmission generally comprises a multimode extraction by which a multimode signal is extracted from the second port 102b to the third port 104a (or, equivalently, from the first position 108a to the third position 114a) along an extraction direction 122, and a multimode injection by which a multimode signal is injected from the third port 104a to the second port 102b (or, equivalently, from the third position 114a to the first position 108a) along an injection direction 124.

In this example, coupling directions, such as an extraction direction 122 and an injection direction 124, may be defined to extend from either of the first and second optical fibers 102, 104 to the other of the first and second optical fibers 102, 104 through the coupling region 106. As such, it has been found convenient to design fiber optic coupler 100 such that coupling region 106 has a cross-section having an etendue that is maintained along at least one or more of the possible coupling directions.

The etendue is defined by an equation equivalent to:

G_i＝πS_i(NA_i)²，

wherein G is_iRepresents the etendue, S, of the coupling region 106 at a longitudinal position i along the coupling direction_iRepresents the surface of the coupling region 106 in cross-section at the longitudinal position i, and NA_iIndicating the coupling region106 at longitudinal position i.

For example, in one particular embodiment, the etendue G of the coupling region 106 at a first longitudinal position along the direction of the extraction direction 122₁Similar to etendue G of coupling region 106 along extraction direction 122 at a second longitudinal position spaced apart from the first longitudinal position₂。

In this way, as discussed above, the optical loss of a multimode optical signal that would experience propagation from the first optical fiber 102 to the second optical fiber 104 along the extraction direction 122 would be maintained at a satisfactory level compared to a similar multimode optical signal that would experience an existing fiber coupler.

In some embodiments, as will be described in detail below, etendue is not only maintained but also increased along extraction direction 122 or injection direction 124.

It has been found that, because the etendue is kept conservative or minimally affected along one or both of the extraction direction 122 and the injection direction 124, the fiber coupler 100 is not limited by a taper ratio that is less than the ratio of the numerical apertures of the first and second multimode fibers 102, 104, as may be inferred by previous U.S. patent publication 2010/0183261a1 to gothiter et al.

Such a limitation can be demonstrated using the optical expansion equation and the principle of conservation. It is first convenient to distinguish the etendue of the system from the etendue of the propagating light within the system. The etendue of the system is defined as the maximum etendue supported by the structure. For an optical fiber, etendue is related to the Numerical Aperture (NA), which can be calculated based on the refractive index of the optical fiber. However, in order to propagate light, etendue is related to the emitted (or otherwise excited) numerical aperture (referred to herein as the LNA). It can therefore be appreciated that in a fiber with surface area S, the LNA ≦ NA. For the transmission conditions where LNA-NA, the structure is said to be fully satisfied. It is also possible to envisage a configuration with a variable etendue that can be increased and then decreased to its original value without losses as long as the LNA remains unchanged.

For having etendue G in non-tapered region₁And an etendue G in the tapered region_1TOf the first taper fiber, conservation law stipulating G₁Must be less than or equal to G_1T. Thus, a tapered region LNA (referred to herein as an LNA)_1T) Will increase in proportion to the taper ratio ITR as shown by the following equation:

G₁≤G_1T (1)

using G₁＝πS₁(NA₁)²And G_1T＝πS_1T(LNA_1T)²Equation (1) can become:

by using the definition of the taper ratio,equation (2) may become:

in order to transmit light from a first optical fiber to a second optical fiber without loss, the second optical fiber must allow propagation of all modes of the first optical fiber. More specifically, the modes supported by the optical fiber are directly related to the numerical aperture at the coupling region, and the numerical aperture of the first optical fiber should preferably be less than or equal to the numerical aperture of the second optical fiber. Thus, in the case where the first optical fiber is a tapered optical fiber, the tapered region LNA_1tShould preferably be smaller than or equal to the second optical fiber NA₂The numerical aperture of (2). According to this constraint, the inventors have shown that the taper ratio ITR_1→1TShould preferably be greater than or equal to the first NA₁Numerical aperture and second optical fiber NA₂The ratio of the numerical apertures of (a) to (b) is as follows:

thus, a multimode extraction and/or injection rate can be achieved that is higher than the ratio obtained with existing fiber couplers. Example 1 presented below illustrates one embodiment of a fiber coupler that is not limited by a taper ratio that is less than a ratio of numerical apertures of a first multimode fiber and a second multimode fiber.

Example 1

Fig. 2A shows a first multimode optical fiber 202 and a second multimode optical fiber 204 each having a corresponding down taper and up taper. As shown, the first multimode optical fiber 202 and the second multimode optical fiber 204 are shown as they would be expected to be positioned prior to optical coupling, which in this case includes thermal fusion.

More specifically, as shown in this embodiment, the first multimode optical fiber 202 and the second multimode optical fiber 204 are parallel to each other, with the rising tapered portion 208 of the first multimode optical fiber 202 longitudinally aligned with the falling tapered portion 214 of the second multimode optical fiber 204.

In this particular embodiment, the first multimode optical fiber 202 and the second multimode optical fiber 204 have an overall diameter of 125 μm and a multimode core diameter of 105 μm, and an NA of 0.22. The descending and ascending tapered portions of each of the first and second multimode optical fibers 202 and 204 are tapered at 30% of their initial diameters d1 and d 2.

Fig. 2B shows an example of a fiber coupler 200 having a first multimode optical fiber 202 and a second multimode optical fiber 204. As shown, the coupled optical fiber 200 has a coupling region 206 in which the rising tapered portion 208 of the first multimode optical fiber 202 is thermally fused to the falling tapered portion 214 of the second multimode optical fiber 204. After fusing, additional tapering (not shown) of the coupling region 206 may be used to facilitate mode transfer from one multimode optical fiber to another.

As shown in this embodiment, the coupling region 206 extends over the length of the rising tapered portion 208 of the first multimode optical fiber 202 and the length of the falling tapered portion 204 of the second multimode optical fiber 204.

It was found that fiber coupler 200 exhibited a multimode extraction ratio of 84% in an extraction direction 222 extending from second port 202b to third port 204a of fiber coupler 200 and an multimode injection ratio of 84% in an injection direction 224 extending from third port 204a to second port 202b of fiber coupler 200. The excess loss of the fiber coupler 200 in both the extraction direction 222 and the injection direction 224 was found to be less than 7%.

It was found that the multimode extraction in only one coupling direction of a fiber coupler made using two identical, but non-tapered multimode fibers made based on the prior art technique was limited to 43% and the excess loss of the non-tapered multimode fiber was 16%. For a fiber coupler made using two identical multimode fibers, where the tapered down section of one multimode fiber is optically coupled to the non-tapered section of the other multimode fiber, only 59% multimode extraction is achieved in only one coupling direction, with an excess loss of 35%. Accordingly, the inventors believe that such results indicate that, in addition to enabling effective multimode bidirectionality, the fiber coupler 200 described herein enables higher multimode extraction and/or injection rates.

The bottom of fig. 2B presents a cross-section of the first multimode optical fiber 202 and the second multimode optical fiber 204 at a plurality of longitudinal locations along the coupling region 206. It should be appreciated that along the extraction direction 222 and the injection direction 224 of the coupling region 206, etendue is substantially maintained, thereby helping to reduce optical losses as discussed above.

Example 2

Fig. 3A shows a double-clad optical fiber 302 and a multimode optical fiber 304 each having a corresponding down-taper portion and up-taper portion. As shown, the double-clad optical fiber 302 and the multimode optical fiber 304 are shown as they would be expected to be positioned prior to optical coupling.

More specifically, in this embodiment, the double-clad optical fiber 302 and the multimode optical fiber 304 are parallel to each other, with the rising tapered portion 308 of the double-clad optical fiber 302 longitudinally aligned with the falling tapered portion 314 of the multimode optical fiber 304.

In this particular example, double-clad fiber 302 has an overall diameter of 125 μm, an inner cladding diameter of 105 μm, and a core diameter of 9 μm, and an NA of 0.12/0.2. The multimode optical fiber 304 has an overall diameter of 125 μm and a multimode core diameter of 105 μm, and has an NA of 0.22. Both the double-clad optical fiber 302 and the multimode optical fiber 304 are pre-tapered to 35% of their original dimensions d1 and d 2.

FIG. 3B shows an example of a fiber coupler 300 having a double-clad fiber 302 and a multimode fiber 304, where the rising taper portion 308 of the double-clad fiber 302 is thermally fused to the falling taper portion 314 of the multimode fiber 304.

As in the previous example, the coupling region 306 has a length that extends over the length of the rising taper portion 308 and over the length of the falling taper portion 314.

The fiber coupler 300 was found to exhibit a multimode extraction of 77%, which means that 77% of the multimode signal propagating from the inner cladding 330 of the double-clad fiber 302 was extracted into the multimode core 332 of the multimode fiber 304 with an excess loss of 4%. Conventionally, the fiber coupler 300 has a multimode injection rate of 65%, which means that 65% of a multimode signal propagating from the multimode core 332 of the multimode fiber 304 is injected into the inner cladding 330 of the double-clad fiber 302, and the excess loss is 20%. Furthermore, it was found that a single-mode signal propagating along the core 334 of the double-clad fiber 302 would experience only 0.5dB loss from the first port 302a to the second port 302b of the fiber coupler 300 or from the first port 302b to the second port 302a of the fiber coupler 300.

The bottom of fig. 3B presents a cross-section of the double-clad optical fiber 302 and the multimode optical fiber 304 at a plurality of longitudinal locations along and beyond the coupling region 306. It should be appreciated that along the extraction direction 322 and the injection direction 324 of the coupling region 306, the etendue is substantially maintained, thereby helping to reduce optical losses as discussed above.

Example 3

Fig. 4A depicts a double-clad fiber 402 and a multimode fiber 404 each having a corresponding down-taper and up-taper portion. As shown, a double-clad fiber 402 and a multimode fiber 404 are shown as they would be expected to be positioned prior to optical coupling.

In this example, the rising tapered portion 408 of the double-clad fiber 402 is longitudinally aligned with the falling tapered portion 414 of the multimode fiber 414.

More specifically, double-clad fiber 402 has an overall diameter of 125 μm, an inner cladding diameter of 105 μm, and a core diameter of 9 μm, and has an NA of 0.12/0.2. The multimode optical fiber 404 has an overall diameter of 125 μm and a multimode core diameter of 105 μm, and has an NA of 0.15.

As shown in this example, the multimode optical fiber 404 has a taper ratio that is greater than the taper ratio of the double-clad optical fiber 402. Thus, the double-clad fiber 402 is tapered to 36.7% of its original dimension d1, while the multimode fiber 404 is tapered to 17.5% of its original dimension d 2.

FIG. 4B shows an example of a fiber coupler 400 having a double-clad fiber 402 and a multimode fiber 404, where the rising tapered portion 408 of the double-clad fiber 402 is thermally fused to the falling tapered portion 414 of the multimode fiber 404;

the fiber coupler 400 was found to exhibit a multimode extraction of 62.6%, meaning that 62.6% of the multimode signal propagating from the inner cladding 430 of the double-clad fiber 402 was extracted into the multimode core 432 of the multimode fiber 404 with an excess loss of 20%, while providing a multimode injection rate of 86%, meaning that 86% of the multimode signal propagating from the multimode core 432 of the multimode fiber 404 was injected into the inner cladding 430 of the double-clad fiber 402 with an excess loss of 4%. Again, it was found that a single mode signal propagating along the core 434 of the double-clad fiber 402 would experience only 0.5dB of loss from the first port 402a to the second port 302b of the fiber coupler 400 (or vice versa).

Fig. 4B also shows a cross-section of the double-clad fiber 402 and the multimode fiber 404 at a plurality of longitudinal locations along and beyond the coupling region 406. It should be appreciated that along either of the extraction direction 422 and the injection direction 424, etendue is substantially maintained, thereby helping to reduce optical losses as discussed above.

Example 4

Fig. 5A shows a double-clad optical fiber 502 and a multimode optical fiber 504 each having a corresponding down-taper and up-taper portion. As depicted, the double-clad fiber 502 and the multimode fiber 504 are shown as they would be expected to be positioned prior to optical coupling.

In this example, the double-clad optical fiber 502 and the multimode optical fiber 504 are parallel to each other, with the rising tapered portion 508 of the double-clad optical fiber 502 longitudinally aligned with the falling tapered portion 514 of the multimode optical fiber 504.

In this particular example, double-clad fiber 502 has an overall diameter of 125 μm, an inner cladding diameter of 105 μm, and a core diameter of 9 μm, and an NA of 0.12/0.2. The multimode optical fiber 504 has an overall diameter of 220 μm and a multimode core diameter of 200 μm, and has an NA of 0.22. Both the double-clad optical fiber 502 and the multimode optical fiber 504 were pre-tapered to 36.7% of their original dimensions d1 and d 2.

FIG. 5B shows an example of a fiber coupler 500 having a double-clad fiber 502 and a multimode fiber 504, where the rising tapered portion 508 of the double-clad fiber 502 is thermally fused to the falling tapered portion 514 of the multimode fiber 504.

The fiber coupler 500 was found to exhibit a multimode extraction of 85%, meaning that 85% of the multimode signal propagating from the inner cladding 530 of the double-clad fiber 502 was extracted into the multimode core 532 of the multimode fiber 504 with an excess loss of 1%. In addition, the fiber coupler 500 has a multimode injection rate of 22%, which means that 22% of the multimode signal propagating from the multimode fiber 504 is injected into the inner cladding 530 of the double-clad fiber 502 with an excess loss of 50%. In addition, it was found that a single mode signal propagating along the core 534 of the double-clad fiber 502 would experience only 0.5dB of loss from the first port 502a to the second port 502b of the fiber coupler 500 (or vice versa).

It was found that a fiber coupler made using a similar but non-tapered fiber made based on the prior art could achieve 72% multimode extraction and 6% excess loss, while achieving 14% multimode injection and less than 15% excess loss, again supporting the performance of the fiber coupler of this example.

Fig. 5B shows a cross-section of optical fiber 502 and optical fiber 504 at multiple longitudinal locations along and beyond coupling region 506. It will be appreciated in this example that the etendue of coupling region 506 increases along injection direction 524, which may reduce the multi-mode bidirectionality of fiber coupler 500.

As will be discussed below, the fiber optic coupler presented herein has been found to be particularly useful in biomedical fiber-based imaging and/or sensing systems. For example, the fiber optic coupler may be conveniently used for endoscopy, optical coherence tomography, fluorescence imaging, diffuse spectroscopy, raman spectroscopy, confocal microscopy, confocal endoscopy, laser coagulation, laser ablation, or any combination thereof.

Example 5

Fig. 6A depicts a double-clad optical fiber 602 and a multimode optical fiber 604, each having a corresponding down-taper and up-taper portion. As shown, a double-clad fiber 602 and a multimode fiber 604 are shown as they would be expected to be positioned prior to optical coupling.

In this example, the rising tapered portion 608 of the double-clad fiber 602 is longitudinally offset from the falling tapered portion 614 of the multimode fiber 614. As shown, in this example, the rising tapered portion 608 is longer than the falling tapered portion 614.

More specifically, double-clad fiber 602 has an overall diameter of 125 μm, an inner cladding diameter of 102 μm, and a core diameter of 4 μm, and an NA of 0.19/0.24. The multimode fiber 604 had an overall diameter of 125 μm and a multimode core diameter of 105 μm, and an NA of 0.22.

As shown in this example, the multimode optical fiber 604 has a taper ratio that is greater than the taper ratio of the double-clad optical fiber 602. Thus, the double-clad fiber 602 is tapered to 48% of its original dimension d1, while the multimode fiber 604 is tapered to 17.5% of its original dimension d 2.

FIG. 6B shows an example of a fiber coupler 600 having a double-clad fiber 602 and a multimode fiber 604, where an ascending tapered portion 608 of the double-clad fiber 602 is thermally fused to a descending tapered portion 614 of the multimode fiber 604;

the fiber coupler 600 was found to exhibit multimode optical characteristics similar to those of the fiber coupler 400 described with reference to example 3. However, due to the single mode adiabatic sensitivity of the double-clad fiber 602, the rising taper portion 608 of the double-clad fiber 602 must be smoother, which is achieved by increasing the length and taper ratio.

Fig. 6B also shows a cross-section of the double-clad fiber 602 and the multimode fiber 604 at a plurality of longitudinal locations along and beyond the coupling region 606. It should be appreciated that along either of the extraction direction 622 and the injection direction 624, etendue is substantially maintained, thus helping to reduce optical losses as discussed above.

Example 6

Fig. 7A shows a first multimode optical fiber 702 and a second multimode optical fiber 704 each having a corresponding down taper and up taper. As shown, the first and second multimode optical fibers 702 and 704 each have a cleave region 746 in which the outer cladding 742 and 744 of the first and second multimode optical fibers 702 and 704 are removed before the down-taper and up-taper portions. The first multimode optical fiber 702 and the second multimode optical fiber 704 are shown as they would be expected to be positioned prior to optical coupling, which in this case comprises thermal fusion. As depicted, the descending tapered portion of the first multimode optical fiber 702 is adjacent to the stripped portion of the second multimode optical fiber 704, while the ascending tapered portion of the second multimode optical fiber 704 is adjacent to the stripped portion of the first multimode optical fiber 702.

More specifically, as shown in this embodiment, the first multimode optical fiber 702 and the second multimode optical fiber 704 are parallel to each other, with the rising tapered portion 708 of the first multimode optical fiber 702 longitudinally aligned with the falling tapered portion 714 of the second multimode optical fiber 704.

In this particular embodiment, the first and second multimode optical fibers 702, 704 have a total diameter of 125 μm and a multimode core diameter of 105 μm, and an NA of 0.22. A portion of the outer cladding 742 and a portion of the outer cladding 744 are removed within the cleave region 746 such that the first and second multimode optical fibers 702, 704 have an initial diameter of 105 μm in this region. The descending and ascending tapered portions of each of the first and second multimode optical fibers 702 and 704 are tapered at 30% of their initial diameters d1 and d 2.

Fig. 7B shows an example of a fiber coupler 700 having a first multimode optical fiber 702 and a second multimode optical fiber 704. As shown, the coupled optical fiber 700 has a coupling region 706 in which the rising tapered portion 708 of the first multimode optical fiber 702 is thermally fused to the falling tapered portion 714 of the second multimode optical fiber 704. After fusing, additional tapering (not shown) of the coupling region 706 may be performed to facilitate mode transfer from one multimode optical fiber to another.

As shown in this embodiment, the coupling region 706 extends over the length of the rising tapered portion 708 of the first multimode optical fiber 702 and the length of the falling tapered portion 714 of the second multimode optical fiber 704.

In view of the above examples, the inventors have found that such a fiber coupler may exhibit satisfactory performance depending on the embodiment. For example, in some embodiments, the fiber coupler may be designed such that the sum of the multimode injection rate and the multimode extraction rate may be higher than 100%, preferably higher than 110%, more preferably higher than 120%, most preferably higher than 150%. Furthermore, in an alternative embodiment, the fiber coupler may be designed such that both the multimode injection rate and the multimode extraction rate are higher than 50%, preferably both higher than 60%, most preferably both higher than 70%. It should be understood that depending on the embodiment or intended application, the coupling region may be configured to block the multimode injection rate in favor of a given multimode extraction rate, or vice versa.

The following presents an exemplary application in which one or more of the above-described fiber couplers may be advantageously used. The following applications are intended to be examples only, as fiber couplers in accordance with the present disclosure may be used in other applications as well.

Example application 1

Fig. 8 shows an exemplary system 850 that incorporates both a laser marking system 852 and an Optical Coherence Tomography (OCT) monitoring system 854. The illustrated system 850 is a clinical system suitable for intraluminal endoscopy (e.g., gastroscopy or colonoscopy) and allowing real-time monitoring of in vivo thermal effects.

As shown, system 850 has a fiber coupler corresponding to fiber coupler 400 described and illustrated with reference to fig. 4B. In this example, the fiber coupler 400 is used to efficiently inject a multimode signal from a multimode fiber 404 into the inner cladding of a double-clad fiber 402.

As shown, OCT monitoring system 854 may have, for example, a wavelength-scanning laser, a single-mode fiber-based interferometer, and a detection module. A single mode sample arm fiber 756 is connected (e.g., spliced) to port 1 of the fiber coupler 400 to propagate the OCT signal to the sample 858. Port 2 of Fiber coupler 400 is connected to a Fiber Optic Rotating Joint (FORJ) 860, which Fiber optic rotating Joint 860 is disposed on a longitudinal translation stage 862 allowing helical scanning of the Fiber optic probe. Alternatively, a micro-motor may be used at the distal end. Backscattered light from the sample 858 is collected back by the single mode core of the double clad fiber 404, propagates through the fiber coupler 400 and returns to the OCT monitoring system 854 for detection. Therefore, minimal single mode loss of fiber coupler 400 is critical for high sensitivity OCT imaging.

In order to enable robust and efficient coupling of the marking laser into the inner cladding of double-clad fiber 404, fiber coupler 400 is used. This allows for a clinically compatible setup and does not require the use of free space optics, which can be lossy, bulky, and prone to misalignment in at least some cases.

The OCT monitoring system 854 and laser marking system 852 are interfaced through an a/D plate 864 for triggering and signal acquisition. The a/D board 864 may also perform motor control, as the skilled reader will appreciate.

Example application 2

Figure 9 shows an example of a spectral OCT system 950 in accordance with one embodiment. The illustrated fiber coupler corresponds to fiber coupler 300 of fig. 3B and functions as a bi-directional multimode coupler to achieve single fiber illumination and detection of spectral signals. As shown in this example, the fiber output 956 of the spectral OCT system 950 is spliced directly to port 1 of the double-clad fiber coupler 300. Port 2 is used to interface imaging or sensing optics. Depending on the application, such optics may include prisms mounted on micromirrors, prism and GRIN lens assemblies, or others. The OCT signal back-reflected by the sample 958 is collected back by the core of the double-clad fiber 302 and transmitted to the detection module 970 of the spectral OCT system 950.

In this example, light from broadband visible light source 972 is coupled into multimode fiber 304 at port 3 using free-space optics (e.g., beam splitter 974 and lens/objective 976) and injected into the inner cladding of double-clad fiber 302 through fiber coupler 300. Visible light backscattered by the sample 958 and collected by the inner cladding passes through the fiber coupler 300 and is transmitted to port 3. The beam splitter 974 is used to reflect the signal to a detection module 970, which in this example is provided in the form of a spectrometer.

It will be appreciated that this embodiment may be used to combine OCT with diffuse spectral, white light spectral or hyperspectral imaging, as desired.

Example application 3

Fig. 10 shows a system 1050 similar to that shown in fig. 9, but the free-space optics for coupling/decoupling the illumination and detection signals are replaced by a Wideband Multimode Circulator (WMC) 1080. WMC 1080 efficiently transmits light from light source 1072 to fiber coupler 300 and efficiently transmits light back from fiber coupler 300 toward detection module 1070. This arrangement allows the use of a robust all-fiber approach to combining OCT and spectroscopy.

It is to be understood that the above description and illustrated examples are intended to be exemplary only. For example, although the fiber optic couplers described with reference to the figures have two optical fibers coupled to each other, thereby creating four different optical ports, it is contemplated that the fiber optic couplers described herein may be integrated in one or more other fiber optic couplers, or even optically coupled to one or more other multimode fibers or multi-clad fibers, which may result in more than four different optical ports. Thus, the fiber optic couplers described herein are not limited to only two optical fibers being optically coupled to one another. Furthermore, the fiber optic couplers described in the present disclosure may be used in non-biomedical fields, such as light detection and ranging (or lidar), single and multi-mode based telecommunications, gas detection, spectroscopy, and the like. The scope is indicated by the appended claims.