Directional coupler with reduced phase distortion

Abstract

The present disclosure provides a directional coupler with reduced phase deviation. The directional coupler includes a first coupler waveguide and a second coupler waveguide. At least one of the spacing distance between the first coupler waveguide and the second coupler waveguide and the length of the first coupler waveguide and the second coupler waveguide is selected to achieve an acceptable phase deviation and set a coupling ratio. The phase deviation is caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide and a second optical mode in the second coupler waveguide.

Description

Background Art

[0001] Fiber or optical directional couplers are used to transfer energy between adjacent fiber waveguides. When the two waveguides are positioned close enough to each other in a parallel manner, the propagation mode field generated by the light wave (signal) in the first coupler waveguide of the directional coupler overlaps into the other second coupler waveguide, causing the optical modes in these coupler waveguides to couple therein, thereby transferring energy from the light wave in the first waveguide to the light wave in the second waveguide.

[0002] An example of an apparatus implementing a directional coupler is an optical resonator-based gyroscope. The optical gyroscope may implement a first directional coupler that guides a first light wave from a first laser into and out of a ring resonator in a clockwise direction and a second directional coupler that guides a second light wave into and out of the ring resonator in a counterclockwise direction. An important source of error in an optical resonator gyroscope is caused by the directional coupler that guides the light wave into and out of the ring resonator. Specifically, when the loss coefficients between the optical modes in the coupler waveguides of the directional coupler are not equal, the phase difference between the optical modes in the coupler waveguides deviates from the ideal value. This may cause distortion of the optical resonance used to measure the rotation. This distortion may cause errors in the measured rotation rate, and thus this distortion should be minimized or stabilized as much as possible.

[0003] For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a directional coupler having closely matched loss factors. Summary of the invention

[0004] The following summary is made by way of example rather than limitation. The purpose of providing the summary is merely to help the reader understand some aspects of the subject matter. Embodiments provide a directional coupler with reduced phase deviation.

[0005] In one embodiment, a method for designing a directional coupler with reduced phase deviation is provided. The method includes: setting a coupling ratio of the directional coupler; setting a spacing distance between a first coupler waveguide and a second coupler waveguide of the directional coupler; setting the lengths of the first coupler waveguide and the second coupler waveguide of the directional coupler; determining a phase deviation caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide and a second optical mode in the second coupler waveguide; and when the determined phase deviation is unacceptable, changing at least one of the spacing distance between the first coupler waveguide and the second coupler waveguide and the lengths of the first coupler waveguide and the second coupler waveguide until an acceptable phase deviation is achieved while maintaining the set coupling ratio.

[0006] In another embodiment, a directional coupler with reduced phase deviation is provided. The directional coupler includes a first coupler waveguide and a second coupler waveguide. At least one of the spacing distance between the first coupler waveguide and the second coupler waveguide and the length of the first coupler waveguide and the second coupler waveguide is set based on an acceptable phase deviation and a set coupling ratio. The phase deviation is caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide and a second optical mode in the second coupler waveguide.

[0007] In another embodiment, a gyroscope is provided. The gyroscope includes a first laser generating a first light wave, a second laser generating a second light wave, a ring resonator, a first directional coupler, a second directional coupler, a first PDH loop, a second PDH loop and a subtractor. The first directional coupler is configured to guide the first light wave into and out of the ring resonator along a CW direction, and the first directional coupler includes a first coupler waveguide and a second coupler waveguide. At least one of the spacing distance between the first coupler waveguide and the second coupler waveguide and the length of the first coupler waveguide and the second coupler waveguide is selected to achieve an acceptable phase deviation and a set coupling ratio of the first directional coupler. The phase deviation of the first directional coupler is caused by the difference in loss coefficient between the first optical mode in the first coupler waveguide and the second optical mode in the second coupler waveguide. The second directional coupler is configured to guide the first light wave into and out of the ring resonator along a CCW direction. The second directional coupler includes a third coupler waveguide and a fourth coupler waveguide. At least one of the spacing distance between the third coupler waveguide and the fourth coupler waveguide and the length of the third coupler waveguide and the fourth coupler waveguide is selected to achieve an acceptable phase deviation and a set coupling ratio of the second directional coupler. The phase deviation of the second optical coupler is caused by the difference in loss coefficient between the first optical mode in the third coupler waveguide and the second optical mode in the fourth coupler waveguide. The first PDH loop couples the output end of the first directional coupler to the first laser to stabilize the first frequency of the first light wave emitted from the first laser. The second PDH loop couples the output end of the second directional coupler to the second laser to stabilize the second frequency of the second light wave emitted from the second laser. The subtractor is coupled to subtract the second frequency from the first frequency to provide a rate of change output. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention may be more readily understood and further advantages and uses of the present invention will become more apparent when considered in light of the detailed description and the following drawings, in which:

[0009] Figure 1 is a block diagram of an optical gyroscope including a first directional coupler and a second directional coupler according to an example aspect of the present invention;

[0010] Figure 2A is a diagram of a first directional coupler according to an example aspect of the present invention;

[0011] Figure 2B is a diagrammatic representation of a second directional coupler according to an example aspect of the present invention;

[0012] Figure 2C is a diagram of a third directional coupler according to an example aspect of the present invention;

[0013] Figure 3A is a graph of coupling ratio versus length according to an example aspect of the present invention;

[0014] Figure 3B is a graph of phase non-ideality versus length according to an example aspect of the invention;

[0015] Figure 4 is a graph of phase deviation versus coupler waveguide length and spacing according to an example aspect of the invention; and

[0016] Figure 5 is a formation flow chart illustrating a method of forming a directional coupler according to an example aspect of the present invention.

[0017] As is common practice, the various features described are not necessarily drawn to scale, but rather are used to emphasize specific features relevant to the present invention. Reference characters denote similar elements throughout the drawings and text. DETAILED DESCRIPTION

[0018] In the following specific embodiments, reference is made to the accompanying drawings, which form a part of the specific embodiments, and in these drawings, specific embodiments that can implement the present invention are shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to implement the present invention, and it should be understood that other embodiments can be utilized and can be changed without departing from the spirit and scope of the present invention. Therefore, the following specific embodiments are not restrictive, and the scope of the present invention is limited only by the claims and their equivalents.

[0019] Embodiments of the present invention provide a directional coupler with reduced phase distortion between optical modes in coupled waveguides, which can be used in sensing applications, such as but not limited to resonant optical gyroscopes. In an embodiment, the optical loss coefficients between two optical modes in the directional coupler (a first optical mode in a first coupler waveguide and a second optical mode in a second coupler waveguide) are designed to be nearly equal to each other, thereby reducing possible resonance distortion and the resulting offset error.

[0020] In an embodiment, a desired coupling ratio is first established for a directional coupler. Once the desired coupling ratio is selected, several combinations of waveguide spacing and coupler length are tested or modeled until the target value is reached. Typically, the directional coupler that will produce the lowest phase distortion will be a directional coupler with a longer length and a larger spacing (gap) between the coupler waveguides of the directional coupler, because this produces an optical mode with almost exactly equal loss coefficients. In one example, where the length of the directional coupler is a concern, a directional coupler with the shortest coupler waveguide associated with an acceptable phase distortion may be used.

[0021] A block diagram of an optical gyroscope 100 including ideal directional couplers 200-1 and 200-2 in one example is shown. The optical gyroscope 100 includes a first laser 102 and a second laser 106. The first laser 102 generates a first laser signal (a first light wave) that is directed to a first phase modulator 104. The first light wave is then directed to the first ideal coupler 200-1 through a first input waveguide 130, where the first light wave is directed into a closed-loop fiber coil 110 (resonator) in a clockwise (CW) direction.

[0022] The second laser 106 generates a second laser signal (second light wave) which is guided to the second phase modulator 108. The second light wave is then guided to the second ideal directional coupler 200-2 through the second input waveguide 134. The second directional coupler 200-2 guides the second light wave into the fiber coil 110 in a counterclockwise (CCW) direction.

[0023] The first light wave leaves the fiber coil 110 at the first ideal directional coupler 200-1 and is guided to the first detector 105 through the first output waveguide 132. In one example, the first detector 105 includes a photodiode detector. The first detector 105 is used to convert the first light wave into a first electrical output signal. The first electrical output signal from the first detector 105 is then guided back to the first laser 102 through the first pound-Drever-Hall (PDH) loop 112 to stabilize the frequency of the first light wave emitted from the first laser 102. In addition, the output end of the first PDH loop 112 is coupled to the subtractor 116.

[0024] The second light wave leaves the fiber coil 110 at the second ideal directional coupler 200-2 and is guided to the second detector 107 (or sensor) through the second output waveguide 136. In one example, the second detector 107 is a photodiode detector. The second detector 107 is used to convert the second light wave into a second electrical output signal. The second electrical output signal from the second detector is then guided back to the second laser 106 through the PDH loop 114 to stabilize the frequency of the second light wave emitted from the second laser 106. In addition, the output end of the second PDH loop 112 is coupled to the subtractor 116.

[0025] The first PDH loop 112 and the second PDH loop 114 lock the respective first laser 102 and the second laser 106 to resonator resonance at reflection points at the respective first directional couplers 200-1 and the second directional couplers 200-2. The rotation information provided by the optical gyroscope 100 may be determined using a processor 121 in communication with the output of the subtractor 116. The subtractor 116 provides a rate of change output to the processor 121 by subtracting the CCW resonant frequency from the CW resonant frequency.

[0026] Figure 2A , Figure 2B and Figure 2C An example of a directional coupler is shown in . A directional coupler typically includes two coupler waveguides that are positioned close enough to each other that a portion of the optical energy from one coupler waveguide will couple to the other coupler waveguide. Directional couplers can be used as splitters and to direct optical waves into and out of a closed-loop resonator, as illustrated above in the gyroscope 100 example.

[0027] Figure 2A A first directional coupler 150 is shown including a first coupler waveguide 152 and a second coupler waveguide 154. The first coupler waveguide 152 and the second coupler waveguide 154 have a defined length 151. Figure 2A There is a defined spacing 153 between the first coupler waveguide 152 and the second coupler waveguide 154 of the first directional coupler 150 . Figure 2B A second directional coupler 160 is shown having a first coupler waveguide 162 and a second coupler waveguide 164. The first coupler waveguide 162 and the second coupler waveguide 164 have a defined length 161. Figure 2B There is a defined spacing 163 between the first coupler waveguide 162 and the second coupler waveguide 164 of the second directional coupler 160 . Figure 2CA third directional coupler 200 is shown having a first coupler waveguide 202 and a second coupler waveguide 204. The first coupler waveguide 202 and the second coupler waveguide 204 have a defined length 201. Figure 2C There is a defined spacing 203 between the first coupler waveguide 202 and the second coupler waveguide 204 of the third directional coupler 200. The third directional coupler 200 is Figure 1 1 is a general illustration of a first directional coupler 200-1 and a second directional coupler 200-2 in an optical gyroscope. That is, in one example, the same directional coupler can be used for both the first directional coupler and the second directional coupler in the optical gyroscope.

[0028] Ideally, for a closed-loop resonator such as Figure 1 The optical fiber coil 110, the phase shift of the light waves emitted from the first output waveguide 132 and the second output waveguide 136 is ideally π / 2. However, the phase shift will not be exactly π / 2 because the loss coefficients in the optical modes in the coupler waveguides of the directional coupler will generally not be equal. The difference in loss coefficients may translate into a phase relationship deviation from π / 2. A slight deviation from π / 2 may result in resonance distortion. Distortion of resonance may in turn result in output errors from optical sensors that use resonance to track changes in the environment or in inertial applications.

[0029] Embodiments provide a directional coupler that provides a fixed coupling ratio that minimizes the amount of deviation from a desired π / 2 phase offset. In one example, a design of an ideal directional coupler is provided that achieves a splitting ratio in which half of the input power passes through a coupler waveguide in the directional coupler.

[0030] As the spacing between the two coupler waveguides in a directional coupler increases, the energy coupling becomes lower. In order to achieve the same splitting ratio as the spacing increases, the coupler waveguide of the directional coupler needs to increase in length. As the waveguide length increases, the loss coefficients between the modes in the coupler waveguides of the directional coupler become more similar, so the deviation from π / 2 caused by the difference in loss coefficients in the optical modes becomes smaller and smaller. Therefore, the longer coupler waveguides in the directional coupler with larger spacing provide a more ideal directional coupler, thereby getting closer to the ideal phase offset. This is ideal for devices such as the optical gyroscope 100 discussed above.

[0031] Figure 3A A graph 300 showing coupling ratio versus length is shown, Figure 3B An example phase non-ideality versus length graph 320 is shown. Figure 3AThe coupling ratio versus length graph 300 shows how the coupling ratio varies based on the coupling length of a waveguide in a directional coupler having a first gap 302 (or pitch) of 100 nm and a second gap 304 of 400 nm. Figure 3B The phase non-ideality versus length graph 320 shows how the phase non-ideality varies based on the coupling length of the coupled waveguide in the directional coupler having a first gap 302 of 100 nm and a second gap 340 of 400 nm. As shown, the phase non-ideality decreases as the coupler length increases, and the gap (spacing) also has an impact on the phase non-ideality.

[0032] As discussed above, embodiments provide directional couplers with reduced non-idealities, which in turn make the loss coefficients between modes in the waveguides forming the directional coupler more common (equal). Figure 4 A graph 400 of phase deviation versus coupler waveguide length and spacing in an example is shown. In this example, an acceptable phase deviation 404 has been set. Specifically, graph 400 shows where the phase deviation versus length and spacing curve 402 of graph 400 is at or below the acceptable phase deviation 404. In one example, the length and spacing distance of the coupler waveguides can be selected at the intersection of the phase deviation versus length and spacing curve 402 and the acceptable deviation 404 to minimize the possible length (and / or spacing) of the coupler waveguides while still achieving an acceptable phase deviation. This can be used in situations where a small package is required for the application of the directional coupler.

[0033] exist Figure 5 A method of forming a directional coupler is provided in a flowchart 500. Flowchart 500 is provided as a series of sequential blocks. In other examples, these blocks can be performed in a different order or even in parallel. Therefore, the embodiments are not limited to Figure 5 The sequence is given in the flowchart 500.

[0034] At block 502, an acceptable amount of deviation from π / 2 is identified. The acceptable amount of deviation may depend on the application and desired metrics of the directional coupler. As discussed above, applications using ring resonators, such as gyroscopes and inertial reference systems, may require a small amount of deviation.

[0035] At block 504, a fixed coupling ratio is selected. The coupling ratio is the desired amount of energy transfer between coupled waveguides. Here, the fixed coupling ratio can be set based on the application of the directional coupler.

[0036] At box 508, the spacing (separation distance) between the coupler waveguides is selected, and at box 506, the length of the coupler waveguide is selected. At box 509, the phase deviation is determined. This process can be completed by modeling the phase deviation or by constructing and measuring the phase deviation. In one example, a finite difference time domain (FDTD) modeling system is used, which determines the phase deviation based on the waveguide spacing on the assumption that the selected coupling ratio remains constant between models. In one example, at least one of the maximum spacing between the waveguides and the maximum length of the waveguide is applied to the corresponding boxes 508 and 506, which may depend on the intended application of the directional coupler. For example, some intended applications may limit the size of the directional coupler, which can be achieved by limiting the spacing and width of the coupler waveguides.

[0037] Then at block 510, a determination is made as to whether an acceptable deviation has been achieved. Figure 2A The first directional coupler 150 shown in Figure 2B The second directional coupler 160 shown in FIG. Figure 2C The third directional coupler 200 further explains the modeling example. In the example of a 0.5% directional coupler with a 40 nm thick silicon nitride (SiN) waveguide, the model is a 2 μm pitch and a 66 μm length. Figure 2A The directional coupler 150 provides a phase deviation of 0.000061π. The modeling also provides a phase deviation of 0.000061π for a directional coupler with a pitch of 4 μm and a length of 225 μm. Figure 2B The directional coupler 160 provided a determined phase deviation of 0.000052π. The modeling also provided a directional coupler 160 with a pitch of 6 μm and a length of 725 μm. Figure 2C The directional coupler 200 provides a determined phase deviation of 0.000043π. Therefore, in this example, Figure 2C The directional coupler 200 provides the minimum amount of phase deviation.

[0038] In this example, assuming that the acceptable deviation for this application is 0.000043π or less, the process will end at block 512. In this example, if the set acceptable deviation has not been reached at block 510, the process continues by changing or adjusting at least one of the pitch and length of the coupler waveguides at block 506 while maintaining the set coupling ratio.

[0039] Example Implementation

[0040] Embodiment 1 is a method for designing a directional coupler with reduced phase deviation. The method includes: setting a coupling ratio of the directional coupler; setting a spacing distance between a first coupler waveguide and a second coupler waveguide of the directional coupler; setting the lengths of the first coupler waveguide and the second coupler waveguide of the directional coupler; determining a phase deviation caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide and a second optical mode in the second coupler waveguide; and when the determined phase deviation is unacceptable, changing at least one of the spacing distance between the first coupler waveguide and the second coupler waveguide and the lengths of the first coupler waveguide and the second coupler waveguide until an acceptable phase deviation is achieved while maintaining the set coupling ratio.

[0041] Embodiment 2 includes the method according to embodiment 1, further comprising setting the acceptable phase deviation.

[0042] Embodiment 3 includes a method according to any one of Embodiments 1 to 2, wherein changing the spacing distance between the first coupler waveguide and the second coupler waveguide of the directional coupler and one of the lengths of the first coupler waveguide and the second coupler waveguide further includes: when the phase deviation is greater than the acceptable phase deviation, reducing the spacing distance between the first coupler waveguide and the second coupler waveguide of the directional coupler.

[0043] Embodiment 4 includes a method according to any one of Embodiments 1 to 3, wherein changing one of the spacing distance between the first coupler waveguide and the second coupler waveguide of the directional coupler and the length of the first coupler waveguide and the second coupler waveguide further includes: when the phase deviation is greater than the acceptable phase deviation, increasing the length of the first coupler waveguide and the second coupler waveguide of the directional coupler.

[0044] Embodiment 5 includes the method of any one of embodiments 1 to 4, further comprising determining the acceptable phase deviation based on an intended application of the directional coupler.

[0045] Embodiment 6 includes the method according to any one of embodiments 1 to 5, further comprising setting a maximum distance of the spacing distance between the first coupler waveguide and the second coupler waveguide and at least one of the maximum lengths of the first coupler waveguide and the second coupler waveguide based on an application of the directional coupler.

[0046] Embodiment 7 includes a directional coupler with reduced phase deviation. The directional coupler includes a first coupler waveguide and a second coupler waveguide. At least one of a separation distance between the first coupler waveguide and the second coupler waveguide and a length of the first coupler waveguide and the second coupler waveguide is selected to achieve an acceptable phase deviation and set a coupling ratio. The phase deviation is caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide and a second optical mode in the second coupler waveguide.

[0047] Embodiment 8 includes a directional coupler according to embodiment 7, wherein the directional coupler is modeled to determine the phase deviation using at least a set fixed coupling ratio when at least one of the separation distance between the first coupler waveguide and the second coupler waveguide and the length of the first coupler waveguide and the second coupler waveguide changes.

[0048] Embodiment 9 includes the directional coupler of any one of embodiments 7-8, wherein the acceptable phase deviation is selected based on an application of the directional coupler.

[0049] Embodiment 10 includes a directional coupler according to any one of embodiments 7 to 9, wherein the difference in loss coefficient between the first optical mode and the second optical mode in the directional coupler is set based on a spacing distance between the first coupler waveguide and the second coupler waveguide and at least one of the lengths of the first coupler waveguide and the second coupler waveguide.

[0050] Embodiment 11 includes a gyroscope, the gyroscope including a first laser generating a first light wave, a second laser generating a second light wave, a ring resonator, a first directional coupler, a second directional coupler, a first PDH loop, a second PDH loop, and a subtractor. The first directional coupler is configured to guide the first light wave into and out of the ring resonator along a CW direction, and the first directional coupler includes a first coupler waveguide and a second coupler waveguide. At least one of the spacing distance between the first coupler waveguide and the second coupler waveguide and the length of the first coupler waveguide and the second coupler waveguide is selected to achieve an acceptable phase deviation and a set coupling ratio of the first directional coupler. The phase deviation of the first directional coupler is caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide and a second optical mode in the second coupler waveguide. The second directional coupler is configured to guide the first light wave into and out of the ring resonator along a CCW direction. The second directional coupler includes a third coupler waveguide and a fourth coupler waveguide. At least one of the spacing distance between the third coupler waveguide and the fourth coupler waveguide and the length of the third coupler waveguide and the fourth coupler waveguide is selected to achieve an acceptable phase deviation and a set coupling ratio of the second directional coupler. The phase deviation of the second optical coupler is caused by the difference in loss coefficient between the first optical mode in the third coupler waveguide and the second optical mode in the fourth coupler waveguide. The first PDH loop couples the output end of the first directional coupler to the first laser to stabilize the first frequency of the first optical wave emitted from the first laser. The second PDH loop couples the output end of the second directional coupler to the second laser to stabilize the second frequency of the second optical wave emitted from the second laser. The subtractor is coupled to subtract the second frequency from the first frequency to provide a rate of change output.

[0051] Embodiment 12 includes a gyroscope according to embodiment 11, wherein the first directional coupler is modeled to determine the phase deviation using at least a set fixed coupling ratio when at least one of the separation distance between the first coupler waveguide and the second coupler waveguide and the length of the first coupler waveguide and the second coupler waveguide changes.

[0052] Embodiment 13 includes a gyroscope according to any one of Embodiments 11 to 12, wherein the second directional coupler is modeled to determine the phase deviation using at least a set fixed coupling ratio when at least one of the separation distance between the third coupler waveguide and the fourth coupler waveguide and the length of the third coupler waveguide and the fourth coupler waveguide changes.

[0053] Embodiment 14 includes the gyroscope of any one of Embodiments 11 to 13, wherein the acceptable phase deviation in the first directional coupler is set based on an application of the first directional coupler in the gyroscope.

[0054] Embodiment 15 includes the gyroscope of any one of Embodiments 11 to 14, wherein the acceptable phase deviation in the second directional coupler is set based on the application of the first directional coupler in the gyroscope.

[0055] Embodiment 16 includes a gyroscope according to any one of Embodiments 11 to 15, wherein the difference in loss coefficient between the first optical mode and the second optical mode in the first directional coupler is set based on a spacing distance between the first coupler waveguide and the second coupler waveguide and at least one of the lengths of the first coupler waveguide and the second coupler waveguide.

[0056] Embodiment 17 includes a gyroscope according to any one of Embodiments 11 to 16, wherein the difference in loss coefficient between the first optical mode and the second optical mode in the second directional coupler is set based on a spacing distance between the third coupler waveguide and the fourth coupler waveguide and at least one of the lengths of the third coupler waveguide and the fourth coupler waveguide.

[0057] Embodiment 18 includes the gyroscope according to any one of embodiments 11 to 17, further comprising a first phase modulator and a second phase modulator. The first phase modulator is coupled between the output end of the first laser and the first directional coupler. The second phase modulator is coupled between the output end of the second laser and the second directional coupler.

[0058] Embodiment 19 includes the gyroscope of any one of embodiments 11 to 18, further comprising a first detector and a second detector. The first detector is coupled to the output of the first directional coupler to convert the first light wave into a first electrical signal. The second detector is coupled to the output of the second directional coupler to convert the second light wave into a second electrical signal.

[0059] Embodiment 20 includes the gyroscope of any one of embodiments 11 to 19, further comprising a processor in communication with an output of the subtractor, the processor being configured to determine rotation information based on the rate of change output.

[0060] Although specific embodiments have been illustrated and described herein, one of ordinary skill in the art will recognize that any arrangement calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any modifications or variations of the present invention. Therefore, it is apparent that the present invention is limited only by the claims and their equivalents.

Claims

1. A method of designing a directional coupler (200) with reduced phase deviation, the method comprising: Setting the coupling ratio of the directional coupler (200); Setting a spacing distance (203) between a first coupler waveguide (202) and a second coupler waveguide (204) of the directional coupler (200); Setting the lengths (201) of the first coupler waveguide (202) and the second coupler waveguide (204) of the directional coupler (200); determining a phase deviation caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide (202) and a second optical mode in the second coupler waveguide (204); as well as When the determined phase deviation is unacceptable, at least one of the spacing distance (203) between the first coupler waveguide (202) and the second coupler waveguide (204) and the length (201) of the first coupler waveguide (202) and the second coupler waveguide (204) is changed until an acceptable phase deviation is achieved while maintaining the set coupling ratio.

2. A directional coupler (200) with reduced phase deviation, the directional coupler (200) comprising: a first coupler waveguide (202); and A second coupler waveguide (204), at least one of a spacing distance (203) between the first coupler waveguide (202) and the second coupler waveguide (204) and a length (201) of the first coupler waveguide (202) and the second coupler waveguide (204) is selected to achieve an acceptable phase deviation and a set coupling ratio, the phase deviation being caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide (202) and a second optical mode in the second coupler waveguide (204).

3. A gyroscope (100), comprising: a first laser (102) generating a first light wave; a second laser (106), the second laser generating a second light wave; A ring resonator (110); A first directional coupler (200-1), the first directional coupler being configured to guide the first light wave into and out of the ring resonator (110) in a clockwise (CW) direction, the first directional coupler (220-1) comprising: a first coupler waveguide (202), and a second coupler waveguide (204), at least one of a spacing distance (203) between the first coupler waveguide (202) and the second coupler waveguide (204) and a length (201) of the first coupler waveguide (202) and the second coupler waveguide (204) being selected to achieve an acceptable phase deviation and a set coupling ratio of the first directional coupler (200-1), the phase deviation of the first directional coupler (200-1) being caused by a difference in loss coefficient between a first optical mode in the first coupler waveguide (202) and a second optical mode in the second coupler waveguide (204); A second directional coupler (200-2), the second directional coupler being configured to guide the first light wave into and out of the ring resonator (110) in a counterclockwise (CCW) direction, the second directional coupler (200-2) comprising: a third coupler waveguide (202), and a fourth coupler waveguide (204), at least one of a spacing distance (203) between the third coupler waveguide (202) and the fourth coupler waveguide (204) and a length (201) of the third coupler waveguide (202) and the fourth coupler waveguide (204) being selected to achieve an acceptable phase deviation and a set coupling ratio of the second directional coupler (200-2), the phase deviation of the second optical coupler (200-2) being caused by a difference in loss coefficient between a first optical mode in the third coupler waveguide (202) and a second optical mode in the fourth coupler waveguide (204); a first pound-Drever-Hall (PDH) loop (112), the first PDH loop coupling the output end of the first directional coupler (200-1) to the first laser (102) to stabilize the first frequency of the first light wave emitted from the first laser (102); a second PDH loop (114) coupling the output end of the second directional coupler (200-2) to the second laser (106) to stabilize the second frequency of the second light wave emitted from the second laser (106); and A subtractor (116) is coupled to subtract the second frequency from the first frequency to provide a rate of change output.

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