TW202208904A - Push-pull tunable optical delay line and phase shifter - Google Patents

Abstract

The present invention provides a push-pull tunable optical delay line comprising: a first coupler having a first end and a second end, in which the first end includes an input end; a second coupler having a third end and a fourth end, in which the fourth end include an output end; a first waveguide connected between the second end and the third end, and is covered by a first heater; a second waveguide connected between the second end and the third end; and a third waveguide connected between the first end and the fourth end, and is covered by a second heater.

Description

Push-pull dimmable delay line

The invention discloses an optical delay line, particularly a push-pull adjustable optical delay line.

Optical Delay Lines (ODLs) are key components in many applications in the field of optical communication or optics. For example, ODL can be used for processing and buffering optical signals in optical networks, or for beamforming and filtering in microwave optoelectronic systems. Optical delay lines can also be used in biomedical sensing and three-dimensional light detection and ranging (LiDAR). Among them, the Tunable Optical Delay Line can flexibly control the propagation and delay of the optical signal. Furthermore, tunable delay lines are implemented by a large number of optical elements, exchanged fiber spools, or integrated optical chips.

In recent years, with the development of Photonic Integrated Circuits (PICs), on-chips (On-Chip) are integrated with other optical functional components to realize compact modules for various applications. These applications include synchronization, buffering, multiplexing, and real-time latency, among others. Compared to bulk optics or fiber-based optical delay lines, integrated optical delay lines on a single wafer have many advantages. For example, lower manufacturing cost, smaller component size, lower weight and power consumption, etc.

Besides, ring resonators and photonic crystals are the most commonly used PIC optical components to construct controllable delay lines. Ring resonators and photonic crystals can continuously modulate group delays of hundreds of picoseconds or even nanoseconds. Among them, the ring resonator utilizes its resonant spectral characteristics to realize a small optical delay device, but it also brings a narrower bandwidth, and the miniaturization of the optical delay line element improves the control speed. In integrated wafers, optical delay lines can be easily combined with other functional components such as light modulators, filters, lasers, and photodiodes, thus providing more powerful optics than single components and microwave processing capability.

The trade-off between integration density and waveguide propagation loss also needs to be considered before selecting a suitable integration platform. In terms of silicon-on-insulator (SOI) optical integrated circuit platforms, the silicon photonics platform has a large refractive index contrast, a small bend radius, and is fabricated using a standard CMOS circuit die. equipment, and is considered to be one of the most promising technologies for large-scale high-density optical integration.

Generally speaking, there are two ways to tune SOI-based ODL. One of them is the effect of refractive index modulation by heating or by changing the carrier concentration. Both of the above-mentioned refractive index modulation mechanisms change the phase of the light wave passing through the waveguide. Among them, the effect of changing the carrier concentration can be achieved by using carrier injection (applying current) or carrier depletion (applying reverse bias). In order to induce the modulation effect of the refractive index by carrier injection or depletion, a PN junction (P-N junction) needs to be formed in the waveguide, and the change of the refractive index also causes the change of the optical loss. Therefore, as long as modulation speed is not a major consideration, thermal modulation with a simple structure and low optical loss would be the main choice. In fact, for localized heating of compact SOI waveguides, the thermal modulation speed may be as short as a few microseconds. Furthermore, the high thermal resistance of the silicon dioxide capping layer on the SOI structure can also reduce thermal crosstalk and power dissipation.

Tunable ODLs based on ring resonators have been shown to be feasible and can utilize single or multiple stages to implement optical delay lines, while most designs are used for optical signal processing or buffering. However, in these applications, key requirements include a large enough true delay, large bandwidth, and low high-order phase variations to avoid causing chromatic dispersion and signal distortion. Therefore, multi-order ring resonators can achieve the required delay with large bandwidth and low distortion compared to single-order ODL.

Please refer to FIG. 1 , which is a schematic diagram of a conventional two-order delay line. A conventional two-order delay line consists of a Mach-Zehnder interferometer and three tunable couplers. Among them, the coupling ratios of the three adjustable couplers are K1, K2 and K3 respectively. The coupling ratios K1, K2 and K3 can be continuously varied between 0 and 1. In this case, the tunable coupler can be equipped with three balanced Mach-Zehnder phase shifters, but it is also suitable to use any other coupler design. Referring to FIG. 1 again, there are unbalanced delay line phase shifters between the two branches of the two stages to adjust the operating frequency of the components. Again, for a single-order circuit, the design method is similar, including the first two adjustable couplers K1 and K2 and the first unbalanced part. In general, a component may have two input ports and two output ports. However, the conventional two-order delay line is designed as a single-input single-output circuit.

An object of the present invention is to provide a racetrack-type (ring-type) resonator optical delay line based group delay device and method modulated by a thermal heater operating in a push-pull mode. Wherein, in the apparatus and method of the present invention, the group delay peak and the resonance wavelength peak can be independently modulated. The group delay is modulated with the heater of one arm in the balanced MZI coupler to change the effective coupling coefficient with the resonator. Thermal modulation on the racetrack type corrects for wavelength drift without affecting group delay.

According to one of the objectives of the present disclosure, a push-pull adjustable optical delay line is provided, comprising: a first coupler having a first end and a second end, the first end including an input end; a second coupling The device has a third end and a fourth end, the fourth end includes an output end; a first waveguide, the first waveguide is coupled between the second end and the third end, the first The waveguide is covered with a first heater; a second waveguide is coupled between the second end and the third end; and a third waveguide is coupled to the first Between one end and the fourth end, the third waveguide is covered with a second heater. The retardation of a fixed wavelength light wave is changed by adjusting the temperature of the first heater and the second heater.

Preferably, the first coupler and the second coupler are a three-dB coupler (3dB coupler).

Preferably, the input terminal is used for inputting an input optical signal.

Preferably, the first heater is used to modulate a coupling coefficient of the input optical signal.

Preferably, the second heater is used to compensate for a phase change.

Preferably, the second heater is used to correct a resonance wavelength error.

Preferably, the first waveguide and the second waveguide have the same length.

Preferably, the first heater and the second heater are a phase shifter due to the carrier effect.

Preferably, the temperature of the first heater and the second heater is adjusted in such a way that when the first heater is heated, the second heater is cooled, or when the second heater is heated, the first heater is Heater cools.

Please refer to FIG. 2 , which is a schematic structural diagram of a first embodiment of an adjustable optical delay line of the present invention. As shown in FIG. 2 , the push-pull tunable optical delay line 20 of the present invention includes a first coupler 200 , a second coupler 201 , a first waveguide 202 , a second waveguide 203 and a third waveguide 204 .

The first coupler 200 has a first end 2001 and a second end 2002, and the first end 2001 includes an input end Ein. The second coupler 201 has a third terminal 2011 and a fourth terminal 2012, and the fourth terminal 2012 includes an output terminal Eout. The first waveguide 202 is coupled between the second end 2002 and the third end 2011 , and the first waveguide 202 is covered with a first heater 205 . The second waveguide 203 is coupled between the second end 2002 and the third end 2011 . The third waveguide 204 is coupled between the first end 2001 and the fourth end 2012 , and the third waveguide 204 is covered with a second heater 206 .

In this embodiment, the input terminal Ein is used for inputting an input optical signal (not shown in the figure). Furthermore, the first heater 205 is used for modulating a coupling coefficient of the input optical signal, and the second heater 206 is used for compensating for a phase change. In addition, the second heater 206 is further used to correct a resonance wavelength error.

Wherein, in this embodiment, the first waveguide 200 and the second waveguide 200 have the same length. Furthermore, in the embodiment of the present invention, the first heater 205 and the second heater 206 are, but not limited to, a phase shifter based on the carrier effect. In addition, the first coupler 200 and the second coupler 201 are, but not limited to, a three-dB coupler (3dB coupler) in the embodiment of the present invention.

Furthermore, in the embodiment of the present invention, the retardation of a fixed wavelength light wave is changed by adjusting the temperature of the first heater 205 and the temperature of the second heater 206 . In more detail, the way to adjust the temperature of the first heater 25 and the second heater 206 is that when the first heater 205 is heated, the second heater 206 is cooled. Alternatively, when the second heater 206 is heated, the first heater 205 is cooled.

Push-pull operation can also be understood as when one phase regulator (the first heater 205 or the second heater 206 ) increases the current (carrier), the other phase regulator will decrease the current (carrier). In addition, the push-pull operation can also be understood as when one phase regulator increases the reverse voltage, the other phase regulator will increase the reverse voltage (carrier). Among them, the push-pull operation is that when one phase regulator increases the reverse voltage, the other phase regulator will increase the reverse voltage (carrier).

It is worth noting that the tunable optical delay line 20 of the present invention realizes the combination of a racetrack resonator, a balanced Mach-Zehnder interferometer (MZI) and two tunable heaters. The first heater 206 on the first waveguide 202 is used to modulate a coupling coefficient of an input optical signal (as described above, the input optical signal input from the input terminal Ein to the adjustable optical delay line 20 ), so that the input optical signal Signals are coupled in and out of the channel from the straight waveguide.

Furthermore, in the embodiments of the present invention, the maximum group delay occurs under resonance conditions. Among them, the coupling coefficient determines the quality parameter of the loop, and then determines the delay time. However, modulation of the coupling coefficient also causes a phase change, which changes the resonant wavelength. Therefore, in order to achieve large changes in group delay using a single racetrack-type resonator, the operating wavelength needs to be aligned with the resonant peak. Therefore, the phase change in modulating the coupling coefficient must be compensated by the phase change of the racetrack portion of the other thermal heater (ie, the second heater 206). The second heater 206 can also be used to correct for errors in resonance wavelengths due to process errors.

As mentioned above, in this embodiment, the first coupler 200 and the second coupler 201 are a three-dB coupler (3dB coupler). Thereby, the present embodiment consists of two 3 dB couplers to form a Mach-Zehnder interferometer, which is used as an equivalent tunable coupler for the resonator. Furthermore, the structure of the tunable optical delay line 20 (racetrack resonator) of this embodiment is to connect one output port of the MZI to one input port thereof (ie, the third end 2011 and the fourth end 2012 are coupled). The two arms of the MZI are of equal length (the first waveguide 202 and the second waveguide 203 have the same length). The first heater 205 is located above the lower arm of the MZI (the first heater 205 covers the first waveguide 202) to generate a phase change, and changes the propagation of the two arms (the first waveguide 202 and the third waveguide 204) through the thermo-optic effect The phase relationship between the light and therefore the optical power coupled into the racetrack-type resonator can be varied, enabling tunability of the coupling efficiency. The second heater 206 is placed over the racetrack waveguide (the second heater 206 covers the third waveguide 204) to control the resonant wavelength position.

Please refer to FIG. 3 again. FIG. 3 is a schematic structural diagram of a second embodiment of the adjustable optical delay line of the present invention. As shown in FIG. 3 , the adjustable optical delay line 30 of the second embodiment is connected in series with two adjustable optical delay lines 20 of the first embodiment. The adjustable optical delay line 30 of the second embodiment shown in FIG. 3 is a two-order optical delay line. Among them, tunable racetrack-type resonators with tunable couplers on each order. The tunable coupler is a symmetrical Mach-Zehnder interferometer (MZI), and one of the two arms is equipped with a thermal heater as a phase regulator to adjust the output ratio of the MZI. The peak delay occurs at the resonance condition of the racetrack resonator.

where the peak delay depends on the coupling ratio of the MZI output. Generally, as the peak delay increases, the insertion loss of the optical delay line increases, while the wavelength bandwidth decreases. At the same time, the peak wavelength at which the peak delay occurs moves with the coupling ratio. To stabilize the peak wavelength in light sensing applications, the two phase regulators of each order work in a push-pull mode. That is, when one phase regulator heats up, the other phase regulator will cool down. The peak wavelengths of the two orders can be aligned with each other to double the peak group delay or the peak wavelengths of the two orders can be shifted slightly to achieve a larger bandwidth.

To sum up, one objective of the present invention is to design an optical delay line based on a single-order racetrack-type (ring) resonator, wherein the resonator is configured with two tunable heaters. One of the adjustable heaters is used to modulate the coupling coefficient, while the other is used to modulate the phase of the round trip. The design of the present invention can be applied to optical and biomedical sensing applications, where optical signals typically have narrow linewidths. Application examples may include optical phased arrays for LiDAR, laser linewidth measurements, and interferometric biomedical sensing using laser Doppler or swept-frequency coherence tomography techniques.

In conclusion, the present invention proposes a novel method for modulating group delay based on racetrack resonator optical delay lines with thermal heaters operating in push-pull mode. Group delay peak and resonance wavelength peak can be modulated independently. The group delay is modulated with the heater of one arm in the balanced MZI coupler to change the effective coupling coefficient with the resonator. Thermal modulation on the racetrack type corrects for wavelength drift without affecting group delay.

In summary, for a given type of ODL, the bandwidth-delay product (DBP) is limited, while for resonant-type delay lines such as ring-ring resonators, the DBP is smaller. For the above biomedical sensing or optical sensing type applications, the linewidth of the laser is usually less than a few MHz or even about kHz. Therefore, longer delays can be achieved by using simpler resonators with higher quality parameters and smaller bandwidths. In this case, the wavelength of the resonance peak needs to be stabilized and aligned with the laser light wavelength. The ODL design of the present invention can achieve true delays from tens of ps to hundreds of ps at stable wavelengths.

It is worth noting that for optical sensing or biomedical sensing, the light source usually has a stable and narrow linewidth, while the design rule of tunable optical delay line (ODL) may be tuned for its optical communication and buffering. According to the present invention, the resonant cavity-based ODL is stabilized by a push-pull operation to stabilize the resonant wavelength. Furthermore, the present invention simulates the device with thermal modulation effect, integrates the device and verifies the characteristics of the tunable ODL. The present invention uses a simple racetrack-type resonator, simply adjusts the group delay by changing the coupling coefficient of the resonator, and stabilizes the wavelength by adjusting the racetrack-type loop. Furthermore, the present invention utilizes compact components and enables modulation of hundreds of picoseconds with small power consumption.

It can be seen that the present disclosure has indeed achieved the desired enhancement by breaking through the previous technology, and it is not easy for those who are familiar with the technology to think about it. Yuan to file a patent application in accordance with the law.

The above description is exemplary only, not limiting. Any other equivalent modifications or changes without departing from the spirit and scope of the present disclosure should be included in the appended patent application scope.

20,30: Push-pull dimmable delay line 200: First coupler 201: Second coupler 202: First Waveguide 203: Second Waveguide 204: Third Waveguide 2001: First End 2002: Second End Ein: input terminal 2011: The Third End 2012: Fourth End Eout: output terminal 205: First heater 206: Second heater

FIG. 1 is a schematic diagram of a conventional two-order delay line;

FIG. 2 is a schematic structural diagram of a first embodiment of an adjustable optical delay line of the present invention; and

FIG. 3 is a schematic structural diagram of a second embodiment of the adjustable optical delay line of the present invention.

20: Push-pull dimmable delay line

200: First coupler

201: Second coupler

202: First Waveguide

203: Second Waveguide

204: Third Waveguide

2001: First End

2002: Second End

Ein: input terminal

2011: The Third End

2012: Fourth End

Eout: output terminal

205: First heater

206: Second heater

Claims (9)

A push-pull adjustable optical delay line, comprising: a first coupler having a first end and a second end, the first end including an input end; a second coupler having a third end and a fourth end, the fourth end including an output end; a first waveguide, the first waveguide is coupled between the second end and the third end, the first waveguide is covered with a first heater; a second waveguide coupled between the second end and the third end; and a third waveguide, the third waveguide is coupled between the first end and the fourth end, the third waveguide is covered with a second heater; The retardation of a fixed wavelength light wave is changed by adjusting the temperature of the first heater and the second heater. The push-pull tunable optical delay line of claim 1, wherein the first coupler and the second coupler are a three-dB coupler (3dB coupler). The push-pull adjustable optical delay line as claimed in claim 1, wherein the input terminal is used for inputting an input optical signal. The push-pull tunable optical delay line of claim 3, wherein the first heater is used to modulate a coupling coefficient of the input optical signal. The push-pull dimmable delay line of claim 1, wherein the second heater is used to compensate for a phase change. The push-pull tunable optical delay line of claim 5, wherein the second heater is used to correct a resonance wavelength error. The push-pull tunable optical delay line of claim 1, wherein the first waveguide and the second waveguide have the same length. The push-pull tunable optical delay line of claim 1, wherein the first heater and the second heater are phase shifters due to carrier effects. The push-pull dimmable delay line of claim 1, wherein the temperature of the first heater and the second heater are adjusted in such a way that when the first heater is heated, the second heater is cooled, or When the second heater heats, the first heater cools.

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