CN116599596A - On-chip octave rate adjustable DPSK demodulator and tuning method - Google Patents
On-chip octave rate adjustable DPSK demodulator and tuning method Download PDFInfo
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Abstract
The invention relates to an optical communication technology, in particular to a DPSK demodulator with adjustable on-chip octave rate and a tuning method, which aims to solve the defect that the existing demodulator cannot meet the requirements of multi-rate and high-performance demodulation of DPSK signals; the first coupling unit adopts an MZI optical waveguide structure and is provided with two output ends for outputting two demodulation signals, the light splitting ratio of the two demodulation signals is arbitrarily adjustable, and high extinction ratio demodulation is realized. The invention can realize rapid, efficient, accurate and flexible continuous tuning of demodulation speed, and has important significance for wide engineering application of the DPSK demodulator on the push plate.
Description
Technical Field
The invention relates to an optical communication technology, in particular to a DPSK demodulator with adjustable on-chip octave rate and a tuning method.
Background
Differential Phase Shift Keying (DPSK) is one modulation format commonly used in free-space optical and fiber optic communication systems. Compared with other modulation formats, the DPSK modulation has the advantages of stable light intensity after modulation, lower requirement on laser linewidth, higher tolerance to fiber dispersion and nonlinearity, and the like. The DPSK receiver comprises a DPSK demodulator with a simple structure and a balance detector, and compared with an on-off keying (OOK) modulation mode, the system sensitivity is improved by 3dB.
The DPSK signal uses the phase difference between adjacent bits to carry data information, and usually uses a demodulator to convert the phase difference information into amplitude information by the interference of the optical signals of the front and rear symbols. There are three main types of demodulators: free space michelson interference, dual core fiber, planar optical waveguide. Compared with the first two types, the planar optical waveguide has smaller volume, high extinction ratio and more stable performance, and is beneficial to mass production. The working principle of the planar optical waveguide type DPSK demodulator is shown in fig. 1 a, specifically, an on-chip Mach-Zehnder delay line interferometer structure is used, an optical signal is divided into two paths through a beam splitter 01, one path of delay arm waveguide 03 has a fixed delay of 1Bit relative to the other path of reference arm waveguide 02 (the fixed delay of 1Bit is equal to the inverse of the symbol rate of the DPSK signal), and then the two paths of optical signals are interfered through a directional coupler 04, so that the phase difference between adjacent code elements is always 0 or pi, and constructive interference or destructive interference is realized. DPSK receivers built based on traditional discrete components are difficult to meet the size, weight and power consumption (SWaP) requirements of free-space optical communication systems. An on-chip Mach-Zehnder delay line interferometer (MZI) demodulator is a common DPSK demodulator, utilizes an MZI optical waveguide structure, has small volume, is easy to realize full-chip integration of a receiver, can solve the problems faced by the traditional DPSK receiver, and is a main technical scheme for realizing DPSK signal demodulation.
However, since the delay length of the existing demodulator is fixed, only the DPSK signal with a fixed rate can be demodulated, and the demodulator matched with the DPSK signal with a different rate needs to be replaced, the use of the existing MZI demodulator lacks flexibility. In order to meet the requirements of a communication system on multi-rate and high-performance demodulation of DPSK signals, a technical scheme which can be flexibly tuned according to the DPSK signal rate and has good demodulation effect is needed.
Disclosure of Invention
The invention aims to solve the defect that the existing demodulator cannot meet the multi-rate and high-performance demodulation requirements of DPSK signals, and provides a DPSK demodulator with adjustable on-chip octave rate and a tuning method.
In order to achieve the above purpose, the technical solution provided by the present invention is as follows:
the utility model provides an on-chip octave rate adjustable DPSK demodulator, includes first beam splitter, reference arm, delay arm, beam combiner and first coupling unit, and first beam splitter is used for dividing into two-way with DPSK signal, and two-way signal is transmitted to beam combiner through reference arm and delay arm respectively, and the demodulation signal is output by first coupling unit coupling after beam combination, its characterized in that: the system also comprises a controller and two temperature control modules;
the reference arm includes a reference arm waveguide, and the delay arm includes a delay arm waveguide; the reference arm waveguide and the delay arm waveguide are both arranged as long spiral line waveguides and are used for realizing octave rate tuning by adjusting delay; the first coupling unit adopts an MZI optical waveguide structure and is provided with two output ends for outputting two demodulation signals, the light splitting ratio of the two demodulation signals is arbitrarily adjustable, high extinction ratio demodulation is realized, and the high extinction ratio refers to an extinction ratio greater than or equal to 25 dB; the two temperature control modules are respectively used for controlling the temperatures of the reference arm waveguide and the delay arm waveguide; the controller is respectively connected with the first coupling unit and the two temperature control modules and is used for controlling the splitting ratio of the first coupling unit, and the temperature control modules are used for controlling the temperatures of the reference arm waveguide and the delay arm waveguide, so that the delay length is controlled, the optical path difference between the delay arm and the reference arm is 1Bit or a positive integer multiple of 1Bit, and the demodulation rate continuous tuning is realized.
Further, the first coupling unit comprises a second beam splitter, an upper arm waveguide, a lower arm waveguide, two heating electrodes corresponding to the upper arm waveguide and the lower arm waveguide, and a coupler;
the input end of the second beam splitter is connected with the output end of the beam combiner, so that two paths of signals interfere, and the output end of the second beam splitter is respectively connected with the input ends of the upper arm waveguide and the lower arm waveguide;
the output ends of the upper arm waveguide and the lower arm waveguide are respectively connected with the coupler, and the coupler is a coupler with any adjustable spectral ratio and is used for converting phase information of two paths of signals into intensity information and outputting two demodulation signals after coupling;
the two heating electrodes are connected with the output end of the controller, and the temperature change of the upper arm waveguide and the lower arm waveguide is controlled by controlling the heating electrodes, so that the light splitting ratio control of the first coupling unit is realized, and the output demodulation signal has a higher extinction ratio.
Further, the temperature control module comprises a thermistor and a micro-heater;
the thermistor is connected with the input end of the controller and is used for monitoring the temperature of the corresponding reference arm waveguide or delay arm waveguide in real time;
the micro heater is connected with the corresponding output end of the controller, and the working state of the micro heater is controlled by the controller, so that the reference arm waveguide or the delay arm waveguide is heated or cooled.
Further, the heating electrode adopts a metal film heater, and the metal film material is one of Al, cu, ti, W, ni, cr, au;
the coupler adopts an MMI structure or a directional coupler structure;
the micro heater is a semiconductor refrigerator.
Further, the optical fiber optical system further comprises at least one additional cascaded optical waveguide arranged on the reference arm and the delay arm respectively, wherein the additional cascaded optical waveguides are MZI optical waveguide structures, and the number of the additional cascaded optical waveguides on the reference arm and the delay arm is the same;
the additional cascaded optical waveguide comprises a second coupling unit, the structure of the second coupling unit is the same as that of the first coupling unit, one output end of the second coupling unit is connected with the input end of the first coupling unit, and the other output end of the second coupling unit is connected with the input end of the first coupling unit or the input end of the next-stage additional cascaded optical waveguide through a delay waveguide;
the second coupling unit is connected with the corresponding output ends of the controller, and the controller controls the switch states of the two output ends of the second coupling unit to realize switch jump tuning of the delay length.
Further, two additional cascading optical waveguides are arranged on the reference arm and the delay arm;
one output end of the second coupling unit is connected with the first coupling unit, and the other output end of the second coupling unit is connected with the input end of the first coupling unit or the additional cascaded optical waveguide of the next stage through a delay waveguide long spiral line waveguide;
the delay waveguide is a long spiral line waveguide.
Further, the light splitting ratio of the first beam splitter is 5:5;
the reference arm waveguide and the delay arm waveguide are one of silicon, silicon dioxide, high-refractive-index-difference doped glass, lithium niobate, silicon nitride and III-V materials;
the controller is a field programmable gate array or a microprocessor.
Meanwhile, also provided is an on-chip octave rate tuning method, based on the on-chip octave rate adjustable DPSK demodulator, which is characterized by comprising the following steps:
step one, determining a demodulation rate y by the following formula;
Y=b*y
wherein Y is the speed of the DPSK signal, Y is more than or equal to X, X is the lower limit of the demodulation speed, b is the multiple of the demodulation speed, and a positive integer is taken;
step two, adjusting the delay length delta through a controller to enable the demodulation rate of the demodulator to be y;
and thirdly, controlling the first coupling unit by using the controller to output a demodulation signal with a high extinction ratio.
Further, in the second step, the adjusting delay length is specifically:
2.1, controlling a temperature control module to adjust the temperature of the reference arm waveguide and the delay arm waveguide through a controller, and continuously adjusting the delay length delta so as to continuously tune the demodulation rate;
2.2, judging whether the demodulation rate is tuned to y;
if yes, entering a step three;
if not, the extra cascade optical waveguides arranged on the reference arm and the delay arm are controlled to carry out jump adjustment on the delay length, so that the switch jump tuning of the demodulation rate is realized, and the demodulation rate of the demodulator is y by combining the switch jump tuning and the continuous tuning.
Further, the delay length δ is calculated by:
Δl is calculated by the following formula:
wherein c is the speed of light, n g For the group refractive index of the reference arm waveguide and the delay arm waveguide, R is the symbol rate of the DPSK signal, al is the length difference of the delay arm relative to the reference arm,in order to change the free spectrum range along with the temperature, m is the mode number, the value is a positive integer,αthe shift of the resonance peak of the waveguide with temperature is given by Δt, which is the temperature difference between the reference arm waveguide and the delay arm waveguide.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention can realize continuous, accurate, high-efficiency and flexible adjustment of the delay length of the demodulator, thereby realizing multi-rate and high-performance demodulation of the DPSK signal; when the continuous adjustable range of the delay length reaches octaves, the single on-chip demodulator can be used for demodulating the DPSK signal with any rate larger than a certain fixed value, and the method has important significance for the wide engineering application of the DPSK demodulator on the push-plate.
2. The invention uses the thermo-optical effect of the long spiral line waveguide and the controller to change the temperature of the long spiral line waveguide, thereby changing the length difference and demodulation rate of the delay arm relative to the reference arm.
3. The invention realizes octave rate continuous tuning by combining two modes of continuous thermal tuning and switch jump tuning, and solves the problems of high randomness of demodulation rate and difficult tuning caused by process errors.
Drawings
Fig. 1 is a schematic diagram of the operation of a conventional demodulator; wherein a is a structural schematic diagram of a traditional Mach-Zehnder delay line interferometer demodulator, b is an integer multiple rate demodulation schematic diagram, c is an octave rate continuous tuning schematic diagram, and 1,2,3 … are demodulation rates of 1Gbps,2Gbps,3Gbps and …;
fig. 2 is a schematic structural diagram of a DPSK demodulator (beam combiner is not shown) according to a first embodiment of the present invention;
FIG. 3 is a schematic diagram of a first coupling unit according to a first embodiment of the present invention;
fig. 4 is a schematic structural diagram of a DPSK demodulator (beam combiner is not shown) in the second embodiment of the present invention;
FIG. 5 is a tuning schematic diagram in a second embodiment of the invention;
FIG. 6 is a flow chart of a tuning method in a second embodiment of the invention;
FIG. 7 is a waterfall diagram of the resonance peak of a DPSK demodulator based on a high-refractive-index-difference doped glass photonic integrated platform according to an embodiment of the invention as a function of temperature;
FIG. 8 is a graph of simulation results of FSR with temperature change based on a silicon-based photonic integrated platform with different reference arm lengths L in a second embodiment of the present invention;
reference numerals illustrate:
01-beam splitter, 02-reference arm waveguide, 03-delay arm waveguide, 04-directional coupler;
10-first beam splitter, 20-reference arm waveguide, 30-delay arm waveguide, 40-controller, 50-temperature control module, 51-thermistor, 52-micro-heater, 60-first coupling unit, 61-second beam splitter, 62-upper arm waveguide, 63-lower arm waveguide, 64-heating electrode, 65-coupler, 70-second coupling unit.
Detailed Description
Example 1
Aiming at the problem that the existing DPSK demodulator can only demodulate signals with fixed rate and the application value and prospect of a silicon-based optical chip in the field of optical communication, the invention provides the DPSK demodulator with adjustable on-chip octave rate and a tuning method, and the core is to provide a tuning idea of the DPSK demodulator with adjustable octave rate.
As shown in a of fig. 1, a conventional planar optical waveguide type DPSK demodulator usually adopts an unequal arm mach-zehnder delay line interferometer structure, the straight waveguide structure of the reference arm waveguide 02 is short, and the optical path difference between the reference arm waveguide 02 and the two paths of the delay arm waveguide 03 is exactly 1Bit or a positive integer multiple of the delay, so that the DPSK signal corresponding to a fixed rate and the positive integer multiple of the delay arm waveguide can be demodulated, the demodulation principle is shown in b of fig. 1, and if the demodulation rate corresponding to 1Bit delay is 1Gbps, signals such as 2Gbps,3Gbps, 4Gbps, 5Gbps … can be demodulated, and c of fig. 1 is an octave rate continuous tuning principle diagram.
The demodulator is of a planar optical waveguide structure, adopts two paths of long spiral lines to form an MZI optical waveguide structure, continuously adjusts the delay length of the two paths of waveguides based on the thermo-optic effect of waveguide materials through the controller 40 and the temperature control module 50, further realizes the continuous tuning of octave rate, and in addition, can realize high extinction ratio demodulation by changing the voltage of electrodes at two ends of the coupler 65 with any adjustable light splitting ratio.
The on-chip octave rate adjustable DPSK demodulator comprises a first beam splitter 10, a reference arm, a delay arm, a first coupling unit 60, a beam combiner, a controller 40 and two temperature control modules 50, as shown in FIG. 2; the reference arm comprises a reference arm waveguide 20, the delay arm comprises a delay arm waveguide 30, and the reference arm waveguide 20 and the delay arm waveguide 30 are long spiral line waveguides and are used for realizing octave rate tuning of the DPSK demodulator through large-range delay adjustment. The beam combiner is configured to interfere signals of the reference arm and the delay arm to obtain a combined signal, and transmit the combined signal to the first coupling unit 60.
The first coupling unit 60 adopts an MZI optical waveguide structure, as shown in fig. 3, and is provided with two output ends for outputting two demodulation signals, where the splitting ratio of the two demodulation signals is arbitrarily adjustable, so as to implement high extinction ratio demodulation greater than or equal to 25 dB. The first coupling unit 60 includes a second beam splitter 61, an upper arm waveguide 62, a lower arm waveguide 63, two heating electrodes 64 corresponding to the upper arm waveguide 62 and the lower arm waveguide 63, and a coupler 65, where the second beam splitter 61 is configured to split a combined signal into two beams for transmission to the upper arm waveguide 62 and the lower arm waveguide 63, and output ends of the upper arm waveguide 62 and the lower arm waveguide 63 are connected to the coupler 65; the heating electrode 64 is connected with the output end of the controller 40, and the control of the splitting ratio of the first coupling unit 60 is realized by controlling the temperature of the upper arm waveguide 62 and the lower arm waveguide 63 through controlling the heating electrode 64; the coupler 65 is used for converting the phase information of the signals transmitted by the upper arm waveguide 62 and the lower arm waveguide 63 into intensity information, and outputs two demodulation signals after coupling, and adopts a multimode waveguide interference coupler structure (MMI structure) or a directional coupler structure. In order to make the light intensities of the upper and lower arms as equal as possible, and further facilitate demodulation with a high extinction ratio, the light splitting ratio of the first beam splitter 10 and the second beam splitter 61 is 5:5. because the lengths of the upper arm waveguide 62 and the lower arm waveguide 63 are different, and the lengths of the reference arm waveguide 20 and the delay arm waveguide 30 are different, the intensities of the two paths of optical signals are lossy, and the first beam splitter 10 and the second beam splitter 61 are 5: and 5, after the two paths are respectively combined, the extinction ratio obtained by adjusting the splitting ratio through the first coupling unit 60 and the second coupling unit 70 is the largest.
The temperature control module 50 comprises a thermistor 51 and a micro-heater 52, wherein the thermistor 51 is used for monitoring the temperatures of the reference arm waveguide 20 and the delay arm waveguide 30 in real time and feeding back the temperatures to the controller 40, and the micro-heater 52 adopts a semiconductor refrigerator and is used for heating or refrigerating the reference arm waveguide 20 and the delay arm waveguide 30 with high precision; the controller 40 is a Field Programmable Gate Array (FPGA) or a Microprocessor (MCU), and the output end of the controller 40 is respectively connected to the two heating electrodes 64 in the first coupling unit 60 and the micro heater 52 and the thermistor 51 in the temperature control module 50, so as to control the splitting ratio of the first coupling unit 60 to achieve any adjustment of the splitting ratio, and simultaneously, control the delay of the reference arm waveguide 20 and the delay arm waveguide 30 through the temperature adjustment of the temperature control module 50.
The demodulator is made of a material platform with obvious thermal effect, and the optical path or delay of the demodulator is adjusted in a large range by changing the temperature, wherein the heating electrode 64 adopts a metal film heater, and the metal film material is one of Al, cu, ti, W, ni, cr, au and other materials; the reference arm waveguide 20 and the delay arm waveguide 30 are made of one of silicon, silicon dioxide, high-refractive-index-difference doped glass, lithium niobate, silicon nitride and III-V materials.
The working principle of the demodulator is as follows: the DPSK signal is divided into two paths after passing through the first beam splitter 10, one path passes through the reference arm waveguide 20, the other path passes through the delay arm waveguide 30, the temperature control module 50 is used for independently controlling the temperature of the reference arm waveguide 20 and the delay arm waveguide 30, so that the optical path difference of the two paths is exactly 1Bit or the positive integer multiple of the optical path difference is delayed, the first coupling unit 60 is used for interfering the two paths of optical signals, converting the phase information into intensity information, controlling the splitting ratio, and enabling the demodulator to output a demodulation signal with high extinction ratio.
The tuning method of the demodulator comprises the following steps:
step one, determining a demodulation rate Y according to the rate Y (Y is larger than or equal to X) of an input DPSK signal and a formula y=b×y (b e n+);
the demodulation rate y is between the lower demodulation rate limit X and the octave demodulation rate 2X. Because the reference arm and the delay arm are both made of long spiral line waveguides with obvious thermal effects, when the delay length of the reference arm is long enough and the demodulation speed lower limit of the demodulator is X, the octave demodulation speed of the demodulator is 2X, and if the demodulation speed can be continuously tuned between X and 2X, the DPSK signal with any speed higher than X can be demodulated by the demodulator. The principle is as follows: for any DPSK signal with a rate higher than X, a demodulation rate Y can always be found between X and 2X, such that y=b×y, where b is a multiple of the demodulation rate, and a positive integer is taken (i.e., b e n+), thus enabling the DPSK signal with a rate Y to be demodulated by a demodulator with a demodulation rate Y.
Step two, the controller 40 adjusts the temperature of the reference arm waveguide 20 and the delay arm waveguide 30 through the temperature control module 50, and then continuously adjusts the delay length, so that the demodulation rate of the demodulator is y;
and step three, the controller 40 is used for controlling the voltage of the heating electrode 64 in the first coupling unit 60, and the temperatures of the upper arm waveguide 62 and the lower arm waveguide 63 are adjusted so as to output demodulation signals with high extinction ratio.
The relationship between the delay length and the demodulation rate, and the relationship between the delay length and the demodulation rate with temperature change are as follows:
let the symbol rate of DPSK signal be R (Gbps), the length of reference arm be L, the length of delay arm be DeltaL+L, the group refractive index of waveguide be n g C is the speed of light, Δt is the delay time, and the corresponding delay length δ is:
(1)
free spectral rangeConsistent with the demodulation rate, expressed as:
(2)
according to the interference conditions:
(3)
wherein N is the effective refractive index of the waveguide medium, m is the mode number (m.epsilon.N+), lambda m Is the wavelength corresponding to the mode in the waveguide medium. Changing the temperature of the micro-heater to Δt by the temperature control module 50, assuming that the shift of the resonance peak of the waveguide with temperature is α (pm/°c/Δl mm) based on the thermal effect (thermo-optical effect and thermal expansion effect) of the long spiral waveguide, the amount of change in the delay length caused by the temperature is Δδ, expressed by the formula:
（pm/℃/ΔL mm） (4)
the corresponding free spectral range with temperature can be deduced from equation (1), equation (2) and equation (4)The expression is:
(5)
after the target delay length is determined, the target delay length is substituted into the formula (5)And under the condition that one quantity of the delta T or the delta L is determined, the adjusting range of the other quantity can be obtained by solving, and the long spiral line waveguide is adjusted, so that the delay length is adjusted.
In this embodiment, in order to characterize the thermal effect of the long spiral waveguide, fig. 7 shows a waterfall diagram of the resonance peak of the DPSK demodulator according to the temperature change based on the high refractive index difference doped glass photonic integrated process platform. Wherein the free spectral range is 10GHz, the length of the reference arm is 1.67 mm, the length difference DeltaL of the delay arm relative to the reference arm is 18.4mm, when the temperature is gradually changed from 20 ℃ to 32 ℃ at intervals of 1 ℃, the resonance peak moves approximately linearly, the moving speed is 1.67 GHz/DEGC/18.4 mm, and each 6 ℃ just moves by one free spectral range.
Example two
As shown in fig. 4, the DPSK demodulator in this embodiment is based on the DPSK demodulator structure of the first embodiment, and an additional cascaded optical waveguide is disposed on the reference arm and the delay arm, and the delay length is made to reach an octave by combining two modes of switch jump tuning and continuous thermal tuning, and its principle is shown in fig. 5.
The additional cascaded optical waveguide is an MZI optical waveguide structure and comprises a second coupling unit 70, the structure of the second coupling unit 70 is the same as that of the first coupling unit 60, one output end of the second coupling unit 70 is connected with the input end of the first coupling unit 60, and the other output end of the second coupling unit is connected with the input end of the first coupling unit 60 or the input end of the next-stage additional cascaded optical waveguide through a delay waveguide; the second coupling unit 70 is connected with the corresponding output end of the controller 40, and the controller 40 controls the switch states of the two output ends of the second coupling unit 70 to realize the switch jump tuning delay length. The number of additional cascaded optical waveguides on the reference arm and the delay arm is at least one.
To further illustrate the tuning effect of the DPSK demodulator of this example, FIG. 8 shows a graph of simulation results of free spectral range FSR as a function of temperature for different reference arm lengths L based on a silicon-based photonic integrated platform, where when a 50Gbps delayed demodulator is used, the required ΔL is 1.4mm, and the shift of the resonant peak of the silicon-based waveguide with temperature is 10 GHz/. Degree.C/1.4 mm (80 pm/. Degree.C/1.4 mm). Wherein the effective refractive index of the waveguide is 3.476, the group refractive index is 4.36, and when DeltaL is 14mm, the corresponding free spectral range is 5GHz; if the temperature is changed so that the demodulation rate continuous tuning range does not reach the target octave, the delay length is changed stepwise by the second coupling unit 70 by using the additional cascaded optical waveguide, thereby realizing the rate switch jump tuning. Therefore, the continuous thermal tuning based on the waveguide material thermo-optical effect and the switch jump tuning are combined to realize the continuous, accurate, efficient and flexible tuning of the octave rate. For the case of reference arm length of about 1m, when the temperature of the delay arm waveguide 30 is changed from 25 ℃ to 90 ℃, the delay length is doubled, and the demodulation rate is continuously changed from 5Gbps to 2.5Gbps, namely, the demodulation rate octave continuous tuning is realized, so that the demodulation of the DPSK signal with any rate greater than 2.5Gbps can be realized on a single on-chip demodulator. If loss factors are considered, the length of the long helical waveguide can be shortened, but octave continuous tuning is difficult to achieve by continuous thermal tuning alone. As shown in fig. 8, when the reference arm length is 0.5m or 0.2m, it is difficult to achieve a demodulation rate of 2.5Gbps only by continuous thermal tuning, and the demodulation rate can be achieved at 2.5Gbps using the demodulator in the second embodiment. In the embodiment, two additional cascaded optical waveguides are arranged on the reference arm and the delay arm; one output end of the second coupling unit 70 is connected to the first coupling unit 60, and the other output end is connected to the input end of the first coupling unit 60 or the next additional cascaded optical waveguide through a long helical line waveguide, so that the reference arm andthe delay arms each have 3 waveguide length choices and thus a total of 9 Δl combinations. Let the delta L change required to implement octaves be S ΔL (for a lower demodulation rate limit of 2.5Gbps, S ΔL 14 mm), the reference arm and delay arm lengths of the additional cascaded optical waveguide structure of fig. 4 are designed as shown in table 1, with fixed waveguide lengths=S ΔL /9：
Table 1 reference arm, delay arm length and corresponding Δl
For further explanation of the present embodiment, assuming that the target demodulation rate lower limit is 2.5Gbps, the variation S ΔL Fixed waveguide length of 14mmShould be set to 1.56mm, the corresponding reference arm length can be reduced from the original 1m to about 115 mm. The specific implementation process is as follows: ΔL is set to 0, ++L in order by MZI optical waveguide structure>Fixed waveguide length between two adjacent ΔL>The range is realized by continuous thermal tuning, whereby 0 to +.>Continuous tuning, i.e., implementing 2.5Gbps octave continuous tuning.
In use, when the delay length of the reference arm is limited, Δl cannot be octave by temperature tuning, for example, the demodulation rate can be continuously tuned between X and 1.25X, then any DPSK signal with a rate higher than 5X can be demodulated by the demodulator, while there is a portion of the region between X and 5X where the rate cannot be demodulated, as shown by the dark shaded portion and the portion between the two dashed lines in fig. 1 c, respectively. At this time, the demodulator of the embodiment is used for demodulating the optical fiber, and the delay length is changed stepwise through the added extra cascaded optical waveguide structure, so that the jump tuning of the rate switch is realized. The continuous thermal tuning and the switch jump tuning are combined to realize continuous, accurate, efficient and flexible tuning of the X-2X octave rate. In addition, because the lower demodulation rate limit X and the corresponding Δl are inversely related, which is a pair of contradictory amounts, a smaller lower demodulation rate limit X requires a larger range of delay length adjustment, and further requires a longer reference arm and delay arm, which results in a larger optical loss, so that in practical application, the lower demodulation rate limit X should be determined more reasonably by considering the thermo-optical coefficient and waveguide loss of the material in a compromise manner.
The tuning method of the demodulator according to the present embodiment, as shown in fig. 6, includes the steps of:
step one, according to the speed Y (Y is more than or equal to X) of an input DPSK signal and a formula Y=b×y (b epsilon N+), calculating to obtain a demodulation speed Y;
step two, tuning the demodulation rate y
2.1. The controller 40 adjusts the temperature of the reference arm waveguide 20 and the delay arm waveguide 30 through the temperature control module 50, continuously tunes the delay length, and thereby adjusts the demodulation rate;
2.2. judging whether the demodulation rate is tuned to y;
if yes, entering a step three;
if not, controlling the additional cascade optical waveguide arranged on the reference arm and the delay arm to perform switch jump tuning on the delay length, combining continuous thermal tuning and switch jump tuning to ensure that the demodulation rate of the demodulator is y, and entering the step three;
and step three, controlling the voltage of the heating electrode 64 in the first coupling unit 60 by using the controller 40, and adjusting the temperatures of the upper arm waveguide 62 and the lower arm waveguide 63 so as to output demodulation signals with high extinction ratio.
The waveguide loss and the thermo-optic coefficient of different material platforms are different due to the limitation of the prior art. If the thermo-optic coefficient is larger in other embodiments, the octave rate is adjusted in the first embodiment, and if the thermo-optic coefficient is smaller, the second embodiment is recommended. If the waveguide loss is larger, the octave rate is adjustable by adopting the second embodiment, and if the waveguide loss is smaller, the mode of the first embodiment is recommended.
Claims (10)
1. The utility model provides an on-chip octave rate adjustable DPSK demodulator, includes first beam splitter (10), reference arm, delay arm, beam combiner and first coupling unit (60), and first beam splitter (10) are used for dividing into two-way with the DPSK signal, and two-way signal is transmitted to beam combiner through reference arm and delay arm respectively, and the demodulation signal is output by coupling unit (60) coupling after the beam combination, its characterized in that:
also comprises a controller (40) and two temperature control modules (50);
the reference arm comprises a reference arm waveguide (20) and the delay arm comprises a delay arm waveguide (30); the reference arm waveguide (20) and the delay arm waveguide (30) are both arranged as long spiral line waveguides and are used for realizing octave rate tuning by adjusting delay;
the first coupling unit (60) adopts an MZI optical waveguide structure, is provided with two output ends and is used for outputting two demodulation signals, the light splitting ratio of the two demodulation signals is arbitrarily adjustable, high extinction ratio demodulation is realized, and the high extinction ratio refers to an extinction ratio greater than or equal to 25 dB;
the two temperature control modules (50) are respectively used for controlling the temperatures of the reference arm waveguide (20) and the delay arm waveguide (30);
the controller (40) is respectively connected with the first coupling unit (60) and the two temperature control modules (50) and is used for controlling the light splitting ratio of the first coupling unit (60), and controlling the temperatures of the reference arm waveguide (20) and the delay arm waveguide (30) through the temperature control modules (50), so that the delay length is controlled, the optical path difference between the delay arm and the reference arm is 1Bit or a positive integer multiple of 1Bit, and the demodulation rate continuous tuning is realized.
2. An on-chip octave rate adjustable DPSK demodulator in accordance with claim 1, wherein:
the first coupling unit (60) comprises a second beam splitter (61), an upper arm waveguide (62), a lower arm waveguide (63), two heating electrodes (64) arranged corresponding to the upper arm waveguide (62) and the lower arm waveguide (63), and a coupler (65);
the input end of the second beam splitter (61) is connected with the output end of the beam combiner, so that two paths of signals interfere, and the output end of the second beam splitter (61) is respectively connected with the input ends of the upper arm waveguide (62) and the lower arm waveguide (63);
the output ends of the upper arm waveguide (62) and the lower arm waveguide (63) are respectively connected with the coupler (65), the coupler (65) is a coupler (65) with any adjustable spectral ratio, and the coupler is used for converting phase information of two paths of signals into intensity information and outputting two demodulation signals after coupling;
the two heating electrodes (64) are connected with the output end of the controller (40), and the temperature change of the upper arm waveguide (62) and the lower arm waveguide (63) is controlled by controlling the heating electrodes (64), so that the light splitting ratio control of the first coupling unit (60) is realized, and the output demodulation signal has high extinction ratio.
3. An on-chip octave rate adjustable DPSK demodulator in accordance with claim 2, wherein:
the temperature control module (50) comprises a thermistor (51) and a micro heater (52);
the thermistor (51) is connected with the input end of the controller (40) and is used for monitoring the temperature of the corresponding reference arm waveguide (20) or the corresponding delay arm waveguide (30) in real time;
the micro heater (52) is connected with the corresponding output end of the controller (40), and the working state of the micro heater (52) is controlled by the controller (40), so that the reference arm waveguide (20) or the delay arm waveguide (30) is heated or cooled.
4. The on-chip octave rate adjustable DPSK demodulator of claim 3, wherein:
the heating electrode (64) adopts a metal film heater, and the metal film material is one of Al, cu, ti, W, ni, cr, au;
the coupler (65) adopts an MMI structure or a directional coupler structure;
the micro-heater (52) is a semiconductor refrigerator.
5. An on-chip octave rate adjustable DPSK demodulator according to any one of claims 1-4, wherein:
the system also comprises at least one additional cascade optical waveguide arranged on the reference arm and the delay arm respectively, wherein the additional cascade optical waveguides are MZI optical waveguide structures, and the number of the additional cascade optical waveguides on the reference arm and the delay arm is the same;
the additional cascaded optical waveguide comprises a second coupling unit (70), the structure of the second coupling unit (70) is the same as that of the first coupling unit (60), one output end of the second coupling unit (70) is connected with the input end of the first coupling unit (60), and the other output end of the second coupling unit is connected with the input end of the first coupling unit (60) or the input end of the next-stage additional cascaded optical waveguide through a delay waveguide;
the second coupling unit (70) is connected with the corresponding output end of the controller (40), and the controller (40) controls the switch states of the two output ends of the second coupling unit (70) to realize switch jump tuning of the delay length.
6. An on-chip octave rate adjustable DPSK demodulator in accordance with claim 5, wherein:
two additional cascading optical waveguides are arranged on the reference arm and the delay arm;
one output end of the second coupling unit (70) is connected with the first coupling unit (60), and the other output end of the second coupling unit is connected with the input end of the first coupling unit (60) or the next-stage additional cascaded optical waveguide through a delay waveguide;
the delay waveguide is a long spiral line waveguide.
7. An on-chip octave rate adjustable DPSK demodulator in accordance with claim 6, wherein:
the light splitting ratio of the first beam splitter (10) is 5:5;
the reference arm waveguide (20) and the delay arm waveguide (30) are made of one of silicon, silicon dioxide, high-refractive-index-difference doped glass, lithium niobate, silicon nitride and III-V materials;
the controller (40) is a field programmable gate array or a microprocessor.
8. An on-chip octave rate tuning method based on the on-chip octave rate adjustable DPSK demodulator according to any one of claims 1-7, comprising the steps of:
step one, determining a demodulation rate y by the following formula;
Y=b*y
wherein Y is the speed of the DPSK signal, Y is more than or equal to X, X is the lower limit of the demodulation speed, b is the multiple of the demodulation speed, and a positive integer is taken;
step two, the delay length delta is adjusted through a controller (40) so that the demodulation rate of the demodulator is y;
and step three, controlling the first coupling unit (60) by using the controller (40) to output a demodulation signal with a high extinction ratio.
9. The on-chip octave rate tuning method of claim 8, wherein:
the time delay length adjustment in the second step is specifically as follows:
2.1, controlling a temperature control module (50) through a controller (40) to adjust the temperature of the reference arm waveguide (20) and the delay arm waveguide (30), continuously adjusting the delay length delta, and continuously tuning the demodulation rate;
2.2, judging whether the demodulation rate is tuned to y;
if yes, entering a step three;
if not, the extra cascade optical waveguides arranged on the reference arm and the delay arm are controlled to carry out jump adjustment on the delay length, so that the switch jump tuning of the demodulation rate is realized, and the demodulation rate of the demodulator is y by combining the switch jump tuning and the continuous tuning.
10. The on-chip octave rate tuning method of claim 9, wherein the delay length δ is calculated by:
，
wherein ,calculated by the following formula:
，
wherein c is the speed of light, n g For the group refractive index of the reference arm waveguide (20) and the delay arm waveguide (30), R is the symbol rate of the DPSK signal, deltaL is the difference in length of the delay arm relative to the reference arm,in order to change the free spectrum range along with the temperature, m is the mode number, the value is a positive integer,αfor the shift of the resonance peak of the waveguide with temperature, Δt is the temperature difference of the reference arm waveguide (20) and the delay arm waveguide (30).
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