CN101222121A - Integrated optoelectronic devices for high-frequency microwave generation using SOA four-wave mixing effect - Google Patents

Abstract

The invention belongs to the technical field of optoelectronic devices, and in particular relates to an integrated optoelectronic device for generating high-frequency microwaves by using the SOA four-wave mixing effect. The integrated optoelectronic devices all adopt the ridge waveguide structure, and the two sides of the ridge waveguide are filled with SiO ₂ insulating layers, and a distributed feedback DFB laser and a semiconductor optical amplifier SOA are integrated. A layer of P-type electrodes and a layer of N-type electrodes are respectively fabricated on the top and bottom surfaces of the DFB laser and SOA. There is an electrical isolation section where the DFB laser is connected to the SOA, and there is no P-type electrode and ohmic electrode in the electrical isolation section. The contact layer; the two first-order modulation sidebands generated by the output light of the DFB laser are used as the pump light for the four-wave mixing effect of the SOA, and two lights with a larger frequency difference are generated to perform beat difference. The device has the advantages of high integration, low cost, high yield, simple manufacturing method and improved performance.

Description

Integrated optoelectronic devices for high-frequency microwave generation using SOA four-wave mixing effect

technical field

The invention belongs to the technical field of optoelectronic devices, and in particular relates to an integrated optoelectronic device for generating high-frequency microwaves by using the SOA four-wave mixing effect.

Background technique

The invention is a monolithic photonic integrated device based on the four-wave mixing effect of a semiconductor optical amplifier (Semiconductor Optical Amplifier, SOA). control etc. The following first briefly introduces the importance of high-frequency microwave or millimeter wave in wireless communication, and then introduces the application of optoelectronic technology in millimeter wave wireless communication.

In recent years, with the continuous development of optical fiber networks and the Internet (Internet), voice, image, data, video, and multimedia services based on the Internet have greatly stimulated people's demand for communication speed. Optical fiber communication technology based on wavelength division multiplexing technology (WDM) develops rapidly and has been widely used, and has become the most effective means of transmission in the physical layer of the trunk system. But on the other hand, the communication rate of the access network connecting the user terminal and the trunk optical fiber network is still at a relatively low level. At present, the "first mile" (or "last mile") access network in the entire communication link has become the bottleneck of high-speed communication technology, so the next-generation high-speed access technology has become a research hotspot in recent years. Among various access technologies, high-speed wireless access technology has attracted much attention due to its flexibility of terminal mobility. The most effective way to increase the transmission rate is to use electromagnetic waves with higher frequency as the communication carrier. Therefore, the use of millimeter waves (30GHz-300GHz) with a higher frequency than the current mobile communication carrier (800MHz-1.9GHz) can support transmission rates exceeding Gb/s high-speed wireless access. Due to the potential of millimeter-wave microwave in high-speed wireless communication, many countries such as the United States, Japan, and Germany are currently conducting research on millimeter-wave wireless access technology. The core research content includes access network system structure, mm Wave generation and transmission technology.

Due to the extremely high frequency of millimeter wave, if it is transmitted by traditional coaxial cable, its loss will be very serious. At the same time, the generation and processing of millimeter-wave signals using the electrical domain method also has the problem of high cost. An effective solution is to combine millimeter-wave wireless access with light-wave technology. Using light-wave as a carrier to transmit millimeter-wave signals can effectively reduce its transmission loss, which is commonly referred to as RoF (Radio over Fiber) technology. At the same time, light wave technology can be used to realize the generation of millimeter wave signals, thereby effectively reducing the complexity and cost of the wireless access system.

To load a millimeter wave with a frequency f on the light wave, the millimeter wave oscillator can be used directly to generate a signal with a frequency f and a high-speed optical modulator can be used to load the signal onto the light wave. However, the cost of the access system is limited because the millimeter-wave oscillator and the high-speed optical modulator working in the millimeter-wave frequency band are still very expensive. On the other hand, the millimeter-wave signal can be generated by optical heterodyning, that is, two optical signals with a frequency difference of f are used to generate a signal of frequency f in the photodetector through the difference frequency. Since this method does not require millimeter-wave oscillators and high-speed optical modulators, system costs can be greatly reduced. At the same time, the method can also be combined with optical wavelength division multiplexing technology to simultaneously modulate multiple channels, thereby further reducing the cost of the entire system and simplifying the system structure. Therefore, the optical difference frequency millimeter wave wireless access network has a simple structure and low cost, and is very suitable for the next generation broadband wireless access network. At present, technologies for generating microwave or millimeter waves by using optical heterodyne mainly include dual-wavelength lasers, mode-locked lasers, optical phase-locked loops, and sideband injection locking. Recently, literature (T.Wang, M.Chen, H.Chen and S.Xie, "Millimetre-wave signal generation using FWM effect in SOA", Electronics Letters, vol.43, No.1, pp.36～38, 2007 ) reported a method of using the nonlinear effect of SOA to generate millimeter waves, and its system is shown in Figure 1. In this document, a semiconductor laser is first modulated with a LiNbO ₃ modulator, which produces two first-order modulation sidebands. Suppose the frequency of the laser output light is ω ₀ and the modulation frequency is ω, then the frequencies of the two first-order sidebands are ω ₀ +ω and ω ₀ -ω respectively. The two first-order modulation sidebands are sequentially amplified by Erbium-Doped Fiber Ampiifer (EDFA), and the amplified spontaneous radiation generated by EDFA is filtered out by a band-pass filter, and then enters the semiconductor optical amplifier. Since the size of the semiconductor optical amplifier is small, generally below 1 mm, it is easy to meet the condition of phase matching, so that four-wave mixing effect (four-wave mixing, FWM) occurs. When the four-wave mixing effect occurs, the two first-order modulation sidebands can be used as pump light to generate two new frequencies of light, the frequencies of which are ω ₀ +3ω and ω ₀ -3ω respectively. At the same time, the two beams The phase of the newly generated light is correlated and kept constant with the phase of the semiconductor laser as the light source. Therefore, the two beams of light are beat-differenced and then received by a photodetector to obtain a frequency of 6ω and a small phase noise. microwave. SOA is a new research direction for semiconductor laser modulation sideband four-wave mixing to generate millimeter wave signals.

Most of the currently reported articles on microwave generation by optical heterodyne are systems built with discrete devices, while there are few reports on monolithic integrated devices. As we all know, systems built with discrete components are often large and complex, with poor stability and relatively high costs.

Contents of the invention

In order to solve the deficiencies in the prior art, the invention particularly provides an integrated optoelectronic device for generating high-frequency microwaves by using the SOA four-wave mixing effect.

Technical scheme of the present invention is as follows:

An integrated optoelectronic device that uses SOA four-wave mixing effect to generate high-frequency microwaves. The device grows the following epitaxial layers sequentially on an N-type substrate: lower cladding layer 9, lower waveguide layer 10, multi-quantum well active layer 11, The grating layer 12 , the upper waveguide layer 13 , the upper cladding layer 14 , and the ohmic contact layer 16 . An N-type electrode 8 and a P-type electrode 17 are respectively plated on the N-type substrate 9 and the ohmic contact layer 16 .

The device integrates a distributed feedback (Distributed Feedback, DFB) laser 21 and an SOA 23 . A ridge waveguide 20 structure is adopted, and both sides of the ridge waveguide are filled with SiO ₂ insulating layers 17 . A layer of P-type electrode 17 and a layer of N-type electrode 10 are formed on the top and bottom surfaces of DFB laser 21 and SOA 23 respectively. Where the DFB laser 21 is connected to the SOA 23, there is an electrical isolation section 22, which has no P-type electrode 17 and ohmic contact layer 18, so as to realize electrical isolation between devices.

The two first-order modulation sidebands generated by the output light of the DFB laser are used as the pump light for the four-wave mixing effect of the SOA to generate two lights with a larger frequency difference for beating.

The length of the DFB laser section is 300-500 μm, the length of the SOA section is 200-300 μm, and the length of the electrical isolation section is 30-50 μm. The SOA of the integrated optoelectronic device adopts an anti-reflection coating on the end face or a curved waveguide to control the feedback of light The rate is between 0.01% and 10%.

In addition, the integrated optoelectronic device can also integrate an electroabsorption (Electroabsorption, EA) modulator between the DFB laser and the SOA to indirectly modulate the DFB laser.

At this time, the length of the EA modulator segment is 50-150 μm.

The beneficial effect of the present invention is that: the integrated optoelectronic device of the present invention realizes monolithic integration, has the advantages of high integration, low cost, high yield, simple manufacturing method and improved performance.

Description of drawings

Fig.1 Schematic diagram of optically generated microwave system based on SOA four-wave mixing effect

Figure 2 integrates DFB lasers, SOA, and an optically-generated microwave monolithic integrated device based on the four-wave mixing effect of SOA

Figure 3 integrates DFB laser, EA modulator and SOA, based on the four-wave mixing effect of SOA, an optically-generated microwave monolithic integrated device

The numbers in the figure are: 1. DFB laser; 2. LiNbO ₃ modulator; 3. Modulator DC bias; 4. Modulator AC input; 5. EDFA; 6. Band-pass filter; 7. Can Variable attenuator; 8. Isolator; 9. SOA; 10. N electrode; 11. Substrate and lower cladding layer; 12. Lower waveguide layer; 13. Multiple quantum well active layer; 14. Grating layer; 15. Upper waveguide layer; 16. upper cladding layer; 17. SiO ₂ insulating layer; 18. ohmic contact layer; 19. P electrode; 20. ridge waveguide; 21. DFB section; 22. electrical isolation section; 23. SOA section; 24. EA modulator segment.

Detailed ways

The present invention will be described in further detail below with reference to the drawings and specific embodiments.

The invention provides an optically-generated microwave monolithic photon integrated device based on the SOA four-wave mixing effect. The device is integrated by multiple devices, wherein a distributed feedback (DFB) semiconductor laser 21 is used as the light source. The laser is directly modulated, or an electroabsorption modulator (Electroabsorption Modulator, EAM) 24 is integrated for indirect modulation, thereby generating two first-order modulation sidebands. Suppose the frequency of the laser is ω ₀ and the modulation frequency is Ω, then the frequencies of the two first-order sidebands are ω ₀ +Ω and ω ₀ -Ω respectively. Then an SOA 23 is integrated behind it, through the four-wave mixing effect of the SOA 23, light of two new frequencies can be generated, and the frequencies are ω ₀ +3Ω and ω ₀ −3Ω respectively. The phases of the two beams of newly generated light are correlated, and keep constant with the phase of 21 as the light source. Therefore, the beat difference between the two beams of light, and then received by the photodetector, can obtain a frequency of 6Ω and a relatively low phase noise. Small microwave.

In order to further optimize the device performance, the present invention also proposes that the SOA adopts an anti-reflection coating or a curved waveguide on the end face, so that the Fabry-Perot cavity (Fabry-Perot, FP) mode when the laser is lasing can be suppressed on the one hand, and at the same time The light reflected back to the DFB laser by the SOA output end face can be reduced, thereby avoiding the adverse effect of external reflection on the line width of the laser. The reflectance of the end face after the anti-reflection coating ranges from 10 ^-4 to 10%.

Two embodiments of the device of the present invention are introduced below, which are monolithic photonic integrated devices for generating high-frequency microwaves by utilizing SOA four-wave mixing effect on the basis of direct modulation and indirect modulation of DFB lasers.

Example 1

Referring to Figure 2, the working wavelength is in the 1550nm band, on the basis of direct modulation of DFB laser, using SOA four-wave mixing effect to generate high-frequency microwave InGaAsP/InP-based monolithic photonic integrated device.

The device integrates a DFB laser 21 and a SOA 23 on the same chip.

First, the epitaxial materials of the device are as follows. Through the MOCVD method, first epitaxially grow on the n-type substrate material, and then grow the n-type InP lower cladding layer 11 (thickness 200nm, doping concentration about 1×10 ¹⁸ cm ^-2 ), 100nm thick non-doped lattice matching InGaAsP Waveguide layer 12 (photofluorescence wavelength 1.2 μm), strained InGaAsP multiple quantum wells 13 (photofluorescence wavelength 1.52 μm, 7 quantum wells: well width 8nm, 0.5% compressive strain, barrier width 10nm, lattice matching material, photofluorescence wavelength 1.2 μm), 70 nm thick InGaAsP grating material layer 14 . Next, the grating structure is fabricated by holographic interference exposure, and the grating in the SOA area is removed by photolithography and wet etching. Then, a 100nm-thick p-type lattice-matched InGaAsP waveguide layer 15 (photoluminescent wavelength 1.2μm, doping concentration about 1×10 ¹⁷ cm ^-2 ) and a 1.7μm-thick P-type InP upper cladding layer 16 ( The doping concentration gradually changes from 3×10 ¹⁷ cm ^-2 to 1×10 ¹⁸ cm ^-2 ) and a 100nm thick p-type InGaAs ohmic contact layer 18 (doping concentration>1×10 ¹⁹ cm ^-2 ).

The entire device adopts a ridge waveguide 20 structure, and the ridge waveguide is fabricated by photolithography and dry etching. The ridge width is 3 μm and the height is 1.5 μm. The SiO ₂ insulating layer 17 is used to fill up the two sides of the ridge waveguide by plasma enhanced chemical vapor deposition (Plasma Enhanced Chemical Vapor Deposition, PECVD), and then the SiO ₂ on the top of the ridge is etched away. The P electrode 19 and the N electrode 10 are fabricated by sputtering. The material of the P electrode is Cr/Au alloy, and the material of the N electrode is Ti/Au alloy. Wherein, the P electrode includes two parts: one part is 400 μm long and serves as the P electrode of the DFB laser 21 ; the other part is 200 μm long and is used as the P electrode of the SOA part 23 . There is a 40 μm long region 22 between the two parts of the P electrodes, and the ohmic contact layer in this region is etched away to form an electrical isolation segment between the DFB laser and the SOA. The end face anti-reflection coating or curved waveguide is used in the SOA to reduce the light feedback caused by the end face reflection, so that the light feedback rate is between 10 ^-4 and 10%.

The characteristic parameters of this example are: in the manufactured integrated device, the typical value of the threshold current of the DFB laser is 10mA, and the side mode suppression ratio reaches above 40dB. The microwave signal is loaded on the injection current of the DFB laser to realize the direct modulation of the DFB laser. After the output light passes through the photodetector, microwaves with a frequency of 20-60GHz and a phase noise of less than -84dBc/Hz at 100kHz can be obtained.

Example 2

Referring to Figure 3, the working wavelength is in the 1550nm band, on the basis of indirect modulation of DFB lasers, using the SOA four-wave mixing effect to generate high-frequency microwave InGaAsP/InP-based monolithic photonic integrated devices.

The device integrates a DFB laser 21, an EA modulator 24 and an SOA 23 on the same chip.

First, the epitaxial materials of the device are as follows. Through the MOCVD method, first epitaxially grow on the n-type substrate material, and then grow the n-type InP lower cladding layer 11 (thickness 200nm, doping concentration about 1×10 ¹⁸ cm ^-2 ), 100nm thick non-doped lattice matching InGaAsP Waveguide layer 12 (photofluorescence wavelength 1.2 μm), strained InGaAsP multiple quantum wells 13 (photofluorescence wavelength 1.52 μm, 7 quantum wells: well width 8nm, 0.5% compressive strain, barrier width 10nm, lattice matching material, photofluorescence wavelength 1.2 μm), 70 nm thick InGaAsP grating material layer 14 . Next, the grating structure is fabricated by holographic interference exposure, and the gratings in the EA and SOA regions are removed by photolithography and wet etching. Then, a 100nm-thick p-type lattice-matched InGaAsP waveguide layer 15 (photoluminescent wavelength 1.2μm, doping concentration about 1×10 ¹⁷ cm ^-2 ) and a 1.7μm-thick P-type InP upper cladding layer 16 ( The doping concentration gradually changes from 3×10 ¹⁷ cm ^-2 to 1×10 ¹⁸ cm ^-2 ) and a 100nm thick p-type InGaAs ohmic contact layer 18 (doping concentration>1×10 ¹⁹ cm ^-2 ).

The entire device adopts a ridge waveguide 20 structure, and the ridge waveguide is fabricated by photolithography and dry etching. The ridge width is 3 μm, the height of the ridge waveguide in the DFB section and the SOA section is 1.5 μm, and the height of the ridge waveguide in the EA section is 4 μm. . The two sides of the ridge waveguide are filled with SiO ₂ insulating layer 17 by means of plasma enhanced chemical vapor deposition (Plasma Enhanced Chemical Vapor Deposition, PECVD), and then the SiO ₂ on the top of the ridge is etched away. The P electrode 19 and the N electrode 10 are fabricated by sputtering. The material of the P electrode is Cr/Au alloy, and the material of the N electrode is Ti/Au alloy. Wherein, the P electrode includes three parts whose lengths are 400 μm, 100 μm and 200 μm respectively, and they are the P electrodes serving as the DFB laser 21 , the EA modulator 24 and the SOA 23 in turn. There is a 40 μm long region 22 between two adjacent P electrodes, and the ohmic contact layer in this region is etched away to form electrical isolation between the DFB laser and the EA modulator, and between the EA modulator and the SOA. The end face anti-reflection coating or curved waveguide is used in the SOA to reduce the light feedback caused by the end face reflection, so that the light feedback rate is between 10 ^-4 and 10%.

The characteristic parameters of this example are: in the manufactured integrated device, the typical value of the threshold current of the DFB laser is 10mA, and the side mode suppression ratio reaches above 40dB. The microwave signal is loaded on the reverse electric field of the EA modulator to realize the indirect modulation of the DFB laser. After the output light passes through the photodetector, microwaves with a frequency of 20-60GHz and a phase noise of less than -84dBc/Hz at 100kHz can be obtained.

Claims (5)

1. An integrated optoelectronic device that uses SOA four-wave mixing effect to generate high-frequency microwaves. The device grows the following epitaxial layers sequentially on an N-type substrate: lower cladding layer, lower waveguide layer, multi-quantum well active layer, grating Layer, upper waveguide layer, upper cladding layer, ohmic contact layer, N-type electrode and P-type electrode are respectively plated on N-type substrate and ohmic contact layer, it is characterized in that: The integrated optoelectronic devices all adopt a ridge waveguide structure, and both sides of the ridge waveguide are filled with _SiO2 insulating layers, and a distributed feedback DFB laser and a semiconductor optical amplifier SOA are integrated; A layer of P-type electrodes and a layer of N-type electrodes are respectively fabricated on the top and bottom surfaces of the DFB laser and SOA. There is an electrical isolation section where the DFB laser is connected to the SOA, and there is no P-type electrode and ohmic electrode in the electrical isolation section. contact layer; The two first-order modulation sidebands generated by the output light of the DFB laser are used as the pump light for the four-wave mixing effect of the SOA, and two lights with a larger frequency difference are generated for beat difference. 2. A kind of integrated optoelectronic device utilizing SOA four-wave mixing effect to produce high-frequency microwaves according to claim 1, characterized in that: the length of the DFB laser section is 300-500 μm, and the length of the SOA section is 200-500 μm. 300 μm, the length of the electrical isolation section is 30-50 μm. 3. A kind of integrated optoelectronic device utilizing SOA four-wave mixing effect to produce high-frequency microwave according to claim 1, it is characterized in that: described integrated optoelectronic device integrates an EA modulator between DFB laser and SOA, to The DFB laser is indirectly modulated. 4. An integrated optoelectronic device for generating high-frequency microwaves by using SOA four-wave mixing effect according to claim 3, characterized in that: the length of the EA modulator section is 50-150 μm. 5. A kind of integrated optoelectronic device utilizing SOA four-wave mixing effect to generate high-frequency microwaves according to claim 1 or 3, characterized in that: the SOA of the integrated optoelectronic device adopts an anti-reflection coating on an end face or a curved waveguide, and the control The light feedback rate is between 0.01% and 10%.

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