A directly modulated multi-section tunable laser

Abstract

The application discloses a kind of directly modulated multi-section tunable laser, which is composed of multiple laser units in series, both ends of the tunable laser are coated with anti-reflection film, the front end of the first laser unit is integrated with an optical amplifier, the sampling grating periods of adjacent laser units are different, and the lasing wavelength difference is 2-5 nm;The laser unit includes: a laser light-emitting active gain region and a feedback compensation region arranged before and after the laser light-emitting active gain region, the laser light-emitting active gain region is inserted with an equivalent π phase shift sampling grating structure through the reconfiguration-equivalent chirp technology, and the feedback compensation region adopts a uniform sampling grating period, which is different from the sampling period of the sampling grating structure of the laser light-emitting active gain region.This tunable laser is beneficial to improve the modulation rate of the laser and the single-mode output characteristics of the laser, and simultaneously cascades multiple distributed feedback laser units to realize wavelength tuning coverage.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of laser, in particular to a directly modulated multi-section tunable laser. BACKGROUND

[0002] In optical access network, wavelength division multiplexing passive optical network (WDM-PON) combines the advantages of wavelength division multiplexing technology, and can realize further improvement of bandwidth and flexibility of access by using different wavelengths as carriers of information transmission at the same time. Therefore, in the backhaul network of WDM-PON, a wavelength tunable laser is needed to emit different wavelength signal light sources. As one of the most important optical communication network components, optical transmitter, due to the rapid development of access network service capacity and the substantial growth of the number of users, the demand for optical transmitter is the largest in medium and short distance (<30km) communication.

[0003] The commonly used optical transmitter includes directly modulated laser (DML) and electroabsorption modulation laser (EML). For medium and short distance, DML has the advantages of low loss, high output and low cost, but the modulation bandwidth of DML is reduced, i.e. the data transmission rate is reduced, due to the limitation of carrier and photon relaxation oscillation frequency. The directly modulated tunable laser applied to WDM network has strict specifications for the wavelength accuracy of each channel, and the wavelength accuracy of each channel is less than 0.1 nm. At the same time, there are certain protocol network requirements for the output power and side mode suppression ratio of each channel. At the same time, the wavelength tuning needs to cover a wide range.

[0004] Therefore, the directly modulated tunable semiconductor laser with high speed transmission has a wide application in access network and data exchange network, and the high-speed directly modulated laser array meeting the above requirements can be used to constitute the transmitter of the next generation Ethernet, and can be used to design a more intelligent optical communication network architecture and the corresponding optical module. SUMMARY

[0005] Based on this, the present application provides a directly modulated multi-section tunable laser, which solves the problems of bandwidth limitation by relaxation oscillation frequency, low side mode suppression ratio and accurate wavelength emission of all channels in the tunable laser in the prior art. The tunable laser is beneficial to improve the modulation rate of the laser and the single mode output characteristics of the laser, and simultaneously realizes wavelength tuning coverage by cascading multiple distributed feedbacks.

[0006] In order to achieve the above object, the technical scheme adopted by the present application is as follows: a directly modulated multi-section tunable laser is composed of a plurality of laser units in series, both ends of the tunable laser are coated with an anti-reflection film, an optical amplifier is integrated at the front end of the first laser unit, the sampling grating periods of adjacent laser units are different, and the lasing wavelength difference is 2-5 nm; the laser unit comprises a laser light-emitting active gain region and a feedback compensation region arranged in front and back sections of the laser light-emitting active gain region, the laser light-emitting active gain region adopts a sampling grating designed by a reconstruction-equivalent chirp technology, and an equivalent pi phase shift structure is introduced in the laser light-emitting active gain region, and the feedback compensation region adopts a uniform sampling grating structure, which is different from the sampling period of the sampling grating structure of the laser light-emitting active gain region.

[0007] Further, the lower part of the electrode of the laser light-emitting active gain region is etched with InP, and the etched high dielectric constant InP is filled with a benzocyclobutene material.

[0008] Further, the electrode of the laser light-emitting active gain region adopts a circular electrode.

[0009] Further, the length of the laser light-emitting active gain region accounts for 25%-45% of the length of the laser unit.

[0010] Further, the laser light-emitting active gain region is injected with a mixed signal of alternating current and direct current bias, and the feedback compensation region is injected with a transparent current signal.

[0011] Further, InGaAs and InP are etched between the laser units.

[0012] Further, InGaAs and InP are etched between the laser light-emitting active gain region and the feedback compensation region.

[0013] Further, the feedback compensation regions are all connected to the same electrode.

[0014] Further, the equivalent sampling pi phase shift structure is located in the 1 / 2 part of the laser light-emitting active gain region.

[0015] Further, the sampling grating structure is an InGaAsP material, and the duty cycle of the sampling grating is 0.5.

[0016] Compared with the prior art, the application has the following beneficial effects: the application realizes the design of a high-speed directly modulated cascade laser array, and simultaneously meets the functions of high-speed direct modulation and wavelength tunable; meanwhile, the laser unit adopts a multi-section structure, reduces the length of the intermediate active gain region, improves the relaxation oscillation frequency of the laser unit, improves the parasitic parameters of the directly modulated laser, enhances the single-mode output characteristics and improves the side mode suppression ratio through the design of the sampling grating and the design of the feedback compensation region; in addition, an optical amplifier is integrated in the front end of the array to improve the laser output power. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A schematic view of the laser unit of the application parallel to the laser emission direction, wherein 201 is an n-InP substrate, 202 is an n-InP buffer layer, 203 is an n-InAlGaAs lower optical confinement layer, 204 is an InAlGaAs multi-quantum well, 205 is a p-InAlGaAs upper optical confinement layer, 206 is a P-InP buffer layer, 207 is a P-InGaAs ridge waveguide, 208 is a metal electrode, 209 is an isolation region, and 210 is an InGaAsP grating region;

[0018] Figure 2 A sampling grating structure of the serial laser array in the embodiment;

[0019] Figure 3 A light field intensity distribution simulation diagram of the multi-section laser unit in the embodiment;

[0020] Figure 4 An output spectrum schematic view of each laser unit in the serial tunable laser in the embodiment, wherein 501 is the first laser spectrum, 502 is the second laser spectrum, 503 is the third laser spectrum, and 504 is the fourth laser spectrum;

[0021] Figure 5 A side mode suppression ratio of the spectrum output and a temperature range of thermal tuning of the laser array in the embodiment during the tunable process, wherein 601 is the 12-channel wavelength output side mode suppression ratio, and 602 is the temperature range during the thermal tuning of the 12-channel wavelength;

[0022] Figure 6 A 12-channel wavelength output diagram of the high-speed directly modulated laser array in the embodiment;

[0023] Figure 7 A bandwidth diagram of the high-speed directly modulated laser unit in the embodiment under a small signal response;

[0024] Figure 8 A 0 km (back-to-back transmission) eye diagram of the single-channel of the high-speed directly modulated laser array in the embodiment for 25 Gbps direct modulation transmission;

[0025] Figure 9 Eye diagram of 10km transmission of 25Gbps direct modulation for a single channel of high-speed direct modulation laser array in the embodiment. DETAILED DESCRIPTION

[0026] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described below in connection with the specific embodiments of the present application and corresponding drawings. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0027] The present application improves the photoelectric response frequency of the tunable laser by designing a direct modulation multi-section tunable laser. The tunable laser is composed of a plurality of laser units in series, and the two ends of the tunable laser are coated with an anti-reflection film. The end surface emissivity of the anti-reflection film is within the range of 10 -5 to 10%; the front end of the first laser unit is integrated with an optical amplifier, and the optical amplifier injects current for power compensation amplification. The optical amplifier has the same quantum well structure as the laser. When the optical signal passes through, the particle inversion is formed under the action of the applied bias current, the electrons lose energy in the form of photons and return to the ground state, thereby the energy conversion from electrons to photons occurs; the sampling grating periods of adjacent laser units are different, the lasing wavelength difference is 2-5nm, and the lasing wavelength interval of adjacent laser units is greater than 2nm, which can avoid the lasing mode crosstalk between the laser units, ensure the single mode characteristic of the working laser, and at the same time, the wavelength interval of the laser units is within 5nm, which can make the laser cover a continuous wavelength range under the action of temperature tuning.

[0028] The laser unit in the application comprises: a laser light-emitting active gain region and a feedback compensation region arranged in front and back of the laser light-emitting active gain region, the laser light-emitting active gain region adopts a sampled grating designed by a reconstruction-equivalent chirp technology, an equivalent pi phase shift structure is introduced in the laser light-emitting active gain region, a lambda / 4 phase shift is introduced, the precision of the grating phase is improved, meanwhile, the reconstruction-equivalent chirp technology is adopted to simplify the manufacturing process and reduce the manufacturing cost, and the grating phase is accurately controlled; the feedback compensation region and the laser light-emitting active gain region use the same quantum well structure, and the feedback compensation region adopts a uniform sampled grating period which is different from the sampling period of the sampled grating structure of the laser light-emitting active gain region. This is because, in the use process, all the front and back feedback compensation region power electrodes are combined, the transparent current is injected at the front and back, the feedback compensation region power current value is relatively small, and thus the temperature is low, the laser light-emitting active gain region power value is relatively high, and thus the temperature is high, and thus the temperature is about 2-10 DEG C higher than that of the front and back feedback compensation region. In order to control the feedback compensation region lasing wavelength to be consistent with the middle gain active region wavelength in actual use, according to the formula Wherein, m represents the order of the sampled grating, m is selected as +1 order, P represents the period of the sampled grating, Lambda0 represents the period of the seed grating, Lambda m represents the m order seed grating period. Meanwhile, Lambda m = 2n eff Lambda m , wherein n eff is the effective refractive index, Lambda m is the m order seed grating lasing wavelength, i.e. the seed grating lasing wavelength. By controlling the period interval P of the sampled grating, the final lasing output wavelength value of the seed grating can be controlled. In this way, the grating period interval of the front and back feedback compensation region is increased, the low-temperature wavelength is consistent with the high-temperature lasing wavelength of the laser light-emitting active gain region, the center wavelength of the front and back feedback compensation region of the laser unit is 0.2-1 nm higher than the input wavelength of the laser light-emitting active gain region, the overall optical field distribution of the laser can be improved, the single mode characteristic of the laser light-emitting active gain region is increased, and the side mode suppression ratio is improved.

[0029] The principle of the direct modulation laser is that when the distributed feedback laser is injected with the modulation current, the carrier concentration in the cavity changes, the carrier transitions to the ground state to produce photon energy, and the optical gain generated in the active region with different concentrations is also different, and the final output power of the laser changes with time. The key to realizing the direct modulation of the tunable series laser is to increase the modulation bandwidth and reduce the crosstalk between the grating of the series laser. The electro-optic response of the direct modulation distributed feedback laser can be expressed as:

[0030] R(f) = M(f) x L(f)

[0031] Wherein, M(f) is the inherent frequency response of the laser, representing the dynamic response of the interaction between carriers and photons, L(f) is the frequency response of the entire system, depending on the equivalent circuit model of the entire system, mainly affected by the parasitic capacitance and parasitic resistance, L(f) depends on the RC constant of the equivalent circuit model.

[0032]

[0033] Wherein, f r Representing the relaxation oscillation frequency of photons and electrons, f represents the direct modulation frequency, and gamma represents the damping coefficient.

[0034] Therefore, the relaxation oscillation frequency of the laser needs to be improved, and the RC constant caused by the parasitic effect needs to be reduced, wherein the relaxation oscillation frequency can be represented as:

[0035]

[0036] Wherein, v g Representing the group velocity, S0 represents the stable photon density in the cavity, g is the differential gain of the laser, tau P The effective length of the active region is reduced, the electron lifetime is reduced, the photon lifetime is increased, the density in the photon laser cavity is increased, the relaxation oscillation frequency is improved, and the bandwidth of the optoelectric effect is improved.

[0037] In order to further improve the relaxation oscillation frequency bandwidth of the laser, and improve the photoelectric effect bandwidth of the laser, the laser unit in the present application has high differential gain and photon density in the light-emitting active gain region of the laser, increases the relaxation oscillation frequency, and therefore improves M(f), that is, the inherent frequency response of the laser. The length of the modulation part of the light-emitting active gain region of the laser is reduced, and the parasitic capacitance of the electrode structure is also reduced, thereby reducing the RC constant and increasing the L(f) frequency. Therefore, the length of the light-emitting active gain region of the laser in the present application accounts for 25%-45% of the length of the laser unit, the RC constant in the equivalent circuit model of the laser chip is reduced, and the effective length of the light-emitting active gain region of the laser is reduced. The electron lifetime is reduced, the photon lifetime is increased, the density in the photon laser cavity is increased, the relaxation oscillation frequency is improved, and the inherent frequency response of the laser is improved.

[0038] In the application, the InP under the electrode of the laser light-emitting active gain region is etched, the etched high dielectric constant InP is filled with a benzocyclobutene material, the dielectric constant is reduced, and the capacitance between the electrodes is reduced. At the same time, the electrode of the laser light-emitting active gain region adopts a circular electrode to reduce the area and thus reduce the parasitic capacitance. This is conducive to further improving the bandwidth performance of L(f).

[0039] When analyzing the electrical performance of the laser, the frequency response of the entire system is considered, which depends on the equivalent circuit model of the entire system and is mainly affected by the parasitic capacitance and the parasitic resistance. The smaller the parasitic capacitance, the better the bandwidth performance under the electrical performance. Therefore, in the application, the laser light-emitting active gain region is injected with a mixed signal of alternating current and direct current bias, a main mode gain is generated, and a modulation signal is additionally applied; the feedback compensation region is injected with a transparent current signal, which can suppress the side mode, suppress the crosstalk of the side mode when other lasers are powered on, and improve the intensity of the main mode of the light-emitting active gain region of each laser unit and the single mode characteristic.

[0040] In the application, InGaAs and InP are etched between the laser units to generate an electrical isolation region, so as to control the injection current of each laser respectively and prevent the influence of the transparent current of other lasers on the working laser. InGaAs and InP are etched between the laser light-emitting active gain region and the feedback compensation region, so that the feedback region and the gain region are well electrically isolated, and the high-speed signal of the gain region does not interfere with the direct current bias current value of the feedback grating region.

[0041] In the application, the feedback compensation regions are connected to the same electrode, so as to reduce the number of power supplies and the complexity of the packaging process.

[0042] In one technical solution of the application, the compensation of the equivalent sampling pi phase shift is located at 1 / 2 of the laser light-emitting active gain region, and the phase shift of the ordinary Bragg grating containing the pi phase shift is equivalent to the sampling Bragg grating. The equivalent phase shift sampling Bragg grating is implemented by at least one of the following methods:

[0043] (1) The equivalent reconstruction technology is used to insert a pi phase shift in the sampling structure of the grating.

[0044] In the directly modulated tunable laser of the application, the wavelength tuning is achieved by selecting the working laser in the series laser and controlling the temperature of the laser chip. The thermal tuning method includes but is not limited to:

[0045] (A) A semiconductor thermoelectric cooler TEC (containing temperature sensitive material, using the Peltier effect) is installed at the lower part of the laser test table, the temperature of the laser array test table is changed, and thus the working wavelength of the laser is adjusted.

[0046] (B) The laser is packaged on a carrier, and the temperature of the laser carrier is controlled by a TEC, so as to adjust the working wavelength of the laser.

[0047] (C) The contact thermal resistance is plated on the electrode surface of the injection current of the active region, so that the working temperature of the injection current is changed, and the working wavelength of the laser is adjusted.

[0048] In one technical solution of the present application, the sampling grating structure and the uniform sampling grating are both InGaAsP materials, and the duty cycle of the sampling grating structure and the uniform sampling grating is 0.5. This duty cycle is selected because the effective grating feedback coefficient of the +1 order is the largest, the mode feedback is the strongest, and the mode coupling coefficient is the largest.

[0049] As Figure 1 For the laser unit involved in the present application, the epitaxial material is grown by two times of metal organic thin film growth chemical vapor deposition (MOCVD). First, an n-InP buffer layer 202, an n-InAlGaAs lower optical confinement layer 203, an InAlGaAs multi-quantum well 204, a p-InAlGaAs upper optical confinement layer 205, and an InGaAsP grating region 210 are grown on an n-InP substrate 201 in sequence. A traditional grating method is adopted, and the sampling grating is made by conventional exposure combined with holographic exposure. Then, photolithography and subsequent etching processes are performed, and then a p-InP buffer layer 206 and a p-InGaAs ridge waveguide 207 are grown. The waveguide adopts a shallow ridge waveguide design, and there is no carrier surface recombination. At the same time, defects caused by quantum well etching are also avoided. If partial structures need to be electrically isolated, InP and InGaAs need to be etched to form an isolation region 209 to prevent current crosstalk between adjacent structures. Subsequently, silicon oxide is deposited on the waveguide to avoid the contact of the metal electrode with other parts and cause large loss. Then, the silicon oxide on the top of the waveguide is removed, and a metal electrode 208 is plated on the waveguide for electron injection to the waveguide.

[0050] Embodiment

[0051] In the present embodiment, four laser units are connected in series to form a tunable laser, and anti-reflection films are plated on both ends of the tunable laser. In the present embodiment, the end surface emissivity of the anti-reflection film is 10 -4 , which avoids random phase of the end surface and ensures controllable phase of the laser and improves the single-mode characteristics of the laser. Each laser light-emitting active gain region adopts a sampling grating designed by a reconstruction-equivalent chirp technology, an equivalent π phase shift structure is introduced in the middle of the unit, and a λ / 4 phase shift is introduced. Figure 2As shown, 301 represents the tandem structure of the first laser with a π-phase-shift grating, 302 represents the tandem structure of the second laser with a π-phase-shift grating, 303 represents the tandem structure of the third laser with a π-phase-shift grating, and 304 represents the tandem structure of the fourth laser with a π-phase-shift grating. The feedback compensation region uses a uniform sampling grating with a different period than the sampling grating structure of the laser's active gain region. Simultaneously, InP is etched between the laser units to reduce current injection crosstalk between each laser unit. Each laser unit is 500µm long, the active gain region is 200µm long, and the feedback compensation regions at both ends are 150µm long. The total length of this tunable laser is 2500µm, and the total width is 300µm. An optical amplifier (SOA) is integrated at the very front of the first tandem laser unit to adjust the final output power of the tunable laser. The waveguide of the monolithically integrated SOA at the front of the tunable laser is tilted and bent at 7° to reduce reflection from the tandem lasers and prevent light reflection from the output end face from affecting the laser's modulation performance. The temperature of the tunable laser was controlled at 25℃. Currents of 80mA, 80mA, 30mA, and 30mA were injected into the optical amplifier, the active gain region of the laser, and the feedback compensation regions at both ends, respectively. Simultaneously, when the laser at the rear end of the series laser emitted light, a current of 15mA was injected into the front end laser to compensate for light loss in the front end laser cavity during the operation of the rear end laser. This current was defined as the transparent current, thus fabricating a multi-segment tunable laser. Figure 3 The simulation diagram of the light field intensity distribution of the laser unit shows that the photon concentration distribution in the laser cavity in lasing mode is concentrated in the active gain region of the laser, thereby increasing the photon density S0 in the active gain region of the laser in steady state, increasing the relaxation oscillation frequency of the laser, and enhancing the single-mode characteristics.

[0052] like Figure 4 This diagram illustrates the output spectrum of each laser unit in the cascaded tunable laser system of this embodiment. 501 represents the spectrum of the first laser, 502 represents the spectrum of the second laser, 503 represents the spectrum of the third laser, and 504 represents the spectrum of the fourth laser. The interval between each laser unit is 2.47 nm. Each laser unit can output three wavelengths with an interval of 0.08 nm. Furthermore, the side-mode suppression ratio of each laser unit is greater than 50 dB, demonstrating the single-mode characteristics of a single laser operating within the cascaded laser system.

[0053] When the current of the optical amplifier SOA is changed, the output power of the laser also changes accordingly. At the same time, when each laser is working, the output power of the laser is greater than 14mW when 60mA is injected into the optical amplifier.

[0054] The selection of the working laser and the control of the temperature of the external semiconductor cooler TEC affect the temperature of the tunable laser, thereby adjusting the wavelength of the laser output, as shown in Figure 5 , 6 The tunable laser realizes 12-channel wavelength output with a wavelength interval of 0.08 nm, realizes precise wavelength interval design, and the temperature tuning range of the 12 channels is only 16℃.

[0055] The radio frequency packaging carrier for designing the chip test has a high-frequency microstrip line designed and simulated thereon, and meets the requirement of high-speed transmission in the electrical aspect, and a resistance is connected in series on the coplanar waveguide transmission line to achieve the matching resistance value. The small-signal curve of the laser in the series laser is measured by a vector network analyzer, and when the direct current bias current of the middle light-emitting active region is 100 mA, the 3dB bandwidth under the small-signal excitation reaches 20 GHz, which basically meets the high-speed transmission of 25 Gbit / s. Figure 7

[0056] The tunable laser of the embodiment simultaneously performs a systematic transmission experiment to directly prove the high-speed performance of the integrated chip. A high-quality bit test system is built by using a code generator and a bit error instrument. The code generator generates a "0" "1" pseudo-random bit sequence, the high-speed signal and the direct current bias generated by the current source are loaded on the heat sink carrier through AC-DC coupling, the direct current bias AC signal is loaded on the integrated chip through the microstrip line and the gold wire bonding, the output light is directly coupled to an optical oscilloscope through a tapered optical fiber, and relevant high-speed eye diagram information is obtained. The back-to-back eye diagram test of the integrated chip is shown in Figure 8 As can be seen, the eye diagrams of the 12 channels are fully open, and no signal points fall on the standard template, and the test effects of all channels are similar.

[0057] The tunable laser of the embodiment performs a simulation of the actual application scene test of fiber transmission. The high-speed direct modulation laser is usually applied to a short-distance optical transmission network, and therefore the high-speed direct modulation laser array in the application is used for 10 km distance transmission, and then the modulated light after transmission is connected to an optical oscilloscope, and the eye diagram after high-speed transmission of 10 km is shown in Figure 9 As can be seen, the eye diagram is still fully open, and no deformed eye pattern appears, and the test effects of all channels are similar. Therefore, it can be predicted that the cascaded laser array can be used as a component part of a short-distance transmission optical transmitter.

[0058] The above is only a preferred embodiment of the application, and the protection scope of the application is not limited to the above embodiment. Any technical solution falling within the idea of the application belongs to the protection scope of the application. It should be noted that some improvements and refinements without departing from the principle of the application should be considered as the protection scope of the application.​

Claims

1. A directly modulated multi-section tunable laser, characterized by, The tunable laser is composed of a plurality of laser units in series, both ends of the tunable laser are coated with an anti-reflection film, an optical amplifier is integrated at the front end of the first laser unit, the sampling grating periods of adjacent laser units are different, and the difference of the lasing wavelengths is 2-5 nm; the laser unit comprises a laser light-emitting active gain region and a feedback compensation region arranged at the front and rear sections of the laser light-emitting active gain region, the laser light-emitting active gain region adopts a sampling grating designed by a reconstruction-equivalent chirp technology, and an equivalent π phase shift structure is introduced in the laser light-emitting active gain region, and the feedback compensation region adopts a uniform sampling grating structure, which is different from the sampling period of the sampling grating structure of the laser light-emitting active gain region; The lower part of the electrode of the laser light-emitting active gain region is etched InP, and the etched high dielectric constant InP is filled with a benzocyclobutene material; The length of the laser light-emitting active gain region accounts for 25%-45% of the length of the laser unit; The laser light-emitting active gain region injects a mixed signal of alternating current and direct current bias, and the feedback compensation region injects a transparent current signal.

2. The directly modulated multi-section tunable laser of claim 1, wherein, The electrode of the laser light-emitting active gain region adopts a circular electrode.

3. The directly modulated multi-section tunable laser of claim 1, wherein, InGaAs and InP are etched between the laser units.

4. The directly modulated multi-section tunable laser of claim 1, wherein, InGaAs and InP are etched between the laser light-emitting active gain region and the feedback compensation region.

5. The directly modulated multi-section tunable laser of claim 1, wherein, The feedback compensation regions are connected to the same electrode.

6. The directly modulated multi-section tunable laser of claim 1, wherein, The equivalent sampling π phase shift structure is located in the 1 / 2 part of the laser light-emitting active gain region.

7. The directly modulated multi-section tunable laser of claim 1, wherein, The sampling grating structure is InGaAsP material, and the duty cycle of the sampling grating is 0.5.

Images