WO2016206570A1 - 一种短腔长的分布反馈激光器 - Google Patents

一种短腔长的分布反馈激光器 Download PDF

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WO2016206570A1
WO2016206570A1 PCT/CN2016/086143 CN2016086143W WO2016206570A1 WO 2016206570 A1 WO2016206570 A1 WO 2016206570A1 CN 2016086143 W CN2016086143 W CN 2016086143W WO 2016206570 A1 WO2016206570 A1 WO 2016206570A1
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grating
reflection
region
laser
distributed feedback
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国伟华
赵龚媛
陆巧银
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华中科技大学
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    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
    • H01S3/067—Fibre lasers
    • H01S3/0675—Resonators including a grating structure, e.g. distributed Bragg reflectors [DBR] or distributed feedback [DFB] fibre lasers
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    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
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    • H01S3/0632—Thin film lasers in which light propagates in the plane of the thin film
    • H01S3/0635—Thin film lasers in which light propagates in the plane of the thin film provided with a periodic structure, e.g. using distributed feed-back, grating couplers
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    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/028—Coatings ; Treatment of the laser facets, e.g. etching, passivation layers or reflecting layers
    • H01S5/0287—Facet reflectivity
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    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • H01S5/124—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers incorporating phase shifts
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    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/12—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region the resonator having a periodic structure, e.g. in distributed feedback [DFB] lasers
    • H01S5/125—Distributed Bragg reflector [DBR] lasers
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    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10007—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers
    • H01S3/10023—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating in optical amplifiers by functional association of additional optical elements, e.g. filters, gratings, reflectors
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    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/105—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length
    • H01S3/1055—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling the mutual position or the reflecting properties of the reflectors of the cavity, e.g. by controlling the cavity length one of the reflectors being constituted by a diffraction grating
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    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1305—Feedback control systems
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    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/1082—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region with a special facet structure, e.g. structured, non planar, oblique
    • H01S5/1085—Oblique facets
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    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/16—Window-type lasers, i.e. with a region of non-absorbing material between the active region and the reflecting surface

Definitions

  • the invention belongs to the technical field of semiconductor lasers and relates to a distributed feedback laser with ⁇ /4 phase shift.
  • the distributed feedback laser adopts a structure with periodically changing refractive index to realize the feedback of the resonant cavity. Due to the planar process, it can be applied to the integrated optical path for coupling and integration with other devices, and the application range is very wide.
  • ⁇ /4 phase shift distribution feedback laser Utaka K., Akiba S., Sakai K., et al., ⁇ /4 -shifted InGaAsP/InP DFB lasers [J].
  • Quantum Electronics IEEE Journal of, 1986, 22(7): 1042-1051.); the other is a uniform grating with high-reflection and anti-reflection coatings on both ends Distributed feedback laser (Razeghi M., Blondeau R., Krakowski M., et al. Low-threshold distributed feedback lasers fabricated on material grown complete by LP-MOCVD [J]. Quantum Electronics, IEEE Journal of, 1985, 21 (6) ): 507-511.).
  • the ⁇ /4 phase-shifted distributed feedback lasers are used for integration (Kobayashi W., Kanazawa S., Ohno T.). , et al., Monolithically integrated directly modulated DFB laser array with MMI coupler for 100 GBASE-LR4 application [C]//Optical Fiber Communication Conference. Optical Society of America, 2015: Tu3I.2.). Since the two ends are coated with an anti-reflection coating, the output of the laser is equally power at both ends, but typically only one end of the output can be coupled into the fiber to become an externally available power output. This means that the output power of the laser is lost by half.
  • the usual method is to reduce the cavity length of the laser. For example, to achieve a 3dB direct modulation bandwidth above 30 GHz, the cavity length of the laser needs to be reduced to less than 200 microns. If the cavity length is reduced to 200 microns or less, the threshold gain of the conventional ⁇ /4 phase-shifted distributed feedback laser with an antireflection coating on both ends becomes very high, and the laser performance deteriorates.
  • a bandwidth of more than 30 GHz can be achieved with a cavity length of 150 ⁇ m (Kobayashi W., Tadokoro T., Ito T., et Al., High-speed operation at 50Gb/s and 60-km SMF transmission with 1.3- ⁇ m InGaAlAs-based DML[C]//Semiconductor Laser Conference (ISLC), 201223rd IEEE International. IEEE, 2012: 50-51.
  • the side mode suppression ratio and yield of the laser are greatly reduced, that is, some advantages of the conventional ⁇ /4 phase shift distributed feedback laser, such as high side mode suppression ratio, high yield, and lasing wavelength accuracy. The advantages of control have disappeared.
  • the present invention proposes a new method for realizing a short cavity length (200 micron and below) distributed feedback laser.
  • the laser is divided into two parts: an active area and a reflective area, both of which contain a grating; the active area grating has a ⁇ /4 phase shifting region, the active area length is less than or equal to 200 ⁇ m; and the reflective area is at the non-output end of the laser.
  • the reflection area grating is a uniform period grating; the end surface of one end of the laser output is coated with an anti-reflection film.
  • the grating is a Bragg reflection grating whose period ⁇ is calculated according to the following formula:
  • n eff is the effective refractive index of the waveguide.
  • the reflection region and the waveguide of the active region adopt the same core layer structure, and the waveguide transmission layer of the reflection region is also an active quantum well material.
  • the quantum well material in the reflection region can be pumped to the transparent state by the laser's own light, so the absorption loss of the quantum well can be reduced to zero.
  • the loss of the waveguide is the internal loss of the active region waveguide, and the usual loss is 20cm - 1 magnitude.
  • the length of the reflection region can be customized according to the required reflectance.
  • the grating coupling coefficient of the reflection region can be customized according to the required reflectivity, and does not necessarily need to be the same as the grating of the active region.
  • the larger the coupling coefficient, the higher the reflectivity, and the same reflectivity can be Adjusted to 80% or more according to the coupling coefficient.
  • the distributed feedback laser, the reflection region grating can adopt a period different from that of the active region grating, so that even when the effective refractive index of the active region and the reflective region are not the same, the Bragg wavelength of both can be made by adjusting the period.
  • the same, that is, the peak wavelength of the reflection peak is the same.
  • the outer end portion of the reflective region is selected to adopt a window region, or a horizontally inclined end surface, or an anti-reflection coating, or a combination of the above various methods to reduce reflection.
  • the output of the distributed feedback laser is coated with an anti-reflection film, and the reflectance of the anti-reflection film is less than 1% to reduce reflection.
  • the present invention introduces a Bragg grating in the reflection region, which can improve the feedback of the laser cavity, and is equivalent to plating the anti-reflection film on the end surface of the active region, thereby reducing the threshold gain, so that the laser can Achieve short cavity length (200 micron and below) operation, which can improve the direct modulation bandwidth of the laser; in addition, the slope efficiency of the laser output can be improved, and the reflection phase generated by the reflection region grating can be controlled, so that the side mode suppression ratio is not caused.
  • the laser adopts the same waveguide core layer in the active area and the reflection area, and the formation of the reflection area no longer requires a complicated docking regrowth process, and thus has the characteristics of low fabrication difficulty.
  • FIG. 1 is a schematic structural view of a distributed feedback laser of the present invention.
  • Figure 2 is a simulated reflection spectrum of different length reflection regions.
  • Fig. 3A is a graph showing the relationship between the equivalent refractive index difference of the laser gain region and the reflection region of the present invention and the threshold gain.
  • FIG. 3B is a simulated refractive index difference and main reflection difference of the laser gain region and the reflection region of the present invention. A plot of the threshold gain difference between the mode and the side mode.
  • the short cavity length distributed feedback laser of the present invention comprises an active region 1, also referred to as an optical gain region and a reflective region 2, and the active region 1 includes an electrode contact layer 3 and a waveguide upper cover from top to bottom.
  • the reflection region 2 includes a waveguide upper cover layer 4, a grating layer 5, an upper light confinement layer 6, a quantum well layer 7, a lower light confinement layer 8, and a waveguide lower cap layer 9 from top to bottom.
  • the grating of the active region portion includes a phase shift region of ⁇ /4, and the gratings on both sides of the phase shift region are uniform gratings of the same period.
  • the introduction of the phase shift region of ⁇ /4 causes the Bragg wavelength of the grating to be also the resonant wavelength of the resonant cavity, and also the lasing wavelength of the laser. This makes it possible to control the lasing wavelength of the laser by precisely controlling the Bragg wavelength of the grating.
  • the grating can only provide the most effective reflection at the Bragg wavelength, the single model of the laser is good.
  • the additional feedback of the reflection region can reduce the threshold gain of the laser, thereby enabling short cavity length, and the feedback of the reflection region increases the output power of the laser output. Therefore, the slope efficiency of the laser can be improved.
  • the Bragg reflection grating whose period ⁇ is calculated according to the following formula:
  • n eff is the effective refractive index of the waveguide.
  • the feedback of the reflection zone is adjusted in two ways:
  • the above two methods can theoretically improve the reflectivity of the reflection region to close to 1, but the inherent loss of the waveguide itself causes the reflectance to be less than 1, and the loss of the simulated waveguide is 20 cm -1 , the obtained wavelength and the like.
  • the relationship between the effective reflectance is shown in Figure 2. It can be seen that the reflectance at the wavelength of Bragg can reach more than 85%. Since the grating is processed by a high-precision fabrication method, for example, by electron beam exposure technology, the phase of the feedback provided by the grating of the reflection region can be precisely controlled, so that the feedback provided by the reflection region and the feedback provided by the gain region grating can remain the same. Phase, which ensures that the laser can lasing at the maximum feedback wavelength.
  • the threshold of the laser can be maintained at a relatively low level. This is very important when the cavity length of the laser is reduced.
  • the laser's lasing wavelength can be precisely controlled, which is to control the Bragg wavelength of the grating.
  • the laser's single-mode yield is high because the lasing wavelength is always The wavelength of the maximum feedback is the Bragg wavelength of the grating. The feedback of other modes is much weaker than the feedback of the other modes, so the threshold gain is much higher.
  • the cleavage surface of the conventional plated high reflective film can easily provide more than 90% of the reflection, which can lower the laser threshold, but since the position of the cleavage plane of the laser cannot be controlled sufficiently accurately, the reflection cannot be guaranteed with the gain region grating.
  • the reflections are in phase, which causes the laser's lasing wavelength to be not precisely controlled, and has a very serious single-mode yield problem.
  • the reflection region and the gain region of the distributed feedback laser have the same waveguide structure, and the waveguide transmission layer is the active layer 7, except that the metal electrode 3 is not present in the reflection region, no current is injected, and gain cannot be obtained, which is only for the laser.
  • the active and reflective regions of the distributed feedback laser should have the same equivalent refractive index without injecting current.
  • the equivalent refractive index of the active region will be somewhat different from that of the reflective region. If the gratings of the reflective region and the active region are of exactly the same period, the Bragg wavelengths of the two partial gratings will be slightly different, which will result in a lower equivalent reflection of the reflective region. Therefore, in practice, the period of the reflection region grating can be appropriately adjusted to compensate for this difference so that the Bragg wavelengths of the two-part grating continue to be consistent. But even if such compensation measures are not taken, we expect that the effect of the equivalent refractive index change due to the injection current is small.
  • the active region generates a gain due to the injected current.
  • this part of the gain is clamped near the threshold gain.
  • the threshold gain is close to 40 cm -1 .
  • the reflection region is pumped by the laser's own light output to a transparent state in which the quantum well of the waveguide core region produces neither gain nor absorption, and the net gain of the waveguide is -20 cm -1 . Therefore, the difference between the net gain of the reflection area and the gain area is about 60 cm -1 .
  • This part of the gain difference produces a corresponding effective refractive index difference, which is linked by a linewidth enhancement factor.
  • the linewidth enhancement factor is typically between 1 and 2. This means that the equivalent refractive index difference between the reflective and gain regions will be less than 0.005.
  • the relationship between the equivalent refractive index change and the threshold gain, the difference between the main mode and the side mode threshold gain is shown in Fig. 3.
  • the threshold gain is always below 40 cm -1
  • the threshold gain difference of the main mode side mode is always greater than 5 cm -1
  • the threshold gain difference of the main mode side mode is greater than 5 cm -
  • a distributed feedback laser of 1 can achieve a good single model. The simulation shows that even if the gain region and the reflection region adopt the same grating period, the gain threshold and side mode suppression ratio of the laser will not be greatly degraded.
  • a window region, or a cleavage plane inclined in the horizontal direction, or an anti-reflection coating may be used, so that the final cleavage surface reflection does not affect the laser. Performance.
  • the laser output of the distributed feedback laser is coated with an anti-reflection coating, and the reflectivity of the antireflection coating can be less than 1%.
  • the grating can provide additional reflection, so the laser lasing threshold can be reduced overall, so that the laser can be Work in the case where the length of the source portion is further reduced.
  • this part of the grating also generates additional losses and thus reduces the output efficiency of the laser. Therefore, the grating added at the output end face is not suitable for the grating with respect to the grating of the reflection portion.

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Abstract

一种短腔长分布反馈激光器，包含有增益区（1）和反射区（2）两个部分：增益区上存在金属电极，需要注入电流，给激光器提供增益；增益区的光栅层（5）为含有相移区的分布反馈布拉格光栅；增益区的一端为反射区，反射区含有与增益区相同的芯层结构，即波导传输层同样为有源量子阱材料，但是反射区不注入电流；反射区的光栅层为均匀分布的布拉格光栅；反射区的外端面镀增透膜或采用抗反射的结构来降低反射；增益区的另外一端为激光器的输出端面，输出端面镀有增透膜以减小反射。这样，可以使得激光器在腔长短时仍然具有较低的阈值增益和好的边模抑制比，同时具有制作难度小的特点。

Description

一种短腔长的分布反馈激光器 技术领域
本发明属于半导体激光器技术领域，涉及λ/4相移的分布反馈激光器。
背景技术
随着大容量超高速光纤通信技术的发展，高直调制速率的半导体激光器成长为光纤通信系统和下一代光网络的关键器件之一。在密集波分复用(DWDM)系统中，25Gb/s的高速直调制激光器可用于构成100Gb/s的下一代以太网的发射机。在局域网的发展中，25GHz调制带宽的高速直调制激光器则是实现40Gb/s光纤通信的关键器件，其中分布反馈(Distributed Feedback，DFB)激光器由于其高功率和单纵模特性好而受到国内外的广泛关注和研究。
分布反馈激光器采用折射率周期性变化的结构实现谐振腔的反馈，由于采用了平面工艺，可以应用于集成光路中与其他器件进行耦合和集成，适用范围非常广泛。目前分布反馈激光器有多种实现方案，其中最主要的方案有两种：一种是λ/4相移分布反馈激光器(Utaka K.，Akiba S.，Sakai K.，et al.，λ/4-shifted InGaAsP/InP DFB lasers[J].Quantum Electronics，IEEE Journal of，1986，22(7)：1042-1051.)；另一种是均匀光栅，两端面分别镀高反膜和增透膜的分布反馈激光器(Razeghi M.，Blondeau R.，Krakowski M.，et al.Low-threshold distributed feedback lasers fabricated on material grown completely by LP-MOCVD[J].Quantum Electronics，IEEE Journal of，1985，21(6)：507-511.)。
传统的λ/4相移分布反馈激光器都是两端镀增透膜的，这样整个激光器的反馈来自于光栅。因为两个端面现在不提供有效的反馈，导致端面解理面位置相对于光栅的不确定性的影响被消除了。由于采用了λ/4的相移，光栅的反 射峰值波长即光栅的布拉格波长，同时也是激光器腔的谐振波长，同时也将是激光器的激射波长。传统的λ/4相移分布反馈激光器虽然采用了更复杂的光栅增加了光栅制作的复杂性，但它能实现精确的激射波长控制、高的边模抑制比和高的成品率，因此在需要高成品率和精确波长控制的情况下，比如说分布反馈激光器的阵列，都无一例外的采用了λ/4相移的分布反馈激光器进行集成(Kobayashi W.，Kanazawa S.，Ohno T.，et al.，Monolithically integrated directly modulated DFB laser array with MMI coupler for 100 GBASE-LR4application[C]//Optical Fiber Communication Conference.Optical Society of America，2015：Tu3I.2.)。由于两端镀了增透膜，所以激光器的输出在两个端面是等功率的，但通常只有一端的输出能耦合到光纤中成为外界可用的功率输出。这意味着激光器的输出功率损失了一半。
另外，如果需要提高激光器的直调制带宽，通常采用的方法是减小激光器的腔长。比如说为实现30GHz以上的3dB直调制带宽，激光器的腔长需要减小到200微米以下。如果腔长减小到200微米或更短以后，传统的两端面镀增透膜的λ/4相移分布反馈激光器的阈值增益就会变得非常高，激光器性能就会变差。如果采用一端镀高反膜、另一端镀增透膜的方法把阈值增益降低，在腔长为150微米的情况下可以实现超过30GHz的带宽(Kobayashi W.，Tadokoro T.，Ito T.，et al.，High-speed operation at 50Gb/s and 60-km SMF transmission with 1.3-μm InGaAlAs-based DML[C]//Semiconductor Laser Conference(ISLC)，201223rd IEEE International.IEEE，2012：50-51.)，但激光器的边模抑制比和成品率就会大大降低，也就是说传统的λ/4相移分布反馈激光器的一些优势比如说高边模抑制比、高成品率、以及激射波长精确可控的优点就都消失了。
为解决上面的困境，一种比较好的解决方案是在传统λ/4相移分布反馈激光器的两端集成无源的分布布拉格反射区(如，Simoyama T.，Matsuda M.，Okumura S.，et al.，50-Gbps direct modulation using 1.3-μm AlGaInAs  MQW distribute-reflector lasers[C]//European Conference and Exhibition on Optical Communication.Optical Society of America，2012：P2.11.)。这两个反射区增强了整个激光器谐振腔的反馈因而能降低阈值增益，同时这两个反射区的反射也是基于光栅，其反射相位是可以通过无源波导区的材料生长以及后期的制作工艺精确控制的，所以不会降低激光器的边模抑制比与成品率，所以λ/4相移分布反馈激光器的优点仍然能够保留。但这样制作工艺变得非常复杂，主要是两端无源波导的集成需要对接再生长技术，其技术难度大，工艺复杂。
发明内容
为了解决上述技术问题，本发明提出了一种新的实现短腔长(200微米及以下)分布反馈激光器的方法。所述激光器分为两部分：有源区和反射区，两部分都含有光栅；有源区光栅存在λ/4相移区，有源区长度小于等于200微米；反射区在激光器非输出的一端，反射区光栅为均匀周期的光栅；激光器输出的一端的端面镀有增透膜。
所述光栅为布拉格反射光栅，其周期Λ按照以下公式计算：
Figure PCTCN2016086143-appb-000001
其中，m是光栅级数，λ是所述光栅所对应的布拉格波长，在该波长处光栅能产生最高的反射，neff为波导的有效折射率。优选的，所述的光栅级数m＝1。
所述的分布反馈激光器，所述的反射区与有源区的波导采用相同的芯层结构，反射区的波导传输层同样为有源量子阱材料。反射区的量子阱材料可以被激光器自身的出光泵浦到透明状态，因而量子阱的吸收损耗可以减小到零，最后波导的损耗即为有源区波导的内部损耗，通常的损耗在20cm-1量级。
所述分布反馈激光器，其反射区的长度可以根据所需的反射率进行自定义，长度越长，反射率越高，最高反射率可达到80％以上，主要受到波导内部损耗的限制。
所述分布反馈激光器，其反射区的光栅耦合系数可以根据所需的反射率进行自定义，并不一定需要与有源区的光栅相同，耦合系数越大，反射率越高，同样反射率可根据耦合系数调节到80％以上。
所述分布反馈激光器，反射区光栅可以采用与有源区光栅不一样的周期，这样即使当有源区和反射区的有效折射率不一样的时候，通过调整周期也可以使两者的布拉格波长一样，即反射峰的峰值波长一样。
所述分布反馈激光器，其反射区的外端面部分选择采用窗口区，或者带水平倾斜的端面，或者是镀增透膜，或者是上述多种方式的结合，来减小反射。
所述分布反馈激光器输出端镀增透膜，增透膜的反射率小于1％，以减小反射。
与现有的分布反馈激光器相比，本发明在反射区引入了布拉格光栅，能提高激光器谐振腔的反馈，相当于在有源区端面镀增反膜，因此可以减小阈值增益，使得激光器可以实现短腔长(200微米及以下)工作，这样可以提高激光器的直调制带宽；另外可以提高激光器输出的斜率效率，并且反射区光栅所产生的反射相位可控，因此不会导致边模抑制比的降低。同时激光器在有源区和反射区采用相同的波导芯层，反射区的形成不再需要复杂的对接再生长过程，因此具有制作难度小的特点。
附图说明
下面结合附图和具体实施方式对本发明的技术方案作进一步具体说明。
图1为本发明的分布反馈激光器的结构示意图。
图2为模拟得到的不同长度反射区的反射谱。
图3A为模拟得到的本发明激光器增益区和反射区等效折射率差与阈值增益的关系图。
图3B为模拟得到的本发明激光器增益区和反射区等效折射率差与主 模和边模阈值增益差的关系图。
具体实施方式
如图1所示，本发明的短腔长的分布反馈激光器包括有源区1，又称为光增益区和反射区2，有源区1从上而下包括电极接触层3、波导上盖层4、光栅层5、上光限制层6、量子阱层7、下光限制层8，波导下盖层9。反射区2从上而下包括波导上盖层4、光栅层5、上光限制层6、量子阱层7、下光限制层8、波导下盖层9。有源区部分的光栅包含有λ/4的相移区，相移区的两边的光栅为同周期的均匀光栅。λ/4的相移区的引入使得光栅的布拉格波长同时也是谐振腔的谐振波长，同时也是激光器的激射波长。这使得可以通过精确控制光栅的布拉格波长来控制激光器的激射波长。同时由于光栅只能在布拉格波长处提供最有效的反射，所以激光器的单模特性好。
在激光器的非输出的一端引入一段均匀分布布拉格光栅形成反射区，反射区的额外反馈可以使得激光器的阈值增益降低，因此能实现短腔长，同时反射区的反馈使得激光器输出端的输出功率增加，因此激光器的斜率效率可以得到提高。
布拉格反射光栅，其周期Λ按照以下公式计算：
Figure PCTCN2016086143-appb-000002
其中，m是光栅级数，λ是光栅所对应的布拉格波长，在该波长处光栅能产生最高的反射，neff为波导的有效折射率。
通过以下两种方法对反射区的反馈进行调节：
1.调节该段光栅的长度，在一定耦合系数的情况下，长度越长，反射区的等效反射率越大。
2.调节该段光栅的耦合系数，在光栅长度一定的情况下，耦合系数越大，反射区的等效反射率越大。
以上两种方法理论上可以提高反射区的反射率到接近于1，但是由于波导本身存在的固有损耗会使得反射率小于1，模拟设置波导的损耗为典型值20cm-1，得到的波长与等效反射率的关系如图2所示。可以看到在布拉格波长处反射率可以达到85％以上。由于光栅是通过高精度的制作方法加工而成比如说利用电子束曝光技术，反射区的光栅所提供反馈的相位可以精确控制，这样反射区所提供的反馈和增益区光栅提供的反馈可以保持同相位，这样保证了激光器能够在最大反馈波长处激射。这样有三个效果：第一是激光器的阈值可以维持在比较低的水准。当激光器的腔长减小的时候这一点就非常重要；第二是激光器的激射波长可以精确控制，就是控制光栅的布拉格波长；第三是激光器的单模成品率高，因为激射波长总是反馈最大的波长也就是光栅的布拉格波长，其他模式波长的反馈相比之下都要弱很多，因此阈值增益也要高很多。传统的镀高反射膜的解理面很容易提供高于90％的反射，可以降低激光器阈值，但由于激光器的解理面的位置不能足够精确的控制，因而该反射不能保证与增益区光栅的反射同相位，这导致激光器的激射波长不能精确控制，同时有很严重的单模成品率的问题。
该分布反馈激光器的反射区和增益区具有相同的波导结构，波导传输层都为有源层7，区别在于反射区不存在金属电极3，不注入电流，无法得到增益，对于激光器只是起到增大端面反射率的作用，因此在工艺上不需要额外的刻蚀和再生长技术，制作工艺简单。
该分布反馈激光器的有源区和反射区在不注入电流的情况下，其等效折射率应该是相同的。当有源区注入电流时，有源区的等效折射率与反射区会有些不同。如果反射区与有源区的光栅采用完全相同的周期，那么这两部分光栅的布拉格波长会有稍微的不同，这会造成反射区的等效反射降低。所以实际中可以适当调整反射区光栅的周期来补偿这部分差别以使得两部分光栅的布拉格波长继续保持一致。但即使不采取这样的补偿措施， 我们预计由于注入电流引起的等效折射率变化所造成的影响很小。我们做如下模拟：有源区由于注入电流会产生增益，当激光器激射以后这部分增益会被钳制在阈值增益附近，一般情况下阈值增益接近40cm-1。反射区会被激光器自身的光输出泵浦到透明状态，这种状态下波导芯区的量子阱既不产生增益也不产生吸收，这时候波导的净增益在-20cm-1。所以反射区与增益区的净增益的差在60cm-1左右。这部分增益差会产生相应的有效折射率差，它们之间是通过线宽增强因子联系起来的。由于高速直调的激光器通常采用InGaAlAs量子阱材料，线宽增强因子通常在1到2。这意味着反射区和增益区的等效折射率差会小于0.005。模拟有源增益区150微米，反射区长度分别为75微米时，等效折射率变化与阈值增益、主模和边模阈值增益差的关系，结果如图3所示。可以看出当等效折射率变化差n在0.005以内，其阈值增益一直在40cm-1以下，主模边模阈值增益差总是大于5cm-1，一般主模边模阈值增益差大于5cm-1的分布反馈激光器能得到一个好的单模特性。模拟表明即使增益区与反射区采用相同的光栅周期，激光的增益阈值和边模抑制比不会出现大的退化。
在激光器的反射区外端面部分，为减小解理面反射的影响，可以采用窗口区、或水平方向倾斜的解理面、或者镀增透膜等，使得最后得到解理面反射不影响激光器的性能。
分布反馈激光器的激光器输出端镀增透膜，增透膜的的反射率可以小于1％。
为进一步增加激光器谐振腔的反馈，还可以在激光器的输出端面加上一段和反射区相同的光栅，该光栅可以提供额外的反射，所以总体上可以降低激光器的激射阈值，使得激光器可以在有源区部分长度进一步减小的情况下工作。但这一部分光栅也会产生额外的损耗因而降低激光器的输出效率，所以相对于反射区部分的光栅，在输出端面添加的光栅不适合太长。
最后所应说明的是，以上具体实施方式仅用以说明本发明的技术方案 而非限制，尽管参照较佳实施例对本发明进行了详细说明，本领域的普通技术人员应当理解，可以对本发明的技术方案进行修改或者等同替换，而不脱离本发明技术方案的精神和范围，其均应涵盖在本发明的权利要求范围当中。

Claims (9)

  1. 一种短腔长的分布反馈激光器，其特征在于，所述激光器分为两部分：有源区和反射区，两部分都含有光栅；有源区光栅存在λ/4相移区，有源区长度小于等于200微米；反射区在激光器非输出的一端，反射区光栅为均匀周期的光栅；激光器输出的一端的端面镀有增透膜。
  2. 根据权利要求1所述的短腔长的分布反馈激光器，其特征在于，所述光栅为布拉格反射光栅，其周期Λ按照以下公式计算：
    Figure PCTCN2016086143-appb-100001
    其中，m是光栅级数，λ是所述光栅所对应的布拉格波长，在该波长处光栅能产生最高的反射，neff为波导的有效折射率。
  3. 根据权利要求2所述的短腔长的分布反馈式激光器，其特征在于，所述的光栅级数m＝1。
  4. 根据权利要求1所述的短腔长的分布反馈激光器，其特征在于，所述的反射区与有源区的波导采用相同的芯层结构，反射区的波导传输层同样为有源量子阱材料。
  5. 根据权利要求1或2所述的短腔长的分布反馈激光器，其特征在于，所述反射区的长度可以根据所需的反射率进行自定义调节，长度越长，反射率越高，所述反射区的最高反射率可达到80％以上。
  6. 根据权利要求1或2所述的短腔长的分布反馈激光器，其特征在于，所述的反射区的光栅耦合系数可以根据所需的反射率进行自定义调节，耦合系数越大，反射率越高，调节耦合系数能够使反射率达到80％以上。
  7. 根据权利要求1或2所述的短腔长的分布反馈激光器，其特征在于，反射区光栅可以采用与有源区光栅不一样的周期。
  8. 根据权利要求1或2所述的短腔长的分布反馈激光器，其特征在于，反射区后外端面部分选择采用窗口区，或者带水平倾斜的端面，或者是镀增透膜，或者是上述多种方式的结合。
  9. 根据权利要求1所述的短腔长的分布反馈激光器，其特征在于，所 述激光器输出端的增透膜的反射率小于1％。
PCT/CN2016/086143 2015-06-24 2016-06-17 一种短腔长的分布反馈激光器 WO2016206570A1 (zh)

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