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本文中研究的DFB激光器控制系统原理图如图 1所示。主要包括3个部分:参考电压设定模块、温度与电流驱动模块和DFB激光器模块。参考电压设定模块用于激光器温度和电流的设定,并将设定值以电压信号的方式输出到温度与电流驱动模块。温度与电流驱动模块的温度控制端和电流控制端连接DFB激光器模块两个蝶形底座的温度控制和电流控制接口,实时调节DFB激光器系统的温度值和电流值。图 1中的通用串行总线(universal serial bus, USB)数据采集卡(data acquisition, DAQ)用于稳定性试验中的数据采集。
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参考电压设定模块设计原理如图 2所示。主要包括DSP芯片(型号:TMS320F28335)和数模转换器(digital to analog conversion, DAC)(型号:DAC8552)。TMS320F28335的工作频率为150MHz,包含命令寄存器模块、逻辑控制模块和串行外设接口(serial peripheral interface,SPI)模块。DAC8552是16位双路输出数模转换器,包含命令寄存器和通道选择的逻辑电路,具有噪声小、稳定性好的特点。采用C#编写上位机(PC)软件,其与DSP芯片之间以RS-232串口方式进行通讯。
控制器工作时,由PC发送控制命令,设定控制温度T和电流I。DSP芯片根据所接收的命令通过SPI来控制数模转换器DAC8552输出的对应电压值。通过时序设计用通用输入/输出(general purpose input output,GPIO)管脚构建了一个1kHz的低速SPI通讯模块,用于控制两个DAC8552模块。数模转换器上的一个通道用于温度控制,另一个通道用于电流控制。本研究中,DAC8552输出电压幅值设定为5V,则输出电压的分辨率Vmin:
$ {V_{\min }} = \frac{{5{\rm{V}}}}{{{2^{16}}}} = 0.0763{\rm{mV}} $
(1) -
研究中采用蝶形封装DFB激光二极管(型号:NLK1E5GAAA),对激光器进行光隔离后,连接蝶形封装半导体光放大器(semiconductor optical amplifier,SOA)。激光二极管和光放大器分别安装在两个蝶形激光器底座上(型号:LM412)。DFB激光器内部集成热敏电阻与微型半导体制冷片,两者紧贴激光器二极管。热敏电阻的阻值与温度关系符合以下公式[17-18]:
$ {R_{{\rm{NTC}}}} = {R_{{T_0}}}\exp \left[ {B\left( {\frac{1}{T} - \frac{1}{{{T_0}}}} \right)} \right] $
(2) 式中,T为激光器温度,T0为25℃对应的开尔文温度,即298.15K,RT0为25℃时对应的热敏电阻阻值,即10kΩ,B为热敏指数,其值为3410K。
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温度与电流驱动模块控制原理如图 3所示。主要包括电流控制和温度控制两部分,分别对应LDTC0520内部的FL500芯片和WTC3243芯片。
FL500芯片具有高精度、慢启动的特点,用于激光二极管的电流控制。该芯片采用5V驱动电源供电,根据输入端参考电压输出电流,其输入电压转电流参量为250mA/V。研究中,其输入端电压来自DAC8552的流控参考电压设定值,其激光电流正负输出端分别连接DFB激光器模块蝶形底座的正负输入端,用于驱动DFB激光器和光放大器。DAC8552输出电压最小分辨率为0.0763mV,可以计算出FL500芯片最小可设置的输出电流间隔Imin:
$ \begin{array}{l} {I_{\min }} = k{V_{\min }} = \frac{{250{\rm{mA}}}}{{1{\rm{V}}}} \times \\ 0.0763{\rm{mV}} = 0.02{\rm{mA}} \end{array} $
(3) 式中,k是电压到电流的转换系数。
温控芯片WTC3243外围电路简单,采用5V驱动电源供电,主要管脚有参考电压端、PI参量设定端、加热制冷输出端和热敏电阻接入端。其比例系数和积分系数分别为20A/V和2.2s。该芯片加热制冷输出端和热敏电阻接入端分别连接到蝶形底座的对应端口。研究中,WTC3243热敏电阻接入端电流恒定为10μA,则热敏电阻阻值变化与温控参考电压设定值变化之间的关系为:
$ \begin{array}{l} \frac{{\Delta V}}{{\Delta T}} = - \frac{B}{{{T^2}}}\exp \left[ {B\left( {\frac{1}{T} - \frac{1}{{{T_0}}}} \right)} \right] = \\ \;\;\;\frac{{3410}}{{{T^2}}}\exp \left[ {3410\left( {\frac{1}{T} - \frac{1}{{{T_0}}}} \right)} \right] \end{array} $
(4) 式中,ΔV为电压变量,ΔT为温度变量。
由DAC8552输出电压最小分辨率为0.0763mV可以计算出,在25℃时可设置的最小温度变化值为1.99mK。该值随着温度增高而变大,30℃时为2.48mK。
分布反馈激光器温度与电流控制研究
Study on temperature and current control of distributed feedback laser diodes
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摘要: 为了精密控制分布反馈激光器的温度与电流, 采用数字信号处理芯片, 设计了分布反馈激光器驱动装置。通过该装置设定激光器温度和电流的参考电压, 经数模转换, 再通过温度和电流驱动模块, 馈入并驱动分布反馈激光器, 进行了实验验证。结果表明, 40min内温度变化极差与标准差分别不超过5mK和0.7mK, 电流变化极差与标准差不超过40μA和6μA; 驱动半导体光放大器, 关断时间小于1μs, 具有良好的瞬间响应特性; 该装置具有较高的温度和电流稳定性, 流控模块具有良好的瞬态特性, 能够精密控制分布反馈激光器的温度和电流。该控制装置可用于光腔衰荡光谱研究, 控制分布反馈激光器并驱动光放大器来关断激光。Abstract: In order to precisely control the temperature and current of distributed feedback lasers, the drive device of distributed feedback laser was designed by using digital signal processing chip.The device was used to set the reference voltage of the current and the temperature of the laser. After digital-to-analog conversion, through the temperature and current driving module, the reference voltage and the temperature were fed into the distributed feedback laser. And then, the device was used to drive distributed feedback lasers and experiments were carried out to verify the results. The results show that, within 40min, the temperature variation range and the standard difference are no more than 5mK and 0.7mK, respectively. The current variation range and the standard deviation are not more than 40μA and 6μA. When driving the semiconductor optical amplifier, the turn-off time is less than 1μs. The device has good instantaneous response characteristics and high temperature and current stability. The flow control module has good transient characteristics and can precisely control the temperature and current of distributed feedback lasers. The control device can be used to study optical cavity ring-down spectroscopy. It can controll distributed feedback lasers and drive the optical amplifiers to turn off the lasers.
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